Trans-Dihydride Complexes of Ruthenium Bearing N-Heterocyclic Carbene Ligands

  • Caroline Davies

Student thesis: Doctoral ThesisPhD


Addition of 1,3,4,5 tetramethylimidazol 2 ylidene (IMe4) to Ru(PPh3)3HCl (in the presence of H2) or Ru(PPh3)4H2 gave the all trans isomer of Ru(IMe4)2(PPh3)2H2, whereas 1,3 diethyl 4,5 dimethylimidazol 2 ylidene (IEt2Me2) reacted with Ru(PPh3)4H2 to form cis, cis, trans Ru(IEt2Me2)2(PPh3)2H2. The previously reported trans dihydride complex, Ru(IMe4)4H2, was synthesized by a new method involving the reduction of Ru(IMe4)4Cl2 with KC8/H2. CO reacted with Ru(IMe4)2(PPh3)2H2 to give a mixture of Ru(IMe4)2(PPh3)(CO)H2, Ru(IMe4)(PPh3)2(CO)H2 and Ru(IMe4)2(CO)3; Ru(IEt2Me2)2(PPh3)2H2 reacted in a similar manner, although more slowly, allowing isolation of the monocarbonyl species Ru(IEt2Me2)2(PPh3)(CO)H2. Insertion of CO2 into one of the Ru H bonds of Ru(NHC)2(PPh3)H2 (NHC = IMe4, IEt2Me2) generated mixtures of major and minor isomers of the κ2 formate complexes Ru(IMe4)2(PPh3)(OCHO)H and Ru(IEt2Me2)2(PPh3)(OCHO)H. The hydridic nature of Ru(NHC)2(PPh3)2H2 (NHC = IMe4, IEt2Me2) was apparent by their reactivity toward MeI, which gave [Ru(IMe4)2(PPh3)2H]I, Ru(IEt2Me2)2(PPh3)HI, [Ru(IEt2Me2)2(PPh3)2H]I, and Ru(IEt2Me2)(PPh3)2HI. H/D exchange of Ru(IMe4)2(PPh3)2H2 with C6D6 (elevated temperature) or D2 (room temperature) gave Ru(IMe4)2(PPh3)2HD and Ru(IMe4)2(PPh3)2D2. Addition of P(C6D5)3 to a solution of the dihydride precursor resulted in the rapid substitution of the PPh3 ligands. Addition of D2 to Ru(IMe4)4H2 gave Ru(IMe4)4HD and Ru(IMe4)4D2. The dihydride complex activated benzene and toluene at room temperature and could catalyse H/D exchange between the two solvents. The catalytic activity of Ru(NHC)2(PPh3)2H2 (NHC = IMe4, IEt2Me2) for the reduction of CO2 by pinacolborane has been contrasted with that of the all NHC analogue Ru(IMe4)4H2. Despite the latter being substitutionally inert, it affords a far more active catalyst and yields a wider array of reduction products. These findings suggest that access to vacant coordination sites on the metal centre may not be a prerequisite for catalytic activity in the reduction of CO2 and that, in this case, the presence of a highly nucleophilic hydride ligand may be of greater relevance. The dihydridoborate hydride complexes Ru(IMe4)2(PPh3)(H2Bpin)H and Ru(IEt2Me2)2(PPh3)(H2Bpin)H were isolated by reaction of pinacolborane with Ru(NHC)2(PPh3)2H2 (NHC = IMe4, IEt2Me2) and were structurally characterized.
Date of Award23 Dec 2014
Original languageEnglish
Awarding Institution
  • University of Bath
SupervisorMichael Whittlesey (Supervisor)

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