Chemistry ofC-Trimethylsilyl-Substituted Heterocarboranes. 31. New Insights into Reaction Pathways of Carborane Ligand Systems: Synthetic, Structural, Spectroscopic, and Electrochemical Studies on Sandwich and Half-Sandwich Metallacarboranes of Iron, Cobalt, and Nickel

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Abstract

A series of full- and half-sandwich metallacarboranes were synthesized from the reaction of MCl2 (M = Co, Fe) and closo-exo-y,x-Li(L)-1-Li(L)-2,n-(SiMe3)2-2,n-C2B4H4 (y = 4, x = 5, L = 2 THF, n = 3 (1); y = 4, x = 5, L = TMEDA, n = 3 (2); y = 5, x = 6, L = 2 THF, n = 4 (3); y = 5, x = 6, L = TMEDA, n = 4 (4)) in 1:1 molar ratios in benzene. The metallacarborane complexes exo-4,4‘,5,5‘-Fe(TMEDA)-commo-1,1‘-Fe[2,3-(SiMe3)2-2,3-C2B4H4]2 (5), M+[commo-1,1‘-Co{2,3-(SiMe3)2-(2,3-C2B4H4)}2]- (M = CoCl (6a), Co3(TMEDA)3Cl5 (6b)), commo-1,1‘-M[2,4-(SiMe3)2-2,4-C2B4H4]2 (M = FeH (7), CoH (8)), and 1-(TMEDA)-closo-2,4-(SiMe3)2-1,2,4-MC2B4H4 (M = Co (9)) were isolated in yields ranging from 80 to 92%. The reaction of commo-1,1‘-Ni[2,4-(SiMe3)2-2,4-C2B4H4]2 (10) with carefully dried TMEDA produced the charge-compensated commo-1,1‘-NiIII-[(2,4-(SiMe3)2-5,6-B2-{σ-N(Me)(CH2)2N(Me)2}-2,4-C2B2H2)(2‘,4‘-(SiMe3)2-2‘,4‘-C2B4H4)] (12) and 1,2-(SiMe3)2-closo-1,2-C2B4H4. With the exception of 5 and 12, all of the commo-metallacarboranes involved metal oxidation in addition to ligation, with M0 as a coproduct. All compounds were characterized by analysis and infrared spectroscopy, and 5, 6b, 7, 9, and 12 were also characterized by X-ray diffraction analysis. Because of the paramagnetic nature of either the products or their counterions, only 8 could be characterized by 1H, 11B, and 13C NMR spectroscopy; its 1H NMR spectrum showed a broad resonance at δ −18.9 attributable to the Co-bound H. The paramagnetic compounds were characterized by EPR spectroscopy. The electrochemical reduction of 10 produced a Ni(III) metallacarborane that was characterized by EPR spectroscopy and found to be a 3d7 species related to nickelocenium derivatives. All results are consistent with a mechanism involving the initial formation of a half-sandwich metallacarborane, which can be isolated or can undergo a disproportionation process to give the full-sandwich products.

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