Two-electron (2e − )-transfer reactions for monometallic complexes of first-row transition metals are uncommon because of the tendency of these metals to proceed through sequential one-electron (1e − )-transfer pathways. For this chemistry to be observed, structural changes upon electron transfer are often needed to shift the 1e − redox potentials to a condition of potential inversion where 2e − transfer becomes favorable. Nickel(II) dithiocarbamate complexes take advantage of these conditions to drive 2e − oxidation from Ni II to Ni IV . Here, we have studied the electrochemistry of Ni II (dtc) 2 , where dtc − is N,N-diethyldithiocarbamate in an acetonitrile solvent as a function of the scan rate and added pyridine to gain further insight into the mechanism for its 2e − oxidation to [Ni IV (dtc) 3 ] + . The scan rate dependence revealed evidence for an ECE mechanism in which the chemical step constituted ligand exchange between [Ni III (dtc) 2 ] + and Ni II (dtc) 2 . A pseudo-first-order rate constant for this reaction of 34 s −1 was obtained at 1 mM Ni II (dtc) 2 . The addition of pyridine to the electrolyte solution showed pronounced changes to the cyclic voltammetry (CV) that were consistent with the formation of a pyridine-bound Ni III complex, [Ni III (dtc) 2 (py) 2 ] + , which was stable at high scan rates but decomposed to [Ni IV (dtc) 3 ] + at low scan rates. The observed decomposition rate constant was well modeled with two parallel decay pathways, one through the dipyridine [Ni III (dtc) 2 (py) 2 ] + and another through a monopyridine [Ni III (dtc) 2 py] + . Overall, these data point to a mechanism for oxidation from Ni II (dtc) 2 to [Ni IV (dtc) 3 ] + that proceeds through an undercoordinated [Ni III (dtc) 2 ] + complex, which can be trapped on the time scale of CV experiments using pyridine ligands. These studies provide insight into how we may be able to control 1e − versus 2e − redox chemistry using the coordination environment and nickel oxidation state.
The unique redox cycle of NiII(dtc)2, where dtc– is N,N-diethyldithiocarbamate, in acetonitrile displays 2e– redox chemistry upon oxidation from NiII(dtc)2 → [NiIV(dtc)3]+ but 1e– redox chemistry upon reduction from [NiIV(dtc)3]+ → NiIII(dtc)3 → NiII(dtc)2. The underlying reasons for this cycle lie in the structural changes that occur between four-coordinate NiII(dtc)2 and six-coordinate [NiIV(dtc)3]+. Cyclic voltammetry (CV) experiments show that these 1e– and 2e– pathways can be controlled by the addition of pyridine-based ligands (L) to the electrolyte solution. Specifically, the addition of these ligands resulted in a 1e– ligand-coupled electron transfer (LCET) redox wave, which produced a mixture of pyridine-bound Ni(III) complexes, [NiIII(dtc)2(L)]+, and [NiIII(dtc)2(L)2]+. Although the complexes could not be isolated, electron paramagnetic resonance (EPR) measurements using a chemical oxidant in the presence of 4-methoxypyridine confirmed the formation of trans-[NiIII(dtc)2(L)2]+. Density functional theory calculations were also used to support the formation of pyridine coordinated Ni(III) complexes through structural optimization and calculation of EPR parameters. The reversibility of the LCET process was found to be dependent on both the basicity of the pyridine ligand and the scan rate of the CV experiment. For strongly basic pyridines (e.g., 4-methoxypyridine) and/or fast scan rates, high reversibility was achieved, allowing [NiIII(dtc)2(L) x ]+ to be reduced directly back to NiII(dtc)2 + xL. For weakly basic pyridines (e.g., 3-bromopyridine) and/or slow scan rates, [NiIII(dtc)2(L) x ]+ decayed irreversibly to form [NiIV(dtc)3]+. Detailed kinetics studies using CV reveal that [NiIII(dtc)2(L)]+ and [NiIII(dtc)2(L)2]+ decay by parallel pathways due to a small equilibrium between the two species. The rate constants for ligand dissociation ([NiIII(dtc)2(L)2]+ → [NiIII(dtc)2(L)]+ + L) along with decomposition of [NiIII(dtc)2(L)]+ and [NiIII(dtc)2(L)2]+ species were found to increase with the electron-withdrawing character of the pyridine ligand, indicating pyridine dissociation is likely the rate-limiting step for decomposition of these complexes. These studies establish a general trend for kinetically trapping 1e– intermediates along a 2e– oxidation path.
The unique redox cycle of Ni II (dtc)2, where dtcis N,N-diethyldithiocarbamate, in acetonitrile displays 2eredox chemistry upon oxidation from Ni II (dtc)2 → [Ni IV (dtc)3] + but 1eredox chemistry upon reduction from [Ni IV (dtc)3] + → Ni III (dtc)3 → Ni II (dtc)2. The underlying reasons for this cycle lie in the structural changes that occur between four-coordinate Ni II (dtc)2 and sixcoordinate [Ni IV (dtc)3] + . Cyclic voltammetry (CV) experiments show that these 1eand 2e -pathways can be controlled by the addition of pyridine-based ligands (L) to the electrolyte solution. Specifically, the addition of these ligands resulted in a 1eligand-coupled electron transfer (LCET) redox wave which produced a mixture of pyridine-bound Ni(III) complexes, [Ni III (dtc)2(L)] + and [Ni III (dtc)2(L)2] + . Although the complexes could not be isolated, electron paramagnetic resonance (EPR) measurements using a chemical oxidant in the presence of 4-methoxypyridine confirmed the formation of trans-[Ni III (dtc)2(L)2] + . Density functional theory calculations were also used to support the formation of pyridine coordinated Ni(III) complexes through structural optimization and calculation of EPR parameters. The reversibility of the LCET process was found to be dependent on both the basicity of the pyridine ligand and the scan rate of the CV experiment. For strongly basic pyridines (e.g. 4-methoxypyridine) and/or fast scan rates, high reversibility was achieved, allowing [Ni III (dtc)2(L)x] + to be reduced directly back to Ni II (dtc)2 + xL. For weakly basic pyridines (e.g. 3-bromopyridine) and/or slow scan rates, [Ni III (dtc)2(L)x] + decayed irreversibly to form [Ni IV (dtc)3] + . Detailed kinetics studies using CV reveal that [Ni III (dtc)2(L)] + and [Ni III (dtc)2(L)2] + decay by parallel pathways due to a small equilibrium between the two species. The rate constants for ligand dissociation ([Ni III (dtc)2(L)2] + → [Ni III (dtc)2(L)] + + L) along with decomposition of [Ni III (dtc)2(L)] + and [Ni III (dtc)2(L)2] + species were found to increase with 2 the electron-withdrawing character of the pyridine ligand, indicating pyridine dissociation is likely the rate limiting step for decomposition of these complexes. These studies establish a general trend for kinetically trapping 1eintermediates along a 2eoxidation path.
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