Structures of Pb(II) porphyrins: [5,10,15,20-terakis-triisopropylsilylethynylporphinato]lead(II) and [5,15-bis- (3,5-bis-tert-butylphenyl)-10,20-bis-triisopropylsilyl- ethynylporphinato]lead(II)

Article abstract of DOI:10.1016/S0277-5387(01)00932-9

The crystal structures of two lead containing porphyrins are described. Structure 1 contains unprecedented stacks of three metalloporphyrins. Each structure shows disorder of the Pb atoms which are situated out of the mean plane of the porphyrin ring.

Full text of DOI:10.1016/S0277-5387(01)00932-9

Polyhedron 20 (2001) 3219–3224
www.elsevier.com/locate/poly
Structures of Pb(II) porphyrins:
[5,10,15,20-tetrakis-triisopropylsilylethynylporphinato]lead(II) and
[5,15-bis-(3,5-bis-tert-butylphenyl)-10,20-bis-triisopropylsilyl-
ethynylporphinato]lead(II)
M. John Plater a,*, Stuart Aiken a, Thomas Gelbrich b,1, Michael B. Hursthouse b,1,
Grant Bourhill c,2
a Department of Chemistry, Uni6ersity of Aberdeen, Meston Building, Room G103, Meston Walk, Aberdeen AB24 3UE, UK
b Department of Chemistry, Uni6ersity of Southampton, Highfield, Southampton, SO17 1BJ, UK
c Defence Science and Technology Laboratory, Great Mal6ern, St. Andrews Road, Worcestershire WR14 3PS, UK
Received 3 August 2001; accepted 6 September 2001
Abstract
The crystal structures of two lead containing porphyrins are described. Structure 1 contains unprecedented stacks of three
metalloporphyrins. Each structure shows disorder of the Pb atoms which are situated out of the mean plane of the porphyrin ring.
© 2001 Elsevier Science Ltd. All rights reserved.
Keywords: Porphyrin; Lead(II); Optical limiter
tures are of interest since concentrated solutions of
certain lead porphyrins aggregate which diminishes
their optical limiting performance.
1. Introduction
Optical limiters or reverse saturable absorbers are
materials which display diminishing transmittance with
increasing incident light intensity. This attenuation of
the optical throughput can be used to protect various
sensors against damage from pulsed laser sources [1–5].
Porphyrins and phthalocyanines are promising candi-
dates for certain applications. Population of triplet-ex-
cited states is desirable since triplet states are longer
lived than singlet states which enhances the limiters
performance. Hence dyes, which contain heavy atoms,
are of interest because heavy atoms can promote inter-
system crossing. The two lead porphyrins reported here
were prepared for these studies. The solid-state struc-
2. Results and discussion
[5,10,15,20 - Tetrakis - triisopropylsilylethynylporphi-
nato]lead(II), (1) crystallises in the monoclinic space
group Cc. Fig. 1 displays one of the three independent
molecules in the asymmetric unit. Crystallographic data
and selected bond distances and angles for 1 can be
found in Tables 1 and 2, respectively. Pb and the four
nitrogen atoms form a distorted square pyramid with
the metal atom in an apical position and the nitrogen
occupying the corners of the base plane. A similar
arrangement resembling the lead coordination by four
oxygen atoms in PbO is described for Pb(TPrP)
(TPrP=5,10,15,20-tetra-n-propylporphyrin), the only
other Pb(II) porphyrin that has been analysed by X-ray
crystallography so far [6]. The distances between Pb
and the least-squares plane through the four coordinat-
* Corresponding author. Tel.: +44-1224-272-943; fax: +44-1224-
272-921.
E-mail addresses: m.j.plater@abdn.ac.uk (M.J. Plater), gel-
brich@soton.ac.uk (T. Gelbrich), grant.bourhill@sharp.co.uk (G.
Bourhill).
1 Fax: +44-2380-596-723.
2 Fax: +44-1865-747-719.
0277-5387/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0277-5387(01)00932-9

3220
M.J. Plater et al. / Polyhedron 20 (2001) 3219–3224
ing nitrogen atoms are 1.235(4), 1.274(4) and 1.233(4)
parameters for the three major lead positions Pb1, Pb2
and Pb3. The only reasonable interpretation of the
disorder, illustrated in Fig. 2, is that all PbꢀX vectors
within the same stack always point in the same direc-
tion, when X is defined as the centroid of a N4 base
plane of each square pyramid around Pb. The site
occupancy for the major component set was refined to
0.88(1). Furthermore, the staggered arrangement allows
the four ꢀCꢁCꢀSi(Pri)3 moieties of the central molecule
to fill the holes left by the ꢀCꢁCꢀSi(Pri)3 fragments of
its two neighbours. The shape of each stack is domi-
nated, as a consequence, by these voluminous side
chains. Hence, the van der Waals surface of a given
stack is not significantly altered with the inversion of
PbꢀX vector which is essential for the occurrence of the
described Pb disorder. The separation between adjacent
,
A, respectively. The PbꢀN bond distances vary between
,
2.388(8) and 2.449(8) A and fall into the range observed
for Pb(TPrP) [6]. The two different types of NꢀPbꢀN
bond angles have values between 116.3(3) and 118.6(3)°
and between 73.2(3) and 75.1(3)°, respectively. The
,
diagonal N···N separation is 4.10(1)–4.13(1) A, similar
values are observed in metal-free porphyrins. Depend-
ing on the size of the metal-ion accommodated by the
tetradentate ligand, metalloporphyrins with square pla-
nar metal centres show a wide variation in their mean
,
diagonal N···N distances from 4.25 A for Cd(II) to 3.74
,
A for Ni(II), and the macrocycles with shorter separa-
tions are significantly puckered [7,8].
The structure motif of 1, shown in Fig. 2, is a
metal-over-metal stack consisting of three molecules
where all porphyrin mean planes are essentially parallel.
Fig. 3 shows that the three macrocycles are staggered in
such a way that the sandwiched molecule is rotated
with respect to the two outer molecules by approxi-
mately 45° about the Pb1···Pb2···Pb3 axis. The macro-
cycle is slightly distorted from planarity with bond
distances and angles showing the expected values.
Each of the three independent lead atoms is disor-
dered over two positions related by approximate m
symmetry with the porphyrin mean plane serving as a
pseudo-mirror plane. An independent refinement of the
site occupancy factors resulted in almost identical
,
Pb atoms in the same stack of 3.782(4) and 3.768(4) A
do not imply a direct metal–metal interaction, and the
separations between the respective centroids X are
,
3.75(1) and 3.80(1) A. Intermolecular Pb···N distances
,
within a stack lie between 3.147(8) and 3.35(8) A.
The described structure motif of metal-over-metal
stacks of three metalloporphyrins is, to the best of our
knowledge, unprecedented. Fig. 3 shows how this par-
ticular arrangement of porphyrins with four equally
voluminous (5, 10, 15, 20) bonded side chains creates
an effectively filled space around each macrocycle in the
stack. The packing of these stacks in the crystal struc-
Fig. 1. Line drawing and structure of one of the three independent molecules of 1 showing 30% probability ellipsoids.

M.J. Plater et al. / Polyhedron 20 (2001) 3219–3224
3221
surrounded by anions, in most cases I−3 chains. It is not
entirely clear what prevents the formation of infinite
stacks in 1, and both the facts that the metal centre is
not square planar and the bulkiness of the
ꢀCꢁCꢀSi(Pri)3 groups could be the contributing factors.
5,15-Bis-(3,5-bis-tert-butylphenyl)-10,20-bis-triiso-
propylsilylethynylporphyrin (2) crystallises in the tri-
Table 1
Crystal data and structure refinement parameters for 1 and 2
Compound
1
2
Empirical formula
Formula weight
Crystal system
Space group
Unit cell dimensions
C
192H276N12Pb3Si12 C70H92N4PbSi2
3710.90
monoclinic
Cc
1252.85
triclinic
P1
(
(
clinic space group P1 with Z=2. Fig. 5 shows the
molecular structure of 2. Crystallographic data and
selected bond distances and angles for 2 can be found
in Tables 1 and 3, respectively. Similar to 1, the Pb
coordination by four nitrogen atoms is square-pyrami-
dal, and the distance between Pb and the centroid of
,
a (A)
39.4327(6)
19.1701(3)
26.5605(4)
13.2543(4)
15.0949(5)
19.0357(7)
72.367(1)
85.210(1)
64.666(1)
3275.8(2)
2
,
b (A)
,
c (A)
h (°)
i (°)
k (°)
V (A )
104.519(1)
3
,
the base plane is 1.182(2) A.
,
19436.7(5)
4
Z
Crystal
dark-green block
0.20×0.15×0.10
2.9–25.0
colourless block
0.12×0.10×0.10
3.0–25.0
Crystal size (mm)
q Range (°)
Reflections collected
47 857
23 825
Independent reflections/Rint 24 895/0.086
11 392/0.063
98.2
11 392/760
0.054/0.109
0.081/0.116
Data completeness (%)
Data/parameters
99.7
24 895/1872
0.059/0.121
0.099/0.133
0.037(5)
R1/wR2 [F2\2|(F2)]
R1/wR2 (all data)
Absolute structure
parameter
Goodness-of-fit on F2
0.967
1.045
Largest difference peak
1.43 and −0.79
1.30 and −1.56
−3
,
and hole (e A
)
Table 2
,
Selected bond lengths (A) and angles (°) for 1
Fig. 2. Stack of three molecules in 1. i-Pr groups have been omitted
for clarity.
Bond lengths
Pb1ꢀN12
Pb1ꢀN10
Pb1ꢀN11
Pb1ꢀN13
Pb2ꢀN20
Pb2ꢀN21
2.388(8)
2.391(8)
2.410(8)
2.413(8)
2.407(8)
2.411(8)
Pb2ꢀN23
Pb2ꢀN22
Pb3ꢀN32
Pb3ꢀN31
Pb3ꢀN33
Pb3ꢀN30
2.419(8)
2.449(8)
2.384(8)
2.390(8)
2.400(8)
2.404(8)
Bond angles
N12ꢀPb1ꢀN10
N12ꢀPb1ꢀN11
N10ꢀPb1ꢀN11
N12ꢀPb1ꢀN13
N10ꢀPb1ꢀN13
N11ꢀPb1ꢀN13
N20ꢀPb2ꢀN21
N20ꢀPb2ꢀN23
N21ꢀPb2ꢀN23
118.6(3)
73.9(3)
75.7(3)
74.8(3)
74.2(3)
117.6(3)
74.3(3)
73.2(3)
116.7(3)
N20ꢀPb2ꢀN22
N21ꢀPb2ꢀN22
N23ꢀPb2ꢀN22
N32ꢀPb3ꢀN33
N31ꢀPb3ꢀN33
N32ꢀPb3ꢀN30
N31ꢀPb3ꢀN30
N33ꢀPb3ꢀN30
N32ꢀPb3ꢀN31
116.3(3)
74.1(3)
74.1(3)
74.3(3)
117.4(3)
118.6(3)
75.1(3)
74.3(3)
74.8(3)
ture is presented in Fig. 4. This arrangement is not
repeated in the structure of 2 where two of the side
chains are 3,5-bis-tert-butylphenyl groups that have a
shape somewhat different from that of ꢀCꢁCꢀSi(Pri)3.
Infinite metal-over-metal columnar ABAB stacks are
known to be formed by partially oxidised metallopor-
phyrins [9–11]. In these structures the infinite stacks are
Fig. 3. Staggered arrangement of three molecules forming one single
stack of 1.

3222
M.J. Plater et al. / Polyhedron 20 (2001) 3219–3224
Table 3
,
Selected bond lengths (A) and bond angles (°) for 2
Bond lengths
Pb1ꢀN4
Pb1ꢀN1
2.332(4)
2.362(4)
Pb1ꢀN3
Pb1ꢀN2
2.383(4)
2.411(5)
Bond angles
N4ꢀPb1ꢀN1
N4ꢀPb1ꢀN3
N1ꢀPb1ꢀN3
75.8(2)
75.1(2)
120.7(2)
N4ꢀPb1ꢀN2
N1ꢀPb1ꢀN2
N3ꢀPb1ꢀN2
119.7(2)
75.5(2)
76.0(2)
bouring molecule this Pb disorder is accompanied by a
disorder of one of the isopropyl groups (C62\C64)
attached to Si2. Fig. 6 shows the packing arrangement
of 2 which is fundamentally different from that of 1.
Each of the ꢀCꢁCꢀSi(Pri)3 fragments is sandwiched by
the macrocycles of two neighbouring molecules in such
a way that the three ꢀCꢁCꢀSiꢀ fragments involved
arrange themselves in an antiparallel fashion.
Fig. 4. Packing arrangement of stacks in the crystal structure of 1.
i-Pr groups have been omitted for clarity.
The core of the molecule of 2 shows the same basic
features as discussed for 1, though with a wider varia-
3. Experimental
,
tion in PbꢀN bond lengths, 2.332(4)–2.411(5) A. The
Ultraviolet spectra were recorded in a Perkin–Elmer
Lambda 15 UV–Vis spectrophotometer using CHCl3 as
the solvent. Infrared spectra were recorded in an ATI
diagonal NꢀPbꢀN angles are slightly increased to val-
ues between 120.7(2) and 119.7(2)°, and the other
NꢀPbꢀN angles lie between 75.1(2) and 76.0(2)°. This
structure exhibits a disorder of the Pb atom similar to
that described for 1 with the final site occupancy factor
for the major component being 0.90(1). In the neigh-
1
Mattson FTIR spectrometer. H and 13C NMR spectra
were obtained at 250 and 62.9 MHz, respectively, in a
Brucker AC 250 spectrometer. Chemical shifts, l, are
Fig. 5. Line drawing and molecular structure of 2 showing 30% probability ellipsoids.

M.J. Plater et al. / Polyhedron 20 (2001) 3219–3224
3223
given in parts per million relative to the residual sol-
vent. Coupling constants, J, are given in hertz. Low-
resolution mass spectra were obtained using
electrospray ionisation in a Finnigan Navigator Mass
Spectrometer and accurate mass at the University of
Wales, Swansea using fast atom bombardment meth-
ods. Elemental analyses were obtained from Butter-
worth Laboratories using a PE 2400 CHN analyser.
Melting points were carried out using a Kofler hot-
stage microscope and are uncorrected.
7.5 mmol) was added, stirring continued for 10 min and
the solution filtered through silica. The solvent was
removed under reduced pressure and the residue, dis-
solved in light petroleum–DCM (1:1), filtered through
silica. The solvent was removed under reduced pressure
and the residue was crystallised from DCM–MeOH to
give the title compound (3.96 g, 27%) as a purple solid
with metallic lustre, melting point (m.p.) \250 °C;
Found: C, 74.7; H, 9.1; N, 5.6. C64H94N4Si4 requires C,
74.5; H, 9.2; N, 5.4%; umax (CHCl3, nm): 450 (log m=
5.7), 607 (4.9) and 710 (4.3); wmax (KBr, cm−1): 457m,
572m, 669s, 705vs, 794m, 877m, 914w, 987m, 1058w,
1132m, 1238m, 1342w, 1459s, 1502w, 1616w, 2136s,
2356w, 2858s, 2942s, 3407m, 3478m and 3544m; lH
(250 MHz; CDCl3) −1.76 (2H, s, NꢀH), 1.44 and 1.46
(84H, 2×s, i-Pr), 9.55 (8H, s, b-pyrrolic); lC (62.9
MHz; CDCl3) 11.9, 19.1, 100.3, 102.6 and 107.7 (two
resonances are missing); m/z 1031.8 (M+, 100%).
3.1. X-ray structure determinations
Details of the crystal data and a summary of data
collection parameters are given in Table 1. Intensity
data for 1 and 2 were recorded at 150 and 120 K,
respectively, using a Nonius Kappa CCD area-detector
diffractometer mounted at the window of a rotating
anode FR591 generator with a molybdenum anode
,
(0.71073 A). ƒ and ꢀ scans (2° increments) were carried
out to fill the Ewald sphere. Data collection and pro-
3.3. [5,10,15,20-Tetrakis-triisopropylsilylethynyl-
porphinato]lead(II) (1)
cessing were carried out using the programs DIRAX
,
COLLECT, DENZO and MAXUS, and an empirical absorp-
tion correction was applied using SORTAV [12–16]. The
structure was solved by direct methods and refined on
F2 by full-matrix least-squares refinements [17,18]. In
both structures all hydrogen atoms were included in the
refinement in calculated positions using a riding model.
A
mixture of 5,10,15,20-tetrakis-triisopropylsilyl-
ethynylporphyrin (0.71 g, 0.5 mmol) and Pb(OAc)2·
3H2O (2 g, 5 mmol) in DMF (60 ml) was heated at
reflux. After 2 h water was added and the precipitate
filtered, washed with water, MeOH and recrystallised
from DCM–MeOH to give the title compound (0.54 g,
90%) as a green solid, m.p.\250 °C; Found: C, 62.0;
H, 7.4; N, 4.4. C64H92N4Si4Pb requires C, 62.1; H, 7.5;
N, 4.5%; umax (CHCl3, nm): 500 (log m=5.8) and
719 (4.9); wmax (KBr, cm−1): 457m, 516w, 572m,
665s, 709vs, 788m, 877m, 993m, 1058m, 1149s,
1203m, 1238w, 1315w, 1380w, 1463s, 2130s, 2858s,
2935s, 3408m, 3476m and 3547w; lH (250 MHz;
CDCl3) 1.50 and 1.51 (84H, 2×s, i-Pr) and 9.67 (8H, s,
b-pyrrolic); lC (62.9 MHz; CDCl3) 12.0, 19.3, 99.3,
3.2. 5,10,15,20-Tetrakis-triisopropylsilyl-
ethynylporphyrin
Boron trifluoride etherate (1.1 ml) was added to a
stirred solution of pyrrole (3.80 g, 10 mmol) and 3-tri-
isopropylsilylpropargylaldehyde (11.95 g, 10 mmol) in
DCM (5 l) cooled to −78 °C. The solution was al-
lowed to warm to room temperature (r.t.) over 2 h.
Stirring was continued for a further 1 h, DDQ (9.7 g,
Fig. 6. Packing arrangement of four molecules in the crystal structure of 2.

3224
M.J. Plater et al. / Polyhedron 20 (2001) 3219–3224
104.1, 108.7, 131.9 and 151.2; m/z 1236 (M++H,
100%).
MHz; CDCl3) 13.0, 20.2, 32.8, 36.0, 99.5, 103.4, 110.7,
122.2, 126.6, 131.0, 132.0, 134.2, 142.3, 150.3 and 152.5
(one resonance is missing); m/z 1252.6623 (M+
C70H92N4Si2Pb requires 1252.6627), 1252.7 (M+, 100%).
3.4. 5,15-Bis-(3,5-bis-tert-butylphenyl)-
10,20-bis-triisopropylsilylethynylporphyrin
4. Supplementary material
Under argon, BF3·Et2O (0.1 ml) was added to a
stirred solution of meso-(3,5-di-tert-butylphenyl)-2,2%-
dipyrrylmethane (753 mg, 2.2 mmol) and 3-triisopropy-
lsilylpropargylaldehyde (500 mg, 2.4 mmol) in DCM (1
l) and the solution stirred. After 1 min, DDQ (0.768 g,
3.4 mmol) was added and stirring continued for 20 min
more. The solution was filtered through silica, the
solvent removed under reduced pressure and the residue
chromatographed on silica with light petroleum–
dichloromethane (1:1) as eluent. The second fraction
was collected and the solvent removed under reduced
pressure to give the title compound (230 mg, 20%) as a
purple crystalline solid, m.p.\300 °C (dichloro-
methane–methanol). Larger preparations of this com-
pound can be purified by repeated crystallisation.
Found: C, 79.2; H, 9.0; N, 5.0. C70H94N4Si2·CH3OH
requires C, 79.0; H, 9.15; N, 5.2%; umax (CHCl3, nm):
434 (log m=5.7) and 585 (4.9); wmax (KBr, cm−1):
2959vs, 2862s, 2139m, 1591m, 1558w, 1473m, 1427w,
1363m, 1263w, 1245m, 1136m, 1069m, 985m, 915m,
881m, 801s, 715s, 676m and 580w; lH (250 MHz;
CDCl3) −2.11 (s, 2H, NꢀH), 1.42 (42H, m, i-Pr), 1.53
(36H, s, t-Bu), 7.80 (2H, t, J=1.8 Hz, p-Ph), 8.01 (4H,
d, J=1.8 Hz, o-Ph), 8.86 (4H, d, J=4.6 Hz) and 9.64
(4H, d, J=4.6 Hz); lC (62.9 MHz; CDCl3) 11.9, 19.1,
31.8, 35.1, 99.1, 100.9, 108.8, 121.4, 123.1, 129.4, 130.4,
131.9, 140.4 and 148.9 (two resonances are missing); m/z
1047 (M+, 100%).
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic
Data Centre, CCDC Nos. 167552 and 167553 for com-
pounds 1 and 2, respectively. Copies of this information
may be obtained free of charge from The Director,
CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK
(fax: +44-1223-336033; e-mail: deposit@ccdc.cam.
ac.uk or www: http://www.ccdc.cam.ac.uk).
References
[1] R. Crane, K. Lewis, E. van Stryland, M. Khoshnevisan (Eds.),
Materials for optical limiting, Proceedings of the MRS, 1996, p.
374.
[2] K. Mansour, D. Alvarez, K.J. Perry, I. Choong, S.R. Marder, J.W.
Perry, Proc. SPIE 1853 (1993) 132.
[3] K. Mansour, P. Fuqua, S.R. Marder, B. Dunn, J.W. Perry, Proc.
SPIE 2143 (1994) 239.
[4] (a) J.W. Perry, K. Mansour, S.R. Marder, C.-T. Chen, P. Miles,
M.E. Kenney, G. Kwag, Proc. MRS 374 (1996) 257;
(b) P. Le Barny, V. Dentan, P. Robin, F. Soyer, M. Vergnolle,
SPIE 2852 (1996) 201.
[5] P.V. Bedworth, J.W. Perry, S.R. Marder, Chem. Commun. (1997)
1353.
[6] K.M. Barkigia, J. Fajer, A.D. Adler, G.J.B. Williams, Inorg.
Chem. 19 (1980) 2057.
[7] M.O. Senge, M.W. Renner, W.W. Kalisch, J. Fajer, J. Chem. Soc.,
Dalton Trans. (2000) 381.
[8] M.P. Byrn, C.J. Curtis, I. Goldberg, Yu Hsiou, S.I. Khan, P.A.
Sawin, S.K. Tendick, C.E. Strouse, J. Am. Chem. Soc. 113 (1991)
6549.
3.5. [5,15-Bis-(3,5-bis-tert-butylphenyl)-
10,20-bis-triisopropylsilylethynylporphinato]lead(II) (2)
[9] L.J. Pace, J. Martinsen, A. Ulman, B.M. Hoffman, J.A. Ibers, J.
Am. Chem. Soc. 105 (1983) 2612.
[10] T.P. Newcomb, M.R. Godfrey, B.M. Hoffman, J.A. Ibers, Inorg.
Chem. 29 (1990) 223.
[11] K. Liou, T.P. Newcomb, M.D. Heagy, J.A. Thompson, W.B.
Heuer, R.L. Musselman, C.S. Jacobsen, B.M. Hoffman, J.A.
Ibers, Inorg. Chem. 31 (1992) 4517.
[12] A.M. Duisenberg, J. Appl. Crystallogr. 25 (1992) 92 (DIRAX).
[13] R. Hooft, B.V. Nonius, COLLECT: Data Collection Software, 1998.
[14] Z. Otwinowski, W. Minor, in: C.W. Carter Jr., R.M. Sweet (Eds.),
Macromolecular Crystallography, Part A. In: Methods in Enzy-
mology, vol. 276, Academic Press, New York, 1997, pp. 307–326
A suspension of 5,15-bis-(3,5-bis-tert-butylphenyl)-
10,20-bis-triisopropylsilylethynylporphyrin (40 mg, 0.04
mmol) and Pb(OAc)2·3H2O (1.2 g, 3 mmol) in DMF (20
ml) was refluxed for 4 h under argon. The resulting deep
green solution was poured into water (50 ml). The
precipitate was filtered, washed with water, MeOH and
recrystallised from DCM–MeOH to give the title com-
pound (30 mg, 63%) as a deep green, almost black, solid,
m.p.\300 °C (dichloromethane–methanol). Found: C,
67.9; H, 7.5; N, 4.35. C70H92N4PbSi2 requires C, 67.1;
H, 7.4; N, 4.5%; umax (CHCl3, nm): 473 (log m 5.3), 493
(5.3) and 704 (4.6); wmax (KBr, cm−1): 2955vs, 2862s,
2361w, 2332w, 2134m, 1589m, 1470m, 1389w, 1361m,
1280m, 1244m, 1207s, 1157m, 1062w, 995m, 923m,
880m, 794m, 717vs, 670m and 565w; lH (400 MHz;
CDCl3) 1.55 (78H, m, t-Bu and i-Pr), 7.82 (2H, t, J=2
Hz, p-Ph), 7.87 (2H, bs, o-Ph), 8.19 (2H, bs, o-Ph), 8.99
(4H, d, J=4 Hz) and 9.78 (4H, d, J=4 Hz); lC (100.6
(DENZO).
[15] S. Mackay, C.J. Gilmore, C. Edwards, M. Tremayne, N. Stewart,
K. Shankland, MAXUS: A Computer Program for the Solution and
Refinement of Crystal Structures from Diffraction Data.
[16] SORTAV: (a) R.H. Blessing, Acta Crystallogr., Sect. A 51 (1995)
33;
(b) R.H. Blessing, J. Appl. Crystallogr. 30 (1997) 421.
[17] P.T. Beurskens, G. Beurskens, W.P. Bosman, R. de Gelder, S.
Garcia-Granda, R.O. Gould, R. Israe¨l, J.M.M. Smits, DIRDIF-96,
Crystallography Laboratory, University of Nijmegen, The Nether-
lands, 1996.
[18] G.M. Sheldrick, SHELXL-97, University of Go¨ttingen, Go¨ttingen,
Germany, 1997.