Differential reactivity between interconvertible side-on peroxo and bis-μ-oxodicopper isomers using peralkylated diamine ligands [22]

Full text of DOI:10.1021/ja002527h

J. Am. Chem. Soc. 2000, 122, 10249-10250
10249
Scheme 1
Differential Reactivity between Interconvertible
Side-On Peroxo and Bis-µ-oxodicopper Isomers
Using Peralkylated Diamine Ligands
Viswanath Mahadevan,† Mark J. Henson,†
Edward I. Solomon,† and T. D. P. Stack*,†
Department of Chemistry, Stanford UniVersity
Stanford, California 94305
The O2 reaction of Cu(I)-PDL complexes has been generally
thought to exclusively form O at low temperature in aprotic
solvents using weakly coordinating counteranions.3,10 Systematic
structural variation has now allowed a limited subset of PDLs to
be identified whose copper complexes are capable of stabilizing
the µ-η2:η2-peroxodicopper(II) species. Mixtures of P and O are
created. The existence of both species in solution has been verified
using characteristic spectroscopic features in the UV-visible,
resonance Raman (rR), and X-ray absorption (XAS) spectra.15-18
In THF, acetone, and CH2Cl2 solutions, a facile equilibrium exists
as evidenced by a rapid, reversible interconversion between P
and O signaled by opposing changes in the intensity of charac-
teristic optical bands upon temperature variation.19
The position of the P:O equilibrium is most sensitive to
the ligand structure, and to a lesser extent on solvent,20 counter-
anions, and temperature.21 An increase in the equilibrium constant
(Keq ) [P]/[O]; 193 K, CH2Cl2) coincides with increasing
steric bulk of the ligand (Scheme 1).3 Larger N-alkyl sub-
stituents, at parity of diamine backbone, bias the equilibrium
toward P: Keq(L3) ≈ Keq(L2) < Keq(L1) for the 1,2-ethanediamine
(ED) series, and Keq(L4) < Keq(L5) < Keq(L6) for the 1,3-
propanediamine (PD) series. A comparison of the ED and PD
ligands, at parity of N-alkyl substituents, indicates that the PD
ligands also bias the equilibrium toward P (Keq(L6) > Keq(L3))
and (Keq(L5) > Keq(L2)). These trends intimate interligand steric
interactions as the major determinant in the position of the
equilibrium; ligands that enhance such interactions bias the
equilibrium away from O toward P. This interpretation assumes
that the Cu2O2 core of O is intrinsically more stable than that of
P, fully consistent with the simplest Cu(I)-PDL forming only
measurable amounts of O.
ReceiVed July 12, 2000
Interest in metal-mediated dioxygen (O2) bond cleavage and
formation has persisted over several decades due to its biological
relevance and potential application in industrial redox processes.
In biological systems, iron and copper sites play important roles
in many O2-activating enzymes that oxidize substrates.1 Charac-
terization of the oxidant structure is a first-line approach to
understanding the reactivity of such enzymes. In the binuclear
copper enzymes tyrosinase and catechol oxidase, the structurally
and spectroscopically characterized µ-η2:η2-peroxodicopper(II)
species is generally accepted to be the active oxidant.1,2 Recent
reports of a facile interconversion between the isoelectronic µ-η2:
η2-peroxodicopper(II) species (P) and the bis-µ-oxodicopper(III)
species (O) within a model system3-5 offer an alternative oxidative
mechanistic route: O-O bond cleavage preceding substrate
hydroxylation, rather than the generally accepted electrophilic
peroxide attack on the arene ring.6,7 Additional findings that O
is capable of ligand hydroxylation provide chemical plausibility
for such hydroxylation reactivity.8-10 Understanding the P:O
equilibrium systems and determining the relative reactivity of
each isomer with externally added substrates will provide keen
insights into harnessing the oxidizing potential of these complexes.
In this report, several new P:O equilibrium systems are described.
In one particular case, the rate of isomer interconversion is
significantly slower than the rate of the reaction with substrates,
which allows the reactivity of each isomer, at parity of ligand, to
be assessed. In all reactions examined, P is the more reactive
species.
Ligand structure has proven critical in controlling not only the
formation of small-molecule Cu/O2 complexes but also their
subsequent reactivity. Peralkylated diamine ligands (PDLs)
provide sufficient Cu(I) ligation for O2 activation while maintain-
ing potential substrate access to the subsequently formed Cu/O2
cores.10,11 Selection of bidentate chelation follows from the
structures of µ-η2:η2-peroxodicopper(II) complexes; each copper
atom is ligated by a weak axial nitrogen ligand and two stronger
equatorial nitrogen ligands.12-14
Dramatic alterations in the equilibrium position can be achieved
by addition of various weakly coordinating counteranions to
mixtures of P and O. Isosbestic shifts of the equilibrium position
is observed optically upon addition of tetra-n-butylammonium per-
chlorate to a mixture of [(L1)2Cu2O2](OTf)2 in CH2Cl2 [P f O]
(12) Magnus, K. A.; Ton-That, H.; Carpenter, J. E. Chem. ReV. 1994, 94,
727-735.
(13) Kitajima, N.; Fujisawa, K.; Fujimoto, C.; Moro-oka, Y.; Hashimoto,
S.; Kitagawa, T.; Toriumi, K.; Tatsumi, K.; Nakamura, A. J. Am. Chem. Soc.
1992, 114, 1277-1291.
† Stanford University.
(1) Solomon, E. I.; Sundaram, U. M.; Machonkin, T. E. Chem. ReV. 1996,
(14) Kodera, M.; Katayama, K.; Tachi, Y.; Kano, K.; Hirota, S.; Fujinami,
S.; Suzuki, M. J. Am. Chem. Soc. 1999, 121, 11006-11007.
(15) Henson, M. J.; Mukherjee, P.; Root, D. E.; Stack, T. D. P.; Solomon,
E. I. J. Am. Chem. Soc. 1999, 121, 10332-10345.
96, 2563-2606.
(2) Klabunde, T.; Eicken, C.; Sacchettini, J. C.; Krebs, B. Nat. Struct. Biol.
1998, 5, 1084-1090.
(3) Tolman, W. B. Acc. Chem. Res. 1997, 30, 227-237.
(4) Holland, P. L.; Tolman, W. B. Coord. Chem. ReV. 1999, 192, 855-
869.
(16) DuBois, J. L.; Mukherjee, P.; Collier, A. M.; Mayer, J. M.; Solomon,
E. I.; Hedman, B.; Stack, T. D. P.; Hodgson, K. O. J. Am. Chem. Soc. 1997,
119, 8578-8579.
(5) Obias, H. V.; Lin, Y.; Murthy, N. N.; Pidcock, E.; Solomon, E. I.;
Ralle, M.; Blackburn, N. J.; Neuhold, Y. M.; Zuberbuhler, A. D.; Karlin, K.
D. J. Am. Chem. Soc. 1998, 120, 12960-12961.
(17) Pidcock, E.; DeBeer, S.; Obias, H. V.; Hedman, B.; Hodgson, K. O.;
Karlin, K. D.; Solomon, E. I. J. Am. Chem. Soc. 1999, 121, 1870-1878.
(18) The absorbance spectrum exhibits intense bands characteristic of both
O (∼300 nm, ∼400 nm) and P (∼360 nm). Resonance Raman experiments
(6) Holland, P. L.; Rodgers, K. R.; Tolman, W. B. Angew. Chem., Int. Ed.
1999, 38, 1139-1142.
identified an 18O2 isotope sensitive rR band at ∼600 cm-1 (∆ν ) -24 cm-1
;
(7) Mahadevan, V.; Klein Gebbink, R. J. M.; Stack, T. D. P. Curr. Opin.
Chem. Biol. 2000, 4, 228-234.
406.7 nm excitation) characteristic of O and an isotope insensitive band at
294 cm-1 (363.8 and 406.7 nm excitation) in THF, indicating the presence of
P. This mixture has further been identified in solution by XAS.
(19) See Supporting Information.
(8) Itoh, S.; Taki, M.; Nakao, H.; Holland, P. L.; Tolman, W. B.; Que, L.;
Fukuzumi, S. Angew. Chem., Int. Ed. 2000, 39, 398-400.
(9) Mahapatra, S.; Halfen, J. A.; Tolman, W. B. J. Am. Chem. Soc. 1996,
118, 11575-11586.
(20) As different solvents generate various P:O ratios, interconversion
between P and O can be conveniently achieved by 10-fold dilution of the
mixture in THF with acetone (P to O) and vice-versa (O to P) when the
mixture in acetone is diluted 10-fold with THF.
(21) Cahoy, J.; Holland, P. L.; Tolman, W. B. Inorg. Chem. 1999, 38,
2161-2168.
(10) Mahadevan, V.; Hou, Z. G.; Cole, A. P.; Root, D. E.; Lal, T. K.;
Solomon, E. I.; Stack, T. D. P. J. Am. Chem. Soc. 1997, 119, 11996-11997.
(11) Mahadevan, V.; DuBois, J. L.; Hedman, B.; Hodgson, K. O.; Stack,
T. D. P. J. Am. Chem. Soc. 1999, 121, 5583-5584.
10.1021/ja002527h CCC: $19.00 © 2000 American Chemical Society
Published on Web 09/30/2000

10250 J. Am. Chem. Soc., Vol. 122, No. 41, 2000
Communications to the Editor
Table 1
and upon addition of tetra-n-butylammonium triflate to a mixture
of [(L1)2Cu2O2](SbF6)2 in acetone [P r O].19 Similar shifts have
also been confirmed by rR spectroscopy. Variable temperature
studies of the optical features associated with several P:O mixtures
at equilibrium consistently show that O is enthalpically, but not
entropically, favored relative to P.22 It is the combination of the
small magnitude and opposing nature of the enthalpic and entropic
contributions that allows significant quantities of each isomer to
form at low temperatures with specific ligands.21
Figure 1. Reaction of [(L1)2Cu2O2](OTf)2 (0.05 mM Cu, ∼50:50 mixture
of P:O) in 2-MeTHF with 2,4-di-tert-butylphenol (20 equiv) at 193 K.
Preferential decay of P is observed.
In general, the rates of formation for P and O are equal, as are
their rates of thermal decomposition. Such behavior is con-
sistent with an equilibrium in which isomer interconversion is
faster than the rate-determining step (RDS) of both formation
and decomposition.5,24 In contrast, the isomer interconversion
rate between O and P for [(L1)2Cu2O2]2+ in 2-MeTHF is signifi-
cantly slower.23 The decay of the optical features associated
with P, while comparable to rates determined in other solvents,
is an order of magnitude faster than the decomposition rate for
O (Table 1).19 Deuteration of the methyl substituents of L1,
d6-L1, allows the role of methyl C-H bond cleavage in the RDS
of the decomposition pathway of each isomer to be assessed. Only
the decomposition of O is significantly impacted with a kinetic
isotope effect of 4.58-10,19,25 at 233 K, suggesting independent
decomposition pathways for P and O. However, the observed
differential decay indicates that P is the more reactive of the two
isomers at parity of ligand.26
3.5-3.6 Å), a reaction requiring access to the Cu2O2 core would
likely proceed at a slower rate with O. Combined, these results
implicate that accessibility of substrate to the Cu2O2 core is as
important as the electronic composition of the isomers in
determining oxygen-atom transfer reactivity.
2,4-Di-tert-butylphenol also reacts preferentially with P at 193
K (Figure 1), yet the decay of P does not track with the rate of
formation of the product 3,3′,5,5′-tetra-tert-butyl 2,2′-biphenol.
A possible P decay pathway is peroxide displacement by the
phenol resulting in a phenoxide-bound Cu(II) complex.27 The
oxidative coupling of the phenol closely tracks the decay of O,19
and the yield (40%) closely matches the initial estimate of O in
the mixture. Thus, at parity of ligands, P appears to be a better
oxygen-atom transfer reagent28 while O is a better hydrogen-atom
acceptor,29 consistent with theoretical studies.15
Various PDLs that generate a mixture of isomeric Cu/O2
binuclear complexes, P and O, have been identified. While less
bulky PDLs preferentially form the bis-µ-oxodicopper(III) com-
plex, increasing the steric demands of the N-substituents favors
P. Isomers P and O can be interconverted by temperature, solvents
and counteranions. In contrast to a previously reported system
with rapid equilibrium between P and O, differential reactivity
for P and O with [(L1)2Cu2O2]2+ in 2-MeTHF can be observed
due to slow interconversion. Slow equilibration is observed during
thermal decomposition of P and O as well as during reactions
with PPh3 and phenols. Under all reported conditions P reacts
faster, a result that may have potential mechanistic relevance for
enzymes such as tyrosinase and catechol oxidase and for the
design of organic oxidants.
The slow equilibration between O and P for [(L1)2Cu2O2]2+ in
2-MeTHF also allows the examination of each isomer’s reactivity
with externally added substrates such as PPh3 and 2,4-di-tert-
butylphenol. Addition of PPh3 at 193 K results in preferential
decay of the optical features of P, at a rate equaling the formation
rate of PPh3O; ∼50% yields of PPh3O were found upon workup.19
This contrasts with the low yield (5%) of PPh3O obtained using
[(L2)2Cu2O2]2+, the most closely related complex, except that
only measurable quantities of O are formed. One might conclude
that O is ineffective at oxygen-atom transfer were it not for the
result that the complex with the least bulky ligand in this study,
[(L4)2Cu2O2]2+, which also only forms measurable amounts of
O, nearly quantitatively converts PPh3 to PPh3O (>95%). A
possible explanation for the observed reactivity involves the
structures of P and O. Since O is significantly more compact
than P at parity of ligand (O: Cu-Cu, 2.75-2.83 Å; P: Cu-Cu,
Acknowledgment. We thank Dr. J. L. DuBois, Dr. B. Hedman, and
Professor K. O. Hodgson for XAS data. Funding was provided by NIH
GM50730 (T.D.P.S) and NIH DK31450 (E.I.S.).
(22) ∆H° ) ∼0.6 kcal/mol; ∆S° ) ∼2 cal/mol in CH2Cl2 for [(L1)2Cu2O2]-
(OTf)2. The enthalpic difference is similar for the TACN based P:O
equilibrium (ref 21).
Supporting Information Available: Synthetic details, optical spec-
trum of [(L1)2Cu2O2](OTf)2, experimental details for kinetic, ligand
product analysis (PDF). This material is available free of charge via the
Internet at http://pubs.acs.org.
(23) The slow interconversion rate is independent of the anion used
(CF3SO3-, ClO4-).
(24) Halfen, J. A.; Mahapatra, S.; Wilkinson, E. C.; Kaderli, S.; Young,
V. G.; Que, L., Jr.; Zuberbu¨hler, A. D.; Tolman, W. B. Science 1996, 271,
1397-1400.
JA002527H
(25) The KIE was calculated from rate constants based on a P f C;
O f C model. kO (Table 1) is the largest contributor to the KIE (∼4.5).
(26) Mono-demethylated ligand is the only significant ligand decay product
of this reaction, but the yields of this oxidized ligand account for only ∼10%
of the required reducing equivalents to give Cu(II) products. The exact
chemical nature of the decay process thus remains unclear.
(27) Fujisawa, K.; Iwata, Y.; Kitajima, N.; Higashimura, H.; Kubota, M.;
Miyashita, Y.; Yamada, Y.; Okamoto, K.; Morooka, Y. Chem. Lett. 1999,
739-740.
(28) Pidcock, E.; Obias, H. V.; Zhang, C. X.; Karlin, K. D.; Solomon, E.
I. J. Am. Chem. Soc. 1998, 120, 7841-7847.
(29) O likely reacts by abstracting a hydrogen atom from the phenol.