In Situ Formation of Heterobimetallic Salen Complexes Containing Titanium and/or Vanadium Ions

A combination of high-resolution electrospray mass spectrometry and 1H NMR spectroscopy has been used to prove that when a mixture of [(salen)TiO]2 complexes containing two different salen ligands (salen and salen′) is formed, an equilibrium is established between the homodimers and the heterodimer [(salen)TiO2Ti(salen′)]. Depending upon the structure and stereochemistry of the two salen ligands, the equilibrium may favor either the homodimers or the heterodimer. Extension of this process to mixtures of titanium(salen) complexes [(salen)TiO]2 and vanadiumV(salen′) complexes [(salen′)VO]+Cl− allowed the in situ formation of the heterobimetallic complex [(salen)TiO2V(salen′)]+X− to be confirmed for all combinations of salen ligands studied except when the salen ligand attached to titanium contained highly electron-withdrawing nitro-groups. The rate of equilibration between heterobimetallic complexes is faster than that between two titanium complexes as determined by line broadening in the 1H NMR spectra. These structural results explain the strong rate-inhibiting effect of vanadiumV(salen) complexes in asymmetric cyanohydrin synthesis catalyzed by [(salen)TiO]2 complexes. It has also been demonstrated for the first time that the titanium and vanadium complexes can undergo exchange of salen ligands and that this is catalyzed by protic solvents. However, the ligand exchange is relatively slow (occurring on a time scale of days at room temperature) and so does not complicate studies aimed at using heterobimetallic titanium and vanadium salen complexes as asymmetric catalysts. Attempts to obtain a crystal structure of a heterobimetallic salen complex led instead to the isolation of a trinuclear titanium(salen) complex, the formation of which is also consistent with the catalytic results obtained previously.