Study of the replacement of weak ligands on square-planar organometallic nickel(II) complexes. Organo-nickel aquacomplexes

Article abstract of DOI:10.1021/ic061933k

When trans-[NiRf₂L₂] (Rf = 3,5-C₆Cl ₂F₃; L = group 15 soft monodentate weak ligand such as SbPh₃ or AsPh₃) is dissolved in wet (CD₃) ₂CO, isomerization (to give cis-[NiRf₂L₂]) and subsequent substitutions of L by (CD₃)₂CO or by water occur, and several complexes containing acetone and aqua ligands are formed. The isomerization takes place in a few seconds at room temperature. The substitution reactions on the cis isomer formed are faster. The kinetics of the equilibria between all of the participating species have been studied by ¹⁹F exchange spectroscopy experiments at 217 K, and the exchange rates and rate constants have been calculated. These data reflect the weakness of acetone compared to water and AsPh₃. The data obtained are the first available for square-planar nickel(II) aquacomplexes. The bulkier AsCyPh₂ ligand slows down the exchange processes while the displacement of AsMePh₂ is clearly disfavored. Activation entropy studies support an associative ligand substitution. All of these data fit well with the previously reported relative activity of these complexes as catalysts in norbornene polymerization.

Full text of DOI:10.1021/ic061933k

Inorg. Chem. 2007, 46, 1027−1032
Study of the Replacement of Weak Ligands on Square-Planar
Organometallic Nickel(II) Complexes. Organo-Nickel Aquacomplexes
Juan A. Casares, Pablo Espinet,* Jesu´s M. Mart´ınez-Ilarduya, Jesu´s J. Mucientes, and Gorka Salas
Qu´ımica Inorga´nica, Facultad de Ciencias, UniVersidad de Valladolid, E-47011 Valladolid, Spain
Received October 10, 2006
When trans-[NiRf2L2] (Rf
) 3,5-C6Cl2F3; L ) group 15 soft monodentate weak ligand such as SbPh3 or AsPh3) is
dissolved in wet (CD3)2CO, isomerization (to give cis-[NiRf2L2]) and subsequent substitutions of L by (CD3)2CO or
by water occur, and several complexes containing acetone and aqua ligands are formed. The isomerization takes
place in a few seconds at room temperature. The substitution reactions on the cis isomer formed are faster. The
kinetics of the equilibria between all of the participating species have been studied by 19F exchange spectroscopy
experiments at 217 K, and the exchange rates and rate constants have been calculated. These data reflect the
weakness of acetone compared to water and AsPh3. The data obtained are the first available for square-planar
nickel(II) aquacomplexes. The bulkier AsCyPh2 ligand slows down the exchange processes while the displacement
of AsMePh2 is clearly disfavored. Activation entropy studies support an associative ligand substitution. All of these
data fit well with the previously reported relative activity of these complexes as catalysts in norbornene polymerization.
Introduction
behavior quite similar to that of the palladium complex is
observed for trans-[NiRf2(SbPh3)2] (1a), an excellent catalyst
for vinylic polymerization of norbornene.4 Square-planar
coordination in Ni(II) is favored in organometallic com-
plexes. On the other hand, organo-nickel aquacomplexes are
extremely uncommon due to the inherent high reactivity of
Ni-C bonds toward hydrolysis.5 For this reason, while water
and solvent exchange processes are well-studied for octa-
hedral complexes [Ni(solv)6]2+ (solv ) H2O, DMF, CH3-
CN),6,7 no data have been reported for nickel(II) square-
planar organometallic compounds. Moreover, no crystal
structures have been reported for organo-nickel aquacom-
plexes although the structures of some hydroxo-derivatives
have been determined.8
Aquacomplexes of group 10 elements are of importance
because of their involvement as catalysts or as intermediates
in a number of processes.1 The increasing interest in metal-
mediated synthesis in water adds relevance to these aqua-
complexes.2
In the course of our work on polyfluorophenyl com-
plexes of group 10 elements, we have synthesized several
complexes containing weakly coordinating neutral ligands
that can be replaced by water.3 In fact, the (CD3)2CO
solutions of cis-[MRf2(THF)2] (M ) Pd, Pt; Rf ) 3,5-
C6Cl2F3; THF ) tetrahydrofuran) consist of a mixture of cis-
[MRf2{OC(CD3)2}2], cis-[MRf2{OC(CD3)2}(OH2)], and cis-
[MRf2(OH2)2] in ratios that depend on the H2O content in
the deuterated solvent. The exchange between these com-
plexes is fast in the NMR time scale at room temperature
for M ) Pd and too slow to be detected for M ) Pt. A
In contrast to the high reactivity of conventional Ni-C
bonds, the Ni-Rf bonds are exceptionally inert toward
hydrolysis, which makes them perfectly suited substrates for
* To whom correspondence should be addressed. E-mail: espinet@
qi.uva.es.
(4) Casares, J. A.; Espinet, P.; Mart´ın-Alvarez, J. M.; Mart´ınez-Ilarduya,
J. M.; Salas, G. Eur. J. Inorg. Chem. 2005, 3825-3831.
(5) Koelle, U. Coord. Chem. ReV. 1994, 135/136, 623-650.
(6) (a) Ducommun, Y.; Newmann, K. E.; Merbach, A. E. Inorg. Chem.
1980, 19, 3696-3703. (b) Ducommun, Y.; Earl, W. L.; Merbach, A.
E. Inorg. Chem. 1979, 18, 2754-2758.
(7) (a) Wilkins, R. G. Kinetics and Mechanism of Reactions of Transition
Metal Complexes, 2nd ed.; VCH: Weinheim, Germany, 1991. (b)
Dunand, F. A.; Helm, L.; Merbach, A. E. AdV. Inorg. Chem. 2003,
54, 1-69. (c) Helm, L.; Merbach, A. E. Chem. ReV. 2005, 105, 1923-
1959.
(1) Vicente, J.; Arcas, A. Coord. Chem. ReV. 2005, 249, 1135-1154.
(2) For interest in water as the solvent for reactions see: (a) Aqueous-
Phase Organometallic Catalysis: Concepts and Applications, 2nd ed.;
Cornils, B., Herrmann, W. A., Eds.; Wiley-VCH: Weinheim,
Germany, 2004. (b) Shaughnessy, K. H.; DeVasher, R. B. Curr. Org.
Chem. 2005, 9, 585-604. (c) Leadbeater, N. E. Chem. Commun. 2005,
2881-2902.
(3) Espinet, P.; Mart´ınez-Ilarduya, J. M.; Pe´rez-Briso, C.; Casado, A. L.;
Alonso, M. A. J. Organomet. Chem. 1998, 551, 9-20.
10.1021/ic061933k CCC: $37.00
Published on Web 01/10/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 3, 2007 1027

Casares et al.
Scheme 1
the study of other reactions, particularly substitution reactions
involving water in square-planar Ni complexes. This prompted
us to study the equilibria and exchange rates between the
different species that coexist in solution when 1a or other
related trans-[NiRf2L2] complexes (L ) AsPh3, 2a; AsCyPh2,
3a; AsMePh2, 4a; PPh3, 5a) are dissolved in wet (CD3)2CO.
These are shown in Scheme 1.
19F EXSY (exchange spectroscopy) experiments were used
for the quantitative evaluation of the kinetics of these
equilibria, since two-dimensional (2D) EXSY is appropriate
to study multisite exchanges and dynamic processes that are
slow in the chemical shift time scale.9-11 Examples of the
application of EXSY in organometallic and transition metal
chemistry, often including their use to obtain quantitative
kinetic data, are available in the literature.12
Figure 1. 19F NMR spectra of a solution of trans-[NiRf2(SbPh3)2] (1a) in
wet (CD3)2CO at 293 K (a) and 217 K (b).
at 213 K from solid samples dissolved at low temperature
(only the trans isomer is present in the solid state), it can be
concluded that these solutions contain the trans isomer only,
at least in a detectable concentration. Related complexes with
aryl phosphines also show preference for trans stereochem-
istry, as found for [Ni(C6F5)2(PMePh2)2] and [Ni(4-C6BrF4)2
(PMePh2)2] in solution (CDCl3 or CD2Cl2) and in the solid
state.13-15 In contrast, only the cis isomers are obtained for
complexes involving less sterically demanding ligands,
such as [PPh3Me][Ni(C6F5)2Br(CO)], [Ni(C6F5)2(CO)2], and
[Ni(C6F5)2(THF)2].16
Behavior in Acetone-d6 Solution. The 19F NMR spectra
of 1a-3a in (CD3)2CO at 293 K show two sharp singlets
(2:1) and two broad signals also with relative integrals 2:1
(see Figure 1a for the representative case of 1a). The sharp
singlets are due to the Fortho (Fo) and Fpara (Fp) nuclei of the
starting material (complexes a). The broad signals were
resolved at low temperature showing the presence of several
complexes (see Figure 1b for 1a), which are gathered in
Table 1.
Results and Discussion
The complexes trans-[NiRf2L2] (1a-5a) are stable in the
solid state, but they slowly decompose in a solution of CDCl3
or (CD3)2CO at 298 K to give Rf-Rf, RfH, and small
amounts of other non-characterized products. The synthesis
and behavior as catalysts of these complexes have been
reported, as well as the X-ray structures of 1a, 2a, and 5a
that show a square-planar trans geometry for all of them.4
Since the room-temperature spectra of complexes 1a-5a
in the CDCl3 solution were the same as the spectra recorded
(8) (a) Carmona, E.; Mar´ın, J. M.; Palma, P.; Paneque, M.; Poveda, M.
L. Inorg. Chem. 1989, 28, 1895-1900. (b) Carmona, E.; Marin, J.
M.; Paneque, M.; Poveda, M. L. Organometallics 1987, 6, 1757-
1765. (c) Carmona, E.; Marin, J. M.; Palma, P.; Paneque, M.; Poveda,
M. L. Organometallics 1985, 4, 2053-2055. (d) Lo´pez, G.; Garc´ıa,
G.; Sa´nchez, G.; Garc´ıa, J.; Ruiz, J.; Hermoso, J. A.; Vegas, A.;
Martinez-Ripoll, M. Inorg. Chem. 1992, 31, 1518-1523.
The colors of the solutions are pale yellow, and they do
not show any variation with the temperature. In addition,
the 1H and 19F NMR chemical shifts observed for the
different species (similar to the ones found for Pd and Pt)3
are those expected for diamagnetic compounds. We do not
observe any indication for the presence of paramagnetic
species (as should be expected for tetrahedral or octahedral
complexes).
(9) Jeener, J.; Meier, B. H.; Bachmann, P.; Ernst, R. R. J. Chem. Phys.
1979, 71, 4546-4553.
(10) Perrin, C. L.; Gipe, R. K. J. Am. Chem. Soc. 1984, 106, 4036-4038.
(11) Perrin, C. L.; Dwyer, T. J. Chem. ReV. 1990, 90, 935-967.
(12) See, for instance, (a) Geldbach, T. J.; Ru¨egger, H.; Pregosin, P. S.
Magn. Reson. Chem. 2003, 41, 703-708. (b) Evans, D. R.; Huang,
M.; Seganish, W. M.; Chege, E. W.; Lam, Y.-F.; Fettinger, J. C.;
Williams, T. L. Inorg. Chem. 2002, 41, 2633-2641. (c) Frick, A.;
Schulz, V.; Huttner, G. Eur. J. Inorg. Chem. 2002, 3129-3147. (d)
Catellani, M.; Meali, C.; Motti, E.; Paoli, P.; Perez-Carren˜o, E.;
Pregosin, P. S. J. Am. Chem. Soc. 2002, 124, 4336-4346. (e) Cronin,
L.; Higgitt, C. L.; Perutz, R. N. Organometallics 2000, 19, 672-683.
(f) Alonso, M. A.; Casares, J. A.; Espinet, P.; Soulantica, K. Angew.
Chem., Int. Ed. 1999, 38, 533-535. (g) Valentini, M.; Selvakumar,
K.; Wo¨rle, M.; Pregosin, P. S. J. Organomet. Chem. 1999, 587, 244-
251. (h) Higgitt, C. L.; Klahn, A. H.; Moore, M. H.; Oelckers, B.;
Partridge, M. G.; Perutz, R. N. J. Chem. Soc., Dalton Trans. 1997,
1269-1280. (i) Wik, B. J.; Lersch, M.; Krivokapic, A.; Tilset, M. J.
Am. Chem. Soc. 2006, 128, 2682-2696.
(13) Rausch, M. D.; Chang, Y. F.; Gordon, H. B. Inorg. Chem. 1969, 8,
1355-1362.
(14) Churchill, M. R.; Veidis, M. V. J. Chem. Soc., Dalton Trans. 1972,
670-674.
(15) McDonald, R.; Sturge, K. C.; Hunter, A. D.; Shilliday, L. Organo-
metallics 1992, 11, 893-900.
(16) Fornie´s, J.; Mart´ın, A.; Menjo´n, B.; Kalamarides, H. A.; Rhodes, L.
F.; Day, C. S.; Day, V. W. Chem.sEur. J. 2002, 8 (21), 4925-4934.
1028 Inorganic Chemistry, Vol. 46, No. 3, 2007

Organo-Nickel Aquacomplexes
Table 1. 19F NMR Chemical Shifts (ppm) for the Observed Species in
(CD3)2CO at 217 K
complex
δFï (T1, s)
-86.57
δFp
trans-[NiRf2(SbPh3)2] (1a)
trans-[NiRf2(AsPh3)2] (2a)
cis-[NiRf2(AsPh3)2] (2b)
-119.05
-120.10
-120.90
-120.02,
-88.21 (0.45)
-90.25 (0.71)
-89.66 (0.78),
-91.51 (0.66)
-88.14
cis-[NiRf2(AsPh3){OC(CD3)2}] (2c)
-121.34
trans-[NiRf2(AsCyPh2)2] (3a)
cis-[NiRf2(AsCyPh2)2] (3b)
cis-[NiRf2(AsCyPh2){OC(CD3)2}] (3c) -88.68,
-119.96
-119.43
-120.22,
-88.35
-91.58
-121.12
trans-[NiRf2(AsMePh2)2] (4a)
cis-[NiRf2(AsMePh2)2] (4b)
trans-[NiRf2(PPh3)2] (5a)
cis-[NiRf2{OC(CD3) 2}2] (6)
cis-[NiRf2{OC(CD3)2}(OH2)] (7)
-88.92
-119.90
-121.42
-120.99
-120.30
-120.49,
-89.81
-87.60
-90.97 (0.71)
-89.84 (0.75),
-90.03 (0.75)
-89.20 (0.78)
-120.69
-120.96
cis-[NiRf2(OH2)2] (8)
We could assign some of the signals to cis-[NiRf2-
{OC(CD3)2}2] (6), cis-[NiRf2{OC(CD3)2}(OH2)] (7), and cis-
[MRf2(OH2)2] (8), taking into account that their signals
appeared for the three starting complexes 1a-3a and their
intensities were sensitive to the amount of water in solution.17
The presence of uncoordinated ligand L was detected in
each case by 13C{1H} NMR which confirmed a high
degree of dissociation in all cases (98% for 1a, 71% for 2a,
48% for 3a).
Figure 2. 19F EXSY experiment in the Fo region for complex 1a in wet
(CD3)2CO at 217 K (tm ) 0.2 s) and scheme of the exchange processes
for 6/7/8.
The assignment of the complexes detected in solution was
made as follows: The resonances of coordinated H2O were
19F EXSY Experiments for 1a-3a. When trans-
[NiRf2(SbPh3)2] (1a) is dissolved in (CD3)2CO, almost all
of the complex is transformed into 6, 7, 8, and uncoordinated
SbPh3. Only 2% remains as 1a. The 19F EXSY experiment
at 217 K shows that 1a does not participate in the exchange
of 6-8 at this temperature (Figure 2). Therefore, we can
analyze the latter, neglecting the presence of 1a. In order to
carry out this study, several phase-sensitive 19F EXSY
experiments were recorded in the Fo region at 217 K, with
different mixing times (tm ) 0-0.3 s). The contour map of
the spectrum obtained with a mixing time of 0.2 s is shown
in Figure 2. The four resonances appearing along the diagonal
at -89.2 (singlet), -89.8 and -90.0 (AB spin system), and
-90.9 (singlet) correspond to 8, 7, and 6, respectively. The
stronger cross-peaks correlate with the single-step conversion
of 8/7 and 7/6. Lower intensity cross-peaks are due to the
exchange between 8 and 6. Since a simultaneous double
substitution reaction is extremely unlikely, the latter exchange
must proceed through 7 in a two-step process. In concordance
with this, these cross-peaks do not appear in the spectra
registered with very short mixing times, which is a commonly
observed EXSY effect in multistep reactions.20
1
found in the H NMR spectra at 5.32 and 5.34 ppm for 7
and 8, respectively. The stereochemical assignment of a cis
geometry for 7 was straightforward considering the inequiva-
lence of the fluoroaryl rings and the strong coupling between
the Fo of the two aryls.18,19 Assignment of the cis geometry
to 6 and 8 was also based on the exchanges observed between
6, 7, and 8 by 19F EXSY experiments (see below) and the
preference of the NiRf2 moiety to form cis complexes with
hard ligands.16
The trans isomer of [NiRf2(AsMePh2)2] (4a) is in equi-
librium with a small amount of the cis isomer (4b) in
(CD3)2CO at 217 K. In the 19F NMR spectrum, the two
fluoroaryl rings of 4b are equivalent, and the ratio 4a/4b
did not change when small amounts of AsMePh2 or H2O
were added to the solution. This discarded other formulations
for 4b such as [NiRf2(AsMePh2)L] (L ) (CD3)2CO, H2O).
No exchange, isomerization, or substitution processes were
detected for trans-[NiRf2(PPh3)2] (5a) in (CD3)2CO.
(17) The formation of 7 and 8 is a consequence of the presence of small
amounts of water in the commercial (CD3)2CO used to prepare the
NMR samples, which implies the presence of DHO and D2O in
equilibrium. Since Ni complexes with coordinated H2O, DHO, or D2O
are not differentiated in the 19F NMR, we consider them in practice
as one compound. The same applies to (CD3)(CHD2)CO molecules
in the solvent. See also Experimental Section.
When trans-[NiRf2(AsPh3)2] (2a) is dissolved in
(CD3)2CO, the 19F NMR spectrum at 217 K shows three Fo
and three Fp signals in addition to those due to 2a and 6-8.
These three new signals for each kind of F (ortho or para)
are due to two new products 2b (one signal) and 2c (two
signals with the same intensity). Altogether, 71% of AsPh3
is dissociated. The addition of H2O shifts the equilibria
(18) Inter-aryl (including fairly large through-space) F-F couplings between
Fo nuclei belonging to different fluoroaryl groups in the PdR2 fragments
have been reported: (a) Albe´niz, A. C.; Casado, A. L.; Espinet, P.
Organometallics 1997, 16, 5416-5423. (b) Alonso, M. A.; Casares,
J. A.; Espinet, P.; Mart´ınez-Ilarduya, J. M.; Pe´rez-Briso, C. Eur. J.
Inorg. Chem. 1998, 1745-1753. (c) De la Cruz, R.; Espinet, P.;
Gallego, A. M.; Mart´ın-Alvarez, J. M.; Mart´ınez-Ilarduya, J. M. J.
Organomet. Chem. 2002, 663, 108-177.
(20) Heise, J. D.; Raftery, D.; Breedlove, B. K.; Washington, J.; Kubiak,
(19) Casares, J. A.; Espinet, P. Inorg. Chem. 1997, 36, 5428-5431.
C. P. Organometallics 1998, 17, 4461-4468.
Inorganic Chemistry, Vol. 46, No. 3, 2007 1029

Casares et al.
completely to the formation of 7 and 8 while the addition of
AsPh3 shifts the equilibria toward 2a and 2b. Taking this
into account, we assigned the above-mentioned signals to
the following products: cis-[NiRf2(AsPh3)2] (2b) with two
equivalent fluoroaryl groups and cis-[NiRf2(AsPh3)-
{OC(CD3)2}] (2c) with two inequivalent fluoroaryl groups
and strong coupling between the Fo of the two aryls.
As observed for 1a, the exchange between 2a and any
other complex is slow in the NMR time scale, but substitu-
tions on its isomer 2b take place easily (Figure 3). Appar-
ently, the high trans effect of the haloaryl groups provides
an efficient pathway for ligand substitution, and AsPh3 is
partially displaced by water or acetone.
Further support for the assignment of equilibria among
2b, 2c, and 6-8 came from the 19F EXSY spectra. With a
short tm (0.15 s, Figure 3a), chemical exchange cross-peaks
correlating 2c signals with 2b and 6 (corresponding to
acetone-AsPh3 and AsPh3-acetone substitution processes,
respectively) were observed. The cross-peaks due to AsPh3-
water exchange (2c-7) were not observable, indicating a
very slow reaction. Using a longer tm (0.45 s, Figure 3b),
we obtained additional cross-peaks which correlate signals
that exchange at a slower rate. Note that the new signals
include the exchange of complexes involving the substitution
of two ligands and also the exchange of the two nonequiva-
lent fluoroaryl groups in complex 2c.
The species observed when trans-[NiRf2(AsCyPh2)2] (3a)
is dissolved in (CD3)2CO are similar to those observed for
trans-[NiRf2(AsPh3)2] (2a). Altogether, less dissociated As-
CyPh2 is produced (48%) than AsPh3 (71%) for the case of
2a, although the first substitution of L by (CD3)2CO in the
cis isomer is more favored for 3b than for 2b (see K3 values
in Table 2). The arsine substitution reactions (3b-3c and
3c-6 or 7) are slower, and consequently no cross-peaks
correlating these complexes were observed in the EXSY
spectrum at 217 K, even at very large mixing times
(Figure 4).
Considering the equilibria depicted in Figure 3 and the
molar concentration of the different species detected in
acetone solutions of 1a, 2a, and 3a, we can calculate the
equilibrium constants K1-K5 at 217 K (Table 2).
Rate Constant Calculations. The rate constants at 217
K for the dynamic exchange processes observed in solutions
of 1a and 2a in commercial (CD3)2CO and in (CD3)2CO/
H2O mixtures have been obtained (see Experimental Section)
and are gathered in Table 2. The values obtained for 1a in
solutions with different amounts of water showed that the
exchange rate k7,8 is first order in free water (k7,8 ) k-1 [free
water]). We assume that the dependence of k6,7 on free water
is the same, although this point cannot be experimentally
supported because the signals due to 6 were not observable
after the addition of small amounts of water. The larger k1,
k-1, k2, and k-2 values reported for 2a compared to those
reported for 1a could arise in part from small differences in
temperature and mostly from the limits of precision in the
determination of the water concentration in commercial
(CD3)2CO.
Figure 3. 19F EXSY experiments in the Fo region of a solution of complex
2a in wet (CD3)2CO at 217 K ((a) tm ) 0.15 s, (b) tm ) 0.45 s) and scheme
of the exchange processes between the different species formed. Arrows in
(b) indicate the cross-peaks due to indirect exchanges (2b-6, 2c-7, 6-8,
and the intramolecular exchange of the two nonequivalent fluoroaryls groups
in 2c).
The rate constant for the substitution of acetone by water
in 7 to give 8 (k-1) is two orders of magnitude higher than
that for the substitution of water by acetone in 7 to give 6
(k2/[acetone] ) 0.074 M-1 s-1). This reflects the higher
lability of the coordinated acetone as compared to water. A
1030 Inorganic Chemistry, Vol. 46, No. 3, 2007

Organo-Nickel Aquacomplexes
Table 2. Thermodynamic and Kinetic Data at 217 K for the Dynamic
Exchange Processes Observed when 1a-3a Were Dissolved in
(CD3)2CO (Errors in Parentheses)
The complexes reported here afford the only study
available to date on square-planar Ni(II) aquacomplexes.
However, the substitution rates observed should be used with
some caution when comparing to other systems, as the high
trans effect of the Rf groups is probably reducing the
activation energy toward associative ligand substitution,
compared to the classical inorganic ligands used in Ni(II)
octahedral complexes (these Rf ligands were, however,
needed to induce a square-planar coordination in Ni(II)). In
this respect, it is interesting to note that the fast ligand
substitutions take place on the cis isomers (where the leaving
ligand is trans to Rf) and not on the initial trans isomers.
equilibrium constantsa
rate constantsb
calculated from calculated from
concentration
data
the rate constants
for 2a
1ac
2ad
K1 0.45 (0.12)
0.30 (0.04)
k1
3.7 (0.8) 5.3 (0.4)
k-1 7.9 (0.6) 17.9 (0.9)
K2 0.0106 (0.0007) 0.0081 (0.0006)
k2
0.9
1.08 (0.06)
133 (3)
k-2 70.6
K3 99 (4) (2a)
382 (15) (3a)
K4 26.4 (1.1) (2a)
8.0 (0.4) (3a)
140 (50)
40 (11)
k3
k-3
k4
149 (4)
1.1 (0.4)
31 (3)
k-4
0.77 (0.14)
The use of a better donor and less labile ligand than
AsPh3 (AsCyPh2) slows the substitution processes down to
the point that they cannot be detected experimentally in the
same conditions. For similar electronic characteristics,
displacement of a less bulky ligand (AsMePh2) is clearly
disfavored.
K5 2.00 (0.04) (2a)
19.0 (0.4) (3a)
a Equilibrium constants with units in parentheses: K1 (M) ) [7][H2O]/
[8] ) k1/k-1, K2 (M) ) [6][H2O]/[7] ) k2/k-2, K3 (M-1) ) [c]/[6][L] )
k3/k-3, K4 (M-1) ) [b]/[c][L] ) k4/k-4, K5 ) [a]/[b]. b Constants k1, k2,
k-3, and k-4 in s-1; k-1, k-2, k3, and k4 in M-1 s-1 c Obtained from EXSY
.
experiments (tm ) 0.15 s, [1a]0 ) 5.15 mM) carried out with different free
water concentrations (47.1, 431.6, and 819.1 mM). d Obtained from eight
EXSY experiments (tm ) 0.04-0.21 s); [2a]0 ) 11.02 mM, [free water] )
43.56 mM; [AsPh3] ) 13.66 mM. Free water concentration was calculated
summing up [H2O] + [DHO] + [D2O] and subtracting the overall
coordinated water for each sample.
All of these data fit well with the behavior observed for
these complexes as catalysts in the insertion polymerization
of norbornene. A prerequisite for insertion polymerization
is efficient ligand substitution by an incoming norbornene,
which then coordinates by its double bond and undergoes
insertion.4 The yield and molecular mass of the polymers
obtained with these catalysts show a strong dependence on
the ligand used and the solvent. High yield and high
molecular mass values were obtained with the ligands shown
here which were easier to displace. Furthermore, solvents
with hard donor atoms which, as shown here, give more
stable complexes were detrimental for polymerization.
Experimental Section
General Methods. All reactions were carried out under N2.
Solvents were distilled using standard methods. The nickel com-
pounds were prepared by published methods.4 1H (300.13 MHz),
19F (282.40 MHz), and 13C{1H} (75.47 MHz) NMR spectra were
recorded on Bruker ARX 300 and AC 300 instruments equipped
with a VT-100 variable-temperature unit. The actual temperature
was measured before spectral accumulation, using Bruker standard
samples of methanol in methanol-d4 for low temperatures and
ethylene glycol in DMDO-d6 (80/20) for high temperatures and
using the equations provided by the manufacturer. Chemical shifts
are reported in parts per million from SiMe4 (1H, 13C{1H}) or CCl3F
(19F) with positive shifts downfield, at an ambient probe temperature
unless otherwise stated. J values are given in hertz. 19F EXSY
experiments were carried out with a standard nuclear Overhauser
effect spectroscopy (NOESY) program operating in phase sensitive
mode, with a 5% random variation of the evolution time to avoid
correlation spectroscopy (COSY) cross-peaks. T1 values were
obtained using a standard inversion recovery sequence.
Figure 4. 19F EXSY experiment in the Fï region for complex 3a in wet
(CD3)2CO at 217 K (tm ) 0.5 s). The cross-peaks only correlate signals of
6, 7, and 8.
similar result is observed when we compare the substitution
of acetone by AsPh3 and vice versa.
A deeper insight into the mechanism of these substitution
reactions was obtained for the reaction 7 + (CD3)2CO f 6
+ H2O. Its activation parameters were obtained using
line shape analysis for 1a to determine the values of k2 at
different temperatures. The values obtained were ∆Hq ) 39.1
( 0.7 kJ mol-1 and ∆Sq ) -64 ( 3 J mol-1 K-1. The
negative value of ∆Sq is indicative of an associative
substitution.21
(CD3)2CO was used as received, and the total water content in
it ([H2O] + [HDO] + [D2O]) was calculated from the integrated
2
area values of H2O (2.84 ppm) and HDO (2.80 ppm, JH-D ) 1.1
Hz) in the 1H NMR spectra of a commercial sample and using the
equilibrium constant Keq ) 3.81 for the self-exchange reaction H2O
+ D2O ) 2HDO.22
(21) Monlien, F. J.; Helm, L.; Abou-Hamdan, A.; Merbach, A. E. Inorg.
Chem. 2002, 41, 1717-1727 and references therein.
(22) Chen, C. L.; Bopp, P.; Wolfsberg, M. J. Chem. Phys. 1982, 77, 579-
580.
Inorganic Chemistry, Vol. 46, No. 3, 2007 1031

Casares et al.
The mathematical analysis to extract rate constants from 2D-
EXSY experiments has been carefully summarized in the review
by Perrin and Dwyer.11 For a multispin system undergoing chemical
exchange, the peak intensities are given by eq 1, where I is the
matrix of cross-peak intensities Iij, M° is the matrix of equilibrium
magnetizations of the nulei, R is the relaxation matrix, and tm is
the mixing time used for acquisition of the spectrum.
The computer simulation of the variable temperature NMR
spectrum of a solution of 1a in (CD3)2CO was carried out by line
shape analysis using the standard program gNMR.23
Acknowledgment. Financial support by the Ministerio
de Ciencia y Tecnolog´ıa (Project No. CTQ2004-07667), the
Junta de Castilla y Leo´n (Project No. VA060/04), and
Consolider Ingenio 2010 (Grant CSD2006-0003) is very
gratefully acknowledged.
I ) M° exp(-Rtm)
(1)
Supporting Information Available: Percentages of the differ-
ent species which arise from the solution of trans-[NiRf2L2] in
(CD3)2CO at 217 K in different conditions, thermodynamic and
kinetic data (exchange rates and rate constants) at 217 K for the
dynamic exchange processes observed when 1a-3a were dissolved
in (CD3)2CO, rate constants (k2) for the reaction 7 + (CD3)2CO f
6 + H2O obtained by line shape analysis, analysis section,
microprogram (Matlab) for rate calculations. This material is
available free of charge via the Internet at http://pubs.acs.org.
The matrix I for each spectrum was obtained with the values of
peak volumes Iij. The experimental values at both sides of the
diagonal (Iij(exp) and Iji(exp)), which were integrated with the
standard Bruker software, were not symmetrized. The matrix M°
was constructed with the integrated values of the diagonal peak
volumes at tm ) 5 µs.
For complex 7, we have considered jointly the AB system
because this simplifies the spin problem (from a four site to a
three site exchange system for the exchange 6/7/8) and also
eliminates the errors introduced from J cross-peaks (which has to
be taken into account particularly if ∆ν < 20J) and from cross-
relaxation.11,12e
IC061933K
(23) gNMR, version 3.6 for Macintosh; IvorySoft, Cherwell Scientific
Publishing, Ltd.: Oxford, U.K., 1995.
1032 Inorganic Chemistry, Vol. 46, No. 3, 2007