Correlation of Structure with Crystalline to Amorphous Phase Transitions of 1,3,6-Substituted Fulvene-Derived Molecular Glasses

Article abstract of DOI:10.1021/acs.joc.0c01014

An investigation into the crystalline to amorphous phase transitions of prepared 1,3,6-substituted pentafulvenes showed the expected reversible heated melt and cooling recrystallization in only a few examples. Systematic incorporation of bulky substituent

Full text of DOI:10.1021/acs.joc.0c01014

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Article
Correlation of Structure with Crystalline to Amorphous Phase
Transitions of 1,3,6-Substituted Fulvene Derived Molecular Glasses
Loren C Brown, Andrew Peloquin, Nicholas P. Godman, Gary James Balaich, and Scott T Iacono
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.0c01014 • Publication Date (Web): 13 Jul 2020
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Correlation of Structure with Crystalline to Amorphous
Phase Transitions of 1,3,6-Substituted Fulvene Derived Molecular Glasses
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Loren C. Brown, Andrew J. Peloquin, Nicholas P. Godman,b
Gary J. Balaich,* Scott T. Iacono*
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aDepartment of Chemistry & Chemistry Research Center, Laboratories for Advanced
Materials, United States Air Force Academy, Colorado Springs, Colorado (USA)
bAir Force Research Laboratory, Materials and Manufacturing Directorate,
Wright-Patterson Air Force Base, Dayton, Ohio (USA)
*Correspondence e-mail: gary.balaich@usafa.edu or scott.iacono@usafa.edu
Abstract:
An investigation into the crystalline to amorphous phase transitions of prepared 1,3,6-
substituted pentafulvenes showed the expected reversible heated melt and cooling
recrystallization in only a few examples. Systematic incorporation of bulky substituents
at the 6-position of the fulvene ring led to non-reversible thermal behavior, rendering
phases that were locked into glassy, vitrified states. These molecular glasses produced
physically translucent and amorphous features with glass transition temperatures in the
range of 61–77 °C, comparable with high-strength plastics such as polyethylene
terephthalate. Additionally, the melting point transitions and the resulting heat of fusion
values were found to be directly influenced by the nature of the 6-position substituent.
Single crystal X-ray crystallography showed that in some cases fulvenes possessing fused
aromatics exhibited a high degree of intermolecular π-π stacking. These results point to a
class of molecular glass formers as host materials possessing tunable bulk properties for
potentially new optical applications.
Keywords: Fulvene; Crystallinity; Amorphous; Vitrogen; Molecular Glass; Calorimetry
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Molecular Pentafulvene Glasses – Melt conversion of crystalline 1,3,6-substituted
fulvenes to a locked, vitreous state affords new translucent, colored amorphous
materials. The crystalline and amorphous properties are correlated with their structure
using single crystal X-ray diffraction and calorimetric techniques.
Introduction
Small molecule crystallization is driven by intermolecular forces, a process that is well
understood and generally accepted as reversible upon additional heating and cooling
cycles. However, a phenomenon that is less understood occurs when low molecular
weight compounds fail to crystallize under conditions below those known for their
thermodynamic solid state phase changes.1,2 Although 1,3,6-triaryl substituted fulvenes
(Figure 1) undergo crystalline melt transitions, to our surprise, this was followed by the
formation of amorphous glassy states which could be validated on visual inspection as
well as by calorimetric measurements. Such a process is caused by vitrification, a
phenomenon that allows low molecular weight molecules to become structurally
interlocked in random configurations by overcoming intermolecular forces that would
normally induce crystallization. The requirements for small molecules to possess glass-
forming properties include a combination of large molecular weight, the presence of
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aromatic rings, molecular asymmetry, a large number of rotatable bonds, branched
carbon skeletons, and/or a combination of electronegative atoms.3–6 Vitrification is
common in high molecular weight, chain-extended linear or network architectures such
as oligomers or polymers and quantified by the glass transition temperature (Tg).
However, rheological studies on super-cooled small molecule liquids such as glycerol and
ortho-terphenyl indicate that long range order of these molecules can also be achieved in
the glassy state.7 Further, in a recent comprehensive study of industrially important
compounds, small molecules of the phenyl propanol family such as 1-phenyl-1-propanol,
glycerol, and 2-isopropylphenol were found to exhibit Tg’s slightly below or above room
temperature at standard atmospheric pressure.8 However, since the introduction of
organic monomeric glasses by Molaire and Johnson9, molecules that exhibit a Tg above
room temperature have been reported only in narrow fields of interest and likely
overlooked for their promising applications. For example, amorphous solid dispersions
(ASD), drug formulations composed of active pharmaceutical ingredients (API’s) dispersed
in an amorphous glassy matrix, have been shown to improve important variables of
biological drug efficacy such as transport and dissolution.9–14 The value of the glassy state
has also been exploited for incorporating liquid crystal mesogens into low molecular
weight matrices for information storage materials.15 Randomly organized, glassy
materials have also been used as host matrices to lock in chromophore guests in the
design of optoelectronic materials.16–18 More recently, small molecules have been
designed for well-defined fluorescent glasses,19 non-linear optics,20 charge transporting
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devices,21 and radioluminescence.22 Related to fulvenes of interest to this work, it has
been reported that 1,2,3,4-tetraphenyl substituted fulvenes exhibited Tg’s of over 100 °C,
and correlate with the nature of the 6-substituent (Figure 1).23–25
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To our knowledge, there have been no systematic thermal studies of substituted
fulvenes that involve structural correlations between crystallinity and their amorphous
nature. Motivated by this lapse, it is the purpose of this work to present a study of the
melting phase transitions of 1,3,6-substituted fulvenes as a function of the 6-substituent
on the fulvene core as supported by DSC and single crystal X-ray diffraction. The
amorphous nature of 1,3,6-substituted fulvenes that can achieve a vitrified state was
studied, and their corresponding Tg values were correlated to the nature of the
substituent at the 6-position. Insight gained from this study expands the application of
these fulvenes as low molecular weight glass formers for a new class of amorphous host
matrix materials, obtainable from operationally simple and high yielding synthetic
procedures.
Ar
Ar (or Alkyl)
previous work:
this work:
1,3,6-substitued fulvenes
1,2,3,4-tetraphenyl fulvenes
(10 examples)
(4 examples)
Figure 1. Generic structural depiction of fulvenes:
previously reported work24-26 compared with the
present investigation.
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Results and Discussion
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Fulvene Synthesis
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The synthesis of 1,3,6-substituted fulvenes 2–11 was carried out by the coupling of 1,3-
diphenylcyclopentadiene 1 with the corresponding substituted aldehydes in pyrrolidine-
promoted ethanolic media according to a modified literature procedure (Scheme 1).26
Good overall isolated yields (52–97%) of high purity 1,3,6-substituted fulvenes 2–11 were
afforded as microcrystalline solids of varying color depending on the nature of the 6-
position, several of which were previously reported for their optical27,28 as well as
electrochemical29 properties. For the purposes of this study, selected 6-substituted
fulvenes were chosen and are grouped according to branched and cyclic aliphatic (2–4),
functionalized aromatic (5–8), and fused aromatic of varying architecture (9–11).
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R
6
RCHO
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pyrrolidine
EtOH
r.t.
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2 11

1
Tm 131°C
2
Tdecomp >133°C
Tg n.o.
3
Tm 100°C
Tg n.o.
(74%)
(52%)
S
Tm 91°C
Tg n.o.
Tm 111°C
Tg n.o.
4
5
(63%)28
(80%)
6
Tm 123°C
Tg n.o.
Tm 77°C
Tg n.o.
7
(65%)28
(81%)36
8
9
Tm 166°C
Tg 74°C
Tm 167°C
Tg 75°C
(97%)30
(87%)30
OMe
MeO
11
(95%)30
Tm 168°C
Tg 61°C
Tm 217°C
Tg 77°C
10
(89%)
Scheme 1. Synthesis of 1,3,6-substituted fulvenes
(2–11) from 1,3-diphenylcyclopentadiene 2 and
their corresponding isolated yield (%), melting
point (Tm), and glass transition temperature (Tg) as
observed by DSC (second scan).
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Crystalline Properties
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The melting point (Tm) and enthalpy of fusion (∆Hf) values for fulvene compounds 2–11
are plotted in Figure 2. There are distinct differences in both Tm and ∆Hf that depend on
the nature of the substitution at the 6-position (Figure 2). Aliphatic fulvenes 2 and 3 and
aromatic fulvene compounds 5 and 6 have melting points above 100 °C, narrowly ranging
between 100–131 °C with the highest melting 6-aliphatic substituted fulvene being the
cyclopropyl-functionalized 3. Fulvenes 4 and 7 fall significantly below the observed mp
trend (91 °C (4) and 77 °C (7)). This is presumably due to the highly flexible nature of the
cyclohexyl functionalized fulvene 4 and the bulky nature of the 2,4,6-trimethylphenyl
substituted fulvene 7, each structure causing disruptions of the predominating van der
Waals interactions in the solid state. The perturbation of the intermolecular forces is
further validated given the lower ∆Hf observed for 4 (53 J/g) and 7 (57 J/g), compared to
the series of all other fulvenes studied wherein the ∆Hf values fall in the narrow range of
95–109 J/g. Interestingly, as bulky aromatic groups are substituted (fulvenes 8–11), there
is a significant melting point increase from fulvene 3. While the biphenyl- 8, naphthyl- 9,
and 2,3-dimethoxynaphthyl- 10 substituted fulvenes were observed to have similar melt
transitions (166–168 °C), the anthracene analogue 11 produced the highest Tm (217 °C)
which is 49–51 °C higher among this series. The large increase in melting point for fulvene
11 appears to be a direct result of observed π–π stacking induced by the anthracene units
(vide infra, Figure 4). Curiously, fulvene 8 was observed to have distinct, but overlapping
melting points at 167.3 and 167.9 °C, likely due to two polymorphs generated by the
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rotational
motion
of
the
biphenyl
unit
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the
6-position.
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Figure 2. DSC derived melting points and heat of
fusion data for fulvene compounds 2–11.
In the crystal structures of fulvenes 2, 8 and 11, some correlation of crystal packing to
observed thermal behavior could be made. Single crystal X-ray structures for fulvenes 4
and 6 (Scheme 1) were previously reported,28 and the structures for fulvenes 2, 8 and 11
are included and discussed as part of this work. Each fulvene molecular structure
exhibited the expected alternating short-long bond distances within the fulvene ring, and
the exocyclic bond distances and angles were found to be consistent with those obtained
from similar 1,3-diphenyl-6-substituted structures.28 The correlation between the crystal
structures and their respective melts lies within the type of intermolecular forces at play
and the contact area over which these forces act. In this study the data (vide infra)
demonstrate that the size, shape and type of the 6-position substituent of the fulvene can
have significant effects in a correlation comparing aliphatic to linked aromatic and fused
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aromatic groups at the 6-position of 1,3-diphenylfulvenes. In particular, there is an
approximately linear increase in mp from fulvene 2 to 8 to 11 (Figure 2), and this trend is
herein related to crystal packing effects in the solid state structures of these fulvenes.
Molecules of the 6-isopropyl fulvene 2 pack in a zig-zag pattern along the c-direction
with weak van der Waals contacts between 6-iPr and 1-Ph substituents (H, head) that
orient head-to-head (H-H) and weak - interactions of the 3-Ph rings (T, tail) that orient
tail-to-tail (T-T) (Figure S1). The 3-Ph ring slipped - stacking (distance between mean
planes of 3-Ph rings, 3.377 Å, Figure S2) orients fulvene molecules in a T-T fashion in the
middle of columns that run perpendicular to the c-direction. Adjacent columns that are
separated along the c-direction interact via the H-H interactions, producing a repeating
zig-zag H-T-T-H packing sequence (Figure S1). Although the intermolecular forces holding
crystals of fulvene 2 are overcome at the mp of 100° C, the mp’s of fulvenes 4 (91° C) and
7 (77° C) are significantly lower (Scheme 1), producing a break in the mp trend between
fulvenes 2 and 8. The crystal structures of fulvenes 428 and 736 indicate that very weak
intermolecular interactions of the C-H--- type dominate. In addition, we believe that
the conformational flexibility of the 6-cyclohexyl group (4) and the shape of the larger 6-
mesityl group (7) disrupt stronger intermolecular forces and result in significantly lower
mp’s for these fulvenes.
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In contrast, the extended shape and rigidity of the 6-biphenyl substituent of fulvene
8 contributes to a significantly higher mp (167° C). In the crystal structure of the 6-
biphenyl fulvene 8, the biphenyl groups participate in an interleaved packing along the c-
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direction, with the long axes of the biphenyl groups (H, head) oriented H-H and
maximizing van der Waals contacts in the space between the 6-biphenyl and 3-Ph
substituents on adjacent fulvene rings (Figure S3). This packing arrangement leaves the
1,3-diphenylfulvene rings (T, tail) oriented T-T and the entire structure devoid of any -
stacking between phenyl or biphenyl groups on adjacent fulvene units. Each biphenyl
group has 6 nearest neighbor biphenyl groups that are distributed in layers that run along
the c-direction and that bisect the b-direction. Although there are no - stacking
interactions in the 6-biphenyl fulvene solid state structure, the energy required to disrupt
this packing and to reach the melting point transition is higher compared to the 6-
isopropyl fulvene and provides one of the higher mp values in this series of fulvenes
(Figure 2). We correlate this difference mainly to the effect of the long biphenyl group at
the 6-position with its resulting significantly larger van der Waals contact area.
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The structure of fulvene 11 illustrates how the van der Waals contact area can be
maximized by extended and fused ring aromatics that drive intermolecular - stacking
at the 6-position of 1,3-diphenylfulvenes.28 A view down the b-axis in the structure of
fulvene 11 reveals chains of fulvene molecules running along the c-direction with an H-H
(6-anthracene rings) and a T-T (1,3-diphenylfulvene rings) orientation and linked via a
slipped - stacking of the 6-anthracene rings (Figure 3, 4). Although the 6-anthracene
ring planes on adjacent fulvene molecules are oriented parallel to each other, the 1-Ph
and 3-Ph rings from adjacent molecules are positioned nearly perpendicular to each other
running along the b-direction and nearly perpendicular to the 6-anthracene rings of
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fulvene molecules in separate chains. This packing gives rise to a herringbone pattern in
the ac plane, maximizing slipped - overlap of the larger 6-anthracene units (distance
between mean planes of 6-anthracene rings, 3.491 Å, Figure 4) as well as C-H---
interactions of the 1-Ph and 3-Ph rings with each other and with nearest neighbor
anthracene units. The larger ring tilt angle for the plane of the 3-Ph ring from the fulvene
ring plane (30.38(4)°) as well as the very large ring tilt angle of the plane of the 6-
anthracene units from the fulvene plane (80.51(4)°) are probably due to the significant
effect of this crystal packing on the molecular structure of 11 in the solid state. This solid
state structure gives rise to the highest observed melting point (217° C) in the series of
fulvenes highlighted in this paper.
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Figure 3. Packing view of 11 showing 6-anthracene
ring - interactions. Thermal ellipsoids shown at
50%. Hydrogen atoms were omitted for clarity.
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Figure 4. Thermal ellipsoid view of 11 depicting -
overlap between adjacent molecules (distance
shown is separation of 6-anthracene ring mean
planes). Thermal ellipsoids shown at 50%. Hydrogen
atoms were omitted for clarity.
Amorphous Properties
The amorphous thermal properties of fulvenes 2–11 were investigated utilizing DSC,
and the results varied significantly depending on the nature of the substituent at the 6-
position. Initial study of the aliphatic series beginning with isopropyl fulvene 2 showed
typical behavior of a microcrystalline solid by revealing a Tm at 100 °C and, upon cooling,
a late developing crystallization temperature (Tc) at 50 °C. Partial vitrification of 2 was
shown by the observed suppression of the exothermic heat of crystallization (∆Hc from -
43 J/g to -11 J/g) after only one additional heating/cooling cycle (Figure 5). Upon the third
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heating/cooling cycle, no Tg or Tc was evident, indicating that fulvene 2 was transformed
into an entirely amorphous phase. Observation of the pellet in the DSC pan after these
thermal cycles showed a translucent glass-like coating with poor mechanical properties.
For comparison, cyclopropyl fulvene 3 could not be converted to a glass state by thermal
annealing of the solid sample. Fulvene 3 gave a sharp melt transition at 131 °C, but was
immediately followed by an exothermic event (onset at 131 °C) affording a dark brown
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1
solid. In the H NMR (CDCl3) spectrum of the brown solid, indeterminable peaks were
observed, presumably due to oligomerized products from ring opening of the cyclopropyl
ring. The obvious target for comparison in the thermal analysis of the aliphatic series is
the 6-cyclopentyl fulvene. However, attempts to prepare cyclopentyl fulvene 12a
(Scheme 2) using the same synthetic procedure for fulvenes 2–11 did not afford the
desired fulvene product. Instead, a [1,7]-hydride shift from fulvene 12a resulted in the
formation of the cyclopentadiene isomer 12b as the preferred product (Scheme 2).
Computational work has shown that similar [1,7]-hydride shifts occur for 6,6-
dimethylfulvene either by a unimolecular or bimolecular process both having the same
activation energies.31 X-ray crystallographic analysis showed a pattern in bond lengths
for the cyclopentadiene ring and exocyclic C-C bonds consistent with the structure of 12b
(Figure S6, 1.467(2), C5-C6 long bond; 1.342(2), C6-C19 short bond). A singlet peak at 
3.82 in the 1H NMR spectrum pointed to the cyclopentadiene ring CH2 further confirming
the isolated structure of 12b.
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Although not a fulvene, the thermal analysis of 12b proved insightful. The
cyclopentadiene adduct 12b produced a reversible Tm (145 °C ) and Tc (87 °C ) after
repeated heating-cooling cycles as shown in Figure 5. This reflects the structural rigidity
of 1,3,6-substituted fulvenes compared with 12b. By shifting the exocyclic double bond
from C5–C6 in a fulvene to C6–C19 in 12b, this double bond is no longer constrained to
the same plane as the cyclopentadiene system. By allowing rotation around the C5–C6
bond, extended π conjugation is disturbed, decreasing the potential for the long-range
order necessary for formation of a glassy state, and this results in reversible Tm and Tc
values.
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Figure 5. First and second DSC cycles for fulvenes 2
and 4 (top and bottom thermograms) and
cyclopentadiene 12b (middle thermogram).
DSC analysis of the cyclohexyl functionalized fulvene 4 revealed that an entirely
vitrified, glassy state could be achieved by a single heating cycle past the melting
transition (Tm = 91 °C). There was no evidence of recrystallization during the cooling part
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of the first cycle, and upon further heating after the second cycle, there was no observed
melting transition. This behavior was driven by the increased entanglement of the
cyclohexyl rings in the 6-position of the fulvene. Inspection of the DSC pan revealed a
translucent dark orange and brittle solid circular disk that could be re-dissolved in
common organic solvents, further validating the glassy state achieved by this fulvene.
NMR and GC-MS analysis of this formed fulvene glass showed no indication of
degradation. However, the cyclohexyl ring, which is capable of adopting multiple
conformers in the solid state, did not sufficiently induce long-range order, resulting in a
non-observable glass transition temperature (Tg) even after the third heating scan.
Furthermore, fulvenes substituted with thiophene 5, phenyl 6, and mesityl 7 also do not
exhibit a Tg after reaching their melting points.
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CyCHO
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Pyr
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1
12a
desired,
not observed
[1,7]-H shift
(42%)
12b
Scheme 2.
cyclopentadiene 12b via a [1,7]-hydride shift.
Isomerization of fulvene 12a –
In the DSC study of fulvenes with linked or fused ring aromatic substituents at the 6-
position (8-11), well defined glass transition events were observed (Figure 6). Comparison
of the DSC scan for the biphenyl fulvene 8 with the phenyl fulvene 6 showed a well-
defined glass transition event at 75 °C, which was observed upon heating after one heat-
melt-cool cycle. Additionally, the naphthyl 9 and anthracene 11 substituted fulvenes had
similar Tg’s at 74 °C and 77 °C, respectively. Although not entirely anomalous, the 2,3-
dimethoxynaphthyl fulvene 10 produced a slightly lower Tg (61 °C), but this was expected
due to the freely rotating methoxy groups. Similar phenomena were observed with
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1,2,3,4-substituted fulvenes (Figure 1), whereby the same correlation of an increase in Tg
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was observed with bulkier functional groups at the 6-position.24-26
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Figure 6. DSC trace of second scan immediately after
each fulvene melting point, showing selected Tg
values for fulvenes 6 and 8–11.
In the DSC thermograms of fulvenes 2 and 4-7, exotherms were observed when
heated beyond their melting transitions, and very broad Tg’s (60-70 °C) were observed
upon reheating.
Fulvene 6 served as a model substrate to demonstrate this
phenomenon. As depicted in Figure 7, an initial heating to 200 °C produced a broad
exothermic event (Tmax = 163 °C, H = –29 J/g). The second re-heating showed a slight
inflection in the thermogram (Figure 7, broad Tg) indicative of a developing glass
transition, but the fulvene exists in an entirely amorphous state. Additional heating to
200 °C and holding at this temperature for 5 h induced the formation of a defined Tg at
90 °C (Figure 7).
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Figure 7. DSC thermogram overlay of fulvene 6:
initial melting and broad exotherm to 200 ⁰C (top),
second cooling and re-heating displaying a broad Tg
(middle), and thermogram with defined Tg after 5h
isothermal hold at 200 °C (bottom).
The observed development of a glass transition temperature upon thermal cycling of
fulvenes 2 and 4-7 at 200° C promoted further investigation. A [6 + 4] cycloaddition to
afford dimerized adducts is consistent with the development of these defined Tg values
and is further validated by results previously published.30,31
For example,
cyclodimerization of phenyl fulvene 6 affords twice the molecular weight increase of
612.82 g/mol, a value in the range for which glass transition temperatures are expected
for small molecules. These ideas are supported by the reported crystal structure of the
6-(3,5-dimethylphenyl) substituted fulvene dimer 13b as shown in Scheme 3 (X-ray
structure in Figure S7).32 Given the published experimental conditions by which 13b was
formed, we postulate the dimerization occurs via a [6 + 4] cycloaddition of 13, with the
initial formation of bicyclic 13a followed by a [1,5]-hydride shift to the most stable adduct
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13b. Also relevant to this cyclodimerization is the observation of intractable networks that
were obtained from the thermal heating of a 6-(3-styryl) fulvene at 125 °C. The
installation of the styryl moiety at the 6-position induced simultaneous vinyl-vinyl [2 + 2]
as well as vinyl-fulvene [2 + 4] cyclodimerizations, the former [2 + 2] being preferred based
on significant crystalline organization and the ability to obtain free spatial orientation in
the solid state.28
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Ph
Ph
[6+4]
Ph
Ph

Ph
H
Ph
13
13a
[1,5]-H shift
• 2 CH3CN
Ph
Ph
Ph
Ph
H
13b
Scheme
3.
Thermally
promoted
[6+4]
cyclodimerization of fulvene 13 followed by
isomerization to the most stable adduct 13b.
Thermal ellipsoids are shown at the 50% probability
level.
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Conclusion
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In this work, we carried out a systematic study relating crystalline to amorphous phase
transitions of a series of 1,3,6-substituted fulvenes with varying degree of substitution at
the 6-position. Similar Hf data for these fulvenes suggest that analogous intermolecular
forces predominate the melt transition, while data from single crystal X-ray diffraction
demonstrated that fulvenes with linked or fused aromatic groups exhibited significantly
higher melting points due to higher van der Walls contact areas and - stacking effects.
We have also shown that the vitrified state could easily be achieved upon substitution
with a cyclohexyl group at the 6-position, and that substitution at the 6 position with
bulkier aromatic groups produced glassy materials with more well defined Tg values.
This work expands the previous scope of applicability of highly colored 1,3-diphenyl-
6-substituted fulvenes28 to fulvenes with application as precursors for the formation of
glassy materials having tunable solid state optical properties. In the consideration of
composite systems with other organic or inorganic fillers of interest, it is possible to blend
various 6-substituted fulvenes in order to adjust their spectral absorbance properties as
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well as potentially tuning the Tg.
Additionally, the synthesis of 1,3-
diphenylcyclopentadiene compounds possessing reactive bromine functionalities35 offers
the ability to prepare a diverse pool of 1,3,6-substituted fulvene substrates that would
further alter bulk physical properties of interest for molecular glasses. Finally,
consideration of chain extended systems brings the opportunity to significantly influence
solid state and solution state polymer architecture and microstructure by studying the
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influence of the 1,3-diphenyl-6-substituted fulvenes or related 1,3,6-substituted fulvene
repeat units in the main chain.
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Experimental Section
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General Methods
Synthesis of 1,3-diphenylcyclopentadiene 1 was carried out using a previously
published procedure.27 All solvents were purchased from commerical suppliers and used as
received unless otherwise noted. Syntheses of 2, 3, 5, and 10 using 1 as the cyclopentadiene
starting material were carried out using a modified literature procedure as detailed
below.28 The synthesis of fulvenes 4, 6–9, 11, 13, and fulvene dimer 13b were previously
reported and successfully reproduced as necessary for this study.27,29,32,33
Instrumentation
1H and 13C NMR data were obtained from a JEOL 500 MHz NMR spectrometer under
ambient temperature conditions and chemical shifts reported in parts per million (δ ppm).
Chemical shifts were referenced using the peak for residual CHCl3 (δ 7.25) or TMS (δ 0.00)
for 1H NMR and the peak for CDCl3 (δ 77.0) for 13C NMR. Coupling constants for all spectra
are reported in Hertz (Hz).
Differential scanning calorimetry (DSC) analyses were performed on a TA Instruments
Q20 instrument utilizing aluminum hermetic pans. The analyses were carried out using a
5°C/min temperature gradient from 20 °C to 100-150 °C (for 2–7 and 12) or to 250 °C (for
8–11), cooled to 20 °C under nitrogen, and repeated two additional times for three overall
scans. HRMS data of purified samples was obtained using a Synapt G2 HDMS time-of-
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flight (MALDI-TOF) mass spectrometer located at the Central Analytical Mass
Spectrometry Facility at the University of Colorado at Boulder. Crystals of fulvenes 2, 8,
11, and cyclopentadiene 12b were mounted on CryoLoops with Paratone oil, and data
collected at 100 K on a Bruker SMART APEX II CCD diffractometer equipped with Cu Ka
radiation (l = 1.54178)
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1,3-diphenyl-6-(isopropyl)fulvene 2
To a vigorously stirred mixture of 1,3-diphenylcyclopentadiene (3.07 g, 14.1 mmol) in
absolute EtOH (30 mL) under N2 was added isobutyraldehyde (1.22 g, 16.9 mmol) and
pyrrolidine (1.50 g, 21.1 mmol), and the reaction mixture maintained at room
temperature for 24 hrs. The orange precipitate from the reaction mixture was vacuum
filtered, washed with cold absolute EtOH (3 × 10 mL), and vacuum dried to give 2 as a
bright orange, microcrystalline solid (2.84 g, 74%). Crystals suitable for single crystal X-ray
diffraction were obtained from petroleum ether by slow evaporation. 1H NMR (500 MHz,
CDCl3) 7.69 – 7.24 (m, 10H), 6.94 (s, 1H), 6.86 (s, 1H), 6.33 (d, 1H, J = 10.0 Hz), 3.15 (m,
1H), 1.17 (d, 6H, J = 6.7 Hz). 13C NMR (125 MHz, CDCl3) 149.7, 144.2, 142.5, 140.6, 136.3,
135.0, 129.3, 128.8, 128.5, 128.2, 127.8, 127.0, 126.1, 114.7, 30.4, 22.2. HRMS−ESI (m/z):
calcd for C21H21 [M+H]+ 273.1643, found 273.1647.
1,3-diphenyl-6-(cyclopropyl)fulvene 3
To a vigorously stirred mixture of 1,3-diphenylcyclopentadiene (3.70 g, 16.9 mmol) in
absolute EtOH (30 mL) under N2 was added cyclopropyl carboxaldehyde (1.43 g, 20.3
mmol) and pyrrolidine (1.80 g, 25.4 mmol), and the reaction mixture maintained at room
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temperature for 24 hrs. The orange precipitate from the reaction mixture was vacuum
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filtered, washed with cold absolute EtOH (3 × 10 mL), and vacuum dried to give 3 as a
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bright orange, microcrystalline solid (2.38 g, 52%). H NMR (500 MHz, CDCl3) 7.71 – 7.27
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(m, 10H), 7.02 (s, 1H), 6.91 (s, 1H), 5.86 (d, 1H, J = 10.8 Hz), 2.22 (m, 1H), 1.15, 0.77 (m,
13
4H). C NMR (125 MHz, CDCl3) 149.4, 144.2, 142.8, 139.7, 136.4, 135.8, 129.3, 128.8,
128.5, 127.6, 126.9, 126.8, 126.1, 113.8, 15.0, 10.5. HRMS−ESI (m/z): calcd for C21H19
[M+H]+ 271.1487, found 271.1493.
1,3-diphenyl-6-(2-thiophene)fulvene 5
To a vigorously stirred mixture of 1,3-diphenylcyclopentadiene (1.25 g, 5.7 mmol) in
absolute EtOH (30 mL) under N2 was added 2-thiophene carboxaldehyde (0.767 g, 6.8
mmol) and pyrrolidine (0.61 g, 8.6 mmol), and the reaction mixture maintained at room
temperature for 24 hrs. The red precipitate from the reaction mixture was vacuum
filtered, washed with cold absolute EtOH (3 × 10 mL), and vacuum dried to give 5 as a dark
red, microcrystalline solid (1.42 g, 80%). 1H NMR (500 MHz, CDCl3) 7.74 – 7.24 (m, 14H),
13
7.08 (m, 1H), 6.91 (s, 1H). C NMR (125 MHz, CDCl3) 146.7, 142.2, 141.5, 140.9, 136.2,
135.4, 133.7, 131.0, 130.3, 129.5, 128.8, 128.6, 128.1, 128.0, 127.7, 127.2, 126.3, 113.8.
HRMS−ESI (m/z): calcd for C22H17S [M]+ 313.1051, found 313.1055.
1,3-diphenyl-6-(2,3-dimethoxynaphthyl)fulvene 10
To a vigorously stirred mixture of 1,3-diphenylcyclopentadiene (1.01 g, 4.6 mmol) in
absolute EtOH (30 mL) under N2 was added 2,3-dimethoxynaphthaldehyde (1.20 g, 5.6
mmol) and pyrrolidine (0.49 g, 6.9 mmol), and the reaction mixture maintained at room
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temperature for 24 hrs. The red precipitate from the reaction mixture was vacuum
filtered, washed with cold absolute EtOH (3 × 10 mL), and vacuum dried to give 10 as a
red, microcrystalline solid (1.70 g, 89%). 1H NMR (500 MHz, CDCl3) 7.91 – 7.21 (m, 16H),
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7.03 (s, 1H), 6.43 (s, 1H), 4.01 (s, 3H), 3.82 (s, 3H). C NMR (125 MHz, CDCl3) 152.0, 148.0,
147.6, 145.6, 140.6, 136.1, 135.1, 132.3, 131.5, 129.3, 129.0, 128.9, 128.7, 128.6, 128.0,
127.2, 127.0, 126.8, 126.2, 125.8, 125.7, 124.4, 116.7, 108.1, 61.2, 55.9. HRMS−ESI (m/z):
calcd for C30H25O2 [M+H]+ 417.1855, found 417.1859.
(4-(cyclopentylmethyl)cyclopenta-1,3-diene-1,3-diyl)dibenzene 12b
To a vigorously stirred mixture of 1,3-diphenylcyclopentadiene (1.00 g, 4.6 mmol) in
absolute EtOH (30 mL) under N2 was added cyclopentyl carboxaldehyde (0.54 g, 5.5
mmol) and pyrrolidine (0.49 g, 6.9 mmol), and the reaction mixture maintained at room
temperature for 24 hrs. The yellow precipitate from the reaction mixture was vacuum
filtered, washed with cold absolute EtOH (3 × 10 mL), and vacuum dried to give 12b as a
yellow, microcrystalline solid (0.57 g, 42%). Crystals suitable for single crystal X-ray
1
diffraction were obtained from DCM by slow evaporation. H NMR (500 MHz, CDCl3) 7.57
– 7.15 (m, 10 H), 7.06 (s, 1H), 6.58 (s, 1H), 3.87 (s, 2H); 2.58, 2.42, 1.84, 1.66 (m, 8H). 13C
NMR (125 MHz, CDCl3) 145.7, 144.1, 142.1, 139.7, 137.1, 136.2, 131.0, 128.8, 128.4,
126.9, 126.7, 124.9, 116.9, 44.4, 36.6, 32.7, 28.6, 26.3. HRMS−ESI (m/z): calcd for C23H21
[M]+ 297.1638, found 297.1647.
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Supporting Information
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The supporting information is available free of charge on the ACS Publications website,
http://www.ccdc.cam.ac.uk/cont/retrieving.html or from the CCDC, 12 Union Road,
Cambridge CB21EZ, UK. Fax: +44 1223 336033. E-mail: deposit@ccdc.cam.ac.uk.
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1H and C NMR spectra for 2, 3, 5, 10, and 12b, text describing crystallographic details,
and tables of crystallographic data for 2 (CCDC 1988531), 8 (CCDC 1988532), 11 (CCDC
1988529) and 12b (CCDC 1988530).
Acknowledgement
We thank Professor Arnold L. Rheingold and Dr. Milan Gembicky of the University of
California at San Diego (UCSD) for allowing GJB the use of state of the art X-ray
diffractometers, one of which was used for the analysis of 2, 8, 11 and 12b while GJB was
a crystallographer in residence for a 2019 fall semester sabbatical working in the UCSD
Small Molecule Crystallography Laboratory. LCB is a National Research Council (NRC)
Postdoctoral Fellow funded by the Air Force Office of Scientific Research (AFOSR). AJP
gratefully acknowledges the United States Air Force Institute of Technology Civilian
Institutions program for graduate sponsorship. STI and GJB acknowledge financial support
from the Defense Threat Reduction Agency (DTRA) and the Air Force Office of Scientific
Research (AFOSR). We finally thank Dr. Thomas Lee at the University of Colorado at
Boulder for high resolution mass spectrometry analysis.
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