Synthesis of cyclo-2,4,6-triarsa-1,3,5-triazanes from cyclo-2,4-diarsa-1,3- diazanes demonstrating the general influence of substituent steric strain on the relative stability of pnictazane oligomers

Article abstract of DOI:10.1021/ic050532m

2,4-Dichloro-1,3-diaryl-cyclo-2,4-diarsa-1,3-diazanes (aryl = 2,6-dimethylphenyl, Dmp, or 2,6-diisopropylphenyl, Dipp) have been transformed into the corresponding 2,4,6-trichloro-cyclo-2,4,6-triarsa-1,3,5-triazanes on reaction with GaCl₃ followed by 4-(dimethylamino)pyridine (DMAP). The nitrogen bound Dmp and Dipp substituents impose medium substituent steric strain on the heterocycles influencing the relative thermodynamic stability of potential oligomers in favor of the trimers. This ring expansion disproportionation reaction is initiated by chloride ion abstraction, and the intermediate 2,4-dichloro-1,3,5-tris(2,6-diisopropylphenyl)-cyclo-2,4-diarsa-1, 3,5-triazane-6-arsenium tetrachlorogallate has been isolated and structurally characterized. Subsequent reaction with 4-(dimethylamino)pyridine (DMAP) effects release of chloride ion from the gallate anion and consequential formation of a covalent As-Cl bond in the trimer. The observations are analogous to those for the phosphorus derivatives demonstrating a general applicability of this new synthetic procedure for the development and diversification of cyclopnictazane chemistry.

Full text of DOI:10.1021/ic050532m

Inorg. Chem. 2005, 44, 5897−5902
Synthesis of cyclo-2,4,6-Triarsa-1,3,5-triazanes from
cyclo-2,4-Diarsa-1,3-diazanes Demonstrating the General Influence of
Substituent Steric Strain on the Relative Stability of Pnictazane
Oligomers
Neil Burford,*,† Jeff C. Landry,† Michael J. Ferguson,‡ and Robert McDonald‡
Department of Chemistry, Dalhousie UniVersity, Halifax, NoVa Scotia B3H 4J3, Canada, and
X-ray Crystallography Laboratory, Chemistry Department, UniVersity of Alberta,
Edmonton, Alberta T6G 2G2, Canada
Received April 7, 2005
2,4-Dichloro-1,3-diaryl-cyclo-2,4-diarsa-1,3-diazanes (aryl
) 2,6-dimethylphenyl, Dmp, or 2,6-diisopropylphenyl, Dipp)
have been transformed into the corresponding 2,4,6-trichloro-cyclo-2,4,6-triarsa-1,3,5-triazanes on reaction with
GaCl3 followed by 4-(dimethylamino)pyridine (DMAP). The nitrogen bound Dmp and Dipp substituents impose
“medium” substituent steric strain on the heterocycles influencing the relative thermodynamic stability of potential
oligomers in favor of the trimers. This ring expansion disproportionation reaction is initiated by chloride ion abstraction,
and the intermediate 2,4-dichloro-1,3,5-tris(2,6-diisopropylphenyl)-cyclo-2,4-diarsa-1,3,5-triazane-6-arsenium tetra-
chlorogallate has been isolated and structurally characterized. Subsequent reaction with 4-(dimethylamino)pyridine
(DMAP) effects release of chloride ion from the gallate anion and consequential formation of a covalent As−Cl
bond in the trimer. The observations are analogous to those for the phosphorus derivatives demonstrating a general
applicability of this new synthetic procedure for the development and diversification of cyclopnictazane chemistry.
Introduction
(RPnNR′)2 have been reported for all of the heavier pnic-
togens (Pn ) P,10-13 As,21-24 Sb,25-32 Bi33,34), including an
homologous series,13 highlighting the potential for extrapola-
tion of synthetic procedures from phosphorus to bismuth.
The N-P heterocatenated cyclophosphazenes (R2PN)n are
known to exist in a wide range of ring sizes, and some have
been transformed into useful polymeric materials.1-3 Analo-
gous compounds (“cyclopnictazenes”) containing the other
pnictogen elements (As, Sb, Bi,) are rare.4-7 Cyclophos-
phazanes represent (RPNR′)n isomers of cyclophosphazenes
and are known for n ) 2-4,8-20 but examples of dimers
(9) Shaw, R. A. Phosphorus, Sulfur, Silicon Relat. Elem. 1978, 4, 101-
121.
(10) Keat, R. Top. Curr. Chem. 1982, 102, 89-116.
(11) Chen, H.-J.; Haltiwanger, R. C.; Hill, T. G.; Thompson, M. L.; Coons,
D. E.; Norman, A. D. Inorg. Chem. 1985, 24, 4725-4730.
(12) Stahl, L. Coord. Chem. ReV. 2000, 210, 203-250.
(13) Burford, N.; Cameron, T. S.; Lam, K.-C.; LeBlanc, D. J.; Macdonald,
C. L. B.; Phillips, A. D.; Rheingold, A. L.; Stark, L.; Walsh, D. Can.
J. Chem. 2001, 79, 342-348.
* Author to whom correspondence should be addressed. E-mail:
neil.burford@dal.ca.
† Dalhousie University.
‡ University of Alberta.
(14) Burford, N.; Conroy, K. D.; Landry, J. C.; Ragogna, P. J.; Ferguson,
M.; McDonald, R. Inorg. Chem. 2004, 43, 8245-8251.
(15) Zeiss, W.; Barlos, K. Z. Naturforsch. 1979, 34b, 423-426.
(16) Zeiss, W.; Pointner, A.; Engelhardt, C.; Klehr, H. Z. Anorg. Allg. Chem.
1981, 475, 256-270.
(1) Manners, I. Angew. Chem., Int. Ed. Engl. 1996, 35, 1602-1621.
(2) Gates, D. P.; Manners, I. Dalton Trans. 1997, 2525
(3) Allcock, H. R.; West, R.; Mark, J. E. Inorganic Polymers; Prentice
Hall: Englewood Cliffs, NJ, 1992.
(4) Krannich, L. K.; Thewalt, U.; Cook, W. J.; Jain, S. R.; Sisler, H. H.
Inorg. Chem. 1973, 12, 2304-2313.
(5) Krieg, V.; Weidlein, J. Angew. Chem., Int. Ed. Engl. 1971, 10, 516-
518.
(6) Preiss, H.; Hass, D. Z. Anorg. Allg. Chem. 1974, 404, 190-198.
(7) Begley, M. J.; Sowerby, D. B.; Tillott, R. J. Dalton Trans. 1974, 2530
(8) Niecke, E.; Gudat, D. Angew. Chem., Int. Ed. Engl. 1991, 30, 217-
237.
(17) Harvey, D. A.; Keat, R.; Rycroft, D. S. Dalton Trans. 1983, 425-
431.
(18) Murugavel, R.; Krishnamurthy, S. S.; Chandrasekhar, J.; Nethaji, M.
Inorg. Chem. 1993, 32, 5447-5453.
(19) Malavaud, C.; M’Pondo, T. N.; Lopez, L.; Barrans, J.; Legros, J.-P.
Can. J. Chem. 1984, 62, 43-50.
(20) Zeiss, W.; Schwarz, W.; Hess, H. Angew. Chem., Int. Ed. Engl. 1977,
16, 407-408.
10.1021/ic050532m CCC: $30.25
Published on Web 07/02/2005
© 2005 American Chemical Society
Inorganic Chemistry, Vol. 44, No. 16, 2005 5897

Burford et al.
drybox techniques. Chemicals and reagents were obtained from
Aldrich Chemical Co. All solvents were dried on an MBraun solvent
purification system and stored over molecular sieves prior to use.
4-(Dimethylamino)pyridine (DMAP), 2,6-dimethylaniline (Dmp-
NH2), and 2,6-diisopropylaniline (DippNH2) were used as received.
Arsenic trichloride was distilled and gallium chloride sublimed prior
to use, while triethylamine was purified by fractional distillation
from potassium hydroxide and calcium hydride. Solvent volumes
in reaction mixtures are approximate.
We envisage the potential for ring-opening oligomerization
and polymerization (that has been well established for
cyclophosphazenes1,2) of these cyclodipnictadiazanes and the
possibility to acquire an array of NP, NAs, NSb, and NBi
polymer backbones, with a consequential variety of proper-
ties. Therefore, we are devising general synthetic methods
for cyclopnictazanes and have exploited the steric influence
of organic substituents at nitrogen to manipulate the relative
thermodynamic stability of the pnictazane oligomers. The
imposition of large substituent steric strain35 in derivatives
of (RPNR′)2 can destabilize the dimer enthalpically with
respect to the monomer.36 However, we have noted that the
medium-sized substituents 2,6-dimethylphenyl- (Dmp) and
2,6-diisopropylphenyl- (Dipp) provide suitable steric strain
to favor and promote the formation of trimeric cyclophos-
phazanes 2P from the corresponding dimers 1P.14 We now
illustrate the generality of this substituent steric tuning of
cyclopnictazane oligomerization with the preparation and
characterization of analogous heterocycles for Pn ) As. In
addition, six-membered cationic intermediates 3As have been
isolated and comprehensively characterized.
Infrared spectra were recorded as Nujol mulls on CsI plates using
a Bruker Vector 22 FT-IR and are presented as wavenumber (cm-1
)
maxima with ranked intensity for each absorption given in
parentheses and the most intense peak given a ranking of 1. Melting
points were obtained using an Electro-thermal apparatus. Elemental
analyses were performed by Desert Analytics, Tuscon, AZ. Solution
1H NMR spectra were obtained at room temperature on a Bruker
AC-250 NMR spectrometer. Chemical shifts are reported in ppm
relative to SiMe4 and are calibrated to an internal reference signal
of CHCl3, 7.26 ppm. X-ray diffraction data were obtained on a
Bruker PLATFORM diffractometer with a sealed tube generator
and a SMART 1000 CCD detector using graphite-monochromated
Mo KR (λ ) 0.710 73 Å) radiation on samples cooled to 193(2)
K. The structures were solved by direct methods and refined by
full-matrix least squares. Unit cell parameters were obtained from
the refinement of the setting angles of reflections from the data
collection. The choice of space groups was based on systematically
absent reflections and was confirmed by the successful solution
and refinement of the structures. Crystal data are presented in Table
1, and selected bond lengths and angles are presented in Table 2.
Preparation/Isolation Procedures and Characterization Data.
General Method for (RNAsCl)2 (1AsR; R ) Dmp, Dipp). RNH2
was added slowly over 10 min to an ice-cooled solution of AsCl3
and NEt3 in benzene (∼300 mL). A yellow precipitate formed
almost immediately. The solution was stirred for 18 h and then
filtered and concentrated by removal of solvent in vacuo. Addition
of pentane gave a white precipitate that was separated from the
yellow solution, dried, and redissolved; vapor diffusion of pentane
into a CH2Cl2 solution (R ) Dmp) or slow solvent evaporation of
benzene (R ) Dipp) yielded plate crystals.
(DmpNAsCl)2 (1AsDmp): DmpNH2 (7.3 mL), AsCl3 (3.0 mL),
NEt3 (10 mL); yield 1.7 g (14%, not optimized); mp 207-209 °C;
1H NMR (CDCl3) 2.76 (s), 7.12 (m); IR 489 (11), 521 (5) 699 (4),
774 (3), 810 (1), 882 (6), 914 (15), 974 (12), 1030 (10), 1096 (7),
1166 (9), 1203 (2), 1256 (8), 1535 (14), 1587 (13), 1624 (20), 1748
(19), 1867 (18), 1944 (17). Anal. Calcd (found) for C16H18N2As2-
Cl2: C, 41.86 (42.89); H, 3.95 (4.78); N, 6.10 (6.11).
Experimental Section
(DippNAsCl)2 (1AsDipp): DippNH2 (6.7 mL), AsCl3 (5.0 mL),
NEt3 (13 mL); yield 3.4 g (34%, not optimized); mp 231-234 °C;
General Information. All manipulations were carried out under
oxygen- and moisture-free conditions using standard Schlenk or
(29) Beswick, M. A.; Harmer, C. N.; Hopkins, A. D.; Paver, M. A.; Raithby,
P. R.; Wright, D. S. Polyhedron 1998, 17, 745-748.
(30) Bryant, R.; James, S. C.; Jeffrey, J. C.; Norman, N. C.; Orpen, A. G.;
Weckenmann, U. Dalton Trans. 2000, 4007-4009.
(31) Beswick, M. A.; Elvidge, B. R.; Feeder, N.; Kidd, S. J.; Wright, D.
S. Chem. Commun. 2001, 379-380.
(21) Bohra, R.; Roesky, H. W.; Noltemeyer, M.; Sheldrick, G. M. Acta
Crystallogr. 1984, C40, 1150-1152.
(22) Ahlemann, J.-T.; Roesky, H. W.; Murugavel, R.; Parisini, E.; Nolte-
meyer, M.; Schmidt, H.-G.; Mu¨ller, O.; Herbst-Irmer, R.; Markovskii,
L. N.; Shermolovich, Y. G. Chem. Ber. 1997, 130, 1113-1121.
(23) Avtomonov, E. V.; Megges, K.; Li, X.; Lorberth, J.; Wocadlo, S.;
Massa, W.; Harms, K.; Churakov, A. V.; Howard, J. A. K. J.
Organomet. Chem. 1997, 544, 79-89.
(32) Haagenson, D. C.; Stahl, L.; Staples, R. J. Inorg. Chem. 2001, 40,
4491-4493.
(33) Wirringa, U.; Roesky, H. W.; Noltemeyer, M.; Schmidt, H. G. Inorg.
Chem. 1994, 33, 4607-4608.
(24) Raston, C. L.; Skelton, B. W.; Tolhurst, V.-A.; White, A. H. Dalton
Trans. 2000, 1279-1285.
(25) Ross, B.; Belz, J.; Nieger, M. Chem. Ber. 1990, 123, 975-978.
(26) Garbe, R.; Pebler, J.; Dehnicke, K.; Fenske, D.; Goesmann, H.; Baum,
G. Z. Anorg. Allg. Chem. 1994, 620, 592-598.
(34) Briand, G. G.; Chivers, T.; Parvez, M. Can. J. Chem. 2003, 81, 169-
174.
(35) Burford, N.; Clyburne, J. A. C.; Chan, M. S. W. Inorg. Chem. 1997,
36, 3204-3206.
(27) Edwards, A. J.; Leadbeater, N. E.; Paver, M. A.; Raithby, P. R.;
Russell, C. A.; Wright, D. S. Dalton Trans. 1994, 1479-1482.
(28) Edwards, A. J.; Paver, M. A.; Rennie, M.-A.; Raithby, P. R.; Russell,
C. A.; Wright, D. S. Dalton Trans. 1994, 2963-2966.
(36) Burford, N.; Cameron, T. S.; Conroy, K. D.; Ellis, B.; Lumsden, M.
D.; Macdonald, C. L. B.; McDonald, R.; Phillips, A. D.; Ragogna, P.
J.; Schurko, R. W.; Walsh, D.; Wasylishen, R. E. J. Am. Chem. Soc.
2002, 124, 14012-14013.
5898 Inorganic Chemistry, Vol. 44, No. 16, 2005

cyclo-2,4,6-Triarsa-1,3,5-triazanes
Table 1. Crystal Data
param
1AsDmp
1AsDipp.2C6 H6
2AsDmp
2AsDipp
[3AsDipp][GaCl4]‚CH2Cl2
empirical formula
fw
cryst system
space group
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
C16H18N2As2Cl2
459.06
monoclinic
P21/c
15.591(1)
11.425(1)
20.719(1)
C36H46N2As2Cl2
727.49
orthorhombic
Pnma
22.845(1)
10.217(1)
15.694(2)
C24H27N3As3Cl3
634.85
monoclinic
I2/a
31.883(3)
9.263(1)
18.738(1)
C36H51N3As3Cl3
856.91
monclinic
P21/n
12.305(1)
21.386(2)
16.328(1)
C37H53N3P3Cl8Ga
1117.90
triclinic
P1h
10.508(1)
14.803(1)
16.591(1)
87.36(1)
81.20(1)
79.07(1)
2381.9(2)
2
104.25(1)
96.36(1)
111.85(1)
3576.8(4)
8
1.705
0.0367
0.0.690
1.045
3663.0(4)
4
1.319
0.0337
0.0652
1.053
5499.9(8)
8
1.663
0.0648
0.1100
1.020
3955.7(8)
4
1.439
0.0495
0.0752
1.034
Z
D
C (Mg m-3
)
1.559
0.0317
0.0742
1.049
R (I > 2σ(I))a
wR (all data)a
goodness-of-fit Sa
a R(F [I > 2σ(I)]) ) Σ||Fo| - |Fc||/Σ|Fo|; wR(F2 [all data]) ) [Σw(Fo - Fc )2/Σw(Fo )2]1/2; S (all data) ) [Σw(Fo - Fc )2/(n - p)]1/2 [n ) no. of data;
2
2
2
2
2
2
2
2
p ) no. parameters varied, w ) 1/ [σ2(Fo ) + (aP)2 + bP], where P ) (Fo + 2Fc )/3 and a and b are constants suggested by the refinement program (see
Supporting Information)].
Table 2. Selected Bond Lengths (Å) and Angles (deg) for Derivatives of 1-3
38
(DmpNAsCl)2 (DippNAsCl)2 (DmpNAsCl)3 (DippNAsCl)3 (MeNAsCl)3
[Dipp3N3As3Cl2][GaCl4] [Dipp3N3P3Cl2][GaCl4]14
param
(1AsDmp)
(1AsDipp)
(2AsDmp)
(2AsDipp)
(2AsMe)
([3AsDipp][GaCl4])
([3PDipp][GaCl4])
N1-Pn1
N(2/3)-Pn1
N1-Pn2
N2-Pn2
N2-Pn3
N3-Pn3
1.841(2)
1.847(2)
1.838(2)
1.842(2)
1.845(1)
1.843(1)
1.845(1)
1.843(1)
1.834(3)
1.836(3)
1.846(3)
1.826(3)
1.828(3)
1.829(3)
1.848(2)
1.831(2)
1.826(2)
1.835(2)
1.827(2)
1.837(2)
1.79
1.85
1.83
1.88
1.76
1.86
1.829(2)
1.881(2)
1.834(2)
1.864(2)
1.793(2)
1.786(2)
1.697(2)
1.753(2)
1.703(2)
1.736(2)
1.656(2)
1.650(2)
N-Pn1-N
Pn1-Cl1
79.7(1)
2.243(1)
2.244(1)
79.9(1)
2.239(1)
2.239(1)
100.2(1)
2.255(1)
2.245(1)
2.232(1)
98.5(2)
99.5(1)
102.5
2.28
2.24
2.28
99.7
105.5
98.4(1)
99.4(1)
2.230(1)
2.270(1)
2.235(1)
101.4(1)
100.3(1)
2.207(1)
2.209(1)
2.708(1)
99.1(1)
2.081(1)
2.088(1)
2.704(1)
100.8(1)
105.0(1)
Pn2-Cl2
Pn3-Cl3
N-Pn2-N
N-Pn3-N
80.0(1)
79.9(1)
100.7(1)
103.8(1)
Σ° at N1
Σ° at N2
Σ° at N3
Σ° at Pn1
Σ° at Pn2
Σ° at Pn3
353.9(1)
354.3(2)
352.8(1)
352.1(1)
354.7(3)
359.9(2)
360.0(2)
300.2(1)
299.1(1)
300.3(1)
356.6(2)
359.5(2)
359.4(2)
301.4(1)
302.4(1)
300.5(1)
359.1
359.6
358.3
303.9
295.2
301.1
359.4(1)
358.8(1)
358.6(1)
301.5(1)
298.1(1)
359.7(2)
359.0(2)
359.6(2)
304.5(9)
302.6(7)
258.2(1)
284.7(1)
284.6(1)
284.6(1)
1H NMR (CDCl3) 1.345 (d), 4.08 (m), 7.28 (s); IR 329 (4), 354
(2), 419 (11), 455 (15), 517 (13), 702 (19), 715 (13), 788 (1), 828
(6), 873 (9), 893 (12), 933 (17), 1041 (18), 1055 (14), 1101 (8),
1191 (3), 1241 (7), 1316 (5), 1384 (10), 1578 (20). Anal. Calcd
(found) for C24H34N2As2Cl2: C, 50.46 (50.36); H, 6.00 (5.75); N,
4.90 (5.06).
(20). Anal. Calcd (found) for C36H51N3As3Cl6Ga: C. 41.86 (41.19);
H, 4.98 (4.75); N, 4.07 (3.60).
General Method for (RNAsCl)3 (2AsR; R ) Dmp, Dipp).
(RNAsCl)2 and GaCl3 were combined and dissolved in toluene (∼3
mL); the formation of a dark red solution was immediate but faded
after 2 h, with stirring for 18 h. Addition of DMAP gave a light
yellow solution. Partial removal of the solvent in vacuo deposited
a yellow oil, which was separated from the solution.
General Method for [R3N3As3Cl2][GaCl4] ([3AsR][GaCl4] R
) Dmp, Dipp). (RNAsCl)2 and GaCl3 were combined and dissolved
in CH2Cl2 (∼2 mL), immediately forming a dark red solution, which
faded to light orange after hours. Vapor diffusion of pentane into
CH2Cl2 gave orange plate crystals.
(DmpNAsCl)3 (2AsDmp): (DmpNAsCl)2 (0.300 g), GaCl3
(0.0767 g), DMAP (0.052 g). The solvent was removed under
vacuum to give a white solid, which was dissolved in minimal CH2-
Cl2, and vapor diffusion with pentane gave rod shaped crystals:
yield 0.140 g (47%); mp 215-217 °C; 1H NMR (CDCl3) 2.59 (s),
2.69 (s), 2.74 (s), 2.79 (s), 7.20 (s), 7.22 (s); IR 310 (1), 336 (3),
384 (9), 449 (14), 524 (9), 534 (19), 700 (11), 718 (8), 782 (5),
868 (2), 915 (10), 979 (15), 1025 (12), 1098 (6), 1163 (4), 1256
(13), 1559 (17), 1628 (18), 1797 (20), 1954 (16). Anal. Calcd
(found) for C24H27N3As3Cl3: C, 41.86 (41.59); H, 3.95 (3.94); N,
6.10 (5.87).
[Dmp3N3As3Cl2][GaCl4] ([3AsDmp][GaCl4]): (DmpNAsCl)2
(0.20 g), GaCl3 (0.052 g); yield 0.12 g (48%); mp 85-95 °C; IR
445 (16), 487 (8), 523 (3), 551 (17), 628 (14), 640 (15), 700 (7),
718 (6), 736 (9), 874 (1), 914 (4), 980 (12), 1025 (11), 1096 (5),
1164 (2), 1262 (10), 1567 (15), 1799 (20), 1875 (19), 1954 (18).
Anal. Calcd (found) for C24H27N3As3Cl6Ga: C, 33.34 (33.51); H,
3.15 (3.39); N, 4.86 (4.61).
[Dipp3N3As3Cl2][GaCl4] ([3AsDipp][GaCl4]): (DippNAsCl)2
(0.10 g), GaCl3 (0.021 g); yield 0.049 g (40%); mp 182-185 °C;
IR 463 (19), 520 (1), 537 (9), 575 (17), 693 (13), 737 (8), 799 (4),
839 (14), 877 (16), 932 (5), 1037 (7), 1053 (12), 1097 (3), 1159
(2), 1180 (10), 1234 (15), 1264 (11), 1314 (18), 1385 (16), 1581
(DippNAsCl)3 (2AsDipp): (DippNAsCl)2 (0.150 g), GaCl3
(0.090 g), DMAP (0.065 g). The solution was filtered through a
small column of silica, which was washed repeatedly with toluene
(∼1 mL). Slow solvent evaporation of the toluene gave plate
Inorganic Chemistry, Vol. 44, No. 16, 2005 5899

Burford et al.
crystals: yield 0.065 g (43%); mp 244-247 °C; 1H NMR (CDCl3)
1.14 (d), 1.28-1.39 (m), 3.49 (sep), 3.83 (sep), 3.99 (sep), 4.10
(sep), 7.32-7.41 (m); IR 303 (6), 327 (16), 355 (4), 425 (7), 523
(11), 748 (18), 800 (2), 838 (9), 877 (1), 933 (13), 1036 (12), 1052
(15), 1098 (5), 1159 (3), 1235 (16), 1255 (17), 1310 (14), 1346
(8), 1365 (8), 1592 (20). Anal. Calcd for C36H51N3As3Cl3 (found):
C, 50.46 (49.65); H, 6.00 (5.74); N, 4.90 (4.85).
Discussion
Discovery and development of new polymeric materials
involving elements of group 15 requires realization of
effective and versatile synthetic procedures and understand-
ing of processes relating monomers, dimers, and oligomers.
Compounds and materials containing phosphorus can be
monitored by the sensitive 31P NMR probe, but elucidation
of the chemistry for compounds of As, Sb, and Bi is less
efficient. Therefore, comparisons in reactivity between
homologous series of compounds are vital. In this context,
synthetic methodologies devised for phosphorus can some-
times be extrapolated to analogous compounds for arsenic,
antimony, and bismuth.
Figure 1. Solid-state structure of [3AsDipp][GaCl4]. Hydrogen atoms and
isopropyl groups have been omitted. Elipsoids are drawn at 50%.
We have embarked on the development of processes that
provide access to oligomeric pnictazanes with the ultimate
goal of discovering new polymeric materials offering a series
of chemically varied but related backbones. Cyclodipnicta-
diazanes are known for all pnictogens and represent potential
starting materials for ring-opening oligomerization and
polymerization. In this context, we have recently discovered
a high yield ring expansion reaction for cyclodiphospha-
diazanes 1P to give the cyclotriphosphatriazanes 2P,14
demonstrating the influence of substituent steric strain
imposed by Dmp and Dipp on the relative stability of the
oligomers. This process has now been extrapolated to the
analogous arsenic derivatives providing high yield prepara-
tions of 2As.
As first recognized by Olah for alkyl derivatives, cyclo-
diarsadiazanes 1As are readily prepared by direct addition
of excess primary amine to AsCl3.37 This process can be
facilitated and generalized in the presence of NEt3, and we
have exploited this to obtain the new derivatives 1AsDmp
and 1AsDipp (see below for discussion of structural and
spectroscopic features). Both derivatives of 1As react rapidly
with 0.66 equiv of GaCl3 to give dark red solutions, and
crystalline materials have been isolated and characterized as
[3AsR][GaCl4]. In addition, crystals of [3AsDipp][GaCl4]
were suitable for X-ray crystallography studies (Figure 1).
The cation [3AsDipp] is a six-membered As-N heterocat-
enated ring with two chloroarsine centers and one arsenium
center, and its formation can be described as a ring expansion
disproportionation that is induced by chloride ion abstraction.
Equimolar combinations of DMAP and [3As][GaCl4]
(including in situ mixtures of 1As with 0.66 equiv of GaCl3)
give yellow solutions, from which the corresponding cyclo-
triarsatriazanes 2As have been isolated. The reactions are
envisaged to involve release of chloride ion from the gallate
anion by formation of DMAP-GaCl3 and consequential
formation of a covalent As-Cl bond in 2As. While the
Figure 2. Solid-state structure of 2AsDmp. Hydrogen atoms have been
omitted. Elipsoids are drawn at 50%.
reaction stoichiometry (3:2 1As:GaCl3) is appropriate for the
formation of [3As][GaCl4], the ring expansion process is
facilitated by increasing the relative amount of GaCl3, from
0.66 to 2 equiv, given suitable reactions times (>2 h) before
addition of DMAP. Addition of DMAP to the reaction
mixture after a short (<30 min) reaction time gives 1As
quantitatively. We speculate that the increase in the sto-
ichiometric ratio of GaCl3 promotes the formation of
[RNAs+] by chloride ion abstraction and ring cleavage, and
drives the equilibrium toward the six-membered heterocyclic
cation 3As.
Selected solid-state structural features for 1AsDmp, 1As-
Dipp, 2AsDmp, 2AsDipp, and [3AsDipp][GaCl4] are listed
in Table 2, together with analogous features for 2AsMe,38
representing the only previous structural report of an arsazane
trimer. Both 1AsDmp and 1AsDipp adopt a syn configuration
of the chlorine substituents, consistent with the phosphorus
analogues 1PDmp14 and 1PDipp39 and previous structural
reports for other derivatives of 1As,21,24 although one
derivative of 1As with R ) C6H2(CF3)3 exhibits an anti
configuration.22 The geometries at nitrogen and arsenic in
1AsDmp and 1AsDipp are distinguished by the sums of the
three angles at each site (Σ°), defining an almost planar
(38) Weiss, J.; Eisenhuth, W. Z. Naturforsch. 1967, 22b, 454-455.
(39) Burford, N.; Cameron, T. S.; Conroy, K. D.; Ellis, B.; Macdonald, C.
L. B.; Ovans, R.; Phillips, A. D.; Ragogna, P. J.; Walsh, D. Can. J.
Chem. 2002, 80, 1404-1409.
(37) Olah, G. A.; Oswald, A. A. Can. J. Chem. 1960, 38, 1428-1430.
5900 Inorganic Chemistry, Vol. 44, No. 16, 2005

cyclo-2,4,6-Triarsa-1,3,5-triazanes
Figure 3. 1H NMR spectrum of 2AsDmp in CDCl3 at 298 K.
geometry at each nitrogen center (almost 360°; Table 2) and
a distinctly pyramidal geometry at each arsenic center
(substantially less than 360°). The slight distortion of the
nitrogen centers from planarity is due to a combination of
ring strain and substituent steric strain.
maintained in solution. Four distinct ortho-methyl signals
in the 1H NMR spectra of (DmpNPBr)3 and 2AsDmp (Figure
3), with relative intensity of 1:2:1:2, suggest restricted
rotation of the 2,6-dimethylphenyl substituents. In contrast,
2PDmp14 exhibits only two signals in the 1H NMR spectrum
consistent with free rotation about the N-C bonds.
A combination of ring strain and substituent steric strain
is likely responsible for the preferred anti configuration in
derivatives of 2, which enables maintenance of the almost
planar geometry at each nitrogen center and the distinctly
pyramidal geometry at each arsenic or phosphorus center
(substantially less than 360°; Table 2). All bond lengths
(N-As, N-P, As-Cl, and P-Cl) are essentially identical
with those observed in the corresponding dimers and are
representative of single bonds.
Although numerous mechanistic pathways can be consid-
ered for the disproportionation of 1Pn to 2Pn, dissociation
of the dimer to provide the corresponding monomer is
consistent with the observed equilibrium between Mes*NPOTf
and (Mes*NPOTf)2, which have been isolated as a mixture
in the solid state.36 Isolation of the cationic heterocyclic
intermediates 3PDipp14 and 3AsDipp indicates that GaCl3
facilitates the dissociation and possibly the insertion process.
As for [3PDipp][GaCl4],14 the solid-state structure of
[3AsDipp][GaCl4] (Figure 1) is distinctly ionic with two
covalent As-Cl bonds [As-Cl 2.207(1) and 2.209(1) Å] in
a syn configuration about the six-membered As3N3 hetero-
cyclic frame and a long anti-configured As3---Cl3 contact
[2.708(1) Å] representing the closest interaction between
cation and complex anion. The N-As3 distances are
correspondingly distinct and are shorter [1.793(2) and
1.786(2) Å] than the other four N-As bonds in the
heterocycle [1.829(2), 1.834(2), 1.864(2), 1.881(2) Å],
highlighting cation 3As as an arsazane/arsenium hybrid and
indicating a significant π interaction in the N-As3 bonds.
In this context, the N2-As3 and N3-As3 bonds compare
with those in previously reported examples of azarsenium
salts [4, 1.776(4) Å;40 5, 1.752(5), 1.949(4) Å, this cation
adopts a dimeric arrangement in the solid-state involving
intermolecular N-As interactions that effect disruption of
one N-As π interaction;41,42 6, 1.68(3), 1.67(2)].42
Conclusion
The substituents 2,6-dimethylphenyl (Dmp) and 2,6-
diisopropylphenyl (Dipp) impose “medium” substituent steric
strain on derivatives of (RNAsCl)2 influencing the relative
thermodynamic stability of potential oligomers in favor of
the trimers (RNAsCl)3. Extrapolation of observations for
phosphazanes to arsazanes demonstrates the general ap-
plicability of this new synthetic procedure for the develop-
ment and diversification of pnictazane chemistry.
Interesting similarities and distinctions are apparent be-
tween 2PDmp,14 (DmpNPBr)3,14 and 2AsDmp (Figure 2).
All adopt an anti configuration of the halogens that is
Acknowledgment. We thank the Natural Sciences and
Engineering Research Council of Canada, the Killam Pro-
gram of the Canada Council for the Arts, the Canada
(40) Burford, N.; Parks, T. M.; Royan, B. W.; Richardson, J. F.; White, P.
S. Can. J. Chem. 1992, 70, 703-709.
(41) Burford, N.; Parks, T. M.; Royan, B. W.; Borecka, B.; Cameron, T.
S.; Richardson, J. F.; Gabe, E. J.; Hynes, R. J. Am. Chem. Soc. 1992,
114, 8147-8153.
(42) Burford, N.; Macdonald, C. L. B.; Parks, T. M.; Wu, G.; Borecka,
B.; Kwiatkowski, W.; Cameron, T. S. Can. J. Chem. 1996, 74, 2209-
2216.
Inorganic Chemistry, Vol. 44, No. 16, 2005 5901

Burford et al.
Supporting Information Available: CIF file for the crystal
structures. This material is available free of charge via the Internet
at http://pubs.acs.org.
Research Chairs Program, the Canada Foundation for In-
novation, and the Nova Scotia Research and Innovation Trust
Fund for funding, the Atlantic Region Magnetic Resonance
Center for use of instrumentation, and C. A. Dyker for
performing preliminary experimental work.
IC050532M
5902 Inorganic Chemistry, Vol. 44, No. 16, 2005