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METAL-BASED MOLECULARCHAINS: DESIGN BYCOORDINATION CHEMISTRYGuillem Aromí aa Departament de Quimica Inorgànica, Universitatde Barcelona, Barcelona, Spain
To cite this article: Guillem Aromí (2011) METAL-BASED MOLECULAR CHAINS:DESIGN BY COORDINATION CHEMISTRY, Comments on Inorganic Chemistry: AJournal of Critical Discussion of the Current Literature, 32:4, 163-194, DOI:10.1080/02603594.2011.642086
To link to this article: http://dx.doi.org/10.1080/02603594.2011.642086
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METAL-BASED MOLECULAR CHAINS: DESIGN
BY COORDINATION CHEMISTRY
GUILLEM AROMI
Departament de Quimica Inorganica, Universitat deBarcelona, Barcelona, Spain
Molecular extended metal atom chains (EMACs) or ‘‘molecular
chains’’ are discrete molecules exhibiting arrays of closely spaced
metals in a nearly straight fashion. Such species are gaining momen-
tum as possible crucial components of molecular functional devices.
The synthetic strategies for the preparation of such species have
diversified over the years. The existing strategies rely either on the
formation of ‘‘unsupported’’ metal-metal bonds or depend entirely
on the ability of specifically designed ligands to promote the assembly
of metals in desired topologies. In this review, a summary of the meth-
ods employed for the preparation of molecular EMACs is presented,
and several important examples are described.
Keywords: coordination chemistry, electric, magnetism conductivity,
metallic chains, molecular wires, spintronics
1. INTRODUCTION
For a long time, coordination chemists have used their synthetic skills for
the preparation of so-called ‘‘metallic molecular wires’’ or molecular
extended metal atom chains (EMACs). These species may be defined
as discrete assemblies of closely spaced metals in the form of linear
strings. Such systems have recently attracted considerable interest in
the context of nanotechnology.[1,2] Indeed, molecular metal chains are
Address correspondence to Guillem Aromı, Departament de Quimica Inorganica,
Universitat de Barcelona, Diagonal 647, Barcelona 08028, Spain. E-mail: guillem.aromi@
qi.ub.es
Comments on Inorganic Chemistry, 32: 163–194, 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 0260-3594 print
DOI: 10.1080/02603594.2011.642086
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promising candidates to constitute the connecting ‘‘wires’’ in future
molecular devices. In this respect, one of their advantages over organic-
based, p-extended systems is that the conducting metal core is embedded
inside an organic shell formed by ligands that could act as an insulator,
thus preventing eventual lateral current leakages.[3] Some advances have
been made in probing the potential of this category of molecules for
technological purposes; for instance, the electric conductivity of individ-
ual molecules of several aligned metals has been tested, as well as their
ability to act as on=off switches.[4] These functions depend on the nature
of the metals and their mode of interaction, the latter being a direct conse-
quence of the molecular structure imposed by the ligands responsible for
the assembly. Most of the current relevant systems involve ligands
that ensure the formation of metal-metal bonds by locating the metals
sufficiently close to each other. In such species, the electron transport
efficiency is directly related to the electron delocalization within the chain
and thus on the electronic structure of the metals and the nature of the
interactions between them.[5] Other ligands have the metals connected
by mono- or diatomic bridges or by longer spacers. This serves to modu-
late the nature of the (possible) electronic exchange between metals.
Other important properties can also be influenced by the ligands, such
as the redox properties of the metallic centers or the magnetic interaction
between them. Another category of discrete metallic chains rely on the
establishment of ‘‘unsupported’’ metal-metal bonds. We begin this survey
on molecular chains with a small summary of this last category of
molecules, and subsequently revise more extensively, in a non-exhaustive
manner, the various families of molecular EMACs resulting directly from
the ligand structure. An outlook with the future challenges faced by this
field of synthetic coordination chemistry is included at the end.
2. MOLECULAR CHAINS FEATURING ‘‘UNSUPPORTED’’
M���M BONDS
Metal-metal interactions may suffice on their own to stabilize molecular
chains of metals. An early example is the RhI=RhII cation [Rh4(dicp)8
Cl]5þ (dicp¼ 1,3�diisocyanopropane), consisting of two dimetallic
‘‘paddlewheel’’ units, each exhibiting four dicp bridges, and linked to
each other exclusively via a (formally) RhII�RhII metal bond (with a dis-
tance of 2.775 A).[6] The distance between metals within dinuclear units
(2.932 A) is consistent with that expected for a d7d8 interaction, with a
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(formal) bond order of one half. In fact, the [Rh4] cations are linked to
each other by Cl� bridges, forming infinite cationic chains. A group of
related tetranuclear complexes is that of pairs of dinuclear metal units,
exhibiting two bridging carboxylates each. The M2 units are connected
to each other solely through unsupported interactions involving the
metals. This assembly has been featured with Cu(I) in the formation of
[Cu4(OBz)4(tic)4] (tic¼ p�tolylisocyanide),[7] Ru(I) with [Ru4(tfmb)4
(CO)10] (Htfmb¼ 3,5�bis�trifluoromethylbenzoic acid),[8] Rh(I,II) in
[Rh4(O2CH)4(bpy)4](PF6)2,[9] and Pt(II) as [Pt4(O2iPr)4(NH3)8](ClO4)4.[10]
The Cu4 compound relies on weak interactions between the metal of one
Cu2 unit and the isocyanide carbon atom of the neighboring pair. The Ru4
system exhibits an interdimer Ru � � �Ru contact 2.9065 A long, in line with
unsupported bonds between Ru(I) metals. The unbridged Rh � � �Rh bond
is 2.7797 A in length, which reflects the particular stability of the Rh46þ
core. The d8-d8 unsupported interaction of the Pt4 complex leads to an
interdimer metal�metal distance of 3.2619 A. Pyridonate (Opy) has
also served as a bridging ligand within dinuclear metal units that in turn
ensemble into molecular chains through metal�metal bonds. Thus, the
dinuclear complex [Ir2(Opy)2(CO)4] was found to be, in solution, an equi-
librium between the head-to-head (HH) and the head-to-tail (HT) con-
figuration. Oxidation of this solution with I2 produces the mixed valence
IrI2IrII
2 chains HH,HH�[Ir4(Opy)4I2(CO)8] and HH,HT�[Ir4(Opy)4I2
(CO)8], the former being the thermodynamic product and the latter being
the kinetic one. Control of the reaction thermal conditions allows isolating
either one or the other, so that each of them has been crystallographically
characterized.[11] More remarkably, the use of a deficient amount of
oxidant produces a hexanuclear IrI4IrII
2 chain (average oxidation state
Figure 1. Representation of complex [Pt8(amd)8(NH3)16](NO3)10 (amd¼ acetamidate). Only
H atoms on the N-carriers of amd are represented. dPt���Pt (most ext.)¼ 2.880 and 2.900 A,
dNi���Ni (sec. ext.)¼ 2.900 and 2.778 A, dNi���Ni (cent.)¼ 2.934 A. (Color figure available online.)
METAL-BASED MOLECULAR CHAINS 165
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of þ1.33) with formula HH,HT,HH�[Ir6(Opy)6I2(CO)12].[12] Several
related tetranuclear complexes involving pyridonate and other similar
bridging donors also exist with platinum (belonging to the group of
so-called ‘‘platinum blue’’ compounds), such as the complex salt contain-
ing the cation [Pt4(Opy)4(NH3)8]5þ,[13] which features the oxidation state
sequence PtII3PtIII. An impressive octanuclear string of this family of
complexes was isolated and characterized using acetamidate (amd) as a
bridging ligand; [Pt8(amd)8(NH3)16](NO3)10 (Figure 1).[14] In this complex,
the Pt atoms are formally in the oxidation state þ2.25.
The complex [Rh4(s-pqdi)2(pqdi)4(CO)4]2þ (Figure 2; pqdi¼ 9,10-
phenanthroquinonediimine; Scheme 1),[15] and its Ir analogue (both
metals in the oxidation state þ1, d8) exhibit so-called d8-d8 metal-metal
bonds, which are supported in a synergistic manner by p-p interactions
between the aromatic ligand. The stacking is in fact believed to occur
Figure 2. Representation of complex cation [Rh4(s-pqdi)2(pqdi)4(CO)4]2þ (pqdi¼ 9,10-
phenanthroquinonediimine). H atoms not shown. dRh���Rh (ext.)¼ 2.848 A, dRh���Rh (cent.)¼2.858 A. (Color figure available online.)
Scheme 1. 9,10-phenanthrosemiquinonediimine (s-pqdi) and 9,10-phenanthroquinonedii-
mine (pqdi).
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between the p orbitals of the five-member chelate rings, which are
disposed in an eclipsed manner to facilitate this interaction.
Gold(I) to gold(II) electronic donation (d10-d9) is proposed to be the
driving force leading to the penta- and hexanuclear chains of this metal
with formulae [(RAu(CH2PPh2CH2)2Au)2AuR2]ClO4 and [(RAu(CH2
PPh2CH2)2Au)2(Au(CH2PPh2CH2)2Au)](ClO4)2 (R¼C6F5 and
C6F3H2), holding together as unsupported Au�Au bonds dinuclear and
mononuclear building blocks.[16] These compounds were obtained by
reacting Au(II) dinuclear precursors of the type [Au2(CH2PPh2CH2)2]2þ
with Au(I) mononuclear or dinuclear units, respectively.
3. METALLIC MOLECULAR CHAINS MADE WITH
DESIGNED POLYDENTATE LIGANDS
The distinctive feature of the various classes reviewed in this section will
be the type of ligands employed.
3.1. Polypyridylamine-Based Metal Chains
The group of discrete 1D metal arrays wrapped by four oligo�a�pyridy-
lamine type ligands (Scheme 2) certainly constitutes one of the most
extensive, varied, and studied classes of molecular wires. Indeed, a very
large number of molecules (around 260) of nuclearity ranging from three
to eleven have been prepared and studied for a large group of metals (Cr,
Co, Ni, Cu, Ru, Rh, Pd, Pt), of which trinuclear complexes represent
71%. The latter have been of paramount importance for understanding
the electronic structure within this family of metal chains and the metal-
metal bonds that exist within most of them.[17] The heptanuclear complex
[Ni7(teptra)4Cl2] (Figure 3, H3teptra¼ tetrapyridyltriamine) will serve to
describe the main structural features in this family of compounds.[18] This
complex features a central core of seven Ni(II) ions almost perfectly
aligned and wrapped helically by four teptra3� ligands in a syn-syn-syn-
syn-syn-syn configuration. Such arrangement ensures the alternative
Scheme 2. Oligo–pyridylamine type ligands.
METAL-BASED MOLECULAR CHAINS 167
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presence of either four pyridyl or four amido N-donors around the met-
allic string, resulting on square planar environments, except for the two
most external ions, which exhibit apical Cl� ligands and thus square pyr-
amidal geometries. Such an arrangement locates adjacent Ni(II) ions at
an average distance of 2.302 A, including the third shortest distance ever
observed on any Ni complex, 2.214 A (only seen shorter on a Ni7 and a
Ni6 complex of the same family).[19,20] The electronic configuration of
the 3d orbitals in the [Nix] wires leads in general to only weak metal-metal
interactions, which is detrimental to the conductive properties of the mol-
ecular wire. Indeed, conductance studies using STM (scanning tunneling
microscopy) show that analogous [M(II)x] molecular chains exhibit grow-
ing conductivity along the sequences Ni(II), Co(II), and Cr(II), resulting
from increasing metal-metal bond orders of 0, 0.5, and 1, respectively.[4]
Modifications of the backbone in the oligo�a�pyridylamine ligand have
allowed the preparation of long-chain derivatives more resistant to oxi-
dation and versatile. For example, replacement of one pyridine group
by pyrazine within the ligand (H4mpz; Scheme 3, top) has led to the non-
achromium complexes, [Cr9(mpz)4(X)2] (X�¼Cl�, NCS�).[21] These two
Scheme 3. H4mpz and H2napany.
Figure 3. Two perpendicular views of complex [Ni7(teptra)4Cl2] (H3teptra¼ tetrapyridyl-
triamine). H atoms not shown. dNi���Ni (most ext.)¼ 2.383 and 2.374 A, dPt���Pt (sec. ext.)¼ 2.304
2.304 and 2.310 A, dPt���Pt (cent.)
¼ 2.214 and 2.226 A. (Color figure available online.)
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chain compounds are 23 and 25 A long, respectively, and exhibit two
types of Cr�Cr distances; one corresponds to quadruple metal-metal
bonds (ranging 1.973 to 2.097 A, with four such contacts per molecule)
and another within the 2.397 to 2.497 A range, which might involve a weak
Cr�Cr bond of r-character. Another variation to the a�pyridylamine
sequence is, for example, the introduction of naphthyridine units within
the ligand backbone (see H2napany, Scheme 3, bottom). This has allowed
us to stabilize (Ni2)3þ fragments within the molecular chain, such as in
[Ni9(napany)4Cl](BF4)2.[5] Such metal pairs contain Ni(I) and Ni(II)
and feature a high level of delocalization, such that in the ground state
these fragments are carriers of an S¼ 3=2 spin magnetic moment. These
properties are likely to confer enhanced electron conductivity to the
chain, and have been observed for chains with other nuclearities, such
as [Ni5][22] and [Ni9].[23] The longest chains of this impressive family exhi-
bit eleven Ni ions and have the formula [Ni11(tentra)4X2](PF6)4 (X¼Cl�
or SCN�; H3tentra¼ tetranaphthyridyltriamine).[24]
3.2. Metal Chains Sandwiched within Conjugated
pp-extended Ligands
A fascinating family of molecular metallic chains is made by aggregates of
metal atoms sandwiched by the pp backbone of extended aromatic ligands.
In this class of linear organometallic complexes, the shape of the chain
may vary, depending on the structure of the organic ligand. Thus the first
complex of this kind was prepared from the simple sp2 carbon atom chain
all�trans�1,8�diphenyl�1,3,5,7�octatetraene (dpot, Scheme 4), leading
to the highly straight tetranuclear complex [Pd4(dpot)2]X2 (X¼various anions), which could also be made with two pyridine ligands,
Scheme 4. Various polyaryl-polyene ligands.
METAL-BASED MOLECULAR CHAINS 169
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bound at end metals of the chain; [Pd4(dpot)2(py)2]X2 (Figure 4, top).[25]
Binding of pyridine causes a slippage of the dpot ligand along the direction
of the Pd4 chain, going from the g3:g2:g2:g3 coordination mode to
g2:g2:g2:g2. The electronic structure of a [Pdn]2þ metal chain is described
as resulting from 1 Pd(II)þ (n� 1) Pd(0), thus leaving 2(n� 1) electrons
for the establishment of n� 1 metal-metal bonds (six electrons for the case
of Pd4). In fact, a theoretical study demonstrates that the resulting 2þcharge is delocalized over the metallic chains as well as both polyenes of
the cationic assembly, providing the system with high stability.[26] The
linear pentanuclear chain [Pd5(dpdh)2]2þ was obtained from the longer
ligand all�trans�1,12�diphenyl�1,3,5,7,9,11�dodecahexaene (dpdh).
As a remarkable extension of this work, it was demonstrated that the poly-
cyclic aromatic compound perylene (pery) could also serve to sandwich
linear Pd4 chains in form of complex cations such as [Pd4(pery)2(py)2]2þ,
acting in the l�g2:g2:g2:g2 coordination mode.[27] The electronic structure
Figure 4. Representation of the complex cations (top) [Pd4(dpot)2(py)2]2þ (dpot¼ all�trans�1,8�diphenyl�1,3,5,7�octatetraene, dPd���Pd (ext.)¼ 2.746 A, dPd���Pd (cent.)¼2.721 A), and (bottom) [Pd5(p-bpbb)2(py)2]2þ (p-bpbb¼ 1,4�bis(4�phenyl�1,3�butadie-
nyl)benzene, dPd���Pd (ext.)¼ 2.733 and 2.722 A, dPd���Pd (cent.)¼ 2.721 and 2.713 A). (Color
figure available online.)
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of the metal skeleton in this compound is the same as in the analogous
tetrametallic chain formed with the linear polyene dpot and, in fact, the
latter was shown to displace completely the perylene as ligand to form
quantitatively the corresponding tetranuclear chain of Figure 4. The bis-
perylene sandwich complex was then used as starting material in reactions
with other ligands to obtain new EMACs with unprecedented structures.
This was the case for ligands featuring a central ortho� or para�phenylene
inserted within a polyene chain, such as 1,2�bis(4�phenyl�1,3�butadie-
nyl)benzene (o�bpbb) or 1,4�bis(4�phenyl�1,3�butadienyl)benzene
(p�bpbb) (Figure 4, bottom), which exhibit a bent configuration
(Scheme 4). These reactions conduce to the formation of [Pd4] and
[Pd5] sandwiched metal chains with an arched and an angular topology,
respectively, demonstrating that the shape and topology of the polymetallic
chains may be controlled by the structure of the carbon backbone of the psystem, which plays the role of a template. A surprising recent addition to
this successful research program is the demonstration that the mixed-
valence chain [Pd4(dpot)2]2þ may be reversibly reduced by two electrons
with two equivalents of Cp2Co to the corresponding neutral [Pd�4] ana-
log.[28] The structure of this species could be obtained using the ligandtBu-dpot (a derivative of dpot with tert-butyl substituents on the aryl
groups), and revealed two different hapticities for both ligands in the
molecule; l�g3:g2:g2:g3 and l�g2:g2:g2:g2, respectively.
3.3. Trinuclear Linear Complexes with Pyrazole, Triazole,
or Tetrazole Ligands
All five-membered heterocycles derived from pyrazole, triazole, or tetra-
zole moieties (Scheme 5) share the presence of two bonded nitrogen
atoms [�N�N�], which constitutes a l:g2 bridging moiety very common
in coordination chemistry. These rings have originated a large family of
(more than 80 structurally characterized) trinuclear linear metal clusters
exhibiting either triple (class I; �66%) or double (class II; �14%) [M�N�N�M] bridges between metals, or two such bridges combined with
a third monoatomic bridge (class III; �20%). These compounds are
Scheme 5. Five membered heterocyclic azole-type ligands.
METAL-BASED MOLECULAR CHAINS 171
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structurally related to their (extremely abundant) dinuclear counterparts.
Of these, the majority are made with 1,2,3� or 1,2,4�triazoles (around 60
examples), and only one case with a tetrazole[29] was found on the CSD
database (version 5.32, May 2011 update). The rest are made with
pyrazole derivatives. Complexes representative for classes I, II, and III
are, respectively, [Ni3(Htz)6(H2O)3](NO3)6 (Htz¼ 1,2,4�triazole),[30]
[Co3(AcO)2(dmpz)4(Hdmpz)2] (Hdmpz¼ 3,5�dimethylpyrazole)[31] and
[Co3F2(SCN)4(detz)6] (Hdetz¼ 3,5�diethyl�1,2,4�triazole),[32] shown
in Figure 5.
Class I complexes may be described as two fused [M2(azole)3] units
sharing the central metal of the complex. Each metal pair is bridged by
Figure 5. Representation of complexes (top) [Ni3(Htz)6(H2O)3](NO3)6 (Htz¼ 1,2,4�triazole, dNi���Ni¼ 3.737 A), (middle) [Co3(AcO)2(dmpz)4(Hdmpz)2] (Hdmpz¼ 3,5�dimethyl-
pyrazole, dCo���Co¼ 3.373 A) and (bottom) [Co3F2(SCN)4(detz)6] (Hdetz¼ 3,5�diethyl�1,2,4�triazole, dCo���Co¼ 3.609 and 3.601 A). H atoms not shown. (Color figure available
online.)
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three heterocycles, their respective idealized planes being disposed 120�
from each other, in the form of a ‘‘paddlewheel.’’ The three ligands of
each [M2] unit are oriented in a mutually staggered manner. This causes
the central metal to adopt an octahedral coordination geometry. In
general, the external metal ions of the chain exhibit also octahedral
geometry. The three rings of the ‘‘paddlewheel’’ leave on each of the
outer metals three free positions for coordination that are usually
completed by solvent monodentate ligands (Figure 5; top),[30] anionic
groups,[33] the corresponding azole group acting as neutral terminal
ligand or additional groups attached to the azole five-membered ring.[34]
It is interesting to note that only the dinuclear and trinuclear analogues
of fused ‘‘paddlewheel’’ oligomers exist, and not longer discrete members
of the series. By contrast, several 1D [Cu(azole)3]n polymers have been
structurally characterized[35] and the corresponding [Fe]n derivative,
whose structure has been inferred by other methods, exhibits very inter-
esting spin crossover behavior.[36] Almost all complexes from this class
are homometallic (featuring mainly Ni, Fe, Co, Cu, Cd, and to a lesser
extent Mn and Zn). Most of them have been studied for their magnetic
properties.[37–39] The intramolecular exchange between paramagnetic
metal centers as mediated through the three [�N�N�] pathways was
always found to be antiferromagnetic. Many of the (numerous) Fe
analogs display interesting spin-crossover behavior.[40] A recent review
summarizes the properties of most trinuclear complexes of class I.[41]
A rare exception to the homometallic nature of these complexes is the
[ReIMnIIReI] compound (NHEt3)2[MnRe2(pz)6(CO)6].[42] A beautiful
molecule related to the latter is the tetranuclear EMAC with formula
(NHEt3)2[Mn2Re2O3(pz)6(CO)6] (Figure 6, Hpz¼ pyrazole), which
Figure 6. Representation of the complex anion [Mn2Re2O3(pz)6(CO)6]2� (Hpz¼pyrazole).
H atoms not shown. dRe���Mn¼ 3.897 A, dMn���Mn¼ 2.394 A. (Color figure available online.)
METAL-BASED MOLECULAR CHAINS 173
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exhibits a ReIMnIVMnIVReI sequence, with the central Mn centers
bridged by two l�O2� ligands and the Re � � �Mn pairs linked by triple
l�pz� bridges. This complex is obtained from the heterometallic dinuc-
lear [MnIReI] analogue (NHEt3)[MnRe(CO)6], by reaction with O2.
Class II trinuclear complexes are made with pyrazole derivatives and
exhibit double l1,2�N,N�azole bridges between metals (Figure 5, mid-
dle). This group of complexes is less numerous than Class I, but it is much
more versatile. Metals involved span Ni, Co, Cu, Zn, Rh, Pd, and Ag and
some cases are heterometallic (such as complexes [PdIIPdIICoII][31] and
[RhIPdIIRhI]).[43] The central metal may be either in tetrahedral (CoII,
ZnII, AgII) or square planar (NiII, PdII) coordination. In the first case,
the cyclic ligands within each metal pair are approximately coplanar,
and approximately perpendicular to the rings from the adjacent M2 unit
(Figure 5, middle). When square planar, both pyrazole groups within each
M2 pair form a ‘‘butterfly’’ shape, with an angle (approximately between
60 and 70�). The ‘‘butterflies’’ from both M2 pairs point to opposite direc-
tions. The coordination geometry of the external metals in this class of
complexes comprises tetrahedral, square planar, and triangular. One
related complex made with pyrazole-based compartmental ligands
together with another pyrazole derivative is a [Cu4], which comprises
three fused [Cu2(azole)2] moieties yielding a tetranuclear chain.[44] It is
clear from the coordination geometries observed and the metals involved
in the compounds just reviewed that when the cations exhibit preferen-
tially coordination number six, Class I complexes may be expected. If
the metals favor square planar or tetrahedral geometry, the complexes
obtained will be of Class II. Only metals with very versatile coordination
geometries (such as ZnII or CoII) may exhibit either of both Class I and II
configurations, as well as metals with two strong preferences (e.g., NiII,
which exhibits stable complexes in an octahedral environment, but also
as square planar centers).
In Class III complexes (Figure 5, bottom), the bridging moiety
between metals is heteroleptic; it contains two l1,2�N,N�azole fragments
together with a monoatomic bridge (F�, Cl�, OH�, or NCS�). The struc-
ture and properties of this type of complexes have been recently summar-
ized.[41] The majority are made with CuII, while examples with Ni, Co,
Mn, and Cd also exist. The presence of a different ligand, other than
the azole groups, adds versatility to the magnetic exchange between
metals within the chain. For example, it was demonstrated that the mag-
netic coupling may be tuned by selectively changing the variable ligand
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X�. Thus, in going from NCS� to F�, the coupling becomes more
antiferromagnetic (as seen for the Ni ant the Co cases).[32] Also, the effect
of the second (variable) ligand has allowed us to see ferromagnetic-type
couplings and therefore high-spin molecules.[45]
3.4. Use of Diazines to Link Chains of Metals
Cyclic diazines, such as pyridazine (pdz), could act in many ways as the
azole ligands discussed in the previous section. As it turns out, there is
not an analogous extensive family of complexes such as that discussed
above; however, there exists a small group of trinuclear complexes with
pyridazine that exhibit the same type of structure as complexes of Class
III of section 2.3, where essentially the five-membered ring is replaced
by pdz. The metals involved are NiII,[46] CuII,[47] and CoII,[48] and the
coordination geometry in all the metals is octahedral. Susceptibility mea-
surements on the complex [Ni3(pdz)6(NCS)6] unveil a quasi Curie law
behavior. This is interpreted as the result of the almost exact compen-
sation of the antiferromagnetic coupling mediated by the diazine bridges
by the ferromagnetic interaction induced by end-on l�NCS� groups.[46]
On the other hand, the comparison between complexes [Co3(pdz)6
(OH)2(NO3)2(H2O)2] and [Co3(pdz)6(NCS)6] is very interesting. The
difference of variable bridging ligand (OH� vs NCS�) causes the central
Co atom to be in a different spin state. Thus, for the OH� complex, the
central metal is in the low-spin state (S¼ 1=2) with the external Co
ions being high-spin (S¼ 3=2), whereas the complex with bridging NCS�
Scheme 6. Trinuclear metal complexes formed with diazine-type ligands.
METAL-BASED MOLECULAR CHAINS 175
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exhibits high-spin states at the three metal centers. This is corroborated
crystallographically and by magnetic susceptibility measurements.
A beautiful series of triple stranded helicates hosting [M4] metal
chains is obtained from ligands combining a pyridazine ring with two
non-cyclic (flexible) azine groups, in addition to pyridyl groups at both
ends (L; Scheme 6). Thus complexes with CuII,[49] MnII,[50] and NiII[51]
have been prepared, exhibiting the formula [M4L3](ClO4)n (n¼ 2 or 8,
depending on whether the ligand is doubly deprotonated or not,
Figure 7). In general, the magnetic coupling between metals, as mediated
through triple l�NN diazine groups, is weakly antiferromagnetic and
modulated by the M�N�N�M torsion angles. In the MnII complex,
the open-chain diazine moieties seem to facilitate weakly ferromagnetic
interactions. The family of coordination chemistry metallohelicates is
relatively large;[52] however, unlike the above examples, the vast majority
of systems locate the metals too far away from each other for these centers
to exhibit any significant magnetic or electronic interaction.
Another important group of trinuclear linear complexes linked by
diazine groups is obtained with R�N�N�R0 ligands where R and R0 con-
tain donors capable of forming chelates together with the N-atom to
which they are attached. Thus, with divalent ions (mainly CuII[53] and
NiII[54] with some examples of CoII,[54] ZnII,[55] and PdII[56]), two such
ligands chelate a central metal with one N-atom of the azine and then each
ligand binds another metal through the other N-atom, forming trinuclear
structures, usually with an overall flat structure (Scheme 6).[57] If the
metals involved are trivalent (MIII), instead of linear structures, these
ligands usually form metalladiazamacrocycles of various nuclearities.[57]
Figure 7. Representation of the complex anion [Mn4L3]8þ. H atoms not shown. dMn���Mn
(ext)¼ 3.802 A, dMn���Mn (cent)¼ 3.850 A. (Color figure available online.)
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3.5. Ferromagnetic Metal Chains with Oligo�m�phenyleneoxalamide Ligands
A series of ðCuIInÞ (n¼ 2–4) linear metal arrays has been synthesized with
a class of polytopic ligands containing two outer oxamato groups and x
(x¼ 0, 1, 2) inner oxamidato moieties, the various donor units being sepa-
rated by m�phenylene spacers (Scheme 8).[58] These wires exhibit the
well-established ferromagnetic interaction observed between CuII centers
separated by m�phenylene groups, as mediated by a spin polarization
mechanism, with coupling constants spanning þ15 to þ17 cm�1 (in the
H¼�JSiSj convention).[59] These linear species are described as double
stranded anionic complexes of 2, 3, or 4 metals (Figure 8), respectively
(and charges�4, �6 and �8), each metal being located within two chelat-
ing pockets of two ligands disposed in front of each other. The helical
chirality around the metals alternates between M and P thus providing
for an overall meso-helicate-type architecture. The configuration of the
ligands is all-syn, of the oxamidato or oxamato groups with respect to
the phenylene spacer, both types of planes forming angles with each other
that span 62 to 82 degrees. As a result of the ferromagnetic coupling
between adjacent metals, these [Cun] arrays exhibit maximum possible
molecular spin (S¼ n=2). In addition, the CuII centers of the chains
can be sequentially and reversibly oxidized to diamagnetic CuIII, thus
providing for a mechanism to externally interrupting the ferromagnetic
interaction along the chain and thus switching ON and OFF the electron-
exchange throughout the wire.
Scheme 8. Bis-oxamato-polyoxamidato ligands.
Scheme 7. A bis-diazine-pyridazine ligand.
METAL-BASED MOLECULAR CHAINS 177
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3.6. Phosphine-Derived Ligands
The tri-phosphine ligand (diphenylphosphinomethyl)phenylphosphine
(dpmp, Scheme 9) exhibits quite versatile coordination chemistry. The
reaction of dinuclear [Pt(I)2] precursors [Pt2(dpmp)2(RNC)2]2þ (RNC¼isocyanide ligands) with mononuclear compounds of d10 M(0) centers
(M¼Pd, Pt) produces the corresponding PtPtM linear homo- and het-
erometallic chains with formulae [Pt2M(dpmp)2(RNC)2]2þ.[60] In these
compounds, the electronic structure was described as d9�d10�d9, follow-
ing an electron transfer from the external atom joining the cluster to the
central atom of the final assembly, leading to two single metal-metal r�bonds within the molecule. The reactivity of the [Pt(I)2] precursor was
also investigated with [MCl(cod)]2 species (M¼Rh(I), Ir(I); d8 metals),
which produced the corresponding linear heterometallic assemblies with
formula [Pt2MCl(dpmp)2(RNC)2].[61] The bonding within the backbone
of these chains occurs through one covalent (d9�d9) Pt�Pt bond and
one dative (d8!d9) M�Pt interaction. Reactivity studies of the Pt2Rh
Figure 8. Representation of the complex anion [Cu4L2]8þ (H8L¼ 2�methyl�1,3�phenylenebis�[N0�(2�methyl�3�phenylamine)oxamide]). H atoms not shown. dCu���Cu
(ext)¼ 7.294 A, dCu���Cu (cent)¼ 7.395 A. (Color figure available online.)
Scheme 9. Various posphine-type ligands.
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system suggest that in this assembly, the Pt2 unit exhibits nucleophilic
character and the Rh center is electrophilic. In the presence of NaBH4
or NaOMe, the Pt2M species (M¼Pt, Pd) react to form hydride bridged
hexanuclear species [Pt4M2(l�H)(dpmp)4(RNC)2]3þ (Figure 9).[62] The
bridge between both trinuclear entities in these compounds features a
MHM three-center, two-electron unit. These species may be chemically
or electrochemically oxidized, leading to the linear chains with a Pt�Pt�M�M�Pt�Pt sequence, exhibiting Pt�Pt and Pt�M bonds, as a
result of a migration of bonding electrons from the Pt�M bonds to the
central M�M unit.
The use of a ligand analogous to dpmp, with a central As donor instead
of P (bis-(diphenylphosphinomethyl)phenylarsine (bdppa, Scheme 9),
allows the preparation of heterotrinuclear metal chains of the type MM0M,
by selectively locating at the center of the assembly a metal distinctly differ-
ent from the end metals. In this manner, a large variety of combinations
have been obtained and studied, with a great diversity of electronic config-
urations such as Ir2M (d8s2d8; M¼Pb(II), Sn(II), Ge(II), In(I), Tl(I),
Sb(III), Bi(III)),[63] Ir2M0 (d8d10d8; M0 ¼Cu(I), Ag(I), Au(I)),[64] Ir2M00
(d8d8d8; M00 ¼Pd(II), Rh(I), Ir(I)),[65] etc., the majority of which also
obtained with Rh(I) as external metals.
The phosphine ligand Hpyphos (6�diphenylphosphino�2�pyri-
done; Scheme 9) also contains donor atoms of different types (O,N,P),
aligned for the assembly of linear metal chains. These properties have
Figure 9. Representation of the complex cation [Pt4Pd2(m�H)(dpmp)4(RNC)2]3þ (dpmp¼(diphenylphosphinomethyl)phenylphosphine; RNC¼ isocyanide ligands. H atoms and H�
not shown. dPt���Pt¼ 2.723 and 2.714 A, dPt���Pd¼ 2.749 and 2.750 A, dPd���Pd¼ 3.355 A.
(Color figure available online.)
METAL-BASED MOLECULAR CHAINS 179
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been employed for the formation of heterometallic tetranuclear arrays,
prepared in a sequential manner. This was done by obtaining first quad-
ruple bonded Mo2 complexes surrounded by four pyphos� ligands in a
‘‘paddlewheel’’ arrangement; [Mo2(pyphos)4]. The steric demands of
the ligand cause the P donors of different ligands to be oriented, pairwise,
to opposite sides of the Mo�Mo axis. This provides for two suitably dis-
posed axial sites on each molecular end, well tailored for coordination to
two additional, late-transition metal centers in the formation of [Mo2M2]
linear clusters (M¼PdII, PtII, IrI, RhI), of the type [Mo2M2(pyphos)4X4]
(X¼Cl, Br, I) for Pt or Pd,[66] and [Mo2M2(pyphos)4(RNC)4]Cl2 for Ir
and Rh.[67] Following the electronic configuration of the metallic back-
bone, none of these compounds exhibit M�Mo interactions. Interest-
ingly, reduction of the MII (Pd, Pt) centers leads to the formation of
Mo�M bonds with concomitant decrease of the Mo�Mo interaction
bond order (from 4 to 3) to produce chains exhibiting a fully bonded
[M�Mo�Mo�M] axis within the new complexes [Mo2M2(pyphos)4X4]
(M¼Pd, Pt; X¼Cl, Br, I). Likewise, oxidation of the MI metals (IR,
Rh) causes a very similar effect, by facilitating the interaction of the relectrons of the Mo2 core with the dz orbitals of the resulting MII (d7)
centers, producing the complexes, with a fully metal-metal bonded tetra-
metallic chain, [Mo2M2(pyphos)4X2(RNC)4]2þ (M¼ Ir, Rh; X¼Cl, Br, I;
Figure 10).
Figure 10. Representation of the complex cation [Mo2Rh2(pyphos)4Cl2(RNC)4]2þ
(Hpyphos¼ 6�diphenylphosphino�2�pyridone). H atoms not shown. dRh���Mo¼ 2.731 A,
dMo���Mo¼ 2.124 A. (Color figure available online.)
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3.7. Homo and Heterometallic Chains with m�phenylene
Spaced Bis�b�diketones
A family of molecular linear chains of metallic cations has been synthe-
sized with ligands consisting of two b�diketone groups separated by a
1,3�phenylene�type moiety (derived from benzene, pyridine or phenol,
Scheme 10) in combination with other donor groups. 1,3�Diketonates
are very good chelating functions. The family of bis�b�diketones is
prone to form dinuclear complexes. If the metals are 3d trivalent (FeIII,
MnIII, etc.) these complexes are described as triple-stranded helicates
(or mesocates),[68,69] whereas with MII cations two bis�b�diketonate
ligands sandwich two metals and their (axial) coordination sites are com-
pleted by solvent molecules.[70,71] In this family of ligands, the diketone
groups are accompanied by a combination of phenol and=or pyridine-
type groups, disposed in a linear fashion, thus favoring the assembly of
coordination metal chains (Scheme 10). The preparation of these ligands
involves well-established reactions in organic chemistry synthesis and
always goes through one key step, the Claisen condensation between
one ester and one ketone, which yields a 1,3�diketone.[72,73] In this case,
Scheme 10. Bis-beta-diketone ligands.
METAL-BASED MOLECULAR CHAINS 181
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two such condensations occur simultaneously in one molecule, producing
the corresponding bis�1,3�diketone.
Ligand H3L1 (Scheme 10) displays two b�diketones separated by a
phenol group, thus disposing five regularly spaced oxygen atoms in a
row, in a good arrangement to favor the assembly of coordination metal
chains. It was found that this ligand reacts with M(AcO)2 salts (MII¼CoII,
MnII), leading to trinuclear asymmetric linear chains, [M3(HL1)3],
described as triple stranded pseudo helicates (Figure 11).[74,75] The
chirality of the metals along the chain alternates between K and D(Scheme 11); thus both sequences KDK and DKD are found within the
crystal by virtue of the inversion center present within these systems’ sym-
metry space group. Therefore, the helices invert their sense of rotation
along the molecular axis. In these complexes, three metals are located
within three coordination pockets from each of the twice deprotonated
ligands, displaying a quasi trigonal prismatic coordination geometry. The
fourth potential coordination pocket of each ligand is, in fact, filled by
the proton remaining on each of them, which leads to the formation of
intramolecular hydrogen bonds. The bulk magnetization properties of
these complexes have both been modeled as a pair of coupled metals next
to a third isolated ion. For the Mn(II) system, the coupled pair exhibits a
coupling constant of �2.75 cm�1 (in the convention H¼�2JS1S2). The
complex of cobalt was modeled considering three isolated Co(II) ions sub-
ject to a considerable spin-orbit coupling effect, experiencing the effect of
magnetic coupling at low temperatures, which was treated as a pertur-
bation in form of interaction between effective S ¼ 12
centers, leading to
a coupling constant J ¼�4.9 cm�1 in the best fit.[75] One interesting
Figure 11. Representation of the complex [Co4(L3)2(py)6]. H atoms not shown. dCo���Co
(ext)¼ 3.160 A, dCo���Co (cent)¼ 6. 962 A. (Color figure available online.)
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feature of the [M3(HL1)3] trinuclear chains is that they dissolve and react
in coordinating solvents, S, transforming into the corresponding solvate
dinuclear complex [M2(HL1)2(S)2] (Figure 11). This transformation is
reversible, thus isolated crystals of the dinuclear species dissolve into
non-coordinating solvents to generate the corresponding [M3(HL1)3]
compound.[75,76]
If in place of a central phenol spacer as in H3L1, there is a pyridyl
group, as in the H2RL4 type ligands (Scheme 11), the donor exhibits a
central larger coordination pocket instead of two smaller ones. These
unique structural properties have allowed the preparation of a remarkable
series of more than 30 complexes with a [M2Ln(MeL4)2]3þ backbone
(MII¼CuII, NiII; LnIII¼ all lanthanoids except Pm; Scheme 12 and
Figure 12).[77,78] In these 3d�4f heterometallic complexes, the axial sites
of square pyramidal Cu(II) or octahedral Ni(II) are occupied by solvent
molecules (DMF, MeOH, or H2O). The central Ln(III) ion, besides hav-
ing six coordination sites occupied by donors from two MeL42� ligands
(forming a hexagonal equatorial plane), is bound to three No �3 ligands,
completing coordination numbers of 10 or 12 (the latter only seen for
the [Cu2La] complex). This incredible synthetic and crystallographic
work allowed a systematic and quasi comprehensive study of the nature
of the magnetic interaction between lanthanides and Cu(II) or Ni(II).
Scheme 12. Schematic structure of [MLaM] complexes with ligand H2MeL4. (Color figure
available online.)
Scheme 11. Stereochemistry within [M3(HL1)3] complexes.
METAL-BASED MOLECULAR CHAINS 183
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This was done by subtracting to the variable temperature molar suscepti-
bility of the [MLnM] complexes, the contribution from a [MLaM] (LaIII
being diamagnetic) and that from the corresponding [ZnLnZn] complex.
The resulting DvMT value (vM being the molar paramagnetic suscepti-
bility) is the contribution from the Ln � � �M coupling. If vMT< 0, the
coupling is antiferromagnetic; otherwise, it is ferromagnetic. These
studies have confirmed that for such systems the coupling with Cu(II)
is antiferromagnetic for Ce, Pr, Nd, and Sm and ferromagnetic for Gd,
Tb, Dy, Ho, and Er. For the case of Ni(II), the coupling was found to
be antiferromagnetic for Ce, Pr, and Nd, whereas it is ferromagnetic for
Gd, Tb, Dy, Ho, and Er. Interestingly, this extensive work allows us to
infer that larger Ln(III) ions tend to favor antiferromagnetic interactions
with 3d metals, while smaller cations seem to facilitate ferromagnetic
exchange. The equivalent Gd(III) and La(III) systems have also been
prepared with Co(II),[79] whereas with Fe(III), instead of two ligands
sandwiching the metal chain, a total of three ligands wrap the molecular
axis in a pseudo helical manner, while non-coordinated [FeCl4]� counter
ions compensate the three positive charges of the complex.[80]
Ligand H5L2 is related to H3L1, but includes two additional phenol
groups located at both ends of the ligand backbone, thus featuring an
array of six adjacent coordination pockets (or seven aligned oxygen
donors). This multidentate ligand was synthesized with the aim of
promoting the assembly of long metal chains. Initial reactions with
Mn(AcO)2 in DMF or pyridine facilitate the formation of the chain tetra-
nuclear aggregates [Mn4(H2L2)2(AcO)2(dmf)4] and [Mn4(H2L2)2
(AcO)2(py)5], respectively (Figure 13). These two related complexes
exhibit four Mn(II) ions sandwiched and chelated by two l4�(H2L2)3�
Figure 12. Representation of the complex cation [Ni2Gd(NO3)2(MeL4)2]þ. H atoms not
shown. dNi���Pr¼ 3.688 A. (Color figure available online.)
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donors and exhibit two additional syn,syn�AcO� bridges, with coordi-
nation sites completed by terminal solvent ligands. The topology of the
metals in these compounds, however, is removed from straight linear;
instead, the metals describe a zigzag (Figure 13). It is very likely that
the reason for this is related to structural constraints imposed by the
multidentate ligand. Indeed, the backbone’s rigidity of H5L2 may prevent
restraining the metals within a straight line and instead the ligand rotates
around some of its �C�C�bonds, thus allowing the metals to externally
coordinate other ligands. This provides more flexibility and enables the
formation of M�O bonds with the bis�b�diketonate of the appropriate
length. If the reaction is performed in conditions favoring the oxidation to
Mn(III), another linear chain is formed which exhibits the unprecedented
sequence [MnIIIMnIIIMnII]; [Mn3(HL2)2(py)6] (Figure 14). In this
unique complex, the metals are within a quasi straight line (Mn�Mn�Mn angle of 172.92�), perhaps because they are more spaced and also
because MnIII favors shorter Mn�O equatorial bonds. Magnetic mea-
surements show that the coupling between the metals within the molecule
is antiferromagnetic, leading to a ground state of S¼ 5=2. Fitting of
Figure 13. Two views of the complex [Mn4(H2L2)2(AcO)2(dmf)4]. H atoms not shown.
dMn���Mn (ext)¼ 3.318 A, dMn���Mn (cent)¼ 3.388 A. (Color figure available online.)
METAL-BASED MOLECULAR CHAINS 185
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the susceptibility data reveals that the coupling between Mn(III) ions is
stronger by one order of magnitude than the MnIII � � �MnII exchange.
Therefore, the unusual asymmetry of this linear disposition of metals does
not only relate to the sequence in oxidation states, but also in the magni-
tude of the magnetic exchange.
Another interesting ligand is H4L3, which is similar to H5L2 without
the central OH group. The structure of H4L3 (Scheme 10) suggests that
this donor may favor the assembly of linear complexes in the form of
two well-defined, separate groups. Indeed, in the presence of a strong
base (such as Bu4NOH), pyridine solutions of H4L3 and MX2 salts
(MII¼NiII, CoII, CuII; X¼ counter anions) produce tetranuclear chain
assemblies with formula [M4(L3)2(py)x] (x¼ 4 or 6).[81,82] These mole-
cules exhibit a linear disposition of four metals of the type MM � � �MM
(Figure 11), i.e., divided in two dimers. The structure of the assembly
causes both distinct metal sites to embody different stereochemical
properties. This has been exploited for the formation of heterometallic
metal chains with the same structure. Thus, the complex [Cu2Ni2(L3)2
(py)6] has been isolated and well characterized,[82] whereas the structure
of [Cu2Co2(L3)2(py)6] and [Co2Ni2(L3)2(py)6] has also been deter-
mined.[83] Within each dimer of the chain, the metals are coupled antifer-
romagnetically. If the cluster is homometallic, this leads to a diamagnetic
ground state; however, if both metals of the dimer are different, the mag-
netic exchange causes a quasi independent non-zero ground state on each
side of the cluster. This has allowed us to propose this kind of system as a
possible carrier of two spin-based qubits and thus constituting a proto-
type of 2qubit quantum gate for quantum computing.[84] Represented in
Figure 16 are DvMT vs T plots for a series of tetranuclear clusters of this
kind, reflecting these properties at the thermodynamic level.
Figure 14. Representation of the complex [Mn3(HL2)2(py)6]. H atoms not shown.
dMn(III)���Mn(III)¼ 5.246 A, dMn(II)���Mn(III)¼ 5.155 A. (Color figure available online.)
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Ligand H2L5 displays one 1,3�pyridyl group separating two b�dike-
tones and two more lateral pyridyl fragments. The structure of this donor
suggests that it can accommodate rows of various metals, exhibiting dif-
ferent coordination geometry. This has been observed from reactions of
H2L5 with Co(NO3)2 in MeOH, which led to the tetranuclear chain
[Co4(L5)2(MeOH)8](NO3)4 (Figure 17).[85] In this complex, the metals
are accommodated within two unconventional coordination environ-
ments: pentagonal bipyramidal and very distorted octahedral
(Figure 17). This system was found to exhibit significant intramolecular
magnetic exchange. The [Co4(L5)2] backbone structure of this EMAC
constitutes a very flat platform with axial MeOH ligands perpendicular
to it, which proved to be very labile. This affords the opportunity to
replace these axial ligands by bridging species with the aim of linking
[Co4] units to each other.[86]
Figure 15. Representation of the complexes [Mn3(HL1)3] (dMn���Mn¼ 3.032 and 5.044 A)
and [Mn2(HL1)2(py)2] (dMn���Mn¼ 9.302 A), showing their solvent-driven interconversion.
H atoms not shown. (Color figure available online.)
Figure 16. 4vMT vs T plots for complexes [M2M02(L3)2(py)x] (x¼ 4 or 6; M, M0 ¼Ni, Co or
Cu). vM is the molar paramagnetic susceptibility.
METAL-BASED MOLECULAR CHAINS 187
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The family of 1,3�diketones may be readily converted into a new
family of interesting ligands by converting the dicarbonyl groups into pyr-
azoles through cyclization with hydrazine (N2H4)[87] (Scheme 13). The
resulting species also exhibits a series of donor atoms in a row, potentially
capable of assembling metals chains. However, because of the bent struc-
ture of such ligands, these are not suited to obtain straight EMACS. In
fact, multinuclear complex structures instead of linear molecules are
often obtained.[88,89] Nevertheless, some unique metallic chains with a
bent structure have been prepared with these polypyrazolyl derivatives.
Examples are the following: i) The trinuclear complex [Mn3(HL10)2
(OAc)3(MeOH)3], made with H3L10 (the bis-pyrazol derived from
H3L1)[90]; ii) two mixed CoII=CoIII tetranuclear chains with formula
[Co4(OR)(OAc)(L30)2(py)4] (R¼H, CH3; H4L30 ¼ bis-pyrazolyl from
H4L3; Figure 18)[91]; iii) a unique tetranuclear manganese complex with
the unprecedented sequence of oxidation states (MnIIIMnIIIMnIIMnIII) and
formula [Mn4(L20)2(OAc)(MeOH)5] (H5L20 ¼ bispyrazolyl of H5L2).[92]
Figure 17. Representation of the complex cation [Co4(L5)2(MeOH)8]4þ, and the coordi-
nation environment of the CoII centers. H atoms not shown. dCo���Co (ext)¼ 3.550 A, dCo���Co
Co (cent)¼ 3.692 A. (Color figure available online.)
Scheme 13.
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4. CONCLUSIONS AND OUTLOOK
The main conclusion extracted from this overview on molecular EMACS
is that the methods and strategies employed for the preparation of this
type of species have reached a high degree of diversity and versatility.
From this work, it follows that the limit to the length that can be attained
for discrete molecular chains will likely be imposed, either by the struc-
ture of the supporting ligands or by a fine balance between thermodyn-
amic stability and solubility (for the case of many ‘‘unsupported’’
chains). The progress made has not only allowed accessing linear clusters
of a large variety of metals with many types of chemical and electronic
properties; the various methods of molecular assembly should also open
ways for introducing specific properties to these architectures such as
precise conductivity, redox-behavior, magnetic properties, etc. Examples
of this are the capacity to induce spin cross-over properties to [Co3]
chains[93] or influence the conductance of [Ni5] and [Cr5] chains[94]
through chemical or electrochemical oxidation. The capacity of linear
chains to respond in desired manners to external stimuli or the ability
of integrating these into more complex molecular devices remain future
challenges for their potential applications in nanotechnology. One excit-
ing proposal awaiting such control on addressability and processing
suggests that spin chains could be used for the realization of one- or
two-qubit quantum gates.[95] For this and other potential applications,
the ability to fix linear molecules on surfaces of various types is of
paramount importance. This is being achieved to a large degree with
single-chain, 1D, molecule-based systems.[96] As great progress is being
made on these areas, a growing tendency for interdisciplinary activity
Figure 18. Representation of the complex [Co4(OMe)(OAc)(L30)2(py)4] (H4L30 ¼bis-
pyrazolyl from H4L3). H atoms not shown. dCo(III)���Co(II) (ext.)¼ 3.691 A, dCo(II)���Co(II)
(cent.)¼ 3.193 A. (Color figure available online.)
METAL-BASED MOLECULAR CHAINS 189
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within molecular materials science will lead to major breakthroughs in the
use of molecular chains as part of nanoscopic functional assemblies.
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