Stephen F. Woollam

Oxidation of diphosphazane-bridged derivatives of diruthenium nonacarbonyl by silver(I) salts in protic solvents: synthesis, structural characterization and protonation of the adduct [Ru2{µ-η2-OC(O)}(CO)4{µ-(RO)2PN(Et)P(OR)2}2](R = Me or Pri) involving a novel mode of co-ordination of carbon dioxide

Treatment of [Ru2(µ-CO)(CO)4{µ-(RO)2PN(Et)P(OR)2}2](R = Me or Pri) with AgSbF6 in methanol, ethanol or tetrahydrofuran–water resulted in the formation of the solvento species [Ru2(CO)5(R′OH){µ-(RO)2PN(Et)P(OR)2}2][SbF6]2 which is isolable for R′= H but which spontaneously deprotonates to the alkoxycarbonyl-bridged derivative [Ru2{µ-η2-OC(OR′)}(CO)4{µ-(RO)2PN(Et)P(OR)2}2]SbF6 for R′= Me or Et. The aqua species [Ru2(CO)5(H2O){µ-(RO)2PN(Et)P(OR)2}2][SbF6]2 was readily deprotonated in consecutive steps by appropriate bases to afford respectively the hydroxycarbonyl-bridged species [Ru2{µ-η2-OC(OH)}(CO)4{µ-(RO)2PN(Et)P(OR)2}2]SbF6 and the adduct [Ru2{µ-η2-OC(O)}(CO)4{µ-(RO)2PN(Et)P(OR)2}2] in which the carbon dioxide molecule adopts a novel bridging co-ordination mode; this deprotonation is reversible and treatment of the latter with HBF4·OEt2 leads to stepwise regeneration of the aqua species. The co-ordinated water molecule in [Ru2(CO)5(H2O){µ-(PriO)2PN(Et)P(OPri)2}2][SbF6]2 was readily displaced by acids HA derived from conjugate bases with potential co-ordinating properties such as thiolate ions R″S–(R″= H or Ph) or carboxylate ions R‴CO2–(R‴= H, Me, Ph or CF3), to produce monocationic pentacarbonyl species of the type [Ru2A(CO)5{µ-(PriO)2PN(Et)P(OPri)2}2]SbF6; detection of an intermediate, presumably [Ru2(CO)5(HA){µ-(PriO)2PN(Et)P(OPri)2}2][SbF6]2, was possible for HA = HCO2H and MeCO2H. The sulfido derivatives [Ru2(SR″)(CO)5{µ-(PriO)2PN(Et)P(OPri)2}2]SbF6(R″= H or Ph) rapidly decarbonylate in solution to afford the tetracarbonyl products [Ru2(µ-SR″)(CO)4{µ-(PriO)2PN(Et)P(OPri)2}2]SbF6 in which the sulfido group bridges the two ruthenium atoms. On the other hand the carboxylato derivatives [Ru2{OC(O)R‴}(CO)5{µ-(PriO)2PN(Et)P(OPri)2}2]SbF6(R‴= H, Me, Ph or CF3) are stable to decarbonylation in solution at room or elevated temperatures but can be decarbonylated to the carboxylato-bridged products [Ru2{µ-η2-OC(R‴)O}(CO)4{µ-(PriO)2PN(Et)P(OPri)2}2]SbF6 by irradiation with ultraviolet light. The water molecule in [Ru2(CO)5(H2O){µ-(PriO)2PN(Et)P(OPri)2}2][SbF6]2 was also readily displaced by the conjugate bases of the above acids HA, but in contrast to that observed for the carboxylic acids R‴CO2H (R‴= H, Me or Ph), reaction of the aqua species with the corresponding carboxylate ions R‴CO2– led to direct formation of the carboxylato-bridged species [Ru2{µ-η2-OC(R‴)O}(CO)4{µ-(PriO)2PN(Et)P(OPri)2}2]SbF6. Possible mechanisms for the formation of the various products are discussed as are the structures of [Ru2(CO)5(H2O){µ-(PriO)2PN(Et)P(OPri)2}2][SbF6]2·OCMe2, [Ru2{µ-η2-OC(OEt)}(CO)4{µ-(MeO)2PN(Et)P(OMe)2}2]SbF6, [Ru2{µ-η2-OC(Me)O}(CO)4{µ-(PriO)2PN(Et)P(OPri)2}2]PF6 and [Ru2{µ-η2-OC(O)}(CO)4{µ-(PriO)2PN(Et)P(OPri)2}2], established X-ray crystallographically.

Ready deprotonation of the protic solvento species [Ru2(CO)5(R′OH){µ-(RO)2PN(Et)P(OR)2}2]2+(R = Me or Pri; R′= H, Me, Et, etc.) and the formation of [Ru2{µ-OC(O)}(CO)4{µ-(RO)2PN(Et)P(OR)2}2] containing carbon dioxide in a bridging co-ordination mode

Treatment of [Ru2(µ-CO)(CO)4{µ-(RO)2PN(Et)P(OR)2}2](R = Me or Pri) with silver(I) salts in the presence of protic solvents of the type R′OH (R′= H, Me, Et, etc.) leads to the formation of the solvento species [Ru2(CO)5(R′OH){µ-(RO)2PN(Et)P(OR)2}2]2+ which is readily deprotonated and rearranges to afford [Ru2{µ-OC(OR′)}(CO)4{µ-(RO)2PN(Et)P(OR)2}2]+; [Ru2{µ-OC(OH)}(CO)4{µ-(RO)2PN(Et)P(OR)2}2]+ can be deprotonated further to give [Ru2{µ-OC(O)}(CO)4{µ-(RO)2PN(Et)P(OR)2}2] containing a bridging carbon dioxide group, confirmed X-ray crystallographically.

Synthesis and reactivity of the unsaturated diruthenium diphosphazane-bridged species [Ru2(CO)4{µ-(RO)2PN(Et)P(OR)2}2](R = Me or Pri)

Thermolysis of [Ru2(µ-CO)(CO)4{µ-(RO)2PN(Et)P(OR)2}2], [Ru2H2(CO)4{µ-(RO)2PN(Et)P(OR)2}2] or [Ru2{µ-OC(O)}(CO)4{µ-(RO)2PN(Et)P(OR)2}2](R = Me or Pri) under appropriate reaction conditions affords the formally unsaturated species [Ru2(CO)4{µ-(RO)2PN(Et)P(OR)2}2], which have been established by X-ray crystallography for both R = Me and R = Pri to contain two complementary semi-bridging carbonyl groups as well as two terminal carbonyls. These compounds are highly labile and react under mild conditions with a range of small molecule nucleophiles and electrophiles including carbon monoxide, isonitriles, nitrosyl ions, alkynes, sulfur, hydrogen sulfide, dioxygen, sulfur dioxide, tin(II) chloride, dihydrogen, protons, halogens and carbon tetrachloride. The compound [Ru2H2(CO)4{µ-(PriO)2PN(Et)P(OPri)2}2], the product of the reaction involving dihydrogen and the tetraisopropoxy-diphosphazane derivative and in which, as determined by X-ray crystallography, the hydrogens are situated equatorially and trans to each other on different ruthenium atoms, is also highly reactive, typically reductively eliminating dihydrogen in its reactions with nucleophiles including alkynes.

Synthesis and reactivity of the formally unsaturated diruthenium diphosphazane-bridged species [Ru2(CO)4{µ-(RO)2PN(Et)P(OR)2}2](R = Me or Pri)

Thermolysis of [Ru2(µ-CO)(CO)4{µ-(RO)2PN(Et)P(OR)2}2] or [Ru2H2(CO)4{µ-(RO)2PN(Et)P(OR)2}2](R = Me or Pri) under appropriate reaction conditions affords the formally unsaturated species [Ru2(CO)4{µ-(RO)2PN(Et)P(OR)2}2], which reacts spontaneously at room temperature with various substrates; the crystal structure of [Ru2(CO)4{µ-(PriO)2PN(Et)P(OPri)2}2] as well as that of [Ru2H2(CO)4{µ-(PriO)2PN(Et)P(OPri)2}2] is reported.

Electrochemical behaviour of ditertiary phosphine and diphosphazane ligand-bridged derivatives of di-iron and diruthenium nonacarbonyl

Cyclic voltammetric studies in benzonitrile, dichloromethane and acetone show that the oxidation of the di-iron derivatives [Fe2(μ-CO)(CO)4(μ-R2PYPR2)2] (Y = CH2, R = Me or Ph; Y = NEt, R = OMe, OEt, OiPr or OPh) generally proceeds via an EEC mechanism, the only exception being the oxidation of the Y = CH2, R = Ph derivative in acetone, which proceeds via an EE mechanism. The chemical step in the EEC mechanism involves solvent attack at an iron atom with formation of a dicationic solvento species of the type [Fe2(CO)5(solvent)(μ-R2PYPR2)2]2+. The electrochemical oxidation of the diruthenium tetramethoxydiphosphazane ligand-bridged derivative [Ru2(μ-CO)(CO)4{μ-(MeO)2PN(Et)-P(OMe)2}2] only proceeds via an EEC mechanism in the very weakly coordinating solvent dichloromethane; in benzonitrile and acetone oxidation is via an ECE mechanism for which the potential required to remove the second electron is lower than that for the removal of the first electron giving rise to an overall 2e-transfer reaction. Again the end-product of the oxidation process is a dicationic solvento species. Electrochemical oxidation in all three solvents of the diruthenium tetraisopropoxydiphosphazane ligand-bridged derivative [Ru2(μ-CO)(CO)4{μ-(iPrO)2PN(Et)P(OiPr)2}2] is proposed to proceed via an ECEC mechanism for which the first chemical step involves a structural rearrangement and the second solvent attack at a ruthenium atom to form the dicationic solvento species. Significantly, the separation between the potentials required to remove the first and second electrons is small, i.e., < 0.5 V. Two pathways are utilized in the electrochemical oxidation of the mixed-ligand complex [Ru2(μ-CO)(CO)4{μ-(MeO)2PN(Et)P(OMe)2}{μ-(iPrO)2PN(Et)P(OiPr)2}], their nature being dependent on the choise of solvent. The ECE mechanism is adopted in all three solvents benzonitrile, acetone and dichloromethane; however, in the first solvent the second pathway is the EEC process whereas the second pathway adopted in acetone and dichloromethane is the ECEC process. Thus, the overall mechanism proposed for the electrochemical oxidation of the above derivatives of [Fe2(CO)9] and [Ru2(CO)9] allows for three pathways to a dicationic solvento species, the pathway adopted being dependent on the metal, the bridging ligand, in particular on its size, and on the coordinating ability of the solvent.

Electrophilic attack on diphosphazane-bridged derivatives of diruthenium nonacarbonyl by halogens. Crystal structure of [Ru2(µ-I)I(CO)3{µ-(PriO)2PN(Et)P(OPri)2}2]

Treatment of [Ru2(µ-CO)(CO)4{µ-(RO)2PN(Et)P(OR)2}2](R = Me or Pri) with halogens results in the ready formation of [Ru2X(CO)5{µ-(RO)2PN(Et)P(OR)2}2]+(X = Cl, Br or I) with the halogen atom co-ordinating terminally. These pentacarbonyl species, isolated as their hexafluorophosphate salts, decarbonylate in solution, rapidly in the presence of trimethylamine N-oxide dihydrate but slowly in the absence of this decarbonylating agent, to produce the tetracarbonyl species [Ru2(µ-X)(CO)4{µ-(RO)2PN(Et)p(OR)2}2]+ in which the halogen bridges the two ruthenium atoms. Substitution of the carbonyl groups in these tetracarbonyl species can be effected further by halide ions, either photochemically or by promoting the process using Me3NO·2H2O and thus reaction of [Ru2(µ-X)(CO)4{µ-(RO)2PN(Et)P(OR)2}2]+ with chloride, bromide or iodide ions in the presence of Me3NO·2H2O readily affords [Ru2(µ-X)X(CO)3{µ-(RO)2PN(Et)P(OR)2}2]. Significantly, treatment of [Ru2I(CO)5{µ-(RO)2PN(Et)P(OR)2}2]I3, or [Ru2I(CO)5{µ-(RO)2PN(Et)P(OR)2}2]PF6 in the presence of iodide ions, with an excess of Me3NO·2H2O leads solely to the neutral tetracarbonyl derivative [Ru2I2(CO)4{µ-(RO)2PN(Et)P(OR)2}2]. On the basis of an X-ray crystallographic study on [Ru2(µ-I)I(CO)3{µ-(PriO)2PN(Et)P(OPri)2}2], the tricarbonyl derivatives have structures related to that of [Ru2(µ-X)(CO)4{µ-(RO)2PN(Et)P(OR)2}2]+ with an axial carbonyl group having been replaced by a halide ion.

Synthesis of the solvento species [Ru2(CO)5(solvent){μ- (RO)2PN(Et)P(OR)2}2]2+ and its potential as a source for a wide range of dinuclear derivatives of ruthenium

Treatment of [Ru2(μ-CO)(CO)4{μ-(RO)2PN(Et)P(OR)2}2] (R = Me or Pri), electron-rich derivatives of [Ru2(CO)9], with a twice molar amount of a silver(I) salt in aprotic, weakly co-ordinating solvents such as acetone, acetonitrile or benzonitrile leads to the formation of the solvento species [Ru2(CO)5(solvent)- {μ-(RO)2PN(Et)P(OR)2}2]2+. The structure of the benzonitrile derivative, [Ru2(CO)5(PhCN){μ-(PriO)2PN(Et)P(OPri)2}2](SbF6)2, has been established by X-ray crystallography. The acetone molecule in [Ru2(CO)5(acetone){μ- (RO)2PN(Et)P(OR)2}2]2+ is readily replaced by various nucleophiles to afford products of the type [Ru2(CO)5L{μ-(RO)2PN(Et)P(OR)2}2]2+, where L is a neutral ligand such as CO, Me2C6H3NC, PhCN, C5H5N, H2O, Me2S or SC4H8, [Ru2Y(CO)5{μ-(RO)2PN(Et)P(OR)2}2]2+, where Y− is an anionic ligand such as Cl−, Br−, I−, CN−, SCN−, MeCO2−, CF3CO2− or [Ru2(μ-Y)(CO)4{μ-(RO)2- PN(Et)P(OR)2}2]+ where Y− is an anionic ligand such as Cl−, Br−, I−, SPh−, S2CNEt2−, MeCO2− or CF3CO2−.

Variable Co-ordination behaviour of ethyne and other alkynes towards the diruthenium complexes [Ru2(µ-CO)-(CO)4{(RO)2PN(Et)P(OR)2}2] and [Ru2(µsb-CO)2(CO)2-{(RO)2PN(Et)P(OR)2}2](sb = semi-bridging, R = Me or Pri)

The reaction of ethyne with [Ru2(µ-CO)(CO)4{(RO)2PN(Et)P(OR)2}2] in toluene at 80 °C or with [Ru2(µsb-CO)2(CO)2{(RO)2PN(Et)P(OR)2}2](R = Me or Pri, sb = semi-bridging), in toluene at room temperature, affords almost exclusively the ethenediyl-bridged species [Ru2(µ-σ2-HCCH)(CO)4{(RO)2-PN(Et)P(OR)2}2] for R = Me and the vinylidene-bridged product [Ru2(µ-σ-CCH2)(CO)4{(RO)2-PN(Et)P(OR)2}2] for R = Pri; a second, minor product is also formed in each of these reactions and was identified as [Ru2(µ-σ-CCH2)(CO)4{(MeO)2PN(Et)P(OMe)2}2] and [Ru2(µ-σ2-HCCH)(CO)4-{(PriO)2PN(Et)P(OPri)2}2] respectively. The reactions of [Ru2(µ-CO)(CO)4{(RO)2PN(Et)P(OR)2}2] and [Ru2(µsb-CO)2(CO)2{(RO)2PN(Et)P(OR)2}2] with the terminal alkynes MeCCH, PhCCH and MeO2CCCH also afford mixtures of alkenediyl- and vinylidene-bridged products with the relative yields of these isomers being dependent on the identity of the alkyne and of the bridging diphosphazane ligand. On the other hand reaction with the internal alkyne MeO2CCCCO2Me gives solely the alkenediyl-bridged product [Ru2(µ-σ2-MeO2CCCCO2Me)(CO)4{(RO)2PN(Et)P(OR)2}2] irrespective of the diphosphazane ligand involved. Possible mechanisms for the formation of the two types of products are described. The crystal structure of [Ru2(µ-σ2-HCCH)(CO)4{(MeO)2PN(Et)P(OMe)2}2] is reported.

Synthesis and reactivity of the formally co-ordinatively unsaturated diruthenium hydride [Ru2(µ-H)(µ-CO)(CO)3{µ-(PriO)2PNEtP(OPri)2}2]+ and its co-ordinatively saturated parent [Ru2H(CO)5{µ-(PriO)2PNEtP(OPri)2}2]+

Protonation of the co-ordinatively unsaturated species [Ru2(µsb-CO)2(CO)2(µ-etipdp)2][sb = semi-bridging, etipdp =(PriO)2PNEtP(OPri)2] by acids of non-co-ordinating conjugate bases, e.g. HBF4·OEt2, produced [Ru2(µ-H)(µ-CO)(CO)3(µ-etipdp)2]+ which, as established X-ray crystallographically for the PF6– salt, contains both a bridging carbonyl and a bridging hydride ligand. This cationic species is very susceptible to attack by both neutral and anionic nucleophiles affording a range of product types. For instance, its reactions with anions X– which are capable of functioning as monodentate bridging ligands and which preferentially adopt the closed bridging co-ordination mode, e.g. halide and hydrogensulfide ions, afforded products of the type [Ru2(µ-X)H(µsb-CO)(CO)2(µ-etipdp)2](X = Cl, Br, I, SH, etc.), resulting from the substitution of a carbonyl group by the nucleophile. On the other hand, anionic nucleophiles such as H– and CN– gave addition products of the type [Ru2HX(CO)4(µ-etipdp)2](X = H, CN, etc.) in which the hydride and the X– ligand occupy equatorial sites trans disposed with respect to each other, as established in a separate study for [Ru2H2(CO)4(µ-etipdp)2]. Carbon monoxide also afforded a simple addition product, viz.[Ru2H(CO)5(µ-etipdp)2]+, but the majority of the other neutral nucleophiles studied, particularly the unsaturated systems, yielded products resulting from formal insertion of the nucleophile into the Ru–H bond. Thus sulfur produced [Ru2(µ-SH)(CO)4(µ-etipdp)2]+, while unsaturated nucleophiles of general formula X′Y′, e.g. PhCN and RCCH (R = H, Ph, etc.), gave products of the type [Ru2{µ-X′Y′(H)}(CO)4(µ-etipdp)2]+, e.g.[Ru2{µ-NC(H)Ph}(CO)4(µ-etipdp)2]+ or of the type [Ru2{µ-η2-X′Y′(H)}(CO)4(µ-etipdp)2]+, e.g.[Ru2(µ-η1 : η2-CHCHR)(CO)4(µ-etipdp)2]+. Heterocumulenes X″Y″Z″ such as CS2 and PhNCS behaved similarly affording products of general formula [Ru2{µ-η2-X″Y″(H)Z″}(CO)4(µ-etipdp)2]+ containing five-membered RuX″Y″Z″Ru rings. The co-ordinatively saturated pentacarbonyl [Ru2H(CO)5(µ-etipdp)2]PF6 gave products similar to those afforded by [Ru2(µ-H)(µ-CO)(CO)3(µ-etipdp)2]PF6 on reaction with systems of the type X′Y′ and X″Y″Z″ except that, for terminal alkynes such as PhCCH, alkenylcarbonyl-bridged products, e.g.[Ru2{µ-η2-OC(CHCHPh)}(CO)4(µ-etipdp)2]PF6, are produced. The crystal structures of the following compounds were determined : [Ru2(µ-H)(µ-CO)(CO)3(µ-etipdp)2]PF6, [Ru2(µ-I)H(µsb-CO)(CO)2(µ-etipdp)2], [Ru2{µ-N(CHPh)}(CO)4(µ-etipdp)2]PF6, [Ru2(µ-η1 : η2-CHCH2)(CO)4(µ-etipdp)2]PF6, [Ru2{µ-η2-OC(CHCHPh)}(CO)4(µ-etipdp)2]PF6 and [Ru2{µ-η2-SC(H)NPh}(CO)4(µ-etipdp)2]PF6.

Reactions of diphosphazane-bridged derivatives of diruthenium nonacarbonyl with metal-containing electrophiles: formation of solvento species [Ru2(CO)5(solv){µ-(RO)2PN(Et)P(OR)2}2]2+(R = Me or Pri) and their reactivity towards various nucleophiles

Reaction of [Ru2(µ-CO)(CO)4{µ-(RO)2PN(Et)P(OR)2}2](R = Me or Pri) with equimolar quantities of metal-containing electrophiles such as [AuCl(PPh3)], silver(I) salts, [Cu(MeCN)4]PF6 and HgCl2 has afforded cationic products in which the metal substrate are co-ordinated either terminally as in [Ru2(σ-HgCl)(CO)5{µ-(RO)2PN(Et)P(OR)2}2]+ or in the bridging mode as in [Ru2{µ-Au(PPh3)}(µ-CO)(CO)4{µ-(RO)2PN(Et)P(OR)2}2]+ and [Ru2(µ-AgLx)(µ-CO)(CO)4{µ-(RO)2PN(Et)P(OR)2}2]+(L = pyridine or MeCN, x= 1 or 2). The silver adducts readily rearrange in solution with the nature of the product(s) formed being dependent on the identity of the ligand L and/or the solvent employed For non-protic weakly co-ordinating ligands such as acetonitrile, the silver-bridged cations disproportionate in solution to the parent compound [Ru2(µ-CO)(CO)4{µ-(RO)2PN(Et)P(OR)2}2]and the solvento species [Ru2(CO)5(Solv){µ-(RO)2PN(Et)P(OR)2}2]2+ thereby establishing that the formation of [Ru2-(CO)5(Solv){µ-(RO)2PN(Et)P(OR)2}2]2+ by treatment of [Ru2(µ-CO)(CO)4{µ-(RO)2PN(Et)P(OR)2}2] with a two-fold molar amount of silver(I) ions in non-protic weakly co-ordinating solvents occurs via an inner-sphere mechanism. In the case of the aqua and acetone solvento species [Ru2(CO)5(solv)-{µ-(RO)2PN(Et)P(OR)2}2]2+(solv = H2O or Me2CO), both the solvent molecule and the carbonyl groups are labile with one or both being readily displaced by a wide range of neutral and ionic nucleophiles including carbon monoxide, isonitriles, nitriles, pyridine, 4,4′-bipyridine, dimethyl sulfide and tetra-hydrothiophene, and halide, thiocyanate, benzenethiolate, trifluoroacetate, acetate and hydride ions. The crystal structures of [Ru2{µ-Au(PPh3)}(µ-CO)(CO)4{µ-(MeO)2PN(Et)P(OMe)2}2]SbF6, [Ru2-(CO)5(PhCN){µ-(PriO)2PN(Et)P(OPri)2}2][SbF6]2 and [Ru2(CO)5(SC4H8){µ-(PriO)2PN(Et)P(OPri)2};2]-[SbF6]2 have been determined.