A density functional study of S(N)2 substitution at square-planar platinum(II) complexes

The energetics and reaction path in a series of S(N)2 substitution reactions at square-planar Pt(II) complexes have been studied by the application of density functional theory (DFT). Calculated free energies show excellent correlation with their experimental counterparts, while the enthalpic and entropic contributions individually indicate the presence of weak intermolecular interactions not accounted for in the present model. The nature of the leaving ligand has been shown to be much more significant in determining the activation barrier than that of the entering ligand; it is inferred (and confirmed by analysis of individual bond energies) that the reaction is driven by the dissociation of the leaving ligand, with the entering ligand playing a more passive role. Analysis of the intrinsic reaction coordinate indicates, further, that the trans ligand plays an unexpectedly dynamic role in stabilizing the transition state due to competition between stabilization and the steric effects of the entering and leaving ligands. The cis ligands, by contrast, are shown to move only slightly through the course of the reaction.     

Iodo-methyl ligand exchange reaction in platinum complexes: A density functional study

A theoretical investigation at the gradient-corrected density functional (BP86) level of theory on the iodo-methyl ligand exchange reaction in platinum-diphosphine complexes is discussed. The reaction consists of two elementary steps: the oxidative addition of methyl-iodide, and reductive elimination of ethane from the intermediate Pt(bdpp)(CH₃)₃I complex which is the rate determining step with a free energy of activation of 19.5 kcal/mol in acetonitrile phase. The oxidative addition step takes place with S_N2 mechanism via a transition state with a collinear arrangement of the I-CH₃-Pt moiety.     

A density functional theory study of uranium(VI) nitrate monoamide complexes

Density functional theory calculations were performed on uranyl complexed with nitrate and monoamide ligands (L) [UO(2)(NO(3))(2)·2L]. The obtained results show that the complex stability is mainly governed by two factors: (i) the maximization of the polarizability of the coordinating ligand and (ii) the minimization of the steric hindrance effects. Furthermore, the electrostatic interaction between ligands and uranium(vi) was found to be a crucial parameter for the complex stability. These results pave the way to the definition of (quantitative) property/structure relationships for the in silico screening of monoamide ligands with improved extraction efficiency of uranium(vi) in nitrate acidic solution.     

Supramolecular cobalt(II) complexes: syntheses,crystal structures and solvent effects

Two Co^II complexes, namely {[CoL(MeOH)(μ-OAc)]₂Co}·2MeCN·2MeOH (1) and {[CoL(EtOH)(μ-OAc)]₂Co}·3EtOH (2) (H₂L=3,3′-dimethoxy-2,2′-[(1,3-propylene)dioxybis(nitrilomethylidyne)]diphenol), have been synthesized and characterized by X-ray crystallography. Both complexes contain octahedral coordination geometries, comprising three Co^II atoms, two deprotonated bisoxime L²⁻ units in which four μ-phenoxo oxygen atoms form two [CoL(X)] (X = MeOH or EtOH) units, two acetate ligands coordinated to three Co^II centers through Co–O–C–O–Co bridges, and coordinated and non-coordinated solvent. Both complexes exhibit 2D supramolecular networks through different intermolecular hydrogen-bonding interactions.     

Contrasting solvent and capping ligand effects directing the photochemistry of uranyl(VI) Schiff base complexes

Photolysis of the uranyl(VI) Schiff base complex UO2(tBu4-salphen)(THF) (1a) with cobaltocene in THF affords [Cp2Co][UO2(tBu4-salphen)(OH)] (2) in high yield while irradiation in toluene yields no reaction. Electronic emission spectra of 1a reveal a large Stokes' shift in toluene similar to that observed in the free ligand, while in THF the structural rearrangement responsible for this shift is blocked. Instead, the ligand-centered excited state is redirected to the uranyl(VI) center by way of energy transfer, thus generating 2 from the intramolecular activation of a coordinated THF molecule.     

Gold(I) and platinum(II) complexes with a new diphosphine ligand based on the cyclotriphosphazene platform

A new diphosphine ligand assembled on the cyclotriphosphazene platform forms linear chelate and dimetallic bridged complexes with Au(I) and a cis-chelate complex with Pt(II).     

Gold(III) and platinum(II) complexes of hexamethylenetetramine

Gold(III) trichloride and platinum(II) dichloride form 1:1 complexes with hexamethylenetetramine. The compounds prepared were characterized by the elemental analysis, infrared,Raman,¹H-NMR, and¹³C-NMR spectroscopy, and conductivity measurements.     

Tuning redox potentials of bis(imino)pyridine cobalt complexes: an experimental and theoretical study involving solvent and ligand effects

The structure and electrochemical properties of a series of bis(imino)pyridine Co(II) complexes (NNN)CoX(2) and [(NNN)(2)Co][PF(6)](2) (NNN = 2,6-bis[1-(4-R-phenylimino)ethyl]pyridine, with R = CN, CF(3), H, CH(3), OCH(3), N(CH(3))(2); NNN = 2,6-bis[1-(2,6-(iPr)(2)-phenylimino)ethyl]pyridine and X = Cl, Br) were studied using a combination of electrochemical and theoretical methods. Cyclic voltammetry measurements and DFT/B3LYP calculations suggest that in solution (NNN)CoCl(2) complexes exist in equilibrium with disproportionation products [(NNN)(2)Co](2+) [CoCl(4)](2-) with the position of the equilibrium heavily influenced by both the solvent polarity and the steric and electronic properties of the bis(imino)pyridine ligands. In strong polar solvents (e.g., CH(3)CN or H(2)O) or with electron donating substituents (R = OCH(3) or N(CH(3))(2)) the equilibrium is shifted and only oxidation of the charged products [(NNN)(2)Co](2+) and [CoCl(4)](2-) is observed. Conversely, in nonpolar organic solvents such as CH(2)Cl(2) or with electron withdrawing substituents (R = CN or CF(3)), disproportionation is suppressed and oxidation of the (NNN)CoCl(2) complexes leads to 18e(-) Co(III) complexes stabilized by coordination of a solvent moiety. In addition, the [(NNN)(2)Co][PF(6)](2) complexes exhibit reversible Co(II/III) oxidation potentials that are strongly dependent on the electron withdrawing/donating nature of the N-aryl substituents, spanning nearly 750 mV in acetonitrile. The resulting insight on the regulation of redox properties of a series of bis(imino)pyridine cobalt(II) complexes should be particularly valuable to tune suitable conditions for reactivity.     

Dicarbonyl dithio ligand complexes of tungsten(II)

Reaction of LWI(CO)_n [L=hydrotris(3,5-dimethylpyrazol-1-yl)borate, n=2, 3] with NH₄[S₂PR₂] [R=OEt, OPrⁱ, (−)-mentholate (R*), Ph] in acetonitrile or THF results in the formation of the dithio ligand complexes LW(S₂PR₂-S)(CO)₂. The yellow–orange, diamagnetic complexes exhibit IR spectra featuring two ν(CO) bands at ca. 1950 and 1840 cm⁻¹ and ¹H-NMR spectra consistent with fluxional behavior in solution. Crystallographic characterisation of LW{S₂P(OPrⁱ)₂-S}(CO)₂ revealed a six-coordinate, distorted octahedral complex composed of a tungsten center coordinated by a monodentate dithiophosphate ligand, two cis carbonyl ligands, and a facial, tridentate L ligand. Unlike analogous complexes bearing strictly monodentate sulfur donor ligands, the LW(S₂PR₂)(CO)₂ complexes undergo reactions with oxygen atom donors to produce (carbonyl)oxo complexes of the type LWO(S₂PR₂-S)(CO).     

Catalysis and inhibition of ligand substitution in palladium(II) square-planar complexes: effects of DNA.

The kinetics of substitution, by thiourea, of ethylenediamine (en) or N,N'-dimethylethylenediamine (Me(2)en) coordinated to palladium(II) in the complexes [Pd(4,4'-R(2)bpy)(en)](PF(6))(2) (bpy = 2,2'-bipyridine; R = H or Me), [Pd(en)(2)](PF(6))(2) and [Pd(Me(2)en)(2)](PF(6))(2) have been studied at 25 degrees C, pH 7 and various ionic strength values, in the presence of calf thymus DNA. The rate of the reaction in water depends on ionic strength, pH, and nucleophile concentration; at fixed pH and ionic strength the k(obsd) values are correlated to the square of the thiourea concentration. This rate law is not altered by the presence of DNA, but the rate of reaction is influenced, depending on the nature of ancillary ligand, L-L, bound to palladium. DNA inhibits the substitution process when L-L is bpy or 4,4'-Me(2)bpy and catalyzes the same reaction when L-L is en or Me(2)en. These opposite kinetic effects can be related to the noncovalent interactions of the various complexes with the DNA double helix. Inhibition of the reactivity of the complexes [Pd(4,4'-R(2)bpy)(en)](2+) is due to protection of the reaction center from nucleophile attack by DNA. Acceleration of the reaction when L-L is en or Me(2)en is related to the dependence of the rate of reaction on pH. If, due to the higher activity of water under the electric field of phosphate groups, hydronium ion concentration on DNA surface is higher than in the bulk solution, the enzyme-like dependence of the rate of reaction on [DNA] is due to progressive accumulation of the complexes around the double helix. Regardless of the complexes' nature, the rate constant values obtained in DNA at pH 7 correspond to values determined in water at pH 5. This pH value on the DNA surface, lower by about two units with respect to the bulk solution, is in good agreement with theoretical predictions. Acceleration of ethylenediamine substitution has been observed for all of the complexes studied in the presence of sodium polyvinylsulfonate.     

o-Iminobenzosemiquinonato(1-) and o-amidophenolato(2-) complexes of palladium(II) and platinum(II): a combined experimental and density functional theoretical study

From the reaction mixture of [M(II)(bpy)Cl(2)], the ligand 2-anilino-4,6-di-tert-butylphenol, H[L(AP)], and 2 equiv of a base (NaOCH(3)) in CH(3)CN under anaerobic conditions were obtained the blue-green neutral complexes [M(II)(L(AP)-H)(bpy)] (M = Pd (1), Pt (2)). (L(AP)-H)(2)(-) represents the o-amidophenolato dianion, (L(AP))(1)(-) is the o-aminophenolate(1-), (L(ISQ))(1)(-) is its one-electron-oxidized, pi-radical o-iminobenzosemiquinonate(1-), and (L(IBQ))(0) is the neutral quinone. Complexes 1 and 2 can be oxidized by ferrocenium hexafluorophosphate, yielding the paramagnetic salts [M(II)(L(ISQ))(bpy)]PF(6) (S = (1)/(2)) (M = Pd (1a), Pt (2a)). The reaction of PtCl(2), 2 equiv of H[L(AP)], and 4 equiv of base in CH(3)CN in the presence of air yields diamagnetic [Pt(L(ISQ))(2)] (3), which is shown to possess an electronic structure that is best described as a singlet diradical. Complexes 1, 1a, 2, 2a, and 3 have been structurally characterized by X-ray crystallography at 100 K. It is clearly established that O,N-coordinated (L(AP)-H)(2)(-) ligands have a distinctly different structure than the corresponding O,N-coordinated (L(ISQ))(1)(-) radicals. It is therefore possible to unambiguously assign the protonation and oxidation level of o-aminophenol derived ligands in coordination compounds. All complexes have been investigated by cyclic voltammetry, spectroelectrochemistry, EPR, and UV-vis spectroscopy. Complexes 1 and 2 can be reversibly oxidized to the [M(II)(L(ISQ))(bpy)](+) and [M(II)(L(IBQ))(pby)](2+) mono- and dications, respectively, and reduced to the [M(L(AP)-H)(bpy(*))](-) anion, where (bpy(*))(1)(-) is the radical anion of 2,2'-bipyridine. Complex 3 exhibits four reversible one-electron-transfer waves (two oxidations and two reductions) which are all shown to be ligand centered. The EPR spectra of the one-electron-reduced species [Pt(L(AP)-H)(L(ISQ))](-) (S = (1)/(2)) and of the one-electron-oxidized species [Pt(L(ISQ))(L(IBQ))](+) (S = (1)/(2)) in CH(2)Cl(2) solutions have been recorded. To gain a better understanding of the electronic structure of 3 and its monooxidized and reduced forms, relativistic DFT calculations have been carried out. Magnetic coupling parameters and hyperfine couplings were calculated and found to be in very good agreement with experiment. It is shown that both the one-electron oxidation and reduction of 3 are ligand centered. A simple MO model is developed in order to understand the EPR properties of the monocation and monoanion of 3.     

A density functional theory study of the conformational properties of 1,2-ethanediamine: protonation and solvent effects

 The structures and the conformational energies of nonprotonated, monoprotonated and diprotonated 1,2-ethanediamine have been investigated through density functional theory. The relative performance of local and gradient-corrected functionals is discussed. The existence of hydrogen-bond formation has been determined with electron localisation function calculations. Proton affinities for nonprotonated and monoprotonated 1,2-ethanediamine have been calculated and are in agreement with experimental data. The influence of solvation has been accounted for through the self-consistent isodensity polarisable continuum model. The results for the nonprotonated conformers show that solvation stabilises those conformers which have the lone pair in an antiperiplanar conformation. Solvation of the monoprotonated conformer stabilises significantly the “anti” conformation, which is unstable in the gas phase. For the di-protonated species, solvation stabilises slightly the gauche conformer, which is unstable in the gas phase. Received: 28 September 1999 / Accepted: 2 May 2000 / Published online: 27 September 2000     

Biological assays and theoretical density functional theory calculations of Rh(I), Ir(III), and Ru(II) complexes of chiral phosphinite ligand

Four metal complexes, IL-OPPh₂-Ru-p-cymene (3) , IL-OPPh₂-Ru-benzene (4) , IL-OPPh₂-Ir-Cp* (5) , IL-OPPh₂-Rh-COD (6) , have been evaluated for in vitro antioxidant activity such as 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging and reducing power activity. Maximum scavenging activity (71.43%) was obtained with IL-OPPh₂-Ru-p-cymene, whereas IL-OPPh₂-Rh-COD showed the highest reducing power ability. The complexes were also studied for their antimicrobial activity against three Gram-positive and three Gram-negative bacteria. In addition, DNA binding of the complexes was evaluated using calf thymus DNA. Both Ru(II) complexes exhibited good DNA-binding activity while the other complexes did not have any activity. Furthermore, ab initio quantum calculations of four complexes were also carried out using density functional theory to better understand their chemical behaviors.     

Combined ligand field and density functional theory analysis of the magnetic anisotropy in oligonuclear complexes based on Fe(III)-CN-M(II) exchange-coupled pairs

Magnetic anisotropy in cyanide-bridged single-molecule magnets (SMMs) with Fe(III)-CN-M(II) (M = Cu, Ni) exchange-coupled pairs was analyzed using a density functional theory (DFT)-based ligand field model. A pronounced magnetic anisotropy due to exchange was found for linear Fe(III)-CN-M(II) units with fourfold symmetry. This results from spin-orbit coupling of the [Fe(III)(CN)6](3-) unit and was found to be enhanced by a tetragonal field, leading to a (2)E g ground state for Fe(III). In contrast, a trigonal field (e.g., due to tau 2g Jahn-Teller angular distortions) led to a reduction of the magnetic anisotropy. A large enhancement of the anisotropy was found for the Fe(III)-CN-Ni(II) exchange pair if anisotropic exchange combined with a negative zero-field splitting energy of the S = 1 ground state of Ni(II) in tetragonally compressed octahedra, while cancellation of the two anisotropic contributions was predicted for tetragonal elongations. A recently developed DFT approach to Jahn-Teller activity in low-spin hexacyanometalates was used to address the influence of dynamic Jahn-Teller coupling on the magnetic anisotropy. Spin Hamiltonian parameters derived for linear Fe-M subunits were combined using a vector-coupling scheme to yield the spin Hamiltonian for the entire spin cluster. The magnetic properties of published oligonuclear transition-metal complexes with ferromagnetic ground states are discussed qualitatively, and predictive concepts for a systematic search of cyanide-based SMM materials are presented.     

Cyclometallated Pt(II) and Pd(II) complexes with a trithiacrown ligand

We report the synthesis and full characterization for a series of cyclometallated complexes of Pt(II) and Pd(II) incorporating the fluxional trithiacrown ligand 1,4,7-trithiacyclononane ([9]aneS3). Reaction of [M(C insertion mark N)(micro-Cl)]2 (M = Pt(II), Pd(II); C insertion mark N = 2-phenylpyridinate (ppy) or 7,8-benzoquinolinate (bzq)) with [9]aneS3 followed by metathesis with NH4PF6 yields [M(C insertion mark N)([9]aneS3)](PF6). The complexes [M(C insertion mark P)([9]aneS3)](PF6) (M = Pt(II), Pd(II); Cinsertion markP = [CH2C6H4P(o-tolyl)2-C,P]-) were synthesized from their respective [Pt(C insertion mark P)(micro-Cl)]2 or [Pd(C insertion mark P)(micro-O2CCH3)]2 (C insertion mark P) starting materials. All five new complexes have been fully characterized by multinuclear NMR, IR and UV-Vis spectroscopies in addition to elemental analysis, cyclic voltammetry, and single-crystal structural determinations. As expected, the coordinated [9]aneS3 ligand shows fluxional behavior in its NMR spectra, resulting in a single 13C NMR resonance despite the asymmetric coordination environment of the cyclometallating ligand. Electrochemical studies reveal irreversible one-electron metal-centered oxidations for all Pt(II) complexes, but unusual two-electron reversible oxidations for the Pd(II) complexes of ppy and bzq. The X-ray crystal structures of each complex indicate an axial M-S interaction formed by the endodentate conformation of the [9]aneS3 ligand. The structure of [Pd(bzq)([9]aneS3)](PF6) exhibits disorder in the [9]aneS3 conformation indicating a rare exodentate conformation as the major contributor in the solid-state structure. DFT calculations on [Pt([9]aneS3)(ppy)](PF6) and [Pd([9]aneS3)(ppy)](PF6) indicate the HOMO for both complexes is primarily dz2 in character with a significant contribution from the phenyl ring of the ppy ligand and p orbital of the axial sulfur donor. In contrast, the calculated LUMO is primarily ppy pi* in character for [Pt([9]aneS3)(ppy)](PF6), but dx2-y2 in character for [Pd([9]aneS3)(ppy)](PF6).     

X-Ray and EPR study on copper (II) complexes with an enamine ligand

The enamine (HEAID) obtained from aniline and 2-acetyl-1,3-indandione (2AID) behaves as a bidentate ligand in coordination with copper (II) ion. Two types of crystals, apparently different in shape, were isolated and studied by single-crystal X-ray diffraction. The X-ray data for the brown rhombic crystals of compound 1 shows a mononuclear complex of Cu(II) coordinated with two EAID-anions, Cu(EAID)₂. The X-ray data for the green crystals of compound 2 shows a dinuclear Cu(II) complex with two OH⁻ groups acting as bridging ligands, [Cu₂(μ-OH)₂(EAID)₂]. In both cases the ligand coordinates after deprotonation of the amine group.     

Alkyl group dependence of C-Si reductive eliminations from alkyl(silyl) Pt(II) complexes: a density functional study

C-Si reductive elimination from Pt(R)(SiPh₃)(PMe₃)₂ (R=Me, Pr) was theoretically studied with the density functional theory. For comparisons with the experiment, substitution of PMe₃ with diphenylacetylene was taken into account. The calculated activation barriers in the C-Si elimination step after the ligand exchange were 22.0 and 28.9 kcal mol⁻¹ for R=Me and Pr, respectively, which explains the reactivity difference reported experimentally. In order to analyze the energy difference, we optimized transition states of several model complexes, and examined the influence of the steric repulsion between R and the other ligands. Comparisons of the geometries and the barrier heights reveal that the steric repulsion and the Si-alkyl bond energy are important factors controlling the reaction rate.