Boryl Complexes of Ruthenium and Osmium

Abstract

The synthesis and chemistry of new ruthenium and osmium complexes containing single covalent bonds to boron (LnM-BR2) is the subject of this thesis. In chapter one a thorough review of transition metal-boron chemistry is presented, covering the bonding, preparative methods, reactivity and molecular structures of transition metal-boryl complexes. Included in this review are the results of an SCF-Xα calculation, carried out by P. Hunt and P. A. Schwerdtfeger of this department, on the model compound OsCl(B[OH]2)(CO)2(PH3)2 to ascertain the level, if any, of π-bonding between boron and osmium in the structurally characterised complex OsCl(B[OH]2)(CO)2(PPH3)2 described in chapter six. The results of this calculation revealed that π-bonding between osmium and boron is modest in this compound. In chapter two the synthesis of the first coordinatively unsaturated boryl complexes of ruthenium and osmium is describes. These species were produced by either reaction of a ruthenium-hydride RuHCl(CE)(PPh3)3 (E=O, S, Np-tolyl), or an osmium σ-aryl complex OsCl(Ph)(CE)(PPh3)2 (E=O, S) with a hetero-substituted benzannelated borole or the borane HBO2C10H6 derived from 2,3-dihydroxynaphthalene. These complexes were found to generally exhibit a high tolerance to both oxygen and moisture. Furthermore the boryl ligands of these complexes displayed a remarkable resistance to derivatisation. The synthesis and properties of a novel 14 electron catecholboryl complex RuCl(BO2C6H4)-(PPh3)2 are also described. The synthesis and reactivity of a number of new coordinatively saturated boryl complexes forms the subject matter of chapter three. As anticipated, the newly formed five coordinate carbonyl-containing boryl complexes described in chapter two, react rapidly with Lewis bases such as CO and CNp-tolyl, to afford the corresponding six coordinate octahedral species. Mixtures of cis and trans dicarbonyl isomers were produced for the complexes RuCl(B{NH}2C6H4)(CO)2(PPh3)2 and RuCl(B{NH}SC6H4)(CO)2(PPh3)2. Two isomers of RuCl(BO2C6H4)(CO)(CNp-tolyl)(PPh3)2 were also obtained. The crystal structures of the three closely related osmium dicarbonyl complexes, OsCl(BO2C6H4)(CO)2(PPh3)2, OsCl(B{NH}2C6H4)(CO)2(PPh3)2 and OsCl(B{NH}SC6H4)(CO)2(PPh3)2 were determined. Reaction of MCl(B{NH}SC6H4)(CO)(PPh3)2 (M=Ru, Os) with AgSbF6 in the presence of acetonitrile affords the cations [M(B{NH}SC6H4)(CO)(CH3CN)2-(PPh3)2]SbF6. The five coordinate complexes MCl(BO2C6H4)(CO)(PPh3)2(M = Ru, Os) react with anionic ligands such as dimethyldithiocarbamate to form the coordinatively saturated complexes M(BO2C6H4)(η2-S2CNMe2)(PPh3)2. Oxidative addition of catecholborane and 1,3-dimethyl-2H-1,3,2-benzodiazaborole to Os(CO)2(PPh3)3 and OsCl(NO)(PPh3)3 gave the complexes OsH(BO2C6H4)(CO)2(PPh3)2, OsH(B{NMe}2C6H4)(CO)2(PPh3)2 and OsHCl(BO2C6H4)(NO)(PPh3)2. Two isomers of OsHCl(BO2C6H4)(NO)(PPh3)2 were generated. Chapter four begins with a brief synopsis of the migratory insertion rearrangement for transition metal complexes in general. The five coordinate thiocarbonyl-containing boryl complexes, MCl(B(X)(Y)C6H4)(CS)(PPh3)2 (M=Ru, X=Y=O, X=NH, Y=S; M=Os, X=Y=O) react with CO. Coordination of the sixth ligand is followed by a migratory insertion rearrangement to give the novel dihapto bora-thioacyl compounds MCl(η2-C[S]-B(X)(Y)C6H4)(CO)(PPh3)2. In the case of OsCl(BO2C6H4)(CS)(PPh3)2, the intermediate six coordinate complex OsCl(BO2C6H4)(CS)(CO)(PPh3)2 was isolated prior to rearrangement. The crystal structure of RuCl(η2-C[S]-B{NH}SC6H4)(CO)(PPh3)2 has been obtained, confirming the dihapto mode of coordination for the bora-thioacyl ligand in these complexes. Chapter five begins with a brief review of transition metal catalysed hydroboration of olefins and alkynes. Reaction of the five coordinate boryl complex RuCl(BO2C6H4)(CO)(PPh3)2 with acetylene affords the compound RuCl(CH=CH-{BOC6H4O})(CO)(PPh3)2 resulting from stoichiometric insertion of the alkyne into the Ru-B bond. The insertion of unsaturated molecules, such as alkynes, into M-B bonds is thought to be an important step in some transition metal catalysed hydroborations. The geometry about the double bond is cis, with an oxygen of the catecholboryl group weakly interacting with the metal. In solution, this compound was found to exhibit facile rotation about the C(olefin)-B bond. Variable temperature 1H and 13C NMR spectra were obtained to probe this phenomenon. The X-ray crystal structure of this compound has been determined, confirming the cis geometry of the ethenyl subunit and the weak Ru-O interaction. Reaction of RuCl(CH=CH{BOC6H4O})(CO)(PPh3)2 with 1,2-dihydroxyethane led to the formation of the complex RuCl(CH=CH[BOCH2CH2O)(CO)(PPh3)2. Carbonylation of RuCl(CH=CH[BOCH2CH2O](CO)(PPh3)2 followed by treatment with ethanol gave RuCl(CH=CH[B(OEt){OEt}](CO)(PPh3)2. Both of these derivatives retain the cis geometry at the double bond and the Ru-O interaction. By an extension of the synthetic methodology described in chapter two the dichloroboryl complexes OsCl(BCl2)(CO)(PPh3)2 and OsCl(BCl2)(CNp-tolyl)(PPh3)2 have been prepared. The synthesis and reaction chemistry of these compounds are the subject of chapter six. For the compound OsCl(BCl2)(CO)(PPh3)2 substitution reactions at the boron atom with various oxygen and nitrogen nucleophiles is presented. X-ray crystal structures of OsCl(B[OEt]2)(CO)(PPh3)2, OsCl(B[OH]2(CO)(PPh3)2 and OsCl(B{NMe}2C6H4)(CO)-(PPh3)2 have been obtained. A variable temperature 1H NMR study of OsCl(B{NMe}2C6H4)(CO)(PPh3)2 showed the presence of a dynamic process ascribed to hindered rotation about the Os-B bond. The results of this study which include a determination of ΔG≠ for the process are presented. In addition to substitution chemistry centred at boron, the reactions of OsCl(BCl2)(CO)(PPh3)2 and its five coordinate derivatives with neutral Lewis bases such as CO and CNCH2-p-tosyl are described. The structure of the compound OsCl(B[OH]2)(CO)2(PPh3)2 has been verified by X-ray crystallography. Attempts to develop a rational synthesis leading to a species containing a metallated difluoroboryl moiety resulted in the generation of an unusual compound, which contained a hydroxytrifluoroborate ligand, OsCl(OH•BF3)(CO)2(PPh3)2. The synthesis of this compound, together with an X-ray structural determination are reported. The preparation, characterisation and reactivity of the unusual complexes MCl(η3-C6H7)(CO)(PPh3)2 (M=Os, Ru) are the subject of chapter seven. Initially the osmium compound was prepared by the attempted insertion of acetylene into the metal-boron bond of OsCl(BO2C6H4)(CO)(PPh3)2. The ruthenium and osmium compounds were later prepared by reaction of a metal-hydride MHCl(CO)(PPh3)3 with excess acetylene. The osmium complex OsCl(η3-C6H7)(CO)(PPh3)2 was thoroughly characterised by 1D and 2D multinuclear NMR spectroscopies enabling the framework of the C6H7 ligand to be deciphered. The novel 5-methylene-2-cyclopenten-1-yl (C6H7) ligand resulted from the metal catalysed trimerisation of three acetylene molecules. A mechanism for the formation of these complexes is proposed. A number of cationic derivatives, [Os(η3-C6H7)(CO)(L)(PPh3)2]+ (L = CH3CN, CO, CNp-tolyl) generated by abstraction of the chloride ligand with silver salts are described. [Os(η3-C6H7)(CO)(CH3CN)(PPh3)2]SbF6 was structurally characterised by an X-ray diffraction study, confirming the identity of the 5-methylene-2-cyclopenten-1-yl ligand, C6H7, and its exocyclic η3-mode of coordination. Reaction of OsCl(η3-C6H7)(CO)(PPh3)2 with AgClO4 produced the neutral species Os(η3-C6H7)(CO)(OClO3)(PPh3)2. Abstraction of the metal bound chloride in OsCl(η3-C6H7)(CO)(PPh3)2 with AgSbF6, in the absence of a suitable coordinating ligand, afforded [Os(η5-C5H4CH3)(CO)(PPh3)2]SbF6. This compound, which contains a methylcyclopentadienyl ligand, resulted from a 1,3-hydrogen shift involving the 5-methylene-2-cyclopenten-1-yl ligand. A possible mechanism to account for its formation is discussed. The structure of this compound has been confirmed by X-ray crystallography.