Electronic Structure of Oxidized Complexes Derived from cis-[Ru^II(bpy)₂(H₂O)₂]²⁺ and Its Photoisomerization Mechanism

Abstract

The geometry and electronic structure of cis-[Ru(II)(bpy)(2)(H(2)O)(2)](2+) and its higher oxidation state species up formally to Ru(VI) have been studied by means of UV-vis, EPR, XAS, and DFT and CASSCF/CASPT2 calculations. DFT calculations of the molecular structures of these species show that, as the oxidation state increases, the Ru-O bond distance decreases, indicating increased degrees of Ru-O multiple bonding. In addition, the O-Ru-O valence bond angle increases as the oxidation state increases. EPR spectroscopy and quantum chemical calculations indicate that low-spin configurations are favored for all oxidation states. Thus, cis-[Ru(IV)(bpy)(2)(OH)(2)](2+) (d(4)) has a singlet ground state and is EPR-silent at low temperatures, while cis-[Ru(V)(bpy)(2)(O)(OH)](2+) (d(3)) has a doublet ground state. XAS spectroscopy of higher oxidation state species and DFT calculations further illuminate the electronic structures of these complexes, particularly with respect to the covalent character of the O-Ru-O fragment. In addition, the photochemical isomerization of cis-[Ru(II)(bpy)(2)(H(2)O)(2)](2+) to its trans-[Ru(II)(bpy)(2)(H(2)O)(2)](2+) isomer has been fully characterized through quantum chemical calculations. The excited-state process is predicted to involve decoordination of one aqua ligand, which leads to a coordinatively unsaturated complex that undergoes structural rearrangement followed by recoordination of water to yield the trans isomer.

Experimental demonstration of radicaloid character in a RuV=O intermediate in catalytic water oxidation

“…2C reflects the isotopic composition for Ru 99,101 (I = 5/2) (details of EPR simulation in SI Materials and Methods). The EPR results are in agreement with earlier experiments for the BD complex prepared in the highly oxidized conditions at pH = 0.5, where apparent Ru 99,101 hyperfine splitting (hfs) on the order of 40 G were reported (30,41 (42). Notable Ru 99,101 hfs in BD [4,5] is in agreement with DFT analysis (Table 2), showing considerable spin density on at least one Ru center in BD [4,5].…”

Mechanistic Analysis of Water Oxidation Catalyst cis-[Ru(bpy)2(H2O)2]2+: Effect of Dimerization

“…of Ce IV . This signal is characteristic for intermediates containing a Ru V =O fragment [20] and has been previously reported for cis-[Ru(bpy)2(H2O)2] 2+ [19]. The sample that was oxidized with 3 equiv.…”

“…Interestingly, in the same work, trans-[Ru(bpy) 2 2 ] 2+ is believed to involve a water nucleophilic attack on the Ru V =O species produced in a series of proton-coupled electron transfer reactions, which occur upon oxidation [18]. cis-[Ru(bpy) 2 (H 2 O) 2 ] 2+ and various products of its oxidation have been studied by density functional theory (DFT), EPR, and X-ray Absorption Spectroscopy, allowing for the establishment of their geometric and electronic structure [19].…”

Spin in density‐functional theory

“…ðs i Þ Áŝðs j Þ; (17) where the first term emerges because any many-electron wavefunction is an eigenfunction of ŝ 2 (s i ) with eigenvalue ð3=4Þh 2 (i.e., electrons are spin-1/2 particles). Note that Ŝ 2 couples different electrons, that is, it is a two-electron operator.…”

“…In particular, the complete active space self-consistent field method, usually in combination with second-order perturbation theory (CASPT2), has been employed to study transition metal complexes (for examples, see Refs. [13][14][15][16][17][18]). However, the factorial scaling with the size of the active space puts rather severe limits on the size of the active space, which prevents most applications to polynuclear transition metal complexes and clusters.…”