The synthesis and coordination chemistry of boranes and boronium reagents
Creators: name: Ward, James
Publisher: name: The Australian National University
Year: 2015

description: This thesis describes investigations into highly reactive boron-functionalised reagents and the reactions they undergo with appropriate organometallic precursors. Trifluoroborate-functionalised alkynes failed to yield isolable products on reaction with various organoruthenium substrates. However, B-ethynyl-N-methyliminoacetatoborane (HCCBMIDA) proved to be a more versatile reagent. The reaction of HCCBMIDA with Ru(CO)2(PPh3)3 or Ru(C2H4)(CO)2(PPh3)2 afforded the sigma-ethynyl hydrido complex Ru(H)(CCBMIDA)(CO)2(PPh3)2 via reversible C-H activation of the terminal alkyne. The reaction of the hydride complex with chloroform gave the corresponding chloro complex RuCl(CCBMIDA)(CO)2(PPh3)2. Reaction of HCCBMIDA with [RuCl(dppe)2][PF6] (dppe = 1,2-bis(diphenylphosphino)ethane) yielded the vinylidene complex [Ru(=C=CHBMIDA)Cl(dppe)2][PF6], deprotonation of which with NEt3 gave the sigma-alkynyl complex [Ru(CCBMIDA)Cl(dppe)2]. The reaction of HCCBMIDA with mer-Ru(H)Cl(CO)(PPh3)3 proceeded via alkyne hydrometallation to yield the 16-electron complex Ru(E-CH=CHBMIDA)Cl(CO)(PPh3)2 as the major product. Reaction of Ru(E-CH=CHBMIDA)Cl(CO)(PPh3)2 with CO, CNC6H2Me3-2,4,6 and NCMe afforded the coordinatively saturated derivatives Ru(E-CH=CHBMIDA)Cl(CO)(L)(PPh3)2 (L = CO, CNC6H2Me3-2,4,6, NCMe), whilst treatment with [NH2Et2][S2CNEt2] and K[Tp] (Tp = hydrotris(pyrazol-1-yl)borate) provided Ru(CH=CHBMIDA)(S2CNEt2)(CO)(PPh3)2 and Ru(CH=CHBMIDA)Tp(CO)(PPh3), respectively. The reaction of HCCBMIDA with Ru(H)(S2CNEt2)(CO)(PPh3)2 afforded the sigma-alkynyl complex Ru(CCBMIDA)(S2CNEt2)(CO)(PPh3)2. The stabilisation of highly reactive boron halides was explored via the synthesis and structural characterisation of a range of new phosphine-borane adducts, R3P.BHnX3-n (n = 1, 2, 3; X = Cl, Br; R = Ph, Cy) via reactions of monodentate phosphines with labile dimethylsulphide borane adducts BHnX3-n.SMe2. Modifications of the procedure were conducted to include the use of multidentate heteroatom donors. The diphosphines dppe, dcpe (1,2-bis(dicyclohexylphosphino)ethane), dmpe (1,2-bis(dimethylphosphino)ethane) and dppf (1,1'-bis(diphenylphosphino)ferrocene) reacted with BH2Br.SMe2 to afford chelated boronium salts, [H2B(diphosphine)]Br. Similarly, triphos (1,1,1-tris(diphenylphosphinomethyl)ethane) was found to react with BH2Br.SMe2 to give [H2B(P,P'-triphos)]Br, which featured a pendant phosphine arm. Whilst this pendant donor was found not to substitute the labile PPh3 ligand in a range of organometallic precursors, halide bridge-splitting reactions ensued with Rh2Cl4Cp*2, Ru2Cl4(C6H4Me-1-iPr-4)2 and Rh2Cl2(COD)2 (COD = 1,5-cyclo-octadiene) to provide monomeric complexes [RhCl2Cp*(P-H2B{triphos})]Br, [RuCl2(C6H4MeiPr-1,4)(P-H2B{triphos})]Br and [RhCl(COD)(P-H2B{triphos})]Br. The mixed phosphine/arsine ligand arphos (Ph2AsCH2CH2PPh2) reacted with BH2Br.SMe2 to generate P-arphos.BH2Br. The pendant arsine arm of arphos.BH2Br was potentially able to coordinate to suitable organometallic precursors, but was not observed to do so experimentally. Instead, loss of the BH2Br substituent facilitated conventional bidentate fashion of the arphos ligand. Thus the reactions of arphos.BH2Br with Rh2Cl4Cp*2 afforded the complex [RhClCp*(As,P-arphos)]Cl, and reaction of arphos.BH2Br with Ru(Ph)Cl(CO)(PPh3)2 afforded RuBr2(CO)(As,P-arphos)(PPh3) and RuBr2(As,P-arphos)2. Synthetic routes to phosphine ligated B-bromo-boronium salts [BrHB(L)2]Br were developed via the reactions of BHBr2.SMe2 with appropriate phosphines (L), though the formation of [BrHB(L)2]+ boronium species proved much less general (cf. [H2B(L)2]+ salts) and was often accompanied by subsequent decomposition. The reactions of BHBr2.SMe2 with dcpe or PMe2Ph provided [BrHB(dcpe)]Br and [BrHB(PMe2Ph)2]Br, respectively, whilst the heterodentate amine/phosphine donor compound amphos (N,N-dimethyl-2-diphenylphosphinoaniline) gave [BrHB(amphos)]Br. Rare examples of borane adducts and boronium salts [H2B(L)2]Br (L = PHCy2, PHPh2, PH2Cy, PH2Mes, PMe2Ph, PHEtPh, arphos) ligated by primary, secondary and tertiary phosphines were obtained via reactions of the phosphines PHCy2, PHPh2, PH2Cy, PH2Mes, PMe2Ph, PHEtPh and arphos with BH2Br.SMe2. In contrast, the formation of [BrHB(L)2]Br was never unequivocally observed upon reaction of PHCy2 and PH2Cy with BHBr2.SMe2 due to their facile decomposition to afford the respective phosphonium salts.