Comparison of density functionals for energy and structural differences between the high- [5T2g: (t2g)4(eg)2] and low- [1A1g: (t2g)6(eg)0] spin states of the hexaquoferrous cation [Fe(H2O)6]2+

A comparison of density functionals is made for the calculation of energy and geometry differences for the high- [(5)T(2g): (t(2g))(4)(e(g))(2)] and low- [(1)A(1g): (t(2g))(6)(e(g))(0)] spin states of the hexaquoferrous cation [Fe(H(2)O)(6)](2+). Since very little experimental results are available (except for crystal structures involving the cation in its high-spin state), the primary comparison is with our own complete active-space self-consistent field (CASSCF), second-order perturbation theory-corrected complete active-space self-consistent field (CASPT2), and spectroscopy-oriented configuration interaction (SORCI) calculations. We find that generalized gradient approximations (GGAs) and the B3LYP hybrid functional provide geometries in good agreement with experiment and with our CASSCF calculations provided sufficiently extended basis sets are used (i.e., polarization functions on the iron and polarization and diffuse functions on the water molecules). In contrast, CASPT2 calculations of the low-spin-high-spin energy difference DeltaE(LH)=E(LS)-E(HS) appear to be significantly overestimated due to basis set limitations in the sense that the energy difference of the atomic asymptotes ((5)D-->(1)I excitation of Fe(2+)) are overestimated by about 3000 cm(-1). An empirical shift of the molecular DeltaE(LH) based upon atomic calculations provides a best estimate of 12 000-13 000 cm(-1). Our unshifted SORCI result is 13 300 cm(-1), consistent with previous comparisons between SORCI and experimental excitation energies which suggest that no such empirical shift is needed in conjunction with this method. In contrast, after estimation of incomplete basis set effects, GGAs with one exception underestimate this value by 3000-4000 cm(-1) while the B3LYP functional underestimates it by only about 1000 cm(-1). The exception is the GGA functional RPBE which appears to perform as well as or better than the B3LYP functional for the properties studied here. In order to obtain a best estimate of the molecular DeltaE(LH) within the context of density functional theory (DFT) calculations we have also performed atomic excitation energy calculations using the multiplet sum method. These atomic DFT calculations suggest that no empirical correction is needed for the DFT calculations.     

Relative energy of the high-(5T2g) and low-(1A1g) spin states of [Fe(H2O)6]2+, [Fe(NH3)6]2+, and [Fe(bpy)3]2+: CASPT2 versus density functional theory

High-level ab initio calculations using the CASPT2 method and extensive basis sets were performed on the energy differences of the high-[(5)T(2g):t(2g) (4)e(g) (2)] and low-[(1)A(1g):t(2g) (6)] spin states of the pseudo-octahedral Fe(II) complexes [Fe(H(2)O)(6)](2+), [Fe(NH(3))(6)](2+), and [Fe(bpy)(3)](2+). The results are compared to the results obtained from density functional theory calculations with the generalized gradient approximation functional BP86 and two hybrid functionals B3LYP and PBE0, and serve as a calibration for the latter methods. We find that large basis set CASPT2 calculations may provide results for the high-spin/low-spin splitting DeltaE(HL) that are accurate to within 1000 cm(-1), provided they are based on an adequately large CAS[10,12] reference wave function. The latter condition was found to be much more stringent for [Fe(bpy)(3)](2+) than for the other two complexes. Our "best" results for DeltaE(HL) (including a zero-point energy correction) are -17 690 cm(-1) for [Fe(H(2)O)(6)](2+), -8389 cm(-1) for [Fe(NH(3))(6)](2+), and 3820 cm(-1) for [Fe(bpy)(3)](2+).     

Density functional theory study of trans-dioxo complexes of iron, ruthenium, and osmium with saturated amine ligands, trans-[M(O)2(NH3)2(NMeH2)2]2+ (M=Fe, Ru, Os), and detection of [Fe(qpy)(O)2]n+ (n=1, 2) by high-resolution ESI mass spectrometry

Density functional theory (DFT) calculations on trans-dioxo metal complexes containing saturated amine ligands, trans-[M(O)2(NH3)2(NMeH2)2]2+ (M=Fe, Ru, Os), were performed with different types of density functionals (DFs): 1) pure generalized gradient approximations (pure GGAs): PW91, BP86, and OLYP; 2) meta-GGAs: VSXC and HCTH407; and 3) hybrid DFs: B3LYP and PBE1PBE. With pure GGAs and meta-GGAs, a singlet d2 ground state for trans-[Fe(O)2(NH3)2(NMeH2)2]2+ was obtained, but a quintet ground state was predicted by the hybrid DFs B3LYP and PBE1PBE. The lowest transition energies in water were calculated to be at lambda approximately 509 and 515 nm in the respective ground-state geometries from PW91 and B3LYP calculations. The nature of this transition is dependent on the DFs used: a ligand-to-metal charge-transfer (LMCT) transition with PW91, but a pi(Fe-O)-->pi*(Fe-O) transition with B3LYP, in which pi and pi* are the bonding and antibonding combinations between the dpi(Fe) and ppi(O(2-)) orbitals. The FeVI/V reduction potential of trans-[Fe(O)2(NH3)2NMeH2)2]2+ was estimated to be +1.30 V versus NHE based on PW91 results. The [Fe(qpy)(O)2](n+) (qpy=2,2':6',2':6',2':6',2'-quinquepyridine; n=1 and 2) ions, tentatively assigned to dioxo iron(V) and dioxo iron(VI), respectively, were detected in the gas phase by high-resolution ESI-MS spectroscopy.     

Relative energy of the high-(5T2g) and low-(1A1g) spin states of the ferrous complexes [Fe(L)(NHS4)]: CASPT2 versus density functional theory

High-level ab initio calculations using multiconfigurational perturbation theory [complete active space with second-order perturbation theory (CASPT2)] were performed on the transition energy between the lowest high-spin (corresponding to (5T2g) in Oh) and low-spin (corresponding to 1A1g in Oh) states in the series of six-coordinated Fe(II) molecules [Fe(L)(NHS4)], where NHS4 is 2,2'-bis(2-mercaptophenylthio)diethylamine dianion and L=NH3, N2H4, PMe3, CO, and NO+. The results are compared to (previous and presently obtained) results from density functional theory (DFT) calculations with four functionals, which were already shown previously by Casida and co-workers [Fouqueau et al., J. Chem. Phys. 120, 9473 (2004); Ganzenmuller et al., ibid. 122, 234321 (2005); Fouqueau et al., ibid. 122, 044110 (2005); Lawson Daku et al., ChemPhysChem 6, 1393 (2005)] to perform well for the spin-pairing problem in these and other Fe(II) complexes, i.e., OLYP, PBE0, B3LYP, and B3LYP*. Very extended basis sets were used both for the DFT and CASPT2 calculations and were shown to be necessary to obtain quantitative results with both types of method. This work presents a sequel to a previous DFT/CASPT2 study of the same property in the complexes [Fe(H2O)6]2+, [Fe(NH3)6]2+, and [Fe(bpy)3]2+ [Pierloot et al., J. Chem. Phys. 125, 124303 (2006)]. The latter work was extended with new results obtained with larger basis sets and including the OLYP functional. For all considered complexes, the CASPT2 method predicts the correct ground state spin multiplicity. Since experimental data for the actual quintet-singlet (free) energy differences are not available, the performance of the different DFT functionals was judged based on the comparison between the DFT and CASPT2 results. From this, it was concluded that the generalized gradient OLYP functional performs remarkably well for the present series of ferrous compounds, whereas the success of the three hybrid functionals varies from case to case.     

The reaction of Li[Al(OR)4] R = OC(CF3)2Ph, OC(CF3)3 with NO/NO2 giving NO[Al(OR)4], Li[NO3] and N2O. The synthesis of NO[Al(OR)4] from Li[Al(OR)4] and NO[SbF6] in sulfur dioxide solution

NO[Al(OC(CF(3))(2)Ph)(4)] 1 and NO[Al(OC(CF(3))(3))(4)] 2 were obtained by the metathesis reaction of NO[SbF(6)] and the corresponding Li[Al(OR)(4)] salts in liquid sulfur dioxide solution in ca 40% (1) and 85% (2) isolated yield. 1 and 2, as well as Li[NO(3)] and N(2)O, were also given by the reaction of an excess of mixture of (90 mol%) NO, (10 mol%) NO(2) with Li[Al(OR)(4)] followed by extraction with SO(2). The unfavourable disproportionation reaction of 2NO(2)(g) to [NO](+)(g) and [NO(3)](-)(g)[DeltaH degrees = +616.2 kJ mol(-1)] is more than compensated by the disproportionation energy of 3NO(g) to N(2)O(g) and NO(2)(g)[DeltaH degrees =-155.4 kJ mol(-1)] and the lattice energy of Li[NO(3)](s)[U(POT)= 862 kJ mol(-1)]. Evidence is presented that the reaction proceeds via a complex of [Li](+) with NO, NO(2)(or their dimers) and N(2)O. NO(2) and Li[Al(OC(CF(3))(3))(4)] gave [NO(3)(NO)(3)][Al(OC(CF(3))(3))(4)](2), NO[Al(OC(CF(3))(3))(4)] and (NO(2))[Al(OC(CF(3))(3))(4)] products. The aluminium complex [Li[AlF(OC(CF(3))(2)Ph)(3)]](2) 3 was prepared by the thermal decomposition of Li[Al(OC(CF(3))(2)Ph)(4)]. Compounds 1 and 3 were characterized by single crystal X-ray structural analyses, 1-3 by elemental analyses, NMR, IR, Raman and mass spectra. Solid 1 contains [Al(OC(CF(3))(2)Ph)(4)](-) and [NO](+) weakly linked via donor acceptor interactions, while in the SO(2) solution there is an equilibrium between the associated [NO](+)[Al(OC(CF(3))(2)Ph)(4)](-) and separated solvated ions. Solid 2 contains essentially ionic [NO](+) and [Al(OC(CF(3))(3))(4)](-). Complex 3 consists of two [Li[AlF(OC(CF(3))(2)Ph)(3)]] units linked via fluorine lithium contacts. Compound 1 is unstable in the SO(2) solution and decomposes to yield [AlF(OC(CF(3))(2)Ph)(3)](-), [(PhC(CF(3))(2)O)(3)Al(mu-F)Al(OC(CF(3))(2)Ph)(3)](-) anions as well as (NO)C(6)H(4)C(CF(3))(2)OH, while compound 2 is stable in liquid SO(2). The [small nu](NO(+)) in 1 and [NO](+)(toluene)[SbCl(6)] are similar, implying similar basicities of [Al(OC(CF(3))(2)Ph)(4)](-) and toluene.     

Synthesis and Chemical and Electrochemical Characterization of Fe-S Carbonyl Clusters. X-ray Crystal Structures of [N(PPh(3))(2)](2)[Fe(5)S(2)(CO)(14)] and [N(PPh(3))(2)](2)[Fe(6)S(6)(CO)(12)

A reinvestigation of the redox behavior of the [Fe(3)(&mgr;(3)-S)(CO)(9)](2)(-) dianion led to the isolation and characterization of the new [Fe(5)S(2)(CO)(14)](2)(-), as well as the known [Fe(6)S(6)(CO)(12)](2)(-) dianion. As a corollary, new syntheses of the [Fe(3)S(CO)(9)](2)(-) dianion are also reported. The [Fe(5)S(2)(CO)(14)](2)(-) dianion has been obtained by oxidative condensation of [Fe(3)S(CO)(9)](2)(-) induced by tropylium and Ag(I) salts or SCl(2), or more straightforwardly through the reaction of [Fe(4)(CO)(13)](2)(-) with SCl(2). The [Fe(6)S(6)(CO)(12)](2)(-) dianion has been isolated as a byproduct of the synthesis of [Fe(3)S(CO)(9)](2)(-) and [Fe(5)S(2)(CO)(14)](2)(-) or by reaction of [Fe(4)(CO)(13)](2)(-) with elemental sulfur. The structures of [N(PPh(3))(2)](2)[Fe(5)S(2)(CO)(14)] and [N(PPh(3))(2)](2)[Fe(6)S(6)(CO)(12)] were determined by single-crystal X-ray diffraction analyses. Crystal data: for [N(PPh(3))(2)](2)[Fe(5)S(2)(CO)(14)], monoclinic, space group P2(1)/c (No. 14), a = 24.060(5), b = 14.355(6), c = 23.898(13) ?, beta = 90.42(3) degrees, Z = 4; for [N(PPh(3))(2)](2)[Fe(6)S(6)(CO)(12)], monoclinic, space group C2/c (No. 15), a = 34.424(4), b = 14.081(2), c = 19.674(2) ?, beta = 115.72(1) degrees, Z = 4. The new [Fe(5)S(2)(CO)(14)](2)(-) dianion shows a "bow tie" arrangement of the five metal atoms. The two Fe(3) triangles sharing the central Fe atom are not coplanar and show a dihedral angle of 55.08(3) degrees. Each Fe(3) moiety is capped by a triply bridging sulfide ligand. The 14 carbonyl groups are all terminal; two are bonded to the unique central atom and three to each peripheral iron atom. Protonation of the [Fe(5)S(2)(CO)(14)](2)(-) dianion gives reversibly rise to the corresponding [HFe(5)S(2)(CO)(14)](-) monohydride derivative, which shows an (1)H-NMR signal at delta -21.7 ppm. Its further protonation results in decomposition to mixtures of Fe(2)S(2)(CO)(6) and Fe(3)S(2)(CO)(9), rather than formation of the expected H(2)Fe(5)S(2)(CO)(14) dihydride. Exhaustive reduction of [Fe(5)S(2)(CO)(14)](2)(-) with sodium diphenyl ketyl progressively leads to fragmentation into [Fe(3)S(CO)(9)](2)(-) and [Fe(CO)(4)](2)(-), whereas electrochemical, as well as chemical oxidation with silver or tropylium tetrafluoroborate, in dichloromethane, generates the corresponding [Fe(5)S(2)(CO)(14)](-) radical anion which exhibits an ESR signal at g = 2.067 at 200 K. The electrochemical studies also indicated the existence of a subsequent one-electron anodic oxidation which possesses features of chemical reversibility in dichloromethane but not in acetonitrile solution. A reexamination of the electrochemical behavior of the [Fe(3)S(CO)(9)](2)(-) dianion coupled with ESR monitoring enabled the spectroscopic characterization of the [Fe(3)S(CO)(9)](-) radical monoanion and demonstrated its direct involvement in the generation of the [Fe(5)S(2)(CO)(14)](n)()(-) (n = 0, 1, 2) system.     

Preparation and characterization of [Hg[P(C6F5)2]2], [Hg[(mu-P(C6F5)2)W(CO)5]2], and [Hg[(mu-P(CF3)2)W(CO)5]2] and the X-ray crystal structure of [Hg[(mu-P(C6F5)2)W(CO)5]2].2DMF

The thermally unstable compound [Hg[P(C(6)F(5))(2)](2)] was obtained from the reaction of mercury cyanide and bis(pentafluorophenyl)phosphane in DMF solution and characterized by multinuclear NMR spectroscopy. The thermally stable trinuclear compounds [Hg[(mu-P(CF(3))(2))W(CO)(5)](2)] and [Hg[(mu-P(C(6)F(5))(2))W(CO)(5)](2)] are isolated and completely characterized. The higher order NMR spectra exhibiting multinuclear satellite systems have been sufficiently analyzed. [Hg[(mu-P(CF(3))(2))W(CO)(5)](2)].2DMF crystallizes in the monoclinic space group C2/c with a = 2366.2(3) pm, b = 1046.9(1) pm, c = 104.0(1) pm, and beta = 104.01(1) degrees. Structural, NMR spectroscopic, and vibrational data prove a weak coordination of the two DMF molecules. Structural, vibrational, and NMR spectroscopic evidence is given for a successive weakening of the pi back-bonding effect of the W-P bond in the order [W(CO)(5)PH(R(f))(2)], [Hg[(mu-P(R(f))(2))W(CO)(5)](2)], and [W[P(R(f))(2)](CO)(5)](-) with R(f) = C(6)F(5) and CF(3). The pi back-bonding effect of the W-C bonds increases vice versa.     

Fe2(SR)2(mu-CO)(CNMe)6]2+ and analogues: a new class of diiron dithiolates as structural models for the H(ox)Air state of the fe-only hydrogenase

Low-temperature oxidation of Fe(2)(S(2)C(n)H(2n)(CNMe)(6-x)(CO)x (n = 2, 3; x = 2, 3) affords a family of mixed carbonyl-isocyanides of the type [Fe(2)(S(2)C(n)H(2n)(CO)x(CNMe)(7-x)](2+). The degree of substitution is controlled by the RNC/Fe ratio, as well as the degree of initial substitution at iron, with tricarbonyl derivatives favoring more highly carbonylated products. The structures of the monocarbonyl derivatives [Fe(2)(S(2)C(n)H(2n))(mu-CO)(CNMe)(6)](PF(6))(2) (n = 2,3) established crystallographically and spectroscopically, are quite similar, with Fe---Fe distances of ca. 2.5 A, although the mu-CO is unsymmetrical in the propanedithiolate derivative. Isomeric forms of [Fe(2)(S(2)C(3)H(6))(CO)(CNMe)(6)](PF(6))(2) were characterized where the CO is bridging or terminal, the greatest structural difference being the 0.1 A elongation of the Fe---Fe distance when MeNC (vs CO) is bridging. In the dicarbonyl species, [Fe(2)(S(2)C(2)H(4))(mu-CO)(CO)(CNMe)(5)](PF(6))(2), the terminal CO ligand is situated at one of the basal sites, not trans to the Fe---Fe vector. Oxidation of Fe(2)(S(2)C(2)H(4))(CNMe)(3)(CO)(3) under 1 atm CO gives the deep pink tricarbonyl [Fe(2)(S(2)C(2)H(4))(CO)(3)(CNMe)(4)](PF(6))(2). DFT calculations show that a bridging CO or MeNC establishes a 3-center, 2-electron bond within the two Fe(II) centers, which would otherwise be nonbonding.     

New class of diiron dithiolates related to the Fe-only hydrogenase active site: synthesis and characterization of [Fe2(SR)(2)(CNMe)7]2+

Electron-rich polyisocyano derivatives Fe(2)(S(2)C(n)H(2n)(CO)(6-x)(CNMe)(x) (x approximately 4) undergo oxidatively induced (FeCp(2)(+)) reaction with additional CNMe to give [Fe(2)(SR)(2)(CNMe)(7)](PF(6))(2), a new class of iron thiolates. Crystallographic characterization established that the 34 e(-) dinuclear core resembles the oxidized (H(2)-binding) form of the active sites of the Fe-only hydrogenases, key features being the face-sharing bioctahedral geometry, the mu-CX ligand, and an Fe-Fe separation of 2.61 A. Oxidation of the phenylthiolate Fe(2)(SPh)(2)(CO)(2)(CNMe)(4) led to mononuclear [Fe(SPh)(CNMe)(5)](PF(6)), which is analogous to [Fe(2)(SR)(2)(CNMe)(10)](PF(6))(2) formed upon treatment of [Fe(2)(S(2)C(3)H(6))(CNMe)(7)](PF(6))(2) with excess CNMe.     

A high-pressure iron K-edge x-ray absorption spectral study of the spin-state crossover in (Fe[HC(3,5-(CH(3))(2)pz)(3)](2))I(2) and (Fe[HC(3,5-(CH(3))(2)pz)(3)](2))(BF(4))(2)

The room temperature iron K-edge X-ray absorption near edge structure spectra of (Fe[HC(3,5-(CH(3))(2)pz)(3)](2))I(2) and (Fe[HC(3,5-(CH(3))(2)pz)(3)](2))(BF(4))(2) have been measured between ambient and 88 and 94 kbar, respectively, in an opposed diamond anvil cell. The iron(II) in (Fe[HC(3,5-(CH(3))(2)pz)(3)](2))I(2)undergoes the expected gradual spin-state crossover from the high-spin state to the low-spin state with increasing pressure. In contrast, the iron(II) in (Fe[HC(3,5-(CH(3))(2)pz)(3)](2))(BF(4))(2) remains high-spin between ambient and 78 kbar and is only transformed to the low-spin state at an applied pressure of between 78 and 94 kbar. No visible change is observed in the preedge peak in the spectra of (Fe[HC(3,5-(CH(3))(2)pz)(3)](2))I(2) with increasing pressure, whereas the preedge peak in the spectra of ((e[HC(3,5-(CH(3))(2)pz)(3)](2))(BF(4))(2) changes as expected for a high-spin to low-spin crossover with increasing pressure. The difference in the spin-state crossover behavior of these two complexes is likely related to the unusual behavior of (Fe[HC(3,5-(CH(3))(2)pz)(3)](2))(BF(4))(2) upon cooling.     

Grid expansion: a rhombiclike [L4Fe2(Ag2)2] complex containing Ag2 dumbbells at two vertices

The pyrazolate-based ditopic ligand HL forms a strongly hydrogen-bonded corner complex dimer [Fe(II)(HL)(2)](2)(BF(4))(4) (1) with a [2 × 2] gridlike arrangement of four ligand strands. The two empty vertices can then be filled with {Ag(2)}(2+) dumbbells, yielding the unprecedented diferric complex [L(4)Fe(III)(2)(Ag(I)(2))(2)](BF(4))(6) (2) that features a rhombiclike structure with an almost planar hexagon of metal ions.     

Recognition of topological isomerism: synthesis,structure, and magnetic properties of two pentanuclear high-spin molecules of the type [NiI(N-N)2]3[FeIII(CN)6]2

The syntheses, structures, and magnetic properties of two pentanuclear cyanide-bridged compounds are reported. The trigonal bipyramidal molecule [[Ni(tmphen)(2)](3)[Fe(CN)(6)](2)].14H(2)O, (1).14H(2)O (tmphen = 3,4,7,8-tetramethyl-1,10-phenanthroline) crystallizes in the space group P2(1)/c (No. 14) with unit cell parameters a = 19.531(4) A, b = 24.895(5) A, c = 24.522(5) A, beta = 98.68(3) degrees, V = 11787(4) A(3), and Z = 4. The pi-pi interactions between the tmphen ligands provide the closest intermolecular contacts of 3.37 A leading to large intermolecular M...M distances (> 8.68 A). The dc magnetic susceptibility of 1 indicates a ferromagnetically coupled S = 4 ground state best fit to the parameters g = 2.23, J = +4.3 cm(-1), and D(Ni) = +8.8 cm(-1) for the Hamiltonian H = -2J [(S(Fe(1)) + S(Fe(2))).(S(Ni(1)) + S(Ni(2)) + S(Ni(3)))] + D[S(Ni(1))(z)(2) + S(Ni(2))(z)(2) + S(Ni(3))(z)(2)]. The extended square molecule [Ni(bpy)(2)(H(2)O)][[Ni(bpy)(2)](2)[Fe(CN)(6)](2)].12H(2)O, (2).12H(2)O (bpy = 2,2'-bipyridine) crystallizes in the space group P1 (No. 2) with unit cell parameters a = 13.264(3) A, b = 17.607(4) A, c = 18.057(4) A, alpha = 94.58(3) degrees, beta = 103.29(3) degrees, gamma = 95.18(3) degrees, V = 4065(2) A(3), and Z = 2. The pi-pi interactions of 3.29 A between the bpy ligands are the closest intermolecular contacts, and the intermolecular M...M separations are greater than 7.76 A. The dc magnetic susceptibility data for 2 are also in accord with an S = 4 ground state arising from intramolecular ferromagnetic coupling. The data were best fit to the parameters g = 2.25, J = J' = +3.3 cm(-1), and D(Ni) = +5.8 cm(-1) for the Hamiltonian H = -2J[(S(Fe(1)) + S(Fe(2))).(S(Ni(1)) + S(Ni(2)))] - 2J'[(S(Fe(2)).S(Ni(3)))] + D[S(Ni(1))(z)(2) + S(Ni(2))(z)(2) + S(Ni(3))(z)(2)]. No evidence for long-range magnetic ordering was observed for crystalline samples of 1 or 2.     

Dinitrosyl iron complexes (DNICs) [L(2)Fe(NO)2]- (L = thiolate): interconversion among {Fe(NO)2}9 DNICs, {Fe(NO)2}10 DNICs, and [2Fe-2S] clusters, and the critical role of the thiolate ligands in regulating NO release of DNICs

Dinitrosyl iron complex [(-SC(7)H(4)SN)(2)Fe(NO)(2)](-) (1) was prepared by reaction of [S(5)Fe(NO)(2)](-) and bis(2-benzothiozolyl) disulfide. In synthesis of the analogous dinitrosyl iron compounds (DNICs), the stronger electron-donating thiolates [RS](-) (R = C(6)H(4)-o-NHCOCH(3), C(4)H(3)S, C(6)H(4)NH(2), Ph), compared to [-SC(7)H(4)SN](-) of complex 1, trigger thiolate-ligand substitution to yield [(-SC(6)H(4)-o-NHCOCH(3))(2)Fe(NO)(2)](-) (2), [(-SC(4)H(3)S)(2)Fe(NO)(2)](-) (3), and [(SPh)(2)Fe(NO)(2)](-) (4), respectively. At 298 K, complexes 2 and 3 exhibit a well-resolved five-line EPR signal at g = 2.038 and 2.027, respectively, the characteristic g value of DNICs. The magnetic susceptibility fit indicates that the resonance hybrid of {Fe(+)((*)NO)(2)}(9) and {Fe(-)((+)NO)(2)}(9) in 2 is dynamic by temperature. The IR nu(NO) stretching frequencies (ranging from (1766, 1716) to (1737, 1693) cm(-)(1) (THF)) of complexes 1-4 signal the entire window of possible electronic configurations for such stable and isolable {Fe(NO)(2)}(9) [(RS)(2)Fe(NO)(2)](-). The NO-releasing ability of {Fe(NO)(2)}(9) [(RS)(2)Fe(NO)(2)](-) is finely tuned by the coordinated thiolate ligands. The less electron-donating thiolate ligands coordinated to {Fe(NO)(2)}(9) motif act as better NO-donor DNICs in the presence of NO-trapping agent [Fe(S,S-C(6)H(4))(2)](2)(2-). Interconversion between {Fe(NO)(2)}(9) [(RS)(2)Fe(NO)(2)](-) and {Fe(NO)(2)}(10) [(Ph(3)P)(2)Fe(NO)(2)] was verified in the reaction of (a) [(RS)(2)Fe(NO)(2)](-), 10 equiv of PPh(3) and sodium-biphenyl, and (b) 2 equiv of thiol, [RS](-), and [(Ph(3)P)(2)Fe(NO)(2)], respectively. The biomimetic reaction cycle, transformation between {Fe(NO)(2)}(9) [(RS)(2)Fe(NO)(2)](-) and {Fe(NO)(2)}(9) [(R'S)(2)Fe(NO)(2)](-), reversible interconversion of {Fe(NO)(2)}(9) and {Fe(NO)(2)}(10) DNICs, and degradation/reassembly of [2Fe-2S] clusters may decipher and predict the biological cycle of interconversion of {Fe(NO)(2)}(9) DNICs, {Fe(NO)(2)}(10) DNICs, and the [Fe-S] clusters in proteins.     

Synthesis and reactivity of bridging and terminal hydrosulfido palladium and platinum complexes. Crystal structures of [NBu4]2[(Pt(c6F5)2(mu-SH)]2], [Pt(C6F5)2(PPh3)[S(H)AgPPh3]], and [Pt(C6F5)2(PPh3)[S(AuPPh3)2]

The reactions of the hydroxo complexes [M(2)R(4)(mu-OH)(2)](2)(-) (M = Pd, R = C(6)F(5), C(6)Cl(5); M = Pt, R = C(6)F(5)), [[PdR(PPh(3))(mu-OH)](2)] (R = C(6)F(5), C(6)Cl(5)), and [[Pt(C(6)F(5))(2)](2)(mu-OH)(mu-pz)](2-) (pz = pyrazolate) with H(2)S yield the corresponding hydrosulfido complexes [M(2)(C(6)F(5))(4)(mu-SH)(2)](2-), [[PdR(PPh(3))(mu-SH)](2)], and [[Pt(C(6)F(5))(2)](2)(mu-SH)(mu-pz)](2-), respectively. The monomeric hydrosulfido complexes [M(C(6)F(5))(2)(SH)(PPh(3))](-) (M = Pd, Pt) have been prepared by reactions of the corresponding binuclear hydrosulfido complexes [M(2)(C(6)F(5))(4)(mu-SH)(2)](2-) with PPh(3) in the molar ratio 1:2, and they can be used as metalloligands toward Ag(PPh(3))(+) to form the heterodinuclear complex [(C(6)F(5))(2)(PPh(3))[S(H)AgPPh(3)]], and toward Au(PPh(3))(+) yielding the heterotrinuclear complexes [M(C(6)F(5))(2)(PPh(3))[S(AuPPh(3))(2)]]. The crystal structures of [NBu(4)](2)[[Pt(C(6)F(5))(2)(mu-SH)](2)], [Pt(C(6)F(5))(2)(PPh(3))[S(H)AgPPh(3)]], and [Pt(C(6)F(5))(2)(PPh(3))[S(AuPPh(3))(2)]] have been established by X-ray diffraction and show no short metal-metal interactions between the metallic centers.     

Synthesis and Characterization of Four Consecutive Members of the Five-Member [Fe(6)S(8)(PEt(3))(6)](n)()(+) (n = 0-4) Cluster Electron Transfer Series

The clusters [Fe(6)S(8)(PEt(3))(6)](+,2+) have been shown by other investigators to be formed by the reaction of [Fe(OH(2))(6)](2+) and H(2)S, to contain face-capped octahedral Fe(6)S(8) cores, and to be components of the five-membered electron transfer series [Fe(6)S(8)(PEt(3))(6)](n)()(+) (n = 0-4) estalished electrochemically. We have prepared two additional series members. Reaction of [Fe(6)S(8)(PEt(3))(6)](2+) with iodine in dichloromethane affords [Fe(6)S(8)(PEt(3))(6)](3+), isolated as the perchlorate salt (48%). Reduction of [Fe(6)S(8)(PEt(3))(6)](2+) with Na(Ph(2)CO) in acetonitrile/THF produces the neutral cluster [Fe(6)S(8)(PEt(3))(6)] (65%). The structures of the four clusters with n = 0, 1+, 2+, 3+ were determined at 223 K. The compounds [Fe(6)S(8)(PEt(3))(6)](ClO(4))(3), [Fe(6)S(8)(PEt(3))(6)] crystallize in trigonal space group R&thremacr;c with a = 21.691(4), 16.951(4) ?, c = 23.235(6), 19.369(4) ?, and Z = 6, 3. The compounds [Fe(6)S(8)(PEt(3))(6)](BF(4))(2), [Fe(6)S(8)(PEt(3))(6)](BF(4)).2MeCN were obtained in monoclinic space groups P2(1)/c, C2/c with a = 11.673(3), 16.371(4) ?, b = 20.810(5), 16.796(4) ?, c = 12.438(4), 23.617(7) ?, beta = 96.10(2), 97.98(2) degrees, and Z = 2, 4. [Fe(6)S(8)(PEt(3))(6)](BPh(4))(2) occurred in trigonal space group P&onemacr; with a = 11.792(4) ?, b = 14.350(5) ?, c = 15.536(6) ?, alpha = 115.33(3) degrees, beta = 90.34(3) degrees, gamma = 104.49(3) degrees, and Z = 1. Changes in metric features across the series are slight but indicate increasing population of antibonding Fe(6)S(8) core orbitals upon reduction. Zero-field M?ssbauer spectra are consistent with this result, isomer shifts increasing by ca. 0.05 mm/s for each electron added, and indicate a delocalized electronic structure. Magnetic susceptibility measurements together with previously reported results established the ground states S = (3)/(2) (3+), 3 (2+), (7)/(2) (1+), 3 (0). The clusters [Fe(6)S(8)(PEt(3))(6)](n)()(+) possess the structural and electronic features requisite to multisequential electron transfer reactions. This work provides the first example of a cluster type isolated over four consecutive oxidation states. Note is also made of the significance of the [Fe(6)S(8)(PEt(3))(6)](n)()(+) cluster type in the development of iron-sulfur-phosphine cluster chemistry.     

Structure and Properties of [Fe(4)S(4){2,6-bis(acylamino)benzenethiolato-S}(4)](2)(-) and [Fe(2)S(2){2,6-bis(acylamino)benzenethiolato-S}(4)](2)(-): Protection of the Fe-S Bond by Double NH.S Hydrogen Bonds

Iron-sulfur clusters containing a singly or doubly NH.S hydrogen-bonded arenethiolate ligand, [Fe(4)S(4)(S-2-RCONHC(6)H(4))(4)](2)(-) (R = CH(3), t-Bu, CF(3)), [Fe(4)S(4){S-2,6-(RCONH)(2)C(6)H(3)}(4)](2)(-), [Fe(2)S(2)(S-2-RCONHC(6)H(4))(4)](2)(-) (R = CH(3), t-Bu, CF(3)), and [Fe(2)S(2){S-2,6-(RCONH)(2)C(6)H(3)}(4)](2)(-), were synthesized as models of bacterial [4Fe-4S] and plant-type [2Fe-2S] ferredoxins. The X-ray structures and IR spectra of (PPh(4))(2)[Fe(4)S(4){S-2,6-(CH(3)CONH)(2)C(6)H(3)}(4)].2CH(3)CN and (NEt(4))(2)[Fe(2)S(2){S-2,6-(t-BuCONH)(2)C(6)H(3)}(4)] indicate that the two amide NH groups at the o,o'-positions are directed to the thiolate sulfur atom and form double NH.S hydrogen bonds. The NH.S hydrogen bond contributes to the positive shift of the redox potential of not only (Fe(4)S(4))(+)/(Fe(4)S(4))(2+) but also (Fe(4)S(4))(2+)/(Fe(4)S(4))(3+) in the [4Fe-4S] clusters as well as (Fe(2)S(2))(2+)/(Fe(2)S(2))(3+) in the [2Fe-2S] clusters. The doubly NH.S hydrogen-bonded thiolate ligand effectively prevents the ligand exchange reaction by benzenethiol because the two amide NH groups stabilize the thiolate by protection from dissociation.     

Postfunctionalization of periodic mesoporous silica SBA-1 with magnesium(II) and iron(II) silylamides

Magnesium silylamide complexes Mg[N(SiHMe(2))(2)](2)(THF)(2) and Mg[N(SiPhMe(2))(2)](2) were synthesized according to transsilylamination and alkane elimination protocols, respectively, utilizing Mg[N(SiMe(3))(2)](2)(THF)(2) and [Mg(n-Bu)](2) as precursors. Cage-like periodic mesoporous silica SBA-1 was treated with donor solvent-free dimeric [Mg{N(SiHMe(2))(2)}(2)](2), [Mg{N(SiMe(3))(2)}(2)](2) and monomeric Mg[N(SiPhMe(2))(2)](2), producing hybrid materials [Mg(NR(2))(2)]@SBA-1 with magnesium located mainly at the external surface. Consecutive grafting of [Mg{N(SiHMe(2))(2)}(2)](2) and [Fe(II){N(SiHMe(2))(2)}(2)](2) onto SBA-1 led to heterobimetallic hybrid materials which exhibit complete consumption of the isolated surface silanol groups, evidencing intra-cage surface functionalization. All materials were characterized by DRIFT spectroscopy, nitrogen physisorption and elemental analysis.     

New nickel(II) and iron(II) helicates and tetrahedra derived from expanded quaterpyridines

As an extension of prior studies involving the linear quaterpyridine ligand, 5,5'-dimethyl-2,2':5',5':2',2'-quaterpyridine 1, the synthesis of the related expanded quaterpyridine derivatives 2 and 3 incorporating dimethoxy-substituted 1,4-phenylene and tetramethoxy-substituted 4,4'-biphenylene bridges between pairs of 2,2'-bipyridyl groups has been carried out via double-Suzuki coupling reactions between 5-bromo-5'-methyl-2'-bipyridine and the appropriate di-pinacol-diboronic esters using microwave heating. Reaction of 2 and 3 with selected Fe(II) or Ni(II) salts yields a mixture of both [M(2)L(3)](4+) triple helicates and [M(4)L(6)](8+) tetrahedra, in particular cases the ratio of the products formed was shown to be dependent on the reaction conditions; the respective products are all sufficiently inert to allow their chromatographic separation and isolation. Longer reaction times and higher concentrations were found to favour tetrahedron formation. The X-ray structures of solvated [Ni(2)(2)(3)](PF(6))(4), [(PF(6)) ? Fe(4)(2)(6)](PF(6))(7), [Fe(4)(3)(6)](PF(6))(8) and [Ni(4)(3)(6)](PF(6))(8) have been determined, while the structure of the parent Fe(II) cage in the series, [(PF(6)) ? Fe(4)(1)(6)](PF(6))(7), was reported previously. The internal volumes of the Fe(II) tetrahedral cages have been calculated and increase from 102 ?(3) for [Fe(4)(1)(6)](8+) to 227 ?(3) for [Fe(4)(2)(6)](8+) to 417 ?(3) for [Fe(4)(3)(6)](8+) and to an impressive 839 ?(3) for [Ni(4)(3)(6)](8+). The corresponding void volume in the triple helicate [Ni(2)(2)(3)](4+) is 29 ?(3).