Generation, characterization, and electrochemical behavior of the palladium-hydride cluster [Pd3(dppm)3(mu3-CO)(mu3-H)]+ (dppm=Bis(diphenylphosphinomethane)

Addition of formate on the dicationic cluster [Pd(3)(dppm)(3)(mu(3)-CO)](2+) (dppm=bis(diphenylphosphinomethane) affords quantitatively the hydride cluster [Pd(3)(dppm)(3)(mu(3)-CO)(mu(3)-H)](+). This new palladium-hydride cluster has been characterised by (1)H NMR, (31)P NMR and UV/Vis spectroscopy and MALDI-TOF mass spectrometry. The unambiguous identification of the capping hydride was made from (2)H NMR spectroscopy by using DCO(2) (-) as starting material. The mechanism of the hydride complex formation was investigated by UV/Vis stopped-flow methods. The kinetic data are consistent with a two-step process involving: 1) host-guest interactions between HCO(2) (-) and [Pd(3)(dppm)(3)(mu(3)-CO)](2+) and 2) a reductive elimination of CO(2). Two alternatives routes to the hydride complex were also examined : 1) hydride transfer from NaBH(4) to [Pd(3)(dppm)(3)(mu(3)-CO)](2+) and 2) electrochemical reduction of [Pd(3)(dppm)(3)(mu(3)-CO)](2+) to [Pd(3)(dppm)(3)(mu(3)-CO)](0) followed by an addition of one equivalent of H(+). Based on cyclic voltammetry, evidence for a dual mechanism (ECE and EEC; E=electrochemical (one-electron transfer), C=chemical (hydride dissociation)) for the two-electron reduction of [Pd(3)(dppm)(3)(mu(3)-CO)(mu(3)-H)](+) to [Pd(3)(dppm)(3)(mu(3)-CO)](0) is provided, corroborated by digital simulation of the experimental results. Geometry optimisations of the [Pd(3)(H(2)PCH(2)PH(2))(3)(mu(3)-CO)(mu(3)-H)](n) model clusters were performed by using DFT at the B3 LYP level. Upon one-electron reductions, the Pd--Pd distance increases from a formal single bond (n=+1), to partially bonding (n=0), to weak metal-metal interactions (n=-1), while the Pd--H bond length remains relatively the same.     

Synthesis and characterisation of nu3-octahedral [Ni36Pd8(CO)48]6- and [Ni35Pt9(CO)48]6- clusters displaying unexpected surface segregation of Pt atoms and molecular and/or crystal substitutional Ni/Pd and Ni/Pt disorder

The synthesis and structure, as well as the chemical and electrochemical characterisation of two new nu(3)-octahedral bimetallic clusters with the general [Ni(44-x)M(x)(CO)(48)](6-) (M = Pd, x = 8; M = Pt, x = 9) formula is reported. The [Ni(35)Pt(9)(CO)(48)](6-) cluster was obtained in reasonable yields (56 % based on Pt) by reaction of [Ni(6)(CO)(12)](2-) with 1.1 equivalents of Pt(II) complexes, in ethyl acetate or THF as the solvent. The [Ni(36)Pd(8)(CO)(48)](6-) cluster was obtained from the related reaction with Pd(II) salts in THF, and was isolated only in low yields (5-10 % based on Pd), mainly because of insufficient differential solubility of its salts. The unit cell of the [NBu(4)](6)[Ni(35)Pt(9)(CO)(48)] salt contains a substitutionally Ni-Pt disordered [Ni(24)(Ni(14-x)Pt(x))Pt(6)(CO)(48)](6-) (x = 3) hexaanion. A combination of crystal and molecular disorder is necessary to explain the disordering observed for the Ni/Pt sites. The unit cell of the corresponding [Ni(36)Pd(8)(CO)(48)](6-) salt contains two independent [Ni(30)(Ni(8-x)Pd(x))Pd(6)(CO)(48)](6-) (x = 2) hexaanions. The two display similar substitutional Ni-Pd disorder, which probably arises only from crystal disorder. The structure of [Ni(36)Pd(8)(CO)(48)](6-) establishes the first similarity between the chemistry of Ni-Pd and Ni-Pt carbonyl clusters. A comparison of the chemical and electrochemical properties of [Ni(35)Pt(9)(CO)(48)](6-) with those of the related [Ni(38)Pt(6)(CO)(48)](6-) cluster shows that surface colouring of the latter with Pt atoms decreases redox as well as protonation propensity of the cluster. In contrast, substitution of all internal Pt and two surface Ni with Pd atoms preserves the protonation behaviour and is only detrimental with respect to its redox aptitude. A qualitative rationalisation of the different surface-site selectivity of Pt and Pd, based on distinctive interplays of M--M and M--CO bond energies, is suggested.     

Synthese und Struktur von [(Ph3PAu)6Co(CO)2](PF6) und [(Ph3PAu)7Co(CO)2](PF6)2

Synthesis and Structure of [(Ph₃PAu)₆Co(CO)₂](PF₆) and [(Ph₃PAu)₇Co(CO)₂](PF₆)₂ By the reaction of (Ph₃PAu)₄Co[(CO)₃]⁺ with OH^? in the presence of excess Ph₃PAuCl the larger cluster cations [(Ph₃PAu)₆Co(CO)₂]⁺ ( 1 ) and [(Ph₃PAu)₇Co(CO)₂]²⁺ ( 2 ) can be built up with 1 being the main product. 1 crystallizes with PF^?₆ as counterion in the monoclinic space group C2/c with a = 3008.3(6); b = 1339.1(2); c = 2909.4(6) pm; β = 103.08(1)°; Z = 4. The inner core of the cluster cation 1 with the symmetry C₂ has the form of a bicapped trigonal bipyramid with the heteroatom in equatorial position, and distances Au? Au between 280.4(1) and 288.4(1) pm and Co? Au between 254.9(1) and 257.1(2) pm. 2 · (PF₆)₂ crystallizes in the triclinic space group P1 with a = 2155.7(1); b = 1720.6(1); c = 3543.6(1) pm; α = 91.89(1)°; β = 97.51(1); γ = 89.92(1)°; Z = 4. The unit cell contains two symmetry independent cluster cations 2 of about the same geometry. The cluster skeleton Au₇Co can be described as fragment of an icosahedron formed by seven gold atoms with the Co atom in its center. The Au? Au distances range from 274.8(3) to 332.6(3) pm, and the Co? Au distances are 256.8(6) to 264.7(5) pm. The bonding in 1 and 2 is discussed.     

Niobium Alcoholate Clusters with an Octahedral Arrangement of Metal Atoms: [K(CH3OH)4]2 [Nb6(OCH3)18] and [Na([18]crown‐6)(C2H5OH)2]2 [Nb6(OC2H5)12(NCS)6]

Complete exchange : [M₆X₁₂] type cluster compounds with an octahedral M₆ metal atom arrangement, which is completely surrounded by alcoholato ligands, were unknown until now. The first representatives are prepared containing a [Nb₆(OR)₁₂]⁴⁺ unit (R=CH₃ or C₂H₅). They are accessible at elevated temperatures from strongly basic alcoholate solutions of [Nb₆Cl₁₂]²⁺‐containing precursors. C gray, H white, K turquoise, Nb blue, O red.

     

Synthesis, molecular structure and properties of the [H6-nNi30C4(CO)34(CdCl)2]n- (n=3-6) bimetallic carbide carbonyl cluster: a model for the growth of noncompact interstitial metal carbides

Reaction of the [Ni(9)C(CO)(17)](2-) dianion with CdCl(2)2.5 H(2)O in THF affords the novel bimetallic Ni--Cd carbide carbonyl clusters [H(6-n)Ni(30)C(4)(CO)(34)(micro(5)-CdCl)(2)](n-) (n=3-6), which undergo several protonation-deprotonation equilibria in solution depending on the basicity of the solvent or upon addition of acids or bases. Although the occurrence in solution of these equilibria complicates the pertinent electrochemical studies on their electron-transfer activity, they clearly indicate that the clusters [H(6-n)Ni(30)C(4)(CO)(34)(micro(5)-CdCl)(2)](n-) (n=3-6), as well as the structurally related [H(6-n)Ni(34)C(4)(CO)(38)](n-) (n=4-6), undergo reversible or partially reversible redox processes and provide circumstantial and unambiguous evidence for the presence of hydrides for n=3, 4 and 5. Three of the [H(6-n)Ni(30)C(4)(CO)(34)(micro(5)-CdCl)(2)](n-) anions (n=4-6) have been structurally characterized in their [NMe(3)(CH(2)Ph)](4)[H(2)Ni(30)C(4)(CO)(34)(CdCl)(2)]2 COMe(2), [NEt(4)](5)[HNi(30)C(4)(CO)(34)(CdCl)(2)]2 MeCN and [NMe(4)](6)[Ni(30)C(4)(CO)(34)(CdCl)(2)]6 MeCN salts, respectively. All three anions display almost identical geometries and bonding parameters, probably because charge effects are minimized by delocalization over such a large metal carbonyl anion. Moreover, the Ni(30)C(4) core in these Ni-Cd carbide clusters is identical within experimental error to those present in the [HNi(34)C(4)(CO)(38)](5-) and [Ni(35)C(4)(CO)(39)](6-) species, suggesting that the stepwise assembly of their nickel carbide cores may represent a general pathway of growth of nickel polycarbide clusters. The fact that the [H(6-n)Ni(30)C(4)(CO)(34)(micro(5)-CdCl)(2)](n-)(n=4-6) anions display two valence electrons more than the structurally related [H(6-n)Ni(34)C(4)(CO)(38)](n-) (n=4-6) species has been rationalized by extended Hückel molecular orbital (EHMO) analysis.     

Unexpected reaction of the unsaturated cluster host and catalyst [Pd3(mu3-CO)(dppm)3]2+ with the hydroxide ion: spectroscopic and kinetic evidence of an inner-sphere mechanism

The title cluster, [Pd(3)(mu(3)-CO)(dppm)(3)](2+) (dppm=bis(diphenylphosphino)methane), reacts with one equivalent of hydroxide anions (OH(-)), from tetrabutylammonium hydroxide (Bu(4)NOH), to give the paramagnetic [Pd(3)(mu(3)-CO)(dppm)(3)](+) species. Reaction with another equivalent of OH(-) leads to the zero-valent compound [Pd(3)(mu(3)-CO)(dppm)(3)](0). From electron paramagnetic resonance analysis of the reaction medium using the spin-trap agent 5,5-dimethyl-1-pyrroline-N-oxide (DMPO), the 2-tetrahydrofuryl or methyl radicals, deriving from the tetrahydrofuran (THF) or dimethyl sulfoxide (DMSO) solvent, respectively, were detected. For both [Pd(3)(mu(3)-CO)(dppm)(3)](2+) and [Pd(3)(mu(3)-CO)(dppm)(3)](+), the mechanism involves, in a first equilibrated step, the formation of a hydroxide adduct, [Pd(3)(mu(3)-CO)(dppm)(3)(OH)]((n-1)+) (n=1, 2), which reacts irreversibly with the solvent. The kinetics were resolved by means of stopped-flow experiments and are consistent with the proposed mechanism. In the presence of an excess of Bu(4)NOH, an electrocatalytic process was observed with modest turnover numbers (7-8). The hydroxide adducts [Pd(3)(mu(3)-CO)(dppm)(3)(OH)]((n-1)+) (n=1, 2), which bear important similarities to the well-known corresponding halide adducts [Pd(3)(mu(3)-CO)(dppm)(3)(mu(3)-X)](n) (X=Cl, Br, I), have been studied by using density functional theory (DFT). Although the optimised geometry for the cluster in its +2 and 0 oxidation states (i.e., cation and anion clusters, respectively) is the anticipated mu(3)-OH form, the paramagnetic species, [Pd(3)(mu(3)-CO)(dppm)(3)(OH)](0), shows a mu(2)-OH form; this suggests an important difference in electronic structure between these three species.