Synthesis, electronic structures, and reactivity of mononuclear and dinuclear low-valent molybdenum complexes in iminopyridine and bis(imino)pyridine ligand environments.

Molybdenum in redox non-innocent ligand environments features prominently in biological inorganic systems. While Holm and coworkers, along with many other researchers, have thoroughly investigated formally high-oxidation-state molybdenum (Mo(IV)-Mo(VI)) ligated by dithiolenes, less is known about molybdenum in other formal oxidation states and/or different redox-active ligand environments. This work focuses on the investigation of low-valent molybdenum in four different redox non-innocent nitrogen ligand type environments (mononucleating and dinucleating iminopyridine, mononucleating and dinucleating bis(imino)pyridine). The reaction of iminopyridine N-(2,6-diisopropylphenyl)-1-(pyridin-2-yl)methanimine (L) with Mo(CO)(NCMe) produced Mo(L)(CO)(NCMe). Mo(L)(CO)(NCMe) undergoes transformation to Mo(L)(CO) upon treatment with CS or prolonged stirring in dichloromethane. The reaction of the open-chain dinucleating bis(iminopyridine) ligand N,N'-(2,7-di-tert-butyl-9,9-dimethyl-9H-xanthene-4,5-diyl)bis(1-(pyridin-2-yl)methanimine) (L) similarly produced an hexacarbonyl complex Mo(L)(CO)(NCMe) which also underwent transformation to the octacarbonyl Mo(L)(CO). Both complexes featured anti-parallel geometry of the chelating units. The oxidation of Mo(L)(CO)(NCMe) with I resulted in Mo(L)(CO)I. The reaction of mononucleating potentially tridentate bis(imino)pyridine ligand (L) (N-mesityl-1-(6-((E)-(mesitylimino)methyl)pyridin-2-yl)methanimine) with both Mo(CO)(NCMe) and Mo(CO)(NCMe) produced complexes Mo(L)(CO)(NCMe) and Mo(L)(CO) in which L was coordinated in a bidentate fashion, with one imino sidearm unbound. The reaction of dinucleating macrocyclic di(bis(imino)pyridine) analogue (L) led to the similar chemistry of Mo(L)(CO)(NCMe) and Mo(L)(CO) complexes. Treatment of Mo(L)(CO)(NCMe) with I formed a mono(carbonyl) complex Mo(L)(CO)I in which molybdenum was formally oxidized and L underwent coordination mode change to tridentate. The electronic structures of formally Mo(0) complexes in iminopyridine and bis(imino)pyridine ligand environments were investigated by density functional theory calculations.