[P(μ-NTer)]2: A biradicaloid that is stable at high temperature

Article abstract of DOI:10.1002/anie.201103742

Radical protection makes a biradical: High-temperature-stable biradicaloids [P(μ-NR)]₂ (R=Ter, Hyp) were isolated from [ClP(μ-NR)] ₂ when mild reducing agents were employed. The bulky substituents prevent dimerization. Ter=2,6-Mes

Full text of DOI:10.1002/anie.201103742

Communications
DOI: 10.1002/anie.201103742
P,N Biradicaloids
[P(m-NTer)]2: A Biradicaloid That Is Stable at High Temperature**
Torsten Beweries, Rene Kuzora, Uwe Rosenthal, Axel Schulz,* and Alexander Villinger
In memory of Kurt Dehnicke
As early as 1894 Michaelis and Schroeter isolated the first
phosphorus(III)–nitrogen heterocycle from the reaction of
aniline hydrochloride with an excess of PCl3 (Scheme 1).[1]
Interestingly, the authors assumed to have isolated the
ꢀ = ꢀ
monomeric species, C6H5 N P Cl, which they called “phos-
phazobenzene chloride” but already speculated on the
existence of the dimer. Today we know that the dimer is the
stable form, and such four-membered rings of the type [XP(m-
NR)]2, which contain alternating phosphorus(III) and nitro-
gen centers, are called cyclo-1,3-diphospha(III)-2,4-diazanes
(X = halogen, R = organic group; old name: 1,3-diaza-2,4-
diphosphetidines).[2] They play a major role in preparative
phosphorus–nitrogen chemistry because such species are
Scheme 2. Four-membered heterocycles with alternating N and PIII
atoms bearing 6p electrons derived from cyclo-1,3-diphospha(III)-2,4-
diazanes.
good starting materials for polycyclic inorganic and organo-
metallic compounds.[3,4]
diphospha-2,4-diazane biradicaloid, [P(m-NR)]2 (3; R =
bulky group) which should formally be generated in a two-
electron reduction process upon chloride ion abstraction.
Different reducing reagents, such as lithium metal, [Cp2Ti-
(btmsa)][6] (btmsa = bis(trimethylsilyl)acetylene, Me3Si C
ꢀ ꢁ
Scheme 1. Synthesis of 1,3-dichloro-2,4-diphenyl-cyclo-1,3-diphospha-
(III)-2,4-diazane.
II
ꢀ
C SiMe3) with the reactive {Cp2Ti } fragment concealed in
the h2-bounded btmsa complex, and [{Cp2TiIIICl}2] were
utilized. Furthermore, it was of interest to study the effect
Cyclo-1,3-diphospha(III)-2,4-diazanes (1) can exist as cis
or trans isomers with trigonal-pyramidal P and trigonal-planar
N atoms.[3] Both the N and the P atoms have one localized
lone pair. Thus, formally eight electrons are found for this
type of electron-rich heterocycles (Scheme 2). To the best of
our knowledge, four-membered P2N2 rings bearing 6 p elec-
trons are unknown. As illustrated in Scheme 2, three possible
target molecules (2, 3, and 4) with electronic structures that
are related to those of aromatic hydrocarbons ([4n + 2]p
electrons),[5] can be considered. The most promising candi-
date for synthesis seemed to be the neutral cyclo-1,3-
of the bulky group on the reduction process. Thus, the
terphenyl (Ter= 2,6-Mes2 C6H3, Mes = 2,4,6-Me3C6H2)[7] and
ꢀ
hypersilyl group (Hyp = (Me3Si)3Si)[8] were used for kinetic
stabilization.
Following our interest in Group 15 heterocycle chemis-
try,[9] we describe herein the synthesis, isolation and full
characterization of a formal aromatic P2N2 heterocycle of the
type [P(m-NR)]2 (R = Hyp, Ter) with an unusual biradicaloid
bond situation.
Biradicals can be described as molecules bearing two
unpaired electrons (in two almost degenerate non-bonding
molecular orbitals) that act almost independently of each
other.[10] Species in which the two radical centers interact
significantly are often referred to as biradicaloids.[11,12] In
contrast to very short-lived radical intermediates of organic
reactions, numerous biradicaloids of heavy main-group ele-
ments were isolated in the last two decades, some of which
formally are intermediates in the s-bond-formation pro-
[*] R. Kuzora, Prof. Dr. A. Schulz, Dr. A. Villinger
Universitꢀt Rostock, Institut fꢁr Chemie
Albert-Einstein-Strasse 3a, 18059 Rostock (Germany)
E-mail: axel.schulz@uni-rostock.de
Homepage: http://www.schulz.chemie.uni-rostock.de/
Dr. T. Beweries, Prof. Dr. U. Rosenthal, Prof. Dr. A. Schulz
Leibniz-Institut fꢁr Katalyse e.V. an der Universitꢀt Rostock
Albert-Einstein-Strasse 29a, 18059 Rostock (Germany)
cess.[12] Pioneering work in this field was carried out by Niecke
[13]
et al. for [ClC(m-PMes*)]2
(and later for several deriva-
[**] Support by the Deutsche Forschungsgemeinschaft is highly
acknowledged. We are indebted to Dr. Haijun Jiao (LIKAT) and
Prof. Dr. Frank Breher (KIT) for helpful advice. Ter=2,6-Mes2C6H3
(with Mes=2,4,6-Me3C6H2).
tives), which might be regarded as the isolobal analogues of
the well-studied S2N2 heterocycle.[14] Along with these carbon-
based 1,3-biradicaloids, a 1,3-dibora-2,4-diphosphacyclobu-
tane-1,3-diyl compound [tBuB(m-PiPr2)]2 was reported and
systematically studied by Bertrand et al.[15] Isolobal replace-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201103742.
8974
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 8974 –8978

ment of the nitrogen constituents in S2N2 by ER moieties and
the sulfur atoms by NR groups leads to isovalence-electronic,
experimentally known species [RE(m-NR’)]2 (E = Group 14
Our new method to synthesize 1,3-diazaphosphaallyl-
lithium complexes is fast for 1Ter (< 60 min), but requires
48 h in case of 1Hyp for a complete conversion (2 mmol).[24a]
To study intermediates, 31P NMR spectroscopy experiments
were carried out, which revealed for the reduction of 1Hyp
(singlet at d(31P) = 257 ppm)[19] the intermediate formation of
[16]
element; [ClSn(m-N-SiMe3)]2 and [RGe(m-NSiMe3)]2 (R =
2,6-Dipp2C6H3, Dipp = 2,6-iPr2C6H3)).[17] Recently, the com-
pound [RAl(m-PtBu2)]2 (R = PtBu2) from Schnçckel et al.
extended the area of intermediates of s-bond-formation
processes.[18]
2
biradicaloid 3Hyp (quintet at d = 334 ppm with J(31P-14N) =
42 Hz). For the analogous (but fast) reaction with 1Ter, no
intermediate but rather the 1,3-diazaphosphaallyl species
5Ter was observed (5Ter : d(31P) = 357.6; cf. 5Hyp: d(31P) =
Using lithium (or magnesium) as reducing agent in the
reaction with 1R (R = Hyp,[19] Ter[20]) leads to the unexpected
formation of the diazaphosphaallyl system 5R (Scheme 3,
Figure 1) in good yields (> 90%). Such 1,3 diazaphospha-allyl
lithium complexes had been discussed by Lappert[21] and
ꢀ ꢀ
401.5 and 350–380 Li[R N P=N-Aryl] (R = CPh3, 1-Ad, tBu,
Aryl = tBu3C6H3)).[22] Unfortunately, it was impossible to
isolate 3Hyp from the reaction mixture, but it led to the
idea of using milder reducing agents, such as [Cp2Ti(btmsa)]
and [{Cp2TiCl}2]. The reaction of [ClP(m-NR)]2 with [Cp2Ti-
(btmsa)] yields, depending on the bulky group, either 1) for
R = Hyp the btmsa-bridged addition product 6Hyp; or 2) for
R = Ter the intriguing four-membered biradicaloid hetero-
cycle 3Ter (Scheme 3). In the latter case the larger Ter group
protects biradicaloid 3Ter from addition of the acetylene
(btmsa) and from the dimerization observed for R = tBu.[23]
To avoid addition of btmsa to 3Hyp, the reduction was again
carried out this time utilizing [{Cp2TiCl}2], and indeed
biradicaloid 3Hyp could be observed. 3Ter can also be
prepared when [{Cp2TiCl}2] is used as reducing reagent. To
show that our assumption of a two-step reaction to 6Hyp
when [Cp2Ti(btmsa)] was used is correct, freshly prepared
3Hyp was reacted with free btmsa, which also afforded 6Hyp
as the only product (Scheme 3), while the analogous reaction
of 3Ter with btmsa afforded only the starting materials
(Scheme 3). Therefore, the kinetic protection is more sub-
stantial in 3Ter than in 3Hyp. The larger steric shielding in
3Ter can also be deduced from its reactivity. While 3Ter can
be isolated in large quantities and is thermally stable up to
2248C, 3Hyp once formed decomposes quickly in solution,
and it is extremely difficult to isolate 3Hyp as a pure
substance. Single-crystal studies of 3Hyp
isolated by Niecke et al.[22] in the reaction of R(H)N P = N
Aryl (R = CPh3, 1-Ad, tBu; Aryl = tBu3C6H3)) with nBuLi. In
the only paper on metal reduction of [XP(m-NR)]2 (X = hal-
ogen) species, Paine et al. reported the isolation of an eight-
membered cage compound P4(tBuN)4 , which is a structural
analogue of S4N4.[23] Here the reduction of [ClP(m-NtBu)]2
was carried out with magnesium chips and a dimer of the four-
membered heterocycle 3tBu was found (Scheme 2).
ꢀ
ꢀ
ꢀ ꢁ ꢀ
Scheme 3. Reduction of 1R (btmsa=Me3Si C C SiMe3).
revealed the correct connectivity, but the poor
data set does not allow a detailed discussion.
The different reactivity of 3Hyp in contrast to
3Ter can also be partly attributed to electronic
effects (apart from a larger steric protection in
3Ter). The electronic interactions with substitu-
ents control the extent of biradical character
markedly,[12c] and is increased upon pushing
more electron density to the ring system, which
is the case in 3Hyp compared to 3Ter (cf. charge
of the P2N2 ring: Sq(N2P2) = ꢀ1.22 and ꢀ0.41 e,
respectively).[24b,c]
Diazaphosphaallyllithium complex 5Hyp,
btmsa-bridged 6Hyp, and the biradicaloid 3Ter
were fully characterized by NMR, IR, and
Raman spectroscopy, elemental analysis, and
Figure 1. ORTEP of the molecular structure of 5Hyp (left) and 6Hyp (right) in the
crystal. Hydrogen atoms omitted for clarity. Ellipsoids are set at 30% probability at
173 K. Selected bond lengths [ꢂ] and angles [8]: 5Hyp: Li1–N2 2.01(7), Li1–N1 2.05(6),
single-crystal structure elucidation (Figure 1
and Figure 2).[24a] Here we will focus on biradi-
caloid 3Ter, which is the first neutral all-
pnictogen 6 p-electron four-membered hetero-
cycle.
ꢀ
Li1–P1 2.53(6), N1–P1 1.585(2), N2–P1 1.586(2), P1–N1 Li1 87(2), N1–P1 N2
106.9(1). 6Hyp: P1–N1 1.742(1), P1–N2 1.723(1), P2–N1 1.733(3), P2–N2 1.739(2),
P1–P2 2.442(1), Si1–N1 1.759(1), Si5–N2 1.765(1); N1-P1-N2 83.5(1), P1-N1-P2
89.3(1), P1-N2-P2 89.7(1), P1-N1-Si1 133.6(1), P2-N1-Si1 137.0(1).
Angew. Chem. Int. Ed. 2011, 50, 8974 –8978
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8975

Communications
ꢀ
(a(NPNP) = ꢀ27.3(1)8). Both P N bonds (1.715(1) and
1.718(1) ꢀ) are almost equivalent and lie in the range found
for 1Ter (1.709(2) and 1.731(2) ꢀ), which are shorter than the
ꢀ
sum of the covalent radii for a single bond (dcov(N P) = 1.8,
[28]
=
dcov(N P) = 1.6 ꢀ).
Significantly shorter bonds are
ꢀ
observed in the five-membered tetrazaphospholes Mes*
N4P (1.631(4) and 1.664(3) ꢀ).[9b] Comparison of the struc-
tural data of 1Ter (cis isomer) with those of 3Ter (Table 1)
reveal only small differences. Even the P···P distance is almost
ꢀ
identical (1Ter: 2.612(1) vs. 3Ter: 2.6186(8) ꢀ, cf. Srcov(P
P) = 2.22 ꢀ), indicating no significant transannular interac-
tion. It is interesting to note that the compound [P(m-PMes*)]2
of Fluck et al., which is the isovalence-electronic phosphorus
analogue of 3Ter (both N atoms substituted by two P atoms),
ꢀ
shows a strong transannular interaction with a P P distance of
2.166(2) ꢀ and thus must be referred to as a bicyclotetra-
phosphane.[29] This major difference can be attributed to the
fact that tricoordinated P atoms prefer a pyramidal environ-
ment with a lone pair containing a significant amount of
s orbital character, while nitrogen atoms prefer a trigonal
planar environment when delocalization of the lone pair
(occupying a p orbital) is possible.[24] Thus delocalization of
the 6p electrons (N: 2eꢀ, P: 1eꢀ) stabilizes the biradicaloid
and prevents transannular through-space interaction resulting
in the formation of a bicycle.
Figure 2. ORTEP of the molecular structure of 3Ter in the crystal. H
atoms omitted for clarity; ellipsoids are set at 30% probability at
233 K. Selected bond lengths [ꢂ] and angles [8]: N1–C1 1.423(2), N1–
P2 1.715(1), N1–P1 1.718(1), P1···P2 2.6186(8); C1-N1-P2 130.8(1),
C1-N1-P1 129.6(1), P2-N1-P1 99.44(6), N1i-P1-N1 80.48(8), N1-P2-N1i
i
ꢀ
80.64(8), P2-N1-P1 N1 0.0. Symmetry code: (i) ꢀx, y, ꢀz+3/2.
As illustrated in Scheme 3, both synthetic routes with
either [Cp2Ti(btmsa)] or [{Cp2TiCl}2] as reducing agents
afford in good yields (> 80%) 3Ter as orange microcrystalline
solid. Biradicaloid 3Ter is air- and moisture-sensitive but
stable under argon atmosphere over a long period as solid and
in solvents such as thf, diethyl ether, or toluene, even at
ambient temperatures. The orange color of 3Ter vanishes
rapidly when traces of H2O are present. Like 5Hyp and
btmsa-bridged 6Hyp, biradicaloid 3Ter is easily prepared in
bulk and almost indefinitely stable when stored in a sealed
tube. 31P NMR spectroscopy in particular is suitable to follow
the reduction process and to distinguish between 1Ter
Table 1: Selected bond lengths [ꢂ] and angles [8] along with NAO partial
charges [e] for 1Ter, 3Ter, 5Hyp, and 6Hyp.
1Ter[a]
3Ter[b]
5Hyp[c]
6Hyp
d(N–P)
1.720(2)[c]
2.612(1)
80.93(8)[c]
98.83(8)
ꢀ5.4(1)
+1.32
1.716(1)[c]
2.6186(8)
80.56(8)[c]
99.44(6)
0.0
1.585(2)[c]
2.53(6)
106.9(1)[c]
88.0(2)
4.28(4)
1.29
1.734(3)[c]
2.442(1)
83.5(1)
89.5(1)[c]
27.8(1)
ꢀ1.49
d(P···E)[b]
a(NPN)
a(PNE)[b]
a(NPNE)[b]
q(P)
(d(31P) = 227.4 ppm (cis
isomer), 264.1 ppm (trans
+0.83
isomer)),[20] and 3Ter (d(31P) = 289.8 ppm). The characteristic
deshielding of the P atoms in central P2N2 unit indicates a P,N
p-bonding system. The resonance signals lie in the range
q(N)
ꢀ1.18
ꢀ1.03
ꢀ1.55
+1.16
Sq(NPNE)[b]
+0.28
ꢀ0.41
ꢀ1.22
ꢀ0.67
[a] Taken from reference [20]. [b] E=P for 1Ter, 3Ter, 6Hyp; E=Li for
5Hyp. [c] Averaged values are presented.
ꢀ
typical for cyclic p-delocalized P,N aromatics (cf. Ter N4P:
d(31P) = 217.2 ppm,[9d] Mes* N4P: d(31P) = 226.7 ppm,[9b]
ꢀ
31
[9a]
ꢀ
(Me3Si)2N N3P2: d( P) = 292.1/317.2 ppm), which is con-
siderably more deshielded compared to the compound [Li-
Computations of the electronic structure, and also MO
and NAO analyses (NAO = natural atomic orbital)[24b,c,30]
support the assumption that 3Ter may be regarded as a
biradicaloid with six delocalized p electrons (Figure 3).[24c]
NAO/MO analyses indicate an electron-rich 6p-electronic
system with a pp orbital occupation of 1.65 for the N (2pp) and
1.26e for the P atoms (3pp).[24b,c] The overall charge of the
P2N2 ring amounts to ꢀ0.41e (Table 1). Full optimization at
the UB3LYP/6-31G(d,p) level of theory shows a singlet
ground state with a planar N2P2 ring and no P···P bond. The
singlet state of 3Ter is 22.6 kcalmolꢀ1 lower in energy than the
triplet state (UB3LYP/6-311 + G(d,p)//6-31G(d,p)).[24b,c] UHF
and CASSCF(2,2)/6-31G(d) calculations indicate open-shell
character. The two dominant contributions to the CI wave
2ꢀ
(dme)]+2 [Me3Si C(m-P)]2
(d[31P] = 200.3) from Niecke
ꢀ
et al.[25] The presence of aromaticity is supported by the
calculated NICS(0)[24b,26] value of ꢀ6 ppm (cf. ꢀ7 ppm for
2ꢀ [25]
[Li(dme)]+2 [Me3Si C(m-P)]2
,
and + 5 ppm in 4p elec-
ꢀ
tronic, antiaromatic [TerN(m-Si)]2.[27]
Biradicaloid 3Ter crystallizes as orange crystals from
toluene (or Et2O) without solvent molecules in the mono-
clinic space group C2/c and with four units per cell. The
phenyl rings on the amino nitrogen are twisted to each other,
with a dihedral angle of 27.638 forming a pocket with four aryl
groups (in the 2 and 6 position) in which the N2P2 ring is very
well sterically protected (Figure 2). Thus, the molecular
structure shows, in contrast to P4(tBuN)4, a monomer
consisting of a planar, Cs-symmetric four-membered P2N2
ring (a(P1NP2) = 99.44(6)8, a(N1P1N1’) = 80.48(8)8, and
a(N1P2N1’) = 80.64(8)8). For comparison a butterfly con-
formation is found for 1Ter (a(NPNP) = ꢀ5.3(1)8) and 6Hyp
2
2
2
function for the 1A state are F(1A) = 0.93 j p1 p2 p3 > ꢀ0.35 j
2
2
2
p1 p2 p4 > (Figure 3 left, HOMO = p3, LUMO = p4).[24c] This
corresponds to an occupation of the nonbonding p3 orbital
with 1.7 electrons and thus species 3 possesses considerably
8976 www.angewandte.org
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Angew. Chem. Int. Ed. 2011, 50, 8974 –8978

3Hyp forms alkyne-bridged species 6Hyp upon addition of
alkynes. Thus, highly reactive 3Hyp prepared in situ can be
used as a quenching reagent for further synthesis.
Received: June 1, 2011
Published online: August 19, 2011
Keywords: biradicaloids · heterocycles · main-group elements ·
.
multiple bonds · structure elucidation
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[ClC(m-PMes*)]2 biradicaloid from Niecke et al.[13] In the
VB picture, compound 3 is best described by resonance
between biradical structure (A) and dipole structures (B–E)
as shown in Figure 3 (right). The calculated phosphorus and
nitrogen NAO net charges are qP =+ 0.83 and qN = ꢀ1.03e,
respectively, which would seem to support the conclusion that
structures B–E are rather unlikely VB structures. However
ꢀ
this conclusion ignores 1) the P N s bond polarization that
occurs for the valence shell s electrons: the total spd
populations for these electrons are 2.91 (P) and 4.38e (N),
respectively; and 2) the values of the valence-shell pp AO
populations, 1.26 (phosphorus 3pp) and 1.65 (nitrogen 2pp).
ꢀ
Therefore the P N s-bond polarization rather than the p-
electron distribution is primarily responsible for the magni-
tudes of the atomic net charges.
Over the last decades only a very small number of planar
four-membered ring molecules with an aromatic 6p-elec-
tronic structure have been isolated.[12] Apart from the well-
studied S2N2, which possesses 6% biradical character,[12c,31]
2ꢀ[32]
2+ [33]
and its isoelectronic analogues P4
and S4
,
only one
anion of the type [P(m-CR)]2 has been reported.[25] Biradi-
caloid 3 opens a new class with a N2P2 skeleton that can be
derived from [RC(m-P)]2 by isoelectronic substitution of
2ꢀ
2ꢀ
“Cꢀ” by N. Only recently, Roesky, Frenking et al. reported an
almost planar but antiaromatic 4p-electron-containing stable
dimeric silaisonitrile of the type [Si(m-NTer)]2.[27] It can be
assumed that a two-electron-reduction process should yield
another biradicaloid of the type [Si(m-NTer)]22ꢀ, which would
be isoelectronic to 3Ter.
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