On the Nature and Incidence of β-Agostic Interactions in Ethyl Derivatives of Early Transition Metals:  Ethyltitanium Trichloride and Related Compounds

Abstract

In light of the structures and spectroscopic properties of the ethyltitanium compounds EtTiCl3 (1), and EtTiCl3(dmpe) (dmpe = Me2PCH2CH2PMe2) (2), extensive DFT calculations have been carried out to explore the structures, energetics, potential energy surfaces, and spectroscopic properties not only of these but also of related ethyl derivatives of early transition metals, M. The analysis has sought to assess the true nature of so-called β-agostic interactions in these systems and how such interactions depend on the atomic number, coordination number, valence electron (VE) count, and net charge of M. The calculations reproduce well the experimentally determined properties of 1 and 2. Analysis of wave functions indicates that, where significant β-interactions occur, the M−Cα bonding electrons are delocalized over the entire ethyl group, the marked reduction of the MCC valence angle allowing the metal atom to establish covalent bonding interactions with the β-C atom and, perhaps to a lesser extent, with its appended proximal H atom. A β-agostic alkyl group may be considered a two-electron ligand, and the main contribution to the stabilization arises from Ti···Cβ bonding. Barriers to conformational change of the M−Et unit depend inter alia on the space available in the coordination sphere of the central M atom, with rearrangement of the ethyl group to accommodate significant β-interaction being opposed by interligand repulsion. Such interaction is found not in 1, but in its dmpe adduct 2 because the increase in coordination number is offset by elongation of the Ti−C and Ti−Cl bonds and by the extra pliability of the EtTiCl3 moiety resulting from complexation by the dmpe ligand.

Agostic Interactions in d0Metal Alkyl Complexes

“…[39] In [15] the IPR technique has been applied to a wide variety of systems which are expected to exhibit agostic interactions, or which have been shown by other methods to be agostic. [41][42][43] In general, the results of such experiments have been at best inconclusive, with many systems showing only a small or zero IPR effect, and others which are unambiguously agostic in fact displaying an inverse effect. [42] The drawbacks and limitations of the technique have been noted elsewhere.…”

“…[35, 42, 55b] Furthermore, calculations in the case of d 0 complexes with no nonbonding electron density associated with the metal center have shown both the metal and the agostic H atom to carry a net positive charge. [42] Whilst factors other than atomic charges play a role in determining chemical shifts, the resulting repulsion between M and H will contribute to a deshielded H nucleus in the agostic site, and consequently an inverse IPR effect will be observed: such a situation has been shown to pertain for [EtTiCl 3 (dmpe)] (11), the archetypal example of a b-agostic complex. [42] 2.2.2.…”

“…[42] Whilst factors other than atomic charges play a role in determining chemical shifts, the resulting repulsion between M and H will contribute to a deshielded H nucleus in the agostic site, and consequently an inverse IPR effect will be observed: such a situation has been shown to pertain for [EtTiCl 3 (dmpe)] (11), the archetypal example of a b-agostic complex. [42] 2.2.2. Solid-State NMR Spectroscopy…”

“…The relative structural and spectroscopic simplicity of this complex and its base-free precursor [EtTiCl 3 ] (14) make it an excellent benchmark system to explore the nature of the interaction by a variety of physical and theoretical approaches. [35,42,55,58,118] In seeking to explain the paradox whereby 14, with 8 VE and CN = 4 is clearly not agostic, whilst the complex 11 (12 VE; CN = 6) is agostic (Figure 7), we have ultimately developed a revised bonding concept to rationalize agostic interactions. Table. Examples and evidence in support of our revised model are provided in Section 5.…”

“…For example, quantum chemical calculations predict a non-agostic structure (a V-C-C = 117. [42,84] 5. Evidence for the Revised Bonding Model…”