Note

Carbon�/phosphorous bond cleavage on a platinum centre.Crystal structure of sym-cis-[Pt(m-PPh2)(PPh2CH2CH2S�/P,S)]2

Josep Duran a, Alfonso Polo a,*, Julio Real b,*, Angel Alvarez-Larena c,J. Francesc Piniella c

a Departament de Quımica, Universitat de Girona. Campus de Montilivi s/n, E-17071 Girona, Spainb Departament de Quımica, Universitat Autonoma de Barcelona, E-08193, Cerdanyola, Barcelona, Spain

c Servei de Difraccio de Raigs-X, Universitat Autonoma de Barcelona, E-08193, Cerdanyola, Barcelona, Spain

Received 13 November 2002; accepted 28 February 2003

Inorganica Chimica Acta 353 (2003) 280�/283

www.elsevier.com/locate/ica

Abstract

The complex [Pt(dppet�/P ,S )2] (Hdppet�/PPh2CH2CH2SH) that had been synthesized previously from K2PtCl4 and Hdppet in

the presence of base with moderate yields (ca. 50%), has been prepared in high yield (ca. 95%) in the absence of base. [Pt(dppet�/

P ,S )2] is stable in the air, in the presence of acid (2 M HCl) and in refluxing toluene, but in the sun light it turns into binuclear sym -

cis -[Pt(m-PPh2)(dppet�/P ,S )]2 (1). The crystal structure of 1 revealed a non-crystallographically planar Pt2P4S2 core with open m-P�/

Pt�/P(dppet) angles (102, 1048) and similar m-P�/Pt distances of 2.311(4), 2.318(5), 2.302(5), 2.324(5) A, little influenced by the trans

ligand.

# 2003 Elsevier B.V. All rights reserved.

Keywords: Platinum; Phosphinothiolate; Phosphide; Crystal structures

1. Introduction

The stability of ligands is an important factor in any

practical application of metal complexes [1]. In the case

of phosphinothiols, it has been reported that this ligands

may decompose into phosphine sulphides, as in certain

conditions there is cleavage of the carbon�/sulphur bond

and formation of the strong phosphorous�/sulphur bond[2]:

PMe(Ph)CH2CH2SH 0 S�PMe(Ph)CH2CH3

Phosphinothiols are also subjected to oxidation, first

on phosphorous but also on sulphur. Furthermore,

thiolates are basic and this constitutes a difference

with the important class of diphosphine ligands, which

are obviously less basic, for any application in acidic

media. However, we have reported that some palladi-

um(II) phosphinothiolate complexes, such as the bische-late [Pd(dppet)2] (Hdppet�/PPh2CH2CH2SH) [3], are

best prepared in acidic conditions rather than in basic

conditions. This observation prompted us to study the

case of platinum, i.e., the synthesis and stability of

phosphinothiolate complexes of platinum in acid con-

ditions.

2. Results and discussion

2.1. Synthesis

The bischelate [Pt(dppet)2] is perhaps the simplest

phosphinothiolate complex of platinum [4,5], it can be

prepared in reproducible low yields (ca. 50%) from

K2PtCl4 or PtCl2 and the ligand in the presence of bases

such as NEt3. The use of a base seems necessary to

remove the proton from the weakly acidic thiol, but the

presence of a base throughout the synthesis favours the

oxidation of both the phosphorous and the sulphur.Basic conditions are commonly used in the synthesis of

platinum thiolates and even dithiolates [6�/8], but it is

difficult to decide if this responds to the actual need for

* Corresponding authors. Fax: �/34-93-581 3101.

E-mail addresses: alfons.polo@udg.es (A. Polo), juli.rcal@uab.es (J.

Real).

0020-1693/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved.

doi:10.1016/S0020-1693(03)00227-5

a base or to the idea that the thiol sulphur will not form

complexes unless the proton is removed previously.

The direct, base free reaction of K2PtCl4 with the

phosphinothiol Hdppet afforded [Pt(dppet)2] in highyield, always above 85%. Apparently, the phosphi-

nothiolato chelate forms a very strong bond to platinum

and the presence of HCl and excess chloride is no

problem. Also, the acidic conditions seem beneficial for

the stability of the ligand. The spectroscopic data,

specifically the large coupling constant of phosphorous

to platinum (1JP�Pt�/2811 Hz), together with the strong

preference of platinum for a cis -P,P coordination,should support a cis -P,P structural assignment as

reported [5], although other authors have assumed a

trans structure [4,11�/13]. Chemical shifts for cis - and

trans -P,P complexes of this type can be very similar, as

shown in the case of the comparable palladium com-

plexes [Pd(sp)2] (Hsp�/PPh2(6-R�/C6H3�/2-SH, R�/H,

SiMe3) that exist in both cis and trans forms [9,10].

Bischelate [Pt(dppet)2] showed high stability, it isstable in hydrochloric acid (2 M), in air as a solid or

in solution for moderate periods of time, and in

refluxing toluene. However, when solutions of

[Pt(dppet)2] were allowed to stand in the sun light, the

formation of crystals was observed. This was a new

material, as it was only sparingly soluble or insoluble in

all tested solvents and its elemental analysis did not

correspond to [Pt(dppet)2], but to a composition high inphosphorous and low in sulphur: P2PtS. It later proved

to be the phosphido-bridged binuclear [Pt(m-

PPh2)(dppet�/P ,S )]2 (1).

In this new complex the ligand diphenylphosphido

has formed by cleavage of the phosphorous�/carbon

bond of dppet ligand and loss of the C2H4S fragment.

The thiolate sulphur, a group that usually forms strong

bonds to transition metals, has been effectively displaced

from the coordination sphere of platinum by thebridging phosphide.

2.2. Crystal structure

The structure of 1 consists of discrete molecules of

sym -cis -[Pt(m-PPh2)(dppet�/P ,S )]2 as depicted in Fig. 1.

Comparable bridging phosphido structures with nega-

tive (X) and neutral terminal ligands (PR3), be them

open or chelate tend to adopt sym �/trans geometries of

type A (see Scheme 1), rather than type C [14�/18].However, binuclear 1 is unique in that it prefers a cis

structure (D) rather than the apparently less sterically

encumbered trans configuration (B). It is interesting to

note that in the distribution of bridging versus terminal

coordination positions, two ligands (the phosphido

PR2� and the thiolato RS� groups) with a strong

preference to form bridges between transition metal ions

are competing, the result is that the phosphide phos-

phorous takes the bridging position and structure E is

not observed [3].

The six phenyl rings on phosphorous P1, P2 and P3

are accommodated on the same side, overcoming the

steric interactions by adopting conformations in some

Fig. 1. ORTEP plots of sym -cis -[Pt(m-PPh2)(dppet�/P ,S )]2, in the

above projection carbon atoms have been represented by small circles

(all non-hydrogen atoms have been refined anisotropically), below all

carbon atoms (except C1�/C4) have been omitted, all for the sake of

clarity.

Scheme 1. Structural possibilities of phosphido complexes relevant to

the discussion.

J. Duran et al. / Inorganica Chimica Acta 353 (2003) 280�/283 281

cases (notably phenyl groups on P2, see Fig. 1) almost

perpendicular to the core plane. Also, the interfering

phenyl rings on P2 and P3 are almost parallel to each

other. The angles P1�/Pt1�/P2 (1048, see Table 2) andP2�/Pt2�/P3 (1028) are large compared to the angles

formed by the less bulky thiolato ligand: S1�/Pt1�/P4

(948) and S2�/Pt2�/P4 (958). This deformation can be

compared to that of cis -[Pd(PPh2(C6H4�/2-S))2], where a

similar effect was observed [9]. As this congestion could

be avoided by adopting the more common sym �/trans

geometry, it must be assumed that the sym �/cis arrange-

ment is caused by electronic factors.Bridging phosphido phosphorous to platinum(II)

distances trans to sulphur in 1 are only very slightly

shorter than those trans to the phosphine-type phos-

phorous of ligand dppet, which seems exceptional as

phosphorous should exert a larger trans influence. The

consequences of the lower trans influence of chloride are

important in comparable platinum complexes (see Table

3), making m-P�/Pt distances very different.

3. Experimental

3.1. Materials and measurements

Conventional oxygen-free synthetic methods have

been employed in order to prevent the oxidation of the

ligand. The solid complexes are air stable but thesolutions were kept under nitrogen atmosphere. 31P

NMR spectra are reported in the d scale and referred to

external 85% H3PO4. Elemental analyses (C, H and S)

were performed with a Carlo Erba CHNS EA-1108.

Phosphorous content was determined after mineraliza-

tion by a colorimetric method on phosphomolybdova-

nadate, at the S.A.Q. of the U.A.B. Platinum content

was determined by repeated cycles of mineralizationwith conc. HNO3 in a heated crucible and ignition at

temperatures over 900 8C (open crucible), which gave

platinum metal. The ligand Hdppet was prepared as

described [19]. The synthesis of [Pt(dppet)2] in basic

conditions was done as reported [5], using triethylamine,

sodium hydroxide and potassium tert -butoxyde, the

yields ranged from 40 to 60% of a dull yellow material.

The preparation of [Pt(dppet)2] in acidic conditions wasdone in the same way but omitting the base, the yields

were reproducibly higher and ranged from 85 to 98%, of

a bright yellow material with 31P{1H} NMR (121.4

MHz, CD2Cl2, r.t.) d 61.7, 1J(P�/Pt)�/ 2811 Hz.

3.2. Preparation of [Pt(m-PPh2)(dppet�/P ,S)]2 (1)

A solution of [Pt(dppet)2] (200 mg, 0.29 mmol) indichloromethane (ca. 100 ml) and n-buthylether (ca. 40

ml) was prepared under nitrogen in a side arm flask. The

stoppered flask was allowed to rest in the sun for 6

weeks, after which time a crop of small yellow prismatic

crystals had formed. The crystals were isolated by

filtration and washed with ethyl ether (3�/10 ml). The

yield was 52 mg (28%) of 1. Elemental Anal. Found

(Calc. for C52H48P4Pt2S2): C, 49.53 (49.92); H, 4.13

(3.87); P, 9.53 (9.90); Pt, 30.85 (31.18); S, 4.88 (5.13)%.

Compound 1 was insoluble in organic solvents; other

platinum phosphido complexes such as [PtCl(m-

PHMes)(PH2Mes)] [16] are also insoluble.

Table 2

Selected bond lengths (A) and bond angles (8) for 1

Bond lengths

Pt(1)�/P(1) 2.287(5) Pt(2)�/P(3) 2.292(5)

Pt(1)�/P(2) 2.311(4) Pt(2)�/P(2) 2.302(5)

Pt(1)�/P(4) 2.318(5) Pt(2)�/P(4) 2.324(5)

Pt(1)�/S(1) 2.324(5) Pt(2)�/S(2) 2.316(5)

P(1)�/C(21) 1.827(9) P(3)�/C(4) 1.82(2)

P(1)�/C(11) 1.836(8) P(3)�/C(61) 1.831(15)

P(1)�/C(1) 1.847(17) P(3)�/C(51) 1.850(12)

P(2)�/C(41) 1.812(11) P(4)�/C(81) 1.835(11)

P(2)�/C(31) 1.854(9) P(4)�/C(71) 1.839(10)

S(1)�/C(2) 1.776(19) C(1)�/C(2) 1.52(2)

S(2)�/C(3) 1.80(2) C(3)�/C(4) 1.54(3)

Bond angles

Pt(2)�/P(2)�/Pt(1) 103.45(17) Pt(1)�/P(4)�/Pt(2) 102.54(17)

P(1)�/Pt(1)�/P(2) 104.30(16) P(3)�/Pt(2)�/P(2) 102.17(18)

P(1)�/Pt(1)�/P(4) 176.50(15) P(3)�/Pt(2)�/S(2) 85.8(2)

P(2)�/Pt(1)�/P(4) 76.86(16) P(2)�/Pt(2)�/S(2) 171.94(18)

P(1)�/Pt(1)�/S(1) 84.38(17) P(3)�/Pt(2)�/P(4) 175.16(19)

P(2)�/Pt(1)�/S(1) 170.77(17) P(2)�/Pt(2)�/P(4) 76.91(16)

P(4)�/Pt(1)�/S(1) 94.66(17) S(2)�/Pt(2)�/P(4) 95.24(18)

Table 1

Crystallographic data for complex 1

Formula C52H48P4Pt2S2

M 1251.08

T (K) 293(2)

Crystal system monoclinic

Space group P21/c

Unit cell dimensions

a (A) 9.572(10)

b (A) 21.437(9)

c (A) 23.716(9)

b (8) 93.16(6)

Z 4

V (A3) 4859(6)

Dcalc (g cm�3) 1.710

m (mm�1), Tmax, Tmin 6.004, 0.999, 0.705

Crystal dimensions (mm3) 0.07�/0.04�/0.04

Data/restraints/parameters 8521/0/445

Goodness-of-fit 0.850

R (Fo) [I �/2s (I )] a 0.0751

Rw(Fo2) (all data) b 0.1618

a R (Fo)�/SjjFoj�/jFcjj/SjFoj.b Rw(Fo

2)�/[S w (Fo2�/Fc

2)2/S w (Fo2)2]1/2.

J. Duran et al. / Inorganica Chimica Acta 353 (2003) 280�/283282

3.3. X-ray crystallography

All crystals of 1 were very small yellow prisms. Data

collection and determination of the unit cell on the

selected specimen was performed on a Enraf NoniusCAD 4 diffractometer, operating with graphite-mono-

chromated Mo Ka radiation (l�/0.71069 A) at 293(2)

K. The u range was 1�/258, with h from �/11 to 11, k

from 0 to 25 and l from 0 to 28, using the v �/2u scan

method. The crystal structure was solved by direct

methods using the SHELXS-86 program [20] and refined

by full-matrix least-squares methods on F2 for all

reflections with SHELXL-97 [21]. The function minimisedwas S w (jFoj2�/jFcj2)2 where w�/1/[s2(Fo

2)�/

(0.0825P )2], with P�/(Fo2�/2Fc

2)/3. Non-hydrogen

atoms were refined anisotropically and phenyl rings

were refined as rigid groups. Hydrogen atoms were

included in calculated positions. Crystallographic data

are collected in Table 1 and selected distances and angles

in Table 2.

Acknowledgements

This work was financially supported by the M.E.C.

(Spain) through project BQU2002-04070-C02.

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Table 3

Representative m-phosphido phosphorous�/platinum(II) distances relevant to trans influence effects

d (m-P�/Pt) [A] trans to P d (m-P�/Pt) [A] trans to Cl or S Ref., CSD code

[Pt(m-PPh2)(dppet�/P ,S )]2 2.318(5), 2.324(5) 2.311(4), 2.302(5) This work

[PtCl(m-PPh(CH2)3PCy2)]2 2.316(2), 2.330(2) 2.243(2), 2.255(2) [14], BOPPEB20

[PtCl(m-PPh(CH2)3PCy2)]2 2.312(1) 2.247(1) [14], CIBDAS10

[PtCl(m-PHMes)(PEt3)]2 2.323(2) 2.265(2) [16], ZUQWOX

[PtCl(m-PHMes)(PPh3)]2 2.318(2) 2.268(2) [16], ZUQWUD

[PtCl(m-PPh2)(PHPh2)]2 2.329(3) 2.260(3) [15], BEXYAE

[Pt(m-PPh2)(dppe)]22� 2.362(3), 2.335(3) [15], BEXYEI

[Pt(m-PHMes)(dppe)]22� 2.356(2) [16], ZUQXAK

J. Duran et al. / Inorganica Chimica Acta 353 (2003) 280�/283 283