Capture of NO by a frustrated lewis pair: A new type of persistent N-oxyl radical

Article abstract of DOI:10.1002/anie.201101622

NO your pairs: The intramolecular frustrated Lewis pair (FLP) Mes ₂PCH₂CH₂B(C₆F₅) ₂ captures NO to give the novel N-oxyl radical P/B-FLP-NO^. (see scheme). Coordination of NO to the FLP incites H-atom abstraction reactivity with cyclohexene and ethylbenzene to give P/B-FLP-NOH and the O-functionalized P/B-FLP-NOR species. Copyright

Full text of DOI:10.1002/anie.201101622

DOI: 10.1002/anie.201101622
Frustrated Lewis Pairs
Capture of NO by a Frustrated Lewis Pair: A New Type of Persistent
N-Oxyl Radical**
Allan Jay P. Cardenas, Brooks J. Culotta, Timothy H. Warren,* Stefan Grimme,* Annika Stute,
Roland Frꢀhlich, Gerald Kehr, and Gerhard Erker*
Frustrated Lewis Pairs (FLPs) show a rapidly increasing
spectrum of interesting chemical reactions.[1] They have been
reported to activate dihydrogen under mild conditions and to
serve as metal free hydrogenation catalysts toward specific
organic substrates.[2] FLPs add to numerous unsaturated
substrates such as alkenes and alkynes, carbonyl compounds,
azides and even CO2 or N2O.[3,4] For instance, the intra-
molecular P/B-FLP 1 adds to nitrosobenzene to form the six-
membered heterocycle 2.[5]
Treatment of
a
yellow solution of the FLP
Mes2PCH2CH2B(C6F5)2 (1)[10] (in situ generated from
Mes2PCH CH2 and Piersꢀ borane [HB(C6F5)2][11]) in fluoro-
=
benzene with 1 equiv NOgas gave rise to an intense green
solution from which blue crystals of the P/B-FLP-NOC product
3 were isolated in 58% yield by precipitation with pentane
(Scheme 1). X-ray crystal structure analysis of 3 (Figure 1)
Nitric oxide (NO) is an important messenger molecule
and regulator in biological systems.[6] We find that the reactive
frustrated Lewis pair 1 cleanly and rapidly reacts with this
essential small molecule to form the persistent heterocyclic N-
oxyl radical “P/B-FLP-NOC” (3). Compound 3 represents a
novel type of N-oxyl radical related to the ubiquitous
TEMPO radical (4) and its congeners (e.g. 5 (PINO)[7] and
6,[8] see Scheme 1).[9] Herein we describe the synthesis,
characterization, and O-based reactivity of this novel type
of N-oxyl radical derived from a frustrated Lewis pair and
nitric oxide.
Figure 1. X-ray structure of P/B-FLP-NOC (3).[33]
revealed that both the phosphorus and the boron atoms have
formed bonds to the nitrogen atom of the incorporated NO
ꢀ
ꢀ
molecule (P N: 1.7127(14) ꢁ, B N: 1.592(2) ꢁ, P-N-B:
ꢀ
114.27(10)8). The N O bond distance in 3 of 1.2962(17) ꢁ is
significantly lengthened relative to that of free NO
(1.151 ꢁ);[12] it is similar to that found in N-oxyl radicals
R2N-OC such as TEMPO (4: 1.284(8) ꢁ)[13] and tBu2N-OC
(1.28(2) ꢁ (gas phase)[14]).
Scheme 1. Synthesis of P/B-FLP-NOC (3) with related N-oxyl radicals.
The odd-electron species 3 exhibits an X-band EPR signal
at room temperature in fluorobenzene solution consistent
with coupling of the electron spin to the 14N nucleus as well as
to 31P and 11/10B nuclei (Figure 2). Simulation of the symmetric
pattern about g = 2.0089 gives A(14N) = 18.5 MHz, A(31P) =
48.5 MHz, and A(11B) = 9.1 MHz (contribution from 10B
neglected). While A(14N) is markedly lower than in the
related nitroxides TEMPO[15] and tBu2NO[16] (43.5 and
43.3 MHz in toluene, respectively), DFT calculations at the
TPSS0[17] level employing a def2-TZVP AO basis (fully de-
contracted for P, N, B atoms) predict very similar hyperfine
interactions (A(14N) = 16.7, A(31P) = 46.9, A(11B) = 10 MHz)
to those used to successfully simulate the experimental EPR
spectrum. To experimentally confirm these assignments, we
prepared the isotopomer P/B-FLP-15NOC (3-15N) by addition
of 15NO (generated from the reaction of tBuO15NO, [Cp2Fe],
and TMS-OTf; see Supporting Information, Scheme S1) to a
[*] A. J. P. Cardenas, B. J. Culotta, Prof. T. H. Warren
Department of Chemistry, Georgetown University
Box 571227, Washington, DC 20057-1227 (USA)
E-mail: thw@georgetown.edu
Prof. Dr. S. Grimme, Dipl.-Chem. A. Stute, Dr. R. Frꢀhlich, Dr. G. Kehr,
Prof. Dr. G. Erker
Organisch-Chemisches Institut der Universitꢁt Mꢂnster
Corrensstrasse 50, 48149 Mꢂnster (Germany)
E-mail: grimmes@uni-muester.de
erker@uni-muenster.de
[**] We acknowledge funding from the National Science Foundation
(CHE-0957606 to T.H.W.; CHE-0840453 to Georgetown for an EPR
spectrometer) and the Deutsche Forschungsgemeinschaft (S.G.,
G.E.).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201101622.
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7567

Communications
such as TEMPO (lmax = 460 nm (10.3mꢀ1 cmꢀ1) in MeCN).[18]
This band exhibits detailed vibrational fine structure with an
average spacing of 1109(11) cmꢀ1 for the six most intense
peaks. Since difference IR spectra of 3-14N and 3-15N identify
the n(14NO) and n(15NO) stretches at 1474 and 1457 cmꢀ1
(DFT predicts n(14NO) in a range 1428-1472 cmꢀ1 (delocal-
ized)), the vibronic spacing observed in the visible spectrum
likely reflects the excited state of 3. Time-dependent DFT
calculations indicate that this weak transition (f = 0.0002)
possesses n!p* character (Supporting Information, Fig-
ure S12).
While NO itself is a very poor H-atom acceptor owing to
ꢀ
the very modest H NO bond strength of approximately
47 kcalmolꢀ1 in nitroxyl (HNO),[19] the FLP-NO adduct 3
engages readily in H-atom abstraction reactions (Scheme 2).
ꢀ
Reaction of P/B-FLP-NOC (3) with 1,4-cyclohexadiene (C H
[19]
bond dissociation energy (BDE) ca. 76 kcalmolꢀ1
)
rapidly
quenches the blue color of 3 to give the corresponding
diamagnetic product P/B-FLP-NOH (7). This species may be
isolated in 67% yield as colorless crystals by vapor diffusion
of heptane into a benzene solution of 7. X-ray crystal
structure analysis of 7 reveals a significant lengthening of
Figure 2. X-band EPR spectra (fluorobenzene, room temperature) and
simulations for P/B-FLP-NOC (3-14N, bottom; 3-15N, top).
ꢀ
solution of Mes2PCH2CH2B(C6F5)2. The EPR spectrum of 3-
15N (Figure 2) is best simulated with A(15N) = 25.5 MHz
very close to the anticipated value of 26.0 MHz based on
the relative gyromagnetic ratios of 15N and 14N.
the N O distance to 1.422(2) ꢁ in 7 (from 1.2962(17) ꢁ in 3)
ꢀ
consistent with a decrease in N O bond order. A similar
ꢀ
lengthening of the N O distance occurs upon reduction of
TEMPO to TEMPOH (N O: 1.450(2)–1.462(2) ꢁ). The
conversion of 3 into 7 is accompanied by a decrease in the P
N bond length in 7 to 1.6315(18) ꢁ while the B N distance of
1.561(3) ꢁ in 7 remains similar to that found in 3, slightly
opening the P-N-B angle to 118.39(15)8. The hydroxy H-atom
was refined and shows a close contact to one of the Fatoms on
an adjacent C6F5 ring (H1···F6 = 2.218 ꢁ).
[20]
ꢀ
ꢀ
The optical absorption spectrum of 3 in fluorobenzene
ꢀ
(Figure 3)
features
a
maximum
at
l = 705 nm
(6.7(1)mꢀ1 cmꢀ1) that is quite distinct from typical nitroxides
NMR spectroscopic characterization of the diamagnetic
species 7 in [D1]chloroform reveals multiplets in the 1H NMR
spectrum at d = 2.83 and 1.76 ppm corresponding to the
backbone PCH2 and BCH2 groups along with a broad singlet
at d = 4.76 ppm for the NOH group. 31P{1H} and 11B{1H} NMR
spectra possess signals at d = 46.8 ppm (n1/2 = 40 Hz) and ꢀ5.8
(n1/2 = 150 Hz) ppm, respectively. The IR spectrum of 7 in the
solid state shows a relatively sharp band for the n(14NO-H)
stretch at 3527 cmꢀ1 along with the n(14NO) stretch at
1110 cmꢀ1 (n(15NO) = 1082 cmꢀ1).
Figure 3. Visible spectrum of P/B-FLP-NOC (3) in fluorobenzene at
room temperature.
Scheme 2. H-atom abstraction/O-functionalization reactivity of P/B-FLP-NOC (3) and the X-ray structures of products 7 and 8. (Only one of two
positions shown for the disordered 2-cyclohexenyl group of 8.)[33]
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Angew. Chem. Int. Ed. 2011, 50, 7567 –7571

The P/B-FLP-NOC radical 3 abstracts H-atoms from
ꢀ
stronger C H bonds as well. Exposure of 3 to excess
ꢀ1 [19]
ꢀ
cyclohexene (C H BDE ꢁ 82 kcalmol ) results in clean
H-atom abstraction (HAA) to form 7 along with the new
product 8 (31P NMR: d = 47.4 ppm; 11B NMR: d = ꢀ5.5 ppm
in [D1]chloroform) in a 1:1 ratio (Scheme 2). With excess
cyclohexene, the loss of 3 follows clean first-order kinetics in
which the observed rate constant kobs essentially doubles from
9.04 ꢂ 10ꢀ4 sꢀ1 to 2.16 ꢂ 10ꢀ3 sꢀ1 with a doubling of the
concentration of cyclohexene from 2.47m to 4.94m in
fluorobenzene to give a second order rate constant kcyclohexene
(258C) = 4.0(4) ꢂ 10ꢀ4 mꢀ1 sꢀ1. Isolation of the new product 8
by crystallization (Scheme 2) of the mixture from methanol
allows for its characterization as P/B-FLP-NO(2-cyclohex-
enyl). This species is similar in structure to P/B-FLP-NOH (7)
ꢀ
Figure 4. a) Spin density plot of 3 and TEMPO (isospin=0.001).
b) Alternative structures considered for 3 and 7 with P-N-O-B linkages.
with a N O bond length of 1.445(2) ꢁ and similar metrical
ꢀ
ꢀ
parameters about the P-N-B linkage (P N 1.6516(17) ꢁ, B N
1.586(3) ꢁ, P-N-B 116.99(13)8).
Mechanistic studies employing the reactive N-oxyl radical
PINO (5) in cyclohexene functionalization had revealed two
concurrent pathways. The first is a stepwise HAA/radical
combination sequence that results in the formation of a new
has a singly occupied p* SOMO and a concomitantly small
WBO of 1.15 (1.21 in TEMPO). The Mulliken spin-density
populations at O and N in 3 are 0.54 and 0.34eꢀ, respectively,
slightly more biased towards O as compared to the corre-
sponding values in TEMPO (0.50 and 0.44eꢀ, respectively).
Identified through both simulation and calculation, the
hyperfine couplings present in the EPR spectrum of 3
(Figure 2) clearly reflect the lower spin-density at the
N atom in 3. This perturbation in electronic structure
enhances the H-atom abstracting ability of 3 to form 7. P/B-
ꢀ
=
O C bond. Addition of the PINO radical to the C C double
bond, however, followed by abstraction of an allylic H-atom
to give the functionalized product represents a competing
pathway.[7] Without such mechanistic ambiguity, 3 also under-
ꢀ
goes HAA with the significantly stronger C H bonds of
ethylbenzene (C-H BDE ꢁ 85 kcalmolꢀ1)[19] following clean
pseudo first-order kinetics in 3 employing excess ethylben-
zene substrate (4–7m) in fluorobenzene to give kethylbenzene
(258C) = 5.6(3) ꢂ 10ꢀ6 mꢀ1 sꢀ1. The reaction of 3 with excess
ethylbenzene gives a 1:1 ratio of 7 along with the new O-
functionalized product P/B-FLP-NO(1-CH(Me)Ph) (9;
Scheme 2) as monitored by 31P NMR (d = 48.1 ppm in
[D1]chloroform).
ꢀ
FLP-NOH (7) has a calculated O H bond dissociation
enthalpy (BDH) of 77.3 kcalmolꢀ1 (at 298 K) which is about
10 kcalmolꢀ1 larger than the corresponding values for
TEMPO-H at the same level of theory (BDH = 67.2 kcal
molꢀ1). These calculated values for TEMPO-H compare well
with experimental values in benzene (bond dissociation free
energy (BDFE) 70.2, BDE = 65.2 kcalmolꢀ1).[26]
To better understand the electronic structure of 3 along
with the heightened HAA reactivity of 3 relative to typical
nitroxides such as TEMPO,[21] we examined 3 and 7 by
theoretical methods. The DFT structural optimizations have
been performed at the dispersion corrected (DFT-D3[22]
method) TPSS[17] level using very large quadruple-zeta AO
basis sets.[23] The computed structures and predicted spectro-
scopic properties agree very well with experimental data. For
Inspired by the nitrosobenzene adduct 2 that features a
six-membered ring with a P-N-O-B linkage (Scheme 1), we
computationally considered related k2-isomers of 3 and 7
(Figure 4b). These species are only 8.1 and 5.1 kcalmolꢀ1
higher in free energy than the experimentally observed
species 3 and 7 that feature k1-NO units. Interestingly, 3-k2
undergoes a reversal in the spin-density distribution (O:
0.32eꢀ, N: 0.52eꢀ) as compared to 3. Isomer 7-k2 suggests the
possibility of trapping the elusive molecule HNO[27] with 1,
supported by calculations that reveal a particularly tight
interaction to give the experimentally observed P/B-FLP-
NOH ground state 7 with DE = ꢀ57.3 kcalmolꢀ1 (DG(298) =
ꢀ41.9 kcalmolꢀ1).
ꢀ
ꢀ
ꢀ
instance, the computed N O, P N, and B N distances for 3
are 1.283, 1.732, and 1.608 ꢁ compared to values of
1.2962(17), 1.7127(14), and 1.592(2) ꢁ determined by X-ray.
Single-point thermochemical calculations at the even higher
B2PLYP-D3/def2-QZVP level of theory[24] reveal the binding
of NO to P/B-FLP (1) in its closed (quenched) four-
membered ring conformation to give P/B-FLP-NOC (3) is
quite exothermic with DE = ꢀ24.6 kcalmolꢀ1 (DG(298) =
ꢀ11.9 kcalmolꢀ1).
As a point of comparison, we briefly investigated the
reaction of the intermolecular FLP tBu3P/B(C6F5)3 with NO.
Bubbling 1 equiv NOgas through a bromobenzene solution of
tBu3P and B(C6F5)3 at room temperature resulted in 40%
conversion into a 1:1 mixture of known products FLP-N2O 10
and phosphine oxide 11 (Scheme 3; 31P NMR: d = 65.8 and
92.2 ppm in [D6]benzene, respectively).[4] This reaction fol-
lows a course related to the well studied disproportionation of
While the steric environment of the N-oxyl moiety of 3 is
more congested than in TEMPO, analysis of the electronic
structure of 3 reveals a basic similarity to TEMPO (Figure 4a
ꢀ
and Supporting Information S11–S12). The N P bond in 3 is
essentially a single bond with a Wiberg Bond Order
(WBO)[3c,f,25] of 1.07 while the N–B interaction is of donor–
acceptor type (WBO = 0.73). As in TEMPO, the N–O unit
=
NO to give N2O and R3P O upon reaction of NO with
phosphines PR3.[28] Thus the ability of the intramolecular FLP
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Scheme 3. Reaction of “open” FLP system tBu3P/B(C6F5)3 with NO.
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Keywords: frustrated Lewis pairs · H-atom abstraction ·
nitric oxide · nitroxides · radicals
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