Coordination and Reactivity of Ligands with Unconventional 'Donors'

Abstract

This thesis describes investigations into tridentate facial or meridional ligand frameworks that feature unconventional ‘donors’ including boron, aluminium and silicon and their coordination chemistry with transition metals. Inclusion of electropositive elements within the ligand framework are of interest as they impart unique properties to their corresponding complexes and contrasts that of conventional donors such as phosphorus, nitrogen and sulfur. Chapter one provides an introduction that includes a survey of relevant literature of pincer and scorpionate chemistry. In Chapter two, the series of complexes [Ru(X)(CO)(PPh3){3-H,S,S’-H2B(mt)2}] (X = H, Cl, SePh, BCat, SiCl3, SiMe3) was synthesised for spectroscopic and structural comparisons to gain insight into perturbations on the B–H–Ru interaction affected by the trans X ligand. The trans influence of the X ligand on the H chemical shift of the borohydride B–H–Ru group was assessed. Ligands of strong trans influence resulted in a B–H…Ru interaction consisting of more borohydride character while those of weaker trans influence resulted in a B…H–Ru interaction with metallohydridic character. The osmium complex [OsH(CO)(PPh3){3-H,S,S’-H2B(mt)2}] served as a further (5d transition metal) point of comparison. When the trans ligand was a -organyl (X = Ph, CH=CHPh), the complexes [Ru(X)(CO)(PPh3){3-H,S,S’-H2B(mt)2}] were observed as transient species en route to the ruthenaboratrane [Ru{3-B,S,S’-BH(mt)2}(CO)(PPh3)2]. In Chapter three, a convenient one-pot synthesis from [RuHCl(CO)(PPh3)3] was developed for the first doubly-bridged ruthenaboratrane [Ru{3-B,S,S’-BH(mt)2}(CO)(PPh3)2] (Ru→B), which can alternatively be obtained using [Ru(Ph)Cl(CO)(PPh3)2]. Trace amounts of [Ru(C≡CPh)(CO)(PPh3){3-H,S,S’-BH(mt)2}] and [Ru(2-N,S-mt)(Ph)(CO)(PPh3)2] were crystallographically identified during the syntheses. The reactivity of the Ru→B bond was explored for oxidative conversion to the 3-H,S,S mode of coordination. The reactions revealed either a plethora of products indicative of ligand degradation or no reaction, which suggests a robust M→B interaction. The complex [Ru{3-B,S,S’-BH(mt)2}(CO)(PPh3)2] was shown to serve as a useful precursor to access further examples of doubly-bridged ruthenaboratrane complexes. Phosphine substitution reactions of [Ru{3-B,S,S’-BH(mt)2}(CO)(PPh3)2] afforded the monosubstituted products [Ru{3-B,S,S’-BH(mt)2}(CO)(PPh3)(L)] (L = CO, PMe2Ph) and the disubstituted products [Ru{3-B,S,S’-BH(mt)2}(CO)(L)2] (L = PMe2Ph, P(OMe3)) and [Ru{3-B,S,S’-BH(mt)2}(CO)(Z-Ph2PCH=CHPPh2)]. These complexes consistently feature an elongated ruthenium phosphorus bond trans to boron compared to that trans to sulfur, confirming a pronounced trans influence exerted by the Ru→B bond. Comparisons were made to sulfur-based metallaboratranes, including the triply-bridged series [Ru{4-B,S,S’,S’’-B(mt)3}(CO)(PR3)] (PR3 = PMe2Ph, PCy3, P(OMe)3), which were synthesised and characterised. No general trends concerning the M→B interaction were identified from the complexes assessed, reflecting the constraints of chelation in different ligand frameworks and metals. In Chapter four, the synthesis of aluminium based ligands was explored and yielded the novel aluminium pro-ligand, Li[H2Al(mt)2].THF. Coordination of the [H2Al(mt)2]‒ ligand to transition metal precursors allowed spectroscopic observation of the desired products on ruthenium, osmium and rhenium, but ligand degradation precluded isolation of complexes with an intact [H2Al(mt)2]‒ unit. The greater reactivity and metal-mediated lability of Li[H2Al(mt)2].THF compared to the Na[H2B(mt)2] ligand was a recurrent feature. In Chapter five, coordination of the o-phenylenediamine-based silane HPhSi(NCH2PPh2)2C6H4-1,2 to metal precursors afforded rhodium(III), rhodium(I), iridium and osmium complexes. A third preparatory method toward the reported complex [RhHCl{SiPh(NCH2PPh2)2C6H4-1,2}(PPh3)] was discovered with use of [RhCl(CO)(PPh3)2]. The iridium analogue [IrHCl{SiPh(NCH2PPh2)2C6H4-1,2}(PPh3)] and [RhHCl{SiCl(NCH2PPh2)2C6H4-1,2}(PPh3)] were similarly synthesised. Preparative routes to rhodium(I) square planar complex [Rh{SiPh(NCH2PPh2)2C6H4-1,2}(PPh3)] were explored and include dehydrohalogenation of octahedral rhodium(III) complexes and a direct synthesis pathway from [RhH(PPh3)4]. The strong -donating properties of the silyl unit was evident by elongation of the bond trans to silicon in crystallography studies. Complex [Rh{SiPh(NCH2PPh2)2C6H4-1,2}(PPh3)] displayed facile ligand addition and substitution with dihydrogen, carbon monoxide and norbornadiene to give [RhH2{SiPh(NCH2PPh2)2C6H4-1,2}(PPh3)], [Rh{SiPh(NCH2PPh2)2C6H4-1,2}(CO)(PPh3)], [Rh{SiPh(NCH2PPh2)2C6H4-1,2}(CO)2] and [Rh{SiPh(NCH2PPh2)2C6H4-1,2}(C7H8)]. Direct synthesis resulted in the first benzosiladiazole based osmium complex [OsCl{SiPh(NCH2PPh2)2C6H4-1,2}(PPh3)] from [OsCl2(PPh3)3] and HPhSi(NCH2PPh2)2C6H4. The Appendix Chapter details the reaction of pro-ligands H2C(NCH2PR2)2C10H6-1,8 (R = Cy, Ph) with iridium precursors. Pro-ligand H2C(NCH2PCy2)2C10H6-1,8 was found to readily react with iridium(I) chloro-bridged dimers through double C–H activation to afford iridium(III) NHC complex trans-[IrH2Cl{C(NCH2PCy2)2C10H6}]. The trans hydrides in [IrH2Cl{C(NCH2PCy2)2C10H6}] hindered reductive elimination of dihydrogen. However, the complex underwent spontaneous hydride substitution upon solvation in chloroform, which afforded cis-[IrHCl2{C(NCH2PCy2)2C10H6}]. Hydride replacement reactions were similarly observed for the diphenylphosphine supported trihydride complex, [IrH3{C(NCH2PPh2)2C10H6}]. Show more