The two-coordinate cationic Ni(I) bis-N-heterocyclic carbene complex [Ni(6-Mes)2]Br (1) [6-Mes =1,3-bis(2,4,6-trimethylphenyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene] has been structurally characterized and displays a highly linear geometry with a C-Ni-C angle of 179.27(13)°. Density functional theory calculations revealed that the five occupied metal-based orbitals are split in an approximate 2:1:2 pattern. Significant magnetic anisotropy results from this orbital degeneracy, leading to single-ion magnet (SIM) behavior.

Comproportionation of [Ni(cod)(2)] (cod = cyclooctadiene) and [Ni(PPh(3))(2)X(2)] (X = Br, Cl) in the presence of six-, seven- and eight-membered ring N-aryl-substituted heterocyclic carbenes (NHCs) provides a route to a series of isostructural three-coordinate Ni(I) complexes [Ni(NHC)(PPh(3))X] (X = Br, Cl; NHC = 6-Mes 1, 6-Anis 2, 6-AnisMes 3, 7-o-Tol 4, 8-Mes 5, 8-o-Tol 6, O-8-o-Tol 7). Continuous wave (CW) and pulsed EPR measurements on 1, 4, 5, 6 and 7 reveal that the spin Hamiltonian parameters are particularly sensitive to changes in NHC ring size, N substituents and halide. In combination with DFT calculations, a mixed SOMO of ∣3d z 2〉 and ∣3d x 2-y 2〉 character, which was found to be dependent on the complex geometry, was observed and this was compared to the experimental g values obtained from the EPR spectra. A pronounced (31)P superhyperfine coupling to the PPh(3) group was also identified, consistent with the large spin density on the phosphorus, along with partially resolved bromine couplings. The use of 1, 4, 5 and 6 as pre-catalysts for the Kumada coupling of aryl chlorides and fluorides with ArMgY (Ar = Ph, Mes) showed the highest activity for the smaller ring systems and/or smaller substituents (i.e., 1>4≈6≫5).

Activation of O 2 by the three-coordinate Ni(I) ring-expanded N-heterocyclic carbene complexes Ni(NHC)(PPh 3 )Br (NHC = 6-Mes, 7-Mes) produced the dimeric Ni(II) complexes Ni(6-Mes)(Br)(µ-OH)(µ-O-6-Mes′)NiBr (3) and Ni(7-Mes)(Br)(µ-OH)(µ-O-7-Mes′)NiBr (4) containing oxidized ortho-mesityl groups from one of the carbene ligands. Mass spectrometry was consistent with bis-µ-aryloxy compounds also being formed in these reactions. Low temperature UV-visible spectroscopy showed that the reaction between Ni(6-Mes)(PPh 3 )Br and O 2 was too fast even at ca. -80 °C to yield any observable intermediates. Addition of O 2 to Ni(I) precursors containing a less donating diamidocarbene (6-MesDAC) or the less bulky 6-/7-membered ring diaminocarbene ligands (6-/7-o-Tol) proceeded quite differently, affording phosphine and carbene oxidation products (Ni(O=PPh 3 ) 2 Br 2 and (6-MesDAC)=O) and mononuclear dibromido Ni(II) complexes (Ni(6-/7-o-Tol)(PPh 3 )Br 2 ) respectively.3

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

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

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

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

Bromide abstraction from the three-coordinate Ni(i) ring-expanded N-heterocyclic carbene complex [Ni(6-Mes)(PPh)Br] (1; 6-Mes = 1,3-bis(2,4,6-trimethylphenyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene) with TlPF in THF yields the T-shaped cationic solvent complex, [Ni(6-Mes)(PPh)(THF)][PF] (2), whereas treatment with NaBAr in EtO affords the dimeric Ni(i) product, [{Ni(6-Mes)(PPh)}(μ-Br)][BAr] (3). Both 2 and 3 act as latent sources of the cation [Ni(6-Mes)(PPh)], which can be trapped by CO to give [Ni(6-Mes)(PPh)(CO)] (5). Addition of [(EtSi)(μ-H)][B(CF)] 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)(η-CHMe)][B(CF)] (4), and [6MesHCHMe][B(CF)]. Treatment of 1 with sodium salts of more strongly coordinating anions leads to substitution products. Thus, NaBH yields the neutral, diamagnetic dimer [{Ni(6-Mes)}(BH)] (6), whereas NaBH(CN) gives the paramagnetic monomeric cyanotrihydroborate complex [Ni(6-Mes)(PPh)(NCBH)] (7). Treatment of 1 with NaOBu/NHPh affords the three-coordinate Ni(i) amido species, [Ni(6-Mes)(PPh)(NPh)] (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.