Synthesis and structure of isomeric palladium(II)-pyrazole chelate complexes with and without an N-H group as hydrogen bond donor

Article abstract of DOI:10.1021/ic990995q

Four new ligands containing a pyrazole ring and either a phosphine or thioether were prepared and converted to their cis-dichloropalladium(II) complexes. Two of the ligands are especially notable for the attachment of a side chain at pyrazole carbon, rather than at nitrogen. The new metal complexes include dichloro[3-(diphenylphos-phinomethyl)pyrazole]palladium(II) (1-PdCl₂) and dichloro[3-(methylthiomethyl)pyrazole]palladium(II) (2-PdCl₂), which both feature an N-H group as a potential proton or hydrogen bond donor. For comparison, isomeric complexes lacking an NH group were prepared: Dichloro[1-(diphenylphosphinomethyl)pyrazole]palladium(II) (3-PdCl₂) and dichloro[1-(methylthiomethyl)pyrazole]palladium(II) (4.-PdCl₂). As determined by X-ray crystallography, all four complexes were found to have slightly distorted square planar geometry. Complexes 1-PdCl₂ and 2-PdCl₂, which contain an NH group, exhibit both intermolecular and intramolecular hydrogen bonding, whereas isomers 3-PdCl₂ and 4-PdCl₂ do not. Single-crystal X-ray structure determinations on the following compounds are reported: 1-PdCl₂, space group P1, a = 8.4488(9) A, b = 8.9175(13) A, c = 12.731(2) A, Z = 2, V = 871.8(2) A³; 2-PdCl₂, space group Pbca, a = 10.8827(10) A, b = 11.7721(7) A, c = 14.874(2) A, Z = 8, V = 1905.6 A³; 3-PdCl₂, space group P2₁/c, a = 20.520(2) A, b = 12.549(2) A, c = 13.9784(13) A, Z = 8, V = 3401.1(6) A³; 4-PdCl₂, space group Pbca, a = 10.6545(10) A, b = 12.0205(11) A, c = 14.6474(14) A, Z = 8, V = 1875.9(3)A³.

Full text of DOI:10.1021/ic990995q

2080
Inorg. Chem. 2000, 39, 2080-2086
Synthesis and Structure of Isomeric Palladium(II)-Pyrazole Chelate Complexes with and
without an N-H Group as Hydrogen Bond Donor
Douglas B. Grotjahn,* David Combs, and Sang Van
Department of Chemistry, San Diego State University, 5500 Campanile Drive,
San Diego, California 92182-1030
Gerardo Aguirre and Fernando Ortega
Centro de Graduados e Investigacio´n, Instituto Technolo´gico de Tijuana, Apartado Postal 1166,
22000 Tijuana, Baja California, Mexico
ReceiVed August 18, 1999
Four new ligands containing a pyrazole ring and either a phosphine or thioether were prepared and converted to
their cis-dichloropalladium(II) complexes. Two of the ligands are especially notable for the attachment of a side
chain at pyrazole carbon, rather than at nitrogen. The new metal complexes include dichloro[3-(diphenylphos-
phinomethyl)pyrazole]palladium(II) (1-PdCl2) and dichloro[3-(methylthiomethyl)pyrazole]palladium(II) (2-PdCl2),
which both feature an N-H group as a potential proton or hydrogen bond donor. For comparison, isomeric
complexes lacking an NH group were prepared: dichloro[1-(diphenylphosphinomethyl)pyrazole]palladium(II)
(3-PdCl2) and dichloro[1-(methylthiomethyl)pyrazole]palladium(II) (4-PdCl2). As determined by X-ray crystal-
lography, all four complexes were found to have slightly distorted square planar geometry. Complexes 1-PdCl2
and 2-PdCl2, which contain an NH group, exhibit both intermolecular and intramolecular hydrogen bonding,
whereas isomers 3-PdCl2 and 4-PdCl2 do not. Single-crystal X-ray structure determinations on the following
compounds are reported: 1-PdCl2, space group P1h, a ) 8.4488(9) Å, b ) 8.9175(13) Å, c ) 12.731(2) Å, Z )
2, V ) 871.8(2) Å3; 2-PdCl2, space group Pbca, a ) 10.8827(10) Å, b ) 11.7721(7) Å, c ) 14.874(2) Å, Z )
8, V ) 1905.6 Å3; 3-PdCl2, space group P21/c, a ) 20.520(2) Å, b ) 12.549(2) Å, c ) 13.9784(13) Å, Z ) 8,
V ) 3401.1(6) Å3; 4-PdCl2, space group Pbca, a ) 10.6545(10) Å, b ) 12.0205(11) Å, c ) 14.6474(14) Å, Z
) 8, V ) 1875.9(3) Å3.
Introduction
complexes should allow the study of cooperativity between
metal ions and proton donors or acceptors at near-physiological
pH: the free ligand exhibits a pKa of 14.2,7 whereas pyrazole
complexes of CrIII(NH3)5, CoIII(NH3)5, and RuIII(NH3)5 have pKa
values in the range 5.98-7.21.8
In order to anchor a metal ion firmly on a pyrazole derivative,
we have designed polydentate ligands (1 and 2, Scheme 1) to
form stable chelates involving one soft ligating atom (P or S)
and one pyrazole N, while leaving the other pyrazole N and its
attached hydrogen available for donating a hydrogen bond or a
proton to another ligand on the metal. Although hundreds of
polydentate ligands containing pyrazoles are known,9 virtually
all of them feature a pyrazole ring substituted at one nitrogen,
probably because of the ease of synthesizing such systems. For
Many metalloenzymes activate substrates using a combination
of one or more metal ions and nearby functional groups capable
of donating or accepting one or more hydrogen bonds or
protons.1,2 In recent years, a variety of artificial metal-ligand
complexes have been prepared to study the effects of such
cooperativity on reactions as diverse as the hydrolysis of
phosphate esters3 and carboxylic acid amides,4 the heterolysis
of dihydrogen,5 or the interaction of dioxygen and hemes.6
Pyrazole complexes have not been studied in this context.
However, very limited literature data on the acidifying effect
of metal complexation suggest that suitably designed pyrazole
* Correspondence author. E-mail: grotjahn@sundown.sdsu.edu.
(1) Lippard, S. J.; Berg, J. M. Principles of Bioinorganic Chemistry;
University Science Books: Mill Valley, CA, 1994; pp 257-281.
(2) For a discussion of carboxypeptidase, see: Creighton, T. E. Proteins:
Structures and Molecular Properties, 2nd ed.; W. H. Freeman: New
York, 1993; pp 431-434.
(3) Leading references: (a) Wall, M.; Linkletter, B.; Williams, D.; Lebuis,
A.-M.; Hynes, R. C.; Chin, J. J. Am. Chem. Soc. 1999, 121, 4710-
4711. (b) Ko¨va´ri, E.; Kra¨mer, R. J. Am. Chem. Soc. 1996, 118, 12704-
12709. (c) Kimura, E.; Kodama, Y.; Koike, T.; Shiro, M. J. Am. Chem.
Soc. 1995, 117, 8304-8311. (d) Morrow, J. R.; Aures, H.; Epstein,
D. J. Chem. Soc., Chem. Commun. 1995, 2431-2432. (e) Chu, F.;
Smith, J.; Lynch, V. M.; Anslyn, E. V. Inorg. Chem. 1995, 34, 5689-
5690. (f) Suh, J. Acc. Chem. Res. 1992, 25, 273-279. (g) Breslow,
R.; Berger, D.; Huang, D.-L. J. Am. Chem. Soc. 1990, 112, 3686-
3687.
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1987, 109, 1814-1826. (b) Groves, J. T.; Baron, L. A. J. Am. Chem.
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(5) (a) Lee, D.-H.; Patel, B. P.; Clot, E.; Eisenstein, O.; Crabtree, R. H.
Chem. Commun. (Cambridge) 1999, 297-298. (b) Lee, J. C., Jr.; Peris,
E.; Rheingold, A. L.; Crabtree, R. H. J. Am. Chem. Soc. 1994, 116,
11014-11019. (c) Lough, A. J.; Park, S.; Ramachandran, R.; Morris,
R. H. J. Am. Chem. Soc. 1994, 116, 8356-8357.
(6) Momenteau, M.; Reed, C. A. Chem. ReV. 1994, 94, 659-698.
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1984, 23, 2754-2763. Winter, J. A.; Caruso, D.; Shepherd, R. A.
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A. P.; Basson, S. S. Coord. Chem. ReV. 1996, 147, 247-297.
10.1021/ic990995q CCC: $19.00 © 2000 American Chemical Society
Published on Web 04/21/2000

Palladium(II)-Pyrazole Chelate Complexes
Inorganic Chemistry, Vol. 39, No. 10, 2000 2081
MHz for 13C) spectrometer. 1H and 13C NMR chemical shifts are
reported in parts per million downfield from tetramethylsilane and
referenced to residual solvent resonances (1H NMR, 7.27 for CHCl3
and 2.50 for CHD2SOCD3; 13C NMR, 77.23 for CDCl3 and 39.51 for
for CD3SOCD3), where 1H NMR chemical shifts are followed by
multiplicity, coupling constants J in hertz, and integration in parentheses.
For complex coupling patterns, e.g., “(dt, J ) 3.2, 7.6, 1H)”, the first
doublet (d) represents the smaller coupling, and the second triplet (t)
indicates the larger coupling. In some compounds, a doublet of doublets
Scheme 1
1
(dd) in H NMR for the pyrazole protons is reported as an apparent
triplet (app t) due to the signal appearance. Assignments are provided
for key moieties only. 31P{1H} NMR chemical shifts are referenced to
external 85% H3PO4 (aq).
IR spectra were obtained on a Perkin-Elmer 1600 FTIR spectrometer.
Samples were prepared either neat (NaCl plates) or in a solid (KBr
pellets), and absorptions are reported in wavenumbers (cm-1). Elemental
analyses were performed at NuMega Resonance Labs, San Diego, CA.
Chart 1
Chromatography was carried out with a Harrison Research Chro-
matotron under N2 atmosphere; silica gel (SiO2) and deoxygenated
solvents were used.
Bis(acetonitrile)palladium(II) dichloride was either purchased or
prepared in a manner similar to that reported for the benzonitrile
analogue.11
3-(Diphenylphosphinomethyl)pyrazole (1). To a solution of THF
(50 mL) and triphenylphosphine (1.011 g, 3.86 mmol) at room
temperature was added lithium (0.0533 g, 7.68 mmol), and after 2 h of
stirring the lithium had dissolved. The bright red solution was cooled
to 0 °C, and 3-(chloromethyl)pyrazole hydrochloride (12) (0.1942 g,
1.26 mmol) was added at once as a solid. The ice bath was removed
and the reaction solution was stirred for an additional 2 h. Deoxygenated
water (25 mL) was added to the reaction mixture followed by
deoxygenated Et2O (50 mL). The organic phase was separated and the
aqueous phase extracted with deoxygenated Et2O (2 × 25 mL). The
organic phases were combined and dried over MgSO4, filtered, and
concentrated. The crude residue was purified by chromatography (SiO2,
50% ethyl acetate/petroleum ether) to give 1 (0.246 g, 0.92 mmol, 73%)
example, deprotonation of pyrazole and subsequent reaction with
an alkyl halide lead to an N-alkylated pyrazole, and one-step
reaction of a borohydride salt and the appropriate amount of a
pyrazole derivative creates a poly(pyrazolyl)borate ion, where
anywhere from one to four pyrazolyl groups can be incorporated.
However, these and all other N-substituted pyrazole derivatives
are unsuitable for our purpose because, after complexation to
the remaining unsubstituted nitrogen atom, neither nitrogen is
available for proton- or hydrogen-bond acceptance. Therefore,
in this study, a pyrazole substituent containing a ligating P or
S atom must be attached to a carbon of the heterocycle. The
surprisingly few such compounds which have been prepared
and coordinated to transition metals are portrayed in Chart 1.10
Compounds 8a-c feature soft ligating atoms but readily form
binuclear complexes with a bridging pyrazolate ligand, an
undesirable situation in our work. Therefore, here new ligands
1 and 2 featuring one pyrazole ring substituted at carbon with
a single -CH2SCH3 or -CH2PPh2 group are reported, along
with their square-planar, mononuclear complexes to a cis-
dichloropalladium(II) fragment. Finally, for comparative pur-
poses isomeric ligands (3 and 4) and complexes without an NH
group are also presented.
1
as a clear colorless air-sensitive oil. H NMR (CDCl3, 500 MHz): δ
7.50-7.43 (m, 4 H), 7.41 (d, J ) 2.0 Hz, 1 H), 7.37-7.25 (m, 6 H),
5.99 (d, J ) 2.0 Hz, 1 H), 3.47 (s, 2 H) ppm. 13C{1H} NMR (CDCl3,
125.7 MHz): δ 144.52, 138.16 (d, J ) 14.3 Hz), 134.13, 132.97 (d, J
) 18.6 Hz), 129.11, 128.73 (d, J ) 6.6 Hz), 105.18 (d, J ) 5.1 Hz),
27.14 (d, J ) 6.2 Hz) ppm. 31P{1H} (CDCl3, 81.0 MHz): δ -14.45
ppm. IR (neat, NaCl) 3179, 3060, 2976, 2924, 1480, 1433 cm-1
.
cis-Dichloro[3-(diphenylphosphinomethyl)pyrazole]palladium-
(II) (1-PdCl2). To 1 (0.124 g, 0.46 mmol) and bis(acetononitrile)-
palladium(II) dichloride (0.121 g, 0.46 mmol) was added deoxygenated
methanol (10 mL). The reaction slurry was stirred for 14 h at room
temperature. The reaction slurry was filtered, and the solid was washed
with CH2Cl2 (2 × 10 mL). The solid residue was placed under vacuum
to give 1-PdCl2 (0.192 g, 0.43 mmol, 93%) as a yellow solid. Slow
evaporation of methanol from a dilute solution of 1-PdCl2 in methanol
afforded crystals suitable for X-ray analysis. 1H NMR (DMSO-d6, 200
MHz): δ 12.90 (s, 1 H), 7.88 (m, 5 H), 7.60 (m, 6 H), 6.54 (bs, 1 H),
4.03 (d, J ) 13 Hz, 2 H) ppm. 13C{1H} NMR (DMSO-d6, 50.3 MHz):
δ 152.38 (d, J ) 6.5 Hz), 134.09, 133.09 (d, J ) 11.0 Hz), 132.28 (d,
J ) 3.1 Hz), 129.14 (d, J ) 11.8 Hz), 127.67 (d, J ) 55.4 Hz), 104.56
(d, J ) 12.9 Hz), 28.68 (d, J ) 31.9 Hz) ppm. 31P{1H} NMR (DMSO-
d6, 81.0 MHz): δ 46.67 ppm. IR (KBr): 3286, 3170, 3047, 2937, 1509,
1434, 1199, 1109 cm-1. Anal. Calcd for C16H15Cl2N2PPd (443.60): C,
43.32; H, 3.41; N, 6.31. Found: C, 43.19; H, 3.27; N 6.02.
Experimental Section
General. Unless otherwise specified, 1H, 13C, and 31P data were
measured at room temperature on a 200 MHz (50.3 MHz for 13C and
1
81.0 MHz for 31P) or nominal 500 MHz (499.9 MHz for H, 125.7
(10) (a) Deters, R.; Kra¨mer, R., Inorg. Chim. Acta 1998, 269, 117-124.
(b) Singh, K.; Long, J. R.; Stavropoulos, P., Inorg. Chem. 1998, 37,
1073-1079. (c) Zadycowicz, J.; Potvin, P. G., J. Org. Chem. 1998,
63, 235-240. (d) Meyer, F.; Ruschewitz, U.; Schober, P.; Antelmann,
B.; Zsolnai, L. J. Chem. Soc., Dalton Trans. 1998, 1181-1186. (e)
Satake, A.; Nakata, T. J. Am. Chem. Soc. 1998, 120, 10391-10396.
(f) Ward, M. D.; Fleming, J. S.; Psillakis, E.; Jeffery, J. C.; McCleverty,
J. A. Acta Crystallogr., Sect. C 1998, C54, 609-612. (g) Meyer, F.;
Jacobi, A.; Zsolnai, L. Chem. Ber./Recueil 1997, 130, 1441-1447.
(h) Schenck, T. G.; Downes, J. M.; Milne, C. R. C.; Mackenzie, P.
B.; Boucher, H.; Whelan, J.; Bosnich, B. Inorg. Chem. 1985, 24,
2334-2337.
3-(Methylthiomethyl)pyrazole (2). 3-(Chloromethyl)pyrazole hy-
drochloride (12) (1.77 g, 11.6 mmol) was partially dissolved in THF
(100 mL) under a nitrogen atmosphere. At room temperature, lithium
thiomethoxide (1.25 g, 23.2 mmol) was added to the mixture. The
reaction mixture became slightly pink. The reaction slurry was stirred
for 10 h and quenched with water (3 mL). The organic phase was
extracted with ethyl acetate (3 × 10 mL). The organic phases were
(11) Doyle, J. R.; Slade, P. E.; Jonassen, H. B. Inorg. Synth. 1960, 6, 216-
219.

2082 Inorganic Chemistry, Vol. 39, No. 10, 2000
Grotjahn et al.
combined, dried over MgSO4, filtered, and concentrated. The crude
residue was purified by Kugelrohr distillation at 140 °C under oil pump
vacuum to 2 (1.22 g, 9.52 mmol, 82%) as a clear oil. 1H NMR (CDCl3,
500 MHz): δ 9.20 (bs, 1 H), 7.55 (d, J ) 2.5 Hz, 1 H), 6.24 (d, J )
2.5 Hz, 1 H), 3.77 (s, 2 H), 2.06 (s, 3 H) ppm. 13C{1H} NMR (CDCl3,
125.7 MHz): δ 146.31, 132.85, 104.78, 30.08, 15.42 ppm. IR (neat,
NaCl): 3180, 3163, 2971, 2910, 1571, 1529, 1468, 1432, 1143, 1101,
Scheme 2
1054, 981, 783 cm-1
.
cis-Dichloro[3-(methylthiomethyl)pyrazole]palladium(II) (2-P-
dCl2). To a solution of 2 (0.67 g, 5.25 mmol) in methanol (10 mL)
under nitrogen atmosphere was added at room temperature bis-
(acetonitrile)palladium(II) dichloride (1.36 g, 5.25 mmol). The pal-
ladium complex was dissolved in 5 min with stirring. After 12 h of
stirring, an orange precipitate formed. The reaction mixture was filtered,
and the solid was washed with methanol (2 × 5 mL). The solid residue
was placed under vacuum to give 2-PdCl2 (1.36 g, 4.46 mmol, 80%)
as a yellow solid. Crystals suitable for X-ray analysis were grown from
the diffusion of acetone into a solution of 2-PdCl2 in DMSO. 1H NMR
(DMSO-d6, 500 MHz): δ 12.5 (s, 1 H), 7.89 (d, J ) 2.0 Hz, 1 H),
6.54 (d, J ) 2.0 Hz, 1 H), 4.31 (d, J ) 16.5 Hz, 1 H), 3.99 (d, J )
16.5 Hz, 1 H), 2.62 (s, 3 H) ppm. 13C{1H} (DMSO-d6, 125.7 MHz):
δ 154.92, 133.44, 104.61, 35.06, 23.15 ppm. IR (KBr): 3329, 3129,
2921, 2850, 1511, 1420, 1371, 778 cm-1. Anal. Calcd for C5H8Cl2N2PdS
(305.52): C, 19.66; H, 2.64; N, 9.17. Found: C, 19.39; H, 2.34; N,
8.94.
1-(Diphenylphosphinomethyl)pyrazole (3). Diphenylphosphine
(2.642 g, 14.2 mmol) was placed into a Schlenk flask with degassed
THF (50 mL). The solution was cooled to -78 °C, and n-butyllithium
(8.4 mL, 1.6 M in hexanes, 15.0 mmol) was added dropwise. The red
solution was stirred at -78 °C for an additional 1 h. The ice bath was
removed, and the solution was stirred for an additional 3 h. The red
solution was cooled to 0 °C, and 13 (0.698 g, 4.56 mmol) was added
at once as a solid. The ice bath was removed, and the reaction mixture
was stirred for 11 h before addition of deoxygenated methanol (25 mL)
and water (20 mL). The organic phase was separated, and the aqueous
phase was extracted with deoxygenated Et2O (3 × 10 mL). The organic
phase was dried over MgSO4, filtered, and concentrated. The crude
material was purified by chromatography (SiO2, 10% ethyl acetate/
petroleum ether) to give 3 (0.574 g, 2.16 mmol, 47%) as a white solid.
1H NMR (CDCl3, 500 MHz): δ 7.49 (dd, J ) 2.0, 0.5 Hz, 1 H), 7.45-
7.40 (m, 4 H), 7.40-7.35 (m, 6 H), 7.25 (dd, J ) 2.5, 0.5 Hz, 1 H),
6.19 (dd, J ) 2.5, 2.0 Hz, 1 H), 4.91 (d, J ) 4.5 Hz, 2 H) ppm. 13C-
{1H} NMR (CDCl3, 125.7 MHz): δ 139.50, 136.04 (d, J ) 13.4 Hz),
133.18 (d, J ) 19.3 Hz), 129.34, 129.49, 128.95 (d, J ) 6.4 Hz), 106.11,
53.01 (d, J ) 16.0 Hz) ppm. 31P{1H} NMR (CDCl3, 81.0 MHz): δ
-14.98 ppm. IR (KBr): 3129, 3111, 3107, 3069, 3049, 3025, 3015,
3002, 2969, 2905, 1427, 1386, 1089, 1042 cm-1. Anal. Calcd for
C16H15N2P (266.28): C, 72.17; H, 5.68; N, 10.52. Found: C, 72.04;
H, 5.56; N, 10.28.
and quenched with water (3 mL). The organic phase was extracted
with ethyl acetate (3 × 10 mL). The organic phases were combined,
dried over MgSO4, filtered, and concentrated. The crude residue was
purified by Kugelrohr distillation at 60-70 °C under oil-pump vacuum
to give 4 (1.67 g, 11.7 mmol, 70%) as a yellow oil.
Method B. Pyrazole (9) (2.00 g, 29.0 mmol) was dissolved in THF
(20 mL), and the reaction solution was cooled to 0 °C. Sodium hydride
(0.70 g, 29.0 mmol) was slowly added. After the bubbling ceased,
(chloromethyl)methyl sulfide (2.80 g, 29.0 mmol) was added at 0 °C
and the reaction mixture was warmed to room temperature by removal
of the ice bath. The reaction slurry was stirred for an additional 30
min before it was concentrated. The crude mixture was purified as
described in method A to give the 4 (1.72 g, 13.3 mmol, 46%) as a
1
yellow oil. H NMR (CDCl3, 500 MHz): δ 7.58 (d, J ) 2 Hz, 1 H),
7.51 (d, J ) 1.5 Hz, 1 H), 6.32 (app t, J ) 2.0 Hz, 1 H), 5.15 (s, 2 H),
2.12 (s, 3 H) ppm. 13C{1H} NMR (CDCl3, 125.7 MHz): δ 139.79,
128.94, 106.87, 54.79, 15.03 ppm. IR (neat, NaCl): 3103, 2983, 2923,
1510, 1432, 1083, 1047, 964, 752 cm-1
.
cis-Dichloro[1-(methylthiomethyl)pyrazole]palladium(II) (4-Pd-
Cl2). To a solution of 4 (0.160 g, 1.25 mmol) in methanol (10 mL)
was added bis(acetonitrile)palladium(II) dichloride (0.325 g, 1.25 mmol)
at room temperature. The reaction was stirred for 12 h, during which
time a yellow precipitate formed. The reaction mixture was filtered,
and the solid was washed with methanol (2 × 5 mL). The solid residue
was placed under vacuum to give pure 4-PdCl2 (0.297 g, 0.98 mmol,
78%) as a yellow solid. Crystals suitable for X-ray analysis were grown
from the diffusion of acetone into a solution of 4-PdCl2 in DMSO. 1H
NMR (DMSO-d6, 500 MHz): δ 8.33-8.30 (m, 1 H), 7.92-7.89 (m,
Hz, 1 H), 6.63-6.60 (m, 1 H), 5.51 (d, J ) 12.5 Hz, 2 H), 5.38 (d, J
) 12.5 Hz, 2 H), 2.53 (s, 3 H). 13C{1H} (DMSO-d6, 125.7 MHz): δ
141.44, 134.32, 108.15, 54.83, 21.16. IR (KBr): 3120, 2975, 2915,
1404, 1278, 1056, 767 cm-1. Anal. Calcd for C5H8Cl2N2PdS (305.52):
C, 19.66; H, 2.64; N, 9.17. Found: C, 19.42; H, 2.34; N, 8.88.
3-(Hydroxymethyl)pyrazole hydrochloride (11) is a known com-
pound,12 but this is a new procedure for making it. To a solution of
1-(hydroxymethyl)pyrazole (3.49 g, 35.6 mmol) in THF (150 mL) was
added a solution of n-BuLi in hexanes (32 mL, 2.5 M, 80.0 mmol) at
-78 °C. After addition was completed, a white precipitate formed. The
reaction slurry was warmed to -20 °C for 2 h before the addition of
paraformaldehyde (1.34 g, 44.5 mmol). The reaction slurry was warmed
to room temperature and stirred for 10 h, after which time 2 N HCl
(aq) was added until the pH of the mixture reached 4. After stirring
for 4 h, the solution was neutralized with saturated NaHCO3 (aq) until
the pH of the mixture reached 7-8. The solvents were removed by
cis-Dichloro[1-(diphenylphosphinomethyl)pyrazole]palladium-
(II) (3-PdCl2). A flask was charged with 3 (0.049 g, 0.184 mmol) and
bis(acetonitrile)palladium(II) dichloride (0.048 g, 0.186 mmol). Deoxy-
genated methanol (5 mL) was added. The resulting yellow solution
instantaneously became cloudy. The slurry was stirred for 5 h at room
temperature and then filtered through a glass frit. The precipitate was
washed with CH2Cl2 and then dried under vacuum (0.05 mmHg), giving
3-PdCl2 (0.068 g, 0.153 mmol, 83%) as a yellow solid. Crystals suitable
for X-ray analysis were grown from the slow evaporation from a
1
solution of 3-PdCl2 in CH2Cl2. H NMR (DMSO-d6, 200 MHz): δ
8.26-8.22 (m, 1 H), 8.12-8.08 (m, 1 H), 8.00-7.80 (m, 4 H), 7.75-
7.40 (m, 6 H), 6.63-5.99 (m, 1 H), 5.47 (d, J ) 8.2 Hz). 13C{1H}
NMR (CDCl3, 125.7 MHz): δ 141.50, 133.68, 133.43 (d, J ) 11.4
Hz), 132.93 (d, J ) 3.0 Hz), 129.39 (d, J ) 12.0 Hz), 125.98 (d, J )
58.4 Hz), 108.67, 49.98 (d, J ) 37.6 Hz); 31P{1H} NMR (CDCl3, 81.0
MHz): δ 43.43 ppm. IR (KBr): 3117, 3036, 2933, 2899, 2841, 1434,
1405, 1101 cm-1. Anal. Calcd for C16H15Cl2N2PPd (443.60): C, 43.32;
H, 3.41; N, 6.31. Found: C, 43.54; H, 3.40; N, 6.06.
1-(Methylthiomethyl)pyrazole (4). Method A. 1-(Chloromethyl)-
pyrazole hydrochloride (13) (2.00 g, 13.0 mmol) was mixed with dry
THF (100 mL), and lithium thiomethoxide (1.40 g, 26.0 mmol) was
added at room temperature. The reaction slurry was stirred for 12 h
(12) Jones, R. G. J. Am. Chem. Soc. 1949, 71, 3994-4000.

Palladium(II)-Pyrazole Chelate Complexes
Inorganic Chemistry, Vol. 39, No. 10, 2000 2083
2-PdCl2
Table 1. Crystallographic Data for 1-PdCl2, 2-PdCl2, 3-PdCl2, and 4-PdCl2
1-PdCl2
empirical formula
fw
C16H15Cl2N2PPd
C5H8Cl2N2PdS
305.49
443.57
temp
291(2) K
294(2) K
wavelength
cryst syst
space group
unit cell dimens
0.71073 Å
0.71073 Å
triclinic
orthorhombic
P1h
Pbca
a ) 8.4488 (9) Å , R ) 97.758 (13)°
b ) 8.9175 (13) Å, â ) 93.558(10)°
c ) 12.731 (2) Å, γ ) 112.409 (9)°
871.8(2) Å3
a ) 10.8827(10) Å, R ) 90°
b ) 11.7721(7) Å, â ) 90°
c ) 14.874(2) Å, γ ) 90°
1905.6 Å3
vol
Z and F(000)
2 and 440
8 and 1184
density (calcd)
1.690 Mg/m3
2.130 Mg/ m3
abs coeff
1.459 mm-1
2.666 mm-1
abs corr
none
semiempirical from ψ-scans
0.60 × 0.50 × 0.40
2.74-30.00°
cryst size
0.38 × 0.35 × 0.12 mm
θ range for data collection
scan type and scan width
scan time/background time
index ranges
1.63-25.00°
1
2
1
2
2θ-θ and KR - 1° to KR + 1°
2θ-θ and KR - 1° to KR + 1°
2:1
2:1
-1 e h e 9, -10 e k e 10, -15 e l e 15
0 e h e 15, 0 e k e 16, 0 e l e 20
2785
reflns collected
indep reflns
3747
3066 (Rint ) 0.0295)
2785
max and min transm
refinement meth
data/restraints/params
goodness of fit on F2, (S)
final R indices [I>2σ(I)]
R indices (all data)
largest diff peak and hole
0.3129 and 0.2335
full-matrix least squares on F2
2782/0/100
full-matrix least squares on F2
3063/0/199
1.081
R1 ) 0.0298, wR2 ) 0.0736
R1 ) 0.0403, wR2 ) 0.0972
0.433 and -0.429 e Å-3
0.998
R1 ) 0.0358, wR2 ) 0.0917
R1 ) 0.0554, wR2 ) 0.1189
1.070 and -0.602 e Å-3
3-PdCl2
4-PdCl2
empirical formula
fw
C16H15Cl2N2PPd
443.57
C5H8Cl2N2PdS
305.49
temp
294(2) K
294(2) K
wavelength
cryst sys
space group
unit cell dimens
0.71073 Å
0.71073 Å
monoclinic
orthorhombic
P21/c
Pbca
a ) 20.520 (2) Å, R ) 90°
b ) 12.549 (2) Å, â ) 109.112 (7)°
c ) 13.9784 (13) Å, γ ) 90°
3401.1(6) Å3
a ) 10.6545 (10) Å, R ) 90°
b ) 12.0205 (11) Å, â ) 90°
c ) 14.6474 (14) Å, γ ) 90°
1875.9(3) Å3
vol
Z and F(000)
8 and 1760
8 and 1184
density (calcd)
1.733 Mg/m3
2.163 Mg/m3
abs coeff
1.496 mm-1
2.708 mm-1
abs corr
semiempirical from ψ-scans
0.50 × 0.45 × 0.32 mm
2.10-25.00°
semiempirical from ψ-scans
0.45 × 0.35 × 0.28 mm
2.78-30.00°
cryst size
θ range for data collection
scan type and scan width
scan time/background time
index ranges
1
2
1
2
2θ-θ and KR - 1° to KR + 1°
2θ-θ and KR - 1° to KR + 1°
2:1
2:1
-24 e h e 23, -14 e k e 0, 0 e l e 16
6264
0 e h e 14, 0 e k e 16, 0 e l e 20
2743
reflns collected
indep reflns
5992 (Rint ) 0.0425)
2743
max and min transm
refinement meth
data/restraints/params
goodness of fit on F2, (S)
final R indices [I>2σ(I)]
R indices (all data)
largest diff peak and hole
0.4197 and 0.3506
0.3575 and 0.2019
full-matrix least squares on F2
2742/0/100
full-matrix least squares on F2
5985/0/397
1.020
1.07
R1 ) 0.0559, wR2 ) 0.1127
R1 ) 0.1250, wR2 ) 0.1496
0.707 and -0.666 e Å-3
R1 ) 0.0356, wR2 ) 0.0937
R1 ) 0.0447, wR2 ) 0.1024
0.715 and -1.640 e Å-3
rotary evaporation, leaving a thick oil, which was subjected to Kugelrohr
distillation under vacuum (0.05 mmHg, 120-140 °C), affording a clear
colorless oil, which was dissolved in methanol (2 mL). Concentrated
HCl (aq) was added. After 15 min, solvents were removed by rotary
evaporation and the resulting solid residue was redissolved in methanol
(2 mL), followed by the addition of diethyl ether (10 mL), which caused
a white precipitate to form. The white precipitate was collected by
vacuum filtration and dried under vacuum to yield 3-(hydroxymethyl)-
pyrazole hydrochloride (2.20 g, 46%).
the acidic pyrazole NH function was protected with a group
which would also direct deprotonation of the pyrazole ring with
strong base. Katritzky and co-workers13 have previously reported
the synthesis of 1-hydroxymethylpyrazole (10, Scheme 2) from
pyrazole (9) and aqueous formaldehyde in THF. We have found
that organic solvent was unnecessary; moreover, the yield of
10 exceeded 95%. Protected pyrazole 10 was then lithiated as
reported with 2 equiv of n-butyllithium.13 We found that the
resulting species generated in situ could be hydroxyalkylated
with formaldehyde. Subsequent deprotection in aqueous hydro-
chloric acid gave 3-hydroxymethylpyrazole12 as its hydrochlo-
ride salt 11 in 46-64% yield. Conversion of the alcohol
Results and Discussion
Synthesis and Properties of 3-Phosphino- and 3-Thio-
methyl Pyrazole Palladium(II) Dichlorides. To introduce a
functionalized side chain at carbon of the pyrazole ring, first
(13) Katritzky, A. R.; Lue, P.; Akutagawa, K. Tetrahedron 1989, 45, 4243-
4262.

2084 Inorganic Chemistry, Vol. 39, No. 10, 2000
Grotjahn et al.
PCH2 side chain in ligand 1 was identified by a singlet in the
31P{1H} NMR spectrum at -14.45 ppm and a carbon doublet
in the 13C{1H} NMR spectrum at 27.14 ppm, while the MeSCH2
1
side chain in 2 was identified by two singlets in the H NMR
spectrum at 2.06 and 3.77 ppm, ascribed to the methyl and
methylene protons, respectively.
Pyrazole ligands 1 and 2 were cleanly complexed to the PdCl2
fragment in yields of 93% and 80%, respectively (Scheme 2).
1
The H NMR spectra of 1-PdCl2 and 2-PdCl2 exhibit slightly
broad singlets at 12.90 and 12.50 ppm, respectively, verifying
the anticipated lack of oxidative addition of the pyrazole N-H
bond to Pd(II). Complexation of phosphorus to palladium in
complex 1-PdCl2 was indicated by a signal at 46.67 ppm in the
31P{1H} NMR spectrum, significantly downfield of the chemical
shift shown by the free phosphine. Finally, complex 2-PdCl2
was easily identified by two mutually coupled doublets (3.99
and 4.31 ppm, J ) 16.5 Hz) in its 1H NMR spectrum, ascribed
to two diastereotopic protons on the methylene carbon.
Crystals of the two complexes suitable for X-ray diffraction
were grown in the case of 1-PdCl2 by the slow evaporation of
methanol at room temperature and for 2-PdCl2 by the diffusion
of acetone into a DMSO solution of the complex. The solid
state structures of 1-PdCl2 and 2-PdCl2 were solved as described
in Table 1, and the ORTEP views of both complexes from above
the square plane are shown in Figures 1 and 2. Figures 3 and 4
provide views of the closest intermolecular hydrogen bonds for
the two complexes.
Palladium dichloride complex 1-PdCl2 is a slightly distorted
square planar complex with the sum of four angles around the
palladium equal to 359.9(1)°. Selected bond lengths and angles
are shown in Table 2. The distance between Pd and the Cl trans
to P [Cl(1), 2.3941(10) Å] is about 0.11 Å longer than the Pd-
Cl(2) bond length, presumably because of the greater trans
influence of the phosphorus ligand. In comparison, the sum of
four angles around the palladium in complex 2-PdCl2 is 360.0-
(1)°. The Pd-Cl(1) and Pd-Cl(2) bond lengths are 2.3127(12)
Figure 1. Molecular structure of 1-PdCl2.
Figure 2. Molecular structure of 2-PdCl2.
functional group in 11 to a chloride in 12 was accomplished
with thionyl chloride12 in 93% yield.
Using a procedure similar to that reported by Bosnich10h and
co-workers for the preparation of 1,3-bis(diphenylphosphino-
methyl)pyrazole (8a) from 1,3-bis(chloromethyl)pyrazole hy-
drochloride salt and excess lithium diphenylphosphide, we were
able to obtain the desired phosphine ligand 1 in 73% yield using
3 equiv of LiPPh2 and 3-chloromethylpyrazole hydrochloride.
In addition, thioether ligand 2 could be prepared in 82% yield
by treatment of chloride 12 with 2 equiv of LiSMe. The Ph2-
Figure 3. Diagram of part of the unit cell of 1-PdCl2, showing closest intermolecular contacts of chloride and the atoms of the nearest NH group.
A: N2A-Cl1C ) 3.280 Å. B: H2A-Cl1C ) 2.587 Å.

Palladium(II)-Pyrazole Chelate Complexes
Inorganic Chemistry, Vol. 39, No. 10, 2000 2085
Table 2. Selected Bond Lengths (Å) and Angles (deg) for Palladium Complexes 1-PdCl2, 2-PdCl2, 3-PdCl2, and 4-PdCl2
3-PdCl2
bond lengths and angles
1-PdCl2
2-PdCl2
molecule 1
molecule 2
4-PdCl2
Pd-N(1)
2.011(3)
2.3941(10)
2.2812(11)
2.2097(10)
2.796
1.987(4)
2.3127(12)
2.2858(12)
2.2716(12)
2.738
2.017(7)
2.368(2)
2.276(2)
2.202(2)
na
2.024(7)
2.363(3)
2.286(2)
2.205(2)
na
2.007(3)
2.3078(9)
2.2911(10)
2.2661(9)
na
Pd-Cl(1)
Pd-Cl(2)
Pd-L, L ) P(1), S(1)
Cl(1)-H(2A)
Cl(1)-H intermolecular
N(1)-Pd-L, L ) P(1), S(1)
P(1)-Pd-Cl(2)
N(1)-Pd-Cl(1)
Cl(1)-Pd-Cl(2)
2.587
2.772
na
na
na
81.10(10)
90.99(4)
91.86(10)
95.96(4)
83.93(11)
91.28(5)
91.37(11)
93.42(5)
85.1(2)
89.17(2)
92.8(2)
92.87(2)
84.6(2)
89.49(9)
92.7(2)
93.21(11)
85.02(8)
90.53(4)
91.31(8)
93.11(4)
Scheme 3
identification of the hydrogen-chlorine contacts seen in struc-
tures of 1-PdCl2 and 2-PdCl2 as hydrogen bonds.
Figure 4. Diagram of the unit cell of 2-PdCl2, showing the closest
intermolecular contacts of chloride and the atoms of the nearest NH
group. A: H2AA-Cl2D ) 2.772 Å.
Synthesis and Properties of 1-Substituted Pyrazole Ligands
3 and 4 and Their Complexes. The isoelectronic ligands 3
and 4 lacking an N-H functional group because of the
attachment of the side chain at the 1-position on the pyrazole
ring have been prepared in two steps from 1-hydroxymeth-
ylpyrazole (10). These ligands give access to complexes not
capable of hydrogen bonding when chelated to a metal through
phosphorus or sulfur and the unsubstituted nitrogen, and allow
for a direct comparison with their N-H derivatives 1-PdCl2
and 2-PdCl2. The synthesis of chelating ligands 3 and 4 is
outlined in Scheme 3. Literature methods were used to convert
the alcohol functionality in 10 to a chloride in hydrochloride
salt 13.17 Chloride 13 was then treated with lithium diphen-
ylphosphide and lithium thiomethoxide in the same way as was
isomer 12 to give either phosphine 3 or thioether 4 in 47% or
70% yield, respectively. Thioether 3 could also be prepared from
the treatment of pyrazole with sodium hydride18 and ClCH2SCH3
in 46% yield. The Ph2PCH2 side chain in ligand 3 could be
identified by a singlet in the 31P{1H} NMR spectrum at -14.98
ppm, a doublet in the 13C{1H} NMR spectrum at 49.98 ppm,
and 2.2858(12) Å, respectively, which are quite consistent with
the similar trans influence for nitrogen and sulfur. In the
structure of 1-PdCl2, the intramolecular bond distance between
Cl(1) and N-H hydrogen is 2.796 Å, but there is an even closer
intermolecular contact between Cl(1) and an N-H hydrogen
on another molecule in the unit cell (2.587 Å, see Figure 3). In
thioether complex 2-PdCl2, the intramolecular bond distance
between Cl(1) and N-H hydrogen is 2.738 Å (Figure 2),
whereas the nearest intermolecular contact between Cl(1) and
an N-H hydrogen on another molecule is 2.772 Å (Figure 4).
Similar intramolecular14 and intermolecular14e,15 contacts be-
tween halide and pyrazole ligands have been invoked as proof
of hydrogen bonding. For comparison, van der Waals radii of
H and Cl are 1.20 and 1.75 Å, respectively,16 consistent with
(14) Intramolecular hydrogen bonding between halide and pyrazole
ligands: (a) Reedijk, J.; Stork-Blaisse, B. A.; Verschoor, G. C. Inorg.
Chem. 1971, 10, 2594-2599. (b) Mighell, A. D.; Reimann, C. W.;
Santoro, A. Acta Crystallogr., Sect. B 1969, B25, 595-599. (c)
Reimann, C. W. J. Chem. Soc., Chem. Commun. 1969, 145-146. (d)
Reimann, C. W.; Mighell, A. D.; Mauer, F. A. Acta Crystallogr. 1967,
23, 135-141. (e) Esteruelas, M. A.; Oliva´n, M.; On˜ate, E.; Ruiz, N.;
Tajada, M. A. Organometallics 1999, 18, 2953-2960.
1
and a doublet in the H NMR spectrum at 5.47 ppm for the
methylene bridging unit. The MeSCH2 side chain of 4 was
identified by two singlets in the 1H NMR spectrum at 2.12 and
5.15 ppm for the methyl and methylene protons, respectively.
Coordination of the PdCl2 fragment to ligands 3 and 4 gave
complexes 3-PdCl2 and 4-PdCl2 in 83% and 78% yields,
(15) Intermolecular hydrogen bonding involving pyrazole ligands: (a)
Reference 10a (to ionic chloride). (b) Reference 10f (Pd-Cl‚‚‚H-N
with H‚‚‚Cl ) 2.40 A). (c) For a related complex, see: Redmore, S.
M.; Rickard, C. E. F.; Webb, S. J.; Wright, L. J. Inorg. Chem. 1997,
36, 4743-4748.
(17) Katritzky, A. R.; Lam, J. N. Can. J. Chem. 1989, 67, 1144-1147.
(18) Begtrup, M.; Larsen, P. Acta Chem. Scand. 1990, 44, 1050-1057.
(16) Bondi, A. J. Phys. Chem. 1964, 68, 441-451.

2086 Inorganic Chemistry, Vol. 39, No. 10, 2000
Grotjahn et al.
in the bond lengths Pd-Cl(1) [2.368(2) Å] and Pd-Cl(2)
[2.276(2) Å] is ascribed to the stronger trans influence of the
phosphorus. In contrast to the complexes with an N-H moiety,
there are no apparent intermolecular interactions in the unit cell
of 3-PdCl2 or 4-PdCl2.
The sum of the four angles around the palladium in 4-PdCl2
is 359.9(1)°. The Pd-Cl(1) and Pd-Cl(2) bond lengths are
2.3078(9) Å and 2.2911(10) Å, respectively, almost identical
to the analogous bond lengths in complex 2-PdCl2. However,
there are no intermolecular interactions between Cl(1) or Cl(2)
and C-H hydrogens on other molecules, which is consistent
with the absence of an N-H hydrogen bond donor. Selected
bond lengths and angles are shown in Table 2.
Figure 5. Molecular structure of 3-PdCl2.
Comparing isomeric complexes (1-PdCl2 vs 3-PdCl2, 2-PdCl2
vs 4-PdCl2), the Pd-ligand distances are within 0.03 Å. As for
interligand bond angles, complexes 1-PdCl2 and 2-PdCl2 have
N-Pd-L and Cl(1)-Pd-Cl(2) angles smaller than do 3-PdCl2
and 4-PdCl2, respectively. These more acute bond angles may
be attributed to different five-membered rings in complexes
1-PdCl2 and 2-PdCl2 (P/S, C, C, N, Pd) vs the five-membered
rings in 3-PdCl2 and 4-PdCl2 (P/S, C, N, N, Pd) and not due to
a chemical difference from the direct coordination of the ligands
on palladium.
Conclusion
Figure 6. Molecular structure of 4-PdCl2.
Four new ligands designed to form bidentate complexes using
a soft donor atom and a pyrazole nitrogen have been synthesized
and their corresponding cis-dichloropalladium(II) complexes
prepared. In the solid state, the two palladium complexes (1-
PdCl2 and 2-PdCl2) with a pyrazole N-H group show intra-
and intermolecular hydrogen bonding to the chloride ligands.
The intramolecular distances between chloride and the NH
proton are comparable to the closest intermolecular contacts
found in the solid state structures. In contrast, isomeric
complexes lacking an N-H group (3-PdCl2 and 4-PdCl2) show
no hydrogen bonding. Future work will explore the cooperativity
of the metal center and the coordinated pyrazole ligand.
respectively (Scheme 3), using the same conditions developed
for their isomers. NMR spectral changes on complexation were
similar to those seen in the reactions of 1 and 2. Coordination
of phosphorus to the metal in 3-PdCl2 was revealed by a
significant downfield 31P chemical shift (43.43 ppm). Binding
of sulfur in 4-PdCl2 with the formation of a stereogenic center
and a pair of diastereotopic protons at the methylene carbon
was shown by the two mutually coupled doublets at 5.38 and
1
5.51 ppm (J ) 16.5 Hz) in the H NMR spectrum.
Crystals of the new Pd(II) complexes suitable for X-ray
diffraction were grown by the slow evaporation of dichlo-
romethane at room temperature (3-PdCl2) or by diffusion of
acetone into a solution of 4-PdCl2 and DMSO. The solid state
structures of 3-PdCl2 and 4-PdCl2 were solved as described in
Table 1, and the top ORTEP views of both complexes are shown
in Figures 5 and 6.
Acknowledgment. NSF funding (CHE 9413802) for a 500
MHz NMR spectrometer for SDSU and CONACYT Proyecto
Infraestructura funding (F264-E9207) for an X-ray diffracto-
meter are gratefully acknowledged.
Supporting Information Available: Tables of all crystal bond
distances and angles, anisotropic displacement coefficients, H atom
coordinates, and anisotropic displacement coefficients and diagrams
of intermolecular contacts for 1-PdCl2 and 2-PdCl2. This material is
available free of charge via the Internet at http://pubs.acs.org.
The unit cell of 3-PdCl2 contains two independent molecules,
whose metrical parameters are virtually the same, within
experimental uncertainty. For simplicity only the bond lengths
and angles for molecule 1 are described (Table 2). Complex
3-PdCl2 is a square planar complex with the sum of four angles
around the palladium equal to 359.9(2)°. The large difference
IC990995Q