Alkynyldicobalt derivatives of dibenzosuberenol and dibenzocyclooctatrien-5-ol: Ring conformations, ease of carbonyl elimination and relevance to pauson-khand cyclization

Article abstract of DOI:10.1002/ejic.201601538

Under very mild conditions, (5-alkynyl-5H-dibenzo-[a,d]cycloheptatrien-5-ol)Co₂(CO)₆ clusters 1 lose a carbon monoxide ligand and form the corresponding η2-alkene-μ-alkyne-Co₂(CO)₅ complexes 2 (the first isolated examples of the initial phase of the Pauson-Khand reaction). This behaviour is not paralleled by the homologue in which an additional methylene group has been inserted within the seven-membered ring, as in the [5-alkynyl-6H-dibenzo[a,e]cyclooctatrienol]Co₂-(CO)₆ system 8, for which several X-ray crystal structures are reported. The apparent additional flexibility of this molecular framework does not translate into conformations appropriate for the formation of η²-alkene-μ-alkyne-Co₂(CO)₅ complexes. The relative inertness of the ethynyl complex 1a, compared with the ease of carbonyl loss in the trimethylsilylethynyl-and the tri(isopropyl)silylethynyl-substituted hexacarbonyldicobalt clusters, 1b and 11, respectively, is discussed in terms of outof-plane angle strain within the dibenzo[a,d]cycloheptatrienyl moiety. Comparison of the changing conformations of the seven-membered ring in a series of free and complexed molecules reveals a marked flattening of the organic framework, as bulkier substituents are incorporated at C₅. The possibility of η²-alkene-μ-alkyne-Co₂(CO)₅ clusters functioning as hemilabile systems was explored by monitoring the ease of carbonyl substitution by triphenylphosphine in 2b (alkynyl substituent C≡CSiMe₃).

Full text of DOI:10.1002/ejic.201601538

A Journal of
Accepted Article
Title: Alkynyl-dicobalt derivatives of dibenzosuberenol and
dibenzocyclooctatrien-5-ol: ring conformations, the ease of
carbonyl elimination, and the relevance to Pauson-Khand
cyclization
Authors: Sarah Brusey, Wen Shen, Helge Muller-Bunz, Yannick Ortin,
Paul Evans, and Michael James McGlinchey
This manuscript has been accepted after peer review and appears as an
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201601538
Link to VoR: http://dx.doi.org/10.1002/ejic.201601538

10.1002/ejic.201601538
European Journal of Inorganic Chemistry
Alkynyl-dicobalt derivatives of dibenzosuberenol and dibenzocyclooctatrien-5-ol:
ring conformations, the ease of carbonyl elimination, and the relevance to
Pauson-Khand cyclization
Sarah A. Brusey, Wen Shen, Helge Müller-Bunz, Yannick Ortin,
Paul Evans* and Michael J. McGlinchey*
School of Chemistry, University College Dublin, Belfield, Dublin 4, Ireland
Abstract: Under very mild conditions 5-alkynyl-5H-dibenzo[a,d]cycloheptatrien-5-ol)-
Co2(CO)6 clusters, 1, lose a carbon monoxide ligand and form the corresponding η2-alkene-μ-
alkyne-Co2(CO)5 complexes, 2, (the first isolated examples of the initial phase of the Pauson-
Khand reaction). This behaviour is not paralleled by the homologue in which an additional
methylene group has been inserted within the seven-membered ring, as in the [5-alkynyl-6H-
dibenzo[a,e]cyclooctatrienol]Co2(CO)6 system, 8, for which several X-ray crystal structures
are reported. The apparent additional flexibility of this molecular framework does not
translate into conformations appropriate for formation of η2-alkene-μ-alkyne-Co2(CO)5
complexes. The relative inertness of the ethynyl complex, 1a, compared to the ease of
carbonyl loss in the trimethylsilylethynyl- and the tri(isopropyl)silylethynyl-substituted
dicobalt-hexacarbonyl clusters, 1b and 11, respectively, is discussed in terms of out-of-plane
angle strain within the dibenzo[a,d]cycloheptatrienyl moiety. Comparison of the changing
conformations of the seven-membered ring in a series of free and complexed molecules
reveals a marked flattening of the organic framework as bulkier substituents are incorporated
at C(5). The possibility of η2-alkene-μ-alkyne-Co2(CO)5 clusters functioning as hemi-labile
systems was explored by monitoring the ease of carbonyl substitution by triphenylphosphine
in 2b (alkynyl substituent C≡CSiMe3).
Keywords: 7-membered ring conformations • 8-membered ring conformations • steric
hindrance • silyl ligands • alkyne-dicobalt complexes
Dr. S. Brusey, W. Shen, Dr. H. Müller-Bunz, Dr. Y. Ortin, Dr. P. Evans, Prof. Dr. M. J. McGlinchey
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Introduction
In earlier work we reported that tetrahedral clusters in the series (5-alkynyl-5H-
dibenzo[a,d]cycloheptatrien-5-ol)hexacarbonyldicobalt (1) readily undergo loss of a carbonyl
ligand whereupon the C(10)=C(11) double bond coordinates at the vacant site on cobalt, as in
2 (Scheme 1).[1,2] These η2-alkene-μ-alkyne-Co2(CO)5 complexes provided the first examples
whereby the initial events in the Magnus-Schore mechanism[3] of the Pauson-Khand (PKR)
cyclopentenone synthesis[4] could be trapped and unequivocally characterized by X-ray
crystallography.
Co2(CO)8
OH
OH
R
+
OH
THF, RT, 15 h
R
(OC)2Co
Co(CO)3
Co
Co
R
(CO)3
(CO)3
1a: R = H
2b: R = SiMe3
1b: R = SiMe3
1c: R = C6H4X
2c: R = C6H4X
(X = H, F, CN, CF3)
Scheme 1. Reaction of Co2(CO)8 with 5-alkynyl-5H-dibenzo[a,d]cycloheptatrien-5-ols.
One can surmise that the rigidity of the pentacarbonyl complexes, 2, in which the
alkene cannot rotate into the preferred pseudoaxial site, together with the potentially severe
steric strain in the product, prevents continuation along the Pauson-Khand pathway. With the
aim of studying a more flexible system, to see whether it might be possible to complete an
intramolecular PKR, the propynyl analogue, 3, possessing an additional methylene group
between the seven-membered ring and the alkyne unit, was prepared.[2] In the event, there
was no evidence of facile decarbonylation of 3 to the corresponding alkene-pentacarbonyl, 4,
nor to any PKR products; however, the intermolecular reaction with norbornadiene
proceeded readily to give the anticipated PKR product 5 (94%) (Scheme 2).
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10.1002/ejic.201601538
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Co2(CO)6
OH
HO
PhMe
110ºC
OH
(10 equiv.)
O
(OC)2Co
(OC)3Co
H
PhMe, 110ºC,
2 h, 94%
5
3
4
H
Scheme 2. Intermolecular PKR reaction of 3 with norbornadiene.
Herein we describe another approach towards the introduction of additional flexibility
by enlarging the ring system from dibenzo[a,d]cycloheptatrienyl to dibenzo[a,e]cycloocta-
trienyl, a ligand that, to the best of our knowledge, has not been widely used in
organometallic chemistry. In addition, we discuss the reaction of triphenylphosphine with
(5-trimethylsilyl-5H-dibenzo[a,d]cycloheptatrien-5-ol)pentacarbonyldicobalt complexes (2b)
and also the different possible mechanisms involved.
Results and Discussion
Preparation and structures of (alkynyl-dibenzo[a,e]cyclooctatrienol)Co2(CO)6
complexes. As previously reported by Wudl,[5] the reaction of dibenzosuberenone with
trimethylsilyldiazomethane forms 6H-dibenzo[a,e]cyclooctatrien-5-one (6) (Scheme 3). Its
structure has now been determined by X-ray crystallography; Figure 1a provides a bird’s-eye
view of 6, whereas, in Figure 1b, it is presented as a side view emphasizing the
unsymmetrical nature of the central eight-membered ring.
BF3·OEt2,
TMSCHN2
O
O
6
Scheme 3. Preparation of 6H-dibenzo[a,e]cyclooctatrien-5-one (6).
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10.1002/ejic.201601538
European Journal of Inorganic Chemistry
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O
C9
C3
C7
C6
C8
C2
C5
C4a
C12a
C12
C4
C8
C7
C1
C4
C6a
C3
C2
C10
C9
C10a
O
C5
C10
C6
C11
C12
C1
C11
(a)
(b)
Figure 1. Molecular structure of 6H-dibenzo[a,e]cyclooctatrien-5-one (6); (a) bird’s-eye
view, (b) side view. Thermal ellipsoids are shown at 50%. Selected bond distances (Å) and
angles (deg): C(4a)-C(12a) 1.404(3), C(12a)-C(12) 1.477(3), C(12)-C(11) 1.331(4), C(11)-
C(10a) 1.453(4), C(10a)-C6a) 1.389(3), C(6a)-C(6) 1.507(3), C(6)-C(5) 1.478(3), C(5)-C(4a)
1.498(3), C(5)-O 1.216(3), C(6a)-C(6)-C(5) 110.0(2), C(6)-C(5)-C(4a) 120.5(2).
Unlike dibenzo[a,e]cyclooctatetraene (DBCOT), which adopts C2v symmetry with the
benzo rings folded up approximately 45º from the central plane containing the two double
bonds,[6] the presence of the CH2-C=O fragment in molecule 6 breaks both the mirror and
two-fold axial symmetry and brings about a twisting of the eight-membered ring. In DBCOT
the C(5)=C(6) and C(11)=C(12) double bonds are parallel and the interplanar angle, φ,
between the benzo rings is 89º.[6] However, in the ketone 6 it is the bonds C(12)-C(12a) and
C(5)-C(6) that are now almost parallel (torsion angle 3°), the angle φ (see Figure 2) has
widened to 106º, and the torsion angle C(12)-C(11)-C(6)-C(5) is now 38º.

O
Figure 2. The interplanar angle, φ, between benzo rings in 6.
The reaction of 6 with R-C≡C-Li (R = SiMe3, Si(i-Pr)3 or Ph) led to the alkynols, 7a
to 7c, that reacted with dicobalt octacarbonyl to furnish the tetrahedral clusters 8a to 8c,
(Scheme 4); the X-ray crystal structures of 8a (R = TMS) and of 8b (R = TIPS) are shown in
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10.1002/ejic.201601538
European Journal of Inorganic Chemistry
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Figures 3 and 4, respectively. However, numerous attempts to form the corresponding
alkene-bonded pentacarbonyls, 9, including extended thermolysis in toluene, were
unsuccessful.
C9
C8
C10
C11
C2
O
C1
C3
C12
C6
C5
Co2
C13
C14
Si
Co1
Figure 3. Molecular structure of [5-(trimethylsilylethynyl)-6H-dibenzo[a,e]cyclooctatrien-
5-ol]hexacarbonyldicobalt (8a); thermal ellipsoids are shown at 50%. Selected bond
distances (Å) and angles (deg): C(4a)-C(12a) 1.407(5), C(12a)-C(12) 1.486(5), C(12)-C(11)
1.335(5), C(11)-C(10a) 1.464(5), C(10a)-C6a) 1.394(5), C(6a)-C(6) 1.502(5), C(6)-C(5)
1.531(5), C(5)-C(4a) 1.533(5), C(5)-O 1.430(4), Co(1)-Co(2) 2.4661(7), Co(1)-C(13)
1.972(4), Co(2)-C(13) 1.960(4), Co(1)-C(14) 1.993(4), Co(2)-C(14) 1.987(3), C(13)-C(14)
1.327(5), C(5)-C(13) 1.511(5), C(14)-Si 1.847(4), C(5)-C(6)-C(6a) 115.3(3), C(4a)-C(5)-
C(6) 116.4(3), C(13)-C(14)-Si 151.0(3).
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10.1002/ejic.201601538
European Journal of Inorganic Chemistry
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Co2(CO)8
-CO
R
Li
OH
OH
6
R
(OC)2Co
Co(CO)3
OH
R
Co
(CO)3
Co
(CO)3
R
7a: R = SiMe3 (90%)
7b: R = Si(i-Pr)3 (40%)
7c: R = Ph (84%)
8a: R = SiMe3 (76%)
8b: R = Si(i-Pr)3 (62%)
8c: R = Ph (85%)
9
Scheme 4. Preparation of [5-alkynyl-6H-dibenzo[a,e]cyclooctatrien-5-ol]hexacarbonyl-
dicobalt complexes, 8a,8b,8c.
C9
C10
C8
C7
C2
C3
C1
C11
C12
C4
O
C6
C5
C13
Co1
C14
Si
Co2
Figure 4. Molecular structure of [5-(tri(isopropyl)silylethyl)-6H-dibenzo[a,e]cyclooctatrien-
5-ol]hexacarbonyldicobalt (8b); thermal ellipsoids are shown at 50%. Selected bond
distances (Å) and angles (deg): C(4a)-C(12a) 1.400(2), C(12a)-C(12) 1.487(3), C(12)-C(11)
1.332(3), C(11)-C(10a) 1.473(2), C(10a)-C6a) 1.400(2), C(6a)-C(6) 1.505(2), C(6)-C(5)
1.549(2), C(5)-C(4a) 1.536(2), C(5)-O 1.4413(18), Co(1)-Co(2) 2.4569(3), Co(1)-C(13)
1.9527(15), Co(2)-C(13) 1.9630(15), Co(1)-C(14) 2.0244(14), Co(2)-C(14) 2.0159(15),
C(13)-C(14) 1.346(2), C(5)-C(13) 1.525(2), C(14)-Si 1.8820(16), C(5)-C(6)-C(6a)
113.75(12), C(4a)-C(5)-C(6) 116.22(12), C(13)-C(14)-Si 150.27(12).
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10.1002/ejic.201601538
European Journal of Inorganic Chemistry
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In complex 8a the trimethylsilyl substituent is located almost directly below the
C(11)=C(12) double bond such that the torsion angle between bonds O-C(5) and C(14)-Si is
169°. In contrast, in complex 8b the bulk of the tri(iso-propyl)silyl unit results in a rotation of
the C2Co2 tetrahedral core relative to the dibenzo[a,e]cyclooctatrienol moiety, thus
positioning a cobalt vertex almost directly below C(12), such that the torsion angle O-C(5)-
C(14)-Si is now 71°. Moreover, the presence of the TIPS group brings about a marked
increase in the length of the C(14)-Si linkage from 1.847(4) Å in 8a to 1.882(2) Å in 8b. As a
consequence of the change in the character of C(5) from sp2 to sp3, the interplanar angle, φ,
between the benzo rings has contracted from 106° in the ketone, 7, to 85° in 8a and to 89° in
8b. It is noteworthy that the methylene protons at C(6) appear in the NMR as a pair of 13 Hz
doublets separated by 0.11 ppm in the alkynols, 7, but this difference increases to 0.9 ppm in
the cobalt complexes, 8, reflecting the strikingly changed environment. Based on the marked
differences in the solid state structures of 8a and 8b, we decided to re-visit the
dibenzocyclotrienyl series to investigate whether this had any bearing on their reactivity.
Preparation and structure of [5-(tri(isopropyl)silylethynyl)-5H-dibenzo[a,d]cyclohepta-
trien-5-ol]pentacarbonyldicobalt, (12). The reaction of dibenzosuberenone with
tri(isopropyl)silylethynyl lithium (TIPS-C≡CLi) furnished the alkynol, 10, in 53% yield. As
shown in Scheme 5, subsequent addition of dicobalt octacarbonyl yielded directly the
pentacarbonyldicobalt cluster, 12, with no evidence for the presumed hexacarbonyl
precursor, 11. The mass balance for this process was unreacted recovered alkynol 10. The
X-ray structure of 12 is depicted in Figure 5 and once again illustrates the boat conformation
of the seven-membered ring, and the attachment of a dicarbonylcobalt vertex to the
C(10)=C(11) double bond. In general, the bond lengths closely resemble those found in the
TMS analogue, 2b, except for the marked lengthening of the C(13)-Si linkage (1.855 Å in
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10.1002/ejic.201601538
European Journal of Inorganic Chemistry
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2b, versus 1.888 Å in 12), a pattern already noted in 8a and 8b. The isopropyl methyl groups
in 12 are diastereotopic and give rise to two peaks in the 13C NMR spectrum.
Co2(CO)8
- CO
OH
OH
HO
10
(i-Pr)3Si
(OC)2Co
Co(CO)3
Si(i-Pr)3
Si(i-Pr)3
Co
(CO)3
Co
(CO)3
11
12 (49%)
Scheme 5. Reaction of Co2(CO)8 with 5-(tri(isopropyl)silylethynyl)-5H-dibenzo[a,d]-
cycloheptatrien-5-ol] to form only the pentacarbonyl complex (12).
C8
C7
C9
C3
C4
C6
C2
C1
C11
O
C10
C5
C12
Co1
C13
Si
Co2
Figure 5. Molecular structure of [5-(tri(isopropyl)silylethynyl)-5H-dibenzo[a,d]cyclohepta-
trien-5-ol]pentacarbonyldicobalt (12); thermal ellipsoids are shown at 50%. Selected bond
distances (Å) and angles (deg C(4a)-C(11a) 1.4065(13), C(11a)-C(11) 1.4723(13), C(11)-
C(10) 1.4039(14), C(10)-C(9a) 1.4710(13), C(9a)-C5a) 1.4057(13), C(5a)-C(5) 1.5193(12),
C(5)-C(4a) 1.5293(13), C(5)-O 1.4295(11), Co(1)-Co(2) 2.49090(18), Co(1)-C(12)
1.9471(9), Co(2)-C(12) 1.9581(9), Co(1)-C(13) 2.0302(9), Co(2)-C(13) 2.0590(9), Co(1)-
C(10) 2.1437(9), Co(1)-C(11) 2.1839(9), C(12)-C(13) 1.3344(12), C(5)-C(12) 1.5114(12),
C(13)-Si 1.8883(9), C(4a)-C(5)-C(5a) 108.48(7), C(12)-C(13)-Si 144.61(7).
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10.1002/ejic.201601538
European Journal of Inorganic Chemistry
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Comparison of the conformations of the seven- and eight-membered rings in 1b and 8a.
The successful isolation of the η2-alkene-μ-alkyne-Co2(CO)5 complexes (2) that represent the
first fully characterized examples of the initial phase of the Pauson-Khand reaction,[1,2]
prompted us to investigate a potentially more flexible system that could yield either an
intramolecular product, or possibly another intermediate in the PKR pathway.[7] As noted
above, 6H-dibenzo[a,e]cyclooctatrien-5-one (6) (Figure 1) was converted into the alkynyl-
Co2(CO)6 complexes 8 (Figures 3 and 4). However, even when heated at reflux in toluene,
there was no indication of formation of the corresponding pentacarbonyl complexes, 9.
Indeed, on prolonged heating only variable amounts of de-complexed alkynol 7 were
detected in addition to unchanged hexacarbonyl complexes 8.
The known organometallic chemistry of dibenzo[a,e]cyclooctatetraene, DBCOT,
includes a range of complexes in which silver or palladium,[8] chromium or molybdenum,[9]
rhodium or iridium,[10,11] or gold,[11] are π-bonded to the 1,5-diene unit; the interplanar angle
(φ) between the benzo rings decreases from 89º in DBCOT to 86º-76º (Ir) and 68º (Rh) when
the 1,5 diene unit is complexed to different metal centres.[10,11] Moreover, it has been shown
that an η6-Cr(CO)3 fragment can be attached to one or both of the benzo rings.[12] In contrast,
6H-dibenzo[a,e]cyclooctatrien-5-one has been used primarily as an intermediate en route to
DBCOT derivatives.[5,10,11] Upon formation of the alkynols, 7, and complexation to a
hexacarbonyldicobalt unit, the geometry of the eight-membered ring in 8 changes
considerably and is more akin to that of DBCOT.[6] The torsion angle C(12)-C(11)-C(6)-C(5)
decreases from 38° in 6 to 25.7º (26.6°) in 8a (8b), and the interplanar angle, φ, between the
benzo rings is now 85º (89º) in 8a (8b) from 106° in 6.
Our experimental observations on the ease, or difficulty, of releasing a carbon
monoxide ligand from the dicobalt hexacarbonyl derivatives of 5-alkynyl-5H-dibenzo[a,d]-
cycloheptatrien-5-ol or of 5-alkynyl-6H-dibenzo[a,e]cyclooctatrien-5-ol, 1, and most notably
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10.1002/ejic.201601538
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11, or 8, respectively, raise a number of issues. Why are the eight-membered ring systems so
resistant to loss of CO? In the seven-membered ring complexes, the TMS derivative, 1b,
loses CO readily to yield 2b, even at room temperature; more strikingly, in the TIPS system
the hexacarbonyl, 11, cannot be isolated and the only product is the pentacarbonyl, 12. In
contrast, starting from 1a (R = H) the corresponding pentacarbonyl, 2a, was only detectable
upon treatment with N-methylmorpholine N-oxide monohydrate (NMO) or when photolysed
at 557 nm.[2]
Comparison of the structures of the alkynyl-cycloheptatrienyl and alkynyl-
cyclooctatrienyl hexacarbonyldicobalt complexes reveals a number of significant differences.
As shown in Figure 6a, the seven-membered ring complex is much flatter; φ, the interplanar
angle between the benzo rings in the TMS-hexacarbonyl system, 1b, is 142º, closing to 127º
upon formation of the corresponding alkene-linked TMS-pentacarbonyl complex, 2b. This
transformation requires the cluster 1b to undergo rotation about the C(5)-C(12) axis so as to
position a cobalt atom directly below the potentially coordinating alkene fragment, as in
Scheme 6; such a rotation places the cobalt at a distance (d) approximately 3.1 Å from the
centre of the C(10)-C(11) double bond. Decarbonylation and coordination of the alkene to
form the pentacarbonyldicobalt complex 2b requires further bending of the seven-membered
ring towards the cluster such that this distance, d, shortens to 2.04 Å. In contrast, in the eight-
membered ring complex, 8a, not only is the corresponding bond-centroid-to-cobalt distance,
d, considerably longer (ca. 4.2 Å) but also the C(11)-C(12) double bond is twisted away from
the horizontal, as depicted in Figure 6b.
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10.1002/ejic.201601538
European Journal of Inorganic Chemistry
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O
O
C10
C11
C12
Co
Co
Co
Co
Si
Si
(a)
(b)
Figure 6. Molecular structures of (a) 1b (7-membered ring) and (b) 8a (8-membered ring)
viewed from the perspective of their potential to form a bond between the alkene unit and a
cobalt vertex upon rotation of the cluster fragment.
-CO
OH
OH
R
d
R
(OC)3Co
Co(CO)3
Co
Co
(CO)3
(CO)3
Scheme 6. Rotation and positioning of a tricarbonylcobalt vertex below the alkene unit prior
to loss of a carbonyl ligand.
These factors, together with the likelihood that the interplanar angle φ would
presumably have to contract even further, may account for the failure of the 5-alkynyl-6H-
dibenzo[a,e]cyclooctatrien-ol complexes, 8, to undergo facile loss of a carbon monoxide
ligand and form an alkene-coordinated molecule, 9 (analogous to 2b).
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10.1002/ejic.201601538
European Journal of Inorganic Chemistry
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On the relative stability of (5-alkynyl-5H-dibenzo[a,d]cycloheptatrien-5-ol)Co2(CO)6
complexes (1) versus [5-alkynyl-(η2-5H-dibenzo[a,d]cycloheptatrien-5-ol)]Co2(CO)5 (2).
It is now apparent that the ready isolation of the alkene-dicobaltpentacarbonyl systems 2b
and 2c was unexpected, and to some extent serendipitous; in fact, the original goal of the
experiment was to prepare and then protonate the (alkynyl-dibenzo[a,d]cycloheptatrien-5-
ol)Co2(CO)6 clusters 1a, 1b and 1c so as to study the behaviour of the corresponding metal-
stabilized cations, and that was indeed the focus of a subsequent manuscript.[13] The stability
of the η2-alkene-μ-alkyne-pentacarbonyldicobalt clusters 2b,c is a result of the particular
combination of rigidity and flexibility provided by the dibenzo[a,d]cycloheptatrienyl
fragment that is lacking in other slightly modified analogues, in which an additional
methylene unit has been incorporated either externally, as in the propynyl system 3, or within
the seven-membered ring in the form of the dibenzo[a,e]cyclooctatrienyl clusters 8a,b,c.
However, this still begs the question as to why the parent ethynyl-cluster, 1a, is so
reluctant to lose the initial CO ligand. The kinetics of the conversion of the TMS derivative
1b into 2b have been followed over a range of temperatures and allowed the evaluation of the
activation energy for the decarbonylation process (~29 kcal mol-1), and for the overall
reaction which was endothermic by ~14 kcal mol-1.[2] These data agree well with theoretical
predictions[14] that gave a range of 26-33 kcal mol-1 for Eact and 12-15 kcal mol-1 for ΔH.
Based on these data, the entropy gained by elimination of CO is presumably the overall
driving force for the process.
The molecular structures of all the relevant hexacarbonyl clusters, 1a through 1c,
have been determined by X-ray crystallography and reveal, in each case, the boat
conformation of the seven-membered ring.[1,2] As is normal for (alkyne)Co2(CO)6 tetrahedral
clusters, the axial cobalt-carbonyl bonds are shorter and stronger than the corresponding
equatorial linkages; nevertheless, displacement of a carbonyl by a phosphine normally leads
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10.1002/ejic.201601538
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to the isomer with the new ligand in an axial position,[15] as was indeed found for the
triphenylphosphine-containing complex 13 (see below). The corresponding axial or
equatorial Co-C(O) bond distances in 1a (R = H), 1b (R = TMS) and 1c (R = C6H4X) are not
significantly different, and their ν(CO) stretching vibrations are also very similar.
The clue may come in the changes to the interplanar angles upon complexation of the
free alkyne to a hexacarbonyldicobalt fragment; as illustrated in Figure 7, φ1 and φ2 are the
out-of-plane bend angles within the seven-membered ring, and φ3 is the previously
mentioned dihedral angle between the benzo rings. In the free ligands (5-ethynyl)- and (5-
trimethylsilylethynyl)-dibenzo[a,d]cycloheptatrien-5-ol, the values of φ1, φ2, φ3 are 123º,
158º, 126º and 127º, 153º, 126º, respectively.[1,16] After reaction to form the corresponding
hexacarbonyldicobalt clusters, 1a and 1b, these angles become 129º, 155º, 127º and 135º,
161º, 142º, respectively. Thus, in 1a (R = H), the angular distortion is minimal; φ1 and φ2,
the out-of-plane bend angles within the seven-membered ring, change by 6º and 3º,
respectively, and φ3, the interplanar angle between the benzo rings increases by only 1º. In
contrast, in 1b (R = TMS), φ1 and φ2 both increase by 8º, and φ3 is widened by an additional
16º; that is, the seven-membered ring is markedly flattened, clearly a result of the presence of
the bulky trimethylsilyl substituent. However, upon decarbonylation and coordination of the
C(10)-C(11) double bond to cobalt, as in 2b, the values of φ1, φ2 and φ3 revert to 123º, 152º,
127º, respectively; that is, the seven-membered ring conformation has essentially returned to
its previous sterically less strained geometry. Thus, in 1a (R = H), where there is essentially
no conformational change of the central ring when the free ligand reacts to form the
tetrahedral alkyne-Co2(CO)6 cluster, there is no pressure for steric relief by decarbonylation
and subsequent coordination by the alkene fragment to drive the process.
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10.1002/ejic.201601538
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The changing conformation of cycloalkynols upon incorporation of a Co2(CO)6
moiety has been noted earlier in both five- and six-membered ring systems.[17] Thus,
treatment of axial alkynyl-cyclohexanols with dicobalt octacarbonyl brings about a ring flip
to place the cluster fragment in the equatorial position.
3
OH

2 1
Figure 7. Definition of the interplanar angles φ1, φ2 and φ3 in the
dibenzo[a,d]cycloheptatrien-5-ol system.
It is interesting to compare the gradually changing values of the angles φ1, φ2 and φ3
in the dibenzo[a,d]cycloheptatrienyl boat skeleton as determined X-ray crystallographically
in uncomplexed and cluster-complexed systems. In general, increasing the bulk of
substituents (from H to aryl to TMS) leads to larger values of all three interplanar angles.
Thus, in a scissoring-type motion of orthogonal units, widening the bend angles φ1 and φ2
within the seven-membered ring also pushes the benzo wingtips outwards; in the limit, in the
zwitterionic dibenzotropylium-containing adduct of dibenzosuberenone and AlCl3, the
7-membered aromatic ring is planar and the angles φ1, φ2 and φ3 all become 180º.[18]
As shown in Figure 8, a plot (in green) of (φ1 + φ2) – the sum of the out-of-plane
bend angles in the seven-membered ring – versus φ3 – the interplanar angle between benzo
rings – for a number of non-complexed 5H-dibenzo[a,d]cycloheptatrienols is rectilinear with
a correlation coefficient of 0.97; several of these molecules (as do their cobalt complexes)
adopt multiple independent structures within the same unit cell, thus giving additional data
points; the alkynols have also been studied when co-crystallized with a range of other
aromatics.[2,16,19] The analogous plot (in red) for the structurally characterized (5-alkynyl-5H-
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10.1002/ejic.201601538
European Journal of Inorganic Chemistry
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dibenzo[a,d]cycloheptatrien-5-ol)Co2(CO)6 clusters[1,2] is also linear (R2 = 0.95) but is clearly
displaced to larger dihedral angles, and with a markedly steeper slope (0.81 vs 0.44). This
again supports the assertion that in the dicobalt-hexacarbonyl clusters the steric crowding
increases markedly going from R = H, 1a, through R = C6H4X, 1c, culminating in R = TMS,
1b, which undergoes decarbonylation in solution readily even at room temperature.
Gratifyingly, the plot (in blue) of (φ1 + φ2) versus φ3 for the structurally characterized
decarbonylated systems, 2b,c, has an R2 value of 0.92 with a slope (0.33) much closer to that
found for the uncomplexed dibenzo[a,d]cycloheptatrienols.
We note the particular behaviour of (5-tri(isopropyl)silylethynyl-5H-dibenzo[a,d]-
cycloheptatren-5-ol)Co2(CO)5 (12) (labelled A) which lies on the blue line. Generally, the
values of (φ1 + φ2) and φ3 increase with the size of the substituent R; that is to say, the
tricyclic ligand tends to flatten. However, despite the enhanced volume of the (i-Pr)3Si group
relative to Me3Si, these angles decrease. Apparently, increased flattening of the tricyclic
ligand is incompatible with the proximity of a wing-tip benzo ring to such a bulky
substituent, and so the ring conformation adjusts, but still lies on the blue line, clearly
demonstrating the correlated nature of the interplanar bend angles in this system.
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10.1002/ejic.201601538
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298
296
294
292
290
288
286
284
282
280
278
g
OH
R
Co(CO)2L
Co
(CO)2L
f
e
c
d
b
a
OH
11
R
9
10
OH
6
5
8
276
1
3
7
F
4
D
E
(OC)2Co
274
R
2
Co
C
B
272
270
(CO)2L
A
110
115
120
125
130
135
140
145
φ 3
Figure 8. Plots of (φ1 + φ2) versus φ3: (red) for (5-alkynyl-5H-dibenzo[a,d]cycloheptatrien-
5-ol)Co2(CO)4L2 clusters: a, R = H, L = CO;[2] b, R = Ph, L = CO;[1] c, R = H, L = CO;[2] d, R
= C6H4-p-CF3, L = CO;[2] e, R = Ph, L = CO;[1]; f, R = H, L-L = dppm;[13] g, R = SiMe3, L =
CO:[2] (green) for 5-substituted-5H-dibenzo[a,d]cycloheptatrien-5-ols (DBZ(OH)R: 1,
DBZ(OH)-C≡C-C≡C-(HO)DBZ•pyridine;[19a]; 2, DBZ(OH)-C6H4-p-OMe•xylene;[19b] 3,
DBZ(OH)-C6H4-p-OMe•bromobenzene;[19b]; 4, DBZ(OH)-C6H4-p-OMe•chlorobenzene;[19b]
;
5, DBZ(OH)-C≡C-C≡C-(HO)DBZ•acetone;[19a] 6, DBZ(OH)-C6H4-p-OMe•benzene;[19b] 7,
DBZ(OH)-C6H4-p-OMe• benzene;[19b] 8, DBZ(OH)-C6H4-p-OMe;[19b] 9, DBZ(OH)-C6H4-p-
OMe•pyridine;[19b] 10, DBZ(OH)-C≡C-SiMe3;[2] 11, DBZ(OH)-C≡C-H:[16] (blue) for (5-
alkynyl-5H-dibenzo[a,d]cycloheptatrien-5-ol)Co2(CO)4L clusters: A, R = Si(CHMe2)3, L =
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10.1002/ejic.201601538
European Journal of Inorganic Chemistry
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CO;[this work] B, R = C6H4-m-F, L = CO;[2] C, R = Ph, L = CO;[1] D, R = SiMe3, L = PPh3;[this
work] E, R = C6H4-p-CN, L = CO;[2] F, R = SiMe3, L = CO. [2]
Reaction of 5-(trimethylsilylethynyl)-(dibenzo[a,d]cycloheptatrienol)Co2(CO)5, 2b, with
triphenylphosphine. The ability of alkynyl-(dibenzo[a,d]cycloheptatrienol)Co2(CO)6
complexes, such as 1b, to lose a carbonyl ligand, even at ambient temperature, and distort the
seven-membered ring so as to allow the C(10)-C(11) double bond to bind at the vacant
coordination site has already been discussed.[1,2] Apart from its relevance to the mechanism
of the PKR, this also raises the possibility that the 5-alkynyl-5H-dibenzo[a,d]cyclohepten-5-
ol unit could function as a hemi-labile ligand by reversible decoordination of the alkene
moiety, thus allowing temporary access to a vacant site, analogous to the behaviour of
numerous bisphosphine monoxide (BPMO) complexes.[20-22]
To this end, in a preliminary investigation, the alkyne cluster 2b (R = Me3Si) was
treated with triphenylphosphine to determine whether the reaction resulted merely in ligand
addition, or instead substitution of a carbonyl ligand occurred. As depicted in Scheme 7, the
reaction of 2b with triphenylphosphine led to formation of the phosphine-dicobalt-
tetracarbonyl derivative 13, slowly, even at ambient temperature, and more rapidly in
refluxing THF.
OH
OH
PPh3, THF
(-CO)
(OC)2Co
(OC)2Co
SiMe3
SiMe3
(CO)2PPh3
Co
(CO)3
Co
13 (78%)
2b
Scheme 7. Reaction of 2b with triphenylphosphine.
The product contains one Co(CO)2PPh3 vertex while the other dicarbonylcobalt
fragment is coordinated to the C(10)=C(11) double bond. The X-ray crystal structure of 13
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10.1002/ejic.201601538
European Journal of Inorganic Chemistry
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appears as Figure 9 and reveals that, as with all the previously characterized pentacarbonyl
complexes, the alkene adopts a pseudo-gauche position; the phosphine has replaced an axial
carbonyl in the other cobalt vertex. As found in the other complexes bearing sterically
demanding substituents (such as TIPS in 8b and 12) the presence of the triphenylphosphine
fragment in 13 brings about a marked lengthening of the C(13)-Si bond.
C7
C8
C6
O
C9
C4
C3
C2
C5
C12
C10
C1
C11
Co2
P
Co1
Si
Figure 9. Molecular structure of (5-trimethylsilylethynyl-5H-dibenzo[a,d]cyclohepten-5-ol)-
tetracarbonyl(triphenylphosphine)dicobalt (13); thermal ellipsoids are shown at 50%.
Selected bond distances (Å) and angles (deg C(4a)-C(11a) 1.403(3), C(11a)-C(11) 1.473(3),
C(11)-C(10) 1.402(3), C(10)-C(9a) 1.464(3), C(9a)-C5a) 1.398(3), C(5a)-C(5) 1.522(2),
C(5)-C(4a) 1.527(2), C(5)-O 1.426(2), Co(1)-Co(2) 2.4743(3), Co(1)-C(12) 1.9812(16),
Co(2)-C(12) 1.9569(17), Co(1)-C(13) 2.0534(18), Co(2)-C(13) 2.0046(16), Co(1)-C(10)
2.163(2), Co(1)-C(11) 2.111(2), C(12)-C(13) 1.312(3), C(5)-C(12) 1.517(2), C(13)-Si
1.8612(18), Co(2)-P 2.2351(5), C(4a)-C(5)-C(5a) 109.76(14), C(12)-C(13)-Si 144.57(14).
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10.1002/ejic.201601538
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The most closely related structure to 13 of which we are aware is 14, the product
derived by heating (trimethylsilylethyne)Co2(CO)6 with dibenzosuberenyldiphenylphosphine.
In that case, however, the phosphine and alkene are both bonded to the same cobalt.
Interestingly, even though the alkene is well aligned with the alkyne in 14, and the potential
PKR product would not be particularly strained, the process halts at this point.[23] Moreover,
we note that the particularly favourable geometry of the 5H-dibenzo[a,d]cycloheptatrienyl
unit that allows coordination of the C(10)=C(11) double bond to a metal center, as in 2, 12 or
13, has also been elegantly exploited by Grützmacher, especially in a very extensive series of
5-phosphino derivatives exemplified by molecules such as 15 or 16, shown in Figure 10.[24]
H
H
H
o
CO
PPh2
C
o
3C
)
Ru
C
PPh2
O
Ph2P
H
(
Cl Pd
Me
PPh2
Me3Si
16
15
14
Figure 10. Metal complexes exhibiting coordination to the C(10)=C(11) bond in the dibenzo-
suberene system.
One can envisage two mechanistic scenarios leading to the formation of the
tetracarbonyl 13: (i) the phosphine displaces a carbonyl from the remaining Co(CO)3 vertex
in 2b to yield 13 directly; (ii) displacement of the weakly bonded alkene unit by the
phosphine, to form 17, is followed by rotation and a second decarbonylation leading to 13.
Although it would be non-trivial experimentally, in principle these mechanisms are
distinguishable. If one could start with a single enantiomer of 2b, then direct substitution at
the Co(CO)3 vertex – mechanism (i) – versus metal-alkene detachment, addition of the
phosphine to the Co(CO)2 fragment, rotation of the tetrahedral unit and loss of CO from the
Co(CO)3 moiety – mechanism (ii) – would lead to opposite enantiomers, as shown in Scheme
8.
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OH
OH
(i)
(OC)2Co
(OC)2Co
R
R
Co
(CO)2PPh3
Co
PPh3
(CO)3
13: R = SiMe3
(ii)
PPh3
OH
OH
rotate
- CO
(OC)2Co
Ph3P
(OC)2Co
Co(CO)2PPh3
R
Co
(CO)3
R
17
ent-13: R = SiMe3
Scheme 8. Possible mechanistic routes from 2b to 13 upon reaction with PPh3.
Conclusions
The ability of (5-alkynyl-5H-dibenzo[a,d]cycloheptatrien-5-ol)Co2(CO)6 clusters, 1,
to lose a carbon monoxide ligand and form the corresponding η2-alkene-μ-alkyne-Co2(CO)5
complexes, 2, under very mild conditions is not paralleled by the behaviour of their
homologues in which an additional methylene group has been inserted externally, as in 3, or
within the seven-membered ring, as in the 5-alkynyl-6H-dibenzo[a,e]cyclooctatrienyl system
9. The apparent additional flexibility of these latter molecular frameworks does not translate
into conformations appropriate for formation of η2-alkene-μ-alkyne-Co2(CO)5 complexes.
The relative inertness of (5-ethynyl-5H-dibenzo[a,d]cycloheptatrien-5-ol)Co2(CO)6,
1a, compared to the ease of carbonyl loss in the substituted silyl or aryl clusters, 1b, 1c and
12, has been discussed in terms of out-of-plane bend angle strain within the dibenzo[a,d]-
cycloheptatrienyl moiety. Comparison of the changing conformations of the seven-membered
ring in a series of free and complexed molecules reveals a marked flattening of the organic
framework as bulkier substituents are incorporated at C(5). In this context, we note also the
work of Hjelmencrantz, Friberg and Berg who studied the changing conformation of the
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10.1002/ejic.201601538
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5H-dibenzo[a,d]cycloheptene ring system as the electronic and steric properties of the
substituent at the 5-position were varied.[25] A comprehensive study of the seven-membered
ring conformations of all molecules containing a dibenzo[a,d]cycloheptatrienyl fragment
whose structures are available from the Cambridge Crystallographic Data Centre will be the
subject of a future report.
The possibility of η2-alkene-μ-alkyne-Co2(CO)5 clusters functioning as hemi-labile
systems was explored in a preliminary fashion by following the reaction of
(5-trimethylsilylethynyl-5H-dibenzo[a,d]cyclohepten-5-ol)-pentacarbonyldicobalt, 2b, with
triphenylphosphine. It is possible that the reaction proceeds via initial attack at the alkene-
coordinated cobalt in 2b by the incoming phosphine with displacement of the C(10)=C(11)
double bond to form intermediate 16; subsequent rotation of the tetrahedral fragment, loss of
a carbonyl from the Co(CO)3 vertex, and reattachment of the alkene moiety to the
dicarbonylcobalt vertex could lead to 13. However, direct substitution at the Co(CO)3 vertex
cannot be ruled out; this question will be addressed in a future study.
Experimental Section
General methods. All reactions were carried under a nitrogen atmosphere, and solvents were
1
13
dried by standard procedures. H and C NMR spectra were recorded on Varian 400 MHz,
500 MHz or 600 MHz spectrometers. Assignments were based on standard 2-dimensional
1
NMR techniques (1H-1H COSY, H-13C HSQC and HMBC, NOESY). Infrared spectra were
recorded on a Perkin-Elmer Paragon 1000 FT-IR spectrometer and were calibrated with
polystyrene. Merck silica gel 60 (230-400 mesh) or alumina was used for flash
chromatography. (5-Ethynyl-5H-dibenzo[a,d]cyclohepten-5-ol)Co2(CO)6 (1a), (5-trimethyl-
silylethynyl-5H-dibenzo[a,d]cyclohepten-5-ol)Co2(CO)6 (1b) and 6H-dibenzo[a,e]cyclo-
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octatrien-5-one (6) were prepared according to literature procedures.[1,2,5] A sample of 6
suitable for X-ray diffraction was obtained by crystallization from pentane/dichloromethane.
Preparation of 5-trimethylsilylethynyl-6H-dibenzo[a,e]cyclooctatrien-5-ol (7a). To a
solution of trimethylsilylethyne (4.85 mL, 34 mmol) in THF (200 mL) at -78 ºC was added
n-BuLi (21.25 mL of a 1.6 m soln., 34 mmol) and the resulting mixture was stirred at this
temperature for 30 min. It was then brought to room temperature, stirred for a further 30
min., then brought back to -78 ºC. To this was added a solution of 6H-dibenzo[a,e]-
cyclooctatrien-5-one (6.30 g, 28.6 mmol) in THF (100 mL) and the mixture stirred for 2 h.
The reaction was quenched using distilled water (200 mL), extracted with diethyl ether (3 x
100 mL), washed with brine, dried and the solvent removed to yield a pale brown oil (8.9 g).
Chromatography on an alumina column, using pentane/dichloromethane as eluent yielded
5-trimethylsilylethynyl-6H-dibenzo[a,e]cyclooctatrien-5-ol, 7a, (8.19 g, 25.8 mmol; 90%).
1H NMR (400 MHz, CDCl3): δ 8.03 (dd, 1H, J = 6 Hz, J = 3.6 Hz, H4), 7.54 (dd, 1H, J =
6.0 Hz, J = 2.8 Hz, H7), 7.2-7.1 (m, 6H, H1-3,8-10), 6.95 (d, 1H, J = 12.4 Hz, H12), 6.78 (d, 1H,
J = 12.4 Hz, H11), 3.72 (d, 1H, J = 13.2 Hz, H6a), 3.61 (d, 1H, J = 13.2 Hz, H6b), 2.45 (s, 1H,
13
OH), 0.21 (s, 9H, SiMe3). C NMR (100 MHz, CDCl3): δ 138.9 (C4a), 137.7 (C10a), 135.1
(C6a), 133.5 (C12a), 132.0 (C1), 131.5 (C12), 130.8 (C7), 130.5 (C4), 130.0 (C11), 127.5 (C3),
127.4 (C2), 127.3 (C10), 127.1 (C9), 126.9 (C8), 109.5 (C13), 90.5 (C14), 74.9 (C5), 46.0 (C6),
0.15 (SiMe3).
Preparation of 5-tri(isopropyl)silylethynyl-6H-dibenzo[a,e]cyclooctatrien-5-ol (7b). To a
o
solution of TIPS-acetylene (1.00 mL, 4.5 mmol) in THF (20 mL) at -78 C was added a 1.5
M solution of n-BuLi (2.78 mL, 4.2 mmol) in hexane. The resulting mixture was stirred at
this temperature for 30 min. It was then brought to room temperature and stirred for further
o
30 min. before re-cooling to -78 C. To this was added a solution of 6H-dibenzo[a,e]
cyclooctatrien-5-one, 6, (0.66 g, 3.0 mmol) in THF (7 mL) and stirred for 2 h. The reaction
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was quenched with distilled water (30 mL), extracted with diethyl ether (3 x 15 mL). The
solution was washed with brine (30 mL), dried over MgSO4 and the solvent was removed
under reduced pressure. Purification by flash column chromatography (c-Hex/EtOAc; 19:1)
afforded 7b (490 mg, 40%) as a colourless solid. Rf = 0.2 (c-Hex/EtOAc; 19:1). M.p. 72-74
1
°C. H NMR (500 MHz, CDCl3): δ 8.04-8.02 (m, 1H, H4), 7.50-7.47 (m, 1H, H7), 7.08-7.01
(m, 6H), 6.87 (d, 1H, J = 12.2 Hz, H12), 6.69 (d, 1H, J = 12.2 Hz, H11), 3.64 (d, 1H, J = 13.2
Hz, H6a), 3.59 (d, 1H, J = 13.2, H6b), 2.41 (s, 1H, OH), 1.02-1.00 (m, 21H, CH, CH3) ppm;
13C NMR (125 MHz, CDCl3): δ 140.3 (C4a), 137.7 (C6a), 135.3 (C10a), 133.6 (C12a), 133.3
(C12), 131.3, 130.7 (C7), 130.2 (C4), 130.1 (C11), 127.3, 127.2, 127.1, 126.9, 126.6, 111.9
(C13), 87.2 (C14), 75.0 (C5), 46.4 (C6), 18.6 (CH3), 11.3 (CH) ppm; HRMS (ES+):
C27H34ONaSi (M+Na+) calcd. 425.2277; found 425.2256; IR: 3433, 3059, 3013, 2941, 2864,
2164, 1488, 1064, 1049, 883, 794, 676 cm-1.
Preparation of 5-phenylethynyl-6H-dibenzo[a,e]cyclooctatrien-5-ol (7c). Analogously to
7a, 5-phenylethynyl-6H-dibenzo[a,e]cyclooctatrien-5-ol, 7c, was prepared from phenyl-
ethyne, n-BuLi and 6H-dibenzo[a,e]cyclooctatrien-5-one in 84% yield. 1H NMR (500 MHz,
CDCl3): δ 8.15 (dd, 1H, J = 6.0, 3.5 Hz, H4), 7.63 (dd, 1H, J = 6.0, 3.5 Hz, H7), 7.53-7.51 (m,
3H, o-,p-Ph), 7.38-7.35 (m, 2H, m-Ph), 7.27-7.25 (m, 2H, H2,9), 7.22-7.19 (m, 3H, H3,8,10),
7.17 (d, 1H, J = 6.0 Hz, H1), 7.02 (d, 1H, J = 12.0 Hz, H12), 6.84 (d, 1H, J = 12.0 Hz, H11),
3.82 (d, 1H, J = 13.0 Hz, H6a), 3.70 (d, 1H, J = 13.0 Hz, H6b), 2.81 (1H, OH); 13C NMR (125
MHz, CDCl3): δ 139.9 (C4a), 137.8 (C10a), 135.1 (C6a), 133.4 (C12a), 133.3 (C12), 131.9 (C1),
131.8 (Ph, o-C’s), 131.0 (C7), 130.4 (C4), 130.1 (C11), 128.7 (Ph, p-C), 128.5 (Ph, m-C’s),
127.8 (C2), 127.6 (C3), 127.2 (C10), 127.0 (C9), 126.9 (C8), 122.5 (Ph, ipso-C), 93.2 (C13),
86.1 (C14), 75.0 (C5), 48.5 (C6) ppm; HRMS (ES+): C24H18ONa (M+Na+) calcd. 345.1255;
found 345.1259; IR: 3431, 3057, 3021, 2932, 2160, 1597, 1489, 1431, 1041, 754 cm-1.
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Preparation of (5-trimethylsilylethynyl-6H-dibenzo[a,e]cyclooctatrien-5-ol)hexacarb-
onyldicobalt (8a). To a solution of 5-trimethylsilylethynyl-6H-dibenzo[a,e]cyclooctatrien-5-
ol (2.09 g, 6.57 mmol) in THF (50 mL) was added dicobalt octacarbonyl (2.32 g, 6.78 mmol)
and the resulting mixture was stirred overnight at room temperature. Chromatography on a
silica column, using pentane/dichloromethane as eluent, yielded (5-trimethylsilylethynyl-6H-
dibenzo[a,e]-cyclooctatrien-5-ol)hexacarbonyldicobalt, 8a, as a brown solid (3.02 g, 4.99
mmol; 76%). A sample of 8a suitable for X-ray diffraction was obtained by crystallization
from pentane/dichloromethane. 1H NMR (600 MHz, CDCl3): δ 7.93 (d, 1H, J = 7.8 Hz), 7.47
(d, 1H, J = 7.2 Hz), 7.24-7.19 (m, 2H), 7.17-7.11 (m, 2H), 7.06-7.02 (m, 2H), 6.95 (d, 1H, J
= 11.4 Hz, H11), 6.84 (d, 1H, J = 11.4 Hz, H12), 4.15 (d, 1H, J = 13.2 Hz, H6a), 3.20 (d, 1H,
13
J = 13.2 Hz, H6b), 2.83 (s, 1H, OH), 0.0 (s, 9H, SiMe3). C NMR (150 MHz, CDCl3): δ
200.2 (CO), 144.1 (C6a), 138.1 (C10a), 136.6 (C4a), 134.6, 133.8 (C12a), 130.6, 130.2, 129.7,
129.2, 127.7, 127.4, 127.2, 126.8, 121.9, 79.7 (C5), 78.9 (alkyne C), 50.7 (C6), 0.30 (SiMe3).
IR (CH2Cl2): 2088, 2051, 2053 cm-1 (νCO).
Preparation of (5-tri(isopropyl)silylethynyl-6H-dibenzo[a,e]cyclooctatrien-5-ol)hexa-
carbonyldicobalt (8b). To a solution of 5-tri-isopropylsilyethynyl-6H-dibenzo[a,e]-
cyclooctatrien-5-ol 7b (52 mg, 0.13 mmol) in THF (3 mL) was added dicobalt octacarbonyl
(45 mg, 0.13 mmol) in THF (2 mL) and the resulting mixture was stirred overnight (approx
20 h) at room temperature under nitrogen. Additional dicobalt octacarbonyl (46 mg, 0.13
mmol) was added and the resulting solution was left to stir for a further 2 h. Silica (approx 2
g) was added and the solvent removed under reduced pressure. Purification by flash column
chromatography (pentane/dichloromethane; 19:1 to 9:1) yielded 8b (55 mg 62%) as a brown
solid. Rf = 0.25 (pentane/dichloromethane; 19:1); 1H NMR (600 MHz, CDCl3):  7.91 (d, 1H,
J = 7.5 Hz), 7.44 (d, 1H, J = 7.5 Hz), 7.21-7.18 (m, 3H), 7.14-7.13 (m, 2H), 7.05 (d, 1H, J =
5.5 Hz), 6.99 (d, 1H, J = 12.5 Hz), 6.78 (d, 1H, J = 12.5 Hz), 4.19 (d, 1H, J = 13.25 Hz), 3.30
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13
(d, 1H, J = 13.25 Hz), 2.87 (s, 1H, OH), 1.34-1.32 (m, 3H), 1.20-1.16 (m, 18H) ppm; C
NMR (150 MHz, CDCl3):  200.3 (CO), 143.7 (C4a), 138.2 (C10a), 136.6 (C6a), 134.6 (C12),
134.4 (C12a), 130.4 (C1), 130.1 (C11), 129.9 (C7), 127.7 (C10), 127.4 (C8), 127.2 (C2), 127.1
(C3), 126.9 (C9), 124.7 (C13), 79.8 (C5), 74.9 (C14), 50.2 (C6), 19.2 (CH3), 14.5 (CH) ppm; IR:
3565, 3060, 2944, 2866, 2083, 2042, 2000, 1488, 1322, 1270, 1068, 953, 830, 707, 519 cm-1
Preparation of (5-phenylethynyl-6H-dibenzo[a,e]cyclooctatrien-5-ol)hexacarbonyl-
dicobalt (8c). To a solution of 7c (241 mg, 748 mmol) in THF (5.5 mL) was added dicobalt
octacarbonyl (268 mg, 784 mmol) and the resulting mixture was stirred overnight (approx. 15
h) at room temperature under nitrogen. Silica (approx. 2 g) was added and the solvent
removed under reduced pressure. Purification by flash column chromatography (SiO2,
gradient elution pentane/dichloromethane; 19:1 to 4:1) yielded 8c (387 mg, 637 mmol; 85%),
1
as a brown solid. Rf = 0.2 (pentane/dichloromethane; 19:1); H NMR (500 MHz, CDCl3): 
7.92 (d, 1H, J = 7.9 Hz), 7.61 (d, 2H, J = 6.9 Hz, Ph, o-H’s), 7.51 (d, 1H, J = 7.2 Hz), 7.37-
7.29 (m, 3H, Ph, m,p-H’s), 7.24-7.18 (m, 2H), 7.15 (d, 1H, J = 7.3 Hz), 7.12-7.08 (m, 1H),
7.05 (t, 2H, J = 7.0 Hz), 6.89 (d, 1H, J = 12.0 Hz) and 6.75 (d, 1H, J = 12.0 Hz), (H11,H12),
13
4.21 (d, 1H, J = 13.4 Hz, H6a), 3.32 (d, 1H, J = 13.4 Hz, H6b), 3.03 (s, 1H, OH); C NMR
(125 MHz, CDCl3):  199.3 (CO), 143.9 (C6a), 138.5, 138.2 (C10a), 136.3, 134.3, 133.8,
130.5, 130.2, 129.9, 129.7, 129.2, 128.5, 127.8, 127.4, 127.3, 127.2, 126.9, 108.1 (C13), 93.3
(C14), 80.1 (C5), 48.7 (C6). IR: 2088, 2047, 2002 cm-1 (νCO).
Preparation of (5-tri(isopropyl)silylethynyl-5H-dibenzo[a,d]cycloheptatrien-5-ol) 10.
Under nitrogen in oven dried glassware a solution of TIPS-acetylene (0.82 mL, 3.64 mmol)
in dry THF (15 mL) was treated dropwise with a 1.5 M solution of n-BuLi in hexanes (2.26
mL, 3.39 mmol) at -78 oC. This solution was stirred for 15 min at -78 oC and then warmed to
o
rt over 0.5 h. The resulting acetylide was re-cooled to -78 C and a solution of 5-
dibenzosuberenone (0.50 g, 2.42 mmol) in dry THF (5 mL) was added. The mixture was
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10.1002/ejic.201601538
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o
stirred at -78 C for 15 min before warming to rt over 0.5 h. TLC (c-Hex/EtOAc; 3:1)
indicated consumption of starting material. Aqueous saturated NH4Cl (20 mL) was added and
the mixture was extracted with Et2O (2 x 25 mL). The combined organic extracts were dried
over MgSO4. Filtration and solvent removal under reduced pressure gave crude product,
which was further purified by flash column chromatography (c-Hex/EtOAc; 6:1) to afford 10
(0.70 g, 74%) as a white solid. Rf = 0.5 (c-Hex/EtOAc; 3:1); M.p. 68 °C; 1H NMR (400 MHz,
CDCl3):  8.12 (d, 2H, J = 7.8 Hz, H4, H6), 7.40–7.37 (m, 4H), 7.30-7.27 (m, 2H), 7.12 (s,
13
2H, H10, H11), 2.91 (s, 1H, OH), 1.09–1.05 (m, 21H, CH, CH3) ppm; C NMR (100 MHz,
CDCl3):  140.1 (C4a, C5a), 132.8 (C9a, C11a), 131.5 (C10, C11), 129.2 (C1, C9), 128.2 (C3, C7),
127.1 (C2, C8), 124.2 (C4, C6), 108.1 (C13), 94.8 (C12), 85.2 (C5), 18.5 (CH3), 11.3 (CH) ppm;
IR: 3597, 3471, 2941, 2862, 1484, 1365, 984, 765 cm-1; HRMS (ES+): C26H32ONaSi
(M+Na+) calcd. 411.2120; found 411.2136.
Preparation of (5-tri(isopropyl)silylethynyl-5H-dibenzo[a,d]cycloheptatrien-5-ol)penta-
carbonyldicobalt (12). Method 1: Under nitrogen a solution of compound 10 (0.57 g, 1.47
mmol) and dicobalt octacarbonyl (0.71 g, 2.08 mmol) in dry THF (15 mL) was stirred at
room temperature for 15 h. Additional dicobalt octacarbonyl (0.59 g, 1.73 mmol) was added
and the solution was left stirring for another 2 days. The solvent was removed under reduced
pressure and crude product was purified by chromatography on silica gel (pentane/
dichloromethane; 19: 1) this gave compound 12 (0.326 g 33%) as a brown solid which was
subsequently recrystallized from pentane/dichloromethane to generate crystals suitable for
X-ray diffraction. Further elution gave 10 (0.30 g, 53%). Method 2: Under nitrogen, a
solution of 10 (1.03 g, 2.65mmol) and dicobalt octacarbonyl (0.981 g, 2.66 mmol) in dry
DCM (25 mL) was stirred at room temperature for 4 days. The solvent was removed under
reduced pressure and, as above, the crude product was purified by chromatography to give
compound 12 (0.85 g, 49%) as brown solid. Further elution gave 10 (0.40 g, 39%). Rf = 0.3
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1
(pentane/DCM; 19:1); H NMR (400MHz, CDCl3):  7.81 (d, 1H, J = 8.0 Hz, H6), 7.76 (d,
1H, J = 7.8 Hz, H4), 7.53-7.51 (m, 2H), 7.41-7.21 (m, 3H), 7.08 (d, 1H, J = 8.0 Hz), 5.45 (d,
1H, J = 9.5 Hz, H11), 5.40 (d, 1H, J = 9.5 Hz, H10), 2.57 (s, 1H, OH), 1.07 (s(br), 3H, CH),
0.97-0.93 (m, 18H, CH3) ppm; 13C NMR (100 MHz, CDCl3):  198.3 (CO), 143.2 (C5a),
142.8 (C4a), 134.6 (C9a), 133.8 (C11a), 129.8 (C1), 128.8 (C9), 127.8 (C3), 127.7 (C7), 127.1
(C2), 126.6 (C8), 121.8 (C4), 121.6 (C6), 78.1, 77.2, 75.9 and 73.8 (C5,C10,C11,C12), 18.9
(CH3), 18.7 (CH3), 13.31 (CH) ppm; IR: 3575, 3062, 2890, 2063, 1990, 1574 cm-1.
Preparation of (5-trimethylsilylethynyl-5H-dibenzo[a,d]cycloheptatrien-5-ol)tetracarb-
onyl(triphenylphosphine)dicobalt (13). To a solution of (5-trimethylsilylethynyl-5H-
dibenzo[a,d]cycloheptatrien-5-ol)pentaacarbonyldicobalt, 2b, (261 mg, 0.46 mmol) in THF
(20 mL) was added triphenylphosphine (121 mg, 0.46 mmol) and the reaction mixture was
stirred initially at room temperature and then at reflux overnight. Filtration through celite,
and chromatography on a silica column, using pentane/dichloromethane as eluent, yielded 13,
as a red-brown solid (286 mg, 0.36 mmol; 78%). A sample suitable for X-ray diffraction was
1
obtained by crystallization from pentane/dichloromethane. H NMR (400 MHz, CDCl3): 
8.49 (d, 1H, J = 7.2 Hz), 8.34 (d, 1H, J = 8.0 Hz), 8.1 – 7.2 (m, 23H), 3.29 (s, 1H, OH), 0.0
(s, 9H, SiMe3); 13C NMR (100 MHz, CDCl3):  205.6, 202.0 (CO), 133.5, 134.4 (C1,C9),
2
132.3 (d, JC-P = 11 Hz, Ph, o-C’s), 132.1 (Ph, p-C’s), 132.0, 131.7 (C3,C7),, 129.4, 128.8
3
(C2,C8), 128.7 (d, JC-P = 9 Hz, Ph, m-C’s), 128.5, 127.4 (C4,C6), 76.5 (C5), 74.2 (C11), 70.9
(C10), 0.0 (SiMe3). 31P NMR (202 MHz, CDCl3): 54.8.
X-ray measurements for 7, 9a, 9b, 12 and 13.
Data for 6, 8a and 13 were collected using a Bruker SMART APEX CCD area detector
diffractometer. A full sphere of reciprocal space was scanned by phi-omega scans. Pseudo-
empirical absorption correction based on redundant reflections was performed by the
program SADABS.[26] Data for 8b and 12 were collected on Mo-Kα (0.71073 Å) using an
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10.1002/ejic.201601538
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Rigaku Oxford Diffraction (former Agilent Technologies, former Oxford Diffraction)
SuperNova A diffractometer fitted with an Atlas detector. An eightfold redundant dataset was
collected, assuming that the Friedel pairs are not equivalent. An analytical absorption
correction based on the shape of the crystal was performed.[27] The structures were solved by
direct methods using SHELXS-97,[28] and refined by full matrix least-squares on F2 for all
data using SHELXL-97.[28] Anisotropic displacement parameters were refined for all non-
hydrogen atoms. Further details are listed in Table 1. CCDC 1509159 (6), 1509160 (8a),
1509162 (8b), 1509161 (12) and 1509158 (13) contain supplementary X-ray crystallographic
data for this paper. These data can be obtained free of charge from the Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Corresponding Authors
 P.E. e-mail, paul.evans@ucd.ie; phone +353-1-716-2291.
 ORCID: 0000-0002-0584-876X. www.ucd.ie/chem/staff/drpaulevans.
 M.J.M., e-mail, michael.mcglinchey@ucd.ie; fax, +353-1-716-1178
Notes
The authors declare no competing financial interest.
Acknowledgements
We thank Science Foundation Ireland, University College Dublin, and the Centre for
Synthesis and Chemical Biology funded by the Higher Education Authority’s Programme for
Research in Third-Level Institutions (PRTLI) for generous financial support, and the
reviewers for their helpful comments.
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