Binucleating Ligand Structural Effects on (μ-Peroxo)- and Bis(μ-oxo)dicopper Complex Formation and Decay:  Competition between Arene Hydroxylation and Aliphatic C−H Bond Activation

Abstract

The reactivity of dicopper(I) complexes of the ligands α,α‘-bis(4,7-diisopropyl-1,4,7-triazacyclononan-1-yl)-p- and m-xylene (p- and m-XYLiPr4) with dioxygen was examined by spectroscopic and rapid stopped-flow kinetics methods. Only bis(μ-oxo)dicopper(III) core formation was observed with p-XYLiPr4, but both (μ-η2:η2-peroxo)dicopper(II) and bis(μ-oxo)dicopper(III) species were generated in the m-XYLiPr4 case, their relative proportions being dependent on the solvent, concentration of the dicopper(I) precursor, and temperature. Subsequent decomposition under conditions that favored bis(μ-oxo) core formation resulted in oxidative N-dealkylation of isopropyl groups, whereas μ-η2:η2-peroxo decay led to the product resulting from hydroxylation of the bridging arene, [(m-XYLiPr4-O)Cu2(μ-OH)](SbF6)2. Evidence from kinetics studies, decomposition product analyses, and comparison to the chemistry exhibited by complexes of other substituted 1,4,7-triazacyclonane ligands support a model for the oxygenation of the m-XYLiPr4 compound involving initial, essentially rate-limiting 1:1 Cu:O2 adduct formation followed by partitioning between intra- and intermolecular pathways. At low temperature and high starting material concentrations, the latter route that yields tetranuclear “dimer-of-dimer” species and/or higher order oligomers with bis(μ-oxo) cores is favored, while at higher temperatures and dilution, intramolecular reaction predominates to afford a (peroxo)dicopper(II) species. The course of the subsequent decompositions of these oxygenated products correlates with their proposed formulations. Thus, analysis of final products and kinetics data, including with selectively deuterated compounds, showed that N-dealkylation arises from the high-nuclearity bis(μ-oxo) species and arene hydroxylation occurs upon decay of the intramolecular peroxo complex. Geometric rationales for the divergent oxygenation and decomposition reactions supported by p- and m-XYLiPr4 are proposed.

Synthesis and reactivity of (μ-η2:η2-peroxo)dicopper(II) complexes with dinucleating ligands: Hydroxylation of xylyl linker with a NIH shift

“…The IR spectra of the complexes showed strong m(C-O) bands at 2096 and 2085 cm À1 for 1-CO, 2089 cm À1 for 2-CO, and 2092 cm À1 for 3-CO, which are comparable to those of [Cu(Me 2 -pybn)(CO)] 2+ (5-CO, 2100 and 2088 cm À1 ) that has a half part of the dinucleating ligand H-L-H (see Scheme 1) [16] and 7-CO (2079 cm À1 ) [7], indicating that the electron donor properties of these monoand dinucleating ligands are similar [7,16,17]. The coordinated CO ligands in [Cu 2 (X-L-R)(CO) 2 ] 2+ can be removed by refluxing the methanol solutions under N 2 to afford [Cu 2 (X-L-R)] 2+ (1, 2, and 3) as air-sensitive yellow powders.…”

“…Crystal structure of 1a-CH 3 CN shows that each Cu(I) site has a tetrahedral structure with two pyridyl nitrogens, one tertiary amine nitrogen, and an additional CH 3 CN as shown in Fig. 1 [7]. H-XYL-H forms a tri-coordinate complex, whereas m-XYL iPr4 and X-L-R form four-coordinate complexes with an additional CH 3 CN.…”

“…H-XYL-H forms a tri-coordinate complex, whereas m-XYL iPr4 and X-L-R form four-coordinate complexes with an additional CH 3 CN. However, dioxygen reactivities of 7-CH 3 CN and [Cu 2 (X-L-R)(CH 3 CN) 2 ] 2+ significantly differ; oxygenation of 1-CH 3 CN, 2-CH 3 CN, and 3-CH 3 CN is very slow and not fully completed probably due to strong coordination of CH 3 CN, whereas 7-CH 3 CN is readily oxygenated [7]. Thus, for full oxygenation, we prepared [Cu 2 (X-L-R)] 2+ (1, 2, and 3) from [Cu 2 (X-L-R)(CH 3 CN) 2 ] 2+ through [Cu 2 …”

“…It has been shown that the oxygenation of the dicopper(I) complexes having a xylyl linker such as [Cu 2 (NO 2 -XYL-H)] 2+ (NO 2 -6) and [Cu 2 (m-XYL iPr4 )] 2+ (7) generate both intramolecular (l-g 2 :g 2 -peroxo)Cu(II) 2 species and intermolecular (l-g 2 :g 2 -peroxo)Cu(II) 2 or bis(l-oxo)Cu(III) 2 species such as a dimer of dimer depending on the concentrations of the complexes and the nature of the side arms [5,7]. NO 2 -6 generates an intramolecular (l-g 2 :g 2 -peroxo)Cu(II) 2 species at the concentrations bellow 4 mM, whereas 7 generates an intermolecular bis(l-oxo)Cu(III) 2 species even at the concentration of 0.1 mM.…”

“…The Cu n /O 2 -mediated arene hydroxylation is of current interest for understanding the reaction mechanisms of dioxygen activating copper proteins in biological systems such as tyrosinase and utilizing metal complexes as oxidation catalysts [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. Karlin and co-workers have first demonstrated that the (l-g 2 :g 2 -peroxo)Cu(II) 2 complexes, [Cu 2 (O 2 )(X-XYL-R)] 2+ , are capable of performing hydroxylation of the xylyl linkers of the supporting ligands [3][4][5].…”