β-diketiminate ligand backbone structural effects on Cu(I)/O2 reactivity: Unique copper-superoxo and Bis(μ-oxo) complexes

Article abstract of DOI:10.1021/ja017820b

Copper(I) and -(II) complexes of β-diketiminate ligands with identical flanking 2,6-diisopropylphenyl groups but divergent backbone substitution patterns were prepared and structurally characterized, and reactions of the Cu(I) species with O₂ a

Full text of DOI:10.1021/ja017820b

Published on Web 02/16/2002
â-Diketiminate Ligand Backbone Structural Effects on Cu(I)/O2 Reactivity:
Unique Copper-Superoxo and Bis(µ-oxo) Complexes
Douglas J. E. Spencer, Nermeen W. Aboelella, Anne M. Reynolds, Patrick L. Holland, and
William B. Tolman*
Department of Chemistry, UniVersity of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455
Received December 19, 2001
Reactive intermediates derived from the reaction of Cu(I) species
and O2 play a central role in many biological and catalytic
processes.1 Delineation of the structures and properties of these
intermediates has been addressed through detailed studies of
synthetic complexes, which have led to the isolation and charac-
terization of a variety of superoxo, peroxo, and bis(µ-oxo) species.2
Of particular importance to the involvement of these structures in
biology and catalysis are their relative stabilities and interconversion
pathways,3 and understanding how these aspects are influenced by
Figure 1. X-ray structures of (a) (2)Cu(MeCN) and (b) (3)Cu(MeCN),
with all nonhydrogen atoms shown as 50% thermal ellipsoids.
supporting ligand structural features is an important research
objective. Herein, we report the results of an exploration of the O2
that of previously reported (2)CuCl,10 but the same reaction using
Li(3) formed [(3)CuCl]n. This complex adopts a three-coordinate
structure in CH2Cl2 (UV-vis, n ) 1), but exhibits temperature-
dependent UV-vis spectra in toluene (Figure S2). In addition, the
EPR spectrum (X-band, 4 K, toluene) is quite different from that
of (1 or 2)CuCl; it contains a broad signal at g ≈ 2 with hyperfine
features in both high- and low-field portions, as well as a signal at
g ) 4.4 attributable to a ∆MS ) (2 transition of a magnetically
coupled dicopper system (Figure S3).11 These combined data for
[(3)CuCl]n in toluene indicate an equilibrium between a three-
coordinate monomer (n ) 1, predominant at high temperature) and
a chloride-bridged dimer (n ) 2, preferred at low temperature),
and are consistent with 3 exerting less steric hindrance about a
coordinated metal ion than 1 or 2.
reactivity of Cu(I) complexes of a set of â-diketiminate anions,4
which by virtue of their charge and specific steric properties differ
from the neutral N-donor ligands that have been more widely used
in such studies.5 Ligands 1-3 have identical 2,6-diisopropylphenyl
flanking groups, but divergent backbone substituents which,
although far removed from the coordinated metal ion, nonetheless
significantly impact the course of Cu(I) complex oxygenations and
yield novel superoxo or bis(µ-oxo) intermediates.
Most significantly, these steric differences are manifested in the
formation of divergent intermediates upon low-temperature oxy-
genation of the Cu(I) complexes.12,13 Treatment of a yellow solution
of (3)Cu(MeCN) in THF with O2 at -80 °C resulted in a color
change to yellow-brown, with new UV-vis spectral features at λmax
≈ 380 nm (ꢀ ≈ 20 000 M-1 cm-1 per Cu) and ≈ 420 nm (sh, ꢀ ≈
5000 M-1 cm-1) (Figure S4). The former band is most likely a
ligand-based π f π* transition,14 while the latter resembles a CT
transition exhibited by bis(µ-oxo)dicopper complexes.3a,15 Consistent
with this assignment, the oxygenated THF solution is EPR silent
(9.61 GHz, 20 K) and spectrophotometric titration data showed a
Cu:O2 stoichiometry of 2.0(2):1. Finally, in resonance Raman
spectra (λex ) 457.9 nm, THF, 77 K) the only O-isotope sensitive
feature seen was a single peak at 580 cm-1 that shifted to 559 cm-1
when 18O2 was used (Figure 2a).15,16 In an alternate synthetic route,
addition of 1-5 equiv of a 1:1 solution of H2O2(aq) (31.3%) and
Et3N to a solution of [(3)CuCl]n in THF or toluene at -40 °C
yielded spectral features similar to those seen upon oxygenation
of (3)Cu(MeCN). The combined data allow unequivocal identifica-
tion of the intermediate as [(3)2Cu2(µ-O)2], a rare example of a
neutral bis(µ-oxo)dicopper complex.5
Reaction of the lithium salts of 1,6 2,7 and 38 with [Cu(MeCN)4]-
CF3SO3 in THF or toluene yielded complexes LCu(MeCN) (Figures
1 and S1). All the complexes display three-coordinate geometries,
but subtle differences in the structures suggest disparities in the
effective steric bulk imposed by their respective supporting ligands.
i
In (2)Cu(MeCN), the Pr group methine hydrogen atoms point
inward toward the â-diketiminate, in a typical orientation that
presumably arises from steric interactions between the backbone
i
R-Me and the Pr groups. In contrast, in (3)Cu(MeCN) that lacks
i
i
the R-Me groups, one of the Pr groups (bottom left Pr in Figure
1b) is rotated by 180° in a unique orientation suggestive of an
increased conformational flexibility and, thus, decreased steric
encumbrance in 3 relative to 2. In (1)Cu(MeCN) (Figure S1), CR-
N-Caryl angles of 128-129° attest to a canting of the 2,6-
diisopropylphenyl flanking groups toward the metal ion due to the
R-tert-butyl substituents (cf. previously reported Fe complexes of
1).9 This drastic steric influence is absent in the complexes of 2
and 3, which have similar CR-N-Caryl angles of 116-119°. In
sum, the structural data imply an effective steric bulk order 1 > 2
> 3.
In contrast, oxygenation of (2)Cu(MeCN) in THF or acetone at
-80 °C resulted in a color change to green, with development of
UV-vis features at 385 nm (sh, ꢀ ≈ 2400 M-1 cm-1) and 600 nm
Consistent with this order, treatment of Li(1) with CuCl2 yielded
three-coordinate (1)CuCl with similar spectroscopic properties to
9
2108 VOL. 124, NO. 10, 2002 J. AM. CHEM. SOC.
10.1021/ja017820b CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
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Figure 2. Resonance Raman spectra of oxygenated solutions of (a) (3)Cu-
(MeCN) ((λex ) 457.9 nm, THF, 77K) and (b) (2)Cu(MeCN) (λex ) 413
nm, acetone, 77 K). In (a) and (b, bottom) data obtained using 16O2 or 18O2
are denoted by a solid or a dashed line, respectively. The spectrum in (b,
top) was obtained using a mixture of 18O2, 16O18O, and 16O2. The * indicates
a solvent band.
(4) Cu(I) complexes of other â-diketiminates: (a) Dai, X.; Warren, T. H.
Chem. Commun. 2001, 1998. (b) Yokota, S.; Tachi, Y.; Nishiwaki, N.;
Ariga, M.; Itoh, S. Inorg. Chem. 2001, 40, 5316.
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amide ligand has been reported: Straub, B. F.; Rominger, F.; Hofmann,
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(br, 200) (Figure S5). This new species is EPR silent and
spectrophotometric titration data showed a Cu:O2 uptake ratio of
1.0(2):1. The resonance Raman spectrum (λex ) 413 nm, acetone,
77 K) contained no O-isotope sensitive peaks in the region typical
for peroxo or bis(µ-oxo)dicopper complexes (550-850 cm-1).
Instead, a peak was observed at 968 cm-1 that shifted by 51 cm-1
when 18O2 was used (Figure 2b, bottom), consistent with an O-O
stretch [∆18O2(calcd) ) 55 cm-1]. When a mixture of 18O2, 16O18O,
and 16O2 (40% 18O, statistical) was used, a single peak was observed
at 943 cm-1 with a line width identical to those of the peaks arising
from 18O2 and 16O2 (Figure 2b, top). Similar UV-vis and Raman
data were obtained for the product of oxygenation of (1)CuMeCN.17
The combined data are consistent with formulation of the inter-
mediates as unprecedented low-coordinate (superoxo)copper(II)
complexes (LCuO2) with symmetric “side-on” (η2) superoxo
ligation.2,18 The vibrational spectral data are similar to those reported
for a structurally characterized Co(η2-O2) compound,19 but differ
from the only available data for a Cu complex20 postulated to
contain an end-on superoxo ligand on the basis of the presence of
two peaks for 16O18O.
(7) (a) Feldman, J.; McLain, S. J.; Parthasarathy, A.; Marshall, W. J.;
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(8) For synthetic procedures and characterization data, see the Supporting
Information. The procedure used for the synthesis of 3 was adapted from
one kindly provided to us by R. F. Jordan (University of Chicago).
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(12) By cyclic voltammetry (MeCN, 0.1 M Bu4NPF6), we observed an
irreversible wave for (1)Cu(MeCN) with Epc ) -294 mV (vs Fc/Fc+)
and reversible waves for the compounds supported by 2 and 3 with E1/2
) -215 and -256 mV, respectively. Rationales for the divergent O2
reactivity that invoke electronic differences are not obvious from these
data.
(13) All of the oxygenated intermediates decay upon warming, as indicated
by bleaching of their spectral features.
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In sum, despite having identical diisopropylphenyl flanking units,
the different backbone substitution patterns in the ligands 1-3 result
in structural variation among their Cu(I) and Cu(II) complexes and
the formation of divergent intermediates in reactions of their Cu(I)
complexes with O2. These intermediates were identified as (super-
oxo)copper or bis(µ-oxo)dicopper species, the formation of which
depends on the steric encumbrance of the supporting â-diketiminate.
This tuning of the effective steric bulk at the metal center by the
distant backbone substituent arrangement represents a notable ligand
structural influence on biologically relevant Cu(I)/O2 reactivity.21
(17) See Supporting Information. Interestingly, the oxygenation of (1)Cu-
(CH3CN) was significantly slower than those of the complexes supported
by 2 or 3 (to completion in ∼40 min vs s, respectively).
(18) The only superoxocopper complex to be characterized by X-ray diffraction
is reported in: Fujisawa, K.; Tanaka, M.; Moro-oka, Y.; Kitajima, N. J.
Am. Chem. Soc. 1994, 116, 12079. The ν(O-O) value of 1111 cm-1
described in this work is not associated with the title superoxocopper
complex, however (K. Fujisawa, personal communication).
(19) Egan, J. W., Jr.; Haggerty, B. S.; Rheingold, A. L.; Sendlinger, S. C.;
Theopold, K. H. J. Am. Chem. Soc. 1990, 112, 2445. FTIR: ν(O-O) )
Acknowledgment. We thank the NIH (GM47365) and the NSF
(predoctoral fellowships to N.W.A. and A.M.R.) for funding, and
L. Que, Jr., and J. D. Lipscomb for providing access to their
respective Raman and EPR facilities.
961 (16O2), 937 (16O18O), 908 (18O2) cm-1
.
(20) Chaudhuri, P.; Hess, M.; Weyhermu¨ller, T.; Wieghardt, K. Angew. Chem.,
Int. Ed. 1999, 38, 1095. FTIR: ν(O-O) ) 964 (16O2), 932/942 (16O18O),
909 (18O2) cm-1
.
(21) Conceptually related remote steric effects have been reported. For example,
see (a) Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem. ReV. 2000, 100,
1169. (b) Dagorne, S.; Jordan, R. F.; Young, V. G., Jr. Organometallics
1999, 18, 4619.
Supporting Information Available: Text giving synthetic proce-
dures and characterization data, Figures S1-S4 (PDF) and X-ray
crystallographic data files, in CIF format. This material is available
free of charge via the Internet at http://pubs.acs.org.
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