An unusual binding mode of a guanidinate ligand in a Ti26+ species with a short metal–metal single bond

A compound having the formula Ti2(η2-hpp)2(μ–η2, η2-hpp)2Cl2, 1, (hpp = the anion of 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2a]pyrimidine) has been made by reduction of the mononuclear complex Ti(hpp)2Cl2 with KC8 in THF. Compound 1 has a short Ti–Ti bond distance of 2.5944(9) Å. DFT calculations suggest that there is a single bond and the electronic configuration of σ2 may be assigned to the Ti26+ unit. The structure shows one chloride group and a chelating hpp ligand bound to each titanium atom. There are also two hpp ligands that link the Ti units in a unusual μ–η2, η2-coordination. This type of coordination for a bicyclic guanidinate ligand (hpp) was also observed in [Nb2(η2-hpp)4(μ–η2, η2-hpp)]PF6, 2. Solution studies of 1 by 1H NMR spectroscopy suggest that the structure in solution and that in the solid state may be different, with that in solution being much more symmetrical.