Six-coordinate high-spin iron(ii) complexes with bidentate PN ligands based on 2-aminopyridine-new Fe(ii) spin crossover systems

Article abstract of DOI:10.1039/c4dt00186a

Several new octahedral iron(ii) complexes of the type [Fe(PN ^R-Ph)₂X₂] (X = Cl, Br; R = H, Me) containing bidentate PN^R-Ph (R = H, Me) (1a,b) ligands based on 2-aminopyridine were prepared. ⁵⁷Fe Moess

Full text of DOI:10.1039/c4dt00186a

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Six-coordinate high-spin iron(II) complexes with
bidentate PN ligands based on 2-aminopyridine –
new Fe(II) spin crossover systems†
Cite this: Dalton Trans., 2014, 43,
11152
Christian Holzhacker,a Maria José Calhorda,b Adrià Gil,b Maria Deus Carvalho,b
Liliana P. Ferreira,c,d Berthold Stöger,e Kurt Mereiter,e Matthias Weil,e Danny Müller,a
Peter Weinberger,a Ernst Pittenauer,e Günter Allmaiere and Karl Kirchner*a
Several new octahedral iron(II) complexes of the type [Fe(PNR-Ph)2X2] (X = Cl, Br; R = H, Me) containing
bidentate PNR-Ph (R = H, Me) (1a,b) ligands based on 2-aminopyridine were prepared. 57Fe Mössbauer
spectroscopy and magnetization studies confirmed in all cases their high spin nature at room temperature
with magnetic moments very close to 4.9μB reflecting the expected four unpaired d-electrons in all these
compounds. While in the case of the PNH-Ph ligand an S = 2 to S = 0 spin crossover was observed at low
temperatures, complexes with the N-methylated analog PNMe-Ph retain an S = 2 spin state also at low
temperatures. Thus, [Fe(PNH-Ph)2X2] (2a,3a) and [Fe(PNMe-Ph)2X2] (2b,3b) adopt different geometries. In
the first case a cis-Cl,P,N-arrangement seems to be most likely, as supported by various experimental
data derived from 57Fe Mössbauer spectroscopy, SQUID magnetometry, UV/Vis, Raman, and ESI-MS as
well as DFT and TDDFT calculations, while in the case of the PNMe-Ph ligand a trans-Cl,P,N-configuration
is adopted. The latter is also confirmed by X-ray crystallography. In contrast to [Fe(PNMe-Ph)2X2] (2b,3b),
[Fe(PNH-Ph)2X2] (2a,3a) is labile and undergoes rearrangement reactions. In CH3OH, the diamagnetic
dicationic complex [Fe(PNH-Ph)3]2+ (5) is formed via the intermediacy of cis-P,N-[Fe(κ2-P,N-PNH-Ph)2-
(κ1-P-PNH-Ph)(X)]+ (4a,b) where one PN ligand is coordinated in a κ1-P-fashion. In CH3CN the dia-
magnetic dicationic complex cis-N,P,N-[Fe(PNH-Ph)2(CH3CN)2]2+ (6) is formed as a major isomer where
the two halide ligands are replaced by CH3CN.
Received 19th January 2014,
Accepted 10th March 2014
DOI: 10.1039/c4dt00186a
www.rsc.org/dalton
blies are able to coordinate reversibly to a metal center provid-
ing or protecting temporarily a vacant coordination site, a
Introduction
Heterodifunctional ligands are intensively studied and applied feature very desirable for catalysts. In this context, we have
in coordination and organometallic chemistry owing to the become interested in heterodifunctional PN ligands based on
often unique properties of their metal complexes and their 2-aminopyridine in which the aromatic pyridine ring is separ-
ability to generate hemilabile systems which often display ated from the phosphine moiety by an amino group. These
enhanced reactivity.1 In particular, soft/hard, e.g., P/N assem- types of ligands are easily constructed by reacting 2-aminopyri-
dines with R2PCl (R = alkyl, aryl) in the presence of a base.2
Owing to the relative ease of phosphorus–nitrogen bond
forming reactions compared to those involving phosphorus–
carbon bonds, it is not surprising that the synthesis and reac-
tivity of many examples of transition metal complexes featur-
ing this type of PN ligands have been reported over the last
few decades.3–14 The most prominent member of this ligand
family, with a few exceptions,15 is N-diphenylphosphino-
2-aminopyridine (PNH-Ph). Examples of iron complexes con-
taining the PNH-Ph ligand are very scarce.16
As far as iron complexes are concerned, we have recently
shown3a that anhydrous FeX2 (X = Cl, Br) reacts with the bulky
and strongly electron donating PN ligands N-di-iso-propylphos-
phino-2-aminopyridine (PNH-iPr) and N-di-tert-butylphosphino-
aInstitute of Applied Synthetic Chemistry, Vienna University of Technology,
Getreidemarkt 9, A-1060 Vienna, Austria. E-mail: kkirch@mail.tuwien.ac.at
bCentro de Química e Bioquímica/DQB, Faculdade de Ciências, Universidade de
Lisboa, 1749-016 Lisboa, Portugal
cCentro de Física da Matéria Condensada/DF, Faculdade de Ciências,
Universidade de Lisboa, 1749-016 Lisboa, Portugal
dDepartamento Física, Faculdade Ciências e Tecnologia, Universidade de Coimbra,
3004-516 Coimbra, Portugal
eInstitute of Chemical Technologies and Analytics, Vienna University of Technology,
Getreidemarkt 9, A-1060 Vienna, Austria
†Electronic supplementary information (ESI) available: Atomic coordinates and
energies for all optimized species. Calculated and experimental UV/Vis, FIR and
Raman spectra of 2a. CCDC 981196–981198 for 2b, 5, and 6. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/c4dt00186a
11152 | Dalton Trans., 2014, 43, 11152–11164
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synergy with theoretical calculations utilizing DFT and TDDFT
methods.
Results and discussion
In sharp contrast to reactions with the bulky ligands PNH-iPr
and PNH-tBu (Scheme 1), treatment of anhydrous FeCl2 with 2
equiv. of the PN ligands PNH-Ph (1a) and PNMe-Ph (1b) in THF
at room temperature for 12 h afforded pale yellow octahedral
complexes of the general formula [Fe(PNR-Ph)2Cl2] (2a,b) in 85
and 92% isolated yields (Scheme 3). The analogous bromide
complexes [Fe(PNR-Ph)2Br2] (3a,b) were obtained in a similar
fashion by straightforward complexation of the respective free
PN ligands with anhydrous ferrous dibromide in 83 to 91%
yields. The formation of these complexes is independent of
whether 1 or 2 equiv. of ligands are used. However, in the first
case substantial amounts of FeX2 remained unreacted. There
was no evidence for the formation of tetra-coordinate [Fe(PNR-
Ph)X2] as in the case of the more electron donating and
Scheme 1
Scheme 2
2-aminopyridine (PNH-tBu) to form stable coordinatively un- bulkier PNH-iPr and PNH-tBu ligands (Scheme 1). In the solid
saturated 14e iron(II) complexes of the type [Fe(PNH-R)X2] state all complexes are thermally stable if air is excluded. In
(Scheme 1). The formation of these complexes is independent solution, 2a and 3a turned out to be highly unstable, forming
of whether 1 or 2 equiv. of ligands are used. Interestingly, different decomposition products depending on the solvent
despite being highly unsaturated, these complexes turned out (vide infra). On the other hand, 2b and 3b are stable in most
to be inert towards the addition of CO.
common solvents such as CH3OH, THF, CH2Cl2, and CH3CN
Here we report the reactions of anhydrous FeX2 with the for several days without showing any noticeable decompo-
ligands PNH-Ph and PNMe-Ph (Scheme 2) which are sterically sition. In fact, even single crystals suitable for X-ray diffraction
less demanding as well as less electron donating than their iPr measurements could be grown for 2b. This behavior clearly
and tBu congeners. These modifications result in the synthesis suggests that 2a and 3a are structurally different from 2b and
of octahedral high spin complexes of the general formula 3b. It has to be kept in mind that for complexes of the general
[Fe(PNR-Ph)2X2]. Surprisingly, additional modification of the formula [Fe(PNR-Ph)2X2], in principle five different coordi-
ligand scaffold by introducing a methyl group instead of nation isomers are conceivable as illustrated in Scheme 4.
H leads to the formation of isomers with strikingly different
The molecular structure of 2b, depicted in Fig. 1, reveals
chemical properties. In fact, [Fe(PNH-Ph)2X2] is substitutionally that all Cl, P and N atoms are in trans position to one another.
labile in solution and exhibits a high spin/low spin crossover Accordingly, in the case of complexes bearing the PNMe-Ph
in the solid state, contrasting with the behavior of [Fe(PNMe
-
(1b) ligand, isomer A in its high spin form is the thermodyna-
Ph)2X2]. Notably, octahedral Fe(II) spin crossover complexes mically most stable species. This finding is also supported by
with phosphine ligands which are able to switch between the DFT calculations (vide infra).
low-spin and high-spin states are rare.17 This work is aimed at
In order to establish the nature of the isomer formed in the
exploring and understanding the structural and electronic case of complexes 2a and 3a featuring the PNH-Ph ligand (1a),
properties of [Fe(PNR-Ph)2X2] complexes by means of various reactivity studies in different solvents were carried out.
experimental data derived from 57Fe Mössbauer spectro- Additionally, a variety of measurements including SQUID
scopy, SQUID magnetometry, UV/Vis, Raman, and ESI-MS in magnetometry, 57Fe Mössbauer spectroscopy, UV/Vis, Raman,
Scheme 3
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Scheme 4 DFT calculated geometries and electronic energies in kcal mol−1 for complex 2 (X = Cl and PN = PNH-Ph).
ESI-MS as well as DFT calculations were performed. Due to the
instability of 2a and 3a in solution as well as the paramagnetic
nature of complexes 2a,b and 3a,b, bearing basically aromatic
protons and carbon atoms, 1H NMR spectra exhibited only
very broad and featureless signals and were thus not very infor-
mative. 13C{1H} and 31P{1H} NMR signals could not be
detected at all.
In CH3OH complexes 2a and 3a are substitutionally labile
and undergo rearrangement reactions to yield the diamagnetic
dicationic tris-PNH-Ph complex [Fe(PNH-Ph)3]2+ (5) (Scheme 5,
the reaction is not balanced) together with free ligand PNH-Ph,
the paramagnetic tetrahaloferrate anion FeX42− and intractable
paramagnetic materials. Monitoring this reaction by 31P{1H}
NMR spectroscopy revealed that this reaction proceeds via the
Fig. 1 Structural view of trans-Cl,P,N-[Fe(PNMe-Ph)2Cl2] (2b) showing
50% thermal ellipsoids (H atoms omitted for clarity). The complex is
centrosymmetric. Selected bond lengths (Å) and bond angles (°): Fe1–N1
2.1910(13), Fe1–P1 2.6046(4), Fe1–Cl1 2.3637(4), P1–N2 1.7195(14), N2–
C18 1.469(2), N2–C1 1.394(2), N1–Fe1–P1 75.80(4), N1–Fe1–P1i 104.20(4),
N1–Fe1–Cl1 88.67(4), N1–Fe1–Cl1i 91.33(4), P1–Fe1–Cl1 94.83(1),
P1–F1–Cl1i 85.17(1), Cl1–Fe1–Cl1i = P1–Fe–P1i = N1–Fe1–N1i = 180.00.
intermediates
cis-P,N-[Fe(κ2-P,N-PNH-Ph)2(κ1-P-PNH-Ph)(X)]+
(4a,b) where one PN ligand is coordinated in a κ1-P-fashion.
This reaction involves an intermolecular ligand transfer
between two [Fe(PNH-Ph)2X2] complexes and is complete
within about 15 minutes at room temperature. In the 31P{1H}
Scheme 5
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NMR spectrum (measured using a 600 MHz NMR spectro-
meter), intermediate 4a displays an ABX pattern providing
further evidence for a cis-Cl,P,N-structure (isomer B) of the
starting materials 2a and 3a (Scheme 4). Intermediate 4b still
exhibits a simpler A2X spin system (δA ≈ δB). A rational syn-
thesis of 5 (as chloride 5a or bromide salt 5b) can be achieved
in 90 and 94% isolated yields by reacting anhydrous FeX2 with
3 equiv. of PNH-Ph in CH3OH.
This complex gives rise to a singlet in the 31P{1H} NMR
spectrum at 104.5 ppm, indicating that all P and N atoms,
respectively, are cis to one another. The structure of 5 (as
bromide salt) was unequivocally established by X-ray crystallo-
graphy. A structural view is depicted in Fig. 2 with selected
bond distances given in the caption.
Scheme 6
Upon dissolution of 2a and 3a in acetonitrile a mixture of
the diamagnetic dicationic complex cis-N,P,N-[Fe(PNH-Ph)2-
(CH3CN)2]2+ (6) and an unidentified complex, presumably
a symmetrical isomer of 6, in a 2 : 1 molar ratio was afforded
within 24 h. Complex 6 displays an AB pattern for the two
nonequivalent 31P nuclei centered at 114.6 ppm (Δν = 533.2 Hz)
with a coupling constant JPP of 54.8 Hz (Scheme 6). The small
coupling constant is consistent with a cis-P,P arrangement. The
minor isomer gives rise to a singlet at 110.8 ppm (for complexes
of the formula [Fe(PNH-Ph)2(CH3CN)2]2+, four isomers are
conceivable which would all give rise to a singlet in the 31P{1H}
NMR spectrum, cf. isomers A, C, D, and E in Scheme 4).
It has to be noted that in the presence of NaBF4 as a halide
scavenger the same complexes were formed in about the same
molar ratio. From this mixture, crystals of 6 suitable for an
X-ray diffraction study were obtained. A molecular view of 6 is
depicted in Fig. 3 with selected bond distances given in the
caption. The dicationic iron complex adopts a slightly dis-
Fig. 3 Structural view of cis-P,N,N-[Fe(PNH-Ph)2(CH3CN)2](BF4)2 (6)
showing 50% thermal ellipsoids (H atoms, BF4 anions, and the second
−
independent Fe complex omitted for clarity). Selected bond lengths (Å)
and bond angles (°): Fe1–N1 2.034(2), Fe1–N3 2.006(2), Fe1–N5
1.966(3), Fe1–N6 1.918(3), Fe1–P1 2.2241(9), Fe1–P2 2.2056(8), P1–N2
1.693(3), P2–N4 1.680(3), N2–C1 1.379(4), N4–C18 1.383(4), N1–Fe1–P1
81.03(8), N3–Fe1–P2 82.03(7), N1–Fe1–N3 91.18(10), N1–Fe1–N5
91.28(11), N1–Fe1–N6 88.61(10), N5–Fe1–N6 88.03(12), P1–Fe1–P2
96.86(3), N1–Fe1–P2 172.97(8), N3–Fe1–N6 175.89(12), N5–Fe1–P1
171.28(8), P1–N2–C1 116.8(2), P2–N4–C18 116.5(2).
torted octahedral geometry around the metal center with all
donor atoms of the CH3CN and PNH-Ph ligands being in cis
position to one another. If the formation of 6 is the result of
two consecutive dissociative substitution processes, the start-
ing materials 2a and 3a most likely adopt the all cis form BHS
(Scheme 4) as in this case no further rearrangement steps are
required.
Upon cooling to 77 K the color of complexes 2a and 3a
changed from pale yellow to dark violet suggesting possible
high-spin/low-spin transitions. In agreement with this obser-
vation, the UV/Vis spectrum of 2a and 3a in the solid state at
293 K did not show any absorption in the visible range, while
at 123 K these complexes exhibited two bands at 562 and
690 nm and 565 and 716 nm, respectively. On the other hand,
all other complexes did not change their color at low tempera-
ture again suggesting that the structures of 2a and 3a are sig-
nificantly different from those of 2b and 3b.
Fig. 2 Structural view of cis-P,N-[Fe(PNH-Ph)3]Br2 (5b) showing 50%
thermal ellipsoids (H atoms and bromide anions omitted for clarity).
Selected bond lengths (Å) and bond angles (°): Fe1–N1 2.042(2), Fe1–N3
2.048(2), Fe1–N5 2.058(2), Fe1–P1 2.2396(7), Fe1–P2 2.2546(8), Fe1–P3
2.2401(7), P1–N2 1.697(2), P2–N4 1.684(2), P3–N6 1.679(2), N1–Fe1–
P1 83.44(6), N3–Fe1–P2 83.16(6), N5–Fe1–P3 82.72(6), N1–Fe1–N3
90.43(8), N1–Fe1–N5 89.33(8), N3–Fe1–N5 90.01(8), P1–Fe1–P2
102.40(3), P1–Fe1–P3 98.54(3), P2–Fe1–P3 100.63(3), N1–Fe1–P3
171.53(7), N3–Fe1–P1 171.43(6), N5–Fe1–P2 172.18(6).
The magnetic properties of all compounds were studied by
SQUID magnetometry and 57Fe Mössbauer spectroscopy. The
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mol−1 K at 10 K, corresponding to a high fraction of LS Fe(II)
and a residual content of HS Fe(II), as confirmed by Mössbauer
spectra. Both samples were measured on cooling and warming
sequences displaying gradual spin transitions without thermal
hysteresis, maintaining the low temperature LS Fe(II) fraction
up to approximately 125 K (2a) or 150 K (3a). At those tempera-
tures the population of the HS state gradually increases,
achieving full conversion over a temperature range of 125 K
(2a) or 150 K (3a). The ST temperatures, T1/2, can be assumed
to be 185 K for 2a and 222 K for 3a (Fig. 4, inset). These first-
order spin transitions, somewhat gradual, not complete at low
temperatures and displaying no thermal hysteresis, are more
likely to be associated with weak intermolecular cooperative
effects, without structural phase transitions.18,19 Although
hydrogen-bonded networks can induce cooperative inter-
actions, and their existence cannot be discarded in the pres-
ence of the PNH-Ph ligand, they should be relatively weak since
the complexes are neutral and there is no charge assistance.19
57Fe Mössbauer results are in good agreement with magne-
tization data, as can be seen from Fig. 5 and Table 1. At room
temperature only one quadrupole doublet characteristic of HS
Fe(II) with isomer shift values (δ) of 0.87(1) mm s−1 and quad-
Fig. 4 Temperature dependence of χmT for [Fe(PNH-Ph)2Cl2] (2a) (blue
squares) and [Fe(PNH-Ph)2Br2] (3a) (red circles) upon cooling (solid
symbols) and warming (open symbols). The inset shows the temperature
variation of the HS Fe(II) mole fraction for both compounds, assuming
that at 300 K there is only HS.
temperature dependent magnetic behavior of complexes 2a rupole splitting values (ΔEQ) around 3.1(1) mm s−1 are
and 3a (Fig. 4), in agreement with the color changes, shows detected for both complexes 2a and 3a. A second quadrupole
that both compounds exhibit a thermal and almost complete doublet attributed to LS Fe(II) appears and its fraction
spin transition (ST) with χmT values near 3.0 cm3 mol−1 K at increases as the temperature decreases. At 78 K the hyperfine
300 K consistent with HS Fe(II), and χmT values below 0.4 cm3 parameters δ = 0.48(1) mm s−1 and ΔEQ ≈ 1.0(1) mm s−1
Fig. 5 Temperature dependence of 57Fe Mössbauer spectra of [Fe(PNH-Ph)2Cl2] (2a) (left) and [Fe(PNH-Ph)2Br2] (3a) (right).
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Table 1 Estimated hyperfine parameters from the 57Fe Mössbauer
spectra of [Fe(PNH-Ph)2Cl2] (2a) and [Fe(PNH-Ph)2Br2] (3a) collected at
different temperatures
T
δ
ΔEQ
Γ
I
Fe(II)
site
Compound
(K) (mm s−1
)
(mm s−1
)
(mm s−1
)
(%)
[Fe(PNH-Ph)2Cl2] (2a) 290 0.87(1)
3.11(1)
3.22(1)
1.00(1)
3.18(1)
0.99(1)
2.95(5)
0.98(1)
3.09(1)
3.25(1)
1.04(1)
3.23(3)
1.04(1)
3.32(4)
1.03(1)
0.27(1)
0.28(2)
0.29(2)
0.26(2)
0.25(1)
0.63(9)
0.24(1)
0.26(1)
0.30(1)
0.31(1)
0.39(4)
0.26(1)
0.41(7)
0.25(1)
100
58.5 HS
41.5 LS
30.4 HS
69.6 LS
13.3 HS
86.7 LS
HS
195 0.92(1)
0.44(1)
170 0.96(1)
0.45(1)
78 1.10(3)
0.48(1)
[Fe(PNH-Ph)2Br2] (3a) 290 0.87(1)
100
HS
44.8 HS
55.2 LS
20.9 HS
79.1 LS
8.5 HS
215 0.91(1)
0.44(1)
185 0.92(1)
0.46(1)
78 1.08(2)
0.48(1)
Fig. 6 Temperature dependence of the inverse molar susceptibility
(circles) and of χmT (squares) for [Fe(PNMe-Ph)2Cl2] (2b) (solid symbols)
and [Fe(PNMe-Ph)2Br2] (3b) (open symbols). The straight lines correspond
to Curie law fitting to the experimental data.
91.5 LS
δ – isomer shift; ΔEQ – quadrupole splitting; Γ – FWHM line width;
I – relative area with uncertainties <2%.
confirm the LS state of Fe(II) with relative fractions of 87% for
the Cl compound and 92% for the Br one, assuming the same
Debye–Waller factor for LS and HS species. According to the
magnetization data, the HS fraction existing at this tempera-
ture should remain until 10 K.
The thermal variation of the inverse molar magnetic sus-
ceptibility and of χmT for complexes 2b and 3b is shown in
Fig. 6. Upon replacement of the NH moiety by a NMe unit the
spin transition completely vanishes, indicating an increased
energy difference between the HS and LS forms in the 2b and
3b complexes when compared with 2a and 3a ones. The
thermal variation of χm−1 is well described by a modified Curie
law (χm = Cm/T + K, where Cm is the molar Curie constant and
K is a temperature independent constant). Iron effective mag-
netic moments of 5.0(1) and 5.1(1)μB were extracted from the
Cm values of the Cl and Br complexes, respectively, in good
agreement with the effective magnetic moment of HS Fe(II) in
the spin-only approximation (4.9μB).
57Fe Mössbauer spectra collected at room temperature for
2b and 3b also agree with magnetization data since only one
iron site (one quadrupole doublet) with hyperfine parameters
characteristic of HS Fe(II) is observed: δ = 0.85(1) and 0.88(1)
mm s−1 and ΔEQ = 3.04(1) and 3.14(1) mm s−1 for the Cl and
Br complexes, respectively (Fig. 7, Table 2). Comparing room
temperature hyperfine parameters obtained for all octahedral
compounds, it became clear that the substitution of the NH
moiety by a NMe unit does not change significantly either the
s-electron density at the iron nuclei (related with δ) or the
quadrupole splitting values. The latter parameter depends on
the electric field gradient (EFG) surrounding the Fe nuclei
with two contributing sources: the lattice (charges in a non-
cubic symmetry) surrounding the Mössbauer probe and the
iron valence electrons. As similar values were obtained for
trans and cis complexes and rather different values were
obtained for the cis compound in high and low spin states, it
Fig. 7 57Fe Mössbauer spectra of [Fe(PNMe-Ph)2Cl2] (2b) (top) and
[Fe(PNMe-Ph)2Br2] (3b) (bottom) collected at 290 K.
Table 2 Estimated hyperfine parameters from the 57Fe Mössbauer
spectra of [Fe(PNMe-Ph)2Cl2] (2b) and [Fe(PNMe-Ph)2Br2] (3b) collected
at 290 K
δ
ΔEQ
Γ
Fe(II)
site
Compound
(mm s−1
)
(mm s−1
)
(mm s−1
)
[Fe(PNMe-Ph)2Cl2] (2b)
[Fe(PNMe-Ph)2Br2] (3b)
0.85(1)
0.88(1)
3.04(1)
3.14(1)
0.26(1)
0.27(1)
HSa
HS
δ – isomer shift; ΔEQ – quadrupole splitting; Γ – FWHM line width.
a A small impurity due to oxidation during Mössbauer sample
preparation is also present.
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can be deduced that the main contribution to the EFG comes
from the iron valence electrons.
In all cases the quintet ground state (S = 2) is thermodyna-
mically more stable and the energies of isomers AHS–DHS are
The 57Fe Mössbauer parameters were calculated for the very similar ranging from −0.8 to 2.2 kcal mol−1, while EHS is
isomer B (see below) of complex 2a with a DFT approach less stable than AHS by 11.1 kcal mol−1. All of these four
(using ORCA20 and two basis sets, b1 and b2; see Experimental isomers are possible high spin species. It is interesting to
section for details). The electronic density at the nucleus (ρ) note, however, that isomers DHS and EHS are structurally sig-
was converted in isomer shifts (δ) using the method of nificantly different from the other isomers. These complexes
Neese,21 and the quadrupole splitting was obtained directly. are five coordinate species adopting a distorted square pyrami-
The isomer shifts are 0.835 (HS) and 0.611 (LS) mm s−1 with dal geometry. While in DHS one pyridine moiety is no longer
the b1 basis set, and 0.736 and 0.515 mm s−1, respectively, coordinated to the iron center (the Fe⋯Npy distance is 3.86 Å),
with the better b2 basis set. The quadrupole splittings were in EHS one phosphine moiety is weakly bound with a Fe–P
calculated to be 0.579, 3.009 (b1) and 0.579, 3.026 (b2) mm bond distance of 3.10 Å. In fact, isomer DHS strongly resembles
s−1, for LS and HS, respectively. Both are good estimates of the five coordinate PNP pincer complexes of the type [Fe(PNP)Cl2]
experimental parameters. The same calculation (b1) for isomer which form stable HS complexes with a quintet ground state
C led to similar isomer shifts (0.830 and 0.631 mm s−1, for HS that shows no spin crossover.22 The corresponding singlet
and LS) and quadrupole splitting for the LS isomer (0.620 mm ground states ALS–ELS, all with an octahedral geometry, are less
s−1). However, the quadrupole splitting for the HS room temp- stable than the corresponding HS states. The energy differ-
erature species is calculated to be 2.398 mm s−1, very far from ences between HS and LS spin states (ΔEHS/LS) of isomers A–E
the experimental value. This suggests that isomer B is the are 20.3, 9.7, 14.7, 18.3, and 7.3 kcal mol−1. It is, thus, reason-
most likely species to be present in the solid.
able that at low temperatures no spin-crossover takes place in
Moreover, all complexes were also investigated by means of the case of isomer A, which displays a very high energy differ-
ESI-MS (Table 3). Solutions of complexes 2a, 2b, 3a, and 3b in ence. Accordingly, a HS/LS crossover is more likely to take
CH3OH in the presence of NaX (X = Cl, Br) were subjected to place in the case of B and to a lesser extent of C where (i) the
ESI-MS analysis in the positive ion mode. By means of the so HS species has low energy and (ii) ΔEHS/LS is small. In contrast,
called “soft ionization” technique ESI at atmospheric pressure, spin transitions in D and E would be associated with severe
the most abundant signals are observed at m/z 647.10, 675.13, structural changes, e.g., changes of both geometry and coordi-
691.05, and 719.08, respectively, which correspond to the cat- nation number (Scheme 4), which appear to be unlikely to
ionic complexes [Fe(PNR-Ph)2X]+ ([M − X]+), where one halide occur in the solid state and are not suggested by any experi-
ligand is dissociated. Further abundant fragments are [M − X mental data. The high-spin state exhibits typically longer
− HX]+ (only for 2a and 3a), [M − X − 1a]+, and [M − X − 1b]+. metal ligand bonds, since the partially occupied e*g states are
Representative corresponding positive ion ESI full scan mass metal–ligand anti-bonding. In the present case of the [Fe(PNR-
and MS/MS (low energy CID) spectra of 3a are depicted in Ph)2X2] complexes this effect is very pronounced for the Fe–P
Fig. 8. Furthermore in the inset of Fig. 8(A), the isotopic and Fe–N bonds (for instance, Fe–N1 shortens from 2.388 to
pattern of the [M − Br]+ ion is compared with the theoretical 1.967 Å, and Fe–P1 from 2.529 to 2.151 Å) but modest for
pattern, which turned out to correlate quite well.
the Fe–Cl bonds (Table 4). There are several literature examples
The geometries and energies (in kcal mol−1) of all possible of octahedral Fe(II) diphosphine complexes of the types
isomers of 2a and 3a in two different spin states (S = 2 and S = [Fe(PP)2X2] (PP = cis-1,2-bis(diphenylphosphino)ethylene, X = Cl,
0) were determined by means of DFT/OPBE calculations Br) where the Fe–P bonds are abnormally long in the quintet
(Scheme 4, electronic energies shown for all isomers of 2a). ground state.17 Based on these calculations, we propose that
Isomer AHS was used as a reference point for all calculations. 2a and 3a adopt a cis-Cl,P,N-geometry (B). The structures of the
Stable minima in both S = 2 and S = 0 spin states could be HS and LS isomers of B are shown in Fig. 9. However, a cis-P,N–
obtained for all five isomers.
trans-Cl arrangement (C) cannot be conclusively ruled out.
Table 3 Elemental compositions of the neutral compounds 2, 3, 5 and calculated m/z values of [M − X]+ (compounds 2 and 3), [M − X − HX]+
(compounds 2, 3, 5) and [M − X − HX − 1a/b) ions (compounds 2, 3, 5; X = Cl, Br)
Elemental composition/
Compound
molecular weighta
Elemental composition
Elemental composition
2a
C
34H30Cl2Fe1N4P2 682.07
C34H30Cl1Fe1N4P2 [M − Cl]+ = 647.10
C34H29Fe1N4P2 [M − Cl − HCl]+ = 611.12
C17H15Cl1Fe1N1P1 [M − Cl − 1a]+ = 369.00
C18H17Cl1Fe1N1P1 [M − Cl − 1b]+ = 383.02
C34H29Fe1N4P2 [M − Br − HBr]+ = 611.12
C17H15Br1Fe1N1P1 [M − Br − 1a]+ = 412.95
C18H17Br1Fe1N1P1 [M − Br − 1b]+ = 426.97
C34H29Fe1N4P2 [M − Cl − HCl − 1a]+ = 611.12
C34H29Fe1N4P2 [M − Br − HBr − 1a]+ = 611.12
2b
3a
C36H34Cl2Fe1N4P2 710.10
C34H30Br2Fe1N4P2 769.97
C36H34Cl1Fe1N4P2 [M − Cl]+ = 675.13
C34H30Br1Fe1N4P2 [M − Br]+ = 691.05
3b
5a
5b
C36H34Br2Fe1N4P2 798.00
C51H45Cl2Fe1N6P3 960.16
C51H45Br2Fe1N6P3 1048.06
C36H34Br1Fe1N4P2 [M − Br]+ = 719.08
C51H44Fe1N6P3 [M − Cl − HCl]+ = 889.22
C51H44Fe1N6P3 [M − Br − HBr]+ = 889.22
a Mass calculations are based on the lowest mass chlorine (35Cl) and bromine isotope (79Br), respectively, and the most abundant iron isotope (56Fe).
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Fig. 8 Positive-ion ESI full scan mass spectrum of [Fe(PNH-Ph)2Br2] (3a) (A) and the corresponding MS/MS (low energy CID)-spectrum of the
in-source-generated [M − Br]+ precursor ion (m/z 691.0) (B). The inset shows the calculated and measured isotopic patterns of [M − Br]+. In
both spectra only signals containing the Fe-isotope of highest abundance (56Fe) are annotated.
Table 4 Mean calculated Fe–P, Fe–Npy, and Fe–Cl bond lengths (Å) for
complexes 2b, [trans-Fe(PP)2Cl2] (PP = cis-1,2-bis(diphenylphosphino)
ethylene),17 AHS, ALS, BHS, BLS, 5, and 6
Spin
state
Complexes
Fe–P
Fe–N
Fe–Cl
2b
HS
HS
LS
HS
LS
HS
LS
LS
LS
2.605
2.584
2.301
2.687
2.253
2.612
2.159
2.245
2.215
2.191
2.364
2.363
2.329
2.332
2.313
2.350
2.337
[Fe(PP)2Cl2]
[Fe(PP)2Cl2]
AHS
ALS
BHS
BLS
5
2.325
2.012
2.397
2.033
2.049
2.020
Fig. 9 Optimized geometries of cis-Cl,P,N-[Fe(PNH-Ph)2Cl2] (B) in spin
states S = 2 (BHS, left) and S = 0 (BLS, right). Selected bond lengths (Å) for
BHS: Fe–P1 2.529, Fe–P2 2.695, Fe–N1 2.388, Fe–N2 2.406, Fe–Cl1
2.379, Fe–Cl2 2.320. BLS: Fe–P1 2.151, Fe–P2 2.167, Fe–N1 1.967, Fe–N2
2.098, Fe–Cl1 2.351, Fe–Cl2 2.322.
6
As was shown above, the Mössbauer parameters calculated
for isomer B of complex 2a are in good agreement with those
determined experimentally for the HS and the LS species, con- (647.0 and 709.1 nm) for C. The agreement between calculated
trary to isomer C. Since there is a pronounced color change and experimental spectra (562 and 690 nm) is slightly better
from pale yellow to dark violet, we also calculated the elec- for isomer B reinforcing the idea that it corresponds to the
tronic spectrum of the two spin states of isomers B and C of observed isomer of 2a.
complex 2a. The high spin forms exhibit very weak absorptions
To get further evidence for the above suggested cis-configur-
in the visible, with the bands at 392.7 nm (B) or 368.7 nm (C) ation, vibrational spectroscopy (FIR, Raman) was performed.
representing the beginning of the UV strong absorptions, and While the Raman spectra did not reveal characteristic
both are consistent with the pale color of the complex. On the vibrations allowing
a distinction between the different
other hand, the spectra of the low spin forms are dominated isomers, in the FIR characteristic bands associated with Fe–Cl
by the strong absorptions at 545.5 nm (B) or 559.8 nm (C), and Cl–Fe–Cl vibrations could be identified. In the theoreti-
with another weaker one (684.5 nm) for B and two weaker ones cally obtained FIR of the cis-isomer BHS, two characteristic
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Fe–Cl stretching vibrations are found at 252 and 276 cm−1
,
this dual theoretical/experimental approach provides a consist-
whereas for the trans-isomer CHS vibrations at 141 and ent picture of the chemistry of complexes [Fe(PNH-Ph)2X2] and
298 cm−1 are observed which are assignable to the Cl–Fe–Cl [Fe(PNMe-Ph)2X2] with respect to their geometries and elec-
bending mode and the asymmetric Cl–Fe–Cl stretching fre- tronic properties.
quency, respectively. In the experimentally obtained spectrum,
despite the weak signals, the two expected bands for the cis-
product are observed at 253 and 278 cm−1, while the character-
Experimental section
General considerations
istic asymmetric Cl–Fe–Cl stretching mode expected for CHS
(and AHS) is missing. Thus, the experimental FIR data seem to
support the cis-configuration, as the respective spectral fea- All manipulations were performed under an inert atmosphere
tures in the two trans isomers are missing. One has to keep in of argon by using Schlenk techniques. The solvents were puri-
mind, however, that the experimental IR and Raman spectra fied according to standard procedures.23 The deuterated sol-
are obtained for bulk solid samples, while the calculated vents were purchased from Aldrich and dried over 4 Å
spectra correspond to single isolated molecules in the gas molecular sieves. The ligand N-diphenylphosphino-2-amino-
phase.
pyridine (PNH-Ph) (1a)24 was prepared according to the litera-
1
ture. H, 13C{1H}, and 31P{1H} NMR spectra were recorded on
Bruker AVANCE-250, AVANCE-400, and AVANCE DRX 600 spec-
trometers. 1H and 13C{1H} NMR spectra were referenced
internally to residual protio-solvent and solvent resonances,
Conclusion
In the present work we have prepared stable octahedral 18e respectively, and are reported relative to tetramethylsilane (δ =
iron(II) complexes of the general formula [Fe(PNR-Ph)2X2] (R = 0 ppm). 31P{1H} NMR spectra were referenced externally to
H, Me) by reacting anhydrous FeX2 with 2 equiv. of PNR-Ph H3PO4 (85%) (δ = 0 ppm).
ligands. Tetrahedral complexes of the type [Fe(PNR-Ph)X2] as in
Magnetization measurements as a function of temperature
the case of the bulkier PN-iPr and PN-tBu ligands were not were performed on powder samples using a SQUID magneto-
formed. 57Fe Mössbauer spectroscopy and magnetization meter (Quantum Design MPMS). The curves were obtained
studies confirmed their high spin nature with magnetic under 0.1 T, for temperatures ranging from 10 to 300 K, using
moments very close to 4.9μB at room temperature reflecting cooling and warming sequences with a temperature variation
the expected four unpaired d-electrons in all these com- rate of 1 K per minute and 1 minute waiting time at each
pounds. While in the case of the PNH-Ph ligand an S = 2 to S = measurement temperature. The molar susceptibility values
0 spin crossover was observed at low temperatures; complexes (χm) were corrected for diamagnetism of the constituent atoms
with the N-methylated analog PNMe-Ph maintain the high spin using Pascal constants.
state also at low temperatures. It should be emphasized that
The 57Fe Mössbauer spectra were recorded in transmission
this is the first example of spin crossover in this type of com- mode at several temperatures between room temperature and
plexes with heterodifunctional PN ligands. Accordingly, com- 78 K using a conventional constant-acceleration spectrometer
plexes [Fe(PNH-Ph)2X2] and [Fe(PNMe-Ph)2X2] adopt different and a 50 mCi 57Co source in a Rh matrix. The low temperature
coordination geometries. For complexes of the general measurements were performed using a liquid nitrogen flow
formula [Fe(PNR-Ph)2X2], in principle five different coordi- cryostat with a temperature stability of 0.5 K. The velocity
nation isomers are conceivable. Whereas in [Fe(PNMe-Ph)2X2] scale was calibrated using an α-Fe foil. The spectra were fitted
all donor atoms of the two PNMe-Ph ligands are trans to one to Lorentzian lines using the WinNormos software program,
another, i.e., a trans-Cl,P,N-geometry is adopted, in [Fe(PNH- and the isomer shifts reported are relative to metallic α-Fe
Ph)2X2] the PN ligands most likely exhibit a cis-Cl,P,N-arrange- at room temperature.
ment. The structural assignments are based on reactivity
All mass spectrometric measurements were performed on
studies and various experimental data derived from 57Fe Möss- an Esquire 3000plus 3D-quadrupole ion trap mass spectrometer
bauer spectroscopy, SQUID magnetometry, UV/Vis, Raman, (Bruker Daltonics, Bremen, Germany) in positive-ion mode
and ESI-MS as well as theoretical considerations by means of electrospray ionization (ESI-MS). Mass calibration was done
DFT and TDDFT calculations. In the case of [Fe(PNMe-Ph)2Cl2] with a commercial mixture of perfluorinated trialkyl-triazines
the geometry is unequivocally established by X-ray crystallogra- (ES Tuning Mix, Agilent Technologies, Santa Clara, CA, USA).
phy. Moreover, in contrast to [Fe(PNMe-Ph)2X2], [Fe(PNH- All analytes were dissolved in methanol “hypergrade for LC-MS
Ph)2X2] is very labile and undergoes rearrangement reactions. Lichrosolv” quality (Merck, Darmstadt, Germany) to a concen-
In CH3OH the diamagnetic dicationic complex [Fe(PNH-Ph)3]2+ tration of roughly 1 mg mL−1 and doped with sodium chloride
is formed via the intermediacy of cis-P,N-[Fe(κ2-P,N-PNH- or sodium bromide (Merck, Darmstadt, Germany) to avoid dis-
Ph)2(κ1-P-PNH-Ph)(X)]+ where one PN ligand is coordinated in a sociation of both halogen substituents from the iron cation
κ1-P-fashion. This reaction involves an intermolecular ligand promoting the corresponding [M − X]+ ion formation (X = Cl,
transfer. In CH3CN the diamagnetic dicationic complex cis- Br) as previously described for molybdenum and titanium
N,P,N-[Fe(PNH-Ph)2(CH3CN)2]2+ is formed as a major isomer complexes.25,26 Direct infusion experiments were carried out
where the two halide ligands are replaced by CH3CN. In sum, using a Cole Parmer model 74900 syringe pump (Cole Parmer
11160 | Dalton Trans., 2014, 43, 11152–11164
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Instruments, Vernon Hills, IL, USA) at a flow rate of 2 μL py5), 2.91 (s, 3H, NCH3). 13C{1H} NMR (δ, CD2Cl2, 20 °C): 161.2
2
min−1. Full scan and MS/MS-scans were measured in the range (d, JCP = 26.8 Hz, py2), 147.5 (s, py6), 136.9 (s, py4), 136.8
1
3
m/z 100–1100 with the target mass set to m/z 1000. Further (d, JCP = 14.0 Hz, Ph1), 131.9 (d, JCP = 20.9 Hz, Ph3,5), 128.9
experimental conditions include: drying gas temperature: (s, Ph4), 128.5 (d, JCP = 4.6 Hz, Ph2.6), 114.6 (s, py5), 110.4
2
3
2
150 °C; capillary voltage: −4 kV; skimmer voltage: 40 V; octa- (d, JCP = 20.9 Hz, py3), 34.1 (d, JCP = 8.25 Hz, NCH3). 31P{1H}
pole and lens voltages: according to the target mass set. NMR (δ, CD2Cl2, 20 °C): 62.7.
Helium was used as a buffer gas for full scans and as a col-
cis-Cl,P,N-[Fe(PNH-Ph)2Cl2] (2a). A solution of anhydrous
lision gas for MS/MS-scans in the low energy CID (collision FeCl2 (111 mg, 0.88 mmol) and PNH-Ph (1a) (500 mg,
induced dissociation) mode. The activation and fragmentation 1.80 mmol) was stirred in THF (15 mL) at room temperature
width for tandem mass spectrometric (MS/MS) experiments for 12 h. The solvent was then removed under reduced
was set to 6 Da to cover the main isotope cluster for fragmenta- pressure. The remaining white solid was collected on a glass
tion. The corresponding fragmentation amplitude ranged from frit, washed with toluene (15 mL) and twice with diethyl ether
0.4 to 0.6 V in order to keep a low abundant precursor ion (15 mL) and dried at 50 °C under vacuum for 3 h. Yield:
intensity in the resulting MS/MS spectrum. As precursor ions 510 mg (85%). Anal. calcd for C34H30Cl2N4P2Fe (683.34): C,
for tandem mass spectrometric experiments the ions [M − X]+ 59.76; H, 4.42; N, 8.20%. Found: C, 59.88; H, 4.48; N, 8.24%.
could be selected as precursor ions. All mass calculations are ESI-MS (m/z, CH3OH, NaCl) positive ion: 647.10 [M − Cl]+.
based on the lowest mass (i.e. most abundant) iron isotope
trans-Cl,P,N-[Fe(PNMe-Ph)2Cl2] (2b). This complex was pre-
(56Fe-isotope). Mass spectra and tandem spectra were averaged pared analogously to 2a with anhydrous FeCl2 (100 mg,
during a data acquisition time of 1 to 2 min and one analytical 0.79 mmol) and PNMe-Ph (1b) (473 mg, 1.62 mmol) as starting
scan consisted of five successive micro scans resulting in 50 materials. Yield: 519 mg (92%). Anal. calcd for
and 100 analytical scans, respectively, for the final mass spec- C36H34Cl2N4P2Fe (711.39): C, 60.78; H, 4.82; N, 7.88%. Found:
trum or MS/MS spectrum.
C, 60.65; H, 4.88; N, 7.94%. ESI-MS (m/z, CH3OH, NaCl) posi-
Raman spectra were collected on a Horiba Jobin Yvon tive ion: 675.13 [M − Cl]+.
Micro-Raman spectrometer (LabRam 800 HR) equipped with
cis-Cl,P,N-[Fe(PNH-Ph)2Br2] (3a). This complex was prepared
an integral Olympus BX 41 microscope (20f objective) and a analogously to 2a with anhydrous FeBr2 (189 mg, 0.88 mmol)
Peltier-cooled CCD detector, using the 632.8 nm line of a He– and 1a (500 mg, 1.80 mmol) as starting materials. Yield:
Ne laser (1.5 mW) for excitation. A 600 line grating was used 562 mg (83%). Anal. calcd for C34H30Br2N4P2Fe (772.24): C,
for obtaining the Raman spectrum. The Raman–Stokes spectra 73.96; H, 5.86; N, 9.58%. Found: C, 73.72; H, 5.91; N, 9.50%.
were recorded in the 4000–200 cm−1 range of Raman shifts at ESI-MS (m/z, CH3OH, NaBr) positive ion: 691.05 [M − Br]+.
1.3 cm−1 spectral resolution; ˜ν in cm−1; relative intensities are
trans-Cl,P,N-[Fe(PNMe-Ph)2Br2] (3b). This complex was pre-
given in % of the most intense peak. The spectrograph was pared analogously to 2b with anhydrous FeBr2 (176 mg,
calibrated using a Si-wafer at 520 cm−1 Raman-shift. Far IR 0.81 mmol) and 1b (488 mg, 1.67 mmol) as starting materials.
spectra were recorded within the range 700 cm−1 to 100 cm−1 Yield: 595 mg (91%). Anal. calcd for C36H34Br2N4P2Fe (800.29):
on a Perkin-Elmer 400 FIR FTIR spectrometer, equipped with a C, 73.96; H, 5.86; N, 9.58%. Found: C, 73.85; H, 5.79;
Pike Technologies GladiATR using a diamond crystal plate.
N, 9.68%. ESI-MS (m/z, CH3OH, NaBr) positive ion: 719.08
Electronic spectra of the undiluted powder samples have [M − Br]+.
been measured using a Perkin Elmer Lambda 900 UV-VIS-NIR
cis-P,N-[Fe(PNH-Ph)3]Cl2 (5a). A solution of anhydrous FeCl2
spectrometer equipped with a thermostatable powder sample (44 mg, 0.35 mmol) and PNH-Ph (1a) (300 mg, 1.08 mmol) was
holder in diffuse reflection geometry (Praying Mantis acces- stirred in CH3OH (15 mL) at room temperature for 12 h. The
sory®) between 335 nm and 1200 nm within the temperature red solution was then filtered and the solvent was removed
range of 123 K and 298 K.
under reduced pressure. The remaining pink solid was washed
twice with diethyl ether (15 mL) and dried at 50 °C under
vacuum for 3 h. Yield: 301 mg (90%). Anal. Calcd for
Syntheses
PNMe-Ph (1b). A solution of n-BuLi in n-hexane (2.5 M, C51H45Cl2N6P3Fe (961.63): C, 63.70; H, 4.72; N, 8.74%. Found:
16.5 mL, 41.11 mmol) was added slowly to a solution of C, 63.82; H, 4.69; N, 8.64%. 1H NMR (δ, CD3OD, 20 °C):
N-methylpyridine-2-amine (4.23 g, 39.15 mmol) in toluene 7.64–7.55 (m, 6H, py, Ph), 7.36–7.23 (m, 15H, ph), 7.16–7.11
(150 mL) at −10 °C. After stirring at room temperature for 3 h, (m, 9H, py, Ph), 7.05–6.97 (bs, 6H, Ph), 6.61 (t, J = 6.5 Hz, 3H,
the red solution was cooled to −30 °C, chlorodiphenylphos- py), 6.48 (d, J = 5.9 Hz, 3H, py), 4.91 (s, 3H, NH). 13C{1H} NMR
2
3
phine was added and the mixture was stirred at 80 °C for (δ, CD3OD, 20 °C): 163.9 (dd, JCP = 3.9 Hz, JCP = 7.6 Hz, py2),
12 h. The LiCl was filtered over pre-dried Celite® and the 148.8 (s, py6), 140.1 (s, py4), 134.97 (dd, JCP = 16.0 Hz, JCP
remaining clear yellow solution was concentrated under 31.1 Hz, Ph), 133.9 (dd, JCP = 14.7 Hz, JCP = 27.6 Hz, Ph),
=
1
3
1
3
2
2
vacuum. The product was isolated as a white solid. Yield: 131.58 (s, Ph), 131.28 (d, JCP = 7.2 Hz, Ph), 131.3 (d, JCP = 7.3
3
8.89 g (78%). Anal. calcd for C18H17N2P (292.32): C, 73.96; H, Hz, Ph), 130.0 (s, Ph), 129.2 (d, JCP = 19.8 Hz, Ph), 129.2 (d,
3
3
5.86; N, 9.58%. Found: C, 73.85; H, 5.98; N, 9.61%. 1H NMR (δ, 3JCP = 19.8 Hz, Ph), 129.1 (d, JCP = 19.7 Hz, Ph), 129.1 (d, JCP
CD2Cl2, 20 °C): 8.21 (d, 1H, J = 2.9 Hz, py6), 7.53 (t, 1H, J = 7.7 = 19.5 Hz, Ph), 128.2 (d, JCP = 6.4 Hz, Ph), 128.2 (d, JCP = 6.2
2
2
Hz, py4), 7.47–7.35 (m, 11H, py3, ph), 6.75 (t, 1H, J = 5.5 Hz, Hz, Ph), 117.9 (s, py5), 112.2 (s, py3). 31P{1H} NMR (δ, CD3OD,
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20 °C): 104.5. ESI-MS (m/z, CH3OH, NaCl) positive ion: 889.22 were independent; Rint = 0.042; final R values: R1 = 0.0356
[M − Cl − HCl]+.
(I > 2σ(I)), wR2 = 0.0806 (all data).
cis-P,N-[Fe(PNH-Ph)3]Br2 (5b). This complex was prepared
5b: C51H45Br2FeN6P3, Mr = 1050.5, orthorhombic, space
analogously to 5a with anhydrous FeBr2 (75 mg, 0.35 mmol) group Fdd2 (no. 43), a = 32.0762(14) Å, b = 35.738(2) Å, c =
and 1b (300 mg, 1 : 08 mmol) as starting materials. Yield: 35.738(2) Å, α = β = γ = 90°, V = 22 070(3) Å3, Z = 16, μ =
343 mg (94%). Anal. calcd for C51H45Br2N6P3Fe (1050.53): C, 1.845 mm−1. Of 43 656 reflections collected (θ ≤ 30°), 15 186
58.31; H, 4.32; N, 8.00%. Found: C, 58.12; H, 4.39; N, 8.12%. were independent; Rint = 0.042; final R values: R1 = 0.0373 (I >
ESI-MS (m/z, CH3OH, NaBr) positive ion: 889.22 [M − Br − 3σ(I)), wR2 = 0.0819 (all data). Flack parameter = −0.008(4).
HBr]+.
6: C38H36B2F8FeN6P2, Mr = 868.14, triclinic, space group P ī
Reaction of cis-Cl,P,N-[Fe(PNH-Ph)2Cl2] (2a) in CD3CN. For- (no. 2), a = 10.7266(13) Å, b = 20.317(3) Å, c = 21.130(3) Å, α =
mation of cis-N,P,N-[Fe(PNH-Ph)2(CD3CN)2]2+ (6). Upon dissolu- 64.129(2)°, β = 83.943(2)°, γ = 89.942(2)°, V = 4115.1(9) Å3, Z =
tion of 2a (20 mg) in ca. 0.5 mL of CD3CN, a mixture of the 4, μ = 0.516 mm−1. Of 67 127 reflections collected (θ ≤ 30°),
dicationic complex cis-N,P,N-[Fe(PNH-Ph)2(CD3CN)2]2+ (6) and a 23 584 were independent; Rint = 0.048; final R values: R1
symmetrical isomer thereof (7) in a 2 : 1 molar ratio was 0.0606 (I > 2σ(I)), wR2 = 0.1649 (all data).
afforded within 24 h as monitored by 31P{1H} NMR spec-
=
Computational details
troscopy. 31P{1H} NMR (δ, CD3CN, 20 °C): Isomer 6: 114.6 (AB
spin system, Δν = 533.2 Hz, JPP = 54.8 Hz). Isomer 7: Calculations were performed using the Gaussian 09 software
110.8 ppm. Crystals of 6 suitable for X-ray crystallography package,31 and the OPBE32 functional without symmetry con-
could be obtained by reacting 2a in CH3CN followed by slow straints. This functional was shown to perform well in
diffusion of diethyl ether.
mechanistic studies of spin forbidden reactions in related Fe
systems.33 It has to be noted however that while DFT is very
successful at predicting geometries of both high-spin and low-
spin complexes, obtaining the correct ground state represents
X-ray structure determination
Single crystals of the complexes 2b, 5b, and 6 suitable for X-ray a major challenge since GGA (generalized gradient approxi-
diffraction were obtained by the solvent/antisolvent vapor mation) functionals (e.g., OPBE and PBE) tend to favor the low-
diffusion method at room temperature using CH2Cl2–diethyl spin states, while the hybrid functionals (e.g., B3LYP and
ether (2b), CH3OH–diethyl ether (5b), and CH3CN–diethyl PBE0) artificially favor the high-spin states.34,35 For example,
ether (6). X-ray diffraction data were collected at T = 100 K on a the energy difference between the HS and LS states of cis-Cl,P,N-
Bruker Kappa APEX-2 CCD diffractometer using graphite- [Fe(PNH-Ph)2Cl2] (B) is 4.5 kcal mol−1 when calculated with
monochromated Mo-Kα radiation (λ = 0.71073 Å) and φ- and PBE,36 changing to 24.1 kcal mol−1 with PBE037 and to
ω-scan frames covering complete spheres of the reciprocal 18.4 kcal mol−1 if B3LYP38 is employed. With the exception of
space with θmax = 30°. Corrections for absorption and λ/2 the PBE functional data, in all cases the HS state is more
effects were applied using the program SADABS.27 After struc- stable than the LS state which is in agreement with the experi-
ture solution with the program SHELXS97, refinement on F2 mental data. The optimized geometries were obtained with the
was carried out with the program SHELXL9728 for 2b and 6. Stuttgart/Dresden ECP (SDD) basis set39 to describe the elec-
Non-hydrogen atoms were refined anisotropically. Hydrogen trons of the iron atom. For all other atoms a standard 6-31g**
atoms were placed in calculated positions and thereafter basis set was employed.40 Frequency calculations were per-
treated as riding. Prior to final refinement of 6 a disordered formed to confirm the nature of the stationary points yielding
CH3CN solvent molecule (ca. 0.5 CH3CN per Fe complex) was no imaginary frequency for the minima.
removed with the procedure SQUEEZE of the program
PLATON.29
TDDFT41 calculations were performed to obtain the UV/Vis
spectra of isomers B and C of complex 2a (S = 0 and S = 2)
The structure of 5b was solved with charge-flipping using the formalism implemented in Gaussian 09. The OPBE
implemented in SUPERFLIP and refined with JANA2006.30 functional was found to reproduce experimental spectra more
Non-hydrogen atoms were refined anisotropically. Hydrogen reliably than others tested (CAM-B3LYP). The basis set was the
atoms were placed in calculated positions and thereafter same as that for geometry optimization, with one added f
treated as riding except for N-bonded H atoms, which were polarization function.42
refined freely. The N–H bond lengths were restrained to 0.870(1)
Mössbauer parameters (quadrupole splitting – ΔEQ – and
Å. Due to disorder, the solvent molecules in 5b could not isomer shift – δ) were evaluated by performing single-point cal-
be resolved and the electron density in the solvent voids culations at the B3LYP34 level of theory with the ORCA software
corresponding to approximately 2.5 CH3OH molecules per (Version 2.9.0),20 on the optimized geometries. The value for
asymmetric unit was removed with the procedure SQUEEZE of ΔEQ is directly given by the program, while the isomer shift
the program PLATON.25
was evaluated from the electron density at the Fe nucleus
2b: C36H34Cl2FeN4P2, Mr = 711.36, monoclinic, space group using the approach of Neese.21 Fe was described by the triply
P21/n (no. 14), a = 10.1491(3) Å, b = 12.8821(3) Å, c = 12.8714(3) polarized core properties basis set CP(PPP)43 and the other
Å, α = 90°, β = 103.291(2)°, γ = 90°, V = 1637.75(7) Å3, Z = 2, μ = atoms by the SV(P) basis set44 with the inner s-functions
0.755 mm−1. Of 33 383 reflections collected (θ ≤ 30°), 4749 uncontracted; the auxiliary basis set SV/J45 was also used for
11162 | Dalton Trans., 2014, 43, 11152–11164
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Paper
these calculations (basis set b1). For the Fe atom, an enhanced
integration grid was used, and the overall integration accuracy
was increased to 7.21 Taking into account the results of a
recent benchmark study46 about the prediction of 57Fe Möss-
bauer parameters by DFT, we also calculated both Mössbauer
parameters, ΔEQ and δ, with the Partridge-1 basis set47 for Fe
and the cc-pVDZ basis set48 for the other atoms (basis set b2).
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Acknowledgements
Financial support by the Austrian Science Fund (FWF) (project
no. P24202-N17) is gratefully acknowledged. The ESI mass 11 G. Sanchez, J. Garcia, D. Meseguer, J. L. Serrano, L. Garcia,
spectrometer was made available by the Austrian Science Fund
(FWF) (project no. P15008-N03 to GA). The X-ray center of the
J. Perez and G. Lopez, Inorg. Chim. Acta, 2004, 357,
4568.
Vienna University of Technology is acknowledged for financial 12 (a) E. Smolensky, M. Kapon, J. D. Woollins and M. S. Eisen,
support and for providing access to the single-crystal diffracto-
meter. MJC, AG, MDC and LPF acknowledge FCT (PEst-OE/QUI/
Organometallics, 2005, 24, 3255; (b) E. Smolensky, M. Kapon
and M. S. Eisen, Organometallics, 2007, 26, 4510.
UI0612/2013, PEst-OE/QUI/UI0536/2011, PEst-OE/FIS/UI0261/ 13 S. M. Nabavizadeh, E. S. Tabei, F. N. Hosseini, N. Keshavarz,
2011) for financial support and AG for a grant (SFRH/BPD/ S. Jamali and M. Rashidi, New J. Chem., 2010, 34, 495.
89722/2012). We also thank Christian Knoll, Johannes Ofner 14 B. Blank, G. Glatz and R. Kempe, Chem. Asian J., 2009, 4,
and Bernhard Lendl for the recording of Raman spectra. The 321.
support and sponsorship by COST action CM1205 “Catalytic 15 (a) I. Hyder, M. Jimenez-Tenorio, M. C. Puerta and
Routines for Small Molecule Activation (CARISMA)” are also
acknowledged.
P. Valerga, Organometallics, 2011, 30, 726; (b) I. Macias-Arce,
M. C. Puerta and P. Valerga, Eur. J. Inorg. Chem., 2010,
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11164 | Dalton Trans., 2014, 43, 11152–11164
This journal is © The Royal Society of Chemistry 2014