Isolation of a metal-to-ligand charge-transfer (MLCT) state of a tris 1,4-diimine complex of iron in the solid state: X-ray crystal structure and EPR

Article abstract of DOI:10.1039/C6NJ02874H

Using 5,6-dihydro-5,6-epoxy-1,10-phenanthroline as the ligand L, the X-ray crystal structure of [FeL₃](ClO₄)₂·2H₂O is determined. Its EPR and magnetic properties indicate entrapment of an MLCT state of the type Fe(iii)-L^-?.

Full text of DOI:10.1039/C6NJ02874H

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Isolation of a metal-to-ligand charge-transfer
(MLCT) state of a tris 1,4-diimine complex of iron
in the solid state: X-ray crystal structure and EPR†
Cite this:DOI: 10.1039/c6nj02874h
Received (in Victoria, Australia)
12th September 2016,
Nirmal K. Shee,a Michael G. B. Drewb and Dipankar Datta*a
Accepted 21st December 2016
DOI: 10.1039/c6nj02874h
www.rsc.org/njc
Using 5,6-dihydro-5,6-epoxy-1,10-phenanthroline as the ligand L,
the X-ray crystal structure of [FeL3](ClO4)2ꢀ2H2O is determined.
Its EPR and magnetic properties indicate entrapment of an MLCT
state of the type Fe(III)–Lꢁ
.
ꢂ
Metal-to-ligand charge-transfer (MLCT) bands, observed in
electronic spectra, are one of the salient features of transition
metal complexes. The corresponding excited states have never
been crystallized, though their electronic nature can be discerned
from photo-luminescence (if observed). Herein we report the
X-ray crystal structure of the MLCT state in the [FeL3]2+ ion where
the ligand
L is 5,6-dihydro-5,6-epoxy-1,10-phenanthroline.
Epoxidation of the 5,6 double bond in 1,10-phenanthroline
(phen), a versatile ligand in coordination chemistry, yields L.
The 10 Dq of phen1 with respect to Ni(II) is 12 745 cmꢁ1 and
that2 of L is 12 515 cmꢁ1. This small difference in the crystal
field strength of phen and L results in a dramatic change in the
electronic properties of their iron complexes.
Fig. 1 Structure of the cation in [FeL3](ClO4)2ꢀ1.5CH3CN with ellipsoids at
30% probability. The oxygen atoms O(25) and O(65) are disordered over
two sites on either side of the ligand plane. Only the major components
O(25a) and O(65a) with occupancy factors 0.65(1) and 0.58(1) are shown.
Selected bond distances (Å) and angles (1): Fe(1)–N(22) 1.956(4),
Fe(1)–N(51) 1.972(4), Fe(1)–N(11) 1.976(5), Fe(1)–N(62) 1.977(4), Fe(1)–N(31)
1.977(4), Fe(1)–N(42) 1.983(4), N(22)–Fe(1)–N(51) 93.7(2), N(22)–Fe(1)–N(11)
82.4(2), N(51)–Fe(1)–N(11) 175.3(2), N(22)–Fe(1)–N(62) 89.7(2), N(51)–Fe(1)–
N(62) 82.2(2), N(11)–Fe(1)–N(62) 95.1(2), N(22)–Fe(1)–N(31) 93.8(2), N(51)–
Fe(1)–N(31) 93.9(2), N(11)–Fe(1)–N(31) 89.0(2), N(62)–Fe(1)–N(31) 174.9(2),
N(22)–Fe(1)–N(42) 175.2(2), N(51)–Fe(1)–N(42) 89.7(2), N(11)–Fe(1)–N(42)
94.3(2), N(62)–Fe(1)–N(42) 94.3(2), N(31)–Fe(1)–N(42) 82.4(2).
The reaction of L with Fe(ClO4)2ꢀxH2O in methanol in 3 : 1
molar proportions at room temperature in air leads to the
formation of [FeL3](ClO4)2ꢀ2H2O (1ꢀ2H2O). Single crystals suitable
for X-ray diffraction were obtained by slow diffusion of diethyl
ether into a dilute solution of the compound in acetonitrile as
[FeL3](ClO4)2ꢀ1.5CH3CN (1ꢀ1.5CH3CN). The X-ray crystal structure
of the cation [FeL3]2+ is shown in Fig. 1 with crystallographic data
given in Table 1. The structure of 1ꢀ1.5CH3CN contains discrete
ꢁ
[FeL3]2+ and 2ClO4 ions together with solvent MeCN; one
molecule with full occupancy and another with 50% occupancy
disordered over a center of symmetry. The metal atom is bonded
to three ligands and is six-coordinate with a distorted octahedral
environment. In the three ligands only one of the oxygen atoms
O(45) is in an ordered position as the other two oxygen atoms
O(25) and O(65) are disordered over two sites on either side of
the ligand plane. The major components O(25a) and O(65a) have
a Department of Inorganic Chemistry, Indian Association for the Cultivation of Science,
Calcutta 700 032, India. E-mail: icdd@iacs.res.in
b Department of Chemistry, University of Reading, Whiteknights,
Reading RG6 6AD, UK
† Electronic supplementary information (ESI) available: Fig. S1 (EPR of 1ꢀxCH3CN)
and Fig. S2 (solution electronic spectra of 1ꢀ2H2O and 1ꢀ3H2O). CCDC 1429593
contains the supplementary crystal data for [FeL3](ClO4)2ꢀ1.5CH3CN. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/c6nj02874h refined occupancy factors of 0.65(1) and 0.58(1). The Fe–N bond
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Table 1 Selected crystallographic data for [FeL3](ClO4)2ꢀ1.5CH3CN
[Fe(5,6-dimethyl phen)3](ClO4)2 (by one electron) at 77 K in
acetonitrile is similar with g = 1.994, which has been attributed to
a phen based radical.5 The single crystals of [FeL3](ClO4)2ꢀ1.5CH3CN
display an EPR spectrum (Fig. S1; ESI†) similar to that of 1ꢀ2H2O
in the solid state at room temperature where the weak signal at
gJ = 2.387 is not observed. The various observations made from
the EPR spectra indicate that an electron has moved over to a
ligand moiety from the metal center in [FeL3](ClO4)2ꢀ2H2O, i.e. a
species resulting from a metal-to-ligand charge transfer (MLCT)
has been captured in [FeL3](ClO4)2ꢀ2H2O. A consequence of this
is the observation of a broad inter-valence charge transfer band
in the electronic spectrum of [FeL3](ClO4)2ꢀ2H2O at 1160 nm with
a half-width of 5160 cmꢁ1 in the solid state (Fig. 3).
Formula
C39H28.5N7.5FeCl2O11
M
904.94
Monoclinic, C2/c
Space group
Cell dimensions
a (Å)
24.8882(11)
9.1632(4)
b (Å)
c (Å)
33.9677(12)
101.789(4)
7583.1(5)
8, 1.585
0.614
b (deg)
V [Å3]
Z, dcalc (gm cmꢁ3
)
m (mmꢁ1
)
Tmin/Tmax
F(000)
ymax (deg)
0.914/1.000
3704
30
Unique/observed reflections
Parameters
10 951/3492 [R(int) = 0.0832]
543
0.819
Whether the valence is localized or delocalized in systems
displaying inter-valence charge transfer can be discerned from
Hush’s theory,6–8 where for a class II (valence localized) system
GOF on F2
R1, wR2 [I 4 2s(I)]
R1, wR2 (all data)
0.0695, 0.1536
0.2166, 0.2000
1.059/ꢁ0.710
the observed bandwidth should be around (2310 ꢃ nmax
) .
1/2 cmꢁ1
Largest peak/hole (e Åꢁ3
)
When the bandwidth is much smaller than this, the system
becomes class III signifying complete valence delocalization.
lengths are in the expected range at 1.956(4)–1.983(4) Å. The bite In our case, a lmax of 1160 nm yields a bandwidth of 4463 cmꢁ1
angles of 82.4(2), 82.2(2), and 82.46(2)1 are primarily responsible close to our value of 5160 cmꢁ1. Thus the valence is somewhat
for the small deviations from octahedral geometry. The dimen- localized in [FeL3](ClO4)2ꢀ2H2O. This is also apparent from the
sions in the structure compare closely to the values found in the EPR spectrum of [FeL3](ClO4)2ꢀ2H2O where signals from two
analogous complex3 [Fe(phen)3](ClO4)2ꢀ0.5H2O where the iron distinct locations of the unpaired electrons are observed.
atom has a similar distorted octahedral coordination sphere with To check the identity of the molecular orbitals involved in the
Fe–N bond lengths in the range 1.973–1.984 Å. Again it is the bite inter-valence charge transfer, we have examined the nature of the
angles of 82.1, 82.9, and 82.91 that are primarily responsible for the HOMO and LUMO in the cation FeL32+ by means of DFT calcula-
deviation from octahedral geometry. A notable physical property tions at the BP86/6-311G(2d,p) level, using the GAUSSIAN09 suite
of [FeL3](ClO4)2ꢀ1.5CH3CN is that the single crystals slowly lose the of programs,9 assuming spin multiplicity 2S + 1 = 1. The necessary
solvent molecules resulting in a loss of transparency.
input was given from the CIF of [FeL3](ClO4)2ꢀ1.5CH3CN (it should
1ꢀ2H2O is paramagnetic with a magnetic moment of 1.94 BM be noted that the positions of the oxygen atoms were taken
at room temperature. It gives a unique EPR spectrum at room as shown in Fig. 1). It is found (Table 2) that the HOMO is
temperature in the solid state showing two infused signals metal based and the LUMO is ligand based. Thus a metal-to-
(Fig. 2). One of them is of a low-spin iron(III) species with ligand or ligand-to-metal charge transfer process is feasible in
g> = 2.072 and gJ = 2.387 (gav = 2.175). The other one is sharp [FeL3](ClO4)2ꢀ2H2O. For comparison, we have also performed
and isotropic with g = 1.993. By comparison [Fe(phen)3](ClO4)3 similar DFT calculations at the BP86/6-311G(2d,p) level on low
gives4 an EPR in the solid state at 77 K with g> = 2.65 and spin [Fe(phen)3]2+, where the input has been given from the CIF
gJ = 1.49 (gav = 2.26). Our complex is EPR silent at 77 K. The signal of [Fe(phen)3](ClO4)2ꢀ0.5H2O.3 While the HOMO shows a major
with g = 1.993 (Fig. 2) is due to an unpaired electron residing on contribution from the metal, the LUMO in [Fe(phen)3]2+ is
one of the three L moieties. The EPR of electrochemically reduced found to be spread over all three phen moieties. On the other
2+
hand, the LUMO in FeL3 is distributed over only two of
the three ligand fragments. Furthermore the HOMO–LUMO
Fig. 2 X-band EPR spectrum of [FeL3](ClO4)2ꢀ2H2O in the solid state at
room temperature. Spectrometer settings: microwave frequency, 9.445 GHz;
microwave power, 1.0 mW; modulation frequency, 100 kHz; modulation
amplitude, 1 mT.
Fig. 3 Absorption spectrum of 1ꢀ2H2O (blue) and 1ꢀ3H2O (black) in nujol
mull at room temperature.
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2+
2+
Table 2 Frontier orbitals of FeL3 and Fe(phen)3 as obtained by DFT
calculations at the BP86/6-311G(2d,p) level. For both the complexes
multiplicity 2S + 1 is taken as 1
Complex
HOMO
LUMO
De (eV)
2+
FeL3
1.63
Fig. 5 Segmented cyclic voltammogram of 1ꢀ2H2O (blue) and 1ꢀ3H2O
(black) in CH3CN at a Pt electrode under an N2 atmosphere. Solute
concentration, 1 mmol dmꢁ3. Supporting electrolyte, 0.1 mol dmꢁ3 tetra-
2+
Fe(phen)3
1.75
ethyl ammonium perchlorate (TEAP). Scan rate, 50 mV sꢁ1
.
gap De in low spin [Fe(phen)3]2+ is higher than that in low spin
2+
FeL3 by 0.12 eV.
Now the question is what is the nature of the excited state in
[FeL3](ClO4)2ꢀ2H2O in terms of spin multiplicity. Is it 1MLCT
Scheme 1 Possible valence tautomerism in [FeL3](ClO4)2ꢀ2H2O.
3
or MLCT? Our magnetic susceptibility measurements at 77 K
show that [FeL3](ClO4)2ꢀ2H2O is diamagnetic. We have mentioned
above that the complex [FeL3](ClO4)2ꢀ2H2O is EPR silent at 77 K that there exists a valence tautomerism of the type described in
implying that the magnetic exchange between the unpaired Scheme 1. But this is not tenable, as a quick precipitation of
electron on the metal and that on the ligand is anti-ferromagnetic 1ꢀ3H2O from its acetonitrile solution by adding diethyl ether in air
1
leading to an MLCT ground state with a thermally accessible does not yield the paramagnetic compound 1ꢀ2H2O but rather
3MLCT state.
gives rise to 1ꢀ3H2O only. On the other hand, similar precipitation
When the synthesis of 1ꢀ2H2O is carried out under an N2 of 1ꢀ2H2O from acetonitrile solution gives the paramagnetic
atmosphere, a diamagnetic complex, which is identified as species. Incidentally, valence tautomerism in complexes having
[FeL3](ClO4)2ꢀ3H2O (1ꢀ3H2O) through analysis, is obtained. This ligand radicals (non-innocent ligands) is quite common. Metal
complex does not show the low energy band at 1160 nm in the solid complexes of non-innocent ligands are of much current interest
state electronic spectra (Fig. 3). However its solution properties are due to their potential for applications in catalysis and as functional
the same as those of 1ꢀ2H2O. The H NMR spectra of both the materials.12–17 The generation of diamagnetism of 1ꢀ2H2O in solu-
1
compounds at room temperature in CD3CN are identical (Fig. 4). tion follows the pathway 1,3MLCT - 3T - 5T2 - 1A1 (T indicates
The 5/6 protons of the chelated L resonate10 at 4.85 ppm in metal centered states), which is known to be prevalent in iron
[CuL2]ClO4 in CD2Cl2. These appear at 4.88 ppm in Fig. 4. complexes displaying spin crossover.18,19 The conversion of the
3
1
No paramagnetic shift or broadening is observed in the solution paramagnetic MLCT state to diamagnetic A1 occurs in a sub-
NMR of 1ꢀ2H2O and 1ꢀ3H2O. The electronic spectra of 1ꢀ2H2O picosecond scale in solutions of 1ꢀ2H2O.
and 1ꢀ3H2O in acetonitrile are also identical (Fig. S2, ESI†). The
It is speculated that the three water molecules in 1ꢀ3H2O are
cyclic voltammograms of the two compounds in acetonitrile on H-bonded to the three epoxide O atoms of the ligand while in
the positive side of the NHE (normal hydrogen electrode) are 1ꢀ2H2O, two of the three epoxide O atoms are engaged in
essentially similar with the observed Fe(III)/Fe(II) couple being H-bonding with the water molecules. This subtle difference
somewhat more reversible in 1ꢀ2H2O with a half-wave potential seems to lead to the different properties of the two compounds
2+
of 1.37 V (Fig. 5) close to that (1.41 V vs. NHE)11 of Fe(phen)3
.
observed in the solid state.
The NMR spectrum of 1ꢀ2H2O shows that it becomes diamag-
Previously in iron complexes, it has been possible to trap the
netic in solution. A simple interpretation of this observation is metal centered excited state 5T2 at low temperatures.18–22 But it has
never been isolated in the solid state. Here we have crystallised the
3MLCT state in Fe(II)L32+. As mentioned earlier, our L is a congener of
phen and also a 1,4-diimine. But the corresponding iron chemistry is
not known with phen or 2,20-bipyridine. In fact a complex such as
1ꢀ2H2O is not known in the chemistry of any transition metal.
1. Experimental
1.1. Materials and physical measurements
Fig. 4 300 MHz 1H NMR spectrum of [FeL3](ClO4)2ꢀ2H2O or [FeL3](ClO4)2ꢀ3H2O
L was purchased from Aldrich. TEAP was prepared by the action
of perchloric acid on tetraethyl ammonium bromide in water.
in CD3CN. Reference, TMS.
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Microanalyses were performed using a Perkin-Elmer 2400II using the Oxford Diffraction X-Calibur CCD System. The crystal
elemental analyser. Magnetic susceptibility measurements were was positioned at 50 mm from the CCD. 321 frames were
carried out in a Curie-type balance using the Faraday method.23 measured with a counting time of 10 s. Data analysis was carried
The susceptibility data were corrected for diamagnetism by using out with the CrysAlis program.24 The structure was solved using
Pascal’s constants. UV-Vis spectra (in CH3CN) were recorded on direct methods with the Shelxs97 program.25 The non-hydrogen
a Perkin Elmer Lambda 950 spectrophotometer, FTIR spectra atoms were refined with anisotropic thermal parameters. The
(KBr) on a Shimadzu FTIR-8400S spectrometer, 300 MHz NMR hydrogen atoms bonded to carbon were included in geometric
spectra on a Bruker DPX300 spectrometer in CD3CN, X-band EPR positions and given thermal parameters equivalent to 1.2 times
spectra on a JEOL JES-FA200 spectrometer and ESI mass spectra those of the atom to which they were attached. An absorption
on a Waters Qtof Micro YA263 spectrometer in CH3CN. Cyclic correction was carried out using the ABSPACK program.26 Two
voltammetry was performed using an EG&G PARC electrochemical of the three ligands contained oxygen atoms in two alternate
analysis system (Model 273A) under a dry N2 atmosphere in positions above and below the plane. These were refined with
purified CH3CN in a conventional three electrode configuration population parameters x and 1 ꢁ x respectively to give values of
using TEAP as the supporting electrolyte. A planar EG&G PARC 0.65(1) and 0.58(1) for x. The structure was refined on F2 using
G0228 Pt milli electrode was used as the working electrode and the Shelxl97.25
reference electrode was Ag/AgCl. A Pt wire was used as the auxiliary
electrode. Under the same experimental conditions, the ferrocene–
Acknowledgements
ferrocenium couple appears at 0.71 V vs. NHE with a peak-to-peak
separation of 70 mV at a scan rate of 50 mV sꢁ1
.
M. G. B. D. thanks EPSRC and the University of Reading for funds
for the X-Calibur system.
1.2. Synthesis of [FeL3](ClO4)2ꢀ2H2O (1ꢀ2H2O)
Fe(ClO4)2ꢀxH2O (0.025 g, 0.1 mmol) and L (0.059 g, 0.3 mmol)
were dissolved in methanol (20 ml). The resulting dark red
solution was stirred for 9 h. The reddish brown compound pre-
cipitated was filtered out and dried in vacuo over fused CaCl2.
Yield: 0.032 g (36%). Anal. calc. for C36H28N6FeCl2O13: C, 49.17;
H, 3.21; N, 9.56. Found: C, 49.11; H, 3.31; N, 9.51%. meff/mB: 1.94
(at 298 K). FTIR n/cmꢁ1: 3425br, 2922w, 2852w, 1627w, 1435m,
1086vs, 897w, 800m, 739m, 625m. 1H NMR d/ppm: 4.88
(6H, epoxide), 7.21–7.48 (12H, phen), 8.36 (6H, phen). UV-Vis
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atmosphere. The resulting dark red solution was stirred for 9 h
under an N2 atmosphere. The reddish brown compound pre-
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Yield: 0.027 g (30%). Anal. calc. for C36H30N6FeCl2O14: C, 48.18;
H, 3.37; N, 9.36. Found: C, 48.04; H, 3.24; N, 9.25%. Diamagnetic.
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895w, 800s, 739m, 625m. H NMR d/ppm: 4.88 (6H, epoxide),
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,
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Caution! Though while working with our iron complexes we
have not met with any incident, care should be taken in handling
them, as perchlorates are potentially explosive. These should not
be prepared and stored in large amounts.
1.4. X-ray crystallography
Crystal data for [FeL3](ClO4)2ꢀ1.5CH3CN are given in Table 1.
Reflection data were collected with MoKa radiation at 150 K
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