Evaluation of the relative importance of Ti-Cl···H-N hydrogen bonds and supramolecular interactions between perfiuorophenyl rings in the crystal structures of [Ti(NR)Cl2(NHMe2)2] (R = iPr, C6H5</

Article abstract of DOI:10.1039/b109251k

The three closely-related compounds [Ti-(NⁱPr)Cl₂(NHMe₂)₂] 1, [Ti(N₆H₅)Cl₂(NHMe₂)₂] 2 and [Ti(NC₆F₅)Cl₂(NHMe₂)

Full text of DOI:10.1039/b109251k

Evaluation of the relative importance of Ti–Cl…H–N hydrogen bonds
and supramolecular interactions between perfluorophenyl rings in the
i
crystal structures of [Ti(NR)Cl2(NHMe2)2] (R = Pr, C6H5 or C6F5)†
Nico Adams, Andrew R. Cowley, Stuart R. Dubberley, Andrew J. Sealey, Michael E. G. Skinner
and Philip Mountford*
Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, UK OX1 3QR.
E-mail: philip.mountford@chemistry.oxford.ac.uk
Received (in Cambridge, UK) 10th October 2001, Accepted 14th November 2001
First published as an Advance Article on the web 6th December 2001
i
The
three
closely-related
compounds
[Ti-
these compounds, namely where R = Pr 1, C6H5 2 or C6F5 3.†‡
The solid state molecular structures are fully consistent with the
solution 1H and 13C-{1H} NMR and solid state IR (Nujol mull)
spectra given as ESI.†
(NiPr)Cl2(NHMe2)2] 1, [Ti(NC6H5)Cl2(NHMe2)2] 2 and
[Ti(NC6F5)Cl2(NHMe2)2] 3 all crystallize in the space group
C2/c with the titanium atoms lying on two-fold axes at (0, y,
1/4); in compounds 1 and 2 the molecules are linked in one-
dimensional infinite chains by intermolecular Ti–Cl…H–N
hydrogen bonds along the direction of the crystallographic c
axis, whereas in 3 offset face-to-face interactions between the
C6F5 rings break down the hydrogen bonded chains.
i
The molecular structures of [Ti(NR)Cl2(NHMe2)2] (R = Pr
1, C6H5 2 or C6F5 3) are presented in Fig. 1.‡ All adopt
approximately trigonal bipyramidal geometries (equatorial NR
and Cl groups) with linear or near-linear TiNN–R linkages; the
intramolecular distances and angles in 1–3 are unexceptional in
comparison with other titanium imido complexes.9 Hydrogens
were placed in calculated positions (N–H 0.87 Å, C–H 1.00 Å).
The structures are approximately isomorphous, with all three
compounds crystallizing in the space group C2/c and the Ti
atoms lying on crystallograhpic two-fold axes (passing through
the TiNN bond) at (0, y, 1/4) or a symmetry-equivalent position.
While the molecular structures of 1, 2 and 3 are very similar,
their supramolecular structures differ significantly.
Molecules of [Ti(NiPr)Cl2(NHMe2)2] 1 form hydrogen
bonded chains in the solid state as shown in Fig. 2(a). Hydrogen
bonds form between the N–H groups of NHMe2 ligands and Ti–
Cl group hydrogen bond acceptors on neighbouring titanium
complexes, and propagate in the direction of the crystallo-
graphic c axis. The Ti–Cl…H–N contacts of 2.51 Å may within
error be considered ‘short’ according to Brammer and Orpen’s
classification10 based on a detailed analysis of crystallographic
data.9 Moving on to the phenylimido system
[Ti(NC6H5)Cl2(NHMe2)2] 2 [Fig. 2(b)] we encouter the same
hydrogen-bonded supramolecular arrangement with experi-
mentally comparable Ti–Cl…H–N contacts of 2.46 Å and
Cl…H–N angles (158° in 2 vs. 160° in 1). There are single C–
The importance of non-covalent interactions between hydro-
gen-substituted arene rings in supramolecular chemistry is now
well-recognised.1 Arene–perfluoroarene interactions represent
a special case and are, for instance, responsible for the 24 °C
melting point of the 1+1 benzene–perfluorobenzene complex
(while those of neat benzene and perfluorobenzene are 5.5 and
4 °C, respectively).2 Such mixed arene–perfluoroarene inter-
actions have recently attracted renewed attention in a range of
supramolecular contexts.3 More recently, reports of the possible
importance of supramolecular interactions between pairs of
perfluoroarene rings in organic and inorganic/organometallic
contexts have appeared.4 Hunter and Sanders were the first to
propose a simple model for the interpretation of intermolecular
p-interactions between aromatic molecules.5 Also recently,
Dance and coworkers6 reported density funtional calculations
on the gas-phase dimers (C6H6)2 and (C6F6)2 and concluded that
(i) intermolecular interactions between perfluorinated aromatic
rings are slightly more attractive than those of the hydro
analogues (due mainly to an increased van der Waals compo-
nent); (ii) calculated intermolecular potentials for offset face-to-
face (off) interactions between pairs of C6F6 rings are twice as
favourable as edge-to-face or vertex-to-face interactions; and
that (iii) there are no major differences between the supramo-
lecular embraces adopted by poly-phenyl and poly-fluoro-
phenyl systems. Despite these recent, promising indicators of
such supramolecular interactions, no clear-cut, ‘head-to-head’
study has been reported showing the relative importance of
fluoroarene–fluoroarene interactions in comparison with possi-
ble arene–arene or other (e.g. hydrogen bonded) supramolecular
motifs. Here we report the molecular and supramolecular
structures of a series of closely related compounds that further
demonstrate the importance of fluoroarene–fluoroarene inter-
actions in crystal engineering.
H
arene…C pairwise contacts (C–H…C = 3.17 Å) between rings
of one hydrogen-bonded chain of 2 and the neighbouring one.
However, these are considered to be secondary consequences of
the favoured Ti–Cl…H–N hydrogen bonded arrangement
established for 1 (where no arene–arene interactions are
possible). The a [16.913(1) vs. 16.260(1) Å] and c [12.044(1)
vs. 11.935(1)] unit cell lengths for 1 and 2 are fairly similar
while the b unit cell dimensions of 9.101(1) and 10.303(1) Å,
As part of our research programme in transition metal imido
chemistry7 we found that the reaction of a very wide range of
primary amines RNH2 (R = alkyl or aryl) with [TiCl2(NMe2)2]
in benzene leads to the highly air- and moisture-sensitive imido-
bis(dimethylamino) titanium complexes [Ti(NR)Cl2(NHMe2)2]
in good to excellent yields.8 In this communication we
focus on the molecular and supramolecular structures of three of
i
Fig. 1 Structures of [Ti(NR)Cl2(NHMe2)2] [ (a) R = Pr 1; (b) R = C6H5
† Electronic supplementary information (ESI) available: characterisation
and crystal data for compounds 1–3. See http://www.rsc.org/suppdata/cc/
b1/b109251k/
2 and (c) R = C6F5 3]. C-bound H atoms omitted; displacement ellipoids
drawn at the 25% probablility level; H atoms drawn as spheres of arbitrary
radius.
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Chem. Commun., 2001, 2738–2739
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b109251k

respectively, show the greatest difference (being the direction in
which the larger imido substitutent, iPr vs. C6H5, is oriented).
Turning now to the perfluorophenylimido complex
[Ti(NC6F5)Cl2(NHMe2)2] 3 [Fig. 3(a)] we see immediately that
the unit cell has considerably distorted to disrupt the hydrogen
bonded chain and arrange the neighbouring perfluorophenyl
rings in a close, offset face-to-face (off) arrangement. The Ti–
Cl…H–N distances of 2.93 Å (associated C…H–N angles =
136°) may now be classified10 as ‘long’, and are approximately
equal to the sum of the van der Waals radii for H (1.2 Å) and Cl
(1.75 Å).11 The solid state (Nujol mull) IR spectra of 1, 2 and 3
feature NHMe2 ligand n(N–H) stretches of 3228, 3220 and 3275
cm21, respectively, consistent with the variations in Cl…H–N
interactions determined by X-ray diffraction. While the unit cell
a length [15.696(3) Å] in 3 is comparable to those in 1 and 2
(unsurpringly since it is perpendicular to the direction of
propagation of the (formerly) hydrogen-bonded chain), there is
a substantial expansion in the b direction [unit cell length =
15.569(1) Å] and a concomitant large contraction in the c unit
cell length [6.7371(9) Å]. The separation between neighbouring
titanium centres is c/2 = 3.365 Å which is just slightly larger
than the interplanar separation of 3.23 Å between neighbouring
C6F5 rings.
Fig. 3(b) shows in projection the off arrangement of two
adjacent C6F5 rings. Each of the rings shown is involved in four
close contacts to the neighbouring ring, namely two with C…C
3.264(3) Å and two with F…C 3.256(4) Å. In the p-stacked
motif in crystals of 3 this gives each ring eight contacts in total
(four with each of its two neigbours). The C…C and C…F
contacts can be favourably compared to Dance and coworkers’
calculated values of 3.19 and 3.14 Å, respectively, for gas-phase
off-(C6F6)2 (being the most stable supramolecular arrangement
for this dimer).6
In summary, the crystal structures of the three compounds
1–3, when taken together, provide further evidence for the
comparatively strong driving force of supramolecular
C6F5…C6F5 p-stacking interactions in the solid state. Such
interactions appear to be at least as strong as other, well-
documented examples such as M–Cl…H–N–M hydrogen
bonding.10 We are continuing to investigate the supramolecular
and crystal engineering roles of these types of interactions
between C6F5 rings for both imido and non-imido systems.
This work was supported by the EPSRC, DSM Research and
Millenium Pharmaceuticals Ltd. We thank Professor Ian Dance
for disclosing some additional computational results.
Fig. 2 Portion of the hydrogen-bonded chains of [Ti(NR)Cl2(NHMe2)2] [(a)
R = Pr 1; (b) R = C6H5 2] with carbon-bound H atoms omitted and other
i
atoms drawn as spheres of arbitrary radius.
Notes and references
‡ CCDC reference numbers 172499–172501. See http://www.rsc.org/
suppdata/cc/b1/b109251k/ for crystallographic data in CIF or other
electronic format.
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Fig. 3 (a) Portion of the p-stacked chains of [Ti(NC6F5)Cl2(NHMe2)2] 3
with carbon-bound H atoms omitted. Intermolecular Ti–Cl…H–N distances
= 2.93 Å and Cl…H–N angle = 136°. (b) Relationship between the C6F5
groups bonded to N(1) and N(1C). Projected strictly onto the
{C(1),C(2),C(3),C(4),C(2B),C(3B)} least-squares plane. Separation be-
tween C6F5 planes = 3.23 Å.
11 A. J. Bondi, J. Chem. Phys., 1974, 68, 441.
Chem. Commun., 2001, 2738–2739
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