Synthesis, characterisation and magnetic properties of octahedral chromium(iii) compounds with six C-donor ligands

Article abstract of DOI:10.1039/c0dt00891e

The homoleptic, square pyramidal organochromium(iii) compound [NBu ₄]₂[Cr(C₆F₅)₅] (1) reacts with excess organic isocyanides, CNR [R = ^tBu, 2,6-dimethylphenyl (Xy)], under dissociation of the apical C₆F₅ ligand to give the more saturated, singly charged complexes [NBu₄][trans- Cr(C₆F₅)₄(CNR)₂] [R = ^tBu (2), Xy (3)], containing six monodentate C-donor ligands. These compounds exhibit an axially distorted octahedral structure (single-crystal X-ray diffraction) with the four C₆F₅ groups defining the equatorial positions and the CNR ligands occupying the axial ones. Compounds 2 and 3 both behave as spin quartet species (S = 3/2) at microscopic level (EPR spectroscopy), their macroscopic magnetic properties depending upon the nature of the terminal R group, as established by magnetisation measurements. When the R substituent is the saturated alkyl group ^tBu, the compound (2) behaves as a simple paramagnet, with no magnetic interaction between individual Cr^III centres along the whole temperature range measured (1.8-265 K). By contrast, a weak antiferromagnetic interaction is detected for compound 3 at low temperature with T_N = 0.19(1) K. Since the closest intermetallic distances are similar in the crystals of 2·CH₂Cl₂ and 3·1.75CH₂Cl₂ (ca. 1.1 nm), we conclude that the insaturation of the aromatic Xy group together with the extended intermolecular π-π stacking interactions between Xy rings observed in the crystal lattice of 3·1.75CH₂Cl₂ (centroid-to-centroid distance: 0.35 nm) favour magnetic interaction between the individual magnetic centres. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c0dt00891e

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PAPER
Synthesis, characterisation and magnetic properties of octahedral
chromium(III) compounds with six C-donor ligands†
Pablo J. Alonso,a Ana B. Arauzo,a M. Angeles Garc´ıa-Monforte,a Ine´s Garc´ıa-Rubio,b Antonio Mart´ın,a
Babil Menjo´n*a and Conrado Rilloa
Received 21st July 2010, Accepted 29th October 2010
DOI: 10.1039/c0dt00891e
The homoleptic, square pyramidal organochromium(III) compound [NBu4]2[Cr(C6F5)5] (1) reacts with
t
excess organic isocyanides, CNR [R = Bu, 2,6-dimethylphenyl (Xy)], under dissociation of the apical
C6F5 ligand to give the more saturated, singly charged complexes [NBu4][trans-Cr(C6F5)4(CNR)2] [R =
tBu (2), Xy (3)], containing six monodentate C-donor ligands. These compounds exhibit an axially
distorted octahedral structure (single-crystal X-ray diffraction) with the four C6F5 groups defining the
equatorial positions and the CNR ligands occupying the axial ones. Compounds 2 and 3 both behave
as spin quartet species (S = 3/2) at microscopic level (EPR spectroscopy), their macroscopic magnetic
properties depending upon the nature of the terminal R group, as established by magnetisation
measurements. When the R substituent is the saturated alkyl group tBu, the compound (2) behaves as a
simple paramagnet, with no magnetic interaction between individual CrIII centres along the whole
temperature range measured (1.8–265 K). By contrast, a weak antiferromagnetic interaction is detected
for compound 3 at low temperature with TN = 0.19(1) K. Since the closest intermetallic distances are
similar in the crystals of 2·CH2Cl2 and 3·1.75CH2Cl2 (ca. 1.1 nm), we conclude that the insaturation of
the aromatic Xy group together with the extended intermolecular p–p stacking interactions between Xy
rings observed in the crystal lattice of 3·1.75CH2Cl2 (centroid-to-centroid distance: 0.35 nm) favour
magnetic interaction between the individual magnetic centres.
Organochromium compounds, in particular, find extensive use
in organic synthesis2 as well as in a number of technically
Introduction
important catalytic processes.3 In these processes, however, the
added Cr reagent only seldom coincides with the really active
species. As a rule, changes in the stereochemistry and the
coordination number occur frequently. Even the oxidation state
of the Cr centre in the active species involved in a given process is
usually a much-debated subject.4 Mononuclear Cr compounds
in intermediate oxidation states, CrII (d4), CrIII (d3) and CrIV
(d2), should be magnetically active species. Amongst them, CrIII
compounds are half-integer spin (S = 1/2 or 3/2) Kramers
systems and are therefore especially suited for EPR spectroscopic
detection. Moreover, in the high-spin case (S = 3/2), zero-field
splitting (ZFS) is expected to have an effect on the electronic
structure.5 This effect is very sensitive to the chemical and magnetic
environment of the paramagnetic centre, as has been demonstrated
in Werner-type CrIII coordination compounds.6 An extension of
this well-established body of knowledge to organochromium(III)
derivatives would be desirable because of its potential application
to detect the in situ generated transient species involved in a given
stoichiometric or catalytic process.
The development of synthetic procedures to obtain non-
metallocene or even Cp-free organo-derivatives of early- and mid-
transition metals (TM) is currently attracting much interest in
chemistry.1 The presence of p-coordinated aromatic ligands that
comply with the Hu¨ckel rule, most typically those based on the
(C5R5)- or C6R6 frames, usually endow the corresponding organo-
TM derivatives with enhanced stability. In the absence of that kind
of ligand, organo-TM compounds are, in general, less stable but,
in turn, more reactive. This latter effect is usually much desired
and is one of the main reasons behind the rapid upsurge in this
research area.
aInstituto de Ciencia de Materiales de Arago´n, Facultad de Ciencias, Uni-
versidad de Zaragoza–C.S.I.C., C/Pedro Cerbuna 12, E-50009 Zaragoza,
Spain. E-mail: menjon@unizar.es
bLaboratorium fu¨r Physikalische Chemie, ETH-Ho¨nggerberg, Wolfgang-
Pauli-Straße 10, CH-8093 Zu¨rich, Switzerland
† Electronic supplementary information (ESI) available: X-ray crystallo-
graphic data (CIF), graphic representations of the bulk magnetic properties
of 3 (isothermal magnetisation measurements and thermal dependence of
its magnetic susceptibility), as well as graphic comparisons between cal-
culated and experimental EPR spectra. CCDC reference numbers: 777393
(2·CH2Cl2) and 777394 (3·1.75CH2Cl2). For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c0dt00891e
We now report on the synthesis and structural characterisation
of two closely related Cp-free organochromium(III) compounds,
t
[NBu4][trans-Cr(C6F5)4(CNR)2] [R = Bu (2), Xy (3)], deriving
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Table 1 Selected interatomic distances (pm) and angles (◦) and their
estimated standard deviations for the trans-[Cr(C6F5)4(CNtBu)2]- aniona
as found in single crystals of 2·CH2Cl2
from the unusual five-coordinate species [NBu4]2[Cr(C6F5)5] (1).7
The coordination environment of the new compounds 2 and
3 comprises exclusively monodentate C-donor ligands. Their
magnetic properties both at microscopic (EPR) and macroscopic
levels (magnetisation measurements) are related to chemical and
structural changes in the second coordination sphere of the
magnetically active centre.
Cr–C(1)
Cr–C(7)
Cr–C(13)
Cr–C(19)
Cr–C(25)
Cr–C(30)
C(25)–N(1)
N(1)–C(26)
C(30)–N(2)
N(2)–C(31)
C(1)–Cr–C(7)
C(1)–Cr–C(13)
C(1)–Cr–C(19)
C(1)–Cr–C(25)
C(1)–Cr–C(30)
216.1(5)
214.6(5)
215.7(5)
216.1(5)
205.9(6)
206.9(6)
114.6(7)
144.5(7)
113.5(7)
145.9(8)
89.7(2)
C(7)–Cr–C(13)
C(7)–Cr–C(19)
C(7)–Cr–C(25)
C(7)–Cr–C(30)
C(13)–Cr–C(19)
C(13)–Cr–C(25)
C(13)–Cr–C(30)
C(19)–Cr–C(25)
C(19)–Cr–C(30)
C(25)–Cr–C(30)
Cr–C(25)–N(1)
C(25)–N(1)–C(26)
Cr–C(30)–N(2)
C(30)–N(2)–C(31)
89.0(2)
178.7(2)
91.2(2)
89.6(2)
92.2(2)
89.5(2)
87.9(2)
89.1(2)
90.2(2)
177.2(2)
177.9(5)
175.5(5)
174.7(5)
173.9(6)
Synthesis and structural characterisation
The
five-coordinate
organochromium(III)
compound
[NBu4]2[Cr(C6F5)5] (1) was found to react with excess organic
t
isocyanides, CNR [R = Bu, 2,6-dimethylphenyl (Xy)], to afford
177.4(2)
89.1(2)
88.4(2)
the singly charged complexes [NBu4][trans-Cr(C6F5)4(CNR)2]
t
[eqn (1); R = Bu (2), Xy (3)]. These reactions can be easily
94.3(2)
followed by IR spectroscopy in solution. They proceed slowly
at room temperature and, given the thermal instability of the
starting material 1, in competition with further decomposition
processes that lower the overall yield of the final products 2 and 3.
In order to minimise the effect of those undesired side-reactions
while maintaining the formation of 2 and 3 at reasonable pace,
the process is conveniently carried out at 4 ◦C. Under such
conditions, and after 30 h, fairly good yields of isolated 2 and 3
are obtained (> 60%).
a Average ∠Cortho–Cipso–Cortho¢ within the C6F5 rings: 112.4(5)◦.
[NBu4 ]2[CrIII (C6F )5 ]+C ≡ NR
→
[NBu4 ][trans-CrIII (C6F )4 (CNR)2 ]
5
5
1
2,R = t Bu
3,R =Xy
(1)
The reaction outlined in eqn (1) involves not just isocyanide
coordination to 1, but also the loss of one of the C6F5 groups
with no change in the metal oxidation state. The formal dis-
sociation of an aryl carbanion had already been observed in
organochromium(III) chemistry. Thus, the homoleptic compound
Na3[CrPh6]·xEt2O was only found to be stable in the presence
of excess NaPh, otherwise it readily gave the five-coordinate
compound Na2[CrPh5]·xEt2O.8 This tendency to release phenyl
groups precluded the isolation of Na3[CrPh6]·xEt2O in pure form,
while the corresponding lithium salt Li3[CrPh6]·xEt2O proved to
be more stable.9 In our case, the corresponding (C6F5)- carbanion
would not be stable under the reaction conditions and would
therefore evolve into other unidentified species.‡ The reaction of
1 (d3) with CNR bears a close relationship to that undergone
by the isoleptic species [P(CH2Ph)Ph3]2[Rh(C6F5)5] (d6) with CO,
which afforded the dicarbonyl derivative [P(CH2Ph)Ph3][trans-
Rh(C6F5)4(CO)2].11 In spite of the isoelectronic and isolobal
character of the CO and CNR ligands, compound 1 failed to
react with CO, at least under similar conditions to those affording
2 and 3. No better results were obtained in the presence of
Me3SiOSO2CF3, which was added as an aryl group-sequestering
reagent. Compound 1 did not react with typical small-sized, s-
donor ligands, such as MeCN or py, either.
Fig. 1 Thermal ellipsoid diagram (50% probability) of the anion of
[NBu4][trans-Cr(C6F5)4(CNtBu)2] as found in single crystals of 2·CH2Cl2.
t
[Cr(C6F5)4(CNR)2]- anions of 2 (R = Bu; Fig. 1; Table 1) and 3
(R = Xy; Fig. 2; Table 2) are very similar and can be described
as axially-distorted octahedral (OC-6). The equatorial plane is
defined in each case by the C donor atoms of the helically
arranged C6F5 groups. Average Cr–C(aryl) values of 215.6(5) and
215.3(5) pm are observed for 2 and 3, respectively, which do
not significantly deviate from the value found for the equatorial
C6F5 groups in the square-pyramidal (SPY-5) parent compound
1 (average Cr–C(eq) = 216.1(2) pm).7 Similar Cr–C bond dis-
tances (average Cr–C = 215.8(6) pm) were also found in the
homoleptic square-planar (SP-4) organochromium(II) derivative
[Mg(thf)6][CrII(C6F5)4]·2thf, regardless of the different oxidation
state of the metal centre.12 It is reasonable to assume that the
change in coordination number would compensate for the decrease
in the Cr–C distance expected to occur on going from CrII to CrIII.
The C6F5 groups in 2 and 3 adopt a heavily tilted disposition with
respect to the equatorial plane (tilt angles: 44.0–55.8◦), while in
The crystal and molecular structures of compounds 2 and 3
have been established by single-crystal X-ray diffraction methods.
The local coordination environment of the Cr atom in the trans-
‡ The (C6F5)- carbanion is a thermally unstable species prone to undergo
b-fluoride elimination, with the formation of the highly reactive benzyne
species C6F4 (see Ref. 10).
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Table 2 Selected interatomic distances (pm) and angles (◦) and their
estimated standard deviations for the trans-[Cr(C6F5)4(CNXy)2]- aniona
as found in single crystals of 3·1.75CH2Cl2
206.4(6) pm for 2 and 207.7(5) pm for 3) are similar to
those found in the homoleptic isocyanide CrIII derivative
[Cr(CNPh)6][SbCl6]3 (average Cr–C = 207(1) pm).15 Signifi-
cantly shorter distances have been found in the mononuclear
Cr–C(1)
Cr–C(7)
Cr–C(13)
Cr–C(19)
Cr–C(25)
Cr–C(34)
C(25)–N(1)
N(1)–C(26)
C(34)–N(2)
N(2)–C(35)
C(1)–Cr–C(7)
C(1)–Cr–C(13)
C(1)–Cr–C(19)
C(1)–Cr–C(25)
C(1)–Cr–C(34)
215.3(5)
215.5(5)
214.9(4)
215.7(4)
206.8(5)
208.6(5)
116.8(6)
139.3(6)
115.9(6)
140.6(6)
89.99(17)
179.18(17)
90.40(17)
91.04(18)
91.96(17)
C(7)–Cr–C(13)
C(7)–Cr–C(19)
C(7)–Cr–C(25)
C(7)–Cr–C(34)
C(13)–Cr–C(19)
C(13)–Cr–C(25)
C(13)–Cr–C(34)
C(19)–Cr–C(25)
C(19)–Cr–C(34)
C(25)–Cr–C(34)
Cr–C(25)–N(1)
C(25)–N(1)–C(26)
Cr–C(34)–N(2)
C(34)–N(2)–C(35)
90.43(17)
179.14(17)
88.66(17)
91.47(17)
89.17(17)
88.26(17)
88.74(17)
90.56(17)
89.28(17)
177.00(17)
174.9(4)
5
5
chromocenium-type derivatives [Cr{(h -C5H4)CHPhCHPh(h -
C5H4)}(CNtBu)][B(C6F5)4] (Cr–C(isocyanide) = 195.6(3) pm)
5
5
and [Cr{(h -C5H4)CMe2CMe2(h -C5H4)}(CNXy)][B(C6F5)4] (Cr–
C(isocyanide) = 193.3(5) pm).16 It is interesting to note, how-
ever, that the low magnetic moments obtained for these ansa-
metallocenium complexes (meff ~1.8 mB), indicate an electronic
configuration that is substantially different from that found in our
Cp-free compounds (see below). The coordination of the CNXy
ligand to the “Cr(C6F5)4” fragment in 3 produces no significant
change in the structural features associated with the C N–R unit.
Thus, the C N triple bond length (av. 116.3(6) pm) as well as the
C–N single bond length (av. 140.0(6) pm) are identical to those
found in the free ligand: 116.0(3) and 139.9(2) pm, respectively.17
This negligible structural effect is in keeping with the high n(C N)
values observed in the IR spectra of 2 (nCN = 2225 cm-1 in CH2Cl2
solution) and 3 (nCN = 2192 cm-1 in CH2Cl2 solution).
176.5(5)
177.1(4)
178.1(5)
a Average ∠Cortho–Cipso–Cortho¢ within the C6F5 rings: 112.6(4)◦; average
∠Cortho–Cipso–Cortho¢ within the Xy rings: 123.5(5)◦.
An important feature in the molecular structures of 2 and 3
is the relative orientation of the terminal organic substituents
R along the almost linear R–N C–Cr–C N–R chain. Two
equally populated, staggered and periplanar conformations have
t
been found for the terminal Bu groups in 2 due to 0.5 : 0.5
disorder observed in only one of the tBu groups over two
positions.§ In contrast, the Xy groups in 3 were found to only
adopt the eclipsed conformation (Fig. 3). This arrangement is
similar to that found, for instance, in the diamagnetic compounds
trans-[ZrCl4(NHC)2], where the N-heterocyclic carbene ligands
Fig. 2 Thermal ellipsoid diagram (50% probability) of the anion of
[NBu4][trans-Cr(C6F5)4(CNXy)2] in single crystals of 3·1.75CH2Cl2.
the aforementioned homoleptic CrII derivative they were found to
be roughly perpendicular to the coordination plane (tilt angles:
74.8–88.8◦). The presence of axial ligands seems to result in
more pronounced tilting of the equatorial C6F5 groups, as has
been found in the six-coordinate transition-metal compounds
with formula [M(C6F5)4X2]q- for which the molecular struc-
ture is known, viz. [Li(12-crown-4)2]2[Zr(C6F5)4Cl2] (tilt angles:
30.8–66.9◦),13 [P(CH2Ph)Ph3][Rh(C6F5)4(CO)2] (tilt angles: 51.1–
52.5◦)11 and [NBu4][Pt(C6F5)4Br(CO)] (tilt angles: 43.0–49.0◦).14
All these examples are diamagnetic species with either d0 (ZrIV) or
low-spin d6 (RhIII and PtIV) electron configurations.
Fig. 3 Schematic projection of the trans-[Cr(C6F5)4(CNXy)2]- anion
along the normal to the equatorial plane showing the virtually eclipsed
arrangement of the Xy rings (mutual dihedral angle: ca. 3◦).
The axial isocyanide ligands in 2 and 3 are bound in
an approximately linear way, with Cr–C N angles between
174.7◦ and 177.9◦ and C N–C angles in the range 173.9–
178.1◦. The Cr–C(isocyanide) bond lengths (average value =
§ The tBu group is known to be highly mobile even in the solid state (see
Ref. 18).
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(NHC = 1,3-diisopropyl-, 1-methyl-3-(1¢-methylpropyl)- and 1-
methyl-3-(2¢,4¢,6¢-trimethylbenzyl)imidazol-2-ylidene) also adopt
an eclipsed conformation regardless of the size of the N-alkyl
substituent.19 The overall structural pattern found in compounds
2 and 3 also bears a strong resemblance to a number of known
bis(isocyanide) derivatives of iron in various oxidation states with
the formula trans-[Fe{N4}(CNR)2]q+, where {N4} is a macrocyclic
ligand that defines the equatorial plane.20
At the supramolecular level, the eclipsed conformation of the
terminal isocyanide substituents found in 3 leads to an extended
parallel arrangement of the Xy rings of different Cr units along
the a-axis with a ladder-like disposition (Fig. 4). The centroid-to-
centroid distance between nearest-neighbour Xy rings is 0.35 nm,
which is well in the range typically considered as denoting the
existence of non-covalent intermolecular p–p stacking interactions
(0.33–0.36 nm).21 The interplanar distance within the stack is
much smaller than the shortest intermetallic separation: Cr ◊ ◊ ◊ Cr =
1.18 nm. A similar nearest intermetallic distance (Cr ◊ ◊ ◊ Cr = 1.17
nm) has also been found in 2.
sample was observed and the system was not studied any further.
The latter species, D, seems to be more stable than the former
ones, which apparently are kinetically-favoured metastable stages.
The spectral parameters associated with each set of signals A–D
are very similar (see below), and their differences did not have
any perceptible effect on the macroscopic magnetic properties.
The IR spectrum of the sample was periodically checked and it
was found to remain unaltered throughout the whole evolution
described. The invariability of the vibrational behaviour allows us
to conclude that compound 2 does not undergo any chemical
transformation nor does it suffer any significant change at a
molecular level. The observed evolution can therefore be ascribed
to minor modifications in the CrIII environment, as well as to
possible changes in the crystal packing. We tentatively assign the
existence of transient species A–C to the presence of different
conformers of the type detected in the crystal structure of
2·CH2Cl2 deriving from the relative orientation of the terminal
tBu groups (see above). The long-term change of type C into type
D entities entails a change in crystal packing, as evidenced by X-
ray powder diffraction experiments. Our attempts to carry out a
three-dimensional X-ray diffraction analysis on single crystals of
2·CH2Cl2 at 55 ◦C failed due to loss of crystallinity upon warming.
EPR spectra of polycrystalline samples of 3 are also consistent
with a CrIII quartet species (Fig. 7). The broader nature of the
signals in this case, however, does not enable us to confirm (or to
discard) the presence of different magnetic sites in the samples.
Furthermore, no evidence of evolution on standing was observed
in this case.
Magnetic properties
EPR spectra of 2 and 3 were measured both at X-band and Q-
band. The EPR spectra of 2 (Fig. 5 and 6) are in keeping with
the presence of CrIII (d3) quartet species (S = 3/2). However, the
fine structure of the spectra was found to depend upon the aging
of the sample as well as on its thermal treatment. Consequently,
we undertook a systematic study of the evolution in order to gain
as thorough a description of the system as possible. A selection
of representative stages observed along the evolution is given in
Fig. 5 and 6. The spectra of a freshly prepared sample of 2 (Fig. 5a
and 6a) mainly consist of two dominant types of signals, labelled
A and B, from which only the type B contribution remained after
two weeks at room temperature (Fig. 5b and 6b). The spectra
evolved further over an eight-month period into a new type of
contribution, labelled C in Fig. 5c and 6c. Yet another type
of contribution, labelled D, emerged in the long run (1 year;
Fig. 5d and 6d) and eventually became dominant on warming
the sample for one week at 55 ◦C (Fig. 5e and 6e). When the
temperature was raised to 60 ◦C, extensive decomposition of the
The experimental EPR spectra of 2 have been analyzed using
the spin-Hamiltonian given in eqn (2).
⎧
⎫
⎪
1
⎪
2
Z
2
X
H = m H⋅g ⋅S + D S − S S +1 +h S −SY2
⎪
⎪
ꢀ
(2)
(
)
⎨
(
)
⎬
s
B
⎪
⎪
3
⎪
⎪
⎩
⎭
The first term in eqn (2) stands for the Zeeman contribution,
where mB is the Bohr magneton and g˜ is the giromagnetic tensor
that accounts for the coupling of the spin with the applied magnetic
field, H. The second term is responsible for the zero field splitting
(ZFS) of the S = 3/2 quartet in two Kramers doublets with the
2
following energy separation: D0 = 2D(1 + 3h )1/2, where D gives
Fig. 4 Extended arrangement of the trans-[Cr(C6F5)4(CNXy)2]- anions in the crystal lattice of 3·1.75CH2Cl2, showing the p–p stacking interaction
between neighbouring Xy rings.
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Fig. 5 Room temperature X-band EPR spectra of polycrystalline samples
of 2, with labelling of the different CrIII entities detected (see text) for: (a)
a freshly prepared sample; (b) after 2 weeks at room temperature; (c) after
8 months at room temperature; (d) after 1 year at room temperature; (e)
after 1 week warming at 55 ◦C. The corresponding Q-band spectra are
shown in Fig. 6.
Fig. 6 Room temperature Q-band EPR spectra of polycrystalline samples
of 2, with labelling of the different CrIII entities detected (see text) for: (a)
a freshly prepared sample; (b) after 2 weeks at room temperature; (c) after
8 months at room temperature; (d) after 1 year at room temperature; (e)
after 1 week warming at 55 ◦C. The corresponding X-band spectra are
shown in Fig. 5.
Table 3 Relevant spin-Hamiltonian parameters derived from the EPR
spectroscopic data of 2a
the strength of the ZFS and h its orthorhombicity (in a canonical
election of axes 0 £ h £ 1/3).
The g˜ tensor of CrIII centres in approximately OC-6 environ-
ments, as is the case for 2 and 3, is expected to be practically
isotropic (g) with a value close to that of the free electron (ge =
2.0023). Structural distortions usually have no significant effect on
the g value. In contrast, the ZFS contribution is very sensitive to
small changes inthesurroundings ofthe magneticallyactive centre.
It therefore acts as a highly sensitive probe to distinguish among
different magnetic loci that may be difficult to detect by other
experimental techniques. Thus, assuming an isotropic g factor,
the analysis of the EPR spectra of 2 yields the spin-Hamiltonian
parameters given in Table 3. A satisfactory agreement between the
experimental and calculated spectra has been attained in all cases
(see ESI†), by following the same simulation procedure used to
analyze the spectra of the parent species 1.7 Use of the referred
procedure is also justified in the present case, because of the D0
values for type A–D magnetic loci (Table 3) are comparable to the
microwave exciting frequency (n ª 9.5 GHz in X-band; n ª 34 GHz
in Q-band). It is worth noting that similar D0 values are obtained
Centre
g
D/GHz
h
D0/GHz
Type A
Type B
Type C
Type D
1.99(1)
1.99(1)
1.99(1)
1.99(1)
2.24(2)
2.22(2)
2.20(1)
2.93(1)
0.150(10)
0.190(5)
0.240(5)
0.266(5)
4.63(5)
4.67(5)
4.76(3)
6.45(3)
a Included in the analyses were both X-band (Fig. 5) and Q-band (Fig. 6)
spectra. See the referred figures for labelling.
for type A–C centres (4.6–4.7 GHz), which we have tentatively
assigned to different conformers. A slightly higher value (D0 ª
6.5 GHz) is obtained for the type D centre, in keeping with our
assignment (see above).
A similar analysis could not be carried out in the case of
compound 3 due to the poorly-resolved fine structure in the
corresponding EPR spectra (Fig. 7). However, the overall spectral
pattern (Fig. S4†) strongly suggests, for the CrIII in 3, a ZFS value
comparable with that determined for compound 2.
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Fig.
8
(a) Magnetisation vs. the reduced applied magnetic field,
Fig. 7 Room temperature EPR spectra of a polycrystalline sample of 3:
M(m0H/kBT), of a powder sample of 2 at 1.8 K (dotted line corresponds
to the evolution calculated with a Brillouin function with S = 3/2 and g =
1.99); (b) the thermal dependence of the magnetic susceptibility, c (empty
circles, ordinate scale on the left) and its inverse, c-1 (full circles, ordinate
scale on the right) of the same sample (dotted lines correspond to the
predicted evolution following the Curie law).
(a) in X-band; (b) in Q-band.
The spin-quartet state of compounds 2 and 3 has been confirmed
by isothermal magnetisation measurements, M(m0H), at different
temperatures. As an example, the results obtained for compound
2 at 1.8 K are shown in Fig. 8a. No deviations are observed in the
data collected at 5 and 77 K, which appear to be superimposed
with those given in Fig. 8a (not portrayed for clarity). Excellent
agreement is observed between the experimental results and the
ideal evolution (Brillouin function), corresponding to a S = 3/2
system with a value of g = 1.99, identical to that derived from
EPR spectroscopic measurements (Table 3). A similar evolution is
observed for compound 3 (Fig. S5a†). A good agreement with a
Brillouin function is equally obtained in this case, using a g = 1.92
value as derived from detailed analysis of the thermal dependence
of the magnetic susceptibility (see below).
The thermal dependence of the magnetic susceptibility, c(T),
in the range 1.8–273 K for compound 2 follows the simple Curie
law (Fig. 8b) with an effective magnetic moment m = 3.85(2) mB.
Although roughly similar behaviour was observed for compound
3 (Fig. S5b†), a small, but consistent deviation from the Curie
law is observed at low temperature (several samples from different
preparative batches were checked). The difference between 2 and
3 can be more clearly appreciated by representing the product cT
vs. T (Fig. 9): While for compound 2 this product is independent
of temperature, as required by the Curie law, a decrease in the cT
value is observed for 3 below 25 K. It could, in principle, be argued
that the existence of ZFS might be responsible for this deviation.
The effect of the ZFS on the thermal evolution of the magnetic
Fig. 9 Thermal evolution of the cT product for: (a) compound 2 (solid
circles; ordinate scale on the left) and (b) compound 3 (empty circles;
ordinate scale on the right). Dashed lines represent the predicted evolution
according to the model suggested in each case (see text).
susceptibility of an S = 3/2 system with an isotropic g factor can
be quantified as given in eqn (3).22
858 | Dalton Trans., 2011, 40, 853–861
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as a simple paramagnet between 1.8 and 273 K. In contrast,
a weak antiferromagnetic interaction has been observed at low
temperature for compound 3, in which the R substituent is the
insaturated, aromatic Xy group. Since the closest intermetallic
separation is similar in both cases (ca. 1.1 nm), we conclude that
the delocalised nature of the Xy substituent, together with the
extended intermolecular p–p stacking interactions between Xy
rings that were observed in the crystal (Fig. 4), favour the detected
intermolecular magnetic interaction between individual magnetic
centres.
The results presented here nicely evidence the parallelism
between the magnetic properties of Cp-free organochromium(III)
derivatives and those exhibited by Werner-type CrIII coordination
compounds. Moreover, it becomes apparent that spin-quartet
Kramers metal-systems are highly sensitive to changes, not only
in the local environment of the magnetic centre, but also in the
second coordination sphere.
⎡
⎤
⎞
⎛
⎜
⎜
C
2k T
⎟
B
⎢
⎥
⎟
c T = 1−F
( )
(3)
⎟
⎥
⎟
⎢
⎜
⎜
⎝
T
D0
⎠
⎢
⎥
⎦
⎣
In the previous equation, C is the Curie constant, C =
2
2
NmB g2S(S + 1)/3kB = 5NmB g2/4kB, D0 is the energy separation
between the two Kramers doublets and U is a function defined as
U(x) = 2{1 - x·th(x-1)}/5 that accounts for the departure from the
Curie law. U(x) is a monotonously decreasing function of x, which
tends to zero as x goes to infinity. In the case of compound 2, for
which the D0 value has been determined (Table 3), it follows that
2kBT/D0 > 11 for all the temperatures in the measuring range (1.8–
273 K) and hence U(2kBT/D0) < 10-3. The departure associated
with the ZFS is negligible in this case and thus, the Curie law
would indeed be a good approximation for describing the thermal
dependence of the magnetic susceptibility, c(T), of 2. Accordingly,
the cT product should be independent of the temperature, as has
been experimentally found (Fig. 9a).
Although in the case of compound 3 the D0 value could not be
directly determined, it can be considered to be comparable to that
found for 2 (see above). A comparable ZFS effect cannot explain
the significant drop in cT observed for compound 3 at tempera-
tures below 25 K (Fig. 9b). An alternative explanation would be
the existence of magnetic interactions among the magnetic entities,
favoured in this case by the extended intermolecular p–p stacking
interactions between the Xy rings that was observed in the crystal
of 3·1.75CH2Cl2 (Fig. 4). In order to check this possibility we have
fitted the experimental data to a Curie–Weiss law or, equivalently,
the data shown in Fig. 9b to the expression given in eqn (4).
Experimental
General procedures and materials
All the manipulations and operations were carried out under an
inert atmosphere (Ar) using Schlenk techniques. Solvents were
dried using an MBraun SPS-800 System. The isocyanide ligands,
CNtBu and CNXy, were purchased (Aldrich) and used as received.
The starting material [NBu4]2[Cr(C6F5)5] (1) was prepared using a
previously reported method.7 Elemental analyses were carried out
with a Perkin-Elmer 2400-Series II microanalyzer. IR spectra of
KBr discs were recorded on a Perkin-Elmer Spectrum One (4000–
350 cm-1) spectrophotometer.
CT
c T ⋅T =
( )
(4)
T +TN
CAUTION: Compound 1 has been found to explode by
percussion and sometimes for no obvious reason.
The dashed line in Fig. 9b corresponds to the predicted evolu-
tion given by the preceding equation with TN = 0.19(1) K.¶ This
would imply the existence of a weak antiferromagnetic interaction
between the trans-[Cr(C6F5)4(CNXy)2]- entities. Moreover, such
an interaction would also explain the broadening of the EPR
spectral features measured in the paramagnetic phase (Fig. 7).
Synthesis of [NBu4][trans-Cr(C6F5)4(CNtBu)2] (2)
◦
To a CH2Cl2 (20 cm3) solution of 1 (0.25 g, 0.18 mmol) at 0 C
was added CNtBu (5◦1 mm3, 0.45 mmol) and the mixture was
allowed to react at 4 C for 30 h. The, by then, brown solution
was concentrated up to ca. 5 cm3 and covered with an Et2O layer
(15 cm3). The slow diffusion of the two-solvent system at -30 ◦C
gave rise to a yellow solid, which was filtered, washed with Et2O
(3 ¥ 3 cm3) and vacuum dried (2: 0.12 g, 0.11 mmol, 61% yield).
Anal. found: C 53.0, H 4.6, N 3.8; C50H54CrF20N3 requires: C 53.2,
Conclusions
The mononuclear organochromium(III) derivatives [NBu4][trans-
t
Cr(C6F5)4(CNR)2] [R = Bu (2), Xy (3)] have been prepared and
isolated in reasonable yields (>60%) by reaction of the homoleptic
five-coordinate precursor [NBu4]2[Cr(C6F5)5] (1) with the corre-
sponding organic isocyanide, CNR. These are open-shell species
(15 valence electrons), in which the CrIII centre is surrounded by
six monodentate C-donor ligands in an approximately octahedral
environment (Fig. 1 and 2). The first coordination sphere of
the metal atom is virtually identical in both compounds 2 and
3. According to their stereochemistry and electronic structure
(d3), they behave as spin-quartet (S = 3/2) species, as it has
been established by EPR spectroscopy as well as by magnetic
measurements. Their fine magnetic properties, however, have been
found to be affected by changes in the second coordination sphere.
Thus, when the distal substituent R is the saturated alkyl group
tBu, the respective organochromium(III) compound 2 behaves
H 4.8, N 3.7%. IR (KBr): nmax/cm-1 = 2968 (m), 2880 m, 2229 (s;
˜
C
N), 1627 (m), 1493 (vs), 1475 (m), 1432 (vs), 1416 (vs), 1371
(m), 1323 (m), 1304 (w), 1271 (w), 1236 (m), 1213 (m), 1057 (s),
1042 (vs), 999 (w), 945 (vs; C–F), 881 (w; NBu4 ), 742 (m; C6F5:
X-sensitive),24 528 (w), 478 (w).
Crystals suitable for single-crystal X-ray diffraction analysis,
with the formula [NBu4][trans-Cr(C6F5)4(CNtBu)2]·CH2Cl2, were
obtained by slow diffusion of a layer of Et2O (30 cm3) into a
solution of 50 mg of 2 in 10 cm3 of CH2Cl2 at -30 ◦C.
+
Synthesis of [NBu4][trans-Cr(C6F5)4(CNXy)2] (3)
Compound 3 was prepared from 1 (0.265 g, 0.19 mmol) and CNXy
(63 mg, 0.48 mmol) by using a similar procedure to that just
described for the synthesis of 2. Complex 3 was obtained as a
yellow solid (0.18 g, 0.15 mmol, 79% yield). Anal. found: C 56.7,
¶ Similar Ne´el temperatures have been found in other CrIII antiferromag-
netic systems (see Ref. 23).
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Table 4 The crystal data and structure refinement for 2·CH2Cl2 and 3·1.75CH2Cl2
2·CH2Cl2
3·1.75CH2Cl2
Formula
FW
T/K
C50H54CrF20N3·CH2Cl2
C58H54CrF20N3·1.75CH2Cl2
1213.89
1373.66
100(1)
100(1)
Monoclinic
Cc
1401.88(19)
1907.42(25)
2058.13(26)
90
92.079(3)
90
5.4998(12)
4
1.466
0.411
2484
Crystal system
Space group
a/pm
Triclinic
¯
P1
1196.90(2)
1423.68(2)
1837.19(3)
88.589(1)
b/pm
c/pm
a/◦
b/◦
87.877(1)
88.956(1)
3.12701(9)
2
1.459
0.433
1401
7.6–52.5
-14 £ h £ 14
-17 £ k £ 17
-22 £ l £ 22
48 256, Rint = 0.0365
99.7%
max 0.9340
min 0.8812
12 586/48/827
R1 = 0.0954, wR2 = 0.3208
R1 = 0.1184, wR2 = 0.3389
1.064
g /◦
V/nm3
Z
Dc/g cm-3
m/mm-1
F(000)
2q range/◦
Index range
3.6–50.2
-16 £ h £ 16
-22 £ k £ 20
-24 £ l £ 16
14 925, Rint = 0.0457
99.5%
max 0.9798
min 0.8867
8005/20/721
R1 = 0.0579, wR2 = 0.1189
R1 = 0.0692, wR2 = 0.1245
1.072
no. of reflns collected
completeness to q = 25.07◦
transmission
no. of data/restrts/params
Final R indices (I > 2s(I))a
R indices (all data)
GOFb on F2
2
a R1 = R(|Fo| - |Fc|)/R|Fo|; wR2 = [Rw(F2o - F2c )2/Rw(Fc2)2]1/2; w = [s (Fo2 ) + (g1P)2 + g2P]-1; P = (1/3)·[max{F2o,0} + 2Fc2]. b Goodness-of-fit = [Rw(Fo2
- F2c ))2/(nobs - nparam)]1/2
H 4.4, N 3.6; C58H54CrF20N3 requires: C 56.9, H 4.4, N 3.4%. IR
geometries and assigned isotropic displacement parameters equal
to 1.2 times the Uiso values of their attached parent atoms (1.5 times
for the CH3 atoms). In the structure of 2·CH2Cl2, the tBu groups
of one the isocyanide ligands was found to have its Me groups
disordered over two positions with 0.5 : 0.5 occupancy. The C–
C distances in both isocyanide ligands were restrained to be
approximately the same. In the structure of 3·1.75CH2Cl2, very
diffuse solvent in the form of several dichloromethane molecules
was found in the density maps. These were modelled as four moi-
eties, with a total occupancy of 1.75 CH2Cl2 molecules, for which
constraints in the geometric parameters and anisotropic thermal
parameters were used. Full-matrix least-squares refinement of
these models against F2 converged to final residual indices given
in Table 4.
(KBr): nmax/cm-1 = 2969 (m), 2880 (w), 2191 (s; C N), 1629 (m),
˜
1495 (s), 1432 (vs), 1416 (vs), 1370 (m), 1323 (m), 1305 (m), 1270
(m), 1240 (m), 1177 (w), 1056 (s), 1042 (vs), 998 (w), 945 (vs; C–F),
880 (w; NBu4 ), 783 (m), 746 (m; C6F5: X-sensitive),24 479 (w).
+
Crystals suitable for single-crystal X-ray diffraction analysis
with formula [NBu4][trans-Cr(C6F5)4(CNXy)2]·1.75CH2Cl2 were
obtained by slow diffusion of a layer of Et2O (30 cm3) into a
solution of 50 mg of 3 in 10 cm3 of CH2Cl2 at -30 ◦C.
X-Ray structure determination†
The crystal data for 2·CH2Cl2 and 3·1.75CH2Cl2 and other details
of the structure analyses are presented in Table 4. The crystals
were mounted at the end of a quartz fibre. The radiation used
in both cases was graphite monochromated Mo-Ka (l = 71.073
pm). For 2·CH2Cl2, the X-ray intensity data were collected on
a Bruker Smart Apex diffractometer. The diffraction frames
were integrated using the SAINT program25 and the reflections
corrected from absorption with SADABS.26 For 3·1.75CH2Cl2,
the X-ray intensity data were collected on an Oxford Diffraction
Xcalibur diffractometer. The diffraction frames were integrated
and corrected from absorption by using the CrysAlis RED
program.27
The structures were solved by Patterson and Fourier methods
and refined by full-matrix least squares on F2 with SHELXL-97.28
All non-hydrogen atoms were assigned anisotropic displacement
parameters and refined without positional constraints, except as
noted below. All hydrogen atoms were constrained to idealised
EPR measurements
EPR data were taken in a Bruker Elexsys E580 spectrometer
working at X-band or Q-band. The magnetic field was measured
with a Bruker ER035M gaussmeter. The samples were introduced
in fused quartz tubes.
Magnetic measurements
Magnetic measurements of polycrystalline samples were carried
out using a Quantum Design SQUID-Based Magnetometer
MPMS-XL5. The magnetometer was calibrated using standard
palladium and dysprosium oxide reference samples supplied by
Quantum Design. The accuracy of the measurements is better
than 1%. Isothermal dc magnetisation curves, M(m0H), at T = 1.8,
860 | Dalton Trans., 2011, 40, 853–861
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5 and 78 K were taken in the magnetic field range 0 < m0H <
5 T. Magnetic susceptibility measurements were performed from
1.8 K to 265 K at 10 Hz and 4.0 Oe amplitude.
10 D. D. Callander, P. L. Coe, J. C. Tatlow and A. J. Uff, Tetrahedron,
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11 M. P. Garc´ıa, L. A. Oro and F. J. Lahoz, Angew. Chem., Int. Ed. Engl.,
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Acknowledgements
12 J. Fornie´s, A. Mart´ın, L. F. Mart´ın, B. Menjo´n, H. Zhen, A. Bell and
L. F. Rhodes, Organometallics, 2005, 24, 3266.
This work was supported by the Spanish MICINN (DG-
PTC)/FEDER (Projects CTQ2008-06669-C02-01/BQU and
MAT2008-03461/MAT) and the Gobierno de Arago´n (Grupo
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