The Striking Stabilization of Ni⁰(η⁶‐Arene) Complexes by an Ylide‐Like Silylene Ligand

Abstract

The right mix does the trick: Elusive {Ni(0)(eta(6)-arene)} moieties can be dramatically stabilized by the N-heterocyclic silylene ligand 1, which has a zwitterionic mesomeric structure. The sigma, pi-acid-base synergism between nickel and 1 explains the unexpectedly high stability of the new silylene complexes 2, which enables arene exchange studies at a Ni(0) center. Addition of B(C(6)F(5))(3) to 2 affords the zwitterionic silylene complex 3 (see scheme, R=2,6-iPr(2)C(6)H(3)).

Recent Advances in Silylene Chemistry: Small Molecule Activation En-Route Towards Metal-Free Catalysis

“…For excellent reviews on these complexes, see: [36,37] In 2009 the latent ability of the zwitterionic NHSi 3 to coordinate with Ni(0) upon reaction with the precursor Ni(cod) 2 in aromatic solvents, resulting in (η 6 -arene)Ni NHSi complexes 41-43 ( Fig. 4b), was shown [40,41]. These complexes have proven to readily exchange the η 6 -arene for three CO ligands upon reaction with CO, resulting in 44.…”

Striking Reactivity of a Stable, Zwitterionic Silylene Towards Substituted Diazomethanes, Azides, and Isocyanides

“…This implies the presence of two reactivity sites in the molecule with an electrophilic silicon site and a nucleophilic exocyclic methylene group. In fact, the zwitterionic nature of 1 causes a fascinating reactivity pattern: i) 1,4‐addition with HX (X=B(C 6 F 5 ) 4 ,5a OH,5b or triflate4), that is, electrophilic addition of H + to the exocyclic methylene group in 1 and addition of the nucleophile X to the silicon(II) center; ii) facile insertion reactions of silicon (II) into element–element bonds, for example, the reaction of 1 with white phosphorus,5c halosilanes and halocarbons,5d ammonia,5e terminal alkynes,5f as well as primary NHC to Si addition and subsequent oxygenation of Si to form isolable silanones5g and dioxasiliranes, [5 h] respectively; iii) cycloadditions with unsaturated organic substrates, for example, the [2+1] cycloaddition of 1 to alkynes5f and benzylideneacetone;5i iv) facile coordination to transition‐metal centers in low oxidation state;5j, k and v) silicon‐mediated CH activation and tautomerization processes 5l. m The unusual electronic features of 1 prompted us to include reactivity studies employing unsaturated organic nitrogen substrates for the exploitation of novel building blocks.…”

Synthesis of Mixed Silylene–Carbene Chelate Ligands from N‐Heterocyclic Silylcarbenes Mediated by Nickel

“…This is likely due to the stronger p-acidity of CO than phosphine ligands, which also results in decreasing the p backdonation from the Ni center to the silicon and NHC carbon centers. This consequently leads to longer Ni1 À Si1 (2.2131 (13) ) and Ni1ÀC33 (1.947(4) ) bond lengths of 6 ( Figure 3) in comparison to those in 5 (2.1740(18) and 1.918 (7) , respectively). The IR stretching vibrations for the CO ligands are observed at n = 1952 and 1887 cm À1 , which are blue-shifted in comparison to those observed in [{(dmpm)Ni(CO) 2 } 2 ] (n = 1991 and 1927 cm À1 ; dmpm = bis(dimethylphosphino)methane) [22] and [(NHC Mes ) 2 Ni(CO) 2 ] (n = 2050 and 1877 cm À1 , NHC Mes = (CHNMes) 2 CD, Mes = 2,4,6-Me 3 C 6 H 2 ).…”

“…[2d] Single crystals of 2 suitable for X-ray diffraction analysis were obtained from concentrated toluene solutions at À20 8C ( Figure 1). [12] The complex crystallizes in the orthorhombic space group P2 1 2 1 2 1 . Noteworthy is that the nickel center exhibits a somewhat distorted square-planar geometry, with a sum of the angles around nickel of 365.038.…”

“…The Ni1ÀSi1 distance of 2.1553(8) is similar to those in a pincer-type bis(NHSi) Ni II complex (2.1737 (7) ; Scheme 1), [2d] whereas it is longer than those in NHSi-supported Ni 0 (h 6 -arene) complexes (2.0369(6)-2.0936(10) ), which have a somewhat stronger Ni!Si II p back-donation. [13] The substantially longer Ni1ÀBr2 distance of 2.4437 (6) in comparison to that of Ni1ÀBr1 (2.3538(5) ) might be attributed to a stronger trans effect of the NHSi compared to that of the NHC moiety.…”