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

Abstract

The Ni(II) -mediated tautomerization of the N-heterocyclic hydrosilylcarbene L(2) Si(H)(CH2 )NHC 1, where L(2) =CH(CCH2 )(CMe)(NAr)2 , Ar=2,6-iPr2 C6 H3 ; NHC=3,4,5-trimethylimidazol-2-yliden-6-yl, leads to the first N-heterocyclic silylene (NHSi)-carbene (NHC) chelate ligand in the dibromo nickel(II) complex [L(1) Si:(CH2 )(NHC)NiBr2 ] 2 (L(1) =CH(MeCNAr)2 ). Reduction of 2 with KC8 in the presence of PMe3 as an auxiliary ligand afforded, depending on the reaction time, the N-heterocyclic silyl-NHC bromo Ni(II) complex [L(2) Si(CH2 )NHCNiBr(PMe3 )] 3 and the unique Ni(0) complex [η(2) (Si-H){L(2) Si(H)(CH2 )NHC}Ni(PMe3 )2 ] 4 featuring an agostic SiH→Ni bonding interaction. When 1,2-bis(dimethylphosphino)ethane (DMPE) was employed as an exogenous ligand, the first NHSi-NHC chelate-ligand-stabilized Ni(0) complex [L(1) Si:(CH2 )NHCNi(dmpe)] 5 could be isolated. Moreover, the dicarbonyl Ni(0) complex 6, [L(1) Si:(CH2 )NHCNi(CO)2 ], is easily accessible by the reduction of 2 with K(BHEt3 ) under a CO atmosphere. The complexes were spectroscopically and structurally characterized. Furthermore, complex 2 can serve as an efficient precatalyst for Kumada-Corriu-type cross-coupling reactions.

A Bis(silylene)‐Substituted ortho‐Carborane as a Superior Ligand in the Nickel‐Catalyzed Amination of Arenes

“…Currently, the chelating bis(silylene) ligands A and D , and the SiXSi pincer‐type NHSi ligands B (X=CH) and C (X=N) have been synthesized in our laboratory (Scheme ) . TM complexes of the latter type of ligands can act as very active precatalysts in the borylation of benzenes, Sonogashira coupling, Kumada–Corriu coupling, the cyclotrimerization of alkynes, the co‐cyclotrimerization of alkynes and organocyanides to form pyridines, the hydrosilylation of ketones,– and the reduction of amides to amines . However, compared to the rich structural diversity of phosphine and NHC ligands, multidentate NHSi ligands are still scarce.…”

“…Single‐crystal X‐ray analysis of [SiCCSi]NiBr 2 revealed that the Ni II center is, as expected, in a square‐planar environment (Figure , bottom). The Ni(1)−Si(1) (2.1378(5) Å) and Ni(1)−Si(2) (2.1447(5) Å) distances are shorter than those in NHSi‐Ni II complexes supported by pincer‐type B (2.1737(7) Å) and related NHC‐NHSi ligands …”

Isolable Silylene Ligands Can Boost Efficiencies and Selectivities in Metal‐Mediated Catalysis

“…[24] TheN ic omplex 6 was obtained by the reaction of NiBr 2 (dme) with the N-heterocyclic silylcarbene LSi(H)(CH 2 )NHC (L = CH{C = CH 2 }(CMe)(NAr) 2 , Ar = 2,6-iPr 2 C 6 H 3 ,N HC = 3,4,5-trimethylimidazol-2-ylidene) in toluene via proton migration. [25] TheC o III complex 7 was synthesized through the SiÀHb ond activation of the [PSiP] pincer ligand [HSiMe(NCH 2 PPh 2 ) 2 C 6 H 4 ]w ith the Co I precursor bearing silylene ligand L3. [26] TheR h III hydride 8 was obtained through the oxidative addition reaction of ligand L19 with Rh I precursor [RhCl(coe)] 2 .…”

“…[28] It was shown that 3 efficiently catalyzes the CÀCcoupling reaction of aryl halides with Grignard reagents or organozinc reagents.M oreover, complex 6 proved to be most effective in Kumada-Tamao-Corriu coupling reactions with promising catalytic performances up to 99 %. [25] Very recently,L ia nd Sun et al demonstrated that 7 is an efficient precatalyst for similar cross-coupling reactions [26] (Scheme 6c,d). Reported by Dri-ess and Hartwig et al in 2013, the Ni II complex 4 was found to be an active precatalyst for the Sonogashira coupling reaction of phenylacetylene with 1-octenyl iodide (Scheme 6e), and investigations of stoichiometric reactions shed light on the mechanism of the astonishing role of the heterobimetallicmediated coupling process (see also discussion below).…”

Chelating N-Heterocyclic Silylenes as Steering Ligands in Catalysis

“…A C-H bond is oxidatively added into the Si(II) center of the zwitterionic NHSi stabilized by NHC 28, NHC ! 26 Complex 31 is hence the first example of a complex featuring the mixed ligand system of type III. 25 Compound 30 represents an intriguing starting material for access to a mixed NHC-NHSi system of type III (Figure 1).…”