Construction of a stable N-heterocyclic phosphenium cation with an electron-rich framework and its complexation to rhodium

Article abstract of DOI:10.1021/om060466b

The synthesis and characterization of a new N-heterocyclic phosphenium cation captured in a six-membered, electron-rich framework is presented. The cation reacts quantitatively with 1 equiv of Wilkinson's catalyst, Rh(PPh ₃)₃Cl, to form a cationic rhodium - phosphenium complex, the first to be characterized in the solid state. Computational and X-ray diffraction studies support the existence of a strong dπ - pπ bond between rhodium and phosphorus.

Full text of DOI:10.1021/om060466b

Organometallics 2006, 25, 3541-3543
3541
Construction of a Stable N-Heterocyclic Phosphenium Cation with
an Electron-Rich Framework and Its Complexation to Rhodium
Heather A. Spinney,† Glenn P. A. Yap,‡ Ilia Korobkov,† Gino DiLabio,§ and
Darrin S. Richeson*,†
Department of Chemistry and Center for Catalysis Research and InnoVation, UniVersity of Ottawa,
Ontario, Canada K1N 6N5, Department of Chemistry and Biochemistry, UniVersity of Delaware,
Newark, Delaware 19716, and National Institute for Nanotechnology, National Research Council of
Canada, W6-010 ECERF, 9107-116th Street, Edmonton, Alberta, Canada T6G 2V4
ReceiVed May 28, 2006
Summary: The synthesis and characterization of a new N-
heterocyclic phosphenium cation captured in a six-membered,
electron-rich framework is presented. The cation reacts quan-
titatiVely with 1 equiV of Wilkinson’s catalyst, Rh(PPh3)3Cl, to
form a cationic rhodium-phosphenium complex, the first to be
characterized in the solid state. Computational and X-ray
diffraction studies support the existence of a strong dπ-pπ bond
between rhodium and phosphorus.
(3).9 This molecule possesses a novel topology and electronic
framework in which a divalent carbon sits in a formally seven-
π-electron, six-membered heterocyclic ring.10 The utilization of
this scaffold to support group 15 compounds, and phosphenium
cations in particular, promises to yield novel compounds with
applications in coordination chemistry and catalysis.
The diaminochlorophosphine 4 provides a precursor for
phosphenium synthesis and was prepared via the reaction of
1,8-bis(isopropylamino)naphthalene with an excess of phos-
phorus trichloride and triethylamine in hexane at room temper-
ature (eq 1). Compound 4 was obtained as a colorless crystalline
The heteroatom-stabilized, cyclic phosphenium cations 1 are
isoelectronic analogues of the N-heterocyclic carbenes (NHCs)
2. The strong σ-donor properties of NHCs have led to their now
prominent role as ligands for catalytically relevant transition-
metal complexes.1 The fundamental electronic differences
between 1 and 2 lead to reciprocal coordination behavior
between NHCs and N-heterocyclic phosphenium cations, with
their excellent π-acceptor and poor σ-donor properties.2 These
features suggest that coordinated phosphenium cations may
enhance the electrophilicity of a metal center during catalysis.3,4
While a number of N-heterocyclic phosphenium cations have
been reported, these are dominated by five-membered 1,3,2-
diazaphospholenium species (1).2-8 Recently, we reported the
successful synthesis of a stable N-heterocyclic carbene pos-
sessing a 1,8-bis(alkylamido)naphthalene (iPr2DAN) framework
solid in excellent yield and exhibited a 31P NMR chemical shift
of 102 ppm, typical of diaminochlorophosphines.11 The identity
of 4 was confirmed through spectroscopic, microanalytical, and
single-crystal X-ray analyses.12 Reaction of 4 with chloride
abstraction agents provided ready access to the phosphenium
salts [P(iPrN)2C10H6][GaCl4] (5) and [P(iPrN)2C10H6][OTf] (6;
OTf ) OSO2CF3 (triflate)). For example, addition of GaCl3 to
a toluene solution of 4 resulted in immediate precipitation of
the dark purple solid 5, which exhibited a 31P NMR chemical
shift of 239 ppm in CD2Cl2. This value is consistent with the
localization of a positive charge at phosphorus and is similar
to 31P NMR chemical shift values obtained for other N-
heterocyclic phosphenium cations.8 Dark red 6 is soluble in
arene solvents and has a 31P NMR chemical shift of 142 ppm
* To whom correspondence should be addressed. E-mail:
darrin@science.uottawa.ca.
† University of Ottawa.
‡ University of Delaware.
§ National Research Council of Canada.
(1) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1290.
(2) Gudat, D.; Haghverdi, A.; Hupfer, H.; Nieger, M. Chem. Eur. J. 2000,
6, 3414.
(3) (a) 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. (b) Abrams, M. B.; Scott, B. L.; Baker, R. T. Organometallics 2000,
19, 4944.
(4) (a) Sakakibara, K.; Yamashita, M.; Nozaki, K. Tetrahedron Lett. 2005,
46, 959. (b) Breit, B. J. Mol. Catal. A 1999, 143, 143.
(5) Reeske, G.; Hoberg, C. R.; Hill, N. J.; Cowley, A. H. J. Am. Chem.
Soc. 2006, 128, 2800.
(9) Bazinet, P.; Yap, G. P. A.; Richeson, D. S. J. Am. Chem. Soc. 2003,
125, 13314.
(10) The 1,8-bis(alkylamido)naphthalene framework has also been used
to construct heavier stable N-heterocyclic germylenes and stannylenes:
Bazinet, P.; Yap, G. P. A.; Richeson, D. S. J. Am. Chem. Soc. 2001, 123,
11162. Bazinet, P.; Yap, G. P. A.; DiLabio, G. A.; Richeson, D. S. Inorg.
Chem. 2005, 44, 4616.
(11) Burford, N.; Cameron, T. S.; Conroy, K. D.; Ellis, B.; Macdonald,
C. L. B.; Ovans, R.; Phillips, A. D.; Ragogna, P. J.; Walsh, D. Can. J.
Chem. 2002, 80, 1404.
(12) Characterization and structural details for compound 4 are provided
in the Supporting Information.
(6) Gudat, D. Coord. Chem. ReV. 1997, 163, 71.
(7) Carmalt, C. J.; Lomeli, V.; McBurnett, B. G.; Cowley, A. H. Chem.
Commun. 1997, 2095.
(8) Cowley, A. H.; Kemp, R. A. Chem. ReV. 1985, 85, 367.
10.1021/om060466b CCC: $33.50 © 2006 American Chemical Society
Publication on Web 06/16/2006

3542 Organometallics, Vol. 25, No. 15, 2006
Communications
Figure 1. Thermal ellipsoid plot showing the molecular structure and atom-numbering scheme for the phosphenium cation in 5. Hydrogen
-
atoms and the GaCl4 anion have been omitted for clarity. The insets show the LUMO (left) and P-centered lone pair HOMO-4 (right)
obtained from DFT calculations.
in C6D6 and 179 ppm in CD2Cl2, suggesting significant cation/
anion contacts in less polar solvents.
The structure of the phosphenium salt [P(iPrN)2C10H6][GaCl4]
(5) (Figure 1) displays a well-separated cation-anion pair
(closest P‚‚‚Cl > 4 Å), demonstrating that the iPr2DAN ligand
is effective at stabilizing the electron-deficient phosphenium
center. Associated with the formation of the cation are a
contraction of the P-N bonds in 5 (P-N(1) ) 1.609(8) Å;
P-N(2) ) 1.629(7) Å; N(1)-P-N(2) ) 105.4(3)°) relative to
those in phosphine 4 (P-N(1) ) P-N(2) ) 1.670(2) Å; N(2)-
P(1)-N(1) ) 99.7(1)°) and formation of a more planar
heterocyclic ring, indicating increased P-N π-bonding in the
cations. The cations in salts 5 and 6 represents a unique example
of a six-membered N-heterocyclic phosphenium cation pos-
sessing a π-conjugated carbon backbone.13 Furthermore, at-
tempts by Burford and co-workers to prepare related â-diketim-
inate-supported phosphorus compounds resulted in the isolation
Figure 2. Thermal ellipsoid plot showing the molecular structure
of a product in which the phosphorus center added to the
and partial atom-numbering scheme for the phosphenium-rhodium
complex 8. Hydrogen atoms, the disordered triflate anion, and a
disordered solvent molecule have been omitted for clarity.
backbone of the ligand (7).14 Our use of a 1,8-bis(alkylamido)-
showed liberation of free triphenylphosphine along with forma-
tion of complex 8 and a small amount of the diaminochloro-
phosphine 4. Complex 8 appears as an AX2 spin system
consisting of an upfield doublet of doublets for Rh-bonded PPh3
1
2
(31 ppm, JRh-P ) 104 Hz, JPP ) 51 Hz) and a downfield
1
doublet of triplets for the phosphenium (240 ppm, JRh-P
)
326 Hz, JPP ) 51 Hz). The 31P NMR spectrum is consistent
with the topology for 8 shown above and is reminiscent of
spectra previously reported for phosphenium-rhodium com-
plexes by Baker and co-workers.3b A nearly identical AX2
pattern in the 31P NMR spectrum is observed when Rh(PPh3)3-
Cl is reacted with the phosphenium gallate salt 5. A further
notable observation upon coordination of the phosphenium
2
naphthalene framework prevents this possible reaction pathway.
The results of density functional theory calculations15 on 5
indicate that the nitrogen lone-pairs are delocalized into the P
pz-orbital, forming strong π-bonding interactions in the hetero-
cyclic ring. The lone-pair of electrons localized on P resides in
a low-energy (HOMO-4) orbital that is composed of substantial
s character, is oriented in the molecular plane, and points away
from the ring (Figure 1).
1
cations is a dramatic upfield shift in the H NMR resonance
for the methine septets of the NCH(CH3)2 groups from 3.8 ppm
in free 6 to 6.0 ppm for 8. We attribute this to the proximity of
the NCH protons to the electron-rich metal center.9
i
The ability of the Pr2DAN phosphenium cation to stabilize
catalytically relevant metal complexes is demonstrated by the
reaction of the triflate salt [P(iPrN)2C10H6][OTf] (6) with
Wilkinson’s catalyst, Rh(PPh3)3Cl, in CH2Cl2 at room temper-
ature to yield the orange metal-phosphenium complex 8. The
reaction was monitored by 31P NMR spectroscopy, which
Red-orange crystals of the phosphenium-rhodium complex
8 were isolated, and single-crystal X-ray analysis produced a
structure consistent with the spectroscopic data (Figure 2). This
structure, the first solid-state characterization of a phosphenium
cation bound to rhodium, displays the shortest reported phos-
phorus-rhodium(I) bond (Rh-P(1) ) 2.089(2) Å).16 The bound
phosphenium exhibits a planar (∑(P angles) ) 360.0°), three-
coordinate phosphorus center, militating against a stereochemi-
cally active lone pair of electrons on the phosphorus atom and
consistent with a π-accepting phosphenium ligand. Visualization
(13) Six-membered cations derived from 2-phospha-l,3-diazacyclohex-
anes are reported in: Maryanoff, B. E.; Hutchins, R. T. J. Org. Chem. 1972,
37, 3475.
(14) Burford, N.; D’eon, M.; Ragogna, P. J.; McDonald, R.; Ferguson,
M. J. Inorg. Chem. 2004, 43, 734.
(15) Results of our calculations are presented in the Supporting Informa-
tion.

Communications
Organometallics, Vol. 25, No. 15, 2006 3543
of the orbitals of 8 confirmed that there are both σ- and
π-bonding interactions between Rh and the P center of the
phosphenium.15
has been shown to be effective at stabilizing a low-coordinate,
electrophilic phosphorus center, despite the low steric bulk of
the substituents at nitrogen. The novel topology and electronic
framework of the resultant phosphenium cation make it an ideal
ligand for use in transition-metal coordination chemistry, as
illustrated by the successful isolation of a rhodium-phosphen-
ium complex. Both computational and structural features support
combined σ- and π-bonding interactions that produce a remark-
ably short Rh-P bond. Studies are currently underway to assess
the catalytic potential of the complex 8 and related phosphenium
complexes of other late-metal centers.
Interestingly, there are only minor changes in the structural
features of the coordinated phosphenium cation in 8 (P(1)-
N(1) ) 1.629(4) Å; P(1)-N(2) ) 1.631(5) Å; N(1)-P(1)-
N(2) ) 106.8(2)°) in comparison to those of the free phosphen-
ium cation in 5. The planar phosphenium ligand is perpendicular
to the square plane of the Rh center (∑(Rh angles) ) 359.9°),
which positions the NiPr groups on either side of this plane.
Consistent with the 1H NMR observations, the iPr substituents
are oriented to position the methine protons to point directly
above and below the rhodium center. The Rh-Cl and Rh-
PPh3 bond lengths (Rh-P(2) ) 2.354(2) Å; Rh-P(3) ) 2.363-
(1) Å; Rh-Cl ) 2.351(2) Å) are similar to those observed in
the solid-state structure of the red form of Wilkinson’s catalyst.17
The P(2)-Rh-P(3) (172.81(6)°) and P(1)-Rh-Cl (177.20(6)°)
angles in 8 deviate slightly from linearity, most likely as a
consequence of steric crowding around the rhodium center.
In conclusion, the 1,8-bis(alkylamido)naphthalene framework
Acknowledgment. This work was supported by NSERC.
H.A.S. is the recipient of an NSERC Postdoctoral Fellowship.
G.D. thanks the Centre of Excellence in Integrated Nanotools
for access to computational resources.
Supporting Information Available: Text giving experimental
procedures and characterization data for new compounds, the
structural details for compounds 4, 5, and 8, a figure for the structure
of 4, and text and tables giving details of the DFT calculations on
5 and 8. This material is available free of charge via the Internet at
http://pubs.acs.org.
(16) A search of the Cambridge CSD yielded 2.138 Å as the shortest
reported RhI-P distance: Schumann, H.; Stenzel, O.; Dechert, S.; Girgsdies,
F.; Blum, J.; Gelman, D.; Halterman, R. L. Eur. J. Inorg. Chem. 2002,
211.
(17) Bennett, M. J.; Donaldson, P. B. Inorg. Chem. 1977, 16, 655.
OM060466B