Spectroelectrochemical Investigation of the One‐Electron Reduction of Nonplanar Nickel(II) Porphyrins

Abstract

The electrochemical reduction of a series of nickel porphyrins with an increasing number of substituents was investigated in acetonitrile. A one-electron reduction of [5,15-bis(1-ethylpropyl)porphyrinato]nickel(II) leads to π-anion radicals and to efficient formation of phlorin anions, presumably by disproportionation and subsequent protonation of the doubly reduced species. The phlorin anion was identified by using cyclic voltammetry and UV/Vis and resonance Raman spectroelectrochemistry, complemented by quantum-chemical calculations to assign the spectral signatures. The theoretical analysis of the potential-energy landscape of the singly reduced species suggests a thermally activated intersystem crossing that populates the quartet state and thus lowers the energy barrier towards disproportionation channels. Structure-reactivity correlations are investigated by considering different substitution patterns of the investigated nickel(II) porphyrin cores, that is, for the porphyrin with additional β-aryl ([5,15-bis(1-ethylpropyl)-2,8,12,18-tetra(p-tolyl)porphyrinato]nickel(II)) and meso-alkyl substitution ([5,10,15,20-tetrakis(1-ethylpropyl)porphyrinato]nickel(II)), no phlorin anion formation was observed under electrochemical conditions. This observation is correlated either to kinetic inhibition of the disproportionation reaction or to lower reactivity of the subsequently formed doubly reduced species towards protonation.

Sterically induced distortions of nickel(II) porphyrins – Comprehensive investigation by DFT calculations and resonance Raman spectroscopy

“…However, A represents a constant offset of the resonance Raman frequencies with respect to structurally induced shifts of the Raman bands, which is apparently not exactly covered by the scaling factor of 0.98 applied for the DFT calculations [77,99]. The overall good agreement, which also holds true for the joint DFT-experimental analysis of  3 (A=1394(8) cm -1 , B=95(11) cm -1 r 2 =0.90, fits only for tetrasubstituted porphyrins) and  4 (A=1327(5) cm -1 , B=42(6) cm -1 , r 2 =0.79, values of NiT(4-MeOPh)P and NiT(Ph)P excluded for fit; see Figure 11A), indicates that the calculated structures and the calculated ruffling angle are overall correctly predicted by DFT.…”

“…All quantum chemical calculations have been performed using the Gaussian 09 program. Equilibrium structures and electronic structures of all 18 porphyrins were obtained at the DFT level of theory by means of a functional based on B3LYP [109,110], comprising 15 % of exact-exchange, 58.5 % of nonlocal B88 exchange and the LYP correlation, which is denoted B3LYP(15) [77,[111][112][113]. The 10electron non-relativistic effective core potential MDF-10 [114] was used with its basis set for the nickel atom, that is, 3s, 3p, 3d and 4s electrons are treated explicitly, whereas the two first inner shells are described by the core potential.…”

“…To correct for the lack of anharmonicity and for the approximate description of electron correlation, the harmonic frequencies were scaled by a factor of 0.98 [77,99]. Solvent effects (dichloromethane:  = 8.93) with respect to equilibrium structures and vibrational frequencies were taken into account by means of the integral equation formalism of the polarizable continuum model [116].…”

“…The resonance Raman spectra of porphyrins are also highly sensitive to chemical alteration of the macrocycle. In this regard, we applied resonance Raman spectroscopy to probe the reactivity of singly-reduced nickel(II) porphyrins towards the formation of nickel(II) phlorin anions [77], which are discussed as a key intermediate in electrocatalytic hydrogen evolution reactions [5].…”

pysisyphus: Exploring potential energy surfaces in ground and excited states

“…All quantum chemical calculations were performed using Gaussian 16. [15] GS equilibrium structures were obtained at the DFT level of theory utilizing the B3LYP35 functional, [92][93][94][95] comprising 35% of exact-exchange, 58.5% of nonlocal B88 [79] exchange, and the LYP correlation. [81] The effective core potential MWB-60 [96] and its corresponding valence basis set were used for platinum, and the 6-31G(d) basis set was used for all other atoms.…”