Accessibility and selective stabilization of the principal spin states of iron by pyridyl versus phenolic ketimines: Modulation of the 6 A 1 ? 2 T 2 ground-state transformation of the [FeN4O2]+ chromophore

Article abstract of DOI:10.1021/ic300732r

Several potentially tridentate pyridyl and phenolic Schiff bases (apRen and HhapRen, respectively) were derived from the condensation reactions of 2-acetylpyridine (ap) and 2′-hydroxyacetophenone (Hhap), respectively, with N-R-ethylenediamine (RNHCH₂CH₂NH₂, Ren; R = H, Me or Et) and complexed in situ with iron(II) or iron(III), as dictated by the nature of the ligand donor set, to generate the six-coordinate iron compounds [Fe^II(apRen)₂]X₂ (R = H, Me; X ^- = ClO₄^-, BPh₄^-, PF ₆^-) and [Fe^III(hapRen)₂]X (R = Me, Et; X^- = ClO₄^-, BPh₄^-). Single-crystal X-ray analyses of [Fe^II(apRen)₂](ClO ₄)₂ (R = H, Me) revealed a pseudo-octahedral geometry about the ferrous ion with the Fe^II-N bond distances (1.896-2.041 ?) pointing to the ¹A₁ (d_π ⁶) ground state; the existence of this spin state was corroborated by magnetic susceptibility measurements and M?ssbauer spectroscopy. In contrast, the X-ray structure of the phenolate complex [Fe^III(hapMen) ₂]ClO₄, determined at 100 K, demonstrated stabilization of the ferric state; the compression of the coordinate bonds at the metal center is in accord with the ²T₂ (d_π⁵) ground state. Magnetic susceptibility measurements along with EPR and M?ssbauer spectroscopic techniques have shown that the iron(III) complexes are spin-crossover (SCO) materials. The spin transition within the [Fe ^IIIN₄O₂]⁺ chromophore was modulated with alkyl substituents to afford two-step and one-step ⁶A ₁ ? ²T₂ transformations in [Fe ^III(hapMen)₂]ClO₄ and [Fe^III(hapEen) ₂]ClO₄, respectively. Previously, none of the X-salRen- and X-sal₂trien-based ferric spin-crossover compounds exhibited a stepwise transition. The optical spectra of the LS iron(II) and SCO iron(III) complexes display intense d_π → p_π* and p_π → d_π CT visible absorptions, respectively, which account for the spectacular color differences. All the complexes are redox-active; as expected, the one-electron oxidative process in the divalent compounds occurs at higher redox potentials than does the reverse process in the trivalent compounds. The cyclic voltammograms of the latter compounds reveal irreversible electrochemical generation of the phenoxyl radical. Finally, the H₂salen-type quadridentate ketimine H₂hapen complexed with an equivalent amount of iron(III) to afford the μ-oxo-monobridged dinuclear complex [{Fe^III(hapen)}₂(μ-O)] exhibiting a distorted square-pyramidal geometry at the metal centers and considerable antiferromagnetic coupling of spins (J ≈ -99 cm^-1).

Full text of DOI:10.1021/ic300732r

Article
pubs.acs.org/IC
Accessibility and Selective Stabilization of the Principal Spin States
6
of Iron by Pyridyl versus Phenolic Ketimines: Modulation of the A1
2
↔ T2 Ground-State Transformation of the [FeN4O2]+ Chromophore
Musa S. Shongwe,*,† Usama A. Al-Zaabi,† Faizah Al-Mjeni,† Carla S. Eribal,‡ Ekkehard Sinn,‡
Imaddin A. Al-Omari,§ Hussein H. Hamdeh, Dariusz Matoga,∥ Harry Adams,⊥ Michael J. Morris,⊥
Arnold L. Rheingold,# Eckhard Bill,≠ and David J. Sellmyer¶
†Department of Chemistry, College of Science, Sultan Qaboos University, PO Box 36, Al-Khod 123, Muscat, Sultanate of Oman
‡Department of Chemistry, Western Michigan University, Kalamazoo, Michigan 49008, United States
§Department of Physics, College of Science, Sultan Qaboos University, PO Box 36, Al-Khod 123, Muscat, Sultanate of Oman
Department of Physics, Wichita State University, 1845 Fairmount St, Wichita, Kansas 67260, United States
∥Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Krakow
⊥Department of Chemistry, University of Sheffield, Sheffield S3 7HF, U.K.
́
, Poland
#Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, California 92093-0358, United States
≠Max-Planck-Institut fur Bioanorganische Chemie, Stiftstrasse 34-36, 45470 Mulheim an der Ruhr, Germany
̈
̈
¶Department of Physics and Astronomy and Nebraska Center for Materials and Nanoscience, University of Nebraska, Lincoln,
Nebraska 68588, United States
S
* Supporting Information
ABSTRACT: Several potentially tridentate pyridyl and
phenolic Schiff bases (apRen and HhapRen, respectively)
were derived from the condensation reactions of 2-
acetylpyridine (ap) and 2′-hydroxyacetophenone (Hhap),
respectively, with N-R-ethylenediamine (RNHCH2CH2NH2,
Ren; R = H, Me or Et) and complexed in situ with iron(II) or
iron(III), as dictated by the nature of the ligand donor set, to
generate the six-coordinate iron compounds [FeII(apRen)2]X2
−
−
−
−
−
(R = H, Me; X− = ClO4 , BPh4 , PF6 ) and [FeIII(hapRen)2]X (R = Me, Et; X− = ClO4 , BPh4 ). Single-crystal X-ray analyses of
[FeII(apRen)2](ClO4)2 (R = H, Me) revealed a pseudo-octahedral geometry about the ferrous ion with the FeII−N bond
distances (1.896−2.041 Å) pointing to the 1A1 (dπ ) ground state; the existence of this spin state was corroborated by magnetic
6
susceptibility measurements and Mossbauer spectroscopy. In contrast, the X-ray structure of the phenolate complex
̈
[FeIII(hapMen)2]ClO4, determined at 100 K, demonstrated stabilization of the ferric state; the compression of the coordinate
5
bonds at the metal center is in accord with the 2T2 (dπ ) ground state. Magnetic susceptibility measurements along with EPR and
Mossbauer spectroscopic techniques have shown that the iron(III) complexes are spin-crossover (SCO) materials. The spin
̈
6
transition within the [FeIIIN4O2]+ chromophore was modulated with alkyl substituents to afford two-step and one-step A1 ↔
2T2 transformations in [FeIII(hapMen)2]ClO4 and [FeIII(hapEen)2]ClO4, respectively. Previously, none of the X-salRen- and X-
sal2trien-based ferric spin-crossover compounds exhibited a stepwise transition. The optical spectra of the LS iron(II) and SCO
iron(III) complexes display intense dπ → pπ* and pπ → dπ CT visible absorptions, respectively, which account for the spectacular
color differences. All the complexes are redox-active; as expected, the one-electron oxidative process in the divalent compounds
occurs at higher redox potentials than does the reverse process in the trivalent compounds. The cyclic voltammograms of the
latter compounds reveal irreversible electrochemical generation of the phenoxyl radical. Finally, the H2salen-type quadridentate
ketimine H2hapen complexed with an equivalent amount of iron(III) to afford the μ-oxo-monobridged dinuclear complex
[{FeIII(hapen)}2(μ-O)] exhibiting a distorted square-pyramidal geometry at the metal centers and considerable antiferromagnetic
coupling of spins (J ≈ −99 cm−1).
phenomenon was first recognized in iron-based dithiocarba-
INTRODUCTION
■
mato complexes eight decades ago.3 Since then spin crossover
The ease with which iron exhibits multiple oxidation states and
an array of accessible spin states (S = 0−5/2) is held in some
measure responsible for the richness of the coordination
chemistry of this bioactive metal.1,2 The spin-crossover
Received: April 16, 2012
Published: July 18, 2012
© 2012 American Chemical Society
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Scheme 1. Synthetic Pathways and Ligand Designations
has been observed in complexes of other transition metals that
meet the basic criteria in terms of the ground-state electron
configuration of the metal center (d4−d7, octahedral) and the
ligand-field strength (P ≈ Δo), notably manganese(III)4 and
cobalt(II)5 complexes. Spin crossover is of immense
fundamental and technological interest with applications
envisaged in the fabrication of molecule-based electronic
devices for visual displays and information storage.
solvents of crystallization, counterions, sample type and, in
rare cases, configurational isomerism.11
It is rather surprising that the seemingly attractive
heterodonor apRen and HhapRen Schiff bases have received
very little attention, some derivatives none at all, in
coordination chemistry. The only literature report on the
iron chemistry of apHen is the classic paper of Krumholz on
MLCT spectra of low-spin iron(II)−imine complexes.12 Only
nickel(II)13 and copper(II)14 complexes with apHen have ever
been structurally characterized. As far as we are aware, the
coordination chemistry of apMen and the phenolic ligands
HhapRen has never been explored previously. Of the analogous
ferric phenolate SCO materials [FeIII(X-salRen)2]Y15 and
[FeIII(X-sal)2trien]Y16 only a few exhibited abrupt or hysteretic
Spin crossover is by far more prevalent for iron(II)6,7 than
for iron(III),7,8 whereas there is rapid development of ferrous
spin crossover borne out by the ever-increasing research output
on such iron(II) SCO materials, there are only a mere handful
of examples of ferric spin-crossover compounds reported in
recent times. A preponderance of iron(II) spin-crossover
substances possess the [FeN6]2+ coordination sphere;6
however, there is a growing number of examples of ferrous
spin-crossover complexes supported by an [N4O2] donor set.9
In contrast, the vast majority of iron(III) spin transitions occur
within the [FeN4O2] core8 with the oxygen atoms mostly
phenolic in nature; ferric spin-crossover materials featuring an
all-N-donor environment are extremely rare.10
6
2
spin transitions; none displayed two-step A1 ↔ T2 trans-
formations. In this work, the use of apMen and HhapMen
sought to compare and contrast the iron-coordination proper-
ties of the pyridyl and phenolate moieties on an equal footing.
The compounds [FeIII(hapEen)2]ClO4, [FeIII(hapMen)2]ClO4,
and [FeIII(hapMen)2]BPh4 were intended to tune the spin
crossover in this ketimine system.
Exposure of SCO molecular materials to external perturba-
tions, such as temperature, pressure, or electromagnetic
radiation, induces a variety of spin transitions,6−10 namely,
abrupt, gradual, complete, incomplete (at either or both ends of
the spin-transition curve), one-step, two-step one-sited, two-
step two-sited, symmetry-breaking and hysteretic, as well as
various combinations of some of these. It is now well
established that strong cooperativity of spin-crossover centers
in the crystal lattice causes abrupt reversible spin transitions
with a relatively large thermal hysteresis loop, a property highly
desired for applications in molecular electronics.7 Such
cooperativity tends to arise from intermolecular forces
including hydrogen bonding and π−π stacking interactions.
Spin transitions are influenced by several factors,6−10 including
ligand substituent groups (steric and electronic effects),
RESULTS AND DISCUSSION
■
Synthetic Routes, Chemical Formulations, and Spec-
troscopic Identification. The Schiff bases apRen (R = H,
Me) and HhapRen (R = Me, Et) were generated by the
condensation reaction of stoichiometric amounts of either 2-
acetylpyridine or 2′-hydroxyacetophenone (a ketone), respec-
tively, with the appropriate primary amine (N-R-ethylenedi-
amine) in refluxing MeOH or EtOH (Scheme 1).
With the exception of H2hapen, these ligands were not
isolated as solids but rather were complexed in situ with
iron(II) or iron(III) to afford the desired iron compounds.
However, for spectroscopic characterization, the solutions of
HhapRen and apRen were stripped of the solvent to give
viscous orange liquids. The chemical identity of H2hapen, a
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Table 1. Selected Crystallographic Data for H2hapen, [FeII(apRen)2](ClO4)2 (R = H, Me), [FeIII(hapMen)2]ClO4, and
[{FeIII(hapen)}2(μ-O)]
H2hapen
[Fe(apHen)2](ClO4)2
[Fe(apMen)2](ClO4)2
[Fe(hapMen)2]ClO4
[{FeIII(hapen)}2(μ-O)]
empirical formula
molar mass (g/mol)
T (K)
C18H20N2O2
296.36
C18H26N6O8Cl2Fe
581.20
C20H30N6O8Cl2Fe
609.25
C22H30N4O6ClFe
537.80
C36H36N4O5Fe
716.39
150
150
208
100
100
crystal system
space group
a (Å)
monoclinic
P21/n
monoclinic
P21/c
monoclinic
P21/c
monoclinic
P21/c
triclinic
P1
̅
5.6177(2)
20.4136(8)
6.7833(3)
90
15.7479(19)
9.8664(12)
15.2144(18)
90
16.3596(18)
10.4130(12)
15.3533(17)
90
8.0933(8)
8.6263(8)
33.904(3)
90
11.500(4)
11.623(4)
14.007(4)
81.618(4)
73.509(4)
61.073(3)
1571.2(8)
2
b (Å)
c (Å)
α (deg)
β (deg)
104.412(3)
90
95.654(2)
90
97.8400(10)
90
90.7700(10)
90
γ (deg)
V (Å3)
753.41(5)
2
2352.4(5)
4
2591.0(5)
4
2366.8(4)
4
Z
ρcalcd (g/cm3)
1.306
1.641
1.557
1.504
1.514
μ (mm−1
)
0.086
0.926
0.845
0.796
0.975
F(000)
316
1200
1256
1116
744
crystal size (mm)
θ range (deg)
reflns collected
independent reflns
Rint
0.26 × 0.24 × 0.09
2.00−36.53
15 539
0.34 × 0.32 × 0.21
1.30−28.59
26 199
0.40 × 0.40 × 0.32
1.26−25.93
23 732
0.41 × 0.38 × 0.34
2.40−25.40
13 623
0.28 × 0.22 × 0.17
1.52−25.00
26 214
3699
5611
4972
4337
5532
0.0263
0.0705
0.0597
0.0275
0.0990
GOF on F2
0.998
1.020
1.073
1.008
1.071
R1, R2 [I > 2σ(I)]
R1, R2 (all data)
0.0487, 0.1331
0.0677, 0.1463
0.0553, 0.1360
0.0969, 0.1527
0.0555, 0.1293
0.0774, 0.1380
0.0365, 0.0983
0.0415, 0.1014
0.0690, 0.1696
0.1111, 0.1925
potentially quadridentate Schiff-base ligand, was established by
microanalysis (C, H, and N) and EI mass spectrometry. Its
characteristic functional features, namely, azomethine CN
and phenolic OH groups, were readily identified by their
vibrational stretches at 1611 and 3450 cm−1, respectively. The
1H NMR spectra of the ligands (see Supporting Information)
are comparable with those of closely related Schiff bases.17 That
of H2hapen is displayed in Supporting Information Figure S1.
The signature color of the Schiff bases appears to originate
from π → π* electronic transitions occurring within the imine
bonds; this assertion is supported by the observation that
reduction of these ligands with NaBH4 in refluxing EtOH
causes disappearance of both the color and the visible (or near-
visible) absorption band. This spectroscopic feature of the
ligands is exemplified by HhapMen in Supporting Information
Figure S2.
the iron(II) was spontaneously oxidized to iron(III), indicating
that the donor set N4O2 stabilizes the ferric state preferentially.
These iron(III)−phenolate complexes are purple-pink or violet
in solution. The complex of the quadridentate Schiff-base
ligand H2hapen with iron(III), [{Fe(hapen)}2(μ-O)], was
synthesized by reaction of the ligand (produced in situ or
isolated as crystals) with Fe(ClO4)2·xH2O or Fe(ClO4)3·xH2O
as the source of the iron.
The chemical formulations of the iron(II) complexes
−
−
[Fe(apRen)2]X2 (R = H, Me; X− = ClO4 , PF6 ), [Fe-
(apHen)2](BPh4)2·2H2O, and [Fe(apMen)2](BPh4)2·4H2O, as
well as the iron(III) complexes [Fe(hapMen)2]X (X− = ClO4 ,
−
−
BPh4 ) and [Fe(hapEen)2]ClO4, were verified by elemental
analyses (C, H, and N) and FAB mass spectrometry.
Supporting Information Figure S3 illustrates the mass spectral
characterization of the iron(II) and iron(III) bis-chelate
complexes and reveals an interesting difference between the
pyridyl and phenolate complexes. The FAB mass spectrum of
[Fe(apHen)2](ClO4)2 [Supporting Information Figure S3a] in
the positive mode displays peaks at m/z = 382 and 218
representing the complex cation [Fe(apHen)2]2+ and the
fragment “[Fe(apHen)]2+”, respectively. Surprisingly, there is
an additional peak at m/z = 481 corresponding to the formula
unit “[Fe(apHen)2]ClO4”, which indicates retention of one of
the perchlorate counterions. In contrast, this behavior is not
observed in the case of the iron(III)−phenolate complexes as
illustrated by the FAB mass spectrum of [Fe(hapMen)2]ClO4
[Supporting Information Figure S3b], which exhibits a
molecular peak for the complex cation at m/z = 438 and
reveals dissociation of one ligand to give the fragment
“[Fe(hapMen)]2+”. Finally, according to mass spectrometry,
the dinuclear iron(III) complex [{Fe(hapen)}2(μ-O)] ruptures
asymmetrically at the μ-oxo bridge to give the structural units
“[Fe(hapen)(O)]” (m/z = 366) and “[Fe(hapen)]” (m/z =
The iron(II)−pyridyl compounds [FeII(apHen)2−]X2 and
[FeII(apMen)2]X2 (X = ClO4 , BPh4 , or PF6 ) were
synthesized by reaction of the appropriate Schiff base produced
in situ with half molar equivalent of iron(II) or iron(III) ion.
That these all-N-donor ligands have a preference for iron(II)
over iron(III) has been demonstrated by their reaction with the
latter which resulted in spontaneous reduction to the ferrous
ion. The varying of the counterions was motivated principally
by the quest to probe the effect of counterions on iron spin
crossover.18 On going from apHen to apMen, there occur
discernible color differences between the complex cations
[FeII(apHen)2]2+ (navy-blue) and [FeII(apMen)2]2+ (purple-
tinged royal blue).
−
−
The iron(III)−phenolate compounds [Fe(hapMen)2]X (X−
−
−
= ClO4 or BPh4 ) and Fe(hapEen)2]ClO4 were produced
from the reactions of stoichiometric amounts of HhapRen (R =
Me or Et) with FeII or FeIII ion as described for the analogous
aforementioned iron(II)−pyridyl complexes, but in this case
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Article
350). This bridge breakage is reminiscent of that which was
observed in the vanadium(IV) thiosemicarbazonato dimer
[{V(daptsc)(MeOH)}2(μ-O)](ClO4)2.19
In the IR spectra of the iron compounds, the vibrational
bands of importance that stand out are those of the azomethine
bond, the amino group, the aromatic ring and the counterions.
The ν(CN) vibrations occurring typically in the range
1596−1601 cm−1 verified the existence of the Schiff-base
ligands in these complexes. The amino groups exhibit ν(N−H)
absorptions between 3100 and 3400 cm−1 whereas the pyridyl
and phenolate ring vibrations are the dominant features in the
region 1400−1590 cm−1. As for the iron−phenolate complexes,
prominent absorption bands associated with ν(C−O),
conspicuously absent from the spectra of the iron−pyridyl
complexes, are observed between 1200 and 1300 cm−1. The
spectrum of the dinuclear complex [{Fe(hapen)}2(μ-O)]
reveals stretches at 842 and 422 cm−1 consistent with
νas(Fe−Ooxo) and νs(Fe−Ooxo), respectively, of the angular
bridge [Fe−O−Fe]4+. For closely related μ-oxo-monobridged
diiron(III) complexes, the asymmetric stretch lies typically in
the range 730−880 cm−1, whereas the symmetric vibration
occurs between 360 and 545 cm−1; the latter is forbidden in the
IR region for linear [Fe−O−Fe] cores.1b,c Each counterion
presents a unique and conspicuous feature at the lower-energy
Figure 1. X-ray structure of the complex cation in [Fe(apHen)2]-
(ClO4)2.
8a,b
−
end of the spectrum. Generally, ν(ClO4 )
gives rise to
strong absorptions around 1145, 1120, and 1090 cm−1 along
with a relatively weaker one at ∼625 cm−1. The tetraphenylbo-
rate ion16a,20 is characterized by intense vibrational bands in the
ranges 730−740 and 700−710 cm−1 accompanied by an
absorption band of medium intensity between 610 and 615
cm−1. The stretches of the PF6− ion16a are readily recognizable
by their characteristic absorptions at 842 (s) and 558 (m) cm−1.
Elucidation of Molecular Structures. The 3-D structures
of [Fe(apHen)2](ClO4)2, [Fe(apMen)2](ClO4)2, [Fe-
(hapMen)2](ClO4)2, H2hapen, and [{Fe(hapen)}2(μ-O)]
have been determined by single-crystal X-ray crystallography.
Blocks of [Fe(apHen)2](ClO4)2 and [Fe(apMen)2](ClO4)2
amenable to single-crystal X-ray analyses were grown from
methanol solutions of these iron compounds by slow
evaporation of the solvent. Both compounds crystallized in
the monoclinic space group P21/c. A summary of the
crystallographic data is provided in Table 1. As expected,
[Fe(apHen)2](ClO4)2 and [Fe(apMen)2](ClO4)2 are isostruc-
tural and possess comparable crystallographic parameters. Each
crystal structure is composed of a mononuclear [Fe(apRen)2]2+
(R = H or Me) complex cation and two disordered perchlorate
counterions. In the case of [Fe(apMen)2](ClO4)2, these
counterions were eliminated using the PLATON SQUEEZE
function21 as indicated in the relevant CIF. The X-ray analyses
gave definitive evidence for the stabilization of the divalent state
of iron by the pyridyl ketimines. Figures 1 and 2 depict the X-
ray structures of the complex cations [Fe(apHen)2]2+ and
[Fe(apMen)2]2+, respectively, with selected pertinent bond
distances and angles compiled in Table 2. In each structure, the
iron(II) ion is in a distorted octahedral environment created by
the two tridentate apRen ligands which are oriented nearly
orthogonally to each other. Each apRen ligand provides three
types of nitrogen donor atoms for coordination, namely,
pyridyl, imine, and amine. As is often the case with tridentate
Schiff bases, the apRen ligand is oriented such that each donor
set adopts a meridional arrangement with the imine nitrogens
occupying trans positions relative to each other. In [Fe-
(apHen)2](ClO4)2, the cis angles range from 80.47(13)° to
Figure 2. X-ray structure of the complex cation in [Fe(apMen)2]-
(ClO4)2.
99.94(12)°, and the trans ones are 164.21(12)°, 164.29(13)°,
and 179.30(13)°; these compare favorably with those of the
[Fe(apMen)2](ClO4)2 congener [cis angles = 80.40(11)−
98.61(11)°; trans angles = 163.26(12)°, 164.20(11)°, and
178.36(11)°].
The chelate angles formed by the pyridyl and imine nitrogens
[80.47(13) and 81.07(13)°] in [Fe(apHen)2](ClO4)2 and
[80.40(11) and 80.57(11)°] in [Fe(apMen)2](ClO4)2] are
comparable with the bite angles observed in related low-spin
pseudo-octahedral iron(II) Schiff-base complexes with the
pyridyl-imine structural unit.22 A direct comparison can be
made between the Np−M−Ni [M = Fe2+ (LS) or Ni2+] bite
angles in [Fe(apHen)2]2+ and [Ni(apHen)2]2+:13 the angles are
significantly smaller for the latter complex cation [77.5(2)° and
77.8(2)°] because Ni2+ is larger than LS Fe2+ [ionic radii rFe(II)
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Table 2. Selected Bond Distances (Å) and Angles (deg) for [FeII(apHen)2](ClO4)2, [FeII(apMen)2](ClO4)2, and
[FeIII(hapMen)2]ClO4
[FeII(apHen)2](ClO4)2
[FeII(apMen)2](ClO4)2
[FeIII(hapMen)2]ClO4
Fe(1)−N(5)
1.897(3)
1.901(3)
1.952(3)
1.958(3)
2.019(3)
2.024(3)
1.296(5)
1.470(4)
1.291(5)
1.466(5)
Fe(1)−N(2)
1.896(3)
1.902(3)
1.959(3)
1.964(2)
2.041(3)
2.041(3)
1.292(4)
1.462(4)
1.290(4)
1.470(4)
Fe(1)−O(1) 1.8649(16)
Fe(1)−N(2)
Fe(1)−N(3)
Fe(1)−N(6)
Fe(1)−N(4)
Fe(1)−N(1)
C(3)−N(2)
C(2)−N(2)
C(11)−N(5)
C(10)−N(5)
Fe(1)−N(5)
Fe(1)−N(4)
Fe(1)−N(1)
Fe(1)−N(6)
Fe(1)−N(3)
C(6)−N(2)
C(8)−N(2)
C(16)−N(5)
C(18)−N(5)
Fe(1)−O(2)
Fe(1)−N(1)
Fe(1)−N(3)
Fe(1)−N(4)
Fe(1)−N(2)
C(7)−N(1)
C(9)−N(1)
C(18)−N(3)
C(20)−N(3)
1.8652(16)
1.9397(19)
1.9405(19)
2.048(2)
2.050(2)
1.292(3)
1.472(3)
1.298(3)
1.478(3)
a
a
a
∑ = 71.73°
∑ = 72.73°
∑ = 29.74°
N(5)−Fe(1)−N(2)
N(6)−Fe(1)−N(4)
N(3)−Fe(1)−N(1)
179.30(13)
164.21(12)
164.29(13)
N(2)−Fe(1)−N(5)
N(4)−Fe(1)−N(6)
N(1)−Fe(1)−N(3)
178.36(11)
163.26(12)
164.20(11)
N(1)−Fe(1)−N(3)
O(2)−Fe(1)−N(4)
O(1)−Fe(1)−N(2)
176.37(8)
178.15(8)
176.26(7)
a
NB: ∑ is the angular distortion parameter, which represents the sum of the deviations of the cis angles from the idealized angle.
= 75 pm, rNi(II) = 83 pm). The Np−M−Ni chelate angle is a
useful distinguishing feature between HS FeII and LS FeII. This
geometric parameter typically falls within the range ∼73−77°
for the 5T2 ground state.22a,23 The average FeII−N bond lengths
1.955 Å (FeII−Npyridyl), 1.899 Å (FeII−Nimine) and 2.022 Å
(FeII−Namine) for [Fe(apHen)2](ClO4)2, and 1.962 Å (FeII−
N
pyridyl), 1.899 Å (FeII−Nimine), and 2.041 Å (FeII−Namine) for
[Fe(apMen)2](ClO4)2 are in keeping with the low-spin state of
the complex cations and are comparable with those of related
low-spin iron(II) complexes.18a,b,22−24 LS FeII is favored over
HS FeII by the greater ligand-field stabilization energy (t2g vs
6
t2g4eg2 configuration).
X-ray data collection on a single crystal of [Fe(hapMen)2]-
ClO4 was performed at 100 K. The crystal system and space
group are identical to those of the corresponding pyridyl
analogue, [Fe(apMen)2](ClO4)2. Crystal data along with
structure solution and refinement parameters for [Fe-
(hapMen)2]ClO4 are summarized in Table 1. Selected bond
distances and angles are given in Table 2. The crystal structure
of [Fe(hapMen)2]ClO4 consists of a mononuclear [Fe-
(hapMen)2]+ complex cation with a perchlorate counteranion,
pointing to the trivalent state of the central metal atom. The
structure of the complex cation is depicted in Figure 3. Each of
the two uninegative tridentate Schiff-base ligands provides a
phenolate oxygen, an imine nitrogen and a secondary amine
nitrogen as donor atoms to create a six-coordinate geometry
about the iron(III) ion. The azomethine (CN) bonds
(average distance =1.295 Å) confer rigidity to the ligands and
influence a meridional arrangement of the donor atoms with
the imine nitrogen atoms of the two ligands oriented trans to
each other [N(1)−Fe(1)−N(3) = 176.37(8)°]. Each of the
other two pairs of trans bonds is Ophenolate−Fe−Namine
[176.26(7)° and 178.15(8)°] from the same ligand. The
phenolate moieties of the two ligands are adjacent to each other
[O(1)−Fe(1)−O(2) = 91.59(7)°] as are the secondary amines
[N(2)−Fe(1)−N(4) = 88.90(8)°]. Incidentally, recently,
Verani et al.25 demonstrated crystallographically that the
rigidity or flexibility of the framework of a tridentate ligand
and the nature of a substituent group dictate the geometric
isomer (fac or mer) to be adopted by an octahedral complex. A
ligand structural feature of interest in [Fe(hapMen)2]ClO4 is
the extension of the delocalization of π-electrons to the
Figure 3. X-ray structure of the cation in [Fe(hapMen)2]ClO4.
phenolate oxygen atom as evidenced by the relatively short
phenolate C−O bond [1.326(3) and 1.324(3) Å].
The cis angles in the coordination sphere range from
84.84(8)° to 94.42(8)° with the octahedral angular distortion
parameter, ∑ = 29.74° (the sum of the deviations of the cis
2
angles from the idealized angle), indicative of the T2 ground
state. In the closely related salicylaldimine ferric complex-
es,15a,f,h,i the value of ∑ lies approximately within the ranges
40−50° and 65−75° for the 2T2 and 6A1 ground states,
respectively. Generally, the Nim−FeIII−Nam bite angle of the 5-
membered en-chelate ring is more reliable in predicting the spin
state: HS, 78−80°; LS, 83−85°. For [Fe(hapMen)2]ClO4, this
angular parameter has an average value of ∼85.0° (LS). The
average distances of the bonds FeIII−Ophenolate (1.8651 Å),
FeIII−Nimine (1.9401 Å) and FeIII−Namine (2.049 Å) are
consistent with the low-spin state of iron(III) in a
pseudooctahedral geometry. Typically, octahedral LS distances
of FeIII−Ophenolate, FeIII−Nimine, and FeIII−Namine are in the
ranges 1.85−1.89,8,15,16,26 1.92−1.96,8,15,16,26 and 2.02−2.08
Å,15,16,26a respectively, whereas the corresponding distances for
HS iron(III) are in the ranges 1.89−1.93,8,15,16,26,27 2.09−
2.15,8,15,16,26 and 2.18−2.26 Å,15,16,26a,27a respectively. These
variations of bond distances with spin state are readily explained
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Figure 4. X-ray structure of H2hapen.
using MO theory which shows an antibonding (dσ*) HOMO in
HS octahedral iron(III) complexes, but a nonbonding (dπ)
HOMO in the corresponding LS complexes. Alternatively, the
disparity in the metal−ligand bond lengths upon spin
conversion can be explained in terms of the variation of the
ionic radius at the metal center. HS FeIII in an octahedral field is
expected to exhibit a larger ionic radius, hence longer FeIII−L
bonds, than the corresponding LS ferric ion.
The potentially quadridentate ligand H2hapen crystallized in
the monoclinic space group P21/n. The crystallographic data
are compiled in Table 1. A conspicuous feature of the molecular
structure of H2hapen (Figure 4) is the centrosymmetry with the
inversion center located in the middle of the C−C bond of the
ethylenediimine backbone. The enolimine tautomeric form of
this Schiff base is stabilized by the intramolecular hydrogen
bonding O(1)−H(1)phenolic···N(1)imine [O(1)−H(1) = 0.84 Å,
H(1)···N(1) = 1.90 Å, O(1)···N(1) = 2.5071(10) Å, O(1)−
H(1)−N(1) = 128.2°]. H2hapen is isostructural with the
analogous Schiff-base ligand 2,2′-[(1,2-ethanediyl)bis-
(nitrilopropylidyne)]bisphenol (H2hppen).28 However, the
molecular structure of the analogue 2,2′-{(1,2-ethanediyl)bis-
[nitrilo(phenyl)methylidyne]}bisphenol (H2hbpen)28 exhibits
a gauche conformation.
[{Fe(hapen)} (μ-O)] crystallized in the triclinic P1 space
̅
2
group. The crystallographic data for this complex are
summarized in Table 1 and selected geometric parameters are
presented in Supporting Information Table S1. The molecular
structure of [{Fe(hapen)}2(μ-O)] is depicted in Figure 5. The
structural feature that stands out is the μ-oxo-monobridged
diiron(III), [Fe−O−Fe]4+, core. The Fe−O−Fe bridge bond
lengths Fe(1)−O(5) = 1.787(4) Å and Fe(2)−O(5) =
1.776(5) Å are consistent with high-spin iron(III) centers
and lie in the literature range 1.73−1.82 Å for similar iron(III)
dinuclear complexes.2,29 The FeIII···FeIII separation (3.484 Å)
compares favorably with the corresponding distances observed
in related μ-oxo-monobridged dinuclear complexes of iron(III)
(3.39−3.62 Å).2,29 The bridge angle Fe(1)−O(5)−Fe(2) =
155.9(3)° is considerably bent. The corresponding angles in the
structure of [{Fe(salen)}2(μ-O)] from two independent
crystallographic studies are 144.6°29m and 147.8°.29n Interest-
ingly, for the complex [{Fe(3,5-tBu2-salen)}2(μ-O]29e with the
bulky tert-butyl substituent groups on the ligand framework, the
Fe−O−Fe angle is 171.63(17)°. Sterically encumbering groups
on salen-based ligands impose linearity on the Fe−O−Fe
linkage.29e This bridge angle is somewhat larger in [{Fe-
Figure 5. X-ray structure of [{Fe(hapen)}2(μ-O)].
(hapen)}2(μ-O)] than in [{Fe(salen)}2(μ-O)] possibly due to
the presence of the azomethine methyl groups on the backbone
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of hapen2−. In the recent work of Glaser et al.29q on μ-oxo-
monobridged dinuclear iron(III) complexes with the variously
substituted tetradentate ligand system N,N′-dimethyl-1,2-
diaminoethane, the magnitude of the bridge angle was
correlated solely with the electronic properties of the
substituent groups on the phenolic rings. Whereas the strongly
electron-donating tert-butyl and methyl groups favored the
formation of a linear bridge, the electron-withdrawing chloro
group influenced the bending of the FeIII−O−FeIII core.29q
Each of the two FeIII centers is five-coordinate with four
donor atoms provided by the doubly deprotonated (hapen2−)
ligand and the fifth donor atom being the μ-oxo atom. The
hapen2− donor atoms are two phenolate oxygens and two imine
nitrogens. Although the coordination spheres are similar they
are not identical as revealed by the values of the angular
geometric parameter τ = [(β−α)/60]:30 for Fe(1), τ = 0.075,
and for Fe(2), τ = 0.116. Hence the geometry at the two metal
centers is best described as distorted square pyramidal.29m,n,q
The distorted basal plane is defined by the four donor atoms of
hapen2− with the μ-O atom occupying the apical position. As is
often the case with such a coordination sphere, the central
metal atom is displaced out of the mean basal plane toward the
bridging oxygen atom.32 Since other salen-based μ-oxo-
monobridged diiron(III) complexes exhibit the same coordi-
nation sphere, it can be assumed that the distorted square-
pyramidal geometry is imposed by the nature of the
tetradentate Schiff base. The relatively more flexible tripodal
ligands have been shown to favor trigonal-bipyramidal
structures.31
Figure 6. Plots of effective magnetic moment vs absolute temperature
for [Fe(hapMen)2]BPh4 (A), [Fe(hapMen)2]ClO4 (B), and [Fe-
(hapEen)2]ClO4 (C).
transition in this system is illustrated by the magnetic property
of [Fe(hapMen)2]BPh4 (Figure 6, graph A), whereby
replacement of perchlorate by tetraphenylborate stabilizes the
HS state (μeff = 5.61−5.88 μB) over the temperature range 5−
400 K. On the other hand, the effect of ligand substituents is
exemplified by [Fe(hapEen)2]ClO4, whose spin transition curve
(Figure 6, graph C) differs markedly from that of [Fe-
(hapMen)2]ClO4 as a consequence of the seemingly trivial
replacement of the secondary amine methyl group by an ethyl
group. The spin transition in [Fe(hapEen)2]ClO4 is one-step
and follows an incomplete sigmoidal curve. At 400 K, this
compound is predominantly high spin (μeff = 5.08 μB), but at
room temperature γLS is just over 80%; T1/2 ≈ 355 K. Below
165 K, [Fe(hapEen)2]ClO4 is purely LS (μeff = 1.99−1.77 μB).
̈
Magnetic Susceptibility Measurements and Mossba-
uer Spectroscopy. The values of χMT for [Fe(apMen)2]-
(ClO4)2 obtained from SQUID measurements varied steadily
between 0.008 and 0.090 cm3 K mol−1 over the temperature
range 5−300 K, confirming the crystallographic observation
that this iron(II) compound exists in the LS state. Magnetic
measurements of the other iron(II) compounds [Fe(apMen)2]-
−
−
−
X2 (X− = BPh4 , PF6 ) and [Fe(apHen)2]X2 (X− = ClO4 ,
−
−
BPh4 , PF6 ) with a Gouy balance at room temperature gave an
effective magnetic moment below 0.90 μB (1A1 ground state).
The Mossbauer spectra of [Fe(hapMen) ]ClO (Figure 7)
̈
2
4
The Mossbauer spectrum of [Fe(apMen) ](ClO ) recorded at
̈
are in accord with the magnetic behavior of this SCO
compound. At 20 K (where μeff = 1.80 μB), the spectrum
exhibits an asymmetric LS doublet (ΔEQ = 2.74 mm s−1, δ =
0.16 mm s−1). NB: the X-ray structure of this compound at 100
K is also consistent with the 2T2 ground state. At room
2
4 2
78 K (Supporting Information Figure S4) exhibits a LS doublet
(ΔEQ = 1.00 mm s−1, δ = 0.29 mm s−1), corroborating the
magnetic data of this compound.
Whereas the pyridyl ketimine apMen stabilizes the LS state
of iron(II) the corresponding phenolic ketimine HhapMen
temperature, the Mossbauer spectrum consists of an outer
̈
6
2
asymmetric LS doublet (ΔEQ = 2.47 mm s−1, δ = 0.08 mm s−1)
and an inner HS doublet (ΔEQ = 0.33 mm s−1, δ = 0.10 mm
s−1); HS/LS proportions ∼30: 70% according to the ratio of
the peak areas, which is comparable to the spin composition
predicted from magnetic measurements at this temperature.
The magnetic data of the dinuclear iron(III) complex
[{Fe(hapen)}2(μ-O)] were recorded over the temperature
range 2−290 K. A graph of μeff versus T has been plotted in
Figure 8 to determine parameters for spin−spin coupling. The
appearance of this magnetic curve resembles graphs of
magnetic data for similar μ-oxo-monobridged diiron(III)
complexes exhibiting antiferromagnetic coupling of spins at
the metal centers.29d,f−i,o,p At room temperature the value of μeff
for [{Fe(hapen)}2(μ-O)] is 2.63 μB, which is considerably
smaller than the expected spin-only value of 8.37 μB for two
uncoupled high-spin iron(III) centers. As can be seen from
Figure 8, the magnetic moment decreases with temperature
down to 0.38 μB around 40 K where the curve levels off to form
favors iron(III) and promotes the A1 ↔ T2 ground-state
transformation. The magnetic data of three iron(III)-phenolate
compounds are presented in Figure 6. The plot of the variation
of the effective magnetic moment of [Fe(hapMen)2]ClO4 as a
function of absolute temperature (graph B) reveals a very
nearly complete stepwise S = 5/2 ↔ S = 1/2 crossover (μeff
=
5.50 μB at 400 K and 1.74 μB at 5 K). The magnetic moment
drops sigmoidally from 400 to 250 K, followed by a steady
decrease to 215 K and then a sharper drop to 195 K. The spin
transition curve begins to level off at 190 K where the LS state
has been fully accessed (μeff = 1.99 μB). T1/2 is approximately
330 K and the HS/LS proportion at RT is about 35: 65% (i.e.,
predominantly LS). Iron(III)-based stepwise spin transitions
are rare; to the best of our knowledge none of the closely
related X-salRen15 or (X-sal)2trien16 iron spin-crossover
materials exhibited a two-step spin transition, making [Fe-
(hapMen)2]ClO4 unique in this family of ferric spin-crossover
complexes. The influence of counterions on the ferric spin
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μ-oxo-monobridged complexes, decreasing the Fe−O−Fe
bridge angle from 180° causes a small but significant decrease
in the strength of the spin-exchange antiferromagnetic coupling.
For example, [{Fe(salen)}2(μ-O)] has an Fe−O−Fe angle of
2
∼145° with a J value of −92 cm−1 and yet the sterically
encumbered complex [{Fe(3-tBu-saltmen)}2O] has an Fe−O−
Fe angle of ∼173° with a J value of −100 cm−1.2 In this work,
the J value of −99 cm−1 for [{Fe(hapen)}2(μ-O)] is consistent
with the Fe−O−Fe angle of ∼155°. Dinuclear iron(III)
complexes with a linear (180°) Fe−O−Fe linkage can have J
values close to (or above) −200 cm−1.2,29b,h
Electron Paramagnetic Spectroscopy. The X-band EPR
spectrum of a frozen solution of [Fe(hapMen)2]ClO4 in
MeOH at 77 K (Supporting Information Figure S5) shows that
the low-spin state of this compound at low temperature is
retained in solution (g = 2.28, 2.24, 2.19, 1.94).8a,b The axial LS
signals imply that the unpaired electron of iron(III) resides in a
HOMO of dxy character. That [Fe(hapMen)2]ClO4 and
[Fe(hapMen)2]ClO4 are isostructural at the metal center is
demonstrated by the virtually identical EPR spectra (Support-
ing Information Figure S5). Although in the solid state
[Fe(hapMen)2]BPh4 is HS (5−400 K), in frozen methanol
solution at 77 K, this compound is purely in the doublet ground
state.
Figure 7. Mossbauer spectra of [Fe(hapMen) ]ClO .
̈
2
4
Electrochemistry. The one-electron reversible oxidative
responses for the iron(II)-pyridyl complexes [Fe(apHen)2]-
(ClO4)2 and [Fe(apMen)2](ClO4)2 at E1°/2 = 0.72 and 0.82 V
(vs SHE), respectively, are attributable to the FeIII/FeII redox
couple. The cyclic voltammogram of [Fe(apMen)2](ClO4)2 is
displayed in Figure 9a. During these redox processes the ligands
remained intact. The difference in the redox potentials of these
compounds can only be associated with the different amine
donor atoms: primary amine for [Fe(apHen)2](ClO4)2 and
methyl-bearing secondary amine for [Fe(apMen)2](ClO4)2.
Evidently, the oxidative process of [Fe(apMen)2](ClO4)2 is
significantly more spontaneous than that of [Fe(apHen)2]-
(ClO4)2 (ΔG° = −nFE°). This observation can be explained in
terms of the amount of electron density at the metal center of
each of these compounds. The methyl group is electron-
donating, thus its effect is to stabilize the ferric state and
facilitate the oxidation of the ferrous state during the
electrochemical process; by so doing the redox potential
becomes more positive.33
Figure 8. Temperature dependence of the effective magnetic moment
of [{Fe(hapen)}2(μ-O)].
The cyclic voltammogram of the iron(III)−phenolate
complex [Fe(hapMen)2]ClO4 [Figure 9b] shows two elec-
tron-transfer processes in the potential range of −1.1 to 2.0 V
(vs SHE). The metal-based reversible reductive wave at E°1/2
=
a plateau probably due to the presence of a paramagnetic
−0.56 V (vs SHE) is associated with the FeIII/FeII redox couple.
At a higher potential (Ea = 1.23 V), a phenoxyl species is
generated. Electrochemical oxidations of phenolates and
naphtholates to the corresponding phenoxyl and naphthoxyl
radicals, respectively, are quite common and are well
documented.34 The electrochemical behavior of the ethyl-
substituted complex [Fe(hapEen)2]ClO4 is identical to that of
[Fe(hapMen)2]ClO4, given the indistinguishable cyclic voltam-
mograms. This result is consistent with the closely matching
Hammett parameters35 of the methyl and ethyl substituents (σp
= −0.17 and −0.15, respectively).
monomeric impurity (∼0.3%).29b,d,g−i,o,p
Spin−spin coupling in [{Fe(hapen)}2(μ-O)] was determined
̂
by the general isotropic spin-exchange Hamiltonian Hex
−2JS1S2
=
2,29a,b,d,f−i,o,p
(S1 = S2 = 5/2). The best fit of the
experimental magnetic data gave a J value of −99 cm−1 and g =
2.00 with TIP = 1.00 × 10−6 cm3 mol−1.32 The magnitude and
sign of the exchange coupling constant imply strong
antiferromagnetic interactions. J values for the vast majority
of μ-oxo-monobridged diiron(III) complexes range from ∼−65
to ∼−220 cm−1.2,29a,b,d,f−i,o,p Previous studies have correlated
the magnitude of the J value with the size of the Fe−O−Fe
angle and the Fe−Ooxo bond length. The transmission of the
spin−spin interactions is believed to be through orbitals on
each iron(III) atom and the bridging oxygen atom.2,29g,i
Therefore, π-bonding across the bridge is often suggested to
be the major pathway for antiferromagnetic coupling. In most
Electronic Absorption Spectroscopy. The iron(II)−
pyridyl compounds [Fe(apHen)2]X2 (X− = ClO4 , BPh4 , or
−
−
−
−
−
−
PF6 ) and [Fe(apMen)2]X2 (X = ClO4 , BPh4 , or PF6 ) are
characterized by intense navy blue and purple-tinged royal blue
solutions, respectively, in methanol whereas the iron(III)−
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Figure 9. Cyclic voltammograms for (a) [Fe(apMen)2](ClO4)2 and
(b) [Fe(hapMen)2]ClO4 in MeCN (clockwise scans).
−
phenolate compounds [Fe(hapMen)2]X (X− = ClO4 or
−
BPh4 ) and [Fe(hapEen)2]ClO4 are deep purple-pink in the
Figure 10. Electronic absorption spectra of [Fe(apMen)2](ClO4)2 (A;
0.125 mM, 1-cm path length) and [Fe(hapMen)2]ClO4 (B; 0.188 mM,
1-cm path length) in MeOH.
same solvent. These intense colors originate from the strong
visible absorptions (Figure 10a). For each complex cation, the
counterion has no effect on the color of the iron compound.
The UV−visible spectra of the iron(II) compounds [Fe-
(apHen)2](ClO4)2 and [Fe(apMen)2](ClO4)2 resemble each
other in accord with the closeness of their colors (navy blue for
the former and purplish royal blue for the latter). The strong
lobsided absorption bands between 440 and 640 nm [506 nm
(ε ≈ 3500 M−1 cm−1) and 595 nm (ε ≈ 8200 M−1 cm−1)],
responsible for the colors, are attributable to charge-transfer
transitions in these low-spin iron(II) compounds. These
MLCT transitions represent transference of charge from filled
iron(II) dπ orbitals to vacant low-lying p*π ligand orbitals.
According to the Tanabe−Sugano diagram for octahedral d6
complexes, several LS spin-allowed ligand-field transitions are
were not observed for [Fe(apHen)2](ClO4)2 and [Fe-
(apMen)2](ClO4)2 presumably because they were obscured
by the hugely intense MLCT absorptions.36 At higher energies
[272 nm (ε ≈ 12 000 M−1 cm−1) and 373 nm (ε ≈ 3350 M−1
cm−1)], there are strong absorptions associated with ligand π →
π* transitions. The electronic absorption spectra of the iron(II)
compounds presented in this work bear a close resemblance to
those of related pyridyl-containing iron(II) complexes.8a,12,37
The electronic absorption spectra of the iron(II)−pyridyl
ketimines and the corresponding iron(III)−phenolate keti-
mines contrast as starkly as do the colors of these two classes of
iron compounds. The visible spectrum of [Fe(hapMen)2]ClO4
possible, but only A1g → T1g and A1g → T2g occur at
relatively low energies.6b However, as is often the case with
pyridyl-containing LS iron(II) complexes, these d−d transitions
[Figure 10b] displays two LMCT absorptions,8b,25,27,33,38
a
1
1
1
1
band at 515 nm (εmax = 2330 M−1 cm−1) and a shoulder at 630
nm (εmax ≈ 1150 M−1 cm−1) ascribable to charge transfer from
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vigilance in the laboratory. Although problems with such materials
were not experienced in the course of this work, only minor explosions
occurred during determinations of melting points of the iron
compounds possessing perchlorate as counteranion).
a phenolate pπ orbital to an iron(III) dπ orbital in the HS and
LS states,15b respectively. The intense UV absorption (λmax
=
330 nm, εmax = 9010 M−1 cm−1) corresponds to the pπ → dσ*
CT transition. Given that Δo > P for LS Fe(III) and Δo < P for
HS Fe(III), there is greater stabilization of the dπ orbitals in LS
Fe(III) compounds. Hence the energy separation between the
phenolate pπ and iron(III) dπ orbitals in any given octahedral
iron(III)−phenolate compound is smaller for the LS state than
for the HS state. For this reason, the LS LMCT absorption
occurs at longer wavelength than that for the HS state. The
same observation was made for [Fe(hapEen)2]ClO4. The
relative intensities of the two LMCT bands indicate the
position of the S = 5/2 ↔ S = 1/2 spin equilibrium at 298 K. A
similar study has been undertaken previously on the iron(III)
salicylaldimine complexes [Fe(X-salmeen)2]PF6 by Wilson et
al.15b In the case of [Fe(hapMen)2]BPh4, the pure crystalline
solid is HS down to 4 K, but undergoes spin conversion in
MeOH solution at room temperature; in frozen MeOH
Melting points were measured with a Gallenkamp melting point
apparatus. Infrared spectra were recorded on a Perkin-Elmer Spectrum
BX FT-IR spectrophotometer in the range 4000−400 cm−1 using KBr
1
disks of the samples compressed with a Carver hydraulic press. H
NMR spectra were run on an Avance Bruker 400 DPX spectrometer
with DMSO-d6 as solvent and TMS as internal reference standard.
Measurements of UV−visible spectra were carried out on a Hewlett-
Packard 8453 diode-array UV−visible spectrophotometer in the range
190−1100 nm using freshly prepared solutions. Microanalyses were
performed on a CE440 CHN elemental analyzer. Electron-impact (EI)
and fast-atom bombardment (FAB) mass spectra were recorded on a
VG 70-SE mass spectrometer with nitrobenzyl alcohol as the matrix.
Variable-temperature magnetic susceptibility measurements were
carried out on a Quantum Design MPMS-5S or MPMS-7 SQUID
magnetometer operating at a magnetic field of 0.5−1.0 T with
HgCo(NCS)4 or palladium as the calibrant. The magnetic data were
corrected for diamagnetism the usual way using Pascal’s constants. The
susceptibility and magnetization of the iron(III) dinuclear complex
[{Fe(hapen)}2(μ-O)] were simulated with the program julX for
exchange coupled systems designed by E. Bill (Max-Planck-Institut,
6
2
solution at 77 K, the transition A1 → T2 is complete as
revealed by EPR spectroscopy.
CONCLUSION
Mulheim an der Ruhr, Germany).32 An expression of the Hamilton
■
̈
The iron compounds [Fe(apRen)2]X2 and [Fe(hapRen)2]X
have been generated by reaction of the appropriate tridentate
Schiff base (apRen or HhapRen, respectively) with a ferrous or
ferric salt in stoichiometric amounts. Illustrative X-ray analyses
of [Fe(apHen)2](ClO4)2, [Fe(apMen)2](ClO4)2, and [Fe-
(HapMen)2]ClO4 have provided conclusive evidence for the
existence of these iron compounds. The physicochemical
properties of [Fe(apRen)2X2 and [Fe(hapRen)2]X have been
compared and contrasted, and the differences between the
pyridyl and phenolate moieties highlighted. Whereas in the
former, the ferrous LS (1A1) state is preferentially stabilized by
the pyridyl ketimine irrespective of the type of counterion, in
the latter both ferric HS (6A1) and LS (2T2) states are
supported by the corresponding phenolic ketimine albeit to
different extents depending on the type of counterion. In the
case of [Fe(hapMen)2]ClO4 and [Fe(hapEen)2]ClO4, a
seemingly trivial change of amino group, Ren, triggers a
dramatic difference in spin crossover behavior, namely, two-
step and one-step 6A1 ↔ 2T2 transitions, respectively. However,
operator is as follows:
̂
̂
̂
̂
⇀
⇀ ⇀
⇀
̂
̂
̂
H = −2J S1· S2 + HZFS( S1) + HZFS(S2)
Mossbauer spectra were measured with a conventional constant-
̈
acceleration spectrometer equipped with a 50 mCi 57Co(Rh) source of
γ-rays and a low-temperature accessory. The spectrometer was
calibrated with α-Fe at room temperature. X-band EPR spectra were
recorded on a Bruker ELEXSYS E-500 CW spectrometer. Cyclic
voltammetric experiments were carried out in the range 1100−2000
mV on a model EA9 electrochemical analyzer in distilled MeCN with
[Bu4N][PF6] (0.10 M) as the supporting electrolyte using a three-
electrode cell made up of a platinum working electrode, a Ag/AgCl
reference electrode and a platinum counter electrode. The redox
potentials were calibrated with ferrocene as the internal standard (Fc+/
Fc) and are reported herein relative to the standard hydrogen
electrode (SHE) potential.
Syntheses of Schiff Bases and [{Fe(hapen)}2(μ-O)]. The
preparative routes to the Schiff-base ligands and the μ-oxo-
monobridged iron(III) dimer are described in the Supporting
Information.
−
−
replacement of the counterion ClO4 by BPh4 renders the
resultant iron(III) compound high spin. Interestingly, ESR
spectroscopy shows that in frozen MeOH solution, all the
Synthesis of [Fe(apHen)2](ClO4)2. A colorless mixture of 2-
acetylpyridine (0.0969 g, 0.800 mmol) and ethylenediamine (0.0481 g,
0.800 mmol) in MeOH (30 mL) was heated under reflux for 2 h
whereupon a light yellow solution was formed. Addition of
Fe(ClO4)2·xH2O (0.1019 g, 0.4000 mmol) or Fe(ClO4)3·xH2O
(0.1417 g, 0.4000 mmol) yielded a purple-tinged royal blue solution
which was heated under reflux for 15 min. The resultant reaction
mixture was filtered and kept standing at room temperature for slow
evaporation. Black blocks were deposited within three days and
isolated by decantation of the mother liquor. Thereafter, this product
was washed with ice-cold EtOH and dried in a desiccator over P4O10.
Yield: 0.1125 g (48.39%); m.p.: 228−230 °C (explosive). Anal. Calcd
for C18H26N6Cl2O8Fe: C, 37.20; H, 4.51; N, 14.46. Found: C, 37.11;
H, 4.45; N, 14.47. FAB-MS (+ve mode): m/z = 481, 382, 218. IR data
(KBr/cm−1): 3400, 3253, 3208, 3112, 3060, 3042, 2997−2840, 1596,
1589−1453, 1144, 1116, 1090, 627.
−
iron(III)−phenolate complexes [Fe(hapRen)2]X (X− = ClO4
−
or BPh4 ) convert quantitatively to the low-spin state. The
intense navy and purplish blue colors of [FeII(apRen)2]2+ derive
from strong visible absorptions (440−640 nm) attributable to
iron(II) (dπ) → pyridyl (p*π ) charge-transfer transitions. On the
other hand, the deep purple-pink color of [FeIII(hapRen)2]+ is
associated with the phenolate (pπ) → iron(III) (dπ) charge-
transfer absorption centered around 515 nm. All the iron(II)
and iron(III) compounds are redox-active with reversible metal-
centered redox processes. Finally, the dimeric iron(III) complex
[{Fe(hapen)}2(μ-O)] exhibits antiferromagnetic coupling of
spins mediated by the μ-oxo bridge with a J value of −99 cm−1.
Synthesis of [Fe(apHen)2]X2 (X− = BPh4−, PF6−). The
compounds [Fe(apHen)2]BPh4·H2O and [Fe(apHen)2]PF6 were
synthesized by an analogous procedure to that described above except
that the source of the iron(II) ion was FeCl2·4H2O or FeCl3·6H2O
EXPERIMENTAL SECTION
■
Materials and Physicochemical Techniques. The pertinent
ketones, primary amines, salt precursors, and solvents were
commercially available from Sigma-Aldrich at the highest levels of
purity possible and used as received. (Caution: Perchlorate salts are
infamous for explosiveness; hence they must be handled with extreme
−
−
and the counterion NaX (X− = BPh4 , PF6 ). The appropriate sodium
salt, NaBPh4 (0.4107 g, 1.200 mmol) or NaPF6 (0.2015 g, 1.200
mmol), was added to a light yellow solution of FeCl2·4H2O (0.0795 g,
0.400 mmol) or FeCl3·6H2O (0.1081 g, 0.4000 mmol) in MeOH (15
8250
dx.doi.org/10.1021/ic300732r | Inorg. Chem. 2012, 51, 8241−8253

Inorganic Chemistry
Article
mL). The resultant mixture was swirled vigorously and then filtered
directly into the hot solution of apHen in MeOH (30 mL), prepared
as in the synthesis of [Fe(apHen)2](ClO4)2, causing instantaneous
color change to purple-tinged royal blue. [Fe(apHen)2]PF6 was
isolated as large black blocks overnight. Yield: 0.1140 g (42.40%). mp:
269−272 °C. Anal. Calcd for C18H26N6P2F12Fe: C, 32.16; H, 3.90; N,
12.50. Found: C, 32.18; H, 3.88; N, 12.47. FAB-MS (+ve mode): m/z
= 527, 382, 218. IR data (KBr/cm−1): 3337, 3297, 3227, 3193, 3087,
3014, 2975−2857, 1601, 1590−1465, 842, 558. [Fe(apHen)2]-
(BPh4)2·2H2O was obtained as a purple powder immediately after
cooling the reaction mixture to room temperature. Yield: 0.2519 g
(59.59%). m.p.: 269−273 °C. Anal. Calcd for C66H70N6B2O2Fe: C,
75.01; H, 6.68; N, 7.95. Found: C, 75.13; H, 6.57; N, 7.98. FAB-MS
(+ve mode): m/z = 701, 382, 218. IR data (KBr/cm−1): 3400br, 3289,
3244, 3055, 2997−2902, 1597, 1589−1460, 736, 708, 613.
(KBr/cm−1): 3245, 2966−2862, 1595, 1540−1428, 1230, 1120, 1090,
1060, 623.
Single-Crystal X-ray Crystallography. Single-crystal X-ray
analyses were performed on Bruker SMART 1K and Bruker
SMART APEX-II diffractometers equipped with graphite-monochro-
mated Mo−Kα radiation (λ = 0.71073 Å). The structures were solved
by direct methods (SHELXS-97)39 and refined by full-matrix least-
squares based on F2 (SHELXL-97).39 Hydrogen atoms were
positioned geometrically and allowed to ride on their respective
parent atoms. The severely distorted perchlorate counterions of
[Fe(apMen)2](ClO4)2 were eliminated by employing the PLATON
SQUEEZE function.21 Details are provided in the relevant CIF.
ASSOCIATED CONTENT
* Supporting Information
■
S
Syntheses of [Fe(apMen)2]X2 (X− = ClO4−, BPh4−, PF6−). These
iron(II) compounds were produced as described for the corresponding
Synthetic procedures for ligands and the complex [{Fe-
(hapen)}2(μ-O)], X-ray crystallographic files of [Fe(apHen)2]-
(ClO4)2, [Fe(apMen)2](ClO4)2, [Fe(hapMen)2]ClO4,
H2hapen, and [{Fe(hapen)}2(μ-O)] in CIF format, table of
−
−
−
series [Fe(apMen)2]X2 (X− = ClO4 , BPh4 , PF6 ), but using N-
methylethylenediamine instead of ethylenediamine. The very pale
yellow solution of apMen turned navy blue on treatment with the
solution of the appropriate iron(II) or iron(III) salt in MeOH. After
brief heating under reflux, the reaction mixture was allowed to stand at
room temperature and slowly evaporate. [Fe(apMen)2](ClO4)2
crystallized as black blocks within three days. Yield: 0.0955 g
(38.1%). m.p.: 212−215 °C (explosive). Anal. Calcd for
C20H30N6Cl2O8Fe: C, 39.43; H, 4.96; N, 15.76. Found: C, 39.23; H,
4.77; N, 15.61. FAB-MS (+ve mode): m/z = 509, 410, 232. IR data
(KBr/cm−1): 3266, 3169, 3048 3002, 2975−2852, 1598, 1590−1460,
1145, 1119, 1089, 626. [Fe(apMen)2](PF6)2 was obtained as black
blocks after a fortnight of solution evaporation. Yield: 0.0563 g
(20.1%). m.p.: 188−191 °C. Anal. Calcd for C20H30N6P2F12Fe: C,
34.30; H, 4.32; N, 12.00. Found: C, 34.41; H, 4.38; N, 12.02. FAB MS
(+ve mode): m/z = 555, 410, 232. IR data (KBr/cm−1): 3400br, 3151,
3040, 3010, 2973−2891, 1600, 1589−1463, 842, 559. [Fe(apMen)2]-
(BPh4)2·4H2O was deposited by the solution as a black powder
immediately. Yield: 0.1564 g (34.89%). m.p.: 264−267 °C. Anal. Calcd
for C68H78N6B2O4Fe: C, 72.87; H, 7.01; N, 7.50. Found: C, 72.65; H,
6.98; N, 7.52. FAB-MS (+ve mode): m/z = 729, 410, 232. IR data
(KBr/cm−1): 3500br, 3232, 3218, 3160, 3054, 3036, 2997−2891,
1598, 1580−1461, 735, 708, 612.
1
bond distances and angles for [{Fe(hapen)}2(μ-O)], H NMR
spectrum of H2hapen, EPR spectra of [Fe(hapRen)2]ClO4 (R =
Me, Et) in MeOH at 77 K and Mossbauer spectrum of
̈
[Fe(apMen)2](ClO4)2. This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: musa@squ.edu.om.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are greatly indebted to Sultan Qaboos University for
financial support (IG/SCI/CHEM/07/02). Research in the
Department of Physics and Nebraska Center for Materials and
Nanoscience of the University of Nebraska-Lincoln is
supported in part by NSF MRSEC Grant DMR-0820521.
Syntheses of [Fe(hapMen)2]X (X− = ClO4−, BPh4−). A yellow
mixture of 2′-hydroxyacetophenone (0.1089 g, 0.8000 mmol) and N-
methylethylenediamine (0.0593 g, 0.800 mmol) in MeOH (30 mL)
was heated under reflux for 2 h to afford an intense canary yellow
solution. Thereafter, Fe(ClO4)3·xH2O (0.1417 g, 0.4000 mmol) or the
filtrate of the mixture of FeCl3·6H2O (0.1081 g, 0.4000 mmol) and
NaBPh4 (0.4107 g, 1.200 mmol) in MeOH (10 mL) was added, giving
an intense purple solution which was heated under reflux for 10 min.
Then this reaction mixture was filtered and left standing at room
temperature for crystallization. The solution of [Fe(hapMen)2]ClO4
gave black blocks after slow evaporation over a period of five days.
Yield: 0.0623 g (27.1%). m.p.: 206−208 °C. Anal. Calcd for
C22H30N4O6ClFe: C, 49.13; H, 5.62; N, 10.42. Found: C, 49.15; H,
5.62; N, 10.44. FAB-MS (+ve mode): m/z = 438, 247, 191. IR data
(KBr/cm−1): 3280, 3256, 3008, 2980−2880, 1596, 1580−1435, 1259,
1235, 1120, 1090, 1057, 626. On the other hand, [Fe(hapMen)2]BPh4
crystallized overnight as large irregular-shaped shiny black crystals.
Yield: 0.1512 (49.90%). m.p.: 216−217 °C. Anal. Calcd for
C46H50N4O2BFe: C, 72.93; H, 6.65; N, 7.40. Found: C, 72.87; H,
6.62; N, 7.43. FAB MS (+ve ion): m/z = 438, 247, 191. IR data (KBr/
cm−1): 3260, 3238, 3055, 3036, 3010, 2981−2873, 1596, 1577−1433,
1267, 1233, 741, 730, 702, 610.
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