An organometallic complex revealing an unexpected, reversible, temperature induced SC-SC transformation

Article abstract of DOI:10.1039/c4ce00070f

A reversible, temperature driven phase transformation that takes place at ca. 180 K, in a single-crystal to single-crystal manner, has been observed for a monometallic transition metal coordination complex based on a fac-Re(CO) ₃ core, with a c

Full text of DOI:10.1039/c4ce00070f

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An organometallic complex revealing an unexpected,
reversible, temperature induced
Cite this: CrystEngComm, 2014, 16,
SC–SC transformation†
4641
*
Rupert G. D. Taylor, Benjamin R. Yeo, Andrew J. Hallett, Benson M. Kariuki
*
and Simon J. A. Pope
A reversible, temperature driven phase transformation that takes place at ca. 180 K, in a single-crystal to
single-crystal manner, has been observed for a monometallic transition metal coordination complex
based on a fac-Re(CO)3 core, with a chelated 2,2′-bipyridine unit and a halogenated N-(4-iodo-
phenyl)nicotinamide axial co-ligand. A range of closely related analogues that varied the halogen and its
position of substitution exemplified the rarity of such an observation: none of the eight structurally
related examples demonstrated such interesting behavior in the temperature range of study. The
structures of all the complexes also revealed that the trifluoromethanesulfonate counter anion plays an
important role through hydrogen bonding, but no isomorphism or polymorphism was observed. The
complexes are also fully characterised from a spectroscopic perspective, with IR, UV-vis, time-resolved
luminescence and NMR spectroscopies providing complementary solution-state data.
Received 10th January 2014,
Accepted 1st April 2014
DOI: 10.1039/c4ce00070f
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mechanisms. There has been a recent upsurge in the interest
Introduction
in SC–SC processes in metal–organic compounds3 with reports
Transformations in the solid state are associated with a vari-
ety of processes, including elimination or incorporation of
guest molecules, solid-state chemical reactions, and transi-
tions between polymorphs. In the crystalline state, chemical
reactions are generally highly specific and efficient. This
specificity and efficiency can be rationalised on the basis
of the topochemical principal, developed several decades ago
through the study of reactions in the solid state of organic
substances, and specifically the photochemistry of the cinnamic
acids.1 In addition to application in clean solvent-free syn-
thesis, solid state reactions are also potential synthetic routes
to new materials that are inaccessible by conventional solu-
tion methods.2
On rare occasions, transformation can proceed without
compromising crystal integrity i.e. in a single-crystal to single-
crystal (SC–SC) manner. This retention of three-dimensional
order often requires the structural difference between the
initial and final states to be small, and any steric barrier dur-
ing the process to be low. SC–SC processes continue to draw
interest because they can provide information about reaction
of processes driven by a variety of methods, including thermal,3a,i,p
photochemical,3g mechanochemical3a treatment, exposure to
vacuum3l conditions, and to gases3b,o as well as soaking3c,e,h
in solvents or solutions. The reported examples have predomi-
nantly involved incorporation or expulsion of reagents and
products into, or out of the crystal, including processes such as
solvation and desolvation,3d,j,l ligand and solvent exchange3b,k,m
as well as cation exchange.3c,o
Polymorphic phase transformation is not accompanied by
a change in the chemical composition or bonding and is the
result of a switch in the relative thermodynamic stabilities of
the polymorphs. The switch may be driven by a number of
factors, including changes in temperature and pressure as
well as mechanical treatment, and can often be accompanied
by the loss of single crystallinity. Polymorphic phase transfor-
mation represents a special situation where the delicate bal-
ance between interactions within the crystal can be perturbed
by a relatively small change in environmental conditions, and
therefore provides an opportunity to obtain some insight into
the subtle interplay between intra-crystalline interactions.4
During our routine structural characterisation of a lumi-
nescent organometallic rhenium(I) ligand complex, it was dis-
covered that this species (designated Re–p-I) displayed a very
rare and remarkable thermal phase transformation, occur-
ring in a SC–SC manner. The discovery of the transformation
was particularly interesting because polymorphism can be a
route to materials with different physical properties for a
School of Chemistry, Cardiff University, Park Place, Cardiff, UK.
E-mail: popesj@cardiff.ac.uk, kariukib@cardiff.ac.uk; Fax: +44 (0)29 208 74030;
Tel: +44 (0)29 208 74023
† Electronic supplementary information (ESI) available: Cif files and structural
characterisation of the ligands. CCDC 893632–893645. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c4ce00070f
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in physical behaviour to the structures. The aim was to gener-
ate isomorphous structures by exploitation of halogen–halogen
similarity6 and methyl–halogen interchangeability7 to explore
the effect of sterics, polarisability and electronic properties
of the ligand substituent on the thermal behaviour of the
solid material.
Ligands based on the substituted phenylnicotinamide were
prepared, namely, 2-fluoro, 2-bromo, 2-iodo, 4-flouro, 4-chloro,
4-bromo and 4-methyl derivatives (Scheme 1) and the corre-
sponding fac-[Re(CO)3bipy(L)]+ complexes synthesised. For con-
sistency, the same counter-anion was retained throughout and
the recrystallisation method was generally performed at room
temperature using the same solvent/anti-solvent combinations
(acetonitrile/diethyl ether). The exception was the methyl-
derivative, for which additional solvents were attempted, as
discussed later.
Scheme 1 General structure of the halogenated rhenium complexes:
Re–p-F R = H, R′ = F; Re–o-F R = F, R′ = H; Re–p-Cl R = H, R′ = Cl;
Re–o-Cl R = Cl, R′ = H; Re–p-Br R = H, R′ = Br; Re–o-Br R = Br, R′ = H;
Re–p-I R = H, R′ = I; Re–o-I R = I, R′ = H.
The nicotinamide-derived ligands were easily obtained by
reacting an excess (ca. 2 eq.) of freshly isolated nicotinoyl
chloride with a range of 2- (ortho) and 4-substituted (para)
aniline derivatives. Purification was achieved by repeated extrac-
tion of the crude reaction mixture with water thereby removing
unreacted nicotinoyl chloride as nicotinic acid. The ligands
(labeled o-X and p-X for the ortho- and para-substituted species)
were coordinated to rhenium using the convenient precursor
fac-[Re(CO)3bipy(MeCN)](OTf) in chloroform (at reflux) as sol-
vent. Typically reactions were completed within 24 hours.8
The resultant complexes fac-[Re(CO)3bipy(L)](OTf) are abbre-
viated to Re–o-X or Re–p-X in the subsequent discussions.
given molecular composition,5 as illustrated by the different
UV-vis absorption and emission properties of the two poly-
morphs of [Re2(μ-Cl)2(CO)6(μ-4,5-(Me3Si)2pyridazine)].3r
As phase transformations are the result of subtle shifts in
the balance of molecular interactions, one approach to inves-
tigating such phenomena would be to introduce structural
modifications to a system that demonstrates such a phase
transformation. Therefore this paper focuses on the synthesis
and characterization of a systematic series of structural ana-
logues (Scheme 1) of Re–p-I with the potential to display
comparable structural and thermal properties. The outcome
of this study revealed that despite this methodological ratio-
nale, no isomorphism or additional phase transformations
were discovered for the structural variants. The investigation
is therefore instructive as it highlights the inherent challenges
associated with engineering crystalline materials with specific
structural and physical properties. The paper firstly describes
the synthesis and structural characterisation of the complete
series of rhenium complexes and subsequently focuses on
the unique SC–SC transformation of the anomolous complex,
Re–p-I.
Spectroscopic characterisation of the complexes
The complexes were characterised in the solution state using
a variety of spectroscopies. IR spectroscopy confirmed the
geometric arrangement of the carbonyl groups at rhenium
(Table 1): typically two stretches were observed at around
2030 and 1920 cm−1, consistent with a pseudo C3v local sym-
metry at rhenium. An additional lower energy carbonyl stretch
(ca. 1700 cm−1) was observed that corresponds to the presence
1
of the amide carbonyl within the axial ligand. H NMR spec-
troscopic studies (CDCl3) confirmed the 1 : 1 stoichiometric
formulation of the complexes whilst revealing a downfield
shift of the proton resonances associated with the pyridine
moiety upon complexation with rhenium. The integrities of the
Results and discussion
The systematic study on a series of closely related complex
derivatives was initiated in an attempt to elucidate any trends
Table 1 IR, UV-vis and luminescence spectroscopic data for the complexes
Compound
CO νa/cm−1
1MLCT λabsb/nm
3MLCT λemc/nm
τd/ns
Re–p-F
Re–o-F
Re–p-Cl
Re–o-Cl
Re–p-Br
Re–o-Br
Re–p-I
2026, 1928
2026, 1928
2037, 1933
2038, 1933
2037, 1933
2037, 1929
2037, 1933
2026, 1900
2036, 1939
396 sh
358 sh
365 sh
392 sh
363 sh
358 sh
369 sh
370 sh
362 sh
549
542
538
548
542
542
545
542
543
324
361
321
333
359
351
348
334
339
Re–o-I
Re–p-Me
a
c
d
Solid. b CHCl3. CHCl3, λem = 370 nm. CHCl3, λem = 295 nm.
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complexes were also established using LR and HR MS methods,
each confirming the presence of the cationic complex moiety
and the 185/187Re isotopic distribution of the complexes.
one N–H⋯O hydrogen bond (labelled 1 in Fig. 2b) with dis-
tances (H⋯O = 2.10 Å, N⋯O = 2.88(2) Å, and angle N–H⋯O =
147.3°). In the N-(4-chlorophenyl)nicotinamide ligand the
angle between the planes of chlorophenyl and pyridyl groups
is 40.2(9)°. Some π–π interaction is observed between the
bipyridyl rings of neighbouring cations (centroid–centroid
separation of 4.04(2) Å), but the closest centroid–centroid con-
tact between the chlorophenyl rings is 4.30(2) Å (labelled 2 in
Fig. 2b), with minimal ring overlap.
The crystal structure of Re–p-Br (Fig. 3a) is monoclinic
and contains two crystallographically unique complex cations
and two triflate counterions. The angles between the planes
of bromophenyl and pyridyl groups in the two independent
N-(4-bromophenyl)nicotinamide ligands are 50.5(6)° and 58.2(6)°,
respectively. Triflate anions are accommodated in pairs within
the crystal structure, with each ion accepting one amide hydro-
gen bond. The bonds are labelled 1a (H1⋯O10 = 2.18 Å,
N1⋯O10 = 2.99(1) Å, N–H⋯O = 152.1°) and 1b (H5a⋯O12 =
2.13 Å, N5⋯O12 = 2.96(1) Å, N–H⋯O = 157.8°) in Fig. 3b.
The closest π–π contact (labelled 2 in Fig. 3b) involves two
independent bromophenyl rings, which are not parallel (the
inter-planar angle is 26.6(6)°) and have a centroid-to-centroid
distance of 4.23(1) Å.
Unlike the other structures of complexes presented in this
study, the crystals of the 4-methyl derivative, Re–p-Me, were
obtained only in solvated form. Initial recrystallisation using
the standard conditions (room temperature and acetonitrile/
diethyl ether) produced Re–p-Me-A. Subsequent attempts using
other solvents did not produce usable crystals except for a second
solvated form, Re–p-Me-B, from chloroform/diethyl ether.
For Re–p-Me-A (Fig. 4a) the crystal structure is monoclinic
and contains two crystallographically unique complex cations
and two triflate ions. The conformations of the N-(4-methyl-
phenyl)nicotinamide ligands in the two independent complex
units are similar, as evidenced by the angles between the
planes of methylphenyl and pyridyl groups (25.(2)° and
26.(2)°). The structure contains ordered acetonitrile solvent
molecules, which, together with the anions and cations,
create channels around sites occupied by disordered water
molecules as well as voids in which no solvent has been
located. The two independent triflate anions are isolated from
each other, with just one participating in hydrogen bonding
(Fig. 4b). The anion accepts two N–H⋯O hydrogen bonds
from amide groups of ligands of two neighbouring indepen-
dent cations; the hydrogen bonds are labelled 1a (H1⋯O9 =
2.12 Å, N1⋯O9 = 2.97(2) Å, N1–H1⋯O9 = 162.4°) and 1b
(H5a⋯O10 = 2.23 Å, N5⋯O10 = 3.08(2) Å, N5–H5a⋯O10 =
162°) in Fig. 4b. The planes of the two ligands hydrogen
bonded to one triflate are roughly parallel to each other, with
ring centroid separations of 3.63(2) Å and 3.72(2) Å (labelled
2a and 2b, respectively, in Fig. 4b).
The electronic properties of the complexes (Table 1) were
assessed in chloroform solution and revealed characteristic
features common to this class of complex. The pale yellow
1
colour of the complexes can be attributed to a MReLbpyCT
transition that broadly absorbs around 330 nm, tailing to
400 nm. Ligand-centred absorptions dominate to higher energy
(<320 nm) with 1π–π* contributions from both the 2,2′-bipyridine
and the aryl axial ligand. Each of the complexes possessed
classical 3MLCT emission: a broad, featureless emission
dominating at around 539–549 nm, with only very minor
differences observed through structural variations of the
axial ligand.9 The corresponding luminescence lifetimes
confirmed the phosphorescent nature of the emission, typical
of cationic complexes of this type.8a,g The lifetime data suggest
that the presence of the heavier halide atoms in the axial
nicotinamide does not impart any excited state quenching.
Structural characterisation of the ligands
A number of ligands yielded crystals (Fig. S1, ESI†) suitable
for crystal structure determination during the course of the
study and the structures are presented in the ESI.† Selected
crystallographic data are presented in Table S1 ESI.†
The structures suggest a tendency towards planarity accom-
panied by pyridyl ring participation in hydrogen bonding of
the p-substituted ligands. Unsurprisingly, the o-substituted
ligands are less planar due to steric effects and favour hydro-
gen bonding between amide groups.
Structural characterisation of the complexes
As the purpose of the discussion is to compare the general
characteristics of a range of structures, the following analysis
is restricted mainly to conformational, hydrogen bonding
and π–π interaction properties, rather than a detailed description
of each structure.
The para-substituted complexes, Re–p-X
Selected crystallographic data are presented in Table 2. The
ReI coordination geometry in the monoclinic structure of
Re–p-F (Fig. 1a) is distorted octahedral. Pairs of triflate anions
occupy pockets in the structure with each triflate anion
accepting one N–H⋯O hydrogen bond (labelled 1 in Fig. 1b)
from the ligand amide group, with distances H⋯O = 2.11 Å,
N⋯O = 2.96(2) Å, and angle N–H⋯O = 161.1°. The N-(4-
fluorophenyl)nicotinamide ligand is almost planar with an
angle of 8.6° between the planes of the fluorophenyl and pyridyl
groups. In the structure, π–π type interactions occur between
the fluorophenyl rings and the bipyridyl rings of neighbouring
cations (labelled 2 in Fig. 1b, with a centroid-to-centroid
separation of 3.70(2) Å) to form chains parallel to the a-axis.
The triclinic structure of Re–p-Cl (Fig. 2) also has distorted
octahedral coordination geometry for ReI. In the structure,
triflate anions also occur in pairs, with each anion accepting
The second solvate, Re–p-Me-B (Fig. 4c) is also monoclinic
and contains two crystallographically unique complex cations
and two triflate ions. The conformations of the two indepen-
dent N-(4-methylphenyl)nicotinamide ligands in the complex
units are also fairly similar, with angles of 26(1)° and 24(1)°
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Fig. 1 (a) The structure of the 4-fluoro complex, Re–p-F, and (b) a
segment of the structure. Hydrogen atoms have been omitted for clarity.
Fig. 3 (a) The structure of the 4-bromo complex, Re–p-Br, and (b) a
segment of the structure. Hydrogen atoms have been omitted for clarity.
other type by the second triflate ion which is not involved in
hydrogen bonding. The second triflate occupies a site dis-
playing two-fold disorder (occupancy: 0.59/0.41), in contrast
to Re–p-Me-A in which both anions are ordered.
The crystal structures Re–p-Me-A and Re–p-Me-B both
contain the same number of anions and cations and consist
of channels. However, they are not polymorphs in the true
sense as they incorporate additional solvent and/or water
molecules in different proportions. Despite the differences
in solvent incorporation, both structures contain a common
motif involving one triflate anion and two complex cations
(Fig. 4b and d), indicating that the hydrogen bonding,
π–π, electrostatic and other interactions involved generate a
robust unit.
Fig. 2 (a) The structure of the 4-chloro complex, Re–p-Cl, and (b) a
segment of the structure. Hydrogen atoms have been omitted for clarity.
The ortho-substituted complexes, Re–o-X
Selected crystallographic data are presented in Table 2. The
crystal structure of Re–o-F (Fig. 5a) is triclinic, with the
2-fluorophenyl group of the N-(2-fluorophenyl)nicotinamide
ligand showing disorder between two orientations (0.83/0.17
occupancy) related roughly by a two-fold rotation about the
N–C bond. For the major component, the angle between
the planes of fluorophenyl and pyridyl groups is 76.24°. Pairs
of triflate anions occupy pockets in the structure and one
O atom of every triflate ion accepts two amide N–H⋯O
hydrogen bonds from ligands of two complexes. Each ligand
is involved in bifurcated hydrogen bonding as shown in
Fig. 5b (labelled 1a (H⋯O = 2.29 Å, N⋯O = 3.01(1) Å,
N–H⋯O = 139.7°) and 1b (H⋯O = 2.56 Å, N⋯O = 3.25(1) Å,
and angle N–H⋯O = 136.1°)). The closest π–π contact involves
nonparallel fluorophenyl and bipyridyl rings separated by a
between the planes of methylphenyl and pyridyl groups. In
a similar manner to Re–p-Me-A, the two triflate anions are
isolated from each other, with just one participating in
hydrogen bonding (Fig. 4d) by accepting two N–H⋯O hydro-
gen bonds from two independent ligands – labelled 1a
(H1⋯O9 = 2.05 Å, N1⋯O9 = 2.90(4) Å, N1–H1⋯O9 = 162°) and
1b (H5a⋯O11 = 2.22 Å, N5⋯O11 = 3.02(4) Å, N5–H5a⋯O11 =
151°) in Fig. 4d. The two ligands involved in hydrogen bond-
ing to one triflate are also roughly parallel to each other, with
ring centroid separations of 3.71(4) Å and 3.61(4) Å (labelled
2a and 2b, respectively, in Fig. 4d).
The structure can be visualised as consisting of two
types of channels running parallel to the a-axis (Fig. 4a) with
one type occupied by disordered water molecules and the
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Fig. 5 (a) Crystal packing in the 2-fluoro substituted complex (Re–o-F)
and (b) a segment of the structure. Hydrogen atoms have been omit-
ted for clarity.
The contact between pyridyl rings is longer (4.46(2) Å, labelled
2b in Fig. 6c).
The structure of the iodo-analogue, Re–o-I, is triclinic
(Fig. 7a). The angle between the planes of iodophenyl and
pyridyl groups in the N-(2-iodophenyl)nicotinamide ligand is
87.9(5)°. Unlike in the structure of Re–o-Br, individual triflate
ions are isolated from each other and just one triflate O atom is
involved in N–H⋯O hydrogen bonding (labelled 1 in Fig. 7b,
H⋯O = 2.05 Å, N⋯O = 2.91(1) Å, N–H⋯O = 162.5°). Contacts
of π–π type are observed between the bipyridine rings of
neighbouring units with centroid-to-centroid separation dis-
tances of 3.89(1) Å (labelled 2a in Fig. 8b) and 4.44(1) Å
(labeled 2b in Fig. 7b). The closest contact between N-(2-iodo-
phenyl)nicotinamide ligands involves pyridyl rings with a dis-
tance of 4.49(1) Å (labelled 2b in Fig. 7c). Crystal packing in
Re–o-Br (Fig. 6a) and Re–o-I (Fig. 7a) indicates that the chloro
and bromo substituents are not directly interchangeable in
these complexes although local similarities do exist, as illus-
trated in Fig. 6b, c and 7b, c.
Fig. 4 (a) Crystal packing in the 4-methyl substituted complex (Re–p-Me-A)
and (b) a segment of the structure. (c) Crystal packing in 4-methyl
substituted complex (Re–p-Me-B) and (d) a segment of the structure.
Hydrogen atoms have been omitted for clarity.
centroid-to-centroid distance of 3.96(2) Å (labelled 2 in Fig. 5b)
with an interplanar angle of 25.1(9)°.
Structural characterisation of Re–p-I
The Re–o-Br crystal structure (Fig. 6a) is monoclinic. The
angle between the planes of bromophenyl and pyridyl groups in
the N-(2-bromophenyl)nicotinamide ligand is 46.3(7)°. Pairs of
triflate ions occupy spaces in the structure and just one triflate
O atom is involved in N–H⋯O hydrogen bonding (labelled 1 in
Fig. 6b, H⋯O = 2.21 Å, N⋯O = 2.87(1) Å, N–H⋯O = 131.4°).
The closest π–π contact involves non-parallel bipyridine rings
separated by a centroid-to-centroid distance of 4.21(2) Å
(labelled 2a in Fig. 6b) with an interplanar angle of 18.1(7)°.
During the structural characterisation of the 4-iodo-substituted
complex, Re–p-I, different unit cells were obtained at 294 K
and 150 K and full diffraction data were collected at these
temperatures in order to determine the crystal structures of
the high temperature (Re–p-I-HT) and low temperature forms
(Re–p-I-LT) of the complex. Both crystal structures are mono-
clinic forms of Re–p-I. Selected crystallographic data are
presented in Table 2.
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Fig. 6 (a) Crystal packing in the 2-bromo substituted complex (Re–o-Br)
and (b), (c) segments of the structure. Hydrogen atoms have been omit-
ted for clarity.
Fig. 7 (a) Crystal packing in the 2-iodo substituted complex (Re–o-I)
and (b), (c) segments of the structure. Hydrogen atoms have been
omitted for clarity.
A description of the crystal structures separately highlight-
ing common structural characteristics will be followed by a
discussion of the changes that accompany a phase transition
between them.
I⋯O = 3.09(2) Å, angle C–I⋯O = 170.4(9)°), an example of a
halogen bond.10 The N-(4-iodophenyl)nicotinamide ligand is
not planar, as indicated by the angle between the planes of
the iodophenyl and pyridyl groups (20.2(9)°). π–π type interac-
tions also occur in the structure between the iodophenyl ring
of a complex unit and the bipyridine ligand of a neighbouring
complex (labelled 2 in Fig. 8b). The centroid-to-centroid sepa-
ration for the rings involved is 3.86(2) Å (for the major compo-
nent of the iodophenyl ring) and with an angle of 14.3(9)°
between the planes of the rings.
The high temperature form, Re–p-I-HT
In the structure (Fig. 8a) the cation consists of a ReI coordi-
nated by three carbonyl groups, one bidentate 2,2′-bipyridine
ligand and one N-(4-iodophenyl)nicotinamide ligand. The coor-
dination geometry of Re is distorted octahedral (N(2)–Re(1) =
2.226(9) Å, N(3)–Re(1) = 2.181(9) Å, N(4)–Re(1) = 2.157(8) Å,
C(23)–Re(1) = 1.911(12) Å, C(24)–Re(1) = 1.959(12) Å, C(25)–Re(1) =
1.905(13) Å), the longest contact being to the nicotinamide ligand.
The arrangement of complex cations in the crystal struc-
ture creates pockets in the lattice in which triflate anions are
accommodated in pairs. The CF3 and SO3 groups in the triflate
ion are in a staggered conformation. One of the triflate oxygen
atoms accepts an amide N–H⋯O hydrogen bond (labelled 1 in
Fig. 8b, with distances H⋯O = 2.10 Å, N⋯O = 2.94(2) Å, and
angle N–H⋯O = 160.3°). Another triflate oxygen is involved in
a short C–I⋯O interaction (labelled 3 in Fig. 8b, with distance
The low temperature form, Re–p-I-LT
The coordination geometry of the ReI ion is closely comparable
to that in Re–p-I-HT (N(2)–Re(1) = 2.237(10) Å, N(3)–Re(1) =
2.181(11) Å, N(4)–Re(1) = 2.162(10) Å, C(23)–Re(1) = 1.931(14) Å,
C(24)–Re(1) = 1.935(16) Å, C(25)–Re(1) = 1.923(14) Å). Simi-
larly to Re–p-I-HT, one triflate oxygen accepts an N–H⋯O
hydrogen bond (labelled 1 in Fig. 8d, with distances H⋯O =
2.15 Å, N⋯O = 2.95(2) Å, and angle N–H⋯O = 151°) and
another is involved in a short C–I⋯O interaction (labelled 3
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between Re–p-I-HT and Re–p-I-LT. It is therefore unsurprising
that transformation between the two forms has been observed.
Indeed, the process takes place in a SC–SC manner and the
exact same crystal was used to determine the two structures
discussed above.
Determination of the unit cell parameters at 20–50 K inter-
vals between 340 K and 105 K indicated that the transforma-
tion occurred between 200 K and 180 K; additional data at 5 K
intervals within this range narrowed the transition tempera-
ture to 180–185 K. The cell lengths change as would be
expected on transformation of the unit cell (Fig. 9a), but more
informative is the sudden change in the cell volume clearly
indicating a phase transformation (Fig. 9b). Upon cooling, the
transition results in a ca. 4% decrease in volume with a corre-
sponding increase in density (from 2.024 g cm−3 to 2.114 g cm−3).
Additionally, recycling between high and low temperatures
showed that the process is reversible for this material.
This indicates that the free energy curves of the polymorphs
cross before the melting points and hence the process is
enantiomorphic.
The retention of crystal integrity during the phase trans-
formation is an indication: (i) that high steric barriers do not
need to be overcome during transformation; or (ii) that the
Fig. 8 (a) Crystal packing in the high temperature form of the 4-iodo
substituted complex (Re–p-I-HT) and (b) a segment of the structure.
(c) Crystal packing in the low temperature form of the 4-iodo
substituted complex (Re–p-I-LT) and (d) a segment of the structure.
Hydrogen atoms have been omitted for clarity.
in Fig. 8d, with distance I⋯O = 3.21(2) Å, angle C–I⋯O =
162.7(9)°). The angle between the planes of the iodophenyl
and pyridyl groups in the N-(4-iodophenyl)nicotinamide
ligand is 60(1)°. The centroid-to-centroid distance between
the iodophenyl ring and the bipyridine rings involved in π–π
interaction (labelled 2 in Fig. 8d) is 4.23(2) Å and the angle
between the planes of the rings is 10.8(9)°.
Transformation of Re–p-I-HT to Re–p-I-LT
Fig. 9 Plot showing the changes in (a) unit cell parameters [blue crosses:
a-axis, red boxes: b-axis, black triangles: c-axis], and (b) unit cell volume
with temperature.
The discussion of the crystal structures above and particu-
larly an examination of Fig. 8 reveal a striking similarity
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material is flexible enough to accommodate large intracrystal
displacements. In the case of Re–p-I-HT and Re–p-I-LT, a
comparison of the structures indicates similarities consistent
with relatively small structural changes. The high temperature
unit cell can be roughly transformed to the low temperature
cell by the matrix [0.934 0 0.065, 0 1.084 0, 0 0 0.933], but
structural transformation involves both molecular reorientation
and conformational change.
Intramolecularly, as would be expected, the most notable
changes involve the more flexible N-(4-iodophenyl)nicotinamide
co-ligand. We consider the ligand as consisting of three planes:
the pyridyl group, the amide link and the iodobenzene group.
Fig. 8b and d) is retained on transformation but lengthens
from 3.09(2) Å to 3.21(2) Å.
The phase transition behaviour of Re–p-I is the result of
mutual influence of many factors as there is no single obvi-
ous structural feature to which the process can be attributed.
Experiments using the rest of the complexes studies did not
reveal any transformation over the same temperature range,
confirming that the enantiotropic transformation of the Re–p-I
is an example of an extremely rare reversible, temperature
driven phase transformation for a mononuclear organometallic
coordination complex.
On transformation, the orientation of the pyridyl group remains Experimental
the same (i.e. minimal rotation about the Re–N bond). The
amide link rotates by ca. 6° from the plane of the pyridyl
General information
All reactions were performed with the use of vacuum line and
group and the iodobenzene group rotates by a further 34°.
The changes in intramolecular geometry are accompanied by
relatively small, but significant, adjustments in intermolecular
contacts. An example is illustrated in Fig. 10. Interactions
between the iodobenzene and pyridyl rings of adjacent mole-
cules are labelled 1 for Re–p-I-HT, (Fig. 10a) and Re–p-I-LT.
(Fig. 10b). For interaction 1, an increase in the ring centroid–
centroid distance from 4.26(2) Å to 4.52(2) Å is observed
on transformation from the high temperature to the low-
temperature form. The increase in separation is accompanied
by an increase in the angle between the planes of the two
rings, from ca. 20° to ca. 60°.
Interaction 2 in Fig. 10(a) and (b) involves the iodo-
benzene ring and a bipyridine ring of a neighbouring
molecule. On transformation from Re–p-I-HT to Re–p-I-LT
the centroid–centroid distance also increases from 3.86(2) Å
to 4.23(2) Å but the interplanar angle decreases slightly
from 14.3(9) to 10.8(9)°. The short I⋯O contact (3 in
Schlenk techniques. Reagents were commercial grade and
1
were used without further purification. H and 13C-{1H} NMR
spectra were run on a NMR-FT Bruker 400 and 250 MHz spec-
trometers and recorded in MeOD or CDCl3. NMR chemical
shifts (δ) were determined relative to internal Si(CH3)4 and
are given in ppm. Low-resolution mass spectra were obtained
by the staff at Cardiff University. High-resolution mass spec-
tra were carried out by the EPSRC National Mass Spectrometry
Service at Swansea University. Infrared spectra were run on a
Perkin-Elmer FT-1600 spectrophotometer as CHCl3 solutions.
UV-vis studies were performed on a Jasco V-570 spectropho-
tometer as CHCl3 solutions (ca. 10−5 M−1). Photophysical data
were obtained on a JobinYvon-Horiba Fluorolog spectrometer
fitted with a JY TBX picosecond photodetection module as
CHCl3 solutions. Emission spectra were uncorrected and exci-
tation spectra were instrument corrected. The pulsed source
was a Nano-LED configured for 372 nm output operating at
500 kHz. Luminescence lifetime profiles were obtained using
the JobinYvon-Horiba FluoroHub single photon counting
module and the data fits yielded the lifetime values using the
provided DAS6 deconvolution software.
For crystal structure determination, diffraction data
were collected on a Nonius KappaCCD diffractometer using
graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) and
equipped with an Oxford Cryostream cooler. The structures
were solved by direct methods and refined using SHELX-97.11
Generally, all non-H atoms were refined anisotropically and a
riding model was used for hydrogen atoms. Crystallographic
data are shown in Tables 2, 3 and S1.† The iodobenzene ring
in Re–p-I-HT was refined using a disordered model with
major/minor components of occupancy 0.67(2)/0.33(2). The
crystals of Re–p-Me-A and Re–p-Me-B were thin needles,
which gave weak data and low data-to-parameter ratios. Con-
tributory factors include large asymmetric units and disor-
dered solvent molecules necessitating parameter restraints
during refinement.
Materials and procedures
Other than N-(4-iodophenyl)nicotinamide ( p-I), each of the ligands
[N-( p-tolyl)nicotinamide ( p-Me);12 N-(2-chlorophenyl)nicotinamide
Fig. 10 Segments of the structures of (a) Re–p-I-HT (major component)
and (b) Re–p-I-LT.
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CrystEngComm
Table 3 Crystal data collection and refinement details for the o-substituted complexes
Re–o-F
Re–o-Br
Re–o-I
C26H17F4N4O7ReS
791.70
C26H17BrF3N4O7ReS
852.61
C26H17F3IN4O7ReS
899.60
FW
T/K
210(2)
150(2)
150(2)
λ/Å
0.71073
0.71073
Monoclinic
P21/c
12.2430(3)
12.0173(2)
19.2931(4)
90
101.549(1)
90
2781.08(10)
4
2.036
0.40 × 0.12 × 0.12
10 634
6359
0.0345
0.71073
System
Space grp
a/Å
b/Å
c/Å
Triclinic
Triclinic
¯
¯
P1
P1
8.5187(3)
11.9891(4)
13.4341(4)
77.040(2)
89.004(2)
84.019(2)
1329.79(8)
2
1.977
0.30 × 0.25 × 0.03
8828
6007
0.0411
10.0523(2)
11.6515(4)
13.0988(4)
69.740(1)
78.794(2)
82.263(2)
1408.05(7)
2
2.122
0.25 × 0.15 × 0.10
9805
6443
0.0240
α/°
β/°
γ/°
V/Å3
Z
σcalc/Mg m−3
Size/mm3
Reflections
Unique
R(int)
Gof
1.043
1.054
1.046
R1 [I > 2σ(I)]
wR2
0.0403
0.0970
0.0410
0.0933
0.0324
0.0716
(o-Cl);13 N-(4-bromophenyl)nicotinamide ( p-Br);14 N-(4-fluoro-
phenyl)nicotinamide ( p-F);12b N-(2-chlorophenyl)nicotinamide
(o-Cl);15 N-(2-bromophenyl)nicotinamide (o-Br);16 N-(2-iodo-
phenyl)nicotinamide (o-I);17 N-(2-fluorophenyl)nicotinamide
(o-F)12b,18] have been previously reported.
The N-(halophenyl)nicotinamide (0.111 mmol) and chloroform
(10 mL) were added to the reaction flask and flushed with
nitrogen. The reaction was heated at 60 °C for 24 h. A yellow
solution was afforded; solvent was then removed and the
residue dried in vacuo. The title product was afforded in
powdered form via re-precipitation from CHCl3–Et2O to give
the title complex in each case.
A
typical synthesis for the ligands: preparation of
N-(4-iodophenyl)nicotinamide ( p-I). Nicotinic acid (1.00 g,
8.12 mmol) in CHCl3 (5 ml) was added to a round bottom
flask fitted with a condenser and flushed with nitrogen. NEt3
(1.25 ml, 8.98 mmol) was added, followed by dropwise addition
of SOCl2 (2.5 ml, 34.4 mmol). The reaction mixture was stirred
under nitrogen at 60 °C. After 24 h a red solution was afforded,
which was dried in vacuo (removing excess SOCl2, NEt3 and HCl)
to give nicotinoyl chloride (1.12 g, 97%) as a red/brown solid
which was used without further purification. 4-Iodoaniline
(2.01 g, 9.18 mmol), Et2NiPr (4 ml, 23.0 mmol) and MeCN
(10 ml) were added to a round bottom flask containing nicotinoyl
chloride (1.12 g, 7.91 mmol) fitted with a condenser, and then
flushed with nitrogen. The dark red-brown solution was stirred
at 60 °C. After 24 h the solution was dried in vacuo (removing
MeCN and excess Et2NiPr), dissolved in CH2Cl2 (15 ml), washed
with water (3× 15 ml), dried over MgSO4, filtered and dried in vacuo.
Further purification was achieved using column chromatogra-
phy (silica : CH2Cl2). Unreacted 4-iodoaniline was firstly eluted
with CH2Cl2, the pure product eluted with CH2Cl2/MeOH (95: 5)
to give N-(4-iodophenyl)nicotinamide (yield: 1.95 g, 74%) as a
Synthesis
of
fac-[Re(CO)3(2,2′-bipyridine)(N-(4-iodo-
phenyl)nicotinamide)][OTf ] (Re–p-I). Obtained as a yellow
1
solid (yield: 89%). H NMR (CDCl3, 250 MHz, 298 K) δH: 7.30
(1H, m), 7.40 (2H, d, 3JHH = 8.8 Hz), 7.51 (2H, d, 3JHH = 8.0 Hz),
3
7.59 (1H, d, JHH = 8.3 Hz), 7.69 (2H, m), 8.15 (2H, m), 8.21
(1H, d, 3JHH = 5.4 Hz), 8.26 (1H, d, 3JHH = 8.0 Hz), 8.33 (3H, m,
3
ArH, NH), 9.10 (2H, d, JHH = 6.3 Hz), 9.67 (1H, s) ppm.
IR (CHCl3) νmax: 2037, 1933, 1680 cm−1. UV-vis (CH3Cl) λmax
(ε = M−1 cm−1): 251 (30 400), 279 (30 800), 369 sh (10 300) nm.
Emission (CHCl3) λem (τ/ns): 545 (348) nm. ESMS m/z: calcu-
lated 750.4, found 750.9 [M − OTf ]+; HRMS m/z: calculated
748.9819, found 748.9822 [C25H17IN4O4185Re]+.
Synthesis
of
fac-[Re(CO)3(2,2′-bipyridine)(N-(2-iodo-
phenyl)nicotinamide)][OTf ] (Re–o-I). Obtained as a yellow
1
solid (yield: 89%). H NMR (CDCl3, 250 MHz, 298 K) δH: 6.91
(1H, d, 3JHH = 8.9 Hz), 7.31 (1H, d), 7.49 (1H, d, 3JHH = 8.9 Hz),
7.6–7.9 (3H, m), 7.9–8.3 (4H, m), 8.4–8.6 (3H, m), 8.71
(1H, br s), 9.0–9.2 (2H, overlapping m) ppm. IR (CHCl3) νmax
:
2026, 1928, 1900, 1681 cm−1. UV-vis (CH3Cl) λmax (ε = M−1 cm−1):
250 (25 600), 273 (23600), 370 sh (6300) nm. Emission (CHCl3)
λem (τ/ns): 542 (334) nm. ESMS m/z: calculated 751.0, found
751.0 [M − OTf ]+; HRMS: m/z calculated 748.9819, found
748.9817 [C25H17IN4O4185Re]+.
1
light brown solid. H NMR (CDCl3, 400 MHz, 298 K) δH: 7.38
(3H, m), 7.63 (2H, d, 3JHH = 8.8 Hz), 7.86 (1H, s, NH), 8.13 (1H, m),
3
8.72 (1H, d, JHH = 3.2 Hz), 9.01 (1H, s) ppm. IR (CHCl3) νmax
1685, 1590, 1514, 1488 cm−1. EIMS m/z: found 324.0 [M]+.
:
Synthesis
of
fac-[Re(CO)3(2,2′-bipyridine)(N-(4-fluoro-
General method for the synthesis of the complexes
phenyl)nicotinamide)][OTf ] (Re–p-F). Obtained as a yellow
1
Typically fac-[Re(CO)3(2,2′-bipyridine)(CH3CN)][OTf ] (0.0993 mmol)
was added to a round bottom flask fitted with a condenser.
solid (yield: 81%). H NMR (CDCl3, 250 MHz, 298 K) δH: 7.30
(1H, m), 7.40 (2H, d, 3JHH = 8.1 Hz), 7.51 (2H, d, 3JHH = 8.0 Hz),
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3
3
7.59 (1H, d, JHH = 7.9 Hz), 7.69 (2H, m), 8.15 (2H, m), 8.21
(1H, br s), 9.11 (2H, d, JHH = 8.2 Hz) ppm. IR (CHCl3)
νmax: 2037, 1929 cm−1. UV-vis (CHCl3) λmax (ε = M−1 cm−1): 250
(29 600), 273 (29 600), 322 (16 300), 358 sh (6600) nm. Emis-
sion (CHCl3) λem (τ/ns): 542 (351) nm. ESMS m/z: calculated
703.0, found 703.0 [M − OTf ]+; HRMS m/z: calculated
700.9957, found 700.9956 [C25H17O4N4Br185Re]+.
3
3
(1H, d, JHH = 5.4 Hz), 8.26 (1H, d, JHH = 8.0 Hz), 8.30–8.37
(3H, m), 9.10 (2H, d, JHH = 6.3 Hz), 9.67 (1H, s) ppm. IR
3
(CHCl3) νmax: 2026, 1928, 1899, 1682 cm−1. UV-vis (CH3Cl) λmax
(ε = M−1 cm−1): 244 (11 050), 292 (10 000), 396 (1600) nm. Emis-
sion (CHCl3) λem (τ/ns): 549 (324) nm. ESMS m/z: calculated
643.1, found 643.1 [M − OTf ]+; HRMS m/z: calculated 641.0758,
found 641.0751 [C25H17O4N4F185Re]+.
Synthesis
of
fac-[Re(CO)3(2,2′-bipyridine)(N-( p-
tolyl)nicotinamide)][OTf] (Re–p-Me). Obtained as a yellow
1
Synthesis
of
fac-[Re(CO)3(2,2′-bipyridine)(N-(2-fluoro-
solid (yield: 74%). H NMR (CDCl3, 250 MHz, 298 K) δH: 2.23
3
phenyl)nicotinamide)][OTf ] (Re–o-F). Obtained as a yellow
solid (yield: 85%). H NMR (CDCl3, 250 MHz, 298 K) δH: 7.03
(2H, d, JHH = 7.92 Hz), 7.33 (1H, br s), 7.47 (2H, d, JHH
8. 0 Hz), 7.69 (2H, br s), 8.1–8.5 (7H, m), 9.03 (1H, d, JHH
5.1 Hz), 9.11 (2H, d, JHH = 5.1 Hz) ppm. IR (CHCl3)
νmax: 2026, 1928, 1900, 1681 cm−1. UV-vis (CH3CN) λmax (ε =
M−1 cm−1): 249 (28 200), 275 (28 600), 358 sh (4600) nm. Emis-
sion (CHCl3) λem (τ/ns): 542 (341) nm. ESMS (ES) m/z: calcu-
lated 643.1, found 643.1 [M − OTf ]+; HRMS m/z: calculated
641.0758, found 641.0754 [C25H17O4N4F185Re]+.
(3H, s, CH3), 7.03 (2H, d, JHH = 7.9 Hz), 7.28 (1H, br s), 7.47
1
3
3
(2H, d, JHH = 8.5 Hz), 7.67 (2H, t, JHH = 6.55 Hz), 8.1–8.4
(8H, m), 9.08 (2H, d, JHH = 5.1 Hz), 9.44 (1H, br s) ppm.
3
3
3
=
=
IR (CHCl3) νmax: 2036, 1939 cm−1. UV-vis (CHCl3) λmax (ε =
M−1 cm−1): 247 (25 900), 277 (26 200), 322 (15 200), 362 sh
(6700) nm. Emission (CHCl3) λem (τ/ns): 543 (339) nm. ESMS
m/z: calculated 639.1, found 639.1 [M − OTf ]+. HRMS m/z:
calculated 637.1009, found 637.1006 [C26H20N4O4185Re]+.
3
3
Conclusions
Synthesis
of
fac-[Re(CO)3(2,2′-bipyridine)(N-(4-chloro-
A systematic structural survey of a series of monometallic
complexes (twelve examples in total) based on the core for-
mulation fac-[Re(CO)3bipy(L)](OTf) revealed that one of the
species, Re–p-I, with an iodine-substituted axial co-ligand,
undergoes an extremely rare, temperature driven phase trans-
formation. This transformation process is reversible and pro-
ceeds in a single-crystal to single-crystal manner. Somewhat
surprisingly, this species was found to be anomolous within
the series of closely related molecular analogues.
phenyl)nicotinamide)][OTf ] (Re–p-Cl). Obtained as a yellow
1
solid (yield: 65%). H NMR (CDCl3, 250 MHz, 298 K) δH: 7.14
3
3
(2H, d, JHH = 8.1 Hz), 7.51 (2H, d, JHH = 8.10 Hz), 7.60–7.75
3
(3H, m), 8.10–8.40 (8H, m), 9.11 (2H, d, JHH = 6.3 Hz), 9.69
(1H, s) ppm. IR (CHCl3) νmax: 2037, 1933, 1680 cm−1. UV-vis
(CH3Cl) λmax (ε = M−1 cm−1): 245 (17 800), 278 (18 400), 321
(7500), 365 sh (2600) nm. Emission (CHCl3) λem: 538 nm. ESMS
m/z: calculated 659.1, found 659.1 [M − OTf ]+. HRMS m/z:
calculated 657.0462, found 657.0462 [C25H17O4N4Cl185Re]+.
Clearly, the other crystalline materials obtained in this
study do not undergo transformation. However, the premise
for a meaningful detailed discussion on the effect of system-
atic chemical modification is the existence of isomorphism.
No isomorphism was found between the derivatives discussed
in this study. It is therefore not possible to predict how the
other derivatives would behave were alternative crystal struc-
tures to be obtained.
It is also noteworthy that apart from the two forms of Re–p-I,
polymorphism was not observed for any other species in the
series, further exemplifying the unique behaviour of Re–p-I in
this study. Of all the structures, only Re–p-Me is solvated and
two solvates have been obtained which, although not poly-
morphs, have many similar structural features.
Synthesis
of
fac-[Re(CO)3(2,2′-bipyridine)(N-(2-chloro-
phenyl)nicotinamide)][OTf ] (Re–o-Cl). Obtained as a yellow
solid (yield: 71%). 1H NMR (CDCl3, 250 MHz, 298 K) δH:
6.9–7.2 (2H, m), 7.4–7.6 (2H, overlapping m), 7.59 (1H, d,
3JHH = 8.8 Hz), 7.69 (2H, m), 7.9–8.2 (5H, m), 8.7–8.9 (2H, m),
3
9.02 (2H, d, JHH = 6.3 Hz), 9.41 (1H, br s) ppm. IR (CHCl3)
νmax: 2038, 1933, 1680 cm−1. UV-vis (CH3CN) λmax (ε = M−1 cm−1):
247 (26 100), 294 (27 400), 392 (4400) nm. Emission (CHCl3)
λem (τ/ns): 548 (333) nm. ESMS m/z: calculated 659.1, found
659.0 [M − OTf ]+. HRMS m/z: calculated 657.0462, found
657.0457 [C25H17O4N4F185Re]+.
Synthesis
of
fac-[Re(CO)3(2,2′-bipyridine)(N-(4-bromo-
phenyl)nicotinamide)][OTf ] (Re–p-Br). Obtained as a yellow
1
solid (yield: 84%). H NMR (CDCl3, 250 MHz, 298 K) δH: 7.28
3
3
(2H, d, JHH = 7.9 Hz), 7.51 (2H, d, JHH = 8.10 Hz), 7.60–7.75
3
Acknowledgements
(3H, m), 8.10–8.40 (8H, m), 8.96 (1H, d, JHH = 8.7 Hz), 9.11
3
(2H, d, JHH = 6.3 Hz), 9.61 (1H, s) ppm. IR (CHCl3) νmax
:
We thank Cardiff University and EPSRC for financial support
and the staff of the NMSCC at Swansea University.
2037, 1933, 1680 cm−1. UV-vis (CH3CN) λmax (ε = M−1 cm−1):
248 (24 200), 283 (25 300), 363 sh (5300) nm. Emission
(CHCl3) λem (τ/ns): 542 (359) nm. ESMS m/z: calculated 703.0,
found 703.0 [M − OTf ]+; HRMS m/z: calculated 700.9957,
found 700.9956 [C25H17O4N4Br185Re]+.
Notes and references
1 G. M. J. Schmidt, Pure Appl. Chem., 1971, 27, 647.
Synthesis
of
fac-[Re(CO)3(2,2′-bipyridine)(N-(2-bromo-
2 C. R. Theocharis and W. Jones, in Organic Solid State
Chemistry, ed. G. R. Desiraju, Elsevier, 1987, ch. 2, p. 47.
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O. V. Zenkina, E. C. Keske, R. Wang and C. M. Crudden,
phenyl)nicotinamide)][OTf ] (Re–o-Br). Obtained as a yellow
1
solid (yield: 73%). H NMR (CDCl3, 250 MHz, 298 K) δH: 6.94
3
3
(1H, d, JHH = 8.3 Hz), 7.32 (1H, d), 7.49 (1H, d, JHH
=
7.7 Hz), 7.6–8.3 (7H, overlapping m), 8.40–8.60 (3H, m), 8.71
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