Copper(II) Complexes of Cyclams Containing Nitrophenyl Substituents: Push–Pull Behavior and Scorpionate Coordination of the Nitro Group

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 X-ray diffraction studies.
This study documents the first examples of effective metal coordination
by the nitro group.