Structural interconversion and regulation of optical properties of stable hypercoordinate dipyrrin-silicon complexes

Article abstract of DOI:10.1021/ja2001924

Novel pentacoordinate dipyrrin-silicon complexes showed efficient red or near-IR fluorescence, and the structural interconversion between silanol and siloxane derivatives resulted in significant changes in the optical properties.

Full text of DOI:10.1021/ja2001924

COMMUNICATION
pubs.acs.org/JACS
Structural Interconversion and Regulation of Optical Properties of
Stable Hypercoordinate DipyrrinÀSilicon Complexes
Naoya Sakamoto, Chusaku Ikeda, Masaki Yamamura, and Tatsuya Nabeshima*
Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan
S Supporting Information
b
Scheme 1. Synthesis of Silicon Complexes 3-R and 4-R
ABSTRACT: Novel pentacoordinate dipyrrinÀsilicon
complexes showed efficient red or near-IR fluorescence,
and the structural interconversion between silanol and
siloxane derivatives resulted in significant changes in the
optical properties.
ipyrrins are widely used as monoanionic bidentate ligands,
D
and their complexes show unique optical properties.1 In
particular, dipyrrinÀboron complexes (BODIPYs) have at-
tracted much attention because of their strong and tunable
fluorescence properties.2 Dipyrrin complexes with other ele-
ments such as Zn,3 Cu,4 In,5 Ga,5 and Sn6 are also intriguing
because different coordination geometries and/or complexation
stoichiometries lead to versatile properties and applications.
Although many kinds of dipyrrin complexes have been synthe-
sized, dipyrrinÀsilicon complexes have not been reported
to date.
fluorescence properties of the dipyrrin complexes by using the
peculiar reactivity and properties of hypercoordinate silicon
compounds.
Herein we describe the first synthesis and characterization of
hypercoordinate silicon complexes with N2O2-type dipyrrins,
which are interconvertible between the pentacoordinate sila-
nol and siloxane. These dipyrrin silicon complexes exhibit
different fluorescence properties in the red and NIR regions.
Consequently, the unusual dynamic interconversion between
the hypercoordinate silicon compounds by reversible hydro-
lysis/dehydration results in significant changes in the optical
properties.
Silicon atoms often exert a considerable influence on the
optical properties of fluorophores.7 In addition, hypercoordinate
silicon compounds such as pentacoordinate and hexacoordinate
silicon complexes generally exhibit completely different or
reverse reactivity (e.g., more facile bond cleavage) than the usual
tetracoordinate silicon compounds.8 For example, pentacoordi-
nate siloxane derivatives, which have SiÀOÀSi units, easily
undergo hydrolysis to give the corresponding silanol (SiÀOH)
derivatives, whereas tetracoordinate siloxanes are much more
stable than the corresponding silanols.9 Reversible bond forma-
tion and dissociation between the hypercoordinate and the usual
tetracoordinate silicon compounds are responsible for switching
of the fluorescence properties.10
The N2O2-type dipyrrins, which contain 2-hydroxyphenyl
groups at the pyrrole R-positions, adopt six-membered-ring
coordination and provide trianionic tetradentate chelation sites
to B and Al, affording red-fluorescent B and Al complexes in high
yield.11 We are convinced that the N2O2-type dipyrrins are
strong candidates for functional modulation of hypercoordinate
silicon compounds on the basis of the pentacoordinate sila-
nolÀsiloxane structural interconversion because the pentacoor-
dinate structure of silicon compounds is favored by five- or six-
membered-ring chelation. We also expect N2O2-type dipyr-
rinÀSi complexes to show near-IR (NIR) fluorescence because
of the expected planar structure of the fluorophores. Recent
interest in NIR emission has grown because of its potential
application to biosensors and solar batteries. The combination of
dipyrrin and silicon would enable the switching of red or NIR
Ligands 1 and 2 were synthesized according to a reported
procedure.11b The silicon complexes 3-R and 4-R (R = Me, Ph),
in which the silicon atom bears a methyl or phenyl group, were
obtained in moderate yields by the reactions with MeSiCl3 or
PhSiCl3, respectively, in the presence of NEt(iPr)2 (Scheme 1).
In the case of the N2-type 3,5,8-triphenyldipyrrin, however, the
starting material was recovered in the reactions with Me3SiCl,
Me2SiCl2, MeSiCl3, and SiCl4 under similar conditions.
Silicon complexes 3-R and 4-R were stable under aerobic
conditions and readily purified by column chromatography on
silica gel. The obtained complexes were soluble in chloroform
and characterized by spectroscopic measurements (1H, 13C, and
1
29Si NMR; ESI-MS) and elemental analysis. The H NMR
spectra in CDCl3 showed a Cs-symmetric spectral pattern. 3-R
exhibited a single set of proton resonances for the two hydro-
xyphenylpyrrole moieties, while the two o-methyl groups of the
mesityl ring appeared as two singlets. The proton resonances of
the methyl groups on the silicon atoms appeared at 0.09 and 0.04
ppm for 3-Me and 4-Me, respectively. In addition, the 29Si NMR
spectra showed singlet signals at À117.4, À116.6, À130.5, and
À129.8 ppm for 3-Me, 4-Me, 3-Ph, and 4-Ph, respectively.
These shifts are characteristic of pentacoordinate silicon atoms.
Received: January 8, 2011
Published: March 14, 2011
r
2011 American Chemical Society
4726
dx.doi.org/10.1021/ja2001924 J. Am. Chem. Soc. 2011, 133, 4726–4729
|

Journal of the American Chemical Society
COMMUNICATION
Table 1. Optical Properties of the DipyrrinÀSilicon
Complexes in CHCl3
λabs/nm
λ
fluor/nm
ΦF
0.76
3-Me
4-Me
3-Ph
4-Ph
3-OH
32-O
606
649
606
648
0.012
0.72
612
662
613
660
0.03
605
626
664,a 671b
0.81
0.44,a 0.37b
573, 601
a After excitation at 601 nm. b After excitation at 573 nm.
Scheme 3. Synthesis of Silicon Complexes 3-OH and 32-O
Figure 1. Crystal structure of one enantiomer of 4-Ph in the solid state:
(a) top view; (b) side view. Thermal ellipsoids are displayed at the 50%
level. Colors: C, gray; N, blue; O, red; Si, green. Hydrogen atoms have
been omitted for clarity.
Scheme 2. Pseudorotation between Two Structures of 3-R
1
The H NMR spectra of 3-OH and 32-O indicated Cs-
symmetric dipyrrin moieties. The 29Si NMR spectra showed
singlets at À141.2 and À148.6 ppm for 3-OH and 32-O,
respectively. These spectra also suggested the formation of
pentacoordinate silicon complexes. The IR spectrum of 3-OH
showed the OÀH stretching vibration peak at 3440 cmÀ1 and no
X-ray structural analysis of single crystals of 4-Ph showed that
the silicon atom is in a distorted trigonal-bipyramidal (TBP)
coordination environment (Figure 1).12,13 The atoms N1 and
O2 form the axis of the bipyramid, with an O2ÀSi1ÀN1 angle of
177.80(9)°. The atoms N2, O1, and C28 are in the equatorial
plane of the bipyramid, and the angles N2ÀSi1ÀC28,
O1ÀSi1ÀN2, and C28ÀSi1ÀO1 are ∼120° each.14 4-Ph has
a chiral structure and exists as a racemate in the crystals. Although
the C1-symmetric pentacoordinate structure was observed in the
asymmetric SiÀOÀSi stretching vibration band at 1112 cmÀ1
,
which was observed in 32-O. The electrospray ionization (ESI)
mass spectrum of 32-O showed a peak at m/z 959.3, consistent
with a disiloxane structure. The diffusion constants obtained
using diffusion-ordered NMR spectroscopy (DOSY) are 6.0 Â 10À10
and 7.3 Â 10À10 m2/s for 32-O and 3-OH, respectively. These
results are consistent with 3-OH being the monomeric product,
as the diffusion constant of 3-Me is 7.7 Â 10À10 m2/s. Finally,
X-ray structural analysis revealed that the two silicon dipyrrin
moieties of 32-O were connected by a μ-oxo oxygen (O3)
located at the inversion center, indicating a linear SiÀOÀSi
arrangement (Figure 2).13,16,17
1
crystal structure, the H NMR spectrum suggested a Cs-sym-
metric spectral pattern as seen for 3-R. Since pentacordinate
silicon compounds are known to undergo intramolecular iso-
merization according to the widely accepted Berry pseudorota-
tion mechanism,15 the observed Cs-symmetric 1H NMR spectral
pattern was attributed to fast interconversion between the two
structures on the NMR time scale (Scheme 2).
The obtained silicon complexes showed intense fluorescence
in the NIR region (Table 1). 3-Me and 3-Ph showed higher
fluorescence quantum yields than 4-Me and 4-Ph. This is most
likely due to the restricted rotation of the mesityl group.3 The
substituents on the silicon atom also affected the optical proper-
ties. 3-Ph and 4-Ph showed absorption and fluorescence peaks at
wavelengths longer than those of 3-Me and 4-Me.
We then examined the reactions of ligand 1 with SiCl4 to make
a pentacoordinate silicon complex with a chlorine atom, which
could then be replaced by another substituent. Indeed, the
monodipyrrin complex 3-OH and disiloxane derivative 32-O
were obtained after quenching the reaction with wet MeOH
(Scheme 3). The silanol 3-OH (41%) was almost exclusively
isolated, and the yield of 32-O was extremely low (<1%). This
unusual formation of the silanol is ascribed to the intrinsic
properties of the hypercoordinate silicon compounds.
Although the usual tetracoordinate siloxanes are resistant
toward hydrolysis, disiloxane 32-O was hydrolyzed to 3-OH by
1
refluxing in wet MeOH. The H NMR spectrum in CDCl3
showed the formation of 3-OH and 32-O in a ratio of 95:5. Very
interestingly, this hydrolysisÀdehydration process was reversi-
ble. The dehydration of 3-OH by refluxing in hexane gave 32-O
with high efficiency (32-O:3-OH = 90: 10).
Disiloxane 32-O showed two absorption bands at 573 and
601 nm (Table 1). This spectral pattern may suggest an inter-
action between the two siliconÀdipyrrin moieties. 32-O showed
two different fluorescence peaks at 671 and 664 nm upon
excitation at 573 and 601 nm, respectively.18 In contrast, 3-OH
showed much sharper absorption and fluorescence spectral
bands than 32-O (Figure 3). 3-OH had twice the fluorescence
quantum yield (0.81) as 32-O (∼0.4). Thus, the definitely
different optical properties can be well-controlled in an off/on
switching fashion by the facile dehydration/hydration process on
4727
dx.doi.org/10.1021/ja2001924 |J. Am. Chem. Soc. 2011, 133, 4726–4729

Journal of the American Chemical Society
COMMUNICATION
by utilizing their peculiar reactivities provided unprecedented
regulation of the optical properties. The results obtained here
promise versatile applications using this off/on switching ability
by interconversion between the hypercoordinate silicon compounds.
’ ASSOCIATED CONTENT
S
Supporting Information. Crystallographic data (CIF),
b
synthetic procedures, and spectral data. This material is available
free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
nabesima@chem.tsukuba.ac.jp
’ ACKNOWLEDGMENT
This research was financially supported by Grants-in-Aid for
Scientific Research from the Ministry of Education, Culture,
Sports, Science, and Technology of Japan.
’ REFERENCES
(1) Wood, T. E.; Thompson, A. Chem. Rev. 2007, 107, 1831.
(2) (a) Loudet, A.; Burgess, K. Chem. Rev. 2007, 107, 4891. (b)
Ulrich, G.; Ziessel, R.; Harriman, A. Angew. Chem., Int. Ed. 2008,
47, 1184.
Figure 2. Crystal structure of 32-O: (a) top view; (b) side view.
Thermal ellipsoids are displayed at the 50% level. Colors: C, gray; N,
blue; O, red; Si, green. Hydrogen atoms have been omitted for clarity.
(3) Sazanovich, I. V.; Kirmaier, C.; Hindin, E.; Yu, L.; Bocian, D. F.;
Lindsey, J. S.; Holten, D. J. Am. Chem. Soc. 2004, 126, 2664.
(4) (a) Halper, S. R.; Malachowski, M. R.; Delaney, H. M.; Cohen,
S. M. Inorg. Chem. 2004, 43, 1242. (b) Halper, S. R.; Cohen, S. M. Angew.
Chem., Int. Ed. 2004, 43, 2385.
(5) Stork, J. R.; Thoi, V. S.; Cohen, S. M. Inorg. Chem. 2007,
46, 11213.
(6) Kobayashi, J.; Kushida, T.; Kawashima, T. J. Am. Chem. Soc.
2009, 131, 10836.
(7) Kyushin, S.; Ikarugi, M.; Goto, M.; Hiratsuka, H.; Matsumoto, H.
Organometallics 1996, 15, 1067.
(8) (a) Holmes, R. R. Chem. Rev. 1990, 90, 17. (b) Chuit, C.; Corriu,
R. J. P.; Reye, C.; Young, J. C. Chem. Rev. 1993, 93, 1371. (c) Holmes,
R. R. Chem. Rev. 1996, 96, 927.
(9) Tacke, R.; Burschka, C.; Richter, I.; Wagner, B.; Willeke, R. J. Am.
Chem. Soc. 2000, 122, 8480.
(10) Yamaguchi, S.; Akiyama, S.; Tamao, K. J. Am. Chem. Soc. 2000,
122, 6793.
(11) (a) Kim, H.; Burghart, A.; Welch, M. B.; Reibenspies, J.;
Burgess, K. Chem. Commun. 1999, 1889. (b) Ikeda, C.; Ueda, S.;
Nabeshima, T. Chem. Commun. 2009, 2544.
(12) Crystal data for 4-Ph: C33H22N2O2Si, M = 506.62; triclinic;
a = 10.0066(5) Å, b = 11.3661(6) Å, c = 12.8556(9) Å, R = 62.181(2)°,
β = 77.287(2)°, γ = 70.1211(14)°; U = 1212.79(12) Å3; T = 120(2) K;
space group P1 (No. 2); Z = 2; 12001 reflns measured, 5484 unique
(Rint = 0.0395); R1 = 0.0627 [I > 2σ(I)], wR2 = 0.1564 (all data),
GOF(F2) = 1.101.
(13) Sheldrick, G. M. SHELXL 97: Program for Crystal Structure
Refinement; University of G€ottingen, G€ottingen, Germany, 1997.
(14) The pentacoordination characters %TBPa and %TBPe were
determined as 95.3 and 99.7%, respectively, indicating a nearly ideal TBP
structure for 4-Ph. See: Tamao, K.; Hayashi, T.; Ito, Y. Organometallics
1992, 11, 2099.
Figure 3. (a) Absorption and (b) fluorescence spectra of 3-OH (solid
line) and 32-O (dashed line) in CHCl3. The samples were excited at
605 nm (3-OH), 573 nm (32-O, blue line), and 601 nm (32-O, red line).
(c, d) Photographs of 3-OH (left) and 32-O (right) were taken under
(c) ambient light and (d) UV light (365 nm).
the basis of the unique reactivities of the hypercoordinate
dipyrrinÀsilicon complexes.
(15) Berry, R. S. J. Chem. Phys. 1960, 32, 933.
In conclusion, the first synthesis of hypercoordinate silicon
complexes with N2O2-type dipyrrins, red/NIR-fluorescent sili-
con analogues of BODIPY, has been accomplished. Structural
interconversion between the pentacoordinate silanol and siloxane
(16) Crystal data for 32-O: C60H46N4O5Si2, M = 959.20; triclinic;
a = 10.6926(12) Å, b = 10.9105(9) Å, c = 11.2603(12) Å, R = 65.661(2)°,
β = 76.871(3)°, γ = 87.781(3)°; U = 1161.2(2) Å3; T = 120(2) K;
space group P1 (No. 2); Z = 1,; 11621 reflns measured, 5296 unique
4728
dx.doi.org/10.1021/ja2001924 |J. Am. Chem. Soc. 2011, 133, 4726–4729

Journal of the American Chemical Society
COMMUNICATION
(Rint = 0.0621); R1 = 0.0562 [I > 2σ(I)], wR2 = 0.1617 (all data),
GOF(F2) = 1.095.
(17) %TBPa and %TBPe were determined as 95.5 and 99.8%,
respectively, for 32-O.
(18) This probably resulted from interconversion of the two isomers
by pseudorotation and/or two different transitions of 32-O.
4729
dx.doi.org/10.1021/ja2001924 |J. Am. Chem. Soc. 2011, 133, 4726–4729