spin crossover behavior
Post on 17-Feb-2018
223 Views
Preview:
TRANSCRIPT
-
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
top related