Structural Diversity of Copper(I) Complexes Formed by Pyrrole- and Dipyrrolylmethane-Based Diphosphine Ligands with Cu–X···HN Hydrogen Bonds

Abstract

Metal complexes containing hydrogen bond donor/acceptor groups are interesting because of their applications in several areas. In the course of our investigation on the synthesis of metal complexes using newly developed pyrrole-based diphosphine ligands, a few structurally interesting copper(I) complexes containing the pyrrolic NH hydrogen bond donors were synthesized. The reaction of 2,5-bis(diphenylphosphinomethyl)pyrrole (PNP) with an equimolar quantity of CuX (X = Cl, Br, and I) afforded the binuclear copper(I) complexes [Cu(μ-X)(μ-PNP-P,P)]2 (1-3) in very good yields (87-90%). Conversely, the analogous reaction between 1,9-bis(diphenylphosphinomethyl)diphenyldipyrrolylmethane (PNNP) and CuX (X = Cl, Br, and I) yielded the mononuclear Cu(I) complexes [CuX(PNNP-P,P)] (4-6) in very good yields (∼88%), in which the diphosphine ligand is chelated to the copper metal atom. Interestingly, when this reaction was carried out with a 1:2 mol ratio of ligand/metal, the cubane-like tetranuclear Cu(I) complex, [Cu4I4{μ-Ph2C(C4H3N)2-1,9-(CH2PPh2)2-P,P}2] 7, was isolated in 68% yield. In addition, the reaction between the dipyrrolyldiphosphine ligand (PNNP) and CuCl in the presence of 1 equiv of 1,10-phenanthroline monohydrate and NaBF4 afforded a novel ionic binuclear mixed-ligand Cu(I) complex, [Cu2(μ-X)(μ-PNNP-P,P)(NN)2]BF4 8, where NN = 1,10-phenanthroline in 57% yield. The structures of all these complexes were confirmed by the single-crystal X-ray diffraction method and are supported by spectroscopic data. In contrast to the PNP pincer ligand, the dipyrrolyl-diphosphine ligand (PNNP) adopts chelation as well as bridging coordination modes with Cu(I) atoms, indicating its flexibility of bonding. In all the structures, the Cu-X···HN type of hydrogen bonds involving the metal halide ion as acceptor and the pyrrolic NH as donor are present with the Cu-X···H angles, which deviate from the favored 90°, as observed in their solid state structures. Further, the presence of this type of hydrogen bond was confirmed by NBO, AIM, and Hirshfeld analyses.