Complexation of nitrous oxide by frustrated lewis pairs

Article abstract of DOI:10.1021/ja904377v

(Figure Presented) Frustrated Lewis pairs comprised of a basic yet sterically encumbered phosphine with boron Lewis acids bind nitrous oxide to give intact PNNOB linkages. The synthesis, structure, and bonding of these species are described.

Full text of DOI:10.1021/ja904377v

Published on Web 07/01/2009
Complexation of Nitrous Oxide by Frustrated Lewis Pairs
Edwin Otten, Rebecca C. Neu, and Douglas W. Stephan*
Department of Chemistry, UniVersity of Toronto, 80 St. George Street, Toronto, Ontario, Canada, M5S 3H6
Received June 4, 2009; E-mail: dstephan@chem.utoronto.ca
Anthropogenic disturbance of Earth’s atmospheric composition
is a source of great environmental concern due to its contribution
to global warming. Of the trace gases that have increased steadily
relative to preindustrial levels, carbon dioxide has attracted the most
attention because of its established relationship to human activity.
Although nitrous oxide (N2O) is only a minor constituent of the
atmosphere (319 ppb), it is ∼300 times more potent as a greenhouse
gas than CO2.1 N2O is also a potentially strong and yet environ-
mentally benign oxidant; however its high kinetic stability has
hampered its use.2 Nonetheless, Nature uses a Cu4S cluster in
nitrous oxide reductase (NOR) to convert N2O to dinitrogen and
water, a process that is part of microbial denitrification.3 A synthetic
NOR analogue has recently been shown to reduce N2O.4 Transition
metal mediated reactivity of N2O includes O-transfer reactions to
low-valent metal centers,5 insertion of the oxygen atom into M-R
(R ) alkyl, hydride) bonds,6 N-N bond cleavage,7 and hydrogena-
tion to give N2 and H2O.8 In addition, (catalytic) oxidation of
organic substrates using N2O has received renewed attention.9 A
possible reason for its generally sluggish reactivity is the fact that
N2O is a very poor ligand. Despite being comprised of N2 and NO
fragments, both of which are well-established as ligands in transition
metal chemistry, only a few N2O complexes are described in the
literature.10 None of these species have been structurally character-
ized although computational studies have probed the interactions
of N2O with various metal systems. Perhaps the most well-studied
N2O complex is [Ru(NH3)5(N2O)]2+ reported by Armor and Taube
in 1969, which is thought to contain a linear Ru-NNO fragment
based on spectroscopic10a,11 and computational studies.11c,12
We have recently developed the concept of “frustrated Lewis
pairs” (FLPs) in which steric congestion precludes formation of
classical Lewis adducts so that the unquenched acidity/basicity can
be exploited for further reactivity.13 This has led to unique main-
group systems capable of (reversible) H2 activation,14 hydrogena-
tion,15 and unprecedented small-molecule reactivity.16 In this report,
we demonstrate FLP binding of N2O and describe the first
crystallographic characterizations of bound N2O species.
within the N2O fragment. The infrared spectrum of 1 showed no
bands that could be unambiguously assigned to NsN or NsO
vibrations. However, comparison with 1-15N reveals an additional
band isotopically shifted to 1410 cm-1 which was attributed to the
NsN stretch. The corresponding absorption in 1 is obscured by a
broad peak due to aromatic CsC vibrations. A crystal structure
determination confirmed the proposed formulation, with a N2O
molecule bridging the P and B fragments in a 1,3 mode (Figure
1).18 The NsN and NsO bonds in the N2O fragment (1.2570(17)
and 1.3361(15) Å, respectively) are significantly elongated in
comparison to free N2O (1.127 and 1.186 Å).19 For related
phosphazides R3P(NRNꢀNγ)R′ the NRsNꢀ bond lengths (∼1.34 Å)
are generally longer than those observed for 1.20 The PsN bond
in 1 (1.7088(12) Å) is also significantly longer than the corre-
sponding PsN bond lengths in phosphazides.21 Collectively, these
metrical data indicate the bonding in 1 to be best described as
PsNdNsOsB (Vide infra) in which the tBu3P and OB(C6F5)3
fragments adopt a transoid disposition with respect to the NdN
double bond.
Figure 1. Synthesis and POV-ray depictions of the molecular structures
of 1 and 2. Selected bond distances (Å) and angles (deg): (1): P-N
1.7087(12); N-N 1.2573(17); N-O 1.3362(15); O-B 1.5428(18); P-N-N
117.04(10); N-N-O 109.11(11); N-O-B 114.43(10); (2): P-N 1.7107(6);
N-N 1.2602(8); N-O 1.3270(8); O-B 1.5475(9); P-N-N 112.85(5);
N-N-O 111.68(6); N-O-B 111.61(5).
The reaction of an equimolar mixture of tBu3P and B(C6F5)3 with
N2O (1 bar) in bromobenzene results in the precipitation of a white
solid 1, which was isolated in 76% yield after recrystallization from
CH2Cl2/hexane (Figure 1). NMR spectroscopic analysis in CD2Cl2
showed a single 31P resonance at 68.5 ppm. The observed 11B (0.4
ppm) and 19F (∆δ(p,m-F) ) 5.7 ppm) resonances are indicative of
a four-coordinate boron center. The 15N isotopomer 1-15N was
synthesized from 15N15NO. The observation of 15N NMR signals
at 566.6 and 381.7 ppm which exhibit NsP coupling of 58.7 and
19.6 Hz, respectively, and 1JNN ) 15.6 Hz establishes the presence
of two inequivalent nitrogen atoms. Taken together, these spectro-
scopic data are consistent with the formulation of 1 as tBu3P(NNO)-
B(C6F5)3. Comparing the 15N NMR parameters for 1-15N to those
A dependence on the combined Lewis acidity and basicity has
been previously noted in the interaction of small molecules with
FLPs.13 Attempts to generate N2O complexes with less basic
phosphines have so far not been successful. For example, the
frustrated Lewis pair (o-tolyl)3P/B(C6F5)3 does not react with N2O.
On the other hand, variation of the Lewis acid shows that N2O
binding using FLPs is not restricted to the very acidic borane
B(C6F5)3. Treatment of tBu3P and B(C6F5)2Ph, a substantially
weaker Lewis acid than B(C6F5)3,22 with N2O (1 bar) gave the
product 2 in 76% yield (Figure 1). This species exhibited 31P and
for free N2O (218 and 135 ppm, JNN ) 8.1 Hz)17 or the N2O
1
complex cis-RuCl2(η1-15N14NO)(PsN)(PPh3) (PsN ) 1-Ph2P-2-
Me2NC6H4; Nt 125.8 ppm)10b suggests substantial perturbations
9
9918 J. AM. CHEM. SOC. 2009, 131, 9918–9919
10.1021/ja904377v CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
11B NMR resonances at 67.3 and 3.3 ppm respectively, suggesting
a formulation similar to that of 1. This was confirmed crystallo-
graphically (Figure 1).18 While the general structural features of 2
are similar to those of 1, the P-N and N-N bonds are slightly
longer at 1.7107(6) and 1.2602(8) Å, respectively, while the N-O
bond distance is slightly shorter at 1.3270(8) Å. The O-B distance
in 2 is similar to that in 1. These perturbations demonstrate that
the Lewis acidity at B has a greater impact on the remote N-N
and P-N interactions without a dramatic effect on the B-O bond.
Additional insight into the bonding in 1 was obtained from DFT
calculations at the B3LYP/6-31G(d) level of theory. The optimized
geometry 1calc is in good agreement with the crystallographically
determined structure, with a somewhat longer NsN (1.273 Å) and
shorter NsO bond (1.298 Å). A frequency analysis for 1calc revealed
NsN and NsO infrared frequencies in the fingerprint region at
1483 and 1257 cm-1, respectively, corroborating the experimental
observation that NNO vibrations are obscured in the IR spectrum
of 1. Natural Bond Orbital analysis shows the bonding within the
NNO fragment to consist of a NdN double bond and a NsO single
bond (Figure 2), consistent with the crystallographic data.
Research (NWO). R.C.N. is grateful for the award of an NSERC
of Canada scholarship.
Supporting Information Available: Experimental and computa-
tional details, NMR spectra of 1-15N and X-ray crystallographic details
of 1 and 2. This material is available free of charge via the Internet at
http://pubs.acs.org.
References
(1) Hansen, J.; Sato, M. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 16109.
(2) (a) Lee, D.-H.; Mondan, B., Karlin, K. D. Nitrogen Monoxide and Nitrous
Oxide Binding and Reduction. In ActiVation of Small Molecules; Tolman,
W. B., Ed. WILEY-VCH Verlag GmbH: Weinheim, 2006. (b) Leont’ev,
A. V.; Fomicheva, O. A.; Proskurnina, M. V.; Zefirov, N. S. Russ. Chem.
ReV. 2001, 70, 91.
(3) Chen, P.; Gorelsky, S. I.; Ghosh, S.; Solomon, E. I. Angew. Chem., Int.
Ed. 2004, 43, 4132.
(4) Bar-Nahum, I.; Gupta, A. K.; Huber, S. M.; Ertem, M. Z.; Cramer, C. J.;
Tolman, W. B. J. Am. Chem. Soc. 2009, 131, 2812.
(5) (a) Bottomley, F. Polyhedron 1992, 11, 1707. (b) Howard, W. A.; Parkin,
G. J. Am. Chem. Soc. 1994, 116, 606. (c) Groves, J. T.; Roman, J. S. J. Am.
Chem. Soc. 1995, 117, 5594. (d) Harman, W. H.; Chang, C. J. J. Am. Chem.
Soc. 2007, 129, 15128. (e) Yu, H. Z.; Jia, G. C.; Lin, Z. Organometallics
2009, 28, 1158.
(6) (a) Vaughan, G. A.; Rupert, P. B.; Hillhouse, G. L. J. Am. Chem. Soc.
1987, 109, 5538. (b) Vaughan, G. A.; Sofield, C. D.; Hillhouse, G. L.;
Rheingold, A. L. J. Am. Chem. Soc. 1989, 111, 5491. (c) Vaughan, G. A.;
Hillhouse, G. L.; Rheingold, A. L. J. Am. Chem. Soc. 1990, 112, 7994. (d)
Kaplan, A. W.; Bergman, R. G. Organometallics 1997, 16, 1106. (e) Yu,
H. Z.; Jia, G. C.; Lin, Z. Y. Organometallics 2007, 26, 6769. (f) Yu, H. Z.;
Jia, G. C.; Lin, Z. Y. Organometallics 2008, 27, 3825.
(7) (a) Laplaza, C. E.; Odom, A. L.; Davis, W. M.; Cummins, C. C.;
Protasiewicz, J. D. J. Am. Chem. Soc. 1995, 117, 4999. (b) Johnson, A. R.;
Davis, W. M.; Cummins, C. C.; Serron, S.; Nolan, S. P.; Musaev, D. G.;
Morokuma, K. J. Am. Chem. Soc. 1998, 120, 2071. (c) Cherry, J.-P. F.;
Johnson, A. R.; Baraldo, L. M.; Tsai, Y.-C.; Cummins, C. C.; Kryatov,
S. V.; Rybak-Akimova, E. V.; Capps, K. B.; Hoff, C. D.; Haar, C. M.;
Nolan, S. P. J. Am. Chem. Soc. 2001, 123, 7271.
Figure 2. NBO orbitals for the NdN σ- and π-bond (left, middle) and
NsO σ-bond (right).
(8) Lee, J.-H.; Pink, M.; Tomaszewski, J.; Fan, H.; Caulton, K. G. J. Am. Chem.
Soc. 2007, 129, 8706.
(9) (a) Parmon, V. N.; Panov, G. I.; Uriarte, A.; Noskov, A. S. Catal. Today
2005, 100, 115. (b) Poh, S.; Hernandez, R.; Inagaki, M.; Jessop, P. G.
Org. Lett. 1999, 1, 583.
(10) (a) Armor, J. N.; Taube, H. J. Am. Chem. Soc. 1969, 91, 6874. (b) Pamplin,
C. B.; Ma, E. S. F.; Safari, N.; Rettig, S. J.; James, B. R. J. Am. Chem.
Soc. 2001, 123, 8596.
(11) (a) Bottomley, F.; Crawford, J. R. J. Am. Chem. Soc. 1972, 94, 9092. (b)
Bottomley, F.; Brooks, W. V. F. Inorg. Chem. 1977, 16, 501. (c) Paulat,
F.; Kuschel, T.; Nather, C.; Praneeth, V. K. K.; Sander, O.; Lehnert, N.
Inorg. Chem. 2004, 43, 6979.
(12) Tuan, D. F. T.; Hoffmann, R. Inorg. Chem. 1985, 24, 871.
(13) (a) Stephan, D. W. Org. Biomol. Chem. 2008, 1535. (b) Stephan, D. W.
Dalton Trans. 2009, 3129.
(14) (a) Welch, G. C.; Juan, R. R. S.; Masuda, J. D.; Stephan, D. W. Science
2006, 314 (5802), 1124. (b) Spies, P.; Erker, G.; Kehr, G.; Bergander, K.;
Fröhlich, R.; Grimme, S.; Stephan, D. W. Chem. Commun. 2007, 5072.
(c) Sumerin, V.; Schulz, F.; Atsumi, M.; Wang, C.; Nieger, M.; Leskela¨,
M.; Repo, T.; Pyykko¨, P.; Rieger, B. J. Am. Chem. Soc. 2008, 130, 14117.
(15) Chase, P. A.; Welch, G. C.; Jurca, T.; Stephan, D. W. Angew. Chem., Int.
Ed. 2007, 46, 8050.
The formation of 1calc is exothermic by 17.4 kcal/mol. However,
1calc is shown to be a kinetic product as extrusion of N2 and
formation of tBu3PdO and B(C6F5)3 is thermodynamically favorable
by 60.4 kcal/mol relative to 1. This notion was confirmed
experimentally. Heating an NMR sample of 1 in C6D5Br at 135 °C
for 44 h resulted in the liberation of N2 and formation of the Lewis
acid-base adduct (tBu3PdO)B(C6F5)3 3 as the main product (∼80%
at 95% conversion) as evidenced by spectroscopic data and
independent synthesis (Figure 3). In addition, photolysis of 1 readily
afforded 3 (5 min of irradiation gives ∼75% at 90% conversion),
but prolonged photolysis resulted in decomposition to unidentified
species. These observations suggest that isomerization of 1 placing
P and O cis to one another prompts loss of N2. This notion is
reminiscent of proposed transition states in Staudinger oxidations
of phosphines.23
(16) (a) McCahill, J. S. J.; Welch, G. C.; Stephan, D. W. Angew. Chem., Int.
Ed. 2007, 46, 4968. (b) Dureen, M. A.; Lough, A.; Gilbert, T. M.; Stephan,
D. W. Chem. Commun. 2008, 4303. (c) Ullrich, M.; Seto, K. S.-H.; Lough,
A. J.; Stephan, D. W. Chem. Commun. 2009, 2335. (d) Dureen, M. A.;
Stephan, D. W. J. Am. Chem. Soc. 2009, 131, 8396. (e) Mo¨mming, C. M.;
Otten, E.; Kehr, G.; Fro¨hlich, R.; Grimme, S.; Stephan, D. W.; Erker, G.
Angew. Chem., Int. Ed. 2009, 48, ASAP. (f) Stephan, D. W. Dalton Trans.
2009, 3129.
(17) Pileio, G.; Carravetta, M.; Hughes, E.; Levitt, M. H. J. Am. Chem. Soc.
2008, 130, 12582.
j
(18) X-ray data: 1: P1, a ) 9.5265(4) Å, b ) 11.6603(5) Å, c ) 14.3458(7) Å,
R ) 76.6040(10)°, ꢀ ) 89.0710(10)°, γ ) 87.1940(10)°, V ) 1548.32(12)
Å3, data (>3σ) ) 5446, var 451, R ) 0.0286, Rw ) 0.0778, GOF 1.013. 2:
Figure 3. Proposed mechanism of thermolysis or photolysis of 1 to 3.
j
P1, a ) 10.3832(8) Å, b ) 11.9066(9) Å, c ) 14.5601(12) Å, R )
70.621(4)°, ꢀ ) 76.818(4)°, γ ) 65.912(4)°, V ) 1541.2(2) Å3, data (>3σ)
) 16452, var 415, R ) 0.0377, Rw ) 0.1043, GOF 1.021.
(19) Griggs, J. J. L.; Narahari Rao, K.; Jones, L. H.; Potter, R. M. J. Mol.
Spectrosc. 1968, 25, 34.
In summary, frustrated Lewis pairs of a basic yet sterically
encumbered phosphine with Lewis acids bind nitrous oxide to give
intact PNNOB linkages. The reactivity of these new N2O species
and the utility of FLPs in the activation of small molecules continue
to be the focus of efforts in our laboratory.
(20) (a) Bebbington, M. W. P.; Bourissou, D. Coord. Chem. ReV. 2009, 253,
1248. (b) Fortman, G. C.; Captain, B.; Hoff, C. D. Inorg. Chem. 2009, 48,
1808.
(21) (a) Courtenay, S.; Wei, P. R.; Stephan, D. W. Can. J. Chem. 2003, 81,
1471. (b) Courtenay, S.; Walsh, D.; Hawkeswood, S.; Wei, P.; Das, A. K.;
Stephan, D. W. Inorg. Chem. 2007, 46, 3623.
(22) Morrison, D. J.; Piers, W. E. Org. Lett. 2003, 5, 2857.
(23) Staudinger, H.; Hauser, E. HelV. Chim. Acta 1921, 4 (1), 861.
Acknowledgment. D.W.S. gratefully acknowledges the financial
support of NSERC of Canada and the award of a Canada Research
Chair. E.O. is grateful for the support of a Rubicon postdoctoral
fellowship from The Netherlands Organisation for Scientific
JA904377V
9
J. AM. CHEM. SOC. VOL. 131, NO. 29, 2009 9919