Advances in guanidine ligand design: synthesis of a strongly electron-donating, imidazolin-2-iminato functionalized guanidinate and its properties on iron

Imidazolin-2-imines (Im^RN-), derived from N-heterocyclic 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 guanidine ligand class, the effect of appending an Im^RN- backbone onto a guanidinate scaffold was investigated. Addition of 1 equiv of [Li(Et₂O)][Im^tBuN] to the aryl carbodiimide (dippN)₂C (dipp = 2,6-diisopropylphenyl) cleanly affords the lithium Im^tBuN-functionalized guanidinate [Li(THF)₂][(Im^tBuN)C(Ndipp)₂] (1). Subsequent metalation of the ligand with FeBr₂ gives the yellow Fe(II) complex {[(Im^tBuN)C(Ndipp)₂]FeBr}₂ (4) in good yield. Solid-state structural analyses of 1 and 4 show the Im^tBuN- group acts as a non-coordinating backbone substituent. Direct structural comparison of 4 to the closely related guanidinate and ketimine-guanidinate complexes {[(X)C(Ndipp)₂]FeBr}₂ (X = ^tBu₂C=N (5); N(ⁱPr)₂ (6)), differing only in their backbone, reveals a resonance contribution of the Im^tBuN- group to the guanidinate ligand electronic structure. Moreover, the Fe(II)/Fe(III) redox couple of 4 (E_1/2 = −0.67 V) is cathodically shifted by greater than 200 mV from the oxidation potentials of 5 (E_1/2 = −0.42 V) and 6 (E_1/2 = −0.45 V), demonstrating the [(Im^tBuN)C(Ndipp)₂]⁻ system to be a quantifiably superior electron donor.