Structural investigations of a lead(IV) tetraacetate-pyridine complex

Article abstract of DOI:10.1039/b506366c

A 1 : 1 crystalline complex of lead(IV) tetraacetate and pyridine (LTA-py) has been prepared. The single-crystal X-ray structure, at 296 and 150 K, establishes the presence of a relatively short Pb-N bond (2.307 A) within an intriguing seven-coordinate lead inner sphere consisting of the pyridine ligand and two bidentate and two monodentate acetate ligands. The pyridine occupies a surprising amount of the available coordination space and has induced a dramatic change in coordination compared to the four chelating acetate ligands found in lead tetraacetate (LTA). Thermal measurements (TGA/DSC) indicate the de-coordination of pyridine and its loss from the solid between 360 and 380 K. ²⁰⁷Pb CP/MAS NMR spectroscopy also demonstrates the existence of the Pb-N bond through observation of ¹( ²⁰⁷Pb,¹⁴N) = 63 Hz and a ²⁰⁷Pb-¹⁴N dipolar coupling constant, of 149 Hz. The solid-state ²⁰⁷Pb NMR parameters are used to give insight into the coordination environment of Pb(IV) in LTA-py. In solution, ligand exchange is rapid on chemical shift and J-coupling time scales. A ²⁰⁷Pb NMR study of the titration of an LTA solution by pyridine yields a stability constant for LTA-py of K = 1.5 M ^-1 and predicts it to have a ²⁰⁷Pb NMR chemical shift essentially identical to that observed by CP/MAS NMR in the solid state. This correlation between the solid state and solution indicates that the seven-coordinate LTA-py structure found in the crystalline state does persist in solution, and this could further explain why the addition of pyridine has such profound effects on lead(IV) carboxylate-mediated organic reactions. Simulations of exchange-broadened line shapes of ¹³C CP/MAS NMR spectra in the temperature regime above 280 K indicate local motion of the pyridine rings in the form of 180° jumps (activation energy 72.5 kJ mol^-1); these are first such ring flips reported for a coordinated pyridine ligand. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b506366c

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F U L L P A P E R
Structural investigations of a lead(IV) tetraacetate–pyridine complex
a
a
b
a
Jonathan E. H. Buston, Tim D. W. Claridge, Stephen J. Heyes,* Michael A. Leech,
a
a
b
Mark G. Moloney,* Keith Prout and Maya Stevenson
a
Department of Chemistry, University of Oxford, Chemistry Research Laboratory,
Mansfield Rd, Oxford, UK OX1 3TA. E-mail: mark.moloney@chem.ox.ac.uk;
Fax: +44 1865 275674; Tel: +44 1865 275656
b
The Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford,
South Parks Rd, Oxford, UK OX1 3QR. E-mail: stephen.heyes@chem.ox.ac.uk
Received 6th May 2005, Accepted 20th July 2005
First published as an Advance Article on the web 18th August 2005
A 1 : 1 crystalline complex of lead(IV) tetraacetate and pyridine (LTA–py) has been prepared. The single-crystal
˚
X-ray structure, at 296 and 150 K, establishes the presence of a relatively short Pb–N bond (2.307 A) within an
intriguing seven-coordinate lead inner sphere consisting of the pyridine ligand and two bidentate and two
monodentate acetate ligands. The pyridine occupies a surprising amount of the available coordination space and has
induced a dramatic change in coordination compared to the four chelating acetate ligands found in lead tetraacetate
(
LTA). Thermal measurements (TGA/DSC) indicate the de-coordination of pyridine and its loss from the solid
2
07
between 360 and 380 K. Pb CP/MAS NMR spectroscopy also demonstrates the existence of the Pb–N bond
1
207
14
207
14
through observation of J( Pb, N) = 63 Hz and a Pb– N dipolar coupling constant, of 149 Hz. The solid-state
Pb NMR parameters are used to give insight into the coordination environment of Pb(IV) in LTA–py. In solution,
2
07
2
07
ligand exchange is rapid on chemical shift and J-coupling time scales. A Pb NMR study of the titration of an LTA
−
1
207
solution by pyridine yields a stability constant for LTA–py of K = 1.5 M and predicts it to have a Pb NMR
chemical shift essentially identical to that observed by CP/MAS NMR in the solid state. This correlation between the
solid state and solution indicates that the seven-coordinate LTA–py structure found in the crystalline state does
persist in solution, and this could further explain why the addition of pyridine has such profound effects on lead(IV)
carboxylate-mediated organic reactions. Simulations of exchange-broadened line shapes of C CP/MAS NMR
spectra in the temperature regime above 280 K indicate local motion of the pyridine rings in the form of 180 jumps
1
3
◦
−
1
(
activation energy 72.5 kJ mol ); these are first such ring flips reported for a coordinated pyridine ligand.
6
–8
Introduction
in cross-coupling reactions is also well known. Whilst it has
been speculated that pyridine could be acting as a ligand, a base,
or indeed both in these reactions, no direct evidence for its role
has been established. However, indirect evidence for its role as a
ligand has come from recent observations that pyridine catalyses
ligand redistribution between aryllead triacetates and diaryllead
Lead(IV) compounds have been shown to mediate both a wide
1–5
6–8
range of oxidizing and coupling processes. There is now
considerable evidence that the latter processes occur by ligand
9,10
11,12
coupling involving a reductive elimination at the lead atom
1
3
rather than a radical or ionic mechanism. The ability to
attenuate, enhance or otherwise modify the reactivity of metal
cations by controlling their coordination environment is well
27
2
diacetates, and that it, along with other sp -hybridized amines,
effectively catalyzes lead(IV) mediated arylation reactions. That
this coordinative effect could be put to valuable synthetic effect
was recently indicated by Yamamoto and co-workers, who
showed that useful enantiocontrol and conversion yields could
be achieved in biaryl coupling reactions mediated by lead(IV) in
the presence of chiral amines (e.g. brucine) although incorpora-
28
14,15
known,
and has recently been exploited extensively, for
22
example, in asymmetric bond forming reactions (e.g. catalytic
16
17
18
reduction, Sharpless allylic epoxidation, dihydroxylation
19
and aminohydroxylation ). It has therefore been of interest to
us to examine the effect of ligands on the reactions mediated by
lead(IV) that have been investigated in our laboratory.
A recurring theme in the reactions of lead(IV) in much of the
literature has been the influence of pyridine (py) in directing
the course of a wide range of reactions, including oxidation
and ligand coupling processes (vide supra). It has been widely
tion of such r-donors within a ligand neither enhanced yields
20,21
29,30
nor enantioselectivity in arylation reactions.
Furthermore,
the value of imine-containing lead(IV) ligands for enantioselec-
tive aziridination reactions has recently been demonstrated.
Direct evidence for the existence of aryllead(IV)–pyridine and
aryllead(IV)–acetonitrile adducts was recently established using
31
6,7,22,23
speculated in the literature
that pyridine acts as a r-donor,
32
ESI mass spectrometry.
thereby facilitating ligand exchange, particularly replacement of
acetate by other enolic-type substrates, but no direct evidence
has thus far been produced. For example, oxidations of alcohols
by LTA are reported to occur in good yield only in neat pyridine;
in other solvents such as benzene, competing reactions leading to
In the light of the obvious importance of pyridine, and
more generally amine ligands in the chemistry of lead(IV), we
were intrigued by a report of the isolation of a LTA–pyridine
33
complex, and present here a re-investigation of that work,
including results from a detailed structural analysis of this
complex using a combination of crystallographic, and solid-
state and solution multinuclear NMR techniques.
24
a variety of products are predominant. Furthermore, the rate
of oxidation of lactic acid is found to be greatly increased by the
2
5
presence of pyridine or 1,10-phenanthroline. The kinetics of
the oxidation of benzylic alcohols by lead(IV) acetate have been
26
investigated, and in the absence of pyridine the reaction is
second order in the alcohol. However, with pyridine present, the
reaction is first order in both the alcohol and pyridine; clearly the
mechanism changes under these conditions. Similarly, the need
for pyridine (or a similar nitrogen containing heterocyclic base)
Results
Preparation
The lead(IV) tetraacetate–pyridine complex (LTA–py) could be
easily crystallized from a solution of lead(IV) tetraacetate and
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
D a l t o n T r a n s . , 2 0 0 5 , 3 1 9 5 – 3 2 0 3
3 1 9 5

View Online
◦
Table 1 Selected bond lengths ( A˚ ) and angles ( ) at 150 K
symmetry (2.218(3), 2.499(3) A˚ , DPb–O = 0.281 A˚ ) and a
˚
significantly longer average Pb–O distance (by 0.066 A). Even
Pb1–N18
Pb1–O2
Pb1–O4
Pb1–O6
Pb1–O8
Pb1–O10
Pb1–O12
Pb1–O20
Pb1–O22
C3–O2
2.307(3)
2.497(3)
2.218(3)
2.162(3)
2.844(3)
2.304(3)
2.276(3)
2.160(3)
2.776(3)
1.244(4)
1.285(4)
1.325(4)
C7–O8
1.206(4)
1.263(4)
1.265(4)
1.297(4)
1.232(4)
1.367(3)
1.375(3)
1.392(3)
1.342(3)
1.339(3)
1.397(3)
the more symmetrical chelate, which shows similar asymmetry
C11–O10
C11–O12
C21–O20
C21–O22
C14–C15
C15–C16
C16–C17
C17–N18
N18–C19
C19–C14
34
˚
in its binding (DPb–O = 0.028 A) to lead(IV) tetraacetate
˚
(
LTA) (DPb–O = 0.009–0.059 A), has a longer average Pb–O
35
distance than those in LTA or lead tetra(o-benzoyl benzoate)
˚
by 0.015 A. The other two acetate ligands are even more
distorted, bonded to the lead atom through only one oxygen
atom, with the other oxygen atom falling outside the van der
Waals radius of the lead atom normally considered for bonding.
The bonded Pb–O distances in these ligands are significantly
C3–O4
C7–O6
˚
shorter (2.160(3), 2.162(3) A) than those in the chelating ligands
av. 2.324(3) A). The distortion in the lead–oxygen bond lengths
˚
(
O2–Pb1–O4
O6–Pb1–O8
O10–Pb1–O12
O20–Pb1–O22
O2–C3–O4
O6–C7–O8
O10–C11–O12
55.0(1)
50.4(1)
57.0(1)
51.1(1)
120.1(2)
122.5(2)
119.7(2)
O20–C21–O22
C14–C15–C16
C15–C16–C17
C16–C17–N18
C17–N18–C19
N18–C19–C14
C19–C14–C15
121.1(2)
119.0(3)
119.7(3)
120.6(3)
120.3(3)
120.6(3)
119.8(3)
is also reflected in the oxygen-carbon bonds (C7–O6 is notably
long at 1.328(5) A), such that the acetate group is deformed
˚
by the mode of binding. Pictorial representations of the inner
coordination sphere of the lead atom of LTA, and of the
LTA–pyridine adduct are shown in Fig. 2. That of LTA–
py can be described as a distorted pentagonal bipyramidal
coordination, with chelation across two edges of the pentagon,
the two monodentate carboxylates being axial and the pyridine
being the final vertex of the pentagon. Such an arrangement is
pyridine in dry acetonitrile. The yellow crystals are moisture
sensitive, decomposing on contact with atmospheric moisture
considerably more rapidly than lead(IV) tetraacetate itself, to
give a brown powder, and hence were normally handled under
a nitrogen or argon atmosphere. The complex is readily soluble
in many non-protic organic solvents.
expected for complexes of the type M(bidentate)
2
6
(unidentate)
3
3
with low normalized bite, b < 1.2, for the chelate. The pyridine
ligand occupies a larger cone volume than any other ligand and
interestingly the ring plane lies roughly in the approximate plane
of the distorted pentagon. This structure is in contrast to that for
X-Ray structure analysis
The solid-state structure of the LTA–py complex was found by
single-crystal X-ray diffraction at 150 K, and is shown in Fig. 1,
with selected bond lengths and angles in Table 1. The presence of
a lead-nitrogen bond, can be seen immediately, together with the
large amount of ‘coordination space’ that the pyridine takes up.
˚
This lead–nitrogen bond, at 2.304(3) A, is the shortest amongst
the crystal structures of lead(IV) reported in the CCDB, and is
of the same order as a lead–oxygen bond (e.g. Pb1–O10 2.302(3)
˚
A), consistent with a strong coordination effect. It is also the
first example of an uncharged but non-chelating N-ligand for
lead(IV).
Fig. 1 Crystal structure of LTA–py with thermal ellipsoids.
It can be seen that the lead atom is seven coordinate,
with only two of the acetate ligands bonding in a bidentate
fashion. Of these, one is virtually symmetrically bound, with
oxygen atoms almost equidistant from the lead atom (2.302(3),
Fig. 2 Selected views of the coordination sphere of (a) LTA; (b) LTA–py
showing all ligands and (c) LTA–py showing only bonds involving the
lead(IV) cation.
˚
2
.277(3) A), whereas the other shows significant distortion of
3
1 9 6
D a l t o n T r a n s . , 2 0 0 5 , 3 1 9 5 – 3 2 0 3

View Online
3
4
35
13
13
207
the eight-coordinate LTA, and other lead tetracarboxylates,
Table 2
C NMR Chemical shifts and J( C, Pb) for LTA and
LTA–py
in which both oxygen atoms of each carboxylate are bonded
to the lead atom, with a rather smaller variation in lead–
oxygen bond lengths. The structure of this complex was also
determined at 293 K, and except for the expected increase in the
lattice dimensions and atomic displacement parameters, shows
no discernable differences from the low-temperature structure.
An X-ray powder diffraction pattern of a bulk sample of the
LTA–py complex was obtained and showed good correlation
with that predicted from the single-crystal lattice parameters and
Solution d (ppm)
CP/MAS d (ppm) [J/Hz]
a
LTA
18.3
LTA–py
LTA
LTA–py
CO
2
CH
3
3
18.1
17.0 [75]
8.1 [71]
16.7 [73]
20.1 [48]
1
2
1.4 (2) [46]
CO
2
CH
180.0
180.0
182.1 [56]
83.4 [65]
173.5 [59]
176.6 [67]
1
37
atomic positions suggesting that the single-crystal structure is
indeed representative of the bulk sample.
1
78.7 [98]
180.0 [73]
28.4
1
Pyridine
124.0
129.4
142.3
145.6
147.6
Thermal measurements
1
36.7
TGA and DSC were performed on LTA–py in a flowing nitrogen
gas atmosphere from 293 to 473 K with a temperature ramp of
149.1
For a 1 : 1 solution mixture of LTA and pyridine.
◦
−1
2
C min . Several features of interest are observed in the DSC
a
and relate to two specific regions of mass loss (Fig. 3). The first
distinct region of mass loss between 70 and 110 C may be split
◦
into two processes by observation of two separate endotherms
in the DSC. The first and smaller endotherm is attributed to loss
3
8
◦
of residual solvent, the second (from 87 to 110 C) represents a
relative 15% mass loss, which is consistent with the loss of all the
◦
pyridine from the LTA–py complex. Between 120 and 165 C, a
further relative mass loss of 22% occurs, which may be compared
to the expected mass loss of 26.5% for the complete loss of
two acetate groups (LTA decomposing to lead(II) diaacetate
(
LDA)). The DSC exotherm which accompanies this mass loss
has a distinct shoulder to it, perhaps suggesting two separate
◦
processes. Beyond 165 C a further endotherm is detected in a
region of only slow mass loss.
207
Fig. 4 Solution Pb NMR data for (a) a 1 : 1 mixture of LTA and B
in CDCl ; and (b) LTA–py in CDCl
3
3
.
of exchange of these carboxylates is increased. In contrast, the
2
07
addition of pyridine to LTA alone decreases the Pb shielding
2
07
but does not significantly broaden the Pb NMR resonance.
2
07
The Pb NMR chemical shift was recorded as pyridine was
titrated into a solution of LTA. From these chemical shifts
the binding constant of the pyridine to the lead atom was
Fig. 3 Combined TGA and DSC of LTA–py, performed under a flow
of dry nitrogen gas.
41
calculated by means of the Associate program, assuming a
Solution NMR
−
1
1
: 1 binding model. This was found to be 1.5 (± 0.1) M , with
1
13
In chloroform solution the H and C NMR spectra of the
LTA–pyridine complex are identical to those of an equimolar
a good fit of the data, as demonstrated in Fig. 5. This indicates
a weakly bound species in solution, which is consistent with the
other observations. Extrapolating the observed chemical shifts
allows the solution chemical shift of the LTA–py complex to be
predicted to be −1855.0 ppm, a downfield shift of 18 ppm from
1
3
solution of LTA and pyridine. The C NMR spectrum, for
example, shows only very slight deshielding of the acetate carbon
1
3
atoms compared to LTA (see Table 2). In the C NMR spectrum
of LTA at low temperatures (225 K), carbon–lead J-couplings
2
13
207
3
13
207
(
J( C, Pb) = 131 Hz, J( C, Pb) = 141 Hz) can be observed
39
in satellites for the acetate carbon atoms; these are not observed
at higher temperatures due to a higher rate of intermolecular
exchange of the acetate ligands. In the C NMR spectra of LTA–
py these couplings are not seen at all, suggesting that the rate of
acetate exchange is increased by the presence of the pyridine.
Further evidence for this phenomenon arises from an investi-
1
3
2
07
gation of the Pb NMR spectroscopy of these compounds. We
4
0
have previously shown that in a mixture of LTA (PbA
benzoic acid (B), the rate of exchange of the carboxylate ligands
PbA etc.) is rapid at room temperature,
but by cooling the solution to 225 K, five distinct Pb NMR
resonances, one for each species, can be observed. Upon the
addition of pyridine, even at low temperatures, only a single very
broad resonance is observed (Fig. 4). This suggests that the rate
4
) and
(
4
ꢀ PbA
3
B ꢀ PbA
2
B
2
2
07
Fig. 5 207Pb NMR Binding curve, determined by titration of LTA with
py in CDCl .
3
D a l t o n T r a n s . , 2 0 0 5 , 3 1 9 5 – 3 2 0 3
3 1 9 7

View Online
2
07
Table 3
Pb CP/MAS NMR Chemical shifts and anisotropies
NMR spectra of silicon-nitrogen ceramics containing locally
46,47 207
tetrahedral SiNO
3
units.
The Pb resonance profile for LTA–
48
LTA
LTA–py
py was simulated using the computer program WSolids. Using
˚
the Pb–N bond distance of 2.30 A obtained from the single-
d
d
d
d
iso
11
22
33
−1872.7
−1692
−1938
−1988
−1854.9
−1729
−1862
−1973
207
14
crystal determination a Pb– N dipolar coupling constant of
1
207
14
1
48.7 Hz is calculated. This, together with J( Pb, N) = 63 Hz,
with J-anisotropy D
J
≈ 0 Hz, an axial quadrupolar coupling
1
4
j
d
0.66
0.09
constant for N of ≈ −1.0 MHz with coincident dipolar
14
X
296
244
and quadrupolar tensor frames at N, and appropriate line-
broadening (both Gaussian and Lorentzian functions) yields
an excellent simulation, shown together with the experimental
profile in Fig. 7. An identical simulation was generated from a
simpler procedure which models the system simply in terms of
the J-coupling (J = 63 Hz) and a residual dipolar coupling (d =
LTA, which is still not a large change compared to the known
Pb chemical shift range of over 6000 ppm.
2
07
42
2
07
Pb CP/MAS NMR
2
9
3
Hz). The precedent from Si MAS NMR and the excellent
207
2
07
The
Pb CP/MAS NMR of LTA–py shows apparently
three isotropic resonances of approximately equal intensity at
fit of the simulations to the experimental Pb CP/MAS NMR
spectrum of the LTA–py indicates that it is reasonable to suggest
−
1853.3, −1854.9 and −1856.3 ppm (all with their associated
207
that the three resonances observed in the Pb CP/MAS NMR
spinning sidebands) (Fig. 6). These are different from the single
resonance manifold recorded for LTA (Fig. 6), both in isotropic
and anisotropic (obtained from an analysis of the sideband
spectrum are caused by a scalar coupling and a residual dipolar
14
coupling to the N nucleus of the bound py ligand. This confirms
the very real nature of the lead-nitrogen bond in this complex and
43
intensities through the Herzfeld–Berger method ) chemical
shifts (Table 3). The pattern of these three peaks is highly
reproducible between samples; the same peak envelope is seen
in each sideband, and the pattern is conserved throughout the
temperature range of 213–330 K, with merely a slight (ca.
207
14
is the first report of J( Pb, N) in the solid state and also the first
207
observation of residual dipolar coupling in Pb MAS NMR.
1
0 ppm) upfield shift in the isotropic resonance observed as
the temperature is increased through this range. The average
chemical shift at 296 K, −1854.9 ppm, is essentially identical
2
07
to the shift predicted for the LTA–py complex by Pb solution
NMR, −1855.0 ppm.
207
Fig. 7 (a) Isotropic region of the Pb CP/MAS NMR spectrum of
the LTA–py complex; and (b) simulated spectrum with scalar coupling
207
14
J( Pb, N) = 63 Hz and residual dipolar coupling, d = 3 Hz. An
207
14
identical simulation is produced with J( Pb, N) = 63 Hz, dipolar
coupling of D = 148.7 Hz (calculated from Pb–N = 2.307 A˚ ) and
1
4
N quadrupolar coupling constant of v = −1.0 MHz with coincident
dipolar and quadrupolar tensor orientations.
CSA analysis of the integrated intensities of the pattern of
the spinning sidebands observed at several MAS rates was
performed using the Herzfeld–Berger method and the resultant
principle values of the CSA tensor, along with those of LTA,
are given in Table 3. The most notable feature of the LTA–
py CSA tensor is that the anisotropy is small and the skew is
close to zero, indicative of an essentially spherical distribution
of electron density around the lead atom. This is very different
from the general pattern for lead tetracarboxylates which have
axial or close to axial CSA tensors. The span of the CSA is,
2
07
Fig. 6
Pb CP/MAS NMR spectra of (a) LTA and (b) LTA–py at a
MAS rate of 2.5 Hz. iso indicates the region of the isotropic resonances,
the profile of which is also shown in expanded form for LTA–py.
2
07
14
J-Coupling of Pb to N (I = 1) should result in a 1 :
2
07
1
1
: 1 triplet of the sort recently reported in the Pb{ H}
solution NMR spectra of trisaryllead amide compounds where
the slow quadrupolar relaxation rates allow the measurement
1
207
14
t
of J( Pb, N) = 224 Hz for Bu
3
PbNH
2
and 265 Hz for
49
however, within the range typical for lead tetracarboxylates.
t
44
207
BuPbN(H)SiMe If the triplet of lines in the Pb CP/MAS
3
.
NMR spectrum of LTA–py reflects such a scalar coupling, it
is clearly smaller in value than the lead amide examples and
the spectrum is distorted by a further interaction such that
the intra-triplet peak separations are not symmetrical. The
spectral distortion reflects a residual dipolar coupling between
1
3
C CP/MAS NMR
The structures of both LTA and the LTA–py complex were
1
3
further investigated by C CP/MAS NMR spectroscopy at
ambient temperature. The spectrum of LTA (Fig. 8(a)) shows
two resonances for each of the distinct acetate carbon atoms.
This is interpreted as reflecting the presence of two independent
molecules of LTA in the asymmetric unit of the X-ray crystal
structure. Although in principle each of the independent
carbon atoms should give rise to a separate carbon resonance it
is assumed that because the acetate ligands within each molecule
2
07
14
Pb and N which is not removed by MAS. Such residual
1
3
dipolar couplings are commonly observed in C CP/MAS
45
spectra of carbon atoms directly bonded to nitrogen atoms.
34
Resonance patterns similar to that observed in our LTA–py
spectrum and shown to be due to scalar coupling and residual
1
4
29
dipolar coupling to N have been observed in the Si MAS
3
1 9 8
D a l t o n T r a n s . , 2 0 0 5 , 3 1 9 5 – 3 2 0 3

View Online
1
3
1
the C– H dipolar couplings from their static values. Therefore,
on raising the temperature, regions of spectral broadening are
expected when the rate of exchange reaches the order of the
51
MAS rate and then subsequently the frequency of the proton-
52
decoupling field. The projected rates of exchange beyond 337 K
are such that a MAS-broadening effect is likely, and since the
simulation process assumes only an exchange-broadening effect,
it will underestimate the true rate of exchange; spectra above
3
37 K were omitted from the analysis for this reason.
Discussion
The evidence in the crystalline state for the existence of a
coordinative complex LTA–py containing a Pb–N bond is
overwhelming. Specifically the single-crystal X-ray structure
2
07
˚
shows a Pb–N bond distance of 2.307 A, and Pb CP/MAS
1
207
14
NMR shows J( Pb, N) = 63 Hz and a residual dipolar
coupling constant consistent with the X-ray bond length. The
conclusion that a Pb–N bond is formed in solution relies on
more circumstantial evidence. Firstly, it is to be noted that the
crystalline material is obtained directly from solution, but this
does not prove that the association between Pb(IV) and py is
specific outside of the crystalline environment. NMR studies
in solution indicate that exchange of py and acetate between
the lead coordination sphere and bulk solution is rapid on
the NMR time scales determined by chemical shift differences
1
3
Fig. 8
C CP/MAS NMR data of (a) LTA and (b) LTA–py recorded
at MAS rate = 3 Hz (* indicates spinning side bands).
type, although crystallographically independent, are so similar
1
3
13
207
they are not distinguished through C NMR chemical shifts.
The spectrum for the LTA–py complex (Fig. 8(b)) has four
distinct carboxylate carbon resonances and three methyl carbon
resonances, with one almost twice as intense as the others. Thus
the four crystallographically distinct acetate groups in LTA–
py are distinguished by C NMR spectroscopy. Five slightly
broader resonances in the aromatic region of the spectrum
are assigned to the py carbon atoms and attribution to the
carbons o-, m- and p- to the nitrogen atom is made partially
by comparison with typical spectra of py and its adducts, and
also by the observation that the peaks at 145.6 and 147.6 ppm
are the broadest and so can be assigned to C17 and C19 (o- to
nitrogen), since they have the largest residual dipolar coupling
to N. There is evidence of Pb coupling satellites of each of
the resonances associated with the acetate groups in both LTA
and LTA–py. C NMR chemical shifts and coupling constants
are collected in Table 2.
A spectrum taken at 213 K showed no appreciable difference
to that recorded at room temperature. Heating of the LTA–
py complex in the range 262 to 337 K, however, causes a
dramatic change in the pyridine peaks of the C CP/MAS
NMR spectrum. On increase in temperature, pairs of resonances
attributed to C17/C19 (145.6 and 147.6 ppm) and C14/C16
and coupling constants. Indeed, that J-coupling ( C, Pb)
is observed at low temperature for acetate carbons in LTA,
but not LTA–py (1 : 1) solutions, indicates that the rate of
ligand exchange at Pb(IV) is accelerated by the presence of py.
2
07
Observation of Pb NMR as LTA is titrated by py provides the
crucial piece of evidence for an LTA–py complex in solution.
1
3
−
1
Although the derived binding constant of 1.5 M is not in itself
convincing as to a specific Pb · · · N interaction, the fact that the
2
07
Pb NMR shift for the LTA–py complex is predicted from the
titration to be −1855.0 ppm as compared to the measured shift in
2
07
the solid-state Pb CP/MAS NMR spectrum at −1854.9 ppm
is highly significant. Because the solid-state NMR is consistent
both with the unequivocal evidence from X-ray crystallography
and with the solution NMR data, it acts as a bridge between
the solid state and solution, allowing us to postulate that the
LTA–py complex exists in solution in a form substantially
unaltered from its manifestation in the crystalline solid. Hence,
also in solution, the addition of the py ligand has a profound
influence on the coordination sphere of the lead atom, and it
is perhaps therefore not surprising that LTA when dissolved in
py has a very different reactivity than when dissolved in a non-
coordinating solvent, and this offers insight into the observation
that py catalysis is often required in a wide range of reactions of
lead(IV).
1
4
207
1
3
1
3
(
128.4 and 129.4 ppm) were observed to first broaden and then
coalesce in a pairwise fashion (Fig. 9). The resonance assigned
to C15 (142.3 ppm) broadens only very slightly, whilst the
resonances for the acetate groups remain essentially unchanged.
On cooling, the spectral changes are fully reversible and there is
no hysteresis observed.
Comparison of the nature of the coordination of Pb(IV) in
LTA and LTA–py is of great interest. In LTA, Pb(IV) has a
distorted dodecahedral PbO inner coordination sphere in which
8
all the acetate groups are chelating, with each acetate in the two
molecules found in the crystallographic asymmetric unit having
slight, but different, asymmetry in their chelation. LTA–py has
a single molecule in the asymmetric unit with an unusual and
intriguing geometry. Two of the acetate ligands are chelating, one
similar to those in LTA though less tightly bound, the other more
considerably distorted. The other two acetate ligands are clearly
monodentate. Together with the coordination of the py molecule
These observations suggest a local dynamic process of an
◦
activated 180 ring flip motion of the py about the N–C15 axis,
which is slow on the NMR exchange-broadening time scale at
room temperature, but at higher temperatures is of a greater rate,
causing coalescence of first the C17 and C19 resonances and then
1
3
the C14 and C16 resonances through pairwise exchange. The C
CP/MAS NMR spectra between 305 and 337 K were further
this leads to a distorted pentagonal-bipyramidal PbO N inner
6
50
analysed by modelling, using the gNMR program, the spectral
line shapes of the py carbon atoms on the above assumption of
exchange, in order to obtain rates for the dynamic process at each
temperature, as shown in Fig. 9. The exchange data are shown
on an Arrhenius plot in Fig. 10. Fitting to the rate data from
coordination sphere. Since the py ring is observed to lie in an
approximate plane with the two bidentate acetate ligands (Fig. 2)
there is no obvious steric reason why the monodentate acetate
ligands could not chelate; the reason that they do not must be
electronic in origin.
−
1
207
3
08 to 337 K yields an activation barrier of E1
a
4
= 72.5 kJ mol
Pb NMR in the solid state can offer some insight into
−1
with a pre-exponential factor of A = 1.5 × 10 s . The py ring
the electronic environment of the Pb atoms and to the reasons
1
3
207
flip process is expected to significantly average the C CSAs and
for this interesting structure. The Pb NMR isotropic shift
D a l t o n T r a n s . , 2 0 0 5 , 3 1 9 5 – 3 2 0 3
3 1 9 9

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13
Fig. 9 Region of C CP/MAS NMR spectra of LTA–py, showing the pyridine peaks at a range of temperatures indicated from 296 to 328 K, and
the corresponding fit to the exchange model of pyridine ring flips at the temperature specified.
of LTA–py is, at −1854.9 ppm, just 16.5 ppm downfield of
Because the ideal coordination sphere of Pb in LTA–py would
have only C2v point symmetry, in comparison with the D2d
symmetry approximated by lead tetracarboxylates, there is no
2
07
LTA. In the context of an overall range of Pb NMR shifts
of ca. 6000 ppm and of a range of observed shifts for 20
40
207
lead(IV) tetracarboxylates of −1816.5 to −2137.0 ppm, the
expectation of an axial shielding tensor, so the skew of the Pb
2
07
average shielding of the Pb nucleus in LTA and LTA–py can
be considered to be virtually identical (though note that in the
preceding argument the difference is considered significant and
reproducible in respect of proving that the LTA–py complex
persists in solution). This suggests that the combined donor
power of the two bidentate acetates, two monodentate acetates
and py ligand in LTA–py is effectively the same as that of
the four bidentate acetates in LTA. This offers a potential
explanation for the unusual Pb coordination environment in
LTA—it would seem to be unnecessary for the two monodentate
acetate ligands to become bidentate, because the Pb(IV) atom is
already receiving the optimum amount of electron donation.
Presumably the electron density created by the alternative
mono-capped square antiprismatic nine-coordinate geometry
around Pb, in a structure where all the acetate ligands remained
chelating, could not be tolerated, even by Pb(IV).
NMR CSA, j
d
= 0.09, found for LTA–py is indicative of the
207
changed geometry. The span of the Pb NMR CSA, X =
244 ppm is within the range of 187–449 ppm found for lead
40
tetracarboxylates, but may be placed in the context of spans
of over 2000 ppm reported for some other lead compounds.
Clearly, despite initial appearances, the distorted pentagonal-
bipyramidal PbO
6
N coordination environment of the Pb in
LTA–py provides as uniform an electron distribution around the
2
07
Pb nucleus as does the slightly distorted PbO
around Pb in LTA.
8
dodecahedron
◦
The observation of a local dynamic motion, namely the 180
flips of the py ring, is also interesting. Similar flip-type motions
are well-known for a variety of phenyl groups in solids, but
reports of activated dynamic processes of py rings within solids
are restricted to inclusion and intercalation compounds in which
53
54–57
58,59
it is a guest molecule
and when adsorbed on surfaces,
in
3
2 0 0
D a l t o n T r a n s . , 2 0 0 5 , 3 1 9 5 – 3 2 0 3

View Online
first hard evidence towards the development of a mechanistic
approach to this class of reagent. The X-ray crystal structure
˚
indicates a Pb–N bond of 2.307 A and an intriguing seven-
2
07
coordination of the Pb(IV). Solid-state Pb CP/MAS NMR
studies act as a bridge to the solution NMR measurements,
showing that this structure of the complex persists in solution,
despite a relatively rapid rate of ligand exchange. The structure
of this product is totally unexpected; that lead tetracetate might
associate with pyridine seems at first glance to unremarkable,
but that the neutral N-donor pyridine ligand should displace
two O-donor atoms from two chelating and charged acetate
ligands from the Pb(IV) coordination sphere is unprecedented.
2
07
Analysis of the solid-state Pb NMR spectra suggests that the
driving rationale for the coordination environment of Pb(IV) is
for it, in all cases, to receive from the combination of ligands
a similar electron density in as symmetrical a distribution as
possible; the huge structural diversity and apparent adaptability
of known Pb(IV) environments seems to have masked this simple
principle. All these apparently dissimilar environments achieve
essentially the same result for the lead atom: Pb(IV) controls
its electronic distribution and the ligands must adapt, hence
the potentially interesting modifications in their chemistry. It
seems clear that when pyridine is added to solutions for the
enhancement of the exploitation of lead(IV) carboxylate-assisted
organic reactions, it can act as a ligand to Pb(IV) with a profound
effect on the coordination sphere of the lead atom, and not just as
a Brønsted base. The coordination structure, geometry and local
pyridine ring dynamics of the crystalline LTA–py coordination
compound are of great intrinsic interest too, offering insight into
the delicate balance of forces involved in the construction of
these compounds. The link between the solid state and solution
Fig. 10 Arrhenius plot (ln(rate) vs 1/T) for pyridine ring flip exchange
data.
all of which cases the dynamics are not confined to a simple
◦
flip motion. The simple 180 ring flip does not, of course,
lead to equilibrium positional disorder within the crystal and is
hence essentially “diffraction-invisible”. The localized electron
density seen in the X-ray structure at 293 K confirms that the
1
3
exchange motion detected by C CP/MAS NMR is indeed a
flipping process rather than a continuous rotation. This is the
first example of such a motion for a coordinated py molecule in
the solid state and is the first full analysis of a flip motion for a
ring bonded to Pb(IV).
2
07
provided by the Pb NMR is possibly the most compelling
example to date of using NMR to show that a relatively weakly
bound and labile complex in solution has as a highly defined
and unforeseen structure that is identical to that determined
crystallographically.
That the py ring in this complex is unique in demonstrating
ring flips under conditions where many other py coordination
60
complexes do not can almost certainly be rationalized through
its unusual structure. All the previously studied py coordination
complexes would be expected to have significant intramolecular
and intermolecular steric barriers to a ring flip process. As we
have previously remarked, the py ring occupies a large volume
of the Pb(IV) coordination sphere, and because the axial acetate
ligands are mono- not bi-dentate, we would expect little, if any,
intramolecular steric barrier to the ring flip process. Here, as in
the many reported examples of phenyl ring flips, it would appear
that the barrier to the flip motion is determined primarily by
intermolecular steric factors. It is far more likely that part of
a neighboring molecule can move away for a sufficient time to
allow a ring flip to occur than it is for part of another ligand in
the same coordination sphere to do so. It seems that in the other
py coordination complexes studied the intermolecular barrier to
ring flips proves prohibitive.
Experimental
Preparation of the complex
Lead tetraacetate (4 mmol, 1.77 g) was dissolved in acetonitrile
(
15 ml) at room temperature under an inert argon atmosphere.
Upon the addition of pyridine (20 mmol, 1.56 ml) the solution
turned to a deep red color, and a yellow crystalline solid started
◦
to precipitate. Precipitation was completed by standing at 4 C
for 4 days. The supernatant liquid was then decanted and the
crystals washed with cold hexane under an argon atmosphere
and dried in vacuo, to yield the lead(IV) tetraacetate–pyridine
(LTA–py) complex (1.49 g, 72%). Elemental analysis: C, 28.9;
H, 3.00; N, 2.50; Pb, 38.7%. C13 Pb requires C, 29.9; H,
H
17NO
3
It is probably also significant that we observe de-coordination
and loss of py from the solid on heating to 360 K, only 25 K above
the maximum of the temperature range in which ring flips are
studied by CP/MAS NMR spectroscopy. Presumably the loss
of the py molecules is presaged by some rearrangement within
the complex, and most particularly one might expect the Pb–
N bond to lengthen with increasing temperature, and that the
crystal should become less dense prior to the de-coordination of
py above ca. 360 K. Such behavior must have an effect in regard
to the rate of py ring flips and means that caution should be
observed in respect of the Arrhenius analysis as the activation
barrier to ring flips might be expected not to be independent of
temperature.
61
3
.28; N, 2.68; Pb, 39.66%.
Solution NMR spectroscopy
13
Broadband proton-decoupled C NMR spectra at 125 MHz
were recorded at 300 K on a Bruker AM500 spectrometer with
207
a 5 mm broadband probe. Broadband proton-decoupled Pb
NMR spectra in solution were recorded at 52.2 MHz on a
Bruker AM250 spectrometer with a 10 mm broadband probe,
at a temperature of 295 K unless otherwise stated. The spectra
were referenced externally to 80% Me Pb in toluene, whereby
4
62
the reference frequency of this standard was calculated from
207
62,63
1
N( Pb) = 20.920597 MHz
(when the H NMR frequency
of Me Si is 100.000 MHz). For each sample about 64 scans were
4
required to obtain a satisfactory signal-to-noise ratio.
Conclusions
We have prepared a complex of lead tetraacetate and py;
complexes of this type have become of greater synthetic interest
in recent years, although their chemistry has developed in an
empirical fashion. The structure reported here represents the
Solid-state CP/MAS NMR spectroscopy
1
3
207
C and
Pb CP/MAS NMR spectra were acquired, at
50.32 and 41.868 MHz respectively, on a Bruker MSL200
D a l t o n T r a n s . , 2 0 0 5 , 3 1 9 5 – 3 2 0 3
3 2 0 1

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2
07
spectrometer, with a multinuclear, proton-enhanced, double
bearing magic angle sample spinning probe, using a dry nitrogen
gas supply. Inside an inert atmosphere box, approximately
The isotropic Pb CP/MAS NMR resonance was simulated
48
using the WSolids computer program to fit the chemical
shift, line width, J( Pb, N) and the residual dipolar coupling
constant D( Pb, N). The residual dipolar coupling constant
was calculated from the value of the Pb–N bond distance
obtained from the X-ray crystal structure and also independently
obtained by fitting the spectrum using the assumptions of
an axial quadrupolar coupling and similar orientation of the
2
07
14
2
07
14
2
50 mg of sample was packed into 7 mm zirconia rotors with Kel-
F caps. MAS was performed at typical rates of 2–3 kHz. A single
64
contact spin-locked cross polarization (CP) sequence was used
65
with alternate cycle spin-temperature inversion and flip-back
1
66
1
◦
of H magnetization. The H rf field strength was 55 kHz (a 90
pulse length of 4.5 ls). Free induction decays were defined by 2 K
73
dipolar and quadrupolar tensor frames.
1
3
data points over a spectral width of 20 kHz for C spectra and
2
07
Thermal measurements
3
0 kHz for Pb and were zero filled to 16 K data points before
13
Fourier transformation. For C NMR experiments, typically
00 transients, with a contact time of 2 ms and a relaxation delay
Simultaneous thermogravimetric analysis (TGA) and differen-
tial scanning calorimetry (DSC) were performed on a Stanton
Redcroft STA 785 Thermal Analyzer. The dry powdered sample
20 mg) was loaded into a platinum crucible and heated under
a flow of dry nitrogen at a rate of 2 C min ; data points were
recorded at 5 s intervals. Calibration of the DSC was against
the solid-state transition of KNO (taking due note of the
particular properties of this change
of naphthalene).
5
1
3
of 5 s, were recorded for each spectrum. C NMR chemical shifts
are reported on the scale with respect to TMS = 0 ppm and were
referenced externally to the upfield resonance of adamantane
at 29.5 ppm. For Pb NMR experiments, typically 120
transients, with a contact time of 5 ms and a relaxation delay
of 10 s, were recorded for each spectrum. Pb NMR chemical
shifts were referenced externally to a 1 M aqueous solution of
lead nitrate at 298 K as 0 ppm. Temperature measurement and
regulation of the bearing gas were controlled with a Bruker B-
TV1000 unit equipped with a copper–constantan thermocouple
and digital reference. Temperature calibration was achieved both
(
◦
−1
67
207
3
2
07
74,75
and against the fusion
42
Powder X-ray diffraction
The sample was sealed in a 0.3 mm soda-glass capillary tube.
Powder diffraction patterns were collected using a Siemens
D5000 diffractometer operating in transmission geometry using
1
3
with the samarium ethanoate tetrahydrate Curie Law C NMR
68
chemical shift thermometer, previously set against the phase
transitions of D-camphor, cobaltocenium hexafluorophosphate
and 1,4-diazobicyclo[2.2.2]octane, and with the samarium
O ) Curie Law Sn NMR chemical shift
thermometer. The sample was allowed to equilibrate at each
˚
monochromatic Cu-Ka
1
radiation (k = 1.54056 A). Data was
◦ ◦
collected for 5 ≤ 2h ≤ 50 in steps of 0.02 . The X-ray powder
1
19
pattern was simulated from the single-crystal X-ray parameters
stannate (Sm
2
Sn
2
7
37
69
using the CrystalDiffract computer program.
new temperature for ca. 20 min before spectral acquisition. Non-
Single-crystal X-ray diffraction
70
quaternary suppression (NQS) dipolar dephasing experiments,
using delays of 20–200 ls, were carried out with the pulse
For the low-temperature determination a crystal of the LTA–
py complex was selected and mounted on a nylon fibre using
a drop of perfluoropolyether oil. It was then rapidly cooled to
150 K in a stream of cold nitrogen using an Oxford Cryosystems
CRYOSTREAM cooling system. The crystal for the ambient
temperature determination was mounted using silicone grease
in a Lindemann capillary. The data were collected on an Enraf-
Nonius DIP2020 image-plate diffractometer using graphite-
71
sequence of Alemany et al. Chemical Shift Anisotropy (CSA)
calculations were performed on integrated intensities of the
sidebands and analyzed with the Herzfeld-Berger method using
48
the HBA routine of the WSolids computer program. The CSA
72
tensors are reported following the suggestions of Harris with
d
j
11 > d22 > d33 a )l o/ nX g. with the span, X = d11 − d33, and the skew,
d
= 3(diso − d22
˚
To obtain the exchange rates for the pyridine ring flips,
monochromated Mo-Ka radiation (k = 0.71070 A). The images
1
3
the C CP/MAS NMR spectral line shapes were modeled
were processed using the DENZO and SCALEPACK suite of
50
76
using the gNMR program. The chemical shifts and Gaussian
and Lorentzian line widths of the pyridine carbon peaks were
simulated from the room temperature spectrum, which is in the
low-temperature limiting region of the exchange broadening
timescale. The rates of exchange at higher temperatures were
then fitted iteratively to the experimental data assuming a two-
fold exchange model between C17 and C19, and between C14
and C16. Rate data from the exchange broadening simulations
were used to determine Arrhenius parameters.
programs. Data were corrected for Lorentz and polarization ef-
fects and a partial absorption correction applied by multi-frame
scaling of the image-plate data using equivalent reflections.
Selected crystal data, data collection and refinement parameters
are given in Table 4. The structure was solved by direct
methods using the SIR92 program, and refined using full-
matrix least squares procedures, based on F, with anisotropic
thermal parameters for all non-hydrogen atoms. The hydrogen
atoms were placed in calculated positions during the final
77
Table 4 Crystal data, data collection and refinement parameters
At 150 K
At 293 K
Crystal formula
Crystal dimensions/mm
Symmetry
C
13
H
17PbNO
8
13 8
C H17PbNO
Pale yellow, 0.3 × 0.3 × 0.3
Pale yellow, 0.35 × 0.25 × 0.25
Monoclinic, P2
8.971(1)
13.870(1)
12.950(1)
1611.14, 4
90.901(3)
5921
1
/n
Monoclinic, P2
9.103(1)
14.036(1)
13.122(1)
1676.49, 4
90.625(2)
12572
1
/n
a/A˚
b/A˚
c/A˚
V/ A˚ , Z
3
◦
b/
Measured reflections
Independent reflections
Reflections with I > 3r(I)
Parameters refined
2783
2415
209
3452
2759
209
Final Dq/e A˚
Final R and R
−
3
−1.03, 1.22
−1.86, 1.52
w
0.0242, 0.0273
0.0295, 0.0372
3
2 0 2
D a l t o n T r a n s . , 2 0 0 5 , 3 1 9 5 – 3 2 0 3

View Online
cycles of refinement. A three-parameter Chebychev weighting
R. D. Gillard, G. Wilkinson and J. A. McCleverty, Pergamon, Oxford,
1987, vol. 1, pp. 31–107.
78
scheme was applied. All crystallographic calculations were
79
37 D. C. Palmer, CrystalDiffract Version 1.02b2, CrystalMaker Soft-
performed using the CRYSTALS program. Neutral atom
scattering factors were taken from International Tables for X-
ray Crystallography (1974, Vol. IV, Table 2.2B).
ware, 1995–2004 (www.crystalmaker.co.uk).
3
8 The LTA–py complex is prepared from commercially available lead
tetraacetate which is supplied wet with acetic acid for reasons of
stabilization; this is incompletely removed during the purification
process, and is not detected by NMR spectroscopy due to fast
CCDC reference numbers 260943 and 260944.
See http://dx.doi.org/10.1039/b506366c for crystallographic
data in CIF or other electronic format.
40
exchange in solution but becomes evident as a rapid initial mass
loss in TGA.
3
4
4
4
4
9 T. D. W. Claridge, E. J. Nettleton and M. G. Moloney, Magn. Reson.
Acknowledgements
Chem., 1997, 35, 159.
0 J. E. H. Buston, T. D. W. Claridge and M. G. Moloney, J. Chem.
Soc., Perkin Trans. 2, 1995, 639.
1 B. R. Peterson and F. N. Diederich, Associate Program Version 1.6,
Zurich, 1994 (www.diederich.chem.ethz.ch).
2 R. K. Harris and B. E. Mann, NMR and the Periodic Table, Academic
Press, London, 1978.
We thank EPSRC (Grant GR/K93167) and the Associated
Octel Company for funding, and Dr N. H. Rees for help with
the simulations of the solid-state NMR line shapes.
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