Titanium pincer complexes

Pugh et al. [486] published an equally facile synthesis for the transition metal pincer carbene complexes of titanium(IV), vanadium(ll), chromium(ll), manganese(II), niobium(III) and uranium(IV). It consists of treating the free carbene with the respective transition metal halide or the pincer imidazolium salt with the respective metal bis-trimethylsilylamide (see Figure 3.160). 

Bis-NHC CNC-type pincer ligands were first used with Pd and, later on, coordinated onto early-transition metal complexes. The titanium (III) complex 35 was readily prepared from (THF)3TiCl3 and the free bis(carbene)pyridine, whereas the imido Ti complex 36 was obtained from Ti(NtBu)Cl2(pyridine)3 (Scheme 14.19) [66,67]. The pincer complex 35 was tested in ethylene polymerization. With 500 equivalents of MAO cocatalyst, an activity of 791kgmor bar h was observed. 

In 2003, Danopoulos et al. successfully synthesized a CNC pincer ligand and used it to prepare a Pd complex. This ligand was later applied to the synthesis of ETM and actinoid complexes. Substitution of tmeda with the CNC pincer yielded [(NHC)VCl2(THF)] 37 (Equation (6.8)). This route was alternatively applied to the preparation of titanium, chromium, niobium and uranium complexes. ... 

Helgert TR, Hollis TK, Valente EJ. Synthesis of titanium CCC-NHC pincer complexes and catalytic hydroamination of unactivated alkenes. Organometallks. 2012 31 3002-3009. 

This example shows the extreme influence of the nature of each of the substituents at P on the stabilization of the gem-dianion as well as on the formation of carbene complexes. Most recently, we were able to synthesize either the monocarbene or the homoleptic biscarbene of titanium using dianion 7Li2 (route A) [93]. In 2009, a related class of pincer ligand was developed by Martin-Vaca, Bourissou et al. who used route B to obtain the first example of indenylidene complexes of Zr (Scheme 29) [94]. Here also, the corresponding isolable dianionic ligands are not accessible. It is to be noted here that the in situ coordination/deprotonation required forcing conditions (ca. 100°C for several hours). 

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