kass
kdiss
coordinate monohydrated species, while the slower process
occurs from the short-lived eight-coordinate intermediate. The
rate of decay of europium emission from [Eu.4] was
3+
[
Ln.4]3+ + H2O
[Ln.4(H O)]
2
3
+
kem
k′em
characterised by a single exponential decay profile, both in
3
21
water (kH O = 1.72 3 10 s ) and in dry acetonitrile (kMeCN
=
2
3
21
0
.76 3 10 s 293 K). The observed mono-exponential decay
Scheme 2
rate in MeCN increased as a function of added water and from
a titration measuring kobs as a function of added water
concentration, the equilibrium constant for dissociation of water
d
from the hydrated species was assessed to be K = 42 ±5 mM
(
298 K). Between 240 and 0 °C the rate of water exchange was
sufficiently slow that independent (bi-exponential) decay was
observed from the eight- and nine-coordinate species, under
2
conditions where sufficient H O had been added to allow each
species to be significantly populated. The proportion of the
shorter-lived component (i.e. the species with water bound to
Eu) in the bi-exponential decay increased as the temperature
was lowered.
Taken together, this information is consistent with rate-
limiting dissociative water exchange which occurs ca. 500
times faster at ambient temperature at Yb, in a twisted square-
antiprismatic isomer, than at Eu in the related square-
antiprismatic complex. The experiments also suggest that water
(
proton) exchange may be amenable to analysis by variable
Fig. 2 View of the molecular structure of (R)-L-[Yb.4(H
SO ·3H O at 120 K, showing the hydrogen bonding of the inner and
second sphere water molecules.
2 3
O)](CF -
temperature luminescence measurements.
We thank EPSRC for support, and the Royal Society for a
Leverhulme Trust Senior Research Fellowship (D. P.).
3
)
3
2
for the water signal at 283 K. Line-shape analysis for the water
signal in the temperature range 293–343 K correcting for the
Notes and references
1 D. H. Powell, O. M. Ni Dhubhghaill, D. Pubanz, L. Helm, Y. S.
Lebedev, W. Schlaepfer and A. E. Merbach, J. Am. Chem. Soc., 1996,
118, 9333.
2 G. Gonzàles, D. H. Powell, V. Tissierès and A. E. Merbach, Inorg.
Chem., 1994, 32, 3844; D. Pubanz, G. Gonzalez, D. H. Powell and A. E.
Merbach, Inorg. Chem., 1995, 34, 4457.
predominant dipolar T dependence of the chemical shift,7
2
2
allowed the rate of water proton exchange to be measured (Dn
4
4
Yb
2
=
H O
(
9.6 3 10 Hz at 233 K, 6.2 3 10 Hz at 283 K; p
= 0.11,
O
H
free
p = 0.89). At 298 K, the forward rate of association
2
5
21
Scheme 2) was 2.5 3 10 s , and the rate of dissociation from
6
21
ytterbium was 1.9 ± 0.7 3 10 s with a free energy of
3
S. Aime, A. Barge, M. Botta, A. S. de Sousa and D. Parker, J. Am. Chem.
Soc., 1997, 119, 4767.
2
1
activation of 34.6 ± 2.6 kJ mol
A single crystal X-ray diffraction analysis of (R)-[Yb-
4(H O)]CF SO ·3H O at 120 K revealed a nine-coordinate
.
4
R. S. Dickins, J. A. K. Howard, C. L. Maupin, J. M. Moloney, D. Parker,
J. P. Riehl, G. Siligardi and J. A. G. Williams, Chem. Eur. J., 1999, 5,
1095.
.
2
3
3
2
8
complex (Fig. 2) with ligand Yb–N and Yb–O distances
averaging 2.62 and 2.28 Å, some 0.08 Å longer than in the
5 S. Aime, A. Barge, M. Botta, A. S. De Sousa and D. Parker, Angew.
Chem. Int., Ed., 1998, 37, 2673.
4
related europium complex. The Yb–OH
2
distance of 2.44 Å
6
S. Aime, A. S. Batsanov, M. Botta, R. S. Dickins, S. Faulkner, C. E.
Foster, A. Harrison, J. A. K. Howard, J. M. Moloney, T. J. Norman, D.
Parker, L. Royle and J. A. G. Williams, J. Chem. Soc., Dalton Trans.,
was 0.01 Å longer than in the Eu analogue, but is similar to the
only other Yb–water bond length reported for a nine-coordinate
species.10 The N–C–C–N and N–C–C–O torsional angles
averaged +59.0 and 228.9°, consistent with a L(dddd)
1
997, 3623.
22
7
The T temperature dependence of the chemical shift of the bound
4 4 4
configuration. The twist of the N /O planes about the C axis
water resonance (2.9 Å from Yb) is an approximation, as it assumes that
was 239.7°, typical of a regular square-antiprismatic geometry
at the metal centre. The selective crystallisation from water of
this minor solution isomer at 293 K may seem surprising, but
related square-antiprismatic complexes also crystallise prefer-
the contact shift (varying as T21) is not significant. Earlier work has
2
2
shown that a T dependence represents a good approximation (valid to
±10%) for the dipolar term: B. McGarvey, J. Magn. Reson., 1979, 33,
4
45 and references therein. The most shifted axial ring proton (3.5 Å
9
from the Yb centre), resonating at d 108 at 293 K (CD OD), showed a
3
entially.
2
2
T
dependence of its chemical shift.
Crystal data for C46 Yb(CF
monoclinic, space group P2 ; a = 14.390(1), b = 11.928(1), c =
9.300(1) Å; b = 102.67°, U = 3232.1(4) Å , D ,
Luminescence from the 2F5/2 and D
5
excited states of the Yb
o
8
66
H N
8
O
1
5
3 3 3 2
SO ) .3H O: M = 1509.29,
and Eu ions occurs on the micro- and milli-second timescale,
respectively. The slowness of water exchange suggested that
measurements of the rate constant for decay of the luminescent
lanthanide excited state might allow simultaneous observation
3
23
1
c
= 1.551 g cm
21
l(Mo-Ka) = 0.71073 Å, Z = 2, m = 1.640 mm . Data were collected
on a SMART at 120(2) K. Refinement of 999 parameters by full matrix
2
of decay from the shorter-lived nine-coordinate (Ln·OH
2
)
least squares on F (SHELX96) converged at R = 0.021, wR
2
= 0.051
species (in which vibrational quenching to OH oscillators
occurs) and from the eight-coordinate lanthanide species. The
for 16442 observed reflections with I > 2s(I). The disordered phenyl
ring in the structure was isotropically refined with two sets of partially
occupied atoms. CCDC 182/1231. See http://www.rsc.org/suppdata/cc/
3+
rate constant for decay of Yb luminescence in [Yb.4] was 1.6
1
999/1011/ for crystallographic files in .cif format.
5
21
3
10 s at 295 K in D
curve was observed. In H O, kobs = 1.4 3 10 s at 295 K, but
bi-exponential decay was apparent at temperatures below 280
K. In D O there are no OH (or NH) oscillators available to
quench the luminescence and a single exponential decay is
observed: in H O the faster decay may be ascribed to the nine-
2
O, and a single exponential decay
9
M. Woods, J. A. K. Howard, A. M. Kenwright, J. M. Moloney, D.
6
21
2
Parker, M. Navet and O. Rousseau, Chem. Commun., 1998, 1381.
1
0 I. N. Polyakova, T. A. Senina, T. N. Polynova and M. A. Porai-Koshits,
Koord. Khim., 1983, 9, 1131: Yb–OH = 2.41 Å.
2
2
2
Communication 9/02260K
1012
Chem. Commun., 1999, 1011–1012