published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

The crystal structure of Ru₃ (C0)₁₀(C₆H₅)₂PN(C₂H₅) P(C₆H₅)₂ (RUC)PNP) has been determined by single crystal X-ray analysis. The crystals are tri-- clinic with space group PĪ. The unit cell of dimensions a = 14,732; b = 12,386; c = 10,982 Å; α = 104,52; β = 100,64; γ = 94,89° contains two formula units. The unit cell and space group were determined by photographic deJong-Bouman and precession techniques using CuKα radiation. A procedure was developed for more accurate alignment of the crystal for rotating crystal techniques. Intensity data were collected on a Philips PW1100 four-circle diffractometer with MoKα radiation. The positions of the Ru atoms were determined from Patterson syntheses and the remaining atoms located using successive Fourier synthesis. The structure was refined by blocked full-matrix least-squares methods to a residual of R = 0,0537 for 3538 independent reflections with I > 5σ(I) with 300 parameters in the final refinement. The phenyl rings and the CH₂ and CH₃ moities were refined as rigid groups with H-atoms included at fixed positions. A difference Fourier synthesis was done and showed no significant peaks. RUCOPNP is derived from Ru₃(CO)₁₂ by substitution of two equatorial carbonyl ligands by the P atoms of a single edge-bridging (C₆H₅)₂PN(C₂H₅)P( C₆H₅)₂ ligand (PNP). In both complexes the Ru atoms are arranged in a triangle and have distorted octahedral geometry. The introduction of the PNP ligand to the symmetrical parent Ru₃(CO)₁₂ has the following effects: (a) It causes a contraction of the ligand-bridged Ru-Ru bond distance to 2,798 Å whereas the other Ru-Ru distances are 2,860 and 2,848 Å which are close to the distances in the parent compound; (b) It causes the adjacent equatorial carbonyl ligands to rotate towards the PNP ligand by 11,4°. (c) It causes considerable deviations of some of the axial carbonyl ligands from the normal to the plane containing the Ru atoms. / Thesis (M.Sc.)-University of Durban-Westville, 1984.

Cyclometalated phosphine-based PNP and PCP ‘pincer’ complexes of the platinum group metals have been the subject of intense research in recent years, owing to the remarkable stoichiometric and catalytic reactivity exhibited by such complexes. With the goal of discovering new metal-mediated reactivity patterns and extending the versatility of metal pincer chemistry, significant effort has been devoted to the synthesis of structurally and/or electronically related systems where strategic alterations have been introduced to the pincer ligand architecture, including variation of the central and peripheral donor fragments, as well as the ancillary ligand backbone. In this context, the synthesis and study of Ru and Pd complexes supported by pincer-like tridentate ancillary ligands that feature a central anionic phosphorus ([NPN]) or silicon ([PSiP]) donor in the pincer ligand backbone are described herein. The decreased propensity for forming ?-bonds to P was anticipated to lead to a higher degree of electronic unsaturation in complexes supported by tridentate phosphido ligation relative to structurally related metal amido (M-NR2) species. In the case of [PSiP] ligation, the reduced electronegativity of Si relative to C should promote the formation of electron-rich late metal species that can readily undergo oxidative addition reactions. The trans-labilizing silyl donor was also expected to stabilize coordinatively and electronically unsaturated late metal complexes. The synthesis and reactivity of Ru complexes featuring bis(phosphino)silyl ligation of the type [?3-(2-R2PC6H4)2SiMe] ([R-PSiP]; R = Ph, Cy) are described. The 5-coordinate complex [Ph-PSiP]RuCl(PPh3) was shown to be catalytically active for the transfer hydrogenation of ketones in basic isopropanol. These transfer hydrogenation studies are among the first catalytic studies of silyl-pincer complexes and establish [R-PSiP]M species as viable candidates for catalysis. The synthesis and reactivity of 4- and 5-coordinate RuII complexes featuring the [Cy-PSiP] ligand were explored. Reaction of [Cy-PSiP]H with [(p-cymene)RuCl2]2 in the presence of NEt3 and PCy3 resulted in the formation of ([Cy-PSiP]RuCl)2, which serves as a precursor to a series of unprecedented 4-coordinate, formally 14-electron [Cy-PSiP]RuX (X = NHAr, N(SiMe3)2, OtBu) complexes that feature an unusual trigonal pyramidal geometry at Ru. The reactivity of these novel diamagnetic complexes is described, including the reaction of [Cy-PSiP]RuOtBu with amine-boranes resulting in the formation of rare bis(?-BH) complexes. Computational studies confirmed the key role of the strongly ?-donating silyl group of the Cy-PSiP ligand in facilitating the synthesis of such low-coordinate Ru species and enforcing the unusual trigonal pyramidal geometry. The mechanism of ammonia-borane activation was also examined computationally. Lastly, the synthesis and structural characterization of PdII complexes supported by the pincer-like bis(amino)phosphido ligand [?3-(2-Me2NC6H4)2P]- ([NPN]) is described. Examples of ?1-, ?2-, and ?3-NPN coordination to Pd are described, as is the catalytic activity of ([NPN]PdX)2 (X = Cl, OAc, OTf) complexes in the Heck olefin arylation reaction. In an effort to discourage the formation of phosphido-bridged dinuclear complexes, pre-coordination of the Lewis acid BPh3 to [NPN] was pursued. Upon reaction of [N(P?BPh3)N]K with [PdCl(C3H5)]2, the ?1-allyl complex [?3-N(P?BPh3)N]Pd(?1-C3H5) was isolated, which establishes the coordination of a Lewis acid to the phosphido donor of the [NPN] ligand as a viable strategy for encouraging the formation of mononuclear ?3-NPN complexes.

No description available.

No description available.

The coordination chemistry of eight chelating heterocyclic ligands is described. These ligands all contain heterocyclic ring systems with bridgehead nitrogens, and have received little attention in the literature. The ring systems examined are, specifically, imidazo[1,2-a]pyridine, imidazo[1,5-a]pyridine and triazolo[1,5-a]pyridine. The coordination complexes synthesised are mononuclear ruthenium(II) complexes, of the type [Ru(bpy)2(L)]2+. Complexes have been examined by a combination of 1H and 13C NMR, UV-visible spectroscopy, cyclic voltammetry, and X-ray crystallography, in order to study their metal-ligand interactions. A total of six complexes were analysed by single crystal X-ray diffractometry, and the resulting structures are described herein. In general, the inclusion of these ligands had the effect of raising the HOMO and lowering the LUMO, relative to the [Ru(bpy)3]2+ complex. As a result, the complexes were easier to oxidise, harder to reduce, and absorb visible light at longer wavelengths.

No description available.

This thesis describes a broad range of coordination and organometallic chemistry on a series of ruthenium and rhodium complexes towards the aim of activating unsaturated N Ξ N, C=O, and C=C bonds. The dinitrogen complex [RuTp(pzP)(N2)]BPh4 (5) (where Tp= tris(pyrazolyl)borate, and PzP= 1-(2-diphenylphosphinoethyl)pyrazole) was synthesized via displacement of the chloride ligand from the complex [RuTp(pzP)Cl] (1). It was found that N2 coordination proceeded through an unusual oxidation/reduction cycle with the Ru(III) intermediate, [RuTp(pzP)CI]BF4 (6), isolated upon reaction of 1 with AgBF4 in THF. Investigations of the coordination chemistry of the related Tp complex [RuTp(Bp)PPh3] (4) (where Bp= bis(pyrazolyl)borate), resulted in several unusual reactions occurring on the Bp chelate. Reaction of 4 with AgBF4 gave the unusual product [RuTp(BpF')PPh3] (6), which had the Bp B-hydride substituents replaced by fluoride substituents from the BF4 anion, (i.e. BpF,). Alternatively, reaction of 4 with AgOTf, or SOCb, led to the synthesis of the products [RuTp(BpoH)PPh3]OTf (34), and [RuTp(BpoH)PPh3]Cl (35), respectively, which have a single hydroxyl substituent substituted in place of the two B-hydrides to yield a highly unusual neutral borane chelate (BpOH). A series of ruthenium tris(pyrazolyl)methane (Tpm) complexes [RuTpm(PPh3)2Cl]BPh4 (44.BPh4) RuTpm(PPh3CI2] (46), [RuTpm(PPh3)2CI]BPh4 (44.BPh4) [RuTpm(PPh3)(MeCN)Cl]BPh4 (50), [RuTpm(PPh3HMeCN)](BF4)2 (51), [RuTpm(PPh3)(MeCNh](BF4)2 (52), and [RuTpm(MeCNhCI]BPh4 (54.BPh4) were synthesized. These complexes varied in the number of labile acetonitrile ligands they contained, the net charge of the complex, and the presence or absence of strongly coordinating phosphine coligands on the complex. The influence of these properties on the catalytic activity of the complexes for the transfer hydrogenation of acetophenone was investigated. It was shown that the net charge and number of labile MeCN donors on the complex had little influence on the activity of the catalyst. It was also observed that the catalyst [RuTpm(MeCN)2CI]BPh4 (54.BPh4), which does not contain a strongly coordinating PPh3 ligand, would rapidly decompose during catalysis. A series of bis(tert-butylthiomethyl)pyridyl (SNS tBU ) pincer complexes [Ru(SNStBU)(PPh3)Cb] (65), [Ru(SN StBU)(PPh3)(MeCN)CI]BPh4 (66), [Ru(SNStBU)(PPh3)(MeCN)2](BF4h (67), and [Ru(SNStBU)(MeCN)Cb] (68) were synthesized and their as catalysts for the transfer hydrogenation of acetophenone was investigated. The activity of these complexes for as transfer hydrogenation catalysts was shown to increase as the number of labile coligands on the complex increased, with an extremely high transfer hydrogenation activity obtained using 68. The catalytic activity of 68 is one of the fastest to be reported in the literature, achieving a superlative TOF (turnover frequency) of 87360 h- I. The coordination of SNStBu in 66 and the related complexes [Ru(SNS (MeCN)2CI]BPh4 (70), and [Ru(SNS)(MeCN)2CI]2[??-Ag(MeCN)2]2(BF4)4(71) was observed to yield a range of different conformational isomers. These isomers were studied in detail using low temperature NMR and 20 NOESY and COSY IH_1H correlation experiments. The complex 71 was also characterized crystallographically and was shown to have an unusual tetrametallic macrocyclic structure with two [Ru(SNS)(MeCN)2C1r moieties bridged by two [Ag(MeCN)2r ions through a chloro and thioether donor group. The hydrogenation of unsaturated olefinic bonds was achieved using a series of Rh N-heterocyclic carbene (NHC) complexes of the type [Rh(L)(COD)]BPh4 (where L= and NHC-pyrazolyl chelate). A series of NHC-pyrazoly ligands (L) were synthesized that contained varying degrees of steric bulk on the pyrazolyl and NHC donor group. The influence of these steric parameters on the rhodium complex structure and activity of the complexes as catalysts for the hydrogenation of styrene was investigated. It was found that increasing the steric bulk around Rh decreased the activity of the catalyst.