Synthesis and Characterization of Palladium(II) Complexes with New Polydentate Nitrogen Ligands. Dynamic Behavior Involving Pd-N Bond Rupture. X-ray Molecular Structure of [{Pd(η3-C4H7)} 2(Me-BPzTO)](4-MeC6H4SO3) [Me-BPzTO = 4,6-Bis(4-methylpyrazol-1-yl)-1,3,5-triazin-2-olate]

Article abstract of DOI:10.1021/ic980307n

The ligands 2,4,6-tris(4-methylpyrazol-1-yl)-1,3,5-triazine (Me-TPzT), 2,4,6-tris(4-bromopyrazol-1-yl)-1,3,5-triazine (Br-TPzT), and 2-methoxy-4,6-bis(4-methylpyrazol-1-yl)-1,3,5-triazine (Me-BPzTOMe) have been synthesized and their reactions with some palladium derivatives explored. The palladium fragment [Pd(η³-2-Me-C₃H₄)(S)₂] ⁺, S = acetone, reacts in acetone with Me-TPzT or Br-TPzT in a 3:1 molar ratio to generate new complexes in which two allylpalladium fragments are present and the TPzT ligands have been partially hydrolyzed: [{Pd(η³-C₄H₇)}₂(X-BPzTO)]A, X-BPzTO = 4,6-bis[4-methyl(or bromo)pyrazol-1-yl]-1,3,5-triazin-2-olate (X = Me, A = BF₄, 1; A = PF₆, 2; A = CF₃SO₃, 3; A = p-MeC₆H₄SO₃, 4; X = Br, A = CF₃SO₃, 5). When the ligand Me-BPzTOMe is made to react with only 1 equiv of the palladium solvate, compound 6, [Pd(η³-2-Me-C₃H₄)(Me-BPzTOMe)]CF ₃SO₃, is isolated. Reaction of 6 with another 1 equiv of the palladium derivative leads to 3. The intermediate 7, [{Pd(η³-2-Me-C₃H₄)} ₂(Me-BPzTOMe)]CF₃SO₃, has been isolated as an almost pure compound. The reaction of Me-BPzTOMe with 1 equiv of [Pd(C₆F₅)₂(cod)] (cod = 1,5-cyclooctadiene) leads to the complex [Pd((C₆F₅)₂(Me-BPzTOMe)], 8. Attention has been focused on the dynamic behavior, related with metallotropic phenomena, of the new complexes. ¹H NMR variable-temperature studies of complexes 1, 6, and 8 have been carried out. For 8, only one static species is observed, while, for 1 and 6, two isomers are detected at low temperature. Different ΔG_C^? activation energies at the coalescence temperature have been determined and are ascribed to processes implying Pd-N bond ruptures. For 6, two different barriers are detected, corresponding to Pd-N(triazine) or Pd-N(pyrazole) bond ruptures. From the ΔG_C^? data, it is concluded that the main driving force of the hydrolysis process is the formation of a better coordinating ligand. The molecular structure of 4 has been determined by X-ray diffraction. The meso isomer, in which the two C-Me axes of the allylic groups are oriented in the same direction, is found in the solid state.

Full text of DOI:10.1021/ic980307n

6606
Inorg. Chem. 1998, 37, 6606-6614
Synthesis and Characterization of Palladium(II) Complexes with New Polydentate Nitrogen
Ligands. Dynamic Behavior Involving Pd-N Bond Rupture. X-ray Molecular Structure of
[{Pd(η3-C4H7)}2(Me-BPzTO)](4-MeC6H4SO3) [Me-BPzTO )
4,6-Bis(4-methylpyrazol-1-yl)-1,3,5-triazin-2-olate]
Felipe Go´mez-de la Torre, Antonio de la Hoz, Fe´lix A. Jalo´n,* Blanca R. Manzano,*,†
Antonio Otero, Ana M. Rodr´ıguez, and M. Carmen Rodr´ıguez-Pe´rez
Departamento de Qu´ımica Inorga´nica, Orga´nica y Bioqu´ımica, Facultad de Qu´ımicas, Campus
Universitario, Universidad de Castilla-La Mancha, 13071 Ciudad Real, Spain
Aurea Echevarr´ıa and Jose´ Elguero
Instituto de Qu´ımica Me´dica, CSIC, Juan de la Cierva, 3, 28006-Madrid, Spain
ReceiVed March 19, 1998
The ligands 2,4,6-tris(4-methylpyrazol-1-yl)-1,3,5-triazine (Me-TPzT), 2,4,6-tris(4-bromopyrazol-1-yl)-1,3,5-triazine
(Br-TPzT), and 2-methoxy-4,6-bis(4-methylpyrazol-1-yl)-1,3,5-triazine (Me-BPzTOMe) have been synthesized
and their reactions with some palladium derivatives explored. The palladium fragment [Pd(η3-2-Me-C3H4)(S)2]+,
S ) acetone, reacts in acetone with Me-TPzT or Br-TPzT in a 3:1 molar ratio to generate new complexes in
which two allylpalladium fragments are present and the TPzT ligands have been partially hydrolyzed: [{Pd(η3-
C4H7)}2(X-BPzTO)]A, X-BPzTO ) 4,6-bis[4-methyl(or bromo)pyrazol-1-yl]-1,3,5-triazin-2-olate (X ) Me, A
) BF4, 1; A ) PF6, 2; A ) CF3SO3, 3; A ) p-MeC6H4SO3, 4; X ) Br, A ) CF3SO3, 5). When the ligand
Me-BPzTOMe is made to react with only 1 equiv of the palladium solvate, compound 6, [Pd(η3-2-Me-C3H4)(Me-
BPzTOMe)]CF3SO3, is isolated. Reaction of 6 with another 1 equiv of the palladium derivative leads to 3. The
intermediate 7, [{Pd(η3-2-Me-C3H4)}2(Me-BPzTOMe)]CF3SO3, has been isolated as an almost pure compound.
The reaction of Me-BPzTOMe with 1 equiv of [Pd(C6F5)2(cod)] (cod ) 1,5-cyclooctadiene) leads to the complex
[Pd(C6F5)2(Me-BPzTOMe)], 8. Attention has been focused on the dynamic behavior, related with metallotropic
1
phenomena, of the new complexes. H NMR variable-temperature studies of complexes 1, 6, and 8 have been
carried out. For 8, only one static species is observed, while, for 1 and 6, two isomers are detected at low
q
temperature. Different ∆Gc activation energies at the coalescence temperature have been determined and are
ascribed to processes implying Pd-N bond ruptures. For 6, two different barriers are detected, corresponding to
q
Pd-N(triazine) or Pd-N(pyrazole) bond ruptures. From the ∆Gc data, it is concluded that the main driving
force of the hydrolysis process is the formation of a better coordinating ligand. The molecular structure of 4 has
been determined by X-ray diffraction. The meso isomer, in which the two C-Me axes of the allylic groups are
oriented in the same direction, is found in the solid state.
Introduction
In particular, recent contributions4 in the field of alkene
polymerization and copolymerization using Pd and Ni com-
plexes with N-donor ligands should be emphasized.
Polydentate nitrogen-donor ligands with sp2-hybridized ni-
trogens are widely used, especially polypyrazolylborates5 and
polypyridines.5b,6 Other aromatic polydentate N-donor ligands
such as TPT7 (2,4,6-tris-2-pyridyl-1,3,5-triazine) have also
In a coordination complex the electron-donor ability of the
ligand is considered to be an essential aspect in its stabilization
and reactivity modulation.1 In this context, ligands such as
phosphines, which allow a high versatility in donor ability and
steric hindrance, have frequently been used in complex forma-
tion with transition metals of the last periodic series.2 N-donor
ligands that show less studied structural and reactivity patterns
are receiving increased attention with these types of metals.3
(4) (a) Brookhart, M.; Rix, F. C.; De Simone, J. M. J. Am. Chem. Soc.
1992, 114, 5894. (b) Brookhart, M.; Wagner, M. I. J. Am. Chem. Soc.
1994, 116, 3641. (c) Johnson, L. K.; Killian, C. M.; Brookhart, M. J.
Am. Chem. Soc. 1995, 117, 6414. (d) Johnson, L. K.; Macking, S.;
Brookhart, M. J. Am. Chem. Soc. 1996, 118, 267. (e) Rix, F. C.;
Brookhart, M.; White, P. S. J. Am. Chem. Soc. 1996, 118, 4746. (f)
Brookhart, M.; Wagner, M. I. J. Am. Chem. Soc. 1996, 118, 7219. (g)
Killian, C. M.; Tempel, D. J.; Johnson, L. K.; Brookhart, M. J. Am.
Chem. Soc. 1996, 118, 11664. (h) Milani, B.; Anzilutti, A.; Vicentini,
L.; Sessanta, A. S.; Zangrando, E.; Geremia, S.; Mestroni, G.
Organometallics 1997, 16, 5064.
† E-mail: bmanzano@qino-cr.uclm.es.
(1) ComprehensiVe Coordination Chemistry; Wilkinson, G., Gillard, R.
D., McCleverty, J. A., Eds.; Pergamon: Oxford, U.K., 1987; Vol. 2.
(2) (a) McAuliffe, C. A.; Levason, W. Phosphine, Arsine and Stibine
Complexes of the Transition Elements; Elsevier: Amsterdam, 1979.
(b) Alyea, E. C. Catalytic Aspect of Metal Phosphine Complexes. AdV.
Chem. Ser. 1982, No. 196. (c) Homogeneous Catalysis with Metal
Phosphine Complexes; Pignolet, L. H., Ed.; Plenum: New York, 1983.
(3) Togni, A.; Venanzi, L. M. Angew. Chem., Int. Ed. Engl. 1994, 33,
497 and references therein.
(5) (a) Trofimenko, S. Chem. ReV. 1993, 93, 943. (b) Ward, M. D. Annu.
Rep. Prog. Chem. Inorg. Chem. Sect. A 1994, 91, 317.
10.1021/ic980307n CCC: $15.00 © 1998 American Chemical Society
Published on Web 12/01/1998

Pd(II) Complexes with Polydentate Nitrogen Ligands
Inorganic Chemistry, Vol. 37, No. 26, 1998 6607
received considerable attention, especially in complex formation
for charge-transfer studies and related properties. However,
pyrazolyl-substituted triazines8 have been explored to a lesser
extent.
Scheme 1
A relevant aspect in the reactivity of transition metal
complexes is the effect that the metal may induce on organic
molecules in such a way that a better ligand which is stabilized
after its coordination is formed. The template-mediated mac-
rocyclic synthesis9 is a classical example but numerous cases
related with this property of metals may be quoted.1,10 Recently,
several of us have reported11 that the [(η3-2-Me-C3H4)Pd]+
cation is able to stabilize the PO2F2- anion, an intermediate in
the hydrolysis of PF6-, by means of its coordination to the
palladium center. In this paper we describe work that demon-
strates that the [Pd(η3-2-Me-C3H4)(S)2]+ solvate induces the
hydrolysis of 1,3,5-triazine derivatives in such a way that these
triazines are converted into ligands with better coordinating
ability. The ligands used are pyrazole derivatives and contain
several asymmetric chelating coordination sites. These ligands
are 2,4,6-tris(4-methylpyrazol-1-yl)-1,3,5-triazine (Me-TPzT),8b
2,4,6-tris(4-bromopyrazol-1-yl)-1,3,5-triazine (Br-TPzT) (see
Scheme 1), and 2-methoxy-4,6-bis(4-methylpyrazol-1-yl)-1,3,5-
triazine (Me-BPzTOMe) (see Scheme 2). Considering the less
basic character of the pyrazolyl fragments as compared with
those of pyridine derivatives, the ligands used must lead, in
principle, to a more facile fluxional behavior than that observed
for TPT complexes. We have studied12 the dynamic behavior
of allylpalladium complexes, some of which contain chelate
pyrazole-derived ligands, and we have shown the existence of
Pd-N bond rupture, observable in the NMR time scale, that
takes place preferentially at the less basic nitrogen atoms. In
this paper, we report interesting examples in which, for a single
compound, two simultaneous fluxional processes are detected
with different energy barriers. We have also found that the
difference lies in the Pd-N bond strength.
(6) (a) Juris, A.; Barigelletti, S.; Campagna, S.; Balzani, V.; Belser, P.;
von Zelewsky, A. Coord. Chem. ReV. 1988, 27. Some recent papers
with terpyridine as a ligand in Pd or Pt complexes are: (b) Kvam, P.
I.; Puzyk, M. V.; Cotlyr, V. S.; Balashev, K. P.; Songstad, J. Acta
Chem. Scand. 1995, 29, 645. (c) Bailey, J. A.; Hill, M. G.; Marsh, R.
E.; Miskowski, V. M.; Schaefer, W. P.; Gray, H. B. Inorg. Chem.
1995, 34, 4591. (d) Hill, M. G.; Bayley, J. A.; Miskowski, V. M.;
Gray H. B. Inorg. Chem. 1996, 35, 4585. (e) Kvam, P. I.; Engebretsen,
T.; Maartmannmoe, K.; Songstad, J. Acta Chem. Scand. 1996, 50,
107. (f) Kvam, P. I.; Puzyk, M. V.; Cotlyr, V. S.; Songstad, J.;
Balashev, K. P. Acta Chem. Scand. 1996, 50, 6. (g) Abel, E. W.;
Gelling, A.; Orrell, K. G.; Osborne, A. G.; Sik, V. J. Chem. Soc.,
Chem. Commun. 1996, 2329. (h) Abel, E. W.; Orrell, K. G.; Osborne,
A. G.; Pain, H. M.; Sik, V.; Husthouse, M. B.; Malik, K. M. A. J.
Chem. Soc., Chem. Commun. 1996, 253. (i) Abel, E. W.; Dimitrov,
V. S.; Long, N. J.; Orrell, K. G.; Osborne, A. G.; Sik, V.; Hursthouse,
M. B.; Mazid, A. A. J. Chem. Soc., Chem. Commun. 1993, 291. (j)
Schiavo, S. L.; Tresoldi, G.; Mezzasahna, A. M. Inorg. Chim. Acta
1997, 36, 251. (k) Yang, L.; Wimmer, F. L.; Wimmer, S., Zhao, J.
X.; Braterman, P. S. J. Organomet. Chem. 1996, 525, 1.
Experimental Section
(7) (a) Thomas, N. T.; Foley, B. L.; Rheingold, A. L. Inorg. Chem. 1988,
27, 3426. (b) Faus J.; Julve, M.; Amigo´, J. M.; Debaerdemaeker, T.
J. Chem. Soc., Dalton Trans. 1989, 1681. (c) Folgado, J.-V.; Henke,
W.; Allmann, R.; Stratemeier, H.; Beltra´n-Porter, T.; Reinen, D. Inorg.
Chem. 1990, 29, 2035. (d) Chirayil, S.; Hedge, V.; Jahng, Y.;
Thummel, R. P. Inorg. Chem. 1991, 30, 2821. (e) Gupta, N.; Grover,
N.; Neyhart, G. A.; Singh, P.; Thorp, H. Inorg. Chem. 1993, 32, 310.
(f) Granifo, J. Polyhedron 1995, 14, 1593. (g) Berger, R. M.;
Holcombe, J. R. Inorg. Chim. Acta 1995, 232, 217. (h) Berger, R.
M.; Ellis, D. D., II. Inorg. Chim. Acta 1996, 241, 1. (i) Granifo, J.
Polyhedron 1996, 15, 203. (j) Byers, P.; Chan, G. Y. S.; Drew, G. B.;
Hudson, M. J.; Madic, C. Polyhedron 1996, 15, 2845. (k) Chan, G.
Y. S.; Drew, G. B.; Hudson, M. J.; Isaacs, N. S.; Byers, P.; Madic, C.
Polyhedron 1996, 15, 3385.
(8) (a) Reimlinger, H.; Noels, A.; Jadot, J.; van Overstraeten, A. Chem.
Ber. 1979, 103, 1954. (b) Echevarr´ıa, A.; Elguero, J.; Llamas-Saiz,
A. L.; Foces-Foces, C.; Schultz, G.; Hargittai, I. Struct. Chem. 1994,
5, 255. (c) Yang, C. Y.; Chen, X.-M.; Zhang, W.-H.; Chen, J.; Yang,
Y.-S.; Gong, M.-L. J. Chem. Soc., Dalton Trans. 1996, 1767. (d)
Gelling, A.; Orrell, K. G.; Osborne, A. G.; Sik, V. J. Chem. Soc.,
Dalton Trans. 1996, 3371.
General Comments. All manipulations were carried out under an
atmosphere of dry oxygen-free nitrogen using standard Schlenk
techniques. Solvents were distilled from the appropriate drying agents
and degassed before use. [(η3-2-Me-C3H4)Pd(µ-Cl)]213 and [Pd(C6F5)2-
(cod)]14 were prepared as described in the literature. Elemental analyses
were performed with a Perkin-Elmer 2400 microanalyzer. IR spectra
were recorded as KBr pellets with a Perkin-Elmer PE 883 IR
spectrometer. Mass spectra: VG Autospec instrument with FAB
technique and nitrobenzyl alcohol as matrix. 1H and 13C NMR spectra
were recorded on a Varian UNITY 300 spectrometer. Chemical shifts
(ppm) are given relative to TMS. COSY spectra: standard pulse
sequence with an acquisition time of 0.214 s, pulse width 10 µs,
relaxation delay 1 s, number of scans 16, number of increments 512.
The NOE difference spectra were recorded with the following acquisi-
tion parameters: spectral width 5000 Hz, acquisition time 3.27 s, pulse
width 18 µs, relaxation delay 4 s, irradiation power 5-10 dB, number
of scans 240. For variable temperature spectra, the probe temperature
((1 K) was controlled by a standard unit calibrated with a methanol
reference. Free energies of activation were calculated15 from the
coalescence temperature (Tc) and the frequency difference between the
(9) (a) Philp, D.; Stoddart, J. F. Angew. Chem., Int. Ed. Engl. 1996, 35,
1154. (b) Sessler, J. L.; Murai, T.; Lynch, V. Inorg. Chem. 1989, 28,
133. (c) Acholla, F. V.; Takusagawa, F.; Mertes, K. B. J. Am. Chem.
Soc. 1985, 107, 6902. (d) Curtis, N. F. In ComprehensiVe Coordination
Chemistry; Wilkinson, G., Gillard, R. D., McCleverty, J. A., Eds.;
Pergamon: Oxford, U.K., 1982; Vol. 2, Chapter 21.1, p 899. (e)
Ktakoviak, K.; Bradshaw, J. S.; Jiang, W.; Dalley, N. K.; Wu, G.;
Izatt, R. M. J. Org. Chem. 1991, 56, 2675. (f) Melson, G. A.; Busch,
D. H. J. Am. Chem. Soc. 1964, 86, 4834. (g) Blake, A. J.; Schro¨der,
M. AdV. Inorg. Chem. 1990, 35, 1.
(12) (a) Jalo´n, F. A.; Manzano, B. R.; Otero, A.; Rodr´ıguez-Pe´rez, M. C.
J. Organomet. Chem. 1995, 494, 179. (b) Elguero, J.; Fruchier, A.;
de la Hoz, A.; Jalo´n, F. A.; Manzano, B. R.; Otero, A.; Go´mez-de la
Torre, F. Chem. Ber. 1996, 129, 589. (c) Ferna´ndez-Gala´n, R.; Jalo´n,
F. A.; Manzano, B. R.; Rodr´ıguez-de la Fuente, J.; Weissensteiner,
W.; Kratky, C. Organometallics 1997, 16, 3758.
(10) Reactions of Coordinated Ligands; Braterman, P. S., Ed.; Plenum
Press: New York, London, 1986.
(11) Ferna´ndez-Gala´n, R.; Manzano, B. R.; Otero, A.; Lanfranchi, M.;
Pellinghelli, M. A. Inorg. Chem. 1994, 33, 2309.
(13) (a) Dent, W. T.; Long, R.; Wilkinson, G. J. Chem. Soc. 1964, 1585.
(b) Tatsuno, Y.; Yoshida, T.; Seiotsuha. Inorg. Synth. 1979, 19, 220.

6608 Inorganic Chemistry, Vol. 37, No. 26, 1998
Go´mez-de la Torre et al.
Scheme 2
coalescing signals in the limit of slow exchange with the formula
[{(η3-2-Me-C3H4)Pd}2(Me-BPzTO)]PF6‚(CH3)2CO (2), [{(η3-2-
Me-C3H4)Pd}2(Me-BPzTO)]CF3SO3‚1/2(CH3)2CO (3), [{(η3-2-Me-
C3H4)Pd}2(Me-BPzTO)](p-MeC6H4SO3) (4), and [{(η3-2-Me-C3H4)-
Pd}2(Br-BPzTO)]CF3SO3 (5). These compounds were prepared in a
similar way to 1 from [(η3-2-Me-C3H4)Pd(µ-Cl)]2, the corresponding
silver salt, and Me-TPzT (2-4) or Br-TPzT (5). 2: Yield 84%. Anal.
Calcd for C19H24F6N7OPPd2‚C3H6O: C, 33.78; H, 3.87; N, 12.53.
Found: C, 33.77; H, 3.83; N, 12.14. 3: Yield 88%. Anal. Calcd for
C20H24F3N7O4Pd2S‚1/2C3H6O: C, 34.10; H, 3.59; N, 12.95. Found: C,
33.85; H, 3.62; N, 13.00. 4: Yield 92%. Anal. Calcd for C26H31N7O4-
Pd2S: C, 41.62; H, 4.16; N, 13.06. Found: C, 41.42; H, 3.89; N, 12.76.
5: Yield: 68%. Anal. Calcd for C18H18Br2F3N7O4Pd2S: C, 27.50; H,
2.64; N, 10.70; S, 3.50. Found: C, 27.10; H, 2.27; N, 10.88; S, 3.61.
[(η3-2-Me-C3H4Pd)(Me-BPzTOMe)]CF3SO3 (6). [(η3-2-Me-C3H4)-
Pd(µ-Cl)]2 (143 mg, 0.36 mmol) was dissolved in 15 mL of distilled
acetone. To the above solution was added AgCF3SO3 (186 mg, 0.73
mmol). After being stirred for 6 h at room temperature in the dark, the
solution was filtered through a plug of Celite, and Me-BPzTOMe (197
mg, 0.73 mmol) was added. The mixture was left stirring for 12 h;
then the solvent was removed under vacuum and the residue washed
with 10 mL of diethyl ether. Yield: 323 mg, 77%. Anal. Calcd for
C17H20F3N7O4PdS: C, 35.09; H, 3.46; N, 16.85. Found: C, 35.42; H,
4.01; N, 16.85.
[{(η3-2-Me-C3H4)Pd}2(Me-BPzTOMe)](CF3SO3)2 (7). An enriched
sample of this compound was obtained by the following procedure.
[(η3-2-Me-C3H4)Pd(µ-Cl)]2 (20.3 mg, 0.051 mmol) was dissolved in
15 mL of distilled acetone. To the above solution was added AgCF3-
SO3 (26.4 mg, 0.102 mmol). After being stirred for 12 h at room
temperature in the dark, the solution was filtered through a plug of
Celite and cooled to -78 °C with a dry ice/acetone bath, and then 6
(50 mg, 0.086 mmol) was added. After the solution was stirred for 1
min, the solvent was removed under vacuum and the product washed
with 5 mL of cooled diethyl ether. Satisfactory analysis could not be
obtained.
∆Gcq ) aT(10.319 + log T/δν). The estimated error in the calculated
free energies of activation is 1.0-1.1 kJ mol-1
.
Preparation of Compounds. 2,4,6-Tris(4-methylpyrazol-1-yl)-
1,3,5-triazine (Me-TPzT). An alternative method to that previously
reported8b was used. A solution of 4-methylpyrazole (1.23 g, 15 mmol)
in 20 mL of freshly distilled THF was added to a stirred suspension of
sodium hydride (360 mg, 15 mmol) in THF. The mixture was stirred
at room temperature for 21 h in order to complete deprotonation. 2,4,6-
Trichloro-1,3,5-triazine (920 mg, 5 mmol) in 20 mL of THF was added
to the above solution. A white precipitate formed immediately. After
being stirred for 4 h, the solution was evaporated to dryness. The residue
was extracted with distilled toluene (50 mL, 3 times). The solvent was
removed under vacuum, and the resulting pale yellow solid washed
with 25 mL of diethyl ether. Yield: 514 mg, 32%.
2,4,6-Tris(4-bromopyrazol-1-yl)-1,3,5-triazine‚1/2CH2Cl2 (Br-
TPzT). Br-TPzT was prepared from 4-bromopyrazole (500 mg, 3.4
mmol), sodium hydride (82 mg, 3.4 mmol), and 2,4,6-trichloro-1,3,5-
triazine (209 mg, 1.13 mmol) by following the same procedure as
described for Me-TPzT. Yield: 105 mg, 18%. Anal. Calcd for C12H6-
Br3N9‚1/2CH2Cl2: C, 26.89; H, 1.26; N, 22.5. Found: C, 27.38; H, 0.95;
N, 22.48.
2-Methoxy-4,6-bis(4-methylpyrazol-1-yl)-1,3,5-triazine (Me-BPz-
TOMe). Me-BPzTOMe was prepared from 4-methylpyrazole (1.0 g,
12 mmol), sodium hydride (290 mg, 12 mmol), and 2,4-dichloro-6-
methoxy-1,3,5-triazine (1.11 g, 6 mmol) by following the same
procedure as described for Me-TPzT. Yield: 600 mg, 37%. Anal. Calcd
for C12H13N7O: C, 53.13; H, 4.83; N, 36.14. Found: C, 52.97; H, 4.91;
N, 36.22.
[{(η3-2-Me-C3H4)Pd}2(Me-BPzTO)]BF4 (1). [(η3-2-Me-C3H4)Pd-
(µ-Cl)]2 (120 mg, 0.3 mmol) was dissolved in 15 mL of distilled
acetone. AgBF4 (120 mg, 0.6 mmol) was then added to the above
solution. After being stirred for 6 h at room temperature in the dark,
the solution was filtered through a plug of Celite and cooled with an
ice bath, and then Me-TPzT (65 mg, 0.2 mmol) was added. The product
appeared as a precipitate, and the mixture was left stirring for 5 h. The
solid was filtered off and the residue washed with 10 mL of diethyl
ether. Yield: 91 mg, 68%. Anal. Calcd for C19H24BF4N7OPd2: C, 34.26;
H, 3.63; N, 14.72. Found: C, 34.20; H, 3.61; N, 14.27.
[(Pd(C6F5)2)(Me-BPzTOMe)] (8). [Pd(C6F5)2(cod)] (92.5 mg, 0.17
mmol, cod ) 1,5-cyclooctadiene) was dissolved in 20 mL of distilled
THF. To the above solution was added Me-BPzTOMe (45.7 mg, 0.17
mmol). After the solution was stirred for 45 min at room temperature,
the solvent was removed and the residue washed with 10 mL of hexane.
Yield: 97.8 mg, 82%. Anal. Calcd for C24H13F10N7OPd: C, 40.50; H,
1.84; N, 13.77. Found: C, 40.61; H, 1.50; N, 14.11. 19F NMR at 295
K in acetone-d6: Fortho, -118.04 (m), -117.67 (m); Fpara, -160.92 (t),
-162.54 (t), JFF ) 21.4 Hz; Fmeta, -164.05 (m), -165.84 (m).
(14) Espinet, P.; Mart´ınez-de Ilarduya, J. M.; Pe´rez-Briso, C.; Casado, A.
L.; Alonso, M. A. J. Organomet. Chem. 1998, 551, 9.
(15) Sandstro¨m, J. Dynamic NMR Spectroscopy; Academic Press: London,
1982.

Pd(II) Complexes with Polydentate Nitrogen Ligands
Inorganic Chemistry, Vol. 37, No. 26, 1998 6609
Table 1. 1H NMR Data for Me-TPzT, Br-TPzT, and Me-BpzTOMe Ligands and 1-8 Complexesa (δ, ppm; Solvent, Acetone-d6)
compd
temp (K)
H3
7.81
7.88
7.74
8.24
8.25
8.29
8.56
H5
8.64
8.77
8.52
8.78
8.76
8.79
9.35
X
OMe
Hs
Hs′
Ha
Ha′
CH3
Me-TPzT
Br-TPzT
Me-BPzTOMe
1-4
1-4 M
1-4 m
5
295
295
295
295
218
218
295
295
2.18
2.18
2.19
2.11
2.15
4.18
4.63 (bs)
4.51
4.55
4.41 (bs)
4.43
4.43
3.31 (bs)
3.36
3.34
2.26
2.15
2.18
2.21
2.23
3.30
3.28
4.61
4.60
3.39
3.44
6
7.93c (bs)
8.37 (bs)
7.98c
8.43
8.74c (bs)
8.87 (bs)
8.88c
9.16
2.19
2.26
2.11
2.20
2.12
2.26
2.24
2.18
2.21
4.40
4.30
4.33
6Mb
6mb
183
183
4.63
4.64
3.45
3.45
2.17
2.17
2.27
8.02c
8.45
8.73c
8.79
7
8
295
295
8.37
9.07
4.56
3.87
4.42 (bs)
3.39 (bs)
7.23c
7.83
8.30c
8.46
a See Scheme 1 for numbering scheme. b M ) major isomer; m ) minor isomer. c Signals corresponding to free pyrazol groups (H3′ or H5′).
Table 2. 13C NMR Data for Me-TPzT, Br-TPzT, and Me-BPzTOMe Ligands and 1-8 Complexesa (T, 295 K; δ, ppm; Solvent, Acetone-d6 If
Not Indicated)
allylic signals
compd
C2
C4
C6
C3′
C4′
C5′
X
OMe
-Cd
CH2
CH3
Me-TPzT
162.9
128.8
131.3
129.2
126.9
129.5
133.2
130.4
130.8
129.7
130.7
120.6
98.7
146.5
146.0
147.5
146.8
148.6
149.1
149.7
150.9
148.1
149.4
7.8
Br-TPzTb
c
c
c
c
c
c
Me-BPzTOMe
121.5
120.4
122.2
100.4
123.5
124.1
123.1
123.9
9.3
8.7
7.7
56.6
55.7
Me-BPzTOMed
172.7
156.0
c
c
162.3
161.4
1-4
5
6
131.4
122.3
c
59.7 (bs), 61.5 (bs)
62.7 (bs)
62.7 (bs)
22.2
23.3
23.1
c
c
c
c
8.8
8.9
58.5
57.5
8
171.3
163.3
163.0
C6F5 ring
120-150 (bs)
a See Scheme 1 for numbering scheme. b In dmso-d6. c Not observed. d In chloroform-d.
Table 3. Observed Coalescences and Calculated Free Activation
Table 4. Crystal Data for Compound 4
1
Energies after the Variable-Temperature H NMR Spectra of 1
chem formula
fw
C26H31N7O4Pd2S‚CH2Cl2
835.36
(Left) and 6 (Right)a
q
q
T
λ
25(2) °C
0.710 70 Å
A2/m
Tc
∆Gc
Tc
∆Gc
coalescenceb
H3(pz)
(K)c (kJ/mol)d coalescenceb (K)c (kJ/mol)d
space group
243
253
253
263
53.6
54.4
54.6
58.0
60.9
56.0
60.5
H3 (pz)
H3′ (pz)
MeO
H5′ (pz)
H5 (pz)
H5 + H5′
H3 + H3′
208
213
218
228
238
303
328
46.0
46.1
46.4
46.6
46.9
62.8
65.1
a
b
c
â
V
Z
9.7640(10) Å
16.1890(10) Å
21.175(5) Å
99.040(10)°
3305.5(9) Å3
8
H5(pz)
1 + 2 Hsyn
3 + 4 Hsyn
(1 + 2) + (3 + 4) Hsyn 302
1 + 2 Hanti
258
(1 + 2) + (3 + 4) Hanti 288
Fcalcd
1.679 g/cm3
13.56 cm-1
0.0523
a Acetone-d6 solution. For the allylic protons 1-4 refer to the signals
in the order of increasing frequency. b See Schemes 1 and 2 for labeling
scheme. c Coalescence temperatures. d Free activation energies at the
coalescence temperatures calculated by ∆Gcq ) aT[9.972 + log(T/δν)]
(a ) 1.914 × 10-2).
µ
R1a
wR2a
0.1328
a R1 ) ∑||Fo| - |Fc||/∑|Fo|; wR2 ) [∑[w(Fo2 - Fc2)2]/∑[w(Fo )2]]0.5
.
2
methods (SIR 92)16 and refinement on F2 was carried out by full-matrix
least-squares analysis (SHELXL-93).17 A number of cycles of refine-
ment with isotropic thermal parameters, followed by difference
synthesis, enabled location of all the non-hydrogen atoms along with
three unexpected peaks, which were attributed to CH2Cl2 from the
recrystallization mixture. The occupation factors for these sites were
fixed at 0.5 in the subsequent refinement procedure. The CH2Cl2 half
molecule was found to be disordered across the mirror plane, and as a
result, its geometry is imprecise. For the final cycles of refinement all
non-hydrogen atoms were refined anisotropically, while hydrogen atoms
were included in calculated positions but not refined. No extinction
X-ray Structure Determination of 4. Crystals of compound 4
suitable for X-ray diffraction were obtained by crystallization from
methylene chloride/hexane and are air stable. Crystallographic data are
given in Table 4. Reflections were collected at 25 °C on a Nonius
MACH3 diffractometer equipped with graphite-monochromated radia-
tion (λ ) 0.7107 Å). Unit cell dimensions were obtained by least-
squares fit of the 2θ values of 25 high-order reflections by using the
MACH3 centering routines. The intensities of 4121 reflections were
collected (2 < θ < 28); of these only 2330 reflections obeyed the
condition I > 2 σ(I). Data were corrected in the usual fashion for
Lorentz and polarization effects, and an empirical absorption correction
(Ψ scan) was applied (range of transmission factor 0.459-1.000). The
space group was determined from the systematic absences (hkl, k + l
) 2n + 1; 0kl, k + l ) 2n + 1; h0l, l ) 2n + 1; hk0, k ) 2n + 1; 0k0,
k ) 2n + 1; 00l, l ) 2n + 1). The structure was solved by direct
(16) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.; Burla,
M. C.; Polidori, G.; Camalli, M. J. Appl. Crystallography 1994, 435.
(17) Sheldrick, G. M. Program for the Refinement of Crystal Structures
from Diffraction Data; University of Go¨ttingen: Go¨ttingen, Germany,
1993.

6610 Inorganic Chemistry, Vol. 37, No. 26, 1998
Go´mez-de la Torre et al.
correction was found to be necessary. The scattering factors used,
corrected for the real and imaginary parts of the anomalous dispersion,
were taken from the literature.18 Upon convergence the final Fourier
difference map showed no significant peaks. Final disagreement indices
are R ) 0.052, Rw ) 0.1328, GOF ) 1.061, and largest difference
Scheme 3
peak and hole 0.756 and -0.704 e‚Å-3
.
Results and Discussion
Synthesis of the New Ligands and Complexes. Me-TPzT,8b
Br-TPzT, and Me-BPzTOMe were prepared from the corre-
sponding pyrazolate anion and 2,4,6-trichloro-1,3,5-triazine (in
the cases of Me-TPzT and Br-TPzT) or 2,4-dichloro-6-methoxy-
1,3,5-triazine (Me-BPzTOMe).
Reaction of [Pd(η3-2-Me-C3H4)(S)2]+ (S ) acetone), prepared
by reaction of [Pd(η3-2-Me-C3H4)(µ-Cl)]213 with the appropriate
silver salt, with Me-TPzT or Br-TPzT in acetone in a 3:1 molar
ratio generated the complexes [{Pd(η3-C4H7)}2(X-BPzTO)]A
(X-BPzTO ) 4,6-bis(4-methylpyrazol-1-yl)-1,3,5-triazin-2-olate
(Me-BPzTO), A ) BF4 (1), PF6 (2), CF3SO3 (3), 4-Me-C6H4-
SO3 (4); X-BPzTO ) 4,6-bis(4-bromopyrazol-1-yl)-1,3,5-tri-
azin-2-olate (Br-BPzTO), A ) CF3SO3 (5)) (Scheme 1) in high
yields (80%). These complexes were slightly soluble in acetone,
and they were air stable in the solid state. However, on standing
of solutions for prolonged periods, an evolution to give free
4-Me- or 4-Br-pyrazole was observed. When the corresponding
reaction was carried out in a 1:1 or 2:1 molar ratio, the same
complexes were always obtained, although in lower yields and
with free ligand remaining in solution.
Surprisingly, two allylpalladium fragments were incorporated
and the TPzT ligands were partially hydrolyzed, with the
corresponding pyrazole being detected by 1H NMR in solution.
Although N-pyrazole fragments are good leaving groups, the
X-TPzT ligands were stable in solution. Consequently, the
allylpalladium fragment must induce, in some way, the hy-
drolysis process by adventitious water19 present in the reaction
mixture. In 1,3,5-triazine derivatives it is known that such a
hydrolysis takes place by nucleophilic displacement in a
stepwise process20 or by a single transition state consistent with
simultaneous bond formation and fission (ANDN).21 An inter-
mediate suggesting this type of nucleophilic attack has been
isolated in pyridine-substituted triazines.21d The ease of substitu-
tion of pyrazole in these complexes may be explained (i) as a
consequence of the steric hindrance of the allyl group when
two palladium fragments are coordinated (see Scheme 1,
intermediate II) or (ii) by a special stability of the hydrolyzed
ligand coordinated to two allylpalladium groups. To obtain
complementary information about the driving force for this
hydrolysis we performed the reaction with a new ligand,
2-methoxy-4,6-bis(4-methylpyrazol-1-yl)-1,3,5-triazine (Me-
BPzTOMe), in which a pyrazolyl group has been replaced by
a methoxy group. The methoxy group is less sterically demand-
ing than the pyrazole ring and also a poorer leaving group.
However, on using a Pd:ligand ratio of 2:1 (A ) CF3SO3), the
same complex (3) was obtained and only the loss of the methoxy
group was observed. If one takes into account the smaller steric
hindrance induced by the methoxy group in a dinuclear
intermediate (type II, Scheme 1), the special stability of the
X-BPzTO cationic compound must play a predominant role in
the evolution of these reactions.
The intermediate 7, [{Pd(η3-C4H7)}2(Me-BPzTOMe)](CF3-
1
SO3)2, was detected by monitoring the reaction by H NMR
spectroscopy at low temperature as the major component in a
mixture with 3. It was observed that 7 evolves to 3 and
methanol. Complex 7 was isolated as an almost pure compound
after only a short reaction time at low temperature, but in
solution 7 evolves quickly to 3.
In contrast to the behavior observed with Me-TPzT, when
Me-BPzTOMe is reacted with 1 equiv of the palladium solvate,
complex [Pd(η3-C4H7)(Me-BPzTOMe)]CF3SO3 (6) with one
palladium center and a nonhydrolyzed ligand was isolated (see
Scheme 2). Reaction of 6 with a second 1 equiv of the palladium
derivative leads to 3. In agreement with all these observations,
complexes 6 and 7 are reasonable intermediates in the formation
of 3 when ligand Me-BPzTOMe is used, as shown in Scheme
2, the dicationic complex 7 being more prone to nucleophilic
attack than the monocationic complex 6. A similar pathway may
be assumed for reactions with X-TPzT ligands. The difference
in behavior between BPzTOMe and X-TPzT complexes must
be due to the better leaving character of Pz- in comparison with
MeO-.
(18) International Tables for X-ray Crystallography; Kynoch Press: Bir-
mingham, England, 1974; Vol. IV.
(19) The water necessary for the hydrolysis must arise from the silver salts
and perhaps also from traces in the solvent. We have tried to
completely remove all water from AgPF6, but it has proved impossible,
even after drying over P2O5 with a vacuum higher than 10-2 mmHg
during 5 days. Similar results have been reported: Horn, E.; Snow,
M. R. Aust. J. Chem. 1980, 33, 2369.
(20) (a) Illuminati, G. AdV. Heterocycl. Chem. 1964, 3, 285. (b) Illuminati,
G.; Stegel, F. AdV. Heterocycl. Chem. 1983, 34, 305. (c) Cramton,
M. R. AdV. Phys. Org. Chem. 1969, 7, 211. (d) Terrier, F. Chem.
ReV. 1982, 82, 78. (e) Bernasconi, C. F.; Muller, M. C. J. Am. Chem.
Soc. 1978, 100, 5530. (f) Manderville, R. A.; Buncel, E. J. Chem.
Soc., Perkin Trans. 2 1993, 1887. (g) Manderville, R. A.; Buncel, E.
J. Am. Chem. Soc. 1993, 115, 8985. (h) Terrier, F. Nucleophilic
Aromatic Displacement: Influence of the Nitro group; VCH: Wein-
heim, Germany, 1991. (i) Buncel, E.; Crampton, M. R.; Strauss, M.
J.; Terrier, F. Electron Deficient Aromatic and Heterocyclic-Base
Interactions; Elsevier: Amsterdam, 1984. (j) Pietra, F. Q. ReV. Chem.
Soc. 1969, 23, 504. (k) Miller, J. Aromatic Nucleophilic Substitution;
Elsevier: Amsterdam, 1968.
To show the influence of the Pd fragment on the hydrolysis
process, the reactions of Me-BPzTOMe with 1 and 2 equiv of
[Pd(C6F5)2(cod)] (cod ) 1,5-cyclooctadiene) were carried out.
Reactions in THF with a 1:1 ratio produced a complex with
the formula [Pd(C6F5)2(Me-BPzTOMe)] (8) containing the
unaltered triazine ligand (see Scheme 3). However, when 2 equiv
of palladium was added, compound 8, which is instantaneously
formed in the solution, slowly evolves to give unidentified
decomposition products of the ligand according to a study
(21) (a) Renfrew, A. H. M.; Taylor, J. A.; Whitmore, J. M. J.; Williams,
A. J. Chem. Soc., Perkin Trans. 2 1994, 2383. (b) Renfrew, A. H.
M.; Rettura, D.; Taylor, J. A.; Whitmore, J. M. J.; Williams, A. J.
Am. Chem. Soc. 1995, 117, 5484. (c) Cullum, N. R.; Renfrew, A. H.
M.; Rettura, D.; Taylor, J. A.; Whitmore, J. M. J.; Williams, A. J.
Am. Chem. Soc. 1995, 117, 9200. (d) Thomas, N. C.; Foley, B. L.;
Rheingold, A. L. Inorg. Chem. 1988, 27, 3426.
1
monitored by H NMR in acetone-d6 solution. The reaction of
[Pd(C6F5)2(cod)] with Me-TPzT, both in 1:1 and 2:1 ratios, leads
to unidentified complex mixtures.

Pd(II) Complexes with Polydentate Nitrogen Ligands
Inorganic Chemistry, Vol. 37, No. 26, 1998 6611
Characterization of 1-8. Complexes 1-8 were character-
ized by NMR and IR spectroscopy, by elemental analysis
1
(except for 7), and, in some cases, by FAB MS. H and 13C-
{1H} NMR data of the complexes at room temperature, along
with those of the starting ligands, are compiled in Tables 1 and
2. For complex 1, pyrazolic and allylic signals have been
assigned after NOE experiments and a one-bond carbon-proton
correlation has allowed the assignment of the 13C resonances.
The resonances of the other complexes have been assigned by
comparison with 1. Complexes 1-4 only differ in the nature
of the anion, and their 1H and 13C{1H} NMR spectra are
identical. All data for 1-5 and 7 indicate a highly symmetric
structure with only one type of pyrazole. Besides, for 1-4 the
triazine C4 and C6 carbons appear equivalent (these carbons are
1
not observed in 5). The H NMR spectrum of 6 shows four
broad signals for the aromatic protons, indicating that two
different pyrazolyl groups are present. The corresponding carbon
resonances are also broad. The H NMR resonances of one
1
1
Figure 1. Variable-temperature H NMR study of complex 1 in the
aromatic and allylic region. Temperatures are in K. In the aromatic
region the meso T dl isomer exchange, which is represented in Scheme
4, is shown. In the allylic region a simultaneous syn-syn, anti-anti
exchange of these two isomers is visualized.
pyrazole are shifted to higher frequencies as a consequence of
its coordination to the palladium center.
In the complexes described thus far, although asymmetric
allylic groups are expected, the Hanti, Hsyn, and terminal allylic
carbons give rise to either two broad signals or a unique signal
in each case. These observations have been shown to be
indicative of a fluxional behavior in similar allylpalladium
complexes with N-donor ligands.12b The observation of broad
proton and carbon signals for the two different pyrazolyl groups
in 6 also indicates a possible dynamic process that interconverts
unchanged ligand ([M - C6F5]+ and [M - 2(C6F5)]+). This is
consistent with the expected decrease in the acceptor character
of the metallic center when C6F5 fragments are lost and, as a
consequence, the triazine ring is then less sensitive to nucleo-
philic attack by water.
Fluxional Behavior of Complexes 1 and 6. Complex 1. The
1H NMR spectrum of 1 at room temperature in acetone-d6
exhibits the signals for the Hsyn and Hanti protons of the allyl
group as two broad signals and a broad singlet, respectively.
When the temperature is increased, the Hsyn signals evolve to
give a singlet beyond its coalescence. The Hsyn signals are split
and finally four resonances, which are not completely separated,
are seen (see Figure 1) at low temperature in such a way that
several coalescences are observed. The same effect is observed
for Hanti. Similarly, the signals due to the pyrazolic H3 and H5
protons, the methyl allyl groups, and the Me-BPTO group are
split into two signals at low temperatures. The calculated free
1
both groups. Consequently, a variable-temperature H NMR
study was carried out for 1 and 6 (see below).
1
Both the H and 13C{1H} NMR of 8 indicate the existence,
at room temperature, of two distinct pyrazolyl groups. One of
these groups must be free and the other coordinated to the
palladium center. In the 19F NMR spectrum two different and
freely rotating C6F5 groups are seen (see the Experimental
Section).
A significant fact observed in the IR spectra of 1-5 is the
presence of a ν(CdO) band at about 1790 cm-1. This is a
consequence of the partial hydrolysis process of the TPzT
ligands. This is a very high wavelength considering the amidic
character of the functional group; for instance, it represents a
shift of 95 cm-1 from cyanuric acid.22 This marked shift
confirms the double bond character of the CdO group due to
the coordination of the adjacent nitrogen atoms to the palladium
centers. The existence of this bond has also been confirmed by
the X-ray structure determination of this compound (see below).
The FAB mass spectra of 1 and 5 both show a similar pattern.
The more characteristic peaks are those corresponding to [M]+,
[M - C4H7]+ and [M - C4H7Pd]+. Unfortunately, the mass
spectrum of a highly enriched sample of 7 is identical to that
of 1. This indicates the ease of hydrolysis of the Me-BPzTOMe
ligand when coordinated to two allyl-Pd fragments and also
the high stability of the corresponding cation formed. In contrast,
the FAB mass spectrum of 6 does not show peaks corresponding
to fragments containing hydrolyzed ligand. The most relevant
peaks are those corresponding to the cation [M]+ and [M -
C4H7]+. The FAB mass spectrum of 8 provides new data about
the origin of the hydrolysis in this type of ligand. In this
spectrum some of the peaks that appear correspond to a
hydrolyzed ligand ([M - Me]+, [M - Me - pz]+, and [M -
Me - pz - C6F5]+) whereas two other fragment peaks,
corresponding to the loss of C6F5 groups, are due to the
q
activation energies15 at the coalescences (∆Gc ) and the corre-
sponding temperatures are shown in Table 3. The different
q
activation energies ∆Gc determined in this way are related in
a linear fashion with the coalescence temperatures (r2 ) 0.92).
This fact agrees with the existence of a unique fluxional process.
To account for these observations we propose the existence of
three stereoisomers, a meso form and a d,l pair of enantiomers
(see Scheme 4). Each isomer has two equivalent asymmetric
allyl groups and two identical pyrazole rings. The molecular
structure of the meso form has been determined by an X-ray
diffraction study (see below). Other isomers, of lower symmetry,
containing one allylpalladium fragment bonded to a pyrazole
group and to the N5 of the triazine may be excluded on the
basis of the steric hindrance of the allyl group with the pyrazole
ring coordinated to the other palladium center. Besides, con-
sidering that the meso form is present, the simultaneous
existence of the asymmetric isomer would lead to a higher
1
number of signals than is actually observed in the H NMR
spectrum.
When the variable-temperature 1H NMR study of 1 was
carried out in dichloromethane-d2, similar changes were ob-
q
served in the resonances and the calculated ∆Gc values are
slightly higher, indicating a possible coordinative participation
of the acetone molecules in the fluxional process. Moreover,
the addition of chloride ions to the dichloromethane solution
(22) Pouchet, C. J. The Aldrich Library of Infrared Spectra, 3rd ed.; Aldrich
Chemical Co., Inc.: Milwaukee, WI, 1981; p 1380 G.

6612 Inorganic Chemistry, Vol. 37, No. 26, 1998
Go´mez-de la Torre et al.
Scheme 4
noticeably accelerates the interchange. The formation of the
known [Pd(η3-C4H7)Cl]2 dimer is also observed after this
addition as a consequence of a dissociative process.
Several mechanisms have been proposed for the isomerization
or syn-syn and anti-anti interchange in (η3-allyl)palladium
complexes.23 A simple rotation of the allyl ligand in its plane
about an axis containing the metal center is usually excluded
in square-planar geometries.23b Some authors24 support an
associative mechanism for this apparent allyl rotation, while in
some complexes with N-donor ligands a dissociative pathway
has been proposed25 by others and also by ourselves.12 The rate
enhancement effect produced by addition of chloride anions (and
also of the acetone solvent) has been used as an argument
supporting both dissociative (stabilization of tricoordinate
intermediates)25b and associative (formation of the pentacoor-
dinated intermediate) pathways.24b We think that in complexes
such as 1 the Pd-N bond rupture is a very probable occurrence
for the following two reasons. First, in two allyl complexes with
pyrazole-containing ligands,12b which are similar to those
reported in this work, we have proved without ambiguity,
through the observation of NOEs between Hanti and protons
situated far away from the coordination center, that the Pd-N
bond rupture and internal rotation of the ligand take place. In
addition, in complex 6, whose dynamic behavior is described
below, it is clear that Pd-N bond cleavage occurs. Second, the
free activation parameters for 1 resemble the values described
for these types of complexes where dissociative pathways are
proposed.25,12 The whole process may be explained as depicted
in Scheme 4 and proceeds according to the following steps:
(a) dissociation of a Pd-N bond, (b) isomerization and Pd-N
bond rotation of the tricoordinated intermediate,26 and (c) re-
formation of the Pd-N bond. Cleavage of the Pd-N(triazine)
Figure 2. Variable-temperature 1H NMR study of complex 6 (see Table
3) in the aromatic region. Temperatures are in K. At 283 K equilibrium
i (see Scheme 5) exchanges the corresponding 6a,b signals. At 328 K
equilibrium ii exchanges coordinated and noncoordinated pyrazole
resonances. M and m refers to the major and minor isomers of 6.
bond is reflected in Scheme 4, but we have no data to exclude
the possibility of the rupture of the Pd-N(pyrazole) bond. The
proposed mechanism simultaneously explains the syn-syn and
anti-anti interchange and also the observed isomerization
process.
Complex 6. The broad resonances of the aromatic protons
at room temperature become even broader when the temperature
of the acetone solution is increased, and this continues until
the corresponding coalescences between the signals of the
coordinated and noncoordinated pyrazoles are reached (see Table
3 and Figure 2). At low temperatures (205 K) (Figure 2), two
different species in an approximately 60:40 ratio are seen with
resonances for two distinct pyrazoles in each case (see Table
1). The identification of the protons and methyl groups that
belong to the same pyrazolyl group has been performed by a
COSY experiment. Two signals are also observed for the
methoxy group, but the expected resonances for the allylic
protons are not resolved. We propose that these two species
are 6a,b and that these are in fast interconversion at room
temperature (see Scheme 5). Although we have made great
efforts, by means of NOE studies at low temperature, we have
not been able to assign the observed resonances to a particular
species. One reason could be the low solubility of 6 at low
temperature. However, it is also reasonable that the noncoor-
dinated pyrazole in 6b will be twisted out of the triazine plane
in order to avoid steric interaction with the allylpalladium
fragment, thus making it difficult to observe NOE between these
groups.27
(23) (a) Maitlis, P. M.; Espinet, P.; Russell, M. J. H. In ComprehensiVe
Organometallic Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E.
W., Eds.; Pergamon Press: Oxford, U.K., 1982; Vol. 6, p 385. (b) K.
Vrieze In Dynamic Nuclear Magnetic Resonance Spectroscopy;
Jackman, L. M., Cotton, F. A., Eds.; Academic Press: New York,
1975.
(24) (a) Crociani, B.; Di Bianca, F.; Giovenco, A.; Boschi, T. Inorg. Chim.
Acta 1987, 127, 169. (b) Hansson, S.; Norrby, P. O.; Sjo¨gren, M. P.
T.; A° kermark, B.; Cucciolito, M. E.; Giordano, F.; Vitagliano, A.
Organometallics 1993, 12, 4940. (c) Crociani, B.; Antonaroli, S.; Paci,
M.; Di Bianca, F.; Canovese L. Organometallics 1997, 16, 384.
(25) (a) Albinati, A.; Kunz, R. W.; Amman, C. J.; Pregosin, P. S.
Organometallics 1991, 10, 1800. (b) Gogoll, A.; O¨ rnebro, J.; Gren-
nberg, H.; Ba¨ckvall, J. E. J. Am. Chem. Soc. 1994, 116, 3631. (c)
Gogoll, A.; Grennberg, H.; Axe´n, A. Organometallics 1997, 16, 1167.
(26) (a) Thorn, D. L.; Hoffmann, R. J. Am. Chem. Soc. 1978, 100, 2079.
(b) Alibrandi, G. Scolaro, L. M.; Romeo, R. Inorg. Chem. 1991, 30,
4007. (c) Tatsumi, K.; Hoffmann, R.; Yamamoto, A.; Still, J. K. Bull.
Chem. Soc. Jpn. 1981, 54, 1857.
(27) For example, in the X-ray structure of the complex [PtIMe3(dmpbipy)]
[dmpbipy ) 6-(3,5-dimethylpyrazol-1-yl)-2,2′-bipyridine], the pyra-
zolyl ring that it is not involved in the coordination is twisted out of
the bipy plane: Gelling, A.; Orrell, K. G.; Osborne, A. G.; Sik, V.;
Hursthouse, M. B.; Coles, S. J. Polyhedron 1996, 15, 3203.

Pd(II) Complexes with Polydentate Nitrogen Ligands
Inorganic Chemistry, Vol. 37, No. 26, 1998 6613
be excluded because it would lead to a unique activation barrier
for both types of interchange. From the evidence described
above it is reasonable to propose that the Pd-N bond rupture
is the process that has most influence on the activation barrier
of each process and that this bond is stronger when the pyrazolyl
nitrogen atom is involved. This hypothesis fits in well with the
corresponding basicities29 for the parent heterocycles. A similar
metallotropic process has been reported by Orrell et al.30 for
2,4,6-tris(2-pyridyl)-1,3,5-triazine (tpt) and 2,4,6-tris(2-pyridyl)-
pyrimidine coordinated to palladium or platinum fragments
containing fluorinated aryl groups. In these cases, the activation
barriers are considerably higher, both for paths i and ii, than
those found by us and they have only been measured on the
basis of EXSY experiments. In our case, the difference in the
energy barriers between the two processes is considerably
higher.
Another interesting conclusion can be drawn after the
comparison of the ∆Gcq data for complexes 1 and 6 (see Table
3). The low barrier corresponding to the Pd-N(triazine) bond
rupture of complex 6 does not correspond with any ∆Gcq value
of complex 1. In fact, all the values for this last derivative are
above the barriers related to the Pd-N(pyrazolyl) bond rupture
in complex 6. This denotes that in 1 the Pd-N(triazine) bond
is notably stronger that in 6. The reason for this must be the
anionic character of the Me-BPzTO ligand and the delocalization
of the electronic charge in the triazine ring. Consequently, the
ease of hydrolysis of the Me-TPzT, Br-TPzT, and Me-
BPzTOMe ligands, in the presence of the allylpalladium
fragment, may have its origin in the bond reinforcement of the
triazine with the palladium center.
Figure 3. Linear plot of ∆Gcq (kJ mol-1) versus Tc (K) for complex
6 (see Table 3): (0) exchange of 6a,b resonances; ([) exchange of
free and coordinated pyrazole resonances.
Scheme 5
Complex 8. To assess the influence of the electrophilic
character of the metallic fragment on the fluxional behavior, a
variable temperature study in toluene-d8 solution was carried
out for complex 8. Only one compound was observed over the
whole temperature range studied (183-373 K), which does not
indicate interchange of free and coordinated pyrazole rings. This
reflects the rigid character of complex 8. The main difference
between 8 and 6 is the electrophilic character of the palladium
center and, as a consequence, the strength of the Pd-N bond.
This again suggests a significant influence of the Pd-N bond
rupture in the mechanism for this type of dynamics. Although
a structure similar to 6a has been proposed for the static structure
of 8 (see Scheme 3), the possibility of having the metallic center
in a disposition similar to that in 6b cannot be totally rejected.
The former structure seems to be more reasonable considering
steric reasons and the higher basicity of the triazine N1 atom in
comparison to N5.
X-ray Structure of 4. To support our studies of the
complexes in solution, we determined the molecular structure
of 4 by X-ray diffraction. The crystal structure consists of a
dinuclear Pd cation and p-toluenesulfonate counterion. An
ORTEP plot of the cation is shown in Figure 4, and a selected
list of bond lengths and angles is given in Table 5.
The cation lies on a crystallographic mirror plane containing
the atoms O(5), C(13), and N(8), so that only half of the
molecule is crystallographically independent. The geometry
Between 208 and 238 K several coalescences of the pyrazolyl
signals are observed (see Table 3) that correspond to interchange
of resonances of the major isomer with those of the minor
isomer, without changing the character of the free or coordinated
pyrazolyl groups. The plot of the ∆Gcq values against Tc fits a
straight line very well, suggesting the same activation barrier
q
for these exchanges (Figure 3). However, the ∆Gc values
corresponding to the interchange between free and coordinated
pyrazolyl protons (Tc above room temperature) are considerably
higher than expected for this linear plot (see Figure 3). This
fact is very conclusive and clearly indicates a different pathway
for the two types of observed exchanges. If we consider Scheme
5, it is possible to observe that the interchange between 6a and
6b (path i) can occur through a Pd-N(triazine) bond rupture
accompanied by a 180° rotation of the metal-pyrazolyl group
about the C-N bond, while the change of the allylpalladium
fragment from one pyrazolyl group to the other (path ii), which
can take place by a 1,4 metallotropic shift,28 needs the cleavage
of the Pd-N(pyrazolyl) bond. A full dissociation of the metal
center followed by a recombination in a different position can
(29) As a reference, pyrazole is 56.0 kJ/mol more basic than triazine: (a)
Meot-Ner, M. J. Am. Chem. Soc. 1979, 101, 2396. (b) Abboud, J. L.
M.; Cabildo, P.; Can˜ada, T.; Catala´n, J.; Claramunt, R. M.; de Paz, J.
L. G.; Elguero, J.; Homan, H.; Toiron, C.; Yranzo, G. I. J. Org. Chem.
1992, 57, 3938.
(30) (a) Gelling, A.; Olsen, M. D.; Orrell. K. G.; Osborne, A. G.; Sik, V.
J. Chem. Soc., Chem. Commun. 1997, 587. (b) Gelling, A.; Oken, M.
D.; Orrell, K. G.; Osborne, A. G.; Sik, V. Inorg. Chim. Acta 1997,
264, 257.
(28) For some examples of 1,4-metallotropic shifts in square-planar
complexes see: (a) Abel, E. W.; Orrell, K. G.; Osborne, A. G.; Pain,
H. M.; Sik, V.; Hursthouse, M. B.; Malik, K. M. A. J. Chem. Soc.,
Dalton Trans. 1994, 3441. (b) Abel, E. W.; Gelling, A.; Orrell, K.
G.; Osborne, A. G.; Sik, V. J. Chem. Soc., Chem. Commun. 1996,
2329. (c) Rotondo, E.; Giordano, G.; Minniti, D. J. Chem. Soc., Dalton
Trans. 1996, 253 (letter).

6614 Inorganic Chemistry, Vol. 37, No. 26, 1998
Go´mez-de la Torre et al.
and the central allyl carbon [C(19)] is 0.647 Å below it. The
dihedral angle between the allylic plane and the palladium
coordination plane is 111.3°, with the C(central) to -CH3 vector
pointing away from the metal. The bond length of C(13)-O(5)
[1.21(1) Å] indicates the double-bond character of the C-O
linkage. The triazine ring C-N distances at C(13) are typical
single-bond values (1.40 Å), and the remaining four C-N
distances are narrowly spread in the double-bond range [the
C(12)-N(9) and C(12)-N(8) distances are 1.323(8) and 1.314-
(7) Å, respectively]. In addition, the central C3N3 ring is nearly
planar, supporting pentadienide-delocalized bonding for the five
atoms N(9), C(12), N(8), C(12A), and N(9A). The pyrazolyl
rings coordinated to the palladium atoms are nearly coplanar
with the triazine ring (dihedral angle of 1.81°).
Figure 4. ORTEP view with atomic numbering of the cation [{(η3-
2-Me-C3H4)Pd}2(Me-BPzTO)]+ (cation of 4).
Conclusions
Table 5. Selected Bond Distances (Å) and Bond Angles (deg) of
Compound 4
We have shown that allylpalladium fragments may induce
the partial hydrolysis of some pyrazole derivatives of triazine.
The presence of two coordinated metallic moieties facilitates
the process, and the better leaving character of pyrazolate in
comparison to methoxy groups is observed. From the free
activation energies at the coalescence temperatures, calculated
Pd(1)-N(10)
Pd(1)-N(9)
Pd(1)-C(19)
Pd(1)-C(18)
Pd(1)-C(20)
O(5)-C(13)
N(8)-C(12)
N(8)-C(12A)
N(9)-C(12)
N(9)-C(13)
N(10)-C(16)
2.092(5)
2.106(5)
2.129(7)
2.119(7)
2.113(7)
1.212(11)
1.314(7)
1.314(7)
1.323(8)
1.402(7)
1.311(8)
N(10)-N(11)
N(11)-C(14)
N(11)-C(12)
C(13)-N(9A)
C(14)-C(15)
C(15)-C(16)
C(15)-C(17)
C(18)-C(19)
C(19)-C(20)
C(19)-C(21)
1.376(7)
1.345(8)
1.396(8)
1.402(7)
1.360(9)
1.411(9)
1.506(10)
1.423(10)
1.402(12)
1.526(13)
1
from H NMR studies, it is concluded that the main driving
force for the hydrolysis is the improvement in the coordination
ability of ligands. For complex 6 we have found an example in
which the metal moiety moves completely around the outside
of the triazine ring, adopting all the coordination possibilities
q
involving nitrogen atoms. The calculation of the different ∆Gc
N(10)-Pd(1)-N(9)
N(9)-Pd(1)-C(18)
N(10)-Pd(1)-C(20)
C(18)-Pd(1)-C(20)
77.8(2) O(5)-C(13)-N(9)
121.7(4)
116.6(8)
values for the observed coalescences in a variable temperature
1H NMR study shows that two different barriers are involved
in the fluxional phenomenon. The corresponding energies for
these two barriers correlate with the different strengths of the
Pd-N bond to be broken (Pd-triazine or Pd-pyrazole) in the
fluxional process and thus strongly support the importance of
the Pd-N bond rupture in fluxional processes of allylpalladium
complexes with N-donor ligands.
106.8(3) N(9)-C(13)-N(9A)
107.1(3) C(20)-C(19)-C(18) 114.9(8)
68.5(3) C(20)-C(19)-C(21) 120.8(8)
C(12)-N(8)-C(12A) 112.0(7) C(18)-C(19)-C(21) 122.8(8)
C(12)-N(9)-C(13)
N(9)-C(12)-N(8)
117.2(6) C(21)-C(19)-Pd(1)
117.4(6)
128.5(6)
around the Pd(1) atom is approximately square planar, and the
five-membered chelate ring, including the Pd atom, shows only
very small deviations from planarity. The Pd(1)-N(9) and Pd-
(1)-N(10) bonds form the basis for the five-membered chelate
ring, and the distances are essentially equivalent [Pd(1)-N(9),
2.106(5) Å; Pd(1)-N(10), 2.092(8) Å]. The Pd-η3-allyl
distances are the following: Pd(1)-C(18) ) 2.119(8) Å; Pd-
(1)-C(19) ) 2.128(8) Å; Pd(1)-C(20) ) 2.112(7) Å. These
latter values are in the range expected for a Pd(II)-allyl with
nitrogen ligands in trans positions. If we define a plane
containing the Pd and the two nitrogen atoms N(9) and N(10)
(coordination plane), then the terminal carbons are 0.065 Å
[C(18)] below the plane and 0.175 Å [C(20)] above the plane
Acknowledgment. We gratefully acknowledge financial
support from the Direccio´n General de Investigacio´n Cient´ıfica
y Te´cnica (DGICYT) (Grants No. PB95-0901 and PB94-0742)
of Spain. We also thank Dr. Mariano Laguna from the
University of Zaragoza, Zaragoza, Spain, who recorded the mass
spectra.
Supporting Information Available: X-ray crystallographic files,
in CIF format, for complex 4 are available on the Internet only. Access
information is given on any current masthead page.
IC980307N