Spectroscopic properties of trapped electrons on the surface of MgO nanoparticles.

To characterise electron-trapping sites on the surface of MgO nanoparticles, surface colour centres were generated using UV light in conjunction with selected hydrogen-based electron sources. Four different colour-centre species, including the characteristic (e-)(H+) or F(S)+(H) centre, were identified due to the distinct shape of the respective electron paramagnetic resonance (EPR) signals. The analysis of the EPR saturation behaviour down to microwave powers of 5 x 10(-3) mW reveals an enhanced spin-relaxation probability of the (e-)(H+) centre compared to all other F(S)+ centres that do not exhibit significant magnetic interactions with hydroxylic protons. Beside the dipolar magnetic interaction in the (e-)(H+) centre observed by EPR, the electronic interaction between the unpaired electron and the proton of a closely spaced OH group produces a redshift of the OH stretching band by about 70 to 170 cm(-1), as observed by infrared spectroscopy. EPR and IR spectroscopic data obtained after the selective address of individual reaction channels for surface colour-centre formation point to the fact that (e-)(H+) centres are formed by trapping electrons from H atoms. Consequently, the underlying surface defect does not belong to the sites of the MgO surface, which chemisorb hydrogen via a heterolytic splitting process.     

pH-Dependent spectroscopic and luminescent properties, and metal-ion recognition studies of Re(I) complexes containing 2-(2'-pyridyl)benzimidazole and 2-(2'-pyridyl)benzimidazolate

A series of Re(I) complexes, [Re(CO)(3)Cl(HPB)] (1), [Re(CO)(3)(PB)H(2)O] (2), [Re(CO)(3)(NO(3))(PB-AuPPh(3))] (3), and [Re(CO)(3)(NO(3))(PB)Au(dppm-H)Au](2) (4) [HPB = 2-(2'-pyridyl)benzimidazole; dppm = 2,2'-bis(diphenylphosphinomethane)], have been synthesized and characterized by X-ray diffraction. Complex 1, which exhibits interesting pH-dependent spectroscopic and luminescent properties, was prepared by reacting Re(CO)(5)Cl with an equimolar amount of 2-(2'-pyridyl)benzimidazole. The imidazole unit in complex 1 can be deprotonated to form the imidazolate unit to give complex 2. Addition of 1 equiv of AuPPh(3)(NO(3)) to complex 2 led to the formation of a heteronuclear complex 3. Addition of a half an equivalent of dppm(Au(NO(3)))(2) to complex 2 yielded 4. In both 3 and 4, the imidazolate unit acts as a multinuclear bridging ligand. Complex 4 is a rare and remarkable example of a Re(2)Au(4) aggregate in combination with μ(3)-bridging 2-(2'-pyridyl)benzimidazolate. Finally, complex 2 has been used to examine the Hg(2+)-recognition event among group 12 metal ions. Its reversibility and selectivity toward Hg(2+) are also examined.     

New Cu(II) complexes based on the hydroxo-bridged dinuclear [Cu(OH)2Cu] units: step-like di- and trimerizations of [Cu(OH)2Cu] units

Four new Cu(II) complexes {[Cu(4)(bpy)(4)(OH)(4)(H(2)O)(2)]}(NO(3))(2)(C(7)H(5)O(2))(2)·6H(2)O 1, {[Cu(4)(bpy)(4)(OH)(4)(H(2)O)(2)]}(NO(3))(2)(C(5)H(6)O(4))·8H(2)O 2, {[Cu(4)(bpy)(4)(OH)(4)(H(2)O)(2)]}(C(5)H(6)O(4))(2)·16H(2)O 3 and {[Cu(6)(bpy)(6)(OH)(6)(H(2)O)(2)]}(C(8)H(7)O(2))(6)·12H(2)O 4 were synthesized (bpy = 2,2'-bipyridine, H(2)(C(5)H(6)O(4)) = glutaric acid, H(C(7)H(5)O(2)) = benzoic acid, H(C(8)H(7)O(2)) = phenyl acetic acid). The building units in 1-3 are the tetranuclear [Cu(4)(bpy)(4)(H(2)O)(2)(μ(2)-OH)(2)(μ(3)-OH)(2)](4+) complex cations, and in 4 the hexanuclear [Cu(6)(bpy)(6)(H(2)O)(2)(μ(2)-OH)(2)(μ(3)-OH)(4)](6+) complex cations, respectively. The tetra- and hexanuclear cluster cores [Cu(4)(μ(2)-OH)(2)(μ(3)-OH)(2)] and [Cu(6)(μ(2)-OH)(2)(μ(3)-OH)(4)] in the complex cations could be viewed as from step-like di- and trimerization of the well-known hydroxo-bridged dinuclear [Cu(2)(μ(2)-OH)(2)] entities via the out-of-plane Cu-O(H) bonds. The complex cations are supramolecularly assembled into (4,4) topological networks via intercationic ππ stacking interactions. The counteranions and lattice H(2)O molecules are sandwiched between the 2D cationic networks to form hydrogen-bonded networks in 1-3, while the phenyl acetate anions and the lattice H(2)O molecules generate 3D hydrogen-bonded anionic framework to interpenetrate with the (4,4) topological cationic networks with the hexanuclear complex cations in the channels. The ferromagnetic coupling between Cu(II) ions in the [Cu(4)(μ(2)-OH)(2)(μ(3)-OH)(2)] cores of 1-3 is significantly stronger via equatorial-equatorial OH(-) bridges than via equatorial-apical ones. The outer and the central [Cu(2)(OH)(2)] unit within the [Cu(6)(μ(2)-OH)(2)(μ(3)-OH)(4)] cluster cores in 4 exhibit weak ferromagnetic and antiferromagnetic interactions, respectively. Results about i.r. spectra, thermal and elemental analyses are presented.     

Probing Surface Sites of TiO2: Reactions with [HRe(CO)5] and [CH3Re(CO)5]

Two carbonyl complexes of rhenium, [HRe(CO)₅] and [CH₃Re(CO)₅], were used to probe surface sites of TiO₂ (anatase). These complexes were adsorbed from the gas phase onto anatase powder that had been treated in flowing O₂ or under vacuum to vary the density of surface OH sites. Infrared (IR) spectra demonstrate the variation in the number of sites, including Ti⁺³? OH and Ti⁺⁴? OH. IR and extended X‐ray absorption fine structure (EXAFS) spectra show that chemisorption of the rhenium complexes led to their decarbonylation, with formation of surface‐bound rhenium tricarbonyls, when [HRe(CO)₅] was adsorbed, or rhenium tetracarbonyls, when [CH₃Re(CO)₅] was adsorbed. These reactions were accompanied by the formation of water and surface carbonates and removal of terminal hydroxyl groups associated with Ti⁺³ and Ti⁺⁴ ions on the anatase. Data characterizing the samples after adsorption of [HRe(CO)₅] or [CH₃Re(CO)₅] determined a ranking of the reactivity of the surface OH sites, with the Ti⁺³? OH groups being the more reactive towards the rhenium complexes but the less likely to be dehydroxylated. The two rhenium pentacarbonyl probes provided complementary information, suggesting that the carbonate species originate from carbonyl ligands initially bonded to the rhenium and from hydroxyl groups of the titania surface, with the reaction leading to the formation of water and bridging hydroxyl groups on the titania. The results illustrate the value of using a family of organometallic complexes as probes of oxide surface sites.     

Tuning the reactivity in classic low-spin d6 rhenium(I) tricarbonyl radiopharmaceutical synthon by selective bidentate ligand variation (L,L'-Bid; L,L'= N,N', N,O, and O,O' donor atom sets) in fac-[Re(CO)3(L,L'-Bid)(MeOH)]n complexes

A range of fac-[Re(CO)(3)(L,L'-Bid)(H(2)O)](n) (L,L'-Bid = neutral or monoanionic bidentate ligands with varied L,L' donor atoms, N,N', N,O, or O,O': 1,10-phenanthroline, 2,2'-bipydine, 2-picolinate, 2-quinolinate, 2,4-dipicolinate, 2,4-diquinolinate, tribromotropolonate, and hydroxyflavonate; n = 0, +1) has been synthesized and the aqua/methanol substitution has been investigated. The complexes were characterized by UV-vis, IR and NMR spectroscopy and X-ray crystallographic studies of the compounds fac-[Re(CO)(3)(Phen)(H(2)O)]NO(3)·0.5Phen, fac-[Re(CO)(3)(2,4-dQuinH)(H(2)O)]·H(2)O, fac-[Re(CO)(3)(2,4-dQuinH)Py]Py, and fac-[Re(CO)(3)(Flav)(CH(3)OH)]·CH(3)OH are reported. A four order-of-magnitude of activation for the methanol substitution is induced as manifested by the second order rate constants with (N,N'-Bid) < (N,O-Bid) < (O,O'-Bid). Forward and reverse rate and stability constants from slow and stopped-flow UV/vis measurements (k(1), M(-1) s(-1); k(-1), s(-1); K(1), M(-1)) for bromide anions as entering nucleophile are as follows: fac-[Re(CO)(3)(Phen)(MeOH)](+) (50 ± 3) × 10(-3), (5.9 ± 0.3) × 10(-4), 84 ± 7; fac-[Re(CO)(3)(2,4-dPicoH)(MeOH)] (15.7 ± 0.2) × 10(-3), (6.3 ± 0.8) × 10(-4), 25 ± 3; fac-[Re(CO)(3)(TropBr(3))(MeOH)] (7.06 ± 0.04) × 10(-2), (4 ± 1) × 10(-3), 18 ± 4; fac-[Re(CO)(3)(Flav)(MeOH)] 7.2 ± 0.3, 3.17 ± 0.09, 2.5 ± 2. Activation parameters (ΔH(k1)(++), kJmol(-1); ΔS(k1)(), J K(-1) mol(-1)) from Eyring plots for entering nucleophiles as indicated are as follows: fac-[Re(CO)(3)(Phen)(MeOH)](+) iodide 70 ± 1, -35 ± 3; fac-[Re(CO)(3)(2,4-dPico)(MeOH)] bromide 80.8 ± 6, -8 ± 2; fac-[Re(CO)(3)(Flav)(MeOH)] bromide 52 ± 5, -52 ± 15. A dissociative interchange mechanism is proposed.     

Lanthanide-tungstobismuthate clusters based on [BiW9O33]9- building units: synthesis, crystal structures, luminescent and magnetic properties

The reaction of Na(12)[Bi(2)W(22)O(74)(OH)(2)]·44H(2)O, Na(9)[BiW(9)O(33)]·16H(2)O, lanthanide chloride and Na(2)CO(3) in aqueous solution at a pH value of about 7.0 resulted in the three unprecedented giant lanthanide-tungstobismuthate clusters Na(x)H(22-x)[(BiW(9)O(33))(4)(WO(3)){Bi(6)(μ(3)-O)(4)(μ(2)-OH)(3)}(Ln(3)(H(2)O)(6)CO(3))]·nH(2)O {Ln = Pr(3+) (1), Nd(3+) (2), La(3+) (3), x = 22 (1), 22 (2), 20 (3), n = 95 (1), 91 (2), 73 (3)}. These three complexes represent the first examples of lanthanide ions encapsulated in polyoxotungstobismuthates and the largest polytungstobismuthates so far. Furthermore, a [{Bi(6)(μ(3)-O)(4)(μ(2)-OH)(3)}](7+) polyoxo cation was incorporated into the structure of these compounds. All complexes are characterized by single-crystal X-ray diffraction, IR spectra, electronic spectroscopy, thermogravimetric and elemental analysis. Magnetic investigation revealed that the progressive depopulation of excited Stark sublevels of the lanthanide ions at low temperature and the weak antiferromagnetic interaction between the neighboring metal centres are responsible for the magnetic properties of 1 and 2. The original synthesis strategy in this work may open a gateway to assembly of large lanthanide-tungstobismuthates clusters and novel multifunctional solid materials in aqueous solution under mild conditions.     

Reactivity of molybdenum and rhenium hydroxo-carbonyl complexes toward organic electrophiles

The hydroxo compounds [Re(OH)(CO)(3)(N-N)] (N-N=bipy, 2 a; Me(2)-bipy, 2 b) were prepared in a biphasic H(2)O/CH(2)Cl(2) medium by reaction of [Re(OTf)(CO)(3)(N-N)] with KOH. In contrast, when anhydrous CH(2)Cl(2) was used, the binuclear hydroxo-bridged compound [[Re(CO)(3)(bipy)](2)(mu-OH)]OTf (3-OTf) was obtained. Compound [Re(OH)(CO)(3)(Me(2)-bipy)] (2 b) reacted with phenyl acetate or vinyl acetate to afford [Re(OAc)(CO)(3)(Me(2)-bipy)] (4) and phenol or acetaldehyde, respectively. The reactions of [Mo(OH)(eta(3)-C(3)H(4)-Me-2)(CO)(2)(phen)] (1), 2 a, and 2 b toward several unsaturated organic electrophiles were studied. The reaction of 1 with (p-tolyl)isocyanate afforded an adduct of N,N'-di(p-tolyl)urea and the carbonato-bridged compound [[Mo(eta(3)-C(3)H(4)-Me-2)(CO)(2)(phen)](2)(mu-eta(1)(O),eta(1)(O)-CO(3))] (5). In contrast, the reaction of 2 a with phenylisocyanate afforded [Re(OC(O)NHPh)(CO)(3)(bipy)] (6); this results from formal PhNCO insertion into the O-H bond. On the other hand, compounds [Mo[SC(O)NH(p-tolyl)](eta(3)-C(3)H(4)-Me-2)(CO)(2)(phen)] (7), [Re[SC(O)NH(p-tolyl)](CO)(3)(Me(2)-bipy)] (8 a), and [Re[SC(O)NHEt](CO)(3)(Me(2)-bipy)] (8 b) were obtained by reaction of 1 or 2 b with the corresponding alkyl or aryl isothiocyanates. In those cases, RNCS was inserted into the M-O bond. The reactions of 1, 2 a, and 2 b with dimethylacetylenedicarboxylate (DMAD) gave the complexes [Mo[C(OH)-C(CO(2)Me)C(CO(2)Me)-O](eta(3)-C(3)H(4)-Me-2)(CO)(phen)] (9) and [Re[C(OH)C(CO(2)Me)C(CO(2)Me)O](CO)(2)(N-N)] (N-N=bipy, 10 a; Me(2)-bipy, 10 b). The molecules of these compounds contain five-membered metallacycles that are the result of coupling between the hydroxo ligand, DMAD, and one of the CO ligands. The new compounds were characterized by a combination of IR and NMR spectroscopy, and for [[Re(CO)(3)(bipy)(2)(mu-OH)]BF(4) (3-BF(4)), 4, 5, 6, 7, 8 b, 9, and 10 b, also by means of single-crystal X-ray diffraction.     

A reversible SCSC transformation from a blue metamagnetic framework to a pink antiferromagnetic ordering layer exhibiting concomitant solvatochromic and solvatomagnetic effects

A [Co(6)(μ(3)-OH)(4)(datrz)(2)](6+) ribbon-based blue framework with a metamagnetic transition from an antiferromagnetic ordering to a weak spontaneous magnetization state, {[Co(3)(CH(3)OH)(μ(3)-OH)(2)(datrz)(sip)]·2.25H(2)O}(n) (1, Hdatrz = 3,5-diamino-1,2,4-triazole, sip(3-) = 3,5-dicarboxybenzenesulfonate), was solvothermally synthesized. 1 exhibits a reversible single-crystal-to-single-crystal transformation by solvent exchange to generate a pink antiferromagnetic ordering coordination layer with a similar [Co(6)(μ(3)-OH)(4)(datrz)(2)](6+) ribbon to 1, {[Co(3)(H(2)O)(3)(μ(3)-OH)(2)(datrz)(sip)]·2.125H(2)O}(n) (2). Such concomitant solvatochromic and solvatomagnetic effects are scarcely observed and are significantly due to the coexistence of differently distorted metal coordination spheres and the cleavage/generation of the weak coordination bond.     

Hydrogen atom abstraction by a mononuclear ferric hydroxide complex: insights into the reactivity of lipoxygenase

The lipoxygenase mimic [Fe(III)(PY5)(OH)](CF3SO3)2 is synthesized from the reaction of [Fe(II)(PY5)(MeCN)](CF3SO3)2 with iodosobenzene, with low-temperature studies suggesting the possible intermediacy of an Fe(IV) oxo species. The Fe(III)-OH complex is isolated and identified by a combination of solution and solid-state methods, including EPR and IR spectroscopy. [Fe(III)(PY5)(OH)](2+) reacts with weak X-H bonds in a manner consistent with hydrogen-atom abstraction. The composition of this complex allows meaningful comparisons to be made with previously reported Mn(III)-OH and Fe(III)-OMe lipoxygenase mimics. The bond dissociation energy (BDE) of the O-H bond formed upon reduction to [Fe(II)(PY5)(H2O)]2+ is estimated to be 80 kcal mol(-1), 2 kcal mol(-1) lower than that in the structurally analogous [Mn(II)(PY5)(H2O)]2+ complex, supporting the generally accepted idea that Mn(III) is the thermodynamically superior oxidant at parity of coordination sphere. The identity of the metal has a large influence on the entropy of activation for the reaction with 9,10-dihydroanthracene; [Mn(III)(PY5)(OH)]2+ has a 10 eu more negative DeltaS++ value than either [Fe(III)(PY5)(OH)]2+ or [Fe(III)(PY5)(OMe)]2+, presumably because of the increased structural reorganization that occurs upon reduction to [Mn(II)(PY5)(H2O)]2+. The greater enthalpic driving force for the reduction of Mn(III) correlates with [Mn(III)(PY5)(OH)]2+ reacting more quickly than [Fe(III)(PY5)(OH)]2+. Curiously, [Fe(III)(PY5)(OMe)]2+ reacts with substrates only about twice as fast as [Fe(III)(PY5)(OH)]2+, despite a 4 kcal mol(-1) greater enthalpic driving force for the methoxide complex.     

Investigation of the zinc(II)-benzoate-2-pyridinealdoxime reaction system

The employment of pyridine-2-carbaldehyde oxime (paoH) in zinc(II) benzoate chemistry, in the absence or presence of azide ions, is described. The syntheses, crystal structures and spectroscopic characterization are reported for the complexes [Zn(O(2)CPh)(2)(paoH)(2)] (1), [Zn(12)(OH)(4)(O(2)CPh)(16)(pao)(4)] (2) and [Zn(4)(OH)(2)(pao)(4)(N(3))(2)] (3). The Zn(II) centre in octahedral 1 is coordinated by two monodentate PhCO(2)(-) groups and two N,N'-chelating paoH ligands. The metallic skeleton of 2 describes a tetrahedron encapsulated in a distorted cube. The {Zn(12)(μ-OH)(4)(μ(3)-ΟR)(4)}(16+) core of the cluster can be conveniently described as consisting of a central {Zn(4)(μ(3)-ΟR)(4)}(4+) cubane subunit (RO(-) = pao(-)) linked to four {Zn(2)(μ-OH)}(3+) subunits via the OH(-) group of each of the latter, which becomes μ(3). The molecule of 3 has an inverse 12-metallacrown-4 topology. Two triply bridging hydroxido groups are accommodated into the metallacrown ring. Each pao(-) ligand adopts the η(1)?:?η(1)?:?η(1)?:?μ coordination mode, chelating one Zn(II) atom and bridging a Zn(II)(2) pair. Complexes 1 and 2 display photoluminescence with maxima at ～355 nm and ～375 nm, upon maximum excitation at 314 nm; the origin of the photoluminescence is discussed.     

Self-assembly, crystal structures, and properties of metal-2-sulfoterephthalate frameworks based on [M4(μ3-OH)2]6+ clusters (M = Co, Mn, Zn and Cd)

Hydro- and solvo-thermal reactions of d-block metal ions (Mn(2+), Co(2+), Zn(2+) and Cd(2+)) with monosodium 2-sulfoterephthalate (NaH(2)stp) form six 3D coordination polymers featuring cluster core [M(4)(μ(3)-OH)(2)](6+) in common: [M(2)(μ(3)-OH)(stp)(H(2)O)] (M = Co (1), Mn (2) and Zn (3)), [Zn(2)(μ(3)-OH)(stp)(H(2)O)(2)] (4), [Zn(4)(μ(3)-OH)(2)(stp)(2)(bpy)(2)(H(2)O)]·3.5H(2)O (5) and [Cd(2)(μ(3)-OH)(stp) (bpp)(2)]·H(2)O (6) (stp = 2-sulfoterephthalate, bpy = 4,4'-bipyridine and bpp = 1,3-di(4-pyridyl)propane). All these coordination polymers were characterized by single crystal X-ray diffraction, IR spectroscopy, thermogravimetric and elemental analysis. Complexes 1-3 are isostructural coordination polymers with 3D frameworks based on the chair-like [Zn(4)(μ(3)-OH)(2)](6+) core and the quintuple helixes. In complex 4, there exist double helixes in the 3D framework based on the chair-like cluster cores. Complex 5 possesses a 2-fold interpenetration structure constructed from boat-like cluster core and the bridging ligands stp and bpy. For complex 6, the chair-like cluster cores and stp ligands form a 2D (4,4) network which is further pillared by bpp linkers to a 3D architecture. Magnetic studies indicate that complex 1 exhibits magnetic ordering below 4.9 K with spin canting, and complex 2 shows weak antiferromagnetic coupling between the Mn(II) ions with g = 2.02, J(wb) = -2.88 cm(-1), J(bb) = -0.37 cm(-1). The fluorescence studies show that the emissions of complexes 3-6 are attributed to the ligand π-π* transition.     

Novel iso-reticular Zn(II) metal-organic frameworks constructed by trinuclear-triangular and paddle-wheel units: synthesis, structure and gas adsorption

Two novel Zn(II) metal-organic frameworks (MOFs) constructed by trinuclear-triangular and paddle-wheel units, namely {[Zn(5)(dmtrz)(3)(IPA)(3)(OH)]·DMF·H(2)O}(n) (MAC-4, Hdmtrz = 3,5-dimethyl-1H-1,2,4-triazole, H(2)IPA = isophthalic acid, DMF = dimethyl formamide) and {[Zn(5)(dmtrz)(3)(OH-IPA)(3)(OH)]·DMF·5H(2)O}(n) (MAC-4-OH, OH-H(2)IPA = 5-hydroxyisophthalic acid), were solvothermally synthesized. Single-crystal analyses reveal that MAC-4-OH is an iso-reticular framework of MAC-4 with channels functionalized by hydroxyl groups. Gas adsorption reveals that MAC-4-OH shows a significant enhancement for CO(2) uptake compared with that of MAC-4 due to the existence of electrostatic attractive interactions, though its surface area is lower than that of MAC-4.     

Antisymmetric exchange in triangular tricopper(II) complexes: correlation among structural, magnetic, and electron paramagnetic resonance parameters

Two new trinuclear copper(II) complexes, [Cu(3)(μ(3)-OH)(daat)(Hdat)(2)(ClO(4))(2)(H(2)O)(3)](ClO(4))(2)·2H(2)O (1) and [Cu(3)(μ(3)-OH)(aaat)(3)(H(2)O)(3)](ClO(4))(2)·3H(2)O (2) (daat = 3,5-diacetylamino-1,2,4-triazolate, Hdat = 3,5-diamino-1,2,4-triazole, and aaat = 3-acetylamino-5-amino-1,2,4-triazolate), have been prepared from 1,2,4-triazole derivatives and structurally characterized by X-ray crystallography. The structures of 1 and 2 consist of cationic trinuclear copper(II) complexes with a Cu(3)OH core held by three N,N-triazole bridges between each pair of copper(II) atoms. The copper atoms are five-coordinate with distorted square-pyramidal geometries. The magnetic properties of 1 and 2 and those of five other related 1,2,4-triazolato tricopper(II) complexes with the same triangular structure (3-7) (whose crystal structures were already reported) have been investigated in the temperature range of 1.9-300 K. The formulas of 3-7 are [Cu(3)(μ(3)-OH)(aaat)(3)(H(2)O)(3)](NO(3))(2)·H(2)O (3), {[Cu(3)(μ(3)-OH)(aat)(3)(μ(3)-SO(4))]·6H(2)O}(n) (4), and [Cu(3)(μ(3)-OH)(aat)(3)A(H(2)O)(2)]A·xH(2)O [A = NO(3)(-) (5), CF(3)SO(3)(-) (6), or ClO(4)(-) (7); x = 0 or 2] (aat =3-acetylamino-1,2,4-triazolate). The magnetic and electron paramagnetic resonance (EPR) data have been analyzed by using the following isotropic and antisymmetric exchange Hamiltonian: H = -J[S(1)S(2) + S(2)S(3)] - j[S(1)S(3)] + G[S(1) × S(2) + S(2) × S(3) + S(1) × S(3)]. 1-7 exhibit strong antiferromagnetic coupling (values for both -J and -j in the range of 210-142 cm(-1)) and antisymmetric exchange (G varying from to 27 to 36 cm(-1)). At low temperatures, their EPR spectra display high-field (g < 2.0) signals indicating that the triangles present symmetry lower than equilateral and that the antisymmetric exchange is operative. A magneto-structural study showing a lineal correlation between the Cu-O-Cu angle of the Cu(3)OH core and the isotropic exchange parameters (J and j) has been conducted. Moreover, a model based on Moriya's theory that allows the prediction of the occurrence of antisymmetric exchange in the tricopper(II) triangles, via analysis of the overlap between the ground and excited states of the local Cu(II) ions, has been proposed. In addition, analytical expressions for evaluating both the isotropic and antisymmetric exchange parameters from the experimental magnetic susceptibility data of triangular complexes with local spins (S) of (1)/(2), (3)/(2), or (5)/(2) have been purposely derived. Finally, the magnetic and EPR results of this work are discussed and compared with those of other tricopper(II) triangles reported in the literature.     

Structures and magnetic properties of an antiferromagnetically coupled polymeric copper(II) complex and ferromagnetically coupled hexanuclear nickel(II) clusters

Reactions between 2,6-diformyl-4-methylphenol (DFMF) and tris(hydroxymethyl) aminomethane (THMAM = H(3)L2) in the presence of copper(II) salts, CuX(2) (X = CH(3)CO(2)(-), BF(4)(-), ClO(4)(-), Cl(-), NO(3)(-)) and Ni(CH(3)CO(2))(2) or Ni(ClO(4))(2)/NaC(6)H(5)CO(2), sodium azide (NaN(3)), and triethylamine (TEA), in one pot self-assemble giving a coordination polymer consisting of repeating pentanuclear copper(II) clusters {[Cu(2)(H(5)L(2-))(μ-N(3))](2)[Cu(N(3))(4)]·2CH(3)OH}(n) (1) and hexanuclear Ni(II) complexes [Ni(6)(H(3)L1(-))(2)(HL2(2-))(2)(μ-N(3))(4)(CH(3)CO(2))(2)]·6C(3)H(7)NO·C(2)H(5)OH (2) and [Ni(6)(H(3)L1(-))(2)(HL2(2-))(2)(μ-N(3))(4)(C(6)H(5)CO(2))(2)]·3C(3)H(7)NO·3H(2)O·CH(3)OH (3). In 1, H(5)L(2-) and in 2 and 3 H(3)L1(-) and HL2(2-) represent doubly deprotonated, singly deprotonated, and doubly deprotonated Schiff-base ligands H(7)L and H(4)L1 and a tripodal ligand H(3)L2, respectively. 1 has a novel double-stranded ladder-like structure in which [Cu(N(3))(4)](2-) anions link single chains comprised of dinuclear cationic subunits [Cu(2)(H(5)L(2-))(μ-N(3))](+), forming a 3D structure of interconnected ladders through H bonding. Nickel(II) clusters 2 and 3 have very similar neutral hexanuclear cores in which six nickel(II) ions are bonded to two H(4)L1, two H(3)L2, four μ-azido, and two μ-CH(3)CO(2)(-)/μ-C(6)H(5)CO(2)(-) ligands. In each structure two terminal dinickel (Ni(2)) units are connected to the central dinickel unit through four doubly bridging end-on (EO) μ-azido and four triply bridging μ(3)-methoxy bridges organizing into hexanuclear units. In each terminal dinuclear unit two nickel centers are bridged through one μ-phenolate oxygen from H(3)L1(-), one μ(3)-methoxy oxygen from HL2(2-), and one μ-CH(3)CO(2)(-) (2)/μ-C(6)H(5)CO(2)(-) (3) ion. Bulk magnetization measurements on 1 show moderately strong antiferromagnetic coupling within the [Cu(2)] building block (J(1) = -113.5 cm(-1)). Bulk magnetization measurements on 2 and 3 demonstrate that the magnetic interactions are completely dominated by ferromagnetic coupling occurring between Ni(II) ions for all bridges with coupling constants (J(1), J(2), and J(3)) ranging from 2.10 to 14.56 cm(-1) (in the ? = -J(1)(?(1)?(2)) - J(1)(?(2)?(3)) - J(2)(?(3)?(4)) - J(1)(?(4)?(5)) - J(1)(?(5)?(6)) - J(2)(?(1)?(6)) - J(3)(?(2)?(6)) - J(3)(?(2)?(5)) - J(3)(?(3)?(5)) convention).     

Two novel Dy8 and Dy11 clusters with cubane [Dy4(μ3-OH)4]8+ units exhibiting slow magnetic relaxation behaviour

Two unique octa- and hendeca-nuclear dysprosium(III) clusters incorporating [Dy(4)(μ(3)-OH)(4)](8+) cubane units have been synthesized with the 1,10-phenanthroline-2,9-dicarbaldehyde dioxime (H(2)phendox) ligand and DyCl(3)·6H(2)O or Dy(OAc)(3)·4H(2)O. They are [Dy(8)(OH)(8)(phendox)(6)(H(2)O)(8)]Cl(2)(OH)(2)·18H(2)O·18MeOH (1) and [Dy(11)(OH)(11)(phendox)(6)(phenda)(3)(OAc)(3)](OH)·40H(2)O·7MeOH (2). Adjacent Dy(8) in 1 or Dy(11) in 2 motifs are packed by off-set π-π interactions of the aromatic rings on phendox(2-) to generate a 3D supramolecular architecture in the honeycomb topology and with 1D or 3D channels along the c-axis. Adsorption research shows that complex 1 has selective adsorption ability for H(2)O over small gas molecules (H(2), N(2), CO(2)). Complex 2 is stable upon the removal of guest molecules and the desolvated compound absorbed a considerable amount of CO(2). Furthermore, the oximes underwent hydrolysis to carboxylic acid and the resulting 1,10-phenanthroline-2,9-dicarboxylate link the dysprosium atoms to form a hendecanuclear cluster of 2. Magnetic studies reveal that both clusters exhibit slow magnetic relaxation behavior, expanding upon the recent reports of the pure 4f type single-molecule magnets (SMMs).     

Effect of cyanato, azido, carboxylato, and carbonato ligands on the formation of cobalt(II) polyoxometalates: characterization, magnetic, and electrochemical studies of multinuclear cobalt clusters

Five Co(II) silicotungstate complexes are reported. The centrosymmetric heptanuclear compound K(20)[{(B-beta-SiW(9)O(33)(OH))(beta-SiW(8)O(29)(OH)(2))Co(3)(H(2)O)}(2)Co(H(2)O)(2)]47 H(2)O (1) consists of two {(B-beta-SiW(9)O(33)(OH))(beta-SiW(8)O(29)(OH)(2))Co(3)(H(2)O)} units connected by a {CoO(4)(H(2)O)(2)} group. In the chiral species K(7)[Co(1.5)(H(2)O)(7))][(gamma-SiW(10)O(36))(beta-SiW(8)O(30)(OH))Co(4)(OH)(H(2)O)(7)]36 H(2)O (2), a {gamma-SiW(10)O(36)} and a {beta-SiW(8)O(30)(OH)} unit enclose a mononuclear {CoO(4)(H(2)O)(2)} group and a {Co(3)O(7)(OH)(H(2)O)(5)} fragment. The two trinuclear Co(II) clusters present in 1 enclose a mu(4)-O atom, while in 2 a mu(3)-OH bridging group connects the three paramagnetic centers of the trinuclear unit, inducing significantly larger Co-L-Co (L=mu(4)-O (1), mu(3)-OH (2)) bridging angles in 2 (theta(av(Co-L-Co))=99.1 degrees ) than in 1 (theta(av(Co-L-Co))=92.8 degrees ). Weaker ferromagnetic interactions were found in 2 than in 1, in agreement with larger Co-L-Co angles in 2. The electrochemistry of 1 was studied in detail. The two chemically reversible redox couples observed in the positive potential domain were attributed to the redox processes of Co(II) centers, and indicated that two types of Co(II) centers in the structure were oxidized in separate waves. Redox activity of the seventh Co(II) center was not detected. Preliminary experiments indicated that 1 catalyzes the reduction of nitrite and NO. Remarkably, a reversible interaction exists with NO or related species. The hybrid tetranuclear complexes K(5)Na(3)[(A-alpha-SiW(9)O(34))Co(4)(OH)(3)(CH(3)COO)(3)]18 H(2)O (3) and K(5)Na(3)[(A-alpha-SiW(9)O(34))Co(4)(OH)(N(3))(2)(CH(3)COO)(3)]18 H(2)O (4) were characterized: in both, a tetrahedral {Co(4)(L(1))(L(2))(2)(CH(3)COO)(3)} (3: L(1)=L(2)=OH; 4: L(1)=OH, L(2)=N(3)) unit capped the [A-alpha-SiW(9)O(34)](10-) trivacant polyanion. The octanuclear complex K(8)Na(8)[(A-alpha-SiW(9)O(34))(2)Co(8)(OH)(6)(H(2)O)(2)(CO(3))(3)]52 H(2)O (5), containing two {Co(4)O(9)(OH)(3)(H(2)O)} units, was also obtained. Compounds 2, 3, 4, and 5 were less stable than 1, but their partial electrochemical characterization was possible; the electronic effect expected for 3 and 4 was observed.     

Expanded Prussian blue analogues incorporating [Re6Se8(CN)6](3-/4-) clusters: adjusting porosity via charge balance

Face-capped octahedral [Re(6)Se(8)(CN)(6)](3-/4-) clusters are used in place of octahedral [M(CN)(6)](3-/4-) complexes for the synthesis of microporous Prussian blue type solids with adjustable porosity. The reaction between [Fe(H(2)O)(6)](3+) and [Re(6)Se(8)(CN)(6)](4-) in aqueous solution yields, upon heating, Fe(4)[Re(6)Se(8)(CN)(6)](3).36H(2)O (4). A single-crystal X-ray analysis confirms the structure of 4 to be a direct expansion of Prussian blue (Fe(4)[Fe(CN)(6)](3).14H(2)O), with [Re(6)Se(8)(CN)(6)](4-) clusters connected through octahedral Fe(3+) ions in a cubic three-dimensional framework. As in Prussian blue, one out of every four hexacyanide units is missing from the structure, creating sizable, water-filled cavities within the neutral framework. Oxidation of (Bu(4)N)(4)[Re(6)Se(8)(CN)(6)] (1) with iodine in methanol produces (Bu(4)N)(3)[Re(6)Se(8)(CN)(6)] (2), which is then metathesized to give the water-soluble salt Na(3)[Re(6)Se(8)(CN)(6)] (3). Reaction of [Co(H(2)O)(6)](2+) or [Ni(H(2)O)(6)](2+) with 3 in aqueous solution affords Co(3)[Re(6)Se(8)(CN)(6)](2).25H(2)O (5) or Ni(3)[Re(6)Se(8)(CN)(6)](2).33H(2)O (6). Powder X-ray diffraction data show these compounds to adopt structures based on the same cubic framework present in 4, but with one out of every three cluster hexacyanide units missing as a consequence of charge balance. In contrast, reaction of [Ga(H(2)O)(6)](3+) with 3 gives Ga[Re(6)Se(8)(CN)(6)].6H(2)O (7), wherein charge balance dictates a fully occupied cubic framework enclosing much smaller cavities. The expanded Prussian blue analogues 4-7 can be fully dehydrated, and retain their crystallinity with extended heating at 250 degrees C. Consistent with the trend in the frequency of framework vacancies, dinitrogen sorption isotherms show porosity to increase along the series of representative compounds 7, Ga(4)[Re(6)Se(8)(CN)(6)](3).38H(2)O, and 6. Furthermore, all of these phases display a significantly higher sorption capacity and surface area than observed in dehydrated Prussian blue. Despite incorporating paramagnetic [Re(6)Se(8)(CN)(6)](3-) clusters, no evidence for magnetic ordering in compound 6 is apparent at temperatures down to 5 K. Reactions related to those employed in preparing compounds 4-6, but carried out at lower pH, produce the isostructural phases H[cis-M(H(2)O)(2)][Re(6)Se(8)(CN)(6)].2H(2)O (M = Fe (8), Co (9), Ni (10)). The crystal structure of 8 reveals a densely packed three-dimensional framework in which [Re(6)Se(8)(CN)(6)](4-) clusters are interlinked through a combination of protons and Fe(3+) ions.     

The organometallic fac-[(CO)3Mn(H2O)3]+ aquaion: base-hydrolysis and kinetics of H2O-substitution

The novel organometallic aqua complex [(CO)(3)Mn(H(2)O)(3)](+) (1(+)) was obtained through hydrolysis of the analogous acetone complex. IR [nu(CO) = 2051, 1944 cm(-)(1)] and (17)O NMR spectroscopy revealed the presence of a fac tricarbonyl unit. Potentiometric titrations established that the trimer [(CO)(3)Mn(3)(OH)(4)](-) was the principal condensation product in the pH range >6 prior to slow formation of the tetramer [[(CO)(3)Mn](OH)](4). Water exchange in 1(+), determined by NMR line broadening as k(ex) = 19 +/- 4 s(-)(1) at 298 K, is four orders faster than with the analogous Re complex. The activation volume DeltaV(++) = -4.5 +/- 0.4 cm(3) mol(-1) is indicative of an associatively activated (I(a)) process.     

Reactivity of the organometallic fac-[(CO)3ReI(H2O)3]+ aquaion. Kinetic and thermodynamic properties of H2O substitution

The water exchange process on [(CO)(3)Re(H(2)O)(3)](+) (1) was kinetically investigated by (17)O NMR. The acidity dependence of the observed rate constant k(obs) was analyzed with a two pathways model in which k(ex) (k(ex)(298) = (6.3 +/- 0.1) x 10(-3) s(-1)) and k(OH) (k(OH)(298)= 27 +/- 1 s(-1)) denote the water exchange rate constants on 1 and on the monohydroxo species [(CO)(3)Re(I)(H(2)O)(2)(OH)], respectively. The kinetic contribution of the basic form was proved to be significant only at [H(+)] < 3 x 10(-3) M. Above this limiting [H(+)] concentration, kinetic investigations can be unambiguously conducted on the triaqua cation (1). The variable temperature study has led to the determination of the activation parameters Delta H(++)(ex) = 90 +/- 3 kJ mol(-1), Delta S(++)(ex) = +14 +/- 10 J K(-1) mol(-1), the latter being indicative of a dissociative activation mode for the water exchange process. To support this assumption, water substitution reaction on 1 has been followed by (17)O/(1)H/(13)C/(19)F NMR with ligands of various nucleophilicities (TFA, Br(-), CH(3)CN, Hbipy(+), Hphen(+), DMS, TU). With unidentate ligands, except Br(-), the mono-, bi-, and tricomplexes were formed by water substitution. With bidentate ligands, bipy and phen, the chelate complexes [(CO)(3)Re(H(2)O)(bipy)]CF(3)SO(3) (2) and [(CO)(3)Re(H(2)O)(phen)](NO(3))(0.5)(CF(3)SO(3))(0.5).H(2)O (3) were isolated and X-ray characterized. For each ligand, the calculated interchange rate constants k'(i) (2.9 x 10(-3) (TFA) < k'(I) < 41.5 x 10(-3) (TU) s(-1)) were found in the same order as the water exchange rate constant k(ex), the S-donor ligands being slightly more reactive. This result is indicative of I(d) mechanism for water exchange and complex formation, since larger variations of k'(i) are expected for an associatively activated mechanism.