Catching an Entatic State—A Pair of Copper Complexes

Abstract

The structures of two types of guanidine-quinoline copper complexes have been investigated by single-crystal X-ray crystallography, K-edge X-ray absorption spectroscopy (XAS), resonance Raman and UV/Vis spectroscopy, cyclic voltammetry, and density functional theory (DFT). Independent of the oxidation state, the two structures, which are virtually identical for solids and complexes in solution, resemble each other strongly and are connected by a reversible electron transfer at 0.33 V. By resonant excitation of the two entatic copper complexes, the transition state of the electron transfer is accessible through vibrational modes, which are coupled to metal-ligand charge transfer (MLCT) and ligand-metal charge transfer (LMCT) states.

One‐ versus Two‐Electron Oxidation of Complexed Guanidino‐Functionalized Aromatic Compounds

“…However, the complex turned out to be highly redox active, and small amounts of O 2 quickly led to rapid oxidation. [3] The superconducting quantum interference device (SQUID) magnetometric curve could be fitted by assuming a weak antiferromagnetic coupling between two Cu II centers with a coupling constant J [H = -2JS(1)·S(2)] of -5 cm -1 (see Figure 1c and the Supporting Information for the curve fit as well as information on the spin Hamiltonian H and the applied fit formula). The C-C distances (1.393-1.406 Å) within the C 6 ring argue for an intact aromatic system.…”

“…The new compound was crystallized from a CH 3 hedral angle between the N1-Cu-N4 and O1-Cu-O3 planes measures 18°. Oxidation with I 2 led to the salt [1{Cu(OAc) 2 } 2 ](I 3 ) 2 (see Scheme 4).…”

“…[25] In such studies, EPR spectroscopy delivered useful information about the location of the unpaired electrons, especially in cases of valence tautomerism. Values of χ at 300 K from SQUID measurements: 3.34 ϫ 10 -3 cm 3 2+ , which is green (see Figure 8a). Both [1{Cu(OAc) 2 } 2 ] ·+ (PF 6 -) and [1{Cu(OAc) 2 } 2 ] ·+ -(SbF 6 -) exhibit strong absorptions at 576 and 536 nm (see Figure 7a), which are the principal contributors to the red color of the salts (in contrast to the green of the salts of the dicationic complexes).…”

“…Crystals of the product were grown from a CH 3 (14) (14) Then the reaction mixture was stirred at room temperature for a period of 3 h. After removal of the solvent under vacuum the remaining dark green solid was washed with toluene (3.5 mL).…”

Hydrogen Bonds Dictate the Coordination Geometry of Copper: Characterization of a Square‐Planar Copper(I) Complex

“…[1b, 6] Concomitant with mutagenesis studies on BCPs, other groups have investigated the entatic state hypothesis by synthesizing small molecule complexes that exhibit minimal structural reorganization upon oxidation and reduction. [3, 7] Typically, the ligand frameworks for these studies exploit steric interactions or a macrocyclic ligand to achieve fast ET rates. [8] In contrast to the covalent coordination strategies most commonly employed by small molecule models, synthetic frameworks incorporating secondary sphere H-bonding interactions as a means to stabilize entatic states are exceedingly rare, yet are critical to the ‘rack’ in BCPs.…”