A hexadecanuclear copper(I)-copper(II) mixed-valence compound: structure, magnetic properties, intervalence charge transfer, EPR, and NMR

Hexadecanuclear copper mixed-valence complex 2 containing 10 Cu-II, centers and 6 Cu-I centers was isolated with N,O donor ligands. From the X-ray crystal structure, 2 was found to contain a centrosymmetric dimeric cation - each monomeric unit composed of eight copper centers. It displays a very broad and weak intervalence charge-transfer band around 1100 nm at room temperature in the solid state. Variable-temperature magnetic susceptibility measurements indicate an S = 1/2 ground state for half of 2, explicitly, each Cu-8 moiety has a g value around 2.26. Complex 2 was examined by NMR spectroscopy at room temperature in solution and by EPR at low temperature; the data indicates that the valence is delocalized in 2 at room temperature but localized at low temperature. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

A new emissive Ru(II)N5O core

From the reaction of cis-Ru(1,10-phenanthroline)(2)Cl(2 center dot)2H(2)O with 2-picolinic acid in 1:1 molar ratio in degassed methanol-water mixture, [Ru(1,10-phenanthroline)(2)(2-picolinate)]PF6 center dot H2O (1) has been isolated as a red compound by adding excess of NH4PF6. Single crystal X-ray crystallography shows that the metal in 1 has an octahedral N5O coordination sphere. Complex 1 displays (MLCT)-M-1 bands in the 400-500 nm region in acetonitrile. Upon excitation at 435 nm, complex 1 gives rise to a broad emission band at 675 nm in acetonitrile at room temperature with a quantum yield of 0.0022. The energy of the MLCT state in 1 is estimated as 1.99 eV. Since, from cyclic voltammetry, the ground state potential of the Ru(II/III) couple in 1 is found to be 1.01 V vs NHE, the potential of the same couple in the excited state is calculated as -0.98 V vs NHE. The emissive state in 1 seems to be the triplet Ru(II) -> 1, 10-phenanthroline charge transfer state.

The kinetics of the 2π+2π photodimerisation reactions of single-crystalline derivatives of trans-cinnamic acid: a study by infrared microspectroscopy

The kinetics of the photodimerisation reactions of the 2- and 4-β-halogeno-derivatives of trans-cinnamic acid (where the halogen is fluorine, chlorine or bromine) have been investigated by infrared microspectroscopy. It is found that none of the reactions proceed to 100% yield. This is in line with a reaction mechanism developed by Wernick and his co-workers that postulates the formation of isolated monomers within the solid, which cannot react. β-4-Bromo and β-4-chloro-trans-cinnamic acids show approximately first order kinetics, although in both cases the reaction accelerates somewhat as it proceeds. First order kinetics is explained in terms of a reaction between one excited- and one ground-state monomer molecule, while the acceleration of the reaction implies that it is promoted as defects are formed within the crystal. By contrast β-2-chloro-trans-cinnamic acid shows a strongly accelerating reaction which models closely to the contracting cube equation. β-2-Fluoro- and β-4-fluoro-trans-cinnamic acids show a close match to first order kinetics. The 4-fluoro-derivative, however, shows a reaction that proceeds via a structural intermediate. The difference in behaviour between the 2-fluoro- and 4-fluoro-derivative may be due to different C–HF hydrogen bonds observed within these single-crystalline starting materials.

Vibrations in the B-4 rhombic structure

A double minimum six-dimensional Potential energy surface (PES) is determined in symmetry coordinates for the most stable rhombic (D-2h) B-4 isomer in its (1)A(g) electronic ground state by fitting to energies calculated ab initio. The PES exhibits a barrier to the D-4h square structure of 255 cm(-1). The vibrational levels (J=0) are calculated variationally using an approach which involves the Watson kinetic energy operator expressed in normal coordinates. The pattern of about 65 vibrational levels up to 1600 cm-1 for all stable isotopomers is analyzed. Analogous to the inversion in ammonia-like molecules, the rhombus rearrangements lead to splittings of the vibrational levels. In B-4 it is the B-1g (D-4h mode which distorts the square molecule to its planar rhombic form. The anharmonic fundamental vibrational transitions of B-11(4) are calculated to be (splittings in parentheses): G(O) = 2352(22) cm(-1), v(1)(A(1g)) - 1136(24) cm(-1,) v(2)(B-1g)=209(144) cm(-1) v(3)(B-2g)=1198(19)cm(-1), v(4)(B-2u) = 271(24) cm(-1), and v(5) (E-u) = 1030( 166) cm(-1) (D-4h notation). Their variations in all stable isotoporners were investigated. Due to the presence of strong anharmonic resonances between the B-1g in-plane distortion and the B-2u, out-of-plane bending modes. the hiaher overtones and combination levels are difficult to assign unequivocally. (C) 2005 American Institute of Physics.

Quantum studies of the vibrations in H3O2- and D3O2-

The vibrations of H3O2- and D3O2- are investigated using diffusion Monte Carlo (DMC) and vibrational configuration-interaction approaches, as implemented in the program MULTIMODE. These studies use the potential surface recently developed by Huang [ J. Am. Chem. Soc. 126, 5042 (2004)]. The focus of this work is on the vibrational ground state and fundamentals which occur between 100 and 3700 cm(-1). In most cases, excellent agreement is obtained between the fundamental frequencies calculated by the two approaches. This serves to demonstrate the power of both methods for treating this very anharmonic system. Based on the results of the MULTIMODE and DMC treatments, the extent and nature of the couplings in H3O2- and D3O2- are investigated. (C) 2005 American Institute of Physics.

Structure determination, magnetic and optical properties of a new chromium(II) thioantimonate, [Cr((NH2CH2CH2)(3)N)]Sb4S7

The chromium(II) antimony(III) sulphicle, [Cr((NH2CH2CH2)(3)N)]Sb4S7, was synthesised under solvothermal conditions from the reaction of Sb2S3. Cr and S dissolved in tris(2-aminoethyl)amine (tren) at 438 K. The products were characterised by single-crystal X-ray diffraction. elemental analysis, SQUID magnetometry and diffuse reflectance spectroscopy. The compound crystallises in the monoclinic space group P2(1)/n with a = 7.9756(7), b = 10.5191(9), c = 25.880(2) angstrom and beta = 90.864(5)degrees. Alternating SbS33- trigonal pyramids and Sb36 semi-cubes generate Sb4S72- chains which are directly bonded to Cr(tren pendant units. The effective magnetic moment of 4.94(6)mu(B) shows a negligible orbital contribution, in agreement with expectations for Cr(II):d(4) in a (5)A ground state. The measured band gap of 2.14(3) eV is consistent with a correlation between optical band gap and framework density that is established from analysis of a wide range of antimony sulphides. (C) 2007 Elsevier Ltd. All rights reserved.

Theoretical calculation of absorption intensities of C2H and C2D

The theory of dipole-allowed absorption intensities in triatomic molecules is presented for systems with three close-lying electronic states of doublet multiplicity. Its derivation is within the framework of a recently developed variational method [CARTER, S., HANDY, N. C., PUZZARINI, C., TARRONI, R., and PALMIERI, P., 2000, Molec. Phys., 98,1967]. The method has been applied to the calculation of the infrared absorption spectrum of the C2H radical and its deuterated isotopomer for energies up to 10000 cm(-1) above the ground state, using highly accurate ab initio diabatic potential energy and dipole moment surfaces. The calculated spectra agree very well with those recorded experimentally in a neon matrix [FORNEY, D., JACOX, M. E., and THOMPSON, W. E., 1995, J. molee. Spectrosc., 170, 178] and assignments in the high energy region of the IR spectra are proposed for the first time.

Ab initio prediction of the infrared-absorption spectrum of the C2Cl radical

The three lowest (1(2)A('), 2(2)A('), and 1(2)A(')) potential-energy surfaces of the C2Cl radical, correlating at linear geometries with (2)Sigma(+) and (2)Pi states, have been studied ab initio using a large basis set and multireference configuration-interaction techniques. The electronic ground state is confirmed to be bent with a very low barrier to linearity, due to the strong nonadiabatic electronic interactions taking place in this system. The rovibronic energy levels of the (CCCl)-C-12-C-12-Cl-35 isotopomer and the absolute absorption intensities at a temperature of 5 K have been calculated, to an upper limit of 2000 cm(-1), using diabatic potential-energy and dipole moment surfaces and a recently developed variational method. The resulting vibronic states arise from a strong mixture of all the three electronic components and their assignments are intrinsically ambiguous. (c) 2005 American Institute of Physics.

Ab initio prediction of the infrared absorption spectrum of the C2Br radical

The first three electronic states (1(2)A', 2(2)A', 1(2)A '') of the C2Br radical, correlating at linear geometries with (2)Sigma(+) and (2)Pi states, have been studied ab initio, using Multi Reference Configuration Interaction techniques. The electronic ground state is found to have a bent equilibrium geometry, R-CC = 1.2621 angstrom, R-CBr = 1.7967 angstrom, < CCBr 156.1 degrees, with a very low barrier to linearity. Similarly to the valence isoelectronic radicals C2F and C2Cl, this anomalous behaviour is attributed to a strong three-state non-adiabatic electronic interaction. The Sigma, Pi(1/2), Pi(3/2) vibronic energy levels and their absolute infrared absorption intensities at a temperature of 5K have been calculated for the (CCBr)-C-12-C-12-Br-79 isotopomer, to an upper limit of 2000 cm(-1), using ab initio diabatic potential energy and dipole moment surfaces and a recently developed variational method.

Mono- and heterodimetallic FeII and RuII complexes based on a novel heteroditopic 4′-{bis(phosphanyl)aryl}-2,2′:6′,2″-terpyridine ligand

In the search for a versatile building block that allows the preparation of heteroditopic tpy-pincer bridging ligands, the synthon 14'-[C6H3(CH2Br)(2)-3,5]-2,2':6',2 ''-terpyridine was synthesized. Facile introduction of diphenylphosphanyl groups in this synthon gave the ligand 14'-[C6H3(CH2PPh2)2-3,5]-2,2':6',2"-terpyridine) ([tpyPC(H)Pj). The asymmetric mononuclear complex [Fe(tpy){tpyPC(H)P}](PF6)(2), prepared by selective coordination of [Fe(tpy)Cl-3] to the tpy moiety of [tpyPC(H)P], was used for the synthesis of the heterodimetallic complex [Fe(tpy)(tpyPCP)Ru(tpy)](PFC,)3, which applies the "complex as ligand" approach. Coordination of the ruthenium centre at the PC(H)P-pincer moiety of [Fe(tpy){tpyPC(H)P}](PF6)(2) has been achieved by applying a transcyclometallation procedure. The ground-state electronic properties of both complexes, investigated by cyclic and square-wave voltammetries and UV/Vis spectroscopy, are discussed and compared with those of [Fe(tPY)(2)](PF6)(2) and [Ru(PCP)(tpy)]Cl, which represent the mononuclear components of the heterodinuclear species. An in situ UV/Vis spectroelectrochemical study was performed in order to localize the oxidation and reduction steps and to gain information about the Fe-II-Ru-II communication in the heterodimetallic system [Fe(tpy)(tpyPCP)Ru(tpy)](PF6)(3) mediated by the bridging ligand [tpyPCP]. Both the voltammetric and spectroelectrochemical results point to only very limited electronic interaction between the metal centres in the ground state.

A square-planar nickel(II) monoradical complex with a bis(salicylidene)diamine ligand

The new square-planar Ni-II-N2O2 complex [Ni(L-Me)] (1(Me)), where L-Me, stands for the dianionic phenolato form of N,N'bis(3,5-di-tert-butyl-salicylidene)-4,5-dimethyl-1,2-phenyl- enediamine ((LH2)-L-Me), has been synthesised and fully characterised. X-ray crystallography was also used for the characterisation. The electrochemical one-electron oxidation of 1(Me) produces the thermally stable (within the temperature range 10-295 K) cationic species (1(Me))(+). The UV/Vis and X-band EPR experimental data, supported by DFT calculations, indicate that (1(Me))(+), is best described as a Ni-II monoradical complex and, thus, does NOT exist in a Ni-III ground state, in contrast to its demethylated counterpart [Ni(L-H)](+) (1(H))(+) below 170 K.

The synthesis, structure, reactivity and electrochemical properties of ruthenium complexes featuring cyanoacetylide ligands

The complex [Ru(C&3bond; CC&3bond; N)(dppe)Cp*] (1) is readily obtained (ca. 70%) from the sequential reaction of [Ru(C=CH2)(dppe)Cp*]PF6 with (BuLi)-Bu-n and phenyl cyanate. The complex behaves as a typical transition metal acetylide upon reaction with tetracyanoethene, affording a metallated pentacyanobutadiene. Complex I is a useful metalloligand, and its reactions with [W(thf)(CO)5], [RuCl(PPh3)(2)Cp], [RuCl(dppe)Cp*] or cis-[RuCl2(dppe)(2)] all afforded products featuring the M-C&3bond; CC&3bond; N-M' motif, for which ground state structures indicate a degree of polarisation. Electrochemical and spectroelectrochemical studies reveal moderate interactions between the metal centres in the 35-electron dications [{Cp*(dppe)Ru}(mu-C&3bond; CC&3bond; N){RuL2Cp'}](2+) Ru(PPh3)(2)CP, Ru(dppe)Cp*).

Ultrafast charge separation in a photoreactive rhenium-appended porphyrin assembly monitored by picosecond transient infrared spectroscopy

Rhenium(bipyridine)(tricarbonyl)(picoline) units have been linked covalently to tetraphenylmetalloporphyrins of magnesium and zinc via an amide bond between the bipyridine and one phenyl substituent of the porphyrin. The resulting complexes, abbreviated as [Re(CO)(3)(Pic)Bpy-MgTPP][OTf] and [Re(CO)(3)(Pic)Bpy-ZnTPP][OTf], exhibit no signs of electronic interaction between the Re(CO)(3)(bpy) units and the metalloporphyrin units in their ground states. However, emission spectroscopy reveals solvent-dependent quenching of porphyrin emission on irradiation into the long-wavelength absorption bands localized on the porphyrin. The characteristics of the excited states have been probed by picosecond time-resolved absorption (TRVIS) spectroscopy and time-resolved infrared (TRIR) spectroscopy in nitrile solvents. The presence of the charge-separated state involving electron transfer from MgTPP or ZnTPP to Re(bpy) is signaled in the TRIR spectra by a low-frequency shift in the nu(CO) bands of the Re(CO)(3) moiety similar to that observed by spectroelectrochemical reduction. Long-wavelength excitation of [Re(CO)(3)(Pic)Bpy-MTPP][OTf] results in characteristic TRVIS spectra of the S-1 state of the porphyrin that decay with a time constant of 17 ps (M = Mg) or 24 ps (M = Zn). The IR bands of the CS state appear on a time scale of less than 1 ps (Mg) or ca. 5 ps (Zn) and decay giving way to a vibrationally excited (i.e., hot) ground state via back electron transfer. The IR bands of the precursors recover with a time constant of 35 ps (Mg) or 55 ps (Zn). The short lifetimes of the charge-transfer states carry implications for the mechanism of reaction in the presence of triethylamine.

The synthesis, structure, and electrochemical properties of Fe(C CC N)(dppe)Cp and related compounds

The cyanoacetylide complex Fe(CCCN)(dppe)Cp (3) is readily obtained from sequential reaction of Fe(CCSiMe3)(dppe)Cp with methyllithium and phenyl cyanate. Complex 3 is a good metalloligand, and coordination to the metal fragments [RhCl(CO)(2)], [Ru(PPh3)(2)Cp](+), and [Ru(dppe)Cp*](+) affords the corresponding cyanoaceylide-bridged heterobimetallic complexes. In the case of the 36-electron complexes [Cp(dppe)Fe-CCCN-MLn](n+), spectroscopic and structural data are consistent with a degree of charge transfer from the iron centre to the rhodium or ruthenium centre via the C3N bridge, giving rise to a polarized ground state. Electrochemical and spectroelectrochemical methods reveal significant interactions between the metal centres in the oxidized (35 electron) derivatives, [Cp(dppe)Fe-CCCN-MLn]((n+1)+).

Redox control of light-induced charge separation in a transition metal cluster: photochemistry of a methyl viologen-substituted [Os-3(CO)10(a-diimine)] cluster

Sub)picosecond transient absorption (TA) and time-resolved infrared (TRIR) spectra of the cluster [OS3(CO)(10-) (AcPy-MV)](2+) (the clication AcPy-MV = Acpy-MV2+ = [2-pyridylacetimine-N-(2-(1'-methyl-4,4'-bipyridine-1,1'-diium-1-yl) ethyl)] (PF6)(2)) (1(2+)) reveal that photoinduced electron transfer to the electron-accepting 4,4'-bipyridine-1,1'diium (MV2+) moiety competes with the fast relaxation of the initially populated sigmapi* excited state of the cluster to the ground state and/or cleavage of an Os-Os bond. The TA spectra of cluster 12 in acetone, obtained by irradiation into its lowest-energy absorption band, show the characteristic absorptions of the one-electron-reduced MV*(+) unit at 400 and 615 nm, in accordance with population of a charge-separated (CS) state in which a cluster-core electron has been transferred to the lowest pi* orbital of the remote MV2+ unit. This assignment is confirmed by picosecond TRIR spectra that show a large shift of the pilot highest-frequency nu(CO) band of 1(2+) by ca. +40 cm(-1), reflecting the photooxidation of the cluster core. The CS state is populated via fast (4.2 x 10(11) s(-1)) and efficient (88%) oxidative quenching of the optically populated sigmapi* excited state and decays biexponentially with lifetimes of 38 and 166 ps (1:2:1 ratio) with a complete regeneration of the parent cluster. About 12% of the cluster molecules in the sigmapi* excited state form long-lived open-core biradicals. In strongly coordinating acetonitrile, however, the cluster core-to-MV2+ electron transfer in cluster 12+ results in the irreversible formation of secondary photoproducts with a photooxidized cluster core. The photochemical behavior of the [Os-3(CO)(10)(alpha-diimine-MV)](2+) (donor-acceptor) dyad can be controlled by an externally applied electronic bias. Electrochemical one-electron reduction of the MV2+ moiety prior to the irradiation reduces its electron-accepting character to such an extent that the photoinduced electron transfer to MV*+ is no longer feasible. Instead, the irradiation of reduced cluster 1(.)+ results in the reversible formation of an open-core zwitterion, the ultimate photoproduct also observed upon irradiation of related nonsubstituted clusters [Os-3(CO)(10)(alpha-diimine)] in strongly coordinating solvents such as acetonitrile.