The Photochemistry of [CpMo(CO)3(η1-C3H5)] in Frozen Gas Matrices at 10 K and the Behaviour of [CpMo(CO)2(η3-C3H5)] Isomers in Different Media

Abstract

On photolysis of [CpMo(CO)3(η1‐C3H5)] in solid inert matrices at low temperature, one CO molecule is eliminated yielding the exo and endo isomers of [CpMo(CO)2(η3‐C3H5)] in almost 1:1 proportions, although the fraction of the exo conformer is increased by prolonged broad‐band UV irradiation. If gaseous [CpMo(CO)2(η3‐C3H5)] is isolated in an Ar matrix the proportions of the two conformers are again about 1:1, indicating that they possess almost equal energies, as confirmed by DFT calculations (B3LYP/SDD). This situation changes in a solvent environment, where increasing the polarity of the solvent favours the exo isomer, as indicated by the ratios exo/endo = 1.7:1 (pentane or cyclohexane), 3.0:1 (benzene) and 3.3:1 (CH2Cl2) determined by IR and NMR measurements. Such a finding is consistent with the results of DFT calculations, which reveal that the exo isomer is more polar.

Organic Matrix and Solution Photochemical Studies of (Allyl)dicarbonyl(cyclopentadienyl)molybdenum Compounds

“…The comparative ease in stimulating the hapticity change in the case of this pendant allyl is attributed to the relief of strain in the open form. [5] Our work and that of Limberg et al [7] require a reinterpretation of the observations reported by Rest et al [6] on the gas matrix photolysis of [W(η 5 -C 5 H 5 )(CO) 3 (η 1 -C 3 H 5 )]. These workers observed loss of CO and the appearance of four bands attributable to metal carbonyl species.…”

“…[5] A brief study of the photochemistry of the tungsten analog of 1 in a methane matrix has been reported by Rest. [6] The results of the present study along with those of Limberg et al [7] on the photolysis of 1 in a frozen argon matrix reported in this issue, shed new light on the interpretation of the earlier spectroscopic data.…”

“…[7] Our interpretation of the observed photochemical transformations in Nujol and MeTHF is presented in Scheme 1. As also observed by Limberg in a frozen Ar matrix, [7] the major feature of the photolysis of 2 at longer wavelengths is the photochemical interconversion of the endo to the exo rotamers. At high energy a species formed by CO loss is observed at 1842 cm Ϫ1 that we assign as [Mo(CO)(η 3 -C 3 H 5 )(η 5 -C 5 H 5 )].…”

Fluxional Processes in Diamagnetic and Paramagnetic Allyl Dicarbonyl and 2-Methylallyl Dicarbonyl Molybdenum Histidinato Complexes as Revealed by Spectroscopic Data and Density Functional Calculations

“…Even under cryogenic conditions, no h 1 intermediate could be detected. [57] Our work demonstrates that substituents on the allyl ligand have a decisive influence on the energetic order of different isomers. Moreover, rotation about the Mo ± Cp axis-which is the equivalent of a trigonal twist in 1 or 3-is a low-energy process too rapid to be observed in Mo(Cp) compounds.…”

“…The behavior of [Mo(Cp)(h 3 -allyl)(CO) 2 ] and [Mo(Cp)(h 1 -allyl)(CO) 3 ] at cryogenic temperatures, as well as their interconversion, was studied in detail by Limberg and co-workers. [57] Poli et al have studied equilibria between different isomers of [Mo(Cp)(allyl)(h 4 -butadiene)]. [16,19] Again, allyl rotation is observed along with isomerization of the butadiene ligand (s-cis and s-trans).…”