A Sound Sequence to Triphenylphosphino Dibromoplatinum(II) Complexes – Solvothermal Preparation of trans-[PtBr(μ-Br)(PPh3)]2

Article abstract of DOI:10.1002/ejic.201900711

A sound synthetic procedure for the preparation of trans-[PtBr(μ-Br)(PPh₃)]₂ is described. The species was fully characterized and used to obtain [PtBr₂(PPh₃)(L)] complexes (L = DMSO, p-toluidine, pyridine) by a bridge-splitting reaction. All products were fully characterized by NMR spectroscopy, together with cis-[PtBr₂(PPh₃)(NCCH₃)], obtained as an intermediate in the synthesis of the dinuclear precursor. Cis-[PtBr₂(PPh₃)(NCCH₃)] was also studied by X-ray diffraction.

Full text of DOI:10.1002/ejic.201900711

DOI: 10.1002/ejic.201900711
Full Paper
Dibromoplatinum(II) Complexes
A Sound Sequence to Triphenylphosphino Dibromoplatinum(II)
Complexes – Solvothermal Preparation of trans-[PtBr(μ-Br)-
(PPh3)]2
David Fioco,[a] Daniela Belli Dell'Amico,[a] Luca Labella,[a] Fabio Marchetti,[a] and
Simona Samaritani*[a]
Abstract: A sound synthetic procedure for the preparation of products were fully characterized by NMR spectroscopy, to-
trans-[PtBr(μ-Br)(PPh3)]2 is described. The species was fully char- gether with cis-[PtBr2(PPh3)(NCCH3)], obtained as an intermedi-
acterized and used to obtain [PtBr2(PPh3)(L)] complexes (L = ate in the synthesis of the dinuclear precursor. Cis-
DMSO, p-toluidine, pyridine) by a bridge-splitting reaction. All [PtBr2(PPh3)(NCCH3)] was also studied by X-ray diffraction.
Introduction
mation of [PtBr(μ-Br)(PPh3)]2 by decomposition of a reaction
intermediate, but the compound was not characterized. Consid-
ering the existing literature and the importance of the product
as precursor in the high yield synthesis of mixed ligand di-
bromide platinum(II) complexes, we describe here a high yield
convenient preparation of trans-[PtBr(μ-Br)(PPh3)]2 from
[PtBr2(NCMe)2] and triphenylphosphine, in solvothermal condi-
tions. The synthetic sequence is described starting from com-
mercial K2PtCl4.
Phosphane complexes of platinum find application in many
fields of inorganic chemistry, from catalysis[1] to bioactive com-
pounds.[2] Among the last compounds, dichlorotriphenylphos-
phino derivatives [PtCl2(PPh3)(L)] have found interesting appli-
cations in the field of anticancer compounds.[2a–2g,2j] Since anti-
cancer properties can be modulated changing the coordination
sphere of the metal, besides varying L, we have been interested
in varying the nature of the coordinating halide from chloride
to bromide. In general, [PtX2(L)(PR3)] complexes {X = Cl, Br can
be readily obtained by bridge splitting reactions of the suitable
dinuclear precursors [PtX(μ-X)(PR3)]}2.[3] Some years ago a con-
venient synthetic procedure for trans-[PtCl(μ-Cl)(PPh3)]2 was de-
scribed, making this chlorinated dinuclear derivative formally
accessible from commercial K2PtCl4 aqueous solution.[4] For the
corresponding bromo derivative, trans-[PtBr(μ-Br)(PPh3)]2, there
is a lack of data in the literature and its molecular structure was
determined only recently[5] by single-crystal X-ray diffraction. In
this literature paper, the complex formed from PtBr2 and PPh3
in the presence of an excess of Bu4PBr, a system catalyzing a
hydroamination reaction, but its isolated yield was not re-
ported. From a synthetic point of view, literature usually refers[6]
to the reaction of PtX2 with [PtX2(PR3)2] in high boiling solvents,
mostly for chlorinated dinuclear complexes.[7] Moreover these
reactions usually report good isolated yields but long purifica-
tion workups to eliminate by-products formed at high tempera-
ture. As an alternative route, [PtBr(μ-Br)(C2H4)]2 has been pro-
posed as precursor in exchange reactions with PR3,[8] but steri-
cally demanding phosphines often afford mononuclear bridge-
splitting products. Moreover, Gilchrist et al.[9] observed the for-
Furthermore, in this text, considerable attention has been
paid to differences in the reactivity of bromide/chloride ana-
logues.
Results and Discussion
The synthesis of the brominated dinuclear complex (Scheme 1)
has been optimized exploiting the consolidated procedure[4,10]
employed for the analogous chlorinated derivative.
Scheme 1. Synthesis of Pt2Br4(PPh3)2.
Step1: K2PtBr4 can be prepared by an exchange reaction from
the commercially available K2PtCl4. Since the commonly used
preparation[11,12] that involves the treatment of aqueous solu-
tions of K2PtCl4 with excess KBr, often yields an anionic bromo
complex contaminated with KBr, we preferred to follow another
procedure,[13] where the exchange reaction is carried out using
aqueous HBr (48 %). Since experimental details were not previ-
[a] Dipartimento di Chimica e Chimica Industriale, Università di Pisa,
Via Giuseppe Moruzzi 13, 56124 Pisa, Italy
E-mail: simona.samaritani@unipi.it
https://people.unipi.it/simona_samaritani/
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejic.201900711.
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ously reported, we have to mention here that although the and –4143 (1JP-Pt = 3944 Hz) for cis and trans isomers respec-
procedure is very simple, consisting in mixing two aqueous so- tively, with an expected upfield shift[14] due to the Br/Cl substi-
lutions, great care has to be taken in excluding atmospheric O2, tution. The crystalline cis product forms by slowly cooling the
which can oxidize the bromide ion to bromine, leading to non- solution to room temperature. Its molecular structure was con-
negligible amounts of K2PtBr6. The oxidation by-product is rec- firmed by single-crystal X-ray diffraction (Figure 1).
ognizable by 195Pt-NMR, affording a singlet signal at –1860 ppm
(solvent: H2O).[14] Anyway, if the reaction is carried out under
nitrogen with deoxygenated reagents, potassium tetrabromo-
platinate(II) is obtained as a single product, affording a singlet
signal at –2700ppm (in H2O).[12] The product was recovered by
removing the solvent under reduced pressure and further re-
crystallization from 0.5
M
HBr.[13]
Step 2: Preparation of [PtBr2(NCMe)2], obtained from K2PtBr4
in acetonitrile, has also been already described[13,15] but we
would like to add some important synthetic and spectroscopic
details. Compared to the chlorinated species the product is
more soluble in water and requires a larger excess of aceto-
nitrile. To push the reaction forward after roughly 50 % of the
product has been collected the solution needs to be concen-
trated by removing the solvent and restoring the acetonitrile
lost during the process. Collecting a few fractions of the prod-
uct an overall yield of 81 % was achieved. The complex (yellow
crystalline powder) was characterized by spectroscopy (IR and
NMR) and elemental analysis. As for IR (ATR) spectroscopy, coor-
dinated nitrile afforded a weak, but visible absorption band at
2340 cm–1, while 1H-NMR in CD3NO2 allowed us to detect a
single singlet signal at 2.67 ppm, with satellites (4JH-Pt = 15 Hz),
which could be ascribed to methyl group of coordinated aceto-
nitrile. This data indicates the preferential formation of one iso-
mer as confirmed by a single signal registered in the 195Pt-NMR
spectrum (–2800 ppm).
Figure 1. Crystal structure of cis-[PtBr2(PPh3)(NCMe)]. Selected bond lengths
[Å]: Pt(1)–N(1) 1.986(3); Pt(1)–P(1) 2.2485(9); Pt(1)–Br(1) 2.4096(4); Pt(1)–Br(2)
2.4780(5). Selected bond angles (°): N(1)–Pt(1)–P(1) 92.92(9); N(1)–Pt(1)–Br(1)
174.41(9); P(1)–Pt(1)–Br(1) 90.01(2); N(1)–Pt(1)–Br(2) 87.14(9); P(1)–Pt(1)–Br(2)
174.35(3); Br(1)–Pt(1)–Br(2) 90.412(17).
It is also possible to prepare [PtBr2(PPh3)(NCMe)] using 1
equivalent of [PtBr2(NCMe)2] and 1 equivalent of [PtBr2-
(PPh3)2].[15a] The last compound is readily accessible by the syn-
thetic sequence depicted in Equation (2) and Equation (3) and
it involves: preliminary extraction of platinum into dichloro-
methane solution as [TBA]2[PtBr4] (TBA = tetrabutylammonium;
DCM = dichloromethane)[16] followed by a reaction with PPh3
(phosphine/Pt = 2 in mol)
[PtBr2(NCMe)2] could also be obtained by halogen exchange
reaction between the corresponding chlorinated compound
and a tenfold excess of tBuBr in acetonitrile solution [Equa-
tion (1)] The reaction is quite slow and solvothermal conditions
are required (120 °C in Carius tube, 24 h).
[PtCl2(NCMe)2] + tBuBr(exc) → [PtBr2(NCMe)2] + tBuCl
(1)
When prepared by this method, the product was recovered
as a poorly soluble orange powder, which turned yellow when
it was washed with water. Its elemental analysis, IR and 1H NMR
spectra were in good agreement with those shown by the prod-
uct recovered from the aqueous synthesis, for an overall yield
of 67 %.
K2PtCl4aq + 4TBABr(DCM) → [TBA]2[PtBr4](DCM) + 2TBACl + 2KCl
(2)
[TBA]2[PtBr4] + 2PPh3 → [PtBr2(PPh3)2] + 2TBABr
(3)
Step 3: For the synthesis of [PtBr2(PPh3)(NCMe)],
[PtBr2(NCMe)2] was treated with a stoichiometric amount of
When a sample of pure cis-[PtBr2(PPh3)(NCMe)] (0.100 g) was
PPh3, in acetonitrile solution and in solvothermal conditions dissolved in CH3CN (10 mL) equilibrium was reached in 24 h
(150 °C in a Carius tube) (Scheme 1). Compared to the analo- (31P NMR) and the mixture contained 68 % of cis- and 32 % of
gous chlorinated species[4] this reaction is much faster requiring trans isomers. [PtBr2(PPh3)(NCMe)] is stable in CH3CN diluted
only 3 hours to react [PtBr2(NCMe)2] with PPh3, compared to solution, while it rapidly releases acetonitrile and affords an or-
the 120 hours required by [PtCl2(NCMe)2].
31P-NMR analysis was carried out on the solution, showing
no residual PPh3 and two new signals at 5.34 (1JP-Pt = 3457 Hz)
ange solid identified as trans-[PtBr(μ-Br)(PPh3)]2 when dissolved
in other solvents or when heated (60 °C) under vacuum.
As already described for the acetonitrile complex
and 1.73 (1JP-Pt = 3944 Hz) ppm. The two signals were assigned [PtBr2(NCMe)2], the formation of [PtBr2(PPh3)(NCMe)] was ob-
to cis and trans isomers respectively by comparison with the served when the corresponding chlorinated compound was
chlorinated species.[4] In the 195Pt-NMR spectrum of the mix- treated with an excess of tBuBr in acetonitrile solution [Equa-
ture, two resonances were observed at –3924 (1JP-Pt = 3467 Hz) tion (4)], 150 °C in Carius tube, 72 h.
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[PtCl2(PPh3)(NCMe)] + tBuBr(exc) → [PtBr2(PPh3)(NCMe)]+ tBuCl
were assigned by comparison with the analogous chlorinated
complexes.[10]
(4)
For ease of comparison, the most significant 31P NMR signals
in CDCl3 {except for [PtX2(PPh3)(NCMe)] and [PtX2(PPh3)(DMSO)]
which were registered in MeCN and DMSO respectively} are re-
ported in Table 1 for chlorinated and brominated complexes.
When the preparation of [PtBr2(PPh3)(amine)] (amine = Py,
pTol) complexes was attempted by exchange reaction between
[PtCl2(PPh3)(amine)] and excess tBuBr, under the same experi-
mental conditions affording [PtBr2(NCMe)2] (Carius tube,
120 °C), decomposition was observed. In the 1H-NMR spectrum
main signals were attributed to amine·HX species. These data
suggest that in the experimental conditions used, tBuBr is par-
tially converted into isobutene and HBr. While in the previously
discussed cases the presence of hydrogen bromide does not
prevent the formation of the desired brominated products, this
route cannot be used with acid sensitive complexes.
Nevertheless, in this case it was not possible to isolate the cis
isomer and the product was obtained as a mixture of cis,trans-
[PtBr2(PPh3)(NCMe)] and trans-[PtBr(μ-Br)(PPh3)]2.
Step 4: The final dinuclear product trans-[PtBr(μ-Br)(PPh3)]2
was formed refluxing the toluene solution of [PtBr2(PPh3)-
(NCMe)] (Scheme 1). The brominated dinuclear derivative was
obtained in high yield (90 %) as a light orange solid, sparingly
soluble in chlorinated solvents and was characterized by IR
(ATR), 31P-NMR and elemental analysis. Due to the scarce solu-
bility of the species, a complete spectroscopic characterization
of the dinuclear species was not carried out. Nevertheless, its
prompt reactivity towards coordinating solvents could be used
to confirm indirectly its nature; as a matter of fact, when a
sample of the orange solid was dissolved in acetonitrile, a pale
yellow solution was obtained, showing the 31P NMR signals of
cis- and trans-[PtBr2(PPh3)(NCMe)] (Table 1). Analogously, the
dissolution of the sample in DMSO afforded a colorless solu-
tion, showing a single 31P NMR signal, ascribed to cis-
[PtBr2(PPh3)(DMSO)], on the basis of the comparison with the
signal of known cis-[PtCl2(PPh3)(DMSO)][2d] (Table 1). Compari-
son with the reactivity and the spectroscopic data of trans-
[PtCl(μ-Cl)(PPh3)]2 allowed to assign a trans configuration to the
present brominated system.
Conclusions
A stepwise synthetic sequence to prepare trans-[PtBr(μ-Br)-
(PPh3)]2 was described. Solvothermal conditions (Carius tube,
acetonitrile at 150 °C) were conveniently used, but in this case
the reaction was much faster respect to the analogous chlorin-
ated system. Since cis,trans-[PtBr2(PPh3)(NCMe)] is an intermedi-
ate in the formation of the dinuclear compound, it can be rea-
sonably assumed that acetonitrile elimination in the last step
proceeds from the trans isomer, which is easily formed due to
Table 1. 31P NMR signals in CDCl3 for [PtX2(PPh3)(L)]: δ [ppm] (1JP-Pt [Hz]).
L
X = Cl
X = Br[b]
bromide ion steric hindrance. As
a
matter of fact,
PPh3
cis 16.9 (3660)
trans 21.1 (2615)
cis 4.8 (3530)
trans 1.6 (4100)
trans 4.05 (3590)
cis 7.2 (3907)
cis 13.7 (3610)
trans 18.7 (2573)
cis 5.3 (3467)
trans 1.7 (3944)
trans 3.0 (3653)
cis 7.8 (3810)
[PtBr2(NCMe)(PPh3)] is present, in acetonitrile solution, as a mix-
ture of geometric isomers, where the concentration of trans
complex is much higher than in the chlorinated counterpart
(32 % vs. 5 % at room temperature, respectively). The possible
use of tert-butyl bromide as exchange brominating agent in
non-aqueous environment was explored, but its use appears
limited to non-acid-sensitive complexes. Thus, as exemplified by
the reported reactivity, trans-[PtBr(μ-Br)(PPh3)]2 is an important
precursor to a series of structural analogues of known antiprolif-
erative platinum compounds.
MeCN[a]
pTol
Py
trans 2.6 (3582)
cis 16.2 (3720)
trans 0.8 (3458)
cis 17.2 (3730)
DMSO[b]
[a] Solvent MeCN. [b] Solvent DMSO.
As already mentioned for coordinating solvents, the reactiv-
ity displayed by trans-[PtBr(μ-Br)(PPh3)]2 towards nucleophiles
was also remarkably similar to its chlorine bearing counterpart.
In all cases tested, a suspension of the dinuclear precursor in
chloroform, when treated with a suitable ligand, afforded a
clear solution of the product of the bridge-splitting reaction
(Scheme 2). As expected, the reaction is directed by the strong
trans-effect exerted by the phosphine ligand, with the fast
formation of the kinetic trans product, sometimes followed by
isomerization in solution.
Experimental Section
Reactions were performed under dinitrogen atmosphere. Unless
otherwise specified all solvents were previously purified according
to reported procedures.[17] Elemental analyses were collected with
an Elementar “vario MICRO CUBE” CHNOS elemental analyzer. Solid
state IR spectra were collected with Perkin Elmer “Spectrum One”
spectrometer outfitted with an Attenuated Total Reflectance (ATR)
accessory. Abbreviations used to describe signal shape and inten-
sity: w = weak; m = medium; s = strong; br = broad band. NMR
spectra were collected with a Bruker “Avance DRX 400” spectrome-
ter with a 400 MHz 1H frequency and with a Varian “Gemini 200”
spectrometer with a 200 MHz 1H frequency. CDCl3 was used, unless
otherwise stated. 31P-and 195Pt-NMR spectra were also acquired
without deuterated solvents, using a capillary containing C6D6 to
allow for locking by the spectrometer. Chemical Shifts [ppm] are
referenced to Si(CH3)4 for 1H and 13C, H3PO4 (85 % in D2O) and
Scheme 2. Synthesis of [PtBr2(PPh3)(L)].
Specifically, the product obtained by reaction with p-tolu-
idine was trans with no trace of isomerization (31P NMR), while
DMSO afforded a stereochemically pure cis complex and pyrid-
ine yielded a mixture of isomers. 31P- and 195Pt-NMR signals H2PtCl6 were employed for 31P- and for 195Pt-, respectively. Abbrevi-
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ations used to describe signal multiplicity: s = singlet; d = doublet;
t = triplet; td = triple doublet; m = multiplet.
Table 2. Crystal data and structure refinement for cis-[PtBr2(PPh3)(NCMe)].
Compound
cis-[PtBr2(PPh3)(NCMe)]
Synthesis of K2PtBr4: A sample (1.00 g) of K2PtCl4 (2.41 mmol) was
dissolved in 250mL of deoxygenated 48 %HBraq. After 24h reaction
progress was tested via 195Pt-NMR and found complete (195Pt-NMR:
–2662[12]). Solvent was removed under reduced pressure at 40 °C
and a dark brown solid was obtained, which was recrystallized from
aqueous HBr (1.23g, 86 % yield).
Empirical formula
Formula weight
Temperature
Wavelength
Crystal system
Space group
C20H18Br2NPPt
658.23
296(2) K
0.71073 Å
Monoclinic
P21/c
Unit cell dimensions
a = 14.8719(10) Å
b = 8.9060(5) Å
c = 17.3852(11) Å
α = 90°
Synthesis of [PtBr2(NCMe)2]: Method A.
A sample (1.23 g,
2.08 mmol) of K2PtBr4 was dissolved in 80 mL of deoxygenated
water and 4 mL of deoxygenated acetonitrile were added, maintain-
ing the system in nitrogen atmosphere. The first greenish/black pre-
cipitate was discarded. From the bright red filtrate yellow crystals
formed over the course of several days. More crystalline product
was collected, through fractional crystallization. The overall yield
(0.708 g) was 81 %. El. anal. calcd. for [PtBr2(NCMe)2],
C4H6Br2N2Pt, %: C = 11.0; H = 1.4; N = 6.4; found C = 11.3; H = 1.4;
N = 6.6. IR (ATR, cm–1): 2920 w, 2340 m, 1354 m, 1351 m, 1012
ꢀ = 114.827(2)°
γ = 90°
Volume
Z
Density (calculated)
Absorption coefficient
F(000)
Crystal size
Theta range for data collection
Index ranges
2089.8(2) Å3
4
2.092 Mg/m3
10.617 mm–1
1232
0.155 × 0.155 × 0.095 mm3
2.381 to 32.504°
–14 ≤ h ≤ 22, –12 ≤ k ≤ 13,
–26 ≤ l ≤ 24
26438
4
m. 1H NMR (CD3NO2): 2.67 (s, 3H, JH-Pt = 15 Hz, CH3). 13C NMR
(CD3NO2):122.0, 5.0. 195Pt NMR (CD3NO2): –2805.
Method B. A sample (0.267 g, 0.77 mmol) of [PtCl2(NCMe)2][18] was
introduced into a Carius tube, suspended in acetonitrile (5 mL) and
tert-butyl bromide (2 mL) was added. The mixture was stirred at
120 °C (5 h), then cooled. An orange precipitate was obtained,
which turned yellow upon washing with water. IR (ATR) and elemen-
tal analyses were in good agreement with those collected for
Method A samples. (0.210 g, 67 %).
Reflections collected
Independent reflections
Completeness to theta = 25.242° 99.7 %
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2sigma(I)]
R indices (all data)
Extinction coefficient
Largest diff. peak and hole
7371 [R(int) = 0.0234]
Full-matrix least-squares on F2
7371/0/227
1.094
R1 = 0.0305, wR2 = 0.0634
R1 = 0.0469, wR2 = 0.0689
n/a
Synthesis of cis-[PtBr2(PPh3)(NCMe)]: In a Carius tube under nitro-
gen atmosphere, 0.298 g (0.68 mmol) of [PtBr2(NCMe)2] were added
to 3 mL of acetonitrile. The suspension was stirred and a stoichio-
metric amount of PPh3 (0.179 g, 0.68 mmol) was added. The tube
was sealed and heated (150 °C) for 2–3 hours with vigorous stirring.
Reaction progress was monitored with 31P-NMR. Once the reaction
1.549 and –1.275 e Å–3
precursor [5.34 (1JP-Pt 3457 Hz)]. The orange precipitate was filtered
and washed with pentane, then dried under reduced pressure.
Yield: 0.109 g (90 %). El. anal. calcd. for [Pt2Br4(PPh3)2],
1
1
was found to be complete (5.34, JP-Pt 3457 Hz, cis; 1.72, JP-Pt
3944 Hz, trans), the tube was left overnight to cooldown and yellow
crystals of [PtBr2(PPh3)(NCMe)] were recovered (0.244 g, 55 %). The
crystals were collected for X-ray analysis and found to be in cis
configuration (see Table 2 for experimental details). Correlation be-
tween the crystalline structure and the NMR data was established
by comparing the product with its chlorine equivalent. Using the
same solvothermal conditions it is possible to obtain cis-
[PtBr2(PPh3)(NCMe)] from [PtBr2(NCMe)2] and [PtBr2(PPh3)2] in a 1:1
ratio. The bright yellow solution was evaporated at reduced pres-
sure and afforded a yellow-orange residue (0.201 g). A sample of
this solid was dissolved in CHCl3 and analysed (31P-NMR): 5.17
(1JP-Pt 3480 Hz, 30 %), 1.5 (1JP-Pt not detectable, traces), 7.61 (1JP-Pt
3940 Hz, 69 %). The signal at 7.61 progressively became the only
observable signal and an orange solid appeared. The signal at
7.61 ppm was attributed to dinuclear trans-[PtBr(μ-Br)(PPh3)]2 (cfr.
infra), while the signals at 5.17 and 1.5 were ascribed to cis- and
trans-[PtBr2(PPh3)(NCMe)] respectively. IR (ATR, cm–1): 3072 w, 3061
w, 3045 w, 2909 w, 2355 w, 2323 w, 1588 w, 1574 w, 1482 m, 1433
s, 1357 m, 1100 s, 993 m, 744 s, 684 s. Isomer cis: 31P-NMR (CH3CN):
5.34 (1JP-Pt 3457 Hz); 195Pt-NMR: –3924 (1JP-Pt 3457 Hz). Isomer trans:
31P-NMR (CH3CN): 1.73 (1JP-Pt 3940 Hz); 195Pt-NMR: –4143 (1JP-Pt
3940 Hz).
C
36H30Br4P2Pt2, %: C 35.0, H 2.5; found C 35.4, H 2.4 %. IR (ATR,
cm–1): ν = 3077 w, 3063 w, 3041, 2913 m, 2363 w, 2322 w, 1583 w,
˜
1571 w, 1481 m, 1433 s, 1094 s, 1000 m, 741 s, 690 s. Due to the
very limited solubility, only 31P-NMR is reported. 31P-NMR(CD2Cl2):
7.7 (1JP-Pt 3906 Hz). When a sample of the orange solid was dis-
solved in acetonitrile, a pale yellow solution was obtained. 31P NMR:
5.34 {1JP-Pt 3457 Hz, cis-[PtBr2(PPh3)(NCMe)], 68 %}; 1.73 {1JP-Pt
3940 Hz, trans-[PtBr2(PPh3)(NCMe)], 32 %}.
Synthesis of [PtBr2(PPh3)2]:[11] This product was synthesized from
1.034 g (1.034 × 10–3 mol) of [TBA]2[PtBr4][8] by adding triphenyl-
phosphine (0.542 g, 2.066 × 10–3 mol) in dichloromethane at room
temperature and with magnetic stirring (PPh3/Pt = 2 in mol). The
system was maintained in nitrogen atmosphere for the duration of
the synthesis. The solution changed from a dark reddish brown to
a very pale yellow. Reaction progress was monitored by 31P-NMR
and found to have reached completeness in 12 h. The product was
precipitated by removing most of the solvent under reduced pres-
sure and adding heptane. The precipitate was then filtered and
dried in vacuo. Yield: 0.861 g (95 %) El. anal. calcd. for [PtBr2(PPh3)2],
C36H30Br2PPt, %: C 51.0, H 3.6 %; found C 50.2, H 3.6 %. IR (ATR,
cm–1): ν = 3045 w, 1482 m, 1433 s, 1265 m, 1091 s, 730 s, 689 s.
˜
31P-NMR for cis isomer: 13.7 (1JP-Pt = 3610 Hz).[11] 31P-NMR for trans
isomer: 18.7 (1JP-Pt = 2573 Hz).[11]
Synthesis of [PtBr(μ-Br)(PPh3)]2:
A
sample of cis-
[PtBr2(PPh3)(NCMe)] (0.130 g, 0.19 mmol) was heated in refluxing
Bridge-splitting examples of [Pt2Br4(PPh3)2]: All reactions were
toluene (110 °C) for about 3 h, whilst stirring. The initially yellow carried out in NMR test tubes using CDCl3 as
a
solvent.
[Pt2Br4(PPh3)2] precursor (10–20 mg) was suspended in CDCl3 (1 mL)
and the suitable ligand was added (Ligand/Pt = 1.0 in mol). The
suspension turned to orange. Reaction was followed by 31P-NMR,
showing the disappearance, in the liquid phase, of the signal of
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system turned clear yellow in most cases in a 5 minutes time span.
A micro syringe was used to add the reagents as the quantities
involved were in the range of 3 to 10μL. Products were not isolated.
Spectroscopic NMR characterizations is reported.
trans-[PtBr2(PPh3)(p-Toluidine)]: The reaction was complete (31P-
NMR) after half an hour later and displayed only one signal, attribu-
1
table to kinetic trans product. H-NMR: 7.72 (m, 6H, Harom), 7.41 (m,
2
11H, Harom), 7.14 (m, 2H, Harom), 5.41 (m 2H, JH-Pt = 36 Hz, NH2),
2.35 (s, 3H, CH3). 13C-NMR: 136.7, 135.4, 135.0 (d, JC-P = 10 Hz), 130.8
(d, JC-P = 2 Hz), 129.8, 126.9 (d, 1JC-P = 66 Hz), 127.8 (d, JC-P = 11 Hz),
122.0, 21.0. 31P-NMR: 3.0 (1JP-Pt = 3653 Hz). 195Pt-NMR: –4106
1
(d, JP-Pt = 3653 Hz).
[PtBr2(PPh3)(Py)]: The reaction was complete (31P-NMR) after half an
hour later and displayed two signals, attributable to a mixture of
cis and trans products. Configuration was assigned comparing
31P-NMR chemical shift and coupling constants data with those of
the chlorinated isomers.[10] Isomer cis: 31P-NMR: 7.8 (1JP-Pt
=
1
3810 Hz). 195Pt-NMR: –3721 (d JP-Pt = 3810 Hz). Isomer trans:
1
31P-NMR: 0.8 (1JP-Pt = 3458 Hz). 195Pt-NMR: –4008 (d JP-Pt
=
3458 Hz).
cis-[PtBr2(PPh3)DMSO]: Pt2Br4(PPh3)2 is dissolved in [D6]DMSO, the
reaction was complete (31P-NMR) in a few minutes and displayed
only one signal, attributable to isomer cis. Configuration was as-
signed comparing 31P-NMR chemical shift and coupling constants
data with those of the known cis-[PtCl2(PPh3)(DMSO)].[2d] 31P-NMR:
17.1 (1JP-Pt = 3730 Hz). 195Pt-NMR: –4162 (1JP-Pt = 3730 Hz).
CCDC 1937596 {for cis-[PtBr2(PPh3)(NCMe)]} contains the supple-
mentary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Centre.
Acknowledgments
The authors thank the Università di Pisa (Fondi di Ateneo 2017–
2018) and Ministero dell′Università e della Ricerca (MIUR) for
grant PRIN 2015, “Design of Innovative Metal-Ligand Systems
for Catalysis and Energy Applications” as well as the Interuniver-
sity Consortium CHEMICAL REACTIVITY and CATALYSIS (CIRCC)
and Consorzio Interuniversitario di Ricerca in Chimica dei
Metalli nei Sistemi Biologici (C.I.R.C.M.S.B.), for financial support.
S. S. is grateful for the financial support provided by Università
di Pisa – Progetti di Ricerca di Ateneo 2017 “Composti di metalli
di transizione come possibili agenti antitumorali” (PRA_2017_
25).
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Keywords: Platinum · Exchange reactions · Dinuclear
complexes · Synthesis design
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