Homologous series of heavier element dipnictenes 2,6- Ar2H3C6E=EC6H3-2,6-Ar2 (E = P, As, Sb, Bi; Ar = Mes = C6H2-2,4,6- Me3; or trip = C6H2<

Article abstract of DOI:10.1021/ja983999n

The synthesis and characterization of several m-terphenyl heavier main group 15 (P, As, Sb, or Bi) dihalides, together with their reduction to give a homologous series of double-bonded dipnictenes, are reported. Reaction of LiC₆H₃-2,

Full text of DOI:10.1021/ja983999n

J. Am. Chem. Soc. 1999, 121, 3357-3367
3357
Homologous Series of Heavier Element Dipnictenes
2,6-Ar2H3C6E)EC6H3-2,6-Ar2 (E ) P, As, Sb, Bi; Ar ) Mes )
C6H2-2,4,6-Me3; or Trip ) C6H2-2,4,6-iPr3) Stabilized by m-Terphenyl
Ligands
Brendan Twamley,† Chadwick D. Sofield,‡ Marilyn M. Olmstead,† and Philip P. Power*,†
Department of Chemistry, UniVersity of California, DaVis, California, 95616, and Department of
Chemistry, UniVersity of California, Berkeley, California, 94720
ReceiVed NoVember 19, 1998
Abstract: The synthesis and characterization of several m-terphenyl heavier main group 15 (P, As, Sb, or Bi)
dihalides, together with their reduction to give a homologous series of double-bonded dipnictenes, are reported.
Reaction of LiC6H3-2,6-Mes2 (Mes ) C6H2-2,4,6-Me3) or LiC6H3-2,6-Trip2 (Trip ) C6H2-2,4,6-iPr3) with the
appropriate trihalide affords 2,6-Mes2H3C6ECl2 (E ) As, 1; Sb, 2; Bi, 3) and 2,6-Trip2H3C6ECl2 (E ) P, 4;
As, 5; Sb, 6; Bi, 7). The compounds 1-7 were characterized by 1H and 13C NMR spectroscopy as well as by
31P NMR spectroscopy in the case of 4. In addition, the structures of 3, 5, and 6 were determined. Reduction
of the phosphorus species 4 with potassium in hexane gives a mixture of the diphosphene 2,6-Trip2H3C6Pd
PC6H3-2,6-Trip2, 12, and the phosphafluorene species, 1-(2,4,6-triisopropylphenyl)-5,7-diisopropyl-9-phos-
phafluorene, 11. The compound 11, which results from the insertion of a phosphorus into a C(Ar)-C(i-Pr)
bond was synthesized in higher yield by the reduction of 4 with magnesium. The simple reduction of 1-4, 6,
and 7 with potassium, and of 5 with magnesium, yielded the new series of dipnictenes, 2,6-Mes2H3C6E)EC6H3-
2,6-Mes2 (E ) As, 8; Sb, 9; Bi, 10) and 2,6-Trip2H3C6E)EC6H3-2,6-Trip2 (E ) P, 12; As, 13; Sb, 14a; Bi,
15), as well as the partially reduced species 2,6-Trip2H3C6(Cl)SbSb(Cl)C6H3-2,6-Trip2 (14b). The compounds,
which displayed high thermal stability, were characterized by 1H, 13C, and 31P NMR and UV-vis spectroscopy.
The structures of 8-11, 13, 14a, and 14b were determined. These compounds constitute the first homologous
series of dipnictene structures for all the heavier group 15 elements. The E-E bond shortenings observed for
the heaviest antimony or bismuth derivatives lead to the conclusion that π overlap is quite important in the
fifth- and sixth-period elements of this group.
Introduction
recently, stable-multiple-bonding character in the heaviest
elements, antimony and bismuth, was represented only by a few
The first stable diphosphene that featured an unsupported
phosphorus-phosphorus double bond was reported in 1981 by
Yoshifuji and co-workers.1 This compound, Mes*PdPMes*
(Mes* ) C6H2-2,4,6-tBu3), represented a significant landmark
in the study of multiple bonding in the heavier main group
elements. In the ensuing period, structural data for approximately
20 organo diphosphenes have been published.2 In addition, there
are numerous examples of metallodiphosphenes, in which the
organic group at phosphorus is replaced by a transition-metal
moiety.3 Also, there are several related complexes in which an
organosubstituted diphosphene with an unsupported phosphorus-
phosphorus bond behaves as an end-on, lone-pair electron donor
through one or both phosphorus atoms to a transition-metal
fragment.4 In contrast, there are only two published structures
(2) (a) Cowley, A. H.; Kilduff, J. E.; Newman, T. H.; Pakulski, M. J.
Am. Chem. Soc. 1982, 104, 5820. (b) Escudie´, J.; Couret, C.; Andriamizaka,
J. D.; Satge´, J. J. Organomet. Chem. 1982, 228, C76. (c) Bertrand, G.;
Couret, C.; Escudie´, J.; Majid, S.; Majoral, J. P. Tetrahedron Lett. 1982,
23, 3567. (d) Couret, C.; Escudie´, J.; Satge´, J. Tetrahedron Lett. 1982, 23,
4941. (e) Niecke, E.; Ruger, R. Angew. Chem. 1983, 95, 154. (f) Smit, C.
N.; van der Kaap, Th. A.; Bickelhaupt, F. Tetrahedron Lett. 1983, 24, 2031.
(g) Cowley, A. H.; Kilduff, J. E.; Mehrotra, S. K.; Norman, N. C.; Pakulski,
M. J. Chem. Soc., Chem. Commun. 1983, 528. (h) Jaud, J.; Couret, C.;
Escudie´, J. J. Organomet. Chem. 1983, 249, C25. (i) Niecke, E.; Ruger,
R.; Lysek, M.; Pohl, S.; Schoeller, W. Angew. Chem., Int. Ed. Engl. 1983,
22, 486. (j) Cowley, A. H.; Kilduff, J. E.; Lasch, J. G.; Mehrotra, S. K.;
Norman, N. C.; Pakulski, M.; Whittlesey, B. R.; Atwood, J. L.; Hunter, W.
E. Inorg. Chem. 1984, 23, 2582. (k) Jutzi, P.; Meyer, U.; Krebs, B.;
Dartmann, M. Angew. Chem., Int. Ed. Engl. 1986, 25, 919. (l) Niecke, E.;
Kramer, B.; Nieger, M. Angew. Chem., Int. Ed. Engl. 1989, 28, 215. (m)
Cowley, A. H.; Knuppel, P. C.; Nunn, C. M. Organometallics 1989, 8,
2490. (n) Busch, T.; Schoeller, W. W.; Niecke, E.; Nieger, M.; Westermann,
H. Inorg. Chem. 1989, 28, 4334. (o) Hanssgen, D.; Aldenhoven, H.; Nieger,
M. J. Organomet. Chem. 1989, 375, C9. (p) Chernega, A. N.; Antipin, M.
Yu.; Struchkov, Yu. T.; Romanenko, V. D. Zh. Strukt. Khim. 1990, 31,
108. (q) Niecke, E.; Altmeyer, O.; Nieger, M. Angew. Chem., Int. Ed. Engl.
1991, 30, 1136. (r) Yoshifuji, M.; Abe, M.; Toyota, K.; Miyahara, I.; Hirotsu,
K. Bull Chem. Soc. Jpn. 1993, 66, 3831. (s) Zanin, A.; Karnop, M.; Jeske,
J.; Jones, P. G.; du Mont, W.-W. J. Organomet. Chem. 1994, 475, 95. (t)
Weber, L. Chem. ReV. 1992, 92, 1839 and references therein. (u) Lu¨bben,
T.; Roesky, H. W.; Gornitzka, H.; Steiner, A.; Stalke, D. Eur. J. Solid State
Inorg. Chem. 1995, 32, 121. (v) Cowley, A. H.; Decken, A.; Norman, N.
C.; Kruger, C.; Lutz, F.; Jacobsen, H.; Ziegler, T. J. Am. Chem. Soc. 1997,
119, 3389. (w) Yoshifuji, M. J. Chem. Soc., Dalton Trans. 1998, 3343 and
references therein.
of diarsenes,5 i.e., those of Mes*AsAsCH(SiMe3)2 and [As-
5a
{C(SiMe3)3}]2 by Cowley and co-workers.5c Similarly, there
exists just a handful of structurally characterized unsymmetrical
species featuring PdAs double bonds,6 e.g., Mes*PAsCH-
(SiMe3)26a and {(η5-C5H5)(CO)2Fe}{(CO)5Cr}AsPMes*.6e Until
* To whom correspondence should be addressed. Tel.: (530) 752-6913.
Fax: (530) 752-8995. E-mail: pppower@ucdavis.edu.
† University of California, Davis.
‡ University of California, Berkeley.
(1) Yoshifuji, M.; Shima, I.; Inamoto, N.; Hirotsu, K.; Higuchi, T. J.
Am. Chem. Soc. 1981, 103, 4587.
10.1021/ja983999n CCC: $18.00 © 1999 American Chemical Society
Published on Web 03/30/1999

3358 J. Am. Chem. Soc., Vol. 121, No. 14, 1999
Twamley et al.
transition-metal complexes7,8 such as {RSb}2{W(CO)5}3 (R )
tBu, Me, Ph)7a or {Bi}2{W(CO)5}3,8a in which the multiple bond
was supported by metal bridging. Apart from the above-
mentioned phosphaarsenes, compounds having double bonding
between two different heavier pnictogens were confined to two-
solution species featuring phosphorus-antimony double
bonds,2k,6a,9 i.e., Mes*PdSbCH(SiMe3)2 and Mes*PdSbN-
strated that bulky m-terphenyl ligands, e.g., -C6H3-2,6-Mes2,
could be used to stabilize diphosphene derivatives,11 and recently
it has been shown that related m-terphenyl ligands can effect
the stabilization of unsymmetric dipnictenes such as 2,6-Mes2-
4-MeH2C6PdAsC6H2-2,6-Mes2-4-Me6h and MesPdEC6H3-2,6-
Trip2, (E ) As or Sb; Mes ) C6H2-2,4,6-Me3).12 It is now
shown that this type of ligand is generally applicable to the
stabilization of the first complete homologous series of double
bonds involving all the heavier group 15 elements. Moreover,
it is shown that such compounds are accessible in acceptable
yields by the simple and straightforward direct reduction of their
halide precursors.
t
(SiMe2 Bu)2, which, however, decomposed at room temperature.
Nonetheless, Okazaki, Tokitoh, and co-workers have shown that
stable examples of distibenes and dibismuthenes with unsup-
ported SbdSb or BidBi double bonds can be isolated by using
the very crowding -C6H2-2,4,6-{CH(SiMe3)2}3 ligand. A
synthetic route which involves deselenation of the six-membered
[E(Se)(C6H2-2,4,6-{CH(SiMe3)2}3)]3 (E ) Sb or Bi) precursor
was used to isolate these species.10 Parallel work has demon-
Experimental Section
General Procedures. All manipulations were carried out by using
modified Schlenk techniques under an atmosphere of N2 or in a Vacuum
Atmospheres HE 553 drybox. All solvents were distilled from a Na/K
alloy and degassed twice immediately before use. The compounds
LiC6H3-2,6-Mes2,13a Et2O‚LiC6H3-2,6-Trip2,13b and KC814 were prepared
according to literature procedures. Commercial AsCl3 and SbCl3 were
distilled before use, and commercial anhydrous BiCl3 was used as
received. 1H and 13C NMR data were recorded on a GE 300 MHz
instrument and referenced to the deuterated solvent (C6D6). Infrared
spectral data were recorded on a Perkin-Elmer PE-1430 instrument.
UV-vis spectral data were recorded on a Hitachi U 2000 instrument.
2,6-Mes2H3C6AsCl2 (1). A solution of LiC6H3-2,6-Mes2 (2.30 g, 7.20
mmol) in pentane/Et2O (2:1, 60 mL) was added dropwise (10 min) to
a stirred solution of AsCl3 (1.30 g, 7.20 mmol, freshly distilled) in
pentane/Et2O (2:1, 30 mL) with cooling to ca. -78 °C. The low
temperature was maintained for 2 h, and the mixture was allowed to
come to room temperature overnight. The solvent was removed under
reduced pressure, and the residue was extracted with C6H6 (2 × 100
mL) and filtered (Celite). The solvent was concentrated to approximately
40 mL and reheated to redissolve any precipitate. Cooling to room
temperature afforded colorless crystals of 1. The supernatant liquid was
separated, and its volume was reduced further to incipient crystalliza-
tion; combined yield 1.89 g, 57%; Mp: 206-208 °C; IR ν(As-Cl) 380,
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1
370 s, br cm-1; H NMR (300 MHz, C6D6) δ 2.10 (s, 12H, o-CH3),
2.14 (s, 6H, p-CH3), 6.79 (d, 2H, m-C6H3) 3JHH ) 7.5 Hz, 6.80 (s, 4H,
m-Mes), 7.09 (t, 1H, p-C6H3) 3JHH ) 7.5 Hz; 13C{1H} NMR (75 MHz,
C6D6) δ 21.18 (p-CH3), 21.51 (o-CH3), 128.61 (m-Mes), 130.70 (m-
C6H3), 132.53 (p-C6H3), 135.41 (p-Mes), 136.89 (o-Mes), 138.21 (i-
Mes), 146.71 (o-C6H3).
2,6-Mes2H3C6SbCl2 (2). A solution of LiC6H3-2,6-Mes2 (2.55 g, 7.96
mmol) in hexane/Et2O (1:1, 60 mL) was added dropwise (15 min) to
a stirred suspension of SbCl3 (1.82 g, 7.96 mmol) in hexane/Et2O (1:1,
20 mL) at -78 °C. The reaction mixture was kept at ca. -78 °C (3 h)
before it was allowed to come to room temperature, whereupon it was
stirred for a further 2.5 d. The solvent was then removed under reduced
pressure, and the residue was extracted with hexane (60 mL). Filtration
at 60 °C through Celite afforded a pale yellow solution. The solvent
volume was reduced slightly to incipient crystallization and was cooled
to afford the product 2 as pale yellow crystals: 1.72 g, 43%; mp: 163-
1
165 °C; IR ν(Sb-Cl) 340, 320 s, br cm-1; H NMR (300 MHz, C6D6) δ
2.09 (s, 12H, o-CH3), 2.14 (s, 6H, p-CH3), 6.79 (s, 4H, m-Mes), 6.87
3
3
(d, 2H, m-C6H3) JHH ) 7.8 Hz, 7.13 (t, 1H, p-C6H3) JHH ) 7.8 Hz;
13C{1H} NMR (75 MHz, C6D6) δ 21.19 (p-CH3), 21.51 (o-CH3), 128.82
(10) (a) Tokitoh, N.; Arai, Y.; Sasamori, T.; Okazaki, R.; Nagase, S.;
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117.

Homologous Series of HeaVier Element Dipnictenes
J. Am. Chem. Soc., Vol. 121, No. 14, 1999 3359
(m-Mes), 130.37 (m-C6H3), 131.98 (p-C6H3), 136.39 (p-Mes), 137.14
(o-Mes), 138.43 (i-Mes), 148.52 (o-C6H3).
o-CH(CH3)2) 3JHH ) 6.6 Hz, 7.11 (t, 1H, p-C6H3) 3JHH ) 6.9 Hz, 7.19
3
(s, 4H, m-Trip), 7.23 (d, 2H, m-C6H3) JHH ) 6.9 Hz; 13C{1H} NMR
(75 MHz, C6D6) δ 23.07 (o-CH(CH3)2), 24.27 (p-CH(CH3)2), 26.06
(o-CH(CH3)2), 31.32 (o-CH(CH3)2), 34.81 (p-CH(CH3)2), 121.25 (m-
Trip), 129.69 (p-C6H3), 131.84 (m-C6H3), 134.51 (i-Trip), 146.97 (p-
Trip), 147.98 (o-Trip), 150.46 (o-C6H3), 152.95 (i-C6H3).
2,6-Mes2H3C6BiCl2 (3). A solution of LiC6H3-2,6-Mes2 (2.30 g, 7.20
mmol) in toluene/Et2O (1:1, 60 mL) was added dropwise (15 min) to
a stirred suspension of BiCl3 (2.26 g, 7.20 mmol) in toluene/Et2O (1:1,
50 mL) at -78 °C. The reaction mixture was kept at ca. -78 °C for 2
h before it was allowed to come to room temperature and stirred
overnight. The solvent was then removed under reduced pressure, and
the residue was extracted with hot (70 °C) benzene (70 mL) and filtered
(Celite) to give a pale yellow solution. The solvent volume was reduced
slightly, and cooling at ca. 6 °C overnight yielded pale yellow X-ray
diffraction quality crystals of 3: 2.79 g, 65%; mp 220-224 °C; IR
2,6-Trip2H3C6BiCl2 (7). A solution of Et2O‚LiC6H3-2,6-Trip2 (2.00
g, 3.55 mmol) in Et2O (40 mL) was added dropwise (10 min) to a
stirred suspension of BiCl3 (1.12 g, 3.55 mmol) in Et2O (30 mL) at ca.
-78 °C. The mixture was stirred at ca. -78 °C (1 h) before the solution
was allowed to come to room temperature. The solvent was then
removed under reduced pressure, and the residue was extracted with
toluene (60 mL) and filtered (Celite) to give a clear yellow solution.
Reduction of the solvent to ca. 20 mL and cooling in a ca. -20 °C
freezer overnight yielded 7 as X-ray quality pale yellow crystals:
combined yield 1.91 g, 71%; mp 255-257 °C; IR ν(Bi-Cl) 310, 295 s,
br cm-1; 1H NMR (300 MHz, C6D6) δ 1.05 (d, 12H, o-CH(CH3)2) 3JHH
ν
(Bi-Cl) 280, 240 s, br cm-1; 1H NMR (300 MHz, C6D6) δ 2.13 (s, 12H,
o-CH3), 2.14 (s, 6H, p-CH3), 6.80 (s, 4H, m-Mes), 7.23(t, 1H, p-C6H3)
3JHH ) 7.8 Hz, 7.42(d, 2H, m-C6H3) 3JHH ) 7.8 Hz; 13C{1H} NMR (75
MHz, C6D6) δ 21.17 (p-CH3), 21.46 (o-CH3), 128.92 (m-Mes), 130.30
(p-C6H3), 135.06 (m-C6H3), 136.13 (p-Mes), 137.35 (o-Mes), 138.54
(i-Mes), 150.13 (o-C6H3).
3
) 6.6 Hz, 1.25 (d, 12H, p-CH(CH3)2) JHH ) 6.9 Hz, 1.38 (d, 12H,
3
3
o-CH(CH3)2) JHH ) 6.9 Hz, 2.83 (septet, 2H, p-CH(CH3)2) JHH
)
2,6-Trip2H3C6PCl2 (4). A solution of Et2O‚LiC6H3-2,6-Trip2 (2.00
g, 3.55 mmol) in hexane (50 mL) was added dropwise (15 min) to a
stirred solution of freshly distilled PCl3 (488 mg, 3.55 mmol) in hexane
(30 mL) at -78 °C. The mixture was stirred (1 h) and gradually allowed
to come to room temperature overnight. The supernatant liquid was
filtered (60 °C, Celite), and the solution was concentrated (ap-
proximately 40 mL) before cooling in a -20 °C freezer for 20 h.
Colorless crystals of 4 were obtained: yield 1.55 g, 75%; mp sweats
3
6.9 Hz, 2.99 (septet, 4H, o-CH(CH3)2) JHH ) 6.6 Hz, 7.21 (s, 4H,
3
m-Trip), 7.22 (t, 1H, p-C6H3) JHH ) 7.5 Hz, 7.77 (d, 2H, m-C6H3)
3JHH ) 7.5 Hz; 13C{1H} NMR (75 MHz, C6D6) δ 22.76 (o-CH(CH3)2),
23.62 (o-CH(CH3)2), 25.59 (p-CH(CH3)2), 30.66 (o-CH(CH3)2), 34.22
(p-CH(CH3)2), 120.83 (m-Trip), 128.32 (p-C6H3), 133.83 (i-Trip),
136.15 (m-C6H3), 147.75 (o-Trip), 148.14 (p-Trip), 150.01 (o-C6H3),
214.63 (i-C6H3).
at 239 °C and melts from 245 to 247 °C; IR ν(P-Cl) 480, 442 s, br cm-1
;
2,6-Mes2H3C6AsdAsC6H3-2,6-Mes2 (8). A solution of 1 (1.50 g,
3.27 mmol) in hexane (50 mL) was added dropwise (15 min) to a stirred
suspension of finely divided potassium (260 mg, 6.53 mmol) in hexane
(20 mL) at room temperature. An immediate reaction was observed,
and the solution became a pale yellow color. Stirring overnight afforded
a deep orange solution with some unreacted potassium. The solution
was filtered (medium frit), and the solvent was concentrated to ca. 20
mL. Cooling in a ca. -20 °C freezer overnight yielded X-ray diffraction
quality yellow crystals of 8: yield 0.365 g, 30%; mp 181-186 °C; 1H
NMR (300 MHz, C6D6) δ 1.92 (s, 24H, o-CH3), 2.23 (s, 12H, p-CH3),
6.74 (s, 8H, m-Mes), 6.89 (d, 4H, m-C6H3) 3JHH ) 7.5 Hz, 7.08 (t, 2H,
1H NMR (300 MHz, CDCl3) δ 1.08 (d, 12H, p-CH(CH3)2) 3JHH ) 6.9
3
Hz, 1.25 (d, 12H, o-CH(CH3)2) JHH ) 6.9 Hz, 1.38 (d, 12H, o-CH-
(CH3)2) 3JHH ) 6.6 Hz, 2.83 (septet, 6H, o-CH(CH3)2) 3JHH ) 6.9 Hz,
7.04 (t, 1H, p-C6H3), 7.11 (d, 2H, m-C6H3) J ) 7.35 Hz, 7.2 (s, 4H);
13C{1H} NMR (75 MHz, CDCl3) δ 22.64 (o-CH(CH3)2), 24.10 (p-CH-
(CH3)2), 25.68 (o-CH(CH3)2), 31.09 (o-CH(CH3)2), 34.23 (p-CH(CH3)2),
120.49 (m-Trip), 130.57 (p-C6H3), 131.69 (m-C6H3), 133.66 J ) 8.3
Hz (i-C6H3), 136.15 (i-Trip), 145.09 (p-Trip), 146.69 (o-Trip), 148.99
(o-C6H3); 31P{1H}NMR (121 MHz, CDCl3) δ 156.47 (referenced to
H3PO4).
3
p-C6H3) JHH ) 7.5 Hz; 13C{1H} NMR (75 MHz, C6D6) δ 21.17 (p-
2,6-Trip2H3C6AsCl2 (5). A solution of Et2O‚LiC6H3-2,6-Trip2 (2.00
g, 3.55 mmol) in hexane/Et2O (2:1, 40 mL) was added dropwise (15
min) to a stirred solution of freshly distilled AsCl3 (644 mg, 3.55 mmol)
in hexane/Et2O (2:1, 20 mL) with cooling in a dry ice/acetone bath at
ca. -78 °C. Immediate precipitation of LiCl occurred. After the
addition, the mixture was kept at -78 °C (3 h) and stirred overnight
with gradual warming to room temperature. The solvent was removed
under reduced pressure, and the residue was extracted with hexane (3
× 100 mL), heated to 60 °C, and filtered through Celite to give a clear
colorless solution. The volume was reduced slightly and upon cooling
in a ca. -20 °C freezer, 5 was obtained as X-ray diffraction quality
colorless crystals. The supernatant liquid was isolated, and the volume
was reduced to incipient crystallization: combined yield 1.65 g, 74%;
mp sweats at 241 °C, melts at 259-260 °C; IR ν(As-Cl) 380, 360 s, br
CH3), 21.28 (o-CH3), 128.62 (m-Mes), 129.34 (m-C6H3), 130.40 (p-
C6H3), 135.64 (o-Mes), 136.49 (p-Mes), 139.60 (i-Mes), 144.97 (o-
C6H3); UV-vis (benzene) 462 nm (650), 400 nm (6970).
2,6-Mes2H3C6SbdSbC6H3-2,6-Mes2 (9). A solution of 2 (1.50 g,
2.96 mmol) in pentane (90 mL) was added to a stirred suspension of
finely divided potassium (232 mg, 5.93 mmol) in pentane (10 mL).
The supernatant liquid developed a dark orange-red color, and a green
precipitate appeared. The mixture was stirred (3 d) and the solvent was
removed under reduced pressure. The green residue was extracted with
C6H6 (100 mL) and filtered at ca. 60 °C through Celite. The resulting
dark orange-red solution was concentrated (approximately 40 mL), and
on standing overnight at room temperature, X-ray diffraction quality
orange cystals of the product 9 were obtained: yield 0.340 g, 26%;
1
3
1
cm-1; H NMR (300 MHz, C6D6) δ 1.07 (d, 12H, o-CH(CH3)2) JHH
mp sweats at 170 °C and melts at 200-210 °C; H NMR (300 MHz,
3
) 6.9 Hz, 1.25 (d, 12H, p-CH(CH3)2) JHH ) 6.9 Hz, 1.38 (d, 12H,
C6D6) δ 2.02 (s, 24H, o-CH3), 2.23 (s, 12H, p-CH3), 6.74 (s, 8H,
3
3
3
o-CH(CH3)2) JHH ) 6.9 Hz, 2.84 (septet, 2H, p-CH(CH3)2) JHH
)
m-Mes), 6.97 (d, 4H, m-C6H3) JHH ) 7.5 Hz, 7.10 (t, 2H, p-C6H3)
6.9 Hz, 2.897 (septet, 4H, o-CH(CH3)2) 3JHH ) 6.9 Hz, 7.14-7.16 (m,
3H, p-C6H3, m-C6H3) 7.198 (s, 4H, m-Trip); 13C{1H} NMR (75 MHz,
C6D6) δ 23.01 (o-CH(CH3)2), 24.27 (p-CH(CH3)2), 25.94 (o-CH(CH3)2),
31.49 (o-CH(CH3)2), 34.77 (p-CH(CH3)2), 121.06 (m-Trip), 130.25 (p-
C6H3), 132.11 (m-C6H3), 133.40 (i-Trip), 142.89 (i-C6H3), 145.25 (p-
Trip), 147.73 (o-Trip), 150.14 (o-C6H3).
3JHH ) 7.5 Hz; 13C{1H} NMR (75 MHz, C6D6) δ 21.34 (p-CH3), 21.67
(o-CH3), 127.86 (p-C6H3), 128.27 (m-C6H3), 128.81 (m-Mes), 135.46
(o-Mes), 136.59 (p-Mes), 141.35 (i-Mes), 148.36 (o-C6H3); UV-vis
(benzene) 451 nm (5450).
2,6-Mes2H3C6BidBiC6H3-2,6-Mes2 (10). A solution of 3 (1.00 g,
1.77 mmol) in toluene/hexane (3:1, 60 mL) was added dropwise (10
min) to a stirred suspension of finely divided potassium (150 mg, 3.54
mmol) in toluene/hexane (3:1, 10 mL) at ca. -78 °C. The mixture
was kept at ca. -78 °C for 6 h before gradual warming to room
temperature overnight, during which time the supernatant liquid
developed a deep purple color. The mixture was stirred for a further 3
d, and the solvent was removed under reduced pressure. The residue
was extracted with C6H6 (60 mL), filtered (fine frit), and concentrated
to ca. 20 mL. On standing at room temperature for 0.5 h, red crystals
of the product 10 were obtained: yield 0.280 g, 30%; mp darkens at
100 °C and melts at 215-219° dec; 1H NMR (300 MHz, C6D6) δ 2.11
(s, 24H, o-CH3), 2.29 (s, 12H, p-CH3), 6.77 (s, 8H, m-Mes), 6.92 (t,
2,6-Trip2H3C6SbCl2 (6). Compound 6 was synthesized using the
same procedure outlined above for 2. A solution of Et2O‚LiC6H3-2,6-
Trip2 (2.00 g, 3.55 mmol) in hexane/Et2O (2:1, 40 mL) was added
dropwise (15 min) to a stirred solution of freshly distilled SbCl3 (0.81
g, 3.55 mmol) in hexane/Et2O (2:1, 20 mL) with cooling in a dry ice/
acetone bath at ca. -78 °C. After workup, colorless crystalline X-ray
diffraction quality 6 was obtained in a combined yield of 1.68 g, 70%:
mp 255-259 °C; IR ν(Sb-Cl) 345, 330 s, br cm-1; 1H NMR (300 MHz,
C6D6) δ 1.06 (d, 12H, o-CH(CH3)2) 3JHH ) 6.6 Hz, 1.25 (d, 12H, p-CH-
3
3
(CH3)2) JHH ) 6.9 Hz, 1.37 (d, 12H, o-CH(CH3)2) JHH ) 6.6 Hz,
3
2.83 (septet, 2H, p-CH(CH3)2) JHH ) 6.9 Hz, 2.94 (septet, 4H,

3360 J. Am. Chem. Soc., Vol. 121, No. 14, 1999
Twamley et al.
2H, p-C6H3) 3JHH ) 7.5 Hz, 7.13 (d, 4H, m-C6H3) 3JHH ) 7.5 Hz; 13C-
{1H} NMR (75 MHz, C6D6) δ 21.198 (p-CH3), 22.58 (o-CH3), 126.396
(m-C6H3), 128.306 (p-C6H3), 129.09 (m-Mes), 135.79 (o-Mes), 136.62
(p-Mes), 144.65 (i-Mes), 153.24 (o-C6H3); UV-vis (benzene) 970 nm
(27), 505 nm (5012).
90%) and 2,6-Trip2H3C6 (Cl)Sb-Sb(Cl)C6H3-2,6-Trip2, 14b, by X-ray
crystallography. Method b. A solution of 6 (2.40 g, 3.56 mmol) in
C6H6 (100 mL) was added to finely divided potassium (0.278 g, 7.12
mmol). The pale yellow solution immediately darkened to orange. The
mixture was stirred (3 d), and the red solution was filtered through
Celite. The solvent was removed under reduced pressure, leaving a
green residue (14a): yield 2.74 g, 81%; mp sweats and darkens at 222
1-(2,4,6-Triisopropylphenyl)-5,7,9-triisopropyl-9-phosphafluo-
rene (11). THF (50 mL) was added to a mixture of 4 (1.00 g, 1.71
mmol) and Mg (42 mg, 1.71 mmol). There was no apparent reaction,
and the solution remained colorless. The mixture was stirred (24 h)
during which time it became a bright golden yellow color. After further
stirring (12 h) the solution lightened to pale yellow. The solvent was
removed under reduced pressure, and the residue was extracted with
hexane (60 mL) and filtered (Celite). The solution was concentrated
to ca. 15 mL before cooling in a ca. -20 °C freezer. Colorless X-ray
diffraction quality crystals of 11 were obtained after 2 d: yield 0.600
g, 68%; mp sweats at 132 °C and melts at 149-152 °C; 1H NMR (300
MHz, CDCl3) δ 0.25 (t, 2H, PCH(CH3)2, J ) 6.6 Hz), 0.92-1.52 (m,
30H, CH(CH3)2), 1.78 (m, 1H, PCH(CH3)2, JHH ) 7 Hz, JHP ) 2.2
Hz), 2.57 (septet, 1H, CH(CH3)2), 2.696 (septet, 1H, CH(CH3)2), 3.01
(septet, 2H, o-Trip CH(CH3)2), 4.07 (septet, 1H, o-CH(CH3)2), 7.10
1
°C and melts at 280-282 °C; H NMR (300 MHz, C6D6) δ 1.06 (d,
3
3
24H, o-CH(CH3)2) JHH ) 6.9 Hz, 1.08 (d, 24H, o-CH(CH3)2) JHH
)
6.6 Hz, 1.31 (d, 24H, p-CH(CH3)2) 3JHH ) 6.0 Hz, 2.83-2.99 (m, 12H,
o,p-CH(CH3)2), 7.10 (s, 8H, m-Trip), 7.14-7.16 (m, 6H, p-C6H3,
m-C6H3); 13C{1H} NMR (75 MHz, C6D6) δ 24.28 (o-CH(CH3)2), 26.09
(p-CH(CH3)2), 31.16 (o-CH(CH3)2), 34.53 (p-CH(CH3)2), 121.66 (m-
Trip), 127.72 (p-C6H3), 129.94 (m-C6H3), 140.06 (i-Trip), 146.00 (o-
Trip), 148.25 (p-Trip), 148.61 (o-C6H3), 150.57 (i-C6H3); UV-vis
(hexane) 470 nm (6570).
2,6-Trip2H3C6BidBiC6H3-2,6-Trip2 (15). A solution of 7 (1.00 g,
1.31 mmol) in hexane/toluene (3:1, 80 mL) was added to a stirred
suspension of finely divided potassium (103 mg, 2.63 mmol), and the
mixture was stirred overnight. The solution developed a deep purple
color, and all the potassium was consumed. The solution was filtered
(fine frit) and concentrated to ca. 40 mL under reduced pressure.
Cooling in a -20 °C freezer overnight afforded deep purple crystalline
(d, 1H, H8 JHP ) 1.8 Hz), 7.16 (s, 2H, m-Trip), 7.33 (d, 1H, H6 JHP
)
1.5 Hz), 7.38 (dd, 1H, H2, J ) 5.7, 1.8 Hz), 7.50 (t, 1H, H3, JHH ) 7.5
Hz), 8.16 (d, 1H, H4, JHH ) 8.4 Hz); 31P{1H} NMR (121 MHz, C6D6)
δ -3.08.
1
15: yield 0.90 g, 83%; mp >250 °C dec; H NMR (300 MHz, C6D6)
3
2,6-Trip2H3C6PdPC6H3-2,6-Trip2 (12). A solution of 4 (470 mg,
0.81 mmol) in hexane (50 mL) was added to freshly divided potassium
(63 mg, 1.60 mmol). The initially colorless solution rapidly developed
a yellow color. The mixture was stirred for 12 h, during which time
the color deepened to brown. The supernatant liquid was filtered (fine
frit), and the solvent was removed to afford a deep orange residue.
NMR analysis of this product showed it to be a mixture of 12 and 11.
The crude product was chromatographed (SiO2, hexane, N2 atm),
yielding 12 with trace amounts of 11: yield ca. 0.09 g, 20%; mp sweats
δ 1.09 (d, 24H, p-CH(CH3)2) JHH ) 6.6 Hz, 1.23 (d, 24H, o-CH-
3
3
(CH3)2) JHH ) 6.9 Hz, 1.36 (d, 24H, o-CH(CH3)2) JHH ) 6.9 Hz,
3
2.86 (septet, 8H, o-CH(CH3)2) JHH ) 6.9 Hz, 2.94 (septet, 4H,
o-CH(CH3)2) 3JHH ) 6.6 Hz, 6.88 (t, 2H, p-C6H3) 3JHH ) 7.5 Hz, 7.13
(s, 8H, m-Trip), 7.29 (d, 4H, m-C6H3) JHH ) 7.5 Hz; 13C{1H} NMR
3
(75 MHz, C6D6) δ 24.10 (p-CH(CH3)2), 25.73 (o-CH(CH3)2), 26.03
(o-CH(CH3)2), 30.84 (o-CH(CH3)2), 33.97 (p-CH(CH3)2), 121.04 (m-
Trip), 127.51 (m-C6H3), 127.93 (p-C6H3), 142.33 (i-Trip), 146.29 (o-
Trip), 148.19 (p-Trip), 153.09 (o-C6H3); UV-vis (hexane) 1025 nm
(80), 913 nm (200), 518 nm (6920).
1
at 128 °C and melts at 160-165 °C; H NMR (300 MHz, C6D6) δ
1.21 (d, 24H, o-CH(CH3)2) 3JHH ) 6.9 Hz, 1.36 (d, 24H, p-CH(CH3)2)
X-ray Crystallographic Studies. The crystals were removed from
the Schlenk tube under a stream of N2 and immediately covered with
a layer of hydrocarbon oil. A suitable crystal was selected, attached to
a glass fiber, and immediately placed in the low-temperature nitrogen
stream.15a All data were collected near 130 K using the following
instruments: a Siemens R3 (Mo KR radiation, 5, 8, and 10), a Syntex
P21 (Cu KR radiation, 3b, 11, and 13), a Siemens P4-RA (Cu KR
radiation, 14), and a Siemens SMART system (Mo KR radiation and
a CCD area detector, 3a, 6, and 9). In the case of 3b, 5, 8-11, 13, and
14, the diffractometers were equipped with a locally modified Enraf-
Nonius universal low-temperature device for low-temperature work.
The SHELXTL version 5.03 program package was used for the structure
solutions and refinements.15b An absorption correction was applied using
the program XABS2.15c For 3a and 9 the programs SIR9216a and
teXsan,16b respectively, were used for solution and refinement. An
absorption correction was applied using XPREP.16c SHELXTL, version
5.03 was used for the refinement of 6, and an absorption correction
was applied using SADABS.16d The crystal structures were solved by
direct methods and refined by full-matrix least-squares procedures. All
non-hydrogen atoms were refined anisotropically, except those involved
in disorder or in 3b (due to absorption effects) which were maintained
isotropic. Hydrogen atoms in 3b, 5, 6, 8-11, 13, and 14 were included
in the refinement at calculated positions using a riding model included
in the SHELXTL program. Some hydrogen atoms in 3a and 9 were
located on the Fourier difference maps; the rest were placed in
calculated positions and were not refined. The structures of 5, 6, and
3
3JHH ) 6.6 Hz, 1.59 (d, 24H, o-CH(CH3)2) JHH ) 6.9 Hz, 3.00 (m,
6H, o,p-CH(CH3)2), 3.14 (m, 6H, o,p-CH(CH3)2), 7.34 (s, 8H, m-Trip),
7.44 (d, 4H, m-C6H3); 13C{1H} NMR (75 MHz, C6D6) δ 23.85 (o-CH-
(CH3)2), 24.35 (p-CH(CH3)2), 25.68 (o-CH(CH3)2), 31.20 (o-CH(CH3)2),
34.61 (p-CH(CH3)2), 121.54 (m-Trip), 129.07 (p-C6H3), 131.29 (m-
C6H3), 137.93 (i-Trip), 144.17 (p-Trip), 146.41 (o-Trip), 148.27 (o-
C6H3); 31P{1H}NMR (121 MHz, C6D6) δ 521.36 (referenced to H3PO4);
UV-vis (benzene) 502 nm (570), 393 nm (7270).
2,6-Trip2H3C6AsdAsC6H3-2,6-Trip2 (13). A mixture of 5 (1.00 g,
1.59 mmol) and magnesium (40 mg, 1.59 mmol) were stirred (2.5 d)
in THF (40 mL). The initially colorless solution became deep orange,
and all the magnesium was consumed. The solvent was removed under
reduced pressure, and the residue was extracted with hexane (60 mL),
stirred overnight, and filtered (Celite). The deep orange solution was
reduced in volume to ca. 30 mL, and cooling in a -20 °C freezer for
several days afforded 13 as X-ray diffraction quality orange crystals:
1
combined yield 0.706 g, 80%; mp 256-260 °C; H NMR (300 MHz,
C6D6) δ 0.99 (d, 24H, o-CH(CH3)2) 3JHH ) 6.9 Hz, 1.07 (d, 24H, o-CH-
3
3
(CH3)2) JHH ) 6.9 Hz, 1.32 (d, 24H, p-CH(CH3)2) JHH ) 6.9 Hz,
2.79-2.91 (m, 12H, o,p-CH(CH3)2), 6.99-7.02 (m, 6H, p-C6H3,
m-C6H3) 7.09 (s, 8H, m-Trip); 13C{1H} NMR (75 MHz, C6D6) 24.12
(o-CH(CH3)2), 24.32 (o-CH(CH3)2), 25.88 (p-CH(CH3)2), 31.18 (o-CH-
(CH3)2), 34.55 (p-CH(CH3)2), 121.66 (m-Trip), 127.79 (p-C6H3), 131.07
(m-C6H3), 138.33 (i-Trip), 144.87 (p-Trip), 146.17 (o-Trip), 148.27 (o-
C6H3), 151.91 (i-C6H3); UV-vis (hexane) 504 nm (570), 409 nm
(4060).
2,6-Trip2H3C6SbdSbC6H3-2,6-Trip2 (14a). Method a. A solution
of 6 (0.40 g, 0.59 mmol) in THF (40 mL) was added dropwise (10
min) to a stirred slurry of KC8 (0.16 g, 1.19 mmol) in THF (30 mL) at
ca. -78 °C. The solution turned a yellow color immediately. The
mixture was stirred at ca. -78 °C for 1.5 h, during which time the
solution became orange. After gradual warming to room temperature,
the mixture was stirred a further 3.5 d. Filtration, followed by
concentration of the orange solution to ca. 30 mL and cooling to ca.
-25 °C (2 weeks), gave a mixture of orange and colorless crystals.
The orange crystalline product was shown to be a mixture of 14a (80-
(15) (a) Hope, H. Prog. Inorg. Chem. 1995, 41, 1. (b) SHELXTL PC,
version 5.03; Siemens Analytical X-ray Instruments Inc.: Madison, WI,
1994. (c) Parkin, S. R.; Moezzi, B.; Hope, H. J. Appl. Crystallogr. 1995,
28, 53.
(16) (a) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, M.;
Giacovazzo, C.; Guagliardi, A.; Polidori, G. J. Appl. Crystallogr. 1993,
26, 343. (b) teXsan: Crystal Structure Analysis Package; Molecular
Structure Corporation: 1985 and 1992. (c) XPREP, version 5.03, part of
the SHELXTL Crystal Structure Determination Package; Siemens Industrial
Automation, Inc.: Madison, WI 1995. (d) SADABS (Siemens Area Detector
ABSorption correction program); George Sheldrick 1996.

Homologous Series of HeaVier Element Dipnictenes
J. Am. Chem. Soc., Vol. 121, No. 14, 1999 3361
Table 1. Selected Crystallographic Data for 3a‚0.5C6H6, 3b‚Et2O, 5, 6, 8-11, 13‚0.5n-C6H14, and 14‚n-C6H14
3a‚0.5C6H6
C27H28BiCl2
632.405
3b‚Et2O
5
6
formula
fw
C52H60Bi2OCl4
1260.76
C36H49AsCl2
627.57
C36H49SbCl2
674.40
color, habit
cryst syst
space group
a, Å
b, Å
c, Å
pale yellow, brick
monoclinic
P21/n
11.0928(3)
16.5753(5)
13.7509(4)
colorless, parallelepiped
monoclinic
P21
10.474(2)
17.278(4)
colorless, parallelepiped
monoclinic
P21/c
11.063(4)
12.079(4)
colorless, parallelepiped
orthorhombic
Pnma
25.651(5)
11.110(5)
13.870(3)
25.634(8)
12.087(5)
R, deg
â, deg
95.344(1)
92.04(3)
90.03(3)
γ, deg
V, Å3
2517.3(1)
4
2508.5(9)
2
3425(2)
4
3445(2)
4
Z
cryst dim, mm
0.16 × 0.18 × 0.25
0.40 × 0.30 × 0.20
0.30 × 0.10 × 0.10
0.27 × 0.18 × 0.07
d
calc, g cm-3
1.565
1.669
1.217
1.300
µ, mm-1
7.209
4651
3511 (I > 3σ(I))
0.024
15.832
3517
3328 (I > 2σ(I))
0.0605
1.170
7862
6277 (I > 2σ(I))
0.0599
0.978
15 954
3334 (I > 2σ(I))
0.0455
no. of refln
no. of obsd refln
R1 obsd refln
wR2 all
0.1530
0.1524
0.0959
8
9
10
11
formula
fw
color, habit
cryst syst
space group
a, Å
C48H50As2
776.72
pale yellow, parallelepiped
monoclinic
C2/m
9.821(2)
23.018(5)
C48H50Sb2
870.42
orange, brick
monoclinic
C2/m
9.9691(3)
22.8395(5)
8.7017(3)
C48H50Bi2
1044.84
red, hexagonal
monoclinic
C2/m
10.052(2)
22.784(5)
8.7387(17)
C36H49P
512.72
colorless, parallelepiped
triclinic
P1h
9.877(3)
13.185(6)
13.560(4)
62.54(3)
77.13(3)
83.66(3)
1527.5(9)
2
b, Å
c, Å
8.7090(17)
R, deg
â, deg
101.16(3)
99.6250(1)
99.00(3)
γ, deg
V, Å3
1931.5(7)
2
1953.39(8)
2
1976.7(7)
2
Z
cryst dim, mm
0.20 × 0.10 × 0.04
0.10 × 0.16 × 0.21
0.22 × 0.12 × 0.10
0.38 × 0.25 × 0.25
d
calc, g cm-3
1.335
1.48
1.755
1.115
µ, mm-1
1.763
1755
1032 (I > 2σ(I))
0.0967
1.413
3560
1628 (I > 3σ(I))
0.023
8.922
1811
1459 (I > 2σ(I))
0.0527
0.936
4055
2954 (I > 2σ(I))
0.0756
no. of refln
no. of obsd refn
R1 obsd refln
wR2 all
0.2475
0.1297
0.2252
13‚0.5n-C6H14
14‚n-C6H14
formula
fw
C75H105As2
1156.44
C78H110Cl0.42Sb2
1307.89
color, habit
cryst syst
space group
a, Å
b, Å
c, Å
bright orange, parallelepiped
monoclinic
P2/n
15.097(18)
12.420(13)
20.424(12)
red, parallelepiped
monoclinic
P21/c
13.129(11)
15.969(6)
17.397(5)
R, deg
â, deg
γ, deg
V, Å3
90.61(7)
104.06(5)
3829(6)
2
3538(3)
2
Z
cryst dim, mm
0.40 × 0.40 × 0.30
0.26 × 0.22 × 0.04
d
calc, g cm-3
1.003
1.228
µ, mm-1
1.326
6168
6.482
4461
no. of refln
no. of obsd refn
R1 obsd refln
wR2 all
4532 (I > 2σ(I))
3338 (I > 2σ(I))
0.1417
0.0908
0.4211
0.2386
13 were subject to disorder which could be modeled successfully using
partial occupancies for the disordered atoms as described in the
Supporting Information. The structure of 14 displays partial occupancy
of 2 species, the distibene 2,6-Trip2H3C6SbdSbC6H3-2,6-Trip2, 14a,
with Sb occupancies of 79%, and the partially reduced 1,2-dichloro-
distibane 2,6-Trip2H3C6(Cl)Sb-Sb(Cl)C6H3-2,6-Trip2, 14b, with Sb and
Cl occupancies of 21%. Some details of the data collection and
refinement are given in Table 1. Further details are provided in the
Supporting Information.
Results and Discussion
The synthesis and structural characterization of the stable
dipnictene compounds [EC6H2-2,4,6-{CH(SiMe3)2}3]2 (E )

3362 J. Am. Chem. Soc., Vol. 121, No. 14, 1999
Twamley et al.
Table 2. Selected Spectroscopic Data for Compounds 1-7
IR ν(E-Cl)
Sb10a or Bi10b), which have SbdSb and BidBi double bonded
distances of 2.642(1) and 2.8208(8) Å (cf. Sb-Sb and Bi-Bi
single bond, distances of 2.837 and 2.990(2) Å, respectively,
in E2Ph4 (E ) Sb17a or Bi17b) molecules), have added new vigor
to the study of multiple bonding of the heavier pnictogen
elements. Unlike related heavier main group 13 and 14
analogues of alkenes or alkynes, these compounds bear con-
siderable structural similarity to their lighter, congeneric di-
imines, diphosphenes, and diarsenes.10,18 For example, they
generally have a trans geometry featuring an essentially planar
C-E-E-C array as well as bond lengths that are ca. 6-7%
shorter than single bonds (cf. ca. 7-10% shortening in diphos-
phenes and diarsenes). These parameters suggest that, although
π bonding is weakened with increasing atomic number, it
remains quite appreciable in strength even in the fifth and sixth
periods. The only notable structural change going down the
group is that the angle at the pnictogen atom in dipnictenes
narrows from ca.. 113.6(2)° in PhNNPh19 to 102.8(9)° in
Mes*PPMes*1 and 100.5(2)° in 2,4,6-{(Me3Si)2CH}3H2C6Bid
BiC6H2-2,4,6-{CH(SiMe3)2}3.10a As a result it has been pro-
posed10,20 that the s-orbital character of the lone pairs also
increases with increasing atomic number such that at antimony
or bismuth the lone pairs have mostly s-orbital character.
Accordingly, the σ and π bonds comprising the SbdSb and
BidBi double bonds are mostly composed from p-orbitals. It
is noteworthy that the existence of significant π-orbital overlap
between the heaviest group members (at least in the group 15
elements) is contrary to one of the major justifications of the
so-called “double bond rule” which accounted for the weakness
of heavier main group multiple bonds in terms of greatly
diminished p overlap upon descending the group.21
The synthesis of the SbdSb and BidBi double-bonded
dipnictenes also highlights the fact that even though there exists
a fair amount of structural data for diimines and diphosphenes,
data for the rest of the heavier main group 15 elements analogues
are quite scant. The literature shows that, besides the above-
mentioned SbdSb and BidBi species, the structures of only
two diarsenes,5a,5c one phosphaarsene,6a one metallaphospha-
arsene,6e and the recently reported pair of unsymmetric species
MesPEC6H3-2,6-Trip2 (E ) As or Sb)12 are known at present.
To study the dipnictene series further, it was thought desirable
to have one or more complete series of heavier element
dipnictenes in which the stabilizing ligand remains unchanged
throughout the group. In addition, a simple synthetic route
(preferably with the use of one step and beginning with the
easily synthesized halide precursors) to the heaviest antimony
and bismuth derivatives would enable convenient accessibility
to these compounds as well as promote the study of their
chemistry. It was found that the use of the terphenyl ligands
-C6H3-2,6-Mes2 and -C6H3-2,6-Trip2 adequately enables the
first of these objectives to be achieved.
compound
mp (°C)
(cm-1
)
Cipso (ppm)
1
2
3
4
5
6
7
2,6-Mes2H3C6AsCl2 206-208 380, 370 n.o.
2,6-Mes2H3C6SbCl2 263-265 340, 320 n.o.
2,6-Mes2H3C6BiCl2 220-224 280, 240 n.o.
2,6-Trip2H3C6PCl2
245-247 480, 442 133.66 J ) 8.3 Hz
2,6-Trip2H3C6AsCl2 259-260 380, 360 142.89
2,6-Trip2H3C6SbCl2 255-259 345, 330 152.95
2,6-Trip2H3C6BiCl2 255-257 310, 295 214.63
a n.o. ) not observed.
element terphenyl compounds were reported only recently. The
compound 2,6-Mes2H3C6PCl2 was synthesized by the reaction
of the organolithium reagent with PCl3, and hydrolysis affords
the phosphinic acid derivative 2,6-Mes2H3C6P(O)(OH)H.11a
Reduction of the dihalide by magnesium with sonication affords
the diphosphene 2,6-Mes2H3C6PdPC6H3-2,6-Mes2 which has
a remarkably short PP distance of 1.985(2) Å. The phosphine
dihalide and diphosphene derivatives of -C6H2-2,6-Mes2-4-Me
and -C6H3-2,6{C6H3-2,6-Me2}2 have also been synthesized and
spectroscopically characterized in order to study their reduction
potentials.11b For the heavier elements the arsenic compound
2,6-Mes2-4-Me-H2C6AsCl2 has been reported,6h and the phos-
phaarsene 2,6-Mes2-4-Me-H2C6AsdPC6H2-2,6-Mes2-4-Me and
the corresponding radical anion were generated, although no
structures were determined owing to disorder and instability
problems. Unusually, the attempted synthesis of the correspond-
ing diarsene was also unsuccessful.6h Some related bismuth
derivatives have also been studied. Reaction of LiC6H2-2,4,6-
Ph3 with BiCl3 in a 2:1 or 3:1 ratio affords (2,4,6-Ph3H2C6)2-
BiCl or Bi(C6H3-2,4,6-Ph3)3. The monoaryl 2,4,6-Ph3H2C6BiCl2
could be synthesized by the reaction of BiCl3 with the triaryl-
or diarylbismuth halide in the appropriate ratio.23a However,
direct reaction of BiCl3 with LiC6H2-2,4,6-Ph3 in a 1:1 ratio
gave only a low yield (20%) of the monosubstituted species.
The monoaryl 2,4,6-Ph3H2C6BiCl2 is a dimer in the solid with
Bi-Cl distances of 3.074(1) Å (bridging) and 2.532(1) Å
(terminal) and a Bi-C distance of 2.226(4) Å.23b
Aryldihalopnictogen Synthesis. In the systems studied in
this paper it was found that the reaction of LiC6H3-2,6-Mes2
with ECl3 (E ) As, Sb, or Bi) or LiC6H3-2,6-Trip2 with ECl3
(E ) P, As, Sb, or Bi) yields the monosubstituted products 1-7
in accordance with eqs 1 and 2.
2,6-Mes2H3C6Li + ECl3 f 2,6-Mes2H3C6ECl2 + LiCl (1)
E ) As, 1; Sb, 2; Bi, 3
2,6-Trip2H3C6Li + ECl3 f 2,6-Trip2H3C6ECl2 + LiCl (2)
E ) P, 4; As, 5; Sb, 6; Bi, 7
The products 1-7 were obtained in moderate to good yield
(usually 50-80%) by the dropwise addition of an ether solution
of the lithium reagent to a cooled ether solution or suspension
of the pnictogen trihalide. All the products 1-7 are colorless
or pale yellow crystals and are air- and moisture-sensitive. They
possess excellent thermal stability and display no sign of an
orthometalation reaction such as that observed in some Mes*-
As derivatives.2k,6a For example, the compounds 4-7 melt
without decomposition within the temperature range 245-260
°C. Selected spectroscopic data for 1-7 are given in Table 2.
It can be seen that two IR absorptions are observed for the
Oddly, terphenyl or related ligands such as -C6H2-2,4,6-Ph3
have rarely been used in association with the group 15 elements.
Although the nitro and amine derivatives O2NC6H2-2,4,6-Ph3
or H2NC6H2-2,4,6-Ph3 have been known since 1957,22 heavier
(17) (a) Burger, H.; Eujen, R. J. Mol. Struct. 1983, 98, 265. (b)
Calderazzo, F.; Poli, R.; Pelizzi, G. J. Chem. Soc., Dalton Trans. 1984,
2365.
(18) Power, P. P. J. Chem. Soc., Dalton Trans. 1998, 2939.
(19) Brown, C. J. Acta Crystallogr. 1966, 21, 146.
(20) Nagase, S.; Suzuki, S.; Kurakake, T. J. Chem. Soc., Chem. Commun.
1990, 1724.
(21) For short critical discussions see: (a) Dasent, W. E. Nonexistent
Compounds; Marcel Dekker: New York, 1965, Chapter 4. (b) Norman, N.
C. Polyhedron 1993, 12, 2431.
(22) Dimroth, K.; Brauninger, G.; Neubauer, G. Chem. Ber. 1957, 90,
1634.
(23) (a) Li, X.-W.; Lorberth, J.; Massa, W.; Wocadlo, S. J. Organomet.
Chem. 1995, 485, 141. (b) Avtomonov, E. V.; Li, X.-W.; Lorberth, J. J.
Organomet. Chem. 1997, 530, 71.

Homologous Series of HeaVier Element Dipnictenes
J. Am. Chem. Soc., Vol. 121, No. 14, 1999 3363
Figure 1. Thermal ellipsoid plot (30%) of 3a. H atoms are omitted
for clarity.
Figure 4. Thermal ellipsoid plot (30%) of 6. H atoms are omitted for
clarity.
in 3b. When it is crystallized from benzene, 3a is obtained as
a monomer with Bi-C and terminal Bi-Cl distances that are
essentially the same as those seen in the weakly dimerized form.
The most unusual feature of 3 (both structures) is the existence
of a Menshutkin interaction24 between the bismuth and one of
the ortho aromatic rings. The strength of this interaction can be
expressed as a difference in the C(2)-C(1)-Bi(1) and the C(6)-
C(1)-Bi(1) angles at the ipso carbon which in 3a is 23.4°. The
distance from the C(7) ring centroid to bismuth is 3.472 Å. The
arsenic and antimony compounds 5 and 6, which are derivatives
of the -C6H3-2,6-Trip2 rather than the -C6H3-2,6-Mes2 ligand,
display very similar structures to the monomeric bismuth species
3a and feature a strongly pyramidalized geometry at the
pnictogen atom. The monomeric structure of the antimony
compound, in particular, may be contrasted with those of other
organoantimony dihalide species which display a tendency to
associate, even in the presence of bulky substituents such as
-CH(SiMe3)2, as in the compound (Me3Si)2HCSbCl2.25 The
structure of 6 is also characterized by the existence of a plane
of symmetry that incorporates the plane of the central C(1) ring.
There is also a strong Menshutkin interaction as evidenced by
a difference of 22.5° in the C(2)-C(1)-Sb and C(6)-C(1)-
Sb angles and an antimony-C(7) ring centroid distance of 3.474
Å. The Sb-Cl distance 2.365(av) Å is comparable to those
observed in other organoantimony dihalides and the Sb-C
distance, 2.187(5) Å, is within the experimentally known range
of ca. 2.13-2.20 Å.25 The arsenic compound 5 has a structure
that is very similar to that of its antimony analogue. A strong
Menshutkin interaction is also observed, as evidenced by the
As-C(7) ring centroid distance of 3.43 (av) Å and a difference
in the C(2)-C(1)-As and C(6)-C(1)-As angles of 23.3°. The
As-C and As-Cl distances are normal. The Menshutkin
interactions observed for these group 15 aryldihalides are,
apparently, a common feature of heavier main group terphenyl
derivatives, and similar interactions have already been observed
for germanium and tin derivatives of these ligands.27
Figure 2. Thermal ellipsoid plot (30%) of 3b. H atoms are omitted
for clarity.
Figure 3. Thermal ellipsoid plot (30%) of 5. H atoms are omitted for
clarity.
pnictogen halide stretching vibrations. The 13C chemical shift
of the ipso-C atom moves to lower field with increasing atomic
number of the pnictogen. The 31P NMR chemical shift (156.47
ppm) of 4 is almost identical to that observed for 2,6-Mes2H3C6-
PCl2 (160.4 ppm).11a
The structures of 3a, 3b, 5, and 6 were also determined, and
these are illustrated in Figures 1-4. Selected structural param-
eters are given in Table 3. The bismuth dihalide 3 can be
crystallized in either a monomeric form (3a) from benzene or
as a weakly associated dimer (3b) from Et2O. The dimeric form
has a structure similar to that of {2,4,6-Ph3H2C6BiCl2}2 23a with
the terminal and one of the bridging Bi-Cl distances averaging
about 2.52 Å in both compounds. The “long” bridging Bi-Cl
distances in 3b, 3.104(6) and 3.138(6) Å, are just marginally
longer than those seen in {2,4,6-Ph3H2C6BiCl2}2, 3.009(5) and
3.078(12) Å, perhaps as a result of the greater steric congestion
Reduction of the Aryl Pnictogen Dihalides. Reduction of
the halide derivatives 1-7 always produces a doubly bonded
(24) For example, see 23(b) and the following: (a) Mootz, D.; Handler,
V. Z. Anorg. Allg. Chem. 1986, 533, 23 and references therein. (b)
Menshutkin, B. N. Zh. Russ. Fiz. Khim. OVa. 1911, 43, 1298, 1785.
(25) (Me3Si)2HCSbCl2 crystallizes as a monomer25a from hexane but as
a dimer25b from the melt: (a) Cowley, A. H.; Norman, N. C.; Pakulski,
M.; Bricker, D. L.; Russell, D. H. J. Am. Chem. Soc. 1985, 107, 8211. (b)
Mohammed, M. A.; Ebert, K. E.; Breunig, H. Z. Naturforsch., B: Chem.
Sci. 1996, 51, 149.
(26) Wardell, J. L. ComprehensiVe Organometallic Chemistry; Wilkinson,
G., Stone F. G. A., Abel, E. W., Eds.; Pergamon Oxford: 1982; Vol. 2,
Chapter 13. ComprehensiVe Organometallic Chemistry II; Wilkinson, G.,
Stone, F. G. A., Abel, E. W., Davies, A. W., Eds.; Pergamon-Elsevier:
Oxford, 1995; Vol. 2, Chapter 8.
(27) Simons, R. S.; Pu, L.; Olmstead, M. M.; Power, P. P. Organome-
tallics 1997, 16, 1920.

3364 J. Am. Chem. Soc., Vol. 121, No. 14, 1999
Twamley et al.
Table 3. Selected Structural Parameters for 3a, 3b, 5, 6, 11, and 14a
Compound
E-C(1) (Å) E-X (Å) E-Centa (Å) C(2)-C(1)-E (deg) C(6)-C(1)-E (deg) C(1)-E-X (deg) X-E-X (deg)
9-phosphafluorene, 11
(X ) C)
1.805(11) 1.817(4)
1.867(5)
2.005(av) 2.164(av)
2.131(av)
112.8(3)
137.8(av)
131.5(4)
141.8(7)
131.7(3)
130.7(16)
132.2(17)
126.1(3)
103.2(av)
109.0(4)
100.8(6)
108.3(3)
110.9(15)
108.5(17)
89.4(2)
104.2(2)
101.9(av)
100.1(av)
97.40(8)
103.04(13)
101.0(5)
101.56(19)
105.9(av)
94.43(12)
2,6-Trip2H3C6AsCl2, 5
(X ) Cl)
3.43(av)
3.474
3.613
3.472
3.485
3.584
2,6-Trip2H3C6SbCl2, 6
(X ) Cl)
[2,6-Trip2H3C6(Cl)Sb]2, 14b 2.172(10) 2.427(16)
(X ) Cl)
2,6-Mes2H3C6BiCl2, 3a
(X ) Cl)
{2,6-Mes2H3C6BiCl2}2, 3b
(X ) Cl)
2.187(5)
2.384(3)
2.346(2)
2.267(5)
2.29(2)
2.21(2)
2.539(1)
2.501(1)
2.496(6)
2.544(5)
2.516(6)
2.531(5)
102.3(1)
94.3(1)
94.3(6)
88.20(5)
88.78(19)
88.87(19)
96.7(6)
a Cent ) Centroid.
Scheme 1. Possible Reduction Pathways of REX2 (E ) P, As, Sb, or Bi; X ) Halogen) with Magnesium
dipnictene species among the products. However, the type of
reductant used may influence the product obtained, and the
reduction of 4 illustrates this point. If 4 is treated with
magnesium in THF, the cyclized phosphafluorene 11 is produced
in ca. 70% yield as illustrated in eq 3.
(Cl)P-P(Cl)Mes*28a and 14b (vide infra) suggests that one of
the two initial reduction pathways illustrated by Scheme 1 may
exist. One pathway involves insertion of magnesium into the
E-X bond to give RE(X)MgX which may then, either eliminate
MgX2 to generate a phosphanediyl(phosphinidene), or react
further with REX2 to give the 1,2-dihalodipnictane R(X)E-
E(X)R. The alternate radical pathway has received strong
support from EPR studies.28b Cyclized products related to 11
have also been observed in the reduction of 2,6-Trip2H3C6BX2
(X ) Cl or Br) species.29
The crystal structure of 11 (Figure 5) shows that phosphorus
center has quite pyramidal coordination, Σ°P ) 295.2°, sug-
gesting little delocalization of the phosphorus lone pair in the
five-membered ring, at least in the ground state.30 However, it
is notable that the P(1)-C(1) and P(1)-C(18) distances (Table
3) are significantly, ca. 0.05-0.06 Å, shorter than the P(1)-
C(34) distance. The difference in the P-C bond lengths may
be partially due to the approximate sp2 hybridization of the
(28) (a) Goldwhite, H.; Kaminski, J.; Millhauser, G.; Ortiz, J.; Vargas,
M.; Vertal, L.; Lappert, M. F.; Smith, S. J. J. Organomet. Chem. 1986,
310, 21. (b) Cetinkaya, B.; Hudson, A.; Lappert, M. F.; Goldwhite, H. J.
Chem. Soc., Chem. Commun. 1982, 609.
(29) Grigsby, W. J.; Power, P. P. J. Am. Chem. Soc. 1996, 118, 7981.
(30) (a) Egan, W.; Tang, R.; Zon, G.; Mislow, K. J. Am. Chem. Soc.
1971, 93, 6205. (b) Mathey, F. Coord. Chem. ReV. 1988, 88, 429.
When 4 is reduced with potassium, however, the diphosphene
12 is produced together with a minor amount of 11. The exact
mechanism of the reduction pathway of organophosphorus
dihalides by magnesium is unknown. However, the observation
of 11 in addition to partially reduced dipnictanes such as Mes*-

Homologous Series of HeaVier Element Dipnictenes
J. Am. Chem. Soc., Vol. 121, No. 14, 1999 3365
Figure 5. Thermal ellipsoid plot (30%) of 11. H atoms are omitted
for clarity.
Figure 8. Thermal ellipsoid plot (30%) of 10. H atoms are omitted
for clarity.
phosphorus compound,11a in addition to the arsenic, antimony,
and bismuth analogues, are presented in Table 4. The two main
features of interest in these molecules are the E-E bond distance
and the E-E-C bond angle. In the diphosphene two different
P-P-C angles, 97.5(1) and 109.8(1) (average 103.7°), together
with a P-P distance of 1.985(2) Å (the shortest P-P distance
in a diphosphene) are observed. The reason for the differing
P-P-C angles is probably steric. It appears that the size of
the -C6H3-2,6-Mes2 ligand does not allow a parallel alignment
of central C6H3 rings. In fact, these rings are almost perpen-
dicular to each other in the diphosphene, and a wider angle at
one of the phosphorus centers is due to steric conflict between
one of the ortho mesityl groups and the PdP double bond.
Crystals of the arsenic, antimony, and bismuth compounds are
isomorphous with each other and have a different space group
from that of the phosphorus species.11a The structures of 8, 9,
and 10 all have a center of symmetry in the middle of the E-E
bond, and thus, the planar C6H3 rings in each -C6H3-2,6-Mes2
substituent are parallel. This configuration is permitted by the
steric relaxation caused by the larger central atom size. In
addition, the two E-E-C angles are identical and decrease from
98.5(4)° in the diarsene, to 94.1(1)° in the distibene, and finally
to 92.5(4)° in the dibismuthene (Table 4). The As-As distance
in 8, 2.276(3) Å is somewhat longer than the average distance
Figure 6. Thermal ellipsoid plot (30%) of 8. H atoms are omitted for
clarity.
5c
of 2.244(1) Å in (Me3Si)3CAsdAsC(SiMe3)3 (As-As-C )
106.4(2)°) and 2.224(2) Å in Mes*AsdAsCH(SiMe3)25a (As-
As-C(Mes*) ) 93.6(3)°; As-As-CH(SiMe3)2 ) 99.9(3)°). In
addition, the Sb-Sb (2.6558(5) Å) and Bi-Bi (2.8327(14) Å)
distances in 9 and 10 are slightly longer than the 2.642(1) and
2.8208(8) Å in the recently reported 2,4,6-{Me3Si)2CH}3H2C6Ed
EC6H3-2,4,6-{CH(SiMe3)2}3 (E ) Sb10a or Bi10b, respectively)
species. These compounds have Sb-Sb-C and Bi-Bi-C
angles of 101.4(1) and 100.5(2)°, respectively. The variety of
ligand sizes and paucity in numbers of these previously known
compounds make angular trends more difficult to assess.
However, it is clear from the data in Table 4 that the E-E-C
angles decrease in the sequence 103.7°(P) > 98.5(4)°(As) >
94.1(1)°(Sb) > 92.5(4)°(Bi). The angles measured in the
antimony and bismuth compounds 9 and 10 are in quite good
agreement with the 93.0 and 91.8° values calculated20 for the
hypothetical hydrogen derivatives HSbdSbH and HBidBiH.
The experimental angles in the known diphosphenes and
diarsenes are significantly wider than those calculated for the
hydrogen derivatives, however. This is probably a result of the
greater steric congestion in the smaller, lighter congeners. In
contrast, the calculated PdP and AsdAs double-bonded dis-
Figure 7. Thermal ellipsoid plot (30%) of 9. H atoms are omitted for
clarity.
σ-bonding orbitals of the C(1) and C(18) carbons (cf. ap-
proximate sp3 hybridization for C(34)), but it may also be a
reflection of the incipient delocalization of the phosphorus lone
pair in a 6 π electron aromatic ring in the transition state for
the inversion of the phosphorus the geometry.
The structure of the diphosphene species 12 is probably very
similar to that of the diphosphene 2,6-Mes2H3C6PdPC6H3-2,6-
Mes2 reported by Protasiewicz and co-workers.11a The related
diarsene 8 (Figure 6), distibene 9 (Figure 7), and dibismuthene
10 (Figure 8) were synthesized in ca. 30% yield by reduction
of the corresponding dihalide. Selected structural data for the

3366 J. Am. Chem. Soc., Vol. 121, No. 14, 1999
Twamley et al.
Table 4. Selected Structural Parameters for 8-10, 13, and 14a
Compound
E-C (Å)
E-E (Å)
E-E-C (deg)
C-E-E-C (deg)
a
2,6-Mes2-H3C6PdPC6H3-2,6-Mes2
1.842(3)
1.840(3)
1.963(13)
1.986(7)
1.980(7)
2.169(4)
2.190(9)
2.257(17)
1.985(2)
109.8(1)
97.5(1)
98.5(4)
96.4(2)
107.8(2)
94.1(1)
98.9(2)
92.5(4)
177.1(2)
2,6-Mes2-H3C6AsdAsC6H3-2,6-Mes2, 8
2,6-Trip2-H3C6AsdAsC6H3-2,6-Trip2, 13
2.276(3)
2.285(3)
180
176.6
2,6-Mes2-H3C6SbdSbC6H3-2,6-Mes2, 9
2,6-Trip2-H3C6SbdSbC6H3-2,6-Trip2, 14a
2,6-Mes2-H3C6BidBiC6H3-2,6-Mes2, 10
2.6558(5)
2.668(2)
2.8327(14)
180
180
180
a Reference 11a.
Figure 9. Thermal ellipsoid plot (30%) of 13. H atoms are omitted
for clarity.
Figure 11. Thermal ellipsoid plot (30%) of 14b. H atoms are omitted
for clarity.
two different As-As-C angles of 96.4(2) and 107.8(2)°. These
values are very close to the two different P-P-C angles
observed in 2,6-Mes2H3C6PPC6H3-2,6-Mes2.11a The cause of the
difference in 13 is probably the same as that argued earlier for
the phosphorus compound. The good agreement between the
three experimentally determined SbdSb double-bonded dis-
tances and the two BidBi double-bonded distances demonstrates
beyond doubt that the amount of the shortening in these bonds,
in comparison to the single bond lengths in Ph2EEPh2 (E )
Sb17a or Bi17b), is both consistent and significant. Furthermore,
calculations have shown that rotational barriers of the order of
12-15 kcal mol-1 can be expected for the SbdSb and BidBi
double bonds.20 These values are in reasonable agreement with
the π-bond strengths expected from spectroscopic measurements
and data extrapolation.18 The experimental results together with
the theoretical data provide strong evidence that p-p orbital
overlap to give π bonding is important even in the heaviest
elementssat least those in group 15. It has often been suggested
that side-on p-p π-orbital overlap diminishes at a much more
rapid rate than head-to-head orbital overlap in σ bonding.31 The
presently available data suggest that the difference in the rates
at which σ- and π-bonds decrease in strength is not as great as
was once thought.
Figure 10. Thermal ellipsoid plot (30%) of 14a. H atoms are omitted
for clarity.
tances are in good agreement with experimentally measured
values, whereas the SbdSb and BidBi experimental distances
are 0.04-0.11 Å longer than the calculated values.
Reduction of the bulkier 2,6-Trip2H3C6ECl2 (E ) As, 5; Sb,
6; Bi, 7) dihalides affords the dipnictenes 13-15 in 80-90%
yield. The almost quantitative yields of these products may be
due to the greater steric crowding which may prevent side
reactions leading to other products. The structures of the diarsene
and distibene 13 (Figure 9) and 14a (Figure 10) have slightly
longer (0.01 Å) As-As and Sb-Sb distances than in their less
sterically crowded -C6H3-2,6-Mes2 counterparts. The average
As-As-C (102.1°) and Sb-Sb-C (99.0(2)°) angles in 13 and
14a are wider than the corresponding ones in 8 and 9 for the
same reason. The structures of 14a and the partially reduced
1,2-dihalodipnictane species 14b (Figure 11), which have similar
degrees of steric crowding since they have the same aryl
substituents, provide a further gauge of the shortening caused
by reduction to a double bond. A difference of 0.223 Å or 7.7%
was observed in the Sb-Sb bond lengths (Sb-Sb: 2.668(2)
Å, 14a; 2.892(8) Å, 14b). It is notable that the diarsene 13 has
The arsenic and antimony species 13 and 14a are isoelectronic
with the recently synthesized germanium and tin species Na2-
{2,6-Trip2H3C6GedGeC6H3-2,6-Trip2} and K2{2,6-Trip2H3C6-
SndSnC6H3-2,6-Trip2}, which have GedGe and SndSn double-
bonded distances of 2.394(1) and 2.7763(9) Å.32 These distances
are 0.1 Å longer than those in 13 and 14a, and some of this
increase can be attributed to the slightly larger covalent radius
(31) Pitzer, K. S. J. Am. Chem. Soc. 1948, 70, 2140.
(32) Pu, L.; Senge, M. O.; Olmstead, M. M.; Power, P. P. J. Am. Chem.
Soc. 1998, 120, 12682.

Homologous Series of HeaVier Element Dipnictenes
J. Am. Chem. Soc., Vol. 121, No. 14, 1999 3367
Table 5. UV-vis Spectroscopic Data for Dipnictenes
Compound
λ1 (nm, ꢀ)
λ2 (nm, ꢀ)
λ3 (nm, ꢀ)
a
2,6-Mes2-H3C6PdPC6H3-2,6-Mes2
371, 8000
393, 7270
400, 6970
409, 4060
451, 5450
470, 6570
505, 5010
518, 6920
455, 501
502, 570
462, 650
504, 120
2,6-Trip2-H3C6PdPC6H3-2,6-Trip2, 12
2,6-Mes2-H3C6AsdAsC6H3-2,6-Mes2, 8
2,6-Trip2-H3C6AsdAsC6H3-2,6-Trip2, 13
2,6-Mes2-H3C6SbdSbC6H3-2,6-Mes2, 9
2,6-Trip2-H3C6SbdSbC6H3-2,6-Trip2, 14a
2,6-Mes2-H3C6BidBiC6H3-2,6-Mes2, 10
2,6-Trip2-H3C6BidBiC6H3-2,6-Trip2, 15
970, 30
913, 200
1025, 80
a Reference 11a.
of germanium in comparison to arsenic (1.22 vs 1.21 Å).33
However, the covalent radii of tin and antimony are very
similar,33 and it seems likely that the primary reason for the
longer distances in the tin and germanium species is the presence
of a double negative charge located mainly at the GedGe and
SndSn moieties. This causes an increase in interelectronic
repulsion, and hence, a lengthening of the Ge-Ge and Sn-Sn
bonds. This phenomenon has also been observed in single bonds.
For example, the Sb-Sb single bond distance in Ph2Sb-SbPh2
(Sb-Sb ) 2.837 Å)17a is shorter than the corresponding Sn-
Sn distance (2.905(3) Å) in the isoelectronic dianion [Ph2Sn-
thousand). It can be seen that, for the π-π* absorptions of the
-C6H3-2,6-Mes2 derivatives, there is a shift to longer wave-
length (cf. 371 nm (P), 400 (As), 451 (Sb), 505 (Bi)) as the
group is descended. Very similar maxima are observed for the
π-π* transitions for the -C6H3-2,6-Trip2 derivatives 12, 13,
14a, and 15 which are all at slightly longer (by 9-19 nm)
wavelength (i.e., lower energy) than their -C6H3-2,6-Mes2
substituted counterparts. Longer wavelengths were also reported
for the π-π* absorption in the 2,4,6-{(Me3Si)2CH}3H2C6Ed
EC6H2-2,4,6-{CH(SiMe3)2}3 derivatives. The wavelengths mea-
sured for these compounds, Sb (466 nm) and Bi (525 nm), are
within 4 and 7 nm, respectively, of those measured for 14a and
15. These trends are consistent with a weakening of the π bond
as the group is descended and consistent with calculations on
these systems.
SnPh2]2- 34
.
The dipnictene compounds 8-10 and 12-15 are intensely
colored. Their color ranges from yellow or orange yellow in
the phosphorus and arsenic species to deep red (purple solution)
in the case of the bismuth derivative 15. The color arises from
chromophores (usually two) in the visible region. The UV-vis
data are summarized in Table 5; also included are data for the
phosphorus species 2,6-Mes2H3C6PdPC6H3-2,6-Mes2. Generally
speaking, the two absorption maxima are thought to correspond
to the π-π* (symmetry allowed) and n-π* (symmetry forbid-
den) electronic transitions. The π-π* absorption maxima are
thus significantly more intense (ꢀ value is usually several
Acknowledgment. We thank the National Science Founda-
tion for financial support and Profs. R. A. Andersen and T. L.
Allen for useful discussions.
Supporting Information Available: Tables of data-collec-
tion parameters, atom coordinates, bond distances and angles,
anisotropic thermal parameters and hydrogen coordinates. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(33) Wells, A. F. Structural Inorganic Chemistry, 5th ed.; Clarendon:
Oxford, 1984; pp 376 and 1279.
(34) Scotti, N.; Zachawieja, U.; Jacobs, J. Z. Anorg. Allg. Chem. 1997,
623, 1503.
JA983999N