New Aspects of Ruthenium-Mediated Polyhedral Contraction of Monocarbollides

Abstract

It has been shown that the interaction of tris(triphenylphosphine)ruthenium dichloride RuCl₂(PPh₃)₃ (1) with 10-vertex monocarborane [6-Ph-nido-6-CB₉H₁₁]⁻[Et₄N]⁺ (2) under mild thermolysis conditions is not selective due to the undesired coordination of ruthenium to a phenyl substituent in the carborane and phosphine ligands, giving the series of new classical and non-classical metallacarborane complexes. In contrast, the reaction of 1 and monocarborane [arachno-6-CB₉H₁₄]⁻[Et₄N]⁺ (3) proceeds more selectively with the formation of the only one product, a isocloso-structured metallacarborane. The structures of two ruthenacarboranes were resolved by X-ray diffraction.

1. Introduction

Metallacarboranes are of particular interest due to their unique structures in which the metal atom is covalently bonded to the carborane polyhedron [1,2,3]. In the last decade, they have found a wide range of applications in catalysis [4,5]. In particular, the introduction of substituents into the carborane ligand allows to tune on the electronic, steric, and enantioselective properties of the catalyst. Thus, metallacarboranes showed good catalytic efficiency for amination of carbonyl compounds [6], radical polymerization of methyl methacrylate [7,8,9,10], oxidative coupling of benzoic acids and arenes with internal alkynes [11,12,13,14], carbene transfer reactions [15], as well as hydrogenation, isomerization, and metathesis of olefins [16,17,18]. Very recently, Romero and Teixidor with co-workers demonstrated the use of the cobalt bis(dicarbollide) anion and its derivatives as photocatalysts in various organic transformations [19,20,21].

Another way to modify electronic and steric parameters of metallacarboranes are the polyhedral contraction reactions, which occur with the removal of BH vertices [22,23,24]. However, many such reactions are not clean and selective, which reduces their use in synthesis. In continuation of our studies on the polyhedral contraction [25,26], in this work, we studied reactions of the electron-deficient complex RuCl₂(PPh₃)₃ (1) with 10-vertex monocarboranes [6-Ph-nido-6-CB₉H₁₁]⁻[Et₄N]⁺ (2) and [arachno-6-CB₉H₁₄]⁻[Et₄N]⁺ (3) to compare their outcome and selectivity. It turned out that the presence of phenyl substituent in 2 significantly complicates the polyhedral contraction due to undesirable coordination of this substituent, while carborane 3 reacts smoothly, giving only one ruthenacarborane.

2. Results and Discussion

We found that refluxing of carborane 2 and ruthenium complex 1 in MeOH gives new 10-vertex clusters 4a,b and 5a,b, zwitterionic ruthenium arene complex 6, as well as 8-vertex ruthenacarboranes 7a,b with a total yield of ~25% (Scheme 1). We have previously shown that a similar reaction of 2 with the osmium complex OsCl₂(PPh₃)₃ is more selective and gives the isocloso-{OsCB₈} cluster as the only stable product [25].

According to the X-ray diffraction data (Figure 1), diruthenacarborane 1-Ph-2-[5,9-exo-RuClPPh₃(μ,η⁶-C₆H₅PPh₂)]-5,9-(μ-H)₂-closo-2,1-RuCB₈H₅(OMe) (4a) has the geometry of a bicapped tetragonal antiprism and belongs to the classical 10-vertex clusters (22 skeletal electrons). The Ru(1) atom coordinates one chloride and two triphenylphosphine ligands, forming an exo-RuClPPh₃(PPh₂-μ,η⁶-C₆H₅)]⁺ fragment, which is bonded to the carborane frame via two B-H···Ru bonds involving boron atoms located in the α- and β-positions relative to the carbon atom. Notably, the formation of 4a is accompanied by the incorporation of methoxy to the 6-positioned boron atom and the loss of one boron vertex. Probably, the methoxylation of carborane cage is the first step of the polyhedral contraction, and methanol plays a key role in this fascinating process. For example, Stone with co-workers showed that the reaction of 2 with [ReBr(thf)₂(CO)₃] in non-alcoholic solvent (THF) leads to 11-vertex {ReCB₉} species without loss of BH vertexes [27]. Moreover, we recently found that polyhedral contraction can occur not only simultaneously upon coordination with a metal atom, but also upon simple boiling of a preliminarily prepared metallacarborane in methanol [26].

A similar structure was proposed for 1-Ph-2-[5,9-exo-RuClPPh₃(μ,η⁶-C₆H₅PPh₂)]-5,9-(μ-H)₂-closo-2,1-RuCB₈H₄(OMe)₂ (4b), which has two methoxy-substituents in the carborane ligand in contrast to 4a. Thus, the ¹H NMR spectra of 4a,b contain signals of MeO groups with δ 3.88 ppm (4a) and 3.46, 3.86 ppm (4b) with a relative intensity of 3H. Unfortunately, the exact position of the substituents in 4b has not been determined, since we were not able to obtain single crystals suitable for X-ray crystallography.

The ¹H NMR spectra of 4a,b also contain a set of broad high-field signals corresponding to the bridging protons of B-H···Ru groups (δ −3.34, −16.34 and −3.30, −15.83 ppm for 4a and 4b, respectively). It can be seen that the chemical shifts in the bridging H atoms differ greatly. A similar phenomenon was previously observed in “three-bridged” exonido-osmacarboranes exonido-5,6,10-[Cl(Ph₃P)₂Os]-5,6,10-(μ-H)₃-10-H-7-R-8-R¹-7,8-C₂B₉H₆ (R, R¹ = H, Alk) [28], where it was rigorously proven that the downfield B-H···Os bond signal corresponds to the bridging hydrogen atom that occupies the trans-position relative to the Os-Cl bond in the octahedral environment of the metal atom. By analogy, the signals B-H···Ru(1) with δ −16.34 and −15.83 ppm in ¹H NMR spectra of 4a,b were attributed to H(9) atoms, and the signals with δ −3.34 and −3.30 ppm were attributed to H(5).

In addition, the ¹H NMR spectra of both complexes contain signals characteristic of a coordinated arene ligand: a set of four well-resolved multiplets in the range δ 6.0−4.0 ppm. Each of these signals corresponds to one proton; the fifth signal overlaps with proton signals of uncoordinated phenyl rings. The downfield shift in the signal of one of the hydrogen atoms of the bridging μ,η⁶-phenyl ligand can be explained by its participation in the intramolecular Ph-o-H···Cl hydrogen bond. According to X-ray diffraction data for complex 4a, the distance between H(23A) and the Cl atom (2.48 Å) is shorter than the sum of the corresponding Van der Waal radii (2.86 Å [29]) and the C-H···Cl angle is 141.4° (average values are given for two independent molecules). Such μ,η⁶-coordination of triphenylphosphine ligands is known for a number of complexes [30]. Among them, several crystallographically studied clusters belonging to polyhedral boron-containing complexes are described: PMe₂Ph-μ-η⁶(Ru)-η¹(Pt)-(C₆H₅PPh₂)-closo-PtRuB₉H₉ [31] and μ,η⁶-C₆H₅PPh₂)]-7,11-(μ-H)₂-closo-2,1-RuCB₁₀H₈R (R = H, 6-OMe, 3-OMe) [32], in which the phosphine ligand on the one metal acts as an η⁶-coordinated ligand with respect to another metal atom.

Other products of the reaction discussed are mononuclear 10-vertex isocloso complexes: 1,3-(PPh₃)₂-1-H-1-Cl-2-Ph-4-OMe-isocloso-1,2-RuCB₈H₆ (5a) and 1-PPh₃-1-H-1-Cl-2-Ph-4-OMe-isocloso-1,2-RuCB₈H₅(OMe)(PPh₃) (5b), which belong to electron-deficient 2n clusters (20 skeletal electrons). We used label “isocloso” for these compounds to emphasize their non-conventional structure, which is quite distinct from the cluster geometry of classically structured closo 10-vertex metallacarbaboranes [33]. Similar to 4a,b, the formation of 5a,b is also accompanied by the deboronation of the carborane framework and the methoxylation process, which leads to a mixture of mono- and dimethoxy-substituted derivatives. It should be noted that monomethoxy substituted compound 5a is indefinitly stable and does not transform into 5b upon boiling in methanol.

According to X-ray diffraction analysis (Figure 2), complex 5a has the geometry of a 10-vertex polyhedron with the {CB₈} carborane ligand substituted with the MeO group and triphenylphosphine at the boron atoms. The ruthenium atom coordinates the six-membered {CB₅} open face of carborane in the hexahapta chair conformation. The metallacarborane cage has idealized C_3v symmetry with the ruthenium atom in the axial position. In the molecule, there are three short distances from the Ru(1) atom to the C(2), B(3), and B(4) atoms with a cluster coordination number of 4 [2.188(14), 2.063(13), 2.090(14) Å) and three longer distances to B(5), B(6), and B(7) atoms with a coordination number of 5 [2.448(14), 2.364(11), 2.514(14) Å].

The ¹¹B NMR spectra of complexes 5a and 5b contain three groups of signals in the range δ −30–+83 ppm. The signals of the first group are in a low field (δ from +64 to +83 ppm) and, in accordance with [34], are assigned to B(3) and B(4) atoms, which have a low coordination number equal to 4. Since one of the signals is present as a singlet, it can be concluded that the boron atom [B(3) or B(4)] corresponding to this signal has a MeO substituent. The evidence that in complexes 5a and 5b one of the PPh₃ groups is also bound to the boron atom is the presence of doublet splitting with ¹J(P,B) = 160 and 138 Hz, respectively, in one of the signals in the ¹¹B/¹¹B{¹H} NMR spectra of these compounds. Despite the presence of two different PPh₃ groups in complexes 5a and 5b (at the metal atom and in the carborane ligand), in the ¹H NMR spectra, the signal of the terminal hydride appears as a broadened doublet with a spin–spin coupling constant ²J(P,H) = 63 and 43 Hz, respectively. However, in the ¹H{¹¹B} NMR spectrum of complex 5a (with complete decoupling from ¹¹B), the hydride signal already has the form of a doublet of doublets, and the long-range spin–spin coupling constant ³J(P,H) = 16 Hz can be observed experimentally. The position of the triphenylphosphine substituent as well as the second methoxy group in 5b is not defined.

The zwitterionic mononuclear complex 6-[η⁶-C₆H₅RuH(PPh₃)₂-nido-6-CB₉H₁₁ (6) was also isolated in the reaction in low yield. The structure of complex 6 was determined from the ¹H and ³¹P{¹H} NMR spectra. According to the C_s symmetry, the ¹¹B NMR spectrum of complex 6 contains a set of six signals with a relative intensity of 2:1:2:2:1:1. In the ³¹P{¹H} NMR spectrum, there is one singlet signal from two equivalent PPh₃ groups with δ 53.8 ppm. In the ¹H NMR spectrum, a high-field broad signal was found corresponding to two bridging (B-H-B) hydrogen atoms of carborane (δ −3.17 ppm) and a terminal hydride triplet signal [δ −9.95 ppm, J(P,H) = 38 Hz].

The ruthenium atom η⁶-coordinates the phenyl group of the carborane, as well as one hydride and two triphenylphosphine ligands. At the same time, the carborane ligand in 6 retains two B–H–B bridging hydrogen atoms in the same position as in the starting carborane 2, i.e., it formally has a negative charge. Taking into account that the ruthenium atom coordinates only one hydride ligand, it should be concluded that complex 6 is zwitterionic, and the metal atom has a formal oxidation state of +2. A similar example of zwitterionic metallacarboranes, 6-[η⁶-C₆H₅RuH(PPh₃)₂-nido-7,9-C₂B₉H₁₁, is described in the literature [35].

Among the reaction products, another group of metallacarboranes with an extraordinary structure was found, which belongs to rare 8-vertex capped closo-clusters with a boron vertex built over a triangular face. Two compounds of this type were isolated by column chromatography: capped closo-l-Ph-2,2-(PPh₃)2-2-H-3,8-(OMe)₂-6-R-2,1-RuCB₆H₃ [R = H (7a), R = OMe (7b)], which are products of a deeper degradation (the loss of two boron vertices) of the initial carborane 2.

Structure of compounds 7a,b was confirmed using the ¹H, ³¹P{¹H}, ¹¹B/¹¹B{¹H} NMR and mass spectroscopic data by analogy with the carbon-unsubstituted analogue capped closo-2,2-(PPh₃)₂-2-H-3,6,8-(OMe)₃-2,1-RuCB₆H₄ (

Table 1. Selected NMR Data for 7a,b and capped closo-2,2-(PPh₃)₂-2-H-3,6,8-(OMe)₃-2,1-RuCB₆H₄.

NMR		7a	7b	(PPh3)2HRuCB6H4(OMe)3 [36]
1H	OMe	4.24 s (3H); 2.96 s (3H)	4.27 s (3H); 2.80 s (6H)	4.17 s (3H); 3.05 s (6H)
	HRu	−9.84 br t (1H), 2J(H,P) = 24 Hz	−10.43 br t (1H), 2J (H,P) = 24 Hz	−9.02 br t (1H), 2J(H,P) = 20.7 Hz
31P{1H}	PPh3	46.2 d (1P), 2J = 10 Hz; 46.0 d (1P), 2J = 10 Hz	48.3 s (2P)	49.4 s (2P)
11B{1H}	B-8	67.5 s (1B)	70.1 s (1B)	68.9 s (1B)
	B-3,6	33.7 s (1B); 24.1 s (1B)	32.8 s (2B)	33.7 s (2B)
	B-7	14.0 s (1B)	14.1 s (1B)	12.8 s (1B)
	B-4,5	−26.3 (1B); −30.1 (1B)	−32.4 s (2B)	−31.8 s (2B)

), whose structure was unambiguously confirmed previously by X-ray diffraction [36].

In the ¹H NMR spectrum of complex 7b, two of the three MeO groups appear as a single signal (δ 2.8 ppm) with an intensity of 6H, which indicates their equivalence and, therefore, the location at the B(3) and B(6) (both are in the α-position relative to the carbon atom). Taking into account the preservation of symmetry in this complex, the third MeO-group should be bound to the B(8) atom, which is located symmetrically over the Ru(2)-B(4)-B(5) face. In the ¹H NMR spectrum, the signal of the terminal hydride has the form of a broadened triplet, while in the ³¹P{¹H} NMR spectrum, only one singlet signal (δ 48.3 ppm) from two equivalent PPh₃ ligands bound to the metal atom is observed. In the ¹¹B{¹H} NMR spectrum of 7b, in accordance with the C_s symmetry, a set of four signals with a relative intensity of 1:2:1:2 is observed. Extremely weak field signal with δ 70.1 ppm confirms the presence of a coordinatively unsaturated B(8) atom in the complex.

Complex 7a with two MeO groups has C₁ symmetry. Therefore, in the ¹¹B NMR spectrum, six signals appear, each with an intensity of 1B. Two of these signals are singlets, which indicates two MeO substituents associated with the carborane ligand. Here, as in spectrum of 7b, one signal from the B(8) atom appears in a much weaker field than the others (the assignment of some of them was performed by analogy with complex 7b). The ³¹P{¹H} NMR spectrum of 7a contains two doublet signals with δ 46.2 and 46.0 ppm with ²J(P,P) = 10 Hz. Accordingly, in the ¹H NMR spectrum, the hydride signal appears as a broadened triplet with ²J(P,H) = 24 Hz.

In contrast to the previous reaction, RuCl₂(PPh₃)₃ under similar conditions selectively reacts with monocarbon carborane [arachno-6-CB₉H₁₄]⁻[Et₄N]⁺ (3), forming 10-vertex ruthenacarborane 1,1-(PPh₃)₂-1-H-3-OMe-isocloso-1,2-RuCB₈H₇ (8) as the only stable product in 26% yield (Scheme 2). The higher selectivity in this case can be explained by the absence of phenyl substituent in 3, which significantly complicates the reactions due to the strong coordination affinity of ruthenium to arene ligands [37].

The structure of 8 was established on the basis of the ¹H, ³¹P{¹H}, and ¹¹B/¹¹B{¹H} NMR spectra. The ¹H NMR spectrum contains a signal of the hydrogen atom of the CH group of the carborane ligand (δ 7.83 ppm), which is a doublet with ²J(P,H) = 10 Hz, and one singlet signal of the MeO group (δ 4.03 ppm). The hydride signal with δ −5.30 ppm appears as a broadened doublet of doublets with ²J(P^a,H) = 22 Hz and ²J(P^b,H) = 46 Hz. In the ³¹P{¹H} NMR spectrum, one of the two signals of the phosphorus atoms of triphenylphosphine ligands is a broadened singlet (δ 49.2 ppm), and the second is a doublet (δ 35.9 ppm) with ²J_AB = 15 Hz, probably due to the trans-location of the first ligand relative to one of the boron atoms of the carborane fragment. In the ¹¹B NMR spectra of complex 8, as well as in the spectra of ruthenacarboranes 5a,b, there is a broad singlet signal (δ 83.1 ppm) with doubled intensity, which we attributed to two coordinatively unsaturated atoms B(3) and B(4). Since the remaining signals in the spectrum have the form of doublets with their usual spin–spin coupling constant ¹J(B,H) in the range of 130–145 Hz, it can be concluded that the MeO group is located precisely at one of these boron atoms, B(3) or B(4) (the positions are equivalent).

3. Materials and Methods

3.1. General Procedures

The reaction described above was carried out using standard Schlenk equipment under an atmosphere of dry argon. All solvents, including those used for column chromatography as eluents, were dried under appropriate drying agents and distilled under argon prior to use. The reactions products were isolated by column chromatography and purified by crystallization in air. Chromatography was performed on silica gel (Merck, 70–230 mesh). The NMR spectra were obtained with a Bruker AMX-400 spectrometer (¹H, 400.13 MHz; ³¹P, 161.98 MHz; ¹¹B, 128.33 MHz) using TMS as an internal reference and 85% H₃PO₄ and BF₃·Et₂O as the external references, respectively. Microanalyses were performed at the Analytical Laboratory of the Institute of Organoelement Compounds of the RAS. The starting material, tris(triphenylphosphine)ruthenium dichloride (1) was prepared by methods published elsewhere [38]. Tetraethylammonium 6-phenyl-nido-6-carbadecaborate (2) and tetraethylammonium arachno-6-carbadecaborate (3) were obtained from the laboratory of Prof. Kennedy J.D. (University of Leeds, Leeds, UK).

3.2. Reaction of RuCl₂(PPh₃)₃ (1) with Tetraethylammonium 6-Phenyl-Nido-6-Carbadecaborate (2)

A solution of complex 1 (1 g, 1.04 mmol) in 50 mL of benzene was added dropwise over 2 h to a stirred solution of nido-carborane 2 (0.38 g, 1.16 mmol) in 50 mL of anhydrous methanol under argon at low boiling. The use of toluene as a co-solvent instead of benzene leads to a significant decrease in the yield of products. The solution was refluxed with stirring for another 3 h, while the color of the reaction mixture changed from brown to dark red. After cooling the reaction mixture and removing the solvent, the solid residue was placed on a silica gel column 63–210 μm, eluting with CH₂Cl₂. The first light yellow fraction containing PPh₃, 6, and 7a,b, and the next two red fractions, 4a, 5a and 4b, 5b. The mixture of products from the light yellow fraction was separated again on a column of silica gel in benzene, washing off, successively, 7a, 7b, and 6. The products from the two red fractions were also chromatographed on a column of silica gel using CH₂C1₂/n-hexane (2:1) as the eluent; from the first red fraction, an orange substance 5a and a red substance 4a were obtained; from the second red fraction, an orange product 5b and a red product 4b. The complexes were finally purified by recrystallization from a CH₂C1₂/n-hexane solvent mixture.

1-Ph-2-[5,9-exo-RuClPPh₃(μ,η⁶-C₆H₅PPh₂)]-5,9-(μ-H)₂-closo-2,1-RuCB₈H₅(OMe)(4a). Yield 64 mg, 6%. IR, ν/cm⁻¹: 2546 (B-H). ¹H NMR (400.13 MHz, CD₂Cl₂): δ 7.83, 7.60–7.18, 6.84 m, 30H + 1H (H_Ph); 5.91 t, 1H, J = 6 Hz (η-Ph); 5.53 t, 1H, J = 6 Hz (η-Ph); 5.05 t, 1H, J = 6 Hz (η-Ph); 4.16 t, 1H, J = 6 Hz (η-Ph); 3.88 s, 3H (OMe); −3.34 br m, 1H (H-5); −16.34 br m, 1H (H-9). ³¹P{¹H} NMR (161.98 MHz, CD₂Cl₂): δ 64.1 d, 1P, ²J_AB = 32 Hz; 59.2 d, 1P, ²J_AB = 32 Hz. ¹¹B NMR (128.33 MHz, CD₂Cl₂): δ 42.5 s, 1B; 16.6 d, 1B, ¹J(B,H) = 137 Hz; 10.9 d, 2B, ¹J(B,H) = 145 Hz; 8.6 d, 1B, ¹J(B,H) = 137 Hz; −6.5 d, 1B, ¹J(B,H) = 143 Hz; −20.2 d, 2B, ¹J(B,H) = 131 Hz. For C₄₄H₄₅B₈ClOP₂Ru₂ (975.86) calculated (%): C, 54.15; H, 4.65; B, 8.86; P, 6.35. Found (%): C, 54.05; H, 4.52; B, 8.97; P, 6.28.

1-Ph-2-[5,9-exo-RuClPPh₃(μ,η⁶-C₆H₅PPh₂)]-5,9-(μ-H)₂-closo-2,1-RuCB₈H₄(OMe)₂(4b). Yield 54 mg, 5%. IR, ν/cm⁻¹: 2541 (B-H). ¹H NMR (400.13 MHz, CD₂Cl₂): δ 7.83–7.15, 6.94 m, 30H + 1H (H_Ph); 5.76 t, 1H, J = 6 Hz (η-Ph); 5.44 t, 1H, J = 6 Hz (η-Ph); 4.88 t, 1H, J = 6 Hz (η-Ph); 3.98 t, 1H, J = 6 Hz (η-Ph); 3.86 s, 3H (OMe); 3.46 s, 3H (OMe); −3.30 br m, 1H (H-5); −15.83 br m, 1H (H-9). ³¹P{¹H} NMR (161.98 MHz, CD₂Cl₂): δ 63.1 d, 1P, ²J_AB = 32 Hz; 58.3 d, 1P, ²J_AB = 32 Hz. ¹¹B NMR (128.33 MHz, CD₂Cl₂): δ 38.0 br s, 1B; 27.5 br s, 1B; 4.8 br s, 2B; −12.4 br s, 1B; −22.6 br s, 2B; −31.4 br s, 1B. For C₄₅H₄₇B₈ClO₂P₂Ru₂ (1005.88) calculated (%): C, 53.73; H, 4.71; B, 8.60; P, 6.16. Found (%): C, 53.54; H, 4.69; B, 8.71; P, 6.21.

1,3-(PPh₃)₂-1-H-1-Cl-2-Ph-4-OMe-isocloso-1,2-RuCB₈H₆ (5a). Yield 35 mg, 4%. IR, ν/cm⁻¹: 2531 (B-H); 1989 (Ru-H). ¹H{¹¹B} NMR (400.13 MHz, CD₂Cl₂): δ 7.92–7.06 m, 6.03 t, 28H + 2H (H_Ph); 3.64 br s, 1H (BH); 3.44 br s, 1H (BH); 3.16 br s, 1H (BH); 2.99 s, 3H (OMe); 2.85 br s, 1H (BH); 2.69 br s, 1H (BH); 0.00 br s, 1H (BH); −5.44 dd, 1H, ²J(P,H) = 16 Hz, ²J(P,H) = 63 Hz (H_Ru). ³¹P{¹H} NMR (161.98 MHz, CD₂Cl₂): δ 50.1 s, 1P (Ru-PPh₃); 8.4 q, 1P, ¹J(P,B) = 160 Hz (B-PPh₃). ¹¹B NMR (128.33 MHz, CD₂Cl₂): δ 82.9 s, 1B (B-4); 64.5 d, 1B, ¹J(B,P) = 160 Hz (B-3); 14.1 d, 1B, ¹J(B,H) = 141 Hz (B-9); 12.5 d, 1B, ¹J(B,H) = 137 Hz (B-10); 1.7 d, 1B, ¹J(B,H) = 146 Hz (B-8); −6.9 d, 1B, ¹J(B,H) = 118 Hz (B-7); −9.8 d, 1B, ¹J(B,H) = 128 Hz (B-5); −21.8 d, 1B, ¹J(B,H) = 131 Hz (B-6). ¹³C{¹H} NMR (100.51 MHz, CD₂Cl₂): δ 169.8 br s (C_carb); 147.5, 136.0−126.6, 123.5, 122.9 (C_Ph); 57.8 s (OMe). For C₄₄H₄₅B₈ClOP₂Ru (874.79) calculated (%): C, 60.41; H, 5.18; B, 9.89; P, 7.08. Found (%): C, 60.36; H, 5.22; B, 10.02; P, 6.98.

1-PPh₃-1-H-1-Cl-2-Ph-4-OMe-isocloso-1,2-RuCB₈H₅(OMe)(PPh₃)(5b). Yield 24 mg, 3%. IR, ν/cm⁻¹: 2536 (B-H); 1999 (Ru-H). ¹H NMR (400.13 MHz, CD₂Cl₂): δ 7.53-7.23 m, 35H (H_Ph); 4.15 s, 3H (OMe); 3.71 s, 3H (OMe); −3.11 br d, 1H, ²J(H,P) = 43 Hz (H_Ru). ³¹P{¹H} NMR (161.98 MHz, CD₂Cl₂): δ 45.9 s, 1P (Ru-P); 5.3 br q, 1P (B-P). ¹¹B NMR (128.33 MHz, CD₂Cl₂): δ 79.4 s, 1B (B-4); 73.5 d, 1B, ¹J(B,H) = 96 Hz (B-3); 16.6 d, 1B, ¹J(B,H) = 122 Hz [B-8 (9 or 10)]; 13.4 s, 1B [B-5 (6 or 7)]; 8.41 d, 2B, ¹J(B,H) = 112 Hz [B-9,10 (8,10 or 8,9)]; −26.7 d, 1B, ¹J(B,H) = 151 Hz [B-6 (7 or 5)]; −29.8 d, 1B, ¹J(B,P) = 138 Hz [B-7 (5 or 6)]. For C₄₅H₄₇B₈ClO₂P₂Ru (904.81) calculated (%): C, 59.73; H, 5.24; B, 9.56; P, 6.85. Found (%): C, 60.01; H, 5.21; B, 9.58; P, 6.76. m/z 904.2 [M]⁺.

6-[(η⁶-C₆H₅)RuH(PPh₃)₂]-nido-CB₉H₁₁(6). Yield 14 mg, 2%. IR, ν/cm⁻¹: 2534 (B-H); 1985 (Ru-H). ¹H NMR (400.13 MHz, CD₂Cl₂): δ 7.34−7.16 m, 30H (PPh₃); 6.13 t, 1H, ³J(H,H) = 5.5 Hz (H_para, η-Ph); 5.05 d, 2H, ³J(H,H) = 6.5 Hz (H_ortho, η-Ph); 4.85 t, 2H, ³J(H,H) = 6 Hz (H_meta, η-Ph); −3.17 br s, 2H (μ-H); −8.95 t, 1H, ²J(H,P) = 38 Hz (H_Ru). ³¹P{¹H} NMR (161.98 MHz, CD₂Cl₂): δ 53.8 s, 2P (PPh₃). ¹¹B NMR (128.33 MHz, CD₂Cl₂): δ 4.0 d, 2B, ¹J(B,H) = 142 Hz; 2.1 d, 1B, ¹J(B,H) = 80 Hz; −3.2 d, 2B, ¹J(B,H) = 141 Hz; −11.5 d, 2B, ¹J(B,H) = 115 Hz; −25.7 d, 1B, ¹J(B,H) = 158 Hz; −35.6 d, 1B, ¹J(B,H) = 143 Hz. For C₄₃H₃₇B₉P₂Ru (824.15) calculated (%): C, 62.67; H, 5.75; B, 11.81; P, 7.52. Found (%): C, 62.61; H, 5.71; B, 11.92; P, 7.48. m/z 824.3 [M]⁺.

Capped closo-1-Ph-2,2-(PPh₃)₂-2-H-3,8-(OMe)₂-2,1-RuCB₆H₄(7a). Yield 24 mg, 3%. IR, ν/cm⁻¹: 2522 (B-H); 2050 (Ru-H). ¹H NMR (400.13 MHz, CD₂Cl₂): δ7.30−6.97 m, 35H (H_Ph); 4.24 s, 3H (OMe); 2.96 s, 3H (OMe); −9.84 br t, 1H, ²J(H,P) = 24 Hz (H_Ru). ³¹P{¹H} NMR (161.98 MHz, CD₂Cl₂): δ 46.2 d, 1P, ²J_AB = 10 Hz; 46.0 d, 1P, ²J_AB = 10 Hz. ¹¹B NMR (160.46 MHz, CD₂Cl₂): δ 67.5 s, 1B (B-8); 33.7 s, 1B (B-3); 24.1 br s, 1B [B-6 (7)]; 14.0 d, 1B, ¹J(B,H) = 147 Hz [B-7 (6)]; -26.3 d, 1B, ¹J(B,H) = 149 Hz [B-4 (5)]; −30.1 d, 1B, ¹J(B,H) = 147 Hz [B-5 (4)]. For C₄₅H₄₆B₆O₂P₂Ru (846.73) calculated (%): C, 63.83; H, 5.48; B, 7.66; P, 7.32. Found (%): C, 63.75; H, 5.52; B, 7.51; P, 7.16. m/z 847.3 [M]⁺.

Capped closo-1-Ph-2,2-(PPh₃)₂-2-H-3,6,8-(OMe)₃-2,1-RuCB₆H₃(7b). Yield 10 mg, 1%. IR, ν/cm⁻¹: 2517 (B-H); 2043 (Ru-H). ¹H NMR (400.13 MHz, CD₂Cl₂): δ 7.24−6.96 m, 6.48 d, 35H (H_Ph); 4.27 s, 3H (OMe); 2.80 s, 6H (OMe); −10.43 br t, 1H, ²J(H,P) = 24 Hz (H_Ru). ³¹P{¹H} NMR (161.98 MHz, CD₂Cl₂): δ 48.3 s, 2P. ¹¹B{¹H} NMR (128.33 MHz, CD₂Cl₂): δ 70.1 s, 1B (B-8); 32.8 s, 2B (B-3,6); 14.1 s, 1B (B-7); −32.4 s, 2B (B-4,5). For C₄₆H₄₈B₆O₃P₂Ru (876.76) calculated (%): C, 63.02; H, 5.52; B, 7.40; P, 7.07. Found (%): C, 63.38; H, 5.49; B, 7.48; P, 7.34. Found: m/z 876.4 [M]⁺.

3.3. Reaction of RuCl₂(PPh₃)₃ (1) with Tetraethylammonium Arachno-6-Carbadecaborate (3)

Complex 1 (0.5 g, 0.52 mmol) was added to a stirred solution of arachno-carborane 3 (0.16 g, 0.63 mmol) in 15 mL of absolute methanol under argon. The mixture was refluxed with stirring for 14 h. Gradually, the precipitate turned from brown to yellow. The reaction mixture was cooled, and after removing the solvent, the resulting oily residue was chromatographed through a silica-gel column (63–210 μm) in a mixture of solvents CH₂Cl₂/n-hexane (2:1). After crystallization from a mixture of solvents CH₂Cl₂/n-hexane, yellow crystals of complex 8 were obtained.

1,1-(PPh₃)₂-1-H-3-OMe-isocloso-1,2-RuCB₈H₇(8). Yield 0.11 g, 26%. IR, ν/cm⁻¹: 2538 (B-H); 1990 (Ru-H). ¹H NMR (400.13 MHz, CD₂Cl₂): δ 7.83 br d, 1H, ³J(H,P) = 10 Hz (CH_carb); 7.40−7.16 m, 30H (Ph); 4.03 s, 3H (OMe); −5.30 br dd, 1H, ²J(P^a,H) = 22 Hz, ²J(P^b,H) = 46 Hz (H_Ru). ³¹P{¹H} NMR (161.98 MHz, CD₂Cl₂): δ 49.2 br s, 1P; 35.9 d, 1P, ²J_AB = 15 Hz. ¹¹B NMR (128.33 MHz, CD₂Cl₂): δ 83.1 br s, 2B (B-3,4); 16.6 d, 1B, ¹J(B,H) = 137 Hz [B-8 (9 or 10)]; 10.9 d, 1B, ¹J(B,H) = 145 Hz [B-9 (10 or 8)]; 8.6 d, 1B, ¹J(B,H) = 137 Hz [B-10 (8 or 9)]; −6.5 d, 1B, ¹J(B,H) = 143 Hz [B-5 (6 or 7)]; −20.2 d, 2B, ¹J(B,H) = 131 Hz [B-6,7 (5,7 or 5,6)]. For C₃₈H₄₂B₈OP₂Ru (764.25) calculated (%): C, 59.72; H, 5.54; B, 11.32; P, 8.11. Found (%): C, 59.48; H, 5.58; B, 11.50; P, 8.32.

3.4. X-ray Diffraction Study of 4a and 5a

The X-ray single crystal data were collected using Mo Kα radiation (λ = 0.71073 Å) on Bruker SMART 1000 diffractometer equipped with the Cryostream (Oxford Cryosystems) open-flow nitrogen cryostat. The structures were solved by direct methods and refined by the full-matrix least squares technique against F² with anisotropic thermal parameters for all non-hydrogen atoms using the SHELXL software [39]. Hydrogen atoms of the carborane ligands, as well hydride ligand in 5a, were located from the Fourier syntheses. The remaining hydrogen atoms were placed geometrically and included in the structure factors calculation in the riding motion approximation. Crystal data and parameters of the refinements are listed in

Table 2. Crystal data, data collection and structure refinement parameters for 4a and 5a.

Compound	4a	5a
CCDC No.	2201207	2201208
Empirical formula	C44H45B8ClOP2Ru2	C44H45B8ClOP2Ru · 2(CH2Cl2)
Molecular weight	975.81	1044.59
Temperature (K)	120(2)	110(2)
Crystal system	triclinic	monoclinic
Space group	$P\overline{1}$	P21/c
a (Å)	10.560(2)	18.467(11)
b (Å)	20.205(4)	15.515(9)
c (Å)	22.352(4)	18.611(11)
α (deg)	114.814(4)	90
β (deg)	101.570(4)	100.170(11)
γ (deg)	90.753(4)	90
V (Å3)	4215.4(14)	5249(5)
Z	4	4
Dcalc (g cm−3)	1.538	1.322
linear absorption μ (cm−1)	8.92	6.47
Tmin/Tmax	0.659/0.862	0.560/0.928
2θmax (deg)	58	52
Reflections collected	54553	45233
Independent reflections (Rint)	22374 (0.0329)	10275 (0.1263)
Observed reflections (I > 2σ(I))	15232	5005
Number of parameters	1102	602
R1 (on F for I > 2σ(I)) a	0.0462	0.0987
wR2 (on F2 for all data) b	0.1000	0.2767
GOOF	1.075	1.009
Largest diff. peak/hole (e Å−3)	1.792/−0.643	2.931/−1.039

^a R₁ = Σ ||F_o| − |F_c||/Σ|F_o|, ^b wR₂ = {Σ[w(F_o² – F_c²)²]/Σw(F_o²)²}^1/2.

. Crystallographic data for the structures have been deposited with the Cambridge Crystallographic Data Centre as supplementary publications CCDC 2201207–2201208 and can be found in the Supplementary Materials.

4. Conclusions

In summary, we showed that the interaction of arachno-carborane 3 with ruthenium complex 1 leads to the selective formation of a 10-vertex 20-electron isocloso-cluster as the only stable product, whereas an analogous reaction of the phenyl substituted nido-carborane 2 produces a complex mixture of cluster compounds. For example, a number of products with the (η⁶-Ph)Ru motif were isolated along with other 8- and 10-vertex clusters, and this can be explained by the strong coordination affinity of ruthenium to arene ligands. By contrast, we have shown previously that the related osmium complex OsCl₂(PPh₃)₃ provides a more selective polyhedral contraction of monocarbollides [25,26].