Facile preparation of neutral monoporphyrinate lanthanide complexes with strong near-infrared emission

Article abstract of DOI:10.1016/j.inoche.2008.08.004

Ytterbium (III) acetylacetonate reacts with 5,10,15,20-tetraphenylporphyrin (H₂TPP) and 5,10,15,20-tetra(4-bromophenyl)porphyrin (H₂TBrPP) in 1,2,4-trichlorobenzene and forms unexpected acetate-bridged dimer and propionate-coordinate

Full text of DOI:10.1016/j.inoche.2008.08.004

Inorganic Chemistry Communications 11 (2008) 1304–1307
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Facile preparation of neutral monoporphyrinate lanthanide complexes with
strong near-infrared emission
b
Hongshan He a, , Andrew G. Sykes
*
a Center for Advanced Photovoltaics, South Dakota State University, Brookings, SD 57007, USA
b Department of Chemistry, University of South Dakota, Vermillion, SD 56069, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Ytterbium (III) acetylacetonate reacts with 5,10,15,20-tetraphenylporphyrin (H2TPP) and 5,10,15,20-
tetra(4-bromophenyl)porphyrin (H2TBrPP) in 1,2,4-trichlorobenzene and forms unexpected acetate-
bridged dimer and propionate-coordinated monomer of monoporphyrinate ytterbium (III) complexes,
respectively, whereas interaction of ytterbium (III) acetate with H2TPP gives stable acetate-coordinated
monomer with two labile methanol binding directly to metal center. This compound reacts readily with
1,10-phenanthroline derivatives to give eight-coordinate monoporphyrinate ytterbium (III) complexes
with strong near infrared emission efficiency.
Received 3 June 2008
Accepted 3 August 2008
Available online 11 August 2008
Keywords:
Porphyrin
Lanthanide
Near-infrared
Emission
Published by Elsevier B.V.
Synthesis
Developing facile preparation methods for near-infrared emis-
sive monoporphyrinate lanthanide complexes with high coordina-
tion number is critical for the application of lanthanide complexes
in diagnostics, photonics and photovoltaics [1–16]. One method
that has been widely used was reported by Wong et al. in 1974
[17–21]:
dimer,
[Yb(TPP)(
l
-OOCCH3)(CH3OH)]2
(1)
is
obtained
(H2TPP = 5,10,15,20-tetraphenylporphyrin) [27]. The crystal struc-
ture of 1 is shown in Fig. 1, in which two ytterbium (III) ions are
bridged by two acetate anions. Each ytterbium (III) ion is coordi-
nated by four nitrogen atoms from the porphyrinate dianion plus
one oxygen atom from methanol, yielding a seven-coordinate
ytterbium (III) center. The two ytterbium (III) TPP units are centro-
symmetric and 5.321 Å away from each other indicating there is no
direct bond interaction. In another experiment, a propionate-coor-
dinated monomer, [Yb(TBrPP)(OOCCH2CH3)(CH3OH)2] (2), is iso-
lated when H2TBrPP and ytterbium (III) acetylacetonate hydrate
are used (H2TBrPP = 5,10,15,20-tetrat(4-bromophenyl)porphyrin)
[27]. The crystal structure of 2 is shown in Fig. 2. The ytterbium
(III) ion is also seven-coordinate with four nitrogen atoms from
the porphyrinate dianion, two oxygen atoms from methanol mole-
cules and one oxygen atom from the propionate anion bonded to
the metal. The propionate is monocoordinate as the distances be-
tween Yb1–O7 and Yb2–O8 are 3.674 Å and 3.705 Å, respectively,
indicating this ligand does not chelate the Yb. Two TPP units in
the unit cell are hydrogen bonded (dashed lines) to each other with
relatively short O–Hꢂ ꢂ ꢂO hydrogen bonds ꢃ2.55 Å. Methanol is
present as the elutant used during column chromatography.
This structure diversity led us to explore the possibility of other
alternative synthetic methods for production of usable Yb-contain-
ing starting materials that could be substituted by other ancillary
ligands. One method we pursued is the reaction between porphy-
rin free base and lanthanide acetate [28]:
H2Por þ ½LnðacacÞ3ꢀ ! ½LnðPorÞðg2 ꢁ acacÞðSÞnꢀ þ 2Hacac
ð1Þ
where Ln is the trivalent lanthanide ion, Por is the pophyrinate
dianion, acac is the acetylacetonate anion, and S is the solvent.
The number of solvent molecules is one as we determined recently
[22]. One disadvantage of this method is the difficulty in substitut-
ing the acetylacetonate group for other ancillary ligands, which lim-
its the modification of these complexes for other functionalized
materials. Another disadvantage is the pyrolysis of acetylacetonates
at high temperature [23,24]. Acetate, the major pyrolytic product,
can coordinate to the lanthanide ion in situ and form acetate-coor-
dinated monoporphyrinates as reported by Spyroulias et al. [25,26]
for halogenated porphyrins:
H2Por þ ½TbðacacÞ3ꢀ ! ½TbðporÞðg2 ꢁ OOCCH3ÞðDMSOÞ2ꢀ
ð2Þ
In an attempt to utilize this pyrolytic reaction for non-haloge-
nated porphyrinate complexes, we found the reaction products
were difficult to predict. When H2TPP and ytterbium (III) acetyl-
acetonate hydrate are employed in this reaction, a bridging acetate
* Corresponding author.
E-mail address: Hongshan.he@sdstate.edu (H. He).
H2Por þ ½LnðOOCCH3Þ3ꢀ ! ½LnðPorÞðOOCCH3ÞðSÞ2ꢀ
ð3Þ
1387-7003/$ - see front matter Published by Elsevier B.V.
doi:10.1016/j.inoche.2008.08.004

H. He, A.G. Sykes / Inorganic Chemistry Communications 11 (2008) 1304–1307
1305
Fig. 3. The ORTEP diagram of [Yb(TPP)(OOCH3)(CH3OH)2] with 50% thermal
ellipsoid probability. The hydrogen atoms are omitted for clarity. Bond lengths
(Å): Yb1–N1 2.322(3), Yb1–N2 2.331(3), Yb1–N3 2.325(3), Yb1–N4 2.313(3), Yb1–
O1 2.272(2), Yb1–O4 2.351(3), Yb1–O5 2.361(3).
Fig. 1. The ORTEP diagram of [Yb(TPP)(
l-OOCCH3)(CH3OH)]2 ꢂ 2CHCl3 with 50%
thermal ellipsoid probability. The CHCl3 and hydrogen atoms are omitted for clarity.
Bond lengths (Å): Yb01–N1 2.31454(19), Yb01–N2 2.318(2), Yb01–N3 2.3039(19),
Yb01–N4 2.305(2), Yb01–O1 2.3135(18), Yb01–O2 2.3722(18), Yb01–O3
2.3201(17).
tage as
a platform to build other highly functionalized Ln
complexes:
3 þ LL ! ½YbðTPPÞðg2 ꢁ OOCCH3ÞðLLÞꢀ þ 2CH3OH
ð4Þ
The resulting complexes are bright purple, soluble in dichlorometh-
ane, chloroform, toluene, etc. and slightly soluble in methanol. The
crystal structure of 4-methyl-1,10-phenanthroline Yb(III) deriva-
tive, [Yb(TPP)(
g
2-OOCCH3)(4-Me-phen)]ꢂMeOH (4), is shown in
Fig. 4. Different from its monodentate coordination geometry in 3,
the acetate anion this time chelates to the ytterbium (III) ion, form-
ing an eight-coordinate complex [27]. The phenanthroline backbone
is slightly bent to minimize steric interaction with two adjacent
phenyl groups of the TPP ring. The ytterbium ion is 1.1671 Å above
the mean plan of N1/N2/N3/N4, which is larger than found in the
seven-coordinate complexes. One solvated methanol molecule is
also present in the asymmetric unit and is hydrogen-bonded to
the carbonyl oxygen (O2) of the acetate.
The photoluminescence properties of all complexes have been
investigated and reveal emission enhancement in the eight-coordi-
nate diimine complex 4. All complexes exhibit characteristic near-
infrared emission upon irridiation as shown in Table 1. A strong
peak centered at 974 nm is observed for all complexes, which
can be assigned to characteristic F5=2 ! F7=2 transition. Two
shoulders [9,29,30] on the right side of this peak in 1–3 have
merged into one peak at 1005 nm in 4 with intensity greater than
Fig. 2. The ORTEP diagram of [Yb(TBrPP)(OOCCH2CH3) (CH3OH)2]2 ꢂ 2CH2Cl2 with
50% thermal ellipsoid probability. The CH2Cl2 and hydrogen atoms are omitted for
clarity. Bond lengths (Å): Yb1–N1 2.323(10), Yb1–N2 2.317(9), Yb1–N3 2.329(11),
Yb1–N4 2.331(10), Yb1–O3 2.364(10), Yb1–O4 2.279(9), Yb1–O5 2.339(9), Yb2–O1
2.353(10), Yb2–O2 2.356(11), Yb2–O6 2.243(9).
2
2
The reactions were carried out in 1,2,4-trichlorobenzene at 210 °C
under N2 for 48 h, followed by stirring with methanol at 70 °C for
2 h. The reaction is very easy to handle and only one product is ob-
tained [27]. The Fig. 3 shows the crystal structure of the product
[Yb(TPP)(OOCCH3)(CH3OH)2] (3) from the reaction between H2TPP
and Yb(acetate)3 ꢂ 3H2O. The ytterbium (III) ion is also seven-coor-
dinate, with four nitrogen atoms from the porphyrinate dianion,
two oxygen atoms from coordinated methanol and one oxygen
atom from acetate anion occupying the coordination sphere. The
distance between Yb1 and O2 is 3.759 Å, indicating the acetate is
monocoordinate. The ytterbium (III) is 1.078 Å above the mean plan
of N1/N2/N3/N4. One solvated methanol molecule is present in the
asymmetric unit, through which two TPP units are hydrogen-
bonded. The coordination of the second methanol and its hydrogen
bonding with solvated methanol in 3 probably prevents dimeriza-
tion as was observed in the structure of 1.
Fig. 4. The ORTEP diagram of [Yb(TPP)(
g
2-OOCCH3)(4-Me-Phen)] ꢂ CH3OH with
Our results show that two methanol molecules in 3 are labile
and can be easily substituted by a variety of bidentate ligands
(LL) which form 1:1 stiochiometric adducts. This is of great advan-
50% thermal ellipsoid probability. The hydrogen atoms are omitted for clarity. Bond
lengths (Å): Yb1–N1 2.351(4), Yb1–N2 2.371(4), Yb1–N3 2.352(4), Yb1–N4
2.359(4), Yb1–N5 2.529(4), Yb1–N6 2.509(4), Yb1–O1 2.368(4), Yb1-O2 2.395(4).

1306
H. He, A.G. Sykes / Inorganic Chemistry Communications 11 (2008) 1304–1307
[4] H. He, W.-K. Wong, J. Guo, K.-F. Li, W.-Y. Wong, W.-K. Lo, K.-W. Cheah, Inorg.
Table 1
Chim. Acta 357 (2004) 4379–4388.
Emission and lifetimes of 1–4 in CH2Cl2 at room temperature
[5] H. He, X. Zhu, A. Hou, J. Guo, W.-K. Wong, W.-Y. Wong, K.-F. Li, K.-W. Cheah,
Dalton Trans. (2004) 4064–4073.
[6] H. He, W.K. Wong, J. Guo, K.F. Li, W.Y. Wong, W.K. Lo, K.W. Cheah, Austr. J.
Chem. 57 (2004) 803–810.
[7] H.-S. He, Z.-X. Zhao, W.-K. Wong, K.-F. Li, J.-X. Meng, K.-W. Cheah, Dalton Trans.
(2003) 980–986.
[8] B.S. Harrison, T.J. Foley, A.S. Knefely, J.K. Mwaura, G.B. Cunningham, T.-S. Kang,
M. Bouguettaya, J.M. Boncella, J.R. Reynolds, K.S. Schanze, Chem. Mater. 16
(2004) 2938–2947.
[9] T.J. Foley, B.S. Harrison, A.S. Knefely, K.A. Abboud, J.R. Reynolds, K.S. Schanze,
J.M. Boncella, Inorg. Chem. 42 (2003) 5023–5032.
[10] H. Tsukube, S. Shinoda, H. Tamiaki, Coord. Chem. Rev. 226 (2002) 227–234.
[11] Z.-X. Zhao, Q.-H. Xu, D.-M. Li, G.-F. Liu, L.-S. Li, R.-R. Xu, Solid State Sci. 3 (2001)
339–345.
Compound
No. of OH
CNa
Em in VISb
(nm, ns)
Em in NIRb
(nm, s)
UYb(%)c
l
1
2
3
4
1
2
2
0
7
7
7
8
650 (8.42), 717
652 (7.32), 716
647 (8.20), 713
651(6.70), 716
974 (1.27)
974 (2.40)
974 (1.56)
0.064
0.12
0.078
0.86
980 (17.29), 1006
a
CN: coordination number.
b
The excitation wavelength is 375 nm from Xe lamp for steady state and from
pulsed diode laser for decay, respectively.
c
UYb(%) = ꢄ 100 sobs/s0, where s0 = 2000 l
s for YbIII ion.
[12] W.-K. Wong, A. Hou, J. Guo, H. He, L.W. Zhang, Wai-Yeung, K.-F. Li, K.-W.
Cheah, F.M. Xue, T.C. W, J. Chem. Soc. Dalton Trans. (2001) 3092–3098.
[13] W.-K. Wong, L. Zhang, F. Xue, T.C.W. Mak, J. Chem. Soc. Dalton Trans. (1999)
3053–3062.
[14] G.A. Spyroulias, D. de Montauzon, A. Maisonat, R. Poilblanc, A.G. Coutsolelos,
Inorg. Chem. Acta 275–276 (1998) 182–191.
[15] J. Jiang, R.L.C. Lau, T.W.D. Chan, T.C.W. Mak, D.K.P. Ng, Inorg. Chim. Acta 255
(1997) 59–64.
[16] G.A. Spyroulias, A.G. Coutsolelos, Polyhedron 14 (1995) 2483–2490.
[17] C.-P. Wong, R.F. Venteicher, W.D. Horrocks Jr., J. Am. Chem. Soc. 96 (1974)
7149–7150.
[18] W.D. Horrocks Jr., C.-P. Wong, J. Am. Chem. Soc. 98 (1976) 7157–7162.
[19] M. Yu, G.-J. Chen, G.-F. Liu, J. Phys. Chem. Solids 68 (2007) 541–548.
[20] X. Zhu, W.-K. Wong, W.-K. Lo, W.-Y. Wong, Chem. Commun. (2005) 1022–
1024.
[21] J. Jiang, L. Wei, P. Gao, Z. Gu, K.-I. Machida, G.-Y. Adachi, J. Alloys Comp. 225
(1995) 363–366.
[22] H. He, A.G. Sykes, D. Galipeaua, S.W. Ng, M. Ropp, Inorg. Chem. Commun., in
press.
[23] E.P. Kohler, J.L.E. Erickson, J. Am. Chem. Soc. 53 (1931) 2301–2309.
[24] W.M. Kutz, H. Adkins, J. Am. Chem. Soc. 52 (1930) 4036–4042.
[25] G.A. Spyroulias, A. Despotopoulos, C.P. Raptopoulou, A. Terzis, A.G. Coutsolelos,
Chem. Commun. (1997) 783–784.
[26] G.A. Spyroulias, A.P. Despotopoulos, C.P. Raptopoulou, A. Terzis, D. de
Montauzon, R. Poilblanc, A.G. Coutsolelos, Inorg. Chem. 41 (2002) 2648–2659.
[27] Synthesis of 1: The solution of 5,10,15,20-tetraphenylporphyrin (H2TPP)
(0.50 g, 0.80 mmol) and ytterbium(III) acetylacetonate hydrate (1.0 g,
2.1 mmol) in TCB (50 mL) was refluxed (ꢃ220 °C) under N2 atmosphere for
4 h. The solvent was then removed under vacuum. The residue was re-
dissolved in chloroform, loaded on column (silica gel) and eluted with
chloroform-methanol (v/v: 100:5). Collected the second band (the first band
was unreacted H2TPP) and third band. The second band was identified as 1 and
third band as [Yb(TPP)(acac)(H2O)]. Yield for 1: 82 mg, 12%. Calc. for
C94H70N8O6Yb2: C, 64.38; H, 4.02; N, 6.39. Found: C, 64.30; H, 3.95; N, 6.43.
Synthesis of 2: The solution of 5,10,15,20-tetra(4-bromophenyl)porphyrin
(H2TBrPP) (0.22 g, 0.35 mmol) and ytterbium(III) acetate hydrate (0.12 g,
0.34 mmol) in TCB (25 mL) was refluxed (ꢃ220 °C) under N2 atmosphere for
4 h. The solvent was then removed under vacuum. The residue was re-
dissolved in chloroform, loaded on column (silica gel) and eluted with
chloroform–methanol (v/v: 100:5). Collected the second band and third bands
(the first band was unreacted H2TBrPP). The second band was identified as
[Yb(TBrPP)(acac)(CH3OH)] and third band as 2. Yield for 2: 52 mg, 18%. Anal.
Calc. for C49H37Br4N4O4Yb [Yb(TBrPP)(OOCCH2CH3)(OHCH3)2]: C, 47.52; H,
3.01; N, 4.52; Found: C, 47.58; H, 3.09; N, 4.44. Synthesis of 3: To a TCB
solution (25 mL) of H2TPP (21.57 mg, 0.35 mmol) was added Yb(OAc)3 ꢂ 3H2O
(12.86 mg, 0.35 mmol). The resulting solution was then heated to 210 °C and
magnetically stirred for 48 h under N2. The solution was cooled to ꢃ70 °C and
methanol (10 mL) was added. The resulting solution was stirred for another
2 h at 70 °C. Then all solvent was removed under vacuum on rotary
evaporator. The purple solid was re-dissolved in chloroform and loaded on
column using silica gel. After removing free base with chloroform as elute, the
silica gel was washed with chloroform–methanol (v:v: 100:5) and major band
was collected. The final product was re-crystallized from dichloromethane-
methanol at room temperature and dried under vacuum at room temperature
overnight. Yield for 3: 12.5 mg, 65%. Anal. Calcd. for C48H39N4O4Yb
[Yb(TPP)(OOCCH3)(OHCH3)2]: C, 63.43; H, 4.33; N, 6.16; found: C, 63.49, H,
4.32, N, 6.52. Synthesis of 4: To a dichloromethane solution (20 mL) of 3
(28.7 mg, 31.5 mmol) was added excess of 4-methyl-1,10-phenanthroline
(67.9 mg, 35.0 mmol). The solution was magnetically stirred at room
temperature for 4 h. Then all solvent was removed. The residual was
dissolved in chloroform and loaded on column (silica gel). The column was
washed with chloroform first to remove tiny amount of H2TPP from
decomposition and then with chloroform–methanol (v:v: 250:3). The major
band gave the target complex 4. Yield for 4: 26 mg, 81%. Anal Calcd. for
C59H45N6O4Yb [Yb(TPP)(OOCCH3)(4-Me-1,10-Phen)(2H2O)]: C, 65.92; H, 4.22;
N, 7.82; found: C, 65.79; H, 4.20; N, 8.00. Single crystal X-ray diffraction data
Fig. 5. Near-infrared emission spectra of 1–4 in dichloromethane at room temper-
ature. The excitation wavelength is 375 nm. The concentration of the sample is
1.4 ꢄ 10ꢁ4 mol/L.
in 1–3. The appearance of shoulders can be explained by Stark
splitting of 2F7=2 level due to the variation of an asymmetric ligand
field in these complexes [29]. The NIR emission intensity and life-
time of 4 are 6- and 12-fold larger compared to those of 3 in CH2Cl2
at room temperature (see Fig. 5).
In conclusion, a more facile preparation method for monopor-
phyrinate lanthanide complexes with high coordination number
has been developed beginning from ytterbium (III) acetate trihy-
drate, a commonly available reagent. The resulting octa-coordinate
diimine complex exhibits intense near-infrared emisson and a long
lifetime. The application of this method for self-assembly of heter-
onuclear lanthanide complexes as up-conversion materials is
ongoing.
Acknowledgements
This work was supported by the National Science Foundation/
EPSCoR Grant No. 0554609, the State of South Dakota, and South
Dakota State University.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.inoche.2008.08.004.
References
[1] W.-K. Wong, X. Zhu, W.-Y. Wong, Coord. Chem. Rev. 251 (2007) 2386–2399.
[2] S. Fu, X. Zhu, G. Zhou, W.-Y. Wong, C. Ye, W.-K. Wong, Z. Li, Eur. J. Inorg. Chem.
(2007) 2004–2013.
[3] F.-L. Jiang, W.-K. Wong, X.-J. Zhu, G.-J. Zhou, W.-Y. Wong, P.-L. Wu, H.-L. Tam,
K.-W. Cheah, C. Ye, Y. Liu, Eur. J. Inorg. Chem. (2007) 3365–3374.
were collected on a Bruker SMART CCD diffractometer (kMo Ka = 0.71073 Å),
T = 125(2) K, and refined by full-matrix least-squares refinements on F2 using
all data with SHELXL programs. An empirical absorption correction was
applied to the data using SADABS. Hydrogen atoms were generated

H. He, A.G. Sykes / Inorganic Chemistry Communications 11 (2008) 1304–1307
1307
(Mo
geometrically and refined with isotropic thermal parameters. Crystal data for
c
K
= 80.6060(10)°, V = 2028.0(2) Å3, Z = 2, Dc = 1.541 g cmꢁ3
) = 2.360 mmꢁ1
T = 125(2) K, F(000) = 950, 19,785 reflections measured
,
l
[Yb(TPP)(OOCCH3)(CH3OH)]2ꢂ2CHCl3 (1): C96H72Cl6N8O6Yb2, MW = 1992.4,
a
,
monoclinic, P21/n, a = 13.2404(5) Å, b = 14.9951(6) Å, c = 21.3514(8) Å,
with 7185 unique reflections. The final R = 0.0246, wR = 0.0563 for 6409
observed reflections (I > 2 (I)). CCDC 681181. Crystal data for
a
K
= 90°, b = 105.29°,
a
c
= 90°, V = 4089.0(3) Å3, Z = 2, Dc = 1.618 g cmꢁ3
,
l(Mo
r
) = 2.532 mmꢁ1, T = 125(2) K, F(000) = 1988, 48,020 reflections measured
[Yb(TPP)(OOCCH3)(4-Me-1,10-Phen)]ꢂCH3OH (4): C60H44N6O3Yb, MW =
with 9900 unique reflections. The final R = 0.0225, wR = 0.0578 for 7233
observed reflections (I > 2 (I)). CCDC 681180. Crystal data for
[Yb(TBrPP)(OOCCH2CH3)(OHCH3)2] ꢂ CH2Cl2 (2): C50H37Br4Cl2N4O4Yb,
MW = 1324.42, triclinic, P-1, a = 12.1450(16) Å, b = 19.033(3) Å, c = 21.636(3)
Å, = 98.831(2)°, b = 94.109(2)°,
= 95.744(2)°, V = 4898.0(11) Å3, Z = 4,
Dc = 1.792 g cmꢁ3 ) = 5.330 mmꢁ1
(Mo T = 125(2) K, F(000) = 2564,
46,905 reflections measured with 17,728 unique reflections. The final
R = 0.0824, wR = 0.1996 for 10,726 observed reflections (I (I)). CCDC
682486. Crystal data for
[Yb(TPP)(OOCH3)(MeOH)2] ꢂ CH3OH
C49H43N4O5Yb, MW = 940.91, triclinic, P-1, a = 11.2696(7)
b = 12.5577(8) Å, c = 15.4978(10) Å,
1071.06, triclinic, P-1, a = 9.8315(5) Å, b = 13.7173(7) Å, c = 17.6627(9) Å,
r
a
= 96.5870(10)°, b = 90.4030(10)°,
Dc = 1.506 (Mo
cmꢁ3 ) = 2.035 mmꢁ1
23,291 reflections measured with 8293 unique reflections. The final
R = 0.0364, wR = 0.0955 for 7190 observed reflections (I > 2 (I)). CCDC 682748.
c
= 93.1670(10)°, V = 2362.5(2) Å3, Z = 2,
g
,
l
K
a
, T = 125(2) K, F(000) = 1082,
a
c
r
,
l
Ka
,
[28] M.A. Garcia-Sanchez, A. Campero, J. Non-Cryst. Solids 296 (2001) 50–56.
[29] M.P.O. Wolbers, F.C.J.M.V. Veggel, B.H.M. Snellink-Ruël, J.W. Hofstraat, F.A.J.
Geurts, D.N. Reinhoudt, J. Chem. Soc. Perkin Trans. 2 (1998) 2141–2150.
[30] M. Gouterman, C.D. Schumaker, T.S. Srivastava, T. Yonetani, Chem. Phys. Lett.
40 (1976) 456–461.
>
2r
(3):
Å,
a
= 73.9610(10)°, b = 75.2230(10)°,