Mixed metal bis(μ-oxo) complexes with [CuM(μ-O)2] n+ (M = Ni(III) or Pd(II)) cores

Article abstract of DOI:10.1039/b404640d

Two highly reactive heterodinuclear bis(μ-oxo) complexes were prepared by combining mononuclear peroxo species with reduced metal precursors at -80°C and were identified by UV-vis, EPR/NMR, and resonance Raman spectroscopy, with corroboration in the case

Full text of DOI:10.1039/b404640d

Mixed metal bis(m-oxo) complexes with [CuM(m-O)2]n+ (M = Ni(III) or
Pd(II)) cores†
Nermeen W. Aboelella,a John T. York,a Anne M. Reynolds,a Koyu Fujita,b Christopher R. Kinsinger,a
Christopher J. Cramer,*a Charles G. Riordan*b and William B. Tolman*a
a Department of Chemistry, Center for Metals in Biocatalysis, Supercomputer Institute, University of
Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota, 55455. E-mail: tolman@chem.umn.edu.
E-mail: cramer@chem.umn.edu; Fax: 612-624-7029; Tel: 612-625-4061
b Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716.
E-mail: riordan@udel.edu; Fax: 302-831-6335; Tel: 302-831-1073
Received (in West Lafayette, IN, USA) 30th March 2004, Accepted 10th June 2004
First published as an Advance Article on the web 29th June 2004
Two highly reactive heterodinuclear bis(m-oxo) complexes were
prepared by combining mononuclear peroxo species with
reduced metal precursors at 280 °C and were identified by UV-
vis, EPR/NMR, and resonance Raman spectroscopy, with
corroboration in the case of the CuPd system from density
functional calculations.
product, but selective decomposition of the bis(m-oxo) Raman
feature in the laser beam was observed, thus precluding attainment
of an excitation profile and rendering the use of these intensities
ineffective for assessing the product ratio. Indeed, the X-band EPR
spectrum of the red solution (9.6 GHz, 2K) is dominated by a
rhombic signal (Fig. 1b, apparent gx = 2.03, gy = 2.19, and gz =
2.26) that lacks hyperfine splitting characteristic of Cu(II) (I = 3/2),
is a clear indicator of the presence of S = 1/2 Ni(III), and
corresponds by integration to ~ 90% of the amount of 2 added.∑ The
The characterization of reactive metal–oxygen intermediates is an
important objective in research aimed at understanding the
mechanisms of oxidation catalysis in synthetic and biological
systems.1 As part of such efforts, bis(m-oxo)dimetal complexes2 of
Cu,2,3 Fe,2,4 Co,5 Ni,5–7 and Pt8 have been prepared, characterized,
and shown in many instances to exhibit interesting oxidative
reactivity. The syntheses of these compounds generally involve the
reaction of an oxidant (e.g., O2 or H2O2) with mononuclear or
homodinuclear precursors, and inevitably lead to complexes
comprising [M2(m-O)2]n+ or isomeric [M2(O2)]n+ cores with
identical metal ions (M). Mixed-metal species [MMA(m-O)2]n+ are
also of significant interest due to their potentially unique properties
and their possible involvement in various catalytic processes,9,10
yet they are exceedingly rare,11 arguably because of intrinsic
challenges associated with their preparation by the methods usually
used to obtain symmetric [M2O2]n+ complexes. Bis(m-oxo)di-
copper complexes asymmetric by virtue of divergent N-donor
supporting ligands on each metal were recently obtained by
reacting a solution of a monocopper–peroxide complex (1)12 with
combined data thus indicate that bis(m-oxo)copper(III)nickel(III
)
complex 3 is the primary reaction product, with a 1,2-peroxo
species being a minor component despite the Raman data, which
are deceptive due to the laser-induced decay of 3. The peroxo
species may be a precursor to 3, but this notion has yet to be
confirmed.
Using a complementary strategy to that implemented to prepare
the CuNi compound 3, the known18 complex 5 (which we
characterized by X-ray diffraction**) was treated with the Cu( )
I
compound 419 in THF at 280 °C. A dark yellow-brown color and
a new absorption band at lmax = 448 nm (e 5900 M21cm21, Fig.
2a) developed immediately, which bleached upon warming. A
spectrophotometric titration revealed that the maximum absorbance
is attained when the molar ratio of 4 and 5 is 1 : 1. The product is
EPR silent at 2K and exhibits a sharp 1H NMR spectrum (Fig. S4†)
and a single peak in the 31P{1H} spectrum (26.8 ppm, THF, 280
°C) distinct from that exhibited by 5 (34.2 ppm) and a decomposi-
tion product generated upon warming, ONPPh3 (24.7 ppm). Finally,
various LCu(
) compounds in the absence of free O2.13,14 Herein,
I
we report the use of this type of sequential building block
methodology to prepare novel complexes with [CuNi(m-O)2]2+ and
[CuPd(m-O)2]1+ cores, thus demonstrating the applicability of a
more general strategy for the preparation of heterodinuclear bis(m-
oxo) species.15
Addition of yellow [PhTttBu]Ni(CO) (2, PhTttBu = phenyl-
tris((tert-butylthio)methyl)borate), a compound previously shown
to react with O2 to yield a bis(m-oxo)dinickel(III) complex,6 to an
anaerobic (purged with Ar) solution of green 1 in THF at 280 °C
resulted in the gradual formation (t1/2 ~ 4.5 h, [1] = [2] = 0.7 mM)
of a deep red color and a broad, intense, and multifeatured
absorption in the 400–600 nm region (Fig. 1a). This absorption
bleaches upon warming to ambient temperature.‡ The resonance
Raman spectrum of the initial red solution (l = 457.9 nm, frozen
THF) contains peaks at 847 and 625 cm21 that shift to 800 and 595
cm21, respectively, when 1 prepared with 18O2 was used (Fig. 1c).
A (1,2-peroxo)dimetal complex is implicated by the 847/800
features (nO–O),§16 which are inconsistent with monomeric 1 : 1 M/
O2 species (M = Cu12 or Ni14). The 625/595 peaks are signatures
of a bis(m-oxo)dimetal core (symmetric MMAO2 vibration).2,17
Importantly, the latter peak positions differ from those due to
[(PhTttBuNi)2(m-O)2] (590/560 cm21), thus supporting their attribu-
tion to a new mixed metal [CuNi(m-O)2]2+ core (3).¶ The relative
peak intensities suggest that the peroxo species is the major reaction
Fig. 1 (a) Optical absorption spectra of a THF solution of 1 (dotted line) and
the product of addition of equimolar 2 (solid line). (b) EPR spectrum of THF
solution of 1 + 2 (1 mM, 9.6 GHz, 2K). (c) Resonance Raman spectrum of
THF solution of 1-16O2 + 2 (solid line) and 1-18O2 + 2 (dotted line).
† Electronic supplementary information (ESI) available: experimental and
calculation details. See http://www.rsc.org/suppdata/cc/b4/b404640d/
1716
C h e m . C o m m u n . , 2 0 0 4 , 1 7 1 6 – 1 7 1 7
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

§ The value Dobs
49 cm21 for a O–O harmonic oscillator.
¶ Oxygenation of the Cu(
( (
16O–18O) = 47 cm21 closely matches Dcalc 16O–18O) =
I
) precursor of 1 does not yield a [Cu2(m-O)2]2+
complex, thus arguing against the possibility that the 625/595 cm21 features
are due to such a complex (which would also be diamagnetic and thus EPR
silent).
∑ EPR spectra recorded over a range of temperatures (2–65 K), microwave
powers (5.0 3 1025–0.8 mW), and reaction times (31–90 min.) contained
the same features as shown in Fig. 1b. The small low field feature adjacent
to the major rhombic signal is associated with the product of sample
decomposition (supporting information†).
** X-ray data for 5·1.5C7H8: C46.50H42O2P2Pd, MW = 801.14 g mol21
,
monoclinic, a = 9.2639(14), b = 23.168(4), c = 18.260(3) Å, b =
92.962(3)°, V = 3913.9(11) Å3, T = 173(2) K, P2(1)/n, Z = 4, m = 0.593
mm21, 25447 reflections, 6916 independent reflections [R(int) = 0.0562],
R1 = 0.0368, wR2 (for I > 2s(I)) = 0.0831, 484 parameters, 43 restraints.
CCDC 235181. See http://www.rsc.org/suppdata/cc/b4/b404640d/ for crys-
tallographic data in .cif or other electronic format.
1 (a) Metal-Oxo and Metal-Peroxo Species in Catalytic Oxidations, ed. B.
Meunier, Springer: Berlin, 2000; (b) Biomimetic Oxidations Catalyzed
by Transition Metal Complexes, ed. B. Meunier, Imperial College Press:
London, 2000.
2 L. Que, Jr. and W. B. Tolman, Angew. Chem. Int. Ed., 2002, 41,
1114.
3 (a) L. M. Mirica, X. Ottensaelder and T. D. P. Stack, Chem. Rev., 2004,
104, 1013; (b) E. A. Lewis and W. B. Tolman, Chem. Rev., 2004, 104,
1047.
Scheme 1
4 (a) L. Que, Jr. and Y. Dong, Acc. Chem. Res., 1996, 29, 190; (b) L. Que,
Jr., J. Chem. Soc., Dalton Trans., 1997, 3933.
5 (a) S. Hikichi, M. Yoshizawa, Y. Sasakura, M. Akita and Y. Moro-oka,
J. Am. Chem. Soc., 1998, 120, 10567; (b) S. Hikichi, M. Yoshizawa, Y.
Sasakura, H. Komatsuzaki, Y. Moro-oka and M. Akita, Chem. Eur. J.,
2001, 7, 5012.
6 (a) B. S. Mandimutsira, J. L. Yamarik, T. C. Brunold, W. Gu, S. P.
Cramer and C. G. Riordan, J. Am. Chem. Soc., 2001, 123, 9194; (b) R.
Schenker, B. S. Manimutsira, C. G. Riordan and T. C. Brunold, J. Am.
Chem. Soc., 2002, 124, 13842.
7 S. Itoh, H. Bandoh, M. Nakagawa, S. Nagatomo, T. Kitagawa, K. D.
Karlin and S. Fukuzumi, J. Am. Chem. Soc., 2001, 123, 11168.
8 (a) P. R. Sharp, J. Chem. Soc., Dalton Trans., 2000, 2647; (b) J. J. Li,
W. Li and P. R. Sharp, Inorg. Chem., 1996, 35, 604.
9 (a) T. Hosokawa and S.-I. Murahashi, Acc. Chem. Res., 1990, 23, 49; (b)
P. L. Alsters, J. Boersma and G. v. Koten, Organometallics, 1993, 12,
1629; (c) T. Hosokawa, M. Takano and S.-I. Murahashi, J. Am. Chem.
Soc., 1996, 118, 3990.
Fig. 2 (a) Optical absorption spectra of a THF solution of 5 (dotted line) and
the product of addition of equimolar 4 (solid line). (b) Resonance Raman
spectrum of THF solution of 5-16O2 + 4 (solid line) and 5-18O2 + 4 (dotted
line).
excitation at l = 457.9 nm led to resonance enhancement of a peak
at 660 cm21 in the Raman spectrum that shifted to 631 cm21 when
5 prepared with 18O2 was used (Fig. 2b). On the basis of the
combined data, we postulate that the reaction of 4 with 5 produces
the diamagnetic bis(m-oxo)copper(III)palladium(II) complex 6.
Notably diagnostic for this formulation are the charge transfer
transition at lmax = 448 nm and a MMA(m-O)2 feature in the
resonance Raman spectrum.2,17 Further corroboration was provided
by DFT calculations on 6,† which yielded as an energy minimum a
C2 structure with parameters typical for bis(m-oxo)dimetal com-
plexes (e.g., Cu–O = 1.848 Å, Pd–O = 2.029 Å, Pd…Cu = 2.994
Å) and a frequency for a CuPd(m-O)2 rhomb mode of 594 cm21
(D18O = 26 cm21). The isotope shift is in good agreement with the
experimentally observed value, although the predicted frequency is
somewhat low. Further theoretical work on 6 is underway.
In sum, we have successfully prepared heterodinuclear bis(m-
oxo) complexes by combining mononuclear peroxo species with
reduced metal precursors. Both the CuNi and CuPd compounds are
highly reactive, as shown by their tendency to decompose rapidly
upon warming. Studies of their reactivity with exogeneous
substrates are underway, as are extensions of the synthetic
methodology to other heterometal combinations.
10 Of relevance are FeCuO2 species prepared as models of cytochrome c
oxidase: (a) R. A. Ghiladi, K. R. Hatwell, K. D. Karlin, H.-w. Huang, P.
Moënne-Loccoz, C. Krebs, B. H. Huynh, L. A. Marzilli, R. J. Cotter, S.
Kaderli and A. D. Zuberbühler, J. Am. Chem. Soc., 2001, 123, 6183; (b)
T. Chishiro, Y. Shimazaki, F. Tani, Y. Tachi, Y. Naruta, S. Karasawa,
S. Hayami and Y. Maeda, Angew. Chem. Int. Ed., 2003, 42, 2788.
11 (a) [PdGe(m-O)2]2+ complex: Z. T. Cygan, J. E. Bender IV, K. E. Litz,
J. W. Kampf and M. M. Banaszak-Holl, Organometallics, 2002, 21,
5373; (b) [PtGe(m-O)2]2+ complex: K. E. Litz, M. M. Banaszak-Holl, J.
W. Kampf and G. B. Carpenter, Inorg. Chem., 1998, 37, 6461.
12 D. J. E. Spencer, N. W. Aboelella, A. M. Reynolds, P. L. Holland and
W. B. Tolman, J. Am. Chem. Soc., 2002, 124, 2108.
13 N. W. Aboelella, E. A. Lewis, A. M. Reynolds, W. W. Brennessel, C. J.
Cramer and W. B. Tolman, J. Am. Chem. Soc., 2002, 124, 10660.
14 A similar sequence has been used to prepare a LNi(m-O)2NiLA
compound: K. Fujita, R. Schenker, W. Gu, T. C. Brunold, S. P. Cramer
and C. G. Riordan, Inorg. Chem., 2004, 43, 3324.
15 Related strategies have been implemented. For example, see: (a) E. E.
Chufán and K. D. Karlin, J. Am. Chem. Soc., 2003, 125, 16160; (b) M.
Hunger, C. Limberg and P. Kircher, Organometallics, 2000, 19, 1044;
(c) M. S. Rau, C. M. Kertz, L. A. Mercando, G. L. Geoffroy and A. L.
Rheingold, J. Am. Chem . Soc., 1991, 113, 7420.
16 M. J. Baldwin, P. K. Ross, J. E. Pate, Z. Tyeklar, K. D. Karlin and E. I.
Solomon, J. Am. Chem. Soc., 1991, 113, 8671.
We thank the NIH (GM47365 to W.B.T.), the NSF (CHE-
0213260 to C.G.R., CHE-0203346 to C.J.C. and predoctoral
fellowship to N.W.A.), and Profs. J. Lipscomb and L. Que, Jr., for
access to EPR and Raman instrumentation, respectively.
17 (a) P. L. Holland, C. J. Cramer, E. C. Wilkinson, S. Mahapatra, K. R.
Rodgers, S. Itoh, M. Taki, S. Fukuzumi, L. Que, Jr. and W. B. Tolman,
J. Am. Chem. Soc., 2000, 122, 792; (b) M. J. Henson, P. Mukherjee, D.
E. Root, T. D. P. Stack and E. I. Solomon, J. Am. Chem. Soc., 1999, 121,
10332.
18 C. J. Nyman, C. E. Wymore and G. Wilkinson, J. Chem. Soc. (A), 1968,
561.
Notes and references
‡ Reliable spectrophotometric titration data with which to confirm a 1 : 1
reaction stoichiometry were unavailable because of the slow rate of reaction
at 280 °C and competing decomposition at higher temperatures.
19 D. J. E. Spencer, A. M. Reynolds, P. L. Holland, B. A. Jazdzewski, C.
Duboc-Toia, L. L. Pape, S. Yokota, Y. Tachi, S. Itoh and W. B. Tolman,
Inorg. Chem., 2002, 41, 6307.
C h e m . C o m m u n . , 2 0 0 4 , 1 7 1 6 – 1 7 1 7
1717