Mono N,C,N-pincer complexes of titanium, vanadium and niobium : synthesis, structure and catalytic activity in olefin polymerisation

Transmetallation of 4,4′-bis{(2,6-bis[(dimethylamino)methyl]phenylgold)diphenyl-phosphino}biphenyl (3) with MCl4 (M = Ti, NbCl, V) in benzene gave the corresponding transition metal pincer complexes (4) and insoluble 4,4′-bis[P-(chloro gold(I))diphenylphosphino]biphenyl (2), which can be quantitatively recovered and recycled. Interestingly, 3 did not react with TiCl3. However, reaction of 2,6-bis[(dimethylamino)methyl]phenyllithium (1) with TiCl3 resulted in formation of the novel diaryltitanium(IV) compound 5 (16% yield), comprising one N,C,N-mer bound NCN-pincer ligand and a second NCN-pincer ligand that is rearranged from a 1,2,6-isomer to a 1,2,4 one. The latter NCN-ligand is dianionic and is bidentate bonded; one of the CH2NMe2 substituents (para to C′ipso) is non-coordinated, while the second CH2NMe2 group, after C–H activation of one of the Me groups, is η2-C,N-bonded to the titanium centre trans to Cipso of the mer-NCN ligand. The new NCN-pincer metal complexes 2,6-bis[(dimethylamino)methyl]phenylTiCl3 (4a) and 2,6-bis[(dimethylamino)methyl]-phenylVCl2 (4d) gave, after immobilization on MgCl2-based supports, very high activity in ethene polymerisation.