Carbon–Nitrogen Bond Construction and Carbon–Oxygen Double Bond Cleavage on a Molecular Titanium Oxonitride: A Combined Experimental and Computational Study

New
carbon–nitrogen bonds were formed on addition of isocyanide
and ketone reagents to the oxonitride species [{Ti­(η5-C5Me5)­(μ-O)}3(μ3-N)] (1). Reaction of 1 with XylNC
(Xyl = 2,6-Me2C6H3) in a 1:3 molar
ratio at room temperature leads to compound [{Ti­(η5-C5Me5)­(μ-O)}3­(μ-XylNCCNXyl)­(NCNXyl)]
(2), after the addition of the nitrido group to one coordinated
isocyanide and the carbon–carbon coupling of the other two
isocyanide molecules have taken place. Thermolysis of 2 gives [{Ti­(η5-C5Me5)­(μ-O)}3­(XylNCNXyl)­(CN)] (3) where the heterocumulene
[XylNCCNXyl] moiety and the carbodiimido [NCNXyl] fragment in 2 have undergone net transformations. Similarly, tert-butyl isocyanide (tBuNC) reacts with the starting
material 1 under mild conditions to give the paramagnetic
derivative [{Ti3(η5-C5Me5)3­(μ-O)3­(NCNtBu)}2­(μ-CN)2] (4). However, compound 1 provides the oxo ketimide
derivatives [{Ti3(η5-C5Me5)3­(μ-O)4}­(NCRPh)]
[R = Ph (5), p-Me­(C6H4) (6), o-Me­(C6H4) (7)] upon reaction with benzophenone, p-methylbenzophenone, and o-methylbenzophenone,
respectively. In these reactions, the carbon–oxygen double
bond is completely ruptured, leading to the formation of a carbon–nitrogen
and two metal–oxygen bonds. The molecular structures of complexes 2–4, 6, and 7 were determined by single-crystal X-ray diffraction analyses. Density
functional theory calculations were performed on the incorporation
of isocyanides and ketones to the model complex [{Ti­(η5-C5H5)­(μ-O)}3­(μ3-N)] (1H). The mechanism involves the coordination
of the substrates to one of the titanium metal centers, followed by
an isomerization to place those substrates cis with respect to the
apical nitrogen of 1H, where carbon–nitrogen bond
formation occurs with a low-energy barrier. In the case of aryl isocyanides,
the resulting complex incorporates additional isocyanide molecules
leading to a carbon–carbon coupling. With ketones, the high
oxophilicity of titanium promotes the unusual total cleavage of the
carbon–oxygen double bond.