Assessing the Character of the C6F5 Ligand from the Electrochemical and Photophysical Properties of [Ni(C6F5)2(N∧N)] Complexes

Abstract

Organonickel complexes containing α-diimine ligands [Ni(C₆F₅)₂(N^∧N)] (N^∧N = 2,2'-bipyridine (bpy), 2,9-dimethyl-1,10-phenanthroline (dmphen), 3,4,7,8-tetramethyl-1,10-phenanthroline (tmphen), dipyrido[3,2-a:2',3'-c]phenazine (dppz), 1,4-bis(isopropyl)-1,4-diazabutadiene (iPr-DAB), and 1,4-bis(2,6-dimethylphenyl)-1,4-diazabutadiene (Xyl-DAB) were prepared and studied structurally, spectroscopically, and electrochemically. Their molecular structures from single-crystal X-ray diffraction show near-perfect square planar Ni(II) coordination except in the case of dmphen. Primary reversible electrochemical reductions in the range from -1 to -2 V vs ferrocene/ferrocenium couple lead to mainly diimine-localized radical anion complexes, while secondary reductions in the range from -2 to -2.5 V lead to dianion complexes, as shown through spectroelectrochemistry. Irreversible metal-centered oxidations at around 0.7 V result in rapid aryl-aryl reductive elimination and formation of decafluorobiphenyl. No photoluminescence was detected for the complexes containing chromophoric α-diimine ligands at room temperature. At 77 K in frozen glassy 2-Me-THF matrices, weak photoluminescence was detected for the dmphen and tmphen derivatives, with broad emission bands peaking around 570 nm. All results are rationalized with the support of (TD-)DFT calculations, highlighting the role of the C₆F₅ ligand in different systems.