Experimental Electrochemical Potentials of Nickel Complexes

Qiao Lin; Gregory Dawson; Tianning Diao

doi:10.1055/s-0040-1719829

Synlett, Inhaltsverzeichnis

Synlett 2021; 32(16): 1606-1620
DOI: 10.1055/s-0040-1719829

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Modern Nickel-Catalyzed Reactions

Experimental Electrochemical Potentials of Nickel Complexes

Qiao Lin

,

Gregory Dawson

,

Tianning Diao ∗

Abstract

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Key words

redox potentials - nickel - cyclic voltammetry - reduction - oxidation

Introduction

1

Introduction

1.1

Scope

In recent years, nickel-catalyzed cross-coupling,[1] photoredox-dual catalysis,[2] and electrocatalytic[3] reactions have emerged as versatile tools to enable challenging transformations and construct organic molecules. Reaction development is dependent on delicate design and intricate arrangement of redox-active organonickel species to accomplish the catalytic cycle. A proper selection of the nickel catalyst and the corresponding organic substrates delivers selective electron-transfer processes. The thermodynamic driving force of an outer-sphere electron-transfer event is often estimated by the Gibbs free energy change, which can be calculated by the standard potentials of the donor and the acceptor (ΔG° = –nFE°).

A formal potential, sometimes referred to as a conditional potential, is the reduction potential that applies to a half reaction under a specific set of conditions, as opposed to the standard-state conditions.[4] Nicewicz and co-workers measured and summarized the formal potentials of organic molecules with common functional groups.[5] In this review article, we tabulate the redox potentials of nickel complexes that have been reported in the literature. Since nickel can accommodate oxidation states ranging from 1^– to 4⁺,[6] more than one redox processes can occur at a certain nickel center. We organize nickel complexes according to their isolated oxidation states and indicate the directions of the redox transformations in the ‘process’ column.

1.2

Measurement of Formal Redox Potentials

Cyclic voltammetry (CV) is a common tool for determining the formal potential for a redox-active compound.[7] [8] As described by the Nernst equation, the electrode potential (E) is determined by the formal potential (E⁰′) and the concentrations of the oxidized and reduced analyte, where R is the gas constant, T is the temperature, F is the Faraday's constant, n is the number of electrons transferred, and [ox] and [red] are the concentrations of the oxidized and reduced species, respectively (Equation 1).

Equation 1

For example, in the CV scan of ferrocene (Fc), an electric potential is applied linearly to the sample (Figure [1]A). The line in the voltammogram is the current passed-per-unit time (Figure [1]B). Current is dependent on the concentration of the substrate at the electrode per unit time, which is determined by the rate of diffusion, caused by the concentration gradient near the electrode. As the potential is scanned in a positive direction, current starts to build and increases from point A to point C, due to the increasingly faster diffusion of Fc to the electrode, as Fc is oxidized to ferrocenium (Fc⁺) on the electrode surface. The higher the oxidation potential applied to the electrode, the higher the current and higher ratio of [Fc⁺]/[Fc] till the electrode potential reaches point B, whose potential is E_1/2, where [Fc] = [Fc⁺]. The diffusion rate continues to grow until arriving point C, where the current reaches maximum at point C (E_p). When the applied potential travels from point C to D, [Fc] far away from the electrode starts to deplete and [Fc⁺] far away from the electrode increases. The current decreases, as the electrooxidation becomes diffusion controlled. At point D, the current converges to the value of ‘diffusion-limited current’.

Figure 1 (A) Applied potential as a function of time for a generic cyclic voltammetry experiment, with the initial, switching, and end potentials represented (A, D, and G, respectively). (B) Cyclic voltammogram of the reversible oxidation of a 1 mM Fc solution to Fc⁺, at a scan rate of 100 mV/s.

The same process occurs when scanning Fc in a negative direction, resulting in a reduction peak.[9] For an electrochemically reversible process, E_1/2 is determined as the midpoint of anodic and cathodic peak potentials and is typically regarded as the formal potential E⁰′.[8] For an irreversible CV, when the reverse peak is not observed, the half-peak potential E_p/2, defined as the potential at the half-peak current, is used as an alternative to estimate E⁰′.[5] E_p/2 values must be considered in the context of the detailed conditions at which the CVs are measured.[5] In this review, we focus on nickel complexes with available E_1/2 data.

1.3

Redox Potentials in Nonaqueous Solution

Formal potentials (E⁰′) estimated by averaging the forward and backward peak potentials from reversible redox-active species are documented in this review vs. the Fc/Fc⁺ couple, as recommended by IUPAC.[10] Aqueous reference electrodes such as saturated calomel electrode (SCE) or saturated Ag/AgCl could cause the generation of liquid junction potentials, a potential difference built up due to the tendency of electrolytes to diffuse between two different solutions, when applied to the organic media. The resulting liquid junction potentials could shift the observed potential from the inherent redox potential to various extents according to the solvent.[11] Table [1] summarizes potentials of the Fc/Fc⁺ couple, measured in different solvents and with supporting electrolytes.[12] Given the good reproducibility of SCE in nonaqueous solutions, Table [1] can be used to calibrate potentials measured in different solvents and using different electrolytes.

Table 1 Formal Potentials (V) of the Ferrocene/Ferrocenium Redox Couple vs SCE with Selected Electrolytes^a
Solvent	TBABF₄	TBAPF₆	TBAClO₄	Et₄NPF₆	Et₄NClO₄	Et₄NBF₄
MeCN	0.39[14]	0.40	0.38	0.38	0.39[15]
DCM	0.46[16]	0.46	0.48		0.49[26]	0.59[17]
THF		0.56	0.53
DMF	0.55[18]	0.45	0.47	0.46
Acetone		0.48	0.50	0.46
PhCN			0.50[19]		0.47[19]

^a Supporting electrolyte concentration, 0.1 M. Data are extracted from ref. 12 unless specified otherwise.

In this article, we extract CV data from the literature and convert the potential values of a certain reference electrode into Fc/Fc⁺ based on Table [1] or the Fc/Fc⁺ potentials reported in the original paper; the parameters used for the conversion are listed underneath each table. In this regard, we unify the redox potentials to the same reference for direct comparison. As shown in Table [1], the potential of Fc/Fc⁺ is sensitive to the experimental conditions, such as electrolytes and their concentration, solvent, etc.[13] Thus, it is strongly recommended to specify solvent and electrolyte conditions when reporting CV data against Fc/Fc⁺.[12]

Redox Potentials of Nickel Complexes

2

Redox Potentials of Nickel Complexes

2.1

Redox Potentials of (Phosphine)Ni Complexes

Most phosphine ligands applied to support nickel complexes are strong σ-donors.[20] Table [2] summarizes the one-electron oxidation potentials of (phosphine)Ni(0) or the one-electron reduction of (phosphine)Ni(I) complexes. Redox potentials are directly related to the valence orbitals, sensitive to both the identity of the ligands and the molecular geometry. In general, electron-rich substituents on the ligand framework shift the redox potentials to the negative direction. For example, [PhB^–(CH₂P ⁱ Pr₂)₃]Ni 4 and 6, with a borate on the ligand, have E[Ni(I/0)] as negative as –1.95 V, whereas (B₂P₂)Ni 27, a complex supported on an electron-deficient borane ligand, has the most positive E[Ni(I/0)] in Table [2]. Outer-sphere counterions, on the other hand, only have subtle impact on the redox potentials. The E[Ni(I/0)] of [HN(P ⁱ Pr₂)₂]₂NiX₂ (X = NO₃ 7, ClO₄ 9, BF₄ 10) are almost identical. Generally, Ni(I)–aryl and halide complexes exhibit significantly more negative redox potentials than Ni(diphosphine)₂ complexes.

Table 2 Formal Potentials of the Ni(I)/Ni(0) Transformation for (Phosphine)Ni Complexes
Complex		Process	Solvent	Electrolyte (M)	Potential reference	E_1/2 (V vs. Fc/Fc⁺)
(^tBuXantphos)Ni(2,4-xylene)[21]	1	Ni(I) → Ni(0)	THF	TBAPF₆ (0.4)	Fc/Fc⁺	–2.78
(^tBuXantphos)Ni(o-Tol)[21]	2	Ni(I) → Ni(0)	THF	TBAPF₆ (0.4)	Fc/Fc⁺	–2.70
(dppb)Ni[(CN)₂C₂S₂][40]	3	Ni(I) → Ni(0)	DMF	TBABF₄ (0.1)	Fc/Fc⁺	–2.22
[PhB^–(CH₂P ⁱ Pr₂)₃]Ni(PMe₃)[22]	4	Ni(I) → Ni(0)	THF	TBAPF₆ (0.35)	Fc/Fc⁺	–1.95
(^acriPNP)Ni(CO)[23]	5	Ni(I) → Ni(0)	THF	TBAPF₆ (0.3)	Fc/Fc⁺	–1.87
[PhB^–(CH₂PⁱPr₂)₃]Ni(CN^tBu)[22]	6	Ni(I) → Ni(0)	THF	TBAPF₆ (0.35)	Fc/Fc⁺	–1.85
[HN(P ⁱ Pr₂)₂]₂Ni(NO₃)₂ [24]	7	Ni(I) → Ni(0)	THF	TBAPF₆ (0.1)	Fc/Fc⁺	–1.53
Ni(P^Cy ₂N^tBu ₂)₂ [25]	8	Ni(0) → Ni(I)	PhCN	TBAPF₆ (0.2)	Fc/Fc⁺	–1.49
[HN(P ⁱ Pr₂)₂]₂Ni(ClO₄)₂ [24]	9	Ni(I) → Ni(0)	THF	TBAPF₆ (0.1)	Fc/Fc⁺	–1.49
[HN(P ⁱ Pr₂)₂]₂Ni(BF₄)₂ [24]	10	Ni(I) → Ni(0)	THF	TBAPF₆ (0.1)	Fc/Fc⁺	–1.45
Ni(dmpp)₂ [26]	11	Ni(0) → Ni(I)	MeCN	Et₄NBF₄ (0.3)	Fc/Fc⁺	–1.33
Ni(PMe₃)₄ [27]	12	Ni(0) → Ni(I)	1,2-C₆H₄F₂	TBAPF₆ (0.1)	Fc/Fc⁺	–1.31
(P^Me ₂N^Ph ₂)₂Ni(BF₄)₂ [28]	13	Ni(I) → Ni(0)	PhCN	TBAPF₆ (0.2)	Fc/Fc⁺	–1.30
Ni(depe)₂ [26]	14	Ni(0) → Ni(I)	MeCN	Et₄NBF₄ (0.3)	Fc/Fc⁺	–1.29
(P^Ph ₂N^Me(CH)Ph ₂)₂Ni(BF₄)₂ [29]	15	Ni(I) → Ni(0)	MeCN	TBABF₄ (0.1)	Fc/Fc⁺	–1.27
Ni(NHC^MesCH₂P^Cy2)(cod)[30]	16	Ni(0) → Ni(I)	THF	TBAPF₆ (0.1)	Fc/Fc⁺	–1.26
Ni(dppf)₂ [31]	17	Ni(0) → Ni(I)	THF	TBAPF₆ (0.2)	Fc/Fc⁺	–1.18
(P^Ph ₂N^Ph(CH)Ph ₂)₂Ni(BF₄)₂ [29]	18	Ni(I) → Ni(0)	MeCN	TBABF₄ (0.1)	Fc/Fc⁺	–1.14
(P^Ph ₂N^Bn ₂)₂Ni(BF₄)₂ [29]	19	Ni(I) → Ni(0)	MeCN	TBABF₄ (0.1)	Fc/Fc⁺	–1.13
(P^Ph ₂N^p-Tol ₂)₂Ni(BF₄)₂ [29]	20	Ni(I) → Ni(0)	MeCN	TBABF₄ (0.1)	Fc/Fc⁺	–1.08
(triphos)(PEt₃)Ni(BF₄)₂ [32]	21	Ni(I) → Ni(0)	MeCN	Et₄NBF₄ (0.2)	SCE	–1.05^a
Ni(dcype)(cod)[30]	22	Ni(0) → Ni(I)	THF	TBAPF₆ (0.1)	Fc/Fc⁺	–0.95
Ni(dppp)₂ [26]	23	Ni(0) → Ni(I)	MeCN	Et₄NBF₄ (0.3)	Fc/Fc⁺	–0.91
(triphos)Ni(PPh₃)[33]	24	Ni(0) → Ni(I)	THF	TBAPF₆ (0.1)	NHE	–0.90^b
Ni(dppe)₂ [26]	25	Ni(0) → Ni(I)	MeCN	Et₄NBF₄ (0.3)	Fc/Fc⁺	–0.88
(dppv)₂Ni(BF₄)₂ [34]	26	Ni(I) → Ni(0)	MeCN	Et₄NBF₄ (0.3)	Fc/Fc⁺	–0.83
(B₂P₂)Ni[35]	27	Ni(0) → Ni(I)	MeCN	TBAPF₆ (0.1)	Fc/Fc⁺	0.06

^a Fc = 0.40 V vs SCE (MeCN/Et₄NBF₄).[32]

^b Fc = 0.56 V vs SCE (THF/TBAPF₆).[12] Converted into NHE by adding 0.24 V.

Complexes with halide ligands generally do not have reversible reduction CV, due to the fast dissociation of halides. Monodentate phosphine ligands can be labile and may result in geometry reorganization upon oxidation or reduction.[36] The one-electron oxidation of Ni(PMe₃)₄ 12 shows a reversible CV, whereas the following oxidation to Ni(II) is electrochemically quasireversible, reflecting a change of geometry from tetrahedral to square planar.[27] The synthesis of a series of Ni(I) complexes has enabled the measurement of redox potentials starting from the +1 oxidation state. A (^tBuXantphos)Ni(I)–aryl complex 1 exhibits a very negative reversible reduction peak at –2.78 V.[21] In contrast, Ni(I)–bromide and Ni(I)–chloride complexes, supported on an isopropyl phosphine ligand with a dibenzofuran backbone, give irreversible reduction peaks due to the fast halide dissociation.[37]

[(Cy)N(Ph₂P)₂]Ni(ClO₄)₂ 43 exists as an equilibrium between the tetrahedral and square planar geometries in acetonitrile. Two E[Ni(II/I)] peaks are observed at –0.97 V and –1.77 V, responding to the tetrahedral and the square planar isomers, respectively. This data is consistent with the lower-energy LUMO in a tetrahedral field. Sometimes, the CV data of certain complexes cannot be used to estimate the potentials of their analogues. Bis(diphosphine)Ni complexes exhibit a wide range of redox potentials, from –1.16 V of Ni(depe)₂ 14 to –0.19 V of Ni(dppp)₂ 23, responding to the substituents and the chain length between the two phosphines. Dithiolate and catecholate are good electron-donating ligands. E[Ni(II/I)] of [(Me)N(Et₂PCH₂)₂]Ni(C₂H₄S₂) 28 is –2.34 V (Table [3]), whereas that of [(Me)N(Et₂PCH₂)₂]Ni(BF₄)₂ 48 is –0.64 V. With the same spectator ligand, varying the halide down the group shifts the potential to a more positive direction (E[Ni–Cl] < E[Ni–Br] < E[Ni–I], E[Ni–OR] < E[Ni–SR] < [Ni–SeR]).

The two-electron reduction, E[Ni(II/0)], is also observed, in some cases, due to the overlap of two redox events.[50] (Triphos)(P(OMe)₃)Ni(BF₄)₂ shows a two-electron reduction peak at –0.85 V, while its analogue (triphos)(PEt₃)Ni(BF₄)₂ 21 gives two sequential one-electron reductions at –0.77 V and –1.05 V.[32] Theoretically, the half-peak separation, |E_pa –E_pc |/2, of the two-electron redox processes should be 30 mV, narrower than that of a one-electron event, 60 mV. Since peak separation is also dependent on kinetics and the resistance, the peak-to-peak separation alone is indefinitive for determining the electron stoichiometry for redox events.[51] Formal potentials for high-valence (phospines)Ni mostly are obtained via oxidation of isolated Ni(II) complexes (Table [4]).

Table 3 Formal Potentials of the Ni(II)/Ni(I) Transformation for (Phosphine)Ni Complexes
Complex		Process	Solvent	Electrolyte (M)	Potential reference	E_1/2 (V vs. Fc/Fc⁺)
[(Me)N(Et₂PCH₂)₂]Ni(C₂H₄S₂)[47]	28	Ni(II) → Ni(I)	MeCN	Et₄NBF₄ (0.3)	Fc/Fc⁺	–2.34
(dppe)Ni(3,4-CH₃C₆H₃S₂)[38]	29	Ni(II) → Ni(I)	DCM	TBAClO₄ (0.1)	Ag/AgCl	–2.05^a
[2,6-( ^t Bu₂PCH₂)₂C₆H₃]NiCl[39]	30	Ni(II) → Ni(I)	MeCN	TBABF₄ (0.1)	NHE	–1.88^b
[(cyclohexyl)N(Ph₂P)₂]₂ Ni(ClO₄)₂ [44]	43	Ni(II) → Ni(I)	MeCN	TBABF₄ (0.45)	Fc/Fc⁺	–1.77^c
(dppe)Ni[(CN)₂C₂S₂][38]	31	Ni(II) → Ni(I)	DCM	TBAClO₄ (0.1)	Ag/AgCl	–1.66^a
(^tBuXantphos)Ni(2,4-xylene)[21]	1	Ni(I) → Ni(II)	THF	TBAPF₆ (0.4)	Fc/Fc⁺	–1.59
(^tBuXantphos)Ni(o-Tol)[21]	2	Ni(I) → Ni(II)	THF	TBAPF₆ (0.4)	Fc/Fc⁺	–1.51
[PhB^–(CH₂PPh₂)₃]Ni(OSiPh₃)[22]	32	Ni(II) → Ni(I)	THF	TBAPF₆ (0.35)	Fc/Fc⁺	–1.47
[PhB^–(CH₂P ⁱ Pr₂)₃]NiCl[22]	33	Ni(II) → Ni(I)	THF	TBAPF₆ (0.35)	Fc/Fc⁺	–1.44
(dae)Ni[(CN)₂C₂S₂][38]	34	Ni(II) → Ni(I)	DCM	TBAClO₄ (0.1)	Ag/AgCl	–1.44^a
(dppb)Ni[(CN)₂C₂S₂][40]	3	Ni(II) → Ni(I)	DMF	TBABF₄ (0.1)	Fc/Fc⁺	–1.43
[PhB^–(CH₂PPh₂)₃]Ni(O-p- ^t Bu-Ph)[22]	35	Ni(II) → Ni(I)	THF	TBAPF₆ (0.35)	Fc/Fc⁺	–1.36
(d ^t bpe)Ni(CH₂CMe₃)[41]	36	Ni(I) → Ni(II)	THF	TBAPF₆ (0.4)	Fc/Fc⁺	–1.25
(^acriPNP)Ni(CO)[23]	5	Ni(II) → Ni(I)	THF	TBAPF₆ (0.3)	Fc/Fc⁺	–1.20
[PhB^–(CH₂PPh₂)₃]NiCl[22]	37	Ni(I) → Ni(II)	THF	TBAPF₆ (0.35)	Fc/Fc⁺	–1.20
(PPh₃)₂Ni[(CN)₂C₂S₂][38]	38	Ni(II) → Ni(I)	DCM	TBAClO₄ (0.1)	Ag/AgCl	–1.20^a
[( ⁿ Bu)N(Ph₂PCH₂)₂]NiCl₂ [42]	39	Ni(II) → Ni(I)	DCM	TBAPF₆ (0.1)	Ag/AgCl	–1.18^d
Ni(depe)₂ [26]	14	Ni(I) → Ni(II)	MeCN	Et₄NBF₄ (0.3)	Fc/Fc⁺	–1.16
[(Me₄PNP^tBu)NiMe](BPh₄)[43]	40	Ni(I) → Ni(II)	MeCN	TBAPF₆ (0.1)	Fc/Fc⁺	–1.14
[PhB^–(CH₂PPh₂)₃]NiI[22]	41	Ni(II) → Ni(I)	THF	TBAPF₆ (0.35)	Fc/Fc⁺	–1.12
[PhB^–(CH₂PPh₂)₃]Ni(S-p- ^t Bu-Ph)[22]	42	Ni(II) → Ni(I)	THF	TBAPF₆ (0.35)	Fc/Fc⁺	–1.12
[HN(P ⁱ Pr₂)₂]₂Ni(NO₃)₂ [24]	7	Ni(II) → Ni(I)	THF	TBAPF₆ (0.1)	Fc/Fc⁺	–1.06
(P^Me ₂N^Ph ₂)₂Ni(BF₄) ₂ [28]	13	Ni(II) → Ni(I)	PhCN	TBAPF₆ (0.2)	Fc/Fc⁺	–1.01
[HN(P ⁱ Pr₂)₂]₂Ni(ClO₄)₂ [24]	8	Ni(II) → Ni(I)	THF	TBAPF₆ (0.1)	Fc/Fc⁺	–1.01
[(cyclohexyl)N(Ph₂P)₂]₂ Ni(ClO₄)₂ [44]	43′	Ni(II) → Ni(I)	MeCN	TBABF₄ (0.45)	Fc/Fc⁺	–0.97^e
[HN(P ⁱ Pr₂)₂]₂Ni(BF₄)₂ [24]	10	Ni(II) → Ni(I)	THF	TBAPF₆ (0.1)	Fc/Fc⁺	–0.97
(P^Ph ₂N^Me(CH)Ph ₂)₂Ni(BF₄)₂ [29]	15	Ni(II) → Ni(I)	MeCN	TBABF₄ (0.1)	Fc/Fc⁺	–0.93
(tdppme)Ni(S^tBu)[45]	44	Ni(I) → Ni(II)	DCM	TBAPF₆ (0.1)	SCE	–0.93^f
Ni(dmpp)₂ [26]	11	Ni(I) → Ni(II)	MeCN	Et₄NBF₄ (0.3)	Fc/Fc⁺	–0.89
(dppp)NiBr₂ [46]	45	Ni(II) → Ni(I)	THF	TBAPF₆ (0.1)	Ag/AgNO₃	–0.89^g
(triphos)(MeCN)Ni(BF₄)₂ [32]	46	Ni(II) → Ni(I)	MeCN	Et₄NBF₄ (0.2)	SCE	–0.88^h
Ni(P^Cy ₂N^tBu ₂)₂ [25]	9	Ni(I) → Ni(II)	PhCN	TBAPF₆ (0.2)	Fc/Fc⁺	–0.87
(P^Ph ₂N^p-Tol ₂)₂Ni(BF₄)₂ [29]	20	Ni(II) → Ni(I)	MeCN	TBABF₄ (0.1)	Fc/Fc⁺	–0.83
(triphos)(PEt₃)Ni(BF₄)₂ [32]	21	Ni(II) → Ni(I)	MeCN	Et₄NBF₄ (0.2)	SCE	–0.77^h
(tdppme)Ni(S^Ph)[45]	47	Ni(I) → Ni(II)	DCM	TBAPF₆ (0.1)	SCE	–0.75^f
(P^Ph ₂N^Ph(CH)Ph ₂)₂Ni(BF₄)₂ [29]	18	Ni(II) → Ni(I)	MeCN	TBABF₄ (0.1)	Fc/Fc⁺	–0.72
Ni(dppe)₂ [26]	25	Ni(I) → Ni(II)	MeCN	Et₄NBF₄ (0.3)	Fc/Fc⁺	–0.70
[(Me)N(Et₂PCH₂)₂]₂Ni(BF₄)₂ [47]	48	Ni(II) → Ni(I)	MeCN	Et₄NBF₄ (0.3)	Fc/Fc⁺	–0.64
(tdppme)Ni(Se^Ph)[45]	49	Ni(I) → Ni(II)	DCM	TBAPF₆ (0.1)	SCE	–0.64^f
(tdppme)NiCl(ClO₄)[48]	50	Ni(II) → Ni(I)	MeCN	Et₄NClO₄ (0.1)	SCE	–0.63ⁱ
(tdppme)NiBr(ClO₄)[48]	51	Ni(II) → Ni(I)	MeCN	Et₄NClO₄ (0.1)	SCE	–0.57ⁱ
(dppv)₂Ni(BF₄)₂ [34]	26	Ni(II) → Ni(I)	MeCN	Et₄NBF₄ (0.3)	Fc/Fc⁺	–0.52
(tdppme)NiI(ClO₄)[48]	52	Ni(II) → Ni(I)	MeCN	Et₄NClO₄ (0.1)	SCE	–0.47ⁱ
Ni(PMe₃)₄ [27]	12	Ni(I) → Ni(II)	1,2-C₆H₄F₂	TBAPF₆ (0.1)	Fc/Fc⁺	–0.33
Ni(dppp)₂ [26]	23	Ni(I) → Ni(II)	MeCN	Et₄NBF₄ (0.3)	Fc/Fc⁺	–0.19

^a Fc = 0.46 V vs Ag/Ag⁺ 0.1 M LiCl in DCM (DCM/TBAClO₄).[38]

^b Fc = 0.69 V vs NHE (MeCN/TBABF₄).[39]

^c E⁰ for Sp isomer.

^d Fc = 0.33 V vs Ag/AgCl (DCM/TBAPF₆).[42]

^e E⁰ for Td isomer.

^f Fc = 0.46 V vs SCE (DCM/TBAPF₆).[12]

^g Fc = 0.176 V vs Ag/0.01 M AgNO₃ (THF/TBAPF₆).[49]

^h Fc = 0.40 V vs SCE (MeCN/Et₄NBF₄).[32]

ⁱ Fc = 0.38 V vs SCE (MeCN/Et₄NClO₄).[48]

Table 4 Formal Potentials of the Ni(III)/Ni(II) and Ni(IV)/Ni(III) Transformations for Selected (Phosphine)Ni Complexes
Complex		Process	Solvent	Electrolyte (0.1 M)	Potential reference	E_1/2 (V vs. Fc/Fc⁺)
(dppe)Ni(3,4- ^t BuC₆H₃O₂)[38]	53	Ni(II) → Ni(III)	DCM	TBAClO₄	Ag/AgCl	–0.25^a
(dppb)Ni[(Me)₂C₂S₂][40]	54	Ni(II) → Ni(III)	DCM	TBAPF₆	Fc/Fc⁺	–0.20
(dppb)Ni[(C₆H₄-p-OMe)₂C₂S₂][40]	55	Ni(II) → Ni(III)	DCM	TBAPF₆	Fc/Fc⁺	–0.15
[o-C₆H₄(PMe₂)₂]₂NiCl₂ [52]	56	Ni(II) → Ni(III)	MeCN	TBABF₄	SCE	–0.03^b
[o-C₆F₄(PMe₂)₂]₂NiCl₂ [52]	57	Ni(II) → Ni(III)	MeCN	TBABF₄	SCE	0.08^b
[o-C₆H₄(PMe₂)₂]₂NiBr₂ [52]	58	Ni(II) → Ni(III)	MeCN	Et₄NClO₄	SCE	0.10^c
(dppe)Ni(o-C₆Cl₄O₂)[38]	59	Ni(II) → Ni(III)	DCM	TBAClO₄	Ag/AgCl	0.25^a
(dcpf)NiCl₂ [53]	60	Ni(II) → Ni(III)	DCM	TBAPF₆	Fc/Fc⁺	0.30
[2,6-( ^t Bu₂PO)₂C₆H₃]NiH[54]	61	Ni(II) → Ni(III)	MeCN–THF	TBABF₄	Fc/Fc⁺	0.33
(dppe)NiCl₃ [55]	62	Ni(III) → Ni(II)	MeCN	TBAPF₆	Fc/Fc⁺	0.40
[2,6-( ⁱ Pr₂PO)₂C₆H₃]Ni(OAc)[56]	63	Ni(II) → Ni(III)	DCM	TBAPF₆	Fc/Fc⁺	0.43
[2,6-(Ph₂PO)₂C₆H₃]Ni(OAc)[56]	64	Ni(II) → Ni(III)	DCM	TBAPF₆	Fc/Fc⁺	0.55
[( ^t Bu₂PO)₂C₆H₃]NiCl[54]	65	Ni(II) → Ni(III)	MeCN–THF	TBABF₄	Fc/Fc⁺	0.72
[2,6-( ^t Bu₂PO)₂C₆H₃]NiBr[57]	66	Ni(II) → Ni(III)	DCM	TBAPF₆	Fc/Fc⁺	0.75
[2,6-(Ph₂PO)₂C₆H₃]Ni(OTf)[56]	67	Ni(II) → Ni(III)	DCM	TBAPF₆	Fc/Fc⁺	0.81
[2,6-( ⁱ Pr₂PO)₂C₆H₃]Ni(OTf)[56]	68	Ni(II) → Ni(III)	DCM	TBAPF₆	Fc/Fc⁺	0.98
(dppb)Ni[(Me)₂C₂S₂][40]	54	Ni(III) → Ni(IV)	DCM	TBAPF₆	Fc/Fc⁺	0.50^d
(dppb)Ni[(C₆H₄-p-OMe)₂C₂S₂][40]	55	Ni(III) → Ni(IV)	DCM	TBAPF₆	Fc/Fc⁺	0.44
[o-C₆H₄(PMe₂)₂]₂NiCl₂ [52]	56	Ni(III) → Ni(IV)	MeCN	TBABF₄	SCE	0.79^b
[o-C₆H₄(PMe₂)₂]₂NiBr₂ [52]	58	Ni(III) → Ni(IV)	MeCN	Et₄NClO₄	SCE	0.84^c
[o-C₆H₄(AsMe₂)₂]₂NiCl₂ [52]	69	Ni(III) → Ni(IV)	MeCN	Et₄NClO₄	SCE	0.91^c
[o-C₆F₄(PMe₂)₂]₂NiCl₂ [52]	57	Ni(III) → Ni(IV)	MeCN	TBABF₄	SCE	1.01^b

^a Fc = 0.46 V vs Ag/Ag⁺ 0.1 M LiCl in DCM (DCM/TBAClO₄).[38]

^b Fc = 0.39 V vs SCE (MeCN/TBABF₄).[14]

^c Fc = 0.38 V vs SCE (MeCN/Et₄NClO₄).[48]

^d Potentials estimated from differential pulse voltammetry by width-at-half-height analysis.

2.2

Redox Potentials of (Nitrogen)Ni Complexes

Ni-catalyzed cross-coupling reactions proceeding through radical pathways has benefited from various bidentate and tridentate N-ligands, including bipyridine (bpy), bioxazoline (biOx), terpyridine (terpy), pyridine-oxazoline (pyox), and pyridine-bioxazoline (pybox).[6] [58] N-Ligands are π-acceptors and generally stronger σ-donors than phosphines.[59] The redox activity of π-acceptor ligands greatly contributes to the stability of radical complexes. Redox processes may occur on the ligand rather than the metal center. Data in Table [5] and Table [6] refer to the formal oxidation state of the nickel complexes, but do not distinguish the change of oxidation state due to ligand redox activity.

Table 5 Formal Potentials of the Ni(I)/Ni(0) Transformation for Selected Ni/Nitrogen Complexes
Complex		Process	Solvent	Electrolyte (0.1 M)	Potential reference	E_1/2 (V vs. Fc/Fc⁺)
[(–)-i-Pr-pybox]Ni(Ph)B(Ar^F)₄ [60]	70	Ni(I) → Ni(0)	THF	TBAPF₆	Fc/Fc⁺	–2.36
(dtbbpy)(C^Prop2C)Ni(PF₆)₂ [61]	71	Ni(I) → Ni(0)	MeCN	TBAPF₆	Fc/Fc⁺	–2.06
(bpy)(C^Prop2C)Ni(PF₆)₂ [61]	72	Ni(I) → Ni(0)	MeCN	TBAPF₆	Fc/Fc⁺	–2.00
(^Prbimiiql)Ni(PF₆)₂ [62]	73	Ni(I) → Ni(0)	MeCN	TBAPF₆	SCE	–1.60^a
(6,6′-Mebpy)NiBr₂ [63]	74	Ni(I) → Ni(0)	MeCN	TBABF₄	SCE	–1.56^b
(^DippBIAN)NiCl₂ [64]	75	Ni(I) → Ni(0)	MeCN	TBABF₄	Fc/Fc⁺	–1.52
(^DippBIAN)NiBr₂ [64]	76	Ni(I) → Ni(0)	MeCN	TBABF₄	Fc/Fc⁺	–1.47
(^DippBIAN)NiI₂ [64]	77	Ni(I) → Ni(0)	MeCN	TBABF₄	Fc/Fc⁺	–1.46
(^DippBIAN)Ni(NCMe)₄(BF₄)₂ [64]	78	Ni(I) → Ni(0)	MeCN	TBABF₄	Fc/Fc⁺	–1.45
(^DippN^PyN^DippN)NiCl₂ [65]	79	Ni(I) → Ni(0)	MeCN	TBAPF₆	Fc/Fc⁺	–1.23
(^DippN^PyN^DippN)NiBr₂ [65]	80	Ni(I) → Ni(0)	MeCN	TBAPF₆	Fc/Fc⁺	–1.22
(bpy)Ni(cod)[30]	81	Ni(0) → Ni(I)	THF	TBAPF₆	Fc/Fc⁺	–1.17
(2-OMe-Ph-Me₂DAB)(Cp)NiBF₄ [66]	82	Ni(I) → Ni(0)	MeCN	TBAPF₆	Fc/Fc⁺	–1.17
(Ph-Me₂DAB)(Cp)NiBF₄ [66]	83	Ni(I) → Ni(0)	MeCN	TBAPF₆	Fc/Fc⁺	–1.05
(2-CF₃-Ph-Me₂DAB)(Cp)NiBF₄ [66]	84	Ni(I) → Ni(0)	MeCN	TBAPF₆	Fc/Fc⁺	–0.80

^a Fc = 0.40 V vs SCE (MeCN/TBAPF₆).[12]

^b Fc = 0.38 V vs SCE (MeCN/TBABF₄).[63]

The electronic effect of ligands on the redox potential is evident by comparing a series of (4,4′-Mebpy)Ni (4,4′-Mebpy = 92), (bpy)Ni (bpy = 95), and DAB(Ni) (DAB = 105) complexes (Table [6]). The first reduction of Ni(II) complexes can be ligand centered, depending on the coordination number, geometry, and the ligand. For example, the first electron reduction of (dtbbpy)(C^Prop2C)Ni(PF₆)₂ 71 and (bpy)(C^Prop2C)Ni(PF₆)₂ 72 is ligand centered, and the second electron reduction is metal centered.[61] The nature of ligands can affect the reversibility of CV. Halides can easily dissociate upon reduction and give rise to irreversible CVs. Terpy complex 104 shows a reversible CV at room temperature, but the CVs of bidentate nitrogen-ligated Ni(Mes)Br complexes in Table [6] were measured at –60 °C to prevent bromide dissociation.[69]

Table 6 Formal Potentials of the Ni(II)/Ni(I) Transformation for (Nitrogen)Ni Complexes
Complex		Process	Solvent	Electrolyte (0.1 M)	Potential reference	E_1/2 (V vs. Fc/Fc⁺)
(bme-daco)Ni[67]	85	Ni(II) → Ni(I)	MeCN	TBAPF₆	NHE	–2.58^a
(en)Ni(acac)₂ [68]	86	Ni(II) → Ni(I)	DMF	TBAClO₄	SCE	–2.57^b
(3,4,7,8-tmphen)Ni(Mes)₂ [69]	87	Ni(II) → Ni(I)	DMF	TBAPF₆	Fc/Fc⁺	–2.22
(Phbpy)Ni(CF₃) [70]	88	Ni(II) → Ni(I)	THF	TBAPF₆	Fc/Fc⁺	–2.04
(bpy)Ni(Mes)₂ [69]	89	Ni(II) → Ni(I)	DMF	TBAPF₆	Fc/Fc⁺	–2.02
(cyclam)NiBr₂ [71]	90	Ni(II) → Ni(I)	DMF	TBABF₄	Fc/Fc⁺	–2.00
(Phbpy)NiBr[70]	91	Ni(II) → Ni(I)	THF	TBAPF₆	Fc/Fc⁺	–1.90
(4,4′-Mebpy)Ni(Mes)Br[69]	92	Ni(II) → Ni(I)	DMF	TBAPF₆	Fc/Fc⁺	–1.87
(cyclam)Ni(BF₄)₂ [71]	93	Ni(II) → Ni(I)	DMF	TBABF₄	Fc/Fc⁺	–1.85
(Me-bme-daco)NiI[67]	94	Ni(II) → Ni(I)	MeCN	TBAPF₆	NHE	–1.84^a
(bpy)Ni(Mes)Br[69]	95	Ni(II) → Ni(I)	DMF	TBAPF₆	Fc/Fc⁺	–1.79
(α,α′-Me₂salen)Ni[72]	96	Ni(II) → Ni(I)	DMF	TBAClO₄	Fc/Fc⁺	–1.71
(bpy)Ni(Fmes)Br[69]	97	Ni(II) → Ni(I)	THF	TBAPF₆	Fc/Fc⁺	–1.68
(saltMe)Ni[72]	98	Ni(II) → Ni(I)	DMF	TBAClO₄	Fc/Fc⁺	–1.67
(salen)Ni[72]	99	Ni(II) → Ni(I)	DMF	TBAClO₄	Fc/Fc⁺	–1.60
(dtbbpy)(C^Prop2C)Ni(PF₆)₂ [61]	71	Ni(II) → Ni(I)	MeCN	TBAPF₆	Fc/Fc⁺	–1.59
(terpy)₂Ni(PF₆)₂ [73]	100	Ni(II) → Ni(I)	MeCN	TBAClO₄	SCE	–1.58^c
(bpy)NiCl₂ [74]	101	Ni(II) → Ni(I)	DMF	TBAPF₆	Ag/AgCl	–1.52^d
(bpy)(C^Prop2C)Ni(PF₆)₂ [61]	72	Ni(II) → Ni(I)	MeCN	TBAPF₆	Fc/Fc⁺	–1.50
(bpz)Ni(Mes)₂ [69]	102	Ni(II) → Ni(I)	DMF	TBAPF₆	Fc/Fc⁺	–1.48
(bpym)Ni(Mes)Br[69]	103	Ni(II) → Ni(I)	DMF	TBAPF₆	Fc/Fc⁺	–1.47
(terpy)Ni(Mes)Br[69]	104	Ni(II) → Ni(I)	DMF	TBAPF₆	Fc/Fc⁺	–1.45
[(–)-i-Pr-pybox]Ni(Ph)B(Ar^F)₄ [60]	70	Ni(II) → Ni(I)	THF	TBAPF₆	Fc/Fc⁺	–1.37
(i-Pr-DAB)Ni(Mes)Br[69]	105	Ni(II) → Ni(I)	DMF	TBAPF₆	Fc/Fc⁺	–1.37
(Cl₂-saltMe)Ni[72]	106	Ni(II) → Ni(I)	DMF	TBAClO₄	Fc/Fc⁺	–1.37
(trans-III-Me₄-cyclam)Ni(ClO₄)₂ [75]	107	Ni(II) → Ni(I)	MeCN	TBABF₄	Ag/AgNO₃	–1.36^e
(bpz)Ni(Mes)Br[69]	108	Ni(II) → Ni(I)	DMF	TBAPF₆	Fc/Fc⁺	–1.34
(Bz₂-bme-daco)NiBr₂ [67]	110	Ni(II) → Ni(I)	MeCN	TBAPF₆	NHE	–1.31^a
(saloph-Cl₂)Ni[72]	109	Ni(II) → Ni(I)	DMF	TBAClO₄	Fc/Fc⁺	–1.30
(6,6′-Mebpy)NiBr₂ [63]	74	Ni(II) → Ni(I)	MeCN	TBABF₄	SCE	–1.26
(trans-I-Me₄-cyclam)Ni(ClO₄)₂ [75]	111	Ni(II) → Ni(I)	MeCN	TBABF₄	Ag/AgNO₃	–1.23^e
(bpm)Ni(Mes)Br[69]	112	Ni(II) → Ni(I)	DMF	TBAPF₆	Fc/Fc⁺	–1.20
(^Prbimiiql)Ni(PF₆)₂ [62]	73	Ni(II) → Ni(I)	MeCN	TBAPF₆	SCE	–1.14^f
(Me₂-bme-daco)NiI₂ [67]	113	Ni(II) → Ni(I)	MeCN	TBAPF₆	NHE	–1.12^a
(^DippBDI)Ni(η²-O₂) [76]	114	Ni(II) → Ni(I)	THF	TBAPF₆	Fc/Fc⁺	–0.98
(^DippBIAN)NiCl₂ [64]	75	Ni(II) → Ni(I)	MeCN	TBABF₄	Fc/Fc⁺	–0.97
(^DippN^PyN^DippN)NiCl₂ [65]	79	Ni(II) → Ni(I)	MeCN	TBAPF₆	Fc/Fc⁺	–0.86
(^DippBIAN)NiBr₂ [64]	76	Ni(II) → Ni(I)	MeCN	TBABF₄	Fc/Fc⁺	–0.81
(^DippBIAN)NiI₂ [64]	77	Ni(II) → Ni(I)	MeCN	TBABF₄	Fc/Fc⁺	–0.80
(^DippBIAN)Ni(NCMe)₄(BF₄)₂ [64]	78	Ni(II) → Ni(I)	MeCN	TBABF₄	Fc/Fc⁺	–0.77
(bpy)Ni(cod) [30]	81	Ni(I) → Ni(II)	THF	TBAPF₆	Fc/Fc⁺	–0.76
(^DippN^PyN^DippN)NiBr₂ [65]	80	Ni(II) → Ni(I)	MeCN	TBAPF₆	Fc/Fc⁺	–0.68

^a Fc = 0.64 V vs NHE (MeCN/TBAPF₆).[12]

^b Fc = 0.47 V vs SCE (DMF/TBAClO₄).[12]

^c Fc = 0.38 V vs SCE (MeCN/TBAClO₄).[12]

^d Fc = 0.50 V vs Ag/0.1 M NaCl (DMF).[72]

^e Fc = 0.037 V vs Ag/0.1 M AgNO₃ (MeCN/TBAPF₆).[77]

^f Fc = 0.40 V vs SCE (MeCN/TBAPF₆).[12]

Ligand effects in the oxidation of Ni(II) to Ni(III) states follow the typical trend: electron-withdrawing para substituents of the NCN pincer ligand shift the oxidation potential to the positive direction: E[Ni–NH₂ 117] < E[Ni–OMe 121] < E[Ni–H 123] < E[Ni–Cl 126] < E[Ni–Ac 127] (Table [7]). The reduction potentials of (porphyrin)Ni(III) complexes also reflect the electronic trend of the ligands. Reduction requires a more negative potential for complexes with electron-donating substitutions: E[(T ^t BuP)Ni 132] < E[(T ⁱ PrP)Ni 133] < E[(T^EtPrP)Ni 134] < E[(T ⁱ BuP)Ni 137] < E[(TPP)Ni 138].[86]

Table 7 Formal Potentials of Ni(III)/Ni(II) and Ni(IV)/Ni(III) Transformations for Selected (Nitrogen)Ni Complexes
Complex		Process	Solvent	Electrolyte (M)	Potential reference	E_1/2 (V vs. Fc/Fc⁺)
(N^tBu ₂PyPh)Ni(MeCN)₂PF₆ [78]	115	Ni(II) → Ni(III)	MeCN	TBABF₄ (0.1)	Fc/Fc⁺	–0.66
(N^tBu ₂PyPh)NiBr(MeCN)PF₆ [78]	116	Ni(III) → Ni(II)	MeCN	TBABF₄ (0.1)	Fc/Fc⁺	–0.65
[2,6-(Me₂NCH₂)₂-(4-NH₂)C₆H₂]NiBr[79]	117	Ni(II) → Ni(III)	DCM	TBABr (0.1)	Ag/AgCI	–0.45^a
(N^tBu ₂Py₂)Ni(p-F-Ph)Cl[80]	118	Ni(II) → Ni(III)	MeCN	TBAClO₄ (0.1)	Fc/Fc⁺	–0.45
[2,6-(Me₂NCH₂)₂C₆H₃]NiCl₂ [81]	119	Ni(III) → Ni(II)	Acetone	TBACl (0.1)	Ag/AgCl	–0.44^b
(Tp)(Cp)Ni(PF₆)[82]	120	Ni(II) → Ni(III)	DCM	TBAPF₆ (0.1)	Fc/Fc⁺	–0.42
[2,6-(Me₂NCH₂)₂-(4-OMe)C₆H₂]NiBr[79]	121	Ni(II) → Ni(III)	DCM	TBABr (0.1)	Ag/AgI	–0.40^a
(N^tBu ₂Py₂)Ni(p-F-Ph)Br[80]	122	Ni(II) → Ni(III)	MeCN	TBAClO₄ (0.1)	Fc/Fc⁺	–0.40
[2,6-(Me₂NCH₂)₂C₆H₃]NiBr[79]	123	Ni(II) → Ni(III)	DCM	TBABr (0.1)	Ag/AgI	–0.39^a
(Tp)(Cp*)Ni(PF₆)[82]	124	Ni(II) → Ni(III)	DCM	TBAPF₆ (0.1)	Fc/Fc⁺	–0.39
[2,6-(Me₂NCH₂)₂C₆H₃]Ni(NO₃)₂ [81]	125	Ni(III) → Ni(II)	Acetone	TBACl (0.1)	Ag/AgCl	–0.38^b
[2,6-(Me₂NCH₂)₂-(4-Cl)C₆H₂]NiBr[79]	126	Ni(II) → Ni(III)	DCM	TBABr (0.1)	Ag/AgI	–0.33^a
[2,6-(Me₂NCH₂)₂-(4-Ac)C₆H₂]NiBr[79]	127	Ni(II) → Ni(III)	DCM	TBABr (0.1)	Ag/AgI	–0.32^a
(Phbpy)Ni(CF₃)[70]	88	Ni(II) → Ni(III)	THF	TBAPF₆ (0.1)	Fc/Fc⁺	–0.08
(dtbbpy)Ni(C₄F₈)[83]	128	Ni(II) → Ni(III)	MeCN	TBABF₄ (0.1)	Fc/Fc⁺	–0.02
(Phbpy)NiBr[70]	91	Ni(II) → Ni(III)	THF	TBAPF₆ (0.1)	Fc/Fc⁺	0.08
(TACN)₂Ni(ClO₄)₃ [84]	129	Ni(III) → Ni(II)	MeCN	TBAClO₄ (0.1)	Fc/Fc⁺	0.56
(Me₂Ac₂Me₂malen)Ni[85]	130	Ni(II) → Ni(III)	MeCN	TBABF₄ (0.1)	Ag/AgNO₃	0.57^c
(Me₂Ac₂H₂malen)Ni[85]	131	Ni(II) → Ni(III)	MeCN	TBABF₄ (0.1)	Ag/AgNO₃	0.68^c
(bpy)Ni(Fmes)Br[69]	97	Ni(II) → Ni(III)	DCM	TBAPF₆ (0.1)	Fc/Fc⁺	0.81
(Bz₂-bme-daco)NiBr₂ [67]	109	Ni(II) → Ni(III)	MeCN	TBAPF₆ (0.1)	NHE	0.93^d
(Me₂-bme-daco)NiI₂ [67]	113	Ni(II) → Ni(III)	MeCN	TBAPF₆ (0.1)	NHE	0.93^d
(T ^t BuP)Ni[86]	132	Ni(II) → Ni(III)	PhCN	TBAClO₄ (0.1)	SCE	1.08^e
(T ⁱ PrP)Ni[86]	133	Ni(II) → Ni(III)	PhCN	TBAClO₄ (0.1)	SCE	1.14^e
(trans-III-Me₄-cyclam)Ni(ClO₄)₂ [75]	107	Ni(II) → Ni(III)	MeCN	TBABF₄ (0.1)	Ag/AgNO₃	1.18^c
(trans-I-Me₄-cyclam)Ni(ClO₄)₂ [75]	111	Ni(II) → Ni(III)	MeCN	TBAClO₄ (0.1)	Ag/AgNO₃	1.23^c
(T^EtPrP)Ni[86]	134	Ni(II) → Ni(III)	PhCN	TBABF₄ (0.1)	SCE	1.23^e
(bpy)₃Ni(BF₄)₂ [87]	135	Ni(II) → Ni(III)	MeCN	TBABF₄ (0.1)	Fc/Fc⁺	1.23
(terpy)₂Ni(PF₆)₂ [73]	100	Ni(II) → Ni(III)	MeCN	TBAClO₄ (0.1)	SCE	1.27^f
(bpy)₃Ni(ClO₄)₂ [88]	136	Ni(II) → Ni(III)	MeCN	TBAClO₄ (0.2)	SCE	1.30^f
(T ⁱ BuP)Ni[86]	137	Ni(II) → Ni(III)	PhCN	TBAClO₄ (0.1)	SCE	1.32^e
(TPP)Ni[86]	138	Ni(II) → Ni(III)	PhCN	TBAClO₄ (0.1)	SCE	1.33^e
(bpy)₂Ni[88]	139	Ni(II) → Ni(III)	MeCN	TBAClO₄ (0.2)	SCE	1.34^f
(OEP)Ni[86]	140	Ni(II) → Ni(III)	PhCN	TBAClO₄ (0.1)	SCE	1.38^e
(Tp)Ni(CF₃)₃ [89]	141	Ni(IV) → Ni(III)	MeCN	TBAPF₆ (0.1)	SCE	–0.80^g
(N^Me ₂Py₂)NiMe₂(PF₆) [90]	142	Ni(III)→ Ni(IV)	MeCN	TBAPF₆ (0.1)	Fc/Fc⁺	–0.03
(N^Me ₂Py₂)Ni(cycloneophyl)(PF₆)[90]	143	Ni(III)→ Ni(IV)	MeCN	TBAPF₆ (0.1)	Fc/Fc⁺	0.21
(Tp)(Cp)Ni(PF₆)[82]	120	Ni(III)→ Ni(IV)	DCM	TBAPF₆ (0.1)	Fc/Fc⁺	0.44
[2,6-(Me₂NCH₂)₂C₆H₃]NiBr₂ [91]	144	Ni(III)→ Ni(IV)	MeCN	TBAPF₆ (0.1)	Fc/Fc⁺	0.69
(dtbbpy)Ni(C₄F₈)[83]	128	Ni(III)→ Ni(IV)	MeCN	TBABF₄ (0.1)	Fc/Fc⁺	1.16
(bpy)₃Ni(BF₄)₂ [87]	135	Ni(III)→ Ni(IV)	MeCN	TBABF₄ (0.1)	Fc/Fc⁺	1.98

^a Fc = 0.87 V vs Ag/AgI (0.4 M TBAClO₄ and 0.05 M TBAI in DCM) (DCM/TBABr).[79]

^b Fc = 0.63 V vs Ag/Ag⁺ 0.1 M LiCl in acetone (acetone/TBACl).[81]

^c Fc = 0.037 V vs Ag/0.1 M AgNO₃ (MeCN/TBAPF₆).[77]

^d Fc = 0.64 V vs NHE (MeCN/TBAPF₆).[12]

^e Fc = 0.50 V vs SCE (PhCN/TBAClO₄).[19]

^f Fc = 0.38 V vs SCE (MeCN/TBAClO₄).[12]

^g Fc = 0.40 V vs SCE (MeCN/TBAPF₆).[12]

2.3

Redox Potentials of (NHC)Ni Complexes

The use of N-heterocyclic carbenes (NHC) in homogeneous nickel catalysis has dramatically expanded over the past two decades as a modular, strongly σ-donating, and nonlabile alternative to phosphines.[20] (NHC)Ni complexes have found a wide range of applications in cross-coupling reactions, in which nickel is stabilized in both open and closed-shell electron configurations.[20] [92] A wide range of oxidation states can be supported on (NHC)Ni complexes. Data collected in Table [8], Table [9], and Table [10] cover single-electron transformations from Ni(0) up to Ni(IV). As a strong σ-donor, NHC drastically shifts the redox potentials of nickel complexes to the negative direction. E[Ni(I/0)] of 145 is as negative as –2.50 V. Nickel(0) complexes carrying more NHC ligands, or electron-donating substituents, are oxidized at a more negative potential (Table [8]). The better σ-donor (SIPr)Ni(0) 153 is oxidized at a more negative potential relative to (IPr)Ni(0) 154.

In summary, we tabulate the redox potentials of nickel complexes experimentally measured by CV and convert data to a unified Fc/Fc⁺ reference electrode for direct comparison. The redox potentials are clearly determined by the oxidation state, the electronic effect of the ligand, the coordination geometry, the solvent, and the electrolyte conditions. This article is meant to assist synthetic organic and organometallic chemists to evaluate the feasibility and kinetics of redox events occurring at the nickel center, when designing catalytic reactions and preparing nickel complexes.

Table 8 Formal Potentials of the Ni(I)/Ni(0) Transformation for Selected (NHC)Ni Complexes
Complex		Process	Solvent	Electrolyte (0.1 M)	Potential reference	E_1/2 (V vs. Fc/Fc⁺)
(TIMEN^tBu)Ni[93]	145	Ni(0) → Ni(I)	THF	TBAClO₄	Fc/Fc⁺	–2.50
(IPr)Ni(NHDipp)[94]	146	Ni(I) → Ni(0)	THF	TBAPF₆	Fc/Fc⁺	–2.41
(SPMes)₂NiBr[95]	147	Ni(I) → Ni(0)	THF	TBAPF₆	NHE	–2.12^a
(IMes)₂Ni[96]	148	Ni(0) → Ni(I)	THF	TBAPF₆	Fc/Fc⁺	–1.90
(^MeC^PropC^Me)Ni(dtbbpy)(PF₆)₂ [61]	149	Ni(I) → Ni(0)	MeCN	TBAPF₆	Fc/Fc⁺	–1.85
(^MeC^PropC^Me)Ni(bpy)(PF₆)₂ [61]	150	Ni(I) → Ni(0)	MeCN	TBAPF₆	Fc/Fc⁺	–1.79
(^Prbimiql)Ni(PF₆)₂ [62]	151	Ni(I) → Ni(0)	MeCN	TBAPF₆	SCE	–1.78^b
(^Prbzbimpy)Ni(PF₆)₂ [62]	152	Ni(I) → Ni(0)	MeCN	TBAPF₆	SCE	–1.62^b
(SIPr)Ni(Cp)[97]	153	Ni(0) → Ni(I)	THF	TBAPF₆	Fc/Fc⁺	–0.75
(IPr)Ni(Cp)[98]	154	Ni(0) → Ni(I)	THF	TBAPF₆	Fc/Fc⁺	–0.66
I ⁱ Pr(bzim)Ni(Cp)[98]	155	Ni(0) → Ni(I)	DCM	TBA[B(Ar^F ₄)]	Fc/Fc⁺	0.22
(benzo)I ⁱ Pr(bzim)Ni(Cp)[99]	156	Ni(0) → Ni(I)	DCM	TBA[B(Ar^F ₄)]	Fc/Fc⁺	0.27
IMe(bzim)Ni(Cp)[99]	157	Ni(0) → Ni(I)	DCM	TBA[B(Ar^F ₄)]	Fc/Fc⁺	0.32
(benzo)IMe(bzim)Ni(Cp)[99]	158	Ni(0) → Ni(I)	DCM	TBA[B(Ar^F ₄)]	Fc/Fc⁺	0.40

^a Fc = 0.80 V vs NHE (THF/TBAPF₆).[12]

^b Fc = 0.40 V vs SCE (MeCN/TBAPF₆).[12]

Table 9 Formal Potentials of the Ni(II)/Ni(I) Transformation for Selected (NHC)Ni Complexes
Complex		Process	Solvent	Electrolyte (0.1 M)	Potential reference	E _1/2 (V vs. Fc/Fc⁺)
[(benzo)I(CH₂Py)₂]₂NiBr₂ [100]	159	Ni(II) → Ni(I)	DMF	TBAPF₆	Ag/AgCl	–1.56^a
(^MeC^PropC^Me)Ni(dtbbpy)(PF₆)₂ [61]	149	Ni(II) → Ni(I)	MeCN	TBAPF₆	Fc/Fc⁺	–1.54
[(benzo)I(CH₂Py)(Bz)]₂NiBr₂ [99]	160	Ni(II) → Ni(I)	DMF	TBAPF₆	Ag/AgCl	–1.51^a
(IMes)(Cp)NiCl[100]	161	Ni(II) → Ni(I)	MeCN	TBABF₄	NHE	–1.51^b
(^MeC^PropC^Me)Ni(bpy)(PF₆)₂ [61]	150	Ni(II) → Ni(I)	MeCN	TBAPF₆	Fc/Fc⁺	–1.42
(^Prbimiql)Ni(PF₆)₂ [62]	151	Ni(II) → Ni(I)	MeCN	TBAPF₆	SCE	–1.32^c
(IPr)Ni(Cp*)[101]	162	Ni(I) → Ni(II)	THF	TBAPF₆	Fc/Fc⁺	–1.18
(^Prbzbimpy)Ni(PF₆)₂ [62]	152	Ni(II) → Ni(I)	MeCN	TBAPF₆	SCE	–1.03^c
(TIMEN^tBu)Ni[94]	145	Ni(I) → Ni(II)	THF	TBAClO₄	Fc/Fc⁺	–1.09
(IMes)Ni(Cp)[102]	163	Ni(I) → Ni(II)	THF	TBAPF₆	Fc/Fc⁺	–1.06
(IPr)Ni(Cp)[102]	154	Ni(I) → Ni(II)	THF	TBAPF₆	Fc/Fc⁺	–1.02
(IPr)Ni(NHDipp)[95]	146	Ni(II) → Ni(I)	THF	TBAPF₆	Fc/Fc⁺	–0.84
(I ⁿ Bu)(Cp)NiBr[102]	164	Ni(II) → Ni(I)	DCM	TBAPF₆	SCE	0.22^d
(pyrene-I ⁿ Bu)(Cp)NiBr[103]	165	Ni(II) → Ni(I)	DCM	TBAPF₆	SCE	0.22^d
(benzo-I ⁿ Bu)(Cp)NiBr[103]	166	Ni(II) → Ni(I)	DCM	TBAPF₆	SCE	0.24^d

^a Fc = 0.51 V vs Ag/AgCl (DMF/TBAPF₆).[100]

^b Fc = 0.69 V vs NHE (MeCN/TBABF₄).[101]

^c Fc = 0.40 V vs SCE (MeCN/TBAPF₆).[12]

^d Fc = 0.44 V vs SCE (DCM/TBAPF₆).[103]

Table 10 Formal Potentials of the Ni(III)/Ni(II) and Ni(IV)/Ni(III) Transformations for Selected (NHC)Ni Complexes
Complex		Process	Solvent	Electrolyte (0.1 M)	Potential reference	E _1/2 (V vs. Fc/Fc⁺)
(IPr)Ni(S₂C₂Ph₂)(MeCN)[103]	167	Ni(II) → Ni(III)	DCM	TBAPF₆	Fc/Fc⁺	–0.13
(IMe)(Cp)NiI[101]	168	Ni(II) → Ni(III)	MeCN	TBABF₄	NHE	–0.13^a
(IMes)(Cp)NiCl[101]	161	Ni(II) → Ni(III)	MeCN	TBABF₄	NHE	0.03^a
[I(2-oxy-3,5- ^t Bu₂Ph)]Ni(Py)[104]	169	Ni(II) → Ni(III)	DCM	TBAPF₆	Fc/Fc⁺	0.10
(^iPrCNN)Ni(CCPh)[105]	170	Ni(II) → Ni(III)	DCM	TBAPF₆	Fc/Fc⁺	0.21
trans-[(IMes)₂NiF(2,3,5-F-Ph)][97]	171	Ni(II) → Ni(III)	MeCN	TBAPF₆	Fc/Fc⁺	0.40
(^DIPPCCC)NiCl[106]	172	Ni(II) → Ni(III)	DCM	TBAPF₆	Fc/Fc⁺	0.57
[(benzo)I(2-oxy-3,5- ^t Bu₂Ph)]Ni(Py)[105]	173	Ni(II) → Ni(III)	DCM	TBAPF₆	Fc/Fc⁺	0.71
[I(2-oxy-3,5- ^t Bu₂Ph)]Ni(Py)[105]	169	Ni(III) → Ni(IV)	DCM	TBAPF₆	Fc/Fc⁺	0.70
[(benzo)I(2-oxy-3,5- ^t Bu₂Ph)]Ni(Py)[105]	173	Ni(III) → Ni(IV)	DCM	TBAPF₆	Fc/Fc⁺	1.30

^a Fc = 0.69 V vs NHE (MeCN/TBABF₄).[101]

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Abbildungen

Equation 1

Figure 1 (A) Applied potential as a function of time for a generic cyclic voltammetry experiment, with the initial, switching, and end potentials represented (A, D, and G, respectively). (B) Cyclic voltammogram of the reversible oxidation of a 1 mM Fc solution to Fc⁺, at a scan rate of 100 mV/s.