Phenanthroline-Based Molecular Switches for Prospective Chemical Grafting: A Synthetic Strategy and Its Application to Spin-Crossover Complexes

Article abstract of DOI:10.1021/acs.inorgchem.9b01424

1,10-Phenanthroline represents a well-known versatile ligand system finding many applications in chemistry, biology, and material science. The properties and thus the use of these molecules are determined by coordinating metal ions and ligand substituents. Advanced ligand systems that, for instance, feature simultaneously an integrated photochrome and a surface anchoring group require the introduction of several differing substituents and the synthesis of asymmetric derivatives. In spite of a long history of the ligand system - and to our great surprise - a general synthetic approach allowing the introduction of differing substituents at positions (3,8) and (5,6) of 1,10-phenanthroline is not known. Here, we present a general approach for the synthesis of such phenanthrolines. The approach is used to integrate a diarylethene photochrome into a functionalized phenanthroline and thus to synthesize a novel photoswitchable phenanthroline and a corresponding spin-crossover molecular photoswitch. The functionality of both the ligand and its iron(II) complex at room temperature has been demonstrated. The importance of this work for chemical grafting of molecular switches based on phenanthrolines is emphasized.

Full text of DOI:10.1021/acs.inorgchem.9b01424

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Phenanthroline-Based Molecular Switches for Prospective Chemical
Grafting: A Synthetic Strategy and Its Application to Spin-Crossover
Complexes
Max Mortel, Thomas Lindner, Andreas Scheurer, Frank W. Heinemann,
̈
and Marat M. Khusniyarov*
̈
Department of Chemistry and Pharmacy, Friedrich-Alexander University Erlangen-Nurnberg (FAU), Egerlandstraße 1, 91058
Erlangen, Germany
*
S Supporting Information
ABSTRACT: 1,10-Phenanthroline represents a well-known ver-
satile ligand system finding many applications in chemistry,
biology, and material science. The properties and thus the use of
these molecules are determined by coordinating metal ions and
ligand substituents. Advanced ligand systems that, for instance,
feature simultaneously an integrated photochrome and a surface
anchoring group require the introduction of several differing
substituents and the synthesis of asymmetric derivatives. In spite of
a long history of the ligand systemand to our great surprisea
general synthetic approach allowing the introduction of differing
substituents at positions (3,8) and (5,6) of 1,10-phenanthroline is
not known. Here, we present a general approach for the synthesis of such phenanthrolines. The approach is used to integrate a
diarylethene photochrome into a functionalized phenanthroline and thus to synthesize a novel photoswitchable phenanthroline
and a corresponding spin-crossover molecular photoswitch. The functionality of both the ligand and its iron(II) complex at
room temperature has been demonstrated. The importance of this work for chemical grafting of molecular switches based on
phenanthrolines is emphasized.
INTRODUCTION
Scheme 1. Photoisomerizable phen-DAE Ligand Featuring a
Diarylethene Photochrome
■
1
,10-Phenanthroline (phen) is a classical chelating ligand in
coordination chemistry. Its physicochemical properties, relative
easy access to derivatives, and the ability to form complexes
with most metal ions resulted in its appearance in many
branches of science. Molecular recognition and sensing,
luminescent and magnetic materials, photosensitizers, DNA/
RNA cleaving reagents, catalysis, and self-assembly are some
representative research areas with a strong contribution of phen
1
−3
and its metal complexes.
Introduction of photochromic units into phen renders the
4
−6
latter photoisomerizable,
which provides access to versatile
7
8,9
10
molecular switches. Thus, spirooxazine and diarylethene
photochromes have been successfully integrated
ligands, and spectacular photoswitching of luminescent
11−13
into phen
spin (LS) states with light at room temperature (RT) via the
photoisomerization of the diarylethene unit. Unique reversible
photoswitching was demonstrated in solution and in the solid
13−15
1
6−20
and magnetic
properties of corresponding metal com-
2
4−30
plexes, as well as light-controlled binding of metal-free ligands
state. For many practical applications,
functional molecules are required.
thin films of
2
1
31−33
to DNA, have been demonstrated in recent years.
Thus, using a physical
vapor deposition technique, corresponding thin films were
prepared, which, however, showed relatively poor photo-
switchability. This is presumably due to unfavorable
interactions between molecules and the surface, which is a
19,20
In our previous work,
a photoisomerizable phen-DAE
ligand, which comprises phen and an electronically coupled
diarylethene (DAE) photochrome (Scheme 1), was used to
achieve an unprecedented remote control of magnetism in
1
7
2
2,23
spin-crossover
(SCO) metal complexes. Thus, an iron(II)
complex with a phen-DAE ligand was successfully switched
Received: May 15, 2019
between paramagnetic high-spin (HS) and diamagnetic low-
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.9b01424
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
3
4,35
χLT T + χHT Te(
−ΔH+TΔS)/RT
known phenomenon in SCO research.
A possible
χT =
alternative to physisorptionchemisorptionmight provide
a great opportunity to diminish or even eliminate such
destructive interactions by decoupling the absorbed molecules
from the substrate.
+ e(−ΔH+TΔS)/RT
1
(1)
5
7
Fe Mossbauer spectra were recorded on a WissEl Mossbauer
̈
̈
57
spectrometer (MRG-500) in constant-acceleration mode. Co/Rh
was used as the radiation source. The temperature of the samples was
For successful chemisorption, the parent SCO molecular
switch should be modified: an appropriate anchoring group
should be introduced at a proper position of the molecule
without functionality loss. Very recently, we showed that
positions 3,8 of phen (Scheme 1) should be suitable for
introducing anchors. However, and to our great surprise, a
general synthetic scheme allowing introduction of substituents
at positions 3,8 (prospective anchors) with simultaneous
functionalization at positions 5,6 (photoactive site) of phen is
not known.
controlled by an MBBC-HE0106 Mossbauer He/N cryostat within
̈
2
an accuracy of ±0.3 K. Isomer shifts were determined relative to α-
iron at 298 K. The program MFIT was used for the quantitative
44
analysis of spectra. NMR spectra were recorded with JEOL
ECX400, JEOL ECZ400S, JEOL EX270, and Bruker Avance III
HD 600 in rotating 5 mm o.d. tubes and processed with Delta V4.0
software provided by JEOL Ltd. Electronic absorption spectra were
recorded with a Shimadzu UV 3600 spectrophotometer.
Photochemisty. All solutions for photoexperiments were
prepared under inert conditions and sealed in custom-made Quartz
SUPRASIL cells (QS). An L.O.T.-Oriel Xe(OF) 150 W arc lamp
equipped with appropriate filters was used as a light source. Water and
bandpass (λ = 282 ± 5 nm) filters were used for cyclization with UV,
while a set of water, heat absorbing (KG-5), and long-pass (λ ≥ 400
nm) filters was used for cycloreversion with visible. Further
cyclization experiments were also conducted with the sample
positioned between two opposing UV analytical lamps (Herolab
GmBh, 15W, λ = 254 nm).
Crystallographic Data. The material consisted exclusively of very
thin and weakly diffracting crystals (see Figure S12). A suitable single
crystal was embedded in protective perfluoropolyalkyether oil on a
microscope slide, and a single specimen was selected and
subsequently transferred to the cold nitrogen gas stream of the
diffractometer. Intensity data were collected using Mo Kα radiation (λ
18
Here, we present a general approach for the synthesis of
phen derivatives with unequal substituents at positions (5,6)
and 3 (Scheme 2). Note that the preparation of some phen
derivatives with equal substituents was mentioned in old
3
6,37
literature.
We have successfully used this approach to
38
introduce a pro-anchoring phenyl group into phen-DAE. A
novel Ph-phen-DAE ligand (Scheme 3) and a corresponding
SCO molecular switch [Fe (Ph-phen-DAE)(H Bpz ) ]
II
2
2 2
(
H Bpz = bis(1-pyrazolyl)dihydroborate(1−)) have been
2
2
synthesized, and their switchability at RT has been
demonstrated. The developed synthetic strategy opens the
route toward novel asymmetric phen derivatives including
phen-DAE-based molecular switches capable of chemisorption.
=
0.71073 Å) on a Bruker Kappa PHOTON II IμS Duo
diffractometer equipped with QUAZAR focusing Montel optics at
temperatures of 100 and 273 K. Data were corrected for Lorentz and
polarization effects, and semiempirical absorption corrections were
performed on the basis of multiple scans using SADABS. The
structures were solved by direct methods (SHELX XT) and refined
45
46
Scheme 2. A Successful Synthetic Approach toward 3,5,6-
2
by full-matrix least-squares procedures on F using SHELXL 2016/
1
2
Trisubstituted phen’s with Differing Substituents R and R
47
6.
All non-hydrogen atoms were refined with anisotropic displace-
ment parameters. All hydrogen atoms were placed in positions of
optimized geometry, and their isotropic displacement parameters
were tied to those of the corresponding carrier atoms by a factor of
48
either 1.2 or 1.5. Olex2 was used to prepare material for publication.
Crystallographic data, data collection, and structure refinement details
are given in Table S1. At both temperatures the thiophene ring C25,
C26, C27, S2, and C28 seemed to be slightly disordered with a
change in positions of S2 and C27 (and consequently C29). However,
attempts to refine corresponding split positions remained unsuccess-
ful, probably due to the fact that the alternative orientation was
occupied by only a small percentage. The compound crystallized with
a total of 1.5 molecules of toluene per formula unit with half a toluene
molecule being situated disordered on the crystallographic inversion
center (C201−C204). The other toluene molecule was also
disordered. Two alternative orientations were refined and resulted
at 100 K in site occupancies of 47.4(9) and 52.6(9)% and at 273 in
site occupancies of 49(2) and 51(2)% for the atoms C101−C107 and
C111−C117, respectively. Similarity and pseudoisotropic restraints
(with esd’s of 0.01 and 0.02) were applied to the anisotropic
displacement parameters of the disordered atoms. Fixed distance
restraints (with esd’s of 0.01) were applied to some of the distances in
the disordered toluene molecules.
EXPERIMENTAL SECTION
■
Materials. All starting materials and solvents were utilized as
received without further purification unless otherwise noted. Pure
anhydrous solvents were collected from a solid-state solvent
purification system, Glass Contour (Irvine, CA). (2,5-Dimethylth-
3
9,40
41
iophen-3-yl)boronic acid,
bis(1-pyrazolyl)borohydride, and 5,6-
4
2
dibromo-1,10-phenanthroline were prepared according to literature
methods with minor modifications.
Instrumentation. Elemental analyses were carried out with a
EURO EA analyzer from EuroVector. Magnetic susceptibility data on
solid samples were collected with a Quantum Design MPMS 3
Magnetometer. DC susceptibility data were collected on powder
samples restrained within a polycarbonate gel capsule in the applied
magnetic field of 1 T at 1 K min− heating/cooling rate and 2 K
intervals. The magnetic susceptibility data were corrected for
Synthesis. 5,6-Dibromo-3-iodo-1,10-phenanthroline (5). 5,6-
Dibromo-1,10-phenanthroline (1.338 g, 4.1 mmol) and iodine
(3.642 g, 14.35 mmol) were dissolved in 32 mL of 1,2-dichloroethane.
The reaction mixture was stirred vigorously and heated to reflux
1
−
6
3
diamagnetism using an estimation χ
= 1/2 M ·10 cm
before tert-butyl hydroperoxide (TBHP) (70% in H O, 36.0 mL,
m,diamag
w
2
−
1
43
mol with Mw being the molar mass of the compound. The χT
product was fitted using the van’t Hoff equation (eq 1) to give the
enthalpy and entropy changes ΔH and ΔS for the LS → HS
conversion with χ and χ being low- and high-temperature limits
262.8 mmol) was added. After 7 h more TBHP (18.0 mL, 131.4
mmol) was added. After 24 h, the reaction mixture was cooled down
to RT and poured into a saturated Na S O solution. The solution
2
2
3
turned yellow after stirring for 15 min and was extracted with CHCl3
LT
HT
for molar magnetic susceptibility, respectively.
(1 × 150 mL, 4 × 50 mL). The combined organic phases were dried
B
DOI: 10.1021/acs.inorgchem.9b01424
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Scheme 3. Explored Synthetic Routes and the Final Synthesis of a Photoisomerizable Ph-phen-DAE Ligand 7
in vacuo. The remaining solid was purified via SiO2 column
(C ), 127.9 (C , 11), 125.7 (C , 6/5), 125.5 (C , 6/5), 124.5 (C , 2)
q
t
q
q
t
chromatography using CHCl :EtOAc (1:1) with 5% triethylamine
ppm.
3
as an eluent. Pure 5 was obtained as a white powder after
recrystallization from boiling toluene. Yield: 381 mg, 20%. Elemental
analysis calcd. for C H Br N : C 31.07, H 1.09, N 6.04; found: C
5,6-Bis(2,5-dimethylthiophen-3-yl)-3-phenyl-1,10-phenanthro-
line (7). A Schlenk flask with inert atmosphere was charged with 6
(640 mg, 1.54 mmol), 2,5-dimethylthiophenyl-3-boronic acid (1.441
12
5
2
2
1
3
9
9
7
0.80, H 1.04, N 5.98. H NMR (269.71 MHz, CDCl , 22.9 °C) δ:
g, 9.24 mmol), Na CO (1.71 g, 9.24 mmol), and [Pd(tpp) ] (356
2
3
4
3
4
3
4
.31 (d, J = 1.9 Hz, 1 H, 1), 9.19 (dd, J = 4.3, J = 1.6 Hz, 1 H, 5),
mg, 0.308 mmol). Degassed 1,4-dioxane (24 mL) and degassed water
(12 mL) were added, and the reaction mixture was heated to 100 °C
under vigorous stirring for 36 h. The biphasic solution was extracted
4
3
4
.06 (d, J = 1.9 Hz, 1 H, 2), 8.73 (dd, J = 8.4, J = 1.6 Hz, 1 H, 3),
3
3
13
.74 (dd, J = 8.4, J = 4.3 Hz, 1H, 4) ppm. C NMR (67.8 MHz,
CDCl , 22.7 °C) δ: 156.6 (C , 1), 151.1 (C , 10), 145.1 (C , 12),
with CHCl
brine (2 × 100 mL) before it was dried over Na
removed in vacuo, and the crude product was purified via column
chromatography on silica using EtOAc/CHCl 1:1 with 5%
3
(5 × 50 mL), and the organic extract was washed with
3
t
t
q
1
44.8 (C , 3), 143.9 (C , 11), 137.5 (C , 8), 129.8 (C , 4), 128.8 (C ,
2
SO . The solvent was
4
t
q
t
q
q
7
), 126.1 (C , 5), 124.8, (C , 9), 123.7 (C , 6), 94.9 (C , 2) ppm.
q t q q
5
,6-Dibromo-3-phenyl-1,10-phenanthroline (6). A Schlenk flask
3
was charged with 5 (552 mg, 1.19 mmol), phenylboronic acid (170
triethylamine as eluent. The resulting oil was suspended in hexanes
and sonicated for 5 min in an ultrasonic bath. The final compound
was obtained after extensive drying in vacuo as a beige powder. Yield:
mg, 1.31 mmol), anhydrous Na CO (378 mg, 3.57 mmol), and
2
3
[
(
Pd(dppf)]Cl ·DCM (147 mg, 0.18 mmol, dppf = 1,1′-bis-
2
diphenylphosphino)ferrocene) before it was cycled with nitrogen.
357 mg, 49%. Elemental analysis calcd. for C30
5.08, N 5.88, S 13.45 found: C 74.71, H 5.05, N 5.79, S 13.31. H
NMR (600.13 MHz, CDCl , 24.9 °C, contains signals of parallel/
H N S : C 75.60, H
24 2 2
1
Then 14 mL of degassed THF, 14 mL of degassed toluene, and 6.5
mL of degassed H O were added. While stirring, the reaction was
3
2
heated to 95 °C. After 26 h the reaction mixture was extracted with
antiparallel conformers with an indiscernible 0.8:1.0 ratio. The minor
isomeric species is labeled with a star (*))δ 9.43 (m, J = 2.3 Hz, 2H.
4
CHCl (1 × 50 mL, 3 × 25 mL) and washed with water and brine (2
3
3
4
×
50 mL). The combined organic phases were dried over Na SO . All
12/12*), 9.20 (d, J = 4.1 Hz, 2H, 1, 1*), 8.17 (d, J = 2.3 Hz, 1H,
4 3
2
4
volatiles were removed and the dark red powder was dried in vacuo
before it was purified via SiO2 column chromatography using
8), 8.11 (d, J = 2.2 Hz, 1H, 8*), 8.03 (dd, J = 8.3 Hz, 1.6 Hz, 1H,
3 3
3), 7.98 (dd, J = 8.3, 1.6 Hz, 1H, 3*), 7.66 (t, J = 7.5 Hz, 4H, 9, 9*),
3
4
3
CHCl :EtOAc 1:1 with 5% added triethylamine as an eluent.
7.57 (m, J = 8.3 Hz, J = 4.1 Hz, 2H, 2, 2*), 7.50 (t, J = 7.6 Hz, 4H,
3
3
Recrystallization from the eluent mixture yields 6 as colorless needles.
Yield: 390 mg, 79%. Elemental analysis calcd. for C H Br N · 0.45
10, 10*), 7.42 (t, J = 7.4 Hz, 2H, 11, 11*), 6.37 (m, 4 H, 4/4*, 7/
7*) 2.40 (m, 12H, 5′/5′*, 6/6*), 2.05 (s, 3H, 5), 2.02 (s, 3H, 6’),
1
8
10
2
2
1
13
CHCl : C 47.37, H 2.25, N 5.99; found: C 47.44, H 2.17, N 5.87. H
NMR (270 MHz, CDCl , 22.9 °C) δ 9.45 (d, J = 2.1 Hz, 1H, 8),
.24 (dd, J = 4.3 Hz, J = 1.5 Hz, 1H, 1), 8.89 (d, J = 2.2 Hz, 1H, 4),
.77 (dd, J = 8.4 Hz, J = 1.6 Hz, 1H, 3), 7.81 (d, J = 6.9 Hz, 2H, 5),
.74 (dd, J = 8.4, J = 4.3 Hz, 1H, 3), 7.59 (t, J = 7.2 Hz, 2H, 6),
.51 (t, J = 7.3 Hz, 1H, 7). C NMR (100.53 MHz, CDCl ,22.8 °C)
2.01 (s, 3H, 5*), 1.98 (s, 3H, 6’*). C NMR (100.53 MHz, CDCl ,
3
3
4
24.7 °C) δ: 149.98 (C ), 149.97 (C ), 149.01 (C ), 148.99 (C ),
3
t
t
t
t
3
4
4
9
8
7
7
145.54 (C ), 145.49 (C ), 144.57 (C ), 144.52 (C ), 137.84 (C ),
q q q q q
3
4
3
137.82 (C ), 135.97 (C ), 136.00 (C ), 135.74 (C ), 135.66 (C ),
q q q q q
3
3
3
135.6 (C ), 135.5 (C ), 135.26 (C ), 135.12 (C ), 134.28 (C ), 134.22
q q t t q
3
13
(C ), 133.97 (C ), 133.94 (C ), 133.91 (C ), 133.87 (C ), 133.75
q q q q q
3
δ: 151.3 (C , 1), 150.2 (C , 14), 145.6 (C , 15/16), 145.5 (C , 15/
(C ), 133.66 (C ), 133.61 (C ), 133.59 (C ), 133.5 (C ), 132.5 (C ),
q q q q q t
t
t
q
q
1
6), 137.6 (C , 9), 137.5 (C , 3), 136.9 (C , 10), 134.8 (C , 8), 129.5
132.4 (C ), 129.19 (C ), 128.7 (C ), 128.67 (C ), 128.64 (C ), 128.5
t t q q q
q
q
q
t
(
C , 12), 129.0 (C , 13), 128.73 (C , 4/7), 128.70 (C , 4/7), 128.7
(C ), 128.3 (C ), 128.24 (C ), 128.21 (C ), 127.65 (C ), 127.63 (C ),
q t t t t t
t
t
q
q
C
DOI: 10.1021/acs.inorgchem.9b01424
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
1
1
27.15 (C ), 127.10 (C ), 123.12 (C ), 123.07 (C ), 15.23 (CH ),
7 becomes red due to photocyclization. The photoreaction is
accompanied by decreasing intense absorption bands at 247
and 278 nm and appearance of two novel bands at 363 and
t
t
t
t
3
5.21 (CH ), 15.18 (CH ), 14.14 (CH ), 14.05 (CH ), 13.97 (CH ).
3
3
3
3
3
II
[
(Ph-phen-DAE)Fe (H Bpz ) )] (8). A solution of potassium bis(1-
2 2 2
pyrazolyl)borohydride (0.156 g, 0.84 mmol) in dry MeOH (5 mL)
was added dropwise to a stirred dry MeOH solution (3 mL) of
FeSO ·7H O (0.116 g, 0.42 mmol) at RT, whereupon a very fine
∼
515 (broad) nm, which is characteristic for a closed-ring
isomer (Figure 1). The photostationary state was reached
4
2
white solid precipitated. The fine powder was removed by
centrifugation followed by a decantation of the colorless solution of
II
the in situ formed “[Fe (H B(pz) ) ]”. To this solution, 7 (0.2 g, 0.42
2
2 2
mmol) dissolved in dry MeOH (15 mL) was added, and the resulting
violet suspension was stirred for 1.5 h at RT. The suspension was
concentrated to ∼1/4 volume and stored at −35 °C for 3 days. The
violet precipitate was filtered off (glass filter G4) and dried in vacuo to
give 8 as a fine blue powder. Yield: 238 mg, 69%. Elemental analysis
calcd. for C H B FeN S : C 61.04, H 4.88, N 16.95, S 7.76; found:
42
40
2
10 2
C 61.02, H 4.90, N 16.86, S 7.68.
RESULTS AND DISCUSSION
■
We pursued three synthetic routes toward the desired Ph-phen-
DAE ligand (Scheme 3). 1,10-Phenanthroline was regioselec-
tively brominated according to Yamamoto et al. to yield 3,8-
4
9
dibromo-1,10-phenanthroline 1, which was successfully
converted into 3,8-diphenyl derivative 2 via a Suzuki cross-
50
Figure 1. Photocyclization of Ph-phen-DAE ligand 7 with UV at RT
coupling with 2 equiv of phenylboronic acid. The subsequent
bromination at positions 5 and 6 of phen, which requires harsh
conditions (oleum), was unsuccessful, leading to polymers
insoluble in common organic solvents. Alternatively, 3,5,6,8-
tetrabromo-1,10-phenanthroline 3 can be easily prepared
−
5
(
λ = 282 nm, MeCN, c = 6.0 × 10 M).
within 180 s under our conditions (MeCN solution, c = 6.0 ×
−
5
10 M, RT, see Supporting Information for details). The
reverse reaction, photocycloreversion, is accomplished by
irradiation with visible light (λ > 400 nm), which leads to a
nearly complete restoration of the original spectrum of the
open-ring form within 1 min (Supporting Information). The
photocyclization−cycloreversion cycle can be repeated at least
3 times revealing a very good resistance to fatigue (Supporting
Information). The closed-ring isomer shows moderate thermal
stability with a half-life of 29.8(8) h at RT in solution. Thus,
the closed-ring 7 is less stable than the parent nonsubstituted
51
according to Rau et al. The attempted regioselective cross-
coupling with 2 equiv of phenylboronic acid was not successful
in our hands. Moreover, 3 is poorly soluble in common organic
solvents, which hinders the search for optimal conditions for
the anticipated regioselective coupling.
Finally, 1,10-phenanthroline was regioselectively brominated
42
to yield 5,6-dibromo-1,10-phenanthroline 4. Adopting the
52
procedure of Jain et al. and using tert-butyl hydroperoxide as
a radical source, we successfully iodinated 4 to obtain a novel
20
5
,6-dibromo-3-iodo-1,10-phenanthroline 5. A common by-
phen-DAE (137 h) but much more stable than a derivative
18
productpoorly soluble 5,6-dibromo-3,8-diiodo-1,10-phenan-
throlineand unreacted 4 can be removed via column
chromatography followed by recrystallization. The raw yield
before the column was 30−35% as determined by NMR
spectroscopy, and the pure product was isolated with the yield
of 20%. The relatively low yields are mainly due to the
formation of intractable polymers in the free-radical reaction
and due to the product loss upon purification. It is very
important to note that the novel phen derivative 5 possesses
two different types of halogens (Br and I), which provides an
exciting opportunity to accomplish two subsequent C−C
coupling reactions regioselectively, thus leading to a new
library of asymmetric phen ligands.
with phenyl-substituted thiophenes (12 min).
Once the photoswitchability of Ph-phen-DAE was confirmed,
we used this ligand to control magnetic properties with light at
II
RT. Thus, a SCO complex [(Ph-phen-DAE)Fe (H Bpz ) ] 8
2
2 2
was synthesized: A straightforward reaction of Ph-phen-DAE
II
with in situ formed “[Fe (H Bpz ) ]” in absolute MeOH under
2
2 2
anaerobic conditions gave 8 as a blue powder with yields up to
69%.
Single-crystal structure determination performed on a rather
small crystal of 8 obtained from a toluene/n-hexane mixture
53
reveals exclusively a parallel conformer (Figure 2). The
complex crystallizes in the triclinic P1 group and contains 1.5
̅
toluene molecules per molecular unit. At 100 K, all Fe−N
Thus, a cross-coupling of 5 with 1 equiv of phenylboronic
acid yielded exclusively 5,6-dibromo-3-phenyl-1,10-phenan-
throline 6 with yields up to 79%. A subsequent C−C coupling
with 2 equiv of 2,5-dimethylthiophenyl-3-boronic acid gave the
target Ph-phen-DAE ligand 7 as a light-beige powder. Due to its
asymmetry and the presence of two conformers (parallel and
bond distances are short (1.990−2.013 Å), suggesting a LS
54
iron(II) center. Upon increasing the temperature to 273 K,
these bond distances become significantly larger (2.150−2.171
Å), which confirms a thermal switching to a HS state. The π
system of phen is essentially electronically uncoupled from the
thiophene π systems as confirmed by dihedral angles between
1
0
55
antiparallel), 7 reveals sophisticated NMR spectra (Support-
ing Information). For instance, seven different methyl groups
and four thiophene-protons are observed within 1.21−1.40 and
the corresponding unit planes at 64.8 and 70.8°. The phenyl
group is rotated from the phen plane by 38.9° due to repulsion
between α-hydrogen atoms of phenyl and the corresponding
hydrogen atoms of phen.
6
.34−6.37 ppm range, respectively.
The ligand was obtained as an open-ring isomer (ground
The complex reveals thermally induced SCO both in solid
state and solution, which was investigated by SQUID
state), and its photochemical properties were tested. Thus,
upon irradiation with UV (λ = 282 nm), a colorless solution of
5
7
magnetometry, Fe Mo
̈
ssbauer spectroscopy, and Evans
D
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Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
seems to be even smoother in the solid state than in a toluene
solution (vide infra). Similar anticooperative behavior was
18
previously observed for a closely related complex. Finally,
magnetic data obtained in the heating and cooling modes are
nearly identical, pointing to the absence of thermal hysteresis
in the solid 8.
̈
Thermal SCO is further confirmed by Mossbauer spectros-
copy (Supporting Information), revealing one quadrupole
−
1
−1
doublet (δ = 0.54 mm·s , |ΔE | = 0.49 mm·s ) due to a LS
Q
−
1
state at 77 K. A new doublet (δ = 1.50 mm·s , |ΔE | = 1.00
mm·s ) appears at RT due to a HS state. In toluene solution,
adopts a pure HS state at RT. Upon lowering the
temperature, the χT decreases gradually, reaching 2.07 cm
mol K at 188 K, which points to incomplete thermal
switching to a LS state at this temperature (Figure 4). The
Q
−
1
8
3
−
1
Figure 2. Molecular structure of open-ring 8 in crystals of 8·1.5
toluene at 100 K showing a parallel conformer. The H atoms and
solvent molecules are omitted for clarity; thermal ellipsoids are drawn
at the 50% probability level.
Figure 4. Temperature dependent χT product of 8 measured with the
Evans NMR method in toluene:toluene-d :TMS = 10:2:1 solution.
8
−
1
Referenced on TMS. Fitting parameters: ΔH = 22.4(8) kJ mol , ΔS
−1
−1
=
111(4) J K mol .
Figure 3. Variable temperature χT product of 8 measured on a
powder sample at an external magnetic field of 1 T in the heating
transition temperature T1/2 determined using the average χT =
−
1
3
−1
mode (2 K steps, 1 K min ). Fitting parameters: ΔH = 13.2(1) kJ
(χHTT − χ T)/2 = 1.914 cm mol K is 186 K, which is
LT
−
1
−1
−1
mol , ΔS = 45.3(5) J K mol .
lower than in the solid state. The van’t Hoff fit yields
thermodynamic parameters for the thermal SCO transition,
and the corresponding T1/2 = ΔH/ΔS = 200.8(4) K.
NMR method. In the solid state, χT product (χ is molar
magnetic susceptibility) of 2.66 cm3 mol K at RT is
consistent with the majority of 8 being in a HS state (Figure
−1
Photoswitching of 8 was investigated at RT in acetonitrile
solution. Upon UV irradiation the intense bands in the UV
region decrease, while two new distinct bands at 381 and ∼550
(broad) nm emerge (Figure 5). The new bands resemble those
of the closed-ring metal-free ligand 7 but appear bathochromi-
cally shifted, which points to a successful photocyclization of a
coordinated ligand. Photocycloreversion is accomplished by
irradiation with visible light, where the parent spectrum of the
3
−1
3
). Upon heating to 360 K, χT increases until 3.09 cm mol K
due to increasing HS fraction. Upon lowering the temperature,
3
χT decreases very gradually reaching a plateau at ∼0.27 cm
−
1
mol K due to the thermal switching to a LS state. The
nonzero χT at low temperatures is likely due to a residual HS
fraction (∼7%); however, the contribution of the temperature
independent paramagnetism of the LS state cannot be
excluded. A van’t Hoff fit reveals thermodynamic parameters
ΔH = 13.2(1) kJ·mol and ΔS = 45.3(5) J K mol for
thermal SCO. The transition temperature T1 is 244 or 236 K
−
1
−1
−1
/2
as determined by the maximum of the first derivative d(χT)/
3
dT or from the average χT = (χ T − χ T)/2 = 1.622 cm
HT
LT
−
1
mol K, respectively. Notice that the transition temperature
derived through the fitted thermodynamic parameter as T1/2
=
ΔH/ΔS is artificially too high due to the limited applicability
of van’t Hoff fits to solid samples having intermolecular
interactions. The HS fraction is estimated to amount 75 and
8
6% at RT and 360 K, respectively.
The shape of the transition curve points to substantially
weak intermolecular interactions in the solid. Interestingly,
although very gradual SCO transitions for ferrous complexes in
Figure 5. Photocyclization of spin-crossover complex 8 with UV at
RT (λ = 282 nm, MeCN, c = 1.3 × 10 M).
56−58
−5
the solid state are not uncommon,
the transition in 8
E
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Inorganic Chemistry
Article
open-ring 8 is nearly completely restored (Supporting
Information). Reversible photocyclization of 8 can be repeated
several times (Figure 6 and Supporting Information). The
Accession Codes
CCDC 1897471−1897472 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
E-mail: marat.khusniyarov@fau.de.
■
*
ORCID
Andreas Scheurer: 0000-0002-2858-9406
Frank W. Heinemann: 0000-0002-9007-8404
Marat M. Khusniyarov: 0000-0002-2034-421X
Notes
The authors declare no competing financial interest.
Figure 6. Multiple photoswitching of 8 at RT (MeCN, c = 1.3 × 10−
M). The evolution of absorption at 380 nm upon alternating
irradiation with UV (λ = 282 nm, blue, photocyclization) and visible
light (λ > 400 nm, red, photocycloreversion) is shown. The data were
fitted according to first-order kinetics.
5
ACKNOWLEDGMENTS
This work was financially supported by the Deutsche
Forschungsgemeinschaft (DFG Research Grant KH 279/3).
Dr. Jorg Sutter is acknowledged for measuring Mossbauer
̈ ̈
spectra. We thank Jochen Schmidt for acquiring some of the
NMR data. MMK is grateful to Prof. Karsten Meyer for
providing access to spectroscopic facilities and his continuous
support. Maria Mateusch is acknowledged for the assistance
with the cover art. AS thanks Prof. Karsten Meyer and German
Federal Ministry of Education and Research (BMBF Grant
■
exceptionally long half-life of the closed-ring 8 of 533(19) h
22 days) at RT in solution exceeds that of the metal-free
ligand 7 and all analogues’ SCO photoswitches reported
(
1
8,20
before.
The photomagnetic effect was investigated in MeCN/
0
3SF0502).
MeCN-d solution at RT using the NMR Evans method.
3
3
−1
3
The χT product of 3.21 cm mol K, dropped to 2.93 cm
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variable-temperature Evans measurements on a UV irradiated
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