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A delicate balance of complexation vs. activation of alkanes interacting with [Re(Cp)(CO)(PF3)] studied with NMR and time-resolved IR spectroscopy.

ABSTRACT: The organometallic alkane complexes Re(Cp)(CO)(PF(3))(alkane) and Re(Cp)(CO)(2)(alkane) have been detected after the photolysis of Re(Cp)(CO)(2)(PF(3)) in alkane solvent. NMR and time-resolved IR experiments reveal that the species produced by the interaction of n-pentane with [Re(Cp)(CO)(PF(3))] are an equilibrium mixture of Re(Cp)(CO)(PF(3))(pentane) and Re(Cp)(CO)(PF(3))(pentyl)H. The interaction of cyclopentane with [Re(Cp)(CO)(PF(3))] most likely results in a similar equilibrium between cyclopentyl hydride and cyclopentane complexes. An increasing proportion of alkane complex is observed on going from n-pentane to cyclopentane to cyclohexane, where only a small amount, if any, of the cyclohexyl hydride form is present. In general, when [Re(Cp)(CO)(PF(3))] reacts with alkanes, the products display a higher degree of oxidative cleavage in comparison with [Re(Cp)(CO)(2)], which favors alkane complexation without activation. Species with the formula Re(Cp)(CO)(PF(3))(alkane) have higher thermal stability and lower reactivity toward CO than the analogous Re(Cp)(CO)(2)(alkane) complexes.

SUBMITTER: Ball GE

PROVIDER: S-EPMC1855363 | biostudies-other | 2007 Apr

REPOSITORIES: biostudies-other

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A delicate balance of complexation vs. activation of alkanes interacting with [Re(Cp)(CO)(PF3)] studied with NMR and time-resolved IR spectroscopy.

Ball Graham E GE Brookes Christopher M CM Cowan Alexander J AJ Darwish Tamim A TA George Michael W MW Kawanami Hajime K HK Portius Peter P Rourke Jonathan P JP

Proceedings of the National Academy of Sciences of the United States of America 20070413 17

The organometallic alkane complexes Re(Cp)(CO)(PF(3))(alkane) and Re(Cp)(CO)(2)(alkane) have been detected after the photolysis of Re(Cp)(CO)(2)(PF(3)) in alkane solvent. NMR and time-resolved IR experiments reveal that the species produced by the interaction of n-pentane with [Re(Cp)(CO)(PF(3))] are an equilibrium mixture of Re(Cp)(CO)(PF(3))(pentane) and Re(Cp)(CO)(PF(3))(pentyl)H. The interaction of cyclopentane with [Re(Cp)(CO)(PF(3))] most likely results in a similar equilibrium between cyc ...[more]

PMID: 17435163

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Project description:We report a study of the photocatalytic reduction of CO2 to CO by zinc porphyrins covalently linked to [ReI(2,2'-bipyridine)(CO)3L]+/0 moieties with visible light of wavelength >520 nm. Dyad 1 contains an amide C6H4NHC(O) link from porphyrin to bipyridine (Bpy), Dyad 2 contains an additional methoxybenzamide within the bridge C6H4NHC(O)C6H3(OMe)NHC(O), while Dyad 3 has a saturated bridge C6H4NHC(O)CH2; each dyad is studied with either L = Br or 3-picoline. The syntheses, spectroscopic characterisation and cyclic voltammetry of Dyad 3 Br and [Dyad 3 pic]OTf are described. The photocatalytic performance of [Dyad 3 pic]OTf in DMF/triethanolamine (5?:?1) is approximately an order of magnitude better than [Dyad 1 pic]PF6 or [Dyad 2 pic]OTf in turnover frequency and turnover number, reaching a turnover number of 360. The performance of the dyads with Re-Br units is very similar to that of the dyads with [Re-pic]+ units in spite of the adverse free energy of electron transfer. The dyads undergo reactions during photocatalysis: hydrogenation of the porphyrin to form chlorin and isobacteriochlorin units is detected by visible absorption spectroscopy, while IR spectroscopy reveals replacement of the axial ligand by a triethanolaminato group and insertion of CO2 into the latter to form a carbonate. Time-resolved IR spectra of [Dyad 2 pic]OTf and [Dyad 3 pic]OTf (560 nm excitation in CH2Cl2) demonstrated electron transfer from porphyrin to Re(Bpy) units resulting in a shift of ?(CO) bands to low wavenumbers. The rise time of the charge-separated species for [Dyad 3 pic]OTf is longest at 8 (±1) ps and its lifetime is also the longest at 320 (±15) ps. The TRIR spectra of Dyad 1 Br and Dyad 2 Br are quite different showing a mixture of 3MLCT, IL and charge-separated excited states. In the case of Dyad 3 Br, the charge-separated state is absent altogether. The TRIR spectra emphasize the very different excited states of the bromide complexes and the picoline complexes. Thus, the similarity of the photocatalytic data for bromide and picoline dyads suggests that they share common intermediates. Most likely, these involve hydrogenation of the porphyrin and substitution of the axial ligand at rhenium.

Dataset Information

A delicate balance of complexation vs. activation of alkanes interacting with [Re(Cp)(CO)(PF3)] studied with NMR and time-resolved IR spectroscopy.

Publications

A delicate balance of complexation vs. activation of alkanes interacting with [Re(Cp)(CO)(PF3)] studied with NMR and time-resolved IR spectroscopy.

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