Solvent-free mechanochemical synthesis of two Pt complexes: Cis-(PH3P)2PtCl2 and cis-(Ph3P)2PtCO3

Article abstract of DOI:10.1039/b203694k

Cis-(Ph₃P)₂PtCl₂ and cis-(Ph₃P)₂PtCO₃ were prepared mechanochemically from solid reactants in the absence of a solvent; cis-(Ph₃P)₂PtCl₂ was obtained in 98% yield after ball-milling of polycrystalline PtCl₂ and Ph₃P; the mechanically induced solid-state reaction of cis-(Ph₃P)₂PtCl₂ with an excess of anhydrous K₂CO₃ produced cis-(Ph₃P)₂PtCO₃ in 70% yield; the formation of transition metal complexes as a result of mechanochemical solvent-free reactions has been confirmed by means of solid-state ³¹P MAS NMR spectroscopy, X-ray powder diffraction and differential thermal analysis.

Full text of DOI:10.1039/b203694k

Solvent-free mechanochemical synthesis of two Pt complexes:
cis-(Ph3P)2PtCl2 and cis-(Ph3P)2PtCO3†
Viktor P. Balema,*a Jerzy W. Wiench,a Marek Pruskia and Vitalij K. Pecharsky*ab
a Ames Laboratory, Iowa State University, Ames, IA 50011-3020, USA. E-mail: balema@ameslab.gov
b Department of Materials Science and Engineering, Iowa State University, Ames, IA 50011, USA.
E-mail: vitkp@ameslab.gov
Received (in Corvallis, OR, USA) 15th April 2002, Accepted 10th June 2002
First published as an Advance Article on the web 26th June 2002
Cis-(Ph3P)2PtCl2 and cis-(Ph3P)2PtCO3 were prepared me-
chanochemically from solid reactants in the absence of a
solvent; cis-(Ph3P)2PtCl2 was obtained in 98% yield after
ball-milling of polycrystalline PtCl2 and Ph3P; the mechani-
cally induced solid-state reaction of cis-(Ph3P)2PtCl2 with an
excess of anhydrous K2CO3 produced cis-(Ph3P)2PtCO3 in
70% yield; the formation of transition metal complexes as a
result of mechanochemical solvent-free reactions has been
confirmed by means of solid-state 31P MAS NMR spectros-
copy, X-ray powder diffraction and differential thermal
analysis.
molten Ph3P. In addition to 1, the reaction in the melt produces
at least 20% of trans-bis(triphenylphosphine)platinum(II) di-
chloride as a by-product.5 Carbonate 2 is often synthesized in
solution by ligand exchange between 1 and Ag2CO3.6 The latter
has been proven to be the only metal carbonate effective in this
transformation. To the best of our knowledge, mechanochem-
ical solvent-free synthesis of Pt-complexes was not reported in
the literature before.
Ball-milling of crystalline PtCl2 with two molar equivalents
of Ph3P at rt for 1 h produces an X-ray amorphous powder†. Its
solid-state 31P MAS NMR spectrum, shown in Fig. 1a, is in
excellent agreement with the previously reported solid-state
NMR data of pure crystalline 1,7–9 indicating the formation of
compound 1 during mechanical processing.
The differential thermal analysis of the ball-milled powder
confirms the completion of this reaction because no endo-
thermic events due to the melting of Ph3P have been observed.†
Endothermic anomaly in the DTA trace of the mechanically
processed sample (onset at 280 °C) agrees well with melting and
decomposition of pure 1.5,10† The DTA trace of the mecha-
nochemically prepared complex 1 contains two additional
exothermic effects between 104 and 180 °C, which are absent in
the crystalline cis-(Ph3P)2PCl2. The origin of these anomalies
was studied using in-situ high temperature X-ray powder
diffraction,† which revealed crystallization of the ball-milled
amorphous sample between 100 °C and 180 °C. It is worth
noting that similar exothermic events were also observed in the
DTA trace of the pure crystalline cis-(Ph3P)2PCl2 ball-milled
for 2 h. Hence, the exothermic anomalies in the DTA trace of
Mechanochemical transformations of transition metal com-
plexes containing organic molecules as ligands have been
reported in a number of recent publications.1,2 In all known
cases, however, mechanical treatment of the reactants was
followed by a subsequent heating and/or the processed samples
were dissolved in appropriate solvents for characterization.
Consequently, reports1,2 provide little, if any, evidence to
confirm that observed reactions occur during mechanical
processing and not as the result of a successive treatment. The
latter was demonstrated in experiments with chromium acet-
ylacetonate, which did not form during ball-milling of CrCl3
and NaO2C5H7, but it was obtained after heating of the
mechanically processed mixture of chromium trichloride and
sodium acetylacetonate in a vacuum.2
Recently we demonstrated that high energy mechanical
processing triggers a variety of efficient organic reactions in the
solid-state without a solvent.3 This was made possible by the
means of a unique experimental approach, where solid-state 31
P
magic angle spinning nuclear magnetic resonance (31P MAS
NMR) spectroscopy, X-ray powder diffraction, and differential
thermal analysis (DTA) were combined to investigate both
chemical and physical events that occur in organic materials
during mechanical processing. Here we show that the same
modus operandi can be extended towards different classes of
molecular solids, for example metal coordination compounds.
For the first time, two platinum complexes — cis-bis-
(triphenylphosphine)platinum(II) dichloride 1 and cis-bis(tri-
phenylphosphine)platinum(II) carbonate 2 — were prepared
from solid reactants during ball-milling in a solvent-free
environment, as depicted in Scheme 1.
Complex 1 is usually prepared in solution by ligand
substitution in platinum (II) compounds or by oxidative addition
of chlorine to (Ph3P)4Pt.4,5 It also forms when PtCl2 reacts with
Fig. 1 Solid-state 31P MAS NMR spectra: powders obtained during ball-
milling of Ph3P with PtCl2 for 1 h {lit. for 1: d31P/JPt–P (ppm/Hz) =
12.5/3727, 10.9/3910, 7.8/35967 and 12.6/3750, 7.8/35808} (a), and 2 h (b);
crystalline 1 ball-milled for 2 h (c); reaction mixture formed during ball-
milling of 1 with 2.5 equiv. of K2CO3. The broad peak at d31P = 25 ppm
includes both the signal of Ph3PO and the resonance signals due to 195Pt–31
P
interactions in complex 2 (d); crystalline complex 2 (e), and Ph3PO (f). The
values of the isotropic chemical shifts (ppm) and the coupling constants
(Hz, in parenthesis) are shown at the corresponding peaks. The phosphorus
resonances are accompanied by doublets due to Pt–P J coupling, denoted by
asterisks, with intensities consistent with the natural abundance of 195Pt
(33.8%).
Scheme 1
† Electronic supplementary information (ESI) available: differential ther-
mal analysis data, X-ray diffraction data. See http://www.rsc.org/suppdata/
cc/b2/b203694k/
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This journal is © The Royal Society of Chemistry 2002

the mechanochemically prepared complex 1 represent amor-
phous to crystalline transitions in the powder, which are
exothermic processes.
Pure complex 1, identical to that described in the lit-
erature,5,8,11 was obtained in 98% yield after dissolution of the
ball-milled powder in CH2Cl2, filtration and evaporation of the
solvent in a vacuum.9 According to the conventional 31P NMR
spectroscopy in CDCl3, compound 1 was the only product of the
mechanochemical reaction, i.e. no triphenylphosphine oxide or
other phosphorus containing species were detected.
mental data suggest that mechanical processing enables inter-
actions of reacting centers in the solid state by first breaking the
crystallinity of the reactants and second by providing the mass
transfer in the absence of a solvent. Our experiments on the ball-
milling of pure PtCl2 and 1, both of which lose crystallinity and
become essentially amorphous powders during mechanical
processing, support this assumption.‡
In summary, we showed that platinum (II) complexes can be
prepared in high yields during mechanically induced solid-state
reactions at solvent-free conditions. The completion of the
reactions during ball-milling has been confirmed directly using
a combination of solid-state 31P MAS NMR spectroscopy, X-
ray powder diffraction and DTA. The mechanochemical
technique appears to be a convenient and extremely efficient
experimental tool, which opens a pathway to new processes
where chemical transformations are performed in an envir-
onmentally benign manner.
Ball-milling of PtCl2 with two molar equivalents of Ph3P for
2 h results in the same product 1 (98% yield). On the other hand,
prolonged mechanical treatment has a significant effect on the
shape of the solid-state 31P MAS NMR spectrum of the resulting
powder, which however, remains in excellent agreement with
the spectrum of pure 1 ball-milled at the same experimental
conditions (Figs. 1b and c). It is worth noting that this
observation is in line with the previous reports about the
Ames Laboratory is operated by Iowa State University for the
U.S. Department of Energy (DOE) under contract No. W-
7405-ENG-82. Different aspects of this work were supported by
the Office of Basic Energy Sciences, Materials Sciences
Division (VPB, and VKP) and Chemical Sciences Division
(JWW and MP) of the U.S. DOE.
extreme sensitivity of solid-state 31P NMR spectra of Pt(II
)
complexes to the crystallinity of the samples.7,8
The mechanical processing of 1 with 2.5 equiv. of anhydrous
K2CO3 produced a powder mixture containing complex 2,
crystalline KCl (identified by X-ray powder diffraction†) and
minor amounts of Ph3PO plus unidentified Pt-specie(s) as side-
products.9 According to the conventional 31P NMR spectra of
the samples extracted from the milling equipment during the
mechanochemical experiment, the complete transformation of
the starting complex 1 into the reaction products requires 6.5 h
of ball-milling. The solid-state 31P MAS NMR spectrum of the
mechanochemically prepared sample and the spectra of the pure
crystalline 2 and Ph3PO are shown in Figs.1d–f. Once again, the
solid-state NMR spectroscopy clearly indicates the formation of
the carbonate 2 during mechanical processing prior to any
further treatment.
Notes and references
‡ PtCl2, anhydrous K2CO3 and Ph3P were purchased from Alfa Aesar,
Fisher or Aldrich. Solid-state 31P MAS NMR spectra were collected on a
Chemagnetics Infinity 400 MHz system using relaxation delay of 45 s, 20
kHz MAS and 1H decoupling of 30 kHz. 1H and 31P NMR spectra in CDCl3
were performed on Varian VXR-300 and 400 spectrometers. Chemical
shifts are reported with respect to an 85% solution of H3PO4 in water (31P)
or TMS (1H). Ball-milling was performed as described in ref. 3.
1 P. J. Nichols, C. L. Raston and J. W. Steed, Chem. Commun., 2001,
1062; D. Orsa, D. M. Ho, L. Takacs and S. K. Mandal, Abstr. Pap.–Am.
Chem. Soc., 2000, 220; A. Paneque, J. Fernandez-Bertran, E. Reguera
and H. Yee-Madeira, Transition Met. Chem., 2001, 25, 76; V. D.
Makhaev, A. P. Borisov and L. A. Petrova, J. Organomet. Chem., 1999,
590, 222; M. M. Mostafa, E. A. H. Gomaa, M. A. Mostafa and F. I. El-
Dossouki, Spectrochim. Acta A, 1999, 55, 2869; A. P. Borisova, L. A.
Petrova and V. D. Makhaev, Zh. Obshch. Khim., 1992, 62, 15.
2 V. D. Makhaev, A. P. Borisova, L. A. Petrova and T. P. Karpova, Russ.
J. Inorg. Chem., 1996, 41, 394.
3 V. P. Balema, J. W. Wiench, M. Pruski and V. K. Pecharsky, Chem.
Commun., 2002, 724; V. P. Balema, J. W. Wiench, M. Pruski and V. K.
Pecharsky, J. Am. Chem. Soc., 2002, 124, 6244.
4 C. A. McAuliff and W. Levason, Phosphine, Arsine and Stibine
Complexes of the Transition Elements, Elsivier, Amsterdam, 1979; P. J.
Stang, L. Song and B. Halton, J. Organomet. Chem., 1990, 388, 215.
5 R. D. Gillard and M. F. Pilbrow, J. Chem. Soc., Dalton Trans., 1974,
2320.
6 M. A. Andrews, G. L. Gould, W. T. Klooster, K. S. Koenig and E. J.
Voss, Inorg. Chem., 1996, 35, 5478.
7 T. Allman, J. Magn. Res., 1989, 83, 637; C. A. Fyfe, Solid State NMR
for Chemists, CFC Press, Guelph, Ontario, Canada, 1983; J. A. Davies
and S. Dutremez, Coord. Chem. Rev., 1992, 114, 61.
8 W. P. Power and R. E. Wasylishen, Inorg. Chem., 1992, 31, 2176.
9 Mechanochemically prepared platinum complexes. 1: yield 98%; solid-
state 31P MAS NMR: d31P/JPt-P (ppm/Hz): 12.5/3765, 10.6/3850,
7.8/3600; liquid-state NMR (CDCl3): d31P/JPt-P (ppm/Hz): 15.3/3672;
d1H (ppm): 7.52–7.47 (m, 2H, Ph), 7.35–7.32 (m, 1H, Ph), 7.19–7.16
(m, 2H, Ph); mp 298–303 °C (decomp.); 2: yield 70%; solid-state 31P
MAS NMR: d31P/JPt-P (ppm/Hz): 11.4/3818, 2.8/3685; liquid-state
NMR (CDCl3): d31P/JPt-P (ppm/Hz): 7.2/3704; d1H (ppm/Hz):
7.42–7.33 (m, 3H, Ph), 7.22–7.18 (m, 2H, Ph); IR (KBr): 1672(CO),
1629 (CO); mp 193–195 °C (decomp.). The amount of Ph3PO formed as
a side-product in 2 was ~ 20% as estimated from the liquid-state 31P
NMR (corrected for T1 of the reaction products).
The ball-milled powder was dissolved in CH2Cl2, filtered and
the complex 2 was then isolated by precipitation with diethyl
ether in 70% yield. The conventional NMR and IR spectra, and
the melting point of the thus obtained compound 29 were
identical with literature data reported for cis-
(Ph3P)2PtCO3.6,12
Carbonate 2 also forms as a major product during ball-milling
of the mixture of PtCl2, Ph3P and anhydrous K2CO3 without a
solvent (Scheme 1). Apparently, this transformation is a two-
stage process involving the intermediate formation of complex
1, which was detected in the ball-milled powder using
conventional 31P NMR spectroscopy. Contrary to the previous
reaction, this transformation did not result in a complete
consumption of 1 even after mechanical treatment for 14 h.
We found no evidence that the studied reactions proceed in
the absence of the mechanical treatment. At least two different
scenarios could explain the role of mechanical energy in the
transformations described above. First, the reactions may occur
in the melt, which possibly forms locally and momentarily in
the areas where the balls collide with the walls of the reaction
vial and with one another. Previously, we showed that the local
temperature in a material during mechanical processing at the
experimental conditions identical to those used in the current
study does not exceed 110 °C.3 Thus, it is unlikely that complex
2 forms as a result of a liquid-state reaction between high-
melting complex 1 (mp 193 °C) and anhydrous K2CO3 (mp 891
°C). On the other hand, the possibility of the reaction between
the transient Ph3P liquid (mp 79–82 °C) and solid PtCl2 (mp 581
°C) could not be entirely excluded. However, since one of the
major products of the reaction between PtCl2 and molten Ph3P
— trans-(Ph3P)2PtCl25 — does not form in a detectable amount
in the mechanochemical process, this mechanism is rather
doubtful.
10 C. E. Scott and S. H. Mastin, Thermochim. Acta, 1976, 14, 141.
11 G. Bacher, D. M. Grove, L. Venanzi, F. Bachechi, P. Mura and L.
Zambonelli, Helv. Chim. Acta, 1980, 63, 2519.
The second scenario reflects the occurrence of truly solid-
state mechanochemical transformations. The available experi-
12 M. Ebner and H. Werner, Chem. Ber., 1986, 119, 482.
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