Nonclassical Titanocene Silyl Hydrides



The titanocene silyl hydride complexes [Ti(Cp)2(PMe3)(H)(SiR3)] [SiR3=SiMePhCl (6), SiPh2Cl (7), SiMeCl2 (8), SiCl3 (9)] were prepared by HSiR3 addition to [Ti(Cp)2(PMe3)2] and were studied by NMR and IR spectroscopy, X-ray diffraction (for 6, 8, and 9), and DFT calculations. Spectroscopic and structural data established that these complexes exhibit nonclassical Ti-H-Si-Cl interligand hypervalent interactions. In particular, the observation of silicon–hydride coupling constants J(Si,H) in 69 in the range 22–40 Hz, the signs of which we found to be negative for 8 and 9, is conclusive evidence of the presence of a direct Si[BOND]H bond. The analogous reaction of [Ti(Cp)2(PMe3)2] with HSi(OEt)3 does not afford the expected classical silyl hydride complex [Ti(Cp)2(PMe3)(H){Si(OEt)3}], and instead NMR-silent titanium (apparently TiIII) complex(es) and the silane redistribution product Si(OEt)4 are formed. The structural data and DFT calculations for the compounds [Ti(Cp)2(PMe3)(H)(SiR3)] show that the strength of interligand hypervalent interactions in the chlorosilyl complexes decreases as the number of chloro groups on silicon increases. However, in the absence of an Si-bound electron-withdrawing group trans to the Si[BOND]H moiety, a silane σ complex is formed, characterized by a long Ti[BOND]Si bond of 2.658 Å and short Si[BOND]H contact of 1.840 Å in the model complex [Ti(Cp)2(PMe3)(H)(SiMe3)]. Both the silane σ complexes and silyl hydride complexes with interligand hypervalent interactions exhibit bond paths between the silicon and hydride atoms in Atoms in Molecules (AIM) studies. To date a classical titanocene phosphane silyl hydride complex without any Si[BOND]H interaction has not been observed, and therefore titanocene silyl hydrides are, depending on the nature of the R groups on Si, either silane σ complexes or compounds with an interligand hypervalent interaction.