Chemistry:Ni(IV) organometallic complex

Ni(IV) organometallic complexes (Nickel(IV)-centered organometallic complexes) are a form of organonickel compounds which feature a central nickel atom at the +4 oxidation state. These complexes typically need to be supported by non-canonical ligand scaffolds which are usually developed fit-for-purpose. These types of complexes are rare, with only few reports in the chemical literature when compared to lower oxidation state Ni species. These types of high-valent nickel compounds have risen in popularity only in the past decade due to their general instability, challenging characterization, and unique reactivity with organic molecules.  Organic reactions exploiting the reactivity of nickel typically occur via organometallic Ni⁰, Ni^I, Ni^II, and Ni^III intermediate species. These types of species are well studied and have been the subject of several literature reviews, with few regarding Ni^IV species.

Representative preparation

Typically, Ni^IV complexes are prepared from a much more accessible precursory Ni^II complex via a two electron oxidation process. The nature of the oxidant required relies heavily upon the ligands surrounding the Ni center, but can range from air to more exotic oxidants, such as S-(trifluoromethyl)dibenzothiophenium triflate, also known as the Umemoto reagent. For example, one of the first isolated Ni^IV complexes that was used in a cross coupling transformation was reported by Linden and Dimitrov in 2003 and was prepared by simple air-induced oxidation of the following anionic Ni^II complex:

Linden and Dimitrov Ni(II) to Ni(IV)

Reactivity and synthetic transformations

Ni^IV, although elusive, features an interesting and unique reactivity profile compared to its lower valent Ni counterparts. While the cross-coupling reactivity of lower valent nickel species have been thoroughly investigated and have found vast historical and contemporary applications, Ni^IV provides an orthogonal mode of reactivity, enabling carbon-heteroatom bonds to be formed, which would be difficult through a lower-valent Ni intermediate. Only recently has the synthetic potential of Ni^IV been explored. There are currently no examples in total synthesis that use Ni^IV to enable a particular transformation.

Carbon-heteroatom bond formation

Studies reported by Sanford and Camasso in 2015 revealed the ability of a Ni^IV organometallic complex to deliver carbon(sp³)-oxygen, carbon(sp³)-nitrogen, and carbon(sp³)-sulfur bonds. The Ni^IV species was formed using Umemoto reagent as an oxidant and a trispyrazolylborate ligand.

Sanford Ni(IV) 2015

C-H activation

In 2014, Chatani and coworkers demonstrated that a Ni^IV intermediate was formed during an aminoquinoline-directed aliphatic C-H activation process using diaryliodonium salts as an oxidant.

AQ-directed C-H activation

Trifluoromethylations

A handful of examples exist in the literature demonstrating how Ni^IV can perform challenging trifluoromethylation reactions on (hetero)arenes. A representative method is shown below, reported by Nebra and coworkers in 2017. Here, a six-coordinate Ni^IV complex undergoes stoichiometric CF₃ transfer to a functionalized arene.

Stoichiometric trifluromethylation by Nebra and coworkers

A catalytic variant was reported by Sanford and coworkers in 2019. The mechanism for this transformation is shown in the following scheme. Here, the key Ni^IV intermediate undergoes a slow homolysis to produce a Ni^III intermediate and a trifluoromethyl radical, which can promote propagation of the catalytic cycle.

Sanford Catalytic Trifluoromethylation of Arenes