Abiological Ironminus signSulfur Clusters

Generation and Reactivity of Polychalcogenide Chains in Binuclear Cobalt(II) Complexes

A series of six binuclear Co(II)-thiolate complexes, [Co2(BPMP)(S-C6H4-o-X)2]1+ (X = OMe, 2; NH2, 3), [Co2(BPMP)(μ-S-C6H4-o-O)]1+ (4), and [Co2(BPMP)(μ-Y)]1+ (Y = bdt, 5; tdt, 6; mnt, 7), has been synthesized from [Co2(BPMP)(MeOH)2(Cl)2]1+ (1a) and [Co2(BPMP)(Cl)2]1+ (1b), where BPMP1- is the anion of 2,6-bis[[bis(2-pyridylmethyl)amino]methyl]-4-methylphenol. While 2 and 3 could allow the two-electron redox reaction of the two coordinated thiolates with elemental sulfur (S8) to generate [Co2(BPMP)(μ-S5)]1+ (8), the complexes, 4-7, could not undergo a similar reaction. An analogous redox reaction of 2 with elemental selenium ([Se]) produced [{Co2(BPMP)(μ-Se4)}{Co2(BPMP)(μ-Se3)}]2+ (9a) and [Co2(BPMP)(μ-Se4)]1+ (9b). Further reaction of these polychalcogenido complexes, 8 and 9a/9b, with PPh3 allowed the isolation of [Co2(BPMP)(μ-S)]1+ (10) and [Co2(BPMP)(μ-Se2)]1+ (11), which, in turn, could be converted back to 8 and 9a upon treatment with S8 and [Se], respectively. Interestingly, while the redox reaction of the polyselenide chains in 9a and 11 with S8 produced 8 and [Se], the treatment of 8 with [Se] gave back only the starting material (8), thus demonstrating the different redox behavior of sulfur and selenium. Furthermore, the reaction of 8 and 9a/9b with activated alkynes and cyanide (CN-) allowed the isolation of the complexes, [Co2(BPMP)(μ-E2C2(CO2R)2)]1+ (E = S: 12a, R = Me; 12b, R = Et; E = Se: 13a, R = Me; 13b, R = Et) and [Co2(BPMP)(μ-SH)(NCS)2] (14), respectively. The present work, thus, provides an interesting synthetic strategy, interconversions, and detailed comparative reactivity of binuclear Co(II)-polychalcogenido complexes.

Cation binding of Li(I), Na(I) and Zn(II) to cobalt and iron sulphide clusters - electronic structure study

The binding of alkaline (Li⁺ and Na⁺) and zinc (Zn²⁺) cations to mononuclear disulphides MS₂ and to persulphides, containing an S-S bond, M(S₂), to binuclear disulphides M₂S₂ and persulphides M₂(S₂) and to cubic tetranuclear sulphides M₄S₄ where M = Fe, Co, is examined by density functional theory with the B3LYP functional, and dispersion corrections were applied. For the small-sized clusters (up to two transition metal centres), the energy gaps between different configurations were verified by CCSD(T) calculations. Persulphides M(S₂) are more stable than disulphides MS₂ as bare clusters, upon carbonyl and chloride ligand coordination and upon cation binding (Li⁺, Na⁺, Zn²⁺). The one-electron reduction of alkali cations and two-electron reduction of Zn²⁺ reverses order of stability and the planar disulphides (MS₂-reduced cation) become more stable; the energy gap disulphide to persulphide increases. In all reduced clusters, zinc ions form bonds with sulphur and with the transition metal centre (Co or Fe). Lithium cations also form bonds to cobalt or iron, but only in the M₂S₂ clusters, upon reduction. Energy barriers were calculated for the disulphide to persulphide reaction in the Zn-Co-S₂ system in the isolated clusters (gas-phase), in water, acetonitrile and 1-Cl-hexane solution. Most significant decrease in the energy barriers were obtained with less-polar solvents, acetonitrile, and particularly, 1-Cl-hexane. In M₄S₄ clusters, the cations do not reach optimal coordination to the sulphur centres. The global minima of M₂S₂ clusters are antiferromagnetic; in the reduced Zn-M₂S₂ clusters, magnetic moment is induced at zinc centres as a result of charge transfer between Zn and Co or Zn and Fe.

Evidence for Low-Valent Electronic Configurations in Iron-Sulfur Clusters

Although biological iron-sulfur (Fe-S) clusters perform some of the most difficult redox reactions in nature, they are thought to be composed exclusively of Fe²⁺ and Fe³⁺ ions, as well as mixed-valent pairs with average oxidation states of Fe^2.5+. We herein show that Fe-S clusters formally composed of these valences can access a wider range of electronic configurations─in particular, those featuring low-valent Fe¹⁺ centers. We demonstrate that CO binding to a synthetic [Fe₄S₄]⁰ cluster supported by N-heterocyclic carbene ligands induces the generation of Fe¹⁺ centers via intracluster electron transfer, wherein a neighboring pair of Fe²⁺ sites reduces the CO-bound site to a low-valent Fe¹⁺ state. Similarly, CO binding to an [Fe₄S₄]⁺ cluster induces electron delocalization with a neighboring Fe site to form a mixed-valent Fe^1.5+Fe^2.5+ pair in which the CO-bound site adopts partial low-valent character. These low-valent configurations engender remarkable C-O bond activation without having to traverse highly negative and physiologically inaccessible [Fe₄S₄]⁰/[Fe₄S₄]^- redox couples.

Chalcogen-atom abstraction reactions of a Di-iron imidophosphorane complex

Reaction of the complexes [Fe2(μ2-NP(pip)3)2(NP(pip)3)2] (1-Fe) and [Co2(μ2-NP(pip)3)2(NP(pip)3)2] (1-Co), where [NP(pip)3]1- is tris(piperidinyl)imidophosphorane, with nitrous oxide, S8, or Se0 results in divergent reactivity. With nitrous oxide, 1-Fe forms [Fe2(μ2-O)(μ2-NP(pip)3)2(NP(pip)3)2] (2-Fe), with a very short Fe3+-Fe3+ distance. Reactions of 1-Fe with S8 or Se0 result in the bridging, side-on coordination (μ-κ1:κ1-E22-) of the heavy chalcogens in complexes [Fe2(μ-κ1:κ1-E2)(μ2-NP(pip)3)2(NP(pip)3)2] (E = S, 3-Fe, or Se, 4-Fe). In all cases, the complex 1-Co is inert.

Reactivity of Binary and Ternary Sulfur Halides towards Transition-Metal Compounds

Binary sulfur fluorides exhibit an interesting reactivity towards transition metal complexes. They open up routes for the generation of sulfur‐containing building blocks. Often ligands with particular properties can be constructed. This includes their ability to transfer sulfur atoms or polysulfide units as well as fluorination reactions. This Minireview provides an insight into the reactivity of the binary and ternary sulfur halides S₂Cl₂, SCl₂, SF₄, SF₆ and SF₅Cl towards transition‐metal compounds.

A combined experimental and theoretical study of bimetallic bis- and tris-homocubane analogues

Various bimetallic bis- and tris-homocubane analogues of group 9 transition metals have been isolated and structurally characterized employing Li[BH₂E₃] (E = S or Se).

Homocubane Chemistry: Synthesis and Structures of Mono- and Dicobaltaheteroborane Analogues of Tris- and Tetrahomocubanes

Room-temperature reactions between [Cp*CoCl]2 (Cp* = η5-C5Me5) and large excess of [BH2E3]Li (E = S or Se) led to the formation of homocubane derivatives, 1-7. These species are bimetallic tetrahomocubane, [(Cp*Co)2(μ-S)4(μ3-S)4B2H2], 1; bimetallic trishomocubane isomers, [(Cp*Co)2(μ-S)3(μ3-S)4B2H2], 2 and 3; monometallic trishomocubanes, [M(μ-E)3(μ3-E)4B3H3] [4: M = Cp*Co, E = S; 5: M = Cp*Co, E = Se and 6: M = {(Cp*Co)2(μ-H)(μ3-Se)2}Co, E = Se], and bimetallic homocubane, [(Cp*Co)2(μ-Se)(μ3-Se)4B2H2], 7. As per our knowledge, 1 is the first isolated and structurally characterized parent prototype of the 1,2,2',4 isomer of tetrahomocubane, while 3, 4, and 5 are the analogues of parent D 3-trishomocubane. Compounds 2 and 3 are the structural isomers in which the positions of the μ-S ligands in the trishomocubane framework are altered. Compound 6 is an example of a unique vertex-fused trishomocubane derivative, in which the D 3-trishomocubane [Co(μ-Se)3(μ3-Se)4B3H3] moiety is fused with an exopolyhedral trigonal bipyramid (tbp) moiety [(Cp*Co)2(μ-H)(μ3-Se)2}Co]. Multinuclear NMR and infrared spectroscopy, mass spectrometry, and single crystal X-ray diffraction analyses were employed to characterize all the compounds in solution. Bonding in these homocubane analogues has been elucidated computationally by density functional theory methods.

Chalcogen stabilized trimetallic clusters: synthesis, structures, and bonding of [(Cp*M)3(E)6+m(BH)n] (M = Nb or Ta; E = S or Se; m = 0 or 1 or 2; n = 0 or 1)

An unusual trimetallic octaselenide complex, [(Cp*Ta)₃(μ-Se)₄{μ-Se₂(Se₂)}], has been synthesized and structurally characterized.

Cubane-Type [Mo3 S4 M] Clusters with First-Row Groups 4-10 Transition-Metal Halides Supported by C5 Me5 Ligands on Molybdenum

A synthetic protocol was developed for a series of cubane-type [Mo₃ S₄ M] clusters that incorporate halides of first-row transition metals (M) from Groups 4-10. This protocol is based on the anionic cluster platform [Cp*₃ Mo₃ S₄ ]^- ([1]^- ; Cp*=η⁵ -C₅ Me₅ ), which crystallizes when K(18-crown-6) is used as the counter cation. Treatment of in situ-generated [1]^- with such transition-metal halides led to the formation of [Mo₃ S₄ M] clusters, in which the M/halide ratio gradually changes from 1:2 to 1:1.5 and to 1:1, when moving from early to late transition metals. This trend suggests a tendency for early transition metals to tolerate higher oxidation states and adopt larger ionic radii relative to late transition metals. The properties of the [Mo₃ S₄ Fe] cluster 6 a were investigated in detail by using ⁵⁷ Fe Mössbauer spectroscopy and computational methods.

Origin and phylogenetic relationships of [4Fe-4S]-containing O2 sensors of bacteria

The advent of environmental O₂ about 2.5 billion years ago forced microbes to metabolically adapt and to develop mechanisms for O₂ sensing. Sensing of O₂ by [4Fe-4S]²⁺ to [2Fe-2S]²⁺ cluster conversion represents an ancient mechanism that is used by FNR_Ec (Escherichia coli), FNR_Bs (Bacillus subtilis), NreB_Sa (Staphylococcus aureus) and WhiB3_Mt (Mycobacterium tuberculosis). The phylogenetic relationship of these sensors was investigated. FNR_Ec homologues are restricted to the proteobacteria and a few representatives from other phyla. Homologues of FNR_Bs and NreB_Sa are located within the bacilli, of WhiB3 within the actinobacteria. Archaea contain no homologues. The data reveal no similarity between the FNR_Ec , FNR_Bs , NreB_Sa and WhiB3 sensor families on the sequence and structural levels. These O₂ sensor families arose independently in phyla that were already present at the time O₂ appeared, their members were subsequently distributed by lateral gene transfer. The chemistry of [4Fe-4S] and [2Fe-2S] cluster formation and interconversion appears to be shared by the sensor protein families. The type of signal output is, however, family specific. The homologues of FNR_Ec and NreB_Sa vary with regard to the number of Cys residues that coordinate the cluster. It is suggested that the variants derive from lateral gene transfer and gained other functions.

Fe-V sulfur clusters studied through photoelectron spectroscopy and density functional theory

Iron-vanadium sulfur cluster anions are studied by photoelectron spectroscopy (PES) at 3.492 eV (355 nm) and 4.661 eV (266 nm) photon energies, and by density functional theory (DFT) calculations. The structural properties, relative energies of different structural isomers, and the calculated first vertical detachment energies (VDEs) of different structural isomers for cluster anions FeVS1-3- and FemVnSm+n- (m + n = 3, 4; m > 0, n > 0) are investigated at a BPW91/TZVP theory level. The experimental first VDEs for these Fe-V sulfur clusters are reported. The most probable ground state structures and spin multiplicities for these clusters are tentatively assigned by comparing their theoretical and experiment first VDE values. For FeVS1-3- clusters, their first VDEs are generally observed to increase with the number of sulfur atoms from 1.45 eV to 2.86 eV. The NBO/HOMOs of the ground state of FeVS1-3- clusters are localized in a p orbital on a S atom; the partial charge distribution on the NBO/HOMO localized site of each cluster anion is responsible for the trend of their first VDEs. A less negative localized charge distribution is correlated with a higher first VDE. Structure and steric effect differences for FemVnSm+n- (m + n = 3, m > 0, n > 0) clusters are suggested to be responsible for their different first VDEs and properties. Two types of structural isomers are identified for FemVnSm+n- (m + n = 4, m > 0, n > 0) clusters: a tower structure isomer and a cubic structure isomer. The first VDEs for tower like isomers are generally higher than those for cubic like isomers of FemVnSm+n- (m + n = 4, m > 0, n > 0) clusters. Their first VDEs are can be understood through: (1) NBO/HOMO distributions, (2) structures (steric effects), and (3) partial charge numbers on the NBO/HOMO's localized sites. EBEs for excited state transitions for all Fe-V sulfur clusters are calculated employing OVGF and TDDFT approaches at the TZVP level. The OVGF approach for these Fe/V/S cluster anions is better for the higher transition energies than the TDDTF approach. The experimental and theoretical results for these Fe/V/S cluster anions are compared with their related pure iron sulfur cluster anions. Properties of the NBO/HOMO are essential for understanding and estimating the different first VDEs for Fe/V/S, and comparing them to those of the pure Fe/S cluster anions.

Photoelectron spectroscopy and density functional theory studies of (FeS)mH- (m = 2-4) cluster anions: effects of the single hydrogen

Single hydrogen containing iron hydrosulfide cluster anions (FeS)_mH^- (m = 2-4) are studied by photoelectron spectroscopy (PES) at 3.492 eV (355 nm) and 4.661 eV (266 nm) photon energies, and by Density Functional Theory (DFT) calculations. The structural properties, relative energies of different spin states and isomers, and the first calculated vertical detachment energies (VDEs) of different spin states for these (FeS)_mH^- (m = 2-4) cluster anions are investigated at various reasonable theory levels. Two types of structural isomers are found for these (FeS)_mH^- (m = 2-4) clusters: (1) the single hydrogen atom bonds to a sulfur site (SH-type); and (2) the single hydrogen atom bonds to an iron site (FeH-type). Experimental and theoretical results suggest such available different SH- and FeH-type structural isomers should be considered when evaluating the properties and behavior of these single hydrogen containing iron sulfide clusters in real chemical and biological systems. Compared to their related, respective pure iron sulfur (FeS)_m^- clusters, the first VDE trend of the diverse type (FeS)_mH_0,1^- (m = 1-4) clusters can be understood through (1) the different electron distribution properties of their highest singly occupied molecular orbital employing natural bond orbital analysis (NBO/HSOMO), and (2) the partial charge distribution on the NBO/HSOMO localized sites of each cluster anion. Generally, the properties of the NBO/HSOMOs play the principal role with regard to the physical and chemical properties of all the anions. The change of cluster VDE from low to high is associated with the change in nature of their NBO/HSOMO from a dipole bound and valence electron mixed character, to a valence p orbital on S, to a valence d orbital on Fe, and to a valence p orbital on Fe or an Fe-Fe delocalized valence bonding orbital. For clusters having the same properties for NBO/HSOMOs, the partial charge distributions at the NBO/HSOMO localized sites additionally affect their VDEs: a more negative or less positive localized charge distribution is correlated with a lower first VDE. The single hydrogen in these (FeS)_mH^- (m = 2-4) cluster anions is suggested to affect their first VDEs through the different structure types (SH- or FeH-), the nature of the NBO/HSOMOs at the local site, and the value of partial charge number at the local site of the NBO/HSOMO.

Synthesis, structural characterization and conversion of dinuclear iron-sulfur clusters containing the disulfide ligand: [Cp*Fe(μ-η2:η2-bdt)(cis-μ-η1:η1-S2)FeCp*], [Cp*Fe(μ-S(C6H4S2))(cis-μ-η1:η1-S2)FeCp*], and [{Cp*Fe(bdt)}2(trans-μ-η1:η1-S2)]

The treatment of [Cp*Fe(μ-η²:η⁴-bdt)FeCp*] (1, Cp* = η⁵-C₅Me₅, bdt = benzene-1,2-dithiolate) with 1/4 equiv. of elemental sulfur (S₈) gave a dinuclear iron-sulfur cluster [Cp*Fe(μ-η²:η²-bdt)(cis-μ-η¹:η¹-S₂)FeCp*] (2), which contains a cis-1,2-disulfide ligand. When complex 2 further interacted with 1/8 equiv. of S₈, another sulfur atom inserted into an Fe-S bond to give a rare product [Cp*Fe(μ-S(C₆H₄S₂))(cis-μ-η¹:η¹-S₂)FeCp*] (3). Unexpectedly, a trans-1,2 disulfide-bridged diiron complex [{Cp*Fe(bdt)}₂(trans-μ-η¹:η¹-S₂)] (4) was isolated from the reaction of complex 1 with 1/2 equiv. of S₈, which represents a structural isomer of [2Fe-2S] ferredoxin-type clusters. In addition, cis-1,2-disulfide-bridged complex 3 can slowly convert into trans-1,2-disulfide-bridged complex 4 and the complex [Cp*Fe(μ-η²:η²-S₂)(cis-μ-η¹:η¹-S₂)FeCp*] (5) by self-assembly reaction at ambient temperature, which is evidenced by time-dependent ¹H NMR spectroscopy.

Mechanisms of the S/CO/Se interchange reactions at FeMo-co, the active site cluster of nitrogenase

The active site of the N2 fixing enzyme nitrogenase is a C-centred Fe7MoS cluster (FeMo-co) containing a trigonal prism of six Fe atoms connected by a central belt of three doubly-bridging S atoms. The trigonal faces of the prism are capped via triply-bridging S atoms to Fe1 at one end and Mo at the other end. One of the central belt atoms, S2B, considered to be important in the chemical mechanism of the enzyme, has been shown by Spatzal, Rees et al. to undergo substitution by CO, and also substitution by Se in the presence of SeCN(-), under turnover conditions. Further, when turning over under C2H2 or N2/CO there is migration of Se to the other two belt bridging positions. These reactions are extraordinary, and unprecedented in metal chalcogenide cluster chemistry. Using density functional simulations, mechanisms for all of these reactions have been developed, involving the small molecules SCO, SeCO, C2H2S, C2H2Se, SeCN(-), SCN(-) functioning as carriers of S and Se atoms. The possibility that the S2B bridge position is vacant is discounted, because the barrier to formation of a bridge-void intermediate with two contiguous three-coordinate Fe atoms is too large. A bridging ligand is retained throughout the proposed mechanisms. Intermediates with Fe-C(O)-S/Se-Fe cycles and with SCO/SeCO C-bound to Fe are predicted. The energetics of the reaction trajectories show them to be feasible and easily reversible, consistent with experiment. Alternative mechanisms involving intramolecular differential rotatory rearrangements of the cluster to scramble the Se bridges are also examined, and shown to be very unlikely. The implications of these new facets of the reactivity of the FeMo-co cluster are discussed: it is considered that they are unlikely to be part of the mechanism of the physiological reactions of nitrogenase.

Synthesis of Diiron(I) Dithiolato Carbonyl Complexes

Virtually all organosulfur compounds react with Fe(0) carbonyls to give the title complexes. These reactions are reviewed in light of major advances over the past few decades, spurred by interest in Fe2(μ-SR)2(CO)x centers at the active sites of the [FeFe]-hydrogenase enzymes. The most useful synthetic route to Fe2(μ-SR)2(CO)6 involves the reaction of thiols with Fe2(CO)9 and Fe3(CO)12. Such reactions can proceed via mono-, di-, and triiron intermediates. The reactivity of Fe(0) carbonyls toward thiols is highly chemoselective, and the resulting dithiolato complexes are fairly rugged. Thus, many complexes tolerate further synthetic elaboration directed at the organic substituents. A second major route involves alkylation of Fe2(μ-S2)(CO)6, Fe2(μ-SH)2(CO)6, and Li2Fe2(μ-S)2(CO)6. This approach is especially useful for azadithiolates Fe2[(μ-SCH2)2NR](CO)6. Elaborate complexes arise via addition of the FeSH group to electrophilic alkenes, alkynes, and carbonyls. Although the first example of Fe2(μ-SR)2(CO)6 was prepared from ferrous reagents, ferrous compounds are infrequently used, although the Fe(II)(SR)2 + Fe(0) condensation reaction is promising. Almost invariably low-yielding, the reaction of Fe3(CO)12, S8, and a variety of unsaturated substrates results in C-H activation, affording otherwise inaccessible derivatives. Thiones and related C═S-containing reagents are highly reactive toward Fe(0), often giving complexes derived from substituted methanedithiolates and C-H activation.

A [3Fe–3S]3+ cluster with exclusively μ-sulfide donors

A [3Fe–3(μ-S)]³⁺ cluster is reported in which each ferric center has a distorted trigonal pyramidal geometry, with an S = 1/2 ground state for the cluster and unusually anisotropic hyperfine coupling constants as determined by variable temperature magnetometry and Mössbauer spectroscopy.

Crystal structure of [μ2-3,3-dimethyl-4-(propan-2-yl-idene)thietane-2,2-dithiol-ato-κ(4) S:S':S:S']bis[tricarbonyl-iron(I)](Fe-Fe)

The title complex, [Fe2(C8H12S3)(CO)6] or [{Fe(CO)3}2(μ-L)] [L = 3,3-dimethyl-4-(propan-2-yl-idene)thietane-2,2-bis-(thiol-ato)], consists of two Fe(CO)3 moieties double-bridged by a di-thiol-ate ligand. Each of the two Fe(I) atoms has a distorted anti-prismatic coordination environment consisting of three carbonyl groups, two S atoms of the di-thiol-ate ligand and the neighboring Fe(I) atom [Fe-Fe = 2.4921 (4) Å]. Weak C-H⋯O inter-molecular inter-actions result in the formation of dimers. This is the second crystal structure reported with the 3,3-dimethyl-4-(propan-2-yl-idene)thietane-2,2-bis-(thiol-ate) ligand and the first in which it bridges two metal atoms.

A Low-Valent Iron Imido Heterocubane Cluster: Reversible Electron Transfer and Catalysis of Selective C-C Couplings

Enzymes and cofactors with iron-sulfur heterocubane core structures, [Fe4 S4 ], are often found in nature as electron transfer reagents in fundamental catalytic transformations. An artificial heterocubane with a [Fe4 N4 ] core is reported that can reversibly store up to four electrons at very negative potentials. The neutral [Fe4 N4 ] and the singly reduced low-valent [Fe4 N4 ](-) heterocubanes were isolated and fully characterized. The low-valent species bears one unpaired electron, which is localized predominantly at one iron center in the electronic ground state but fluctuates with increasing temperatures. The electrons stored or released by the [Fe4 N4 ]/[Fe4 N4 ](-) redox couple can be used in reductive or oxidative CC couplings and even allow catalytic one-pot reactions, which show a remarkably enhanced selectivity in the presence of the [Fe4 N4 ] heterocubanes.

Reactivity of an All-Ferrous Iron-Nitrogen Heterocubane under Reductive and Oxidative Conditions

The reactivity of the all-ferrous FeN heterocubane [Fe4 (Ntrop)4 ] (1) with i) Brønsted acids, ii) σ-donors, iii) σ-donors/π-acceptors, and iv) one-electron oxidants has been investigated (trop = 5H-dibenzo[a,d]cyclo-hepten-5-yl). 1 showed self-re-assembling after reactions with i) and proved surprisingly inert in reactions with ii) and iii), with the exception of CO. Reductive and oxidative cluster degradation was observed in reactions with CO and TEMPO, respectively. These reactions yielded new cluster compounds, namely a trinuclear bis(μ3 -imido) 48 electron complex in the former case and a tetranuclear all ferric μ-oxo-μ-imido species in the latter case. Characterization techniques include NMR and in situ IR spectroscopy, single crystal X-ray analysis, Mössbauer spectroscopy, cyclic voltammetry, magnetic susceptibility measurements, and DFT calculations.

Synthesis, Characterization, and Reactivity of Functionalized Trinuclear Iron-Sulfur Clusters - A New Class of Bioinspired Hydrogenase Models

The air- and moisture-stable iron-sulfur carbonyl clusters Fe3S2(CO)7(dppm) (1) and Fe3S2(CO)7(dppf) (2) carrying the bisphosphine ligands bis(diphenylphosphanyl)methane (dppm) and 1,1'-bis(diphenylphosphanyl)ferrocene (dppf) were prepared and fully characterized. Two alternative synthetic routes based on different thionation reactions of triiron dodecacarbonyl were tested. The molecular structures of the methylene-bridged compound 1 and the ferrocene-functionalized derivative 2 were determined by single-crystal X-ray diffraction. The catalytic reactivity of the trinuclear iron-sulfur cluster core for proton reduction in solution at low overpotential was demonstrated. These deeply colored bisphosphine-bridged sulfur-capped iron carbonyl systems are discussed as promising candidates for the development of new bioinspired model compounds of iron-based hydrogenases.

Diiron Azamonothiolates via Scission of Dithiadiazacyclooctanes by Iron Carbonyls

The reaction of Fe₃(CO)₁₂ with the dithiadiazacyclooctanes [SCH₂N(R)CH₂]₂ affords Fe₂[SCH₂N(Me)CH₂](CO)₆ (R = Me, Bn). The methyl derivative 1^Me was characterized crystallographically (Fe-Fe = 2.5702(5) Å). Its low symmetry is verified by variable temperature ¹³C NMR spectroscopy which revealed that the turnstile rotation of the S(CH₂)Fe(CO)₃ and S(NMe)Fe(CO)₃ centers are subject to very different energy barriers. Although 1^Me resists protonation, it readily undergoes substitution by tertiary phosphines, first at the S(CH₂)Fe(CO)₃ center, as verified crystallographically for Fe₂[SCH₂N(Me)CH₂](CO)₅(PPh₃). Substitution by the chelating diphosphine dppe (Ph₂PCH₂CH₂PPh₂) gave Fe₂[SCH₂N(Me)CH₂](CO)₄(dppe), resulting from substitution at both the S(CH₂)Fe(CO)₃ and S(NMe)Fe(CO)₃ sites.

Lessons from Nature: A Bio-Inspired Approach to Molecular Design

Metalloproteins contain actives sites with intricate structures that perform specific functions with high selectivity and efficiency. The complexity of these systems complicates the study of their function and the understanding of the properties that give rise to their reactivity. One approach that has contributed to the current level of understanding of their biological function is the study of synthetic constructs that mimic one or more aspects of the native metalloproteins. These systems allow individual contributions to the structure and function to be analyzed and also permit spectroscopic characterization of the metal cofactors without complications from the protein environment. This Current Topic is a review of synthetic constructs as probes for understanding the biological activation of small molecules. These topics are developed from the perspective of seminal molecular design breakthroughs from the past that provide the foundation for the systems used today.

Electronic, magnetic structure and water splitting reactivity of the iron-sulfur dimers and their hexacarbonyl complexes: A density functional study

The iron sulfide dimers (FeS)2 and their persulfide isomers with S-S bonds are studied with the B3LYP density functional as bare clusters and as hexacarbonyls. The disulfides are more stable than the persulfides as bare clusters and the persulfide ground state lies at 3.2 eV above the global minimum, while in the hexacarbonyl complexes this order is reversed: persulfides are more stable, but the energy gap between disulfides and persulfides becomes much smaller and the activation barrier for the transition persulfide → disulfide is 1.11 eV. Carbonylation also favors a non-planar Fe2S2 ring for both the disulfides and the persulfides and high electron density in the Fe2S2 core is induced. The diamagnetic ordering is preferred in the hexacarbonyls, unlike the bare clusters. The hexacarbonyls possess low-lying triplet excited states. In the persulfide, the lowest singlet-to-triplet state excitation occurs by electron transition from the iron centers to an orbital located predominantly at S2 via metal-to-ligand charge transfer. In the disulfide this excitation corresponds to ligand-to-metal charge transfer from the sulfur atoms to an orbital located at the iron centers and the Fe-Fe bond. Water splitting occurs on the hexacarbonyls, but not on the bare clusters. The singlet and triplet state reaction paths were examined and activation barriers were determined: 50 kJ mol(-1) for HO-H bond dissociation and 210 kJ mol(-1) for hydrogen evolution from the intermediate sulfoxyl-hydroxyl complexes Fe2S(OH)(SH)(CO)6 formed. The lowest singlet-singlet excitations in the hexacarbonyls, the water adsorption complexes and in the reaction intermediates, formed prior to dihydrogen release, fall in the visible light region. The energy barrier of 210 kJ mol(-1) for the release of one hydrogen molecule corresponds to one visible photon of 570 nm. The dissociation of a second water molecule, followed by H2 and O2 release via hydro-peroxide intermediate is a two-step process, with activation barriers of 218 and 233 kJ mol(-1), which also fall in the visible light region. A comparison of the full reaction path with that on diiron dioxide hexacarbonyls Fe2O2(CO)6 is traced.

An overview of selected current approaches to the characterization of aqueous inorganic clusters

This Perspective highlights several modern characterization techniques used to identify nanoscale inorganic clusters in the context of multiple case studies.

μ2-Acetato-κ(2) O:O'-(4,4'-bipyridyl-κN)tris-(diethyl di-thio-phosphato-κ(2) S,S')-μ3-sulfido-tri-μ2-sulfido-trimolybdenum(IV) diethyl ether monosolvate

In the title compound, [Mo₃(CH₃COO)(C₄H₁₀O₂PS₂)₃S₄(C₁₀H₈N₂)]·C₄H₁₀O, the complex mol­ecule has a trinuclear incomplete cuboidal structure which is coordinated by three kinds of ligands, namely, diethyl di­thio­phosphate, acetate and 4,4′-bipyridyl. If Mo—Mo bonds are ignored, each Mo atom can be considered as six-coordinated in a distorted octa­hedral geometry. The Mo—Mo distance of 2.6880 (5) Å for two the Mo atoms bridged by the acetate ligand is shorter than the other two Mo—Mo distances [2.7490 (5) and 2.7566 (5) Å]. One ethyl group is disordered between two conformations in a 0.65 (3):0.35 (3) ratio. In the crystal, weak C—H⋯O inter­actions link the trinuclear mol­ecules related by translation in [100] into chains. The crystal packing exhibits short inter­molecular S⋯S contacts of 3.1886 (13) Å. In other words, in this crystal packing, a supramolecular structure is constructed by the C—H⋯O and S⋯S interactions.