Controlled protonolysis of (N f -N f )Pt(CH 3 ) 2 (1; N f -N f ) ArNdCMesCMedNAr, Ar ) 3,5-(CF 3 ) 2 C 6 H 3 ) with HBF 4 ‚Et 2 O in dichloromethane in the presence of small quantities of water gives the BF 4salt of the aqua complex (N f -N f )Pt(CH 3 )(H 2 O) + (6). When dissolved in trifluoroethanol (TFE), 6(BF 4 -) effects the activation of methane and benzene C-H bonds under very mild conditions. Thus, 6 reacted with benzene in TFE-d 3 at ambient temperature to quantitatively yield (N f -N f )Pt(C 6 H 5 )(H 2 O) + and methane after 2-3 h. The use of C 6 D 6 led to multiple incorporation of deuterium into the methane produced and suggests the involvement of methane σ-complex and benzene σ-or π-complex intermediates. When the solution of 6(BF 4 -) was exposed to 13 CH 4 , an exchange reaction produced ca. 50% of (N f -N f )Pt( 13 CH 3 )(H 2 O) + and CH 4 after ca. 48 h at 45 °C. The reaction was inhibited by added water, suggesting that water is reversibly lost from 6 before C-H activation takes place. The use of CD 4 resulted in multiple deuterium incorporation into the methane produced, again implying a Pt-methane σ-complex intermediate. Low-temperature protonation of 1 in dichloromethane-d 2 generated observable Pt(IV) hydride species (N f -N f )Pt(CH 3 ) 2 (H)(L) + . These decomposed via methane elimination, raising the possibility that the observed C-H activation proceeds by an oxidative addition pathway. The reaction between 6 and CH 4 was investigated by DFT calculations using a model system with the HNdCHsCHdNH ligand. The C-H activation was investigated for oxidative addition and σ-bond metathesis pathways starting from the four-coordinate methane complex (N-N)Pt(CH 3 )(CH 4 ) + . The oxidative addition pathway, thermodynamically uphill by 23 kJ/mol (ZPE-corrected data), was favored by 12 kJ/mol relative to the σ-bond metathesis. When a H 2 O ligand was added to the five-coordinate oxidative addition product, the overall oxidative addition reaction was thermodynamically downhill by 33 kJ/mol (partially ZPEcorrected) starting from an H 2 O adduct of (N-N)Pt(CH 3 )(CH 4 ) + with H 2 O electrostatically bonded at the diimine moiety. In this case, the oxidative addition pathway was favored by 20 kJ/mol. The calculations indicated that reductive elimination of methane from the six-coordinate (N-N)Pt(CH 3 ) 2 (H)(H 2 O) + with the hydride and H 2 O ligands trans disposed occurred in concert with dissociation of the aqua ligand.

The C-H activation of toluene and p-xylene at cationic Pt(II) diimine complexes (N-N)Pt(CH(3))(H(2)O)(+)BF(4)(-) (N-N = Ar-N=CMe-CMe=N-Ar; 1(BF(4)(-)), N(f)-N(f), Ar = 3,5-(CF(3))(2)C(6)H(3)); 2(BF(4)(-)), N'-N', Ar = 2,6-(CH(3))(2)C(6)H(3)) has been investigated. The reactions were performed at ambient temperature in 2,2,2-trifluoroethanol (TFE), and after complete conversion of the starting material to mixtures of Pt-aryl/Pt-benzyl complexes and methane, acetonitrile was added to trap the products as more stable acetonitrile adducts. In the reactions with toluene, the relative amounts of products resulting from aromatic C-H activation were found to decrease in the order (N-N)Pt(m-tolyl)(NCMe)(+) > (N-N)Pt(p-tolyl)(NCMe)(+) > (N-N)Pt(o-tolyl)(NCMe)(+) for both 1 and 2. Unlike the reaction at 1, significant amounts of the benzylic activation product (N'-N')Pt(benzyl)(NCMe)(+) were concurrently formed in the C-H activation of toluene at 2. The C-H activation of p-xylene revealed an even more remarkable difference between 1 and 2. Here, the product ratios of (N-N)Pt(xylyl)(NCMe)(+) and (N-N)Pt(p-methylbenzyl)(NCMe)(+) were found to be 90:10 and 7:93 for reactions at 1 and 2, respectively. The elimination of toluene from (N(f)-N(f))Pt(Tol)(2) species (3a-c; a, Tol = o-tolyl; b, Tol = m-tolyl; c, Tol = p-tolyl) after protonolysis with 1 equiv of HBF(4) was investigated. Most notably, protonation in neat TFE followed by addition of acetonitrile gave a 77:23 mixture of (N(f)-N(f))Pt(m-tolyl)(NCMe)(+) (4b) and (N(f)-N(f))Pt(p-tolyl)(NCMe)(+) (4c) from all three isomeric bis(tolyl) complexes 3a-c. The presence of acetonitrile during the protonation reactions resulted in considerably less isomerization. This behavior is explained by an associative mechanism for the product-determining displacement of toluene by the solvent. For the C-H activation reactions, our findings suggest the existence of a dynamic equilibrium between the isomeric intermediates (N-N)Pt(aryl)(CH(4))(+) (aryl = tolyl/benzyl from 1; xylyl/p-methylbenzyl from 2). The observed selectivities might then be explained by steric and electronic effects in the pentacoordinate transition-state structures for the solvent-induced associative elimination of methane from these intermediates.

Exploiting charge-transfer complexes in visible light-promoted single-electron redox reactions is a promising route for opening novel synthetic pathways, and catalytic approaches to complex formation are critical for facilitating this chemistry. This report describes the use of a substituted hydroquinone catalyst to promote radical perfluoroalkylation reactions. Mechanistic studies indicate that the reaction is initiated through formation of a visible light-absorbing halogen bonding complex between the hydroquinone catalyst and the perfluoroalkyl halide radical precursor.

Despite the many recent advances in exploiting stoichiometric charge-transfer complexes in visible light promoted single-electron redox reactions, catalytic approaches to charge-transfer complex formation remain limited. This report describes the radical perfluoroalkylation of electron-rich (hetero)arenes and iodoperfluoroalkylation of alkenes and alkynes promoted by a substituted hydroquinone catalyst. Mechanistic and computational studies indicate that the reaction is initiated by the formation of a visible light absorbing halogen bonding complex between the hydroquinone catalyst and the perfluoroalkyl iodide radical precursor.