A Planar Nickelaspiropentane Complex with Magnesium-Based Metalloligands: Synthesis, Structure, and Synergistic Dihydrogen Activation

Abstract

The nature of transition-metal–olefin bonding has been explained by the Dewar–Chatt–Duncanson model within a continuum of two extremes, namely, a π-complex and a metallacyclopropane. The textbook rule suggests that a low-spin late-transition-metal–ethylene complex more likely forms a π-complex rather than a metallacyclopropane. Herein, we report a low-spin late-transition-metal–bis-ethylene complex forming an unprecedented planar metalla-bis-cyclopropane structure with magnesium-based metalloligands. Treatment of LMgEt (L = [(DippNCMe)2CH]−, Dipp = 2,6- i Pr2C6H3) with Ni­(cod)2 (cod = 1,5-cyclooctadiene) formed the heterotrimetallic complex (LMg)2Ni­(C2H4)2, which features a linear Mg–Ni–Mg linkage and a planar coordination geometry at the nickel center. Both structural features and computational studies strongly supported the Ni­(C2H4)2 moiety as a nickelaspiropentane. The exposure of (LMg)2Ni­(C2H4)2 to 1 bar H2 at room temperature produced a four-hydride-bridged complex (LMg)2Ni­(μ-H)4. The profile of H2 activation was elucidated by density functional theory calculations, which indicated a novel Mg/Ni cooperative activation mechanism with no oxidation occurring at the metal center, differing from the prevailing mono-metal-based redox mechanism. Moreover, the heterotrimetallic complex (LMg)2Ni­(C2H4)2 catalyzed the hydrogenation of a wide range of unsaturated substrates under mild conditions.

Reversible Dihydrogen Activation and Catalytic H/D Exchange with Group 10 Heterometallic Complexes

“…[33,34] H2 addition to a Ni complex related to 1 has been calculated to occur by a sbond metathesis pathway. [21] Despite these precedents, NBO analysis of key intermediates is consistent with the assignment as a ligand-assisted oxidative addition. Throughout the pathway the NPA charge on Pd becomes more positive (Int-1 = -0.37; Int-2 = -0.17; TS-1 = -0.16; Int-3 = -0.07; TS-2 = -0.05) suggestive of an increase in formal oxidation state of the transition metal on addition of H2.…”

“…[24,25] A closely related species Ni-Mg has very recently been reported by Xu and coworkers. [21] Scheme 2. Synthesis of tetrahydride complexes Pt-Mg•TMEDA and Pt-Zn Both Pt-Mg•TMEDA and Pt-Zn could be isolated as crystalline solids.…”

“…The disclosure of our results was promoted by a very recent publication from Xu and coworkers that demonstrated H2 activation by a Ni-Mg heterometallic complex. [21]…”

Amplifying Reactivity of Metal Hydrides: A Heterotrimetallic NiAl2(μ2-H)2 Catalyst for the Facile Dearomatization of N-Heterocycles

“…[9][10][11] Welldefined heterometallic hydrides have recently been reported, including recent examples featuring Ni-Mg and Ni-Zn bridged hydrides, yet their utilization in catalysis is in their infancy. [12][13][14] Various transition metal complexes are known to facilitate hydroboration of N-heterocycles, such as Rh, 15 Ru, 16 Fe, 17 and a recent example by the Findlater group featuring Ni. 18 In addition, complexes bearing redox-inactive metals such as La, Mg, and Zn as well as phospholenes are known to facilitate hydroboration of heterocycles.…”

Reversible Dihydrogen Activation and Catalytic H/D Exchange with Group 10 Heterometallic Complexes**

“…[26,27] A closely related species Ni-Mg has very recently been reported by Xu and co-workers. [21] Both Pt-Mg•TMEDA and Pt-Zn could be isolated as crystalline solids. X-ray diffraction studies confirmed that the coordination geometry at the transition metal is square planar (Figure 1).…”