Benzimidazole-diamide (bida) Pincer Chromium Complexes: Structures and Reactivity

Serendipitous discovery of bida (i.e., N¹-Ar-N²-((1-Ar-1-benzo[d]imidazol-2-yl)methyl)benzene-1,2-diamide; Ar = 2,6-Pr-C₆H₃), a potentially redox noninnocent, hemilabile pincer ligand with a methylene group that may facilitate proton/H atom reactivity, prompted its investigation. Chromium was chosen for study due to its multiple stable oxidation states. Disodium salt (bida)Na₂(THF) was prepared by thermal rearrangement of (dadi)Na₂(THF)₄ (i.e., (N,N'-di-2-(2,6-diisopropylphenylamine)phenylglyoxaldiimine)-Na₂(THF)₄). Salt metathesis of (bida)Na₂(THF) (generated in situ) with CrCl₃(THF)₃ or Cl₃V═NAr (Ar = 2,6-Pr₂C₆H₃) afforded (bida)CrCl(THF) (1-THF) and (bida)ClV═NAr, respectively. Substitutions provided (bida)CrCl(PMe₂Ph) (1-PMe₂Ph) and (bida)CrR(THF) (2-R, where R = Me, CH₂CMe₂Ph (Nph)). Oxidation of 1-THF with ArN₃ (Ar = 2,6-Pr₂C₆H₃) or AdN₃ (Ad = 1-adamantyl) generated (bida)ClCr═NAr (3═NAr) and (bida)ClCr═NAd (3═NAd) and subsequent alkylation converted these to (bida)R'Cr═NR (R' = Me, R = Ad, Ar, 5═NR; R' = CH₂CMe₂Ph (Nph), R = Ad, Ar, 6═NR). In contrast, the addition of AdN₃ to 2-Nph gave the insertion product (bida)Cr(κ²-N,N-ArN₃Nph) (7). Addition of N-chlorosuccinimide to 1-THF produced (bia)CrCl₂(THF) (8), where bia is the pincer derived via hydrogen atom loss from bida methylene. A similar HAT afforded (bia)ClCr(CNAr')₂ (9, Ar' = 2,6-Me₂C₆H₃) when 3═NAd was exposed to Ar'NC. An empirical equation of charge was applied to each bida species, whose metric parameters are unchanging despite formal oxidation state conversions from Cr(III) to Cr(V). Calculations and Mulliken spin density assessments reveal several situations in which antiferromagnetic (AF) coupling and admixtures of integer ground states (GSs) describe a complicated electronic structure.