Copper(I) and -(II) Complexes of Neutral and Deprotonated N-(2,6-Diisopropylphenyl)-3-[bis(2-pyridylmethyl)amino]propanamide

As part of a study of atom-transfer radical polymerization (ATRP) catalysts, four new copper(I) and -(II) compounds of a new monoanionic, tripodal tetradentate ligand, N-(2,6-diisopropylphenyl)-3-[bis(2-pyridylmethyl)amino]propanamide (DIPMAP), were prepared. Ligand synthesis followed from the addition−elimination reaction of 2,6-diisopropylaniline with acryloyl chloride and then a Lewis acid catalyzed Michael addition of bis(2-pyridylmethyl)amine to this product. The ligand was complexed to CuCl to yield monomeric Cu(DIPMAP)Cl featuring an intramolecular hydrogen bond between the free amide hydrogen and the coordinated chloride ligand. Deprotonation of the amide hydrogen in Cu(DIPMAP)Cl using n-BuLi led to the incorporation of LiCl in the resulting product, Li2Cu2(DIPMAP)2Cl2. This complex exhibited an unusual dimeric structure, with the amine nitrogens of one ligand coordinated to a lithium ion, the amide oxygen of the same ligand bridging between the lithium ions, and the amidate nitrogen of that ligand coordinated to a CuCl unit that has a structure analogous to dihalocuprate ions. Deprotonation of Cu(DIPMAP)Cl using KOtBu yielded an alkali-metal chloride free product, Cu2(DIPMAP)2, that also exhibited a dimeric structure in which the three amine nitrogens of one ligand were coordinated to one CuI ion and the amidate nitrogen of the same ligand was coordinated to the other CuI ion. Cu2(DIPMAP)2 was effective in abstracting halogen atoms from organic halides, but in the attempted ATRP of tert-butyl acrylate, molecular weight versus conversion behavior reminiscent of a redox-initiated polymerization was observed. DIPMAP was coordinated to CuBr2 to yield [Cu(DIPMAP)Br]Br with a square-pyramidal structure. The amide hydrogen in this complex could be deprotonated using KOtBu to form complex [DIPMAP]CuBr (6). Spectral characterization of complex 6 confirmed deprotonation of the ligand and that it most likely had an axially distorted trigonal-bipyramidal structure, although crystals suitable for X-ray analysis could not be obtained. Solution oxidation of Cu2(DIPMAP)2 using CBr4 yielded a product, complex 4, whose spectral signatures did not match those of complex 6. The dimeric structure of Cu2(DIPMAP)2 might be a significant contributing factor to the slow rate of deactivation observed in atom-transfer reactions using Cu2(DIPMAP)2 as the catalyst.