REVERSIBLE DIOXYGEN BINDING AND AROMATIC HYDROXYLATION IN O-2-REACTIONS WITH SUBSTITUTED XYLYL DINUCLEAR COPPER(I) COMPLEXES - SYNTHESES AND LOW-TEMPERATURE KINETIC THERMODYNAMIC AND SPECTROSCOPIC INVESTIGATIONS OF A COPPER MONOOXYGENASE MODEL SYSTEM/
Kd. Karlin et al., REVERSIBLE DIOXYGEN BINDING AND AROMATIC HYDROXYLATION IN O-2-REACTIONS WITH SUBSTITUTED XYLYL DINUCLEAR COPPER(I) COMPLEXES - SYNTHESES AND LOW-TEMPERATURE KINETIC THERMODYNAMIC AND SPECTROSCOPIC INVESTIGATIONS OF A COPPER MONOOXYGENASE MODEL SYSTEM/, Journal of the American Chemical Society, 116(4), 1994, pp. 1324-1336
The binding and subsequent reactivity of dioxygen (O-2) upon binding t
o copper ion centers is of fundamental interest in chemical and biolog
ical processes. We provide here a detailed account of the reaction of
O-2 with dicopper(I) complexes, involving O-2-reversible binding, foll
owed by the stoichiometric aromatic hydroxylation of the ligand. Thus,
tricoordinated dicopper(I) complexes [Cu-2(R-XYL)](2+) (R = H, MeO, t
-Bu, F, CN, NO2; 1a-f) possess dinucleating meta-substituted xylylene
ligands with two chelating tridentate bis [2-(2-pyridyl)ethyl] amine (
PY2) moieties and a 5-R substituent. Upon reaction with O-2, dioxygen
adducts [Cu-2(R-XYL)(O-2)](2+) (2a,c-f) form reversibly, and these sub
sequently yield 2-xylylene-hydroxylated products [Cu-2(R-XYL-O-)(OH)](
2+)(3a-f), which are phenoxo- and hydroxobridged copper(II) complexes.
The products 3 have been characterized via the X-ray structure of the
parent complex 3a, and by their UV-visible, infrared, and room-temper
ature magnetic properties. Incorporation of the O-atom from dioxygen i
nto the phenolic products has been proven by isotopic labeling experim
ents, except in the case of 3f; where workup results in an exchange re
action causing loss of the oxygen label. In read-ions of O-2 With 1 in
dichloromethane at room temperature, 10-25% yields of unhydroxylated
complexes [Cu-2(R-XYL)(OH)](3+) (5) are obtained. A stopped-flow kinet
ics study of O-2 reactions of 1 in CH2Cl2 demonstrates that [Cu-2(R-XY
L)(O-2)](2+) (2a,c-f) complexes form reversibly, proceeding via the re
action 1 + O-2 reversible arrow 2 (K-1 = k(1)/k(-1); this is followed
by the irreversible reaction 2 - 3 (k(2)). Analysis of temperature-dep
endent data which is accompanied by spectrophotometric monitoring yiel
ds both kinetic and thermodynamic parameters for R = H, t-Bu, F, and N
O2. Dioxygen binding to 1 occurs in a single observable step with low
activation enthalpies. (6-29 kJ mol(-1) and large, negative activation
entropies (-66 to -167 J K-1 mol(-1). The remote R-substituent has a
significant effect on the dioxygen binding process and this is explain
ed in terms of its multistep nature. Strong binding (K-1) occurs at lo
w temperature (e.g. -80 degrees C), and thermodynamic parameters indic
ate a large enthalpic contribution (Delta H degrees = -52 to -74 kJ me
l(-1), but room-temperature stabilities of the dioxygen adducts are pr
ecluded by very large unfavorable entropies (Delta S degrees - -156 to
-250 J K-1 mol(-1)). Electron-releasing R-substituents cause a small
but significant enhancement of k(2), the hydroxylation step, consisten
t with a mechanism involving electrophilic attack of the Cu2O2 interme
diate 2 upon the xylyl;ylyl aromatic ring. The influence of substituen
t upon the various rates of reaction allows for stabilization (similar
to minutes), allowing the bench-top observation of 2d,e,f using W-vis
ible spectroscopy at -80 degrees C. ''Vacuum-cycling'' experime:nts ca
n be carried out on 1f/2f, i.e., the repetitive oxygenation of 1f at-8
0 degrees C, followed by removal of O-2 from 2f by a application of a
vacuum. Dicopper(I) complexes I have been characterized by H-1 and C-1
3 NMR spectroscopy, along with analogs in which an ethyl group has bee
n placed in the 5-position of the pyridyl ring donor groups, i.e., [Cu
-2(I)(R-XYL-(5-Et-PY))](2+) (1g, R = H; 1h, R = NO2). Variable-tempera
ture H-1 NMR spectroscopic studies provide clues as to why [Cu-2(MeO-X
YL)](2+) (1b) does not oxygenate (i.e., bind O-2 and/or hydroxy)rlate)
at low temperature, the conclusion being that significant interaction
s of the coordinately unsaturated copper(I) ion(s) with the chelated m
ethoxybenzene group result in conformations unsuitable for O-2-reactiv
ity. The biological implications of the biomimetic chemistry described
here are discussed, as a system effecting oxidative C-H functionaliza
tion using O-2 under mild conditions and as a monooxygenase model syst
em for tyrosinase (phenol o-monooxygenase), with its dinuclear active
site.