Bimetallic complexes supported by bis(diphenylphosphino)methane anti and syn to the Mn-Pd bonds

Article abstract of DOI:10.1016/S0020-1693(02)01546-3

Redox condensation of PPN[Mn₂(μ-PPh₂)(CO) ₈] and PdCl₂(η²-dppm) gives bimetallic PdMn(μ-PPh₂)(μ-dppm)(CO)₃(PPh₃) (1) with an unexpected formation of PPh₃. The latter can be displaced when 1 reacts with free diphosphines (dppm, dppe, dppf) giving PdMn(μ-PPh ₂)(μ-dppm)(CO)₃(Ph₂P-X-P(=O)Ph₂) (2, X=CH₂ (2a), C₂H₄ (2b), C₅H ₄FeC₅H₄ (2c)) and [PdMn(CO)₃(μ- PPh₂)(μ-dppm)]₂(μ-Ph₂P-X-PPh ₂) (3a). Complexes 2 are A-frame -type bimetallic complexes with an syn-dppm bridging the Mn-Pd bond. In contrast, complex 3a is a double A-frame anti-bridged by dppm. As a result, the latter is trans to the Mn-Pd bonds. Both types of dppm bridges are substitutionally inert. Either bridging role is best served by dppm (and not dppe or dppf). The structures of the complexes were derived from NMR analyses of all complexes and X-ray single-crystal diffraction analyses of 1, 2a and 2b. The common and stable A-frame -type core in these bimetallics provides a thermodynamic driving force for the opposite transmetallation migration of phosphide and phosphine (dppm) when 1 is formed.

Full text of DOI:10.1016/S0020-1693(02)01546-3

Inorganica Chimica Acta 350 (2003) 86ꢀ92
/
www.elsevier.com/locate/ica
Bimetallic complexes supported by bis(diphenylphosphino)methane
anti and syn to the MnÃPd bonds
/
Ye Liu, Sheau Wei Chien, Siok Bee Koh, Jagadese J. Vittal, T.S. Andy Hor *
Faculty of Science, Department of Chemistry, National University of Singapore, Kent Ridge 119260, Singapore
Received 3 May 2002; accepted 23 August 2002
We dedicate this paper to Prof. Pierre Braunstein for his pioneering contributions to the development of heterometallic science.
His chemistry provided an inspiration for the work described in this paper
Abstract
Redox condensation of PPN[Mn2(m-PPh2)(CO)8] and PdCl2(h2-dppm) gives bimetallic PdMn(m-PPh2)(m-dppm)(CO)3(PPh3) (1)
with an unexpected formation of PPh3. The latter can be displaced when 1 reacts with free diphosphines (dppm, dppe, dppf) giving
PdMn(m-PPh2)(m-dppm)(CO)3(Ph2PÃ
dppm)]2(m-Ph2PÃXÃPPh2) (3a). Complexes 2 are ‘‘A-frame’’-type bimetallic complexes with an syn-dppm bridging the MnÃ
bond. In contrast, complex 3a is a ‘‘double A-frame’’ anti-bridged by dppm. As a result, the latter is trans to the MnÃPd bonds.
/
XÃ
/
P(Ä
/
O)Ph2) (2, XÄ
/
CH2 (2a), C2H4 (2b), C5H4FeC5H4 (2c)) and [PdMn(CO)3(m-PPh2)(m-
/
/
/Pd
/
Both types of dppm bridges are substitutionally inert. Either bridging role is best served by dppm (and not dppe or dppf). The
structures of the complexes were derived from NMR analyses of all complexes and X-ray single-crystal diffraction analyses of 1, 2a
and 2b. The common and stable ‘‘A-frame’’-type core in these bimetallics provides a thermodynamic driving force for the opposite
transmetallation migration of phosphide and phosphine (dppm) when 1 is formed.
# 2003 Elsevier Science B.V. All rights reserved.
Keywords: Palladium; Manganese; Bimetallic; Heterometallic; Phosphine; Phosphide; Carbonyl; A-frame
1. Introduction
plexes in the fixation of CO2 in waterꢀ
[8] also provides an added impetus. Similar use of
anionic bridges like phosphide (Ã ) is also
Ph2Pꢁ Ã
/
gas shift reaction
Considerable interest in heterometallic clusters is
promoted by their potential applications in areas such
as nanotechnology [1] and catalysis [2]. When two or
more metal sites with different electronic properties are
brought into a single entity, the synergic effect could
lead to enhanced performance, e.g., in bifunctional
catalysis [3]. A simple way to lock two metals within
/
/
effective [5]. Our interest and success in the cooperative
use of these bridges are found in the assembly of some
multimetallic complexes, e.g., [AuMn2(CO)8(m-PPh2)]2-
(m-dppf), (PÃ
dppe, dppp, dppb, dppf) [6] and PdMn(m-PPh2)(CO)4-
(h2-PÃ
P) (PÃPꢂdppm, dppe, dppf), Mn2Pd2Ag(m-
/
P)AuMn2(CO)7(m-PPh2) (PÃ
/
Pꢂ
/
dppm,
/
/
/
Cl)(m-PPh2)2(m-dppm)(CO)8 [7]. In this paper, we
shall extend these ideas to examine the unique properties
of dppm compared with other ligands in this family
and subtle relationship of syn or anti bridging with
close proximity, with or without active MꢀM interac-
/
tion, is through a diphosphine such as bis(diphenylpho-
sphino)methane (dppm), e.g., in A-frame-type
complexes with the [MM?(m-dppm)2] core [4]. The
respect to the MÃM? bond. The syn dppm bridge
/
demonstrated catalytic activity of some Pdꢀdppm com-
/
strengthens the MM? entity whereas the anti one brings
together two dimetal units in a process that marks the
birth of a heterometalÃ
/metal bonded coordination
* Corresponding author. Tel.: ꢃ
E-mail address: andyhor@nus.edu.sg (T.S.A. Hor).
/
65-874 2917; fax: ꢃ65-777 4279.
/
polymer.
0020-1693/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0020-1693(02)01546-3

Y. Liu et al. / Inorganica Chimica Acta 350 (2003) 86ꢀ
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87
2. Experimental
PD, JPÃP
ꢂ
/
95 Hz); 186.5, 188.0 (d, br, PA, JPÃP
ꢂ
/
209
Hz). IR (CO, cmꢁ1): 1963 s, 1884 vs, 1723 s (solid KBr).
2.1. General procedures and materials
2.3.2. PdMn(m-PPh2)(m-dppm)(CO)3(Ph2PCH2P(Ä
/
All reactions were performed under pure nitrogen
using standard Schlenk techniques. Mn2(CO)8(m-H)(m-
PPh2) [9] and PPN[Mn2(CO)8(m-PPh2)] [10] were pre-
pared by literature methods. PdCl2(h2-dppm) was pre-
pared from PdCl2(CH3CN)2 and dppm in CH2Cl2. All
other reagents were commercial products and were used
O)Ph2) (2a) and [PdMn(m-PPh2)(m-
dppm)(CO)3]2(m-ppm) (3a)
To a red solution of PdMn(m-PPh2)(m-dppm)(CO)3-
(PPh3) (1, 0.067 g, 0.062 mmol) in toluene (60 ml) was
added dppm (0.012 g, 0.031 mmol). The mixture was
then refluxed for 24 h. After filtration and removal of
solvent under vacuum, the residue was coated onto TLC
plates and eluted with acetone/hexane (3:7). The first red
band was 1. The following two bands were [PdMn(m-
PPh2)(m-dppm)(CO)3]2(m-dppm) (3a) (yield: 41%) and
as received. FT-IR spectra were recorded on a Perkinꢀ
/
Elmer 1600 spectrometer. All NMR spectra were
recorded in CDCl3 solution on a Bruker ACF 300
MHz spectrometer. 31P NMR chemical shifts were
externally referenced to 85% H3PO4. Elemental analyses
were performed by the analytic service of this depart-
ment. Pre-coated silica TLC plates of layer thickness
0.25 mm were obtained from Merck or Baker.
PdMn(m-PPh2)(m-dppm)(CO)3(Ph2PCH2P(Ä
/
O)Ph2) (2a)
(yield: 23%). Good red crystals of 2a were grown in
acetone/hexane mixture.
Anal. Calc. for 2a: C, 64.24; H, 4.48. Found: C, 64.52;
H, 4.57%. 31P NMR (d): 3.5, 3.7, 3.8, 4.0, 4.1,
2.2. X-ray crystallography
2
2
4.3
(ddd; PD; cis JPÃP ꢂ15 Hz; JPÃP ꢂ30 Hz;
/
C
E
2
/
cis JPÃP ꢂ47 Hz); 12.6, 12.7, 13.4, 13.5, 14.2, 14.3,
A
The diffraction experiments for samples 1, 2a and 2b
were carried out on a Bruker AXS CCD diffractometer
˚
2
15.1,
15.2
(ddd; PC; cis JPÃP ꢂ15 Hz; 2 JPÃP
ꢂ
D
B
2
103 Hz; trans JPÃP ꢂ202 Hz); 23.8, 24.0 (d; PE;/
A
with a Mo Ka (lꢂ
/
0.71073 A). The program SMART [11]
/
2JPÃP ꢂ24 Hz); 71.0, 71.8 (d, br, PB, JPÃP
ꢂ107 Hz);
/
D
was used for collection of data frames, indexing reflec-
tion and determination of lattice parameters, SAINT [11]
for integration of intensity of reflection and scaling,
SADABS [12] for absorption correction and SHELXTL [13]
for space group, and structure determination and
refinements. In 2a, the dangling phosphine (P5) was
partially oxidized with occupancy of oxygen (O4) fixed
at 0.5. Complex 2b crystallized with half a molecule of
acetone and half a molecule of water in the crystal
lattice. Crystal data and refinement details are given in
Tables 1 and 2.
184.7, 184.9, 186.2, 186.5 (dd; PA; JPÃP ꢂ35 Hz;
/
2
/
trans JPÃP ꢂ194 Hz): IR (CO, cmꢁ1): 1965 s, 1880
C
vs, 1858 vs.
Anal. Calc. for 3a: C, 62.61; H, 4.30. Found: C, 63.41
H, 4.35%. 31P NMR (d): ꢁ
9.8 (s, PD); 12.7, 13.5, 14.3,
/
2
15.2
(dd; PC; JPÃP ꢂ103 Hz; trans JPÃP ꢂ202 Hz);
A
71.3, 72.1 (d, br, PB, JPÃP
ꢂ
/
100 Hz); 185.9, 187.6 (d,
PA, JPÃP
1858 vs.
ꢂ
206 Hz). IR (CO, cmꢁ1): 1964 s, 1890 vs,
/
2.3.3. PdMn(m-PPh2)(m-dppm)(CO)3(Ph2PC2H4P(Ä
/
O)Ph2) (2b)
2.3. Syntheses and characterization
A similar procedure as in 2a using PdMn(m-PPh2)(m-
dppm)(CO)3(PPh3) (1, 0.116 g, 0.108 mmol) and dppe
(0.022 g, 0.054 mmol) gave PdMn(m-PPh2)(m-
2.3.1. PdMn(m-PPh2)(m-dppm)(CO)3(PPh3) (1)
To a suspension of PdCl2(h2-dppm) (0.318 g, 0.56
mmol) in toluene (150 ml) was added PPN[Mn2(CO)8(m-
PPh2)] (0.300 g, 0.28 mmol). The mixture was refluxed
with vigorous stirring for 4 h, filtered, and stripped of
solvent to give a reddish-brown residue, which was
dissolved in a minimum amount of acetone, loaded onto
silica TLC plates and eluted with acetone/hexane (3:7, v
vꢁ1). The first major red band was removed and
extracted with acetone to give 1 (yield: 31%). Careful
recrystallization from acetone/hexane gave good crystals
of 1.
dppm)(CO)3(Ph2PC2H4P(Ä
/
O)Ph2) (2b, yield: 46%).
Good red crystals of 2b were grown from CH2Cl2/
hexane mixture.
Anal. Calc. for 2b: C, 64.48; H, 4.59. Found: C, 64.52;
H, 4.67%. 31P NMR (d): 8.6, 8.8, 8.9, 9.0, 9.2,
2
9.3
(ddd; PD; cis JPÃP ꢂ19 Hz; 2 JPÃP ꢂ29 Hz; cis
/
C
E
/
2JPÃP ꢂ45 Hz); 12.5, 13.4, 14.2, 15.0 (dd; PC; JPÃP
ꢂ
A
2
105 Hz; trans JPÃP ꢂ200 Hz); 31.5, 31.9 (d; PE;/
A
/
2JPÃP ꢂ53 Hz); 70.1, 70.8 (d, br, PB, JPÃP
ꢂ95 Hz);
/
D
185.9, 187.7 (d, br, PA, JPÃP
1963 s, 1882 vs, 1861 vs (solid KBr).
ꢂ
213). IR (CO, cmꢁ1):
/
Anal. Calc. for 1: C, 64.67; H, 4.40. Found: C, 64.11;
H, 4.33%. 31P NMR (d): 12.3, 12.4, 13.2, 13.3, 14.0,
2
14.1, 14.8, 14.9 (ddd; PC; trans JPÃP ꢂ204 Hz;
/
2.3.4. PdMn(m-PPh2)(m-dppm)(CO)3(Ph2PC5H4-
A
2
/
2JPÃP ꢂ101 Hz; cis JPÃP ꢂ15 Hz); 15.9, 16.0, 16.1,
FeC5H4P(Ä
/
O)Ph2) (2c)
D
B
2
16.2,
2
16.4,
16.5
(ddd; PB; cis JPÃP ꢂ38 Hz;
/
A similar procedure as in 2a using PdMn(m-PPh2)-
(m-dppm)(CO)3(PPh3) (1, 0.054 g, 0.050 mmol) and
A
/
cis JPÃP ꢂ15 Hz; 2 JPÃP ꢂ15 Hz); 69.8, 70.6 (d, br,
C
D

88
Y. Liu et al. / Inorganica Chimica Acta 350 (2003) 86ꢀ92
/
Table 1
Data collection and structure refinement of PdMn(PPh3)(CO)3(m-PPh2)(m-dppm) (1), PdMn(Ph2PCH2P(ÄO)Ph2)(CO)3(m-PPh2)(m-dppm) (2a) and
/
PdMn(Ph2PC2H4P(Ä
/
O)Ph2)(CO)3(m-PPh2)(m-dppm) (2b)
1
2a
2b×[(CH3)2CO]0.5[H2O]0.5
/
Empirical formula
Formula weight
Temperature (K)
Crystal system
C58H47MnO3P4Pd
1077.18
223(2)
C65H54MnO3.5P5Pd
1207.27
223(2)
C67.5H60MnO5P5Pd
1267.34
223(2)
monoclinic
P21/c
orthorhombic
Pbca
monoclinic
P21/c
Space group
Unit cell dimensions
˚
a (A)
12.319(3)
22.284(5)
18.065(4)
90
11.9379(18)
24.190(4)
39.297(6)
90
14.6600(9)
17.6585(12)
25.2702(16)
90
˚
b (A)
˚
c (A)
a (8)
b (8)
g (8)
92.765(6)
90
90
90
99.839(2)
90
3
˚
V (A )
4953.4(18)
4
1.444
11348(3)
8
6445.6(7)
4
1.306
Z
Dcalc (g cmꢁ3
Absorption coefficient (mmꢁ1
Reflections collected
)
1.413
0.728
63840
)
0.792
28598
0.646
36951
Independent reflections
8736 [Rint
ꢂ
/
0.0688]
9994 [Rint
R1ꢂ0.0495, wR2ꢂ
R1ꢂ0.0872, wR2ꢂ
ꢂ
/
0.1112]
11343 [Rint
R1ꢂ0.0404, wR2ꢂ
R1ꢂ0.0494, wR2ꢂ
ꢂ0.0347]
/
Final R indices [I ꢀ
/
2s(I)]
R1ꢂ/0.0494, wR2ꢂ
/
0.1075
/
/
0.0970
/
/0.1287
R indices (all data)
R1ꢂ/0.0837, wR2ꢂ
/0.1196
/
/0.1086
/
/0.1346
dppf (0.014 g, 0.025 mmol) gave PdMn(m-PPh2)-
(m-dppm)(CO)3(Ph2PCH2P(ÄO)Ph2) (2c, yield:
38%).
11.9, 12.8, 13.6, 14.5 (dd; PC; JPÃP ꢂ103 Hz; trans
/
2
/
/
2JPÃP ꢂ204 Hz); 28.6, 29.2 (d; PE; JPÃP ꢂ73 Hz);
A
D
69.2, 70.0 (d, br, PB, JPÃP
ꢂ
/
96 Hz); 185.6, 187.3 (d, br,
PA, JPÃP
(solid KBr).
ꢂ
206). IR (CO, cmꢁ1): 1971 s, 1882 vs, 1869 vs
/
Anal. Calc. for 2c: C, 64.91; H, 4.42. Found: C, 65.17;
H, 4.52%. 31P NMR (d): 4.2, 4.6 (d, PD, JPÃP
ꢂ38 Hz,);
/
Table 2
˚
Selected bond length (A) and angles (8) for 1, 2a and 2b
MnPd(m-PPh2)(m-dppm)(CO)3(PPh3), 1
Mn(1)Ã
Mn(1)Ã
Pd(1)Ã
/Pd(1)
2.7068(8)
2.2224(14)
2.2959(12)
Pd(1)Ã
/
P(2)
2.3384(13)
2.3056(13)
2.2533(14)
Mn(1)Ã
/
C (average)
1.803
/
P(1)
Pd(1)Ã
/
P(3)
/
P(1)
Mn(1)Ã
P(3)ÃPd(1)Ã
P(1)ÃPd(1)Ã
P(1)ÃPd(1)Ã
Mn(1)Ã
/P(4)
Mn(1)Ã
Pd(1)ÃMn(1)Ã
Mn(1)ÃPd(1)Ã
Mn(1)ÃPd(1)Ã
/
P(1)Ã
/
Pd(1)
73.59(4)
54.45(3)
51.96(4)
89.46(4)
/
/
P(2)
P(2)
P(3)
105.14(5)
114.16(5)
140.65(5)
163.57(4)
P(4)Ã
P(1)Ã
P(3)Ã
/
Mn(1)Ã
Mn(1)Ã
C(12)Ã
/Pd(1)
97.44(4)
151.88(5)
109.3(2)
/
/
P(1)
P(1)
P(3)
/
/
/
/P(4)
/
/
/
/
/
/
P(4)
/
/
/
Pd(1)Ã
/P(2)
MnPd(m-PPh2)(m-dppm)(CO)3(Ph2PCH2P(Ä
/
O)Ph2), 2a
Pd(1)Ã
Pd(1)Ã
Mn(1)Ã
Mn(1)Ã
Pd(1)Ã
/Pd(1)
2.6670(7)
2.2166(13)
2.2958(11)
/
P(4)
2.3234(11)
2.3031(11)
2.2361(13)
Mn(1)Ã
P(5)ÃO(4)
/
C (average)
1.814
1.362(8)
/
P(1)
/P(3)
/
/
P(1)
Mn(1)Ã
P(3)ÃPd(1)Ã
P(1)ÃPd(1)Ã
P(1)ÃPd(1)Ã
Mn(1)Ã
/P(2)
Mn(1)Ã
Pd(1)ÃMn(1)Ã
Mn(1)ÃPd(1)Ã
Mn(1)ÃPd(1)Ã
/
P(1)Ã
/
Pd(1)
72.44(3)
55.15(3)
52.41(3)
91.20(3)
/
/
P(4)
P(4)
P(3)
105.67(4)
111.59(4)
142.73(4)
158.94(3)
P(2)Ã
P(1)Ã
P(3)Ã
/
Mn(1)Ã
Mn(1)Ã
C(5)Ã
/Pd(1)
98.15(4)
153.18(5)
113.0(2)
/
/
P(1)
P(1)
P(3)
/
/
/
/P(2)
/
/
/
/
/
/
P(2)
/
/
/Pd(1)ꢀ
/
P(4)
MnPd(m-PPh2)(m-dppm)(CO)3(Ph2PC2H4P(Ä
/
O)Ph2), 2b
Pd(1)Ã
Pd(1)Ã
Mn(1)Ã
Mn(1)Ã
Pd(1)Ã
/Pd(1)
2.6610(5)
2.2100(10)
2.2920(8)
/
P(4)
2.3202(8)
2.3152(9)
2.2415(10)
Mn(1)Ã
P(5)ÃO(4)
/
C (average)
1.803
1.482(3)
/
P(1)
/P(3)
/
/
P(1)
Mn(1)Ã
P(3)ÃPd(1)Ã
P(1)ÃPd(1)Ã
P(1)ÃPd(1)Ã
Mn(1)Ã
/P(2)
Mn(1)Ã
Pd(1)ÃMn(1)Ã
Mn(1)ÃPd(1)Ã
Mn(1)ÃPd(1)Ã
/
P(1)Ã
/
Pd(1)
72.44(3)
55.20(2)
52.36(2)
92.41(2)
/
/
P(4)
104.40(3)
110.91(3)
144.56(3)
160.16(3)
P(2)Ã
P(1)Ã
P(3)Ã
/
Mn(1)Ã
Mn(1)Ã
C(6)ꢀ
/Pd(1)
97.92(3)
152.99(4)
112.63(18)
/
/
P(1)
P(1)
P(3)
/
/P(4)
/
/P(2)
/
/
/
/P(3)
/
/P(2)
/
/
/Pd(1)-P(4)

Y. Liu et al. / Inorganica Chimica Acta 350 (2003) 86ꢀ
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89
3. Results and discussion
although there is no free or coordinated PPh3 in the
substrates or reaction system. These can be understood
from the structural and mechanistic standpoints given
below.
Formation of 1 is largely driven by the thermody-
namic stability of its A-frame structure. During the
process (Scheme 1), dppm and phosphide have swapped
their metal hosts and, in doing so, release presumably a
The decarbonylative condensation of PdCl2(h2-
dppm) and PPNꢃ[Mn2(CO)8(m-PPh2)]ꢁ (2:1) in hot
toluene gave a dinuclear bimetallic complex analyzed as
MnPd(CO)3(PPh3)(m-PPh2)(m-dppm) (1) in 31% yield.
Its 31P NMR spectrum (Fig. 1) is consistent with a
dimetal structure that has a low symmetry; it shows four
discrete resonances, corresponding to bridging phos-
phide (PA), heterometallic-bridging phosphorus (PD and
PC) and the Pd-bound phosphine (PB). The magnitude
of the J(PP) coupling of the two high-field resonances
[MnCl(CO)4] entity. This oxidation of Mn from OSꢂ
to ꢃ1 serves as an internal reducing source for the
reduction of Pd from formally ꢃ2 [in PdCl2(dppm)] to
1 (in 1). This intramolecular redox behavior provides
/0
/
/
ꢃ
/
supports a transoid PCÃ
/
PdÃ
/
(m-PA) arrangement, with
a driving force for the reaction and its product forma-
tion. Upon losing a dppm arm, the unsaturated Pd
center regains its stability through the acquisition of a 2-
e donor ligand, which is best served by a ligand like
PPh3 [14]. We have discounted the possibility that
PPNꢃ is the source of PPh3. Not only that PPh3 on
PPNꢃ is kinetically stable, we also obtained 1 when
Naꢃ[Mn2(CO)8(m-PPh2)]ꢁ was used as the substrate.
PB cis to PdÃPA/C bonds.
/
This reaction has three unusual features: (1) the
condensation is not a simple cluster build-up process
that results in a trinuclear [PdMn2] core even though
there is a pre-existing MnÃ
has migrated from an MnÃ
bridge, although the driving force is not obvious; (3) this
is most surprising*there is a new PPh3 ligand on Pd,
/Mn bond; (2) the phosphide
/
Mn bridge to an MnÃPd
/
/
The unexpected PPh3 is hence traced to phosphide (Ã
/
Fig. 1. 31P{1H} NMR spectra of the title bimetallic complexes (1, 2a, 2b, 2c and 3a), illustrating the lack of symmetry and presence of chemically
different phosphorus nuclei.

90
Y. Liu et al. / Inorganica Chimica Acta 350 (2003) 86ꢀ92
/
Scheme 1. Formation pathway of PdÃMn bimetallic complexes by
/
redox condensation.
PPh2Ã
yet to be identified. The formation process could also
lead to a by-product MnPd(m-PPh2)(CO)4(h2-PÃ
P) (1a),
/
), through a complex phenyl scrambling that is
Scheme 2. Formation of syn-bridged bimetallic complexes 1 and 2aꢀ
2c, and anti-bridged 3a.
/
/
boring 55Mn quadrupolar nucleus. The dppm bridge
which we have described, albeit in low yield, in a
condensation reaction between PdCl2(h2-dppm) and
[Mn2(CO)8(PPh2)]ꢁ in the presence of AgNO3 [7].
Complexes 1 and 1a are isoelectronic but not isostruc-
tural, since dppm is chelating in 1a but bridging in 1.
Although it seems reasonable to suggest that 1 and 1a
are synthetically interconvertible, we could not convert 1
to 1a even upon heating and in the presence of excess
CO. Nevertheless, the independent isolation of these two
structural relatives in two different but related reactions
suggests that the {MnPd} dimetal core is a thermo-
dynamic sink in this type of condensation and that
dppm has sufficient flexibility to stabilize even a much
less stable ‘‘coordination isomer’’ (1a). When dppm is
replaced by dppe or dppf, i.e., when PdCl2(h2-dppe) or
PdCl2(h2-dppf) is the substrate in this condensation
reaction, the major product is the dppe or dppf analogue
of 1a. This illustrates further not only that dppe or dppf
is more suited as a chelating ligand in this environment,
but also the need to form PPh3 to stabilize the Pd center
is related to the departure of a phosphine arm on it.
Phosphine (PPh3) displacement reactions of 1 with
free diphosphines give the phosphine-oxide complexes
over the MnÃ
/
Pd bond occurs at Â70 ppm (PB) and
/
Â
/
14 ppm (PC). The diphosphine oxides contribute to
the high-field resonances (PD and PE). Their coupling
patterns are consistent with the proposed structures and
length of the diphosphine backbone. 31P NMR spec-
trum of the anti-bridged product [PdMn(m-PPh2)(m-
dppm)(CO)3]2(m-dppm) (3a) shows four resonances
(Fig. 1). Its PA/B/C resonances are very similar to those
of 1 and 2, thus suggesting the retention of the [PdMn(m-
PPh2)(m-dppm)] core. The uniquely high-field PD singlet
points to the unique anti- and inter-M2 role played by
dppm in this inter-bridge arrangement. These signals are
consistent with the structure proposed.
X-ray single-crystal diffractometric analyses of 1 (Fig.
2), 2a (Fig. 3) and 2b (Fig. 4) suggest that the solid-state
structures are consistent with the solution structures
proposed above. Essentially, they contain a heterome-
tallic A-frame core with bridging dppm and phosphide,
MnPd(m-PPh2)(m-dppm)(CO)3(Ph2PÃ
/
XÃ
/
P(Ä
/
O)Ph2)
(XÄCH2 (2a), C2H4 (2b), C5H4FeC5H4 (2c)) and an
/
anti-bridged product, viz. [PdMn(m-PPh2)(m-dppm)-
(CO)3]2(m-dppm) (3a) when the diphosphine used is
dppm (Scheme 2). Complexes 2 are direct substitution
products, but the dangling terminal phosphine is easily
oxidized. Such easy oxidation for a dangling end of a
monodentate diphosphine is known [15]. It is notable
that the anti-bridged tetrametallic structure is only
isolated when dppm is used as a ligand. This again
shows that dppm is a particularly suitable bridge in
multimetallic coordination network.
31P NMR spectra of 2 (Fig. 1) invariably show five
discrete resonances, suggesting that all the phosphorus
centers are chemically inequivalent. The downfield
bridging phosphide resonance is broadened by neigh-
Fig. 2. Molecular structure of MnPd(m-PPh2)(m-dppm)(CO)3(PPh3)
(1) (ellipsoids at 50% probability level).

Y. Liu et al. / Inorganica Chimica Acta 350 (2003) 86ꢀ
/92
91
dppm)(CO)3]2(m-ppm) (3a) are the only bimetallics
obtained. The absence of 1 is understood since there is
no longer a need to create a stabilizing source (PPh3)
when free dppm is present. In fact, excess dppm also
could not result in the analogue of 1, viz.
MnPd(CO)3(h1-dppm)(m-PPh2)(m-dppm), since such
species would easily convert to 3a, which is isolated.
Reaction with free dppe gives MnPd(m-PPh2)(CO)4(h2-
dppe) [7] (major), which is the analogue of 1a, as well as
PdMn(m-PPh2)(m-dppm)(CO)3(Ph2PCH2P(Ä
major), and PdMn(m-PPh2)(m-dppm)(CO)3(Ph2PC2H4P-
(ÄO)Ph2) (2b, major). Formation of the former high-
/
O)Ph2) (2a,
/
lights that dppe is more adapted than dppm as a
chelating ligand in this environment. For dppf,
PdMn(m-PPh2)(m-dppm)(CO)3(Ph3P) (1, minor), MnPd-
(m-PPh2)(CO)4(h2-dppf) [7] (major), PdMn(m-PPh2)(m-
Fig. 3. Molecular structure of MnPd(m-PPh2)(m-dppm)(CO)3-
(Ph2PCH2P(ÄO)Ph2) (2a) (ellipsoids at 50% probability level).
/
dppm)(CO)3(Ph2PCH2P(Ä
/
O)Ph2) (2a, major), and
PdMn(m-PPh2)(m-dppm)(CO)3(Ph2PC5H4FeC5H4P(Ä
/
O)Ph2) (2c, major) could be obtained. These consistently
suggest that both syn and anti bridging positions are
best served by dppm and that it is not substitutionally
labile. The other diphosphines could, however, force
their way to the chelating end or as monodentate
diphosphine oxide.
phosphine on Pd and carbonyls on Mn. Both anti- and
syn-bridges can be served only by dppm, which is not
substitutionally labile, whereas the monodentate dipho-
sphine is less sensitive towards the length of the back-
bone in diphosphine. The stability of this A-frame
underpins the formation of all the complexes described.
˚
Pd bond [2.7068(8) A in 1,
The loss of a [Mn(CO)4] fragment, presumably in
form of MnCl(CO)4 or its derivative during the forma-
tion of 1, is a blessing in disguise. Its departure is
undesirable in a cluster build-up reaction, but it provides
an internal mechanism for a redox condensation, which
is the key to the formation of 1 and subsequently 2 and
3. Eventually, this leads to the formation of a stable
The length of the MnÃ
/
˚
˚
2.6670(7) A in 2a and 2.6610(5) A in 2b] is in agreement
Pd bonds [16].
with other typical MnÃ
/
When the condensation of PdCl2(h2-dppm) and
PPN[Mn2(CO)8(m-PPh2)] is done in the presence of
free diphosphines, the relative abundance of the bime-
tallic species depends on the nature of the diphosphine.
In the case of dppm, PdMn(m-PPh2)(m-dppm)(CO)3-
heterometallic Mn0Ã
/
PdI A-frame. This cleavage and
departure of the Mn fragment was not observed in our
similar work with the AuI/Mn0 system. We attribute this
(Ph2PCH2P(ÄO)Ph2) (2a) and [PdMn(m-PPh2)(m-
/
Fig. 4. Molecular structure of MnPd(m-PPh2)(m-dppm)(CO)3(Ph2PC2H4P(ÄO)Ph2) (2b) (ellipsoids at 50% probability level).
/

92
Y. Liu et al. / Inorganica Chimica Acta 350 (2003) 86ꢀ92
/
(d) R. Uso´n, M.A. Uso´n, S. Herrero, Inorg. Chem. 36 (1997)
5959.
to the ease of reduction from Pd(II) to Pd(I), the
stability of Pd(I)-based A-frame and the difficulty in
[5] (a) J.A. Iggo, M.J. Mays, P.R. Raithby, K.J. Hendrick, J. Chem.
Soc., Dalton Trans. (1983) 205;
the stabilization of Au(0)ꢀMn(0) systems. Our next
/
venture is to test these ideas in the use of Pd(I) as
starting substrates. They may behave very differently
from the current Pd(II) substrate since reduction of
Pd(I) to Pd(0) may not be achievable or desirable and
the implication is that Mn cleavage may not be
necessary. These could pave an alternative pathway for
(b) J.A. Iggo, M.J. Mays, P.R. Raithby, K.J. Hendrick, J. Chem.
Soc., Dalton Trans. (1984) 633;
(c) J.A. Iggo, M.J. Mays, J. Chem. Soc., Dalton Trans. (1984)
643;
(d) P. Braunstein, E. de Je´sus, J. Organomet. Chem. 368 (1989)
C5.
[6] (a) K.H. Lee, P.M.N. Low, T.S.A. Hor, Organometallics 20
(2001) 3250;
the formation of a different series of PdÃ
/
Mn clusters.
(b) P.M.N. Low, A.L. Tan, T.S.A. Hor, Y.S. Wen, L.K. Liu,
Organometallics 15 (1996) 2595.
[7] Y. Liu, K.H. Lee, J.J. Vittal, T.S.A. Hor, J. Chem. Soc., Dalton
Trans., (2002) 2747.
Acknowledgements
[8] (a) B. Denise, R.P.A. Sneedon, J. Organomet. Chem. 221 (1981)
111;
The authors acknowledge the National University of
Singapore (NUS) for financial support and its Chem-
istry Department for technical support.
(b) A.A. Frew, R.H. Hill, L. Manojlovic´-Muir, K.W. Muir, R.J.
Puddephatt, J. Chem. Soc., Chem. Commun. (1982) 198.
[9] J.A. Iggo, M.J. Mays, P.R. Raithby, K.J. Hendrick, J. Chem.
Soc., Dalton Trans. (1983) 205.
[10] J.A. Iggo, M.J. Mays, P.R. Raithby, K.J. Hendrick, J. Chem.
Soc., Dalton Trans. (1984) 633.
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