Stabilisation of high oxidation-state niobium using ‘electron-rich’ bicyclic-guanidinates

Synthetic procedures to high oxidation-state complexes of niobium incorporating the bicyclic guanidinate 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidinate, [hpp]⁻, are described. The ligand source was either the N-trimethylsilylated guanidine or the lithium guanidinate and the reaction proceeded via elimination of trimethylsilylchloride or transmetallation from NbCl₅, respectively. A 1:1 ratio of reagents afforded the mono-ligand product, Nb(hpp)Cl₄ (1) and crystallisation from acetonitrile afforded the solvated species Nb(hpp)Cl₄(MeCN) (1a), demonstrating the ability of the metal centre in 1 to bind small substrate molecules. A 2:1 ratio of lithium guanidinate to NbCl₅ resulted in formation of the seven-coordinate, bis-ligand compound, Nb(hpp)₂Cl₃ (2). These products represent the first examples of guanidinate compounds in which niobium is stable in the +5 oxidation-state, believed to result from enhanced electron donation caused by the bicyclic framework of the ligand. The molecular structures of 1, 1a and 2 are reported, presenting for the first time an opportunity to describe bonding parameters within compounds of this type.