Enantiomeric disorder in racemic cis-dichlorobis(pentane-2,4-dionato)-titanium(IV)

Article abstract of DOI:10.1107/S0108270100019181

The title compound, [Ti(C₅H₇O₂)₂Cl₂], adopts the cis configuration. The racemic compound crystallizes in space group P1 and each molecular site has 0.50 occupancy by each of the two enantiomorphs. The

Full text of DOI:10.1107/S0108270100019181

metal-organic compounds
Acta Crystallographica Section C
Crystal Structure
Communications
Compound (I) crystallizes in the triclinic space group P1
with Z = 2. The molecules have the cis con®guration (Fig. 1)
and the centrosymmetric space group accommodates equal
numbers of à and Á enantiomers. However, each molecular
site is occupied with equal probability by the two enantiomers:
ISSN 0108-2701
Enantiomeric disorder in racemic
cis-dichlorobis(pentane-2,4-dionato)-
titanium(IV)
George Ferguson² and Christopher Glidewell*
only the Ti and one of the Cl have sites common to both
enantiomers, and several pairs of corresponding atoms in the
two enantiomers occupy closely similar positions (Fig. 2).
Because of this, a number of restraints were necessary in the
re®nement but, subject to these, the mean values of the
School of Chemistry, University of St Andrews, St Andrews, Fife KY16 9ST, Scotland
Correspondence e-mail: cg@st-andrews.ac.uk
Received 21 November 2000
Accepted 30 November 2000
Ê
leading geometric parameters are: TiÐCl = 2.282 (4) A; TiÐ
The title compound, [Ti(C5H7O2)2Cl2], adopts the cis con®g-
uration. The racemic compound crystallizes in space group P1
and each molecular site has 0.50 occupancy by each of the two
enantiomorphs. The enantiomeric disorder is correlated in two
dimensions.
Ê
O(trans to Cl) = 1.973 (7) A; TiÐO(trans to O) = 1.930 (7) A;
Ê
Ê
1.266 (13) A; CÐC(ring)
Ê
1.374 (9) A; CÐ
CÐO
=
=
Ê
C(methyl) = 1.480 (10) A. The facial arrangement of the Cl
sites (Fig. 2) precludes the presence of any trans isomer.
The nature of the disorder (Fig. 2) necessarily raises the
question (Marsh, 1999), P1 or P1 ? Using coordinates derived
from the disordered P1 re®nement, a re®nement in P1 with
one à and one Á enantiomorph in the unit cell led to R values
above 0.10 accompanied by wholly unreasonable anisotropic
displacement parameters and unsatisfactory intermolecular
contacts; the ordered P1 model was therefore decisively
rejected.
Comment
Pentane-2,4-dione (CH3COCH2COCH3, Hacac) reacts with
both titanium(IV) chloride and titanium(IV) alkoxides to
yield neutral products [Ti(acac)2X2], where X = Cl (Dilthey,
1904) or X = OR (Yamamoto & Kambara, 1957). NMR studies
suggest that, in solution, these products and the analogous
complexes derived from other 1,3-diketones are all octahe-
dral, containing bidentate O,O0-chelating diketonate ligands
with a cis arrangement of the two ligands X (Fay & Lowry,
1967; Serpone & Fay, 1967; Bradley & Holloway, 1969). We
have recently reported the structure of dichlorobis(2,2,6,6-
tetramethyl-3,5-heptanedionato)titanium(IV), [(Me3CCOC-
HCOCMe3)2TiCl2] (Glidewell et al., 1996), where the mol-
ecules adopt the cis con®guration with equal numbers of Ã
and Á enantiomers present in space group P21/c. However,
apart from this, the only analogous structure recorded in the
Cambridge Structural Database (Allen & Kennard, 1993) is
that of [(PhCOCHCOPh)2TiCl2] (TOPZUT; Matilainen et al.,
1996); here, the molecules were again shown to adopt the cis
con®guration, but the ®nal R value was 0.089 for a data-to-
parameter ratio of only 7.6. It is striking that the structure of
the simplest member of this series [(CH3COCHCOCH3)2-
TiCl2], (I), has not yet been reported; an attempted structure
analysis of [Ti(acac)2Cl2] was thwarted by hydrolysis, and the
compound actually studied was [{Ti(acac)2Cl}2O] (Waten-
paugh & Caughlin, 1967), where the chloride and bridging
oxide ligands occupy cis sites. We report here the structure of
(I) which shows an uncommon form of disorder, which itself
may have hampered earlier attempts at structure analysis.
Equal occupancy by the two enantiomers of the average
molecular site may be a re¯ection of spacial or temporal
disorder, or of a combination of these. In compound (I),
spacial disorder cannot however be merely a random distri-
bution of the two enantiomers amongst all the molecular sites.
The short intermolecular contacts C3Á Á ÁC3i [symmetry code:
(i) 1 + x, y, z] and C23Á Á ÁC33ii [symmetry code: (ii) x, 1 + y,
Ê
z] of 1.94 (2) and 2.15 (2) A, respectively, preclude the
presence of the same enantiomer at adjacent sites in both the
[100] and [010] directions; hence, in the ab plane, the à and Á
enantiomers must alternate in checkerboard fashion. There
Figure 1
The Á enantiomer of compound (I) showing the atom-labelling scheme.
Displacement ellipsoids are drawn at the 30% probability level and H
atoms have been omitted for the sake of clarity.
² GF is on leave from the Department of Chemistry and Biochemistry,
University of Guelph, Guelph, Ontario, Canada N1G 2W1.
ꢀ
264 # 2001 International Union of Crystallography
Printed in Great Britain ± all rights reserved
Acta Cryst. (2001). C57, 264±265

metal-organic compounds
Re®nement
are no prohibitively short contacts in the [001] direction and
hence there is no correlation of the arrangements in neigh-
bouring (001) planes; thus, the structure is correctly described
in terms of the present unit cell with Z = 2, rather than of a
larger cell having Z = 8.
Solution studies using NMR have indicated that the intra-
molecular Ã/Á isomerization in (I) has a low activation
barrier and hence is rapid at ambient temperature (Bradley &
Holloway, 1969). It is possible that the disordered model
derived from the X-ray diffraction data may also re¯ect, at
least in part, rapid intramolecular exchange; again, because of
the short intermolecular contacts, exchange events at adjacent
sites in the ab plane would necessarily be correlated.
Re®nement on F2
H-atom parameters constrained
R[F2 > 2ꢅ(F2)] = 0.053
wR(F2) = 0.171
S = 0.942
w = 1/[ꢅ2(Fo2) + (0.0894P)2]
where P = (Fo2 + 2Fc2)/3
(Á/ꢅ)max < 0.001
3
Ê
3012 re¯ections
277 parameters
Áꢆmax = 0.35 e A
3
Ê
0.35 e A
Áꢆmin
=
Compound (I) crystallized in the triclinic system; space group P1
was assumed and con®rmed by the analysis. It was apparent at the
structure-solution stage that there was signi®cant disorder in the
structure and the density maps could only be interpreted in terms of
two (Ã and Á) enantiomers occupying the same site with one Ti (Ti1)
and one Cl (Cl1) in common. Careful selection of peaks from elec-
tron-density maps allowed all sites for all the non-H atoms to be
determined. Because of the disorder, we imposed several restraints
and re®ned as free variables the distances Csp3ÐCsp2, Csp2ÐCsp2,
Csp2ÐO, TiÐO(trans to O), TiÐO(trans to Cl) and TiÐCl. H atoms
were positioned on geometrical grounds and treated as riding atoms,
Ê
with CÐH distances of 0.93 (ring H) and 0.96 A (methyl H). The
methyl groups were each modelled using six H-atom sites, each with
occupancy 0.50, mutually offset by 60ꢁ. Examination of the structure
with PLATON (Spek, 2000) showed that there were no solvent-
accessible voids in the crystal lattice.
Data collection: CAD-4-PC (Enraf±Nonius, 1992); cell re®nement:
SET4 and CELDIM in CAD-4-PC; data reduction: DATRD2 in
NRCVAX96 (Gabe et al., 1989); program(s) used to solve structure:
SHELXS97 (Sheldrick, 1997); program(s) used to re®ne structure:
SHELXL97 (Sheldrick, 1997); molecular graphics: PLATON (Spek,
2000); software used to prepare material for publication: SHELXL97
and WordPerfect macro PREP8 (Ferguson, 1998).
Figure 2
The contents of a single molecular site in (I) showing the relative
orientations of the two enantiomers. All sites except Ti1 and Cl1 have
occupancy 0.50. Atoms are depicted as spheres whose radii are ranked
thus: Ti > Cl > O > C. H atoms have been omitted for the sake of clarity.
Experimental
Compound (I) was prepared by slow addition, under N2, of tita-
nium(IV) chloride to a threefold molar excess of pentane-2,4-dione in
sodium-dried toluene, followed by heating under re¯ux for 20 min.
After removal of the solvent under reduced pressure, crystals suitable
for single-crystal X-ray diffraction were grown by slow evaporation of
a solution in toluene.
Supplementary data for this paper are available from the IUCr electronic
archives (Reference: SK1440). Services for accessing these data are
described at the back of the journal.
References
Crystal data
Allen, F. H. & Kennard, O. (1993). Chem. Des. Autom. News, 8, 1,
31±37.
Bradley, D. C. & Holloway, C. E. (1969). J. Chem. Soc. A, pp. 282±285.
Dilthey, W. (1904). Chem. Ber. 37, 588±592.
Enraf±Nonius (1992). CAD-4-PC. Version 1.1. Enraf±Nonius, Delft, The
Netherlands.
Fay, R. C. & Lowry, R. N. (1967). Inorg. Chem. 6, 1512±1517.
Ferguson, G. (1998). PREP8. University of Guelph, Canada.
Gabe, E. J., Le Page, Y., Charland, J.-P., Lee, F. L. & White, P. S. (1989). J. Appl.
Cryst. 22, 384±387.
Glidewell, C., Turner, G. M. & Ferguson, G. (1996). Acta Cryst. C52,
11±14.
Marsh, R. E. (1999). Acta Cryst. B55, 931±936.
È
Matilainen, L., Mutikainen, I. & Leskela, M. (1996). Acta Chem. Scand. 50,
755±758.
Serpone, N. & Fay, R. C. (1967). Inorg. Chem. 6, 1835±1843.
È
Sheldrick, G. M. (1997). SHELXS97 and SHELXL97. University of Gott-
ingen, Germany.
Spek, A. L. (2000). PLATON. Utrecht University, The Netherlands.
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[Ti(C5H7O2)2Cl2]
Mr = 317.01
Triclinic, P1
Z = 2
Dx = 1.514 Mg m
Mo Kꢀ radiation
3
Ê
a = 7.764 (3) A
b = 7.8810 (15) A
Cell parameters from 25
re¯ections
Ê
ꢃ = 15.80±27.72ꢁ
Ê
c = 13.0470 (15) A
ꢀ = 79.507 (6)ꢁ
ꢁ = 78.790 (11)ꢁ
ꢂ = 63.308 (11)ꢁ
1
ꢄ = 0.998 mm
T = 293 (2) K
Block, orange
0.40 Â 0.20 Â 0.20 mm
3
Ê
V = 695.5 (3) A
Data collection
Nonius CAD-4 diffractometer
ꢃ/2ꢃ scans
Absorption correction: Gaussian
(ABSCOR in NRCVAX; Gabe
et al., 1989)
Tmin = 0.691, Tmax = 0.825
3012 measured re¯ections
3012 independent re¯ections
1283 re¯ections with I > 2ꢅ(I)
ꢃ
max = 26.97ꢁ
h = 9 ! 9
k = 9 ! 9
l = 0 ! 16
3 standard re¯ections
frequency: 120 min
intensity variation: 0.4%
ꢀ
Acta Cryst. (2001). C57, 264±265
Ferguson and Glidewell
[Ti(C5H7O2)2Cl2] 265