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

Azolium-2-dithiocarboxylates as redox active ligands in nickel chemistry

The coordination chemistry of carbene-CS2 adducts of selected NHCs and cAACs and their redox active nature in nickel complexes is reported. These azolium-2-dithiocarboxylate ligands can be considered as 1,1-isomeric dithiolene analogues bearing a 2π electron reservoir. Depending on the co-ligands attached to nickel, square planar mono- or bis-(carbene-CS2) complexes of the types [Ni(IiPr)2(carbene-CS2)] (2a-g) (carbene = cAACMe, IDipp, IMes, BIMe, BIiPr, IiPr, and IiPrMe) and [Ni(carbene-CS2)2] (3a-c) (carbene = cAACMe, IDipp, and IMes) are accessible by alkene substitution using [Ni(IiPr)2(ƞ2-C2H4)] or [Ni(COD)2] as the starting material (cAACMe = 1-(2,6-di-iso-propylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-ylidene, IR = 1,3-diorganylimidazolin-2-ylidene, IRMe = 1,3-diorganyl-4,5-dimethylimidazolin-2-ylidene, and BIR = 1,3-diorganylbenzimidazolin-2-ylidene). In the complexes 2a-g and 3a-c, all carbene-CS2 ligands are coordinated in a κ2-S,S' fashion to nickel(ii) and are ligated either in their formally one electron reduced (3a-c) or two electron reduced (2a-g) redox states. The complexes 3a-c reveal intense NIR absorptions, which shift upon metallic reduction to the nickelate salts of the type [Cat]+[Ni(carbene-CS2)2]˙- (4a-bcat). In these nickelates, an additional electron is shared across a ligand-centered SOMO of π-symmetry which is delocalized over both azolium-2-dithiocarboxylate ligands and results in carbene-CS2 moieties with a formal -1.5 charge per ligand, further demonstrating the flexible redox active nature of these azolium-2-dithiocarboxylate ligands.

On the reactivity of complexes [Ni(NHC)2] with CS2

A study of the reaction of [Ni(NHC)2] synthons with the heterocumulene CS2 is reported. Nickel complexes of η2-(C-S) side-on coordinated CS2, [Ni(NHC)2(η2-CS2)] (NHC = IiPrMe (1a), IiPr (1b)) were obtained from the reaction of CS2 with precursors of [Ni(NHC)2] (NHC = IiPrMe, IiPr). The result of this reaction critically depends on the NHC employed, as [Ni(IMes)2], the complex of the sterically more demanding N-aryl substituted NHC IMes, led to formation of the dinuclear complex [{Ni(IMes)(μ2-CS2)}2] (2d). The related compound [{Ni(IDipp)(μ2-CS2)}2] (2c) was isolated from the reaction of [Ni(IDipp)(η6-C7H8)] with CS2. If [Ni(cAACMe)2] was used, insertion of CS2 into the cAAC carbon nickel bonds occurs and the bis-(azolium-2-dithiocarboxylate) complex [Ni(cAACMe-CS2)2] (3e) was formed.

Nickel-catalyzed, silyl-directed, ortho-borylation of arenes via an unusual Ni(II)/Ni(IV) catalytic cycle

Nickel-catalyzed C-H bond functionalization reactions provide an impressive alternative to those with noble metal catalysts due to their unique reactivity and low cost. However, the regioselective C(sp2)-H borylation reaction of arenes accomplished by nickel catalyst remains limited. We herein disclose a silyl-directed ortho C(sp2)-H borylation of substituted arenes with a Ni(cod)2/PMe3/KHMDS catalyst system. Using readily available starting materials, this protocol provides easy access to ortho-borylated benzylic hydrosilanes bearing flexible substitution patterns. These products can serve as versatile building blocks for the synthesis of sila or sila/borine heterocycles under mild conditions. Control experiments and DFT calculations suggest that a catalytic amount of base prompts the formation of Ni(II)-Bpin-ate complex, likely related to the C(sp2)-H bond activation. This borylation reaction might follow an unusual Ni(II)/Ni(IV) catalytic cycle.

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

[Ni(IMes)2] reacts with chloroboranes via oxidative addition to form rare unsupported Ni-boryls. In contrast, the oxidative addition of hydridoboranes is not observed and products from competing reaction pathways are identified. Computational studies relate these differences to the mechanism of oxidative addition: B-Cl activation proceeds via nucleophilic displacement of Cl-, while B-H activation would entail high energy concerted bond cleavage.