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.

A series of binuclear Fe I Fe I complexes, (µ-SEt) 2 [Fe(CO) 2 L] 2 (L = CO (1), PMe 4), PMe 3 (4-P)), that serve as structural models for the active site of Fe-hydrogenase are shown to be electrocatalysts for H 2 production in the presence of acetic acid in acetonitrile. The redox levels for H 2 production were established by spectroelectrochemistry to be Fe 0 Fe 0 for the all-CO complexes and Fe I Fe 0 for the PMe 3 -substituted derivatives. As electrocatalysts, the PMe 3 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 H 2 . An (η 2 -H 2 )Fe II -Fe 0/I intermediate is suggested and probable electrochemical mechanisms are discussed.

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.

Reactions of [Cp*Ir(η 3 -CH 2 CHCHPh)(NCMe)]OTf (1) with protic amines, alcohols, and water produce amidine complexes [Cp*Ir(η 3 -CH 2 CHCHPh)(NHdC Et (b), i-Pr (c)), and amido complex Cp*Ir(η 3 -CH 2 CHCHPh)(NHC(dO)Me) (5-K), respectively. The keto form amido complex 5-K undergoes tautomerization to give the enol form complex Cp*Ir(η 3 -CH 2 CHCHPh)(Nd C(OH)Me) (5-E) in polar solvents. Tertiary amines (NMe 3 , NEt 3 ) react with 1 in chlorinated solvents (XCl) to give the chloro complex Cp*IrCl(η 3 -CH 2 CHCHPh) (3) and quaternary ammonium salts [R 3 NX]OTf (R ) Me, Et and X ) CH 2 Cl, CH 3 , CHCl 2 , CCl 3 , PhCH 2 ). Crystal structures of 2a, 4a, 5-K, and [Cp*Ir(NHdC(OH)Me)(OH 2 )(PPh 3 )]OTf 2 (6) have been determined by single-crystal X-ray diffraction analysis, which lead us to suggest hybrid structures, Ir--NH-C(dN + Me 2 )Me (2a′) for 2a and Ir--NH-C(dO + Me)Me (4a′) for 4a to some extent. Complexes 2 and 4 react with PPh 3 to give an iridium(III) complex [Cp*Ir(η 3 -CH 2 CHCHPh)(PPh 3 )]OTf (7) and the free amidines NHdC(NR 2 )Me ( 8) and imino-ethers NHdC(OR′)Me (9), respectively. Nitrile complexes 1 and [Cp*Ir(η 3 -CH 2 CHCHPh)(NCCHd CHMe)]OTf (10) catalyze the hydration of the nitriles in the presence of Na 2 CO 3 to produce amides, and the benzonitrile complex [Cp*Ir(η 3 -CH 2 CHCHPh)(NCPh)]OTf (11) catalyzes the methanolysis of benzonitrile in the presence of Na 2 CO 3 to produce NHdC(OMe)Ph. Plausible mechanisms for these catalytic reactions are suggested with the amido and imino-ether complexes such as 4 and 5 being involved.

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Stable alkynyl complexes of iridium(III) (L(n)Ir-triple bond-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 beta carbon of the alkynyl ligand to increase the electrophilicity of the alpha carbon of the alkynyl ligand. The C-C bond is then formed between the alpha 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.