Dataset Information

Mechanistic Investigations of the Photochemical Isomerizations of [(CO)5MC(Me)(OMe)] (M = Cr, Mo, and W) Complexes.

ABSTRACT: The mechanisms for the photochemical isomerization reactions are determined theoretically using group 6 Fischer carbene complexes (CO)5M=C(Me)(OMe) (M = Cr, Mo, and W) and the complete-active-space self-consistent field (CASSCF) (10-orbital/8-electron active space) and second-order Møller-Plesset perturbation (MP2-CAS) methods with the Def2-SVPD basis set. The structures and energies of the singlet/singlet conical intersections and the triplet/singlet intersystem crossings, which play a decisive role in these photoisomerizations, are determined. The former is applied to the chromium and molybdenum systems because their photoproducts are essentially from the singlet excited states. The latter is applied to the tungsten complex because its photoproducts are formed from a low-lying triplet excited state. Two reaction pathways are examined in this work: photocarbonylation (path I) and CO-photoextrusion (path II). The model studies strongly indicate that in the photochemistry of Cr and Mo Fischer carbene systems, the formation of metallaketene intermediates may occur at higher excitation wavenumbers, whereas the five-coordinated complexes that are attached by a solvent molecule are obtained at lower excitation wavenumbers. However, in the W analogue, because the activation barriers for path I are greater than that for path II and path I has more reaction steps than path II, the quantum yields for the metallaketene intermediate should be smaller than those for the five-coordinated species, which is also attached by a solvent molecule. These theoretical studies also suggest that the conical intersection and the spin crossover mechanisms that are identified in this work explain the process well and support the experimental observations.

SUBMITTER: Su MD

PROVIDER: S-EPMC6644460 | biostudies-literature | 2017 Sep

REPOSITORIES: biostudies-literature

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Mechanistic Investigations of the Photochemical Isomerizations of [(CO)<sub>5</sub>MC(Me)(OMe)] (M = Cr, Mo, and W) Complexes.

Su Ming-Der MD

ACS omega 20170905 9

The mechanisms for the photochemical isomerization reactions are determined theoretically using group 6 Fischer carbene complexes (CO)<sub>5</sub>M=C(Me)(OMe) (M = Cr, Mo, and W) and the complete-active-space self-consistent field (CASSCF) (10-orbital/8-electron active space) and second-order Møller-Plesset perturbation (MP2-CAS) methods with the Def2-SVPD basis set. The structures and energies of the singlet/singlet conical intersections and the triplet/singlet intersystem crossings, which play ...[more]

PMID: 31457808

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Project description:The mechanisms of photoextrusion reactions are determined theoretically for the model system of six-coordinated M(CO)5(CS) (M = Cr, Mo, and W), using both CASSCF and MP2-CAS methods and the Def2-SVPD basis set. Three types of elimination reaction pathways (i.e., path I, path II, and path III for axial CO extrusion, equatorial CO extrusion, and CS ligand extrusion, respectively) are considered in this study. Theoretical findings show that the photoextrusion mechanism for Cr and Mo complexes proceeds as follows: M-S0-Rea + hν → M-S1-FC → M-CI → M-Pro + CO. This study shows that when the reactant, M(CO)5(CS) (M-S0-Rea), is photoirradiated by UV light, it is excited vertically to many low-lying singlet excited states. It then relaxes to the first singlet excited state from the Franck-Condon point (M-S1-FC). After passing through a conical intersection point (M-CI), this species eliminates a CO group to yield a five-coordinated product, M(CO)4(CS) (M-S0-Pro). However, for the W analogue, the photolysis mechanism is represented as W-S0-Rea + hν → W-T1-Min → W-T1-TS → W-T1/S0 → W-S0-Pro + CO. That is to say, when the reactant, W(CO)5(CS) (W-S0-Rea), absorbs UV light, it is excited to its several low-lying excited states by a vertical excitation. This species may then return to an intermediate at the first triplet excited state (W-T1-Min) by means of intersystem crossings or conical intersections. After passing through a triplet transition state (W-T1-TS) and a subsequent intersystem crossing (W-T1/S0), this molecule finally loses a CO ligand to produce a photoproduct at the ground singlet state (W-S0-Pro). In other words, conical intersections and intersystem crossings play a decisive role in these photoextrusion reactions for M(CO)5(CS). Theoretical evidence from a kinetic viewpoint strongly supports the theory that the photolysis of M(CO)5(CS) only produces CO-loss photoproducts rather than the CS-loss photoproduct. Theoretical analysis using the results of this study allows a good interpretation of the available experimental observations.

Project description:Ruthenium(II) complexes feature prominently in the development of agents for photoactivated chemotherapy; however, the excited-state mechanisms by which photochemical ligand release operates remain unclear. We report here a systematic experimental and computational study of a series of complexes [Ru(bpy)2(N∧N)]2+ (bpy = 2,2'-bipyridyl; N∧N = bpy (1), 6-methyl-2,2'-bipyridyl (2), 6,6'-dimethyl-2,2'-bipyridyl (3), 1-benzyl-4-(pyrid-2-yl)-1,2,3-triazole (4), 1-benzyl-4-(6-methylpyrid-2-yl)-1,2,3-triazole (5), 1,1'-dibenzyl-4,4'-bi-1,2,3-triazolyl (6)), in which we probe the contribution to the promotion of photochemical N∧N ligand release of the introduction of sterically encumbering methyl substituents and the electronic effect of replacement of pyridine by 1,2,3-triazole donors in the N∧N ligand. Complexes 2 to 6 all release the ligand N∧N on irradiation in acetonitrile solution to yield cis-[Ru(bpy)2(NCMe)2]2+, with resultant photorelease quantum yields that at first seem counter-intuitive and span a broad range. The data show that incorporation of a single sterically encumbering methyl substituent on the N∧N ligand (2 and 5) leads to a significantly enhanced rate of triplet metal-to-ligand charge-transfer (3MLCT) state deactivation but with little promotion of photoreactivity, whereas replacement of pyridine by triazole donors (4 and 6) leads to a similar rate of 3MLCT deactivation but with much greater photochemical reactivity. The data reported here, discussed in conjunction with previously reported data on related complexes, suggest that monomethylation in 2 and 5 sterically inhibits the formation of a 3MCcis state but promotes the population of 3MCtrans states which rapidly deactivate 3MLCT states and are prone to mediating ground-state recovery. On the other hand, increased photochemical reactivity in 4 and 6 seems to stem from the accessibility of 3MCcis states. The data provide important insights into the excited-state mechanism of photochemical ligand release by Ru(II) tris-bidentate complexes.

Dataset Information

Mechanistic Investigations of the Photochemical Isomerizations of [(CO)5MC(Me)(OMe)] (M = Cr, Mo, and W) Complexes.

Publications

Mechanistic Investigations of the Photochemical Isomerizations of [(CO)<sub>5</sub>MC(Me)(OMe)] (M = Cr, Mo, and W) Complexes.

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