Mechanisms of α-, β-, and γ-H(D) Exchange Processes in the α-Agostic Alkyltitanocene(IV) Complexes [Cp2TiCH2CH(CH3)(CMe3)]+and [Cp2TiCH2CH(CD3)(CMe3)]+: Stark Contrasts with Their γ-SiMe3Analogues

The α-agostic alkyltitanocene(IV) complex [Cp2TiCH2CH(CH3)(CMe3)]+ (II) and its D3 isotopologue [Cp2TiCH2CH(CD3)(CMe3)]+ (II-CD3) undergo reversible, intramolecular H−H(D) exchange among the α-, β-, and γ- (β-Me) positions of the alkyl ligands in addition to concomitant intermolecular H−H(D) exchange with the vinylic and 2-methyl sites of the product of β-hydrogen elimination, CH2=CMeCMe3, results which are in strong contrast to the exchange behavior of the γ-agostic silyl analogues [Cp2TiCH2CH(CH3)(SiMe3)]+ (III) and [Cp2TiCH2CH-(CD3)(SiMe3)]+ (III-CD3). As has been previously shown, III undergoes tunnelling-expedited exchange of the β-H (but not α-H) with the hydrogen atoms of the β-methyl group (β-H/γ-H exchange) while III-CD3 isomerizes reversibly but specifically to the isotopomer [Cp2TiCD2CD(CH3)(SiMe3)]+, forgoing β- H/γ-H exchange completely. In this paper we show that all of the exchange processes of II/II-CD3 and III/III-CD3 initially involve conventional β-H elimination processes and thus, at some stage, intermediacy of the corresponding hydrido alkene complexes [Cp2TiH(CH2=CMeCMe3)]+ (VI) or [Cp2TiH(CH2=CMeSiMe3)]+ (or their D3 isotopologues). The major reason for the vastly different exchange behaviors of the two structurally very similar alkyltitanocene systems II and III is simply that, for steric and electronic reasons, the alkene in VI dissociates reversibly to the hydride, [Cp2TiH]+, and free alkene while that in [Cp2TiH(CH2=CMeSiMe3)]+ does not; CH2=CMeSiMe3 remains coordinated at the temperature and on the time scale of the experiments. DFT calculations generally support our mechanistic conclusions and furthermore point to a subtle ion pairing effect which hinders intramolecular η2-alkene rotation in VI and [Cp2TiH(CH2=CMeSiMe3)]+, thereby exerting a surprisingly important, heretofore unanticipated, influence on the nature of the chemistry involved.