INTRAMOLECULAR BINDING OF NITROGEN BASES TO A COFACIAL BINUCLEAR COPPER(II) COMPLEX

Coordination of pyrazines and pyridines to the cofacial binuclear complex Cu2(NBA)2 (NBAH2 = 3,3′-[2,7-naphthalenediyl-bis(methylene)]bis(2,4-pentanedione)) has been studied by structural and spectroscopic methods. Pyrazines bind more strongly than the corresponding pyridines, suggesting intramolecular, or endo, coordination, which X-ray analysis of the adduct with 2, 5-dimethylpyrazine (2, 5-Me2pyz) confirms: Cu2(NBA)2(μ-2,5-Me2pyz)-4CH2Cl2, monoclinic, space group C2/m (No. 12); a = 22.941 (6), b = 22.432 (4), c = 11.677 (2) Å; β = 97.32 (2)°; V = 5960 (4) Å3; Z = 4; R = 0.096, Rw = 0.093 for 372 parameters and 3358 reflections with I > 1σ(I). The structure contains two independent Cu2(NBA)2(μ-2,5-Me2pyz) molecules (CU⋯CU = 7.596 (2), 7.559 (2) Å), each with a disordered 2,5-Me2pyz moiety and overall 2/m (C2h) symmetry. Equilibrium constants for binding of substituted pyrazines to Cu2(NBA)2 range from ca. 0.2 M−1 (2,3-diethylpyrazine) to 93 M−1 (2-aminopyrazine); binding constants for comparably substituted pyridines are significantly smaller in all cases. Hydrogen bonding between the NH2 group of 2-aminopyrazine and the O atoms of the Cu2(NBA)2 host is probably responsible for its unusually large binding constant. © 1990, American Chemical Society. All rights reserved.