(Amido)- and (Chlorido)titanium and -zirconium Complexes Coordinated by ansa-Bis(amidinate) Ligands with a Rigid o- Phenylene Linker

Bis-amido TiIV and ZrIV complexes stabilized by ansa bis(amidinate) ligands {C6H4-1,2-[NC(tBu)N(2,6-R2C6H3)]2?M(NMe2)2 [M = ZrIV, R = Me (3), R = iPr (4); M = TiIV, R = Me (5), R = iPr (6)] and ?C6H4-1,2-[NC(tBu)N(2,6-Me2C6H3)]2}Zr(NMe2)3H (3?HNMe2) were prepared in fairly good yields by reacting a (bis)amidine ligand [C6H4-1,2-{NC(tBu)N(2,6-R2C6H3)H}2 [R = Me (1), iPr (2)] with an equimolar amount of the metal precursor [M(NMe2)4 (M = ZrIV, TiIV)]. The salt metathesis reaction between an equimolar amount of sodium amidinates ?C6H4-1,2-[NC(tBu)N(2,6-R2C6H3)]2Na2? and ZrCl4(thf)2 was also scrutinized to synthesize the corresponding ZrIV bis(amidinate) dichlorides ?C6H4-1,2-[NC(tBu)N(2,6-R2C6H3)]2?ZrCl2 [R = Me (7), iPr (8)]. The ligands coordination mode to MIV ions was found to be strongly affected by the steric hindrance of the amidinate ligands (Me vs. iPr substituted aniline moieties) as well as by the nature of the ancillary groups bound to the metal center (NMe2 vs. Cl). Bis(amido) ligands with the 2,6-Me2C6H3 substituents at the amidinate nitrogens coordinate the zirconium ion in a tetradentate fashion both in solution and in the solid state (3 and 3?HNMe2). Compounds containing the bulkier 2,6-iPr2C6H3 units prefer a tridentate coordination mode (4). With the smaller TiIV ion, all the bis(amidinate) ligands from this series are tridentate in the solid state (compounds 5 and 6) while they reversibly switch their denticity from tridentate to tetradentate (?3 vs. ?4) in solution depending on the temperature. The ZrIV complex 4, featuring with the bulkier bis(amidinate) ligand 2, shows a similar behavior to that of its TiIV analogue, with a tridentate ligand coordination at the solid state and a temperature induced denticity change in solution as a temperature function. Thermodynamic standard parameters for the ?3 vs. ?4 interconversion on the two model compounds 4 and 5 in toluene-d8 solution have been measured from the respective linear van't Hoff plots fitting. Finally, ZrIVbis(chloride) complexes 7 and 8 invariably show a tetracoordination mode of their bis(amidinate) ligands (1 and 2) both in solution and in the solid state.