A photochemical metallocene route to anionic enediynes: Synthesis, solid-state structures, and ab initio computations on cyclopentadienidoenediynes

Article abstract of DOI:10.1021/ja105149k

The first demonstration of photochemical enediyne liberation from a metal complex has led to a new class of enediynes, the cyclopentadienidoenediynes, which are demonstrated to exist as air-stable solids with low ionization potentials and large dipole moments. NMR and IR spectroscopy, X-ray crystallography, and ab initio computations enable a comparison with the ubiquitous benzoenediynes.

Full text of DOI:10.1021/ja105149k

Published on Web 07/22/2010
A Photochemical Metallocene Route to Anionic Enediynes: Synthesis,
Solid-State Structures, and ab Initio Computations on
Cyclopentadienidoenediynes
Joseph M. O’Connor,*,† Kim K. Baldridge,*,‡ Betsy L. Rodgers,† Marissa Aubrey,† Ryan L. Holland,†
W. Scott Kassel,† and Arnold L. Rheingold*,†
Department of Chemistry, UniVersity of California, San Diego, La Jolla, California 92093, and Institute of Organic
Chemistry, UniVersity of Zu¨rich, Winterthurerstrasse 190, CH-8057 Zu¨rich, Switzerland
Received June 12, 2010; E-mail: jmoconnor@ucsd.edu; kimb@oci.uzh.ch; arheingold@ucsd.edu
In this manner, 5-Ph and 5-OMe were prepared as red crystalline
Abstract: The first demonstration of photochemical enediyne
solids in 49 and 67% isolated yields. In the IR spectra (NaCl)
liberation from a metal complex has led to a new class of
of 5-Ph and 5-OMe, the carbonyl stretch is observed at 1633
enediynes, the cyclopentadienidoenediynes, which are demon-
and 1701 cm-1, respectively. For the series of diyne complexes
strated to exist as air-stable solids with low ionization potentials
(η5-C5H5)Fe[η5-C5H3(Ct CMe)2] (5-Me),7 5-OMe, and 5-Ph,
and large dipole moments. NMR and IR spectroscopy, X-ray
ν(Ct C) decreases from 2235 to 2215 to 2190 cm-1, consistent
crystallography, and ab initio computations enable a comparison
with increasing delocalization as the alkyne substituent is varied
with the ubiquitous benzoenediynes.
1
along the series. A similar trend was observed in the H NMR
spectra (CDCl3), for which the hydrogen resonances of both
Conjugated enediynes (1; Figure 1) are widely utilized as
precursors to parabenzyne intermediates1 and as key components
in carbon-rich materials, such as bowl-shaped fullerene fragments,
conjugated polymers, and carbon allotropes.2 In all of these,
advances have been greatly facilitated by the design and synthesis
of novel enediyne structures. A widely employed strategy for
stabilization of enediynes toward spontaneous polymerization3
involves incorporation of the ene function into an aromatic ring,
as in the benzoenediynes (2). Here we report the first photochemical
liberation of an enediyne from a metal complex to give a new class
of enediynes, the cyclopentadienidoenediynes (3), in which the ene
function is incorporated into a cyclopentadienide ring.
cyclopentadienyl ligands are observed to shift progressively
downfield as the alkyne substituent becomes more electron-
withdrawing: 5-Me (δ 4.22, s, 5H; 4.10, t, 1H; 4.53, t, 2H);
5-OMe (δ 4.33, s, 5H; 4.48, t, 1H; 4.73, d, 2H); 5-Ph (δ 4.40,
s, 5H; 4.65, t, 1H; 4.93, d, 2H). In the solid-state structures of
5-Me, 5-OMe, and 5-Ph, the iron-C3/C9 average distances are
3.130(5), 3.097(1), and 3.078(1) Å, respectively.8,9 Thus, the
spectroscopic and crystallographic data are consistent with
contributions from cumulene resonance forms 5-A and 5-B
(Scheme 1). These resonance structures are similar to the
metal-ligand charge-transfer (MLCT) excited state I that has
been proposed for the photochemical loss of the substituted
cyclopentadienyl ligand in benzoylferrocene.5a
Scheme 1. Resonance Contributors for 5 and 6 and the
Proposed5a Excited State I for Ligand Loss upon Photolysis of
CpFe[C5H3(COR)2]
Figure 1. Acyclic enediynes (1), benzoenediynes (2), cyclopentadienidoene-
diynes (3), and iron enediynes (4). The numbering used in the text is also shown.
Despite the well-established photochemical stability of ferrocene
derivatives in nonhalogenated solvents,4 the long-standing observation
that ferrocenes with conjugating substituents (e.g., -CO2H, -COR)
may undergo photochemical ligand loss5 led us to examine a metal-
locene route toward cyclopentadienido analogues of benzoenediynes.
The desired precursors 5-Ph and 5-OMe were readily prepared by
sequential reaction of 1,2-diethynylferrocene (2.1 mmol, THF)6 with
n-BuLi (5.2 mmol) followed by quenching of the dianion with benzoyl
chloride and methyl chloroformate, respectively, in slight excess (eq 1):
Unlike cyclic iron-enediyne complexes 4 (Figure 1) and 5-Me,
both 5-OMe and 5-Ph are light-sensitive compounds. Initial attempts
at generating a cyclopentadienidoenediyne by photolysis of 5-Ph
in THF solution led to the formation of an intractable brown
precipitate, ferrocene, and free cyclopentadiene, with no clear
evidence of the desired product. However, when a THF-d8 solution
containing 5-Ph (0.045 mmol) and 1 equiv of tris(diphenylphos-
phinomethyl)ethane (triphos) was irradiated with a medium-pressure
Hanovia lamp (2 h), nearly quantitative conversion to 6-Ph was
† University of California, San Diego.
‡ University of Zu¨rich.
1
observed by H NMR spectroscopy (eq 2):
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11030 J. AM. CHEM. SOC. 2010, 132, 11030–11032
10.1021/ja105149k  2010 American Chemical Society

C O M M U N I C A T I O N S
A preparative scale reaction led to the isolation of 6-Ph as a dark-
red (almost black) crystalline solid. In a similar fashion, compound
6-OMe was isolated as dark-brown air-stable crystals by irradiation
of 5-OMe.10
In the 1H NMR spectra (THF-d8) of 6-Ph and 6-OMe, the
substituted cyclopentadienyl ring hydrogens resonate 1.2-1.7 ppm
downfield of those for the iron precursors, with the C5/C7
hydrogens significantly more deshielded relative to the C6 hydro-
gen: 6-Ph (δ 5.84, t, J ) 3 Hz, 1H; 6.43, d, J ) 3.5 Hz, 2H);
6-OMe (δ 5.87, m, 1H; 6.45, d, J ) 2.5 Hz, 2H). The C5H5
hydrogens of the iron cation in 6-Ph (δ 4.95) exhibit a different
chemical shift than those in 6-OMe (δ 4.84), suggesting that the
anion and cation are not completely solvent-separated in THF
solution. The addition of D2O to an acetonitrile-d3 solution of
6-OMe results in the very slow incorporation of deuterium into the
three ring-hydrogen positions of the substituted five-membered ring.
In the IR spectrum (KBr, thin film) of 6-OMe, bands are observed
at 2124 (Ct C), 2155 (Ct C), and 1671 (CdO) cm-1. These can
be compared with the values of 2215 (Ct C) and 1701 (CdO) cm-1
1
for 5-OMe (KBr, thin film). Taken together, the H NMR and IR
data indicate significant contributions from cumulene resonance
forms 6-A and dicumulene forms 6-B (Scheme 1).
The solid-state structures of 6-OMe and 6-Ph were determined
by X-ray crystallographic analyses (Figure 2, Table 1).11 There are
two independent molecules within the unit cell of 6-Ph, indicated
here as 6-Ph-A and 6-Ph-B. Within experimental uncertainty, the
C4-C8, C2-C3, and C9-C10 bond distances of the enediyne
framework are the same in the three structures and similar to the
related values in the solid-state structures of iron precursors 5-OMe
and 5-Ph. The C4-C8 “ene” distances of 1.439(5)-1.457(4) Å in
6 are significantly longer than the distance of 1.346(3) Å in the
one reported example of a structurally characterized cyclopentene-
1,2-diyne.12 The cyclopentadienido ring in all three structures is
essentially planar, with the largest deviation of the sp carbons from
the mean plane occurring at C10 in 6-OMe (0.16 Å) and C2 in
6-Ph-B (0.31 Å). The most significant bond angle distortions are
in the C9-C10-C11 angles, which range from 168.0° in 6-Ph-A
to 171.2° in 6-OMe.13 The significant bending at C10, along with
minor bending at C9 (174.1-178.3°), and large displacements of
the O2 carbonyl oxygen atom from the ring plane (0.48-0.68 Å)
result in significant curvature along C8-C9-C10-C11-O2. In
the crystal lattice of 6-OMe, the concave face of the curve is
presented toward the triphos ligand of the cation, whereas in the
lattice of 6-Ph-A, the convex side of the curve is presented toward
the triphos ligand. The C3-C4-C8-C9 torsion angles in 6-Ph-A
and 6-Ph-B are 0.81(52) and -6.70(55)°, respectively, and the
closest nonbonded distances between benzoyl groups are the
C14A· · · O2A distance of 3.308(4) Å in 6-Ph-A and the corre-
sponding distance of 3.085(5) Å in 6-Ph-B. The O2A · · · H14A
distance of 2.63 Å in 6-Ph-A is 0.27 Å shorter than the O2B · · ·H14B
distance in 6-Ph-B.
Figure 2. ORTEP drawings of (top) 6-OMe and (center) 6-Ph-A, in which
hydrogen atoms have been omitted for clarity, and (bottom) the anion of
6-Ph-B. Red and blue numbers indicate atom displacements from the
C4-C5-C6-C7-C8 mean plane (e0.003 Å).
Table 1. Bond Lengths (Å) and Angles (deg) for the Anions in
6-Ph, 6-OMe, 6-OMe-calc, and 7-calc
6-Ph-A
6-Ph-B
6-OMe
6-OMe-calc
7-calc
C2· · ·C10
C1-C2
4.449(4) 4.256(5) 4.565(4)
1.426(5) 1.417(5) 1.439(4)
1.215(5) 1.220(5) 1.202(4)
1.392(4) 1.394(5) 1.411(4)
1.457(4) 1.439(5) 1.438(4)
1.414(4) 1.406(6) 1.400(4)
1.206(4) 1.221(6) 1.206(4)
1.439(4) 1.436(5) 1.423(4)
175.0(4) 176.8(3) 176.5(3)
178.4(3) 177.0(3) 179.8(3)
176.3(3) 174.1(4) 178.3(3)
4.608
1.419
1.214
1.369
1.440
1.396
1.214
1.419
178.28
178.97
178.97
178.26
4.137
1.449
1.201
1.426
1.409
1.426
1.201
1.450
177.57
179.09
179.09
177.57
C2-C3
C3-C4
C4-C8
C8-C9
C9-C10
C10-C11
C1-C2-C3
C2-C3-C4
C8-C9-C10
C9-C10-C11 168.0(3) 171.2(4) 171.2(3)
Of particular interest for enediyne structure-activity relationships
is the so-called critical distance (or [cd] distance), corresponding
to C2· · · C10 in 6, which has been correlated with the ease of
Bergman cycloaromatization.1b,c The C2 · · · C10 distance of 4.565
Å in 6-OMe is significantly longer than that in 6-Ph-A (4.449 Å),
which is in turn longer than the 4.256 Å distance in 6-Ph-B. These
values can be compared with the distances of 3.442(12) Å in the
10-membered ring enediyne 47 and 4.535(6) and 4.587(6) Å in the
two independent molecules in the unit cell of 6-Me. The large ran-
ge of [cd] distances in the cyclopentadienidoenediyne anions is the
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C O M M U N I C A T I O N S
result of crystal packing forces and is an indication of the
remarkable flexibility of the enediyne framework.
The reactions reported herein represent the first examples of
photochemical enediyne liberation from a metal as well as the
formation of a new class of enediynes, the cyclopentadienidoene-
diynes. We believe that photochemical dissociation of enediyne
ligands will prove to be general with respect to other types of
organometallic enediyne complexes (dienes, alkynes, etc.). Efforts
are underway to prepare photoactive strained-ring enediynes that
will spontaneously cycloaromatize upon enediyne dissociation from
the metal.
M06-2X/TZVP density functional theory (DFT) computational
studies were undertaken to elucidate the fundamental structural and
electronic differences between cyclopentadienidoenediynes and
benzoenediynes, as represented by the structures 6-OMe-calc and
7-calc (Figures 3 and 4, Table 1). The validity of the computational
results is supported by the remarkably close agreement between
the crystallographic and computational data for 6-OMe and anion
6-OMe-calc. The largest structural differences between 6-OMe and
6-OMe-calc are the 0.045 Å difference in the [cd] distance and the
conformation about the C10-C11 bond, for which the carbonyl
oxygen (O2) is oriented endo (pointing toward the other alkyne)
in the solid-state structure. The exo-exo gas-phase structure is
calculated to be the lowest-energy conformer by 0.5 kcal/mol. Both
structural differences are attributed to crystal lattice packing forces.
The C4-C8, C2-C3, and C9-C10 distances as well as the [cd]
distance are longer in cyclopentadienidoenediyne 6-OMe-calc than
in 7-calc, leading to the prediction of a significantly higher
activation energy for Bergman cycloaromatization in the former.
We have observed only decomposition to uncharacterized products
upon heating 6-OMe in THF at 150°. It is anticipated that
incorporation of the enediyne into a 10-membered ring (as in 4)
will lead to accelerated cycloaromatization rates in the metal-free
system relative to strained-ring benzoenediynes as a result of
significant strain-induced destabilization of the cyclopentadieni-
doenediyne ground-state structures. The ∆SCF (Koopmans’) theory
gas-phase ionization potentials for 6-OMe-calc and 7-calc are 3.70
(3.05) and 9.10 (8.37) eV, respectively.14 These can be compared
with the values of 2.03 (1.44) and 9.53 (8.41) eV for cyclopenta-
dienide and benzene, respectively. The dipole moments are 6.4 D
for 6-OMe-calc and 0.44 D for 7-calc. A perspective of the frontier
orbital energies (in eV) for 6-OMe-calc is shown in Figure 4.
Acknowledgment. Financial support by the National Science
Foundation (CHE-0518707, CHE-0911765, and instrumentation
grant CHE-9709183) is gratefully acknowledged. M.A. acknowl-
edges the award of a Kamen/Kaplan Fellowship. K.K.B. acknowl-
edges the Swiss National Science Foundation for support of this
work.
Supporting Information Available: Experimental details for all new
compounds, computational details for 6-OMe-calc and 7-calc, and
crystallographic data for 5-Ph, 5-OMe, 5-Me, 6-Ph, and 6-OMe (CIF).
This material is available free of charge via the Internet at http://
pubs.acs.org.
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Figure 3. DFT structures of (left) 7-calc and (right) 6-OMe-calc.
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Figure 4. Frontier orbitals for (left) 7-calc and (right) 6-OMe-calc [MP2/
DZ(2d,p) for 7-calc: -9.09, 0.87 eV; ∆ ) 9.96 eV. For 6-OMe-calc: -3.73,
5.29 eV; ∆ ) 9.02 eV.]
JA105149K
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11032 J. AM. CHEM. SOC. VOL. 132, NO. 32, 2010