Intrinsic hydrogen evolution capability and a theoretically supported reaction mechanism of a paddlewheel-type dirhodium complex

The intrinsic capability of the paddlewheel-type dirhodium tetraacetate complex, [Rh₂(O₂CCH₃)₄(H₂O)₂] ([1(H₂O)₂]), as a hydrogen evolution catalyst (HEC) for photochemical hydrogen evolution from aqueous solution was illustrated. This was achieved by using an optimized artificial photosynthesis (AP) system with a cyclometalated iridium complex [Ir(ppy)₂(bpy)](PF₆) ([Ir-PS-1]) and triethylamine (TEA) serving as a photosensitizer (PS) and a sacrificial donor, respectively. The total amount of hydrogen evolution and the turnover number (TON) of catalysis using this AP system were 385.7 μmol and 3857 (per Rh ion), respectively; these values are higher than those of [Rh(dtBubpy)₃](PF₆)₃, which is the most efficient HEC among the mononuclear rhodium complexes, and RhCl₃. Moreover, the catalytic performance of [1(H₂O)₂] was further accelerated by using [Ir(ppy)₂(dtBubpy)](PF₆) [Ir-PS-3] as a PS; 9886 TON (H₂ per Rh ion) was verified after 12 h of irradiation. In addition, the detailed mechanism of hydrogen evolution catalyzed by [1(H₂O)₂] was clarified by combining electro- and photochemical analyses and DFT calculations. The optimized geometries of [1(H₂O)₂], [1], hydride intermediates [H-Rh₂(O₂CCH₃)₄] ([H-1]), and their reduced species were theoretically verified by DFT calculations. Moreover, their redox potentials were theoretically estimated and compared with the observed potentials. Their combination analyses indicated that (i) the formation of [1], which has an open-metal site for hydrogen evolution and can be reduced by the one-electron reduced species of [Ir-PS-1], is a trigger for hydrogen evolution; (ii) [H-1] and its reduced species, which are verified by CV analyses, are key intermediate species in this reaction; and (iii) photochemical hydrogen evolution catalyzed by [1(H₂O)₂] occurred by two-electron reduction processes.

Optoelectronic properties and structural effects of the incremental addition of pyridyl moieties on a rhodium dimer

The synthesis and characterization of five C-C coupling products obtained from the reaction of a paddlewheel tetrakis 4-bromo-N,N'-diphenylbenzamidinate dirhodium dimer with 4-pyridineboronic acid pinacol ester are reported. The coupling reactions occur on one to four amidinate ligands, leading to rhodium dimers containing [tetrakis, tris, cis-bis, trans-bis, or mono]-N,N'-diphenyl-4-(pyridin-4-yl)benzamidinate ligands, effectively creating new binding sites on the metal complexes. The new compounds were isolated by column chromatography, and the exact conformations were verified by X-ray crystallography. Redox processes showed only a small variation within the coupling products and included two oxidations (1.30 ± 0.02 V, 0.27 ± 0.01 V vs SCE) and one reduction (-1.55 ± 0.02 V vs SCE), all centered on the Rh-Rh core. Time-dependent density functional theory (TD-DFT) was used to analyze this series with four other fully characterized N,N'-diphenyl-aryl-amidinate rhodium dimers that were found in the literature. The two main absorption bands of these nine rhodium dimers were compared to TD-DFT calculations, both giving excellent correlation. The first, a metal-to-metal (MM) transition around 11800 cm(-1) (845 nm) was blue-shifted in the calculation, with an average difference of 1378 cm(-1) but had only a 15 cm(-1) standard deviation, showing a strong correlation despite the energy difference. The second, a metal-to-ligand charge transfer (MLCT) transition around 18900 cm(-1) (530 nm) was a near perfect match with only a 64 cm(-1) average difference and a 35 cm(-1) standard deviation. The electronic transition, redox potentials, and HOMO and LUMO energies of all dimers were plotted versus the Hammett parameter (σ) of the aryl group and Taft's model with 2 components: field effects (σF) and resonance (σR). The properties involving only the Rh-Rh core (MM band, all oxidation potentials, HOMO and LUMO) were fit with a single set of σF and σR contributions (73% and 27%), with a goodness-of-fit (R(2)) value ranging from 90% to 99.7%. The metal-dimer to ligand charge-transfer band, involving the amidinate ligand, displayed different values of contribution with 45% and 55% for the σF and σR, respectively, with a fit of 94.8%. The accuracy of these fits enables the designed modification of amidinate-based dirhodium complexes to achieve desirable redox and spectroscopic properties.

Synthesis, structure, and electrochemical characterization of a mixed-ligand diruthenium(III,II) complex with an unusual arrangement of the bridging ligands

A mixed-ligand metal-metal bonded diruthenium complex having the formula Ru(2)(2,4,6-(CH(3))(3)ap)(3)(O(2)CCH(3))Cl where ap is the anilinopyridinate anion was synthesized from the reaction of Ru(2)(O(2)CCH(3))(4)Cl and H(2,4,6-(CH(3))(3)ap), after which the isolated product was structurally, spectroscopically and electrochemically characterized. The crystal structure reveals an unusual arrangement of the bridging ligands around the dimetal unit where one ruthenium atom is coordinated to one anilino and two pyridyl nitrogen atoms while the other ruthenium atom is coordinated to one pyridyl and two anilino nitrogen atoms. To our knowledge, Ru(2)(2,4,6-(CH(3))(3)ap)(3)(O(2)CCH(3))Cl is the only example of a mixed-ligand diruthenium complex of the type [Ru(2)L(3)(O(2)CCH(3))](+), where L is an unsymmetrical anionic bridging ligand that has been structurally characterized with a "(2,1)" geometric conformation of the bridging ligands, all others being "(3,0)". The initial Ru(2)(5+) compound in CH(2)Cl(2) or CH(3)CN containing 0.1 M tetra-n-butylammonium perchlorate (TBAP) undergoes up to four one-electron redox processes involving the dimetal unit. The Ru(2)(5+/4+) and Ru(2)(5+/6+) processes were characterized under N(2) using thin-layer UV-visible spectroelectrochemistry and this data is compared to UV-visible spectral changes obtained during similar electrode reactions for related diruthenium compounds having the formula Ru(2)L(4)Cl or Ru(2)L(3)(O(2)CCH(3))Cl where L is an anionic bridging ligand. Ru(2)(2,4,6-(CH(3))(3)ap)(3)(O(2)CCH(3))Cl was also examined by UV-visible and FTIR spectroelectrochemistry under a CO atmosphere and two singly reduced Ru(2)(4+) species, [Ru(2)(2,4,6-(CH(3))(3)ap)(3)(O(2)CCH(3))(CO)Cl](-) and Ru(2)(2,4,6-(CH(3))(3)ap)(3)(O(2)CCH(3))(CO) were in situ generated for further characterization. The CO-bound complexes could be further reduced and exhibited additional reductions to their Ru(2)(3+) and Ru(2)(2+) oxidation states.