β-Diketiminate Ligand Backbone Structural Effects on Cu(I)/O₂ Reactivity:  Unique Copper−Superoxo and Bis(μ-oxo) Complexes

Abstract

Copper(I) and -(II) complexes of beta-diketiminate ligands with identical flanking 2,6-diisopropylphenyl groups but divergent backbone substitution patterns were prepared and structurally characterized, and reactions of the Cu(I) species with O(2) at low temperature were explored. Despite being far removed from the coordinated metal ion, the different backbone patterns significantly influence the steric encumbrance exerted by the ligands, as revealed by differences in (a) the structural features of the Cu(I) and Cu(II) complexes and (b) the course of the oxygenation reactions of the Cu(I) compounds. With the less hindered ligand, a rare example of a neutral bis(mu-oxo)dicopper complex was identified on the basis of its diagnostic spectral features (UV-vis, resonance Raman, EPR) and the stoichiometry of O(2) uptake (Cu:O(2) = 2:1). In contrast, oxygenation of the Cu(I) complexes supported by the more hindered ligands yielded novel (superoxo)copper complexes, identified by a Cu:O(2) ratio of 1:1, a lack of an EPR signal, and O-isotope sensitive resonance Raman spectral features (nu(O)(-)(O) = 968 cm(-1), Delta(18)O(2) = 51 cm(-1)). Symmetric coordination of the superoxo ligand is proposed on the basis of Raman data acquired using (16)O(18)O (single peak at 943 cm(-1)).

Variable character of O—O and M—O bonding in side-on (η 2 ) 1:1 metal complexes of O 2

“…As summarized in Scheme 1 and Table 1, our theoretical models (labeled with a t to facilitate discussion distinguishing them from the experimentally characterized systems) include all substituents and thus are exact replicas for the cases 2 (11), 3 (K. Qin (14,15), and 7 (26). In the case of 1 (10), the theoretical model simplifies the 5-isopropyl groups on the pyrazole rings for computational efficiency, and for the same reason the phenyl rings pendant on the porphinato ligands in 8 (27) Value is an estimate from experimental data, for anion well separated from any counterion.…”

“…In the course of the investigation of 6, for which intermediate character of the dioxygen unit was found (14,15), we noted puzzling incongruities among the data reported for these complexes (Table 1). For example, whereas 1 and 2 (as reported in ref.…”

Intermediates in the oxygenation of a nonheme diiron(II) complex, including the first evidence for a bound superoxo species

“…Such species have been found in the reactions of O 2 with heme proteins and models (6,7) as well as with one copper enzyme (8) and several synthetic copper and nickel complexes (9)(10)(11)(12)(13)(14)(15)(16). Such species have also been invoked in the mechanisms for nonheme iron enzymes, but only indirect evidence has been obtained to date (39)(40)(41).…”

“…Crystallographic evidence for an analogous formulation has also recently been reported for a copper enzyme peptidylglycine ␣-hydroxylating monooxygenase (PHM) (8). Furthermore, synthetic copper(I) and nickel(I) complexes react with O 2 to give rise to copper and nickel superoxo species (9)(10)(11)(12)(13)(14)(15)(16) 2 was prepared by using the same procedure in the presence of 10 eq of D 2 O. All manipulations with complex 1 and solutions thereof were carried out under inert atmosphere by using a glovebox filled with nitrogen.…”

Binuclear β-diketiminate complexes of copper(i)

“…[3][4][5][6][7][8][9][10][11][12][13][14][15] Warren and coworkers have expanded the oxidation chemistry of copper(I) β-diketiminate complexes to include diazoalkanes, PhI=NTs, organoazides and tert-butylperoxide. [16][17][18][19][20][21][22][23][24][25][26][27][28] These stoichiometric studies are part of a broader programme, developing exciting synthetic protocols for the functionalization of carbon-hydrogen bonds in hydrocarbons.…”