Low-Valent Iron Carbonyl Complexes with a Tripodal Carbene Ligand

A bulky tris­(carbene)­borate ligand allows several low-valent iron carbonyl complexes to be isolated. One-electron reduction of the cationic iron­(II) complex [PhB­(MesIm)3Fe­(CO)3]­[B­(C6F5)4] (1) ([PhB­(MesIm3)]− = phenyltris­(1-mesitylimidazol-2-ylidene)­borate) yields the low-spin (S = 1/2) iron­(I) complex PhB­(MesIm)3Fe­(CO)2 (2), as determined by structural and spectroscopic methods. This complex can in turn be reduced to provide the anionic dicarbonyl complex [K]­[PhB­(MesIm)3Fe­(CO)2] (3), which crystallizes as a dimer in which the potassium cation coordinates in a side-on fashion to one CO ligand. Protonation of 3 yields the weakly acidic iron hydride PhB­(MesIm)3Fe­(CO)2H (4), which can also be isolated by treating the κ3-coordinated alkylborohydrido complex PhB­(MesIm)3­Fe­(κ3-BH­(CH2CH3)3) (5) with CO. The strong donor ability of the tris­(carbene)­borate ligand results in significant reduction of the CO bonds, as measured by IR spectroscopy.