Controlling Metal–Ligand–Metal Oxidation State Combinations by Ancillary Ligand (L) Variation in the Redox Systems [L2Ru(μ‐boptz)RuL2]n,boptz=3,6‐bis(2‐oxidophenyl)‐1,2,4,5‐tetrazine,and L=acetylacetonate, 2,2′‐bipyridine,or 2‐phenylazopyridine

The new compounds [(acac)₂Ru(μ‐boptz)Ru(acac)₂] ( 1 ), [(bpy)₂Ru(μ‐boptz)Ru(bpy)₂](ClO₄)₂ ( 2 ‐(ClO₄)₂), and [(pap)₂Ru(μ‐boptz)Ru(pap)₂](ClO₄)₂ ( 3 ‐(ClO₄)₂) were obtained from 3,6‐bis(2‐hydroxyphenyl)‐1,2,4,5‐tetrazine (H₂boptz), the crystal structure analysis of which is reported. Compound 1 contains two antiferromagnetically coupled (J=?36.7 cm^?1) Ru^III centers. We have investigated the role of both the donor and acceptor functions containing the boptz^2? bridging ligand in combination with the electronically different ancillary ligands (donating acac^?, moderately π‐accepting bpy, and strongly π‐accepting pap; acac=acetylacetonate, bpy=2,2′‐bipyridine pap=2‐phenylazopyridine) by using cyclic voltammetry, spectroelectrochemistry and electron paramagnetic resonance (EPR) spectroscopy for several in situ accessible redox states. We found that metal–ligand–metal oxidation state combinations remain invariant to ancillary ligand change in some instances; however, three isoelectronic paramagnetic cores Ru(μ‐boptz)Ru showed remarkable differences. The excellent tolerance of the bpy co ‐ ligand for both Ru^III and Ru^II is demonstrated by the adoption of the mixed ‐ valent form in [L₂Ru(μ‐boptz)RuL₂]³⁺, L=bpy, whereas the corresponding system with pap stabilizes the Ru^II states to yield a phenoxyl radical ligand and the compound with L=acac^? contains two Ru^III centers connected by a tetrazine radical‐anion bridge.     

Three Bis‐BODIPY Analogous Diruthenium Redox Series: Characterization and Electronic Structure Analysis

The dianion derived from (2Z,6Z)‐3,7‐diphenyl‐N2,N6‐di(pyridin‐2‐yl)pyrrolo[2,3‐f]indole‐2,6(1H,5H)‐diimine (H₂BL), a modified BODIPY ligand precursor, is shown to be capable of bridging two metal complex fragments RuL₂, L=2,4‐pentanedionato (acac^?), 2,2’‐bipyridine (bpy) or 2‐phenylazopyridine (pap) in [Ru(acac)₂Ru(μ‐BL)Ru(acac)₂] ( 1 / 2 ), [Ru(bpy)₂Ru(μ‐BL)Ru(bpy)₂](ClO₄)₂ ([ 3 ](ClO₄)₂) and [Ru(pap)₂Ru(μ‐BL)Ru(pap)₂](ClO₄)₂ ([ 4 ](ClO₄)₂). The compounds, including a diastereoisomeric pair 1 (meso) and 2 (rac) were spectroscopically and structurally characterized. Reversible electron transfers as revealed by cyclic and differential pulse voltammetry allowed for an EPR and UV‐vis‐NIR spectroelectrochemical investigation of several neighboring charge states. Together with susceptibility measurements and TD‐DFT calculations the assignment of oxidation states reveals that 1 , 2 are diruthenium(III) species which can be oxidized or reduced by one electron whereas 3 ²⁺ and 4 ²⁺ contain ruthenium(II) and get reduced or oxidized mainly at the dianionic bridge ( 3 ²⁺) or are reduced at the ancillary ligands pap ( 4 ²⁺).     

Application of a Structure/Oxidation‐State Correlation to Complexes of Bridging Azo Ligands

Based on data from more than 40 crystal structures of metal complexes with azo‐based bridging ligands (2,2′‐azobispyridine, 2,2′‐azobis(5‐chloropyrimidine), azodicarbonyl derivatives), a correlation between the N? N bond lengths (d_NN) and the oxidation state of the ligand (neutral, neutral/back‐donating, radical‐anionic, dianionic) was derived. This correlation was applied to the analysis of four ruthenium compounds of 2,2′‐azobispyridine (abpy), that is, the new asymmetrical rac‐[(acac)₂Ru1(μ‐abpy)Ru2(bpy)₂](ClO₄)₂ ([ 1 ](ClO₄)₂), [Ru(acac)₂(abpy)] ( 2 ), [Ru(bpy)₂(abpy)](ClO₄)₂ ([ 3 ](ClO₄)₂), and meso‐[(bpy)₂Ru(μ‐abpy)Ru(bpy)₂](ClO₄)₃ ([ 4 ](ClO₄)₃; acac^?=2,4‐pentanedionato, bpy=2,2′‐bipyridine). In agreement with DFT calculations, both mononuclear species 2 and 3 ²⁺ can be described as ruthenium(II) complexes of unreduced abpy⁰, with 1.295(5)<d_NN<1.320(3) Å, thereby exhibiting effects from π back‐donation. However, the abpy ligand in both the asymmetrical diamagnetic compound 1 ²⁺ (d_NN=1.374(6) Å) and the symmetrical compound 4 ³⁺ (d_NN=1.360(7), 1.368(8) Å) must be formulated as abpy^.?. Remarkably, the addition of [Ru^II(bpy)₂]²⁺ to mononuclear [Ru^II(acac)₂(abpy⁰)] induces intracomplex electron‐transfer under participation of the noninnocent abpy bridge to yield rac‐[(acac)₂Ru1^III(μ‐abpy^.?)Ru2^II(bpy)₂]²⁺ ( 1 ²⁺) with strong antiferromagnetic coupling between abpy^.? and Ru^III (DFT (B3LYP/LANL2DZ/6‐31G*)‐calculated triplet–singlet energy separation E_S=1?E_S=0=11739 cm^?1). Stepwise one‐electron transfer was studied for compound 1 ⁿ, n=1?, 0, 1+, 2+, 3+, by UV/Vis/NIR spectroelectrochemistry, EPR spectroscopy, and by DFT calculations. Whereas the first oxidation of compound 1 ²⁺ was found to mainly involve the central ligand to produce an (abpy⁰)‐bridged Class I mixed‐valent Ru1^IIIRu2^II species, the first reduction of compound 1 ²⁺ affected both the bridge and Ru1 atom to form a radical complex ( 1 ⁺), with considerable metal participation in the spin‐distribution. Further reduction moves the spin towards the {Ru2(bpy)₂} entity.     

Experimental and DFT Evidence for the Fractional Non‐Innocence of a β‐Diketonate Ligand

New compounds [Ru(pap)₂(L)](ClO₄), [Ru(pap)(L)₂], and [Ru(acac)₂(L)] (pap=2‐phenylazopyridine, L^?=9‐oxidophenalenone, acac^?=2,4‐pentanedionate) have been prepared and studied regarding their electron‐transfer behavior, both experimentally and by using DFT calculations. [Ru(pap)₂(L)](ClO₄) and [Ru(acac)₂(L)] were characterized by crystal‐structure analysis. Spectroelectrochemistry (EPR, UV/Vis/NIR), in conjunction with cyclic voltammetry, showed a wide range of about 2 V for the potential of the Ru^III/II couple, which was in agreement with the very different characteristics of the strongly π‐accepting pap ligand and the σ‐donating acac^? ligand. At the rather high potential of +1.35 V versus SCE, the oxidation of L^? into L^. could be deduced from the near‐IR absorption of [Ru^III(pap)(L^.)(L^?)]²⁺. Other intense long‐wavelength transitions, including LMCT (L^?→Ru^III) and LL/CT (pap^.?→L^?) processes, were confirmed by TD‐DFT results. DFT calculations and EPR data for the paramagnetic intermediates allowed us to assess the spin densities, which revealed two cases with considerable contributions from L‐radical‐involving forms, that is, [Ru^III(pap⁰)₂(L^?)]²⁺?[Ru^II(pap⁰)₂(L^.)]²⁺ and [Ru^III(pap⁰)(L^?)₂]⁺?[Ru^II(pap⁰)(L^?)(L^?)]⁺. Calculations of electrogenerated complex [Ru^II(pap^.?)(pap⁰)(L^?)] displayed considerable negative spin density (?0.188) at the bridging metal.     

Solvent‐Mediated Functionalization of Benzofuroxan on Electron‐Rich Ruthenium Complex Platform

An unprecedented reactivity profile of biochemically relevant R‐benzofuroxan (R=H, Me, Cl), with high structural diversity and molecular complexity on a selective {Ru(acac)₂} (acac=acetylacetonate) platform, in conjugation with EtOH solvent mediation, is revealed. This led to the development of monomeric [Ru^III(acac)₂(L^1R)] ( 1 a – 1 c ; L^1R=2‐nitrosoanilido derivatives) and dimeric [{Ru^II(acac)₂}₂(L^2R)] ( 2 a – 2 b ; L^2R=(1E,2E)‐N¹,N²‐bis(2‐nitrosophenyl)ethane‐1,2‐diimine derivatives) complexes in one pot with a change in the metal redox conditions. The functionalization of benzofuroxan in 1 and 2 implied in situ reduction of N=O to NH^? in the former and solvent‐assisted multiple N?C coupling in the latter. The aforesaid transformation processes were authenticated through structural elucidation of representative complexes, and evaluated by their spectroscopic/electrochemical features, along with C₂D₅OD labeling and monitoring of the impact of substituents (R) in the benzofuroxan framework on the product distribution process. The noninnocent potential of newly developed L¹ and L² in 1 and 2 , respectively, was also probed by spectroelectrochemistry in combination with DFT calculations.     

Koordinativ ungesättigte Dirutheniumkomplexe: Synthese und Kristallstrukturen von [Ru2(CO)n(μ‐H)(μ‐PtBu2)(μ‐Ph2PCH2PPh2)] (n = 4; 5) und [Ru2(CO)4(μ‐CH2)(μ‐H)(μ‐PtBu2)(μ‐Ph2PCH2PPh2)]

Coordinatively Unsaturated Diruthenium Complexes: Synthesis and X‐ray Crystal Structures of [Ru₂(CO)_n(μ‐H)(μ‐P^tBu₂)(μ‐Ph₂PCH₂PPh₂)] (n = 4; 5) and [Ru₂(CO)₄(μ‐CH₂)(μ‐H)(μ‐P^tBu₂)(μ‐Ph₂PCH₂PPh₂)] The reaction of [Ru₂(μ‐CO)(CO)₅(μ‐H)(μ‐P^tBu₂)(^tBu₂PH)] ( 2 ) with dppm yields the dinuclear species [Ru₂(μ‐CO)(CO)₄(μ‐H)(μ‐P^tBu₂)(μ‐dppm)] ( 3 ) (dppm = Ph₂PCH₂PPh₂). Under thermal or photolytic conditions 3 loses very easily one carbonyl ligand and affords the corresponding electronically and coordinatively unsaturated complex [Ru₂(CO)₄(μ‐H)(μ‐P^tBu₂)(μ‐dppm)] ( 4 ). 4 is also obtainable by an one‐pot synthesis from [Ru₃(CO)₁₂], an excess of ^tBu₂PH and stoichiometric amounts of dppm via the formation of [Ru₂(CO)₄(μ‐H)(μ‐P^tBu₂)(^tBu₂PH)₂] ( 1 ). 4 exhibits a Ru–Ru double bond which could be confirmed by addition of methylene to the dimetallacyclopropane [Ru₂(CO)₄(μ‐CH₂)(μ‐H)(μ‐P^tBu₂)(μ‐dppm)] ( 5 ). The molecular structures of 3 , 4 and 5 were determined by X‐ray crystal structure analyses.     

Four‐Center Oxidation State Combinations and Near‐Infrared Absorption in [Ru(pap)(Q)2]n (Q=3,5‐Di‐tert‐butyl‐N‐aryl‐1,2‐benzoquinonemonoimine,pap=2‐Phenylazopyridine)

The complex series [Ru(pap)(Q)₂]ⁿ ([ 1 ]ⁿ–[ 4 ]ⁿ; n=+2, +1, 0, ?1, ?2) contains four redox non‐innocent entities: one ruthenium ion, 2‐phenylazopyridine (pap), and two o‐iminoquinone moieties, Q=3,5‐di‐tert‐butyl‐N‐aryl‐1,2‐benzoquinonemonoimine (aryl=C₆H₅ ( 1⁺ ); m‐(Cl)₂C₆H₃ ( 2⁺ ); m‐(OCH₃)₂C₆H₃ ( 3⁺ ); m‐(tBu)₂C₆H₃ ( 4 ⁺)). A crystal structure determination of the representative compound, [ 1 ]ClO₄, established the crystallization of the ctt‐isomeric form, that is, cis and trans with respect to the mutual orientations of O and N donors of two Q ligands, and the coordinating azo N atom trans to the O donor of Q. The sensitive C? O (average: 1.299(3) Å), C? N (average: 1.346(4) Å) and intra‐ring C? C (meta; average: 1.373(4) Å) bond lengths of the coordinated iminoquinone moieties in corroboration with the N?N length (1.292(3) Å) of pap in 1 ⁺ establish [Ru^III(pap⁰)(Q^.?)₂]⁺ as the most appropriate electronic structural form. The coupling of three spins from one low‐spin ruthenium(III) (t_2g⁵) and two Q^.? radicals in 1 ⁺– 4 ⁺ gives a ground state with one unpaired electron on Q^.?, as evident from g=1.995 radical‐type EPR signals for 1 ⁺– 4 ⁺. Accordingly, the DFT‐calculated Mulliken spin densities of 1 ⁺ (1.152 for two Q, Ru: ?0.179, pap: 0.031) confirm Q‐based spin. Complex ions 1 ⁺– 4 ⁺ exhibit two near‐IR absorption bands at about λ=2000 and 920 nm in addition to intense multiple transitions covering the visible to UV regions; compounds [ 1 ]ClO₄–[ 4 ]ClO₄ undergo one oxidation and three separate reduction processes within ±2.0 V versus SCE. The crystal structure of the neutral (one‐electron reduced) state ( 2 ) was determined to show metal‐based reduction and an EPR signal at g=1.996. The electronic transitions of the complexes 1 ⁿ– 4 ⁿ (n=+2, +1, 0, ?1, ?2) in the UV, visible, and NIR regions, as determined by using spectroelectrochemistry, have been analyzed by TD‐DFT calculations and reveal significant low‐energy absorbance (λ_max>1000 nm) for cations, anions, and neutral forms. The experimental studies in combination with DFT calculations suggest the dominant valence configurations of 1 ⁿ– 4 ⁿ in the accessible redox states to be [Ru^III(pap⁰)(Q^.?)(Q⁰)]²⁺ ( 1 ²⁺– 4 ²⁺)→[Ru^III(pap⁰)(Q^.?)₂]⁺ ( 1 ⁺– 4 ⁺)→[Ru^II(pap⁰)(Q^.?)₂] ( 1 – 4 )→[Ru^II(pap^.?)(Q^.?)₂]^? ( 1 ^?– 4 ^?)→[Ru^III(pap^.?)(Q^2?)₂]^2? ( 1 ^2?– 4 ^2?).     

One‐Dimensional Ferromagnetically Coupled Bimetallic Chains Constructed with trans‐[Ru(acac)2(CN)2]−: Syntheses,Structures, Magnetic Properties,and Density Functional Theoretical Study

Four cyano‐bridged 1D bimetallic polymers have been prepared by using the paramagnetic building block trans‐[Ru(acac)₂(CN)₂]^? (Hacac=acetylacetone): {[{Ni(tren)}{Ru(acac)₂(CN)₂}][ClO₄]?CH₃OH}_n ( 1 ) (tren=tris(2‐aminoethyl)amine), {[{Ni(cyclen)}{Ru(acac)₂(CN)₂}][ClO₄]? CH₃OH}_n ( 2 ) (cyclen=1,4,7,10‐tetraazacyclododecane), {[{Fe(salen)}{Ru(acac)₂(CN)₂}]}_n ( 3 ) (salen^2?=N,N′‐bis(salicylidene)‐o‐ethyldiamine dianion) and [{Mn(5,5′‐Me₂salen)}₂{Ru(acac)₂(CN)₂}][Ru(acac)₂(CN)₂]? 2 CH₃OH ( 4 ) (5,5′‐Me₂salen=N,N′‐bis(5,5′‐dimethylsalicylidene)‐o‐ethylenediimine). Compounds 1 and 2 are 1D, zigzagged NiRu chains that exhibit ferromagnetic coupling between Ni^II and Ru^III ions through cyano bridges with J=+1.92 cm^?1, z J′=?1.37 cm^?1, g=2.20 for 1 and J=+0.85 cm^?1, z J′=?0.16 cm^?1, g=2.24 for 2 . Compound 3 has a 1D linear chain structure that exhibits intrachain ferromagnetic coupling (J=+0.62 cm^?1, z J′=?0.09 cm^?1, g=2.08), but antiferromagnetic coupling occurs between FeRu chains, leading to metamagnetic behavior with T_N=2.6 K. In compound 4 , two Mn^III ions are coordinated to trans‐[Ru(acac)₂(CN)₂]^? to form trinuclear Mn₂Ru units, which are linked together by π–π stacking and weak Mn???O* interactions to form a 1D chain. Compound 4 shows slow magnetic relaxation below 3.0 K with ?=0.25, characteristic of superparamagnetic behavior. The Mn^III???Ru^III coupling constant (through cyano bridges) and the Mn^III???Mn^III coupling constant (between the trimers) are +0.87 and +0.24 cm^?1, respectively. Compound 4 is a novel single‐chain magnet built from Mn₂Ru trimers through noncovalent interactions. Density functional theory (DFT) combined with the broken symmetry state method was used to calculate the molecular magnetic orbitals and the magnetic exchange interactions between Ru^III and M (M=Ni^II, Fe^III, and Mn^III) ions. To explain the somewhat unexpected ferromagnetic coupling between low‐spin Ru^III and high‐spin Fe^III and Mn^III ions in compounds 3 and 4 , respectively, it is proposed that apart from the relative symmetries, the relative energies of the magnetic orbitals may also be important in determining the overall magnetic coupling in these bimetallic assemblies.     

Kristallstruktur,Schwingungsspektren und Normalkoordinatenanalyse von (n‐Bu4N)2[{Ru(NO)ClI2}2(μ‐I2)] · 2 I2

Crystal Structure, Vibrational Spectra, and Normal Coordinate Analysis of ( n ‐Bu₄N)₂[{Ru(NO)ClI₂}₂(μ‐I₂)] · 2 I₂ By treatment of (n‐Bu₄N)₂[Ru(NO)I₅] with (n‐Bu₄N)Cl in dichloromethane (n‐Bu₄N)₂[{Ru(NO)ClI₂}₂(μ‐I₂)] is formed. The X‐Ray structure determination on a single crystal of (n‐Bu₄N)₂[{Ru(NO)ClI₂}₂(μ‐I₂)] · 2 I₂ (monoclinic, space group I 2/a, a = 20.446(6), b = 11.482(8), c = 27.225(3) Å, β = 107.51(4)°, Z = 4) reveals a dinuclear iodine bridged structure, in which the chlorine atoms are trans positioned to the nitrosyl groups. The low temperature IR and Raman spectra have been recorded of (n‐Bu₄N)₂[{Ru(NO)ClI₂}₂(μ‐I₂)] · 2 I₂ and are assigned by normal coordinate analysis. A good agreement between observed and calculated frequencies is achieved. The valence force constants are f_d(NO) = 14.08, f_d(RuN) = 5.58, f_d(RuCl) = 1.52, f_d(RuI_t) = 0.90 and f_d(RuI_b) = 0.76 mdyn/Å.     

Aktivierung von Schwefelkohlenstoff an Trirutheniumclustern: Synthese und Kristallstruktur von [Ru3(CO)5(μ‐H)2(μ‐PCy2)(μ‐Ph2PCH2PPh2){μ‐η2‐PCy2C(S)}(μ3‐S)] und [Ru3(CO)5(CS)(μ‐H)(μ‐PtBu2)(μ‐PCy2)2(μ3‐S)]

Activation of Carbon Disulfide on Triruthenium Clusters: Synthesis and X‐Ray Crystal Structure Analysis of [Ru₃(CO)₅(μ‐H)₂(μ‐PCy₂)(μ‐Ph₂PCH₂PPh₂){μ‐η²‐PCy₂C(S)}(μ₃‐S)] and [Ru₃(CO)₅(CS)(μ‐H)(μ‐P^tBu₂)(μ‐PCy₂)₂(μ₃‐S)] [Ru₃(CO)₆(μ‐H)₂(μ‐PCy₂)₂(μ‐dppm)] ( 1 ) (dppm = Ph₂PCH₂PPh₂) reacts under mild conditions with CS₂ and yields by oxidative decarbonylation and insertion of CS into one phosphido bridge the opened 50 VE‐cluster [Ru₃(CO)₅(μ‐H)₂(μ‐PCy₂)(μ‐dppm){μ‐η²‐PCy₂C(S)}(μ₃‐S)] ( 2 ) with only two M–M bonds. The compound 2 crystallizes in the triclinic space group P 1 with a = 19.093(3), b = 12.2883(12), c = 20.098(3) Å; α = 84.65(3), β = 77.21(3), γ = 81.87(3)° and V = 2790.7(11) ų. The reaction of [Ru₃(CO)₇(μ‐H)(μ‐P^tBu₂)(μ‐PCy₂)₂] ( 3 ) with CS₂ in refluxing toluene affords the 50 VE‐cluster [Ru₃(CO)₅(CS)(μ‐H)(μ‐P^tBu₂)(μ‐PCy₂)₂(μ₃‐S)] ( 4 ). The compound cristallizes in the monoclinic space group P 2₁/a with a = 19.093(3), b = 12.2883(12), c = 20.098(3) Å; β = 104.223(16)° and V = 4570.9(10) ų. Although in the solid state structure one elongated Ru–Ru bond has been found the complex 4 can be considered by means of the ³¹P‐NMR data as an electron‐rich metal cluster.     

Reduktive Dimerisierung von NO an Dirutheniumkomplexen und Bildung eines tridentaten 2, 2‐Bis(diphenylphosphanyl)ethanolato‐Liganden

The reaction of the trans‐hyponitrito complex [Ru₂(CO)₄(μ‐η²‐ONNO)(μ‐H)(μ‐PtBu₂)(μ‐dppen)] ( 1 , dppen = Ph₂PC(=CH₂)PPh₂) with tetrafluorido boric acid afforded the new complex salt [Ru₂(CO)₄(μ‐η²‐ONNOH)(μ‐H)(μ‐PtBu₂)(μ‐dppen)]BF₄ ( 2 ) containing the monoprotonate hyponitrous acid as the ligand in the cationic complex. Complex 1 showed a nucleophilic reactivity towards the trimethyloxonium cation resulting in the monoester derivative of the hyponitrous acid [Ru₂(CO)₄(μ‐η²‐ONNOMe)(μ‐H)(μ‐PtBu₂)(μ‐dppen)]BF₄ ( 3 ). During heating of compound 2 in ethanol under reflux for a short time nitrous oxide was liberated affording unexpectedly a new tridentate 2, 2‐bis(diphenylphosphanyl)ethanolato ligand formed by an intramolecular attack of an intermediate hydroxido ligand towards the unsaturated carbon carbon double bond in the bridging dppen ligand. Thus the complex salt [Ru₂(CO)₄{μ‐η³‐OCH₂CH(PPh₂)₂}(μ‐H)(μ‐PtBu₂)]BF₄ ( 4 ) was formed in good yields. The new compounds 2 , 3 , and 4 were characterized by spectroscopic means as well as their molecular structures were determined in the crystal.     

Asymmetric 1,3‐Dipolar Cycloadditions of 2‐Diazocyclohexane‐1,3‐diones and Alkyl Diazopyruvates

The 1,3‐dipolar cycloaddition reactions of 2‐diazocyclohexane‐1,3‐dione ( 7a ; Table 1) and of alkyl diazopyruvates ( 11a – e ; Table 3) to 2,3‐dihydrofuran and other enol ethers have been investigated in the presence of chiral transition metal catalysts. With Rh^II catalysts, the cycloadditions were not enantioselective, but those catalyzed by [Ru^IICl₂( 1a )] and [Ru^IICl₂( 1b )] proceeded with enantioselectivities of up to 58% and 74% ee, respectively, when diazopyruvates 11 were used as substrates. The phenyliodonium ylide 7c yielded the adduct 8a in lower yield and poorer selectivity than the corresponding diazo precursor 7a (Table 2) upon decomposition with [Ru(pybox)] catalysts. This suggests that ylide decomposition by Ru^II catalysts, contrary to that of the corresponding diazo precursors, does not lead to Ru‐carbene complexes as reactive intermediates. Our method represents the first reproducible, enantioselective 1,3‐cycloaddition of these types of substrates.     

Metal‐catalyzed reversible conversion between chemical and electrical energy designed towards a sustainable society

Proton dissociation of an aqua‐Ru‐quinone complex, [Ru(trpy)(q)(OH₂)]²⁺ (trpy = 2,2′ : 6′,2″‐terpyridine, q = 3,5‐di‐t‐butylquinone) proceeded in two steps (pK_a = 5.5 and ca. 10.5). The first step simply produced [Ru(trpy)(q)(OH)]⁺, while the second one gave an unusual oxyl radical complex, [Ru(trpy)(sq)(O^?.)]⁰ (sq = 3,5‐di‐t‐butylsemiquinone), owing to an intramolecular electron transfer from the resultant O^2? to q. A dinuclear Ru complex bridged by an anthracene framework, [Ru₂(btpyan)(q)₂(OH)₂]²⁺ (btpyan = 1,8‐bis(2,2′‐terpyridyl)anthracene), was prepared to place two Ru(trpy)(q)(OH) groups at a close distance. Deprotonation of the two hydroxy protons of [Ru₂(btpyan)(q)₂(OH)₂]²⁺ generated two oxyl radical Ru‐O^?. groups, which worked as a precursor for O₂ evolution in the oxidation of water. The [Ru₂(btpyan)(q)₂(OH)₂](SbF₆)₂ modified ITO electrode effectively catalyzed four‐electron oxidation of water to evolve O₂ (TON = 33500) under electrolysis at +1.70 V in H₂O (pH 4.0). Various physical measurements and DFT calculations indicated that a radical coupling between two Ru(sq)(O^?.) groups forms a (cat)Ru‐O‐O‐Ru(sq) (cat = 3,5‐di‐t‐butylcathechol) framework with a μ‐superoxo bond. Successive removal of four electrons from the cat, sq, and superoxo groups of [Ru₂(btpyan)(cat)(sq)(μ‐O₂^?)]⁰ assisted with an attack of two water (or OH^?) to Ru centers, which causes smooth O₂ evolution with regeneration of [Ru₂(btpyan)(q)₂(OH)₂]²⁺. Deprotonation of an Ru‐quinone‐ammonia complex also gave the corresponding Ru‐semiquinone‐aminyl radical. The oxidized form of the latter showed a high catalytic activity towards the oxidation of methanol in the presence of base. Three complexes, [Ru(bpy)₂(CO)₂]²⁺, [Ru(bpy)₂(CO)(C(O)OH)]⁺, and [Ru(bpy)₂(CO)(CO₂)]⁰ exist as an equilibrium mixture in water. Treatment of [Ru(bpy)₂(CO)₂]²⁺ with BH₄^? gave [Ru(bpy)₂(CO)(C(O)H)]⁺, [Ru(bpy)₂(CO)(CH₂OH)]⁺, and [Ru(bpy)₂(CO)(OH₂)]²⁺ with generation of CH₃OH in aqueous conditions. Based on these results, a reasonable catalytic pathway from CO₂ to CH₃OH in electro‐ and photochemical CO₂ reduction is proposed. A new pbn (pbn = 2‐pyridylbenzo[b]‐1,5‐naphthyridine) ligand was designed as a renewable hydride donor for the six‐electron reduction of CO₂. A series of [Ru(bpy)_3‐n(pbn)_n]²⁺ (n = 1, 2, 3) complexes undergoes photochemical two‐ (n = 1), four‐ (n = 2), and six‐electron reductions (n = 3) under irradiation of visible light in the presence of N(CH₂CH₂OH)₃. © 2009 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Chem Rec 9: 169–186; 2009: Published online in Wiley InterScience ( www.interscience.wiley.com ) DOI 10.1002/tcr.200800039     

Visible‐Light‐Induced Photocatalytic Hydrogen Production over Binuclear RuII–Bipyridyl Dye‐Sensitized TiO2 without Noble Metal Loading

Highly efficient, visible light induced photocatalytic H₂ production was achieved over a TiO₂ system sensitized by binuclear Ru^II bipyridyl (bpy) complex [Ru₂(bpy)₄(BL)](ClO₄)₂ (BL=bridging ligand) without Pt loading, which is almost unaffected by pH in aqueous solution in the wide range from pH 5.00 to 10.50, although the dye molecules can only be loosely attached to TiO₂ due to the absence of terminal carboxyl groups. The photocatalyst shows remarkable long‐term stability and reproducibility of H₂ evolution even after exchanging the aqueous triethanolamine solution. The amount of H₂ evolved over 100 mg of photocatalyst in 27 h of irradiation corresponds to a turnover number of about 75 340, and the apparent quantum yields are estimated to be 16.8 and 7.3 % under 420 and 475 nm monochromatic light irradiation, respectively. A comparative study shows that the loosely attached dye [Ru₂(bpy)₄(BL)](ClO₄)₂ has higher photosensitization efficiency than tightly linked dyes with terminal carboxyl groups, such as [Ru₂(dcbpy)₄(BL)](ClO₄)₂ and N719. It can be rationalized by their different coordination, physicochemical, electron‐injection, and back‐transfer properties.     

Redox Non‐Innocence and Isomer‐Specific Oxidative Functionalization of Ruthenium‐Coordinated β‐Ketoiminate

This article deals with isomeric ruthenium complexes [Ru^III(L^R)₂(acac)] (S=1/2) involving unsymmetric β‐ketoiminates (AcNac) (L^R=R‐AcNac, R=H ( 1 ), Cl ( 2 ), OMe ( 3 ); acac=acetylacetonate) [R=para‐substituents (H, Cl, OMe) of N‐bearing aryl group]. The isomeric identities of the complexes, cct (cis‐cis‐trans, blue, a ), ctc (cis‐trans‐cis, green, b ) and ccc (cis‐cis‐cis_, pink, c ) with respect to oxygen (acac), oxygen (L) and nitrogen (L) donors, respectively, were authenticated by their single‐crystal X‐ray structures and spectroscopic/electrochemical features. One‐electron reversible oxidation and reduction processes of 1 – 3 led to the electronic formulations of [Ru^III(L)(L ^? )(acac)]⁺ and [Ru^II(L)₂(acac)]^? for 1 ⁺‐ 3 ⁺ (S=1) and 1^? – 3^? (S=0), respectively. The triplet state of 1 ⁺‐ 3 ⁺ was corroborated by its forbidden weak half‐field signal near g≈4.0 at 4 K, revealing the non‐innocent feature of L. Interestingly, among the three isomeric forms ( a – c in 1 – 3 ), the ctc ( b in 2 b or 3 b ) isomer selectively underwent oxidative functionalization at the central β‐carbon (C?H→C=O) of one of the L ligands in air, leading to the formation of diamagnetic [Ru^II(L)(L ′ )(acac)] (L ′ =diketoimine) in 4 / 4′ . Mechanistic aspects of the oxygenation process of AcNac in 2 b were also explored via kinetic and theoretical studies.     

Chelation versus Binucleation: Metal Complex Formation with the Hexadentate all‐cis‐N1,N2‐Bis(2,4,6‐trihydroxy‐3,5‐diaminocyclohexyl)ethane‐1,2‐diamine

The hexadentate ligand all‐cis‐N¹,N²‐bis(2,4,6‐trihydroxy‐3,5‐diaminocyclohexyl)ethane‐1,2‐diamine (L^e) was synthesized in five steps with an overall yield of 39 % by using [Ni(taci)₂]SO₄?4 H₂O as starting material (taci=1,3,5‐triamino‐1,3,5‐trideoxy‐cis‐inositol). Crystal structures of [Na_0.5(H₆L^e)](BiCl₆)₂Cl_0.5?4 H₂O ( 1 ), [Ni(L^e)]‐ Cl₂?5 H₂O ( 2 ), [Cu(L^e)](ClO₄)₂?H₂O ( 3 ), [Zn(L^e)]CO₃?7 H₂O ( 4 ), [Co(L^e)](ClO₄)₃ ( 5 c ), and [Ga(H_?2L^e)]‐ NO₃?2 H₂O ( 6 ) are reported. The Na complex 1 exhibited a chain structure with the Na⁺ cations bonded to three hydroxy groups of one taci subunit of the fully protonated H₆(L^e)⁶⁺ ligand. In 2 , 3 , 4 , and 5 c , a mononuclear hexaamine coordination was found. In the Ga complex 6 , a mononuclear hexadentate coordination was also observed, but the metal binding occurred through four amino groups and two alkoxo groups of the doubly deprotonated H_?2(L^e)^2?. The steric strain within the molecular framework of various M(L^e) isomers was analyzed by means of molecular mechanics calculations. The formation of complexes of L^e with Mn^II, Cu^II, Zn^II, and Cd^II was investigated in aqueous solution by using potentiometric and spectrophotometric titration experiments. Extended equilibrium systems comprising a large number of species were observed, such as [M(L^e)]²⁺, protonated complexes [MH_z(L^e)]^2+z and oligonuclear aggregates. The pK_a values of H₆(L^e)⁶⁺ (25 °C, μ=0.10 m ) were found to be 2.99, 5.63, 6.72, 7.38, 8.37, and 9.07, and the determined formation constants (log β) of [M(L^e)]²⁺ were 6.13(3) (Mn^II), 20.11(2) (Cu^II), 13.60(2) (Zn^II), and 10.43(2) (Cd^II). The redox potentials (vs. NHE) of the [M(L^e)]^3+/2+ couples were elucidated for Co (?0.38 V) and Ni (+0.90 V) by cyclic voltammetry.     

Comparative Study of Ruthenium(II) and Ruthenium(III) Complexes with the Ligand dmbpy (dmbpy = 4, 4′‐Dimethyl‐2, 2′‐bipyridine)

The complex cis‐[Ru^III(dmbpy)₂Cl₂](PF₆) ( 2 ) (dmbpy = 4, 4′‐dimethyl‐2, 2′‐bipyridine) was obtained from the reaction of cis‐[Ru^II(dmbpy)₂Cl₂] ( 1 ) with ammonium cerium(IV) nitrate followed by precipitation with saturated ammonium hexafluoridophosphate. The ¹H NMR spectrum of the Ru^III complex confirms the presence of paramagnetic metal atoms, whereas that of the Ru^II complex displays diamagnetism. The ³¹P NMR spectrum of the Ru^III complex shows one signal for the phosphorus atom of the PF₆^– ion. The perspective view of each [Ru^II/III(dmbpy)₂Cl₂]^0/+ unit manifests that the ruthenium atom is in hexacoordinate arrangement with two dmbpy ligands and two chlorido ligands in cis position. As the oxidation state of the central ruthenium metal atom becomes higher, the average Ru–Cl bond length decreases whereas the Ru–N (dmbpy) bond length increases. The cis‐positioned dichloro angle in Ru^III is 1.3° wider than that in the Ru^II. The dihedral angles between pair of planar six‐membered pyridyl ring in the dmbpy ligand for the Ru^II are 4.7(5)° and 5.7(4)°. The observed inter‐planar angle between two dmbpy ligands in the Ru^II is 89.08(15)°, whereas the value for the Ru^III is 85.46(20)°.     

Density Functional Theory Calculations on Oxidative CC Bond Cleavage and NO Bond Formation of [RuII(bpy)2(diamine)]2+ via Reactive Ruthenium Imide Intermediates

DFT calculations are performed on [Ru^II(bpy)₂(tmen)]²⁺ ( M1 , tmen=2,3‐dimethyl‐2,3‐butanediamine) and [Ru^II(bpy)₂(heda)]²⁺ ( M2 , heda=2,5‐dimethyl‐2,5‐hexanediamine), and on the oxidation reactions of M1 to give the C?C bond cleavage product [Ru^II(bpy)₂(NH=CMe₂)₂]²⁺ ( M3 ) and the N?O bond formation product [Ru^II(bpy)₂(ONCMe₂CMe₂NO)]²⁺ ( M4 ). The calculated geometrical parameters and oxidation potentials are in good agreement with the experimental data. As revealed by the DFT calculations, [Ru^II(bpy)₂(tmen)]²⁺ ( M1 ) can undergo oxidative deprotonation to generate Ru‐bis(imide) [Ru(bpy)₂(tmen‐4 H)]⁺ ( A ) or Ru‐imide/amide [Ru(bpy)₂(tmen‐3 H)]²⁺ ( A′ ) intermediates. Both A and A′ are prone to C?C bond cleavage, with low reaction barriers (ΔG^≠) of 6.8 and 2.9 kcal mol^?1 for their doublet spin states ² A and ² A′ , respectively. The calculated reaction barrier for the nucleophilic attack of water molecules on ² A′ is relatively high (14.2 kcal mol^?1). These calculation results are in agreement with the formation of the Ru^II‐bis(imine) complex M3 from the electrochemical oxidation of M1 in aqueous solution. The oxidation of M1 with Ce^IV in aqueous solution to afford the Ru^II‐dinitrosoalkane complex M4 is proposed to proceed by attack of the cerium oxidant on the ruthenium imide intermediate. The findings of ESI‐MS experiments are consistent with the generation of a ruthenium imide intermediate in the course of the oxidation.     

Factors Affecting DNA–DNA Interstrand Cross‐Links in the Antiparallel 3′–3′ Sense: A Comparison with the 5′–5′ Directional Isomer

Reported herein is a study of the unusual 3′–3′ 1,4‐GG interstrand cross‐link (IXL) formation in duplex DNA by a series of polynuclear platinum anticancer complexes. To examine the effect of possible preassociation through charge and hydrogen‐bonding effects the closely related compounds [{trans‐PtCl(NH₃)₂}₂(μ‐trans‐Pt(NH₃)₂{NH₂(CH₂)₆NH₂}₂)]⁴⁺ (BBR3464, 1 ), [{trans‐PtCl(NH₃)₂}₂(μ‐NH₂(CH₂)₆NH₂)]²⁺ (BBR3005, 2 ), [{trans‐PtCl(NH₃)₂}₂(μ‐H₂N(CH₂)₃NH₂(CH₂)₄)]³⁺ (BBR3571, 3 ) and [{trans‐PtCl(NH₃)₂}₂{μ‐H₂N(CH₂)₃‐N(COCF₃)(CH₂)₄}]²⁺ (BBR3571‐COCF₃, 4 ) were studied. Two different molecular biology approaches were used to investigate the effect of DNA template upon IXL formation in synthetic 20‐base‐pair duplexes. In the “hybridisation directed” method the monofunctionally adducted top strands were hybridised with their complementary 5′‐end labelled strands; after 24 h the efficiency of interstrand cross‐linking in the 5′–5′ direction was slightly higher than in the 3′–3′ direction. The second method involved “postsynthetic modification” of the intact duplex; significantly less cross‐linking was observed, but again a slight preference for the 5′–5′ duplex was present. 2D [¹H, ¹⁵N] HSQC NMR spectroscopy studies of the reaction of [¹⁵N]‐ 1 with the sequence 5′‐d{TATACATGTATA}₂ allowed direct comparison of the stepwise formation of the 3′–3′ IXL with the previously studied 5′–5′ IXL on the analogous sequence 5′‐d(ATATGTACATAT)₂. Whereas the preassociation and aquation steps were similar, differences were evident at the monofunctional binding step. The reaction did not yield a single distinct 3′–3′ 1,4‐GG IXL, but numerous cross‐linked adducts formed. Similar results were found for the reaction with the dinuclear [¹⁵N]‐ 2 . Molecular dynamics simulations for the 3′–3′ IXLs formed by both 1 and 2 showed a highly distorted structure with evident fraying of the end base pairs and considerable widening of the minor groove.     

A novel one‐dimensional zinc coordination polymer containing 4‐methylbenzoate and 4,4‐bipyridine

The title compound, catena‐poly[[tris(μ‐4‐methylbenzoato)‐κ²O:O;κ⁴O:O′‐(4‐methylbenzoato‐κ²O,O′)dizinc(II)]‐μ‐4,4′‐bipyridine‐κ²N:N′], [Zn₂(C₈H₇O₂)₄(C₁₀H₈N₂)]_n, is a novel coordination polymer. The asymmetric unit contains two unique Zn^II ions, four 4‐methylbenzoate ligands and one 4,4′‐bipyridine (4,4′‐bpy) ligand, all in general positions. The four 4‐methylbenzoate ligands link the two Zn^II centres to form a dinuclear unit, with a Zn...Zn separation of 3.188 (2) Å, which can be regarded as a supramolecular secondary building unit (SBU). These SBUs are further bridged by 4,4′‐bpy ligands, forming a novel one‐dimensional infinite chain. There are π–π stacking interactions between the benzene rings of the 4‐methylbenzoate ligands and the pyridyl rings of the 4,4′‐bpy ligands, leading to the formation of a corrugated layer. These layers are further assembled via C—H...O hydrogen bonds into a three‐dimensional supramolecular network structure. Coordination polymers such as the title compound are of interest for their potential applications as functional materials.