A bis(μ-oxo)dicopper(III) complex with aromatic nitrogen donors: Structural characterization and reversible conversion between copper(I) and bis(μ-oxo)dicopper(III) species

Full text of DOI:10.1021/ja992680f

2124
J. Am. Chem. Soc. 2000, 122, 2124-2125
A Bis(µ-oxo)dicopper(III) Complex with Aromatic
Nitrogen Donors: Structural Characterization and
Reversible Conversion between Copper(I) and
Bis(µ-oxo)dicopper(III) Species
Hideki Hayashi,1a Shuhei Fujinami,1a Shigenori Nagatomo,1b
Seiji Ogo,1b Masatatsu Suzuki,*,1a Akira Uehara,1a
Yoshihito Watanabe,1b and Teizo Kitagawa1b
Department of Chemistry, Faculty of Science
Kanazawa UniVersity, Kakuma-machi
Kanazawa, Ishikawa 920-1192, Japan
Institute for Molecular Science
Myodaiji, Okazaki 444-8585, Japan
ReceiVed July 29, 1999
ReVised Manuscript ReceiVed December 16, 1999
Transformation of (µ-η2:η2-peroxo)dicopper(II) complexes
bearing sterically bulky tridentate N,N′,N′′-trisubstituted tacn2 to
square pyramidal bis(µ-oxo)dicopper(III) complexes has been
reported by Tolman et al.3 In certain instances, they have observed
a monooxygenase activity of the bis(µ-oxo)dicopper(III) com-
plexes for the coordinated ligand as substrate. A different type
of square planar bis(µ-oxo)dicopper(III) complexes having per-
alkylated-1,2-cyclohexanediamine ligands have been also prepared
by Stack et al.4 Very recently, partial formation of a bis(µ-oxo)-
dicopper(III) complex with a tridentate ligand containing two
pyridyl sidearms5 and a bis(µ-oxo)dicopper(III) complex with a
bidentate ligand containing a pyridyl group have been reported.6
However, there is no crystallographically characterized bis(µ-oxo)-
dicopper(III) complex having aromatic nitrogen donors. Thus, it
is important to explore how the nature of the donor atoms and
the stereochemistry of supporting ligands influence the formation,
structure, and reactivity of bis(µ-oxo)dicopper(III) complexes.
Karlin et al. have demonstrated that a copper(I) complex having
a tetradentate tripodal tpa ligand, [Cu(tpa)(NCCH3)]+, reacts with
O2 to form a trans-(µ-1,2-peroxo)dicopper(II) complex ([Cu2(O2)-
Figure 1. ORTEP view (50% probability) of the complex cation of
[Cu2(µ-O)2(Me2-tpa)2](PF6)2‚2(CH3)2CO (1b). Hydrogen atoms are omit-
ted for clarity. Selected bond distances (Å) and angles (deg): Cu-O1,
1.806(9); Cu-O1*, 1.799(8); Cu-N1, 1.97(1); Cu-N2, 1.91(1); Cu-
N3, 2.48(1); Cu-N4, 2.55(1); O1‚‚‚O1*, 2.32(1); Cu‚‚‚Cu*, 2.758(4);
Cu-O1-Cu*, 99.8(4); O1-Cu-O1*, 80.2(4).
378 nm ( ∼22 000, 0.1 mM), 494 nm (330, 10 mM)).8 Thus,
introduction of two 6-methylpyridyl groups into the tpa ligand
prevents the formation of trans-(µ-1,2-peroxo)dicopper(II) species
in a trigonal bipyramidal structure, probably due to a steric
requirement of two 6-methylpyridyl groups. However, it is
difficult to presume the structure of 1b from its electronic spec-
trum, since the spectral feature of 1b is somewhat different from
those of (µ-η2:η2-peroxo)dicopper(II)9 and bis(µ-oxo)dicopper-
(III) complexes.3,4 Herein, we report a crystal structure of a brown
bis(µ-oxo)dicopper(III) complex, [Cu2(O)2(Me2-tpa)2](PF6)2‚
2(CH3)2CO (1b) and reversible conversion between 1a and 1b.
Complex 1a has a trigonal pyramidal structure with three
pyridyl groups in the trigonal plane and tertiary amine in the
apex.10 Reaction of 1a with O2 in acetone/MeOH (10:1) at -78
°C gave a brown solution, from which brown crystals suitable
for X-ray crystallography were obtained.11 Figure 1 shows a
crystal structure of the complex cation of 1b which consists of a
centrosymmetric Cu2(µ-O)2 core with the Me2-tpa nitrogens. Each
copper ion has a square planar structure composed of a N2O2
donor set with two 6-methyl-2-pyridylmethyl sidearms which
interact weakly with each copper ion in the axial positions (2.48(1)
and 2.55(1) Å). The average Cu-O (1.803 Å) and Cu‚‚‚Cu*
(2.758(4) Å) distances are substantially shorter than those of bis-
(µ-hydroxo)dicopper(II) complex, [Cu2(OH)2(Me2-tpa)2](ClO4)2
(1c)10 (1.942 Å and 2.9368(9) Å, respectively), and are comparable
to those of bis(µ-oxo)dicopper(III) complexes, [Cu2(O)2(Bn3-
tacn)2]2+ (3; 1.806 and 2.794 Å)3a,c and [Cu2(O)2(LME)2]2+ (4;
1.806 and 2.743 Å).4 The resonance Raman spectrum of 1b
measured in acetone ( ∼10 mM) at -80 °C with 488.0 nm laser
excitation showed an isotope-sensitive band at 590 cm-1 with
16O2 (564 cm-1 with 18O2) shown in Figure 2 (inset), characteristic
of those observed for the bis(µ-oxo)dicopper(III) complexes.3c,4
(tpa)2]2+) in a trigonal bipyramidal structure (λmax (ꢀ, M-1 cm-1
)
) ∼440 nm (4000), 525 nm (11500), and ∼590 nm (7600)).7
Previously we found that [Cu(Me-tpa)]+ in acetone at -70 °C
generates a trans-(µ-1,2-peroxo)dicopper(II) species, whereas the
reaction of [Cu(Me2-tpa)]+ (1a) with O2 (Cu:O2 ) 2:1) in acetone
at -70 °C does not form a trans-(µ-1,2-peroxo)dicopper(II)
species, but produces a brown species (1b, λmax (ꢀ, M-1 cm-1) )
* Correspondence author. Telephone: +81-76-264-5701. Fax: +81-76-
264-5742. E-mail: suzuki@cacheibm.s.kanazawa-u.ac.jp.
(1) (a) Kanazawa University. (b) Institute for Molecular Science.
(2) Abbreviations of ligands used: tpa ) tris(2-pyridylmethyl)amine; Me-
tpa ) bis(2-pyridylmethyl)(6-methyl-2-pyridylmethyl)amine; Me2-tpa ) bis-
(6-methyl-2-pyridylmethyl)(2-pyridylmethyl)amine; tacn ) 1,4,7-triazacyclo-
nonane; Bn3-tacn ) 1,4,7-tribenzyl-1,4,7-triazacyclononane; LME ) N,N′-
dimethyl-N,N′-diethyl-trans-(1R,2R)-cyclohexanediamine; HB(3,5-R2pz)3
hydrotris(3,5-dialkyl-1-pyrazolyl)borate.
)
(3) (a) 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. (b) Mahapatra, S.; Halfen, J. A.; Tolman, W. B. J. Am.
Chem. Soc. 1996, 118, 11575-11586. (c) 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, 11555-11574. (d)
Tolman, W. B. Acc. Chem. Res. 1997, 30, 227-237. (e) Mahapatra, S.; Young,
V. G., Jr.; Kaderli, S.; Zuberbu¨hler, A. D.; Tolman, W. B. Angew. Chem.,
Int. Ed. Engl. 1997, 36, 130-133.
(4) 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
(5) (a) 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. (b) 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.
(8) Uozumi, K; Hayashi, Y; Suzuki, M; Uehara, A. Chem. Lett. 1993, 963-
966.
(9) (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, 1277-1291; (b) Baldwin, M. J.; Root, D. E.; Pate, J. E.; Fujisawa,
K.; Kitajima, N.; Solomon, E. I. J. Am. Chem. Soc. 1992, 114, 10421-10431.
(10) Synthetic procedures, analytical data, and details for X-ray crystal-
lography for 1a-BPh4, 1b, and 1c are available as Supporting Information.
(11) Crystal data for 1b, [Cu2(O)2(Me2-tpa)2](PF6)2‚2(CH3)2CO, at -120
°C; monoclinic, P21/c (No. 14), a ) 11.426(5) Å, b ) 15.616(8) Å, c )
15.768(4) Å, â ) 107.05(2)°, V ) 2689(1) Å3, Z ) 2, R(Rw) ) 0.078 (0.107)
based on 1780 reflections (I > 3.00σ(I)) and 335 variable parameters.
(6) Holland, P. L.; Rodgers, K. R.; Tolman, W. B. Angew. Chem., Int. Ed.
1999, 38, 1139-1142.
(7) (a) Jacobson, R. R.; Tyekla´r, Z.; Farooq, A.; Karlin, K. D.; Liu. S;
Zubieta, J. J. Am. Chem. Soc. 1988, 110, 3690-3692. (d) Tyekla´r, Z.;
Jacobson, R. R.; Wei, N.; Murthy, N. N.; Zubieta, J.; Karlin, K. D. J. Am.
Chem. Soc. 1993, 115, 2677-2689.
10.1021/ja992680f CCC: $19.00 © 2000 American Chemical Society
Published on Web 02/18/2000

Communications to the Editor
J. Am. Chem. Soc., Vol. 122, No. 9, 2000 2125
Scheme 1. Reaction Pathways of 1a and 1b
to regenerate 1a under N2 atmosphere. Reaction of 1b with PPh3
in acetone at -78 °C under Ar atmosphere facilitated the O-O
bond formation, and evolved O2 in ∼88% yield based on a dimer
with production of [Cu(Me2-tpa)(PPh3)]+, and no measurable
1
OdPPh3 was detected, which were confirmed by GC and H
NMR measurements (Supporting Information). In the absence of
PPh3, about 20% of O2 evolution was observed in the same
conditions. There is a possibility that if (µ-η2:η2-peroxo)-
dicopper(II) species is present,16 reaction with PPh3 could release
O2 to give the corresponding copper(I) complex as observed for
[Cu2(O2)(HB(3,5-iPr2pz)3)2],9a although a (µ-η2:η2-peroxo)dicop-
per(II) species, [Cu2(O2)(N4)]2+, has been shown not to react with
PPh3 at low temperature.17 Complex 1b has a monooxygenase
activity for the coordinated ligand as substrate (Supporting
Information). Further study is in progress (Scheme 1).
In summary, the tetradentate Me2-tpa ligand having aromatic
nitrogen donors has been shown to produce the bis(µ-oxo)-
dicopper(III) species 1b, that can be reversibly converted to the
copper(I) species 1a at -80 °C in CH2Cl2 by bubbling of N2.
Thus, reactivity patterns for copper-dioxygen chemistry signifi-
cantly vary with ligand system. Me2-tpa has a unique ability to
stabilize both copper(I) and copper(III) oxidation states: it can
take not only a square planar structure having weak ligation from
the axial positions which can fit to the copper(III) oxidation state
but also a trigonal pyramidal structure suitable for the copper(I)
oxidation state.
Figure 2. Electronic spectra of 1a (a and c) and 1b (b and d) in CH2Cl2
at -80 °C measured by an optical fiber apparatus with the corrected light
path length of 0.315 cm (0.11 mM/Cu2). The spectra, a, b, c, and d show
the repetition of the reversible conversion cycle. See text for details
(Supporting Information). Inset: Resonance Raman spectra of 1b in
acetone at -80 °C ( ∼10 mM) with 488.0 nm laser excitation. The asterisk
bands are solvent bands.
There is no evidence for the presence of a (µ-η2:η2-peroxo)-
dicopper(II) species which is expected to exhibit the O-O
stretching vibration at 720-765 cm-1 and with an 18O2 shift of
Acknowledgment. Financial support of this research by the Ministry
of Education, Science, and a Culture Grant-in-Aid for Scientific Research
(Priority Area, Molecular Biometallics) to M.S., Y.W., and T.K. is
gratefully acknowledged.
40-50 cm-1 9,12,13
These data indicate that 1b is assigned as a
.
bis(µ-oxo)dicopper(III) complex, [Cu2(O)2(Me2-tpa)2]2+. Thus,
introduction of two 6-methyl group(s) into tpa has significant
influence on the reactivity patterns.
Supporting Information Available: Experimental details including
Figures S1-S2, Tables S1-S12 listing crystallographic experimental
details, final atomic coordinates, thermal parameters, and full bond
distances and angles for 1a-BPh4, 1b, and 1c, and Figures S3-S5
displaying fully labeled ORTEP drawings for 1a-BPh4, 1b, and 1c (PDF).
This material is available free of charge via the Internet at http://pubs.acs.org.
It has been shown that the electronic spectra of all of the bis-
(µ-oxo)dicopper(III) complexes exhibit two intense absorption
bands at 390-448 nm and 296-324 nm.3,4,6 The spectrum of 1b
in CH2Cl2 at -80 °C exhibits an intense absorption band at 378
nm (ꢀ ) ∼19 000 M-1 cm-1, 0.11 mM/Cu2) and a shoulder at
∼490 nm, very similar to that in acetone, and shows an additional
intense band at 258 nm (ꢀ ) ∼36 000 M-1 cm-1) with a low-
energy shoulder at ∼300 nm (ꢀ ) ∼12 000 M-1 cm-1) as shown
in Figure 2. The band at 378 nm in 1b is higher in energy than
those of the corresponding bands of the other bis(µ-oxo)-
dicopper(III) complexes, indicating that the coordination environ-
ment can modulate the transition energy of this band in a wide
range (378-448 nm).
It should be noted that a reversible conversion between 1a and
1b was observed in CH2Cl2 at -80 °C as exemplified in Figure
2; bubbling of N2 gas into a brown solution of 1b (spectrum b)
for ∼30 min regenerates 1a (spectrum c) with some decomposi-
tion.14 This cycle can be repeated several times. Thus, 1b is the
first example which exhibits the reversible conversion between
copper(I) and bis(µ-oxo)dicopper(III) species depending on the
dioxygen partial pressure, although a reversible conversion
between copper(I) species and (µ-peroxo)dicopper(II) species,7,15
and that between (µ-η2:η2-peroxo)dicopper(II) species and bis-
(µ-oxo)dicopper(III)3a by changing the solvent have been well
demonstrated. A possible O-O bond formation pathway in 1b
may involve a preequilibrium between bis(µ-oxo)dicopper(III) and
(µ-η2:η2-peroxo)dicopper(II) species, the latter of which appears
JA992680F
(12) Pidcock, E.; Obias, H. V.; Abe, M.; Liang, H.-C.; Karlin, K. D.;
Solomon, E. I. J. Am. Chem. Soc. 1999, 121, 1299-1308.
(13) We cannot eliminate a possibilty of the presence of (µ-η2:η2-peroxo)-
dicopper(II) species only from the resonance Raman spectrum, since the
intensity of O-O stretching band is generally low.
(14) Previously, reversible conversion between 1a and 1b could not be
found in acetone at -70 °C,8 because of significant decomposition by bubbling
N2 gas. Reversibility in acetone is poor compared to that in CH2Cl2.
(15) (a) Karlin, K. D.; Cruse, R. W.; Gultneh, Y.; Farooq, A.; Hayes, J.
C.; Zubieta, J. J. Am. Chem. Soc. 1987, 109, 2668-2679. (b) 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. (c) 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.
(d) Wei, N.; Murthy, N. N.; Chen, Q.; Zubieta, J.; Karlin, K. D. Inorg. Chem.
1994, 33, 1953-1965. (e) Mahapatra, S.; Halfen, J. A.; Wilkinson, E. C.;
Que, L., Jr.; Tolman, W. B. J. Am. Chem. Soc. 1994, 116, 9785-9786. (f)
Lynch, W. E.; Kurtz, D. M., Jr.; Wang, S.; Scott, R. A. J. Am. Chem. Soc.
1994, 116, 11030-11038.
(16) Similar dioxygen evolution was observed for the reaction of [Cu2-
(O2)(tpa)2]2+ with PPh3,7,17 although trans-(µ-1,2-peroxo) species may not be
an active species for O2 evolution in this system because its formation seems
to be difficult due to steric requirement of Me2-tpa mentioned above.
(17) (a) Paul, P. P.; Tyekla´r, Z.; Jacobson, R. R.; Karlin, K. D. J. Am.
Chem. Soc. 1991, 113, 5322-5332. (b) 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.