7/23/2019 Spin Crossover Behavior

1/8

Inorg. Chem.

1994,33, 3587-3594

3587

Spin-Crossover Behavior in the Fe( tap)z( N C S)2 dH 3C N System (tap =

1,4,5,8-Tetraazaphenanthrene; n

=

1, '/2). Crystal Structures and Magnetic Properties of Both

Solvates

J d

ntonio Ral,*J.

M.

Carmen

M ~ i i o z , ~ ~

nrique Andrb, Thierry Granier,*Jcand

Bernard GalloislC

Departament de Qulmica Inorganica, Universitat de Valtncia, Dr. Moliner

50,

46 100 Burjassot (Valtncia), Spain, Departamento de Rsic a Aplicada, U niversidad Poli thnica de

Valencia, Camino de Vera s/n,

46071

Valencia, Spain , and L aboratoire de Cristallographie et de

Physique Cristalline, Universitd Bordeaux I,

35 1

Cours de la Liberation,

33405

Talence, France

Received February

9,

1994

The synthesis, structure, and magnetic characterization of two solvates of bis( 1,4,5&tetraazaphenanthrene)bis-

(thiocyanato)iron(II), [Fe(tap)2(NCS)z].nCH3CN with n

=

1 (solvate A) and n =

/z

(solvate B) ar e reported. A

shows a continuous high-spin-low-spin conversion over the temperature ran geca . 1 10-280 K, while B is paramagnetic

over the temperature range 4.2-290 K. The X-ray structure for A was solved at 290 and 135 K. It crystallizes

in the triclinic space group

Pi

with

2

=

2

at both temperatures. The lattice constants are

u

=

8.920(3)

A,

b

=

9.372(3)

A, c =

16.838(4)

A, =

96.32(2) ,

B =

100.47(3)O,

y =

112.14(2) , and V = 1257.3 A a t 290 K and

u

=

8.742(2)

A, b =

9.265(2) A, c

=

16.535(3)

A, =

96.56(2)', j3

=

100.15(3) ,

y =

112.43(3)O,

and

V = 1194.07

A3 t 135 K. The data were refined to

R =

5.67 (290 K) and 7.57% (135 K).

B

crystallizes in the monoclinic space

group

C2 / c ,

with u = 22.636(4) A,

b

= 16.810(3) A,

c

= 18.528(3)

A,

0 = 138.55 (3) , and V = 4666,90 A3 a t

290 K. The final reliability factor was

R

= 5.93%. Molec ular structures for both solvates are very similar at room

temper ature where iron(I1) lies in a distorted octahedron with NC S- liga nds in the cis position. The most significant

structural features which could account for the different magnetic behavior of A and B are found to be the metal-

to-ligand bond distance s and trigona l distortion. Struc tural modifications associated with the spin change in A

mainly consist of a large reorganization of the metal environment: the F e N ( ta p ) and Fe-N(CS) distances decrease

by 0.23 (mean value) and 0.12 (mean value) A, respectively, when the tem perature is lowered from 290 to 135 K,

and a m ore regular sha pe of the [FeN 6] octahed ron is achieved through a modification of the trigona l deformation

from 8 to 3' along with a remarkable variation of the N - F e N angles. The gradual temperature dependence of

XMTfor A was considered as a Boltzmann distribution of molecules in the low-spin ground state and in the therma lly

accessible high-spin excited state reflecting the 'Al - T2 spin equilibrium . The entha lpy and entropy changes

associated with the spin equilibrium were estimated as

AH

=

15.5

kJ mol-' and

A S

=

92

J

mol-'

K-1.

Analysis

of the magnetic data versus temperature for B by using the zero- field-splitting spin Ham iltonian for S =

2

leads

to D =

7.4

cm-1 and g =

2.09.

Introduction

The spin-crossover phenomenon require s the ligand field

strength to

be

of the same order of magnitud e as the mean electron-

pairing energy. Then, high-spin (hs) and low-spin (1s) forms

may interconvert, their proportion varying with temperature?

pressure,3 and electroma gnetic radiation? In the 1s state, the eB

orbitals, which have an antibonding character, are depopulated

and the hs- s crossover results in a shortening of metal-ligand

bond lengths.5

Two closely related aspects are to

be

taken into account to

unders tand the spin-crossover mechan ism: (i) how the spin change

occurs at a molecular level and (ii) how this change spreads in

the solid to result in th e diffe rent kinds of spin conversions. From

a molecular point of view, due to the absence of cooperativeeffects,

the determination of the factors that control the rate and

Abstract publiihcd in Advance ACS Abstracts. June 15, 1994.

(1) (a) Universitat de Valtncia. (b) Universidad Polit&tica de Valtncia.

(c) Universitt de Bordeaux

I.

(2) GQtlich,P.

Struct.

Bodin g (Berlin)

1981, 44,

83.

3)

(a)

Adam, D. M.; Long,G. J.; Williams, A . D. Inorg. Chem. 1982,21,

1049. (b) Pebler, J. Inorg. Chem. 1982, 22, 4135. (c)

Usha, S.;

Srinivasan, R.; Rao,

C.

N. R. Chem. Phys. 1985,100,447.

(4) GQtlich,

P.; Hauser,

A .

Coord.

Chem. Rev. 1990, 97, 1 ,

(5) Kbnig,

E.

Prog. Inorg. Chem. 1987, 35, 527.

0020-1669/94/ 1333-3587%04.50/0

mechanism of the spin-state interconversion in an isolated spin-

crossover complex was at the origin of the spin conversion studies

in solution, which are based on the observation of the relaxation

of theperturbedequilibrium.6In thesolid state, the tem perature-

depen dent spin conversion is coope rative n natu re, involving long-

range interactions among the changing spin state metal complex

molecules themselves as well as the latt er and the la ttice. Thus,

spin conversion rates depend on su btle solid-state effects induced

by noncoordinating c ounterion s, noncoordinating solvent mol-

ecules, preparativ e methods, or ligand substitution. Henc e, such

factors can affect drastically he shape of the temperature variable

order parameter (usually the high-spin molar fraction) and the

critical temperature Tc at which the

50%

of conversion takes

place. So, it is possible to get spin-crossover transform ations

which occur abruptly in a narrow temperature range (less than

10 K) and others which occur very gradually.' In the latter case,

each m olecule in the crysta l acts independently of its neighbors

and there is a simple Boltzmann distribution of the high- and

low-spin forms. Som e papers dealing with the understandin g of

~~~~~~ ~

(6) (a) Beattie,

J.

K. Adu. Inorg. Chem. 1988, 32, 2. (b) Toftlund, H.

Coord. Chem. Rev. 1989,94,67.

( c )

Konig, E.

Struct .

Bodin g (Berlin)

1991,

76, 51.

(7) Kanig,

E.;

Ritter, G.; Kulshrestha,

S.

K.

Chem. Reu. 1985,

85, 219.

1994

American C hemical Society

7/23/2019 Spin Crossover Behavior

2/8

3588

Inorganic

Chemistry, Vol. 33 No.

16 1994

the

molecular mechanism

involved in fast

h s-

s

interconversion

leading

to

spin equi l ibr ium

were reported

recently.*

T h e

most abrupt

spin

conversions

reported up

t o now

are

those

exhibited by

a

number

of

iron(I1) complexes.2

Among these

systems, [Fe(phen)2(NCS)2] (phen

1

, lo-phenanthrol ine) , which

undergoes

an

abrupt hs

- s

t rans i t ion

a t a

t em pera tu r e

T ,

= 176

K, has cer ta inly been one of t he most investigated iron(I1) spin

t ransi t ion complexes .

Only

very

recently,

i t ha s

been

possible to

ca r ry

out

on this

system

a single-crystal

X-ray

s t ruc tu r a l

studyg

a n d

a

t he rm a l

expansion

investigation10

as

well as t h e first X-ray

single-crys ta l s t ructu re determina t ion

as

a function

of

pressure"

a t

room

t em pera tu r e

for

a spin-crossover ystem.

All theses tudies

a im ed

at

finding

the factors

which

determine

th e spin in tercon-

version mechanism.

T h e

present

w o r k was unde r t aken m a in ly

to

s tudy

the influence

of a

modification

of

t he phen l i gand

on the

spin t ransi t ion

of

[Fe(phen)z(NCS)2] .

In

th is

respect,

it deserves to be noted

t h a t

although t h e

effects

of

replacing hydrogen atoms

by electron-

donating, electron-withdrawing,

or

s imply bulky l igands

in

phen

were

widely studied

a long

t ime ago,2 the subst i tu t ion

of

two C-H

groups

of

phen

by

two addi t ional

more

elect ronegat ive ni t rogen

atoms was

unknown. Along this

line,

w e

report

here he synthesis ,

magnet ic character izat ion,

and

s t ructu ra l invest igat ion

of

t h e

system [Fe(tap)2(NCS)2]-nCH3CN,

h e r e t a p

is

t h e

1,4,5,8-

t e t r aazaphenan th rene ( s ee

scheme I) and n

=

1

( A )

and 0.5

(B).

Real

e t

al.

A is a

spin-crossover

system whereas B

is

a

high-spin

one.

A s

t h e

crystal structures

of

both solvates

have been solved,

th is work

is a

good oppor tuni ty

to

ana lyze

and

discuss th e re la t ionships

between

s t ructura l fac tors

on the Occurrence of

spin-crossover.

Experimental Section

Materials.

[

Fe(py)o(NCS)z] (py

=

pyridine) was prepared according

to the method described by Erickson and S utin,Iz the hydrated iron(I1)

perchlorate s alt being replaced by the hy drated iron(I1) sulfate one. Ta p

ligand was purchased from commercial sources and used without fu rther

purification.

Complex Preparation.

[Fe(tap)2(NCS)z]*nCH3CNwas synthesized

under argon atmosph ere as follows: previously deoxygenated acetonitrile

solutions of [Fe(py)d(NCS)z] (0.27

mmol,

40 mL) and tap

(0.55

mmol,

10 mL) were mixed under stirring at room temperature. Polyhedral

( n

=

1, A) and prismatic

( n

=

0.5, B)

dark single crystals were obtained

by slow evaporation of the purple solution 2 weeks lat er. They were dried

under argon atmosph ere and were used for X-ray diffraction and magnetic

studies.

Magnetic Susceptibility Measurements. They were performed on

crystalline sam ples weighing 5.48 and 4.59 mg for A and B, respectively,

over the temperatu re range 295-4.5 K, by using a Faraday-typ e cryostat.

The independence

of

susceptibility on the applied magnetic field was

(8) (a) Chang, H.; McCusker,

J.

K.; Toftlund, H.; Wilson, S. .; Trautwein,

A.

X.; Winkler, H.; Hendrickson, D. N.

J . Am. Chem.

Soc. 1990,112,

6814. (b) Oshio, H.; Toriumi, K.; Maeda , Y.; Takash ima, Y.

Inorg.

Chem.

1991.30, 4252. (c) Conti,

J. A.;

Chadha, R. K.; Sena, K. M.;

Rheingold,A.

L.;

Hendrickson,

D.

N.

Inorg. Chem.

1993,32,2670. (d)

Conti,

J. A.;

Kaji,

K.;

Nagano,

Y.;

ena, K. M.; Yumoto, Y.; Chadha,

R. K.; Rheingold,

A. L.;

Sorai, M.; Hendrickson,

D.

N.

Inorg. Chem.

1993, 32, 2681.

(9)

Gallois,

B.;

Real,

J. A.;

Hauw, C.; Zarembowitch,

J. Inorg. Chem.

1990,

20,

1152.

(10) Real,

J. A.;

Gallois, B.; G ranier, T.; Sue z-Panaml,

F.;

Zarembowitch,

J.

Inorg. Chem.

1992, 31, 4972.

(1 1)

Gran ier,T.; Gallois , B.; Gau ltier, J.;Real, J.

A.;

Zarembowitch,J.

Inorg.

Chem.

1993,32,

5305.

(12) Erickson, N. E.; Sutin,

N . Inorg. Chem.

1966,5, 1834.

Table

1.

Crystallographic Data for Fe(tap)z(NCS)z.CH,CN

(C24HlsN11SzFe;M 576.85)

290 K 135

K

space group

k u K m

A

p0br g

cm3

p , cm-1

R

Rw

R = Z [ I F o I - IFCll/CIFd.

8.742(2)

.920(3)

9.372(3) 9.265 (2)

16.838(4) 16.535(3)

96.32(2) 96.56(2)

100.47(2) 100.15(3)

112.14(2) 112.14(2)

1257.30 1194.07

2 2

Pi Pi

1.5418

1.525 1.605

32.60 32.60

0.0567

0.0757

0.0583 0.0790

Rw = Zw / [IFoI

-

IF~~I/CW'/~IF~I.

Table 2.

Crystallographic Data for Fe(tap)z(NCS)zJ/2CH,CN

space group

C2/c

T =

290 K

p o h = 1.585 g cm-3

p

= 34.87 cm-l

Ra

= 0.0593

chemical formula C Z ~ H ~ ~ . ~ N I O . ~ S Z F ~

a =

22.636(4)

A

c

= 18.528(3)

A

8 = 138.55(3)'

V = 4666.90

A3

Z = 8

RWa

0.0626

MI

556.35

b =

16.810(3)

A ~~

.5418

A

I? =

C[lFol-

lFJJ/ElFd.w = EW'/2[lFcJ

-

Fc]]/Zw / lFd,

checked for each compound at room temperature. Mercury tetrakis-

(thiocyanato)cobaltate(II)

was used as a susceptibility standard. Dia-

magnetic correction^^^ were estimated to be -332

X

1o d and -321

X

10-6

cm3

mol-1 for A and

B,

respectively. The tempera ture was varied at a

rate of

1

K m i d .

Solution and Refinement

of

the X-ray Structures.

Preliminary X-ray

investigations have been performed by usual photographic methods.

Concerning crystal solvate A, low-temperature X-ray diffraction experi-

ments were cond ucted by cooling the sa mpl e with a cold nitrogen gas flow

surrounded by a jacket of dry nitrogen ga s at room temperature , which

prevents frost from growing around th e sample. Data collections were

carried out on an Enraf-N onius CAD 4 diffractometer with monochro-

matizedCuKaradiation. Crystalsizeswere0.10 XO.10 X0.30and0.20

X

0.12

X

0.30

mm

for

A

and B, respectively. Details concerning crystal

data, data collection characteristics, and structure refinement are

summ arized in Tables 1 and 2. Lattice para meters were obtained from

least squares refinement of the setting angles of 25 reflections in the

range

15 < 0