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Multielectron transfer reactions, which are important in many biological and technological contexts, are paradoxical, because transfer of a second unit of charge should be energetically more difficult than the first. Straightforward changes in structure or composition often are offered as an explanation for the redox potential inversion that leads to multielectron behavior. We have explored the basis of this phenomenon by carrying out density functional theory studies of two-electron redox systems that have been carefully characterized by electrochemical methods. Principal findings include the fact that the energy of vertical electron attachment predominates that of structural or compositional change and thus controls the energetics of a given redox event. In addition, decisive energy changes in electron attachment processes often occur in lower-lying rather than frontier orbitals. Thus, simple tracking of LUMO/SOMO/HOMO energy changes (Walsh’s rule) is sometimes inadequate for complete understanding of redox-triggered events. These concepts are illustrated in our combined experimental/computational examination of the following two-electron redox systems: (i) reduction of binuclear, ligand-bridged complexes of Mo and W accompanied by metal−metal bond cleavage and structural rearrangement, (ii) reduction of bis(hexamethylbenzene) complexes of Fe, Ru, and Os accompanied by a change in ligand hapticity, and (iii) reduction of Pt(IV) antitumor prodrugs, whose reductive elimination of axial ligands generates the active Pt(II) form of the drug.

Reductive activation of six-coordinate Pt(IV) complexes to afford square-planar Pt(II) complexes has exhibited surprisingly divergent and unpredictable cathodic peak potentials during cyclic voltammetry (CV) measurements under widely employed experimental conditions. A systematic, detailed investigation reveals that glassy carbon (GC) electrodes are responsible for this erratic behavior. More reproducible CVs are obtained with platinum metal electrodes, which display cathodic responses at much more positive potentials. The unreliable and negatively shifted peak potentials observed at GC are attributed to a non-uniform oxide layer that is formed on the electrode surface causing slow electron transfer. A simple procedure of repetitive scanning to reducing potentials is found to be effective for cleaning and activating the GC surface, such that it exhibits the more consistent and accurate peak potential responses seen with a Pt electrode.

Amino terminal peptides of the general form Gly-Gly-His have been used to introduce single sites of metal binding and redox activity into a wide range of biomolecules to create bioactive compounds and conjugates capable of substrate oxidation. We report here that Gly-Gly-His-like peptides linked in a tandem fashion can also be generated leading to multi-metal binding arrays. While metal binding by the native Gly-Gly-His motif (typically to Cu2+, Ni2+, or Co2+) requires a terminal peptide amine ligand, previous work has demonstrated that an ornithine (Orn) residue can be substituted for the terminal Gly residue to allow solid-phase peptide synthesis to continue via the side chain N-δ. This strategy thus frees the Orn residue N-α for metal binding and permits placement of a Gly-Gly-His-like metal binding domain at any location within a linear, synthetic peptide chain. As we show here, this strategy also permits the assembly of tandem arrays of metal binding units in linear peptides of the form: NH2-Gly-Gly-His-[(δ)-Orn-Gly-His]n-(δ)-Orn-Gly-His-CONH2 (where n = 0, 1, and 2). Metal binding titrations of these tandem arrays monitored by UV-vis and ESI-MS indicated that they bind Cu2+, Ni2+, or Co2+ at each available metal binding site. Further, it was found that these systems retained their ability to modify DNA oxidatively and to an extent greater than their parent M(II)•Gly-Gly-His. These findings suggest that the tandem array metallopeptides described here may function with increased efficiency as "next generation" appendages in the design of bioactive compounds and conjugates.

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.

Relationships between structure and electron transfer reactivity underlie many important electrochemical applications and provide fundamental insight to chemical and biological processes. The vast array of experimental techniques developed during the latter half of the twentieth century helped greatly to foster progress in this area, and the advent of powerful computational techniques such as density functional theory promises even more far-reaching developments. It is evident that molecular composition, geometric and electronic structure, and changes in these features influence the thermodynamics and kinetics of transition metal electron transfer reactions in predictable and understandable ways. Several examples drawn from the author’s research program to illustrate this premise include the influence of sulfur versus oxygen donation on molybdenum-centered electron transfer, reactions in which a change in metal atom spin state accompanies electron transfer, and concomitant multi-electron transfer and metal–metal bond cleavage in binuclear, ligand-bridged complexes.

The manganese(III)-bis[poly(pyrazolyl)borate] complexes, Mn(pzb)2SbF6, where pzb- = tetrakis(pyrazolyl)borate (pzTp) (1), hydrotris(pyrazolyl)borate (Tp) (2), or hydrotris(3,5-dimethylpyrazolyl)borate (Tp*) (3), have been synthesized by oxidation of the corresponding Mn(pzb)2 compounds with NOSbF6. The Mn(III) complexes are low-spin in solution and the solid state (μeff = 2.9−3.8 μB). X-ray crystallography confirms their uncommon low-spin character. The close conformity of mean Mn−N distances of 1.974(4), 1.984(5), and 1.996(4) A in 1, 2, and 3, respectively, indicates absence of the characteristic Jahn−Teller distortion of a high-spin d4 center. N−Mn−N bite angles of slightly less than 90o within the facially coordinated pzb- ligands produce a small trigonal distortion and effective D3d symmetry in 1 and 2. These angles increase to 90.0(4)o in 3, yielding an almost perfectly octahedral disposition of N donors in Mn(Tp*)2+. Examination of structural data from 23 metal−bis(pzb) complexes reveals systematic...

Temperature-dependent measurements of potential, E degrees', and electron-transfer rate constant, k(s,h), are reported for electrochemical reduction (in 0.3 M TBAPF(6)/CH(3)CN) of a series of oxomolybdenum(V) complexes, [(Tp)MoO(X,Y)], where Tp = hydrotris(3,5-dimethyl-1-pyrazolyl)borate and X,Y is a series of bidentate 1,2-disubstituted aliphatic or aromatic ligands in which oxygen donors are replaced sequentially by sulfur. E degrees' values shift in the positive direction, and k(s,h) values increase as O is replaced by S and as the framework of the ligand is changed from aliphatic to aromatic. The electrochemical enthalpy of activation, measured under conditions of zero driving force as DeltaH= -R partial differential[ln(k(s,h))]/ partial differential(1/T) and corrected for an outer-shell component by the mean spherical approximation, is approximately 10 kJ mol(-1) larger for complexes with O versus S donors and with an aliphatic versus aromatic ligand framework. Thus, the rate of Mo(V/IV) electron transfer is modulated primarily by differences in inner-shell reorganization. Following a recent description of electronic structure contributions to electron-transfer reactivity (Kennepohl, P.; Solomon, E. I. Inorg. Chem. 2003, 42, 679 ff), it is concluded that more effective charge distribution over the entire molecular structure, as mediated by electronic relaxation in S versus O and aromatic versus aliphatic systems, is responsible for the influence of ligand structure on the kinetics and thermodynamics of Mo-centered electron transfer. There is no evidence, based on experimentally measured pre-exponential factors, that sulfur donors or an aromatic ligand framework are more effective than their structural counterparts in facilitating electronic coupling between the electrode and the Mo d(xy) redox orbital.

Coupled electron-transfer and spin-exchange reactions of metal(II)–bis[tris(pyrazolyl)methane] complexes, [M(tpm)2]2+, with M=Mn, Fe, Co, or Ni and tpm=HC(pz)3 or HC(3,5-Me2pz)3 are reported. Apparent heterogeneous electron-transfer rate constants, (ks,h)app, for [M(tpm)2]2+ to [M(tpm)2]3+ oxidations are determined from the scan rate dependence of cyclic voltammetric peak potential separations, ΔEp. Consistent with the expectation that electron-transfer reactions that are accompanied by a change in spin-state are slower than those that are not, (ks,h)app for oxidation of high-spin (HS) {Fe[HC(3,5-Me2pz)3]2}2+ to low-spin (LS) {Fe[HC(3,5-Me2pz)3]2}3+ is 10 times smaller than the value for oxidation of predominantly LS {Fe[HC(pz)3]2}2+ to LS {Fe[HC(pz)3]2}3+. In addition, very small values of (ks,h)app are observed for the one-electron oxidations of HS {Co[HC(pz)3]2}2+ and HS {Mn[HC(3,5-Me2pz)3]2}2+. The electrochemical, magnetic, and spectroscopic properties of [M(tpm)2]2+ complexes are similar to those of the corresponding tris(pyrazolyl)borate (pzb−) complexes with the exception that, because of net charge considerations, the M(III/II) potential is ∼1 V more positive for [M(tpm)2]3+/2+ than for [M(pzb)2]+/0 couples. The stability of [M(tpm)2]2+ complexes in CH3CN and DMF solution is investigated by positive ion electrospray ionization mass spectrometry. It is demonstrated that {Fe[HC(pz)3]2}2+ undergoes ligand dissociation in DMF.

Complexes of molybdenum(VI) with the catecholamide siderophore ligands aminochelin, azotochelin and protochelin undergo electrochemical reduction by sequential Mo(VI)→Mo(V) and Mo(V)→Mo(III) electron transfers in pH 5.40–6.74 pyridine (py) buffer in a manner closely analogous to the cis -dioxo Mo(VI)–catechol complex, MoO 2 (cat) 2 2− . The pH dependence of the electrode half-reaction potentials for Mo–azotochelin and Mo–aminochelin reveals that between one and two protons are transferred in conjunction with Mo(VI) to Mo(V) reduction and between two and three protons in conjunction with Mo(V) to Mo(III) reduction. It appears that the acidity of the protons on the coordinated H 2 O in the Mo V O(OH 2 )(L) 2 − product is enhanced by the amide substituents on the siderophore ligands. Electrode potentials shift in the positive direction with increasing py concentration, consistent with coordination of one py to Mo(V) and two py to Mo(III). The Mo(VI)/Mo(V) and Mo(V)/Mo(III) potentials of Mo–siderophore complexes fall in the ranges of −0.45–−0.55 and −0.75–−0.8 V versus Ag/AgCl (−0.2–−0.3 and −0.5–−0.55 V vs. NHE), respectively, in pH 6.74 py buffer.

Electrochemical, magnetic, and spectroscopic properties are reported for homoleptic divalent (M = Mn, Fe, Co, Ni, Ru) and trivalent (M = Cr, Mn, Fe, Co) metal-bis[poly(pyrazolyl)borate] complexes, [M(pzb)(2)](+/0), where pzb(-) = hydrotris(pyrazolyl)borate (Tp), hydrotris(3,5-dimethylpyrazolyl)borate (Tp), or tetrakis(pyrazolyl)borate (pzTp). Ligand field strengths in metal-pzb complexes increase as TpTppzTp, which reflects the importance of steric rather than electronic effects on spectroscopic properties. However, metal-centered redox potentials become more negative as pzTpTpTp, which follows the electron-donating ability of the ligands. Co(III)/Co(II) and Mn(III)/Mn(II) electrode reactions are accompanied by a change in metal atom spin-state; i.e., (S = 0) [Co(pzb)(2)](+) + e(-)==(S = 3/2) [Co(pzb)(2)] and (S = 1) [Mn(pzb)(2)](+) + e(-)==(S = 5/2) [Mn(pzb)(2)]. Apparent heterogeneous electron-transfer rate constants derived from sweep-rate dependent cyclic voltammetric peak potential separations in 1,2-dichloroethane are small and decrease as pzTpTpTp for the Co(III)/Co(II) couples. Slow electron transfer is characteristic of coupled electron transfer and spin exchange. [M(Tp)(2)](+/0) redox potentials relative to values for other homoleptic MN(6)(3+/2+) couples change as M varies from Cr to Ni. For early members of the series, [M(Tp)(2)](+/0) potentials nearly equal those of complexes with aliphatic N-donor ligands (e.g., triazacyclononane, sarcophagine). However, [M(Tp)(2)](+/0) potentials approach those of [M(bpy)(3)](3+/2+) for later members of the series. The variation suggests a change in the nature of the metal-pzb interaction upon crossing the first transition row.

The kinetics and thermodynamics of concerted two-electron transfer and metal−metal bond cleavage in the binuclear phosphido-bridged complexes [M2(μ-PPh2)2(CO)8]0/2- [M = Mo (10/2-), W(20/2-)] have been determined by variable scan-rate cyclic voltammetry in 0.3 M TBAPF6/acetone. The reductions of 1° and 2° are accompanied by an increase of 1.08 A in M−M distance and expansion and contraction by 29° of the M−P−M and P−M−P angles, respectively, within an intact M2(μ-PPh2)2 unit. The one-electron electrode potentials of these systems are highly inverted: ΔE°‘ = − = +0.17 V for 10/2- and +0.18 V for 20/2-, and the rate constant of the second heterogeneous electron-transfer reaction is smaller than the first: ksh,2/ksh,1 = 0.1 for Mo and 0.018 for W. Results are consistent with progressive cleavage of the metal−metal bond in two one-electron steps, of which the second is rate-limiting, because it is accompanied by a larger part of the structural change. EHMO calculations reveal that the redox-active orbital i...

Expressions are derived for rate constants and activation parameters of electrochemical reactions that are accompanied by a change in spin-state. Predictions based on an electrochemical scheme of squares are compared with experimental results for [M(tacn)2]3+/2+ (M = Fe, Co, Ni, Ru; tacn = 1,4,7-triazacyclononane) and [Fe(pzb)2]+/0 (pzb- = hydrotris(pyrazol-1-yl)borate [HB(pz)3-], tetrakis(pyrazol-1-yl)borate [B(pz)4-] or hydrotris(3,5-dimethylpyrazol-1-yl)borate [HB(Me2pz)3-]) redox couples. The electrochemical rate constants, (ks,h)obs, of [Co(tacn)2]3+/2+ and [Fe{HB(Me2pz)3}2]+/0 are decreased by the presence of spin-exchange. Electrochemical enthalpies (ΔH‡obs) and entropies (ΔS‡obs) of activation, determined from the temperature dependence of (ks,h)obs under nonisothermal cell conditions, provide evidence regarding the mechanism of these reactions. [Co(tacn)2]3+/2+, [Fe(tacn)2]3+/2+, and [Fe{HB(Me2pz)3}2]+/0 exhibit values of ΔH‡obs and ΔS‡obs that are larger than anticipated for simple electron-tran...

Preparation, spectroscopic characterization and electrochemical behavior are reported for the six-coordinate monooxo Mo(VI) complexes, MoOL(cat), where L2−=2,2′-thiobis(4-methyl-6-tert-butylphenolate) or 2,2′-thiobis(4,6-di-tert-butylphenolate) and cat2−=catecholate (Cat2−), 4-methylcatecholate (Mecat2−), 4-nitrocatecholate (NO2cat2−), or 3,5-di-tert-butylcatecholate (DTBcat2−). The compounds are prepared by reaction of catechol with Mo2O3L2, the μ-oxo Mo(V) dimer produced by reaction of MoO2(acac)2 with H2L. MoOL(cat) complexes exhibit a MoO stretching vibration (νMoO) at 949–961 cm−1 and a reversible Mo(VI/V) reduction at −0.3 to −0.6 V versus Fc+/0. These features are characteristic of a mononuclear MoO4+ center. The kinetics of the metal-centered electrode reaction is sensitive to the catechol substituent. Under identical experimental conditions, the apparent electron-transfer rate indicated by the voltammetric peak potential separation (ΔEp) increases in the following sequence: DTBcat2−

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+ .

Background The purpose of the study described here was to determine the in vivo degradation rate of 10 percent carbamide peroxide, or CP, gel in bleaching trays. The degradation rate indicates the remaining concentration of the active agent on the facial surfaces at various intervals. Methods The researchers fabricated bleaching trays with 0.5-millimeter reservoirs and loaded them with a 10 percent CP whitening gel. The tray was seated in place in 15 patients for six different intervals that ranged from 15 seconds to 10 hours. When the tray was removed, three samples were collected from each patient: the gel remaining in the tray; the adherent gel scraped from the teeth; and a “grab” sample from the reservoir of tooth no. 8. The researchers analyzed these samples for CP according to the method specified by the U.S. Pharmacopeial Convention. Results The percentage of CP recovered decreased as the intervals increased: 87 percent at 15 seconds, 10 percent at 10 hours. Log of tray, teeth and grab samples, respectively, at 15 seconds were 0.94, 0.98 and 0.96 and at 10 hours were −0.13, −0.38 and 0.11. The first-hour degradation rate for tray, teeth and grab samples, respectively, was 2.0 times, 3.6 times and one time the rate during the next nine hours. The within-subject repeatability of the samples was excellent. Conclusion The degradation rate of CP during the bleaching process is biexponential. In the tray and teeth samples, the degradation rate was accelerated during the first hour. Further research is needed to determine the cause of this acceleration. Clinical Implications The active agent in CP bleaching gel is available in bleaching trays for more than 10 hours. After two hours, more than 50 percent of the active agent is available, and 10 percent is available after 10 hours.

Electrode half-reaction entropies, ΔS°rc, are measured as a function of solvent and electrolyte type and concentration for four M(tacn)23+/2+ (M = Fe, Ni, Co, Ru; tacn = 1,4,7-triazacyclononane) redox couples that experience different amounts of structural change in conjunction with electron transfer. Metal dependent values of ΔS°rc are observed for these couples and are shown to arise primarily from vibrational and electronic contributions to intramolecular entropy. Vibrational terms become important when frequencies are small and change significantly with a change in oxidation state, as occurs when an increase in number of antibonding electrons weakens metal−ligand bonds upon reduction. Entropy measurements are referenced to the Ru3+/2+ couple, which is characterized by small inner-shell reorganization. Experimentally, mean values of Δ(ΔS°rc)M-Ru = 27 (7), 30 (6), and 69 (14) J mol-1 K-1 obtained from data in seven solvents are observed for M = Fe, Ni, and Co, respectively. The computed sums of vibratio...

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− ).

Organic and biological electrochemistry is concerned with the application of electrochemical techniques to the study and utilization of organic and biological systems.

The aqueous electrochemical behavior of 10 Cr(III) complexes with potentially tri- and hexadentate amino carboxylate ligands is reported and is shown to depend on the composition and spatial arrangement of the donor atom set. Complexes with two amine and four carboxylate donors (N(2)O(4)) and two amine, one aquo, and three carboxylate donors (N(2)O(3)O') in which the N atoms are coordinated cis to one another undergo chemically and electrochemically reversible reduction at ca. -1.4 and ca. -1.2 V vs SCE, respectively. However, complexes with a trans-N(2)O(4) donor atom set, as exemplified by Cr(MIDA)(2)(-) (MIDA(2)(-) = N-methyliminodiacetate), undergo quasi-reversible Cr(III/II) reduction at ca. -1.4 V that is followed by a sequence of reactions which establishes an electrochemical square scheme. The chemical reactions in the scheme involve displacement of a bound carboxylate group following reduction to Cr(II) and its reattachment after reoxidation to Cr(III). This mechanistic sequence is analyzed by digital simulation, and values of formal potentials, transfer coefficients, and chemical and electrochemical rate constants are reported for Cr(MIDA)(2)(-) and its N-ethyl homolog. The difference in electrochemical behavior between cis- and trans-N(2)O(4) complexes is attributed to differences in the Jahn-Teller distortions experienced by these structures upon reduction to Cr(II). It is proposed that simultaneous N-Cr-N bond elongation, which is possible only for trans species, leads to greater strain in the facially coordinated N-alkyliminodiacetate ligand and thus increases the barrier to electron transfer and facilitates Cr-carboxylate bond cleavage after reduction.

The polyoxometalate ions PMo12O403−, PW12O403−, and SiW12O404−are incorporated in polymeric ruthenium(II)(vinyl)bipyridine (poly-Ru(vbpy)32+) films from aqueous and dioxane–water electrolytes. Despite their large mass the ions exist as freely diffusing species that compensate for up to 30% of the charge in poly-Ru(vbpy)32+. An investigation of the effect of environmental conditions on electrochemical behavior reveals that the first two one-electron reduction waves of SiW12O404−coalesce into a single two-electron reaction and those of PW12O403−shift significantly in potential upon a change from pure aqueous to 50(v/v)% dioxane/water solvent. The observation is attributed to destabilization of the one-electron reaction products as the solvent is enriched is dioxane. Incorporation of polyoxometalates in protonated poly(vinyl)pyridine and poly-Ru(vbpy)32+films from dioxane–water solvent results in differences in electrochemical behavior. Polyoxometalate anions incorporated in poly-Ru(vbpy)32+films catalyze the electrochemical reduction of hydrogen ion. Keywords: polyoxometalate, electrochemistry, poly-Ru(vbpy)32+, electrocatalysis, immobilization.

Heterogeneous electron transfer rate constants (ks,h) and formal potentials (Eo′) are measured for the one-electron reduction of a series of oxomolybdenum(V) complexes, HB(Me2pz)3MoO(LL), where HB(Me2pz)3−=hydrotis(3,5-dimethyl-l-pyrazolyl)borate and LL2− is a bidentate 1,2-disubstituted ethane or benzene ligand whose donor atoms can be either O or S. Eo′ shifts in the negative direction by about 1 V and ks,h decreases by about 1.5 orders of magnitude as S is replaced by O in the inner coordination sphere of these compounds. These observations are rationalized in terms of a hard acid-hard base interaction between the HB(Me2pz)3MoO2+ fragment and LL2− which stabilizes the metal-ligand bond and increases the energy of the metal-centered redox orbital as the hard base character of the bidentate ligand increases. Electron transfer rate differences are believed to arise primarily from larger inner-shell contributions to the activation energy barrier for MoO versus MoS complexes, but contributions from differences in outer-shell energies and adiabaticities cannot be eliminated at this time.

Bis(hexamethylbenzene) complexes of the group 8 metals (Fe, Ru, Os) show surprising diversity in their electron-transfer mechanisms and associated thermodynamics for the M(II) → M(I) → M(0) redox series. In electrochemical experiments, the Fe complex exhibits normally ordered potentials separated by ∼1 V, the Ru system shows nearly overlapping one-electron redox events, and Os demonstrates a one-step, two-electron transfer with a peak potential separation suggestive of highly inverted potentials. It has been conjectured that the sequential one-electron transfers observed for Fe are due to the lack of an accessible η(4):η(6) Fe(0) state, destabilizing the fully reduced species. Using an established model chemistry based on DFT, we demonstrate that the hapticity change is a consequence of the bonding throughout this transition metal triad and that apparent multielectron behavior is controlled by the vertical electron attachment component of the M(II) → M(I) redox event. Furthermore, the η(6):η(6) Fe(0) triplet state is more favorable than the hypothetical η(4):η(6) singlet state, emphasizing that the hapticity change is not sufficient for multielectron behavior. Despite both displaying two-electron redox responses, Ru and Os traverse fundamentally different mechanisms based on whether the first (Os) or second (Ru) electron transfer induces the hapticity change. While the electronic structure analysis is limited to the Fe triad here, the conceptual model that we developed provides a general understanding of the redox behavior exhibited by d(6) bis(arene) compounds.

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.

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

Iron(II) bis(1,4,7-triazacylononane), Fe(tacn)22+, exhibits 1A1g ⇌ 5T2g spin-crossover behavior in solution. Values of ΔH°se = 23 kJ mol-1 and ΔS°se = 67 J mol-1 K-1 are obtained for the reaction by variable-temperature UV−visible and 1H NMR spectroscopy. The thermodynamic parameters reflect intramolecular structural changes that accompany LS → HS conversion and depend only to a small extent on solvent, counterion, and the presence of excess electrolyte. A procedure is described for quantitative correction of Fe(tacn)23+/2+ thermodynamics for the accompanying Fe(II) spin equilibrium.

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.

The UV-vis and IR spectroelectrochemistry of W 2 (SBz) 2 (CO) 8 2- (1) in 0.1 M (TBA)PF6/CH 2 Cl 2 is reported (where SBz - =benzyl mercaptide).

The ability of axial ligation to modulate redox potentials and electrode half-reactions of metalloporphyrins is illustrated by coordination of methanol to Mn(TPP)Cl (1) and its reduced and oxidized products in 1,2-dichloroethane. Visible spectrophotometry, spectroelectrochemistry, and cyclic and microelectrode voltammetry are used to characterize reactions whereby six-coordinate Mn(III) (Mn(TPP)(CH{sub 3}OH)Cl (3) and Mn(TPP)(CH{sub 3}OH){sub 2}{sup +} (4)) and five-coordinate Mn(II) (Mn(TPP)(CH{sub 3}OH)(6)) species are formed from 1. The axial ligand preferences of these two oxidation states lead to a biphasic behavior in the potential of Mn{sup III/II} reduction. At low CH{sub 3}OH concentrations, five-coordinate 1 is reduced to five-coordinate 6, causing E{sub 1/2} to shift to positive values with increasing (CH{sub 3}OH); at larger methanol concentrations, reduction of 4 to 6 predominates and the shift is in the opposite direction. In addition, the ring- and metal-centered oxidations of 1 at 1.22 and 1.56 V vs Ag/AgCl, respectively, coalesce into a single two-electron process at 1.31 V upon CH{sub 3}OH coordination. The negative shift in the metal-centered oxidation potential is an anticipated consequence of the greater donor strength of two CH{sub 3}OH ligands versus one Cl{sup {minus}} ligand. The positive shift in the ring-centered potential is believed to result from reduced deformationmore » of the porphyrin core and consequent stabilization of the porphyrin HOMO as axial coordination by CH{sub 3}OH returns the Mn atoms to an in-plate position from its significant out-of-plane displacement (0.27 {angstrom}) in 1.« less

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.

Oxidation of the thiolate-bridged tungsten(0) dimer, W2(CO)8(μ-SBz)22−, occurs by transfer of two electrons at the same potential. The process appears paradoxical because the reaction exhibits facile heterogeneous kinetics despite large changes in structure and formation of a metal-metal bond within the W2S2 core. However, the electrode reaction actually consists of overlapping one-electron transfers of which the second is slightly more favorable than the first; e.g., W2(CO)8(μ-SBz)22− ⇌ W2(CO)8(μ-SBz)2−2+e2− (at E1o') and W2(CO)8(μ-SBz)22− ⇌ W2(CO)8(μ-SBz)2 + e2− (at Eo'2) [E1o' = −0.859 and E2o' = −0.881 V vs. Ag/Ag+ (0.01 M)]. This conclusion is supported by a qualitative Marcus theory analysis of the heterogeneous electron transfer kinetics, which indicates that acceptable values of ks,h are obtained if the structural and solvational changes required for two-electron transfer are partitioned equally between the one-electron steps, and by digital simulations, which give a good fit to experimental curves if it is assumed that the one-electron transfer kinetics are facile and ΔEo' = −0.022 V. Electron transfer takes place to and from a metal-metal antibonding orbital whose energy varies significantly with the angle of the W-S-W bridge. Thus, the structural changes that accompany the first electron transfer reduce the thennodynamic requirements of the second and lead to transfer of two electrons at the same potential. The heterogeneous electron transfer kinetics are less sensitive to solvent properties than predicted by the dielectric continuum model of the Marcus theory, but ks,h decreases from 2 × 10−2 to 6 × 10−4 cm s−1 as electrode material is changed in the following order: Hg/Au ⪢ glassy C ⪢ Au ⪢ Pt. A similar sequence of electrode material dependence is exhibited by other electrochemical reactions accompanied by large structural change; possible reasons for this behavior are discussed.

An electrochemical cell is described for measuring electrode reaction kinetics at low and variable temperatures in nonaqueous solvents. The cell contains a working electrode of conventional size (at which the standard heterogeneous electron transfer rate constant is determined by cyclic voltammetry) and a microelectrode (at which the diffusion coefficient is measured by steady-state microelectrode voltammetry). Uncompensated solution resistance is minimized by a combination of close placement of the reference electrode salt bridge tip and the working electrode surface and instrumental positive feedback. Cell performance is evaluated by an examination of ferrocene oxidation and manganese(III) (tetraphenylporphyrin) chloride reduction at temperatures between 229 and 298 K. Comparison of the responses from these reversible and quasi-reversible redox couples in the same solution demonstrates that the effects of uncompensated solution resistance are absent from the experimental measurements and that increases in the cyclic voltammetric peak potential separation with sweep rate are due entirely to the kinetics of the Mn(III)/Mn(II) electrode reaction.

Electrochemical examination of the iron-molybdenum cofactor (FeMoco) extracted from the MoFe protein of Azotobacter vinelandii nitrogenase reveals that this important biological cluster exists in a variety of chemical forms whose numbers, proportions and properties depend on oxidation state, N -methylformamide solvent acidity and presence of thiophenolate ion. The distribution of oxidized (ox) species, determined from voltammetric responses, and of semi-reduced (s-r) species, determined from EPR spectra, show that FeMoco exists as a 2:1 proportion of populations in its ox state in alkaline NMF and in its ox and s-r states in acidic NMF. A single form of FeMoco(s-r) is found in alkaline NMF. Addition of thiophenol converts all populations of FeMoco into single PhS − -coordinated ox and s-r species under all conditions; these forms exhibit quasi-reversible [FeMoco(ox)-SPh] + e − ⇌ [Femoco(s-r)-SPh] electrochemistry. The variety of cofactor species observed is attributed to various states of ligation, protonation or structural arrangement of the metal cluster. The greater distribution of forms observed in the absence versus presence of thiophenol, in acidic versus alkaline solvent and in the oxidized versus semi- reduced oxidation state suggests that FeMoco exhibits greater structural or compositional heterogeneity under the former of each of these conditions.

Cell designs, experimental protocols, and results for electrochemical investigation of small quantities of biological materials under anaerobic conditions are reported. Three types of electrochemical experiments are considered: (i) cyclic voltammetry of 20- to 100-μl samples; (ii) direct coulometry of 0.5- to 1.5-ml samples; and (iii) an electrochemically initiated protein activity assay which includes provision for analysis of gaseous reaction products and correlation with electron flux. The first two procedures are illustrated by measurement of the formal electrode potential ( E o′ ) and number of electrons transferred ( n ) in redox reactions of small quantities of biological and inorganic materials. The third procedure is illustrated by assaying the activity of the MoFe protein plus Fe protein complex from Azotobacter vinelandii nitrogenase for reduction of C 2 H 2 to C 2 H 4 .

The manganese(III)-bis[poly(pyrazolyl)borate] complexes, Mn(pzb)2SbF6, where pzb- = tetrakis(pyrazolyl)borate (pzTp) (1), hydrotris(pyrazolyl)borate (Tp) (2), or hydrotris(3,5-dimethylpyrazolyl)borate (Tp*) (3), have been synthesized by oxidation of the corresponding Mn(pzb)2 compounds with NOSbF6. The Mn(III) complexes are low-spin in solution and the solid state (microeff = 2.9-3.8 microB). X-ray crystallography confirms their uncommon low-spin character. The close conformity of mean Mn-N distances of 1.974(4), 1.984(5), and 1.996(4) A in 1, 2, and 3, respectively, indicates absence of the characteristic Jahn-Teller distortion of a high-spin d4 center. N-Mn-N bite angles of slightly less than 90 degrees within the facially coordinated pzb- ligands produce a small trigonal distortion and effective D3d symmetry in 1 and 2. These angles increase to 90.0(4)degrees in 3, yielding an almost perfectly octahedral disposition of N donors in Mn(Tp*)2+. Examination of structural data from 23 metal-bis(pzb) complexes reveals systematic changes within the metal-(pyrazolyl)borate framework as the ligand is changed from pzTp to Tp to Tp*. These deformations consist of significant increases in M-N-N, N-B-N, and N-N-B angles and a minimal increase in Mn-N distance as a consequence of the steric demands of the 3-methyl groups. Less effective overlap of pyrazole lone pairs with metal atom orbitals resulting from the M-N-N angular displacement is suggested to contribute to the lower ligand field strength of Tp* complexes. Mn(pzb)2+ complexes undergo electrochemical reduction and oxidation in CH3CN. The electrochemical rate constant (ks,h) for reduction of t2g4 Mn(pzb)2+ to t2g3eg2 Mn(pzb)2 (a coupled electron-transfer and spin-crossover reaction) is 1-2 orders of magnitude smaller than that for oxidation of t2g4 Mn(pzb)2+ to t2g3 Mn(pzb)22+. ks,h values decrease as Tp*pzTpTp for the Mn(pzb)2+/0 electrode reactions, which contrasts with the behavior of the comparable Fe(pzb)2+/0 and Co(pzb)2+/0 couples.

In this communication we report that observation of another electrochemical parameter, the standard heterogeneous rate constant and its temperature dependence can assist in assigning the site of electron transfer in metalloporphyrins

Marcus-Hush theory predicts that a large change in molecular structure accompanying an electrode reaction will result in a large inner-shell barrier and a slow rate of electron transfer. Described here is an attempt to establish relationships between molecular structural features and electron transfer rates for two systems of bioinorganic interest: metalloporphyrins and model complexes for molybdenum-containing enzymes. For these compounds structural change occurs when an electron is added to an orbital that interacts strongly with donor atoms in the coordination environment of the metal. For the porphyrins, this results in increases in the metal atom out-of-plane distance and axial ligand bond length. For the molybdenum complexes, it results in cleavage of metal-ligand bonds in conjunction with electron transfer.

Two inorganic chemistry laboratory exercises are described. The first involves parameterization of an empirical force field for molecular mechanics calculations of coordination complexes. Students ...

Complex formation between tungsten and aminopolycarboxylic acids is found to require facial, tridentate orientation of ligand dentates. Coordination of hydroxyl groups is found to result in loss of the hydroxyl proton and formation of a W2O5 core structure. Hydroxyl coordination is found to be only slightly less favored than carboxylate coordination, but the resulting alkoxy bond is much less labile than that of carboxylate. Different coordinating ability is found for the two nitrogens of 1, 2-PDTA and the formation of a ‘proton chelate’ is proposed for this molecule.

The electrochemical reduction of Fe2(salen)2O in dimethylsulfoxide has been investigated by cyclic voltammetry, chronoamperometry and coulometry. At short times the complex is reduced by sequential one-electron transfers to a stable Fe(III)−Fe(II) dimer and to an unstable Fe(II) dimer. At long times the Fe(III)−Fe(II) product dimerizes, and the resultant tetramer undergoes successive one-electron transfers loading to consumption of 1.5 and 2 electrons per Fe2 unit. The electrochemical behavior of Fe2(salen)2O is compared to that of other μ-oxo Fe(III) dimers and Fe(III) monomers and to a molecular orbital description of oxobridged metal dimers. The electrochemical results reflect a strong interaction between metal centers in the Fe−O−Fe unit which is not apparent from spectroscopic and magnetic studies.

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.

The number of electrons involved in the more positive of the two redox couples of the iron-molybdenum cofactor of Azotobacter vinelandii nitrogenase has been investigated by controlled potential coulometry in both the oxidizing and reducing directions. A n value of 1 was determined for interconversion of the oxidized and semi-reduced states of the cofactor. This electron count was confirmed by double integration of the S=3/2 electron paramagnetic resonance signal exhibited by the semi-reduced state.