Synthesis and NMR solution properties of Rhenium(I) and Platinum(IV) complexes of oligopyridines

Article abstract of DOI:10.1016/S0277-5387(98)00431-8

Syntheses and characterisation data are reported for the following Re^I and Pt^IV bidentate chelate complexes of oligopyridines, [{ReBr(CO)₃}₂L] (L=2,2′:6′,2″:6″,2?-quaterpyridine (QP), or 4′,4″-bis(4-tert-butylph

Full text of DOI:10.1016/S0277-5387(98)00431-8

Polyhedron 18 (1999) 1285–1291
Synthesis and NMR solution properties of Rhenium(I) and Platinum(IV)
complexes of oligopyridines
ˇ
*
Andrew Gelling, Keith G. Orrell , Anthony G. Osborne, Vladimir Sik
Department of Chemistry, The University, Exeter EX4 4QD, UK
Received 21 September 1998; accepted 30 November 1998
Abstract
Syntheses and characterisation data are reported for the following ReI and PtIV bidentate chelate complexes of oligopyridines,
[hReBr(CO)3j2L] (L52,29:69,20:60,2--quaterpyridine (QP), or 49,40-bis(4-tert-butylphenyl)-2,29:69,20:60,2--quaterpyridine (BPQP)),
[(PtIMe3)2 (QP)], [ReBr(CO)3(QP)] and [ReBr(CO)3(BPQNP)] (BPQNP549,4--bis(4-tert-butylphenyl)-2,29:69,20:60,2-:6-,2+-quin-
quepyridine). The mononuclear complexes do not exhibit metallotropic shifts in solution as occur in the three-ring 2,29:69,20-terpyridine
complex analogues. This is attributed to the steric effects of the uncoordinated portions of these ligands, which also lead to severe
restrictions to rotation of these pendant arms. Such movements were investigated by low temperature NMR spectra.
Science Ltd. All rights reserved.
1999 Elsevier
Keywords: Rhenium; Platinum; Quaterpyridine; Quinquepyridine; Dynamic NMR
1. Introduction
To investigate the generality of the 1,4-metallotropic
shift and restricted ring rotation fluxions that were ob-
served [1] in some kinetically inert metal complexes of
2,29:69,20-terpyridine (terpy), we extended our studies of
these modes of fluxionality to metal complexes of analo-
gous planar N3 ligands such as 2,6-bis(pyrazol-1-
yl)pyridine (bppy) [2] and to the more complex ligands
2,4,6-tris(2-pyridyl)-1,3,5-triazine (TPT) and 2,4,6-tris(2-
pyridyl)-pyrimidine (TPP). Metal complexes of TPT and
TPP were found to exhibit metal-hurdling fluxions in
addition to the 1,4-metallotropic shifts and restricted ring
rotation [3].
We now report the results of a study of some rhenium(I)
and platinum(IV) complexes of the oligopyridines
2,29:69,20:60,2--quaterpyridine
(QP),
49,40-bis(4-tert-
butylphenyl)-2,29:69,20:60,2--quaterpyridine (BPQP), and
49,4--bis(4-tert-butylphenyl)-2,29:69, 20:60, 2-:6-, 2+-quin-
quepyridine (BPQNP).
*
Corresponding author.
0277-5387/99/$ – see front matter
PII: S0277-5387(98)00431-8
1999 Elsevier Science Ltd. All rights reserved.

1286
A. Gelling et al. / Polyhedron 18 (1999) 1285–1291
2. Experimental
2.4. 2,29:69,20:60,2--Quaterpyridine-bis-
hbromotricarbonylrhenium (I)j
2.1. Materials
The compound [ReBr(CO)5] (0.3 g, 0.7 mmol) and QP
(0.1 g, 0.3 mmol) were dissolved with stirring in a mixture
of benzene (80 cm3) and tetrahydrofuran (20 cm3) and the
solution was heated under reflux for 18 h. The solution was
treated as for the 1:1 complex to give the desired product
as an orange crystalline solid. Yield 0.16 g (21%).
Previously described methods were used to prepare the
complexes [ReBr(CO)5] [4] and [(PtIMe3)4] [5] and the
ligands QP [6], BPQP and BPQNP [7].
2.2. Synthesis of complexes
2.5. Physical methods
All preparations were performed by standard Schlenk
techniques [8] under purified nitrogen using freshly dis-
1H NMR spectra were recorded on Bruker AC300 or
DRX400 spectrometers operating at 300.13 and 400.13
MHz respectively. Spectra were recorded as CDCl3,
CD2Cl2 or CDCl2.CDCl2 solutions of the complexes with
SiMe4 as an internal standard. A B-VT 2000 variable
temperature unit was used to control the probe temperature
(DRX-400). Temperatures, based on calibration against a
Comark digital thermometer, are considered accurate to
618C.
tilled
and
degassed
solvents.
The
complexes
[ReBr(CO)3L] (L5QP or BPQNP) and [M2L] [M5
ReBr(CO)3, L5QP or BPQP; M5PtIMe3, L5QP] were
prepared in an analogous manner, synthetic data being
given in Table 1. The preparations of the 1:1 complex
[ReBr(CO)3QP] and the 2:1 complex [hReBr(CO)3j2(QP)]
are given below as representative examples.
Infrared spectra of the complexes were recorded as
solutions on a Perkin-Elmer 881 spectrometer calibrated
from the signal of polystyrene at 1602 cm21. Elemental
analyses (C, H, N) were carried out by Butterworth
Laboratories, Teddington, Middlesex, UK. Fast atom bom-
bardment (FAB) mass spectra were recorded on a V.G.
Autospec instrument using Cs1 bombardment at 250 kV
energy on the complexes dissolved in a matrix of 3-
nitrobenzyl alcohol.
2.3. 2,29:69,20:60,2--
Quaterpyridinebromotricarbonylrhenium (I).
The compound [ReBr(CO)5] (0.08 g, 0.2 mmol) and QP
(0.08 g, 0.26 mmol) were dissolved in benzene (80 cm3)
with stirring and the mixture was heated under reflux for
1
]
1 2 h. The solution was concentrated under reduced
pressure to |20 cm3 and light petroleum (b.p. 40–608C,
80 cm3) added to precipitate a yellow solid. The almost
colourless solvent was decanted, the solid was washed
with light petroleum (50 cm3) and then dried in vacuo. A
recrystallization from tetrahydrofuran-hexane gave the
product as orange crystals. Yield 0.06 g (48%).
3. Results and discussion
The oligopyridines were prepared by following earlier
Table 1
Synthetic and analytical data for the complexes [ReBr(CO)3L] (L5QP or BPQNP), [hReBr(CO)3j2L] (L5QP or BPQP) and [hPtIMe3j2(QP)]
Complex
Yielda
(%)
n(CO)b
Analysisc (%)
m/z
/cm21
C
H
N
[M]1
660
[M2X]1
[ReBr(CO)3(QP)]
[hReBr(CO)3j2(QP)]
[ReBr(CO)3(BPQNP)]
[hReBr(CO)3j2(BPQP)]
[hPtIMe3j2(QP)]
a
48
21
39
32
20
2025 s 1925 m
1895 m
41.2
(41.8)
29.5
(30.9)
57.5
(57.5)
43.4
(43.3)
30.6
2.2
(2.1)
1.6
(1.4)
4.2
(4.1)
2.8
(2.9)
3.5
(3.1)
7.8
(8.5)
4.3
(5.5)
6.8
(6.9)
4.4
(4.4)
5.5
(5.4)
581d
2029 s,br 1924 m
1010
1001
1274
–
931
920d
1195
917
1903 me
2023 s 1922 m
1892 m
2028 s 2021 sh
1920 m 1903 m
–
(29.9)
Yield quoted relative to metal containing reactant.
Recorded in benzene; s5strong, m5medium, sh5shoulder.
Calculated values in parentheses.
b
c
d
e
Additional peaks due to [M–3CO]1 and [M–3CO–Br]1 also observed.
Recorded in CH2Cl2; br5broad.

A. Gelling et al. / Polyhedron 18 (1999) 1285–1291
1287
procedures [6,7], (see Experimental section). An improved
synthesis of 2,29:69,20:60,2--quaterpyridine (QP) has since
been published [9]. The five complexes were isolated as
air-stable crystalline solids. Difficulty was experienced in
obtaining the rhenium complexes of QP in a completely
pure state. This is reflected in the analytical data (Table 1)
and in the 1H NMR spectra (see later). In solution the
rhenium complexes displayed carbonyl stretching bands in
the infrared (Table 1) indicative of a fac-stereochemistry
for the ReX(CO)3 moiety. The FAB mass spectra of the
complexes showed peaks that are consistent with the
molecular ions M1 or molecular ions minus halogen [M2
X]1.
co-ordination and all 14 ring hydrogens become NMR
distinguishable. Assuming that QP is acting as a bidentate
chelate ligand, such a result can only be interpreted as
involving co-ordination of the pyridyl rings 1 and 2 (or the
equivalent rings 3 and 4). The assignments of the signals
due to environments A–N (see Table 2) were made with
the aid of a COSY spectrum and by comparison with
1
corresponding shifts in [ReBr(CO)3(terpy)], [10]. The H
spectrum at room temperature is shown in Fig. 1. The
additional weak set of signals is the result of slight
contamination with the 2:1 complex [hReBr(CO)3j2(QP)]
(see later). In previous studies of bidentate chelate ReI
complexes of terpy and other ligands containing a pendant
group with a third nitrogen donor [10–14], 1,4-metallot-
ropic shifts occur in solution, which involve a switching of
the metal co-ordination sites between adjacent pairs of N
donors, by an associative process [1]. An analogous
fluxional process was expected to occur in the present
mononuclear QP complex with switching of metal co-
ordination between the N donors of rings 1 and 2 to those
3.1. [ReBr(CO)3(QP)]
Co-ordination of QP to rhenium (I) led to an increased
complexity in its NMR spectrum. Whilst QP itself ex-
1
hibited seven H chemical shifts arising from equivalent
pairs of ring hydrogens, this symmetry is lost on metal
Table 2
Low temperature (223 K, CD2Cl2 solution) 1H NMR data for the two rotamers of [ReBr(CO)3(QP)]a
H label
d (I)
H label
d (II)
uDdub
A
B
C
D
E
F
G
H
I
9.07
7.59
8.18
8.37
8.64
8.09
7.73
7.76
8.29
8.41
8.49
7.79
7.37
8.71
A
B
C
D
E9
F9
G9
H9
I9
9.07
7.59
8.18
8.37
8.60
8.08
7.76
7.95
8.25
8.41
8.53
7.89
7.41
8.72
0.04
0.03
0.19
0.04
c
J
J9
K
L
M
N
a
K9
L9
M9
N9
0.04
0.10
0.04
The planar structures shown are for labelling purposes only and do not represent the preferred solution conformations. See Fig. 2 for a more accurate
representation of the rotameric forms.
b
Significant rotameric shift differences.
c
Very small (|5 Hz) rotamer splitting observed.

1288
A. Gelling et al. / Polyhedron 18 (1999) 1285–1291
1
of the complex (in CD2Cl2 solvent) was carried out. H
spectra were recorded in the range 303–193 K and
numerous changes associated with the pyridyl rings 2, 3
and 4 were detected (Fig. 1). Whilst the magnitudes of the
changes varied greatly, all these signals exhibited broaden-
ing on cooling, followed by eventual splitting into slightly
unequal intensity pairs at ca 223 K. The signals most
affected were those of HH and HL where the low tempera-
ture splittings (Dd ) were 0.19 and 0.10 respectively, (Table
2). Other signals significantly affected were HE, HG, HI,
HK and HM with splittings of 0.03–0.04. These spectral
changes (Fig. 1) can be interpreted in terms of a ‘freezing
out’ of two distinct rotameric forms of the complex
(Structures I and II, Table 2), arising from restricted
rotation about the C–C bond linking pyridyl rings 2 and 3.
Rotation about the bond linking rings 3 and 4 is presumed
to be fast at all temperatures, as it is in the free ligand,
with rings 3 and 4 preferentially adopting a co-planar
transoid arrangement. It is proposed that in the two
rotamers of [ReBr(CO)3(QP)], (Fig. 2), the pseudo plane
of the pendant 2,29-bipyridyl (bipy) lies at an angle,
relative to the plane containing the co-ordinated rings
similar to that adopted by the pendant pyridyl group in the
Fig. 1. 400 MHz 1H NMR spectra of [ReBr(CO)3(QP)] in CD2Cl2
solvent at various temperatures. Signals are labelled according to the
structures at the head of Table 3. Only those signals which show
significant changes with temperature are labelled. The minor set of signals
which does not change with temperature is due to
a trace of
[hReBr(CO)3j2(QP)].
of rings 2 and 3 and thence possibly to rings 3 and 4. Other
possible ring switches were excluded from consideration
on spatial grounds. Accordingly, 1H spectra of
[ReBr(CO)3(QP)] in the high boiling solvent
CDCl2.CDCl2 were recorded in the temperature range
303–403 K. However, no spectral changes associated with
any type of metallotropic shifts were detected. Metal
chelate co-ordination can occur only when adjacent pyridyl
rings adopt a cis orientation with respect to their donor N
atoms. Such an orientation is likely to be greatly dis-
favoured in the case of the QP complex since ring 4 of the
pendant 2,29-bipyridyl group will then exhibit strong steric
interaction with the ReBr(CO)3 moiety. This also implies
that the barrier to rotation about the C–C bonds linking
pyridyl rings 2 and 3 is likely to be much higher than in
terpy complexes, and the preferred conformers will be
those with the pendant pyridyl rings 3 and 4 twisted
considerably out of the plane containing rings 1 and 2. In
order to test this hypothesis a low temperature NMR study
Fig. 2. Structures of the likely static rotamers of [ReBr(CO)3(QP)]
showing the ring and hydrogen labelling.

A. Gelling et al. / Polyhedron 18 (1999) 1285–1291
1289
corresponding terpy complex, viz. 52.98 [10]. In rotamer I
the pendant bipy is shown oriented anti with respect to the
Br group, whereas in rotamer II it is syn to Br. The
hydrogen most affected by this change in relative orienta-
tion is HH since in rotamer I it is close to the axial Br
ligand whereas in rotamer II it lies close to the axial CO
ligand. In the crystal structure of [ReBr(CO)3(terpy)] [10]
the pendant ring N is trans to Br. In the solution of the
present complex the two rotamers are in an abundance
ratio of 60:40 and so, by analogy with the terpy complex,
the more abundant form is attributed to that with the N of
pendant ring 3 trans to Br, (i.e. Structure I).
An estimate was made of the energy barrier to rotation
in [ReBr(CO)3(QP)] by measuring the coalescence tem-
peratures of the pairs of signals most affected by the
process. These were as follows, 288 K (H/H9), 268 K
(E/E9 and I/I9) and 263 K (K/K9 and M/M9). Using the
usual formulae for exchange rates and activation energies
an average energy barrier, DG‡, of 5761 kJ mol21 was
evaluated. This is significantly higher than for
[ReX(CO)3(terpy)] (X5Cl or I) where values in the range
36–43 kJ mol21 have been reported [10]. It lends support
to the postulate that the energy of the structure, where
rings 2 and 3 adopt a cis co-planar arrangement (vital for
the occurrence of 1,4-metallotropic shifts), is much greater
than in those structures where the pendant bipy arm has
rotated well out of the plane containing rings 1 and 2. Such
a situation will militate against any metallotropic shifts
occurring.
3.2. [hReBr(CO)3 j2 L] (L5QP, BPQP) and [(PtIMe3 )2
(QP)]
These three dinuclear chelate complexes were more
easily synthesised than the mononuclear complex above on
account of the strong electron donor properties of the two
pairs of pyridyl N atoms. The formation of dinuclear
chelate metal complexes of QP or BPQP requires rings 1
and 2 and rings 3 and 4 to adopt coplanar cis relationships.
Such a structure differs from the preferred fully transoid
free ligand structure by 1808 rotations about the C–C
bonds between rings 1 and 2 and between rings 3 and 4.
No rotation about the C–C bond linking rings 2 and 3 is
required. Indeed, the trans relationship of these two rings
will favour the co-ordination of two metal moieties from
opposite sides of the ligand.
1
These three complexes gave rather similar H spectra,
differing mainly in the coordination shifts of ReI and PtIV.
Assignments of all signals were firmly established (Table
3). No metallotropic shifts are possible in these complexes.
However, low temperature studies were performed to
investigate any evidence for a restricted bond rotation
Table 3
1H NMR dataa of the dinuclear complexes [hReBr(CO)3j2(QP)], [(PtIMe3)2(QP)] and [hReBr(CO)3j2(BPQP)]b
Label
M5ReBr(CO)3
M5PtIMe3
M5ReBr(CO)3
A
B
C
D
E
F
G
9.19
7.64
8.16
8.34
8.61
8.29
8.41
–
8.91 (13.6)c
7.75
8.27
8.69
9.13
7.65
8.18
8.47
8.87
8.51
8.09
8.66
8.08d, 7.62e, 1.55f
8.62
PtMe(eq)
PtMe(eq)
PtMe(ax)
a
1.32 (72.4)c
0.44 (72.9)c
20.04 (70.2)c
–
–
–
–
–
In CD2Cl2 at 303 K. d values relative to Me4Si.
b
The planar structure shown is for labelling only. The preferred solution conformation is likely to have the two metal-chelated portions considerably
angled to each other.
c
2J(PtH)/Hz values in parentheses.
ortho-ring H.
meta-ring H.
t-Bu H.
d
e
f

1290
A. Gelling et al. / Polyhedron 18 (1999) 1285–1291
Table 4
process. Rotation about the C–C bond linking rings 2 and
3 is theoretically possible but likely to be severely hin-
dered by the interaction between the two metal moieties.
The spectra did not change in any significant way so the
conclusion reached is that no measurable rate of rotation
occurs and the two metal-chelated portions of the complex
are locked with their planes being considerably angled to
each other to minimise steric interactions.
1H NMR dataa for the complex [ReBr(CO)3(BPQNP)]b
3.3. [ReBr(CO)3(BPQNP)]
This is a mononuclear metal complex of a substituted
quinquepyridine ligand [7]. The free ligand appears to
exist in solution preferentially as a near-planar structure
with a transoid arrangement of adjacent pyridyl rings,
since NOE effects between hydrogens of adjacent rings are
absent. A series of NOE difference experiments performed
on the ReI complex [ReBr(CO)3(BPQNP)] also showed no
NOEs between the hydrogen pairs HG /HH, HJ /HK and
HM /HN implying trans orientations of rings 3, 4 and 5 of
the pendant terpyridine chain in this complex. A strong
NOE between HD and HE was observed, however, con-
firming the cis relationship of the coordinated pyridyl rings
1 and 2. This observation also excluded the possible
Label
d a
R
1
alternative chelate coordination of rings 2 and 3. Full H
signal assignments of [ReBr(CO)3(BPQNP)], based on
COSY experiments, are given in Table 4.
A
B
C
D
E
F
G
H
I
9.11
7.54
8.13
8.48
8.54
–
7.95
7.87
8.17
8.97f
8.92
–
8.81
7.64
7.95
7.40
8.76
A (CDCl2)2 solution of the complex was examined in
the above-ambient temperature range 303–403 K, but no
changes associated with any metallotropic shift were
observed. This absence of metallotropic shifts is analogous
to the case of the mononuclear quaterpyridine complex and
is attributed to the low tendency for rings 2 and 3 to adopt
a cis-planar configuration due to the steric effect of the
2,29-bipyridyl group attached to the 60 position of ring 3.
Such a structure might also be associated with a high
barrier to rotation about the C–C bond linking rings 2 and
3, as was found in the case of [ReBr(CO)3(QP)].
A low temperature NMR study of the BPQNP complex
in CD2Cl2 solvent was therefore carried out, and signifi-
cant broadening of many of the signals, notably those due
to HG, HH, HJ and HK, were noted on cooling the solution.
These hydrogens are the ones likely to be most affected by
the rotation about the bond linking rings 2 and 3. Other
signals, notably HM and HN, showed no such broadening
suggesting that rings 4 and 5 remain in their preferred
time-averaged coplanar trans orientation. Similarly, ring 4
will prefer to be coplanar and trans to ring 3 as in the free
ligand structure. However, the environments of the ring 4
hydrogens are expected to be sensitive to the restricted
rotation about the C–C bond linking rings 2 and 3 as this
rotation will modify greatly their proximity to the
ReBr(CO)3 moiety. It is proposed that there are two
rotameric structures distinguishable at low temperatures
(see Table 4). Some evidence of these can be gleaned from
7.82c, 7.49d, 1.39e
J
K
L
M
N
P
7.82c, 7.63d, 1.34e
Q
R
a
In CD2Cl2 at 303 K, where there is rapid rotation about the C–C bond
linking rings 2 and 3. d values relative to Me4Si.
b
The planar structures shown are for labelling purposes only and do not
represent the preferred solution conformations in which the rings 3–5 are
likely to be rotated out of the plane containing rings 1 and 2.
c
ortho-ring Hs.
meta-ring Hs.
t-Bu Hs.
d
e
f
At 263 K this signal splits into two (d58.90, 8.94).
1
the H spectrum at 263 K, where the HJ signal has clearly
split into an unequal intensity pair of signals at d58.90
(rel. int. 65%) and 8.94 (|35%). Other signal splittings are
also evident but these are less easily assigned. Further
cooling below 263 K resulted in a general broadening of
all signals, probably associated with the slow rates of

A. Gelling et al. / Polyhedron 18 (1999) 1285–1291
1291
molecular tumbling of this bulky complex species in
solution at lower temperatures.
us (A.G.) and for use of the mass spectrometry service at
the University of Wales, Swansea.
From the decoalescence of the HJ signal on cooling, a
rough estimate was made of the energy barrier to rotation.
A value for DG‡ of ca. 61 kJ mol21 was calculated, which
compares closely with the value of 57 kJ mol21 obtained
for [ReBr(CO)3(QP)].
References
ˇ
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V. Sik, Polyhedron 15 (1996) 371.
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gives rise to a higher degree of restriction to rotation about
the bond linking rings 2 and 3. The preferred rotamers for
this process are those in which ring 3 is twisted con-
siderably out of the plane containing the coordinated rings
1 and 2, such that its N heteroatom is not suitably placed
for metal chelate coordination, and this precludes any
metallotropic shifts.
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Acknowledgements
We thank the EPSRC for postdoctoral support to one of