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] (<b>1</b>) ([PhB­(MesIm₃)]⁻ = phenyltris­(1-mesitylimidazol-2-ylidene)­borate) yields the low-spin (<i>S</i> = 1/2) iron­(I) complex PhB­(MesIm)₃Fe­(CO)₂ (<b>2</b>), 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] (<b>3</b>), which crystallizes as a dimer in which the potassium cation coordinates in a side-on fashion to one CO ligand. Protonation of <b>3</b> yields the weakly acidic iron hydride PhB­(MesIm)₃Fe­(CO)₂H (<b>4</b>), which can also be isolated by treating the κ³-coordinated alkylborohydrido complex PhB­(MesIm)₃­Fe­(κ³-BH­(CH₂CH₃)₃) (<b>5</b>) 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.