4,5-Bis(diphenylphosphanyl)-1,2,3-triazole and its conversion to 1,1,3,3,-tetraphenyl-1,3-diphospha-2,4,5,6-tetraazapentalene

Article abstract of DOI:10.1002/anie.200351437

Towards a new family of ligands: The synthesis of the ligand HLT (depicted left) is described. Although H LTappears to be a simple chelating diphosphane, the tiazole proton can be removed with a base to produce an anionic diphosphane. Thus HL^T

Full text of DOI:10.1002/anie.200351437

Communications
Bis(diphenylphosphanyl) Ligands
4,5-Bis(diphenylphosphanyl)-1,2,3-triazole and Its
Conversion to 1,1,3,3,-Tetraphenyl-1,3-diphospha-
2,4,5,6-tetraazapentalene
Swiatoslaw Trofimenko,* Arnold L. Rheingold, and
Christopher D. Incarvito
The novel ligand system, based on the anion of a 4,5-
bis(diphenylphosphanyl)-1,2,3-triazole core, which may con-
tain substituents, such as O, S, Se, NR or NAr, on the two
phosphorus atoms, was reported for the case where the
substituent was O.[1] This ligand was capable of coordinating
in two different ways forming either a five- (O,N) or a seven-
membered (O,O) chelate ring, depending on the chelated
metal. More recently, the related (LT-S2)ꢀ ligand and some of
its complexes were also described.[2] This general ligand type
is abbreviated as (LT-E2)ꢀ in its anionic form, or as the free acid
HLT-E2, 1, in which E is the particular heteroatom substituent
on phosphorus, with HLT being the abbreviation for the core
molecule, the hitherto unknown 4,5-bis(diphenylphos-
phanyl)-1,2,3-triazole.[3]
Herein we report a) the synthesis of HLT, 2, the core
molecule of the whole HLT-E2 system, by deselenation of
HLT-Se2, and b) an unprecedented reaction of 2 with Me3SiN3,
in the course of which 2 is transformed into the novel
heterocycle, 1,1,3,3-tetraphenyl-1,3-diphospha-2,4,5,6-tetra-
azapentalene, 3.
ꢁ
The reaction of Ph2PC CPPh2 with selenium formed the
[4,5]
ꢁ
known diselenated derivative Ph2P(Se)C CP(Se)Ph2,
which reacted with NaN3 to produce the sodium salt of the
ligand, NaLT-Se2, and this on acidification yielded the free acid,
HLT-Se2. The NaN3 reaction required heating, and longer
reaction times, than in the case of the oxo- and thio- analogues
ꢁ
of Ph2P(Se)C CP(Se)Ph2, commensurate with the lower
ꢀ
¼
electron withdrawing power of the PPh2 Se substituents in
[*] Dr. S. Trofimenko, Prof. Dr. A. L. Rheingold, C. D. Incarvito
Department of Chemistry and Biochemistry
The University of Delaware
Newark, DE 19716 (USA)
Fax : (+1)302-831-6335
E-mail: trofimen@udel.edu
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DOI: 10.1002/anie.200351437
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Angewandte
Chemie
ꢀ
¼
ꢀ
¼
ꢀ
¼
the series PPh2 O > PPh2 S > PPh2 Se and, conse-
quently, decreased activation of the triple bond. The new
ligand was characterized as the triethylammonium salt,
HNEt3LT-Se2, and its structure was determined by X-ray
crystallography[6] and found to be similar to the previously
Compound 2 underwent an unprecedented “pseudo-
Staudinger” reaction with Me3SiN3. Instead of the expected
ꢀ
¼
1,2,3-triazole, which contains two PPh2( NSiMe3) substitu-
ents in the 4,5-positions, as is typical in the reaction of various
diphosphane species with Me3SiN3,[10,11] we obtained a
product that melts at high temperature, 3, which was stable
to air and moisture unlike the very sensitive known RPPh2
reported HNEt3LT-S2 [3]
.
The free acid itself, HLT-Se2 was an important starting
[12]
¼
material in the synthesis of the core heterocycle 2 [Eq. (1)].
( NSiMe3) compounds.
Compound 3 also contained no
SiMe3 signals in the proton NMR spectrum, while its
31P NMR had a single peak at d = 22.3 ppm, a region more
characteristic of phosphorus(v) compounds. For instance, the
HLT-Se2 þ 2 PðOEtÞ3 ! 2 þ 2 Se¼PðOEtÞ3
ð1Þ
31P resonances for Et3NH[LT-E2
] compounds were d =
Treatment of HLT-Se2 with P(OEt)3 at room temperature
resulted in double deselenation and the formation of HLT, 2,
in good yield.[7] The related compounds HLT-S2 and HLT-O2
failed to react with P(OEt)3 at room temperature, and under
conditions designed to force the reaction gave rise only to 1N-
and 2N-ethylated derivatives. The core molecule, 2, has been
structurally characterized (Figure 1). Its molecular dimen-
sions in the triazole ring are similar to those found in HLT-O2
and HLT-S2 with the nonbonding distance between the two P
atoms being 3.488 Š.
19.2 ppm (E = O), 32.6 (E = S) and 22.2 ppm (E = Se), while
HLT had its 31P resonance at d = ꢀ36.1 ppm. Structural
characterization by X-ray crystallography showed 3 to be a
novel molecule, 1,1,3,3-tetraphenyl-1,3-diphospha-2,4,5,6-tet-
raazapentalene, which belongs to the class of polyazapenta-
lenes. Polyazapentalenes are known to be very stable due to
the multiplicity of charge-separated zwitterionic forms that
they can adopt,[13] and this seems to be the first example of a
polyazapentalene containing phosphorus atoms incorporated
into the pentalene ring system. In compound 3, abbreviated as
LT-N, one can write canonical structures with alternating single
and double bonds, in which both phosphorus atoms are in
their highest, pentavalent oxidation state, as shown below.
One can also write numerous charge-separated struc-
tures:
Figure 1. The X-ray crystal structure of HLT, 2. Selected bond distances
ꢀ
ꢀ
ꢀ
[Š] and angles[ 8]: P(1) C(1) 1.820(3); P(1) C(8) 1.827(2); P(1) C(2)
1.84393); C(1A)-C(1)-P(1) 124.95(9); C(1)-C(1A)-P(1A) 124.2(5).
Although, at first glance, 2 may be regarded as a simple
chelating diphosphane, it has the special feature of containing
the triazole proton, which upon removal by base will produce
a chelating anionic diphosphane capable of forming neutral
bisdiphosphane complexes M[LT]2 with divalent metals, and
thus obviating the need for a counterion, ubiquitous in
complexes with analogous neutral vicinal diphosphanes. Thus,
2 can chelate in two modes: as a neutral rigid diphosphane, or
as an anionic rigid diphosphane, in each case forming
identical five-membered chelate rings of C2v symmetry.
Both, the neutral HLT and the anionic (LT)ꢀ ligands are
isoelctronic and isostructural. Such anionic chelating diphos-
phanes are unknown, although chelating diphosphanes form-
ing six-membered rings as, for instance, [Ph2B(CH2PPh2)2]ꢀ
and [PhB(CH2PPh2)3]ꢀ were reported.[8,9]
As in all these charge-separated descriptions of the
molecule the positive charge resides in the diphospha portion
of the pentalene molecule, and the negative charge is
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Communications
distributed on the nitrogen atoms of
the triazole-derived part, it might be
appropriate to write the structure of 3
in delocalized zwitterionic form. Sev-
eral simple cyclic salts, containing
unprecedented fashion with Me3SiN3, and is transformed into
the stable novel heterocycle, 1,1,3,3-tetraphenyl-1,3-diphos-
pha-2,4,5,6-tetraazapentalene, 3. Various aspects of the coor-
dination chemistry of the new ligands (LT-Se2)ꢀ, (LT)ꢀ and
HLT, are being investigated and will be reported in due
course.
within
moiety
a five-membered ring the
+
ꢀ
¼ ꢀ
ꢀ
[ P(Ph2)P N P(Ph)2 ] ,
associated with a separate bromide
anion, were reported.[14]
Experimental Section
The structure of 3 (Figure 2) is a
molecule with C2v symmetry, both
All NMR spectra were determined in [D]chloroform, and the IR
spectra in KBr. The deselenation reaction of HLT-Se2 and the reaction
of HLT with Me3SiN3 were carried out under nitrogen. Each of the
products described was air-stable at ambient temperature.
phosphorus atoms being in identical environments. In agree-
ꢀ
ment with this observation, the two equal P N distances of
HNEt3LT-Se2: A stirred slurry of Ph2P(Se)C CP(Se)Ph2 (19 g,
ꢁ
¼
ꢀ
1.62 Š fell between the typical P N and P N values (1.59 and
34 mol) and NaN3 (2.5 g, 39 mol) in DMF (200 mL) was stirred and
heated to 1008C for until most of the NaN3 had dissolved. The
solution was decanted from a small amount of undissolved NaN3, and
poured slowly into HCl (1 L, 4m). The precipitated crude HLT-Se2 was
separated by filtration off and dried (yield 19.5 g, 96%), and was used
without further purification for the subsequent formation of 2.
HNEt3LT-Se2 was obtained by stirring a sample of HLT-Se2 in methanol
with excess triethylamine, followed by evaporation of this solution,
and recrystallization of the product from acetonitrile. M.p. 196–
1988C, decomp; IR: n˜ = 3048 (m), 2975 (m), 2621 (m), 2478 (m), 2358
(m), 1479 (s), 1436 (s), 1096 (s), 755 (s), 692 (s), 593 (s), 564 (s),
525 cmꢀ1 (s). 1H NMR: d = 7.63 (m, 8H, phenyl), 7.26 (m, 4H,
phenyl), 7.16 (8H, phenyl), 3.00 (quartet, 6H, CH2), 1.01 ppm (t, 9H,
Me). 31P NMR: d = 22.2 ppm, J(31P-77Se) 361 Hz. Elemental analysis
calcd (%) for C32H36N4P2Se2: C 55.2, H 5.17, N 8.05; found: C 55.0,
H 5.09, N 8.00.
¼
1.68 Š, respectively), but closer to a P N double bond. The
triazolyl N(1)-N(2) and N(2)-N(3) bonds were slightly longer
ꢀ
in 3 than in 2 (1.35 versus 1.33 Š), while the triazolyl C N and
ꢀ
C C bonds lengths were essentially the same in 2 and 3 (1.35
and 1.40 Š, respectively). The linking of the two phosphorus
atoms in 3 by the bridging nitrogen resulted in the shortening
of the nonbonded P···P distance by 0.8 Š (2.696 Š in 3 versus
3.488 Š in 2).
2: An excess of P(OEt)3 was added to a stirred slurry of HLT-Se2
(41.0 g, 68.9 mmol) in 100 mL chloroform, whereupon the solid
dissolved within a few minutes. After 2 h the volatiles were removed
by distillation, ultimately under high vacuum, and the crude 2 thus
obtained (yield 29.8 g, 83.3%), was recrystallized from acetonitrile;
m.p. 137–1388C; IR: n˜ = 3175 (b), 3067 (s), 2998 (m), 1476 (s), 1430
(s), 1365 (m), 1092 (s), 1069 (s), 746 (vs), 692 (center of triplet, vs),
505 cmꢀ1 (s). 1H NMR: d = 7.28 ppm (unresolved m, phenyl); 31P: d =
ꢀ36.1 ppm. Elemental analysis calcd (%) for C26H21N3P2: C 71.4,
H 4.81, N 9.69; found: C 71.5, H 4.97, N 9.55%.
3: A mixture of 2 (12.8 g, 0.03 mol) and Me3SiN3 (10 mL,
0.08 mol) in 20 mL toluene was refluxed for 3 h, then the volatiles
were removed by distillation, ultimately under high vacuum. The
residue was washed with toluene/heptane and isolated by filtration.
Crude 3 (yield 10.2 g, 77%) was recrystallized from acetonitrile; m.p.
234–2358C; IR: n˜ = 3051 (s), 1557 (w), 1478 (m), 1434 (s), 1299 (m),
1118 (s), 746 (s), 727 (s), 697 (s), 570 (s), 509 cmꢀ1 (s). 1H NMR: d =
7.45 (four symmetrical m, 8H, o-H), 7.51 (m, 4H, p-H), 7.87 ppm (m,
8H, m-H); 31P NMR: d = 22.3 ppm. Elemental analysis calcd (%) for
C26H20N4P2: C 69.3, H 4.44, N 12.4; found C 69.5, H 4.53, N 12.2%.
Crystal data for 2 and 3: All data were collected by using Bruker
CCD-equipped diffractometers, MoK’’ radiation. For C26H21N3P2 (2),
monoclinic, C2/c, a = 16.3997(13), b = 6.3423(5), c = 22.3242(18) Š,
Figure 2. The X-ray crystal structure of LTꢀN, 3. Selected distances [Š]
ꢀ
ꢀ
and angles[ 8]: P(1) N(100) 1.6147(13); P(2) N(100) 1.6101(13);
ꢀ
ꢀ
ꢀ
P(1) C(1) 1.7809(15); P(2) C(2) 1.7779(15); C(1) C(2) 1.394(2);
N(100)-P(1)-C(1) 101.78(7); N(100)-P(2)-C(2) 101.75(7); P(1)-N(100)-
P(2) 113.32(8).
Whatever the reaction mechanism of 2 with Me3SiN3 may
be, the driving force for the formation of 3 appears to be the
oxidation of both P atoms to the pentavalent state and the
formation of the resonance-stabilized novel heterocycle, since
in numerous other reactions of chelating diphosphanes with
Me3SiN3 such oxidative bridging of the two phosphorus atoms
was never observed. This net transfer of a single bridging
nitrogen atom by Me3SiN3 in this “pseudo-Staudinger”
reaction appears to be truly unprecedented.
b = 104.5810(10)8, V= 2247.2(3) Š3, Z = 4, Z’’ = 1, T= 173 K, 1calc
=
1.293 Mgmꢀ3. Of 4424 data collected, 1968 were independent. R(F) =
5.38%, wR(F2) = 14.61%. There are two orientations for the
molecule in an 83/17 ratio that are inversionally related. For
C26H20N4P2C·0.5CH3CN (3), monoclinic, P21/c, a = 18.6594(11), b =
13.9544(8), c = 18.4924(11) Š, b = 102.7540(10)8, V= 4696.3(5) Š3,
Z = 8, Z’ = 2, T= 153 K, 1calc = 1.336 Mgmꢀ3. The asymmetric unit
contains two molecules of 3 and a molecule of acetonitrile disordered
over two sites in approximately equal distribution. Of 53778 data
collected, 11234 were independent. R(F) = 4.58%, wR(F2) = 13.65%.
For both molecules, all non-hydrogen atoms were refined with
anisotropic thermal parameters and hydrogen atoms were treated as
idealized contributions. All software is contained in libraries distrib-
In summary, we have prepared the selenium analogue of
ꢀ
ꢀ
the known (LT-O2
) )
and (LT-S2 ligands, (LT-Se2)ꢀ, and con-
verted its free acid to the core molecule of the whole HLT-E2
ligand system, HLT, 2. We have also shown that 2 reacts in
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uted by Bruker AXS (Madison, WI). CCDC-179388 (2) and CCDC-
179389 (3) contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge via www.ccdc.
cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallo-
graphic Data Centre, 12, Union Road, Cambridge CB21EZ, UK; fax:
(+ 44)1223-336-033; or deposit@ccdc.cam.ac.uk).
Received: March 18, 2003 [Z51437]
Keywords: · ligand design · N,P ligands · zwitterions
.
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ꢀ ¼
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