Synthesis, Properties and Characterization of N-Alkyl Substituted beta-Diketiminato Copper(I) Complexes

The ligand nacnacxylH (xyl = C6Me2H3) and the N-alkyl substituted diketimine ligands (nacnac CH(Me)PhH, nacnacBnH and nacnaciPrH) have been prepared in good yields except nacnaciPrH (23%) using a one-step procedure with the help of a Dean-Stark apparatus. Reaction of S,S-nacnacCH(Me)PhH and nacnacBnH with nBuLi in THF gave S,S nacnacCH(Me)PhLi(THF) and nacnacBnLi(THF). Attempts to brominate these THF adducts with N-bromosuccinimide gave instead the beta-carbon substituted succinimido ligands S,S-succnacnac CH(Me)PhH and succ nacnacBnH (succ = succinimido). Chlorination with N-chlorosuccinimide, afforded the desired product albeit with significant amounts of impurities.;NMR studies of the complex (nacnacxylCu)2(mu-toluene) in C6D6, showed no toluene adduct but an equilibrium mixture of (nacnacxylCu)2(i-C 6D6) and nacnacxylCu(C 6D6) in a ratio of 2:1. While addition of up to 50 equiv of either toluene or THF did not cause any significant change in the 1H NMR spectrum, addition of 2 equiv MeCN gave instantaneously the nacnacxylCu(MeCN) complex. In addition, ( nacnacxylylCu)2(mu-C6D 6) did not coordinate or react with N2O even after heating at 60° C for thirteen days.;In the presence of DPA (diphenylacetylene), reaction of nacnac BnH with CuOtBu yields the bridged dimer (nacnacBnCu)2(mu-DPA). Addition of excess DPA (10-12 equiv) converts the bridged dimer to the terminally bound complex nacnacBnCuDPA. Nacnac RH (R = CH(Me)Ph and i-Pr) did not form complexes with olefins or with DPA. Similar reactivity was observed in nacnacCH(Me)PhCu(NCMe) and nacnaci -PrCu(NCMe) complexes. While the terminally bound MeCN complex was isolated and characterized, equilibrium in solution led us to suspect the formation of a bridged acetonitrile adduct.;Reactivity and comparative studies were performed with several copper complexes. Morpholine did not react with nacnacBn Cu(acrylonitrile) while free acrylonitrile does. Olefin exchange experiment showed that acrylonitrile (an electron withdrawing olefin) binds stronger than the other olefins examined, showing the importance of pi-backbonding relative to sigma-donation. pi-Backbonding is, however, still low when compared other structurally characterized transition metal styrene complexes. Complexes nacnacCH(Me)PhCuL (L = PPh 3 and MeCN) have been employed in catalytic cyclopropanation of styrene and the conjugate addition of ZnEt2 to 2-cyclohexenone, but results indicate that the diketimine ligand is lost before it enters the catalytic cycle. Hence, there was no chiral induction.;Reaction of these ligands with CuOtBu (or MesCu and catalytic amounts of CuOtBu, Mes = C6Me3 H2) in the presence of Lewis bases gave (nacnac xylCu)2(mu-toluene), nacnacxyl CuCNC6H3(Me)2, nacnac CH(Me)PhCuL (L = PPh3, PMe3, CNC6H3(Me)2, DMAP, lutidine, Py, MeCN), nacnacBnCuL (L = PPh3, CNC6H3(Me) 2, styrene, trans-stilbene, phenylvinylether, acrylonitrile, diphenylacetylene), nacnaciPrCuL (L = PPh3, CNC6H3(Me)2, MeCN) and succnacnacCH(Me)PhCuL (PPh 3, CNC6H3(Me)2, pyridine). All complexes are yellow and sensitive to air and moisture. There was no reaction between the copper precursors and the N-alkyl substituted ligands in the absence of strong Lewis bases.;Four-coordinate copper(I) complexes of the form nacnac RCu(phen) (R = Bn, CH(Me)Ph and Phen = 1,10-phenanthroline, 2-Mes-1,10-phenanthroline, 2,9-dimethyl-1,10-phenanthroline (dmp) and 2,9-diphenyl-1,10-phenanthroline (dpp)) were also prepared. The complexes are intensely blue in colour and intramolecular pi-stacking interactions between one of the phenyl rings of nacnac ligand with the phenanthroline were observed in the solid state structures. UV-vis absorption measurements were performed in toluene and the MLCT bands are red-shifted relative to those of bisphenanthroline copper complexes. All compounds are emissive in the solid state, but 1,10-phenanthroline and 2-Mes-1,10-phenanthroline complexes do not emit in solution.;To buttress the pi-stacking interactions, the new ligands nacnacRH (R = CH2C6H2(OMe) 3, CH2C6F5) and their respective copper complexes with dmp and dpp were prepared. For the sake of comparison, nacnaciBu(dmp) was prepared. While the dmp complexes showed enhanced pi-pi intramolecular interactions with both phenyl substituents of the diketimine ligand and the phenanthroline, dpp revealed no such interactions. The perfluorinated complex showed a significant blue-shift in absorption and emission spectra when compared to the other complexes, while the isobutyl substituted complex displayed red-shifted transitions. While luminescence intensities and lifetimes were low, reduced Stoke shifts and comparable sharp luminescence peaks indicate reduced distortions in the excited state.;Key words: copper, beta-diketimine, chiral nacnac, pi back-bonding, pi-pi intramolecular interactions, X-ray.