Coordination of an N-heterocyclic phosphenium containing pincer ligand to a Co(CO) 2 fragment allows oxidation to form an unusual N-heterocyclic phosphinito species

Article abstract of DOI:10.1021/om200816p

A tridentate pincer ligand featuring a central N-heterocyclic phosphenium (NHP⁺) donor has been coordinated to a Co(CO) ₂ fragment to generate the Co NHP complex [PPP]Co(CO) ₂ (2). The NHP unit adopts an unusual pyramidal geometry with a relatively long Co-P distance, suggesting a stereochemically active nonbonding phosphorus lone pair. Interestingly, treatment of 2 with trimethylamine N-oxide affords [P(P=O) P]Co(CO) ₂ (3), in which the Co-bound central phosphorus donor has been oxidized to an unprecedented N-heterocyclic phosphinito species. The bonding and electronic properties of these complexes are discussed in the context of DFT and NBO computational data.

Full text of DOI:10.1021/om200816p

Communication
pubs.acs.org/Organometallics
Coordination of an N-Heterocyclic Phosphenium Containing Pincer
Ligand to a Co(CO)2 Fragment Allows Oxidation To Form an Unusual
N-Heterocyclic Phosphinito Species
Baofei Pan, Mark W. Bezpalko, Bruce M. Foxman, and Christine M. Thomas*
Department of Chemistry, Brandeis University, 415 South Street, Waltham, Massachusetts 02454, United States
S
* Supporting Information
ABSTRACT: A tridentate pincer ligand featuring a central N-
heterocyclic phosphenium (NHP+) donor has been coordi-
nated to a Co(CO)2 fragment to generate the Co NHP
complex [PPP]Co(CO)2 (2). The NHP unit adopts an
unusual pyramidal geometry with a relatively long Co−P
distance, suggesting a stereochemically active nonbonding
phosphorus lone pair. Interestingly, treatment of 2 with
trimethylamine N-oxide affords [P(PO)P]Co(CO)2 (3), in
which the Co-bound central phosphorus donor has been
oxidized to an unprecedented N-heterocyclic phosphinito
species. The bonding and electronic properties of these complexes are discussed in the context of DFT and NBO computational
data.
over their noninnocent nitrosyl counterparts are (1) the ability
to modify their steric and electronic properties via derivatiza-
tion and (2) the ability to incorporate these ligands into
chelating frameworks. These strategies may also be effective at
imparting stability in transition-metal NHP complexes and
protecting them from nucleophilic attack. Transition-metal
complexes of multidentate ligands featuring NHPs are
noticeably absent from the literature, particularly in comparison
to the growing number of NHC-containing chelating
ligands.20,21
We recently reported the synthesis of a cationic N-
heterocyclic phosphenium-containing pincer ligand [PPP]+ in
which the central NHP unit is linked to two phosphine side
arms via aryl linkers.22 As a result of the electrophilicity of the
central phosphenium center, coordination to transition metal
halide starting materials proved problematic, leading to
chlorophosphine complexes. Herein, we turn our attention to
a different synthetic strategy, utilizing NaCl extrusion from a
monoanionic transition metal starting material and a
chlorophosphine precursor as a driving force to synthesize a
bona fide metal NHP complex featuring our new chelating
ligand. A similar metal coordination strategy has been utilized
previously to synthesize metal NHP complexes, including
Cp*Fe(CO)2(NHPMe).23
hile N-heterocyclic carbene ligands (NHCs) have
become ubiquitous in the field of transition metal and
W
organo-catalysis,1−5 far less focus has been placed on the
potential applications of their isovalent group 15 analogues, N-
heterocyclic phosphenium cations (NHPs).6−10 First reported
in 1972,11,12 theoretical investigations of the electronic
structure of NHPs have shown that they are poor σ donors
but excellent π acceptors, in contrast to NHCs, which are very
good σ-donors and poor π-acceptors. Recent advancements in
the chemistry of NHPs include new preparative methods13−16
and new reactivity patterns,13,17−19 but the coordination
chemistry and catalytic capabilities of NHPs are relatively
unexplored compared to their NHC counterparts.
One of the more interesting aspects of NHP ligands is their
ability to adopt different binding modes toward transition
metals depending on the electronic nature of the transition
metal center. Most commonly, when binding to an electron-
poor fragment, NHPs act as both σ-donors and π-acceptors,
leading to a planar geometry at the phosphorus atom (NHP+
description). However, when binding to an electron-rich
fragment, NHPs adopt a pyramidal geometry indicative of a
nonbonding phosphorus lone pair. Two limiting descriptions
for the bonding in such pyramidal NHPs have been suggested:
(1) the Lewis acidic NHP phosphorus acts solely as an electron
pair acceptor (NHP+/Mn) and (2) the bonding between the
metal and the NHP is covalent and the NHP is best described
as a phosphido ligand (NHP−/Mn+2). In this sense, a
convincing analogy between NHPs and nitrosyls, which can
adopt either linear or bent geometries, can be made,17
highlighting the potential noninnocent character of these
ligands and bringing about potential ambiguities in the formal
metal oxidation state. Key advantages of phosphenium ligands
Treatment of the PPP-Cl precursor 1 with Na[Co(CO)4] in
THF cleanly generates the neutral red complex [PPP]Co(CO)2
(2) in 95% yield (Scheme 1). Complex 2 is characterized by
two 31P NMR signals at 286.4 (triplet) and 29.9 ppm
(doublet), corresponding to a Co-bound central NHP and
Received: August 31, 2011
Published: October 5, 2011
© 2011 American Chemical Society
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Communication
In an effort to promote carbonyl dissociation, complex 2 was
treated with trimethylamine N-oxide at room temperature.
Rather than oxidation of CO and extrusion of CO2, the new
dicarbonyl product 3 was obtained (Scheme 2). The triplet
Scheme 1
Scheme 2
two Co-bound phosphine donors, respectively. The infrared
spectrum of 2 reveals two ν(CO) stretches at 1981 and 1926
cm−1, implying that two CO ligands remain bound to Co.
X-ray crystallography of single crystals of 2 confirms the
tridentate binding mode of the NHP-diphosphine ligand and
the presence of two remaining carbonyl ligands (Figure 1). The
signal for the central phosphorus in the 31P NMR spectrum of 3
is shifted substantially upfield (81.8 ppm) compared to that of
2, while the doublet for the phosphine side arm donors is
similar (27.8 ppm). X-ray diffraction of single crystals revealed
3 to be the product of oxidation of the central NHP
phosphorus, [P(PO)P]Co(CO)2 (Figure 1), to generate an
N-heterocyclic phosphinito complex. While there are many
examples of metal-bound phosphorus oxidation by trimethyl-
amine N-oxide,27−29 to our knowledge, this is the first isolated
example of an oxidized NHP. The most striking feature of the
solid-state structure of 3 is a Co−PNHP distance significantly
shorter than that observed in the phosphorus(III) precursor 2
(2.1675(5) Å vs 2.2386(6) Å). We attribute this phenomenon
to increased Co to PNHP back-bonding upon phosphorus
oxidation, and a similar unusually short M−P distance has been
observed in a Pd−P(O)(NR2)2 species.30 Consistent with this
explanation, the Co(CO)2-derived IR stretches of 3 are shifted
∼20 cm−1 higher in energy than those of 2 (2000, 1949 cm−1),
suggesting decreased Co→CO π back-bonding as a result of
increased Co→P back-bonding. Similarly, the CO stretching
frequencies of CpFe(CO)2(P(CF3)2) have been shown to
increase upon oxidation to CpFe(CO)2(OP(CF3)2) with a
corresponding contraction of the Fe−P bond by 0.07 Å.31
While one would typically compare P−N distances between 2
and 3 to assess the extent of metal−phosphorus back-donation,
this comparison is rendered invalid by the different oxidation
states of phosphorus in these two compounds. The P(PO)P
ligand in 3 is similar to an oxidized phosphido PP(O)P pincer
ligand recently reported by Bourrisou.32
To investigate the electronic structure and bonding in 2 and
3, a computational investigation was undertaken using density
functional theory (DFT, Gaussian 09) and natural bond orbital
(NBO) analysis (Figure 2). The Co−PNHP NBO in complex 2
is predicted to be essentially covalent, with 46.3% contribution
from Co and 53.7% contribution from P. Upon phosphorus
oxidation in complex 3, the NBO remains covalent, but with
slightly more contribution from P (Co, 41.0%; P, 59.0%).
Notably, there is considerably more phosphorus s character
involved in the Co−P NBO of 3 than in 2. While the natural
charges of Co in both complexes are predicted to be identical
(−1.52), the charge on phosphorus increases from 1.12 in 2 to
2.05 in 3, consistent with oxidation from phosphorus(III) to
phosphorus(V).
Figure 1. Displacement ellipsoid representations of 2 and 3. For
clarity, only one of the two independent molecules in the asymmetric
unit of 2 is shown and hydrogen atoms and a THF solvate molecule
have been omitted. Relevant interatomic distances (Å) are as follows.
For 2: Co1−P102, 2.2386(6); Co1−P101, 2.2083(6); Co1−P103,
2.1988(5); Co1−C139, 1.792(2); Co1−C140, 1.765(2). For 3: Co1−
P2, 2.1675(5); Co1−P1, 2.1791(5); Co1−P3, 2.1853(5); Co1−C39,
1.7773(19); Co1−C40, 1.7793(19); P2−O1, 1.4875(13).
geometry about Co is intermediate between square pyramidal
and trigonal bipyramidal (τ = 0.56),24 with the central NHP
occupying a position essentially trans to a CO ligand (P−Co−
C ≈ 167°). More interesting, however, is the geometry about
the central phosphorus of the NHP unit. Unlike the standard
planar geometry expected for an N-heterocyclic phosphenium
moiety, the NHP adopts a distinctly pyramidal geometry. Of
the two molecules in the asymmetric unit, the average angle
between the N−P−N plane and the Co−P bond vector is
∼114°. Such a pyramidal geometry implies the presence of a
lone pair on the central NHP phosphorus, suggesting that the
complex may be described as either an NHP− phosphido ligand
coordinated to a CoI center or an NHP+ phosphenium ligand
acting as an electron pair acceptor to Co−I. Several Co NHP
complexes have been structurally characterized to date: an
asymmetric Co2(CO)5(μ-NHPMe)2 dimer25 and (CO)3Co-
26
t
(NHPR) (R = Bu, Mes). The Co−PNHP distance in 2
(2.2386(6) Å) is relatively long compared to the Co−PNHP
distances in the planar phosphenium complexes Co2(CO)5(μ-
NHPMe)2 (2.05 Å)25 and (CO)3Co(NHPR) (2.04 and 2.00 Å
for R = tBu, Mes).26 This longer distance is compatible with the
absence of multiple Co−P bond character. Interestingly, of the
∼20 structurally characterized transition-metal terminal NHP
complexes, complex 2 joins Cp*Fe(CO)2(NHPMe) as only the
second example of a pyramidal NHP coordination mode.23 The
latter compound was described by Paine and co-workers as an
adduct between an Fe0 [Cp*2Fe(CO)2]− fragment and an
NHPMe+ phosphenium ligand.
For comparison, calculations were also performed on two
model compounds: (Me2P)Co(PPh3)2(CO)2 (4), a bona fide
phosphido analogue of 2, and (NHPMe)Co(PPh3)2(CO)2 (5;
see the Supporting Information). The phosphide Co−P NBO
of 4 has a constitution remarkably similar to that in 2, with
46.2% contribution from Co and 53.8% contribution from P.
These observations are in line with assignment as an NHP
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Communication
ASSOCIATED CONTENT
* Supporting Information
■
S
Text, tables, and figures giving experimental procedures,
computational data, and additional spectral data and CIF files
giving crystallographic details and data for 2 and 3. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: thomasc@brandeis.edu.
■
ACKNOWLEDGMENTS
C.M.T. is grateful for a 2011 Sloan Research Fellowship. We
also thank Brandeis University for funding this project.
■
Figure 2. Computed Co−P natural bond orbitals (NBOs) of 2 and 3.
2: 46.3% Co, 53.74% P (85.5% p, 14.2% s). 3: 41.0% Co, 59.0% P
(64.1% p, 35.4% s).
REFERENCES
■
phosphido ligand, since a dative donor−acceptor interaction
from Co to P would be expected to have significantly more
contribution from Co. Interestingly, the Co−PNHP NBO in 5 is
calculated to be made up of 53.2% Co and 46.8% P character
and, thus, more in line with a donor/acceptor bonding
description. The natural charge on Co in 2 is computed to
be −1.52, only slightly more negative than that of the Co center
in 4 (−1.31).
(1) Crudden, C. M.; Allen, D. P. Coord. Chem. Rev. 2004, 248,
2247−2273.
(2) Bourissou, D.; Guerret, O.; Gabbai, F. P.; Bertrand, G. Chem. Rev.
1999, 100, 39−92.
(3) Marion, N.; Nolan, S. P. Acc. Chem. Res. 2008, 41, 1440−1449.
(4) Enders, D.; Balensiefer, T. Acc. Chem. Res. 2004, 37, 534−541.
(5) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1290−1309.
(6) Cowley, A. H.; Kemp, R. A. Chem. Rev. 1985, 85, 367−382.
(7) Gudat, D. Coord. Chem. Rev. 1997, 163, 71−106.
(8) Nakazawa, H. Transition Metal Complexes Bearing a
Phosphenium Ligand. In Adv. Organomet. Chem., Academic Press:
New York, 2004; Vol. 50, pp 107−143.
An additional convincing piece of evidence for NHP−
phosphido character comes from the calculated Co−CO
vibrational frequencies for 2 compared to those in 4 and 5.
While the computed values do not match experimental values,
comparisons between other computed values remain valid. The
infrared CO stretches calculated for 2 are 1928 and 1885 cm−1,
while those predicted for phosphido complex 4 are 1925 and
1856 cm−1, suggesting a similar Co oxidation state (CoI) in
these two complexes. On the other hand, the CO stretching
frequencies predicted for 5 are 1894 and 1812 cm−1, indicative
of a more electron-rich Co center involved in a donor/acceptor
interaction with the NHP without fully undergoing two-
electron oxidation. Thus, our findings from DFT and NBO
studies suggest that the bonding in 2 is best described as an
NHP− phosphide ligand bound to a formally CoI center. The
NHP− phosphido bonding description renders these PPP
ligands analogous to a family of bis(phosphine)phosphido
ligands reported in recent years.33−36 The difference between
the bonding of the chelating NHP-diphosphine ligand and the
monodentate NHPMe in model complex 5 is intriguing and may
be related to the enforced planarity of the N-aryl substituents,
delocalizing the nitrogen lone pairs responsible for stabilizing
the singlet state of the phosphenium.
(9) Bansal, R. K.; Gudat, D., Recent Developments in the Chemistry
of N-Heterocyclic Phosphines. In Phosphorus Heterocycles II, Springer:
Berlin/Heidelberg, 2010; Vol. 21, pp 63−102.
(10) Gudat, D. Acc. Chem. Res. 2010, 43, 1307−1316.
(11) Fleming, S.; Lupton, M. K.; Jekot, K. Inorg. Chem. 1972, 11,
2534−2540.
(12) Maryanoff, B. E.; Hutchins, R. O. J. Org. Chem. 1972, 37, 3475−
3480.
(13) Caputo, C. A.; Price, J. T.; Jennings, M. C.; McDonald, R.;
Jones, N. D. Dalton Trans. 2008, 3461−3469.
(14) Dube, J. W.; Farrar, G. J.; Norton, E. L.; Szekely, K. L. S.;
Cooper, B. F. T.; Macdonald, C. L. B. Organometallics 2009, 28,
4377−4384.
(15) Reeske, G.; Cowley, A. H. Inorg. Chem. 2006, 46, 1426−1430.
(16) Abrams, M. B.; Scott, B. L.; Baker, R. T. Organometallics 2000,
19, 4944−4956.
(17) Caputo, C. A.; Jennings, M. C.; Tuononen, H. M.; Jones, N. D.
Organometallics 2009, 28, 990−1000.
(18) Caputo, C. A.; Brazeau, A. L.; Hynes, Z.; Price, J. T.; Tuononen,
H. M.; Jones, N. D. Organometallics 2009, 28, 5261−5265.
(19) Hardman, N. J.; Abrams, M. B.; Pribisko, M. A.; Gilbert, T. M.;
Martin, R. L.; Kubas, G. J.; Baker, R. T. Angew. Chem., Int. Ed. 2004,
43, 1955−1958.
In summary, sodium−halogen exchange has proven to be a
useful synthetic route to a cobalt complex coordinated by our
previously reported NHP-diphosphine chelating ligands. Upon
NHP coordination, however, the NHP unit adopts an unusual
pyramidal geometry indicative of a NHP− phosphido
description and a two-electron-oxidized Co center. Consistent
with this description, oxidation with trimethylamine N-oxide
leads exclusively to oxidation of the central NHP phosphorus to
generate an unprecedented metal-bound N-heterocyclic
phosphinito species. Future investigations will focus on the
coordination chemistry of the NHP-diphosphine ligand and its
oxidized derivative with additional transition metal fragments in
an effort to evaluate the factors that govern planar vs pyramidal
coordination modes in these sterically and electronically
modifiable nitrosyl analogues.
(20) Mata, J. A.; Poyatos, M.; Peris, E. Coord. Chem. Rev. 2007, 251,
841−859.
(21) Peris, E.; Crabtree, R. H. Coord. Chem. Rev. 2004, 248, 2239−
2246.
(22) Day, G. S.; Pan, B.; Kellenberger, D. L.; Foxman, B. M.;
Thomas, C. M. Chem. Commun. 2011, 47, 3634−3636.
(23) Hutchins, L. D.; Duesler, E. N.; Paine, R. T. Organometallics
1982, 1, 1254−1256.
(24) Addison, A. W.; Rao, T. N.; Reedijk, J.; van Rijn, J.; Verschoor,
G. C. J. Chem. Soc., Dalton Trans. 1984, 1349−1356.
(25) Hutchins, L. D.; Light, R. W.; Paine, R. T. Inorg. Chem. 1982,
21, 266−272.
(26) Burck, S.; Daniels, J.; Gans-Eichler, T.; Gudat, D.; Nattinen, K.;
Nieger, M. Z. Anorg. Allg. Chem. 2005, 631, 1403−1412.
(27) Le Floch, P.; Mathey, F. Synlett 1990, 171−172.
̈
5562
dx.doi.org/10.1021/om200816p|Organometallics 2011, 30, 5560−5563

Organometallics
Communication
(28) Fan, W.; Zhang, R.; Leong, W. K.; Chu, C. K.; Yan, Y. K. J.
Organomet. Chem. 2005, 690, 3765−3773.
(29) Fan, W.; Zhang, R.; Leong, W. K.; Yan, Y. K. Inorg. Chim. Acta
2004, 357, 2441−2450.
́
́
(30) Angurell, I.; Martinez-Ruiz, I.; Rossell, O.; Seco, M.; Gomez-Sal,
́
P.; Martin, A.; Font-Bardia, M.; Solans, X. J. Organomet. Chem. 2007,
692, 3882−3891.
(31) Barrow, M. J.; Sim, G. A. J. Chem. Soc., Dalton Trans. 1975,
291−295.
(32) Derrah, E. J.; Ladeira, S.; Bouhadir, G.; Miqueu, K.; Bourissou,
D. Chem. Commun. 2011, 47, 8611−8613.
(33) Bauer, R. C.; Gloaguen, Y.; Lutz, M.; Reek, J. N. H.; de Bruin,
B.; van der Vlugt, J. I. Dalton Trans. 2011, 40, 8822−8829.
(34) D’Auria, I.; Mazzeo, M.; Pappalardo, D.; Lamberti, M.;
Pellecchia, C. J. Polym. Sci., Part A: Polym. Chem. 2011, 49, 403−413.
(35) Mazzeo, M.; Lamberti, M.; D’Auria, I.; Milione, S.; Peters, J. C.;
Pellecchia, C. J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 1374−
1382.
(36) Mazzeo, M.; Strianese, M.; Kuhl, O.; Peters, J. C. Dalton Trans.
2011, 40, 9026−9033.
5563
dx.doi.org/10.1021/om200816p|Organometallics 2011, 30, 5560−5563