Copper−Dioxygen Adducts and the Side-on Peroxo Dicopper(II)/Bis(μ-oxo) Dicopper(III) Equilibrium:  Significant Ligand Electronic Effects

Abstract

The variation of ligand para substituents on pyridyl donor groups of tridentate amine copper(I) complexes was carried out in order to probe electronic effects on the equilibrium between mu-eta2:eta2-(side-on)-peroxo [Cu(II)2(O2(2-))]2+ and bis(mu-oxo) [Cu(III)2(O(2-))2] species formed upon reaction with O2. [Cu(I)(R-PYAN)(MeCN)n]B(C6F5)4 (R-PYAN = N-[2-(4-R-pyridin-2-yl)-ethyl]-N,N',N'-trimethyl-propane-1,3-diamine, R = NMe2, OMe, H, and Cl) (1R) vary over a narrow range in their Cu(II)/Cu(I) redox potentials (E(1/2) vs Fe(cp)2(+/0) = -0.40 V for 1(NMe2), -0.38 V for 1(OMe), -0.33 V for 1H, and -0.32 V for 1Cl) and in C-O stretching frequencies of their carbonyl adducts, 1R-CO: nu(C-O) = 2080, 2086, 2088, and 2090 cm(-1) for R = NMe2, OMe, H, and Cl, respectively. However, within this range of electronic properties for 1R, dioxygen reactivity is significantly affected. The reaction of 1Cl or 1H with O2 at -78 degrees C in CH2Cl2 gives UV-vis and resonance Raman spectra indicative of a mu-eta2:eta2-(side-on)-peroxo dicopper(II) adduct (2R). Compound 1(OMe) reacts with O2, yielding equilibrium mixtures of side-on peroxo (2(OMe)) and bis(mu-oxo) (3(OMe)) species. Oxygenation of 1(NMe2) leads to the sole generation of the bis(mu-oxo) dicopper(III) complex (3(NMe2)). A solvent effect was also observed; in acetone or THF, increased ratios of bis(mu-oxo) relative to side-on peroxo complex are observed. Thus, the equilibrium between a dicopper side-on peroxo and bis(mu-oxo) species can be tuned by ligand design-specifically, more electron donating ligands favor the formation of the latter isomer, and the peroxo/bis(mu-oxo) equilibrium can be shifted from one extreme to the other within the same ligand system. Observations concerning the reactivity of the dioxygen adducts 2H and 3(NMe2) toward external substrates are also presented.

Density functional theory studies of oxygen and carbonate binding to a dicopper patellamide complex

“…To assist experimental identification of critical intermediates in reaction processes, we calculated the IR and Raman spectrum of , which is in good agreement with frequencies found for similar dioxo-dicopper complexes but with different ligand systems [13,30,31]. Well lower lying are the is very large, whereas this band is much less active in the IR spectrum.…”

Theoretical Characterization of End-On and Side-On Peroxide Coordination in Ligated Cu2O2 Models

“…Moreover, large variations in relative energies were observed when four, five, or six roots were employed in multistate calculations (see Table 2). Interestingly, reducing the symmetry to C i (which permits a greater mixing of the active and inactive orbitals in the CAS) leads to a smooth convergence and good agreement between active spaces of (12,12), (12,14), and (16,14) and also to good agreement between CASPT2 and MS-CASPT2 (i.e., interactions between multiple roots are rendered negligible). However, the converged energy of 1 0 relative to that of 1 100 is predicted to be -16.7 kcal mol -1 , which is very far from the CR-CC results.…”

“…This gives an active space of 12 electrons in 10 orbitals, but predicted energetics are Very different from those computed at the CR-CC levels (and also substantially different from DFT). We have extended this (12,10) active space in various ways, up to at most a (16,14) active space, without achieving any systematic agreement with the CR-CC models. In general, no convergence in relative energies was achieved with increasing active space size, suggesting that none were adequate.…”

“…[1][2][3][4][5][6][7][8] Tyrosinase 1,5,6,9 is one example of an enzyme whose active site incorporates a Cu 2 O 2 core, and substantial effort has also been devoted to the characterization of smaller inorganic models characterized by the same core. 2, [10][11][12][13][14][15][16][17][18] There are several motifs for the binding of molecular O 2 to two supported copper(I) atoms ( Figure 1). 12,19 Best characterized experimentally (because they have been observed for many different supporting ligand sets) are the side-on µ-η 2 :η 2 peroxo and the bis(µ-oxo).…”

Reversible O−O Bond Cleavage in Copper−Dioxygen Isomers: Impact of Anion Basicity

“…Such a basicity effect is counter-intuitive, as more electron donation to the Cu 2 O 2 core is anticipated to stabilize the higher oxidation state of the copper centers and hence favor the O isomer. 10 Titration experiments with competing anions highlight the importance of anion basicity and reveal the existence of specific anion/dication interactions. Addition of a more coordinating anion Y − (CF 3 SO 3 − , TsO − , CH 3 SO 3 − , CF 3 CO 2 − , PhCO 2 − ) to a preformed P/O solution with a "weaker" anion X − (SbF 6 − , CF 3 SO 3 − ) results in a rapid, isosbestic isomerization in the direction O → P. Spectroscopically pure P species are obtained by addition of 1.0 equivalent of TsO − , CH 3 2 provides structural evidence of the close association between the anion and the Cu 2 O 2 core in this spectroscopically pure P species.…”