Copper(II) and ruthenium(II)/(III) Schiff base complexes

Metal complexes of general formula [Cu(L)](ClO4)2, [Ru(L)(PPh3)2]Cl2 and [Ru(L)(PPh3)Cl]Cl2[L = 1,4-di- (o-benzylidiminophenoxy/benzylidiminophenylthio)butane] containing N2O2 or N2S2 donor atoms have been prepared and characterised by spectral, magnetic and cyclic voltammetric studies. The rhombic nature of the e.s.r. spectra of the RuIII complexes indicates an asymmetry in the electronic environment around the Ru atom. e.s.r. spectra of the CuII complexes show a typical four-line spectrum with approximate tetrahedral distortion. The observed low A values in the CuII complexes, of the order of 132–160 × 10–4cm–1, indicates a tetrahedrally distorted square planar structure.The influence of modified ligands is reflected in the metal-centered redox potentials. CuII complexes having the N2S2 chromophore, in MeCN on a glassy carbon electrode, undergo quasi-reversible reduction in the 540–680 mV range. A depression in E1/2 values for the open chain N2S2 chromophoric macrocyclic CuII complexes, compared to electronically similar cyclic tetradentate CuII analogues, is due to the increased stabilization of the CuI state by added flexibility provided through the open chain donor sites.     

Ruthenium(II)/(III) complexes of bidentate acetyl hydrazide Schiff bases

A series of octahedral Ru^II/Ru^III complexes of the type [Ru(Y)(CO)(BAX)(PPh₃)₂] and [RuCl₂(BAX)(PPh₃)₂] (Y = H or Cl; BAX = benzaldehydeacetylhydrazone anion; X = H, Me, OMe, OH, Cl or NO₂) have been prepared and characterised by spectral, magnetic and cyclic voltammetric studies. The Ru^II complexes are low spin diamagnetic (S = 0) whereas the Ru^III complexes are low spin and paramagnetic (S = 1/2). These Ru^II and Ru^III complexes absorb in the visible region respectively at ca. 16,000 and 28,000 cm^–1 which bands are assigned to the MLCT. The correlation of the _max values of the Ru^III complexes with the ⁺ Hammett parameter, is linear, indicating the profound effect of substituents on the electron density of the central metal. I.r. spectral data reveals that the hydrazone is chelated to ruthenium through the hydrazinic nitrogen and the deprotonated enolic oxygen. The rhombic nature of the e.s.r. spectra of the Ru^III complexes indicates an asymmetry in the electronic environment around the Ru atom. Ru^II complexes in CH₂Cl₂ show an irreversible Ru^II/III redox couple at ca. 0.9–0.5 V, while the Ru^III complexes show two reversible redox couples in the –0.1–0.1 and 0.8–0.6 V range, indicating that the higher oxidation state of ruthenium is stabilised by hydrazones.     

Electrochemical and thermodynamic properties of hexacyanoferrate(II)/(III) redox system on multi-walled carbon nanotubes

Novel films consist of multi-walled carbon nanotubes (MWCNT) were fabricated by means of catalytic chemical vapor deposition (CVD) technique with decomposition of either acetonitrile (ACN) or benzene (BZ) using ferrocene (FeCp₂) as catalyst. The electrochemical and thermodynamic behavior of the ferrocyanide/ferricyanide, [Fe(CN)₆]^3−/4− redox couple on synthesized MWCNT-based films was investigated by means of cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) techniques at T = (278.15, 283.15, 293.15, and 303.15) K. The redox couple [Fe(CN)₆]^3−/4− behaves quasi-reversibly on fabricated MWCNT-based films and its reversibility is enhanced upon increasing temperature. Namely, the findings establish that with the rise in temperature the barrier for interfacial electron transfer decreases, leading, consequently, to an enhancement of the kinetics of the charge transfer process. According to thermodynamics the equilibrium of the redox process is shifted towards the formation of [Fe(CN)₆]³⁻ at elevated temperatures.     

Mixed-ligand complexes of osmium(III)/(IV) 8-quinolinolates: synthesis,characterization and redox behaviour

Summary A group of mixed-tris chelates of osmium(III) and osmium(IV) of type [OsAQ₂]^{0/1 +} [HA = glycine (glyH), picolinic acid (PicH) and quinaldic acid (qndH); HQ = 8-quinolinol] were prepared and characterized by physicochemical, magnetic and spectroscopic methods. The complexes exhibit several spin-allowed and spin-forbidden absorption bands and shoulders in the 200–700 nm region. The new chelates are electroactive and display nearly reversible Os^IV-Os^III and Os^III-Os^II couples in the ca. –1.1 to +0.3 V range versus s.c.e. The stability of metal oxidation states is discussed in terms of the electrochemical results.     

Novel dioximato-bridged chromium(III)-copper(II)-chromium(III) trinuclear complexes with ferromagnetic exchange interactions

Summary Two new dioximato-bridged trinuclear Cr^II-Cu^II-Cr^III complexes, [Cr(salen)-Cu(-BD)₂-Cr(salen)] (1) and [Cr(salen)-Cu(-FD)₂-Cr(salen)]-H₂O (2), have been prepared and characterized [salen^2– = N,N-ethylene- bis (salicylideneiminate), (-BD)^2– = -benzyldioximato and (-FD)^2– = -furildioximato]. Magnetic susceptibility measurements in the 4.2–300 K range demonstrated the operation of a ferromagnetic interaction between the adjacent Cr^III and Cu^II ions through oximato bridges in both (1) and (2). Based on spin Hamiltonians =–2J(₁₂+₂₃)(S₁=S₃=3/2,S₂=1/2) the exchange integrals (J) were evaluated as 3.19 and 5.38 cm^–1 for (1) and (2), respectively.     

Redox conversions in crystalline compounds based on macrocyclic complexes of nickel(II)/(III) and aromatic carboxylates

It is shown that conversion of nickel(II) complexes, in the composition of crystalline compounds of various structures, into nickel(III) leads to the preparation of new redox-active metal-organic skeletons. The structure of the chromophore of the metal ion and the chemical properties of the compounds formed by this process (thermal stability, reactivity) depend to an important extent on the nature of the construction blocks which makes possible the creation of new materials with controlled properties.     

Syntheses,characterization, and crystal structures of ruthenium(II)/(III) complexes with tridentate salicylaldiminato ligands

Treatment of [Ru(PPh₃)₃Cl₂] with one equivalent of tridentate Schiff base 2-[(2-dimethylamino-ethylimino)-methyl]-phenol (HL) in the presence of triethylamine afforded a ruthenium(III) complex [RuCl₃(κ²-N,N-NH₂CH₂CH₂NMe₂)(PPh₃)] as a result of decomposition of HL. Interaction of HL and one equivalent of [RuHCl(CO)(PPh₃)₃], [Ru(CO)₂Cl₂] or [Ru(tht)₄Cl₂] (tht = tetrahydrothiophene) under different conditions led to isolation of the corresponding ruthenium(II) complexes [RuCl(κ³-N,N,O-L)(CO)(PPh₃)] (2), [RuCl(κ³-N,N,O-L)(CO)₂] (3), and a ruthenium(III) complex [RuCl₂(κ³-N,N,O-L)(tht)] (4), respectively. Molecular structures of 1·CH₂Cl₂, 2·CH₂Cl₂, 3 and 4 have been determined by single-crystal X-ray diffraction.     

Evaluation of electrochemical charge-transfer rates by using (real) laplace space analysis: Single potential-step chronoamperometry of hexacyanoferrate(III)/(II) and Europium(III)/(II) couples

The one-electron hexacyanoferrate(III)/(II) and europium(III)/(II) redox couples were evaluated by using single-pulse chronoamperometry at a planar glassy carbon electrode (and also a mercury-pool electrode for europium) to test Wijnen's method for calculating heterogeneous electron-transfer rate constants by using (real-axis) Laplace space analysis. The k⁰ values obtained for the former couple in potassium or lithium chloride supporting electrolytes agreed well with published constants obtained by diverse real-time techniques. Transfer coefficients (α⁰) obtained from (? in k^a/?η)_η=0 for LiCl electrolyte were 0.4–0.5, rather than 0.22 previously reported. The cathodic rate values calculated for europium(III) reduciton (in NaClO₄/HClO₄) on both glassy carbon and mercury agreed very well with each other and with published values obtained by d.c. polarography and faradaic impedance measurements. Owing to several factors, including its ability to utilize virtually any set of recorded i(t) data points, Wijnen's Laplace technique offers an attractive alternative to conventional single-pulse analysis in real-time.     

Decarboxylation syntheses of transition metal organometallics. III polyfluorophenylplatinum(II) complexes with nitrogen donor ligands

Summary The platinum(II) carboxylates,trans-Pt(O₂CR)₂(py)₂ and Pt(O₂CR)₂bpy (R=C₆F₅,p-HC₆F₄,m-HC₆F₄, oro-HC₆F₄; bpy=2,2-bipyridyl), have been prepared by reactions oftrans-Pt(OH)₂(py)₂ or Pt(OH)₂bpy with the appropriate polyfluorobenzoic acids, whilst [Pt(py)₄](O₂CC₆F₅)₂ has been obtained from reaction oftrans-PtCl₂(py)₂ with thallous pentafluorobenzoate in pyridine at room temperature. In boiling pyridine, the platinum(II) polyfluorobenzoates undergo either decarboxylation givingtrans-PtR₂(py)₂ and PtR₂bpy (R= C₆F₅,p-HC₆F₄, orm-HC₆F₄) complexes or substitution, giving [Pt(py)₄](O₂CC₆F₄H-o)₂ and [Ptbpy(py)₂](O₂CC₆F₄H-o)₂. Reactions oftrans-PtX₂(py)₂ and PtX₂bpy (X=Cl or Br) with appropriate thallous polyfluorobenzoates in boiling pyridine have yielded the complexestrans-PtR₂(py)₂, PtR₂bpy, PtCl(R)bpy (R=C₆F₅,p-HC₆F₄, orm-HC₆F₄ in each case),trans-PtCl(R)(py)₂ (R = C₆F₅ orm-HC₆F₄),trans-PtBr(C₆F₅)(py)₂, and PtBr(C₆F₅)bpy. The complexestrans-PtR₂(py)₂ (R=C₆F₅ orp-HC₆F₄) have also been prepared from potassium tetrachloroplatinate(II) and the appropriate thallous polyfluorobenzoate in boiling py, andtrans-Pt(C₆F₅)₂(py)₂ has been similarly obtained fromcis-PtCl₂(py)₂ and C₆F₅CO₂Tl. Significant decarboxylation was not observed on reaction oftrans-PtCl₂(py)₂ or PtCl₂bpy with thallous 2,3,4,5-tetrafluorobenzoate.Part II, ref. 4;Preliminary communication, ref. 3;     

Solvent effects on the redox properties of tris-(1,10-phenanthroline)iron(II/III) complexes

The redox properties of the system Fe(tmphen)₃(II/III) (tmphen=3,4,7,8-tetramethyl-1,10-phenanthroline) have been studied in the solvents nitromethane, acetonitrile, propanediol-1,2-carbonate, dimethylformamide, dimethylacetamide, dimethylsulfoxide and of the systems Fe(phen)₃(II/III) (phen=1,10-phenanthroline) and Fe(niphen)₃(II/III) (niphen=5-nitro-1,10-phenanthroline) in the solvents nitromethane, acetonitrile, propanediol-1,2-carbonate and acetone. The redox potentials of Fe(tmphen)₃(II/III) are nearly independent of the solvent suggesting that the system might be used as a reference redox couple similar to the systems ferrocene/ferricinium or bisbiphenylchromium(0/I). In contrast the redox potentials of Fe(niphen)₃(II/III) show a significant decrease with increasing donor number of the solvent which can be explained by nucleophilic attack of solvent molecules at the iron. It is shown that such a mechanism is consistent with the known solvent and salt effects on the kinetics of dissociation of ferroin and ferriin type complexes.     

Reversible light-driven redox switching of multifunctional dipolar ruthenium(III/II) pentaammine(4,4'-bipyridinium) complexes

We report the first example of a molecular switch of multifunctional dipolar ruthenium(III/II) pentaammine-N-methyl-(4,4'-bipyridinium) complexes, exclusively driven by light. This is achieved by using a two-phase (water/benzene) system in which RuIII/II complexes are soluble only in the water phase. The reversible redox switching is triggered by the selective irradiation of the water and the benzene compartments with 254 and 528 nm light, respectively.     

Synthesis and studies of a tetradecanuclear manganese(II)/(III) cage

A novel Mn14 cluster is reported; this is a new nuclearity for manganese cages and highly unusual in that the ligands are not exclusively oxygen donors.     

Chemiluminescence of Mn(II) complexes in oscillatory redox processes

It has been shown that the red chemiluminescence (CL) in the Belousov-Zhabotinskii system with Mn(II, III) cannot be attributed to the formation of excited oxygen or its dimers during the process. Chemiluminescence has been discovered upon the reduction of Mn(III) by malonic acid, and its spectrum coincides with the spectrum of the CL in the oscillatory system (with a maximum at 680–720 nm) and is found in the same region as the photoluminescence of Mn(II),- which is known from the literature data. It has been concluded that the principal emitter of the oscillatory CL is Mn(II), which is formed in an excited state upon the reduction of Mn(III).Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 25, No. 2, pp. 191–197, March–April, 1989.     

Determination of Electrochemical Electron‐Transfer Reaction Standard Rate Constants at Nanoelectrodes: Standard Rate Constants for Ferrocenylmethyltrimethylammonium(III)/(II) and Hexacyanoferrate(III)/(II)

The most common approximation of electroneutrality is inappropriate for analyzing the voltammetric response of nanoelectrodes. Therefore, the microelectrode theory for extracting the standard rate constant k⁰ for electron transfer from steady‐state voltammograms is invalid for nanoelectrodes. Unlike previous approaches, we considered the influence of the interfacial potential distribution caused by the absence of electroneutrality. We estimated the magnitude of the error at low overpotential incurred as a result of ignoring the absence of electroneutrality and found that it was small. In this region, electrochemical reaction appears to be limited by the rate of electron transfer. Under these conditions, k⁰ can be obtained from steady‐state voltammogram data in a low overpotential region according to an approximate form of the Butler–Volmer equation. This procedure can greatly simplify analysis and calculation of the rate constant k⁰ at nanoelectrodes. Steady‐state voltammogram of equal‐concentration hexacyanoferrate(III)/(II) (Fe(CN) /Fe(CN) ) and ferrocenylmethyltrimethylammonium(III)/(II) (FcTMA²⁺/FcTMA⁺) redox couples were investigated at Pt? Ir nanoelectrodes in the presence of a support electrolyte. k⁰ for Fe(CN) /Fe(CN) and FcTMA²⁺/FcTMA⁺ at Pt? Ir nanoelectrodes were evaluated.     

Mixed-ligand copper(II) complexes with positive redox potentials

Summary Mixed-ligand complexes formed by reaction of Cu(ClO₄)₂ with 3-(2-pyridyl)-5,6-diphenyl-1,2,4-triazine (dppt) as primary ligand and 2,2-bipyridine (bipy), 1,10-phenanthroline (phen), 2,9-dimethyl-1,10-phenanthroline (dmp), N,N-bis(pyrid-2-ylmethyl)amine (dipica), N,N-bis(benzimidazol-2-ylmethyl)amine (bba), 1,3-bis(2-benzimidazolyl)-2-thiapropane (bbms) and 1,5-bis(benzimidazolyl)-3-thiapentane (bbes) as the secondary ligands have been isolated. They are of the type [Cu(dppt)L](ClO₄)₂·nH₂O, where n = 0 or 2. All complexes exhibit only one ligand field band and their cryogenic solution e.p.r. spectra are axial, with v_max and g values diagnostic of a square-based geometry. The spectral and redox data are consistent with facial coordination of the tridentate ligands. All the complexes exhibit a positive redox potential (versus n.h.e.). The weak -bonding of dppt, caused by the highly electron-withdrawing phenyl rings, the strong -back bonding involving phen and dmp, and interligand repulsions appear to be responsible for the relatively positive Cu^II/Cu^I redox potentials.     

Axial Ligand exchange in chiral macrocyclic ytterbium(III) complexes

We investigate the role of axial ligands on the near-IR-optical and paramagnetic NMR spectra of the complex [YbL](+3) where L is the stereodefined enantiopure chiral macrocycle (L = hexaazapentacyclo[25.3.1.1(12,24).0(4,9).0(19,24)]dotriaconta-1(31),2,10,12,14,16(32),17,25,27,29-decaene). The conformation in solution of the lanthanide complex is characterized by analyzing the pseudocontact 1H NMR shifts and is consistent with X-ray data of single crystal of analogue systems. The macrocycle is confined within a thin equatorial disk, leaving the cation open to at least two axial sites, on the opposite hemispheres. We recorded, assigned, and analyzed the 1H NMR spectra of several species upon changing the anion in solution, calculating the magnetic susceptibility anisotropy tensor for each. Near-IR circular dichroism is used to investigate the solution equilibria involving the competing ligands and to derive a spectroscopic series for Yb.     

Selective low-temperature syntheses of facial and meridional tris-cyclometalated iridium(III) complexes

We have developed a selective low-temperature synthesis of fac and mer tris-cyclometalated Ir(III) complexes. The chloro-bridged dimers [Ir(CwedgeN)2Cl]2 (CwedgeN = cyclometalating ligand) are cleaved in coordinating solvents like acetonitrile to give neutral Ir(CwedgeN)2(NCCH3)Cl species which in turn are reacted with AgPF6 to give hexafluorophosphate salts of the bis-acetonitrile species [Ir(CwedgeN)2(NCCH3)2]PF6 for CwedgeN = 2,2'-thienylpyridine (thpy) and 2-phenylpyridine (ppy). These bis-acetonitrile complexes are excellent starting materials for the synthesis of tris-Ir(III) complexes. The complexes of the general formula fac-Ir(CwedgeN)3 were synthesized with the ligands thpy and ppy at 100 degrees C in o-dichlorobenzene from the corresponding [Ir(CwedgeN)2(NCCH3)2]PF6 complexes. The reaction of [Ir(CwedgeN)2(NCCH3)2]PF6 with thpy at room temperature did not give the expected tris complex but instead gave [Ir(thpy)2(N,S-thpy)]PF6, with the third chelating ligand complexed through the sulfur atom of the thiophene ring. [Ir(thpy)2Cl]2, [Ir(ppy)2Cl]2, Ir(thpy)2(NCCH3)Cl, [Ir(thpy)2(NCCH3)2]PF6, [Ir(ppy)2(NCCH3)2]PF6, and [Ir(thpy)2(N,S-thpy)]PF6 were structurally characterized by X-ray crystallography. Additionally, hydroxy-bridged dimers, [Ir(CwedgeN)2(OH)]2, were synthesized as starting materials for the selective synthesis of mer-Ir(CwedgeN)3 complexes at 100 degrees C in o-dichlorobenzene. A mechanism is proposed that may account for the selectivity observed in the formation of the mer-Ir(CwedgeN)3 and fac-Ir(CwedgeN)3 isomers in previous studies and the studies presented here.