Through-space electrostatic interaction between the electron-donating 1,3-diphosphacyclobutane-2,4-diyl units

Article abstract of DOI:10.1021/ol401456h

Catenation of stable 1,3-diphopsphacyclobutane-2,4-diyl units with biphenyl-, diphenyl ether-, and diphenylmethane-based bridging groups were utilized for evaluation of the distinct through-space interaction between the electron-donating P-heterocyclic biradical chromophores. Structural properties of the oligo(biradicals), characterized by X-ray crystallography and DFT calculation, suggest a possible distance between the biradical units for the through-space interaction.

Full text of DOI:10.1021/ol401456h

ORGANIC
LETTERS
XXXX
Vol. XX, No. XX
000–000
Through-space Electrostatic Interaction
between the Electron-donating
1,3-Diphosphacyclobutane-2,4-diyl Units
Shigekazu Ito,* Makoto Kobayashi, and Koichi Mikami
Department of Applied Chemistry, Graduate School of Science and Engineering,
Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo 152-8552, Japan
ito.s.ao@m.titech.ac.jp
Received May 23, 2013
ABSTRACT
Catenation of stable 1,3-diphopsphacyclobutane-2,4-diyl units with biphenyl-, diphenyl ether-, and diphenylmethane-based bridging groups were
utilized for evaluation of the distinct through-space interaction between the electron-donating P-heterocyclic biradical chromophores. Structural
properties of the oligo(biradicals), characterized by X-ray crystallography and DFT calculation, suggest a possible distance between the biradical
units for the through-space interaction.
The air-tolerant 1,3-diphosphacyclobutane-2,4-diyl unit
can be utilized as a good electron-donating molecular
unit,1 which is expected to be available for development
of electro-functional materials based on the chemistry of
phosphorus heterocycles.2,3 We previously reported that
catenation of such electron-rich P-heterocyclic biradical
units using xylyl spacers (A) induces a through-space
interaction even in the absence of a π-conjugative structure
between the biradicalunits, and stepwiseelectron-releasing
processes would generate the corresponding mixed-
valence intermediates (Figure 1).4,5 Such a nonconjugative
through-space interaction would be based on the particu-
lar properties of a singlet biradical,6 and further molecular
designs for accumulation of the redox-active P-heterocyc-
lic biradicals will provide novel useful compounds for such
organic electronics. On the other hand, synthesis of novel
oligo(biradicals) is desirable to discuss the through-space
interaction in detail, because the previous catenation using
the xylyl-type spacing groups4 might contain the possibi-
lity of a homoconjugativeinteraction between the biradical
units via PꢀCH2 sigma bonds.
(1) Yoshifuiji, M.; Arduengo, A. J., III; Kobovalova, T. A.; Kispert,
L. D.; Kikuchi, M.; Ito, S. Chem. Lett. 2006, 35, 1136–1137.
ꢀ
(2) Baumgartner, B.; Reau, R. Chem. Rev. 2006, 106, 4681–4727.
(3) Niecke, E.; Fuchs, A.; Baumeister, F.; Nieger, M.; Schoeller,
W. W. Angew. Chem., Int. Ed. Engl. 1995, 34, 555–557.
(4) Ito, S.; Miura, J.; Morita, N.; Yoshifuji, M.; Arduengo, A. J., III
Angew. Chem., Int. Ed. 2008, 47, 6418–6421. See also: Rodriguez, A.;
Tham, F. S.; Schoeller, W. W.; Bertrand, G. Angew. Chem., Int. Ed.
2004, 43, 4876–4880.
Figure 1. Previous examples of dually catenated 1,3-diphospha-
cyclobutane-2,4-diyl units with xylyl spacers. Mes* = 2,4,6-
tBu3C6H2.
(5) (a) Ito, S.; Miura, J.; Morita, N.; Yoshifuji, M.; Arduengo, A. J.,
III Z. Anorg. Allg. Chem. 2009, 635, 488–495. (b) Ito, S.; Miura, J.;
Morita, N.; Yoshifuji, M.; Arduengo, A. J., III Heteroat. Chem 2010, 21,
404–411.
In this paper we demonstrate dual catenation of the
P-heterocyclic biradical units using biphenyl-, diphenyl
ether-, and diphenylmethane-based spacing groups in
which the possible homoconjugative effect would be
(6) (a) Breher, F. Coord. Chem. Rev. 2007, 251, 1007–1043. (b) Power,
€
P. P. Nature 2010, 463, 171–177. (c) Grutzmacher, H.; Breher, F. Angew.
Chem., Int. Ed. 2002, 41, 4006–4011.
r
10.1021/ol401456h
XXXX American Chemical Society

effectively reduced. Structural aspects of the novel bis-
(biradicals) are studied using X-ray crystallographic anal-
ysis and DFT calculation. Electrochemical measurements
areutilizedforevaluationofthe through-spaceinteraction.
Furthermore, linear-triple catenation of the biradical units
is also performed.
to generate cyclic anion 2 (Figure 2)8 and subsequently
treated with bis(bromomethyl)biphenyls to afford air-
tolerant crystalline bis(biradicals) 3ꢀ5. Isolated yields
of 3 and 4 were relatively low in comparison with that of 5,
indicating effects of the steric encumbrance. Furthermore,
di(methylphenyl)ether and di(2-methylphenyl)methane
moieties were also nonproblematic to combine the biradical
units, and air-stable 6ꢀ9 were isolated in moderate to good
yields (Figure 3).
Figure 2. Precursors for synthesis of the air-stable P-heterocyclic
biradical derivatives.
Figure 4. An ORTEP drawing of X-ray structure of 3 (20%
probability ellipsoids). Hydrogen atoms and solvent molecules
(dichloromethane) are omitted for clarity. Three p-t-butyl groups
are disordered and the atoms with predominant occupancy factors
˚
(0.72, 0.57, 0.55) are displayed. Bond lengths (A): P1ꢀC1 1.722(4),
P1ꢀC2 1.713(4), P2ꢀC1 1.773(4), P2ꢀC2 1.778(4), P1ꢀtBu
1.875(5), P2ꢀCH2 1.848(4), C1ꢀMes* 1.503(5), C2ꢀMes*
1.484(5), P3ꢀC3 1.713(4), P3ꢀC4 1.714(3), P4ꢀC3 1.789(3),
P4ꢀC4 1.794(4), P3ꢀtBu 1.885(4), P4ꢀCH2 1.858(4), C3ꢀMes*
1.490(5), C4ꢀMes* 1.497(5). Sums of angles (deg): P1 342.3(2), P2
317.9(2), C1 359.5(3), C2 357.6(3), P3 346.9(2), P4 317.4(2), C3
358.3(3), C4 358.6(3). Torsion angle (τ) = 64.0(5)°.
Compounds 3ꢀ9 were characterized by the spectro-
scopic data. In the 13C NMR spectra of 3, due to the
helical conformation of the bridging biayl skeleton, two
signals for the diastereotopic skeletal sp2-C atoms were
observed. Furthermore, we examined X-ray crystallo-
graphic analysis of the single crystals of 3 (Figure 4,
P21/n space group). Probably due to unsolved disordered
structures, quality of the structure refinement was slightly
insufficient, and was not improved even in analysis
with lower-symmetrical space group such as P21. How-
ever, the data seemed to be reasonable enough to
confirm the skeletal parameters. The gross structure of
3 indicates that the two biradical units cannot interact
through any rigid π-conjugated skeleton.9 The metric
parameters around the four-membered cycles are com-
parable to those of the previously reported air-stable
Figure 3. Novel bis(biradicals) 3ꢀ9 and the related known
compound 10.
One of the structural modifications of A for minimizing
the possible homoconjugative interaction is to employ
nonconjugative skeletons as the spacing group. In this
study we first employed biphenyl skeleton for the spacers
because biphenyl inherently displays twisted conforma-
tions.7 Next, as an attempt to the minimization of the
homoconjugative interaction, we also examined catena-
tion of the biradical units with bis(methylenephenyl)ether
and bis(methylphenyl)methane spacers.
According to our established procedures, phosphaalkyne
1 was allowed to react with 0.5 equiv of t-butyllithium
(7) Vonlanthen, D.; Rotzler, J.; Neuburger, M.; Mayor, M. Eur. J.
Org. Chem. 2010, 120–133.
(8) Sugiyama, H.; Ito, S.; Yoshifuji, M. Angew. Chem., Int. Ed. 2003,
42, 3802–3804.
B
Org. Lett., Vol. XX, No. XX, XXXX

1,3-diphosphacyclobutane-2,4-diyls.8,10 On the other hand,
interestingly, in spite of the remarkable steric hindrance,
torsion angle of the biaryl linker (64°) is not perpendicular
completely in the crystalline state. The conformation of the
spacer indicates the presence of an attractive interaction
between the biradical units probably due to electrostatic
effects. The distance between the skeletal centroids of 3
bis(biradicals). Figure 5 displays the cyclic voltammo-
grams and DPV (differential pulse votammetry) data
of 3ꢀ5. Compounds 3 and 4 exhibit two-step reversible
electron-releases, indicating that the electron-donating
biradical units electrostatically interact each other. On
the other hand, quite small separation of oxidation poten-
tials exhibiting almost single-step electron release was
observed in the votammetric measurements of 5. Therefore,
it is plausible that the through-space interaction between the
biradical chromophores can be operable by the bridging
structures. The DFT-optimized structure of 3 characterized
the HOMO and HOMO-1 orbitals with the quite small
energetic difference (Figure S2, Supporting Information),
which might relate to the elecrochemical properties in
solution.
Table 1 summarizes the voltammetric parameters of
3ꢀ9. All the voltammograms indicated reversible oxida-
tion processes (Figures S3, S4, Supporting Information).
Compounds 6ꢀ8 exhibit the through-space interaction
between the biradical chromophores inducing the step-
wise oxidations. Potential difference between the first and
second oxidations considerably depends on the spacer, and
the larger distance between the biradical units reduces
magnitude of the though-space electrostatic interaction.
The similar ΔEox parameter of 3 to that of 10 (0.13 V)4
corresponds to the structural characteristics discussed as
above. On the other hand, the ΔEox data of 6 is also close to
that of 10, whereas 7 exhibits smaller ΔEox data. The DFT-
optimized structure of 7 indicated the longer centroid-cen-
˚
(8.494 A) is comparable or rather shorter in comparison with
4
˚
that of 10 (8.847 A). The Mes* aryl groups are considerably
distorted due to the steric encumbrance,11 whereas the dihe-
dral angles between the 4-membered planar heterocycles and
Mes* aryl planes are 68ꢀ70°.
Such structural characters of 3 were supported by DFT
calculations (Figure S1, Supporting Information) [M06ꢀ
2X/6-31G(d); as the enantiomeric form].12,13 The typical
metric parameters around the P-heterocyclic skeletons, in-
cluding the sp2-type planar skeletal carbons and the sp3-type
pyramidalized phosphorus atoms, are characterized.10 The
distance between the centroids of the P2C2 four-membered
˚
rings is within 1 nm (DFT: 8.175 A), which would enable the
through-space communication.14
In order to evaluate the through-space interaction,
we examined electrochemical measurements of the novel
˚
troid distance (9.905 A, see Supporting Information) in
comparison with those of 3 and 10. Although detailed reason
for the considerable through-space interaction of 6 is unclear,
the diaryl ether structure would be effective to induce the
stepwise oxidations. Stabilization energy of the correspond-
ing radical cations is estimated as 1.9ꢀ3.1 kcal molꢀ1.15 DFT
calculations of 4 and 8 indicated the centroid-centroid dis-
˚
tances of ca. 1 nm (10.34 and 10.82 A, respectively), whereas 5
exhibits 1.2 nm (see Supporting Information).
Table 1. Redox Properties of 3ꢀ9a
compound
b
Kc
Eox11/2/mV
Eox21/2/mV
ΔEox/mV
3
4
5
6
7
8
9
176
172
214
158
192
168
216
312
268
136
96
200
42
Figure 5. Cyclic voltammograms of (a) 3, (b) 4, and (c) 5, and
DPV charts of (d) 3, (e) 4, and (f) 5. Conditions: 1 mM in CH2Cl2;
0.1 M tetrabutylammonium perchlorate (TBAP); working elec-
trode, GC; counter electrode, Pt; reference electrode, Ag/AgCl;
scan rate 100 mVsꢀ1. T = 298 K. Fc/Fcþ = 0.52 V.
292
294
252
134
102
84
185
53
26
a Conditions: 1 mM in CH2Cl2; 0.1 M TBAP; working electrode, GC;
(9) Although the axial bond might be able to rotate, conjugation
effect between the biradical units would be considerably reduced.
(10) Yoshifuji, M.; Sugiyama, H.; Ito, S. J. Organomet. Chem. 2005,
690, 2515–2520.
(11) Ito, S.; Miyake, H.; Yoshifuji, M. Phosphorus, Sulfur, Silicon
2009, 184, 917–927.
counter electrode, Pt; reference electrode, Ag/AgCl; scan rate, 100 mVsꢀ1
.
T = 298 K. Fc/Fcþ = 0.52 V. b Determined by ꢀRTlnKc = ꢀFΔEox. F =
Faraday const.
(12) Frisch, M. J. et al. Gaussian 09, Revision B.01, Gaussian, Inc.:
Wallingford, CT, 2010. See Supporting Information for full reference.
(13) Zhao, Y.; Truhlar, D. G. Theor. Chem. Acc. 2008, 120, 215–241.
(14) (a) Miller, J. R.; Calcaterra, L. T.; Class, G. L. J. Am. Chem. Soc.
1984, 106, 3047–3049. (b) Johnson, M. D.; Miller, J. R.; Green, N. S.;
Closs, G. L. J. Phys. Chem. 1989, 93, 1173–1176.
The redox properties of 5 and 8 correspond to the results
of linearly tricatenated biradicals (Figure 6). Taking the
(15) Richardson, D. E.; Taube, E. Inorg. Chem. 1981, 20, 1278–1285.
Org. Lett., Vol. XX, No. XX, XXXX
C

respectively (Figure S5, Supporting Information). In
contrast to our previous results of tris(biradicals) bearing
1,3,5-tris(methylene)benzene spacer, the linearly catenated
12 exhibited only two observable oxidation potentials, puta-
tively due to one centroid chromophore and two almost
identical terminal biradical units. Separation of the oxidation
potentials of 12 (ΔEox 78 mV) is comparable to that of 8.16
Inconclusion, the scopeof thethrough-spaceinteraction
between the electron-donative P-heterocyclic biradical
units is evaluated by the biphenyl-, diphenyl ether-, and
diphenylmethane-type spacing groups, and it has been
obvious that positioning of the biradical units within ca.
1 nm approximately enables generation of the correspond-
ing mixed-valent radical cation intermediates. Attempts to
analyze the mixed-valent chemical species and application
to electro-functional materials are in progress.
Acknowledgment. This work was supported in part
by Grants-in-Aid for Scientific Research (Nos. 22350058
and 23655173) from the Ministry of Education, Culture,
Sports, Science, and Technology, Japan, Sumitomo Chemi-
cal Co., Ltd., and Nissan Chemical Industries, Ltd. We
thank Prof. Hiroharu Suzuki and Dr. Masataka Oishi of
Tokyo Institute of Technology for supports of X-ray
crystallographic analyses. Prof. Yuji Wada and Dr. Masato
Maitani of Tokyo Institute of Technology supported
UVꢀvis spectroscopic measurements.
Figure 6. Tris(biradicals) bearing linear bridging groups.
predicted synthetic accessibility based on the dual catena-
tion into account, here we employed 3,300,50-trimethyl-1,
10:30,100-terphenyl and 2-methyl-1-(m-tolyloxy)-4-(o-tolyloxy)-
benzene skeletons as the linear-type spacers. Voltammetric
measurements of 11 and 12, successfully synthesized
according to the established procedures, displayed
neglecting and substantial through-space interactions,
Supporting Information Available. Experimental details,
characterization data, X-ray crystallography of 3, DFT
calculations of 3ꢀ5, 7 and 9, and voltammograms of 6ꢀ9,
11 and 12. This material is available free of charge via the
Internet at http://pubs.acs.org.
(16) The second oxidation step did not indicate simultaneous two-
electron release from the two terminal biradical units. This property
might be due to effect of the putative Coulombic repulsion.
The authors declare no competing financial interest.
D
Org. Lett., Vol. XX, No. XX, XXXX