Stoichiometric and catalytic reactions of ruthenium multi N-heterocyclic carbene complexes

Abstract

This thesis describes the stoichiometric and catalytic C-F bond activation of fluoroarenes using a series of N-heterocyclic carbene (NHC) containing trans-dihydride complexes of ruthenium; the tetrakis-carbene complexes [Ru(NHC)4H2] (NHC = IMe4, IMe2) and the mixed carbene-phosphine species [Ru(NHC)2(PPh3)2H2] and [Ru(NHC)2(P-P)H2] (NHC = IMe4, IEt2Me2; P-P = dppe, dppp, dppm). On the basis of a combination of experimental and computational evidence that these complexes react via attack of their nucleophilic hydride ligands, related and altogether serendipitous discoveries involving bond cleavage of DPEphos and tris(pentafluorophenyl)phosphine are also described.The tetrakis-NHC complex, [Ru(IMe4)4H2] (1), proved to be a remarkably efficient and regioselective catalyst for the hydrodefluorination (HDF) of C6F6 to the 1,4-substituted isomer of C6F2H4 at room temperature. Experimental studies showed that 1 reacted without any creation of a vacant coordination site at the metal involving loss of an NHC. DFT calculations provided complete support for this and showed that reactivity involved nucleophilic attack of the Ru-H in a concerted manner to account for the observed selectivities. [Ru(NHC)2(PPh3)2H2] (NHC = IEt2Me2 (8), IMe4 (9)) were capable of bringing about up to 5 HDF steps on C6F6 to afford C6FH5, but only at elevated temperature. The activity was compromised by poor regioselectivity, which was attributed to the reaction occurring through both five- and six-coordinate Ru species, as well as competitive C-H activation and PPh3/HSiR3 substitution processes. This was circumvented by use of bidentate phosphines, which allowed for almost quantitative HDF of C6F6 to C6FH5 using [Ru(IMe4)2(P-P)HF] (P-P = dppe (22), dppp (23)).C-O bond cleavage of DPEphos was observed upon thermolysis with 9 to afford the phosphinophenolate product, [Ru(IMe4)2(PPh3)(Ph2PC6H4O)H] (29). In the case of the N-ethyl substituted precursor 8, C-O activation was accompanied by C-N cleavage of the carbene to give the phosphinocarbene phosphinophenolate complex, [Ru(IEt2Me2)(IEtMe2(C6H4)PPh2)(Ph2PC6H4O)H] (31). Whereas the reactivity of both 8 and 9 (suggested computationally) is believed to arise as a result of the nucleophilic trans-H-Ru-H geometry, DPEphos activation was also found to occur with the cis-dihydride [Ru(DPEphos)2H2] (33) to give [Ru(DPEphos)(Ph2PC6H4O)H] (34).P(C6F5)3 underwent facile C-F activation with both 9 and [Ru(PPh3)4H2] (32) to give [Ru(IMe4)2(PF2(C6F5))(C6F5)H] (36) and [Ru(PPh3)3HF] (37) respectively. The latter reacted with tertiary silanes and HBpin to afford the silyl trihydride complexes [Ru(PPh3)3(SiR3)H3] (R= Et (38), Ph (39)) and σ-borane dihydride species ([Ru(PPh3)3(HBpin)H2]) respectively.

Date of Award	22 Jan 2018
Original language	English
Awarding Institution	University of Bath
Supervisor	Michael Whittlesey (Supervisor)