Linear 6,6′-biazulenyl framework featuring isocyanide termini: Synthesis, structure, redox behavior, complexation, and self-assembly on Au(111)

Article abstract of DOI:10.1021/ja108202d

The key step in accessing the title species (5), the first nonbenzenoid diisocyanobiaryl, involved an unexpected homocoupling of a 6-bromoazulene derivative. The reversible 2e^- reduction of 5 was addressed electrochemically and computationally. The shifts in energies of the S ₀→S₁ and S₀→S₂ transitions for a series of related 6,6′-biazulenyl derivatives correlate with the e^--donating/-withdrawing strength of their 2,2′-substituents but follow opposite trends. Species 5 adsorbs end-on (η¹) to the Au(111) surface via one of its -NC groups to form a 2-nm-thick film. In addition, bimetallic coordination of 5's -NC termini can be readily achieved.

Full text of DOI:10.1021/ja108202d

Published on Web 10/26/2010
Linear 6,6′-Biazulenyl Framework Featuring Isocyanide Termini: Synthesis,
Structure, Redox Behavior, Complexation, and Self-Assembly on Au(111)
Tiffany R. Maher, Andrew D. Spaeth, Brad M. Neal, Cindy L. Berrie, Ward H. Thompson,
Victor W. Day, and Mikhail V. Barybin*
Department of Chemistry, UniVersity of Kansas, 1251 Wescoe Hall DriVe, Lawrence, Kansas 66045
Received September 10, 2010; E-mail: mbarybin@ku.edu
is formally derived by linking two 1,3-diethoxycarbonyl-2-isocy-
anoazulene (1) molecules. To the best of our knowledge, the title
Abstract: The key step in accessing the title species (5), the first
9
nonbenzenoid diisocyanobiaryl, involved an unexpected homo-
species is not only the first structurally characterized linear
-
coupling of a 6-bromoazulene derivative. The reversible 2e
10
biazulenyl compound but also the sole example of a crystallo-
reduction of 5 was addressed electrochemically and computa-
graphically addressed biazulenyl motif of any connectivity not
tionally. The shifts in energies of the S
0
fS
1
and S
0
fS
2
transitions
11
embedded into a larger rigid framework.
for a series of related 6,6′-biazulenyl derivatives correlate with
-
the e -donating/-withdrawing strength of their 2,2′-substituents
a
Scheme 1
1
but follow opposite trends. Species 5 adsorbs end-on (η ) to the
Au(111) surface via one of its -NC groups to form a 2-nm-thick
film. In addition, bimetallic coordination of 5’s -NC termini can
be readily achieved.
8
Azulene is an unusual aromatic hydrocarbon (C10H ) that
comprises an edge sharing combination of five- and seven-
2
membered sp -carbon rings. The azulenic and polyazulenic motifs
a
(
i) B
2
pin
2
(0.26 equiv), 10 mol % Pd(dppf)Cl
2
·CH
2
Cl
2
, KOAc, DMSO,
N, 20 °C.
constitute attractive building blocks in the design of redox addres-
1
00 °C, argon atm.; (ii) HC(O)OAc, 50 °C, (iii) POCl
3
, Et
3
1
–4
sable, optoelectronic, and conductive materials.
Unlike the
frontier molecular orbitals of benzenoid aromatics, the HOMO and
LUMO of azulene are not mirror related and feature mutually
complementary density distributions (HOMO ) Highest Occupied
8
Combining the 6-bromoazulene derivative 2 with bis(pina-
colato)diboron (B pin ) in the presence of Pd(dppf)Cl (dppf )
2
2
2
Molecular Orbital, LUMO ) Lowest Unoccupied Molecular
bis(diphenylphosphino)ferrocene) under the conditions specified in
Scheme 1 afforded brick-red 2,2-diamino-6,6′-biazulenyl 3 in a 91%
yield. Surprisingly, the best yields of this unexpected, “one-pot”
homocoupling of 2 were achieved by employing a substoichiometric
3
Orbital). This leads to a remarkably low S
0
fS
1
excitation energy
for azulene derivatives and enables topological asymmetry in the
1–4
electron and hole transport regimes for azulene-based frameworks.
amount (ca. 0.25 equiv) of B
when the reaction was conducted in the absence of B
2
pin
2
. No formation of 3 was observed
pin under
otherwise identical conditions. Interestingly, our attempts to use
.5 equiv of B pin (the stochiometric quantity of B pin typically
employed in one-pot homocoupling of organohalides via sequential
2
2
1
2
0
2
2
2
2
13
Miyaura borylation/Suzuki cross-coupling) invariably led to much
lower yields (e37%) of 3. The above quite efficient protocol for
the preparation of 3 evolved from our initial efforts to improve its
Coordination and surface chemistry of linear benzenoid diiso-
cyanoarenes containing one or more linked aromatic rings has been
10c
original synthesis by Mutafuji, Sugihara et al. The latter involved
a Pd(dppf)Cl -catalyzed borylation of 2 with 1.1 equiv of B pin
to isolate the corresponding 6-azulenyl-boronic ester, which was
then cross-coupled with 2 using a different catalyst, Pd(PPh Cl
5
,6
the subject of growing experimental and theoretical interest,
2
2
2
particularly in the context of developing advanced materials that
7
may support charge delocalization and transport at the nanoscale.
3
)
2
2
,
1
0c
We have recently engaged in the quest for a novel class of linear
diisocyanoarene linkers based on the nonbenzenoid 2,6-azulenic
to give 3 in a 17% overall yield.
Formylation of 3 with acetic-formic anhydride afforded a 79%
yield of sparingly soluble chestnut-colored 2,2′-diformamido-6,6′-
biazulenyl 4, the double dehydration of which gave lavender needles
of 2,2′-diisocyano-1,1′,3,3′-tetraethoxycarbonyl-6,6′-biazulenyl 5 in
1
,8,9
framework as represented by the homologous series I.
For n )
1
(R ) -CO Et) in I, the orientation of the azulenic dipole can be
2
controlled through regioselective installation and coordination of
8
13
the isocyanide junction groups. For n ) 2, three different linear
a 34% isolated yield. The FTIR and C NMR spectra of 5 exhibit
signature peaks at νCtN ) 2130 cm- (in Nujol mull) and δ )
1
diisocyanobiazulenyl scaffolds can be envisioned: two symmetric
featuring the 6,6′ or 2,2′ connectivity of the azulenic moieties and
one asymmetric having the 2,6′ central C-C bond. Currently, very
few 2,2′-, 6,6′-, and 2,6′-biazulenyl derivatives are synthetically
3
178.6 ppm (in CDCl ), respectively, that correspond to the
isocyanide termini of this air-stable compound.
It is well argued that varying the nature of the substituent in a
1
0
accessible. In this Communication, we introduce the chemistry
of the first member of the linear diisocyanobiazulenyl family that
2-substituted azulene chiefly affects the energy of its LUMO but
3
not HOMO. The λmax value for the relatively weak S
0
fS
1
transition
1
5924 9 J. AM. CHEM. SOC. 2010, 132, 15924–15926
10.1021/ja108202d  2010 American Chemical Society

C O M M U N I C A T I O N S
19
in the electronic spectra of 3, 4, and 5 in CH
2
Cl
2
appears to increase
redox studies of various biazulenic motifs. The persistence of 52-,
at least on the electrochemical time scale, can be attributed to the
closed-shell nature of its 6,6′-biazulenide dianion framework (Figure
1
4
upon proceeding from 3 to 4 (474 nm) to 5 (509 nm). This trend
-
-
parallels the order of decreasing e -donating/increasing e -
10b,19d,20
2-
withdrawing strength of the groups at the 2,2′-positions in these
3, left).
The singlet electronic configuration of 5 is also
suggested by our DFT examination of its model 5a . The singlet (S)
state of 5a is predicted to be nearly 0.7 eV less energetic than the
triplet (T) state. The DFT calculations show that the reduction process
5af5a (S) is accompanied by appreciable shortening of the central
C-C bond, as well as by a 30° decrease in the interplanar angle
between the two azulenic moieties (Table 1). The HOMO of 5a2 (S)
illustrated in Figure 3 clearly implies the significant double bond
character of the dianion’s central C-C linkage. Similar to 5, the CV
of 3 also features one reversible reduction wave, which occurs at a
more negative potential (E1/2 ) -1.64 V) compared to that of 5 due
2-
6
,6′-biazulenyls: NH > sNHCHO > sNtC. At the same time,
2
2-
however, λmax of the more intense higher energy band, which we
tentatively assign as S fS , increases in reverse order 5 (390 nm)
4 (421 nm) < 3 (459 nm). Thus, the 2,2′-substitution of the 6,6′-
biazulenyl scaffold provides an opportunity to simultaneously tune
the wavelengths of both S fS and S fS excitations in mutually
opposing directions in the visible region.
0
2
2-
<
-
0
1
0
2
to the electron-donating nature of the -NH
2
termini.
Figure 1. Molecular structure of 5 (50% thermal ellipsoids).
The solid state structure of 5 depicted in Figure 1 is remarkably
symmetric with only 1/4 of the molecule being crystallographically
n
Figure 2. Cyclic voltammogram of 5 in 0.1 M [ Bu
4
N][PF
]/CH Cl
6 2 2
vs
independent. The C3-N1 bond length of 1.165(3) Å observed for
+
2 2
internal Cp Fe /Cp Fe (1 equiv) at 25 °C. Scan rate ) 100 mV/s.
8
5
is typical for an isocyano NtC triple bond. Every carboxylate
unit in 5 is essentially coplanar with the azulenic moiety to which
it is attached. The long axis of 5 spans 17.1 Å, as defined by the
C3· · ·C3′ distance. The C6-C6′ bond connecting the azulenyl rings
in 5 is 1.512(4) Å long. This distance is statistically shorter than
3
3
the C(sp )-C(sp ) bond of 1.535(4) Å connecting the two seven-
membered rings in 1,1′,6,6′-tetrahydro-6,6′-biazulene-1,1′-diide,
Figure 3. Left: bis(cyclopentadienide)-like resonance form of 52-. Right:
2
-
DFT-generated HOMO of 5a (S).
2
-
15
[
H
8
C
10-C10
H
8
]
,
but only marginally longer than the central
Table 1. DFT-Generated Relative Energies in the Gas Phase
C-C bond length documented for biphenyl (1.494(3)-1.507
1
6,17
(∆Egas) and in Dichloromethane (∆EDCM), Interplanar Dihedral
Å).
Angles (R), and the Central C-C Bond Length (d) for 5a, 5a2-(S),
2
-
The 66.9° torsion angle between the azulenic planes in crystalline
is almost certainly significantly influenced by crystal packing
forces. Our density functional theory (DFT) analysis of 2,2′-
diisocyano-6,6′-biazulenyl (5a), a truncated analogue of 5 that lacks
all ester substituents, predicts the equilibrium interplanar angle of
and 5a (T)
5
Model species
∆Egas, eV
∆EDCM, eV
R, deg
d, Å
5a
5a
5a
0
0
52.0
21.9
58.5
1.50
1.43
1.51
2
-(S)
(T)
-1.58
-0.91
-5.92
-5.23
2-
5
2.0° for this model compound with the barriers to internal rotation
about the C6-C6′ bond to achieve the planar and orthogonal
conformations being ∆E(0°) ) 8.2 kcal/mol and ∆E(90°) ) 1.3
kcal/mol, respectively. Notably, both experimental and recent DFT
The molecule of 5 can be readily used to bridge metal centers.
For example, treatment of in-situ-generated W(CO) (THF) with 0.5
equiv of 5 in THF provided fuchsia-colored [(OC) W] (µ-5) that
features two “(OC) W” units linked through the 6,6′-biazulenyl
bridge by means of the NtC junctions. Complex [(OC) W] (µ-5)
undergoes a reversible reduction at E1/2 ) -1.01 V in CH Cl . This
reduction potential is almost identical to that of 5 thereby indicating
that the LUMO of [(OC) W] (µ-5) is largely bridge-based. The
lowest energy band (λmax ) 496 nm) in the electronic spectrum of
[(OC) W] (µ-5) can be assigned to the metal-to-bridge charge
transfer (MBCT), and its molar extinction coefficient (ε) is ca. 35
times greater than that documented for the S fS transition for 5.
Notably, the analogous MBCT for [(OC) W] (µ-2,6-diisocyano-
1,3-diethoxycarbonylazulene) has a λmax value of 515 nm, whereas
the corresponding transition for [(OC) W] (µ-1,4-diisocyanoben-
zene) occurs in the UV region (λmax ) 370 nm).
Exposure of a gold-coated mica substrate to a 2 mM solution of
5
studies of biphenyl indicate that the H
5
C
6
-C
6
H
5
molecule exhibits
5
2
the torsional angle of ca. 45° with the rotational barriers ∆E(0°) ≈
5
1
8
∆
E(90°) e 2.0 kcal/mol in the gas phase. While the ∆E(90°)
5
2
values for both 5a and (C are similar, the higher ∆E(0°) value
6
H
5
)
2
2
2
for 5a reflects greater steric congestion about the central C-C bond
connecting the two seven-membered rings in the planar conforma-
tion of 5a compared to the environment of the central C-C linkage
in the planar orientation of biphenyl.
5
2
5
2
Compound 1, the structure of which may be viewed as one-half of
that of 5, undergoes an irreversible one-electron reduction at Ep,c
)
0
1
+
2 2 2 2
-1.55 V vs Cp Fe /Cp Fe in CH Cl . In sharp contrast, the cyclic
5
2
8
voltammogram (CV) of 5 in the same solvent features a nicely
reversible (ip,c/ip,a ) 1.0) two-electron reduction wave at the substan-
tially less negative potential of E1/2 ) -1.02 V (Figure 2). This
observation echoes the reduction behavior of the “parent” 6,6′-
biazulenyl addressed by H u¨ nig and Ort in a series of their pioneering
5
2
2
1
5 in CH
2
Cl
2
without protection from air led to adsorption of 5 on
J. AM. CHEM. SOC. 9 VOL. 132, NO. 45, 2010 15925

C O M M U N I C A T I O N S
the Au(111) surface. The ellipsometric thickness of the resulting
film, measured at multiple spots on the substrate, was 20.5 ( 2.4
Å. This value is consistent with the molecular monolayer nature
of the self-assembled film featuring approximately parallel orienta-
tion of the long molecular axis of 5 with respect to the surface
Bruker AXS for collecting the X-ray diffraction data for 5 and
Professors Malinakova and Tunge for helpful discussions.
Supporting Information Available: Experimental procedures;
spectroscopic and analytical data; details of the electrochemical, X-ray,
surface, and DFT studies (PDF and CIF). This material is available
free of charge via the Internet at http://pubs.acs.org.
5
1
normal. Indeed, for the perfectly upright η coordination of 5 to
the gold surface (Figure 4), the monolayer thickness can be expected
to be approximately 19.1 Å. This estimate is obtained by adding
5
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Acknowledgment. This work was supported by the NSF
CAREER Award (CHE-0548212) and DuPont Young Professor
Award to M.V.B. The authors thank Dr. Matthew Benning of
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JA108202D
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5926 J. AM. CHEM. SOC. 9 VOL. 132, NO. 45, 2010