This journal is c the Owner Societies 2012 Phys. Chem. Chem. Phys., 2012, 14, 14905–14910 14905

Cite this: Phys. Chem. Chem. Phys., 2012, 14, 14905–14910

Chemical bonding in supermolecular flowersw

Abril C. Castro,aMikael P. Johansson,

bGabriel Merino

cand Marcel Swart*

bd

Received 19th June 2012, Accepted 22nd August 2012

DOI: 10.1039/c2cp42045g

We report here a systematic study on the ability of molecular cages to bind (transition) metals.

Starting from the superferrocenophane we investigate the incorporation of first-row transition

metal (Sc–Zn) and alkaline-earth metal (Mg, Ca) double cations into these

supermetallocenophane (super[5]phane) cages, and compare them with the corresponding

metallocenes (Inorg. Chim. Acta, 2007, 360, 179). Moreover, we also investigate the binding of

neutral and double-cationic metals inside supermetallocyclophane (super[6]phane) cages.

The heterolytic and homolytic associations show preferences for different metals, and new

metal-containing cages are proposed that should be viable candidates for synthesis.

I. Introduction

One of the most intriguing molecules was presented in 1951 by

the publication of ferrocene, the first of a large series of

sandwich compounds.1–4 Interestingly, although the parent

ferrocene has D5h symmetry (with D5d symmetry for the

transition states connecting the minima) with other transition

metals and with methyl group substituents it may be actually

the D5d form that corresponds to the minimum.5 In the case of

titanium or vanadium,6 the coordination around the central

metal is in fact quite different with additional ligands in a bent

form (see Fig. 1). In contrast, the case of manganocene stands

on its own as one of the cyclopentadienyl (Cp) rings slips away

and a long polymer is formed.7

Previously, one of us studied the chemical bonding in a series

of metallocenes comprising first-row transition metals (Sc–Zn),

extended with alkaline-earth metals (Mg, Ca) and several second-

row transition metals (Ru, Pd, Ag, Cd).8 The heterolytic associa-

tion enthalpy was computed for the reaction:

M2+ + 2Cp� - MCp2 (metallocene) (1)

An idealized D5h symmetry was used for all compounds, to

obtain an honest comparison of the metal–ligand bonding strength

for the different metallocene compounds. Although the electrostatic

interactions between the M2+ cation and the two anionic Cp�

rings were substantial (�480 to �675 kcal mol�1), it was

actually the covalent orbital interactions (DEorbint) that dis-

tinguished between the different metal ions. For instance, for

Ca2+ the DEorbint was only �156 kcal mol�1, while for Fe2+ it

was �635 kcal mol�1.8 The computed heterolytic association

enthalpy for ferrocene (�658 kcal mol�1)8 was ca. 23 kcal mol�1

larger than the experimental value,9 but was in excellent agreement

with the extrapolated ab initio value (�655� 15 kcal mol�1) from

previous ab initio studies by Pierloot, Persson and Roos10 and

Klopper and Luthi.11 As suggested earlier,8 the difference between

theory (ca. �655 kcal mol�1) and experiment (�635 kcal mol�1)

might in fact be resulting from uncertainties in the experimental

data, or assumptions made in their deduction. More recently,

Pierloot and co-workers re-investigated12 some metallocenes

(M = V, Mn, Fe, Ni) using idealized structures without

additional coordinating ligands to the metal. Their CASPT2

data confirmed the �655 kcal mol�1 association enthalpy,

while using a different active space (RASPT2) led to a

reduction by ca. 16 kcal mol�1 (to �639 kcal mol�1). Given

the intrinsic uncertainty of the experimental value, it is not yet

Fig. 1 Metallocene structures.

aUniversidad de Guanajuato, Noria Alta s/n, Guanajuato, Guanajuato,C.P. 36050, Mexico

b Institut de Quımica Computacional and Departament de Quımica,Universitat de Girona, Campus Montilivi, Facultat de Ciencies,17071 Girona, Spain

cDepartamento de Fısica Aplicada, Centro de Investigacion y deEstudios Avanzados, Unidad Merida Km. 6 Antigua carretera aProgreso Apdo. Postal 73, Cordemex, 97310, Merida, Yuc., Mexico

d Institucio Catalana de Recerca i Estudis Avancats (ICREA),Pg. Lluıs Companys 23, 08010 Barcelona, Spain.E-mail: marcel.swart@icrea.cat; Fax: +34 972 418356;Tel: +34 972 418861

w Electronic supplementary information (ESI) available: Coordinatesof all species. See DOI: 10.1039/c2cp42045g

PCCP Dynamic Article Links

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14906 Phys. Chem. Chem. Phys., 2012, 14, 14905–14910 This journal is c the Owner Societies 2012

clear whether the RASPT2 or the OPBE/SSB–D/CASPT2/

CCSD(T) data correspond to the correct value.

It would be interesting to have the opportunity to study

metallocene structures where all metals are coordinated in a

Z5 : Z5 fashion to both rings, as is the case for ferrocene. A

possible manner to achieve this has been reported in a series of

papers on ferrocenophane compounds, in which the hydrogens

at the Cp rings have been replaced by alkyl-chain ‘‘arms’’ that

connect the two rings. The number of ‘‘arms’’ can be only two,

but also five in the case of superferrocenophane (from now

on referred to as super[5]phane), see Fig. 2.13–15 This super-

molecule has been synthesized only with iron, but through,

e.g., transmetallation other super[5]phane structures might be

possible. Since only the ‘‘arms’’ are added without any change

in the axial Cp rings, one might expect that the chemical

bonding between the metal ion and the rings will be similar to

that of the metallocenes.

Apart from the super[5]phane molecular cage with axial Cp

rings, another possible molecular cage able to encapsulate

metal atoms is posed by supercyclophane14,16–18 molecules

(see Fig. 2). In these latter cage compounds, the Cp rings are

replaced by phenyls, analogously to the bis-benzene metallocene-

like bis-benzene-chromium,19,20 and it has therefore six arms

that connect the two rings (from now these cages will

be referred to as super[6]phane molecules). In contrast to

the super[5]phane cages that on their own have a total

molecular charge of �2, the super[6]phane cages are neutral

and therefore might be able to capture both metal(II) cations

and neutral metals.

The encapsulation of metal atoms in these supermolecular

flowers (when looked upon from above, these molecules have

the form of ice-flowers) might be quite different because of the

type of ring present, because of the metal charge (either +2

or 0), because of the metal size and because of its ability to

form covalent bonds through suitable metal (3d) orbitals.

These questions are investigated here, and compared to

the results for metallocenes and also for diphenyl-metal sand-

wich compounds.

II. Computational methods

All of the density functional calculations were carried out with

the AmsterdamDensity Functional (ADF, version 2010.01)21,22

program. Uncontracted basis sets of Slater-type orbitals (STOs)

of triple-z quality plus double polarization functions (TZ2P)

were used, using the frozen-core approach.22 The energies were

obtained using the SSB-D functional,23 using geometries

obtained at the PBE-D224,25 level. Both functionals include

Grimme’s dispersion (D2) corrections.25 The geometry optimi-

zations were carried out with the QUILD program26 that uses

superior optimization routines based on adapted delocalized

coordinates.27

Energy decomposition analyses (EDA)28–31 were performed

using the fragment approach in ADF,22 starting for the metal

either in its +2 or 0 state, and with appropriate occupation of

the orbitals32 that correspond to the occupations in the

molecular cages. The total energy (DEtotal) is then obtained

as the sum of preparation energy (DEprep) and interaction

energy (DEint).32 The former contains three terms: the defor-

mation energy of the cage as it goes from its equilibrium

geometry to the geometry it attains in the complex with the

metal inside (DEdef); the restricted fragment preparation

energy (DEresfrag), i.e. the energy difference between the spin-

polarized metal atom in its ground-state with maximum multi-

plicity (Hund’s rule) and the spin-restricted non-polarized

(singlet) form used for the metal fragment (note that in the

case of e.g. zinc that has a singlet state as a maximum multiplet

state this energy is zero); and finally the fragment polarization

energy (DEfragpol), i.e. the energy gain from changing the

occupations of the spin-restricted fragment to prepare with

e.g. a triplet state. These latter two energy terms together make

up the valence excitation energy (DEvalexc), which is zero for

metals with a singlet spin ground-state (e.g. zinc), and positive

(destabilizing) otherwise. The effect of using non-relaxed (spin-

restricted) fragment orbitals is usually small, of the order of a

few kcal mol�1. In other words, when we take an average-of-

configuration (AOC) spherical representation as the ground-

state for e.g. Fe(II), and prepare a triplet Fe(II) fragment, the

energy needed to prepare this fragment is ca. 2–3 kcal mol�1

larger than that obtained for a truly spin-unrestricted triplet

Fe(II) with the same occupation of the irreps. The only difference

between these two triplet states is that in the case of the fragment

preparation, the a and b orbitals are the same, while in the truly

unrestricted calculation the orbitals have been allowed to relax.

The interaction energy in the EDA analysis consists of four

terms: Pauli repulsion (DEPauli), electrostatic interactions

(DVelstat), dispersion energy (DEdisp), and orbital interactions

(DEorbint).28 These latter might be further subdivided into

contributions from different irreps in case symmetry has

been used.

III. Results and discussion

We have obtained the molecular structures of super[5]phane

and super[6]phane complexes with a series of metals (Mg, Ca,

Sc–Zn) encapsulated inside. For the super[5]phanes, the +2

cationic form of the metal was used (heterolytic association)

giving an overall neutral compound; for super[6]phanes, we

Fig. 2 Super[5]phane and super[6]phane structures.

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studied both the neutral and +2 cationic form of the metals,

leading to an overall molecular charge of 0 and +2, respec-

tively. With these structures, we then performed an energy

decomposition analysis (EDA) of the association energy to

estimate the metal–ligand bonding strength.

A. Molecular structures in D5h/D6h symmetry

We started from the idealized D5h-symmetric structure for

super[5]phane with Fe(II), replaced the metal and reoptimized

the geometries. Compared to the metallocenes structures, the

metal–ring distance is considerably smaller (see Table 1).

Within the super[5]phane cage structure the metal is con-

siderably closer to the ring than in the corresponding metallo-

cene. This shortening of the metal–ring distance is of the order

of 0.2–0.4 A. However, there is sufficient space for most of the

metals to fit inside, as is evident from the rather similar

metal–ring distances of singlet Fe(II) (1.518 A) compared to the

isolated cage (1.491 A). Moreover, adding a fourth methylene

group at the arms leads to a relatively small change in the

metal–ring distance (0.06–0.07 A, data not shown). The only

metals that are too large to fit in are calcium and scandium,

where the metal–ring distance increases by almost 0.5 A.

It is surprising that even though the Cp rings are negatively

charged (with a total MDC-d charge of �1.1 on the five

carbons), the electrostatic repulsion between the two rings

actually seems to be of minor importance, given the rather short

ring–ring distance. This is even more obvious by comparing the

ring–ring distances in the empty super[5]phane and super[6]-

phane cages. The former has a total charge of �2, while the

latter is overall neutral. Nevertheless, the difference in ring–ring

distance is only 0.033 A (see Tables 1 and 2).

There is a large similarity between the metal–ring distances

for the super[5]phane and super[6]phane cages. In most cases,

the super[5]phane distances are a bit larger (0.01–0.10 A), with

again calcium and scandium not really fitting inside the cage.

For the super[6]phane cages, the neutral metals sometimes

fit better inside leading to shorter metal–ring distances (as is

the case for titanium, vanadium or chromium), while for

others the cationic metals lead to shorter metal–ring distances

(e.g. iron, cobalt, see Table 2).

B. Spin state energies with D5h/D6h symmetry

With the geometries obtained at PBE-D2/TZ2P, we then

performed single-point energy calculations to compute the

spin-state splittings of all systems (see Table 3). From this

table it is evident that there is hardly any difference between

the metallocene and super[5]phane splittings.

Although the differences in energy are sometimes larger, the

general trends remain the same, and the same spin ground-

state is obtained. The only real differences are found for the

high-spin state of super[5]phane with Cr2+ and Mn2+: in

contrast to the corresponding metallocenes where the relative

energy compared to the ground state is ca. 11 kcal mol�1, in

the case of the super[5]phanes, they are observed at 53 and

75 kcal mol�1 (see Table 3). This is probably due to the

reduced space inside the super[5]phane cage, with metal–ring

distances that are ca. 0.3 A smaller than for metallocenes (see

Table 1). As a result, the population of the anti-bonding metal

d-orbitals with its usual accompanying increase in metal–

ligand distance is not favoured anymore. It is also interesting

Table 1 Metal–ring distancesa (A) for metallocenes and super[5]phanes

Metal

Metallocenes Super[5]phanes

LSb ISc HSd LSb ISc HSd

Mg2+ 2.016 1.711Ca2+ 2.372 1.976Sc2+ 2.131 1.848Ti2+ 1.958 1.980 1.742 1.750V2+ 1.902 1.919 1.657 1.696Cr2+ 1.713 1.782 1.998 1.578 1.614 1.621Mn2+ 1.673 1.864 2.063 1.546 1.648 1.757Fe2+ 1.631 1.761 1.951 1.518 1.587 1.689Co2+ 1.714 1.859 1.892 1.560 1.638 1.639Ni2+ 1.812 1.821 1.607 1.612Cu2+ 1.926 1.662Zn2+ 1.972 1.702None N 1.491

a D5h, at PBE-D2/TZ2P.b Low spin. c Intermediate spin. d High spin.

Table 2 Metal–ring distancesa (A) for super[6]phanes

Metal

M2+ M0

LSb ISc HSd LSb ISc HSd

Mg2+/0 1.638 1.594Ca2+/0 1.890 1.842Sc2+/0 1.742 1.685Ti2+/0 1.621 1.631 1.567 1.611V2+/0 1.540 1.598 1.524 1.574Cr2+/0 1.542 1.528 1.626 1.491 1.514 1.562Mn2+/0 1.482 1.559 1.660 1.462 1.560 1.626Fe2+/0 1.475 1.515 1.592 1.511 1.532 1.563Co2+/0 1.508 1.554 1.562 1.549 1.543 1.557Ni2+/0 1.544 1.549 1.577 1.570Cu2+/0 1.591 1.612Zn2+/0 1.632 1.615None 1.458 1.458

a D6h, at PBE-D2/TZ2P.b Low spin. c Intermediate spin. d High spin.

Table 3 Spin-state splittingsa (kcal mol�1) for metallocenes,super[5]phanes and super[6]phanes

Metal

Metallocenes Super[5]phanes

LSb ISc HSd LSb ISc HSd

Ti2+ 16.8 0.0 13.3 0.0V2+ 25.0 0.0 20.6 0.0Cr2+ 36.3 0.0 11.0 22.9 0.0 53.1Mn2+ 0.0 27.9 10.5 0.0 55.1 75.4Fe2+ 0.0 46.8 47.1 0.0 64.0 97.9Co2+ 0.0 23.8 0.0 48.4Ni2+ 18.0 0.0 15.1 0.0

Metal

Super[6]phanes, M2+ Super[6]phanes, M0

LSb ISc HSd LSb ISc HSd

Ti2+/0 10.3 0.0 0.0 31.2V2+/0 6.8 0.0 0.0 63.1Cr2+/0 45.4 0.0 53.6 0.0 44.2 115.0Mn2+/0 0.0 54.0 79.0 0.0 29.0 88.5Fe2+/0 0.0 49.5 82.4 57.4 0.0 43.7Co2+/0 0.0 38.3 8.9 0.0 68.6Ni2+/0 14.9 0.0 0.0 13.9

a At SSB-D/TZ2P. b Low spin. c Intermediate spin. d High spin.

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to note that the SSB-D splittings reported here are very similar

to the OPBE ones from the previous study.8

The spin-state splittings of the super[5]phane and cationic

super[6]phane metallocages are remarkably similar. Both

cages give for each M2+ the same spin ground-state (see

Table 3), and similar splittings. The relative energies may be

found with differences of some 10–20 kcal mol�1, but the main

trends are quite the same. This is a clear indication that for

these cages the spin-state is completely determined by the

metal, and the cages are in fact ‘‘innocent’’ ligands.33 The

ground-state changes completely for the neutral super[6]phane

cages, because of the two additional electrons. For instance,

while the intermediate spin state (S = 1, S = 3/2) is the spin

ground state for titanium and vanadium with a +2 formal

charge, in the neutral form the low-spin state is the ground-

state. In contrast for iron with a low-spin ground-state for

super[5]phane and super[6]phane with a formal charge of +2,

an intermediate (S= 1, triplet) ground-state is observed in the

neutral state (see Table 3). The only exception is manganese,

for which both in the neutral and charged +2 state with the

super[6]phane cage the low-spin state is the ground-state (see

Table 3).

C. Energy decomposition analysis D5h/D6h symmetry

In order to measure the metal–ligand bonding strength, and

get an estimate for the viability of the synthesis of the super-

molecular structures with the different metals, we have com-

puted the heterolytic (for M2+ systems) and homolytic (for M0

systems) association energy (see also ref. 34). Similar to

eqn (1), we have therefore the following reactions:

M2+ + super[5]phane2� - M–super[5]phane

M2+ + super[6]phane - M–super[6]phane2+

M0 + super[6]phane - M–super[6]phane

For the metallocenes the strongest interaction energy was

found for iron,8 primarily because of the drastic increase of

the covalent orbital interactions. For the super[5]phane cage,

these covalent interactions are even larger (see Table 4). The

total interaction energy is however of similar magnitude

(ca. �905 kcal mol�1), because of a simultaneous increase of

the Pauli repulsion, reduction of electrostatic interactions and

inclusion of dispersion interactions (see Table 4). These changes

are mainly resulting from a smaller metal–ring distance.

For the super[6]phane structure, the heterolytic association

energy is much smaller (see Table 4). The decrease is coming

entirely from the interaction energy DEint, which decreases

by ca. 300 kcal mol�1. Although intuitively one would

have expected a lowering of the electrostatic interactions

to be behind this decrease, which is indeed happening

(ca. �150 kcal mol�1). However, even more important is a

reduction of the covalent orbital interactions by some

250 kcal mol�1 (see Table 4). This difference in DEorbint

between the super[5]phane and super[6]phane cages comes mainly

from the E2g irrep for super[6]phane, where the orbital interactions

lead to a destabilizing energy of +177.5 kcal mol�1. Similar to

ferrocene and Fe2+–super[5]phane, also Fe2+–super[6]phane is

therefore a p-bonded system.

For the neutral cage, i.e. the homolytic association of Fe0

and super[6]phane, the interaction energy is much smaller, as

was to be expected. Note that in Table 4 we report the

interaction energies for the singlet state of Fe0–super[6]phane,

even though this is not the spin ground-state (the triplet is

ca. 57 kcal mol�1 lower in energy, see Table 3). However, for

comparison purposes it is more convenient to report the

singlet association energies. The largest difference in the

association energy is coming mainly from the interaction

energy, and not as much the preparation energy. Because both

the metal (Fe0) and the super[6]phane cage are now neutral,

the electrostatic interactions (�359 kcal mol�1) are again

smaller than was the case for the heterolytic association of

Fe2+ to either the super[5]phane (�552 kcal mol�1) or

super[6]phane (�400 kcal mol�1) cages. Likewise, the orbital

interactions (�430 kcal mol�1) are also reduced slightly (14%)

compared to the heterolytic association of Fe2+, and shows

now a more or less equal mixture of p- and d-bonding (see

Table 4). The largest effect is however coming from Pauli

repulsion between occupied orbitals, which more than double

from 312 kcal mol�1 to 629 kcal mol�1.

D. Symmetry lowering

So far we have focused on the D5h/D6h symmetric structures in

order to facilitate a direct comparison with the structure and

metal–ligand bonding strength of the metallocenes. However,

for both the super[5]phane and super[6]phane cages the highly

symmetric is in fact not the equilibrium structure.16,17 This is

indeed indicated by the computed vibrational frequencies,

which show a number of imaginary frequencies (five in the

case of super[5]phane), corresponding to a higher-order saddle

point. The movements corresponding to these imaginary

frequencies show that the middle methylene group of the

side-arms prefers to move out of the plane. There are a number

of possible ways for the methylene groups to move: either all in

parallel to lead to a C5h symmetric structure (‘‘rrrrr’’, indicating

that all five methylenes are moving to the right, see Fig. 3), or

some methylenes moving to the right and others to the

left (‘‘rrll0’’). The most stable structure are however by far

the C5h/C6h symmetric structures, which do not show any

Table 4 Energy decomposition analysisa (kcal mol�1)

Ferrocene s[5]pb s[6]p2+c s[6]p0 d

DEprep 214.8 134.8 129.6 136.7

DEdef 2.8 10.0 4.8 17.0DEcyc-cyc

e 87.1 — — —DEvalexc 124.8 124.8 124.8 119.7DEint �907.1 �904.0 �597.6 �171.7DEPauli 286.5 402.9 311.5 628.8DVelstat �605.2 �551.9 �399.8 �358.6DEorbint �583.8 �744.0 �497.6 �430.3pf �372.0 �452.8 �414.8 �251.5dg �17.6 �29.2 +177.5 �200.3DEdisp �4.6 �11.0 �11.7 �11.6DEtotal �692.3 �769.2 �468.0 �35.0a D5h/D6h, at SSB-D/TZ2P, singlet state. b Fe2+–super[5]phane.c Fe2+–super[6]phane. d Fe0–super[6]phane. e Repulsive interaction

between anionic cyclopentadiene rings (see ref. 8). f p Contribution,

coming from irrep E100 (D5h) or E1g (D6h).

g d Contribution, coming

from irrep E200 (D5h) or E2g (D6h).

(2)

(3)

(4)

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longer any imaginary frequencies. Upon lowering the symmetry

from D5h to C5h, the energy goes down by ca. 15 kcal mol�1,

corresponding to roughly 1 kcal mol�1 per methylene group. At

room temperature, the methylenes therefore can move freely

and an average D5h-like structure is obtained.15

The lowering of the symmetry has only a small effect on the

structure, leading to only a small increase in the metal–ring distance

(typically 0.01–0.02 A). Also the energy decomposition analysis

shows only minor changes compared to the high-symmetry cases.

For instance in the case of FeII–super[5]phane, the total interaction

energy lowers ca. 10 kcal mol�1 to �914.4 kcal mol�1.

E. Viability of new molecules

In order to measure the metal–ligand bonding strength, and get

an estimate for the viability of the synthesis of the supermolecular

structures with the different metals, we have computed the

heterolytic and homolytic association energies (see eqn (2)–(4)

above). The resulting energies are given in Table 5.

The (heterolytic/homolytic) association energies for the for-

mation of the supermolecular flowers show some interesting trends.

First of all, the formation of ferrocene and FeII–super[5]phane

should be feasible, while the association energy for the FeII–

super[6]phane cage (�387.5 kcal mol�1) is probably not sufficiently

large compared to the solvation energy in water of an isolated FeII

cation (�441 kcal mol�1).35 Nevertheless, for all super[6]phane

cages the binding of a FeII cation is by far favoured the most (see

Table 5). Instead, the super[5]phane cage favours the binding of a

NiII cation (�782 kcal mol�1), albeit that the difference with either

a CoII or FeII cation is minimal (less than 10 kcal mol�1).

Interestingly, for homolytic association with the super[6]-

phane cage the preference is shifted again. There are only four

neutral metals with an exothermic association (Ti0, V0, Cr0,

Fe0), and of those four one clearly stands out: vanadium

(�162 kcal mol�1). The V0–super[6]phane cage has a low-

spin doublet ground-state, and is therefore EPR-active. Given

its large exothermic association energy, its synthesis might

be feasible.

Conclusions

We have established the molecular structures and corresponding

homolytic/heterolytic association energies for metal-containing

super[5]phane and super[6]phane cages. The different spin-states

for all systems were obtained using the SSB-D functional, which

is a reliable density functional for this property. The binding of

M2+ cations to super[5]phane cages is found to be similar to the

binding of these cations in (idealized) metallocene structures.

Heterolytic binding of M2+ cations to super[6]phane cages is

significantly less favoured. Surprisingly enough, this destabiliza-

tion is not mainly coming from electrostatic interactions, but

from a reduction of orbital interactions. Most likely, the place-

ment of a metal cation inside the super[6]phane cage interferes

with the p-electrons of the phenyl rings present in these cages.

The binding of a neutral metal is favourable only for four metals,

and is predicted to be favoured most for vanadium. The corres-

ponding homolytic association energy is sufficiently large that the

V0–super[6]phane cage might be synthesized.

Acknowledgements

The following organizations are thanked for financial support:

theMinisterio de Ciencia e Innovacion (MICINN, project number

CTQ2011-25086/BQU), and the DIUE of the Generalitat de

Catalunya (project number 2009SGR528). Financial support from

MICINN (Ministry of Science and Innovation, Spain) and the

FEDER fund (European Fund for Regional Development) was

provided by grant UNGI08-4E-003. The authors are grateful to

the Research Executive Agency of the European Research Council

for financial support through the PIRSES-GA-2009-247671

project of the FP7-PEOPLE-2009-IRSES program. MPJ

acknowledges support from the Ministerio de Ciencia e

Innovacion (project JCI-2009-05953) and the Academy of

Finland (project 136079). This work was supported in

Guanajuato by Conacyt (Grant 57892); ACC thanks Conacyt

for the Master fellowship.

Notes and references

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Fig. 3 C5h symmetric equilibrium structure of super[5]phane.

Table 5 Association energiesa (kcal mol�1) for metallocenes,super[5]phanes, and super[6]phanes (spin ground-states)

Metalb Metcene2+ s[5]p2+ s[6]p2+ Diphen0 s[6]p0

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script is the formal charge on the metal.

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