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The H-cluster of [Fe–Fe] hydrogenase consists of a [4Fe]H subcluster linked by the sulfur of a cysteine residue to an organometallic [2Fe]H subcluster that utilizes terminal CO and CN ligands to each Fe along with a bridging CO and a bridging SCH2NHCH2S azadithiolate (adt) to catalyze proton reduction or hydrogen oxidation. Three Fe–S "maturase" proteins, HydE, HydF, and HydG, are responsible for the biosynthesis of the [2Fe]H subcluster and its incorporation into the hydrogenase enzyme to form this catalytically active H-cluster. We have proposed that HydG is a bifunctional enzyme that uses S-adenosylmethione (SAM) bound to a [4Fe–4S] cluster to lyse tyrosine via a transient 5′-deoxyadenosyl radical to produce CO and CN ligands to a unique cysteine-chelated Fe(II) that is linked to a second [4Fe–4S] cluster via the cysteine sulfur. In this "synthon model", after two cycles of tyrosine lysis, the product of HydG is completed: a [Fe(CN)(CO)2(cysteinate)]− organometallic unit that is vectored directly into the synthesis of the [2Fe]H sub-cluster. However our HydG-centric synthon model is not universally accepted, so further validation is important. In this Frontiers article, we discuss recent results using a synthetic "Syn-B" complex that donates [Fe(CN)(CO)2(cysteinate)]− units that match our proposed HydG product. Can Syn-B activate hydrogenase in the absence of HydG and its tyrosine substrate? If so, since Syn-B can be synthesized with specific magnetic nuclear isotopes and with chemical substitutions, its use could allow its enzymatic conversions on the route to the H-cluster to be monitored and modeled in fresh detail. [ABSTRACT FROM AUTHOR]

The reaction of Fe2S2(CO)6 and PPh3 affords Fe2S2(CO)4(PPh3)2 by an unprecedented mechanism involving the intermediacy of SPPh3 and Fe2S(CO)6(PPh3)2. [ABSTRACT FROM AUTHOR]

We report the solution and structural chemistry of nickel(ii) complexes of the phosphine-oxime Ph2PC6H4-2-CH=NOH (PCH=NOH). PCH=NOH invariably binds in a bidentate manner as illustrated by cis-Ni(PCH=NOH)2Cl2 and cis-[Ni(PCH=NOH)2]2+ (as its BF4− salt). Treatment of PCH=NOH with Ni(OAc)2(H2O)4 gave charge-neutral trans-[Ni(PCH=NO)2]0. Treatment of trans-[Ni(PCH=NO)2]0 with BF3 gave [Ni(PCH=NO)2BF2]BF4. The cation features a planar NiP2N2 center wherein the pair of oximate groups are linked by the difluoroboryl center. The 1 : 1 complexes of the oxime and the oximate are illustrated by [Ni(PCH=NOH)Cl2]2 and [Ni(C6F5)(PCH=NO)]2, which feature five- and four-coordinate Ni(ii) centers, respectively. All complexes in this series hydrolyze to give the trinickel oxo-phosphine-oximate complex [Ni3(PCH=NO)3O]+. One feature of the PCH=NOH ligand is its wide bite angle combined with its protic OH center. These aspects are manifested in the structures of Ni(PCH=NOH)2Cl2 and [Ni(PCH=NOH)Cl2]2, which show intramolecular hydrogen bonding to terminal chloride ligands. [ABSTRACT FROM AUTHOR]

The paper describes three methods for the preparation of methoxysiloxide complexes, a rare class of complexes of relevance to room temperature vulcanization (RTV) of polysiloxanes. The salt metathesis reaction involves the use of the recently described reagent NaOSi(OMe)2Me with various metal chlorides to give Cp*2Ti[OSi(OMe)2Me](OMe), (Me,MeN2N)NiOSi(OMe)2Me, (IPr)CuOSi(OMe)2Me, and (triphos)CoOSi(OMe)2Me (Cp* = C5Me5, triphos = Me(CH2PPh2)3). Several attempted reactions gave methoxide complexes instead, a pathway that is attributed to the intermediacy of κ2-OSi(OMe)2Me species. The diol Cp*2Zr(OH)2 reacts with excess (MeO)3SiMe to give Cp*2Zr[OSi(OMe)2Me]2. In contrast the less nucleophilic Cp*2Ti(OH)2 was unreactive. The third route to methoxysiloxide complexes involves the reaction of Cp*2M(O)(py) with (MeO)3SiMe to give Cp*2M[OSi(OMe)2Me](OMe) in nearly quantitative yield (M = Ti, Zr). The structures of Cp*2Ti[OSi(OMe)2Me](OMe), Cp*2Zr[OSi(OMe)2Me](OMe), (IPr)CuOSi(OMe)2Me, and (triphos)CoOSi(OMe)2Me were confirmed by single crystal X-ray diffraction. [ABSTRACT FROM AUTHOR]

The H-cluster of [Fe-Fe] hydrogenase consists of a [4Fe] H subcluster linked by the sulfur of a cysteine residue to an organometallic [2Fe] H subcluster that utilizes terminal CO and CN ligands to each Fe along with a bridging CO and a bridging SCH 2 NHCH 2 S azadithiolate (adt) to catalyze proton reduction or hydrogen oxidation. Three Fe-S "maturase" proteins, HydE, HydF, and HydG, are responsible for the biosynthesis of the [2Fe] H subcluster and its incorporation into the hydrogenase enzyme to form this catalytically active H-cluster. We have proposed that HydG is a bifunctional enzyme that uses S -adenosylmethione (SAM) bound to a [4Fe-4S] cluster to lyse tyrosine via a transient 5'-deoxyadenosyl radical to produce CO and CN ligands to a unique cysteine-chelated Fe(II) that is linked to a second [4Fe-4S] cluster via the cysteine sulfur. In this "synthon model", after two cycles of tyrosine lysis, the product of HydG is completed: a [Fe(CN)(CO) 2 (cysteinate)] - organometallic unit that is vectored directly into the synthesis of the [2Fe] H sub-cluster. However our HydG-centric synthon model is not universally accepted, so further validation is important. In this Frontiers article, we discuss recent results using a synthetic "Syn-B" complex that donates [Fe(CN)(CO) 2 (cysteinate)] - units that match our proposed HydG product. Can Syn-B activate hydrogenase in the absence of HydG and its tyrosine substrate? If so, since Syn-B can be synthesized with specific magnetic nuclear isotopes and with chemical substitutions, its use could allow its enzymatic conversions on the route to the H-cluster to be monitored and modeled in fresh detail.

The reaction of Fe 2 S 2 (CO) 6 and PPh 3 affords Fe 2 S 2 (CO) 4 (PPh 3 ) 2 by an unprecedented mechanism involving the intermediacy of SPPh 3 and Fe 2 S(CO) 6 (PPh 3 ) 2 .

We report the solution and structural chemistry of nickel(ii) complexes of the phosphine-oxime Ph2PC6H4-2-CH[double bond, length as m-dash]NOH (PCH[double bond, length as m-dash]NOH). PCH[double bond, length as m-dash]NOH invariably binds in a bidentate manner as illustrated by cis-Ni(PCH[double bond, length as m-dash]NOH)2Cl2 and cis-[Ni(PCH[double bond, length as m-dash]NOH)2]2+ (as its BF4- salt). Treatment of PCH[double bond, length as m-dash]NOH with Ni(OAc)2(H2O)4 gave charge-neutral trans-[Ni(PCH[double bond, length as m-dash]NO)2]0. Treatment of trans-[Ni(PCH[double bond, length as m-dash]NO)2]0 with BF3 gave [Ni(PCH[double bond, length as m-dash]NO)2BF2]BF4. The cation features a planar NiP2N2 center wherein the pair of oximate groups are linked by the difluoroboryl center. The 1 : 1 complexes of the oxime and the oximate are illustrated by [Ni(PCH[double bond, length as m-dash]NOH)Cl2]2 and [Ni(C6F5)(PCH[double bond, length as m-dash]NO)]2, which feature five- and four-coordinate Ni(ii) centers, respectively. All complexes in this series hydrolyze to give the trinickel oxo-phosphine-oximate complex [Ni3(PCH[double bond, length as m-dash]NO)3O]+. One feature of the PCH[double bond, length as m-dash]NOH ligand is its wide bite angle combined with its protic OH center. These aspects are manifested in the structures of Ni(PCH[double bond, length as m-dash]NOH)2Cl2 and [Ni(PCH[double bond, length as m-dash]NOH)Cl2]2, which show intramolecular hydrogen bonding to terminal chloride ligands.

The paper describes three methods for the preparation of methoxysiloxide complexes, a rare class of complexes of relevance to room temperature vulcanization (RTV) of polysiloxanes. The salt metathesis reaction involves the use of the recently described reagent NaOSi(OMe) 2 Me with various metal chlorides to give Cp* 2 Ti[OSi(OMe) 2 Me](OMe), ( Me,Me N 2 N)NiOSi(OMe) 2 Me, (IPr)CuOSi(OMe) 2 Me, and (triphos)CoOSi(OMe) 2 Me (Cp* = C 5 Me 5 , triphos = Me(CH 2 PPh 2 ) 3 ). Several attempted reactions gave methoxide complexes instead, a pathway that is attributed to the intermediacy of κ 2 -OSi(OMe) 2 Me species. The diol Cp* 2 Zr(OH) 2 reacts with excess (MeO) 3 SiMe to give Cp* 2 Zr[OSi(OMe) 2 Me] 2 . In contrast the less nucleophilic Cp* 2 Ti(OH) 2 was unreactive. The third route to methoxysiloxide complexes involves the reaction of Cp* 2 M(O)(py) with (MeO) 3 SiMe to give Cp* 2 M[OSi(OMe) 2 Me](OMe) in nearly quantitative yield (M = Ti, Zr). The structures of Cp* 2 Ti[OSi(OMe) 2 Me](OMe), Cp* 2 Zr[OSi(OMe) 2 Me](OMe), (IPr)CuOSi(OMe) 2 Me, and (triphos)CoOSi(OMe) 2 Me were confirmed by single crystal X-ray diffraction.

A new route to iron carbonyls has enabled synthesis of (57)Fe-labeled [NiFe] hydrogenase mimic (OC)3(57)Fe(pdt)Ni(dppe). Its study by nuclear resonance vibrational spectroscopy revealed Ni-(57)Fe vibrations, as confirmed by calculations. The modes are absent for [(OC)3(57)Fe(pdt)Ni(dppe)](+), which lacks Ni-(57)Fe bonding, underscoring the utility of the analyses in identifying metal-metal interactions.

Described herein are preparations of synthetic models for the deactivated Ni(II)Fe(II) states of the [NiFe]-hydrogenases. Iodination of the S = ½ species [(dppe)Ni(pdt)Fe(CO)(3)](+) afforded the diamagnetic iodo complex [(dppe)Ni(pdt)IFe(CO)(3)](+). Crystallographic analysis of this species confirmed the presence of square-pyramidal Ni linked to an octahedral Fe centre. The NiFe separation of 3.018 Å indicated the absence of metal-metal bonding. This complex could be reduced to give (dppe)Ni(pdt)Fe(CO)(3) and, in the presence of iodide, decarbonylated to afford (dppe)Ni(pdt)FeI(2). Derivatives of the type [(diphosphine)Ni(dithiolate)XFe(CO)(2)L](+) (X = Cl, Br, I) were prepared by halogenation of mixed-valence precursors [(diphosphine)Ni(dithiolate)Fe(CO)(2)L](+) (diphosphine = dppe, dcpe; L = tertiary phosphine or CO). The Fe(CO)(2)(PR(3))-containing derivatives are more robust than the related tricarbonyl derivatives. Exploiting this greater stability, we characterised examples of chloride and bromide derivatives. Related fluorides could be prepared by F(-) abstraction from BF(4)(-). Spectroscopic evidence is presented for the hydroperoxide [(dppe)Ni(pdt)(OOH)Fe(CO)(2)L](+), which represents a model for the Ni-SU state.

Experimental and computational experiments show that the electrophile MeS(+) attacks a single Fe center in Fe(2)(propanedithiolate)(CO)(4)(PMe(3))(2) followed by isomerization of this terminal thiolato complex to the corresponding μ-SMe derivative., (This journal is © The Royal Society of Chemistry 2011)

The [Fe]-hydrogenase is an ideal system for studying the electronic properties of the low spin iron site that is common to the catalytic centres of all hydrogenases. Because they have no auxiliary iron-sulfur clusters and possess a cofactor containing a single iron centre, the [Fe]-hydrogenases are well suited for spectroscopic analysis of those factors required for the activation of molecular hydrogen. Specifically, in this study we shed light on the electronic and molecular structure of the iron centre by XAS analysis of [Fe]-hydrogenase from Methanocaldococcus jannashii and five model complexes (Fe(ethanedithiolate)(CO)(2)(PMe(3))(2), [K(18-crown-6)](2)[Fe(CN)(2)(CO)(3)], K[Fe(CN)(CO)(4)], K(3)[Fe(III)(CN)(6)], K(4)[Fe(II)(CN)(6)]). The different electron donors have a strong influence on the iron absorption K-edge energy position, which is frequently used to determine the metal oxidation state. Our results demonstrate that the K-edges of Fe(II) complexes, achieved with low-spin ferrous thiolates, are consistent with a ferrous centre in the [Fe]-hydrogenase from Methanocaldococcus jannashii. The metal geometry also strongly influences the XANES and thus the electronic structure. Using in silico simulation, we were able to reproduce the main features of the XANES spectra and describe the effects of individual donor contributions on the spectra. Thereby, we reveal the essential role of an unusual carbon donor coming from an acyl group of the cofactor in the determination of the electronic structure required for the activity of the enzyme.

The stepwise formation of bridging (mu-) hydrides of diiron dithiolates is discussed with attention on the pathway for protonation and subsequent isomerizations. Our evidence is consistent with protonations occurring at a single Fe center, followed by isomerization to a series of mu-hydrides. Protonation of Fe(2)(edt)(CO)(4)(dppv) (1) gave a single mu-hydride with dppv spanning apical and basal sites, which isomerized at higher temperatures to place the dppv into a dibasal position. Protonation of Fe(2)(pdt)(CO)(4)(dppv) (2) followed an isomerization pathway similar to that for [1H](+), except that a pair of isomeric terminal hydrides were observed initially, resulting from protonation at the Fe(CO)(3) or Fe(CO)(dppv) site. The first observable product from low temperature protonation of the tris-phosphine Fe(2)(edt)(CO)(3)(PMe(3))(dppv) (3) was a single mu-hydride wherein PMe(3) is apical and the dppv ligand spans apical and basal sites. Upon warming, this isomer converted fully but in a stepwise manner to a mixture of three other isomeric hydrides. Protonation of Fe(2)(pdt)(CO)(3)(PMe(3))(dppv) (4) proceeded similarly to the edt analogue 3, however a terminal hydride was observed, albeit only briefly and at very low temperatures (-90 degrees C). Low-temperature protonation of the bis-chelates Fe(2)(xdt)(CO)(2)(dppv)(2) produced exclusively the terminal hydrides [HFe(2)(xdt)(mu-CO)(CO)(dppv)(2)](+) (xdt = edt and pdt), which subsequently isomerized to a pair of mu-hydrides. At room temperature these (dppv)(2) derivatives convert to an equilibrium of two isomers, one C(2)-symmetric and the other C(s)-symmetric. The stability of the terminal hydrides correlates with the (C(2)-isomer)/(C(s)-isomer) equilibrium ratio, which reflects the size of the dithiolate. The isomerization was found to be unaffected by the presence of excess acid, by solvent polarity, and the presence of D(2)O. This isomerization mechanism is proposed to be intramolecular, involving a 120 degrees rotation of the HFeL(3) subunit to an unobserved terminal basal hydride as the rate-determining step. The observed stability of the hydrides was supported by DFT calculations, which also highlight the instability of the basal terminal hydrides. Isomerization of the mu-hydride isomers occurs on alternating FeL(3) via 120 degree rotations without generating D(2)O-exchangeable intermediates.

The new complexes Fe2(S2CnH2n)(CO)2(dppv)2 (n = 2, 3; dppv = cis-1,2-C2H2(PPh2)2) form adducts with AlBr3 and B(C6F5)3, which adopt the "rotated structure" proposed for the active site of the Fe-only hydrogenases--the propanedithiolate is significantly more Lewis basic due to nonbonded interactions between the dithiolate strap and the ligands on Fe.

Condensation of [Cp*Rh(CH(3)NO(2))(n)](2+) and the tricyanoborate [PhB(CN)(3)](-) affords the hexagonal bipyramidal cage [[PhB(CN)(3)](6)[Cp*Rh](6)](6+), demonstrating that tetrahedral tricyanide building blocks can lead to novel cage structures.