1,2-Metallotropic shifts in tricarbonylrhenium(I) complexes of pyridazine (pydz): A dynamic NMR investigation of the effects of cis-chelating ligands

Article abstract of DOI:10.1016/S0020-1693(96)05339-X

Complexes of general formula fac-[Re(CO)₃(L-L)(pydz)][BF₄] (L-L=neutral bidentate chelate ligand; pydz=pyridazine) were prepared in high yield from fac-[ReBr(CO)₃(L-L)]. The pyridazine ligand coordinates to the metal moiety in a monodentate fashion, and undergoes a facile intramolecular 1,2-Re-N shift. The kinetics of the 1,2-metallotropic shift were measured by either one-dimensional NMR bandshape analysis or two-dimensional exchange spectroscopy. The free energies of activation, ΔG^≠ (298 K), were found to be dependent on the nature of the chelate ligand, and were in the range 77-87 kJ mol^-1. The origins of the chelate ligand effects are discussed.

Full text of DOI:10.1016/S0020-1693(96)05339-X

Inorganica Chimica Acta 255 (1997) 65–71
1,2-Metallotropic shifts in tricarbonylrhenium(I) complexes of
pyridazine (pydz): a dynamic NMR investigation of the effects
of cis-chelating ligands
Edward W. Abel, Peter J. Heard 1, Keith G. Orrell U
Department of Chemistry, The University, Exeter EX4 4QD UK
Received 18 January 1996; revised 24 April 1996
Abstract
Complexes of general formula fac-[Re(CO)3(L–L)(pydz)][BF4] (L–Lsneutral bidentate chelate ligand; pydzspyridazine) were
prepared in high yield from fac-[ReBr(CO)3(L–L)]. The pyridazine ligand coordinates to the metal moiety in a monodentate fashion, and
undergoes a facile intramolecular 1,2-Re–N shift. The kinetics of the 1,2-metallotropic shift were measured by either one-dimensional NMR
bandshape analysis or two-dimensional exchange spectroscopy. The free energies of activation, DG/ (298 K), were found to be dependent
on the nature of the chelate ligand, and were in the range 77–87 kJ moly1. The origins of the chelate ligand effects are discussed.
Keywords: Rhenium complexes; Chelate ligand complexes; Fluxional pyridazine complexes; Dynamic NMR
1. Introduction
2. Experimental
2.1. Materials
The stereochemical non-rigidityassociatedwithmonoden-
tate diazine–transition metal complexes is well established
[1–8]. In these complexes, the diazine ligand undergoes a
facile 1,2-metallotropic shift between the two contiguous N
donor atoms, via a mechanism which is believed to involve
a 20-electron N,N9 s-bonded transition state [4,5,7,9]. The
authors embarked recently on a systematic approach to the
study of the factors which affect the energetics of transition
metal–diazine rearrangements; we report here the results of
our studies on the effect of cis-chelate ligands in complexes
of general formula fac-[Re(CO)3(L–L)(pydz)][BF4]
(L–Lsneutral bidentate chelate ligand; pydzspyridazine).
These complexes, which are readily prepared from fac-
[ReBr(CO)3(L–L)] by the action of AgBF4 in the presence
of pyridazine, are kinetically inert and thermally stable. Thus
they provide an excellent substrate for the quantitative inves-
tigation of the effects of the cis-chelating ligands on the
fluxional rearrangement.
Bromopentacarbonylrhenium(I) was prepared from di-
rheniumdecacarbonyl as previously described [10,11]. The
proligands and pyridazine were purchased from standard
sources (Aldrich or Lancaster Syntheses Ltd.) and were used
as supplied.
2.2. Synthesis of the complexes
Manipulations were performed using standard Schlenk
techniques [12], under an atmosphere of dry, oxygen-free
nitrogen. Solvents were dried [13] and degassed prior to use.
The precursor complexes, [ReBr(CO)3(L–L)] (L–Ls
2,29-bipyridine (bipy), 2,29-bipyrimidine (bpym), 1,10-
phenanthroline (phen), N,N,N9,N9-tetramethylethylenedi-
amine (tmen) or bis(diphenylphosphino)ethane (dppe)),
were prepared in an analogous fashion to that reported for the
2,29:69,20-terpyridine complex, [ReBr(CO)3(terpy)] [14],
as illustrated by the synthesis of [ReBr(CO)3(bipy)].
2.2.1. [(Bipyridyl)bromotricarbonylrhenium(I)]
Bromopentacarbonylrhenium(I) (1.0 g, 2.46 mmol) was
dissolved in 30 cm3 of 50:50 (vol./vol.) benzene/80–100
light petroleum. 2,29-Bipyridine (0.48 g, 2.56 mmol) was
U Corresponding author.
1 Present address: Department of Chemistry, University of Wales,
Swansea SA2 8PP, UK.
0020-1693/97/$17.00 q 1997 Elsevier Science S.A. All rights reserved
PII S0020-1693(96)05339-X

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E.W. Abel et al. / Inorganica Chimica Acta 255 (1997) 65–71
added to the stirred solution and the reaction mixture was
then refluxed for ;2.5 h. Concentration of the benzene at
reduced pressure yielded a yellow solid, which was isolated
by filtration and dried in vacuo. No further purification was
necessary. Yield: 1.18 g (94%).
The five complexes, fac-[Re(CO)3(L–L)(pydz)][BF4]
(L–Lsbipy, bpym, phen, tmen or dppe), were all prepared
similarly. The preparation of [Re(CO)3(bipy)(pydz)]-
[BF4] is detailed below to illustrate the synthetic procedure.
complexes, [Re(CO)3(L–L)(pydz)][BF4] (L–Lsbipy,
bpym, phen or dppe). Rate data for the complex
[Re(CO)3(tmen)(pydz)][BF4] were obtained by the sim-
ulation of standard one-dimensional variable temperature
spectra. 13 spectra were recorded in the temperature range
297–368 K, and the band shape changes were analysed using
the authors’ version of the program DNMR3 [17]. Computer
simulated spectra were visually fitted with those experimen-
tally obtained, until a ‘best-fit’ was achieved. Rate data for
the five complexes, [Re(CO)3(L–L)(pydz)][BF4] (L–L
sbipy, bpym, phen, tmen or dppe), were used to calculate
the activation parameters from least-squares fits of the Eyring
and Arrhenius plots. The errors quoted are those defined by
Binsch and Kessler [18].
2.2.2. [(Bipyridyl)(pyridazine)tricabonylrhenium(I)]-
[tetrafluoroborate]
(Bipyridyl)bromotricarbonylrhenium(I) (1.0 g, 1.97
mmol) and AgBF4 (0.50 g, 2.56 mmol) were refluxed for
3
;1hni30cm
of tetrahydrofuran (THF). The reaction
mixture was then filtered (to remove AgBr), and added to a
stirred THF solution of pyridazine (0.25 g, 3.12 mmol in 5
cm3). The THF solution was refluxed for ;1 h, then the
volatile components were removed in vacuo. The resultant
orange oil was crystallised from ethylethyl ketone–diethyl
ether to yield 0.70 g (60%) of solid [Re(CO)3(bipy)-
(pydz)][BF4].
3. Results
The five complexes, [Re(CO)(L–L)(pydz)][BF ](L–L
3
4
sbipy, bpym, phen, tmen or dppe), were isolated as crys-
talline orange solids as described (vide supra). The com-
plexes (except L–Lsbpym) are air-stable for periods of
several weeks under ambient conditions; however, they do
decompose slowly on prolonged exposure to air. The IRspec-
tra displayed three bands in the carbonyl stretching region,
and up to nine bands each due to the Re–C stretching and
deformation modes. These data are consistent with a fac-
octahedral coordination geometry for the ReBr(CO)3 metal
moiety [19]. Fast atom bombardment (FAB) mass spec-
trometry was performed on the five complexes; in all cases
(except L–Lsdppe), molecular ions were observed for
the species [Re(CO)3(L–L)(pydz)]q. The complex [Re-
(CO)3(dppe)(pydz)][BF4] exhibited a molecular ion
attributable to [MyBF4yCH2]q. In all cases, the observed
isotope distribution patterns were consistent with those cal-
culated for the formulated species. The elemental analyses
obtained for the complexes [Re(CO)3(L–L)(pydz)][BF4]
(L–Lsbipy, phen, tmen or dppe) were consistent with the
formation of analytically pure samples. The somewhat low
C and N analyses for the 2,29-bipyrimidine complex,
[Re(CO)3(bpym)(pydz)][BF4], probably arise because of
some partial decomposition of the complex (vide supra).
Analytical data for the five complexes, [Re(CO)3(L–L)-
(pydz)][BF4] (L–Lsbipy, bpym, phen, tmen or dppe), are
reported in Table 1.
2.3. Physical methods
IR spectra were recorded as pressed CsI discs on a Nicolet
Magna 550 FT-IR spectrometer, operating in the region
4000–200 cmy1. Fast atom bombardment (FAB) massspec-
tra were obtained on a VG AutoSpec instrument, using Csq
bombardment at 25 kV energy, on the complexes dissolved
in a matrix of 3-nitrobenzyl alcohol. Elemental analyseswere
performed by Butterworth Laboratories Ltd., Teddington,
Middlesex.
1H NMR spectra were recorded as d6-DMSO solutions on
a Bruker AM250 Fourier Transform spectrometer, operating
at 250.13 MHz. Chemical shifts are quoted in ppm, relative
to tetramethylsilane as an internal standard. Probe tempera-
tures were controlled by a standard B-VT 1000 unit. Tem-
peratures were periodically checked against a (Comark)
digital thermometer, and are considered accurate to within
"1 8C. Two-dimensional exchange (EXSY) spectra were
obtained using the Bruker automation program NOESYPH
[15], which generates the pulse sequence D1-908-D0-908-
D0-908 -free induction decay. The relaxation delay, D1, was
2.0 s and the evolution time, D0, was initially set at 3 ms. The
mixing time, D9, was varied according to the temperature of
the experiment and complex under investigation; typically
D9 was in the range 0.05–1.0 s. The spectra were recorded
with 512 words of data in f1 and f2, and transformed with
1024 words. The total experiment times were typically ;2.5
h. Peak intensities were measured by volume integration of
the resulting two-dimensional spectra. Five sets of integra-
tions were performed on each of the spectra; mean values
were then used to calculate the exchange rate constants using
the D2DNMR program [16]. Five or six EXSY experiments
were performed at different temperatures on each of the
3.1. NMR studies
3.1.1. Complexes [Re(CO)3(L–L)(pydz)][BF4] (L–Lsbipy,
bpym or phen)
The ambient temperature (303 K) 1H NMR spectra of the
complexes [Re(CO)3(L–L)(pydz)][BF4] (L–Lsbipy,
bpym or phen) in d6-DMSO solution displayed well resolved
signals. The spectra comprised two overlapping sub-spectra
in the region ca. d 7.5–9.5, due to (i) the proligands (bipy,
bpym or phen) and (ii) the hydrogens of the pyridazine ring.

E.W. Abel et al. / Inorganica Chimica Acta 255 (1997) 65–71
67
Table 1
Synthetic and analytical data for the complexes [Re(CO)3(L–L)(pydz)]qBF4 y (L–Lsbipy, bpym, phen, tmen or dppe)
Complex
(chelate ligand)
Yield a
(%)
n(C–O) b
(cmy1
n(Re–C) b
(cmy1
Anal. Found (Calc.)
(%)
mq/z c
)
)
bipy
60
12
75
70
70
1920.8 b
2030.3
385.0, 408.4, 420.0,
484.0, 521.8, 539.7,
628.9, 642.6
C, 34.54 (34.41)
H, 2.01 (2.04)
N, 9.46 (9.44)
C, 25.34 (30.26)
H, 1.71 (1.69)
N, 11.67 (14.12)
C, 36.62 (36.97)
H, 1.76 (1.96)
N, 9.12 (9.08)
C, 27.49 (28.22)
H, 3.27 (3.64)
N, 9.84 (10.13)
C, 46.81 (47.44)
H, 3.15 (3.38)
N, 3.21 (3.55)
507
509
531
467
735 d
bpym
phen
tmen
dppe
1925.4
1959.7
2040.0
1916.6 b
2029.9
386.1, 398.6, 413.1,
420.4, 479.6, 522.1,
539.8, 627.0, 644.7
384.9, 414.1, 473.3,
482.4, 522.0, 540.3,
546.1, 625.8, 645.9
395.0, 417.8, 455.5,
481.3, 520.0, 534.9,
641.1, 652.0
1904
1935
2032
1934
1967
2039
370.6, 383.2, 435.2,
495.4, 515.5, 604.9,
618.4
a Yield quoted relative to [ReX(CO)3(L–L)].
b IR spectra recorded as CsI discs; not all bands observed, bsbroad.
c FAB mass spectral data; ions observed correspond to [MyBF4]q, except for dppe d.
d Observed ion corresponds to the species [MyBF4yCH2]q
.
The signals due to the proligands were assigned on the basis
of their scalar coupling networks (determined from a series
of selective homonuclear decoupling experiments) and by
comparison with the spectra for the analogous PtIV com-
plexes, [PtMe3(L–L)(pydz)][BF4] (L–Lsbipy, bpym or
phen) [8]. The sub-spectra due to the pyridazine ring dis-
played three resonances in a 1:1:2 intensity ratio, indicative
of the ligand acting as a non-exchanging, monodentateligand
[4,5]. The 3- and 6-position hydrogen atoms, HA and HB
(Fig. 1), were assigned on the basis that HA would be
expected to resonate to higher frequency as a result of its
close proximity to the metal-coordinated N atom [5,7]. The
4- and 5-position hydrogens, HC and HD, have almost iden-
tical chemical shifts and could not be distinguished unambig-
uously. The spectrum of [Re(CO)3(phen)(pydz)][BF4]is
shown in Fig. 2. 1H NMR data are reported in Table 2.
On warming solutions of the complexes above ;380 K,
the resonances due to HA and HB displayed dynamic line
broadening, indicating the onset of the expected 1,2-Re–N
fluxional shift (Fig. 3) at a measurable rate. Magnetisation
transfers also occur between HC and HD as a result of the 1,2-
Re–N shift; however, the close proximity of these signals
(vide supra) renders them insensitive to the rearrangement.
On warming the solutions further, considerable thermal
decomposition of the complexes was evident, and it was not
possible to acquire a reliable set of standard one-dimensional
variable temperature spectra. Kinetic data were therefore
sought by two-dimensional exchange spectroscopy (2D-
EXSY). Accordingly, five EXSY experiments were per-
formed on each of the complexes, [Re(CO)3(L–L)-
(pydz)][BF4] (L–Lsbipy, bpym or phen), and accurate
Fig. 1. The complexes [Re(CO)3(L–L)(pydz)][BF4] (L–Lsbipy, bpym
or phen) showing the H atom labelling in the ligands.
Fig. 2. 250 MHz 1H NMR spectrum of [Re(CO)3(phen)(pydz)][BF4]in
(CD3)2SO solution at 303 K. Labelling refers to Fig. 1.

68
E.W. Abel et al. / Inorganica Chimica Acta 255 (1997) 65–71
Table 2
Table 3
1H NMR data a for the complexes [Re(CO)3(L–L)(pydz)]qBF4 y (L–L
sbipy, bpym, phen, tmen or dppe)
2D EXSY data for the complexes [Re(CO)3(L–L)(pydz)]qBF4 y (L–L
sbipy, bpym, phen or dppe)
Complex
(chelate ligand)
Temperature
(K)
d(pydz–H) b,c
d(chelate) b,c
Complex
(chelate ligand)
Temperature
(K)
Rate a
(sy1
)
bipy
303
303
303
297
303
A, 9.51 (4.9)
B, 8.96 (4.8)
C, 7.9
E, 9.34 (5.5)
F, 7.9
G, 8.44 (7.7, 8.2)
H, 8.75 (8.2)
E, 9.3
bipy
338
343
348
353
358
338
343
348
353
358
363
338
343
348
358
363
368
353
358
368
378
383
0.52
0.85
1.15
2.03
3.47
0.31
0.44
0.99
2.15
3.02
3.47
0.35
0.46
0.97
2.97
3.35
4.23
0.51
1.13
2.37
4.84
5.01
D, 7.9
bpym
phen
tmen
dppe
A, 9.54 (4.4)
B, 8.82 (4.5)
C, 7.9
F, 7.98
G, 9.3
bpym
D, 7.9
A, 9.24 (4.2) d
B, 8.31 (4.2) d
C, 7.7
E, 9.62 (5.2)
F, 8.20 (8.3, 5.2)
G, 9.00 (8.3)
H, 8.29
Me, 2.66
Me, 3.25
CH2, 2.93
CH2, 3.47
CH2, 5.6
D, 7.7
phen
dppe
A, 9.47 (4.4)
B, 9.65 (4.3)
C, 8.0
D, 8.1
A, 9.97 (5.6)
B, 8.05 (4.5)
C, 7.83 (8.1
CH2, 6.1
JCD
D, 7.50 (8.1
JCD
)
)
a1 H NMR spectra recorded in (CD3)2SO solution; chemical shifts quoted
relative to SiMe4 as an internal standard.
a First order rate constants for the 1,2-metal–nitrogen shift process; uncer-
tainties are ca. "10%.
b3 J(HH)/Hz in parentheses; not all scalar couplings resolved.
c Labelling refers to Figs. 1 and 4.
proligand phenyl rings, and three signals in a 1:1:2 intensity
ratio due to the four different hydrogen environments of the
non-exchanging pyridazine ring. The pyridazine hydrogen
signals were assigned to HA,H B and HC/D, respectively(vide
supra). 1H NMR data are reported in Table 2.
d Also 4J(HH)s2.9 Hz, 5J(HH)s1.3 Hz.
On warming the d6-DMSO solution of [Re(CO)3(dppe)-
(pydz)][BF4] above ;358 K, the signals due to the 3- and
6-position hydrogen atoms of the pyridazine ring, HA and
HB, exhibited reversible dynamic line broadening, as a result
of the onset of the 1,2-Re–N shift at a measurable rate on the
NMR chemical shift time scale. At higher temperatures, the
complex started to decompose, thus frustrating attempts to
acquire a reliable set of variable temperature spectra. Kinetic
data were therefore sought by 2D EXSY NMR. Five EXSY
experiments were performed in the temperature range 353–
383 K and accurate rate data obtained for the metallotropic
shift (Table 3). The activation parameters calculated from
the rate data are reported in Table 4.
Fig. 3. Effects of the 1,2-metallotropic shift in [Re(CO)3(L–L)-
(pydz)][BF4] complexes on the pydz hydrogen environments.
rate data extracted from the magnetisation transfers between
signals due to HA and HB (Table 3). The activation param-
eters calculated from the rate data are reported in Table 4.
3.1.2. Complex [Re(CO)3(dppe)(pydz)][BF4]
1
The H NMR spectrum of [Re(CO)3(dppe)(pydz)]-
[BF4]ind 6-DMSO 303 K was well resolved, and comprised
two regions: (i) the proligand (dppe) methylene region (ca.
d 5.6–6.1) and (ii) the aromatic region (ca. d 7.4–10.0). The
proligand methylene region displayed two complex second-
order multiplets in a 1:1 intensity ratio. The lower frequency
resonance was tentatively assigned to the hydrogen atoms cis
with respect to pyridazine (Fig. 4), as these might be
expected to be slightly shielded (relative to the hydrogen
environments trans to pyridazine) by the aromatic ring cur-
rent. The spectrum in the aromatic region comprised a com-
plex overlapping signal due to the hydrogen atoms of the
3.1.3. Complex [Re(CO)3(tmen)(pydz)][BF4]
1
The H NMR spectrum of [Re(CO)3(tmen)(pydz)]-
[BF4]ind 6-DMSO solution at 297 K exhibited well resolved
signals. The spectrum consisted of regions due to (i) the
proligand (tmen) hydrogen atoms (ca. d 2.6–3.5), and (ii)
the pyridazine ring hydrogen atoms (ca. d 8.0–9.7). The
pyridazine region of the spectrum displayed three signals,
characteristic of the ligand acting as a non-exchanging mon-
odentate ligand (vide supra). These signals were tentatively

E.W. Abel et al. / Inorganica Chimica Acta 255 (1997) 65–71
69
Table 4
Eyring and Arrhenius activation parameters a for the complexes [Re(CO)3(L–L)(pydz)]qBF4 y (L–Lsbipy, bpym, phen, tmen or dppe)
Complex
(chelate ligand)
Ea
log10
A (sy1))
DH/
DS/
DG/
(kJ moly1)(
(kJ moly1
)
(J moly1 Ky1
)
(kJ moly1
)
bipy
93.9 (5.8)
109 b
93.4 (8.2)
113.8 (2.2)
84.6 (9.9)
14.2 (0.9)
16.4 (1.7)
14.0 (1.2)
19.1 (0.3)
12.3 (1.4)
91.0 (5.8)
106 b
90.5 (8.3)
110.0 (2.2)
81.6 (9.9)
17.6 (16.6)
58.4 (31.9)
12.8 (23.4)
111.8 (6.4)
y19.0 (27.0)
85.8 (0.8)
88.8 (1.7)
86.7 (1.3)
77.7 (0.3)
87.2 (1.9)
bpym
phen
tmen
dppe
a Errors given in parentheses; DG/ values quoted at 298.15 K.
b Error uncertain.
Fig. 4. The complexes [Re(CO)3(L–L)(pydz)][BF4] (L–Lsdppe or
tmen) showing the hydrogen labelling.
assigned to HB,H A and HC/D (Fig. 4), respectively, by com-
parison with the 1H NMR spectrum of the corresponding PtIV
complex, [PtMe3(tmen)(pydz)][BF4] [8]. It should be
noted that, in contrast to the ReI complexes described here,
the assignment of HA and HB in the PtIV complexes is unam-
biguous because HA displays measurable 3J scalar coupling
to 195Pt (Is1/2, 33% abundance). Thus it is not possible to
establish if the anomalously high frequency shift of HB
observed in [PtMe3(tmen)(pydz)][BF4] also occurs in the
complex [Re(CO)3(tmen)(pydz)][BF4]; the assignments
of HA and HB may therefore be reversed. This uncertainty in
the assignment of HA and HB does not affect the measurement
of the exchange kinetics (vide infra). The proligand (tmen)
region comprised two complex second-order multiplets due
to the methylene hydrogen atoms, and two singlets due to
the N-methyls. In each case, the lower frequency signal
was tentatively assigned to the cis-hydrogen environments,
Fig. 4. 1H NMR data are reported in Table 2.
Fig. 5. Experimental and computer simulated variable temperature 1H NMR
spectra of [Re(CO)3(tmen)(pydz)][BF4]. The ‘best-fit’ rate constants are
given with the computer simulated spectra. ‘Best-fit’ rate constants for other
temperatures are as follows: k (sy1)s1.0 (313 K), 2.8 (318 K), 12 (328
K), 32 (338 K), 325 (358 K), 550 (363 K) and 900 (368 K).
On raising the NMR probe temperature, magnetisation
transfers resulting from the expected 1,2-Re–N metallotropic
shift lead to broadening of the pyridazine ring hydrogen sig-
nals. These line shape changes can be followed by standard
one-dimensional bandshape analysis according to the
dynamic spin problem ABCD|BADC [4], and the tem-
perature dependent spectra were simulated on this basis. The
signals due to HC and HD are insensitive to the exchange
process owing to their close proximity; therefore, only the
line shape changes associated with HA and HB were subject
to analysis (Fig. 5). Some thermal decomposition of the
complex [Re(CO)3(tmen)(pydz)][BF4]isevidentfromits
1H NMR spectra, but this did not adversely affect the band
shape fittings and twelve reliable rate constants were
obtained, six of which are shown in Fig. 5. The Eyring and
Arrhenius activation parameters calculated for the 1,2-
metallotropic shift are reported in Table 4.
4. Discussion
The activation parameters obtained for the 1,2-Re–N flux-
ional shift in the complexes [Re(CO)3(L–L)(pydz)][BF4]
(L–Lsbipy, bpym, phen, tmen or dppe) are reported in
Table 4. Examination of the magnitudes of the free energies

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E.W. Abel et al. / Inorganica Chimica Acta 255 (1997) 65–71
of activation, DG/ (298 K), reveals a dependence on the
chelate ligand, the trend being bpym)dppe)phen)
bipy4tmen. The same trend was observed in the analogous
platinum(IV) complexes, [PtMe3(L–L)(pydz)][BF4]
(L–Lsbipy, bpym, phen or tmen) [8], and may be ration-
alised in terms of a decrease in the M–N(pydz) interaction
concomitant with an increase in the metal–chelate ligand
interaction. Thus the observed trend in DG/ (298 K) would
be expected to be inversely related to the relative trans
influence of the chelate ligands; i.e. the magnitude of DG/
(298 K) would be expected to decrease as the trans influence
of the chelate ligand increases. It isnot possibletocorroborate
this hypothesis in the present complexes, since the relative
trans influences of the chelate ligands cannot be measured.
would be observed on going from a neutral species to a
cationic metal moiety. However, the effect of the positive
charge on the metal centre can be largely offset by a decrease
in the dp–pp backbonding to the carbonyls. In accord with
previously noted trends, the present ReI complexes have a
higher free energy of activation than the analogous PtIV com-
plexes [PtMe3(L–L)(pydz)][BF4] (L–Lsbipy, bpym,
phen or tmen) [8]. This presumably reflects the relative
strengths of the fluxional M–N(pydz) bonds.
5. Conclusions
The energetics of the Re–N 1,2-metallotropic shift in the
complexes [Re(CO)3(L–L)(pydz)][BF4] (L–Lsbipy,
bpym, phen or dppe) are sensitive to the nature of the chelate
ligand. This dependence may be rationalised in terms of
changes in the metal–chelate ligand bonding interactions,and
is in accord with the trends noted previouslyfortheanalogous
platinum(IV) complexes. The substantial decrease in the rel-
ative magnitude of DG/ (298 K) when tmen is employed
as the chelate ligand is presumed to result from large steric
interactions between the N-methyl groups and the fluxional
pyridazine ring.
2
In contrast, measurements of the J(Pt–CH3) scalar cou-
plings in the analogous PtIV complexes enables a qualitative
estimate of the relative trans influences of the chelate ligands
to be obtained [20,21]. It was found [8] that the 2J(PtCH3)
scalar couplings were in the order bpym)phen)bipy)
tmen, indicating that the trans influence increases from 2,29-
bipyrimidine to N,N,N9,N9-tetramethylethylenediamine.The
observed trend in DG/ (298 K) is also in accord with the
pKa values for the free proligands, pKa bpym-dppe-
phen-bipy-tmen. However, it should be noted that
pKa values are not always a reliable indicator of the relative
coordinating abilities of ligands [22].
Since the chelate ligands are cis to the fluxional Re–
N(pydz) bond, no strong dependence was anticipated, and
changes in DG/ (298 K) are generally small (D(DG/) ;3
kJ moly1 for L–Lsbipy, bpym, phen or dppe). However,
the substantial decrease in the observed free energy of acti-
vation for the 1,2-metallotropic shift in the tmen complex,
[Re(CO)3(pydz)][BF4], clearly cannot be rationalised
solely in terms of strong Re–N(tmen) interactions. It seems
likely that the considerable decrease in DG/ (298 K) stems
primarily from a destabilisation of the Re–N(pydz) bond, as
a result of steric interactions between the tmen N-methyl
groups and the fluxional pyridazine ring. The sizable entropy
of activation, DS/, for the complex [Re(CO)3(tmen)-
(pydz)][BF4] helps to support this hypothesis. An anom-
alously low magnitude for the free energy of activation was
also obtained for the analogous PtIV complex, [PtMe3-
(tmen)(pydz)][BF4] [8]. A similar lowering of the free
energy of activation might be expected for the bis(diphenyl-
phosphino)ethane complex, [Re(CO)3(dppe)(pydz)]-
[BF4], as a result of steric interactions between the dppe
phenyl rings and the pyridazine ligand. However, the planar
phenyl rings are able to orient in such a way as to minimise
such interactions; thus, in contrast to the tmen complex, the
observed magnitude of DG/ (298 K) is not dominated by
the steric requirements of the proligand.
Acknowledgements
The authors thank the EPSRC for use of the mass spec-
trometry service at the University of Wales, Swansea. The
University of Exeter is also gratefully acknowledged for a
studentship (to P.J.H.).
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