Tetrakis(imino)pyracene complexes exhibiting multielectron redox processes

Article abstract of DOI:10.1021/ja2102828

The differences in redox behavior of the monofunctional bis(imino)acenaphthene (BIAN) and bifunctional tetrakis(imino)pyracene (TIP) ligands have been explored by treatment of the latter with PI₃, TeI₄, or BI₃. These reactions result in the formation of products involving the transfer of three or four electrons. Accompanying DFT calculations reveal that in each case the extent of electron transfer from each p-block element into the TIP ligand is dependent upon the element-TIP bonding interactions.

Full text of DOI:10.1021/ja2102828

Communication
pubs.acs.org/JACS
Tetrakis(imino)pyracene Complexes Exhibiting Multielectron Redox
Processes
Kalyan V. Vasudevan,† Michael Findlater,‡ Ignacio Vargas-Baca,§ and Alan H. Cowley*
†Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States
‡Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-1061, United States
§Department of Chemistry and Chemical Biology, McMaster University, Hamilton, Ontario, Canada L8S 4M1
S
* Supporting Information
ABSTRACT: The differences in redox behavior of the
monofunctional bis(imino)acenaphthene (BIAN) and
bifunctional tetrakis(imino)pyracene (TIP) ligands have
been explored by treatment of the latter with PI3, TeI4, or
BI3. These reactions result in the formation of products
involving the transfer of three or four electrons.
Accompanying DFT calculations reveal that in each case
the extent of electron transfer from each p-block element
into the TIP ligand is dependent upon the element−TIP
bonding interactions.
ecently, the tetrakis(imino)pyracene (TIP) ligand was
developed as a bifunctional analog of the bis(imino)-
R
Figure 1. POV-Ray view of 1 with thermal ellipsoids shown at 40%
probability. All hydrogen atoms and two molecules of lattice CH2Cl2
have been removed for clarity.
acenaphthene (BIAN) ligand class.1 Subsequently, it was
demonstrated that TIP ligands are capable of undergoing
single-electron reduction at both diimine functionalities when
treated with 2 equiv of the one-electron reductants K, GeCl2-
dioxane, or EuCp*2-OEt2, resulting in a pairing of electrons
over the pyracene backbone.2 Given that BIAN ligands are
capable of accepting up to four electrons,3 we were curious to
discover whether TIP ligands are capable of undergoing further
reduction.
solvent removal. After washing the crude solid with hexanes to
remove the residual iodine, crystals of 2 were obtained by
storage of a 4:1 dichloromethane/hexanes solution at −15 °C
for several days. The reaction of TeI4 with tetrakis(imino)-
pyracene also involves a four-electron reduction. However,
contrary to the ligand-based reduction that occurred in the case
of 1, the reduction takes place at the Te centers to afford the
bis(TeI2) complex 2. No reduction of the TIP ligand occurred
as evidenced by the X-ray crystal structure (Figure 2) which
revealed the presence of a C(1)C(5) single bond and two
CN double bonds within each of the C2N2Te rings. It has
been elegantly demonstrated by Ragogna et al. that the
analogous (TeI2)BIAN compound can serve as a synthon for a
novel Te(II) dication upon halide abstraction with Ag(OTf).5
Current efforts are ongoing to determine whether a similar
synthetic pathway is accessible for 2.
In general, the formation of these p-block derivatives involves
the interplay of several factors, namely the strengths of the σ
and π E−N (E = P, Te) bonding interactions, the ability of the
TIP ligand to accept and delocalize electron density, the
dissociation of E−I bonds with the consequent formation of I2
and/or I−, and the lattice energies in the case of the charged
species. The first two types of contributions were examined by
means of electronic structure calculations (DFT) in order to
Since it had been established previously that the monofunc-
tional 2,6-diisopropylphenyl-substituted (dpp) BIAN ligand
undergoes two-electron reduction with PI3 to afford the
phosphenium cation [(BIAN)PI]+ as its [I3]− salt,4 we decided
to explore the corresponding reaction of the bifunctional dpp-
substituted TIP ligand with 2 equiv of PI3. After the green-
brown dichloromethane solution was stirred overnight, the
crude solid was isolated, dissolved in a 9:1 dichloromethane/
hexanes solution, and stored at −15 °C for several days, thereby
affording a crop of dark red crystals of 1. The 31P NMR
chemical shift of δ 237.7 for the product implied that it
represented the first example of a bis(phosphenium) cation and
inferentially that four-electron reduction had occurred.
Confirmation of this view was provided by a single-crystal X-
ray diffraction study of 1 (Figure 1). Noteworthy structural
changes that take place upon ligation to the P+ centers include
shortening of the C(1)−C(5) distance from 1.549(4) Å in the
uncoordinated TIP ligand to 1.392(8) Å in 1 and elongation of
the average C−N bond distance from 1.268(3) Å to 1.350(7)
Å.
The corresponding reaction of the TIP ligand with 2 equiv of
TeI4 resulted in the isolation of a blue-green solid following
Received: November 1, 2011
Published: November 30, 2011
© 2011 American Chemical Society
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Journal of the American Chemical Society
Communication
Table 1. Calculated Nalewajski−Mrozek Bond Indices
around the C2N2E Chelate Ring
1
2
3
C−C
C−N
N−E
E−X
1.30
1.35
1.19
1.05
1.68
0.40
0.93
1.24
1.27
1.14
1.32
efficient electron sharing between the π manifold of the TIP
ligand and the chalcogen. Therefore, while in the case of 1 the
electron acceptor ability of the ligand is manifested in changes
of bond length that are consistent with a four-electron
reduction, in compound 2 two lone pairs remain centered on
each tellurium atom. The calculated bond orders for 1 are
consistent with the multiple bond character of the P−N link
and conjugation throughout the C2N2P ring. These values are
in clear contrast with the corresponding parameters calculated
for 2, namely a small TeN bond order and localized CC
and CN bonds.
Figure 2. POV-Ray view of 2 with thermal ellipsoids shown at 40%
probability. Two molecules of lattice CH2Cl2 and all hydrogen atoms
have been removed for clarity.
interpret the observed differences in the redox behavior of 1
and 2. Geometry optimizations, which were carried out under
the general gradient approximation with the PW91 functional,
provided an excellent reproduction of the molecular
dimensions. The calculated electron density was then analyzed
on the basis of computed Nalewajski−Mrozek bond indices
(orders)6 and electron localization functions (ELFs).7 Relevant
sections of the calculated ELFs are presented in Figures 3 and
4, and pertinent bond indices are compiled in Table 1.
Attention was turned next to the reactions of the TIP and
BIAN ligands with BI3. In previously reported work, we8 and
others9 have shown that the reactions of the lighter boron
trihalides BCl3 and BBr3 with the TIP or BIAN ligands result in
straightforward heterolytic cleavage of a B−Cl or B−Br bond
and formation of the anticipated boron halide salts. No
evidence was found for redox behavior in any of these reactions.
In sharp contrast, the reaction of the TIP ligand with BI3 in
dichloromethane solution affords the novel radical cation salt 3
(Figure 5) by transfer of three electrons into the TIP ligand.
Figure 3. Contour plots of the ELF of 1 in the TIP plane and a
perpendicular plane containing the P−N bond.
Figure 5. POV-Ray view of 3 with thermal ellipsoids shown at 40%
probability. Three molecules of lattice CH2Cl2 and all hydrogen atoms
have been removed for clarity.
Figure 4. Contour plots of the ELF of 2 in the TIP plane and a
perpendicular plane containing the Te−N bond.
Ligation of the “BI” fragments results in ligand reduction as
evidenced by a shortening of the average C(1)−C(5) bond
length to 1.399(7) Å and extension of the average C−N bond
distance to 1.372(6) Å. A fourth electron is associated with an
iodide anion which, in turn, binds to two I2 molecules to form
the pentaiodide counterion.
Unfortunately, it was not possible to record a satisfactory
ESR spectrum for compound 3. However, the observed
magnetic moment of 1.74 μB is consistent with the presence
of one unpaired electron. Unrestricted DFT calculations
The ELF contour plots in the TIP plane provide a map of the
localization of the σ bonding electron pairs. In this respect,
there is a significant difference between the molecules of 1 and
2. The positions of the ELF maxima along the EN
internuclear axes indicate that the TeN bond is considerably
more polarized than the corresponding PN bond. The
weaker bonding interaction of 2 also results in the formation of
more localized π electron pairs along the TeN bonds and less
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Journal of the American Chemical Society
Communication
indicate that the spin density (Figure 6) is concentrated on
atoms B1, C1, and C5. As before, bond-order calculations were
the case of 2, reduction occurs at the Te centers. The
preparation of the (BIAN)BI complex 4, which displays a
doubly reduced ligand architecture, provides further evidence
that the TIP ligand class possesses a unique electronic behavior
and range of reactivities in comparison with those of the
monofunctional BIAN ligand.
ASSOCIATED CONTENT
* Supporting Information
■
S
Full experimental data, spectroscopic characterization, and
crystallographic data are provided. This material is available free
of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
cowley@mail.utexas.edu
■
Figure 6. Projection of the spin density of 3 on the 0.03 au isosurface
of the electron density.
ACKNOWLEDGMENTS
employed to examine the bonding in 3, the results of which are
consistent with strong σ bonding interactions and efficient
delocalization of π electron density onto the ligand.
■
Financial support from the Robert A. Welch Foundation
(A.H.C.) and the Natural Sciences and Engineering Research
Council of Canada (I.V.-B.) is gratefully acknowledged. This
work was made possible by access to the facilities of the Shared
Hierarchical Academic Research Computing Network
(SHARCNET: www.sharcnet.ca) and Compute/Calcul Cana-
da.
Given the unanticipated course of the reaction of the TIP
ligand with BI3, we were curious to discover the outcome of the
reaction of the analogous monofunctional dpp-BIAN ligand
with this boron trihalide. Accordingly, dpp-BIAN was treated
with 1 equiv of BI3 in dichloromethane solution which resulted,
after workup, in the isolation of dark red, solid 4.
Recrystallization of the crude material was effected by slow
evaporation of a THF solution of 4. X-ray crystallographic
analysis of 4 revealed that it is a neutral complex in which the
BIAN ligand is coordinated to a BI moiety (Figure 7). Scrutiny
REFERENCES
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Figure 7. POV-Ray view of 4 with thermal ellipsoids shown at 40%
probability. All hydrogen atoms and one molecule of lattice CH2Cl2
have been removed for clarity.
of the metrical parameters leads to the conclusion that two-
electron reduction has taken place. Diagnostic in this regard are
the C(1)−C(12) and average C−N bond distances of 1.354(6)
and 1.409(6) Å, respectively. A similar conclusion has been
reached by Nozaki et al. on the basis of the metrical parameters
for the corresponding diazabutadiene complex, (DAB)BI.10
In conclusion, three novel TIP-supported main group
complexes have been prepared that feature three- and four-
electron redox events. To the best of our knowledge,
compounds 1 and 3 represent the first crystallographically
characterized examples of ligands that involve reduction by
more than two electrons from p-block sources. In complexes 1
and 3, the TIP ligand serves as the electron acceptor while, in
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