Ruthenium dihydride and hydride fluoride complexes with N-heterocyclic carbenes: A route to C-F bond activation

Abstract

This thesis describes the synthesis of a range of ruthenium dihydride and hydride fluoride complexes and investigates their role in both stoichiometric and catalytic C-F bond activation. The reaction of Ru(PPh3)3(CO)H2 with excess Et3N·3HF at elevated temperature affords the hydride fluoride complex Ru(PPh3)3(CO)HF (1), which has been fully characterised by NMR and IR spectroscopy and X-ray crystallography. The N-heterocyclic carbene (NHC) complexes Ru(NHC)(PPh3)2(CO)HF (NHC = IMe4 (3), IEt2Me2 (4), IiPr2Me2 (5), ICy (6)) are formed at ambient temperature upon addition of an excess of the corresponding NHC ligand to 1. Complexes 3, 4 and 6 isomerise in solution from the trans to cis-phosphine isomers (11-13) over several weeks (relative rates 3 > 4 >> 6), while 5 undergoes both isomerisation and disproportionation in a matter of hours to yield 1, cis-Ru(IiPr2Me2)(PPh3)2(CO)HF (20) and Ru(IiPr2Me2)2(PPh3)(CO)HF (9).

The catalytic hydrodefluorination (HDF) of hexafluorobenzene, pentafluorobenzene, pentafluoropyridine and octafluorotoluene with alkylsilanes was catalysed by the ruthenium NHC complexes Ru(NHC)(PPh3)2(CO)H2 (NHC = IMes (29) SIMes (39), IPr (47), SIPr (48)). The reaction proceeds via formation of the 16-electron hydride fluoride species Ru(NHC)(PPh3)(CO)HF (NHC = IMes (27) SIMes (32), IPr (40), SIPr (41)). Catalytic activity follows the order 47 > 39 > 29 > 48, with 47 able to catalyse the HDF of C6F5H with Et3SiH with a turnover number (TON) of up to 200 and a turnover frequency (TOF) of up to 0.86 h-1. The catalytic reactions revealed (i) a novel selectivity for substitution at the 2-position in C6F5H and C5F5N, (ii) formation of deuterated fluoroarene products when reactions were performed in C6D6 or C6D5CD3 and (iii) a first-order dependence on [fluoroarene] and zero-order relationship with respect to [R3SiH]. Mechanisms are proposed for the HDF of C6F6 and C6F5H; the principal difference being that the latter occurs by initial C-H rather than C-F activation.

The ruthenium hydride fluoride complex Ru(dppp)(PPh3)(CO)HF (62), formed by reaction of 1 with dppp, reacted with IiPr2Me2 and IMes to give the expected complexes Ru(NHC)(dppp)(CO)HF (NHC = IiPr2Me2 (65), IMes (68)) as well as the C-H activated species Ru(NHC)’(dppp)(CO)H (NHC = IiPr2Me2 (66), IMes (67)). The formation of the latter products resulted from the reaction of 65 and 68 with a base (NHC or Et3N). Displacement of PPh3 from Ru(PPh3)(dppp)(CO)H2 (61) by IEt2Me2 yields Ru(IEt2Me2)(dppp)(CO)H2 (74) and the C-H activated complex Ru(IEt2Me2)’(dppp)(CO)H (73), with full conversion to 74 or 73 obtained upon treatment with dihydrogen or trimethylvinylsilane respectively. Reaction of 61 with IMe4 and ICy results in the dihydride species Ru(NHC)(dppp)(CO)H2 (NHC = IMe4 (76), ICy (78)). The dihydride complexes Ru(NHC)(dppp)(CO)H2 (NHC = IMes (69), IEt2Me2 (74), IMe4 (76), ICy (78)) react with Et3N·3HF at room temperature to give the hydride fluoride complexes Ru(NHC)(dppp)(CO)HF (NHC = IMes (68), IEt2Me2 (75), IMe4 (77), ICy (79)). Thermolysis of 78 with C6F6 at elevated temperature generates 79 and Ru(ICy)(dppp)(CO)(C6F5)H (80). The related fluoroaryl complexes Ru(ICy)(dppp)(CO)(C6F4CF3)H (81) and Ru(ICy)(dppp)(CO)(C5F4N)H (82) are formed upon the room temperature C-F activation of C6F5CF3 and C5F5N by 78. Treatment of 78 with the partially fluorinated arene C6F5H results in both C-F and C-H activation to give 79, 80 and Ru(ICy)(dppp)(CO)(C6F4H)H (83). Reaction of 78 with the partially fluorinated substrates 2,3,5,6-C6F4HCF3 and 2,3,5,6-C5F4HN at 393 K results in the formation of 81 and 82 respectively via C-H bond activation.

Date of Award	1 May 2009
Original language	English
Awarding Institution	University of Bath
Supervisor	Michael Whittlesey (Supervisor)