Heterocyclopentanediyls vs heterocyclopentadienes: A question of silyl group migration

Article abstract of DOI:10.1021/acs.joc.0c00460

The reaction of the singlet biradical [P(μ-NHyp)]2 (Hyp = hypersilyl, (Me3Si)3Si) with different isonitriles afforded a series of five-membered N2P2C heterocycles. Depending on the steric bulk of the substituent at the isonitrile, migration of a Hyp group was observed, resulting in two structurally similar but electronically very different isomers. As evidenced by comprehensive spectroscopic and theoretical studies, the heterocyclopentadiene isomer may be regarded as a rather unreactive closed-shell singlet species with one localized N=P and one C=P double bond, whereas the heterocyclopentanediyl isomer represents an open-shell singlet biradical with interesting photochemical properties, such as photoisomerization under irradiation with red light to a [2.1.0]- housane-type species.

Full text of DOI:10.1021/acs.joc.0c00460

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Article
Heterocyclopentanediyls vs Heterocyclopentadienes: A Question of
Silyl Group Migration
̈
Henrik Beer, Jonas Bresien,* Dirk Michalik, Anne-Kristin Rolke, Axel Schulz,* Alexander Villinger,
and Ronald Wustrack
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ABSTRACT: The reaction of the singlet biradical [P(μ-NHyp)]2 (Hyp =
hypersilyl, (Me3Si)3Si) with different isonitriles afforded a series of five-membered
N2P2C heterocycles. Depending on the steric bulk of the substituent at the isonitrile,
migration of a Hyp group was observed, resulting in two structurally similar but
electronically very different isomers. As evidenced by comprehensive spectroscopic
and theoretical studies, the heterocyclopentadiene isomer may be regarded as a
rather unreactive closed-shell singlet species with one localized NP and one CP
double bond, whereas the heterocyclopentanediyl isomer represents an open-shell
singlet biradical with interesting photochemical properties, such as photoisomerization under irradiation with red light to a [2.1.0]-
housane-type species.
of a doublet radical or triplet biradical. Due to the near
degeneracy of their frontier orbitals, singlet biradicals can easily
undergo (cyclo)addition reactions (vide infra),3 which
typically involve the interaction between HOMO and
LUMO of the reaction partners.
The presence of heteroatoms such as N and P in cyclic
biradicals tends to stabilize the singlet ground state by
increasing the antiferromagnetic coupling (and thus decreasing
the biradical character), resulting in improved stability.
Sterically demanding substituents further provide kinetic
protection, rendering the isolation of such species possible
(Scheme 1).16−31 In particular, our own group could
demonstrate the versatile reactivity of the stable four-
membered cyclic biradical [P(μ-NR)]2 (1, R = Ter [2,6-
dimesitylphenyl] or Hyp [tris(trimethylsilyl)silyl]), which is
based solely on group 15 elements (Scheme 2).32−40
Previously, we reported on the insertion of CO or isonitriles
into one of the N−P bonds, leading to the formation of
intensely colored, five-membered cyclic biradicals (e.g.,
heterocyclopentanediyls 2Ter; Scheme 3).34,35 Only recently
we could demonstrate that these five-membered ring systems
are light-sensitive molecular switches41−45 that can be utilized
to control a chemical equilibrium by turning the biradical
character on and off (Scheme 3, right).46
INTRODUCTION
■
Biradicals (sometimes also called “biradicaloids” if the
interaction between the two radical sites is significant) are an
interesting research topic for both theoretical and synthetic
chemists, as they exhibit unusual electronic structures,
intriguing properties, and high reactivity.1−11 They are
therefore worthwhile targets for the study of structure−
property relationships, but their isolation is a challenge due to
their intrinsic instability.
The high reactivity of biradicals arises from their character-
istic electronic structure, which comprises two electrons in two
(nearly) degenerate orbitals.1 Thus, the electrons are strongly
correlated (i.e., nondynamically or “statically” correlated),12
and multireference methods are needed to correctly describe
the wave function of these systems.13 Depending on the
magnitude and sign of the phenomenological Heisenberg
exchange coupling constant J = 1/2 ΔEST (ΔEST = singlet−
triplet energy gap),14,15 the two electrons can either be
ferromagnetically (J > 0) or antiferromagnetically (J < 0)
coupled,8 resulting in a triplet or singlet ground state,
respectively. Solely in the borderline case J ≈ 0, a “perfect”
biradical is formed, that is, a molecule with two independent
radical centers (a so-called “two-doublet species”),15 which for
all intents and purposes behaves like a radical species.3 In all
other cases, one should note that the terms “biradical” or
“biradicaloid” can sometimes be misleading, since the reactivity
especially of singlet species may differ fundamentally from the
reactivity of radicals (doublet species).
Special Issue: The New Golden Age of Organo-
phosphorus Chemistry
Here, we shall be only concerned with singlet biradicals (J <
0). It is vital to understand that in singlet biradicals, the spin
density is exactly zero at every point in space. Thus, the
reactivity of a singlet biradical may be very different from that
Received: March 10, 2020
Published: May 12, 2020
https://dx.doi.org/10.1021/acs.joc.0c00460
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rearrangement reactions,47,48 which turned out to play an
important role in the outcome of the reactions in case of 1Hyp.
Scheme 1. Some Examples of Open-Shell Singlet Biradicals
Based on Four-Membered Main-Group Heterocycles
RESULTS AND DISCUSSION
■
Reactions with Isonitriles. We started our investigation
by treating the four-membered cyclic biradical [P(μ-NHyp)]2
(1Hyp) with the isonitriles tBuNC, DmpNC (Dmp = 2,6-
dimethylphenyl), MesNC (Mes = mesityl), and TerNC. To
this end, a red solution of 1Hyp in n-hexane was reacted with
the respective isonitrile at ambient temperature (25 °C),
resulting in an immediate color change to yellow (tBu, Dmp,
Mes) or dark blue (Ter). These different colors were
somewhat unexpected, seeing that all reactions of the
analogous biradical 1Ter with isonitriles had resulted in blue
heterocyclopentanediyls (2Ter-R2, Scheme 3).
Crystallization of the products and subsequent single-crystal
X-ray diffraction (SC-XRD) analysis revealed that in case of
tBuNC, DmpNC, and MesNC, one Hyp group had 1,3-
migrated from one of the nitrogen atoms in the five-membered
ring system to the exocyclic nitrogen atom of the former
isonitrile group (3Hyp-R2, Scheme 4), resulting in the
a
Scheme 2. Reactivity of Singlet Biradicals 1R
Scheme 4. Reaction of 1Hyp (R1 = Hyp) with Isonitriles
(CN-R2) Led to the Formation of Heterocyclopentanediyl
2Hyp-Ter or Heterocyclopentadienes 3Hyp-R2, Depending
on the Steric Bulk of the Substituent R2
a
formation of a heterocyclopentadiene rather than a hetero-
cyclopentanediyl. Only in the case of TerNC, the expected
product 2Hyp-Ter was formed, as evidenced by 31P NMR and
UV−vis spectroscopy (vide infra).
The reactions with S, Se, and multiple-bond systems (XCY) were
reported for both R = Ter (1Ter) and R = Hyp (1Hyp). All other
reactions were only investigated for the Ter system.
Scheme 3. Blue Heterocyclopentanediyls (2Ter-R2) Are
Formed by Insertion of Isonitriles into an NP Bond of
Biradical 1Ter
Molecular and Electronic Structures. Due to their
similar structures, the dienes 3Hyp-Dmp and 3Hyp-Mes
formed isomorphous crystals (space group: C2/c). Their
congener 3Hyp-tBu crystallized in the space group P212121
with a distinctly different unit cell (cf. Supporting Information
(SI)). Still, the molecular structures of all three derivatives are
similar with respect to the nearly planar five-membered N2P2C
ring system and exocyclic N atom, with one Hyp substituent
being connected directly to the ring system and the other being
linked to the exocyclic N atom which also bears the tBu, Dmp,
or Mes group (Figure 1).
The structural parameters of the five-membered ring systems
are nearly identical in all three cases (Table 1). All N−P, P−C,
and C−N bond lengths range between typical single and
double bonds (cf. ∑rcov: N−P 1.82, NP 1.62, P−C 1.86,
PC 1.69, C−N 1.46, CN 1.27 Å),49 indicating a
delocalized π-bonding system.
a
a
Under irradiation, they isomerize to the closed-shell housane-type
species 2′Ter-R2 (R1 = Ter; R2 = tBu, Dmp, Mes).
To explore the influence of the substituent R1 at the N2P2
ring system on the reactivity, we started to investigate the
analogous biradical system stabilized by Hyp groups (Hyp =
(Me3Si)3Si, 1Hyp).39 Substitution of Ter by Hyp resulted in a
larger charge transfer of electron density from the substituent
R into the P2N2 ring (Ter 0.4 vs Hyp 1.2 e) and hence to a
higher reactivity in case of 1Hyp.39 Of course, we were also
interested in the possible utilization of this system as a
molecular switch, so it was obvious to investigate its reactivity
toward different isonitriles. However, contrary to aryl-based
groups such as Ter, silyl groups such as Hyp can easily undergo
Several attempts at crystallizing the blue Ter derivative
2Hyp-Ter remained unsuccessful, due to its instability.
Nonetheless, it could be unequivocally identified by NMR
and UV−vis spectroscopy (vide infra). In the following, we will
thus compare the calculated structures (PBE-D3/def2-
SVP)50−52 of the isomers 3Hyp-R2 and 2Hyp-R2 (R2
=
Dmp, Mes, tBu, Ter) to point out differences (Figure 2). First,
B
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electrons were performed to investigate the biradical character
of both isomers (Figure 3). The calculations indicate that the
Figure 1. Molecular structures of 3Hyp-Dmp, 3Hyp-Mes, and 3Hyp-
tBu in the crystal. Thermal ellipsoids set at 50% probability (123 K).
Selected distances and angles can be found in Table 1.
Table 1. Selected Bond Lengths (Å) and Dihedral Angles
(°) of 3Hyp-Dmp, 3Hyp-Mes, and 3Hyp-tBu in the Crystal
3Hyp-Dmp
3Hyp-Mes
3Hyp-tBu
N1−P1
N1−P2
P1−C1
P2−N2
C1−N2
C1−N3
1.728(2)
1.663(2)
1.773(2)
1.617(2)
1.344(2)
1.726(1)
1.671(1)
1.777(2)
1.620(1)
1.345(2)
1.727(4)
1.670(4)
1.782(4)
1.612(4)
1.363(5)
Figure 3. Frontier orbitals of the cyclopentadiene 3SiH3-Me and
cyclopentanediyl 2SiH3-Me (CAS(8,6)/def2-TZVP//PBE-D3/def2-
SVP, isovalue = 0.03). Only the largest contributions to the wave
function are shown. For a depiction of all active orbitals, please see SI,
p. S45f.
1.376(2)
1.377(2)
1.371(5)
N1−P1−P2−N2
N1−P1−C1−N3
−179.8(1)
−179.0(1)
−179.6(1)
−178.1(1)
−179.0(3)
−179.2(4)
wave functions of both isomers have significant contributions
from at least two determinants, the first one being the
hypothetical closed-shell configuration, and the second one
being the double substitution of two electrons from the
HOMO into the LUMO, meaning that the two frontier
orbitals are strongly correlated. However, the coefficient of the
second determinant is significantly higher in the case of the
cyclopentanediyl 2SiH3-Me, resulting in a larger biradical
2
2
2
character. The quantity β = 2c2 /(c1 + c2 ) was proposed to
estimate the biradical character1,62 and amounts to 24% for
2SiH3-Me, which compares nicely with similar singlet
biradicals such as 2Ter-R2 (27%),46 1Ter (27%),63 or 1Hyp
(21%).39 The biradical character of the cyclopentadiene
3SiH3-Me (β = 10%) is significantly smaller and at the edge
of what might still be regarded as a singlet biradical.62,64 (For
example, benzene has been attributed 5−10% biradical
character,65,66 in accordance with our own computations, cf.
SI.)
Moreover, the CAS orbitals indicate that the HOMO of
3SiH3-Me contributes to the P1−C1 and P2−N2 double-bond
character (the LUMO being the corresponding antibonding
orbital), whereas the HOMO of 2SiH3-Me is mainly localized
at the P atoms, forming a π-type antibond between the two P
atoms (and thus describing the “radical sites” at P in
conjunction with the partly occupied, transannularly bonding
LUMO, cf. Figure 3). In line with these considerations, NRT67
analyses of the DFT as well as CAS densities identified the diyl
and diene Lewis resonance formulas as the most important
contributions to 2SiH3-Me and 3SiH3-Me, respectively
(Scheme 5). Note that in the NRT formalism, the “biradical”
structures do not represent the multireference character of the
wave function, though, but merely imply that the electrons in
the HOMO are rather localized in a π-type antibond between
the two P atoms. As such, it gives qualitative information about
the localization of the radical sites but does not provide a
measure of the biradical character!
Figure 2. Calculated molecular structures of the experimentally
observed species 3Hyp-Dmp and 2Hyp-Ter (PBE-D3/def2-SVP; cf.
Tables S13 and S14).
it is worth noting that the calculated and experimental
structures of 3Hyp-R2 (R2 = Dmp, Mes, tBu) are in reasonable
agreement (mean difference in N−P/N−C/P−C bond
lengths: 0.021 Å). When comparing the two sets of computed
structures of 3Hyp-R2 and 2Hyp-R2, it becomes evident that in
the case of 2Hyp-R2, both Hyp groups are attached to the ring
system (N1, N2), as opposed to 3Hyp-R2, where one Hyp
substituent is connected to the exocyclic N3 atom.
Furthermore, the P1−C1 (av. 1.84 Å), P2−N2 (av. 1.71 Å),
and C1−N2 (av. 1.41 Å) bond lengths are significantly longer
in 2Hyp-R2, now more in the range of polarized single bonds,
whereas the C1−N3 bonds are shortened to an average of 1.30
Å, which corresponds to nearly isolated CN double bonds.
Thus, the bond lengths of both 2Hyp-R2 and 3Hyp-R2 are in
nice agreement with the Lewis depictions in Scheme 4,
indicating a rather different bonding situation in both species.
To compare the bonding situation in the cyclopentanediyl
(2Hyp-R2) and cyclopentadiene derivatives (3Hyp-R2),
calculations with the model systems 2SiH3-Me and 3SiH3-
Me were performed. The calculated structural parameters of
these models compare well with those of 2Hyp-R2 and 3Hyp-
R2 (SI). CASSCF(8,6)53−61 computations correlating all π
Why, now, do we observe the formation of the biradical
2Hyp-R2 only in case of the Ter substituent, but formation of
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a
biradical character (which depends, inter alia, on the HOMO−
LUMO gap) and also corresponds to calculated values (TD-
DFT PBE-D3/def2-TZVP, 2Hyp-Ter 635 and 562, 3Hyp-
Dmp 494, 3Hyp-Mes 494, 3Hyp-tBu 516 nm).
Scheme 5. NRT Analyses of 2SiH3-Me and 3SiH3-Me
In the 31P NMR spectrum, diyl (2Hyp-R2) and diene
(3Hyp-R2) are most readily distinguished by the JPP coupling
constant. Calculations for all isomers of 2Hyp-R2 and 3Hyp-R2
(R2 = Dmp, Mes, tBu, Ter; cf. SI) predicted rather small
coupling constants (10−17 Hz) for the dienes 3Hyp-R2,
whereas the diyls 2Hyp-R2 were predicted to have substantially
larger coupling constants (76−93 Hz). This could be verified
experimentally: Only 2Hyp-Ter displayed a large coupling
constant of 118 Hz (calcd 93 Hz), while the other three
species had small coupling constants of 10−15 Hz (calcd 10−
13 Hz), in accordance with their diene character. While theory
and experiment also agreed very nicely in case of the 31P NMR
shifts, these were not suited to distinguish between diyl and
diene, as they were quite similar for all species (Table 2).
a
R1 = SiH3, R2 = Me; PBE-D3/def2-SVP, delocalization threshold: 50
kcal/mol; please refer to the SI for the results using the CAS densities.
Note that the “biradical” structures merely give information about the
localization of the radical sites but not about the biradical character of
the system!
the diene 3Hyp-R2 in all other cases? To investigate this
matter, comprehensive computations covering all derivatives of
2Hyp-R2 and 3Hyp-R2 (R2 = tBu, Dmp, Mes, Ter) were
performed (see SI for a complete set of structures). At the
DLPNO-CCSD(T)68−70/def2-TZVP//PBE-D3/def2-SVP
level of theory, it is apparent that the diene 3Hyp-R2 is the
thermodynamically favored isomer for all substituents R2
except Ter (Scheme 6). This is easily understood in terms of
Table 2. Experimental 31P NMR Data of 2Hyp-R2 and
a
3Hyp-R2
δ(PA) [ppm]
δ(PB) [ppm]
JPP [Hz]
2Hyp-Dmp
2Hyp-Mes
2Hyp-tBu
2Hyp-Ter
3Hyp-Dmp
3Hyp-Mes
3Hyp-tBu
3Hyp-Ter
− (211.8)
− (213.6)
− (176.5)
− (257.8)
− (256.4)
− (245.5)
− (79)
− (79)
− (76)
118 (93)
15 (10)
10 (10)
15 (13)
− (17)
diyl
211.7 (222.7)
209.0 (203.7)
209.1 (204.4)
230.7 (191.6)
− (226.3)
275.1 (249.5)
297.3 (277.1)
297.5 (278.0)
281.9 (248.9)
− (253.6)
Scheme 6. Relative Gibbs Free Energies of Isomerization of
2Hyp-R2 to 3Hyp-R2 for Different Substituents R2
(DLPNO-CCSD(T)/def2-TZVP//PBE-D3/def2-SVP)
diene
Calculated data given in brackets (GIAO method,72−76 PBE-D3/
a
def2-SVP).
Behavior under Irradiation. When a solution of the
biradical 2Hyp-Ter was irradiated with red light (638 nm), it
photoisomerized to the corresponding housane-type species
2′Hyp-Ter (Scheme 7). In the 31P NMR spectrum, the
Scheme 7. While the Diyl 2Hyp-Ter (R1 = Hyp) Could Be
Photoisomerized to the Housane-Type Species 2′Hyp-Ter
in Solution, the Same Is Not True of the Dienes 3Hyp-Dmp,
3Hyp-Mes, and 3Hyp-tBu
large Pauli repulsion between the sterically demanding Hyp
and Ter groups in the diene 3Hyp-R2, where these two
substituents are connected to the same N atom. In agreement
with this consideration, the diene 3Hyp-tBu is also less
stabilized than the derivatives 3Hyp-Dmp and 3Hyp-Mes,
whose aromatic substituents have the smallest steric demand
due to their rather flat structure (see the SI for buried
volumes71 of all four substituents). Thus, the equilibrium
between the diyl 2Hyp-R2 and diene 3Hyp-R2 formally
depends on the steric demand of the substituent R2.
Spectroscopic Properties. The different electronic
structures of the derivatives of 2Hyp-R2 and 3Hyp-R2 could
most easily be proven experimentally by UV−vis spectroscopy
(cf. SI, p S30). While the biradical 2Hyp-Ter showed broad
absorption bands in the range of 500−680 nm (cf. 2Ter-Dmp:
λmax = 643 nm), the dienes 3Hyp-Dmp, 3Hyp-Mes, and 3Hyp-
tBu could be characterized by a single absorption maximum at
399−433 nm. This is in agreement with the calculated
quantitative isomerization was evidenced by a large upfield
shift of the two resonances to −128.1 and −69.2 ppm,
respectively. In contrast, the dienes 3Hyp-Dmp, 3Hyp-Mes,
and 3Hyp-tBu could not be isomerized under irradiation,
irrespective of the wavelength of the light (638, 520, and 445
nm). Even at −150 °C, we did not observe a change in the
crystal structure when the samples were irradiated with white
D
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light (in contrast to the previously published molecular switch
2Ter-Dmp, whose crystals disintegrated under light due to
isomerization to the housane 2′Ter-Dmp).46 Only in case of
3Hyp-Mes, a B layer containing approximately 2% of the
housane 3′Hyp-Mes could be refined in the structural model,
indicating either a very low quantum yield of the photo-
isomerization or a very fast thermal reverse reaction even at
very low temperatures.
exclusive formation of a new AX spin system (4Hyp-tBu) in a
1:1 ratio with the starting material was observed in the 31P
NMR spectrum (Figure 5). Upon warming, the intensity of the
The photoproduct 2′Hyp-Ter isomerized back to the
biradical 2Hyp-Ter under thermal regime. The reaction
could be traced by in situ 31P NMR spectroscopy at different
temperatures (Figure 4). The kinetic data allowed us to
Figure 5. In situ 31P NMR spectra of the reaction of 1Hyp with
tBuNC at −40 and 25 °C. For spectra at additional temperatures,
please see Figure S15.
new AX spin system decreased, while formation of yet another
AX spin system was observed, which could be attributed to the
diene 3Hyp-tBu. At ambient temperature, the diene 3Hyp-tBu
was the main product, although traces of the starting material
1Hyp as well as the intermediate 4Hyp-tBu were still present
(10:1:3 ratio).
DFT and ab initio calculations indicated that the
intermediate 4Hyp-tBu was in fact the tBuNC adduct of the
transient biradical 2Hyp-tBu; that is, the unstable diyl 2Hyp-
tBu could be trapped by adduct formation with tBuNC
(Scheme 8; for calcd NMR data, please see Table S6). Upon
warming, the adduct 4Hyp-tBu redissociated owing to the gain
in entropy, and the transient biradical 2Hyp-tBu isomerized to
the more stable diene 3Hyp-tBu. The hypothetical adduct
5Hyp-tBu was not observed (vide infra).
In case of the dienes 3Hyp-Dmp and 3Hyp-Mes, no adduct
formation was observed. The same holds true for the stable
diyl 2Hyp-Ter. All these observations nicely agree with
calculated thermodynamic data (DLPNO-CCSD(T)/def2-
TZVP//PBE-D3/def2-SVP) for the reactions shown in
Scheme 8. While reaction (i) is exergonic for all derivatives,
reaction (ii), that is, the adduct formation step, is strongly
exergonic solely in case of the tBu derivative (ΔG° = −40.5 kJ/
mol; cf. Table 3). As mentioned above, reaction (iii) is
exergonic for R2 = Dmp, Mes, and tBu. The lowest energy gain
is predicted for the tBu derivative (ΔG° = −29.5 kJ/mol), in
line with the observed equilibrium between the adduct 4Hyp-
tBu and the diene 3Hyp-tBu at ambient temperature. Reaction
(iv), that is, the formation of the hypothetical isonitrile adduct
Figure 4. Thermal equilibration of the housane 2′Hyp-Ter at 25 °C.
For a full set of data at different temperatures, please refer to the SI, p.
S36ff.
estimate the activation barrier of the reaction, which amounts
to ΔG⧧ = 93 2 kJ/mol at 25 °C (cf. SI). This compares to
the activation barriers of thermal equilibration of the housanes
2′Ter-Dmp (88 4 kJ/mol) and 2′Ter-tBu (ca. 94 kJ/mol).46
Adduct Formation with a Second Equivalent of
Isonitrile. The tBu derivative 3Hyp-tBu proved to be most
difficult to isolate. While 3Hyp-Dmp and 3Hyp-Mes could be
isolated in 70−90% yield, only a few crystals of 3Hyp-tBu
could be obtained. This was due to the formation of an adduct
with a second equivalent of tBuNC (cf. Scheme 8), as
evidenced by variable-temperature NMR spectroscopy. When
1Hyp was treated with 1 equiv of tBuNC at low temperatures,
Scheme 8. Trapping of the Biradical 2Hyp-R2 (R1 = Hyp)
a
with a Second Equivalent of Isonitrile
Table 3. Gibbs Free Energies ΔG° (c° = 1 mol/L, 298 K) for
Reactions (i)−(iv) in kJ/mol (DLPNO-CCSD(T)/def2-
a
TZVP//PBE-D3/def2-SVP, see also SI)
reaction
R2
(i)
(ii)
(iii)
(iv)
Dmp
Mes
tBu
−63.5
−58.8
−15.2
−47.4
−8.6
−2.3
−62.1
−62.7
−29.5
+25.8
+59.8
+71.0
+52.2
−40.5
b
b
Ter
+168.4
+137.6
a
Experimentally observed reaction pathways are printed in italics, and
values printed in bold refer to those reactions whose products were
detected experimentally. ΔG° based on DLPNO-CCSD energy (the
triples correction was not computed due to the size of the system).
a
b
The adduct 4Hyp-tBu could be detected spectroscopically. The
hypothetical adduct 5Hyp-R2 is in gray.
E
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3.58 ppm), or externally (31P: 85% H3PO4 δref = 0 ppm). All
measurements were carried out at ambient temperature (25 °C)
unless denoted otherwise. NMR signals were assigned using
experimental data (e.g., chemical shifts, coupling constants, integrals
where applicable) in conjunction with computed NMR data (GIAO
method, cf. Computational Details).
NMR Spectra under Irradiation. Spectra were recorded by
means of an adopted setup previously published by the Gschwind
group,81 who used a fiber-coupled light emitting diode (LED) to
direct light into the NMR spectrometer. Instead of LEDs, a laser
diode (Oclaro HL63193MG, 638 nm, 700 mW) was used. To ensure
inert conditions, all samples were prepared in a glovebox, and the
tubes were sealed with custom-made PTFE caps as well as 2−3 layers
of PTFE tape.46
5Hyp-R2, is strongly endergonic for all derivatives, and it was
not observed. This is yet another indication for the low
biradical character of the dienes 3Hyp-R2.
CONCLUSION
■
In conclusion, we could demonstrate that the Hyp substituent
introduced a new aspect into the chemistry of diphosphadia-
zanediyls (1R), namely the migration of a Hyp group
depending on the stabilization and steric encumberment of
the product species. Accordingly, treatment of 1Hyp with
different isonitriles either led to the formation of a five-
membered biradical 2Hyp-Ter (in analogy to known 2Ter-R2
derivatives) or afforded heterocyclopentadienes (3Hyp-R2),
which to the best of our knowledge represent the first isolated
derivatives of 3H-1,3-diaza-2,4-diphosphole.
IR Spectra. Spectra of crystalline samples were recorded on a
Nicolet 380 FT-IR spectrometer equipped with a Smart Orbit ATR
unit or a Bruker Alpha II FT-IR spectrometer with an ATR unit at
ambient temperature (25 °C).
In our study, the diyl 2Hyp-Ter displayed typical biradical
reactivity, such as activation of small molecules or photo-
isomerization upon irradiation with visible light. In contrast,
despite its similar molecular structure, the diene 3Hyp-R2 was
rather unreactive, as it represented the thermodynamically
favored isomer for smaller substituents at the isonitrile.
Comprehensive computations were carried out to understand
the differences in structure, bonding, and reactivity between
both isomers, revealing that the biradical character of the diyl
2Hyp-Ter is comparable to related compounds (β = 24%),
whereas the diene 3Hyp-R2 does not possess a significant
biradical character (β = 10%). Experimental techniques could
be applied to verify the conclusions drawn from the theoretical
models.
Raman Spectra. Spectra of crystalline samples were recorded
using a LabRAM HR 800 Horiba Jobin YVON Raman spectrometer
equipped with an Olympus BX41 microscope with variable lenses.
The samples were excited by a red laser (633 nm, 17 mW, air-cooled
HeNe laser) or blue laser (473 nm, 20 mW, air-cooled solid-state
laser). All measurements were carried out at ambient temperature (25
°C) unless stated otherwise.
Elemental Analyses. Results were obtained using an Elementar
vario Micro cube CHNS analyzer or a LECO TruSpec Micro CHNS
analyzer.
Melting Points. Results (uncorrected) were determined using a
Stanford Research Systems EZ Melt at a heating rate of 20 °C/min.
Mass Spectra. Spectra were recorded on a Thermo Electron MAT
95-XP sector field mass spectrometer using crystalline samples.
UV−vis Spectra. Spectra were acquired on a PerkinElmer
Lambda 19 UV−vis spectrometer.
X-ray Structure Determination. Single crystals of 3Hyp-Dmp,
3Hyp-Mes, and 3Hyp-tBu were obtained by crystallization from n-
hexane at −40 °C. X-ray quality crystals were selected in Fomblin YR-
1800 perfluoro-ether (Alfa Aesar) at ambient temperature (25 °C).
The samples were cooled to 123(2) K or 298(2) K during
measurements. The data were collected on a Bruker D8 Quest
diffractometer or a Bruker Kappa Apex II diffractometer using Mo Kα
radiation (λ = 0.71073 Å). The structures were solved by iterative
methods (SHELXT)82 and refined by full matrix least-squares
procedures (SHELXL).83 Semiempirical absorption corrections were
applied (SADABS).84 All non-hydrogen atoms were refined
anisotropically, and hydrogen atoms were included in the refinement
at calculated positions using a riding model.
Computational Details. Computations were carried out using
Gaussian 0985 or ORCA 4.1.186 and the standalone version of NBO
6.0.87−92 Structure optimizations were performed using the pure DFT
functional PBE50,51 in conjunction with Grimme’s dispersion
correction D3(BJ)93,94 and the def2-SVP basis set52 (notation PBE-
D3/def2-SVP). All structures were fully optimized and confirmed as
minima by frequency analyses. The validity of the single-
determinantal Kohn−Sham DFT approach was verified by stability
analyses of the KS wave functions. Chemical shifts and coupling
constants were derived by the GIAO method.72−76 The calculated
absolute shifts (σcalc,X) were referenced to the experimental absolute
shift of 85% H3PO4 in the gas phase (σref,1 = 328.35 ppm),95 using
PH3 (σref,2 = 594.45 ppm) as a secondary standard:96
The isomerization of the diyl 2Hyp-R2 to the diene 3Hyp-R2
demonstrates that steric bulk alone is not necessarily sufficient
to stabilize biradicals and that apparently small structural
changes can lead to quite a different electronic situation.
Consequently, these results will help us to devise new, stable
singlet biradicals and molecular switches in the future.
EXPERIMENTAL SECTION
■
General Information. If not stated otherwise, all manipulations
were carried out under oxygen- and moisture-free conditions under an
inert atmosphere of argon using standard Schlenk or Drybox
techniques.
Solvents were obtained from commercial sources. CH2Cl2 was
purified according to a literature procedure,77 dried over P4O10, and
stored over CaH2 and was freshly distilled and degassed (by at least
three freeze−pump−thaw cycles) prior to use. n-Hexane was dried
over Na/benzophenone/tetraglyme. CD2Cl2 was dried over P4O10
and stored over CaH2. C6D6 was dried over Na. THF-d8 was dried
over Na and stored over molecular sieves (4 Å) after distillation. All
solvents were freshly distilled prior to use.
Reactants and starting materials were either purchased or
synthesized according to literature procedures. NEt3 (Sigma-Aldrich,
99%) was dried over Na and distilled. Mg turnings (abcr, 99.8%, for
Grignards) were activated by stirring for several days under argon
atmosphere using a glass-coated magnetic stir bar. POCl3 (old stock)
was dried over P4O10, distilled and degassed. DmpNH2 (Acros, 99%)
was distilled. [NEt4]Br (Fluka, 98%), [PhNEt3]Br (Sigma-Aldrich,
δ
= (σref,1 − σref,2) − (σ
− σ
)
98%), and NaOH (Gru
̈
ssing, 99%) were used as received.
calc,X
calc,X
calc,PH3
78,79
80
TerNH2
and [ClP(μ-NHyp)]2 were synthesized according to
= σ
− σ
− 266.1ppm
calc,PH3
calc,X
literature procedures.
NMR Spectra. Spectra were obtained on Bruker spectrometers
AVANCE 250, 300, or 500 and were referenced internally to the
signals of deuterated solvents (13C: CD2Cl2 δref = 54.0 ppm, C6D6 δref
= 128.4 ppm, THF-d8 δref,1 = 25.4 ppm, δref,2 = 67.6 ppm), to the
At the PBE-D3/def2-SVP level of theory, σcalc,PH amounts to +617.15
3
ppm. Thermal corrections to the total energy were computed at the
PBE-D3/def2-SVP level of theory. More accurate estimates of the
electronic energy were obtained by single-point DLPNO-CCSD(T)/
def2-TZVP or DLPNO-CCSD/def2-TZVP68−70 computations (nota-
signals of protic species in the deuterated solvents (1H: CHDCl2 δref
5.32 ppm, C6HD5 δref = 7.16 ppm, THF-d7 δref,1 = 1.73 ppm, δref,2
=
=
F
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The Journal of Organic Chemistry
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Article
tion DLPNO-CCSD(T)/def2-TZVP//PBE-D3/def2-SVP or
DLPNO-CCSD/def2-TZVP//PBE-D3/def2-SVP). The T1 diagnostic
was evaluated in each case to ensure reliable results. (Empirically,
CCSD(T) results with T1 values smaller than 0.02 are considered
reliable.)13 To investigate the electronic structures of the diyls 2Hyp
and dienes 3Hyp in more detail, the model systems 2SiH3-Me and
3SiH3-Me were used. CASSCF(8,6)/def2-TZVP53−61 computations
including all π-type electrons in the active space were performed to
investigate the biradical character. NRT analyses67 of the DFT and
CAS densities were computed to localize the electrons and obtain
Lewis-type descriptions of the bonding patterns. It should be
emphasized that all computations were carried out for single, isolated
gas phase molecules. There may be significant differences between gas
phase, solution, and solid state.
2116 (vs), 2083 (w), 1935 (w), 1902 (w), 1789 (w), 1754 (w), 1605
(m), 1556 (w), 1477 (m), 1445 (m), 1434 (m), 1377 (m), 1311 (w),
1284 (w), 1255 (w), 1199 (m), 1146 (w), 1127 (w), 1039 (m), 1020
(w), 956 (w), 944 (w), 931 (w), 896 (w), 859 (s), 816 (w), 719 (m),
707 (w), 666 (w), 598 (w), 573 (m), 503 (w), 422 (w), Raman (473
nm, 4 s, 20 scans, cm−1): ν
̃
= 3033 (1), 3021 (1), 2976 (1), 2922 (2),
2861 (1), 2737 (1), 2114 (10), 2080 (1), 1606 (3), 1476 (1), 1438
(1), 1418 (1), 1386 (1), 1307 (2), 1196 (1), 1143 (1), 1124 (1), 953
(1), 703 (1), 595 (1), 576 (2), 499 (1), 422 (1), 283 (1). MS (GC-
MS) m/z (%): 103 (17) [C7H5N]+, 115 (12) [C8H5N]+, 130 (100)
[C9H8N]+, 145 (50) [M]+.
TerN(H)CHO. Terphenyl formamide was synthesized according to a
modified literature procedure.97 The synthesis was carried out under
non-inert conditions. TerNH2 (1.698 g, 5.150 mmol), ZnCl2 (0.627 g,
4.601 mmol), and formic acid (4.063 g, 88.27) are stored for 86 h at
60 °C (drying oven). The crystals are washed two times with water,
and the aqueous phase is discarded. The product is dissolved in
CH2Cl2 (20 mL) and separated from impurities and water residues in
a separating funnel. The organic solvent is evaporated, and the solid is
dried in vacuo (1 × 10−3 mbar). Yield: 1.320 g, 4.001 mmol, 80%.
C25H27NO (357.50 g/mol). Mp. 305 °C. Anal. calcd for C25H27NO:
Syntheses. DmpNC. DmpNC was synthesized according to a
modified literature procedure.34 The synthesis was carried out under
non-inert conditions. In a three-necked flask equipped with a reflux
condenser and a dropping funnel, DmpNH2 (3.36 g, 29.7 mmol),
chloroform (3.55 g, 29.7 mmol), and [NEt4]Br (90 mg, 0.43 mmol)
are dissolved in CH2Cl2 (30 mL). Under vigorous stirring, a solution
of NaOH (30 g, 0.75 mol) in water (30 mL) is added slowly over a
period of 15 min. The mixture is stirred for 18 h without an external
heat source, but with the reflux condenser turned on as to prevent
evaporation of the organic solvents due to the exothermic reaction.
Afterward, the reaction mixture is diluted with water (200 mL). The
aqueous phase is extracted three times with CH2Cl2 (50 mL). The
solvents of the combined organic phases are evaporated, and the solid
residue is sublimed three times (1 × 10−3 mbar, 70 °C, oil bath). A
final sublimation at ambient temperature (1 × 10−3 mbar, 25 °C)
yields colorless crystals of the product (yield: 2.05 g, 15.6 mmol,
53%). C9H9N (131.18 g/mol). Mp. 77 °C. Anal. calcd for C9H9N: C,
1
C, 83.69; H, 7.61; N, 3.92. Found: C, 83.40; H, 7.66; N, 3.82. H
NMR (CD2Cl2, 250.1 MHz): δ 2.01 (s, 12H), 2.32 (s, 6H), 6.47 (br
s, 1H), 6.98 (s, 4H), 7.13 (d, 2H, J = 7.4 Hz), 7.35 (t, 1H, J = 7.4 Hz),
7.58 (d, 1H, J = 10 Hz). 13C{1H} NMR (CD2Cl2, 62.9 MHz): δ 20.7,
21.4, 126.6, 128.5, 128.9, 129.2, 130.8, 132.9, 134.4, 135.3, 136.4,
138.3, 162.6. IR (ATR, 25 °C, 32 scans, cm−1): ν
̃
= 3358 (w), 2972
(w), 2937 (w), 2912 (m), 2855 (w), 2733 (w), 1678 (s), 1612 (m),
1455 (m), 1408 (m), 1377 (m), 1272 (s), 1239 (s), 1206 (m), 1177
(m), 1101 (m), 1070 (m), 1033 (m), 1012 (m), 983 (m), 888 (m),
857 (s), 812 (s), 765 (m), 742 (m), 688 (m), 629 (m), 585 (m), 575
(m), 552 (m), 507 (m), 453 (m). Raman (633 nm, 10 s, 20 scans,
1
82.41; H, 6.92; N, 10.68. Found: C, 82.75; H, 6.44; N, 10.90. H
NMR (CD2Cl2, 300.1 MHz): δ 2.41 (s, 6H), 7.12 (d, 1H, J = 8 Hz),
cm−1): ν
̃
= 3360 (1), 3056 (1), 3023 (2), 2973 (2), 2915 (6), 2855
7.19 (t, 2H, J = 7 Hz). 13C{1H} NMR (CD2Cl2, 75.5 MHz): δ 19.1,
(2), 2735 (1), 1685 (2), 1613 (5), 1588 (2), 1575 (2), 1486 (2),
1458 (2), 1384 (4), 1308 (9), 1208 (1), 1168 (2), 1072 (2), 1009
(3), 947 (2), 889 (2), 743 (1), 690 (1), 577 (10), 563 (5), 522 (5),
456 (2), 374 (2), 327 (2), 269 (3), 244 (3).
128.1, 129.0, 135.3, 168.7. IR (ATR, 32 scans, cm−1): ν
̃
= 3233 (w),
3184 (w), 2984 (w), 2947 (w), 2920 (w), 2881 (w), 2739 (w), 2120
(m), 2085 (w), 1949 (w), 1879 (w), 1811 (w), 1655 (m), 1591 (w),
1525 (w), 1490 (w), 1471 (m), 1440 (m), 1379 (m), 1302 (w), 1282
(w), 1228 (w), 1170 (m), 1084 (m), 1036 (m), 991 (w), 977 (w),
923 (w), 800 (w), 775 (vs), 721 (m), 637 (w), 548 (w). Raman (633
TerNC. Terphenyl isonitrile was synthesized according to a
modified literature procedure.97 To a solution of terphenyl formamide
(0.603 g, 1.687 mmol) in CH2Cl2 (13 mL), NEt3 (1.697 g, 16.77
mmol) is added at 0 °C. Under stirring and cooling, POCl3 (0.680 g,
4.44 mmol) is added slowly. The orange solution is stirred for 16 h at
ambient temperature (25 °C). Afterward, the reaction mixture is
quenched with water at 0 °C. The organic phase is extracted two
times with water (20 mL) and three times with saturated aqueous
NaHCO3 (20 mL). The pale yellow organic layer is separated, dried
over MgSO4, and filtered. The organic solvent is removed under
reduced pressure (1 × 10−3 mbar). The raw product is washed three
times with n-hexane, and the colorless crystals are dried in vacuo (1 ×
10−3 mbar). Yield: 0.231 g, 0.680 mmol, 40%. C25H25N (339.48 g/
mol). Mp. 266 °C. Anal. calcd for C25H25N: C, 88.45; H, 7.42; N,
nm, 20 s, 20 scans, cm−1): ν
̃
= 3071 (3), 3043 (3), 2985 (3), 2947
(4), 2919 (10), 2911 (9), 2885 (3), 2882 (3), 2873 (3), 2863 (4),
2740 (3), 2735 (3), 2119 (7), 1600 (1), 1590 (2), 1471 (1), 1464
(1), 1437 (1), 1423 (1), 1408 (1), 1383 (1), 1373 (1), 1264 (1),
1254 (2), 1171 (2), 1092 (1), 1078 (2), 990 (1), 796 (1), 779 (1),
727 (1), 719 (1), 636 (8), 565 (1), 542 (1), 518 (1), 505 (1), 491
(1), 458 (1), 361 (4), 284 (1), 240 (2). MS (GC-MS) m/z (%): 103
(30) [C7H4N]+, 116 (62) [C8H8N]+, 130 (100) [C9H8N]+, 131 (68)
[M]+.
MesNC. MesNC was synthesized according to a modified literature
procedure.34 The synthesis was carried out under non-inert
conditions. In a three-necked flask equipped with a reflux condenser
and a dropping funnel, MesNH2 (4.39 g, 32.4 mmol), chloroform
(3.85 g, 33.0 mmol), and [PhNEt3]Cl (0.18 mg, 0.8 mmol) are
dissolved in CH2Cl2 (50 mL). Under vigorous stirring, a solution of
NaOH (30 g, 0.75 mol) in water (30 mL) is added slowly over a
period of 15 min. The mixture is stirred for 22 h without external heat
source, but with the reflux condenser turned on as to prevent
evaporation of the organic solvents due to the exothermic reaction.
Afterward, the reaction mixture is diluted with water (200 mL). The
aqueous phase is extracted three times with CH2Cl2 (50 mL). The
solvents of the combined organic phases are evaporated, and the solid
residue is sublimed (1 × 10−3 mbar, 60−70 °C, oil bath). The product
was then crystallized from dry CH2Cl2 (yield: 2.50 g, 17.22 mmol,
53%). C10H11N (145.20 g/mol). Mp. 42 °C. Anal. calcd for C10H11N:
1
4.13. Found: C, 88.64; H, 7.48; N, 3.90. H NMR (CD2Cl2, 300.1
MHz): δ 2.02 (s, 12H), 2.34 (s, 6H), 7.00 (s, 4H), 7.22 (d, 2H, J =
7.7 Hz), 7.54 (t, 1H, J = 7.7 Hz) 13C{1H} NMR (CD2Cl2, 75.5
MHz): δ 20.4, 21.4, 128.8, 130.0, 130.1, 134.9, 136.2, 138.4, 139.8,
185.4. IR (ATR, 25 °C, 32 scans, cm−1): ν
̃
= 2999 (w), 2974 (w),
2943 (w), 2914 (m), 2853 (w), 2733 (w), 2624 (w), 2605 (w), 2496
(w), 2119 (m), 1614 (m), 1573 (w), 1455 (m), 1416 (m), 1377 (m),
1274 (w), 1183 (w), 1068 (w), 1035 (m), 1014 (m), 981 (w), 919
(w), 862 (m), 851 (m), 806 (s), 787 (m), 759 (s), 738 (m), 608 (m),
575 (m), 565 (m), 519 (m), 497 (m). Raman (633 nm, 10 s, 10
scans, cm−1): ν
̃
= 3069 (1), 3058 (1), 3012 (1), 2918 (1), 2857 (1),
2734 (1), 2121 (10), 2087 (1), 1614 (1), 1584 (1), 1490 (1), 1416
(1), 1381 (1), 1308 (4), 1288 (1), 1273 (1), 1188 (2), 1184 (2),
1161 (1), 1067 (3), 1000 (1), 947 (1), 612 (2), 579 (4), 564 (1), 524
(1), 499 (1), 406 (1), 380 (1), 336 (1), 255 (1), 233 (1).
1
C, 82.72; H, 7.64; N, 9.65. Found: C, 82.47; H, 7.51; N, 9.77. H
NMR (CD2Cl2, 250.1 MHz): δ 2.29 (s, 6H), 2.36 (s, 3H), 6.92 (s,
[P(μ-NHyp)]2 (1Hyp). [PCl(μ-NHyp)]2 was synthesized according
to a modified literature procedure.39 [ClP(μ-NHyp)]2 (1.912 g, 2.913
mmol) and Mg turnings (0.725 g, 26.8 mmol) are combined in a
Schlenk flask. It is paramount to ensure that no grease finds its way into
2H). 13C{1H} NMR (CD2Cl2, 250.1 MHz): δ 18.9, 21.2, 128.7,
129.0, 134.9, 139.3, 168.0. IR (ATR, 25 °C, 32 scans, cm−1): ν
̃
= 3031
(w), 2976 (w), 2947 (w), 2916 (w), 2854 (w), 2739 (w), 2256 (w),
G
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Article
the reaction vessel, as this may deactivate the Mg turnings. DME is added
(50 mL), and the reaction mixture is stirred at ambient temperature
(25 °C). The colorless mixture gradually turns dark red. The progress
of the reaction must by monitored by 31P NMR spectroscopy, as over-
reduction and dimerization of the product occur very quickly. When
the reaction is completed, the solvent is removed in vacuo (1 × 10−3
mbar), and the solids are dried at 50 °C (oil bath) for 30 min. n-
Hexane (15 mL) is added, and the insoluble material is separated by
filtration. If necessary, the cloudy suspension is filtered a second time
over a Celite-padded frit. The product is crystallized from a minimal
amount of n-hexane at −40 °C. After crystallization overnight, red
crystals of the product are obtained. The supernatant is removed by
syringe, concentrated, and stored at −40 °C, yielding a second crop of
product. The isolated crystals are dried in vacuo (1 × 10−3 mbar).
Yield: 1.338 g, 2.286 mmol, 70%. C18H54N2P2Si8 (585.27 g/mol). Mp.
205 °C. Anal. calcd for C18H54N2P2Si8: C, 36.94; H, 9.30; N, 4.79.
immediate color change from orange to green to yellow is observed.
After 2 h, the solvent is removed, and the yellow residue is dried in
vacuo (1 × 10−3 mbar). The product is crystallized from a minimal
amount of fresh n-hexane at −40 °C. The supernatant is removed by
syringe, and the crystals are dried in vacuo (1 × 10−3 mbar). Yield:
315 mg, 0.43 mmol, 93%. C28H65N3P2Si8 (730.48 g/mol). Mp. 207
°C. Anal. calcd for C28H65N3P2Si8: C, 46.04; H, 8.97; N, 5.75. Found:
C, 46.01; H, 9.09; N, 5.50. 1H NMR (CD2Cl2, 300.1 MHz): δ 0.13 (s,
27H), 0.21 (s, 27H), 2.18 (s, 6H), 2.29 (s, 3H), 6.91 (s, 2H).
13C{1H} NMR (C6D6, 75.5 MHz): δ 0.0, 1.9, 18.5 (d, J = 3 Hz), 20.2,
127.2, 130.1, 135.5 (d, J = 4 Hz), 136.5. 14N NMR: No signals
observed. 29Si-INEPT NMR (C6D6, 59.6 MHz): δ −13.2 (m, 3Si),
−14.4 (m, 3Si), −31.0 (m, 1Si), −33.6 (m, 1Si). 31P{1H} NMR
2
(C6D6, 121.5 MHz): δ = 209.1 (d, 1P, CPN, J(31P,31P) = 10 Hz),
297.5 (d, 1P, NPN, 2J(31P,31P) = 10 Hz). IR (ATR, 32 scans, cm−1): ν
= 2949 (w), 2891 (w), 1529 (w), 1477 (w), 1439 (w), 1396 (w),
1333 (m), 1304 (w), 1259 (m), 1240 (m), 1200 (m), 1188 (w), 1157
(w), 1034 (w), 1012 (w), 984 (w), 957 (w), 930 (w), 908 (w), 825
(vs), 771 (m), 744 (m), 710 (m), 683 (m), 621 (m), 567 (m), 528
̃
1
Found: C, 36.59; H, 9.34; N, 4.72. H NMR (C6D6, 300.1 MHz): δ
0.21 (s, 54H). 13C{1H} NMR (C6D6, 125.5 MHz): δ 1.13. 14N NMR:
No signals observed. 29Si-INEPT NMR (C6D6, 59.6 MHz): δ −13.02
(m, 6Si), −35.02 (m, 2Si). 31P{1H} NMR (C6D6, 121.5 MHz): δ
(m). Raman (633 nm, 10 s, 20 scans, cm−1): ν
̃
= 3010 (1), 2974 (2),
320.4 (quin, 2P, J = 44 Hz). IR (ATR, 32 scans, cm−1): ν
̃
= 2949 (w),
2952 (3), 2930 (2), 2894 (8), 2824 (1), 2779 (1), 2734 (0), 1610
(1), 1573 (0), 1484 (0), 1443 (0), 1407 (0), 1382 (1), 1374 (1),
1332 (1), 1304 (1), 1263 (1), 1244 (1), 1200 (4), 1188 (4), 1158
(0), 1015 (2), 957 (0), 908 (2), 859 (1), 823 (1), 774 (2), 747 (1),
736 (0), 712 (1), 690 (2), 633 (10), 579 (2), 570 (1), 568 (1), 549
(4), 530 (2), 504 (0), 454 (1), 445 (1), 425 (2), 401 (1), 366 (2),
327 (1), 301 (1), 240 (1), 220 (1), 177 (6), 133 (2), 102 (2). MS (CI
pos., isobutane); m/z (%): 350 (1) [HypN2P2C]+, 482 (1)
[HypN3P2CMes]+, 583 (1) [HypN3P2CSi2(CH3)3]+, 656 (28)
[HypN3P2CSi3(CH3)6]+, 714 (28) [Hyp2N3P2CC8H8]+, 715 (19)
[Hyp2N3P2CC8H9]+, 716 (13) [Hyp2N3P2CC8H10]+, 729 (100)
[M]+, 730 (83) [M + H]+. Single crystals suitable for X-ray structure
determination can be obtained by recrystallization from fresh n-
hexane at −40 °C.
2893 (w), 2818 (w), 2794 (w), 2652 (w), 2368 (w), 2326 (w), 2085
(w), 2050 (w), 1984 (w), 1927 (w), 1863 (w), 1595 (w), 1495 (w),
1439 (w), 1396 (w), 1240 (m), 1053 (w), 982 (w), 947 (w), 816
(vs), 789 (s), 760 (s), 750 (s), 681 (s), 623 (s), 559 (m). Raman
(633 nm, 10 s, 10 scans, cm−1): ν
̃
= 2949 (5), 2927 (3), 2890 (10),
1396 (1), 1259 (1), 1240 (1), 1138 (1), 1116 (1), 1070 (2), 865 (1),
834 (1), 802 (1), 744 (1), 686 (3), 625 (7), 587 (1), 464 (1), 431
(1), 370 (1), 304 (2), 263 (1), 215 (2), 178 (8), 173 (8), 164 (9),
161 (9). MS (CI pos., isobutane), m/z (%): 247 (1) [Si(Si-
(CH3)3)3]+, 264 (2) [Si(Si(CH3)3)3NH3]+, 292 (2) [Si(Si-
(CH3)3)3NP]+, 555 (2) [Si2(Si(CH3)3)6NP]+, 584 (100) [M]+, 585
(67) [M + H]+.
3Hyp-Dmp. To a solution of [P(μ-NHyp)]2 (1Hyp; 274 mg, 0.470
mmol) in hexane (5 mL), DmpNC (60 mg, 0.47 mmol) is added. An
immediate color change from orange to green to yellow is observed.
After 2 h, the solvent is removed, and the yellow residue is dried in
vacuo (1 × 10−3 mbar). The product is crystallized from a minimal
amount of fresh n-hexane at −40 °C. The supernatant is removed by
syringe, and the crystals are dried in vacuo (1 × 10−3 mbar). Yield:
230 mg, 0.32 mmol, 68%. C27H63N3P2Si8 (715.45 g/mol). Mp. 205
°C. Anal. calcd for C27H63N3P2Si8: C, 45.26; H, 8.86; N, 5.87. Found:
3Hyp-tBu. To a solution of [P(μ-NHyp)]2 (1Hyp; 305 mg, 0.521
t
mmol) in hexane (5 mL), BuNC (58 μL, 43 mg, 0,521 mmol) is
added via microliter syringe. An immediate color change from orange
to blue to green to yellow is observed. After 2 h, all volatiles are
removed, and the yellow residue is dried in vacuo (1 × 10−3 mbar).
The product is crystallized from a minimal amount of fresh n-hexane
at −40 °C. Only a few crystals can be obtained due to the equilibrium
chemistry described above. C23H63N3P2Si8 (668.40 g/mol). 1H NMR
(CD5CD3, 300.1 MHz): δ 0.24 (s, 27H), 0.27 (s, 27H), 1.48 (s, 9H).
31P{1H} NMR (C6D5CD3, 121.5 MHz): δ 230.7 (d, 1P, J = 15 Hz,
CPN), 281.9 (d, 1P, J = 15 Hz, NPN). Single crystals suitable for X-
ray structure determination can be obtained by recrystallization from
fresh n-hexane at −40 °C. Due to the dynamic equilibrium chemistry, no
further analysis of the pure species was possible.
1
C, 45.36; H, 9.04; N, 5.70. H NMR (C6D6, 300.1 MHz): δ 0.21 (s,
27H), 0.33 (s, 27H), 2.39 (s, 6H), 6.94 (s, 3H). 13C{1H} NMR
(C6D6, 75.5 MHz): δ 0.0, 1.9, 18.6 (d, J = 3 Hz), 127.2, 129.3, 136.0
(d, J = 3 Hz), 144.7 (d, J = 4 Hz). 14N NMR: No signals observed.
29Si-INEPT NMR (C6D6, 59.6 MHz): δ −13.2 (m, 3Si), −14.3 (m,
3Si), −30.8 (m, 1Si), −33.8 (m, 1Si). 31P{1H} NMR (C6D6, 121.5
MHz): δ = 209.0 (d, 1P, J = 15 Hz, CPN), 297.3 (d, 1P, J = 15 Hz,
4Hyp-tBu. To a solution of [P(μ-NHyp)]2 (1Hyp; 301 mg, 0.520
mmol) in hexane (5 mL), tBuNC (0.58 mL, 430 mg, 5.212 mmol) is
added via microliter syringe. An immediate color change from orange
to blue to green to yellow is observed. After 2 h, the solvent and
NPN). IR (ATR, 32 scans, cm−1): ν
̃
= 2949 (w), 2893 (w), 1599 (w),
1589 (w), 1539 (m), 1470 (w), 144 (w), 1398 (w), 1325 (w), 1242
(m), 1203 (m), 1194 (m), 1097 (w), 99 (m), 937 (m), 910 (m), 825
(vs), 791 (s), 744 (m), 685 (s), 621 (m), 553 (m), 540 (m). Raman
t
excess BuNC are removed, and the yellow residue is dried in vacuo
(633 nm, 20 s, 20 scans, cm−1): ν
̃
= 3068 (1), 3036 (1), 3017 (1),
(1 × 10−3 mbar). A mixture of several species is obtained, with the
main components being 3Hyp-tBu and 4Hyp-tBu in a 2:5 ratio.
2952 (3), 2895 (7), 1594 (1), 1467 (1), 1441 (1), 1406 (1), 1380
(1), 1335 (2), 1332 (2), 1263 (2), 1241 (2), 1222 (1), 1197 (10),
1164 (1), 1101 (1), 1032 (1), 1017 (4), 913 (3), 894 (1), 890 (1),
862 (1), 828 (2), 794 (4), 780 (1), 748 (2), 738 (1), 726 (1), 713
(3), 690 (3), 632 (9), 623 (5), 556 (5), 544 (3), 530 (2), 490 (1),
472 (1), 455 (1), 440 (2), 415 (4), 397 (1), 375 (1), 348 (1), 320
(3), 295 (2), 272 (2), 253 (2), 241 (2), 221 (3), 176 (7), 127 (3), 81
(6). MS (CI pos., isobutane), m/z (%): 424 (18) [DmpNHypCNP]+,
1
C28H72N4P2Si8 (751.58 g/mol). H NMR (C6D5CD3, 300.1 MHz):
diene 3Hyp-tBu: δ 0.29 (s, 27H), 0.30 (s, 27H), 1.45 (s, 9H); adduct
4Hyp-tBu: δ 0.31 (s, 27H), 0.39 (s, 27H), 1.48 (s, 9H). 31P{1H}
NMR (C6D5CD3, 121.5 MHz): diene 3Hyp-tBu: δ 230.7 (d, 1P, J =
15 Hz, CPN), 281.9 (d, 1P, J = 15 Hz, NPN); adduct 4Hyp-tBu: δ
177.7 (d, 1P, J = 22 Hz, NPN), 233.1 (d, 1P, J = 22 Hz, NPN).
Separation of the products and further analysis of the pure species was not
possible.
2Hyp-Ter. To a solution of [P(μ-NHyp)]2 (1Hyp; 139 mg, 0.236
mmol) in hexane (6 mL), TerNC (80 mg, 0.24 mmol) is added. An
immediate color change from orange to blue is observed. After 2 h,
the solvent is removed, and the blue residue is dried in vacuo (1 ×
10−3 mbar). Due to the product’s high sensitivity, some byproducts
are obtained that prevented recrystallization, and the sample was used
for further experiments. C43H79N3P2Si8 (925.75 g/mol). 1H NMR
4 9 7 ( 2 6 ) [ D m p N H y p C N 2 P 2 S i ] +
,
6 4 5 ( 4 )
[DmpNHypCN2P2SiSi2(CH3)3]+, 700 (21) [C7H6N3P2CHyp2]+,
701 (14) [C7H7N3P2CHyp2]+, 702 (10) [C7H8N3P2CHyp2]+, 715
(100) [M]+, 716 (86) [M + H]+. Single crystals suitable for X-ray
structure determination can be obtained by recrystallization from
fresh n-hexane at −40 °C.
3Hyp-Mes. To a solution of [P(μ-NHyp)]2 (1Hyp; 271 mg, 0.464
mmol) in hexane (8 mL), MesNC (66 mg, 0.46 mmol) is added. An
H
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(C6D6, 500.1 MHz): δ 0.15 (s, 27H), 0.26 (s, 27H), 2.22 (s, 6H),
2.29 (s, 6H), 2.52 (s, 6H), 6.82 (m, 4H), 7.04 (s, 3H). 13C{1H} NMR
(C6D6, 125.8 MHz): δ 1.1, 1.9 (d, J = 3 Hz), 21.3, 23.2, 27.4, 123.4,
129.1, 129.4, 130.9, 132.5, 135.5, 136.3, 137.3, 137.6, 139.8, 155.6.
14N NMR: No signals observed. 29Si-INEPT NMR (C6D6, 99.4
MHz): δ −11.9 (m, 3Si), −12.8 (m, 3Si), −26.2 (m, 1Si), −42.4 (m,
1Si). 31P{1H} NMR (C6D6, 202.5 MHz): δ 211.7 (br d, 1P, J = 118
Hz, CPN), 275.1 (br d, 1P, J = 118 Hz, NPN). The product was highly
labile. Therefore, isolation and further analysis of the pure species were not
possible.
Irradiation of 2Hyp-Ter. 1Hyp (25.8 mg, 0.043 mmol) and
TerNC (17.8 mg, 0.043 mmol) are dissolved in THF-d8 (0.45 mL).
The blue solution was irradiated in the NMR spectrometer with red
light (638 nm). After full conversion to the housane 2′Hyp-Ter, the
light source was shut off, and the thermal reverse reaction was traced
by 31P NMR spectroscopy. 31P{1H} NMR of 2′Hyp-Ter (THF-d8,
101 MHz): δ −128.0 (d, 1P, J = 91 Hz, NPC), −69.0 (d, 1P, J = 91
Hz, NPN).
ACKNOWLEDGMENTS
■
We are indebted to the ITMZ of the University of Rostock for
access to the high-performance computing facilities. Especially,
we wish to thank Malte Willert for his continuous support with
all software-related issues. Furthermore, we wish to thank the
DFG (SCHU/1170/12-2) for financial support.
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AUTHOR INFORMATION
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Corresponding Authors
Jonas Bresien − Institute of Chemistry, University of Rostock,
18059 Rostock, Germany; orcid.org/0000-0001-9450-
3407; Email: jonas.bresien@uni-rostock.de
Axel Schulz − Institute of Chemistry and Department Life, Light,
and Matter, University of Rostock, 18059 Rostock, Germany;
Leibniz Institute for Catalysis, 18059 Rostock, Germany;
orcid.org/0000-0001-9060-7065; Email: axel.schulz@uni-
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Dirk Michalik − Institute of Chemistry, University of Rostock,
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Anne-Kristin Rolke − Institute of Chemistry, University of
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Alexander Villinger − Institute of Chemistry, University of
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Ronald Wustrack − Institute of Chemistry, University of
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