Organic and organometallic derivatives of pentaphenylbenzene, C 6Ph5X: Correlation of peripheral phenyl ring orientations with the steric bulk of "x"

Article abstract of DOI:10.1139/cjc-2013-0257

A series of C₆Ph₅X compounds, including X = H, Br, CO₂H, CO₂R, CCPh, cis-BrC=C(Br)Ph, 2-bornenyl, and ferrocenyl, have been characterized by use of X-ray crystallography. Also, the first organometallic complexes of pentaphenylbenzene have been prepared by reaction with chromium hexacarbonyl to yield (η⁶-C ₆Ph₅H)Cr(CO)₃ complexes in which the metal tripod is attached either to an ortho peripheral ring or to the central ring. Crystalline pentaphenylbenzoic acid exists as a hydrogen-bonded dimer; however, the steric bulk of the substituents does not allow the carboxylic acid moieties to be linked directly but instead via two bridging methanol molecules. In the solid state, the orientation of the peripheral rings in bulky C ₆Ph₅X systems is very sensitive to the size of the "X" substituents, such that the twist angle of the para ring responds inversely with increasing bulk of "X", which drives the ortho rings farther out of the plane of the central ring. The relevance of these observations to correlated motion in molecular propellers, and ultimately to molecular machines, is discussed.

Full text of DOI:10.1139/cjc-2013-0257

1098
ARTICLE
Organic and organometallic derivatives of pentaphenylbenzene,
C6Ph5X: correlation of peripheral phenyl ring orientations with the
steric bulk of “X”
Stacey Brydges, Brendan Gildea, John P. Grealis, Helge Müller-Bunz, Mark Stradiotto, Michael Casey, and Michael J. McGlinchey
Abstract: A series of C6Ph5X compounds, including X = H, Br, CO2H, CO2R, C'CPh, cis-BrC=C(Br)Ph, 2-bornenyl, and ferrocenyl,
have been characterized by use of X-ray crystallography. Also, the first organometallic complexes of pentaphenylbenzene have
been prepared by reaction with chromium hexacarbonyl to yield (␩6-C6Ph5H)Cr(CO)3 complexes in which the metal tripod is
attached either to an ortho peripheral ring or to the central ring. Crystalline pentaphenylbenzoic acid exists as a hydrogen-
bonded dimer; however, the steric bulk of the substituents does not allow the carboxylic acid moieties to be linked directly but
instead via two bridging methanol molecules. In the solid state, the orientation of the peripheral rings in bulky C6Ph5X systems
is very sensitive to the size of the “X” substituents, such that the twist angle of the para ring responds inversely with increasing
bulk of “X”, which drives the ortho rings farther out of the plane of the central ring. The relevance of these observations to
correlated motion in molecular propellers, and ultimately to molecular machines, is discussed.
Key words: polyarylbenzenes, substituent orientations, rotational barriers, correlated rotations, arene chromium complexes.
Résumé : À l’aide de la cristallographie aux rayons X, on a caractérisé une série de composés de formule C6Ph5X, où X = H, Br,
CO2H, CO2R, C'CPh, cis-BrC=C(Br)Ph, 2-bornényl et ferrocényl. En outre, on a préparé les premiers complexes organométal-
liques du pentaphényl-benzène en le faisant réagir avec de l’hexacarbonyle de chrome pour obtenir les complexes (␩6-
C6Ph5H)Cr(CO)3 dans lesquels le tripode métallique est attaché a` un noyau périphérique en ortho ou au noyau central. L’acide
pentaphénylbenzoïque existe a` l’état de dimère formé par liaisons hydrogènes; toutefois, l’encombrement stérique des substi-
tuants ne permet pas la liaison directe des groupements carboxyliques qui sont plutôt reliés par un pont constitué de deux
molécules de méthanol. À l’état solide, l’orientation des noyaux périphériques dans les volumineux systèmes C6Ph5X est très
sensible a` la taille des substituants « X », de sorte que l’angle de torsion du noyau en para répond inversement a` l’encombrement
croissant de « X » qui repousse de plus en plus les noyaux en ortho hors du plan du noyau central. Une discussion de l’intérêt de
ces observations en ce qui concerne le mouvement corrélé dans les hélices moléculaires, et en bout de ligne les machines
moléculaires, est offerte. [Traduit par la Rédaction]
Mots-clés : polyarylbenzènes, orientation des substituants, barrières de rotation, rotations corrélées, complexes aréniques du
chrome.
gated. Until very recently, its known reactivity was limited to
Introduction
electrophilic substitutions at the para positions of the peripheral
Polyarylbenzene derivatives have been intensively studied in re-
phenyls,9 with the goal of generating networks possessing multi-
cent years because of their applications in conducting polymers,1 UV
ple specific hydrogen bond interactions,10 dehydrogenation to
organic lasers,2 and nonlinear optical devices and field effect transis-
form a planar “graphite-type” system,11 the design of bulky phos-
tors.3 Their use as organic light-emitting diodes arises from the
phine ligands for use in palladium coupling processes,12 and
marked nonplanar character of systems containing one or more
Birch reduction to 2,3,5,6-tetraphenyl-1,1=-bicyclohexylidene.13
C6Ph5 moieties, which prevent aggregation by minimizing ␲−␲
Many other functionalized C6Ph5X molecules, examples of which
are depicted in Scheme 1, have been prepared by Diels–Alder
addition reactions of appropriate alkynes with tetraphenylcyclo-
pentadienone (tetracyclone) (1). These include a benzo[2,1,3]
thiadiazole with important organic light-emitting diode proper-
ties,4 a platinum coordination complex based on a terpyridine,14
and variously substituted hexaarylbenzenes.8
stacking, thus enhancing fluorescence.4,5 Some polyphenylbenzene
derivatives have exhibited biological activity, behaving as immune
suppressants, antioxidants, antithrombosis agents, and anticoagu-
lants.6 Very recently, a number of their thermodynamic properties,
e.g., enthalpy and entropy of sublimation, have been rationalized in
terms of the torsional freedom of the peripheral phenyl substituents
in the gas phase.7 Additionally, Pascal and Müllen have each reported
a fascinating series of polyaryl systems that adopt picturesque struc-
tures and exhibit almost unprecedented steric strain.8
We here report the syntheses and X-ray crystallographic characteriza-
tion of a series of C6Ph5X systems and also several organometallic deriv-
atives incorporating chromium or cobalt carbonyls. In particular, we
note the influence of the X group on the orientations of the peripheral
phenyls relative to the central ring.
Although pentaaryl-phenyl, C6Ar5, substituents have been in-
corporated into a wide range of molecules, the chemistry of
pentaphenylbenzene itself has been surprisingly little investi-
Received 7 June 2013. Accepted 23 July 2013.
S. Brydges. Department of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093, USA.
B. Gildea, J.P. Grealis, H. Müller-Bunz, M. Casey, and M.J. McGlinchey. School of Chemistry and Chemical Biology, University College Dublin, Belfield, Dublin 4, Ireland.
M. Stradiotto. Department of Chemistry, Dalhousie University, Halifax, NS B3H 4J3, Canada.
Corresponding authors: Stacey Brydges (e-mail: sbrydges@ucsd.edu) and Michael J. McGlinchey (e-mail: michael.mcglinchey@ucd.ie).
Can. J. Chem. 91: 1098–1111 (2013) dx.doi.org/10.1139/cjc-2013-0257
Published at www.nrcresearchpress.com/cjc on 31 July 2013.

Brydges et al.
1099
Scheme 1. A selection of C6Ph5X molecular systems.
O
S
S
Ph
Ph
Ph
Ph
N
N
N
N
Ph
S
Ph
S
S
S
C6Ph5
C6Ph5
- CO
O
Cl
-
Cl
+
Ph
Ph
Ph
Ph
Pt
N
N
N
N
N
N
N
Pt(DMSO)Cl2
N
N
- CO
C6Ph5
O
C6Ph5
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
- CO
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Results and discussion
Fig. 1. Molecular structure of pentaphenylbenzene (2). (For ease of
comparison, all C6Ph5X molecules are numbered here such that C1
is the C-X carbon).
Structures of organic C6Ph5X systems
Pentaphenylbenzene (2) is readily obtainable by Diels–Alder ad-
dition of phenylacetylene to tetracyclone with subsequent chele-
tropic loss of carbon monoxide.15 It has also been prepared by
treatment of 7-borabicyclo[2.2.1]heptadiene with methyl-lithium
and then aqueous hydrolysis of the intermediate anion.16 1-
Phenylethynyl-2,3,4,5,6-pentaphenylbenzene (3) was originally
synthesized by Dilthey et al. in 193517 by heating together tetracy-
clone and 1,4-diphenyl-1,3-butadiyne at 250 °C under a nitrogen
atmosphere; the loss of carbon monoxide from the Diels–Alder
adduct yields Ph-C'C-C6Ph5 (3) directly. A second, albeit sterically
very demanding, [4+2] cycloaddition furnishes decaphenylbiphenyl,
Ph5C6-C6Ph5 (4), as reported by Ogliaruso and Becker in 1965;18 the
acute steric crowding was confirmed by Pascal and co-workers
only in 1988 as part of a comprehensive X-ray crystallographic
study of highly congested polycyclic aromatic hydrocarbons.19
Surprisingly, perhaps, the X-ray crystal structure of pentaphen-
ylbenzene (2) appears not to have been previously reported and is
shown in Fig. 1. It reveals that the central ring has been slightly
distorted whereby the C−C−C angles within the central ring are
118.2° at C2, 123.3° at C1, and 118.7° at C6, whereas the angles at the
other three carbons are almost 120°, as expected. The bond distances
C1−C2 and C1−C6 average 1.387 Å, whereas the other four ring bonds
are slightly longer, average 1.409 Å. The flattening at C1 presumably
minimizes steric repulsions between the phenyl rings. Unlike solid-
state hexaphenylbenzene, which adopts a propeller-type geometry
with a peripheral phenyl average twist angle of ϳ75°,20 or ϳ79°,21
relative to the central ring, the phenyl substituents in 2 exhibit a
quite different pattern. The phenyls positioned ortho and meta to
the central ring hydrogen relax to dihedral angles of 57° 1°,
C25
C26
C63
C62
C24
C64
C23
C22
C21
C61
C65
C66
C1
C2 C6
C3 C5
C4
C36
C51
C56
C52
C35
C34
C31
C53
C41
C32
C33
C55
C54
C46
C45
C42
C43
C44
whereas the para phenyl lies at an angle of 73° to the central ring
(Table 1).
The peripheral phenyl twist angles in 2 may be compared
with those found in the X-ray crystal structure of 1-phenylethynyl-
2,3,4,5,6-pentaphenylbenzene (Fig. 2), which revealed the pres-
ence of two independent, and geometrically unique, molecules in
the asymmetric unit. In the approximately C2-symmetric struc-
ture 3, the central and alkyne-linked rings are essentially copla-
nar (at 3.4°), and the phenyl rotors attached to the central arene
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Can. J. Chem. Vol. 91, 2013
Table 1. Peripheral substituent dihedral angles (°) in pentaphenylbenzene derivatives.
C6Ph5X
ipso
ortho
meta
para
meta
ortho
C6Ph5H (2)
55.8
58.0
73.0
58.0
57.0
C6Ph5CϵCPh
Linear alkyne (3)
Curved alkyne (3=)
C6Ph5-C6Ph5 (4)
67.8
−83.3
65.1
−75.4
78.4
71.9
−76.8
64.2
77
73.9
72.6
67.3
61.6a
48.2
−75.5
−67.2
69.9
68.4
63.5
60.6
84.9
72.3
84.5
82.6
63.3
68.3
69.8
65
70.4
77.5
59.6
63.3
64.8
88.0
76.8
66.9
59.4
66.1
69.9
61.5
76.5
64.0
65.9
71.5
65.2
81.2
66
69.5
69.9
52.0
63.5
53.9
82.2
87.3
69.7
67.6
63.9
56.2
66.3
70.2
56.5
69.0
69.6
77.4
89.3
79
75.0
58.3
60.4
70.7
64.2
65.1
77.8
63.1
55.5
63.2
60.8
62.7
57.0
56.8
−80.1
83.0
79.3
−59.5
−83
66.4
62.2
71.1
50.9
56.1
76.5
89.7
65.5
63.5
64.9
86.5
−86.5
C6Ph5-C2B10H11 (5)
C6Ph5Br (6)
C6Ph5(C5H4)Fe(C5H5) (7)
C6Ph5-2-bornenyl (9)
C6Ph5CBr=CBrPh (11)
50.9
87
69.4
72.6
71.8
C6Ph5CO2H·CH3OH (13)
[(␩6-C6H5)-C6Ph4H]Cr(CO)3 (25)
(␩6-C6Ph5H)Cr(CO)3 (26)
21a
C6Ph6
C6Ph6
89.6
83.3
70.5
64.9
64.0
21b
(␩6-C6Ph6)Co(PhCϵCPh) (21)37
(␩6-C6Ph6)Mo(␩5-C5H5) (22)38
(p-C6Ph5-C6H4)4C (19)34
aChromium-complexed ring.
Fig. 2. X-ray crystal structures of 1-phenylethynyl-2,3,4,5,6-pentaphenylbenzene (3 and 3=).
C80
C8
C7
C70
C10
C1
C60
C50
C6
C5
C2
C20
C3
C30
C4
C40
3
3'
consecutively adopt dihedral angles of 68°, 61°, 70°, 56°, and 61°
from C2 to C6. In the other solid-state conformer 3=, the alkyne
phenyl is oriented 24° relative to the central plane, which is sur-
rounded by a “picket fence” of phenyl groups in a nonpropeller
arrangement marked by individual torsional angles of −83°, 85°,
61°, 66°, and 63° from C20 to C60. These structural features are
most likely coupled to the bending deformation of the acetylenic
unit, which is characterized by bond angles of 173.2° and 175.6° at
C70 and C80, respectively. There is clearly a correlation between
the alkyne curvature and twist of the terminal phenyl group with
the orientations of the adjacent phenyl rings found in the solid
state. Comparing the structures of 3 and 3=, the orientations of the
rings ortho, meta, and para to the alkyne substituent differ by +29°,
+25°, and −9°, respectively, reflecting the progressive transmis-
sion of nonbonded repulsions.
in cyclohexane rather than the “end-to-end” interplay in biaryls. If
“X” is less sterically demanding than a phenyl substituent, it is
reasonable to assume that the aryl groups ortho and meta to “X”
will experience less steric hindrance to rotation than will an aryl
group in the para position, which in turn will tend to adopt a
larger twist angle. Such is clearly the case in 3, where “X” is a
needle-like phenylalkynyl moiety. In contrast, when “X” is particularly
bulky, as in Pascal’s 1-(o-carboran-9-yl)-2,3,4,5,6-pentaphenylbenzene,
C2B10H11-C6Ph5 (5) (Fig. 3), the two ortho phenyls are rotated
through ϳ80° in opposite directions and the spherical carboranyl
cluster (molecular diameter 5.62 Å) is nestled between them.
Moreover, the C1−B linkage is unusually long.23
Likewise, Fig. 4 depicts the structure of 1-bromo-2,3,4,5,6-
pentaphenylbenzene (6) in which the ortho phenyls are twisted
through ϳ77° in opposite directions. Once again, as in pentaphen-
ylbenzene, the angles within the central ring of 6 are distorted with
internal angles at C2, C1, and C6 of 117.6°, 123.4°, and 118.1°, respec-
tively, while the angles at C3, C4, and C5 are each 120°.
As noted by Patton, Dirks, and Gust,22 the steric interaction
between “X” and the adjacent phenyl rings in C6Ph5X is mainly a
“side-by-side” contact such as is observed between 1,3-axial groups
Published by NRC Research Press

Brydges et al.
1101
Fig. 3. (a) Molecular structure of 1-(o-carboran-9-yl)-2,3,4,5,6-pentaphenylbenzene, C2B10H11-C6Ph5 (5); (b) spacefill view of 5. (Data taken from
reference 23.)
a
b
Fig. 4. Molecular structure and spacefill view of 1-bromo-2,3,4,5,6-pentaphenylbenzene (6).
Br
C25
C63
C24
C62
C26
C21
C64
C61
C6
C23
C22
C1
C65
C66
C2
C5
C52
C36
C3
C31
C53
C54
C51
C35
C34
C4
C41
C32
C46
C56
C55
C33
C42
C43
C45
C44
A more extreme example from our earlier work24 is provided by
1-ferrocenyl-2,3,4,5,6-pentaphenylbenzene (7) whereby these de-
formations are enhanced; in this case, the substituents do not
adopt a propeller conformation and the peripheral rings exhibit
an incremental progression of twist angles relative to the central
ring of 51° (Fc group), 64°, 70°, 81°, 89°, and 120° from C1 to C6,
respectively. Figure 5 depicts a comparison of the peripheral ring
orientations in C6Ph5H (2) and C6Ph5Fc (7).
As shown in Scheme 2, the project was then extended to
include a bulky chiral substituent by addition of lithium phenylace-
tylide to 1R-(+)-camphor, which furnished 2-endo-phenylethynyl-2-
exo-hydroxybornane (8). The propensity of reagents to approach
theketoniccarbonfromtheless-hindered␣-face(oppositetothedimeth-
yl bridge) is well established,25 and this was confirmed X-ray crystallo-
graphically for 8, whose molecular structure appears as Fig. 6a.
2-Endo-phenylethynyl-2-exo-hydroxybornane crystallizes as two inde-
pendent, yet closely related, molecules that appear as (O−H···O) hy-
drogen-bonded dimers in the asymmetric unit of the P21 space
group. As featured in Fig. 6b, atom O2A in molecule 1 acts as an
hydrogen bond donor to O2B in molecule 2, yielding values of 2.05 Å
(H···A), 2.872(6) Å (D···A), and 165.9° (D−H···A).
2,3,4,5,6-pentaphenylbenzene (9) whose X-ray crystal structure is
shown in Fig. 7. Despite numerous attempts to obtain better quality
crystals, the carbons in 9 could not be refined anisotropically, and so
we prefer not to quote more precise geometric parameters. Never-
theless, the general features are evident and, as listed in Table 1, the
C6Ph5-bornenyl system 9 adopts a nonpropeller arrangement of pe-
ripheral phenyls. Analogous to the carboranyl- and bromo-C6Ph5 sys-
tems 5 and 6, the ortho phenyls in 9 are rotated by ϳ85° in opposite
directions, thus exhibiting the most heightened conformational re-
sponse to the presence of the “X” substituent in molecules of the
C6Ph5X type surveyed thus far.
With the goal of combining organometallic fragments with bulky
pentaarylphenyl units, 1-phenylethynyl-2,3,4,5,6-pentaphenyl ben-
zene 3 was treated with dicobalt octacarbonyl, the standard route to
alkyne-dicobalt tetrahedral clusters such as (10), but to no avail. This
is presumably attributable to the potentially severe steric crowding
that would be developed upon cluster formation, especially since the
alkyne moiety in such systems is no longer linear but instead adopts
R−C'C−R angles of ϳ135°–140°. It is relevant to note that when
Pascal26 attempted the thermal reaction of 3 with (C5H5)Co(CO)2 as a
route to sterically crowded tetraarylcyclobutadiene complexes,27
the major product was 1,2,3,4-tetraphenylfluorene, along with
Subsequent reaction of the isoborneol 8 with tetracyclone at
200 °C proceeded with elimination of water to give 1-(2-bornenyl)-
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Can. J. Chem. Vol. 91, 2013
Fig. 5. Views along successive peripheral ring – central ring bond axes in pentaphenylbenzene (2) (upper) and ferrocenylpentaphenylbenzene
(7) (lower).
C6
C5
C1 C2
C1
C3
C2
C5
C3
C6
C4
C4
C3
C6
C1
C2
C4
C5
Scheme 2. Syntheses of organic and organometallic derivatives of pentaphenylbenzene.
Ph
Ph
O
O
OEt
Ph
Ph
H
Li
ClCO2Et
Cr(CO)6
Ph
Ph
Ph
Ph
H
Ph
Ph
Ph
Ph
Ph
Ph
Ph 12
Ph
Ph
Cr(CO)3
n-Bu2O / THF
reflux
25
26
+
2 Ph
Ph
Ph
1. CO2
2. H+
Ph
Ph
H
Ph
H
Ph
Cr(CO)3
n-BuLi
THF
Ph
OH
Ph2O / 210 oC
O
Ph
Ph
Ph
Br
Ph
Br
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
C6Ph5
toluene, reflux
3
Ph
Ph
13
H2SO4
Ph
O
1
6
Br2 / CCl4
Co2(CO)8
Ph
H
CH2OH
Ph
Ph
OH
Ph
DMAP / DCC
Ph
- H2O
Br
Ph
C6Ph5
8
H
Ph
O
O
14
Co2(CO)6
C6Ph5 Br
H
Ph
Ph
Co2(CO)8
Ph
Ph
10
11
O
O
Co2(CO)6
Ph
Ph
Ph
Ph
Ph
C6Ph5
16
9
Ph
18
other minor products, apparently arising via initial activation by
cobalt complexation with subsequent ortho-metallation.
methyltrioxorhenium-catalyzed stoichiometric reaction of bromide,
hydrogen peroxide, and diphenylacetylene.29 Subsequently, Barnes
and Chudek30 reported the X-ray crystal structure of cis-1,2-dibro-
mostilbene−diphenylacetylene (2:1). Although details of its original
preparation (ca. 1965) were unknown, it was conjectured that spon-
taneous and partial debromination of the less stable cis isomer had
occurred over the sample’s 30 year span of storage in the dark.
In the present case, the major product of the bromination of 3 was
identified X-ray crystallographically as the cis isomer (11) (Fig. 8) in
which the two independent molecules adopt similar propeller con-
formations. In the upper dynamic limit, the 13C NMR spectrum of 11
should exhibit 22 signals in the aromatic region, of which eight
correspond to aromatic non-hydrogen-bearing sp2 carbons (four ipso
As a dienophile, 3 is not so sterically shrouded that it resists both
single and double Diels−Alder reactions, so one could envisage
halogenation of the C'C bond as an extension of its reactivity. Mass
spectrometric and NMR analysis of the products of reaction of 3 with
bromine in CCl4 indicated the formation of two dibromo adducts in
a 1:4 ratio. Electrophilic bromine addition to aryl-substituted sub-
strates proceeds via a vinyl cation intermediate that permits both syn
and anti products, with the proportion of the latter increased in the
presence of excess bromide ion and bulky substituents.28 In 1999,
Espenson et al. provided definitive solid-state evidence for the exclu-
sive formation of the trans isomer of dibromostilbene from the
Published by NRC Research Press

Brydges et al.
1103
Fig. 6. (a) Molecular structure of 2-endo-phenylethynyl-2-exo-hydroxybornane (8) showing the disordered phenyl substituent in the solid state;
(b) intermolecular hydrogen bonding between hydroxyl substituents to form dimeric units of 8.
a
b
Fig. 7. Molecular structure of 1-(2-bornenyl)-2,3,4,5,6-pentaphenyl
benzene (9).
Fig. 8. Molecular structure of 1-(1=,2=-Z-dibromo-2=-phenyl)vinyl-2,3,
4,5,6-pentaphenylbenzene (11).
Br2
C8
Br1
C7
C1
C6
C5
C2
C3
C4
phenyls and four in the central ring, each set of intensities 1:2:2:1), 12
CH carbons (four ortho (2:4:4:2), four meta (2:4:4:2), and 4 para (1:2:2:1)),
and finally the two vinyl carbons. Since this number of resonances
(and relative intensities) is exceeded at ambient temperature in the
125 MHz 13C regime, some form of conformational rigidity is sug-
gested. We note that, whether the angular rotation of the vinyl group
is unrestricted (generating time-averaged molecular symmetry C2v),
or even limited to a “windshield wiper” motion (time-averaged Cs), it
will not break the symmetry of the molecule unless rotation of one
or more of the phenyl rings is inhibited.
In our hands, attempted direct lithiation of pentaphenylbenzene
gave very poor results. However, noting that iodo-pentaarylbenzenes
can be lithiated and subsequently functionalized,31 we chose to
prepare 1-bromo–2,3,4,5,6-pentaphenylbenzene 6 from 1-bromo-
2-phenylacetylene32 and tetracyclone 1. Treatment of 6 with n-bu-
tyllithium and then with ethyl chloroformate, or carbon dioxide
and acidic workup, led to ethyl 2,3,4,5,6-pentaphenylbenzoate
(12) or 2,3,4,5,6-pentaphenylbenzoic acid (13) in yields of 52% and
68%, respectively, based on pentaphenylbenzene recovered.
As illustrated in Fig. 9, pentaphenylbenzoic acid crystallized as a
hydrogen-bonded dimer. Although carboxylic acids are generally di-
rectly connected via their CO2H linkages, the steric bulk of the C6Ph5
fragment militates against such a situation. Instead, the hydrogen
bonding motif in 13 comprises two carboxylic acid molecules
bridged by two molecules of methanol; the carbonyl group interacts
with the methanol hydrogen (C=O···HOCH3 = 1.703 Å), while the
hydroxyl moiety is hydrogen bonded to the oxygen of the second
methanol (C(=O)O–H···O(H)CH3 = 1.513 Å).
The attempted acid-catalyzed esterification of 13 with
propargyl alcohol, using dilute sulfuric acid, led not to the ester
but instead proceeded with ring closure to form 1,2,3,4-
tetraphenylfluorenone (14) that was identified unequivocally
by X-ray crystallography. One can readily envisage (Scheme 3)
initial generation of the sterically protected acylium ion (15),
analogous to the known behaviour of other 2,6-disubstituted ben-
zoic acids or esters bearing bulky substituents.33
As shown in Fig. 10, the fluorenone skeleton is planar and, to
minimize steric repulsions, the four phenyl substituents are
twisted out of the central plane by an average of 73°. Moreover,
the C−C bonds in the substituted ring were slightly longer (aver-
age 1.402 versus 1.390 Å) than in the other benzo ring.
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1104
Can. J. Chem. Vol. 91, 2013
Fig. 9. Hydrogen-bonding pattern in the dimer [C6Ph5CO2H·CH3OH]2 (13).
•
•
•
•
•
•
•
•
Scheme 3. Reaction of C6Ph5CO2H with acid and formation of its propargyl ester.
O
OH
O
C
+
O
Ph
Ph
H2SO4
- H2O
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
13
15
14
HOCH2-C
DMAP
DCC
CH
CH
H
C
OCH2-C
O
OCH2-C
O
Co(CO)3
Co2(CO)8
Co(CO)3
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
16
18
diate is depicted in Scheme 4. The lengthening of the side chain
in 16 removes the steric problems associated with the directly
attached alkyne in 1-phenylethynyl-2,3,4,5,6-pentaphenylben-
zene 3 and allows ready formation of the tetrahedral dicobalt
cluster (C6Ph5-CO2-CH2-C'CH)Co2(CO)6 (18).
Fig. 10. Molecular structure of 1,2,3,4-tetraphenylfluorenone (14).
O
C8
C8a
C7
C11
C9
Correlation of ortho and para phenyl twist angles
C9a
C1
C2
C6
An early X-ray study on the hexaphenylbenzene-anisole clathrate
indicated a propeller-type (D6d) structure with peripheral phenyl
twist angles of ϳ75°.20 However, more recent reports by Spek (21a)
and by Munakata (21b) revealed that in hexaphenylbenzene itself,
the dihedral angles ranged from 65° to 89° (average 79.5°). In the
symmetrical polyaryl system 4,4=,4== ,4ٞ-tetrakis(pentaphenylphen-
yl)tetraphenylmethane, (p-C6Ph5-C6H4)4C (19), which effectively con-
tains four hexaphenylbenzene units, the peripheral phenyls adopt
an almost ideal propeller conformation with somewhat smaller
twist angles of 64° 2°. This was rationalized by regarding the C6Ph5
fragments as rigid groups with only a few degrees of rotational free-
dom such that the molecule maintains its tetrahedral symmetry. The
packing is a distorted body-centered cubic lattice typical for the crys-
tallization of solid spheres.34 In contrast, in cases of severe steric
encumbrance such as 3,3=,5,5=-tetrakis(pentaphenylphenyl)biphen-
yl (20), the phenyl orientations range from 26° to 89°.34 However, the
average of these 24 dihedral angles is 66°, again showing how the
molecules adjust their conformations to find the minimum energy
structure.
C4b
C4a
C5
C4
C3
C21
C41
C31
In contrast, the corresponding base-catalyzed esterification
furnished the desired propargyl 2,3,4,5,6-pentaphenylbenzoate
(16) and also a Meyer−Schuster rearrangement type product
identified spectroscopically as 2-oxopropyl 2,3,4,5,6-pentaphen-
ylbenzoate (17). It is likely that 17 arose during the acidic
workup, and a possible mechanism involving a cyclic interme-
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Brydges et al.
1105
Scheme 4. Mechanistic proposal for the acid-mediated conversion of 16 into 17.
+
H
H
H
O H2
H
O H
O
O
O
O
O
+
O
O
O
+
O
H
- H
Ph
Ph
Ph
Ph
H
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
16
17
C6Ph5
Ph
Ph
Fig. 11. Correlation of peripheral ortho and para phenyl twist angles
in bulky C6Ph5X compounds.
Ph
Ph
Ph
Ph
Ph
Ph
85
Ph
Ph
Ph
Ph
Br (6)
C
carboranyl (5)
80
C6Ph5
Ph5C6
CBr=C(Br)Ph (11)
75
Ph
Ph
Ph
Ph
Ph Ph
bornenyl (9)
Br (6)
Ph
Ph
C6Ph5
70
65
60
55
19
20
C6Ph5 (4)
PhC(Br)=CBr (11)
Returning to the C6Ph5X system, solid-state structural data
(Table 1) reveal that, in those cases where the bulky “X” groups
most severely distort the propeller conformation, increasing the
volume of “X” drives the ortho phenyls farther out of the plane of
the central ring, and this effect is transmitted via the meta to the
para ring. As shown in Fig. 11, for bulky “X” substituents, there is
a clear correlation of the twist angle of the ortho phenyl ring with
that of its para counterpart whereby the ortho twist angle can
range from 80° to 60° and vice versa for the para phenyl.
ferrocenyl (7)
75
60
65
70
80
85
Para phenyl twist angles (o)
separation, two yellow products (25 and 26), each with infrared
absorptions at 1970 and 1897 cm−1, typical of (␩6-arene)Cr(CO)3
complexes. The 1H NMR spectrum of 25, the first material to elute,
exhibited signals at 5.30 and 5.18 ppm, clearly indicating compl-
exation to a peripheral phenyl ring,42 and a singlet at 7.66 ppm,
typical of the unique ring proton in pentaphenylbenzene. In con-
trast, in the second product 26, the lack of 1H NMR resonances in
the region 5–6 ppm clearly suggested that the metal was bonded
to the central ring. These structures were confirmed X-ray crystal-
lographically, as illustrated in Figs. 12 and 13.
In 25, the tricarbonylchromium moiety is sited on an ortho
phenyl ring and is oriented towards the open space adjacent to
the central ring hydrogen, thus avoiding contact with the ad-
jacent meta phenyl substituent. The twist angles of the periph-
eral phenyls (average 62°) are listed in Table 1, and the greatest
deviation (70.7°) from coplanarity with the central ring is ex-
hibited by the phenyl substituent at C5 i.e., the group sited para
relative to the most bulky substituent (the metal-complexed
ring).
The second fraction to elute was the centrally coordinated
complex 26; it was also characterized X-ray crystallographically
and, similar to the situation in 2, 5, and 25, the central ring was
again flattened with internal angles C5−C6−C1, C6−C1−C2, and
C1−C2−C3 of 118.8°, 122.5°, and 119.3°, respectively. The periph-
eral phenyl twist angles in 26 range from 48° to 65° (average
58°), and the C−C bonds in the central ring are somewhat lon-
ger (average 1.423 Å) than in pentaphenylbenzene itself
(1.387 Å); they are also longer than those in the ortho phenyl
bearing the chromium tricarbonyl in 25 (1.405 Å). As shown in
Fig. 14, in the solid state, (␩6-C6Ph6)Cr(CO)3 26 adopts the piano-
stool geometry whereby the three carbonyls are positioned be-
tween the phenyl rings, so as to minimize repulsions. Interestingly,
Chromium tricarbonyl complexes of pentaphenylbenzene
We are unaware of any organometallic complexes of penta-
phenylbenzene, but several derivatives of hexaphenylbenzene
appear in the literature. Since the standard route to [(␩6-arene)RuX2]2
systems involves Birch reduction of the arene to the corresponding
cyclohexadiene and subsequent reaction with RuCl3,35 hexaphen-
ylbenzene was treated with sodium in ethanol but, surprisingly, this
afforded all-cis-1,2,3,4,5,6-hexaphenylcyclohexane.13 There is a report
of a sandwich complex [(C6Ph6)Ru(C5H5)][BF4] in which the metal is
thought to be bonded to the central ring, but no X-ray crystallo-
graphic data are available.36 Other organometallic complexes
of hexaphenylbenzene include (␩6-C6Ph6)Co(␩2-C2Ph2) (21),37 (␩6-
C6Ph6)Mo(␩5-C5H5) (22),38 (␩6-C6Ph6)V(CO)3 (23),39 and (C6Ph6)Ag2(ClO4)2
(24).40 The cobalt and molybdenum complexes have been crystal-
lographically characterized as being centrally bonded; in the sil-
ver complex 24, the metals bridge pairs of adjacent peripheral
rings via ␩ and ␩ linkages, but the identification of the vana-
dium system 23 rests only on spectroscopic and microanalytical
data. In (␩6-C6Ph6)Co(␩2-C2Ph2) 21, the peripheral phenyls form an
almost ideal propeller with twist angles of 67° 3.5°; presumably
the presence of the alkyne-cobalt moiety forces the hexaphen-
ylbenzene structure into a rather flatter propeller. This effect is
amplified somewhat in (␩6-C6Ph6)Mo(␩5-C5H5) 22 whereby the di-
hedral angles range from 55.5° to 68.4° (average 63.2°). Finally, an
attempt to coordinate a tricarbonylchromium unit to the central
ring of hexaphenylbenzene led instead to (␩6-C6Ph5-C6H5)Cr(CO)3
in which the metal was bonded to a peripheral ring; the barrier to
rotation of the (␩6-C6H5)Cr(CO)3 fragment relative to the central
ring was found to be 12.2 kcal mol−1.41
1
2
The reaction of pentaphenylbenzene 2 with chromium hexacar-
bonyl in refluxing n-dibutyl ether yielded, after chromatographic
Published by NRC Research Press

1106
Can. J. Chem. Vol. 91, 2013
Fig. 12. Molecular structure of [(␩6-C6H5)-C6Ph4H]Cr(CO)3 (25).
Fig. 14. Bird’s eye view of (␩6-C6Ph5H)Cr(CO)3 (26) showing the
staggered orientation of the Cr(CO)3 tripod relative to the peripheral
phenyl substituents.
O8
C8
O9
O7
C7
C9
C64
C63
C61
Cr
C24
C65
C26
C2
C62
C1
C23
C66
C52
C22
C21
C3
C6
C36
C5
C51
C53
C35
C34
C4
C31
C32
C56
C55
C41
C54
C33
C46
C42
C43
C45
C44
C6Ar6,45 or as metal complexes of the type (␩n-CnArn)MLx, where
n is 4,27,46 5,47 or 6.35–41
Fig. 13. Molecular structure of (␩6-C6Ph5H)Cr(CO)3 (26).
In the 1970s, in a series of now classic publications, Gust et al.
established that isomerization in C6Ar6 systems occurs via a
mechanism wherein an individual peripheral ring rotates in an
uncorrelated fashion by approximately ␲ radians.45 Rotational
barriers of 33 or 38 kcal mol−1, respectively, are engendered by
the incorporation of ortho methoxy or methyl substituents;
these barriers fall to 17 kcal mol−1 for meta substituents. In-
creasing the ring size, as in heptaphenyltropylium, [C7Ph7]+,
might have been expected to enhance the barrier to phenyl
rotation but, in fact, the system adopts a shallow boat confor-
mation so as to minimize such interactions.48
As is evident from the X-ray crystallographic data presented
above (Table 1; Fig. 11), rotating rings also detect the nonbonding
repulsions from remote sites around the circumference of the
central benzene ring. Such buttressing effects were evinced by the
free energies of activation for pentaphenylbenzene derivatives,
C6Ph5X, bearing substituents of varying size; these range from
15.5 (X = H, 1) to 18.7 kcal mol−1 (X = CMe3).49 In all of the C6Ph6 and
C6Ph5X systems reported to date, the ⌬G≠ values have been as-
cribed to diastereomeric interconversions, since enantiomeriza-
tion necessarily involves rotation of all substituted rings and is
therefore energetically more demanding.
C43
C53
C52
C35
C44
C45
C42
C36
C31
C54
C34
C33
C41
C51
C5
C3
C62
C56
C25
C2
C6
C21
C24
C23
C61
C66
C22
Cr
C64
C7
O7
C65
C8
C9
O9
O8
the average chromium-to-ring-carbon distances are identical for
both complexes (2.220 Å).
Phenyl orientations and nano-machinery
Our original idea was to construct a molecular brake that would
block peripheral phenyl rotation by means of a base-promoted
␩6- to ␩5-haptotropic shift50 of an organometallic unit across a
fluorenyl framework (Scheme 5), thus moving the bulky M(CO)3
fragment closer to the C6Ph5 moiety.
In the event, the attempted synthetic route to the desired
9-fluorenyl-pentaphenylbenzene, by Diels−Alder addition of
9-phenylethynylfluorene51 to tetracyclone 1, led instead to a diin-
denotetracene by a complex rearrangement mechanism that has
been fully elucidated elsewhere.52 The successful construction of an
organometallic molecular brake that utilized a haptotropic shift pro-
cess was finally achieved by incorporation of a triptycene paddle-
wheel rather than a pentaphenyl group (Scheme 6).53 Migration of
the metal tricarbonyl moiety from the six-membered to the five-
membered ring of the indenyl fragment severely hinders rotation of
the paddlewheel.54,55
Molecular architectures comprised of contiguous groups at-
tached to a planar central framework continue to attract the
attention of stereochemists. Molecules of the type CnRnx (n =
3−7) endowed with a propeller form (R = aryl) or gear-meshed
ground states (R = alkyl) present torsional circuits that may be
greatly influenced by the nonbonded repulsions of voluminous
substituents. One may view this particular series in terms of
the reciprocal relationship between n and the angle ␻ subten-
ded by adjacent moieties at the centre of the internal ring (␻ =
360°/n = 120°, 90°, 72°, 60°, and 51.4°, respectively). Although
increasing the ring size lengthens the radial distance of the
external groups from the centre of the molecule, this is more
than compensated for by the diminishing value of ␻. As a result
of cyclic expansion, the peripheral rotors are successively
placed in a more crowded locale. This steric design has been
explored in terms of correlated rotations,43 a prospect that has
gained more relevance with the intense current interest in
nano-machinery.44 Towards the construction of gears and
brakes, several studies have shed light on the structural attri-
butes and dynamic behaviour of the peripheral substituents in
multi-n-rotor molecular propellers, either as hexaarylbenzenes
Finally, we note that, as emphasized by Mislow56 and by Biali,57
transmission of cooperative torsional motion along chains of suit-
ably meshed N-chemical gears is governed by the parity rule for
gear trains, which dictates correlated disrotation of the terminal
gears when N is even and conrotatory interactions when N is odd.
Thus, in arranging an odd number of gears in a cyclic array, dy-
Published by NRC Research Press

Brydges et al.
1107
Scheme 5. Potentially reversible haptotropic shift of an organometallic moiety to block peripheral phenyl rotations.
-
MLn
MLn
?
- H+
H
H
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
H+
Ph
Ph
Ph
Preparation of 1-bromo-2,3,4,5,6-pentaphenylbenzene (6)
Scheme 6. An organometallic molecular brake mediated by an
␩6- to ␩5-haptotropic shift.
Tetraphenylcyclopentadienone (22.0 g, 63.13 mmol) and 1-bromo-
2-phenylacetylene (16.63 g, 91.87 mmol) were heated at reflux in
toluene (250 mL) for 20 h. After removal of the toluene in vacuo
and addition of cyclohexane (30 mL), the solution was filtered and
the white solid washed with cyclohexane until the filtrate was
colourless. The residue was dried to yield bromopentaphenylben-
base
acid
zene 6 (25.77 g, 47.95 mmol, 76%), mp 289–290 °C (lit. 281–
1
282 °C17). H NMR (500 MHz, CDCl3) ␦: 7.21 (4H, m, H23, H25, H63
,
H65), 7.19 (4H, m, H22, H26, H62, H66), 7.15 (2H, m, H24, H64), 6.88 (2H,
m, H34, H54), 6.86 (4H, m, H33, H35, H53, H55), 6.83 (4H, m, H32, H36
H52, H56), 6.83 (2H, m, H43, H45), 6.80 (2H, m, H42, H46), 6.80 (1H, m,
H44). 13C NMR (125 MHz, CDCl3) ␦: 142.1 (C31, C51), 142.0 (C2, C6),
141.7 (C21, C61), 140.7 (C41), 140.1 (C3, C5), 139.8 (C4), 131.2 (C42, C46),
,
-
Cr(CO)3
Cr(CO)3
131.0 (C33, C35, C53, C55), 130.6 (C22, C26, C62, C66), 127.5 (C23, C25
,
C63, C65), 126.8 (C34, C54), 126.8 (C32, C36, C52, C56), 126.7 (C24, C64),
125.8 (C43, C45), 125.6 (C44) 125.3 (C1). IR (solid, KBr): 3477, 3415,
3077, 3054, 3023, 1598, 1494, 1440, 1382, 1276, 1074, 1027, 773,
696 cm−1. Elemental analysis calcd. (%) for C36H25Br: C 80.45,
H 4.69, Br 14.87; found: C 80.58, H 4.83, Br 14.75. A sample of 6
suitable for an X-ray diffraction study was obtained by recrystalli-
zation from dichloromethane/hexane.
namic gearing is disallowed and immobilization of the rotors can
be expected. This is precisely the situation in pentaphenylben-
zenes, C6Ph5X, and so introduction of a voluminous “X” group, in
conjunction with suitably substituted phenyl rings, may be ex-
pected to lead to restricted peripheral ring rotation.
Conclusions
We here report the syntheses and structures of a series of
pentaphenylbenzenes, C6Ph5X, bearing substituents encom-
passing a wide range of steric demand. When the “X” substitu-
ent is very bulky, there is a clear transmission of nonbonded
interactions as indicated by the orientations of the ortho and
para phenyls relative to the central ring plane. Moreover, in
contrast with the hexaphenylbenzene case, the X-ray crystal
Preparation of 1-(2-bornenyl)-2,3,4,5,6-
pentaphenylbenzene (9)
2-(Phenylethynyl)-isoborneol 8 (0.666 g, 2.62 mmol) and tetracy-
clone (0.600 g, 1.56 mmol) were combined neat in an open flask
and flame heated (>200 °C). After approximately 40 min. the mol-
ten mixture appeared to have changed from magenta to dark
brown in colour, and diphenyl ether (10 mL) was added to the
melt. To effect precipitation, hexanes (20 mL) were added drop-
wise to the cooled reaction mixture. The product was filtered
under suction, reprecipitated in ethanol until the red colour of
unreacted tetracyclone had disappeared, and then dried in vacuo
to give 1-(2-bornenyl)-2,3,4,5,6-pentaphenylbenzene 9 as an off-
white solid (0.157 g, 0.26 mmol, 17%). 1H NMR (200 MHz, CDCl3) ␦:
6.59–7.09 (25H, m), 5.91 (1H, m, H3), 2.15 (2H, m), 1.51 (2H, m), 1.09
(1H, m), 0.75 (3H, s, Me), 0.40 (3H, s, Me), 0.2 (3H, s, Me). 13C NMR
(50 MHz, CDCl3) ␦: 140.9, 136.5 (ipso), 131.7, 131.4, 130.6, 126.7, 126.4,
125.9, 124.8 (ortho, meta, para, and C=C), 29.7, 25.3, 20.3, 17.7, 13.7,
12.8 (bornene ring system). LRMS (DEI), m/z (%), 592 (100) [M]+, 564
(45) [M – C2H4]+, 549 (25) [M – C3H7]+. LRMS (DCI) m/z (%): 592 (100)
[M]+, 564 (30) [M – C2H4]+, 469 (10) [C7Ph5]+. HRMS: m/z calcd. for
C46H40: 592.3101 [M]+; found: 592.3130. Anal. calcd. for C46H40 (%):
C 93.20, H 6.80; found: C 92.98, H 6.94. The growth of appropri-
ately sized crystals of 9 suitable for an X-ray diffraction study
proved to be difficult, although data were eventually collected on
a sample extracted from an NMR tube (deuterated chloroform)
stored in refrigeration.
structure of 20 showed unequivocally that
a tricarbon-
ylchromium fragment can be attached to the central ring of
pentaphenylbenzene, thus making available a bulky organo-
metallic fragment for incorporation into potentially bioactive
or fluorescent molecules while expanding our repertoire of
prototypes for potentially tunable, sterically controlled
molecular-scale propellers and gears.
Experimental
All reactions were carried out under a nitrogen atmosphere
unless otherwise stated. Merck silica gel 60 (230–240 mesh) was
1
used for flash chromatography. H and 13C NMR spectra were
acquired on a Bruker AC-200, Bruker Avance DRX-500, or Var-
ian INOVA 400, 500, or 600 MHz spectrometer at 30 °C, unless
1
otherwise stated. Assignments were based on standard H−1H
and 1H−13C two-dimensional techniques and NOE measure-
ments. Low- and high-resolution mass spectra were acquired on
VG ZAB-E or ZAB-R instruments, respectively. Melting points
were determined on a Gallenkamp instrument in air and are
uncorrected. Elemental analyses were carried out by the Micro-
analytical Laboratory at the University College Dublin or by
Guelph Chemical Laboratories (Guelph, Ontario). Pentaphen-
ylbenzene (2), 1-phenylethynyl–2,3,4,5,6-pentaphenylbenzene
(3), 1-bromo-2-phenylethyne, and 2-(phenylethynyl)-isoborneol
(8) were prepared as described previously.15,17,32,58
Preparation of 1-(1=,2=-Z-dibromo-2=-phenyl)vinyl-2,3,4,5,6-
pentaphenylbenzene (11)
To 1-phenylethynyl-2,3,4,5,6-pentaphenylbenzene 3 (0.279 g,
0.50 mmol) dissolved in CCl4 (10 mL) were added two to three
drops (large excess) of bromine, and the solution was stirred vig-
Published by NRC Research Press

1108
Can. J. Chem. Vol. 91, 2013
orously at room temperature for 5 days. Following solvent re-
moval, the isolated solid was washed successively with CCl4 and
acetone and then dried in vacuo. Recrystallization from CCl4/ace-
tone yielded 11 as a yellow solid (358 mg, 0.49 mmol, 98%). 1H NMR
(200 MHz, CDCl3). The presence of two components was verified
when a sample dissolved in 90:10 acetonitrile−water was injected
Preparation of 2,3,4,5,6-pentaphenylbenzoic acid (13)
To bromopentaphenylbenzene 6 (13.0 g, 24.19 mmol) in dry THF
(250 mL) under nitrogen at −78 °C was added BuLi (17.0 mL of a
1.6 mol L−1 solution, 27.20 mmol). The solution was stirred for
20 min, warmed to room temperature, and lumps of dry ice (ex-
cess) were added. The solution was then stirred until it reached
room temperature, quenched with dilute HCl, and the product
extracted using ethyl acetate. The organic layer was washed with
water and brine, dried over MgSO4 to give a white solid, and
chromatographed on silica using dichloromethane as eluent to
obtain recovered pentaphenylbenzene (1.86 g, 4.06 mmol). The
eluent was then changed to ethyl acetate to yield pentaphenyl-
benzoic acid 13 as a white solid (6.90 g, 13.73 mmol, 57%), mp
352–353 °C (lit. 342 °C17), 68% based on recovered pentaphenylben-
zene. 1H NMR (500 MHz, DMSO-d6) ␦: 12.40 (1H, s, OH), 7.16 (4H, dd,
1
into a 4.6 × 150 mm HPLC analytical column. H NMR (500 MHz,
CD2Cl2)* ␦: 7.29–7.26 (2H, m), 7.25–7.14 (8H, m), 7.09 (2H, m), 6.93–
6.90 (4H, m), 6.86–6.82 (8H, m), 6.77–6.75 (4H, m), 6.51–6.49 (2H,
m). 13C NMR (125 MHz, CDCl3) ␦: 143.4 (1C), 142.3 (2C), 140.8 (1C),
140.6 (2C), 140.5 (2C), 139.7 (1C), 138.6 (1C), 137.3 (1C) (ipso and cen-
tral ring), 132.0, 131.8, 131.4, 131.1, 130.7, 130.7, 129.1, 129.0, 128.7,
128.6, 127.5, 127.4, 127.2, 127.1, 126.9, 126.4, 126.0 (ortho, meta, para,
and C=C)*. LRMS (DEI) m/z (%): 716 (10) [M]+, 637 (5) [M – Br]+, 558
(100) [M – 2Br]+, 481 (25) [M – 2Br – Ph]+, 404 (20) [M – 2Br – 2Ph]+.
LRMS (DCI) m/z (%): 716 (15) [M]+, 637 (5) [M – Br]+, 558 (100) [M –
2Br]+, 481 (30) [M – 2Br – Ph]+ (peaks quoted correspond to the
lower mass (79Br); isotopic distribution is evident. HRMS: m/z
calcd. for C44H30Br2: 716.0714 [M]+; found: 716.0616. (*Resonances
corresponding to a second isomer (ϳ18%) were also observed in
J = 7 Hz, J = 2 Hz, H22, H26, H62, H66), 7.11 (4H, t, J = 7 Hz, H23, H25
,
H63, H65), 7.07 (2H, m, H24, H64), 6.88 (4H, m, H32, H36, H52, H56),
6.87 (2H, m, H34, H54), 6.86 (4H, m, H33, H35, H53, H55), 6.85 (2H, m,
H43, H45), 6.82 (2H, m, H42, H46), 6.80 (1H, m, H44). 13C NMR
(125 MHz, DMSO-d6) ␦: 169.3 (C11), 141.1(C41), 140.1 (C31, C51), 139.6
(C4), 139.2 (C3, C5), 138.8 (C21, C61), 136.4 (C2, C6), 135.8 (C1), 130.8
(C32, C36, C52, C56), 130.7 (C42, C46), 130.2 (C22, C26, C62, C66), 127.0
(C23, C25, C63, C65), 126.6 (C33, C35, C53, C55), 126.5 (C43, C45), 126.4
(C24, C26), 125.6 (C34, C54), 125.5 (C44). IR (solid, KBr): 3484, 3403,
1701 (CO), 1637, 1494, 1403, 1324, 1261, 1074, 1027, 700 cm−1. A
sample of 13 suitable for an X-ray diffraction study was obtained
by recrystallization from dichloromethane/methanol/hexane.
1
the H NMR spectrum.) Anal. calcd. for C44H30Br2 (%): C 73.55, H
4.21, Br 22.24; found: C 73.80, H 4.35, Br 22.07. X-ray quality crys-
tals of the cis isomer, presumably the major product, were isolated
from the CH2Cl2/hexanes supernatant.
Preparation of ethyl 2,3,4,5,6-pentaphenylbenzoate (12)
To bromopentaphenylbenzene 6 (1.00 g, 1.86 mmol) in dry THF
(20 mL) at −78 °C was added BuLi (1.30 mL of a 1.6 mol L−1 solution
in hexane, 2.08 mmol). The solution was stirred for 20 min,
warmed slowly to room temperature, and ethyl chloroformate
(0.22 mL, 2.30 mmol) was added. The solution was heated at reflux
for 20 h, cooled to room temperature, quenched with H2O, and
the product extracted with ethyl acetate. After being washed with
water and brine and dried over MgSO4, the residue was chromato-
graphed on silica, using ethyl acetate (20%) in pentane as eluent,
to give two fractions. The first was pentaphenylbenzene (621 mg,
1.35 mmol); the second was a white solid identified as ethyl
pentaphenylbenzoate 12 (141 mg, 0.27 mmol, 14%), mp 318–319 °C,
52% based on recovered pentaphenylbenzene. 1H NMR (500 MHz,
CDCl3) ␦: 7.16 (4H, dd, J = 8 Hz, J = 2 Hz, H22, H26, H52, H56), 7.12 (4H,
Preparation of 1,2,3,4-tetraphenylfluorenone (14)
To pentaphenylbenzoic acid 13 (500 mg, 0.99 mmol) and propar-
gyl alcohol (0.08 mL, 1.37 mmol) in toluene (20 mL) was added
concentrated H2SO4 (2 mL) and the mixture heated at reflux for
3 h. After removal of the toluene, the resulting yellow solid was
dissolved in dichloromethane, washed with NaHCO3, water, and
brine, dried over MgSO4, and concentrated to give a yellow solid.
Chromatography on silica using a gradient of ethyl acetate (5% ¡
10% ¡ 50%) in pentane yielded 1,2,3,4-tetraphenylfluorenone 14 as
a yellow solid (470 mg, 0.97 mmol, 98%), mp 303–304 °C (lit.
298 °C59,60). 1H NMR (600 MHz, CDCl3) ␦: 7.55 (1H, d, J = 7 Hz, H8),
7.28 (2H, m, H43, H45), 7.26 (1H, t, J = 7.5 Hz, H44), 7.22 (2H, m, H42
,
H46), 7.20 (2H, m, H23, H25), 7.19 (1H, m, H24), 7.16 (1H, t, J = 7 Hz, H7),
7.13 (2H, dd, J = 7 Hz, J = 1.5 Hz, H22, H26), 7.10 (1H, dt, J = 7.5 Hz, J =
1.5 Hz, H6), 6.88 (4H, m, H13, H15, H32, H36), 6.87 (1H, m, H34), 6.86
(1H, m, H14), 6.84 (2H, t, J = 6.5 Hz, H33, H35), 6.79 (2H, d, J = 6.5 Hz,
H12, H16), 6.13 (1H, d, J = 7.5 Hz, H5). 13C NMR (150 MHz, CDCl3) ␦:
193.0 (C9), 147.8 (C31), 143.7 (C4b), 142.9 (C1), 141.7 (C4a), 140.6 (C2),
139.1 (C3), 138.7 (C11), 138.6 (C41), 136.9 (C21) 136.9 (C4), 135.2 (C8a),
134.2 (C6), 131.2 (C12, C16), 130.7 (C33, C35), 130.1 (C9a), 130.0 (C42, C46),
m, H23, H25, H63, H65), 7.11 (2H, m, H24, H64), 6.88 (4H, m, H33, H35
H53, H55), 6.87 (2H, m, H34, H54), 6.85 (1H, m, H44), 6.84 (2H, m, H43
,
,
H45), 6.80 (4H, dd, J = 7 Hz, J = 2 Hz, H32, H36, H52, H56), 6.76 (2H, dd,
J = 7 Hz, J = 2 Hz, H42, H46), 3.67 (2H, q, J = 7 Hz, H12), 0.72 (3H, t, J =
7 Hz, H13). 13C NMR (125 MHz, CDCl3) ␦: 169.1 (C11), 142.4 (C4), 140.6
(C3, C5), 140.0 (C41), 139.5 (C31, C51), 139.3 (C21, C61), 138.0 (C2, C6),
135.0 (C1), 131.4 (C32, C36, C52, C56), 131.2 (C42, C46), 130.2 (C22, C26
,
129.9 (C22, C26), 128.7 (C7), 128.4 (C43, C45), 127.5 (C44), 127.3 (C23
,
C62, C66), 127.4 (C23, C25, C63, C65), 126.9 (C33, C35, C53, C55), 126.8
(C43, C45), 126.7 (C24, C64), 125.7 (C34, C54), 125.6 (C44), 60.8 (C12), 13.6
(C13). IR (solid, KBr): 3449, 3056, 3025, 2925, 2850, 1730 (CO), 1600,
1495, 1441, 1326, 1227, 1158, 1070, 908, 788 cm−1. Anal. calcd. for
C39H30O2·(EtOAc)0.25 (%): C 86.92, H 5.85; found: C 86.88, H 5.84.
C25), 127.0 (C24), 126.9 (C13, C15), 126.9 (C32, C36), 126.1 (C34), 125.9
(C14), 123.8 (C8), 123.3 (C5). IR (solid, KBr): 3423, 3241, 3056, 2919,
2850, 1708 (CO), 1602, 1440, 1205, 1087, 954, 698 cm−1. A sample of
14 suitable for an X-ray diffraction study was obtained by recrys-
tallization from dichloromethane.
H
O
14
13
12
13
14
63
12
25
63
25
O
2
O
6
O
2
O
26
21
64
65
24
23
64
65
26
21
24
23
62
61
62
61
11
1
1
1
6
66
52
22
36
22
66
52
36
3
5
3
5
51
31
31
51
53
54
35
34
53
54
35
34
4
4
41
41
56
42
56
32
46
32
46
42
33
55
33
55
43
45
45
43
16
17
44
44
Published by NRC Research Press

Brydges et al.
1109
Table 2. Crystallographic data for 2, 3, 6, and 8.
2
3
6
8
Formula
M
Crystal system
Space group
a (Å)
b (Å)
c (Å)
␣ (°)
␤ (°)
C36H26
458.57
C44H30
558.68
C36H25Br
537.47
C18H22O
234.36
Monoclinic
P21 (#4)
7.476(2)
11.482(4)
17.239(6)
90
90.340(5)
90
1479.8(8)
4
Triclinic
Triclinic
Triclinic
ˉ
ˉ
ˉ
P1 (#2)
P1 (#2)
P1 (#2)
10.158(3)
12.101(3)
13.017(4)
63.241(5)
68.374(5)
65.662(5)
1268.3(6)
2
11.0833(3)
13.3548(3)
21.94640(10)
103.1890(10)
94.800(2)
93.643(2)
3140.17(11)
4
11.409(2)
12.085(2)
19.780(4)
83.909(3)
77.544(4)
79.535(4)
2612.2(8)
4
␥ (°)
V (Å3)
Z
␳
calcd. (g cm−3
)
1.201
1.182
1.367
1.142
T (K)
100(2)
0.068
484
299(2)
0.067
1176
100(2)
1.587
1104
100(2)
0.068
552
␮ (mm−1
F(000)
)
␪ range for data collection
Index ranges
1.80 to 23.36
−11 ≤ h ≤ 11
−13 ≤ k ≤ 13
−14 ≤ l ≤ 14
8342
1.57 to 22.50
−11 ≤ h ≤ 11
−14 ≤ k ≤ 14
−23 ≤ l ≤ 23
17565
1.06 to 24.00
−13 ≤ h ≤ 13
−13 ≤ k ≤ 13
−22 ≤ l ≤ 22
37592
1.77 to 25.35
−9 ≤ h ≤ 8
−13 ≤ k ≤ 13
−20 ≤ l ≤ 18
9727
Reflections measured
Reflections used
3656
8192
8199
4555
Rint
0.0482
3656/97/325
0.0291
8192/0/794
0.0391
8199/0/667
0.0324
4555/1/335
Data/restraints/parameters
Final R values (I > 2␪(I))
R1
wR2
0.1039
0.2711
0.0447
0.1041
0.0353
0.0900
0.0814
0.2744
R values (all data)
R1
wR2
0.1171
0.2817
1.081
0.0779
0.1218
1.008
0.0433
0.0947
1.044
0.0838
0.2763
1.195
Goodness-of-fit on F2
Table 3. Crystallographic data for 11, 13, 14, 25, and 26.
11
13
14
25
26
Formula
M
Crystal system
Space group
a (Å)
b (Å)
c (Å)
␣ (°)
␤ (°)
C44H30Br2
718.50
C37H26O2·CH3OH (C37H24O)4·CH2Cl2 C39H26O3Cr·(C6H6)3 C39H26O3Cr
534.62
Monoclinic
P21/c (#14)
11.676(2)
5.7835(10)
42.903(7)
90
95.696(3)
90
2882.9(9)
4
2023.17
Tetragonal
I41/a (#88)
24.7661(10)
24.7661(10)
16.7547(13)
90
828.92
594.60
Triclinic
Triclinic
Monoclinic
P21/c (#14)
12.7827(10)
18.5716(14)
13.5031(10)
90
115.214(1)
90
2900.2(4)
4
ˉ
ˉ
P1 (#2)
P1 (#2)
11.5433(5)
16.1621(8)
21.5017(10)
73.733(2)
87.728(2)
71.871(2)
3654.7(3)
4
10.2723(8)
14.4582(12)
15.9124(13)
72.443(2)
80.507(2)
76.123(2)
2176.5(3)
2
90
90
␥ (°)
V (Å3)
Z
10276.7(10)
4
␳
calcd. (g cm−3
)
1.306
1.232
1.308
1.265
1.362
T (K)
299(2)
2.246
1456
100(2)
0.077
1128
100(2)
0.127
4232
100(2)
0.309
868
100(2)
0.433
1232
␮ (mm−1
F(000)
)
␪ range
Index ranges
1.38 to 22.50 1.75 to 26.00
−12 ≤ h ≤ 11 −14 ≤ h ≤ 14
−17 ≤ k ≤ 17 −7 ≤ k ≤ 7
−13 ≤ l ≤ 23 −52 ≤ l ≤ 52
1.47 to 26.39
−30 ≤ h ≤ 29
−30 ≤ k ≤ 30
−20 ≤ l ≤ 11
31492
5263
0.0257
5263/0/466
2.05 to 26.00
−12 ≤ h ≤ 12
−17 ≤ k ≤ 17
−19 ≤ l ≤ 19
18860
8519
0.0226
8519/0/550
1.76 to 30.56
−18 ≤ h ≤ 18
−26 ≤ k ≤ 26
−19 ≤ l ≤ 19
32157
8772
0.0360
8772/0/492
Reflections measured
Reflections used
Rint
18159
23131
9565
5644
0.0886
0.0301
5654/0/479
Data/restraints/parameters 9565/0/830
Final R values (I > 2␪(I))
R1
wR2
0.0600
0.1314
0.0536
0.1327
0.0413
0.1039
0.0430
0.1115
0.0473
0.1279
R values (all data)
R1
wR2
0.1443
0.1520
0.849
0.0599
0.1370
1.092
0.0487
0.1091
1.026
0.0489
0.1166
1.035
0.0566
0.1335
1.051
Goodness-of-fit on F2
Published by NRC Research Press

1110
Can. J. Chem. Vol. 91, 2013
Preparation of propargyl 2,3,4,5,6-pentaphenylbenzoate (16)
To pentaphenylbenzoic acid 13 (600 mg, 1.19 mmol) and propar-
gyl alcohol (0.27 mL, 4.64 mmol) in dry toluene (50 mL) under
nitrogen was added dimethylaminopyridine (40 mg, 0.33 mmol)
and dicyclohexylcarbimide (300 mg, 1.45 mmol), and the solution
was heated at reflux for 20 h. Upon cooling to room temperature,
the solution was acidified using diluted HCl and extracted with
ethyl acetate, the organic layer was washed with water and brine,
dried over MgSO4, and concentrated to give a white solid. Chro-
matography on silica using first a gradient of dichloromethane in
pentane and subsequently ethyl acetate yielded three fractions.
The first fraction was identified as 2-oxopropyl pentaphenyl-
benzoate 17 (30 mg, 0.054 mmol, 5%), mp 266–267 °C, resulting
(Co-CO), 2057 (Co-CO), 2023 (Co-CO), 1733 (CO), 1600, 1442, 1322,
1220, 1153, 1072, 698 cm−1. Anal. calcd. for C46H28Co2O8·(H2O)0.25
(%): C 66.47, H 3.46; found: C, 66.44, H, 3.50.
Preparation of 2-[(␩6-phenyl)tricarbonylchromium]-3,4,5,6-
tetraphenylbenzene (25) and (␩6-pentaphenylbenzene)
tricarbonylchromium (26)
To pentaphenylbenzene (1.5 g, 3.27 mmol) in n-butyl ether
(20 mL) and dry THF (3 mL) under nitrogen was added Cr(CO)6
(720 mg, 3.27 mmol) and the solution was heated at reflux for 12 h.
After allowing the solution to cool to room temperature, the sol-
vent was removed to leave a yellow solid (2.26 g) that was chro-
matographed on silica using a gradient of diethyl ether in pentane
as eluent to yield two fractions, each of which was successfully
recrystallized from dichloromethane/hexane, under nitrogen, in
the absence of light. 2-[(␩6-Phenyl)tricarbonylchromium]-3,4,5,6-
tetraphenylbenzene (25): 1H NMR (400 MHz, CDCl3) ␦: 7.75 (1H, s,
H1), 7.26–6.74 (20H, m, noncomplexed aromatic rings), 5.30 (2H,
m, H62, H66), 5.18 ppm (3H, m, H63, H64, H65). IR (solid, KBr): 1970,
1897 cm−1. Anal. calcd. for C39H26O3Cr·3(C6H6) (%): C 82.59,
1
from the hydrolysis of the desired ester 16. H NMR (500 MHz,
CDCl3) ␦: 7.16 (4H, m, H22, H26, H62, H66), 7.13 (4H, m, H23, H25, H63
,
H65), 7.13 (2H, m, H24, H64), 6.87 (4H, m, H33, H35, H53, H55), 6.84
(2H, m, H34, H54), 6.84 (1H, m, H44) 6.82 (4H, m, H32, H36, H52, H56),
6.77 (2H, m, H43, H45), 6.75 (2H, m, H42, H46), 3.98 (2H, s, H12), 1.70
(1H, s, H14). 13C NMR (125 MHz, CDCl3) ␦: 203.2 (C13), 168.4 (C11), 142.9
(C4), 140.8 (C3, C5), 139.8 (C41), 139.3 (C31, C51), 139.0 (C21, C61), 138.3
(C2, C6), 133.8 (C1), 131.3 (C32, C36, C52, C56), 131.1 (C42, C46), 130.2
(C22, C26, C62, C66), 127.6 (C23, C25, C63, C65), 127.0 (C24, C64), 126.9
(C33, C35, C53, C55), 126.8 (C34, C54), 125.8 (C43, C45), 125.7 (C44), 68.9
(C12), 26.3 (C14). IR (solid, KBr): 3555, 3485, 3413, 3055, 3028, 2924,
1734 (CO), 1617, 1441, 1217, 1154, 1072, 698 cm−1. Anal. calcd. for
C40H30O3·0.5(H2O) (%): C 84.63, H 5.50; found: C 84.99, H 5.56. The
second fraction to elute was identified as propargyl pentaphenyl-
benzoate 16 (185 mg, 0.34 mmol, 29%), mp 310–311 °C, 82% based
on acid reacted. 1H NMR (500 MHz, CDCl3) ␦: 7.18 (4H, dd, J = 8 Hz,
J = 2 Hz, H22, H26, H62, H66), 7.14 (4H, m, H23, H25, H63, H65), 7.12 (2H,
m, H24, H64), 6.91 (4H, m, H33, H35, H53, H55), 6.90 (2H, m, H34, H54),
6.86 (2H, t, J = 7 Hz, H43, H45), 6.86 (1H, m, H44), 6.83 (4H, dd, J =
7.5 Hz, J = 2 Hz, H32, H36, H52, H56), 6.78 (2H, dd, J = 7.5 Hz, J = 2 Hz,
H42, H46), 4.17 (2H, d, J = 2.5 Hz, H12), 2.25 (1H, t, J = 2.5 Hz, H14).
13C NMR (125 MHz, CDCl3) ␦: 166.2 (C11), 142.8 (C4), 140.7 (C3, C5),
139.9 (C41), 139.4 (C31, C51), 139.1 (C21, C61), 138.3 (C2, C6), 134.1 (C1),
131.4 (C32, C36, C52, C56), 131.2 (C42, C46), 130.2 (C22, C26, C62, C66),
127.5 (C23, C25, C63, C65), 126.9 (C24, C64), 126.9 (C33, C35, C53, C55),
126.8 (C34, C54), 125.8 (C43, C45), 125.7 (C44), 77.1 (C13), 74.7 (C14), 52.1
(C12). IR (solid, KBr): 3498, 3027, 2931, 2854, 2127, 1957, 1733 (CO),
1600, 1494, 1442, 1324, 1216, 1153, 1060, 698 cm−1. Anal. calcd. for
C40H28O2·0.25(Et2O) (%): C 88.05, H 5.51; found: C 88.03, H 5.45. The
third fraction was unreacted pentaphenylbenzoic acid (389 mg,
0.774 mmol).
H
5.35; found: C 82.88, H
5.47. (␩6-Pentaphenylbenzene)-
tricarbonylchromium (26): 1H NMR (400 MHz, CDCl3) ␦: 7.25–6.75
(25H, m, noncomplexed aromatic rings). IR (solid, KBr): 1970,
1897 cm−1. Anal. calcd. for C39H26O3Cr (%): C 78.77, H 4.41; found: C
78.98, H 4.62.
X-ray measurements for 2, 3, 6, 8, 9, 11, 13, 14, 25, and 26
(Tables 2 and 3)
Crystal data for 2, 6, 13, 14, 25, and 26 were collected on a D8
Bruker diffractometer equipped with a Bruker Smart Apex CCD
area detector and graphite monochromated Mo-K␣ radiation (␭ =
0.71073 Å). Data for 3, 8, 9, and 11 were acquired on a P4 Siemens
diffractometer equipped with a Siemens SMART 1K CCD area de-
tector and a rotating anode, also utilizing graphite monochro-
mated Mo-K␣ radiation. A full sphere of reciprocal space was
scanned by phi-omega scans. Data processing was carried out by
use of the program SAINT,61 while the program SADABS62 was
utilized for the scaling of diffraction data and an absorption based
on redundant reflections. Structures were solved by using the
direct-methods procedure in the Bruker SHELX63 program library
and refined by full-matrix least-squares methods on F2. Depending
on the crystal quality, hydrogen atoms were either located in the
Fourier difference map and allowed to refine freely or were added
at calculated positions and refined using a riding model. For com-
pound 9, no significant diffraction was observed beyond 1.05 Å
resolution, resulting in an incomplete refinement of the corre-
sponding data; the phenyl rings were modelled as ideal hexagons
and all atoms were treated isotropically.64
Preparation of (propargyl 2,3,4,5,6-pentaphenylbenzoate)
hexacarbonyldicobalt (18)
A solution of propargyl pentaphenylbenzoate 16 (385 mg,
0.712 mmol) and Co2(CO)8 (360 mg, 1.05 mmol) was stirred in dry
THF (15 mL) at room temperature under nitrogen for 20 h and then
filtered through celite and washed with THF until the filtrate was
clear. Chromatography on silica using a gradient of dichloro-
methane in pentane as eluent yielded a red solid that was identi-
fied as (propargyl pentaphenylbenzoate)hexacarbonyldicobalt 18
(433 mg, 0.53 mmol, 74%), mp 242–243 °C (decomposed). 1H NMR
Supplementary material
Supplementary material is available with the article through
the journal Web site at http://nrcresearchpress.com/doi/suppl/
10.1139/cjc-2013-0257. CCDC 940512 (2), 940513 (26), 940514 (25),
940515 (6), 940516 (14), and 940517 (13) contain crystallographic
data for this manuscript. These data can be obtained free of
charge from the Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK (fax: + 44 1223 336033 or
e-mail: deposit@ccdc.cam.ac.uk.
(500 MHz, CDCl3) ␦: 7.19 (4H, dd, J = 7.5 Hz, J = 1.5 Hz, H22, H26, H62
,
H66), 7.14 (4H, t, J = 7.5 Hz, H23, H25, H63, H65), 7.11 (2H, t, J = 7.5 Hz,
H24, H64), 6.88 (4H, m, H33, H35, H53, H55), 6.87 (2H, m, H34, H54),
6.85 (2H, m, H43, H45), 6.84 (1H, m, H44), 6.83 (4H, m, H32, H36, H52
,
Acknowledgements
H56), 6.80 (2H, m, H42, H46), 5.66 (1H, s, H14), 4.68 (2H, s, H12).
13C NMR (125 MHz, CDCl3) ␦: 199.1 (CO’s), 169.2 (C11), 142.6 (C4),
140.6 (C3, C5), 139.9 (C41), 139.5 (C31, C51), 139.2 (C21, C61), 138.1 (C2,
We thank the Natural Sciences and Engineering Research
Council of Canada, Science Foundation Ireland, and University
College Dublin for generous financial support, the Higher Ed-
ucation Authority through PRTLI Cycle 3 for instrumentation
funding, and Dr. Hilary Jenkins, McMaster University, for crys-
tallographic assistance.
C6), 134.4 (C1), 131.4 (C32, C36, C52, C56), 131.3 (C42, C46), 130.4 (C22
C26, C62, C66), 127.4 (C23, C25, C63, C65), 127.0 (C24, C64), 126.9 (C33
,
,
C35, C53, C55), 126.8 (C34, C54), 125.8 (C43, C45), 125.7 (C44), 87.6 (C13),
73.1 (C14), 66.6 (C12). IR (solid, KBr): 3471, 3405, 3083, 3027, 2096
Published by NRC Research Press

Brydges et al.
1111
Müllen, K. Chem. Eur. J. 2002, 8, 3858. doi:10.1002/1521-3765(20020902)8:
17<3858::AID-CHEM3858>3.0.CO;2-5.
(35) Zelonka, R. A.; Baird, M. C. Can. J. Chem. 1972, 50 (18), 3063. doi:10.1139/v72-
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