Five-coordinate Metal Complexes Research Articles

Effect of Molecular Distortion of Ligand Field Splitting in Five-Coordinated Metal Complex

Metal complex indicates the significant physical properties such as magnetism, electrical conductivity and an optical property due to the electronic interaction between the ligand molecule and the central metal ion. We have evaluated the magnetism of the metal complex by controlling the ligand field splitting, which is the energy splitting of the d-orbitals. The magnetic phenomenon of such compounds are realized as a result of the electronic state based on the transition metal coordination complexes, and which can be switched between high spin (HS) and low spin (LS), that is called a spin-crossover (SCO) phenomenon. Our group have been investigated the way how to control the spin state according to the concept of a molecular distortion. The five-coordinated metal complex ML5 has been experimentally confirmed to be a molecular distortion called the Berry pseudorotation (BPR). In the BPR process, both the axial and equatorial ligands can move at the same rate of increasing the angle between the other axial or equatorial ligands in five-coordinated metal complex. The classical BPR mechanism changes its molecular symmetry between two independent trigonal bipyramidals (TBPs) of D3h symmetry via a square pyramidal (SP) of C4v symmetry. The distortion parameters in the five-coordinated metal complexes τ5 are proposed in order to represent the distortion of the BPR process quantitatively. In this work, we predicted the possibility of the SCO phenomenon in intramolecular exchange mechanism through the BPR process in a five-coordinated metal complexes.

QTAIM study of transition metal complexes with cyclophosphazene-based multisite ligands I: Zinc(II) and nickel(II) complexes

The metal–ligand bonds in [(ZnCl 2) 2L], [NiLCl] + and two isomers of [NiLCl 2] complexes with multimodal ligand L = hexakis(2-pyridyloxy)cyclotriphosphazene, cyclo-[NP(NC 5H 4O) 2] 3, are investigated at DFT level of theory using hybrid B3LYP functional. Electron density is evaluated in terms of QTAIM (quantum theory of atoms-in-molecule) topological analysis of electron density. The bonds of central transition metal atoms with phosphazene nitrogens are shorter, stronger and more polar than with the aromatic pyridine nitrogens. The atomic charges of phosphazene nitrogens are ca twice more negative than at the pyridine ones. The higher mechanical strain in the five-coordinate metal complexes than in the six-coordinate ones may be concluded.

Five- and Six-Coordinate Group 4 Compounds Stabilized by beta-Ketiminate and Diketiminate Ligands: Syntheses and Comparisons between Solid-State and Solution Structures.

The preparation and reaction chemistry of beta-diketiminate titanium and zirconium complexes is described. Amine elimination reactions work well for introducing Tolnacnac or Tolnacac to the metal centers (TolnacnacH = 2-(p-tolylamino)-4-(p-tolylimino)-2-pentene; TolnacacH = 4-p-toluidinopent-3-en-2-one). In certain cases, the iminium salt of the diketimine can be used to circumvent the unfavorable reaction kinetics. Salt elimination reactions starting from group 4 metal halides and beta-diketiminate lithium reagents are the most versatile method for introducing beta-diketiminate ligands to the metal. For (beta-diketiminate)MCl(3) compounds (M = Ti, Zr) eta(5)- and eta(2)-coordination modes can be controlled by modifying the diketiminate ligands. Several structures of five- and six-coordinate metal complexes were solved by X-ray diffraction methods. Five-coordinate metal complexes adopt both trigonal bipyramidal and square pyramidal geometries, and the six-coordinate metal complexes possess pseudooctahedral metal centers. For (Tolnacnac)(2)ZrX(2) (X = Cl, OR, NMe(2)) the activation parameters for Lambda/Delta conversion have been probed by dynamic NMR and are consistent with a Bailar-twist mechanism. At a common temperature, the isomerization rates follow the order Cl > OR > NMe(2).

A Route to Bis(benzimidazole) Ligands with Built-In Asymmetry: Potential Models of Protein Binding Sites Having Histidines of Different Basicity

A synthetic strategy has been developed for bis(benzimidazole) ligands in which the two halves are different (4−11), and consequently of different basicity, which could be important for biomimicry and metal ion transport. Their coordination chemistry toward copper(II) has been studied in the solid state via X-ray crystallography. The ligands were complexed with copper(II) bromide and perchlorate salts to yield complexes of a 1:1 stoichiometry. Crystal structures have been determined and compared for [Cu(4)(NCCH3)(OH2)](ClO4)2 (complex A, [C20H23CuN5O2](ClO4)2, monoclinic, space group P21/c, a = 10.2168(7) Å, b = 30.740(2) Å, c = 8.3403(6) Å, β = 105.960(2)°, V = 2518.4(3) Å3, Z = 4), [Cu(6)Br2]·DMF (complex B, [C17H16Br2CuN4O]·C3H7NO, monoclinic, space group P21/c, a = 8.3348(11) Å, b = 18.165(2) Å, c = 14.140(2) Å, β = 91.646(3)°, V = 2140.0(5) Å3, Z = 4), [Cu(8)Br2]·DMF·H2O (complex C, [C16H13Br2CuN5O3]·C3H7NO·H2O, monoclinic, space group P21/c, a = 8.7241(10) Å, b = 18.172(2) Å, c = 14.506(2) Å, β = 97.376(2)°, V = 2280.7(4) Å3, Z = 4), [Cu(3)Br2]·MeOH (complex D, [C16H14Br2CuN4O]·CH4O, orthorhombic, space group Pbca, a = 14.325(2) Å, b = 13.919(2) Å, c = 18.837(2) Å, V = 3756.0(9) Å3, Z = 8), [Cu(4)Br2]·MeOH (complex E, [C18H18Br2CuN4O]·CH4O, triclinic, space group P1̄, a = 7.3120(11) Å, b = 9.9460(15) Å, c = 15.189(2) Å, α = 87.476(4)°, β = 89.093(4)°, γ = 68.673(3)°, V = 1028.0(3) Å3, Z = 2), and Cu(10)Br2 (complex F, C16H13Br2CuN3OS, monoclinic, space group P21/c, a = 7.3130(9) Å, b = 15.861(2) Å, c = 14.846(2) Å, β = 98.318(2)°, V = 1704.0(4) Å3, Z = 4). The Cu(II) complexes were found to be five coordinate, lying between perfect square-based-pyramidal (SBP) and trigonal-bipyramidal (TBP) extremes; in each case the ligands act as tridentate donors coordinating through the pyridine-like nitrogens of the benzimidazole moieties and the ether donor atom of the linking bridge. Use of the structural index parameter (τ) for five-coordinate metal complexes indicated that all the copper(II) complexes exhibit a greater tendency toward square-based-pyramidal geometry (i.e. τ < 0.5). Comparison of the symmetrical bis(benzimidazole) complex with the other, asymmetric, complexes revealed no significant change in the geometrical parameters around the copper(II) ion consequent on introduction of asymmetry and a change of pKa within the bis(benzimidazole) fragment. The degree of hydrogen bonding, solvent of crystallization, and the nature of the anion have a greater impact on the geometrical parameters and coordination environment of the copper(II) ion. The import for biological metal coordination is considered.

Five-coordinated metal complexes of bis(2-hydroxy-1-naphthylideneimine-3-propyl)amine and their reactivity towards dioxygen. Part II. EPR evidence for the role of the solvent on the stability of the Co(III)O 2 − species

Dioxygen adducts of bis(2-hydroxy-1-naphthyl-ideneimine-3-propyl)amine cobalt(II) complexes, in which either a hydrogen atom or a methyl group are present on the amino nitrogen atom of the ligand, were obtained and studied by EPR spectroscopy. The study shows an unexpectedly strong effect of the solvent(toluene or dimethylsulfoxide) on the stability of the adducts. The EPR study carried out at room and lower temperatures gave indications that an intramolecular process plays a major role in the decay of the cobaltoxygen species.

Five-coordinated metal complexes of bis(2-hydroxy-1-naphthylideneimine-3-propyl)amine and their reactivity towards dioxygen. Part I. An electrochemical investigation on manganese(II), iron(II), cobalt(II), nickel(II) and copper(II) complexes

The electrochemical behaviour in aprotic solvent of the complexes {M[bis-(2-hydroxy-l-naphthylideneimine-3-propyl)amine]}, where M = Mn(II), Co(II), Fe(II), Ni(II) and Cu(II) is reported. The complexes were prepared and characterized by elemental analysis, infrared and visible spectroscopy and magnetic susceptibility measurements. In addition the reactivity towards dioxygen of the Mn(II), Fe(II) and Co(II) derivatives was investigated, mainly by cyclic voltammetry and gas-volumetric uptake measurements. The results indicate that the Co(II) complexes are able to add dioxygen reversibly, while Mn(II) and Fe(II) compounds undergo an irreversible oxygenation process. The pathway of the dioxygenation processes is tentatively interpreted on the basis of the electrochemical responses. The results confirm that the location of the oxidation potential allows one to predict whether a compound is able to react with dioxygen, but it is not sufficient to predict whether the dioxygenation reaction proceeds reversibly.

Synthesis and reactivity towards dioxygen of a series of five-coordinated complexes between N,N′-(3,3′-dipropylmethylamine)bis(2-hydroxy-1-naphthylidenamine) and the metal ions Mn(II), Fe(II), Co(II), Ni(II) and Cu(II)

The study of metal complexes able to react with dioxygen has aroused great interest in recent years. t001 Oxidation Potential of the [M(NAPRDPT)] Complexes from Cyclic Voltammetry. Complex E° II′/III/V [Mn(NAPHDPT)] −0.31 [MN(NAPMeDPT)] −0.31 [Fe(NAPHDPT)] −0.32 [Fe(NAPMeDPT)] −0.27 [Co(NAPHDPT)] −0.37 [Co(NAPMeDPT)] −0.23 [Ni(NAPHDPT)] +0.35 [Ni(NAPMeDPT)] +0.40 [Cu(NAPHDPT)] +0.75 1 Cu(NAPMeDPT)] +0.86 1 1 Peak potential values at 0.2 V sec −1. Particular attention has been paid to those derivatives which take up dioxygen reversibly because they can mimic biological oxygen carriers [1]. In an attempt to find the parameters able to give some rationalization on the oxygenation reactions we have previously studied the behaviour towards dioxygenation of a series of five-coordinated metal complexes with O 2N 3 donor set (namely [M(SALRDPT)]) by electrochemical and X-ray diffraction techniques [2–5]. We wish to report here on the behaviour of the complexes [M(NAPRDPT)] having the same O 2N 3 donor set. ▪ M = Mn(II), Fe(II), Co(II), Ni(II), Cu(II) R = H, Me As in the case of SAL derivatives, all the NAP compounds undergo in DMSO solution a quasi-reversible one-electron anodic oxidation, except for copper(II) compounds, which are irreversibly oxidized (Table I). The comparison with SAL complexes shows that NAP derivatives are more easily oxidizable. The reaction with dioxygen in DMSO solution was followed by voltammetric techniques. The results indicate that [Co(NAPRDPT)] compounds are able to add dioxygen reversibly. For the manganese(II) and iron(II) complexes irreversible addition occurs, with formation of different oxygenated products. Copper(II) and nickel(II) complexes do not react with dioxygen The equilibrium constant of the reaction: [Co(NAPMeDPT)] + O 2 ⇄ [Co(NAPMeDPT)] · O 2 in DMSO at 20 °C resulted (1.0 ± 0.5) × 10 4 dm 3 mol −1. At least for these types of pentadentate complexes it is possible to state that the location of the oxidation potential allows one to predict whether a compound is able to react with dioxygen but it is not sufficient to predict whether the dioxygenation reaction proceeds reversibly. The manganese(II), iron(II) and cobalt(II) complexes were ESR inactive both as powders or in degassed solutions. By exposure to O 2 of toluene solutions of the [Co(NAPMeDPT)] or [Co(NAPHDPT)] complexes strong ESR signals were recorded, characterized by g-factor values of 2.022 and 2.037 respectively. The [Co(NAPHDPT)] adduct signal exhibited to Co hyperfine interaction, whereas the [Co(NAPMeDPT)] one did not, due to the rotation of the methyl group in this latter system. The ESR spectra have been related to metal species reversibly coordinating O 2 which also showed a different spontaneous decaying process. The IR spectra of solid samples or homogeneous solutions of the different complexes showed significant changes due to O 2 coordination. Hypotheses on the nature of the reversibly and irreversibly coordinating O 2 species have been made on the basis of the frequencies of the IR absorption.

Apparent Five-Coordinate Metal Complexes: Evidence for Trifunctional Bridge Bonding with the 3-Cyano-2,4-Pentanedionate Ligand in the Trichloro(3-Cyano-2,4-Pentanedionato) Complexes of Tin(IV) and Titanium(IV)

Abstract Sir: We wish to report evidence for an oligomeric structure in trichloro(3-cyano-2,4-pentanedionato) titanium(IV), C13TiL, and trichloro(3-cyano-2,4-pentanedionato)tin(IV), C13SnL. Evidence is presented that oligomerization is accomplished by a novel bridging interaction involving the planar trifunctional 3-cyano-2,4-pentanedionate anion.

Relative bond dissociation energies and heats of formation of two series of five-coordinated metal complexes from manganese(II) to zinc(II). A calorimetric study

Relative bond dissociation energies and heats of formation of two series of five-coordinated metal complexes from manganese(II) to zinc(II). A calorimetric study