4,6-Dihydroxypyrimidine: a selective bridging ligand for controlled Group IV metal alkoxide structures

The generation of controlled metal alkoxide structures has been explored using 4,6-dihydroxypyrimidine (DHP-H2) as a selective bridging ligand. DHP-H2 was reacted with Ti(OPri)4 in pyridine (py). Crystallization from this solution led to the isolation of [(OPri)2(µ-OPri)Ti(µ-DHP)Ti(OPri)3 (py)]2(1). The two dinuclear subunits of 1 possess a bridging DHP ligand bound through both the O and N of the ring, forming an M–O–C–N ring on each metal center. The first Ti possesses three terminal OR ligands and one coordinated pyridine solvent molecule. The second Ti is connected to a µ-OPri group from the other subunit, forming a tetranuclear species. In contrast, recrystallization of 1 from toluene provided the solvent-free, cyclic adduct {[(OPri)2(µ-OPri)Ti]2(µ-DHP)}2(2). An identical reaction using more sterically hindering alkoxy precursors yielded products of the general structure [(py)(RO)3M]2(µ4-DHP), where M = Ti, OR = OCH2CMe3(3), OPri/OCMe3(4); M = Zr, OR = OCMe3(5). Each metal center adopts an octahedral geometry, with one py, three terminal alkoxide ligands, and one bridging DHP. Solid-state 13C MAS NMR studies indicate the bulk materials are consistent with the crystal structures and solution 1H NMR is consistent with retention of the structures in solution for 2–5.