Bio-inspired Photocatalytic Ruthenium Complexes: Synthesis, Optical Properties, and Solvatochromic Effect.

We report the synthesis, characterization, and photo-physical properties of two new rutheniumII -phenol-imidazole complexes. These bio-mimetic complexes have potential as photocatalysts for water splitting. Owing to their multiple phenol-imidazole groups, they have a higher probability of light-induced radical formation than existing complexes. The newly synthesized complexes show improved overlap with the solar spectrum compared to other rutheniumII -phenol-imidazole complexes, and their measured lifetimes are suitable for light-induced radical formation. In addition, we conducted solvatochromic absorption measurements, which elegantly follow Marcus theory, and demonstrate the symmetry differences between the two complexes. The solvatochromic measurements further imply electron localization onto one of the ligands. The new complexes may find applications in photocatalysis, dye-sensitized solar cells, biomedicine, and sensing. Moreover, their multiple chelating units make them promising candidates for light-activated metal organic radical frameworks, i.e. metal-organic frameworks that contain organic radicals activated by light.

Preparation and Properties of Metal Organic Framework/Activated Carbon Composite Materials.

Metal organic frameworks (MOFs) have unique properties that make them excellent candidates for many high-tech applications. Nevertheless, their nonconducting character is an obstacle to their practical utilization in electronic and energy systems. Using the familiar HKUST-1 MOF as a model, we present a new method of imparting electrical conductivity to otherwise nonconducting MOFs by preparing MOF nanoparticles within the conducting matrix of mesoporous activated carbon (AC). This composite material was studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), gas adsorption measurements, and electron paramagnetic resonance (EPR) spectroscopy. We show that MOF nanoparticles grown within the carbon matrix maintain their crystalline characteristics and their surface area. Surprisingly, as a result of the composition process, EPR measurements revealed a copper signal that had not yet been achieved. For the first time, we could analyze the complex EPR response of HKUST-1. We demonstrate the high conductivity of the MOF composite and discuss various factors that are responsible for these results. Finally, we present an optional application for using the conductive MOF composite as a high-performance electrode for pseudocapacitors.

A versatile water-soluble chelating and radical scavenging platform.

The phenol-diamide compound, 5-(tert-butyl)-2-hydroxy-N1,N3-bis(2-hydroxyethyl)isophthalamide (), is water-soluble, non-cytotoxic, and capable of both, trapping ROS species and chelating Cu(ii) and Fe(iii) ions; these combined properties confer a protective effect against ROS induced cell death.

SOD mimetic activity and antiproliferative properties of a novel tetra nuclear copper (II) complex.

The search for novel anticancer therapeutic agents is an urgent and important issue in medicinal chemistry. Here, we report on the biological activity of the copper-based bioinorganic complex Cu4 (2,4-di-tert-butyl-6-(1H-imidazo- [1, 10] phenanthrolin-2-yl)phenol)4]·10 CH3CN (2), which was tested in rat L6 myotubes, mouse NSC-34 motor neurone-like cells, and HepG-2 human liver carcinoma. Upon 96 h incubation, 2 exhibited a significant cytotoxic effect on all three types of cells via activation of two cell death mechanisms (apoptosis and necrosis). Complex 2 exhibited better potency and efficacy than the canonical cytotoxic drug cisplatin. Moreover, during shorter incubations, complex 2 demonstrated a significant SOD mimetic activity, and it was more effective and more potent than the well-known SOD mimetic TEMPOL. In addition, complex 2 was able to interact with DNA and, cleave DNA in the presence of sodium ascorbate. This study shows the potential of using polynuclear redox active compounds for developing novel anticancer drugs through SOD-mimetic redox pathways.

X-ray structure of a Ni(II)-tri-phenoxyl radical complex.

The diimino-diphenolato neutral square-planar Ni(ii) complex, NiL2, is readily oxidised with 2 equiv. of Ag[SbF6], to produce an unprecedented octahedral Ni(ii) tris(phenoxyl) radical complex, [Ni(L˙)3][SbF6]2. This study reveals, for the first time, the X-ray structure of a metal-tri-phenoxyl radical complex.

Nickel(III) complexes of di-amidato-di-phenolato ligands: effect of H-bonding.

The one-electron oxidation of the dianionic diamido-diphenoxo Ni(II) complexes involving H-bonding (1(2-)), or not (2(2-)), yields the corresponding Ni(III) species; the formation, stability and electronic structures of which are affected by the H-bonding interactions.

Redox-switchable tetra-copper assembly of N,N-, N,O-phenolate-phenanthroimidazolate bridging ligands.

A redox-active dianionic N,O-phenolate-imidazolate/N,N-phenanthroline bridging ligand is used to form unique square-like neutral tetra-Cu(II) assemblies, the structural, magnetic, electronic and redox properties of which are herein described.

A novel di-compartmental bis-(2-hydroxyisophtalamide) macrocyclic ligand and its mononuclear Cu(II) and Ni(II) complexes.

The synthesis and characterisation of the new di-compartmental bis-(2-hydroxyisophtalamide) macrocyclic pro-ligand, LH(6), which comprises two phenol-diamide units linked by ethylene bridges, is herein reported, together with its corresponding di-phenolate salt, [NBu(4)](2)[LH(4)]. The three macrocyclic compounds, [LH(4)(OMe)(2)] (protected ligand), LH(6) and [LH(4)][NBu(4)](2) were fully characterised including X-ray crystallography for [LH(4)(OMe)(2)] and [NBu(4)](2)[LH(4)]. The results of solid-state and solution studies have indicated that the macrocycle can adopt specific conformations, which are influenced by H-bonding interactions as well as the deviation of the amide carbonyl relative to the phenol plane. LH(6) reacts with M(II)(acetate)(2)·(H(2)O)(6) (M = Ni, Cu) in a 1 : 1 ratio in the presence of 4 eq of [NBu(4)](OH) in methanol to afford the dianionic [M(LH(2))](2-) complexes, (2-) and (2-), respectively. The X-ray crystallography, EPR, NMR and UV-vis spectroscopic data, combined with DFT calculations, indicate that (2-) and (2-) are unique unsymmetrical square planar mononuclear complexes that are intramolecularly H-bonded. Thus, one macrocyclic compartment contains a M(II)-N(2)O(2) centre resulting from the tetra-anionic di-phenolato di-amidato ligation; the other compartment possesses two protonated amide N-H groups that are H-bonded the coordinated phenolate O atoms. This represents a unique example in which a phenolate is both coordinated and intramolecularly H-bonded. This H-bonding appears unusually strong as revealed by N(H/D) exchange experiments; and may be responsible for the stability of the mononuclear complex, and the difficulty in isolating the corresponding dinuclear complex [M(2)(L)](2-).

Geometric and electronic structures of phenoxyl radicals hydrogen bonded to neutral and cationic partners.

Two di-tert-butylphenols incorporating an N-methylbenzimidazole moiety in the ortho or para position have been synthesised ((Me)OH and (pMe)OH, respectively). Their X-ray structures evidence a hydrogen bond between the phenolic proton and the iminic nitrogen atom, whose nature is intra- and intermolecular, respectively. The present studies demonstrate that (Me)OH is readily oxidised by an intramolecular PET mechanism to form the hydrogen-bonded phenoxyl-N-methylbenzimidazolium system ((Me)OH)(.+) , whereas oxidation of (pMe)OH occurs by intermolecular PET, affording the neutral phenoxyl benzimidazole ((pMe)O)(.) system. The deprotonations of (Me)OH and (pMe)OH yield the corresponding phenolate species ((Me)O)(-) and ((pMe)O)(-), respectively, whilst that of the previously reported (H)OH (analogous to (Me)OH but lacking the N-methyl group) produces an unprecedented hydrogen-bonded phenol benzimidazolate species, as evidenced by its X-ray structure. The latter is believed to be in equilibrium in solution with its tautomeric phenolate form, as suggested by NMR, electrochemistry and DFT studies. The one-electron oxidations of the anions occur by a simple ET process affording phenoxyl radical species, whose electronic structure has been studied by HF-EPR spectroscopy and DFT calculations. In particular, analysis of the g(1) tensor shows the order 2.0079>2.0072>2.0069>2.0067 for ((Me)O)(.), ((H)O)(.), ((Me)OH)(.+) and ((H)OH)(.+), respectively. ((Me)O)(.) exhibits the largest g(1) tensor (2.0079), consistent with the absence of intramolecular hydrogen bond. The g(1) tensor of ((H)O)(.) is intermediate between those of ((Me)OH)(.+) and ((Me)O)(.) (g(1)=2.0072), indicating that the phenoxyl oxygen is hydrogen-bonded with a neutral benzimidazole partner.

H-bonding and steric effects on the properties of phenolate and phenoxyl radical complexes of Cu(II).

Herein, the N-R substituted N,O-phenol-pyrazole redox-active pro-ligands, (R)LH (R = Me, Pr) are reported together with their corresponding bis-Cu(R)L(2) complexes (2 and 3, respectively). The latter are reversibly oxidised to the corresponding stable Cu(II)-phenoxyl radical complexes 2(+) and 3(+). The properties of the tetrahedrally distorted complexes 2 and 3 (and those of 2(+) and 3(+)) are being compared to those of the square-planar H-bonded complex 1 (bis-Cu(H)L(2)) and those of 1(+). These studies have permitted H-bonding and steric effects on the redox, spectroscopic and chemical properties of Cu(II)-phenolate and Cu(II)-phenoxyl radical species to be established.

Persistent hydrogen-bonded and non-hydrogen-bonded phenoxyl radicals.

The production of stable phenoxyl radicals is undoubtedly a synthetic chemical challenge. Yet it is a useful way to gain information on the properties of the biological tyrosyl radicals. Recently, several persistent phenoxyl radicals have been reported, but only limited synthetic variations could be achieved. Herein, we show that the amide-o-substituted phenoxyl radical (i.e. with a salicylamide backbone) can be synthesised in a stable manner, thereby permitting easy synthetic modifications to be made through the amide bond. To study the effect of H-bonding on the properties of the phenolate/phenoxyl radical redox couple, simple H-bonded and non-H-bonded o,p-tBu-protected salicylamidate compounds have been prepared. Their redox properties were examined by cyclic voltammetry and showed a fully reversible one-electron oxidation process to the corresponding phenoxyl radical species. Remarkably, the redox potential appears to be correlated, at least partially, with H-bond strength, as relatively large differences (ca. 300 mV) in the redox potential between H-bonded and non-H-bonded phenolate salts are observed. The corresponding phenoxyl radicals produced electrochemically are persistent at room temperature for at least an hour; their UV/Vis and EPR characterisation is consistent with that of phenoxyl radicals, which makes them excellent models of biological tyrosyl radicals. The analyses of the experimental data coupled with theoretical calculations indicate that both the deviation from planarity of the amide function and intramolecular H-bonding influence the oxidation potential of the phenolate. The latter H-bonding effect appears to be predominantly exerted on the phenolate and not (or only a little) on the phenoxyl radical. Thus, in these systems the H-bonding energy involved in the phenoxyl radical appears to be relatively small.

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.

Efficient near-UV photosensitization of the Tb(III) green luminescence by use of 2-hydroxyisophthalate ligands.

The first examples of lanthanide complexes with a 2-hydroxyisophthalate ligand are reported; the blue-emitting ligand acts as a very efficient sensitizer of the Tb(III) green emissions and does so in the near-UV region.

Insights into the nature of the hydrogen bonding of *Tyr272 in apo-galactose oxidase.

The synthesis and structure of an o-methylthio-phenol-imidazole, 2-(2'-(4'-tert-butyl-6'-methylsulfanyl)-hydroxyphenyl))-4,5-diphenyl-imidazole ((MeS)LH), is reported; X-ray crystallographic studies have shown that (MeS)LH involves an O-H...N(+) hydrogen bond between the phenol and an imidazole nitrogen. (MeS)LH undergoes a reversible, one-electron, oxidation to form the radical cation [(MeS)LH](*)(+) the EPR spectrum of which is remarkably similar to that of (*)Tyr(272) in Cu-free, oxidized, apo-GO. Density Functional Theory calculations, have shown that the proton-transferred (R-O(*)...H-N(+)) form of [(MeS)LH](*)(+) has a spin density distribution--with a substantial delocalization of the unpaired electron spin density onto the ortho sulfur atom--and EPR properties that are in good agreement with those of (*)Tyr(272) in Cu-free, oxidized, apo-GO whereas the non-proton-transferred (R-O(*)(+)-H...N) form does not. The results reported herein are a further demonstration of the influence of hydrogen bonding on the nature and properties of phenoxyl radicals and strongly suggest that the phenoxyl oxygen of (*)Tyr(272) in Cu-free, oxidized, apo-GO is involved in a O(*)...H-O/N hydrogen bond.

The self-assembly between C2-symmetric (methanol)6 or S6-symmetric (ethanol)6 cyclohexamers and paddle-wheel dinuclear copper units leads to unique 1D polymer chains.

The first crystallographic evidences of the theoretically suggested C2-symmetric cyclohexameric form of liquid methanol and the S6-symmetric cyclohexameric form of liquid ethanol are reported, trapped inside a hydrophilic pocket shielded by tert-butyl groups.

Monomeric iron(II) hydroxo and iron(III) dihydroxo complexes stabilized by intermolecular hydrogen bonding.

Using the multidentate ligand bis(N-methylimidazol-2-yl)-3-methylthiopropanol (L), the mononuclear iron(II) hydroxo and iron(III) dihydroxo complexes [Fe(II)(L)2(OH)](BF4) (1) and [Fe(III)(L)2(OH)2](BF4) (2) have been synthesized and characterized by X-ray diffraction and spectroscopic methods. The X-ray data suggest that the remarkable stability of the Fe-OH bond(s) in both compounds results from intermolecular hydrogen-bonding interactions between the hydroxo ligand(s) and the tertiary hydroxyl of the L ligands, which prevent further intermolecular reactions.

Phenoxyl radicals: H-bonded and coordinated to Cu(II) and Zn(II).

Two pro-ligands ((R)LH) comprised of an o,p-di-tert-butyl-substituted phenol covalently bonded to a benzimidazole ((Bz)LH) or a 4,5-di-p-methoxyphenyl substituted imidazole ((PhOMe)LH), have been structurally characterised. Each possesses an intramolecular O-H[dot dot dot]N hydrogen bond between the phenolic O-H group and an imidazole nitrogen atom and (1)H NMR studies show that this bond is retained in solution. Each (R)LH undergoes an electrochemically reversible, one-electron, oxidation to form the [(R)LH] (+) radical cation that is considered to be stabilised by an intramolecular O...H-N hydrogen bond. The (R)LH pro-ligands react with M(BF(4))(2).H(2)O (M = Cu or Zn) in the presence of Et(3)N to form the corresponding [M((R)L)(2)] compound. [Cu((Bz)L)(2)] (), [Cu((PhOMe)L)(2)] (), [Zn((Bz)L)(2)] and [Zn((PhOMe)L)(2)] have been isolated and the structures of .4MeCN, .2MeOH, .2MeCN and .2MeCN determined by X-ray crystallography. In each compound the metal possesses an N(2)O(2)-coordination sphere: in .4MeCN and .2MeOH the {CuN(2)O(2)} centre has a distorted square planar geometry; in .2MeCN and .2MeCN the {ZnN(2)O(2)} centre has a distorted tetrahedral geometry. The X-band EPR spectra of both and , in CH(2)Cl(2)-DMF (9 : 1) solution at 77 K, are consistent with the presence of a Cu(ii) complex having the structure identified by X-ray crystallography. Electrochemical studies have shown that each undergo two, one-electron, oxidations; the potentials of these processes and the UV/vis and EPR properties of the products indicate that each oxidation is ligand-based. The first oxidation produces [M(II)((R)L)((R)L )](+), comprising a M(ii) centre bound to a phenoxide ((R)L) and a phenoxyl radical ((R)L ) ligand; these cations have been generated electrochemically and, for R = PhOMe, chemically by oxidation with Ag[BF(4)]. The second oxidation produces [M(II)((R)L )(2)](2+). The information obtained from these investigations shows that a suitable pro-ligand design allows a relatively inert phenoxyl radical to be generated, stabilised by either a hydrogen bond, as in [(R)LH] (+) (R = Bz or PhOMe), or by coordination to a metal, as in [M(II)((R)L)((R)L )](+) (M = Cu or Zn; R = Bz or PhOMe). Coordination to a metal is more effective than hydrogen bonding in stabilising a phenoxyl radical and Cu(ii) is slightly more effective than Zn(II) in this respect.

Phenolate and phenoxyl radical complexes of Co(II) and Co(III).

The new phenol-imidazole pro-ligands (R)LH react with Co(BF(4))(2).6H(2)O in the presence of Et(3)N to form the corresponding [Co(II)((R)L)(2)] compound (R = Ph (1), PhOMe (2), or Bz (3)). Also, (Bz)LH, reacts with Co(ii) in the presence of Et(3)N and H(2)O(2) to form [Co(III)((Bz)L)(3)](4). The structures of 1.2.5MeCN, 2.2DMF, 3.4MeOH, and 4.4DMF have been determined by X-ray crystallography. 1, 2, and 3 each involve Co(II) bound to two N,O-bidentate ligands with a distorted tetrahedral coordination sphere; 4 involves Co(III) bound to three N,O-bidentate ligands in a mer-N(3)O(3) distorted octahedral geometry. [Co(II)((R)L)(2)](R = Ph or PhOMe) undergo two, one-electron, oxidations. The products of the first oxidation, [1](+) and [2](+), have been synthesised by the chemical oxidation of 1 and 2, respectively; these cations, formulated as [Co(II)((R)L*)((R)L)(2)](+), comprise one phenoxyl radical and one phenolate ligand bound to Co(II) and are the first phenoxyl radical ligand complexes of tetra-coordinated Co(II). 4 undergoes two, one-electron, ligand-based oxidations, the first of which produces [4](+), [Co(III)((Bz)L*)((Bz)L)(2)](+). Unlike [1](+) and [2](+), product of the one-electron oxidation of [Co(II)((Bz)L)(2)], [3](+), is unstable and decomposes to produce [4](+). These studies have demonstrated that the chemical properties of [M(II)((R)L*)((R)L)(2)](+)(M = Co, Cu, Zn) are highly dependent on the nature of both the ligand and the metal centre.