The catalytic hydrodefluorination (HDF) of hexafluorobenzene, pentafluorobenzene, and pentafluoropyridine with alkylsilanes is catalyzed by the ruthenium N-heterocyclic carbene (NHC) complexes Ru(NHC)(PPh(3))(2)(CO)H(2) (NHC = SIMes (1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene) 13, SIPr (1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) 14, IPr (1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) 15, IMes (1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) 16). Catalytic activity follows the order 15 > 13 > 16 > 14, with 15 able to catalyze the HDF of C(6)F(5)H with Et(3)SiH with a turnover number of up to 200 and a turnover frequency of up to 0.86 h(-1). The catalytic reactions reveal (i) a novel selectivity for substitution at the 2-position in C(6)F(5)H and C(5)F(5)N, (ii) formation of deuterated fluoroarene products when reactions are performed in C(6)D(6) or C(6)D(5)CD(3), and (iii) a first-order dependence on [fluoroarene] and zero-order relationship with respect to [R(3)SiH]. Mechanisms are proposed for HDF of C(6)F(6) and C(6)F(5)H, the principal difference being that the latter occurs by initial C-H rather than C-F activation.

Transmetalation of the structurally characterized silver N-heterocyclic carbene (NHC) salt [Ag(IpFMe) 2 ] + (1, IpFMe ) 1-pentafluorobenzyl-3-methylimidazol-2-ylidene) to [Rh(COD)Cl] 2 affords Rh(IpFMe)-(COD)Cl (2). The analogous neutral iridium complex, Ir(IpFMe)(COD)Br (5), may be formed upon elimination of MeOH from [Ir(COD)(OMe)] 2 in the reaction with 1-pentafluorobenzyl-3-methylimidazolium bromide. Thermolysis of [Ag(IpFMe) 2 ] + with either 5 or [Ir(COD)Cl] 2 yields the cationic bis-NHC complex [Ir(IpFMe) 2 (COD)](AgX 2 ) (6, X ) Cl, Br), which exists as a mixture of conformers in solution. Addition of CO or AgOTf to 2 gives Rh(IpFMe)(CO) 2 Cl (3) and Rh(IpFMe)(COD)(OTf) (4), respectively. The molecular structures of compounds 1-6 have been determined by X-ray crystallography.

The reaction of Ru(PPh 3 ) 3 (CO)H 2 with excess Et 3 N‚3HF at elevated temperature affords the hydride fluoride complex Ru(PPh 3 ) 3 (CO)HF (1). This reacts with a series of N-heterocyclic carbenes (NHCs) at ambient temperature to form the mono-NHC products Ru(NHC)(PPh 3 ) 2 (CO)HF (NHC ) IMe 4 (2), IEt 2 -Me 2 (3), ICy (4), I i Pr 2 Me 2 (5)). Complexes 2-4 convert from the trans-to cis-phosphine isomers in solution over weeks (relative rates 2 > 3 . 4), while 5 undergoes both isomerization and disproportionation to yield cis-Ru(I i Pr 2 Me 2 )(PPh 3 ) 2 (CO)HF ( 6), 1, and Ru(I i Pr 2 Me 2 ) 2 (PPh 3 )(CO)HF ( 7) in a matter of hours. The molecular structures of compounds 1-4 have been determined by X-ray crystallography.

The hydrido fluorido ruthenium(II) complex [Ru(PPh3)(dppp)(CO)HF] [1, dppp = 1,4‐bis(diphenylphosphanyl)propane], which forms upon reaction of [Ru(PPh3)3(CO)HF] with dppp, reacts with IMes [1,3‐bis(2,4,6‐trimethylphenyl)imidazol‐2‐ylidene] to give the expected carbene‐containing hydrido fluorido complex [Ru(IMes)(dppp)(CO)HF] (2), as well as the C–H activated species [Ru(IMes)′(dppp)(CO)H] (3). The formation of the latter product results from the reaction of 2 with a base (IMes or Et3N). Displacement of PPh3 from [Ru(PPh3)(dppp)(CO)H2] by ICy (1,3‐dicyclohexylimidazol‐2‐ylidene) yields [Ru(ICy)(dppp)(CO)H2] (7), which upon reaction with Et3N·3HF, gives [Ru(ICy)(dppp)(CO)HF] (8). Thermolysis of 7 with C6F6 at elevated temperature generates 8 and [Ru(ICy)(dppp)(CO)(C6F5)H] (9). The related fluoroaryl complexes [Ru(ICy)(dppp)(CO)(C6F4CF3)H] (10) and [Ru(ICy)(dppp)(CO)(C5F4N)H] (11) are formed upon the room temperature C–F activation of C6F5CF3 and C5F5N by 7, but also by C–H activation of the partially fluorinated substrates p‐C6F4HCF3 and p‐C5F4HN.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

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