Chemistry of C-trimethylsilyl-substituted heterocarboranes .21. Syntheses, structures, EPR spectra, and reactivities of bent-sandwich and half-sandwich titanacarboranes. Full analysis of spin-spin coupling in two structurally characterized titanium(III)-carborane dimers

The reaction of Cp(2)TiCl(2) with the unsolvated dilithium compounds closo-exo-Li-1-Li-2-(R)-3-(SiMe(3))-2,3-C2B4H4 (R = SiMe(3), Me, H) produced dimeric mixed-ligand sandwich titanacarboranes [commo-1-Cp-1-Ti(III)-2-(R)-3-(SiMe(3))-2,3-C2B4H4](2) (R = SiMe(3) (1), Me (2), H (3)) in yields of 60%, 54%, and 60%, respectively. The chemical oxidation of these titanacarboranes in the presence of TiCl4 in THF resulted in the formation of the corresponding diamagnetic Ti(IV) species 1-(Cp)-1-(CI)-1-(THF)-1-Ti-2-(R)-3-(SiMe(3))-2,3-C2B4H4) (R = SiMe(3) (7), Me (8), H (9)) in 86%, 58%, and 45% yields, respectively. When the bis(trimethylsilyl)-substituted dilithiacarborane precursor was reacted with TiCl3, only the monomeric full-sandwich chlorotitanacarborane [Li(TMEDA)](2)[1-Cl-1,1'-Ti-(2,3-(SiMe(3))(2)-2,3-C2B4H4)(2)] (4) was produced, while replacement of a SiMe(3) group with a less sterically demanding Me group afforded the dimeric titanacarborane [Li(TMEDA)](2)[1,1'-Ti-(2-Me-3-SiMe(3)-2,3-C2B4H4)(2)](2) (5A). On the other hand, when the TMEDA-solvated closo-exo-4,5-[(mu-H)(2)Li(TMEDA)]-1-Li[(TMEDA)-2,4-(SiMe(3))(2)-2,4-C2B4H4 was the precursor, reaction with TiCl3 yielded only the corresponding monomeric half-sandwich chlorotitanacarborane 1-(TMEDA)-1-Cl-1-Ti(III)-2,4-(SiMe(3))(2)-2,4-C2B4H4 (6) In addition to the X-ray analyses of 1, 5A, 6, and 7, the Ti(III) compounds and an electrochemically generated Ti(TV)/Ti(III) mixed-valence complex (5B) were characterized by EPR spectroscopy. The dimers 1 and 5A exhibit unusually well-resolved triplet EPR features, which were fully analyzed and correlated with the structural results. Magnetic susceptibility measurements reveal a rather; small d(1)-d(1) exchange coupling constant of -45.8 cm(-1) (antiferromagnetic interaction) for 1. The one-electron oxidation of 5A produced an anion (S = 1/2) with rhombic EPR features, which was tentatively identified as a delocalized [Ti(+3.5)](2) mixed-valence species (5B).