A triazadiphosphole

Article abstract of DOI:10.1002/anie.200502475

Two GaCl₃ adducts of 4-bis(trimethylsilyl)amino-1,2,4,3,5- triazadiphosphole were formed in simple high-yielding reactions. The adducts were fully characterized (see structure of the monoadduct) and shown to be stable at ambient temperature. The structure, bonding, and NMR spectra of the triazadiphosphole adducts are discussed on the basis of MO, NBO, and DFT calculations. (Chemical Equation Presented).

Full text of DOI:10.1002/anie.200502475

Communications
P,N Heterocycles
DOI: 10.1002/anie.200502475
A Triazadiphosphole**
Sebastian Herler, Peter Mayer,
Jörn Schmedt auf der Günne, Axel Schulz,*
Alexander Villinger, and Jan J.Weigand
In memory of Erhard Kurras
Phosphorus–nitrogen compounds with low-coordinated
phosphorus(iii) atoms are a well-established group of com-
pounds.[1–3] However,covalent azide analogues such as R-
PNN and R-NNP (R = organic group) are not yet known. As
part of our research directed toward the synthesis of
(TMS)PNN and (TMS)NNP species,we have recently inves-
tigated the elimination of trimethylsilylchloride (TMSCl) in
the thermal decomposition of N,N’,N’-[tris(trimethylsilyl)hy-
drazino]dichlorophosphane (1).[4] This experimental and
theoretical study revealed rather large activation barriers
(38–45 kcalmolꢀ1) for the successive intrinsic elimination of
TMSCl in 1 and led to the conclusion that Lewis acids should
be used to eliminate TMSCl at ambient temperature. In this
context,we have studied the reactions of 1 with gallium
trichloride as a Lewis acid.[5]
Compound 1 is stable in dichloromethane at ambient
temperature for several days as shown by 31P NMR studies
(d(31P) = 166.6 ppm).[4] However,the 31P NMR spectra for
reaction mixtures of 1 (2 equiv) and GaCl3 (1 equiv) in
CH2Cl2 at ambient temperature show rapid,quantitative
formation of the new phosphorus species 2 within 2 h
(Scheme 1).[6] Two new different phosphorus resonances
belonging to only one species are observed in the range
typical for dicoordinated phosphorus(iii) compounds (dou-
2
blets at d(31P) = 317.2 (P1) and 292.1 ppm (P2),[7a] JP,P
=
22.1 Hz; cf. 265.1 ppm, 2JP,P = 22.7 Hz for a 1,2,3,5-diazadi-
phosphole (RN2P2C,R = Me)[8]). Removal of the solvent
resulted in a yellow polycrystalline powder. 31P MAS NMR
studies proved the existence of the same species in the solid
state as that observed in the solvent 31P NMR experiment.
Because there are two independent molecules in the unit cell,
four resonances at d(31P) = 318/312 (P1 or P3) and 296/
285 ppm (P2 or P4) were found in the 31P MAS NMR
spectrum (Figures 1 and 2). Interestingly,we were able to
[*] S. Herler, Dr. P. Mayer, Dr. J. Schmedt auf der Günne, Dr. A. Schulz,
A. Villinger, Dr. J. J. Weigand
Department Chemie undPharmazie
Ludwig-Maximilians-Universität München
Butenandtstrasse 5–13 (Haus D), 81377 München (Germany)
Fax: (+49)89-2180-77492
E-mail: axel.schulz@cup.uni-muenchen.de
[**] A.S. thanks Prof. Dr. T. M. Klapötke (LMU München) for his
generous support andProf. Dr. A. Schmidpeter, Dr. K. Karaghiosoff,
andDr. B. Krumm for their interest, help, andadvice.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Chemie
of solvent at ꢀ108C. Single-crystal X-ray studies revealed a
surprising GaCl3 adduct 2 of the hitherto unknown 4-
bis(trimethylsilyl)amino-1,2,4,3,5-triazadiphosphole
(Fig-
ure 2).[7b]
The GaCl3-induced elimination of TMSCl (Scheme 1)
from species 1 at ambient temperature leads to 2 quickly and
in high yields (reaction time 2 h,yield > 95%). A solution of
2 in CH2Cl2 is stable for at least four weeks (according to
31P NMR experiments). Adding yet another equivalent of
GaCl3 to a solution of 2 results immediately in the formation
of a new species: the GaCl3 diadduct 3 of 4-bis(trimethylsi-
lyl)amino-1,2,4,3,5-triazadiphosphole (Scheme 1) as shown
by 31P NMR studies (CH2Cl2 solution: 304.6 ppm,solid
(MAS): 298 ppm). The same product is formed from the
reaction of 1 (1 equiv) and GaCl3 (1 equiv). Removal of the
solvent again results in a yellow polycrystalline powder (3,
yield > 95%). X-ray quality crystals were obtained from a
saturated solution of 3 in CH2Cl2 (Figure 3).[7b]
Scheme 1. Synthesis of GaCl3 adducts 2 and 3 of 4-bis(trimethylsilyl)-
amino-1,2,4,3,5-triazadiphosphole.
Figure 1. 31P MAS spectrum of 3. The four resonances arise from two
independent molecules in the unit cell (spinning side bands are
markedwith asterisks).
Figure 3. ORTEP drawing of the molecular structure of 3 in the crystal.
Thermal ellipsoids with 50% probability at 200 K (hydrogen atoms
ꢀ
omittedfor clarity). Selectedbondlengths [Š] andangles [ 8]: Ga1 N3
ꢀ
ꢀ
ꢀ
ꢀ
2.036(4), Ga2 N4 2.035(4), P1 N1 1.682(3), P1 N4 1.630(4), P2 N1
ꢀ
ꢀ
ꢀ
1.675(3), P2 N3 1.637(4), N1 N2 1.414(5), N3 N4 1.379(5); N1-P1-
N4 94.4(2), N1-P2-N3 94.4(2), P1-N1-P2 117.1(2), P1-N4-N3 117.1(3),
P2-N3-N4 116.9(3), P1-N1-N2 121.5(3), P2-N1-N2 121.4(3).[7b]
Both species 2 and 3 are air and moisture sensitive but
stable under argon atmosphere over a long period as solids
and in common organic solvents (e.g. benzene,CH 2Cl2,
ether). The yellow color of 2 and 3 vanishes rapidly when
traces of H2O are present. Compounds 2 and 3 are easily
prepared in bulk and are infinitely stable when stored in a
sealed tube and kept cool at 48C in the dark. This together
with the very good solubility in almost all common organic
solvents makes both compounds good precursors for further
synthesis.[7a–d] Compound 2 is thermally stable up to over
1308C,while 3 can be heated only up to 968C. At these
temperatures decomposition starts,accompanied by a slow
elimination of TMSCl (MS experiments).
Figure 2. ORTEP drawing of the molecular structure of one independ-
ent molecule of 2 in the crystal.[9] Thermal ellipsoids with 50%
probability at 200 K (hydrogen atoms omitted for clarity). Selected
ꢀ
ꢀ
bondlengths [Š] andangles [ 8]: Ga1 N4 1.978(3), P1 N1 1.670(4),
ꢀ
ꢀ
ꢀ
ꢀ
P1 N4 1.633(4), P2 N1 1.694(4), P2 N3 1.603(4), N1 N2 1.451(5),
ꢀ
N3 N4 1.380(5); N1-P1-N4 93.2(2), N1-P2-N3 96.1(2), P1-N1-P2
116.4(2), P1-N4-N3 119.6(3), P2-N3-N4 114.8(3), P1-N1-N2 120.5(3),
P2-N1-N2 122.9(3).[7b]
assign these two groups of resonances on the basis of the
excellent agreement between theoretically and experimen-
¯
Compound 2 crystallizes in the triclinic space group P1
tally obtained values of the 31P chemical shift tensor (diso, daniso
,
with two independent molecules in the unit cell consistent
with the 31P MAS NMR data (Figure 1).[7b,9] In agreement
with our DFT calculations,[7c] the molecule adopts a staggered
configuration with the planar five-membered P2N3 ring
see the Supporting Information).[7a–b,9]
Crystals suitable for X-ray diffraction were obtained from
a yellow hexane/CH2Cl2 solution of 2 after stepwise removal
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almost perpendicular to the Si1-N2-Si2 plane (P1-N1-N2-Si2
92.3(3),P2-N1-N2-Si1 84.7(3) 8). Upon formation of the GaCl3
adduct,the C2v symmetry of the P2N3 ring is slightly distorted,
whereas,the p-AO lone pair of N1 is part of the 6 p-electron
system of the P2N3 ring,as indicated by an investigation of the
noncovalent effects.[17] There are no significant intramolecu-
lar interactions (noncovalent effects) between the bis(trime-
thylsilyl)amino group and the ring. Hence,neither the lone
pair of the amino group nor the TMS groups stabilize the P2N3
ring significantly.
Over the past 50 years only a very small number of planar
five-membered-ring molecules and ions containing only the
elements phosphorus(iii) and/or nitrogen have been isolated
and fully characterized (Scheme 2). Only three are known:
ꢀ
ꢀ
ꢀ
with two shorter P N bonds (P1 N4 1.633(4),P2 N3
ꢀ
ꢀ
1.603(4) Š) and two longer P N bonds (P1 N1 1.670(4),
ꢀ
ꢀ
P2 N1 1.694(4) Š). These P N distances between 1.63 and
1.70 Š are substantially shorter than the sum of the covalent
[10,11]
ꢀ
=
radii (dcov: N P 1.8,N P 1.6 Š),
which indicates partial
ꢀ
double-bond character for the P N bonds. A similar situation
is found for the N3 N4 bond (1.380(5) Š) of the P2N3 ring,
which also lies in the range between a single and a double
ꢀ
ꢀ
=
ꢀ
bond (dcov.: N N 1.4,N N 1.2 Š). The N1 N2 bond is a
ꢀ
typical single bond,with a length of 1.451(5) Š. The Ga
N
ꢀ
bond length of 1.978(3) Š (Ga1 N4) is slightly shorter than
those found for the adducts Cl3Ga·NMe2SiMe2NMe2
(2.003(5) Š),[12] MeCl2Ga·H2N-NHtBu (2.023(7) Š),[13] and
ꢀ
the dimeric gallium amide [{Me2Ga N(Ph)SiMe3}2]
(2.071(2) Š).[14] The P-N-N angles (114–1238) are rather
large compared to the N-P-N angles (93–968),and the Cl-
Ga-Cl angles decrease upon adduct formation (110–1158).
The structural features of 3 are very similar to those of 2
(Figure 3).[7b] The P N,N N,and Si N bond lengths and all
angles within the {P2N3-N(TMS)2} fragment do not change
much upon addition of a second GaCl3 molecule. However,as
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
expected,the two Ga N distances in 3 (Ga1 N3 2.036(3) and
Scheme 2. Binary P-N five-memberedrings with P III atoms. Species in
blue are known and fully characterized, the triazadiphosphole is in red.
(R=organic or inorganic group).
ꢀ
Ga2 N4 2.034(4) Š) are slightly longer than those in 2 (cf.
[7b,9]
ꢀ
ꢀ
Ga1 N4 1.978(3),Ga2 N8 1.968(3) Š)
repulsion and a weaker donor–acceptor bond (smaller
charge transfer,see below). It is interesting to note that for
due to steric
both adducts 2 and 3 noticeable HTMS···ClGaCl contacts are
The neutral pentazole derivatives,RN 5,[18] the pentaphospha-
3
ꢀ
[19]
found (2: 2.855(1); 3: 2.840(1) Š),which are shorter than the
sum of the van der Waals radii (dvdW: H···Cl 2.95 Š).[7b,15] This
intermolecular interaction links the molecules as a chainlike
structure in the solid.
cyclopentadienide anion,P
,
and the 1-alkyl-4-aryltetra-
5
azaphospholium cations,R 2N4P+ (as tetrachloroalumi-
nate).[20] Additionally,the pentazolate anion, cyclo-N5ꢀ,has
been detected recently.[21] Since the electronic structures of
these three classes of planar ring systems and the novel
triazadiphosphole all have (4n + 2) p electrons and therefore
formally obey the Hückel rule,these compounds are related
to aromatic hydrocarbons.[22]
Adducts such as 2 and 3 are typical charge-transfer
complexes,and the bond between the GaCl and the RP2N3
3
fragments can be considered a donor–acceptor bond.[16] The
calculated gas-phase Gibbs energy[7c] for the formation of the
monoadduct,P 2N3-N(TMS)2
+
GaCl3
!
2
(DG298
=
Formally,the formation of the P 2N3 ring in 2 and 3 might
ꢀ7.8 kcalmolꢀ1),is in agreement with the experiment since
no equilibrium between 2 and the dissociated fragments could
be observed in 31P NMR studies. The diadduct formation, 2 +
GaCl3 ! 3,is a true equilibrium reaction at the B3LYP level
of theory[7c] (DG298 = 0.6 kcalmolꢀ1,Scheme 1). However,this
was not evident in the 31P NMR experiments since only
species 3 was detected.[16] According to NBO analysis
(NBO = natural bond orbital),[17] the charge transfer is
about 0.16e in 2 and 0.11e per GaCl3 fragment in 3.[7e] The
be regarded as a
[3+2] cycloaddition of NNPꢀ and
(TMS)2NNP+ ions or as a dimerization of an azide analogue
of (TMS)NNP,which has been stabilized by cycloaddition
followed by a 1,4-shift of one TMS group. We assume that in
the first step a 1,2-elimination of TMSCl in 1 occurs assisted
by GaCl3. All together two TMSCl molecules are eliminated
from 1,and two condensation steps followed by adduct
formation are needed. The exclusive formation of 2 and 3
demonstrates the thermodynamic preference for the five-
membered framework over the six-membered alternatives
and is consistent with the prominence of the known binary
five-membered-ring PN cations,and polyphosphorus
anions.[19,20]
In conclusion,we present here an easy,high-yielding
(> 95%) synthetic procedure and full characterization (see
the Supporting Information) of the first triazadiphosphole
stabilized as a GaCl3 mono- and diadduct.[7a–d] Moreover, 2
and 3 are rare examples of structurally characterized Lewis
acid–base adducts involving a ring atom within a PN ring.[23]
Because of the astonishing stability of 2 and 3 in solution as
ꢀ
P N s and p bonds are highly polarized,while the adjacent
ꢀ
ꢀ
N1 N2 and N3 N4 in 2 and 3 are almost ideally covalent
(Figure 2 and Figure 3).[7e] The nitrogen atoms N1 and N2
have almost planar surroundings (e.g. 2: Si2-N2-N1-Si3 174.9,
Si1-N1-P1-N2 174.78; all N-N-X (X = P,Si) angles are
between 113 and 1238). Hence,as indicated by NBO analysis,
the lone pair (LP) on each nitrogen atom in 2 and 3 is
localized in a pure p-type atomic orbital (p-AO). The two lone
pairs are also perpendicular to each other. As a consequence
the p-AO lone pair of N2 atom lies in the ring plane and does
not contribute to the p-electron system of the P2N3 ring
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[9] The structural parameters of second independent molecule are
well as in solid,both compounds may have great synthetic
potential,for example,as sources for binary PN or ternary
PNGa species.
almost identical to those of the first; hence,we have only
discussed the data of the first molecule.
[10] A. F. Holleman,E. Wiberg,N. Wiberg, Lehrbuch der Anorga-
nischen Chemie,101. Aufl.,de Gruyter,Berlin, 1995,Anhang V.
Received: July 15,2005
Revised: August 17,2005
Published online: October 31,2005
[11] E. Niecke,O. Altmeyer,M. Nieger,
J.Chem.Soc.Chem.
Commun. 1988,945 – 946.
[12] W. R. Nutt,J. S. Blanton,A. M. Boccanfusco,L. A. Silks III,
A. R. Garber,J. D. Odom, Inorg.Chem. 1991, 30,4136 – 4140.
[13] H. Nöth,T. Seifert, Eur.J.Inorg.Chem. 2002,602 – 612.
[14] S. D. Waezsada,C. Rennekamp,H. W. Roesky,C. Röpken,E.
Parisini, Z.Anorg.Allg.Chem. 1998, 624,987 – 990.
Keywords: bonding analysis · Lewis acid–base adducts ·
.
P,N heterocycles · structure determination ·
triazadiphospholes
[15] a) A. Bondi, J.Phys.Chem. 1964, 68,441 – 451; b) see Figure S3
and S4 in the Supporting Information.
[16] It is known that for adducts in the solid state and gas phase,
structural data and donor–acceptor energies can be quite
different. Leopold et al. have indicated that the donor–acceptor
bond is much shorter in the solid state than in the gas phase,and
this change has been associated with the substantial dipole
moment of the adduct. D. L. Fiacco,Y. Mo,S. W. Hunt,M. E.
Ott,A. Roberts,K. R. Leopold, J.Phys.Chem.A 2001, 105,484 –
493,and references therein.
[17] a) E. D. Glendening,A. E. Reed,J. E. Carpenter,F. Weinhold,
NBO Version 3.1; b) A. E. Reed,L. A. Curtiss,F. Weinhold,
Chem.Rev. 1988, 88,899 – 926.
[18] a) R. Huisgen,I. Ugi, Chem.Ber. 1957, 90,2914 – 2927; b) I. Ugi,
H. Perlinger,L. Behringer, Chem.Ber. 1958, 91,2324 – 2330; c) I.
Ugi,R. Huisgen, Chem.Ber. 1958, 91,531 – 537; J. D. Wallis,J. D.
Dunitz, J.Chem.Soc.Chem.Commun. 1983,910.
[19] a) M. Baudler,D. Düster,D. Z. Ouzounis, Z.Anorg.Allg.Chem.
1987, 544,87 – 94; b) O. J. Scherer,T. Brück, Angew.Chem. 1987,
99,59; Angew.Chem.Int.Ed.Engl. 1987, 26,59; c) M. Baudler,
D. Z. Ouzounis, Naturforsch.B 1989, 44,381 – 382; d) M.
Baudler, Angew.Chem. 1987, 99,429 – 451; Angew.Chem.Int.
Ed.Engl. 1987, 26,419 – 441.
[20] G. David,E. Niecke,M. Nieger,V. van der Goenna,W. W.
Schöller, Chem.Ber. 1993, 126,1513 – 1517.
[21] a) A. Hahma,E. Holmberg,N. Hore,R. Tryman,S. Wallin,H.
ꢀstmark, 33rd International Annual Conference of ICT,Karls-
ruhe,Germany, 2002,62/1; b) A. Vij,J. G. Pavlovich,W. W.
Wilson,V. Vij,K. O. Christe, Angew.Chem. 2002, 114,3177 –
[1] a) E. Niecke,W. Flick, Angew.Chem. 1973, 85,586 – 587; Angew.
Chem.Int.Ed.Engl. 1973, 12,585 – 586; b) E. Niecke,W. Flick,
Angew.Chem. 1974, 86,128 – 129; Angew.Chem.Int.Ed.Engl.
1974, 13,134 – 135; c) O. J. Scherer,N. Kuhn, Chem.Ber. 1974,
107,2123 – 2125; d) O. J. Scherer,N. Kuhn, J.Organomet.Chem.
1974, 82,C3 – C6; e) O. J. Scherer,N. Kuhn, Angew.Chem. 1974,
86,899 – 900; Angew.Chem.Int.Ed.Engl.
f) E. Niecke,W. Flick, Angew.Chem. 1975, 87,355 – 356; Angew.
Chem.Int.Ed.Engl. 1975, 14,363 – 364; g) R. Appel,M.
1974, 13,811 – 812;
Halstenberg, Angew.Chem. 1975, 87,810; Angew.Chem.Int.Ed.
Engl. 1975, 14,768; h) E. Niecke,R. Kröher, Angew.Chem.
1976, 88,758 – 759; Angew.Chem.Int.Ed.Engl.
1976, 15,692 –
693; i) S. Pohl, Angew.Chem. 1976, 88,723 – 724; Angew.Chem.
Int.Ed.Engl. 1976, 15,687 – 688; j) A. Schmidpeter,J. Luber,H.
Tautz, Angew.Chem. 1977, 89,554 – 555; Angew.Chem.Int.Ed.
Engl. 1977, 16,546 – 547; k) R. O. Day,A. Schmidpeter,R. R.
Holmes, Inorg.Chem. 1983, 22,3696 – 3699; l) N. Burford,
J. A. C. Clyburne,P. K. Bakshi,T. S. Cameron, J.Am.Chem.Soc.
1993, 115,8829 – 8830; m) N. Burford,T. S. Cameron,J. A. C.
Clyburne,K. Eichele,K. N. Robertson,S. Sereda,R. E. Wasy-
lishen,W. A. Whitla, Inorg.Chem. 1996, 35,5460 – 5467; n) A.
Schmidpeter, Heteroat.Chem. 1999, 10,529 – 537; o) N. Burford,
A. D. Phillips,H. A. Spinney,M. Lumsden,U. Werner-Zwan-
ziger,M. J. Ferguson,R. McDonald, J.Am.Chem.Soc.
127,3921 – 3927.
2005,
[2] a) E. Niecke,D. Gudat, Angew.Chem. 1991, 103,251 – 270;
Angew.Chem.Int.Ed.Engl. 1991, 30,217 – 237; b) A. Schmid-
peter, Phosphorus Sulfur 1986, 28,71 – 89,and references
therein.
[3] Multiple Bonds and Low Coordination in Phosphorus Chemistry
3180; Angew.Chem.Int.Ed.
2002, 41,3051 – 3054; c) H.
ꢀstmark,S. Wallin,T. Brinck,P. Carlqvist,R. Claidge,E.
Hedlun,L. Yudina, Chem.Phys.Lett. 2003, 379,539 – 546; d) C.
John,L. A. Burke, Chem.Commun. 2003, 8,1016 – 1017; e) L.
Belau,Y. Haas,S. Zilberg, J.Phys.Chem.A 2004, 108,11715 –
11720; f) P. Carlqvist,H. ꢀstmark,T. Brinck, J.Org.Chem. 2004,
69,3222 – 3225.
(Ed.: M. Regitz,O. J. Scherer),Thieme,Stuttgart,
references therein.
1990,and
[4] G. Fischer,S. Herler,P. Mayer,A. Schulz,A. Villinger,J. J.
Weigand, Inorg.Chem. 2005, 44,1740 – 1751.
[22] a) A. Schmidpeter,K. Karaghiosoff, Heterophospholes in Multi-
ple Bonding and Low Corrdination in Phosphorus Chemistry
[5] N. Burford,K. D. Conroy,J. C. Landry,P. J. Ragogna,M. J.
Ferguson,R. McDonald, Inorg.Chem. 2004, 43,8245 – 8251.
[6] If less than 1 equiv of GaCl3 is used,the reaction is not complete,
i.e. both compounds 2 and 1 are observed.
(Eds.: M. Regitz,O. J. Scherer),Thieme,Stuttgart,
pp. 258 – 286; b) J. D. Woolins, Non-Metal Rings, Cages, and
Clusters,Wiley,Chichester, 1988.
1990,
[23] E. Rivard,A. Lough,T. Chivers,I. Manners, Inorg.Chem. 2004,
43,802 – 811.
[7] See the Supporting Information: a) Chemical shifts (1H, 13C,
29Si{1H}, 31P, 31P-MAS NMR),assignment was based the excel-
lent agreement between theoretically and experimentally
obtained shifts (d(31P)) and coupling constants; b) crystal data;
c) computational details,calculated structural and vibrational
data of all described species (B3LYP),approximate assignment
and trends; d) full description of the experimental data,tech-
nique and details; e) summary of the NBO analysis for 2 and 3:
1) partial charges,2) hybridization effects,3) polarization,and
4) Wiberg bond indices.
[8] a) A. Schmidpeter,C. Leyh,K. Karaghiosoff, Angew.Chem.
1985, 97,127 – 128; Angew.Chem.Int.Ed.Engl.
1985, 24,124 –
125; b) J. C. Tebby, Handbook of Phosphorus-31 Nuclear
Magnetic Resonance Data,CRC,Boston, 1991.
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