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

Abstract

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.