Synthesis, structure, and redox properties of the extremely crowded triarylpnictogens: Tris(2,4,6-triisopropylphenyl)phosphine, arsine, stibine, and bismuthine

Article abstract of DOI:10.1021/ja027493n

A Series of the extremely crowded triarylpnitogens, tris(2,4,6-triisopropylphenyl)phosphine (1), arsine (2), stibine (3), and bismuthine (4), were synthesized by the reaction of 2,4,6-triisopropylphenylcopper(I) with the corresponding pnictogen trichlorid

Full text of DOI:10.1021/ja027493n

Published on Web 11/20/2002
Synthesis, Structure, and Redox Properties of the Extremely Crowded
Triarylpnictogens: Tris(2,4,6-triisopropylphenyl)phosphine, Arsine, Stibine,
and Bismuthine
Shigeru Sasaki,* Katsuhide Sutoh, Fumiki Murakami, and Masaaki Yoshifuji*
Department of Chemistry, Graduate School of Science, Tohoku UniVersity, Aoba, Sendai 980-8578, Japan
Received June 27, 2002
Crowded triarylpnictogens such as trimesitylphosphine1,2 and
Scheme 1
arsine3,4 have large bond angles and lengths around the pnictogen
atoms and are reversibly oxidized to the stable cation radicals at
significantly low oxidation potentials due to both structural changes
and steric protection by introduction of bulky aryl groups.5 Recently,
we synthesized aminophosphinobenzenes carrying the trimesi-
tylphosphine structure as one of the redox sites.6 However, the
phosphorus redox center was not stable enough to afford a reversible
two-step redox system. To construct more stable phosphorus redox
center, we pursued the introduction of the more sterically demanding
aryl groups on phosphorus, and herein we report the synthesis,
structure, and redox properties of some of the most crowded
triarylpnictogens, tris(2,4,6-triisopropylphenyl)phosphine (1), arsine
(2), stibine (3), and bismuthine (4).
Since the reaction of 2,4,6-triisopropylphenylmagnesium bromide
with phosphorus trichloride only afforded the corresponding tet-
raaryldiphosphane7 unlike the mesityl Grignard reagent,6,8 aryl
copper(I) reagents, which are less reactive as a reducing agent and
have been utilized to prepare chlorophosphines9 and the crowded
phosphine ligand,10 were employed. Phosphine 1 was synthesized
by the reaction of 2,4,6-triisopropylphenylcopper(I)11 with phos-
phorus trichloride (0.30 equiv) at -78 °C followed by refluxing
for 24 h and isolated as colorless prisms (35%) after column
chromatography (Al2O3/n-hexane) followed by recrystallization
from n-hexane (Scheme 1).12 Tris(2,4,6-triisopropylphenyl)arsine
(2) (75%),13 stibine (3) (58%),14 and bismuthine (4) (3%),15 were
synthesized similarly. These crowded triarylpnictogens except for
4 were stable in air in the solid state.
Table 1. Average C-Pn Bond Lengths, C-Pn-C Bond Angles,
and Oxidation Potentials of the Crowded Triarylphosphine, Arsine,
Stibine, and Related Compounds.
a
compounds
bond lengths/Å
bond angles/deg
E
1/2/V
triphenylphosphine
trimesitylphosphine
1
triphenylarsine
trimesitylarsine
2
triphenylstibine
trimesitylstibine
3
1.828b
1.837d
1.845
103.0b
109.7d
111.5
99.8e
1.03c
0.41
0.16
1.18c
0.73
0.50
1.05c
0.76c
0.57
1.949e
1.976f
1.986
107.6f
109.2
(97.3)g
105.3h
106.7
(2.141)g
2.183h
2.197
a Reversible unless otherwise stated, V vs Ag/Ag+ in dichloromethane
with 0.10 M n-Bu4NClO4, scan rate: 30 mVs-1 b Reference 22. c Irrevers-
.
ible, peak potential. d Reference 1. e Reference 23. f Reference 3. g Tris(4-
methylphenyl)stibine, ref 24. h Reference 25.
The 31P NMR (80 MHz, CDCl3) chemical shifts of the crowded
tris(2,4,6-trialkylphenyl)phosphine showed an upfield shift as the
substituents became more sterically demanding from triphenylphos-
phine (δ -6), and trimesitylphosphine (δ -36) to 1 (δ -53). The
1H NMR (200 MHz, CDCl3) and 13C NMR (50 MHz, CDCl3)
spectra of 1 at 295 K were rather simple and two o-CH(CH3)2
protons and four aromatic carbons were observed. On the other
hand, four o-CH(CH3)2 protons and six aromatic carbons were
observed for 2 and 3. The molecular structures of 1, 2, and 3 were
further investigated by X-ray crystallography.16 The triarylpnicto-
gens 1 (Figure 1), 2, and 3 have a crowded propeller shape and the
pnictogen atoms are covered with six ortho isopropyl groups.
To compare the structure and properties of 1, 2, and 3, with the
related compounds, the average bond lengths and angles around
pnictogen atoms, and oxidation potentials measured by cyclic
voltammetry are summarized in Table 1. The bond lengths and
angles become longer and larger as the ortho substituents of the
aryl groups become more bulky in these series of compounds, and
Figure 1. ORTEP drawing of tris(2,4,6-triisopropylphenyl)phosphine (1)
with 50% thermal ellipsoids, (a) top view, (b) side view.
the tris(2,4,6-triisopropylphenyl)pnictogens have the longest bond
length and largest bond angle for each element. Steric repulsion
among the bulky substituents was revealed by elongation of the
bond lengths and expansion of the bond angles around the pnictogen
atoms. Similar trend was recently reported for trisilylpnictogens
with enhanced planarity around pnictogens.17 The bond angles and
length of 1 are slightly larger than those of tris(9-anthracenyl)phos-
phine18 (1.839 Å, 110.4°), which is one of the most crowded
triarylphosphines known to exist. The oxidation potentials of the
triarylpnictogens become lower as the molecules become more
crowded since the energy level of the lone pair (HOMO) is raised
by enhanced planarity of the pnictogen atoms induced by steric
congestion5 as well as the electron donation by the alkyl groups.
* To whom correspondence should be addressed. E-mail: yoshifj@
mail.cc.tohoku.ac.jp.
9
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J. AM. CHEM. SOC. 2002, 124, 14830-14831
10.1021/ja027493n CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
(9) Ja¨kle, F.; Manners, I. Organometallics 1999, 18, 2628-2632.
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(11) Nobel, D.; van Koten, G.; Spek, A. L. Angew. Chem., Int. Ed. Engl. 1989,
28, 208-210.
The 2,4,6-triisopropylphenyl derivatives have the lowest potentials,
especially, that of 1 was comparable to ferrocene. The dichlo-
romethane solution of the phosphine 1 and arsine 2 can be oxidized
by silver perchlorate and afforded a purple (λmax ) 528 and 516
nm for 1+• and 2+•, respectively) solution of the corresponding
cation radical, which were also characterized by the EPR param-
eters19 similar to those of the trimesityl derivatives.3,5 A similar
reaction of trimesitylphosphine was reported to lead to the metal
complex formation.20 Although 1 and 2 were inert to hydrogen
peroxide or elemental sulfur, 1+• and 2+• reacted slowly with oxygen
to result in incorporation of oxygen. Interestingly, not only 1 and
2, but also stibine 3 displayed a reversible redox wave with the
ratio of anodic and cathodic peaks as unity at the 30 mVs-1 scanning
rate at 293 K, which suggests considerable stability of the
corresponding cation radical.21 A detailed study of the cation
radicals and application of these structures to the multistep redox
systems is in progress.
(12) 1: colorless prisms, mp 165-166 °C; 1H NMR (200 MHz, CDCl3, 293
K) δ 6.88 (6H, d, J ) 3.12 Hz), 3.48 (6H, m, J ) 6.70, 5.43 Hz), 2.82
(3H, sept, J ) 6.89 Hz), 1.21 (18H, d, J ) 6.90 Hz), l.14 (18H, d, J )
6.70 Hz), 0.66 (18H, d, J ) 6.65 Hz); 13C NMR (50 MHz, CDCl3, 293
K) δ 152.89 (d, J ) 18.02 Hz), 148.99 (s), 132.20 (d, J ) 24.01 Hz),
121.85 (d, J ) 4.56 Hz), 34.04 (s), 31.92 (d, J ) 17.98 Hz), 24.52 (s),
23.94 (s), 23.00 (s); 31P NMR (80 MHz, CDCl3, 293 K) δ -52.4 (s).
(13) 2: colorless prisms, mp 145-146 °C; 1H NMR (200 MHz, CDCl3, 293
K) δ 6.90 (6H, s), 3.39 (3H, sept, J ) 6.71 Hz), 3.32 (3H, sept, J ) 6.72
Hz), 2.83 (3H, sept, J ) 6.69 Hz), 1.26 (9H, d, J ) 6.68 Hz), 1.21 (18H,
d, J ) 6.91 Hz), 1.07 (9H, d, J ) 6.79 Hz), 1.01 (9H, d, J ) 6.74 Hz),
0.45 (9H, d, J ) 6.57 Hz); 13C NMR (50 MHz, CDCl3, 293 K) δ 153.08,
152.83, 148.70, 136.36, 122.76, 120.86, 34.01, 33.43, 32.67, 25.37, 24.22,
24.02, 24.00, 23.91, 22.78.
(14) 3: colorless prisms, mp 145-146 °C; 1H NMR (200 MHz, CDCl3, 294
K) δ 6.93 (6H, s), 3.32 (6H, sept, J ) 6.46 Hz), 2.84 (3H, sept, J ) 7.04
Hz), 1.32 (9H, d, J ) 6.54 Hz), 1.22 (18H, d, J ) 6.99 Hz), 1.08 (18H,
m), 0.49 (9H, d, J ) 6.32 Hz); 13C NMR (50 MHz, CDCl3, 294 K) δ
154.85, 154.78, 148.92, 137.70, 122.52, 120.97, 36.81, 35.97, 34.02, 25.92,
24.85, 24.44, 24.03, 23.91, 23.26.
(15) 4: colorless prisms, mp 149-150 °C; 1H NMR (200 MHz, CDCl3, 330
K) δ 7.09 (6H, s), 3.18 (6H, sept, J ) 6.69 Hz), 2.84 (3H, sept, J ) 6.85
Hz), 1.24 (18H, d, J ) 6.90 Hz), 1.03 (36H, brs); 13C NMR (50 MHz,
CDCl3, 330 K) δ 159.36, 155.92, 148.44, 122.54, 39.54, 34.17, 24.97,
24.00.
Acknowledgment. Financial support in part by Grants-in-Aid
for Scientific Research from the Ministry of Education, Culture,
Sports, Science, and Technology (Nos. 12740335, 08454193 and
09239101), the JSPS Postdoctoral Fellowship for F.M. by the Japan
Society for the Promotion of Science, and the elemental analysis
and mass spectral measurements by the Instrumental Analysis
Center for Chemistry, Graduate School of Science, Tohoku
University, are gratefully acknowledged. This work was partially
carried out in the Advanced Instrumental Laboratory for Graduate
Research of Department of Chemistry, Graduate School of Science,
Tohoku University.
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R ) 100.96(2)°, â ) 91.82(1)°, γ ) 96.24(2)°, V ) 2140.8(9) Å3, Z )
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Å, b ) 18.44(1) Å, c ) 14.890(2) Å, R ) 99.54(1)°, â ) 110.68(1)°, γ
) 107.37(2)°, V ) 4130(2) Å3, Z ) 4, Dcalc ) 1.101 gcm-1, µ(Mo KR)
) 0.850 mm-1, T ) 123 K, R1; R; Rw ) 0.047; 0.066; 0.064, GOF )1.65.
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(4) Å, â ) 111.049(3)°, V ) 8402(1) Å3, Z ) 8, Dcalc ) 1.157 gcm-1
,
µ(Mo KR) ) 0.683 mm-1, T ) 115 K, R1; R; Rw ) 0.034; 0.041; 0.048,
GOF )1.34.
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