Controlling One-Electron vs Two-Electron Pathways in the Multi-Electron Redox Cycle of Nickel Diethyldithiocarbamate

Abstract

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.

Zinc-Catalyzed Two-Electron Nickel(IV/II) Redox Couple for Multi-Electron Storage in Redox Flow Batteries

“…Ni II (dtc) 2 was synthesized by adding two equivalents of sodium diethyldithiocarbamate trihydrate (Na­(dtc)·3H 2 O, Sigma-Aldrich, >99%) to one equivalent of nickel­(II) chloride hexahydrate (Alfa Aesar, 98%) in DI water as described previously. − A light-green solid precipitated instantly and was filtered under vacuum and washed with cold distilled water, ethanol, and ether to obtain 96% yield. Characterization of the light-green solid was performed by 1 H NMR (acetonitrile- d 3 , CD 3 CN, Cambridge Isotopes): δ 3.60 (q, −CH 2 −), 1.21 (t, −CH 3 ), UV–visible absorbance spectroscopy: λ max (ε) = 388 nm (5,600 M –1 cm –1 ) and 323 nm (26,700 M –1 cm –1 ), and X-ray crystallography.…”

“…Figure shows a thermochemical cycle, or square scheme, to describe the electron transfer (horizontal) and ligand transfer (vertical) reactions for the [Ni IV (dtc) 3 ] + /Ni II (dtc) 2 redox couple. The oxidation reaction, first reported in the 1970’s, , proceeds through an RSD/ECE mechanism described by eqs – and the green dashed line shown in Figure . , RSD refers to radical substrate dimerization, a term derived from 2e – electrochemistry with organic molecules, ,, which indicates that the chemical step involves reaction with a second equivalent of the substrate molecule (i.e., Ni II (dtc) 2 ). Following oxidation of Ni II (dtc) 2 to [Ni III (dtc) 2 ] + (eq ), the chemical step involves ligand transfer from Ni II (dtc) 2 to [Ni III (dtc) 2 ] + (eq ) to produce the tris-chelated Ni III (dtc) 3 , whereby it can be oxidized a second time (eq ) or disproportionate (eq ) to result in [Ni IV (dtc) 3 ] + .…”

“…Following oxidation of Ni II (dtc) 2 to [Ni III (dtc) 2 ] + (eq ), the chemical step involves ligand transfer from Ni II (dtc) 2 to [Ni III (dtc) 2 ] + (eq ) to produce the tris-chelated Ni III (dtc) 3 , whereby it can be oxidized a second time (eq ) or disproportionate (eq ) to result in [Ni IV (dtc) 3 ] + . It should be noted that [Ni III (dtc) 2 ] + is expected to be solvent-coordinated (omitted here for clarity) based on experimental studies and theoretical calculations which suggest a five-coordinate [Ni III (dtc) 2 (MeCN)] + complex. , On the basis of the thermodynamic values for eqs –, a reduction potential for eq can be calculated to be E °(2e – ) = 0.02 V vs Fc +/0 . Despite 2e – transfer being observed in the oxidation direction, sequential 1e – transfer reactions are observed for reduction of [Ni IV (dtc) 3 ] + back to Ni II (dtc) 2 in MeCN (green dashed line in Figure ) due to the mild stability of Ni III (dtc) 3 . The ligand transfer reaction shown in eq is therefore critical to 2e – chemistry as it allows rapid formation of Ni III (dtc) 3 in the oxidation direction.…”

“…The ligand transfer reaction shown in eq is therefore critical to 2e – chemistry as it allows rapid formation of Ni III (dtc) 3 in the oxidation direction. Previous work has shown that strongly coordinating ligands such as pyridine can slow this reaction considerably by coordinating to the [Ni III (dtc) 2 ] + intermediate, resulting in only 1e – oxidation during cyclic voltammetry (CV) experiments. , The fact that 2e – reduction of [Ni IV (dtc) 3 ] + is not observed in MeCN suggests that the reverse reaction in eq is kinetically slow. Therefore, efficient 2e – reduction requires rapid conversion from Ni III (dtc) 3 to [Ni III (dtc) 2 ] + .…”

Two-Electron Redox Tuning of Cyclopentadienyl Cobalt Complexes Enabled by the Phenylenediamide Ligand

“…In fact, only a handful of mononuclear first-row metal complexes with redox-active ligands exhibit a 2e − redox couple, often accompanied by ligand coordination or dissociation that promotes potential inversion. [16][17][18][19][20][21][22][23][24][25][26] Among these reports, 2e − reductions are more common; to our knowledge, only one reported system has shown a reversible 2e − oxidation. 18 A classic redox-active ligand is o-phenylenediamide (opda), which has three readily accessible oxidation states when bound to a metal center: the dianionic opda, the monoanionic semibenzoquinonediimine (s-bqdi), and the neutral benzoquinonediimine (bqdi) (Scheme 1a).…”