M. David Curtis

Metalmetal multiple bonds : IV. The chemistry of bis(cyclopentadienyldicarbonyl-molybdenum) (MoMo)

Abstract The synthesis and chemistry of Cp 2 Mo 2 (CO) 4 (I) a coplex containing a reaction MoMo triple bond, are described. The formation of I in refluxing xylene is shown to occur via odd-electron intermediates from homolysis of the MoMo bond in the Cp 2 Mo 2 (CO) 6 precursor. Numerous soft nucleophiles add to I to give complexes, e.g., trans -(CpMo(CO) 2 L) 2 (L = Ph 3 P, P(OMe) 3 ), (μ-RC 2 R′)Cp 2 Mo 2 (CO) 4 , and (μ-SR) 2 Cp 2 Mo 2 (CO) 4 . (Et 4 N)(Cp 2 Mo 2 (CO) 4 CN) may be isolated from the reaction of cyanide ion with I. Iodine and hydrogen chloride react to give Cp 2 Mo 2 (CO) 4 I 2 and (μ-H)(μ-Cl)Cp 2 Mo 2 (CO) 4 , respectively. Silver and mercuric ions are reduced to the metal by I, but Ph 2 Hg gives (CpMo(CO) 3 ) 2 Hg in addition to metallic mercury. With tetracyanoethylene, I forms [CpMo(CO) 4 ] + [TCNE] − . Complex I mimics the reaction of acetylenes with L 4 Pt (L = Ph 3 P) and forms the triangulo-cluster, (μ-L 2 Pt)Cp 2 Mo 2 (CO) 4 . With Na 2 Fe(CO) 4 , Cp 2 Mo 2 (CO) 4 I 2 forms the analogous cluster, (μ-Fe(CO) 4 )Cp 2 Mo 2 (CO) 4 . By electron counting rules, these clusters should contain MoMo double bonds. Evidence is presented which indicates I and Co 2 (CO) 8 form an unstable tetrahedrane cluster. At 100°C in toluene, I may reversibly dimerize to give a tetrahedrane Mo 4 -cluster, as shown by scrambling experiments. Complex I reacts slowly with Mn 2 (CO) 10 to give CpMo(CO) 3 Mn(CO) 5 as the only isolable mixed-metal product.

Redistribution Reactions on Silicon Catalyzed by Transition Metal Complexes

Publisher Summary This chapter discusses redistributions catalyzed by transition-metal complexes. Redistribution, or disproportionation, constitutes an important class of reactions in organosilicon chemistry. Redistributions on silicon may be initiated by thermolysis or by catalytic activation at somewhat lower temperatures. In the gas phase, methyl is transferred from Me n SiH 4–n , to CH 3 + almost as readily as hydrogen. Silacyclobutanes with metal substituents may also be polymerized to give novel polymers with pendant metal groups. By using chiral silyl hydrides, it has been demonstrated that the exchanges occur with the retention of configuration at silicon. The complete lack of isomerization of the p -tolyl group during the disproportionation of Ar 3 SiH is noteworthy. Similarly, the vinyl group of trimethylvinylsilane is hydrolyzed by a catalytic amount of Zeises salt [(C 2 H 4 )PtC1 3 ] - in moist acetone. The redistribution reactions of hydridodisilanes of the type R 3 SiSiR 2 H are more varied than those of the corresponding monosilanes. In redistribution reactions catalyzed by transition-metal complexes, di- or polysiloxanes are expected to share some characteristics of both monosilanes and di- or polysilanes. Thus, the availability of the four-membered metallacycle in the catalytic cycle greatly enhances the rate of disproportionation of tetramethyldisiloxane. Some of these catalytic cycles lead to products that appear to arise from divalent silylenoid species.

The Crystal and Molecular Structure of Bis(cyclopentadienyldicarbonylchromium) (CrCr)

Abstract The crystal structure of bis(cyclopentadienyldicarbonyl-chromium) has been determined by x-ray diffraction. The compound crystalizes in the triclinic system, space group P1¯(C1i, No. 2) with unit cell parameters: a, 7.829(3); b, 14.543(6); c, 6.588(2)A; α, 94.67(3), β, 110.70(3); γ, 104.04°(3); V, 699.1(4)A3; z=2. There are two independent molecules per unit cell located at the inversion centers at O,O,O and O, 1/2, O. The Cr Cr bond distances are, respectively 2.200(3) and 2.230(3), thus supporting their formulation as triple bonds. The Cp Cr Cr angles in the two molecules are 165.0° and 158.7°, respectively. The structural features are compared with those of Cp2Mo2(CO)4, which has a linear Cp Mo Mo Cp axis; and the differences rationalized in terms of electronic interactions of the Cp-ligand with the orbitals of the M2 unit. The differences observed in the structures of the two independent molecules are also related to the proposed bonding model and to packing considerations.

Reactions of the metal-metal triple bond in Cp2Mo2(CO)4 and related complexes

Abstract The reactions of the MM triple bonds in compounds of type Cp 2 M 2 (CO) 4 (M = Cr, Mo or W) are reviewed. These reactions are grouped under the headings of synthesis and structures of Cp 2 M 2 (CO) 4 -type compounds, nucleophilic additions to the MM bonds, reactions with 1,3-dipoles, oxidative reactions with nonmetals, and cluster-building reactions. Literature coverage is until the end of 1985 with 102 references.

A C/MoS2 mixed-layer phase (MoSC) occurring in metalliferous black shales from southern China, and new data on jordisite

Abstract A new phase composed mainly of Mo, S, and C and referred to herein as MoSC occurs widely in organic-rich, metalliferous Cambrian black shales in south China. MoSC, which has previously been referred to as jordisite, has been studied by scanning electron microscopy (SEM), electron microprobe analysis (EMPA), transmission electron microscopy (TEM), powder X-ray diffraction, (XRD), extended X-ray absorption fine structure (EXAFS), and catalytic activity. TEM data show MoSC to have a layered structure, with packets resembling molybdenite and graphite-like carbon that average five layers in thickness. Analytical data are consistent with an idealized formula of Mo3S6C10 but it commonly contains 1-3 wt% each of Fe, Ni, and As so that its composition may be better approximated by the formula (Mo,Fe,Ni)3(S,As)6C10. Selected area electron diffraction (SAED) patterns show a small number of broad, inhomogeneous rings corresponding to randomly oriented layers arranged in subspherical cells. A single broad, weak peak corresponds to a 10-11 Å layer spacing in powder XRD diffraction patterns. Pseudomorphism after fossil bacteria implies an origin by replacement of sedimentary organic material. In its chemical properties and structure, MoSC resembles synthetic compounds used as catalysts for hydrodesulfurization (HDS) in the petrochemical industry. The large surface-to-volume ratio for MoSC may be an important factor in its relatively strong HDS catalytic activity. Cotype samples of jordisite from Germany, previously thought to be amorphous MoS2, were also studied by SEM and TEM. Jordisite occurs as sequences of a few curved layers that form subspherical units, with an appearance remarkably like that of MoSC. However, the layer spacing is ~6 Å, like that of molybdenite. The ratio of Mo:S is ~1:2, and no carbon was detected, although it coexists with kerogen. Jordisite is thus confirmed to be a form of MoS2, but because powder diffraction- like SAED patterns were obtained, it is not amorphous.

pxpx Bonding in silicon compounds. Ehmo and cndo calculations

Abstract Extended Hūckel (EHMO) calculations on the molecule H 2 CSiH 2 (silaethylene) and H 2 SiSiH 2 (disilaethylene) have been performed and the results subjected to a Mulliken population analysis to elucidate the factors responsible for the instability of such molecules. These calculations indicate that the CSi π-bond is exceedingly polar, and that energy mismatching of carbon and silicon p -orbitals is in large part reponsible for the weakness of the π-bond. The relatively high overlap population of the SiSi π-bond suggests that compounds containing such bonds might be amenable to isolation. These conclusions were reinforced by calculating barriers to rotation about the π-bond via EHMO and CNDO methods; the barrier increases in the order CSi 2 H 4 and Si 2 H 4 in which the triplet state of the 90°-twisted molecule has lowest energy, the singlet state of twisted H 2 CSiH 2 is lowest and corresponds to the configuration, H 2 C − Si + H 2 . Although Si d -orbitals strengthen π-bonds by the formation of p - d hybrids, inclusion of d -orbitals in the basis set decreases the rotational barrier by providing greatly increased bonding capabilities in the excited states.

Syngas and HDS catalysts derived from sulphido bimetallic clusters

The clusters, CP2′Mo2Fe2S2(CO)8 (MoFeS) and Cp2′Mo2CO2S3(CO)4 (MoCoS) (Cp′ = η-C5H4Me) have been supported on the refractory oxides, Al2O3, SiO2, TiO2, and MgO, and subjected to temperature programmed decomposition (TPDE) under flowing H2. Typically, CO evolution commences near 100°C, followed by evolution of 1–2 Cp-ligands from 180 to 400°C along with small amounts of CO2, CH4, and H2S or Me2S. The resulting compositions are shown to be active catalysts for CO hydrogenation and hydrodesulphurization (HDS) of thiophene. Methane is the principal hydrocarbon product from CO hydrogenation except for MoFeS/MgO where high selectivity for C2 products was observed. The activity and selectivity of MoCoS/Al2O3 for thiophene HDS closely resembles those of conventionally prepared “cobalt molybdate” catalysts. The cluster derived catalysts have been characterized by Mossbauer and X-ray absorption (XANES) and EXAFS) spectroscopies. It is concluded that the clusters undergo oxidation by the surface upon loss of organic ligands. The results obtained to date show that sulphido bimetallic clusters are excellent precursors for the formation of uniform catalytic surfaces. The uniformity of the surface species facilitates physical characterization of the active site(s). Our results show that the supported clusters are transformed to surface oxo-ensembles which are active for CO hydrogenation and HDS of organic sulphur compounds.

Is indium tin oxide a suitable electrode in organic solar cells? Photovoltaic properties of interfaces in organic p∕n junction photodiodes

The charge generation properties at all interfaces of a p∕n junction, bilayer photodiode have been investigated by means of the photoaction spectrum (PAS) as a function of applied bias. The organic photodiode was fabricated with a low-glass transition temperature (Tg) polysiloxane with pendant hydrazone groups as the p-type material and a perylene diimide derivative as the n-type material. The PAS under short circuit and reverse bias showed an antibatic response at the high-energy region (3.0–3.5eV), and a symbatic response at the low-energy region (2.0–3.0eV). However, under forward bias, the PAS showed the opposite behavior. These results are interpreted in terms of the band structure of tin-doped indium oxide (ITO) that prevents effective photoinjection of electrons at the polymer/ITO interface and the relative energy levels of the constituent materials.

Electron-Deficient Molybdenum/Cobalt/Sulfido Clusters: Chemistry Related to Hydrodesulfurization (HDS) Catalysis

The authors work on the reactions of Cp2Mo2Co2S3(CO)4 (1) and Cp2Mo2 Co2S4(CO)2 (2) with organosulfur compounds is reviewed. Reaction pathways that lead to C S bond scission are proposed, and the relexance of these results to HDS reactions mer commercial “CoMoS” catalysts is discussed.

Small ring metallocycles : V. Crystal and molecular structure of hydrido-1,3-(1,1,3,3-tetramethyldisiloxanediyl)carbonylbis(triphenylphosphine)iridium(III), Me2SiOSiMe2Ir(H)(CO)(PPh3)2 · EtOH

Abstract The structure of the cyclo-metalladisiloxane, Me2SiOSiMe2Ir(H)(CO)(PPh3)2, has been determined by single crystal X-ray diffraction using Mo-Kα radiation. Data were collected to 20 = 45 ° giving 6060 unique reflections,of which 4582 had I ⩾3σ(I). The latter were used in the full-matrix refinement. Crystallographic data: space group, P 1 ; cell constants: 12.604(7),12.470(4), 15.821(6) A, 66.93(6)°, 105.34(7)°, 112.41(8)°;V 2095(3) A3; p(obs) 1.45 g/cm3; p(calc) 1.46g/cm3 (Z=2). The asymmetric unit consists of one iridium complex and one molecule of ethanol of salvation. The structure was solved by standard heavy atom methods and refined with all non-hydrogen atoms anisotrophic to final R factors, R1 0.034 and R2 0.042. The iridium metallocycle has approximate Cs symmetry with the mirror plane passing through the four-membered IrSiOSi ring. The average IrP, IrSi and SiO bond lengths are 2.38, 2.41, and 1.68 A, respectively. The IrCO and CO bond lengths are 1.903(8) and 1.133(8). The H atom bonded to Ir was not located.The Ir atom is raised out of the basal, P2Si2 plane toward the carbonyl by about 0.26 A. The most striking feature of the structure is the strain apparent in the four-membered ring. The internal angels are: 64.7 (SiIrSi), 96.8 (IrSiO), 97.8 (IrSiO), and 99.8 (SiOSi). In an unstrained molecule, the SiOSi angle is normally in the 130–150° range. It is proposed that the strain in the ring is consistent with the catalytic activity of the metallocycle.