Structure and properties of a new double-stranded tetranuclear [Cu(II)2]2 helicate

A novel double-stranded tetranuclear helicate composed of a pair of [Cu(II)(2)] dimers has been prepared and characterized by exploiting the flexibility, chelating ability and bridging potential of a hexadentate bis-oximate ligand.     

Synthesis and characterization of Cu(I) and Cu(II) complexes containing ketiminate ligands

A series of Cu(I) and Cu(II) complexes containing substituted ketiminate ligands was synthesized. Reaction of CuCl₂ with 2 equiv. of Li[OC(Me)CHC(Me)N(Ar)] in toluene generated dark green solid of Cu[OC(Me)CHC(Me)N(Ar)]₂ (1). Similarly, Cu(I) complex, {Cu[OC(Me)CHC(Me)N(Ar)]Li[OC(Me)CHC(Me)N(Ar)]}₂ (2) was synthesized by reacting 2 equiv. of Li[OC(Me)CHC(Me)N(Ar)] with CuCl in toluene at room temperature for 12 h. While the reaction of CuCl with Li[OC(Me)CHC(Me)N(Ar)] in the presence of triphenylphosphine in THF solution at room temperature, a three-coordinated Cu[OC(Me)CHC(Me)N(Ar)](PPh₃) (3) can be isolated in high yield. Replacing the PPh₃ of 3 with N-heterocarbene (NHC) generates Cu[OC(Me)CHC(Me)N(Ar)](NHC) (4) in low yield. Complexes 2, 3, and 4 were characterized by ¹H and ¹³C NMR spectroscopies and all molecules were structurally characterized by X-ray diffractometry. Two coordination modes of ketiminate ligands were found in the molecular structure of 2, one of which is copper-coordinated terminal ketiminates and the other is lithium-copper-coordinated bridging ketiminates.     

Asymmetric transformation of a double-stranded, dicopper(I) helicate containing achiral bis(bidentate) Schiff bases

Reactions of the bis(bidentate) Schiff-bases N,N'-bis(6-alkyl-2-pyridylmethylene)ethane-1,2-diamine (where alkyl = H, Me, iPr) (L) with tetrakis(acetonitrile)copper(I) hexafluorophosphate and silver(I) hexafluorophosphate afforded, respectively, the double-stranded, dinuclear metal helicates [T-4-(R,R)]-(+/-)-[M2L2](PF6)2 (M = Cu, Ag). The helicates were characterized by 1H and 13C NMR spectroscopy, conductivity, microanalysis, and single-crystal X-ray structure determinations on selected compounds. Intermolecular ligand exchange and intramolecular inversion rates for the complexes were investigated by 1H NMR spectroscopy. Reversible intermolecular ligand exchange between two differently substituted helicates followed first-order kinetics. The rate constants (k) and corresponding half-lives (t(1/2)) for ligand exchange for the dicopper(I) helicates were k = (1.6-1.8) x 10(-6) s(-1) (t(1/2) = 110-120 h) in acetone-d6, k = 4.9 x 10(-6) s(-1) (t(1/2) = 40 h) in dichloromethane-d2, and k > 2 x 10(-3) s(-1) (t(1/2) < 5 min) in acetonitrile-d3. Ligand exchange for the disilver(I) helicates occurred with k > 2 x 10(-3) s(-1) (t(1/2) < 5 min). Racemization of the dicopper(I) helicate by an intramolecular mechanism was investigated by determination of the coalescence temperature for the diastereotopic isopropyl-Me groups in the appropriate complex, and DeltaG() > 76 kJ mol(-1) was calculated for the process in acetone-d6, nitromethane-d3, and dichloromethane-d2 with DeltaG() = 75 kJ mol(-1) in acetonitrile-d3. Complete anion exchange of the hexafluorophosphate salt of a dicopper(I) helicate with the enantiomerically pure Delta-(-)-tris(catecholato)arsenate(V) ([As(cat)3]-) in the presence of Dabco gave the two diastereomers (R,R)-[Cu2L2][Delta-(-)-[As(cat)3]]2 and (S,S)-[Cu2L2][Delta-(-)-[As(cat)3]]2 in up to 54% diastereomeric excess, as determined by (1)H NMR spectroscopy. The diastereomerically and enantiomerically pure salt (R,R)-[Cu(2)L2][Delta-(-)-[As(cat)3]]2 crystallized from the solution in a typical second-order asymmetric transformation. The asymmetric transformation of the dicopper(I) helicate is the first synthesis of a diastereomerically and enantiomerically pure dicopper(I) helicate containing achiral ligands.     

X-ray-excited optical luminescence (XEOL) and X-ray absorption fine structures (XAFS) studies of gold(I) complexes with diphosphine and bipyridine ligands

Synchrotron techniques, X-ray-excited optical luminescence (XEOL) combined with X-ray absorption fine structures (XAFS), have been used to study the electronic structure and optical properties of a series of luminescent gold(I) complexes with diphosphine and bipyridine ligands using tunable X-rays (in the regions of the C and P K-edges and the Au L3-edge) and UV from synchrotron light sources. The effects of gold-ligand and aurophilic interactions on the luminescence from these gold(I) complexes have been investigated. It is found that the luminescence from these complexes is phosphorescence, primarily due to the decay of the Au (5d) --> PR3 (pi*), metal to ligand charge transfer (MLCT) excitation as well as contributions from the conjugated pi-system in the bipyridine ligands via the gold-nitrogen bond. The large Au 5d spin-orbit coupling enhances the intersystem crossing. The elongation of the hydrocarbon chain of the diphosphine ligand does not greatly affect the spectral features of the luminescence from the gold(I) complexes. However, the intensity of the luminescence was reduced significantly when the bipyridine ligand was replaced with 1,2-bis(4-pyridylamido)benzene. The aurophilic interaction, as investigated by EXAFS at the Au L3-edge, is shown to be only one of the factors that contribute to the luminescence of the complexes.     

Complexes of Cu(I) supported by a tris(ketimine) tripod

Arylation of tris(2-benzylnitrile)amine with PhLi, followed by aqueous work-up, results in the formation of a tripodal tris(ketimine) scaffold, N(ArCNHPh)(3). N(ArCNHPh)(3) readily coordinates a number of Cu(I) salts, generating complexes that exhibit trigonal pyramidal geometries in the solid-state.     

Synthesis, characterization and some properties of amide-linked porphyrin-ruthenium(II) tris(bipyridine) complexes

A new molecular dyad, comprised of a zinc-porphyrin and a ruthenium(II) tris(bipyridine) complex linked through an amide bond has been synthesized and characterized by ¹H, ¹³C NMR, UV-vis, mass-spectrometry and elemental analysis. The electrochemistry as well as the steady-state emission properties were investigated. The redox behavior of the dyad exhibits a favorable reversible characteristic. Substantial quenching of porphyrin emission was found when the Q band of 5 and 5-Zn was selectively photoexcited. This observation suggests a quenching mechanism with possible intramolecular electron transfer or energy transfer between the Ru(bpy)₃ moiety and the porphyrin free-base or Zn porphyrin moieties.     

Modular bipyridine ligands coupled with Cinchona alkaloids for Cu(II)-catalyzed asymmetric Henry reactions

A new series of bipyridine ligands coupled with Cinchona alkaloids were prepared in relatively high yields in only one step starting from commercially or readily available materials, and have been developed as chiral ligands for copper(II)-catalyzed asymmetric Henry reactions. The catalytic system works well with a wide range of aromatic, aliphatic, and heteroaromatic aldehydes to afford the corresponding nitroalkanols with high enantioselectivity (up to 92%) in good yields (up to 89%). A transition state model was proposed based on ¹H NMR spectroscopy, X-ray structure, and computational calculations.     

A novel ruthenium(II) tris(bipyridine)-zinc porphyrin-rhenium carbonyl triad: synthesis and optical properties

A novel ruthenium(II) tris(bipyridine)-zinc porphyrin-rhenium carbonyl triads and its free base porphyrin derivative were synthesized and characterized by ¹H, ¹³C NMR, UV-vis, mass-spectrometry and elemental analysis. The redox potentials of the two compounds were measured and compared to their corresponding reference complexes. The fluorescence and transient absorption spectra of the two complexes revealed the features of two different pathways for possible photoinduced intramolecular electron transfer or energy transfer in the triads.     

Theoretical spectroscopic study of europium tris(bipyridine) cryptates

A series of europium cryptates are studied, using semiempirical methods to predict electronic and spectroscopic properties. The results are compared with theoretical (DFT) and experimental results published by Guillaumont and co-workers (ChemPhysChem2007, 8, 480). Triplet energies calculated by semiempirical methods have errors similar to those obtained by TD-DFT methodology but hundreds of times faster. Moreover, the semiempirical results not only reproduce well the experimental values but also help explain the low values of quantum efficiency observed for these complexes.     

Metal complexes with polymeric ligands: Chelation and metalloinitiation approaches to metal tris(bipyridine)‐containing materials

Synthetic approaches to metal complexes with polymeric ligands are described. The development of efficient methods for preparing simple bipyridine (bpy) derivatives and their corresponding metal complexes has facilitated their use as initiators and coupling agents in polymer syntheses. Ligand reagents were utilized as initiators in controlled polymerization reactions to form poly(2‐R‐2‐oxazolines) (R = methyl, ethyl, phenyl, undecyl), polystyrenes, poly(methyl methacrylates) (PMMA)s, poly(ϵ‐caprolactone)s, and poly(lactic acid)s with bipyridine chelates at the end or centers of the chains. Poly(ethylene glycol) macroligands were formed by a chain‐coupling method. Detailed studies of reaction kinetics were performed to determine the scope and limitations of each reaction type with different catalysts and reaction conditions. These results are illustrated for bpyPMMA_n (n = 1 or 2), which was prepared by atom transfer radical polymerization with a CuBr/1,4,4,7,7,10‐hexamethyltriethylenetetraamine catalyst system. Results of the kinetics investigations performed with other ligands and metalloinitiators are summarized. Macroligands thus prepared were coordinated to a labile metal ion, Fe(II), with standard protocols. Ultraviolet–visible spectral data for selected Fe‐centered polymers are provided that confirm the production of the targeted polymeric iron complex products. An inert metal, Ru(II), was used as a template for generating architectural diversity; polymeric complexes with one to six chains emanating from the central core, as well as different heteroarm star products, were prepared. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 4704–4716, 2000     

Synthesis and characterization of a neutral titanium tris(iminosemiquinone) complex featuring redox-active ligands

The neutral tris(semiquinonate) complex [Ti(dmp-BIAN(isq))(3)] [dmp-BIAN(isq) = N,N'-bis(3,5-dimethylphenylimino)acenaphthenesemiquinonate] was structurally, spectroscopically, and electrochemically characterized. Solid-state magnetism experiments reveal field-quenchable, enhanced temperature-independent paramagnetism (TIP). Density functional theory calculations employing the experimental geometry predicts a strong antiferromagnetic coupling, leading to an S = 0 ground state, but they also hint at spin frustration and concomitant close-lying, excited states, which cause the observed large TIP by admixture into the ground state. The dmp-BIAN(isq) ligand, which facilitates intramolecular electron transfer, was shown to undergo four quasi-reversible redox processes, demonstrating the ability of the ligand to act as an electron reservoir in complexes of early metals.     

The optical resolution of tris(pentane-2,4-dionato)metal(III) complexes

A facile large-scale optical resolution of neutral [M(pd)₃] complexes, M = Cr(III), Co(III), Ru(III), Rh(III) and Ir(III), through enantioselective complex formation with (2R, 3R)-(-)-dibenzoyltartaric acid, is described.     

Synthesis of sterically hindered tris(4-imidazolyl)carbinol ligands and their copper(I) complexes related to metalloenzymes

Tris(4-imidazolyl)carbinol, which has close coordination environment to the active site of metalloenzymes, has not been utilized as a biomimetic ligand because of its instability. We have synthesized stable tris(4-imidazolyl)carbinol derivatives having a methyl group as the NH protective group and a bulky substituent on the imidazole ring for stabilizing reactive species bound to the metal center. These ligands provide stable monomeric copper(I) complexes whose coordination environment are very close to the active site of metalloenzymes.     

Copper(I) complexes of tripodal tris(imidazolyl) ligands: potential mimics of the Cu(A) site of hydroxylase enzymes

Copper(I) complexes of tripodal tris(N-methyl-4,5-diphenyl-imidazolyl)methane ligands, N3CR (1a-c, R = OH, OMe, H), have been prepared as models for the Cu(A) site of copper hydroxylase enzymes. In the absence of additional donors, the ligands 1 react with [Cu(CH3CN)4]PF6 (2) to produce dinuclear complexes [(N3CR)2Cu2](PF6)2 (3) in which the tripodal ligands bridge two trigonal Cu centers; the structures of 3b and 3c are established by X-ray diffraction. Mononuclear adducts [(N3CR)CuL]Z are produced with L = acetonitrile (4), carbon monoxide (5), and t-BuNC (6, 7). The carbonyl complexes 5 are in dynamic equilibrium with the dimeric complexes 3, but 5c (R = H) can be isolated. The structures of the isocyanide derivatives depend critically on the tripod methane substituent, R. Thus, the X-ray structures of 6 (R = OMe) and 7 (R = H) show trigonal and tetrahedral geometries, respectively, with bi- or tridentate coordination of the tripod. A trinuclear complex [Cu3(N3COH)2(t-BuNC)2](PF6)3 (8) is formed from N3COH (1a) which features both three-coordinate and two-coordinate Cu atoms and bidentate tripod coordination. Reactions of dioxygen with dinuclear 3c or mononuclear [(N3CR)CuL]Z are sluggish, producing from the latter in acetone [(N3CH)CuII(L)(L')](PF6)2 (9, L = acetone, L' = H2O).     

Highly coupled dyads based on phthalocyanine-ruthenium(II) tris(bipyridine) complexes. Synthesis and photoinduced processes

A new series of multicomponent ZnPc-Ru(bpy)(3) systems, 1a-c, consisting of a zinc-phthalocyanine linked through conjugated and/or nonconjugated connections to a ruthenium(II) tris(bipyridine) complex, has been synthesized. The ruthenium complexes 1a-c were prepared from phthalocyanines 2a-c, bearing a 4-substituted-2,2'-bipyridine ligand by treatment with [Ru(bpy)2Cl2].2H2O. Different synthetic strategies have been devised to prepare the corresponding dyad precursors (2a-c). Compound 2a, for example, with an ethenyl bridge, was synthesized by statistical condensation of 4-tert-butylphthalonitrile and 5-[(E)-2-(3,4-dicyanophenyl)ethenyl]-2,2'-bipyridine (3) in the presence of zinc chloride. Compounds 2b and 2c, having, respectively, an amide or an ethynyl bridge, were prepared following a different synthetic approach. The method involves the coupling of an appropriate 5-substituted-2,2'-bipyridine to an unsymmetrical phthalocyanine suitably functionalized with an amino (4) or an ethynyl group (5). The photophysical properties of the dyads that are ZnPc-Ru(bpy)3 1a-c and related model compounds have been determined by a variety of steady-state (i.e., fluorescence) and time-resolved methods (i.e., fluorescence and transient absorption). Clearly, intramolecular electronic interactions between the two subunits dominate the photophysical events following the initial excitation of either chromophore. These intramolecular interactions lead, in the case of photoexcited ZnPc, to faster intersystem crossing kinetics compared to a ZnPc reference, while photoexcited [Ru(bpy)3]2+) undergoes a rapid and efficient transduction of triplet excited-state energy to the Pc.     

Synthesis, photophysical and nonlinear optical properties of macromolecular architectures featuring octupolar tris(bipyridine) ruthenium(II) moieties: evidence for a supramolecular self-ordering in a dentritic structure

The synthesis, photophysical and nonlinear optical properties of several new multi-octupolar tris(bipyridine) ruthenium complexes are reported. The preparation on these complexes is based on the initial construction of multipodal 4,4'-dialkylaminostyryl-2,2'-bipyridine ligands (DAAS-bpy). Thermally stable polyimides featuring octupolar ruthenium trisbipyridyl complexes have been readily obtained by a polycondensation reaction. The controlled coordination strategy of dipodal and tripodal bipyridines to ruthenium(II) has also been successfully used to build bimetallic, trimetallic as well as the first metallodendrimer made of seven metallo-octupoles. These polymetallic species exhibit very intense absorption bands in the visible and long-lived luminescence. The quadratic NLO-susceptibilities beta of these macromolecules have been characterized by harmonic light scattering at 1.91 microm and compared with those of the corresponding monometallic species. The NLO studies clearly demonstrates a quasi-supramolecular ordering in the metallodendrimer.