Spectroscopic,Electrochemical and Computational Characterisation of Ru Species Involved in Catalytic Water Oxidation: Evidence for a [RuV(O)(Py2Metacn)] Intermediate

A new family of ruthenium complexes based on the N‐pentadentate ligand Py₂^Metacn (N‐methyl‐N′,N′′‐bis(2‐picolyl)‐1,4,7‐triazacyclononane) has been synthesised and its catalytic activity has been studied in the water‐oxidation (WO) reaction. We have used chemical oxidants (ceric ammonium nitrate and NaIO₄) to generate the WO intermediates [Ru^II(OH₂)(Py₂^Metacn)]²⁺, [Ru^III(OH₂)(Py₂^Metacn)]³⁺, [Ru^III(OH)(Py₂^Metacn)]²⁺ and [Ru^IV(O)(Py₂^Metacn)]²⁺, which have been characterised spectroscopically. Their relative redox and pH stability in water has been studied by using UV/Vis and NMR spectroscopies, HRMS and spectroelectrochemistry. [Ru^IV(O)(Py₂^Metacn)]²⁺ has a long half‐life (>48 h) in water. The catalytic cycle of WO has been elucidated by using kinetic, spectroscopic, ¹⁸O‐labelling and theoretical studies, and the conclusion is that the rate‐determining step is a single‐site water nucleophilic attack on a metal‐oxo species. Moreover, [Ru^IV(O)(Py₂^Metacn)]²⁺ is proposed to be the resting state under catalytic conditions. By monitoring Ce^IV consumption, we found that the O₂ evolution rate is redox‐controlled and independent of the initial concentration of Ce^IV. Based on these facts, we propose herein that [Ru^IV(O)(Py₂^Metacn)]²⁺ is oxidised to [Ru^V(O)(Py₂^Metacn)]²⁺ prior to attack by a water molecule to give [Ru^III(OOH)(Py₂^Metacn)]²⁺. Finally, it is shown that the difference in WO reactivity between the homologous iron and ruthenium [M(OH₂)(Py₂^Metacn)]²⁺ (M=Ru, Fe) complexes is due to the difference in the redox stability of the key M^V(O) intermediate. These results contribute to a better understanding of the WO mechanism and the differences between iron and ruthenium complexes in WO reactions.     

Oxidation of linear trinuclear ruthenium complexes [Ru(3)(dpa)(4)Cl(2)] and [Ru(3)(dpa)(4)(CN)(2)]: synthesis, structures, electrochemical and magnetic properties

The neutral, monocationic, and dicationic linear trinuclear ruthenium compounds [Ru(3)(dpa)(4)(CN)(2)], [Ru(3)(dpa)(4)(CN)(2)][BF(4)], [Ru(3)(dpa)(4)Cl(2)][BF(4)], and [Ru(3)(dpa)(4)Cl(2)][BF(4)](2) (dpa=the anion of dipyridylamine) have been synthesized and characterized by various spectroscopic techniques. Cyclic voltammetric and spectroelectrochemical studies on the neutral and oxidized compounds are reported. These compounds undergo three successive metal-centered one-electron-transfer processes. X-ray structural studies reveal a symmetrical Ru(3) unit for these compounds. While the metal--metal bond lengths change only slightly, the metal--axial ligand lengths exhibit a significant decrease upon oxidation of the neutral complex. The electronic configuration of the Ru(3) unit changes as the axial chloride ligands are replaced by the stronger "pi-acid" cyanide axial ligands. Magnetic measurements and (1)H NMR spectra indicate that [Ru(3)(dpa)(4)Cl(2)] and [Ru(3)(dpa)(4)Cl(2)][BF(4)](2) are in a spin state of S=0 and [Ru(3)(dpa)(4)Cl(2)][BF(4)], [Ru(3)(dpa)(4)(CN)(2)], and [Ru(3)(dpa)(4)(CN)(2)][BF(4)] are in spin states of S=1/2, 1, and 3/2, respectively. These results are consistent with molecular orbital (MO) calculations.     

Organic-inorganic hybrids based on novel bimolecular [Si2W22Cu2O78(H2O)]12- polyoxometalates and the polynuclear complex cations [Cu(ac)(phen)(H2O)]n n+ (n=2, 3)

The reaction of a monosubstituted Keggin polyoxometalate (POM) generated in situ with copper-phenanthroline complexes in excess ammonium or rubidium acetate led to the formation of the hybrid metal organic-inorganic compounds A7[Cu2(ac)2(phen)2(H2O)2][Cu3(ac)3(phen)3(H2O)3][Si2W22Cu2O78(H2O)].approximately 18 H2O (A=NH4+ (1), Rb+ (2); ac=acetate; phen=1,10-phenanthroline). These compounds are constructed from inorganic and metalorganic interpenetrated sublattices containing the novel bimolecular Keggin POM, [Si2W22Cu2O78(H2O)]12-, and Cu-ac-phen complexes, [Cu(ac)(phen)(H2O)]n n+ (n=2, 3). The packing of compound 1 can be viewed as a stacking of open-framework layers parallel to the xy plane built of hydrogen-bonded POMs, and zigzag columns of pi-stacked Cu-ac-phen complex cations running along the [111] direction. Magnetic and EPR results are discussed with respect to the crystal structure of the compounds. DFT calculations on [Cu(ac)(phen)(H2O)]n n+ cationic complexes have been performed, to check the influence of packing in the complex geometry and determine the magnetic exchange pathways.     

Early Relaxation Dynamics in the Photoswitchable Complex trans-[RuCl(NO)(py)4]2+

The design of photoswitchable transition metal complexes with tailored properties is one of the most important challenges in chemistry. Studies explaining the underlying mechanisms are, however, scarce. Herein, the early relaxation dynamics towards NO photoisomerization in trans-[RuCl(NO)(py)₄]²⁺ is elucidated by means of non-adiabatic dynamics, which provided time-resolved information and branching ratios. Three deactivation mechanisms (I, II, III) in the ratio 3:2:4 were identified. Pathways I and III involve ultrafast intersystem crossing and internal conversion, whereas pathway II involves only internal conversion.     

Structure Sensitivity of CO Oxidation on Co3O4: A DFT Study

The reaction mechanism of CO oxidation on the Co₃O₄ (110) and Co₃O₄ (111) surfaces is investigated by means of spin‐polarized density functional theory (DFT) within the GGA+U framework. Adsorption situation and complete reaction cycles for CO oxidation are clarified. The results indicate that 1) the U value can affect the calculated energetic result significantly, not only the absolute adsorption energy but also the trend in adsorption energy; 2) CO can directly react with surface lattice oxygen atoms (O_2f/O_3f) to form CO₂ via the Mars–van Krevelen reaction mechanism on both (110)‐B and (111)‐B; 3) pre‐adsorbed molecular O₂ can enhance CO oxidation through the channel in which it directly reacts with molecular CO to form CO₂ [O₂(a)+CO(g)→CO₂(g)+O(a)] on (110)‐A/(111)‐A; 4) CO oxidation is a structure‐sensitive reaction, and the activation energy of CO oxidation follows the order of Co₃O₄ (111)‐A(0.78 eV)>Co₃O₄ (111)‐B (0.68 eV)>Co₃O₄ (110)‐A (0.51 eV)>Co₃O₄ (110)‐B (0.41 eV), that is, the (110) surface shows higher reactivity for CO oxidation than the (111) surface; 5) in addition to the O_2f, it was also found that Co³⁺ is more active than Co²⁺, so both O_2f and Co³⁺ control the catalytic activity of CO oxidation on Co₃O₄, as opposed to a previous DFT study which concluded that either Co³⁺ or O_2f is the active site.     

Synthesis, spectroscopic characterization, X-ray structure and DFT calculations of [ReOCl2(8-Sqn)(OPPh3)]

The reaction of [ReOCl₃(PPh₃)₂] with 8-quinolinethiol (8-HSqn) has been examined, and the [ReOCl₂(8-Sqn)(OPPh₃)] complex has been obtained. It was characterized by IR, UV–Vis spectroscopy and single crystal X-ray analysis. The nature of the frontier orbitals and the electronic transitions involved in the absorption spectrum have been studied by means of the density functional and time-dependent density functional methods.     

Predictable Electronic Tuning By Choice of Azine Substituent in Five Iron(II) Triazoles: Redox Properties and DFT Calculations

Five new mononuclear iron(II) tris‐ligand complexes, and four solvatomorphs, have been made from the azine‐substituted 1,2,4‐triazole ligands ( L^azine ): [Fe^II( L^pyridazine )₃](BF₄)₂ ( 1 ), [Fe^II( L^pyrazine )₃](BF₄)₂ ( 2 ), [Fe^II( L^pyridine )₃](BF₄)₂ ( 3 ), [Fe^II( L^2pyrimidine )₃](BF₄)₂ ( 4 ), and [Fe^II( L^4pyrimidine )₃](BF₄)₂ ( 5 ). Single‐crystal XRD and solid‐state magnetometry reveal that all of them are low‐spin (LS) iron(II), except for solvatomorph 5 ?4 H₂O. Evans method NMR studies in CD₂Cl₂, (CD₃)₂CO and CD₃CN show that all are LS in these solvents, except 5 in CD₂Cl₂ (consistent with L^4pyrimidine imposing the weakest field). Cyclic voltammetry in CH₃CN vs. Ag/0.01 m AgNO₃ reveals an, at best quasi‐reversible, Fe^III/II redox process, with E_pa increasing from 0.69 to 0.99 V as the azine changes: pyridine< pyridazine<2‐pyrimidine<4‐pyrimidine< pyrazine. The observed E_pa values correlate linearly with the DFT calculated HOMO energies for the LS complexes.     

Density functional theory study of trans-dioxo complexes of iron, ruthenium, and osmium with saturated amine ligands, trans-[M(O)2(NH3)2(NMeH2)2]2+ (M=Fe, Ru, Os), and detection of [Fe(qpy)(O)2]n+ (n=1, 2) by high-resolution ESI mass spectrometry

Density functional theory (DFT) calculations on trans-dioxo metal complexes containing saturated amine ligands, trans-[M(O)2(NH3)2(NMeH2)2]2+ (M=Fe, Ru, Os), were performed with different types of density functionals (DFs): 1) pure generalized gradient approximations (pure GGAs): PW91, BP86, and OLYP; 2) meta-GGAs: VSXC and HCTH407; and 3) hybrid DFs: B3LYP and PBE1PBE. With pure GGAs and meta-GGAs, a singlet d2 ground state for trans-[Fe(O)2(NH3)2(NMeH2)2]2+ was obtained, but a quintet ground state was predicted by the hybrid DFs B3LYP and PBE1PBE. The lowest transition energies in water were calculated to be at lambda approximately 509 and 515 nm in the respective ground-state geometries from PW91 and B3LYP calculations. The nature of this transition is dependent on the DFs used: a ligand-to-metal charge-transfer (LMCT) transition with PW91, but a pi(Fe-O)-->pi*(Fe-O) transition with B3LYP, in which pi and pi* are the bonding and antibonding combinations between the dpi(Fe) and ppi(O(2-)) orbitals. The FeVI/V reduction potential of trans-[Fe(O)2(NH3)2NMeH2)2]2+ was estimated to be +1.30 V versus NHE based on PW91 results. The [Fe(qpy)(O)2](n+) (qpy=2,2':6',2':6',2':6',2'-quinquepyridine; n=1 and 2) ions, tentatively assigned to dioxo iron(V) and dioxo iron(VI), respectively, were detected in the gas phase by high-resolution ESI-MS spectroscopy.     

Oxorhenium(V) complexes with quinoline-2-carboxylate ligand. X-ray structure of [ReOCl2(quin-2-c)(PPh3)] and [ReOBr2(quin-2-c)(AsPh3)] complexes: DFT and TD-DFT calculations for [ReOCl2(quin-2-c)(PPh3)]

Novel [ReOX₂(quin-2-c)(EPh₃)] complexes (X = Cl, Br; E = As, P; quin-2-c = quinoline-2-carboxylate ion) have been prepared by treatment of [ReOX₃(EPh₃)₂] with quinoline-2-carboxylic acid in acetone at room temperature. All the complexes were characterised by IR, UV–Vis spectroscopy and elemental analysis. The crystal and molecular structures have been determined for [ReOCl₂(qiun-2c)(PPh₃)] (1) and [ReOBr₂(qiun-2c)(AsPh₃)] (4). The electronic structure of 1 has been calculated with the density functional theory (DFT) method. The spin-allowed electronic transitions of 1 have been calculated with the time-dependent DFT method.     

The Influence of N‐Heterocyclic Carbenes (NHC) on the Reactivity of [Ru(NHC)4H]+ With H2, N2, CO and O2

The five‐coordinate ruthenium N‐heterocyclic carbene (NHC) hydrido complexes [Ru(IiPr₂Me₂)₄H][BAr^F₄] ( 1 ; IiPr₂Me₂=1,3‐diisopropyl‐4,5‐dimethylimidazol‐2‐ylidene; Ar^F=3,5‐(CF₃)₂C₆H₃), [Ru(IEt₂Me₂)₄H][BAr^F₄] ( 2 ; IEt₂Me₂=1,3‐diethyl‐4,5‐dimethylimidazol‐2‐ylidene) and [Ru(IMe₄)₄H][BAr^F₄] ( 3 ; IMe₄=1,3,4,5‐tetramethylimidazol‐2‐ylidene) have been synthesised following reaction of [Ru(PPh₃)₃HCl] with 4–8 equivalents of the free carbenes at ambient temperature. Complexes 1 – 3 have been structurally characterised and show square pyramidal geometries with apical hydride ligands. In both dichloromethane or pyridine solution, 1 and 2 display very low frequency hydride signals at about δ ?41. The tetramethyl carbene complex 3 exhibits a similar chemical shift in toluene, but shows a higher frequency signal in acetonitrile arising from the solvent adduct [Ru(IMe₄)₄(MeCN)H][BAr^F₄], 4 . The reactivity of 1 – 3 towards H₂ and N₂ depends on the size of the N‐substituent of the NHC ligand. Thus, 1 is unreactive towards both gases, 2 reacts with both H₂ and N₂ only at low temperature and incompletely, while 3 affords [Ru(IMe₄)₄(η²‐H₂)H][BAr^F₄] ( 7 ) and [Ru(IMe₄)₄(N₂)H][BAr^F₄] ( 8 ) in quantitative yield at room temperature. CO shows no selectivity, reacting with 1 – 3 to give [Ru(NHC)₄(CO)H][BAr^F₄] ( 9 – 11 ). Addition of O₂ to solutions of 2 and 3 leads to rapid oxidation, from which the Ru^III species [Ru(NHC)₄(OH)₂][BAr^F₄] and the Ru^IV oxo chlorido complex [Ru(IEt₂Me₂)₄(O)Cl][BAr^F₄] were isolated. DFT calculations reproduce the greater ability of 3 to bind small molecules and show relative binding strengths that follow the trend CO ? O₂ > N₂ > H₂.     

Catalytic Water Oxidation by Ruthenium(II) Quaterpyridine (qpy) Complexes: Evidence for Ruthenium(III) qpy‐N,N′′′‐dioxide as the Real Catalysts

Polypyridyl and related ligands have been widely used for the development of water oxidation catalysts. Supposedly these ligands are oxidation‐resistant and can stabilize high‐oxidation‐state intermediates. In this work a series of ruthenium(II) complexes [Ru(qpy)(L)₂]²⁺ (qpy=2,2′:6′,2′′:6′′,2′′′‐quaterpyridine; L=substituted pyridine) have been synthesized and found to catalyze Ce^IV‐driven water oxidation, with turnover numbers of up to 2100. However, these ruthenium complexes are found to function only as precatalysts; first, they have to be oxidized to the qpy‐N,N′′′‐dioxide (ONNO) complexes [Ru(ONNO)(L)₂]³⁺ which are the real catalysts for water oxidation.     

Oxorhenium complexes with the 8-quinolinolato ligand. X-ray structure and DFT calculations for [ReOX2(hqn)(AsPh3)] and [ReOX2(hqn)(PPh3)] complexes

The reactions of [ReOX₃(AsPh₃)₂] and [ReOX₃(PPh₃)₂] with 8-hydroxyquinoline (Hhqn) have been examined and the complexes [ReOX₂(hqn)(AsPh₃)] and [ReOX₂(hqn)(PPh₃)] (X = Cl, Br) have been obtained, respectively. The crystal and molecular structures of [ReOCl₂(hqn)(AsPh₃)] (1) and [ReOBr₂(hqn)(PPh₃)] (4) have been determined. The electronic structure of 1 has been calculated with the density functional theory (DFT) method. The spin-allowed electronic transitions of 1 have been calculated with the time-dependent DFT method, and the UV–Vis spectrum of [ReOCl₂(hqn)(AsPh₃)] has been discussed on this basis.     

Synthesis,spectroscopic characterization,crystal and molecular structure of [ReOX2(bopyH)(PPh3)] complexes: DFT calculations for [ReOCl2(bopyH)(PPh3)]

The reactions of [ReOX₃(PPh₃)₂] (X = Cl, Br) with benzoylpyridine (bopy) have been examined and novel [ReOX₂(bopyH)(PPh₃)] oxocompounds have been obtained. The complexes were structurally and spectroscopically characterised. In the both structures two-electron reduced form of benzoylpyridine is coordinated to the central ion. The electronic structure of [ReOCl₂(bopyH)(PPh₃)] has been calculated with the density functional theory (DFT) method, and additional information about binding has been obtained by NBO analysis. The UV–Vis spectrum of the [ReOCl₂(bopyH)(PPh₃)] has been discussed on the basis of TDDFT calculations.     

Synthesis, spectroscopic characterization, X-ray structure, and DFT calculations of [ReOBr2(hmquin-7-COOH)(AsPh3)]

The reaction of [ReOBr₃(AsPh₃)₂] with 8-hydroxy-2-methylquinoline-7-carboxylic acid (Hhmquin-7-COOH) has been examined and the [ReOBr₂(hmquin-7-COOH)(AsPh₃)] complex has been obtained. It was characterized by IR, UV–vis spectroscopy, and single crystal X-ray analysis. The nature of the frontier orbitals and the electronic transitions involved in the absorption spectrum have been studied by means of density functional and time-dependent density functional calculations.     

Synthesis and Characterization of Terminal [Re(XCO)(CO)2(triphos)] (X=N,P): Isocyanate versus Phosphaethynolate Complexes

The terminal rhenium(I) phosphaethynolate complex [Re(PCO)(CO)₂(triphos)] has been prepared in a salt metathesis reaction from Na(OCP) and [Re(OTf)(CO)₂(triphos)]. The analogous isocyanato complex [Re(NCO)(CO)₂(triphos)] has been likewise prepared for comparison. The structure of both complexes was elucidated by X‐ray diffraction studies. While the isocyanato complex is linear, the phosphaethynolate complex is strongly bent around the pnictogen center. Computations including natural bond orbital (NBO) theory, natural resonance theory (NRT), and natural population analysis (NPA) indicate that the isocyanato complex can be viewed as a classic Werner‐type complex, that is, with an electrostatic interaction between the Re^I and the NCO group. The phosphaethynolate complex [Re(P?C?O)(CO)₂(triphos)] is best described as a metallaphosphaketene with a Re^I–phosphorus bond of highly covalent character.     

Dinitrogen Activation by Fryzuk’s [Nb(P2N2)] Complex and Comparison with the Laplaza–Cummins [Mo{N(R)Ar}3] and Schrock [Mo(N3N)] Systems

The reaction profile of N₂ with Fryzuk’s [Nb(P₂N₂)] (P₂N₂=PhP(CH₂SiMe₂NSiMe₂CH₂)₂PPh) complex is explored by density functional calculations on the model [Nb(PH₃)₂(NH₂)₂] system. The effects of ligand constraints, coordination number, metal and ligand donor atom on the reaction energetics are examined and compared to the analogous reactions of N₂ with the three‐coordinate Laplaza‐Cummins [Mo{N(R)Ar}₃] and four‐coordinate Schrock [Mo(N₃N)] (N₃N=[(RNCH₂CH₂)₃N]^3?) systems. When the model system is constrained to reflect the geometry of the P₂N₂ macrocycle, the N? N bond cleavage step, via a N₂‐bridged dimer intermediate, is calculated to be endothermic by 345 kJ mol^?1. In comparison, formation of the single‐N‐bridged species is calculated to be exothermic by 119 kJ mol^?1, and consequently is the thermodynamically favoured product, in agreement with experiment. The orientation of the amide and phosphine ligands has a significant effect on the overall reaction enthalpy and also the N? N bond cleavage step. When the ligand constraints are relaxed, the overall reaction enthalpy increases by 240 kJ mol^?1, but the N₂ cleavage step remains endothermic by 35 kJ mol^?1. Changing the phosphine ligands to amine donors has a dramatic effect, increasing the overall reaction exothermicity by 190 kJ mol^?1 and that of the N? N bond cleavage step by 85 kJ mol^?1, making it a favourable process. Replacing Nb^II with Mo^III has the opposite effect, resulting in a reduction in the overall reaction exothermicity by over 160 kJ mol^?1. The reaction profile for the model [Nb(P₂N₂)] system is compared to those calculated for the model Laplaza and Cummins [Mo{N(R)Ar}₃] and Schrock [Mo(N₃N)] systems. For both [Mo(N₃N)] and [Nb(P₂N₂)], the intermediate dimer is calculated to lie lower in energy than the products, although the final N? N cleavage step is much less endothermic for [Mo(N₃N)]. In contrast, every step of the reaction is favourable and the overall exothermicity is greatest for [Mo{N(R)Ar}₃], and therefore this system is predicted to be most suitable for dinitrogen cleavage.