Structural and functional analogues of the active sites of the [Fe]-, [NiFe]-, and [FeFe]-hydrogenases

Ultrafast photodriven intramolecular electron transfer from a zinc porphyrin to a readily reduced diiron hydrogenase model complex

Diiron complexes modeled on the active site of the [FeFe] hydrogenases having the general formula [Fe(2)(mu-R)(CO)(6-n)(L)(n)], where commonly R = alkyl or aryl dithiolate and L = CO, CN(-), or PR(3), are a promising class of catalysts for use in photodriven H(2) production. However, many of these catalysts are difficult to photoreduce using chromophores that absorb visible light. Here we report the synthesis and spectroscopic characterization of a naphthalene-4,5-dicarboximide-1,8-dithiolate diiron complex [NMI-Fe(2)S(2)(CO)(6), 1] and a covalently linked, fixed-distance zinc 5,10,15-tri-n-pentyl-20-phenylporphyrin-NMI-Fe(2)S(2)(CO)(6) donor-acceptor dyad (2). The electron-withdrawing nature of the NMI group makes the diiron complex among the most easily reduced hydrogenase mimics reported to date (-0.74 V vs SCE). In the presence of triflic acid, the cyclic voltammogram of 1 showed an increase in current at the first reduction wave at -0.78 V and a new reduction wave at -1.4 V. As the acid concentration was increased, the current at -0.78 V remained constant while the current at -1.4 V increased significantly, which is consistent with a catalytic proton reduction process. Selective photoexcitation of the Zn porphyrin in 2 with 553 nm, 110 fs laser pulses in both toluene and CH(2)Cl(2) yielded transient absorption spectra showing a distinct peak at 616 nm, which has been assigned to [NMI-Fe(2)S(2)(CO)(6)](-*) on the basis of spectroelectrochemical measurements on 1. The 616 nm peak was used to monitor the charge separation (CS) and charge recombination (CR) dynamics of 2, which yielded tau(CS) = 12 +/- 1 ps and tau(CR) = 3.0 +/- 0.2 ns in toluene and tau(CS) = 24 +/- 1 ps and tau(CR) = 57 +/- 1 ps in CH(2)Cl(2). Photoexcitation of the disulfide precursor to 2 in both toluene and CH(2)Cl(2) produced only the singlet and triplet excited states of the Zn porphyrin, showing that electron transfer is favorable only when the diiron complex is present. Photoexcitation of 2 in the presence of trifluoroacetic acid was shown to generate H(2).

A structural and functional mimic of the active site of NiFe hydrogenases

The structural mimic of the active site of NiFe hydrogenases, [Ni(xbsms)FeCp(CO)](BF(4)), is an electrocatalyst for hydrogen evolution from trifluoroacetic acid in DMF.

Directing protonation in [FeFe] hydrogenase active site models by modifications in their second coordination sphere

Subtle changes in the second coordination sphere of [Cl(2)bdtFe(2)(CO)(4)(Ph(2)P-CH(2)-X-CH(2)-PPh(2))] (bdt = benzene-1,2-dithiolate, X = NCH(3), NCH(2)CF(3), CH(2)) that do not influence the electronic character of the Fe(2) center can however direct protonation to three different sites: the N in the bis-phosphane, the Fe-Fe bond or the bdt-S.

Photocatalytic hydrogen evolution from rhenium(I) complexes to [FeFe] hydrogenase mimics in aqueous SDS micellar systems: a biomimetic pathway

To offer an intriguing access to photocatalytic H(2) generation in an aqueous solution, the hydrophobic photosensitizer, Re(I)(4,4'-dimethylbpy)(CO)(3)Br (1) or Re(I)(1,10-phenanthroline)(CO)(3)Br (2), and [FeFe] H(2)ases mimics, [Fe(2)(CO)(6)(mu-adt)CH(2)C(6)H(5)] (3) or [Fe(2)(CO)(6)(mu-adt)C(6)H(5)] (4) [mu-adt = N(CH(2)S)(2)], have been successfully incorporated into an aqueous sodium dodecyl sulfate (SDS) micelle solution, in which ascorbic acid (H(2)A) was used as a sacrificial electron donor and proton source. Studies on the reaction efficiency for H(2) generation reveal that both the close contact and the driving force for electron transfer from the excited Re(I) complexes and [FeFe] H(2)ases mimics are crucial for efficient H(2) generation with visible light irradiation. Steady-state and time-resolved investigations demonstrate that the electron transfer takes place from the excited Re(I) complex 1 or 2 to [FeFe] H(2)ases mimic catalyst 3, leading to the formation of the long-lived Fe(I)Fe(0) charge-separated state that can react with a proton to generate Fe(I)Fe(II).H, an intermediate for H(2) production. As a result, a reaction vessel for the photocatalytic H(2) production in an aqueous solution is established.

Analysis of a pentacoordinate iron dicarbonyl as synthetic analogue of the Hmd or mono-iron hydrogenase active site

Pentacoordinate iron dicarbonyls, (NS)Fe(CO)(2)P (NS=2-amidothiophenylate, P=PCy(3) (4), PPh(3), (5), and P(OEt)(3) (6)) were prepared as potential biomimetics of the active site of the mono-iron hydrogenase, [Fe]-H(2)ase. Full characterization including X-ray diffraction, density functional theory (DFT) computations, and Mössbauer studies for complexes 5 and 6 find that, despite similar infrared v(CO) pattern and absorption frequencies as the active site of the [Fe]-H(2)ase, the geometrical distortions towards trigonal bipyramidal, the negative isomer shift parameters, and the differences in CO-uptake reactivity are due to the "non-innocence" of the NS ligand. Ligand-based protonation with a strong acid, HBF(4).Et(2)O, interrupted the extensive pi-delocalization over Fe and NS ligand of complex 4 and switched on CO uptake (1 bar) to form a CO adduct, mer-[(H-NS)Fe(CO)(3)(PCy(3))](+) or 4(CO)-H(+). The extrinsic CO is reversibly removed on deprotonation with Et(3)N to regenerate complex 4. In a (13)CO atmosphere, concomitant CO uptake by 4-H(+) and exchange with intrinsic CO groups provide a facile route to (13)C-labeled 4(CO)-H(+) and, upon deprotonation, (13)C-labeled complex 4. DFT calculations show substantial Fe character in the LUMO of 4-H(+) typical of the d(6) Fe(II) in a regular square-pyramidal geometry. Thus, the Lewis acidity of 4-H(+) makes it amenable for CO binding, whereas the dianionic NS ligand renders the iron center of 4 insufficiently electrophilic and largely of Fe(I) character.

Artificial hydrogenases

Decades of biophysical study on the hydrogenase (H(2)ase) enzymes have yielded sufficient information to guide the synthesis of analogs of their active sites. Three families of enzymes serve as inspiration for this work: the [FeFe]-H(2)ases, [NiFe]-H(2)ases, and [Fe]-H(2)ases, all of which feature iron centers bound to both CO and thiolate. Artificial H(2)ases affect the oxidation of H(2) and the reverse reaction, the reduction of protons. These reactions occur via the intermediacy of metal hydrides. The inclusion of amine bases within the catalysts is an important design feature that is emulated in related bioinspired catalysts. Continuing challenges are the low reactivity of H(2) toward biomimetic H(2)ases.

The useful properties of H2O as a ligand of a hydrogenase mimic

This paper investigates the required properties of Ru-coordinated ligands of a Ni-Ru based hydrogenase mimic. A series of ligands, including MeCN, pyridine, H(2)O and OH(-) were coordinated to Ru, with H(2)O being the only ligand to promote H(2)-activation. In addition, a tethered pyridyl moiety was synthesised and found to completely inhibit H(2)-activation. We conclude, therefore, that H(2)O is the ideal ligand for this mimic as a result of both its mild basicity and the availability of two lone pairs for simultaneous binding to Ru and H(2).

A model for the CO-inhibited form of [NiFe] hydrogenase: synthesis of CO3Fe(micro-StBu)3Ni{SC6H3-2,6-(mesityl)2} and reversible CO addition at the Ni site

A [NiFe] hydrogenase model compound having a distorted trigonal-pyramidal nickel center, (CO)(3)Fe(micro-S(t)Bu)(3)Ni(SDmp), 1 (Dmp = C(6)H(3)-2,6-(mesityl)(2)), was synthesized from the reaction of the tetranuclear Fe-Ni-Ni-Fe complex [(CO)(3)Fe(micro-S(t)Bu)(3)Ni](2)(micro-Br)(2), 2 with NaSDmp at -40 degrees C. The nickel site of complex 1 was found to add CO or CN(t)Bu at -40 degrees C to give (CO)(3)Fe(S(t)Bu)(micro-S(t)Bu)(2)Ni(CO)(SDmp), 3, or (CO)(3)Fe(S(t)Bu)(micro-S(t)Bu)(2)Ni(CN(t)Bu)(SDmp), 4, respectively. One of the CO bands of 3, appearing at 2055 cm(-1) in the infrared spectrum, was assigned as the Ni-CO band, and this frequency is comparable to those observed for the CO-inhibited forms of [NiFe] hydrogenase. Like the CO-inhibited forms of [NiFe] hydrogenase, the coordination of CO at the nickel site of 1 is reversible, while the CN(t)Bu adduct 4 is more robust.

Polythiophenes containing in-chain cobaltabisdicarbollide centers

New cobalt(III) bis(dicarbollide) complexes covalently linked to two 2-oligothienyl units have been synthesized and electropolymerized in acetonitrile electrolyte in order to produce the corresponding polythiophene films containing in-chain metallic centers. The polymer films electrogenerated from the bithienyl (4b) and terthienyl (4c) derivatives display redox processes attributed to the Co(III)/Co(II) couple at ca. -1.1 V vs SCE and to the p-doping/undoping of the expected quaterthienyl and sexithienyl segments at ca. 0.8 V vs SCE. In contrast, the anodic oxidation of the thienyl (4a) derivative leads to passivation of the electrode surface. As the length of the oligothiophene substituents increases, the metallic and dicarbollide cage carbon atoms contributions in the HOMO decrease dramatically so that the highest occupied frontier orbitals of 4b and 4c can be considered as almost purely oligothiophene-based. From further UV-vis spectroscopy analysis, it is demonstrated that the polymer incorporating the sexithienyl segments is more conjugated than that with the quaterthienyl segments as the absorption maximum for the interband pi-pi* transition was observed at 410 and 448 nm for poly(4b) and poly(4c) respectively. Furthermore, these polymers display a more extended degree of conjugation than the parent oligothiophenes. Such features indicate a significant electronic delocalization through the cobaltabisdicarbollide moiety. Their conducting probe atomic force microscopy characterization indicates that poly(4b) and poly(4c) behave like heavily doped semiconductors rather than pure semiconductors. Mean conductivity values extracted from the current-voltage profiles are 1.4 x 10(-4) and 7.5 x 10(-4) S cm(-1) for poly(4b) and poly(4c), respectively. Such materials are found to be efficient for the electrocatalytic reduction of protons to dihydrogen, as exemplified for poly(4b). The overpotential for hydrogen evolution is significantly decreased by ca. 230 mV with respect to that obtained with the bare electrode (measured for a current density of 1.4 mA cm(-2) in the presence of 20 mM HBF(4)).

Synthesis and Mössbauer characterization of octahedral iron(II) carbonyl complexes FeI2(CO)3L and FeI2(CO)2L2: developing models of the [Fe]-H(2)ase active site

A series of mono- and disubstituted complexes, FeI(2)(CO)(x)L(4-x), x = 2 or 3, is conveniently accessed from simple mixing of N-heterocyclic carbenes, phosphines, and aromatic amines with FeI(2)(CO)(4), first reported by Hieber in 1928. The highly light sensitive complexes yield to crystallization and X-ray diffraction studies for six complexes showing them to be rudimentary structural models of the monoiron hydrogenase, [Fe]-H(2)ase or Hmd, active site in native (Fe(II)(CO)(2)) or CO-inhibited (Fe(II)(CO)(3)) states. Diatomic ligand (nu(CO)) vibrational and Mossbauer spectroscopies are related to those reported for the Hmd active site. The importance of a serial approach for relating such parameters in model compounds to low spin Fe(II) in the diverse ligation of enzyme active sites is stressed.

S-S bond reactivity in metal-perthiocarboxylato compounds

While M-percarboxylato species are elusive intermediates, their sulfur containing analogues are known in some cases. The feasibility of isolation of M-perthioacetato compounds allowed, in this work, to obtain new insights into the pathways that follow the reactivity of M-E-ER (E = O, S) fragments. Herein we report on the isolation of two new M-perthioacetato compounds: trans-[Pt(CH(3)CS(2)S)(2)] () and [Ni(CH(3)CSS)(CH(3)CS(2)S)] (), which have been fully characterized, including X-ray structures. Reactivity of these compounds towards PPh(3) has been studied combining UV-vis monitorization and NMR measurements. Overall the accumulated data suggest that the evolution of the perthioacetato ligand in complexes and by reaction with PPh(3) consists of a complex multistep pathway in which the sulfur transfer is preceded by electron transfer. Cyclic voltammetry measurements indicate that the transference of two electrons from the phosphorus to the sulfur atom is not concerted, suggesting that the first step of the reaction with PPh(3) is the monoelectronic electron transfer followed by P-S bond formation. The results presented here show a novel pathway in the field of S-S bond reactivity processes relevant in biological, synthetic systems and in hydrocarbon desulfurization processes.

Concerto catalysis--harmonising [NiFe]hydrogenase and NiRu model catalysts

This communication reports the successful merging of the chemical properties of a natural [NiFe]hydrogenase (Desulfovibrio vulgaris Miyazaki F) and our previously reported [NiRu] hydrogenase-mimic. The catalytic activity of both the natural enzyme and the mimic is almost identical, with the exception of working pH ranges, and this allows us to use them simultaneously in the same reaction flask. In such a manner, isotope exchange between D(2) and H(2)O could be conducted over an extended pH range (about 2-10) in one pot under mild conditions at ambient temperature and pressure.