A redox-active ligand combines a PCP pincer site with a bidentate N–N donor in opposition

Abstract

A binucleating ligand (2) combining a monoanionic PCP pincer cleft with a monoanionic N–N cleft has been prepared on the basis of bis(imidazolyl)methane (1). Installation of divalent Pd or Pt into the PCP cleft proceeded smoothly, with the formation of square planar (PCP)MCl complexes (M = Pd or Pt). The “C” in the PCP is the central carbon of the β-diketiminate-like N–N cleft. The N–N cleft in these complexes was obtained in three levels of protonation. In 4-Pd and 4-Pt, the N–N cleft carries one proton and is neutral; in 3-Pd or 3-Pt, it is additionally protonated by HCl, and in 6-Pd or 6-Pt, the proton has been removed by the (Me₃Si)₂NLi base and replaced by Li. The chloride in 4-Pd/Pt was replaced with an iodide to make 6-Pd/Pt in a metathesis with Me₃SiI. Compounds 4-Pd/Pt and 6-Pd/Pt exhibited tautomerism reminiscent of acetylacetone and β-diketimines, possessing either an NH or a CH bond. Compounds 6-Pd/Pt possess a lithiated N–N cleft and serve as convenient transmetallation agents. Their reactions with BF₃(OEt₂), ½ZnCl₂, or TaCl₅ produced new compounds 7-Pd/Pt, 8-Pd/Pt, and 9-Pd/Pt that carry a BF₂, ½Zn, or TaCl₄ units in the N–N cleft. Compounds 8-Pd/Pt are trimetallic with a central Zn coordinated by two N–N clefts each of which carries a (PCP)MCl on the opposite side. Structures of 3-Pd, 7-Pt, 8-Pd, and 9-Pd in the solid state were established by X-ray diffractometry. They demonstrate the remarkable planarity of the extended conjugated organic π-system into which the N–N and the PCP binding sites are incorporated. Electrochemical studies on 7–9 established that each of these extended π-systems can be oxidized twice quasi-reversibly. Compounds 9-Pd/Pt could be additionally reduced quasi-reversibly by a single electron, which was ascribed to the reduction of the Ta^V center to Ta^IV. They also differed from 7 and 8 in possessing apparent lower-energy LMCT bands responsible for the blue or purple colors.