We present a simple approach for the calculation of accurate pKa values in water and acetonitrile based on the straightforward calculation of the gas-phase absolute free energies of the acid and conjugate base with use of only a continuum solvation model to obtain the corresponding solution-phase free energies. Most of the error in such an approach arises from inaccurate differential solvation free energies of the acid and conjugate base which is removed in our approach using a correction based on the realization that the gas-phase acidities have only a small systematic error relative to the dominant systematic error in the differential solvation. The methodology is outlined in the context of the calculation of a set of neutral acids with water as the solvent for a reasonably accurate electronic structure level of theory (DFT), basis set, and implicit solvation model. It is then applied to the comparison of results for three different hybrid density functionals to illustrate the insensitivity to the functional. Finally, the approach is applied to the comparison of results for sets of neutral acids and protonated amine cationic acids in both aqueous (water) and nonaqueous (acetonitrile) solvents. The methodology is shown to generally predict the pKa values for all the cases investigated to within 1 pH unit so long as the differential solvation error is larger than the systematic error in the gas-phase acidity calculations. Such an approach is rather general and does not have additional complications that would arise in a cluster-continuum method, thus giving it strength as a simple high-throughput means to calculate absolute pKa values. This article is part of a Special Issue entitled: Metals in Bioenergetics and Biomimetics Systems.

Reported here is the N 2 cleavage of ao ne-electron oxidation reaction using trans-[Mo(depe) 2 (N 2 ) 2 ]( 1)( depe = Et 2 PCH 2 CH 2 PEt 2 ), which is ac lassical molybdenum(0)-dinitrogen complex supported by two bidentate phosphine ligands. The molybdenum(IV) terminal nitride complex [Mo-(depe) 2 N][BArf 4 ]( 2)( BArf 4 = B(3,5-(CF 3 ) 2 C 6 H 3 ) 4 )i ss ynthesized by the one-electron oxidation of 1 upon addition of amild oxidant, [Cp 2 Fe][BArf 4 ](Cp = C 5 H 5 ), and proceeds by N 2 cleavage from aM o II -N=N-Mo II structure.I naddition, the electrochemical oxidation reaction for 1 also cleaved the N 2 ligand to give 2.The dimeric Mo complex with abridging N 2 is detected by in situ resonance Raman and in situ UV-vis spectroscopies during the electrochemical oxidation reaction for 1.Density-functional theory (DFT) calculations reveal that the unstable monomeric oxidized Mo I species is converted into 2 via the dimeric structure involving az igzag transition state.

To understand the role of the unique equatorial coordination environment at the active center in nickel superoxide dismutase (NiSOD), we prepared a novel Ni(II) complex with an amino-carboxamido-dithiolato-type square-planar ligand (1, [Ni(2+)(L1)](-)) as a model of the NiSOD active site. Complex 1 has a low-spin square-planar structure in all solvents. Interestingly, the absorption wavelength and ν(C═O) stretching vibrations of 1 are affected by solvents. This provides an indication that the carbonyl oxygens participate in hydrogen-bonding interactions with solvents. These interactions are reflected in the redox potentials; the peak potential of an anodic wave (Epa) values of Ni(II)/Ni(III) waves for 1 are shifted to a positive region for solvents with higher acceptor numbers. This indicates that the disproportionation of superoxide anion by NiSOD may be regulated by hydrogen-bonding interactions between the carboxamido carbonyl and electrophilic molecules through fine-tuning of the redox potential for optimal SOD activity. Interestingly, the Epa value of the Ni(III)/Ni(II) couple in 1 in water (+0.303 V vs normal hydrogen electrode (NHE)) is similar to that of NiSOD (+0.290 V vs NHE). We also investigated the superoxide-reducing and -oxidizing reactions of 1. First, 1 reacts with superoxide to yield the superoxide-bound Ni(II) species (UV-vis: 425, 525, and ∼650 nm; electron paramagnetic resonance (EPR) (4 K): g// = 2.21, g⊥ = 2.01; resonance Raman: ν((16)O-(16)O)/ν((18)O-(18)O) = 1020/986 cm(-1)), which is then oxidized to Ni(III) state only in the presence of both a proton and 1-methylimidazole, as evidenced by EPR spectra. Second, EPR spectra indicate that the oxidized complex of 1 with 1-methylimidazole at the axial site can be reduced by reaction with superoxide. The Ni(III) complex with 1-methylimidazole at the axial site does not participate in any direct interaction with azide anion (pKa 4.65) added as mimic of superoxide (pKa 4.88). According to these data, we propose the superoxide disproportionation mechanism in superoxide-reducing and -oxidizing steps of NiSOD in both Ni(II) and Ni(III) states.

Incorporation of the tridentate phosphine-enamidoiminophosphorane onto cobalt(II) produces tetrahedral Co(NpNP iPr )Cl, 1, which upon reduction under dinitrogen generates the T-shaped, paramagnetic Co(I) complex Co(NpNP iPr ), 2. This paramagnetic Tshaped derivative is in equilibrium with the paramagnetic dinitrogen derivative Co(NpNP iPr )-(N 2 ), 3, which can be detected by IR and low-temperature UV−vis spectroscopy. Both 1 and 2 act as homogeneous catalysts for the conversion of molecular nitrogen into tris(trimethylsilyl)amine (N(SiMe 3 ) 3 ) (∼200 equiv, quantified as NH 4 Cl after hydrolysis) in the presence of excess KC 8 and Me 3 SiCl at low temperatures.

The µ-η 2 :η 2 -peroxodicopper(II) (Cu 2 (µ-η 2 :η 2 -O 2 )) and bis(µoxo)dicopper(III) (Cu 2 (µ-O) 2 ) species have been investigated in the model studies of type III copper proteins, and these structures are also considered as important motifs for O 2 -activating metalloproteins in biological systems. 1,2 The interconversion between these species in solution was found to depend on organic solvents and counteranions. 2,3 Recent studies of the interconversion equilibrium in copper-aliphatic diamine complex systems strongly suggested that coordination of counteranions promotes the conversion of Cu 2 (µ-O) 2 to Cu 2 (µ-η 2 :η 2 -O 2 ) species. 2b,3b,c However, direct evidence for formation of the anion-coordinating structure is yet to be presented. Here, we report the formation and crystal structure of a new Cu 2 (µ-η 2 :η 2 -O 2 ) complex with a bridging carboxylate ligand. 4a Structural studies and density functional theory (DFT) calculations suggested factors regulating stepwise-activation of dioxygen by bridging-carboxylate ligation to the dicopper core.