title: Advances in Guanidine Ligand Design: Synthesis of a Strongly Electron-Donating, Imidazolin-2-iminato Functionalized Guanidinate and its Properties on Iron.
abstract: Imidazolin-2-imines (ImRN-), derived fromN-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 ImRN- backbone onto a guanidinate scaffold was investigated. Addition of 1 equiv of [Li(Et2O)][ImtBuN] to the aryl carbodiimide (dippN)2C (dipp = 2,6-diisopropylphenyl) cleanly affords the lithium ImtBuN-functionalized guanidinate [Li(THF)2][(ImtBuN)C(Ndipp)2] (1). Subsequent metalation of the ligand with FeBr2gives the yellow Fe(II) complex {[(ImtBuN)C(Ndipp)2]FeBr}2(4) in good yield. Solid-state structural analyses of both1and4shows the ImtBuN- group acts as a non-coordinating backbone substituent. Direct structural comparison of4to 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 ImtBuN- group to the guanidinate ligand electronic structure. Moreover, the Fe(II)/Fe(III) redox couple of4(E1/2= -0.67 V) is cathodically shifted by greater than 200 mV from the oxidation potentials of5(E1/2= -0.42 V) and6(E1/2= -0.45 V), demonstrating the [(ImtBuN)C(Ndipp)2]-system to be a quantifiably superior electron donor.
authors.affiliation: Department of Chemistry, University of Texas at El Paso, El Paso, TX 79968.
authors.forename: Maximiliano
authors.initials: M
authors.lastname: Castillo
authors.affiliation: Department of Chemistry, University of Texas at El Paso, El Paso, TX 79968.
authors.forename: Omar
authors.initials: O
authors.lastname: Barreda
authors.affiliation: Department of Chemistry, University of Texas at El Paso, El Paso, TX 79968.
authors.forename: Arnab K
authors.initials: AK
authors.lastname: Maity
authors.affiliation: Department of Chemistry, University of Texas at El Paso, El Paso, TX 79968.
authors.forename: Brian
authors.initials: B
authors.lastname: Barraza
authors.affiliation: Department of Chemistry, University of Texas at El Paso, El Paso, TX 79968; Department of Chemistry and Biochemistry, University of California, Santa Barbara, Santa Barbara, CA 93106.
authors.forename: Jeffrey
authors.initials: J
authors.lastname: Lu
authors.affiliation: Department of Chemistry, University of Texas at El Paso, El Paso, TX 79968.
authors.forename: Alejandro J
authors.initials: AJ
authors.lastname: Metta-Magaña
authors.affiliation: Department of Chemistry, University of Texas at El Paso, El Paso, TX 79968.
authors.forename: Skye
authors.initials: S
authors.lastname: Fortier
keywords: guanidinate; imidazolin-2-iminato; iron; ketimine-guanidinate; ligand resonance