Computational Studies of ArPtII(NCN) (NCN = Pincer) Reactivity toward Diiodine: Electrophilic Cleavage by a Singlet Pathway or Oxidative Addition by a Radical Pathway?

Abstract

The pincer complex (2-Tol)PtII(NCN) (NCN = [2,6-bis{(dimethylamino)methyl}phenyl-N,C,N]−) reacts with diiodine via electrophilic cleavage to give PtII(NCN)I and 2-tolyl iodide, together with oxidative addition to give cis-(2-Tol)PtIV(NCN)I2 (van Koten et al. 1990). Herein, it is reported that density functional theory computation for benzene as solvent indicates electrophilic cleavage occurs on a singlet manifold, while oxidative addition occurs via a radical chain mechanism related to that demonstrated for the reaction of Pt(acac)2 (acac = [acetylacetonate]−) with diiodine (Hopgood and Jenkins, 1973). For the radical process, time dependent density functional theory calculations indicate that initiation occurs from ArPtII(NCN)(η1-I2) (Ar = 2-Tol, Ph) giving trans-[ArPtIII(NCN)I]• which isomerizes to cis-[ArPtIII(NCN)I]•, where these doublets are isoelectronic with stable [NiIII(NCN)I2]•. In the propagation reaction, cis-[ArPtIII(NCN)I]• reacts with ArPtII(NCN)(η1-I2) to form bridging complexes based on the motif “[Ar(NCN)Pt–I···I–Pt(Ar)(NCN)I]•” that undergo internal electron transfer to release cis-ArPtIV(NCN)I2 and regenerate trans-[ArPtIII(NCN)I]•. In the oxidative addition process for ArPtII(NCN), computation demonstrates the intimate role of ArPtII(NCN)(η1-I2). The preference for electrophilic cleavage over oxidative addition based upon involvement of “Pt(η1-I2)” species on a singlet surface is consistent with the involvement of spectroscopically undetected but computationally confirmed “Pd(η1-I2)” species in palladium catalysis (Musaev et al. 2015).