Dioxygen activation by a neutral β-diketiminato copper(I) ethylene complex

Article abstract of DOI:10.1039/b105244f

The synthesis and structure of two thermally stable neutral β-diketiminato copper(I) olefin complexes are presented along with the structure of a Cu(II)₂(μ-OH)₂ dimer that results from the reaction of the Cu(I) ethylene complex with

Full text of DOI:10.1039/b105244f

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Dioxygen activation by a neutral b-diketiminato copper( ) ethylene
I
complex†
Xuliang Dai and Timothy H. Warren*
Georgetown University, Department of Chemistry, Box 571227, Washington, D.C. 20057-1227, USA.
E-mail: thw@georgetown.edu
Received (in Cambridge, UK) 14th June 2001, Accepted 15th August 2001
First published as an Advance Article on the web 18th September 2001
The synthesis and structure of two thermally stable neutral
b-diketiminato copper( ) olefin complexes are presented
along with the structure of a Cu(II)2(m-OH)2 dimer that
hindered ethylene derivative 1 (K = 0.33(2)). Backbonding in
I
1 and 2 is likely relatively weak as shown by the ethylene 1JCH
coupling constant of 158 Hz in 1 and facile rotation about the
Cu–styrene bond in 2 (DG‡ = 10.7(3) kcal mol21 at 215 K)
(ESI†).
results from the reaction of the Cu( ) ethylene complex with
I
O2 via the proposed intermediacy of a Cu(III)2(m-O)2 spe-
cies.
The X-ray structures of 1 and 2‡ (Fig. 1) reveal a coplanar
orientation of the olefin with the b-diketiminate backbone
which chelates the copper center with bite angles of 98.68(6)
and 99.0(1)°, respectively. The ethylene and styrene C–C
distances in 1 and 2 (1.365(3) and 1.373(6) Å) are similar to that
found in the iminophosphinamide [But2P(NSiMe3)2-
The diverse biological role copper(
a great deal of interest in synthetic nitrogen-ligated systems. For
example, the Cu( )-based ethylene receptor in ETR 1 regulates
I
) complexes play motivates
I
many facets of plant growth such as fruit ripening, seed
germination and even sex determination in some species.1
k N]Cu(C2H4) (1.362(6) Å)2c and [(bipy)Cu(styrene)][ClO4]
2
Synthetic studies have uncovered that most copper(
I
)-ethylene
(1.358(10) Å),9 slightly elongated as compared to the corre-
sponding free olefins (1.337(1)10a and 1.346(2)10b). While the
complexes are quite labile,2 with the most robust systems based
on very electron-rich neutral donors such as bipyridine2b or a
above data suggest minimal backbonding from the d10 copper(
)
I
recently reported monoanionic iminophosphinamide.2c Cu(
I
)
center, the observed orientation may be favored on electronic
grounds due to an enhanced interaction of the olefin p* orbital
with the filled Cu d orbital destabilized by the opposing b-
diketiminate N donors.
complexes are also responsible for the binding and transport of
dioxygen in invertebrates as exemplified by the ubiquitous
enzyme hemocyanin whose active site consists of two Cu( )
I
centers ligated by three histidine residues each.3 The related
enzyme tyrosine is responsible for the selective C–H activation
of phenols, ultimately oxidizing them to orthoquinones.4
Whereas much effort has been directed toward a detailed
understanding of this biological reactivity,3 model systems
based on bi- and tri-dentate amine donors also offer the
synthetic potential for selective C–H bond functionalization.5
As part of our program to explore the chemistry of electron-rich
b-diketiminates of the later first row metals, we report herein
Treating a diethyl ether solution of ethylene complex 1 with
several equivalents of dry dioxygen at room temperature
ultimately results in the precipitation of paramagnetic
{[Me2NN]Cu(m-OH)]}2 36,7 (meff = 1.39 mB in CDCl3) which is
isolated in 80% yield as brown crystals (ESI†). The X-ray
structure of 3‡ (Fig. 2) shows two square planar Cu(II) centers
separated by 3.0581(3) Å nearly symmetrically bridged by two
hydroxo ligands with Cu–O distances of 1.914(1) and 1.923(1)
Å. The hydroxo H-atom was found in the Fourier difference
map, corroborated by a sharp, high energy band at 3646 cm21
in the solid state IR spectrum of 3. We suggest that the reaction
our initial results describing a neutral Cu(
I
) system that both
binds olefins and activates dioxygen.6–8
Reaction of the thallium b-diketiminate Tl[Me2NN] with
CuBr·SMe2 in the presence of either ethylene (1 atm) or styrene
(1.2 equiv.) in benzene or toluene provides thermally stable
[Me2NN]Cu(alkene) complexes in 60–80% yield as colorless
crystals from pentane (Scheme 1) (ESI†). The 1H NMR
spectrum of 1 in benzene-d6 shows a significantly upfield
shifted resonance (d 2.91) for bound ethylene. Neither 1 nor 2 is
exceptionally labile as exchange with free olefin is slow on the
NMR timescale. Reaction of 1 with 1 equiv. styrene at 25 °C in
a sealed NMR tube, however, does result in a 1+2.3 equilibrium
mixture of 1 and 2 after 3 h. Taking the incomplete solubility of
ethylene in benzene-d6 into account, styrene complex 2 is
actually thermodynamically disfavored over the sterically less
proceeds via the intermediacy of a Cu(III)2(m-O)2 dimer5,11
4
followed by abstraction of some H-atom donor5 to provide 3
Fig. 1 ORTEP diagrams of the solid state structures of 1 and 2.‡ Selected
bond distances (Å) and angles (°) for 1·0.5 C5H12: Cu–N(1) 1.917(2), Cu–
N(2) 1.908(1), Cu–C(22) 1.986(2), Cu–C(23) 1.992(2), C(22)–C(23)
1.365(3), N(1)–Cu–N(2) 98.68(6), C(22)–Cu–C(23) 40.13(9). Selected
bond distances (Å) and angles (°) for 2: Cu–N(1) 1.913(3), Cu–N(2)
1.915(3), Cu–C(22) 1.975(5), Cu–C(23) 2.027(4), C(22)–C(23) 1.373(6),
N(1)–Cu–N(2) 99.0(1), C(22)–Cu–C(23) 40.1(2).
Scheme 1 Synthesis of Cu(I) olefin complexes (ESI†).
† Electronic supplementary information (ESI) available: synthesis of
Tl[Me2NN] and 1–3 with relevant analytical and spectroscopic details. See
http://www.rsc.org/suppdata/cc/b1/b105244f/
1998
Chem. Commun., 2001, 1998–1999
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b105244f

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XSEED.12b Intensities were corrected for Lorentz and polarization effects
and an empirical absorption correction was applied to 3 using Blessing’s
method as incorporated into the program SADABS.12c
Crystal data: 1·0.5 C5H12: C25.5H35CuN2, M = 433.10, orthorhombic,
space group Aba2, a = 13.7801(14), b = 23.792(2), c = 14.665(2) Å, V =
4807.9(9) Å3, Z = 8, one molecule of 1 per asymmetric unit with 0.5
molecule of pentane disordered over two orientations, m = 0.92 mm21
,
19652 reflections collected at –100 °C, 5807 independent reflections (Rint
=
0.0303), 5110 reflections observed (I > 2s(I)), final agreement factors for
308 parameters and 1 restraint: R1 = 0.0288, wR2 = 0.0721 (observed
reflections) and R1
20.005(11).
= 0.0363, wR2 = 0.0759 (all data), Flack c =
2: C29H33CuN2, M = 473.11, orthorhombic, space group Pbca, a =
17.4077(14), b = 15.3217(12), c = 18.5088(15) Å, V = 4936.6(7) Å3, Z =
8, one molecule per asymmetric unit, m = 0.903 mm21, 43388 reflections
collected at 2100 °C, 4355 independent reflections (Rint = 0.1818), 2351
reflections observed (I
> 2s(I)), final agreement factors for 308
parameters: R1 = 0.0495, wR2 = 0.1075 (observed reflections) and R1 =
0.1204, wR2 = 0.1240 (all data).
3: C42H52Cu2N4O2, M = 771.96, monoclinic, space group C2/c, a =
23.5194(15), b = 10.6093(7), c = 15.2554(10) Å, b = 93.1880(1)°, V =
3800.7(4) Å3, Z = 4, one half molecule per asymmetric unit, m = 1.160
mm21
,
20485 reflections collected at 2100 °C, 4581 independent
reflections (Rint = 0.0189), 4253 reflections observed (I > 2s(I)), final
agreement factors for 236 parameters: R1 0.0249, wR2 0.0686
(observed reflections) and R1 = 0.0274, wR2 = 0.0701 (all data).
CCDC reference numbers 168944–168946. See http://www.rsc.org/
suppdata/cc/b1/b105244f/ for crystallographic data in CIF or other
electronic format.
Fig. 2 ORTEP diagram of the solid state structure of 3.‡ Selected bond
distances (Å) and angles (°): Cu–N(1) 1.9446(11), Cu–N(2) 1.9373(11),
Cu–O 1.9142(11), Cu–OA 1.9230(11), Cu–CuA 3.0581(3); N(1)–Cu–N(2)
94.83(5), O–Cu–OA 74.32(5).
=
=
1 F. I. Rodríguez, J. J. Esch, A. E. Hall, B. M. Binder, G. E. Schaller and
A. B. Bleecker, Science, 1999, 283, 996; J. R. Ecker, Science, 1995, 268,
667 and references therein.
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94, 727; E. I. Solomon, U. M. Sundaram and T. E. Machonkin, Chem.
Rev., 1996, 96, 2563.
Scheme 2 Proposed formation of 3.
(Scheme 2). Whereas the source of the hydroxo H-atoms is not
yet known, it should be noted that 1 rapidly reacts with H2O
under anaerobic conditions to give the free ligand H[Me2NN]
and an intractable Cu-containing precipitate.
4 H. S. Mason, W. L. Fowlks and E. Peterson, J. Am. Chem. Soc., 1955,
77, 2914.
Significantly less thermally stable than the only other
reported neutral, iminophosphinamide-based Cu2(m-O)2 com-
5 S. Schindler, Eur. J. Inorg. Chem., 2000, 2311; P. L. Holland and W. B.
Tolman, Coord. Chem. Rev., 1999, 190–192, 855; H. V. Obias, Y. Lin,
N. N. Murthy, E. Pidcock, E. I. Solomon, M. Ralle, N. J. Blackburn, Y.
M. Neuhold, A. D. Zuberbühler and K. D. Karlin, J. Am. Chem. Soc.,
1998, 120, 12960; S. Itoh, M. Taki, H. Nakao, P. L. Holland and W. B.
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Henson, E. I. Solomon and T. D. P. Stack, J. Am. Chem. Soc., 2000, 122,
10249.
6 Much of this work has been presented in oral form: T. H. Warren, X.
Dai, S. Puiu and J. E. McDermott, Abstracts of Papers, 221st National
Meeting of the American Chemical Society, Washington, DC, April 1–5,
2001, No. INOR 394.
7 Some dioxygen reactivity with two related Cu(i) b-diketiminates as well
as the structure of 3 prepared from {[Me2NN]CuCl}2 and LiOH has
been presented: W. B. Tolman, P. L. Holland, B. A. Jazdzewski, A. M.
Reynolds, L. L. Bowen, D. J. E. Spencer, D. C. Price, M. Pink and V.
G. Young, Jr, Abstracts of Papers, 221st National Meeting of the
American Chemical Society, Washington, DC, April 1–5, 2001, No.
INOR 16.
8 Sterically hindered Cu(II) b-diketiminates are known: P. L. Holland and
W. B. Tolman, J. Am. Chem. Soc., 1999, 121, 7270; P. L. Holland and
W. B. Tolman, J. Am. Chem. Soc., 2000, 122, 6331.
9 H. Masuda, K. Machida, M. Munakata, S. Kitagawa and H. Shimono,
J. Chem. Soc., Dalton Trans., 1988, 1907.
10 (a) L. S. Bartell, E. A. Roth, C. D. Hollowell, K. Kuchitsu and J. E.
Young Jr, J. Chem. Phys., 1965, 42, 2683; (b) J. C. Cochran, K. Hagen,
G. Paulen, Q. Shen, S. Tom, M. Traetteberg and C. Wells, J. Mol.
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11 (a) B. F. Straub, F. Rominger and P. Hofmann, Chem. Commun., 2000,
1611; (b) M. J. Henson, P. Mukherjee, D. E. Root, T. D. P. Stack and E.
I. Solomon, J. Am. Chem. Soc., 1999, 121, 10332.
12 (a) SHELXTL-PC, Version 5.10; 1998, Bruker-Analytical X-ray
Services, Madison, WI; G. M. Sheldrick, SHELX-97, Universität
Göttingen, Göttingen, Germany; (b) L. Barbour, XSEED, 1999,
University of Missouri, Columbia, MO; (c) SADABS; G. M. Sheldrick,
1996, based on the method described in R. H. Blessing, Acta
Crystallogr., Sect. A, 1995, 51, 33.
plex which slowly decomposes to an unidentified Cu(II
)
species,11a the enhanced reactivity of proposed m-oxo dimer 4
has hampered its isolation in pure form. Addition of O2 to 1 at
278 °C in diethyl ether allows for the isolation of a green
microcrystalline material that gradually converts into 3, even in
the solid state. A solution of this freshly isolated green material
in CH2Cl2 shows an absorption band at 343 nm (e = 2.4 3 104
M21 cm21) corresponding to the m-hydroxo complex 3 as well
as two new absorbances at 315 and 420 nm, consistent with
characteristic LMCT bands of previously reported Cu(III)2(m-
O)2 complexes.5,11 Furthermore, oxygenation of 1 with air
(THF, 25 °C) results in the initial formation of these bands at
315 and 420 nm followed by their gradual loss in intensity
giving rise to the 343 nm band of the m-hydroxo complex 3
(ESI†).
Low temperature resonance Raman spectroscopy5,11 in
addition to the synthesis of a sterically diverse family of neutral
b-diketiminato Cu( ) complexes are underway to uniquely
I
determine the composition of the green intermediate 4 as well as
the source of H-atoms that lead to the isolated m-hydroxo
compound 3. Further protonation studies of the olefin adducts 1
and 2 to yield new cationic b-diimino Cu(
) olefin complexes6
I
and their use as cyclopropanation catalysts will also be reported
in due course.
We thank the Georgetown University Department of Chem-
istry for financial support of this work as well as Professor
Robert Bachman and Professor K. Travis Holman for assistance
with the X-ray structures of 1 and 2.
Notes and references
‡ Data were collected on a Bruker SMART CCD instrument and the
structure solutions were performed using the SHELXTL/PC suite12a and
Chem. Commun., 2001, 1998–1999
1999