Ni8 mesoporous silicas

8/7/2019 Ni8 mesoporous silicas

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Ordered mesoporous silicas as host for the incorporation and aggregation ofoctanuclear nickel(II) single-molecule magnets: a bottom-up approach tonew magnetic nanocomposite materials{{

Emilio Pardo,a Pedro Burguete,b Rafael Ruiz-Garca,a Miguel Julve,a Daniel Beltran,b Yves Journaux,c

Pedro Amoros*

b

and Francesc Lloret*

a

Received 24th February 2006, Accepted 13th April 2006

First published as an Advance Article on the web 11th May 2006

DOI: 10.1039/b602838a

Silica-based mesoporous materials have been employed as the support host for a suitably designed

small octanuclear nickel(II) guest complex with a moderately anisotropic S = 4 ground spin state

(D = 20.23 cm21), which behaves as a single-molecule magnet at low temperature (TB = 3.0 K).

Both unimodal MCM-41 and bimodal UVM-7 porous silica provide appropriate template

conditions for the incorporation and aggregation of the Ni8 complex precursor into larger

complex aggregates, showing slow relaxation of the magnetization at higher blocking

temperatures than the crystalline material. By playing with the initial complex vs. silica

concentration, two series of samples with varying complex loading amounts have been obtained.The degree of aggregation varies, largely depending on the silica used, being higher for the

bimodal UVM-7 silica series. The mesophasic and porous nature of the Ni8 adsorbed silica

samples has been verified from XRD and TEM images. N2 adsorptiondesorption isotherms

show that incorporation initiates inside the small intra-particle mesopores while subsequent

aggregation occurs at the external particle surface (close to the mesopore entrances). Both DC and

AC magnetic susceptibility measurements have demonstrated the occurrence of such a unique

silica-mediated surface aggregation process of cationic Ni8 molecules into oligomeric [Ni8]x

aggregates of large spin values (S = 4x) and high blocking temperatures (TB = 4.510.5 K). The

existence of a wide distribution of aggregates with different conformation and association degree

(size distribution) and the presence of weak interactions between the aggregates leads to an exotic

spin glass magnetic behavior for this family of hostguest hybrid nanocomposite materials.

Introduction

The morphosynthesis of novel materials from the multilevel

organization of molecules with interesting physical and

chemical properties into suitable ordered host systems is a

major current challenge in materials science and nanotechno-

logy.1 This molecular bottom-up approach appears as an

alternative to the classical top-down one to creating nano-

scopic functional materials displaying redox, magnetic, optical,

sensor, or catalytic activities.2 Organic (polymers) and

inorganic (ceramics, porous solids) materials have been utilized

as support host systems. Among porous inorganic materials,

ordered mesoporous silicas have received much attention as a

promising support family for guest molecules due to their

high surface area and large pore volume. Since its discovery

in 1992,3 the technological importance of M41 silicas and

related derivatives have sparked extensive research interest

in materials science. The number of reports dealing with the

use of mesoporous silicas as inorganic hosts for the incorpora-

tion of magnetic guest molecules is relatively scarce when

compared with other typical applications of M41s and related

materials.4,5

Among the magnetic guest molecules, transition metal

polynuclear coordination compounds have attracted much

attention since the discovery, in 1993, of a dodecanuclear

mixed-valence oxomanganese(III,IV) complex that behavesas a molecular nanomagnet, termed single-molecule magnet

(SMM).6 This compound of formula Mn12O12(O2CMe)16-

(H2O)4, referred to as Mn12, exhibits a slow relaxation of the

magnetization below a blocking temperature (TB) of 4.0 K,

similar to that of superparamagnetic nanoparticles of common

magnetic materials such as metals or metal oxides. The

combination of a large spin ground state (S = 10) with a large

and negative axial magnetic anisotropy (D = 20.5 cm21) in

Mn12 results in an effective energy barrier (Ueff = 2DS2 =

50 cm21) for the magnetization reversal between the two

lowest mS = S states which accounts for the observed

magnetic bistability.7

aInstituto de Ciencia Molecular (ICMOL), Universitat de Valencia,46100 Burjassot, Valencia, Spain. E-mail: [email protected];Fax: 34 96354 4322; Tel: 34 96354 4441bInstitut de Ciencia dels Materials (ICMUV), Universitat de Valencia,P. O. Box 2085, 46071 Valencia, Spain. E-mail: [email protected];Fax: 34 96354 3633; Tel: 34 96354 3617cLaboratoire de Chimie Inorganique et Materiaux Moleculaires, UMRCNRS 7071, Universite Pierre et Marie Curie, 75252 Paris, France{ Electronic Supplementary Information (ESI) available: Fig. S1:Temperature dependence of xM9 and xM0 for 1AI; Fig. S2: Arrheniusand VogelFulcher plots for 1A1G and 1I. See http://dx.doi.org/10.1039/b602838a/.{ This article is part of a themed issue on Molecular MagneticMaterials.

PAPER www.rsc.org/materials | Journal of Materials Chemistry

2702 | J. Mater. Chem., 2006, 16, 27022714 This journal is The Royal Society of Chemistry 2006


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Addressing individual SMMs or their aggregates, as well as

increasing their blocking temperatures, are conditions to be

fulfilled for potential applications of such molecule-based

nanoscopic magnetic materials. The recent success in the

elaboration of oxomanganese SMMs of large dimensions by

Christou and co-workers is illustrated by the nanometer-sized

Mn30 and Mn84 complexes prepared from the self-aggregation

in solution of smaller-sized Mn12 SMMs under differentreaction conditions.8 Nevertheless, these giant SMMs

possess TB values of only 1.5 K due to the relatively small

spin ground state (S = 56) in spite of the moderately large

magnetic anisotropy (D # 20.5 cm21). In fact, these

rather serendipitous, homogeneous self-assembly methods are

severely limited by the lack of control over the structure and

topology of the final complex, properties that ultimately

determine both S and D values of SMMs.

By following a completely different heterogeneous self-

assembly approach, several groups have recently reported the

use of both inorganic and organic materials as support host

systems for the organization of SMMs.915 These include a

variety of surfaces and other confined media such as multi-layered LangmuirBlodgett films,9a mesoporous silicas,9b,9c,10

polycarbonate thin films,11a,11c silicon surfaces,11b,12b highly

oriented pyrolytic graphite (HOPG),11d

gold films,9d,12a,13 self-

assembled monolayers (SAMs),14 and ethyl acrylate poly-

mers.15 There are not a large variety of polynuclear complex

types, and in practically all cases the studies are devoted to the

incorporation of Mn12 SMMs with differently functionalized

carboxylate ligands, which are bonded to the surface through

electrostatic interactions, pp interactions, hydrogen bonds, or

covalent bonds. Although the possibility of addressing isolated

or small nanometer-sized aggregates of Mn12 molecules (from

one to a few hundred) has been demonstrated, increase of the

blocking temperature has not been achieved by either of theordered hosts used. In all the cases, the magnetic properties of

the incorporated neutral Mn12 molecules (whenever reported)

are practically identical to those of the isolated molecules in

the crystalline material.9ac,10,11a,15

In this article, we present a simple synthetic strategy to

prepare new magnetic nanocomposite materials from the

heterogeneous chemical aggregation of preformed polynuclear

building blocks, other than those of the Mn12 family, that

behave themselves as SMMs through the use of ordered

mesoporous silica as host matrix. As the magnetic guest

molecule, we have selected the octanuclear nickel(II) oxamate

complex of formula {[Ni2(mpba)3][Ni(dpt)(H2O)]6}(ClO4)4?

12.5H2O 1, where mpba is the bridging ligand m-phenylene-dioxamate and dpt is the terminal ligand dipropylenetriamine

(Scheme 1).16 This complex has been prepared through a step-

by-step strategy (the so-called complex-as-ligand approach)

which allows the rational design of metal coordination cages

from self-assembled, exchange-coupled metallacyclic com-

plexes with aromatic oligooxamate ligands.17 Complex 1

possesses a moderately anisotropic S = 4 ground spin state

(D = 20.23 cm21) and exhibits slow magnetic relaxation below

TB = 3.0 K in the crystalline state. The structure 1 is built from

octanickel(II) cations with a dimer-of-tetramers structure of

double-propeller topology, separated by perchlorate anions

and water molecules of crystallization. Each cationic Ni8 cage

complex consists of two strong antiferromagnetically coupled,

oxamate-bridged, propeller-like Ni4 units (J = 226.6 cm21)

which are weakly ferromagnetically coupled through the meta-

substituted phenylenediamidate bridges (J9 = +3.1 cm21)

(NiNi separations of ca. 6.8 and 5.5 A, respectively). These

intra-molecular magnetic interactions lead to a peculiar spin

topology for this Ni8 molecule with a spin down orientation

of the two central high-spin NiII ions and a spin up

Scheme 1 (a) Side and top views of the molecular structure and the

spin topology of the cationic Ni8 cage complex 1. (b) Structural

formula of the bridging mpba and terminal dpt ligands.

This journal is The Royal Society of Chemistry 2006 J. Mater. Chem., 2006, 16, 27022714 | 2703


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orientation of the six peripheral high-spin NiII ions

(Scheme 1a). Importantly, one coordination position of each

of the six peripheral octahedral metal ions, partially blocked

with the dipropylenetriamine ligand, is occupied by a labile

water molecule that can be readily exchanged by anchoring

and/or bridging deprotonated silanol groups from the host

matrix during the process of mesoporous incorporation.

Herein, we report on an extensive series of hostguest hybridnanocomposite materials prepared from the self-organization

of a self-assembled metallosupramolecular Ni8 complex pre-

cursor into higher nuclearity aggregates within different SiO2porous hosts. These include two ordered mesoporous silica

supports, either unimodal or bimodal like MCM-41 and

UVM-7, respectively, as well as a non-ordered porous xerogel

denoted UVM-11. UVM-7 silica consists of MCM-41 type

mesoporous nanosized particles (average diameter of ca.

1520 nm) joined together in micrometric aggregates (from

0.5 to 2 mm). In addition to the high surface area typical of M41

silicas, this special topology provides a significant intraparticle

mesopore length decrease (from mm to nm) when compared

with MCM-41 solids, together with an important interparticletextural porosity in the border between meso and macropores

that confers on UVM-7 materials their hierarchical porous

nature. This architecture minimizes possible pore blocking

phenomena inside the intraparticle mesopores by guest species,

and enhances at the same time the accessibility to the mesopore

entrances through the large interparticle pore system. Taking

advantage of the different chemical and physical properties of

these silica materials as host matrix, we try to give further

insights into the mechanism of such a unique mesoporous

silica-mediated aggregation process of cationic Ni8 molecules

into oligomeric [Ni8]x aggregates of larger spin values and

higher blocking temperatures. We show that the basic chemical

integrity of the octanuclear complex precursor is retained,while weak ferromagnetic exchange interactions are estab-

lished between the octanuclear units in the resultant aggregate,

leading to rather exotic magnetic properties for this family of

Ni8 adsorbed nanocomposite materials.

Experimental

Materials

Complex 1 was prepared as reported previously.16 The silica-

based host materials, MCM-41, UVM-7 and UVM-11, were

obtained by using the so-called atrane route as described in

detail elsewhere.18 This strategy consists in the use of silicon

atrane complexes (silatrane) as hydrolytic inorganic precursors

and surfactants as porogen species. MCM-41 and UVM-7

silicas have been isolated through a surfactant-assisted pro-

cedure. Cetyltrimethylammonium bromide (CTABr) has been

used as template agent. Surfactant evolution was carried out

under soft-chemical conditions (chemical exchange in EtOH

HCl mixture). UVM-11 xerogels have been prepared following

a non-surfactant strategy also through the atrane route.

Preparations

Ni8 adsorbed MCM-41 nanocomposites (1AC). In a typical

synthesis leading to sample 1A, 20 mg of complex 1 were

dissolved in 5 ml of a basic water solution (pH = 8.5) (adjusted

by addition of NaOH), giving a pale blue solution. Then,

100 mg of MCM-41 mesoporous silica were added and

the reaction mixture was stirred during 30 min at room

temperature to favor the incorporation of the complex into the

silica. The resulting mesostructured light-blue powder was

separated by filtration, washed with water and hexane, and air

dried. Samples 1B and 1C were synthesized following the sameprocedure by starting from solutions containing 40 and 60 mg

of complex 1, respectively.

Ni8 adsorbed UVM-7 nanocomposites (1DH). Samples 1D

to 1H were synthesized following the aforementioned proce-

dure by starting from solutions containing 100 mg of UVM-7

mesoporous silica and 20, 40, 60, 80, and 100 mg of complex 1,

respectively.

Ni8 adsorbed UVM-11 nanocomposites (1I). Sample 1I was

synthesized following the aforementioned procedure by start-

ing from a solution containing 100 mg of UVM-11 xerogel and

60 mg of complex 1.Table 1 summarizes the initial complex 1 concentration

used for the preparation of samples 1AI, along with selected

physical data for each series of samples compared with those

of their corresponding host.

Physical techniques

The chemical composition of samples 1AI were determined

by elemental analysis (C, H, N) and electron probe micro-

analysis (EPMA) (Philips SEM-515). The preservation of the

nanoscopic organization typical of ordered mesoporous silicas

was confirmed by X-ray diffraction (XRD) (Seifert 3000TT h-h

diffractometer using CuKa radiation), transmission electron

microscopy (TEM) (JEOL JEM-1010 operating at 100 kV),

and N2 adsorptiondesorption isotherms (ASAP2010 analy-

zer). Variable-temperature (1.8300 K) direct current (DC)

and alternating current (AC) magnetic susceptibility measure-

ments and variable-field (05 T) magnetization measurements

were carried out on powdered samples of 1AI with a

Quantum Design SQUID magnetometer. The magnetic data

were corrected for the diamagnetism of the silica content of the

samples and for the sample holder.

Results and discussion

Synthesis and general physical characterization

The influence of the nature of the porous host on the loading

efficiency and the location preferences of the Ni8 molecules

has been investigated for three different types of silica-based

materials: unimodal MCM-41 and bimodal UVM-7 meso-

porous silicas, and nanoparticulated UVM-11 silica xerogels.

In the former two cases, the adsorption of complex 1 within

mesoporous silicas is a very fast and highly efficient process

owing to the different net charge of the silica surface

possessing anionic silanolate groups and the cationic Ni8species, which favors effective complex incorporation through

electrostatic interactions. The use of cationic surfactants

together with the soft chemical exchange method employed



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for their evolution led to a high density of negatively charged

silanol groups on the surface able to interact with complex 1.

We worked in a moderately basic medium (pH = 8.5) in order

to maintain the cage complex structure in solution and to

avoid excessive back-dissolution of the silica support whilemaximizing the host/guest electrostatic interactions and,

consequently, the complex loading amount. Under these

conditions, the adsorption is complete in less than 30 min as

deduced from the disappearance of the pale blue colour of

complex 1 in solution, which acts as an indicator that informs

us about the total or partial complex incorporation.

The evolution of the complex 1 loading at varying initial

concentration values for the different silica hosts is shown in

Fig. 1. UVM-7 silica presents the highest loading capability. In

fact, the adsorption is complete (colorless mother solution

after filtration) even at relatively high complex concentration

in the range 2080% weight (samples 1DG). The highest

loading amount value of 0.120 NiSi molar ratio is achieved at

a relatively high complex concentration of 100% weight under

saturation conditions (sample 1H) (Table 1). The loading

ability of MCM-41 silica is significantly lower and saturation

is achieved at an intermediate complex concentration of 60%

weight, with a maximum loading amount value of 0.091 NiSi

molar ratio (sample 1C) (Table 1). At this same complex

concentration, the UVM-11 silica shows an adsorption ability

of only 0.036 NiSi molar ratio under saturation conditions

(sample 1I) (Table 1).

This behavior indicates a high affinity of UVM-7 and

MCM-41 mesoporous silica surfaces for electrostatic cationic

complex immobilization, which contrasts with the moderate to

low affinity of the UVM-11 silica surface. Indeed, all three

silica-based support materials present almost the same surface

density of silanol groups (ca. 56 SiOH/nm2) estimated from29Si NMR and porosimetry data. Thus, the differences in

complex 1 adsorption should have a topological or templating

origin which are related to the distinct active silica surfaces,

either intra- and/or inter-particle. The cylindrical shape of the

Ni8 molecules with approximate height 6 diameter dimen-

sions of 1.5 6 2.0 nm is adequate to allow their hosting in both

pore systems. Complex diffusion through the small intra-

particle UVM-7 and MCM-41 mesopores (ca. 2.5 nm) can be

anticipated because their sizes are slightly greater than the

complex diameter. The fact that UVM-11 silica surface is

rather inefficient for complex 1 adsorption, in spite of the

similarity in the inter-particle pore morphology with UVM-7

silica, underlines the importance of the intra-particle porosity

on the loading efficiency.

The chemical identity of the adsorbed cationic Ni8 species in

samples 1AI was confirmed by elemental analysis (C, H, N)

and electron probe microanalysis (EPMA). The experimental

values of the CN molar ratio in the range 2.62.9 for 1AI

agree fairly well with the expected value for a 3:6 mpba:dpt

stoichiometry in complex 1 (CN molar ratio = 2.75), thusshowing that the basic integrity of the adsorbed Ni8 cation is

retained. On the other hand, EPMA clearly shows the absence

of ClO42 anions in the final materials. Moreover, EPMA also

shows that all samples are chemically homogeneous (regular

distributions of Ni and Si atoms) at the micrometer level (spot

area ca. 1 mm) with constant and well-defined compositions. In

addition, the mesophasic and porous nature (mesopore

symmetry and occupancy) of the Ni8 adsorbed silica samples

has been verified from transmission electronic microscopy

images and X-ray powder diffraction, while N2 adsorption

desorption isotherms provide valuable information on the

location of the Ni8 molecules.

Fig. 1 Dependence of the NiSi content for 1AI on the initial

relative concentration of 1 (data from Table 1).

Table 1 Selected physical data for 1AI compared with those of their corresponding host

Sample[1]/weightratio

Ni:Sia/molarratio

BETarea/m2 g21

Small poresizeb/nm

Small porevolumeb /cm3 g21

Large poresizeb/nm

Large porevolumeb/cm3 g21

MCM-41 874.2 2.80 0.631A 0.2 0.039 628.9 2.71 0.621B 0.4 0.065 147.9 0.121C 0.6 0.091 17.6 0.04UVM-7 971.6 2.58 0.69 34.89 0.791D 0.2 0.032 166.4 0.15 30.93 0.781E 0.4 0.056 145.9 0.13 30.45 0.721F 0.6 0.080 92.5 0.08 31.57 0.401G 0.8 0.104 90.1 0.07 28.95 0.321H 1.0 0.120 90.4 0.06 29.35 0.29UVM-11 145.9 30.45 0.721I 0.6 0.036 92.5 29.85 0.60a Calculated from elemental analysis (C, H, N) and EPMA. b Values estimated by applying the BJH model on the adsorption branch ofthe isotherms.



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Transmission electronic microscopy and X-ray powder dif-

fraction. The TEM images of samples 1AI confirm that the

mesoscopic architecture of the three silica hosts is maintained

after complex 1 immobilization (Fig. 2). The existence of

aggregates constructed from nanoparticles, and the large inter-

particle pores defined among them, can be clearly observed

from TEM images of the Ni8 adsorbed nanocomposites

prepared from UVM-7 and UVM-11 hosts. The preservationof the small mesopore system is appreciated in the enlarged

TEM images of Ni8 adsorbed UVM-7 and MCM-41

nanocomposites (insets of Fig. 2). Regardless of the silica host

type, TEM images allow us to discard the segregation of

complex 1 as independent crystals in the final materials with

confidence. Moreover, the absence of diffraction peaks at high

2h values in the XRD patterns of samples 1AI confirms the

absence of complex 1 as a secondary phase.

The XRD patterns of samples 1DH are characteristic of

UVM-7 type materials (Fig. 3). They display a strong

diffraction peak together with a small and broad signal in

the low angle region. This pattern indicates the existence of a

certain hexagonal order in the intra-particle mesopore (smallpores) system. Thus, the order degree is maintained and the

XRD peak positions remain practically unchanged after the

incorporation of complex 1. The XRD patterns of the Ni8adsorbed UVM-7 nanocomposites differ only in the peak

intensities, which are significantly reduced when compared

with that of pure UVM-7 silica (inset of Fig. 3). The peak

intensity strongly drops when going from UVM-7 pure silica

to sample 1D and then it decreases in a smoother way for

samples 1E to 1H. This intensity loss must be attributed to a

progressive phase cancellation phenomenon associated with

the introduction of scattering material into the mesopore.

Thus, XRD data confirm at least a partial pore filling by Ni 8

molecules inside the small intra-particle mesopore system of

UVM-7. By contrast, the phase cancellation in the Ni8adsorbed MCM-41 nanocomposites (samples 1AC) occurs

in a more abrupt way (Fig. 4). Within this series, only sample

1A maintains the low-angle signals typical of hexagonal

ordered mesoporous materials (inset of Fig. 4). Samples 1B

and 1C show a complete phase cancellation instead. However,TEM images confirm the preservation of the hexagonal order

in the mesopore array in these two samples also. Overall, XRD

data suggest a higher Ni8 inclusion inside the small mesopores

of MCM-41 silica when compared with UVM-7 host. This

behavior could be attributed to the more regular and

cylindrical shape of the MCM-41 mesopores, which should

favor the diffusion of the Ni8 molecules along the pores.

As expected, the Ni8 adsorbed UVM-11 nanocomposite

(sample 1I) does not display XRD signals at all (not shown).

N2 adsorptiondesorption. The N2 adsorptiondesorption

isotherms of samples 1AI are characteristic of their parent

silica-based porous materials (Figs. 57). The first adsorption

step at intermediate P/P0 values for the Ni8 adsorbed MCM-41

and UVM-7 nanocomposites (typical of the intra-particle pore

system) practically disappears, exception being made for the

MCM-41 nanocomposite with the lowest complex 1 loading

(sample 1A) (Fig. 5). In this regard, sample 1A constitutes a

particular case where only a portion of the mesopores are filled

and/or blocked by Ni8 molecules. As the complex 1 loading

Fig. 2 Representative TEM micrographs for samples 1E (a), 1G (b),

1B (c), and 1I (d). The insets show enlarged (63) TEM images.

Fig. 3 XRD patterns for samples 1D (a), 1E (b), 1G (c), and 1H (d).

The inset shows the XRD pattern of the parent UVM-7 pure silica.

Fig. 4 XRD patterns for samples 1A (a), 1B (b), and 1C (c). The inset

shows the XRD pattern of the parent MCM-41 pure silica.



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increases along the series of Ni8 adsorbed MCM-41 nano-

composites, there is a sufficient amount of the Ni8 molecule to

partially fill or completely block all the mesopores (samples 1B

and 1C). The pore volume estimated from partial pressure data

at P/P0 , 0.4 decreases smoothly as the complex 1 content

increases from 1A to 1C (Table 1). The BET surface area also

decreases in a progressive way from the relatively high value of

1A (628.9 m2 g21) down to the very low value of 1C

(17.6 m2 g21), typical of bulk solids (Table 1).

In the series of Ni8 adsorbed UVM-7 nanocomposites, the

pore volume estimated from partial pressure data at P/P0, 0.4

strongly drops from UVM-7 pure silica to 1D and later

decreases in a smooth way as the complex 1 content increases

for 1E to 1H (Table 1). However, the second adsorption step at

high P/P0 values (typical of the inter-particle textural-like

porosity) is present in all samples (Fig. 6). The pore volume

associated with this last large pore system remains practically

constant for UVM-7 pure silica and the nanocomposites

with low complex loading (samples 1D and 1E), and slowly

decreases as the complex loading increases (samples 1F and

1H) (Table 1). A similar trend is observed for BET surface

areas (Table 1). Samples 1D and 1E present small BET surface

areas (166146 m2 g21) with a typology of their isotherms

characteristic of UVM-7 materials with filled primary meso-

pores, as observed for mesostructured UVM-7. As the complex

content increases, a small additional surface loss is detected for

1F to 1H (92.590.4 m2 g21). By contrast, the small complex 1

loading in the Ni8 adsorbed UVM-11 nanocomposite

(sample 1I) hardly modifies the isotherm, which maintains

the adsorption at high P/P0 pressure values characteristic

of UVM-11 pure silica (Fig. 7). This fact points out the

importance of the mesopore entrances as preferred sites for

hostguest interactions, which explains the inefficiency of the

inter-particle macropore system of the UVM-11 silica surface

for Ni8 adsorption when compared with that of the UVM-7

silica surface.Overall, these features suggest that complex 1 presents a

marked preference to be hosted inside the intra-particle meso-

pores. Then, the Ni8 molecules partially fill, and consequently

block, the intra-particle mesopore system, even for nano-

composites with low complex concentration (samples 1A, 1B,

and 1D). After the blocking of the intrananoparticle pores, the

adsorbed Ni8 molecules must be located at the external particle

surface for nanocomposites with high complex concentration

(samples 1C and 1E1H). The relative proportion of Ni8molecules hosted inside and outside the intra-particle meso-

pores, estimated from the evolution of the complex loading

amount along each series, depends on the type of mesoporous

Fig. 5 Representative N2 adsorptiondesorption isotherms for

MCM-41 (a), 1A (b), 1B (c), and 1C (d). The isotherms are vertically

shifted for clarity.


UVM-7 (a), 1D (b), 1E (c), 1G (d), and 1H (e). The isotherms are

vertically shifted for clarity.


UVM-11 (a) and 1I (b). The isotherms are vertically shifted for clarity.



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host material. Thus, the occupation ratio is 71:29 for the Ni8adsorbed MCM-41 nanocomposite with the highest complex

concentration (sample 1C), whereas it equals 27:73 for

the corresponding Ni8 adsorbed UVM-7 nanocomposite

(sample 1H). Indeed, this inverted distribution in the relative

occupation of the Ni8 molecules reflects the differences

between the two types of mesoporous host materials, which

are associated with the morphology of the silica support(unimodal vs. bimodal pore systems) and the pore length

(a few mm vs. 25 nm) and shape (cylindrical vs. worm-like).

Magnetic properties

DC magnetic measurements. The xMT vs. Tplots of samples

1AI, xM being the magnetic susceptibility per Ni8 unit and T

the temperature, are shown in Figs. 810. The variation in the

magnetic behavior as the complex loading amount increases

for the two series of Ni8 adsorbed MCM-41 and UVM-7 nano-

composites is consistent with the occurrence of an aggregation

process of Ni8 units in the solid state, regardless of the

mesoporous silica nature, either unimodal (samples 1AC) or

bimodal (samples 1DH) (Figs. 8 and 9, respectively). This is

evident when compared with the Ni8 adsorbed UVM-11

nanocomposite (sample 1I), which shows a magnetic behavior

similar to that of complex 1 in the crystalline state (Fig. 10).16

xMT shows a minimum in the temperature range 2444 K

for 1AI, which is characteristic of the two antiferromagne-

tically coupled oxamato-bridged S = 2 NiII4 centered triangles

present in 1 (Tmin = 45 K).16 The xMT values at Tmin for 1AI

(xMTmin = 5.35.6 cm3 K mol21) are almost identical to that of

1 (xMTmin = 5.5 cm3 K mol21)15 (Table 2). Yet, the increase of

xMT in the low-temperature region in the two series of Ni8adsorbed MCM-41 and UVM-7 nanocomposites (Figs. 8

and 9, respectively) is higher than expected for magneticallyisolated S = 4 Ni8 molecules as those present in the UVM-11

nanocomposite (Fig. 10). This behavior supports the presence

of S = 4 Ni8 repeat units in the resultant [Ni8]x aggregates of

large spin values (S = 4x) for all silica-based mesoporous

nanocomposite materials. Finally, xMT reaches a maximum

in the temperature range 310 K for 1AI, as observed in

1 (Tmax = 3 K),16 which is due to magnetic anisotropy

effects and/or antiferromagnetic interactions between the

aggregates. The xMT values at Tmax increase slowly from 1A

to 1C (xMTmax = 6.817.8 cm3 K mol21) (Table 2), as the

complex loading amount increases along this series of Ni8adsorbed MCM-41 nanocomposites (Fig. 11). By contrast,

the xMT values at Tmax increase rapidly from 1D to 1G

Fig. 8 Temperature dependence of xMT for 1A (N), 1B (&), and 1C

() under an applied magnetic field of 1 T (T 25 K) and 250 G

(T, 25 K). The inset shows the hysteresis loop of M at T = 2 K for

1C. The solid lines are eye-guides.

Fig. 9 Temperature dependence of xMT for 1D (N), 1E (&), 1F (),

1G (m), and 1H (n) under an applied magnetic field of 1 T (T 25 K)

and 250 G (T, 25 K). The inset shows the hysteresis loop of M atT = 2 K for 1G. The solid lines are eye-guides.

Fig. 10 Temperature dependence ofxMT for 1I (m) and 1 (n) under

an applied magnetic field of 1 T (T 25 K) and 250 G (T, 25 K).

The inset shows the hysteresis loops of M at T = 2 K. The solid lines

are eye-guides.



8/13

(xMTmax = 11.349.4 cm3 K mol21), and then it decreases

abruptly for 1H (xMTmax = 11.5 cm3 K mol21) (Table 2), as the

complex loading amount increases along the series of Ni8adsorbed UVM-7 nanocomposites (Fig. 11). With only one

exception, the xMT values at Tmax for 1AH are greater than

that of1I (xMTmax = 8.4 cm3 K mol21) (Table 2), which in turn

is almost identical to that of 1 (xMTmax = 8.6 cm3 K mol21)16

(Fig. 11). This behavior indicates the occurrence of an inter-molecular ferromagnetic coupling between the S = 4 N i8units in the resultant aggregate for 1AG, with the onset of a

relatively strong antiferromagnetic interaction between the

aggregates for 1H.

From the available physical and magnetic characterization

data, some preliminary considerations can be made on the

mechanism for the nucleation and growth of the aggregates of

Ni8 molecules adsorbed on the mesoporous silica surface, as

shown in Scheme 2. The adsorptive behavior shows that the

initial incorporation of individual Ni8 molecules takes place

inside the intra-particle mesopores which become the preferen-

tial sites for surface adsorption due to strong capillarity effects.

More likely, the isolated Ni8 molecules are bonded to themesopore walls by anchoring silanolate groups which counter-

balance the cationic charge (perchlorate is absent in all of the

studied materials) (Scheme 2(a)). After the filling of the intra-

particle mesopores, the Ni8 molecules that are located close

to the mesopore entrances start the growing of small [Ni8]xaggregates at the external inter-particle surface (Scheme 2(b)).

The importance of these sites as nucleation centres is

confirmed by the absence of Ni8 aggregation in the UVM-11

material because of the lack of intra-particle porosity. The

poor adsorptive capability and the magnetic phenomenology

of this last material show that the interactions of Ni8 molecules

with a non-porous silica surface are relatively weak and labile.

On the other hand, the high degree of aggregation estimatedfrom the magnetic data in the series of Ni8 adsorbed UVM-7

nanocomposites (samples 1DG) benefits from the small intra-

particle mesopore length and the large inter-particle porosity

Table2

Selectedmagneticdatafor1AI

Sample

xMTmin

a/cm

3K

mol2

1

(Tmin/K)

xMT

max

b/cm

3K

mol2

1

(Tmax/K)

Hc/G

(T/K)

TB/K

(n/Hz)

Ueffc/cm

21

t0

c/s

T0

c/K

kd

ae

(T/K)

x

Se

(T/K)

xT

e

(T/K)

1A

5.5(24.0)

6.8

(6.5)

140(2.0)

5.0

(1000)

33(95)

3.06

10

211(1.36

10

214)

1.6

0.031

1B

5.6(28.0)

12.4

(5.5)

325(2.0)

5.3

(1000)

40(100)

1.56

10

211(0.66

10

215)

1.7

0.028

1C

5.3(30.0)

17.8

(9.0)

800(2.0)

9.5

(1000)

42(435)

0.56

10

211(0.56

10

232)

6.3

0.014

0.81(9.0)

0

.08(9.0)

2.70(9.0)

1D

5.5(30.0)

11.3

(5.3)

140(2.0)

4.6

(1000)

20(170)

2.06

10

211(1.76

10

227)

2.9

0.017

1E

5.4(34.0)

15.6

(7.0)

460(2.0)

7.0

(1000)

21(520)

1.56

10

211(1.36

10

250)

5.5

0.009

1F

5.4(30.0)

31.5

(7.5)

790(2.0)

8.6

(1000)

35(415)

1.06

10

211(1.86

10

234)

5.7

0.014

1G

5.4(44.0)

49.4

(10.0)

2990(2.0)

10.5

(1000)

55(290)

1.06

10

211(2.06

10

224)

5.8

0.019

0.76(9.0)

0

.3(9.0)

10.3(9.0)

1H

5.6(30.0)

11.5

(3.8)

65(2.0)

,1.8

(1000)

1I

5.3(30.0)

8.4

(6.5)

240(2.0)

4.5

(1000)

30(120)

0.96

10

211(1.06

10

220)

1.9

0.014

a

AtH

=

1T.

b

AtH

=

250G.

c

Calcu

latedfrom

themaximaofxM0atthediffe

rentfrequenciesthroughtheequationt=

t0exp[Ueff/kB(T2

T0)](witht

=

1/2pnan

dT=

Tmax).Thevalues

calculatedfrom

theArrheniusplots(T0=

0)aregiveninparentheses.

d

Calculated

from

themaximaofxM0atthedifferentfrequenciesthroughtheequationk=(DT/T)/Dlog(n)(withT=Tmax).

e

Calculatedfrom

thefitofthexM0vs.xM

9plotsthroughtheequationxM0={(xT2

xS)/2tan[(1+3)p/2]}{(xM92

xS)(xT2

x

M9)+(xT2

xS)2/4tan

2[(1+3)p/2]}

1/2.

Fig. 11 Dependence of the xMTvalues at Tmax for 1AI on the initial

relative concentration of 1 (data from Table 2). The horizontal line

corresponds to the xMT value at Tmax for 1.



9/13

when compared with the Ni8 adsorbed MCM-41 ones

(samples 1AC). In fact, the aggregation on the external

inter-particle surface is presumably due to the mediated

effect of the deprotonated silanol groups that allows the Ni8condensation through SiO bridges. The inter-molecular

ferromagnetic exchange interactions are then propagated by

the silanolate bridges between the peripheral high-spin NiII

ions of neighboring Ni8 molecules.The growing of the large [Ni8]x aggregates, probably of

filamentous-like shape because of the interacting topologies

of host surface and guest species, stops when discrete

neighbouring aggregates meet each other and condense

through additional silanolate bridges. The resultant super-

aggregate should be consistent with both the C3h molecular

symmetry of the Ni8 units and the hexagonal order of the

mesoporous silica (Scheme 2(c)). The occurrence of such a

self-organization process with increasing complex aggregation

should be favored by the multi-domain surface morphology

and irregular topography of the large cage-like inter-particle

pore system of UVM-7 silica. In this case, the steric

requirements associated with this process dictate a change inthe nature and/or the magnitude of the inter-molecular

interactions through the silanolate bridges from weakly

ferromagnetic within the aggregates to strongly antiferro-

magnetic between the aggregates. This is ultimately responsible

for the magnetic collapse observed in the Ni8 adsorbed

UVM-7 nanocomposite with the highest complex loading

amount (sample 1H). In fact, it is well-known that the nature

of the magnetic coupling in related m-hydroxo, m-alkoxo,

and m-phenoxo nickel(II) complexes depends critically on the

NiONi bridge angle (c), passing from ferromagnetic to

antiferromagnetic when increasing the c value (for c values

close to 90u).19,20

The M vs. H plots of 1AI at 2.0 K, M being the

magnetization per Ni8 unit and H the applied magnetic

field, show hysteresis loops which are indicative of slow

magnetic relaxation effects (inset of Figs. 810). Moreover, the

coercivities vary differently as the complex loading amount

increases along each series of silica-based mesoporous

materials (Fig. 12). In the series of Ni8 adsorbed MCM-41

nanocomposites, the coercive field values increase con-tinuously as the complex loading amount increases from 1A

to 1C (Hc = 140800 G) (Table 2). Similarly, as the complex

loading amount increases in the series of Ni8 adsorbed UVM-7

Scheme 2 Proposed mechanism for the incorporation and aggregation of cationic Ni8 cage complex 1 into mesoporous silica: (a) intra-

particle adsorption at the mesopore walls by anchoring silanolate groups, (b) and (c) inter-particle adsorption at the external surface by linking

silanolate groups.

Fig. 12 Dependence of the Hc values at T = 2 K for 1AI on the

initial relative concentration of 1 (data from Table 2). The horizontal

line corresponds to the Hc value at T = 2 K for 1.



10/13

nanocomposites, the coercive field first increases from 1D to

1G, reaching extremely high values (Hc = 1402990 G), but

then it drops dramatically for 1H (Hc = 65 G) (Table 2).

Interestingly, the coercive field value for 1I (Hc = 240 G)

(Table 2), which contains magnetically isolated S = 4 N i8molecules adsorbed at the UVM-11 silica surface, is even

greater than that of 1 in the crystalline state, which is almost

non-negligible (Hc = 25 G).

AC magnetic measurements. The xM9 and xM0 vs. T plots

for 1AI, xM9 and xM0 being the in-phase and out-of-phase

magnetic susceptibility per Ni8 unit under zero DC field,

respectively, exhibit a frequency-dependent magnetic behavior,

as expected for SMMs (Fig. 13). Indeed, frequency-dependent

xM9 and xM0 maxima are seen for 1AG and 1I in the

temperature range 4.510.5 K, but no maxima are observed

for 1H above 1.8 K (Fig. S1, ESI{). As the frequency of the1 G AC field increases in the range 11000 Hz, the maxima

of both xM9 and xM0 for 1AG and 1I are shifted toward

higher temperatures. Interestingly, the xM9 and xM0 maxima

for 1AG occur at higher temperatures than those of 1I

and, more importantly, they shift to higher temperatures on

progressive aggregation along both series of mesoporous silica-

based nanocomposite materials.

In fact, the temperature value of the maximum of xM0 at a

given frequency (n) corresponds to the blocking temperature

(TB = Tmax), whereby it is assumed that the switching of

the oscillating AC field matches the relaxation rate of the

magnetization (1/t = 2pn). Thus, as the complex loading

amount increases in the series of Ni8 adsorbed UVM-7nanocomposites, the blocking temperature value at n =

1000 Hz first remains practically constant from 1A to 1B

(TB = 5.05.3 K) and then it suddenly increases for 1C (TB =

9.5 K) (Table 2). By contrast, they increase steadily for 1DG

(TB = 4.610.5 K) (Table 2) as the complex loading amount

increases in the series of Ni8 adsorbed UVM-7 nanocompo-

sites. The different evolution of the blocking temperature

values between the two types of mesoporous host materials

(Fig. 14) reflects the differences in the relative proportion of

Ni8 molecules hosted inside and outside the intra-particle

mesopores along each series, as discussed above. Thus, the

higher TB values for the nanocomposites with high complex

concentration (samples 1C and 1E1G) are associated with thepresence of [Ni8]x aggregates located at the external particle

surface (Scheme 2(b)). They will relax more slowly than the

isolated Ni8 molecules within the intra-particle mesopores

in the nanocomposites with low complex concentration

(samples 1A, 1B, and 1D) (Scheme 2(a)). In this regard, the

nanocomposite 1G, which is obtained from the ferromagnetic

Fig. 13 Temperature dependence of xM9 and xM0 (open symbols) for

1C (a), 1G (b), and 1I (c) in zero DC field and under 1G AC field at

oscillating frequencies of 1 (N), 10 (&), 100 (), and 1000 Hz (m). The

solid lines are eye-guides.

Fig. 14 Dependence of the TB values at n = 1000 Hz for 1AG and 1I

on the initial relative concentration of 1 (data from Table 2). The

horizontal line corresponds to the TB value at n = 1000 Hz for 1.



11/13

arrangement of Ni8 units at the inter-particle pores of UVM-7

bimodal mesoporous silica, possesses the highest blocking

temperature yet reported for well-established nickel SMMs.21

On the other hand, the nanocomposite 1I which contains

magnetically isolated S = 4 N i8 molecules adsorbed at the

UVM-11 silica surface possesses a higher blocking temperature

than that of 1 in the crystalline state. Thus, the TB value at

n = 1000 Hz for 1I is 4.5 K (Table 2), whereas it is 3.0 K for 1.

16

This is likely due to the larger magnetic anisotropy (D) of the

adsorbed Ni8 molecules because of the structural modifications

that would accompany the process of incorporation within

the silica host.

The calculated relaxation time (t = 1/2pn) at Tmax for 1AG

and 1I follows the Arrhenius law characteristic of a thermally-

activated mechanism, t = t0exp(Ueff/kBT) (Fig. S2, ESI{). Yet,

the values of the effective energy barrier for the reversal of the

magnetization are very high (Ueff = 95520 cm21) and the pre-

exponential factor values are abnormally low (t0 = 1.3 6

102141.3 6 10250 s) (Table 2). These anomalous Arrhenius-

fitting parameters are reminiscent of a classical spin glass

system for which the relaxation time follows an Arrhenius-likebehavior but with physical meaningless values for the energy

barrier and the pre-exponential factor.22 In fact, the values

of the relative variation of Tmax per decade of frequency, k =

(DT/T)/Dlog(n), for 1AG and 1I are in the range 0.010.03

(Table 2).22,23 These very low kvalues place these Ni8 adsorbed

silica-based nanocomposites between typical spin glasses and

superparamagnets, suggesting a picture of weakly interacting

clusters (cluster glasses).22

That being so, the thermal variation of the relaxation time at

Tmax for 1AG and 1I has been fitted through the Vogel

Fulcher law for interacting systems, t = t0exp[Ueff/kB(T2 T0)]

(Fig. S2, ESI{).22,24

Thus, the calculated T0 values in the range

1.66.3 K give an estimation of the magnitude of the inter-cluster interaction in these Ni8 adsorbed silica-based nano-

composites (Table 2). As was shown earlier for the blocking

temperature, the T0 values increase for 1AC (T0 = 1.66.3 K)

and 1DG (T0 = 2.95.8 K) (Table 2), in measure with the

increase in the degree of aggregation along both series of Ni8adsorbed MCM-41 and UVM-7 nanocomposites, respectively

(Fig. 15). This is as expected for dipoledipole interactions

between the [Ni8]x aggregates, which increase with the spin

value (S= 4x) and the close packing of the aggregates. The fact

that the T0 value for 1I is small but non-negligible (T0 = 1.9 K)

(Table 2) may also suggest that the S = 4 N i8 molecules

adsorbed at the UVM-11 silica surface are not perfectly iso-

lated but weakly interact through dipoledipole interactions.On the other hand, the calculated values of the effective

energy barrier (Ueff = 2055 cm21) and the pre-exponential

factor (t0 = 0.5 6 102113.0 6 10211 s) for 1AG and 1I

through the VogelFulcher law (Table 2) are now physically

meaningful and in agreement with those obtained in other

well-established SMMs. Notably, the Ueff values for 1AG

increase on progressive aggregation along both series of silica-

based nanocomposite materials (Fig. 16). However, the

relative variation in the effective energy barrier is somewhat

smaller along the series of Ni8 adsorbed MCM-41 nano-

composites (Ueff= 3342 cm21 for 1AC) when compared with

the series of Ni8 adsorbed UVM-7 ones (Ueff= 2055 cm21 for

1DG) (Table 2). In general, the magnitude of the effective

energy barrier (Ueff = 2DS2) depends on a number of factors,

including not only S, which increases monotonically with the

degree of aggregation, but also D, which would vary in an

unpredictable manner from one aggregate to another accord-

ing to the structural modifications (molecular symmetry

lowering) that would accompany the process of incorporation

and aggregation in each mesoporous silica support.

In our case, we suggest that the spin glass behavior stems

from the existence of a wide distribution of [Ni8]x aggregates

with different conformation and association degree (size

distribution) and the presence of weak interactions between

the aggregates as well. These two factors contribute to the

different energy barriers and, consequently, different relaxa-

tion times that will lead to the observed non-Arrhenius law

behavior for this family of Ni8 adsorbed silica-based nano-composites. Thus, for instance, the xM0 vs. xM9 plots (so-called

ColeCole plots) of1C and 1G at 9.0 K in the frequency range

0.11400 Hz yield rather flattened semicircles (Fig. 17), which

supports the occurrence of a wide distribution of single

relaxation processes. This is confirmed by the fit of the

experimental data through a generalized Debye model, which

gives very large values for the a parameter of 0.81 for 1C and

Fig. 15 Dependence of the T0 values for 1AG and 1I on the initial


Fig. 16 Dependence of the Ueff values for 1AG and 1I on the initial




12/13

0.76 for 1G (a = 0 for a single relaxation process, while a = 1

for an infinite distribution of single relaxation processes). The

adiabatic (xS) and isothermal (xT) susceptibility values are

0.08 and 2.70 cm3 mol21 for 1C, and 0.30 and 10.3 cm3 mol21

for 1G, respectively.22,25

Conclusions

In this work, we have demonstrated that ordered mesoporous

silicas provide appropriate template conditions for the surface

(heterogeneous) aggregation of suitably designed small

octanuclear nickel(II) SMMs with a relatively large ground

spin state and a significant magnetic anisotropy. The resultant

complex aggregates obtained from the ferromagnetic

arrangement of Ni8 complex precursors show slow relaxation

of the magnetization at higher blocking temperatures and

unique spin glass behavior. This new synthetic approach

in confined mesoporous media may be an attractive alternative

to solution (homogeneous) aggregation methods for the

development of high-TB slow relaxing magnetic nanocompo-

site materials.

Acknowledgements

This work was supported by the Spanish Ministry of Science

and Technology (Projects MAT2002-04329-C03-02 and

MAT2003-08568-C03-01), the Generalitat Valenciana

(GRUPOS03/099), and the European Union through the

Magmanet Network of Excellence (Contract 515767-2). P. B.

and E. P. thank the Spanish Ministry of Education, Culture,

and Sports for a grant.

References

1 A. P. Alivisatos, P. F. Barbara, A. W. Castleman, J. Chang,D. A. Dixon, M. L. Klein, G. L. McLendon, J. S. Miller,

M. A. Ratner, P. J. Rossky, S. I. Stupp and M. E. Thompson,Adv. Mater., 1998, 10, 1297; E. Dujardin and S. Mann, Adv.Mater., 2004, 16, 1125.

2 Thematic issues on nanoscopic functional materials: Chem.Mater., 2001, 13, 3059; Chem. Rev., 2005, 105, 1023.

3 C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli andJ. S. Beck, Nature, 1992, 359, 710.

4 Special issue on novel porous materials for emerging applications:J. Mater. Chem., 2006, 16.

5 (a) M. Comes, G. Rodrguez-Lopez, M. D. Marcos, R. Martnez-Manez, F. Sancenon, J. Soto, L. A. Villaescusa, P. Amoros andD. Beltra n, Angew. Chem., Int. Ed., 2005, 44, 2 91 8; (b)A. B. Descalzo, K. Rurack, H. Weihoff, R. Martnez-Manez,M. D. Marcos, P. Amoros and J. Soto, J. Am. Chem. Soc., 2005,127, 184; (c) M. Comes, M. D. Marcos, R. Martnez-Manez,F. Sancenon, J. Soto, L. A. Villaescusa, P. Amoros and D. Beltran,Adv. Mater., 2004, 16, 1783.

6 (a) R. Sessoli, H. L. Tsai, A. R. Schake, S. Wang, J. B. Vincent,K. Folting, D. Gatteschi, G. Christou and D. N. Hendrickson,J. Am. Chem. Soc., 1993, 115, 1804; (b) R. Sessoli, D. Gatteschi,A. Caneschi and M. A. Novak, Nature, 1993, 365, 141.

7 D. Gatteschi and R. Sessoli, Angew. Chem., Int. Ed., 2003, 42, 268.8 (a) M. Soler, W. Wernsdorfer, K. Folting, M. Pink and

G. Christou, J. A m. C he m. S oc ., 2004, 126 , 21 56; (b)A. Tasiopoulos, A. Vinslava, W. Wernsdorfer, K. A. Abboudand G. Christou, Angew. Chem., Int. Ed., 2004, 43, 2117.

9 (a) M. Clemente-Leon, H. Soyer, E. Coronado, C. Mingotaud,C. J. Gomez-Garc a and P. Delhaes, Angew. Chem., Int. Ed., 1998,37, 2842; (b) M. Clemente-Leon, E. Coronado, A. Forment-Aliaga,J. M. Martnez-Agudo and P. Amoros, Polyhedron, 2003, 22, 2395;(c) M. Clemente-Leon, E. Coronado, A. Forment-Aliaga,P. Amoros, J. Ramrez-Castellanos and J. M. Gonzalez-Calbet,J. Mater. Chem., 2003, 13, 3089; (d) E. Coronado, A. Forment-Aliaga, F. M. Romero, V. Corradini, R. Biagi, V. De Renzi,A. Gambardella and U. Del Pennino, Inorg. Chem., 2005, 44, 7693.

10 (a) T. Coradin, J. Larionova, A. A. Smith, G. Rogez, R. Clerac,C. Guerin, G. Blondin, R. E. P. Winpenny, C. Sanchez andT. Mallah, Adv. Mater., 2002, 14, 896; (b) S. Willemin,G. Arrachart, L. Lecren, J. Larionova, T. Coradin, R. Clerac,T. Mallah, C. Guerin and C. Sanchez, New J. Chem., 2003, 27,1533; (c) B. Folch, J. Larionova, Y. Guari, C. Guerin, A. Mehdiand C. Reye, J. Mater. Chem., 2004, 14, 2703.

11 (a) D. Ruiz-Molina, M. Mas-Torrent, J. Gomez, A. I. Balana,N. Domingo, J. Tejada, M. T. Martnez, C. Rovira and J. Veciana,Adv. Mater., 2003, 15, 42; (b) M. Cavallini, F. Biscarini, J. Gomez-Segura, D. Ruiz and J. Veciana, Nano Lett., 2003, 3, 1527; (c)M. Cavallini, J. Gomez-Segura, D. Ruiz-Molina, M. Massi,C. Albonetti, C. Rovira, J. Veciana and F. Biscarini, Angew.Chem., Int. Ed., 2005, 44, 888; (d) J. Gomez-Segura, O. Kazakova,J. Davis, P. Josephs-Franks, J. Veciana and D. Ruiz-Molina,Chem. Commun., 2005, 5615.

12 (a) A. Cornia, A. C. Fabretti, M. Pacchioni, L. Zobbi, B. Bonacchi,A. Caneschi, D. Gatteschi, R. Biagi, U. Del Pennino, V. De Renzi,L. Gurevich and H. S. J. Van der Zant, Angew. Chem., Int. Ed.,2003, 42, 1645; (b) G. C. Condorelli, A. Motta, I. L. Fragala,F. Giannazzo, V. Ranieri, A. Caneschi and D. Gatteschi, Angew.Chem., Int. Ed., 2004, 43, 4081.

13 A. N. Abdi, J. P. Bucher, P. Rabu, O. Toulemonde, M. Drillon andP. Gerbier, J. Appl. Phys., 2004, 95, 7345.

14 J. S. Steckel, N. S. Persky, C. R. Martnez, C. L. Barnes, E. A. Fry,J. Kulkarni, J. D. Burgess, R. B. Pacheco and S. L. Stoll, NanoLett., 2004, 4, 399.

15 F. Palacio, P. Oliete, U. Schubert, I. Mijatovic, N. Husing andH. Peterlik, J. Mater. Chem., 2004, 14, 1873.

16 E. Pardo, I. Morales-Osorio, M. Julve, F. Lloret, J. Cano, R. Ruiz-Garca, J. Pasan, C. Ruiz-Perez, X. Ottenwaelder and Y. Journaux,Inorg. Chem., 2004, 43, 7594.

17 (a) E. Pardo, K. Bernot, M. Julve, F. Lloret, J. Cano, R. Ruiz-Garca, F. S. Delgado, C. Ruiz-Perez, X. Ottenwaelder andY. Journaux, Inorg. Chem., 2004, 43, 2768; (b) E. Pardo,K. Bernot, M. Julve, F. Lloret, J. Cano, R. Ruiz-Garca,J. Pasan, C. Ruiz-Perez, X. Ottenwaelder and Y. Journaux,Chem. Commun., 2004, 920; (c) X. Ottenwaelder, J. Cano,Y. Journaux, E. Riviere, C. Brennan, M. Nierlich and R. Ruiz-Garca, Angew. Chem., Int. Ed., 2004, 43, 850.

Fig. 17 ColeCole plots for 1C (n) and 1G (m) at T = 9 K in zero

DC field and 1G AC field at oscillating frequencies in the range

0.11400 Hz. The solid lines are the best-fit curves.



13/13

18 (a) S. Cabrera, J. El Haskouri, C. Guillem, J. Latorre, A. Beltran,D. Beltran, M. D. Marcos and P. Amoros, Solid State Sci., 2000, 2,405; (b) P. Amoros, A. Beltran, D. Beltran, S. Cabrera, J. ElHaskouri and M. D. Marcos, Patent WO 01-72635; (c) J . E lHaskouri, D. Ortiz de Zarate, C. Guillem, J. Latorre, M. Caldes,A. Beltran, D. Beltran, A. B. Descalzo, G. Rodrguez-Lopez,R. Martnez-Manez, M. D. Marcos and P. Amoros, Chem.Commun., 2002, 330.

19 (a) J. E. Andrew and A. B. Blake, J. Chem. Soc. A, 1969, 1456; (b)J. A. Bertrand, A. P. Ginsberg, R. I. Kaplan, C. E. Kirkwood,

R. L. Martin and R. C. Sherwood, Inorg. Chem., 1971, 10, 240; (c)W. L. Gladfelter, M. W. Lynch, W. P. Schaefer, D. N. Hendricksonand H. B. Gray, Inorg. Chem., 1981, 20, 2390; (d) L. Ballester,E. Coronado, A. Gutierrez, A. Monge, M. F. Perpinan, E. Pinillaand T. Rico, Inorg. Chem., 1992, 31, 2053.

20 (a) K. K. Nanda, L. K. Thompson, J. N. Bridson and K. Nag,J. Chem. Soc., Chem. Commun., 1994, 1337; (b) K. K. Nanda,R. Das, L. K. Thompson, K. Venkastsubramanian, P. Paul andK. Nag, Inorg. Chem., 1994, 33, 1188.

21 (a) C. Cadiou, M. Murrie, C. Paulsen, V. Villar, W. Wernsdorferand R. E. P. Winpenny, Chem. Commun., 2001, 2666; (b)H. Andres, R. Basler, A. J. Blake, C. Cadiou, G. Chaboussant,

C. M. Grant, H. Gudel, M. Murrie, S. Parsons, C. Paulsen,F. Semadini, V. Villar, W. Wernsdorfer and R. E. P. Winpenny,Chem.Eur. J., 2002, 8, 4867; (c) S. T. Ochsenbein, M. Murrie,E. Rusanov, H. Stckli-Evans, C. Sekine and H. U. Gudel,Inorg. Chem., 2002, 41, 5133; (d) R. S. Edwards, S. Maccagnano,E. C. Yang, S. Hill, W. Wernsdorfer, D. N. Hendrickson andG. Christou, J. Appl. Phys., 2003, 93, 7807; (e) E. C. Yang,W. Wernsdo rf er, S . H ill , R . S . E dw ards, M. N ak an o,S. Maccagnano, L. N. Zakharov, A. L. Rheingold, G. Christouand D. N. Hendrickson, Polyhedron, 2003, 22, 1 72 7; (f)

E. C. Yang, W. Wernsdorfer, L. N. Zakharov, Y. Karaki,A. Yamaguchi, R. M. Isidro, G. D. Lu, S. A. Wilson,A. L. Rheingold, H. Ishimoto and D. N. Hendrickson, Inorg.Chem., 2006, 45, 529.

22 J. A. Mydosh, Spin Glasses: An Experimental Introduction, Taylorand Francis, London, 1993.

23 M. A. Grtu, J. Optoelectron. Adv. Mater., 2002, 4, 85.24 X. X. Zhang, G. H. Wen, G. Xiao and S. Sun, J. Magn. Magn.

Mater., 2003, 261, 21.25 (a) K. S. Cole and R. H. Cole, J. Chem. Phys., 1941, 9, 341; (b)

C. J. F. Boettcher, Theory of Electric Polarisation, Elsevier,Amsterdam, 1952.

Ni8 mesoporous silicas

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