Stabilisation of a very short Cu-F bond within the protected cavity of a copper(ii) compound from a tris(2-aminoethyl)amine derivative

Article abstract of DOI:10.1039/b902907a

The copper(ii) coordination compound of an N-functionalised derivative of tris(2-aminoethyl)amine forms a cavity that is an excellent fluoride ion host, generating a Cu-F entity with a very short distance (182 pm) and characterised by a fluoride ion devoi

Full text of DOI:10.1039/b902907a

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Stabilisation of a very short Cu–F bond within the protected cavity of a
copper(II) compound from a tris(2-aminoethyl)amine derivative†
Ann Almesa˚ker,a,b Patrick Gamez,b Jan Reedijk,*b Janet L. Scott,a Leone Spiccia*a and Simon J. Teatc
Received 11th February 2009, Accepted 26th March 2009
First published as an Advance Article on the web 8th April 2009
DOI: 10.1039/b902907a
The copper(II) coordination compound of an N-functionali-
sed derivative of tris(2-aminoethyl)amine forms a cavity that
is an excellent fluoride ion host, generating a Cu–F entity with
a very short distance (182 pm) and characterised by a fluoride
ion devoid of any additional intermolecular interactions.
copper(II) fluoride coordination compound, whose single-crystal
X-ray structure reveals the shortest reported Cu–F distance, thus
highlighting the ability of this type of ligand to generate products
with novel structural features and properties.
The copper(II) fluoride compound, [CuLF]BF4, was initially
isolated in 28% yield from a THF solution of Cu(BF4)2·xH2O and
L that had been layered with ethanol. The fluoride was generated
The tripodal ligand, tris(2-aminoethyl)amine, tren, and more elab-
orate or extended tren ligands have found wide application in the
preparation of transition-metal complexes.1–4 Due to the tripodal
arrangement of the four nitrogen donor atoms, these ligands are
ideal candidates for the synthesis of trigonal-bipyramidal metal
complexes, and a variety of copper(II) coordination compounds
with this geometry have been reported.1 These metal complexes
have been used in catalysis and dioxygen binding studies.5–7 We
have used these ligands to stabilise redox-active oxoanions, such
as thiosulfate and thiosulfonates,8 and to prepare cyanido-bridged
heteropolynuclear clusters with novel magnetic properties.9,10 The
availability of a single, exchangeable coordination site on the
CuII centre has provided considerable control over the number
of reaction products, facilitating their isolation, and over the reac-
tivity of thiosulfate and the thiosulfonates. Of the tren derivatives,
the aryl substituted versions have been little explored, although
various of these have been employed in the formation of transition
amido complexes,11–14 of which some Mo compounds have been
demonstrated to act as catalysts in dinitrogen reduction.13,14
Some of us recently reported on a simple multi-component
synthesis of aryl substituted tren derivates containing flexible pen-
dant arms, i.e. tris((2-(4-nitrobenzyl)phenylamino)ethyl) amine,
(p-NO2BP)3tren (L).15 L has a shielded cavity for a metal to
bind to the four nitrogen atoms of the tren moiety leaving an
additional coordination site for binding a small exchangeable
ligand, protected by the extended arms of L.
-
by gradual decomposition of BF4 , a relatively well-known phe-
nomenon observed in a number of copper complexes,17–19 mostly
with bulky substituted pyridines and pyrazoles,18,19 or some amine
ligands.17 The low isolated yield from this reaction led us to develop
an alternative synthesis. The use of tetraethylammonium fluoride
(Et4NF), as source of fluoride anions, also produced the desired
compound, showing identical IR and solid-state EPR spectra to
those of the previously isolated compound, with a yield of 67%.
The X-ray crystal structure of [CuLF]BF4 is depicted in Fig. 1.‡
The copper(II) ion is pentacoordinated with an almost ideal
trigonal-bipyramidal coordination environment (t = 0.99)20, as
expected for complexes of substituted tren ligands. The three
equatorial positions are occupied by the aniline nitrogen atoms,
Neq, and the axial positions are occupied by the tertiary tren
-
nitrogen atom, Nax, and a fluoride anion resulting from BF4
decomposition (see Table 1 for selected bond distances and angles).
-
A BF4 counter-anion is located outside the cavity of the ligand
and is non-coordinating (but is hydrogen bonded to the amine
˚
groups with N(3)–H(N3) ◊ ◊ ◊ F(12b) = 2.979(5) A, Table 2).
Such protected exchangeable ligand binding sites allow the
occurrence of interesting and unusual interactions and novel prop-
erties. For example, previous studies have shown the possibility
of stabilising coordinated small molecules and ions, such as O2
and N2.13,16 We report herein the use of L in the synthesis of a
aSchool of Chemistry and Centre for Green Chemistry, Monash University,
Victoria, 3800, Australia. E-mail: leone.spiccia@monash.edu.au.; Fax: +61
3 9905 4597; Tel: +61 3 9905 4526
As is typical for the copper(II) compounds of these tripodal
ligands, the copper centre is slightly out of the plane, which is made
˚
up of the Neq atoms, displaced towards the fluoride ion (0.192(1) A
bLeiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University,
PO Box 9502, 2300, RA, Leiden, The Netherlands. E-mail: reedijk@
chem.leidenuniv.nl.; Fax: +31 71 527 4671; Tel: +31 71 527 4459
above the plane), and the Cu–Nax distance is shorter than the
Cu–Neq distances.21–23 The general trend for tren complexes is
that the Cu–Neq distances increase while the Cu–Nax remains
constant with the steric bulk, as shown by the series tren <
Me3tren ª Bz3tren < Me3Bz3tren ª Me6tren.21,22 (p-NO2BP)3tren
would fit at the end of this series, since the direct connection
between the aryl group and the secondary amine nitrogen atoms
cAdvanced Light Source, Lawrence Berkeley National Laboratory, Berkeley,
California 94720, USA
† Electronic supplementary information (ESI) available: Fig. S1–S3, ex-
perimental section and crystallographic details. CCDC reference numbers
720265. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/b902907a
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Table 1 Selected bond lengths and angles
Angles/◦
˚
Bond distances/A
Cu(1)–F(1)
Cu(1)–N(1)
Cu(1)–N(2)
Cu(1)–N(3)
Cu(1)–N(4)
1.821(1)
2.001(2)
2.207(2)
2.160(2)
2.176(2)
F(1)–Cu(1)–N(1)
F(1)–Cu(1)–N(2)
F(1)–Cu(1)–N(3)
F(1)–Cu(1)–N(4)
N(1)–Cu(1)–N(2)
N(1)–Cu(1)–N(3)
N(1)–Cu(1)–N(4)
N(2)–Cu(1)–N(3)
N(2)–Cu(1)–N(4)
N(3)–Cu(1)–N(4)
175.92(7)
98.36(8)
95.20(8)
91.99(7)
84.79(7)
85.62(7)
84.45(7)
116.71(7)
110.80(7)
130.16(7)
Fig. 2 Histograms of Cu–F distances found in the CSD (version 5.29, Aug
˚
2008). (a) All distances up to 3.2 A (n = 236). (b) 20 shortest distances.
hydrogen atom of each of the three arms (Fig. 3).29,30 These
Table 2 Hydrogen bonds
˚
C–H ◊ ◊ ◊ F contacts (C ◊ ◊ ◊ F distance 3.044(4) A, Table 2) can be
seen as weak hydrogen bonds, and assemble the pendant arms
into a tightly closed binding pocket. A few structures are known
in which the delocalisation of the high electron density on the
fluoride ion occurs by hydrogen bonding to aromatic and aliphatic
hydrogens. These systems also exhibit short Cu–F distances, but
not as short as that reported herein.18,24,26
˚
˚
Hydrogen bonds
d H ◊ ◊ ◊ F/A
d C ◊ ◊ ◊ F/A
∠C–H ◊ ◊ ◊ F/◦
C(9)–H(9B) ◊ ◊ ◊ F(1)
2.34
2.19
2.40
2.38(3)
2.20(2)
2.22(2)
3.114(4)
3.044(4)
3.192(3)
3.008(5)
2.979(5)
3.000(2)
135
145
137
130(3)
149(3)
148(2)
C(24)–H(24B) ◊ ◊ ◊ F(1)
C(39)–H(39B) ◊ ◊ ◊ F(1)
N(3)–H(N3) ◊ ◊ ◊ F(12a)
N(3)–H(N3) ◊ ◊ ◊ F(12b)
N(4)–H(N4) ◊ ◊ ◊ F(13)
Fig. 1 Diagram of the cation [CuLF]+ shown in thermal ellipsoids at 50%
probability level. Hydrogen atoms except those associated with the amine
groups have been omitted for clarity.
Fig. 3 Diagram showing hydrogen bonding interactions (red dotted lines)
between an aliphatic hydrogen atom of each arm and the fluoride anion.
The packing of the X-ray structure also indicates the presence of
unusual nitro–nitro interactions between neighbouring complexes.
introduces even more steric bulk than is present in the last two
members in this series. The novel and most exciting feature of
the compound is the extremely short copper fluoride coordination
˚
For example, the O ◊ ◊ ◊ O distances, viz., O(1) ◊ ◊ ◊ O(4)¢ 2.883(6) A
˚
and O(3) ◊ ◊ ◊ O(3)¢ 2.998(9) A, are shorter than the sum of the
˚
˚
distance. In fact, the Cu–F distance of 1.821(1) A is the shortest
van der Waals radii (3.04 A), and shorter than many reported
reported so far. A thorough analysis of the Cambridge Structural
Database, CSD (version 5.29, Aug 2008), revealed that the shortest
previously in the CSD (see Fig. S1 and S2, ESI).†
The EPR spectrum of a frozen THF solution of [CuL](BF4)2
shows characteristic features indicative of a distorted octahedral
coordination geometry (Fig. 4, Table 3).31 Addition of one
equivalent of Et4NF caused a drastic change in the spectrum,
which is reflected by major changes of the g and A values.
The spectrum is now representative of a trigonal-bipyramidal
coordination geometry,23,32 and is almost identical to that observed
for [Cu(TPA)F]PF6, whose formation involved the decomposition
of a PF6 anion.18
˚
previously reported Cu–F distance was 1.836(2) A for a bispidine
copper complex,24 followed by 1.852(4) A for a [Cu(TPA)F]PF6
˚
(TPA = tris((2-pyridylmethyl)amine) complex.18 There are several
structures with much longer Cu–F distances (see Fig. 2).
The fluoride ion in [CuLF]BF4 is solely bound to the copper(II)
centre and does not participate in any other coordinative or
hydrogen-bonding interactions. This is extremely unusual as fluo-
ride ions coordinated to copper(II) usually either bridge to a second
copper centre,17,25,26 or their high electron density is stabilised via
hydrogen bonding with water or alcohol molecules.27,28
In [CuLF]BF4, the hydrophobic cavity generated by the pendant
arms of L prevents the fluoride ligand from forming bridged
complexes and from accepting hydrogen bonds from H bond
donors. There are, however, weak interactions with an aliphatic
Similarly, the UV-vis spectrum showed significant changes on
addition of a fluoride source (Fig. S3).† The spectrum of the
THF solution containing L and Cu(BF4)2·H2O in a 1 : 1 ratio
before addition of fluoride exhibited one maximum at 640 nm
(e = 380 M-1cm-1), indicative of a distorted octahedral copper
2
2
complex (dxz, dyz → dx
transition).33 After addition of Et4NF,
-y
4078 | Dalton Trans., 2009, 4077–4080
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Table 3 EPR data for a frozen THF solution of a 1 : 1 mixture of L and
Cu(BF4)2·xH2O, and a powdered sample of the title compound
of Energy under Contract No. DE-AC02–05CH11231. AA ac-
knowledges the award of a Monash Graduate Scholarship and a
Monash International Postgraduate Research Scholarship.
Frozen solution at 77 K
g valuesa
A valuesa/mT
b
[CuL](BF4)2
gx = 2.044
gy = 2.082
gz = 1.255
gx = 2.043
gy = 2.165
gz = 1.908
Ax = 6.1
Ay = 7.9
Az = 10.1
Ax = 0.05
Ay = 2.6
Az = 16.0
Notes and references
c
‡ Crystal structure determination: All non-hydrogen atoms were refined
anisotropically. Geometrical and displacement parameter restraints were
[CuLF]BF4
-
used to model the BF4 group. Displacement parameter restraints were
used in modelling one end of one of the ligand arms, even so the ratio
of the displacement parameters max–min is around 5 : 1. Splitting the
end of the arm was considered, but as it reflected only the movement in
the arm and no new chemical information would be gained, it was left
as it was. Hydrogen atoms were placed geometrically where possible and
refined with a riding model. In the case of the N–H’s, these were found in
the difference map and these were allowed to refine with a restrained on
the N–H distance.
Solid
rt
77 K
[CuLF]BF4
gx = 2.01
gy = 2.18
gz = 2.22
gx = 2.02
gy = 2.17
gz = 2.22
a Values from simulated spectra.36 b Solution prepared by mixing equimolar
amounts of L and Cu(BF4)2·xH2O. c Previous solution after adding 1 equiv.
of tetraethylammonium fluoride.
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Fig. 4 EPR spectra of a frozen THF solution of (a) [CuL](BF4)2 before
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tetraethylammonium fluoride. Simulated spectra are shown in light-grey.36
* indicates a small amount of impurity present due to some unreacted
[CuL](BF4)2.
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Acknowledgements
This work was supported by an AKF grant and the Australian
Research Council through the Centre for Green Chemistry. The
Advanced Light Source is supported by the Director, Office of
Science, Office of Basic Energy Sciences, of the U.S. Department
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4080 | Dalton Trans., 2009, 4077–4080
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©