Divergent reactivity of a new dinuclear xanthene-bridged bis(iminopyridine) di-nickel complex with alkynes

Abstract

The reaction of a dinucleating bis(iminopyridine) ligand L bearing a xanthene linker (L = N,N'-(2,7-di-tert-butyl-9,9-dimethyl-9H-xanthene-4,5-diyl)bis(1-(pyridin-2-yl)methanimine)) with Ni(COD)(DPA) (COD = cyclooctadiene, DPA = diphenylacetylene) leads to the formation of a new dinuclear complex Ni(L)(DPA). Ni(L)(DPA) can also be obtained in a one-pot reaction involving Ni(COD), DPA and L. The X-ray structure of Ni(L)(DPA) reveals two square-planar Ni centers bridged by a DPA ligand. DFT calculations suggest that this species features Ni centers antiferromagnetically coupled to each other and their iminopyridine ligand radicals. Treatment of Ni(L)(DPA) with one equivalent of ethyl propiolate (HCCCOEt) forms the Ni(L)(HCCCOEt) complex. Addition of the second equivalent of ethyl propiolate leads to the observation of cyclotrimerised products by H NMR spectroscopy. Carrying out the reaction under catalytic conditions (1 mol% of Ni(L)(DPA), 24 h, room temperature) transforms 89% of the substrate, forming primarily benzene products (triethyl benzene-1,2,4-tricarboxylate and triethyl benzene-1,3,5-tricarboxylate) in 68% yield, in a ca. 5 : 1 relative ratio. Increasing catalyst loading to 5 mol% leads to the full conversion of ethyl propiolate to benzene products; no cyclotetramerisation products were observed. In contrast, the reaction is significantly more sluggish with methyl propargyl ether. Using 1 mol% of the catalyst, only 25% conversion of methyl propargyl ether was observed within 24 h at room temperature. Furthermore, methyl propargyl ether demonstrates the formation of cyclooctatetraenes in significant amounts at a low catalyst concentration, whereas a higher catalyst concentration (5 mol%) leads to benzene products exclusively. Density functional theory was used to provide insight into the reaction mechanism, including structures of putative dinuclear metallocyclopentadiene and metallocycloheptatriene intermediates.

Pacman Compounds: From Energy Transfer to Cooperative Catalysis

“…Secondly, in 2017 Groysman and co‐workers presented a dinickel compound as an alkyne activation catalyst . They coupled two iminopyridine ligands onto a xanthene linker to finally get the dinickel compound, which is additionally bridged by one diphenylacetylene unit.…”

Cyclotrimerization of alkynes catalyzed by a self-supported cyclic tri-nuclear nickel(0) complex with α-diimine ligands

“…16a Also, to our knowledge, for most transition metal systems, such cyclotrimerization reactions work inefficiently using hexane as the medium due to the poor solubility of the catalyst. 3 According to previous reports by Dieck et al, the nickel(0) compound with a similar α-diimine ( H L iPr-2,6 ) behaves as a catalyst for cyclotetramerization of alkynes. 11a In other cases, for nickel complexes of L iPr-2,6 , only Ni-diimine-alkyne adducts were obtained from the reaction with alkynes, indicating poor catalytic activity of these complexes.…”

Nickel-Catalyzed Alkyne Cyclotrimerization Assisted by a Hemilabile Acceptor Ligand: A Computational Study

“…7 In 2013, the Guan group reported an easily accessible and highly efficient in situ generated Ni system (Ni(cod) 2 + 3 equiv of PPh 3 ), active and regioselective for a wide range of ynoates and other related alkynes, with a TON as high as 2000 for the cyclotrimerization of ethyl propiolate at room temperature and within 2 h (Chart 1A, bottom left). 8 Recently, the dinickel complexes from the Uyeda group 9 and the Groysman and Lord groups 10 were shown to promote the regio-and chemoselective cyclotrimerization of several terminal alkynes, preferentially forming the 1,2,4-trisubstituted arenes over the substituted COT products and outperforming related mononuclear Ni complexes of N-bound-containing ligands and Ni(cod) 2 , for which cyclotetramerization is generally favored (Chart 1A, bottom right).…”