Isolation of a carbene-stabilized phosphorus mononitride and its radical cation (PN+.)

Article abstract of DOI:10.1002/anie.201002889

Carbenes can do better than metals: Phosphorus mononitride transition-metal complexes have never been isolated, but a PN fragment capped by two bulky, strongly electron-releasing singlet carbenes can be prepared at room temperature (see picture; P red, N blue, C gray). The one-electron oxidation produced the corresponding stable radical cation. (Figure Presented)

Full text of DOI:10.1002/anie.201002889

Communications
DOI: 10.1002/anie.201002889
Phosphorus Mononitride
Isolation of a Carbene-Stabilized Phosphorus Mononitride and Its
Radical Cation (PN+C)**
Rei Kinjo, Bruno Donnadieu, and Guy Bertrand*
In contrast to the inert nature of N2, diphosphorus (P2)[1] and
diarsenic (As2)[2] are only persistent in the gas phase at high
temperatures; otherwise, they dimerize to form the stable
tetrahedral white phosphorus (P4) and yellow arsenic (As4).
Phosphorus mononitride (PN) has attracted a lot of interest,[3]
mostly because it is an important component of the inter-
stellar medium and the atmospheres of Jupiter and Saturn.[4]
Scheme 1. Carbene-stabilized main-group elements in their zero oxida-
tion state.
In the laboratory, gaseous PN has been synthesized by
pyrolysis of different precursors, and the phosphorus–nitro-
gen bond length (1.49 ꢀ) determined by microwave spectros-
copy.[5] In the condensed phase, PN is even more prone to
association than P2 and As2. Its dimer, P2N2, has not yet been
detected experimentally, and its trimer has only been
observed spectroscopically in a krypton matrix at 40 K.[6]
Although the combination of two Group 15 elements does
not preclude the isolation of small clusters, as shown by the
recent discovery of AsP3,[7] the smallest discrete binary PN
molecule that has been structurally characterized is P3N21.[8,9]
However, P4N4,[10a] PN9,[10b] and PN15[10c] have been observed.
The nitrogen atom was installed by treatment of NHC 1[20]
with bromine, followed by addition of aqueous ammonia.
Deprotonation of 2 with nBuLi and addition of phosphorus
trichloride afforded compound 3 in 68% yield. The desired
product 5 was then obtained in 88% yield by reacting 3 with
cyclic alkyl amino carbene (CAAC) 4[21] and subsequent
reduction with excess magnesium (Scheme 2).
Even more surprisingly, in contrast to the large number of
N2,[11a,b] P2,[11c–e] and As2
transition-metal complexes that
[11e,f]
have been isolated,[11g] only a single report describes the
reaction of PN with metal atoms of Group 11 at 10 K.[12] The
corresponding complexes have only been characterized by IR
spectroscopy, and the description as monomeric PN com-
plexes has been debated.[3d]
Recently, it was shown that, just like transition metals,
stable singlet carbenes can activate small molecules.[13] They
also coordinate main group elements in their zero oxidation
state,[14] leading to stable species such as carbodicarbenes
Scheme 2. Synthesis of bis(carbene)–PN adduct 5. Dipp=2,6-diisopro-
pylphenyl.
(bent allenes) A[15,16] and diatomic silicon B1,[17a] germanium
B2,[17b] phosphorous C1,[13e,18] and arsenic C2
(Scheme 1).
[19]
Herein we report the isolation of phosphorus mononitride
stabilized by two carbenes. We also describe the one-electron
oxidation of PN, which allows the complete characterization
of the first isolable PN radical cation.
Compound 5 is indefinitely stable, even in air at room
temperature, both in the solid state and in solution. The
31P NMR signal (d = + 134.0 ppm) appears at lower field than
those of C1 (d = + 59.4–ꢀ73.6 ppm),[18] which is expected
because of the difference of electronegativity between nitro-
gen and phosphorus. Single crystals of 5 melt at 2158C, and no
decomposition was observed by heating a toluene solution
under reflux for 24 h. According to the X-ray diffraction
study[22] (Figure 1a), compound 5 has a planar trans-bent
geometry (N1-P1-C4 102.788, C1-N1-P1 122.128, C1-N1-P1-
[*] Dr. R. Kinjo, B. Donnadieu, Prof. G. Bertrand
UCR-CNRS Joint Research Chemistry Laboratory (UMI 2957)
Department of Chemistry, University of California Riverside
Riverside, CA 92521-0403 (USA)
Fax: (+1)951-827-2725
E-mail: guy.bertrand@ucr.edu
Homepage: http://research.chem.ucr.edu/groups/bertrand/
guybertrandwebpage/
ꢀ
ꢀ
C4 torsion angle 179.388). Both the P1 C4 (1.719 ꢀ) and C1
ꢀ
N1 (1.282 ꢀ) bonds are rather short, and the central P N
ꢀ
bond (1.7085 ꢀ) is in the normal range for a P N single bond.
The molecular structure of 5 is thus very similar to that
[**] We are grateful to the NSF (CHE-0924410) and RHODIA Inc. for
financial support of this work, and to the Japan Society for the
Promotion of Science for a Fellowship (R.K.).
reported by Robinson et al. for NHC-stabilized diphosphorus
[18a]
C1
and diarsenic C2.[19] All these carbene adducts do not
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201002889.
feature an EE triple bond, as is found in free P2, As2, and
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comparable to the coupling constant found in radical cations
C1 C (a(31P) = 42–44 G).[23] However, coupling with the nitro-
+
gen atom was not observed, indicating a relatively small
distribution of spin density on the central nitrogen atom. To
determine the anisotropic coupling constant, the frozen
fluorobenzene solution EPR spectrum was recorded at
100 K (Figure 2c). The simulation of the spectrum shows
that the tensors are aligned, which is consistent with the p*
geometry of the SOMO: Az(31P) = 143 G, Ax(31P) = ꢀ10 G,
and Ay(31P) ꢁ 0; gz = 2.0028, gx = 2.0052, gy = 2.0087. These
features are also very comparable to those observed for
Figure 1. ORTEP for 5 (a) and 5+C (b). Ellipsoids set at 50% proba-
bility; hydrogen atoms, solvent molecules, and the counteranion are
omitted for clarity. Selected bond lengths [ꢀ] and angles [8]: 5: P1–N1
1.7085(16), C1–N1 1.282(3), P1–C4 1.719(2); C1-N1-P1 122.12(14),
N1-P1-C4 102.78(9). 5+C: P1–N1 1.645(4), C1–N1 1.313(5), P1–C4
1.788(5); C1-N1-P1 119.3(3), N1-P1-C4 102.1(2).
radical cations C1 C.[23]
+
Single crystals suitable for an X-ray diffraction study were
obtained by layering hexane on a fluorobenzene solution of
5+C at 08C.[22] Similar to 5, radical cation 5+C has an almost
PN,[5] but rather an EE single bond. Similarly to C1 and C2, 5
can be regarded as containing either a phosphinidene–nitrene
fragment or a phosphazabutadiene skeleton, as shown by
canonical structures 5a and 5b, respectively. Note that form
5b is reminiscent of the end-on dinuclear N2 transition-metal
complexes, the historically most common bonding motif of
nitrogen, and a form that is considered as corresponding to a
strong activation of N2.[11a,b]
We have recently shown that C1 can be reversibly
oxidized,[23] and therefore to further demonstrate the analogy
between this compound and 5, we carried out an electro-
chemical study. The cyclic voltammogram of a THF solution
of 5, containing 0.1m nBuNPF6 as electrolyte, shows a
reversible one-electron oxidation at E1/2 = ꢀ0.51 V versus
Fc+/Fc, and a second oxidation at about + 0.60 V, which is
irreversible (Figure 2a). These data prompted us to carry out
planar trans-bent structure (Figure 1b). The P N bond
ꢀ
ꢀ
(1.645 ꢀ) is 3.7% shorter, whereas P1 C4 (1.788 ꢀ) and
ꢀ
C1 N1 (1.313 ꢀ) bonds are significantly longer than in 5.
All these experimental findings are readily rationalized by
ab initio calculations performed on the simplified model
compounds 6 and 6+C that feature the parent NHC and
CAAC. The optimized geometries of both molecules at the
(U)B3LYP/6-31G(d) level of theory agree well with experi-
ment. The frontier orbitals ((U)HF/6-311G(d)) are depicted
in Figure 3. The HOMO of 6 is the p*(PN) orbital, which
Figure 3. Selected molecular orbitals for 6 (left) and 6+C (right)
calculated at the (U)HF/6-311G(d)//(U)B3LYP/6-31G(d) level of
theory.
Figure 2. a) Cyclic voltammogram of a THF solution of 5 (0.1m
nBuNPF6 as electrolyte, scan rate 100 mVsꢀ1, potential versus Fc+/Fc);
b) EPR spectrum (9.3305 GHz) of 5+C in a fluorobenzene solution at
298 K; c) EPR spectrum of 5+C in a fluorobenzene frozen solution at
100 K.
mixes in a bonding fashion with p(p) AO of the carbene
carbon atoms. This indicates that the electronic reference
state of PN in the carbene complex is the doubly excited
1G(p!p’*) state, where the doubly occupied p’* MO back-
donates electronic charge into the formally empty p(p) AO of
the carbene. The carbene!PN donation takes place from the
s lone pairs of carbene donors into the empty in-plane p(PN)II
MO, which yields two low-lying orbitals (not shown). The
p*(PN)? orbital in 6 is doubly occupied (HOMO), and is of
course singly occupied in 6+C (SOMO). The change in the
occupation of the p*(PN)? orbital yields a shortening of the
the chemical synthesis of radical cation 5+C. When toluene was
added at room temperature to an equimolar mixture of 5 and
[Ph3C][B(C6F5)4] in an argon atmosphere, the color immedi-
ately changed to dark brown. After 2 h of stirring, the
31P NMR spectrum was silent, indicating the paramagnetic
character of the resulting product. After work up, radical
cation 5+C was quantitatively obtained as an air-sensitive
microcrystalline powder (m.p. 2168C). Not surprisingly,
treatment of a toluene solution of 5+C with KC8 rapidly gave
back 5, demonstrating a fully reversible redox system. The
room-temperature ESR spectrum of a fluorobenzene solution
of 5+C displays a doublet owing to a large coupling with
phosphorous (g = 2.0048; a(31P) = 44 G; Figure 2b), which is
ꢀ
ꢀ
ꢀ
central P N bond and weakening of both C1 N1 and P1 C4
bonds.
The atomic partial charges and spin distribution, calcu-
lated by NBO method at (U)B3LYP/6-311G(d) level of
theory,[24] provide an interesting information about the
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Communications
Science 2006, 313, 1276 – 1279; d) N. A. Piro, C. C. Cummins, J.
Am. Chem. Soc. 2008, 130, 9524 – 9535.
bonding situation in these molecules. The PN fragments in
neutral 6 and radical cation 6+C carry a negative charge of
ꢀ0.37e and ꢀ0.12e, respectively. The spin density in 6+C is
mainly distributed on the phosphorous atom (0.40e). Inter-
estingly, the spin distribution on a central nitrogen atom
(0.18e) is comparable to the nitrogen atom of CAAC ligand
(0.19e), which is attributed to the higher acceptor strength of
CAAC compared with NHC.
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In summary, these results demonstrate the powerful
ability of bulky singlet carbenes to stabilize very reactive
species, including those for which transition metals have so far
failed. Moreover, as singlet carbenes, and especially NHCs,[25]
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use carbene adducts as PN delivery molecules.
Experimental Section
All manipulations were performed under an atmosphere of dry argon
using standard Schlenk techniques.
5: Hexanes (20 mL) was added at ꢀ788C to a mixture of 3
(1270 mg, 2.50 mmol) and CAAC 4 (814 mg, 2.5 mmol). The reaction
mixture was allowed to warm to room temperature and stirred for 3 h.
The white precipitate was filtered, washed with hexanes, and dried
under vacuum. Magnesium (91 mg, 3.75 mmol) and THF (40 mL)
were added to the solid, and the solution was stirred at room
temperature for 14 h. After the solvent was removed under vacuum,
the residue was extracted with toluene (40 mL) and dried under
vacuum to afford 5 as light orange powder (88% yield). Single
crystals of 5 were obtained by recrystallization from a mixture of
THF/hexane (1:1) solution at room temperature. M.p.: 2168C;
1H NMR (500 MHz, C6D6): d = 7.04–7.00 (m, 4H, CH), 6.96–6.91
(m, 5H, CH), 3.22 (br, 4H, CH2 ꢁ 2), 3.13 (sept, J = 7.0 Hz, 2H,
CH(CH3)2), 2.79 (sept, J = 7.0 Hz, 2H, CH(CH3)2), 2.49 (sept, J =
7.0 Hz, 2H, CH(CH3)2), 1.70 (s, 2H, CH2), 1.27–1.13 (m, 10H, CH2),
1.10 (d, J = 7.0 Hz, 12H, CH(CH3)2 ꢁ 2), 1.07 (d, J = 7.0 Hz, 12H,
CH(CH3)2 ꢁ 2), 1.02 (d, J = 7.0 Hz, 12H, CH(CH3)2 ꢁ 2), 0.85 ppm (s,
1
6H, CH3); 13C NMR (125 MHz, C6D6): d = 199.2 (d, JP-C = 50.0 Hz,
2
NPC), 152.4 (d, JP-C = 21.3 Hz, CNP), 149.3 (o), 148.5 (o ꢁ 2), 138.9
(br, ipso), 135.3 (ipso), 128.5 (p ꢁ 2), 128.3 (p), 125.2 (m), 124.7 (m ꢁ
2), 66.4 (Cq), 53.5 (d, 2JP-C = 9.1 Hz, PCCq), 51.2 (CH2), 49.0 (br, CH2 ꢁ
2), 36.8 (CH2), 30.1 (CH3), 29.1 (CH ꢁ 2), 28.9 (CH), 28.4 (CH3), 28.3
(CH3), 25.7 (CH3 ꢁ 2), 25.2 (CH3 and CH2), 24.3 (CH3), 24.0 ppm
(CH2); 31P NMR (121.4 MHz, C6D6): d = 134.0 ppm.
Radical cation 5+C: Toluene (15 mL) was added at room temper-
ature to a mixture of 5 (400 mg, 0.53 mmol) and Ph3C+B(C6F5)4
ꢀ
(485 mg, 0.53 mmol), and the solution was stirred at room temper-
ature for 2 h. After the solvent was removed under vacuum, the dark
brown residue was washed with hexanes (50 mL) and dried under
vacuum to give radical cation 5+C (623 mg; 82% yield) as a highly air-
sensitive dark brown powder. Single crystals of 5+C were grown by
layering hexanes on top of a fluorobenzene solution at 08C. M.p.:
2158C (dec).
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Received: May 13, 2010
Published online: July 15, 2010
Keywords: carbenes · phosphorus mononitride · radical cations
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