NH/PH isomerization and a lewis pair for carbon dioxide capture

Article abstract of DOI:10.1021/ic401498r

Bis(di-i-propylphosphino)amine 1 reacts with B(C₆F ₅)₃ to form an adduct with concomitant N/P H-isomerization. This species reacts smoothly with carbon dioxide. An attempt to prepare an anionic derivative resulted in the formation of a novel heterocycle derived from the PNP ligand and B(C₆F₅)₃.

Full text of DOI:10.1021/ic401498r

Communication
pubs.acs.org/IC
NH/PH Isomerization and a Lewis Pair for Carbon Dioxide Capture
Brian M. Barry,† Diane A. Dickie,† Luke J. Murphy,‡ Jason A. C. Clyburne,*,‡ and Richard A. Kemp*,†,§
†Department of Chemistry and Chemical Biology, University of New Mexico, Albuquerque, New Mexico 87131, United States
‡The Atlantic Centre for Green Chemistry, Department of Chemistry, Saint Mary’s University, Halifax, NS, B3H 3C3, Canada
§Advanced Materials Laboratory, Sandia National Laboratories, Albuquerque, New Mexico 87106, United States
S
* Supporting Information
genation catalysts.5 Here, we report the use of 1 to prepare a
ABSTRACT: Bis(di-i-propylphosphino)amine 1 reacts
Lewis acid−base adduct capable of reacting selectively with
with B(C6F5)3 to form an adduct with concomitant N/P
H-isomerization. This species reacts smoothly with carbon
dioxide. An attempt to prepare an anionic derivative
resulted in the formation of a novel heterocycle derived
from the PNP ligand and B(C6F5)3.
carbon dioxide. It is anticipated that the isomerization reaction
from 1 to 2 in the presence of a Lewis acid might facilitate
̀
break-up of the complex and be more selective vis-a-vis small
molecule reactivity than the elegant but more reactive frustrated
Lewis pair (FLP) systems.6
Bis(di-i-propylphosphino)amine 1 reacts cleanly with B-
(C6F5)3 in benzene to produce the air-stable zwitterionic
derivative 3, which can be crystallized from benzene solution.
Spectroscopic analysis of the isolated material is consistent with
the formation of compound 3. In contrast to the single
resonance at 68.0 ppm (C6D6) in the 31P{1H} NMR spectrum
of 1, 3 exhibits two signals at 26.0 and 42.6 ppm. The upfield
signal is a doublet of quartets consistent with a combination of
is(diarylphosphino)amines, HN(PAr2)2, and the related
B
bis(dialkylphosphino)amines, HN(PR2)2, are reliable
precursors to the monoanionic ligand, [N(PR2)2]−. The steric
demands of these ligands are reminiscent of the t-
butylmethylphosphino-di-t-butylphosphinomethane or “tri-
chickenfootPHOS” ligand,1 suggesting that bis(phosphine)-
amines may have great potential in facilitating a wide variety of
metal-mediated transformations. These ligands are ambidentate
and are known to bond in a variety of modes with either
transition metal or lanthanide and actinide compounds;2
however, the coordination chemistry of this important ligand
class with main group elements has not been extensively
explored. Our group, having a longstanding interest in CO2
interactions with main group compounds,3 has recently been
successful in using the aliphatic ligand HN(PiPr2)2 (1) to
prepare a novel tin-containing compound that reacts with
carbon dioxide.4
3
2JP−P and JB−P coupling and has been assigned to a
phosphorane resonance. The remaining quartet, assigned to
the boron-bound phosphorus atom, is broadened due to the
1
direct interaction with a quadrupolar B atom. The JB−P
coupling can be resolved, but the broadening does not allow
2
1
for JP−P resolution. The JB−P coupling (97 Hz) observed in
the 31P{1H} spectrum is in accord with observation of a doublet
in the 11B{1H} spectrum with a similar coupling. The proton-
1
coupled 31P spectrum of 3 revealed a signal with a JH−P
coupling constant of 466 Hz. This coupling was supported by
1
the 1H NMR spectrum that also showed the same JP−H
coupling constant for the phosphorus bound proton. Taken
in toto, these data are consistent with a prototropic reaction
with migration of the proton from nitrogen to phosphorus
upon interaction with B(C6F5)3. Consistent with this, the
infrared spectrum of 1 exhibits a strong N−H absorption at
3265 cm−1 that is noticeably absent in the infrared spectrum of
3. For compound 3, a new absorption appears at 2378 cm−1,
and this is assigned as a P−H stretch. Last, high-resolution mass
spectrometry studies are in accord with the formula for 3.
In order to confirm the atomic connectivity, crystals of 3
were isolated, and the structure was determined by single
crystal X-ray diffraction. The results of the study are shown in
Figure 1, and the structure shows the anticipated presence of
the B(C6F5)3 fragment coordinated to one of the phosphorus
centers, leading to the unsymmetric nature of the PNP
fragment. Important structural parameters are included in the
caption of Figure 1.
Bis(dialkylphosphino)amines, HN(PR2)2, have the ability to
engage in a tautomerization reaction with Lewis acids,
generating phosphoranes such as 3, discussed below. Having
bulky alkyl groups such as t-butyl on phosphorus will facilitate
this isomerization, and it is well-known that reaction of an
electrophile with compounds such as 1 occurs predominantly at
phosphorus, resulting in the formation of charged phosphor-
animine-phosphonium salts. Recently, this prototropism has
been cleverly used to prepare chiral and air-stable hydro-
Received: June 13, 2013
Published: July 23, 2013
© 2013 American Chemical Society
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Inorganic Chemistry
Communication
Figure 2. Molecular structure of 4 (50% ellipsoids). Hydrogens
(except H2) omitted for clarity. Selected bond lengths (Å) and angles
(deg): P(1)−N(1) = 1.577(2), P(2)−N(1) = 1.575(2), P(1)−C(31)
= 1.880(3), C(31)−O(1) = 1.319(3), C(31)−O(2) = 1.214(3),
B(1)−O(1) = 1.540(3); P(1)−N(1)−P(2) = 143.98(17), O(1)−
C(31)−O(2) = 126.7(3), O(1)−C(31)−P(1) = 112.39(19), O(2)−
C(31)−P(1) = 120.9(2).
Figure 1. Molecular structure of 3 (50% ellipsoids). Hydrogens
(except H2) omitted for clarity. Selected bond lengths (Å) and angles
(deg): P(1)−N(1) = 1.600(4), P(2)−N(1) = 1.545(4), P(1)−B(1) =
2.083(5); P(1)−N(1)−P(2) = 156.1(3).
Compound 3 is air stable and exhibits neither oxidation nor
reaction with water vapor in the air over several weeks.
Furthermore, reactions of compound 3 were attempted with
diphenylamine and phenyl acetylene, but no reactions were
observed under the reaction conditions used (toluene, 60 °C,
for 4 h). However, in benzene solution and in the presence of
an atmosphere of carbon dioxide, the compound reacts readily
and selectively to produce adduct 4. Reactivity of CO2 with 3 is
indicated by the observation of an absorption in the IR
spectrum at 1679 cm−1 characteristic of a new CO bond.
NMR studies in C6D6 solution of 4 are also informative. In
general, the resonances are sharper than for compound 3. The
31P{1H} spectrum of 4 shows two inequivalent doublets (37.0
and 38.4 ppm), which are coupled to each other (27 Hz) while
the 11B{1H} signal is a singlet. Evidence for the formation of a
P−C bond upon formal CO2 insertion is revealed in the
which upon the addition of B(C6F5)3 was only partially
redissolved. Subsequent addition of tetramethylammonium
chloride led to another white precipitate, presumably LiCl,
that was removed via filtration. Solvent was removed under
vacuum conditions, and the solid residue was recrystallized
from pentane, resulting in an off-white crystalline material. This
material was characterized using multinuclear NMR, high-
resolution mass spectrometry and single crystal X-ray
diffraction, and the structure was determined to be the unusual
cyclic structure 5 (Figure 3) rather than the anticipated
[NMe4][i-Pr2P(B(C6F5)3)−N−P(iPr)2]. We also isolated
5·0.5C6H14, and its crystallographic data are included in the
Supporting Information.
The formation of compound 5 was unexpected, as it requires
the cleavage of a very strong carbon−fluorine bond. The
cleavage of the C−F bond is not observed without the addition
of tetramethylammonium chloride, indicating that cation
exchange of the tightly bound lithium ion for the more weakly
coordinating tetramethylammonium cation is necessary for
1
13C{1H} spectrum as a doublet at 167.9 ppm with a JP−C
coupling constant of 117 Hz. High-resolution mass spectrom-
etry studies are in accord with the formula for 4. Last,
compound 4 was also examined using thermogravimetric
analysis-mass spectrometry. Evolution of carbon dioxide was
observed beginning at 160 °C and completed by 180 °C. Above
225 °C, the material decomposed.
Further characterization of 4 was performed using X-ray
crystallography (Figure 2). The C−O/CO bond lengths of
1.319(3) and 1.214(3) Å, respectively, are in excellent
agreement with previously reported P/B-based CO2 adducts.7
Somewhat surprisingly, given the unsymmetric nature of the
molecule, the P−N bonds were found to be identical (P1−N1
= 1.577(2) Å; P2−N1 = 1.575(2) Å). These measurements are
halfway between the nominal P−N and PN bonds of 3 and
significantly shorter than the true single bonds in the free
ligand, 1 (1.706(4) Å).4
Recently, we have found that weakly coordinating cations can
facilitate the formation of extremely novel and highly reactive
complex anions.8 In this light, we attempted to prepare an
anionic FLP system targeting the tetramethylammonium salt of
3 that we expected could be formed via lithiation of 1, followed
by the addition of B(C6F5)3, and subsequent cation exchange
with tetramethylammonium chloride. The lithiation of 1 in
benzene led to immediate formation of a white precipitate,
Figure 3. Molecular structure of 5 (50% ellipsoids). Hydrogens
omitted for clarity. Selected bond lengths (Å) and angles (deg): P(1)−
N(1) = 1.607(5), P(2)−N(1) = 1.567(5), P(1)−B(1) = 2.039(8),
P(2)−C(30) = 1.814(7); P(1)−N(1)−P(2) = 131.4(4), N(1)−P(1)−
B(1) = 110.6(3).
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Inorganic Chemistry
Communication
(3) (a) Dickie, D. A.; Ulibarri-Sanchez, R. P., III; Jarman, P. J.; Kemp,
R. A. Polyhedron 2013, 58, 92. (b) Dickie, D. A.; Gislason, K. B.;
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fluoride salt formation. FLP systems have previously been
reported to undergo aromatic substitution at the para position
of the fluorinated aryl borane leading to zwitterionic species.9
This can be prevented by using an extremely sterically
congested phosphine, or by replacing the para-F with a
hydrogen atom. In the case of 5, the para position is not
accessible when one phosphorus atom of the PNP ligand is
bound to boron, but the ortho position is within reach of the
other phosphorus atom. To our knowledge, this type of
reaction has not been observed previously for PNP ligands,
although related systems have been reported.10 In those
systems, the formation of a stable, five-membered heterocycle
was speculated to be the driving force for the reaction, and it
seems likely that the six-membered heterocycle formed in 5
would be similarly stabilizing.
In summary, bis(di-i-propylphosphino)amine 1 reacts with
B(C6F5)3 to form an adduct with concomitant N/P H-
isomerization. This species 3 is a mildly frustrated Lewis
acid−base boron−phosphorus pair; however much of the
“frustration” is released through the isomerization reaction in
which the PNP angle broadens, releasing much of the steric
strain. This species reacts smoothly with carbon dioxide. Last,
an attempt to prepare an anionic FLP resulted in the
observation of a novel cyclization of the PNP ligand with
B(C6F5)3.
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̈
ASSOCIATED CONTENT
* Supporting Information
Synthetic procedures and spectroscopic data for compounds 3,
4, and 5. Important X-ray crystallographic data for 3, 4, 5, and
5·0.5C6H14 (CCDC 934157−934160). This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
48, 7253. (c) Harhausen, M.; Frohlich, R.; Kehr, G.; Erker, G.
̈
Organometallics 2012, 31, 2801. (d) Bertini, F.; Lyaskovskyy, V.;
Timmer, B. J. J.; de Kanter, F. J. J.; Lutz, M.; Ehlers, A. W.; Slootweg, J.
C.; Lammertsma, K. J. Am. Chem. Soc. 2012, 134, 201. (e) Zhao, X.;
Stephan, D. W. Chem. Commun. 2011, 47, 1833. (f) Peuser, I.; Neu, R.
C.; Zhao, X.; Ulrich, M.; Schirmer, B.; Tannert, J. A.; Kehr, G.;
AUTHOR INFORMATION
Corresponding Author
*E-mail: jason.clyburne@smu.ca (J.A.C.C.), rakemp@unm.edu
(R.A.K.).
Frohlich, R.; Grimme, S.; Erker, G.; Stephan, D. W. Chem.Eur. J.
̈
■
2011, 17, 9640. (g) Momming, C. M.; Otten, E.; Kehr, G.; Frohlich,
̈
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R.; Grimme, S.; Stephan, D. W.; Erker, G. Angew. Chem., Int. Ed. 2009,
48, 6643. (h) Kroner, J.; Noth, H.; Polborn, K.; Stolpmann, H.; Tacke,
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M.; Thomann, M. Chem. Ber. 1993, 126, 1995.
Notes
(8) MacIntosh, I. S.; Sherren, C. N.; Robertson, K. N.; Masuda, J. D.;
Pye, C. C.; Clyburne, J. A. C. Organometallics 2010, 29, 2063.
(9) (a) Travis, A. L.; Binding, S. C.; Zaher, H.; Arnold, T. A. Q.;
Buffet, J.-C.; O’Hare, D. Dalton Trans. 2013, 42, 2431. (b) Geier, S. J.;
Dureen, M. A.; Ouyang, E. Y.; Stephan, D. W. Chem.Eur. J. 2010,
16, 988. (c) Welch, G. C.; Prieto, R.; Dureen, M. A.; Lough, A. J.;
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by the NSF (Grants
CHE09-11110 and CHE12-13529), the Laboratory Directed
Research and Development program at Sandia National
Laboratories (LDRD 151300), and NSERC of Canada.
J.A.C.C. acknowledges support from the Canada Research
Chairs Program, CFI, and NSRIT. L.J.M. thanks SMU for a
graduate fellowship. The Bruker X-ray diffractometer was
purchased via a NSF CRIF:MU award to UNM (CHE04-
43580), and the NMR spectrometers were upgraded via grants
from the NSF (CHE08-40523 and CHE09-46690). High
resolution mass spectrometry data were obtained by UNM
Mass Spectrometry Facility. Sandia is a multiprogram
laboratory operated by Sandia Corporation, a Lockheed Martin
Company, for the United States Department of Energy under
Contract No. DE-AC04-94AL85000.
Labeodan, O. A.; Holtrichter-Rossmann, T.; Stephan, D. W. Dalton
̈
Trans. 2009, 1559. (d) Welch, G. C.; Holtrichter-Rossmann, T.;
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Stephan, D. W. Inorg. Chem. 2008, 47, 1904. (e) Welch, G. C.;
Cabrera, L.; Chase, P. A.; Hollink, E.; Masuda, J. D.; Wei, P.; Stephan,
D. W. Dalton Trans. 2007, 3407.
(10) (a) Schnurr, A.; Vitze, H.; Bolte, M.; Lerner, H.-W.; Wagner, M.
Organometallics 2010, 29, 6012. (b) Zhao, X.; Gilbert, T. M.; Stephan,
D. W. Chem.Eur. J. 2010, 16, 10304.
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