Chemistry of C-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

A series of full- and half-sandwich metallacarboranes were synthesized from the reaction of MCl 2 (M = Co, Fe) and closo-exo-y,x-Li(L)-1-Li(L)-2,n-(SiMe 3 ) 2 -2,n-C 2 B 4 H 4 (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-(SiMe 3 ) 2 -2,3-C 2 B 4 H 4 ] 2 (5), M + [commo1,1'-Co{2,3-(SiMe 3 ) 2 -(2,3-C 2 B 4 H 4 )} 2 ] - (M = CoCl (6a), Co 3 (TMEDA) 3 Cl 5 (6b)), commo-1,1'-M[2,4-(SiMe 3 ) 2 -2,4-C 2 B 4 H 4 ] 2 (M = FeH (7), CoH (8)), and 1-(TMEDA)-closo-2,4-(SiMe 3 ) 2 -1,2,4-MC 2 B 4 H 4 (M = Co (9)) were isolated in yields ranging from 80 to 92%. The reaction ofcommo1,1'-Ni[2,4-(SiMe 3 ) 2 -2,4-C 2 B 4 H 4 ] 2 (10) with carefully dried TMEDA produced the charge-compensated commo-1,1'-Ni I I I -[(2,4-(SiMe 3 ) 2 -5,6-B 2 -{σ-N(Me)(CH 2 ) 2 N(Me) 2 }-2,4-C 2 B 2 H 2 )(2',4'-(SiMe 3 ) 2 -2',4'-C 2 B 4 H 4 )] (12) and 1,2-(SiMe 3 ) 2 -closo-1,2-C 2 B 4 H 4 . With the exception of 5 and 12, all of the commo-metallacarboranes involved metal oxidation in addition to ligation, with M° 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 1 H, 1 1 B, and 1 3 C NMR spectroscopy; its 1 H NMR spectrum showed a broad resonance at 6 -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 3d 7 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.