Reaction of nido-7,8-C2B9H13 with Dicobalt Octacarbonyl: Crystal Structures of the Complexes [Co2(CO)2(η5-7,8-C2B9H11)2], [Co2(CO)(PMe2Ph)(η5-7,8-C2B9H11)2], and [CoCl(PMe2Ph)2(η5-7,8-C2B9H11)]

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Abstract
The compounds [Co2(CO)8] and nido-7,8-C2B9H13 react in CH2Cl2 to give a complex mixture of products consisting primarily of two isomers of the dicobalt species [Co2(CO)2(η5-7,8-C2B9H11)2] (1), together with small amounts of a mononuclear cobalt compound [Co(CO)2(η5-10-CO-7,8-C2B9H10)] (5) and a charge-compensated carborane nido-9-CO-7,8-C2B9H11 (6). In solution, isomers 1a and 1b slowly equilibrate. However, column chromatography allows a clean separation of 1a from the mixture, and a single-crystal X-ray diffraction study revealed that each metal atom is ligated by a terminal CO molecule and in a pentahapto manner by a nido-C2B9H11 cage framework. The two Co(CO)(η5-7,8-C2B9H11) units are linked by a Co−Co bond [2.503(2) Å], which is supported by two three-center two-electron B−H ⇀ Co bonds. The latter employ B−H vertices in each cage which lie in α-sites with respect to the carbons in the CCBBB rings bonded to cobalt. Addition of PMe2Ph to a CH2Cl2 solution of a mixture of the isomers 1, enriched in 1b, gave isomers of formulation [Co2(CO)(PMe2Ph)(η5-7,8-C2B9H11)2] (2). Crystals of one isomer were suitable for X-ray diffraction. The molecule 2a has a structure similar to that of 1a but differs in that whereas one B−H ⇀ Co bridge involves a boron atom in an α-site of a CCBBB ring coordinated to cobalt, the other uses a boron atom in the β-site. Reaction between 1b and an excess of PMe2Ph in CH2Cl2 gave the complex [CoCl(PMe2Ph)2(η5-7,8-C2B9H11)] (3), the structure of which was established by X-ray diffraction. Experiments indicated that 3 was formed through a paramagnetic CoII species of formulation [Co(PMe2Ph)2(η5-7,8-C2B9H11)]. Addition of 2 molar equiv of CNBut to solutions of either 1a or 1b gave a mixture of two isomers of the complex [Co2(CNBut)2(η5-7,8-C2B9H11)2] (4). NMR data for the new compounds are reported and discussed.