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

Abstract

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.

Stable Nickel(I) Complexes with Electron-Rich, Sterically-Hindered, Innocent PNP Pincer Ligands

“…The EPR spectrum of isolated bromo-complex 5 reveals a rhombic signal with g x and g y values that are close to each other, without discernible superhyperfine splitting due to signal broadening. This signal broadening might be due to unresolved splitting from Br and P nuclei similar to other reported Ni I complexes . By comparison, EPR spectra of an analogous chloro-complex 6 shows a rhombic signal (Figure b), with g x = 2.355; g y = 2.293; and g z = 1.990.…”

“…Most research on pincer complexes has historically focused on second- or third-row transition metals and their catalytic activity, or on stable oxidation states of first-row transition metals. In comparison, interest in the isolation of complexes with first-row transition metals in unusual oxidation states ,− and the stabilization of highly reactive species, − has only gained popularity recently. Over the past decade, pincer-based paramagnetic organometallic complexes of first-row transition metals have played an increasing role in catalysis. − In particular, the Ni I oxidation state is paramagnetic and less common than Ni II and Ni 0 , but has growing importance in catalysis − and in coupling reactions. ,− It has been proposed to play an important role in enzymatic processes. − …”

Paramagnetic species in catalysis research: a unified approach towards (the role of EPR in) heterogeneous, homogeneous and enzyme catalysis

“…224 The driving force in this field, in recent years, has focused on the choice of ligands employed to stabilise the active oxidation states, such as N-heterocyclic carbenes (NHCs) and bisphosphines, and these ligands have also been found to improve the overall selectivity in cross-coupling reactions. [225][226][227][228] For example, Whittlesey et al, 224,228,229 designed a series of air-sensitive [Ni I (PPh 3 )(NHC)X] complexes (X ¼ Br, Cl) using bulky NHC ligands. They investigated the effect of sterically demanding substitution at the central heteroaromatic ring which is required to stabilise the low-coordinate and low-valent Ni 11 complex and to avoid any intermolecular reactions.…”

“…They investigated the effect of sterically demanding substitution at the central heteroaromatic ring which is required to stabilise the low-coordinate and low-valent Ni 11 complex and to avoid any intermolecular reactions. 225,229,230 The authors used a combination of CW and pulsed EPR spectroscopy, with complimentary DFT calculations, to extract information about the fundamental properties (structure and bonding) and catalytic efficiency of the complex in the Kumada coupling of aryl-flourides and aryl-chlorides. The EPR spectra revealed a rhombic symmetry for the low coordinated Ni 11 systems, with a large super-hyperfine coupling to the 31 P and 79,81 Br nuclei.…”

“…EPR has been instrumental in leading this revolution of understanding the role of first-row earth abundant metals in C-C cross coupling catalysis. Significant improvements in the use of iron and nickel species that afford greater stability and selectivity, bearing simple salts like ferric salts, 235,236 or ligands such as bisphosphine, 237 and NHC, 225,238,239 is yet to be achieved.…”