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11. TITLE (Include Security Classification) UNCLASSIFIED: Aplicacion Del Modelo De TraslapeAngular A Ftalocinaninas De Lantanidos (Application of the Angular OverlapModel to Lanthanide Phthalocyanines)12. PERSONAL AUTHOR(S)

Juan Padilla and William E. Hatfield13a. TYPE OF REPORT 113b. TIME COVERED 14. DATE OF REPORT (Year, Month, Day) 15. PAGE COUNTTechnical Report FROM TO__ July 15, 1989 1 14

16. SUPPLEMENTARY NOTATION Technical Report No. 35: The paper was published inSpanish; the unpublished English translation is also included.

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FIELD GROUP SUB-GROUP angular overlap model lanthanide phthalocyaninespraseodymium neodymium holmium erbiummagnetic susceptibility

19 -BSTRACT (Continue on reverse if necessary and identify by block number)-.).The magnetic properties of praseodymium, neodymium, gadolinium, holmium,erbium, and lutetium phthalocyanines are described according to magneticsusceptibility measurements measured in the 80-300 K or 4.2-300K range.Sigma and pi interactions between the pyrrolic nitrogen ligand and the f-orbitals of praseodymium, neodymium, holmium, and erbium were obtained usinthe angular overlap model (AOM). The calculations included the completeground-state manifold of the respective lanthanide ions. The position ofthe pyrrolic nitrogen ligand in the metallic two dimensional spectrochemicaseries is presented. The results show that the pyrrolic nitrogen is a weaksigma donor and a moderate pi donor. (

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Aplicacion Del Medelo De Traslape Angular ABy

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APLICACION DEL MODELO DE TRASLAPE ANGULAR

A FTALOCINANINAS DE LANTANIDOS

Juan Padilla,* (1) y William E. Hatfield (2).

(1) Departamento de Quimica, Universidad Aut6noma Metropolitana,Iztapalapa. A. P. 55-534, Mdxico, D.F. 09340

(2) Department of Chemistry, University of North Carolina atChapel Hill, NC 27599-3290.

Introducc ion.

El modelo de traslape angular (MTA) (1-31 permite obtener en

forma cuantitativa la magnitud relativa y el signo de las

interacciones o- y w de ligantes con iones metilicos. Esto trae a

la teoria del campo ligante el concepto de grupos funcionales.

En este trabajo se us6 el enfoque del MTA desarrollado por

Gerloch et. al.[2]

Siendo el MTA un modelo param~trico, para poder calcular los

valores de los pargmetros se requirere ajustarlos a los datos

experimentales. Un metodo experimental de obtener las

interacci6nes a- y w de los ligantes con metales es la

espectroscopia de absorcio'n (UV-Vis-IR). Las interacci6nes (3,y W

se pueden obtener a partir de las energias de transici6n e

interpretarlas usando el MTA. Sin embargo, en el caso de las

ftalocianinas de los, lantgnidos, tales estudios se dificulian por

dos razones. Primero, proque las transiciones f-f son prohi-

bidas y par de tanto de intensidad d~bil. Segundo, porque las

bandas w --> ;; del macrociclo de la ftalocianina son intensas y

obscurecen las bandas f-f mas d~biles, las cuales se requireren

para el an'alisis. Este obsta-culo se puede vencer con ayuda de

mediciones de susceptibilidad magn6tica.

2

En este trabajo, se determinaron los parirnetros del MTA a

partir de los datos de susceptibilidad magn~tica (80-300 K) para

los compuestos de ftalocianinas de praseodimio, neodimic, holmio

y erbio. Tambi~n se determin6 la posici6n de los comeuestos en

la serie espectroquimica met~lica bidimensional.

Resultados y Discusi6n.

Las ftalocianinas de los lantinidos H(Pc)Ln(PC) (Pc -

dianion ftalocianinato, Ln = Pr, Nd, Gd, Ho y Er) se prepararon

por una modificaci6n de la sintesis descrita en las literatura

[4]. Los productos se caracterizaron con ayuda de los espectros

de IR, UV-Vis y RPE.

Usando el algoritmo SIMPLEX [5], se ajustaron los datos de

susceptibilidad magn6tica a la ecuacio'n de Curie-Weiss resultando

los para~metros de la Tabla 1.

Table 1. Parametros de Curie-Weiss para ftalocianinas lantanoides

Compuesto C (uem K mol 9 (K)

H(Pc)Pr(Pc) 1.90 -110H(Pc)Nd(Pc) 1.17 -61H(Pc)Gd(Pc) 6.61 6H(Pc)Ho(Pc) 13.0 0H(Pc)Er(Pc) 8.83 -4

Las constantes de Weiss, 9, para las ftalocianinas de

praseodimio y neodimio y neodiinio son mas grandes que las

observadas en las otras ftalocianinas. Hay dos explicaciones

posibles para racionalizar las diferencias; ya sea que los iones

3

1antinido muestran feno'menos cooperativos, o bien el

paramagnetismo independiente de la temperature (PIT) es

considerable. El signo negativo para 9 implica acoplaniiento de

intercambio antiferromagn6tico; sin embargo, en tales casos se ha

observado que la temperatura de Neel, T N, es aproximadamente

igual al negativo de la constante de Weiss (9 = -T N) [6]. Eso

quiere decir que el ordenamiento magnetico deberia observarse

alrededor de 110 y 61 K para las ftalocianinas de praseodimio y

neodimia, respectivamente. Sin embargo, las mediciones de

susceptibilidad magn6tica hasta 4.2 K no dan alguna indicacio'n de

acoplamiento de intercambio, y eso descarta la primera

posibilidad.

Por otra parte, el PIT proviene de la mezcla en el estado

fundamental de estados excitados energ~ticamente bajos que no

esta'n poblados t~rmicamente [7]. De los cinco io~nes lanta'nido

considerados en este estudio, praseodimia y neodimio son

precisamente los que tienen el primer estado excitado de energia

mis baja, y de gui que sean los tinicos que muestram PIT

apreciable.

Para hacer los ca"lculos usando el MTA, se diagonaliz6 el

multiplete completo del estado fundmental usando una base del

ion-libre bajo un potencial de campo ligante, y el c~lculo

posterior de las susceptibilidades magne'ticas se hizo de acuerdo

a la teoria de perturbaciones usando la ecuacio'n de Van Vleckc.

Es importante recordar que a pesar de que los efectos del campo

ligante en lantinidos representan solo una perturbaci6n peque'na

desde el punto de vista de la espectro copia electr6nica, ellos

4

son la esencia de sus propiedades magneticas. Los mejores

ajustes de los parimetros se enlistan en la Tabla 2.

Tabla 2. Parimetros del MTA en ntimeros de onda

Parametro H(Pc)Pr(Pc) H(Pc)Nd(Pc) H(Pc)Ho(Pc) H(Pc)Er(PC)

ez117 126 153 155

e 33 41 49 72

e 0.28 0.33 0.32 0.46

% Error 2.8 2.0 0.8 3.2

No fu4 posible obtener buenos ajustes cuando el para~metro

mixto e,, se conserv6 constante e igual a cero. Este no es un

resultado inesperado ya que generalmente se observa en compuestos

quelato [8]. Dicho para'metro representa el grado de mezcla entre

los orbitales c- y w en los itomos de nitr6geno, puesto que en las

ftalocianinas de los lantinidos el orbital o- del nitr6geno no

estAi orientado en la posici6n que favorece el miximo traslape

orbital. Esto se conoce como ha contribucio'n de ha valencia mal

dirigida (91. La contribuci6n se tomi6 en cuenta affadiendo los

t6rminos extra Y, 1~ 1 en ha expansi6n de los arm6nicos

esf~ricos. Los mismos parametros mejor ajustados se obtuvieron

independientemente usando el algoritmo de optimizacio~n GRADX

(10].

Los resultados de este trabajo muestran ha valid6z del MTA

aplicado a ftalocianinas de lantanidos. Los paraimetros del MTA

ref lejan ha naturaleza del enlace metal-ligante en el complejo.

5

El signo positivo de e a y e wy indican las propiedades donadoras

del enlace o- y W, respectivamenta del Atomo de nitro-geno, y l~a

magnitud indica la fortaleza del enlace Ln-N [2]. Los valores de

e 0- y e., son pequehos comparados con los que usualmente se

obtiene para compuestos de metales de transici6n. Esto es un

reflejo d~el efecto de apantallamiento que occure en l~a serie de

los lantinidos, asi coma de l~a covalencia d~bil del enlace Ln-N

las propiedades donadoras del Atomo de nitro'geno pirr6lico se

pueden explicar considerando que en el anillo pirrolico la

deslocalizacio'n en el grupo imina es peque? a, y el sistema no se

altera mucho debido a la donaci6n W.

A partir de lus ajustes se encontr6- que ey es mayor al

valor ideal 0.25 e 0 que se basa en l~a relaci6n de integrales de

traslape o- y w de funciones de ondo s y p puras en los Atomos de

nitr6geno, par 1o que se puede concluir que los atomos de

nitr6geno pirr6licos se compartan coma donadores a- d6biles y

donadores j moderadas.

Bibliografia

1. Schaffer, C. E.; Jorgensen, C. K. Mo.. Phys., 1965, 9, 401.

2. Gerloch, m. Magnetism and Ligand-Field Analysis; Cambridge

University; Cambridge, 1983.

3. Burdett, J. K. Adv. Inorg. Chem. Radiochem., 1987, 21, 113.

4. Kirin, I. S.; Moskalev, P. N.; Makashev, Y. A. Russ. J. of

Inorg. Chem.. (Traduccion al ingles), 1965, 10, 1065.

5. Nelder, J. A.; Mead, R. Computer Journal, 1965, 7, 308.

6

6. Smart, J. S. Effective Field Theories of Magnetism; Saunders:

Philadelphia, 1966.

7. Carlin, R. L. Manetochemistry; Springer-Verlag: Berlin 1977.

8. Deeth, R. J.; Duer, M. J.; Gerloch, M. Inorg. Chem., 1987,

26, 2573.

9. idem. 1987, 26, 2578.

10. Goldfeld, S. M.; Quandt, R. E. Nonlinear Methods in Econome-

trics; North Holland: Amsterdam, 1972.

APLICATION OF THE ANGULAR OVERLAP MODEL TO LANTHANIDE

PHTHALOCYANINES.

Juan Padilla* (1) and William E. Hatfield (2).

(1) Departamento de Quimica, Universidad Autonoma MetropolitanaIztapalapa, A.P. 55-534, Mexico, D.F. 09340.(2) Department of Chemistry, University of North Carolina,Chapel Hill, N.C. 27599-3290.

Abstract.

The magnetic properties of praseodymium, neodymium, gadolinium,holmium, erbium, and lutetium phthalocyanines are describedaccording to magnetic susceptibility measurements measured in the80-300 K or 4.2-300 K range. Sigma and pi interactions betweenthe pyrrolic nitrogen ligand and the f-orbitals of praseodymium,neodymium, holmium, and erbium were obtained using the angularoverlap model (AOM). The calculations included the completeground-state manifold of the respective lanthanide ions. Theposition of the pyrrolic nitrogen ligand in the metallic two-dimensional spectrochemical series is presented. The results showthat the pyrrolic nitrogen is a weak sigma donor and a moderatepi donor.

Introduction.

The angular overlap model (AOM) (1,2] allows to obtain quantita-tively the relative magnitude and the sign of the sigma and piinteractions of ligands with metal ions. This brings to ligand-field theory the concept of functional groups.

Being the AOM a parametric model, in order to calculate theparameters it is required to adjust them to the experimentalvalues. Absorption spectroscopy (UV-VIS-IR) provides an experi-mental tool to obtain the sigma and pi interactions betweenligands and metals. They can be calculated from the transitionenergies, and the interpretation accomplished using the AOM.However, in the case of lanthanide phthalocyanines, such studiesare difficult for two reasons. First, the f-f transitions areparity forbidden and therefore of weak intensity. Secondly, thepi ---> pi* bands of the macrocycle phthalocyanine are intense,and obscure the weaker f-f bands which are required for theanalysis. This obstacle may be overcome with help of magneticsusceptibility measurements.

In this work, magnetic susceptibility measurements (80-300 Kor 4.2-300 K) were used to determine the AOM parameters ofpraseodymium, neodymium, holmium, and erbium phthalocyanines.Also, the position of the compounds in the two-dimensionalspectrochemical series was determined.

2

Results and Discussion.

The lanthanide phthalocyanines H(Pc)Ln(Pc) [Pc = dianion phthalo-cyaninato, Ln = Pr, Nd, Gd, Ho, Er, and Lu] were prepared by amodification of the synthesis described in the literature(3]. The products were characterized with help of the IR, UV-VIS,and EPR spectra.

Using the algorithm SIMPLEX (4], the magnetic susceptibilitydata were fitted to a Curie-Weiss equation resulting in theparameters shown in Table 1.

Table 1. Curie-Weiss parameters for lanthanide phthalocyanines.

Compound C (emu K mole-i) e (K)

H(Pc)Pr(Pc) 1.90 - 110H(Pc)Nd(Pc) 1.17 - 61H(Pc)Gd(Pc) 6.61 6H(Pc)Ho(Pc) 13.0 0H(Pc)Er(Pc) 8.83 - 4

The Weiss constants, 0, for praseodymium and neodymiumphthalocyanines are larger than those observed in the otherphthalocyanines. There are two possible explanations to rational-ize the differences; either the lanthanide ions show cooperativephenomena, or the temperature independent paramagnetism (TIP) isconsiderable. The negative sign for 9 implies antiferromagneticexchange coupling; however, in such cases it has been observedthat the Neel temperature TN is approximately equal to thenegative of the Weiss constant (9 = - TN) [5]. That means themagnetic ordering should be observed at about 110 and 61 K forpraseodymium and neodymium phthalocyanines, respectively. Magne-tic susceptibility measurements collected down to 4.2 K howevershow no indication of exchange coupling, ruling out the formerpossibility.

On the other side, the TIP arises from the mixture into theground state of thermally non populated, yet low lying excitedstates [6]. Of the five lanthanide ions considered in this study,praseodymium and neodymium are precisely the ones with the lowestlying excited-states, and therefore the only ones that shouldshow appreciable temperature independent paramagnetism.

Calculations using the AOM were carried out as follows. Thecalculations involve the diagonalization of the complete ground-state manifold of the free-ion basis under the ligand-fieldpotential, and a subsequent computation of magnetic susceptibili-ties by perturbation theory within the Van Vleck equation. It isimportant to remember that even ligand-field effects in lantha-nide compounds represent only small perturbations from the pointof view of electronic spectroscopy, they are the essence of theirmagnetic behavior.

The best-fit parameters are listed in Table 2.

3

Table 2. AOM best-fit parameters in wavenumbers.

Parameter H(Pc)Pr(Pc) H(Pc)Nd(Pc) H(Pc)Ho(Pc) H(Pc)Er(Pc)

e. 117 126 153 155evy 33 41 49 72e., 10 10 12 2ey/e. 0.28 0.33 0.32 0.46% Error 2.8 2.0 0.8 3.2

It was not possible to get good fittings when the cross-termparameter, e7[a , was set equal to zero, and kept constant duringthe fittings. This is not an unexpected result, since it isgenerally found in chelate compounds [7]. The cross-term para-meter represents the degree of admixture between the sigma- andpi-orbitals in the nitrogen atoms, since in the lanthanidephthalocyanines the nitrogen sigma-orbital is not oriented in theposition that favors the maximum orbital overlap. This is theso-called misdirected valency contribution to the ligand-fieldpotential (8]. The cross-term contribution was taken into accountby adding the extra terms Y21, Y4I, and Y61 in the ligand-fieldspherical harmonics expansion. The same best-fit parameters wereobtained independently with the optimization algorithm GRADX [9].

The results of this work show the validity of the AOM asapplied to lanthanide phthalocyanines. The AOM parameters reflectthe nature of the metal-ligand bond in the complex. The positivesigns of e. and e. y indicate the sigma- and pi-donorproperties of the nitrogen atom, respectively, and the magnitudeindicates the strength of the Ln-N bond (2]. The values of e andeny are small compared to the values usually obtained fortransition metal ions. This is a reflection of the screeningeffect that occurs in the lanthanide series, and of the weakcovalency of the Ln-N bond. The donor properties of the pyrrolicnitrogen atom may be explained considering the small delocaliza-tion in the imine group, and little disruption is caused bypi-donation.

From the fittings, it was found e, y to be greater than theideal value of 0.25 ea based on the ratio of sigma- and pi-over-lap integrals of pure s- and p-wavefunctions on the nitrogenatoms. Therefore, it can be concluded that the pyrrolic nitrogenatoms behave as weak sigma-donors, and moderate pi-donors.

Acknowlgements.

This work was done at the University of North Carolina at ChapelHill and was supported in part by the Office of Naval Research.One of us (J.P.) thanks the Consejo Nacional de Ciencia yTecnologia of Mexico for a partial fellowship.

BibliograDhv.

[1] Schaffer, C.E.; Jorgensen, C.K. Mo PhyL.a1965, 2, 401.[2] Gerloch, M. Magnetism and Ligand-Field Analysis; Cambridge

4

University: Cambridge, 1983.[3] Kirin, I.S.; Moskalev, P.N.; Makashev, Y.A. Russ. J. of

Inorg. Chem.; (English translation) 1965, i0, 1065.[4j Nelder, J.A.; Mead, R. Computer Journal 1965, 7, 308.(5] Smart, J.S. Effective Field Theories of Magnetism; Saunders:

Philadelphia, 1966.(63 Carlin, R.L. Ma~netochemistrv; Springer-Verlag: Berlin, 1977.(7 Deeth, R.J.; Duer, M.J.; Gerloch, M. Inorg. Chem., 1987, 26,

2573.[8) idem. 1987, 2&, 2578.(9] Goldfeld, S.M.; Quandt, R.E. Nonlinear Methods in Econo-

metrics; North Holland: Amsterdam, 1972.

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