Linearly aligned metal clusters: Versatile reactivity and bonding nature of tetrametal M-Mo-Mo-M complexes (M = Pt, Pd, Ir, and Rh) supported by 6-diphenylphosphino-2-pyridonato ligand

Article abstract of DOI:10.1246/bcsj.20090281

A tridentate ligand, pyphos (6-diphenylphosphino-2-pyridonato), was utilized to prepare tetrametal complexes since this ligand has unique coordinating sites comprised of three different elements, i.e., phosphorus, nitrogen, and oxygen, in almost linear fashion. By using pyphos ligand, linearly aligned tetrametal complexes of group 10 metals [Mo₂M₂- (pyphos)₄X_2n] (M= Pt and Pd; n = 0, 1, and 2) were prepared, and for group 9 metals, [Mo₂M₂(pyphos) ₄(RNC)₄X_2n]₂₊ (M = Ir and Rh; n = 0 and 1). Fully metal-to-metal bonded complexes were obtained by reduction of M^II to M^I for group 10 metals and by oxidation of M ^I to M^II for group 9 metals. Both reactions afforded complexes having unique M-Mo≡Mo-M skeletons, i.e., metalla-2-butyne. Structural and chemical properties were systematically investigated for M ⁰ (M = Pt and Pd) and M^I (M = Ir and Rh). Thus, oxidative reactions of Pd⁰ complexes [Mo₂Pd₂(pyphos) ₄] and Ir^I complexes [Mo₂Ir₂(pyphos) ₄(RNC)₄]²⁺ with RX or X₂ were studied and unique 1,4-addition reaction was demonstrated. Dichromium complexes analogous to dimolybdenum complexes were prepared and axial donation of PtMe₂ moiety significantly elongated the Cr-Cr bond, due to the dative bonding interaction between Cr^II and Pt^IIunits.

Full text of DOI:10.1246/bcsj.20090281

© 2010 The Chemical Society of Japan
Bull. Chem. Soc. Jpn. Vol. 83, No. 4, 299–312 (2010)
299
Award Accounts
The Chemical Society of Japan Award for Creative Work for 2008
Linearly Aligned Metal Clusters: Versatile Reactivity and Bonding
Nature of Tetrametal M-Mo-Mo-M Complexes (M = Pt, Pd, Ir, and Rh)
Supported by 6-Diphenylphosphino-2-pyridonato Ligand
Kazushi Mashima
Department of Chemistry, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531
Received October 23, 2009; E-mail: mashima@chem.es.osaka-u.ac.jp
A tridentate ligand, pyphos (6-diphenylphosphino-2-pyridonato), was utilized to prepare tetrametal complexes since
this ligand has unique coordinating sites comprised of three different elements, i.e., phosphorus, nitrogen, and oxygen, in
almost linear fashion. By using pyphos ligand, linearly aligned tetrametal complexes of group 10 metals [Mo2M2-
(pyphos)4X2n] (M = Pt and Pd; n = 0, 1, and 2) were prepared, and for group 9 metals, [Mo2M2(pyphos)4(RNC)4X2n]2+
(M = Ir and Rh; n = 0 and 1). Fully metal-to-metal bonded complexes were obtained by reduction of MII to MI for group
10 metals and by oxidation of MI to MII for group 9 metals. Both reactions afforded complexes having unique M-
Mo¸Mo-M skeletons, i.e., metalla-2-butyne. Structural and chemical properties were systematically investigated for M0
(M = Pt and Pd) and MI (M = Ir and Rh). Thus, oxidative reactions of Pd0 complexes [Mo2Pd2(pyphos)4] and IrI
complexes [Mo2Ir2(pyphos)4(RNC)4]2+ with RX or X2 were studied and unique 1,4-addition reaction was demonstrated.
Dichromium complexes analogous to dimolybdenum complexes were prepared and axial donation of PtMe2 moiety
significantly elongated the Cr-Cr bond, due to the dative bonding interaction between CrII and PtII units.
Three synthetic approaches have been rationally conducted for
one-dimensional homoleptic and heteroleptic transition-metal
clusters with more than three metal ions; thus categorized as
(1) use of non-bridged mixed valent metal complexes like
platinum blue species, (2) use of suitable multidentate ligands
for lapping the metal strings, and (3) use of dative bond or
strong closed-shell interactions among heavier metal ions with
d8, d10, and s2 configurations.
1. Introduction
Direct bonding interaction among metal atoms, in particular,
multiply-bonded dinuclear complexes including the most
characteristic concept of ¤ bonding, has been intensively
studied over the last decades as one of the most attractive
targets in inorganic chemistry.1 As a resemblance to carbon-to-
carbon double and triple bonds, an ideal reaction leading to
one-dimensional metal complexes bearing metal-metal bonds,
which have attracted much interest in terms of not only
bonding nature1 but also potential applications as single-
molecule transistors,2-4 (semi)conductors,5 and so on, is
polymerization of such multiply-bonded dinuclear complexes.
However, it has been known that the aligned metal array in
principle turns to the assembly of dinuclear metal units due to
the Peierls distortion (Scheme 1). In addition, it is necessary
to avoid the perpendicular alignment of multiply-bonded
dinuclear units and the [2 + 2] cyclodimerization of multiply-
bonded dinuclear complexes of Re,6,7 Mo,8-12 and W13 (eq 1).
1.1 Homoleptic Linear Transition-Metal Clusters.
A
typical example of category (1) is the partially oxidized
tetracyanoplatinate salts {K2[PtBr0.3(CN)4]¢3H2O}¨, referred
to as Krogmann salts, which were reported to have unique short
Pt£Pt bonding interactions through overlapping the dz2 orbital
of each platinum atom.14 In the same manner, controlled-
potential electrolysis afforded one-dimensional materials:
M M M M M M M M
n
Peierls distortion
Polymerization?
ð1Þ
M M M M M M M M
n
Many efforts have been devoted to construct nanometer-
scale metal strings connected entirely by metal-metal bonds.
Scheme 1. One-dimensional materials vs. multiply-bonded
dinuclear complexes.
Published on the web March 26, 2010; doi:10.1246/bcsj.20090281

300
Bull. Chem. Soc. Jpn. Vol. 83, No. 4 (2010)
AWARD ACCOUNTS
Chart 1.
Ziessel et al. synthesized a deep blue film of [Ru0(bpy)(CO)2]¨
(1) (bpy = 2,2¤-bipyridine) on a working electrode from trans-
[RuII(bpy)(CO)2C12],15,16 and Dunbar et al. prepared a 1D
rhodium chain, {[Rh(CH3CN)4][BF4]1.5
crystalline solids from electrochemical reduction of [Rh2
}
(2) (Chart 1), as
¨
II,II
-
Chart 2.
(MeCN)10][BF4]4 at a Pt electrode, in which there were bonding
interactions among RhI and RhII atoms with two different
contacts.17 A similar strategy has been utilized for dimerization
of dinuclear complexes: a tetrarhodium complex, [RhI4(CO)4-
(pqdi)4(s-pqdi)2]2+ (4) (s-pqdi = 9,10-phenanthrosemiquinone
diimine; pqdi = 9,10-phenanthroquinone diimine), was pre-
pared by the oxidative dimerization of the dirhodium complex
of [RhI(CO)2(s-pqdi)]2 (3) (eq 2).18 Mixed-valent oligomers of
platinum,19-21 rhodium,22 and iridium23 were also reported.
Another synthetic method was mixing transition-metal com-
plexes with different oxidation states: the reaction of a
dipalladium(I) isonitrile complex, [Pd2(CH3NC)6]2+, with a
monopalladium(0) complex, [Pd(CH3NC)x], resulted in the
selective formation of a mixed-valent linear homotrinuclear Pd3
complex, [Pd3(CH3NC)8]2+ 24
.
Chart 3.
naphthyridine, to give a dimer compound 5 with no direct
bonding interaction between the inner Mo atoms, and an
assembled paddle-wheel or lantern-type M2(O2CR)4 core with
long alkyl chain in the absence of neutral donor ligands to give
liquid crystalline laddered complexes 6, both complexes bear-
ing an intermolecular oxygen-to-metal interaction (Chart 2).
Multidentate ligands have played an important role
in preparing homoleptic one-dimensional transition-metal
complexes.26 Linearly aligned Ni3 and Cu3 complexes
[M3(dpa)4X2] (7) (dpa = di(2-pyridyl)amide) were reported
by Hathaway27 and Pyrka.28 Later, two independent groups of
Peng and Cotton intensively investigated the same type of
homoleptic linear trinuclear M3 complexes 7 (M = Cu, Co, Ni,
Cr, Ru, Rh, and so on) using dpa and related multidentate
ligands (Chart 3).29-32 These complexes were crystallographi-
ð2Þ
In category (2), an approach for constructing one-dimen-
sional materials comprised of multiply-bonded dinuclear units
using appropriate supporting ligands was studied by Chisholm
and his co-workers.25 They connected two quadruply-bonded
Mo2 units by a chelating ligand, the dianion of 2,7-dihydroxy-

K. Mashima
Bull. Chem. Soc. Jpn. Vol. 83, No. 4 (2010)
301
2+
P
P
Pt
P
P
Pt
P
Ph
P
(BArf)2
L
Pt
P
L
Pd Pd Pd
=
P
P
P
Ph2P
PPh2
L = XylNC
9
12
4+
BArf = B(3,5-(CF3)2C6H3)4
P
P
Pt
P
P
Pt
P
P
P
P
Pt
P
L
L
Pt
P
Pt
P
Pt
P
(BArf)2
Pd Pd Pd Pd Pd
10
13
BArf = B(3,5-(CF3)2C6H3)4
Chart 5.
+
Ph2
P
Ph2
P
R
Au
Au R
R
Au
Au
Au
14
Chart 4.
R
P
P
Ph2
Ph2
cally characterized, and some of them are an assembly of a
dimer unit and a mononuclear unit; however, chemical
oxidation induced the bond formation through all three metals.
Peng and Cotton have also developed this line of chemistry
to much longer metal oligomers by using oligo(¡-pyridyl)-
amido ligands as well as their analogs, synthesizing tetra-,33
penta-,34-42 hexa-,43,44 hepta-,45,46 octa-,47 and nonanuclear
chain48,49 of various metal atoms. Among them, a linear nona-
chromium complex [Cr9(®9-N9-mpz)4(NCS)2] (8) (N9-mpz =
N,N¤-bis[6-(2-pyridylamino)-2-pyridyl]pyrazine-2,6-diaminato)
is schematically drawn.48
R = C6F5, C6F3H2
2+
Ph2
P
Ph2
P
Ph2
P
R
Au
Au
Au
Au
Au
Au R
P
P
P
Ph2
Ph2
Ph2
Bis(diphenylphosphinomethyl)phenylphosphine (dpmp) is
a triphosphine ligand that is able to support three metals
in a linear fashion. In fact, [Pt3(dpmp)2(XylNC)2](PF6)2
(9) (XylNC = 2,6-dimethylphenyl isocyanide) was prepared
(Chart 4),50 and complex 9 was converted by excess NaBH4
to a hydride-bridged dimer [Pt6(®-H)(dpmp)4(XylNC)2]3+ and
then followed by chemical oxidation to give the dimer of
9, [Pt6(dpmp)4(XylNC)2]4+ (10).51 A hetero-tridentate P-As-P
ligand, bis(diphenylphosphinomethyl)phenylarsine (bdppa),
was used to prepare a tetranuclear Au4 complex, [Au4Cl2-
15
R = C6F5, C6F3H2
Chart 6.
The third category is based on the metallophilic interaction,
where two closed-shell heavy late transition metals with d8 and
d10 configurations have an attracting bonding interaction.58
Such bonding interaction has been successfully used for
forming a linearly aligned mixed-valent pentanuclear Au5
complex [{RAu(CH2PPh2CH2)2Au}2AuR2]ClO4 (14) and
hexanuclear Au6 complex, [{RAu(CH2PPh2CH2)2Au}2-
{Au(CH2PPh2CH2)2Au}](ClO4)2 (15) (R = C6F5 and C6F3H2)
(Chart 6).59
1.2 Heteroleptic Linear Transition-Metal Clusters. For
the synthesis of linear hetero-multinuclear complexes of more
than three metal atoms, a few non-bridged hetero multi-metal
complexes have been reported. Yamaguchi and Ito prepared
a linear pentanuclear complex, [{Pt(thpy)2}{Ag(acetone)}]n-
[ClO4]n (thpy = the anion of 2-(2-thienyl)pyridine), and an
infinite helical compound, [{Pt(phpy)2}{Ag(acetone)}]n[ClO4]n
(phpy = the anion of 2-phenylpyridine), both of which had a
sequence of Pt and Ag and were connected by a Pt ¼ Ag
(bdppa)2]2+ 52
cyanopropane (dpb) and 1,8-diisocyano-p-menthane (dmb),
yielded a tetrarhodium complex, [Rh4Cl(dpb)8]5+ 53-55
and
.
Bridging bis(isocyanide) ligands, 1,3-diiso-
a
,
tetraplatinum and tetrapalladium compounds [M4(dmb)4-
(PPh3)2]Cl2 (11: M = Pd and Pt).56
Conjugated polyenes and aromatic compounds acted as
supporting ligands of multinuclear palladium complexes.
Murahashi and Kurosawa reported that the combination of
Pd0 and PdII precursors in the required ratio in the presence
of conjugated polyenes afforded multinuclear mixed-valent
palladium complexes 12 and 13 (Chart 5), which had entirely
Pd-Pd bonds through the arrays, and conjugated aromatic
compounds produced linear or planar palladium clusters.57

302
Bull. Chem. Soc. Jpn. Vol. 83, No. 4 (2010)
AWARD ACCOUNTS
2+
N
Pt
N
O
NH3
NH3
Pt
N
O
N
Cu
H3N
O
O
NH3
16
=
N
O
-
O
N
Chart 7.
dative bond.60 In sharp contrast, many research work has been
investigated by appropriate multidentate ligands containing
different elements as coordination sites for supporting two
different kinds of metals (category 2). Pyridonato ligand and its
derivatives have been applied to form a heterotrinuclear Pt2Cu
complex, trans-[(NH3)2Pt(C5H4NO)2Cu(C5H4NO)2Pt(NH3)2]2+
(16) (Chart 7),61 along with some related trinuclear Pt2M
¹
complexes.62 By using [CH2P(S)Ph2] (mtp), a linear trinuclear
complex, [Au2Pt(mtp)4] (17), was formed. Chemical oxidation
of 17 produced a chlorinated complex, [Au2PtCl2(mtp)4] (18)
(eq 3).63 A similar heterotrinuclear complex [Au2Pb(mtp)4]
was also reported.64
Chart 8.
C
Cl Au
C
S
C
Au
C
S
C
C
S
C
S
C
PhICl2
Pt
Au Cl
Pt
Au
ð3Þ
S
S
S
S
17
18
Ph2
P
=
C
S
H2C
-
S
Balch and his co-workers used a tridentate ligand, bis-
(diphenylphosphinomethyl)phenylarsine (bdppa), forming
macrocycles 19 comprised of two metal ions in their ring
system, and then added various metal ions to give heterotri-
nuclear M2M¤-type complexes 20 (eq 4), where M2M¤ =
Au2Rh (d10d8d10), Au2Ir (d10d8d10), Rh2Au (d8d10d8), Ir2Au
(d8d10d8), Ir2Ag (d8d10d8), and so on,65 where closed-shell
interaction is a key attracting force.
Figure 1. Strategy to synthesize
complex, metalla-2-butyne, as decreasing bond order of
multiple bond of M2 and forming new M-M¤ single bonds.
a linear tetranuclear
n+
P
LnM
P
As
M
P
P
LnM
P
As
P
MLn
P
+
Mn+
MLn
ð4Þ
tetrametal complexes with the multiply-bonded unit in an
array and elucidating their fundamental chemical properties.
We have thus chosen a tridentate ligand, Hpyphos (6-diphenyl-
phosphino-2-pyridone), because this has unique coordinating
sites comprised of three elements, i.e., phosphorus, nitrogen,
and oxygen, in almost linear fashion. The most attractive
aspect of our synthetic strategy is illustrated in the reaction
outlined schematically in Figure 1, where the addition of
two unpaired transition metals M¤ to the multiply-bonded
M2 moiety generated new tetrametal compounds with decrease
of bond-multiplicity at the central M2 and the formation of
M-M¤ bonds. This account discloses unique tetranuclear
complexes bearing a skeleton of M¤-M-M-M¤, where M
are molybdenum and chromium and M¤ are group 9 and 10
metals, by using tridentate pyphos (6-diphenylphosphino-2-
pyridonato) ligand.
As
P
As
19
20
M = Au, Rh, Ir, etc
M = Group 9, 10, 11,
13, and 14 metals
=
As
Ph
Ph2P
PPh2
As
P
P
Concerning the strategy depicted in Scheme 1, synthetic
studies of linear heteroleptic complexes bearing multiply-
bonded dinuclear units have not been well-developed yet.
By using di(2-pyridyl)formamidinate (dpyf), heteronuclear
[Mo2Co(dpyf)4][CoCl4] (21) was obtained (Chart 8),66 and
the use of the anion of 2,6-di(phenylimino)piperidine (dPhip)
resulted in the formation of a tetranuclear complex [M2Cu2-
(dPhip)4(CH3CN)][CuCl2]2 (22: M = Cr and Mo), in which the
Cu atoms have only weak interactions with the M atoms.32 We
have focused our attention on synthesizing heterometallic

K. Mashima
Bull. Chem. Soc. Jpn. Vol. 83, No. 4 (2010)
303
2. Synthesis of Dinuclear Complexes Supported
by pyphos Ligands
Based on the soft-hard combination of elements of the
pyphos ligand with metals, the pyridone served as a supporting
ligand of multiply-bonded dinuclear moieties, while the
phosphine favorably coordinated to late transition metals,
leading to heterotetrametal complexes (Figure 1). Further-
more, the P-N bridge has a suitable distance to support
the single bond between transition metals, and the N-O
bridge is best fit to a shorter multiple bond such as a quadruple
bond in the dinuclear complexes of group 6 metals. In fact,
the N-O coordination of mhp ligand (mhp = the anion of
2-hydroxy-6-methylpyridine) was utilized for supporting
quadruply-bonded dinuclear complexes of chromium, molyb-
denum, and tungsten.67-69 A typical N-P bridging ligand, 2-
(diphenylphosphino)pyridine, coordinates to later transition
metal atoms such as rhodium, palladium, and platinum to form
dinuclear complexes with rather long metal-to-metal dis-
tances.70
Chart 10.
At first, we prepared the dinuclear molybdenum complex
23 by treating Mo2(OAc)4 with the sodium salt of the
pyphos ligand. Complex 23 has four pyphos ligands for
supporting a quadruple Mo-Mo bond and two axial positions
are suitable for the coordination of two soft, late transition
metals (eq 5).
Figure 2. Synthetic strategies for tetrametal cluster with a
new triple bond.
+
Mo2(O2CCH3)4
4
NaO
P
N
PPh2
3. Synthesis and Chemical Properties of Tetranuclear
M-Mo-Mo-M Complexes
O
N
P
N
O
Using the complex 23 as a starting material, we prepared two
types of tetrametal complexes Mo2M2 (M = Pt and Pd) and
Mo2M2 (M = Ir and Rh). For group 10 metals, reduction of MII
to MI afforded fully metal-metal bonded tetrametal complexes,
while oxidation of group 9 metals from MI to MII resulted in
the formation of the corresponding tetrametal complexes as
schematically described in Figure 2. Of interest, both products
have the triply-bonded Mo2 core of (³2¤) component, being in
sharp contrast to (·³2) component of the carbon-carbon triple
bond.
3.1 Mo2M2 (M = Pt and Pd) Complexes. Linearly aligned
tetrametal complexes [Mo2Pt2(pyphos)4X2] (29) (a: X = Cl, b:
X = Br, and c: X = I) and [Mo2Pd2(pyphos)4X2] (30) (a:
X = Cl, b: X = Br, and c: X = I) have fully metal-to-metal
bonded fragments of M-Mo¸Mo-M (M = Pt and Pd) sup-
ported by four pyphos ligands (eq 6).74,75 As the first step of
preparing 29 and 30, the corresponding MII tetranuclear
complexes, [Mo2M2(pyphos)4X4] (M = Pt (27), Pd (28); a:
X = Cl, b: X = Br, and c: X = I), were prepared. These
complexes 27 and 28 have two d8 group 10 metals in the
line of the axial vector of the Mo-Mo unit; however, there
were no bonding interaction of two metal ions with the
quadruply-bonded Mo2 and the square planes of two
group 10 ions were perpendicular to the vector of the
Mo2 core. Chemical, thermal, or irradiative reduction of 27
and 28 afforded the corresponding complexes 29 and 30, in
which each metal ion interacts with a molybdenum atom
ð5Þ
+
4 NaO2CCH3
Mo Mo
N
O
P
O
N
P
23
Similarly, reaction of [Re2Cl8]2¹ with an excess amount of
Hpyphos in refluxing acetonitrile solution afforded [Re2Cl4(py-
phos)2] (24) (Chart 9), whose Re-Re bond was determined by
X-ray analysis to be a quadruple bond.71
In contrast, treatment of pyphos ligand with palladium
and platinum precursors resulted in the formation of two
dinuclear complexes of palladium(I) [Pd2(Hpyphos)2(pyphos)2]
(25), which has a Pd-Pd single bond surrounded by two
pyphos ligands and two Hpyphos ligands, and platinum(II),
[Pt2(pyphos)4] (26), which has no metal-metal bond
(Chart 10).72 Our tridentate ligand mainly coordinated by its
phosphorus moiety to a ruthenium(II) center.73
Chart 9.

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Bull. Chem. Soc. Jpn. Vol. 83, No. 4 (2010)
AWARD ACCOUNTS
Figure 3. Schematically equation for reductive formation of a Pd-Mo¸Mo-Pd complex 30b from 28b.
to significantly elongate the Mo-Mo distance and the molyb-
denum atom occupies one corner of the square planer of each
metal ions (Figure 3). Entirely metal-to-metal bonded tetra-
metal complexes with longer conjugated systems show third-
order nonlinear optical properties.76,77 Such reductive trans-
formation was not observed for trinuclear [Mo2Pt(OCOR)2-
(pyphos)2X2].78
O
N
P
N
P
O
N
N
Mo2
N
Pd1
Pd2
N
Mo1
O
P
O
P
O
N
P
M
P
O
N
P
M
P
Et4NBH4
or
X
X
P
N
N
O
P
N
O
ð6Þ
X
M
Mo Mo
X
M
Mo Mo
O
O
300 °C
or
hν
P
N O
P
X
O
N
X
N
27: M = Pt
28: M = Pd
29: M = Pt
30: M = Pd
Figure 4. Crystal
(NCCH3)2]2+ (32).
stucture
of
[Mo2Pd2(pyphos)4-
a: X = Cl
b: X = Br
c: X = I
a: X = Cl
b: X = Br
c: X = I
O
N
P
We tried two approaches for assembling tetrametal units by
2 LiC2SiMe3
P
N
O
alkyne coupling and coordination polymerization. We intro-
duced alkyne groups at both of the axial positions of a linear
Mo2Pt2 fragment: an acetylide complex, [Mo2Pt2(C¸CSiMe3)2-
(pyphos)4] (31), was prepared by treating [Mo2Pt2Cl2(pyphos)4]
(29a) with two equivalents of lithium trimethylsilylacetylide
(eq 7).79 Desilylation followed by spontaneous oxidation
coupling gave solid, which was assumed to be a poly(yne)
polymer containing a Mo2Pt2 moiety in its backbone, though its
characterization was hampered by its low solubility.79 Recent-
ly, Ren reported that bis(ferrocenylethynyl) dinuclear com-
plexes containing a Ru2 unit showed significant electronic
couplings of ferrocenyl moieties through the Ru2 unit.80
Reaction of the lithium salt of 1-ferrocenylacetylene with
[Co3(dpa)4(NCCH3)2][PF6]2 afforded the bis-ferrocenylacet-
ylide derivative bearing a Co3(dpa)4 fragment,81 and its
triruthenium derivative also was reported.82
Cl
Pt
Mo Mo Pt
N O
Cl
P
P
O
N
29a
O
N
P
P
N
O
Me3Si C C
Pt
ð7Þ
Mo Mo Pt
C C SiMe3
P
N
O
P
O
N
31
The second approach was the coordination polymerization
of a dicationic complex, [Mo2Pd2(pyphos)4(NCCH3)2]2+ (32),
whose crystal structure is depicted in Figure 4, with a
bidentate ligand, pyrazine (=pyz), to give [Mo2Pd2(pyphos)4-
(pyz)]n[BF4]2n (33) (eq 8).79

K. Mashima
Bull. Chem. Soc. Jpn. Vol. 83, No. 4 (2010)
305
P
N
O
P
N
O
P
N
O
ArS-
O
N
P
O
N
P
O
N
P
Pd
Pd
Mo Mo
Mo Mo Pd
Pd
ArS
Mo Mo Pd
SAr
SAr
Pd
TCNE
O
P
N
N
P
O
P
N
P
O
P
N
N
PH
H
TCNE
O
O
N
O
Pd(I)/Pd(0)
Pd(I)
TCNE-Pd(0)
Ar-S-S-Ar
N
O
P
TCNE
NaBH4
O
N
P
Pd Mo Mo
Pd
O
N
P
P
N
O
CH2Cl2
CH3I
Pd(0)
P
N
O
O
P
N
O
I
P
N
O
Cl
N
P
O
N
P
I
O
N
P
Cl
Pd Mo Mo
Pd
I
Mo Mo Pd
Pd Mo Mo
I
Pd
Pd
H3C
- C2H6
O
P
N
N
P
O
P
N
N
P
CH3
ClCH2
O
P
N
N
P
O
O
CH2Cl
O
Pd(II)
Pd(II)
Scheme 2. Some reactions of [Mo2Pd2(pyphos)4] (34).
Pd(I)
2+
O
N
P
P
N
O
S
Pd
N
+
Mo Mo Pd
O
S
N
P
N
P
P
O
N
32
2n+
O
N
P
N
O
ð8Þ
Pd
Mo Mo Pd
N
N
1/n
P
N O
P
O
N
n
33
Figure 5. Crystal structure of [Mo2Pd2(pyphos)4(tcne)2]
(35c).
It was to our surprise that coordinatively unsaturated Pd0
tetrametal complex, [Mo2Pd2(pyphos)4] (34), could be obtained
by treating [Mo2Pd2(pyphos)4X2] (30) with appropriate
alkylating reagents, in sharp contrast to the successful isolation
of the alkynyl derivative. Alternatively, complex 34 was
obtained in high yield by the reaction of [Mo2(pyphos)4] (23)
with a Pd0 source such as [Pd(dba)2] (dba = dibenzylidene-
acetone).83 Once isolated, such coordinatively unsaturated Pd0
complex was found to be rather stable due to the presence of
Pd0 ¼ MoII dative bonds. Since there were two zero-valent
palladium atoms at both axial positions of a quadruply-bonded
Mo2 core of 34, some typical reactivities due to the Pd0 species
were observed. The reaction of 34 with various ³-ligands (L)
such as olefins, alkyne, isocyanides, carbon monoxide, and
phosphine afforded the corresponding adducts, [Mo2Pd2(L)2-
(pyphos)4] (35) (a: L = acrylonitrile, b: L = fumaronitrile, c:
L = tetracyanoethylene (=tcne), d: L = diisopropyl fumarate,
e: L = diethyl fumarate, f: L = dimethyl fumarate, g: L =
dimethyl maleate, h: L = 2,6-xylyl isocyanide, i: L = tert-
butyl isocyanide, j: L = dimethyl acetylenedicarboxylate,
k: L = 1,4-benzoquinone, l: L = 1,4-naphthoquinone, m: L =
carbon monooxide, and n: L = triphenylphosphine) (Scheme 2
and Figure 5).83,84 Similar Pt0 complexes coordinated by
phosphine and phosphite have been obtained.85
We examined oxidative addition reactions of 34.83,84
Disulfides and benzoyl peroxide added in a 1,4-fashion to the
Pd0 centers of 34 to give the corresponding PdI complexes
[Mo2Pd2(pyphos)4(SAr)2] (36: Ar = C6H5, 4-Me3CC6H4, 4-
MeC6H4, and 4-NO2C6H4) and [Mo2Pd2(OCOPh)2(pyphos)4]
(37), respectively (Scheme 2). The reactions of alkyl and aryl
halides to 34 gave rise to two different reaction patterns: excess
amounts of benzyl halides BnX (X = Cl, Br, and I), PhCl, and
PhBr, and 2 equiv of PhI reacted with 34 to give PdI complexes
[Mo2Pd2(pyphos)4X2] (30). The most notable reactivity was
that the Pd0 atom surrounded by two phosphine atoms
and one molybdenum exhibited unexpected high reactivity
toward CH2Cl2, resulting in the isolation of double oxidative
addition product of PdII, [Mo2{PdCl(CH2Cl)}2(pyphos)4] (38)
(Scheme 2 and Figure 6), in contrast to oxidative reaction of a
dinuclear Pd0 complex, [Pd2(®-dppm)3] [dppm = bis(diphen-
ylphosphino)methane], with CH2Cl2 resulted in a A-shape
dinuclear PdII product having a bridged methylene group.86 The
reactions of 34 with excess amounts of CH3I and PhI also
afforded similar PdII complexes, [Mo2{Pd(CH3)I}2(pyphos)4]
(39) and [Mo2{Pd(Ph)I}2(pyphos)4] (40), respectively
(Schemes 2 and 3).

306
Bull. Chem. Soc. Jpn. Vol. 83, No. 4 (2010)
AWARD ACCOUNTS
PhCl (excess)
No reaction
O
P
N
P
N
O
PhBr (excess)
Br
P
PdI
N
Mo Mo PdI Br
O
O
N
P
P
O
N
P
N
O
30b
Pd0
Mo Mo
N
O
Pd0
P
P
O
N
P
O
N
P
N
O
2 eq. PhI
X
PdI
N
Mo Mo PdI
O
X
34
P
P
PhCH2X (excess)
N
P
O
30
RX = CH2Cl2,
CH3I, PhI
(excess)
O
N
P
R
R
P
N
O
PdII Mo Mo
PdII
P
N
O
O
X
X
N
38: R = CH2Cl, X = Cl
39: R = CH3, Y = I
40: R = Ph, Y = I
Scheme 3. Reactions of 34 with alkyl and aryl halides.
(pyphos)4][BF4]2 [L¤ = acetonitrile (32), dimethyl sulfoxide,
THF, benzonitrile, p-methoxybenzonitrile, p-trifluoromethyl-
benzonitrile, pyridine, and p-(dimethylamino)pyridine], sug-
gesting that there was an attractive bonding interaction between
a closed-shell d10 Pd0 with Mo2 core. Such solvent effect is
consistent with the known evidence that the metal-metal
bonding nature of paddle-wheel-type dinuclear complexes
are sensitive to axial ligands.87 Chemical oxidation of Pt0
complexes, [Mo2Pt2(PR3)2(pyphos)4], also proceeded in a
stepwise manner.85 Referring to such bonding interaction, it
should be pointed out that there are d10 closed shell dinuclear
palladium complexes, [Pd2(®-dppm)3],88,89 [Pd2(®-dmpm)3]
[dmpm = bis(dimethylphosphino)methane],90 [Pd2(®-MeN-
{P(OPh)2}2)3],91 [Pd2(®-dafo){1,4-C6H4(=O)2}2] (dafo = 4,5-
diazafluorene-9-one),92 and [Pd2{®-CH2(PR2)2}2] (R = iPr
and c-C6H11),93,94 which have been reported to have unique
attractive bonding interaction between two Pd0 atoms through
d-p mixed ·-bonding orbitals.
3.2 Mo2M2 (M = Ir and Rh) Complexes. Other linear
hetero tetrametallic Mo2M2 (M = IrI and RhI) complexes were
prepared and their unique oxidative reactions to form metal-
metal bonding in metal arrays were studied. Reaction of
[Mo2(pyphos)4] (34) with [M2(CO)4Cl2] (M = Ir and Rh) gave
linear tetranuclear complexes of the formula, [Mo2M2(CO)2-
Cl2(pyphos)4] (41: M = Ir and 42: M = Rh), which were
further treated by tBuNC and XylNC to yield the corresponding
dicationic complexes [Mo2M2(pyphos)4(tBuNC)4]Cl2 (43a:
M = Ir and 44a: M = Rh) and [Mo2Ir2(pyphos)4(XylNC)4]Cl2
(43b: M = Ir and 44b: M = Rh) with the release of CO (eq 9).
Crystallographic studies of Mo2M2 complexes 41-44 con-
firmed that there was no direct · bonding interaction between
MI atoms and the Mo2 core.95,96
P
P
Cl
O
O
C
Pd
Cl
Mo
Pd
P
Mo
Cl
Cl
C
N
P
O
Figure 6. Crystal structure of [Mo2{PdCl(CH2Cl)}2-
(pyphos)4] (38).
It was still difficult to explain such unique reaction modes
with oxidants; however we were able to measure the electro-
chemical characteristics of 34. Cyclic voltammetry of 34 in
acetonitrile and THF displayed two reversible waves at ¹490
and ¹650 mV vs. Ag/Ag+ (¦E1/2 = 160 mV) in acetonitrile
and ¹200 and ¹415 mV vs. Ag/Ag+ (¦E1/2 = 215 mV) in
THF, the electrochemical properties depending on the solvent
due to the coordination of solvent molecules to the PdI species
during oxidation. Thus, it was reasonably assumed that the first
one-electron oxidation of 34 gave a mixed-valence mono-
cationic species [34]+, where two palladium atoms electro-
chemically communicated through a Mo2 unit, and the second
one-electron oxidation resulted in the formation of [34]2+, i.e.,
complex 32 in acetonitrile (Scheme 4). Chemical oxidation
of 34 with 2 equiv of [Fe(Cp)2][BF4] in other donor mole-
cules afforded dicationic PdI complexes [Mo2Pd2(L¤)2-

K. Mashima
Bull. Chem. Soc. Jpn. Vol. 83, No. 4 (2010)
307
2+
O
N
P
O
N
P
P
N
O
2 [Fe(Cp)2](BF4)
CH3CN
P
N
O
Pd0
H3CCN
Mo Mo PdI
O
Mo Mo
O
Pd0
P
PdI
N
NCCH3
P
N
P
P
O
N
O
N
32
34
+
Eox
-650 mV
=
Eox =
-490 mV
O
N
P
P
Pd0.5
N
N
O
Mo Mo
O
Pd0.5
P
P
O
N
mixed valence intermediate
Scheme 4. Stepwise oxidation of [Mo2Pd0 (pyphos)4] (34) through a mixed-valence intermediate.
2
Figure 7. Schematically equation for oxidative formation of a Ir-Mo¸Mo-Ir complex 45a from 43a.
bonding interaction between the two MI atoms and the Mo2
cores in these tetranuclear complexes is neglected, though its
distance was not as long as found for the PdII complexes
[Mo2Pd2(pyphos)4X4] (28). Chemical oxidation of both group 9
metals MI (M = Ir and Rh) to MII spontaneously proceeded
with the formation of M-Mo bonds and decreased the bond
order of the Mo2 moiety. The resulting MII ions favorably
adopted octahedral geometry, in which one corner of each MII
ion was occupied by a Mo atom.
2+
O
N
P
MI
P
O
N
P
CO
L
CO
P
N
N
O
L
P
N
N
O
L (excess)
MI
Cl
Mo Mo
O
MI
Mo Mo
O
MI
P
ð9Þ
P
P
Cl
L
O
N
L
O
N
41: M = Ir
42: M = Rh
43: M = Ir
44: M = Rh
a: L = tBuNC
b: L = XylNC
Oxidation of these complexes by either two equiv of
[Fe(Cp)2][PF6] or excess I2 afforded the corresponding MII
complexes, [Mo2M2(L)4(pyphos)4X2]2+ (45: M = Ir and 46:
M = Rh; a: L = tBuNC and b: L = XylNC; X = Cl, Br, and I)
(eq 10), in which the bond order of the Mo-Mo moiety
decreased to three as shown in Figure 7. It is obvious that
the square plane around the MI (M = Ir and Rh) fragments
supported by [Mo2(pyphos)4] system is perpendicular to the
Mo-Mo vector, and hence the contribution of the direct ·-
2+
O
N
P
O
N
P
L
L
2 [Fe(Cp)2][BF4]
L
L
P
N
N
O
P
N
N
O
MI
Mo Mo
O
O
MI
P
Cl
MII
Mo Mo
O
O
MII Cl
P
Cl2
ð10Þ
P
P
L
L
L
N
L
N
43: M = Ir
44: M = Rh
a: L = tBuNC
45: M = Ir
46: M = Rh
a: L = tBuNC
b: L = XylNC
b: L = XylNC

308
Bull. Chem. Soc. Jpn. Vol. 83, No. 4 (2010)
AWARD ACCOUNTS
Some alkyl halides oxidatively reacted with the IrI com-
plexes 43a and 43b to give unique 1,4-oxidative addition
products, while the corresponding rhodium complexes 44a and
44b did not react under the same conditions. Reaction of the IrI
complexes 43a and 43b with CH3I afforded unique “1,4-
oxidative addition” products of methyl iodide, [Mo2Ir2(CH3)-
(tBuNC)4I(pyphos)4]Cl2 (47a) and [Mo2Ir2(CH3)(XylNC)4I-
(pyphos)4]Cl2 (47b), respectively (eq 11). It was of interest
that there were no homo-substituted IrII products, such as
diiodo and dimethyl complexes. Similar 1,4-oxidative addition
product [Mo2Ir2(CH2Cl)(tBuNC)4Cl(pyphos)4]Cl2 (48) was
obtained in the reaction of dichloromethane with 43a. Kinetic
analysis of the reaction of 43a with CH3I suggested that
the 1,4-oxidative addition to the IrI complex proceeded by
a SN2 reaction mechanism. Similarly, oxidative addition of
diaryl disulfides afforded bis(thiolato) IrII complexes [Mo2Ir2-
(tBuNC)4(pyphos)4(SAr)2]Cl2, in accord with the 1,4-oxidative
addition to Pd0 complex.
2+
O
N
P
O
N
P
2+
L
L
L
L
I
P
N
N
O
P
N
N
O
CH3I (excess)
IrI
Mo Mo
O
IrI
H3C
IrII
Mo Mo
O
IrII
ð11Þ
P
P
Figure 8. Plots of ¯(Mo-Mo) (cm¹1) vs. bond distance of
Mo-Mo (¡) for [Mo2(pyphos)4] (23) and linear tetrametal
complexes of M-Mo-Mo-M arrays (M = Pt, Pd, Ir, and
Rh) including Pd0 complex, [Mo2Pd2(pyphos)4] (34).
L
L
L
O
N
P
L
O
N
P
43
a: L =
47
tBuNC
b: L = XylNC
tBuNC
b: L = XylNC
a: L =
Cyclic voltammogram of the Mo2Ir2 complex 45a measured
in TBACl/MeCN solution at room temperature with a
positively-directed sweep showed a reversible, two-electron
wave at ¹970 mV (vs. Fc/Fc+), which was determined to be an
IrIIrI/IrIIIrII redox. Thus, the oxidation of 45a was a more
thermodynamically favorable process in comparison with that
of the corresponding rhodium complex 46a (¹660 mV vs. Fc/
Fc+), which might explain the difference in the oxidative
addition reactions between iridium 45a and rhodium tetrametal
complexes 46a.
3.3 Raman Spectra of M-Mo-Mo-M (M = Pt, Pd, Ir, and
Rh) Complexes. For discussion of the bonding nature of the
linear skeleton comprised of M-Mo-Mo-M (M = Pt and Pd),
single-crystal X-ray analysis is the best method to determine
bonding distances. On the other hand, since we could not
obtain crystals of all of them, Raman spectral data of ¯(Mo-
Mo) frequency served as a highly useful spectroscopic probe of
elucidating the bonding nature. Figure 8 shows the plot of
¯(Mo-Mo) frequency and the corresponding Mo-Mo distance
determined by X-ray analyses. The frequency assigned to the
quadruple Mo-Mo bond of MII complexes and [Mo2(pyphos)4]
(23) were observed at 403-404 and 394 cm¹1, respectively,
while the MI complexes displayed a frequency assignable
to ¯(Mo-Mo) at 381-387 cm¹1, a lower wavenumber, being
consistent with the longer Mo-Mo triple bond. The most
attractive discussion concerns the Pd0 and Pt0 complexes,
whose Raman spectral data are diverged in the range of 389-
404 cm¹1, suggesting that the dative bond, M0 ¼ MoII was
strong enough to elongate the Mo-Mo bond.
¹1
30c) (386-387 cm¹1)} < 35b (399 cm¹1) < 35c {404 cm
,
comparable to that of the quadruply-bonded Mo2 complexes
¹1
such as 23 (394 cm
)
and PdII complexes, [Mo2Pd2-
(pyphos)4X4] (28a-28c) (403 cm¹1), with no Mo-Pd bond}.84
Thus, the increase in the number of electron-withdrawing
substituents on the alkenes, acting as stronger ³-acceptors,
weakened the interaction between Pd0 and MoII, as a
consequence of the deformation of the palladium atoms from
the axial vector of the Mo2 core. Based on these argument, it
is assumed that Pd0 compounds 34 (389 cm¹1) and 35a
¹1 84
¹1 85
(389 cm
)
and Pt0 complexes (382-388 cm
)
were in the
range observed for MI complexes, suggesting that the dative
bond between M0 and MoII is a rather strong bonding
interaction. Compared with the group 10 metal series, plots
for IrI and RhI complexes are close to those for IrII and RhII
complexes, strongly suggesting that dative bonding interaction
between MI and MoII atoms are significantly contributed. One
rational explanation for such interaction between MI with Mo2
core is ascribed to the contribution of dative bonding between
MI and MoII, the same as the bonding interaction between
M0 ¼ MoII found for group 10 metals. Thus, the bonding
nature of the tetrametal cluster systems could be evaluated
based on data taken by Raman spectroscopy.
3.4 Cr2M2 (M = Pt and Pd) Complexes. Dichromium
complex, [Cr2(pyphos)4] (49) (Chart 11), analog to dimolyb-
denum complex 23, was prepared in a similar manner applied
to the synthesis of 23.97-99 The quadruply bonded Cr2 moiety is
known to be quite sensitive to the axially coordinating organic
donor ligands,100 however, there was no precedent example of
axial coordination of metal ions. As a first trial, we started
studying the influence of various d8 PtII ions placed at axial
positions of the Cr2 moiety. Three PtII tetranuclear complexes,
[Cr2Pt2Cl4(pyphos)4] (50), [Cr2Pt2Cl2Me2(pyphos)4] (51), and
Among olefin-coordinated Pd0 complexes [Mo2Pd2(L)2-
(pyphos)4] (35) (a: L = acrylonitrile, b: L = fumaronitrile,
and c: L = tetracyanoethylene), the wavenumber of ¯(Mo-Mo)
increased with an increase in the number of CN groups on the
substrates, in the order of 35a {389 cm¹1, comparable to that of
the triply-bonded Mo2 complexes, [Mo2Pd2(pyphos)4X2] (30a-

K. Mashima
Bull. Chem. Soc. Jpn. Vol. 83, No. 4 (2010)
309
new triple bond at Mo-Mo moiety with a component of (³2¤).
The author thus hopes that this account will become a hint for
new ideas in this research field.
O
N
P
O
N
P
P
Cl
Pt
Cl
P
N
N
O
P
N
N
O
Cr Cr
O
Pt
P
Cr Cr
O
P
P
Cl
Cl
O
N
O
N
The author expresses his profound gratitude to Professors A.
Nakamura, K. Tani, K. Yamaguchi, S. Takeda, and W. Mori for
their discussions and suggestions, and his collaborators listed
in the references, in particular Drs. H. Nakano, M. Tanaka,
T. Rüffer, M. Ohashi, and A. Shima for their considerable
experimental contributions on this work.
49
50
O
N
P
Me
Pt
P
P
N
O
Me
Pt
Me
P
N
O
P
Me
N
O
N
Cr Cr
N
O
Pt
P
Pt
Cr Cr
O
P
P
Cl
Me
Cl
O
N
Me
O
N
References
51
52
1
Multiple Bonds Between Metal Atoms, 3rd ed., ed. by F. A.
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J. Park, A. N. Pasupathy, J. I. Goldsmith, C. Chang, Y.
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W. Liang, M. P. Shores, M. Bockrath, J. R. Long, H. Park,
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4. Conclusion
As briefly summarized in the introduction, over the last
decades, several remarkable achievements in this attractive
research field have been obtained and recently the research has
been oriented toward molecular-based nanotechnology includ-
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Polymerization of multiple bonded dinuclear metal complexes
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Kazushi Mashima received Ph.D. in 1986 from Osaka University under the supervision of
Professor A. Nakamura. He became an assistant Professor at the Institute for Molecular Science,
Okazaki National Institutes in 1983, working with the late Professor H. Takaya. He moved in
1989 to the Faculty of Engineering, Kyoto University as an assistant Professor, working with the
late Professor H. Takaya, and then to the Faculty of Science, Osaka University in 1991, working
with Professor A. Nakamura. He was promoted in 1994 to associate Professor at the Faculty of
Engineering Science, Osaka University, and in 2003 to full Professor at the Graduate School of
Engineering Science, Osaka University. He also worked with Professor M. A. Bennett, Australian
National University in 1992 and then with Professor W. A. Herrmann, Technisch Universität
München in 1993 (Humboldt foundation). He was an invited professor of Bergen University,
Norway in 2006 and Ecole Nationale Supérieure de Chimie de Paris in 2007. He received the
Progress Award in Synthetic Organic Chemistry, Japan in 1994, the BCSJ award (the best paper
award of Bull. Chem. Soc. Jpn.) in 2000, The Chemical Society of Japan Award for Creative
Work for 2008, and The 9th Green and Sustainable Chemistry Award Awarded by the Minister of
Education, Culture, Sports, Science and Technology, Japan. His current research interests focus on
the area of organometallic and inorganic chemistry, emphasizing directions to design and create
catalysts of asymmetric reactions and polymerization, and recently to catalysis composed of
polynuclear assembled metal clusters.