Reactivity of Tuck-over Titanium Oxo Complexes with Isocyanides

The reactivity of the “tuck-over” species [Ti₂(η⁵-C₅Me₅)­(CH₂Ph)₃­(μ-η⁵-C₅Me₄CH₂-κ<i>C</i>)­(μ-O)] (<b>1</b>) and [Ti₂(η⁵-C₅Me₅)­(CH₂CMe₃)­(μ-η⁵-C₅Me₄CH₂-κ<i>C</i>)­(μ-CH₂CMe₂CH₂)­(μ-O)] (<b>2</b>) toward isocyanides has been examined both synthetically and theoretically. Treatment of <b>1</b> with the isocyanides RNC, R = Me₃SiCH₂, 2,6-Me₂C₆H₃, <i>t</i>Bu, <i>i</i>Pr, leads to a series of η²-iminoacyl species (<b>3</b>–<b>6</b>) where the molecule of isocyanide inserts into one of the terminal metal–alkyl bonds. The analogous reaction of the “tuck-over” metallacycle species <b>2</b> with 2,6-Me₂C₆H₃NC and <i>t</i>BuNC results in the initial insertion of one isocyanide into the terminal Ti–alkyl bond to form the iminoacyl complexes <b>7</b> and <b>8</b>, followed by a second insertion into the metallacycle moiety to generate <b>9</b>, in the case of <i>tert</i>-butylisocyanide. DFT calculations support the selective reactivity observed experimentally with a kinetic and thermodynamic preference for RNC insertion on the terminal alkyl groups bound to both metallic centers over the alternative insertion on the “tuck-over” ligand