Copper(II) complexes of cyclams containing nitrophenyl substituents: push−pull behavior and scorpionate coordination of the nitro group

ABSTRACT: The three nitrophenyl−cyclam derivatives (nitrocyclams): 1-(4-nitrophenyl)-1,4,8,11-tetraazacyclotetradecane (2), 1-(2-nitrophenyl)-1,4,8,11-tetraazacyclotetradecane (3), and 1-(2,4-dinitrophenyl)-1,4,8,11-tetraazacyclotetradecane (4), in an MeCN solution, specifically incorporate the CuII ion according to an irreversible process signaled by disappearance of the yellow color for a concentration c < 1 × 10−4 M and by a yellow-to-red color change for c ≥ 1 × 10−3, and must be considered efficient and specific dosimeters of copper(II) salts. When present in the ortho position of the nitrophenyl substituent, the −NO2 group coordinates the CuII according to a scorpionate mode, while the metallocyclam system exhibits a trans-I configuration. In an MeCN solution the red trans-I-[CuII(3)]2+ and trans-I-[CuII(4)]2+ scorpionate complexes slowly convert into the violet trans-III scorpionate complexes. Kinetic aspects of the trans-I-to-trans-III configurational rearrangement were investigated in detail for the [CuII(4)]2+ system. In particular, the conversion is spectacularly accelerated by catalytic amounts of Cl−, NCO−, and F−. While for Cl− and NCO− the effect can be associated with the capability of the anion to stabilize through coordination a possible dissociative intermediate, the amazingly powerful effect of F− must be related to the preliminary deprotonation of one N−H fragment of the macrocycle, driven by the formation of the HF2− ion. Most of the metal complex species studied in solution were isolated in a crystalline form, and their molecular structures were elucidated through Xray diffraction studies. This study documents the first examples of effective metal coordination by the nitro group.