On the Reactivity of a NHC Nickel Bis‐Boryl Complex: Reductive Elimination and Formation of Mono‐Boryl Complexes

Abstract

The synthesis of the first terminal mono‐boryl complexes of nickel, which are not stabilized by a pincer ligand, is reported. The reaction of the nickel bis‐boryl complex cis‐[Ni(iPr2ImMe)2(Bcat)2] 1 (cat = 1,2‐O2C6H4) with the small donor ligand PMe3 led to a complete ligand exchange at nickel with reductive elimination of B2cat2, formation of the bisNHC adduct [B2cat2 · (iPr2ImMe)2] 3 and formation of [Ni(PMe3)4] 2 as the metal‐containing species. Electrophilic attack of MeI on complex 1 or ligand dismutation of 1 with trans[Ni(iPr2ImMe)2Br2] led to loss of only one boryl ligand of 1 and afforded the nickel mono‐boryl complexes trans‐[Ni(iPr2ImMe)2(Bcat)Br] 4a and trans‐[Ni(iPr2ImMe)2(Bcat)I] 4b.

Contrasting reactivity of B–Cl and B–H bonds at [Ni(IMes)2] to form unsupported Ni-boryls

“…11 There have been a number of reports of nickelcatalysed borylations 12 but despite Ni-boryls often being proposed as intermediates in these cycles, examples of characterised Ni-boryls remain rare. [13][14][15][16][17][18][19] This reflects both the reactivity of the Ni-BX 2 bond and the challenges associated with its formation.…”

“…The reactivity of Ni-boryls is receiving increased attention and the nucleophilicity of the boryl ligand has been demonstrated by the borylation of bromobenzene by [(PNP)Ni(Bcat)] 13 and the reactivity of cis -[Ni( i Pr 2 Im Me ) 2 (Bcat) 2 ] towards alkyl halides. 18 Incorporation of the boryl into a PBP-pincer ligand has allowed exploitation of its strong σ-donor and potential π-acceptor capabilities for the activation of H 2 , 15,16 dimerization of ethylene 29 and cleavage of CO 2 . 30 With increased interest in Ni-boryls and their applications in bond activation and catalysis we now report the ready formation of stable unsupported Ni-boryls by the oxidative addition of the B–Cl bond at the Ni(0) precursor [Ni(IMes) 2 ].…”

“…The ligand trans to the boryl clearly influences the Ni–B bond distance as those observed for 2 and 3 are similar to that of [(PNP)Ni(Bcat)] (1.9091(18) Å), 13 in which the boryl ligand is trans to an amide, but within the range defined by being trans to another boryl [(PBP)Ni(Bcat)] (2.015(2) Å) 16 and the heavier halogens in trans -[( i Pr 2 Im Me ) 2 Ni(Bcat)X] (X = Br, 1.872(4) Å; X = I, 1.864(4) Å). 18 Comparison to the Ni–B bond distances observed in cis -[( i Pr 2 Im Me ) 2 Ni(Bcat) 2 ] (1.9231(19) Å and 1.9092(18) Å), 17 for which a B–B interaction is still present and the overall bonding picture is more complex, is also pertinent.…”