Electrocatalytic reduction of CO2 with CCC-NHC pincer nickel complexes.

A CCC-NHC pincer Ni(ii)Cl complex was prepared according to the metallation/transmetallation methodology. It was fully characterized by electrochemical, NMR spectroscopic, theoretical, and X-ray crystallographic methods. The complex and its cation were evaluated for electrocatalytic reduction of CO2 under a variety of conditions and found to provide some of the fastest catalytic rates and highest substrate selectivities (CO2vs. H+) reported. Rates improved in the presence of water and, significantly, catalysis occurred at the first reduction potential, presumably at the Ni(i) state. Controlled potential electrolysis (CPE) was found to yield CO at 34% and formate at 47% Faradaic efficiency (FE).

Comparisons of MN2S2vs. bipyridine as redox-active ligands to manganese and rhenium in (L-L)M'(CO)3Cl complexes.

The bipyridine ligand is renowned as a photo- and redox-active ligand in catalysis; the latter has been particularly explored in the complex Re(bipy)(CO)3Cl for CO2 reduction. We ask whether a bidentate, redox-active MN2S2 metallodithiolate ligand in heterobimetallic complexes of Mn and Re might similarly serve as a receptor and conduit of electrons. In order to assess the electrochemical features of such designed bimetallics, a series of complexes featuring redox active MN2S2 metallodithiolates, with M = Ni2+, {Fe(NO)}2+, and {Co(NO)}2+, bound to M'(CO)3X, where M' = Mn and Re, were synthesized and characterized using IR and EPR spectroscopies, X-ray diffraction, cyclic voltammetry, and density functional theory (DFT) computations. Butterfly type structures resulted from binding of the convergent lone pairs of the cis-sulfur atoms to the M'(CO)3X unit. Bond distances and angles are similar across the M' metal series regardless of the ligand attached. Electrochemical characterizations of [MN2S2·Re(CO)3Cl] showed the redox potential of the Re is significantly altered by the identity of the metal in the N2S2 pocket. DFT calculations proved useful to identify the roles played by the MN2S2 ligands, upon reduction of the bimetallics, in altering the lability of the Re-Cl bond and the ensuing effect on the reduction of ReI to Re0.

A reduced 2Fe2S cluster probe of sulfur-hydrogen versus sulfur-gold interactions.

The Ph3 PAu(+) cation, renowned as an isolobal analogue of H(+) , was found to serve as a proton surrogate and form a stable Au2 Fe2 complex, [(µ-SAuPPh3 )2 {Fe(CO)3 }2 ], analogous to the highly reactive dihydrosulfide [(µ-SH)2 {Fe(CO)3 }2 ]. Solid-state X-ray diffraction analysis found the two SAuPPh3 and SH bridges in anti configurations. VT NMR studies, supported by DFT computations, confirmed substantial barriers of approximately 25 kcal mol(-1) to intramolecular interconversion between the three stereoisomers of [(µ-SH)2 {Fe(CO)3 }2 ]. In contrast, the largely dative SAu bond in µ-SAuPPh3 facilitates inversion at S and accounts for the facile equilibration of the SAuPPh3 units, with an energy barrier half that of the SH analogue. The reactivity of the gold-protected sulfur atoms of [(µ-SAuPPh3 )2 {Fe(CO)3 }2 ] was accessed by release of the gold ligand with a strong acid to generate the [(µ-SH)2 {Fe(CO)3 }2 ] precursor of the [FeFe]H2 ase-active-site biomimetic [(µ2 -SCH2 (NR)CH2 S){Fe(CO)3 }2 ].

Regioselectivity in ligand substitution reactions on diiron complexes governed by nucleophilic and electrophilic ligand properties.

The discovery of a diiron organometallic site in nature within the diiron hydrogenase, [FeFe]-H2ase, active site has prompted revisits of the classic organometallic chemistry involving the Fe-Fe bond and bridging ligands, particularly of the (µ-SCH2XCH2S)[Fe(CO)3]2 and (µ-SCH2XCH2S)[Fe(CO)2L]2 (X = CH2, NH; L = PMe3, CN(-), and NHC's (NHC = N-heterocyclic carbene)), derived from CO/L exchange reactions. Through the synergy of synthetic chemistry and density functional theory computations, the regioselectivity of nucleophilic (PMe3 or CN(-)) and electrophilic (nitrosonium, NO(+)) ligand substitution on the diiron dithiolate framework of the (µ-pdt)[Fe(CO)2NHC][Fe(CO)3] complex (pdt = propanedithiolate) reveals the electron density shifts in the diiron core of such complexes that mimic the [FeFe]-H2ase active site. While CO substitution by PMe3, followed by reaction with NO(+), produces (µ-pdt)(µ-CO)[Fe(NHC)(NO)][Fe(CO)2PMe3](+), the alternate order of reagent addition produces the structural isomer (µ-pdt)[Fe(NHC)(NO)PMe3][Fe(CO)3](+), illustrating how the nucleophile and electrophile choose the electron-poor metal and the electron-rich metal, respectively. Theoretical explorations of simpler analogues, (µ-pdt)[Fe(CO)2CN][Fe(CO)3](-), (µ-pdt)[Fe(CO)3]2, and (µ-pdt)[Fe(CO)2NO][Fe(CO)3](+), provide an explanation for the role that the electron-rich iron moiety plays in inducing the rotation of the electron-poor iron moiety to produce a bridging CO ligand, a key factor in stabilizing the electron-rich iron moiety and for support of the rotated structure as found in the enzyme active site.

Comparisons of zinc with cadmium in N2S2 coordination and as S-bonded adducts to tungsten carbonyls.

The synthesis and characterization of bis-mercaptoethanediazaheptane cadmium(II) is reported and compared to the analogous zinc complex. Of significance is the dimeric form of the [Zn(N(2)S(2))](2) complex achieves penta-coordination about zinc through a bridging thiolate whereas cadmium engages two thiolate as S-bridges resulting in hexa-coordination about cadmium within a coordination polymer whose X-ray crystal structure is reported here. In the presence of W(CO)(5), this polymer breaks up, generating dimeric [Cd(N(2)S(2))](2) with two W(CO)(5) units appended to the terminal thiolates, a feat that is not observed for the zinc dimer analogue. The greater thiophilicity of cadmium over zinc is noted in several features of these complexes.

Reactions of palladium and gold complexes with zinc-thiolate chelates using electrospray mass spectrometry and X-ray diffraction: molecular identification of [Pd(bme-dach)], [Au(bme-dach]+ and [ZnCl(bme-dach)]2Pd.

The reaction between the complexes [MCl(L)]Cl(x) (L = 2,2',2''-terpyridine, terpy and dien, diethylenetriamine; M = Pd, x = 1; M = Au, x = 2) and [Zn(bme-dach)](2), an N(2)S(2)-Zn-thiolate bridged dimer used to mimic zinc finger protein sites, was studied by Electrospray Ionisation Mass Spectrometry and the structures of some of the products confirmed by X-ray crystallography. All reactions investigated in this work gave heteronuclear (Zn-thiolate)-metal products, the predominant species being the trinuclear dithiolate-bridged aggregate {[Zn(bme-dach)](2)M}(n+) (M = Pd, Au). X-Ray diffraction studies verified the molecular structure of [{ZnCl(bme-dach)}(2)Pd], and further confirmed that the zinc within the [Zn(bme-dach)](2) unit was retained within the N(2)S(2) binding site. The Zn-bound thiolates form stable thiolate bridges to Pd(2+) in a stair-step shape, held together by a planar PdS(4) center. In addition, both zinc atoms maintained penta-coordinate coordination with apical chloride ligands rather than the more commonly observed tetrahedral geometry. Further, [Pd(bme-dach)] was directly synthesized for X-ray structural characterization of the metal exchanged product observed in mass spectrometry experiments. In the case of Au compounds, the reactions were very fast and the products were similar for both [AuCl(L)]Cl(2) (L = terpy and dien) starting materials. In addition to the multimetallic Zn,Au,Zn aggregate formation, the predominant species from the reaction between [Zn(bme-dach)](2) and both Au compounds was the [Au(bme-dach](+) cation observable via ESI-MS, suggesting Zn/Au metal exchange immediately after mixing the compounds. The direct synthesis of [Au(bme-dach)]BPh(4) confirmed the molecular structure of this species through X-ray crystallography. The reactivity profile of Pd(2+) and Au(3+) species is compared with previous studies using the isostructural Pt compounds and the biological relevance of the results discussed.

Zinc/nickel exchange and ligand cannibalism in N2S2O(1,2) donor ligand sets.

To explore the displacement of Zn(2+) by Ni(2+) from within N-, S-, and O-chelate ligands, (N-(3-thiabutyl)-N'-(3-thiapentaneoate)-1,4-diazacycloheptane)zinc(II), Zn-1'-Ac, and 1,4-diazacycloheptane-1,4-diylbis(3-thiapentanoato) zinc(II), Zn-1'-Ac(2), were reacted with Ni(BF(4))(2) in a methanol solution and were found to yield, in both cases, the bright blue, hexa-coordinate Ni-1'-Ac(2) metal-exchanged product. The latter conditions imply an intact-ligand unwrapping process as the hexadentate N(2)S(2)O(2) ligand is transferred from zinc to nickel. The former involves transfer of the pentadentate N(2)S(2)O ligand generating a green, penta-coordinate nickel intermediate which engages in CH(2)CO(2) fragment scavenging from a second zinc unit. This conclusion is supported by the observed analogous reformulation of the stable Zn-1'-Ac complex into Zn-1'-Ac(2) and the dithiolato [Zn-1'](2) dimer. To our knowledge, this is a rare (possibly the first) example of cannibalism reported in such ligand systems. The M-1'-Ac(2) complexes were characterized by X-ray diffraction and compared to the directly synthesized products.

Development of five-coordinate zinc mono- and dithiolates as S-donor metalloligands: formation of a Zn-W coordination polymer.

The synthesis and isolation of mono- and dithiolate-bridged Zn(mu-SR)(n)W(CO)(m) (where n = 1, m = 5; n = 2, m = 4) species from the dimeric N,N'-bis(2-mercaptoethyl)-1,4-diazacycloheptanezinc(II), [Zn-1'](2), and the monomeric [N-(3-thiabutyl)-N'-(3-thiapentaneoate)-1,5-diazacycloheptane]zinc(II), Zn-1'-Ac, are described. Upon cleavage of the dimeric [Zn-1'](2) with Na(+)[ICH(2)CO(2)](-), the resulting Zn-1'-Ac product is isolated as a monomeric, five-coordinate Zn(N(2)SS'O) complex equipped with one available Zn-bound thiolate for further reactivity. Cleavage of [Zn-1'](2) with [Et(4)N](+)Cl(-) afforded a monomeric intermediate, [Zn-1'-Cl](-), containing two Zn-bound thiolates. The zinc mono- and dithiolato complexes demonstrated reactivity toward labile-ligand tungsten carbonyl species, (THF)W(CO)(5) and (pip)(2)W(CO)(4), to yield respectively [(Zn-1'-Ac)W(CO)(5)](x) and [(Zn-1'-Cl)W(CO)(4)](-) complexes that were isolated and characterized spectroscopically and via X-ray diffraction. Upon binding to W(CO)(5), the five-coordinate Zn(N(2)SS'O) complex becomes six-coordinate within the coordination polymer [(Zn-1'-Ac)W(CO)(5)](x), in which the acetate tether of each molecule provides an O donor to occupy the octahedral axial position of a neighboring moiety. The [(Zn-1'-Cl)W(CO)(4)](-) dithiolate-bridged complex maintains a five-coordinate, square-pyramidal [Zn(N(2)S(2)Cl)](-) center, utilizing a chloride as the apical donor and resulting in an overall anionic complex. The addition of CO(g) to the [(Zn-1'-Cl)W(CO)(4)](-) complex was monitored by IR spectroscopy, which showed the emergence of [(Zn-1'-Cl)W(CO)(5)](-).