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Inorganic Chemistry Communications 7 (2004) 737–740

Synthesis and structure of a novel Pd(0)–Pd(IV)–Pd(IV)–Pd(0)mixed-valence complex q

Wai-Kwok Wong a,*, Hongze Liang a, Ming-Yan Yung a, Jian-Ping Guo a,Ka-Fu Yung b, Wing-Tak Wong b,*, Peter G. Edwards c

a Department of Chemistry, Hong Kong Baptist University, Waterloo Road, Kowloon Tong, Hong Kong, PR Chinab Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, PR China

c Department of Chemistry, Cardiff University, P.O. Box 912, Cardiff CF10 3TB, UK

Received 28 January 2004; accepted 6 April 2004

Available online 6 May 2004

Abstract

The interaction of PdCl2(PhCN)2 with 1,2-[PhP(H)CH2CH2CH2P(Ph)CH2]2C6H4, generated in situ from the reduction of the

corresponding phosphine oxide with LiAlH4 in the presence of Me3SiCl, gave a complex mixture, from which a di- and a tetra-

nuclear palladium complex 1 and 2 whose solid state structures were ascertained by X-ray crystallography, were isolated. A novel

Pd(0)–Pd(IV)–Pd(IV)–Pd(0) mixed-valence structure was observed in complex 2.

� 2004 Elsevier B.V. All rights reserved.

Keywords: Palladium (II) complex; Crystal structure; P-NMR

Template syntheses have been successfully employed

to replace multi-step synthetic routes used for the

synthesis of macrocycles incorporating a C–P–C linkage

[1–3]. Tri- [4–8] and tetra-phosphorus [2,3,9,10] macro-

cycles with various ring sizes have been synthesized via

template synthesis. We are interested in the preparation

and chemistry of tetra-phosphorus macrocycle. Herein

we report our attempted synthesis of a tetra-phosphorusmacrocycle via template synthesis.

Reduction of 1,2-[PhP(O)(OEt)CH2CH2CH2P(Ph)-

(O)CH2]2C6H4 with LiAlH4 in the presence of Me3SiCl

gave a white viscous oil, which in its 31P NMR spec-

trum, exhibited a broad singlet at d )22.6 and a doublet

at d )52.3 (1JP -H ¼ 207 Hz). The resonance at d )22.6can be assigned to a tertiary phosphine and the reso-

nance at d )52.3 to secondary phosphine. In solution,the white viscous oil was slowly oxidized in air to its

corresponding phosphine oxide, which is a white solid.

qSupplementary data associated with this article can be found, in the

online version, at doi:10.1016/j.inoche.2004.04.004.* Corresponding authors. Fax: +852-2547-2933 (W.-T. Wong).

E-mail address: wtwong@hkucc.hku.hk (W.-T. Wong).

1387-7003/$ - see front matter � 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.inoche.2004.04.004

This is supported by its 31P{1H} NMR spectrum, which

exhibited two broad multiplets at d 31.5 and 46.2 for the

phosphine oxide. Furthermore, its FAB mass spectrum

exhibited a parent peak at m=z 687, which corresponds

to the [M+1]þ of 1,2-[PhP(O)(H)CH2CH2CH2P(Ph)-

(O)CH2]2C6H4. These results suggest that the white

viscous oil is the tetraphosphine ligand 1,2-[PhP(H)-

CH2CH2CH2P(Ph)CH2]2C6H4 (L1). The interaction ofPdCl2(PhCN)2 with L1 gave a complex mixture, from

which a di- and a tetra-nuclear palladium(II) complex 1

and 2 were isolated (Scheme 1). The solid state struc-

tures of both complexes were ascertained by X-ray

crystallography. A perspective drawing of 1 is shown in

Fig. 1. Structural analysis revealed that the tetraphos-

phine has undergone cleavage at one of the two CH2-P

bonds of the o-xylene unit and the terminal secondaryphosphine has been deprotonated to form a [(2-

CH3C6H4CH2)(Ph)P(CH2)3P(Ph)]� unit. Although the

mechanism for the cleavage of the P–C bond remains

unknown, it may be proposed that the present of the

chloride ions facilitated the bond rupture. The two

palladium atoms adopt a slightly distorted square pla-

nar geometry with Pd(1) and Pd(2) situated 0.033 and

P P

PP

Ph Ph

Ph

OEtPh

OEt

Pd

P P

Cl

Ph Ph

O OO O

P P

PP

Ph Ph

Ph

HPh

H

Pd

Cl

P P

Ph Ph

P PPh Ph

Pd

P PPh Ph

P PPh Ph

Pd

P PPh Ph

PdCl

PdCl

+

2+

2[HSO4]-

i) LiAlH4, Me3SiCl

ii) H2O

i) PdCl2(PhCN)2

ii) MgSO4

L4

1 2

Scheme 1.

Pd(1)

Pd(2)

P(1)

P(2)

P(3)

P(4)

Cl(1)

Cl(2)

C(1)

C(2)

C(3) C(4)

C(5)

C(6)

C(7)

C(8)

C(9)

C(10) C(11)

C(12)

C(13)

C(14)

C(15)

C(16)

C(17)

C(18) C(19)

C(20)

C(21) C(22)

C(23) C(24)

C(25)

C(26)

C(27)

C(28)

C(29)

C(30)

C(31)

C(32)

C(33)

C(34)

C(35)

C(36)

C(37)

C(38)

C(39) C(40)

C(41) C(42)

C(43)

C(44) C(45)

C(46)

Fig. 1. A perspective view of compound 1. Selected bond lengths (�A) and angles (�): Pd(1)–Cl(1) 2.383(1), Pd(2)–Cl(2) 2.368(2), Pd(1)–P(1) 2.321(2),Pd(1)–P(2) 2.268(1), Pd(1)–P(3) 2.318(1), Pd(2)–P(2) 2.341(1), Pd(2)–P(3) 2.248(1), Pd(2)–P(4) 2.318(1), Cl(1)–Pd(1)–P(1) 95.6(1), Cl(1)–Pd(1)–P(2)

169.8(1), Cl(1)–Pd(1)–P(3) 95.8(1), P(1)–Pd(1)–P(2) 94.6(1), P(1)–Pd(1)–P(3) 168.0(1), P(2)–Pd(1)–P(3) 74.1(1), Cl(2)–Pd(2)–P(2) 96.5(1), Cl(2)–

Pd(2)–P(3) 170.5(1), Cl(2)–Pd(2)–P(4) 94.5(1), P(2)–Pd(2)–P(3) 74.0(1), P(2)–Pd(2)–P(4) 169.0(1), P(3)–Pd(2)–P(4) 95.0(1), Pd(1)–P(2)–Pd(2) 99.8(1),

Pd(1)–P(3)–Pd(2) 101.1(1).

738 W.-K. Wong et al. / Inorganic Chemistry Communications 7 (2004) 737–740

0.011 �A above the mean plane formed by Cl(1)–P(1)–

P(2)–P(3) and Cl(2)–P(2)–P(3)–P(4), respectively. The

two mean planes form a dihedral angle of 149.3�. The[(2-CH3C6H4CH2)(Ph)P(CH2)3P(Ph)]

� moiety behaves

as a chelating ligand forming a six-membered ring with a

palladium atom and as a bridging ligand with the

phosphido group bonded asymmetrically to the otherpalladium centre. The Pd(1)–P(1) [2.321(2) �A], Pd(1)–

P(3) [2.318(1) �A], Pd(2)–P(2) [2.341(1) �A] and Pd(2)–

P(4) [2.318(1) �A] distances are comparable and slightly

longer than the Pd(1)–P(2) [2.268(1) �A] and Pd(2)–P(3)

[2.248(1) �A] distances. This suggests that P(1) and P(3)

[P(2) and P(4)] are dative-bonded and P(2) [P(3)] is r-bonded to Pd(1) [Pd(2)]. The spectroscopic data of 1 are

consistent with its solid state structure. The 31P NMR

spectrum exhibited two multiplets at d )13.2 and )135.4(m), which could be assigned to the phosphino and

phosphido groups, respectively. The mass spectrum(FAB, +ve mode) exhibited a peak corresponding to the

[(M+1 -Cl)] fragment at m=z 974 for 106Pd and 35Cl.

A perspective drawing of the cation of 2 is shown in

Fig. 2. The structure clearly shows that the two terminal

W.-K. Wong et al. / Inorganic Chemistry Communications 7 (2004) 737–740 739

secondary phosphines of the tetraphosphine had been

deprotonated to form two phosphido groups. The re-

sulting tetradentate ligand, with two phosphino and two

phosphido donor groups, behaves as a bridging ligand

and coordinates asymmetrically to two different palla-dium centres in the complex cation. The cation of

compound 2 is C2v-symmetric and adopts a V-shape

geometry with all the phenyl rings pointing outward. All

the palladium atoms adopt a slightly distorted square

planar geometry with Pd(1) and Pd(2) situated 0.071

and 0.143 �A above the mean plane formed by P(1)–

Cl(1)–Cl(2)–P(4) and P(1)–P(2)–P(3)–P(4), respectively.

The mean plane of P(1)–Cl(1)–Cl(2)–P(4) forms a di-hedral angle of 114.7 and 147.6� with the mean plane of

P(1A)–Cl(1)–Cl(2)–P(4A) and P(1)–P(2)–P(3)–P(4), re-

spectively. Probably due to steric factors, all the phenyl

Pd(1) Pd(2)

P(1) P(2)

P(3) P(4)

Cl(1)

Cl(2)

P

Pd(1)

Pd(2)

P(1)

P(2)

P(3)

P(4)

Cl(1)

Cl(2)

(a)

(b)

Fig. 2. A perspective view (a) and side view (b) of the cation of compound 2.

Cl(2) 2.444(4), Pd(1)–P(1) 2.247(3), Pd(1)–P(4) 2.243(4), Pd(2)–P(1) 2.322(4)

Pd(1)–Cl(2) 85.7(1), Cl(1)–Pd(1)–P(1) 99.9(1), Cl(1)–Pd(1)–P(4) 173.0(1),

75.3(1), P(1)–Pd(2)–P(2) 93.4(1), P(1)–Pd(2)–P(3) 164.3(1), P(1)–Pd(2)–P(4) 7

P(4) 93.5(1), Pd(1)–Cl(1)–Pd(1A) 78.8(2), Pd(1)–Cl(2)–Pd(1A) 78.4(1).

rings adopt a syn configuration. The Pd(2)–P distances

[Pd(2)–P(1) 2.322(4), Pd(2)–P(2) 2.332(4), Pd(2)–P(3)

2.332(4) and Pd(2)–P(4) 2.316(3) �A] are very similar. For

the same P atom, the Pd(1)–P distances [Pd(1)–P(1)

2.247(3) and Pd(1)–P(4) 2.243(4) �A] is shorter than thePd(2)–P distances. This is a reflection of the relative

trans influence of P versus Cl. There are two HSO�4

anions and three chloroform molecules per cation. One

of the anions and chloroform molecules is disordered

and assigned half site occupancy. The spectroscopic data

of 2 are consistent with its solid state structure. The 31P

NMR spectrum shows two broad multiplets at d 40.6

and )19.2 corresponding to the phosphino and phos-phido groups, respectively. The mass spectrum exhibited

a peak at m=z 869 (FAB, +ve mode) and 97 (FAB, )vemode) corresponding to the [(M - 2HSO4)

2þ/2 for 106Pd

d(1A)

P(1A)

P(4A)

Pd(2A)

P(2A)

P(3A)

Pd(1A)

P(1A)

P(4A)

Pd(2A)

P(2A)

P(3A)

Selected bond lengths (�A) and angles (�): Pd(1)–Cl(1) 2.436(4), Pd(1)–, Pd(2)–P(2) 2.332(4), Pd(2)–P(3) 2.332(4), Pd(2)–P(4) 2.316(3), Cl(1)–

Cl(2)–Pd(1)–P(1) 173.8(2), Cl(2)–Pd(1)–P(4) 98.9(1), P(1)–Pd(1)–P(4)

2.5(1), P(2)–Pd(2)–P(3) 99.7(1), P(2)–Pd(2)–P(4) 164.9(1), P(3)–Pd(2)–

740 W.-K. Wong et al. / Inorganic Chemistry Communications 7 (2004) 737–740

and 35Cl] and (HSO4)� fragment, respectively. When

focusing on the four Pd atoms, the two Pd atoms that

bond to four P atoms can be considered as zero valent,

while the remaining two are Pd(IV). Only a few exam-

ples of mixed-valence complexes were reported in liter-ature for transition metal like Pt, Pd [11], Cu [12], Ru

[13] and Fe [14]. This complex can be regaraded as a

novel Pd(0)–Pd(IV)–Pd(IV)–Pd(0) mixed-valence with a

oxidation states difference of 4.

Acknowledgements

We thank the Hong Kong Research Grants Council

(HKBU 874/96P) for financial support.

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