Ramaier Narayanaswamy

Photochemistry of pentacarbonyl(cyclohepta-1,3,5-triene-7-carbonyl)manganese and pentacarbonyl(σ-cycloheptatrienyl)rhenium in frozen gas matrices at ca. 12 K: infrared spectroscopic evidence for carbon monoxide ejection with concomitant polyene ring hapticity changes

Infrared spectroscopic evidence, including 13CO labelling and energy-factored force-field fitting, is presented to show that once a CO ligand has been photochemically ejected from [Mn(C7H7CO)(CO)5] to yield [Mn(σ-C7H7)(CO)5]via a 16-electron species, [Mn(C7H7CO)(CO)4], subsequent photolyses of [M (σ-C7H7)(CO)5] complexes (M = Mn or Re) leads to sequential formation of the 18-electron species [M(η3-C7H7)(CO)4], [M(η5-C7H7)(CO)3], and [M(η-C7H7)(CO)2] in Ar, CH4, N2, and CO matrices at ca. 12 K. The fact that no 16-electron species were observed prior to ring slippage even at such low temperatures indicated that such ring slips occur extremely readily or that they occur as a concerted process with CO ejection. The facility of the ring-slippage process was demonstrated by the failure of intermediate species [M(σ-C7H7)(CO)5] and [M(η3-C7H7)(CO)4] to undergo exchange with 13CO in doped matrices or to take up a N2 ligand when photolyses are carried out in nitrogen matrices. Exchange with 13CO and substitution of CO by N2 does, however, occur once the ring-slippage process is nearing completion for Mn affording [Mn(η5-C7H7)(12CO)n(13CO)3 –n] and [Mn(η5-C7H7)(CO)2(N2)]. The electronic spectra of [Mn(η7-C7H7)(CO)2] shows vibrational splitting which indicates a symmetric structure, cf.[Co(η5-C5H5)(CO)2], rather than a bent geometry. The photophysical and photochemical processes associated with the observed reactions are discussed.

Photochemistry of tricarbonyl(η4-cyclo-octatetraene)iron in a methane matrix at 12 K. Infrared spectroscopic evidence for photoisomerisation and carbon monoxide ejection reactions

Fourier-transform infrared spectroscopic evidence, including 13CO labelling and energy-factored CO force-field fitting, is presented to show that photolysis of [Fe(‘tub’-η4-C8H8)(CO)3] in a CH4 matrix at 12 K leads to the formation of a [Fe(‘chair’-η4-C8H8)(CO)3] species via the [Fe(η2-C8H8)(CO)3] intermediate. An additional photoproduct is the co-ordinatively unsaturated species [Fe(η4-C8H8)(CO)2] formed by photoelimination of a CO ligand. The results are related to the fluxional behaviour of [Fe(η4-C8H8)(CO)3] in solution.

Photochemistry of tricarbonyl(η-cyclopentadienyl)methylmolybdenum in frozen gas matrices at 12 K. Infrared spectroscopic evidence for the formation of dicarbonyl(η-cyclopentadienyl)methylmolybdenum and dicarbonyl(η-cyclopentadienyl)(dinitrogen)methylmolybdenum

Infrared spectroscopic evidence, including 13CO lebelling and energy-factored CO force-field fitting, is presented to show that on photolysis of [Mo(η5-C5H5)(CO)3(CH3)] at high dilution in CH4, Ar, CO, and N2 matrices at 12 K new species [Mo(η5-C5H5)(CO)2(CH3)] and [Mo(η5-C5H5)(CO)2(N2)(CH3)] are produced. The reversibility of the reaction [Mo(η5-C5H5)(CO)3(CH3)]⇌[Mo(η5-C5H5)(CO)2(CH3)]+ CO is interpreted as confirming the existence of a reduced co-ordination number species as the intermediate in the substitution reactions of [Mo(η5-C5H5)(CO)3(CH3)]. The results are discussed in relation to the mechanisms of thermal and photochemical solution reactions.

Photochemistry of methyl- and acetyl-dicarbonyl(η-cyclopentadienyl)-iron in frozen gas matrices at 12 K. Infrared spectroscopic evidence for the formation of tricarbonyl(η-cyclopentadienyl)methyliron and acetylmonocarbonyl(η-cyclopentadienyl)iron

Infrared spectroscopic evidence is presented to show that, on photolysis of [Fe(η5-C5H5)(CO)2(CH3)] at high dilution in CO matrices at 12 K, a new species [Fe(η3-C5H5)(CO)3(CH3)] is produced. No evidence has been found for the formation of [Fe(η5-C5H5)(CO)(CH3)] in Ar, CH4, and N2 matrices nor [Fe(η5-C5H5)(CO)2(COCH3)] in Co matrices, although in 13CO-doped matrices rapid exchange was observed. Wavelength-selective photolysis of [Fe(η5-C5H5)(CO)2(COCH3)] in Ar, CH4, and CO matrices produces [Fe(η5-C5H5)(CO)2(CH3)]via the co-ordinatively unsaturated intermediate [Fe(η5-C5H5)(CO)(COCH3)]. The rapid exchange of 12CO ligands in [Fe(η5-C5H5)(CO)2(CH3)] with 13CO in the matrix, the expanded-co-ordination-number species [Fe(η3-C5H5)(CO)3(CH3)], and the co-ordinatively unsaturated species [Fe(η5-C5H5)(CO)(COCH3)] are discussed in relation to the mechanisms of the insertion reactions of [Fe(η5-C5H5)(CO)2(CH3)] and the thermal and photochemical decarbonylation reactions of [Fe(η5-C5H5)(CO)2(COCH3)].

Matrix isolation studies of the bonding of acetyl groups in the co-ordinatively unsaturated species acetyltetracarbonylmanganese and acetylmonocarbonyl(η-cyclopentadienyl)iron

The infrared band positions of νCO(acetyl) for [Mn(CO)4(COCH3)] and [Fe(η5-C5H5)(CO)(COCH3)] are reported in CH4 matrices at 12 K. It is proposed that the acetyl ligands have σ-acetyl type bonding in these co-ordinatively unsaturated species.

Quenching of the 3B1u→1A1g emission of benzene in low temperature matrices at 10 K

The effects of concentration and environment on the triplet state emission of benzene (3B1u→1A1g) have been investigated in low temperature gas matrices at 10 K. For benzene + argon matrices a Monte Carlo calculation of the mean separation between the interacting centres of benzene molecules at the critical concentration has been shown to be 6.5 ± 0.5 A. The phosphorescence lifetime in a variety of matrices at a concentration beyond the self-quenching limit decreased in the order CH4 > N2 > Ar Kr > Xe in accordance with the heavy atom effect which has been noted previously. The fluorescence lifetime of benzene (1B2u→1A1g) has been measured for the first time in an argon matrix and found to be 45 ± 5 ns for matrix ratios benzene:argon in the range 1 : 20 to 1 : 1000.

Infrared and electronic spectra of matrix-isolated dichlorodioxo- and dibromodioxo-molybdenum(VI) and -tungsten(VI)

The prominent i.r. and u.v.–visible bands of the title compounds isolated as monomers in low-temperature matrices have been obtained and assigned. For MoO2Cl2 and MoO2Br2, the vibrational isotope effect on the antisymmetric stretching mode ν(MoO) leads to estimates of bond angles OMoO of 109 ± 3°, whilst for WO2Cl2 and WO2Br2, relative band intensities predict OWO = 107 ± 2° for both molecules. In the quantitative assignment of the electronic spectra, it is suggested that the positions of the charge transfer bands require somewhat lower values of χopt. than are appropriate for octahedral complexes.

Photochemistry of acetylacetonato-, trifluoroacetylacetonato-, and hexafluoroacetylacetonato-dicarbonyl-rhodium and -iridium complexes in frozen gas matrices at 12 K. Infrared spectroscopic evidence for carbon monoxide dissociation and chelate ring-opening processes

Infrared spectroscopic evidence, including 13CO labelling and energy-factored CO force-field fitting, is presented to show that on photolysis of [M(CO)2L](L = RCOCHCOR′; R,R′= CH3 or CF3; M = Rh or Ir) complexes at high dilution in CH4, Ar, CO, and N2 matrices at 12 K new species [M(CO)L], [M(CO)2(L*)](L*= unidentate form of L), and [M(CO)(N2)L] are produced. The results are discussed in relation to the mechanisms of thermal reactions in solution. In particular, the reversibility of the reaction [M(CO)2L]⇌[M(CO) L]+ CO and the dominance of this reaction indicates that the hitherto discounted dissociative pathway should be reconsidered. Some evidence is also found for the chelate ring opening to give [ M(CO)2(L*)] species, but no evidence was found for expanded-co-ordination-number species corresponding to the previously proposed associative mechanism for ligand-exchange and substitution reactions.

Infrared and electronic spectra of matrix-isolated tetrafluoro- and tetrachloro-oxo-molybdenum(VI) and -oxotungsten(VI) and tetrabromo-oxo-tungsten(VI)

The prominent i.r. bands of the title compounds isolated as monomers in low-temperature matrices have been obtained. Some anomalies in the literature regarding the fundamentals of WOCl4 have been resolved, and in particular, it is shown via high-resolution studies, that whilst argon matrices generally yield multiple trapping sites, nitrogen matrices typically produce only one site. Electronic spectra in N2 matrices have been recorded for all five compounds and compared with the predictions of Xα calculations. The matrix electronic spectra are shown to be superior to gas-phase spectra in terms of resolution, the observation of vibrational fine structure, and the absence of any complications due to the possible presence of decomposition products.

Fluorescence lifetimes and quantum yields of 9,10-diphenylanthracene, benzene, and [2H6]benzene in frozen gas matrices at 12 K

The single photon counting technique has been used to determine the fluorescence lifetimes of 9,10-diphenylanthracene, C6H6, and C6D6 in a variety of frozen gas matrices at 12 K; the fluorescence quantum yields of C6H6 and C6D6 in CH4 at 12 K have been determined as 0·35 and 0·45 respectively.