Advances in guanidine ligand design: synthesis of a strongly electron-donating, imidazolin-2-iminato functionalized guanidinate and its properties on iron. Issue 11 (2nd July 2016)

• Record Type: Journal Article
• Authors: Castillo, Maximiliano; Barreda, Omar; Maity, Arnab K.; Barraza, Brian; Lu, Jeffrey; Metta-Magaña, Alejandro J.; Fortier, Skye
• Is Part Of: Journal of coordination chemistry. Volume 69:Issue 11/13(2016)
• Page Start: 2003
• Page End: 2014
• Publication Date: 2016-07-02
• Subjects: Guanidinate -- ketimine-guanidinate -- iron -- ligand resonance -- imidazolin-2-iminato; Coordination compounds -- Periodicals; 541.2242
• DOI: 10.1080/00958972.2016.1167198
• Languages: English
• ISSNs: 0095-8972

Abstract

Abstract: Imidazolin-2-imines (Im R N-), 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 R N- backbone onto a guanidinate scaffold was investigated. Addition of 1 equiv of [Li(Et2 O)][Im t Bu N] to the aryl carbodiimide (dippN)2 C (dipp = 2, 6-diisopropylphenyl) cleanly affords the lithium Im t Bu N-functionalized guanidinate [Li(THF)2 ][(Im t Bu N)C(Ndipp)2 ] (1 ). Subsequent metalation of the ligand with FeBr2 gives the yellow Fe(II) complex {[(Im t Bu N)C(Ndipp)2 ]FeBr}2 (4 ) in good yield. Solid-state structural analyses of1 and4 show the Im t Bu N- group acts as a non-coordinating backbone substituent. Direct structural comparison of4 to the closely related guanidinate and ketimine-guanidinate complexes {[(X)C(Ndipp)2 ]FeBr}2 (X = t Bu2 C=N (5 ); N( i Pr)2 (6 )), differing only in their backbone, reveals a resonance contribution of the Im t Bu N- group to the guanidinate ligand electronic structure. Moreover, the Fe(II)/Fe(III) redox couple of4 ( E 1/2 = − 0.67 V) is cathodically shifted by greater than 200 mV from the oxidation potentials of5 ( E 1/2 = −0.42 V) and6 ( E 1/2 = −0.45 V), demonstrating the [(Im t Bu N)C(Ndipp)2 ] − system to be a quantifiably superior electron donor. Abstract :