Mono- and dinuclear Ni(I) products formed upon bromide abstraction from the Ni(I) ring-expanded NHC complex [Ni(6-Mes)(PPh₃)Br]

Bromide abstraction from the three-coordinate Ni(I) ring-expanded N-heterocyclic carbene complex [Ni(6-Mes)(PPh3)Br] (1; 6-Mes = 1,3-bis(2,4,6-trimethylphenyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene) with TlPF6 in THF yields the T-shaped cationic solvent complex, [Ni(6-Mes)(PPh3)(THF)][PF6] (2), whereas treatment with NaBArF4 in Et2O affords the dimeric Ni(I) product, [{Ni(6-Mes)(PPh3)}2(μ-Br)][BArF4] (3). Both 2 and 3 act as latent sources of the cation [Ni(6-Mes)(PPh3)]+, which can be trapped by CO to give [Ni(6-Mes)(PPh3)(CO)]+ (5). Addition of [(Et3Si)2(μ-H)][B(C6F5)4] to 1 followed by work up in toluene results in the elimination of phosphine as well as halide to afford a co-crystallised mixture of [Ni(6-Mes)(η2-C6H5Me)][B(C6F5)4] (4), and [6MesH⋯C6H5Me][B(C6F5)4]. Treatment of 1 with sodium salts of more strongly coordinating anions leads to substitution products. Thus, NaBH4 yields the neutral, diamagnetic dimer [{Ni(6-Mes)}2(BH4)2] (6), whereas NaBH3(CN) gives the paramagnetic monomeric cyanotrihydroborate complex [Ni(6-Mes)(PPh3)(NCBH3)] (7). Treatment of 1 with NaOtBu/NHPh2 affords the three-coordinate Ni(I) amido species, [Ni(6-Mes)(PPh3)(NPh2)] (8). The electronic structures of 2, 5, 7 and 8 have been analysed in comparison to that of previously reported 1 using a combination of EPR spectroscopy and density functional theory

Stereoelectronic Effects in C−H Bond Oxidation Reactions of Ni(I) N‑Heterocyclic Carbene Complexes

Activation of O2 by the three-coordinate Ni(I) ring-expanded N-heterocyclic carbene complexes Ni(RE-NHC)(PPh3)Br (RE-NHC = 6-Mes, 1; 7-Mes, 2) produced the structurally characterized dimeric Ni(II) complexes Ni(6-Mes)(Br)(&mu;-OH)(&mu;-O-6-Mes&prime;)NiBr (3) and Ni(7-Mes)(Br)(&mu;-OH)(&mu;-O-7-Mes&prime;)NiBr (4) containing oxidized ortho-mesityl groups from one of the carbene ligands. NMR and mass spectrometry provided evidence for further oxidation in solution to afford bis-&mu;-aryloxy compounds; the 6-Mes derivative was isolated, and its structure was verified. Low-temperature UV&ndash;visible spectroscopy showed that the reaction between 1 and O2 was too fast even at ca. &minus;80 &deg;C to yield any observable intermediates and also supported the formation of more than one oxidation product. Addition of O2 to Ni(I) precursors containing a less electron-donating diamidocarbene (6-MesDAC, 7) or less bulky 6- or 7-membered ring diaminocarbene ligands (6- or 7-o-Tol; 8 and 9) proceeded quite differently, affording phosphine and carbene oxidation products (Ni(O═PPh3)2Br2 and (6-MesDAC)═O) and the mononuclear Ni(II) dibromide complexes (Ni(6-o-Tol)(PPh3)Br2 (10) and (Ni(7-o-Tol)(PPh3)Br2 (11)) respectively. Electrochemical measurements on the five Ni(I) precursors show significantly higher redox potentials for 1 and 2, the complexes that undergo oxygen atom transfer from O2.</p

Stereoelectronic Effects in C–H Bond Oxidation Reactions of Ni(I) N‑Heterocyclic Carbene Complexes

Activation of O₂ by the three-coordinate Ni­(I) ring-expanded N-heterocyclic carbene complexes Ni­(RE-NHC)­(PPh₃)Br (RE-NHC = 6-Mes, <b>1</b>; 7-Mes, <b>2</b>) produced the structurally characterized dimeric Ni­(II) complexes Ni­(6-Mes)­(Br)­(μ-OH)­(μ-O-6-Mes′)­NiBr (<b>3</b>) and Ni­(7-Mes)­(Br)­(μ-OH)­(μ-O-7-Mes′)­NiBr (<b>4</b>) containing oxidized <i>ortho</i>-mesityl groups from one of the carbene ligands. NMR and mass spectrometry provided evidence for further oxidation in solution to afford bis-μ-aryloxy compounds; the 6-Mes derivative was isolated, and its structure was verified. Low-temperature UV–visible spectroscopy showed that the reaction between <b>1</b> and O₂ was too fast even at ca. −80 °C to yield any observable intermediates and also supported the formation of more than one oxidation product. Addition of O₂ to Ni­(I) precursors containing a less electron-donating diamidocarbene (6-MesDAC, <b>7</b>) or less bulky 6- or 7-membered ring diaminocarbene ligands (6- or 7-<i>o</i>-Tol; <b>8</b> and <b>9</b>) proceeded quite differently, affording phosphine and carbene oxidation products (Ni­(OPPh₃)₂Br₂ and (6-MesDAC)O) and the mononuclear Ni­(II) dibromide complexes (Ni­(6-<i>o</i>-Tol)­(PPh₃)­Br₂ (<b>10</b>) and (Ni­(7-<i>o</i>-Tol)­(PPh₃)­Br₂ (<b>11</b>)) respectively. Electrochemical measurements on the five Ni­(I) precursors show significantly higher redox potentials for <b>1</b> and <b>2</b>, the complexes that undergo oxygen atom transfer from O₂

Mechanistic Studies of the Rhodium NHC Catalyzed Hydrodefluorination of Polyfluorotoluenes

The six-membered-ring NHC complexes Rh­(6-NHC)­(PPh₃)₂H (6-NHC <b>=</b> 6-ⁱPr (<b>1</b>), 6-Et (<b>2</b>), 6-Me (<b>3</b>)) have been employed in the catalytic hydrodefluorination (HDF) of C₆F₅CF₃ and 2-C₆F₄HCF₃. Stoichiometric studies showed that <b>1</b> reacted with C₆F₅CF₃ at room temperature to afford <i>cis</i>- and <i>trans</i>-phosphine isomers of Rh­(6-ⁱPr)­(PPh₃)₂F (<b>4</b>), which re-form <b>1</b> upon heating with Et₃SiH. Although up to three consecutive HDF steps prove possible with C₆F₅CF₃, the ultimate effectiveness of the catalysts is limited by their propensity to undergo C–H activation of partially fluorinated toluenes to give, for example, Rh­(6-ⁱPr)­(PPh₃)₂(C₆F₄CF₃) (<b>7</b>), which was isolated and structurally characterized

Copper Diamidocarbene Complexes: Characterization of Monomeric to Tetrameric Species

Treatment of CuCl with 1 equiv of the in situ prepared <i>N</i>-mesityl-substituted diamidocarbene 6-MesDAC produced a mixture of the dimeric and trimeric copper complexes [(6-MesDAC)­CuCl]₂ (<b>1</b>) and [(6-MesDAC)₂(CuCl)₃] (<b>2</b>). Combining CuCl with isolated, free 6-MesDAC in 1:1 and 3:2 ratios gave just <b>1</b> and <b>2</b>, respectively, while increasing the ratio to >5:1 allowed the isolation of small amounts of the tetrameric copper complex [(6-MesDAC)₂(CuCl)₄] (<b>3</b>). Efforts to bring about metathesis reactions of <b>1</b> with MO^tBu (M = Li, Na, K) proved successful only for M = Li to afford the spectroscopically characterized ate product [(6-MesDAC)­CuCl·LiO^tBu·2THF] (<b>5</b>). Attempts to crystallize this species instead gave a 1:1 mixture of <b>1</b> and the monomer [(6-MesDAC)­CuCl] (<b>6</b>). The X-ray structures of <b>1</b>–<b>3</b> and <b>1</b> + <b>6</b>, along with the cation [Cu­(6-MesDAC)₂]⁺ (<b>4</b>), have been determined

Synthesis and Small Molecule Reactivity of <i>trans</i>-Dihydride Isomers of Ru(NHC)₂(PPh₃)₂H₂ (NHC = N‑Heterocyclic Carbene)

Addition of IMe₄ (1,3,4,5-tetra­methyl­imidazol-2-ylidene) to Ru­(PPh₃)₃­HCl (in the presence of H₂) or Ru­(PPh₃)₄­H₂ gave the all-trans isomer of Ru­(IMe₄)₂­(PPh₃)₂­H₂ (<b>1a</b>), whereas 1,3-diethyl-4,5-dimethylimidazol-2-ylidene (IEt₂­Me₂) reacted with Ru­(PPh₃)₄­H₂ to form <i>cis</i>,<i>cis</i>,<i>trans</i>-Ru­(IEt₂­Me₂)₂­(PPh₃)₂­H₂ (<b>2b</b>). H/D exchange of <b>1a</b> with C₆D₆ (elevated temperature) or D₂ (room temperature) gave Ru­(IMe₄)₂­(PPh₃)₂­HD (<b>1a-HD</b>) and Ru­(IMe₄)₂­(PPh₃)₂­D₂ (<b>1a-D</b>_<b>2</b>). CO reacted with <b>1a</b> to give a mixture of Ru­(IMe₄)₂­(PPh₃)­(CO)­H₂ (<b>3</b>) and Ru­(IMe₄)₂­(CO)₃ (<b>4</b>); <b>2b</b> reacted in a similar manner, although more slowly, allowing isolation of the monocarbonyl species Ru­(IEt₂­Me₂)₂­(PPh₃)­(CO)­H₂ (<b>5</b>). Insertion of CO₂ into one of the Ru–H bonds of <b>1a</b> and <b>2b</b> generated mixtures of major and minor isomers of the κ²-formate complexes Ru­(IMe₄)₂­(PPh₃)­(OCHO)H (<b>7</b>/<b>8</b>) and Ru­(IEt₂Me₂)₂(PPh₃)­(OCHO)H (<b>9</b>/<b>10</b>). The hydridic nature of <b>1a</b> and <b>2b</b> was apparent by their reactivity toward MeI, which gave [Ru­(IMe₄)₂­(PPh₃)₂­H]I (<b>11</b>), Ru­(IEt₂­Me₂)₂­(PPh₃)­HI (<b>12</b>), [Ru­(IEt₂­Me₂)₂­(PPh₃)₂­H]I (<b>13</b>), and Ru­(IEt₂­Me₂)­(PPh₃)₂­HI (<b>14</b>). Complexes <b>1a</b>, <b>2b</b>, <b>5</b>, <b>9</b>, <b>11</b>, <b>13</b>, and <b>14</b> were structurally characterized

Rh–FHF and Rh–F Complexes Containing Small <i>N</i>‑Alkyl Substituted Six-Membered Ring N‑Heterocyclic Carbenes

Heating the six-membered ring N-heterocyclic carbenes 6-Me and 6-Et with Rh­(PPh₃)₄H afforded the rhodium monocarbene hydride complexes Rh­(6-NHC)­(PPh₃)₂H as a mixture of cis- and trans-P,P isomers (<b>4a</b>/<b>b</b>, NHC = 6-Me; ratio = 1:20; <b>5a</b>/<b>b</b>, NHC = 6-Et; ratio = 1:9). Reaction of <b>4a</b>/<b>b</b> with Et₃N·3HF gave only the trans-P,P isomer of the bifluoride complex Rh­(6-Me)­(PPh₃)₂(FHF) (<b>6b</b>), whereas <b>5a</b>/<b>b</b> reacted to form Rh­(6-Et)­(PPh₃)₂(FHF) as a mixture of cis- and trans-phosphine isomers (<b>7a</b>/<b>b</b>). Variable temperature ¹H and ¹⁹F NMR spectroscopy showed that <b>6b</b> and the previously reported 6-ⁱPr carbene analogue cis-Rh­(6-ⁱPr)­(PPh₃)₂(FHF) (<b>2a</b>; Organometallics 2012, 41, 8584) were fluxional in solution. ¹⁹F Magnetization transfer experiments revealed F exchange in both compounds and afforded similar Δ<i>H</i>^⧧ values (<b>2a</b>, 51 ± 5 kJ mol^–1; <b>6b</b>, 60 ± 6 kJ mol^–1) but somewhat different values of Δ<i>S</i>^⧧ (<b>2a</b>, −70 ± 17 J mol^–1 K^–1; <b>6b</b>, −27 ± 18 J mol^–1 K^–1). The fluoride complexes cis-Rh­(6-Me)­(PPh₃)₂F (<b>8a</b>), cis-/trans-Rh­(6-Et)­(PPh₃)₂F (<b>9a</b>/<b>b</b>), and the previously reported 6-ⁱPr analogue <b>3a</b> could be formed upon C–F activation of CF₃CFCF₂ by <b>4a</b>/<b>b</b>, <b>5a</b>/<b>b</b>, and Rh­(6-ⁱPr)­(PPh₃)₂H (<b>1a</b>/<b>b</b>), respectively. Complex <b>3a</b> reacted slowly with H₂ to partially reform <b>1a</b>/<b>b</b> but rapidly with CO to give Rh­(6-ⁱPr)­(PPh₃)­(CO)F (<b>10</b>) and Rh­(PPh₃)₂(CO)­F, and also quickly with Me₃SiCF₃ to form cis-Rh­(6-ⁱPr)­(PPh₃)₂(CF₃) (<b>11a</b>). Complexes <b>4b</b>, <b>5b</b>, <b>6b</b>, <b>7b</b>, and <b>11a</b> were structurally characterized

Ring-Expanded N‑Heterocyclic Carbene Complexes of Rhodium with Bifluoride, Fluoride, and Fluoroaryl Ligands

Thermolysis of Rh­(PPh₃)₄H in the presence of the six-membered N-heterocyclic carbene 1,3-bis­(2-propyl)-3,4,5,6-tetrahydropyrimidin-2-ylidine (6-ⁱPr) gave the monocarbene complex Rh­(6-ⁱPr)­(PPh₃)₂H as a 1:2 mixture of the cis- and trans-phosphine isomers <b>1a</b> and <b>1b</b>. This same isomeric mixture was formed as the ultimate product from treating Rh­(PPh₃)₃(CO)H with 6-ⁱPr at room temperature, although pathways involving both CO and PPh₃ loss were observed at initial times. Treatment of <b>1a</b>/<b>1b</b> with Et₃N·3HF generated the bifluoride complex <i>cis</i>-Rh­(6-ⁱPr)­(PPh₃)₂(FHF) (<b>2a</b>), which upon stirring with anhydrous Me₄NF was converted to the rhodium fluoride complex <i>cis</i>-Rh­(6-ⁱPr)­(PPh₃)₂F (<b>3a</b>). Thermolysis of <b>1a</b>/<b>1b</b> with C₆F₆ resulted in C–F bond activation to afford a mixture of <b>3a</b> and the pentafluorophenyl complex <i>trans</i>-Rh­(6-ⁱPr)­(PPh₃)₂(C₆F₅) (<b>5b</b>). Complexes <b>1b</b>,<b> 2a</b>, <b>3a</b>, and <b>5b</b> were structurally characterized