Ammonia Activation by μ3-Alkylidyne Fragments Supported on a Titanium Molecular Oxide Model

Ammonolysis of the μ3-alkylidyne derivatives [{Ti(η5-C5Me5)(μ-O)}3(μ3-CR)] [R = H (1), Me (2)] produces a trinuclear oxonitride species, [{Ti(η5-C5Me5)(μ-O)}3(μ3-N)] (3), via methane or ethane elimination, respectively. During the course of the reaction, the intermediates amido μ-alkylidene [{Ti(η5-C5Me5)(μ-O)}3(μ-CHR)(NH2)] [(R = H (4), Me (5)] and μ-imido ethyl species [{Ti(η5-C5Me5)(μ-O)}3(μ-NH)Et] (6) were characterized and/or isolated. This achievement constitutes an example of characterization of the three steps of successive activation of N–H bonds in ammonia within the same transition-metal molecular system. The N–H σ-bond activation of ammonia by the μ3-alkylidyne titanium species has been theoretically investigated by DFT method on [{Ti(η5-C5H5)(μ-O)}3(μ3-CH)] model complex. The calculations complement the characterization of the intermediates, showing the multiple bond character of the terminal amido and the bridging nature of imido ligand. They also indicate that the sequential ammonia N–H bonds activation process goes successively downhill in energy and occurs via direct hydron transfer to the alkylidyne group on organometallic oxides 1 and 2. The mechanism can be divided into three stages: (i) coordination of ammonia to a titanium center, in a trans disposition with respect to the alkylidyne group, and then the isomerization to adopt the cis arrangement, allowing the direct hydron migration to the μ3-alkylidyne group to yield the amido μ-alkylidene complexes 4 and 5, (ii) hydron migration from the amido moiety to the alkylidene group, and finally (iii) hydron migration from the μ-imido complex to the alkyl group to afford the oxo μ3-nitrido titanium complex 3 with alkane elimination.