Frank P. A. Johnson

Efficient photocatalytic CO2 reduction using [Re(bpy) (CO)3{P(OEt)3}]+

Abstract [Re(bpy) (CO)3{P(OEt)3}]+ (1+) (bpy, 2,2′-bipyridine) is the most efficient homogeneous photocatalyst for the selective reduction of CO2 to CO reported to date. Both the quantum yield and turnover number of the photocatalytic reaction are strongly dependent on the irradiation light intensity and wavelength because of the unusual stability of the one-electron-reduced species [Re(bpy.−) (CO)3{P(OEt)3}] (1).

Excited-state properties and reactivity of [ReCL(CO)3(2,2′-bipy)](2,2′-bipy = 2,2′-bipyridyl) studied by time-resolved infrared spectroscopy

Fast time-resolved infrared spectroscopy shows that the v(CO) bands of the important CO2-reducing complex, [ReCl(CO)3(2,2′-bipy)]1(2,2′-bipyridyl), shift up in frequency in the metal to ligand charge-transfer excited state and that the anion 1–, generated by reaction of the excited state with triethylamine, shows a lowering in frequency of the v(CO) bands significantly different from those of related but non-reducing complexes.

Time-resolved infrared spectroscopy of excited states of transition metal species

Abstract This article describes examples of the application of time-resolved infrared Spectroscopy (TRIR) to the probing of the excited states of transition metal species. Other “direct” methods for excited states include time-resolved absorption Spectroscopy, and, more importantly, time-resolved resonance Raman Spectroscopy (TR 3 ). Both of these techniques have limitations and hence TRIR is of value in complementing them. The common “indirect” methods are absorption, emission, and excitation spectroscopies, to which has been more recently added, resonance Raman, particularly in the time-dependent formulation. The relevance of these methods is also discussed.

Excited-state properties of [(OC)5W(L)W(CO)5][L = 4,4′-bipyridyl (4,4′-bipy) or pyrazine] and [(OC)5W(4,4′-bipy)]

Fast time-resolved infrared (TRIR) spectroscopy has been employed to probe the electron distribution in the lowest metal-to-ligand charge-transfer excited states of [(OC)5W(4,4′-bipy)W(CO)5]1, [(OC)5W(4,4′-bipy)]2(4,4′-bipy = 4,4′-bipyridyl) and [(OC)5W(pyz)W(CO)5]3(pyz = pyrazine). The excited state of 1(i.e.1*) shows a ν(CO) band pattern, compared with the ground state, in which some ν(CO) bands increase and some decrease in frequency; those which increase match very closely those observed for the excited state of 2, where the increase in frequency is readily assigned to the increase in the effective oxidation state of W on electron transfer to the 4,4′-bipy ligand (schematically W+L–). The ν(CO) IR bands of the electrochemically reduced 1, i.e.1–, are lower in frequency than those of 1, and nearly match the low-frequency bands of 1*. The interpretation is that the excited state of 1 is localised (schematically ≈W+L–W) on the IR time-scale, and that the degree of coupling between the two halves of the excited molecule is very small. Similar conclusions are obtained for the excited state of 3, based on TRIR and spectroelectrochemistry of this complex.

Photochemistry of[Re(bipy)(CO)3(PPh3)]+(bipy = 2,2′-bipyridine) in thepresence of triethanolamine associated with photoreductive fixation ofcarbon dioxide: participation of a chain reaction mechanism

The complex fac-[Re(bipy)(CO) 3 (PPh 3 )] + 1 + (bipy = 2,2′-bipyridine) was converted into a formate complex fac-[Re(bipy)(CO) 3 {OC(O)H}] 2 in a 52.2% yield by irradiation in the presence of triethanolamine (teoa) and CO 2 . This photochemical fixation of CO 2 proceeds via a unique reaction mechanism: (i) irradiation of 1+ in teoa–dimethylformamide (dmf) resulted in the efficient formation of fac-[Re(bipy)(CO) 3 (teoa)] + 3 + and fac-[Re(bipy)(CO) 3 (dmf)] + 4+ in a quantum yield of 16.9; (ii) the ligand substitution was explained in terms of a chain mechanism involving an initial electron transfer from teoa to excited 1+, followed by substitution of the PPh 3 ligand of 1 with teoa and dmf to give 3 and 4; subsequent electron exchange of 3 and 4 with 1+ leads to the formation of 3+ and 4+ accompanied by the regeneration of 1; (iii) the formate complex 2 was formed in a quantum yield of 1.1 × 10 -3 upon excitation of 3+ and 4+ in the presence of CO 2 ; (iv) further irradiation after the formation of 2 resulted in the photocatalytic reduction of CO 2 to CO in a quantum yield of 0.05.

Efficient carbon dioxide photoreduction by novel metal complexes and its reaction mechanisms

Photoreduction of CO2 to CO using the new catalysts, [Re(bpy)(CO)3{P(OEt)3}]+ (1+) and [Re(bpy)(CO)3(PPh3)]+ (2+) is described. The quantum yields of CO formation are 0.23 and 0.05 for 1+ and 2+, respectively. The value for 1+ is one of the highest reported values for homogeneous CO2 reduction photocatalysts. In-situ UV/vis spectra indicate that one-electron reduced species is an important intermediate.

Infrared rigidochromism: a new effect in the IR spectra of the excited states of coordination compounds

The magnitude of the shift in the ν(CO) IR spectrum of the MLCT excited state of [Re(CO)3Cl(bipy)](bipy = 2,2′-bipyridyl) in PrCN–EtCN solution, compared with the ground-state spectrum, decreases on cooling from fluid to glass; this ‘infrared rigidochromic’ effect is explained by the change in character of the MLCT state on glass formation.

The structure of W(CO)5L (L = pyridine, piperidine) in the lowest ligand field excited state determined by fast time-resolved IR spectroscopy; unexpected C–O bond length changes

The frequencies of the ν(CO) bands of W(CO)5L (L = pyridine and piperidine) in the lowest ligand field (LF) excited states have been obtained by fast time-resolved IR spectroscopy of the species dissolved in low-temperature glasses, and the shifts from the ground state indicate that the C–O bonds lengthen on excitation, contrary to the interpretation of the pre-resonance Raman spectroscopy on the ground state.

Photochemistry of [M(η5-C5H5)(CO)3Et](M = Mo or W): a mechanistic study using time-resolved infrared spectroscopy and matrix isolation

Matrix isolation at 13 K and time-resolved infrared (TRIR) spectroscopy at room temperature have been used to study the mechanism of the photochemical conversion of [M(η5-C5H5)(CO)3Et] into [M(η5-C5H5)(CO)3H](M = Mo or W). Ultraviolet photolysis of [M(η5-C5H5)(CO)3Et] isolated in methane matrices produces two distinct dicarbonyl species, [M(η5-C5H5)(CO)2Et ⋯ CH4]1 and [M(η5-C5H5)(CO)2(CH2CH2-µ-H)]2, together with cis-[(M)η5-C5H5)(CO)2(C2H4)H]3(for M = W only), and eventually trans-[M(η5-C5H5)(CO)2(C2H4)H]4. In the matrix 1 and 2 can be interconverted using selective photolysis with visible wavelengths. These two species have also been identified in room-temperature heptane solution, using TRIR spectroscopy. For the tungsten system TRIR has also confirmed the existence of a fast equilibrium between 2 and 3. An activation energy measured for the decay of 2 suggests that the rate-determining step in this decay is the isomerization of 3 to 4. Although 3 was not observed in either matrix or in TRIR experiments with the molybdenum system, the existence of a further equilibrium between [Mo(η5-C5H5)(CO)2(CH2CH2-µ-H)] and trans-[Mo(η5-C5H5)(CO)2(C2H4)H] has been observed. These results are of relevance to current studies on ‘agostic’ hydrogen interactions.

Time-Resolved Infrared Spectroscopy of a CO2 Reductant

Fast Time-resolved Infrared Spectroscopy is used to monitor the initial steps in the photocatalytic reduction of CO2 using [Re(CO)2(bpy)]P(OEt)3 2]+Br−.