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

Article abstract of DOI:10.1016/j.inoche.2004.04.004

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.

Full text of DOI:10.1016/j.inoche.2004.04.004

Inorganic Chemistry Communications 7 (2004) 737–740
www.elsevier.com/locate/inoche
Synthesis and structure of a novel Pd(0)–Pd(IV)–Pd(IV)–Pd(0)
mixed-valence complex q
a,
a
a
a
Wai-Kwok Wong *, Hongze Liang , Ming-Yan Yung , Jian-Ping Guo ,
b
b,
c
Ka-Fu Yung , Wing-Tak Wong *, Peter G. Edwards
a
Department of Chemistry, Hong Kong Baptist University, Waterloo Road, Kowloon Tong, Hong Kong, PR China
b
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.
Ó 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-phosphorus
macrocycle 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 )22.6
can 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.
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 of
PdCl2(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 secondary
phosphine 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
q Supplementary 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 Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2004.04.004

738
W.-K. Wong et al. / Inorganic Chemistry Communications 7 (2004) 737–740
Ph
Ph
Ph
Ph
Ph
Ph
P
P
P
P
P
P
P
P
i) PdCl2(PhCN)2
ii) MgSO4
i) LiAlH4, Me3SiCl
ii) H2O
OEt
Ph
H
Ph
O
O
O O
OEt
H
L4
2+
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Pd
Cl
P
P
P
P
P
P
P
P
P
P
P
Cl
Cl
+
2[HSO4]-
Pd
Pd
Pd
Pd
Pd
Ph
P
Cl
Ph
Ph
1
2
Scheme 1.
C(27)
C(28)
C(26)
C(25)
C(21)
C(22)
C(36)
C(3)
C(4)
C(29)
C(20)
C(37)
C(35)
C(2)
C(24)
C(23)
C(5)
C(38)
C(34)
C(31)
C(18)
Cl(1)
C(19)
P(3)
C(1)
C(6)
C(33)
C(32)
C(30)
P(1)
Pd(1)
P(2)
C(7)
Pd(2)
P(4)
C(42)
C(44)
C(46)
C(41)
C(16)
C(43)
C(17)
C(15)
C(40)
Cl(2)
C(8)
C(39)
C(45)
C(13)
C(14)
C(9)
C(12)
C(10)
C(11)
ꢀ
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).
ꢀ
0.011 A above the mean plane formed by Cl(1)–P(1)–
ꢀ
[2.248(1) A] distances. This suggests that P(1) and P(3)
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 other
[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
ꢀ
palladium 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)

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
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 the
Pd(2)–P distances. This is a reflection of the relative
trans influence of P versus Cl. There are two HSO4ꢀ
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, )ve
mode) corresponding to the [(M - 2HSO4)2þ/2 for 106Pd
ꢀ
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
Cl(1)
P(3A)
P(4A)
P(4)
P(3)
Pd(2A)
Pd(2)
Pd(1)
Pd(1A)
P(1A)
P(1)
P(2)
P(2A)
Cl(2)
(a)
P(2)
P(2A)
P(3)
Pd(2)
P(3A)
Pd(2A)
P(1)
P(1A)
P(4)
P(4A)
Cl(2)
Cl(1)
Pd(1)
Pd(1A)
(b)
ꢀ
Fig. 2. A perspective view (a) and side view (b) of the cation of compound 2. Selected bond lengths (A) and angles (°): Pd(1)–Cl(1) 2.436(4), Pd(1)–
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(2)–P(2) 2.332(4), Pd(2)–P(3) 2.332(4), Pd(2)–P(4) 2.316(3), Cl(1)–
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), Cl(2)–Pd(1)–P(1) 173.8(2), Cl(2)–Pd(1)–P(4) 98.9(1), P(1)–Pd(1)–P(4)
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) 72.5(1), P(2)–Pd(2)–P(3) 99.7(1), P(2)–Pd(2)–P(4) 164.9(1), P(3)–Pd(2)–
P(4) 93.5(1), Pd(1)–Cl(1)–Pd(1A) 78.8(2), Pd(1)–Cl(2)–Pd(1A) 78.4(1).

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.
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ꢁ
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We thank the Hong Kong Research Grants Council
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