Direct two-step spin-crossover through [HS-HS]...[LS-LS] at the plateau in dinuclear diiron(II) complex [{Fe(NCBH3)(4phpy)} 2(μ-bpypz)2]

Article abstract of DOI:10.1039/b411190g

The variable temperature magnetic susceptibility, X-ray crystallography, and JR and Raman spectra of a new dinuclear complex [{Fe^II(NCBH ₃)(4-phpy)}₂(μ-bpypz)₂] demonstrated the first two-step spin-crossover associated with a 1 : 1 mixture of high-spin pair [HS-HS] and low-spin pair [LS-LS] at the plateau.

Full text of DOI:10.1039/b411190g

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…
Direct two-step spin-crossover through [HS–HS] [LS–LS] at the
plateau in dinuclear diiron(II) complex [{Fe(NCBH3)(4phpy)}2(m-
bpypz)2]{
Keisaku Nakano,a Satoshi Kawata,a Ko Yoneda,a Akira Fuyuhiro,a Takashi Yagi,a Saburo Nasu,b
Syotaro Morimotob and Sumio Kaizaki*a
a Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043,
Japan. E-mail: kaizaki@chem.sci.osaka-u.ac.jp; Fax: 181-6-6850-5408
b Department of Physical Science, Graduate School of Engineering Science, Osaka University,
Toyonaka, Osaka 560-0043, Japan
Received (in Cambridge, UK) 22nd July 2004, Accepted 13th September 2004
First published as an Advance Article on the web 9th November 2004
The variable temperature magnetic susceptibility, X-ray
crystallography, and IR and Raman spectra of a new
dinuclear complex
[{FeII(NCBH3)(4-phpy)}2(m-bpypz)2]
demonstrated the first two-step spin-crossover associated with
a 1 : 1 mixture of high-spin pair [HS–HS] and low-spin pair
[LS–LS] at the plateau.
Spin-crossover (SCO) metal complexes exhibit intriguing molecular
bistability driven by external stimuli. Among them, SCO in Fe(II)
complexes with a d6 electron configuration are expected to lead to
promising functional molecular devices such as molecular memory
or switches.1 Integration of an intramolecular magnetic interaction
and SCO in polynuclear Fe(II) complexes could provide auxiliary
functionality, whereas much attention has been paid to two-step
SCO in mono- or dinuclear Fe(II) complexes where cooperativity is
associated with inter- and intramolecular (short- and long-range)
interactions.2,3 However, only a limited number of two-step SCO
dinuclear complexes are known. There have been extensive studies
of a family of bpym (2,2’-bipyrimidine) bridged complexes,4–6
following the latest report on the phdia (4,7-phenanthroline-5,
6-diamine) bridged complex.7 These complexes exhibit two-step
SCO through the mixed spin intermediate [HS–LS] in the plateau
which was directly monitored unambiguously by Mo¨ssbauer
spectroscopy.5c,7 However, there were no X-ray analyses for
both two-step SCO complexes, until the first X-ray structure of the
SCO dinuclear diiron(II) complex was very recently reported for
[{Fe(NCSe)(m-pzpy)(pzpyH)}2] (pzpyH ~ 2-pyrazolylpyridine) by
Murray et al.8 Shortly afterwards, we studied the X-ray structure of
[{FeII(NCBH3)(py)}2(m-bpypz)2] (Hbpypz ~ 3,5-bis(2-pyridyl)-
pyrazole),9 which was followed by the direct light induced SCO
in terms of Raman spectra.10 The bpypz bridged dinuclear complex
with a replaceable axial pyridine is expected to lead to two-step
SCO processes by controlling the intermolecular interactions.
[{FeII(NCBH3)(4phpy)}2(m-bpypz)2] (4phpy ~ 4-phenylpyri-
dine) (complex 1) was prepared by a similar method to the
corresponding pyridine complex.{ The variable temperature
behavior of a product of magnetic susceptibility xm and T shows
the overall SCO from the quintet (xm?T ~ ca. 3.3 emu mol21 K)
fully high spin [HS–HS] to the singlet (0.2–0.1 emu mol21 K)
fully low spin [LS–LS] state, associated with the plateau of ca.
1.5 emu mol21 K at 200 K (Fig. 1), showing the two-step SCO.
The differential scanning calorimetry gives two exo- and
endothermic peaks, respectively, in the cooling and warming
modes (Fig. 1(inset)). The temperatures at the maxima are fairly
close to those estimated from the magnetic susceptibility measure-
ments, supporting the two-step SCO. The Mo¨ssbauer spectra
confirm that the sample consists of a single phase structure
Fig. 1 xmT vs. T for complex 1; inset: DSC in the cooling and heating
modes.
exhibiting the two-step SCO, because only two pairs of the doublets
corresponding to the HS and LS species are observed at the
halfway point of the SCO.
The X-ray analysis of 1 demonstrates that the space group at
296 K is identical to that at 200 K and 100 K.§ The geometrical
configurations around the iron ions (Fig. 2) at 296 and 100 K are
fundamentally the same as those of the corresponding py complex:9
the iron atoms coordinated to two bpypz to form the centrosym-
metric dimer with only one kind of Fe(II) and the ligands in a
distorted octahedral environment belonging to a pseudo C2h point
group. At 200 K, the X-ray analysis shows the existence of each one
{ Electronic supplementary information (ESI) available: Fig. S1: Mo¨ss-
bauer spectra, Fig. S2: ORTEP drawing at 296 K, Fig. S3: ORTEP
drawing at 200 K, Fig. S4: Far IR and Raman spectra. See http://
www.rsc.org/suppdata/cc/b4/b411190g/
Fig. 2 ORTEP drawing for 1 at 100 K.
2 8 9 2
C h e m . C o m m u n . , 2 0 0 4 , 2 8 9 2 – 2 8 9 3
T h i s j o u r n a l i s ß T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

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which behaves like a mononuclear type in contrast to the step-wise
two-step SCO through a mixed spin state [HS–LS] for the bpym
and phdia complexes.4–7 Such a two-step SCO process may result
from the difference in intra- and intermolecular interactions. The
intramolecular interactions in complex 1 are characteristic of the
rigid bridging site comprising two tetradentate bpypz ligands as
compared with the flexible bidentate bridging moiety in the m-bpym
and m-phdia Fe(II) complexes. As for the intermolecular interac-
tions in complex 1, there are additional p–p stackings between the
pyridyl and phenyl of the 4-phpy other than those between the
bpypz ligands as for the pyridine complex.9 Since the synergy
between the inter- and intramolecular interactions is considered to
be responsible for two-step SCO, the strong intermolecular
interactions in complex 1 are likely to favor the like-spin pair
…
([HS–HS] [LS–LS]) rather than the mixed spin one ([HS–LS]).
The two-step SCO occurrence for the present 4-pypy complex
along with the preliminary observation for the corresponding
isoquinoline complex could give a clue to deeper understanding of
two-step SCO phenomena in dinuclear complexes.
Notes and references
Fig. 3 Perspective views of complex 1 at 200 K: views (a) nearly along the
c axis or parallel to the ac plane, (b) and (c) nearly from the negative and
positive side of the a axis, respectively (see text).
{ The complex was prepared from trans-[Fe(NCBH3)2(py)4] and excess
amounts of 4-phenylpyridine in methanol solutions under an N2 stream.
Yield 36%. Anal. Calcd. for C50H42N12B2Fe2 ~ (1): C, 63.60; H, 4.48; N,
17.80%. Found: C, 63.06; H, 4.31; N, 17.28%.
of the high-spin [HS–HS] and low-spin [LS–LS] dinuclear
complexes in the asymmetric unit doubled in size from 296 and
100 K. That is, the bond lengths around the Fe(1) and Fe(2) at
200 K correspond to those around Fe(1) at 296 and 100 K,
§ Crystal data for C50H42N12B2Fe2(1). For T ~ 296 K, a red block crystal,
¯
M ~ 944.28, triclinic, space group P1, Z ~ 1, a ~ 9.4567(2), b ~
˚
c ~ 13.1355(5) A, a ~ 92.519(6), b ~ 108.793(9),
3
10.7576(6),
c ~ 105.710(5)u, V ~ 1205.1(1) A , Dc ~ 1.301 g cm23, m(MoKa) ~
6.50 cm21, R/wR2: 0.056/0.1638, F000 ~ 488.00, total reflections: 8427,
unique (Rint): 5236 (0.036). For T ~ 200 K, a red block crystal, M ~
˚
˚
˚
respectively; average Fe–N ~ 2.116 A (Fe(2)) and 2.0005 A (Fe(1))
˚
at 200 K; 2.1667 A (Fe(1)) at 296 K and 1.9795 A (Fe(1)) at 100 K;
˚
¯
*
Fe(2)–Fe(2) ~ 4.2000(8) A and Fe(1)–Fe(1) ~ 4.0605(8) A at
*
944.28, triclinic, space group P1, Z ~ 2, a ~ 9.3509(2), b ~ 14.9124(1),
˚
˚
˚
*
c ~ 17.1648(4)A, a ~ 99.612(1), b ~ 94.551(1), c ~ 98.174(1)u, V ~
˚
˚
200 K; Fe(1)–Fe(1) ~ 4.2147 (8) A at 296 K and 4.0434(6) A at
100 K. There are p–p stacking interactions between the adjacent
bpypz, by which a layer and a chain are formed as found for the py
complex.9 At 200 K, the [HS–HS] and [LS–LS] complexes are
arranged in alternate chains through the p–p stackings not only
between the 4-phpy, but also between the pyridyls of the bpypz.
3
2322.89(8) A , Dc ~ 1.350 g cm23, m(MoKa) ~ 6.74 cm21, R/wR2: 0.0739/
˚
0.1755, F000 ~ 976, total reflections: 14676, unique (Rint): 10011 (0.073).
For T ~ 100 K, a black block crystal, M ~ 944.28, triclinic, space group
¯
˚
P1, Z ~ 1, a ~ 9.0589(3), b ~ 10.682(1), c ~ 12.6458(8)A, a ~ 93.521(8),
3
23
,
˚
b ~ 109.12(1), c ~ 104.560(8)u, V ~ 1105.2(2) A , Dc ~ 1.419 g cm
m(MoKa) ~ 7.08 cm21, R/wR2: 0.0574/0.1745, F000 ~ 488.00, total
reflections: 5896, unique (Rint): 4314 (0.039). CCDC 241137–241139. See
http://www.rsc.org/suppdata/cc/b4/b411190g/ for crystallographic data in
.cif or other electronic format.
…
The p–p stackings between the bpypz exist for the pz py between
[HS–HS] and [HS–HS] as well as [LS–LS] and [LS–LS] and for the
pyridyl (HS) pyridyl(LS) (Fig. 3(a)) as the py complex. For a
…
given 4-phpy in [HS–HS] or [LS–LS] at 200 K, strong p–p stackings
of the pyridyl and phenyl on one side are made, respectively, with
the phenyl and pyridyl for the 4-phpy in the adjacent [LS–LS] or
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…
[HS–HS] (Fig. 3(b)), and on the other back side weak phenyl
phenyl p–p stackings with the other neighboring 4-phpy in the [LS–
LS] or [HS–HS] (Fig. 3(c)). There are significant voids in the
crystals, but no electron density is found. All these crystallographic
characteristics support the occurrence of a direct two-step spin
transition from [HS–HS] to [LS–LS] through a mixture of [HS–
…
HS] [LS–LS] intermediates.
Moreover, the far IR and Raman spectra confirm the direct two-
…
step SCO through [HS–HS] [LS–LS]. Since the IR and Raman
spectra of complex 1 are different from each other, the mutual
exclusion selection rules in the C2h point group for the vibrational
spectra hold. At 200 K, the IR and Raman bands give the additive
pattern of those at 296 and 100 K, maintaining the selection rule for
IR and Raman spectra. This fact suggests that the molecular
symmetry remains pseudo C2h point group for each unit of [HS–
HS] and [LS–LS], but does not convert to the Cs point group of
mixed spin state [HS–LS] at 200 K. In fact, there is no extra band at
200 K except the IR and Raman bands observed at 296 and
100 K. Each IR active band at 366.5 and 357 cm21 exhibits only
single occupancy at 296 and 100 K, respectively. The corresponding
bands at 200 K are observed at 364.6 and 357 cm21. On the other
hand, the intensities of the Raman active bands at 380 and
250 cm21 increase on decreasing the temperature. Therefore, these
spectral behaviors demonstrate the coexistence of [HS–HS] and
[LS–LS].
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S. Morimoto and S. Kaizaki, Dalton Trans., 2004, 982.
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In conclusion, the two-step SCO of complex 1 is found to
proceed through a mixture of [HS–HS] and [LS–LS] at the plateau
C h e m . C o m m u n . , 2 0 0 4 , 2 8 9 2 – 2 8 9 3
2 8 9 3