John D. Cotton

Substituent effects in dimethylsulfoxide-promoted carbon monoxide insertion into iron-alkyl bonds

The insertion reaction of carbon monoxide, induced by dimethylsulfoxide, into iron-carbon sigma bonds in the complexes, [(η5-C5H5)(CO)2-FeR], has been extended to a wide variety of primary and secondary alkyl substituents. Rate constants for the forward (k1) and reverse (k-1) reactions, together with the second order rate constant (k2), for the further reaction of the solvated acyl complex with triphenylphosphine, have been measured and evaluated. The major substituent effect is seen in k1 and appears largely to reflect steric enhancement of reaction. The relative reactivity of a series (methyl through n-octyl) of straight chain alkyl complexes has been compared with a recent theoretical prediction. Thermodynamic and kinetic parameters for the formation of the solvated acyl have been measured and evaluated.

Alkyl substituent effects in ligand-induced carbon monoxide insertion reactions in [(η5-C5H5)(CO)3Mo(benzyl)] systems

The first order reactions of benzyl-molybdenum complexes, [(η5-C5H5)(CO)3Mo(CH2C6H4X)], with triphenylphosphine in acetonitrile to yield the trans-substituted acyl complexes, [(η-C5H5)(CO)2(PPh3)Mo(COCH2C6H4X)], have an overallrate constant which tends towards a limiting value at high triphenylphosphine concentration. The reactivity of the molybdenum-carbon bond is enhanced by electron donating substituents, X, and can be analysed in terms of Hammett substituent parameters. A marked enhancement of rate is observed in dimethylsulfoxide, in which solvent the cis-acyl complex is the unexpected initial product.

Reactions of TIN(II) halides with compounds containing a transition metalcarbon σ-bond

Tin(II) chloride inserts into the ironcarbon σ-bond in MeFe(CO)2Cp (Cp = cyclopentadienyl), in refluxing methanol or THF, to give MeClSnFe(CO)2- Cp and traces of Cl3SnFe(CO)2Cp, but, with EtFe(CO)2Cp, a mixture of the insertion product, Cl3SnFe(CO)2Cp, and Cl2Sn[Fe(CO)2Cp]2 is formed. Insertion of tin(II) chloride was not observed for MeMo(CO)3Cp and MeMn(CO)5; in these reactions, and in those between tin(II)_bromide and MeFe(CO)2Cp, EtFe(CO)2Cp and Memo(CO)3Cp, mixtures of halo-metal carbonyls and halotin-metal carbonyls result.

Insertion of group ivb metal dihalides into iron—carbon sigma bonds☆

The insertion reaction of GeCl2, SnCl2 and SnBr2 into the iron-carbon sigma bond in the compounds Cp(CO)2FeR (R = methyl, ethyl, n-propyl, p-methoxybenzyl, benzyl and p-trifluoromethyl- benzyl), to yield complexes with the Group IV metal bound to iron, is a radical chain process. A small substituent effect is observed in all cases for the benzyl series, the reaction being favoured by increasing electron donating ability of the para substituent. No clear reactivity trend is noted for the other alkyl complexes.

Steric effects of tertiary phosphines on nucleophile-induced insertion of carbon monoxide into transition metal-carbon bonds

Rate constants associated with the tertiary phosphine-induced formation of the metal acyls, [(η5-C5H5)(CO)(PR3)FeCOCH2Cy] and [(η5-C5H5)(CO)2(PR3)MoCOCH2Ph], show no correlation with the electronic properties of the phosphine, but exhibit a marked dependence on steric effects. Rates decrease slightly with increase in phosphine cone angles in the lower ranges, but drop very rapidly at higher cone angles. The behaviour is consistent with the concept of a pocket of limited size around the metal centre, with large intermolecular effects arising when the sizes of the tertiary phosphine and the pocket become comparable.

Molybdenum-95 nuclear magnetic resonance. Applications to substituted carbonyls

Molybdenum-95 NMR spectra of a variety of substituted molybdenum carbonyl species are reported. The large chemical shift range permits easy resolution of substituent effects, both within similar ligands and in the number of carbonyls replaced. Molybdenum-95—phosphorus-31 spin—phosphorus-31 spin—spin coupling is observed by this technique for the first time. The results are discussed in terms of general applications of the technique to molybdenum organometallic chemistry.

Substituent effects on the alkyl migration reaction in pentacarbonylbenzylmanganese(I) systems

The k1 step of the formal insertion of carbon monoxide into a MnC σ bond in the reaction of substituted benzylmanganese complexes, [(CO)5MnCH2C6H5-nXn], with tertiary phosphines in acetonitrile is enhanced by electron-donating substituents in meta- and para-positions. Ortho-substitution by a methyl group causes a reduction in k1 compared with para-methyl substitution but di-substitution and single ortho-substitution by large alkyl groups increases reactivity. Similar behaviour occurs with Cl, F and CF3 as ortho-substituents. The value of k-1/k2, which reflects 1/k2, increases with increasing cone angle of the tertiary phosphine but the strongly basic phosphine, PCy3, although unreactive towards [(η5-C5H5)(CO)3MoR] and [(η5-C5H5)(CO)2FeR], induces insertion. Steric influences on the cis/trans isomerisation of the acyl product and its decarbonylation are discussed. Direct insertion of CO is observed for 2,4,6-triisopropyl- and 2,4,6-trimethyl-benzylmanganese complexes.

Solvent and nucleophile effects on the carbonyl insertion reaction into metal-carbon bonds

The rate constant, k3, for the direct insertion of carbon monoxide induced by tertiary phosphines into [(η5-C5H5)(CO)3MoCH 2C6H5] in toluene solution decreases steadily with increasing cone angle of the phosphine. In contrast, for [(CO)5MnCH2C6H5], k3 increases with decrease in the Tolman electronic parameter, v, of the phosphine (i.e., with increasing electron donation) and does not correlate with the cone angle. However, k3 reflects the size of the phosphine to a greater degree as the size of the benzyl substituent on manganese increases. An analysis of k1 values in the solvent-assisted insertion pathway has been made for both the molybdenum and manganese systems in THF, 2-MeTHF and 2,5-Me2THF solutions. The general decrease in k1 with increasing size of the solvent for the cyclopentadienyl-molybdenum system becomes less marked as the size of the benzyl substituent increases. The manganese complexes are relatively less sensitive to solvent size. A comparison has been made between the nucleophilic role of a solvent molecule in the k1 step and that of a tertiary phosphine in the k3 step.

Solvent and substituent effects in isocyanide-induced insertion of carbon monoxide into molybdenum−carbon bonds

The mechanism of reaction of alkyl isocyanides with [(η5-C5H5)(CO)3MoCH2C6H5] to yield isocyanide-substituted acyl complexes is similar to that of the corresponding reaction with tertiary phosphines in that two pathways, one an indirect route involving solvent assistance and the other a direct nucleophilic attack by isocyanide, exist. At very high isocyanide concentrations, decreases in reaction rate, which can be related to the lowering of solvent concentration are observed. The reaction is mildly enhanced by electron-donating substituents. The reaction with cyclohexylisocyanide in benzene yields, in addition to the acyl product, a chelated imino-acyl complex which probably forms by photo- decarbonylation of the acyl derivative.

Structures of [tBucpFe(CO)2]2 and [tBu2cpFe(CO)2]2

Abstract The molecular structures of [ t BucpFe(CO) 2 ] 2 ( 1 ) and [ t Bu 2 cpFe(CO) 2 ] 2 ( 2 ) have been determined by single-crystal X-ray diffraction studies. In the solid state, 1 exists solely as the trans -carbonyl bridged isomer, whereas 2 , containing the bulkier t Bu 2 cp ligand, is unexpectedly found to adopt the cis configuration. The FeFe bonds are approximately equal (2.551(1) A for 1 and 2.5544(7) A for 2 ).