Synthesis and dynamic NMR studies of fluxionality in rhenium(I), platinum(II) and platinum(IV) complexes of 'back-to-back' 2,2′:6′,2″-terpyridine ligands

Article abstract of DOI:10.1039/a805300f

Syntheses are given for the following transition metal complexes, [{ReBr(CO)₃}₂L] (L = L¹, L² or L³), [(PtCIMe₃)₂L¹], [(PtIMe₃)₂L] (L = L²

Full text of DOI:10.1039/a805300f

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Synthesis and dynamic NMR studies of fluxionality in rhenium(I),
platinum(II) and platinum(IV) complexes of ‘back-to-back’
2,2Ј:6Ј,2Љ-terpyridine ligands
ˇ
Andrew Gelling, Matthew D. Olsen, Keith G. Orrell,* Anthony G. Osborne and Vladimir Sik
Department of Chemistry, The University, Exeter, UK EX4 4QD.
E-mail: K.G.Orrell@exeter.ac.uk
Received 8th July 1998, Accepted 20th August 1998
Syntheses are given for the following transition metal complexes, [{ReBr(CO)3}2L] (L = L1, L2 or L3), [(PtClMe3)2L1],
[(PtIMe3)2L] (L = L2 or L3), [ReBr(CO)3L1], [Pt(C6F5)2L3], [Pt(C6F4CF3)2L] (L = L1 or L2), [{Pt(C6F4CF3)2}2L] (L = L1
or L2) and [ReBr(CO)3PtIMe3L1] where the ligands, L, are the ‘back-to-back’ terpyridine ligands, 6Ј,6Љ-bis(2-pyridyl)-
2,2Ј:4Ј,4Љ:2Љ,2ٞ-quaterpyridine (L1), 1,4-bis(2,2Ј:6Ј,2Љ-terpyridin-4Ј-yl)benzene (L2) and 6Ј,6Љ-bis{2-(4-methyl-
pyridyl)}-2,2Ј:4Ј,4Љ:2Љ,2ٞ-quaterpyridine (L3). All the complexes undergo 1,4-metallotropic shifts in solution at
above-ambient temperatures and restricted rotations of the pendant pyridyl rings at below-ambient temperatures.
Activation energies for these processes have been computed from variable temperature one-dimensional bandshape
analysis and 2D-exchange spectroscopy (2D-EXSY) NMR experiments. The metallotropic shift energies are very
metal-dependent being in the order PtIV < ReI < PtII, with ∆G‡ (298.15 K) values ranging from 62 to 101 kJ molϪ1
The fluxions are sensitive only to the local metal-coordination environment, there being negligible electronic
interaction between the metal centres in the dinuclear complexes. In the mixed-metal dinuclear complex
[ReBr(CO)3PtIMe3L1] it proved possible to measure the different rates of fluxion of the ReI and PtIV moieties.
.
2,2Ј:6Ј,2Љ-Terpyridine (terpy) has been studied for many years
as a strong chelating agent to metals.1 Its normal bonding
mode to metals is a NNN terdentate ligand but towards certain
types of kinetically inert metal moieties e.g. fac-ReX(CO)3 it
will act as a N,N bidentate chelate. In such bidentate complexes
the ligand is fluxional and switches its metal coordination sites
between adjacent pairs of its three nitrogen atoms by an
associative mechanism.2
We are interested in studying the fluxional motions in metal
complexes derived from nitrogen ligands that have the potential
capability of binding more than one metal moiety in a bidentate
chelate mode. We have reported3 our studies of PdII and PtII
complexes with the ligands 2,4,6-tris(2-pyridyl)-1,3,5-triazine
(TPT) and 2,4,6-tris(2-pyridyl)pyrimidine (TPP) in which the
fluxional processes of 1,4-metallotropic shift, metal-hurdling
and restricted ring rotation were observed. We have now turned
our attention to the study of metal complexes of N6 ligands
that are derived from linked subunits of terpy, each of which is
capable of N,N bidentate or N,N,N terdentate chelation to a
metal moiety.
We have used the ligands, 6,6Љ-bis(2-pyridyl)-2,2Ј:4Ј,4Љ:
2Љ,2ٞ-quaterpyridine (L1), 1,4-bis(2,2Ј:6Ј,2Љ-terpyridin-4-yl)-
benzene (L2), 6Ј,6Љ-bis{2-(4-methylpyridyl)}-2,2Ј:4Ј,4Љ:2Љ,2ٞ-
quaterpyridine (L3) to form the metal complexes [{ReBr-
(CO)3}2L] (L = L1, L2 or L3), [(PtClMe3)2L1], [(PtIMe3)2L]
(L = L2 or L3), [ReBr(CO)3L1], [Pt(C6F5)2L3], [Pt(C6F4CF3)2L]
(L = L1 or L2), [{Pt(C6F4CF3)2}2L] (L = L1 or L2) and [ReBr-
(CO)3PtIMe3L1], in all of which the ligands are acting in a
bidentate chelate mode to the metal atom(s). We now report our
dynamic NMR studies on these complexes.
and 1,4-bis(2,2Ј:6Ј,2Љ-terpyridin-4Ј-yl)benzene10 (L2) were pre-
pared according to literature methods.
6Ј,6Љ-Bis{2-(4-methylpyridyl)}-2,2Ј:4Ј,4Љ:2Љ,2ٞ-quaterpyridine
(L3)
The complex [NiCl2(PPh3)2] (12.8 g, 19.5 mmol) and PPh3 (10.3
g, 39.4 mmol) were added to dried and degassed dimethylform-
amide and stirred for 10 min to give a blue solution. Zinc dust
(1.28 g, 19.2 mmol) was added and the resulting suspension
stirred for 1 h, after which time the colour had changed from
blue to red. 4-Methyl-4Ј-methylthio-2,2Ј:6Ј,2Љ-terpyridine (2.5
g, 8.5 mmol) was added and the resulting suspension stirred for
16 h causing the colour to change to a very dark green. The
solvent was then removed in vacuo, and the resulting black tar
was extracted with CHCl3 (2 × 250 cm3) to leave a brown solid
which was dried and then added to aqueous ammonia (0.88
specific gravity, 400 cm3) and stirred for 24 h. After this time the
grey solid residue was collected by filtration, thoroughly dried
and then extracted with CHCl3 (450 cm3). The green solution
was dried (MgSO4) and then concentrated in volume to ca. 40
cm3, methanol (200 cm3) was added and the resulting solution
cooled (Ϫ20 ЊC) for 16 h. The cream solid that precipitated
was collected and washed with methanol to yield the desired
product as a cream solid. Yield 0.6 g (14%).
Synthesis of complexes
All preparations were carried out using standard Schlenk tech-
niques11 under purified nitrogen using freshly distilled and
degassed solvents. Synthetic and analytical data are given in
Table 1. In some cases difficulties were experienced in securing
good analytical data due to persistent contamination with
either free ligand, solvent or another metal complex.
Experimental
Materials
The compounds [ReBr(CO)5],4 [PtXMe3]4 (X = Cl or I), [Pt-
2:1 Complexes. The complexes [{ReBr(CO)3}2L] (L = L1, L2
or L3), [(PtClMe3)2L1], [(PtIMe3)2L] (L = L2 or L3) and [{Pt-
(C6F4CF3)2}2L] (L = L1 or L2) were all prepared in analogous
manner from the reaction of the appropriate ligand with at least
5
(C6F5)2(diox)2] (diox = 1,4-dioxane),6 [NiCl2(Ph3P)2],7 [Pt(C6-
F4CF3)2(Et2S)2],8 4-methyl-4Ј-methylthio-2,2Ј:6Ј,2Љ-terpyrid-
ine,9 6Ј,6Љ-bis(2-pyridyl)-2,2Ј:4Ј,4Љ:2Љ,2ٞ-quaterpyridine10 (L1)
J. Chem. Soc., Dalton Trans., 1998, 3479–3488
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Table 1 Synthetic and analytical data for mono- and di-nuclear ReI, PtII and PtIV complexes of ligands L1, L2 and L3
Analysisb (%)
Complex
Yield
νCO or νMC a/cmϪ1
C
H
N
m/z
[{ReBr(CO)3}2L1]
[ReBr(CO)3L1]
40
14
22
61
42
88
28
73
35
36
73
31
20
2023s, 1920m, 1912shc
35.8(37.1)
1.4(1.7)
6.4(7.2)
1164[M]ϩ
e
e
e
2025s, 1924m, 1901md
—
[(PtClMe3)2L1]
—
41.9(42.5)
4.1(3.7)
7.8(8.3)
979[M Ϫ Cl]ϩ
[ReBr(CO)3PtIMe3L1]
[Pt(C6F4CF3)2L1]
[{Pt(C6F4CF3)2}2L1]
[Pt(C6F4CF3)2L2]
[{Pt(C6F4CF3)2}2L2]
[{ReBr(CO)3}2L2]
[(PtIMe3)2L2]
2025s, 1924m, 1901md
1182[M ϩ H]ϩ
f
f
f
790m, 711w
791m, 711s
792m, 711s
48.7(48.3)
2.0(1.8)
7.6(7.7)
—
—
—
—
41.7(40.4)
1.6(1.2)
6.1(4.9)
g
g
g
790m, 713s
42.1(42.7)
39.8(40.6)
39.0(39.8)
38.1(38.3)
36.8(37.2)
51.0(51.7)
2.4(1.3)
2.2(1.9)
2.2(2.6)
2.0(2.0)
3.3(3.4)
2.2(2.4)
4.3(4.7)
6.2(6.8)
6.0(6.6)
6.6(7.0)
6.4(6.9)
7.9(8.2)
2021s, 1912m, 1890shh
1241[M ϩ H]ϩ
—
—
[{ReBr(CO)3}2L3]
[(PtIMe3)2L3]
2024s, 1921m, 1900shd
1193[M ϩ H]ϩ
1099[M Ϫ I]ϩ
—
—
—
[Pt(C6F5)2L3]
a s = strong, m = medium, sh = shoulder. b Calculated values in parentheses. c Recorded in benzene solution. d Recorded in CH2Cl2 solution.
e Analytically pure samples could not be produced due to persistent contamination with [{ReBr(CO)3}2L1]. f Analytically pure samples could not be
produced due to difficulty in obtaining a pure sample of [ReBr(CO)3L1]. g Analytically pure samples could not be produced due to persistent
contamination with L2. h Recorded in THF solution.
1
2 molar equivalents of the corresponding metal halide or Et2S
complex. The preparation of the complex [{ReBr(CO)3}2L1] is
described in more detail below by way of illustration.
successful, as the H NMR spectrum indicated the presence
of ca. 3% unreacted ligand which could not be removed by
further extractions.]
Bis{bromotricarbonylrhenium(I)}µ,η4-{6Ј,6Љ-bis(2-pyridyl)-
2,2Ј:4Ј,4Љ:2Љ,2ٞ-quaterpyridine}. The compound [ReBr(CO)5]
(0.2 g, 0.49 mmol) and L1 (0.1 g, 0.22 mmol) were dissolved in
benzene (80 cm3) and the resulting solution heated under reflux
for 16 h, after which time the solvent was concentrated to ca. 20
cm3 and light petroleum (bp 40–60 ЊC, 100 cm3) added to
precipitate an orange solid. This solid was isolated, washed
thoroughly with light petroleum (2 × 150 cm3) and recrystal-
lized from CH2Cl2–hexane to give the desired product as an
orange powder. Yield 0.1 g (40%).
Mixed metal complex. Bromotricarbonylrhenium(I)µ,η4-
{6Ј,6Љ-bis(2-pyridyl)-2,2Ј:4Ј,4Љ:2Љ,2ٞ-quaterpyridine}iodotri-
methylplatinum(IV). A solution of [PtIMe3]4 (0.042 g, 0.15
mmol) heated under reflux in tetrahydrofuran (65 cm3) for 16 h
was added to a solution of [ReBr(CO)3L1] (0.09 g, 0.11 mmol)
in tetrahydrofuran (20 cm3) and the resulting orange solution
stirred at 45 ЊC for 4 h. After this time the colour of the solution
had changed to cherry red. The volume was reduced to ca. 30
cm3 and hexane (100 cm3) added to produce an orange precipi-
tate which was isolated and dried under vacuum to yield the
desired product. Yield 0.08 g (61%).
1:1 Complexes. The complexes [Pt(C6F5)2L3] and [Pt(C6F4-
CF3)2L] (L = L1 or L2) were prepared by the reaction of the
ligand with approximately 1.5 molar equivalents of [Pt(C6-
F5)2(diox)2] or [Pt(C6F4CF3)2(Et2S)2]. The complex [ReBr-
(CO)3L1] was found to be extremely difficult to obtain free from
contamination with ligand L1 or the complex [{ReBr-
(CO)3}2L1]. The preparations for two complexes are described
as examples.
cis-Bis(pentafluorophenyl)[6Ј,6Љ-bis{2-(4-methylpyridyl)}-
2,2Ј:4Ј,4Љ:2Љ,2ٞ-quaterpyridine]platinum(II). The compounds
trans-[Pt(C6F5)2(diox)2] (0.17 g, 0.27 mmol) and L3 (0.085 g,
0.17 mmol) were dissolved in benzene (50 cm3) and the resulting
solution heated under reflux for 16 h. After this time the volume
was reduced to ca. 15 cm3 and light petroleum (bp 40–60 ЊC,
120 cm3) added to precipitate a yellow solid. This solid was
extracted with ice-cold tetrahydrofuran (3 × 10 cm3), hexane
(80 cm3) was added and the resulting solution cooled (at
Ϫ20 ЊC) for 16 h to produce the desired product as a red solid.
Yield 0.035 g (20%).
Bromotricarbonyl{6Ј,6Љ-bis(2-pyridyl)-2,2Ј:4Ј,4Љ:2Љ,2ٞ-
quaterpyridine}rhenium(I). A solution of [ReBr(CO)3(THF)2]
[prepared from [ReBr(CO)5] (0.1 g, 0.26 mmol) heated under
reflux in tetrahydrofuran (20 cm3) for 12 h] was diluted with
hexane (120 cm3) and added dropwise over 1.5 h to a stirred
solution of L1 (0.2 g, 0.43 mmol) in a mixture of chloroform
and hexane (120:85 cm3) at Ϫ70 ЊC. The temperature was
allowed to rise to Ϫ20 ЊC and hexane (500 cm3) added to pro-
duce a yellow precipitate which was isolated by filtration. The
solid was extracted with ice-cold tetrahydrofuran (2 × 15 cm3)
and hexane (100 cm3) added to yield an orange precipitate
which was isolated and dried under vacuum, to give the desired
product as an orange solid. Yield 0.03 g (14%). [The cold
extraction was designed to remove the 1:1 complex from the
less soluble unreacted ligand and as such was reasonably
Physical methods
Hydrogen-1 NMR spectra were recorded on either Bruker
AC-300 or DRX-400 spectrometers operating at 300.13 or
400.13 MHz respectively. 19F NMR spectra were also recorded
on the DRX-400 spectrometer operating at 376.46 MHz. All
chemical shifts are quoted relative to either SiMe4 (1H) or C6F6
(19F) (δ = 0) respectively. Variable temperature NMR spectra
were obtained using the Bruker variable temperature units
B-VT 1000 (AC300) and B-VT 2000 (DRX-400) to control the
probe temperature, calibration being periodically checked
against a Comark digital thermometer. Sample temperatures
are considered accurate to 1 ЊC. The 195Pt spectrum of com-
plex [(PtIMe3)2L3] was recorded as a (CDCl2)2 solution at 273 K
using a Bruker DRX-400 spectrometer operating at 85.63 MHz
with 195Pt shifts quoted relative to Ξ(195Pt) = 21.4 MHz. Two-
dimensional exchange (EXSY) spectra and homonuclear
correlated (COSY) spectra were obtained using the standard
Bruker programs NOESYPH.AU and COSY.AU respectively.
Mixing times in the EXSY experiments were chosen in the
range 0.5–1.5 s according to the nature of the complex and the
temperature of measurement. Rate data were derived from
1
bandshape analysis of the H spectra using a version of the
DNMR3 program,12 or from 2D-EXSY spectra using volume
integration data in the authors D2DNMR program.13 Acti-
vation parameters based on experimental rate data were
calculated using the THERMO program.14
Infrared spectra were recorded on a Perkin-Elmer 881
spectrometer calibrated from the signal of polystyrene at 1602
cmϪ1. Elemental analyses were performed by Butterworth
Laboratories, Teddington, Middlesex, London.
Electron impact mass spectra were measured on a Kratos
3480
J. Chem. Soc., Dalton Trans., 1998, 3479–3488

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Table 2 1H NMR data for the ligand L1 and its dinuclear complexes [M2L1] {M = ReBr(CO)3, PtClMe3 or Pt(C6F4CF3)2}
HA
HJ
HJ
HA
HI
HB
HC
HI
HB
HC
M
M
N
N
N
N
N
N
HH
HH
HD
HG
HG
HG
HD
HD
HE
HF
HF
HE
HE
HF
HF
HG
HE
HD
HC
HB
HH
HI
HH
HI
HC
HB
N
N
N
N
N
N
M
M
HA
HJ
HJ
HA
a
Compound
Solvent
CDCl3
T/K
δAJ
δBI
δCH
δDG
δEF
δMe
L1
303
303
8.76
7.38
7.90
8.70
8.97
[{ReBr(CO)3}2L1]
(CD3)2SO
9.09(A)
8.79(J)
8.94(A)
8.75(J)
7.78(B)
7.63(I)
7.83(B)
7.56(I)
8.36(C)
8.06(H)
8.30(C)
7.95(H)
9.06(D)
7.90(G)b
8.72(D)
8.51(G)
9.36(E)
8.63(F)b
9.02(E)
8.61(F)
[(PtClMe3)2L1]
CD3NO2
273
1.30(A)(74.7)
0.08(B)(73.4)
0.34(C)(77.4)
[{Pt(C6F4CF3)2}2L1]
(CDCl2)2
303
8.48(A)
8.52(J)
7.67(B)
7.28(I)
8.34(C)
7.84(H)
8.40(D)
8.11(G)
8.66(E)
8.46(F)
a Pt–Me shifts, 2JPtH/Hz values in parentheses, MeC axial, MeA,B equatorial. b Signals further split due to two rotameric forms of the pendant pyridyl
rings.
Profile spectrometer in this Department and FAB mass spectra
obtained on a VG AutoSpec instrument (Csϩ bombardment) at
the University of Wales, Swansea.
Results
The solution NMR properties of the three ‘back-to-back’ terpy
ligands L1, L2 and L3 and their metal coordination complexes
(both 2:1 and 1:1 metal:ligand ratios) will be described in
turn, with the high and low temperature studies discussed
separately.
Ambient and high temperature NMR spectra
Ligand L1 and its dinuclear complexes [M2L1] {M ؍ ReBr-
(CO)3, PtClMe3 or Pt(C6F4CF3)2}. The ambient temperature 1H
NMR data of these compounds are collected in Table 2. The
free ligand gives rise to five equal-intensity chemical shifts with
values similar to terpy itself. The main difference concerns the
central pyridyl signals HE/HF which are significantly deshielded
compared to terpy. Ligand L1 is depicted as a planar species
with transoid arrangements of its pyridyl rings since 4-phenyl-
2,2Ј:6Ј,2Љ-terpyridine is known to be planar in the solid state.1
In solution rapid rotation is most likely to be occurring with its
planar structures being the most favoured conformations. Such
conformations account for the considerable deshielding of the
central pyridyl HE/HF signals by the ring currents of the
adjacent rings. If the two terpy units of the ligand were not
rotating relative to each other but were locked at some angle,
then the deshielding effects of the central pyridyl hydrogens
could not be easily explained.
The spectra of the 2:1 metal:ligand complexes comprise 10
Fig. 1 Variable temperature 400 MHz 1H NMR spectra of [{ReBr-
(CO)3}2L1] in (CD3)2SO showing the effects of 1,4-metallotropic shifts.
The signal labelling refers to Table 2. Note the additional splitting of
signals F and G due to rotameric distinction.
aromatic signals at ambient temperatures arising from the
reduced symmetry of the complexes compared to the free
ligands. Chemical environments are labelled A–J and shifts
assigned as in Table 2. The observed number of chemical shifts
implies essentially free rotation in the ligand about the C–C
bond linking the two central pyridyl rings causing there to be no
relative positional dependence of metal coordination on the
two sides of the ligand. For the ReI and PtIV complexes the
coordination shifts are mainly positive, particularly for HA
which is just three bonds removed from the metal centre. For
the PtII complex a mixture of positive and negative coordin-
ation shifts occur with the HA shift being significantly negative.
This has been noted previously15 in the case of terpy complexes
and is attributed to the shielding of the adjacent C6F4CF3 ring
which adopts a near-orthogonal orientation with respect to the
coordinated pyridyl rings.
1
The H NMR spectra of [{ReBr(CO)3}2L1] in (CD3)2SO at
ambient and various above-ambient temperatures are shown in
Fig. 1. The signals show the expected multiplet splittings due to
three-bond scalar couplings, but signals due to HF and HG show
further splittings into unequal intensity doublets (relative inten-
J. Chem. Soc., Dalton Trans., 1998, 3479–3488
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HK
HT
HK
HT
HL
HS
HR
Pt
HL
HS
HR
Pt
N
N
N
N
N
N
N
HM
HM
HN
HQ
HG
HN
HO
HQ
HD
kRe
HO
HP
HF
HP
HE
HE
HF
HG
HD
HC
HB
HH
HI
HH
HI
HC
HB
N
N
N
N
N
Re
Re
HA
HJ
HJ
HA
kPt
kPt
HK
HT
HK
HT
HS
HR
HL
Pt
HL
HS
HR
Pt
N
N
N
N
N
N
HM
HM
HQ
HP
HN
HG
HQ
HP
HN
HD
kRe
HO
HO
HE
HF
HE
HD
HF
HG
HC
HB
HH
HI
HH
HI
HC
HB
N
N
N
N
N
N
Re
Re
HA
HJ
HJ
HA
Pt fluxion
Re fluxion
HK (9.07)
HT (8.79)
HA (9.16)
B (7.62)
HJ (8.86)*
HI (7.62)
HL (7.74)
HM (8.32)
HS (7.54)*
H
HR (7.95)
HC (8.17)
H
H (7.84)
G (8.01)
HF (8.14)
H
HD (9.12)
HE (8.69)
HN (8.48)
HQ (8.65)
HP(8.20)
HO (8.56)
Fig. 2 Interconversion of the four equivalent structures of [ReBr(CO)3PtIMe3L1] as a result of the ReI and PtIV moiety M–N shifts. The chemical
shifts of the exchanging pairs of signals are measured at 253 K in (CDCl2)2 solvent. The asterisks refer to the pairs of signals used for bandshape
analysis.
sities 55:45%). This splitting soon vanished on warming and is
attributed to restricted rotation of the pendant pyridyl rings. At
303 K two distinct rotameric species occur which differ in their
orientations with respect to the central pyridyl ring, and with
one rotamer being rotated, by approximately 180Њ, with respect
to the other rotamer. The environments of HF and HG are most
sensitive to these different rotameric species. On warming the
solution of the complex all bands commence broadening with
pairs of bands coalescing and sharpening at different rates. By
433 K the spectrum consists of five averaged bands of differing
widths, due to the exchanges A
J, B
I, C
H,
D
G and E F which have occurred as a result of the
1,4-metallotropic shifts of either metal moiety. Rates of these
fluxions were measured by bandshape analysis of the
exchanging B and I signals, taking account of the multiplet
splittings of the three-bond scalar couplings. Spectra at nine
different temperatures were fitted and reliable activation energy
data obtained (see later).
Fig. 3 400 MHz 1H spectra of [ReBr(CO)3PtIMe3L1] in (CDCl2)2
measured at 253 and 413 K. The asterisks denote impurity bands of
[{ReBr(CO)3}2L1]. See Fig. 2 for signal labelling.
The variable temperature 1H spectra of [(PtClMe3)2L1]
showed very analogous changes. Spectra were recorded in
CD3NO2 solvent in the range 263 to 368 K and the bandshapes
of the exchanging B and I signals fitted at ten different
temperatures.
Fig. 2. Assuming unrestricted rotation about the central C–C
bond linking the two terpy ligands, all four species are chem-
ically identical but two different rates of fluxion will occur
depending on which metal moiety moves. These can be dis-
tinguished by NMR as they lead to different averagings of the
aromatic hydrogen environments. These different averagings are
1
The variable temperature H NMR spectra of the mixed-
metal dinuclear complex [ReBr(CO)3PtIMe3L1] were more chal-
lenging to interpret. Fluxional shifts of either metal moiety
leads to exchange between the four structural species shown in
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J. Chem. Soc., Dalton Trans., 1998, 3479–3488

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Table 3 1H NMR data for the ligand L1 and its complexes [ReBr(CO)3L1] and [Pt(C6F4CF3)2L1] at 303 K
HM
HM
HM
HM
HL
HK
HN HN
N
HL
HK
HL
HK
HN HN
N
HL
HK
N
N
N
N
HO
HE
HO
HE
HO
HF
HO
HF
HG
HD
HG
HD
HC
HB
HH
HI
HH
HI
HC
HB
N
N
N
N
N
N
M
M
HA
HJ
HJ
HA
Compound
Solvent
δAJK
δBIL
δCHM
7.90
8.19(C)
8.04(H)
7.98(M)
8.28(C)
7.71(H)
7.95(M)
δDGN
δEFO
L1
CDCl3
(CDCl2)2
8.76
7.38
8.70
8.97
[ReBr(CO)3L1]
9.15(A)
8.90(J)
8.77(K)
8.38(A)
8.52(J)
8.76(K)
7.62(B)
7.60(I)
7.45(L)
7.58(B)
7.20(I)
7.44(L)
8.57(D)
7.94(G)
8.74(N)
8.48(D)
7.91(G)
8.72(N)
8.75(E)
8.26(F)
8.91(O)
8.67(E)
8.45(F)
8.92(O)
[Pt(C6F4CF3)2L1]
CDCl3
1
The H spectra of [{Pt(C6F4CF3)2}2L1] were essentially tem-
perature independent in the range ambient to 393 K indicating
stereochemical rigidity. It was therefore apparent that no
reliable kinetic data would be obtainable without recourse to
2D-exchange spectroscopy (2D-EXSY). Accordingly, a 1H
2D-EXSY spectrum was obtained at 393 K (Fig. 4). Using a
mixing time of 0.8 s, cross peaks between the exchanging pairs
B
I, C
H and D
G were clearly detected indicat-
ing that the 1,4-metallotropic shift was indeed proceeding albeit
at a slow rate. Relative intensities of the diagonal and cross
peaks of the above signals were measured and used as input
into the D2DNMR program.13 Three sets of rate constants
were computed, and an average value used to estimate the
Gibbs free energy of activation of the process using the Eyring
equation.
Mononuclear complexes [ReBr(CO)3L1] and [Pt(C6F4CF3)2-
L1]. The synthesis of [ReBr(CO)3L1] was difficult as the reaction
preferentially proceeded to the dinuclear complex. However, its
1H chemical shifts were measured and assigned (Table 3). The
uncoordinated half of the ligand gave rise to five chemical shifts
(due to the environments K–O), indicating rapid rotation about
the C–C bond linking the two halves of the ligand. If the two
halves were locked at some angle then the symmetry of the
fac-ReBr(CO)3 moiety with its differing axial groups would
remove the chemical equivalence of the two outer pyridyl rings
on the uncoordinated side, and this was not observed.
Fig. 4 400 MHz 1H 2D-EXSY NMR spectrum of [{Pt(C6F4-
CF3)2}2L1] in (CDCl2)2 at 393 K. Mixing time was 0.8 s. Signal labelling
refers to Table 2.
listed in Fig. 2 together with chemical shifts measured at 253 K
in (CDCl2)2 solvent. The spectrum at this temperature is shown
in Fig. 3. The metal fluxions are slow on the 1H chemical shift
time scale at this temperature, but are slightly broadened by
restricted rotations of the pendant pyridyl rings. The assign-
ment of the 20 signals is fairly firm as a result of careful
comparison with the previous spectra of the like-metal moiety
analogues. On warming the (CDCl2)2 solution of the com-
plex numerous bandshape changes occurred as a result of the
onset of the fluxions of both metal moieties. By 413 K, the
high temperature limit of the solvent, the spectra had simpli-
fied to 10 averaged signals, two pairs of which had very simi-
lar chemical shifts. The signals still displayed varying
amounts of exchange broadening and little multiplet struc-
ture could be detected. The spectra were too complex for
bandshape analysis to be performed but estimates of flux-
ional rates of both metal moieties were made from the
coalescence temperatures of the appropriate exchanging sig-
nals, namely coalescence of signals L and S at 323 K as a
result of PtIMe3 fluxion and coalescence of signals A and J
at 353 K resulting from ReBr(CO)3 fluxion.
Coordination shifts of the aromatic hydrogens on the
coordinated side of the complex are very similar to those of the
dinuclear Re complex, whilst the chemical shifts on the
uncoordinated side are very similar to those of the free ligand
L1. On warming, the spectra changed in the predicted way, coa-
lescences occurring between the signal pairs A
H, D G and E F. The signals K, L, M, N and
O remained sharp at all temperatures. Bandshape analysis
was performed on the A J exchanging pair, spectra at 11
J, B
I,
C
temperatures in the range 303–413 K being fitted. The spectra
also showed traces of free ligand but these did not affect the
accuracy of the NMR analyses.
The 1H spectra of [Pt(C6F4CF3)2L1] were essentially
unchanged on heating the solution to ca. 393 K. As in the
case of its dinuclear analogue, a 2D-EXSY spectrum was
taken at 393 K (Fig. 5) and again it revealed slow fluxional
exchange. In this case all five expected pairs of cross peaks were
detected. The exchanging pairs B
I and C
H were
chosen for the rate constant calculations. No cross peaks were
detected between signals K–O as these are unaffected by the PtII
fluxions.
J. Chem. Soc., Dalton Trans., 1998, 3479–3488
3483

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Table 4 1H NMR data for the ligand L2 and its dinuclear complexes
HA
HJ
HJ
HA
HI
HB
HC
HI
HB
HC
M
M
N
N
N
N
N
N
HH
HH
HD
HG
HG
HD
HE
HF
HF
HK
HE
HK
HK
HK
HK
HE
HK
HF
HK
HF
HG
HK
HE
HG
HD
HD
HC
HB
HH
HI
HH
HI
HC
HB
N
N
N
N
N
N
M
M
HA
HJ
HJ
HA
a
Compound
Solvent
T/K
δAJ
δBI
δCH
δDG
8.71
8.46(D)
7.91(G)
8.37(D)
8.55(G)b
δEF
δK
δMe
L2
CDCl3
(CDCl2)2
303
303
8.77
7.38
7.90
8.83
8.08
8.02
—
—
[{ReBr(CO)3}2L2]
9.16(A)
8.89(J)
9.03(A)
8.74(J)
7.61(B)
7.61(I)
7.68(B)
7.47(I)
8.18(C)
8.02(H)
8.14(C)
7.89(H)
8.51(E)
8.04(F)
8.42(E)
8.17(F)
[(PtIMe3)2L2]
CDCl2)2
253
7.99
1.62(A)(74.2)
0.34(B)(69.4)
0.55(C)(68.6)
[{Pt(C6F4CF3)2}2L2]
(CDCl2)2
303
8.35(A)
8.49(J)
7.62(B)
7.24(I)
8.29(C)
7.76(H)
8.40(D)
8.00(G)
8.47(E)
8.31(F)
8.11
a Pt–Me shifts, 2JPtH/Hz values in parentheses, MeC axial, MeA,B equatorial. b Broad band.
1
Mononuclear complex [Pt(C6F4CF3)2L2]. The static H spec-
tral data of this complex are given in Table 5. They consist of 17
chemical shifts all of which have been assigned with confidence.
The signals of the central phenyl ring comprise an AAЈBBЈ spin
system although the cross ring couplings are only just resolved.
The fluxion of the PtII moiety was again revealed in a high
temperature 2D-EXSY spectrum (413 K, mixing time 0.8 s).
The dynamic process had no effect on the signals K–Q (see
Table 5 for labelling).
Ligand L3 and its complexes [(PtIMe3)2L3] and [Pt(C6F5)2L3].
Our recent investigation2 into the mechanism of the 1,4-
metallotropic shift in bidentate chelate complexes required the
preparation of complexes of unsymmetrically substituted
terpyridine ligands. We have now utilized the synthetic route
to such ligands to prepare unsymmetrical back-to-back terpy
ligands such as ligand L3. The 1H NMR spectrum of this com-
pound consists of nine aromatic hydrogen signals and one
methyl signal, which imply rapid rotation about the central C–C
bond causing chemical equivalence of both terpy fragments
(Table 6).
This ligand is capable of forming more than one chemically
distinct type of dinuclear or mononuclear complex and these
were examined in detail below.
Fig. 5 400 MHz 1H 2D-EXSY NMR spectrum of [Pt(C6F4CF3)2L1] in
(CDCl2)2 at 393 K. Mixing time was 0.8 s. Signal labelling refers to
Table 3.
The dinuclear complex [(PtIMe3)2L3] gave a broad, ill-defined
1H spectrum at room temperature, which became better
resolved on cooling to 253 K. This is shown in Fig. 6. Some of
the bands are still somewhat broadened but this was attributed
to the effects of restricted rotation of the pendant rings. The
unsymmetrical nature of the complex presents the possibility of
chemically distinct metal complexes according to whether
coordination involves the 4-methylpyridyl ring or not. Previous
studies2 on the complexes [M(C6F5)2(mcpt)] [M = PdII or PtII,
mcpt = 4-methyl-4Ј-(4-chlorophenyl)-2,2Ј:6Ј,2Љ-terpyridine]
have shown that coordination involving the N-heteroatom of a
4-methylpyridyl ring is slightly preferred over the unsubstituted
ring. Thus, the most preferred coordination complex of
[(PtIMe3)2L3] will involve coordination of both 4-methylpyridyl
rings. This structure is labelled major/major in Fig. 7 and is
one of a set of four. A metallotropic shift of either Pt moiety
will lead to a mixed 4-methylpyridyl/pyridyl ring coordinated
species (major/minor or minor/major, Fig. 7). A further flux-
ional shift will produce a species where neither 4-methylpyridyl
Ligand L2 and its dinuclear complexes [M2L2] {M ؍ ReBr-
1
(CO)3, PtIMe3 or Pt(C6F4CF3)2}. The H spectrum of the free
ligand L2 was straightforwardly assigned (Table 4). The hydro-
gens of the central phenyl ‘spacer’ ring produced a sharp
singlet indicative of free rotation with respect to the attached
pyridyl rings. The ReI, PtIV and PtII complexes of L2 gave very
analogous spectra to the corresponding L1 analogues. Their
spectra also changed with temperature in analogous ways. The
fluxional rates of the ReBr(CO)3 and PtIMe3 moieties became
fast on the NMR timescale by 413 K, and bandshape analyses
were performed with accuracy. Spectra at 11 different temper-
atures were fitted in the temperature range 273–413 K, signals
A/J being used for the ReI complex and signals B/I for the PtIV
complex. The fluxion of [{Pt(C6F4CF3)2}2L2] was again very
much slower and could only be revealed by 2D-EXSY experi-
ments at elevated temperatures.
3484
J. Chem. Soc., Dalton Trans., 1998, 3479–3488

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Table 5 1H NMR data for the ligand L2 and its mononuclear metal complex [Pt(C6F4CF3)2L2]
HM
HM
HM
HM
HL
HK
HN HN
N
HL
HK
HN
HN
HL
HK
HL
HK
N
N
N
N
N
HO
HO
HP
HO
HP
HO
HP
HP
HQ
HE
HQ
HF
HQ
HF
HQ
HE
HG
HD
HG
HD
HC
HB
HH
HI
HH
HI
HC
HB
N
N
Pt
N
N
N
N
Pt
HA
HJ
HJ
HA
Compound
Solvent
T/K
δAJK
δBIL
δCHM
δDGN
δEFO
δPQ
L2
CDCl3
CDCl3
303
303
8.77
7.38
7.90
8.71
8.83
8.08
8.04(P)
8.11(Q)
[Pt(C6F4CF3)2L2]
8.36(A)
8.49(J)
8.74(K)
7.54(B)
7.18(I)
7.38(L)
8.23(C)
7.70(H)
7.91(M)
8.36(D)
7.96(G)
8.70(N)
8.41(E)
8.23(F)
8.78(O)
Table 6 1H NMR dataa for ligand L3 and its mono- and di-nuclear complexes
Compound
L3
Solvent
CDCl3
T/K δAT
303 8.58
δBS
δCR
δDQ
δEP
δFO
δGN
δHM
δIL
δJK
Notes
7.15
2.47
8.46
8.92
8.92
8.66
7.87
7.34
8.73
b
b
c
c
d
[(PtIMe3)2L3] (CDCl2)2 253
[Pt(C6F5)2L3] (CDCl2)2 353
8.90(A) 7.55(B) 2.64(C) 8.26(D) 8.60(E) 8.35(F) 8.75(G) 7.95(H) 7.55(I) 8.80(J)
8.63(T) 7.34(S) 2.52(R) 8.60(Q) 8.46(P) 8.42(O) 8.49(N) 8.19(M) 7.74(L) 9.12(K)
8.25(A) 7.35(B) 2.60(C) 8.24(D) 8.62(E) 8.45(F) 8.06(G) 7.74(H) 7.23(I) 8.55(J)
8.42(T) 7.05(S) 2.47(R) 7.85(Q) 8.45(P) 8.64(O) 8.45(N) 8.27(M) 7.55(L) 8.45(K)
8.67
7.29
2.60
8.55
8.96
8.96
8.75
8.00
7.47
8.82
a See Fig. 7 for labelling. b Additional data: δMe, major signals 1.65(A)(74.0), 0.32(B)(68.0), ≈0.45(C), minor signals 1.68(A)(72), 0.37(B)(70.0),
≈0.45(C). 2JPtH/Hz values in parentheses, C axial methyls (signals broadened due to restricted rotation of pendant ring), A,B equatorial methyls. 195Pt
shifts (273 K), rel. to Ξ(195Pt) = 21.4 MHz, 1870.9(major), 1874.3(minor). c Data refer to coordinated side of ligand. d Data refer to uncoordinated
side of ligand and to corresponding nuclei in the free ligand (see first row of table).
1
plexes, Fig. 7. These two sets of signals observed in the H
spectrum at 253 K (Fig. 6) are in the intensity ratio 64:36%,
which leads to the four static structures having solution popu-
lations of (64/4) × 2 = 32% (major/major), (64/4) ϩ (36/4) =
25% (major/minor), (36/4) ϩ (64/4) = 25% (minor/major)
and (36/4) × 2 = 18% (minor/minor). By careful comparison
with previous related spectra, knowledge of the expected
coordination shifts of the PtIV moiety and a close examination
of the higher temperature spectra (Fig. 6) a fairly definitive
assignment of all 20 lines was made (Table 6). The onset of
metallotropic shifts of either PtIV moiety led to exchange
broadening, coalescence and eventual sharpening of the aver-
aged bands of ten pairs of signals. A fast exchange spectrum
was obtained at 393 K from which firm assignments were made
(Fig. 6). The effects of the metallotropic shifts were seen most
clearly in the methyl signals C and R and in the aromatic hydro-
gen signals H and M. From the coalescence of the latter pair
at 313 K, an estimate of the free energy of activation for the
fluxion was made.
1
Variable temperature H spectra of the dinuclear complex
[{ReBr(CO)3}2L3] were recorded but these proved very intract-
able and no definitive assignments could be made. On warming
to 393 K, the spectra did appear to simplify to nine averaged
aromatic signals and one methyl signal as expected for fluxional
shifts of both ReI moieties, but no estimates of fluxional rates
were made.
1
Fig. 6 400 MHz H NMR spectrum of [(PtIMe3)2L3] in (CDCl2)2 at
253 and 393 K, the slow and fast exchange limits of Pt–N metallotropic
shifts. Signal labelling refers to Table 6.
ring is coordinated (the minor/minor species, Fig. 7). These four
structures constitute three chemically distinct types, but only
two are NMR-distinguishable species, since the NMR shifts (1H
or 195Pt) relate only to individual halves of the complexes, there
being no magnetic influence between the two halves. Thus the
1H shifts represent the summations of the hydrogen environ-
ments A–J on the 4-methylpyridyl-coordinated sides of the
major/major, major/minor and minor/major complexes and of
the hydrogen environments K–T on the pyridyl-coordinated
sides of the major/minor, minor/major and minor/minor com-
The spectra of the mononuclear complex [Pt(C6F5)2L3] were
more tractable. They did not change with temperature in the
range ambient to 393 K, and consisted of 27 aromatic signals
and three methyl signals. Chemical shifts were measured at 353
K (Table 6) and firm assignments of all signals made on the
basis of the free ligand spectrum, predicted coordination shifts,
and a COSY spectrum at this elevated temperature (Fig. 8). A
2D-EXSY spectrum was then recorded at 393 K and showed
J. Chem. Soc., Dalton Trans., 1998, 3479–3488
3485

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major/major
minor/major
HA
HJ
HA
HJ
'
HB
HI
Pt
HI
HB
'
N
N
Pt
N
N
N
N
N
MeC
HH
HH
MeC
HD
HE
HG
HG
HD
HE
HG
HN
kPt
HF
HF
HF
HE
HP
HQ
HO
HD
MeC
HB
HH
HI
MeR
HS
HM
N
N
N
N
N
Pt
HL
Pt
HA
HJ
HT
HK
kPt'
kPt'
HK
HT
HK
HT
HS
HL
Pt'
'
HL
HS
Pt
N
N
N
N
N
N
HM
MeR
HM
MeR
HQ
HN
HG
HQ
HP
HP
HN
HN
kPt
HP
HO
HO
HO
HF
HE
HQ
HD
MeC
HB
HH
HI
HM
HL
MeR
HS
N
N
N
N
N
N
Pt
Pt
HA
HJ
HT
HK
major/minor
minor/minor
Pt /Pt' fluxions
HA
HB
HT
HS
HF
HG
HH
HI
HO
HN
HM
HL
HK
MeC
HD
MeR
HQ
HJ
HE
HP
Fig. 7 Interconversion of the four structures of [(PtIMe3)2L3] following PtIV moiety fluxions.
strong cross-peaks due to the following exchanges B
S,
D
Q, G
N, H
M and I
L. The other
changes expected as a result of the 1,4-metallotropic shift,
namely A T, E P and F O were present but
somewhat obscured by signal overlaps. All signals of the
uncoordinated terpy fragment were not associated with any
exchange cross peaks. An estimate was made of the rate of the
fluxional process at 393 K by measuring volume integrals of the
diagonal and cross-peaks associated with the B
S and
H
M exchanges, and using them as input data in the
D2DNMR program.13
Low temperature NMR spectra
The PtII complexes of ligand L1 and L2 were also subjected to
low temperature NMR study with a view to examining the
rotational behaviour of the pendant pyridyl rings. It has been
shown previously with the complexes [M(C6F5)2(terpy)] (M =
PdII, PtII)15 and [M(C6F4CF3)2(TPT)] [M = PdII, PtII, TPT =
2,4,6-tris(2-pyridyl)-1,3,5-triazine]3 that the ortho and meta ring
fluorine nuclei of the perfluoroaromatic rings act as sensitive
probes of the rates of pyridyl rotation. The perfluoroaromatic
rings do not rotate with respect to the 19F chemical shift time
scale at below-ambient temperatures and so the ortho and meta
ring fluorine signals broaden on cooling, and eventually split
into pairs as the pyridyl rotation becomes slow. The 19F spectra
of the mononuclear complex [Pt(C6F4CF3)2L1] display this
effect in the temperature range 303 to 223 K. Bandshape anal-
1
Fig. 8 400 MHz H 2D-COSY45 NMR spectrum of [Pt(C6F5)2L3] in
(CDCl2)2 at 353 K. Labelling refers to Table 6 and Fig. 7. The
unlabelled signals are due to hydrogens on the un-coordinated side of
the ligand.
3486
J. Chem. Soc., Dalton Trans., 1998, 3479–3488

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Table 7 Activation energy data for 1,4-metal-nitrogen shifts in ReI, PtIV and PtII complexes of ligands L1, L2, L3 and related ligands
Complex
Populations (%)
∆H‡/kJ molϪ1
∆S‡/J K-1 mol-1
∆G‡ a/kJ molϪ1
Ref.
[{ReBr(CO)3}2L1]
[(PtClMe3)2L1]
50:50
50:50
50:50
50:50
69.7 3.5
58.7 1.2
64.5 3.6
—
Ϫ8.7 10.2
Ϫ16.7 3.7
Ϫ22.1 10.1
—
72.3 0.5
63.7 0.1
71.2 0.6
Present work
Љ
Љ
Љ
Љ
Љ
Љ
Љ
Љ
Љ
Љ
Љ
[ReBr(CO)3L1]
[ReBr(CO)3PtIMe3L1]
70.4(Re)(353 K)b
65.3(Pt)(323 K)c
97.2 0.1(393 K)d
99.4 0.1(393 K)d
73.3 0.3
[{Pt(C6F4CF3)2}2L1]
[Pt(C6F4CF3)2L1]
[{ReBr(CO)3}2L2]
[(PtIMe3)2L2]
50:50
50:50
50:50
50:50
50:50
50:50
32:25:25:18
58:42
50:50
50:50
50:50
66:34
50:50
63:37
64.5:35.5
—
—
—
—
69.4 1.9
46.1 0.9
—
—
—
Ϫ13.1 5.2
Ϫ57.3 3.0
—
—
—
63.1 0.1
[{Pt(C6F4CF3)2}2L2]
[Pt(C6F4CF3)2L2]
[(PtIMe3)2L3]
100.1 0.1(393 K)d
100.3 0.1(393 K)d
65.6(313 K)e
100.0(393 K)d
71.6 0.1
[Pt(C6F5)2L3]
—
—
Љ
[ReBr(CO)3(terpy)]
[PtClMe3(terpy)]
[Pt(C6F5)2(terpy)]
[ReBr(CO)3(mcpt)]
[ReBr(CO)3(mcpmt)]
[PtIMe3(mcpt)]
79.8 1.7
57.9 1.4
93.0 2.9
74.1 2.9
72.5 2.7
72.8 0.6
—
27.5 4.8
Ϫ13.7 4.3
Ϫ3.2 7.3
Ϫ0.3 0.1
Ϫ0.4 8.0
22.9 1.9
—
16
17
15
2
2
2
2
62.0 0.1
93.9 0.7
74.1 0.9
72.6 0.4
66.0 0.01
[Pt(C6F5)2(mcpt)]
100.6(393 K)d
a At 298.15 K unless otherwise stated. b From coalescence of the exchanging pair A
J. c From coalescence of the exchanging pair L
S.
d From 2D-EXSY spectra. e From coalescence of the exchanging pair H
M.
terpy
terpy
N
N
N
N
Pt
Pt
N
N
CF3
CF3
CF3
CF3
Fig. 10 Proposed solution rotamers of [Pt(C6F4CF3)2L1].
study of the dinuclear complex [{Pt(C6F4CF3)2}2L1] revealed,
in addition to the expected changes in the ring fluorine signals,
a broadening and eventual splitting (of ca. 5 Hz) of the CF3
triplet signal. This is attributed to the fact that the two pendant
pyridyl rings can become locked at low solution temperatures
with their N-heteroatoms either syn or anti with respect to each
other. In principle all the ring fluorine or CF3 signals should
reflect this conformational isomerism. In practice, however, it is
likely to affect only those fluorines on the rings adjacent to the
pendant pyridyl. In the present case this is seen in the CF3
signals only.
Such sensitivity to the long range spatial relationship of the
locked pyridyl rings is lost when a phenyl spacer group is
inserted between the two terpy fragments in the L2 ligand
complexes. Thus, the low temperature 19F spectra of [{Pt(C6F4-
CF3)2}2L2] show the expected splitting effects of the ring-
fluorine signals on cooling but no effect on the CF3 signal
suggesting no magnetic communication between the two sides
of the dinuclear complex.
Fig. 9 376 MHz 19F NMR spectra (meta ring fluorines only)
of [Pt(C6F4CF3)2L1] in CD2Cl2 in the temperature range 223–303 K.
Computer simulated spectra of the signals of the ring adjacent to the
pendant pyridyl ring are shown alongside with the ‘best-fit’ rate
constants.
Discussion
The activation energy data for the 1,4-metallotropic shift in
these ReI, PtIV and PtII complexes of ‘back-to-back’ terpy lig-
ands are collected in Table 7 and corresponding values for
terpy, mcpt and mcpmt complexes included for comparison
ysis was then carried out on the meta-fluorine signal shapes (see
Fig. 9). The equal intensities of the pairs of rotamer signals at
low temperatures suggest that both rotamers are equivalent
and this suggests that the two perfluoroaromatic rings and the
pyridyl ring are approximately parallel to each other and all
rings are orthogonal to the plane containing the chelated pyri-
dyl rings (Fig. 10).
The 4-position ring or CF3 fluorines lie on average in this
plane and so are quite insensitive to the pendant pyridyl
rotation in the mononuclear complexes [Pt(C6F4CF3)2L1] and
[Pt(C6F4CF3)2L2]. However, a comparable low temperature
purposes
[mcpmt = 4Ј-(4-chlorophenyl)-4,4Љ-dimethyl-2,2Ј:
6Ј,2Љ-terpyridine]. It will be seen that the activation energies
(expressed as ∆G‡ values) for all these terpyridyl-based ligand
complexes fall within very narrow ranges for each transition
metal, these ranges being 62–66 kJ molϪ1 for PtIV complexes,
70–74 kJ molϪ1 for ReI complexes and 94–101 kJ molϪ1 for PtII
complexes. Such an energy ordering has been noted before in
other series of complexes2,15 and is clearly a function primarily
of the transition metal rather than the ligand.
J. Chem. Soc., Dalton Trans., 1998, 3479–3488
3487

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Table 8 Activation parameters for restricted rotation of the pendant pyridyl rings in mono- and di-nuclear PtII complexes of L1, L2 and others
Complex
T rangea/K
∆H‡/kJ molϪ1
∆S‡/J KϪ1 molϪ1
∆G‡b/kJ molϪ1
Ref.
[Pt(C6F4CF3)2L1]
203–293c
273d
45.3 1.8
—
27.8 0.8
29.1 1.0
—
Ϫ32.6 6.5
—
Ϫ89.8 3.3
Ϫ86.5 4.1
—
55.0 0.2
54.4 0.1d
54.5 0.2
54.9 0.2
55.9 0.2
42.3 0.7
Present work
Љ
Љ
Љ
15
3
[{Pt(C6F4CF3)2}2L1]
[Pt(C6F4CF3)2L2]
213–283c
213–273c
253–303
193–243
[{Pt(C6F4CF3)2}2L2]
[Pt(C6F5)2(terpy)]
[Pt(C6F4CF3)2(TPT)]
45.1 2.0
9.2 9.3
a Solvent CD2Cl2. b At 298.15 K. c Values based on bandshape analysis of exchanging meta ring F signals. d Value based on the coalescence
temperature given.
There is no systematic difference in activation energies for
corresponding L1, L2 and L3 ligand complexes. However, it
should be noted that values for these ‘back-to-back’ terpy
ligand complexes are generally slightly higher (by up to 6 kJ
molϪ1) than the corresponding terpy ligand complexes. This,
therefore, does imply some slight electronic influence between
the two sides of the complex perhaps influencing the M–N
bond of the central pyridyl ring. However, this point should not
be made too strongly as this activation energy difference does
not apply when comparison is made with the substituted terpy
ligand, mcpt or mcpmt. Furthermore, there is no significant
difference in energies between the fluxions in the mononuclear
and dinuclear complexes, implying that the metallotropic shifts
are primarily independent of the nature of the opposite side of
the ligand, i.e. whether it be metal coordinated or not.
The results of the restricted rotation studies of the pendant
pyridyl rings are quite predictable. Values for PtII complexes
of L1 and L2 ligands are given in Table 8 and compared with
corresponding terpy15 and TPT3 complexes. ∆G‡ values fall
within the narrow range 54–56 kJ molϪ1 with the exception of
TPT which is significantly lower. There was evidence in the
ambient temperature spectra of [{ReBr(CO)3}2L1] of consider-
able restriction to rotation of the pyridyl rings. However,
this was not easily quantified but our observations suggest
that the energy barrier would be significantly higher than in the
corresponding terpy complexes.16
fellowship (to A. G.), for a research studentship (to M. D. O.)
and for use of the national mass spectrometry service at the
University of Wales, Swansea.
References
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ˇ
2 A. Gelling, K. G. Orrell, A. G. Osborne and V. Sik, J. Chem. Soc.,
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ˇ
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ˇ
14 V. Sik, Ph.D. Thesis, University of Exeter, 1979.
The main conclusions of this work are that the well
established 1,4-metallotropic shifts in bidentate chelate
terpyridine-based complexes are sensitive only to the local
metal-coordination environment, and there is negligible steric
or electronic interaction between the two sides of ‘back-to-
back’ terpyridine ligands with or without phenyl spacer groups.
Likewise, rotations of the pendant pyridyl rings are not influ-
enced by the presence of the other terpy fragment whether it be
metal coordinated or not.
ˇ
15 E. W. Abel, K. G. Orrell, A. G. Osborne, H. M. Pain, V. Sik, M. B.
Hursthouse and K. M. A. Malik, J. Chem. Soc., Dalton Trans., 1994,
3441.
16 E. W. Abel, V. S. Dimitrov, N. J. Long, K. G. Orrell, A. G. Osborne,
ˇ
H. M. Pain, V. Sik, M. B. Hursthouse and M. A. Mazid, J. Chem.
Soc., Dalton Trans., 1993, 597.
17 E. W. Abel, V. S. Dimitrov, N. J. Long, K. G. Orrell, A. G. Osborne,
ˇ
V. S ik, M. B. Hursthouse and M. A. Mazid, J. Chem. Soc., Dalton
Trans., 1993, 291.
Acknowledgements
We wish to thank the EPSRC for a postgraduate research
Paper 8/05300F
3488
J. Chem. Soc., Dalton Trans., 1998, 3479–3488