Synthesis and reactivity of titanium 'POCOP' pincer complexes

The 'POCOP' pincer ligand, [2,6-(OPR2)(2)C6H3], has been attached to titanium in both Ti(III) and Ti(IV) complexes for the first time. Using a lithium-halogen exchange route [2,6-(OPR2)(2)C6H3]Li ([(POCOP)-P-R]Li) can be synthesised. Both the iso-propyl and tert-butyl derivatives can be made, but only the latter isolated. These can be reacted with the Ti(III) and Ti(IV) synthons to make a range of [POCOP]TiClx species. In the presence of Ti(IV), THF and [(POCOP)-P-R]Li, an unprecedented ligand rearrangement occurs. ((POCOP)-P-tBu)TiCl2, 1, can be derivatised with alkylating agents to make bis-methyl, phenyl and neopentyl complexes. The last of these can activate H-2 to make a rare example of a titanium chlorohydride, with the metal pincer fragment staying attached. EPR has been used to characterise the paramagnetic complexes and locate their electron spins, which is further validated with DFT calculations. This opens the door for this archetypical pincer ligand to be used with early transition metals.