Synthesis and characterization of novel ferrocene-containing pyridylamine ligands and their ruthenium(II) complexes: electronic communication through hydrogen-bonded amide linkage

Tris(2-pyridylmethyl)amine (TPA) derivatives with one or two ferrocenoylamide moieties at the 6-position of one or two pyridine rings of TPA were synthesized. The compounds, N-(6-ferrocenoylamide-2-pyridylmethyl)-N,N-bis(2-pyridylmethyl)amine (Fc-TPA; L1) and N,N-bis(6-ferrocenoylamide-2-pyridylmethyl)-N-(2-pyridylmethyl)amine (Fc2-TPA; L2), were characterized by spectroscopic methods, cyclic voltammetry, and X-ray crystallography. Their Ru(II) complexes were also prepared and characterized by spectroscopic methods, cyclic voltammetry, and X-ray crystallography. [RuCl(L1)(DMSO)]PF(6) (1) that contains S-bound dimethyl sulfoxide (DMSO) as a ligand and an uncoordinated ferrocenoylamide moiety exhibited two redox waves at 0.23 and 0.77 V (vs ferrocene/ferrocenium ion as 0 V), due to Fc/Fc(+) and Ru(II)/Ru(III) redox couples, respectively. [RuCl(L2)]PF(6) (2) that contains both coordinated and uncoordinated amide moieties showed two redox waves that were observed at 0.27 V (two electrons) and 0.46 V (one electron), assignable to Ru(II)/Ru(III) redox couples overlapped with the uncoordinated Fc/Fc(+) redox couple and the coordinated Fc/Fc(+), respectively. In contrast to 2, an acetonitrile complex, [Ru(L2)(CH(3)CN)](PF(6))(2) (3), exhibited three redox couples at 0.26 and 0.37 V for two kinds of Fc/Fc(+) couples, and 0.83 V for the Ru(II)/Ru(III) couple (vs ferrocene/ferrocenium ion as 0 V). In this complex, the redox potentials of the coordinated and the uncoordinated Fc-amide moieties were discriminated in the range of 0.11 V. Chemical two-electron oxidation of 1 gave [RuIIICl(L1+)(DMSO)](3+) to generate a ferromagnetically coupled triplet state (S = 1) with J = 13.7 cm-1 (H = -JS(1)S(2)) which was estimated by its variable-temperature electron spin resonance (ESR) spectra in CH(3)CN. The electron spins at the Ru(III) center and the Fe(III) center are ferromagnetically coupled via an amide linkage. In the case of 2, its two-electron oxidation gave the same ESR spectrum, which indicates formation of a similar triplet state. Such electronic communication may occur via the amide linkage forming the intramolecular hydrogen bonding.     

Iron(II) complexes with tetradentate bis(aminophenolate) ligands: synthesis and characterization, solution behavior, and reactivity with O(2)

Tetradentate bis(aminophenolate) ligands H(2)salan(X) and H(2)bapen(X) (where X refers to the para-phenolate substituent = H, Me, F, Cl) react with [Fe{N(SiMe(3))(2)}(2)] to form iron(II) complexes, which in the presence of suitable donor ligands L (L = pyridine or THF) can be isolated as the complexes [Fe(salan(X))(L)(2)] and [Fe(bapen(X))(L)(2)]. In the absence of donor ligands, either mononuclear complexes, for example, [Fe(salan(tBu,tBu))], or dinuclear complexes of the type [Fe(salan(X))](2) are obtained. The dynamic coordination behavior in solution of the complexes [Fe(salan(F))(L)(2)] and [Fe(bapen(F))(L)(2)] has been investigated by VT (1)H and (19)F NMR spectroscopy, which has revealed equilibria between isomers with different ligand coordination topologies cis-α, cis-β and trans. Exposure of the iron(II) salan(X) complexes to O(2) results in the formation of oxo-bridged iron(III) complexes of the type [{Fe(salan(X))}(2)(μ-O)] or [{Fe(salan(X))(L)}(2)(μ-O)]. The lack of catalytic activity of the iron(II) salan and bapen complexes in the oxidation of cyclohexane with H(2)O(2) as the oxidant is attributed to the rapid formation of stable and catalytically inactive oxo-bridged iron(III) complexes.     

Structural, spectroscopic and redox properties of uranyl complexes with a maleonitrile containing ligand

The reaction of uranyl nitrate hexahydrate with the maleonitrile containing Schiff base 2,3-bis[(4-diethylamino-2-hydroxybenzylidene)amino]but-2-enedinitrile (salmnt((Et(2)N)(2))H(2)) in methanol produces [UO(2)(salmnt((Et2N)2))(H(2)O)] (1) where the uranyl equatorial coordination plane is completed by the N(2)O(2) tetradentate cavity of the (salmnt((Et(2)N)(2)))(2-) ligand and a water molecule. The coordinated water molecule readily undergoes exchange with pyridine (py), dimethylsulfoxide (DMSO), N,N-dimethylformamide (DMF) and triphenylphosphine oxide (TPPO) to give a series of [UO(2)(salmnt((Et(2)N)(2)))(L)] complexes (L = py, DMSO, DMF, TPPO; 2-5, respectively). X-Ray crystallography of 1-5 show that the (salmnt((Et(2)N)(2)))(2-) ligand is distorted when coordinated to the uranyl moiety, in contrast to the planar structure observed for the free protonated ligand (salmnt((Et(2)N)(2))H(2)). The Raman spectra of 1-5 only display extremely weak bands (819-828 cm(-1)) that can be assigned to the typically symmetric O=U=O stretch. This stretching mode is also observed in the infrared spectra for all complexes 1-5 (818-826 cm(-1)) predominantly caused by the distortion of the tetradentate (salmnt((Et(2)N)(2)))(2-) ligand about the uranyl equatorial plane resulting in a change in dipole for this bond stretch. The solution behaviour of 2-5 was studied using NMR, electronic absorption and emission spectroscopy, and cyclic voltammetry. Complexes 2-5 exhibit intense absorptions in the visible region of the spectrum due to intramolecular charge transfer (ICT) transitions and the luminescence lifetimes (< 5 ns) indicate the emission arises from ligand-centred excited states. Reversible redox processes assigned to the {UO(2)}(2+)/{UO(2)}(+) couple are observed for complexes 2-5 (2: E(1/2) = -1.80 V; 3,5: E(1/2) = -1.78 V; 4: E(1/2) = -1.81 V : vs. ferrocenium/ferrocene {Fc(+)/Fc}, 0.1 M Bu(4)NPF(6)) in dichloromethane (DCM). These are some of the most negative half potentials for the {UO(2)}(2+)/{UO(2)}(+) couple observed to date and indicate the strong electron donating nature of the (salmnt((Et(2)N)(2)))(2-) ligand. Multiple uranyl redox processes are clearly seen for [UO(2)(salmnt((Et(2)N)(2)))(L)] in L (L = py, DMSO, DMF; 2-4: 0.1 M Bu(4)NPF(6)) indicating the relative instability of these complexes when competing ligands are present, but the reversible {UO(2)}(2+)/{UO(2)}(+) couple for the intact complexes can still be assigned and shows the position of this couple can be modulated by the solvation environment. Several redox processes were also observed between +0.2 and +1.2 V (vs. Fc(+)/Fc) that prove the redox active nature of the maleonitrile-containing ligand.     

Fluorescent and electroactive cyclic assemblies from perylene tetracarboxylic acid bisimide ligands and metal phosphane triflates

Tetraaryloxy-substituted perylene tetracarboxylic acid bisimides with one or two 4-pyridyl receptor substituents at the imide functionality were synthesized and employed in transition metal directed self-assembly with Pd(II) and Pt(II) phosphane triflates. Upon mixing of the components, quantitative formation of functional molecular square-type complexes containing four dye molecules and model complexes of a 2:1 (perylene bisimide ligand:transition metal ion) stoichiometry was observed. The isolated metallosupramolecular squares were characterized by 1H and 31P [1H] NMR spectroscopy as well as conventional electrospray ionization (ESI) and ESI-FTICR mass spectrometry, which gave evidence for the structure and the high stability of these giant cyclic dye assemblies (molecular weight (3a) 8172, Pt-Pt corner diagonal ca. 3.4 nm). Studies of the optical absorption and fluorescence properties and the electrochemistry and spectroelectrochemistry of both the perylene bisimide ligands and the perylene bisimide metal complexes show that Pt(II) coordination does not interfere with the optical and electrochemical properties of the perylene bisimide ligands; this gives squares with high fluorescence quantum yields (phiF (3a)=0.88) and three fully reversible redox couples. The latter could be unambiguously related to quantitative formation of perylene bisimide radical cations (E1/2 = +0.93 V vs. Fc/Fc+), radical anions (E1/2= - 1.01 V vs. Fc/Fc+), and dianions (E1/2 = -1.14 V vs. Fc/Fc+); these redox reactions change the charge state of the cyclic assembly from +12 to zero. In contrast, Pd(II) coordination influenced the electrochemical properties of the assembly because of an irreversible palladium reduction at E1/2= -1.15 V versus Fc/Fc+. Finally, dynamic ligand exchange processes between different metallosupramolecular assemblies were investigated by multinuclear NMR and electrospray mass spectrometry. These studies confirmed the reversible nature of the pyridine-Pt(II)/Pd(II) coordination process.     

Binding of nitrate to a CuII-cyclen complex bearing a ferrocenyl pendant: synthesis, solid-state X-ray structure, and solution-phase electrochemical and spectrophotometric studies

The reaction of Cu(NO3)2.3H2O with the ligand 1-(ferrocenemethyl)-1,4,7,10-tetraazacyclododecane (L) in acetonitrile leads to the formation of a blue complex, [Cu(L)(NO3)][NO3] (C1). The X-ray structure determination shows an unexpected binding of a nitrate anion in that the CuII center is surrounded by four N atoms of the 1,4,7,10-tetraazacyclododecane (cyclen) macrocycle and two O atoms from a chelating nitrate anion, both Cu-O distances being below the sums of the van de Waals radii. Hydrogen-bonding interactions in the crystal lattice and a weak interaction between a second nitrate O and the CuII center in C1 give rise to a highly distorted CuII geometry relative to that found in the known structure of [Cu(cyclen)(NO3)][NO3] (C5). Electrochemical studies in acetonitrile containing 0.1 M [Bu4N][NO3] as the supporting electrolyte showed that oxidation of C1 in this medium exhibits a single reversible one-electron step with a formal potential E degrees f of +85 mV vs Fc0/+ (Fc = ferrocene). This process is associated with oxidation of the ferrocenyl pendant group. Additionally, a reversible one-electron reduction reaction with an E degrees f value of -932 mV vs Fc0/+, attributed to the CuII/I redox couple, is detected. Gradual change of the supporting electrolyte from 0.1 M [Bu4N][NO3] to the poorly coordinating [Bu4N][PF6] electrolyte, at constant ionic strength, led to a positive potential shift in E degrees f values by +107 and +39 mV for the CuII/I(C1) and Fc0/+(C1) redox couples, respectively. Analysis of these electrochemical data and UV-vis spectra is consistent with the probable presence of the complexes C1, [Cu(L)(CH3CN)2]2+ (C2), [Cu(L)(CH3CN)(NO3)]+ (C3), and [Cu(L)(NO3)2] (C4) as the major species in nitrate-containing acetonitrile solutions. In weakly solvating nitromethane, the extent of nitrate complexation remains significant even at low nitrate concentrations, due to the lack of solvent competition.     

Ruthenium terpyridine complexes containing a pyrrole-tagged 2,2'-dipyridylamine ligand-synthesis, crystal structure, and electrochemistry

Ruthenium(II) terpyridine complexes containing the pyrrole-tagged 2,2'-dipyridylamine ligand PPP (where PPP stands for N-(3-bis(2-pyridyl)aminopropyl)pyrrole with the general formula [Ru(tpy)(PPP)X](n+) (1, X = Cl(-); 2, X = H(2)O; 3, X = CH(3)CN; tpy = 2,2':6',2"-terpyridine) have been synthesized and characterized by (1)H NMR, IR, UV-vis, mass spectrometry, and elemental analysis. 1 and 2 have been structurally characterized by X-ray crystallography. Both 1 and 2 were successfully immobilized onto glassy carbon electrode via anodic oxidation of the pyrrole moiety on the PPP ligand to give stable and highly electroactive polymer films. Cyclic voltammetric studies of 1 in acetonitrile revealed a Ru(III)/Ru(II) couple at 0.4 V vs Cp(2)Fe(+/0) initially, but another redox couple resulting from chloride substitution by acetonitrile developed at E(1/2) = 0.82 V upon repetitive potential scan. This ligand substitution was induced by the acidic local environment caused by the release of protons during pyrrole polymerization. The electropolymerization of 2 in aqueous medium allowed the observation of the formation of Ru(IV)═O species in polypyrrole film. As the film grew thicker, the size of the Ru(III)/(/)Ru(II) couple (E(1/2) = 0.8 V vs SCE at pH 1) of poly[Ru(tpy)(PPP)(OH(2))](n+) increased accordingly, whereas the growth of the Ru(IV)/Ru(III) couple (E(1/2) = 0.89 V vs SCE at pH 1) leveled off after the film had reached a certain thickness. The Pourbaix diagram of the E(1/2) of the Ru(III) /Ru(II) and Ru(IV)/Ru(III) couples vs pH of the electrolyte medium has been obtained. The resulting poly[Ru(tpy)(PPP)(OH(2))](n+) film is electrocatalytically active toward the oxidation of benzyl alcohol.     

Phenolate and phenoxyl radical complexes of Cu(II) and Co(III), bearing a new redox active N,O-phenol-pyrazole ligand

The synthesis and characterisation of the new N,O-phenol-pyrazole pro-ligand, (pz)LH, comprising a pyrazole covalently linked to an o,p-di-tert-butyl-substituted phenol, are herein reported. In CH(2)Cl(2) at room temperature, the cyclic voltammogram (CV) of (pz)LH exhibits a quasi-reversible one-electron oxidation process (at E(1/2) = 0.66 V vs. Fc(+)/Fc) attributed to the formation of the phenoxyl radical cation [(pz)LH]˙(+). (pz)LH reacts with M(II)(BF(4))(2) (M = Cu, Co) in a 2:1 ratio to afford the bis-Cu(pz)L(2) (1) and tris-Co(pz)L(3) (2) complexes respectively. The X-ray structure of 1 reveals a Cu(II) ion in a square-planar trans-Cu(II)-N(2)O(2) coordination environment whereas that of 2 consists of a Co(III) ion with an octahedral mer-N(3)O(3) coordination sphere; formed by the chelation of two (in 1) or three (in 2) N,O-bidentate phenolate ligands respectively. Both structures are preserved in CH(2)Cl(2) solution, as revealed by their NMR (for 2) and EPR (for 1) data. The CVs of 1 and 2 consist of two (at E(1/2): 0.43 and 0.58 V vs. Fc(+)/Fc) and three (E(1/2) = 0.12, 0.54 and 0.89 V vs. Fc(+)/Fc) reversible one-electron oxidation processes, respectively. The one-electron electrochemical oxidation of 1 and 2 produces the oxidised species, 1(+) and 2(+), which are stable for several hours at room temperature under inert atmosphere in CH(2)Cl(2). The UV/vis and EPR data obtained for 1(+) and 2(+) are unambiguously consistent with the latter being formulated as Cu(II)- and Co(III)-phenoxyl radical complexes, as [Cu(II)((pz)L˙)((pz)L)](+) and [Co(III)((pz)L˙)((pz)L)(2)](+) respectively.     

Syntheses of copper(I)cis-1,3,5-tri-iminocyclohexane complexes

Copper(I) complexes of the ligand cis-1,3,5-tris(cinnamylideneamino)cyclohexane (L) have been prepared from a versatile precursor complex, [Cu(I)(L)NCMe]BF4, which incorporates a labile acetonitrile ligand that can be exchanged to give a range of new Cu(L)X complexes (where X = Cl, Br, NO2, SPh). 1H NMR spectra and X-ray structures of the Cl, Br and NO2 complexes show L coordinated in a symmetric fashion about the copper centre. The complexes have been further characterised using UV/Visible spectroscopy and cyclic voltammetry. CuLCl shows an electrochemically reversible Cu(I/II) redox couple at 0.51 V (vs. Ag/AgCl) while the CuLNO2 complex shows an analogous quasi-reversible wave at 0.41 V (vs. Ag/AgCl).     

Photophysical and electrochemical properties of new ortho-metalated complexes of rhodium(III) containing 2,2'-dipyridylketone and 2,2'-dipyridylamine. An experimental and theoretical study

Two new ortho-metalated rhodium(III) complexes of the formula [Rh(ppy)(2)(L)](+), ppy = 2-phenylpyridine and L = 2,2'-dipyridylketone (dpk) (), 2,2'-dipyridylamine (HDPA) () have been synthesized and subjected to X-ray diffraction crystal structural, photophysical and electrochemical studies. Density functional theory calculations have also been performed to get rationalizations of the optical orbitals and redox orbitals concerning photophysical and electrochemical data. Complex exhibits the triplet ligand-to-ligand charge transfer ((3)LLCT) [pi(ppy)-pi*(dpk)] phosphorescence at 77K (520 nm) and at room temperature (555 nm), while complex shows triplet ligand centred ((3)LC) [pi-pi*(ppy)] phosphorescence only at 77K (460 nm). Both complexes and have similar irreversible oxidation potentials (+1.19 V for and +1.15 V for vs. Fc/Fc(+)). These two complexes show different characteristics in the reduction process: a reversible process occurs for at -1.31 V, while an irreversible process is observed for 2 at -1.85 V.     

A redox series of aluminum complexes: characterization of four oxidation states including a ligand biradical state stabilized via exchange coupling

Electrophilic activation and subsequent reduction of substrates is in general not possible because highly Lewis acidic metals lack access to multiple redox states. Herein, we demonstrate that transition metal-like redox processes and electronic structure and magnetic properties can be imparted to aluminum(III). Bis(iminopyridine) complexes containing neutral, monoanionic, and dianionic iminopyridine ligands (IP) have been characterized structurally and electronically; yellow (IP)AlCl(3) (1), deep green (IP(-))(2)AlCl (2) and (IP(-))(2)Al(CF(3)SO(3)) (3), and deep purple [(IP(2-))Al](-) (5) are presented. The mixed-valent, monoradical complex (IP(-))(IP(2-))Al is unstable toward C-C coupling, and [(IP(2-))Al](2-)(μ-IP-IP)(2-) (4) has been isolated. Variable-temperature magnetic susceptibility and EPR spectroscopy measurements indicate that the biradical character of the ligand-based triplet in 2 is stabilized by strong antiferromagnetic exchange coupling mediated by aluminum(III): J = -230 cm(-1) for ? = -2J(?(L(1))·?(L(2))). Coordination geometry-dependent (IP(-))-(IP(-)) communication through aluminum(III) is observed electrochemically. The cyclic voltammogram of trigonal bipyramidal 2 displays successive ligand-based oxidation events for the two IP(1-/0) processes, at -0.86 and -1.20 V vs SCE. The 0.34 V spacing between redox couples corresponds to a conproportionation constant of K(c) = 10(5.8) for the process (IP(-))(2)AlCl + (IP)(2)AlCl → 2(IP(-))(IP)AlCl consistent with Robin and Day Class II mixed-valent behavior. Tetrahedral 5 displays localized, Class I behavior as indicated by closely spaced redox couples. Furthermore, CV's of 2 and 5 indicate that changes in the coordination environment of the aluminum center shift the potentials for the IP(1-/0) and IP(2-/1-) redox couples by up to 0.9 V.     

An inverted triplesalen ligand by a convergent synthesis and its influence on trinuclear FeIII 3 complexes

The inverted triplesalen ligand H 6 feld Me has been synthesized from 2,4,6-triformyl-phloroglucinol and a ketimine salen half-unit in a convergent synthesis. NMR, IR, and UV-vis spectroscopy reveal that this ligand is not in the O-protonated tautomer but in the N-protonated tautomer with substantial heteroradialene contribution. This ligand and the conventional triplesalen ligand H 6 talen t-Bu 2 have been used to synthesize the trinuclear Fe III complexes [(feld Me )(FeCl) 3 ] and [(talen t-Bu 2 )(FeCl) 3 ], respectively. The molecular structures of these complexes were obtained by single-crystal X-ray diffraction. Two trinuclear Fe III complexes of [(feld Me )(FeCl) 3 ] dimerize via two Fe-phenolate bonds, whereas due to steric hindrance no dimerization is observed for [(talen t-Bu 2 )(FeCl) 3 ]. The structural data also reveal some heteroradialene contribution in the trinuclear complexes. Whereas UV-vis and M bauer spectroscopy are not suitable to distinguish between the two complexes, FT-IR spectra show characteristic features due to the different substitution patterns of the conventional and the inverted triplesalen ligands. Another handle is provided by electrochemistry. Whereas both complexes exhibit an irreversible oxidation wave (0.94 V vs. Fc + /Fc for [(feld Me )(FeCl) 3 ] and 0.84 V vs. Fc + /Fc for [(talen t-Bu 2 )(FeCl) 3 ]), which is assigned to the oxidation of the central backbone, higher potential oxidations are reversible for [(talen t-Bu 2 )(FeCl) 3 ]) but irreversible for [(feld Me )(FeCl) 3 ]. This is attributed to the reversible oxidation of the terminal phenolates in the di-tert-butyl substituted [(talen t-Bu 2 )(FeCl) 3 ] in contrast to the mono-methyl-substituted phenolates in [(feld Me )(FeCl) 3 ]. The magnetic properties of [(talen t-Bu 2 )(FeCl) 3 ] reveal a very small ferromagnetic coupling with significant zero-field splitting of the FeⅢ S = 5/2 ions. In contrast, the dimerization of two trinuclear complexes in [(feld Me )(FeCl) 3 ] results in antiferromagnetic interactions between the two phenolate-bridged Fe III ions, which mask the intra-trinuclear interactions transmitted by the central phloroglucinol backbone.     

Mononuclear oxovanadium complexes of tris(2-pyridylmethyl)amine

Mononuclear oxovanadium(IV) and dioxovanadium(V) complexes of tris(2-pyridylmethyl)amine (tpa) have been prepared for the first time. Crystal structure determinations of three oxovanadium(IV) complexes, [VO(SO4)(tpa)], [VOCl(tpa)]PF6, or [VOBr(tpa)]PF6, and a dioxovanadium(v) complex [V(O)2(tpa)]PF6 disclosed that the tertiary nitrogen of the tpa ligand always occupies the trans-to-oxo site. The structures of an oxo-peroxo complex [VO(O2)(tpa)]Cl that was prepared previously and of a mu-oxo vanadium(III) complex [{VCl(tpa)}2(mu-O)](PF6)2 have also been determined. The tertiary nitrogen is located at a trans site to the peroxo and chloride ligands, respectively. The total sums of the four V-N bond lengths from the tpa ligand are remarkably similar among the six complexes, indicating that the vanadium oxidation states become less influential in tpa bonding due primarily to the coordination of electron-donating oxo ligand(s). Absorption spectra of [VOCl(tpa)]+ in acetonitrile showed a significant change upon addition of p-toluenesulfonic acid and HClO4, but not on addition of benzoic acid. Protonation at the oxo ligand by the former two acids is suggested. Cyclic voltammetric studies in acetonitrile verified the proton-coupled redox behavior of the V(III)/V(IV) process involving the oxo ligand for the first time. From the dependence of the added p-toluenesulfonic acid to the CV, redox potentials for the following species have been estimated: [V(IV)OCl(tpa)]+/[V(III)OCl(tpa)](E1/2=-1.59 V vs. Fc+/Fc), [V(IV)(OH)Cl(tpa)]2+/[V(III)(OH)Cl(tpa)]+(Epc=-1.34 V), [V(IV)(OH2)Cl(tpa)]3+/[V(III)(OH2)Cl(tpa)]2+(Epa=-0.49 V), and [V(IV)Cl2(tpa)]2+/[V(III)Cl2(tpa)]+(E1/2=-0.89 V). The reduction of [V(V)(O)2(tpa)]+ in 0.05 M [(n-Bu)4N]PF6 acetonitrile showed a major irreversible reduction wave V(V)/(IV) at -1.48 V. The metal reduction potentials of the oxovanadium(IV) and dioxovanadium(V) species are very close, reinforcing the significant influence of the oxo ligand(s).     

Synthesis, structures, magnetism and electrochemical properties of triruthenium-acetylide complexes

A series of triruthenium complexes with arylacetylide axial ligands Ru(3)(dpa)(4)(C(2)X)(2)(BF(4))(y)(dpa = dipyridylamido; X = Fc, y= 0 (1); X = Ph, y= 0 (2); X = PhOCH(3), y= 1 (3); X = PhC(5)H(11), y= 1 (4); X = PhCN, y= 0 (5); X = PhNO(2), y= 0 (6)) have been synthesized. The crystal structures show that the Ru-Ru bond lengths (2.3304(9)-2.3572(5)A) of these compounds are longer than those of Ru(3)(dpa)(4)Cl(2)(Ru-Ru=2.2537(1)A). This is ascribed to the formation of the stronger pi-backbonding from metal to axial ligand which weakens the Ru-Ru interactions and the bond order is reduced in the triruthenium unit. Cyclic voltammetry and differential pulse voltammetry show that compound exhibits electronic coupling between the two ferrocenyl units with DeltaE(1/2) close to 100 mV. Compounds 2-6 display three triruthenium-based reversible one-electron redox couples, two oxidations and one reduction, and the electrode potentials shift upon varying the substituents. A linear relationship is observed when the Hammett constants are plotted against the redox potentials.     

Steric and electronic consequences of flexibility in a tetradentate redox-active ligand: Ti(IV) and Zr(IV) complexes

A redox-active, tetradentate ligand, N,N'-bis-(3-dimethylamino-propyl)-4,5-dimethoxy-benzene-1,2-diamide ([N(2)N(2)(cat)](2-)), was developed, and the six-coordinate metal complexes [N(2)N(2)(cat)]TiCl(2) (3) and [N(2)N(2)(cat)]ZrCl(2) (4) were synthesized. The tetradentate ligand was determined to be fluxional in 3 and 4, enabled by reversible dissociation of the neutral amine groups of the [N(2)N(2)(cat)](2-) ligand. Both amine arms of 3 could be replaced by N,N-dimethylaminopyridine with an overall free energy change of -4.64(3) kcal mol(-1) at 298 K. Cyclic voltammetry experiments were used to probe the redox capabilities of the [N(2)N(2)(cat)](2-) ligand: complex 3 exhibited two one-electron oxidations at -0.19 and -0.52 V versus [Cp(2)Fe](+/0) while 4 exhibited a single two-electron oxidation at -0.55 V. Substitution of the chlorides in 3 for an imide afforded the dimer {[N(2)N(2)(cat)]Ti(μ-p-NC(6)H(4)Me)}(2), in which the metal centers are five-coordinate because of dissociation of one amine arm of the [N(2)N(2)(cat)](2-) ligand. While the bis-azide complex [N(2)N(2)(cat)]Ti(N(3))(2) was stable toward elimination of N(2), the bis-phenylacetylide complex [N(2)N(2)(cat)]Ti(C≡CPh)(2) could be oxidized by PhICl(2), resulting in subsequent reductive elimination of 1,4-diphenylbutadiyne.     

Electrochemical and spectroelectrochemical studies on UO(2)(saloph)L (saloph = N,N'-disalicylidene-o-phenylenediaminate,L=dimethyl sulfoxide or N,N-dimethylformamide)

To examine properties of pentavalent uranium, U(V), we have carried out electrochemical and spectroelectrochemical studies on UO(2)(saloph)L [saloph = N,N'-disalicylidene-o-phenylenediaminate, L = dimethyl sulfoxide (DMSO) or N,N-dimethylformamide (DMF)]. The electrochemical reactions of UO(2)(saloph)L complexes in L were found to occur quasireversibly. The reduction processes of UO(2)(saloph)L complexes were followed spectroelectrochemically by using an optical transparent thin layer electrode cell. It was found that the absorption spectra measured at the applied potentials from 0 to -1.650 V versus ferrocene/ferrocenium ion redox couple (Fc/Fc(+)) for UO(2)(saloph)DMSO in DMSO have clear isosbestic points and that the evaluated electron stoichiometry equals 1.08. These results indicate that the reduction product of UO(2)(saloph)DMSO is [U(V)O(2)(saloph)DMSO](-), which is considerably stable in DMSO. Furthermore, it was clarified that the absorption spectrum of the [U(V)O(2)(saloph)DMSO](-) complex has a very small molar absorptivity in the visible region and characteristic absorption bands due to the 5f(1) orbital at around 750 and 900 nm. For UO(2)(saloph)DMF in DMF, the clear isosbestic points were not observed in the similar spectral changes. It is proposed that the UO(2)(saloph)DMF complex is reduced to [U(V)O(2)(saloph)DMF](-) accompanied by the dissociation of DMF as a successive reaction. The formal redox potentials of UO(2)(saloph)L in L (E(0), vs Fc/Fc(+)) for U(VI)/U(V) couple were determined to be -1.550 V for L = DMSO and -1.626 V for L = DMF.     

New synthetic routes to biscarbonylbipyridinerhenium(I) complexes cis,trans-[Re(X2bpy)(CO)2(PR3)(Y)n+ (X2bpy = 4,4'-X2-2,2'-bipyridine) via photochemical ligand substitution reactions, and their photophysical and electrochemical properties

Photochemical ligand substitution of fac-[Re(X2bpy)(CO)3(PR3)]+ (X2bpy = 4,4'-X2-2,2'-bipyridine; X = Me, H, CF3; R = OEt, Ph) with acetonitrile quantitatively gave a new class of biscarbonyl complexes, cis,trans[Re(X2bpy)(CO)2(PR3)(MeCN)]+, coordinated with four different kinds of ligands. Similarly, other biscarbonylrhenium complexes, cis,trans-[Re(X2bpy)(CO)2(PR3)(Y)]n+ (n = 0, Y = Cl-; n = 1, Y = pyridine, PR'3), were synthesized in good yields via photochemical ligand substitution reactions. The structure of cis,trans-[Re(Me2bpy)(CO)2[P(OEt)3](PPh3)](PF6) was determined by X-ray analysis. Crystal data: C38H42N2O5F6P3Re, monoclinic, P2(1/a), a = 11.592(1) A, b = 30.953(4) A, c = 11.799(2) A, V = 4221.6(1) A3, Z = 4, 7813 reflections, R = 0.066. The biscarbonyl complexes with two phosphorus ligands were strongly emissive from their 3MLCT state with lifetimes of 20-640 ns in fluid solutions at room temperature. Only weak or no emission was observed in the cases Y = Cl-, MeCN, and pyridine. Electrochemical reduction of the biscarbonyl complexes with Y = Cl- and pyridine in MeCN resulted in efficient ligand substitution to give the solvento complexes cis,trans-[Re(X2bpy)(CO)2(PR3)(MeCN)]+.     

A highly emissive Cu2N2 diamond core complex supported by a [PNP]- ligand

A Cu2N2 diamond core structure, {(PNP)CuI}2 (2), supported by a [PNP]- ligand (1) ([PNP]- = bis(2-(diisobutylphosphino)phenyl)amide) has been prepared. 2 is highly emissive at ambient temperature in both the solid and solution states and is characterized by a relatively long-lived excited state (tau > 10 mus) and an unusually high quantum yield (phi > 0.65). These observations are consistent with a low degree of structural reorganization between the ground state of 2 and its excited state *2, and also with a high degree of steric protection of the two copper centers of 2 afforded by the bulky [PNP]- ligand. An estimate for the excited-state reduction potential of *2 (ca. -3.2 V vs Fc+/Fc), and the availability of two well-separated and reversible ground-state redox processes, suggests that bimetallic copper systems of these types may be interesting candidates to consider for photochemically driving multielectron redox transformations.     

Highly phosphorescent iridium complexes containing both tridentate bis(benzimidazolyl)-benzene or -pyridine and bidentate phenylpyridine: synthesis, photophysical properties, and theoretical study of Ir-bis(benzimidazolyl)benzene complex

Novel mixed-ligand Ir(III) complexes, [Ir(L)(NwedgeC)X]n+ (L = N/\C/\N or N/\N/\N; X = Cl, Br, I, CN, CH3CN, or -CCPh; n = 0 or 1), were synthesized, where N/\CwedgeN = bis(N-methylbenzimidazolyl)benzene (Mebib) and bis(N-phenylbenzimidazolyl)benzene (Phbib), N/\N/\N = bis(N-methylbenzimidazolyl)pyridine (Mebip), and N/\C = phenylpyridine (ppy) derivatives. The X-ray crystal structures of [Ir(Phbib)(ppy)Cl] and [Ir(Mebib)(mppy)Cl] [mppy = 5-methyl-2-(2'-pyridyl)phenyl] indicate that the nitrogen atom of the ppy ligand is located trans to the coordinating carbon atom in Me- or Phbib, while the coordinating carbon atom in ppy occupies the trans position of Cl. [Ir(Mebip)(ppy)Cl]+ showed a quasireversible Ir(III/IV) oxidation wave at +1.05 V, while the Ir complexes, [Ir(Mebib)(ppy)Cl], were oxidized at +0.42 V versus Fc/Fc+. The introduction of an Ir-C bond in [Ir(Mebib)(ppy)Cl] induces a large potential shift of 0.63 V in a negative direction. Further, the oxidation potential of [Ir(Mebib)(Rppy)X] was altered by the substitution of R, R', and X groups. Compared to the oxidation potential, the first reduction potential revealed an almost constant value at -2.36 to -2.46 V for [Ir(L)(ppy)Cl] (L = Mebib and Phbib) and -1.52 V for [Ir(Mebip)(ppy)Cl. The UV-vis spectra of [Ir(Mebib)(R-ppy)X] show a clear singlet metal-to-ligand charge-transfer transition around 407 approximately 425 nm and a triplet metal-to-ligand charge-transfer transition at 498 approximately 523 nm. [Ir(Mebip)(ppy)Cl]+ emits at 610 nm with a luminescent quantum yield of Phi = 0.16 at room temperature. The phosphorescence of [Ir(Mebib)(ppy)X] was observed at 526 nm for X = CN and 555 nm for X = Cl with the high luminescent quantum yields, Phi = 0.77 approximately 0.86, at room temperature. [Ir(Phbib)(ppy)Cl] shows the emission at 559 nm with a luminescent quantum yield of Phi = 0.95, which is an unprecedentedly high value compared to those of other emissive metal complexes. Compared to the luminescent quantum yields of the Ir(ppy)2(L) derivatives and [Ir(Mebip)(ppy)Cl]+, the neutral Ir complexes, [Ir(L)(R-ppy)X] (L = Me- or Phbib), reveal very high quantum yields and large radiative rate constants (kr) ranging from 3.4 x 10(5) to 5.5 x 10(5) s(-1). The density functional theory calculation suggests that these Ir complexes possess dominantly metal-to-ligand charge-transfer and halide-to-ligand charge-transfer excited states. The mechanism for a high phosphorescence yield in [Ir(bib)(ppy)X] is discussed herein from the perspective of the theoretical consideration of radiative rate constants using perturbation theory and a one-center spin-orbit coupling approximation.     

Synthetic Approaches to (smif)(2)Ti (smif = 1,3-di-(2-pyridyl)-2-azaallyl) Reveal Redox Non-Innocence and C-C Bond-Formation

Attempted syntheses of (smif)(2)Ti (smif =1,3-di-(2-pyridyl)-2-azaallyl) based on metatheses of TiCl(n)L(m) (n = 2-4) with M(smif) (M = Li, Na), in the presence of a reducing agent (Na/Hg) when necessary, failed, but several apparent Ti(II) species were identified by X-ray crystallography and multidimensional NMR spectroscopy: (smif){Li(smif-smif)}Ti (1, X-ray), [(smif)Ti](2)(μ-κ(3),κ(3)-N,N(py)(2)-smif,smif) (2), (smif)Ti(κ(3)-N,N(py)(2)-smif,(smif)H) (3), and (smif)Ti(dpma) (4, dpma = di-2-pyridylmethyl-amide). NMR spectroscopy and K-edge XAS showed that each compound possesses ligands that are redox noninnnocent, such that d(1) Ti(III) centers AF-couple to ligand radicals: (smif){Li(smif-smif)(2-)}Ti(III) (1), [(smif(2-))Ti(III)](2)(μ-κ(3),κ(3)-N,N(py)(2)-smif,smif) (2), [(smif(2-))Ti(III)](κ(3)-N,N(py)(2)-smif,(smif)H) (3), and (smif(2-))Ti(III)(dpma) (4). The instability of (smif)(2)Ti relative to its C-C coupled dimer, 2, is rationalized via the complementary nature of the amide and smif radical dianion ligands, which are also common to 3 and 4. Calculations support this contention.     

Unusual reactivity of the [Re(V)O]3+ core: syntheses and characterization of novel rhenium halide complexes with n-methyl-o-diaminobenzene

The reactions of 1 or 2 equiv of N-methyl-o-diaminobenzene with trans-[ReOX(3)(PPh(3))(2)] (X = Cl, Br) in refluxing chloroform gave oxo-free rhenium complexes [Re(VI)X(4)(NC(6)H(4)NHCH(3))(OPPh(3))] (X = Cl, 3; X = Br, 6), [Re(V)X(2)Y(NC(6)H(4)NHCH(3))(PPh(3))(2)] (X, Y = Cl, 4; X = Br, Y = Cl, 7), [Re(IV)Cl(2)(NHC(6)H(4)NCH(3))(2)] (5), and [Re(IV)Br(3)(NHC(6)H(4)NCH(3))(PPh(3))] (8). All complexes were characterized by elemental analysis, (1)H NMR and IR spectroscopy, cyclic voltammetry, EPR spectroscopy, and X-ray crystallography. The complexes all display distorted octahedral coordination geometry. For Re(IV) complexes 5 and 8, the ligands coordinate in the benzosemiquinone diimine form. In Re(VI) complexes 3 and 6 and the Re(V) complexes 4 and 7, the ligands coordinate in the dianionic monodentate imido form. The EPR spectra of Re(VI) species 3 and 6 in dichloromethane solution at room temperature exhibit the characteristic hyperfine pattern of six lines, with evidence of strong second-order effects. The IR spectra of the complexes are characterized by Re=N and Re-N stretching bands at ca. 1090 and 540 cm(-)(1), respectively. The Re(IV) and Re(V) complexes display well-resolved NMR spectra, while the Re(VI) complexes exhibit no observable spectra, due to paramagnetism. The cyclic voltammograms of complexes 3 and 6 display Re(VII)/ Re(VI) and Re(VI)/Re(V) processes, those of 4 and 7 exhibit Re(VI)/Re(V) and Re(V)/Re(IV) couples, and those of 5 and 8 are characterized by Re(V)/Re(IV) and Re(IV)/Re(III) processes.