Manuel A. S. Aquino

Diruthenium and diosmium tetracarboxylates: synthesis, physical properties and applications

Abstract Dimetal tetracarboxylates, [M 2 (O 2 CR) 4 ] n , have received much coverage in the literature. The metals include Cr, Mo, W, Tc, Re, Ru, Os, Co, Rh, Ir, Ni and Cu with n ranging from −2 to +4 (0 to +2 being most common). Diruthenium and diosmium tetracarboxylates form two of the younger families of these ubiquitous compounds and are known to exist as homovalent Ru 2 (II,II) and Os 2 (III,III) and mixed-valent Ru 2 (II,III) and Os 2 (II,III) species. This paper will provide a comprehensive review of these complexes and include such things as their discovery, synthesis and crystal structures; electronic, vibrational (IR and Raman), ESR and photoelectron spectroscopy; magnetic susceptibility; electrochemistry; kinetics; calorimetry; theoretical calculations such as SCF-Xα-SW and CASSCF; and more recent studies of their mesomorphic behaviour and biological and catalytic applications. The ruthenium compounds, in particular, have shown a potential application as liquid crystals. Current work (including this authors) is also focused on the development of linear chains involving these metal carboxylate units and bidentate bridges with the view to developing ferromagnetic materials and conductive polymers. There have been no previous reviews on ruthenium (and osmium) carboxylates per se . Cotton and Walton give them some coverage in the context of metal–metal bonds in Multiple Bonds Between Metal Atoms (2nd ed., 1993) however the direct coverage of Ru 2 (O 2 CR) 4 and Os 2 (O 2 CR) 4 type complexes is cursory and dispersed throughout the book. As well, the coverage is only comprehensive through December 1990 with some references from 1991. The current review will be complete through to mid-1997.

Structure and electrochemistry of a tetrakis(ferrocenecarboxylato)diruthenium(II,III) diadduct: evidence for ferrocenyl–ferrocenyl communication

Abstract 1-Propanol and ethanol adducts of tetrakis(ferrocenecarboxylato)diruthenium(II,III) hexafluorophosphate have been synthesized and the 1-propanol diadduct has been characterized by X-ray crystallography. The cyclopentadienyl rings of all four ferrocenyl groups showed on average a 13° deviation from the eclipsed conformation. Cyclic voltammetry measurements in 1,2-dichloroethane show an irreversible RuIIIRuII/RuIIRuII reduction and four partially superimposed one-electron ferrocenyl-centred electron transfer processes. The existence of ferrocenyl-based mixed-valent intermediate states posessing FeIII and FeII sites are implied by the splitting of the ferrocenyl wave into two observable cathodic and anodic peaks. The first cathodic wave shows typical desorption behaviour. Osteryoung Square Wave Voltammetry (OSWV) supports the above findings.

CRYSTAL STRUCTURES AND PHYSICO-CHEMICAL PROPERTIES OF A SERIES OF RU2(O2CCH3)4L2 (PF6) ADDUCTS (L = H2O, DMF, DMSO)

Abstract The reaction of Ru 2 (O 2 CCH 3 ) 4 Cl with water in the presence of Ag 2 SO 4 and NH 4 PF 6 leads to the formation of [Ru 2 (O 2 CCH 3 ) 4 (H 2 O) 2 ](PF 6 ) (1). The subsequent reaction of complex 1 with dimethylformamide (DMF) and dimethylsulfoxide (DMSO) results in the formation of [Ru 2 (O 2 CCH 3 ) 4 (DMF) 2 ](PF 6 ) ( 2 ); [Ru 2 (O 2 CCH 3 ) 4 (DMF) 2 ](PF 6 )·DMF ( 2a ) and [Ru 2 (O 2 CCH 3 ) 4 (DMSO) 2 ](PF 6 ) ( 3 ). All complexes were characterized using single crystal X-ray crystallography, IR and UV-Vis spectroscopy, cyclic voltammetry and magnetic susceptibility. The crystallographic data for [Ru 2 (O 2 CCH 3 ) 4 (H 2 O) 2 ](PF 6 )·3H 2 O ( 1a ) are as follows: monoclinic space group C 2/ c with unit cell dimensions a = 19.552(2), b = 12.853(2), c = 8.487(2) A , β = 93.02(2)°, V = 2129.6(6) A 3 , Z = 4 . The structure was refined to R = 0.0244 ( R w = 0.266) with 1165 reflections having I > 3 σ ( I ). The RuRu distance is 2.2648(9) A; Ru distances are 2.023(4), 2.039(3), 2.018(4) and 2.026(3) A; Ru  O ( axial ) = 2.279(4) A . The relevant data for 2 are: orthorhombic, space group P 2 1 2 1 2 1 with unit cell dimensions a = 11.704(2), b = 28.452(10), c = 8.415(3) A , V = 2802(2) A 3 , Z = 4 . The structure was refined to R = 0.0597 ( wR 2 = 0.1520) with 909 reflections having I >2 σ ( I ). The RuRu distance is 2.262(3) A; RuO distances are 1.999(14), 2.003(13), 2.004(13) and 2.009(13) A; the RuO(axial) distances are both 2.22(2) A. The pertinent crystal data for 2a are: monoclinic, space group P 2 1 / c with unit cell dimensions a = 8.382(4), b = 11.918(3), c = 30.715(5) A , β = 96.84(3)°, V = 3046(1) A 3 , Z = 4 . The structure was refined to R = 0.0558 ( wR 2 = 0.1388) with 1317 reflections having I >2 σ ( I ). The RuRu distance is 2.265(2) A; RuO distances are 2.021(13), 2.052(13) and 2.023(14) A; the RuO(axial) distances are both 2.229(14) A. The data for 3 are: triclinic, space group P 1 with unit cell dimensions a = 11.45(1), b = 14.103(4), c = 8.303(3) A , α = 90.46(2), β = 110.15(4), γ = 78.93(4)°, V = 1232(1) A 3 , Z = 2 . The structure was refined to R = 0.0301 ( R w = 0.0382) with 2578 reflections having I >3 σ ( I ). The RuRu bond distances are 2.274(1) and 2.268(1) A; RuO distances range from 2.017(5) to 2.035(5) A; the RuO(axial) distances are 2.240(5) and 2.243(5) A. Increasing the donor number of the axial ligand manifests only very small changes in RuRu bond length, reduction potential and π ∗ ( Ru 2 ) → π( RuO, Ru 2 ) transition energy and no changes in μ eff implying only minor perturbation of σ ∗ and π ∗ orbital energies.

Weak intermolecular antiferromagnetic exchange in {[Ru2(O2CCH3)4(L)]X}n polymers (L = 4,4′-dipyridine and 1,4-diazabicyclooctane, X = PF6− or BPh4−)

Abstract The mixed-valent compounds [Ru 2 (μ-O 2 CCH 3 ) 4 (4,4′-dipy)](PF 6 ) ( 1 ), [Ru 2 (μ-O 2 CCH 3 ) 4 (4,4′-dipy)](BPh 4 )·H 2 O( 2 ) and [Ru 2 (μ-O 2 CCH 3 ) 4 (dabco)](PF 0 ) ( 3 ) (4,4′-dipy = 4,4′-dipyridine and dabco = 1,4-diazabicyclooctrane) were synthesized and characterized by IR spectroscopy, elemental analysis and variable temperature magnetic susceptibility. The magnetic measurements in the range 4–305 K show a significant drop in μ eff at low temperatures which can be accounted for by a large zero-field splitting (ZFS) and weak intermolecular antiferromagnetic exchange. The model previously employed by Cukiernik et al. (Inorg. Chim. Acta, 215 (1994) 203) was used and found to be successful in fitting the magnetic data. The intermolecular coupling constants, zJ , were determined to be −0.996(1), −0.653(1) and −0.593(1) cm −1 for complexes 1,2 and 3 respectively.

Kinetic studies on the reduction of the tyrosyl radical of the R2 subunit of E. coli ribonucleotide reductase

Kinetic studies at 25 degrees C, I = 0.100 M (NaCl), on the reduction of the tyrosyl radical of the R2 protein of E. coli ribonucleotide reductase with hydroxyurea (HU), N-methylhydroxylamine, catechol, and seven hydroxamic acid derivatives are reported. There are no pH-dependences in the range 6.2-8.6 investigated except that introduced with N-methylhydroxylamine which itself protonates in this range. At pH 7.6 the rate constant (0.46 M-1 s-1) for the HU reaction is in agreement with earlier values. Slower reactions are observed with the bulkier acetohydroxamic (0.020 M-1 s-1) and benzohydroxamic acids (0.040 M-1 s-1). In the case of N-methylhydroxylamine the rate constant (0.41 M-1 s-1 at pH 7.6) decreases with pH, and it is concluded that the protonated form CH3NH2+OH(pKa = 6.2) has little or no reactivity with Tyr. For this reaction under air-free conditions a second-stage (0.027 M-1 s-1) corresponding to reduction of Fe(III)2 is observed. Mid-point redox potentials for the reductants and estimates of reduction potentials applying in the case of the protein are considered. The reactions with 1,2-dihydroxybenzene (catechol) and 3,4-dihydroxybenzohydroxamic acid (Didox) also have two stages, when the initial Tyr reduction, rate constants/M-1 s-1 for catechol (3.2) and Didox (0.010), is followed by removal of the Fe(III) to give catechol and catechol like Fe(III)-complexed products. The single stage reactions of the hydroxamic acid derivatives which incorporate charged amino-acid groups L-glutamic acid, L-histidine, L-glycine and L-lysine, are slow, and saturation kinetics are observed consistent with association (small K values) prior to redox. The mechanism of reduction of R2-Tyr by all of the reagents studied is discussed.

Synthesis and crystal structures of urea and thiourea derivatives of diruthenium(II,III) tetraacetate

Diadduct complexes of the mixed-valent form of diruthenium tetraacetate, [Ru-2(mu-O2CCH3)(4)L-2](PF6), with L = urea (1), 1,1,3,3-tetramethylurea (tmu) (2) and 1,1,3,3-tetramethyl-2-thiourea (tmtu) (3) have been synthesized and characterized by elemental analysis, IR and UV-Vis spectroscopy, magnetic susceptibility, cyclic voltammetry and X-ray crystallography. Compound 1 crystallizes as a co-diadduct system from 1-propanol in which both [Ru-2(mu-O2CCH3)(4)(urea)(2)](+) and [Ru-2(mu-O2CCH3)(4)(1-propanol)(2)](+) cations are present in the unit cell. Complex 3 displays a very long Ru-S bond of 2.610(2) Angstrom and is only the second example of a sulfur-donor adduct of a diruthenium tetracarboxylate that has been structurally characterized. Electrochemical measurements on 2 and 3 show them to have reduction potentials of -487 and -541 mV (vs. Fc/Fc(+)), respectively, which is in the range of moderate to strong axial donor adducts despite the long Ru-S bond in 3. Reaction of [Ru-2(mu-O2CCH3)(4)(H2O)(2)](PF6) with thiourea yields a 1:1 complex, [Ru-2(mu-O2CCH3)(4)(thiourea)](PF6), which from solubility and infrared evidence appears to be a polymer in which the thiourea bridges through the sulfur and one of the amine nitrogens.

Synthesis, structure and electrochemistry of nitrogen base adducts of tetraacetatodiruthenium(II,III): dependence of redox potential and Ru–Ru bond length on axial ligand donor strength

Diadduct complexes of the mixed-valent form of diruthenium tetraacetate, [Ru-2(mu-O2CCH3)(4)L-2](PF6), with L = N-heterocyclic axial ligands quinuclidine (quin) (1), 4-methylpyridine (4-Mepy) (2), pyridine (py) (3), 4-cyanopyridine (4-CNpy) (4), 3-cyanopyridine (3-CNpy) (5) and 4-phenylpyridine (4-Phpy) (6) have been synthesized and all but 5 were characterized by X-ray crystallography to study the effect of the variation of the donor number (DN) of L on the Ru-Ru and Ru-L-ax bond lengths, the magnetic moment, the electronic spectral properties and the redox potential. When data from previous studies on O-donor adducts was also included a DN range of 18-61 could be established. Over this range the Ru-Ru bond length increases slightly from 2.265(1) to 2.2917(6) Angstrom as the donor number is increased from 18 (in [Ru-2(mu-O2CCH3)(4)(H2O)(2)](PF6)) to 61 in 1. UV - Vis measurements show a very slight increase in energy of the pi(Ru-O, Ru-2)-->pi*(Ru-2) transition, however, room temperature magnetic susceptibility measurements show no change in the magnetic moment over the same range of donor numbers. Electrochemical measurements in 1,2-dichloroethane of the Ru-2(4+/5+) redox couple show a decrease in the E-1/2 of 292 mV on going from complex 5 (weakest N-donor) to complex 1 (strongest N-donor). The E-1/2 range is over 400 mV when the unligated [Ru-2(mu-O2C(CH2)(6)CH3)(4)] complex is included (DN = 1 for dichloromethane). The variation of axial ligand base strength does not effect, the near-degeneracy of the (pi*delta*)(3) HOMO or the pi-->pi* energy gap, however, the actual (pi*delta*)(3) HOMO energy varies significantly and increases as the basicity of the axial ligand increases allowing selective tuning of the redox potential

Synthesis and structure of trans-dimethylammonium bis(oxalato)diaquaruthenate(III) tetrahydrate

Abstract Trans-Dimethylammonium bis(oxalato)diaquaruthenate(III) tetrahydrate, trans-[(CH3)2NH2][Ru(C2O4)2(H2O)2]·4H2O, was synthesized by diffusion of dimethylamine (from dimethylformamide) into a refluxed aqueous solution of diruthenium(II,III) tetraacetate and oxalic acid. The structure of the complex was determined by X-ray diffraction. The RuO (oxalate) and RuO (water) distances are 2.041(3) and 1.994(3) A, respectively and the oxalate bite angle is 80.6(2)°. The complex displays an intricate pattern of inter- and intra-chain hydrogen bonding involving the axial water molecules, the water molecules of hydration, the dimethylammonium cation, and the carbonyl and carboxyl oxygens of the oxalate groups. Additional characterization using infrared and UV–Vis spectroscopies, magnetic susceptibility (isothermal and variable temperature), and cyclic voltammetry is also reported.

Mechanism of the reaction of different phosphates with the iron(II)iron(III) form of purple acid phosphatase from porcine uteri (uteroferrin)

Reactions of different phosphates (represented here as PO4), including H2PO4–(as prototype), phenylphosphate (and the p-nitro derivative), pyrophosphate, tripolyphosphate, and adenosine 5′-triphosphate (ATP), with the FeIIFeIII form of purple acid phosphatase (PAPr) from porcine uteri (uteroferrin) have been studied by monitoring absorbance changes for the iron(III) chromophore at 620 nm. Stopped-flow rate constants are independent of total [PO4](10–50 mM), and decrease with increasing pH (2.5–6.5). At the lower pH a mechanism of rapid PO4 binding to the FeII, followed by rate-controlling [PO4]-independent bridging to the FeIII with displacement of a co-ordinated H2O, is proposed. Further information comes from experiments on the hydrolysis activity of PAPr monitored by the release of α-naphthol (323 nm) from α-naphthyl phosphate, which maximises at pH 4.9. The full mechanism requires participation of FeIII–OH, which substitutes into the phosphate moiety thus bringing about hydrolysis. The concentration of the latter peaks at pH 4.9, and possible reasons for the decrease in activity at pH >4.9 are given. Rate constants at maximum activity are of magnitude ≈ 0.5 s–1 only, with no very strong discrimination between the reagents used. Equilibration steps in which the phosphate can if necessary be recycled to bring about hydrolysis are proposed. For the pH range studied the final product has a bridging HPO42– ligand. Trimethyl phosphate with only one oxo group does not appear to react at the FeIII, but inhibits reaction with H2PO4– possibly by co-ordinating to the FeII. Reaction with the sterically bulky cation [Co(NH3)6(HPO4)]+ is much slower, k= 1.6 × 10–4 s–1. The HPO4–-bridged FeIIFeIII form is more responsive to air oxidation to FeIIIFeIII consistent with the decrease in reduction potential from 367 and 183 mV. Rate constants are independent of [H2PO4–] and pH.

Chiral Induction via the Disassembly of Diruthenium(II,III) Tetraacetate by Chiral Diphosphines

[Ru(2)(μ-O(2)CCH(3))(4)(MeOH)(2)](PF(6)) reacts with chiral diphosphines (R,R)- and (S,S)-chiraphos, leading to disassembly and production of the enantiomers Λ-[Ru(η(2)-O(2)CCH(3))(η(2)-(R,R)-chiraphos)(2)](PF(6)) and Δ-[Ru(η(2)-O(2)CCH(3))(η(2)-(S,S)-chiraphos)(2)](PF(6)) in high yield and purity. X-ray crystallography and solid-state circular dichroism (CD) show that only the indicated isomers are present in the solid state. Solution CD measurements also indicate their predominance in solution.