Reduced mono-, di-, and tetracubane-type clusters containing the [MoFe3S4]2+ core stabilized by tertiary phosphine ligation

An Iron-Sulfur Cluster with a Highly Pyramidalized Three-Coordinate Iron Center and a Negligible Affinity for Dinitrogen

Attempts to generate open coordination sites for N2 binding at synthetic Fe-S clusters often instead result in cluster oligomerization. Recently, it was shown for Mo-Fe-S clusters that such oligomerization reactions can be prevented through the use of sterically protective supporting ligands, thereby enabling N2 complex formation. Here, this strategy is extended to Fe-only Fe-S clusters. One-electron reduction of (IMes)3Fe4S4Cl (IMes = 1,3-dimesitylimidazol-2-ylidene) forms the transiently stable edge-bridged double cubane (IMes)6Fe8S8, which loses two IMes ligands to form the face-bridged double-cubane, (IMes)4Fe8S8. The finding that the three supporting IMes ligands do not confer sufficient protection to curtail cluster oligomerization prompted the design of a new N-heterocyclic carbene, SIArMe,iPr (1,3-bis(3,5-diisopropyl-2,6-dimethylphenyl)-2-imidazolidinylidene; abbreviated as SIAr), that features bulky groups strategically placed in remote positions. When the reduction of (SIAr)3Fe4S4Cl or [(SIAr)3Fe4S4(THF)]+ is conducted in the presence of SIAr, the formation of (SIAr)4Fe8S8 is indeed suppressed, permitting characterization of the reduced [Fe4S4]0 product. Surprisingly, rather than being an N2 complex, the product is simply (SIAr)3Fe4S4: a cluster with a three-coordinate Fe site that adopts an unusually pyramidalized geometry. Although (SIAr)3Fe4S4 does not coordinate N2 to any appreciable extent under the surveyed conditions, it does bind CO to form (SIAr)3Fe4S4(CO). This finding demonstates that the binding pocket at the unique Fe is not too small for N2; instead, the exceptionally weak affinity for N2 can be attributed to weak Fe-N2 bonding. The differences in the N2 coordination chemistry between sterically protected Mo-Fe-S clusters and Fe-only Fe-S clusters are discussed.

Cubane-type tungsten-iron-sulfur clusters with a nitrogen atom in the core: terminal ligand substitutions and redox behaviors

The lack of M-Fe-S (M = Mo or W) clusters incorporating a second period (2p) atom in the core has resulted in limited investigations and poor understanding of the physical and chemical properties of the M-Fe-S clusters closely related to the FeMo cofactor. In this work, systematic studies have been carried out to explore the chemical reactivities at the terminal ligand sites and the redox properties of a series of clusters comprising a [WFe₃S₃N] cubane core, based on the previously developed cluster [(Tp*)WFe₃S₃(μ₃-NSiMe₃)Cl₃]^1-. Substitutions of the terminal chlorides with ethanethiolate, methanethiolate, thiophenolate, p-thiocresolate and azide occurred smoothly, while the replacement of the chlorides with carbene ligands required the reduction of the precursor into [(Tp*)WFe₃S₃(μ₃-NSiMe₃)Cl₃]^2- first. The reduced cluster core could also be supported by thiophenolates as terminal ligands, but not thiolates or azides. It is remarkable that the thiophenolate ligated reduced cluster can be synthesized from the precursor [(Tp*)WFe₃S₃(μ₃-NSiMe₃)Cl₃]^1-via different synthetic routes, either reduction followed by substitution or substitution followed by reduction, either in situ or stepwise. This work indicates that terminal ligands contribute significantly to determine the chemical and physical properties of the clusters, even though they might affect the cluster core to a limited extent from a structural point of view, which raises the possibility of delicate control in regulating the physical/chemical properties of M-Fe-S clusters with a heteroleptic core incorporating 2p atom(s).

Structural insight into halide-coordinated [Fe4S4XnY4-n]2- clusters (X, Y = Cl, Br, I) by XRD and Mössbauer spectroscopy

Iron sulphur halide clusters [Fe₄S₄Br₄]^2- and [Fe₄S₄X₂Y₂]^2- (X, Y = Cl, Br, I) were obtained in excellent yields (77 to 78%) and purity from [Fe(CO)₅], elemental sulphur, I₂ and benzyltrimethylammonium (BTMA⁺) iodide, bromide and chloride. Single crystals of (BTMA)₂[Fe₄S₄Br₄] (1), (BTMA)₂[Fe₄S₄Br₂Cl₂] (2), (BTMA)₂[Fe₄S₄Cl₂I₂] (3), and (BTMA)₂[Fe₄S₄Br₂I₂] (4) were isostructural to the previously reported (BTMA)₂[Fe₄S₄I₄] (5) (monoclinic, Cc). Instead of the chloride cubane cluster [Fe₄S₄Cl₄]^2-, we found the prismane-shaped cluster (BTMA)₃[Fe₆S₆Cl₆] (6) (P1̄). ⁵⁷Fe Mössbauer spectroscopy indicates complete delocalisation with Fe^2.5+ oxidation states for all iron atoms. Magnetic measurements showed small χ_MT values at 298 K ranging from 1.12 to 1.54 cm³ K mol^-1, indicating the dominant antiferromagnetic exchange interactions. With decreasing temperature, the χ_MT values decreased to reach a plateau at around 100 K. From about 20 K, the values drop significantly. Fitting the data in the Heisenberg-Dirac-van Vleck (HDvV) as well as the Heisenberg Double Exchange (HDE) formalism confirmed the delocalisation and antiferromagnetic coupling assumed from Mössbauer spectroscopy.

Selenium as a structural surrogate of sulfur: template-assisted assembly of five types of tungsten-iron-sulfur/selenium clusters and the structural fate of chalcogenide reactants

Syntheses of five types of tungsten-iron-sulfur/selenium clusters, namely, incomplete cubanes, single cubanes, edge-bridged double cubanes (EBDCs), P(N)-type clusters, and double-cuboidal clusters, have been devised using the concept of template-assisted assembly. The template reactant is six-coordinate [(Tp*)W(VI)S(3)](1-) [Tp* = tris(3,5-dimethylpyrazolyl)hydroborate(1-)], which in the assembly systems organizes Fe(2+/3+) and sulfide/selenide into cuboidal [(Tp*)WFe(2)S(3)] or cubane [(Tp*)WFe(3)S(3)Q] (Q = S, Se) units. With appropriate terminal iron ligation, these units are capable of independent existence or may be transformed into higher-nuclearity species. Selenide is used as a surrogate for sulfide in cluster assembly in order to determine by X-ray structures the position occupied by an external chalcogenide nucleophile or an internal chalcogenide atom in the product clusters. Specific incorporation of selenide is demonstrated by the formation of [WFe(3)S(3)Se](2+/3+) cubane cores. Reductive dimerization of the cubane leads to the EBDC core [W(2)Fe(6)S(6)Se(2)](2+) containing μ(4)-Se sites. Reaction of these species with HSe(-) affords the P(N)-type cores [W(2)Fe(6)S(6)Se(3)](1+), in which selenide occupies μ(6)-Se and μ(2)-Se sites. The reaction of [(Tp*)WS(3)](1-), FeCl(2), and Na(2)Se yields the double-cuboidal [W(2)Fe(4)S(6)Se(3)](2+/0) core with μ(2)-Se and μ(4)-Se bridges. It is highly probable that in analogous sulfide-only assembly systems, external and internal sulfide reactants occupy corresponding positions in the cluster products. The results further demonstrate the viability of template-assisted cluster synthesis inasmuch as the reduced (Tp*)WS(3) unit is present in all of the clusters. Structures, zero-field Mössbauer data, and redox potentials are presented for each cluster type.

Remote effects of axial ligand substitution in heterometallic Cr≡Cr···M chains

The heterometallic complexes CrCrM(dpa)(4)Cl(2) (dpa = 2,2'-dipyridylamide) featuring linear Cl-Cr≡Cr···M-Cl chains can regiospecifically be modified via axial ligand substitution to yield OTf-Cr≡Cr···M-Cl chains (OTf = triflate) with M being Fe, Mn, or Co. The effect of OTf substitution on the Cr side of the molecule has an unusual and profound structural impact on the square-pyramidal transition metal M. Specifically, elongation of the four equatorial M-N(py) bonds and the axial M-Cl bonds by 0.03 and 0.09 Å for Fe and 0.07 and 0.11 Å for Mn is observed. The longer M-Cl and M-N(py) bonds result from subtle interactions between the equatorial dpa ligand and the three metal ions. The equatorial dpa ligand responds to the introduction of the more labile OTf ligand at Cr by binding more strongly to this Cr ion which in turn weakens bonding to M. The ligand field experienced by M can be tuned by changing the Cr axial ligand, and this effect is observed in electrochemical measurements of the iron compounds.

The [MoFe3S4]2+ oxidation state: synthesis, substitution reactions, and structures of phosphine-ligated cubane-type clusters with the S=2 ground state

The cluster [(Tp)MoFe(3)S(4)(PEt(3))(3)](1+) containing the cubane-type [MoFe(3)(μ(3)-S)(4)](2+) reduced core undergoes facile ligand substitution reactions at the iron sites leading to an extensive set of mono- and disubstituted species [(Tp)MoFe(3)S(4)(PEt(3))(3-n)L(n)](1-n) with L = halide, N(3)(-), PhS(-), PhSe(-), R(3)SiO(-), and R(3)SiS(-) and n = 1 and 2. Structures of 10 members of the set are reported. For two representative clusters, Curie behavior at 2-20 K indicates a spin-quintet ground state. Zero-field Mössbauer spectra consist of two doublets in a 2:1 intensity ratio. (57)Fe isomer shifts are consistent with the mean oxidation state Fe(3)(2.33+) arising from electron delocalization of the mixed-valence oxidation state description [Mo(3+)Fe(3+)Fe(2+)(2)]. Reaction of [(Tp)MoFe(3)S(4)(PEt(3))(2)Cl] with (Me(3)Si)(2)S affords [(Tp)MoFe(3)S(4)(PEt(3))(2)(SSiMe(3))], a likely first intermediate in the formation of the tricluster compound {[(Tp)MoFe(3)S(4)(PEt)(2)](3)S}(BPh(4)) from the reaction of [(Tp)MoFe(3)S(4)(PEt(3))(3)](BPh(4)) and NaSSiMe(3) in tetrahydrofuran (THF). The tricluster consists of three cluster units bound to a central μ(3)-S atom in a species of overall C(3) symmetry. Relatively few clusters in the [MoFe(3)S(4)](2+) oxidation state have been prepared compared to the abundance of clusters in the oxidized [MoFe(3)S(4)](3+) state. This work is the first comprehensive study of the [MoFe(3)S(4)](2+) state, one conspicuous feature of which is its ability to bind hard and soft σ-donors and strong to weak π-acid ligands. (Tp = tris(pyrazolyl)hydroborate(1-)).

Cubane-type Co4S4 clusters: synthesis, redox series, and magnetic ground states

The recent demonstration that the carbene cluster [Fe(4)S(4)(Pr(i)(2)NHCMe(2))(4)] (9) is an accurate structural and electronic analogue of the fully reduced cluster of the iron protein of Azotobacter vinelandii nitrogenase, including a common S = 4 ground state, raises the issue of the existence and magnetism of other [M(4)S(4)L(4)](z) clusters, none of which are known with transition metals other than iron. The system CoCl(2)/Pr(i)(3)P/(Me(3)Si)(2)S/THF assembles [Co(4)S(4)(PPr(i)(3))(4)] (3), which is converted to [Co(4)S(4)(Pr(i)(2)NHCMe(2))(4)] (5) upon reaction with carbene. The clusters support the redox series [3](1-/0/1+) and [5](0/1+/2+); monocations (4, 6) have been isolated by chemical oxidation. Redox potentials and substitution reactions indicate that the carbene is the more effective electron donor to tetrahedral Fe(II) and Co(II) sites. Clusters 3-6 have the same overall cubane-type geometry as 9. Neutral clusters 3 and 5 have an S = 3 ground state. As with the S = 4 state of 9 with local spins S(Fe) = 2, the septet spin state can be described in terms of the coupling of three parallel and one antiparallel spins S(Co) = 3/2. The octanuclear clusters [Co(8)S(8)(PPr(i)(3))(6)](0,1+) were isolated as minor byproducts of the formation and chemical oxidation of 3. The clusters exhibit a rhomb-bridged noncubane (RBNC) structure, whereas clusters with the Fe(8)S(8) core possess edge-bridged double-cubane (EBDC) stereochemistry. There are two structural solutions for the M(8)S(8) core in the form of topological isomers whose stability may depend on valence electron count. A conceptual model for the RBNC <--> EBDC interconversion is presented. (Pr(i)(2)NHCMe(2) = C(11)H(20)N(2) = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene).

VFe3S4 single and double cubane clusters: synthesis, structures, and dependence of redox potentials and electron distribution on ligation and heterometal

Both vanadium and molybdenum cofactor clusters are found in nitrogenase. In biomimetic research, many fewer heterometal MFe3S4 cubane-type clusters have been synthesized with M = V than with M = Mo because of the well-established structural relationship of the latter to the molybdenum coordination unit in the enzyme. In this work, a series of single cubane and edge-bridged double cubane clusters containing the cores [VFe3(mu3-S)4]2+ and [V2Fe6(mu3-S)6(mu4-S)2]2+ have been prepared by ligand substitution of the phosphine clusters [(Tp)VFe3S4(PEt3)3]1+ and [(Tp)2V2Fe6S8(PEt3)4]. The single cubanes [(Tp)VFe3S4L3]2- and double cubanes [(Tp)2V2Fe6S8L4]4- (L= F-, N3-, CN-, PhS-) are shown by X-ray structures to have trigonal symmetry and centrosymmetry, respectively. Single cubanes form the three-member electron transfer series [(Tp)VFe3S4L3]3-,2-,1-. The ligand dependence of redox potentials and electron distribution in cluster cores as sensed by 57Fe isomer shifts (delta) have been determined. Comparison of these results with those previously determined for the analogous molybdenum clusters (Pesavento, Berlinguette, and Holm Inorg. Chem. 2007, 46, 510) allows detection of the influence of heterometal M on the properties. At constant M and variable L, redox potentials are lowest for pi-donor ligands and largest for cyanide and relate approximately with decreasing ferrous character in clusters with constant charge z = 2-. At constant L and z and variable M, EV > E(Mo) and delta(av)V < delta(av)Mo, demonstrating that M = Mo clusters are more readily oxidized and suggesting a qualitative relation between lower potentials (greater ease of oxidation) and ferrous character.

Precursors to clusters with the topology of the P(N) cluster of nitrogenase: edge-bridged double cubane clusters [(Tp)2Mo2Fe6S8L4]z: synthesis, structures, and electron transfer series

Members of the cluster set [(Tp)2Mo2Fe6S8L4]z contain the core unit M2Fe6(mu3-S)6(mu4-S)2 in which two MoFe3S4 cubanes are coupled by two Fe-(mu4-S) interactions to form a centrosymmetric edge-bridged double cubane cluster. Some of these clusters are synthetic precursors to [(Tp)2Mo2Fe6S9L2]3-, which possess the same core topology as the P(N) cluster of nitrogenase. In this work, the existence of a three-member electron-transfer series of single cubanes [(Tp)MoFe3S4L3](z) (z = 3-, 2-, 1-) and a four-member series of double cubanes [(Tp)2Mo2Fe6S8L4]z (z = 4-, 3-, 2-, 1-) with L = F-, Cl-, N3, PhS- is demonstrated by electrochemical methods, cluster synthesis, and X-ray structure determinations. The potential of the [4-/3-] couple is extremely low (<-1.5 V vs SCE in acetonitrile) such that the 4- state cannot be maintained in solution under normal anaerobic conditions. The chloride double cubane redox series was examined in detail. The members [(Tp)2Mo2Fe6S8Cl4]4-,3-,2- were isolated and structurally characterized. The redox series includes the reversible steps [4-/3-] and [3-/2-]. Under oxidizing conditions, [(Tp)2Mo2Fe6S8Cl4]2- cleaves with the formation of single cubane [(Tp)MoFe3S4Cl3]1-. The quasireversible [2-/1-] couple is observed at more positive potentials than those of the single cubane redox step. Structure comparison of nine double cubanes suggests that significant dimensional changes pursuant to redox reactions are mainly confined to the Fe2(mu4-S)2 bridge rhomb. The synthesis and structure of [(Tp)2Mo2Fe6S9F2.H2O]3-, a new topological analogue of the P(N) cluster of nitrogenase, is described. (Tp = hydrotris(pyrazolyl)borate(1-)).

Stabilization of reduced molybdenum-iron-sulfur single- and double-cubane clusters by cyanide ligation

Recent work has shown that cyanide ligation increases the redox potentials of Fe(4)S(4) clusters, enabling the isolation of [Fe(4)S(4)(CN)4]4-, the first synthetic Fe(4)S(4) cluster obtained in the all-ferrous oxidation state (Scott, T. A.; Berlinguette, C. P.; Holm, R. H.; Zhou, H.-C. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 9741). The generality of reduced cluster stabilization has been examined with MoFe(3)S(4) clusters. Reaction of single-cubane [(Tp)MoFe(3)S(4)(PEt(3))3]1+ and edge-bridged double-cubane [(Tp)2Mo(2)Fe(6)S(8)(PEt(3))4] with cyanide in acetonitrile affords [(Tp)MoFe(3)S(4)(CN)3]2- (2) and [(Tp)2Mo(2)Fe(6)S(8)(CN)4]4- (5), respectively. Reduction of 2 with KC(14)H(10) yields [(Tp)MoFe(3)S(4)(CN)3]3- (3). Clusters were isolated in approximately 70-90% yields as Et(4)N+ or Bu(4)N+ salts; clusters 3 and 5 contain all-ferrous cores, and 3 is the first [MoFe(3)S(4)]1+ cluster isolated in substance. The structures of 2 and 3 are very similar; the volume of the reduced cluster core is slightly larger (2.5%), a usual effect upon reduction of cubane-type Fe(4)S(4) and MFe(3)S(4) clusters. Redox potentials and 57Fe isomer shifts of [(Tp)MoFe(3)S(4)L3]2-,3- and [(Tp)2Mo(2)Fe(6)S(8)L(4)]4-,3- clusters with L = CN-, PhS-, halide, and PEt3 are compared. Clusters with pi-donor ligands (L = halide, PhS) exhibit larger isomer shifts and lower (more negative) redox potentials, while pi-acceptor ligands (L = CN, PEt3) induce smaller isomer shifts and higher (less-negative) redox potentials. When the potentials of 3/2 and [(Tp)MoFe(3)S(4)(SPh)3]3-/2- are compared, cyanide stabilizes 3 by 270 mV versus the reduced thiolate cluster, commensurate with the 310 mV stabilization of [Fe(4)S(4)(CN)4]4- versus [Fe(4)S(4)(SPh)4]4- where four ligands differ. These results demonstrate the efficacy of cyanide stabilization of lower cluster oxidation states. (Tp = hydrotris(pyrazolyl)borate(1-)).

Borohydride Anions as Terminal Ligands on a Fe/Mo/S Cluster. Synthesis, Structure, and Characterization of the [(Cl4-cat)(PPr3)MoFe3S4(BH4)2]2(Bu4N)4 Double-Fused Cubane

The synthesis and structure of the first Mo/Fe/S/BH(4) cluster is reported. Reaction of (Cl(4)-cat)(2)Mo(2)Fe(6)S(8)(PPr(3))(6) with 4 equiv of Bu(4)NBH(4) results in the formation of [(Cl(4)-cat)(PPr(3))MoFe(3)S(4)(BH(4))(2)](2)(Bu(4)N)(4) (Cl(4)-cat = tetrachloro-catecholate) which has been fully characterized. X-ray structural determination of this double-fused cubane reveals four BH(4)(-) ligands bound to four Fe atoms in a bidentate fashion. A synopsis of the solution characterization as well as the reactivity of this cluster is also presented.

Application of a universal force field to mixed Fe/Mo-S/Se cubane and heterocubane clusters. 2. Substitution of iron by molybdenum in Fe4(S/Se)4 clusters with terminal halide and thiolate ligands

Infrared and Raman spectra of Fe(4)(S/Se)(4) clusters with terminal halide ligands and MoFe(3)S(4) clusters with terminal thiolate and halide ligands are presented and interpreted on the basis of the force fields determined in the accompanying paper. The Raman spectra of halide coordinated Fe(4)(S/Se)(4) clusters are characterized by the fact that vibrations of the terminal ligands appear with little or vanishing intensity. Infrared and Raman spectra of MoFe(3)S(4) clusters with terminal thiolates are correlated to those of corresponding Fe(4)S(4) systems, which were investigated in part 1 of this study and interpreted with normal coordinate analysis. Band assignments are checked by employing MoFeS(4) clusters with terminal halide ligands. Spectra of these systems are in turn compared to those of their Fe(4)S(4) counterparts, i.e., Fe-S cubane clusters with chloro, bromo, and iodo ligands. A consistent interpretation of all spectra is presented. General implications of these results are discussed.

Structural conversions of molybdenum-iron-sulfur edge-bridged double cubanes and P(n)-type clusters topologically related to the nitrogenase P-cluster

Edge-bridged Mo-Fe-S double cubanes are versatile precursors for the synthesis of other clusters of the same nuclearity. Thus, the double cubane [(Tp)(2)Mo(2)Fe(6)S(8)(PEt(3))(4)] sustains terminal ligand substitution with retention of the Mo(2)Fe(6)(micro(3)-S)(6)(micro(4)-S)(2) core structure and rearrangement to the Mo(2)Fe(6)(micro(2)-S)(2)(micro(3)-S)(6)(micro(6)-S) topology of the nitrogenase P(N) cluster upon reaction with certain nucleophiles. Four distinct processes for the conversion of double cubanes to P(N)-type clusters are documented, affording the products [(Tp)(2)Mo(2)Fe(6)S(9)(SR)(2)](3)(-), [(Tp)(2)Mo(2)Fe(6)S(8)(OMe)(3)](3)(-), and [(Tp)(2)Mo(2)Fe(6)S(7)(OMe)(4)](2)(-). In the latter clusters, two methoxides are terminal ligands and one or two are micro(2)-bridging ligands. The reverse transformation of a P(N)-type cluster to an edge-bridged double cubane has been demonstrated by the reaction of [(Tp)(2)Mo(2)Fe(6)S(8)(OMe)(3)](3)(-) with Me(3)SiX to afford [(Tp)(2)Mo(2)Fe(6)S(8)X(4)](2)(-) (X = Cl(-), Br(-)). Edge-bridged double cubanes have been obtained in the oxidation states [Mo(2)Fe(6)S(8)](2+,3+,4+). The stable oxidation state of P(N)-type clusters is [Mo(2)Fe(6)S(9)](+). Structures of five double cubanes and four P(N)-type clusters are reported. The P(N)-type clusters are synthetic representations of the biologically unique topology of the native P(N) cluster. Best-fit superpositions of the native and synthetic cluster cores gives weighted rms deviations in atom positions of 0.20-0.38 A. This study and an earlier investigation (Zhang, Y.; Holm, R. H. J. Am. Chem. Soc. 2003, 125, 3910-3920) provide a comprehensive account of the synthesis of structural analogues of the native P(N) cluster and provide the basis for continuing investigation of the synthesis of weak-field Mo-Fe-S clusters related to nitrogenase. (Tp = tris(pyrazolyl)hydroborate(1-).)

Vanadium-iron-sulfur clusters containing the cubane-type [VFe3S4] core unit: synthesis of a cluster with the topology of the PN cluster of nitrogenase

A synthetic method affording a topological analogue of the electron-transfer P-cluster of nitrogenase (Fe(8)S(7)(mu(2)-S(Cys))(2)) in the P(N) state has been devised, based in part on our previous development of cubane-type VFe(3)S(4) clusters (Hauser, C.; Bill, E.; Holm, R. H. Inorg. Chem. 2002, 41, 1615-1624). The cluster [(Tp)VFe(3)S(4)Cl(3)](2-) (1) is converted to [(Tp)VFe(3)S(4)(PR(3))(3)](1+) (R = Et (2), Bu (3)) by reaction with R(3)P. The phosphine ligands are readily substituted, leading to [(Tp)VFe(3)S(4)(SR)(3)](2-) (R = Ph (4), H (5)). Reduction of 2 or 3 with cobaltocene produces the edge-bridged double cubanes [(Tp)(2)V(2)Fe(6)S(8)(PR(3))(4)] (R = Et (6), Bu (7)), which are readily converted to [(Tp)(2)V(2)Fe(6)S(8)(SPh)(4)](4-) (8). The structures of clusters 3-5 and 8 were proven crystallographically. Cluster 8 has the double-cubane structure previously shown for 6, in which two cubane units are bridged by two Fe-(mu(4)-S) bonds. (57)Fe isomer shifts are consistent with the formulation [VFe(2.33+)(3)S(4)](2+) for the single cubanes and the all-ferrous description 2[VFe(2+)(3)S(4)](1+) for the double cubanes. Reaction of 6 with 4 equiv of (Et(4)N)(HS) in acetonitrile results in the insertion of sulfide with concomitant structural rearrangement and the formation of [(Tp)(2)V(2)Fe(6)S(9)(SH)(2)](4-) (10), obtained in ca. 50% yield as the Et(4)N(+) salt. The cluster has C(2) symmetry, with two all-ferrous VFe(3)S(4) fragments bridged by a common mu(6)-S atom and two mu(2)-S atoms that simulate the bridging atoms in the two Fe-(mu(2)-S(Cys))-Fe bridges of the P(N) cluster. The bridge pattern V(2)Fe(6)(mu(2)-S)(2)(mu(3)-S)(6)(mu(6)-S) and cluster shape match those of the native cluster. A best-fit superposition of the cores of 10 and the P(N) cluster affords a weighted rms deviation in atom positions of 0.33 A. Cluster 10 and [(Tp)(2)Mo(2)Fe(6)S(9)(SH)(2)](3-), prepared by a related route (Zhang, Y.; Holm, R. H. J. Am. Chem. Soc. 2003, 125, 3910-3920), demonstrate that the topology of the P(N) cluster can be achieved in molecular form in the absence of protein structure (Tp = tris(pyrazolyl)hydroborate).

Synthesis of a molecular Mo2Fe6S9 cluster with the topology of the PN cluster of nitrogenase by rearrangement of an edge-bridged Mo2Fe6S8 double cubane

The structures of the P cluster and cofactor cluster of nitrogenase are well-defined crystallographically. They have been obtained only by biosynthesis; their chemical synthesis remains a challenge. Synthetic routes are sought to the P cluster in the P(N) state in which two cuboidal Fe(3)S(3) units are connected by a mu(6)-S atom and two Fe-(mu(2)-S(Cys))-Fe bridges. A reaction scheme affording a Mo(2)Fe(6)S(9) cluster in molecular form having the topology of the P(N) cluster has been devised. Reaction of the single cubane [(Tp)MoFe(3)S(4)Cl(3)](1)(-) with PEt(3) gives [(Tp)MoFe(3)S(4)(PEt(3))(3)](1+) (2), which upon reduction with BH(4)(-) affords the edge-bridged all-ferrous double cubane [(Tp)(2)Mo(2)Fe(6)S(8)(PEt(3))(4)] (4) (Tp = tris(pyrazolylhydroborate(1-)). Treatment of 4 with 3 equiv of HS(-) produces [(Tp)(2)Mo(2)Fe(6)S(9)(SH)(2)](3)(-) (7) as the Et(4)N(+) salt in 86% yield. The structure of 7 is built of two (Tp)MoFe(3)(mu(3)-S)(3) cuboidal fragments bridged by two mu(2)-S atoms and one mu(6)-S atom in an arrangement of idealized C(2) symmetry. The cluster undergoes three one-electron oxidation reactions and is oxidatively cleaved by p-tolylthiol to [(Tp)MoFe(3)S(4)(S-p-tol)(3)](2)(-) and by weak acids to [(Tp)MoFe(3)S(4)(SH)(3)](2-). The cluster core of 7 has the bridging pattern [Mo(2)Fe(6)(mu(2)-S)(2)(mu(3)-S)(6)(mu(6)-S)](1+) with the probable charge distribution [Mo(3+)(2)Fe(2+)(5)Fe(3+)S(9)](1+). Cluster 7 is a topological analogue of the P(N) cluster but differs in having two heteroatoms and two Fe-(mu(2)-S)-Fe instead of two Fe-(mu(2)-S(Cys))-Fe bridges. A best-fit superposition of the two cluster cores affords a weighted rms deviation in atom positions of 0.38 A. Cluster 7 is the first molecular topological analogue of the P(N) cluster. This structure had been prepared previously only as a fragment of complex high-nuclearity Mo-Fe-S clusters.

Synthesis and reactions of cubane-type iron-sulfur-phosphine clusters, including soluble clusters of nuclearities 8 and 16

A family of soluble, reduced iron-sulfur clusters with nuclearities 4, 8, and 16 having tertiary phosphine ligation and based on the Fe(4)S(4) cubane-type structural motif has been synthesized. The results of this investigation substantially extend and improve the results of our original work on iron-sulfur-phosphine clusters (Goh, C.; Segal, B. M.; Huang, J.; Long, J. R.; Holm, R. H. J. Am. Chem. Soc. 1996, 118, 11844). A general property of this cluster family is facile phosphine substitution. The clusters [Fe(4)S(4)(PR(3))(4)](+) are precursors to monosubstituted [Fe(4)S(4)(PR(3))(3)X] (X = Cl-, RS-), homoleptic [Fe(4)S(4)(SR)(4)](3-), and all-ferrous monocubanes [Fe(4)S(4)(PR(3))(4)] (R = Pr(i), Cy, Bu(t); generated in solution). In turn, [Fe(4)S(4)(PPr(i)()(3))(3)(SSiPh(3))] and [Fe(4)S(4)(PPr(i)(3))(4)] can be transformed into the dicubanes [Fe(8)S(8)(PPr(i)()(3))(4)(SSiPh(3))(2)] and [Fe(8)S(8)(PPr(i)((3))(6)], respectively. Further, the tetracubanes [Fe(16)S(16)(PR(3))(8)] are also accessible from [Fe(4)S(4)(PR(3))(4)] under different conditions. X-ray structures are described for [Fe(4)S(4)(PCy(3))(3)X] (X = Cl-, PhS-), [Fe(8)S(8)(PPr(i)(3))(4)(SSiPh(3))(2)], [Fe(8)S(8)(PPr(i)()(3))(6)], and [Fe(16)S(16)(PCy(3))(8)]. The monosubstituted clusters show different distortions of the [Fe(4)S(4)](+) cores from idealized cubic symmetry. The dicubanes possess edge-bridged double cubane structures with an Fe(2)(mu(4)-S)(2) bridge rhomb and idealized C(2)(h)() symmetry. The ready cleavage of these clusters into single cubanes is considered a probable consequence of strained bond angles at the mu(4)-S atoms. Tetracubanes contain four individual cubanes, each of which is implicated in two bridge rhombs so as to generate a cyclic structure of idealized D(4) symmetry. Redox properties and Mössbauer spectroscopic parameters are reported. The species [Fe(4)S(4)(PR(3))(4)] (in solution), [Fe(8)S(8)(PR(3))(6)], and [Fe(16)S(16)(PR(3))(8)] are the only synthetic all-ferrous clusters with tetrahedral iron sites that have been isolated. Their utility as precursors to other highly reduced iron-sulfur clusters is under investigation.

Rearrangement of symmetrical dicubane clusters into topological analogues of the P cluster of nitrogenase: nature's choice?

Reaction schemes have been developed that lead to clusters having the topology of the PN cluster of nitrogenase. The single cubane clusters [(Tp)MFe3S4Cl3]z (M = Mo, z = 1-; M = V, z = 2-) react with PEt3 to give [(Tp)MFe3S4(PEt3)3]1+, which are reduced to the neutral edge-bridged double cubanes [(Tp)2M2Fe6S8(PEt3)4] with highly reduced (2[MFe3S4]1+) cores. Reaction of these clusters in acetonitrile with (Et4N)(HS) results in the formation of [(Tp)2Mo2Fe6S9(SH)2]3- and [(Tp)2V2Fe6S9(SH)2]4-. X-ray structures of the Et4N+ salts reveal the bridging pattern M2Fe6(mu2-S)2(mu3-S)6(mu6-S) in which two cuboidal MFe3(mu3-S)3 units share the common bridge atom mu6-S and are externally bridged by two mu2-S atoms. The M sites possess trigonal octahedral, and the Fe sites, distorted tetrahedral coordination. Hydrosulfide ligands and sulfide atoms simulate terminal cysteinate ligation and mu2 bridges, respectively, in the protein-bound cluster Fe8S7(mu2-SCys)2(SCys)4. The synthetic clusters have the same bridging pattern as the PN cluster and approach congruency with it. These clusters are the first molecular topological analogues of a PN cluster. Like the latter, they are substantially reduced (majority of Fe(II)).

Synthesis and reactions of triphenylsilanethiolato complexes of manganese(II), iron(II), cobalt(II), and nickel(II)

Reactions of Fe[N(SiMe(3))(2)](2) with 1 and 2 equiv of Ph(3)SiSH in hexane afforded dinuclear silanethiolato complexes, [Fe(N(SiMe(3))(2))(mu-SSiPh(3))](2) (1) and [Fe(SSiPh(3))(mu-SSiPh(3))](2) (2), respectively. Various Lewis bases were readily added to 2, generating mononuclear adducts, Fe(SSiPh(3))(2)(L)(2) [L = CH(3)CN (3a), 4-(t)BuC(5)H(4)N (3b), PEt(3) (3c), (LL) = tmeda (3d)]. From the analogous reactions of M[N(SiMe(3))(2)](2) (M = Mn, Co) and [Ni(NPh(2))(2)](2) with Ph(3)SiSH in the presence of TMEDA, the corresponding silanethiolato complexes, M(SSiPh(3))(2)(tmeda) [M = Mn (4), Co (5), Ni (6)], were isolated. Treatment of 3a with (PPh(4))(2)[MoS(4)] or (NEt(4))(2)[FeCl(4)] resulted in formation of a linear trinuclear Fe-Mo-Fe cluster (PPh(4))(2)[MoS(4)(Fe(SSiPh(3))(2))(2)] (7) or a dinuclear complex (NEt(4))(2)[Fe(2)(SSiPh(3))(2)Cl(4)] (8). On the other hand, the reaction of 3a with [Cu(CH(3)CN)(4)](PF(6)) gave a cyclic tetranuclear copper cluster Cu(4)(SSiPh(3))(4) (9), where silanethiolato ligands were transferred from iron to copper. Silicon-sulfur bond cleavage was found to occur when the cobalt complex 5 was treated with (NBu(4))F in THF, and a cobalt-sulfido cluster Co(6)(mu(3)-S)(8)(PPh(3))(6) (10) was isolated upon addition of PPh(3) to the reaction system. The silanethiolato complexes reported here are expected to serve as convenient precursors for sulfido cluster synthesis.

High-nuclearity sulfide-rich molybdenum[bond]iron[bond]sulfur clusters: reevaluation and extension

High-nuclearity Mo[bond]Fe[bond]S clusters are of interest as potential synthetic precursors to the MoFe(7)S(9) cofactor cluster of nitrogenase. In this context, the synthesis and properties of previously reported but sparsely described trinuclear [(edt)(2)M(2)FeS(6)](3-) (M = Mo (2), W (3)) and hexanuclear [(edt)(2)Mo(2)Fe(4)S(9)](4-) (4, edt = ethane-1,2-dithiolate; Zhang, Z.; et al. Kexue Tongbao 1987, 32, 1405) have been reexamined and extended. More accurate structures of 2-4 that confirm earlier findings have been determined. Detailed preparations (not previously available) are given for 2 and 3, whose structures exhibit the C(2) arrangement [[(edt)M(S)(mu(2)-S)(2)](2)Fe(III)](3-) with square pyramidal Mo(V) and tetrahedral Fe(III). Oxidation states follow from (57)Fe Mössbauer parameters and an S = (3)/(2) ground state from the EPR spectrum. The assembly system 2/3FeCl(3)/3Li(2)S/nNaSEt in methanol/acetonitrile (n = 4) affords (R(4)N)(4)[4] (R = Et, Bu; 70-80%). The structure of 4 contains the [Mo(2)Fe(4)(mu(2)-S)(6)(mu(3)-S)(2)(mu(4)-S)](0) core, with the same bridging pattern as the [Fe(6)S(9)](2-) core of [Fe(6)S(9)(SR)(2)](4-) (1), in overall C(2v) symmetry. Cluster 4 supports a reversible three-member electron transfer series 4-/3-/2- with E(1/2) = -0.76 and -0.30 V in Me(2)SO. Oxidation of (Et(4)N)(4)[4] in DMF with 1 equiv of tropylium ion gives [(edt)(2)Mo(2)Fe(4)S(9)](3-) (5) isolated as (Et(4)N)(3)[5].2DMF (75%). Alternatively, the assembly system (n = 3) gives the oxidized cluster directly as (Bu(4)N)(3)[5] (53%). Treatment of 5 with 1 equiv of [Cp(2)Fe](1+) in DMF did not result in one-electron oxidation but instead produced heptanuclear [(edt)(2)Mo(2)Fe(5)S(11)](3-) (6), isolated as the Bu(4)N(+)salt (38%). Cluster 6 features the previously unknown core Mo(2)Fe(5)(mu(2)-S)(7)(mu(3)-S)(4) in molecular C(2) symmetry. In 4-6, the (edt)MoS(3) sites are distorted trigonal bipramidal and the FeS(4) sites are distorted tetrahedral with all sulfide ligands bridging. Mössbauer spectroscopic data for 2 and 4-6 are reported; (mean) iron oxidation states increase in the order 4 < 5 approximately 1 < 6 approximately 2. Redox and spectroscopic data attributed earlier to clusters 2 and 4 are largely in disagreement with those determined in this work. The only iron and molybdenum[bond]iron clusters with the same sulfide content as the iron[bond]molybdenum cofactor of nitrogenase are [Fe(6)S(9)(SR)(2)](4-) and [(edt)(2)Mo(2)Fe(4)S(9)](3-)(,4-).

Single- and double-cubane clusters in the multiple oxidation states [VFe(3)S(4)](3+,2+,1+)

A new series of cubane-type [VFe(3)S(4)](z)() clusters (z = 1+, 2+, 3+) has been prepared as possible precursor species for clusters related to those present in vanadium-containing nitrogenase. Treatment of [(HBpz(3))VFe(3)S(4)Cl(3)](2)(-) (2, z = 2+), protected from further reaction at the vanadium site by the tris(pyrazolyl)hydroborate ligand, with ferrocenium ion affords the oxidized cluster [(HBpz(3))VFe(3)S(4)Cl(3)](1)(-) (3, z = 3+). Reaction of 2 with Et(3)P results in chloride substitution to give [(HBpz(3))VFe(3)S(4)(PEt(3))(3)](1+) (4, z = 2+). Reaction of 4 with cobaltocene reduced the cluster with formation of the edge-bridged double-cubane [(HBpz(3))(2)V(2)Fe(6)S(8)(PEt(3))(4)] (5, z = 1+, 1+), which with excess chloride underwent ligand substitution to afford [(HBpz(3))(2)V(2)Fe(6)S(8)Cl(4)](4)(-) (6, z = 1+, 1+). X-ray structures of (Me(4)N)[3], [4](PF(6)), 5, and (Et(4)N)(4)[6] x 2MeCN are described. Cluster 5 is isostructural with previously reported [(Cl(4)cat)(2)(Et(3)P)(2)Mo(2)Fe(6)S(8)(PEt(3))(4)] and contains two VFe(3)S(4) cubanes connected across edges by a Fe(2)S(2) rhomb in which the bridging Fe-S distances are shorter than intracubane Fe-S distances. Mössbauer (2-5), magnetic (2-5), and EPR (2, 4) data are reported and demonstrate an S = 3/2 ground state for 2 and 4 and a diamagnetic ground state for 3. Analysis of (57)Fe isomer shifts based on an empirical correlation between shift and oxidation state and appropriate reference shifts results in two conclusions. (i) The oxidation 2 --> 3 + e(-) results in a change in electron density localized largely or completely on the Fe(3) subcluster and associated sulfur atoms. (ii) The most appropriate charge distributions are [V(3+)Fe(3+)Fe(2+)(2)S(4)](2+) (Fe(2.33+)) for 1, 2, and 4 and [V(3+)Fe(3+)(2)Fe(2+)S(4)](3+) (Fe(2.67+)) for 3 and [V(2)Fe(6)S(8)(SEt)(9)](3+). Conclusion i applies to every MFe(3)S(4) cubane-type cluster thus far examined in different redox states at parity of cluster ligation. The formalistic charge distributions are regarded as the best current approximations to electron distributions in these delocalized species. The isomer shifts require that iron atoms are mixed-valence in each cluster.

Heterometal cubane-type MFe(3)S(4) clusters (M = Mo, V) trigonally symmetrized with hydrotris(pyrazolyl)borate(1-) and tris(pyrazolyl)methanesulfonate(1-) capping ligands

A series of heterometal cubane-type clusters containing [VFe(3)S(4)](2+) and [MoFe(3)S(4)](3+,2+) cores has been prepared. Ligand substitution of [(DMF)(3)VFe(3)S(4)Cl(3)](-) affords [(Tpms)VFe(3)S(4)L(3)](2)(-) (L = Cl(-) (8), EtS(-) (9), p-MeC(6)H(4)S(-), p-MeC(6)H(4)O(-)). A new procedure for the preparation of molybdenum single cubanes is introduced by the reaction of recently reported [(Tp)MoS(S(4))](-) with FeCl(2)/NaSEt to afford [(Tp)MoFe(3)S(4)Cl(3)](-) (1, 75% yield). This procedure is more efficient that the existing multistep synthesis of single cubanes, which generally affords clusters of mirror symmetry. Also prepared were [(Tp)MoFe(3)S(4)L(3)](-) (L = EtS(-) (2), p-MeC(6)H(4)S(-)). Reduction of 1 with borohydride gives [(Tp)MoFe(3)S(4)Cl(3)](2-) (5, 67%). Owing to the nature of the heterometal ligand, all clusters have idealized trigonal symmetry, reflected in their (1)H NMR spectra. Trigonal structures are demonstrated by crystallography of (Bu(4)N)[1,2], (Bu(4)N)(2)[5] x MeCN, and (Me(4)N)(2)[8,9]. The availability of 1 and 5 allows the first comparison of structures and (57)Fe isomer shifts of [MoFe(3)S(4)](3+,2+) in a constant ligand environment. Small increases in most bond distances indicate that an antibonding electron is added in the reduction of 1. Collective synthetic and electrochemical results from this and other studies demonstrate the existence of the series of oxidation states [VFe(3)S(4)](3+,2+,1+) and [MoFe(3)S(4)](4+,3+,2+) whose relative stabilities within a given series are strongly ligand dependent. Isomer shifts indicate that the reduction of 1 largely affects the Fe(3) subcluster and are consistent with the formal descriptions [MoFe(3+)(2)Fe(2+)S(4)](3+) (1) and [MoFe(3+)Fe(2+)(2)S(4)](2+) (5). Reaction of 1 with excess Li(2)S in acetonitrile affords the double cubane [[(Tp)MoFe(3)S(4)Cl(2)](2)(mu(2)(-)S)](2)(-), whose sulfide-bridged structure is supported by two sequential reductions separated by 290 mV, in analogy with previously reported double cubanes of higher charge. Trigonally symmetric single cubanes eliminate isomers in the formation of double cubanes and other cluster structures, and may be of considerable value in the preparation of new types of M-Fe-S clusters. (Tpms = tris(pyrazolyl)methanesulfonate(1-); Tp = hydrotris(pyrazolyl)borate(1-).)

Synthesis and characterization of sulfur-voided cubanes. Structural analogues for the MoFe(3)S(3) subunit in the nitrogenase cofactor

A new class of Mo/Fe/S clusters with the MoFe(3)S(3) core has been synthesized in attempts to model the FeMo-cofactor in nitrogenase. These clusters are obtained in reactions of the (Cl(4)-cat)(2)Mo(2)Fe(6)S(8)(PR(3))(6) [R = Et (I), (n)Pr (II)] clusters with CO. The new clusters include those preliminarily reported: (Cl(4)-cat)MoFe(3)S(3)(PEt(3))(2)(CO)(6) (III), (Cl(4)-cat)(O)MoFe(3)S(3)(PEt(3))(3)(CO)(5) (IV), (Cl(4)-cat)(Pyr)MoFe(3)S(3)(PEt(3))(2)(CO)(6) (VI), and (Cl(4)-cat)(Pyr)MoFe(3)S(3)(P(n)Pr(3))(3)(CO)(4) (VIII). In addition the new (Cl(4)-cat)(O)MoFe(3)S(3)(P(n)Pr(3))(3)(CO)(5) cluster (IVa), the (Cl(4)-cat)(O)MoFe(3)S(3)(PEt(3))(2)(CO)(6)cluster (V), the (Cl(4)-cat)(O)MoFe(3)S(3)(P(n)Pr(3))(2)(CO)(6) cluster (Va), the (Cl(4)-cat)(Pyr)MoFe(3)S(3)(P(n)Pr(3))(2)(CO)(6) cluster (VIa), and the (Cl(4)-cat)(P(n)Pr(3))MoFe(3)S(3)(P(n)Pr(3))(2)(CO)(6) cluster (VII) also are reported. Clusters III-VIII have been structurally and spectroscopically characterized. EPR, zero-field (57)Fe-Mössbauer spectroscopic characterizations, and magnetic susceptibility measurements have been used for a tentative assignment of the electronic and oxidation states of the MoFe(3)S(3) sulfur-voided cuboidal clusters. A structural comparison of the clusters with the MoFe(3)S(3) subunit of the FeMo-cofactor has led to the suggestion that the storage of reducing equivalents into M-M bonds, and their use in the reduction of substrates, may occur with the FeMo-cofactor, which also appears to have M-M bonding. On the basis of this argument, a possible N(2)-binding and reduction mechanism on the FeMoco-cofactor is proposed.

Rational synthesis of high nuclearity Mo/Fe/S clusters: the reductive coupling approach in the convenient synthesis of (Cl(4)-cat)(2)Mo(2)Fe(6)S(8)(PR(3))(6) [R = Et, (n)Pr, (n)Bu] and the new [(Cl(4)-cat)(2)Mo(2)Fe(2)S(3)O(PEt(3))(3)Cl]-1/2(Fe(PEt(3))(2)(MeCN)(4)) and (Cl(4)-cat)(2)Mo(2)Fe(3)S(5)(PEt(3))(5) clusters

A general method for the synthesis of high nuclearity Mo/Fe/S clusters is presented and involves the reductive coupling of the (Et(4)N)(2)[(Cl(4)-cat)MoOFeS(2)Cl(2)] (I) and (Et(4)N)(2)[Fe(2)S(2)Cl(4)] (II) clusters. The reaction of I and II with Fe(PR(3))(2)Cl(2) or sodium salts of noncoordinating anions such as NaPF(6) or NaBPh(4) in the presence of PR(3) (R = Et, (n)Pr, or (n)Bu) affords (Cl(4)-cat)(2)Mo(2)Fe(6)S(8)(PR(3))(6) [R = Et (IIIa), (n)Pr (IIIb), (n)Bu (IIIc)], Fe(6)S(6)(PEt(3))(4)Cl(2) (IV) and (PF(6))[Fe(6)S(8)(P(n)Pr(3))(6)] (V) as byproducts. The isolation of (Et(4)N)[Fe(PEt(3))Cl(3)] (VI), NaCl, and SPEt(3) supports a reductive coupling mechanism. Cluster IV and V also have been synthesized by the reductive self-coupling of compound II. The reductive coupling reaction between I and II by PEt(3) and NaPF(6) in a 1:1 ratio produces the (Et(4)N)(2)[(Cl(4)-cat)Mo(L)Fe(3)S(4)Cl(3)] clusters [L = MeCN (VIIa), THF (VIIb)]. The hitherto unknown [(Cl(4)-cat)(2)Mo(2)Fe(2)S(3)O(PEt(3))(3)Cl](+) cluster (VIII) has been isolated as the 2:1 salt of the (Fe(PEt(3))(2)(MeCN)(4))(2+) cation after the reductive self-coupling reaction of I in the presence of Fe(PEt(3))(2)Cl(2). Cluster VIII crystallizes in the monoclinic space group P2(1)/c with a = 11.098(3) A, b = 22.827(6) A, c = 25.855(6) A, beta = 91.680(4) degrees, and Z = 4. The formal oxidation states of metal atoms in VIII have been assigned as Mo(III), Mo(IV), Fe(II), and Fe(III) on the basis of zero-field Mössbauer spectra. The Fe(PEt(3))(2)(MeCN)(4) cation of VIII is also synthesized independently, isolated as the BPh(4)(-) salt (IX), and has been structurally characterized. The reductive coupling of compound I also affords in low yield the new (Cl(4)-cat)(2)Mo(2)Fe(3)S(5)(PEt(3))(5) cluster (X) as a byproduct. Cluster X crystallizes in the monoclinic space group P2(1)/n with a = 14.811(3) A, b = 22.188(4) A, c = 21.864(4) A, beta = 100.124(3) degrees, and Z = 4 and the structure shows very short Mo-Fe, Fe-Fe, Mo-S, Fe-S bonds. The oxidation states of the metal atoms in this neutral cluster (X) have been assigned as Mo(IV)Mo(III)Fe(II)Fe(II)Fe(III) based on zero-field Mössbauer and magnetic measurement. All Fe atoms are high spin and two of the three Fe-Fe distances are found at 2.4683(9) A and 2.4721(9) A.

Titanium alkali metal nitrido complexes

Treatment of [(Ti(eta5-C5Me5)(mu-NH))3(mu3-N)] with alkali metal bis(trimethylsilyl)amido reagents in toluene afforded the complexes [M(mu3-N)(mu3-NH)2[Ti3(mu5-C5Me5)3(mu3-N)]]2 (M = Li (2), Na, (3), K (4)). The molecular structures of 2 and 3 have been determined by X-ray crystallographic studies and show two azaheterometallocubane cores [MTi3N4] linked by metal-nitrogen bonds. Reaction of the lithium derivative 2 with chlorotrimethylsilane or trimethyltin chloride in toluene gave the incomplete cube nitrido complexes [Ti3(eta5-C5Me5)3(mu-NH)2(mu-NMMe3)(mu3-N)] (M = Si (5), Sn (6)). A similar reaction with indium(I) or thallium(I) chlorides yielded cube-type derivatives [M(mu3-N)(mu3-NH)2[Ti(eta5-C5Me5)3(mu3-N)] (M=In (7), Tl (8)).

Molybdenum-iron-sulfur clusters of nuclearities eight and sixteen, including a topological analogue of the P-cluster of nitrogenase

Transformations of the edge-bridged double cubane cluster [(Cl4cat)2(Et3P)2Mo2Fe6S8(PEt3)4] (1) under reducing conditions have been investigated as synthetic approaches to the clusters of nitrogenase. Cluster 1 is a versatile precursor to different Mo-Fe-S cluster types. The reaction system 1/K(C14H10) in THF yields the reduced cluster [(Cl4cat)2(Et3P)2Mo2Fe6S8(PEt3)4]1- (2), which as its crystalline Et4N+ salt retains the edge-bridged structure of 1. X-ray structural and Mössbauer spectroscopic results indicate an unsymmetrical electron distribution with localized [MoFe3S4]2+,1+ cubane-type units. The system 1/2K(C14H10)/2HS- in THF/acetonitrile affords [(Cl4cat)4(Et3P)4Mo4Fe12S20K3(DMF)]5- (3), whose structure was determined as the Ph3PMe+ salt. The cluster consists of two isostructural Mo2Fe6S9 fragments connected by two mu 2-S bridges. Three potassium ions are bound between the two fragments. In each fragment, the iron atoms are present in tetrahedral FeS4 and the molybdenum atoms in octahedral MoO2PS3 coordination units, and two MoFe3(mu 3-S)3 cuboidal units are bridged by a common mu 6-S atom. The fragments have idealized mirror symmetry and are isostructural with two of the fragments present in the previously reported high-nuclearity cluster [(Cl4cat)6(Et3P)6Mo6Fe20S30]8- (4) (Osterloh, F.; Sanakis, Y.; Staples, R. J.; Münck, E.; Holm, R. H. Angew. Chem., Int. Ed. Engl. 1999, 38, 2066). On the basis of overall shape, atom connectivities, and metric features, the Mo2Fe6S9 fragment is a topological analogue of the P-cluster of nitrogenase in the PN (reduced) state. A third cluster type, formed as a minor byproduct in the reaction system leading to 2, was crystallographically identified as [(Cl4cat)2(Et3P)2Mo2Fe6S8(PEt3)4]4-, whose core is made up of two MoFe3(mu 3-S)3 cuboidal units bridged by two mu 2-S atoms and connected by a direct Fe-Fe bond. Full structural details and the redox properties of 2 and 3 are reported.