Azadiphosphaindane-1,3-diyls: A Class of Resonance-Stabilized Biradicals

Article abstract of DOI:10.1002/anie.202011886

Conversion of 1,2-bis(dichlorophosphino)benzene with sterically demanding primary amines led to the formation of 1,3-dichloro-2-aza-1,3-diphosphaindanes of the type C₆H₄(μ-PCl)₂N-R. Reduction yielded the corresponding 2-az

Full text of DOI:10.1002/anie.202011886

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Accepted Article
Title: Aza-diphospha-indane-1,3-diyls: A class of resonance-stabilized
biradicals
Authors: Axel Schulz, Jonas Bresien, Dirk Michalik, Alexander Villinger,
and Edgar Zander
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Aza-diphospha-indane-1,3-diyls: A class of resonance-stabilized
biradicals
Jonas Bresien,* Dirk Michalik, Axel Schulz,* Alexander Villinger, Edgar Zander
Dedicated to Prof. Thomas M. Klapötke on the occasion of his 60th birthday
[*]
Dr. J. Bresien, Dr. D. Michalik, Prof. Dr. A. Schulz, Dr. A. Villinger, E. Zander
Institut für Chemie
Universität Rostock
Albert-Einstein-Straße 3a, 18059 Rostock (Germany)
E-mail: jonas.bresien@uni-rostock.de, axel.schulz@uni-rostock.de
Homepage: http://www.schulz.chemie.uni-rostock.de/
Dr. D. Michalik, Prof. Dr. A. Schulz
Leibniz-Institut für Katalyse e.V.
Albert-Einstein-Straße 29a, 18059 Rostock (Germany)
Supporting information for this article is given via a link at the end of the document.
Abstract: Conversion of 1,2-bis(dichlorophosphino)benzene with ste-
rically demanding primary amines led to the formation of 1,3-dichloro-
2-aza-1,3-diphospha-indanes of the type C6H4(μ-PCl)2N-R. Reduction
yielded the corresponding 2-aza-1,3-diphospha-indane-1,3-diyls (1),
which can be described as phosphorus-centered singlet biradi-
cal(oid)s. Their stability depends on the size of the substituent R:
While derivatives with R = Dmp (2,6-dimethylphenyl) or Ter (2,6-
dimesitylphenyl) underwent oligomerisation, the derivative with very
bulky R = tBuBhp (2,6-bis(benzhydryl)-4-tertbutylphenyl) was stable
with respect to oligomerisation in its monomeric form. Oligomerisation
involved activation of the fused benzene ring by a second equivalent
of the monomeric biradical and can be regarded as formal [2+2]
(poly)addition reaction. Calculations indicate that the biradical charac-
ter in 1 is comparable with literature known P-centered biradicals.
Ring current calculations show aromaticity within the entire ring sys-
tem of 1.
In contrast, CO[14] or isonitriles[15] insert into the four-membered
ring system, leading to stable five-membered cyclic biradicals of
type B (Scheme 1). Other pnictogen-based, five-membered cyclic
biradicals (hetero-cyclopentane-1,3-diyls) are synthesized using
the same approach, with varying substituents or pnictogen at-
oms.[15,16] Yet, the activation chemistry of biradicals B is often lim-
ited by the reversibility of the CO or isonitrile insertion, as the uti-
lization of biradicals A and B often lead to the same activation
products (Scheme 2).[14,15,17,18] Still, biradicals of type B are worth-
while target molecules, as they can be reversibly photo-isomer-
ized to a closed-shell housane-type isomer B’ with a transannular
P-P bond, leading to potential applications as molecular switches
(Scheme 1).[19,20]
Singlet biradical(oid)s are molecules with two electrons in two
nearly degenerate orbitals.[1–4] Although their spin density is zero
at every point in space, biradicals can show extraordinary reactiv-
ity that ranges between monoradicals and closed-shell mole-
cules.[5] Starting with pioneering work by Niecke et al., who syn-
thesized the 1,3-diphosphacyclobutane-2,4-diyl [Mes*P(μ-CCl)]2
in 1995,[6] stable main group-centered biradicals came into focus
of many further investigations.[7–11] For example, our group per-
formed comprehensive research on the phosphorus-centered bi-
radical [P(μ-NTer)]2 (A), which was synthesized from a chlorin-
ated precursor by reduction with elemental magnesium (Scheme
1).[12] Biradical A is highly reactive towards polar and non-polar
single, double, and triple bonds (e.g. H2, S8, O2, ketones, alkenes,
alkynes, nitriles, …), typically resulting in addition products with
tri- or penta-valent phosphorus atoms.[13]
Scheme 2. Due to elimination of CY, reactions with biradicals B often lead to
the same reaction products as found for biradical A.[14,15,17,18]
To overcome the instability of B with respect to elimination of
the CY moiety, we chose to investigate structurally related benzo-
fused cyclopentane-1,3-diyls (i.e. hetero-indanediyls 1, Scheme
3), which might also provide aromatic stabilization of the biradical
moiety.
Scheme 1. Synthesis of [P(μ-NR)]2 with R = Ter[12] (A) and ring expansion with
CY (Y = O[14] or NR’[15]) to biradicals of type B (hetero-cyclopentanediyls). B
can be photo-isomerized to the housane-type isomer B’.
Scheme 3. Ortho-quinodimethane[21] (C) and hetero-indane derivatives D,[22–27]
E[28] and F[29,30] with pnictogen atoms in 1,3-position.
1
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(time-dependent 31P NMR spectra cf. Figure S10). This self-acti-
vation process can be regarded as formal [2+2] addition reaction.
The structural motif of 3Ter is yet unknown and represents the
first example of a six-membered carbon cycle substituted by six P
atoms.
An example of closely related, delocalized biradicals are or-
tho-quinodimethanes (C, Scheme 3), which are known as reactive
intermediates in organic synthesis.[21] Furthermore, isoelectronic
hetero-indane derivatives with Group 15 elements in 1,3-position
were reported, such as a variety of stable benzo-2-chalco-1,3-di-
azoles[22–27] (D), 2-substituted benzotriazoles[28] (E), and 2-pnicta-
1,3-diphosphaindenyl anions[29,30] (F). The biradical character of
these compounds (D-F) has not been evaluated yet.
As no reports about target compound 1 were found in the
literature, we opted to synthesize different derivatives with
differently sized subtituents (Dmp, Ter and tBuBhp), in order to
investigate the kinetic stability of 1 towards di- or oligomerization
(for descriptors of steric demand, see Supporting Information, p.
S44ff).[17,31,32] In a first step, a suitable precursor for biradical 1
was synthesized: By analogy with the synthesis of A (Scheme 1),
chlorinated azadiphospha-indanes (2) were prepared by reaction
of primary amines with 1,2-bis(dichlorophosphino)benzene[33,34]
(Scheme 4). For all substituents (Dmp, Ter, tBuBhp), the cis isomer
of 2 (cis with respect to the Cl atoms, Figures S1, S2) was ob-
tained (31P NMR: δ = 147-149 ppm). Only in case of 2Dmp the
trans isomer was observed as side product (31P NMR:
δ = 171 ppm). DFT calculations showed that the cis isomers of 2
are energetically favored for all substituents (ΔGr° = 9-20 kJ/mol,
cf. SI).
Figure 1. Experimental and simulated[37] 31P{1H} NMR spectrum of 3Ter.
In the 31P{1H} NMR spectrum, 3Ter displays an AA’BB’XX’
spin system (Figure 1) due to its C2 symmetry in solution. The shift
of the PX nuclei (287 ppm) is comparable to the resonance of mon-
omeric 1. The three-valent PA (δ = 82 ppm) and PB (δ = 89 ppm)
nuclei show a significant upfield shift, with well resolved JAB
(−31 Hz), JBX (98 Hz) and JXX’ (−18 Hz) coupling constants. The
absolute values of all other coupling constants are significantly
smaller than 5 Hz, but essential for the coupling pattern. The ex-
perimental data agrees well with calculated NMR shifts and cou-
pling constants (cf. Table S3).
The synthesis of hetero-indanediyls 1 (R = Dmp, Ter, tBuBhp)
was achieved by reduction of 2 with elemental Mg analogous to
the synthesis of A.[12,19] During the reaction, the colorless solu-
tions turned orange, indicating the formation of the desired prod-
uct (Scheme 4). This could be confirmed by 31P NMR spectros-
copy; all hetero-indanediyls 1 (R = Dmp, Ter, tBuBhp) could be
identified by a characteristic singlet resonance (δ = 280-285 ppm),
which compares well with related 1,2,5-azadiphospholes
(tBuC)2(μ-P)2NtBu (δ = 286 ppm)[35,36] or biradicals A (276 ppm)[19]
and B (221, 258 ppm).[19] However, depending on the steric de-
mand of R, different follow-up reactions were observed (Scheme
4). Biradical 1Dmp fully converted to an insoluble red polymer
within one day, as evidenced by 31P NMR spectroscopy (Figure
S8). The polymer was isolated and analysed by elemental analy-
sis and vibrational spectroscopy (cf. SI, p. S27ff).
Crystallization of 3Ter from benzene yielded colorless crystals.
The solid-state structure was determined by single-crystal X-ray
diffraction (Figure 2). 3Ter crystallized in the triclinic space group
P1 with two molecules 3Ter and eight highly disordered benzene
molecules per unit cell. The central condensed ring system is
nearly planar (∢(N1-P2-P1-C6) = −179.7(3)°, ∢(P1-C6-C1-C2) =
175.4(3)° and (∢(C1-C2-C5-C4) = 178.6(4)°). The P1-C6 and
P2-C1 bond lengths (1.698(3) and 1.719(3) Å) are almost identi-
cal and lie in the range of the sum of the covalent radii of a P=C
double bond (∑rcov(P−C) = 1.86 Å, ∑rcov(P=C) = 1.69 Å),[38] while
the C1-C6 bond (1.397(5) Å) is slightly longer than the value ex-
pected for a C=C double bond (∑rcov(C−C) = 1.50 Å, ∑rcov(C=C)
= 1.34 Å).[38] These structural parameters indicate a dominant
diene structure with localized P=C double bonds (see computa-
tions below). The transannular P1-P2 distance is 2.921(3) Å and
therefore significantly longer than a P–P single bond (∑rcov(P−P)
= 2.22 Å).[38]
In the case of 1Ter, a selective trimerization to 3Ter was ob-
served. 3Ter was formed via activation of the fused benzene ring
of 1Ter by two further equivalents of the monomeric biradical
Scheme 4. Synthesis of differently substituted 2-aza-1,3-diphosphaindane-1,3-diyls 1. Their stability depends on the sterical demand of the substituent R, as
depicted on the right.
2
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Table 1. LUMO occupancy and biradical character β[3] for selected compounds.
Further descriptors can be found in Table S16.[39,40]
A
B[a]
1tBuBhp
3Ter
LUMO occ.
0.28
0.28
0.27
0.26
0.28
0.28
0.28
0.27
0.18
0.18
0.21
0.14
0.12
0.12
0.12
0.11
CAS(2,2)
β
LUMO occ.
full π
CAS[b]
β
[a] with Y = NDmp. [b] All π-type electrons of the central ring fragment were
included in the active space (A: CAS(6,4), B: CAS(8,6), 1tBuBhp: CAS(10,9),
3Ter: CAS(6,4)).
Theoretical investigations[41–47] were carried out to quantify the
biradical character of compounds 1tBuBhp and 3Ter. CASSCF[48–
56] calculations were performed to obtain a correct description of
the multireference character (cf. SI, p. S41ff). The biradical char-
acter was quantified by the LUMO occupation number and β scale
(defined as β = 2 ⋅ c22 / (c12 + c22) by Xantheas et al.).[3] First, simple
CAS(2,2) calculations were performed, which ignore any dynamic
correlation within the π-bonding system. In this simple picture,
LUMO occupancy and β are identical by definition. The biradical
character of 1tBuBhp amounts to 18%, which is slightly lower in
comparison with other biradicals such as A and B (Table 1). In
contrast, the biradical character of 3Ter (12%) is significantly
smaller, so it is better described as a diene. This is in accord with
other literature reports.[20]
Figure 2. Molecular structure of 3Ter. Ellipsoids are set at 50 % probability
(123 K). Selected bond lengths [Å] and dihedral angles [°]:C1–C2 = 1.509(5),
C1–C6 = 1.397(5), C2–C3 = 1.559(5), C3–C4 = 1.554(5), C4–C5 = 1.558(5),
C5–C6 = 1.505(5), N1–P1 = 1.698(3), N1–P2 = 1.698(3), P1–C6 = 1.698(3),
P2–C1 = 1.719(3), P1–P2 = 2.921(3); C1–C2–C5–C4 = 178.6(4), N1–P2–P1–
C6 = −179.7(3), P1–C6–C1–C2 = 175.4(3).
1tBuBhp, the most sterically demanding derivative, was stable
in benzene solution for several weeks, as verified by NMR spec-
troscopy. 1tBuBhp is intensely yellow and shows absorption max-
ima at 407 and 424 nm in the UV-vis spectrum (benzene solution).
According to time-dependent density functional theory (TD-DFT)
calculations, the main absorption at 424 nm can be attributed to
the formal HOMO→ LUMO transition (λcalcd = 470 nm, PBE-
D3/def2-TZVP).
Figure 3. Molecular structure of 1tBuBhp. Ellipsoids are set at 50 % probability
(123 K). Selected bond lengths [Å] and dihedral angles [°]: P1–N1 = 1.696(2),
P1–C42 = 1.742(2), P2–N1 = 1.692(2), P2–C37 = 1.742(2), C37–C38 =
1.425(3), C37–C42 = 1.428(3), C38–C39 = 1.365(3), C39–C40 = 1.406(3),
C40–C41 = 1.357(3), C41–C42 = 1.426(2); C42–C41–C38–C39 = 179.3(3),
N1–P1–P2–C37 = −179.4(2), P1–C37–C42–C41 = −177.9(2).
Figure 4. Frontier orbitals of 1tBuBhp (CAS(10,9)/def2-TZVP//PBE-D3/def2-
TZVP). Only the main contributions to the wave function are given. For an il-
lustration of all molecular orbitals within the active space see Figure S17.
Secondly, CAS calculations including all π-type orbitals of the
main ring fragment were performed, thus including non-dynamic
and dynamic correlation within the π-bonding system (Figure 4,
Figures S15-S18). In case of 1tBuBhp, this procedure led to sig-
nificantly different values for LUMO occupancy and β, while these
values hardly differed in case of A, B and 3Ter (Table 1). As β is
based on only two coefficients of the CAS wave function, whereas
the LUMO occupancy reflects a sum over many determinants,
large deviations indicate a strongly correlated wave function.
Nonetheless, considering that all coefficients apart from c1 and c2
individually contributed about 1% or less to the CAS wave func-
tion, the difference between LUMO occupancy and β is primarily
attributed to dynamic correlation.
1tBuBhp could be crystallized from toluene and was examined
by single-crystal X-ray diffraction (Figure 3). It crystallized in the
monoclinic space group P21/n with four molecules per unit cell.
Similarly to 3Ter, the hetero-indanediyl moiety is planar within the
margin of error (∢(N1-P1-P2-C37) = −179.4(2)°, ∢(P1-C37-C42-
C41) = −177.9(2)°, ∢(C42-C41-C38-C39) = 179.3(3)°). Yet, both
P-C bonds (1.742(2) Å) are significantly elongated compared to
3Ter, indicating a reduced P-C double bond character, and thus
a delocalized π-bonding system (see computations below). The
transannular P-P distance (2.9574(7) Å) is similar to 3Ter and
type B biradicals (Y = O: 2.961 Å[14];Y = NDmp: 2.944 Å[19]).
3
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Figure 5. Streamline plot of the current density susceptibility[57] for benzene (a), indole (b), [P(μ-NH)]2 (AH) (c) and 1H (d). For color version see Figure S19.
In the localized orbital picture, it is apparent that biradical
1tBuBhp also possesses some zwitterionic character (approx.
80 % covalent, 20 % ionic), as evidenced by contributions of de-
terminants Ψ2 and Ψ3 (Figure 4, right). The “biradical electrons”
are mainly localized at the P atoms, but also somewhat delocal-
ized across the fused benzene ring. This is, of course, a unique
feature of the benzo-fused ring system in 1tBuBhp compared to
biradicals A or B (Scheme 1).
the first stable heteroindane-1,3-diyl. The biradical character of
1tBuBhp is somewhat lower than the biradical character of other
P-centered biradicals, due to its aromatic stabilization. The self-
activation of 1Ter yielding trimer 3Ter demonstrates that this new
substance class has potential for further activation chemistry,
which was limited in case of previously reported five-membered
cyclic biradicals B due to elimination problems.[13] Reactivity stud-
ies and investigation of the photochemistry of 1tBuBhp are under-
way. Moreover, we plan to analyze the effect of substitutions in
the aromatic backbone or replacement of P by heavier pnictogens
on the reactivity and stability of the resulting biradicals.
All these apparent differences in their electronic structures
prompted us to revisit the aromaticity of compounds 1tBuBhp, A,
and B. One essential parameter is the magnetically induced ring
current,[57,58] which was estimated by GIMIC calculations[57,59–62]
using proton substituted model systems (1H, AH, BH). Addition-
ally, benzene, naphthalene, indole, and borazine were computed
as reference molecules (cf. SI, p. S53ff). The current density sus-
ceptibility of selected systems is visualized in Figure 5 by stream-
line representations. The typical aromatic compounds benzene
and indole clearly display a distinct diatropic π ring current, which
encircles the ring system above and below the ring plane. In AH,
on the other hand, only atomic vortices are found, whereas the
current density of biradical 1H is again very similar to benzene
and indole.
Acknowledgments
We thank the University of Rostock for access to the cluster
computer, and especially Malte Willert for his assistance with the
queueing system and software installations. This research was
supported by the Deutsche Forschungsgemeinschaft (DFG,
SCHU 1170/12-2) and the Fonds der chemischen Industrie (FCI).
Keywords: aromaticity • biradicals • heterocycles • molecule ac-
tivation • phosphorus
Table 2. Net induced currents and NICS(1)zz values of selected model systems.
For fused ring systems, values are given for the five-membered (⭓) and six-
membered part (⬢). Further information can be found in Table S16.
[1]
[2]
[3]
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by integration of the current density (Table 2). The net induced
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13.5 nA/T) is very similar to the values of benzene and indole
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The substituent makes the difference: Only one very bulky substituent R is required to stabilize the first stable hetero-indanediyls
(see figure), which represent a group of resonance-stabilized phosphorus-centered biradicals. Different types of oligomers formed by
self-activation of the aromatic backbone were observed for smaller substituents.
6
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