Synthesis,spectroscopic and magnetic properties of copper complexes with a ligand containing the pyridoxal moiety

Summary Seven new copper(II)pyridoxal salicyloylhydrazone complexes have been prepared and characterized by vibrational and electronic spectra and magnetic measurements. The u.v. absorption band maxima are compared with those of metal chelates of Schiff bases obtained from condensation of pyridoxal with amines or amino acids.     

Synthesis,structure, redox property and ligand replacement reaction of ruthenium(II) complexes containing a terpyridyl ligand with a redox active moiety

Ruthenium(II) complexes bearing a redox-active tridentate ligand 4′-(2,5-dimethoxyphenyl)-2,2′:6′,2′′-terpyridine (tpyOMe), analogous to terpyridine, and 2,2′-bipyridine (bpy) were synthesized by the sequential replacement of Cl by CH₃CN and CO on the complex. The new ruthenium complexes were characterized by various methods including IR and NMR. The molecular structures of [Ru(tpyOMe)(bpy)(CH₃CN)]²⁺ and two kinds of [Ru(tpyOMe)(bpy)(CO)]²⁺ were determined by X-ray crystallography. The incorporation of monodentate ligands (Cl, CH₃CN and CO) regulated the energy levels of the MLCT transitions and the metal-centered redox potentials of the complexes. The kinetic data observed in this study indicates that the ligand replacement reaction of [Ru(tpyOMe)(bpy)Cl]⁺ to [Ru(tpyOMe)(bpy)(CH₃CN)]²⁺ proceeds by a solvent-assisted dissociation process.     

Synthesis,spectroscopic and redox properties of nickel(II) salicylaldimine complexes containing sterically hindered phenols

The preparation and spectroscopic characterization of a series of new bis[N-(2,6-di-t-butyl-1-hydroxyphenyl)salicylaldiminato]nickel(II) complexes, [Ni(L^X)₂], bearing one or two OH and MeO substituents on the salicylaldehyde moiety, as well as radical species generated from these compounds by the oxidation with PbO₂, are reported. The [Ni(L^X)₂] chelates, which appear to be tetrahedral in the solid state and in dioxane solution, are converted into a square-planar configuration in non-donor solvents. The OH-substituted complexes, unlike their MeO analogues, form six-coordinate adducts in pyridine, DMF and DMSO. These new compounds, unlike their analogues with electron-withdrawing substituents (Cl, Br, NO₂), are easily oxidized by PbO₂ to produce Ni^II-stabilized phenoxy radicals in which the unpaired electrons are delocalized over the ligand and do not couple with the second radical center. No e.s.r. signals were observed that could be attributed to a M = ±2 transition of the triplet state biradicals.     

Synthesis, structure, and redox and catalytic properties of a new family of ruthenium complexes containing the tridentate bpea ligand

We have prepared a new family of ruthenium complexes containing the bpea ligand (where bpea stands for N,N-bis(2-pyridyl)ethylamine), with general formula [Ru(bpea)(bpy)(X)](n+) (2, X = Cl(-); 3, X = H(2)O; 4, X = OH(-)), and the trisaqua complex [Ru(bpea)(H2O)(3)](2+), 6. The complexes have been characterized through elemental analyses, UV-vis and (1)H NMR spectroscopy, and electrochemical studies. For complex 3, the X-ray diffraction structure has also been solved. The compound belongs to the monoclinic P2(1)/m space group, with Z = 2, a = 7.9298(6) A, b = 18.0226(19) A, c = 10.6911(8) A, and beta = 107.549(8) degrees. The Ru metal center has a distorted octahedral geometry, with the O atom of the aquo ligand placed in a trans position with regard to the aliphatic N atom of the bpea ligand so that the molecule possesses a symmetry plane. NMR spectra show that the complex maintains its structure in aqueous solution, and that the corresponding chloro complex also has a similar structural arrangement. The pH dependence of the redox potential for the complex [Ru(bpea)(bpy)(H2O)](PF(6))(2) is reported, as well as the ability of the corresponding oxo complex to catalyze the oxidation of benzylic alcohol to benzaldehyde in both chemical and electrochemical manners.     

Structural and spectroscopic characterization of tetranuclear iron complexes containing a bridge

A pair of tetranuclear iron complexes consisting of two Fe₂(Cl₂bdt)(CO)₅ subunits (Cl₂bdt?=?3,6-dicholorobenzene-1,2-dithiolate) bridged by different cyclic 1,5-diaza-3,7-diphosphacyclooctane (P₂N₂) ligands were prepared and structurally characterized. In the solid state, the P₂N₂ ligands adopt a boat conformation, which results in rather short distances between the two Fe₂(Cl₂bdt)(CO)₅ clusters that promotes electronic communication across the diphosphine ligand.     

Synthesis, spectroscopic and structural properties of uranyl complexes based on bipyridine N-oxide ligands

By using 2,2′-bipyridine N-oxide (bipyO) and 2,2′-bipyridine N,N′-dioxide (bipyO₂), three new uranyl complexes [UO₂(bipyO)SO₄]·H₂O (1), [UO₂(bipyO)(OH)(NO₃)]₂·H₂O (2) and [UO₂(bipyO₂)H₂O](ClO₄)₂·(3) were synthesized using uranyl salts including non-coordinating or weakly coordinating power of the ClO₄⁻ anion and the strongly coordinating power of NO₃⁻ and SO₄²⁻ anions. All of the compounds were characterized by CHN microanalytical procedures, infrared and luminescence spectroscopy and by single crystal X-ray diffraction. Spectroscopic studies indicate that the bipyO is bound to the uranyl group via the nitrogen and oxygen atoms. Structural analyses revealed that overall bonding pattern is different in each case: 1 is a polymer; in 2 dimeric complex molecules are formed, whereas 3 is composed of monomers. In all of the complexes, the uranium atom is in a seven-coordinate environment.     

Synthesis and spectroscopic properties of four metal complexes with bipyridinium-based ligand

A series of new transitional metal charge-transfer (CT) complexes with a bipyridinium-based ligand [M(PrBpy)₂(SCN)₄] (M = Cd(II), Ni(II), Co(II)) and [Cu(PrBpy)(SCN)₃(H₂O)] (PrBpyCl = N-(2-pyrazinyl)-4,4′-bipyridinium chloride) have been obtained and determined by X-ray diffraction, FT-IR and elemental analysis. The CT absorption bands of the four compounds were observed by diffuse reflection spectroscopy and a photo-induced electron transfer occurs in methanol solution upon light irradiation.     

Predicting the redox properties of uranyl complexes using electronic structure calculations

A plethora of chemical reactions is redox driven processes. The conversion of toxic and highly soluble U(VI) complexes to nontoxic and insoluble U(IV) form are carried out through proton coupled electron transfer by iron containing cytochromes and mineral surfaces such as machinawite. This redox process takes place through the formation of U(V) species which is unstable and immediately undergo the disproportionation reaction. Thus, theoretical methods are extremely useful to understand the reduction process of U(VI) to U(V) species. We here have carried out the structures and reduction properties of several U(VI) to U(V) complexes using a variety of electronic structure methods. Due to the lack of experimental ionization energies for uranyl (UO₂(V)‐UO₂(VI)) couple, we have benchmarked the current and popularly used density functionals and cost effective ab initio methods against the experimental electron detachment energies of [UO₂F₄]^1‐/2‐ and [UO₂Cl₄]^1‐/2‐. We find that electron detachment energy of U(VI) predicted by RI‐MP2 level on the BP86 geometries correlate nicely with the experimental and CCSD(T) data. Based on our benchmark studies, we have predicted the structures and electron detachment energies of U(V) to U(VI) species for a series of uranium complexes at the RI‐MP2//BP86 level which are experimentally inaccessible till date. We find that the redox active molecular orbital is ligand centered for the oxidation of U(VI) species, where it is metal centered (primarily f‐orbital) for the oxidation of U(V) species. Finally, we have also calculated the detachment energies of a known uranyl [UO₂]¹⁺ complex whose X‐ray crystal structures of both oxidation states are available. The large bulky nature of the ligand stabilizing the uncommon U(V) species which cannot be routinely studied by present day CCSD(T) methods as the system size are more than 20–30 atoms. The success of our efficient computational strategy can be experimentally verified in the near future for the complex as the structures are stable in gas phase which can undergo oxidation.     

Structural diversity in metal complexes with a dinucleating ligand containing carboxyamidopyridyl groups

The synthesis of a (carboxyamido)pyridinepyrazolate (H(5)bppap) dinucleating ligand is described. Bimetallic iron and cobalt complexes of H(5)bppap ([M(II)(2)H(2)bppap](+)) showed structural differences in both their primary and secondary coordination spheres. The binding of small molecules into the preorganized ligand cavity is verified by the hydration of [Fe(II)(2)H(2)bppap](+) and [Co(II)(2)H(2)bppap](+), leading to the formation of complexes [{Co(II)(OH)}Co(II)H(3)bppap](+) and [{Fe(II)(OH)}Fe(II)H(3)bppap](+), in which one of the metal centers has a terminal hydroxo ligand.     

Optical and redox properties of ruthenium phthalocyanine complexes tuned with axial ligand substituents

The optical and electrochemical properties of the ruthenium phthalocyanine complexes [[(t-Bu)4Pc]Ru(4-Rpy)2], where R = NO2, Me, NH2, and NMe2, are reported. The electron density at the macrocycle may be adjusted using the axial ligand substituents, which have varying electron-donating/withdrawing strengths. Electrochemical data show that the axial pyridine ligand substituents exert significant influence over the phthalocyanine ring-based redox processes. The axial ligands also influence the electronic absorption properties of the complexes with influence also being observed in the electrogenerated oxidized and reduced species.     

Ruthenium-terpyridine complexes with multiple ethynylpyrenyl or ethynyltoluyl subunits: X-ray structure, redox, and spectroscopic properties

Several heteroleptic and homoleptic ruthenium-terpyridine complexes bearing two and four ethynylpyrenyl or ethynyltoluyl residues have been prepared from complexes carrying reactive bromo functions. Cross-coupling promoted by low-valent palladium(0) on these preformed complexes has advantageously been used to prepare the target complexes. The structure of a bis-terpyridine complex carrying four ethynylpyrenyl subunits was determined by single-crystal X-ray diffraction, showing a distorted octahedral geometry around the metal center, with the ethynylpyrenyl fragment being slightly tilted (about 5 degrees) from the terpyridine plane. The molecular packing is characterized by intermolecular pi...pi interaction within dimers. The counteranions and the solvent molecules are entrapped in well-defined channels spanning along the a-axis. The complexes are redox active with a Ru oxidation overlapping the pyrene oxidation and two well-defined ligand-centered reduction processes. Pyrene reduction is irreversible and strongly cathodic. The new multichromophoric complexes are luminescent both in solution and in rigid matrix at 77 K, with room-temperature lifetimes and quantum yields significantly larger than those of [Ru(terpy)2]2+. At room temperature, the toluyl-substituted complexes are triplet metal-to-ligand charge-transfer (3MLCT) emitters, whereas for the pyrene-grafted complexes pyrene-centered emission is observed. For the latter complexes, the energy gap, DeltaTT, between higher 3MLCT levels and lower ligand-centered (3pipi*, 3LC) levels is in the 640-730 cm(-1) range, which results in the interstate dynamics at the basis of the observed luminescent behavior. At 77 K, for the pyrene-grafted complexes, the emission reveals features that are tentatively ascribed to intraligand interactions involving the pyrene and terpyridine units.     

IR spectroscopic studies of some uranyl tetrachloride complexes

The IR spectra of dipotassium, dirubidium and diammonium uranyl tetrachloride hydrates (K₂UO₂Cl₄·3H₂O, Rb₂UO₂Cl₄·H₂O, (NH₄)₂UO₂Cl₄·4H₂O) have been measured in the region from 4000 down to 30 cm⁻¹. A normal coordinate analysis of the (UO₂Cl₄)²⁻ ion apart from the K⁺, Rb⁺ and NH₄⁺ ions has been made using Wilson's FG matrix method. The force constants associated with the UO and UCl bonds have been obtained on the basis of both the force models of modified Urey-Bradley and valence force fields. An effort has been directed towards the understanding of the uranyl bond weakening with coordination of the ligands. It has been suggested that this bond weakening is caused by charge transfer from the ligands to the uranium atom.     

Structural,spectroscopic, and redox consequences of a central ligand in the FeMoco of nitrogenase: a density functional theoretical study

Broken symmetry density functional and electrostatics calculations have been used to shed light on which of three proposed atoms, C, N, or O, is most likely to be present in the center of the FeMoco, the active site of nitrogenase. At the Mo(4+)4Fe(2+)3Fe(3+) oxidation level, a central N(3-) anion results in (1) calculated Fe-N bond distances that are in very good agreement with the recent high-resolution X-ray data of Einsle et al.; (2) a calculated redox potential of 0.19 eV versus the standard hydrogen electrode (SHE) for FeMoco(oxidized) + e(-) --> FeMoco(resting), in good agreement with the measured value of -0.042 V in Azotobacter vinelandii; and (3) average M?ssbauer isomer shift values (IS(av) = 0.48 mm s(-1)) compatible with experiment (IS(av) = 0.40 mm s(-1)). At the more reduced Mo(4+)6Fe(2+)1Fe(3+) level, the calculated geometry around a central N(3-) anion still correlates well with the X-ray data, but the average M?ssbauer isomer shift value (IS(av) = 0.54 mm s(-1)) and the redox potential of -2.21 eV show a much poorer agreement with experiment. These calculated structural, spectroscopic, and redox data indicate the most likely iron oxidation state for the resting FeMoco of nitrogenase to be 4Fe(2+)3Fe(3+). At this favored oxidation state, oxygen or carbon coordination leads to (1) Fe-O distances in poor agreement and Fe-C distances in good agreement with experiment and (2) calculated redox potentials of +0.97 eV for O(2-) and -1.31 eV for C(4-). The calculated structural parameters and/or redox data suggest either O(2-) or C(4-) is unlikely as a central anion.     

Zirconium complexes containing a diarylamido diphosphine ligand: Structural preferences controlled by ligand electronics and sterics

This work describes the preparation of [PNP]ZrX₃ ([PNP]⁻ = [N(o-C₆H₄PⁱPr₂)₂]⁻; X = Cl, Me, CH₂SiMe₃) whose structural preference is found to be a function of the electronic and steric nature of the monodentate ligand X⁻. The reaction of ZrCl₄(THF)₂ with [PNP]Li in toluene at room temperature generates [PNP]ZrCl₃ as a red solid in 60% yield. Alkylation of [PNP]ZrCl₃ with three equivalents of Grignard reagents in diethyl ether at −35 °C produces cleanly [PNP]ZrR₃ (R = Me, CH₂SiMe₃) as yellow crystalline materials. An X-ray diffraction study of [PNP]ZrCl₃ showed it to be a chloride-bridged binuclear species {[PNP]ZrCl₂(μ−Cl)}₂ in which both zirconium atoms are 7-coordinate whereas that of [PNP]ZrMe₃ revealed a mononuclear, 6-coordinate core structure. Interestingly, with the incorporation of more sterically demanding alkyls, [PNP]Zr(CH₂SiMe₃)₃ is a 5-coordinate compound wherein the amido phosphine ligand is κ²-N,P bound to zirconium. The solution structures of these molecules were also assessed by variable-temperature NMR spectroscopy.     

Synthesis and spectroscopic properties of lanthanide nitrate complexes with a new amide-based quinoxaline-2,3-dione ligand

Solid complexes of lanthanide nitrate with 1,4-di(N,N-di-n-butyl-acetamido)-quinoxaline-2,3-dione (L), [Ln(N03)3L.H2O] (Ln=La, Nd, Eu, Gd, Tb, Er), have been prepared and characterized by elemental analysis, IR, UV-vis spectra and conductivity measurements. The fluorescence property of the europium complex in solid state and in MeCN, acetone, AcOEt and THF was studied. Under the excitation, the europium complex exhibited characteristic emissions of europium. The result indicates that the triplet state energy level of the ligand matches better to the resonance level of Eu(III) than Tb(III) ion.     

Synthesis and properties of a redox active starburst ligand with three bispicorylamino groups and its trinuclear complexes

A starburst-shaped ligand, 4,4′4″-tris[N,N-bis(2-pyridylmethyl) aminomethyl] triphenylamine, and its palladium and copper trinuclear complexes were designed and prepared. NMR techniques, COSY and ROESY, were applied to the palladium complex to examine its conformation in solution. The palladium complex was found to prefer a folded conformation even at 75 °C, indicating the occurrence of strong intramolecular stacking interaction. CV measurements of the palladium complex showed reversible TTA/TTA⁺ redox couples. ETSF measurements showed that the corresponding radical pendant complex is very unstable. Molecular design rules for triarylamine-based spin bearing ligands are discussed.     

Synthesis,structure and properties of stereochemically non-rigid molybdenum pyrazolylborato complexes containing a dihapto-thiocarboxamido ligand

Pyrazolylboratomolybdenum complexes containing the η²-CSNMe₂ ligand have been prepared by treating the appropriate carbonylmetallate anion with Me₂NCSCl. The structure of pzB(pz)₃Mo(CO)₂(η²-CSNMe₂) (IIIb) has been established by X-ray crystallographic methods. The ¹H and ¹³C NMR spectra of IIIb show evidence for two separate intramolecular dynamic processes in solution. Complex IIIb can be alkylated at the sulphur atom and forms 1/1 complexes with mercuric halides.     

Synthesis, structures and spectroscopic properties of platinum complexes containing orthometalated 2-phenylpyridine

The synthesis, structure and spectroscopy of a series of luminescent orthometalated square planar platinum(II) complexes are reported. Reaction of K₂PtCl₄ with one mole equivalent of 2-phenylpyridine (ppyH) in 2-ethoxyethanol and water (1:1 ratio) resulted in the formation of chloro-bridged dimeric precursor [Pt₂(μ-Cl)₂(ppy)₂], which on further reactions with various anionic one-, two- and three-atom ancillary ligands, having O/N/S donors, yielded mono- and bi-nuclear platinum(II) complexes. Platinum(III) complexes of composition [Pt₂Cl₂(μ-Epy)₂(ppy)₂] have been isolated with pyE⁻ (E = O or S) ligands. These complexes have been characterized by elemental analysis, NMR (¹H, ³¹P, ¹⁹⁵Pt) and absorption spectroscopy. The complexes [Pt₂(μ-N^∩N)₂(ppy)₂] (N^∩N = pyrazole and 3,5-dimethylpyrazole); [Pt(S^∩S)(ppy)] (S^∩S = ethylxanthate and diisopropyldithiophosphate); [Pt₂Cl₂(μ-Epy)₂(ppy)₂] (Epy = 2-pyridinol {Opy} and 2-mercaptopyridine {Spy}) and [PtCl(ppy)(PhNC(Me)NHPh)] have been structurally characterized by X-ray crystallography.