Triggering of Valence Tautomeric Transitions in Dioxolene-Based Cobalt Complexes Influenced by Ligand Substituents, Co-ligands, and Anions

Abstract

We report the multistep synthesis of the 1,1′-(piperazine-1,4-diyl)bis(N,N-bis(pyridin-2-ylmethyl)methanamine)(L^tpbap) octadentate ligand, which, in combination with the known 3,5-di-tert-butylcatechol (3,5-dbcat), allowed the preparation of two di-nuclear cobalt complexes [Co₂(L^tpbap)(3,5-dbcat)₂](SO₄)·5.5MeOH·2H₂O (3a) and [Co₂(L^tpbap)(3,5-dbcat)₂ ](ClO₄)₂·1.5H₂O (3b). We also report the synthesis of two mono-nuclear cobalt complexes [Co(3,5-dbsq)(3,5-dbcat)(4-Mepip)₂] (1) with 4-Mepip being 4-methylpiperidine and (Hpip)[Co(tbcat)₂(pip)₂]·CH₃CN (2) where Hpip denotes a piperidinium cation and tbcat is the tetra-bromocatechol ligand. The obtained complexes were characterized by single-crystal X-ray crystallography, SQUID magnetometry, and IR spectroscopy. The structure of the crystalline material in all the cases was determined at 173 K. The magnetic properties of all complexes were measured between 2 and 380 K. The magnetic data clearly show that mono-nuclear complex 1 and di-nuclear complex 3a exhibit valence tautomerism with onset around 300 K and 370 K, respectively, whereas the other two complexes 2 and 3b remain unchanged over the measured temperature range.

1. Introduction

Bistable molecules that are capable of switching the electronic states are recently of a great interest for their potential applications in data storage, sensors, and display devices [1,2,3,4]. The most commonly studied spin switchable molecules are the spin-crossover molecules where the transition of the spin state in the metal ion occurs between low spin (LS) state and high spin (HS) state [5,6,7,8]. Numerous examples with different metal ions are studied in the literature, including the more common Fe(II) and Fe(III) ions, as well rare examples of Co(II) and even more rare examples of Mn(III) [9,10,11,12]. Another less commonly studied class of switchable molecules are Valence Tautomeric (VT) compounds, in which the transition between the two states occurs by an electron transfer within the molecule (intramolecular) between the non-innocent redox-active ligand and the metal ion using an external stimulus, such as temperature, light, and pressure [13,14,15,16,17]. Cobalt complexes coupled with redox-active ligands are excellent candidates to show VT effect and have been studied widely in the literature [13,15,16,17,18,19,20,21]. The class of redox active ligands most frequently studied in the literature triggering the VT property are catechol and their derivatives. Di-nuclear cobalt complexes are comparatively less reported than the mono-nuclear ones exhibiting VT [16,22,23,24]. The effect of the VT transition in solid state is often affected by the lattice solvents, choice of anions, substituents in the ring and crystal packing effects [25,26,27]. In the present work, we report the synthesis, X-ray structure determination, and magnetic properties of two mono-nuclear and two di-nuclear cobalt complexes. The effect of substituents and the choice of anions in triggering the VT behaviour Is investigated.

2. Materials and Methods

All chemicals including 3,5-di-tert-butylcatechol (3,5-dbcat) and tetra-bromocatechol (tbcat) were used without further purification and were purchased from Alfa Aesar, Acros Organics and Fisher Chemicals. The C, H, and N elemental analyses were carried out on a Foss Heraeus Vario EL (Elementar Analysensysteme GmbH, Langenselbold, Germany) at the microanalytical laboratories of the department of chemistry at the Johannes Gutenberg University Mainz. The infrared absorption spectra were collected at room temperature in a range of 3000–450 cm⁻¹ on a Thermo Fischer NICOLET Nexus FT/IR-5700 spectrometer (Thermo Fischer Scientific, Waltham, MA, USA) equipped with Smart Orbit ATR Diamond cell (Thermo Fischer Scientific, Waltham, MA, USA). X-ray diffraction data for the structure analysis were collected from suitable single crystals at 173 K with a STOE IPDS 2T at the Johannes Gutenberg University Mainz. The structures were solved with ShelXT [25,26] and refined with ShelXL [27,28] implemented in the program Olex2 [29]. Magnetic data of a microcrystalline sample were collected using a Quantum Design SQUID magnetometer MPMS-XL in a temperature range between 2 and 380 K with an applied field of 1 kOe (0.1 T). The magnetic contribution from the holder were experimentally determined and subtracted from the measured susceptibility data. The contribution from the underlying diamagnetism was corrected using the Pascals constants [30].

2.1. Ligand Synthesis

2.1.1. N,N′-Bis(cyanomethyl)piperazine

Piperazine (6.45 g, 75 mmol) and chloroacetonitrile (13.59 g, 180 mmol) was dissolved in 300 mL ethanol. To the reaction solution was added sodium carbonate (31.8 g, 300 mmol) in small portions under continuous stirring. The reaction mixture was refluxed overnight. The hot solution was filtered and washed with hot ethanol. The excess solvent was removed, and the filtrate was concentrated under reduced pressure to 120 mL. The yellow solution was cooled at −18 °C to obtain colourless crystalline product after two days which was isolated by filtration in moderate (9.15 g, 55.7 mmol, 74.3%). ¹H-NMR (400 MHz, CDCl₃)δ (ppm) = 3.57 (s, 4 H, H1), 2.71 (s, 8 H, H7). ¹³C-NMR (100 MHz, CDCl₃) 153. δ (ppm) = 114.3 (s), 51.1 (s), 45.8 (s).

2.1.2. N,N′-Bis(2-aminoethyl)piperazine

N,N′-Bis(cyanomethyl)piperazine (2.50 g, 15.21 mmol) was dissolved in 100 mL THF. A suspension of lithium aluminium hydride (2.66 g, 70.00 mmol) in 100 mL THF was prepared separately. The two solutions were mixed together and refluxed for four hours. The reaction solution was cooled to 0 °C using an ice bath and 2.35 mL water was added dropwise to quench the reaction. In addition to this, 2.35 mL sodium hydroxide solution (15%) and 7.05 mL of water were also added. The reaction suspension was filtered and washed with diethyl ether several times. The filtrate obtained was dried for two hours over sodium sulphate and later the filtrate was evaporated to dryness to obtain the desired product as yellow needles in moderate yields (1.58 g, 9.17 mmol, 60.3%). ¹H-NMR (400 MHz, CDCl₃) δ (ppm) = 2.71 (t, J = 6.3 Hz, 4 H, H8), 2.41 (s, 4 H, H1), 2.35 (t, J = 6.3 Hz, 4 H, H7), and 1.47 (s, 4 H, H9). ¹³C-NMR (100 MHz, CDCl₃) δ (ppm) = 61.14 (s, C7), 53.24 (s, C1), and 38.78 (s, C8).

2.1.3. 1,1′-(piperazine-1,4-diyl)bis(N,N-bis(pyridin-2-ylmethyl)methanamine)(L^tpbap)

N,N′-Bis(2-aminoethyl)piperazine (1.05 g, 5.8 mmol) was dissolved in 150 mL dichloromethane. To the reaction mixture was added sodium triacetoxyborohydride (6.15 g, 29.0 mmol) slowly in small portions. The 2-pyridinecarboxaldehyde (2.61 g, 24.4 mmol) was dissolved in 10 mL dichloromethane was added to the reaction mixture slowly dropwise over 30 min. The reaction mixture was stirred for two days in room temperature. To the reaction mixture, 50 mL saturated sodium bicarbonate was added whereupon an immediate evolution of CO₂ gas was seen. The reaction mixture was then stirred for additional 30 min. After which, the desired crude product was obtained by extraction with dichloromethane and ethyl acetate. The organic phase was collected and dried over sodium sulphate which was evaporated to dryness. The solid was purified by washing with acetone several times to obtain the desired product in high yields as yellow powder which was dried under vacuum for four days (3.01 g, 5.6 mmol, 97%). ¹H-NMR (400 MHz, CDCl₃) δ (ppm) = 8.57 to 8.44 (m, 4 H, H22), 7.65 (td, 4 H, H24), 7.52 (d, 4 H, H25), 7.15 (ddd, 4 H, H23), 3.85 (s, 8 H, H13), 2.74 (t, 4 H, H9), and 2.67 to 2.22 (m, 16 H, H1,7). ¹³C-NMR (100 MHz, CDCl₃) δ (ppm) = 159.61 (s, C17), 148.97 (s, C22), 136.45 (s, C24), 123.02 (s, C25), 122.00 (s, C23), 60.70 (s, C13), 56.06 (s, C7), 52.98 (s, C9), and 51.22 (s, C1). IR (cm⁻¹) = 3421 (br), 3057 (w), 3009 (w), 2939 (m), 2811 (m), 1589 (s), 1570 (m), 1471 (m), 1431 (s), 1363 (m), 1318 (m), 1301 (m), 1248 (m), 1145 (m), 1124 (m), 1051 (m), 1013 (m), 982 (m), 958 (w), 939 (w), 890 (w), 822 (w), 766 (s), 731 (w), 610 (w), 463 (w), and 410 (w).

2.2. Complex Synthesis

2.2.1. [Co₂(OH₂)(piv)₄(Hpiv)₄] (P1)

The precursor complex used for the preparation of the mono-nuclear complexes 1 and 2 was synthesized according to the literature [31]. To a solution of pivalic acid (40.0 g, 392 mmol) in 4 mL water was added cobalt carbonate hydrate (8.0 g, 68.0 mmol). The reaction mixture was refluxed at 100 °C overnight. The reaction mixture was cooled to 80 °C for addition of 100 mL acetonitrile. The reaction mixture immediately changed to dark purple solution. The mixture was kept undisturbed overnight to form dark purple crystalline product in moderate yields (19.8 g, 20.9 mmol, 61.4%).

2.2.2. [Co(3,5-dbsq)(3,5-dbcat)(4-Mepip)₂] (1)

To a solution of P1 (237 mg, 0.25 mmol) and 3,5 dbcat (333 mg, 1.5 mmol) in 15 mL acetonitrile was added 4-methylpiperidine (298 mg, 3.0 mmol). The reaction mixture was refluxed for an hour. The reaction mixture was cooled to room temperature and was filtered to remove undissolved insoluble residues and was set to slowly evaporate. After a day, the desired complex was collected as brown crystalline needles in high yields (249 mg, 0.36 mmol, 71.4%) which was filtered and washed with cold acetonitrile. Elemental Analysis: Found: C, 68.59; H, 9.45; N, 4.48. Calculated for C₄₀H₆₆CoN₂O₄: C, 68.84; H, 9.53; and N, 4.01%. IR (cm⁻¹) = 3567 (br), 2951 (s), 2869 (m), 2360 (w), 1573 (m), 1460 (m), 1415 (m), 1358 (m), 1278 (m), 1243 (m), 1199 (m), 1095 (m), 981 (m), 860 (m), 746 (w), 651(m), 577 (w), 545 (w), 493 (w), and 446 (w).

2.2.3. (Hpip)[Co(tbcat)₂(pip)₂]CH₃CN (2)

To a solution of P1 (237 mg, 0.25 mmol) and H₂tbcat (319 mg, 0.75 mmol) in 15 mL acetonitrile was added piperidine 128 mg. The reaction mixture was refluxed for an hour. The reaction mixture was cooled to room temperature and was filtered to remove insoluble residue and was set to slow evaporation. The desired product was obtained in moderate yields as brown crystalline solid (173 mg, 0.14 mmol, 29.8%) which was filtered and washed with cold acetonitrile. Elemental Analysis: Found: C, 29.09; H, 3.01; N, 4.76%. Calculated for C₂₇H₃₄Br₈CoN₃O₄·CH₃CN: C, 28.93; H, 3.10; and N, 4.65%. IR (cm⁻¹) = 3443 (br), 2936 (w), 2854 (w), 1596 (w), 1432 (s), 1348 (w), 1263 (m),1232 (m), 1085 (w), 1026 (w), 1004 (w), 930 (m), 876 (w), 772 (w), 743 (m), 632(w), and 571 (w).

2.2.4. [Co₂(L^tpbap)(3,5-dbcat)₂](SO₄)·5.5MeOH·2H₂O (3a)

To a solution of CoSO₄·7 H₂O (56 mg, 0.2 mmol), we added L^tpbap (54 mg, 0.1 mmol) and 3,5-dbcat (44 mg, 0.2 mmol) in 15 mL of methanol. Triethylamine (40 mg, 0.4 mmol) was added. The solution was heated under reflux for an hour. After cooling to room temperature, the solution was filtered and was allowed to evaporate slowly to yield green plates of complex 3a suitable for X-ray diffraction. The crystals were obtained in high yields (83 mg, 0.07 mmol, 69.7%), separated by filtration and washed with cold methanol. Elemental Analysis: Found: C, 56.01; H, 7.75; N, 8.00; and S, 2.63. Calc. for C₆₀H₈₀Co₂N₈O₈S·5.5MeOH·2H₂O: C, 56.05; H, 7.61; N, 7.98; S, 2.28 %. IR (cm⁻¹) = 3415 (br), 2952 (m), 1612 (m), 1555 (w), 1463 (m), 1441 (m), 1413 (m),1359 (w), 1320 (w), 1281 (m), 1241 (m), 1120 (m), 979 (m), 826 (w), 769 (m), and 620 (m).

2.2.5. [Co₂(L^tpbap)(3,5-dbcat)₂](ClO₄)₂·1.5 H₂O (3b)

To a solution of Co(ClO₄)₂·6H₂O (73 mg, 0.2 mmol), L^tpbap (54 mg, 0.1 mmol) and 3,5-dbcat (44 mg, 0.2 mmol) in 15 mL of ethanol. Triethylamine (40 mg, 0.4 mmol) was added, and the reaction mixture was refluxed for an hour. After cooling to room temperature, the reaction solution was filtered and allowed to evaporate slowly. After a day, green crystals of complex 3b were isolated by filtration in moderate yields (47 mg, 0.036 mmol, 36.3%) and washed with cold ethanol. Elemental Analysis: Found: C, 54.59; H, 6.13; N, and 8.45. Calc. for C₆₀H₈₀Cl₂Co₂N₈O₁₂·1.5 H₂O: C, 54.55; H, 6.33; N, 8.48 %. IR (cm⁻¹) = 3424 (br), 2952 (m), 1611 (m), 1555 (w), 1461 (m), 1440 (m), 1413 (m),1359 (w), 1321 (w), 1281 (m), 1241 (m), 1204 (w), 1145 (m), 1118 (s), 1090 (s), 980 (m), 945 (w), 826 (w), 768 (m), and 626 (m).

Safety note: “Perchlorate complexes are potentially explosive. While we have not experienced any problems with the compounds described, they should be treated with caution and handled in small quantities”.

3. Results and Discussion

The octadentate ligand 1,1′-(piperazine-1,4-diyl)bis(N,N-bis(pyridin-2ylmethyl)methanamine) L^tpbap was synthesized in three steps (Scheme 1). Firstly, piperazine was cyanomethylated with chloroacetonitrile. Followed by reduction of the dinitriles to the corresponding diamine using lithium aluminium hydride. At the final step, a reductive amination of pyridine-2-carboxaldehyde was carried out to form L^tpbap in high yields. The ligand synthesized was characterized by ¹H-NMR, ¹³C-NMR, and IR spectroscopy (Figures S1–S7).

The mononuclear complexes were synthesized by reacting six equivalents of the commercially available 3,5-dbcat ligand with one equivalent of the cobalt precursor complex (P1) and the additional co-ligand 4-methylpiperdin to give [Co(3,5-dbsq)(3,5-dbcat)(4-Mepip)₂] (1), respectively, by reacting three equivalents of the commercially available tbcat with one equivalent of the precursor cobalt complex (P1) to give (Hpip)[Co(tbcat)₂(pip)₂]·CH₃CN (2).

In the case of the dinuclear complexes, the desired complexes were obtained by reacting one equivalence of the octadentate ligand L^tpbap with two equivalents of cobalt sulphate (3a) and cobalt perchlorate salt (3b) and two equivalents of 3,5-dbcat in methanol in the case of 3a and ethanol in the case of 3b. Additionally, four equivalents of triethylamine were added as a base. The complexes [Co₂(L^tpbap)(3,5-dbcat)₂](SO₄)·5.5MeOH·2H₂O (3a) and [Co₂(L^tpbap)(3,5-dbcat)₂ ](ClO₄)₂·1.5 H₂O (3b) were obtained as brown crystalline solid and washed with diethyl ether. The complexes obtained were characterized by elemental analysis, X-ray diffraction at 173 K, infrared spectroscopy in the range of 450–3000 cm⁻¹ (Figures S8–S11) and SQUID magnetometry in the temperature range of 2–380 K.

3.1. Crystal Structures

Brown needles of complex 1 crystallised from ethanolic solution after one day of slow evaporation. The X-ray data collected at 173 K shows that the compound crystallised in the non-centrosymmetric orthorhombic Pca2₁ ferroelectric space group as a desired mono-nuclear cobalt complex. The central metal ion is coordinated to two chelating 3,5-di-tert-butyldioxolene ligands along with two 4-methylpiperidines coordinated in trans position to each other as an ancillary co-ligand, as seen in Figure 1 and fully labelled complex 1 in Figure S13. Pierpont and co-workers have reported an analogue compound with pyridine as co-ligand [32]. The 4-methylpiperdine is more electron donating and stronger in ligand field strength compared to pyridine. In terms of the crystal geometry, the piperidine complex due to the chair conformation of the co-ligands exhibits lower symmetry. Moreover, the two hydrogen atoms at the 4-methylpiperidine N-atoms point to the same side of the complex the inversion symmetry at the central cobalt atom is broken. Both effects together have crucial influence on the non-equivalence of the electronic structure of the two coordinated dioxolene ligands. The coordination sphere around the cobalt metal ion with the bond distances of approximately 1.884 Å and 1.969 Å for Co-N and Co-O axis, respectively, clearly shows the presence of low spin Co(III) ion at the measuring temperature of 173 K, which was additionally confirmed by calculating the bond valence sum values (Table S3). The analysis of the intra-atomic distances of the ligands shows that the two dioxolene ligands are not equivalent. For one of the two ligands the alternating C-C bond length found within the ring and shorter C-O bond lengths of 1.302 Å for C1-O1 and 1.312 Å for C2-O2 clearly shows the existence of a partial double bond character (Table S1). In the case of the second dioxolene ligand longer C-O single bond distances are exhibited (1.348 Å and 1.362 Å for C15-O3 and C16-O4, respectively), as well as regular aromatic C-C bond length are seen. These bond distances help us to assign one dioxolene as a monoanionic semi-quinonate and the other one as an aromatic dianionic catecholate, which is also in agreement with the overall charge balance for the complex. VT at temperatures above 350 K will then lead to 1 being better described as [Co^II(3,5-dbsq)₂(4-Mepip)₂].

Dark brown needles of complex 2 were obtained from slow evaporation of the complex from ethanolic solution. The X-ray data collected at 173 K shows that the compound crystalized in non-centrosymmetric monoclinic P2₁ ferroelectric space group as a desired mono-nuclear complex. The tetra-bromocatechol (tbcat) ligand used is a lot more electron withdrawing compared to the dbcat ligand. The central metal ion coordination sphere is similar to that of the complex 1 with two dioxolene ligands coordinated to the cobalt ion along with the two piperidines co-ligand that are oriented in trans fashion. Interestingly, the hydrogen atoms of the coordinated nitrogen from the piperdine are orientated in the non-meso fashion, in contrast to complex 1 where the hydrogen atoms are oriented in the meso fashion, as seen in Figure 1 and fully labelled complex 2 in Figure S14. Complex 2 also consists of a piperidinium cation loosely bounded over hydrogen bridges to the oxygen atoms from the catecholate ligand with a bond distance of 2.771 Å and 2.819 Å. The bond distances as tabulated in

Table 1. Selected bond length data for mono-nuclear complexes 1 and 2 and di-nuclear complexes 3a and 3b at 173 K.

Mono-Nuclear	1	2	Di-Nuclear	3a	3b
Metal-Donor	Bond Length (Å)	Bond Length (Å)	Metal-Donor	Bond Length (Å)	Bond Length (Å)
Co1-O1	1.901 (9)	1.909 (3)	Co1-O1	1.878 (4)	1.892 (6)
Co1-O2	1.905 (9)	1.901 (3)	Co1-O2	1.888 (4)	1.884 (5)
Co1-O3	1.885 (9)	1.907 (3)	Co1-N1	2.063 (5)	2.056 (5)
Co1-O4	1.884 (9)	1.893 (3)	Co1-N2	1.954 (4)	1.940 (6)
Co1-N1	1.969 (9)	2.014 (4)	Co1-N3	1.936 (5)	1.956 (6)
Co1-N2	1.978 (9)	2.019 (4)	Co1-N4	1.925 (5)	1.908 (7)

and Table S2, indicate the presence of low spin Co(III) complex. The Bond Value Sum (BVS) calculated for the complex 2 for the cobalt³⁺ (Table S3) is slightly lower with a value of 2.8 compared to the other complexes at 173 K might be due to the electron withdrawing nature of the tbcat ligand.

Thin green plates of di-nuclear isostructural complexes 3a and 3b were obtained in moderate yields from slow evaporation of a solution of ethanol. The X-ray data of the complexes was collected in both the cases at 173 K (Figure 2). Complex 3a with a sulphate anion crystallizes in the monoclinic C2/c space group, whereas the complex 3b with a perchlorate anion exhibits lower crystal symmetry and crystalizes in the triclinic, P-1 space group. In both the cases the cobalt–dioxolene fragments are oriented trans to each other. The metal donor distances of the cobalt ions in both complexes 3a and 3b show a strict undistorted octahedral coordination sphere with an average bond length of 1.85 Å and 1.95 Å for Co-O and Co-N, respectively. The bond lengths clearly indicate the presence of the low spin trivalent cobalt ions. The Bond Value Sum (BVS) calculated for the complex 3a and 3b (Table S3) for both cobalt 2+ and 3+ clearly indicate the presence of trivalent cobalt ions. The metal donor distances (Co-O and Co-N) at 173 K are tabulated in Table 1 and other crystallographic parameters in

Table 2. Crystallographic data for mono-nuclear complexes 1 and 2 and di-nuclear complexes 3a and 3b collected at 173 K.

Compound	1	2
Empirical formula	C40H66CoN2O4	C29H37Br8CoN4O4
Formula weight/g mol−1	697.87	1203.83
Crystal size/mm	0.15 × 0.13 × 0.05	0.19 × 0.12 × 0.03
Crystal system	Orthorhombic	Monoclinic
Space group	Pca21	P21
CCDC No.	2202223	2195460
Unit cell dimensions
a/Å	18.7294(3)	10.3888(2)
b/Å	10.7376(2)	17.3101(3)
c/Å	19.4878(4)	10.5887(2)
α/°	90	90
β/°	90	99.6854(9)
γ/°	90	90
Volume/Å	3919.2(14)	1877.04(6)
Z	4	2
ρcalc./g cm−1	1.183	2.130
μ/mm−1	0.478	9.012
F(000)	1516.0	1156.0
Temperature/K	173(2)	173(2)
Radiation	Mo-Kα	Mo-Kα
Index ranges	−22 < h < 20	−15 < h < 16
	−12 < k <12	−26 < k < 26
	−12 < l < 23	−16 < l < 15
Collected reflections	14473	54646
Independent reflections	5030	14399
Rint	0.1705	0.0673
Rsigma	0.2964	0.08060
Data/restraints/parameters	5030/105/439	14399/23/429
Goodness-of-fit on F2	0.622	0.800
Final R1 [I ≥ 2σ(I)]	0.0469	0.0319
Final wR2 [I ≥ 2σ(I)]	0.0666	0.0496
Final R1 [alldata]	0.1652	0.0577
Final wR2 [alldata]	0.0961	0.0529
Compound	3a	3b
Empirical formula	C69H116Co2N8O17S	C60H80Cl2Co2N8O12
Formula weight/g mol−1	1479.61	1294.08
Crystal size/mm	0.46 × 0.34 × 0.3	0.19 × 0.15 × 0.02
Crystal system	Monoclinic	Triclinic
Space group	C2/c	P$\overline{1}$
CCDC No.	2170727	2170728
Unit cell dimensions
a/Å	26.051(3)	11.071(2)
b/Å	17.448(2)	11.332(18)
c/Å	17.215(2)	15.543(3)
α/°	90	107.932(6)
β/°	94.177(3)	97.683(7)
γ/°	90	94.664(7)
Volume/Å	7804.4(18)	1823.1(5)
Z	4	1
ρcalc./g cm−1	1.259	1.179
μ/mm−1	0.519	0.585
F(000)	3168.0	680.0
Temperature/K	173(2)	173(2)
Radiation	Mo-Kα	Mo-Kα
Index ranges	−34 < h < 34	−13 < h < 14
	0 < k <22	−14 < k < 14
	0 < l < 22	−20 < l < 20
Collected reflections	13257	18429
Independent reflections	13257	8470
Rint	0.0207	0.1584
Rsigma	0.1309	0.3032
Data/ restraints/ parameters	13257/38/455	8740/79/379
Goodness-of-fit on F2	1.041	0.941
Final R1 [I ≥ 2σ(I)]	0.0884	0.1072
Final wR2 [I ≥ 2σ(I)]	0.2066	0.2082
Final R1 [alldata]	0.1786	0.2736
Final wR2 [alldata]	0.2753	0.2692

.

It is evident from the literature that the VT conversion, accompanied with the spin transition from the low spin trivalent cobalt to a high spin divalent cobalt, the metal donor bond distances Co-O and Co-N are usually elongated by approximately 0.15 Å [22,24]. The crystalline lattice must be soft to enable the molecules to increase in volume in this way. In the case of 3b, the molecules within the cell are aligned in the same way in the crystal structure. The crystal lattice exhibits only translational symmetry. Due to angularity and the interlocked arrangement a densely packed and rigid lattice environment is seen from crystal packing along the a–c plane as seen in Figure 3. The complex 3b with the perchlorate anions does not show any hydrogen bonding but they exhibit a very weak π–π interaction between the pyridyl rings on the ligand backbone is observed. Thus, the rigid crystal lattice of complex 3b hinders the expansion of the molecule which is usually seen with VT conversion. Interestingly, in the case of complex 3a, a herringbone type packing pattern with an angle of 72.4° is found for the arrangement of [Co₂(L^tpbap)(3,5-dbcat)₂]²⁺ cations. The packing is not symmetrical, and the molecules are not interlocked, as seen in the case of complex 3b. The solvent methanol molecule and the sulphate anions are filled in the void spaces of the crystal packing. This also allows the molecule to have intermolecular interactions and cooperativity through extended hydrogen bonding network. As mentioned earlier, this enables the molecule to have a softer crystal lattice with flexibility to allow the expansion of the coordination sphere of the cobalt ion while exhibiting VT conversion. Similar impact of crystal packing, solvent incorporation and hydrogen bonding as lattice softening components is commonly reported for other mono-nuclear and di-nuclear cobalt dioxolene valence tautomers [25,26,27].

3.2. Magnetic Characterisation

Magnetic susceptibility data for all four complexes were measured on microcrystalline samples using a SQUID magnetometer in a temperature range from 2 K to 380 K, as shown in Figure 4. All four complexes show a low χ_MT values below room temperature, as anticipated for a trivalent cobalt diamagnetic ion, in agreement with the crystallographic data obtained at 173 K. For compound 1 only, X-ray structural analysis reveals one monoanionic semi-quinonate ligand, consistent with the overall charge balance of the complex, which leads to a finite value of 0.37 cm³ K mol⁻¹ at low temperature, fully consistent with an S = ½ spin state for an uncoupled radical ligand. At temperatures above 300 K this value then increases to reach 0.51 cm³ K mol⁻¹ at 380 K. This small raise in the χ_MT value is assigned to a VT as further supported by the variable temperature solid state reflectance measurement carried over in the temperature range of 20 °C to 120 °C (S1212). The presence of the small amount of paramagnetic impurity is seen from the residual magnetic moment below room temperature. The mono-nuclear complex 1 shows a small increase in the χ_MT 0.36 cm³ K mol⁻¹ at 300 K to 0.51 cm³ K mol⁻¹ at 380 K. The small raise in the χ_MT value is further supported by the variable temperature solid state reflectance measurement carried out in the temperature range of 20 °C to 120 °C. At 20 °C for the Co^III (sq)(cat) species, the most prominent absorption bands are found at 244 nm, 310 nm, and 638 nm. By increasing the temperature, the intensity of the former and the latter decreases while new bands at 222 nm and 760 nm appear. This behaviour and the position of the absorption bands are in good agreement with the other thermochromic, valence tautomeric cobalt dioxolene complexes which also confirms the compound 1 exhibiting VT behaviour [32,33] (Figure S12). In the case of the di-nuclear complex with the sulphate anion 3a, interestingly, above 300 K the magnetic moments show a sharp steady raise from a χ_MT value of 0.21 cm³ K mol⁻¹ at 300 K to 1.26 cm³ K mol⁻¹ at 380 K. This can be attributed to the VT interconversion to a cobalt(II) semi-quinonate along with a spin state transition to a high spin Co(II) species. The VT conversion in both the cases are incomplete within the measured temperature range. Literature evidence suggests that each uncoupled Co^II(sq) moiety will have a χ_MT value of 2.25 cm³ K mol⁻¹. In the case of the complexes 2 and 3b the magnetic moment remains unchanged over the measured temperature window which indicates VT interconversion does not exist over the measured temperature window because of the strong electron withdrawing nature of the tbcat ligand in the case of complex 2 and crystal packing effects in the case of complex 3b.

4. Conclusions

Herein, we reported the synthesis of a novel octadentate 1,1′-(piperazine-1,4-diyl)bis(N,N-bis(pyridin-2-ylmethyl)methanamine)(L^tpbap) ligand along with four novel cobalt complexes. [Co₂(L^tpbap)(3,5-dbcat)₂](SO₄)·5.5MeOH·2H₂O (3a) and [Co₂(L^tpbap)(3,5-dbcat)₂](ClO₄)₂·1.5 H₂O (3b). The mono-nuclear complex 1, [Co(3,5-dbsq)(3,5-dbcat)(4-Mepip)₂], and the di-nuclear complex 3a show a gradual incomplete valance tautomerism around 300 K and 370K, respectively. In contrast, the complexes 2 and 3b stay unchanged over the measured temperature window due to the effects of strong electron withdrawing ligand substituent on the catecholate in the case of complex 2 and crystal packing effects in solid state in the case of complex 3b. Ongoing work is carried out to further fine-tune the ligand design by adding different substituents in the pyridine ring of the L^tpbap to study the electronic effects on achieving a complete VT in this ligand system.