Resumen Informe Proyecto SIP 20070832 Director del Proyecto: Dr. Edilso Reguera Unidad: CICATA-IPN, Unidad Legaria Resumen Se prepararon muestras representativas de 2 familias de materiales moleculares porosos potencialmente útiles en el almacenamiento de hidrogeno. Los resultados de ese estudio dieron lugar a la publicación de 9 artículos científicos en revistas internacionales y a la conclusión de 2 Tesis de Maestría en el 2007. El estudio de estos materiales continua en el 2008.

Introducción El agotamiento progresivo de los combustibles fósiles (petróleo y carbón) y la búsqueda de fuentes alternativas de energía que sean compatibles con el medio ambiente (no contaminantes) han estimulado la investigación-desarrollo en la producción y almacenamiento de hidrógeno y de celdas de combustión. El hidrógeno da lugar a una elevada eficiencia en términos de rendimiento energético y a la vez resulta no contaminante puesto que el subproducto de su combustión es agua. De estos tres aspectos de una tecnología energética basada en hidrógeno el primero y el último han alcanzado un satisfactorio nivel de desarrollo, no así el referente al almacenamiento. Varias opciones han sido evaluadas al respecto y se continua investigando, entre ellas formación de hidruros metálicos y adsorción en tamices moleculares (carbones, zeolitas, etc.). Más recientemente se han estado estudiando enrejados porosos moleculares, muy ligeros, con elevada afinidad por el hidrógeno y buena capacidad de adsorción de este gas. Los enrejados moleculares porosos del tipo azul de Prusia han demostrado los más elevados valores en gramos de hidrogeno retenido por gramo de matriz porosa hasta hoy reportados para materiales del tipo polímero de coordinación. Este es un tipo de enrejado poroso en el cual hemos acumulado suficiente experiencia durante más de una década como para proponernos realizar estudios orientados a lograr niveles de almacenamiento de hidrógeno sin precedentes respecto a lo reportado. Este es el objetivo del Proyecto, lograr enrejados porosos moleculares con valores record para la adsorción y retención de hidrogeno en materiales del tipo polímeros de coordinación. Materiales y Métodos Se prepararon muestras representativas de 2 familias de materiales porosos moleculares y se caracterizaron mediante técnicas de análisis estructural y espectroscópicas, que incluyen: difracción de rayos X, espectroscopia Mössbauer, microscopia electrónica, termogravimetría, magnetometría, entre otras. Detalles de los métodos de síntesis se pueden encontrar en los artículos publicados los cuales están disponibles online, o directamente a partir del autor. La primera pagina de esos artículos se adjunta a este reporte. Resultados Meta 1: Síntesis de los enrejados porosos a evaluar. Se prepararon los sólidos de todas aquellas composiciones de nitroprusiatos y hexacianometalatos con una estructura porosa y se caracterizaron en términos de su pureza, composición química, entre otras. Estos resultados están contenidos en los 9 artículos publicados y en las Tesis terminadas. Meta 2: Caracterización estructural y espectroscópica y de estabilidad térmica de los enrejados porosos a estudiar. Se realizó la caracterización de todas las muestras, según lo programado. Los resultados están contenidos en los 9 artículos publicados y en las Tesis terminadas. Meta 3: Estudios de adsorción de hidrogeno y de varias moléculas sondas en los materiales estudiados. Se hicieron estudios de adsorción de agua, dióxido de carbono, nitrógeno y xenón como moléculas sondas en varias composiciones; y en algunas de ellas de hidrogeno. Los resultados están contenidos en los 9 artículos publicados y en las Tesis terminadas. Continúan los estudios de adsorción de hidrogeno que han resultado tener una cinética muy lenta.

Meta 4: Correlacionar el conjunto de datos experimentales disponibles, escritura de 5 artículos para revistas internacionales, de 2 Tesis de Maestría, presentación de ponencias, etc. Todo se cumplió, y en particular se publicaron 9 artículos en lugar de 5. Impacto El impacto del Proyecto sólo se puede evaluar en estos momentos por el conocimiento científico generado y por la formación de recursos humanos. A mediano y largo plazo los resultados logrados trascenderán en el campo del hidrogeno como principal portador energético.

FECHA DE CLASIFICACIÓN: JUNIO DE 2007

UNIDAD RESPONSABLE: DEPTO. DE ORGANIZACIÓN Y PROMOCIÓN DE LA INVESTIGACIÓNCARACTER CONFIDENCIAL

PARTES CONFIDENCIALES: CARÁTULA Y ANEXOSFUNDAMENTO LEGAL: Art. 3 Fracc. II, Art. 18 Fracc. II y 21, LFTAIPG, Lineamiento 32º Fracc. VII, VIII, XVII

FIRMA DEL TITULAR DE LA UNIDAD:___________________________________

INSTITUTO POLITÉCNICO NACIONAL SECRETARÍA DE INVESTIGACIÓN Y POSGRADO

PROTOCOLO PARA PROYECTOS DE INVESTIGACIÓN O PROPUESTAS DE ESTUDIO (Enero 2007 - Diciembre 2007)

PROYECTO INDIVIDUAL X CORTO PLAZO (1 año) X PROPUESTA DE ESTUDIO MEDIANO PLAZO (2 ó 3 años) Información General de Investigación o Propuesta de Estudio

CICATA-LEG. PosGrado en Tecnología Avanzada Escuela, Centro o Unidad Sección. División o Departamento

Título Materiales Moleculares Porosos para Almacenamiento de Hidrógeno

Registro asignado por la SIP: 20070832

Datos del director(a) del Programa, Proyecto de investigación o Propuesta de Estudio:

Reguera Ruiz Edilso Apellido Paterno Apellido Materno Nombres

Tipo de plaza en el IPN: Titular B - (INTERINATO) Grado académico: Doctorado

Horas de nombramiento: 40 RFC: RERE510126 CURP: RERE510126HNEGZD08 SNI: III BECAS: COFFA IV EDD --- EDI IX (Indicar nivel) Teléfono oficina (Ext): 67760 Teléfono particular: 55570830

Nivel académico en el que se realizará el Proyecto de Investigación o Propuesta de Estudio:

Medio Superior Superior Posgrado X Ubique su propuesta solamente en uno de los campos que a continuación se enuncian:

Ingeniería y Tecnología X Ciencias Sociales Ciencias Naturales Ciencias Agrícolas Ciencias Médicas Humanidades

Clasificación CONACyT:

Sector: Sector Medio Ambiente

Subsector: Campos de frontera y tecnologías de vanguardia (aire, agua, suelo)

Tipos de investigación:

Básica X Aplicada Autoequipamiento Desarrollo Tecnológico Educativa

Edilso Reguera Ruiz

Director(a) del Proyecto de Investigación o Propuesta de Estudio Vo. Bo. de la Academia o

Colegio de Profesores Nombre y firma de Director(a) de la Escuela, Centro o Unidad

Nota: El título deberá ser breve, conciso y representativo del tema central de investigación

La presentación deberá hacerse de acuerdo a la siguiente estructura: 1. Descripción del proyecto

1.1 Resumen El agotamiento progresivo de los combustibles fósiles (petróleo y carbon) y la busuqeda de fuentes alternativas de energía que sean compatibles con el medio ambiente (no contaminantes) han estimulado la investigación-desarrollo en la producción y almacenamiento de hidrógeno y de celdas de combustión. El hidrógeno da lugar a una elevada eficiencia en terminos de rendimiento energético y a la vez resulta no contaminate puesto que el subproducto de su combustion es agua. De estos tres aspectos de una tecnología energética basada en hidrógeno el primero y el último han alcanzado un satisfactorio nivel de desarrollo, no así el referente al almacenamiento. Varias opciones han sido evaluadas al respecto y se continuan investigando, entre ellas formación de hidruros metálicos y adsorción en tamices moleculares (carbones, zeolitas, etc.). Más recientemente se han estado estudiando enrejados porosos moleculares, muy ligeros, con elevada afinidad por el hidrógeno y buena capacidad de adsorción de este gas. Los enrejados moleculares porosos del tipo azul de Prusia han demostrado los más elevados valores en gramos de hidrogeno retenido por gramo de matriz porosa hasta hoy reportados para materiales del tipo polímero de coordinación. Este es un tipo de enrejado poroso en el cual hemos acumulado suficiente experiencia durante más de una década como para proponernos realizar estudios orientados a lograr niveles de almacenamiento de hidrógeno sin precedentes respecto a lo reportado. Este es el objetivo del Proyecto, lograr enrejados porosos moleculares con valores record para la adsorción y retención de hidrogeno en materiales del tipo polímeros de coordinación.

1.2 Objetivos Preparar y evaluar enrejados porosos moleculares con valores records para la adsorción y retención de hidrogeno molecular en materiales del tipo polímeros de coordinación.

1.3 Producto final (descripción) 1) Cinco artículos publicados en el 2007 en revistas indexadas, según el Science Citation Index. 2) Dos Tesis de Maestría concluidas en el 2007; avances en otras tres Tesis de Maestría. 3) Cinco presentaciones en Congresos (nacionales y/o internacionales) y varios

seminarios y charlas.

2. Subproductos esperados (escribir número)

Nacional Internacional Nacional Internacional

Artículo de Divulgación 0 1 Artículo Científico 0 5 Congresos 0 5 Seminiarios 5 0 Cursos 0 2 Manuales 0 0 Libros 0 0 Programa de Radio y TV 0 0 Conferencias o Ponencias 4 1 Piloto Laboratorio Piloto Laboratorio

Proceso 0 0 Prototipo 0 0 Patente 0 0 Certificado de Invención 0 0 Hardware 0 0 Software 0 0 Medio Superior Posgrado

Tesistas 0 0 5 Practicantes Profesionales 0 0 0 Alumnos PIFI 0 0 3 Prestante de Servicio Social 0 1

Otros (especificar) 0

3. Monto y distribución del presupuesto solicitado (anexar): honorarios (sólo centros foráneos), materiales y suministros, servicios generales y equipamiento (anexar cotización)

3.1 ¿Cuenta con financiamiento externo?

No X Si 4. Recursos humanos. Investigadores y alumnos participantes.

4.1 Investigadores participantes

Número Nombre e institución Actividad específica a desarrollar 1 Edilso Reguera Ruiz Director del proyecto

2 Monica Centeno Alvarez (IPN)

Sintesis de estructuras moleculares porosas

4.2 Alumnos participantes

Número Nombre Status* Actividad específica a desarrollar

1 Manuel Avila Santos TS Carcaterización Estructural de materiales microporosos moleculares

2 Manuel Avila Santos PF

Resolucion y refinamiento de la estructura cristalina de 12 composiciones de materiales moleculares porosos. Escritura de la Tesis de Maestria, concluida en enero 2008

3 Alvaro Gordillo Sol TS Estudios de estructura hiperfina acerca de la interacción huesped-hospedero en materiales

microporosos moleculares

4 Diana Herrnadez Bravo PF

Montaje y puesta en marcha de un espectrometro de impedancia. Estudio preliminar de muestras de materiales porosos

5 Cristina Perez Krap PF

Estudio por adsorcion de un grupo de muestras representativas de materiales moleculares porosos. Concluyo la Tesis de Maestria en Junio 2007 y comenzo la de Doctorado en Agosto 2007

6 Cristina Pérez Krap TS Estudios de adsorción de H2 en materiales moleculares porosos

7 Claudia Noemi Vargas Hernández TS Síntesis de Materiales Moleculares Microporosos

8 Sofia Vargas Hernández TS Sintensis y estudio de la estructura electronica y cristalina

* PIFIS(PF); Servicio Social (SS), Tesistas (TS), Prácticas Profesionales (PP) 5.Programación de Actividades de Investigación (Enero 2007 - Diciembre 2007)

Número de meta

Valor % de

cada meta Descripción de actividades Mes de inicio Mes de terminación

1 20 Meta 1 Síntesis de los enrejados porosos a evaluar

Enero 2007 Julio 2007

2 20

Meta 2 Caracterización estructural y espectroscópica y de estabilidad térmica de los enrejados porosos a estudiar

Marzo 2007 Octubre 2007

3 35

Meta 3 Estudios de adsorción de H2 y de varias moleculares sondas en los materiales estudiados

Abril 2007 Noviembre 2007

4 25

Meta 4 Correlacionar el conjunto de datos experimentales disponible, escritura de 5 articulos para revistas internacionales, de 2 Tesis de Maestría, presentación de ponencias, etc.

Abril 2007 Diciembre 2007

Porcentaje: 100 %

Nota: La planeación debe hacerse de acuerdo a las actividades del año actual (sumando 100 %), incluso si el proyecto es a mediano plazo. 6. Presupuesto

Capítulo Descripción / Concepto Total

(Pesos) 1000

2000 Para comprar: reactivos químicos para síntesis de materiales; cristaleria de 80000

laboratorio; gases con adecuada pureza para estudios de adsorción (hidrógeno, metano, argon, nitrógeno, Xenon, etc,); muestras patrones para pH, para adsorción, DRX, Calorimetría, etc.; transductores, sensores y otros accesorios para equipo de adsorción; accesorios para equipos de caracterización (UV-Vis, IR, DRX, etc.); material gastable (papel, insumos de impresión, etc.).

3000

Reparación y mantenimiento de equipos preparativos y de caracterización (equipo de pH, mantas de calentamiento, transductores de presión, sistemas de mediciones de vacío y temperatura, equipo IR, equipo de DRX, etc.)

20000

5000 Total: $ 100000

Author's personal copy

Journal of Physics and Chemistry of Solids 68 (2007) 1630–1642

An atypical coordination in hexacyanometallates: Structure andproperties of hexagonal zinc phases

J. Rodrıguez-Hernandeza, E. Regueraa,b,�, E. Limac, J. Balmasedaa,d,R. Martınez-Garcıaa, H. Yee-Madeirae

aInstitute of Science and Technology of Materials, University of Havana, 10400 Havana, CubabCenter of Applied Science and Advanced Technologies of IPN, Legaria 694, Col. Irrigacion, C.P. 11500, Mexico, D.F., Mexico

cMetropolitan Autonomous University, Iztapalapa, Mexico, D.F., MexicodInstitute of Materials Research, National Autonomous University, Mexico, D.F., Mexico

eSchool of Physics and Mathematics of IPN, Mexico, D.F., Mexico

Received 20 January 2007; received in revised form 16 March 2007

Abstract

In hexacyanometallates, the involved transition metals are usually found with octahedral coordination. The exception corresponds to

the hexagonal zinc phases where this metal appears tetrahedrally coordinated to N ends from the CN ligands. Those zinc

hexacyanometallates where such atypical coordination appears were identified and for four of them the crystal structure was refined from

X-ray diffraction powder patterns using the Rietveld method. Zinc hexacyanoferrates (III), hexacyanocobaltate (III), hexacyanoiridate

(III) and the mixed zinc–cesium hexacyanoferrate (II) were found to be dimorphic, cubic (Fm-3m) and hexagonal (R-3c), related to the

zinc atom in octahedral or tetrahedral coordination, respectively. In the absence of an exchangeable cation, the hexagonal phases result

anhydrous. This last feature was attributed to a low polar character for the pores surface. The Mossbauer spectrum of hexagonal zinc

hexacyanoferrate (III) is an unresolved quadrupole splitting doublet (D ¼ 0.18mm/s). The iron nucleus is sensing a weak electric field

gradient related to a relatively high symmetry for its ligands and charge environment. The IR spectrum appears to be an excellent sensor

to identify the coordination for the zinc atom in a given sample. For the tetrahedral coordination, the CN stretching absorption was

found at least 8 cm�1 above the frequency observed for this vibration in the octahedral one. For hydrated phases, the crystal water

evolves on heating preserving the material porous framework. The temperature at which the material becomes anhydrous parallels the

polarizing power of the charge balancing cation sited within the channels. Hexagonal Zn–Cs ferrocyanide becomes anhydrous at 100 1C,

while for the Zn–Na analogue a heating close to 200 1C is required. The stability temperature range for the anhydrous phases depends on

the nature of the engaged hexacyanometallate anion; the higher stability was observed for hexacyanoferrates (II). Zinc ferricyanide shows

the weaker magnetic interaction for the hexagonal modification due to an unfavourable geometry for the overlapping path between the

unpaired electrons on the iron(III) atoms. The open 3D porous network is formed by relatively large ellipsoidal cavities, three per cell,

communicated through elliptical openings (windows), six per cavity. For dimorphic zinc hexacyanometallates (III), the most compact

structure (higher density) corresponds to the hexagonal modification, however, it has the largest cavity windows and cavity (pore) size,

and also the higher thermal stability.

r 2007 Elsevier Ltd. All rights reserved.

Keywords: A. Inorganic compounds; A. Microporous materials; C. Mossbauer spectroscopy; C. X-ray diffraction; D. Crystal structure

1. Introduction

Transition metal hexacyanometallates, commonlyknown as Prussian blue analogues, usually have an open-channel framework appropriate for small moleculesseparation and storage [1–5]. Recent studies havereported their potentiality for hydrogen storage [6–9].

ARTICLE IN PRESS

www.elsevier.com/locate/jpcs

0022-3697/$ - see front matter r 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jpcs.2007.03.054

�Corresponding author. Institute of Science and Technology of

Materials, University of Havana, 10400 Havana, Cuba.

Tel.: +53 573 2096653; fax: +53 573 2096653.

E-mail address: ereguera@yahoo.com (E. Reguera).

Crystal structures of three anhydrous nitroprussides:M†Fe„CN…5NO‡ „M=Mn,Zn,Cd…

J. Rodríguez-HernándezInstitute of Materials Science and Technology, 10400 Havana University, Havana, Cuba

E. Regueraa�

Institute of Materials Science and Technology, 10400 Havana University, Havana, Cuba and Center ofApplied Science and Technology of IPN, CICATA-Unidad Legaria, Mexico, D.F.

M. Mir and Y. P. MascarenhasInstituto de Fisica de Sao Carlos, Universidade Sao Paulo, Sao Carlos, SP, Brazil

�Received 14 June 2006; accepted 27 December 2006�

The crystal structures of Mn, Zn, and Cd nitroprussides in their anhydrous state, M�Fe�CN�5NO��M =Mn,Zn,Cd�, were refined from XRD powder patterns using the Rietveld method. Thesecompounds have a porous framework useful for adsorption and storage of small molecules. Watercrystallization can be removed by heating below 100 °C without disrupting the 3D network byintroducing certain structural modification mainly around the M site �Mn, Zn, Cd�. For M =Mn andCd, the compounds were found to be orthorhombic with space group Pnma �Mn:a=13.7844�1�,b=7.3750�2�, c=10.9470�2� Å, V=1112.8�1� Å3, Z=4; Cd:a=13.9566�3�, b=7.5040�4�, c=11.0230�2� Å, V=1154.4�1� Å3, Z=4�. Anhydrous zinc nitroprusside crystallizes in rhombohedral

with space group R3 �a=b=19.2525�1� ,c=17.7107�2� Å,�=120.0° ,V=5685.1�1� Å3,Z=18�.When exposed to humid air, these anhydrous compounds become hydrated. The XRD powderpatterns were recorded under vacuum on samples dehydrated in situ. The structural informationfrom XRD was complemented with thermo-gravimetric, infrared, and Mössbauer data. © 2007International Centre for Diffraction Data. �DOI: 10.1154/1.2434787�

Key words: nitroprusside, porous material, Rietveld, crystal structure, Prussian blue analogues

I. INTRODUCTION

Divalent transition metal nitroprussides form an interest-ing family of molecular materials. The NO group has�-anti-bonding orbitals at the N end of relatively low energyand in an appropriate disposition to accept electrons fromiron t2g

orbitals. This allows the metal-to-ligand charge trans-fer in nitroprussides by the sample illumination with radia-tion of an appropriate wavelength. Such photo-inducedmetal-to-ligand charge transfer generates excited states,which have been studied in order to explore the potentialapplications of these materials for holographic informationstorage �Gütlich et al., 2001� and also for optically switch-able molecular communication devices �Gu et al., 1997�.These features are closely related to the nature of the NOgroup. The NO ligand remains unbounded at the O end, leav-ing a small space that “absorbs” the variation in the N-Odistance during the metal-ligand electron transfer. However,our main motivation for the crystal structure resolution of theanhydrous phases of nitroprussides is related to their openchannel porous framework. Water crystallization �coordi-nated and zeolitic� can be removed under moderate heatingwithout disrupting the framework, and the free space in thestructure becomes accessible for adsorption and storage ofsmall molecules �Balmaseda et al., 2003�. In this way, nitro-prussides are among the most efficient H2 sorbents amongporous coordination polymers reported to date �Culp et al.,2006�.

The studied materials result from the 3D assembling of amolecular block, the pentacyanonitrosylferrate�II� anion�Fe�CN�5NO�2−, through divalent transition metals �M�linked at the N end of the CN groups. The resulting materialshave the formula unit M�Fe�CN�5NO� ·xH2O. This family ofcoordination compounds shows a pronounced polymorphismrelated to the different ways to accommodate the pseudo-octahedral block to form a 3D framework, and also to thehydration degree of the obtained solid �Reguera et al., 1996�.In the studied nitroprussides, the water crystallization alwaysappears associated to the M metal, as coordinated water mol-ecules or as zeolitic waters stabilized through hydrogenbonding interactions with the coordinated ones. For Mn, Cd,and Zn, the crystals preserved within the mother liquor aretrihydrates, monoclinic P21/n for Mn and Cd �Mullica et al.,

1990� and rhombohedral R3 on a hexagonal lattice for Zn�Mullica et al., 1989�. When these trihydrates are aged in adry atmosphere, a water molecule per formula unit is lost andthe resulting dihydrates are found to be orthorhombic Pnma�Reguera et al., 1996; Gómez et al., 2001�.

In this paper the crystal structures for the anhydrousphases of Mn, Zn, and Cd nitroprussides are reported. Thestructural study, carried out from XRD powder patterns re-corded in vacuum on samples dehydrated in situ, wascomplemented with thermo-gravimetric �TG�, infrared �IR�,and Mössbauer data.

II. EXPERIMENTAL

The studied samples were obtained by mixing 0.01 Maqueous solutions of sodium nitroprusside and Mn�2+ �,

a�Author to whom correspondence should be addressed. Electronic mail:ereguera@yahoo.com

40 40Powder Diffraction 22 �1�, March 2007 0885-7156/2007/22�1�/40/7/$23.00 © 2007 JCPDS-ICDD

Crystal structures of cubic nitroprussides: M†Fe„CN…5NO‡ ·xH2O„M=Fe,Co,Ni…. Obtaining structural information from the background

A. GómezDepartment of Physics, University of Guelph, Guelph, Ontario NIG 2W1, Canada

J. Rodríguez-HernándezInstitute of Materials Science and Technology, 10400 Havana University, Havana, Cuba

E. Regueraa�

Institute of Materials Science and Technology, 10400 Havana University, Havana, Cuba andCenter of Applied Science and Technology of IPN, CICATA-Unidad Legaria, Mexico, D. F.

�Received 21 April 2006; accepted 23 January 2007�

A new structural model is proposed for cubic nitroprussides and the crystal structure for the complexsalts of Fe�2+ �, Co�2+ �, and Ni�2+ � refined in that model. In cubic nitroprussides the building unit,�Fe�CN�5NO�2−, and the assembling metal �M =Fe2+ ,Co2+ ,Ni2+�, have 3

4 occupancy with threeformula units per cell �Z=3�. This leads to certain structural disorder and to different localenvironments for the outer metal. The crystallographic results are supported by the Mössbauer andinfrared data. The XRD powder patterns, index in a cubic cell �Fm3m space group�, show a sinuousbackground because of diffuse scattering from positional disorder of the metal centers. Because ofthis, the crystal structures were refined allowing the metal centers to move from the �0,0,0� and�0,0,1/2� positions �away from positional symmetry restrictions�. The refinement under theseconditions leads to excellent agreement factors �Rwp, Rp, S�, good pattern background fitting, andproduced a refined structural model consistent with the crystal chemistry of nitroprussides. Thestudied materials are obtained as hydrates. On heating, the crystal water evolves, and below 100 °Can anhydrous phase is obtained, preserving the framework of the original hydrates. The loss of thecrystal water leads to cell contraction that represents around 2% of cell volume reduction. Oncooling down from room temperature to 77 and 12 K, a slight expansion for the -M-N�C-Fe-C�N-M- chain length is observed, suggesting that at low temperature and reduction in the metalscharge delocalization on the CN bridges takes place. For M =Fe and Co the crystal structure wasalso refined for the anhydrous phase at 12, 77, and 300 K. © 2007 International Centre forDiffraction Data. �DOI: 10.1154/1.2700265�

Key words: nitroprusside, porous material, Rietveld refinement, crystal structure, Prussian blueanalogues

I. INTRODUCTION

Nitroprussides are salts of the pentacyanonitrosylferrateanion, �Fe�CN�5NO�2−. For divalent transition metals, in-soluble salts, as fine powders, are obtained. Two propertieshave attracted the attention on these molecular materials inthe past few years. Their nanoporous framework can accom-modate small molecules �Balmaseda et al., 2003; Reguera etal., 2004; Culp et al., 2006� and the light induced metal �Fe�to ligand �NO� charge transfer �Gu et al., 1997; Gütlich etal., 2001 �and references therein��. This last effect has beentaken as prototypical for switching on/off molecular devices�Gu et al., 1996�. These properties suggest the convenienceof a proper description of their crystal structure. Fortunatelyit has been carried out for practically all the divalent transi-tion metals �Mullica et al., 1989, 1990, 1991a, b, 1992, 1993;Benavante et al., 1997; Gómez et al., 2001, 2004; Reguera etal., 1996; Rodríguez-Hernández et al., 2005�, except for thecubic phases of Fe�2+ � and Ni�2+ �. For Co�2+ � the re-ported structure �Mullica et al., 1991b� must be reconsidered.

The building unit, �Fe�CN�5NO�2−, of these materials is rela-tively small and the metals remain linked through a strongligand, the CN group, forming a practically rigid block.These are ideal features for a successful crystal structure re-finement using the Rietveld method. The building unit is apseudo-octahedral block with five coordination sites �the Nends of the CN groups�. This introduces certain particularityto the obtained 3D cubic structure when a metal like Fe2+,Co2+, or Ni2+ is used to assemble neighboring buildingblocks. In such a 3D network systematic vacancies of boththe building unit and the metal appear. This leads to certainstructural disorder and to the appearance of a sinuous contri-bution to the XRD powder pattern background. This sinuouscontribution can be attributed to the metal centers out of therestricted symmetry positions. Because of this, the patternbackground could be used to obtain a refined structuralmodel close to the true material crystal structure. The aim ofthis contribution is the crystal structure refinement for thecubic hydrated phases of Fe2+, Co2+, and Ni2+ nitroprussides,considering the diffuse component of the pattern backgroundin the refinement process. For iron and cobalt the crystalstructures of the anhydrous phases at 12, 77, and 300 K werealso refined.

a�Author to whom correspondence should be addressed. Electronic mail:ereguera@yahoo.com

27 27Powder Diffraction 22 �1�, March 2007 0885-7156/2007/22�1�/27/8/$23.00 © 2007 JCPDS-ICDD

Autho

r's

pers

onal

co

py

Porous hexacyanocobaltates(III): Role of the metal onthe framework properties

J. Roque a, E. Reguera a,b,*, J. Balmaseda a,c, J. Rodrıguez-Hernandez a,L. Reguera d, L.F. del Castillo c

a Institute of Science and Technology of Materials, University of Havana, 10400 Havana, Cubab Center for Applied Science and Advanced Technologies of IPN, Mexico, DF, Mexico

c Institute of Materials Research, National Autonomous University of Mexico, Mexico, DF, Mexicod Faculty of Chemistry, University of Havana, 10400 Havana, Cuba

Received 17 August 2006; received in revised form 15 January 2007; accepted 17 January 2007Available online 26 January 2007

Abstract

The extended porous framework of divalent transition metal hexacyanocobaltates(III) was studied from the refined crystal structuresand adsorption isotherms of H2O, CO2 and N2. From the obtained adsorption data the pore accessibility, pore volume, adsorptionpotentials and nature of the guest–host interactions were evaluated. The properties of the porous framework are modulated by the metalused to form the 3D framework from the elemental building block, the hexacyanocobaltate(III) ion. From that fact, this family of micro-porous compounds can be considered as tunable zeolites, with a pore volume and a system of pore windows appropriate for separationand storage of small molecules. The adsorption isotherms also reveal that the electric field gradient at the pore surface and the pore acces-sibility are determined by the metal linked at the N end of the CN groups. These compounds are usually obtained as hydrates. The dehy-dration process and the thermal stability were studied from thermo-gravimetry combined with X-ray diffraction. The crystal water is lostbelow 100 �C and then the anhydrous structure remains stable, preserving its porous features, up to 250 �C. Upon water removal a pro-gressive cell contraction which amounts 4% of cell volume reduction was observed.� 2007 Elsevier Inc. All rights reserved.

Keywords: Adsorption; Porous material; Porous structure; Prussian blue analogs; Crystal structure

1. Introduction

Divalent transition metal hexacyanocobaltates(III) forman interesting family of porous molecular materials, with apore volume and pore windows appropriate for separationand storage of small molecules [1–3]. In this sense, recentstudies have evaluated the molecular hydrogen storage intheir porous framework [4–6]. Hexacyanometallates canbe considered as a 3D assembling of a molecular block,the hexacyanometallate anion, [M(CN)6]n�, through a tran-

sition metal cation (Tm+), in the following the metal, whichlinks the N ends of neighboring blocks. Within hexacyano-metallates, cobalticyanides of divalent cations (T2+),(T3[Co(CN)6]2 ÆxH2O), are particularly attractive as micro-porous materials. The Co(III) ion in low spin state has sixelectrons in the t2g orbitals. These orbitals are filled. Thisprovides a high stability to the building block ([Co(CN)6]).No obvious participation of the hexacyanocobaltate(III)anion in oxidation or reduction processes is expected. Thereported hydrogen adsorption isotherms in this family ofcompounds show certain dependence on the metal involved[4,5], an effect that deserves to be studied since it could beused as a tuning way for the properties of these materials.

Porous cyanometallates are light weight crystalline solids,with a relatively high free volume and with intermediate to

1387-1811/$ - see front matter � 2007 Elsevier Inc. All rights reserved.

doi:10.1016/j.micromeso.2007.01.030

* Corresponding author. Address: Institute of Science and Technologyof Materials, University of Havana, 10400 Havana, Cuba. Tel./fax: +53 72096653.

E-mail address: ereguera@yahoo.com (E. Reguera).

www.elsevier.com/locate/micromeso

Microporous and Mesoporous Materials 103 (2007) 57–71

Autho

r's

pers

onal

co

py

Journal of Physics and Chemistry of Solids 68 (2007) 290–298

Mixed valence states in cobalt iron cyanide

R. Martınez-Garcıaa, M. Knobelb, J. Balmasedaa,c, H. Yee-Madeirad,1, E. Regueraa,e,�,1

aInstitute of Materials Science and Technology, University of Havana, 10400 Havana, CubabInstitute of Physics ‘‘Gleb Wataghin’’, UNICAMP, Campinas, Brazil

cInstitute of Materials Research, UNAM, Mexico, D.F.dSchool of Physics and Mathematics of IPN, Mexico, D.F.

eCentre for Applied Science and Advanced Technology of IPN, Mexico, D.F.

Received 10 May 2006; received in revised form 5 November 2006; accepted 15 November 2006

Abstract

Cobalt iron cyanide with both Co and Fe in mixed valence states were prepared and characterized. In this mixed valence system the

cobalt atom is found both as high spin Co(2+) and low spin Co(III) while iron always appears in low spin state to form two solid

solutions: Co(2+)Co(III) hexacyanoferrates (II,III), and Co(2+)Co(III) hexacyanoferrate (II). Such solid solutions have the following

formula units: (Co2+)x(CoIII)1�xK[(FeII)1�x(Fe

III)x(CN)6] �12H2O and (Co2+)1.5x(Co

III)1�xK[FeII(CN)6] � yH2O (0pxp1, 1pyp14).

Compounds within these two series were characterized from Infrared, Mossbauer, X-ray diffraction and thermo-gravimetric data, and

magnetic measurements at low temperature. A model for their crystal structure is proposed and the structure for a representative

composition refined from XRD powder patterns using the Rietveld method. A simple and reproducible procedure to prepare these solid

solutions is provided. Within hexacyanoferrates, such mixed valence states system in both metal centres shows unique features, which are

discussed from the obtained data.

r 2006 Elsevier Ltd. All rights reserved.

Keywords: A. Magnetic materials; C. Mossbauer spectroscopy; D. Crystal structure; D. Magnetic properties; D. Electronic structure

1. Introduction

Prussian blue analogues have been extensively studied inthe last decade due to their interesting magnetic properties,e.g. high Tc magnet [1–3], magnetic pole inversion [4], spinglass behaviour [5], magnetic refrigeration [6], and so on.Within Prussian blue analogues, cobalt hexacyanoferrates(II, III) have received a particular attention, mainly due tothe observed photo-induced magnetism in cobalt (III)ferrocyanide [7,8]. The illumination of this compoundinduces an inner charge transfer to form cobalt (2+)ferricyanide, which shows magnetic order at low tempera-ture (below 20K). The heating of cobalt (2+) ferricyanide,above 353K, induces the inverse charge transfer, to form

low spin cobalt (III) ferrocyanide [9]. These two chargetransfer processes indicate that in cobalt hexacyanoferrates(II,III) the energy barrier between high spin Co(2+) andlow spin Co(III) is relatively low. Such behaviour forcobalt iron cyanide suggests the possibility of obtainingsolid compounds with both Co and Fe in mixed valencesates as stable phase at room temperature. However, asfar as we know, such possibility has not been explored.The preparation and study of materials based in thesemixed valence states is the aim of this contribution. Twostable solid solutions of Co(2+)Co(III) hexacyanoferrateswere prepared and characterized from X-ray diffrac-tion (XRD), infrared (IR), Mossbauer, thermogravi-metry and magnetic data. A structural model is proposedfor these solid solutions and the crystal structure refined fora typical composition from XRD powder patterns usingthe Rietveld method. The crystal structures of relatedcobalt iron cyanides are provided as SupplementaryInformation.

ARTICLE IN PRESS

www.elsevier.com/locate/jpcs

0022-3697/$ - see front matter r 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jpcs.2006.11.008

�Corresponding author. Institute of Materials Science and Technology,

University of Havana, 10400 Havana, Cuba. Tel./fax: 53 7 2096653.

E-mail address: ereguera@yahoo.com (E. Reguera).1COFAA Fellow.

IOP PUBLISHING JOURNAL OF PHYSICS D: APPLIED PHYSICS

J. Phys. D: Appl. Phys. 40 (2007) 6076–6082 doi:10.1088/0022-3727/40/19/047

Crystal structures of hexacyanometallateswith bifurcated cyano groupsJoelis Rodrıguez-Hernandez1, Ariel Gomez2, andEdilso Reguera1,3,4

1 Institute of Materials Science and Technology, University of Havana, Cuba2 Department of Physics, University of Guelph, ON, N1G 2W1 Canada3 Center for Applied Science and Advanced Technology of IPN, Mexico DF, Mexico

E-mail: ereguera@yahoo.com

Received 19 April 2007, in final form 8 August 2007Published 21 September 2007Online at stacks.iop.org/JPhysD/40/6076

AbstractThe crystal structures for T2[M(CN)6] where T = Mn, Cd; M = Fe, Ru, Os,were refined from the corresponding XRD powder patterns using the Rietveldmethod in the hexagonal P-3 (147) space group with Z = 1. In the structureof these families of anhydrous hexacyanometallates (II) the N end of the CNgroup appears bifurcated, serving as a ligand for two neighbouring T metals.Such a coordination mode has not been reported before for transition metalhexacyanometallates but it is consistent with the magnetic properties andMossbauer, IR and Raman spectra of the studied compounds. The anhydroussolids are obtained by dehydration of the corresponding octahydrates. In thehydrated form the metals (Mn, Cd) linked at the N ends have a mixedcoordination sphere formed by three N atoms and three coordinated waters,with two of these latter forming bridges between two neighbouring metals.The loss of these structural waters leaves the metals (T ) in an unstable statewith only three ligands in their coordination sphere and a structuraltransformation involving a change in the CN group electronic configuration isinduced. The metal coordination through bifurcated CN groups leads to aremarkable increase in the charge overlapping between the metal centres,which appears attractive for molecular magnet design.

(Some figures in this article are in colour only in the electronic version)

1. Introduction

In hexacyanometallates the metal centres are usually foundbridged by CN groups where the C and N ends remainlinked to only one metal. The metal linked at the C end isalways found with octahedral coordination to form the anionichexacyanometallate octahedral block, [Mn(CN)6]6−n. The 3Dframework is formed when neighbouring blocks are linked attheir N ends through a second transition metal (T ). Froma recent study on the magnetic properties of Mn2[M(CN)6],with M = Fe, Ru and Os, conclusive evidence concerning thebonding of a single N end to two manganese atoms has beenobtained [1]. Mn2[M(CN)6] ·8H2O and Cd2[M(CN)6] ·8H2Oform two isomorphous series of hexacyanometallates (II)[2–4], crystallizing in the monoclinic space group P 21/n (14),

4 Author to whom the correspondence should be addressed

and from spectroscopic evidence in their anhydrous state suchisomorphous behaviour is preserved [1–5]. For Cd2[Fe(CN)6]the crystal structure has been solved and refined from XRDpowder patterns and a combination of radial distributionfunction (RDF) and direct methods in the hexagonal P-3 (147)space group [6]. In that structural model the N end of the CNgroups appears coordinating two neighbouring outer metals(Cd). In this contribution the refined crystal structures in theP-3 model for these two series as anhydrous phases are reportedand their physical properties are discussed from the refinedstructures. For comparison, the crystal structure refinementof the cadmium iron anhydrous phase was revised and theresulting parameters are also reported.

In Mn2[M(CN)6] ·8H2O and Cd2[M(CN)6] ·8H2O seriestwo neighbouring assembling metals (Mn, Cd) are linkedby two water bridges to form a small cluster of two metals

0022-3727/07/196076+07$30.00 © 2007 IOP Publishing Ltd Printed in the UK 6076

129Xe NMR Spectroscopy Study of Porous Cyanometallates

E. Lima,†,‡ J. Balmaseda,†,§ and E. Reguera*,§,|

UniVersidad Auto´noma Metropolitana-Iztapalapa (UAM-I), Me´xico, UniVersidad NacionalAutonoma de Me´xico (UNAM), Instituto Polite´cnico Nacional (Me´xico) (IPN), and

UniVersidad de La Habana (UH), Cuba

ReceiVed December 15, 2006

Zinc and cadmium hexacyanocobaltates(III) were prepared, and their porous networks were explored using129Xespectroscopy. The crystal structures of these two compounds are representative of porous hexacyanometallates, cubic(Fm-3m) for cadmium and rhombohedral (R-3c) for zinc. In the cubic structure, the porosity is related to systematicvacancies created from the elemental building block (i.e., the hexacyanometallate anion), whereas the rhombohedral(R-3c) structure is free of vacant sites but has tetrahedral coordination for the zinc atom, which leads to relativelylarge ellipsoidal pores communicated by elliptical windows. According to the Xe adsorption isotherms, these porousframeworks were found to be accessible to the Xe atom. The structure of the higher electric field gradient at the poresurface (Fm-3m) appears and is accompanied by a stronger guest-host interaction for the Xe atoms and a highercapacity for Xe sorption. For cadmium, the129Xe NMR signal is typical of isotropic movement for the Xe atom,indicating that it remains trapped within a spherical cavity. From spectra recorded for different amounts of adsorbedXe, the cavity diameter was estimated. For the zinc complex,129Xe NMR spectra are asymmetric because of the Xeatom movement within an elongated cavity. The line-shape asymmetry changes when the Xe loading within the porousframework increases, which was ascribed to Xe-Xe interactions through the cavity windows. The Xe adsorptionrevealed additional structural information for the studied materials.

1. IntroductionCyanometallates form a family of porous molecular materials

with relatively large amounts of free space (pore volume) andsmall access windows. Such structural features are attractive forthe separation and storage of small molecules. In this sense,porous hexacyanocobaltates have been used, for instance, in thestudy of molecular hydrogen storage in molecular porousmaterials.1-3 Some nitroprussides (pentacyanonitrosylferrates),for instance, are the most efficient H2 storage materials on thebasis of porous coordination compounds reported to date.4

However, the porous framework properties of cyanometallatesremain poorly documented, even when these compounds showa high flexibility to modulate the geometry, size, and physicalfeatures of the pore system. This communication represents acontribution in that sense. The porous nature of the cyanometallatestructure is related to the coordination adopted by the metal (M)that forms the salt of the involved complex cyanometallate anion.In hexacyanometallates(III) of divalent transition metals, forinstance, two types of porous frameworks are found. When themetal adopts an octahedral coordination, one-third of the anionicoctahedral block sites remain vacant to form a network of largepores (ca. 8.5 Å) communicated by relatively small windows(ca. 4.2 Å) (interstitial free spaces).5 However, when the metalis tetrahedrally coordinated to the N ends of the CN groups, aporous framework is also obtained but without vacant sites in

the crystal structure.6 In this case, the resulting pores have anellipsoidal form (ca. 5.1× 12.7× 8.3 Å) and are communicatedby elliptical windows (ca. 3.9× 5.2 Å7). Cadmium and zinchexacyanocobaltates(III) (cubic,Fm-3m) and rhombohedral (R-3c, based on a hexagonal cell) crystal structures, respectively,are representative of these two porous frameworks and also ofporous hexacyanometallates (Prussian blue analogues). Anhy-drous samples of these two compounds were studied from Xeadsorption isotherms and129Xe NMR spectra, and the obtainedresults are discussed in this contribution. Mixed Zn-K hexacy-anoferrate(II) and its analogues of Na, Rb, NH4, and Cs alsocrystallize in theR-3c rhombohedral cell,8 but their pores werefound to be inaccessible to Xe, even under pressure and at hightemperature. Such behavior was attributed to the exchangeablecations that are situated close to the pore windows. Cadmiumand zinc nitroprussides were initially included in this study, buttheir porous frameworks were also found to be inaccessible tothe Xe atom. To the best of our knowledge, this is the first studyusing129Xe NMR spectroscopy on this family of porous materials.

2. Experimental Section

The studied samples were prepared using the precipitationmethod. Hot aqueous solutions (0.01 M at∼90 °C) of zinc andcadmium sulfates and of K3[Co(CN)6] were mixed under stirring,and the precipitate that was formed was aged 2 days within themother liquor. The obtained solids were separated by centrifugation,washed several times with distilled water, and then air-dried at60°C. All the reagents used were analytical grade and were obtainedfrom Sigma-Aldrich. The nature of the obtained powders ashexacyanometallates was verified from infrared (IR) spectra, andtheir stoichiometry was inferred from the atomic ratio of the involvedmetals, which was estimated from X-ray fluorescence analyses. The

* To whom correspondence should be addressed. E-mail: ereguera@yahoo.com.

† Instituto de Investigaciones en Materiales.‡ Universidad Auto´noma Metropolitana-Iztapalapa.§ Instituto de Ciencia y Tecnologı´a de Materiales.| Centro de Investigacio´n en Ciencia Aplicada y Tecnologı´a Avanzada.(1) Kaye, S. S.; Long, J. R.J. Am. Chem. Soc.2005, 127, 6506.(2) Chapman K. W.; Southon P. D.; Weeks C. L.; Kepert C. J.Chem. Commun.

2005, 3322.(3) Hartman, M. R.; Peterson, V. K.; Liu, Y.; Kaye, S. S.; Long, J. R.Mater.

Chem.2006, 18, 3221.(4) Culp, J. P.; Matranga, C.; Smith, M.; Bittner, E. W.; Bockarth, B.J. Phys.

Chem. B2006, 110, 8325.(5) Ludi, A.; Gudel, H. U.Struct. Bonding1973, 14, 1.

(6) Balmaseda, J.; Reguera, E.; Rodriguez-Hernandez, J.; Reguera, L.; Autie,M. Microporous Mesoporous Mater.2006, 96, 222.

(7) Cartraud, P.; Cointot, A.; Renaud, A.J. Chem. Soc., Faraday Trans. 11981, 77, 1561.

(8) Gravereau, P.; Garnier, E.; Hardy, A.Acta Crystallogr., Sect. B1979, 35,2843.

5752 Langmuir2007,23, 5752-5756

10.1021/la063624c CCC: $37.00 © 2007 American Chemical SocietyPublished on Web 04/03/2007

Author's personal copy

Spectrochimica Acta Part A 68 (2007) 191–197

Photo-induced charge transfer in Prussian blue analoguesas detected by photoacoustic spectroscopy

E. Reguera a,b,∗, E. Marın a, A. Calderon a, J. Rodrıguez-Hernandez b

a Centro de Investigacion en Ciencia Aplicada y Tecnologıa Avanzada del IPN, Legaria 694,Col. Irrigacion, Mexico, D.F. C.P. 11500, Mexico

b Instituto de Ciencia y Tecnologıa de Materiales, Universidad de La Habana, Cuba

Received 26 September 2006; received in revised form 7 November 2006; accepted 17 November 2006

Abstract

The photo-induced charge transfer in four series of Prussian blue (PB) analogues was studied from photoacoustic spectra. In cobalticyanidesthe observed signals were assigned to a metal-to-ligand charge transfer, which appears as a shoulder below 450 nm, and to d–d transitions forCo(II), Ni(II) and Cu(II) complex salts. No evidence of metal-to-metal charge transfer was observed for this series, which is probably due to thehigh stability of low spin cobalt(III) in the hexacyanide complex. Photoacoustic spectra for ferricyanides are broad bands, which result particularlyintense up to 750 nm. Such features were attributed to the overlapping of contributions from metal-to-ligand (<600 nm) and metal-to-metal chargetransfer transitions, with probably also a minor contribution from d–d transitions in the outer metal. The spectra for the ferrocyanides series aredominated by the metal-to-ligand charge transfer band below 550 nm, approximately 100 nm above this transition in cobalticyanides. Within thestudied solids, the most intense and broad metal-to-metal charge transfer bands were found for a series of low spin Co(III) high spin Co(II)hexacyanoferrates(II,III) and with similar features also for ferric ferrocyanide (Prussian blue), assigned to Fe(II)→Co(III) and Fe(II)→Fe(III)photo-induced transition, respectively. The first of these transitions requires of more energetic photons to be observed, its maximum falls at 580 nmwhile for Prussian blue it is found at 670 nm. Prussian blue analogues are usually obtained as nanometric size particles and many of them have amicroporous structure. The role of surface atoms on the observed charge transfer bands in the studied series of compounds is also discussed.© 2006 Elsevier B.V. All rights reserved.

Keywords: Charge transfer; Molecular magnets; Photoacoustic; Photo-induced; Porous materials; Prussian blue

1. Introduction

Prussian blue (PB) analogues or hexacyametallates form afamily of coordination compounds that has received a notableattention as prototype of molecular materials in the last decade.The CN group is a strong bridge ligand that allows a pronouncedcharge overlapping between the metal centers linked at its C andN ends. From this fact, PB analogues show interesting prop-erties as molecular magnets, among them, high temperatureof magnetic ordering [1,2], pole-inversion magnets [3], spin-glass behavior [4] and photo-induced magnetism [5]. This lasteffect is related to the photo-induced charge transfer between themetal centers changing the available population of electrons withunpaired spins and, in consequence the material magnetic prop-erties. This is a typical metal-to-metal charge transfer (MM′CT)

∗ Corresponding author. Tel.: +52 57296000x67774.E-mail address: ereguera@yahoo.com (E. Reguera).

but with a relatively large stability for the excited state to attainmagnetic ordering below certain critical temperature (Tc). TheMM′CT transitions in PB analogues have also been studied forpotential applications of these compounds as electrochromicmaterials, e.g. smarts windows and electrochromic displays [6],since the charge transfer transition leads to a color change. TheMM′CT effect in hexacyanometallates is closely related to thenature of the CN ligand. It has a low energy �-antibonding orbitalat the C end which overlaps with the t2g orbitals of the innermetal (M′), providing a low energy route for the electron move-ment between the metal centers. The deep blue color of ferricferrocyanide (Prussian blue) has been ascribed to a MM′CT tran-sition among iron atoms [7]. The existence of that low energy�*-orbitals also allows the occurrence of metal-to-ligand chargetransfer (M′LCT) in this family of compounds. The opticalabsorption spectra of hexacyanometallates are mainly relatedto these two charge transfer mechanisms together with the pos-sibility of d–d transitions in the involved transition metals. Suchspectra are usually recorded by UV–vis spectroscopy.

1386-1425/$ – see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.saa.2006.11.013

INSTITUTE OF PHYSICS PUBLISHING JOURNAL OF PHYSICS: CONDENSED MATTER

J. Phys.: Condens. Matter 19 (2007) 056202 (11pp) doi:10.1088/0953-8984/19/5/056202

Magnetic interaction between manganese (2+) atomsthrough aquo bridges and bifurcated cyano groups

R Martınez-Garcıa1, L Reguera2, M Knobel3 and E Reguera1,4,5

1 Institute of Science and Technology of Materials, University of Havana, 10400 Havana, Cuba2 Faculty of Chemistry, University of Havana, San Lazaro and L, 10400 Havana, Cuba3 Institute of Physics ‘Gleb Wataghin’, UNICAMP, 13083-970 Campinas SP, Brazil4 Center for Applied Science and Advanced Technology of IPN, Mexico, DF, Mexico

E-mail: ereguera@yahoo.com

Received 7 November 2006, in final form 22 December 2006Published 15 January 2007Online at stacks.iop.org/JPhysCM/19/056202

AbstractThe magnetic interaction between adjacent manganese atoms through aquodouble bridges in Mn2[M(CN)6]·xH2O where x = 8 and 2 and M = Fe, Ru,Os, was studied. Through these bridges a relatively weak antiferromagneticinteraction is established with an estimated Curie–Weiss temperature, |θCW|,close to 4 K and a super exchange constant, |J |, of 0.27 cm−1. When thesematerials are dehydrated the antiferromagnetic interaction between the Mnatoms undergoes a dramatic increase, with estimated values for |θCW| and |J |of 61 K and 4.11 cm−1, respectively. Such reinforcement in the magneticinteraction is accompanied by a shift of 32 cm−1 for the ν (CN) vibrationtowards the low frequency region while for the iron compound the Mossbauerspectrum, initially a single line, becomes a quadrupole splitting doublet ofrelatively low isomer shift (δ) value. The Curie constant of the involved Mnatoms shows a negative correlation with the observed shifts in ν (CN) and δ

on dehydration. From the observed magnetic behaviour and the spectroscopicdata a double coordination of an N end of the CN ligand to two Mn atomsis proposed. Such strong magnetic interaction through the N atom of theCN ligand could be used as a prototypical bridge to obtain high Tc molecularmagnets.

S Supplementary data are available from stacks.iop.org/JPhysCM/19/056202

1. Introduction

The magnetic interaction between transition metal ions in cyano complexes, mainly Prussianblue (PB) analogues, has been an active interdisciplinary research area in the last few

5 Author to whom any correspondence should be addressed.

0953-8984/07/056202+11$30.00 © 2007 IOP Publishing Ltd Printed in the UK 1