Franklin A. Schultz

Coupled electron-transfer and spin-exchange reactions

Abstract Electron-transfers accompanied by a change in metal atom spin-state occur widely in chemistry and biology. An appropriate framework for interpreting such behavior is an electrochemical scheme of squares consisting of two one-electron electrode reactions and two spin-equilibrium steps. The presence of coupled electron-transfers and spin-exchanges is reflected in the thermodynamic and kinetic parameters of such events. Oxidation–reduction potentials and electron-transfer rate constants depend on the positions of the spin-state equilibria in the two oxidation states. Electrochemical activation parameters exhibit significant contributions from the large enthalpic and entropic differences that characterize spin-exchange reactions. The entropy of an electrochemical half-reaction, Δ S ° rc , coupled to a spin-exchange reflects the increases in vibrational and electronic entropy that accompany low- to high-spin conversion. Electrochemical enthalpies and entropies of activation, Δ H ‡ and Δ S ‡ , are influenced by the temperature dependence of the accompanying spin-state transitions and are useful in mechanism diagnosis. The present review describes examples from the literature and our own laboratory regarding Fe(III/II) (d 5 /d 6 ) and Co(III/II) (d 6 /d 7 ) couples that exhibit the above behavior.

Spin Crossover-Coupled Electron Transfer of [M(tacn)2]3+/2+ Complexes (tacn = 1,4,7-Triazacyclononane; M = Cr, Mn, Fe, Co, Ni)

The role of spin state equilibria on the thermodynamics of electron transfer in [M(tacn)(2)](3+/2+) complexes (tacn = 1,4,7-triazacyclononane; M = Cr, Mn, Fe, Co, Ni) was examined using density functional theory at the B3LYP*/cc-pVTZ(-f) level coupled to a continuum solvation model to afford excellent agreement between computed and experimental redox properties. An intuitive explanation of the previously observed nonperiodic trend in reduction potentials, which display a sawtooth pattern along the first-row transition metal series, is offered utilizing a novel diagrammatic illustration of the relationship between spin state energetics and reduction potentials. This representation leads to a generalized proposal for analyzing and designing nearly isoenergetic spin states of transition metals in a given ligand environment. A new ligand specific parameter alpha that allows for quantifying the differential reduction potential as a function of the metal identity is introduced, and a novel protocol is presented that divides the ligand-metal interactions into primary and secondary characteristics, which we anticipate will be useful for rationally designing the electronics of transition metal complexes in general.

Six-coordinate monooxo molybdenum(VI) complexes with catecholate and salicylaldehyde thiosemicarbazone ligands

Abstract Preparation and electrochemical and spectroscopic properties are reported for two six-coordinate monooxo Mo(VI) complexes, MoO(Mecat)(PhTSCsal) and MoO(DTBcat)(PhTSCsal), where Mecat 2− =4-methylcatecholate, DTBcat 2− =3,5-di- tert -butylcatecholate and PhTSCsal 2− =the dianion of salicylaldehyde 4-phenylthiosemicarbazone. Each complex exhibits reversible, one-electron reduction of the MoO 4+ center at approximately −0.6 V versus Fc +/0 and irreversible, one-electron oxidations of the coordinated catecholate at approximately 0.8 and 1.1 V versus Fc +/0 in 1,2-dichloroethane. Two infrared absorptions are observed in the MoO stretching region for both complexes. These are thought to arise from geometric isomers produced by meridional coordination of tridentate PhTSCsal 2− and bidentate coordination of an unsymmetrical catechol to MoO 4+ .

Understanding Intrinsically Irreversible, Non-Nernstian, Two-Electron Redox Processes: A Combined Experimental and Computational Study of the Electrochemical Activation of Platinum(IV) Antitumor Prodrugs

Six-coordinate Pt(IV)-complexes are prominent prodrug candidates for the treatment of various cancers where, upon two-electron reduction and loss of two axial ligands, they form more familiar, pharmacologically active four-coordinate Pt(II) drugs. A series of electrochemical experiments coupled with extensive density functional calculations has been employed to elucidate the mechanism for the two-electron reduction of Pt(IV)(NH3)2Cl2L2 to Pt(II)(NH3)2Cl2 (L = CH3COO(-), 1; L = CHCl2COO(-), 2; L = Cl(-), 3). A reliable estimate for the normal reduction potential E(o) is derived for the electrochemically irreversible Pt(IV) reduction and is compared directly to the quantum chemically calculated reduction potentials. The process of electron transfer and Pt-L bond cleavage is found to occur in a stepwise fashion, suggesting that a metastable six-coordinate Pt(III) intermediate is formed upon addition of a single electron, and the loss of both axial ligands is associated with the second electron transfer. The quantum chemically calculated reduction potentials are in excellent agreement with experimentally determined values that are notably more positive than peak potentials reported previously for 1-3.

Two-electron redox energetics in ligand-bridged dinuclear molybdenum and tungsten complexes.

Electron-transfer energetics of bridged dinuclear compounds of the form [(CO)(4)M(mu-L)](2)(0/1-/2-) (M = Mo, W; L = PPh(2)(-), SPh(-)) were explored using density functional theory coupled to a continuum solvation model. The experimentally observed redox potential inversion, a situation where the second of two electron transfers is more thermodynamically favorable than the first, was reproduced within this model. This nonclassical energy ordering is a prerequisite for the apparent transfer of two electrons at one potential, as observed in many biologically and technologically important systems. We pinpoint the origin of this phenomenon to be an unusually unfavorable electrostatic repulsion for the first electron transfer due to the redox noninnocent behavior of the bridging ligands. The extent of redox noninnocence is explained in terms of an orbital energy resonance between the metal-carbonyl and bridging ligand fragments, leading to a general mechanism by which potential inversion could be controlled in diamond-core dinuclear systems.

Preparation and electrochemistry of six-coordinate monooxo molybdenum(VI) complexes containing bidentate catecholate and tridentate NOS-donor Schiff base ligands

Abstract The preparation, characterization and electrochemical properties are reported for monooxo Mo(VI) complexes, MoO(cat)(ssp), containing bidentate catecholate (cat 2 = 3,5-di-tert-butylcatecholate, naphthalene-1,2-diolate, phenanthrene-9,10-diolate) and tridentate NOS-donor Schiff base (ssp 2 = N -salicylidene-2-aminobenzenethiolate) ligands. The intensely colored compounds are formed by oxo abstraction from MoO 2 (ssp) with EtPh 2 P in THF followed by oxidative addition of the appropriate quinone. Oxo abstraction leads to a mixture of MoO(ssp) and Mo 2 O 3 (ssp) 2 rather than either of these single products under the experimental conditions. The six-coordinate MoO 4+ species exhibit reversible Mo(VI/V) electrochemistry at a potential ∼0.1 V more positive than that for analogous complexes with the NOO′-donor Schiff base ligand N -salicylidene-2-aminophenolate (sap 2− ).

Heterogeneous electron transfer at electrodes coated with electronically conducting nickel-tetraaminophthalocyanine polymer films

The kinetics of heterogeneous electron transfer across films of electronically conducting nickel-4,4′,4″,4‴-tetraaminophthalocyanine polymer (poly(NiTAPc)) are reported. Poly(NiTAPc), which is formed by oxidative electropolymerization of NiTAPc, acts as an n-doped electronic conductor between about 0.8 and −2.0 V vs. Ag/AgCl. Within this range it sustains diffusion-limited charge transfer to one-electron bulk solution reactants at their anticipated formal potentials. However, the rates of heterogeneous electron transfer to these species are diminished by a small, uniform amount that is exponentially dependent on film thickness. Results are interpreted in terms of a porous electrode model in which electron transfer occurs at the polymer—solution interface, a large bulk capacitance arises from an interior pore volume that is inaccessible to diffusing reactants and a resistive element (suggested to be pores of electrolyte trapped between aggregrates of the polymer) is present which acts to reduce apparent values of ks,h. It is demonstrated that electronically conducting polymer films do not accelerate the rate of electron transfer to solution reactants but rather restore kinetics to their anticipated values by preventing suface involvement.

Electrochemical reduction of a binuclear dioxo-bridged molybdenum(V) complex with cysteine

Summary Electrochemical reduction of the binuclear dioxo-bridged Mo(V)-cysteine complex, Mo 2 O 4 (cys) 2 2− , has been studied at mercury electrodes in pH 7.5–10 borate, phosphate, and NH 4 + /NH 3 buffers. The complex is reduced in a single diffusion-controlled 4-electron step to a binuclear Mo(III) product at ∼−1.2 V vs . SCE. The product is unstable and dissociates by cleavage of the oxygen bridge bonds. The rate of the decomposition reaction was measured by double potential step chronocoulometry and found to depend on the nature of the buffering medium, but not on pH or buffer concentration. A mechanism consistent with these experimental facts is proposed which involves coordination of the buffer species to the Mo(III) product during electrochemical reduction followed by intra-molecular dissociation of the complex.

Influence of solvent dynamics on the heterogeneous kinetics of a reaction with a large inner-shell barrier: Chloro(tetraphenylporphinato)manganese(III) reduction

Solvent dynamics play an important role in determining the rates of electron transfer reactions whose activation barriers are dominated by the outer-shell reorganization energy [1-3]. Few redox couples with a significant inner-shell barrier have been studied in this regard, but theory predicts that the influence of solvent dynamics will be muted under these circumstances [4-6]. The relationship can be expressed by incorporating a term involving rL, the longitudinal dielectric relaxation time of the solvent, in the pre-exponential factor of the Marcus equation for heterogeneous electron transfer:

Correlation of heterogeneous electron transfer rates with structural change in cobalt tetraaza macrocyclic complexes

Abstract Heterogeneous electron transfer rates are reported for reduction of four [Co[N 4 ](H 2 O) 2 ] 3+ complexes at Pt electrodes in perchloric acid solution and correlated with changes in molecular structure. N 4 is one of the four tetraaza ligands 1,4,8,11-tetraazacyclotetradecane (cyclam); 2,3,9,10-tetramethyl-1,4,8,11-tetraazacyclotetradeca-1,3,8,10-tetraene (TIM); 5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-4,11-diene (Me 6 N 4 ) and 12,14-dimethyl-1,4,8,11-tetraazacyclotetradeca-1,11-diene-13-one (Me 2 N 4 -one). The structural change accompanying reduction of Co(III) to Co(II) is lengthening of the axial Co-OH 2 bonds, which occurs to an extent (30 to 50 ppm) that is dependent on the nature of the N 4 ligand. The rate of electron transfer decreases (TIM > Me 4 N 4 -one > cyclam > Me 6 N 4 ) as the extent of this displacement increases in accordance with the Marcus theory. The double layer corrected rate constants ( k corr s = 8 × 10 −4 to 2.9 × 10 −5 cm s −1 ) are in good agreement with “absolute” values calculated from the Marcus theory and also with values derived from homogeneous solution self-exchange rates. The generally ideal behavior of the [Co[N 4 ](H 2 O) 2 ] 3+/2+ couple is contrasted with other systems which experience large changes in nuclear coordinates during electron transfer but do not follow the Marcus theory as closely.