Gallium(III) catecholate complexes as probes for the kinetics and mechanism of inversion and isomerization of siderophore complexes

Article abstract of DOI:10.1021/ja953545f

The catechol siderophore analog K₃[Ga(3)₃], 4 (H₂3 = 2,3-dihydroxy-N,N'-diisopropylterephthalamide), is D₃-symmetric in aqueous solution, and exists in two enantiomeric forms, Δ-4 and Λ-4. Variable temperature N

Full text of DOI:10.1021/ja953545f

5712
J. Am. Chem. Soc. 1996, 118, 5712-5721
Gallium(III) Catecholate Complexes as Probes for the Kinetics
and Mechanism of Inversion and Isomerization of Siderophore
Complexes1
Berthold Kersting, Jason R. Telford, Michel Meyer, and Kenneth N. Raymond*
Contribution from the Department of Chemistry, UniVersity of California,
Berkeley, California 94720
ReceiVed October 23, 1995X
Abstract: The catechol siderophore analog K3[Ga(3)3], 4 (H23 ) 2,3-dihydroxy-N,N′-diisopropylterephthalamide),
is D3-symmetric in aqueous solution, and exists in two enantiomeric forms, ∆-4 and Λ-4. Variable temperature
NMR experiments demonstrate that the inversion of the enantiomers of 4 in D2O is facile. The rate of inversion is
independent of pH above pH 8. The mechanism is intramolecular. From line-shape analysis the free energy of
activation ∆Gq ) 67.4(9) kJ mol-1 in D2O at pD 12.1, with ∆Hq ) 58.5(6) kJ mol-1 and ∆Sq ) -0.030(9) kJ
298
mol-1 K-1. Below pD 8 the rate of inversion for 4 is pD dependent and initially first order in [D+]. Potentiometric
titrations reveal that 4 protonates in two one-proton steps with log KHML ) 4.66(4) and log KH ML ) 3.99(7). In
3
2
3
DMSO-d6, formation of a tight contact ion pair between K+ and [Ga(3)3]3- ions increases the free energy barrier to
inversion by ∼7 kJ mol-1. The complex K3[Ga(9)3], 10 (H29 ) 2,3-dihydroxy-N-tert-butyl-N′-benzylterephthalamide),
was prepared to elucidate the mechanism of inversion by dynamic NMR spectroscopy, using the fact that 10 exists
in two isomeric forms, cis-10 and trans-10, which are of C3 and C1 symmetry in solution. The ratio cis-10:trans-10
is 0.78(3) at ambient temperature in D2O or DMSO-d6. Two processes are distinguishable on the NMR time scale
in D2O or DMSO-d6, cis-10-trans-10 isomerization and the inversion of the enantiomers of trans-10. Both processes
proceed intramolecularly with Tc ) 295(1) K for Λ-trans-10 to ∆-trans-10 inversion and Tc ) 335(1) K for cis-10
to trans-10 isomerization in D2O at pD 9.5. The discrete exchange pattern of the tert-butyl resonances during inversion
of trans-10 confirms that the reaction proceeds by a trigonal twist mechanism Via a trigonal prismatic transition
state. The free energy barriers to inversion are ∆Gq ) 60 kJ mol-1 in D2O (pD 9.8) and ∆Gq ) 67 kJ mol-1
295
327
in DMSO-d6.
Introduction
acid units to form a tricatechol ligand, is important because of
its wide-spread utilization by bacterial pathogens. It forms a
1:1 ferric complex with an absolute configuration at the metal
center that is ∆.7 Some time ago it was shown that the synthetic,
mirror image enterobactin (which consequently has Λ config-
uration at the metal center) does not promote growth, as does
the natural siderophore.8 It was presumed that this was because
of stereospecific recognition at the outer membrane of ferric
enterobactin by the receptor protein. Subsequently it has been
found that the mirror image compound enantioenterobactin is
nearly as efficient in promoting transport across the outer
membrane as is the natural enterobactin.9 Where the mirror
image compound differs from enterobactin is at the culmination
of the iron transport step, transport across the cytoplasmic
membrane, and release of iron associated with enterobactin
hydrolysis. Is the rate of inversion of configuration at the metal
center of such a complex fast enough to affect these recognition
and transport processes? While the three catechol units of
enterobactin are tethered, the high stability of tricatecholate
complexes at medium to high pH makes metal-ligand dis-
sociative processes unlikely for such inversion and isomeriza-
tion. Hence, simple tricatecholate complexes provide a useful
model in determining the minimal rate, and probable mecha-
nism, associated with the isomerization reaction leading to
inversion at the metal center.
Siderophore-mediated iron uptake in microorganisms is
generally stereospecific.2-6 In general, the pseudooctahedral
metal centers of these complexes are chiral and are differentially
recognized by chirally-specific membrane-bound protein recep-
tors. In the absence of direct structural data, the mechanism of
recognition by these membrane-bound receptors has been
inferred from substrate mapping.2,3 The importance of the metal
complex configuration to recognition has generally been derived
from uptake or inhibition studies of kinetically-inert complexes
formed by replacing the labile, high-spin ferric ion with
kinetically inert metal ion substitutes.5,6
Enterobactin, composed of a cyclic triester of serine in which
the three amino groups are conjugated to 2,3-dihydroxybenzoic
* To whom corrrespondence should be addressed.
X Abstract published in AdVance ACS Abstracts, June 1, 1996.
(1) Coordination Chemistry of Microbial Iron Transport. 59. Part 58:
Hou, Z; Sunderland, C. J.; Nishio, T.; Raymond, K. N. J. Am. Chem. Soc.
1996, 118, 5148.
(2) Ecker, D. J.; Loomis, L. D.; Cass, M. E.; Raymond, K. N. J. Am.
Chem. Soc. 1988, 110, 2457.
(3) Matzanke, B. F.; Mu¨ller-Matzanke, G.; Raymond, K. N. Iron Carriers
and Iron Proteins. In Physical Bioinorganic Chemistry Series; Loehr, T.
M., Ed.; VCH Publishers: New York, 1989; pp 1-121.
(4) CRC Handbook of Microbial Iron Chelates; Winkelmann, G., Ed.;
CRC Press: Boca Raton, FL, 1991.
(5) Raymond, K. N.; Telford, J. R. Siderophore-mediated Iron Transport
in Microbes. In Bioinorganic Chemistry An Inorganic PerspectiVe of Life;
Kessissoglou, D. P., Ed.; NATO ASI Series; Kluwer Academic Publish-
ers: Dordrecht, The Netherlands, 1995; Vol. 459, pp 25-37.
(6) Raymond, K. N.; Mu¨ller, G.; Matzanke, B. F. Top. Curr. Chem. 1984,
123, 49.
(7) Karpishin, T. B.; Dewey, T. M.; Raymond, K. N. J. Am. Chem. Soc.
1993, 115, 1842.
(8) Venuti, M. C.; Rastetter, W. H.; Neilands, J. B. J. Med. Chem. 1979,
22, 123.
(9) Nishio, T.; Bryan, B.; Raymond, K. N. Manuscript in preparation.
S0002-7863(95)03545-1 CCC: $12.00 © 1996 American Chemical Society

Isomerization of Siderophore Complexes
J. Am. Chem. Soc., Vol. 118, No. 24, 1996 5713
Analyses of the rearrangement reactions of tris-bidentate
complexes have occupied an important part of the development
of coordination chemistry.10-13 A concerted twist motion of
the chelates about a trigonal face of the octahedron, without
disconnecting a metal-ligand bond, generates a trigonal-
prismatic transition state. When this is around the trigonal face
that corresponds to the 3-fold axis of the D3 point symmetry of
a symmetrical trichelate, the Bailar twist results.14 Evidence
for inversion reactions proceeding via this trigonal-prismatic
transition state has been obtained in a few complexes in which
the bidentate ligand is small. In these systems, it has been
concluded that the trigonal twist mechanism may be favored
by ground-state distortions toward the transition state.15 An
analysis of isomerization and inversion reactions of trischelate
complexes of siderophore-type ligands has not appeared in the
literature, although a sterically constrained macrobicyclic tri-
catecholatoiron(III) complex has been shown to have a trigonal-
prismatic coordination geometry.16
Scheme 1
Scheme 2
The major limitation to the study of such reactions in iron-
siderophore type complexes is their kinetic lability. This
prevents the separation of isomerically and enantiomerically pure
compounds and studies under nonequilibrium conditions.17 In
addition, the large line widths and accompanying poor spectral
resolution of high-spin Fe(III) complexes tend to rule out
dynamic NMR studies of isomerization. However, gallium-
(III), because of its similar chemistry,18,19 can be used instead
of iron, and the reactions at chemical equilibrium can be
investigated by NMR spectroscopy. Similar protocols have been
employed to characterize the kinetics and mechanism of
inversion of tris(acetylacetonate) and tritropolonate complexes
of various metal ions in nonaqueous solvents.20-23 In the present
study, variable temperature 1H NMR spectroscopic studies have
been employed to investigate the kinetics and mechanism of
inversion and isomerization reactions of these diamagnetic
gallium(III) complexes of synthetic siderophore-type ligands in
aqueous solution. A combination of pD-dependent NMR
spectra and potentiometric titration was carried out to character-
ize these reactions over a broad pH range. The complexes were
also studied in aprotic solvents. Finally, the corresponding iron-
(III) complexes were prepared and their structures in solution
investigated by infrared and vis/UV spectroscopy.
K[H(9)] with Ga(acac)3 or Fe(acac)3 in CH3OH at room
temperature. The complexes K3[M(L)3] (L ) 3, M ) Ga, 4;
M ) Fe, 5; L ) 9, M ) Ga, 10; M ) Fe, 11) were obtained in
high yields and were purified by recrystallization. The anions
[Ga(3)3]3- and [Fe(3)3]3- were also isolated as tetramethylam-
monium salts 12 and 13. All complexes decompose upon
prolonged exposure to air, and the potassium salts are quite
hygroscopic.24 Positive ion fast-atom-bombardment mass spec-
tra showed the molecular ion peaks of the mononuclear 1:3
complexes. The isotope distribution patterns are consistent with
mononuclear metal complexes. Two stereoisomers are possible
Results and Discussion
Syntheses. The ligands used in this study, H23 and H29, were
synthesized according to Schemes 1 and 2. Gallium(III) and
iron(III) complexes of H23 and H29 were obtained by the
reaction of the respective monopotassium salts K[H(3)] and
(10) (a) Basolo, F.; Pearson, R. G. Mechanisms of Inorganic Reactions,
2nd ed.; Wiley: New York, 1967. (b) Wilkins, R. G. Kinetics and
Mechanism of Reactions of Transition Metal Complexes, 2nd ed.; VCH
Publishers: New York, 1991.
(11) Fay, R. C.; Piper, T. S. Inorg. Chem. 1964, 3, 348.
(12) Fortman, J. J.; Sievers, R. E. Inorg. Chem. 1967, 6, 852.
(13) Gordon II, J. G.; Holm, R. H. J. Am. Chem. Soc. 1970, 92, 5319.
(14) Bailar, Jr, J. C. J. Inorg. Nucl. Chem. 1958, 8, 165.
(15) Rodger, A.; Johnson, B. F. G. Inorg. Chem. 1988, 27, 3062.
(16) Garrett, T. M.; McMurry, T. J.; Hosseini, M. W.; Reyes, Z. E.;
Hahn, F. E.; Raymond, K. N. J. Am. Chem. Soc. 1991, 113, 2965.
(17) McArdle, J. V.; Sofen, S. R.; Cooper, S. R.; Raymond, K. N. Inorg.
Chem. 1978, 17, 3075.
1
for 10 (cis-10 and trans-10), and H NMR spectroscopy of 10
in DMSO-d6, CD3OD, or D2O displays two sets of proton
resonances, corresponding to these isomers. At 295 K integra-
tion of the tert-butyl signals indicates the cis to trans ratio for
10 in DMSO-d6 is 0.78(1).
IR and Vis/UV Spectroscopy. IR spectral data of the ligands
H23 and H29 showed strong absorptions in the regions 3450-
3350 and 1600-1550 cm-1, corresponding to NH and CO
stretching modes, respectively.25 Upon coordination to gallium-
(18) Borgias, B. A.; Barclay, S. J.; Raymond, K. N. J. Coord. Chem.
1986, 15, 109.
(19) Loomis, L. D.; Raymond, K. N. J. Coord. Chem. 1991, 23, 361.
(20) Eaton, S. S.; Holm, R. H. J. Am. Chem. Soc. 1971, 93, 4913.
(21) Hutchison, J. R.; Gordon, J. G., II; Holm, R. H. Inorg. Chem. 1971,
10, 1004.
(22) Eaton, S. S.; Hutchison, J. R.; Holm, R. H.; Muetterties, E. L. J.
Am. Chem. Soc. 1972, 94, 6411.
(23) Eaton, S. S.; Eaton, G. R.; Holm, R. H.; Muetterties, E. L. J. Am.
Chem. Soc. 1973, 95, 1116.
(24) Gemlike crystals of 4, 5, 12, and 13 were easily grown by ether
diffusion in alcoholic solutions of the metal complexes. However, all of
the diffraction patterns obtained from rotation photographs showed diffuse
reflections.

5714 J. Am. Chem. Soc., Vol. 118, No. 24, 1996
Kersting et al.
a
Table 1. 1H NMR Data for H23, K3[Ga(3)3], and [NMe4]3[Ga(3)3] in CD3OD and DMSO-d6
H23
K3[Ga(3)3]
DMSO-d6
[NMe4]3[Ga(3)3]
CD3OD
DMSO-d6
CD3OD
CD3OD
DMSO-d6
assignment
n.o.
7.30 (s)
4.22 (spt)
8.60 (d)
7.35 (s)
n.o.
6.98 (s)
4.03 (spt)
10.76 (d)
6.65 (s)
n.o.
10.75 (d)
6.67 (s)
NHCO
ArH
CH(CH3)2
N(CH3)4
CH(CH3)2
CH(CH3)2
6.96 (s)
4.03 (spt)
2.95 (s)
1.22 (d)
1.05 (d)
4.14 (oct)b
3.84 (oct)b
3.85 (oct)b
3.03 (s)
+
1.24 (d)
1.18 (d)
1.19 (d)
1.01 (d)
1.09 (d)
0.95 (d)
1.10 (d)
0.97 (d)
a Chemical shifts are in ppm with respect to Me4Si (0.0 ppm) at 298 K; s ) singlet, d ) doublet, sep ) septet, oct ) octet. Coupling constants
are included in the Experimental Section. b NH/ND exchange in DMSO is slow on the NMR time scale; the similar coupling constants (3JNHCH
3JCHCH) cause overlapping of the two septets to yield an octet for the methine proton.
≈
atoms.27 1H NMR spectra of the [Ga(3)3]3- anion in D2O, CD3-
OD, or DMSO-d6 are all very similar, the only major difference
being the absence of the NH resonance in protic solvents due
to rapid NH/ND exchange. The ring, methine, and methyl
proton resonances of the metal complexes are shifted upfield
when compared to those of the free ligand. The methyl proton
resonances of the isopropyl group, which appear as a singlet in
the spectrum of the free ligand, are inequivalent and together
with the methine proton form an A3B3X system. Correspond-
ingly two signals, one for each inequivalent methyl carbon atom,
are observed in the 13C NMR spectrum of 4. Relative to the
protonated free ligand, the largest shifts are observed for the
carbon atoms bonded to the catecholate oxygens. The other
resonances are only slightly shifted. It is also clear that the
choice of counter cation does not influence the NMR properties
of the [Ga(3)3]3- anion. On the basis of these data the structure
of the [Ga(3)3]3- complex anion in solution is assigned D3
symmetry, with the three 2,3-dihydroxyterephthalamide ligands
chelated to gallium. The same symmetry has been observed
for closely related complexes of gallium(III) in the solid state.28
The complex 4 is chiral and forms the racemate of the two
enantiomeric forms Λ-4 and ∆-4. The interconversion of both
enantiomers would result in a site exchange of the two
inequivalent methyl proton resonances. This dynamic process
Figure 1. Electronic absorption spectra of 5 (I) (5.4 × 10-5 M), 11
(II) (4.9 × 10-5 M), and 13 (III) (4.3 × 10-5 M) in H2O (5% KH2PO4,
pH 9.1).
(III) or iron(III), a slight shift to higher wavenumbers in the
CO stretching modes was observed, while the lower absorption
of the two NH modes was shifted by 150 cm-1 in the opposite
direction. Both NH bands are also significantly broadened.
These effects are indicative of intramolecular hydrogen bonding
between the NH protons and ortho catecholate oxygen atoms
in the metal complexes. This conclusion is supported by the
1H NMR data of the gallium complexes in DMSO-d6 solution
(Vide infra).
1
was analyzed by variable temperature H NMR spectroscopy.
Kinetics of Inversion for 4. At 295 K, 4 displays an A3B3X
pattern for the methyl and methine protons and two 13C
resonances for the methyl groups of the isopropyl residues in
DMSO-d6 or CD3OD. The methyl proton and carbon reso-
nances result from geminal methyl groups, which are diaste-
reotopic because 4 is dissymmetric (the complex is chiral, point
group D3).
As shown in Figure 1, the absorption spectra of 5, 11, and
13 in H2O at pH 9.1 are very similar. Intense absorptions
(>4000 M-1 cm-1) appear in the UV due to ligand to metal
charge transfer (LMCT) and ligand-localized - * electronic
transitions.26 The absorption bands at ∼364 nm are readily
assigned as - * transitions of the ligands, while the bands at
446 and 510 nm are assigned as LMCT transitions. The energy
and intensity of the bands are typical for octahedral or
pseudooctahedral tris(2,3-dihydroxyterephthalamide)-iron(III)
complexes. By comparison, because of a higher degree of
-bonding in related trigonal-prismatic iron(III) complexes, a
shift of the higher energy LMCT to 428 nm is observed.16,26
In D2O, the A3B3X pattern of 4 is observed only above pD
8 and the A3B3 part of the A3B3X system coalesces on warming
1
or on lowering pD. Variable temperature H NMR spectra of
4 in D2O were collected in phosphate-buffered solutions at pD
7.2, 9.8, and 12.1. A set of selected spectra of 4 at pD 9.8
(methyl proton region) is shown in Figure 2. Coalescence is
observed at Tc ) 330 K, and further heating results in a single
doublet for the methyl protons. No significant change in
coalescence temperature is observed when the pD is raised to
12.1 (332 K). However, at pD 7.2 coalescence already occurs
at 288 K.
1
NMR Spectral Data for H23, 4, and 12. The H and 13C
The observed coalescence of the methyl group protons is due
to inversion of configuration of 4. Line-shape analysis of the
experimental spectra afforded the respective calculated spectra
and rate constants (see Figure 2). The rate constants were fitted
as a function of 1/T according to the Arrhenius (k ) A exp(-
NMR data for H23, K3[Ga(3)3], and (NMe4)3[Ga(3)3] are
presented in Table 1. Upon complexation of Ga3+ by 3, the
amide protons are shifted from 8.60 ppm observed in the free
ligand to 10.76 ppm, indicative of intramolecular hydrogen
bonding of the amide protons and ortho catecholate oxygen
(25) Spectroscopic Techniques For Organic Chemists; Cooper, J. W.,
Ed.; John-Wiley & Sons: New York, 1980.
(26) Karpishin, T. B.; Gebhard, M. S.; Solomon, E. I.; Raymond, K. N.
J. Am. Chem. Soc. 1991, 113, 2977.
(27) Albrecht, M.; Franklin, S. J.; Raymond, K. N. Inorg. Chem. 1994,
33, 5785.
(28) Karpishin, T. B.; Stack, T. D. P.; Raymond, K. N. J. Am. Chem.
Soc. 1993, 115, 6115.

Isomerization of Siderophore Complexes
J. Am. Chem. Soc., Vol. 118, No. 24, 1996 5715
infra). We note that 4 is freely soluble in D2O but only slightly
soluble in DMSO-d6, where it probably exists as a tight contact
ion pair.30 In D2O the potassium cations are solvated and
separated from the anion. Accordingly a slight cation depen-
dence on the free energy barrier to inversion is expected for 4
in DMSO-d6.
pD Dependence of Inversion for 4. The ambient temper-
ature 1H NMR spectrum of 4 in D2O at pD 7.2 showed
coalescence with Tc ) 288 K. This behavior prompted the
examination of the pD dependence of the rate of inversion for
4. From pD 8.5 to about pD 6.2, the evolution of the 1H NMR
spectrum of 4 was similar to that shown in Figure 2. The rate
constants at T ) 298 K calculated for each pD value by line-
shape analysis are plotted versus pD in Figure 4. Below pD 8
the rate of inversion for 4 is dependent on, and initially first
order in, [D+]. A determination of the solution thermodynamics
of 4 proceeded on the assumption that the effect is due to a
different species, which is in equilibrium with 4 at low pD.
Refinement of potentiometric titration data yielded two one-
proton steps. The corresponding protonation constants are log
KMHL ) 4.66(4) and log KMH L ) 3.99(7). Figure 5 displays
3
2 3
a species distribution plot calculated for 4 in D2O as a function
of pH. From pH 7 the species [GaH(3)]2- is in equilibrium
with [Ga(3)3]3-, and its concentration increases to approximately
5% at pH 6. Protonation of [GaH2(3)]- results in dissociation
of the complex to give the free ligand, which precipitates. This
behavior can be compared and contrasted with similar com-
plexes of iron(III)31 and gallium(III).19,32 While the kinetically
active species may not be the same as the thermodynamic
species, the rate of inversion for 4 can be described as a function
of pD by taking into account the sequential protonation:
1
Figure 2. Variable temperature H NMR spectra of 4, 0.015 M, in
D2O at pD 9.8. Experimental spectra (left column) and simulated spectra
(right column) of methyl proton resonances in the temperature range
293-370 K.
Ea/RT)) and Eyring (k ) (kBT/h) exp(-∆Gq/RT)) equations to
obtain the values of Ea, log A, ∆Gq, ∆Hq, and ∆Sq. The kinetic
parameters derived from these fits are reported in Table 2.
Analysis of the data shows that in D2O the inversion of
configuration of 4 is proton independent over the range of pD
where the [Ga(3)3]3- complex is stable (pD > 8, Vide infra).
For example, values of the activation energy, Ea, for inversion
of configuration change only slightly from 57.7(4) kJ mol-1 at
pD 9.8 to 61.2(5) kJ mol-1 at pD 12.1.
-
rate ) k0[Ga(L)33-] + k1[GaD(L)32-] + k2[GaD2(L)3 ]
where the rate constants correspond to the rate of inversion of
each of the complex species in the equation. In the pD range
studied (6.2-12) a diprotonated species does not contribute to
the rate. Hence, the rate expression can be written as
3-
rate ) k[Ga(L)3
]
where k ) k0 + k1[D+]
Evidence for an intramolecular mechanism for the Λ-4 T
∆-4 interconversion is provided by the 1H NMR spectra of 4 in
the presence of H23, at pD 9.8. The methyl proton resonances
of H23 would coalesce with the resonances of 4 if the process
were intermolecular. However, even at temperatures above the
coalescence temperature, the observed spectra consist of reso-
nances corresponding to the complex and the free ligand (see
Figure 3). Furthermore, as expected for first-order kinetics, the
rate constants are independent of concentration. A quantitative
study with three different concentrations (0.015, 0.030, and
0.045 M) at pD 9.8 and 12.1 afforded no change in k at 25 °C
or Tc, which remain unchanged within experimental error.
Intramolecular inversion of configuration should show little
or no solvent effects on Tc. Accordingly, when D2O was
replaced with DMSO-d6, the coalescence temperature was found
The rate constants k0 and k1 were fit to the data by a weighted
nonlinear least-squares method (wi ) 1/ki2), to give k0 ) 9.0(6)
s-1 and k1 ) 6.4(2) × 109 M-1 s-1. The equilibrium structural
changes which occur on equilibrium protonation of tris(2,3-
dihydroxyterephthalamide) complexes have been investigated
in detail for iron(III) and gallium(III), and are associated with
linkage isomerism. The complexes rearrange intramolecularly
to a salicylate mode of coordination in which one phenolate
oxygen of the Ga complex is protonated and replaced by the
adjacent amide carbonyl oxygen.33,34 However, the salicylate
complex which is the thermodynamic product of the protonation
reaction must itself result from a bond breaking process that
(presumably) follows protonation of the coordinated catechol
oxygen atom. Hence, it likely follows, rather than being the
q
to be 360 K, corresponding to an increase in ∆G of 7 kJ
mol-1 29
.
A similar increase in ∆Gq was observed in an
(30) A similar, weakly polar interaction involving Me4N+ cations was
observed in the crystal structure of a related gallium(III) complex with tris-
(2,3-dihydroxyterephthalamide) coordination. Stack, T. D. P.; Karpishin,
T. B.; Raymond, K. N. J. Am. Chem. Soc. 1992, 114, 1512.
(31) Garrett, T. M.; Miller, P. W.; Raymond, K. N. Inorg. Chem. 1989,
28, 128.
(32) Pecoraro, V. L.; Wong, G. B.; Raymond, K. N. Inorg. Chem. 1982,
21, 2209.
(33) Cass, M. E.; Garrett, T. M.; Raymond, K. N. J. Am. Chem. Soc.
1989, 111, 1677.
1
analogous variable temperature H NMR study for 10 (Vide
(29) The rate constant kc, (s-1), calculated from the equation kc ) 2.22 ,
at the coalescence temperature is 126 s-1 in D2O ( ) 57 Hz) and 92 s-1
in DMSO-d6
( ) 41 Hz) for the inversion of configuration of (∆,Λ)-4.
The free energy barrier to inversion (kJ mol-1) was calculated from the
equation ∆Gq ) 19.13 × 10-3Tc(9.97 + log (Tc/kc)). This approximation
yielded ∆Gq ) 67 kJ mol-1 in D2O and ∆Gq ) 74 kJ mol-1 in DMSO-d6.
The former value is in good agreement with the value obtained from the
full line-shape analysis.
(34) Loomis, L. D.; Hahn, F. E.; Raymond, K. N. Unpublished results.

5716 J. Am. Chem. Soc., Vol. 118, No. 24, 1996
Kersting et al.
a
Table 2. Kinetic Parameters for the Inversion of K3[Ga(3)3] in D2O and DMSO-d6
solventb
∆Gq298 (kJ mol-1
)
∆Hq (kJ mol-1
)
∆Sq (kJ mol-1 K-1
)
Ea (kJ mol-1
)
log A
k298 (s-1
)
D2O (pD 7.2)
D2O (pD 9.8)
D2O (pD 12.1)
DMSO
60(3)
46(2)
-0.047(5)
-0.039(9)
-0.030(9)
-0.02(1)
0.00(1)
48(2)
10.8(3)
11.2(1)
11.7(1)
12.1(2)
13.5(3)
240(10)
11(1)
10(1)
4(1)
66.8(6)
67.4(9)
73(2)
55.2(3)
58.5(6)
67(1)
57.7(4)
61.2(5)
70(2)
DMSOc
75(2)
77(1)
80(2)
1(1)
a Arrhenius and Eyring activation parameters were obtained from the least-squares fit of log k vs 1/T and log(k/T) vs 1/T plots, respectively. All
errors (in parentheses) are random errors estimated at the 95% confidence level. b Concentration 0.015 M; the D2O solutions are phosphate buffered.
+
c The NMe4 salt.
Figure 4. pD dependence of the rate constants for the interconversion
of the enantiomers of 4 in D2O (circles). Rate constants were obtained
by line-shape analysis of the experimental spectra (isopropyl methyl
proton region) recorded in buffered D2O solutions at 298 K (5%
phosphate buffer; [4] ) 0.015 M). The rate constants were fit to the
equation k ) k0 + k1[D+] by least-squares methods (k0 ) 9.0(6) s-1, k1
-1
) 6.4(2) × 109 M s-1).
Figure 5. Species distribution plot calculated for 4 in H2O as a function
of pH at T ) 298 K. [4] ) 0.015 M.
bidentate ligand can distinguish the different intramolecular
rearrangements.10
Figure 3. Variable temperature 1H NMR spectra of a mixture of H23
and 4, 0.015 M, in D2O at pD 9.8 (temperature range 295-350 K).
Experimental spectra of methyl proton resonances.
An example of such a complex is K3[Ga(9)3], 10, which can
exist in two isomeric forms; cis- and trans-10, with C3 and C1
symmetry, respectively. While cis-10 should display only a
single resonance line for the tert-butyl protons, trans-10 should
give rise to three resonance lines of equal intensity, provided
that the magnetic shift environments of the tert-butyl protons
differ sufficiently. The exchange pattern(s) of the tert-butyl
resonances provide information about the mechanism of inver-
sion. A twist around the pseudo-C3 axis of the trans isomer,
for example, should result in a site exchange of two of the three
tert-butyl resonances. This motion would not interchange the
remaining tert-butyl signals of cis- and trans-10.
2-
rate-determining step of, the inversion reaction for the [GaHL3
]
species. Until the mechanism of the catecholate-salicylate
interchange is also determined, this will not be certain, but it
seems likely that in the protonated intermediate the gallium
remains coordinated by six phenolate oxygen atoms, one of
which has been protonated.
Isomerization and Inversion in 10. The preceding section
established that inversion of the [Ga(3)3]3- complex proceeds
by an intramolecular mechanism. Several pathways for this type
of rearrangement exist, which cannot be distinguished from the
study of complexes like 4, in which the ligand 3 is a symmetric
bidentate ligand. In contrast, the kinetics of isomerization and
inversion reactions of the cis and trans isomers of trischelate
complexes of the type [M(L)3]3- in which L is an asymmetrical
NMR Spectral Data for H29 and K3[Ga(9)3], 10. As
anticipated, 10 exists as an isomeric mixture of the two isomers
cis-10 and trans-10. However, at conditions where isomeriza-
tion and inversion reactions for 4 are slow (i.e., in D2O at 298
K), some of the resonance lines of 10 are exchange broadened
and a discrete, static spectrum for the two isomers is obtained

Isomerization of Siderophore Complexes
J. Am. Chem. Soc., Vol. 118, No. 24, 1996 5717
resonances; the absolute shift differences (∼2.7 and ∼1.9 ppm)
are similar to those observed in 4 (∼2.15 ppm). The (CH3)3-
CNH resonances were assigned to the cis and trans isomers
from their intensity ratios. No assignments were made for the
BzNH and the aromatic proton resonances.
The isomeric mixture of cis- and trans-10 is expected to
generate four AB patterns for the diastereotopic benzylic protons
3
due to the chirality of the adjacent metal centers. JH-H coupling
of the benzylic protons to BzNH in DMSO-d6 solution required
selective homonuclear decoupling to observe all eight spin
doublets. After irradiation in the BzNH region all of the AB
patterns were observed (see Figure 8). Although some of the
resonance lines are overlapping, the single intense AB system
can be readily assigned to the cis isomer (denoted C). The other
three AB systems were assigned to the trans isomer (denoted
T). The geminal coupling constant for all the benzylic protons
of both isomers is 16 Hz. As pointed out earlier, in D2O the
tert-butyl resonances of trans-10, although resolved, are ex-
change broadened at 273.3 K, revealing that inversion is, to
some extent, operative for the trans isomer at this temperature
(Vide infra). Accordingly the three AB systems of trans-10 are
exchange broadenend, and only the AB system of cis-10 is
observed.
Isomerization of cis- and trans-10. Figure 6 shows the
1
variable temperature H NMR spectrum of 10 in D2O at pD
9.8, in the region of the tert-butyl resonances. At or just above
ambient temperature one sharp (B) and one broadened signal
are observed corresponding to cis-10 and trans-10, respectively.
As the temperature is raised this pattern passes through a
coalescence point at Tc ) 340(1) K, and eventually resolves
into a single resonance at 370 K. The collapse of cis- and trans-
10 tert-butyl signals is associated with the interconversion of
both isomers. A control experiment, in which excess H29 was
added to the sample, showed no exchange broadening of the
resonances due to the free ligand and the metal complexes,
indicating that the mechanism of isomerization proceeds in-
tramolecularly (Figure 9). At pD 12.1 the variable temperature
1H NMR spectrum for 10 was unchanged. However, NMR
spectra in the pD range 6-8 were markedly different, with
patterns of the tert-butyl signals similar to those at elevated
temperatures in the experiments performed at pD 9.8 and 12.1.
Analysis of the variable temperature spectrum of 10 in DMSO-
d6 (Figure 7) produces a Tc of 360(1) K for the cis-trans
isomerization. An increase of similar magnitude in Tc on
changing the solvent to DMSO-d6 was observed for inversion
of 4. Accordingly, the kinetic data reported herein support the
proposal that this is due to the formation of contact ion pairs in
DMSO-d6 solution.
Figure 6. Variable temperature 300 MHz 1H NMR spectra of the tert-
butyl region of cis-10 (B) and trans-10 (A1, A2, A3) in D2O (pD 9.8).
[10] ) 0.015 M.
only at temperatures below 276 K and pD > 8 (see Figure 6,
spectrum at 273 K); even under these conditions some of the
resonances are significantly exchange broadened. Table 3
summarizes the observed resonances for H29, cis-10, and trans-
10 and their assignments. In D2O, at 273 K the three resonances
of equal intensity at 1.19, 1.15, and 1.10 ppm are assigned to
the tert-butyl protons of the trans isomer (denoted A1, A2, and
A3 in Figure 6) while the resonance at 1.30 ppm is assigned to
the cis isomer (B). From integration of the respective reso-
nances, the ratio cis-10:trans-10 in D2O was determined to be
0.78(3), whereas a purely statistical isomer distribution would
be 1/3. This value does not change in the temperature range
273-300 K, and yields ∆G°298 ) 0.59(8) kJ mol-1 for the cis
to trans equilibrium.35 The 1H NMR spectrum of 10 in DMSO-
d6 at 298 K displays only partially resolved tert-butyl proton
resonances (see Figure 7). However, from their intensity ratios
the resonances at 1.18 and 1.05 ppm (which has half the intensity
of the resonance observed at 1.18 ppm) were readily assigned
to the trans isomer, indicating that two of the three signals of
trans-10 are overlapping at 1.18 ppm (A1, A2). Again the
equivalent tert-butyl protons of cis-10 appear downfield from
the resonances of trans-10 at 1.26 ppm (B).
Inversion of trans-10 and cis-10. The process which can
be clearly distinguished from cis-trans isomerization is inver-
sion of trans-10. This can be seen from the exchange pattern
of the tert-butyl resonances in the low-temperature region of
the variable temperature spectrum of 10 in D2O (Figure 6, 273-
300 K) or DMSO-d6 (Figure 7, 298-340 K). In D2O, as the
temperature is lowered below 300 K, the broadened pattern of
tert-butyl resonances resolves into three single resonance lines
(A1-A3) at 273.3 K. The 300 MHz 1H NMR spectra (Figures
6 and 9) indicate that only two of the three signals (A1 and A3)
are coalescing while the remaining signals of trans-10 (A2), cis-
10 (B), and ligand (C) are not affected. The coalescence
temperature is 295(1) K with (A1-A3) ) 27 Hz at 273.3 K.
Assuming that at this temperature exchange is slow, an
approximate activation barrier for inversion of configuration of
∆Gq295 ≈ 60 kJ mol-1 is calculated. A similar exchange pattern
of tert-butyl resonances was observed in DMSO-d6. Figure 7
Intramolecular hydrogen bonding in both isomers results in
a substantial downfield shift of both the (CH3)3CNH and BzNH
(35) The cis-10:trans-10 ratios in D2O and DMSO-d6 were found to be
identical.

5718 J. Am. Chem. Soc., Vol. 118, No. 24, 1996
Kersting et al.
Table 3. 1H NMR Data for H29, cis-K3[Ga(9)3], and trans-K3[Ga(9)3] in D2O and DMSO-d6
cis-K3[Ga(9)3]
a,c
trans-K3[Ga(9)3]
H29
DMSO-d6
a,c
a,c
DMSO-d6
D2Ob,c
DMSO-d6
D2Ob,c
assignment
9.43 (t)
8.17 (s)
11.35-11.42 (m)
n.o.
n.o.
11.35-11.42 (m)
10.91 (s)
2 × 10.89 (s)
7.27-6.72 (m)
4.63 (d)
n.o.
n.o.
n.o.
7.24-7.03 (m)
e
BzNHCOd
NHCO
10.86 (s)
7.40-7.22 (m)
4.51 (d)
7.27-6.72 (m)
4.43 (d)
7.24-7.03 (m)
4.41 (d)
ArH, Ar′Hd
CH2C6H5
4.12 (d)
4.29 (d)
4.58 (d)
4.45 (d)
e
e
4.30 (d)
e
e
2 × 4.13 (d)
2 × 1.18 (s)
1.05 (s)
1.39 (s)
1.26 (s)
1.30 (s)
1.19 (s)
1.15 (s)
1.10 (s)
C(CH3)3
a Chemical shifts are in ppm with respect to Me4Si (0.0 ppm) at 298 K. b Chemical shifts are in ppm with respect to dioxane (3.75 ppm) at 273
K, pD 9.8. c Coupling constants are included in the Experimental Section; s ) singlet, d ) doublet, t ) triplet, m ) multiplet, n.o. ) not observed.
d The resonances in the range 7.5-7.0 ppm are unresolved at the spectrometer frequency used, but were assigned to the BzNH (Bz ) benzyl) and
aromatic protons (ArH and Ar′H denote the protons of the 2,3-dihydroxyterephthalamide ring and the protons of the phenyl ring, respectively) from
their correct intensity ratio. e Resonances obscured by solvent resonances.
Figure 8. Experimental 400 MHz 1H NMR spectrum of 10 (benzylic
proton region) in DMSO-d6 after irradiation in the benzyl proton region
at 298 K. C and T denote the AB spin systems of cis-10 and trans-10,
respectively.
DMSO-d6 is estimated to be ∆Gq ≈ 67 kJ mol-1. A similar
increase in ∆Gq by ∼7 kJ mol-1 is obtained for the inversion
of 4 when the solvent is changed from D2O to DMSO-d6.
Again, cis-trans isomerization is slow up to temperatures as
high as 335 K; however, line broadening of the tert-butyl
resonance of cis-10 at 335 K reveals some overlap of the two
kinetic processes in the temperature range 335-345 K.
As shown in Figure 7, the temperature-dependent spectral
changes in the benzylic proton region parallel the changes
observed in the tert-butyl proton region. The three AB spin
systems of trans-10-d636 broaden and coalesce in the temperature
range where inversion of trans-10 was detected by the coales-
cence of the tert-butyl proton resonances. However, due to
poorly resolved spin doublets and overlap with the resonances
of cis-10-d6, a discrete averaging pattern of the three AB systems
was not seen. The two spin doublets of cis-10-d6 start to
broaden at ca. 335 K, revealing that the barrier to inversion for
cis-10 is larger than for trans-10 (Tc ) 327 K). Inversion of
cis-10 occurs in the same temperature interval as the process
of cis-trans isomerization, and both result in complete averag-
ing of all AB systems.
Mechanism of Inversion. The variable temperature 1H NMR
spectra of K3[Ga(9)3] in D2O or DMSO-d6 reveal two kinetic
Figure 7. Variable temperature 400 MHz 1H NMR spectra of the
benzylic proton (left column) and 300 MHz H NMR spectra of the
tert-butyl proton region (right column) of a mixture of trans-10-d6 (A1,
A2, A3), and cis-10-d6 (B) in DMSO-d6. [10] ) 0.015 M. A deuterated
sample was used to record the spectra in the benzylic proton region
(see ref 36).
1
1
shows the variable temperature H NMR spectrum of 10 in
DMSO-d6. The spectrum at 298 K reveals two tert-butyl signals
with a 2:1 intensity ratio and a peak separation of 52 Hz for
trans-10 (A1-A3) and one singlet for cis-10 (B). Upon raising
the temperature, the signal at 420 Hz (A3) coalesces with only
one of the two chemical shift equivalent resonances at 472 Hz
(A1 or A2) and both signals pass through a coalescence point at
Tc ) 327(1) K. The barrier to inversion of trans-10 in
(36) A deuterated sample was used to suppress 3JHH coupling to the amide
proton. Deuteration of the amide groups in 10 was accomplished by repeated
addition and evaporation of D2O. The 1H NMR spectra of 10 and 10-d6 in
DMSO-d6 are identical, except that the NH resonances are absent and the
benzylic protons appear as an AB pattern for 10-d6.

Isomerization of Siderophore Complexes
J. Am. Chem. Soc., Vol. 118, No. 24, 1996 5719
Figure 10. Inversion of configuration for trans-10 by a trigonal twist
about the pseudo-C3 axis via an idealized trigonal prismatic transition
state and corresponding site interchanges of tert-butyl groups in ∆-
and Λ-trans-10. The inequivalent tert-butyl groups are denoted A1, A2,
and A3, and the inequivalent environments are denoted (a), (b), and
(c).
10 in the temperature range where inversion of trans-10 is fast.
However, mechanistic details of the inversion of the cis isomers
could not be deduced from the variable temperature NMR
spectra since cis-trans isomerization starts at higher temper-
atures. In the absence of cis-trans isomerization, a trigonal
twist about the real C3 axis of the cis isomer would result in
pairwise averaging of the two spin doublets of the benzylic
protons of cis-10. This would not affect the corresponding
resonances of trans-10. Instead, cis-trans isomerization results
in simultaneous averaging of all signals rather than discrete
averaging of the respective resonances of cis-10.
The intramolecular rearrangement reactions of tricatecho-
latogallium(III) complexes we have identified are closely related
to the rearrangement reactions of tristropolonato gallium(III)
complexes.23 The temperature dependent NMR spectra of tris-
(R-isopropenyltropolonato)- and tris(R-isopropyltropolonato)-
gallium(III) complexes, for example, exhibit two similar separate
kinetic processes in particular, overlapping temperature intervals.
A trigonal twist mechanism has been established from the
exchange pattern of methyl resonances for the trans isomers in
the low-temperature region, while at elevated temperatures cis-
trans isomerization occurs. The coalescence temperatures and
free energy barriers to inversion for the tritropolonatogallium-
(III) complexes are Tc ) -65 °C and ∆Gq ) 40 kJ mol-1. For
the tricatecholatogallium(III) complexes described in this paper
the free energy barrier to inversion is ∆Gq ) 60 kJ mol-1; the
tropolonates invert much faster than the catecholates.
Figure 9. Variable temperature 300 MHz 1H NMR spectra of the tert-
butyl region of a mixture of trans-10 (A1, A2, A3), cis-10 (B), and H29
(C) in D2O (pD 12.1). [10] ) 0.015 M. [H29] ) 0.008 M.
processes. In the lower part of the temperature range investi-
gated, intramolecular rearrangement reactions are detected for
the trans-10 isomers. The spectral changes in the tert-butyl
proton resonance region display an averaging of two of the three
resonances of trans-10; the remaining resonances of trans-10,
cis-10, or ligand, H29, are not affected. Inspection of the
benzylic proton resonance region in this temperature interval
clearly reveals that this motion involves inversion of configu-
ration of trans-10. The other process is cis-trans isomerization,
which starts to begin in a temperature range where inversion of
trans-10 is already fast and results in the collapse of all tert-
butyl and benzylic proton resonances at higher temperatures.
Eaton et al.22 have reported a permutational analysis of the
averaging patterns expected for the different possible intramo-
lecular rearrangement reactions of ML3-type complexes (L )
unsymmetrical chelate ligand). The observed two-site inter-
change in trans-10, involving inversion of configuration without
isomerization, is fully consistent with only one of these sets,
which corresponds to a trigonal twist mechanism about the
pseudo 3-fold axes (Figure 10). We therefore conclude that
the mechanism of inVersion of trans-10 proceeds through a
trigonal twist Via a trigonal-prismatic transition state. Com-
pared to the trans-10 isomers, the cis-10 isomers are configu-
rationally more rigid. This is indicated by the non-exchange-
broadened AB system of the benzylic proton resonances of cis-
Conclusions
Inversion and isomerization reactions have been characterized
for triscatecholato complexes of gallium(III). Compared to
tropolonates, the catecholate complexes invert much slower,
consistent with their greater thermodynamic stability. The rate
of inversion of the [Ga(L)3]3- complexes (L ) 2,3-dihydroxy-
N,N′-substituted-terephthalamide) is pH independent above pH
9 and proceeds by an intramolecular mechanism. Below this
pH the rate of inversion is first order with respect to [H+], with
this pathway rapidly becoming dominant. This enhancement
in the rate of inversion is due to the formation of a monopro-
tonated complex H[GaL3]2-. The mechanism of inversion of
the [GaL3]3- complex has been elucidated from the independent
site interchange of tert-butyl resonances in trans-10, which is
consistent with only one model: a trigonal twist around the
pseudo-C3 axis, resulting in a trigonal-prismatic transition state.
Cis-trans isomerization also proceeds intramolecularly with a
larger activation barrier. Isomerization and inversion reactions
of [GaL3]3- anions (with L ) bidentate 2,3-dihydroxyterephthal-
amide chelating units similar to the catechol amide chelating
units found in enterobactin, and gallium(III) as an appropriate
structural and rate simulator for Fe(III)) are fast on the NMR
time scale, in particular in the pH region close to in-ViVo pH

5720 J. Am. Chem. Soc., Vol. 118, No. 24, 1996
Kersting et al.
glass electrode.40 Solutions were titrated with degassed, standardized
0.1 M KOH and 0.1 M HCl under argon. Solutions (50 mL,
approximately 0.05 mmol of the compounds) were titrated both from
base to acid (pH range 4-11.2) and vice versa, to check for hysteresis.
The complex protonation constants were modeled with a version of
the program BETA90, described elsewhere.41
Preparation of Compounds.42 Unless otherwise noted, all starting
materials were obtained commercially and used without further
purification. 3,3′-(2,3-Dimethoxyterephthaloyl)bis(1,3-thiazolidine-2-
thione),43 1, and sodium methyl-2,3-dimethoxyterephthalate,44 6, were
synthesized according to literature procedures. Silica gel 60 (Merck,
230-400 mesh) was used for column chromatography. Organic
solvents and mineral acids were of reagent grade; tetrahydrofuran (THF)
was distilled from sodium benzophenone ketyl prior to use. Water
was deionized, and further purified by a Millipore cartridge system
(resistivity 18 MΩ cm). Metal complex syntheses were performed
under an argon atmosphere using Schlenk techniques.
(I) Ligand Syntheses. 2,3-Dimethoxy-N,N′-diisopropylterephthal-
amide (2). Compound 1 (4.29 g, 10 mmol) was dissolved in 100 mL
of CH2Cl2 and was added to a stirred solution of isopropylamine (2.60
mL, 30.3 mmol) dissolved in 100 mL of CH2Cl2. After stirring for 2
h, the volatiles were removed in Vacuo, leaving a pale yellow residue.
The residue was redissolved in 250 mL of CH2Cl2, washed three times
with 1 M KOH, dried over anhydrous MgSO4, and filtered. Removal
of the solvent afforded a white residue which was recystallized from
CH3OH/H2O. Yield: 2.93 g (94%). 1H NMR (300 MHz, CDCl3):
7.89 (s, 2H, ArH), 7.64 (d, J ) 6.8 Hz, 2H, CONH), 4.29 (oct, J ) 6.8
Hz, 2H, CH), 3.92 (s, 6H, OCH3), 1.27 (d, J ) 6.6 Hz, 12H, CH3).
13C{1H} NMR (100 MHz, CDCl3): 22.4, 41.1, 61.1, 125.7, 129.9,
151.0, 162.9. Anal. Calcd (Found) for C16H24O4N2: C, 62.32 (62.00);
H, 7.84 (7.94); N, 9.08 (9.03).
values. A function of the cyclic triserine backbone of entero-
bactin is to fix the cis-∆-configuration at the metal center. The
relatively rapid rate of isomerization seen in the model
complexes indicates that the inversion rate of the metal center
in the ferric complex of enterobactin is probably rapid on the
time scale of microbial iron transport. Hence, the observed
chiral specificity of iron delivery is a thermodynamic, not
kinetic, property of the siderophore.
Experimental Section
Physical Measurements. The 1H NMR and 13C NMR spectra were
obtained on either a Bruker AMX 300 or AMX 400 spectrometer.
Absorption spectra were recorded on an HP 8452A vis/UV diode array
spectrophotometer. Concentrations of the solutions were analyzed for
iron by atomic absorption. The extinction coefficients for the spectra
are based on these concentrations. Infrared spectra were measured as
KBr pellets using a Nicolet Magna IR 550 spectrometer. Melting points
were taken on a Bu¨chi melting apparatus and are uncorrected.
Microanalyses were performed by the Analytical Services Laboratory,
College of Chemistry, University of California, Berkeley. Fast-atom-
bombardment mass spectra were obtained at the Mass Spectrometry
Laboratory at the University of California, Berkeley.
1
Variable Temperature H NMR Measurements. Variable tem-
perature experiments were carried out on a Bruker AMX 300
spectrometer operating at 300 MHz. The temperature was controlled
by the B-VT2000 equipment of the spectrometer that ensures a precision
of (1 °C. The probe temperature was allowed to equilibrate for 10
1
min prior to final magnetic homogeneity optimization on the H FID.
Variation at a given temperature was less than (0.1 °C. For each
sample the temperature was varied in both directions, and in each case
superimposable spectra were obtained. The samples were prepared by
adding the pure metal complexes to buffered D2O solutions (5% KH2-
PO4). Final sample concentrations were approximately 0.015 M. After
sample preparations, pD values were adjusted with D2O solutions of
10% NaOD and determined by use of a Fisher Accumet digital pH
meter fitted with a glass electrode (pD ) pH meter reading + 0.4).37
The electrode was calibrated with standard commercial buffers of pH
4.00 and 10.00. All chemical shifts were referenced to dioxane as an
internal standard.
2,3-Dihydroxy-NN′-diisopropylterephthalamide (H23). Com-
pound 2 (2.90 g, 9.40 mmol) was dissolved in 100 mL of CH2Cl2.
BBr3 (7 mL, 73 mmol) was added Via syringe, and the slurry was stirred
for 12 h. The cloudy, pale yellow mixture turned clear upon the
addition of 50 mL of methanol. Repeated addition of methanol (16 ×
50 mL) followed by evaporation afforded a pale yellow solid. The
solid was dissolved in 20 mL of CH3OH, and precipitation with water
afforded white crystals. Mp: 240 °C. Yield: 1.89 g (72%). FT-IR
(KBr, cm-1): j 3421, 3371 (NH), 1597, 1537 (CO). 1H NMR (300
MHz, DMSO-d6):
8.60 (d, J ) 7.7 Hz, 2H, NHCO), 7.35 (s, 2H,
The temperature behavior of the methyl proton resonances for 4 was
investigated in D2O at different pD values over the range 275-360 K.
The kinetic parameters for the inversion of 4 were determined by line-
shape analysis.38 The experimental spectra were simulated as a two-
site exchange process using the program DNMR3.39 Line widths (2.40
Hz), coupling constants (3JHH ) 6.52 Hz, 4JHH ) 0.0 Hz), and relative
intensities (0.5, 0.5) for both methyl proton resonances at slow exchange
were obtained at 273.3(1) K, pD 12.10, and used as fixed parameters
in the calculations. The separation between the signals at temperatures
at or above coalescence temperature were obtained from an extrapola-
tion of the shift differences in the temperature range 273.3-310 K.
Rate constants for each temperature were determined by comparison
of the calculated and experimental spectra.
Proton NMR spectra of the gallium complex K3[Ga(3)]3 were
obtained in buffered KH2PO4/KHPO4 D2O solutions (pD range 6.2-
9.0). Samples were prepared as indicated above. Data were collected
on a Bruker AMX 300 spectrometer at 295 K. Rate constants for each
pD value were determined as described above.
Potentiometric Titrations. Solution thermodynamic data were
obtained by potentiometric pH titrations. Ligand protonation constants
for several 2,3-dihydroxy-N,N′-disubstituted-terephthalamides are lit-
erature values.31 For H23, the protonation constants were estimated to
be log K011 ) 11.1 and log K012 ) 6.0.31 Complex protonation constants
were determined potentiometrically in a 0.1 M KCl solution at 25.0
°C with a Dosimat automatic titration system using a calibrated KCl
ArH), 4.14 (oct, J ) 6.8 Hz, 2H, CH), 1.18 (d, J ) 6.6 Hz, 12H, CH3).
13C{1H} NMR (100 MHz, CD3OD): 22.5, 42.9, 117.7, 119.5, 150.2,
169.1. Anal. Calcd (Found) for C14H20O4N2: C, 59.99 (59.62); H,
7.19 (7.16); N, 9.99 (9.92).
2,3-Dimethoxy-4-(tert-butylcarbamoyl)benzoic acid (7). Com-
pound 6 (3.14 g, 12 mmol) was added in portions to 16 mL of freshly
distilled SOCl2 (at 0 °C) and stirred for 12 h at 25 °C. The pale yellow
solution was filtered to remove solid NaCl. Coevaporation of the
solution in Vacuo with CCl4 (3 × 30 mL) afforded the acid chloride
which was used without further purification. The acid chloride was
dissolved in 70 mL of dry THF, and a solution of tert-butylamine (1.25
mL, 12.2 mmol) and triethylamine (1.3 mL) in 70 mL of THF was
added at 0 °C. The reaction mixture was stirred for 1 h, during which
time the solution turned yellow, with formation of solid Et3N‚HCl. After
filtration and removal of the solvent on a rotary evaporator, the residue
was taken up in 300 mL of CH2Cl2 and washed three times with 50
mL of 0.2 M HCl. Removal of the solvent by rotary evaporation
afforded a pale yellow solid. The solid was dissolved in 30 mL of
CH3OH, and 30 mL of 4 M NaOH was added. After 1 h, the solution
was brought to pH 2 with HCl (6 M) and the acid was extracted with
(40) (a) Scarrow, R. D. Ph.D. Dissertation, University of California at
Berkeley, 1985. (b) Turowski, P. N.; Rodgers, S. J.; Scarrow, R. C.;
Raymond, K. N. Inorg. Chem. 1988, 27, 474.
(41) Franczyk, T. S. Ph.D. Dissertation, University of California at
Berkeley, 1991.
(42) Abbreviations: acac, 2,4-pentanedione; DCC, 1,3-dicyclohexylcar-
bodiimide; DCU, 1,3-dicyclohexylurea; DMAP, 4-(dimethylamino)pyridine;
NBA, p-nitrobenzyl alcohol; TGG, thioglycerol/glycerol.
(43) Karpishin, T. B.; Stack, T. D. P.; Raymond, K. N. J. Am. Chem.
Soc. 1993, 115, 182.
(44) Weitl, F. L.; Raymond, K. N.; Durbin, P. W. J. Med. Chem. 1981,
24, 203.
(37) Glasoe, P. K.; Long, F. A. J. Phys. Chem. 1960, 64, 188.
(38) Dynamic Nuclear Magnetic Resonance Spectroscopy; Jackman, L.
M., Cotton, F. A., Eds.; Academic Press: New York, 1975.
(39) Binsch, G.; Kleier, D. Quantum Chemistry Program Exchange, No.
165. DNMR3 is part of The University of Manitoba NMR Spectral
Simulation and Analysis Package. X-11/Motif Version 940101.

Isomerization of Siderophore Complexes
J. Am. Chem. Soc., Vol. 118, No. 24, 1996 5721
diethyl ether (three times). The combined ether extracts were dried
over anhydrous MgSO4 and filtrated, and the solvent was removed to
afford a pale yellow solid which was used without further purification
in the next reaction. Yield: 3.20 g (95%). 1H NMR (300 MHz,
DMSO-d6): 7.94 (s, 1H, NH), 7.38, 7.23 (2d, J ) 8.1 Hz, 1H each,
ArH), 3.82 (s, 3H, OCH3), 3.81 (s, 3H, OCH3), 1.35 (s, 9H, C(CH3)3).
2,3-Dimethoxy-N-butyl-N′-benzylterephthalamide (8). Compound
7 (3.20 g, 11 mmol), DCC (2.5 g, 12 mmol), and catalytic DMAP (13
mg) were dissolved in 25 mL of CH2Cl2. After 20 min of stirring,
benzylamine (1.2 mL, 11 mmol) was added and a white precipitate
formed. The reaction mixture was stirred for 2 days, and insoluble
DCU was removed by filtration. The pale yellow oil which was
obtained after rotary evaporation was applied to a silica gel column
and eluted with ethyl acetate/hexanes (v/v ) 0.6, Rf ) 0.26) and used
without further purification in the next reaction. Yield: 2.6 g (64%).
1H NMR (300 MHz, DMSO-d6): 8.78 (t, J ) 7.7 Hz, 1H, NH), 7.93
(s, 1H, NH), 7.35 - 7.21 (m, 5H + 2H, ArH), 4.47 (d, J ) 7.7 Hz, 2H,
CH2), 3.83 (s, 3H, OCH3), 3.80 (s, 3H, OCH3), 1.36 (s, 9H, C(CH3)3).
2,3-Dihydroxy-N-butyl-N′-benzylterephthalamide (H29). Com-
pound 8 (2.6 g, 7 mmol) was dissolved in 80 mL of CH2Cl2. BBr3
(4.7 mL, 49 mmol) was added carefully Via syringe, and the slurry
was stirred for 12 h. The cloudy, pale yellow mixture turned clear
upon the addition of 50 mL of methanol. Repeated addition of methanol
(16 × 50 mL) followed by evaporation afforded a pale yellow solid.
The solid was dissolved in methanol (20 mL) and precipitated with
water to obtain white crystals. Mp: 173 °C. Yield: 1.80 g (75%).
FT-IR (KBr, cm-1) j 3393, 3337 (NH), 1598, 1535 (CO). 1H NMR
(300 MHz, DMSO-d6): 9.43 (t, J ) 6.0 Hz, 1H, NH), 8.17 (s, 1H,
NH), 7.40-7.22 (m, 5H + 2H, ArH), 4.51 (d, J ) 6.0 Hz, 2H, CH2),
1.39 (s, 9H, C(CH3)3). 13C{1H} NMR (100 MHz, D2O/NaOD): 29.7,
44.0, 51.3, 112.9, 113.0, 117.6, 119.4, 128.4, 130.0, 140.3, 165.3, 166.0,
173.1. Anal. Calcd (Found) for C19H22O4N2: C, 66.65 (66.33); H,
6.48 (6.53); N, 8.18 (8.12).
(II) Metal Complex Syntheses. K3[Ga(3)3] (4). To H23 (134 mg,
0.48 mmol) dissolved in MeOH were added a solution of KOH in EtOH
(0.5 M, 1 mL) and solid Ga(acac)3 (60 mg, 0.16 mmol). After the
solution was stirred for 30 min the volatiles were removed in Vacuo,
leaving a pale yellow solid. The solid was recrystallized from
acetonitrile, filtrated, and air-dried. Yield: 141 mg (86%). FT-IR
(KBr, cm-1) j 3439, 3207 (NH), 1610, 1563 (CO). 1H NMR (300
MHz, DMSO-d6): 10.76 (d, J ) 6.6 Hz, 6H, NHCO), 6.65 (s, 6H,
ArH), 3.84 (oct, J ) 6.6 Hz, 6H, CH), 1.09 (d, J ) 6.6 Hz, 18H, CH3),
0.95 (d, J ) 6.6 Hz, 18H, CH3). 13C{1H} NMR (100 MHz, CD3OD):
23.0, 23.4, 42.1, 113.7, 116.3, 159.8, 170.7. +FAB-MS (TGG): m/e
1022 [M + H]+, 983 [M + 2H - K]+, 946 [M + 3H - 2K]+. Anal.
Calcd (Found) for K3GaC42H54O12N6‚(H2O)12: C, 41.35 (41.79); H, 6.28
(5.99); N, 6.89 (6.74).
970 [M + 2H - K]+, 932 [M + 3H - 2H]+. Anal. Calcd (Found)
for K3FeC42H54O12N6‚(H2O)8: C, 43.78 (43.53); H, 6.12 (5.88); N, 7.29
(7.89).
K3[Ga(9)3] (10). Compound 10 was prepared by the procedure
detailed above for 4. H29 (170 mg, 0.50 mmol) was used instead of
H23. Yield: 179 mg (93%), pale yellow solid. FT-IR (KBR, cm-1) j
3411, 3228 (NH), 1628, 1560 (CO). 1H NMR (400 MHz, DMSO-d6):
11.41 (m, 3H, NHCH2), 10.91, 10.89, 10.86 (3 s, 3H, NH), 7.26-
6.72 (m, 15H + 6H, ArH), 4.67-4.08 (m, 6H, CH2), 1.26, 1.18, 1.05
(3 s, 9H, C(CH3)3). +FAB-MS (CH3OH/NBA): m/e 1208 [M]+, 1247
[M
+
K]+. Anal. Calcd (Found) for K3GaC57H60O12N6‚
(CH3CN)(H2O)3: C, 54.38 (54.44); H, 5.34 (5.24); N, 7.52 (7.72).
K3[Fe(9)3] (11). Compound 11 was prepared by the procedure
detailed above for 5. H29 (170 mg, 0.50 mmol) was used instead of
H23 and Fe(acac)3 instead of Ga(acac)3. Yield: 164 mg (83%). UV/
vis spectrum (H2O, 5% K2HPO4)
) 364 nm (ꢀ ) 10 400 M-1
max
cm-1), 448 nm (ꢀ ) 5500 M-1 cm-1), 516 nm (ꢀ ) 4800 M-1 cm-1).
+FAB-MS (NBA) m/e 1194 [M + H]+. Anal. Calcd (Found) for
K3FeC57H60O12N6‚(H2O)5 C, 53.31 (53.28); H, 5.49 (5.49); N, 6.54
(6.25).
[NMe4]3[Ga(3)3] (12). H23 (134 mg, 0.48 mmol) and NMe4OH‚
5H2O (87 mg, 0.48 mmol) were dissolved in 5 mL of CH3OH. Ga-
(acac)3 (59 mg, 0.16 mmol) was added, and the solution was stirred
for 30 min. 12 was precipitated by addition of diethyl ether,
recrystallized from 2-propanol, and air-dried. Yield: 143 mg (80%),
colorless crystals. FT-IR (KBr, cm-1) j 1627, 1545 (CO). 1H NMR
(300 MHz, CD3OD): 6.96 (s, 6H, ArH), 4.03 (spt, 6H, CH), 2.94 (s,
36H, N(CH3)4+), 1.22 (d, 18H, CH(CH3)2), 1.04 (d, 18H, CH(CH3)2).
13C{1H} NMR (100 MHz, CD3OD): 23.2, 23.7, 42.2, 55.7, 113.4,
115.8, 160.2, 170.7. +FAB-MS (NBA): m/e 1126 [M]+, 1127 [M +
H]+, 1200 [M + NMe4]+. Anal. Calcd (Found) for C54H90O12N9Ga•-
(H2O)3: C, 55.34 (55.58); H 8.62 (8.88); N 8.77 (9.26).
[NMe4]3[Fe(3)3] (13). Compound 13 was prepared by the procedure
detailed above for 12. Fe(acac)3 was used instead of Ga(acac)3. The
red crystals obtained were dried at 30 °C in Vacuo overnight. Yield:
144 mg (76%). FT-IR (KBr, cm-1) j 1617, 1557 (CO). Vis/UV
spectrum (H2O, 5% K2HPO4):
364 nm (ꢀ ) 12 000 M-1 cm-1),
max
448 nm (ꢀ ) 6700 M-1 cm-1), 512 nm (ꢀ ) 5800 M-1 cm-1). +FAB-
MS (TGG): m/e 1114 [M + H]+, 1187 [M + NMe4]+. Anal. Calcd
(Found) for C54H90O12N9Fe‚H2O: C, 57.34 (57.09); H 8.20 (7.96); N
11.14 (11.07).
Acknowledgment. This work was supported by NIH Grants
AI11744 and DK32999. B.K. thanks the Deutsche For-
schungsgemeinschaft for a postdoctoral fellowship. We thank
Professor R. C. Fay for helpful comments.
K3[Fe(3)3] (5). Compound 5 was prepared by the procedure detailed
above for 4 using Fe(acac)3 (56 mg, 0.16 mmol) instead of Ga(acac)3.
Yield: 145 mg (89%). FT-IR (KBr, cm-1) j 3408, 3230 (NH), 1605,
Supporting Information Available: Arrhenius plots of
DNMR3 kinetic data from complete line-shape analysis of H
NMR spectra for 4 (1 page). Ordering information is given on
any current masthead page.
1
1565 (CO). Vis/UV spectrum (H2O, 5% K2HPO4):
) 364 nm
max
(ꢀ ) 10 500 M-1 cm-1), 446 nm (ꢀ ) 5200 M-1 cm-1), 512 nm (ꢀ )
4500 M-1 cm-1). +FAB-MS (CH3OH/NBA): m/e 1008 [M + H]+,
JA953545F