Naphthalene-mediated electronic communication in tetrakis(imino)pyracene complexes

Article abstract of DOI:10.1002/anie.200904122

A hot tip: Bifunctional tetrakis(imino)pyracene (tip) ligands undergo one-electron reduction at both diimine functionalities when treated with potassium metal, germanium dichloride, or decamethyleuropocene (see scheme, Cp* = C₅Me₅).

Full text of DOI:10.1002/anie.200904122

Angewandte
Chemie
DOI: 10.1002/anie.200904122
Conjugated Ligands
Naphthalene-Mediated Electronic Communication in Tetrakis-
(imino)pyracene Complexes**
Kalyan V. Vasudevan, Ignacio Vargas-Baca, and Alan H. Cowley*
Previous work with monofunctional bis(imino)acenaphthene
(bian) ligands demonstrated that they are capable of accept-
ing zero, one, or two electrons from lanthanide bis(cyclopen-
tadienides) depending on the choice of the metal and the
steric/electronic characteristics of the bian ligand substitu-
ents.[1] More recently, the first example of a bifunctional
analogue of the bian ligand class has been reported, namely
the tetrakis(imino)pyracene (tip) ligand 1a (Scheme 1).[2] The
the reaction mixture changing color from orange to green.
Recrystallization of the resulting solid from THF at ꢀ158C
afforded dark green crystals of the dipotassium complex
[(thf)3K(dipp-tip)K(thf)3] (2; Scheme 2).
Scheme 2. [M2(tip)] complexes that undergo two-electron reductions.
A single-crystal X-ray diffraction study of 2[6] (Figure 1)
revealed the coordination of a {K(thf)3} moiety to both ends
of the tip ligand. The metrical parameters for the KN2C2 rings,
which are symmetry-related, are consistent with the view that
single-electron transfer had taken place at both diimine
Scheme 1. Tetrakis(imino)pyracene (tip) ligands.
same preparative method has now afforded 1b, an additional
member of this ligand class. Herein we report the interesting
redox behavior of this new type of bridging ligand. While
several redox-active bridging ligands with a variety of
architectures are known,[3] to the best of our knowledge
their electronic and steric properties cannot be readily tuned.
However, molecules with tunable electronic interactions
between redox-active centers are of interest because they
offer the possibility of extended delocalization of electron
density.[4] Apart from being intrinsically interesting, the
mediation of electronic coupling in redox-active bridging
ligands is pertinent to the design of molecular electronic
devices and the understanding of biochemical processes.[5]
Treatment of 1a with two equivalents of potassium metal
in THF solution for 12 h at ambient temperature resulted in
ꢀ
ꢀ
functionalities. Specifically, the C C and C N bond lengths of
1.484(5) and 1.320(6) (av) ꢀ, respectively, for the KN2C2 rings
fall between those of typical single and double bonds.
Confirmation of this assignment was provided by the syn-
thesis and structural analysis of the corresponding mono-
ꢀ
anionic complex [K(dipp-bian)] (3), for which the C C and
ꢀ
C N (av) bond lengths are 1.448(4) and 1.317(4) ꢀ, respec-
tively. The potassium ions of 2 are located on opposite sides of
the tip ligand and the distance between the K+ centers is
12.61 ꢀ. However, a slight deviation from overall skeletal
planarity is evidenced by the modest angle of 7.63(3)8
ꢀ
between the N-C-C-N and N-K-N planes. The average N K
ꢀ
and K O(thf) distances are 2.703(3) and 2.768(2) ꢀ, respec-
tively. Interestingly, 2 is diamagnetic, thus implying that the
[*] Dr. K. V. Vasudevan, Prof. Dr. A. H. Cowley
Department of Chemistry and Biochemistry
The University of Texas at Austin
two otherwise unpaired electrons in the C2N2K rings occupy
1 University Station, Austin, TX 78712 (USA)
E-mail: cowley@mail.utexas.edu
Prof. Dr. I. Vargas-Baca
Department of Chemistry, McMaster University
1280 Main Street West, Hamilton, Ontario L8S4M1 (Canada)
[**] A.H.C. is grateful to the Robert A. Welch Foundation for financial
support (Grant F-0003). I.V.-B. thanks the Natural Sciences and
Engineering Research Council of Canada and McMaster University
for funding. This work was made possible by the facilities of the
Shared Hierarchical Academic Research Computing Network
(SHARCNET: www.sharcnet.ca).
Supporting information for this article (Experimental details for the
synthesis and characterization of the new ligand 1b and complexes
2–6 along with the details of the DFT/B3LYP calculations) is
available on the WWW under http://dx.doi.org/10.1002/anie.
200904122.
Figure 1. POV-RAY diagram for 2 shown with thermal ellipsoids at
50% probability. All hydrogen atoms have been omitted for clarity.
Angew. Chem. Int. Ed. 2009, 48, 8369 –8371
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
8369

Communications
the lowest unoccupied molecular orbital (LUMO) of the
dipp-tip ligand.
To gain inisight into the electronic structure of 2, the
products of the sequential addition of four electrons to the
model H4tip ligand were explored under idealized D2h
symmetry by means of DFT/B3LYP calculations.[7] The
addition of two electrons resulted in a lengthening of the
ꢀ
ꢀ
C N bonds from 1.2750 to 1.3294 ꢀ, shortening of the C C
bonds in the diimine moieties from 1.5480 to 1.5040 ꢀ, and
Scheme 3. Resonance forms for the doubly reduced tip ligand.
ꢀ
contraction of the central C C bond of the naphthalene unit
The reaction of four equivalents of [GeCl2(1,4-dioxane)]
with one equivalent of the dipp-tip ligand 1a resulted, after
workup, in a virtually quantitative yield of the green
bis(chlorogermyl) compound 4, the structure of which is
illustrated in Figure 3. Akin to 2, compound 4 is diamagnetic,
ꢀ
ꢀ
from 1.3760 to 1.3400 ꢀ. The corresponding C N and C C
bond lengths for the H4tip dianion are close to those cited
earlier for the X-ray structure of 2, as is the experimental
ꢀ
bond length of the central C C bond of the naphthalene
ꢀ
ꢀ
moiety (1.352(3) ꢀ). The DFT/B3LYP calculations on the
H4tip ligand also provided valuable insight into the mode of
the interaction between the unpaired electrons of 2. The
composition of the LUMO of H4tip (4b1u; Figure 2) can be
traced back to the in-phase combination of the LUMOs of the
two N-C-C-N fragments. Such strong orbital interaction is
mediated by the p orbitals of the naphthalene rings. The
overall result is that the two highest energy electrons of the
dianion are effectively paired in an orbital which is delocal-
ized over both diazabutadiene moieties and the naphthalene
bridge, and possesses N C antibonding and C C bonding
character. The enhanced delocalization results in the two-
electron reduction having a smaller effect on the bond lengths
of the H4tip ligand than on those of the corresponding
monofunctional H2bian ligand. The spin pairing can also be
understood on the basis of the pair of resonance structures
displayed in Scheme 3.[8] The overall electronic structure
arises from equal weighting of these two canonical forms.
The structure of 2 itself was also investigated at the same
level of theory and found to exhibit all the salient features of
and the C(1) C(5) and average C N bond lengths of 1.452(7)
and 1.322(6) ꢀ, respectively, are comparable with those of 2.
The other product of this reaction is Ge2Cl6. The analogous
monogermanium complex [GeCl(dipp-bian)] has been
reported by Fedushkin et al.,[9] and was shown to possess a
radical anion structure in which the unpaired electron is
located primarily on the N-C-C-N moiety.[10]
An analogous result was obtained when decamethyleur-
opocene was employed as a one-electron reductant in the
complex [(Cp*)2Eu(p-F-tip)Eu(Cp*)2] (Cp* = C5Me5) (5). In
this case, it was necessary to use p-fluorphenyl substituents on
the tip ligand, to decrease the steric interactions at the
coordination sites and to facilitate electron transfer from the
Eu2+ center to the tip ligand.[11] As in the case of 2, single-
electron transfer from both metal sites was evident from the
ꢀ
ꢀ
ꢀ
ꢀ
C C and C N bond lengths of 1.480(10) and 1.327(9) ꢀ,
respectively, which were determined by single-crystal X-ray
diffraction (Figure 4).[6] Comparison of the structure with that
of [Cp*2Eu(p-F-bian)] (6) revealed similar metrical parame-
ꢀ
ꢀ
ters for the corresponding C C and C N (av) bond lengths,
namely 1.453(7) and 1.339(7) ꢀ, respectively. The skeletal
structures of both 5 and 6 are distinctly nonplanar, as
indicated by the angles of 17.698 and 20.818 between the N-
ꢀ
the single-crystal X-ray structure. In particular, the N K
(2.7373 ꢀ) and K O(thf) (2.7443 ꢀ) separations were repro-
duced accurately, and the C C (1.4845 ꢀ) and C N
(1.3292 ꢀ) bond lengths within the diazabutadiene fragments
were found to be very similar to those computed for the H4tip
ꢀ
ꢀ
ꢀ
ꢀ
dianion. The contraction of the central C C bond of the
naphthalene fragment was also modeled satisfactorily
(0.021 ꢀ).
Figure 3. POV-RAY diagram for 4 shown with thermal ellipsoids at
Figure 2. H4tip and H2bian frontier orbitals calculated using B3LYP.
50% probability. All hydrogen atoms have been omitted for clarity.
8370
www.angewandte.org
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2009, 48, 8369 –8371

Angewandte
Chemie
Holder, G. Kinsel, K. Rajeshwar, Inorg. Chem. 2002, 41, 2471 –
2476; T. Moriuchi, X. Shen, K. Saito, S. Bandoh, T. Hirao, Bull.
Chem. Soc. Jpn. 2003, 76, 595 – 599.
[4] See, for example: C. N. Carlson, J. M. Veauthier, K. D. John,
D. E. Morris, Chem. Eur. J. 2008, 14, 422 – 431; C. N. Carlson,
B. L. Scott, R. L. Martin, J. D. Thompson, D. E. Morris, K. D.
John, Inorg. Chem. 2007, 46, 5013 – 5022; C. N. Carlson, C. J.
Kuehl, R. E. Da Re, J. M. Veauthier, E. J. Schelter, A. E.
Milligan, B. L. Scott, E. D. Bauer, J. D. Thompson, D. E.
Morris, K. D. John, J. Am. Chem. Soc. 2006, 128, 7230 – 7241.
[5] See, for example: S. Ghumaan, B. Sarkar, S. Maji, V. G. Puranik,
J. Fiedler, F. A. Urbanos, R. Jiminez-Aparicio, W. Kaim, G. K.
Lahiri, Chem. Eur. J. 2008, 14, 10816 – 10828; F. F. DeBiani, A.
Dei, C. Sangregorio, L. Sorace, Dalton Trans. 2005, 3638 – 3873;
S. K. Min, A. G. Dipasquale, A. L. Rheingold, H. S. White, J. S.
Miller, J. Am. Chem. Soc. 2009, 131, 6229 – 6236, and references
therein.
Figure 4. POV-RAY diagram for 5 with thermal ellipsoids at 50%
probability. All hydrogen atoms have been omitted for clarity.
[6] Crystallographic data for 2: C86H120K2N4O6, Mr = 1384.06, T=
153(2) K, monoclinic, space group P21/n, a = 13.581(5), b =
17.668(5), c = 18.964(5) ꢀ, b = 107.043(5)8, V= 4351(2) ꢀ3, Z =
2, 1calcd = 1.057 Mgmꢀ3, m = 0.158 mmꢀ1, 16516 reflections col-
lected, 9963 independent reflections (Rint = 0.0435), final R
indices [I > 2s(I)]: R1 = 0.0821, wR2 = 0.2351. 3: C36H40KN2,
Mr = 539.80, T= 153(2) K, monoclinic, space group P21/n, a =
12.368(3), b = 24.327(5), c = 12.812(3) ꢀ, b = 109.32(3)8, V=
C-C-N and N-Eu-N planes, respectively. The reason for this
deviation from planarity stems primarily from the minimiza-
tion of the steric interactions between the p-fluoroaryl rings
and the methyl groups of Cp*. For both complexes, the
average distances of 2.450 ꢀ between the ring centroid and
the europium center are indicative of a Eu oxidation state of
+ 3.[12] The magnetic moment value of 3.86 BM, as measured
by the Evans method, is close to those of the biseuropium(III)
complex [{EuF(hfac)3K(diglyme)}2] (3.7 BM)[13] and the
monoeuropium(III) complex [Cp*2Eu(p-F-bian)] (2.80 BM),
which suggests that there is little or no interaction between
the Eu3+ centers of 5.
3637.7(13) ꢀ3, Z = 4, 1calcd = 0.986 Mgmꢀ3
,
m = 0.168 mmꢀ1
,
12892 reflections collected, 6902 independent reflections
(Rint = 0.0660), final R indices [I > 2s(I)]: R1 = 0.0715, wR2 =
0.1790. 4: C76H88Cl2Ge2N4, Mr = 1273.58, T= 153(2) K, mono-
clinic, space group P21/n, a = 11.689(2), b = 17.192(3), c =
19.130(4) ꢀ, b = 107.54(3)8, V= 3665.8(13) ꢀ3, Z = 2, 1calcd
=
1.154 Mgmꢀ3, m = 0.934 mmꢀ1, 11704 reflections collected, 6452
independent reflections (Rint = 0.1087), final R indices [I >
2s(I)]: R1 = 0.0713, wR2 = 0.1820. 5: C106H112Eu2F4N4, Mr =
Further evidence that Eu!ligand electron transfer had
ꢀ
1821.92, T= 153(2) K, triclinic, space group P1, a = 13.648(3),
taken place in complexes 5 and 6 stemmed from the absence
b = 13.792(3), c = 14.410(3) ꢀ, a = 82.45(3)8, b = 66.98(3)8, g =
ꢀ1
=
70.30(3)8, V= 2350.5(8) ꢀ3, Z = 1, 1calcd = 1.287 Mgmꢀ3
, m =
of C N stretching vibrations at 1620 cm in the IR spectra
1.377 mmꢀ1, 15893 reflections collected, 10647 independent
reflections (Rint = 0.0596), final R indices [I > 2s(I)]: R1 = 0.0782,
wR2 = 0.1835. 6: C44H44EuF2N2, Mr = 790.77, T= 153(2) K,
monoclinic, space group Cc, a = 21.972(5), b = 10.238(5), c =
and the detection of signals at d = ꢀ21.9 ppm (5) and
ꢀ21.7 ppm (6), which are attributable to the Cp* groups, by
1H NMR spectroscopy. These values are similar to that of d =
ꢀ19.7 ppm reported for the Eu3+ complex [Cp*Eu(OCMe3)-
(m-OCMe3)2] by Evans et al.[14]
In summary, bifunctional terakis(imino)pyracene (tip)
ligands have been shown to undergo single-electron reduction
at both diimine functionalities when treated with potassium
metal, germanium dichloride dioxanate, or decamethyleu-
ropocene. The transferred electrons pair up in an orbital that
is delocalized over both diazabutadiene moieties and the
18.282(5) ꢀ, b = 119.046(5)8, V= 3595(2) ꢀ3, Z = 4, 1calcd
=
1.461 Mgmꢀ3, m = 1.789 mmꢀ1, 11969 reflections collected, 7709
independent reflections (Rint = 0.0297), final R indices [I >
2s(I)]: R1 = 0.0370, wR2 = 0.0807. CCDC 732618 (1b), 732617
(2), 732620 (3), 739106 (4), 732619 (5), and 732616 (6) contain
the supplementary 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.
ꢀ
ꢀ
naphthalene bridge and possesses N C antibonding and C C
bonding character. Future work will focus on electronic
communication mediated by larger aromatic systems.
[7] See the Supporting Information for details.
[8] We are grateful to a referee for this suggestion.
[9] I. L. Fedushkin, N. M. Khvoinova, A. Y. Baurin, G. K. Fukin,
V. K. Cherksaov, M. P. Bubnov, Inorg. Chem. 2004, 43, 7807 –
7815.
Received: July 24, 2009
Published online: September 24, 2009
[10] For a recent review of related bian-supported main-group
complexes, see N. J. Hill, I. Vargas-Baca, A. H. Cowley, Dalton
Trans. 2009, 240 – 253.
Keywords: bifunctional ligands · density functional calculations ·
.
[11] Cyclic voltammetry measurements indicate that the first two
reductions of ligand 1a occur at ꢀ1.12 and ꢀ1.68 V, while those
of ligand 1b occur at ꢀ0.83 and ꢀ1.22 V.[7]
electronic communication · ligand effects · redox chemistry
[12] P. Sobota, J. Utko, S. Szafert, Inorg. Chem. 1994, 33, 5203.
[13] W. J. Evans, D. G. Giarikos, M. A. Johnston, M. A. Greci, J. W.
Ziller, J. Chem. Soc. Dalton Trans. 2002, 520 – 526.
[14] W. J. Evans, J. L. Shreeve, J. W. Ziller, Organometallics 1994, 13,
731.
[1] K. V. Vasudevan, A. H. Cowley, Chem. Commun. 2007, 3464 –
3466.
[2] K. V. Vasudevan, M. Findlater, A. H. Cowley, Chem. Commun.
2008, 1918 – 1919.
[3] See, for example: M.-J. Kim, R. Konduri, H. Ye, F. M.
MacDonnell, F. Puntoriero, S. Serroni, S. Campagna, T.
Angew. Chem. Int. Ed. 2009, 48, 8369 –8371
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
8371