Reactivity of Cu^IN₄ Flattened Complexes: Interplay between Coordination Geometry and Ligand Flexibility

Abstract

The relation between redox activity and coordination geometry in CuIN4 complexes indicates that more flattened structures tend to be more reactive. Such a preorganization of the ligand confers to the complex geometries closer to a transition state, which has been termed the “entatic” state in metalloproteins, more recently extending this concept for copper complexes. However, many aspects of the redox chemistry of CuI complexes cannot be explained only by flattening. For instance, the role of ligand flexibility in this context is an open debate nowadays. To analyze this point, we studied oxidation properties of a series of five monometallic CuI Schiff-base complexes, [CuI(L n )]+, which span a range of geometries from a distorted square planar (n = 3) to a distorted tetrahedron (n = 6, 7). This stepped control of the structure around the CuI atom allows us to explore the effect of the flattening distortion on both the electronic and redox properties through the series. Experimental studies were complemented by a theoretical analysis based on density functional theory calculations. As expected, oxidation was favored in the flattened structures, spanning a broad potential window of 370 mV for the complete series. This orderly behavior was tested in the reductive dehalogenation reaction of tetrachloroethane (TCE). Kinetic studies show that CuI oxidation by TCE is faster as the flattening distortion is higher and the oxidation potentials of the metal are lower. However, the most reactive complex was not the more planar, contradicting the trend expected from oxidation potentials. The origin of this irregularity is related to ligand flexibility and its connection with the atom/electron transfer reaction path, highlighting the need to consider effects beyond flattening distortion to better understand the reactivity of this important class of complexes.

Conformationally dynamic copper coordination complexes

“…The Cu–N pyridine bond distances experience a slight contraction upon metal oxidation, consistent with chelating pyridine arms bound to Cu. 10,23,28,29 The Cu I –N aniline bond distance of >2.4 Å is well above the sum of the covalent radii. Therefore, in the solid-state, there appears to be no bond between the Cu I ion and aniline (dashed bond in Fig.…”

Effect of non-covalent self-dimerization on the spectroscopic and electrochemical properties of mixed Cu(i) complexes

“…The possibility of Cu( i ) complexes to reach a more flattened conformation is also responsible for their oxidation to cupric species in solution. 4 d ,6…”

Versatile Activity of a Copper(II) Complex Bearing a N4‐Tetradentate Schiff Base Ligand with Reduced Oxygen Species

“…It is known that [Cu(py 2 pn)] + adopts a flattened geometry, and presents two allowed electronic transitions between d xz and d yz orbitals of Cu(I) and two π* orbitals of the Schiff-base ligand of the tetracoordinated metal ion, at 380 nm (ɛ = 3.3 × 10 3 M À 1 cm À 1 ) and 650 nm (ɛ = 1.1 × 10 3 M À 1 cm À 1 ), respectively. [29] Besides, [Cu(py 2 pn)] + can dimerize and the monomer/dimer equilibrium depends on the total Cu(I) concentration. [52] In the dimer the geometrical arrangement around each Cu(I) is tetrahedral, exhibiting an intense absorption band at 460 nm (ɛ = 11681 M À 1 cm À 1 ).…”

“…[27,28] In copper complexes of the N,N'-bis(2-pyridinylmethylen)alkane-1,n-diamine family, the Cu-(II)/Cu(I) redox potential and geometry can be tuned by the length of the alkyl chain between the two imino N-atoms. [29] Therefore, Cu(II) complexes of N 4 -tetradentate Schiff base ligands are an excellent platform for testing reactivity with superoxide and hydrogen peroxide and evaluating their antioxidant/pro-oxidant activity. To our knowledge, little is known about intermediate [LCuO 2 (H)] + species formed in the reaction of this kind of complexes with either superoxide or peroxide, or catalytic active species involved in oxidation reactions.…”