New thiocyanato and azido adducts of the redox-active Fe(η5- C5Me5)(η2-dppe) center: Synthesis and study of the Fe(II) and Fe(III) complexes

Article abstract of DOI:10.1016/j.ica.2011.03.002

The new thiocyanato- (5) and azido- (6) complexes were synthesized and studied under their Fe(II) and Fe(III) redox states. For the first time among the various [Fe(η⁵-C₅Me₅) (η²-dppe)]-based cationic radicals s

Full text of DOI:10.1016/j.ica.2011.03.002

Inorganica Chimica Acta 374 (2011) 288–301
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New thiocyanato and azido adducts of the redox-active Fe(
g
5-C5Me5)(
g
2-dppe)
center: Synthesis and study of the Fe(II) and Fe(III) complexes
a
a
Floriane Malvolti a, Alexander Trujillo a, Olivier Cador a, , Frédéric Gendron , Karine Costuas ,
⇑
Jean-François Halet a, Arnaud Bondon b, Loic Toupet c, Yann Molard a, Stéphane Cordier a, Frédéric Paul a,
⇑
a Sciences Chimiques de Rennes, UMR CNRS 6226, Université de Rennes 1, 35042 Rennes Cedex, France
b PRISM, UMR CNRS 6026, Université de Rennes 1, 35043 Rennes Cedex, France
c Institut de Physique de Rennes, UMR CNRS 6251, Université de Rennes I, 35042 Rennes Cedex, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 9 March 2011
The new thiocyanato- (5) and azido- (6) complexes were synthesized and studied under their Fe(II) and
Fe(III) redox states. For the first time among the various [Fe(
5-C5Me5)( 2-dppe)]-based cationic radicals
g
g
studied so far, the magnitude and spatial orientation of the g-tensor diagonal values were experimentally
determined for 5[PF6]. These data are in good agreement with those issued from a DFT modelization. The
changes experienced by the electronic structure of the Fe(II) complexes subsequent to oxidation are rem-
iniscent of these previously observed for the known arylalkynyl analogues, albeit some differences can be
pointed out. Thus, the differences observed in the 1H NMR spectra of 5[PF6] and 6[PF6] are attributed to a
slower electronic spin relaxation and to the differently oriented magnetic anisotropy. The sizeable spin
density evidenced by DFT on the terminal atom of the ligands of the Fe(III) complexes renders these
new paramagnetic metallo-ligands quite appealing for accessing larger polynuclear molecular assemblies
with magnetically interacting centers.
Dedicated to Professor W. Kaim.
Keywords:
Organometallics
Thiocyanato complex
Azido complex
Spin delocalization
Magnetic anisotropy
ESR
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
by these functional groups [31–33], we surmised that compounds
such as 5 and 6 would be interesting to synthesize. When used as
For some time we have been involved in the study of various
families of organic ligands functionalized with redox-active orga-
metallo-ligands, these redox-active organometallic building blocks
should open access to polynuclear architectures that might present
a large panel of uses in various fields, encompassing molecular
based-electronics, optics and even spintronics. Beside this general
concern, we were also curious to investigate their electronic prop-
erties and especially these of their open-shell cationic Fe(III) deriv-
atives 5+ and 6+ in order to be able to compare them with known
paramagnetic Fe(III) metallo-ligands, such as 2+–4+.
In this contribution on the thiocyanato and azido complexes, we
will report their synthesis and characterization in both the Fe(II)
and Fe(III) oxidation states. Their electronic structures will also
be investigated experimentally, using usual spectroscopies, cyclic
voltammetry and theoretically, using DFT. In these studies, a close
attention will be brought to the electronic changes induced by oxi-
dation of the redox-active Fe(II) center. Finally, their structures will
be compared to those of well-studied piano-stool analogues such
as 8-Xn+ or 2n+ (n = 0, 1).
nometallic Fe(g g
5-C5Me5)( 2-dppe) centers such as 2–4, (Scheme 1)
[1–4]. Actually, these ‘‘metallo-ligands’’ are all made in several
steps from the chloride precursor complex 1 [5]. They belong to
a fairly new and quite attractive class of compounds [6–10], for
which the control of the oxidation state of the organoiron substitu-
ent provides a simple mean of tuning the electronic properties of
the appended coordinating group, and in turn these of any complex
to which these ligands are coordinated [2,11–13]. While our previ-
ous efforts have mostly focused on the development of metallo-li-
gands featuring organic coordinating units, we have recently
turned our attention to simpler ‘‘inorganic’’ members of this class
of compounds, namely the metal–thiocyanate and metal–azido
complexes 5 and 6.
Indeed, considering the impressive number of complexes built
so far with the related (iso)cyano- [14–24] or cyanoalkynyl-based
[25–29] metallo-ligands (7n), along with the importance of thiocy-
anato and azido ligands in the field of molecular magnetism [30],
and also considering some key organic transformations undergone
2. Results
2.1. Synthesis of the thiocyanato and azido Fe(II) complexes 5 and 6
⇑
Corresponding authors. Tel.: +33 02 23 23 57 12; fax: +33 02 23 23 69 39
(O. Cador), Tel.: +33 02 23 23 59 61; fax: +33 02 23 23 69 39 (F. Paul).
The desired complexes were obtained from the chloride pre-
E-mail address: frederic.paul@univ-rennes1.fr (F. Paul).
cursor complex 1 [34,35] following a ligand metathesis reaction
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.03.002

F. Malvolti et al. / Inorganica Chimica Acta 374 (2011) 288–301
289
Scheme 1. Selected organoiron complexes containing the Fe(g g
5-C5Me5)( 2-dppe) unit.
Scheme 2. Synthesis of the Fe(II) and Fe(III) thiocyanato and azido Fe(g g
5-C5Me5)( 2-dppe) complexes.
in methanol (Scheme 2). Potassium hexafluorophosphate was
added to favor chloride dissociation and formation of a hypo-
more pronounced in the case of 5 (P45 cmꢀ1).2 We shall come
back to this statement later on (see the Section 3).
thetical 16-electron intermediate such as [Fe(
g
5-C5Me5)(g2
-
The NCS ligand can bind metal centers from both its sulfur or
nitrogen ends. For 5, the crystallographic structure of the complex
shows that the NCS ligand is N-bonded to the iron center in the so-
lid state (Fig. 1a). However, considering that several thiocyanato
complexes are known to isomerize in solution from one bonding
mode to the other [38], we were particularly cautious to ascertain
that this bonding mode was preserved in solution. This was sug-
gested by the quite similar IR signature obtained for 5 in dichloro-
methane solutions (Table 1) in comparison to those obtained for
crystalline samples in KBr.3 Further confirmation of that was ob-
tained from the 13C NMR shift of the NCS-carbon atom at
144.2 ppm,4 which is diagnostic of N-bonded thiocyanate [40].
UV–Vis reveals that a set of weak absorptions in the visible range
(Table 1) are responsible for the strong color of these complexes.
Based on previous investigations and considering their intensity,
a metal-to-ligand charge transfer (MLCT) origin appears likely for
these electronic transitions [41]. Finally, cyclic voltammetry reveals
the existence of a chemically reversible metal-centered oxidation at
dppe)]+ [34]. The desired complexes were isolated as light purple
and deep green solids, respectively, and were fully characterized
by usual means. Crystal structures were also obtained for these
compounds which confirmed their identity (Fig. 1a and b). Nota-
bly, we took a particular care to make sure of the purity of the
isolated samples, especially regarding the azido complex 6, since
its color and various other experimental signatures are not very
distinct from those of the starting complex 1 [34,35].1
In infrared (IR), the presence of the coordinated thiocyanate
ligand in
5 was evidenced by a strong absorption near
2099 cmꢀ1 corresponding to one of the mNCS stretching modes
(Table 1), and a weaker one at 807 cmꢀ1 corresponding to the
other mNCS mode of this ligand [36]. For the complex 6, the pres-
ence of the azido ligand was evidenced by a strong absorption at
2049 cmꢀ1 in KBr corresponding to the asymmetric mNNN mode.
Comparison of these data with those reported [36] for the corre-
sponding modes of the ‘‘free’’ anions in potassium thiocyanate
(2053 cmꢀ1
)
and potassium azide (2041 cmꢀ1
)
indicate
a
strengthening of the higher-energy modes upon coordination to
the Fe(
g
5-C5Me5)( 2-dppe) fragment, this strengthening being
g
2
Caution should be used with the azide salts, since their asymmetric stretch is
believed to be strongly dependant on the solid structure of the azide salt [37].
3
By themselves, the rather high mNCS values found are already suggestive of a N-
1
bonded NCS ligand, [36] albeit intensity studies should be performed to remove any
This could be done by checking the complete disappearance of the signal
corresponding to the set of ortho phenyl dppe hydrogen of 1 in the isolated samples of
6. Taking the C5Me5 peak as reference at 1.38 ppm, the previous signal comes out at
8.01 ppm for 1 in C6D6 and at 7.78 ppm for 6 in the same solvent.
ambiguity based on mNCS alone [39].
4
The 135–146 ppm range corresponds to an N-bonded NCS ligand while the 125–
128 ppm range is more characteristic of an S-bonded NCS ligand.

290
F. Malvolti et al. / Inorganica Chimica Acta 374 (2011) 288–301
Fig. 1. ORTEP representations of 5 (a) and 6 (b) at 50% probability level. Hydrogen atoms have been omitted for clarity.
Table 1
Selected spectroscopic data and yields for [Fe(
g
5-C5R5)(
g
2-dppe)(X)]n+ (X = Cl, NCS, N3; R = Me, H; n = 0, 1) complexes.
b
c
d
Cpnd
X,R
IRa mX (cmꢀ1
)
D
mX (Red-Ox) (cmꢀ1
)
31P NMR
CV
UV–Vis–Near IR
ddppe (ppm)
E0 (V vs. SCE)
k in nm (
e
ꢁ 10ꢀ3 in Mꢀ1 cmꢀ1
)
e
6
Cl, Me
Cl, Me
/
/
/
/
63
/
12
/
44
/
92.7
/
93.9
/
94.6
/
n.d.
/
ꢀ0.23
315[sh, 4.7], 411 [sh, 1.6], 538 [0.5], 638 [0.3]
466 [2.4], 550 [sh, ca. 1.0], 1572 [0.01]
396 [sh, 1.3], 508 [0.58], 542 [sh, 0.52]
480 [sh, 2.4], 552 [5.3], 1700 [0.040]
e
6[PF6]
5
5[PF6]
6
6[PF6]
70
NCS, Me
NCS, Me
N3, Me
N3, Me
NCS, H
NCS, H
2104
2041
2050
2038
2084g
2040
0.04
ꢀ0.24
0.32h
320 [sh, 4.8], 418 [1.7], 548 [0.58], 618 [sh, 0.48]
489 [3.5], 568 [sh, 1.4], 1450 [0.025]
510 [0.53]
582 [2.3]
f
i
70[PF6]
a
b
c
d
e
f
In CH2Cl2/KBr windows unless precised.
In C6D6 unless precised.
Conditions: CH2Cl2 solvent, 0.1 M [n-Bu4N][PF6] supporting electrolyte, 20 °C, Pt electrode, sweep rate 0.100 V sꢀ1
.
In CH2Cl2 at 20 °C.
Data from Ref. [42].
Data from Ref. [14].
In KBr pellets.
[n-Bu4N][ClO4] used as supporting electrolyte.
Data from Ref. [43].
g
h
i
Fig. 2. ORTEP representations of 5[PF6] (a) and 6[PF6] (b) at 50% probability level. Hydrogen atoms have been omitted for clarity.

F. Malvolti et al. / Inorganica Chimica Acta 374 (2011) 288–301
291
Table 2
H9
H9
Experimental ESR g-valuesa vs. computed g-values for [Fe(
(X = NCS, N3) complexes 5[PF6] and 6[PF6].
g
5-C5Me5)( 2-dppe)(X)]+
g
H9
Cpnd
R
g1
g2
g3
hgi
Dg
5+
NCS Exp.a
1.979
2.042
2.067
2.050
2.471
2.388
2.442
2.164
2.134
2.162
0.492
0.439
0.449
Ph
H7
Calc.b 1.949
Ph
H8
H4
P
Exp.c
1.993
(X=NCS,NNN)
Fe
P
X
( 0.004) ( 0.003) ( 0.016) ( 0.007) ( 0.020)
Calc.d 1.962
Exp. a 1.996
Calc.b 1.983
Calc.d 1.983
2.083
2.049
2.055
2.057
2.334
2.328
2.220
2.239
2.126
2.124
2.086
2.093
0.372
0.332
0.237
0.256
H5
H6
6+
N3
H1
H3
a
H2
Experimental ESR g-values ( 0.005) determined at 65–70 K in CH2Cl2/1,2-
C2H4Cl2 (1:1) glass.
b
Scheme 3. 1H nuclei numbering corresponding to the proposed assignment for
5[PF6] and 6[PF6].
DFT values of optimized structures.
Experimental (mean) ESR g-value determined at 65 K in the solid state for an
c
oriented single crystal of 5[PF6] (see text).
d
DFT values for solid state geometry.
of 5[PF6] at 293 K, the signal of the C5Me5 protons being hardly dis-
cernible at ambient temperature (see Supporting Information). For
6[PF6], an intense isotropic ESR signal is observed at g = 2.12 in
CH2Cl2/1,2-C2H2Cl2 at 295 K, whereas only a barely discernible sig-
nal shows up at g = 2.08 for 5[PF6] in solution at ambient temper-
atures. We therefore surmised that the unusual broadening
observed in the 1H NMR spectrum of the former Fe(III) complex
was ascribable to a slower relaxation of the electronic spin which
in turn fastens the nuclear spin relaxation of the protons. In line
with this hypothesis, heating of a sample of 6[PF6] in nitroben-
zene-d5 at 363 K (ca. 90 °C) allowed retrieving a comparable reso-
lution than for 5[PF6]. As expected for these low spin (S = ½) Fe(III)
complexes [44], heating also produces a Curie-type (1/T) shift of
the 1H signals towards the diamagnetic region of the spectrum
(see Supporting Information).
0.04 V (versus SCE) for 5 and at ꢀ0.24 V for 6 (Table 1), which indi-
cates that these compounds can be fairly easily oxidized.
2.2. Synthesis of the thiocyanato and azido Fe(III) complexes
Based on the Fe(III)/Fe(II) redox potentials values measured
(Table 1), the chemical oxidation of both 5 and 6 was performed
using ferricinium hexafluorophosphate (Scheme 2), resulting in
apparent color changes of the reaction media [42,44,45]. Thus, 5
darkens whereas 6 switches from deep green to purple upon oxida-
tion. Cyclic voltammetry confirmed the redox parentage between
the isolated products and the starting Fe(II) compounds, while
infrared spectroscopy established their different nature. The iden-
tities of the compounds were definitively established by X-ray dif-
fraction (Fig. 2a and b). Again, 5[PF6] has a N-bonded thiocyanate
ligand in the solid state, which remains N-bonded in solution
according to IR [36]. For both Fe(III) complexes 5[PF6] and 6[PF6],
a decrease in the frequencies of the stretching modes of the thiocy-
anato and azido groups is observed relative to their Fe(II) parents
(Table 1), indicating that oxidation sizeably weakens the bonding
within these inorganic ligands.
The Fe(III) radical cations were then characterized in solution by
UV–Vis–Near-IR (Table 1). In line with the color change accompa-
nying the oxidation reaction of 5[PF6] and 6[PF6], their most in-
tense absorption in the visible range is found at lower energies
and presents an increased intensity relative to those in 5 and 6
(Table 1). In addition, for the Fe(III) complexes, near IR spectros-
copy reveals the presence of a low-intensity electronic transition
near 1500–1700 nm, in a spectral range where the Fe(II) complexes
are silent. Based on previous investigations with 1[PF6] and 8-
X[PF6] [42], the absorptions in the visible range certainly corre-
spond to ligand-to-metal charge transfer (LMCT) transitions, and
that in the near IR range to a forbidden ligand field (LF) transition.
Magnetization measurements have also been performed on
crystalline sample of 5[PF6]. The temperature variation of the mo-
lar magnetic susceptibility vM follows the Curie–Weiss law for a
radical with S = ½ (see Supporting Information). The paramagnetic
(S = ½) nature of 5[PF6] and 6[PF6] was also confirmed by ESR and
NMR measurements. For both compounds, the ESR spectra in sol-
vent glasses at 77 K reveal the presence of an unpaired electron
in a rhombic environment (Table 2), indicative of its metal-centred
2.3. Solid state structures of 5, 6, 5[PF6] and 6[PF6]
The solid state structures reveal that the thiocyanato and azido
complexes are isostructural with one another, featuring each time
four molecules in the elementary cell (Table 6). The Fe(II) com-
plexes crystallize in the monoclinic P21/c system, while the Fe(III)
complexes crystallize in the orthorhombic P212121 system (see
Section 5). The bond lengths and angles for the dppe and C5Me5 li-
gands within the coordination sphere are classic for such ‘‘piano-
stool’’ complexes (Table 3) [35,41,42,46].
For the Fe(II) complexes, the Fe1–N1–X angle is not 180°, but
171.6° and 131.2° for the thiocyanato and azido complexes, respec-
tively. For 5 (X = C37), which features the bulkier S-terminal atom
on the ligand, this angle remains nearly linear. In contrast, in the
case of 6 (X = N1), the angle is closer to the ideal value for a sp2-
hybridized
N atom. The Fe1–N1 bond in the NCS complex
0
(1.927 ÅA) is significantly shorter than those usually found for iron
compounds with a single Fe–N bond (2.063 Å) [47], while bonds
of within the NCS ligand in 5 are slightly longer than those usually
reported for the alkali thiocyanate salts (N–C: 1.15 Å, C–S: 1.63 Å),
in line with a possible dFe
?
p⁄
contribution to the Fe–N bond
NCS
[33]. In contrast, the Fe–N bond of 6 is longer than in 5, and stays
in the range reported for a single bond (2.009(3) Å) [33]. The bond-
ing within the Fe–N3 unit is usual, except perhaps for the bending
angle of the azido ligand which is less important than 120°. Also,
the N–N bonds are slightly shorter and longer, respectively, than
those usually reported for the N3 ligand in organic azides (internal
N–N: 1.216 Å, terminal N–N: 1.124 Å) [48]. As discussed below, the
distinct bending angles adopted by the cumulenic ligands in 5 and
6 can be explained by considering different hybridizations for the
bonded nitrogen (N1) which are induced by different intramolecu-
lar electronic and steric effects. Packing forces might also contrib-
ute to some extent to influence the bonding within the cumulenic
ligand in 5 and 6 in the solid-state, since short contacts are
nature, with a significant anisotropy (Dg), larger for 5[PF6] than for
6[PF6] [42]. Then, the characteristic 1H and 13C NMR isotropic shifts
of these compounds recall those previously observed for related
Fe(III) radicals [44]. Most of the observed signals can be attributed
by comparison with the assignments previously proposed
(Scheme 3, Fig. 3 and Supporting Information). Notably, the 1H
NMR spectrum of 6[PF6] is significantly more broadened than that

292
F. Malvolti et al. / Inorganica Chimica Acta 374 (2011) 288–301
Fig. 3. 1H NMR spectra of 8-NO2[PF6] (a), 5[PF6] (b) and 6 [PF6] (c) in CD2Cl2 at 25 °C with proposed assignment for selected protons according to Scheme 3.
Table 3
Selected bond lengths (Å) and angles (°) for 5, 5[PF6], 6 and 6[PF6].
Cpnd
5
5[PF6]
6
6[PF6]
Selected bond lengths
Fe–(C5Me5)centroid
Fe–P1
Fe–P2
Fe–N1
1.733
1.767
1.728
1.763
2.2282(9)
2.2311(9)
1.927(2)
1.169(3)
1.646(3)
2.2794(8)
2.2896(8)
1.890(2)
1.168(4)
1.608(3)
2.2067(5)
2.2297(6)
2.0267(18)
2.2856(7)
2.2871(7)
2.002(2)
N1–C37
C37–S1
N1–N2
N2–N3
1.183(2)
1.169(3)
1.034(4)
1.250(5)
Selected bond angles
P1–Fe–P2
P1–Fe–N1
86.02(3)
83.11(8)
90.02(8)
171.6(2)
179.8(3)
83.34(3)
83.52(8)
95.10(8)
160.2(2)
179.1(3)
85.52(2)
85.36(5)
83.21(5)
83.05(3)
89.94(6)
83.61(6)
P2–Fe–N1
Fe–N1–C37
N1–C37–S1
Fe–N1–N2
131.19(16)
175.6(2)
129.1(2)
173.2(4)
N1–N2–N3
observed in the solid state between the terminal S or N atom and
one of the dppe aromatic H atoms (H23 and H27, respectively) of
an adjacent molecule (2.983 and 2.718 Å, respectively).
2.4. Solid-state ESR on a monocrystal of the thiocyanato Fe(III)
complex
Regarding the Fe(III) complexes 5[PF6] and 6[PF6], the coordina-
tion sphere formed by the dppe and C5Me5 ligands expands slightly
upon oxidation, as usually observed with related piano-stool com-
plexes [41,42,46,49]. In addition, for the thiocyanato complex
5[PF6], a shortening of the Fe1–N1 bond and a slight shortening
of the C37–S1 bond, concomitant with a slight decrease in the
Fe1–N1–C37 angle takes place upon oxidation. Overall, the NCS li-
gand remains essentially linear after the oxidation. In comparison,
the bending of the N3 ligand in the azido complex is quite unaf-
fected by oxidation. A shortening of the Fe1–N1 bond, along with
a marked shortening of the N1–N2 bond,5 concomitant with a
lengthening of the N2–N3 bond is however observed in 6[PF6].
To obtain additional experimental insight on the electronic
structure of these isostructural radical cations, we decided to
determine the orientation of the diagonal g-tensor components
of 5[PF6] by performing ESR measurements on a single crystal. Sin-
gle crystals of 5[PF6] have a ‘‘shoe box’’ shape with the box axis col-
linear to the crystallographic c axis, the largest face coinciding with
the [1 1 0] plane. The crystal reference frame (XYZ) in which rota-
tions are performed is represented in Scheme 4.
The angular dependences of the ESR spectra have been mea-
sured at 65 K in three perpendicular planes (XY), (XZ) and (YZ). In
the orthorhombic space group P212121 the four molecules are re-
lated by 2-fold axes. Thus, four signals are expected for any orien-
tation of the magnetic field because frozen solution measurements
clearly demonstrate that the ESR signal is rhombic (g1 = 1.979,
g2 = 2.042 and g3 = 2.471). Two resonance lines are expected when
the magnetic field coincides with one of the 2-fold axes. However,
only one resonance field with a strong angular dependence is ob-
served for rotation around Z and two of the same intensity for rota-
5
The extremely short N1–N2 bond in 6[PF6] is quite unusual. The latter bond might
appear artificially shortened due to librational motions of the N3 ligand, since no
packing effet can be invoked to explain it. A related phenomenon has been stated few
year ago with the C–O bond of the [Fe(
complex [50].
g g
5-C5Me5)( 2-dppe)(CO)][PF6] carbonyl

F. Malvolti et al. / Inorganica Chimica Acta 374 (2011) 288–301
293
Z = [0 0 1]
4.8
4.6
4.4
4.2
RZ
Y
4.0
0
0
0
30
30
30
60
90 120 150 180
Rotation angle / °
X = [1 1 0]
6.0
5.6
5.2
4.8
4.4
4.0
RY1
RY2
Scheme 4. Schematic representation of a crystal of compound 5[PF6] with the
reference frame (XYZ) in which rotations are performed.
tions about Y and X (see Figs. S7–S9 of the Supporting Information,
respectively). These symmetric signals with no hyperfine structure
are characteristic of Fe(III) organometallic (S = 1/2) radicals. The
angular variations of the resonance positions by means of g2eff for
the three rotation sets are represented in Fig. 4. Resonance fields
have been determined by the maxima of the numerically inte-
grated ESR spectra.
The resonance positions are labeled RZ for rotation about Z, RX1
and RX2 for rotations about X and RY1 and RY2 for rotation about
Y. In spite of the four distinct molecules in the unit cell, less than
four distinct signals are detected for all orientations. Considering
the symmetry of the unit cell and the different signals detected,
only two combinations of the resonance lines belonging to the
two distinct centers are mathematically possible [(RX1, RY1, RZ),
(RX2, RY2, RZ)] and [(RX1, RY2, RZ), (RX2, RY1, RZ)]. These are,
respectively, noted A and B in the following. In the following, as
a mean to check the consistency of our starting dataset, we have
extracted the experimental g-tensors for every signal separately.6
Least-square fitting procedures were performed for each distinct
center, in each combination, with the following equation: [51]
60
90 120 150 180
Rotation angle / °
6.0
5.6
5.2
4.8
4.4
4.0
RX1
RX2
60
90 120 150 180
Rotation angle / °
g2eff ¼ ggXX sin2 h cos2
u
þ 2ggXY sin2 h cos
u
sin
u
þ ggYY
Fig. 4. Experimental angular variation of the resonance lines (dots or triangles)
with the best fitted curves (full line: black for center 1 and red for center 2) for
rotations around Z (top), Y (middle) and X (bottom). (For interpretation of the
references in color in this figure legend, the reader is referred to the web version of
this article.)
ꢁ sin2 h sin2 u þ 2ggXZ cos h sin h cos
u
þ 2ggYZ
ꢁ cos h sin h sin
u
þ ggZZ cos2 h
ð1Þ
The best-fitted parameters for each combination are given in
Supporting Information. Diagonalization of the resulting matrices
provides principal g-values (combination A: gRX1;
¼
RY1; RZ
agreement is very satisfactory for rotations around X and Y but less
satisfactory for rotations around Z. This might be due to the single
crystal misalignment in the magnetic field but also to the fact that
two signals are contained in one single resonance line. The princi-
pal axes orientation are easily computed in unit cell frame by a
rotation of 37.072° around Z for the resonance lines R1 (RX1,
RY1, RZ) and R2 (RX2, RY2, RZ). The principal g-values and the co-
sine directors in the unit cell frame for the two resonance lines are
given in Table 4 for this combination.
Then, these diagonal g-tensors must be confronted to the crystal
structure. It is important to realize that the two tensors are almost
collinear to the b crystallographic axis. The principal direction cor-
responding to the smallest g value (gx ꢃ 1.99) coincides with b. In
Fig. 5, the four molecules in the unit cell are labeled Fe (the asym-
metric unit), Fei, Feii and Feiii. In looking at the crystal structure
from b axis, one may remark a peculiar feature: C5Me5 rings are al-
most parallel to b which means that the Fe–C5Me5 directions are
½1:997; 2:053; 2:427ꢂ, gRX2;
¼ ½1:989; 2:047; 2:456ꢂ; combi-
RY2; RZ
nation B: gRX1;
¼ ½1:933; 2:135; 2:429ꢂ, gRX2;
¼ ½1:897;
RY2; RZ
RY1; RZ
2:170; 2:404ꢂ). However, only one of these data sets corresponds
to the real g-tensor of the complex. Based on symmetry grounds,
the diagonal g-values found for one center should be similar to
those of the other one6. In this respect, the combination A appears
more self-consistent. Then, considering that the molecular geome-
try should be preserved between molecules in the solid and frozen
solvent glasses, the principal g-values should not change dramati-
cally from the data found in CH2Cl2/1,2-C2H2Cl2 glasses at 65 K
[1.979, 2.042, 2.471] (Table 2). Such an analysis leads to retain
combination A as the correct one (mean g-values between the
two g-sets of this combination are given in Table 2). The best fitted
curves for this combination using Eq. (1) are plotted in Fig. 4. The
6
Since the molecules at the origin of these different signals are symmetry related,
an unique set of solutions should be found for the diagonal components of the g-
tensor in the molecular frame, regardless of the signal initially used.

294
F. Malvolti et al. / Inorganica Chimica Acta 374 (2011) 288–301
Table 4
geometries, which can be considered as being closer to the molec-
ular arrangement in solution. After optimization, the resulting
structures show comparable bond lengths and angles (Supporting
Information) than the experimental X-ray structures (Figs. 1 and
2), except for the N3 fragment in 6[PF6] for which the N1–N2 is
much shorter than computed. In regard to the good match found
experimentally for the other angles and distances and also for
the compounds, this peculiar result suggests that the very short
N1–N2 bond has not an intramolecular origin.5
Principal g-values and cosine directors for the two resonance lines R1 and R2 in the
unit cell frame for combination A.
R1
R2
gx = 1.997
ꢀ0.0245
0.99947
0.02131
0.84352
0.03206
ꢀ0.5361
0.53654
–0.00486
0.84386
gx = 1.989
ꢀ0.15012
ꢀ0.96130
ꢀ0.23100
ꢀ0.81822
0.25195
ꢀ0.51675
ꢀ0.55496
ꢀ0.11143
0.82438
gy = 2.053
gz = 2.427
gy = 2.047
gz = 2.456
Regarding the Fe(II) compounds, the HOMOs of the compounds
result from the anti-bonding interaction of the ‘‘t2g’’ set of metallic
orbitals of the Fe(g g p-orbi-
5-C5Me5)( 2-dppe) fragment with the
tals of the NCS or N3 ligands, while the LUMO and LUMO+2 corre-
spond to empty metal-based MO with a strong dxy and dz2
character and belonging to the ‘‘eꢄg’’ set of frontier orbital in an
octahedral ML6 d6 metal complex (Fig. 6).
Energies of the frontier molecular spin–orbitals (SOs) for 5+ and
6+ complexes are shown in Fig. 7. The lowest unoccupied spin orbi-
tal or LUSO(b) has, in each case, a strong metallic dxz metallic char-
acter which also presents a non-negligible
remote atom of the cumulenic ligand (S for 5+ and N for 6+). The
LUSO( ), LUSO+1( ) and the corresponding LUSO+1(b) and LU-
p character on the
a
a
SO+2(b) are also strongly metallic in character.
Upon oxidation, an electron is formally removed from the
HOMO of the Fe(II) parents followed by an electronic and geomet-
rical relaxation. The resulting ionization potentials are calculated
to be 5.63 and 4.99 eV for 5 and 6, respectively. These values qual-
itatively follow the ordering of the redox potentials experimentally
found for 5 and 6 (Table 1) and confirm that 6 is easier to oxidize
than 5. The computed atomic spin densities are given in Table 5.
Very close spin distributions are found for the two Fe(III) com-
plexes (Fig. 8). The ESR g-tensors were also calculated (Fig. 8)
and the diagonal g-values computed for solid state structures and
for the optimized geometries are given in Table 2, together with
the measured values obtained in the solvent glass. The influence
of the nature of the R ligand (R = NCS, N3) on the g-tensors values
is very well reproduced. The giso values are in each case slightly
smaller than the experimental measurements and the anisotropy
is also slightly underestimated, but overall, the match can be con-
sidered as good. These small theoretical deviations are attributed
to the lack of the solvent and of the counter-ions contribution in
the calculations. Also, the spatial orientation of the three diagonal
values of the g-tensor predicted by DFT for 5+ seem to match much
better with one of the two potential solutions experimentally
Fig. 5. View of the unit cell of compound 5[PF6]. Carbon atoms are in grey, sulfur
atoms in yellow, nitrogen atoms in green, phosphorous atoms in pink and iron
atoms in brown. The arrows represent the orientation of g-tensor components gx, gy
and gz for the
a solution. (For interpretation of the references in color in this figure
legend, the reader is referred to the web version of this article.)
almost perpendicular to b (86.344(3)°) and are contained in the ac
plane. Furthermore, the nitrogen atom is out of the ac plane of only
21° which means that the molecule generated by the 21 axis along
b (Feiii) is very similar to the molecule of the asymmetric unit (Fe).
As a consequence, these two molecules have similar ESR signatures
(i.e. they belong to the same resonance lines and have the same g-
tensor). Similar considerations apply to molecules Fei and Feii.
Therefore, there are two ways of positioning the g-tensors on a gi-
found for the diagonal g-values of 5[PF6] (solution a). Indeed, devi-
ations of 30° and 70° were, respectively, found for the orientation
of the computed principal g-component (g3) compared to the
experimental orientations found for it (gz) in solutions
a and b.
ven molecule (solution
solution b: Fe, Feiii with R2 and Fei, Feii with R1). The first solution
) is given in Fig. 5 while the second solution (b) is given in Sup-
a
: Fe, Feiii with R1 and Fei, Feii with R2, or
3. Discussion
(a
porting information (Fig. S10). The most important features of the
two distributions of g-tensors are the characteristic angles be-
tween gz and the C5Me5 rings. While in the second distribution
the angle between gz and Fe–C5Me5 directions are comprised be-
tween 68.3° and 73.4°, they are comprised between 5.6° and
10.3° in the first distribution. From a strictly experimental point
of view, one cannot discriminate between the two solutions, but
the DFT modelization of these Fe(III) compounds, as shown hereaf-
The new Fe(II)/Fe(III) complexes 5 and 6 were fully character-
ized by usual spectroscopic means. Overall, they exhibit classic
spectral signatures for Fe(II) piano-stool complexes and DFT calcu-
lations reveal electronic properties recalling these of their halide
precursor 1 [35]. In order to gain additional insight into their elec-
tronic structures, we will first discuss the valence bond (VB) repre-
sentation of these compounds. In that connection, the various VB
mesomers of the free thiocyanato and azido ligand are given in
Schemes 5 and 6, respectively. It is important to realize that on
purely electronic grounds, the cumulenic VB forms I and I0 of these
ligands will have a strong tendency to coordinate Lewis-acidic me-
tal centers in a ‘‘bent’’ form due to the sp2 hybridization of the ter-
minal atoms, while all other VB forms will coordinate in an end-on
fashion, due to the sp hybridization of the terminal atoms.
ter, allowed us to retain the
a set as the correct one (Fig. 5).
2.5. DFT computations on 5, 5+, 6 and 6+
These compounds were then studied by DFT both using the so-
lid state geometries (Figs. 1a and b and 2a and b) and optimized

F. Malvolti et al. / Inorganica Chimica Acta 374 (2011) 288–301
295
Fig. 6. Energy diagram of the molecular frontier orbitals of the optimized systems 5 and 6 in their optimized geometry. The MO percentage of metallic, thiocyanato and azido
½
character is indicated in parentheses. The two highest occupied VOs are plotted (iso-contour 0.045 [e/bohr3] ).
y
(%Fe / %N3)
(%Fe / %NCS)
Fe
z
-4
-5
-6
-7
-8
-9
(Ph)2P P(Ph)2
x
(0 / 0)
(0 / 0)
115
β
115
α
115
β
(1 / 0)
(0 / 0)
115
α
(45 / 4)
(46 / 4)
114
β
β
(51 / 4)
(42 / 0)
114
113
β
β
113
(40 / 0)
(43 / 10)
(46 / 4)
(39 / 0)
114
α
α
114
α
α
113
113
)
eV
(
gy
r
2.07 eV
2.09 eV
(62 / 19)
112β
ne
(59 / 25)
E
112
β
0.89 eV
0.60 eV
(50 / 41)
111
110
β
(69 / 17)
111
β
β
(38 / 52)
(30 / 48)
112
α
α
112
α
α
(36 / 37)
(64 / 23)
111
(81 / 0)
110
111
β
(78 / 0)
(72 / 1)
(78 / 0)
110α
110α
(28 / 46)
(16 / 34)
(28 / 13)
(2 / 7)
5[PF6]
6[PF6]
Fig. 7. Energy diagram of the frontier spin–orbitals of the optimized systems 5+ and 6+ in their optimized geometry. The spin–orbital (SO) percentage of metallic, thiocyanato
½
and azido character are indicated in parentheses. The highest occupied SOs are plotted (iso-contour values: 0.045 [e/bohr3] ).
3.1. Valence bond description of the thiocyanato Fe(II) and Fe(III)
complexes
of a VB mesomer such as A. The latter also corresponds to a strong
sp-character of the bound nitrogen, in line with the (quasi-linear)
end-on binding of this ligand. This VB structure is certainly some-
As for the cyclopentadienyl analog of that complex (Table 1;
70[PF6]) reported by Treichel et al. in 1979 [14], the thiocyanato
Fe(II) complex corresponds to the N-bonded isomer both in
solution and in the solid state [52,53]. The quite energetic mNCS
mode of the complex 5 might be attributed to the dominant weight
what stabilized by the strong
p -
-donating character of the Fe(g5
C5Me5)(
g
2-dppe) metal fragment, illustrated by the VB structure
B, which possibly contributes to shorten the Fe–N bond of this
complex. The weight of such a structure in the VB description ni-
cely explains the more positive oxidation potential found for 5 rel-

296
F. Malvolti et al. / Inorganica Chimica Acta 374 (2011) 288–301
Table 5
Computed spin densities (e) on selected fragments or atoms for the [Fe(
g
5-C5Me5)(
g
2-dppe)(X)]+ (X = NCS, NNN and C„C[4-C6H4NO2]) complexes (See Scheme 3 for atom
labeling).
a
Cpnd
Fe
C atoms of C5Me5
P atoms of dppe
X = ABC
A
Refs.
B
C
5+
0.867
0.869
0.899
0.854
0.890
ꢀ0.011
ꢀ0.022
ꢀ0.012
ꢀ0.010
ꢀ0.030
-0.021
ꢀ0.020
ꢀ0.021
ꢀ0.019
ꢀ0.010
0.035
0.028
0.060
-0.004
0.191
0.189
0.148
0.214
0.034f
This work
This work
This work
This work
[44]
5+b
6+
ꢀ0.008
ꢀ0.014
ꢀ0.027
0.238e
6+
7-NO2
0.032
b
ꢀ0.089d
+c
a
b
c
d
e
f
Values below ꢀ0.002 e were found on the carbon atoms of the CH2 groups of dppe.
Data for non-optimized solid state structure.
Mean values between those computed for conformations with parallel and the perpendicular orientations of the nitrophenyl ring [44].
Spin density on C .
a
Spin density on Cb.
Sum for the whole 4-nitrophenyl fragment.
Fig. 8. Plots of the total spin densities and of the spatial orientation of the three computed diagonal values of the g-tensor for [Fe(
complexes 5+ (a) and 6+ (b) (isocontour value: 0.005 [e/bohr3]).
g g
5-C5Me5)( 2-dppe)(X)]+ (X = NCS, N3)
Scheme 5. VB representation of the free thiocyanato ligand (inset) and of its complexes 5n+ in their two stable redox states (n = 0, 1).
ative to 6 or in comparison to alkynyl complexes such as 8-H
(0.15 V versus SCE) [49], featuring more -donating ligands than
The decreased mNCS upon oxidation might be attributed to a
decreasing weight of the VB mesomer D and to the increasing
weight of a VB mesomer such as E or F (and perhaps also G); the
latter VB forms contributing to diminish the N–C bond while rein-
forcing the Fe–N bond strength. The existence of the VB mesomer E
is also supported by the sizeable spin density (0.191 e) found on
the terminal sulfur atom in 5[PF6] in the DFT computations (Ta-
ble 5). As observed on X-ray data and in the DFT calculations,
p
NCS. The participation of a bent VB mesomer such as C is also re-
quired considering the non-strictly linear coordination geometry
adopted by the thiocyanate ligand in 5 (Fig. 1a). Based on the struc-
tural data, it is clear that the conformation presently adopted by
the NCS ligand minimizes any intramolecular interaction with
the cyclopentadienyl methyl groups and the dppe phenyl groups.

F. Malvolti et al. / Inorganica Chimica Acta 374 (2011) 288–301
297
Scheme 6. VB representation of the free azido ligand (inset) and of its complexes 6n+ in their two stable redox states (n = 0, 1).
oxidation leads to a shortening of the Fe–N bond, to a concomitant
lengthening of the N–C bond and perhaps to a slight bending of the
ligand (due to VB forms F and G). The slight modification of the
bending angle (reflecting the terminal nitrogen hybridization)
might be further understood considering the existence of destabi-
lizing intramolecular steric effects between the sulfur atom and
the other ligands, resulting in a small weight for structures F and
G in the VB description.
back to the different atoms constituting these ligands, resulting in
different hybridizations of the metal-bound nitrogen atom, as ratio-
nalized by the VB structures given in Schemes 5 and 6. In this re-
spect, the thiocyanato complex is structurally closer to alkynyl
complexes such as 8-Xn+ (n = 0, 1). In spite of their different
geometry, DFT calculations reveal no marked differences between
5 and 6 regarding the metal versus ligand composition of the fron-
tier molecular orbitals (FMOs), nor in the energy ordering of these
FMOs, and that, even after oxidation. Actually, these features recall
these of classic arylalkynyl complexes, such as 8-Xn+ (n = 0, 1).
3.2. Valence bond description of the azido Fe(II) and Fe(III) complexes
However, a clear difference between 5n+, 6
and the latter com-
n+
pounds resides in the nodal properties of the ligand-based FMOs
which translates into different spin distributions on the ligand for
5[PF6] or 6[PF6] relative to 8-X[PF6], as we will briefly see below.
In the Fe(II) derivative (6), likewise to 5, a slight strengthening
of the high energy stretching mode of the azido ligand is stated
upon coordination2, suggesting an increase of its sp-character upon
complexation. However, based on the Fe1–N1–N2 angle close to
the ideal value of 120° for a sp2 hybridization and on the Fe–N
bond length typical of a single bond, the bonding in the Fe(II) com-
plex 6 must essentially be described by one VB mesomer such as A0
(Scheme 6). Also suggestive of the dominance of a VB form such as
A0 is the shift towards more negative values of the metal-centered
Fe(III)/Fe(II) oxidation potential observed for 6 relative to 5, which
3.4. Spin distribution and magnetic anisotropy of Fe(III) complexes
The Fe(III) complexes 5[PF6] and 6[PF6] possess fairly similar
spin distributions (Table 4). Most of the spin density is located
on the metal center, but a sizeable spin density is also present on
their terminal S (5+) or N (6+) atom, in line with the unsaturated
nature of the ligand. This is an important piece of data since, when
used as metallo-ligands, they might be able to magnetically couple
with any paramagnetic metal ion coordinated to their end. More
precisely, in terms of spin delocalization, DFT computations sug-
gest that the total spin density on the cumulenic ligand compares
with that present on functional 4-arylalkynyl Fe(III) complexes
possessing strongly electron-withdrawing substituents, such as
8-NO2[PF6] (Table 5) [44] or 2[PF6] [1]. However, the spin is now
results from the larger p-donating character of the azido ligand rel-
ative to the thiocyanato ligand.
As for 5, chemical oxidation of 6 produces an isolable low spin
(S = ½) Fe(III) radical cation [14,46,54]. No noticeable change in the
sp-hybridization is indicated by X-rays for 6+, suggesting that the
weight of a mesomer such as D0 remains low in the VB description.
Again, the decrease stated for the N3 asymmetric stretch upon oxi-
dation can be attributed to the presence of VB mesomers such as E0
or F0 which contribute to diminish the N–N bonding, while rein-
forcing the Fe–N bond. Support for the existence of a VB mesomer
such as F0 can be found in DFT computations by the sizeable spin
density evidenced on the terminal nitrogen atom (0.148 e) in 6+
(Table 5, Fig. 7).
preferentially delocalized on the c-atom of the ligand relative to
the metal center in 5[PF6] and 6[PF6], whereas the b-atom was fa-
vored in the case of arylalkynyl Fe(III) complexes. This difference
can be ascribed to the different nature of the highest occupied
FMOs of the thiocyanato and azido ligands. The latter are essen-
tially non-bonding and present a node on the central atom. This in-
duces a partial delocalization of (positive) spin density on the
terminal rather than on the central atom in the SOMO of 5+ and
3.3. Comparison of the ligand bonding in the thiocyanato and azido
complexes
6+. Also, DFT calculations indicate that the larger anisotropy (
Dg)
In line with established trends for these N-coordinated
pseudohalides [33,52], the main difference between the thiocya-
nato and the azido ligands is the tendency of the former to coordi-
nate in a linear (end-on) fashion, whereas the second prefers a bent
coordination geometry. These different behaviors can be traced
experimentally stated by ESR in solvent glasses for 5+ relative to
6+ (Table 2) does not correlate with a larger spin density on the me-
tal center in computed structures,[42] calling for caution when
Dg
is used to estimate the spin density present on metal centers
[1,55].

298
F. Malvolti et al. / Inorganica Chimica Acta 374 (2011) 288–301
Finally, for the first time among all the [Fe(
g
5-C5Me5)(g2
-
property makes 5[PF6] and 6[PF6] much more appealing than long-
er derivatives such as 2–4[PF6] for using them as paramagnetic
metallo-ligands, since they might sustain a sizeable magnetic
interaction between their terminal low-spin Fe(III) center and
any appended paramagnetic ion coordinated to them. Thus, from
the perspective of spintronics, 5[PF6] and 6[PF6] certainly consti-
tute attractive organometallic building blocks for obtaining larger
paramagnetic assemblies, based on coordination reactions.
dppe)(X)]+ radical cations studied so far, the diagonal g-tensor
components were experimentally determined by ESR using a single
crystal of 5[PF6]. This study, consistently with DFT modeling, re-
veals that the orientation of the main diagonal component of the
anisotropy tensor is along the cyclopentadienyl direction, not
along the cumulenic ligand direction, in contrast to what was pre-
viously proposed for 8-X+ cations, based on a perturbative ap-
proach coupled with a classic ligand-field approach. This change
in anisotropy is likely at the origin of the slightly different shifts
presently observed for several cyclopentadienyl and methylene
protons of 5[PF6] and 6[PF6] (Fig. 3), since it should significantly
modify the pseudo-contact contribution to these shifts [44]7. In
line with such an hypothesis, DFT data reveal that only slight
changes take place in the spin-distribution centered on these frag-
ments, suggesting that the contact contribution to the 1H NMR shifts
should not be significantly modified when proceeding from 8-X+ to
5+ to 6+. This shows that 1H NMR can also be used in a straightfor-
ward way to evidence a change in the orientation of the magnetic
anisotropy for such Fe(III) complexes. Another marked difference
with related arylalkynyl cations showing a comparable spin density
present on the metal center such as 8-NO2[PF6] or 2[PF6] [1], is that
both 5[PF6] and 6[PF6] exhibit significantly broadened 1H NMR sig-
nals (Fig. 3). This broadening, particularly striking in the case of
6[PF6], is ascribable to a faster relaxation of the spin of these protons
which might originate (i) from a larger spin density present on the
metal center, rendering the contact relaxation more efficient for nu-
clei close to the metal center, or (ii), from a slower relaxation of the
unpaired electron, resulting in an increase in the dipolar correlation
time [44]. Based on our DFT calculations and ESR studies at 293 K,
the second hypothesis is retained as the major cause of the observed
broadening compared to the 8-X+ cations.
5. Experimental
5.1. General data
All manipulations were carried out under inert atmospheres.
Solvents or reagents were used as follows: Et2O and n-pentane, dis-
tilled from Na/benzophenone; CH2Cl2, distilled from CaH2 and
purged with argon; HN(iPr)2, distilled from KOH and purged with
argon. The [Fe(
previously published procedures [57]. Transmittance-FTIR spectra
were recorded using Bruker IFS28 spectrometer (400–
g
5-C5H5)2][PF6] ferricinium salt was prepared by
a
4000 cmꢀ1). Raman spectra of the solid samples were obtained
by diffuse scattering on the same apparatus and recorded in the
100–3300 cmꢀ1 range (Stokes emission) with a laser excitation
source at 1064 nm (25 mW) and a quartz separator with a FRA
106 detector. Near-infrared (NIR) spectra in solution were recorded
a Cary 5000 spectrometer or a JASCO V-570 spectrometer (4000–
12500 cmꢀ1). UV–Vis spectra were recorded on a Cary 5000 or an
UVIKON 942 spectrometer (250–1000 nm). SQUID measurements
were performed on a MPMS-XL Quantum Design apparatus with
crushed crystalline samples of the compounds [58]. NMR experi-
ments were made on
a Bruker AVANCE 500 operating at
500.15 MHz for 1H, 125.769 MHz for 13C and 201.877 MHz for
31P, with a 5 mm broadband observe probe equipped with a z-gra-
dient coil at the PRISM platform. ESR spectra were recorded on a
Bruker EMX-8/2.7 (X-band) spectrometer equipped with an OX-
FORD cryostat and a goniometer. Cyclic voltammograms were re-
4. Conclusion
The new thiocyanato- (5) and azido- (6) complexes were iso-
lated and studied under their Fe(II) and Fe(III) oxidation states.
Their electronic structures were also modelized in both redox
states by DFT. Moreover, for the first time among the various
corded using
a
EG&G potentiostat (M.263) on platinum
electrodes referenced toward an SCE electrode and were calibrated
with the Fc/Fc+ couple taken at 0.46 V in CH2Cl2 [57]. MS analyses
were performed at the ‘‘Centre Regional de Mesures Physiques de
l’Ouest’’ (CRMPO, University of Rennes) on a high resolution MS/
MS ZABSpec TOF Micromass Spectrometer. Elemental analyses
were performed at the CRMPO or at the Centre for Microanalyses
of the CNRS at Lyon-Solaise, France. The chloro complex 1 was ob-
tained as previously reported [34].
[Fe(g g
5-C5Me5)( 2-dppe)]-based cationic radicals studied so far,
the g-tensor diagonal values of 5[PF6] were experimentally deter-
mined. The changes undergone by the Fe(II) complexes subsequent
to oxidation are reminiscent of these previously observed for other
piano-stool derivatives, such as 1 or 8-X, albeit some notable dif-
ferences in the electronic properties of the resulting Fe(III) radical
cations can be pointed out for 5[PF6] and 6[PF6]. For instance, a
5.2. Synthesis
slower electronic spin relaxation and
a differently oriented
magnetic anisotropy was evidenced in comparison to their
carbon-rich analogues such as 2[PF6], which results in diagnostic
changes in the 1H NMR spectra of these Fe(III) compounds. The
ESR-anisotropy of 5[PF6] and 6[PF6] is thus not simply related to
changes in the spin density present on the metal center, in contrast
to observations previously made with 8-X[PF6] derivatives. The
precise origin of the factors affecting the electronic relaxation
and determining the anisotropy of these Fe(III) piano-stool com-
plexes remain unclear and will make the object of future investiga-
tions. Remarkably, in spite of the larger electronegativity and
5.2.1. Fe(
g
5-C5Me5)(
g
2-dppe)(NCS) (5)
Fe(g
5-C5Me5)(
g
2-dppe)Cl (1, 0.503 g, 0.80 mmol), KPF6
(0.163 g, 0.88 mmol) and KSCN (0.085 g, 0.88 mmol) were sus-
pended in 40 mL of methanol and the mixture was stirred for
12 h at 25 °C to give a violet suspension. After concentration to
10 mL and decantation, the solid that formed was filtrated, washed
with methanol (5 mL) and extracted with 3 ꢁ 10 mL of hot toluene.
After filtration and evaporation of the extract and washing by sev-
eral (2 ꢁ 2 mL) n-pentane fractions, the desired Fe(
g -
5-C5Me5)(g2
dppe)(NCS) complex (5) was isolated as a light violet solid
(0.410 g, 0.63 mmol, 79%). X-ray quality crystals of 5 were grown
by vapor diffusion of n-pentane in a dichloromethane solution of
the complex.
decreased
p-donating ability of the NCS and N3 ligands relative
to functional 4-phenylalkynyl ligands, DFT computations reveal
that a significant amount of spin density is delocalized on the ter-
minal atom of the cumulenic ligand in these Fe(III) complexes. This
Anal. Calc. for C37H39NP2SFe: C, 68.63; H, 6.07; N, 2.16. Found: C,
68.68; H, 6.14; N, 2.08%. MS (positive ESI, CH2Cl2): m/z 647.1622
7
An exact computation of these changes for nuclei lying close to a metal center
[M]+, m/z calc for [C37H39NP2SFe]+ = 647.1628. FT-IR
(
m,
KBr,
exhibiting rhombic anisotropy is very involved and lies outside of the scope of this
paper [56].
cmꢀ1): 2099 (vs, SCN), 808 (s, SCN). Raman (neat,
m
, cmꢀ1): 2101

F. Malvolti et al. / Inorganica Chimica Acta 374 (2011) 288–301
299
(s, SCN), 811 (m, SCN). 31P NMR (d, C6D6, 81 MHz): 93.9 (s, dppe).
1H NMR (d, C6D6, 200 MHz): 7.78 (m, 4H, HAr); 7.36 (m, 4H, HAr);
7.20–6.90 (m, 12H, HAr); 1.99 (m, 2H, CH2/dppe); 1.62 (m, 2H, CH2/
dppe); 1.28 (s, 15H, C5(CH3)5). 13C{1H} NMR (d, CH2Cl2, 50 MHz):
(3 ꢁ 5 mL). Crystals of 6[PF6] were obtained by slow diffusion of
n-pentane into a solution of the complex in dichloromethane.
FT-IR (m
, KBr, cmꢀ1): 2031 (vs, N3), 1310 (m, N3), 840 (vs, PF6).
1H NMR (d, CD2Cl2, 500 MHz): 8.0, 7.7, 4.6, 7.4, 6.5 (s, HAr); 1.5 (very
broad s, HAr); ꢀ8.0 (very broad s, CH2/dppe); ꢀ20.7 (very broad s,
C5(CH3)5); one set of CH2/dppe and one set of HAr not detected.
13C{1H} NMR (d, CD2Cl2, 500 MHz): 145.2, 139.2, 132.1, 123.8,
105.6 (CAr/dppe); 22.1 (C5(CH3)5); 5 signals not detected.
0
144.2 (s, FeSCN); 136.6–134.0 (m, Ci,i /Ar/dppe); 134.2; 133.8, 130.1,
129.8, 128.3, 128.0 (s, CHAr/dppe); 86.0 (s, C5(CH3)5); 29.2 (m, CH2/
dppe); 9.6 (s, C5(CH3)5). C.V. (CH2Cl2, 0.1 M [n-Bu4N][PF6], 20 °C,
0.1 V sꢀ1) E° in V versus SCE (
DEp in V, ipa/ipc) 0.04 (0.08, 1.0).
5.2.2. [Fe(
g
5-C5Me5)(
g
2-dppe)(NCS)][PF6] (5[PF6])
g
5.3. Solution/solvent-glass ESR measurements
0.95 equiv. of [Fe(
5-C5H5)2][PF6] (0.097 g; 0.293 mmol) was
added to a solution of 5 (0.200 g; 0.309 mmol) in 25 mL of dichlo-
romethane resulting in an instantaneous darkening of the purple
solution. Stirring was maintained 1 h at 25 °C and the solution
was concentrated in vacuo to approximately 5 mL. Addition of
50 mL of n-pentane allowed precipitation of a purple solid. Decan-
tation and subsequent washings with toluene (2 ꢁ 2 mL) followed
by diethylether (2 ꢁ 2 mL) and drying under vacuum yielded the
The Fe(III) complexes (1–2 mg) were introduced in a ESR tube
under an argon-filled atmosphere and a 1:1 mixture of degassed
dichloromethane/1,2-dichloroethane was transferred to dissolve
the solid. For solvent glass measurements at 77 K, the solvent mix-
ture was frozen in liquid nitrogen and the tubes were sealed and
transferred in the ESR cavity. The spectra were immediately re-
corded at that temperature and the sample was allowed to come
back to room temperature and solution measurements were
performed.
desired [Fe(g g
5-C5Me5)( 2-dppe)(NCS)][PF6] (5[PF6]) complex as
violet solid (0.200 g; 0.252 mmol; 86%). Crystals of 5[PF6] were ob-
tained by vapor diffusion of n-pentane into a solution of the com-
plex in dichloromethane.
5.4. Solid-state ESR measurements
Anal. Calc. for C37H39N1F6P3S1Fe: C, 56.07; H, 4.96; N, 1.77.
Found: C, 55.58; H, 4.88; N, 1.70%. FT-IR (
m
, KBr, cmꢀ1): 2036 (vs,
A single crystal of 5[PF6] was mounted at room temperature on
SCN), 840 (vs, PF6). 1H NMR (d, CD2Cl2, 500 MHz): 8.4 (very broad
s, CH2/dppe); 7.9, 4.0, 2.5 (s, HAr); 7.3, 5.7, 4.3 (s, HAr); ꢀ3.2 (broad
s, CH2/dppe); ꢀ21.5 (very broad s, C5(CH3)5); one set of CH2/dppe
and one set of HAr not detected. 13C{1H} NMR (d, CD2Cl2,
500 MHz): 257.5 (C5(CH3)5); 152.3, 137.5, 136.1, 124.4, 121.9,
100.3 (CAr/dppe); 22.5 (C5(CH3)5); ꢀ120 (CAr/dppe or FeNCS); ꢀ197.5
(CH2/dppe); 2 signals not detected.
a Nonius four circle diffractometer equipped with a CCD camera
and
a graphite monochromated Mo Ka radiation source
(k = 0.71073 Å), from the Centre de Diffractométrie (CDIFX, Univer-
sité de Rennes 1, France). The unit cell and the orientation of the
crystallographic axes with respect to the crystal shape were then
determined. The single crystal was subsequently transferred on
the goniometer head for ESR measurements.
5.2.3. Fe(
g
5-C5Me5)(
g
2-dppe)(N3) (6)
5.5. Crystallography
Fe(
g
5-C5Me5)(
g
2-dppe)(Cl) (1, 0.630 g, 1.01 mmol), KPF6
(0.200 g, 1.08 mmol) and NaN3 (0.070 g, 1.08 mmol) were sus-
pended in 50 mL of methanol and the mixture was stirred for
12 h at 25 °C to give a dark suspension. After evaporation of the
methanol, the solid that deposited was extracted with 3 ꢁ 10 mL
of toluene to give a greenish solution. After filtration and evapora-
tion of the extract and washing by several n-pentane (3 ꢁ 5 mL)
Crystals of 5, 6, 5[PF6] and 6[PF6] were studied on a Oxford Dif-
fraction Xcalibur Saphir 3 with graphite monochromatized Mo K
a
radiation at low temperature (Table 6). The cell parameters were
obtained with Denzo and Scalepack with 10 frames (psi rotation:
1° per frames) [59]. Details about the data collection [60] (
X rota-
tion and HKL range) are given in Table 6. The structures were
solved with SIR-97 which revealed the non-hydrogen atoms [61].
After anisotropic refinement, the remaining atoms were found by
Fourier difference maps. The complete structures were then re-
fined with SHELXL97 [62] by the full-matrix least-square technique.
Atomic scattering factors were taken from the literature [63]. OR-
TEP views of 5, 6, 5[PF6] and 6[PF6] were realized with PLATON98
[64].
and diethylether (2 ꢁ 5 mL) fractions, the desired Fe(g5
-
C5Me5)(g
2-dppe)(N3) complex (6) was isolated as a dark green so-
lid (0.480 g, 0.760 mmol, 75%). X-ray quality crystals of 6 were
grown by vapor diffusion of n-pentane in a dichloromethane solu-
tion of the complex.
Anal. Calc. for C36H39N3P2Fe: C, 68.47; H, 6.22; N, 6.65. Found: C,
68.39; H, 6.07; N, 6.41%. MS (positive ESI, CH2Cl2): m/z 631.1969
[M]+, m/z calc for [C36H39N3P2Fe]+ = 631.1969. FT-IR
(
m
,
KBr,
cmꢀ1); 2049 (vs, N3). Raman (neat,
m
, cmꢀ1): mN3 not observed
5.6. Computational details
(fluorescence). 31P NMR (d, C6D6, 81 MHz): 94.6 (s, dppe). 1H
NMR (d, C6D6, 200 MHz): 7.85 (m, 4H, HAr); 7.35–7.00 (m, 16H,
HAr); 2.05 (m, 2H, CH2/dppe); 1.70 (m, 2H, CH2/dppe); 1.38 (s, 15 H,
Density functional theory (DFT) calculations were performed
with the Amsterdam Density functional (ADF) program [65–67].
All geometries were optimized without any symmetry constraint
and were successfully validated by vibrational frequency calcula-
tions. Electron correlation was treated within the local density
approximation (LDA) in the Vosko–Wilk–Nusair parametrization
[68]. The non-local correction of Adamo–Barone [69] and of
Perdew–Burke–Ernzerhof [70] were added to the correlation and
exchange energies, respectively. The numerical integration proce-
dure applied for the calculations was that developed by te Velde
et al. [71]. The basis set used for the metal atom was a triple-f sla-
ter-type orbital (STO) basis for Fe 3p, 3d and 4s, with a single-f 4p
polarization function. A triple-f STO basis set was used for 1s of H,
2s and 2p of C and N, 3s and 3p of S and P augmented with a 3d
single-f polarization function for C, N, P and S atoms and with a
2p single-f polarization function for H atoms. A frozen-core
C5(CH3)5). 13C{1H} NMR (d, CH2Cl2, 50 MHz): 137.6–134.0 (m, Ci,i /
0
Ar/dppe); 134.2, 133.8, 129.8, 129.6, 128.3, 128.0 (s, CHAr/dppe); 84.2
(s, C5(CH3)5); 29.0 (m, CH2/dppe); 9.7 (s, C5(CH3)5). C.V. (CH2Cl2,
0.1 M [n-Bu4N][PF6], 20 °C, 0.1 V.sꢀ1) E° in V versus SCE (
V, ipa/ipc): ꢀ0.24 (0.08, 1.0).
DEp in
5.2.4. [Fe(
[Fe(
5-C5H5)2][PF6] (56 mg, 0.168 mmol) was added to a solu-
tion of Fe(
5-C5Me5)( 2-dppe)(N3) (110 mg, 0.175 mmol) in
g g
5-C5Me5)( 2-dppe)(N3)][PF6] (6[PF6])
g
g
g
30 mL dichloromethane. Stirring was maintained for 2 h at 25° C,
and the solution was concentrated in vacuo to ca. 1 mL. Addition
of n-pentane afforded the precipitation of 6[PF6], which could be
isolated as a brown solid (105 mg, 0.135 mmol, 77%), after subse-
quent washings with toluene (2 ꢁ 2 mL) followed by n-pentane

300
F. Malvolti et al. / Inorganica Chimica Acta 374 (2011) 288–301
Table 6
Crystal data, data collection, and refinement parameters for 5, 5[PF6], 6 and 6[PF6].
Cpnd
5
5[PF6]
6
6[PF6]
Formula
fw
T (K)
C
37H39N1P2 S1Fe
C37H39N1P2S1Fe, PF6
792.51
100(2)
C36H39N3P2Fe1
631.49
150(2)
C36H39N3P2Fe1, PF6
776.46
150(2)
647.54
100(2)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
monoclinic
P21/c
11.3320(10)
13.180(2)
21.939(4)
90.0
orthorhombic
P212121
12.5034(2)
16.5322(2)
17.4444(3)
90.0
monoclinic
P21/c
11.1183(2)
12.9922(2)
21.6846(3)
90.0
orthorhombic
P212121
12.6251(2)
16.6827(2)
16.8415(2)
90.0
a
(°)
b (°)
95.4950(10)
90.0
3261.7(8)
4
90.0
90.0
3605.91(10)
4
1.460
93.9570(10)
90.0
3124.90(9)
4
90.0
90.0
3547.17(8)
4
1.454
c
(°)
V (Å3)
Z
Dcalc (g cmꢀ3
)
1.319
1.342
Crystal size (mm)
F(0 0 0)
0.32 ꢁ 0.16 ꢁ 0.16
0.25 ꢁ 0.20 ꢁ 0.20
0.15 ꢁ 0.12 ꢁ 0.10
0.20 ꢁ 0.20 ꢁ 0.20
1360
1636
1328
1604
Absorption coefficient (mmꢀ1
Range HKL
)
0.651
0.669
0.615
0.623
ꢀ14/14
ꢀ16/16
ꢀ28/28
30 737
7104
ꢀ15/15
ꢀ21/21
ꢀ21/22
36 397
ꢀ14/14
ꢀ16/13
ꢀ16/27
14 778
ꢀ16/16
ꢀ21/21
ꢀ21/21
48 822
Number of total reflections
Number of unique reflections
7852
6803
7736
Restraints/parameters
0/379
0/442
0/379
0/442
Final R
Rw
0.0449
0.1075
0.0922
0.1156
0.885
0.0360
0.0813
0.0506
0.0862
0.748
0.392
0.0344
0.0999
0.0459
0.1025
1.033
0.539
0.0341
0.0743
0.0451
0.0762
0.941
0.427
R indices (all data)
Rw (all data)
Sw
Largest difference in peak and hole (e Åꢀ3
)
0.625
ꢀ0.580
ꢀ0.376
ꢀ0.609
ꢀ0.399
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Acknowledgements
F.P. and Y.M. acknowledge the UMR 6226 for a joint research
program. This work was performed using HPC resources from
GENCI-CINES and GENCI-IDRIS (Grant 2010-80649). The C.N.R.S.
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Appendix A. Supplementary material
CCDC 723062, 725071, 745047 and 756190 contain the supple-
mentary crystallographic data for 5 (150 K), 5[PF6] (100 K), 6
(150 K) and 6[PF6] (150 K), respectively. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Centre via http://www.ccdc.cam.ac.uk/data_request/cif. Supple-
mentary data associated with this article can be found, in the on-
line version, at doi:10.1016/j.ica.2011.03.002.
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