“Recapitation” of nido-Metallacarboranes as a Synthetic Tool: Preparation of Apically Substituted CoC2B4 Clusters via Boron Insertion1

Abstract
Derivatives of Cp*Co(2,3-Et2C2B4H4) containing substituents at the apex boron atom [B(7)], the first examples of apically functionalized small metallacarborane clusters, have been prepared in good yield via boron insertion into the nido-Cp*Co(2,3-Et2C2B3H3)2- dianion. Reaction of this substrate with BX3 (X = Cl, Br, I) or PhBCl2 in toluene at room temperature gave the corresponding Cp*Co(2,3-Et2C2B4H3-7-X) derivatives (2a−c and 3 in which X = Cl, Br, I, and Ph, respectively), all of which were isolated via column chromatography as air-stable yellow solids and characterized via 1H, 11B, and 13C NMR, infrared, UV−visible, and mass spectra. Treatment of the same dianion with 1,4-(Br2B)2C6H4 afforded air-stable orange crystalline [Cp*Co(2,3-Et2C2B4H3-7)]2C6H4 (4). The structure of this compound was defined via spectroscopy and X-ray crystallography as a bis(cobaltacarborane) complex linked at the apex borons via a 1,4-phenylene bridge. Crystal data for 4: space group Pbca; a = 15.056(7) Å, b = 21.612 (8) Å, c = 11.641 (3) Å; Z = 4; R = 0.045 for 1582 independent reflections having I > 3σ(I).

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