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Electrochemical oxidation of ReCp(CO)2L (Cp = η5-C5H5; L = PPh3 (2), η2-2-C2Me2H2 (3), η-C2Ph2 (4)) has been studied in CH2Cl2/[NBu4][B(C6F5)4]. All three complexes undergo quasi-Nernstian one-electron oxidations with E1/2 values (vs ferrocene) of 0.49 V (2/2+), 0.45 V (3/3+), and 0.15 V (4/4+). A second reversible one-electron oxidation is observed for 4 at 0.97 V. The radical cation 2+ efficiently forms the Re–Re-bonded dimer dication 222+, which was isolated after chemical oxidation of 2 by [thianthrene][B(C6F5)4]. The 17-electron complex 3+ shows no tendency to dimerize. On the basis of the E1/2 value of 3 and expected potential shifts when replacing a methyl group by a hydrogen, a ligand electronic parameter, EL, of 0.38 was calculated for π-bonded ethylene, which is much lower than the existing literature value of 0.76. IR, ESR, and UV–vis spectra were recorded for 4+. DFT calculations were performed on the neutral complexes, the monocations 2+, 3+, and 4+, and the dimer dication 222+. The SOMO of 2...

The anodic electrochemical behavior of the piano-stool complexes Cr(CO)3(η6-arene), arene = benzene (1) or (3-benzyloxy)-17-hydroxyestra-1,3,5(10)-triene (2), has been studied in dichloromethane containing either [B(C6F5)4]− (TFAB) or [B(C6H3(CF3)2)4]− (BArF24) as the supporting electrolyte anion. Contrary to the behavior of these systems in electrolytes containing the smaller, more traditional, anions such as [PF6]− or [ClO4]−, the radical cations 1+ and 2+ are stable on a bulk electrolysis time scale (30 min or more), allowing systematic study of their spectroscopic and chemical characteristics. IR and ESR spectroscopy confirm that the oxidations of 1 and 2 are highly metal-based. The radical cations were shown to undergo rapid substitution of a single CO by PPh3 (in the case of 1+) or P(OR)3, R = Me or Ph (in the case of 2+). Cathodic rereduction of the oxidized substitution products gave high in situ yields of the neutral substituted complexes. This overall “electrochemically switched” ligand substitu...

The anodic electrochemistry of nickelocene, 1, first probed in ethanolic media by Cotton et al. in 1954, has been examined in 12 different nonaqueous solvents or solvent mixtures using [NBu4][TFAB] (TFAB = [B(C6F5)4]−) as the supporting electrolyte. Two quasi-Nernstian couples were observed for 1/1+ and 1+/12+ with a potential separation (ΔE1/2) that is consistent with a modified Born model for low-donor solvents varying in dielectric constant (e) from 5.6 (chlorobenzene) to 37.3 (nitromethane). In stronger–donor solvents such as acetonitrile, benzonitrile, and THF, the oxidation of nickelocenium ion to the dication 12+ is coupled to chemical follow-up reactions, reversible in the cases of the nitriles, which are likely to involve solvent coordination to the nickel center. The observed complexity of the anodic behavior shows that much of the previous literature on the second oxidation of nickelocene has given oversimplified conclusions. Variations of up to 600 mV in ΔE1/2 for nickelocene can be achieved by solvent variations when using the weakly-coordinating electrolyte anion [B(C6F5)4]−.

The anodic one-electron oxidation of three members of the half-sandwich family of piano-stool compounds MnCp (gamma)(CO) 3, where Cp (gamma) is a generic cyclopentadienyl ligand, has been studied in a CH 2Cl 2/[NBu 4][TFAB] electrolyte (TFAB = [B(C6F5) 4] (-)). The long-sought 17 e (-) radical cation of the parent complex MnCp(CO) 3 (cymantrene, 1, E 1/2 = 0.92 V vs ferrocene) has been shown to be persistent in solutions that use weakly coordinating anions in place of more nucleophilic traditional electrolyte anions. Spectroscopically characterized for the first time, 1 (+) was shown to absorb in the visible (530 nm), near-IR (2066 nm), and IR (2118, 1934 cm (-1)) regions. It was ESR-active at low temperatures (g parallel = 2.213, g perpendicular = 2.079, A parallel (Mn) = 79.2 G, A perpendicular (Mn) = 50 G) and NMR active at room temperature (delta = 22.4 vs TMS). The radical cations of the Cp-functionalized analogues, Mn(eta (5)-C5H 4NH2)(CO) 3, 2, E 1/2 = 0.62 V, and MnCp*(CO) 3 (Cp*= eta (5)-C 5Me 5, 3), E 1/2 = 0.64 V, were generated electrochemically as well by the chemical oxidant [ReCp(CO) 3] (+). The structures of 2 (+) and 3 (+) were determined by X-ray crystallographic studies of their TFAB salts. Compared to the structures of the corresponding neutral compounds, the cations showed elongated Mn-C(O) bonds and shortened C-O bonds, displaying the effect of diminished metal-to-CO backbonding. The bond-length changes in the Mn(CO) 3 moiety were much larger in 3 (+) (avg changes, Mn-C(O) = + 0.142 A, C-O = -0.063 A) than in 2 (+) (avg changes, Mn-C(O) = + 0.006 A, C-O = -0.003 A). Although there were only minor changes in the metal-to-center ring distances upon oxidation of either 2 or 3, there was decidedly less bending of the C(N) atom out of the cyclopentadienyl plane in 2 (+) compared to 2. The optical, vibrational, and magnetic resonance spectra of radicals 2 (+) and 3 (+) were also observed. The spectral data argue for the SOMOs of the 17-electron species being largely located on the Mn(CO) 3 moiety, having 40-50% Mn d-orbital character, with the ground states of the radicals, most likely (2)A'', lying close in energy (within about 6000 cm (-1)) to excited states that are responsible for their rapid electronic relaxations. The cymantrenyl moiety is proposed as an anodic redox tag (or label) having physical and chemical properties that are significantly different from those of its ferrocenyl analogue.

The anodic electrochemical oxidations of ReCp(CO)3 (1, Cp = eta(5)-C5H5), Re(eta(5)-C5H4NH2)(CO)3 (2), and ReCp*(CO)3 (3, Cp* = eta(5)-C5Me5), have been studied in CH2Cl2 containing [NBu4][TFAB] (TFAB = [B(C6F5)4]-) as supporting electrolyte. One-electron oxidations were observed with E(1/2) = 1.16, 0.79, and 0.91 V vs ferrocene for 1-3, respectively. In each case, rapid dimerization of the radical cation gave the dimer dication, [Re2Cp(gamma)2(CO)6]2+ (where Cp(gamma) represents a generic cyclopentadienyl ligand), which may be itself reduced cathodically back to the original 18-electron neutral complex ReCp(gamma)(CO)3. DFT calculations show that the SOMO of 1+ is highly Re-based and hybridized to point away from the metal, thereby facilitating the dimerization process and other reactions of the Re(II) center. The dimers, isolated in all three cases, have long metal-metal bonds that are unsupported by bridging ligands, the bond lengths being calculated as 3.229 A for [Re2Cp2(CO)6]2+ (1(2)2+) and measured as 3.1097 A for [Re2(C5H4NH2)2(CO)6]2+ (2(2)2+) by X-ray crystallography on [Re2(C5H4NH2)2(CO)6][TFAB]2. The monomer/dimer equilibrium constants are between K(dim) = 10(5) M(-1) and 10(7) M(-1) for these systems, so that partial dissociation of the dimers gives a modest amount of the corresponding monomer that is free to undergo radical cation reactions. The radical 1+ slowly abstracts a chlorine atom from dichloromethane to give the 18-electron complex [ReCp(CO)3Cl]+ as a side product. The radical cation 1+ acts as a powerful one-electron oxidant capable of effectively driving outer-sphere electron-transfer reactions with reagents having potentials of up to 0.9 V vs ferrocene.

The syntheses of the new compounds Fe(3-Me3Si-6,6-dmch)2, 2, and Fe[3-(i-Pr)3Si-6,6-dmch]2, 3, are reported, along with X-ray structural studies of these species, and of the previously reported Fe(1,3,5,6-temch)2 (dmch = dimethylcyclohexadienyl; temch = tetramethylcyclohexadienyl). Each species crystallized in something close to the expected gauche-eclipsed conformation. In accord with previous work on Fe(6,6-dmch)2, but in contrast to results for open ferrocenes such as Fe(2,4-C7H11)2 (C7H11 = dimethylpentadienyl), the three species under study undergo reversible one-electron oxidations at room temperature to stable 17-electron cations, with potentials for oxidation being more favorable than that for ferrocene by 0.51–0.78 V. The edge-bridged open ferrocenes also react in a 1:1 ratio with TCNE (TCNE = tetracyanoethylene), yielding salts that were shown spectroscopically to contain the expected cationic 17-electron metal complexes and the TCNE radical anion.

The reduction of 1,1,1-(CO)3-2-Ph-closo-1,2,3,4-MnC3B7H9, 1, to the hyperelectronic dianion 12- has been studied by electrochemistry, spectroscopy, X-ray crystallography, and DFT calculations. Depending on the medium, this cymantrene derivative displays either a single two-electron wave or two separate one-electron waves in cyclic voltammetry scans, always at potentials (e.g., E1/2 = −1.14 V vs ferrocene for 1/1- in THF) that are very far positive of that reported for MnCp(CO)3 (−2.86 V). Reduction of the corresponding Re compound 2 occurs in a single two-electron process. Both 1- and 12- were isolated as their decamethylcobaltocenium salts and characterized by X-ray crystallography. The hapticity of the tricarbadecaboranyl ligand is reduced from hexahapto (η6) in 1 to tetrahapto (η4) in 12-, mimicking an η5/η3 haptotropic rearrangement of a cyclopentadienyl ligand. The dianion 12- retains coordination to the C7−B3−B4−C9 face of the tricarbadecaboranyl ligand, much like that found previously for the neutr...

Cis-Cyclooctene, C8H14, undergoes an efficient electron-transfer (ET) catalyzed cycloaddition reaction in dichloromethane containing [NBu4][B(C6F5)4] as supporting electrolyte. The reactions were initiated by in-situ anodic generation of catalytic amounts of either [N(2,4-C6H3Br2)3]+ (1+, “magic green”) or [ReCp(CO)3]+ (2+, Cp = (η5-C5H5)). The olefin radical chain reaction induced by ET mediation was complete in less than 10 min, some 100–1000 times faster than the optimum photochemically induced preparations of cyclobutane derivatives from cyclic olefins. Yields of up to 87% of mixtures of C16H28 isomers were obtained at an electro-synthetic level. Use of the weakly coordinating [B(C6F5)4]− anion, rather than a traditional small anion such as [PF6]− or [ClO4]−, was required to carry out the cyclization reactions. The more easily oxidized trans-cyclooctene, when anodized directly at an electrode, gave a similar isomeric mixture of cyclobutanes. Cycloheptene and cyclohexene underwent similar [2 + 2] coupling to give cyclobutane products. Cyclopentene gave a [2 + 2 + 2] cyclized trimer. The mechanism appears to involve a key radical–substrate propagation step in which an olefin radical cation reacts with a neutral olefin to give a cyclobutyl radical cation, which then accepts an electron, most likely from another olefin, to form the final neutral compound. Although the E1/2 potentials of the ET mediators are 660–900 mV lower than the estimated oxidation potentials of the olefins, the endergonic nature of the ET initiation step works to the advantage of the chain process by favoring the radical–substrate propagation step.

A series of binuclear FeIFeI complexes, (μ-SEt)2[Fe(CO)2L]2 (L = CO (1), PMe3 (1-P)), (μ-SRS)[Fe(CO)2L]2 (R = CH2CH2 (μ-edt): L = CO (2), PMe3 (2-P); R = CH2CH2CH2(μ-pdt): L = CO (3), PMe3 (3-P); and R = o-CH2C6H4CH2 (μ-o-xyldt): L = CO (4), PMe3 (4-P)), that serve as structural models for the active site of Fe-hydrogenase are shown to be electrocatalysts for H2 production in the presence of acetic acid in acetonitrile. The redox levels for H2 production were established by spectroelectrochemistry to be Fe0Fe0 for the all-CO complexes and FeIFe0 for the PMe3-substituted derivatives. As electrocatalysts, the PMe3 derivatives are more stable and more sensitive to acid concentration than the all-CO complexes. The electrocatalysis is initiated by electrochemical reduction of these diiron complexes, which subsequently, under weak acid conditions, undergo protonation of the reduced iron center to produce H2. An (η2-H2)FeII–Fe0/I intermediate is suggested and probable electrochemical mechanisms are discussed.

Reactions of water-soluble Cp*IrCl2(PAr3) (1) with alkynes HC⋮CR in H2O/C6H6 give new water-soluble alkyl-carbonyl complexes, [Cp*Ir(−CH2R)(CO)(PAr3)]+ (2, R = Ph (a), CH2Ph (b), C(CH3)3 (c), p-C6H4CH3 (d)) by cleaving the C⋮C bond of the alkynes. The CC bond of ethylene is also cleaved in the reaction of complex 1 with ethylene in the presence of Ag+ in water to give [Cp*Ir(−CH3)(CO)(PAr3)]+ (3).

Hydroxide ion and alcohols or amines are added regioselectively to the −C⋮N and −CHCH2 groups of CH2CHCN in [Cp*Ir(η3-CH2CHCHPh)(NCCHCH2)]+ (1a). The unsaturated amido complex Cp*Ir(η3-CH2CHCHPh)(NHCOCHCH2) (2a), alkoxo amido complexes Cp*Ir(η3-CH2CHCHPh)(NHCOCH2CH2OR) (4, R = Me (a), Et (b)), amino amido complexes Cp*Ir(η3-CH2CHCHPh)(NHCOCH2CH2NR2) (6, R2 = Me2 (a), (Me)(H) (b)), and an amino imino−ether complex [Cp*Ir(η3-CH2CHCHPh)(NHC(OMe)CH2CH2NMe2)]+ (8) have been prepared from the reactions of 1a with OH-, OH-/ROH (R = Me, Et), OH-/HNR2 (R2 = Me2, (Me)(H)), and MeOH/NHMe2, respectively. Reactions of 2, 4, 6, and 8 with aqueous HCl yield Cp*IrCl(η3-CH2CHCHPh) and quantitative amounts of H2NCOCHCH2, H2NCOCH2CH2OR, H2NCOCH2CH2NR2, and MeOCOCH2CH2NMe2, respectively. Crystal structures of 1a and 4a have been determined by X-ray diffraction data analysis.

Reactions of [Cp*Ir(η3-CH2CHCHPh)(NCMe)]OTf (1) with protic amines, alcohols, and water produce amidine complexes [Cp*Ir(η3-CH2CHCHPh)(NHC(NR2)Me)]OTf (2) (R2 = (Me)2 (a), (Me)(H) (b), (i-Pr)(H) (c), (−CH2(CH2)3CH2−) (d)), imino−ether complexes [Cp*Ir(η3-CH2CHCHPh)(NHC(OR‘)Me)]OTf (4) (R‘ = Me (a), Et (b), i-Pr (c)), and amido complex Cp*Ir(η3-CH2CHCHPh)(NHC(O)Me) (5-K), respectively. The keto form amido complex 5-K undergoes tautomerization to give the enol form complex Cp*Ir(η3-CH2CHCHPh)(NC(OH)Me) (5-E) in polar solvents. Tertiary amines (NMe3, NEt3) react with 1 in chlorinated solvents (XCl) to give the chloro complex Cp*IrCl(η3-CH2CHCHPh) (3) and quaternary ammonium salts [R3NX]OTf (R = Me, Et and X = CH2Cl, CH3, CHCl2, CCl3, PhCH2). Crystal structures of 2a, 4a, 5-K, and [Cp*Ir(NHC(OH)Me)(OH2)(PPh3)]OTf2 (6) have been determined by single-crystal X-ray diffraction analysis, which lead us to suggest hybrid structures, Ir−-NH−C(N+Me2)Me (2a‘) for 2a and Ir−-NH−C(O+Me)Me (4a‘) for 4a to some extent. Co...

Because of the growing need for more versatile tools in synthetic reactions as well as industrially facile chemical synthesis techniques for the mass production of polymers and pharmaceuticals, a novel approach to the cleavage and coupling of new C-C bonds is crucial. Electrochemistry is a sustainable means of performing a chemical reaction at room temperature without excessive heating; it also offers the possibility of using inexpensive and environmentally benign cross coupling catalysts focused on iron complexes as the organometallic electron-transfer mediator. Several examples are indicated throughout this editorial to give an understanding of the applications of voltammetric techniques as synthetic tools. The role of mandatory high-energy organic methods such as reflux and week-long photolysis can be replaced by sustainable aspects of electrochemical operations using the electron as a proven reagent in synthetic chemistry

An electrochemical reduction process is described for the rapid and efficient conversion of terminal alkynes into useful products. Terminal alkynes (HC≡CR; R = C 6 H 5 , p -C 6 H 4 CH 3 , CH 2 C 6 H 5 , C 6 H 9 ) were hydrated and dimerized by cathodic reduction of catalytic amount (5 %) of [Cp * Ir(NCMe) 2 (PAr 3 )](OTf) 2 , 1 , ( E pc = −620 mV vs Ag/AgCl) through an aqueous/nonaqueous interface. Thin aqueous layer containing water-soluble compound 1 was formed on gold electrode. The cathodic reduction current height of 1 increases as increasing the concentration of terminal alkynes. The electrolysis of 1 while contacting with terminal alkynes containing organic solvent produced ketones, aldehydes and dimerized enyne products from terminal alkyne/water reaction systems. A new micro-scale electrocatalysis technique is depicted.

Stable alkynyl complexes of iridium(III) (LnIr-⋮-R) that are prepared from the reactions of terminal alkynes readily undergo the intramolecular C−C bond-forming reactions between the alkynyl and adjacent hydrocarbyl ligands to yield conjugated olefins. These reactions are initiated by electrophiles (H+, Me+) that attack the β carbon of the alkynyl ligand to increase the electrophilicity of the α carbon of the alkynyl ligand. The C−C bond is then formed between the α carbons of the alkynyl and adjacent hydrocarbyl ligands.

The half-sandwich piano-stool compounds Re(eta5-C5R5)(CO)3 (1, R = H; or 2, R = Me) are oxidized to the corresponding 17-electron Re(II) cations at glassy carbon anodes in CH2Cl2/[NBu4][B(C6F5)4]. Despite the very strongly positive E1/2 values of the couples (1.16 V for 1/1+ and 0.91 V for 2/2+ vs ferrocene/ferrocenium), the radical cations are persistent in this medium and exist in equilibrium with the corresponding dimeric dications, which may be cathodically reduced back to the neutral starting material. DFT calculations show that the dimer of 1+ achieves its stability through formation of a single long (almost 3.3 A) Re-Re bond made possible when the HOMO in 1 is rehybridized away from the metal in the one-electron oxidation process. The pure salts [1][B(C6F5)4]2 and [2][B(C6F5)4]2 were isolated by preparative anodic electrochemistry. The former may be used for storage of the very strong one-electron oxidant 1+, which was used to prepare a number of oxidation products as their [B(C6F5)4]- salts.

As functional biomimics of the hydrogen-producing capability of the dinuclear active site in [Fe]H(2)ase, the Fe(I)Fe(I) organometallic complexes, (mu-pdt)[Fe(CO)(2)PTA](2), 1-PTA(2), (pdt = SCH(2)CH(2)CH(2)S; PTA = 1,3,5-triaza-7-phosphaadamantane), and (mu-pdt)[Fe(CO)(3)][Fe(CO)(2)PTA], 1-PTA, were synthesized and fully characterized. For comparison to the hydrophobic (mu-pdt)[Fe(CO)(2)(PMe(3))](2) and [(mu-H)(mu-pdt)[Fe(CO)(2)(PMe(3))](2)](+) analogues, electrochemical responses of 1-PTA(2) and 1-(PTA.H(+))(2) were recorded in acetonitrile and in acetonitrile/water mixtures in the absence and presence of acetic acid. The production of H(2) and the dependence of current on acid concentration indicated that the complexes were solution electrocatalysts that decreased over-voltage for H(+) reduction from HOAc in CH(3)CN by up to 600 mV. The most effective electrocatalyst is the asymmetric 1-PTA species, which promotes H(2) formation from HOAc (pK(a) in CH(3)CN = 22.6) at -1.4 V in CH(3)CN/H(2)O mixtures at the Fe(0)Fe(I) redox level. Functionalization of the PTA ligand via N-protonation or N-methylation, generating (mu-pdt)[Fe(CO)(2)(PTA-H(+))](2), 1-(PTA.H(+))(2), and (mu-pdt)[Fe(CO)(2)(PTA-CH(3)(+))](2), 1-(PTA-Me(+))(2), provided no obvious advantages for the electrocatalysis because in both cases the parent complex is reclaimed during one cycle under the electrochemical conditions and H(2) production catalysis develops from the neutral species. The order of proton/electron addition to the catalyst, i.e., the electrochemical mechanism, is dependent on the extent of P-donor ligand substitution and on the acid strength. Cyclic voltammetric curve-crossing phenomena was observed and analyzed in terms of the possible presence of an eta(2)-H(2)-Fe(II)Fe(I) species, derived from reduction of the Fe(I)Fe(I) parent complex to Fe(0)Fe(I) followed by uptake of two protons in an ECCE mechanism.

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