Stephen L. Gipson

The reaction of CpFe(CO)2X(X Cl, Br, I) with phosphines catalyzed by [CpFe(CO)2]2: evidence for an electron transfer chain catalysis mechanism

Abstract The catalytic substitution of phosphines for halide on CpFe(CO)2X(X  Cl, Br, I) has been shown to proceed through an electron transfer chain catalysis mechanism. The reaction may be initiated by CpFe(CO)2(PR3) generated by photochemical or thermal cleavage of [CpFe(CO)2]2 or by addition of catalytic amounts of other strong reductants.

New synthesis of [RuH(CO)(PPh3)2L2][BF4] [L2 = 2py, 2(4-Mepy) or bpy] and X-ray crystal structure of [RuH(CO)(PPh3)2(4-Mepy)2][BF4]

Abstract The title compounds were prepared by reaction of RuH2(CO)(PPh3)3 with [C7H7][BF4] in the presence of excess pyridine, 4-picoline or 2,2′-bipyridine. The compounds were characterized by IR, 1 H and 31 P NMR, and X-ray crystallography (for the 4-Mepy complex). The spectroscopic data as well as the results of the X-ray crystal structure show that the complexes contain trans-oriented phosphines and cis-pyridines. Details of the structural content and selected bond distances and angles are internally consistent.

A new route to pentafluorophenylmolybdenum(0) carbonyl complexes. Synthesis and X-ray crystal structure of PPN[Mo(C6F5)(CO)5]

Abstract In order to facilitate studies of the synthesis and reactivity of pentafluorophenylmolybdenum(0) carbonyl complexes, an efficient synthetic route to the pentacarbonyl complex, [Mo(C6F5)(CO)5]−, has been sought. It has now been shown that the known organometallic compounds AgC6F5 and PPN[MoCl(CO)5] [PPN=bis(triphenylphosphoranylidene)ammonium] react to give PPN[Mo(C6F5)(CO)5] in high purity and good yield. This compound has been characterized by IR, NMR, elemental analysis, and X-ray crystallography.

The structural analysis of acetonitrile-cis-dicarbonyl(η5-cyclopentadienyl)(triphe-nylphosphine)molybdenum(II) tetrafluoroborate, cis-[(η5-C5H5)Mo(CO)2(PPh3)(NCCH3)]+[BF4]−

Abstract The crystal structure of the title compound has been determined by means of single crystal X-ray diffraction techniques. The subject compound crystallizes in the monoclinic space group P21/c (No. 14, C2h5. The lattice constants are a=12.997(3), b=11.729(3), c=17.681(4) A and β=100.73(2)°. Anisotropic refinement based on 3655 unique reflections using a full-matrix least-squares program has yielded R=0.046 and Rw=0.051. There are four molecules per unit cell, Dm=1.52(3) and Dx=1.52 Mg m−3. The tetrafluoroborate anion displayed a great deal of disorder. Utilizing cone angle calculations, a ‘ligand profile’ describes the steric bulk of the triphenylphosphine functional group. The resulting maximum cone angle, θ, for the triphenylphosphine group is 120°. Important metrical details are: MP=2.5356(8); MN=2.156(3); MoC=1.978(4) and 1.950(4); CN=1.120(4); and CO=1.156(5) and 1.147(4) A.

Ligand effects on the electrooxidation of molybdenum halide complexes of the type CpMo(CO)3−n(PR3)nX and ChMo(CO)2X

Abstract Complexes of the type CpMo(CO) 3− n (PR 3 ) n X and ChMo(CO) 2 X, where Cp = η 5 -C 5 H 5 , Ch = η 7 -C 7 H 7 and X = halide, can be oxidized electrochemically by one electron in dichloromethane. The potential necessary for oxidation and the rate of decomposition of the resulting cation decrease as n increases or as the phosphine becomes a better electron donor. A linear correlation is observed between the highest energy carbonyl stretching frequency and the formal or peak potential for the oxidation. As the halide is changed from chloride to bromide to iodide the oxidation potential increases but the rate of decomposition of the cation decreases. Both of these trends can be traced to the inverse halide order, in which the oxidation potential increases as the electronegativity of the halide ligand decreases. This effect arises from greater metal to halide backbonding in the complexes of the heavier halogens, which decreases electron density on the metal center and thus increases the oxidation potential. However the added bond order with the heavier halogens apparently also stabilizes those cations toward decomposition.

The effect of cationic surfactant films on the electrochemical oxidation of inorganic anions

Abstract Arsenite, sulfide, iodide, thiosulfate and bromide, which gave no anodic voltammetric curves on platinum electrodes in 2 M NaOH, were observed to give anodic curves when Hyamine 2389 (predominantly methyldodecylbenzyltrimethylammonium chloride) was added to the solution because an oxidized film of surfactant was formed on the platinum, excluding water from the electrode surface. The film was indicated to be formed by adsorption of the anions accompanied by ion pairing of the quaternary salt. It was postulated that the surfactant oxidation proceeded through the phenyl group forming a less labile film, probably by cross-linking. The film was made still less labile by addition of ammonia. Iodide oxidation produced iodate. It was concluded that oxidation of iodide proceeded by anodic oxidation of Hyamine in two steps followed by chemical oxidation of the iodide. Increasing the concentration of Hyamine causes an alternating increase and decrease in the height of the iodide anodic current peak. It was concluded that as a monolayer film with the non-polar end exposed to solution was formed, the current increased reaching a maximum on completion of the monolayer; the current then decreased as the second layer covered the first exposing the polar end to solution. This alternation was shown to repeat through the fifth layer.

The synthesis and characterization of cationic molybdenum-mercury complexes of the type [L′HgMo(CO)2L(η-C5H5)][BF4]. X-ray crystal structure of η-cyclopentadienyl(tricarbonyl)(triphenylphosphinemercuro) molybdenum(II) tetrafluoroborate

The reaction of trimetallic complexes of the type [Mo(CO) 2 L(η-C 5 H 5 )] 2 Hg (L = CO, PPh 3 , PMePh 2 or PMe 2 Ph) with Hg(BF 4 ) 2 in the presence of suitable ligands, L′ = PPh 3 or PCy 3 , has been shown to generate cationic bimetallic complexes of the type [L′HgMO(CO) 2 L(η-C 5 H 5 )][BF 4 ]. Several of these compounds have been synthesized and characterized by elemental analysis, IR and 1 H and 31 P NMR spectroscopy. The structure of the complex [Ph 3 PHgMO(CO) 3 (η-C H 5 )][BF 4 ] ( 1 ) has been determined by X-ray crystallography. This compound crystallizes in the triclinic space group P with two molecules per asymmetric unit ( Z = 4). The compound assumes the usual “four-legged piano stool” geometry about the molybdenum atom. The average MoC(ring) distance is 2.31(2) A with the mean MoHg = 2.686(1) A and MoHgP = 160.5(1) and 169.7(1)°. Other averaged important bond distances and angles are: HgP = 2.483(7); MoC(sp) = 1.963(21); CO = 1.153(12) A; and MoCO = 175.5(17)°.

A THERMODYNAMIC STUDY OF THE ELECTRON TRANSFER CHAIN CATALYZED SUBSTITUTION OF TRIPHENYLPHOSPHINE FOR IODIDE ON CPFE(CO)2I

Abstract The thermodynamic parameters governing the electron transfer chain catalyzed substitution of triphenylphosphine for iodide on CpFe(CO)2I have been studied. The reaction is driven by the much higher stability of the triphenylphosphine complex relative to the iodide complex, and proceeds to completion even though the electron transfer which propagates the catalytic chain is endergonic. The standard reduction potential of CpFe(CO)2I is −1.64 V vs. Fc+/Fc, while that of CpFe(CO)2(PPh3)+ is −1.59 V. Nevertheless, the association constant for triphenylphosphine with the 17-electron CpFe(CO)2 fragment is 4×105 times that for iodide (log KP=1.9±1.9, log KI=−3.7±1.9). The rate of the reaction is accelerated enormously by reduction of the iodide complex, which allows the substitution to proceed through the more labile 17/19-electron complexes. The contrast between the π-basicity of iodide and the π-acidity of triphenylphosphine is proposed to be responsible for the favored complexation of triphenylphosphine by the relatively electron-rich CpFe(CO)2 fragment. The application of redox catalysis and redox equilibration to the study of such 17/19e equilibria shows great promise for obtaining these difficult-to-measure formation constants.

Electron-transfer catalysed substitution of dodecacarbonyltrithenium mediated by dibenzoylethylene: A factorial design experiment

Abstract The electron-transfer catalysed substitution of PPh3 for CO on Ru3(CO)12 mediated by dibenzoylethylene was studied using a factorial design experiment. Within the range of conditions investigated, the efficiency of the reaction was affected exclusively by a two-factor interaction between the Ru3(CO)12 and PPh3 concentrations. Maximum turnover numbers were achieved only when both of these concentrations were at their high levels. The dibenzoylethylene concentration and the current had no significant effect on efficiency. Other phosphines (PR3, R = Me and/or Ph or Bu), P(OMe)3 and ButNC show similar reactivity.

Synthesis and characterization of two new molybdenum(0) σ-aryl complexes of the type [N(PPh3)2][Mo(C6F5)L(CO)2{P(OMe)3}2]: x-ray crystal structure of bis(triphenylphosphine)imminium pentafluorophenyl(mer-tricarbonyl) trans-bis(trimethylphosphite)molybdate(0)

Abstract The complex [ cis-mer -Mo(C 6 F 5 )(CO) 2 {P(OMe) 3 } 3 ] − reacts with the π-acid ligands CO and t BuNC to yield the substitution products [Mo(C 6 F 5 )L(CO) 2 {P(OMe) 3 } 2 ] − , both isolated as the N(PPh 3 ) 2 + salts. Both new complexes have been fully characterized by IR, 1 H and 31 P NMR, and elemental analysis. The electrochemistry of all three complexes has also been examined. The structure of [N(PPh 3 ) 2 ][ mer-trans -MO(C 6 F 5 )(CO) 3 {P(OMe) 3 } 2 ] has been determined by X-ray crystallography. The compound displays essentially octahedral geometry about the molybdenum atom. Averaged important bond distances and angles are: MoC(O) = 1.99(3); CO = 1.18(1); MoP = 2.418(1); PO(C) = 1.614(9); OC = 1.440(11); NP = 1.578(9); P  C(ring) = 1.805(4) A ; and MoPO = 119(4)°; POC = 120.9(7)°; NPC(ring) = 111(2)° .