Versatile Reactivity of Mn^II Complexes in Reactions with N-Donor Heterocycles: Metamorphosis of Labile Homometallic Pivalates vs. Assembling of Endurable Heterometallic Acetates

Abstract

Reaction of 2,2′-bipyridine (2,2′-bipy) or 1,10-phenantroline (phen) with [Mn(Piv)₂(EtOH)]_n led to the formation of binuclear complexes [Mn₂(Piv)₄L₂] (L = 2,2′-bipy (1), phen (2); Piv⁻ is the anion of pivalic acid). Oxidation of 1 or 2 by air oxygen resulted in the formation of tetranuclear Mn^II/III complexes [Mn₄O₂(Piv)₆L₂] (L = 2,2′-bipy (3), phen (4)). The hexanuclear complex [Mn₆(OH)₂(Piv)₁₀(pym)₄] (5) was formed in the reaction of [Mn(Piv)₂(EtOH)]_n with pyrimidine (pym), while oxidation of 5 produced the coordination polymer [Mn₆O₂(Piv)₁₀(pym)₂]_n (6). Use of pyrazine (pz) instead of pyrimidine led to the 2D-coordination polymer [Mn₄(OH)(Piv)₇(µ₂-pz)₂]_n (7). Interaction of [Mn(Piv)₂(EtOH)]_n with FeCl₃ resulted in the formation of the hexanuclear complex [Mn^II₄Fe^III₂O₂(Piv)₁₀(MeCN)₂(HPiv)₂] (8). The reactions of [MnFe₂O(OAc)₆(H₂O)₃] with 4,4′-bipyridine (4,4′-bipy) or trans-1,2-(4-pyridyl)ethylene (bpe) led to the formation of 1D-polymers [MnFe₂O(OAc)₆L₂]_n·2nDMF, where L = 4,4′-bipy (9·2DMF), bpe (10·2DMF) and [MnFe₂O(OAc)₆(bpe)(DMF)]_n·3.5nDMF (11·3.5DMF). All complexes were characterized by single-crystal X-ray diffraction. Desolvation of 11·3.5DMF led to a collapse of the porous crystal lattice that was confirmed by PXRD and N₂ sorption measurements, while alcohol adsorption led to porous structure restoration. Weak antiferromagnetic exchange was found in the case of binuclear Mn^II complexes (J_Mn-Mn = −1.03 cm⁻¹ for 1 and 2). According to magnetic data analysis (J_Mn-Mn = −(2.69 ÷ 0.42) cm⁻¹) and DFT calculations (J_Mn-Mn = −(6.9 ÷ 0.9) cm⁻¹) weak antiferromagnetic coupling between Mn^II ions also occurred in the tetranuclear {Mn₄(OH)(Piv)₇} unit of the 2D polymer 7. In contrast, strong antiferromagnetic coupling was found in oxo-bridged trinuclear fragment {MnFe₂O(OAc)₆} in 11·3.5DMF (J_Fe-Fe = −57.8 cm⁻¹, J_Fe-Mn = −20.12 cm⁻¹).

1. Introduction

Polynuclear coordination compounds of transition metals are widely used as catalysts in various reactions [1,2,3,4,5], as starting materials for the preparation of nanosized oxides or metals [6,7,8], as well as the basis for the creation of new magnetic materials [9,10,11,12,13] and coordination polymers with remarkable properties [14,15,16,17,18,19,20]. In the majority of these applications the stability or reactivity of the polynuclear core play an important role. In many cases polynuclear complexes undergo rearrangement or dissociation in solution because of instability in solvent or upon interaction with the “additional” ligands [21,22,23,24,25,26,27], and the resulting compounds may contain a metal core different from the one existing in the starting complex. Such rearrangement reactions lead to the formation of certain polynuclear cores, which are most stable under the reaction conditions. For example, binuclear cores M₂(O₂CR)₄ (M^II = Cu, Zn, Co, Ni) [28], trinuclear cores Fe^III₃ or Fe^III₂M^II (M = Ni, Co, Mn) [29,30,31,32], M^II₂Ln^III (M^II = Zn, Co, Ni; Ln is lanthanide) [33,34,35] often form due to their exceptional stability in various media. However, the result of such rearrangements is not always predictable, since many factors may influence the reaction pathway.

In this work manganese complexes were chosen as the objects of research due to some specific features of this ion, which are manifested in the reactivity of its polynuclear complexes [25,26]. The reasons for such differences include the wide range of stable oxidation states of manganese with low energy barriers for redox-transformations [36,37], as well as, in the case of Mn^II, the relatively high kinetic lability of this ion [36]. The additional reasons for interest to homo- and heterometallic manganese complexes is that such species have found applications as building blocks for the synthesis of single-molecule magnets [9,15,38] or various coordination polymers capable of absorbing guest molecules [39,40]. It is also known that molecular manganese compounds can oligomerize upon crystallization during solvent changes, giving rise to complexes with higher nuclearity [9,26,26].

The aim of this study was to reveal the stability limits of Mn^II homometallic (pivalate) or heterometallic (Fe₂Mn acetate) complexes in reactions with N-donor ligands (Scheme 1) and to compare the reactivity of Mn^II species with the reactivity of Ni^II and Co^II analogues. An additional aim of the study was to see the influence of the structure of new compounds on their ability to uptake guest molecules or on the magnetic exchange interactions in the polymetallic cores.

In this study we used pivalate manganese(II) polymer [Mn(Piv)₂(EtOH)]_n and trinuclear manganese(II)-iron(III) oxoacetate [MnFe₂O(OAc)₆(H₂O)₃]_n (Piv⁻ is the anion of pivalic acid) as starting compounds. Reactions of [Mn(Piv)₂(EtOH)]_n with both chelating (2,2′-bipyridine (2,2′-bipy) and 1,10-phenantroline (phen)) and non-chelating bridging (pyrimidine (pym), pyrazine (pz)) N-donor ligands in the absence/presence of oxygen as oxidant were investigated. In contrast to Ni^II and Co^II, the reaction of Mn^II pivalate with FeCl₃ led to formation of a compound with a Mn₄Fe₂O₂(Piv)₁₀ core. Reaction of the same Mn^II pivalate with a N-donor ligand led to the assembly of various polynuclear cores, while reaction of Fe^III₂Mn^II acetate with N-donors of the pyridine type led to the formation of new complexes where the trinuclear Fe^III₂Mn^II core was preserved. The crystal and molecular structures of eleven new compounds were determined—[Mn₂(Piv)₄L₂] (L = 2,2′-bipy (1), phen (2)), [Mn₄O₂(Piv)₆L₂]·MeCN (L = 2,2′-bipy (3·MeCN), phen (4·0.5MeCN)), [Mn₆(OH)₂(Piv)₁₀(pym)₄] (5), [Mn₆O₂(Piv)₁₀(pym)₂]_n (6), [Mn₄(OH)(Piv)₇(pz)₂]_n∙2nMeCN (7∙2MeCN), [Mn₄Fe₂O₂(Piv)₁₀(MeCN)₂(HPiv)₂]·2MeCN (8·2MeCN), [MnFe₂O(OAc)₆L₂]_n·2nDMF (L = 4,4′-bipy (9·2DMF), bpe (10·2DMF), [MnFe₂O(OAc)₆(bpe)(DMF)]_n·4nDMF (11·3.5DMF). Sorption of alcohols by porous coordination polymers, as well as the magnetic properties of several polynuclear Mn-containing complexes were studied.

2. Results and Discussion

2.1. Synthesis

The reaction of [Mn(Piv)₂(EtOH)]_n and 2,2′-bipyridine (2,2′-bipy) or 1,10-phenantroline (phen) in MeCN under an argon atmosphere led to formation of molecular complexes [Mn₂^II(Piv)₄L₂] (L = 2,2′-bipy (1), phen (2), Figure 1). Oxidation of these complexes or the initial reaction system resulted in rearrangement of the dinuclear molecules to tetranuclear complexes [Mn₂^IIMn₂^IIIO₂(Piv)₆L₂] (L = 2,2′-bipy (3), phen (4)).

Binuclear complexes 1 and 2 are similar to reported binuclear complexes of other d-metals with +2 charge [41,42,43,44,45,46,47,48]. The complex 3 is known and was obtained earlier by reaction of [Mn(Piv)₂(EtOH)]_n and 2,2′-bipy in THF in air [49].

Reaction of [Mn(Piv)₂(EtOH)]_n and pyrimidine (pym) in MeCN under an argon atmosphere led to the molecular hexanuclear complex [Mn₆(OH)₂(Piv)₁₀(pym)₄] (5). On exposure to air in MeCN compound 5 was oxidized to give the 1D polymer [Mn^II₄Mn^III₂(µ₄-O)₂(Piv)₁₀(µ₂-pym)(pym)]_n (6).

Previously reported carboxylate complexes of transition metals ions formed polymeric compounds with pyrimidine [50], so the formation of the molecular complex 5 is uncommon. At the same time, the formation of the mixed valence hexanuclear fragment {Mn^II₄Mn^III₂(µ₄-O)₂(Piv)₁₀} is similar to that in 5 and quite typical, for example, oxidation of [Mn(Piv)₂(EtOH)]_n (the same starting compound as in formation of 5) by air led to [Mn^II₄Mn^III₂(µ₄-O)₂(Piv)₁₀(HPiv)(EtOH)₃] [51]; a similar hexanuclear Mn₆ core was found in polymeric compounds [40,52]. Introduction of pyrimidine ligands resulted in formation of new coordination polymers.

The use of pyrazine (pz) instead of pyrimidine in the reaction with [Mn(Piv)₂(EtOH)]_n in MeCN under an argon atmosphere led to the formation of the 2D-coordination polymer [Mn₄(OH)(Piv)₇(µ₂-pz)₂]_n (7). This complex was stable in air and was not soluble in MeCN, and this reason probable precluding oxidation of Mn^II. Pyrazine is a quite typical bridging ligand in the chemistry of manganese, forming polymers with Mn^II carboxylates [53,54], as well as with the mixed-valence hexanuclear fragment {Mn^II₄Mn^III₂(µ₄-O)₂(O₂CR)₁₀} [54,55,56]. From an analysis of literature, as well as from the results of this study it can be noted that the tetranuclear unit {Mn^II₂Mn^III₂O₂(O₂CR)₆} usually forms in the presence of chelating ligands, such as 2,2′-bipy and phen, while the hexanuclear unit {Mn^II₄Mn^III₂O₂(O₂CR)₁₀} is produced in reactions with monodentate N-donor ligands or in the absence of such ligands.

It is known that the M^II ions (such as Mn^II, Co^II or Ni^II) and Fe^III ions quite typically form trinuclear acetates [Fe₂MO(OAc)₆(H₂O)₃] [30]. Earlier we reported that Co^II and Ni^II pivalates with FeCl₃·6H₂O in acetonitrile gave similar trinuclear complexes [Fe₂MO(Piv)₆(HPiv)₃] (M = Co, Ni) [19]. Unexpectedly, [Mn(Piv)₂(EtOH)]_n reacted with FeCl₃·6H₂O in MeCN under an argon atmosphere with formation of the hexanuclear complex [Mn^II₄Fe^III₂O₂(Piv)₁₀(MeCN)₃] (8). On the other hand, with an excess of pivalic acid in the reaction of FeCl₃·6H₂O, Mn(NO₃)₂·6H₂O and KOH [Mn^IIFe^III₂O(Piv)₆(HPiv)₃] is formed [57]. The trinuclear fragments {Fe₂MnO(Piv)₆} were also generated in situ in the synthesis of coordination polymers [58].

The synthesis of compounds 9–11 was based on substitution of coordinated water molecules in [Fe₂MnO(OAc)₆(H₂O)₃] by 4,4′-bipyridine (4,4′-bipy) or 1,2-trans-(4-pyridyl)ethene (bpe). In compounds 9 and 10 all vacancies in the coordination spheres of the metal ions are filled by the nitrogen atoms of 4,4′-bipy or bpe ligands, while 4,4′-bipy or bpe molecules act both as bridging and non-bridging (capping) ligands, as will be described in details in the X-ray structures description (vide infra). In compound 11 all bpe molecules link trinuclear blocks but only two of three possible “vacant” positions in the coordination spheres of metal ions are occupied by a pyridine group of bpe; the third position is filled by DMF.

Formation of coordination polymers 9–11 can be formally described as the generation of a 1D-chain of [MnFe₂O(OAc)₆(L)]_n (L = 4,4′-bipy or bpe) and filling of the third positions by terminal 4,4′-bipy and [MnFe₂O(OAc)₆(4,4′-bipy)₃]_n (for 9), by bpe or DMF (for 10 and 11, respectively). Synthesis of compounds [M₃O(RCO₂)₆(4,4′-bipy)₃]^0/+ which can potentially bind metal ions was reported earlier [59]. In contrast to the reaction of [Fe₂MO(OAc)₆(H₂O)₃] (M = Co, Ni) with 4,4′-bipy which led to destruction of the trinuclear blocks or to the formation of porous [MFe₂O(OAc)₆(4,4′-bipy)_1.5]_n coordination polymers [27], compound [Fe₂MO(OAc)₆(H₂O)₃] was stable under the same conditions and formed coordination polymers with a ratio of trinuclear block to bridging ligand equal to 1:1 or 1:2.

2.2. Crystal and Molecular Structures

The crystal structures of molecular complexes 1–5, 8 and coordination polymers 6, 7, 9–11 were determined by single crystal X-ray analysis.

2.2.1. Compounds 1 and 2

Complexes 1 and 2 have dinuclear cores (Mn…Mn 4.448(2) and 4.015(3) Å, respectively) with a common metal-carboxylate fragment {Mn₂(µ-Piv)₂(η-Piv)₂}. Despite their similar composition, the structures of these fragments are different (Figure 2a,b). The binuclear complex in 1 is centrosymmetric (the inversion center lies between the metal atoms), while compound 2 possesses axial symmetry (a 2₁ axis passes between the metal atoms). In both 1 and 2 oxygen donors occupy four positions in the coordination sphere of each of Mn^II ion, and two N atoms from 2,2′-bipy or phen complete the coordination spheres of these ions to form distorted octahedra (Figure 2a,b). The Mn–O and Mn–N bond lengths in 1 and 2 fall in range 2.039(4)–2.607(6) Å and 2.263(4)–2.342(4) Å respectively, which is typical for complexes of Mn^II with carboxylates and 2,2′-bipy or phen [44,45,60,61,62].

To the best of our knowledge, the dinuclear core in 1 is the first example of a Mn₂ structural unit block possessing a syn,syn-binding µ-COO-group and chelating 2,2′-bipy ligands. A compound of similar composition, Mn₂(ad(O₂C)₂)₂(2,2′-bipy)₂·0.5H₂O (where ad(O₂C)₂²⁻ is 1,3-adamantanedicarboxylate) [60] contained a dinuclear Mn₂(µ₂-O₂C)₂(η-O₂C)₂ unit with syn,anti-coordination mode of the µ-O₂C-groups.

The aromatic rings of 2,2′-bipy ligands of the neighboring molecules in 1 are not parallel, and the angle between the mean planes of these molecules is 7.6(3)°. The closest distance between centroids of pairs rings of different 2,2′-bipy ligands is 3.591(4) Å (the slippage is 0.707 Å). This leads to the formation of a supramolecular chain along the c axis (Figure 3b), probably due to π-stacking interactions.

The aromatic rings of two phen ligands in one molecule of 2 are not parallel, and the angle between the mean planes of these molecules is 9.8(2)°. The closest distance between centroids of pairs rings (N2C18-C22, C14-C19), belonging to these different phen ligands, is 3.695(4) Å (the slippage is 1.055 Å). The mean planes of phen ligands from the neighboring different molecules of 2 are parallel and the centroids of these pairs of phen rings are located in 3.730(4) Å (the slippage is 1.561 Å). Such an arrangement of aromatic phen molecules can allow for π-stacking interactions between them and as a result formation of a supramolecular pile structure along the c axis (Figure 3c). However, it cannot be excluded that intramolecular π-stacking between phen molecules in 2 may be the reason for the difference between structures of this compound and 1: the 2,2′-bipy molecules in 1 are located on different sides in respect to the inversion center, located between the Mn^II ions.

2.2.2. Compounds 3 and 4

The molecules of compounds 3 and 4 are centrosymmetric (an inversion center lies between the central metal ions) and possess a similar tetranuclear Mn₄O₂(Piv)₆ core (Figure 3a) of a “butterfly” type, which is quite typical for Mn carboxylates [63,64,65,66]. Each Mn^III ion in the center of the butterfly is located in a square pyramidal coordination polyhedron (MnO₅ chromofore, τ = 0.15 [63,64,65,66]), while Mn^II ions on the wings of the butterfly are hexacoordinate and donor atoms in their coordination environment form highly distorted octahedra (MnO₄N₂ chromophores, where N atoms belong to 2,2′-bipy or phen in 3 and 4, respectively). Mn^III–(µ₃–O) and Mn^II–(µ₃–O) bond lengths in 3 and 4 fall in range 1.843(2)–1.851(2) Å and 2.073(3)–2.099(2) Å, respectively, which is typical for Mn-O distances within trinuclear units Mn^II₂Mn^III₂O₂ [63,64,65,66]. Mn^III–O and Mn^II–O (O atoms from pivalate) bond lengths fall in range 1.957(2)–2.097(2) and 2.116(2)–2.213(3) respectively for 3, and 1.955(3)–2.105(3) Å and 2.105(3)–2.196(3) Å respectively for 4, making these bonds longer than the corresponding bonds of Mn ions with µ₃-O atoms. Terminal Mn^II atoms fill up their coordination sphere by coordination of 2,2′–bipy or phen with Mn–N bond lengths 2.271(3), 2.284(3) Å for 3, and 2.256(4), 2.284(4) Å for 4.

2.2.3. Compound 5

Compound 5 crystallizes in the monoclinic space group P2₁/n as a discrete centrosymmetric hexanuclear complex (the inversion center lies between the central metal ions Mn2, Mn3, Mn2A and Mn3A). The hexanuclear core of 5 can be considered as two identical triangular fragments {Mn₃(OH)(Piv)₃(pym)₂} linked by four carboxylic acid groups (two µ₂-Piv and two µ₃-Piv) (Figure 4a). In each trinuclear fragment Mn^II ions are linked by µ₃-OH (bond lengths Mn–O are equal to 2.049(2)–2.184(2) Å), and the O atom is located on 0.66(2) Å above the Mn1Mn2Mn3 plane, which can be an additional proof that the central oxygen atom belongs to a µ₃-hydroxo group rather than a µ₃-oxo (Mn₃(µ₃-O) unit that is expected to be planar [67,68,69,70]). One µ-O₂C bridging group links Mn1 and Mn2, and two µ₂-O₂C groups link Mn1 with Mn3 (bond lengths Mn–O(Piv) and lie in the 2.095(3)–2.142(3) Å) range. In addition to the oxygen atoms of carboxylate groups, the nitrogen atoms of two pyrimidine molecules are coordinated to Mn1, completing its coordination polyhedron to form a distorted octahedron (Mn1–N bond lengths are 2.293(3) and 2.322(3) Å). Mn2 is located in the distorted tetrahedral donor set O₄ (Mn2–O(Piv) bond lengths are in range 2.067(3)–2.097(3) Å)), and Mn3 is in a distorted octahedral donor set O₆ (Mn3–O(Piv) bond lengths are in range 2.094(3)–2.262(2) Å)).

2.2.4. Compound 6

This compound crystallizes as a 1D polymer in the space group Pn, in which hexanuclear units {Mn^II₄Mn^III₂O₂(Piv)₁₀} are linked by pyrimidine bridges (Figure 4b). Generally, the structure of the Mn₆ core in 6 is similar to that of the hexanuclear units observed in [M^II₄M^III₂(O)₂(O₂CR)₁₀(L)₄] complexes, where L is a neutral N- or O-donor ligands [71,72,73,74] with the difference that one Mn4 atom in 6 possesses coordination number four and is located in a coordination polyhedron, close to a distorted square-pyramid (τ = 0.12) [75]. Central Mn^III ions (Mn1…Mn2 2.835(3) Å) possess O₆ donor sets (Mn–O(µ₄–O) 1.878(7)–1.914(8) Å, Mn–O 1.933(9)–2.267(9) Å), three of four terminal Mn^II ions are located in O₅N donor sets (Mn–O(µ₄–O) 2.019(8)–2.200(8) Å, Mn–O 1.970(10)–2.457(10) Å, Mn3–N1 2.255(12), Mn5–N3 2.511(16), Mn6–N4 2.471(18) Å), where N is an atom of a bridging (N3 and N4) or non-bridging (N1) pyrimidine molecule. In the crystal lattice 1D chains of 6 are arranged parallel to the a axis. The crystal lattice of 6 is retained upon sample storage in air, as confirmed by powder XRD (Figure S1, Supporting information).

2.2.5. Compound 7

This complex crystallizes in the triclinic space group P-1 as a solvate with two molecules of MeCN. The tetranuclear core {Mn^II₄(OH)(Piv)₇} (Figure 5a) in 7 can be described as a trinuclear µ₃-hydroxo-centered unit Mn₃(OH)(Piv)₄ (Mn…Mn 3.371(2)–3.735(2) Å), linked with the fourth Mn4 ion (Mn4…Mn 3.451(2), 4.157(2) Å) by four pivalate anions. Such a Mn₄ unit is not symmetric. The Mn₃(OH)(Piv)₄ bond lengths of Mn–O1M fall in the 2.131(2)–2.134(2) Å range, with atom O1M located above Mn1Mn2Mn3 plane at 0.64(3) Å, d(Mn–O(Piv) = 2.111(3)–2.238(3) Å. The Mn1 ion is bound to two pyrazine ligands (Mn1–N = 2.346(4), 2.389(4) Å), while the Mn3 and Mn4 ions are bound to one molecule of pyrazine (Mn3–N 2.277(4) Å, Mn4–N 2.315(4) Å). Thus, the Mn1, Mn2 and Mn3 ions possess distorted octahedral donor sets O₄N₂, O₆ and O₅N, respectively, and the Mn4 ion is located in a distorted square-pyramidal coordination environment (O₄N donor set; τ = 0.08) [75]. All pyrazine molecules in 7 are bridging. Local symmetry centers of the unit cell are located in the centers of the pyrazine rings which link Mn1-Mn1′ and Mn3-Mn3′ ions.

Each Mn₄ unit is connected with other four Mn₄ units by four pyrazine bridges, while each pyrazine links two tetranuclear units. Such an arrangement results in the formation of a 2D polymer. The 2D-layers are parallel to the bc plane (Figure 5b,c).

2.2.6. Compound 8

The complex 8 crystallizes in the monoclinic space group C2/c as a solvate with four molecules of MeCN. The molecule of 8 has axial symmetry, with axis 2 passing between the Fe1 and Fe1A atoms through the O1M, O2M, C21, C22, C26 and C27 atoms. The structure of the complex is similar to that of the known hexanuclear complexes [Mn^II₄Mn^III₂(O)₂(O₂CR)₁₀(L)₄], where L is a neutral N- or O-donor ligans [71,72,73] but where the central atoms are Fe^III instead of Mn^III. The central Fe^III ions (Fe1…Fe1A 2.8824(10) Å) possess O₆ donor sets (Fe–O(µ₄-O) 1.953(2), 1.962(2) Å, Fe–O(Piv) 2.020(2)–2.070(2) Å), two terminal Mn^II ions (Mn…Fe 3.1806(8)–3.5075(8) Å, Mn…Mn 3.4831(11), 3.4841(11) Å) are located in O₅N donor sets (Mn–O(µ₄-O) 2.085(2) Å, Mn–O(Piv) 2.101(3)–2.492(2) Å, Mn-N 2.343(4), where N is an atom of a MeCN molecule, and two terminal Mn^II ions are located in O₆ donor sets (Mn–O(µ₄-O) 2.086(2) Å, Mn–O(Piv) 2.104(3)–2.522(2) Å, Mn–O(HPiv) 2.240(3) (Figure 6). H-bonds are formed between of a coordinated molecule of acid and the O atom of a bridging carboxylate group (O12…O4 2.603(4) Å, O12–H 0.84 Å, O4…H 1.77 Å, angle O12–H–O4 169°).

2.2.7. Compounds 9–11

Coordination polymers 9–11 are built by linking a neutral trinuclear block, {Fe₂MnO(OAc)₆}, with neutral pyridine-containing bridges. The structure of the {Fe₂MnO(OAc)₆} unit in all these complexes is almost the same (such unit in compound 9 is shown on Figure 7 as example). In this block three metal ions (two Fe^III and Mn^II) are located in the corners of an irregular triangle and generally cannot be distinguished by X-ray crystallography, so the assignment of metal ions was arbitrary. These metal ions are linked by μ₃-O atoms and six bridging acetates, so oxygen donors occupy five positions in the coordination sphere of each metal ion. The sixth positions are taken up by a donor atom (N or O) from other ligands (4,4′-bipy, bpe or DMF), so that each metal ion is located in a distorted octahedral donor set.

M---M separations within trinuclear acetates in compounds 9–11 fall in range 3.240(2)–3.353(2) Å. M–(μ₃-O) bond lengths lie in the range from 1.837(6) to 2.056(7) Å, M–O(carboxylate) bonds vary between 2.011(6) to 2.155(8) Å, which is typical for μ₃-oxocentered carboxylates [30].

In the crystal lattice of 9 trinuclear μ₃-oxocentered units {MnFe₂O(OAc)₆} are connected by 4,4′-bipy molecules. 3/4 of trinuclear blocks are linked by 4,4′-bipy with formation of a 1D zig-zag chain. Two metal ions from each Fe₂Mn unit take part in such chain formation, while the third metal ion is bound to a non-bridging 4,4′-bipy molecule or a “terminal block” {MnFe₂O(OAc)₆(4,4′-bipy)₃}. Thus, there are three types of trinuclear MnFe₂ blocks in 9 (Figure 8):

(1)

MnFe₂ units in 1D chains, bound to two bridging 4,4′-bipy and one additional non-bridging 4,4′-bipy (type A);

(2)

MnFe₂ units in {MnFe₂O(OAc)₆(4,4′-bipy)₃} residues (type B);

(3)

MnFe₂ unit in 1D chains, bound to two bridging 4,4′-bipy and one type B unit (type C).

The distance between µ₃-O atoms of neighboring Fe₂Mn blocks in one 1D chain in 9 is equal to 15.222(8) Å in the case of AA blocks, 15.323 Å for AC blocks and 15.49(1) Å for BC blocks, while the angles between lines connecting the µ₃-O atoms of the neighboring trinuclear blocks, are close to 120° (from 116.83(4)° to 120.99(2)°). Torsion angles between lines connecting four µ₃-O atoms of adjacent Fe₂Mn blocks, are equal to 180 ° for a CAAC fragment (i.e., all µ₃-O atoms for this fragment belong to one plane) and ±119.64(5)° for ACAA and AACA fragments. In other words, one 1D chain of 9 turns clockwise twice by 119.64(5)°, as implied by the torsion angles for ACAA or AACA, then all µ₃-O atoms lie in one plane in the CAAC fragment and finally the chain turns twice counterclockwise by 119.64(5)° in the ACAA and AACA fragments. The main axes of chains in compound 9 are directed along the (c-½a) vector (Figure 9).

The twofold axis passes through µ₃-O atoms of trinuclear blocks B and C. Also the local inversion centers are located between the pyridine groups of 4,4′-bipy molecules which bind two A type trinuclear blocks. The crystal lattice of 9 does not contain continuous channels (see Figure S2, Supporting Information).

2.2.8. Complexes 10 and 11

These are built from trinuclear units {Fe₂MnO(OAc)₆} bound by bpe molecules (Figure 10a). Two metal ions in each trinuclear block coordinate with pyridine rings from bridging bpe, leading to 1D-chain formation. The third metal ion is bound to a nitrogen atom of a terminal (non-bridging) bpe in 10 or oxygen atom of coordinated DMF in 11 (Figure 10b).

Chains of compound 10 are parallel and directed along the a-½b vector. The non-coordinated pyridine ring of bpe in one chain and the pyridine ring of a bridging bpe ligand from the neighboring chain are almost parallel (the angle between mean planes of these rings is 4.1(5)°), and the closest distance between these rings is 3.38(2) Å (the distance between centroids of the rings is 3.634(6) Å), the slippage is 1.179 Å, which can allow for π-interactions (Figure 11a).

Chains of compound 11 are also parallel and directed along the c vector (Figure 11). No specific interactions between different chains are found. Due to this peculiarities of the chain packing channels of dimensions 5 × 12 Å directed along the a axis form in the crystal lattice (Figure 12). Estimation of solvent-accessible volume, performed by PLATON software [76], gives a value of 36% for 11, containing DMF molecules coordinated to metal ions, or 45% for a structure, if coordinated DMF is removed assuming that such removal does not lead to crystal lattice collapse (calculated for a probe molecule with r = 1.4 Å). These values correspond to ca. 0.32 cm³ g⁻¹ pore volume, occupied by solvent in 11, assuming that the volume occupied by coordinated DMF is not included in this value, or 0.40 cm³ g⁻¹, if the volume of coordinated DMF is included.

2.3. Thermal Stability and Sorption Properties of 11·3.5DMF

The thermal stability of 11·3.5DMF was studied by thermogravimetry. Upon heating to 275 °C, compound 11·3.5DMF lost 26% of its weight, which corresponds to the release of both non-coordinated and coordinated solvent (Figure 13). An abrupt weight loss began at 275 °C, which was completed at 400 °C and could be associated with decomposition of the compound. The total weight loss was equal to 76.6% and corresponded to the formation of Fe₂O₃·1/3Mn₃O₄ (Figure 13a). Loss of solvent and coordinated DMF led to significant lattice disorder, as it can be concluded by comparison of the powder XRD pattern of vacuum-dried product at 145 °C and the powder XRD pattern, calculated from the single-crystal structure (Figure 13b).

For sorption experiments compound 11·3.5DMF was heated in vacuum at 153 °C during 6 h, which led to removal of non-coordinated and coordinated DMF. The desolvated sample is hereinafter referred to as 11′.

Compound 11′ showed only surface sorption of N₂ or H₂ at 78 K, which is evidence of crystal lattice collapse and is consistent with the powder XRD data. In contrast, 11′ absorbed significant quantities of methanol and ethanol at 298 K (Figure 14). Such a difference between absorption of gases and alcohols can be caused by expansion of crystal lattice of 11′ upon interaction with methanol and ethanol, similarly to reported gate-opening phenomena [77] and previously reported cases of alcohol absorption by coordination polymers [18]. Both in the cases of methanol and ethanol, the sorption capacity gradually increased to ca. 0.42 cm³·g⁻¹ (methanol) or 0.35 cm³·g⁻¹ (ethanol), which is in good agreement with the value of solvent-accessible volume estimated from the crystallographic data (vide supra).

The plateau in the methanol absorption isotherm at PP_S⁻¹ ca. 0.07–0.3 (V_abs. about 0.05 cm³ g⁻¹) corresponds to a methanol to Fe₂Mn molar ratio 1:1 and can be associated with methanol coordination to the metal ion (in a position which was occupied by coordinated DMF in 11). It can be concluded from the presence of such a plateau that there is a noticeable difference between the energy of methanol coordination to a metal ion in 11′ and the energy of further methanol interaction with 11′·CH₃OH. In contrast, a similar plateau was not found in the ethanol absorption isotherm: ethanol binding by 11′ after filling of unsaturated metal sites seems to be as efficient as ethanol binding due to its coordination (which most probably does occur). Anyhow, the maximal achieved sorption capacity of 11′ corresponds to ca. 10.5 moles of methanol or ca. 5 moles of ethanol per 1 mole of Fe₂Mn, which is significantly higher than the sorption capacity associated with coordination.

2.4. EPR Spectroscopy

X-band EPR experiments for polycrystalline samples 1 and 2 were performed at 293 K. The spectra of 1 and 2 show an intense singlet without hyperfine structure with g ≈ 2.00. In the low magnetic field lines of low intensity are observed; their origin can be explained by the exchange interactions between paramagnetic manganese ions (Figure 15).

Since antiferromagnetic interactions with J = −1.03 cm⁻¹ were found in 1 and 2 by magnetic data analysis (see below), all possible magnetic states of dimer of two Mn²⁺ ions with spins S_1,2 = 5/2, notably, S = 0, 1, 2, 3, 4, 5 (S = S₁ + S₂) were equally populated. Furthermore, since |J| > hν ≈ 0.3 cm⁻¹, transitions between states with different total spin S can be neglected. Thus, the spin Hamiltonian for 1 and 2 is the sum of spin Hamiltonians of five dimers with different total spins. The spin Hamiltonian (1) for single ion in 1 or 2 has a rhombic symmetry:

$${\hat{H}}_i=g_{iz}\beta H_z{S}_{iz}+g_{ix}\beta H_x{S}_{ix}+g_{iy}\beta H_y{S}_{iy}+d_i\cdot (S_{iz}^2-\frac{1}{3}{S}_i(S_i+1))+e_i\cdot (S_{ix}^2-S_{iy}^2),$$

(1)

where g_iz, g_ix, g_iy—z, x, y—g-tensor components of monomer i, where i = 1, 2; S_iz, S_ix, S_iy–projections of spin operator of monomer on coordinate axes, S_i = 5/2; d_i, e_i—component of fine interaction tensor (so-called, single-ion). Mn²⁺ ion has half-filled d⁵ shell and S-state, so g-tensor is isotropic and close to spin-only value, so, g_iz = g_ix = g_iy = g = 2.0023.

Spin Hamiltonian (2) of dimer is the sum of two Hamiltonians for interactions within mononuclear fragments of molecule and the part of their interaction:

$$\begin{array}{l}\widehat{H}={\widehat{H}}_1+{\widehat{H}}_2-2J{S}_1{S}_2+d_{12}\cdot (S_{1z}\cdot S_{2z}-\frac{1}{3}{S}_1\cdot S_2)+\\ e_{12}\cdot (S_{1x}\cdot S_{2x}-S_{1y}\cdot S_{2y})\end{array}$$

(2)

where d₁₂, e₁₂ are the components of fine interaction tensor, caused by dipole interaction of manganese ions.

For the total spin of dimer and neglecting of transitions between multiplets with different total spin S = S₁ + S₂, spin Hamiltonian (3) can be used:

$$\begin{array}{l}{\widehat{H}}_S=g\beta (H_z{S}_z+H_x{S}_x+H_y{S}_y)+D_S\cdot (S_z^2-\frac{1}{3}S(S+1))+E_S\cdot (S_x^2-S_y^2)\\ -2J(S(S+1)-S_1(S_1+1)-S_2(S_2+1))\end{array}$$

(3)

where D and E are the components of fine interaction tensor, associated with parameters d_1,2, e_1,2, d₁₂ и e₁₂ by the following formula [78]:

$$D_S={\alpha }_S{d}_{12}+{\beta }_S{d}_1 E_S={\alpha }_S{e}_{12}+{\beta }_S{e}_1$$

$${\alpha }_S=\frac{S(S+1)+4S_1(S_1+1)}{2(2S-1)(2S+3)} {\beta }_S=\frac{3S(S+1)-3-4S_1(S_1+1)}{(2S-1)(2S+3)}$$

(4)

The spin Hamiltonian (3) was diagonalized numerically. Calculations of resonance fields of spin Hamiltonian (3) required to build a theoretical spectrum were carried out by the Belford method [79], which involves finding the values of magnetic field H, for which two eigenvalues of spin Hamiltonian (3) matrix, corresponding to two different eigenvectors, would differ on the hν.

The spin Hamiltonian parameters for compounds 1 and 2 are given in

Table 1. Spin Hamiltonian parameters for 1 and 2.

Compound	G	d1 (cm−1)	e1 (cm−1)	d12 (cm−1)	e12 (cm−1)
1	2.0023	0.00098	0.0105	0.0038	0.0067
2	2.0023	0.0368	0.0063	0.0347	0.0074

. Thus, while isotropic exchange of two dimers is same, fine interaction tensor causes a noticeable difference in the EPR spectra.

2.5. Magnetic Properties of Complexes 1, 2, 7·2MeCN and 11·3.5DMF

Magnetic properties of the representative complexes from the prepared series—compounds 1, 2, 7·2MeCN and 11 3.5DMF—were characterized by their temperature dependence of the molar magnetic susceptibility, χ_M.

2.5.1. Magnetic Properties of Complexes 1 and 2

For both compounds χ_MT values (here and below χ_M is the magnetic susceptibility per formula unit and T is the temperature in K) monotonously decrease upon lowering the temperature from 8.17 (for 1) or 8.64 (for 2) cm³·K·mole⁻¹ at 300 K to 7.81 (for 1 at 60 K) or 7.74 (for 2 at 68 K) cm³·K·mole⁻¹, after which it falls sharply to 4.01 (for 1 at 5 K) or 0.82 (for 2 at 2 K) cm³·K·mole⁻¹. Room-temperature values of χ_MT are close to the expected spin-only value (8.75 cm³·K·mole⁻¹ for a system with two non interacting magnetic centers with S = 5/2).

The spin Hamiltonian for dinuclear blocks Mn₂ in 1 and 2 is shown as Equation (5).

$$\widehat{H}=-2J_{Mn-Mn}{\widehat{S}}_1{\widehat{S}}_2+\beta H{g}_{Mn}({\widehat{S}}_1+{\widehat{S}}_2)$$

(5)

where the first summand corresponds to the superexchange interactions between Heisenberg spins localized at metal sites (J_Mn-Mn), and the second summand corresponds to the isotropic interactions between local spins and the external field through Zeeman interactions [80].

It should be noted that the spin-Hamiltonian proposed for interpretation of the magnetic properties differed from the one employed for interpretation of the EPR spectra of the same compounds. There was no contradiction between these Hamiltonians, as both of them were “partial” variations of the complete spin-Hamiltonian describing the system of the unpaired electron within the species of the compounds. This complete spin-Hamiltonian had to include all the terms: (i) Zeeman interactions with the external magnetic field; (ii) exchange interactions between the ions; (iii) zero-field splitting. However, the variations of exchange interaction parameters could not notably influence the studied EPR spectra (vide supra), while the influence of zero-field splitting on the magnetization curves was negligible compared to the influence of the exchange interactions. Thus, introduction of the corresponding terms into the spin-Hamiltonians and efforts to extract the corresponding parameters from the data simulations would not produce any reliable values. Regarding Zeeman interactions, their principal parameters—g-factors—were consistent (within accuracy of the methods) for the EPR and magnetochemical data.

Temperature-independent paramagnetism (tip) term was also introduced. Intermolecular interactions were taken into account within molecular field model (zJ’ term).

Analytical expression for the χ_MT values for the Mn₂ unit [80] is the following:

$${\chi }_{M}T=\frac{2N{g}^2{\mu }_B^2}{k}\cdot \frac{e^x+5e^{3x}+14e^{6x}+30e^{10x}+55e^{15x}}{1+3e^x+5e^{3x}+7e^{6x}+9e^{10x}+11e^{15x}},$$

(6)

where x = −J/kT.

The best agreement between experimental and calculated χ_MT curves (Figure 16) was achieved at parameters J_Mn-Mn = −1.03(2) cm⁻¹, g_Mn = 2.0, zJ’ = −0.19(1) cm⁻¹ (R² = 2.44·10⁻⁵) for compound 1 and J_Mn-Mn = −1.03(2) cm⁻¹; g_Mn = 2.0, tip = 7·10⁻⁴ (R² = 9.87·10⁻⁵) for compound 2 (where R² = ∑[(χ_MT)_obs. − (χ_MT)_calc.]²/(∑(χ_MT)_obs.²)).

Absolute values of J_Mn-Mn for 1 and 2 are higher than those reported for benzoato- and phtalato-bridged dinuclear blocks Mn₂(µ-O₂C)₂(η-O₂C)₂ [81,82], which is consistent with the higher electron-donating ability of the tert-butyl groups in pivalates compared to phenyl group.

2.5.2. Magnetic Properties of Complex 7 ·2MeCN

For the compound 7 2MeCN the χ_MT value monotonically decreases upon lowering the temperature from 15.78 cm³·K·mole⁻¹ at 300 K to 0.53 cm³·K·mole⁻¹ at 3 K. The room-temperature value of χ_MT is lower than the expected spin-only value (17.5 cm³·K·mole⁻¹ for a system with four non-interacting magnetic centers with S = 5/2). The coupling scheme within a tetranuclear unit is presented in Figure 17. The spin Hamiltonian for tetranuclear block Mn₄ takes the form:

$$\begin{array}{c}H=-2J_1{S}_{Mn1}\cdot S_{Mn3}-2J_2{S}_{Mn1}{S}_{Mn2}-2J_3{S}_{Mn2}\cdot S_{Mn3}-2J_4{S}_{Mn3}{S}_{Mn4}\\ -2J_5{S}_{Mn2}{S}_{Mn4}+g_{Mn}\beta (S_{Mn1}+S_{Mn2}+S_{Mn3}+S_{Mn4}+S_{Mn5})\cdot H\end{array}$$

(7)

where the first five summands correspond to the superexchange interactions between Heisenberg spins localized at metal sites (J₁–J₅), and the last summand corresponds to the isotropic interactions between local spins and the external field through Zeeman interactions (g_Mn), respectively [80]. A temperature-independent paramagnetism (tip) term was also introduced.

Calculation of the exchange coupling parameters were performed by full-matrix diagonalization using the Mjöllnir software [16,83]. Uncertainty values of simulation parameters were estimated as described previously [33]. Briefly, digits in brackets indicated deviation of the value, which caused 10% increase of R².

The best correspondence between experimental and calculated χ_MT values for compound 7 was achieved for the parameters J₁ = −2.69(2) cm⁻¹, J₂ = −2.38(2) cm⁻¹, J₃ = −0.8(1) cm⁻¹, J₄ = −0.42(2) cm⁻¹, J₅ = −0.8(2) cm⁻¹, g_Mn = 2.0023 (fixed), zJ’ = −0.5(1) cm⁻¹, tip = 0.00129 (R² = 2.3·10⁻⁵).

Exchange coupling parameters have larger values for magnetic interactions of Mn1 ion with Mn2 and Mn3 which agree with the structural data: the Mn1 ion has in its coordination sphere two N atoms from two pyrazine molecules which increase its electron density and amplify the antiferromagnetic interactions. Additionally, the most effective way of magnetic interactions transfer through the OH group in the trinuclear Mn1Mn2Mn3 unit which also agrees with the received data.

The Mn₄ units can be selected in the 2D-coordination polymer [Mn₄(μ₃-OH)(Piv)₇(μ-pz)₂]_n, and exchange coupling within these units can be presented by five integrals J₁–J₅ (Figure 17). From the experimental χ_MT vs. T curve the following values could be calculated by full-matrix diagonalization (performed using the Mjöllnir software [16,83]) (cm⁻¹): J₁ = −2.7, J₂ = −2.4, J₃ = −0.8, J₄ = −0.4, J₅ = −0.8. Signs and magnitudes of these J were estimated using broken-symmetry DFT calculations with TPSSh functional and LANLTZ ECPs (3d ions)/def2-SVP basis set. For calculation of each J value all Mn²⁺ ions in the Mn₄ core except the two ions taking part in the coupling were “substituted” by diamagnetic Zn²⁺ ions, and then calculations of the high-spin (HS) and the broken symmetry (BS) states’ energies (see Experimental part for details of their construction) were performed. The results evidence that two exchange integrals, J₁ and J₂, have the same order of magnitude (−6.9 and −6.4 cm⁻¹, respectively); while J₃, J₄, J₅ fall in the range from −0.9 to −1.2 cm⁻¹. The exchange through pz bridge is estimated as −0.2 cm⁻¹. These results correlate with J values, found from χ_MT vs. T curve simulation (|J₁|, |J₂| the highest and close to each other, |J₃|–|J₅|—lower and close to each other).

It should be noted that fitting of the χ_MT vs. T curve for 7 2MeCN could be fitted with simpler Hamiltonian (8):

$$\widehat{H}=-2J_1(S_1{S}_2+S_3{S}_4)-2J_2 S_2{S}_3$$

(8)

with J₁ = −2.08 ± 0.02 cm⁻¹, J₂ = −3.92 ± 0.07 cm⁻¹, g = 2 (fixed), however there are no reasons to neglect interactions between other Mn^II ions, since structural features of the bridges between them are similar. The χ_MT vs. T curve calculated with these parameters is visually the same as shown on Figure 17.

2.5.3. Magnetic Properties of Complex 11·3.5DMF

For compound 11·3.5DMF, the value of χ_MT at 300 K was 4.66 cm³·K·mol⁻¹, which is significantly lower than the expected spin-only value for three non-interacting spins 5/2 (13.125 cm³·K·mol⁻¹). On cooling, the χ_MT vs. T curve decreased monotonically to 3.20 cm³·K/mol at 50 K, after which it sharply fell to 2.08 cm³·K·mol⁻¹ at 2 K.

The coupling scheme within a trinuclear unit is represented on Figure 18. The spin Hamiltonian for trinuclear blocks Fe₂Mn takes the form:

$$\begin{array}{c}H=-2J_{Fe-Fe}{S}_{F{e}_1}\cdot S_{F{e}_2}-2J_{Fe-Mn}(S_{F{e}_1}+S_{F{e}_2})\cdot S_{Mn}+\\ +g_{Mn}\beta S_{Mn}\cdot H+g_{Fe}\beta (S_{F{e}_1}+S_{F{e}_2})\cdot H\end{array}$$

(9)

where first line corresponds to the superexchange interactions between Heisenberg spins localized at metal sites (J_Fe-Fe and J_Fe-Mn), the second line corresponds to the isotropic interactions between local spins and the external field through Zeeman interactions (g_Fe and g_Mn), respectively [80]. Intermolecular interactions were taken into account within molecular field model.

The best correspondence between experimental and calculated χ_MT values for the compound was achieved at parameters J_Fe-Fe = −57.8(2) cm⁻¹, J_Fe-Mn = −20.12(7) cm⁻¹, g_Fe = g_Mn = 2.0023 (fixed), zJ’ = −0.10(1) cm⁻¹, χ_impurities = 3.2% (S_impurities = 5/2).

Absolute values of J_Fe-Mn and J_Fe-Mn for 11 3.5DMF are higher than those reported for the trifluoroacetate complex [Fe₂MnO(O₂CCF₃)₆(H₂O)₃] (J_Fe-Fe = −56.50(7) cm⁻¹, J_Fe-Mn = −16.23(4) cm⁻¹ [31], which is consistent with the higher electron-donating ability of the methyl group in acetate relative to CF₃-group. However, for {Fe₂MnO(Piv)₆} blocks exchange coupling parameters [58] have close values to exchange coupling parameters for 11·3.5DMF. It can be explained by different nitrogen ligands: pyridine group in 11·3.5DMF in comparison with hexamethylenetetramine [58] compensates lower electron donor efficiency of acetate group in comparison with pivalate group [84].

3. Experimental

3.1. Materials and Methods

Reagents and solvents were commercially available (Sigma-Aldrich, Aldrich, St. Louis, MO, USA) and were used without further purification. Manganese pivalate [Mn(Piv)₂(EtOH)]_n and trinuclear acetate [MnFe₂O(OAc)₆(H₂O)₃], used as starting compound, were prepared as previously reported [30,51]. C,H,N-analyses were performed using a 1106 instrument (Carlo Erba, Instruments, Egelsbach, Germany). IR-spectra were measured in KBr pellets on a Spectrum BX FT-IR spectrometer (Perkin Elmer, Waltham, MA, USA) in 400–4000 cm⁻¹ range. The X-ray powder diffraction analysis of 6 was carried out on a G670 (HUBER, Offenburg, Germany) Guinier camera using CuK_α1 radiation on air. The X-ray powder diffraction analysis of 11 was performed on a D8 Advance instrument (Bruker, Billerica, MA, USA) in air.

Thermogravimetric analyses (TGA) were performed in air on Q1500 instrument, (Paulik-Paulik-Erdey, Budapest, Hungary). The heating rate was 5 °C per minute. Sorption of methanol and ethanol by [MnFe₂O(OAc)₆(dpe)] was studied gravimetrically, using a tungsten microbalance at 293 K. Each point on the absorption and desorption isotherms corresponds to equilibrium conditions (no change of sample weight at certain p·p_S⁻¹, where p_S is the pressure of saturated vapor of the compound at 293 K). This sample was thermally activated at 150 °C in vacuum at 10⁻² Torr. Volume of pores was estimated from the quantity of adsorbed alcohol using its density in liquid phase at 293 K.

Magnetic measurements were performed on a MPMS-XL (for 11), MPMS-5S (for 7) and PPMS (for 1 and 2) SQUID magnetometers (Quantum Design, San Diego, CA, USA) and intrinsic diamagnetic corrections were calculated using Pascal’s constants [80]. The X-band EPR spectra for 1 and 2 were measured on a Bruker Elexsys E680-X spectrometer at T = 293 K.

3.2. Synthesis

3.2.1. Synthesis of [Mn₂(Piv)₄(2,2′-bipy)₂] (1) and [Mn₂(Piv)₄(phen)₂] (2)

The syntheses were carried out under an argon atmosphere. [Mn(Piv)₂(EtOH)]_n (0.150 g, 0.50 mmol) was dissolved in MeCN (30 mL for 1 or 25 mL for 2), followed by the addition of 2,2′-bipyridine (0.078 g, 0.50 mmol) for 1 or 1,10-phenantroline (0.090 g, 0.50 mmol) for 2. The resulting colorless solution was heated at 80 °C during 30 min, then concentrated to 6–8 mL for 1 or 4–6 mL for 2 and kept at 5 °C during 24 h. Crystals were isolated by decantation, washed by cold MeCN and dried under argon stream. Yield of 1: 0.14 g (67%), yellow crystals; 2: 0.162 g (74%), colorless crystals. Anal, calc. for 1, C₄₀H₅₂Mn₂N₄O₈/found, %: C 58.1/58.0, H 6.3/6.5, N 6.8/6.9. IR-spectrum of 1 (cm⁻¹): 3434 m, 3058 w, 2955 s, 2923 m, 2865 w, 1597 vs, 1542 s, 1516 s, 1482 s, 1420 s, 1373 m, 1359 m, 1225 m, 1143 w, 1101 w, 894 w, 864 w, 846 m, 792 b.w, 729 s, 637 w, 601 w. Anal, calc. for 2, C₄₄H₅₂Mn₂N₄O₈/found, %: C 60.4/59.8, H 6.0/6.2, N 6.4/6.4. IR-spectrum of 2 (cm⁻¹): 3434 s, 2956 s, 2923 m, 2866 w, 1586 v.s, 1547 s, 1482 s, 1440 s, 1419 s, 1372 m, 1358 m, 1314 w, 1226 m, 1172 w, 1155 w, 1059 w, 1015 m, 893 m, 805 w, 792 w, 767 s, 738 m, 646 w, 625 w, 603 w, 558 w, 415 m.

3.2.2. Synthesis of [Mn₄O₂(Piv)₆(2,2′-bipy)₂]·MeCN (3·MeCN) (3) and [Mn₄O₂(Piv)₆(phen)₂]· 0.5MeCN (4·0.5MeCN) (4)

The syntheses were carried out under an argon atmosphere. [Mn(Piv)₂(EtOH)]_n (0.150 g, 0.50 mmol) was dissolved in 30 mL of MeCN followed by the addition of 2,2′-bipyridine (0.078 g, 0.50 mmol) for 3·MeCN or 1,10-phenantroline (0.090 g, 0.50 mmol) for 4·0.5MeCN. The colorless solution was heated at 80 °C during 30 min, then kept under air at room temperature during 24 h for 3·MeCN or 10 days for 4·0.5MeCN. The precipitate was isolated by decantation and dissolved in 30 mL of THF, filtered and then kept under air at room temperature during one week. Brown crystals were isolated by decantation, washed by cold MeCN and dried on air. Yield of 3·MeCN: 0.051 g (35%), brown microcrystals; yield of 4·0.5MeCN: 0.045 g (29%), dark-brown crystals. Anal, calc. for 3·MeCN, C₅₀H₇₀Mn₄N₄O₁₄/found, %: C 51.3/51.1, H 6.0/6.2, N 4.8/4.9. IR-spectrum of 3·MeCN (cm⁻¹): 3433 m, 2956 m, 2925 m, 2868 w, 1589 v.s., 1570 s, 1482 s, 1441 m, 1415 s, 1370 s, 1357 m, 1317 w, 1225 m, 1155 w, 1016 w, 890 w, 789 w, 766 m, 740 w, 640 b.m, 431 m, 410 m. Anal, calc. for 4·0.5MeCN, C₅₅H_71.5Mn₄N_4.5O₁₄/found, %: C 53.3/53.4, H 5.8/5.9, N 5.1/5.2. IR-spectrum of 4·0.5MeCN (cm⁻¹): 3435 w, 2956 s, 2924 m, 2868 m, 1590 s, 1560 s, 1514 s, 1482 s, 1456 m, 1411 s, 1371 s, 1358 s, 1261 w, 1226 s, 1141 w, 1101 m, 1028 w, 891 w, 864 m, 853 m, 790 m, 732 s, 656 s, 639 s, 616 s, 420 bm.

3.2.3. Synthesis of [Mn₆(OH)₂(Piv)₁₀(pym)₄] (5) and Synthesis of [Mn₆O₂(Piv)₁₀(pym)₂]_n (6)

Syntheses were carried out under an argon atmosphere. [Mn(Piv)₂(EtOH)]_n (0.6 g, 1.98 mmol) was dissolved in 30 mL of EtOH, followed by the addition of pyrimidine (0.11 g, 1.40 mmol). The colorless solution was heated at 80 °C during 30 min. For the isolation of 5 the solution was concentrated to 2–3 mL and cooled at −18 °C during 24 h. For the isolation of 6 the solution was kept on air at room temperature during 24 h. The crystals were isolated by decantation, washed by cold MeCN and dried under argon stream. Yield of 5: 0.12 g (22%) colorless crystals; yield of 6: 0.43 g (85%), colorless crystals. Anal, calc. for 5, C₆₆H₁₀₈Mn₆N₈O₂₂/found, %: C 46.8/46.7, H 6.4/6.2, N 6.6/6.8. IR-spectrum of 5 (cm⁻¹): 3419 b.m, 2960 s, 2928 s, 1676 s, 1587 b.s, 1484 s, 1640 s, 1422 s, 1362 s, 1227 s, 1169 w, 1077 w, 1030 w, 937 w, 895 m, 790 m, 716 m, 636 m, 601s, 559 m, 540 m, 415 m. Anal, calc. for 6, C₅₈H₉₈Mn₆N₄O₂₂/found, %: C 45.4/45.5, H 6.4/6.2, N 3.7/3.8. IR-spectrum of 6 (cm⁻¹): 3434 b.m, 2958 m, 2926 w, 2870 w, 1582 v.s, 1569 s, 1482 s, 1467 w, 1417 v.s, 1374 s, 1359 s, 1228 s, 1163 w, 1076 w, 1029 w, 1003 w, 892 w, 795 w, 786 w, 715 w, 632 w, 612 m, 557 w, 437 w, 404 w.

3.2.4. Synthesis of [Mn₄(OH)(Piv)₇(pz)₂]_n∙2nMeCN (7∙2MeCN)

The synthesis was carried out under an argon atmosphere. [Mn(Piv)₂(EtOH)]_n (0.1 g, 0.33 mmol) was dissolved in 20 mL of MeCN, then a solution containing pyrazine (0.03 g, 0.33 mmol) in 5 mL MeCN was added dropwise. The colorless solution was kept at 25 °C during 48 h. Yellow precipitate with crystals was isolated by decantation, washed by cold MeCN and dried under argon stream. Yield 0.08 g (82%). Anal, calc. for C₄₇H₇₈Mn₄N₆O₁₅/found, %: C 47.6/47.4, H 6.6/6.8, N 7.1/7.2. IR-spectra (cm⁻¹): 3432 b.m, 2959 s, 2928 m, 2870 w, 1589 v.s, 1569 v.s, 1483 v.s, 1458 m, 1422 s, 1374 s, 1360 s, 1227 s, 1135 w, 1047 m, 893 w, 791 m, 600 m, 564 w, 450 w, 409 m.

3.2.5. Synthesis of [Mn₄Fe₂O₂(Piv)₁₀(MeCN)₂(HPiv)₂]·2MeCN (8·2MeCN)

The synthesis was carried out under an argon atmosphere. A reaction mixture of [Mn(Piv)₂(EtOH)]_n (0.8 g, 2.64 mmol) and FeCl₃ (0.122 g, 0.75 mmol) in 100 mL MeCN was heated (80 °C) during 30 min to complete the dissolution of reagents. The brown solution obtained was concentrated to 50 mL and kept at room temperature during 6 h. The white and brown precipitates formed were separated from the solution. The solution was concentrated to 20 mL and kept at room temperature during 24 h. Brown crystals were isolated by decantation, washed by cold MeCN and dried under argon stream. Yield 0.16 g (15%). Anal, calc. for C₆₄Fe₂H₁₁₆Mn₄O₂₆N₂ (without solvent molecules)/found, %: C 46.3/46.4, H 7.0/6.9, N 1.7/1.6, Fe 6.7/6.6; Mn 13.2/13.0. IR-spectra (cm⁻¹): 3192 w, 2962 s, 2930 s, 2873 m, 1692 m, 1570 s, 1484 s, 1460 m, 1420 s, 1376 s, 1360 s, 1314 w, 1227 s, 1206 m, 1031 w, 938 w, 895 m, 872 w, 787 m, 764 v.w, 604 m, 574 m, 516 m, 420 s.

3.2.6. Synthesis of [MnFe₂O(OAc)₆(4,4′-bipy)₂]_n·2nDMF (9·2DMF)

[Fe₂MnO(OAc)₆(H₂O)₃] (0.1 g, 0.169 mmol) was dissolved in 3 mL of DMF, followed by the addition of 15 mL of MeCN, then 0.1 mL of acetic acid was added, and then bipy (0.250 g, 0.423 mmol, a 25% excess) was dissolved in this solution. After one day black crystals formed, which were collected by filtration, washed with MeCN (2 portions by 3 mL) and dried on air. Yield 0.04 g (25%). Anal, calc. for C₃₈H₄₈N₆O₁₅Fe₂Mn/found, %: C 45.9/45.6, H 4.86/4.50, N 8.44/8.50.

3.2.7. Synthesis of [MnFe₂O(OAc)₆(bpe)₂]_n·2nDMF (10·2DMF)

[Fe₂MnO(OAc)₆(H₂O)₃] (0.1 g, 0.169 mmol) was dissolved in 3 mL of DMF, followed by the addition of 15 mL of MeCN. Next 0.1 mL of acetic acid was added, and then bpe (0.185 g, 0.014 mmol, a 3-fold excess) was dissolved in this solution. After one day black crystals formed, which were collected by filtration, washed with MeCN (2 portions by 3 mL) and dried on air. Yield 0.07 g (40%). Anal, calc. for C₄₂H₅₂N₆O₁₅Fe₂Mn / found, %: C 48.2/48.1, H 5.00/5.15, N 8.02/7.98.

3.2.8. Synthesis of [MnFe₂O(OAc)₆(bpe)(DMF)]_n·3.5nDMF (11·3.5DMF)

[Fe₂MnO(OAc)₆(H₂O)₃] (0.1 g, 0.169 mmol) was dissolved in 3 mL of DMF, followed by the addition of 15 mL of MeCN. Next 0.1 mL of acetic acid was added, and then bpe (0.038 g, 0.211 mmol, a 25% excess) was dissolved in this solution. After one day black crystals formed, which were collected by filtration, washed with MeCN (two portions of 3 mL) and dried on air. Yield 0.02 g (10%). Anal, calc. for C_34.5H_52.5N_5.5O_16.5Fe₂Mn/found, %: C 42.5/42.1, H 5.43/5.35, N 7.90/7.95.

3.3. X-ray Structure Determination

For X-ray structure determination single crystals of the compounds 1–8 were isolated from the mother liquors and mounted on a Bruker APEX II diffractometer equipped with a CCD camera and a graphite monochromated MoK_α radiation source (λ = 0.71073 Å) at the N. S. Kurnakov Institute of General and Inorganic Chemistry (Moscow, Russia). X-ray structure determination for compounds 9–11 was performed using a a Kappa-Nonius four circle diffractometer equipped with a CCD camera and a graphite monochromated MoK_α radiation source (λ = 0.71073 Å), located at the Centre de Diffractométrie (CDIFX), Université de Rennes 1 (Rennes, France).

Effective absorption correction was performed using SCALEPACK. Structures of the complexes were solved by the direct method using SHELXS-97 [85] or Sir-97 [86] software, and refined with a full matrix least squares method on F² using SHELXL-97, SHELX-2014 or SHELX-2018 program [87]. H atoms were treated by a riding model. Solvent molecules, which could not be localized, were removed by SQUEEZE procedure for compounds 9–11 [88]. The structure of 6 was solved taking into account crystal twinning (Flack parameter is 0.42(4)). Crystallographic data and structure refinement parameters for 1–11 are presented in

Table 2. Crystallographic data and structure refinement parameters for 1–4.

	1	2	3·MeCN	4·0.5MeCN
Empirical formula	C40H52Mn2N4O8	C44H52Mn2N4O8	C52H73Mn4N5O14	C55H71.5Mn4N4.5O14
Formula weight (g·mol−1)	826.73	874.77	1211.91	1239.42
T/K	150(2)	296(2)	296(2)	296(2)
Crystal system	Monoclinic	Monoclinic	Monoclinic	Monoclinic
Space group	P21/c	C2/c	P21/c	P21/n
a (Å)	11.676(6)	19.821(16)	12.7090(14)	12.752(4)
b (Å)	18.715(10)	17.592(14)	18.925(2)	16.007(5)
c (Å)	9.859(5)	13.907(11)	14.094(2)	16.088(5)
α (o)	90	90	90	90
β (o)	107.922(8)	115.565(12)	104.947(2)	92.761(5)
γ (o)	90	90	90	90
V (Å3)	2049.9(19)	4374(6)	3275.2(6)	3280.0(16)
Z	2	4	2	2
Dcalc (g·cm−3)	1.339	1.328	1.229	1.255
μ (mm−1)	0.67	0.632	0.811	0.811
F(000)	868	1832	1264	1290
θ range for data collection (o)	1.83 to 26.37	1.62 to 26.70	1.66 to 28.13	1.80 to 26.44
Reflections collected	14,993	16,361	20,634	29,369
Reflections unique	4194	4605	7934	6685
Rint	0.162	0.069	0.044	0.070
Parameters	257	262	409	439
GOF	0.900	1.037	1.010	1.004
R1a [Io > 2σI)]	0.052	0.059	0.053	0.059
wR2b [Io > 2σI)]	0.099	0.155	0.128	0.172

^a R₁ = Σ||Fo| − |Fc||/Σ|Fo|. ^b wR₂ = {Σ[w(Fo² − Fc²)²]/Σ[w(Fo²)²]}^1/2.

,

Table 3. Crystallographic data and structure refinement parameters for 5–8.

	5	6	7·2MeCN	8
Empirical formula	C66H108Mn6N8O22	C58H98Mn6N4O22	C47H78Mn4N6O15	C72H128Fe2Mn4N6O26
Formula weight (g·mol−1)	1695.24	1533.04	1186.91	1825.26
T/K	173(2)	296(2)	173(2)	150(2)
Crystal system	Monoclinic	Monoclinic	Triclinic	Monoclinic
Space group	P21/n	Pn	P‾1	C2/c
a (Å)	13.7821(10)	14.404(3)	13.4652(18)	13.4462(15)
b (Å)	21.676(2)	16.331(3)	13.7324(18)	29.008(3)
c (Å)	13.8218(10)	16.330(3)	18.150(2)	23.868(3)
α (o)	90	90	111.943(2)	90
β (o)	98.7840(10)	103.009(3)	90.612(2)	91.930(2)
γ (o)	90	90	103.879(2)	90
V (Å3)	4080.8(5)	3742.6(14)	3003.2(7)	9304.3(18)
Z	2	2	2	4
Dcalc (g·cm−3)	1.380	1.360	1.313	1.303
μ (mm−1)	0.971	1.050	0.884	0.900
F(000)	1772	1600	1244	3848
θ range for data collection (o)	1.76 to 26.60	1.25 to 28.27	1.22 to 26.90	1.40 to 28.28
Reflections collected	30,266	21,851	28,737	39,848
Reflections unique	8537	14,323	12,922	11,443
Rint	0.052	0.054	0.047	0.097
Parameters	460	813	716	542
GOF	0.990	0.969	1.052	1.025
R1a [Io > 2σI)]	0.046	0.082	0.056	0.058
wR2b [Io > 2σI)]	0.131	0.199	0.148	0.126

^a R₁ = Σ||Fo| − |Fc||/Σ|Fo|. ^b wR₂ = {Σ[w(Fo² − Fc²)²]/Σ[w(Fo²)²]}^1/2.

and

Table 4. Crystallographic data and structure refinement parameters for 9–11.

	9	10	11
Empirical formula	C64H68Fe4Mn2N8O26	C36H37.5Fe2MnN4O13	C27H35Fe2MnN3O14
Formula weight (g·mol−1)	1698.54	900.84	792.22
T/K	293(2)	293(2)	293(2)
Crystal system	Monoclinic	Triclinic	Orthorhombic
Space group	I2/a	P‾1	P212121
a (Å)	20.0570(6)	12.889(5)	8.0410(1)
b (Å)	25.1754(7)	14.469(5)	19.572(1)
c (Å)	34.7618(11)	16.096(5)	30.223(1)
α (o)	90	88.749(5)	90
β (o)	68.184(2)	66.401(5)	90
γ (o)	90	87.004(5)	90
V (Å3)	17,374.0(9)	2747.0(17)	4756.5(7)
Z	8	2	4
Dcalc (g·cm−3)	1.391	1.089	1.106
μ (mm−1)	1.012	0.796	0.912
F(000)	7472	925	1628
θ range for data collection (o)	5.09 to 25.56	2.76 to 26.37	1.24 to 25.68
Reflections collected	43,517	14,357	14,696
Reflections unique	15,023	9780	7253
Rint	0.147	0.041	0.100
Parameters	941	505	425
GOF	1.084	1.001	0.989
R1a [Io > 2σI)]	0.107	0.101	0.095
wR2b [Io > 2σI)]	0.199	0.291	0.248

^aR₁ = Σ||Fo| − |Fc||/Σ|Fo|. ^b wR₂ = {Σ[w(Fo² − Fc²)²]/Σ[w(Fo²)²]}^1/2.

. Supplementary crystallographic data for the compounds synthesized are given in CCDC numbers 2055493-2055503 for 1–11, respectively. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

3.4. DFT Calculations

The signs and the magnitudes of exchange coupling parameters J were independently estimated by DFT calculations similar to the previously reported by us in details [89], brief description of the methodology is provided in this section.

The calculations were performed via ORCA software [90]. TPSSh [91,92,93] exchange-correlation potential was employed for the calculation together with LANLTZ basis sets [94] for 3d ions and def2-SVP [95] basis set for the rest of the elements.

The atomic coordinates were taken from the crystallographic data. For calculation of each J value, all Mn²⁺ ions in Mn₄ core except the two ions taking part in the coupling were “substituted” by diamagnetic Zn²⁺ ions in order to simplify the system of spin states of the species. Broken symmetry DFT approach was applied for the calculation of J: first, a single-point calculation was performed for the high-spin state of the Mn₂Zn₂ species, then the broken symmetry state was constructed by artificial flipping the spin projections of the unpaired electrons localized on one of the Mn²⁺ ions, and a single-point calculation was performed again. J value was obtained using the energies E_HS and E_BS resulted from the two converged single-point calculations as J = − (E_HS − E_BS)/(<S²>_HS−<S²>_BS) [96] (<S²>_HS and <S²>_BS—the total spin operator expectation values derived from the calculations).

4. Conclusions

In this study a variety of transformations of Mn-containing complexes in reactions with N-donor heterocycles was shown. In was found that homometallic Mn pivalates underwent metamorphosis while heterometallic acetates with a Fe₂MnO core preserved their structure, giving rise to coordination polymers. Different behavior of Mn^II or Ni^II and Co^II pivalates in reactions with FeCl₃ was also revealed: while the first complex produced a hexanuclear [Mn^II₄Fe^III₂O₂(Piv)₁₀(MeCN)₂(HPiv)₂] compound, the latter (Ni^II and Co^II) pivalates under similar conditions gave trinuclear pivalates with a Fe₂MnO core.

Several new coordination polymers with bridging pyrimidine (i.e., [Mn₆O₂(Piv)₁₀(pym)₂]_n), 4,4-bipyridine (i.e., [MnFe₂O(OAc)₆(4,4′-bipy)₂]_n) or 1,2-bis-trans-(4-pyridyl)ethylene (i.e., [MnFe₂O(OAc)₆(bpe)₂]_n and [MnFe₂O(OAc)₆(bpe)(DMF)]_n) were prepared. Unexpectedly, the composition of the coordination polymers based on MnFe₂O(OAc)₆ was probably governed by a fine balance between formation and crystallization kinetics and solubility of certain species, in contrast to the ratio of potential vacations in the coordination spheres of metal ions and the number of donor atoms). The crystal lattice of [MnFe₂O(OAc)₆(bpe)(DMF)]_n collapsed upon desolvatation, and the resulting compound was not porous in respect to N₂, however the porosity was restored upon interaction with methanol and ethanol. Notably, the hexanuclear unit {Mn^II₄Fe^III₂O₂(O₂CR)₁₀} can be considered a promising new building block for the creation of coordination polymers.

Magnetic properties of the compounds were in line with those expected for Mn carboxylates. The temperature dependence of the magnetic susceptibility of [Mn₄(μ₃-OH)(Piv)₇(μ-pz)₂]_n could be fitted with five exchange coupling parameters, and their reliability was independently checked by DFT calculations.