Copper(II)- and Proton-Assisted Condensation Reactions of a Tetrapodal Pentaamine with Acetone: Formation of “Podand-cum-Macrocycle” Copper Complexes and a Protonated Bis(aminal)

Abstract

The tetrapodal pentaamine 2,6-C5H3N[CMe(CH2NH2)2]2 (pyN4, 1) forms mononuclear complexes with CuII, as shown for a series of compounds of the type [(1)Cu]X2 (X = Br, SCN, PF6, ClO4). The coordination environment of the copper ion is square-pyramidal, with the pyridine nitrogen atom of 1 in the apical position. The axial bond Cu−Npy is significantly longer than the four Cu−N bonds at the base of the pyramid, average values being 2.16 A and 2.03 A, respectively. When [(1)Cu]X2 (X = PF6, ClO4) is refluxed with an excess of acetone in methanol in the absence of base, the corresponding bis(isopropylideneimine) complexes [{(nac)2pyN4}Cu]X2 are obtained, in which two diametrically opposite primary amino groups of 1 have condensed with the ketone. The complex cations, which are now chiral, again have square-pyramidal coordinated CuII. In the presence of sodium methoxide, the condensation of [(1)Cu]X2 with acetone proceeds further, leading to the formation of one diacetone-amine-imine linkage in the product. A 12-membered 1,5,9-triazamacrocycle, incorporating the 2,6-disubstituted pyridine unit, is thus formed. Two podand nitrogen donors of the NN4 set (which retains its square pyramidal topology) remain: a primary amine and an isopropylidene imine group. The product forms as a mixture of both conceivable isomers (azomethine groups at the base of the pyramid cis or trans), which were separated and structurally characterised as the tribromocuprate(I) and hexafluorophosphate salts, respectively. The condensation of metal-free 1 with acetone in the presence of two equivalents of HBr yields the bis(aminal) salt (2,6-C5H3N{CMe[cyclo-CH2NH2C(CH3)2NHCH2-]}2)Br2, in which one of the secondary amino groups in each diazacyclohexane ring is protonated, as ascertained by X-ray crystallography. The aminal is a metal-free tautomer of the bis(isopropylideneimine) ligand (nac)2pyN4, to which it converts upon reaction with CuII under suitable conditions, providing an alternative synthetic route to complexes of the type [{(nac)2pyN4}Cu]X2.