Photochemical reactions of triplet phenylphosphinidene with carbon monoxide and nitric oxide

Article abstract of DOI:10.1039/c8cc08664h

Here we report the photochemical reactions of triplet phenylphosphinidene with carbon monoxide and nitric oxide. The photolysis of phenylphosphirane in carbon monoxide-doped matrices enabled the first spectroscopic identification of phenylphosphaketene, t

Full text of DOI:10.1039/c8cc08664h

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Photochemical reactions of triplet
phenylphosphinidene with carbon monoxide and
nitric oxide†
Cite this: Chem. Commun., 2018,
4, 13694
5
Received 30th October 2018,
Accepted 12th November 2018
Artur Mardyukov * and Dominik Niedek
DOI: 10.1039/c8cc08664h
rsc.li/chemcomm
Here we report the photochemical reactions of triplet phenyl-
phosphinidene with carbon monoxide and nitric oxide. The photolysis
of phenylphosphirane in carbon monoxide-doped matrices enabled
the first spectroscopic identification of phenylphosphaketene, the
hitherto unreported phosphorus analogue of phenyl isocyanate.
The hitherto undisclosed phosphinimine-N-oxyl radical formed
upon UV irradiation of the phenylphosphinidene with nitric oxide
in argon matrices at 10 K.
Scheme 1 Triplet phosphinidenes (2, 5 and 6), phenyldioxophosphorane
7), and previously prepared heavier congeners of 7, namely 8, and 9.
(
substitution (R N–, R P–) may stabilize the phosphinidene
2
2
Fragmentation of white phosphorus (P ) is a fundamental route singlet over the triplet state. For example, the thermolysis of
4
4
2
to convert P into activated intermediates, and it is the base dibenzo-7-phosphanorbornadiene generates Me NQP whose
2
3–25
chemical for the production of many organophosphorus molecular mass was detected via mass spectrometry.
Although
1
compounds. Understanding the structure, properties and reactivity phosphinidenes are usually not isolable under ordinary reaction
of transient species can help to clarify the mechanisms of reactions conditions, to date, only one stable singlet phosphinidene has been
where these intermediates are involved. Phosphinidenes (R–P%:) are a successfully isolated in the presence of sterically demanding
class of reactive intermediates, which are of considerable impor- substituents. Furthermore, the reaction of singlet phosphinidene
tance in different organic reactions. Aryl and alkyl substituted with electron poor alkenes and isonitriles has been explored. The
25
2–6
25
7–10
phosphinidenes have a triplet ground state
and are therefore chemistry of triplet phosphinidenes remains sparse, in contrast to
highly reactive phosphorus species so that they can only be detected free carbenes and nitrenes, because of the lack of suitable synthetic
7–10
26
in the gas phase and in cryogenic matrices.
Triplet mesitylphos- precursors. Phosphiranes represent potentially useful precursors
phinidene (5) was detected via EPR and IR spectroscopy in the for the generation of free phosphinidenes that can be activated
7
,9
27
photolysis of trans-2,3-dimethyl-1-mesitylphosphirane.
The either thermally or photochemically. Borden and colleagues have
follow-up chemistry of 5, in particular, cycloaddition reactions, computationally explored the energies and electronic structures
1
1,12
has also been studied.
Only in 2017, the recorded X-band associated with the thermal fragmentation of phenylphosphirane
À1
EPR of 5 and a zero-field-splitting (ZFS) parameter, D = 4.116 cm
were determined in glassy methylcyclohexane at 5 K. Triplet CASSCF computations, the formation of triplet 2 from 3 via a
silylphosphilide (6) was prepared and isolated in argon matrices stepwise pathway is 21.2 kcal mol less endothermic and has a
by the reaction of atomic silicon with PH
these diverse roles of phosphinidenes, the majority of systems singlet 2 via a concerted fragmentation pathway.
studied to date are transition metal or Lewis base coordinated.
(3) to phenylphosphinidene (2) and ethylene. According to
8
À1
8
À1
3
(Scheme 1). Despite 12.2 kcal mol lower barrier height than the formation of
2
7
13–19
Recently, we disclosed a photochemical method for the efficient
The transient formation of phosphinidenes has been postu- preparation of previously elusive triplet 2 in argon matrices by light-
2
0,21
lated based on chemical trapping experiments.
computational and experimental studies,
Based on induced elimination of ethylene from the corresponding phenyl-
2
2
23–25
3
strong p-donor phosphirane (3). The reaction of 2 with P–O gave long-elusive
2
10
phenyldioxophosphorane (7). The heavier congeners of phenyl-
dioxophosphorane have recently been prepared and isolated in
argon matrices (8 and 9) (Scheme 1).
Institute of Organic Chemistry, Justus Liebig University, Heinrich-Buff-Ring 17,
28,29
3
5392 Giessen, Germany. E-mail: artur.mardyukov@org.chemie.uni-giessen.de
In continuation of these studies, in this work, we further
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c8cc08664h
examine the reactivity of 2 with small molecules, e.g., carbon
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Scheme 2 Photochemical generation of 1 and 4.
ꢀ
monoxide (CO) and nitric oxide ( NO). The reaction of 2 with
CO will lead to the formation of phenylphosphaketene (1), which
Fig. 1 (a) IR difference spectra showing the photochemistry of 3 in
the presence of 2% CO after irradiation at l = 254 nm (15 min) then at
l = 365 nm (30 min) in argon at 10 K. Downward bands assigned to 3
disappear while upward bands assigned to 1 appear after 30 min of irradiation
represents the phosphorus analogue of phenyl isocyanate.
ꢀ
The photochemical reaction with NO gives an equally novel
phosphinimine-N-oxyl radical (4) (Scheme 2).
Studies describing free phosphinidenes have recently time. (b) IR spectrum of 1 computed at the M06-2X/6-311++G(2d,2p) level.
c) IR difference spectra showing the photochemistry of 3 in the presence
(
garnered attention, but explorations of their chemistry remain
13
13
2
4,25,30
of 2% CO. (d) IR spectrum of computed CO-1 at the M06-2X/6-
11++G(2d,2p) level.
rare.
To the best of our knowledge, compounds 1 and 4
3
have neither been synthesized nor observed using spectro-
scopic methods. For instance, the PCO radical and the simplest
phosphaketene, HPCO (11), have been prepared by the reaction is assigned to the structure of 1 and the IR absorption at
3
1
À1
of CO with P atoms or PH and isolated in argon matrices.
1978 cm is assigned to the nCO stretching vibration. When 3
In contrast to the high stability of isocyanates (R-NCO; R = aryl, is photolyzed in pure CO matrices, phosphaketene, 1, seems to be
alkyl), the phosphaketenes (R-PCO) have been considered formed exclusively via 2. The good agreement between the
extremely reactive species. For example, the butylphosphaketene is computed (M06-2X/6-311++G(2d,2p) and B3LYP/cc-pVTZ) levels
13
stable only below À60 1C. It is known that phosphaketenes dimerize and experimental frequencies of 1 and CO-1 isotopologues
3
2
to cyclic 1,3-diketones (RP)
2
(CO)
2
at room temperature. To underlines the successful preparation of 1 (ESI,† Table S1).
The measured CQO stretching frequency agrees well with
circumvent these problems, the kinetic stability of phosphaketenes
was increased in the presence of bulky (e.g., Ar = 2,4,6-tri-tert- previous IR measurements for other phosphaketenes. In HPCO,
3
3
À1
butylphenyl) aryl substituents (10) (Scheme 2).
a strong absorption at 1997.8 cm was attributed to the CO
À1
Reaction with CO. Phenylphosphinidene 2 (Scheme 2) can be stretching vibration, and a similar frequency at 1918.8 cm
generated through photolysis (l = 254 nm) from 3, and was has been measured for the PCO radical. In a recent gas-phase
easily identified by its matrix IR spectrum with the strongest study, a sharp absorption at 2011 cm was assigned to the nCO
absorption bands at 733 and 683 cm , which nicely match stretching mode of HPCO. In 10, the nCO vibration mode was
3
1
À1
À1
34
10
À1
33
with literature data (ESI,† Fig. S1). Trapping of the products of identified at 1953 cm (Scheme 2).
the photolysis of 3 in argon doped with variable CO concentra-
tions in the range of 0.1–2% at 10 K resulted in the disappearance been thoroughly studied.
of all IR absorption bands assigned to 3, and simultaneously, in form corresponding ketenes.
The reaction of singlet and triplet carbenes with CO has
3
5–37
Carbenes readily react with CO to
3
6,38
The photochemical reaction
the formation of two distinct species formed during photolysis. of triplet pentafluorophenyl nitrene with CO leads to the
3
9
The product distribution observed upon photodissociation of 3 corresponding isocyanate. The reaction of donor stabilized
depends on the initial concentration of CO. In highly diluted borylnitrene with CO analogously results in the formation of
4
0
matrices (41% of CO), 2 was observed as the major product. the corresponding isocyanate. Recently, it was shown that
Irradiation of 3 isolated in an argon matrix containing 2% of CO the triplet phenylborylene (PhB) reacts under photochemical
resulted in the formation of a new compound with a strong IR conditions (l 4 495 nm) with CO to give (PhBCO); the photo-
À1
band at 1978 cm (Fig. 1). The major constituent of the photo- product possesses a triplet ground state with a linear arrange-
4
1
lysis product of the starting material 3 was identified as phenyl- ment at the boron center (Scheme S1 in the ESI†).
À1
phosphaketene (1). The intense absorption at 1978 cm (calc.:
The formation mechanism of 1 is not clear, because it could
104 and 2052 cm ) shows a large isotopic shift of 46 cm (calc.: be a photochemical or a hot ground state reaction. Triplet
0 cm ) for CO-1, and is thus assigned to the CQO stretching phosphinidene 2 does not react thermally with CO at such low
À1
À1
2
5
À1
13
vibration (Table S1 in the ESI†). As expected, annealing of these temperatures since the direct formation of ground state phos-
matrices to 35 K did not, however, result in further growth of the phaketene is spin-forbidden. It should be noted, however,
IR bands of 1 (vide infra).
that cyclopentadienylidene, a triplet carbene, reacts readily
3
8
When the matrix was irradiated (l = 365 nm) at 30 K (and with CO at 25–30 K, yielding the corresponding ketene. This
re-cooled before the measurement), all IR bands assigned to 1 can be explained by the small singlet–triplet energy gap (DEST
)
À1
increased in intensity, while those of 2 decreased (Scheme 2).
in carbenes (less than 5 kcal mol in favor of the triplets),
On the basis of comparison with IR spectra of other phos- and consequently, the triplet state can serve as a reservoir for
phaketenes and DFT computations, the newly formed compound highly reactive singlet carbenes. The singlet–triplet splitting
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À1
in 2 was determined computationally to be in the range of the homodesmotic reaction (DH
r
) of À39.8 kcal mol at the
À1 42
À1
20–25 kcal mol
.
Compound 2 is unreactive towards CO, and M06-2X/6-311++G(2d,2p) level (À38.9 kcal mol at the B3LYP/
annealing the matrices containing 2 and CO at temperatures cc-pVTZ level) as shown in eqn (1).
above 30 K allows diffusion of CO and this results in the
formation of newly formed cage-pairs of 2 and CO that react
photochemically via an excited state of 2. This would probably
be the first singlet state, obtained by intersystem crossing from
the first excited triplet state.
(1)
The photochemical reaction of 2 with CO was also followed
by UV/vis spectroscopy. In line with the IR experiments, the
ꢀ
Reaction with NO. Motivated by the high reactivity of 2 toward
UV/vis spectral analysis of the 365 nm photolysis of 2 in the carbon monoxide upon UV/vis light irradiation, we aimed at
presence of 2% CO shows that its absorption maximum at investigating the photochemical reaction of 2 with nitric oxide
ꢀ
2
65 nm gradually decreases while new absorptions appear. ( NO). For this goal, irradiation of matrix (l = 254 nm) isolated 3
ꢀ
The UV/vis spectrum of matrix isolated 1 reveals two strong containing 1% of NO resulted in the formation of 4 with strong
absorption bands at lmax = 222 and 246 nm, which correlates IR absorptions at 1545 cm . The major constituent of the
À1
well with the electronic excitations computed at the TD-M06- photolysis products was identified as phosphinimine, N-oxyl
À1
2
X/6-311++G(2d,2p) level (ESI,† Fig. S2).
radical 4. The intense absorption at 1545 cm shows an isotopic
À1
À1
15
The formation of 2 and CO from 1 is photochemically shift of À32 cm (calc.: À31 cm ) in NO-4 and is assigned to
reversible. When the sample was irradiated at l = 254 nm the the NQO stretching vibration (Fig. 3). Two other absorptions at
À1
IR bands associated with 1 vanished, whereas the IR bands of 743 and 689 cm are assigned to the CH deformation vibration,
the starting material 2 and CO appeared (Scheme 2). Indeed, respectively. The observed vibrational bands match well with the
recently it has been shown that the thermal reaction of singlet fundamentals of 4 (Table S2, ESI†), computed anharmonically at
(
phosphino)phosphinidene with carbon monoxide gives the the UM06-2X/6-311++G(2d,2p) and UB3LYP/cc-pVTZ level.
2
4
corresponding (phosphino)phosphaketene.
However, to the best of our knowledge there are no experi-
Phosphorus forms weaker and longer bonds than nitrogen, mental reports describing phosphinimine, N-oxyl radical 4
ꢀ
owing to the larger phosphorus atom with larger and more diffuse (Fig. 3). Indeed, the arylazooxyl radical ( ONQNAr) has been
orbitals, resulting in a poorer orbital overlap with the phosphorus generated in solution as a long-lived radical species attached to
orbitals as compared to the nitrogen atom. As expected, the C–P propagating polymer chain ends in radical polymerization
4
3
bond length in 1 is computed to be considerably longer, i.e., reactions. Interestingly, computations at the [UM06-2X/6-
.861 Å at the M06-2X/6-311++G(2d,2p) level and 1.858 Å at the 311++G(2d,2p) and UMP2/cc-pVTZ] level predict a bent struc-
1
MP2/cc-pVTZ level, whereas in 12 the computed C–N bond ture for this radical. The P–N distance is 1.606 Å and 1.664 Å,
length is 1.402 Å at the M06-2X/6-311++G(2d,2p) level and and the N–O distance is 1.203 Å and 1.192 Å, respectively
1.398 Å at the MP2/cc-pVTZ level (Fig. 2a).
(Fig. S3 in the ESI†). A Wiberg bond index of approximately
The larger stability of phenylisocyanate 12 over 1 can be 1.7 is computed for the NO bond in 4, indicating a double-bond
discerned by the examination of molecular orbital plots. character. Note that most nitroxides reveal N–O bond lengths of
The molecular orbitals of 1 that are expected to influence its about 1.23 to 1.29 Å based on X-ray scattering and electron
4
4
reactivity the most are depicted in Fig. 2b. The HOMOÀ1 has diffraction analysis. We also computed the a and b spin
pronounced lone-pair character on the phosphorus atom, and densities at the UM06-2X/6-311++G(2d,2p) level of theory: high
the HOMO is dominantly p bonding between phosphorus and
carbon atoms. In the case of 12, the HOMO and HOMOÀ1 are
mainly localized over the entire phenyl ring. When phosphorus
is present instead of nitrogen, there is larger destabilization of
the HOMO energy, leading to a smaller HOMO–LUMO gap.
Thus, the estimated D(ELUMO À EHOMO) value for 1 (0.24 eV)
is smaller than that of 12 (0.29 eV). Moreover, the obvious
stability of 12 over 1 is evident from the reaction enthalpy of
Fig. 3 (a) IR spectrum of 4 computed at the UM06-2X/6-311++G(2d,2p)
level (unscaled). (b) IR difference spectra showing the photochemistry of 3
ꢀ
in the presence of 1% NO after irradiation at l = 254 nm in argon at 10 K.
Fig. 2 (a) Selected bond lengths (Å) of 1 and 12 at the M06-2X/6-
11++G(2d,2p) level. The values in parentheses were computed at the
Downward bands assigned to 4 appear after 15 min of irradiation time.
15
3
(c) IR spectrum of NO-4 computed at the UM06-2X/6-311++G(2d,2p)
level (unscaled). (d) IR difference spectra showing the photochemistry of 3
MP2/cc-pVTZ level. (b and c) Molecular orbitals of 1 and 12 computed at
the M06-2X/6-311++G(2d,2p) level.
15
in the presence of 1% NO after irradiation at l = 254 nm in argon at 10 K.
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ꢀ
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2
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2
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