Cluster Fusion: Face-Fused Macropolyhedral Tetracobaltaboranes

Publication Date (Web): December 7, 2018 ... cluster that contains icosahedral {Co4B8}, square-pyramidal {CoB4}, tetrahedral {Co2B2}, and nido-{CoB7} ...
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Cluster Fusion: Face-Fused Macropolyhedral Tetracobaltaboranes Mohammad Zafar, Sourav Kar, Chandan Nandi, Rongala Ramalakshmi, and Sundargopal Ghosh* Department of Chemistry, Indian Institute of Technology (IIT) Madras, Chennai 600036, India

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cobaltaboranes, two new compounds were characterized by H{11B}, 11B{1H}, and 13C{1H} NMR, IR spectroscopy, and mass spectrometry. The mass spectrum of 1 showed a molecular-ion peak at m/z 953.4417 ([M + H]+), whereas 2 showed a molecular-ion peak at m/z 835.3724 ([M + H]+). The room temperature 11B{1H} NMR spectrum of 1 shows the presence of 10 chemical shifts between −12.0 and +56.3 ppm. The 1H{11B} NMR of 1 suggests the existence of three different Cp* environments appearing at 1.81, 1.63, and 1.53 ppm with a relative ratio of 1:1:2. The 11B{1H} NMR spectrum of 2 shows a total of 16 resonances between −45.9 and +158.9 ppm. The 1 H{11B} NMR also shows the presence of three different Cp* environments at 1.79, 1.75, and 1.61 ppm with an intensity ratio of 1:1:1. Although a higher molecular-ion peak and 11B{1H} NMR spectra of 1 and 2 specify the formation of large-size clusters, these spectroscopic data were not sufficient to envisage the identity of both 1 and 2. A clear explanation eluded us until solid-state X-ray structure analyses of 1 and 2 were carried out. The molecular structure of 1, shown in Figure 1 (left), is based on a symmetric 19-vertex tetracobaltaborane [(Cp*Co)3B15H10{Cp*CoH}]. The architecture comprises 12-vertex {Co3B9}, tetrahedral {B4}, and 10-vertex {CoB9} units. The 12-vertex moiety has a closo-icosahedral geometry, whereas the 10-vertex moiety has a closo-bicapped squareantiprismatic geometry. As shown in Figure 1 (left), the square faces of the closo-bicapped square-antiprismatic core are made of B1−B2−B11−B15 and Co1−B3−B12−B5. Three of the common triangular faces in 1, B6−B11−B15, B13−B11−B15, and B12−B11−B15, are used for face fusion, which eventually led all of the fused boron atoms to have a higher degree of connectivity. One of the most striking features of 1 is the presence of a symmetry plane that bisects the B8−B9, B11−B15, and B1−B2 bonds and passes through the Co1−B4 bond. All three core clusters that are used for the construction of 1 have a common B11−B15 edge [Figure 1 (left)]. As a result, the B11− B15 bond distance of 1.97 Å is significantly longer compared to other B−B bonds in 1. All other B−B and Co−B bond distances are in the normal range, established for the macropolyhedral cluster. The terminal B−H bond lengths range from 1.05(6) to 1.260(5) Å. As shown in Figure 1 (left), the icosahedral unit in 1 is not a perfect one because the pentagons are distorted. For example, the interior angle of the Co3−B14−Co4−B15−B11 pentagon is in the range of 93.13−120.60° (average 107.2°), less than the expected one (108°) for a planar pentagon. Consistent with the single-crystal X-ray structure analysis, the 11 1 B{ H}, 1H{11B}, and 13C{1H} NMR spectra of 1 evidently

ABSTRACT: In an effort to isolate the 16-vertex supraicosahedral cobaltaborane [(Cp*Co)3B12H12Co{Cp*CoB4H9}] (Cp* = η5-C5Me5), we have pyrolyzed an in situ generated intermediate, obtained from the fast metathesis of [Cp*CoCl]2 and [LiBH4·THF], with an excess amount of [BH3·THF]. Although the objective of isolating the 16-vertex cobalt analogue was not achieved, the reaction yielded a closo-19-vertex face-fused cluster presenting icosahedral {Co3B9}, tetrahedral {B4}, and 10vertex {CoB9} units. The reaction also yielded a 20-vertex face-fused cluster that contains icosahedral {Co4B8}, square-pyramidal {CoB4}, tetrahedral {Co2B2}, and nido{CoB7} units.

1

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eginning with the founding work of Lipscomb1 and Hawthorne et al.,2 followed by many main-group pioneers, the bonding patterns around boron have developed both experimentally3−5 and theoretically.6−8 Concomitantly, the chemistry associated with the supraicosahedral metallaborane and metallacarborane clusters has been augmenting the field of organometallic chemistry. Although, during the last 50 years, Evans and Hawthorne,9 Welch et al.,5a Xie et al.,10,11 and we12 reported various 12−16-vertex single-cage clusters enriched with boron, carbon, and transition metals, the single-cage cluster beyond 16 vertex is still a boron chemist’s dream. Highernuclearity clusters containing large numbers of boron have a wide range of applications starting from the field of polymers, nanomaterials, ceramics, and boron neutron capture therapy.3a−c,13 Apart from the applications, the expansion of higher vertex polyhedral boron clusters allows one to investigate unique bonding and their exceptionally large-size electronic structures. Numerous methodologies for the synthesis of supraicosahedral clusters containing main-group and transition-metal fragments have received substantial attention.14,15 One of the optimal and convenient synthetic methods for the construction of higher-nuclearity metallaborane clusters is mainly based on the reaction of pentamethylcyclopentadienylmetal chlorides and monoborane reagents, for example, LiBH4·THF (THF = tetrahydrofuran), BH3·THF, and BH3·SMe2.16,17 Using this methodology, we have recently reported the 15- and 16-vertex supraicosahedral rhodaborane clusters.12 Although rhodium and cobalt are in the same group, isolation of supraicosahedral cobaltaborane had met with no success to date. As a result, in the progress of our studies for the isolation of higher vertex metallaborane, we have revised the synthetic methodology, i.e., the treatment of [LiBH4·THF] with [Cp*CoCl]2, followed by pyrolysis with an excess amount of [BH3·THF]. In parallel with the formation of some known © XXXX American Chemical Society

Received: September 25, 2018

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DOI: 10.1021/acs.inorgchem.8b02740 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

Figure 1. Molecular structures and labels for 1 (left) and 2 (right). Note that the Cp* ligands attached to cobalt are not shown for clarity; Co4 in molecule 2 does not contain the Cp* ligand. Selected bond lengths (Å) and angles (deg) for 1: B1−B6 1.816(10), B4−Co1 2.003(9), B6−Co2 2.009(7), B11−B13 1.851(10), B12−B13 1.775(11), B13−Co3 2.024(7), B14−Co4 2.100(7); B13−Co3−B11 54.4(3), B13−B11−B12 57.5(4). Selected bond lengths (Å) and angles (deg) for 2: B1−B4 1.749(14), B3−B4 1.732(15), B5−B8 1.84(2), Co4−B9 1.993(11), Co1−B8 1.94(2); B1− B4−Co3 118.8(6), B8−B7−Co3 67.9(8).

indicate the presence of a symmetry plane. The HSQC NMR of 1 implies the presence of no hydrogen atoms at B11, B15, B6, B12, and B13. The 11B chemical shift at 56.3 ppm can be assigned for two boron atoms [B11 and B15; Figure 1 (left)] because the integration is 2-fold compared to other naked boron atoms, B6, B12, and B13, appearing at −12.0, −10.6, and +29.0 ppm. On the basis of the 11B{1H} chemical shift and integration, the resonances at 16.1 and 14.2 ppm have been assigned to the B14 or B4 atoms, which are attached to the hydrogen atom. The 1 H{11B} chemical shift at −21.75 ppm may be assigned to the Co−H proton. Furthermore, to confirm the assignment of this proton, we performed HSQC NMR, which confirms the absence of any correlation between the proton at −21.75 ppm and any boron atoms. Along with the isolation of 1, we have also isolated compound 2 in 27% yield. The solid-state X-ray structure analysis of 2 evidently shows the core geometry as a 20-vertex face-fused tetracobaltaborane [(Cp*Co)3CoB16H17] [Figure 1 (right)]. Three common triangular faces, Co3−Co4−B8, Co4−B5−B9, and Co4−B15−B16, are used for the fusion of four polyhedra, for example, 12-vertex icosahedral {Co3B9}, tetrahedral {Co2B2}, square-pyramidal {CoB4}, and nido-{CoB7} units [Figure 1 (right)]. The nido-{CoB7} may be considered to be generated from a closo-bicapped square antiprism. The icosahedral unit in 2 is not a perfect icosahedron because both pentagonal rings deviate from the plane. Consistent with the single-crystal X-ray structure analysis, the 11 1 B{ H} NMR spectrum of 2 shows 16 chemical shifts for 16 boron atoms (Figure 2). The 1H NMR supports the absence of one Cp* ligand at Co4. As a result, Co4 became the common vertex of all four polyhedra with 10 degree of connectivity. Note that earlier Grimes, Gaines, Fehlner, and we reported similar kinds of higher connectivity of transition metals in a fused cluster, for example, [1,2′- and 2,2′-B5H10FeB5H10],18 [2,2′B 5 H 10 BeB 5 H 10 ], 19 [{1-(Cp*Ru)(μ-H)B 4 H 9 } 2 Ru], 20 and [(Cp*Mo)3MoB9H18].21 The 1H{11B} NMR of 2 suggests the presence of 13 BHt and three B−H−B protons at −2.28, −2.96, and −3.48 ppm. The chemical shift at −11.97 ppm may be due to the presence of one Co−H−B proton.

Figure 2. Structure of 2 featuring icosahedral, tetrahedral, squarepyramidal, and nido-octaborane(12) analogues (shared faces are shown in red).

Although ample fused clusters are known in organometallic and main-group chemistry, these types of faced-fused clusters are very rare.22 One of the key synthetic routes for the synthesis of supraicosahedral clusters is intercluster fusion.23 In the case of boranes, two face-fused polyhedral boranes, such as [B21H18]− and [B20H16(CH3CN)3], are known.24,25 Apart from boranes, only a handful of face-fused polyhedral clusters are known in the carborane, metallacarborane, and metallaborane fields. Recently, Sevov et al. reported two transition-metal main-group clusters, which are believed to be the first entry to the face-fused Zintl ion clusters.22d,e However, the search for face-fused closo-metallaborane clusters met with very little success (Chart 1). Fused clusters had been employed in the advancement of electron-counting rules. The electron-counting rules,8a,b,26,27 presented by Wade, Mingos, and Jemmis, not only linked borane,25 metallaborane,5b−e metallacarborane,4 and metallaheteroborane clusters in a simplified pleasing fashion but also gave a perspective in estimating the electronic requirements of a variety of macropolyhedral clusters. Considering the Mingos fusion formalism, the number of cluster valence electrons required for 1 is calculated by adding the valence electrons of the icosahedron (14 × 3Co + 4 × 9B + 2 = 80), tetrahedron (5 × 4B = 20), and bicapped square antiprism (14 × 1Co + 4 × 9B + 2 = 52) and subtracting the number of valence electrons of common fragments, i.e., butterfly (4 × 4B + 6 = 22) and triangle (6 × 3B = 18). Therefore, the face-fused cluster 1 requires 112 electrons, B

DOI: 10.1021/acs.inorgchem.8b02740 Inorg. Chem. XXXX, XXX, XXX−XXX

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Chart 1. Examples of Known Face-Fused Clusters (A−D), 19-Vertex-Fused Cobaltaborane 1, and 20-Vertex-Fused Cobaltaborane 2a

A:25 [B21H18]−. B:22b [Ge18Pd3(SniPr3)6]2−. C:28 [Ir2B16H14(PMe3)4(CO)2]. D:22a [Sn4Ni(CO)]4.

a

both of these clusters is a bit complex, they follow both Mingos and Jemmis electron-counting rules.

which is satisfied from the available cluster valence electrons of 1 [4 × 9Co + 4 × 5Cp* + 15 × 3B + 11 × 1H = 112]. Considering the mno rule, the number of electron pairs required for 1 is 54 (where m = 7, n = 39, o = 4, p = 4, and q = 0). Here each {BH} fragment will give one electron pair to the cluster, and one extra electron of boron will take part in the formation of an exo twocenter, two-electron (2c-2e) B−H bond, whereas for naked boron atoms, because of the absence of an exo 2c-2e bond, all of the valence electrons are taking part in skeletal bonding.8b,29 The skeletal electron pairs from various sources, for example, 4 cobalt atoms (6 electron pairs), 10 {BH} fragments (10 electron pairs), 5 naked boron atoms (7.5 electron pairs), 1 hydrogen atom (0.5 electron pair), and 20 {C(Me)} fragments (30 electron pairs), are satisfying the requirements. The fusion of 2 is also very interesting in which four polyhedra are involved in the formation of the 20-vertrex fused cluster 2. The number of cluster valence electrons required for 2 is calculated by adding the valence electron contribution of the icosahedron (14 × 4Co + 4 × 8B + 2 = 90), tetrahedron (15 × 2Co + 5 × 2B = 40), square pyramid (14 × 1Co + 4 × 4B + 4 = 34), and nido-{CoB7} (14 × 1Co + 4 × 7B + 4 = 46), followed by subtracting the number of valence electrons of three common faces [(16 × 2Co + 6B) + 2(16 × 1Co + 6 × 2B) = 94]. Therefore, the amount of electrons required for this fused cluster 2 is 116, which is achieved from the available cluster valence electrons in 2, i.e., 4 × 9Co + 3 × 5Cp* + 16 × 3B + 17 × 1H = 116. From the viewpoint of the mno rule, enumeration of the electron pairs also satisfies the electronic requirements.30 Further, in an attempt to compare the electronic property of these two clusters with their electron count, their cyclic voltammograms were recorded (Figures S10 and 11).31 In summary, we have synthesized and structurally characterized two unusual collections of neutral 19- and 20-vertex facefused tetracobaltaboranes. Although the goal of making a 16vertex cobalt analogue of [(Cp*Rh)3B12H12Rh{Cp*RhB4H9}] was not accomplished, we isolated two face-fused clusters that are very unique in terms of cluster fusion. While the fusion of

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b02740. Experimental procedures and additional experimental data (PDF) Accession Codes

CCDC 1866802 and 1869294 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Sundargopal Ghosh: 0000-0001-6089-8244 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Generous support of the Department of Science and Technology, SERB (Project EMR/2015/001274), New Delhi, India, is gratefully acknowledged. M.Z. and S.K. thank IIT Madras, C.N. thanks DST-INSPIRE for fellowships. We thank Dr. Babu Varghese for X-ray data analysis. X-ray support from IIT Indore and SAIF, IIT Madras, is gratefully acknowledged. Some preliminary studies by Sukanya Bagchi are gratefully acknowledged. C

DOI: 10.1021/acs.inorgchem.8b02740 Inorg. Chem. XXXX, XXX, XXX−XXX

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(10) Deng, L.; Chan, H.-S.; Xie, Z. Synthesis, Reactivity, and Structural Characterization of a 14-Vertex Carborane Angew. Angew. Chem., Int. Ed. 2005, 44, 2128−2131. (11) Deng, L.; Zhang, J.; Chan, H.-S.; Xie, Z. Synthesis and Structure of 14- and 15-Vertex Ruthenacarboranes. Angew. Chem., Int. Ed. 2006, 45, 4309−4313. (12) (a) Roy, D. K.; Bose, S. K.; Anju, R. S.; Mondal, B.; Ramkumar, V.; Ghosh, S. Boron Beyond the Icosahedral Barrier: A 16-Vertex Metallaborane. Angew. Chem., Int. Ed. 2013, 52, 3222−3226. (b) Roy, D. K.; Mondal, B.; Shankhari, P.; Anju, R. S.; Geetharani, K.; Mobin, S. M.; Ghosh, S. Supraicosahedral Polyhedra in Metallaboranes: Synthesis and Structural Characterization of 12-, 15- and 16-Vertex Rhodaboranes. Inorg. Chem. 2013, 52, 6705−6712. (13) Tietze, L. F.; Griesbach, U.; Bothe, U.; Nakamura, H.; Yamamoto, Y. Novel Carboranes with a DNA Binding Unit for the Treatment of Cancer by Boron Neutron Capture Therapy. ChemBioChem 2002, 3, 219−225. (14) Kennedy, J. D. The Polyhedral Metallaboranes Part I. Metallaborane Clusters with Seven Vertices and Fewer. Prog. Inorg. Chem. 2007, 32, 519−679. (b) Kennedy, J. D. The Polyhedral Metallaboranes Part II. Metallaboranes Clusters with Eight Vertices and More. Prog. Inorg. Chem. 2007, 34, 211−434. (15) Grimes, R. N. In Metal Interactions with Boron Clusters; Grimes, R. N., Ed.; Plenum: New York, 1982; p 269. (16) (a) Ghosh, S.; Lei, X.; Shang, M.; Fehlner, T. P. Role of the Transition Metal in Metallaborane Chemistry. Reactivity of (Cp*ReH2)2B4H4 with BH3·thf, CO, and Co2(CO)8. Inorg. Chem. 2000, 39, 5373−5382. (b) Ghosh, S.; Beatty, A. M.; Fehlner, T. P. The Reaction of Cp*ReH6, Cp* = C5Me5, with Monoborane to Yield a Novel Rhenaborane. Synthesis and Characterization of arachnoCp*ReH3B3H8. Collect. Czech. Chem. Commun. 2002, 67, 808−812. (17) (a) Lei, X.; Shang, M.; Fehlner, T. P. Chemistry of Dimetallaboranes Derived from the Reaction of [Cp*MCl2]2 with Monoboranes (M = Ru, Rh; Cp* = η5-C5Me5). J. Am. Chem. Soc. 1999, 121, 1275−1287. (b) Ghosh, S.; Noll, B. C.; Fehlner, T. P. Expansion of iridaborane clusters by addition of monoborane. Novel metallaboranes and mechanistic detail. Dalton Trans 2008, 371−378. (c) Geetharani, K.; Kumar Bose, S.; Pramanik, G.; Kumar Saha, T.; Ramkumar, V.; Ghosh, S. An Efficient Route to Group 6 and 8 Metallaborane Compounds: Synthesis of arachno-[Cp*Fe(CO)B3H8] and closo[(Cp*M)2B5H9] (M = Mo, W). Eur. J. Inorg. Chem. 2009, 2009, 1483− 1487. (18) (a) Weiss, R.; Grimes, R. N. New ferraboranes. Structural analogs of hexaborane(10) and ferrocene. A complex of cyclic B5H10−, a counterpart of C5H5−. J. Am. Chem. Soc. 1977, 99, 8087−8088. (b) Weiss, R.; Grimes, R. N. Polyhedral ferraboranes derived from the B5H8− ion. Analogs of ferrocene, hexaborane(10), and nido-B11H15. Inorg. Chem. 1979, 18, 3291−3294. (c) Gilbert, K. B.; Boocock, S. K.; Shore, S. G. In Comprehensive Organometallic Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E., Eds.; Pergamon Press: Oxford, U. K., 1982; Vol. 5, 879. (19) (a) Gaines, D. F.; Walsh, J. L. Chemistry of 2-berylla-nidohexaborane(11) compounds. Insertion of beryllium into a borane cage. Inorg. Chem. 1978, 17, 1238−1241. (b) Gaines, D. F.; Walsh, J. L.; Calabrese, J. C. Low-temperature crystal and molecular structures of 2tetrahydroborato-2-berylla-nido-hexaborane(11) and 2, 2’-commobis[2-berylla-nido-hexaborane(11)]. Inorg. Chem. 1978, 17, 1242− 1248. (20) Lei, X.; Shang, M.; Fehlner, T. P. 2, 2′-commo-Bis[2-ruthenanido-1-(η5-pentamethylcyclopentadienyl)ruthenahexaborane(12)]: An Unusual Ruthenaborane Related to Ruthenocene and Exhibiting a Linear Triruthenium Fragment. Angew. Chem., Int. Ed. 1999, 38, 1986− 1989. (21) Dhayal, R. S.; Sahoo, S.; Reddy, K. H. K.; Mobin, S. M.; Jemmis, E. D.; Ghosh, S. Vertex-Fused Metallaborane Clusters: Synthesis, Characterization and Electronic Structure of [(η5C5Me5Mo)3MoB9H18]. Inorg. Chem. 2010, 49, 900−904. (22) (a) Bose, S. K.; Ghosh, S.; Noll, B. C.; Halet, J.-F.; Saillard, J.-Y.; Vega, A. Linked and Fused Tungstaborane Clusters: Synthesis,

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DOI: 10.1021/acs.inorgchem.8b02740 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry Characterization, and Electronic Structures of bis-{(η 5 C5Me5W)2B5H8}2 and (η5-C5Me5W)2{Fe(CO)3}nB6−nH10−n, n = 0, 1. Organometallics 2007, 26, 5377−5385. (b) Bose, S. K.; Geetharani, K.; Varghese, B.; Ghosh, S. Condensed Tantalaborane Clusters: Synthesis and Structures of [(Cp*Ta)2B5H7{Fe(CO)3}2] and [(Cp*Ta)2B5H9{Fe(CO)3}4]. Inorg. Chem. 2011, 50, 2445−2449. (c) Thakur, A.; Sao, S.; Ramkumar, V.; Ghosh, S. Novel Class of Heterometallic Cubane and Boride Clusters Containing Heavier Group 16 Elements. Inorg. Chem. 2012, 51, 8322−8330. (d) Perla, L. G.; Sevov, S. C. A Stannyl-Decorated Zintl Ion [Ge18Pd3(SniPr3)6]2‑: Twinned Icosahedron with a Common Pd3-Face or 18-Vertex HyphoDeltahedron with a Pd3-Triangle Inside. J. Am. Chem. Soc. 2016, 138, 9795−9798. (e) Perla, L. G.; Sevov, S. C. Cluster Fusion: Face-Fused Nine-Atom Deltahedral Clusters in [Sn14Ni(CO)]4‑. Angew. Chem., Int. Ed. 2016, 55, 6721−6724. (23) (a) Hayward, C.-M. T.; Shapley, J. R.; Churchill, M. R.; Bueno, C.; Rheingold, A. L. Synthesis, characterization, and crystal structure of the [Ru10C2(CO)24] dianion. An edge-fused bioctahedral dicarbide cluster. J. Am. Chem. Soc. 1982, 104, 7347−7349. (b) Ghosh, S.; Fehlner, T. P.; Noll, B. C. Condensed metallaborane clusters: synthesis and structure of Fe2(CO)6(η5-C5Me5RuCO)(η5-C5Me5Ru)B6H10. Chem. Commun. 2005, 3080−3082. (c) Geetharani, K.; Bose, S. K.; Sahoo, S.; Mobin, S. M.; Varghese, B.; Ghosh, S. Cluster Expansion Reactions of Group 6 and 8 Metallaboranes Using Transition Metal Carbonyl Compounds of Groups 7−9. Inorg. Chem. 2011, 50, 5824− 5832. (24) Enemark, J. H.; Friedman, L. B.; Lipscomb, W. N. The Molecular and Crystal Structure of B20H16(NCCH3)2.CH3CN. Inorg. Chem. 1966, 5, 2165−2172. (25) Bernhardt, E.; Brauer, D. J.; Finze, M.; Willner, H. closo[B21H18]−: A Face-Fused Diicosahedral Borate Ion. Angew. Chem., Int. Ed. 2007, 46, 2927−2930. (26) King, R. B. Three-Dimensional Aromaticity in Polyhedral Boranes and Related Molecules. Chem. Rev. 2001, 101, 1119−1152. (27) Mingos, D. M. P. Polyhedral skeletal electron pair approach. Acc. Chem. Res. 1984, 17, 311−319. (28) Barton, L.; Bould, J.; Kennedy, J. D.; Rath, N. P. Macropolyhedral boron-containing cluster chemistry. Isolation and characterisation of the eighteen-vertex nido-5′-iridaoctaborano[3′,8′:1′,2]-closo-4-iridadodec aborane, [(CO)(PMe3)2IrB16H14Ir(CO)(PMe3)2]. J. Chem. Soc., Dalton Trans. 1996, 3145−3149. (29) Burdett, J. K.; Canadell, E. Role of Large but Defective Deltahedra in the Structural Chemistry of Very Complex Solid Borides and Gallides. Inorg. Chem. 1991, 30, 1991−1998. (30) Using the mno rule, the electron pair required for 2 is 48 (m + n + o + p − q = 48, where m = 7, n = 35, o = 2, p = 5, and q = 1). There are 4 cobalt atoms (6 electron pairs), 13 {BH} fragments (13 electron pairs), 3 naked boron atoms (4.5 electron pairs), 4 hydrogen atoms (2 electron pairs), and 15 {C(Me)} fragments (22.5 electron pairs). (31) Compound 1 underwent one irreversible and one reversible redox process, whereas 2 exhibits one reversible redox process.

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DOI: 10.1021/acs.inorgchem.8b02740 Inorg. Chem. XXXX, XXX, XXX−XXX