79931-95-6Relevant academic research and scientific papers
Kinetics and Mechanism of Decomposition of a Benzodicobaltacyclohexene: Reversible Dinuclear Elimination of o-Xylylene via a Dimetalla-Diels-Alder Reaction
Hersh, William H.,Bergman, Robert G.
, p. 5846 - 5859 (2007/10/02)
A detailed mechanistic study of several reactions of the first dimetallacyclohexene, (η5-Cp)2Co2(μ-CO)2(μ-o-xylylene) (1), is described.The predominant reaction pathway is proposed to involve reversible cleavage of 1 In Diels-Alder fashion, into free o-xylylene (3) and the metal-metal double-bonded dimer 9.Several lines of evidence support this conclusion.Crossover experiments demonstrated that loss of 3 from 1 in its reaction with PPhMe2 to give dinuclear monophosphine adduct 2 is a dinuclear elimination process that leaves the cobalt-cobalt bond intact.Kinetic studies showed that sufficiently high concentrations of several ligands (phosphines, bis(methylcyclopentadienyl) metal-metal double-bonded dimer 10, or dimethyl acetylenedicarboxylate (DMAD)) induce decomposition of 1 at the same maximum rate, to give monophosphine adduct 2, bis(methylcyclopentadienyl)dimetallacyclohexene 5, or dinuclear DMAD adduct 16, respectively.Both the phosphine and DMAD reactions exhibited falloff in the observed rates of decomposition at lower ligand concentration.On the basis of the proposal that 1 was in thermal equilibrium with two reactive intermediates (3 and 9), theoretical results suggested, and were experimentally confirmed, that at low DMAD concentration the fallof in decomposition rate could be eliminated by lowering the concentration of 1, to again induce decomposition at the previously observed maximum rate.An Arrenius plot of data collected in this limiting rate regime, representing the rate of the retro-dimetalla-Diels-Alder reaction,gave ΔH = 24.3 kcal/mol and ΔS = +12.1 eu.Preparative reactions of double-bonded dimer 9 with two different o-xylylene precursors gave moderate yields of metallacycle 1, providing additional evidence for the forward dimetalla-Diels-Alder reaction.A minor reaction pathway, thermal decomposition of 1 to CpCo(o-xylylene) (12) and CpCo(CO)2 (13), was also observed.Evidence is presented suggesting that it is mechanistically related to the major decomposition pathway operating in other dinuclear dicobalt systems, involving intramolecular alkyl to cobalt migration.
Synthesis, Crystal and Molecular Structures, and Reactions of a Benzodicobaltacyclohexene, a Thermally Derived Mononuclear o-Xylylene Complex, and an Unsymmetrical Phosphine-Derived Dinuclear Complex
Hersh, William H.,Hollander, Frederick J.,Bergman, Robert G.
, p. 5834 - 5846 (2007/10/02)
Alkylation of Na2 (Cp = η5-cyclopentadienyl) with α,α'-dibromo-o-xylene gives bis(μ-carbonyl)(μ-o-xylylene)bis5-cyclopentadienyl)cobalt>(Co-Co) (1), the first dimetallacycloxene.The X-ray crystal structure of the derivative analogue 8 hyving η5-methylcyclopentadienyl rings was solved (2371 reflections; R = 2.64percent; space group P; a = 9.1045 (11), b = 9.5465 (13), c = 11.8147 (16) Angstroem; α = 83.730 (11) deg; β = 81.414 (10) deg, γ = 63.392 (10) deg; Z = 2, dcalcd = 1.598 g/cm3) and shows a six-membered ring containing two cobalt atoms with a Co-Co ''single bond'' length of 2.438 Angstroem; the dimetallacycle ring adopts a boat conformation with a folding angle of 58.5 deg.Compound 1 undergoes slow decomposition at room temperature in solution to give CpCo(CO)2 (10) and new complex (η5-cyclopentadienyl)(η4-o-xylylene)cobalt (11).The crystal structure of 11 (1370 reflections; R = 3.90percent; space group Pbca; a = 7.6286 (40), b = 11.6094 (16), c = 23.7196 (24) Angstroem; Z = 8, dcalcd = 1.443 g/cm3) is that of a diene complex in which the o-xylylene ligand consists of a delocalized butadiene portion and a nonaromatic six-membered carbocycle.Photolysis of 1 also leads to 10 and 11 as primary products.Carbonylation of mononuclear o-xylylene complex 11 at 70 deg C gives 10 and the CO insertion product 2-indanone (13).Carbonylation of 1 yields 10 and dimers 14 and 15 of free o-xylylene (4).Reactions of 1 with phosphines and phosphites (L) also give 14 and 15 as well as CpCo(CO)L (16), but the unstable dinuclear intermediate 17, proposed to be Cp2Co2(μCO)2(L), was detected in the phosphine reactions.This material can also be prepared from the dinuclear compound 2 (3) and phosphines.Compound 17 (L = PMe3) decomposes via parallel first and second-order pathways in the presence of PMe3, while 17 (L = PPh3) decomposes primarily via a first-order pathway in the presence of PPh3.The X-ray crystal structure of an analogue of 17, (MeCp)2Co2(μ-CO)2(PPhMe2) (20), was determined (2664 reflections; R = 2.76percent; space group P21/c; a = 10.0035 (14), b = 14.0816 (13), c = 14.4761 (18) Angstroem; β = 92.042 (11) deg; Z = 4, dcalcd = 1.533 g/cm3) and indicates the presence of a Co-Co ''single bond'' (2.416 Angstroem); the phosphine is bound to one of the cobalt atoms, and the two carbonyls are semibridging, with the closer contacts being to the cobalt atom not bound to phosphine.
