J. Chim. Phys. (1 998) 95, 112-121
Q EDP Sciences, Les Ulis
1
H NMR studies of the diamagnetic gallium (III)
and paramagnetic iron (III) complexes of a chiral
macrobicyclic ligand of bicapped tris (binaphtol) type
P. Baret, V. Beaujolais, C. Bougault, D. Gaude and J.-L. pierre*
LEDSS, UMR 5616 du CNRS, Université Joseph Fourier, BP. 53,
38041 Grenoble cedex 9, France
(Received 24 July 1997; accepted 24 September 1997)
Correspondence and reprints
RÉSUMÉ
Les complexes du gallium (III) et du fer (III) d'un ligand macrobicyclique chiral
impliquant trois sous-unités de type binaphtol sont étudiés en RMN du proton en
solution méthanolique. L'étude du complexe (diamagnétique) du gallium permet de
montrer que le complexe : (i) ne subit pas d'inversion de la configuration ( A I A ) du
site octaédrique et : (ii) qu'il n'y a pas d'échange entre ligand libre et complexe à la
température ambiante. L'évolution du spectre du complexe paramagnétique du fer
avec la température permet une attribution des protons du ligand et met en Svidence
la symétrie D3 du complexe. Une bonne corrélation est obtenue entre la distance
fer-proton (donnée par la modélisation moléculaire) et le Ti du proton considéré.
ABSTRACT
1~ NMR studies of the diamagnetic gallium (III) and paramagnetic iron (III)
complexes of a chiral macrobicyclic ligand of bicapped tris (binaphtol) type are
described. The study of the gallium complex emphasizes : (i) that the inversion of
the octahedral center is not observed and : (ii) the absence of exchange between free
ligand and complex, at room temperature. In the case of the iron complex,
assignments of the hyperfine shifted resolved resonances are achieved, based on
temperature-behavior studies, which evidence the D3 syrnmetry of the complex.
These assignments are in complere agreement with measured Tl values and protonto-iron distances obtained from molecular modelling.
KEY WORDS : 1H NMR ; gallium (III) and iron (III) complexes ; chiral
macrobicyclic ligand
MOTS CLES : RMN du proton : complexes du gallium (HI) et du fer (III) :
ligand macrobicyclique chiral
'H NMR OF GALLIUM(II1) AND IRON(III) COMPLEXES
113
INTRODCCTION
Some studies using chiral ligands have evidenced the stereoselective formation of
A or A coordination isomers upon octahedral complexation of transition metal ions
[l-121. The biological importance of the absolute configuration of the octahedral
metal center has been demonstrated in the recognition of iron (III) siderophore
complexes by the membrane receptor protein [13]. We described [14] the specific
control of the helical chirality (A or A) in iron (III) and chromium (III) complexes
by the chiral macrobicyclic tris (binaphtol) ligand LH6 (Figure 1). Only the A-as,
as, a s or the A-aR, aR. aR enantiomers were obtained respectively from the a s , as,
a s and the aR, aR, aR enantiomers of LH6. We describe herein new 1~ NMR
studies for the diamagnetic gallium (III) and for the paramagnetic iron (III)
complexes of LH6. Diamagnetic gallium (III) has been extensively used as a
substitute for iron (III) in NMR studies [8, 9, 15-17], both having the same charge
and similar ionic radii in six-coordinate complexes. Furthermore, since neither the
high spin d5 iron (III) nor the d l 0 gallium (III) complexes have any crystal field
stabilization, they are similar in their ligand exchange rates. On another hand, the
proton NMR spectra of paramagnetic transition metal complexes can yield valuable
informations on the nature and extent of metal-ligand covalency [18].
Figure 1 :the hicapped ms (hinaphtol) ligand LH6
P. Baret et al
114
MATERIAL AND METHODS
Materials
Preparation and characterization of the complexes have been previously described
i 141.
IH NMR data
Gallium (III) comp1e.x [LGu] 3- : The NMR spectra in CD30D or in D20-NaOD
were recorded at 300 MHz on a Bruker AM 300 spectrometer using standard pulse
sequence.
Iron (III) complr-c [LFe] 3- : The NMR spectra in CD;OD were recorded at 300
MHz on a Bruker AM 300 spectrometer for temperatures ranging from 223 to
303K. Each Free Induction Decay consisted of 1024 scans over a 20 kHz bandwidth
centered on the residual OH resonance of the solvent. A 410 ms total recycle tirne
was used. The spectra were then apodized using eitlier a standard exponential
function to introduce a 10-50 Hz linebroadening (when integration of the
resonances was considered) or a lorenz-gaussian enhanced transformation
-
1
Tt lb
-[(gb*taq)>
e Zgbt,
il']
) with Ib = -1.1 Hz and gb = 0.1 Hz in order to increase
the signal to noise ratio for the fast relaxing peaks . Nonselective spin-lattice
relaxation times, T I , were collected usiag an inversion-recovery pulse sequence
with a 500 ms recycle time. Tl values were calculated for resolved resonances using
two altemate methods: the nul1 point ( Tl = t(I=O) 1 ln2 ) and the initial slopes of the
magnetization recovery data. where the data are fitted through exponential
t
--
( 1 = Io (1 - 2Ae
)) or logarithrnic (In- 1 0 - 1 -- In A - -)t
functions. The three
210
Tl
methods gave similar results within 20 % of the Tl value and the accuracy of the
intensiiy measurement for very broad and weak resonances on a 60 ppm bandwith
seem to be more crucial than the method used. Given in the tables are the average
T l values. Spin-spin relaxation times, Tl, were estimated based on the linewidth for
each resolved resonance.
The observed chernical shifts Fobs in paramagnetic species consist of three
contributions, the diamagnetic shift Fdia (which could be obtained from an
isostructural diamagnetic compound), the dipolar shift 6dip and the contact shift
Gcon :
sobs = sdia + Fhf = Fdia + Fdip + Gcon (1)
'H NMR OF GALLIUM(II1) AND IRON(III) COMPLEXES
115
For high-spin ~ e ' " complexes with little g-anisotropy, the contact contribution
G c o n shows an inverse linear temperature-dependence, while the dipolar
contribution 8dip is accounted for an inverse quadratic function of the temperature
[19]. As a consequence, the hyperfine shifts vanish at Iiigh temperatures and T-> r*,
intercepts in 6= f(l/T) plots give access to the diamagnetic-only contribution.
The presence of a paramagnetic center results in broad signals and enhanced
spin-lattice relaxation, which is given by :
T-1 = T-1
1
-1
1
1 dia +
para = 'c'dia + T ~ l d i p+
+ TI (-une
where R is the distance to the iron atorn and T l e the electronic spin-lattice
relaxation time.
In most of the iron complexes studied by 1H NMR [20a, 20b], the dipolar
relaxation mechanism appears to be dominant and Tl becomes proportional to Rb,
which leads for nonequivalent protons i and j to :
6
6
(3)
Tli /Tlj = RFe-i iRFe-j
RESULTS AND DISCUSSION
Gallium (III) cornplex [LGa] 3- : Surprisingly, the unique gallium isomers
respectively obtained (LGaK3) from each enantiomer of LH6 revealed to be less
stable than the corresponding iron (III) complexes and decomposed into solution in
a few days ; they are more stable in the solid state. Nevertheless, before
decomposition, no isomerisation is observed (no change in the NMR spectrum) : the
constrained bicapped structure does not allow inversion of the octahedral center, as
observed for simple tris-bidentate complexes [17]. It has to be emphazised that
inversion of the octahedral center would lead to diastereoisomeric species since the
ligand LH6 cannot racemise under the experimental conditions.
The 1~ NMR data (300 MHz) in CD30D for the two enantiomers (A-as, a s , a s
or A-aR. aR, aR) of LGaK3 are the same. The symrnetry of the spectrum reveals
the equivalence of the three binaphtyl subunits. the equivalence of the two naphtyl
parts for a given binaphtyl and the equivalence of the two tren moieties (6 Hq 7.77,
s ; F Hg 6.42. d 8.2 Hz ; 6 Hg 6.75. m ; 6 H7 6.90, m ; 8 Hg 7.60. d 8.3H.z). The
116
P. Baret et al.
two sets of methylenic protons of the tren moieties appear as a complex ABCD
spectrum (6 2.05 and 2.43). The spectrum pattern is similar to that of the hexa
methoxyl-protected free ligand for which extensive 2D NMR studies (COSY and
XHCORR sequences) allowed assignrnents of al1 proton resonances. On the basis of
these data, the structure of the [ L G ~ Icomplex
~anion in solution is assigned D 3
syrnrnetry. The titration of LH6 by Ga(acac)3 (in D20, NaOD) was monitored by
1~ NMR spectroscopy revealing a 1:l stoechiometry for the complex (Figure 2). In
particular. a precise measurement of the ligand to gallium ratio can be achieved
through the singlet H4 resonance. The absence of exchange at ambient temperature
between free and complexed ligand reveals the stability of the complex before
decomposition : a kinetic lability resulting of the absence of ligand field stabilization
would favor the release of the metal and then, the exchange between free and
complexed ions. This exchange has been observed for the simple tris (bidentate)
complexes [8].
O
0.5
1
1.5
2
2.5
equivalent of Ga (acac)
Figure 2 :titration ofLH6 b-YGa(acac)j in D20-KOD
Iron (III) complex [LFe] 3- : The 300 MHz 'H NMR spectrum in CD30D for
the free ligand L at 293 K and for the complex [FeIIILI 3- at 243 K are illustrated in
Figure 3A and 3B, respectively. Both the spectral width and the line broadening of
the complex signals are consistent with the presence of paramagnetic species in
solution, which were identified by EPR as high-spin Fe(II1) c o m p l e x e s
(characteristic signal at g = 4.3).
'H NMR OF GALLIUM(II1) AND IRON(II1) COMPLEXES
l
9.0
1
I
I
I
8.0
7.0
6.0
5.0
l
4.0
1
3.0
117
I
2.0 ppm
Figure 3 : IH NMR spectra for the free ligand L ( A )at 293 K. for the GaiIll) co!nplerc ( B )
or 193 K and for the Fe([[[)comp1e.r (Ci ut 233K. Prororzs are labelled according to Figrire 1.
118
P. Baret et al.
In order to differenciate the contribution of the ligand coordination to the Fe III
center from a dipolar-only interaction. Curie plots (chetnical shift vs reciprocal
temperature) and plots of the chemical shift vs the inverse quadratic temperature
were generated for temperatures ranging from 323 to 303 K. Resulting slopes and
T-> x intercepts are presented in Table 1. Correlation coefficients are very similar
for the two different fits (not shown), nevertheless intercepts in the Curie plots are
more consistent with the diamagnetic shifts observed in the free ligand. An inverse
linear temperature dependence of the chemical shifts then suggests a large
contribution of the contact shift (i.e. coordination of the ligand to the metal center)
and a rather small contribution of the dipolar shift (Le. rather small zero-field
splitting as in the case of "FeO6" trischelate complexes and consequently a rather
symmetric environment for the Fe III ) [21]. Wliile COSY experiments are unadapted
due to extreme relaxation. tentative assignrnents for the hyperfine shifted resolved
resonances can be pursued that take advantage from Curie slopes, Curie intercepts
andlor relaxation considerations. In particular. peaks a. b, f and g in Figure 1B can
be assigned to naphtalene ring protons on the ligand. Arguments in favor of this
assignment are the position of the diamagnetic shifts (deduced from Curie plots) in
the aromatic region for peaks b and f. but also the sizeable sirnilarities in spinlattice relaxation times and Curie slopes for resonances a-b and f-g. Sign and
intensity of Curie slopes together with relative relaxation times can be considered
next, in order to discriminate between these protons. In fact, smaller positive
contact shifts and longer relaxation times (peaks a and b) are assigned to protons 57, while the leftover protons. nurnbered 4 and 8. show shorter T l s and increased
Curie slope (peaks g and f respectively) consistent with a closer through-space and
through-bond distance to the iron center. The remaining unassigned resonances may
arise from protons located on the aliphatic chains of the ligand (c,e) whicli are
expected to experience no contact chemical shifts and moderate dipolar shifts with
respect to their increased distance to the metal center. The obtained assignments are
summarized in Table II. which shows a good correlation between the distance to the
iron atom (from molecular modelling studies [14]) and the T l values. or the
nurnber of bonds to the Fe atom and the hyperfine shifts deduced from the SObs and
the corresponding &dia for the diamagnetic [LGa]3-.
A cornparison between the IH NMR spectra of the free ligand ( Figure 3A ) and
the Ga(II1) complex ( Figure 3B ) places protons I Q , C&NH and C&N as good
candidates for sensitive probes upon metal complexation ( 0.61, 1.22 and 0.96 ppm
upfield shift respectively upon metal ligation ). Similar effects are expected for the
Fe(II1) complex, implemented with the intrinsic paramagnetic shifts due to the iron
'H NMR OF GALLIUM(II1) AND IRON(III) COMPLEXES
119
Table 1 : 'H NMR spectral properties for the [ L F ~ ] complex
~in c D 3 0 D a at 293 K
Resonances
Label
60bs
T1
(PP~)
20.7
18.5
7.5
2.0
O. 1
- 10.2
a
b
c
d
e
f
T2
(ms)
1.1
1.1
2.6
(ms)
0.4
0.4
1.3
0.5
0.3
Curie plots
10' x Slope Intercept
(pprn.~)
52.0
30.3
0.3
O
O
-56.5
(pprn)
3.0
8.1
6.3
2.0
0.2
9.0
Inverse quadratic plots
104 x Slope Intercept
(ppm.~2)
66.8
39.3
0.44
O
-0.05
-71.3
(ppm)
13.O
13.9
6.9
2.0
O. 1
-2.1
g
- 19.9
0.7
0.3
-96.5
13.1
-123.5
-5.5
(a) Chernical shifts referenced to tetrarnethylsilane via residual C m 0 H solvent signal at 3.30 ppm
(b) Average result from the two methods (nul1 point and initial slope of thee rnagnetisation recovev
data)
Table II : Comparison of some 1~ NMR properties for the free ligand L and the two
complexes (LGaj3- and ILFej3- in CD30D at 293K
Assignrnenta
Ligand
6
[LGa]36
Label
(PP~) (PP~)
--
[LFe]3Nurnber of 6hfLGa 6hfL
bonds to Fe (ppm)b (ppm)b
TI
(ms)
R F ~
(A)c
8.38
7.77
g
5
-27.7 -28.0
0.7
6.0
6.85
6.42
b
7
12.1
11.7
1.1
7.4
H6
6.96
6.75
a
7
14.0
14.0
1.1
8.5
H7
6.96
6.90
a
8
13.8
14.0
1.1
7.7
H8
7.59
7.60
f
6
-17.8 -17.6
0.5
5.0
CH2NH
3.65
2.43
c
5.1
4.0
2.6 4.515.6
CH2N
3.01
2.05
(a) According to labelling in Figure 3
O>) 6hfLGa = 60bs - & [ L G ~ ] ~ - 6 h f ~= 60bs -6L
(c) Distances measured on the rninimized A-confomtion of the complex obtained through
molecular modelling calculations using the Insight Discover software.
H4
H5
120
P. Baret et al
center. As a consequence. inequivalences in the three binaphtyl subunits or in the
two tren moities in a non-symmetric complex should give rise to multiple
resonances for proton H j . which is well resol\.ed in the upfield window. The
absence of such patterns is then suggesting a structure where the iron atom is
encaged in the macrobicyclic ligand cavity in a D3 symrnetry. Nethertheless the
lines broadening may indicate local distorsions or dynamic effects that precludes
definitive conclusion. On the other hand. the absence of free ligand resonances
favors the stability of the complex with regard to tris (bidentate) ligands.
REFERENCES
1 Okawa H (1988) Noncovalent interligands interactions in metal complexes.
Coord. Chrm Rev. 92. 1-28 and references cited herein.
2 Okawa H. Tokunaga H. Katsuki T. Koikawa M. Kida S (1988) Noncovalent
interactions in metal complexes 16. Stereoselectivity of 1:3 complexes of Sc, Y,
La. Al. Ga and In ions with 4-(1 - menthy1oxy)-1-phenyl-1.3-butanedioneand 4(1- menth-v1o.q)- 1- p-tolyl-1, 3- butanedione. Inorg. Chem. 27, 4373-4377
3 Tor Y, Librnan J. Shanzer A. Lifson S (1987) Biomirnetic ferric ion camers.
A chiral analogue of enterobactin. J. Am. Chem. Soc. 109. 65 17-65 18
4 Tor Y, Libman J1 Shanzer A (1987) Biomimetic femc ion carriers. Chiral
ferrichrome analogues. J. Am. Chem. Soc. 109, 65 18-65 19
5 Hisaeda Y. Ihara T, Ohno T, Murakarni Y (1991) Absolute configuration of
iron (III) complexes of an enterobactin model. a hexapus cyclophane with
catechol segments. Chem. Lerrers 2 139-2 142
6 Stack TDP, Karpishin TB, Raymond KN (1992) Structural and spectroçcopic
characterization of chiral ferric tris-catecholamides : unraveling the design
of enterobactin. J. Am. Chem. Soc. 1 14, 15 12- 15 14
7 Abu-Dan K. Raymond KN (1992) Synthesis and characterization of chiral
isomers of tris (1-0x0-22( 1 H)-pyridinethionate) iron (III), chromium (III),
and cobalt (III) complexes. J. Coord. Chem. 26,l- 14
8 Karpishin TB, Stack TDP, Raymond KN (1993) Stereoselectivity in chiral Fe(1II)
and Ga (III) tris (catecholate) complexes effected by nonbonded, weakly polar
interactions. J. Am. Chem. Soc. 115, 6 1 15-6 125
9 Dayan 1, Libman J, Agi Y, Shanzer A (1993) Siderophore analogs : ferrichrome.
Inorg. Chem. 32, 1467-1475
10 Dietz SD. Eilerts NW, Heppert JA. Morton MD (1993) Binaphtolate and
binaphtolate chlonde complexes of tungsten (VI). Inorg. Chem. 32.1698- 1705
1 1 Hayoz P. Von Zelewsky A, Stoeckli-Evans H (1993) Stereoselective synthesis of
octahedral complexes witli predetermined helical chirality. J. Am. Chem. Soc.
115. 5111-5114
12 Akiyama M. Hara Y, Gunji H (1995) Artificial siderophores as model for
femchrome. Control of the A- or A- configuration of iron (III) complexes of
tripodal hydroxamates by linking to the C- or N- terminus of the same L-alanylL-alanyl-p-(N-hydroxy) alanine unit. Chem. Lerrers 225-226
J Chim Phvs
'H NMR OF GALLIUM(II1) AND IRON(III) COMPLEXES
121
13 Matzanke B, Muller-Matzanke G, Raymond KN (1 989) Iron Carriers and Iron
Proreins. (Ed. T. M. Loehr), VCH, New-York, 1-121 and reference cited herein.
14 Baret P, Beaujolais V, Gaude D, Coulombeau C, Pierre J-L (1997) Control of the
helical chirality in octahedral complexes by a chiral macrobicyclic cavity
possessing six convergent hydroxyl groups. Chem. Eur. J. 3, 969-973
15 Bergeron RJ. Kline LJ (1984) 300-MHz 1H NMR study of parabactin and its
gallium (III) chelate. J. Am. Chem. Soc. 106, 3089-3098
16 Borgias BA, Barclay SJ, Raymond KN (1986) Structural chernistry of gallium
(III). Crystal structures of K j [Ga (catecholate)3]. 1.5 H 2 0 and
[Ga (benzohydroxamate)3].H20.CH3CH20H.
J. Coord. Chem. 15,109- 123
17 Kersting B, Telford JR, Meyer M, Raymond KN (1996) Gallium (III) catecholate
complexes as probes for the kinetics and mechanism of inversion and isomerization
of siderophore complexes. J. Am. Chem. Soc. 118, 57 12-5721
18 La Mar GN, Horrocks WD, Holm RH Edts (1973) NMR of Paramagnetic
Molecules. Academic Press, New-York, 678 p
19 Kurland RJ, Mc Garvey BR (1970) Isotropic NMR shift in transition metal
complexes : the calculation of the Fermi contact and pseudocontact terms.
J. Magn. Res. 2, 286-301
20a Berliner LJ. Reuben J Edts (1993) .VMR of paramagnetic molecules.
Academic Press, New-York, 440 p
20b Lehmann TE, Ming LJ, Rosen ME, Que L Jr (1997) NMR studies of the
paramagnetic complex Fe(I1)-bleomycin. Biochernistry 36, 2807-28 16
21 Collison D, Powell AK (1990) Electron spin resonance studies of "Fe06" tris
chelate complexes : models for the effect of zero-field splitting in distorded
s=5/2 spin systerns. Inorg. Chem. 29, 4735-4746