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

摘要

Imidazolin-2-imines (Im^RN-), derived from N-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 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 both >1 and >4 shows 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 detectable resonance contribution of the Im^tBuN- group to the guanidinate ligand electronic structure. Moreover, the Fe(II)/Fe(III) redox couple of >4 (E1/2 = −0.67 V) is cathodically shifted by greater than 200 mV from the oxidation potentials of >5 (E1/2 = −0.42 V) and >6 (E1/2 = −0.45 V), demonstrating the [(Im^tBuN)C(Ndipp)2]⁻ system to be a quantifiably superior electron donor.