Excited-state annihilation in a homodinuclear ruthenium complex

Ultrafast excited-state annihilation in a homodinuclear ruthenium complex is observed. This coordination compound constitutes a model system for approaches towards artificial photosynthetic systems. The observation of pump-intensity dependent triplet-triplet annihilation highlights the importance of considering various loss mechanisms in the design of artificial photosynthetic assemblies.     

Proton-coupled electron shuttling in a covalently linked ruthenium-copper heterodinuclear complex

A heterodinuclear complex based on a Ru(II)-TPA [TPA = tris(2-pyridylmethyl)amine] complex having a peripheral Cu(II)(bpy)(2) (bpy = 2,2'-bipyridine) group bonded through an amide linkage displayed reversible intramolecular electron transfer between the Ru and Cu complex units that can be controlled by protonation and deprotonation of the bridging amide moiety.     

Preparation and electrochemical behavior of a macrocyclic heterodinuclear FeIIICoIII complex

A new macrocyclic heterodinuclear Fe^IIICo^III complex, prepared by the condensation of 2,6-diformylpyridine N-oxide with 1,3-diaminopropane in the presence of FeCl₂.4H₂O and CoCl₂.6H₂O, and formulated as FeCoLCl₆.C₂H₅OH.8H₂O, was characterized by elemental analyses, IR and Mössbauer spectra and investigated electrochemically on an ultramicrodisk platinum electrode. The results show that the electrode reaction can be thought as a nearly reversible transfer process. The diffusion coefficient is 1.93×10^-6 m² s^-1 and E^o ̄ is −0.229V (versus s.c.e) and α=0.877. The average electron transfer rate constant k^o′ is 3.21×10^-3 cm s^-1.     

Iron substitution for sodium in a carboxylate-bridged, heterodinuclear sodium-iron complex

The synthesis and structural characterization of the carboxylate-bridged, heterodinuclear iron-sodium complex [NaFe(PIC2DET)(mu-O2CTrp)3] (2), where PIC2DET (1) is a 2,3-diethynyltriptycene-linked dipicolinic methyl ester ligand and Trp is 9-triptycenyl, are described. The metal ions in 2 are bridged by three triptycene carboxylates with an Fe...Na distance of 3.181(2) A, and each is coordinated to a pyridine nitrogen and carbonyl oxygen atom of 1, forming two five-membered chelate rings. A linkage isomer in which Fe1 is bound by the other ester oxygen atom of 1 was identified by X-ray crystallographic analysis. Treatment of 2 with Fe(OTf)2.2MeCN resulted in substitution of sodium by iron(II) to give the cationic diiron(II) complex [Fe2(PIC2DET)(mu-O2CTrp)3][OTf] (3). This reaction was investigated by UV-vis, IR, MS, and stopped-flow spectroscopy. The substitution is first order with respect to 2 and zero order with respect to Fe(OTf)2.2MeCN (kobs = 21 +/- 2 s-1), consistent with a dissociative mechanism. A positive enthalpy of activation (DeltaH = 59 +/- 6 kJ mol-1) and negative entropy of activation (DeltaS = -20 +/- 6 J mol-1 K-1) were calculated from the temperature dependence of the rate-determining dissociation step.     

Magnetic properties and structure of a CuDy heterodinuclear Schiff base complex

The crystal structure of a new heterodinuclear lanthanide complex, L²Cu(Me₂CO)Dy(NO₃)₃ (L²=N,N′-bis(2-hydroxy-3-methoxy-benzylidene)-ethylenediamine) has determined and the magnetic properties of the complex investigated. (C₁₈H₁₈N₂O₄)Cu(C₃H₆O)Dy(NO₃)₃, monoclinic, space group P2₁/c, with a=9.875(2), b=18.870(7), c=15.675(8) Å, β=95.45(3)°, V=2908(2) Å³, Z=4. The structure consists of ordered dinuclear units with Cu^II and Dy^III ions bridged by two phenolato oxygen atoms of the Schiff base ligand. The Cu^II ion has a square-pyramidal geometry involving the basal N₂O₂ donor atoms of the Schiff base ligands and one oxygen atom of the acetone molecule at the apex position. The Dy^III ion is decacoordinated by the four oxygen atoms of L² and six oxygen atoms from the three bidentate nitrate ions. The Cu?Dy separation is 3.461(2) Å. The χT versus T plots, χ being the molar magnetic susceptibility per Cu^IIDy^III unit and T the temperature, has been measured in the 4.5-300 K temperature range. The magnetic properties of the compound are dominated by the crystal field effect on the Dy^III site, masking the magnetic interaction between the paramagnetic centers.     

Ferromagnetic coupling induced by spin-orbit coupling in dipyridylamide linear trinuclear Cu-Pd-Cu and Cu-Pt-Cu complexes

The mechanism of the magnetic coupling in the [CuMCu(pda)₄Cl₂] complexes with M = Pd, Pt is analyzed. First principles calculations based on the wave function theory are performed (DDCI + SO-RASSI): it is shown that the interaction without spin-orbit coupling is antiferromagnetic while the contribution due to the spin-orbit is ferromagnetic. The agreement between calculations and experiment is very good for the CuPdCu complex but is lesser for the CuPtCu complex. A Hubbard model is used in order to rationalize these results: this shows that the two d frontier orbitals of the central atom play a key role in these two interactions. Although the model parameters extracted from CASCI calculations do not permit to recover the order of magnitude of the interactions, it allows understanding the main interactions that give physical insight.     

Intermolecular interaction and magnetic coupling mechanism on a mononuclear CoII complex

Anti-ferromagnetic interaction was observed in a new crystal that consists of mononuclear Co^II complexes, namely [Co(PMP)(N₃)] (PMP?=?2,9-bis(pyridin-2-methoxyl)-1,10-phenanthroline); in the mononuclear complex Co^II has a distorted trigonal-bipyramidal geometry. Analysis for the crystal structure indicates six magnetic coupling pathways among adjacent complexes, in which three involve π–π stacking and the other three deal with intermolecular interactions. The fitting for the variable-temperature magnetic susceptibilities with the Curie–Weiss formula shows an anti-ferromagnetic interaction between adjacent Co^II ions with θ?=??5.49 K?=??3.82?cm^?1. Theoretical calculations on the spin section reveal that the three π–π stacking systems result in magnetic coupling constants 2J?=??0.10?cm^?1, ?0.10?cm^?1, and 1.24?cm^?1, respectively, and the three intermolecular interactions lead to weak anti-ferromagnetic interactions with 2J?=??0.36?cm^?1, ?0.26?cm^?1, and ?0.32?cm^?1, respectively. The theoretical calculations and the experimental magnetic data imply that the anti-ferromagnetic interaction involves the orbital contribution of the relevant Co^II ions.     

Ferromagnetic coupling in the three-dimensional malonato-bridged gadoliniumIII complex [Gd2(mal)3(H2O)6] (H2mal = malonic acid)

The novel gadolinium(III) complex of formula [Gd(2)(mal)(3)(H(2)O)(6)] (1) (H(2)mal = 1,3-propanedioic acid) has been prepared and characterized by X-ray diffraction analysis. Crystal data for 1: monoclinic, space group I2/a, a = 11.1064(10) A, b = 12.2524(10) A, c =13.6098(2) A, beta = 92.925(10) degrees, U = 1849.5(3) A(3), Z = 4. Compound 1 is a three-dimensional network made up of malonate-bridged gadolinium(III) ions where the malonate exhibits two bridging modes, eta(5)-bidentate + unidentate and eta(3):eta(3) + bis(unidentate). The gadolinium atom is nine-coordinate with three water molecules and six malonate oxygen atoms from three malonate ligands forming a distorted monocapped square antiprism. The shortest metal-metal separations are 4.2763(3) A [through the oxo-carboxylate bridge] and 6.541(3) A [through the carboxylate in the anti-syn coordination mode]. The value of the angle at the oxo-carboxylate atom is 116.8(2) degrees. Variable-temperature magnetic susceptibility measurements reveal the occurrence of a significant ferromagnetic interaction through the oxo-carboxylate pathway (J = +0.048(1) cm(-1), H = -JS(Gd(1)) x S(Gd(1a))).     

Higher rank coupling in magnetic clusters with spin = 1

The use of tensor operator techniques is demonstrated for coupling the angular moments of ions with spin = 1, in magnetic clusters. The importance of including second rank coupling as well as the usual first coupling, is emphasized. The symmetry of such coupled spins is explored with some extension to higher spins. The overall objective in this paper is to describe the application of the symmetry of coupled spin angular momentum and of tensor operator techniques to systems of coupled transition ions. Utilization of this symmetry and of tensor operator techniques leads to direct and efficient computer programming that would treat data from such systems. We have chosen trimeric clusters with spin-1 for illustrative discussion. Such systems demonstrate the important points without leading to the mass of obscuring detail that would accompany a discussion of still higher spin in larger clusters, and of course Ni(II) clusters are commonly encountered. One possible effect that is very readily treated in this way is that of higher-order coupling.     

Control of electronic and magnetic coupling via bridging ligand geometry in a bimetallic ytterbocene complex

The ligand 1-methyl-3,5-bis(2,2':6',2' '-terpyridin-4'-yl)benzene has been employed in the synthesis of a new bimetallic ytterbocene complex [(Cp*)2Yb](1-methyl-3,5-bis(2,2':6',2' '-terpyridin-4'-yl)benzene)[Yb(Cp*)2] (1) and the doubly oxidized congener [1]2+ in an attempt to determine the impact of the bridging ligand geometry on the magnetic/electronic properties as compared to the previously reported 1,4-analog [(Cp*)2Yb](1,4-di(terpyridyl)benzene)[Yb(Cp*)2] (2). Electrochemical, electronic, and magnetic data provide compelling evidence that the 1,3-geometry associated with the bridging ligand of 1 has done an effective job of inhibiting electronic communication between metal centers and magnetic coupling of spin carriers at room temperature as compared to 2. In fact, the physical data associated with 1 are quite similar to those reported for the monometallic analog (Cp*)2Yb(tpy) (3). In particular, the f-f profile of [1]2+ is nearly identical to that of [3]+ in its spectral features but with an almost exact doubling of the intensities. Further, the electronic coupling between metal centers as manifested in the potential separation between metal-based reduction waves has for the first time in these bimetallic ytterbocene complexes been found to go to zero for 1. Thus, the linkage isomerism at the phenyl coupling unit has induced a change in the ground-state electronic configuration from the singlet dianion-bridged (4f)13(pi*)2(4f)13 state found in 2 to the diradical-bridged (4f)13(piA*)1(piB*)1(4f)13 state in 1. This diradical formulation on the bridging ligand in 1 is supported by DFT calculations for the uncomplexed doubly reduced ligand that indicate the ground-state configuration is a singlet diradical state with the triplet-diradical state lying to slightly higher energy. Magnetic characterization of 1 is most consistent with the behavior previously observed for monometallic analogs such as 3, and there is no evidence of long-range magnetic ordering such as that observed for 2. In addition, X-ray crystallographic characterization of 1 represents the first case of a structurally characterized 2:1 metal-to-ligand adduct of the 1,3-bis(tpy) framework.     

Ferromagnetic exchange in a dinuclear copper(II) complex mediated by a methanolate bridging ligand

The copper(II) complex Cu(2)L(OMe)(H(2)O)(3), [middle dot]3H(2)O [H(3)L = 2-(2-hydroxyphenyl)-1,3-bis[4-(2-hydroxyphenyl)-3-azabut-3-enyl]-1,3-imidazolidine] was obtained and recrystallised in methanol to yield crystals of [[Cu(2)L(OMe)]](2).2.5MeOH.4H(2)O, 1.2.5MeOH.4H(2)O. Its single X-ray study shows that it contains two crystallographically different but chemically equivalent dinuclear [Cu(2)L(OMe)] 1 molecules in the asymmetric unit cell. The copper atoms of each dinuclear moiety are in distorted square-pyramidal environments, with both pyramids sharing an apical phenolate and a basal methanolate oxygen atom. Magnetic characterisation of 1.3H(2)O shows a quite strong intramolecular ferromagnetic coupling between both metal atoms. Extended Huckel calculations reveal that the intradinuclear magnetic interaction seems to be mediated by the exogenous methanolate bridging ligand.     

Synthesis of SrFe12O19 magnetic nanoparticles by EDTA complex method

A modified polyacrylamide gel route was used to prepare SrFe₁₂O₁₉ magnetic nanoparticles; ethylenediaminetetraacetic acid (EDTA) was used as a carboxyl chelating agent. The phase purity, morphology and magnetic properties of as-prepared samples were analyzed via X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and vibrating sample magnetometery (VSM). XRD analysis indicates that high-purity SrFe₁₂O₁₉ magnetic nanoparticles can be synthesized at 700°C in air. The characteristic peaks of as-prepared sample at 210, 283, 321, 340, 381, 411, 432, 475, 532, 618, 686, and 726 cm^–1 were observed in Raman spectra. SEM and TEM show that the synthesized SrFe₁₂O₁₉ magnetic nanoparticles are uniform with the mean particle size of ~60 nm. VSM measurement shows that the maximum magnetic energy product (BH)_max of sample prepared using EDTA as a chelating agent is higher than that of sample prepared using citric acid as a chelating agent.

     

Enantioconvergent photoredox radical-radical coupling catalyzed by a chiral-at-rhodium complex

Racemic α-chloro imidazol-2-yl-ketones undergo an enantioconvergent photoactivated C–C bond formation with N-aryl glycines catalyzed by a single bis-cyclometalated chiral-at-rhodium catalyst in yields of up to 80% and up to 98% enantiomeric excess(ee). Control experiments support a mechanism which is initiated by a single electron transfer from N-aryl glycinate to the photochemically excited rhodium-bound α-chloro imidazol-2-yl-ketone, followed by chloride fragmentation of the α-chloroketone, decarboxylation of the glycinate, and a subsequent highly stereocontrolled radical-radical coupling. This work showcases the ability of the chiral rhodium catalyst to serve a dual function as chiral Lewis acid and at the same time as the photoredox active species upon substrate binding.     

Intermolecular interaction and magnetic coupling mechanism of a mixed-valence CuIICuI tetranuclear complex with iodide bridge

A tetranuclear Cu^ICu^II mixed oxidation state complex, [Cu^II ₂(μ-I)₂Cu^I ₂(μ-I)₂(phenP)₂I₂] (phenPE: 2-(1H-pyrazol-1-yl)-1,10-phenanthroline), has been prepared and its crystal structure is determined by X-ray crystallography. In the complex, Cu^II is a distorted square pyramid and Cu^I is a distorted trigonal planar coordination environment; Cu^II and Cu^I are bridged by iodide. It is rare to form a Cu^II-iodide bond and for Cu^II and Cu^I to be bridged by iodide. In the crystal, there is a slipped π–π stacking between adjacent Cu^II complexes, which resulted in the formation of the 1-D chain along the c axis. The fitting for the variable-temperature magnetic susceptibility data gave magnetic coupling constant 2J?=??1.16?cm^?1 and it may be ascribed to the intermolecular π–π magnetic coupling pathway.     

Solvent deuterium kinetic isotope effects for the methanolyses of neutral C=O, P=O and P=S esters catalyzed by a triazacyclododecane : Zn(2+)-methoxide complex

The methanolyses of several organophosphate/phosphonate/phosphorothioate esters (O,O-diethyl O-(4-nitrophenyl) phosphate, paraoxon, ; O,O-diethyl S-(3,5-dichlorophenyl) phosphorothioate, ; O-ethyl O-(2-nitro-4-chlorophenyl) methylphosphonate, ; O,O-dimethyl O-(3-methyl-4-nitrophenyl) phosphorothioate, fenitrothion, ; O-ethyl S-(3,5-dichlorophenyl) methylphosphonothioate ) and a carboxylate ester (p-nitrophenyl acetate, ) catalyzed by methoxide and the Zn(2+)((-)OCH(3)) complex of 1,5,9-triazacyclododecane ( : Zn(2+)((-)OCH(3))) were studied in methanol and d(1)-methanol at 25 degrees C. In the case of the methoxide reactions inverse skie's were observed for the series with values ranging from 2 to 1.1, except for where the k(D)/k(H) = 0.90 +/- 0.02. The inverse k(D)/k(H) values are consistent with a direct nucleophilic methoxide attack involving desolvation of the nucleophile with varying extents of resolvation of the TS. With the : Zn(2+)((-)OCH(3)) complex all the skie values are k(D)/k(H) = 1.0 +/- 0.1 except for where the value is 0.79 +/- 0.06. Arguments are presented that the fractionation factors associated with complex : Zn(2+)((-)OCH(3)) are indistinguishable from unity. The skie's for all the complex-catalyzed methanolyses are interpreted as being consistent with an intramolecular nucleophilic attack of the Zn(2+)-coordinated methoxide within a pre-equilibrium metal : substrate complex.     

Crystal structures and magnetic and luminescent properties of a series of homodinuclear lanthanide complexes with 4-cyanobenzoic ligand

A series of homodinuclear lanthanide(III) complexes with the 4-cba ligand, [La2(4-cba)6(phen)2(H2O)6] (1) and [Ln2(4-cba)6(phen)2(H2O)2] (Ln = Pr (2), Nd (3), Sm (4), Eu (5), Gd (6), and Dy (7); 4-Hcba = 4-cyanobenzoic acid; phen = 1,10-phenanthroline), have been synthesized and structurally characterized by single-crystal X-ray diffraction. In 1, two water molecules bridge two nine-coordinated La ions, and six 4-cba ligands coordinate to the two La ions in terminal mode. In the isostructural complexes 2-7, two eight-coordinated Ln ions are connected by four bidentate 4-cba ligands, and another two 4-cba ligands terminate the two Ln ions. The variable-temperature magnetic properties of 2-7 have been investigated. Complex 7 shows a significant ferromagnetic interaction between Dy(III), while no magnetic interaction exists between Gd(III) ions in 6. In 2-5, the value of chi(M)T decreases with decreasing temperature, but the magnetic interactions between the Ln(III) ions cannot definitely be concluded. Notably, the spin-orbit coupling parameters, lambda, for Sm(III) (216(2) cm(-1)) and Eu(III) (404(2) cm(-1)) have been obtained in 4 and 5, respectively. The strong fluorescent emissions of 4, 5, and 7 demonstrate that ligand-to-Ln(III) energy transfer is efficient and that the coordinated water molecules do not quench their luminescence by the nonradiative dissipation of energy.