Iron-amide-sulfide and iron-imide-sulfide clusters: heteroligated core environments relevant to the nitrogenase FeMo cofactor.

Heteroligated cluster cores consisting of weak-field iron, strongly basic nitrogen anions, and sulfide are of interest with respect to observed and conjectured environments in the FeMo cofactor of nitrogenase. Selective syntheses have been developed to achieve such environments with tert-butyl-substituted amide and imide core ligands. A number of different routes were employed, including nominal ligand substitution and oxidative addition reactions, as well as novel transformations involving the combination of different cluster precursors. New cluster products include precursor Fe(2)(µ-NH(t)Bu)(2)[N(SiMe(3))(2)](2) (6), Fe(2)(µ-NH(t)Bu)(2)(µ-S)[N(SiMe(3))(2)](2) (7), which has a rare confacial bitetrahedral geometry previously unknown in iron chemistry, [Fe(2)(µ-N(t)Bu)(µ-S)Cl(4)](2-) (2), and cuboidal [Fe(4)(µ(3)-N(t)Bu)(3)(µ(3)-S)Cl(4)](-) (8), [Fe(4)(µ(3)-N(t)Bu)(2)(µ(3)-S)(2)Cl(4)](2-) (9), and [Fe(4)(µ(3)-N(t)Bu)(µ(3)-S)(3)Cl(4)](2-) (10), as well as selenide-substituted derivatives Fe(2)(µ-NH(t)Bu)(2)(µ-Se)[N(SiMe(3))(2)](2) (7-Se) and [Fe(4)(µ(3)-N(t)Bu)(µ(3)-Se)(3)Cl(4)](2-) (10-Se). The imide-sulfide clusters complete the compositional sets [Fe(2)(µ-N(t)Bu)(n)(µ-S)(2-n)Cl(4)](2-) (n = 0-2) and [Fe(4)(µ(3)-N(t)Bu)(n)(µ(3)-S)(4-n)Cl(4)](z) (n = 0-4), represented previously only by the all-imide and all-sulfide core congeners, and they share chemical and physical properties with the parent homoleptic core species. All imide-sulfide cores are compositionally stable and show no evidence of core ligand exchange over days in solution. Beyond structural differences, the impact of mixed core ligation is most evident in redox potentials, which show progressive decreases of -435 (for z = 1-/2-) or -385 mV (for z = 2-/3-) for each replacement of sulfide by the more potent imide donor, and a corresponding effect may be expected for the interstitial heteroligand in the FeMo cofactor. Cluster 10 presents an [Fe(4)NS(3)] core framework virtually isometric with the isostructural [Fe(4)S(3)X] subunit of the FeMo cofactor, thus providing a synthetic structural representation for this portion of the cofactor core.

Selective syntheses of iron-imide-sulfide cubanes, including a partial representation of the Fe-S-X environment in the FeMo cofactor.

The dinuclear precursors Fe(2)(N(t)Bu)(2)Cl(2)(NH(2)(t)Bu)(2), [Fe(2)(N(t)Bu)(S)Cl(4)](2-), and Fe(2)(NH(t)Bu)(2)(S)(N{SiMe(3)}(2))(2) allowed the selective syntheses of the cubane clusters [Fe(4)(N(t)Bu)(n)(S)(4-n)Cl(4)](z) with [n, z] = [3, 1-], [2, 2-], [1, 2-]. Weak-field iron-sulfur clusters with heteroleptic, nitrogen-containing cores are of interest with respect to observed or conjectured environments in the iron-molybdenum cofactor of nitrogenase. In this context, the present iron-imide-sulfide clusters constitute a new class of compounds for study, with the Fe(4)NS(3) core of the [1, 2-] cluster affording the first synthetic representation of the corresponding heteroligated Fe(4)S(3)X subunit in the cofactor.

[Fe(4)S(4)](q) cubane clusters (q = 4+, 3+, 2+) with terminal amide ligands.

Bis(trimethylsilyl)amide-ligated iron-sulfur cubane clusters [Fe(4)(mu(3)-S)(4)(N{SiMe(3)}(2))(4)](z) (z = 0, 1-, 2-) are accessible by the reaction of FeCl(N{SiMe(3)}(2))(2)(THF) (1) with 1 equiv of NaSH (z = 0), followed by reduction with either 0.25 (z = 1-) or 1 equiv (z = 2-) of Na(2)S as needed. The anionic clusters are obtained as the sodium salts [Na(THF)(2)][Fe(4)S(4)(N{SiMe(3)}(2))(4)] and [Na(THF)(2)](2)[Fe(4)S(4)(N{SiMe(3)}(2))(4)]; in the solid state, these two clusters both possess a unique contact ion pair motif in which individual sodium ions each coordinate to a cluster core sulfide, an adjacent amide nitrogen, and two THF donors. The monoanionic cluster can also be prepared as the lithium salt [Li(THF)(4)][Fe(4)S(4)(N{SiMe(3)}(2))(4)] by the reaction of 1 with 1:0.5 LiCl/Li(2)S. The characterization of the three-membered redox series allows an analysis of redox trends, as well as a study of the effects of the amide donor environment on the [Fe(4)S(4)] core. Bis(trimethylsilyl)amide terminal ligation significantly stabilizes oxidized cluster redox states, permitting isolation of the uncommon [Fe(4)S(4)](3+) and unprecedented [Fe(4)S(4)](4+) weak-field cores.

Binding Orientation of a Ruthenium-Based Water Oxidation Catalyst on a CdS QD Surface Revealed by NMR Spectroscopy.

We report on the binding of a Ru-based water oxidation catalyst (WOC) to CdS quantum dots (QDs) revealed by 1H NMR spectroscopy. Spin centers within the WOC exhibit correlated trends in chemical shift and T2 lifetime shortening upon QD binding. These effects are a highly directional function of proton position within the WOC, thus uncovering orientation information relative to the QD surface. The data suggest that the WOC interacts with the QD surface via the Ru terpyridine ligand, an unexpected orientation that has important implications for interfacial charge transfer and subsequent catalysis. This binding motif enables strong enough donor-acceptor electronic coupling for ultrafast photoinduced hole transfer while maintaining electronically distinct functional subunits.

Synthesis and elaboration of the dinuclear iron-imide cluster core [Fe2(mu-NR)2]2+.

The protolysis of mononuclear ferric amide precursors FeCl[N(SiMe3)2]2(THF) (1) or [FeCl2{N(SiMe3)2}2]- (2) by primary amines provides, under suitable conditions, an effective route to dinuclear weak-field ferric-imide clusters with [Fe2(mu-NR)2]2+ cores. In the synthesis of known arylimide clusters [Fe2(mu-NAr)2Cl4]2- (Ar = Ph, p-Tol, Mes) from 2, the counterion has a major effect on selectivity and yield, and the use of quaternary ammonium salts affords a substantial improvement over earlier, Li+-based chemistry. The new tert-butylimide core is obtained by protolysis of 1 with excess tBuNH2 to give crystalline cis-Fe2(mu-NtBu)2Cl2(NH2tBu)2 (9). Complex 9 can be transformed to other dinuclear species through substitution of the terminal amines by pyridines, PEt3, or chloride, or through protolysis of bridging alkylimides by arylamines, allowing isolation of trans-Fe2(mu-NtBu)2Cl2(DMAP)2 (DMAP = 4-dimethylaminopyridine), cis-Fe2(mu-NtBu)2Cl2(PEt3)2, [Fe2(mu-NtBu)2Cl4]-, and trans-Fe2(mu-NPh)2Cl2(NH2tBu)2. The susceptibility of alkyl substituents to beta-elimination appears to limit the general applicability of protolytic cluster assembly using alkylamines. The dinuclear clusters have been characterized by X-ray, spectroscopic, and electrochemical analyses.

Nanotechnology applications and implications research supported by the US Environmental Protection Agency STAR grants program.

Since 2002, the US Environmental Protection Agency (EPA) has been funding research on the environmental aspects of nanotechnology through its Science to Achieve Results (STAR) grants program. In total, more than $25 million has been awarded for 86 research projects on the environmental applications and implications of nanotechnology. In the applications area, grantees have produced promising results in green manufacturing, remediation, sensors, and treatment using nanotechnology and nanomaterials. Although there are many potential benefits of nanotechnology, there has also been increasing concern about the environmental and health effects of nanomaterials, and there are significant gaps in the data needed to address these concerns. Research performed by STAR grantees is beginning to address these needs.

Iron-arylimide clusters [Fe(m)()(NAr)(n)Cl(4)](2)(-) (m, n = 2, 2; 3, 4; 4, 4) from a ferric amide precursor: synthesis, characterization, and comparison to Fe-S chemistry.

Tetrahedral FeCl[N(SiMe(3))(2)](2)(THF) (2), prepared from FeCl(3) and 2 equiv of Na[N(SiMe(3))(2)] in THF, is a useful ferric starting material for the synthesis of weak-field iron-imide (Fe-NR) clusters. Protonolysis of 2 with aniline yields azobenzene and [Fe(2)(mu-Cl)(3)(THF)(6)](2)[Fe(3)(mu-NPh)(4)Cl(4)] (3), a salt composed of two diferrous monocations and a trinuclear dianion with a formal 2 Fe(III)/1 Fe(IV) oxidation state. Treatment of 2 with LiCl, which gives the adduct [FeCl(2)(N(SiMe(3))(2))(2)](-) (isolated as the [Li(TMEDA)(2)](+) salt), suppresses arylamine oxidation/iron reduction chemistry during protonolysis. Thus, under appropriate conditions, the reaction of 1:1 2/LiCl with arylamine provides a practical route to the following Fe-NR clusters: [Li(2)(THF)(7)][Fe(3)(mu-NPh)(4)Cl(4)] (5a), which contains the same Fe-NR cluster found in 3; [Li(THF)(4)](2)[Fe(3)(mu-N-p-Tol)(4)Cl(4)] (5b); [Li(DME)(3)](2)[Fe(2)(mu-NPh)(2)Cl(4)] (6a); [Li(2)(THF)(7)][Fe(2)(mu-NMes)(2)Cl(4)] (6c). [Li(DME)(3)](2)[Fe(4)(mu(3)-NPh)(4)Cl(4)] (7), a trace product in the synthesis of 5a and 6a, forms readily as the sole Fe-NR complex upon reduction of these lower nuclearity clusters. Products were characterized by X-ray crystallographic analysis, by electronic absorption, (1)H NMR, and Mössbauer spectroscopies, and by cyclic voltammetry. The structures of the Fe-NR complexes derive from tetrahedral iron centers, edge-fused by imide bridges into linear arrays (5a,b; 6a,c) or the condensed heterocubane geometry (7), and are homologous to fundamental iron-sulfur (Fe-S) cluster motifs. The analogy to Fe-S chemistry also encompasses parallels between Fe-mediated redox transformations of nitrogen and sulfur ligands and reductive core conversions of linear dinuclear and trinuclear clusters to heterocubane species and is reinforced by other recent examples of iron- and cobalt-imide cluster chemistry. The correspondence of nitrogen and sulfur chemistry at iron is intriguing in the context of speculative Fe-mediated mechanisms for biological nitrogen fixation.