Multielectron Redox Chemistry of Transition Metal Complexes Supported by a Non‐Innocent N₃P₂ Ligand: Synthesis, Characterization, and Catalytic Properties

Abstract

A new redox-active, diarylamido-based ligand (L N3P2 ) capable of κ 5 -N,N,N,P,P chelation has been used to prepare a series of complexes with the general formula [M II (L N3P2 )]X, where M = Fe (1; X = OTf ), Co (2; X = ClO 4 ), or Ni (3; X = ClO 4 ). The diarylamido core of monoanionic L N3P2 is derived from bis(2-amino-4-methylphenyl)amine, which undergoes condensation with two equivalents of 2-(diphenylphosphanyl)benzaldehyde to provide chelating arms with both arylphosphine and imine donors. X-ray structural, magnetic, and spectroscopic studies indicate that the N 3 P 2 coordination environment generally promotes low-spin configurations. Three quasi-reversible redox couples between +1.0 and -1.5 V (vs. Fc + /Fc) were ob- IntroductionRecent studies have highlighted the ability of cobalt and nickel complexes to function as efficient electrocatalysts for environmentally significant reactions, such as CO 2 reduction [1] and H 2 generation. [2] Continued advances in this field depend upon the rational design of new and sophisticated ligand frameworks to enhance catalytic performance. By adjusting the ligand coordination environment, it is possible to tune the redox properties of the transition metal center, control the binding of substrates, and improve the overall stability of the catalyst. Moreover, these multielectron reactions can be facilitated by redox-active (i.e., "noninnocent") ligands that actively participate in the catalytic mechanism by donating or accepting one or more electrons. [3] Starting with the well-studied cobaloxime-based systems, [4] the large majority of cobalt catalysts for the hydrogen evolution reaction (HER) have employed ligands with only N-donors. [5] Examples include catalysts with tetra-or pentadentate polypyridyl ligands [6] and those featuring tetradentate Schiff-base [a]

Reversible Dioxygen Binding to Co(II) Complexes with Noninnocent Ligands

“…Table provides selected bond distances derived from X-ray crystal structures of the four Tp Me2 -based Co complexes. The Co–N/S bond distances of 3 and 3* , which are dramatically shorter than those of the o -aminothiophenolate precursors, are characteristic of 5C complexes with low-spin cobalt centers. − Modest structural changes are also observed for the bidentate L S,N ligand upon oxidation, including shortening of the S–C bonds and greater variation in C–C bond distances (Table ). Previous studies by the Wieghardt and Pierpont groups demonstrated that these changes are consistent with at least partial oxidation of the ligand to the iminothiosemiquinonate (ITSQ •– ) form. − Indeed, as illustrated in Figure a, the metric parameters of 3* closely match those reported by Herebian and Wieghardt for the dimeric complex, [Co II ( tBu2 ITSQ) 2 ] 2 .…”

“…The EPR spectrum of 5* in frozen THF (Figure 4) features a S = 1/2 signal with anisotropic g-values (g 1 = 1.98, g 2 = 2.15, g 3 = 2.62) and prominent hyperfine splitting from the 59 Co nucleus (I = 7/2). The magnitude of the hyperfine coupling (A = 68 ± 5 G) in the g 1 -region is somewhat smaller than A Co values reported for Co(II) complexes with similar S/N ligation, which typically lie between 80-110 G. 27,32,57 Nevertheless, the A Co value of 5* is still much larger than the values of 10−25 G reported for Co(III) complexes attached to ligand radicals, such as superoxide 27,32 or ITSQ •− . 63 Because A Co values are proportional to the amount of Co-based spin density, the EPR data provide strong evidence that the unpaired electron is localized on the cobalt center.…”