Advances in Guanidine Ligand Design: Synthesis of a Strongly Electron-Donating, Imidazolin-2-iminato Functionalized Guanidinate and its Properties on Iron.

Imidazolin-2-imines (ImN-), derived from -heterocylic carbenes, have been shown to be strong electron donors when directly coordinated to metals or when used as a substituent in larger ligand frameworks. In an attempt to enhance the electron-donating properties of the popular guanidine ligand class, the effect of appending an ImN- backbone onto a guanidinate scaffold was investigated. Addition of 1 equiv of [Li(EtO)][Im N] to the aryl carbodiimide (dippN)C (dipp = 2,6-diisopropylphenyl) cleanly affords the lithium Im N-functionalized guanidinate [Li(THF)][(Im N)C(Ndipp)] (). Subsequent metalation of the ligand with FeBr gives the yellow Fe(II) complex {[(Im N)C(Ndipp)]FeBr} () in good yield. Solid-state structural analyses of both and shows the Im N- group acts as a non-coordinating backbone substituent. Direct structural comparison of to the closely related guanidinate and ketimine-guanidinate complexes {[(X)C(Ndipp)]FeBr} (X = BuC=N (); N( Pr) ()), differing only in their backbone, reveals a detectable resonance contribution of the Im N- group to the guanidinate ligand electronic structure. Moreover, the Fe(II)/Fe(III) redox couple of (E = -0.67 V) is cathodically shifted by greater than 200 mV from the oxidation potentials of (E = -0.42 V) and (E = -0.45 V), demonstrating the [(Im N)C(Ndipp)] system to be a quantifiably superior electron donor.