Structural, magnetic and DFT studies of a hydroxide-bridged
{Cr
8
} wheel
Paul Christian,
a
Gopalan Rajaraman,
a
Andrew Harrison,
b
Joseph J. W. McDouall,
a
James T. Raftery
a
and Richard E. P. Winpenny*
a
a
Department of Chemistry, The University of Manchester, Oxford Road, Manchester,
UK M13 9PL. E-mail: richard.winpenny@man.ac.uk
b
Department of Chemistry, The University of Edinburgh, West Mains Road, Edinburgh,
UK EH9 3JJ
Received 16th March 2004, Accepted 14th April 2004
First published as an Advance Article on the web 19th April 2004
Structural, magnetic and theoretical studies of an octa-
nuclear chromium(III) wheel are reported, containing
hydroxide and pivalate bridges.
There is much interest in the literature in the synthesis and
properties of wheels of 3d-metals as possible models for infinite
chain systems,
1
and some suggestions that related compounds
could be used as Qubits for quantum computing.
2
There are
several examples of wheels with chromium.
3
One such com-
pound is [CrF(O
2
CCMe
3
)
2
]
8
1.
3a,b
Here we report the hydroxide
analogue of this molecule and compare the magnetic properties
of the two.
[Cr(OH)(O
2
CCMe
3
)
2
]
8
2 is synthesised by heating a solution
of chromium nitrate (28 mmol) in water (50 ml) with sodium
pivalate (58 mmol) at 40 °C. Subsequent filtration and dissol-
ution in MeCN–diethyl ether solutions give a purple micro-
crystalline solid which analyses well† for 24H
2
O. Deep purple
crystals suitable for X-ray diffraction were obtained by vapour
diffusion of MeCN into toluene with a yield of crystals of 5%.
Diffraction studies ‡ shows the compound crystallises with
one molecule in the asymmetric unit, and with a regular
octagonal structure with the bridging OH pointing into the
centre of the ring (Fig. 1). The pivalate moieties are arranged in
two conformations, firstly in a plane about the edge of the ring
and secondly in an alternating fashion above and below the
ring. The Cr–O distances in the wheel are unexceptional. The
other hydroxide bridged {Cr
8
} wheels are [Cr(OH)(O
2
CPh)
2
]
8
,
which was made in very low yield via heating a chromium
triangle,
3c
and a similar structure with chlorobenzoate which
co-crystallises with a Cr triangle.
5
We can also make this
Fig. 1 Structure of 2. Bond length ranges (Å): Cr–O (piv) 1.952–1.985,
Cr–O(hydroxide) 1.932–1.953. Bond angle ranges (°): cis at Cr 86.1–
94.0, trans at Cr 176.1–179.5°. Av. esds: 0.007 Å, 0.3°.
benzoate bridged ring reliably by the route described here,
which suggests that the pyrolysis step described previously
3c
was unnecessary.
The ES-MS of 2 in acetone gave a signal at m/z 2204,
probably due to the molecular ion plus one H
2
O and one
sodium. The UV-vis spectra of both 1 and 2 in diethyl ether
show three bands, a charge transfer band at 217 nm, and two
d–d transitions at 409 and 570 nm for 2 and at 426 and 610 nm
for 1. The result of this difference is that 1 is deep green while
2 is deep purple.
The magnetic susceptibility, χ
m
, of 2 has been studied from
2 to 320 K (Fig. 2).§ There is a steady decrease in χ
m
T as the
temperature falls, demonstrating that the coupling between
Cr() centres is antiferromagnetic and the ground state is S = 0.
The room-temperature value of χ
m
T (13.7 cm
3
K mol
-1
) is a
little below the value calculated for eight S = 3/2 centres and
g = 1.99 (calc. 14.9 cm
3
K mol
-1
).
The coupling constant (J) for the Cr–Cr magnetic interaction
was found by treating the wheel as an infinite chain using the
Fisher approximation (eqn. (1)):
6
where u =coth[JS(S + 1)/kT] - [kT/JS(S + 1)].
This gave a value for J as -10.1 cm
-1
(Fig. 2). This value for
J is comparable with literature values for similar compounds.
3c,e
Previous work
7
has shown that DFT calculations using
the hybrid B3LYP functional with Ahlrich’s TZV basis set
implemented on Gaussian 98/03,
8
provides good estimates of
magnetic exchange. We have therefore used this technique to
compare the exchange in 1 and 2.
Dinuclear model complexes of 1 and 2, and variants, have
been used to calculate exchange coupling constants, with water
added as a terminal ligand (Table 1). The computational
Fig. 2 Plot of χ
m
and χ
m
T vs. T for 2. Circles: measured χ
m
; triangles,
measured χ
m
T ; solid line: fit as described in text.
(1)
DOI: 10.1039/ b404031g
1511
This journal is © The Royal Society of Chemistry 2004 Dalton Trans. , 2004, 1511–1512
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