The role of hydrogen-bonding interactions in stabilising trigonal planar copper(I) in Cu(BF4)–pyridazine–nitrile systems †

Abstract

Regardless of conditions, reaction of [Cu(NCMe)4][BF4] with pyridazine (pydz) or 3-methylpyridazine (Mepydz) in MeCN or with pydz in PhCN invariably gave tris-bridged dinuclear cations, [{Cu(NCR)}2(µ-diimine)3]2+. Structural analysis, by single crystal X-ray diffraction, of complexes containing [{Cu(NCMe)}2(µ-pydz)3]2+, [{Cu(NCMe)}2(µ-Mepydz)3]2+ and [{Cu(NCPh)}2(µ-pydz)3]2+ confirmed the presence of two tetrahedral copper(I) centres bridged by three pyridazine molecules and terminally co-ordinated by nitriles. This chemistry contrasts with that for 2-cyanoguanidine (cnge), a planar nitrile with considerable hydrogen-bonding potential, which leads to both bis- and tris-bridged dinuclear cations, [{Cu(cnge)}2(µ-pydz)2]2+ and [{Cu(cnge)}2(µ-pydz)3]2+. Whereas the tris-bridged cation is based on four-co-ordinate tetrahedral copper(I), the bis-bridged cation contains three-co-ordinate trigonal planar copper(I). The unique ability of cnge to stabilise co-ordinatively unsaturated copper(I) in the solid state with pydz bridged dications is attributed to the formation of an extended 2-D sheet architecture based on hydrogen-bonding intermolecular interactions. This type of molecular construction, which is common to all copper(I)–cnge three-co-ordinate structures, suggests that the three-co-ordinate geometry is not an intrinsic property of copper(I) systems but a result of the efficient packing of parallel two-dimensional sheets. Treatment of [{Cu(NCMe)}2(µ-diimine)3]2+ with CO led to [{Cu(CO)}2(µ-diimine)3]2+; reaction of [{Cu(cnge)}2(µ-diimine)2]2+ with CO or PPh3 gave [{Cu(cnge)(L)}2(µ-diimine)2]2+ (L = CO or PPh3). Recrystallisation of [{Cu(cnge)(PPh3)}2(µ-diimine)2]2+ yielded a variety of crystalline products including [Cu(pydz)2(PPh3)2][BF4] and [{Cu(PPh3)}2(µ-pydz)3][PF6]2. Structural studies confirmed the former to be a mononuclear cation with four monodentate (two pydz and two Ph3P) ligands and the latter to be a tris(µ-pydz) dinuclear cation with terminal Ph3P molecules. The copper(I) co-ordination geometries in both complexes are tetrahedral, the three-co-ordinate copper(I) geometry of [{Cu(cnge)}2(µ-diimine)2]2+ being lost on treatment with Ph3P. In the absence of structural data, [{Cu(CO)}2(µ-diimine)3]2+ and [{Cu(cnge)(CO)}2(µ-diimine)2]2+ are considered to comprise tris- and bis-(µ-diimine) dinuclear cations based on tetrahedral copper(I) with terminal CO.