Preparation and characterization of methyl substituted 2,2′-dipyridyl diselenides and -ditellurides: X-ray structure of 6,6′-dimethyl-2,2′-dipyridyl diselenide

Article abstract of DOI:10.1080/10426500214573

A convenient method for the preparation of various methyl substituted 2,2′-dipyridyl diselenides and -ditellurides by the aerial oxidation of lithium 2-pyridylselenolate/tellurolate, prepared from the lithium-halogen exchange between n-butyllithium and 2-bromo methyl substituted pyridines is reported. All the compounds prepared are new and have been characterized by elemental analysis, IR, ¹H, ¹³C, ¹¹Se NMR, and mass spectral studies. Crystal structure of 6,6′-dimethyl-2,2′-dipyridyl diselenide has been determined.

Full text of DOI:10.1080/10426500214573

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Preparation and Characterization of Methyl Substituted
2,2′-Dipyridyl Diselenides and -Ditellurides: X-ray
Structure of 6,6′-Dimethyl-2,2′-dipyridyl Diselenide
K. K. Bhasin a , P. Venugopalan a & J. Singh a
a Department of Chemistry and Centre of Advanced Studies in Chemistry, Panjab University,
Chandigarh, India
Version of record first published: 27 Oct 2010.
To cite this article: K. K. Bhasin , P. Venugopalan & J. Singh (2002): Preparation and Characterization of Methyl Substituted
2,2′-Dipyridyl Diselenides and -Ditellurides: X-ray Structure of 6,6′-Dimethyl-2,2′-dipyridyl Diselenide, Phosphorus, Sulfur,
and Silicon and the Related Elements, 177:11, 2579-2587
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Phosphorus, Sulfur and Silicon, 2002, Vol. 177:2579–2587
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DOI: 10.1080/10426500290110793
PREPARATION AND CHARACTERIZATION
OF METHYL SUBSTITUTED 2,20-DIPYRIDYL
DISELENIDES AND -DITELLURIDES: X-RAY
STRUCTURE OF 6,60-DIMETHYL-2,20-DIPYRIDYL
DISELENIDE
K. K. Bhasin, P. Venugopalan, and J. Singh
Department of Chemistry and Centre of Advanced Studies
in Chemistry, Panjab University, Chandigarh, India
(Received March 1, 2002; accepted April 30, 2002)
A convenient method for the preparation of various methyl substi-
tuted 2,20-dipyridyl diselenides and -ditellurides by the aerial oxidation
of lithium 2-pyridylselenolate/tellurolate, prepared from the lithium-
halogen exchange between n-butyllithium and 2-bromo methyl substi-
tuted pyridines is reported. All the compounds prepared are new and
have been characterized by elemental analysis, IR, 1H, 13C, 11Se NMR,
and mass spectral studies. Crystal structure of 6,60-dimethyl-2,20-
dipyridyl diselenide has been determined.
Keywords: Organochalcogens; selenides; selenium; tellurides;
tellurium
INTRODUCTION
Organoselenium and tellurium compounds have emerged recently as
important reagents and intermediates in organic synthesis,1,2 accord-
ingly, development of convenient methods for the synthesis of these ver-
satile compounds starting from readily available materials has always
remained a subject of current interest.3 In contrast to alkyl, aryl, and
mixed alkylaryl selenium/tellurium compounds, very little is known
about the pyridyl selenium/tellurium compounds. In continuation of
Paper presented in 8th International Conference on the Chemistry of Selenium and
Tellurium at Brazil, August 6–11, 2000.
Thanks are acknowledged to CSIR, New Delhi for financial assistance and to Jeol
Ltd., Tokyo (Japan) for 77Se NMR.
Address correspondence to K. K. Bhasin, Department of Chemistry and Centre of
Advanced Studies, Panjab University, Chandigarh 160014, India. E-mail: kkbhasin@
pu.ac.in
2579

2580
K. K. Bhasin et al.
our research4,5 on the chemistry of organochalcogen compounds, we
report in this article on the preparation and characterization of the
hitherto unknown stable methyl substituted 2,20-dipyridyl diselenides
and -ditellurides along with the crystal structure of 6,60-dimethyl-2,20-
dipyridyl diselenide 1.
RESULTS AND DISCUSSION
Mautner et al.6 were the first to synthesise 2,20-dipyridyl diselenide by
reacting 2-bromo pyridine with hydrogen selenide (generated by ad-
dition of dilute hydrochloric acid to aluminium selenide). This method
involves the use of highly poisonous hydrogen selenide gas. A need was,
therefore, felt to develop a safe and convenient method for its prepa-
ration and subsequently the method was improved upon by reducing
elemental selenium with sodium borohydride in various solvents by
various workers. All the workers developed different methodologies in
the subsequent years7 and explored variations of the published proce-
dures to improve on the yield of 2,20-dipyridyl diselenide. We have now
developed and optimized the conditions for the preparation of hith-
erto unknown stable methyl substituted 2,20-dipyridyl diselenide and
-ditelluride by a different route, which avoids the use of toxic gases and
afford better yields.
The method involves the bromine exchange of 2-bromo methyl sub-
stituted pyridine using n-butyllithium in THF at 78 C as depicted
below.
The reactions were carried out in the presence of oxygen-free ni-
trogen. Addition of elemental selenium/tellurium to the lithiated pyri-
dine was made at 78 C and later allowed to come to room tempera-
ture. It took nearly 20 min to dissolve elemental selenium whereas it
took more than 45 min to dissolve elemental tellurium. This is proba-
bly because commercially available tellurium is known to exhibit low

2,20-Dipyridyl Diselenides and -Ditellurides
2581
chemical reactivity due to its surface oxidation.8 Aerial oxidation of
the above lithium-2-pyridyl selenolate/tellurolate solutions resulted in
the formation of the methyl substituted 2,20-dipyridyl diselenides and
ditellurides respectively. The yields obtained in case of diselenides were
higher than in case of the ditellurides.
The diselenides and -ditellurides thus prepared are stable enough
to be purified by column chromatography (silica gel using hexane-
ethylacetate) on a laboratory bench. The diselenides are light yellow
colored crystalline compounds whereas the corresponding ditellurides
are deep red except 3,30-dimethyl-2,20-dipyridyl ditelluride which is ex-
tremely deep blue in color. All compounds are soluble in conventional
organic solvents. The diselenides and -ditellurides have a shelf life of
several months without any sign of decomposition even at room tem-
perature.
1H NMR of the diselenides and ditellurides display two sets of pro-
tons, one corresponding to the methyl group attached to the ring and
the other corresponding to three types of protons attached to the ring
itself. Methyl protons in case of diselenides appear as a singlet in the
range 2.0–2.05 ( ppm) whereas in case of ditellurides in the range
2.27–2.48 ( ppm). The protons attached to the pyridine ring appear
in the range 6.63–8.29 ( ppm) in these compounds. 13C NMR shows
a set of six signals one corresponding to the methyl carbon and five
corresponding to the carbon of the pyridine ring. Selenium atom is
very sensitive to extremely small electronic and steric changes. There-
fore its chemical shifts are suitable tool to probe its chemical envi-
ronment within the molecule. 77Se (completely decoupled) NMR of
methyl substituted 2,20-dipyridyldiselenides (1, 2 and 4) were recorded
in CDCl3 employing selenic acid as an external reference resonating
at 0 ( ppm). In 6,60-dimethyl-2,20-dipyridyldiselenide, the selenium
signal was observed at 576.767 ( ppm) whereas in 5,50-dimethyl-
2,20-dipyridyldiselenide, it resonated at 584.469 ( ppm) and in 3,30-
dimethyl-2,20-dipyridyldiselenide, it was observed at 594.362 ( ppm)
relative to selenic acid. Thus it appears that the presence of the methyl
group in 3 position (ortho to selenium) reduces the interaction between
the selenium lone pairs and the -electron system of the pyridine ring,
which results in the shielding of the selenium nucleus. On the other
hand the presence of the methyl group at 6 position increases the in-
teraction between the selenium lone pairs and the -electron system of
the pyridine ring and thus deshields the selenium with respect to 5,50-
dimethyl-2,20-dipyridyldiselenide. The fragmentation pattern of all the
diselenides and ditellurides show molecular-ion peak (M+) at m/e 344
and 444 respectively. The base-ion peak in case of diselenides was ob-
served at 183 whereas in case of ditellurides it was observed at 92.

2582
K. K. Bhasin et al.
FIGURE 1 A perspective view of structure of 1 with atom numbering scheme
(thermal ellipsoids are at 40% probability level).
Solid State Structural Feature of
6,60-Dimethyl-2,20-dipyridyldiselenide (1)
The crystals of 1 were obtained by the slow evaporation of mixed solvent
of hexane-ethylacetate. In order to understand structural details, single
crystal x-ray diffraction study was carried out. A perspective view of
structure of 1 with atom numbering scheme is given in Figure 1 with
bond lengths and angles in Table I.
In the crystal lattice the molecule is located on a two fold axis with
˚
Se(1)-Se(1)#1 bond length 2.2935(11)A and angle C(1)-Se(1)-Se(1)#1
104.47 (14). These values are comparable with the values found in other
aromatic diselenides.9 Each pyridine rinag in the molecule is substituted
TABLE I Selected Bond Distances (A) and Angles ( )
for Compound 1
Se(1)-C(1)
N(1)-C(1)
C(1)-C(2)
C(3)-C(4)
1.929(5)
1.314(6)
1.380(7)
1.374(9)
1.484(8)
104.47(14)
124.5(4)
125.6(4)
120.1(5)
119.8(5)
123.2(5)
Se(1)-Se(1)#1
N(1)-C(5)
C(2)-C(3)
2.2935(11)
1.343(6)
1.358(8)
1.378(8)
C(4)-C(5)
C(5)-C(6)
C(1)-Se(1)-Se(1)#1
N(1)-C(1)-C(2)
C(2)-C(1)-Se(1)
C(2)-C(3)-C(4)
N(1)-C(5)-C(4)
C(4)-C(5)-C(6)
C(1)-N(1)-C(5)
N(1)-C(1)-Se(1)
C(3)-C(2)-C(1)
C(3)-C(4)-C(5)
N(1)-C(5)-C(6)
118.9(4)
109.8(3)
116.5(5)
120.1(5)
117.0(4)
Symmetry transformations used to generate equivalent atoms: #1
x + 1, y, z + 1/2.

2,20-Dipyridyl Diselenides and -Ditellurides
2583
˚
at 2 position [Se C bond length 1.929(5)A] and at 6 position [H3C C
˚
bond length 1.484(8)A]. The molecular geometry of the pyridine ring is
normal in terms of bond lengths and angles. It is worth mentioning that
Se-Se is coplanar with both the pyridine rings as can be seen from the
torsion angle Se(1A)-Se(1)-C(1)-C(2) of 1.8(4). Dihedral angle between
two pyridine rings is 90.1 , which are orthogonal to each other. Short
contact is observed between Se(1) and H(2)#1 (2.957).
EXPERIMENTAL
All the reactions were carried out in dry oxygen-free nitrogen atmo-
sphere. Solvents were dried and distilled freshly under nitrogen. Ele-
mental selenium (Aldrich), tellurium (Aldrich) and 2-amino methyl sub-
stituted pyridines (Lancaster) were used as received. 2-Bromo methyl
substituted pyridines were prepared by an adapation of the Craig’s
method.10 The melting points reported are uncorrected. NMR spectra
were recorded in CDCl3 using TMS as an internal standard on Bruker
AC-300F, 300 MHz spectrometer. 77Se-BCM (single pulsed with com-
plete decoupling) NMR spectra of the compounds (1, 2, and 4) were
recorded on JNM-AL400, 400 MHz spectrometer with TH5AT/FG2
probe. The mass spectra (EI, 70 eV) were obtained on a VG-Analytical
11-2505-70 SMS Mass spectrometer. Infrared spectra were recorded
between KBr pellets on a Perkin-Elmer model 1430 ratio recording
spectrometer. Carbon, hydrogen, and nitrogen were estimated miro-
analytically using Perkin-Elmer 2400 CHN elemental analyzer. Sele-
nium/tellurium was estimated by standard method.11
Synthesis of 2-bromo methyl substituted pyridines: To a me-
chanically stirred solution of 2-amino methyl substituted pyridine (54 g,
0.5 mol) in 48% HBr (190 ml), bromine (75 ml, 1.5 mol) was added drop-
wise in an ice-bath followed by the slow addition of aqueous solution of
sodium nitrite (87.1 g, 1.5 mol) in 120 ml water. The temperature must
be maintained below 0 C throughout the entire procedure. The mix-
ture was neutralized with aqueous sodium hydroxide, extracted with
diethyl ether, dried over anhydrous sodium sulphate, and concentrated
in vacuum to give oil, which was distilled.
2-Bromo-6-methyl pyridine: B.p. 60–62 C/4 mm (Lit.,12 62–65 C/
1
25 mm Hg), yield 67.6 g (89%). H NMR: , 2.50 (s, 3H), 7.09 (d, 1H),
7.25 (d, 1H), 7.42 (t, 1H). 13C NMR: , 23.5, 121.5, 124.3, 138.0, 140.5,
159.1.
2-Bromo-5-methyl pyridine: M.p. 41–42 C (Lit.,13 m.p. 49–50 C),
1
yield 69 g (91%). H NMR: , 2.27 (s, 3H), 7.33 (s, 2H), 8.16 (s, 1H).
13C NMR: , 17.6, 127.3, 132.3, 138.7, 139.1, 150.2.

2584
K. K. Bhasin et al.
2-Bromo-4-methyl pyridine: B.p. 68–70 C/4 mm Hg (Lit.,13 b.p.
1
223–224 C/760 mm Hg), yield 64.6 g (85%). H NMR: , 2.31 (s, 3H),
7.07–7.05 (d, 1H), 7.29 (s, 1H), 8.20–8.18 (d, 1H). 13C NMR: , 20.48,
123.68, 128.42, 142.0, 149.4, 150.0.
2-Bromo-3-methyl pyridine: B.p. 212 C/760mm Hg (Lit.,13 b.p.
1
218–219 C/760 mm Hg), yield 66.1 g (80%). H NMR: , 2.32 (s, 3H),
7.18–7.14 (t, 1H), 7.51–7.48 (d, 1H), 8.16–8.14 (s, 1H). 13C NMR:
21.19, 122.1, 138.0, 143.7, 146.4.
,
General Procedure for the Preparation of Methyl
Substituted 2,20-Dipyridyl Diselenides
In a clean, flame dried two-necked round bottom flask was placed 3.49 g
(20.2 mmol) of 2-bromo methyl substituted pyridine in tetrahydrofu-
ran. The solution was cooled to 78 C to which 24.3 mmol of n-BuLi
was added dropwise. The solution turned from colorless to red. The
solution was stirred for 20 minutes and 1.9 g (24.2 mmol) of elemental
selenium was added. The reaction mixture was slowly brought to
room temperature till all the elemental selenium dissolves. The supply
of nitrogen was discontinued and the reaction mixture was exposed
to aerial oxidation for another 30 min. When clear yellow solution
was obtained, the solvent was stripped off on the rota-evaporator
and the product was washed with water and then extracted with
diethyl ether. The ether extract was dried over anhydrous sodium
sulphate and the diethyl ether removed on a rota-evaporator. The
crude product was subjected to column chromatography on silica gel
using hexane-ethyl acetate (10:1) as eluent affording pure methyl
substituted 2,20-dipyridyl diselenide as dark yellow needles.
6,60-Dimethyl-2,20-dipyridyl diselenide (1): Yield, 6.86 g (62%)
1
m.p. 61–62 C. H NMR: , 2.52 (s, 6H), 6.92–6.89 (d, 2H), 7.43–7.36
(t, 2H), 7.61–7.58 (d, 2H). 13C NMR: , 24.2, 120.5, 136.0, 137.5, 153.6,
158.5. 77Se NMR: , 576.767. IR (KBr, cm 1): 3060, 2960, 2920, 1580,
1540, 1430, 1120, 1090, 1020, 840, 780, 660, 540. MS(EI): 344 [M]+
(27.1); 264 [M-Se]+ (43.3); 183 [M-Se2H] (81); 92 [M CH3PySe2]+ (100).
Anal. Calcd. for: C12H12N2Se2; C, 41.86, H, 3.48, N, 8.13. Found: C,
41.08, H, 3.28, N, 8.42.
5,50-Dimethyl-2,20-dipyridyl diselenide (2): Yield, 7.75 g (70%)
m.p. 67–68 C. 1H NMR: , 2.27 (s, 6H), 7.35–7.32 (d, 2H), 7.68–7.66 (d,
2H), 8.27 (s, 2H). 13C NMR: , 17.8, 123.3, 130.7, 136.1, 149.8, 150.7.
77Se NMR: , 584.469. IR (KBr, cm 1): 3030, 2983, 2914, 1579, 1559,
1446, 668, 1219, 1081, 1040, 910, 822, 723, 592, 479. ME(EI): 344 [M]+
(34.3); 263 [M-SeH]+ (6.2); 183 [M-Se2H] (100); 92 [M CH3PySe2]+

2,20-Dipyridyl Diselenides and -Ditellurides
2585
(60.5). Anal. Calcd. for: C12H12N2Se2; C, 41.86, H, 3.48, N, 8.13. Found:
C, 41.12, H, 3.35, N, 8.32.
4,40-Dimethyl-2,20-dipyridyl diselenide (3): Yield, 6.64 g (60%)
m.p. 96–98 C. 1H NMR: , 2.20 (s, 6H), 6.69–6.00 (d, 2H), 7.60 (s, 2H),
8.30–8.29 (d, 2H). 13C NMR: , 21.0, 123.4, 123.9, 148.8, 149.0, 154.0.
IR (KBr, cm 1): 3060, 2960, 2920, 1580, 1540, 1460, 1270, 1120,
1080, 840, 700, 500. ME(EI): 344 [M]+ (36); 263 [M-SeH]+ (10.1); 183
[M-Se2H] (100); 92 [M CH3PySe2]+ (63). Anal. Calcd. for: C12H12N2Se2;
C, 41.86, H, 3.48, N, 8.13. Found: C, 41.72, H, 3.93, N, 8.42.
3,30-Dimethyl-2,20-dipyridyl diselenide (4): Yield, 7.97 g (72%)
m.p. 142–144 C. 1H NMR: , 2.42 (s, 6H), 6.99–6.96 (d, 2H), 7.30–7.27
(d, 2H), 8.26 (s, 2H). 13C NMR: , 20.5, 121.7, 133.5, 136.6, 147.7, 153.0.
77Se NMR: , 594.362. IR (KBr, cm 1): 3030, 2960, 2920, 1664, 1570,
1543, 1460, 1277, 1060, 785, 635, 575, 470. ME(EI): 344 [M]+ (19.6);
263 [M-SeH]+ (24.4); 183 [M-Se2H] (100); 92 [M CH3PySe2]+ (62.2).
Anal. Calcd. for: C12H12N2Se2; C, 41.86, H, 3.48, N, 8.13. Found: C,
41.45, H, 3.62, N, 8.34.
General Procedure for Preparation of Methyl Substituted
2,20-Dipyridyl Ditellurides
Methyl substituted 2,20-dipyridyl ditellurides were prepared by
the same method as applied for the preparation of corresponding
diselenides.
6,60-Dimethyl-2,20-dipyridyl ditelluride (5): Yield, 3.36 g (20%)
m.p. 55–56 C. 1H NMR: , 2.48 (s, 6H), 6.63–6.60 (d, 2H), 7.24–7.19 (t,
2H), 7.63–7.61 (d, 2H). 13C NMR: , 24.1, 121.1, 127.9, 134.9, 158.6,
136.9. IR (KBr, cm 1): 3042, 2916, 1571, 1429, 1376, 1242, 1163, 1083,
988, 826, 781, 655, 545. ME(EI): 444 [M]+ (5.5); 314 [M-Te]+ (5.5); 314
[M-Te]+ (6.5); 183 [M Te2H]+ (30.9); 92 [M CH3PyTe2]+ (100). Anal.
Calcd. for: C12H12N2Te2; C, 32.43, H, 2.70, N, 6.30. Found: C, 32.21, H,
2.52, N, 6.18.
5,50-Dimethyl-2,20-dipyridyl ditelluride (6): Yield, 4 g (25%) m.p.
110–112 C. 1H NMR: , 1.97 (s, 6H), 6.63–6.60 (d, 2H), 7.24–7.19
(t, 2H), 7.63–7.61 (d, 2H). 13C NMR: , 24.1, 121.1, 127.9, 134.9, 136.9,
158.6. IR (KBr, cm 1): 3033, 2969, 2917, 1582, 1544, 1526, 1368, 1272,
1073, 983, 835, 694, 520. ME(EI): 444 [M]+ (8.2); 314 [M-Te]+ (12.1);
183 [M Te2H]+ (67.3); 92 [M CH3PyTe2]+ (100). Anal. Calcd. for:
C12H12N2Te2; C, 32.43, H, 2.70, N, 6.30. Found: C, 32.83, H, 2.12, N, 6.19.
4,40-Dimethyl-2,20-dipyridyl ditelluride (7): Yield, 2.61 g (22%)
1
Semi solid. H NMR: , 2.27 (s, 6H), 6.62–6.61 (d, 2H), 7.85 (s, 2H),
8.29–8.28 (d, 2H). 13C NMR: , 20.7, 122.5, 131.5, 135.1, 146.1, 149.3. IR

2586
K. K. Bhasin et al.
(KBr, cm 1): 3025, 2963, 2908, 1685, 1573, 1546, 1261, 1097, 1020, 865,
685, 473. ME (EI): 444 [M]+ (1.8); 314 [M-Te]+ (41.15); 183 [M Te2H]+
(37.5); 92 [M CH3PyTe2]+ (100). Anal. Calcd. for: C12H12N2Te2; C,
32.43, H, 2.70, N, 6.30. Found: C, 32.74, H, 2.13, N, 6.92.
3,30-Dimethyl-2,20-dipyridyl ditelluride (8): Yield, 8.73 g (52%)
m.p. 120–125 C. 1H NMR: , 2.41 (s, 6H), 7.14–7.10 (m, 2H), 7.47–7.45
(d, 2H), 8.14–8.12 (d, 2H). 13C NMR: , 24.52, 122.5, 134.8, 136.2, 147.1,
154.8. IR (KBr, cm 1): 3060, 2960, 2920, 1580, 1540, 1430, 1120, 1090,
1020, 840, 780, 660, 540. ME(EI): 444 [M]+ (7.33); 314 [M-Te]+ (14.51);
183 [M Te2H]+ (76.62); 92 [M CH3PyTe2]+ (100). Anal. Calcd. for:
C12H12N2Te2; C, 32.43, H, 2.70, N, 6.30. Found: C, 32.60, H, 2.42, N, 6.18.
X-Ray Crystal Structure Determination of
6,60-Dimethyl-2,20-dipyridyl Diselenide (1)
Intensity data were collected on a Siemens P4 single crystal diffrac-
˚
tometer equipped with molybdenum sealed tube ( = 0.71073 A) and
highly-oriented graphite monochromator using crystals of dimen-
sions 0.34 0.28 0.25 mm, mounted in Lindmann glass capillaries
at 293(2) K. The cell parameters and their standard deviation were
TABLE II Crystal Data and Structure Refinement Details for
Compound 1
Empirical formula
Formula weight
C12H12N2Se2
342.16
Crystal system, space group
Unit cell dimensions
Monoclinic, C2/c
a
a = 20.079(2)A
= 90
a
b = 8.649(1)A
= 108.06(1)
= 90
a
c = 7.925(1)A
a
Volume
1308.5 (3)A3
Z, Calculated density
Absorption coefficient
F(000)
4, 1.737 Mg/m3
1
5.626 mm
664
Max. and min. transmission
Theta range for data collection
Index ranges
1.00, 0.804
2.13 to 24.50
23
h
0, 10
k
0,
8
1
9
Reflection collected
1118
Independent reflections
Refinement method
1080[Rint = 0.0268]
Full-matrix least-squares on F2
Data/restraints/parameters
Goodness on fit on F2
Data to parameter ratio
Final R indices, 873 reflections, [I > 2s(I)]
R indices (all data)
1080/0/74
1.065
14.6:1
R1 = 0.0400, wR2 = 0.0991
R1 = 0.0530, wR2 = 0.1062
a
3
Largest diff. peak and hole
0.478 and 0.556e.A

2,20-Dipyridyl Diselenides and -Ditellurides
2587
obtained by least square to 40 reflections. The 2 – scan mode was
used with variable scan speed ranging from 2.0 to 60.0 /min in ω.
Three reflections were used to monitor the stability and orientation of
the crystal and were remeasured after every 97 reflections. All other
relevant information about the data collection and the refinement are
presented in Table II.
The crystal structure of 1 was solved by direct method using SHELX-
9714 and also refined on F2 using the same one. All the nonhydrogen
atoms were refined anisotropically. The hydrogen atoms were included
in the ideal positions with fixed isotropic U values and were riding with
their respective nonhydrogen atoms. A weighting scheme of the form
w = 1/[ 2(F2o) + 0.0534P)2 + 2.34P], P = [max(Fo2, 0) + 2 F2o]/3 was used.
The refinement converged to a final R value of 0.0400, (wR2 = 0.0991
for 873 reflections) [I > 2 (I)].
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Data deposited at CCDC (No. 159119 for compound 1).