Ligand macrocycle structural effects on copper-dioxygen reactivity

Article abstract of DOI:10.1021/ic000248p

With the goal of understanding how the nature of the tridentate macrocyclic supporting ligand influences the relative stability of isomeric μ-η²:η²-peroxo- and bis(μ-oxo)dicopper complexes, a comparative study was undertaken of the O₂ reactivity of Cu(I) compounds supported by the 10- and 12-membered macrocycles, 1,4,7-R₃-1,4,7-triazacyclodecane (R₃TACD; R = Me, Bn, iPr) and 1,5,9-triisopropyl-1,5,9-triazacyclododecane (iPr₃TACDD). While the 3-coordinate complex [(iPr₃TACDD)Cu]SbF₆ was unreactive with O₂, oxygenation of [(R₃TACD)Cu(CH₃CN)]X (R = Me or Bn; X = ClO₄^- or SbF₆^-) at -80 °C yielded bis(μ-oxo) species [(R₃TACD)₂Cu₂(μO)₂]X₂ as revealed by UV-vis and resonance Raman spectroscopy. Interestingly, unlike the previously reported system supported by 1,4,7-triisopropyl-1,4,7-triazacyclononane (iPr₃TACN), which yielded interconverting mixtures of peroxo and bis(μ-oxo) compounds (Cahoy, J.; Holland, P. L.; Tolman, W. B. Inorg. Chem, 1999, 38, 2161), low-temperature oxygenation of [(iPr₃TACD)Cu(CH₃CN)]SbF₆ in a variety of solvents cleanly yielded a μ-η²:η²-peroxo product, with no trace of the bis(μ-oxo) isomer. The peroxo complex was characterized by UV-vis and resonance Raman spectroscopy, as well as an X-ray crystal structure (albeit of marginal quality due to disorder problems). Intramolecular attack at the α C-H bonds of the substituents was indicated as the primary decomposition pathway of the oxygenated compounds through examination of the decay kinetics and the reaction products, which included bis(μ-hydroxo)- and μ-carbonato-dicopper complexes that were characterized by X-ray diffraction. A rationale for the varying results of the oxygenation reactions was provided by analysis of (a) the X-ray crystal structures and electrochemical behavior of the Cu(I) precursors and (b) the results of theoretical calculations of the complete oxygenated complexes, including all ligand atoms, using combined quantum chemical/molecular mechanics (integrated molecular orbital molecular mechanics, IMOMM) methods. The size of the ligand substituents was shown to be a key factor in controlling the relative stabilities of the peroxo and bis(μ-oxo) forms, and the nature of this influence was shown by both theory and experiment to depend on the ligand macrocycle ring size.

Full text of DOI:10.1021/ic000248p

Inorg. Chem. 2000, 39, 4059-4072
4059
Ligand Macrocycle Structural Effects on Copper-Dioxygen Reactivity
Bernice M. T. Lam,† Jason A. Halfen,† Victor G. Young, Jr.,† John R. Hagadorn,†
Patrick L. Holland,† Agust´ı Lledo´s,‡ Lourdes Cucurull-Sa´nchez,‡ Juan J. Novoa,§
Santiago Alvarez,*,| and William B. Tolman*,†
Department of Chemistry and Center for Metals in Biocatalysis, University of Minnesota,
207 Pleasant Street SE, Minneapolis, Minnesota 55455, Departament de Qu´ımica, Universitat Auto`noma
de Barcelona, 08193 Bellaterra, Spain, Departament de Qu´ımica F´ısica and Centre Especial de Recerca
en Qu´ımica Teo`rica, Universitat de Barcelona, Diagonal 647, 08028 Barcelona, Spain, and Departament
de Qu´ımica Inorga`nica and Centre Especial de Recerca en Qu´ımica Teo`rica, Universitat de Barcelona,
Diagonal 647, 08028 Barcelona, Spain
ReceiVed March 6, 2000
With the goal of understanding how the nature of the tridentate macrocyclic supporting ligand influences the
relative stability of isomeric µ-η2:η2-peroxo- and bis(µ-oxo)dicopper complexes, a comparative study was undertaken
of the O2 reactivity of Cu(I) compounds supported by the 10- and 12-membered macrocycles, 1,4,7-R3-1,4,7-
triazacyclodecane (R3TACD; R ) Me, Bn, iPr) and 1,5,9-triisopropyl-1,5,9-triazacyclododecane (iPr3TACDD).
While the 3-coordinate complex [(iPr3TACDD)Cu]SbF6 was unreactive with O2, oxygenation of [(R3TACD)Cu-
-
(CH3CN)]X (R ) Me or Bn; X ) ClO4 or SbF6-) at -80 °C yielded bis(µ-oxo) species [(R3TACD)2Cu2(µ-
O)2]X2 as revealed by UV-vis and resonance Raman spectroscopy. Interestingly, unlike the previously reported
system supported by 1,4,7-triisopropyl-1,4,7-triazacyclononane (iPr3TACN), which yielded interconverting mixtures
of peroxo and bis(µ-oxo) compounds (Cahoy, J.; Holland, P. L.; Tolman, W. B. Inorg. Chem. 1999, 38, 2161),
low-temperature oxygenation of [(iPr3TACD)Cu(CH3CN)]SbF6 in a variety of solvents cleanly yielded a µ-η2:
η2-peroxo product, with no trace of the bis(µ-oxo) isomer. The peroxo complex was characterized by UV-vis
and resonance Raman spectroscopy, as well as an X-ray crystal structure (albeit of marginal quality due to disorder
problems). Intramolecular attack at the R C-H bonds of the substituents was indicated as the primary decomposition
pathway of the oxygenated compounds through examination of the decay kinetics and the reaction products,
which included bis(µ-hydroxo)- and µ-carbonato-dicopper complexes that were characterized by X-ray diffraction.
A rationale for the varying results of the oxygenation reactions was provided by analysis of (a) the X-ray crystal
structures and electrochemical behavior of the Cu(I) precursors and (b) the results of theoretical calculations of
the complete oxygenated complexes, including all ligand atoms, using combined quantum chemical/molecular
mechanics (integrated molecular orbital molecular mechanics, IMOMM) methods. The size of the ligand substituents
was shown to be a key factor in controlling the relative stabilities of the peroxo and bis(µ-oxo) forms, and the
nature of this influence was shown by both theory and experiment to depend on the ligand macrocycle ring size.
Introduction
study of the reactions of O2 with Cu(I) complexes, for example,
important insights into how the O2 molecule may coordinate to
The development of a detailed mechanistic understanding of
dioxygen activation by metalloproteins continues to be an
important objective in bioinorganic chemistry research, both
because of the general significance of oxidase and oxygenase
structure/function relationships in biology and because knowl-
edge of O2 binding and activation pathways may inform efforts
to invent new synthetic oxidation catalysts.1 Through in-depth
and subsequently be activated by copper sites in biology have
been obtained.2 In particular, a pathway by which the dioxygen
O-O bond may be broken and formed within a dimetal protein
active site was identified in studies of the oxygenation of [(iPr3-
TACN)Cu(CH3CN)]+ (iPr3TACN ) 1,4,7-triisopropyl-1,4,7-
triazacyclononane).2b,3 Under certain conditions, the reaction
yields rapidly equilibrating mixtures of isomeric µ-η2:η2-peroxo-
and bis(µ-oxo)dicopper cores in ratios that depend on solvent,
temperature, and counterions (Scheme 1).4
* E-mail: salvarez@kripto.qui.ub.es, tolman@chem.umn.edu.
† University of Minnesota.
‡ Universitat Auto`noma de Barcelona.
§ Departament de Qu´ımica F´ısica and Centre Especial de Recerca en
Qu´ımica Teo`rica, Universitat de Barcelona.
(2) Recent reviews: (a) Karlin, K. D.; Zuberbu¨hler, A. D. In Bioinorganic
Catalysis, 2nd ed.; Reedijk, J., Ed.; Marcel Dekker: Inc.: New York,
1999; pp 469-534. (b) Tolman, W. B. Acc. Chem. Res. 1997, 30,
227. (c) Blackman, A. G.; Tolman, W. B. In Metal-Oxo and Metal-
Peroxo Species in Catalytic Oxidations; Meunier, B., Ed.; Structure
& Bonding Vol. 97; Springer-Verlag: Berlin, 2000; pp 179-211. (d)
Kopf, M.-A.; Karlin, K. D. In Biomimetic Oxidations Catalyzed by
Transition Metal Complexes; Meunier, B., Ed.; Imperial College
Press: London, 2000; pp 309-362.
| Departament de Qu´ımica Inorga`nica and Centre Especial de Recerca
en Qu´ımica Teo`rica, Universitat de Barcelona.
(1) Selected reviews: (a) Solomon, E. I.; Sundaram, U. M.; Machonkin,
T. E. Chem. ReV. 1996, 96, 2563-2605. (b) Klinman, J. P. Chem. ReV.
1996, 96, 2541. (c) Que, L., Jr. Chem. ReV. 1996, 96, 2607-2624.
(d) Wallar, B. J.; Lipscomb, J. D. Chem. ReV. 1996, 96, 2625. (e)
Sono, M.; Roach, M. P.; Dawson, J. H. Chem. ReV. 1996, 96, 2841.
(f) Ferguson-Miller, S.; Babcock, G. T. Chem. ReV. 1996, 96, 2889.
(g) Cytochrome P450: Structure, Mechanism, and Biochemistry, 2nd
ed.; Ortiz de Montellano, P. R., Ed.; Plenum Press: New York, 1995.
(3) 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.
10.1021/ic000248p CCC: $19.00 © 2000 American Chemical Society
Published on Web 08/12/2000

4060 Inorganic Chemistry, Vol. 39, No. 18, 2000
Lam et al.
Scheme 1
disposition of the N-donor groups also must be considered in
any effort to understand the relative stability of the isomers.
Experimental verification of the role of N-donor disposition in
ligand systems that vary only in this regard is lacking, however,
and previous theoretical studies have not adequately considered
the aforementioned key role of the N-donor substituents.
We report herein a combined experimental/theoretical ex-
amination of the Cu(I)/O2 reactivity of compounds supported
by ligands comprising 10- and 12-membered macrocycles, R3-
TACD (TACD ) 1,4,7-triazacyclodecane) and iPr3TACDD
(TACDD ) 1,5,9-triazacyclododecane) (Scheme 2).10 Through
Studies of this system, as well as other related ones,5 have
raised important questions about the potential involvement of
the peroxo/bis(µ-oxo) interconversion in biological and catalytic
oxidations and about the relative reactivity of each isomer
toward hydrocarbon substrates. A significant underlying issue
concerns how the supporting ligand influences the relative
stability of the isomeric cores. From previous work it is clear
that the nature of the substituents on the capping TACN unit
influences the oxygenation chemistry significantly. Thus, with
benzyl- or methyl-substituted TACN ligands5e,f,6 or with a ligand
containing two iPr2TACN units closely tethered by a -CH2-
CH2- chain,7 only bis(µ-oxo) complex formation is observed.
More complicated mixtures of intramolecular peroxo and
intermolecular (dimer-of-dimer or oligomeric) bis(µ-oxo) com-
pounds result with xylyl tethers.8 A survey of these results led
to the hypothesis that the basis for the observed behavior for
the TACN-supported systems is steric in nature.2b According
to this notion, large groups on TACN inhibit formation of the
more compact bis(µ-oxo) core characterized by Cu‚‚‚Cu dis-
tances of ∼2.8 Å and favor µ-η2:η2-peroxo compound formation
(Cu‚‚‚Cu ) ∼3.5 Å); with smaller N-substituents, collapse to
the bis(µ-oxo) core is facilitated. In ligands comprising linked
TACN units, tether length appears to be the controlling factor.
Theoretical studies of the model system [(NH3)3Cu(O2)Cu-
(NH3)3]2+ and related species5f,9 have provided many electronic
structural insights and, in particular, have indicated that the
Scheme 2
this work, and with reference to previous studies of the TACN
system,3,4,5e,f,6 variation of both N-donor disposition and sub-
stituent size in a closely related series was accomplished, thus
enabling a detailed analysis of their conjoined roles in determin-
ing the relative stabilities of the peroxo and bis(µ-oxo) isomers.
We have found that disparities in supporting ligand macrocycle
ring and substituent size correlate with acute differences in the
nature of the products obtained in the oxygenations. To
rationalize these findings, we have analyzed X-ray crystal-
lographic data for the Cu(I) precursors and results of theoretical
calculations of the complete oxygenated complexes, including
all ligand atoms, using combined quantum chemical/molecular
mechanics (integrated molecular orbital molecular mechanics,
IMOMM) methods.
(4) (a) Cahoy, J.; Holland, P. L.; Tolman, W. B. Inorg. Chem. 1999, 38,
2161. (b) Holland, P. L.; Tolman, W. B. Coord. Chem. ReV. 1999,
190-192, 855.
(5) (a) 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.
(b) 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. (c) Itoh, S.; Nakao,
H.; Berreau, L. M.; Kondo, T.; Komatsu, M.; Fukuzumi, S. J. Am.
Chem. Soc. 1998, 120, 2890. (d) 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. (e) Mahadevan, V.; DuBois, J. L.;
Hedman, B.; Hodgson, K. O.; Stack, T. D. P. J. Am. Chem. Soc. 1999,
121, 5583. (f) Henson, M. J.; Mukherjee, P.; Root, D. E.; Stack, T.
D. P.; Solomon, E. I. J. Am. Chem. Soc. 1999, 121, 10332. (g) Hayashi,
H.; Fujinami, S.; Nagatomo, S.; Ogo, S.; Suzuki, M.; Uehara, A.;
Watanabe, Y.; Kitagawa, T. J. Am. Chem. Soc. 2000, 122, 2124. (h)
Decker, H.; Dillinger, R.; Tuczek, F. Angew. Chem., Int. Ed. 2000,
39, 1591.
Results and Discussion
Copper(I) Complexes. (a) Synthesis. The complexes [(R3-
-
TACD)Cu(CH3CN)]X (R ) Me, iPr, or Bn; X ) ClO4 or
(6) Mahapatra, S.; Halfen, J. A.; Wilkinson, E. C.; Pan, G.; Wang, X.;
Young, V. G., Jr.; Cramer, C. J.; Que, L., Jr.; Tolman, W. B. J. Am.
Chem. Soc. 1996, 118, 11555.
(7) Mahapatra, S.; Young, V. G., Jr.; Tolman, W. B. Angew. Chem., Int.
Ed. 1997, 36, 130.
SbF6-) and [(iPr3TACDD)Cu]SbF6 (Scheme 2) were synthesized
by mixing the appropriate salt of [Cu(CH3CN)4]+ with R3TACD
or iPr3TACDD, ligands which were prepared in turn from their
respective parent macrocycles10 by known procedures11 or by
precedented alkylation methods.12 The indicated formulations
for the complexes are supported by the combined NMR and
FTIR spectroscopic, mass spectrometric, elemental analysis, and
(except for the Bn3TACD case) X-ray crystallographic data (vide
(8) Mahapatra, S.; Kaderli, S.; Llobet, A.; Neuhold, Y.-M.; Palanche´, T.;
Halfen, J. A.; Young, V. G., Jr.; Kaden, T. A.; Que, L., Jr.;
Zuberbu¨hler, A. D.; Tolman, W. B. Inorg. Chem. 1997, 36, 6343.
(9) (a) Cramer, C. J.; Smith, B. A.; Tolman, W. B. J. Am. Chem. Soc.
1996, 118, 11283. (b) Be´rces, A. Inorg. Chem. 1997, 36, 4831. (c)
Eisenstein, O.; Getlicherman, H.; Giessner-Prettre, C.; Maddaluno, J.
Inorg. Chem. 1997, 36, 3455. (d) Liu, X.-Y.; Palacios, A. A.; Novoa,
J. J.; Alvarez, S. Inorg. Chem. 1998, 37, 1202. (e) Bernardi, F.; Bottoni,
A.; Casadio, R.; Fariselli, P.; Rigo, A. Int. J. Quantum Chem. 1996,
58, 109. (f) Bernardi, F.; Bottoni, A.; Casadio, R.; Fariselli, P.; Rigo,
A. Inorg. Chem. 1996, 35, 5207. (g) Flock, M.; Pierloot, K. J. Phys.
Chem. A 1999, 103, 95. (h) Lind, T.; Siegbahn, P. E. M.; Crabtree, R.
H. J. Phys. Chem. B 1999, 103, 1193.
(10) (a) Koyama, H.; Yoshino, T. Bull. Chem. Soc. Jpn. 1972, 45, 481. (b)
Richman, J. E.; Atkins, T. J. J. Am. Chem. Soc. 1974, 96, 2268.
(11) Geraldes, C. F. G. C.; Sherry, A. D.; Marques, M. P. M.; Alpoim, M.
C.; Cortes, S. J. Chem. Soc., Perkin Trans. 1991, 137.
(12) Haselhorst, G.; Stoetzel, S.; Strassburger, A.; Walz, W.; Wieghardt,
K.; Nuber, B. J. Chem. Soc., Dalton Trans. 1993, 83.

Ligand Macrocycle Effects on Copper-Dioxygen Reactivity
Inorganic Chemistry, Vol. 39, No. 18, 2000 4061
Table 1. Electrochemical Data for Cu(I) Complexesa and ν(CO)
Bands in FTIR Spectra of Derived Carbonyl Complexes
c
complex
E1/2 (V)b ∆Ep (V) ν(CO) (cm-1
)
[(iPr3TACN)Cu(CH3CN)]+
[(iPr3TACD)Cu(CH3CN)]+
[(Me3TACD)Cu(CH3CN)]+
[(Bn3TACD)Cu(CH3CN)]+
[(iPr3TACDD)Cu]+
0.57
0.60
0.40
0.51
0.18
0.39
0.50
2067d
2063
2086
2074
0.44
(0.91)e
a Conditions: 9:1 (v/v) CH2Cl2/CH3CN, 0.1 M (Bu4N)(PF6), scan
rate 0.100 V/s, Pt electrode. b Reported versus SCE. c Carbonyl stretch-
ing frequencies of CO adducts of indicated complexes, measured as
e
KBr pellets. d Reference 15. Epa value (irreversible oxidation).
Figure 2. Representation of the cationic portion of the X-ray structure
of [(Me3TACD)Cu(CH3CN)]ClO4, shown as 50% ellipsoids with H
atoms omitted for clarity. Selected bond distances (Å) and angles
(deg): Cu(1)-N(1) 2.088(5), Cu(1)-N(2) 2.124(6), Cu(1)-N(3) 2.136-
(6), Cu(1)-N(4) 1.862(5), N(4)-Cu(1)-N(1) 141.2(2), N(4)-Cu(1)-
N(2) 115.7(2), N(1)-Cu(1)-N(2) 86.3(2), N(4)-Cu(1)-N(3) 121.7(2),
N(1)-Cu(1)-N(3) 86.4(2), N(2)-Cu(1)-N(3) 92.5(2).
Figure 1. Representation of the cationic portion of the X-ray structure
of [(iPr3TACD)Cu(CH3CN)]SbF6, shown as 50% ellipsoids with H
atoms omitted for clarity. Selected bond distances (Å) and angles
(deg): Cu(1)-N(1) 2.102(6), Cu(1)-N(2) 2.137(5), Cu(1)-N(3) 2.144-
(7), Cu(1)-N(4) 1.895(6), N(4)-Cu(1)-N(1) 130.4(3), N(4)-Cu(1)-
N(2) 117.5(2), N(1)-Cu(1)-N(2) 87.3(2), N(4)-Cu(1)-N(3) 120.3(3),
N(1)-Cu(1)-N(3) 101.6(3), N(2)-Cu(1)-N(3) 87.7(2).
infra). Metal complexation of the respective ligands was
1
particularly apparent in H NMR spectra, where broad, often
overlapping peaks for the macrocycle backbone hydrogens in
the free macrocycle were replaced by a greater number of
downfield-shifted and sharpened peaks resulting from confor-
mational “locking” upon Cu(I) binding. In one case, [(iPr3-
TACD)Cu(CH3CN)]SbF6, an analysis of 2D 1H NMR data
Figure 3. Two representations of the cationic portion of the X-ray
structure of [(iPr3TACDD)Cu]ClO4, shown as 50% ellipsoids with H
atoms omitted for clarity. Selected bond distances (Å) and angles
(deg): Cu(1)-N(1) 2.019(2), Cu(1)-N(2) 2.022(2), Cu(1)-N(3) 2.026-
(2), N(1)-Cu(1)-N(2) 114.53(9), N(1)-Cu(1)-N(3) 116.96(9), N(2)-
Cu(1)-N(3) 113.09(8).
1
allowed detailed assignment of the H NMR spectrum to be
made (cf. COSY spectrum in Figure S1).13 The carbon monoxide
adducts [(R3TACD)Cu(CO)]SbF6 (R ) Me, iPr, or Bn) were
prepared in straightforward fashion (Scheme 2). Corresponding
ν(CO) values (Table 1) are compared to electrochemical data
below in an attempt to assess the relative electron-donating/
withdrawing capabilities of the substituents.
of [(iPr3TACDD)Cu]ClO4 is unique because of the absence of
a CH3CN coligand. The Cu(I) ion adopts a 3-coordinate
geometry with Cu-N bond distances slightly shorter [2.019-
(2)-2.026(2) Å range] than observed in the other 4-coordinate
compounds. Residing deep within the 12-membered macrocycle
but 0.46 Å above the N1-N2-N3 plane, the metal geometry
is best described as pyramidal; the N-Cu-N angles are similar
(113-117° range) and sum to 345° (vs 360° expected for a
trigonal planar site). The coordination geometry in this com-
pound would be expected to greatly favor the Cu(I) over the
Cu(II) state, which was verified experimentally by a lack of
reactivity with O2 and a high reduction potential (vide infra).
Roughly similar 4-coordinate, C3V-distorted tetrahedral ge-
ometries are adopted by the four remaining complexes of the
R3TACD and iPr3TACN ligands. The Cu-Namine bond distances
fall within a narrow range typical for these types of compounds
(b) X-ray Crystal Structures. The cationic portions of the
structures of [(iPr3TACD)Cu(CH3CN)]SbF6, [(Me3TACD)Cu-
(CH3CN)]ClO4, and [(iPr3TACDD)Cu]ClO4 are shown in
Figures 1, 2, and 3, respectively. For comparison, the structure
of [(iPr3TACN)Cu(CH3CN)]BPh4 was also solved (Figure 4).
Selected crystallographic data are listed in Table 2; see
Supporting Information for complete listings. Consideration of
the structural parameters of these compounds reveals features
of potential importance for understanding their disparate reac-
tivities with O2.
While all of the compounds exhibit tridentate, facial coor-
dination of their respective macrocyclic ligands, the structure
(13) Lam, B. M. T. M.S. Thesis, University of Minnesota, 1998.

4062 Inorganic Chemistry, Vol. 39, No. 18, 2000
Table 2. X-ray Crystallographic Data
Lam et al.
[(iPr3TACD)2-
[(iPr3TACD)-
Cu(CH3CN)]SbF6
[(Me3TACD)-
Cu(CH3CN)]ClO4
[(iPr3TACDD)-
Cu]ClO4
[(iPr3TACN)-
Cu(CH3CN)]BPh4
[(Me3TACD)2-
Cu2(OH)2](ClO4)2
Cu2(CO3)](BPh4)2‚
4CH2Cl2‚1.5C3H6O
empirical formula
C18H38CuF6N4Sb
C12H26ClCuN4O4
C18H39ClCuN3O4
C41H56BCuN4
C20H48Cl2Cu2N6O10 C89.5H127B2Cl8-
Cu2N6O4.5
fw
609.81
orthorhombic
P212121
9.9638(4)
13.3049(6)
19.5424(9)
90
90
90
4
389.36
monoclinic
P21
7.5134(1)
14.2737(3)
8.2653(2)
90
98.264(1)
90
2
460.51
orthorhombic
Pca21
17.4559(3)
8.3552(1)
15.0250(3)
90
90
90
4
679.25
monoclinic
P21/n
12.9986(3)
19.9770(3)
14.7452(1)
90
97.592(1)
90
4
730.62
monoclinic
P21/n
8.7398(6)
8.5565(6)
20.566(2)
90
95.705(1)
90
2
1791.27
triclinic
P1h
12.5429(2)
12.9911(2)
28.0132(1)
85.0661(4)
89.7265(8)
88.4361(6)
2
cryst syst
space group
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
Z
density (calcd;
1.563
1.474
1.396
1.189
1.586
1.309
g cm-3
)
temp (K)
293(2)
173(2)
173(3)
173(2)
173(2)
173(2)
cryst size (mm)
abs coeff (mm-1
θ range (deg)
reflns collected
indep reflns
0.35 × 0.25 × 0.22 0.26 × 0.21 × 0.06 0.40 × 0.19 × 0.06 0.35 × 0.15 × 0.15 0.40 × 0.30 × 0.08 0.38 × 0.29 × 0.05
)
1.917
1.418
1.146
0.608
1.623
0.755
1.85-25.03
13008
2.49-25.11
4498
2.33-25.01
10981
1.73-25.04
17612
1.99-25.03
7219
0.73-25.01
25779
4549
2845
3416
6575
2676
15039
params/restraints
R1 [I > 2σ(I)]a
wR2 [I > 2σ(I)]b
GOF
271/24
0.0461
0.1094
1.040
204/1
250/1
484/0
181/0
1043/0
0.0989
0.2259
1.055
0.0562
0.1561
1.075
0.0285
0.0560
1.074
0.0629
0.0968
1.094
0.0561
0.1231
1.097
largest diff features 0.437/-0.377
1.193/-0.444
0.213/-0.238
0.512/-0.322
0.756/-0.447
2.212/-0.982
(e Å-3
)
a R1 ) Σ||Fo| - |Fc||/Σ|Fo|. b wR2 ) [Σ[w(Fo - Fc2)2/Σ[w(Fo )2]]1/2 where w ) q/σ2(Fo ) + (aP)2 + bP.
2
2
2
may be traced to substituent size variation. Following previously
published analyses of TACD compounds (Figure S3),16 we
assign the conformations of the two 5-membered and one
6-membered chelate rings within each of the R3TACD com-
plexes as δ,λ-chair (R ) Me) and λ,λ-sofa (R ) iPr). The former
conformation matches that found previously to be most stable
(lowest strain energy) for Co and Cu complexes of the parent
TACD ligand.16a,b Thus, whereas methyl substitution appears
to have negligible effects on the conformational energetics of
the coordinated TACD macrocycle, the finding of a different
conformation in the iPr3TACD case implicates more substantial
conformational energy effects of the larger isopropyl substitu-
ents. Still, the energy differences between conformations are
not large, since we observed a chair conformation for the
6-membered ring in a different complex of iPr3TACD in the
solid state (vide infra).
Second, while Cu-N distances are similar in the iPr3TACN
and iPr3TACD complexes comprising identical substituents, the
additional methylene unit in the macrocycle of the latter
influences the disposition of the iPr groups. Thus, in [(iPr3-
TACD)Cu(CH3CN)]+, there is greater crowding about the CH3-
CN ligand compared to the iPr3TACN analogue. This difference
between the two structures may be traced to differences in the
position of the Cu ion relative to the planes defined by the
macrocycle N-donor atoms and the iPr methine carbon atoms,
respectively (Figure 5). The Cu ion in the iPr3TACD case lies
close to the N-donor plane (deeper inside this larger macrocycle)
and approximately within the plane of the methine C atoms,
whereas in the iPr3TACN complex it is farther from the N-donor
atom plane and 0.7 Å aboVe the methine C atom plane. As a
result, access of exogenous molecules or molecular fragments
to the Cu ion in the iPr3TACD case is more hindered than in
the iPr3TACN instance. This key difference provides a possible
rationale for O2 reactivity results described below.
Figure 4. Representation of the cationic portion of the X-ray structure
of [(iPr3TACN)Cu(CH3CN)]BPh4, shown as 50% ellipsoids with H
atoms omitted for clarity. Selected bond distances (Å) and angles
(deg): Cu(1)-N(1) 2.113(3), Cu(1)-N(4) 2.151(3), Cu(1)-N(7) 2.147-
(3), Cu(1)-N(10) 1.865(3), N(10)-Cu(1)-N(1) 135.6(1), N(10)-Cu-
(1)-N(7) 129.4(1), N(1)-Cu(1)-N(7) 85.9(1), N(10)-Cu(1)-N(4)
118.3(1), N(1)-Cu(1)-N(4) 86.2(1), N(7)-Cu(1)-N(4) 85.4(1).
(2.09-2.15 Å)14,15 and the Cu-Nnitrile distances are short, as
expected (range 1.86-1.90 Å). Despite these general similarities,
specific comparisons between the pairs of complexes ligated
by iPr3TACD vs Me3TACD (different substituents, same ring
size) and iPr3TACD vs iPr3TACN (same substituents, different
ring size) reveal important structural disparities.
First, the larger substituents in iPr3TACD vs Me3TACD result
in greater encapsulation of and, thus, more hindered access to
the Cu(I) ion (see space-filling drawings in Figure S2).
Differences in the observed macrocycle ring conformations also
(14) Chaudhuri, P.; Oder, K. J. Organomet. Chem. 1989, 367, 249.
(15) Halfen, J. A.; Mahapatra, S.; Wilkinson, E. C.; Gengenbach, A. J.;
Young, V. G., Jr.; Que, L., Jr.; Tolman, W. B. J. Am. Chem. Soc.
1996, 118, 763.
(16) (a) Dwyer, M.; Searle, G. H. Aust. J. Chem. 1981, 34, 2025. (b) Chen,
X.-M.; Yao, Y.-X.; Shi, K.-L.; Mak, T. C. W. Aust. J. Chem. 1995,
48, 139. (c) DaCruz, M. F.; Zimmer, M. Inorg. Chem. 1998, 37, 366.

Ligand Macrocycle Effects on Copper-Dioxygen Reactivity
Inorganic Chemistry, Vol. 39, No. 18, 2000 4063
iPr3TACN where a small amount of bis(µ-oxo)dicopper isomer
is apparent (UV-vis, resonance Raman) and is in equilibrium
with the peroxo form,3,4 no bis(µ-oxo)dicopper form was seen
for the iPr3TACD case in any solvent. The cleanliness of the
reaction to form the peroxo unit is illustrated in Figure S7, where
absorption spectra for the systems capped by iPr3TACD and
iPr3TACN are overlaid.
In view of the apparent absence of a bis(µ-oxo)dicopper
species that would complicate crystallographic analysis,2b,5d we
attempted to determine the X-ray structure of the (µ-η2:η2-
peroxo)dicopper complex supported by iPr3TACD. After many
trials, suitable crystals were obtained by using ligand with
Figure 5. Comparison of the position of the Cu(I) ions in the X-ray
structures of the indicated complexes relative to the planes defined by
the macrocycle N-donor atoms and the iPr group methine carbon atoms.
(c) Electrochemistry. The results of cyclic voltammetry
experiments are listed in Table 1, with representative CVs for
the iPr3TACDD, iPr3TACD, and Me3TACD cases provided in
Figure S4. After numerous trials, the best results under a single
set of conditions were obtained by using 9:1 (v/v) CH2Cl2/CH3-
CN mixtures with 0.1 M (Bu4N)(PF6) as the supporting
electrolyte. Nevertheless, large ∆Ep and unequal ipc and ipa
values were observed in most instances, indicative of quasire-
versible behavior at best. Irreversible oxidation of [(iPr3-
TACDD)Cu]+ was observed at high potential, consistent with
stabilization of the Cu(I) state suggested by its trigonal structure
and with major rearrangement upon oxidation. Comparison of
the data acquired for the R3TACD and iPr3TACN systems is
not particularly informative, although there appears to be a trend
related to the size of the substituents; higher E1/2 values were
found for the iPr-substituted compounds than for the effectively
smaller Me- and Bn-substituted cases. This trend does not
correlate with the ν(CO) values of the carbonyl complexes,
which should be indicative of the amount of electron density at
the metal center but do not track with the relative electron-
donating properties of the ligand substituents. These seeming
inconsistencies, in conjunction with the lack of cleanly reversible
electrochemical behavior of the complexes in the CV experi-
ments, has complicated efforts to relate their O2 reactivity and
redox properties.
-
perdeuterated iPr substituents, BPh4 counterions, and an
acetone/butanone (1:1 v/v) solvent mixture at -80 °C. Although
the structure is plagued by problems associated with solvent
disorder and pseudosymmetry issues (see Supporting Informa-
tion for details), the data were sufficient to confirm the presence
of a planar (µ-η2:η2-peroxo)dicopper core capped by iPr3TACD
macrocycles (Figure 6). Detailed interpretation of bond distances
is not warranted, but some important structural aspects are
nonetheless worth noting. The data indicate an approximately
planar (µ-η2:η2-peroxo)dicopper topology [Cu‚‚‚Cu 3.5 Å,
O(1)-O(2), 1.4 Å] like those in the only other structurally
characterized examples, [(TpiPr2Cu)2(O2)] [Cu‚‚‚Cu 3.560(3) Å,
O(1)-O(2), 1.412(12) Å],18a [LCu2(O2)](PF6)2 [Cu‚‚‚Cu 3.477-
(7) Å, O(1)-O(2), 1.485(8) Å],18d and oxyhemocyanin [Cu‚‚‚
Cu 3.6(2) Å, O(1)-O(2), 1.4(2) Å].19 However, whereas the
TpiPr2 complex and oxyhemocyanin contain square pyramidal
(SP) Cu(II) sites [τ ) 0.03 and 0.10 (av), respectively]20 with
the axial Cu-N bonds in an anti conformation (Figure 6c), each
Cu(II) ion in the iPr3TACD compound (Figure 6b) adopts a
geometry significantly distorted (pseudorotated) from SP [τ )
0.30 (av)]. Related distortions away from idealized SP Cu
geometries were reported for [LCu2(O2)](PF6)2, although they
probably are caused by constraints imposed by a tether between
the capping N-donor units in this case.18d Another aspect of
the structure of the iPr3TACD complex concerns the angle τ′
between the pseudoaxial Cu-N bonds for each metal center,
where τ′ ) 180° and 0° for the extreme anti and syn
conformations, respectively. The observed angle is 64°, an
important distortion from the idealized conformations that are
more closely approximated in known structures of other µ-η2:
η2-peroxo- and bis(µ-oxo)dicopper compounds.5a,6,7
Reactivity of Copper(I) Complexes with O2. When solu-
tions of [(R3TACD)Cu(CH3CN)]SbF6 (R ) Me or Bn) in CH2-
Cl2, THF, or acetone were oxygenated at -80 °C, an orange-
brown color developed. The UV-vis and resonance Raman
spectral features of the orange-brown solution were diagnostic
for the formation of the bis(µ-oxo)dicopper core and clearly
were inconsistent with the presence of its (µ-η2:η2-peroxo)-
dicopper isomer (Figures S5 and S6, and Table 3).3-8,17 Thus,
we observed a pair of absorption features of similar high
intensity (ꢀ ∼13000-16000 M-1 cm-1) at ∼310 and ∼415 nm
in UV-vis spectra and an 18O-sensitive feature at ∼600 cm-1
(∆18O ) 20-25 cm-1) in the resonance Raman spectrum (λex
) 457 nm) that are signatures of the [Cu2(µ-O)2]2+ core.
In contrast, low-temperature oxygenation of [(iPr3TACD)-
Cu(CH3CN)]SbF6 in CH2Cl2, THF, or acetone yielded a (µ-η2:
η2-peroxo)dicopper complex only. This assignment was sup-
ported by UV-vis and resonance Raman spectroscopic data that
are similar to those in the literature (Table 3);3,18 thus, diagnostic
peroxo f Cu(II) CT bands were observed in the absorption
Decomposition Reactions of Peroxo- and Bis(µ-oxo)-
dicopper Complexes. The oxygenation products supported by
the R3TACD ligands decompose upon standing at temperatures
above -80 °C. Detailed analyses of these reactions were not
(18) Data for representative examples are discussed in the following: (a)
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. (b) Baldwin, M. J.; Root, D. E.; Pate, J.
E.; Fujisawa, K.; Kitajima, N.; Solomon, E. I. J. Am. Chem. Soc. 1992,
114, 10421. (c) Mahapatra, S.; Halfen, J. A.; Wilkinson, E. C.; Que,
L., Jr.; Tolman, W. B. J. Am. Chem. Soc. 1994, 116, 9785. (d) Kodera,
M.; Katayama, K.; Tachi, Y.; Kano, K.; Hirota, S.; Fujinami, S.;
Suzuki, M. J. Am. Chem. Soc. 1999, 121, 11006. L ) 1,2-bis[2-(bis-
(6-methyl-2-pyridyl)methyl)-6-pyridyl]ethane. (e) Solomon, E. I.;
Tuczek, F.; Root, D. E.; Brown, C. A. Chem. ReV. 1994, 94, 827. (f)
Hu, Z.; Williams, R. D.; Tran, D.; Spiro, T. G.; Gorun, S. M. J. Am.
Chem. Soc. 2000, 122, 3556.
spectrum, and a ν(O-O) at 739 cm-1 with the appropriate 18
O
shift was identified in the Raman spectrum. The ν(O-O) is
higher in energy than for the analogue supported by iPr3TACN
(713 cm-1),17 but is similar to that seen for other systems with
heterocyclic donor groups (cf. second and third entries in Table
3). Interestingly, unlike the case for the system supported by
(19) (a) Magnus, K. A.; Hazes, B.; Ton-That, H.; Bonaventura, C.;
Bonaventura, J.; Hol, W. G. J. Proteins 1994, 19, 302-309. (b)
Magnus, K. A.; Ton-That, H.; Carpenter, J. E. Chem. ReV. 1994, 94,
727.
(20) The parameter τ is defined in the following: Addison, A. W.; Rao,
T. N.; Reedjik, J.; von Rijn, J.; Verschoor, G. C. J. Chem. Soc., Dalton
Trans. 1984, 1349. For an idealized square pyramidal geometry τ )
0, while for an idealized trigonal bipyramidal arrangement τ ) 1.0.
(17) 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.

4064 Inorganic Chemistry, Vol. 39, No. 18, 2000
Lam et al.
Table 3. Spectroscopic Data for Products of Oxygenations of Selected Cu(I) Complexes
a
b
oxygenated complex
{[(iPr3TACN)Cu]2(µ-η2:η2-O2)}2+
λ
max (nm)
ꢀ (M-1 cm-1
)
Raman (cm-1
713c
)
∆18O (cm-1
)
ref
366
510
360
532
349
551
380
520
324
418
318
430
304
404
312
428
22 500
1 300
24 700
1 530
21 000
790
22 000
2 300
11 000
13 000
12 000
14 000
16 000
16 200
13 500
13 600
41
17, 18c
[(LCu)2(µ-η2:η2-O2)]2+ d
760
41
43
43
22
23
20
25
18d
[(TpiPr2Cu)2(µ-η2:η2-O2)]
741
18a,b
{[(iPr3TACD)Cu]2(µ-η2:η2-O2)}2+
{[(iPr3TACN)Cu]2(µ-O)2}2+
{[(Bn3TACN)Cu]2(µ-O)2}2+
{[(Me3TACD)Cu]2(µ-O)2}2+
{[(Bn3TACD)Cu]2(µ-O)2}2+
739
this work
3, 6
589
603/595e
595
3, 6, 17
this work
this work
600
a O-O or Cu2O2 vibration in resonance Raman spectrum acquired on frozen solutions at 77 K with λex ) 457 or 514 nm. b Raman shift for
complex prepared with 16O2 minus Raman shift for complex prepared with 18O2. c This value is corrected for a referencing error made in the
original report; see ref 17 for details. d L ) 1,2-bis[2-(bis(6-methyl-2-pyridyl)methyl)-6-pyridyl]ethane. e Fermi doublet in spectrum of 16O2-derived
sample.
Table 4. Activation Parameters for the Decomposition Reactions of
Selected µ-η2:η2-Peroxo- and Bis(µ-oxo)dicopper Complexesa
∆H‡
∆S‡
(eu)
complex
(kcal mol-1
)
ref
{[(iPr3TACN)Cu]2(µ-η2:η2-O2)}2+
{[(iPr3TACD)Cu]2(µ-η2:η2-O2)}2+
{[(iPr3TACN)Cu]2(µ-O)2}2+
{[(Bn3TACN)Cu]2(µ-O)2}2+
{[(Me3TACD)Cu]2(µ-O)2}2+
{[LMeCu]2(µ-O)2}2+ b
13.5(5)
11.9(5)
13.2(5)
13.0(5)
12.2(5)
11.8(3)
-12(1) 18c
-20(1) this work
-14(2) 21
-13(2) 21
-24(2) this work
-25(1) 5a
a Determined from Eyring plots of first-order rate constants (Figure
b
S8). LMe ) N,N ′-diethyl-N,N ′-dimethyl-trans-(1R,2R)-diaminocy-
clohexane.
for Eyring plots). The observed behavior is quite similar to that
noted previously for various TACN derivatives, suggesting
similar pathways involving intramolecular attack of the peroxo
or bis(µ-oxo) core at ligand substituent C-H bonds R to the
N-donor atoms.
Although no effort was made to identify and quantify all of
the decomposition products, small amounts of crystalline
complexes were isolated and subjected to X-ray diffraction
analysis. From the decay of the bis(µ-oxo) compound supported
by Me3TACD we isolated {[(Me3TACD)Cu]2(µ-OH)2}(ClO4)2
(Figure 7), which features a bis(hydroxo)dicopper(II) core
typical for such species [Cu‚‚‚Cu ) 3.015 Å, Cu-OH ) 1.93-
1.95 Å]. The Cu(II) ions adopt slightly distorted SP geometries
(τ ) 0.17), and the Me3TACD macrocycles exhibit a similar
conformation as in the Cu(I) complex (e.g., chair conformation
for the 6-membered chelate ring). Another Cu(II) complex was
isolated as a side product during attempts to crystallize the µ-η2:
η2-peroxo complex supported by iPr3TACD; its X-ray structure
revealed it to be a µ-carbonato species apparently derived from
fixation of atmospheric CO2 by a bis(hydroxo) precursor (Figure
8). Numerous other dicopper carbonato complexes have been
reported.22 The Cu(II) ions in this particular compound adopt
trigonal bipyramidal geometries (τ ) 0.82, 0.75) with one of
the amine donors and one of the carbonate oxygen atoms serving
Figure 6. (a) Representation of the cationic portion of the X-ray
structure of [(d21-iPr3TACD)2Cu2(µ-η2:η2-O2)](BPh4)2‚xsolvate. (b)
View of the coordination spheres of the Cu(II) ions along the Cu‚‚‚Cu
vector, with τ′ shown as the N-Cu-Cu-N torsional angle involving
the pseudoaxial N donor atoms most distant from their respective Cu-
(II) ions. (c) View of the coordination spheres of the Cu(II) ions in
[TpiPr22Cu2(µ-η2:η2-O2)] along the Cu‚‚‚Cu vector, with the indicated
τ′ value.16a
undertaken, but selected data were acquired in order to draw
comparisons to more thoroughly studied cases with other ligand
systems reported previously.5,7,8,21 The decays of {[(Me3TACD)-
Cu]2(µ-O)2}(ClO4)2 and {[(iPr3TACD)Cu]2(µ-η2:η2-O2)}(ClO4)2
follow first-order kinetics with the activation parameters listed
in Table 4 (determined via UV-vis monitoring; see Figure S8
(22) Some selected recent reports: (a) Escuer, A.; Mautner, F. A.; Pen˜alba,
E.; Vicente, R. Inorg. Chem. 1998, 37, 4190. (b) Fusch, G.; Fusch, E.
C.; Erxleben, A.; Hutterman, J.; Scholl, H.-J.; Lippert, B. Inorg. Chim.
Acta 1996, 252, 167. (c) Sorrell, T. N.; Allen, W. E.; White, P. S.
Inorg. Chem. 1995, 34, 952. (d) Kruger, P. E.; Fallon, G. D.;
Moubaraki, B.; Berry, K. J.; Murray, K. S. Inorg. Chem. 1995, 34,
4808.
(21) Mahapatra, S.; Halfen, J. A.; Tolman, W. B. J. Am. Chem. Soc. 1996,
118, 11575.

Ligand Macrocycle Effects on Copper-Dioxygen Reactivity
Inorganic Chemistry, Vol. 39, No. 18, 2000 4065
Table 5. Relative Energies (kcal/mol) of the Peroxo and Bis(µ-oxo)
Isomers of [LCuO2CuL]2+ and of the Transition State for Their
Interconversion, Calculated for Different Sets of Terminal Donors
and at Different Computational Levels
transition
entry N-donors peroxo
state
bis(µ-oxo)
method
B3LYP
BP
B3LYP
bs-B3LYP
CASPT2
CASPT2/RHF 9a
B3LYP
BP
B3LYP
ref
1
2
3
4
5
6
7
8
9
(NH3)3
(NH3)3
(NH3)3
(NH3)3
(NH3)3
(NH3)3
TACN
TACN
TACD
0
0
0
0
12.7
0
0
0
0
0
17.0
12.6
11.7
14.4
19.9
0
0.3
11.3
0.2
7.2
2.9
21.5
30.0b
34.5b
9d
9b
9g
9g
9g
18.7
24.2
17.0
7.9
15.8
8.8
c
9b
c
9b
c
c
Figure 7. Representation of the cationic portion of the X-ray structure
of [(Me3TACD)2Cu2(µ-OH)2)](ClO4)2, shown as 50% ellipsoids with
all H atoms (except those placed on the bridging hydroxides) omitted
for clarity. Selected bond distances (Å) and angles (deg): Cu(1)-O(1)
1.950(3), Cu(1)-O(1a) 1.933(3), Cu(1)-N(2) 2.041(4), Cu(1)-N(3)
2.060(4), Cu(1)-N(1) 2.272(4), Cu‚‚‚Cu 3.016(1), O(1a)-Cu(1)-O(1)
78.1(2), O(1a)-Cu(1)-N(2) 173.1(2), O(1)-Cu(1)-N(2) 97.0(2),
O(1a)-Cu(1)-N(3) 96.8(2), O(1)-Cu(1)-N(3) 162.9(2), N(2)-Cu-
(1)-N(3) 86.6(2), O(1a)-Cu(1)-N(1) 102.7(2), O(1)-Cu(1)-N(1)
100.4(2), N(2)-Cu(1)-N(1) 82.9(2), N(3)-Cu(1)-N(1) 96.7(2), Cu-
(1)-O(1)-Cu(1a) 101.9(2).
10 Tpa
BP
11 Me3TACD
12 iPr3TACN
13 iPr3TACD
0
0
0
B3LYP:MM3
B3LYP:MM3
B3LYP:MM3
c
a Tp ) tris(pyrazolyl)hydroborate. b Not a minimum, evaluated at
O-O ) 2.36 Å. c This work.
atoms within these differently sized macrocyles. Then we will
assess the steric effects associated with the presence of Me or
iPr substituents at the donor atoms, as present in the real
compounds, using combined molecular orbital/molecular me-
chanics IMOMM calculations.
The relative energies of the peroxo and bis(µ-oxo) isomers
at their optimized structures in the model compound [(NH3)3-
CuO2Cu(NH3)3]2+ calculated at different levels of theory are
given in Table 5 (entries 1-6). These previously published
results indicate that the peroxo isomer generally is predicted to
be more stable with DFT calculations, but when electron
correlation is treated at the more precise CASPT2 level the
relative stabilities are inverted.9b,d The B3LYP calculations
probably overestimate the relative stability of the µ-peroxo form.
The use of broken-symmetry (bs) B3LYP calculations would
be indicated for the formally Cu(II)2 peroxo isomer, but these
are expected to further stabilize it relative to the oxo-bridged
form as indeed found by Flock and Pierloot (Table 5, entry 4).9g
We conclude from the combined available results that a
quantitative interpretation of the calculated value for the
difference in energy between the two isomers bound to NH3
ligands is not warranted with the computational methodology
available for the real molecules under study. The best we can
say is that the two isomers probably differ in energy by less
than 15 kcal/mol. Consequently, our emphasis will be on
evaluating the qualitative effects of ligand structure on the
relative stabilities of the two isomeric forms.
Figure 8. Representation of the cationic portion of the X-ray structure
of [(iPr3TACD)2Cu2(µ-CO3)](X)2‚xsolvate, shown as 50% ellipsoids
with all H atoms omitted for clarity. Selected bond distances (Å): Cu-
(1)-O(2) 1.939(5), Cu(1)-N(2) 2.030(6), Cu(1)-N(3) 2.038(6), Cu-
(1)-N(1) 2.053(6), Cu(2)-O(3) 1.940(5), Cu(2)-N(5) 2.018(5),
Cu(2)-N(4) 2.041(5), Cu(2)-N(6) 2.088(6), Cu(2)-O(1) 2.415(6). For
bond angles, see Supporting Information.
as axial ligands in an arrangement different from an iPr3TACN-
supported analogue23 that has SP Cu(II) geometries and a coor-
dinated H2O molecule. Unlike the Cu(I) complex of iPr3TACD,
the 6-membered chelate ring is in a chair conformation in the
carbonato compound, indicating relatively similar energies for
the sofa and chair conformers of this ligand when coordinated.
Theoretical Calculations. We evaluated via theoretical
methods the relative energies and stereochemical differences
between µ-η2:η2-peroxo- and bis(µ-oxo)dicopper complexes
supported by variously N-substituted tridentate 9- and 10-
membered macrocycles. The primary objective was to determine
how the size of the macrocycle and its N-donor substituents
influence the structures and relative amounts of the peroxo and
bis(µ-oxo) products of the respective oxygenation reactions and,
thus, rationalize the experimental findings discussed above. In
the following discussion, we first consider unconstrained systems
supported by NH3 donors that serve as benchmarks for inherent
electronic structure effects and then compare them to compounds
containing the parent, unsubstituted TACN and TACD ligands.
In so doing, we address the results of constraining the donor
(a) Influences of Macrocycle Ring Size. In a previous theo-
retical study important changes in the orientation of the terminal
ligands were found to accompany the peroxo to bis(µ-oxo) trans-
formation.9d In addition, the constraints imposed on the relative
position of the donor atoms in a tridentate ligand were shown
to affect the relative energies of the two isomers significantly.
The main structural differences are summarized in Table 6,
where δ reflects the orientation of the CuN3 pyramids relative
to the Cu2O2 plane as shown and Σ is the sum of the N-Cu-N
bond angles that indicates the degree of pyramidalization around
the Cu atoms. A simple way to summarize the calculated struc-
tural discrepancies is to consider two extreme situations for the
coordination sphere of the copper atoms. At one extreme, a Cu-
(I) ion can be viewed as adopting a tetrahedral geometry with
a η2-O2 ligand and the three N-donors occupying the four ver-
tices (expected values for this arrangement are δ ≈ 0° and Σ ≈
330°). At the other extreme, a Cu(III) ion adopts a square pyram-
(23) Schneider, J. L.; Young, V. G., Jr.; Tolman, W. B. Inorg. Chem. 1996,
35, 5410.

4066 Inorganic Chemistry, Vol. 39, No. 18, 2000
Lam et al.
Table 6. Calculated Structural Parameters for the Peroxo and Oxo
Isomers of [LCuO2CuL]2+ (L ) (NH3)3, TACN, TACD)a
peroxo
bis(µ-oxo)
param
(NH3)3 TACD TACN (NH3)3 TACD TACN
O...O
1.515
1.985
2.055
2.220
19
1.427
1.980
2.044
2.274
21 (17)
273
1.425
1.970
2.064
2.226
14
2.288
1.835
2.003
2.381
29
2.368
1.830
2.004
2.400
23 (20)
263
2.346
1.820
2.008
2.322
19
Cu-O
Cu-Neq
Cu-Nax
δb
Figure 9. Relative energy of [(TACD)CuO2Cu(TACD)]2+ as a function
of the rotation angle τ′ in the peroxo and bis(µ-oxo) forms. The rotation
angle was kept fixed and the rest of the structure optimized for each
point.
Σc
313
256
289
251
a All distances are given in angstroms; the angles δ and Σ are given
in degrees. All values correspond to the anti conformation shown above
(τ′ ) 180°), except for those in parentheses, which indicate the δ values
for τ′ ≈ 0° (all other values are practically the same for this case).b δ
is defined by the drawing above. c Σ is the sum of the N-Cu-N bond
angles.
for the transition states indicate that the barriers for the cleavage
of the O-O bond [peroxo f bis(µ-oxo)] are quite similar with
both macrocycles or the monodentate NH3 ligands, but due to
the divergent stabilities of the bis(µ-oxo) isomers the barriers
for the reverse processes [bis(µ-oxo) f peroxo] differ according
to the series TACD > (NH3)3 > TACN.
idal geometry with one of the N donors occupying the axial
position and two oxo anions in basal sites (expected parameters
δ g 35° and Σ ≈ 270°). The bis(µ-oxo) structure thus
corresponds closely to the latter case, while the peroxo struc-
ture lies intermediate between the two extremes, consistent with
its formal description and experimental data that indicate that
it should be viewed as a (µ-η2:η2-peroxo)dicopper(II,II) unit.18
Comparison of the calculated structural data listed in Table
6 for the complexes [LCuO2CuL]2+, where L ) (NH3)3, TACN,
or TACD, indicates closely related geometric differences
between the respective peroxo and bis(µ-oxo) forms. Thus, for
each supporting ligand system, as the molecule converts from
the peroxo to the bis(µ-oxo) structure (a) the Cu-O distances
significantly shorten, (b) the Cu-Neq distances decrease and
the Cu-Nax distances increase, resulting in much larger differ-
ences between the two types of Cu-N distances in the bis(µ-
oxo) form, (c) the value of Σ decreases, and (d) the CuN3
pyramids are bent further away from the Cu2O2 plane, as
measured by an increase in δ. Importantly, the angle sum Σ for
each isomeric form follows the series TACN < TACD < NH3,
consistent with the expected trend in ligand flexibilities; values
of Σ and δ as well as their differences for the two isomers
increase as the N-donor constraints decrease (NH3 being the
least and TACN being the most constrained).
In terms of energies, computational results for the compound
[(TACN)CuO2Cu(TACN)]2+ (Table 5, entry 7) give nearly the
same difference between the peroxo and the bis(µ-oxo) isomers
as calculated previously at the same level of theory for the
analogue supported by the monodentate NH3 ligands (entry 1).9g
For the TACD complex (entry 9), however, the difference in
energy between the two isomers is significantly smaller, a
finding that we explain by the increased flexibility of the TACD
macrocycle that can better adapt to the stereochemical changes
required for the peroxo to bis(µ-oxo) transformation. This greater
relative stabilization of the bis(µ-oxo) form for TACD compared
to TACN contrasts with the experimental observation of only
the peroxo isomer for iPr3TACD and suggests that the nature
of the N-donor substituents is an important determinant (vide
infra). Finally, we note in passing that the calculated energies
It is important to stress that the syn and anti conformations
(i.e., Nax-Cu-Cu-Nax torsion angles τ′ ) 0 and 180°,
respectively) of both the peroxo and bis(µ-oxo) forms supported
by TACD are essentially isoenergetic at the present level of
calculation (energy difference less than 0.4 kcal/mol, Figure 9).
Rotation between these two conformations affects the Cu-N
distances; an excellent linear correlation (Figure S9) is found
between the position of each Cu-N bond relative to the Cu2O2
plane (given by the N-Cu-Cu-O torsion angle) and the Cu-N
distance, with the shortest distance (1.969 Å) corresponding to
the Cu-N bond coplanar with the Cu2O2 ring and the longest
one to the Cu-N bond perpendicular to the same plane (2.401
Å). Importantly, while the energy of the rotated conformation
(τ′ ) 60°) for the peroxo isomer differs little from those of its
syn and anti conformations, this rotated conformation for the
bis(µ-oxo) form is about 5 kcal/mol less stable than its syn and
anti ones. This point is relevant to the following discussion of
substituent effects on the relative stabilities and structures of
the TACN- and TACD-supported systems. Finally, we note that
previous calculations on the model complex [(NH3)6Cu2(µ-
O)2]2+ with unconstrained monodentate ligands found syn and
anti conformations of the oxo isomer to be almost isoenergetic
at the DFT level, but a more accurate treatment of the electron
correlation (CASPT2 level) predicted the syn conformation to
be some 9 kcal/mol more stable.6
(b) Influences of N-Donor Substituents. The effects of the
substituents in the TACD and TACN complexes were analyzed
via combined molecular orbital/molecular mechanics IMOMM
calculations (B3LYP:MM3), in which both of the entire
macrocyclic ligands together with the Cu2O2 core were treated
quantum-mechanically (QM) and the R groups at the six
nitrogen donors were treated by molecular mechanics (MM)
(see Experimental Section for details). In these calculations, the
geometry of the full molecule is optimized by minimizing the
total energy, which is the sum of the QM and MM contributions
(Etot ) EQM + EMM). The MM term is mostly associated with
the steric interactions between the substituents. In order to
minimize such repulsions, the [LCuO2CuL]2+ core is somewhat

Ligand Macrocycle Effects on Copper-Dioxygen Reactivity
Inorganic Chemistry, Vol. 39, No. 18, 2000 4067
Table 7. Calculated Structural Parameters for the Peroxo and Bis(µ-oxo) Isomers of [(R3TACX)CuO2Cu(R3TACX)]2+, Where R ) H, Me, or
iPra
ligand/isomer
TACD/peroxo
Me3TACD/peroxo
iPr3TACD/peroxo
iPr3TACD/peroxo (expt)
O‚‚‚O
τ′
Cu-O
1.980
1.990
2.012
1.89
Cu-Neq
2.045
2.065
2.067, 2.154
2.05
Cu-Nax
2.274
2.272
2.317, 2.394
2.218
Σ
N‚‚‚Nc
C‚‚‚Cd
δe
1.427
1.416
1.403
1.367
4
273
275
276, 277
276, 281
6.001
5.952
6.464
5.930
21 (17)
18
11
44
64
3.985
3.591
3.620
13
12
TACD/bis(µ-oxo)
Me3TACD/bis(µ-oxo)
iPr3TACD/bis(µ-oxo)
2.367
2.169
2.360b
0
31
58
1.830
1.832
1.84-1.89
2.005
2.068
2.064-2.265
2.400
2.366
2.372, 2.777
263
264
258, 264
5.224
5.506
5.695
23 (20)
16
16
3.768
3.497
TACN/peroxo
1.425
1.411
2.346
2.360b
180
180
180
180
1.970
1.984
2.008
1.852, 1.844
2.064
2.068, 2.115
2.008
2.226
2.247, 2.255
2.322
256
258, 260
251
14
12
19
16
iPr3TACN/peroxo
TACN/bis(µ-oxo)
iPr3TACN/bis(µ-oxo)
6.465
5.899
5.803
4.295
3.912
3.904
2.086
2.452
249
a All distances are given in angstroms; the angles δ and Σ are given in degrees. The angle δ is defined by the drawing above Table 6, Σ is the
sum of the N-Cu-N bond angles, and τ′ is the Nax-Cu-Cu-Nax torsion angle. b O‚‚‚O distance frozen and the remaining parts of the structure
optimized. c Shortest contact between the corresponding atoms of the two TACD ligands. d Shortest contact between the substituents of the two
TACD ligands. e Values in parentheses correspond to the anti conformation (τ′ ) 180°).
To gain some understanding of the reasons for the changes
in relative stability upon substitution, we separately analyze the
QM and MM contributions to the total energy. The energy
partition for the complexes of Me3TACD, iPr3TACD, and iPr3-
TACN is presented in Figure 11 as a function of the O‚‚‚O
distance (∆EQM ) squares, ∆EMM ) triangles, total energy )
circles), together with the calculated electronic energies for the
corresponding unsubstituted macrocycles (crosses). As might
be intuitively obvious, the steric energy reflecting the interligand
repulsions within each complex becomes greater as the O‚‚‚O
distance increases and the Cu‚‚‚Cu distance simultaneously
decreases. Interestingly, however, the extent of MM destabiliza-
tion in the process peroxo f bis(µ-oxo) differs little between
Me3TACD, iPr3TACD, and iPr3TACN. Instead, a more impor-
tant issue is the difference between the QM energies of the
complexes with N-alkylated ligands and those analogues with
the respective unsubstituted, parent macrocycle. In each case,
the QM destabilization of the bis(µ-oxo) form increases upon
N-substitution, with the greatest difference being that between
the complexes supported by iPr3TACD and TACD (Figure 11).
Figure 10. Calculated (B3LYP:MM3) relative energy (∆Etot) for the
R3TACD complexes as a function of the O-O distance (all other
parameters optimized for each distance).
An analysis of the structural perturbations attending N-
substitution in the series of TACD-supported systems helps us
understand these QM energy effects (Table 7). We note initially
the excellent agreement between the main structural features
of the iPr3TACD-capped peroxo isomer obtained from IMOMM
calculations with the experimental data. Second, the calculations
indicate that the incorporation of bulkier substituents produces
an increasing destabilization of the syn conformation (τ′ ) 0°),
resulting in rotated conformations with torsion angles in the
range 11° < τ′ < 56°. This effect is enhanced as the macrocyclic
ligands approach each other (i.e., O‚‚‚O distance increases) and
as the N-substituent size increases (iPr > Me > H). The
qualitative agreement in the large torsion angle for the calculated
and experimentally determined peroxo structures supported by
iPr3TACD is a key finding. Evidently, a significant reorganiza-
tion of the Cu coordination sphere reflected by an increase in
τ′ results from steric repulsions between substituents of the
capping ligands. The reorganization is most drastic for the case
of the bis(µ-oxo) form supported by iPr3TACD and results in
strong desymmetrization of the complex that becomes apparent
when the two oxygen atoms are separated by 2.10 Å. One of
the copper atoms presents a coordination sphere characteristic
of bis(µ-oxo) isomers, with a relatively small Σ value (260°)
and a large difference between the long and the two short Cu-N
distances. The other copper atom, however, has the same value
of Σ as in the peroxo isomer (276°) and a smaller difference
distorted with respect to the analogous molecule with unsub-
stituted ligands, resulting in an increase in EQM
.
Only a minor distortion of the core in the peroxo form is
induced by the presence of the Me groups on the TACD ligand,
as reflected by a small increase of EQM upon methyl substitution
(1.3 kcal/mol). In contrast, the iPr groups impose severe
distortions (see Table 7) and a large increase of EQM (11.2 kcal/
mol). The destabilization introduced by the substituents is more
important as the O‚‚‚O distance increases, the Cu‚‚‚Cu distance
simultaneously decreases, and the substituents of the two
macrocycles approach each other as the peroxo core isomerizes
to the bis(µ-oxo) form. This trend is illustrated in Figure 10,
where the total IMOMM energies for the various TACD
compounds are represented as a function of the O‚‚‚O distance,
taking as the zero for each compound the energy of its most
stable peroxo form (except for the peroxo form, the O‚‚‚O
distance was frozen, and the rest of the molecular structure
reoptimized). Note that the destabilization of the bis(µ-oxo) form
relative to the peroxo increases with the size of the substituent,
iPr > Me > H (Table 5, entries 9, 11, and 13). As a result, the
formation of the peroxo isomer is most favored when the core
is supported by the iPr3TACD ligand, in good agreement with
experiment. We note in passing that the TACD macrocycle
presents the same conformation in all cases with no significant
differences in its bonding parameters.

4068 Inorganic Chemistry, Vol. 39, No. 18, 2000
Lam et al.
Figure 12. Illustration of the relationship between the sum of
N-Cu-N angles (Σ) and the distance between the N-substitutents in
the bis(µ-oxo)dicopper complexes.
lations with the unsubstituted TACN and TACD ligands sug-
gests that the greater flexibility of the larger macrocycle prefer-
entially stabilizes its bis(µ-oxo) isomer. However, consideration
of Figure 11 reveals that it is the lesser QM destabilization of
the TACN derivative by the isopropyl group N-substitution that
makes the bis(µ-oxo) form more stable with this macrocycle.
Hence, the QM destabilization of the bis(µ-oxo) form that is
introduced by the isopropyl groups in iPr3TACN is calculated
to be 5.9 kcal/mol, compared to 11.2 kcal/mol for the iPr3TACD
ligand. What is the structural basis for this difference? One
divergence between the bis(µ-oxo) topologies for the two ligand
systems is the torsional angle, which for iPr3TACN is τ′ ) 180°
(an anti conformation) versus τ′ ) 56° for its ring-expanded
counterpart (Table 7). However, it would be expected that closer
interligand substituent contacts between groups in the equatorial
positions and correspondingly greater energetic destabilization
would attend the 180° angle for the iPr3TACN case, arguing
against this feature being responsible for the observed energetic
discrepancy. Instead, we see that the angle sum Σ is significantly
smaller for TACN than for TACD at a similar O-O distance,
regardless of the nature of the substituent. In other words, TACN
is a tridentate ligand with a smaller cone angle, hence the
nitrogen donor atoms of each macrocyclic ligand and their
substituents are farther away from each other (Figure 12) as
illustrated by longer N‚‚‚N and C‚‚‚C contacts at the same O-O
distance (Table 7). The steric effects of the iPr groups are thus
magnified in TACD relative to TACN, with the result being a
greater destabilization of the more congested, smaller bis(µ-
oxo) core for the ligand with the larger macrocyclic backbone.
These effects are corroborated by the X-ray structural data
obtained for the Cu(I) precursor complexes (vide supra).
Figure 11. Calculated relative energy (IMOMM, kcal/mol) for the
[(R3TACX)Cu(O2)Cu(R3TACX)] complexes as a function of the O-O
distance (∆EQM ) squares, ∆EMM ) triangles, ∆Etot ) circles), together
with the calculated ∆EQM values for the corresponding unsubstituted
macrocycles (crosses). The energy at the shortest O-O distance is taken
as the zero for each curve.
between the long and short Cu-N distances. We infer that these
structural distortions (large τ′ and disparate Σ values) underlie
the large calculated QM destabilization of this bis(µ-oxo) isomer
relative to those capped by less sterically encumbered R3TACD
ligands, thus explaining the experimental observation of only
the peroxo form for the iPr3TACD system.
Summary and Conclusions
Through a combination of experimental and theoretical
studies, we have obtained new insights into the influences of
ligand structure on the relative stability of the isomeric (µ-η2:
η2-peroxo)- and bis(µ-oxo)dicopper cores. We synthesized and
fully characterized a set of Cu(I) complexes of N-substituted
macrocyles with 12- and 10-membered rings, iPr3TACDD and
R3TACD (R ) Me, Bn, and iPr), respectively, and we compared
their O2 reactivity with that of previously reported systems
Comparison of the results of IMOMM calculations for the
systems supported by iPr3TACN and iPr3TACD enables evalu-
ation of the experimental findings, namely, the formation of
both peroxo and bis(µ-oxo) isomers for the former and only
the peroxo form for the latter. These differences in Cu/O2 reac-
tivity are particularly interesting because the systems only differ
by virtue of one methylene unit in their capping ligand back-
bone. As described above, consideration of the results of calcu-

Ligand Macrocycle Effects on Copper-Dioxygen Reactivity
Inorganic Chemistry, Vol. 39, No. 18, 2000 4069
supported by the 9-membered-ring congeners, R3TACN. The
lack of O2 reactivity for the iPr3TACDD system may be
rationalized by considering the structure of its Cu(I) complex,
which due to its trigonal geometry has a prohibitive redox
potential. For the systems capped by Me3TACD and Bn3TACD,
clean formation of bis(µ-oxo) complexes was observed, whereas
sole generation of a peroxo complex was seen when iPr3TACD
was used. This peroxo complex was characterized by X-ray
crystallography, which revealed interesting distortions of the
metal coordination spheres that include a relative orientation
(τ′ ) 64°) of the pseudoaxial N-donors intermediate between
syn (τ′ ) 0°) and anti (τ′ ) 180°). Sole production of the peroxo
form upon oxygenation of the Cu(I) complex of iPr3TACD
contrasts with the behavior found previously for the iPr3TACN-
supported system, where mixtures of rapidly interconverting
peroxo and bis(µ-oxo) isomers were found whose ratios
depended on solvent and temperature.
withstanding this issue, the comparative calculations presented
here do allow the relative influences of key aspects of ligand
structure on the peroxo/bis(µ-oxo) ratio to be understood.
Experimental Section
General Procedures. All reagents and solvents were obtained from
commercial sources and were used as received unless stated otherwise.
Solvents were dried according to published procedures24 and distilled
under N2 prior to use. Air-sensitive reactions were performed either in
a Vacuum Atmospheres inert-atmosphere glovebox or by standard
Schlenk and vacuum line techniques. The starting materials [Cu(CH3-
CN)4]X (X ) ClO4-, SbF6-),25 1,4,7-triazacyclodecane (TACD),10 1,5,9-
triazacyclododecane (TACDD),10 1,4,7-trimethyl-1,4,7-triazacyclode-
cane (Me3TACD),11 and 1,4,7-triisopropyl-1,4,7-triazacyclononane12
were synthesized using published procedures. Elemental analyses were
performed by Atlantic Microlabs of Norcross, GA. Resonance Raman
spectra were collected on a Spex 1403 spectrometer interfaced with a
DM3000 data collection system or on an Acton 506 spectrometer using
a Princeton Instruments LN/CCD-1100-PB/UVAR detector and ST-
1385 controller interfaced with Winspec software. A Spectra-Physics
2030-15 argon ion laser at a power of roughly 40 mW was employed
to give the excitation wavelength at 514.5 or 457.9 nm. The spectra
were obtained at 77 K using a 135° backscattering geometry; samples
were frozen onto a gold-plated copper coldfinger in thermal contact
with a dewar containing liquid nitrogen. Raman shifts were referenced
In order to rationalize this important disparity in O2 reactivity
that resulted from a relatively small ligand structural difference
(a single methylene unit in the macrocycle), we analyzed the
X-ray structures of the Cu(I) precursors and performed com-
parative theoretical studies using combined quantum mechanics/
molecular mechanics methods. Although the calculations did
not necessarily yield good quantitative estimates of the relative
energy of the oxo and peroxo isomers (vide supra), they
provided an excellent indication of how that relative energy is
influenced by ligand reorganization, by the macrocyclic nature
of the ligand, and by the steric effects of the ligand substituents.
The calculations showed that an important rearrangement of the
N-donor orientation of the TACN and TACD ligands relative
to the Cu2O2 core occurs as the peroxo converts to the bis(µ-
oxo) form, as previously found with monodentate NH3 model
ligands.9d While the increased flexibility of the TACD ligand
results in a somewhat more stable bis(µ-oxo) isomer than for
the TACN case, the destabilization of the bis(µ-oxo) relative
to the peroxo isomer by iPr group N-substitution is greater for
TACD than TACN due to cone angle differences. A large part
of the destabilization of the bis(µ-oxo) form is not purely steric,
but instead results from distortion of the central [N3CuO2-
CuN3]2+ core and rotation of the two Cu(macrocycle) units (0°
< τ′ < 180°) that may be traced to avoidance of close
interligand contacts between the iPr groups. The IMOMM
computational results for [(iPr3TACD)CuO2Cu(iPr3TACD)]2+ are
in excellent agreement with the main structural features deduced
from the X-ray diffraction data. In addition, the conclusions
concerning the divergent steric effects of the iPr groups on the
different macrocycles are corroborated by the X-ray structural
data for the Cu(I) precursors, which show clear differences in
steric accessibility due to shifted orientations of the N-donor
substituents on the TACN and TACD macrocycles.
to the intense solvent features at 702 (CH2Cl2) or 797 (acetone) cm-1
.
Other instrumentation and conditions for physicochemical characteriza-
tion of new compounds were used as described previously.15
1,4,7-Triisopropyl-1,4,7-triazacyclodecane (iPr3TACD). The re-
agent 2-bromopropane (11.0 g, 0.0894 mol) was added to a solution
of TACD (2.70 g, 0.0189 mol) in toluene (30 mL). The colorless
reaction mixture was allowed to reflux for 2 h and over time became
light yellow. Powdered KOH (∼10 g) was added to the yellow solution,
and the mixture was refluxed overnight. The solution was then cooled
to room temperature and filtered, and the residue was washed with
toluene (30 mL). The combined filtrates were dried, and the solvent
was removed under reduced pressure, to yield a light yellow oil, which
was distilled (∼85 °C, 0.25 Torr), to give the product as a clear oil
1
(2.11 g, 42%). H NMR (CDCl3, 300 MHz): δ 0.91 (d, J ) 6.6 Hz,
18H), 1.38 (q, J ) 5.7 Hz, 2H), 2.40-2.43 (m, 4H), 2.59-2.62 (m,
4H), 2.67 (t, J ) 5.7 Hz, 4H), 2.82 (h, J ) 6.6 Hz, 3H) ppm. 13C{1H}
NMR (CDCl3, 75 MHz): δ 50.6, 50.4, 49.5, 49.3, 46.9, 27.9, 18.5,
17.9 ppm. Anal. Calcd for C16H35N3: C, 71.31; H, 13.09; N, 15.60.
Found: C, 71.26; H 12.97, N; 15.49. The deuterated ligand d21-iPr3-
TACD was prepared analogously to iPr3TACD using d7-2-bromopro-
pane. 1H NMR (CDCl3, 300 MHz): δ 1.40 (q, J ) 5.7 Hz, 2H), 2.40-
2.48 (m, 4H), 2.60-2.72 (m, 8H) ppm.
1,4,7-Tribenzyl-1,4,7-triazacyclodecane (Bn3TACD). Benzyl chlo-
ride (1.32 g, 0.0104 mol) was added to a solution of TACD (0.372 g,
2.60 mmol) in toluene (15 mL). The light yellow solution was stirred
for 2 h. Powdered KOH (∼4 g) was added to the solution, and the
mixture was refluxed overnight. The solution was then cooled to room
temperature and filtered, and the residue was washed with toluene (20
mL). The combined filtrates were dried, and the solvent was removed
under reduced pressure, to yield a light yellow oil (0.955 g, 88%). The
1
ligand was used without further purification. H NMR (CDCl3, 300
Finally, we recognize that the present computations predict
the peroxo isomer to be more stable than the bis(µ-oxo) one in
all cases, including those supported by Me3TACX (X ) D or
N) that in reality yield bis(µ-oxo) compounds only. This
consistent overestimation of the energy of the bis(µ-oxo) form
may result from a lack of inclusion of solvent effects; we recall
that for the model compound [(NH3)3CuO2Cu(NH3)3]2+ Cramer
et al. found6,9a that application of a simple solvent dielectric
stabilizes a bis(µ-oxo) isomer by 5.6-8.4 kcal/mol relative to
the peroxo. Experimental studies of the iPr3TACN system have
illustrated the importance of solvent, temperature, and counterion
effects and suggest that quantitative assessment via theory of
the stabilities of the isomeric Cu2O2 cores will necessitate
inclusion of entropic and/or “second-sphere” influences.4 Not-
MHz): δ 1.60-1.72 (m, 2H), 2.42-2.65 (m, 6H), 2.69-2.91 (m, 6H),
3.23 (s, 2H), 3.55 (s, 4H), 7.05-7.22 (m, 15H) ppm.
1,5,9-Triisopropyl-1,5,9-triazacyclododecane (iPr3TACDD). This
ligand was prepared analogously to iPr3TACD but with TACDD as
the starting material. The resulting light yellow oil was distilled (∼120
1
°C, 0.25 Torr), to give the product as a colorless oil (55%). H NMR
(CDCl3, 300 MHz): δ 0.91 (d, J ) 6.6 Hz, 18H), 1.47 (q, J ) 6.3 Hz,
6H), 2.40 (t, J ) 6.3 Hz, 12H), 2.83 (h, J ) 6.6 Hz, 3H) ppm. 13C{1H}
NMR (CDCl3, 75 MHz): δ 48.5, 44.7, 21.9, 18.5 ppm. Anal. Calcd
for C18H39N3: C, 72.65; H, 13.22; N, 14.13. Found: C, 72.74; H, 13.32;
N, 14.00.
(24) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory
Chemicals; Pergamon Press: New York, 1988.
(25) Kubas, G. J. Inorg. Synth. 1979, 19, 90-92; 1990, 28, 68.

4070 Inorganic Chemistry, Vol. 39, No. 18, 2000
Lam et al.
[(iPr3TACD)Cu(CH3CN)]SbF6. In the glovebox, solid [Cu(CH3-
CN)4]SbF6 (0.172 g, 0.371 mmol) was added to a stirred solution of
iPr3TACD (0.100 g, 0.372 mmol) in CH3CN (2 mL). The solution was
stirred for 30 min before being added to Et2O (20 mL), which resulted
in the deposition of the product as a white powder (0.181 g, 80%). 1H
NMR (CD3CN, 500 MHz): δ 1.19 (d, J ) 6.5 Hz, 6H), 1.20 (d, J )
6.5 Hz, 6H), 1.21 (d, J ) 6.5 Hz, 6H), 1.58-1.70 (m, 1H), 1.88-2.00
(m, 4H), 2.50-2.64 (m, 6H), 2.76-2.98 (m, 6H), 3.0 (h, J ) 6.5 Hz,
2H), 3.1 (h, J ) 6.5 Hz, 1H) ppm. 13C{1H} NMR (CD2Cl2, 75 MHz):
δ 58.6, 55.2, 51.9, 50.1, 28.4, 18.9, 18.6, 18.3, 2.7 ppm (observed 9
out of 11 expected signals). FTIR (KBr, cm-1): 2980, 2911, 2883,
2853, 2267 (CtN), 1495, 1472, 1461, 1376, 1270, 1183, 1161, 1147,
1123, 1071, 1032, 955, 937, 738, 658 (SbF6-). FAB-MS (MNBA):
m/e (relative intensity) 373 ([M - SbF6]+, 3), 332 ([M - CH3CN -
SbF6]+, 100). Anal. Calcd for CuC18H38N4SbF6: C, 35.45; H, 6.28; N,
9.19. Found: C, 35.65; H, 6.48; N, 9.14. [(iPr3TACD)Cu(CH3CN)]-
ClO4 was prepared analogously to [(iPr3TACD)Cu(CH3CN)]SbF6,
X-ray-quality crystals were obtained with the similarly prepared
perchlorate salt by using CH2Cl2/Et2O (vapor diffusion).
[(iPr3TACD)Cu(CO)]SbF6. Solid [(iPr3TACD)Cu(CH3CN)]SbF6
(0.063 g, 0.103 mmol) was dissolved in acetone (5 mL) in a Schlenk
flask. Carbon monoxide was bubbled through the light yellow solution
for 5 min. After the solution was allowed to equilibrate for an additional
10 min, the solvent was removed under reduced pressure, yielding a
white powder. The product was reprecipitated from acetone/Et2O (1:
1
20) to give a white powder (0.053 g, 85%). H NMR (CD2Cl2, 300
MHz): δ 1.27 (d, J ) 6.6 Hz, 6H), 1.30 (d, J ) 6.6 Hz, 6H), 1.33 (d,
J ) 6.6 Hz, 6H), 1.73-1.86 (m, 1H), 1.98-2.14 (m, 1H), 2.64-2.82
(m, 6H), 2.92-3.08 (m, 6H), 3.17 (h, J ) 6.6 Hz, 2H), 3.23 (h, J )
6.6 Hz, 2H) ppm. 13C{1H} NMR (CD2Cl2, 75 MHz): δ 60.4, 56.8,
53.1, 52.9, 51.0, 27.2, 20.2, 19.3, 18.2 ppm (observed 9 out of 10
expected signals). FTIR (KBr, cm-1): 2980, 2939, 2879, 2063 (Ct
O), 1494, 1465, 1393, 1377, 1349, 1214, 1178, 1142, 1063, 1035, 915,
864, 746, 707, 657 (SbF6). Anal. Calcd for CuN3C17H35OSbF6: C,
34.21; H, 5.88; N, 7.04. Found: C, 33.64; H, 5.77; N, 7.12.
1
except by using [Cu(CH3CN)4]ClO4 (81%). H NMR (CD3CN, 300
MHz): δ 1.18 (d, J ) 6.6 Hz, 6H), 1.19 (d, J ) 6.6 Hz, 6H), 1.22 (d,
J ) 6.6 Hz, 6H), 1.58-1.70 (m, 1H), 1.88-2.00 (m, 4H), 2.50-2.64
(m, 6H), 2.76-2.98 (m, 6H), 3.0 (h, J ) 3.9 Hz, 2H), 3.1 (h, J ) 3.9
Hz, 1H) ppm.
[(Me3TACD)Cu(CO)]SbF6. This compound was prepared analo-
gously to [(iPr3TACD)Cu(CO)]SbF6, except by using [(Me3TACD)-
Cu(CH3CN)]SbF6 (84%). 1H NMR (CD2Cl2, 500 MHz): δ 1.78-1.82
(m, 1H), 2.07-2.16 (m, 1H), 2.46-2.52 (m, 2H), 2.75 (s, 6H), 2.80-
2.85 (m, 4H), 2.88 (s, 3H), 2.98-3.06 (m, 6H) ppm. 13C{1H} NMR
(CD2Cl2, 75 MHz): δ 58.9, 57.0, 55.1, 50.8, 50.7, 24.3 ppm (observed
6 out of 7 expected signals). FTIR (KBr, cm-1): 2949, 2880, 2086
(CtO), 1474, 1365, 1262, 1106, 1065, 1032, 762, 658 (SbF6). Anal.
Calcd for CuN3C11H23OSbF6: C, 25.77; H, 4.52; N, 8.20. Found: C,
25.96; H, 4.52; N, 8.13.
[(Bn3TACD)Cu(CO)]SbF6. This compound was prepared analo-
gously to [(iPr3TACD)Cu(CO)]SbF6, except by using [(Bn3TACD)-
Cu(CH3CN)]SbF6 (91%). 1H NMR (CD2Cl2, 500 MHz): δ 1.74-1.88
(m, 1H), 1.96-2.18 (m, 1H), 2.62-2.76 (m, 4H), 2.76-2.96 (m, 4H),
2.08-3.32 (m, 4H), 3.83 (s, 2H), 2.86 (s, 4H), 7.12-7.24 (m, 4H),
7.32-7.48 (m, 8H), 7.48-7.54 (m, 3H) ppm. 13C{1H} NMR (CD2Cl2,
75 MHz): δ 134.9, 132.29, 131.6, 131.5, 129.3, 129.1, 128.9, 128.7,
65.8, 64.9, 56.1, 55.4, 49.8, 23.5 (observed 14 out of 15 expected
signals). FTIR (KBr, cm-1): 3061, 3032, 2931, 2872, 2358, 2074 (Ct
O), 1499, 1456, 1348, 1204, 1072, 1038, 975, 916, 762, 743, 707, 657
(SbF6). Anal. Calcd for CuN3C29H35OSbF6: C, 47.01; H, 4.76; N, 5.67.
Found: C, 47.75; H, 4.94; N, 5.82.
[(iPr3TACN)Cu(CH3CN)]BPh4. A solution of CuCl (0.20 g, 2.00
mmol) in CH3CN (5 mL) was treated with a solution of iPr3TACN
(0.52 g, 2.04 mmol) in THF (3 mL). After stirring the colorless solution
for 30 min, a solution of NaBPh4 (0.68 g, 1.99 mmol) in CH3CN (5
mL) was added, causing the precipitation of NaCl. After stirring for
an additional 30 min the mixture was filtered through a pad of Celite
and the filtrate treated with Et2O (20 mL). Storage at -20 °C caused
the deposition of colorless plate crystals of the product (1.09 g, 80%).
1H NMR (500 MHz, CD2Cl2): δ 7.34-7.32 (m, 8H), 7.04 (t, J ) 6.5
Hz, 8H), 6.89 (t, J ) 7.5 Hz, 4H), 3.05 (heptet, J ) 6.5 Hz, 3H),
2.79-2.72 (m, 6H), 2.48-2.42 (m, 6H), 1.94 (s, 3H), 1.20 (d, J ) 6.5
Hz, 18H) ppm. 13C{1H} NMR (125 MHz, CD2Cl2): δ 164.6, 135.4,
126.2, 126.1, 122.2, 58.6, 51.2, 20.0, 3.2 ppm. FTIR (KBr, cm-1): 2265
(CtN), 734 (BPh4-), 704 (BPh4-). Anal. Calcd for C41H56N4BCu: C,
72.50; H, 8.31; N, 8.25. Found: C, 72.13; H, 8.33; N, 8.33.
Reactions of the Cu(I) Complexes with Dioxygen. Dry O2 was
bubbled through a precooled solution of [(iPr3TACD)Cu(CH3CN)]X
(X ) SbF6-, ClO4-, or BPh4-) in CH2Cl2, acetone, or THF, [(Me3-
TACD)Cu(CH3CN)]X (X ) SbF6-, ClO4-) in CH2Cl2 or THF, or [(Bn3-
TACD)Cu(CH3CN)]SbF6 in CH2Cl2, THF, or acetone. The time
required for full oxygenation, as indicated by maximization of the
absorbance bands associated with the product(s), differed, with [(iPr3-
TACD)Cu(CH3CN)]X requiring 1 h of vigorous O2 bubbling and [(Me3-
TACD)Cu(CH3CN)]X and [(Bn3TACD)Cu(CH3CN)]SbF6 requiring
only 5 min. Spectroscopic properties of the oxygenation products are
listed in Table 3. The extinction coefficients of the UV-vis features
were obtained from Beer’s law plots (concentration ranges for the cases
with iPr3TACD ) 0.16-0.60 mM; Me3TACD ) 0.04-0.17 mM; Bn3-
TACD ) 0.02-0.25 mM). Samples for resonance Raman spectroscopy
were prepared by first dissolving the Cu(I) complex in the desired
solvent (∼5 mM) in a Schlenk flask. After cooling to -80 °C, dry O2
[(d21-iPr3TACD)Cu(CH3CN)]BPh4. In the glovebox, solid NaBPh4
(0.158 g, 0.462 mmol) was added to a stirred solution of d21-iPr3TACD
(0.149 g, 0.514 mmol) and CuCl (0.046 g, 0.465 mmol) in CH3CN (2
mL). After the cloudy white solution had been stirred for 30 min, it
was filtered through a Celite pad. Additional CH3CN (5 mL) was used
to wash the Celite pad, and the combined filtrates were brought to
dryness under reduced pressure, yielding a white powder. Recrystal-
lization with CH3CN/Et2O (1:20) gave white feathery crystals (0.317
g, 86%). 1H NMR (CD2Cl2, 300 MHz): δ 1.62-1.72 (m, 1H), 1.83-
1.97 (m, 4H), 2.47-2.58 (m, 6H), 2.73-2.94 (m, 6H), 6.91 (t, J ) 7.2
Hz, 4H), 7.06 (t, J ) 7.2 Hz, 8H), 7.32-7.38 (m, 8H) ppm. 13C{1H}
NMR (CD2Cl2, 75 MHz): δ 164.1, 135.9, 125.6, 121.7, 51.9, 50.1,
41.7, 29.9, 21.6, 17.9, 17.0, 2.3 ppm (observed 12 out of 15 expected
signals). FTIR (KBr, cm-1): 3054, 3037, 2983, 2927, 2853, 2259 (Ct
N), 2244, 1582, 1480, 1457, 1429, 1295, 1213, 1185, 1147, 1051, 845,
742, 729 (BPh4-), 702 (BPh4-), 604. FAB-MS (MNBA) m/e (relative
intensity) 394 ([M - BPh4]+, 2), 248 ([M - CH3CN - BPh4]+, 100).
[(Me3TACD)Cu(CH3CN)]SbF6. In the glovebox, solid [Cu(CH3-
CN)4]SbF6 (0.060 g, 0.11 mmol) was added to a stirred solution of
Me3TACD (0.030 g, 0.16 mmol) in CH3CN (2 mL). After the solution
had been stirred for 10 min, it was added to Et2O (20 mL), resulting in
1
the precipitation of the product as a white powder (0.053 g, 78%). H
NMR (CD2Cl2, 500 MHz): δ 1.68-1.76 (m, 1H), 1.94-2.04 (m, 1H),
2.20 (s, 3H), 2.40-2.46 (m, 2H), 2.58 (s, 6H), 2.64-2.74 (m, 7H),
2.74-2.90 (m, 6H) ppm. 13C{1H} NMR (CD2Cl2, 75 MHz): δ 59.0,
55.8, 55.3, 49.2, 48.9, 25.7, 2.6 ppm (observed 7 out of 8 expected
signals). FTIR (KBr, cm-1): 2971, 2866, 2813, 2362, 2342, 2266 (Ct
N), 1494, 1468, 1367, 1273, 1042, 768, 662 (SbF6-). FAB-MS
(MNBA): m/e (relative intensity) 289 ([M - SbF6]+, 10), 248 ([M -
CH3CN - SbF6]+, 100). X-ray-quality crystals were obtained with the
perchlorate salt, which was synthesized in a similar fashion.
[(Bn3TACD)Cu(CH3CN)]SbF6. This was prepared analogously to
[(iPr3TACD)Cu(CH3CN)]SbF6, except by using Bn3TACD (72%) in
CH2Cl2. The light yellow solid was further purified by recrystallization
from CH2Cl2/Et2O (1:20). 1H NMR (CD2Cl2, 300 MHz): δ 1.66-1.79
(m. 1H), 1.90-2.60 (m, 1H), 2.23 (s, 3H), 2.40-2.58 (m, 4H), 2.62-
2.88 (m, 6H), 3.06-3.20 (m, 2H), 3.67 (s, 2H), 3.78 (s, 4H), 7.24-
7.31 (m, 4H), 7.37-7.47 (m, 11H) ppm. 13C{1H} NMR (CD2Cl2, 75
MHz): δ 135.5, 133.5, 131.4, 131.1, 128.6, 128.4, 128.3, 65.1, 63.8,
56.3, 54.6, 49.7, 24.7, 2.87 ppm (observed 14 out of 16 expected
signals). Anal. Calcd for CuC30H38N4SbF6: C, 47.79; H, 5.08; N, 7.43.
Found: C, 48.34; H, 5.18; N, 6.99.
[(iPr3TACDD)Cu]SbF6. This was prepared analogously to [(iPr3-
TACD)Cu(CH3CN)]SbF6, except by using iPr3TACDD in CH2Cl2
1
(74%). H NMR (CD2Cl2, 300 MHz): δ 1.21 (d, J ) 6.6 Hz, 18H),
1.76-1.98 (m, 6H), 2.74 (qd, J ) 2.4, 7.2 Hz, 6H), 2.94 (qd, J ) 2.4,
8.1 Hz, 6H), 3.20 (h, J ) 6.6 Hz, 3H) ppm. 13C{1H} NMR (CDCl3, 75
MHz): δ 56.1, 56.0, 25.8, 18.6 ppm. Anal. Calcd for C18H39CuF6N3-
Sb: C, 36.22; H, 6.59; N, 7.04. Found: C, 36.47; H, 6.61; N, 7.09.

Ligand Macrocycle Effects on Copper-Dioxygen Reactivity
Inorganic Chemistry, Vol. 39, No. 18, 2000 4071
was bubbled through the solution and a drop of the resulting colored
solution was then quickly transferred via cold pipet onto a gold-plated
copper coldfinger cooled to liquid N2 temperature.
halves (including most of the iPr3TACD ligand) are pseudosymmetri-
cally related. The residuals for Pn are better than those for P21/n by
∼3%, and the Hamilton test selects Pn as the better space group at the
99% confidence level. Still, many restraints were imposed to keep this
fully anisotropically refined structure positive definite. The parts related
by the pseudosymmetry were made to have similar σ and â bond
distances by using the SAME restraint. Some groups had unreasonable
thermal ellipsoids so DELU and SIMU restraints were applied where
found necessary. Although the overall topology appears to be correct,
the degree of precision implied by the small esds and low residuals
should be viewed with skepticism, and we therefore refrain from
analyzing interatomic distances and angles in detail.
(e) [(iPr3TACD)2Cu2(CO3)](BPh4)2. Green prisms of this complex
cocrystallized with the (µ-η2:η2-peroxo)dicopper complex mentioned
above. Orientation matrices were determined from 115 reflections, and
the final cell constants were calculated from a set of 8192 strong
reflections. Four solvent molecules of dichloromethane and 1.5 of
acetone were located along with the two tetraphenylborates.
(f) [(Me3TACD)2Cu2(OH)2](ClO4)2. The oxygenation product [(Me3-
TACD)2Cu2(O)2](ClO4)2 in CH2Cl2 was warmed to room temperature.
Slow evaporation of the resulting green solution at room temperature
led to deposition of a few green crystals, which were mounted for data
collection. Orientation matrices were determined from 40 reflections,
and final cell constants were calculated from a set of 3699 strong
reflections.
X-ray Crystallography. Crystals of [(iPr3TACD)Cu(CH3CN)]SbF6,
[(Me3TACD)Cu(CH3CN)]ClO4, [(iPr3TACDD)Cu]ClO4, [(iPr3TACD)2-
Cu2(O2)](BPh4)2, [(iPr3TACD)2Cu2(CO3)](BPh4)2, and [(Me3TACD)2Cu2-
(OH)2](ClO4)2 were attached to a glass fiber with epoxy and mounted
on the Siemens SMART system for data collection. Initial sets of cell
constants were calculated from reflections harvested from three sets of
20 frames oriented such that orthogonal wedges of reciprocal space
were surveyed. Space groups were determined based on systematic
absences and intensity statistics, and successful direct-methods solutions
were calculated, which provided most non-hydrogen atoms. Several
full-matrix least-squares/difference Fourier cycles were performed
which located the remainder of the non-hydrogen atoms. All non-
hydrogen atoms were refined with anisotropic displacement parameters
unless stated otherwise. All hydrogen atoms were placed in ideal
positions and refined as riding atoms with relative isotropic displacement
parameters. The technique of hemisphere collection was used in which
a randomly oriented region of reciprocal space was surveyed to the
extent of 1.3 hemispheres to a resolution of 0.84 Å. Three major swaths
of frames were collected with 0.30° steps in ω. The collection and
refinement data are summarized in Table 2, and selected bond distances
and angles are provided in the captions to Figures 1-4, 7, and 8.
Complete descriptions of all structures, including full tables of positional
parameters, interatomic distances and angles, and thermal parameters,
are included in the Supporting Information. Crystallization methods
and some unique attributes of each structure determination are as
follows:
(a) [(iPr3TACD)Cu(CH3CN)]SbF6. X-ray-quality crystals were
obtained by slow diffusion of Et2O into a solution of the complex in
CH3CN. The data were collected at 293(2) K due to an apparent phase
change (as evidenced by observed shattering of the crystals) at low
temperature. Orientation matrices were determined from 189 reflections,
and the final cell constants were calculated from a set of 7194 strong
reflections. The anisotropic displacement parameters of the iPr carbon
atoms and those of the propyl group linking N(1) and N(2) were
restrained to approximate rigid body motion.
(b) [(Me3TACD)Cu(CH3CN)]ClO4. X-ray-quality crystals were
obtained by slow diffusion of Et2O into a solution of the complex in
CH3CN. The crystal was found to be rotationally twinned (0.87: 0.13);
the structure is reported for the major twin only (twin law by rows 1
0 0, 0 -1 0, -1/3 0 -1).
(c) [(iPr3TACDD)Cu]ClO4. X-ray-quality crystals were obtained
by slow diffusion of Et2O into a solution of the complex in CH2Cl2.
(d) [(d21-iPr3TACD)2Cu2(µ-η2:η2-O2)](BPh4)2. After numerous
attempts to grow crystals of this complex using combinations of solvents
(acetone, CH2Cl2, THF, 2-butanone, CH3OH, and benzene) and
counterions (ClO4-, SbF6-, and BPh4-), the following procedure was
found to be successful. Solid [d21-iPr3TACDCu(CH3CN)]BPh4 (∼0.14
g) was dissolved in CH2Cl2 (1 mL) and transferred into a crystallization
tube. The solution of the Cu(I) complex was cooled to -80 °C, and
O2 was bubbled through the solution for 40 min. A precooled solution
(1 mL) of acetone/2-butanone (1:1 v/v) was then carefully layered on
top of the purple-red CH2Cl2 solution, and the resulting mixture was
stored untouched at -80 °C for 6-10 days, leading to the deposition
of purple-red rectangular-shaped crystals (sometimes mixed with green
crystals, identified as the carbonate compound described in (e) below)
that were carefully mounted so as to ensure no warming (which induced
decomposition as indicated by conversion to green material). Orientation
matrices were determined from 334 reflections, and the final cell
constants were calculated from a set of 7729 strong reflections. All
solvent in the structure was disordered, so PLATON/SQUEEZE26 was
used to remove the effect of the disordered solvent from the data in
order to refine the remainder of the structure as accurately as possible.
The solution was possible in either Pn or P21/n; in the former a pair
of iPr groups are ordered, but in the latter they must be disordered.
This is the only significant obstacle to this structure being centrosym-
metric. As a consequence, the two BPh4- anions and the monocopper
Computational Details. Calculations on [(L)2Cu2(O2)]2+ (L )
(NH3)3, TACN, and TACD) were performed with Gaussian94.27
Calculations on the real systems [(L)2Cu2(O2)]2+ (L ) Me3TACD or
iPr3TACD) were performed with the IMMOM program28 built from
modified versions of Gaussian92/DFT29 and MM3(92).30 In all cases,
quantum mechanical calculations were carried out at the B3LYP level31
with basis sets LANL2DZ for Cu,32 6-31G(d) for O,33 and 6-31G for
H, C, and N.34 The MM3(92) force field34 was used in the molecular
mechanics part of the calculations. The van der Waals parameters for
Cu were taken from the UFF force field35 and torsional contributions
involving dihedral angles with the metal atom in the terminal position
were set to zero. All geometrical parameters were optimized except
the bond distances connecting the QM and MM regions, which were
kept constant: N-H (1.019 Å) and N-C (1.438 Å) in the QM and
MM parts, respectively.
Acknowledgment. Financial support for this research was
provided by the NIH (GM47365 to W.B.T., F32-GM19374 to
J.R.H.), the NSF (National Young Investigator Award to
(27) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson,
B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T. A.; Petersson, G. A.;
Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski,
V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.;
Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen,
W.; Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.;
Martin, R. L.; Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J. P.;
Stewart, J. P.; Head-Gordon, M.; Gonzalez, C.; Pople, J. A. Gaussian
94, revision C.3; Gaussian, Inc.: Pittsburgh, PA, 1995.
(28) Maseras, F.; Morokuma, K. J. Comput. Chem. 1995, 16, 1170.
(29) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson,
B. G.; Wong, M. W.; Foresman, J. B.; Robb, M. A.; Head-Gordon,
M.; Replogle, E. S.; Gomperts, R.; Andres, J. L.; Raghavachari, K.;
Binkley, J. S.; Gonzalez, C.; Martin, R. L.; Fox, D. J.; Defrees, D. J.;
Baker, J. P.; Stewart, J. P.; Pople, J. A. Gaussian 92/DFT: Gaussian,
Inc.; Pittsburgh, PA, 1993.
(30) Allinger, N. L., MM3(92); QCPE: Bloomington, IN, 1992.
(31) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang,
W.; Parr, R. G. Phys. ReV. B 1988, 37, 785. (c) Stephens, P. J.; Devlin,
F. J.; Chabalowski, C. F.; Frisch, M. J. J. Phys. Chem. 1994, 98, 11623.
(32) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299.
(33) (a) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56,
2257. (b) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28,
213.
(34) (a) Allinger, N. L.; Yuh, Y. H.; Lii, J. H. J. Am. Chem. Soc. 1989,
111, 8551. (b) Lii, J. H.; Allinger, N. L. J. Am. Chem. Soc. 1989,
111, 8566. (c) Lii, J. H.; Allinger, N. L. J. Am. Chem. Soc. 1989,
111, 8576.
(35) Rappe´, A. K.; Casewit, C. J.; Cowell, K. S.; Goddard, W. A., III.;
Skiff, W. M. J. Am. Chem. Soc. 1992, 114, 10024.
(26) Spek, A. L. Acta Crystallogr. 1990, A46, C34.

4072 Inorganic Chemistry, Vol. 39, No. 18, 2000
Lam et al.
Supporting Information Available: 1H-1H COSY spectrum of
[(iPr3TACD)Cu(CH3CN)]+ (Figure S1), structural comparisons and
ligand conformational analyses of Cu(I) complexes (Figures S2 and
S3), cyclic voltammograms (Figure S4), spectroscopic and kinetic data
for oxygenated complexes (Figures S5-S8), plot of calculated Cu-N
distances as a function of the orientation of the Cu-N bond in TACD-
supported Cu2O2 compounds (Figure S9), and full X-ray crystal-
lographic data including tables of bond lengths and angles and thermal
parameters. This material is available free of charge via the Internet at
http://pubs.acs.org.
W.B.T.), the Camille and Henry Dreyfus and Alfred P. Sloan
Foundations (fellowships to W.B.T.), and the Direccio´n General
de Ensen˜anza Superior e Investigacio´n Cient´ıfica (DGESIC,
Grants PB98-1166-C02-01 and PB98-0916-CO2-01 to S.A. and
A.L.). Allocation of computer time at the Centre de Supercom-
putacio´ de Catalunya (CESCA) was in part funded through
grants from the Fundacio´ Catalana per a la Recerca (FCR),
Comissio´ Interdepartamental de Recerca i Innovacio´ Tecno-
lo`gica (CIRIT), and the Universitat de Barcelona. The authors
thank F. Maseras for invaluable technical advice on IMOMM
calculations.
IC000248P