Thermally Induced Two-Step, Two-Site Incomplete
6
A
1
T
2
T
2
Crossover
in a Mononuclear Iron(III) Phenolate-Pyridyl Schiff-Base Complex: A
Rare Crystallographic Observation of the Coexistence of Pure S )
5
/
2
and
1
/
2
Metal Centers in the Asymmetric Unit
Musa S. Shongwe,*
,†
Badria A. Al-Rashdi,
†
Harry Adams,
‡
Michael J. Morris,*
,‡
Masahiro Mikuriya,*
,§
and Giovanni R. Hearne
|
Department of Chemistry, College of Science, Sultan Qaboos UniVersity, P.O. Box 36, Al-Khod
123, Muscat, Sultanate of Oman, Department of Chemistry, The UniVersity of Sheffield,
Sheffield S3 7HF, United Kingdom, Department of Chemistry and Open Research Center for
Molecule-Based DeVices, School of Science and Technology, Kwansei Gakuin UniVersity,
2-1 Gakuen, Sanda 669-1337, Japan, and School of Physics, UniVersity of the Witwatersrand,
PriVate Bag 3, Wits 2050, Johannesburg, South Africa
Received March 1, 2007
The six-coordinate mononuclear iron(III) complexes [Fe(salpm)
2
]ClO
4
‚0.5EtOH, [Fe(salpm)
2
]Cl, [Fe{(3,5-
t
Bu
2
)-
salpm}
2
]X (X ) ClO
4
-
or Cl
-
), and [Fe{(3,5-
t
Bu
2
)-salpm}
2
]NO
3
‚2H
2
O [Hsalpm ) N-(pyridin-2-ylmethyl)-
salicylideneamine; H(3,5-
t
Bu
2
)-salpm ) N-(pyridin-2-ylmethyl)-3,5-di-tert-butylsalicylideneamine] have been synthesized
and isolated in crystalline form; their chemical identities have been ascertained by elemental analyses, FAB mass
spectrometry, and infrared spectroscopy. The room-temperature effective magnetic moments [(8
M
T)
1/2
∼ 5.85-
5.90 µ
B
] of these complexes are consistent with the high-spin (S )
5
/
2
) ground state. These complexes are intensely
colored on account of the strong p
π
f d
π
* LMCT visible absorptions. Definitive evidence for the structures of
[Fe(salpm)
2
]ClO
4
‚0.5EtOH and [Fe{(3,5-
t
Bu
2
)-salpm}
2
]NO
3
‚2H
2
O has been provided by single-crystal X-ray
crystallography. The monomeric complex cations in both compounds comprise two uninegative phenolate-pyridyl
tridentate Schiff-base ligands coordinated meridionally to the iron(III) to afford a distorted octahedral geometry with
a trans,cis,cis-[FeO
2
N
4
] core. Whereas [Fe(salpm)
2
]ClO
4
‚0.5EtOH undergoes a thermally induced
6
A
1
T
2
T
2
crossover,
[Fe{(3,5-
t
Bu
2
)-salpm}
2
]NO
3
‚2H
2
O retains its spin state in the solid state down to 5 K. However, EPR spectroscopy
reveals that the latter complex does exhibit a spin transformation in solution, albeit to a much lesser extent than
does the former. The spin crossover in [Fe(salpm)
2
]ClO
4
‚0.5EtOH has resulted in an unprecedented crystallographic
observation of the coexistence of high-spin and low-spin iron(III) complex cations in equal proportions around 100
K. At room temperature, the two crystallographically distinct ferric centers are both high spin; however, one [Fe-
(salpm)
2
]
+
complex cation undergoes a complete spin transition over the temperature range ∼200-100 K, whereas
the other converts very nearly completely between 100 and 65 K; ∼10% of the complex cations in [Fe(salpm)
2
]-
ClO
4
‚0.5EtOH remain in the high-spin state down to 5 K.
Introduction
Designing and synthesizing spin-crossover molecular
materials is an exciting field of vigorous research endeavors
driven largely by the recognition of the potential technolog-
ical applicability of these electronically bistable substances
as materials for information storage, molecular switches, and
visual displays in molecule-based electronics.
1
Over the
* To whom correspondence should be addressed. E-mail: musa@
squ.edu.om (M.S.S.), m.morris@sheffield.ac.uk (M.J.M.), junpei@
ksc.kwansei.ac.jp (M.M.).
†
Sultan Qaboos University.
‡
The University of Sheffield.
§
Kwansei Gakuin University.
|
University of the Witwatersrand.
(1) (a) Gu ¨tlich, P.; Hauser, A.; Spiering, H. Angew. Chem., Int. Ed. Engl.
1994, 33, 2024. (b) Real, J. A.; Andres, E.; Mun ˜oz, M. C.; Julve, M.;
Granier, T.; Bousseksou, A.; Varret, F. Science 1995, 268, 265. (c)
Kahn, O.; Martinez, C. J. Science 1998, 279, 44. (d) Gu ¨tlich, P.; Garcia,
Y.; Goodwin, H. A. Chem. Soc. ReV. 2000, 29, 418.
Inorg. Chem. 2007, 46, 9558-9568
9558 Inorganic Chemistry, Vol. 46, No. 23, 2007 10.1021/ic700397u CCC: $37.00 © 2007 American Chemical Society
Published on Web 10/05/2007