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 Cp2TiCl2 with the unsolvated dilithium compounds closo-exo-Li-1-Li-2-(R)-3-(SiMe3)-2,3-C2B4H4 (R = SiMe3, Me, H) produced dimeric mixed-ligand sandwich titanacarboranes [commo-1-Cp-1-Ti(III)-2-(R)-3-(SiMe3)-2,3-C2B4H4]2 (R = SiMe3 (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-(Cl)-1-(THF)-1-Ti-2-(R)-3-(SiMe3)-2,3-C2B4H4) (R = SiMe3 (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-(SiMe3)2-2,3-C2B4H4)2] (4) was produced, while replacement of a SiMe3 group with a less sterically demanding Me group afforded the dimeric titanacarborane [Li(TMEDA)]2[1,1‘-Ti-(2-Me-3-SiMe3-2,3-C2B4H4)2]2 (5A). On the other hand, when the TMEDA-solvated closo-exo-4,5-[(μ-H)2Li(TMEDA)]-1-Li[(TMEDA)-2,4-(SiMe3)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-(SiMe3)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(IV)/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 d1−d1 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).