Search

New half-sandwich molybdenum complexes [Mo(CpBz)Cl2(O)] (1), [{Mo(CpBz)(O)2}2(μ-Ο)] (5), and [Mo(CpBz)Cl2]2 (6) (CpBz = C5(CH2Ph)5), the tungsten derivative [W(CpBz)Cl(O)2] (3), and a high yield synthesis of [Mo(CpBz)Cl(O)2] (2) are described. The molecular structures and cyclic voltammetry measurements of 1 and 2 and a comparative study of various molybdenum dioxo complexes as olefin epoxidation catalyst precursors are also presented. The performance of [Mo(CpPrⁱ4)Cl(O)2] (9) as cyclooctene epoxidation catalyst using TBHP as oxidant was determined and compared with other [MoCp‘Cl(O)2] complexes (Cp‘ = C5H5 (7), C5Me5 (8), CpBz (2)) to assess the activity dependence on the ring substituents. Under the same experimental conditions, bimetallic compounds [{MoCp‘(O)2}2(μ-Ο)] (Cp‘ = CpBz (5), Cp* (10), CpBu^t3 (11), CpPrⁱ4 (12)) have been tested and [CpMoO2]2O was checked in aqueous solution, with H2O2 and TBHP as oxidant. Finally, [Mo(CpBz)(CO)3CH3] (4) was used as a catalyst precursor for cyclooctene epoxidation in the presence of TBHP. The analogy between the behaviors displayed by this complex and complex 2 is discussed.

The complexes (η⁵-C5R5)MoO2Cl (R = H, CH3 (Me), CH2Ph (Bz)) are readily prepared from the parent carbonyls (η⁵-C5R5)Mo(CO)3Cl upon reaction with t-BuOOH (TBHP) in n-decane. The compounds are characterized by vibrational spectroscopy, ¹H, ¹³C, and ⁹⁵Mo NMR spectroscopy, and elementary analysis and are compared to their (η⁵-C5R5)ReO3 homologues. The Mo−C5R5 force constants have been determined. (η⁵-C5Bz5)MoO2Cl can be stored and handled at room temperature without decomposition, in contrast to the more temperature sensitive Cp (R = H) and Cp* (R = Me) analogues. The (η⁵-C5R5)MoO2Cl complexes catalyze the epoxidation of cyclooctene, styrene, and 1-octene with TBHP as oxidizing agent. The highest activity is found for (η⁵-C5Bz5)MoO2Cl:  TOF 21000 mol/(mol × h) for cyclooctene in CH2Cl2 at 55 °C with a ratio catalyst:substrate:TBHP = 0.0001:1:2.5. This activity even surpasses that of the well-known MeReO3/H2O2 system. The stable parent carbonyls (η⁵-C5R5)Mo(CO)3Cl can be used as catalyst precursors since they are transformed into (η⁵-C5R5)MoO2Cl under the operating catalytic conditions.

The inclusion compounds formed between β-cyclodextrin (β-CD) and the tetrafluoroborate salts [Cp‘Mo(η⁴-C6H8)(CO)2][BF4] and the neutral derivatives Cp‘Mo(η³-C6H7)(CO)2 [Cp‘ = Cp (η⁵-C5H5), Ind (η⁵-C9H7)] were studied by means of elemental analysis, FTIR spectroscopy, thermogravimetric analysis (TGA), powder X-ray diffraction (XRD), and magic-angle spinning (MAS) NMR (¹³C, ¹¹B). Additional information concerning the possible structure of the inclusion compounds was obtained from ab initio calculations using a two-layer approximation. The cationic and neutral η⁵-cyclopentadienyl analogues form stable two-to-one (host-to-guest) channel-type inclusion compounds in a crystalline state. By contrast, the η⁵-indenyl analogues form only weak complexes with β-CD and it is evident that the organometallic guests are easily liberated from the host cavities. The ab initio calculations revealed that the steric hindrance arising from the presence of the indenyl ligand is a possible explanation for the experimentally observed lower stability of these compounds.

The dicationic complexes [IndCpMoL2]²⁺ (Ind = indenyl; Cp = cyclopentadienyl; 1, L2 = dppe; 2, L = PMe3; 4, L2 = Bipy; 5, L2 = ^tBu2bipy; 6, L2 = H2biim; 7, L = CO and CNMe; 8, L = CNMe; 9, L = CN^tBu; 10, L = NCMe and NMF; 11, L = NMF) were synthesized from [IndCpMoL‘2]²⁺ (L‘ = CO, NCMe), [IndCpMo(NCMe)Cl]BF4, or IndCpMoCl2 by ligand substitution. The neutral complexes (η³-Ind)CpMoL2 (13, L2 = dppe; 14, L = PMe3; 15, L2 = Bipy; 16, L = CO and CNMe; 17, L = CN^tBu) were obtained upon reduction of the respective dications with Cp2Co. (η³-Ind)CpMo(dppe) (13) was also prepared by deprotonation of the cyclopentadiene cation [IndMo(η⁴-C5H6)(dppe)]BF4 (12) with NEt3. The cyclic voltammograms of the dications present one reversible 2e reduction step from Mo(IV) to Mo(II) except for 1, 2, and 6, in which cases two reversible 1e waves are observed. The ring-slipped complexes (η³-Ind)CpMoL2 and the parent dications present similar CV's. The X-ray crystal structures of [IndCpModppe][BF4]2 and (η³-Ind)CpMo(dppe) confirm the indenyl ring planar η⁵ coordination and slip-folded η³, respectively.

The complex IndMoCl3(CO)2 (Ind = η⁵-C9H7) has been synthesized by the reaction of IndMo(η³-C3H5)(CO)2 with HCl gas in dichloromethane at 25 °C. Thermal decarbonylation of IndMoCl3(CO)2 in chloroform gives paramagnetic IndMoCl3 (μeff = 1.48 μB). A consistent stepwise route to bis-indenyl derivatives of molybdenum is available upon treatment of IndMoCl3(CO)2 with KInd in THF at low temperature. The complex IndMo(η³-Ind)(CO)2 is obtained in high yield and opens a convenient entry into bis-indenyl molybdenum chemistry. The reaction of IndMo(η³-Ind)(CO)2 with Ph3CBF4, in CH2Cl2, yields [Ind2Mo(CO)2][BF4]2. Solvolysis of [Ind2Mo(CO)2][BF4]2 in acetonitrile affords [Ind2Mo(CO)(NCMe)][BF4]2 and [Ind2Mo(NCMe)2][BF4]2. The reaction of [Ind2Mo(NCMe)2][BF4]2 with [Bu4N][S2CNEt2] forms the dithiocarbamate complex [Ind2Mo(S2CNEt2)][BF4]. The molecular structures of IndMo(η³-Ind)(CO)2 and [Ind2Mo(S2CNEt2)][BF4] have been determined by single-crystal X-ray diffraction.

The influence of several phosphorus(III) ligands upon the activity and the stereoregulation ability of the soluble organometallic complex [Ni(η³-(2-methylallyl))(η⁴-COD)]PF6 (COD = 1,5-cyclooctadiene), 1, is studied. There is a general increase of the catalytic activity which can be associated with the P(III) ligand's lability. A detailed analysis of the mass distribution and microstructure is carried out by GPC and ¹H and ¹³C NMR of the solvent-fractionated polymers. In the majority of cases, regioregular low molecular weight polymers with the structure PhCH&dbd;CH2[CH(Ph)CH2]nCH(Ph)CH3 are obtained. The addition of bulky phosphines, such as P(o-Tol)3 (o-Tol = o-tolyl) and PCy3 (Cy = cyclohexyl), affords highly isotactic oligomers (≈90%). For the latter, the regioselectivity of the chain growth is broken during the insertion of the last monomer, which results in the formation of a tail-to-tail end group, CH2&dbd;C(C6H5)CH(C6H5)CH2−. The superimposed effects of η³-coordination of the chain end and the large stereochemical influence of these ligands seem to be the reason for the observed isospecifity.

The reaction of a series of cationic and neutral complexes [(η⁵-Ind)Mo(CO)2L2]^0,+ (1, L = NCMe; 2, L = CNMe; 3, L = OPPh3; 4, L = OP(OMe)3; 5, L = PMe3; 6, L2 = (PhSCH2)2; 7, L2 = (O2CCF3)^-; 8, L2 = (O2CCH3)^-; 9, L2 = (O2CPh)^-; 10, L2 = [(NPh)2CH]^-; 11, L2 = [(N-p-tolyl)2CH]^-; 12, L2 = (S2CNEt2)^-; 13, L2 = (S2P(OEt)2)^-; 14, L2 = acac^-; 15, L2 = MeC(O)CHC(NPh)CMe) with acetonitrile produces the ring-slipped adducts [(η³-Ind)MoL2(NCMe)(CO)2]^0,+ only in the cases where L = NCMe, OPPh3, OP(OMe)3 or L2 = (O2CCF3)^-, acac^-. The adducts are labile and have been characterized by means of the distinctive chemical shift of the central pseudo-allylic proton of the η³-indenyl ligand. In the other cases no ring slippage is observed. In NCMe solvent 6 undergoes substitution to give 1, but the macrocycle trithiacyclononane (ttcn) reacts with 1 to give [(η³-Ind)Mo(CO)2(κ³-ttcn)]⁺ as the BF4^- salt. PMe3 reacts with 1 to give substitution products in a stepwise fashion. [(η⁵-Ind)Mo(NCMe)(PMe3)(CO)2]BF4 is formed instantaneously at −70 °C, but 5 forms slowly at room temperature without any detectable ring-slipped intermediate. The crystal structure of the isonitrile cation 2 is presented. 2 adds neither NCMe nor CNMe but undergoes CO substitution with excess CNMe. Extended Hückel and density function calculations predict the most stable conformation of [(η⁵-Ind)MoL2(CO)2]⁺ (L = NCR, CNR) to be the one with the ring trans to the carbonyls, while for the [(η³-Ind)MoL2(NCR)(CO)2]⁺ complexes the ring lies over the carbonyls. According to the dft calculations, acetonitrile addition induces η⁵ → η³ slipping of the ring in the NCR derivatives but not in the CNR analogues, because the reaction is enthalpically driven in the former and not in the latter. The dft calculations of part of the reaction path indicate that the indenyl ring starts to slip when the incoming ligand is still far from the metal (~400 pm), thus supporting earlier kinetically based proposals.

The novel (η⁵-Ind)Mo(η³-Flu)(CO)2 (Ind = indenyl, C9H7; Flu = fluorenyl, C13H9) can be readily prepared from (η⁵-Ind)MoCl3(CO)2 and LiFlu in toluene (90% yield). X-ray crystallographic studies show an exocyclic η³ coordination for the fluorenyl ligand, which is also predicted from dft calculations.