Dinuclear Spin-Crossover Complexes Based on Tetradentate and Bridging Cyanocarbanion Ligands

Article abstract of DOI:10.1021/acs.inorgchem.6b01542

Spin-crossover (SCO) Fe(II) dinuclear complexes of formula [Fe₂(tmpa)₂(μ₂-tcpd)₂]·0.8(CH₃OH) (1·MeOH) and [Fe₂(andmpa)₂(μ₂-tcpd)₂]·2CH₃OH (2·MeOH) (tmpa = tris(2-pyridylmethyl)amine, andmpa = bis(2-pyridylmethyl)aminomethyl)aniline, (tcpd)^2- = 2-dicyanomethylene-1,1,3,3-tetracyanopropanediide) have been synthesized and characterized by infrared spectroscopy, X-ray diffraction, and magnetic measurements. The crystal structure determinations of the two complexes (1·MeOH and 2·MeOH) and the desolvated complex 1 (from 1·MeOH) revealed a neutral centrosymmetrical dinuclear structure in which the (tcpd)^2- cyanocarbanion acts as a double μ₂-bridging ligand between two [FeL]²⁺ (L = tmpa (1), andmpa (2)) units involving two free coordination sites in the cis configuration. Examination of the shortest intermolecular contacts in 1·MeOH and 1 reveals no significant hydrogen bonding between the dinuclear units, while in 2·MeOH these units are held together by significant hydrogen bonds between one of the uncoordinated nitrile groups and the anilate function, giving rise to 1D supramolecular structure. The three dinuclear complexes 1, 2·MeOH, and 2 exhibit SCO behaviors which have been evidenced by the thermal evolutions of the χ_mT product and by the average values of the six Fe-N distances for 1 and 2·MeOH, that reveal a gradual conversion with transition temperatures (T_1/2) at ca. 352 K (1), 196 K (2), and 180 K (2·MeOH). For the solvated 1·MeOH, the sharp SCO transition observed around 365 K was induced by the desolvatation process above 330 K during the magnetic measurements.

Full text of DOI:10.1021/acs.inorgchem.6b01542

Article
pubs.acs.org/IC
Dinuclear Spin-Crossover Complexes Based on Tetradentate and
Bridging Cyanocarbanion Ligands
Eric Milin,† Sabrina Belaïd,†,‡ Veronique Patinec,† Smail Triki,*,† Guillaume Chastanet,§
́
and Mathieu Marchivie§
†UMR CNRS 6521, Chimie, Electrochimie Molec
Gorgeu, CS 93837, 29238 Brest Cedex 3, France
́
ulaires, Chimie Analytique, Universite
́
de Bretagne Occidentale, 6 Avenue V. Le
‡Laboratoire de Physico-Chimie des Mater
́
iaux et Catalyse, Universite
de Bejaia, Bejaia, Algeria
́ ́ ́
§CNRS, Universite
́
Bordeaux, ICMCB, 87 Av. Doc. A. Schweitzer, F-33608 Pessac, France
S
* Supporting Information
ABSTRACT: Spin-crossover (SCO) Fe(II) dinuclear complexes of formula
[Fe2(tmpa)2(μ2-tcpd)2]·0.8(CH3OH) (1·MeOH) and [Fe2(andmpa)2(μ2-tcpd)2]·
2CH3OH (2·MeOH) (tmpa = tris(2-pyridylmethyl)amine, andmpa = bis(2-
pyridylmethyl)aminomethyl)aniline, (tcpd)2− = 2-dicyanomethylene-1,1,3,3-tetracya-
nopropanediide) have been synthesized and characterized by infrared spectroscopy,
X-ray diffraction, and magnetic measurements. The crystal structure determinations of
the two complexes (1·MeOH and 2·MeOH) and the desolvated complex 1 (from 1·
MeOH) revealed a neutral centrosymmetrical dinuclear structure in which the
(tcpd)2− cyanocarbanion acts as a double μ2-bridging ligand between two [FeL]2+ (L
= tmpa (1), andmpa (2)) units involving two free coordination sites in the cis
configuration. Examination of the shortest intermolecular contacts in 1·MeOH and 1
reveals no significant hydrogen bonding between the dinuclear units, while in 2·
MeOH these units are held together by significant hydrogen bonds between one of
the uncoordinated nitrile groups and the anilate function, giving rise to 1D
supramolecular structure. The three dinuclear complexes 1, 2·MeOH, and 2 exhibit SCO behaviors which have been evidenced
by the thermal evolutions of the χmT product and by the average values of the six Fe−N distances for 1 and 2·MeOH, that reveal
a gradual conversion with transition temperatures (T1/2) at ca. 352 K (1), 196 K (2), and 180 K (2·MeOH). For the solvated 1·
MeOH, the sharp SCO transition observed around 365 K was induced by the desolvatation process above 330 K during the
magnetic measurements.
polydentate neutral ligand, A− or A2− = terminal or bridging N-
INTRODUCTION
■
donating anion) presents a richer chemical diversity since the
molecular structure of the complex can be designed with a
planned dimension or with a desired nuclearity, with only the
appropriate choice of the anionic ligand, owing to its chemical
structure including its coordination properties.
In this context, we have reported in the past few years the
first SCO series based on cyanocarbanion ligands together with
abpt chelating neutral ligand (Scheme 1).6
In this series, the single charge on the anion induces a
terminal coordination mode for the cyanocarbanion unit,
resulting in neutral discrete SCO complexes. Afterward, we
have shown, in a second report, that the use of the (tcpd)2−
cyanocarbanion ((tcpd)2− = 2-dicyanomethylene-1,1,3,3-tetra-
cyanopropanediide anion) ligand bearing two negative charges
induces bridging coordination modes to lead to an original
thermo- and photoswitchable SCO chain.7 In these reports, we
have shown that coordination of the cyanocarbanion in the
Among the molecular switchable materials, the spin-crossover
(SCO) complexes are by far the most studied during the past
decade, because of their several potential applications such as in
memory display devices and sensing technology.1−3 These
complexes can be switched reversibly from a high spin (HS) to
a low spin (LS) state by external stimuli such as temperature,
pressure, magnetic field, or light irradiation. Although the spin
transition can occur in d4−d7 transition metal complexes, the
most studied examples to date are those based on the Fe(II)
(d6 configuration) metal ion, for which a paramagnetic−
diamagnetic transition from HS (S = 2) to LS (S = 0) is
observed.3,4 The [FeN6] SCO compounds can be roughly
divided into two categories: the cationic complexes in which
the six donating nitrogen atoms arise from one or several
neutral ligands to generate the [FeLn]2+ complexes, and neutral
or anionic complexes for which three or four nitrogen atoms
arise from neutral ligands and the octahedral environment of
the iron centers is completed by two or three nitrogen atoms
arising from monodentate or bridging anionic ligands.3−5 This
last category which can be formulated as [FeLnAm]0/− (L =
Received: June 28, 2016
© XXXX American Chemical Society
A
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Article
to form SCO dinuclear complexes. We describe the syntheses
and structural characterizations, including thermal variation of
the crystallographic data and magnetic properties of the
solvated and desolvated Fe(II) dinuclear neutral complexes
[Fe2(tmpa)2(μ2-tcpd)2]·nCH3OH (1·MeOH and 1) and
[Fe2(andmpa)2(μ2-tcpd)2]·2CH3OH (2·MeOH and 2) (andm-
pa = (bis(2-pyridylmethyl)aminomethyl)aniline) involving the
(tcpd)2− anionic bridges. It should be pointed out that several
SCO systems exhibiting dinuclear structure have been reported.
Most of them involve neutral bridging ligands such as 2,2′-
bpyrimidine,11 4,7-phenanthroline-5,6-diamine,12 or substituted
triazole, pyrazole, and thiadiazole ligands.13 To the best of our
knowledge, only one SCO dinuclear complex involving
bridging anions has been reported.14 Thus, the present work
shows that the use of cyanocarbanion ligands not only leads to
the design of polynuclear discrete complexes based on anionic
bridges, but also allows the development of new neutral
systems without any counterions.
Scheme 1. Cyanocarbanion and Polydentate Ligands
Employed in This Work and in References 6 and 7
trans positions is clearly imposed by the planar [Fe(abpt)2]2+
precursor. With the aim to better explore the potential impact
of such anionic ligands on the structural features of their
complexes, and then on the switching properties (transition
temperatures, hysteresis width, abruptness of the transition,
photoinduced effects, and so forth), we have extended this
work to the use of other polydentate neutral ligands such as the
tris(2-pyridylmethyl)amine (Scheme 1). In contrast to the abpt
bidentate ligands, such flexible tetradentate molecules should
allow free coordination sites in the cis positions which are more
appropriate for the design of polynuclear complexes.8−11 For
the tmpa ligand (tmpa = tris(2-pyridylmethyl)amine), few SCO
systems have been reported.8−10 The [Fe(tmpa)]2+ entity
possesses two free coordination sites in the cis configuration
which has been used to allow the coordination of terminal
ligands as observed in the polymorphic [Fe(tmpa)(NCS)2]
system,8 or the coordination of potentially bridging ligands to
design polynuclear SCO derivatives, such as the tetranuclear
complex [Fe(tmpa)(N(CN))2]4 or the diminoquinonoid-
bridged dinuclear series [Fe2(tmpa)2(μ2-XL)]2+ (XL2− = double
deprotonated form of 3,6-disubstituted-2,5-dianilino-1,4-benzo-
quinone; X = H, Br, Cl, F) in which the authors showed that
the slight chemical modifications of the anionic bridging ligand
(XL2−) affect the transition temperature (T1/2) strongly.9,10
We report herein the use of the tetradentate chelating neutral
ligands in association with a cyanocarbanion bridging coligand
RESULTS AND DISCUSSION
■
Synthesis. The andmpa ligand was synthesized according to
a two-step method described by S. Lippard,15 which was slightly
modified. The starting reactant 2-nitrobenzyl bromide was
obtained as previously published, by reacting the 2-nitrobenzyl
alcohol and PBr3 in Et2O as the solvent in the place of CCl4
(see Figure S1).16 Reaction of this bromo-derivative with
dipicolylamine (dpa) in SN2 conditions gave rise to the
formation of the 2-[bis(2-pyridylmethyl)aminomethyl]-
nitrobenzene intermediate in 67% yield after purification (see
Figures S2−S4). Reduction of the nitro-group into a primary
amine function was carried out using hydrazine hydrate in
ethanol with activated charcoal17 instead of hydrogenation in
Pd/C catalytic conditions15 and yielded the andmpa ligand (see
Figures S5−S7) with 60% overall yield (Scheme 2).
Both Fe(II) complexes (1·MeOH and 2·MeOH) were
synthesized, as single crystals, using a diffusion technique in a
fine glass tube (3.0 mm diameter) by carefully layering 2 mL of
methanolic solution of K2tcpd onto a methanolic solution
containing Fe(BF4)2·6H2O and the tetradentate neutral ligand
in 1:1 ratio (tmpa for 1·MeOH; andmpa for 2·MeOH). In each
case, single crystals suitable for X-ray analysis were formed after
3 days. The single crystals of the unsolvated phase 1
[Fe2(tmpa)2(μ2-tcpd)2] can be obtained either by heating the
sample at 70 °C for 3 h or with time at room temperature since
they easily lose their solvent molecules. This “natural”
a
Scheme 2. Synthesis of the Ligand andmpa
a
(i) 1.1 equiv PBr3, Et2O, 0 °C, 2 h; (ii) K2CO3, CH3CN, reflux, 5 days (67%); (iii) NH2NH2−H2O, EtOH, activated charcoal, 48 h (92%).
B
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Table 1. Crystal Data and Structural Refinement Parameters for Compounds 1·MeOH, 1, and 2·MeOH
1·MeOH
1
1
2·MeOH
2·MeOH
296(2)
T/K
296(2)
296(2)
380(2)
100(2)
a
empirical formula
fw/g mol−1
wavelength/Å
cryst syst
space group
a/Å
(C56H36Fe2N20), 0.8(CH3OH)
1127.49
C56H36Fe2N20
1100.75
(C58H40Fe2N20), 2(CH3OH)
1192.88
0.71073 Å
monoclinic
P21/n
0.71073 Å
monoclinic
P21/n
0.71073 Å
monoclinic
P21/n
11.7269(5)
16.6966(6)
13.9317(6)
104.672(4)
2638.87(19)
2
11.468(4)
17.0057(9)
13.7743(6)
105.098(9)
2593.6(10)
2
10.894(4)
18.7523(9)
13.7750(6)
106.492(9)
2698.3(11)
2
9.6690(10)
24.658(2)
11.5670(10)
102.631(6)
2691.0(4)
2
9.6831(12)
b/Å
25.065(3)
12.0485(16)
103.711(14)
2840.9(7)
2
c/Å
β/deg
V/Å3
Z
D
calc/g cm−3
1.419
1.409
1.355
1.472
1.395
CCDC no.
1413422
1435809
1435810
1413424
1413423
a
The asymmetric unit contains 0.5 of the chemical formula.
Table 2. Hydrogen Bonds in 1·MeOH and 2·MeOH at 296 K
compd
D−H···A
d(D−A)/Å
d(D−H···A)/Å
D−H−A/deg
a
1·MeOH
2·MeOH
2·MeOH
(MeOH)O1−H···N10(a) (tcpd)
(MeOH)O1−H···N9(b) (tcpd)
(an.)N4−H4b···N8(b) (tcpd)
2.849(8)
2.843(7)
3.183(6)
2.079(3)
2.045(5)
2.215(5)
156.4(8)
164.7(4)
175.1(3)
a
3
1
3
Equivalent positions: (a) /2 − x, y − /2, /2 − z; (b) 1 − x, 1 − y, 1 − z. (tcpd) indicates the involved atom belongs to the tcpd anion; (an.)
indicates the involved atom belongs to the anilate moiety. (MeOH) indicates the involved atom belongs to the methanol molecule.
Table 3. Shortest Intermolecular Contacts (Å) in 1·MeOH, 1, and 2·MeOH at 296 K
b
π−π-like Intermolecular Contacts
b
c
d
e
π system
c−c /Å
shift /Å
angle /deg
C20C21C22C23C27N8(a) (1)
C20C21C22C23C27N8(a) (1·MeOH)
C20C21C22C23C29N10(b) (2·MeOH)
3.605(2)
3.658(2)
1.088(4)
1.303(4)
0.488(8)
0.00(2)
0.0(3)
3.672(4)
0.00(3)
f
van der Waals Interactions
1·MeOH
1
2·MeOH
a
(tcpd)N8···C20(a)(tcpd)
3.327(5)
3.328(4)
3.362(4)
3.359(4)
(tcpd)N8···C21(a)(tcpd)
(tcpd)N8···C25(a)(tcpd)
(tcpd)N10···C27(b)(tcpd)
(tcpd)C25···C29(b)(tcpd)
(an)N4···N8(b)(tcpd)
3.278(4)
3.294(4)
3.388(8)
3.590(7)
3.183(6)
(MeOH)O1···N10(c)(tcpd)
(MeOH)O1···C6(b)(am.Me)
(MeOH)C30···C20(tcpd)
(MeOH)C30···C22(tcpd)
(MeOH)C30···C11(d)(py)
(MeOH)O1···C4(b)(py)
(MeOH)O1···C4(e)(py)
(MeOH)O1···C10(f)(py)
(MeOH)C30···N9(b)(tcpd)
(MeOH)C30···C24(g)(tcpd)
(MeOH)C30···C25(g)(tcpd)
(MeOH)C30···C21(g)(tcpd)
(MeOH)C30···C1(g)(py)
(MeOH)O1···N9(b)(tcpd)
2.849(8)
3.273(9)
3.388(10)
3.5366(11)
3.558(12)
3.169(9)
3.265(8)
3.258(7)
3.346(9)
3.557(9)
3.566(9)
3.580(9)
3.581(9)
2.843(7)
a
Codes of equivalent positions: (a) 2 − x, 1 − y, 2 − z; (b) 1 − x, 1 − y, 1 − z; (c) 3/2 − x, y − 1/2, 3/2 − z; (d) 1/2 − x, y − 1/2, 3/2 − z; (e) x, y, 1 +
b
z; (f) 1/2 + x, 3/2 − y, 1/2 + z; (e) −x, 1 - y, 1 − z. (a). The intermolecular interaction involves the defined π system and its (a) and (b) symmetry
c
d
e
f
equivalent. Centroid to centroid distance. Shift distance. Angle between planes. (tcpd) = the involved atom belongs to the tcpd anion; (an.) the
involved atom belongs to the anilate moiety; (am.Me) the involved atom belongs to the amino-methyl moiety; (py) the involved atom belongs to
the pyridine moiety; (MeOH) the involved atom belongs to the methanol molecule.
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Table 4. Selected Bond Lengths (Å) and Distortion Parameters of the Coordination Sphere in 1 and 2
1
2·MeOH
1·MeOH 296 K
296 K
380 K
100 K
296 K
a
Fe1−Fe1(a)
Fe1−N1
Fe1−N2
Fe1−N3
Fe1−N4
Fe1−N5
7.0752(7)
1.985(2)
1.973(2)
1.966(2)
1.975(2)
1.949(3)
1.939(3)
1.965(3)
7.1242(8)
1.991(2)
1.973(2)
1.971(2)
1.973(2)
1.951(3)
1.939(2)
1.966(3)
7.2688(15)
2.191(4)
2.131(6)
2.149(5)
2.121(5)
2.118(6)
2.055(5)
2.128(5)
7.1732(11)
2.025(3)
1.973(3)
1.972(3)
2.068(3)
1.956(3)
1.955(3)
1.992(3)
7.3377(16)
2.232(4)
2.145(4)
2.136(4)
2.200(4)
2.149(4)
2.100(4)
2.160(4)
Fe1−N6(a)
⟨Fe−N⟩
Distortion
b
Σ(Fe1) (deg)
51(2)
56(2)
94(3)
48(2)
75(2)
c
Θ(Fe1) (deg)
133(3)
143(3)
248(5)
141(4)
214(4)
a
b
Equals 1 − x, −y, −z + 2 (1·MeOH or 1) or −x, −y + 1, −z + 1 (2·MeOH). Σ is the sum of the deviation from 90° of the 12 cis angles of the
FeN6 octahedron.23,24 Θ is the sum of the deviation from 60° of the 24 trigonal angles of the projection of the FeN6 octahedron onto its trigonal
c
faces.24,25
desolvatation process is confirmed by single crystal X-ray
diffraction at room temperature at different times and by
elemental analysis since the freshly prepared single crystals of 1·
MeOH reveal only a fraction of ca. 0.3CH3OH (see
Experimental Section).18 In contrast, the two MeOH molecules
in compound 2·MeOH are clearly maintained in the crystal
packing, even after their long exposition at room temperature,
as revealed by the crystal structure at 296 K. Crystals of 2·
MeOH were heated up to try to solve the crystal structure of
desolvated compound 2. At 330 K the crystal structure reveals
no significant difference with the room temperature structure
and confirms the presence of the two solvent molecules. Above
350 K the diffraction pattern of 2 changes drastically,
highlighting the crystal damage that may be caused by the
loss of the solvent molecules. Nevertheless, attempts to solve
the crystal structure of 2 failed. This suggests that the solvent
molecule should be involved with more and/or shorter
intermolecular contacts in the crystal packing of 2·MeOH
(see below). TGA measurements performed on a set of single
crystals of 2·MeOH show a loss of a fraction of ca. 0.27MeOH
(0.7%) at 100 °C (Figure S8). Above this temperature,
additional mass loss of ca. 2.0% has been observed, but
according to the infrared spectrum of the sample maintained at
100 °C for 6 h (see Experimental Section), this mass loss
cannot be attributed to only MeOH molecules (see below).
This implies that 2·MeOH may lose the major part of the
MeOH molecules (ca. 1.73 molecules) during the preliminary
vacuum procedure before the TGA experiment. Compounds 1·
MeOH, 1, 2·MeOH, and 2 display similar infrared patterns
(see Figures S9 and S10). The infrared spectrum of 2·MeOH
displays a broad absorption band around 3468 cm−1. This band
which was not observed in the infrared spectra of the warmed
samples (see Figure S10) can be attributed to OH stretching of
the MeOH molecules in 2·MeOH. As expected, the four
complexes (1·MeOH, 1, 2·MeOH, and 2) show absorption
bands assigned to the νCN stretching vibrations of the
cyanocarbanion (2192 and 2165 cm−1 for 1·MeOH; 2192
and 2166 cm−1 for 1; 2186 and 2161 cm−1 for 2·MeOH; 2187
and 2165 cm−1 for 2). These absorption bands, distinct from
the stretching vibration modes observed for the tetrabutylam-
monium salt ([(C4H9)4N]2(tcpd)) containing the uncoordi-
nated cyanocarbanion units (2094−2174 cm−1),7 can be
assigned to the presence of both bridging (2192−2186 cm−1)
and terminal (2161−2166 cm−1) nitrile groups, in agreement
with the single crystal structure determination (see below).
Description of the Crystal Structures. Pertinent crystal
data and selected bond distances and bond angles for
compounds 1·MeOH, 1, and 2·MeOH are summarized in
Tables 1−4 and Table S2. The molecular structures are
depicted in Figures 1 and 2. The three compounds crystallize in
the monoclinic P21/n space group for all the studied
temperatures (Table 1 and Table S1).
Figure 1. Molecular structure of the dinuclear iron(II) complex of 1:
(a) = 1 − x, −y, −z + 2.
The asymmetric unit of 1·MeOH is built from one Fe(II)
ion, one tmpa neutral ligand, one (tcpd)2− anion, and a
methanol solvent molecule with a partial occupation factor of
0.4 at room temperature, all located on general positions. The
molecular structure of 1·MeOH consists of centrosymmetric
neutral dinuclear entities of the formula [Fe2(tmpa)2(μ2-
tcpd)2] and 0.8 methanol molecule (Figure 1).
Except for the absence of MeOH solvent molecules, the
asymmetric unit of 1 is similar to that observed for compound
1·MeOH, and the two complexes are isostructural with only
slight structural differences. In both complexes, the tmpa
molecule acts as a tetradentate ligand leading to the
[Fe(tmpa)]2+ entities which are connected by two (tcpd)2−
anions acting as μ2-bridging ligands through two nitrile groups
of the same C(CN)2 wing. Each iron(II) metal ion presents a
distorted FeN6 octahedral environment assumed by four
nitrogen atoms (N1, N2, N3, N4) arising from the tmpa
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MeOH and 1 reveals no significant hydrogen bonding between
the dinuclear units, excepting the hydrogen bonding observed
between the dinuclear units and the oxygen of the methanol
molecule (Table 2 and Figure 3a) in 1·MeOH. However, as
depicted in Figure 3a, the packing cohesion in 1·MeOH and 1
is ensured by the π−π-like contacts between the π-delocalized
parts of the tcpd anions of the adjacent dinuclear units (see
Table 3, top). In addition to the hydrogen bond mentioned
above, the MeOH molecule is also stabilized by several van der
Waals interactions involving the carbon atoms of the tcpd anion
and the tmpa ligand (Table 3, bottom). Despite the loss of the
solvent molecule, the packing cohesion in 1 is very similar to
that in 1·MeOH but with a slightly more pronounced π−π
contact between the tcpd anions (Table 3a). In contrast, a
different situation is observed in complex 2·MeOH since the
dinuclear units are held together by significant hydrogen
bonding between the anilate −NH2 group (N4) of the andpma
ligand and one of the uncoordinated nitrile groups (N4···N8,
3.183(6) Å), giving rise to 1D supramolecular structure as
highlighted in Figure 3b. As in 1, the adjacent (tcpd)2− anions
are involved in π−π-like contacts between π-delocalized parts
of tcpd anions, and hydrogen bonding (O1···N9, 2.843(7) Å)
occurs between the methanol molecule (O1) and one of the
noncoordinated nitrile groups (N9) of the cyanocarbanion (see
Tables 2 and 3). According to Table 2, this hydrogen bond is
slightly stronger in 2·MeOH than in 1·MeOH as the O−H···N
angle is more linear in 2·MeOH. It is worth noting that the
Figure 2. Molecular structure of the dinuclear iron(II) complex of 2:
(a) = −x, −y + 1, −z + 1.
ligand and by two nitrogen atoms of two equivalent (tcpd)2−
anions (N5 and N6(a)).
For compound 2·MeOH, the crystallographic parameters are
different from those observed for compounds 1·MeOH and 1
(see Table 1). Despite this difference, the structure of 2·MeOH
is built from a similar asymmetrical unit, leading to a dinuclear
molecular structure of the formula [Fe2(andmpa)2(μ2-tcpd)2]·
2CH3OH, similar to that described for 1·MeOH (Figure 2).
Examination of the shortest intermolecular contacts in 1·
Figure 3. Intermolecular contacts in compounds 1·MeOH (a) and 2·MeOH (b).
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MeOH molecule is also involved in van der Waals interactions
that are more numerous in 2·MeOH than in 1·MeOH. Both
observations are consistent with the fact that the MeOH
molecules remain in the network of 2·MeOH at room
temperature whereas they slowly go out of the crystal of 1·
MeOH as revealed by variable temperature X-ray data
collections and also by elemental analysis.
Magnetic Properties. The thermal dependence of the
product of the molar magnetic susceptibility times the
temperature (χmT) for this dinuclear family has been studied
on single crystal samples in the temperature range 10−395 K.
Concerning compound 1·MeOH, it has been inserted at 150 K
in the SQUID magnetometer to prevent easy solvent loss. The
temperature then has been increased up to 395 K (Figure 4).
desolvated compound 2, with a T1/2 value of 196 K. This spin-
crossover curve is reversible upon cooling and warming (Figure
S13).
The solvent dependent magnetic behavior is commonly
reported in the SCO literature. In the binuclear case, this is less
common. Indeed, apart from the report of new binuclear units,
few examples are related to the influence of counteranion and
solvent.13a,b,14,19 The amount and nature of solvent could
strongly affect the magnetic properties, affecting the presence of
multistep spin-crossover behavior. Among the more than 30
binuclear systems reported, both one- and two-step spin-
crossover behaviors are described.20 To date, there has been no
clear understanding of the occurrence or not of steps along the
transition. The main theoretical approach provided evidence
that a stepped transition could follow from elastic strains within
the molecules and/or the network.21 However, from the
structural point of view this is not always obvious, and this
point deserves more attention.22
Magneto-Structural Relationships. On the basis of the
conclusions derived from the magnetic data, the crystal
structures of both compounds have been determined at
different temperatures (380 K for 1 and 100 K for 2·
MeOH), to know more about the two spin states of each
dinuclear complex. To know how the crystal structure and the
two iron(II) sites are affected by the incomplete SCO transition
observed for 1, the temperature dependence of its crystal
structure was performed in the temperature range 296−380 K.
The corresponding structural parameters are depicted in Table
4 and in Figure 5. These crystallographic data, such as unit cell
●
Figure 4. Thermal variation of the χmT product of 1·MeOH ( ), 1
■
□
○
( ), 2·MeOH ( ), and 2 ( ), measured at 1 K/mn in settle mode for
2·MeOH and 2 and in sweep mode in 1·MeOH and 1.
From 150 to 275 K, the χmT value remains close to zero (0.12
cm3 K mol−1), in agreement with a low spin state of the Fe(II)
ions. From 275 to 330 K, a slight increase of χmT is observed
followed by a sharp increase above 330 K. At 395 K, the χmT
value of 5.95 cm3 K mol−1 is reached, which is slightly lower
than the expected value for two Fe(II) HS species (between 6
and 7 cm3 K mol−1 depending on the g value). This behavior is
relevant with a desolvation occurring above 330 K, inducing a
spin-crossover. The curve’s shape indicates that, even at 395 K,
this spin-crossover is not complete and that a small amount of
Fe(II) should stay in the LS state. Upon cooling, a new curve is
described that presents a gradual decrease of the χmT value
down to 0.37 cm3 K mol−1 at 150 K, without any clear steps.
This curve appears stable upon further temperature cycling
(Figure S11). Regarding the ability of 1·MeOH to lose its
solvent, this second SCO complex, characterized by a T1/2 of
352 K, could be assigned to compound 1.
Compound 2·MeOH shows a room temperature χmT value
of 6.29 cm3 K mol−1, in agreement with the presence of two
isolated HS Fe(II) metal ions with a g value of 2.05. The χmT
product for 2·MeOH smoothly decreases upon cooling down
to 0.21 cm3 K mol−1 at 50 K (Figure 4). This HS ↔ LS SCO
conversion is characterized by a T1/2 value of 180 K and is
reversible upon warming and cooling (Figure S12). After
warming at 350 K for half an hour, the thermal behavior of the
sample was recorded. A slightly more abrupt curve is observed
(Figure 4) that could be related to the spin-crossover of the
▲
Figure 5. Evolution of the unit cell volume ( ) and the mean Fe−N
■
distance ( ) for compound 1 from room temperature to 380 K.
volume, the average Fe−N distances (⟨Fe−N⟩LS ∼ 2.0 Å; ⟨Fe−
N⟩HS ∼ 2.2 Å), and the trigonal distortion parameters Σ and Θ
(see Table 4),23−25 are known to be highly sensitive to the spin
state of the Fe(II) centers, and will thus be used to assign the
spin state of Fe(II) sites in each complex.
The iron centers present an octahedral FeN6 geometry for all
compounds. In good agreement with the magnetic data, the
mean Fe−N distances observed at 296 K for the solvated phase
(1·MeOH) and for the unsolvated phase (1) (1.965(3) and
1.966(3) Å, respectively) are in the range of those expected for
the Fe(II) ion in the LS state.6,7 The octahedral geometry
appears slightly more distorted for compound 1 than for 1·
MeOH since the trigonal distortion parameter calculated for 1
(Θ = 143(3)°) is higher than the corresponding value
calculated for 1·MeOH (Θ = 133(3)°). At 380 K, 1·MeOH
F
DOI: 10.1021/acs.inorgchem.6b01542
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
irreversibly loses its methanol molecule, becoming 1. The
coordination sphere volume of the metal increases as the mean
Fe−N distance rises to 2.128(5) Å, and conjointly the
octahedral geometry becomes more distorted (see Table 4
and Table S2), suggesting an LS → HS transition. Nevertheless,
the mean Fe−N distance (2.128(5) Å) appears significantly
smaller than that observed for the HS Fe(II) in compound 2·
MeOH (2.160(4) Å) at 296 K) and smaller than those
generally observed for other Fe(II) compounds. This can be
due to an incomplete spin transition at 380 K for 1,
corresponding to a small amount of the remaining LS Fe(II)
in the crystal structure. Even if this observation agrees perfectly,
the magnetic observations and the crystal data at 380 K did not
reveal clearly if, at this temperature, the plateau corresponding
to the HS sate is reached. Thus, in order to investigate further
this incomplete spin transition, the crystal structure of
compound 1 has been solved every 5 K between 296 and
380 K (see Table 1 for 296 and 380 K; and CCDC numbers
1442955−1442970 for the temperature range 300−375 K).
These additional structural investigations did not reveal any loss
of symmetry over the whole temperature range, suggesting that
both iron centers of the dinuclear complex are similarly affected
by the spin transition, which is consistent with the observed
one-step transition from [LS−LS] to [HS−HS] states.
According to Figure 5, that shows the evolution of the unit
cell volume and the mean Fe−N bond length between 296 and
380 K, the incomplete spin transition reaches a plateau at 370
K, revealing that about 20% of the complex is trapped in the LS
state.
process (1·MeOH → 1 and 2·MeOH → 2) show only slight
modification of the SCO behaviors while the strong changes of
the transition temperatures have been observed between
compounds 1 and 2 (Δ(T1/2) > 150 K). Finally, it is worth
noting that this drastic evolution of the transition temeprature
cannot be attributed exclusively to the ligand substitution
performed on the tmpa initial ligand but also to the solid-state
packing which affects significantly the elastic interactions and
the SCO behavior as exemplified by several polymorphic
systems.8,26
EXPERIMENTAL SECTION
■
General Remarks. Tetracyanoethylene, urea, potassium tert-
butoxide (C4H9OK), malononitrile (CH2(CN)2), 2-nitrobenzyl
alcohol, dipicolyl amine, PBr3, K2CO3, and Fe(BF4)2·6H2O were
purchased from commercial sources and used without further
purification. Solvents were used and purified by standard procedures.
The organic ligands have been prepared under nitrogen. The
potassium salt K2(tcpd) and tmpa ligand (tris(2-pyridylmethyl)amine)
were prepared according to published procedures (see Figures S14−
S16).27,28 Elemental analyses were performed by the “Service Central
d’Analyses du CNRS”, Gif-sur-Yvette, France. Infrared spectra were
recorded in the range 4000−200 cm−1 on an FT-IR Bruker ATR
Vertex 70 spectrometer. Diffraction analyses were performed using an
Oxford Diffraction Xcalibur κ-CCD diffractometer. NMR spectra were
recorded on a Bruker DRX 300 MHz or Bruker Avance 400 and 500
MHz. TGA measurements were performed on Symmetric TGA
Setaram TAG2400. The sample was preliminarily put under vacuum
and then heated at 3 °C min−1 under an argon atmosphere. Magnetic
measurements were performed with a Quantum Design MPMS-XL-5
SQUID magnetometer in the 10−400 K temperature range with an
applied magnetic field of 2 T on crushed single crystals of compounds
1·MeOH and 2·MeOH. The susceptibility data were corrected for the
sample holders previously measured under the same conditions, and
for the diamagnetic contributions as deduced by using Pascal’s
constant tables (χdia = −547.4 × 10−6 and −571.6 × 10−6 emu mol−1
for 1·MeOH and 2·MeOH, respectively).29
For compound 2·MeOH, the mean Fe−N distance observed
at room temperature (dFe−N = 2.160(4) Å) is characteristic of
the presence of the iron(II) ion in the HS state. At 100 K, the
octahedral geometry of 2·MeOH becomes more regular, and
the mean Fe−N distance decreases drastically to reach 1.992(3)
Å. In agreement with the magnetic data, this feature is the
signature of the change of the spin state from HS to LS at low
temperature.
Crystallographic Data Collection and Refinement. Crystallo-
graphic studies of the two derivatives (1 and 2) were performed at
296, 100, and 380 K, using an Oxford Diffraction Xcalibur κ-CCD
diffractometer equipped with a graphite monochromated Mo Kα
radiation (λ = 0.71073 Å) or a Bruker APEX-II CCD diffractometer (λ
= 0.71073 Å). At 380 K a crystal of 1 was mounted on a glass fiber and
glued with high temperature resisting structural glue (DP700 epoxy
structural adhesive from 3M Scotch-Weld) and then collected on a
Bruker APEX-II CCD diffractometer (λ = 0.71073 Å). The full-sphere
data collections were performed using 1.0° ω-scans with an exposure
time of 200 and 35 s per frame for 1·MeOH and 1, and 60 and 80 s
per frame for 2·MeOH at 296 and 100 K, respectively. Data collection
and data reduction were done with the CRYSALIS-CCD and
CRYSALIS-RED programs or on the Bruker APEX-II program suite
on the full set of data.30 The crystal structures were solved by direct
methods and successive Fourier difference syntheses, and were refined
on F2 by weighted anisotropic full-matrix least-squares methods.31 All
non-hydrogen atoms were refined anisotropically, while the hydrogen
atoms were calculated and therefore included as isotropic fixed
contributors to Fc. All other calculations were performed with standard
procedures (WINGX, OLEX2).32,33 Pertinent crystal data and
structure refinement and collection parameters are listed in Table S1.
Synthesis of Bis(2-pyridylmethyl)aminomethyl)aniline
(andmpa). 2-Nitrobenzyl bromide (1.08 g, 5.0 mmol) was added to
a solution of bis(2-pyridylmethyl)amine (dmpa) (1.00 g, 5.0 mmol) in
distilled CH3CN (20 mL) with K2CO3 (1.38 g, 10.0 mmol). The
reaction mixture was heated to reflux with stirring for 5 days. After the
solution was cooled and filtered, the solvent was removed by
evaporation. Purification of the crude product by column chromatog-
raphy on neutral alumina (CH2Cl2/MeOH from 100/0 to 99/1)
produced a brown oil of bis(2-pyridylmethyl)aminomethyl)-
CONCLUSIONS
■
Four dinuclear complexes of formula [Fe2(L)2(μ2-tcpd)2]·
nCH3OH are obtained from reaction of two tetradentate
neutral ligands (L = tmpa (1·MeOH and 1) and L = andmpa
(2·MeOH and 2)) and the bridging (tcpd)2− anionic ligand.
The complexes display similar molecular structures described as
neutral dinuclear units involving similar double μ2-tcpd bridges.
However, as shown by their crystallographic parameters and
confirmed by the intermolecular contacts, the tmpa (1·MeOH
and 1) and the andmpa (2·MeOH) complexes display different
crystal packing. One of the principle objectives of this study
concerns the investigation of the subtle chemical substitution of
the tetradentate ligand on the SCO behavior. Complex 1·
MeOH based on the tmpa ligand shows the LS state even at
room temperature but undergoes an incomplete spin-crossover
around 365 K which was probably induced by the desolvation
of the sample above 330 K. The crystallographic structural
studies confirm clearly the total desolvatation of 1·MeOH at
380 K, leading to compound 1 without drastic structural
changes. This desolvated complex (1) reveals a slightly more
gradual SCO behavior at ca. 352 K. Complex 2·MeOH
involving a slightly different tetradentate ligand (andmpa)
shows SCO behavior with a transition temperature of ca. 180
K; after desolvatation, a more abrupt SCO occurs at a slightly
higher temperature (T1/2 = 196 K). The two desolvatation
G
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Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
nitrobenzene (1.12 g, 67%). IR data (ν/cm−1): 3060(w), 3008(w),
1588(m), 1569(m), 1523(s), 1432(m), 1358(m), 764(m), 731(m).
1H NMR δ (300 MHz, CDCl3): 3.78 (4H, s, CH2), 4.07 (2H, s, CH2),
7.12 (2H, dd), 7.33 (1H, t), 7.40 (2H, d), 7.48 (1H, t), 7.63 (2H, t),
7.70 (1H, d), 7.75 (1H, d), 8.49 (d, 2H). 13C NMR δ (75 MHz,
CDCl3): 55.7, 60.3 (2C) (CH2); 121.9 (2C), 123.1 (2C), 124.2, 127.8,
131.2, 132.2 (Cquat); 134.2 (2C), 148.8 (2C), 149.8 (Cquat), 158.5
(Cquat) (Caromatic). Oxygen-free hydrazine hydrate (15 mL, excess)
(Caution! Harmful and unstable, should be used carefully!) was added to
a solution of bis(2-pyridylmethyl)aminomethyl)nitrobenzene (1.00 g,
3.0 mmol) in absolute ethanol (50 mL) with activated carbon (1.0 g).
The reaction mixture was heated to reflux with stirring under nitrogen
atmosphere for 48 h. After cooling, the solution was filtered through
Celite, and the filtrate was evaporated under reduced pressure. The
residue was dissolved in CHCl3 (30 mL) and dried with MgSO4. After
filtration, the solvent was removed to yield a brown solid of bis(2-
pyridylmethyl)aminomethyl)aniline (andmpa) (840 mg, 92%). IR data
(ν/cm−1): 3401(m), 3312(m), 3201(m), 1633(m), 1589(s), 1492(m),
1432(m), 752(vs). 1H NMR δ (300 MHz, CDCl3): 3.66 (2H, s, CH2),
3.79 (2H, s, CH2), 6.61−6.64 (2H, m), 7.02−7.05 (2H, m), 7.11−7.15
(m, 2H), 7.36 (d, 2H), 7.62 (t, 2H), 8.53 (d, 2H). 13C NMR δ (75
MHz, CDCl3): 57.6, 59.9 (2C) (CH2), 115.2, 116.9, 121.8 (2C), 122.0
(Cquat), 123.2 (2C), 128.2, 130.9, 136.1 (2C), 146.8 (Cquat), 148.8
(2C), 158.9 (Cquat) (Caromatic).
Complex Syntheses. For [Fe2(tmpa)2(μ2-tcpd)2]·nCH3OH (1·
MeOH), Fe(BF4)2·6H2O (6.7 mg, 0.02 mmol) was added to a
solution of tmpa (0.02 mmol, 5.8 mg) in methanol (2 mL). The
resulting yellow solution was placed in a glass capillary (diameter 0.3
mm). Then a solution of K2tcpd (5.6 mg, 0.02 mmol) in methanol (2
mL) was carefully added in the capillary. Red crystals suitable for X-ray
diffraction were obtained after 3 days (yield: 5.0 mg, 44.5%). Anal.
Calcd (%) for [C56H36Fe2N20](CH3OH)n for n = 0.3: C, 60.9; H, 3.4;
N, 25.2. Found (%): C, 61.3; H, 3.4; N, 25.4. IR data (ν/cm−1) for the
freshly filtered sample: 3417(br), 3079(w), 2192(s), 2165(s),
1606(m), 1433(s), 1399(s), 1287(w), 1243(m), 1157(w), 1034(w),
991(m), 759(s), 735(m), 644(w), 503(w), 466(m), 443(m). The
unsolvated phase of 1·MeOH [Fe2(tmpa)2(μ2-tcpd)2] (1) has been
prepared easily by heating the sample at 70 °C for 3 h. Anal. Calcd (%)
for [C56H36Fe2N20]: C, 61.1; H, 3.3; N, 25.5. Found (%): C, 61.5; H,
3.3; N, 25.8.18 IR data (ν/cm−1): 3073(w), 2925(w), 2192(s),
2166(s), 1606(m), 1481(m), 1433(s), 1400(s), 1304(w), 1288(w),
1244(m), 1156(w), 1097(s), 1055(w), 1025(w), 991(m), 761(s),
735(m), 686(w), 644(w), 541(w), 502(w), 467(m), 442(m), 406(m).
For [Fe2(andmpa)2(μ2-tcpd)2]·2CH3OH (2·MeOH), Fe(BF4)2.6H2O
(6.7 mg, 0.02 mmol) was added to a solution of andmpa (0.02 mmol,
6.1 mg) in methanol (2 mL). The resulting yellow solution became
green after standing overnight, and then was placed in a glass capillary
(diameter 0.3 mm). A solution of K2tcpd (5.6 mg, 0.02 mmol) in
methanol (2 mL) was carefully added in the capillary. Green crystals
suitable for X-ray diffraction were obtained after 3 days (yield: 4.2 mg,
35.5%). Anal. Calcd (%) for C60H48Fe2N20O2: C, 60.4; H, 4.1; N, 23.5.
Found (%): C, 59.9; H, 4.2; N, 23.2. IR data (ν/cm−1): 3468(m),
3268(w), 3213(w), 3134(w), 2922(w), 2854(w), 2186(s), 2161(s),
1442(s), 1390(s), 1294(w), 1241(w), 1035(s), 1020(s), 867(m),
762(s), 736(m), 688(m), 642(w), 619(w), 572(w), 539(m), 509(s),
488(w), 476(w), 424(w). The unsolvated phase of 2·MeOH
[Fe2(andmpa)2(μ2-tcpd)2] (2) has been prepared by heating the
sample at 100 °C for 6 h. Anal. Calcd (%) for [C58H40Fe2N20]: C,
61.7; H, 3.5; N, 24.8. Found (%): C, 62.1; H, 3.7; N, 24.3. IR data (ν/
cm−1): 3221(w), 3129(w), 2922(w), 2857(w), 2187(s), 2165(s),
1606(m), 1436(s), 1387(s), 1295(w), 1151(w), 1081(m), 1020(m),
867(m), 759(s), 737(m), 688(w), 643(w), 539(m), 508(m), 488(w),
419(w).
Additional characterization data (PDF)
X-ray crystallographic data, CCDC numbers 1413422 (1·
MeOH at 296 K), 1413423−1413424 (2·MeOH at 296
and 100 K, respectively), 1435809−1435810 (1 at 296
and 380 K, respectively), and 1442955−1442970 (1 from
300 to 375 K, respectively) (CIF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: smail.triki@univ-brest.fr.
Author Contributions
The manuscript was written through contributions of all
authors.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work is supported by the CNRS (Centre National de la
Recherche Scientifique), Brest University, the “Agence
Nationale de la Recherche” (ANR project BISTA-MAT:
ANR-12-BS07-0030-01), and the Aquitaine Region for support
of the International Center of Photomagnetisme in Aquitaine.
Authors especially thank the “Service Commun” of NMR
facilities of the University of Brest and D. Denux for thermal
measurements of ICMCB for the TGA analysis.
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I
DOI: 10.1021/acs.inorgchem.6b01542
Inorg. Chem. XXXX, XXX, XXX−XXX