Low-Coordinate NHC-Cobalt(0)-Olefin Complexes: Synthesis, Structure, and Their Reactions with Hydrosilanes

Abstract

The one-pot reaction of CoCl with 1 or 2 equiv of N-heterocyclic carbene (NHC) ligands, vinyltrimethylsilane (vtms), and 2 equiv of reducing reagents (KC or Na(Hg)) proved an effective protocol for the synthesis of three-coordinate NHC-cobalt(0)-olefin complexes, and by which the new low-coordinate cobalt(0) complexes [(NHC)Co(vtms)] (NHC = 1,3-di(2',6'-diethylphenyl)-4,5-(CH)-imidazol-2-ylidene (cyIDep), 6; 1,3-diadamantylimidazol-2-ylidene (IAd), 7) and [(NHC)Co(vtms)] (NHC = 1-(2',4',6'-trimethylphenyl)-3-cyclohexylimidazol-2-ylidene (IMesCy), 8; 1,3-dicyclohexylimidazol-2-ylidene (ICy), 9) were prepared in moderate yields and further characterized by spectroscopic methods. The NHC-cobalt(0)-vtms complexes can serve as low-coordinate NHC-cobalt(0) precursors to react with hydrosilanes. From the reactions of PhSiH with the corresponding NHC-cobalt(0)-vtms complexes, the cobalt silyl complexes [(cyIDep)(H)Co(μ-η:η-HSiPh)(μ-η-HSiPh)Co(H)(cyIDep)] (10), [(NHC)Co(μ-η-SiHPh)Co(NHC)] (NHC = IPr, 13; IAd, 14), and [(IMesCy)Co(H)(SiHPh)] (15) were synthesized. Further reactivity studies established the thermal decomposition of 10 to give [(cyIDep)Co(μ-η-HSiPh)Co(cyIDep)] (11), H, and PhSiH, the reaction of 11 with BuNC (4.5 equiv) to afford [(cyIDep)(BuNC)Co(μ-η-HSiPh)Co(CNBu)] (12) and cyIDep, as well as the conversion of 15 to a silyl-functionalized NHC-cobalt(II) complex 16 upon eliminating H. These cobalt silyl complexes have been characterized by single-crystal X-ray diffraction studies, NMR, absorption, and infrared spectroscopies, as well as elemental analyses. The diversified reaction outcomes demonstrate the rich reaction chemistry of low-coordinate NHC-cobalt(0) complexes.

The Stabilization of Three-Coordinate Formal Mn(0) Complex with NHC and Alkene Ligation

“…Synthesis and Characterization of the Three-Coordinate Formal Mn(0) Complexes Low-valent manganese complexes are mostly known for five-and six-coordinate Mn(ÀI), Mn(0), and Mn(I) complexes with CO, isocyanide, cyclopentadienyl, and phosphine ligands, [50][51][52][53][55][56][57][58][59] and low-valent manganese complexes with the lower coordination numbers of 2-4 are rare. 16,17,46,47,[60][61][62][63] Noting this and also the success of a combined ligand set of NHC with the bis(alkene) ligand dvtms in stabilizing three-coordinate Pt(0), 64 Pd(0), 65 Ni(0), 31,32 Co(0), [35][36][37][38] and Fe(0) 39,41 complexes reported by Markó , Beller, Hazari, Johnson and co-workers, and us, we were curious whether this ligand set could allow stabilization of three-coordinate formal Mn(0) species or not. To our delight, our exploration proved its effectiveness.…”

“…After workup and recrystallization, low-coordinate manganese complexes [(NHC)Mn(h 2 :h 2 -dvtms)] (NHC = IPr, 1; IMes, 2; Et 2 -cAAC, 3) were isolated in good yields (62%, 41%, and 65%, respectively) as crystalline solids from the mixtures (Scheme 1). The synthetic protocol is identical to that used for the preparation of the analogous formal Fe(0) and Co(0) complexes, [35][36][37][38][39]41 but the reduction step has to be run at a low temperature (À40 C) as the trials to synthesize 1 at room temperature gave oily material that resisted crystallization. The failure suggests the high activity of the low-coordinate manganese species.…”

Solvent-Free Hydrosilylation of Alkenes Catalyzed by Well-Defined Low-Valent Cobalt Catalysts

“…This is comparable to the ν(Co(I)−H) (1948 cm −1 ) for the silyl cobalt hydride. 28 The 1 H NMR spectrum of 4 provides the evidence for the hydrido ligand at −13.01 ppm as a singlet. In 31 P NMR spectrum of 4, two peaks appear at 2.65 and −3.62 ppm, respectively.…”

“…Deng reported a series of dinuclear cobalt(I) complexes with similar structures in 2017. 28 Compared with the complexes reported by Deng, the Co−Co bond length (2.6451(8) Å) of complex 4 is longer than those complexes 4). This is consistent with Deng's view.…”

Reusable N-Heterocyclic Carbene Complex Catalysts and Beyond: A Perspective on Recycling Strategies

“…However, in our opinion, these two reactions, especially cross-coupling reactions (such as Suzuki reactions), are overly concerned today, and recyclable catalytic applications of other important NHC metal complex catalysts, such as Cu, − Ag, ,− Au, − Rh, − and Ir, − are treasure troves that are worth exploiting further. Recently, the immobilization of NHC-Co, − NHC-Ni, − and NHC-Fe ,− complexes has attracted great attention and NHC complexes containing these low-priced and environmentally benign metals have been a research hot spot, albeit the corresponding applications remain scarce.…”