Chemistry of C-trimethylsilyl-substituted heterocarboranes.: 27.: Synthetic, spectroscopic, structural, and reactivity studies on d0 hafnacarboranes of 2,3-C2B4-Carborane ligands

The sodium/lithium compounds cboso-exo-Li(THF)-1-Na(THF)-2-(SiMe3)-3-(R)-2,3-C2B4H4 (R = SiMe3, Me, H), were found to react with several different hafnium reagents to produce half- and full-sandwich and mixed-ligand chlorohafnacarboranes in good yields. The reaction of the sodium/lithium compounds with HfCl4 in 2:1 molar ratios produced the full-sandwich complexes 1-Cl-1-(THF)-2,2'-(SiMe3)(2)-3,3'-(R)(2)-4',5,5',6-Li(THF)(n)-[1,1'-commo-Hf(2,3-C2B4H4)(2)] (R = SiMe3, n = 2 (I); R = Me, n = 1 (II); R H, n = 1 (III)) in yields of 77%, 65%, and 71%, respectively. When R = SiMe3, a 1:1 molar ratio of carborane to HfCl4 gave the half-sandwich species [Li(THF)(2)][1,1,1-(Cl)(3)-closo-1-Hf-2,3-(SiMe3)(2)-2,3-C2B4H4] (V) in 77% yield. On the other hand, both 2:1 and 1:1 reaction ratios of carborane (R = Me) to Cp*HfCl3 gave only the mixed-ligand sandwich compound 1,1'-(Cl)(2)-2-(SiMe3)-3-(Me)-4,5-Li(THF)-1,1'-commo-Hf[(eta(5)-C5Me5)(eta(5)-2,3-C2B4H4)] (IV) in 82% yield. All compounds were characterized by infrared spectroscopy, H-1, C-13, and B-11 NMR spectroscopy, and chemical analysis. Compounds I-III were characterized further by single-crystal X-ray diffraction. The structures show the hafnacarboranes to have bent-sandwich structures in which a Hf atom, in a formal +4 state, was sandwiched between two carborane ligands and was also bonded to a Cl atom and a THF molecule to give a distorted-tetrahedral arrangement about the metal.