Synthesis, characterization and reactivity of copper(I) imidazole complexes based on a cavitand ligand design

Article abstract of DOI:10.1039/b102480a

Two new cavitand-based imidazolyl ligands and their corresponding copper(I) complexes are reported; the tris(imidazole) complex is substitutionally inert while the bis(imidazole) complex readily reacts with CO, PMe₃, N₃^- a

Full text of DOI:10.1039/b102480a

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Synthesis, characterization and reactivity of copper(I) imidazole
complexes based on a cavitand ligand design†
Janis K. Voo, K. C. Lam, Arnold L. Rheingold and Charles G. Riordan*
Department of Chemistry and Biochemistry, University of Delaware, Newark, DE 19716, USA.
E-mail: riordan@udel.edu
Received 16th March 2001, Accepted 10th May 2001
First published as an Advance Article on the web 25th May 2001
Two new cavitand-based imidazolyl ligands and their
corresponding copper(I) complexes are reported; the
tris(imidazole) complex is substitutionally inert while the
X-Ray structure analysis of the tris(imidazolyl) TriMIm‡
confirmed that the free ligand adopts the expected ababab
(a = above, b = below) conformation in which successive sub-
stituents reside on alternating faces above and below the
hexasubstituted arene ring. The highly predisposed binding
conformations of TriMIm and BiMIm are maintained in
solution as supported by proton NMR analysis. Reaction of
[Cu(CH3CN)4]BF4 with TriMIm proceeded smoothly yielding
[(TriMIm)Cu]BF4 as an analytically pure, microcrystalline
white solid. [(TriMIm)Cu]BF4 has been characterized by X-ray
diffraction, Fig. 1. TriMIm binds to the copper() ion to form a
trigonal planar geometry with an average Cu–N distance of
2.00(1) Å and an average N–Cu–N angle of 119.5(1)Њ. The
copper ion occupies a position above the center of the arene
ring, Cu–arene centroid, 3.02 Å. The imidazolyl groups are
canted in the same direction resulting in a propeller twist and a
chirality of the cation. In solution there is a small barrier to
equilibration of the two enantiomers which can be observed
by variable temperature NMR studies, ∆G‡ = 13 kcal molϪ1
(Tcoal ≈ 285 K). [(TriMIm)Cu]BF4 exhibits a quasi-reversible
oxidation in acetonitrile, E1/2 = 355 mV (vs. Fc/Fcϩ), ∆Ep = 290
mV, Ip,a/Ip,c = 2.0, consistent with formation of the copper()
adduct. The lack of clean electrochemical reversibility high-
lights the structural change, to square planar, required to access
Cu(). Despite the observed electrochemical oxidation [(Tri-
MIm)Cu]BF4 does not react with O2. Furthermore, it is inert
to other small molecules such as CO, NO, or PR3. Therefore
we synthesized the bis(imidazole) analog with the goal of
uncovering such reactivity.
Ϫ
bis(imidazole) complex readily reacts with CO, PMe3, N3
and SCNϪ.
Preorganization of metal binding residues is an important
thermodynamic advantage achieved by proteins that insures
kinetically robust active site compositions. The entatic state is
the classic and most extreme case in which the polypeptide fold
fixes ligand positions resulting in a metal ligation sphere dis-
torted from the thermodynamic ground state structure.1 The
consequence of the distortion is a reduced activation barrier to
electron transfer or to a chemical transformation. Entatic-like
structures are difficult to reproduce outside the protein matrix.
Consequently, ligand preorganization remains a challenging
goal for the synthetic chemist. Herein, we report our initial
contribution to the area with the design, preparation and char-
acterization of two new cavitand ligands preorganized to
provide biologically representative imidazolyl donors for metal
ion coordination. The crystallographically defined copper()
derivatives exhibit markedly different reactivity with the poten-
Ϫ
tial donors CO, PMe3, N3 and SCNϪ. Contemporary with
these studies, several reports of calixarene-2,3 and 1,3,5-tri-
methylbenzene-based4 poly(imidazolyl) ligands have appeared.
Each ligand was prepared in good yield starting from 1,3,5-
triethylbenzene and 3-methyl-1-imidazolyl-2-thione following
the procedures outlined in Scheme 1.† Careful control of tem-
perature during bromomethylation of 1,3,5-triethylbenzene
allowed for clean formation of either the bis(bromomethyl) or
tris(bromomethyl) intermediates required for ligand formation.
Reaction of [Cu(CH3CN)4]BF4 with BiMIm in methylene
chloride afforded [(BiMIm)Cu(CH3CN)]BF4 as an analytically
pure, white solid. [(BiMIm)Cu(CH3CN)]BF4 is soluble in most
organic solvents and decomposes both in solution and in the
solid state upon exposure to moist air. The coordination of
acetonitrile was confirmed by FT-IR spectroscopy, νCN at
† Electronic supplementary information (ESI): complete synthetic
details. See http://www.rsc.org/suppdata/dt/b1/b102480a/
Scheme 1
DOI: 10.1039/b102480a
J. Chem. Soc., Dalton Trans., 2001, 1803–1805
This journal is © The Royal Society of Chemistry 2001
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Scheme 2
distance of 1.99(1) Å and average N–Cu–N angle of 119.1(1)Њ
lie within the range of known three-coordinate Cu() complexes
with nitrogen donors.6 The coordination of acetonitrile
provided an anticipated metal–solvent labile site for ligand
substitution studies. [(BiMIm)Cu(CH3CN)]BF4 exhibits an
irreversible oxidation in acetonitrile at high potential, Ea = 1050
mV, consistent with formation of the copper() adduct. That its
oxidation is ca. >600 mV more positive than [(TriMIm)Cu]BF4
indicates that the three imidazolyl donors of the latter greatly
stabilize the Cu() oxidation state. This observation is well
documented in blue copper protein sites, although the origin
of the redox stabilization is attributed to peptide structural
effects.7
Unlike the TriMIm analog, [(BiMIm)Cu(CH3CN)]BF4 reacts
with a variety of donor ligands, as highlighted in Scheme 2. At
Ϫ78 ЊC, [(BiMIm)Cu(CH3CN)]BF4 binds CO reversibly produc-
ing the three and four coordinate adducts as evidenced by the
appearance of terminal νCO stretching bands in the FT-IR spec-
trum at 2073 cmϪ1 and 2108 cmϪ1. The former band is assigned
to a four coordinate complex, [(BiMIm)Cu(CH3CN)(CO)]BF4,
whilst the latter is consistent with three coordinate [(BiMIm)-
Cu(CO)]BF4.8,9 The bands are appropriately sensitive to iso-
topic substitution; under an atmosphere of 13CO, two bands
appear at 2026 cmϪ1 and 2061 cmϪ1. Under these conditions the
four coordinate complex was formed initially followed by con-
version to the three coordinate species, ultimately yielding an
equilibrium mixture of the two complexes. The reversible
nature of CO binding was evidenced by the disappearance
of the CO bands upon warming the solution to ambient
temperature.
Reaction of [(BiMIm)Cu(CH3CN)]BF4 with PMe3 in methyl-
ene chloride yielded [(BiMIm)Cu(PMe3)]BF4 as a white solid.
PMe3 coordination was indicated by the quartet resonance in
the 31P NMR spectrum, 1JCu–P, 800 Hz (63Cu, I = 3/2). Although
several copper()–PPh3 complexes with nitrogen based ligands
have been reported,10 few of the analogous Cu()–PMe3 com-
pounds have been synthesized.11
Neutral thiocyanato and azido derivatives of [(BiMIm)-
Cu(CH3CN)]BF4 were accessed via metathetical ligand
exchange, Scheme 2. [(BiMIm)Cu(CH3CN)]BF4 reacted with
KSCN in acetone resulting in isolation of [BiMIm]Cu(NCS).
Coordination of SCNϪ was confirmed by a strong FT-IR
absorption band at 2109 cmϪ1 assigned to the CN stretch of the
thiocyanate ligand. No CH3CN derived vibration was present.
The assignment corresponds well with other reported Cu()–
thiocyanate compounds.12 The azido species, [BiMIm]Cu(N3),
was accessed similarly. The diagnostic asymmetric azide stretch
occurs at 2041 cmϪ1, again consistent with values reported for
nitrogen ligated copper()–azido complexes.13
Fig. 1 Molecular structures of the cations of [(TriMIm)Cu]BF4 (A)
and [(BiMIm)Cu(CH3CN)]BF4 (B). Selected bond distances (Å) and
bond angles (Њ) for [(TriMIm)Cu]BF4: Cu(1)–N(2) 2.003(3), Cu(1)–N(4)
2.001(3), Cu(1)–N(6) 1.990(3); N(2)–Cu(1)–N(4) 119.4(1), N(2)–Cu(1)–
N(6) 119.9(1), N(4)–Cu(1)–N(6) 119.1(1) and for [(BiMIm)Cu(CH3-
CN)]BF4: Cu(1)–N(2) 1.956(3), Cu(1)–N(4) 2.021(3), Cu(1)–N(5)
1.987(3); N(2)–Cu(1)–N(4) 126.2(1), N(2)–Cu(1)–N(5) 130.7(1), N(4)–
Cu(1)–N(5) 101.2(1).
2267 cmϪ1.5 [(BiMIm)Cu(CH3CN)]BF4 has been characterized
by X-ray diffraction, Fig. 1. The structure reveals the Cu() ion
residing in a trigonal planar geometry with the BiMIm ligand
bound via both imidazolyl groups, N2 and N4, with coordin-
ation completed by an acetonitrile molecule. The average Cu–N
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In summary, two new imidazolyl cavitand ligands and their
corresponding copper() complexes have been prepared and
structurally characterized. In [(TriMIm)Cu]BF4, the ligand
provides a trigonal array of donors that greatly stabilizes the
copper() with respect to ligand substitution and oxidation.
Removal of one imidazolyl substituent results in isolation of
[(BiMIm)Cu(CH3CN)]BF4 in which the acetonitrile ligand is
sufficiently labile to permit access to three coordinate deriv-
atives with CO, PMe3, N3Ϫ and SCNϪ coordination. Curiously,
[(BiMIm)Cu(CH3CN)]BF4 does not react with NO.
Acknowledgements
We thank the National Institutes of Health (GM-59191-01) for
support of this work.
Notes and references
‡ TriMIm = 1,3,5-tris(3-methyl-1-imidazolyl-2-thione)-2,4,6-triethyl-
benzene;
triethylbenzene.
BiMIm = 1,3-bis(3-methyl-1-imidazolyl-2-thione)-2,4,6-
Crystal data for [(TriMIm)Cu]BF4ؒ0.5CH2Cl2: C27.5H37BClCuF4-
N6S3, M = 733.61, triclinic, a = 8.4150(2) Å, b = 12.8962(2) Å,
c = 15.3722(2) Å, α = 90.4394(6)Њ, β = 99.6345(5)Њ, γ = 93.8131(4)Њ,
3
¯
V = 1640.75(2) Å , T = 173(2) K, space group P1, Z = 2, µ(Mo-
Kα) = 99.0 cmϪ1 (λ = 0.71073 Å), 9720 reflections measured,
R1 = 0.0561.
Crystal data for [(BiMIm)Cu(CH3CN)]BF4: C24H33BCuF4N5S2,
M = 606.02, monoclinic, a = 18.6841(2) Å, b = 8.2017(2) Å, c =
19.1047(2) Å, β = 100.8733(3)Њ, V = 2875.07(6) Å3, T = 193(2) K, space
group P21/n, Z = 4, µ(Mo-Kα) = 95.3 cmϪ1 (λ = 0.71073 Å), 11762
reflections measured, R1 = 0.0514. CCDC reference numbers 163539
and 163540. See http://www.rsc.org/suppdata/dt/b1/b102480a/ for
crystallographic data in CIF or other electronic format.
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