C O M M U N I C A T I O N S
Scheme 2. Energetics of Dimer-Tetramer Interconversion
methyl groups onto this [Cu
steric energy builds up to ∼20 kcal mol () ∆Hdimer-tetramer
metathesis), which is large enough to invert this inherent
4 4
L ] platform, however, the collective
-
1
+
q
12
∆H
1
5
Figure 1. (a) H NMR spectra of an equimolar mixture of [Cu2L 2] (b)
preference for tetramer over dimer (Scheme 2).
6
and [Cu2L 2] (9) in CDCl3 at 25 °C. Methyl proton signals from the
In sum, a delicate interplay between steric constraint and
mechanical stability shapes the energy landscape of self-assembly,
which could be mapped out using thermodynamic and kinetic
parameters of metathesis reactions. As a bonus, steric energies
dictating such ligand-directed assembly and disassembly could be
5
6
heteroleptic [Cu2L L ] complex (/) build up over time with concomitant
diminution of those from b and 9. (b) CI-MS of the solution sample of (a)
after 48 h. (c) An Eyring plot for ligand metathesis reaction between
7
8
[
Cu2L 2] and [Cu2L 2].
1
1
) could readily be obtained from integrated methyl H NMR
resonances. Four independent sets of [Cu L′ ] + [Cu L′′ ] afforded
quantified without resorting to empirical parameters or computa-
tional methods.3
2
2
2
2
K values ranging 3.5-4.2 (Table S2), which are close to the
statistical mixing entropy term of 4. The essentially isoenthalpic
nature of this metathesis reaction shown in Figure 1 allowed us to
use ligand scrambling between differently substituted copper dimers
Acknowledgment. This work was supported by Indiana Uni-
versity. Acknowledgment is also made to the donors of the
American Chemical Society Petroleum Research Fund for support
of this research.
(L′ * L′′) to approximate kinetic parameters dictating genuine self-
exchange (L′ ) L′′).
Supporting Information Available: Experimental details (PDF)
and crystallographic data (CIF). This material is available free of charge
via the Internet at http://pubs.acs.org.
To attain baseline resolutions of methyl 1H NMR signals,
7
8
compounds [Cu
2 2 2 2
L ] (7) and [Cu L ] (8) were deployed in our
kinetic studies. Under pseudo-first-order conditions ([8] > 12[7]),
the formation kinetics of [Cu
7
8
2
L L ] cleanly correlates with the decay
References
kinetics of both 7 and 8, with a second-order rate constant of k )
-
3
-1 -1
(1) Steed, J. W.; Atwood, J. L. Supramolecular Chemistry. John Wiley &
Sons: Chichester, U.K., 2000.
(2) (a) Caulder, D. L.; Raymond, K. N. Acc. Chem. Res. 1999, 32, 975-982.
(b) Lehn, J.-M.; Eliseev, A. V. Science 2001, 291, 2331-2332. (c) Acc.
Chem. Res. 2005, 38, 215-378 (special issue on Molecular Architecture).
(
1
7.0 ( 0.3) × 10
M
s
at 20 °C. From an Eyring plot (Figure
c) constructed from temperature-dependent measurements (Figure
q
-1
S4), activation parameters of ∆H ) 14.5 ( 1.2 kcal mol and
q
-1 -1
∆
S ) -19.3 ( 3.9 cal mol
K
could be obtained. The negative
(
3) Absolute steric energy associated with any substituent needs to be
quantified within the context of its interactions with a specific molecular
entity, making its generalization less than straightforward. Empirical scales
have been commonly used to assess “steric effects” instead, which
activation entropy is consistent with an associative mechanism, in
which formation of the mechanistically required tetrameric inter-
mediate becomes rate-determining. An alternative pathway requiring
complete dissociation of free ligand from the dicopper(I) complex
is less likely, considering the high energy required for complete
charge separation as well as the positive activation entropy
anticipated for this scenario, which is inconsistent with the
experimental observation.
include: (a) Taft parameter, E
ratios of acid-catalyzed ester hydrolysis with methyl as the standard group;
b) Winstein-Holness A value, which is the free energy difference, -∆G°,
s
) log(k/k ), based on the relative rate
o
4
(
5
between axial and equatorial isomers of monosubstituted cyclohexanes;
c) cone angle, θ, defined as the internal angle of an enveloping cone of
(
6
a metal-bound PR ligand with M-P distance of 2.28 Å. Computational
3
7
studies have complemented such efforts.
(
4) Taft, R. W. In Steric Effects in Organic Chemistry, Newman, M. S., Ed.;
Wiley: New York, 1956; Chapter 13.
From the thermodynamic and kinetic studies described above
emerges a unifying energy diagram (Scheme 2). The equilibrium
constants of K ≈ 4 determined for the metathesis of bulky ligands
indicate no enthalpic preference for either reactants’ or products’
side, therefore indicating a symmetric reaction coordinate with the
(
5) Winstein, S.; Holness, N. J. J. Am. Chem. Soc. 1955, 77, 5562-5578.
(6) Tolman, C. A. Chem. ReV. 1977, 77, 313-348.
(
7) (a) Brown, T. L.; Lee, K. J. Coord. Chem. ReV. 1993, 128, 89-116. (b)
Woo, T. K.; Ziegler, T. Inorg. Chem. 1994, 33, 1857-1863. (c) White,
D. P.; Anthony, J. C.; Oyefeso, A. O. J. Org. Chem. 1999, 64, 7707-
7716.
(
8) Jiang, X.; Bollinger, J. C.; Baik, M.-H.; Lee, D. Chem. Commun. 2005,
tetrameric intermediate C in the middle (Scheme 2, right side). The
1043-1045.
q
experimentally determined activation barrier leading to C, ∆H metathesis
,
(9) For use of steric controller groups and reaction stoichiometry to access
discrete tetra-, tri-, and dinuclear Au(I) clusters of amidinate ligands,
see: Abdou, H. E.; Mohamed, A. A.; Fackler, J. P., Jr. Inorg. Chem.
2005, 44, 166-168.
10) See Supporting Information.
11) Unless otherwise noted, H NMR peaks are referenced to (Me
is the upper limit of the dimer-tetramer energy difference for the
bulky ligand system. The corresponding dimer-tetramer energy
difference in the unsubstituted “less-bulky” system (Scheme 2, left
side), ∆Hdimer-tetramer, is measured from the thermodynamics of
dimer-tetramer equilibrium. Without steric constraint, the enthalpic
(
(
1
3
Si)
2
O, which
also served as an internal standard in quantitation.
5
8
(12) This estimation assumes that 2,6-dimethyl substitutions in L -L induce
negligible interligand steric congestion at the dimer level, and therefore,
the two dicopper(I) species in Scheme 2 are of comparable energy. In
support of this notion, X-ray structures of 5-8 show no van der Waals
-
1
gain (∆H ≈ -5 kcal mol ) associated with the formation of
interlinked and thus mechanically more robust [Cu ] structure
overrides the entropic cost (∆S ≈ -10 cal mol K ) of dimerizing
two [Cu ] units. With bulky ligands that introduce a total of 16
4 4
L
1
0
-1
-1
contacts between adjacent methyl groups.
2 2
L
JA055225U
J. AM. CHEM. SOC.
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VOL. 127, NO. 45, 2005 15679