A. Turco

Reactions of organic halides and cyanides with bis(tricyclohexylphosphine)nickel(0)

Abstract The reactions of the complex bis(tricyclohexylphosphine)nickel(0) (Ni(PCy 3 ) 2 ) with organic halides and cyanides RX (R  CH 3 , X  I, CN; R  C 2 H 5 , X  Br, I, CN; R  C 3 H 7 , X  Br, CN; R  C 6 H 5 , X  Cl, CN), all involving fission of the RX bond, have been studied in toluene at various temperatures. Oxidative addition of the bromides produces the stable complexes [Ni I Br(PCy 3 ) 2 ] 2 and Ni II (H)(Br)(PCy 3 ) 2 . Methyl iodide affords phosphonium salts of a nickel(II) complex. The organic products of these reactions involve alkanes, alkenes, benzene, and reductive coupling products RR. Except for the methyl and phenyl derivatives the distribution of organic products is rather insensitive to the functional group X. The reactions are discussed in terms of molecular and radical mechanisms. The reactivity of Ni(PCy 3 ) 2 is compared with that of Ni(PEt 3 ) 3 and Ni(PMe 3 ) 4 .

Oxidative addition of alkanenitriles to nickel(0) complexes via π-intermediates

Abstract The alkanenitriles R(C 6 H 5 )CHCN (R = H, CH 3 ) coordinate rapidly and quantitatively through the CN group to the complexes [Ni(PCy 3 ) 2 ] or [{Ni(PCy 3 ) 2 } 2 N 2 ]. Both the end-on [Ni(PCy 3 ) 2 (σ-R′CN)] and the edge-on [Ni(PCy 34 )(π-R′CN)] adducts are formed,and are present in an equilibrium the position of which is governed by the amount of added PCy 3 . The π-complexes react to give the cyano-organometal complexes [Ni(PCy 3 ) n (R′)(CN)] through an oxidative addition involving splitting of the CCN bond. The complexes obtained are unstable and slowly decompose under the reaction conditions to give the coupling R′-R′ (R = H) or the β-elimination R′(-H) (R = CH 3 ) products. The kinetics of the reaction and the stereochemical result suggest a template mechanism in agreement with the findings above.

Mechanism of reductive elimination of benzonitrile in cyanophenyl complexes of nickel(II) : II. Reactions of [cyano(phenyl)bis(tricyclohexylphosphine)nickel(II)]☆

Abstract The thermal decomposition of the complex Ni(CN)(C 6 H 5 )(PCy 3 ) 2 (Cy = cyclohexyl) in decalin has been examined. The complex reacts with P(OC 2 H 5 ) 3 to give C 6 H 5 CN in quantitative yield. The mechanism of this reaction has been investigated and compared with that of the similar reaction shown by Ni(CN)(C 6 H 5 )(PEt 3 ) 2 . The results indicate that the easiest path for the reaction involves a bimolecular attack of P(OC 2 H 5 ) 3 at the metal atom before reductive elimination of C 6 H 5 CN.

Reductive elimination of benzonitrile from cyanophenyl complexes of nickel(II) : III. Reactions of [cyano(phenyl)bis(triethylphosphine)nickel(II)] with triethylphosphite. General reaction mechanism

Abstract The kinetics of the elimination of C 6 H 5 CN in the reaction of Ni(CN)(C 6 H 5 )-[P(C 2 H 5 ) 3 ] 2 with P(OC 2 H 5 ) 3 have been studied in toluene at 15°C. The results and the rate law show that the elimination mainly takes place by intramolecular decomposition of the 4-coordinate Ni(CN)(C 6 H 5 )[P(OC 2 H 5 ) 3 ] 2 formed in initial substitution steps. A minor contribution to the formation of C 6 H 5 CN comes from the 5-coordinate species Ni(CN)(C 6 H 5 )[P(C 2 H 5 ) 3 ][P(OC 2 H 5 ) 3 ] 2 . The intimate mechanism of these reactions, involving splitting or formation of C—CN bonds in cyanophenyl complexes, is discussed in terms of the peculiar bonding properties of the CN group.

A high spin-low spin conformational equilibrium in the complex dithiocyanatobis(triethylphosphine)cobalt(II)

Abstract The magnetic moments, electronic and infrared spectra of the compound [Co(PEt 3 ) 2 (NCS) 2 ] are found to depend markedly on temperature and concentration in solvents such as dichloromethane and dichloroethane. It is shown that these characteristics depend on a monomeric (high spin) dimeric (low spin) equilibrium. The high spin form is identified with a tetrahedral configuration about the cobalt atom. It is suggested that the low spin species is a dimer in which the cobalt atoms are linked by SCN bridges to give a pentacoordinate, approximately pyramidal, configuration. The values of the equilibrium constants for the solution equilibria derived from the temperature dependence of the magnetic susceptibilities are presented.

The elimination of benzonitrile in the reaction of [cyano-(phenyl)bis(triethylphosphine)nickel(II)] with 1,2-bis-(diethylphosphino)ethane

Abstract Mechanistic studies on the reaction in benzene of the complex Ni(CN)-(C 6 H 5 )[P(C 2 H 5 ) 3 ] 2 with the diphosphine (C 2 H 5 ) 2 P(CH 2 ) 2 P(C 2 H 2 ) 2 (DEE), leading to reductive elimination of C 6 H 5 CN, are reported. The results indicate that ready substitution of P(C 2 H 5 ) 3 by DEE in the substrate complex precedes the rate-determining elimination step. The rate-law indicates that this process involves a 5-coordinate intermediate of the type Ni(CN)(C 6 H 5 )(P) 3 (P = coordinated ligand phsophorus atoms).

Reactions of methyl and ethyl halides with the complexes Ni[P(C2H5)3]4 and Ni(X)[P(C2H5)3]3

Abstract The rates of the thermal reaction of the nickel(0) complex Ni[P(C 2 H 5 ) 3 ] 4 with the alkyl halides CH 3 Br, CH 3 I in toluene have been compared with those of the reactions of the nickel(I) complexes Ni(X)[P(C 2 H 5 ) 3 ] 3 (X  Br,I). The organic products from CH 3 X are methane and ethane, and those from C 2 H 5 I are ethane and ethylene. The reactivity of the nickel(I) complexes is 10–20 times less than that of the nickel(0) complex. The result suggest that the first step of the reaction of nickel(0) with CH 3 I is the expected oxidative addition of the halide to the metal substrate. The intermediate thus formed decomposes to produce ethane (and small amounts of methane) without further reaction with the organic halide. This mechanism is supported by deuterium-labeling experiments.

Reactions of dioxygen with benzylnickel complexes

Abstract The benzyl complexes Ni(X)(CH2C6H5)(PCy3) (X = Cl, CN; Cy = cyclohexyl) react with molecular oxygen to give benzaldehyde and benzyl alcohol as main oxidation products. The ratio of the two products is strongly dependent on the nature of X and is also influenced by the solvent and the temperature. Isotopic labelling and mass spectra show that the hydrogen atoms necessary for the formation of the benzyl alcohol are supplied by the phosphine ligands. Isolation and characterization of the chloride complex by conventional spectroscopic techniques (IR, 1H 31P NMR, visible spectra) provide evidence in favour of a η3-τ-benzyl structure for the compound.

Mechanism of thermolysis of monoalkylplatinum(II) complexes with tertiary phosphine ligands. Methyl radical elimination from trans-Pt(I)(CD3)[(P(C3)2]2

Abstract Thermolysis of the complexes trans -Pt(I)(Me)(PR 3 ) 2 (R = CH 3 , CD 3 , C 2 H 5 , C 6 H 5 , C 6 H 11 , Me = CD 3 ) in deuterated or non-deuterated hydrocarbons at 120°C produces MeH and/or MeD. Appropriate isotopic labeling has revealed the existence of two decomposition pathways. The main route involves homolytic splitting of the platinum-methyl bond to give methyl radicals, which then form methane by abstraction of hydrogen from the R groups of the phosphines or from the solvent. The second, less important, route has a molecular mechanism involving coordinate methyl groups.

Thermal stability of organopalladium compounds: Non-radical methyl elimination from [PdXMe(PEt3)2]

Abstract The thermolysis of the palladium complexes [PdX(Me){P(C 2 H 5 ) 3 } 2 ] (X = Br, I, CN; Me = CH 3 , CD 3 ) in decalin or toluene under argon, in the temperature range 120–160°C, produces methane, ethane and ethylene, in ratios which vary with the temperature. Deuterium labelling shows that the methane is mainly formed through intramolecular abstraction of hydrogen from the phosphine ligands by the coordinated methyl group and not through homolytic fission of the PdMe bond. The thermal stability and the decomposition mechanisms of the organopalladium complexes are compared with those of the platinum analogues, which are remarkably more stable. At the higher temperatures, the thermal decomposition involves cleavage of the PEt bonds in the phosphine ligands, and this leads to the formation of ethane and ethylene. The rate of generation of methane from the PdMe moieties is increased by a factor of 10 by the presence of an excess of dioxygen. Deuterium isotopic labelling shows that the rate increase is accompanied by a change from an intramolecular to a radical mechanism involving the abstraction of hydrogen by the methyl groups.