Contrasting copper-dioxygen chemistry arising from alike tridentate alkyltriamine copper(I) complexes

Article abstract of DOI:10.1021/ja0125265

Copper(I)-dioxygen interactions are of great interest due to their role in biological O₂-processing as well as their importance in industrial oxidation processes. We describe here the study of systems which lead to new insights concerning the factors which govern Cu(II)-μ-η²:η² (side-on) peroxo versus Cu(III)-bis-μ-oxo species formation. Drastic differences in O₂-reactivity of Cu(I) complexes which differ only by a single -CH₃ versus -H substituent on the central amine of the tridentate ligands employed are observed. [Cu(MeAN)]B(C₆F₅)₄ (1) (MeAN = N,N,N′,N′,N′-pentamethyl-dipropylenetriamine) reacts with O₂ at -80 °C to form almost exclusively the side-on peroxo complex [{Cu^II(MeAN)}₂(O₂)]²⁺ (3) in CH₂Cl₂, tetrahydrofuran, acetone, and diethyl ether solvents, as characterized by UV-vis and resonance Raman spectroscopies. In sharp contrast, [Cu(AN)]B(C₆F₅)₄ (2) (AN = 3, 3′-iminobis(N,N-dimethyl-propylamine) can support either Cu₂O₂ structures in a strongly solvent-dependent manner. Extreme behavior is observed in CH₂Cl₂ solvent, where 1 reacts with O₂ giving 3, while 2 forms exclusively the bis-μ-oxo species [{Cu^III(AN)}₂(O)₂]²⁺ (4^Oxo). Stopped-flow kinetics measurements also reveal significant variations in the oxygenation reactions of 1 versus 2, including the observations that 4^Oxo forms much faster than does 3; the former decomposes quickly, while the latter is quite stable at 193 K. The solvent-dependence of the bis-μ-oxo versus side-on peroxo preference observed for 2 is opposite to that reported for other known copper(I) complexes; the factors which may be responsible for the unusual behavior of 1/O₂ versus 2/O₂ (possibly N-H hydrogen bonding in the AN chemistry) are suggested. The factors which affect bis-μ-oxo versus side-on peroxo formation continue to be of interest. Copyright

Full text of DOI:10.1021/ja0125265

Published on Web 03/28/2002
Contrasting Copper-Dioxygen Chemistry Arising from Alike Tridentate
Alkyltriamine Copper(I) Complexes
Hong-Chang Liang,† Christiana Xin Zhang,† Mark J. Henson,‡ Roger D. Sommer,§
Karen R. Hatwell,†,| Susan Kaderli,| Andreas D. Zuberbu¨hler,| Arnold L. Rheingold,§
Edward I. Solomon,‡ and Kenneth D. Karlin*,†
Department of Chemistry, The Johns Hopkins UniVersity, Baltimore, Maryland 21218, Department of Chemistry,
Stanford UniVersity, Stanford, California 94305, Department of Chemistry, UniVersity of Delaware,
Newark, Delaware 19716, and Institut fu¨r Anorganische Chemie, UniVersity of Basel, CH-4056 Basel, Switzerland
Received November 13, 2001
Copper-dioxygen interactions are ubiquitous and essential in
biological and industrial processes. Multidentate ligands with N-
donor-atoms are known to impart O2-reactivity to the resulting
copper(I) complexes.1-5 Spectroscopic and structural studies show
that a diversity of Cun-O2 binding modes exists.1-5 Besides the
Figure 1. ORTEP diagrams of the cationic portions of complexes [CuI-
µ-1,2 (end-on Cu‚‚‚Cu ≈ 4.3 Å) and the µ-η2:η2 (side-on Cu‚‚‚Cu
(MeAN)]B(C6F5)4 (1-B(C6F5)4) and [CuI(AN)]B(C6F5)4 (2-B(C6F5)4).14
≈ 3.6 Å) (as seen in the O2-carrier protein hemocyanins)6
dicopper(II)-peroxo complexes, Cun/O2 reactions can also lead to
Cu(III)-bis-µ-oxo species (Cun-(O)2, n ) 2 or 37). In fact, the
research groups of Tolman8,9 and Stack10,11 have demonstrated that
using different tridentate or bidentate alkylamine ligands, and
depending on conditions (i.e., solvent, counteranion),11,12 a given
Cu(I) compound can react with O2 to form both the µ-η2:η2-
peroxo- or the bis-µ-oxo-dicopper(III) cores (Cu‚‚‚Cu ≈ 2.7 Å)
and that these species may be in rapid equilibrium.8,9,11,13
Figure 2. UV-vis spectra (193 K) in CH2Cl2 of [{CuII(MeAN)}2(O2)]2+
Here, we report on new chemistry utilizing copper(I) complexes
(3) (left) and [{CuIII(AN)}2(O)2]2+ (4Oxo) (right). Insets: kinetic traces
of the tridentate ligands AN and MeAN (AN ) 3,3′-iminobis(N,N-
dimethylpropylamine; MeAN ) N,N,N′,N′,N′′-pentamethyldipro-
pylenetriamine).14 These alkylamine chelates were chosen for study
because of their close analogy to our previously studied derivatives
of bis[(2-(2-pyridyl)ethyl]amine (PY2),15-17 containing six-mem-
bered chelate rings.2,18,19 While AN and MeAN differ by only one
methyl group in their ligand structure, the formation, O2-structure-
type and reactivity of O2-adducts of [CuI(MeAN)]+ (1) and
[CuI(AN)]+ (2) differ drastically, offering new results and insights.
[CuI(MeAN)]B(C6F5)4 (1-B(C6F5)4) and [CuI(AN)](B(C6F5)4) (2-
B(C6F5)4) were synthesized by reaction of 1 equiv of [CuI(MeCN)4]B-
(C6F5)420 with the corresponding ligands.14 The complexes possess
tricoordinate copper(I) centers, even when isolated from acetonitrile
as solvent; their PY2 analogues tenaciously bind RCN donors as
fourth ligands.15,17,21,22 Complexes 1 and 2 possess nearly identical
structures (Figure 1),14 adopting distorted trigonal planar configura-
tions. The “outer” N(1)-Cu(1)-N(3) angles are 151.0(2)° and
152.1(2)° for 1 and 2, respectively, close to that seen for other
tricoordinate structures with the PY2 moiety.23-25 No significant
differences occur in Cu-N bond lengths.14
(179 K) monitored at 366 nm for 3 and 386 nm for 4Oxo (ꢀ, M-1 cm-1).
Scheme 1
[{CuII(MeAN)}2(O2)]2+ (3) formed in CH2Cl2 (Scheme 1) is
formulated as a side-on µ-η2:η2-peroxo complex based on its
typical11,12 absorption bands at 360 nm (ꢀ 22 000 M-1 cm-1) and
540 nm (ꢀ 2500 M-1 cm-1), Figure 2. Confirmation comes from
resonance Raman measurements, which exhibited a characteris-
tic13,27 but very low4 νÃ-à ) 721 cm-1 (16O2 683 cm-1 with 18O2),28
plus a diagnostic29 Cu-Cu band at 268 cm-1 which corresponds to
ν
a vibrational mode involving primarily Cu‚‚‚Cu motion.14 The bis-
µ-oxo-dicopper(II) core in [{CuIII(AN)}2(O)2]2+ (4Oxo) is deduced
from the UV-vis absorptions at 293 nm (ꢀ 15 000 M-1 cm-1) and
393 nm30 (ꢀ 12 000 M-1 cm-1) (Figure 2) and from its character-
istic13,27 resonance Raman Cu2O2 core vibration νCu-O ) 608 cm-1
(16O2 580 cm-1 with 18O2). Both [CuI(MeAN)]+ (1) and [CuI(AN)]+
(2) appear to give only these single products in CH2Cl2.26,31,32
Additional insights have been obtained from stopped-flow
kinetics measurements (CH2Cl2, 350-700 nm monitoring, -94 to
20 °C). [CuI(MeAN)]+ (1) reacts reversibly with O2 (Figure 2 inset)
forming [{CuII(MeAN)}2(O2)]2+ (3) with ∆Hq ) -27 ( 3 kJ/mol,
and ∆Sq ) -335 ( 16 J mol-1 K-1 (kon ) (6.9 ( 0.7) × 102 M-2
s-1, 183 K). No intermediates were observed, but the negative
activation enthalpy and extremely negative activation entropy
presuppose the formation of an unstable superoxo complex
[CuII(MeAN(O2-)]+ in a rapid left-lying preequilibrium. Thermo-
Although 1 and 2 differ by a single -CH3 versus -H substituent,
their reactivities toward dioxygen differ drastically in CH2Cl2,
Scheme 1. [CuI(MeAN)]+ (1) reacts at 193 K giving essentially
only the µ-η2:η2 (side-on) complex [{CuII(MeAN)}2(O2)]2+ (3). By
contrast, only the µ-oxo species [{CuIII(AN)}2(O)2]2+ (4Oxo) is
obtained with [CuI(AN)]+ (2).26
† The Johns Hopkins University.
‡ Stanford University.
§ University of Delaware.
| University of Basel.
9
4170
J. AM. CHEM. SOC. 2002, 124, 4170-4171
10.1021/ja0125265 CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
(4) Blackman, A. G.; Tolman, W. B. Struct. Bonding (Berlin) 2000, 97, 179-
211.
(5) Karlin, K. D.; Zuberbu¨hler, A. D. In Bioinorganic Catalysis, 2nd ed.,
revised and expanded; Reedijk, J., Bouwman, E., Eds.; Marcel Dekker:
New York, 1999; pp 469-534.
(6) Solomon, E. I.; Sundaram, U. M.; Machonkin, T. E. Chem. ReV. 1996,
96, 2563-2605.
(7) Cole, A. P.; Root, D. E.; Mukherjee, P.; Solomon, E. I.; Stack, T. D. P.
Science 1996, 273, 1848-1850.
dynamic parameters derived from the reversible kinetics (overall
forward and back reactions) are ∆H° ) -28 ( 2 kJ/mol, ∆S° )
-61 ( 12 J mol-1 K-1 (K ) (7.6 ( 0.6) × 104 M-2, 183 K);
nearly all other Cu2O2 peroxo complexes measured exhibit con-
siderably more negative (favorable) ∆H° values.15,33 Compared to
[(MePY2)CuI]+ reaction with O2 (in acetone solvent, where the
adduct [((MePY2)Cu}2(O2)]2+ forms),17,31 the 1/O2 reaction is
slower and the peroxo product 3 is somewhat less stable. By
contrast, in the reaction of [CuI(AN)]+ (2) with O2, the bis-µ-oxo
product [{CuIII(AN)}2(O)2]2+ (4Oxo) forms much faster than does
3 (Figure 2), ∆Hq ) -9.9 ( 0.6 kJ/mol (again implicating a steady
state intermediate forms in a preequilibrium), ∆Sq ) -210 ( 3 J
mol-1 K-1, and kon ) (2.7 ( 0.1) × 104 M-2 s-1, 183 K. The
thermodynamic parameters obtained are distinctive; a relatiVely
unfavorable reaction enthalpy (as for formation of 3), ∆H° ) -24
( 1 kJ/mol, is accompanied by peculiarly favorable (compared to
all other systems)15,33 reaction entropy ∆S° ) -14 ( 6 J mol-1
K-1 (K ) (1.02 ( 0.07) × 106 M-2, 183 K). While 3 is quite stable
at reduced temperatures (i.e., hours, at 193 K), 4Oxo decomposes
relatively quickly (cf., Figure 2, right), ∆H° ) 35 ( 2 kJ/mol,
∆S° ) -95 ( 11 J mol-1 K-1 kdecomp ) (3.0 ( 0.2) × 10-1 s-1 at
223 K.
(8) Halfen, J. A.; Mahapatra, S.; Wilkinson, E. C.; Kaderli, S.; Young, V.
G., Jr.; Que, L., Jr.; Zuberbu¨hler, A. D.; Tolman, W. B. Science 1996,
271, 1397-1400.
(9) Holland, P. L.; Tolman, W. B. Coord. Chem. ReV. 1999, 190-192, 855-
869.
(10) Mahadevan, V.; Hou, Z.; 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.; Henson, M. J.; Solomon, E. I.; Stack, T. D. P. J. Am.
Chem. Soc. 2000, 122, 10249-10250.
(12) Cahoy, J.; Holland, P. L.; Tolman, W. B. Inorg. Chem. 1999, 38, 2161-
2168.
(13) Henson, M. J.; Mukherjee, P.; Root, D. E.; Stack, T. D. P.; Solomon, E.
I. J. Am. Chem. Soc. 1999, 121, 10332-10345.
(14) See Supporting Information for more details.
(15) Liang, H.-C.; Karlin, K. D.; Dyson, R.; Kaderli, S.; Jung, B.; Zuberbu¨hler,
A. D. Inorg. Chem. 2000, 39, 5884-5894.
(16) Pidcock, E.; Obias, H. V.; Abe, M.; Liang, H.-C.; Karlin, K. D.; Solomon,
E. I. J. Am. Chem. Soc. 1999, 121, 1299-1308.
(17) Obias, H. V.; Lin, Y.; Murthy, N. N.; Pidcock, E.; Solomon, E. I.; Ralle,
M.; Blackburn, N. J.; Neuhold, Y.-M.; Zuberbu¨hler, A. D.; Karlin, K. D.
J. Am. Chem. Soc. 1998, 120, 12960-12961.
(18) Chelate ring size dramatically affects copper complex redox potentials
and corresponding CuI/O2 reactivity. See refs 2 and 19.
(19) Schatz, M.; Becker, M.; Thaler, F.; Hampel, F.; Schindler, S.; Jacobson,
R. R.; Tyekla´r, Z.; Murthy, N. N.; Ghosh, P.; Chen, Q.; Zubieta, J.; Karlin,
K. D. Inorg. Chem. 2001, 40, 2312-2322.
(20) Liang, H.-C.; Kim, E.; Incarvito, C. D.; Rheingold, A. L.; Karlin, K. D.
Inorg. Chem., 2002, 41. ASAP on web March 21, 2002.
(21) Karlin, K. D.; Haka, M. S.; Cruse, R. W.; Meyer, G. J.; Farooq, A.;
Gultneh, Y.; Hayes, J. C.; Zubieta, J. J. Am. Chem. Soc. 1988, 110, 1196-
1207.
Other remarkable differences are in the solvent dependencies of
formation of O2-adducts of 1 and 2. The side-on peroxo [{CuII-
(MeAN)}2(O2)]2+ (3) is the predominant product of O2 reaction
with 1 in CH2Cl2, acetone, tetrahydrofuran (THF) and diethyl ether
solvents, based on UV-vis or resonance Raman data.34 [{CuIII-
(AN)}2(O)2]2+ (4Oxo) is formed exclusively from 2/O2 reaction in
CH2Cl2. However, both 4Oxo and a µ-η2:η2 (side-on)-peroxo
(22) Karlin, K. D.; Tyekla´r, Z.; Farooq, A.; Haka, M. S.; Ghosh, P.; Cruse, R.
W.; Gultneh, Y.; Hayes, J. C.; Toscano, P. J.; Zubieta, J. Inorg. Chem.
1992, 31, 1436-1451.
complex [{CuII(AN)}2(O2)]2+ (4Peroxo) form in acetone {νCu-Cu
)
271 cm-1; νCu-O ) 604 cm-1; ∆(18O) ) 26 cm-1} and THF {νCu-Cu
) 271 cm-1}, in a roughly 1:1 proportion (UV-vis criterion).14
Yet, in diethyl ether, 80-90% (UV-vis criterion) 4Peroxo forms.
Thus, the AN ligand can support either peroxo-Cu2(O2) or bis-µ-
oxo-Cu2(O)2 in a strongly solvent dependent manner. It is notable
that the bis-µ-oxo versus peroxo preference (THF vs CH2Cl2) for
the AN ligand complex is opposite to the results seen by the groups
of Tolman (with triaza macrocyclic ligands)12 and Stack (with
substituted ethylenediamine ligands).11 It is important to obtain a
further detailed understanding of the factors underlying copper(I)/
O2 chemistry leading to µ-η2:η2 (side-on)-peroxo- versus bis-µ-
oxo-dicopper(III) species, their relative energetics and their
possible differential reactivity toward substrate oxidation.11,13,35-40
Solvent medium effects (here and previously)11,12 may be due to
environmental (dielectric or solvation) influences or coordination
to copper.41 Sterically demanding ligands have been suggested to
favor side-on peroxo-dicopper(II) complex formation,9,11,12 but a
-CH3 versus -H substituent (in MeAN vs AN) is sufficient to
shift the course of reaction. H-bonding in complexes of AN could
also be important. Further studies are needed.
(23) Blackburn, N. J.; Karlin, K. D.; Cocannon, M.; Hayes, J. C.; Gultneh,
Y.; Zubieta, J. J. Chem. Soc., Chem. Commun. 1984, 939-940.
(24) Blackburn, N. J.; Strange, R. W.; Reedijk, J.; Volbeda, A.; Farooq, A.;
Karlin, K. D.; Zubieta, J. Inorg. Chem. 1989, 28, 1349-1357.
(25) Sanyal, I.; Mahroof-Tahir, M.; Nasir, S.; Ghosh, P.; Cohen, B. I.; Gultneh,
Y.; Cruse, R.; Farooq, A.; Karlin, K. D.; Liu, S.; Zubieta, J. Inorg. Chem.
1992, 31, 4322-4332.
(26) Absorption spectra indicate predominant formation of one product, but
resonance Raman spectroscopy can detect minute (<5-10%) amounts
of the other isomer, if present in each case.
(27) Holland, P. L.; Cramer, C. J.; Wilkinson, E. C.; Mahapatra, S.; Rodgers,
K. R.; Itoh, S.; Taki, M.; Fukuzumi, S.; Que, L., Jr.; Tolman, W. B. J.
Am. Chem. Soc. 2000, 122, 792-802.
(28) Data obtained in tetrahydrofuran, as interference from a broad Raman
feature at ∼760 cm-1 precluded obtainng good νO-O data in CH2Cl2.
(29) Henson, M. J.; Mahadevan, V.; Stack, T. D. P.; Solomon, E. I. Inorg.
Chem. 2001, 40, 5068-5069.
(30) This band position is very high energy compared to values observed for
almost all bis-µ-oxo-dicopper(III) complexes. See refs 4 and 10.
(31) This adduct, as a solution (ref 17) or solid (ref 32), is a mixture of peroxo
(major form) and bis-µ-oxo (minor) species.
(32) 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.
(33) Karlin, K. D.; Kaderli, S.; Zuberbu¨hler, A. D. Acc. Chem. Res. 1997, 30,
139-147.
(34) CH2Cl2: 264 cm-1 Cu-Cu. Acetone: 721 cm-1 (∆(18O) ) 38 cm-1).
ν
-1
THF: 721 cm
(∆(18O) ) 38 cm-1). A small amount of bis-µ-oxo-
dicopper(III) species is also observed in this solvent, νCu-O, 586 cm-1
Acknowledgment. We are grateful to the National Institutes
of Health (K.D.K., GM28962; E.I.S., DK31450) and Swiss National
Science Foundation (A.D.Z.) for support of this research.
Supporting Information Available: Synthetic details, kinetics
(UV-vis traces, Eyring and van’t Hoff plots), resonance Raman and
X-ray crystallographic data (PDF). This material is available free of
charge via the Internet at http://pubs.acs.org.
(∆(18O) ) 25 cm-1).
(35) Mahapatra, S.; Halfen, J. A.; Tolman, W. B. J. Am. Chem. Soc. 1996,
118, 11575-11586.
(36) Holland, P. L.; Rodgers, K. R.; Tolman, W. B. Angew. Chem., Int. Ed.
1999, 38, 1139-1142.
(37) Pidcock, E.; Obias, H. V.; Zhang, C. X.; Karlin, K. D.; Solomon, E. I. J.
Am. Chem. Soc. 1998, 120, 7841-7847.
(38) Itoh, S.; Taki, M.; Nakao, H.; Holland, P. L.; Tolman, W. B.; Que, L.,
Jr.; Fukuzumi, S. Angew. Chem., Int. Ed. 2000, 39, 398-400.
(39) Taki, M.; Itoh, S.; Fukuzumi, S. J. Am. Chem. Soc. 2001, 123, 6203-
6204.
References
(40) (3) is relatively unreactive towards added substrates whereas 4Oxo (CH2Cl2
solvent) reacts rapidly with dihydroanthracene and benzyl alcohol.
(41) Results on a system where solvent coordination is clearly indicated will
be included in a manuscript currently under preparation.
(1) Zhang, C. X.; Liang, H.-C.; Humphreys, K. J.; Karlin, K. D. In Catalytic
ActiVation of Dioxygen by Metal Complexes; Simandi, L., Ed.; Kluwer:
Dordrecht, The Netherlands, 2002. In press.
(2) Schindler, S. Eur. J. Inorg. Chem. 2000, 2311-2326.
(3) Suzuki, M.; Furutachi, H.; Okawa, H. Coord. Chem. ReV. 2000, 200-
202, 105-129.
JA0125265
9
J. AM. CHEM. SOC. VOL. 124, NO. 16, 2002 4171