Abstract
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 [6MesHC6H5Me][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.
Highlights
In a very recent review, Lin and Power referred to Ni(I) as a ‘... “rare” oxidation state of growing importance’.1,2 In terms of monodentate ligands, the early dependence on tertiary phosphines to stabilise Ni(I)[3,4] has largely been superseded by the use of N-heterocyclic carbenes (NHCs) and these have facilitated the isolation of a wide range of fully characterised four, three- and even two-coordinate Ni(I) species.[5,6]Over the last few years, we have used so-called ringexpanded NHCs (RE-NHCs; carbenes with ring sizes >5) for the preparation of three- and two-coordinate Ni(I) complexes with interesting stoichiometric[7] and catalytic chemistry,[8] as well as novel magnetic properties.[9]
MO theory predicts that the SOMO in a T-shaped d9 complex will be of dx2−y2 character, whereas in a Y-shaped d9 complex, it will be of dxy character (Fig. 10a), in agreement with the dominant character of the Density functional theory (DFT)-calculated orbitals (Fig. 10b)
Of most interest are the three-coordinate d9 complexes, 2, 5, 7 and 8, of general formula [Ni(6-Mes)(PPh3)X]0/+ that distort from ideal D3h symmetry by forming either T-shaped or Y-shaped geometries. These structural differences manifest in different electronic structure characteristics, namely that the SOMO for a T-shape complex is expected to be of dx2−y2 character, whereas for a Y-shape complex, it will be of dxy character
Summary
In a very recent review, Lin and Power referred to Ni(I) as a ‘... “rare” oxidation state of growing importance’.1,2 In terms of monodentate ligands, the early dependence on tertiary phosphines to stabilise Ni(I)[3,4] has largely been superseded by the use of N-heterocyclic carbenes (NHCs) and these have facilitated the isolation of a wide range of fully characterised four-, three- and even two-coordinate Ni(I) species.[5,6]Over the last few years, we have used so-called ringexpanded NHCs (RE-NHCs; carbenes with ring sizes >5) for the preparation of three- and two-coordinate Ni(I) complexes with interesting stoichiometric[7] and catalytic chemistry,[8] as well as novel magnetic properties.[9]. When 1 was treated with TlPF6 in THF, the three-coordinate cationic THF complex [Ni(6-Mes) (PPh3)(THF)][PF6] (2) was isolated as a pale yellow solid in 85% yield (Scheme 1).
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