Evidence for the Net Addition of Arene C−H Bonds across a Ru(II)−Hydroxide Bond

Abstract

The activation of C-H bonds by transition metal complexes can proceed by multiple pathways, including the 1,2-addition of C-H bonds across early transition metal imido bonds. [1][2][3][4] Such transformations are potentially useful since subsequent N-C reductive elimination (RE) would produce a free amine; however, RE is often a high-energy reaction for early transition metals. In contrast, REs of N-C and O-C bonds form the foundation of routes to aryl amines and ethers using late transition metal catalysts. 5,6 Thus, it is anticipated that accessing the net addition of C-H bonds (regardless of the specific mechanism) across late transition metal M-X bonds (X ) anionic N or O-based ligand) could ultimately lead to the development of routes for hydrocarbon functionalization. Late transition metal oxo and related systems are known to initiate hydrogen atom abstraction; 7 however, these transformations do not involve direct interaction of the metal center with the external C-H substrate. In addition, Ru-amido complexes have been demonstrated to deprotonate "acidic" C-H bonds. 8 The previously reported complex TpRu(PMe 3 ) 2 (OTf) (OTf ) trifluoromethanesulfonate) reacts with CsOH‚H 2 O in refluxing toluene to produce TpRu(PMe 3 ) 2 (OH) (1). A solid-state X-ray diffraction study of 1 has confirmed its structure as a monomeric Ru ( Kinetic studies reveal that H/D exchange at the hydroxide ligand of 1 in C 6 D 6 is first-order with k obs ) 8.0(2) × 10 -5 s -1 (80°C, t 1/2 ≈ 2.4 h). This rate of H/D exchange suggests a more active catalyst for H/D scrambling between H 2 O and C 6 D 6 than has been observed; however, the poor solubility of H 2 O in benzene likely slows the rate of catalysis. The addition of 10 mol % of TpRu(PMe 3 ) 2 (OTf) (precursor to 1) to a solution of 1 and C 6 D 6 does not increase the rate of H/D exchange at the hydroxide ligand of 1. The presence of coordinating ligands suppresses the rate of H/D exchange at the hydroxide ligand as well as the rate of catalysis, which is consistent with a metal-mediated process. For example, heating a solution of complex 1 in C 6 D 6 in the presence of 0.1 equiv (based on 1) of PMe 3 results in no observable H/D exchange at the hydroxide ligand after 168 h at 80°C. The addition of the "non-coordinating" base 2,6-lutidine does not increase the rate of H/D exchange at the hydroxide ligand.In order for the proposed pathway to be viable, complex 1 must access a five-coordinate species on a time scale that is consistent with the observed H/D exchange at the hydroxide ligand. Monitoring the rate of exchange of PMe 3 upon combination of complex 1 with PMe 3 -d 9 at 80°C reveals that k obs ) 1.7(1) × 10 -4 s -1 (t 1/2 ≈ 68 min). Thus, the rate of PMe 3 exchange is greater than the rate of H/D exchange and indicates that external substrates (e.g., C 6 D 6 ) † North Carolina State University. ‡ University of North Texas. § West Virginia University.

Hydrogen−Deuterium Exchange between TpRu(PMe3)(L)X (L = PMe3 and X = OH, OPh, Me, Ph, or NHPh; L = NCMe and X = Ph) and Deuterated Arene Solvents: Evidence for Metal-Mediated Processes

“…Having observed H/D exchange at the Tp 4-positions with 1 and 5, we extended our studies to the complexes TpRu(PMe 3 ) 2 X (X ) Cl, OTf, Me, Ph, NHPh, SH, or NH 2 ). 18,21,22 Table 2 With the observation that TpRu(PMe 3 ) 2 OH (1) undergoes H/D exchange whereas TpRu(PMe 3 ) 2 Cl (7) does not initiate the H/D exchange, we heated (90°C) a 1:1 molar ratio of 1 and 7 in benzene-d 6 . Under these conditions, the 1 H NMR resonances of the Tp 4-positions of both 1 and 7 decrease.…”

“…18,21,22 Some experimental efforts for TpRu(PMe3)2OH (1) have been previously described. 21 Details of computational studies of benzene C-H activation by (Tab)Ru(PH3)2-OH have been reported. 21 TpRu ( TpRu(PMe3)2OPh (5).…”

“…21 Details of computational studies of benzene C-H activation by (Tab)Ru(PH3)2-OH have been reported. 21 TpRu ( TpRu(PMe3)2OPh (5). A mixture of 4 (0.50 g, 0.81 mmol) and NaOPh‚3H2O (0.52 g, 3.1 mmol) was refluxed in approximately 40 mL of toluene for 24 h. The solution was subsequently allowed to cool to room temperature and was filtered through Celite.…”

“…Consistent with this notion, access to TpRu(PMe 3 )(NCMe)Ph (10) decreases the barrier to all H/D exchange processes, and Ru(II) hydroxide and amido complexes with more labile ligand sets will test this hypothesis. In a preliminary report, 21 we disclosed calculated energetics for benzene C-H activation by (Tab)Ru(PH 3 ) 2 OH (Tab ) tris-(azo)borate as a model for the full Tp ligand). The calculated free energy of activation for the conversion of (Tab)Ru(PH 3 )-(OH)(η 2 -C 6 H 6 ) to the C-H activation product (Tab)Ru(PH 3 )-(OH 2 )(Ph) is 17.6 kcal/mol.…”

Mechanisms of C–H bond activation: rich synergy between computation and experiment

“…For the nitrosyl species, SBM is slightly more accessible, while OA/RE is preferred with the anionic CO analogue. 34 The major geometric differences between the SBM transition state and OA/RE intermediate are in the C-W-C angles (40)(41)(42)(43)(44)(45)(46)(47)(48)(49)(50) • wider in the OA intermediate) and W ◊ ◊ ◊ H distances (ca. 0.1 Å shorter in the OA intermediate).…”