Chromium-(II) and -(III) over a planar oxo surface modelled by calix[4]arene anions: Redox chemistry and formation of Cr-C functionalities

Article abstract of DOI:10.1039/a908499a

The metallation of [p-Bu^tcalix[4](ONa)₂(OMe)₂] 1 was performed using [CrCl₃(THF)₃] and the resulting complex [Cr{p-Bu^tcalix[4](O)₂(OMe)₂}(Cl)(THF)] 2 allowed entry into low valent and organometallic chemistry of chromium. In the presence of strong bases or nucleophiles the ligand underwent demethylation exemplified by the reaction of 2 with pyridine, which led to [C₅H₄NMe][Cr{p-Bu^tcalix[4](O) ₃(OMe)}Cr(py)(Cl)] 3. The alkylation of 2 was successful only when the concurrent reduction to Cr^II was irrelevant, in which case the two well characterized organometallic derivatives [Cr{p-Bu^tcalix[4](O)₂(OMe)₂}(Mes)] 4 [Mes = 2,4,6-Me₃C₆H₂] and [Cr{p-Bu^tcalix[4](O)₂(OMe)₂}(η ⁵-C₅H₅)] 5 were obtained. None of them showed, even under photochemical or thermal forced conditions, any sign of reactivity, due to the kinetic inertness of the d³ configuration. Complex 2 was easily reduced to the chromium(II) derivative [Cr{p-Bu^tcalix[4](O)₂(OMe)₂}(THF)] 6, which underwent facile oxidative functionalization. The reaction with O₂ led to an unprecedented di-μ-oxo chromium(IV) derivative, [Cr₂{p-Bu^tcalix[4](O)₂(OMe)₂} ₂(μ-O₂)] 7, which displays a ferromagnetic coupling between the two d² centres. The proposed structures are supported by X-ray analyses of complexes 2-4, 6 and 7. The Royal Society of Chemistry 2000.

Full text of DOI:10.1039/a908499a

View Article Online / Journal Homepage / Table of Contents for this issue
Chromium-(II) and -(III) over a planar oxo surface modelled by
calix[4]arene anions: redox chemistry and formation of Cr–C
functionalities
Joëlle Hesschenbrouck,a Euro Solari,a Carlo Floriani,*a Nazzareno Re,b Corrado Rizzolic and
Angiola Chiesi-Villac
a Institut de Chimie Minérale et Analytique, BCH, Université de Lausanne, CH-1015 Lausanne,
Switzerland. E-mail: carlo.floriani@icma.unil.ch
b Facoltà di Farmacia, Università degli Studi “G. D’Annunzio”, I-66100 Chieti, Italy
c Dipartimento di Chimica, Università di Parma, I-43100 Parma, Italy
Received 25th October 1999, Accepted 24th November 1999
The metallation of [p-Butcalix[4](ONa)2(OMe)2] 1 was performed using [CrCl3(THF)3] and the resulting complex
[Cr{p-Butcalix[4](O)2(OMe)2}(Cl)(THF)] 2 allowed entry into low valent and organometallic chemistry of
chromium. In the presence of strong bases or nucleophiles the ligand underwent demethylation exemplified by the
reaction of 2 with pyridine, which led to [C5H4NMe][Cr{p-Butcalix[4](O)3(OMe)}Cr(py)(Cl)] 3. The alkylation of
2 was successful only when the concurrent reduction to CrII was irrelevant, in which case the two well characterized
organometallic derivatives [Cr{p-Butcalix[4](O)2(OMe)2}(Mes)] 4 [Mes = 2,4,6-Me3C6H2] and [Cr{p-Butcalix[4](O)2-
(OMe)2}(η5-C5H5)] 5 were obtained. None of them showed, even under photochemical or thermal forced conditions,
any sign of reactivity, due to the kinetic inertness of the d3 configuration. Complex 2 was easily reduced to the
chromium() derivative [Cr{p-Butcalix[4](O)2(OMe)2}(THF)] 6, which underwent facile oxidative functionalization.
The reaction with O2 led to an unprecedented di-µ-oxo chromium() derivative, [Cr2{p-Butcalix[4](O)2(OMe)2}2-
(µ-O2)] 7, which displays a ferromagnetic coupling between the two d2 centres. The proposed structures are supported
by X-ray analyses of complexes 2–4, 6 and 7.
Introduction
Calix[4]arene derived anions comprise a set of oxygen donor
atoms preorganized in a quasi-planar fashion particularly
appropriate for studying the redox and organometallic chem-
istry of early transition metals.1–3 The partial alkylation of the
O4 set allows one to tune the charge of the On macrocycle and,
by consequence, the degree of functionalization of the metal.3
A nearly planar preorganized O4 set binding chromium should
be considered as a molecular model of a variety of chromium–
oxide extended structures.4 It is well known that chromium and
chromium oxides supported on other oxides, such as Al2O3,
are important catalysts for a wide variety of reactions.5,6 This
report is focused on the synthesis of CrIII– and CrII–calix[4]-
arene derivatives, which can be considered appropriate starting
materials for developing chromium chemistry over the oxo-
Chart 1
AC-200, DPX-400 Bruker, and Perkin-Elmer FT 1600 instru-
ments, respectively. Magnetic susceptibility measurements were
surface modelled by the calix[4]arene skeleton in its cone con-
formation. The Cr–calix[4]arene chemistry is limited so far to
a single report on octahedral CrIII encapsulated by two fused
made using an MPMS5 SQUID susceptometer (Quantum
calix[4]arene moieties,7 while there is a lack of appropriate start-
Design Inc.) operating at a magnetic field strength of 1 kOe.
Corrections were applied for diamagnetism calculated from
ing compounds for entering the chromium redox and organo-
Pascal constants.8 Effective magnetic moments were calculated
metallic chemistry supported by the calix[4]arene macrocycle.
from µeff = 2.828(χCrT)1/2, where χCr is the magnetic suscepti-
bility per chromium. Fitting of the magnetic data by the
The reactivity of the Cr–calix[4]arene derivatives has been
exemplified, studying a few redox and organometallic reactions.
The supporting ligand, adapted for CrIII and CrII, is the alkyl-
theoretical expression was performed by minimizing the agree-
ment factor, defined as Σ[(χiobsTi Ϫ χicalcTi)2/(χiobsTi)2], through a
ated form of calix[4]arene displayed in Chart 1.
Levenberg–Marquardt routine. The syntheses of p-Butcalix[4]-
(OMe)2(OH)2 9 and its disodium salt 13 were performed accord-
Experimental
ing to the literature.
General
Syntheses
All reactions were carried out under an atmosphere of purified
nitrogen. Solvents were dried and distilled before use by stand-
ard methods. The H NMR and IR spectra were recorded on
Compound 2. The compound [CrCl3(THF)3] (5.53 g, 14.8
mmol) was added to a white suspension of 1ؒTHF (11.71 g,
1
J. Chem. Soc., Dalton Trans., 2000, 191–198
This journal is © The Royal Society of Chemistry 2000
191

View Article Online
14.8 mmol) in THF (200 cm3). The mixture was stirred over-
night and THF was evaporated. Then n-hexane (150 cm3) was
added, and a green product was collected and extracted with
diethyl ether (250 cm3). The green solid was collected and dried
in vacuo (11.38 g, 92%). Crystals suitable for X-ray analysis
were grown in a THF–ether solution (Found C, 71.08; H, 8.22.
and data reductions were performed with the MARHKL ver-
sion 1.9.1 suite of programs.10 Data for 4, 6 were collected on a
KumaCCD diffractometer. Cell measurement, cell refinement
and data reduction were performed with KM4RED.11 The
crystal quality was tested by ψ scans showing that crystal
absorption effects could be neglected for all complexes. The
function minimized during the least-squares refinements was
Σw(∆F2)2. Anomalous scattering corrections were included in
all structure factor calculations.12b Scattering factors for neutral
atoms were taken from ref. 12(a) for non-hydrogen atoms and
from ref. 13 for H. Structure solutions were based on the
observed reflections [I > 2σ(I)] while the refinements were
based on the unique reflections having I > 2σ(I). The structures
were solved by the heavy-atom method starting from a three-
dimensional Patterson map.14 Refinements were done by full
matrix least squares first isotropically and then anisotropically
for all non-H atoms except for the disordered atom. The hydro-
gen atoms of all complexes were put in geometrically calculated
positions and introduced in the refinements as fixed atom con-
tributions (Uiso = 0.05 Å2). For all complexes the final differ-
ence maps showed no unusual features, with no significant
peaks above the general background. All calculations were per-
formed by using SHELXL9315 implemented on a QUANSAN
personal computer equipped with an INTEL PENTIUM II
processor.
2, C50H66ClCrO5 requires C, 71.96; H, 7.97%). IR (Nujol, νmax
/
cmϪ1): 1594w, 1361m, 1333s, 1294m, 1261m, 1206s, 1161m,
1117s, 1090m, 1006s, 922w, 867m, 839s, 798w, 784w and 552w.
Compound 3. A green solution of compound 2 (2.49 g, 3.0
mmol) in pyridine (70 cm3) was refluxed overnight. A black
microcrystalline product was collected, washed with pentane
(50 cm3) and dried in vacuo (1.46 g, 52%). Crystals suitable for
X-ray analysis were grown in a pyridine solution (Found C,
72.24; H, 7.29; N, 3.99. 3, C56H68ClCrN2O4 requires C, 73.06;
H, 7.44; N, 3.04%). IR (Nujol, νmax/cmϪ1): 1630w, 1602m,
1580w, 1359s, 1322s, 1282s, 1256m, 1206s, 1167w, 1120w,
1098m, 1069w, 1042w, 1017s, 989w, 912w, 872m and 844m.
Compound 4. Mesityllithium (0.54 g, 4.29 mmol) was added
to a toluene (150 cm3) solution of compound 2 (3.58 g, 4.29
mmol). The mixture was stirred overnight and LiCl removed by
filtration. Toluene was evaporated in vacuo and pentane (100
cm3) added. The green product was collected and dried in vacuo
(1.98 g, 55%). Crystals suitable for X-ray analysis were grown
in a toluene–hexane solution (Found C, 78.28; H, 8.53. 4,
C55H69CrO4 requires C, 78.07; H, 8.22%). IR (Nujol, νmax/cmϪ1):
1589w, 1361m, 1311s, 1272m, 1206s, 1161m, 1117m, 1091m,
1044w, 1000s, 922w, 872m, 839m, 800m, 767w, 711w and 539m.
CCDC reference number 186/1744.
See http://www.rsc.org/suppdata/dt/a9/a908499a/ for crystal-
lographic files in .cif format.
Results and discussion
The transmetallation of compound 1 is quite straightforward
using [CrCl3(THF)3]. The reaction led to six-co-ordinate CrIII
in complex 2, having a quite distorted octahedral co-ordination.
The reactivity of 2 can be moderately affected by the use of
strong bases or nucleophiles under drastic conditions. In fact,
on refluxing 2 in pyridine the demethylation of one of the meth-
oxy groups and the formation of 3 were obtained (Scheme 1).
Compound 5. The compound NaCpؒDME (0.96 g, 5.4 mmol)
was added to a green solution of 2 (4.50 g, 5.3 mmol) in THF
(150 cm3). The mixture was stirred overnight and NaCl
removed by filtration. A brown solution was obtained; THF
was evaporated in vacuo and pentane (100 cm3) added. The
orange product was collected and dried in vacuo (1.72 g, 40%)
(Found C, 78.16; H, 8.46. 5, C51H63CrO4 requires C, 77.34;
H, 8.02%). IR (Nujol, νmax/cmϪ1): 1600w, 1378w, 1360m, 1309s,
1272m, 1211m, 1156w, 1124w, 1083w, 1016m, 993m, 917w,
870m, 843m, 813m, 779w and 534w.
Compound 6. Sodium (0.52 g, 22.5 mmol) and naphthalene
(1.30 g, 10.1 mmol) were added to a green solution of com-
pound 2 (18.75 g, 22.5 mmol) in THF (300 cm3). The mixture
was stirred overnight and NaCl removed by filtration. The
brown solution was concentrated, n-hexane (100 cm3) added
and a blue product collected and dried in vacuo (10.50 g, 58%).
Crystals suitable for X-ray analysis were grown in a THF–
n-hexane solution (Found C, 75.19; H, 8.75. 6, C50H66CrO5
requires C, 75.16; H, 8.32%). IR (Nujol, νmax/cmϪ1): 1594w,
1389w, 1356m, 1319s, 1300s, 1200m, 1161w, 1117w, 1089w,
1067s, 1037m, 1006s, 894m, 867m, 822w, 800m and 539m.
Compound 7. A solution of compound 6 (1.90 g, 2.4 mmol)
was refluxed in toluene (100 cm3) for 2 h, then toluene was
distilled and n-hexane (100 cm3) added. The blue green solution
was stirred under O2 (27 cm3, 1.2 mmol). A black crystalline
product was collected and dried in vacuo (0.507 g, 25%) (Found
C, 75.22; H, 8.58. 7, C52H72CrO5 requires C, 75.33; H, 8.75%).
IR (Nujol, νmax/cmϪ1): 1587w, 1392w, 1361m, 1284s, 1250s,
1208s, 1121m, 1002m, 977m, 907w, 874w, 807m, 803m, 768w,
756m, 736m, 708m, 641w, 577w, 535s, 482w and 449m.
Scheme 1
Dealkylation of [p-Butcalix[4](O)2(OMe)2]2Ϫ occurs when high
valent metals are used.2f,3a–c The structures of 2 and 3 are shown
in Figs. 1 and 2, respectively. Selected bond distances and angles
are quoted in Table 2. Relevant conformational parameters
within the Cr–calix[4]arene units are given in Table 3. The label-
ling scheme adopted for the calixarene macrocycle is depicted in
Chart 2.
X-Ray crystallography
Suitable crystals were mounted in glass capillaries and sealed
under nitrogen. Crystal data and details associated with data
collection are given in Table 1. Data for complexes 2, 3, 7 were
collected on a Mar345 imaging plate system. Cell refinements
Crystals of complex [Cr{p-Butcalix[4](O)2(OMe)2}(Cl)-
(THF)] (Fig. 1) contain Et2O in a 1:2 molar ratio. The co-
192
J. Chem. Soc., Dalton Trans., 2000, 191–198

View Article Online
Table 1 Experimental data for the X-ray diffraction studies on crystalline complexes 2–4, 6 and 7
2a
3b
4c
6d
7e
Formula
C50H66ClCrO5ؒ
2C4H10O
C56H68ClCrN2O4ؒ
C5H5N
C55H69CrO4ؒ2C7H8
C50H66CrO5ؒ2C4H8O
C92H116Cr2O10ؒ
2C6H14
Formula weight
Crystal system
Space group
a/Å
b/Å
c/Å
α/Њ
β/Њ
982.8
999.7
1030.4
943.3
829.1
Monoclinic
13.529(3)
30.221(5)
13.998(4)
Monoclinic
15.408(3)
23.553(5)
16.322(3)
Triclinic
13.542(2)
15.295(2)
15.682(2)
81.43(2)
81.68(2)
69.06(2)
2985.0(8)
143
Orthorhombic
12.526(2)
19.174(2)
21.933(2)
Monoclinic
14.605(3)
18.269(4)
17.875(4)
103.76(2)
109.26(2)
93.85(3)
γ/Њ
5559(2)
173
P21/c (no. 14)
4
5591.8(20)
185
P21/n (no. 14)
4
5267.7(11)
143
P212121 (no. 18)
4
4758.6(18)
143
P21/c (no. 14)
2
2.76
24743
7423
U/Å3
¯
T/K
P1 (no. 2)
Z
2
µ/cmϪ1
2.95
2.92
2.30
22564
11844
0.026
9595
2.59
Total reflections
Unique total
R(int)
Observed reflections
[I > 2σ(I)]
R
48947
10134
0.063
6982
42565
10930
0.059
38710
12824
0.042
0.090
4187
7865
10284
0.070
0.181
0.067
0.163
0.067
0.166
0.061
0.142
0.066
0.165
wR2
a The C(38)–C(40) methyl carbon atoms of the tert-butyl group associated to the C ring and the C(48) carbon atom of the THF molecule showed
high thermal parameters indicating the presence of disorder. The best fit was found by splitting the atoms over two positions (A and B) isotropically
refined with site occupation factors of 0.5. One Et2O solvent molecule was also affected by disorder, which was solved by splitting the O(7), C(55)–
C(58) atoms over two positions (A and B) isotropically refined with site occupation factors of 0.5. During the refinement the C–O and C–C bond
distances within the disordered solvent molecule were constrained to 1.43(1) and 1.54(1) Å respectively. b The methyl carbon atoms of the tert-butyl
group associated to the A and B rings showed high thermal parameters indicating the presence of disorder. The best fit was found by splitting the
atoms over two positions (A and B) isotropically refined with site occupation factors of 0.7 and 0.3 respectively for C(30)–C(32) and 0.5 for C(34)–
C(36). c The C(34)–C(36) and C(38)–C(40) methyl carbon atoms of the tert-butyl group associated to the B and C rings were statistically distributed
over two positions (A and B) isotropically refined with site occupation factors of 0.6 and 0.4, respectively. The guest toluene molecule was also
affected by disorder, which was solved by splitting the C(56)–C(62) atoms over three positions (A, B and C) isotropically refined with site occupation
factors of 0.5, 0.25 and 0.25, respectively. During the refinement the aromatic rings of the “partial” toluene molecules were constrained to have D6h
symmetry [C–C 1.39(1) Å]. d Refinement was carried out straightforwardly. Since the space group is polar, the crystal chirality was tested by inverting
all the coordinates (x, y, z → Ϫx, Ϫy, Ϫz) and refining to convergence again. The resulting R values (R = 0.067, wR2 = 0.154) indicated the
original choice should be considered the correct one. e The methyl carbon atoms of the tert-butyl group associated to the A ring showed high thermal
parameters indicating the presence of disorder. The best fit was found by splitting the C(30)–C(32) atoms over two positions (A and B) isotropically
refined with site occupation factors of 0.5. The n-hexane solvent molecule was also affected by severe disorder, which was solved by splitting the
C(47), C(48) atoms over three positions (A, B and C) and the C(49)–C(52) atoms over two positions (A and B) isotropically refined with the site
occupation factors of 0.5 for the A and B positions of C(30)–C(32), 0.4 and 0.6 for the A and B positions respectively of C(49)–C(52), 0.2, 0.6 and
0.2 for the A, B and C positions respectively of C(47), C(48). During the refinement the C–C bond distances within the disordered solvent molecule
were constrained to be 1.54(1) Å.
Fig. 1 A SCHAKAL16 view of complex 2. Disorder affecting the C
butyl group and the THF molecule has been omitted for clarity.
Chart 2
ordination polyhedron around chromium is a slightly distorted
octahedron, with the best equatorial plane defined by the O(1),
O(3), O(5), Cl(1) atoms (the deviations from the planarity range
from Ϫ0.036(3) to 0.038(3) Å for O(3) and O(1), respectively)
and the O(2) and O(4) atoms in the axial positions. The metal
is displaced by 0.022(1) Å from the mean equatorial plane
toward O(2). The cis arrangement of the THF and chloride
ligands removes the planarity of the O4 core, which shows
remarkable tetrahedral distortions (Table 3). The very short
Cr–O1 and Cr–O3 [Cr–Oav, 1.868(3) Å] distances17 are consist-
ent with a significant metal–oxygen π bonding (Table 2). The
J. Chem. Soc., Dalton Trans., 2000, 191–198
193

View Article Online
Table 2 Selected bond distances (Å) and angles (Њ) for complexes 2–4, 6, 7
2
Cr(1)–Cl(1)
Cr(1)–O(1)
Cr(1)–O(2)
2.307(1)
1.870(3)
2.072(3)
Cr(1)–O(3)
Cr(1)–O(4)
Cr(1)–O(5)
1.866(3)
2.065(3)
2.181(3)
Cl(1)–Cr(1)–O(5)
85.9(1)
3
Cr(1)–Cl(1)
Cr(1)–O(1)
Cr(1)–O(2)
2.337(1)
1.886(2)
2.239(2)
Cr(1)–O(3)
Cr(1)–O(4)
Cr(1)–N(1)
1.866(3)
1.911(2)
2.162(3)
Cl(1)–Cr(1)–N(1) 89.0(1)
4
Cr(1)–O(1)
Cr(1)–O(2)
1.890(2)
2.084(2)
Cr(1)–O(3)
Cr(1)–O(4)
1.842(2)
2.072(2)
Cr(1)–C(47)
Cr(1)–O(5)
2.068(2)
2.297(2)
6
Cr(1)–O(1)
Cr(1)–O(2)
1.960(2)
2.089(2)
Cr(1)–O(3)
Cr(1)–O(4)
1.961(2)
2.091(2)
7
Cr(1)–O(1)
Cr(1)–O(2)
Cr(1)–O(5)–Cr(1Ј)
1.853(4)
2.462(3)
97.4(2)
Cr(1)–O(3)
Cr(1)–O(4)
1.856(4)
2.058(3)
Cr(1)–O(5)
Cr(1)–O(5Ј)
1.726(4)
1.888(3)
A prime denotes the transformation 1 Ϫ x, Ϫy, 1 Ϫ z.
equatorial plane toward O(4). The six-co-ordination of the
metal removes, as is usual, the planarity of the O4 core (the
deviations from planarity range from Ϫ1.304(3) to 0.274(2)
Å). The Cr–O bond distances involving O(1), O(3) and O(4)
oxygen atoms [mean value 1.893(12) Å] reveal the existence of a
significant π bonding, which is absent for the methoxy group
[Cr–O(2), 2.239 Å]7,17 (Table 2). The elliptical conformation of
the macrocycle is similar to that observed in 2, the main differ-
ence consisting in the narrowing of the dihedral angle between
the “reference” plane and the A and B rings. The methylpyridin-
ium cation guest molecule enters the cavity approximately
parallel to the B and D rings (dihedral angles 30.4(1) and
18.3(1)Њ, respectively) and perpendicular to the A and C rings
(dihedral angles 76.2(1) and 89.0(1)Њ, respectively). This
arrangement could favour CH3–π host–guest interactions
between the protons of the C(56) methyl group of toluene and
the A aromatic ring [C(56) ؒ ؒ ؒ CA, 3.309(4)–3.761(5); C(56) ؒ ؒ ؒ
A
centroid, 3.286(6); H(561) ؒ ؒ ؒ Acentroid, 2.49 Å].
The magnetic behaviour of compounds 2 and 3 does not show
any anomaly, being in agreement with the expected high spin
state for octahedral d3 (see below).
Fig. 2 A SCHAKAL view of complex 3. Disorder affecting the A and
B butyl groups has been omitted for clarity.
Complex 2 is a starting material for access to organometallic
functionalization and the lower oxidation state of chromium.
The alkylation of 2 was successful only when using LiMes
(Mes = mesityl) and NaCp, and led to clean synthesis of the
mesityl and the cyclopentadienyl derivatives 4 and 5, respect-
ively (Scheme 2). In the case of other alkylating agents the con-
current reduction reaction or the intrinsic thermal instability of
the CrIII–alkyl functionality prevented the isolation of clean
compounds. The stability of 4 and 5 could be associated to the
steric protection by the axial mesityl or the cyclopenta-
dienyl ligand. Even under drastic thermal conditions or with
inserting substrates, we did not observe any reactivity of 4, in
accordance with its expected kinetic inertness. The structure of
4 is displayed in Fig. 3.
Complex 4 crystallizes with toluene in a 1:2 molar ratio. One
toluene molecule is hosted in the calixarene cavity. Chromium
exhibits a distorted trigonal bipyramidal co-ordination, with
O(1), O(3) and C(47) defining the equatorial plane and O(2),
O(4) in the axial positions. The metal is displaced by 0.025(1) Å
from the equatorial plane toward O(4). The O4 core shows
remarkable tetrahedral distortions ranging from Ϫ0.398(2) to
0.234(2) Å, the normal to the mean O4 plane being nearly paral-
lel to the Cr–C(47) vector (dihedral angle 7.7(1)Њ). The Cr–O
calixarene macrocycle assumes a flattened elliptical conform-
ation with the C ring roughly parallel to the mean “reference”
plane through the methylene C(7), C(14), C(21), C(28) carbon
atoms (Table 3). This conformation allows an Et2O molecule to
be accommodated inside the calixarene cavity. The host–guest
interactions should be described in terms of CH–π inter-
actions18 occurring between the C(51) methyl and the C(52)
methylene carbons of the guest and the A, B and D rings.
C(51) ؒ ؒ ؒ CA, 3.750(7)–3.919(7); C(51) ؒ ؒ ؒ Acentroid
H(511) ؒ ؒ ؒ Acentroid 2.64; C(52) ؒ ؒ ؒ CB, 3.803(7)–4.213(7);
C(52) ؒ ؒ ؒ Bcentroid
, 3.586(7);
,
,
3.757(5);
H(522) ؒ ؒ ؒ Bcentroid
,
2.78;
C(52) ؒ ؒ ؒ CD, 3.858(6)–4.213(5); C(52) ؒ ؒ ؒ Dcentroid
,
3.746(7);
H(521) ؒ ؒ ؒ Dcentroid, 2.80 Å.
Complex
3 crystallizes from pyridine which is pres-
ent in the crystals in 1:1 molar ratio. The structure of the
anion is shown in Fig. 2. It is quite similar to that of 2, where a
pyridine molecule replaces the THF molecule in the co-
ordination sphere of the metal. The O(1), O(3), Cl(1), N(1) set
of donor atoms defines the equatorial plane of a distorted
octahedron, while O(2) and O(4) atoms are in the axial
positions. The metal protrudes by 0.149(1) Å from the mean
194
J. Chem. Soc., Dalton Trans., 2000, 191–198

View Article Online
Table 3 Comparison of relevant conformational parameters within the Cr(calix[4]arene) units for complexes 2–4, 6 and 7
2
3
4
6
7
(a) Distances (Å) of atoms from the O4 mean plane
O(1)
O(2)
O(3)
O(4)
Cr
Ϫ0.494(3)
0.488(3)
Ϫ0.484(3)
0.491(3)
0.702(1)
Ϫ0.136(2)
0.274(2)
Ϫ1.304(3)
0.247(2)
0.442(1)
Ϫ0.204(2)
0.234(2)
Ϫ0.398(2)
0.223(2)
0.378(1)
Ϫ0.048(2)
0.047(2)
Ϫ0.048(2)
0.048(2)
0.093(1)
0.422(4)
Ϫ0.415(4)
0.422(4)
Ϫ0.492(4)
0.865(1)
(b) Dihedral angles (Њ) between planar moietiesa
E ∧ A
E ∧ B
E ∧ C
E ∧ D
A ∧ C
B ∧ D
138.4(1)
113.5(1)
162.8(1)
113.4(1)
121.2(1)
133.1(1)
157.1(1)
111.7(1)
148.4(1)
116.5(1)
125.5(1)
131.8(1)
131.1(1)
112.4(1)
151.1(1)
112.6(1)
102.1(1)
135.0(1)
125.5(1)
116.0(1)
124.6(1)
117.0(1)
109.8(1)
127.0(1)
120.5(1)
114.2(1)
119.7(1)
173.0(1)
119.7(2)
107.2(2)
(c) Contact distances (Å) between para-carbon atoms of opposite aromatic rings
C(4) ؒ ؒ ؒ C(17)
C(10) ؒ ؒ ؒ C(24)
9.894(6)
7.454(6)
10.090(5)
7.472(5)
9.637(5)
7.276(4)
8.560(6)
7.840(6)
7.865(8)
9.361(8)
a E (reference plane) refers to the least-squeares mean plane defined by the C(7), C(14), C(21), C(28) bridging methylenic carbons.
atives.19 The mean plane through the mesitylene aromatic ring is
oriented to form a dihedral angle of 21.2(1)Њ with the Cr, O(1),
O(3), C(47) mean plane.†
The metal sitting inside a quite protected cavity is rather
inaccessible to any substrate, thus almost completely
unreactive. Although the metal is formally five-co-ordinate, the
steric hindrance of the mesityl group, along with the C–H–O
interactions between the o-methyl groups from the mesityl and
the oxygens from the calix[4]arene, are probably responsible for
the elliptical conformation of the calix[4]arene skeleton. Similar
elliptical conformations have been observed in the case of six-
co-ordinated metals (Table 3).3b As in the latter case, the result-
ing cavity is suitable for hosting a toluene molecule.
The reactivity of the chromium() derivatives so far
reported is very low, even under thermal or photochemical
forced conditions. In order to increase the reactivity of the
metal center in the Cr–calix[4]arene moiety we moved to the
by far less kinetically inert CrII. Compound 2 undergoes one-
electron reduction to 6 using sodium in the presence of naph-
thalene in THF. Complex 6 was isolated as blue crystals, while
all the CrIII–calix[4]arene derivatives are green. The magnetic
measurement is consistent with high-spin d4 with a normal
paramagnetic behaviour (see below). Its structure shows the five-
co-ordination of the metal atom, achieved by the presence of a
THF molecule (Fig. 4).
Complex 6 crystallizes with THF in a 1:2 molar ratio. One of
the two THF molecules of crystallization is hosted in the calix-
arene cavity. The metal exhibits a square pyramidal co-
ordination, the oxygen atoms from the O4 core defining the
equatorial plane. The O4 core shows small but significant tetra-
hedral distortions (Table 3), the metal being displaced by
0.093(1) Å from the mean plane through it. The Cr–O(5) vector
forms a dihedral angle of 8.2(1)Њ with the normal to the O4
mean plane. The trend in the Cr–O bond distances (Table 2),
appearing in two sets, is the same as discussed for complexes
2–4. The macrocycle shows a nearly regular cone conformation
(Table 3) due to the presence of a five-co-ordinate metal.3b
Fig. 3 A SCHAKAL view of complex 4. Disorder affecting the B and
C butyl groups and the toluene guest molecule has been omitted for
clarity.
† Short intramolecular contacts are observed involving the H(551) and
H(531) protons from methyl groups of mesitylene and O(1) and O(3)
atoms respectively (C(55) ؒ ؒ ؒ O(1), 2.922(3); H(551) ؒ ؒ ؒ O(1), 2.06;
C(53) ؒ ؒ ؒ O(3), 3.190(3); H(531) ؒ ؒ ؒ O(3), 2.27 Å). Additional contacts
involve the H(451) and H(461) protons from the methoxy groups of
calixarene, which are oriented to point toward the centroid of the mes-
Scheme 2
distances (Table 2) vary from 1.890(2) to 2.084(2) Å, according
to the presence, for the former value, of π bonding. The Cr–C
bond distance, 2.068(2) Å, is quite normal for Cr–aryl deriv-
itylene ring: C(45) ؒ ؒ ؒ CMes, 3.344(4)–5.154(4); C(45) ؒ ؒ ؒ Mescentroid
,
4.115(4); H(451) ؒ ؒ ؒ Mescentroid, 3.27; C(46) ؒ ؒ ؒ CMes, 3.296(4)–5.053(4);
C(46) ؒ ؒ ؒ Mescentroid, 4.021(4); H(461) ؒ ؒ ؒ Mescentroid, 3.18 Å.
J. Chem. Soc., Dalton Trans., 2000, 191–198
195

View Article Online
Fig. 4 A SCHAKAL view of complex 6.
Fig. 5 A SCHAKAL view of complex 7. Disorder affecting the A
butyl group has been omitted for clarity. A prime denotes the trans-
Complex 6 is air sensitive. It absorbs O2 in the O2 :Cr molar
ratio of 1:2, so exemplifying the tendency of Cr to move up in
oxidation state. The reaction with O2 should be carried out on 6
previously refluxed in toluene. This operation probably removes
the THF molecule and makes 6 more reactive. In addition,
THF can be engaged in an oxidative degradation assisted by the
chromium complex. According to the plausible pathway drawn
in Scheme 3, we suppose the formation of a bridged peroxodi-
formation 1 Ϫ x, Ϫy, 1 Ϫ z.
the C(21), C(28), C(45) carbon atoms and the O(5) oxygen
atoms: C(21) ؒ ؒ ؒ O(5), 3.312(7); H(212) ؒ ؒ ؒ O(5), 2.71;
C(28) ؒ ؒ ؒ O(5), 3.311(7); H(281) ؒ ؒ ؒ O(5), 2.72; C(45) ؒ ؒ ؒ O(5Ј),
3.011(6); H(451) ؒ ؒ ؒ O(5Ј), 2.39 Å (primes indicate the trans-
formation 1 Ϫ x, Ϫy, 1 Ϫ z). The very long Cr–O(2) distance
would suggest the absence of any bonding between the two
atoms, leaving the methyl group free to rotate and be accom-
modated inside the calix cavity. The intramolecular “host–
guest” interactions could be interpreted in terms of CH3–π
interactions18 occurring between the protons of the C(46)
methyl group and the aromatic A, B and C rings: C(46) ؒ ؒ ؒ CA,
3.094(8)–4.068(8); C(46) ؒ ؒ ؒ Acentroid
,
3.336(8); H(462) ؒ ؒ ؒ
A
centroid, 2.81; C(46) ؒ ؒ ؒ CB, 3.670(8)–4.428(8); C(46) ؒ ؒ ؒ Bcentroid
,
3.813(8); H(461) ؒ ؒ ؒ Bcentroid
, 2.89; C(46) ؒ ؒ ؒ CC, 3.074(8)–
3.930(8); C(46) ؒ ؒ ؒ Ccentroid, 3.278(8); H(463) ؒ ؒ ؒ Ccentroid, 2.71 Å.
The conformation of the calix skeleton in 7 is remarkably dif-
ferent from those in complexes 2–4 and 6, in particular the D
ring is nearly parallel to the “reference” plane (Table 3).
The magnetic susceptibilities of complexes 2–7 were meas-
ured in the temperature range 1.9–300 K. The magnetic
moments of 2–5 are essentially constant in the whole range of
temperature, with a value of ca. 3.8 µB at room temperature
showing only an almost negligible decrease below 2 K (due to a
small zero-field splitting), thus indicating a high spin, S = 3/2,
state for these monomeric chromium() compounds. The mag-
netic moment of 6 has a value of ca. 4.2 µB at room temperature
and is essentially constant throughout the whole range of tem-
peratures with a small decrease below 10 K (due to zero-field
splitting), thus indicating a high spin, S = 2, state for this
monomeric chromium() species. The temperature dependence
of the magnetic moment of the bis-µ-oxo dimer 7 shows a
steady increase from room temperature down to about 20 K
and then a slight decrease (Fig. 6). The increase is due to a
ferromagnetic coupling between the two chromium() centres,
while the decrease at low temperatures is due to zero field split-
ting, which may be significant for chromium() species. The
data were fitted using the spin Hamiltonian21 (1) where
Scheme 3
chromium complex A, which undergoes cleavage of the O–O
bond, thus forming a quite unique di-µ-oxo-CrIV derivative, 7.20
The structure of complex 7 consists of centrosymmetric
dimers (Fig. 5), where the two metal ions are asymmetrically
bridged by the oxo groups, thus forming a Cr2O2 ring, which is
planar for reasons of symmetry. Chromium exhibits a severely
distorted octahedral co-ordination, the best equatorial plane
being defined by the O(2), O(4), O(5), O(5Ј) oxygen atoms [devi-
ations ranging from Ϫ0.002(4) to 0.003(4) Å]. The metal lies
exactly in the equatorial mean plane. The Cr–O(5) bond
distances involving the µ-oxo anion are significantly different
[Cr–O(5), 1.726(4); Cr–O(5Ј), 1.888(3) Å]. The Cr–O bond
distances involving the calixarene oxygen atoms fall in a
rather wide range (Table 2). This fact may be the consequence
of remarkable intraligand steric hindrance mainly involving
ˆ
H =
gβH(S1 ϩ S2) ϩ D[S1z Ϫ ] ϩ D[S2z Ϫ ] Ϫ 2J(S1ؒS2) (1)
2
2
ˆ
ˆ
ˆ
ˆ
ˆ ˆ
2
–
2
–
3
3
S1 = S2 = 1 are the spins of the two chromium() ions, D the
zero-field-splitting parameter and J the Heisenberg coupling
constant between the two centres. The magnetic susceptibility
at each temperature point was calculated by use of the
196
J. Chem. Soc., Dalton Trans., 2000, 191–198

View Article Online
Fig. 6 Magnetic susceptibility (᭺) and magnetic moment (᭹) as a
function of the temperature for complex 7.
Van-Vleck equation21 (2) where N is Avogadro’s number, k the
Fig. 7 Orbital interaction diagram for complex 5.
χ = M/H =
2
1a1(dz ). Finally a weak δ interaction is also observed between
the empty e2Љ and the metal 1a2(dxy). Although the metal centre
would be expected to be reactive toward both electrophiles and
nucleophiles, the steric bulkiness of the cyclopentadienyl ligand
prevents any reaction in the exo-calix position.
[NΣi(ϪdEi/dH)exp(ϪEi/kT)]/[HΣi exp(ϪEi/kT)] (2)
Boltzmann constant and H the applied field. The energy levels
of the tetramers, Ei, are evaluated by diagonalizing the Hamil-
tonian matrix in the uncoupled spin functions basis set. The
calculated best-fit parameters are: g = 1.92, J = ϩ40.4 cmϪ1
and DCr = Ϫ5.9 cmϪ1. To the best of our knowledge, bis-µ-oxo-
chromium() dimers have not been magnetically characterized.
However, the ferromagnetic coupling constants have been fre-
quently observed for other µ-oxo bridged vanadium() dimers
(with the same d2–d2 metal configuration)22 and interpreted in
terms of the interaction between the magnetic orbitals.22,23
In order to perform extended Hückel calculations,24 the
Cr{p-Butcalix[4](O2)(OMe)2} fragment was slightly simplified
by replacing it by two phenoxo and two anisole molecules
within a C2v geometry. This simplified model retains the main
features of the whole ligand. In particular, the geometrical con-
straints on the O4 set of donor atoms has been maintained by
fixing the geometry of the phenoxo and ether molecules to the
experimental X-ray values.
Acknowledgements
We thank the Fonds National Suisse de la Recherche
Scientifique (Bern, Switzerland, Grant No. 20-53336.98) and
Action COST D9 (European Program for Scientific Research,
OFES No. C98.008) for financial support.
References
1 C. Floriani, Chem. Eur. J., 1999, 5, 19 and references therein.
2 (a) L. Giannini, E. Solari, S. Dovesi, C. Floriani, N. Re, A. Chiesi-
Villa and C. Rizzoli, J. Am. Chem. Soc., 1999, 121, 2784; (b) L.
Giannini, G. Guillemot, E. Solari, C. Floriani, N. Re, A. Chiesi-
Villa and C. Rizzoli, J. Am. Chem. Soc., 1999, 121, 2797; (c) L.
Giannini, S. Dovesi, E. Solari, C. Floriani, A. Chiesi-Villa and
C. Rizzoli, Angew. Chem., Int. Ed., 1999, 38, 807; (d) L. Giannini,
E. Solari, C. Floriani, N. Re, A. Chiesi-Villa and C. Rizzoli, Inorg.
Chem., 1999, 38, 1438; (e) A. Caselli, E. Solari, E. Scopelliti and
C. Floriani, J. Am. Chem. Soc., 1999, 121, 8296; ( f ) B. Castellano,
E. Solari, C. Floriani, N. Re, A. Chiesi-Villa and C. Rizzoli, Chem.
Eur. J., 1999, 5, 722.
The frontier orbitals of the Cr{p-Butcalix[4](O)2(OMe)2}
fragment are reported on the left of Fig. 7 and consist of four
low-lying metal-based orbitals. We can distinguish two low-
lying almost degenerate orbitals, i.e. the 1b1(dxz), pointing in
2
the plane of the two methoxy ligands, and the 1a1(dz ). Owing
3 (a) B. Castellano, E. Solari, C. Floriani, N. Re, A. Chiesi-Villa and
C. Rizzoli, Organometallics, 1998, 17, 2328; (b) A. Zanotti-Gerosa,
E. Solari, L. Giannini, C. Floriani, N. Re, A. Chiesi-Villa and
C. Rizzoli, Inorg. Chim. Acta, 1998, 270, 298; (c) M. Giusti,
E. Solari, L. Giannini, C. Floriani, A. Chiesi-Villa and C. Rizzoli,
Organometallics, 1997, 16, 5610; (d) A. Caselli, L. Giannini,
E. Solari, C. Floriani, N. Re, A. Chiesi-Villa and C. Rizzoli,
Organometallics, 1997, 16, 5457; (e) L. Giannini, A. Caselli, E.
Solari, C. Floriani, A. Chiesi-Villa, C. Rizzoli, N. Re and A.
Sgamellotti, J. Am. Chem. Soc., 1997, 119, 9198; ( f ) L. Giannini,
A. Caselli, E. Solari, C. Floriani, A. Chiesi-Villa, C. Rizzoli, N. Re
and A. Sgamellotti, J. Am. Chem. Soc., 1997, 119, 9709; (g) B.
Castellano, E. Solari, C. Floriani, R. Scopelliti and N. Re, Inorg.
Chem., 1999, 38, 3406.
4 J. M. Thomas and W. J. Thomas, Principles and Practice of
Heterogeneous Catalysis, VCH, Weinheim, 1997; H. H. Kung,
Transition Metal Oxides: Surface Chemistry and Catalysis, Elsevier,
Amsterdam, 1989; Catalyst Design, Progress and Perspectives, ed.
L. Hegedus, Wiley, New York, 1987; G. C. Bond, Heterogeneous
Catalysis, Principles and Applications, 2nd edn., Oxford University
Press, New York, 1987.
to the stronger interaction with the phenoxo ligands in the yz
plane, the 1b2(dyz) orbital is ca. 0.8 eV higher in energy than the
1b1(dxz). The 1a2(dxy) orbital is also raised in energy by the π
interactions with the p orbitals of the unmethylated O atoms
and lies only 0.2 eV lower than the 1b2.
2
The presence of a low-lying 1a1(dz ) orbital in this fragment
allows the easy co-ordination by σ ligands, as in the mesityl
complex 4. The Cr{p-Butcalix[4](O)2(OMe)2} fragment may
also employ the two dπ orbitals, 1b1(dxz) and 1b2(dyz), in binding
a suitable π ligand. This has been exemplified by the co-
ordination of the cyclopentadienyl anion in 5.
The orbital interaction diagram between the [Cr{p-Butcalix-
[4](O)2(OMe)2}]ϩ cation and the CpϪ ligand is shown in Fig. 7.
On the extreme right we present the frontier orbitals of CpϪ, i.e.
the a2Љ, e1Љ and e2Љ of, respectively, σ, π and δ symmetry with
respect to the Cr–Cp axis. The filled e1Љ interact with the metal
1b1(dxz) and 1b2(dyz) orbitals respectively, while a weaker inter-
action is observed between the lower lying a2Љ and the metal
J. Chem. Soc., Dalton Trans., 2000, 191–198
197

View Article Online
5 A. Clark, Catal. Rev., 1969, 3, 145; F. J. Karol, G. L. Karapinka,
C. Wu, A. W. Dow, R. N. Johnson and W. L. Carrick, J. Polym. Sci.,
Part A-1, 1972, 10, 2621; F. J. Karol, G. L. Brown and J. M. Davison,
J. Polym. Sci., Polym. Chem. Ed., 1973, 11, 413.
6 (a) B. J. Thomas and K. H. Theopold, J. Am. Chem. Soc., 1988, 110,
5902; (b) B. J. Thomas, S.-K. Noh, G. K. Schulte, S. C. Sendlinger
and K. H. Theopold, J. Am. Chem. Soc., 1991, 113, 8983; (c) K. H.
Theopold, R. A. Heintz, S.-K. Noh and B. J. Thomas, in
Homogeneous Transition Metal Catalyzed Reactions, eds. W. R.
Moser and D. W. Slocum, American Chemical Society, Washington,
DC, 1992, p. 591.
7 V. C. Gibson, C. Redshaw, W. Clegg and M. R. J. Elsegood, Chem.
Commun., 1997, 1605.
8 E. A. Boudreaux and L. N. Mulay, Theory and Applications of
Molecular Paramagnetism, Wiley, New York, 1976, pp. 491–495.
9 A. Arduini and A. Casnati, in Macrocycle Synthesis, ed. O. Parker,
Oxford University Press, New York, 1996, ch. 7.
16 SCHAKAL 97, a computer program for the graphic representation
of molecular and crystallographic models, E. Keller, J. Appl.
Crystallogr., 1989, 22, 12.
17 J. Hvoslef, H. Hope, B. D. Murray and P. P. Power, J. Chem. Soc.,
Chem. Commun., 1983, 1438; B. D. Murray, H. Hope and P. P.
Power, J. Am. Chem. Soc., 1985, 107, 169; D. C. Bradley, R. C.
Mehrotra and D. P. Gaur, Metal Alkoxides, Academic Press,
London, 1978; R. C. Mehrotra, Adv. Inorg. Chem. Radiochem.,
1983, 26, 269; M. H. Chisholm, F. A. Cotton, M. W. Extine and
D. C. Redeout, Inorg. Chem., 1979, 18, 120.
18 C. D. Gutsche, Calixarenes Revisited, The Royal Society of Chem-
istry, Cambridge, 1998, ch. 6.
19 G. Bhandari, Y. Kim, J. M. McFarland, A. L. Rheingold and K. H.
Theopold, Organometallics, 1995, 14, 738; P. A. White, J. Calabrese
and K. H. Theopold, Organometallics, 1996, 15, 5473; M. J. Winter
and S. Woodward, in Comprehensive Organometallic Chemistry II,
eds. G. Wilkinson, F. G. A. Stone and E. W. Abel, Elsevier, Oxford,
1995, vol. 5, ch. 5.
10 Z. Otwinowski and W. Minor, Methods Enzymol., 1997, 276,
307.
20 E. S. Gould, Coord. Chem. Rev., 1994, 135/136, 651; D. A. House,
11 KM4RED DATA REDUCTION, version 1.5.8, KUMA diffraction
instruments GmbH, PSE-EPFL module 3.4, Lausanne, 1999.
12 International Tables for X-Ray Crystallography, Kynoch Press,
Birmingham, 1974, vol. IV, (a) p. 99; (b) p. 149.
13 R. F. Stewart, E. R. Davidson and W. T. Simpson, J. Chem. Phys.,
1965, 42, 3175.
14 G. M. Sheldrick, SHELX 76, Program for crystal structure
determination, University of Cambridge, Cambridge, 1976.
15 G. M. Sheldrick, SHELXL 93, Program for crystal structure
refinement, University of Göttingen, Göttingen, 1993.
Adv. Inorg. Chem., 1997, 44, 341.
21 C. J. O’Connor, Prog. Inorg. Chem., 1982, 29, 203; O. Kahn,
Molecular Magnetism, VCH, Weinheim, 1993.
22 P. Knopp and K. Wieghardt, Inorg. Chem., 1991, 30, 4061 and
references therein.
23 R. Hotzelmann and K. Wieghardt, Inorg. Chem., 1993, 32, 114.
24 R. Hoffmann and W. N. Lipscomb, J. Chem. Phys., 1962, 36, 2179;
R. Hoffmann, J. Chem. Phys., 1963, 39, 1397.
Paper a908499a
198
J. Chem. Soc., Dalton Trans., 2000, 191–198