Cobalt(II), copper(II), and zinc(II) isostructural, pseudotetrahedral, racemic complexes of pyrrolo[2,3-b]quinoxaline with bis(2-thienylmethyl)propylenediamine chain: Synthesis, crystal structure, spectroscopy

Abstract

Cobalt(II), copper(II), and zinc(II) cations all have similar effective ionic radii so they form with (E)-1-phenyl-3-{N-[N′,N′-bis(thiophen-2-ylmethyl)-3-aminopropyl]aminophenylmethylidene}-1,3-dihydro-2H-pyrrolo[2,3-b]quinoxalin-2-one (1) isostructural, pseudotetrahedral complexes 2a, 2b, and 2c, respectively. Single crystal X-ray analysis of the synthesized complexes reveals that they are chiral (Λ-M-pS, pS; Δ-M-pR, pR) and crystalize as racemates in P 21/c space group. There is axial chirality connected with tetrahedrally substituted metal center and crystallization induced planar chirality of two symmetry independent ligand molecules (different conformers). In solution only metal center chirality is maintained due to the conformational lability of the 3-[N,N-bis(thien-2-ylmethyl)amino]propyl group. Chirality of zinc complex 2c in solution was confirmed by the analysis of 1D and 2D NMR spectra. NMR data show diastereotopic protons of aliphatic chain and the restriction of phenyl ring rotation at C(30) carbon atom. Complex 2c is fluorescent in solid state as well as in solution (fluorescence quantum yield in acetonitrile, ϕ = 0.1). The NMR titration at room temperature shows that the major product of zinc coordination is not 2c but the complex with a tautomeric form of the ligand having proton located at N(4) nitrogen atom (enamine-imine tautomerism). UV–Vis data for 2a, 2b, and 2c both in solid and in solution are discussed.