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

A free radical chain reaction occurs for the oxidation of PhPt^II(NCN) by diiodine in benzene (NCN = [2,6-bis{(dimethylamino)methyl}phenyl-N,C,N]⁻). Initiation provides [PhPt^III(NCN)]^• from PhPt^II(NCN)(η¹-I₂), followed by formation of binuclear species [PhPt(NCN)Pt–I···I–PtPh(NCN)I]^• that undergo internal electron transfer to form cis-PhPt^IV(NCN)I₂ and regenerate [PhPt^III(NCN)I]^• to continue propagation. In contrast, computation for a singlet pathway in benzene favors electrophilic cleavage to form Pt^II(NCN)I and Ph–I.

The pincer complex (2-Tol)Pt^II(NCN) (NCN = [2,6-bis{(dimethylamino)methyl}phenyl-N,C,N]⁻) reacts with diiodine via electrophilic cleavage to give Pt^II(NCN)I and 2-tolyl iodide, together with oxidative addition to give cis-(2-Tol)Pt^IV(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 ArPt^II(NCN)(η¹-I₂) (Ar = 2-Tol, Ph) giving trans-[ArPt^III(NCN)I]^• which isomerizes to cis-[ArPt^III(NCN)I]^•, where these doublets are isoelectronic with stable [Ni^III(NCN)I₂]^•. In the propagation reaction, cis-[ArPt^III(NCN)I]^• reacts with ArPt^II(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-ArPt^IV(NCN)I₂ and regenerate trans-[ArPt^III(NCN)I]^•. In the oxidative addition process for ArPt^II(NCN), computation demonstrates the intimate role of ArPt^II(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).

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