Comproportionation Constants Research Articles

The stable diiron(2.5) complex ion [(NC)(5)Fe(mu-tz)Fe(CN)5](5-), tz = 1,2,4,5-tetrazine, and its neighboring oxidation states.

The conceptually simple mixed-valent diiron compound (NEt(4))(5)[(NC)(5)Fe(mu-tz)Fe(CN)(5)] with the 1,2,4,5-tetrazine (tz) bridging ligand was obtained as a thermally and air-stable material that displays large and highly variable electrochemical comproportionation constants between about 10(8) (in water) and 10(19.0) (in acetonitrile). Strong metal-metal interaction is also evident from spectroscopic results obtained for the solid and for the dissolved species. The rather intense intervalence charge-transfer band occurs around 2400 nm; infrared and Mössbauer spectra reveal the high spectroscopic symmetry of the system according to an (Fe(2.5))(2) formulation. DFT calculations on the [(NC)(5)Fe(mu-tz)Fe(CN)(5)](6-) ion confirm the presence of very low-lying pi(tz) and high-lying d(Fe) orbitals.

Aqueous solutions of unipositive cadmium; reactions of (CdI)22+(aq)

Aqueous solutions 10−3 mol dm−3 in (CdI)2, prepared by treating O2-free solutions of Cd(ClO4)2 or Cd(O3SCF3)2 with Cd powder at 65 °C, can be handled by conventional methods; the comproportionation constant (Cd2+ + Cd ⇌ Cd22+) is estimated as 0.018 (24 °C, I = 1.14 M) and the formal oxidation potential as −0.45 V; this atypical state readily reduces I3−, IrCl62−, pyridine complexes of (NH3)5RuIII, and superoxo derivatives of (NH3)5CoIII.

A Strongly Coupled Mixed Valence State Between Ru3 Clusters. Intramolecular Electron Transfer on the Infrared Vibrational Time Scale in a Pyrazine (pz) Bridged Dimer of Triruthenium Clusters, [{Ru3(μ3-O)(μ-CH3CO2)6(CO)(abco)}2(μ-pz)] (abco = 1-azabicyclo[2,2,2]octane)

Abstract A pyrazine bridged dimer of triruthenium clusters [{RuIII2RuII(μ3-O)(μ-CH3CO2)6(CO)(abco)}2(μ-pz)] (abco =1-azabicyclo[2,2,2]octane, pz = pyrazine) gives an inter-cluster mixed valence state upon one electron reduction. The splitting of the reduction waves in cyclic voltammetry, ΔE, was 470 mV. This corresponds to a comproportionation constant for formation of the mixed valence state of 9.0 × 107. The IR spectrum of the mixed valence species in the ν(CO) region, measured by reflectance IR spectroelectrochemistry, showed a completely coalesced ν(CO) band. This represents delocalized redox behavior on the infrared vibrational timescale. The rate constant for intramolecular electron transfer was estimated to be 1 × 1012 s-1 at -18 °C by simulating the ν(CO) absorption bandshape. The mixed valence complex showed an intervalence charge transfer (ICT) band at 12500 cm-1. Hush analysis provides a spectroscopic estimate of HAB to be 2490 cm-1. Crystal structures of [{RuIII2RuII(μ3-O)(μ-CH3CO2)6(CO)(abco)}2(μ-pz)]·2CH2Cl2 and its precursor, [RuIII2RuII(μ3-O)(CH3CO2)6(CO)(abco)2]·CH2Cl2, were determined by X-ray crystallography. Trends in Ru-O(oxo) and Ru-O(acetate) distances in these compounds show that the divalent site is localized on the Ru center attached to the carbonyl ligand.

Metal-metal interactions as a function of bridging ligand topology: an electrochemical, spectroelectrochemical, and magnetic study on dinuclear Oxo-Mo(V) complexes with various isomers of dihydroxynaphthalene as bridging ligand.

Reaction of [MoV(TpMe,Me)(O)Cl2] with 1,3-, 1,5-, 1,6-, 2,6-, and 2,7-dihydroxynaphthalene affords the dinuclear complexes [[Mo(TpMe,Me)(O)Cl]2(mu-C10H6O2)], abbreviated as 1,3-Mo2, 1,5-Mo2, 1,6-Mo2, 2,6-Mo2, and 2,7-Mo2, according to the substitution pattern of the bridging ligand. Electrochemical, UV-vis/NIR spectroscopic, and variable-temperature magnetic susceptibility studies have been used to probe the effects of the bridging-ligand topology on the metal-metal electronic and magnetic interactions. The complexes can be split into two classes according to the properties of the bridging ligands. Complexes 1,3-Mo2, 1,6-Mo2, and 2,7-Mo2 all have bridging ligands that are topologically equivalent to meta-substituted bridging ligands such as 1,3-dihydroxybenzene, in that (i) there is an odd number of C atoms separating the two oxygen atoms, regardless of the pathway that is taken through the ligand skeleton, and (ii) the doubly oxidized from of the bridging ligand is a diradical. These complexes are classified as being "T-meta" (= topologically equivalent to meta). Complexes 1,5-Mo2 and 2,6-Mo2 have bridging ligands that are topologically equivalent to para-substituted groups such as 1,4-dihydroxybenzene, in that (i) there is an even number of C atoms separating the two oxygen atoms, whichever pathway is taken through the ligand skeleton, and (ii) the doubly oxidized form of the bridging ligand is a diamagnetic quinone. These complexes are classified as "T-para". Electrochemical studies show that the comproportionation constants for the Mo(V)/Mo(IV) mixed-valence states of the T-meta complexes are smaller than those for the T-para complexes. Spectroelectrochemical studies show that the Mo(V)/Mo(IV) mixed-valence states of the T-para complexes show pronounced Mo(IV)-->Mo(V) IVCT transitions, whereas those of the T-meta complexes do not show these transitions. Magnetic susceptibility studies show that the T-meta complexes all display ferromagnetic exchange between the metal centers, whereas the T-para complexes all display antiferromagnetic exchange. Thus, both the electronic and the magnetic properties of these complexes show a clear demarcation into two sets according to the bridging-ligand topology.

Molecular Wires: Synthesis and Properties of the New Mixed-Valence Complex [Cp*(dppe)Fe−C⋮C−X−C⋮C−Fe(dppe)Cp*][PF6] (X = 2,5 -C4H2S) and Comparison of Its Properties with Those of the Related All-Carbon-Bridged Complex (X = −C4−)

The binuclear μ-bis(ethynyl)thiophene complex [Cp*(dppe)Fe−C⋮C-2,5-C4H2S−C⋮C−Fe(dppe)Cp*] (3, Cp* = pentamethylcyclopentadienyl, dppe = 1,2-bis(diphenylphosphino)ethane) was prepared in a three-step one-pot procedure from the 2,5-bis(trimethylsilylethynyl)thiophene and the chloro organoiron complex Cp*(dppe)FeCl (2). Complex 3, isolated as an orange thermally stable powder (72%), was characterized by 1H, 13C, and 31P NMR, FTIR, UV−vis, and Mössbauer spectroscopies and elemental analysis. CV of 3 from −1.5 to 1.4 V vs SCE displayed two reversible one-electron waves with the (ipa/ipc) current ratio of unity. A third almost reversible process was observed at a more positive potential (ipa/ipc = 0.83, 0.100 V s-1 scan rate). The redox potentials E0 are −0.39, −0.05, and 1.04 V vs SCE. The wave separation (|ΔE0| = 0.34 V) between the two reversible oxidation processes leads to a big comproportionation constant (Kc = 5.8 × 105), establishing the stability of the mixed-valence complex (MV). Addition of 0.95 equiv of [Cp2Fe][PF6] to a CH2Cl2 solution of 3 produces the MV Fe(II)−Fe(III) complex 4 (86%). Analytically pure samples in 4 were characterized by FTIR, Mössbauer, ESR, UV−vis, and NIR spectroscopies. It is established that the two metal sites in 4 are not equivalent on the very fast IR time scale (10-13 s). The Mössbauer spectrum of 4 shows the presence of a sharp unique doublet, indicating that the odd electron is delocalized on both metal centers on the Mössbauer time scale in the range 300 > T > 4 K. In contrast, the Mössbauer spectrum of [Cp*(dppe)Fe−C⋮C−C⋮C−C⋮C−C⋮C−Fe(dppe)Cp*][PF6] (5) displayed three doublets corresponding to Fe(II), Fe(III), and Fe(averaged), corresponding to the simultaneous presence of the trapped and detrapped valence in the same sample. The electronic coupling Vab determined in CH2Cl2 is 2515 and 2520 cm-1 for the complexes 4 and 5, respectively.

Metal-metal interactions in weakly coupled mixed-valence E- and Z-diferrocenylethylene complexes.

To study metal-to-metal interactions in mixed-valence states of two weakly coupling ferrocenyl groups assembled in E or Z conformation on an ethylenic double bond, E-1,2-dimethyldiferrocenylethylene (1), Z-1,2-dimethyldi-ferrocenylethylene (2), and 1,2-diferrocenylcyclohexene (3) were synthesized and structurally characterized. Crystals of 1 are triclinic, P1, with a = 7.494(9) A, b = 10.801(3) A, c = 11.971(2) A, alpha = 102.17(2) degrees, beta = 106.12(9) degrees, gamma = 90.42(2) degrees, V = 907.8 A3, and Z = 2. Crystals of 2 are monoclinic, P2(1)/c, with a = 13.601(8) A, b = 11.104(4) A, c = 13.732(1) A, beta = 114.26(7) degrees, V = 1890.8(3) A3, and Z = 4. Crystals of 3 are orthorhombic, P2(1)2(1)2(1), with a = 5.766(2) A, b = 13.090(1) A, c = 26.695(2) A, V = 2014.9(3) A3, and Z = 4. Intervalence transition spectra (IT) and electrochemical data have been determined and compared with those of diferrocenyl-benzene (para, ortho, and meta). The comproportionation constants in nitrobenzene at 25 degrees C were found to be 490 and 813 for 1 and 3, respectively. That of 2 was not measured because of the fact that 2+ isomerizes rapidly in all solvents tested, yielding nearly a racemic mixture of E and Z conformers. This finding helps to clear the paradoxical phenomenon between experimental results of mixed-valence complexes of E- and Z-1,2-bis(1'-ethyl-1-ferrocenyl)-1,2-dimethylethylene and theories. The stability of the mixed-valence species was discussed in terms of resonance delocalization, Coulomb repulsion energy, inductive effect, magnetic interaction, structural factors, and statistical factor. According to our analysis based on the Hush formalism, the contribution due to Coulomb repulsion energy dominates the overall stability of the mixed-valence state in 1+, 2+, and 3+. Stabilization that arises from resonance delocalization is only minor and contributes less than 4% to the overall stability, even in 3+ where linked Cp rings and the ethylenic plane are coplanar. In calculating the resonance contribution, crystallographic Fe-Fe distances of 7.44 A (1) and 6.68 A (3) were used for 1+, and 3+, respectively.

Vibronic Coupling in Dicyano-Complex-Bridged Mixed-Valence Complexes. Relaxation of Vibronic Constraints in Systems with Degenerate Bridging-Ligand and Electron-Transfer Excited States

Intense near-infrared (NIR) absorption bands have been found in mixed-valence Ru(NH3)5(2+,3+) complexes bridged by trans-Ru(py)4(CN)2 and cis-Os(bpy)2(CN)2, epsilonmax approximately 1.5 x 10(3) cm(-1) and deltav1/2 approximately 5 x 10(3) cm(-1) for bands at 1,000 and 1,300 nm, respectively. The NIR transitions implicate substantial comproportionation constants (64 and 175, respectively) characteristic of moderately strong electronic coupling in the mixed-valence complexes. This stands in contrast to the weakly forbidden electronic coupling of Ru(NH3)5(2+,3+) couples bridged by M(MCL)(CN)2+ complexes (MCL = a tetraazamacrocyclic ligand) (Macatangay; et al. J. Phys. Chem. 1998, 102, 7537). A straightforward perturbation theory argument is used to account for this contrasting behavior. The electronic coupling between a cyanide-bridged, donor-acceptor pair, D-(CN-)-A, alters the properties of the bridging ligand. Such systems are described by a "vibronic" model in which the electronic matrix element, HDA, is a function of the nuclear coordinates, QN, of the bridging ligand: HDA = HDA degrees + bQN. Electronic coupling in the dicyano-complex-bridged, D-[(NC)M(CN)]-A, systems is treated as the consequence of the perturbational mixing of the "local", D(NC)M and M(CN)A, vibronic interactions. If M is an electron-transfer acceptor, then the nuclear coordinates are assumed to be configured so that bQN is larger for D(NC)M but very small (bQN approximately 0) for M(CN)A. When the vertical energies of the corresponding charge-transfer transitions, EDM and EDA, differ significantly, a perturbation theory treatment results in HDA = HDAHAM/Eave independent of M and consistent with the earlier report. When EDM approximately equals EDA, configurational mixing of the excited states leads to HDA proportional to HDM, consistent with the relatively intense intervalence bands reported in this paper. Some implications of the model are discussed.

Metal-assisted unusual hydroxylation at the carbon atom of the triazine ring in dinuclear ruthenium(II) and osmium(II) complexes bridged by 2,4,6-tris(2-pyridyl)-1,3,5-triazine: synthesis, structural characterization, stereochemistry, and electrochemical studies.

The reaction of cis-[M(bpy)2Cl2] (M = Ru(II), and Os(II) with 2,4,6-tris(2-pyridyl)-1,3,5-triazine (tptz) in refluxing ethanol-water resulted in the formation of dinuclear complexes of the composition [(M(bpy)2)2(tptz-OH)](PF6)3.nH2O (n = 1 for Ru and n = 0 for Os). In this reaction an unusual metal-induced hydroxylation at the carbon atom of the triazine ring of bridged tptz occurred. However, hydroxylation did not occur in the corresponding mononuclear complexes under similar reaction condition. A comparative study revealed that sufficient electrophilicity on the carbon atom and free movement of the attached pyridyl ring promoted the hydroxylation reaction. The hydroxylated dinuclear complexes exist in two stereoisomeric forms, a rac form (delta delta/lambda lambda) and a meso form (delta lambda/lambda delta). Both diastereoisomers have been isolated in pure form and characterized. The molecular structures of the rac form of Ru(II) complex (3-II) and meso form of the Os(II) complex (4-I) have been established by single-crystal X-ray studies. Crystal data: complex 3-II, monoclinic, C2/c, a = 24.584(7) A, b = 14.309(4) A, c = 41.044(13) A, beta = 92.84(2) degrees, V = 14420.0(7) A3, Z = 8, R = 0.179, wR2 = 0.479; complex 4-I, triclinic, P1, a = 13.444(7) A, b = 14.576(5) A, c = 19.641(7) A, alpha = 98.21(3) degrees, beta = 101.67(4) degrees, gamma = 105.80(4) degrees, V = 3546.0(3) A3, Z = 2, R = 0.093, wR2 = 0.279. The poor data quality of 3-II did not allow anisotropic refinement of non-hydrogen atoms except Ru and P. A PLUTO drawing of this compound is given only to support the molecular structure. 1H NMR data have been used to characterize the diastereoisomers. The dinuclear complexes exhibit unusual electrochemical behavior; cathodic shifts of the metal-centered oxidations and ligand-based first reduction compared to mononuclear complexes have been observed. There is a splitting in the metal-centered oxidation potentials, indicating strong electronic communication between the metal centers. Comproportionation constants (Kcom) of the mixed-valence species have been calculated; the values are in the range 6.03 x 10(4)-4.7 x 10(6). It appears that a metal-metal interaction occurred by an electron-transfer mode across the low-lying pi* orbital of the bridged tptz.

An Exceedingly Stable Diiron(II,III) Complex Ion [(tz){Fe(CN)5}2]5− with Comproportionation Constants between 108 (in H2O) and 1019 (in CH3CN)

A record equilibrium constant of K(c)=10(19) was found for the comproportionation of the diiron(II,III) complex 1 in acetonitrile. In water the K(c) value is strongly diminished (10(7.9)), but still exceeds that of the Creutz-Taube ion. Intervalence charge transfer bands occur at 2520 nm (in CH(3)CN) and 2250 nm (in D(2)O), and all evidence points to a strongly coupled system with delocalized valences.

An Exceedingly Stable Diiron(II,III) Complex Ion [(tz){Fe(CN)5}2]5 with Comproportionation Constants between 108 (in H2O) and 1019 (in CH3CN)

A record equilibrium constant of Kc=1019 was found for the comproportionation of the diiron(II,III) complex 1 in acetonitrile. In water the Kc value is strongly diminished (107.9), but still exceeds that of the Creutz–Taube ion. Intervalence charge transfer bands occur at 2520 nm (in CH3CN) and 2250 nm (in D2O), and all evidence points to a strongly coupled system with delocalized valences.

Rigid Molecular Rods with Cumulene-Bridged Polyphosphines: Synthesis, Electronic Communication, Molecular Photophysics, Mixed-Valence State, and X-ray Photoelectron Spectroscopic Study.

The synthesis, molecular photophysics, redox characteristics, and electronic interactions, as well as an X-ray photoelectron spectroscopic (XPS) study of a series of Ru(II) and Os(II) complexes with a polyphosphine/cumulene spacer, namely, 1,1',4,4'-tetrakis(diphenylphosphino)cumulene (C(4)P(4)), are studied and compared with the corresponding systems containing spacers with shorter sp carbon chain (C(n)()) lengths. Characterizations of all mono-, homo-, and heterobimetallic complexes with PF(6)(-) counteranions are accomplished using (1)H, (13)C, and (31)P{(1)H} NMR, fast atom bombardment (FAB/MS), and matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF/MS) mass spectroscopy and elemental analysis. From the electrochemical study it is observed that the length of the C(n)() bridges has a profound influence on redox potentials and the electronic interaction between the two metal-based termini. XPS studies reveal that a simple change in carbon chain length affects the electron donation of the phosphine spacer to the metal-based termini. As a result, the redox potential of the Ru(II) or Os(II) center is shifted significantly. The comproportionation constant, K(c), is calculated as 1.3 x 10(7) (M = Ru(II)) or 4.5 x 10(10) (M = Os(II)) for homobimetallic [(bpy)(2)M(C(4)P(4))M(bpy)(2)](4+), suggesting a strong electronic communication across the C(4)P(4) spacer. However, the K(c) value is estimated to be ca. 4 for the corresponding complexes [(bpy)(2)M(C(3)P(4))M(bpy)(2)](4+) (M = Ru, Os; C(3)P(4) = 1,1',3,3'-tetrakis(diphenylphosphino)allene), indicative of a system with electronic isolation between the two termini. In heterobimetallic [(bpy)(2)Ru(C(n)()P(4))Os(bpy)(2)](4+) (n = 3, 4), the energy transfer from Ru(II) to Os(II) is found to be very efficient, with rate constants k(en) of ca. 3 x 10(9) s(-)(1) (n = 3) and 1 x 10(11) s(-)(1) (n = 4). The increased value of k(en) upon the change from C(3) to C(4) can be explained by the increase in the electronic communication across spacers. Detailed studies and calculations have revealed a Dexter-type of mechanism for the triplet energy transfer in the system.

Widely differing stabilities of molecule-bridged cyanodiiron(III, II) species in non-aqueous solvents

The redox systems [(NC) 5Fe(μ-L)Fe(CN) 5]) 6−/5−/4− with L=pyrazine ( 1 n ), 4,4′-bipyridine ( 2 n ) or 1,2-bis(4-pyridyl)ethene ( 3 n ), and [(NC) 4Fe(μ-bmtz)Fe(CN) 4] 5−/4−/3− ( 4 n ) with bmtz=3,6-bis(2-pyrimidyl)-1,2,4,5-tetrazine were studied in acetonitrile and other aprotic solvents. The comproportionation constants K c for the mixed-valent diiron(III, II) intermediates vary from less than 10 for 3 5− via 10 6.5 for 1 5− to 10 14.1 for 4 3−. Intervalence charge transfer band maxima lie between 1270 nm ( 2 5−) and 2475 nm ( 1 5−). In comparison with previous results from aqueous solutions, the system 1 n in particular shows increased metal–metal interaction. UV–Vis and IR spectroelectrochemistry suggest a localized/delocalized borderline sitation for 1 5− in acetonitrile, with only slightly weaker metal–metal coupling than in the related Creutz–Taube ion. The results illustrate how the established strong solvent sensitivity of cyanoiron complexes can be exploited in mixed-valence chemistry.

Synthesis, electrochemical and magnetic properties of new macrocyclic copper(II) complexes derived from 2,6-diacetyl-4-4-methylphenol

A series of binuclear macrocyclic copper(II) complexes [Cu2Lm,n](ClO4)2·xH2O have been prepared in which the two copper(II) ions are placed in two geometrically distinct co-ordination environments. The macrocycles with two 2,6-bis(iminomethyl)-4-methylphenol entities combined through two different lateral chains, –(–CH2–)–m and –(–CH2–)–n (m = 2 or 3, n = 2 to 5) were synthesized by stepwise cyclization. Cyclic voltammetry shows the presence of two reduction couples: CuIICuII CuICuII and CuIICuI CuICuI. The comproportionation constants, Kcom, for the mixed valence CuICuII complexes have been determined electrochemically. The Kcom value increases in the order of the macrocycles: (L2,2)2−<(L2,3)2−<(L2,4)2−<(L2,5)2− and (L3,3)2−<(L3,4)2−<(L3,5)2−. Cryomagnetic investigations (80–300K) reveal a moderately strong antiferromagnetic spin exchange between the copper(II) ions within each complex (J = −210 to −390 cm−1).

Porphyrin Dimers Linked by Conjugated Butadiyne Bridges: Preparations, Spectra, Voltammetry and Reductive Spectroelectrochemistry of {[M(OEP)](μ-C4)[M(OEP)]}(M2≡H4, Co2, Ni2, Cu2, Zn2, Pd2, Pt2, Co/Ni, Ni/Cu, Ni/Zn)

A series of derivatives M 2 P 2 ( M 2 ≡ H 4, Co 2, Ni 2, Cu 2, Zn 2, Pd 2, Pt 2, Co / Ni , Ni / Cu , Ni / Zn ) of the ligand meso,meso′-bis(octaethylporphyrinyl)butadiyne has been prepared and characterized by 1 H NMR, FT Raman and visible absorption spectroscopies as well as by cyclic and a.c. voltammetry in CH 2 Cl 2 solution at 20 and −40 °C. The electronic spectra exhibit multiple Soret bands and the voltammetry reveals successive one-electron reductions indicating the accessibility of ‘mixed valence’ π-radical anions and π-dianions. Using in situ thin layer spectroelectrochemistry, the UV to near-IR spectra of [ M 2 P 2]1− and [ M 2 P 2]2− ( M as above) were recorded at ≤ −40 °C. Apart from the Co complexes (reduced at the metal ion), the bis(porphyrin) anions have spectra which include sharp, intense near-IR bands (ε = 50 000–200 000 M−1cm−1) at c. 4500 and 11 500 cm−1([ M 2 P 2]1−) and 9500 cm−1([ M 2 P 2]2−). An empirically constructed semiquantitative frontier orbital model explains the observed electronic absorption bands. Inter-porphyrin conjugation, mediated by the butadiyne bridge, is responsible for the Soret band multiplicity. The near-IR bands of the anions are assigned to long-axis polarized π → π* transitions within the newly occupied upper manifold of the two-porphyrin eight-orbital framework. The small comproportionation constants found for the diporphyrin monoanions contradict the usual assumption that electronically coupled dimers should have a large voltammetric separation between the first and second redox steps.

Comparison of Hole-Transfer Superexchange in Dinuclear Mixed-Valence Ruthenium Complexes

Three dinuclear complexes [{mer-Ru(NH3)3(bpy)}2(μ-L)][ClO4]4, where L = 2,5-dimethyl- (Me2dicyd2-), 2,5-dichloro- (Cl2dicyd2-), and unsubstituted 1,4-dicyanamidobenzene dianion (dicyd2-), have been synthesized and characterized by magnetic resonance, electrochemical, and electronic absorption spectroscopic techniques. A crystal structure of [{mer-Ru(NH3)3(bpy)}2(μ-dicyd)][ClO4]4·3H2O showed dicyd2- to be approximately planar with the cyanamido groups in an anti configuration. Crystal structure data are space group = P1̄, with a, b, and c equal to 12.5613(1), 12.8738(1), and 16.3267(2) Å, respectively, α, β, and γ equal to 76.756(1), 83.893(1), and 69.053(2)°, respectively, V = 2399.28(4) Å3, and Z = 2. The structure was refined using 6112 independent reflections with I > 2.5σ(I) to a final R factor of 0.0568. The strongly coupled mixed-valence complexes [{mer-Ru(NH3)3(bpy)}2(μ-L)]3+ where L = Me2dicyd2-, dicyd2-, and Cl2dicyd2- had decreasing comproportionation constants of 1.3 × 107, 9.3 × 106, and 3.5 × 105, respectively, which are consistent with a hole transfer superexchange mechanism for metal−metal coupling. The mixed-valence properties of these complexes together with analogous systems were compared in the context of a transformation from a localized to a delocalized mixed-valence state.

Synthesis and Characterization of Mononuclear and Dinuclear Ruthenium Complexes with Tris(2-pyridylmethyl)amine and Tris(5-methyl-2-pyridylmethyl)amine.

Novel Ru(II) and Ru(III) complexes having TPA (tris(2-pyridylmethyl)amine, L1) and 5-Me(3)-TPA (tris(5-methyl-2-pyridylmethyl)amine, L2) were prepared to establish their synthetic routes and to elucidate coordination geometry and interactions between tightly bound tripodal tetradentate ligands and Ru(II)/Ru(III) centers. They include mononuclear Ru(II) complexes [RuCl(DMSO)(L)]ClO(4) (1 (L1), 2 (L2)), dinuclear bis-&mgr;-chloro Ru(II) complexes [RuCl(L)](2)(ClO(4))(2) (3 (L1), 4 (L2)), and mononuclear Ru(III) complexes [RuCl(2)(L)]ClO(4) (5 (L1), 6 (L2)). They were characterized by X-ray crystallography (for 2, 3, and 5), (1)H NMR spectroscopy, and cyclic voltammetry. For compound 2, the crystal structure was determined to possess S-bound DMSO ligand which was trans to pyridine and Cl(-) trans to the tertiary amino group of L2, and this isomer was obtained exclusively. Complex 1 was also isolated as a single isomer. Complex 3 was revealed to be a dinuclear bis-&mgr;-chloro Ru(II) species with the center of symmetry midway between two Cl(-). (1)H NMR spectra of Ru(II) complexes 1-4, the molecular structure of 2, and comparison of the molecular structure of 3 with 5 suggest that the interaction between the Ru(II) center, pyridyl moieties, and the DMSO ligand is strengthened by pi-back-bonding from the Ru(II) center to the ligands in addition to sigma-bonding of the tertiary amino group. Electrochemical measurements on 1-6 in CH(3)CN revealed that the methyl groups on pyridyl rings exert electron-donating effects onto the Ru centers to lower each redox process and such effect strengthens the Ru-S bonding in 2 compared with that in 1, accommodating pi-back-bonding from Ru(II) center to other pi-acceptors such as DMSO in 2 enough to prevent isomerization of DMSO binding mode. The dinuclear complexes 3 and 4 showed relatively large comproportionation constants, which suggest mixed-valent Ru(II)Ru(III) states would be stabilized.

Synthesis, characterization, magnetic and electrochemical studies of homo- and hetero-dinuclear complexes of a macrocyclic ligand with dissimilar compartments

Abstract A series of mononuclear complexes [M (H2L) ] (ClO4) 2 (1–6) [M = MgII, CuII, NiII, CoII, FeII, MnII] , homo-\hetero-dinuclear complexes [MALMB] (ClO4) 2 (7–11) [MA = MB = CuII, NiII ; MA = NiII, MB = CuII, CoII ; MA = MnII, MB = CuII] , and a tetranuclear complex [ {NiIILFeIII (H2O) } 2 (μ-O) ] (ClO4) 4 (12) derived from an asymmetric tetraiminodiphenol macrocyclic ligand H2L have been synthesized and characterized. The dinucleating ligand is formally the condensation product of 2 equiv. of 2,6-diformyl-4-methylphenol and 1 equiv. each of 1,2-diaminoethane and 1,3-diaminopropane. In the complexes [Ni (H2L) ] (ClO4) 2 (3) , [NiLCu] (ClO4) 2 (9) and [NiLCo (H2O) 2] (ClO4) 2 (10) , the Ni2 ion is located at the site having the ethylene chain and has a square planar geometry, while the second metal centre in 9 or 10 is either of square planar or pseudo octahedral geometry. The nickel (II) centre continguous to the propylene chain in [NiLNi (H2O) 2] (ClO4) 2 (8) , however, has octahedral configuration. Variable-temperature magnetic susceptibility data for [MnLCu (H2O) ] (Cl) (ClO4) (11) reveal antiferromagnetic spin-exchange interaction (H = −2J S1 · S2) with J = −31 cm−1. The cyclic voltammograms of [CuLCu] (ClO4) 2 · 2H2O (7) and [NiLNi (H2O) 2] (ClO4) 2 (8) show the occurrence of two stepwise reduction couples MIIMII\MIIMI and MIIMI\MIMI and the corresponding comproportionation constants are 2.4×1012 and 1.7×104, respectively. The heterodinuclear complexes also undergo stepwise reduction, e.g. [NiLCu] 2 with E11\2 = −0.25 V and E21\2 = −1.45 V, and [NiLCo (H2O) 2] 2 with E11\2 = −1.10 V and E21\2 = −1.30 V. In the hetero-tetranuclear complex [ {NiIILFeIII (H2O) } 2 (μ–O) ] (ClO4) 4 (12) the electron transfer reactions take place in three steps at −0.04, −0.40 and −1.20 V.

An asymmetrically-coordinated 2,3-dimethylpyrazine bridging ligand in the (II,II) binuclear ion, {[Ru II(hedta)] 2(2,3-Me 2pz)} 2− (hedta 3− = N-hydroxyethylethylenediaminetriacetate)

[Ru(hedta)(H 2O)] − (hedta 3− = N-hydroxyethylethylenediaminetriacetate) reacts with 2,3-Me 2pz (2,3-dimethylpyrazine) at 1:1, forming [Ru(hedta)(2,3-Me 2pz)] − ( 1), and at 2:1, forming {[Ru(hedta)] 2(2,3-Me 2pz) 2− ( 2). Complex 1 is characterized by a Ru II/III wave at 0.20 V versus NHE which shifts to 0.31 V upon protonation at N-4. Complex 2 exhibits two waves at 0.19 and 0.27 V from which the corrected comproportionation constant, K c ′ = 5.7, is calculated. The value of K c ′ indicates very poor electronic coupling between Ru II/III centers. The origins of the decrease from K c ′ value of 190 for the {[Ru(hedta)] 2(pz)} − complex is discussed in terms of (i) differences in orbital overlap due to hindered rotation in the 2,3-Me 2pz complex versus the pz case, or (ii) differences in structural distance, orientation or solvation in these complexes. The 1H NMR spectrum of complex 1 exhibits resonances at 2.67 (2CH 3), 2.58 (3CH 3), 8.01 (H5) and 8.86 (H6) ppm, respectively. The 1H NMR spectrum of complex 2 requires asymmetric coordination of the two [Ru II(hedta)] − moieties as shown by non-equivalent resonances: 2.74 (2CH 3), 2.68 (3CH 3), 8.45 (H5) and 8.55 (H6) ppm, respectively. On the 13C NMR time scale carbons C2 and C3, and C5 and C6 are pairwise equivalent with respective shifts of 155.81 and 166.44 ppm, downfield of the free ligand by 12.99 and 11.69 ppm. There may be a time-dependent reorganization of bonding at the two Ru II centers which lengthens and shortens opposite bonds at N-1 and N-4 with k ∼ 10 s −1. This motion will also account for the reduced value of K c ′ for the 2,3-Me 2pz complex. The protonated form of 1 migrates to provide η 2-coordinated [Ru II(hedta)(2,3-Me 2pzH)], bound at η 2(1,2) (31.5%), less than the 79.7% η 2(1,2) plus η 2(2.3) species observed for the related pyrazine complex, at equilibrium with the N-1-coordinated complex.

Spectroelectrochemistry of the Multistep Redox System {(μ-bpym)[Ru(NH3)4]2}n+,n= 2−6, in a Nonaqueous Medium

The dinuclear complex ion {(μ-bpym)[Ru(NH3)4]2}4+, isolated as the tetrakis(hexafluorophosphate) salt, undergoes two reversible oxidation and two reversible reduction processes in dry acetonitrile/0.1 M n-Bu4NPF6. The more accessible 3+, 4+, and 5+ states were characterized spectroelectrochemically (UV/vis/NIR). The EPR-active 3+ form (g1 = 2.0089, g2 = 1.9967, g = 1.9308, 〈g〉 = 1.9785) is best described as a complex between the bpym radical anion and two tetraammineruthenium(II) fragments. The 5+ state, on the other hand, is EPR silent down to 3.5 K due to rapid relaxation involving metal-based orbitals; it exhibits a weak absorption in the 3200 cm-1 region which may be associated with the intervalence-transfer (IT) transition. Although the question of (de)localization in the mixed-valent 5+ species cannot be answered from the available data, the comproportionation constant of 105.1 signifies strong metal−metal interaction under these conditions.

Valence delocalisation in a strongly coupled (Kc = 1014) molecule-bridged cyanodiiron(III,II) species

A hybrid between the Prussian Blue and Creutz–Taube mixed valent species is presented in the form of the bmtz-bridged bis[tetracyanoiron(+2.5)] complex 13– [bmtz = 3,6-bis(2-pyrimidyl)-1,2,4,5-tetrazine] which exhibits a very large comproportionation constant of 1014 and a delocalised structure, according to UV–VIS–NIR–IR spectroelectrochemistry in acetonitrile solution.