Synthesis and characterization of 1-(3,5-di-tert-butyl)pyrazolyldiphenylphosphine and its gold(I) complex

Article abstract of DOI:10.1016/j.inoche.2007.08.012

1-(3,5-Di-tert-butyl)pyrazolyldiphenylphosphine was prepared from 3,5-di-tert-butyl)pyrazole and chlorodiphenylphosphine. It reacted with (Me₂S)AuCl to afford a Au(I) complex bearing the pyrazolylphosphine ligand in a monodentate coordination mode.

Full text of DOI:10.1016/j.inoche.2007.08.012

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Inorganic Chemistry Communications 10 (2007) 1339–1341
www.elsevier.com/locate/inoche
Synthesis and characterization of
1-(3,5-di-tert-butyl)pyrazolyldiphenylphosphine and its gold(I) complex
a,
b
a
Aibing Xia *, Corey Seward , Mariza Voges
a
Department of Chemistry, Mount Saint Vincent University, Halifax, Nova Scotia, Canada B3M 2J6
b
Department of Chemistry, Saint Mary’s University, Halifax, Nova Scotia, Canada B3H 3C3
Received 29 June 2007; accepted 16 August 2007
Available online 28 August 2007
Abstract
1-(3,5-Di-tert-butyl)pyrazolyldiphenylphosphine was prepared from 3,5-di-tert-butyl)pyrazole and chlorodiphenylphosphine. It
reacted with (Me2S)AuCl to afford a Au(I) complex bearing the pyrazolylphosphine ligand in a monodentate coordination mode.
Ó 2007 Elsevier B.V. All rights reserved.
Keywords: Unsymmetrical ligands; Hemilabile property; Pyrazolylphosphine; Gold(I) complex
Unsymmetrical P,N ligands, which contain one phos-
phorus donor and one nitrogen donor, have attracted
increasing attention recently due to their bonding versatil-
ity, unique reactivity, and catalytic applications as hemila-
bile ligands [1,2]. Pyrazolylphosphines (Fig. 1), with a P–N
bond between one pyrazolyl group and one phosphino
group, were prepared by Peterson and co-workers over
30 years ago [3]. They are air- and moisture-sensitive,
and react readily with protic species such as alcohols
through the nitrogen atom at the 2-position (N(2)) of the
pyrazolyl group, leading to cleavage of the P–N bond.
Therefore even though they were easy to prepare and iso-
late, their applications as ancillary ligands were scarce
and mainly limited to mid-transition metal carbonyl com-
plexes, in which the pyrazolylphosphine ligands acted as
either monodentate or bidentate donors [4]. In these com-
plexes, the electron-withdrawing carbonyls make the N(2)
atom in the pyrazolyl group less nucleophilic, which stabi-
lizes the pyrazolylphosphine ligands. But stable late transi-
tion-metal complexes bearing pyrazolylphosphine ligands
are even more elusive. Recently Ros and co-workers reac-
ted[RuCl2(p-cymene)]2 with 1-(3,5-di-methyl)pyrazolyldi-
phenylphosphine, PPh2(3,5-Me2pz) [5]. However, the
targeted product RuCl2(p-cymene)(PPh2(3,5-Me2pz)) was
always contaminated by by-products such as [RuCl(p-cym-
ene)(3,5-Me2pzH)(PPh2OH)]Cl, indicating hydrolysis of
the pyrazolylphosphine ligand via P–N bond cleavage. As
electron-rich late transition metal complexes are widely
used in a variety of important catalytic processes [6], it is
desirable to employ electron-rich pyrazolylphosphines as
ancillary ligands in late transition-metals. Here we report
the synthesis, characterization, and application of a novel
pyrazolylphosphine compound, 1-(3,5-di-tert-butyl)pyra-
zolyldiphenylphosphine, which contains bulky tert-butyl
substituents in the pyrazolyl moiety and shows improved
stability over its smaller analogues.
Reaction of chlorodiphenylphosphine with 3,5-di-tert-
butylpyrazole [7] in diethyl ether in the presence of
4-N,N-dimethylaminopyridine (DMAP) afforded 1-(3,5-di-
tert-butyl)pyrazolyldiphenylphosphine, PPh2(3,5-tBu2pz),
in quantitative yield (Eq. (1)). The resultant compound
can be easily obtained after filtering of the pyridinium salt
and removal of solvent under reduced pressure [8].
PPh2(3,5-tBu2pz) demonstrated drastically improved stabil-
ity over its smaller analogues. While PPh2(3,5-Me2pz) and
PPh2pz are very air- and moisture-sensitive, PPh2(tBu2pz)
may be recrystallized from bench diethyl ether in air [9].
*
Corresponding author. Tel.: +1 902 457 6543.
E-mail address: aibing.xia@msvu.ca (A. Xia).
1387-7003/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2007.08.012

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A. Xia et al. / Inorganic Chemistry Communications 10 (2007) 1339–1341
Fig. 1. Pyrazolylphosphines.
In addition, PPh2(3,5-tBu2pz) showed good thermal stabil-
ity and could be sublimed under vacuum. In a GC/MS
analysis of the compound, one single peak was observed
with a molecular mass of 364. As PPh2(3,5-tBu2pz) is more
electron-rich than PPh2(3,5-Me2pz), the improved stability
of the more bulky compound can be attributed to the steric
factor instead of an electronic one.
Fig. 2. Molecular Structure of (PPh2(3,5-tBu2pz))AuCl (thermal ellipsoids
˚
drawn at the 30% probability). Selected bond lengths (A) and angles (°):
Au(1)–P(1) 2.228(1), Au(1)–Cl(1) 2.276(1), P(1)–N(1) 1.725(4), P(1)–C(18)
1.803(5), P(1)–C(12) 1.807(5); P(1)–Au(1)–Cl(1) 176.93(5), N(1)–P(1)–
Au(1) 116.61(14), C(18)–P(1)–Au(1) 114.96(15), C(12)–P(1)–Au(1)
109.43(16), N(1)–P(1)–C(18) 103.7(2), N(1)–P(1)–C(12) 104.97(18),
C(18)–P(1)–C(12) 106.3(2).
ð1Þ
Addition of PPh2(3,5-tBu2pz) to a suspension of
(Me2S)AuCl (prepared from reaction of Me2S and HAuCl4
in MeOH) in CH2Cl2 led to a clear colorless solution at
room temperature (Eq. (2)). The solution showed a single
peak at 75.38 ppm in 31P NMR spectrum, indicating a sin-
gle phosphorus-containing product. The 29.10 ppm down-
field shift after coordination of the ligand to Au(I) is
similar to that of its methyl analogues [4–6]. The 1H
NMR spectrum of the Au(I) complex showed that chemi-
cal shifts of the methine (CH) and two tert-butyl groups
in the pyrazolyl moiety are slightly changed upon coordi-
nation [8,10]. The methine hydrogen showed a doublet with
a coupling constant of 2.75 Hz, reflecting the long-range
interaction between phosphorus and hydrogen nuclei [3,4].
N(2) atom on the pyrazolyl group is flanked by two phenyl
groups and an adjacent bulky tert-butyl group, which gives
rise to improved stability against hydrolysis and oxidation.
In summary, we demonstrated that through steric con-
trol, it is feasible to prepare electron-rich and stable pyraz-
olylphosphine compounds. The desirable steric and
electronic features, combined with the ease of preparation
and isolation of the pyrazolylphosphines, may facilitate
their broader use as ancillary ligands in late transition
metal complexes. The Au(I) complex reported here is the
first fully characterized pyrazolyphosphine-based late tran-
sition metal complex that does not contain electron-with-
drawing carbonyl ligands. The catalytic applications of
the novel pyrazolylphosphine and related compounds are
under way.
Acknowledgement
We thank the Natural Sciences and Engineering
Research Council of Canada and Canada Foundation for
Innovation for financial support.
ð2Þ
Recrystallization of the Au(I) complex in CH2Cl2/hex-
anes at ꢀ30 °C afforded single crystals suitable for X-ray
analysis (Fig. 2) [11]. In solid state, the Au(I) complex is
monomeric with an almost perfectly linear geometry
around Au(I) (P–Au–Cl angle = 176.93(5)°). The Au–P–
N(1) angle of 109.43(16)° is ideal for the tetrahedral phos-
Appendix A. Supplementary material
CCDC 652244 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free
of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.
html, or from the Cambridge Crystallographic Data Cen-
tre, 12 Union Road, Cambridge CB2 1EZ, UK; fax:
(+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
˚
phorus atom. The Au–P length of 2.228(1) A and Au–Cl
˚
bond length of 2.276(1) A are virtually identical to those
of (PPh3)AuCl [12]. In the new pyrazolylphoshine, the

A. Xia et al. / Inorganic Chemistry Communications 10 (2007) 1339–1341
1341
[6] (a) Recent reviews on catalytic applications of electron-rich phos-
phines: A. Zapf, M. Beller, Chem. Commun. (2005) 431;
(b) R.B. Bedford, C.S.J. Cazin, D. Holder, Coord. Chem. Rev. 248
(2004) 2283;
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.inoche.2007.
08.012.
(c) A.F. Littke, G.F. Fu, Angew. Chem. Int. Ed. 41 (2002) 4176.
[7] J. Elguero, E. Gonzalez, R. Jacquier, Bull. Soc. Chim. Fr. (1968) 707.
[8] Characterization of PPh2(3,5-tBu2pz): M.p. 136–138 °C. Anal. Calc.
for C23H29N2P: C, 75.80; H, 8.02; N, 7.69. Found: C, 75.68; H, 8.29;
N, 7.66%. 1H (CDCl3, 250.14 MHz): d 1.20 (s, 9H, CH3), 1.52 (s, 9H,
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´
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˚
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˚
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˚
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´
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´
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