A thermally stable and sterically unprotected terminal electrophilic phosphinidene complex of cobalt and its conversion to an η1-phosphirene

Article abstract of DOI:10.1021/ja028303b

The terminal chloroaminophosphido complex [Co(CO)₃(PPh₃){P(Cl)NⁱPr₂}] is formed via reaction of K[Co(CO)₄] with ⁱPr₂NPCl₂ in the presence of triphenylphosphine. Chloride abstraction by aluminum trichloride leads to the first terminal phosphinidene complex of cobalt, [Co(CO)₃(PPh₃)(PNⁱPr₂)][AlCl₄]. The electrophilicity of the phosphinidene was demonstrated by its reaction with diphenylacetylene to form the phosphirene complex [Co(CO)₃(PPh₃){P(NⁱPr₂)C(Ph)C(Ph)}][AlCl₄]. Copyright

Full text of DOI:10.1021/ja028303b

Published on Web 02/07/2003
A Thermally Stable and Sterically Unprotected Terminal Electrophilic
Phosphinidene Complex of Cobalt and Its Conversion to an η1-Phosphirene
Javier Sa´nchez-Nieves, Brian T. Sterenberg, Konstantin A. Udachin, and Arthur J. Carty*
Steacie Institute for Molecular Sciences, National Research Council of Canada, 100 Sussex DriVe, Ottawa,
Ontario, Canada K1A 0R6, and Ottawa-Carleton Chemistry Research Institute, Department of Chemistry,
UniVersity of Ottawa, Ottawa, Ontario, Canada K1N 6N5
Received August 27, 2002 ; E-mail: arthur.carty@nrc.ca
Since their discovery by Lappert in 1987,1 stable terminal
phosphinidene complexes of transition metals have remained
relatively rare.2 Like carbenes, terminal phosphinidene complexes
can be roughly divided into electrophilic or nucleophilic categories
on the basis of their reactivities.3 Until recently, all examples of
stable phosphinidene complexes could be categorized as nucleo-
philic, although transient electrophilic phosphinidene complexes
have been studied extensively.4,5 We have developed a successful
route to electrophilic aminophosphinidene complexes via chloride
abstraction from chloroaminophosphido complexes and have used
it to synthesize stable electrophilic phosphinidene complexes of
molybdenum, tungsten, and ruthenium.6,7 The ruthenium complex
was the first example of a stable phosphinidene involving a late
Figure 1. ORTEP diagrams of the cations of [Co(CO)3(PPh3)(PNiPr2)]-
[AlCl4] (2) and [Co(CO)3(PPh3){P(NiPr2)C(Ph)C(Ph)}][AlCl4] (3). Hydro-
gen atoms have been omitted, and thermal ellipsoids are shown at the 50%
level.
transition metal. The amino substituent on phosphorus in these
complexes stabilizes the electrophilic phosphorus center via a
nitrogen lone pair to phosphorus π donor interaction, which is
analogous to the heteroatom to carbon donor interaction observed
in Fischer-type carbene complexes.8 Since that report, there has
been an increased interest in the synthesis of terminal late metal
phosphinidene complexes, and examples of sterically congested
nickel9 and iridium10 complexes have recently been reported. In
this communication, we report the synthesis and characterization
of the first terminal phosphinidene complex of cobalt and the first
sterically unhindered late transition metal phosphinidene complex.
The terminal cobalt chloroaminophosphido complex [Co(CO)3-
(PPh3){P(Cl)NiPr2}] (1) was formed via reaction of K[Co(CO)4]
directed away from the proximal isopropyl group. The geometry
at nitrogen is planar within experimental error (Scheme 1).
The 31P NMR spectrum of 2 shows a high-field resonance at δ
861.2 in the typical region for terminal phosphinidene complexes,2,7
as well as a peak at δ 47.6 which corresponds to the triphenyl
phosphine ligand. No coupling is observed between the two
1
phosphorus nuclei despite their trans relationship. The H NMR
spectrum of 2 shows two inequivalent isopropyl groups, indicating
that there is a barrier to rotation about the PN bond.
Like Fischer carbene complexes, electrophilic phosphinidene
complexes are generally considered to be formally derived from
the singlet state of free phosphinidene in which the phosphorus
atom has two lone pairs and an empty p orbital perpendicular to
the plane of the substituent and the lone pairs. Coordination of this
moiety to a transition metal results in donation of one lone pair to
an appropriate empty metal orbital. Metal to phosphorus π-back-
donation to the empty phosphorus p orbital competes with nitrogen
lone pair to phosphorus donation.3 The structural and spectroscopic
parameters of compound 2 are clearly consistent with this formula-
tion. The Co-P bond length is longer than expected for a cobalt-
phosphorus double bond13 and is in fact only slightly shorter than
typical cobalt-trimethylphosphine bond lengths.14 The P-N bond
is intermediate between PN single and PN double bonds and is
similar to those in recently characterized Mo, W, and Ru amino-
phosphinidene complexes.7 This bond length, along with the
planarity at nitrogen and the restricted PN rotation, indicates the
presence of a significant N to P donor interaction.
i
with the dichloroaminophosphine Pr2NPCl2 in the presence of
triphenylphosphine.11 Reaction of 1 with aluminum trichloride
results in abstraction of chloride from the chloroaminophosphido
ligand to form the aminophosphinidene complex [Co(CO)3-
(PPh3)(PNiPr2)][AlCl4] (2).12 Compound 2 has been structurally
characterized, and an ORTEP diagram of the cation is shown in
Figure 1. The geometry at cobalt is trigonal bipyramidal with the
two phosphorus ligands occupying the axial positions and three
carbonyl ligands occupying equatorial positions. The phosphinidene
ligand is staggered with respect to the equatorial carbonyl ligands,
with N-P(1)-Co-C(1) and N-P(1)-Co-C(3) torsion angles of
59.0(1)° and 58.4(1)°. The P(1)-Co-P(2) angle of 172.82(3)° is
distorted from the ideal trigonal bipyramidal geometry, likely to
avoid steric interaction of the di-isopropylamino group with the
equatorial carbonyl ligands.
The triphenylphosphine phosphorus-cobalt distance is 2.2633-
(7) Å, while the phosphinidene P-Co distance is 2.1875(8) Å. Other
than cobalt, the phosphorus is bound only to the nitrogen atom
with a P(1)-N distance of 1.626(2) Å and an angle at phosphorus
of 115.41(8)°. The methyl groups of the isopropyl group proximal
to the metal are directed away from the metal and toward the distal
isopropyl group, minimizing steric interaction with the equatorial
carbonyls. The methyl groups of the distal isopropyl group are
An examination of the structure of 2 reveals that the two-
coordinate phosphorus center has very little steric protection. In
contrast, other late metal phosphinidene complexes of iridium and
nickel required the stabilizing effects of bulky ligands on the metal
and sterically demanding substituents on phosphorus. The successful
isolation of complex 2 clearly shows that given sufficient π-donation
9
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J. AM. CHEM. SOC. 2003, 125, 2404-2405
10.1021/ja028303b CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1
(2) (a) Cowley, A. H. Acc. Chem. Res. 1997, 30, 445. (b) Cowley, A. H.;
Pellerin, B. J. Am. Chem. Soc. 1990, 112, 6734. (c) Breen, T. L.; Stephan,
D. W. J. Am. Chem. Soc. 1995, 117, 11914. (d) Bonanno, J. B.;
Wolczanski, P. T.; Lobkovsky, E. B. J. Am. Chem. Soc. 1994, 116, 11159.
(e) Cummins, C. C.; Schrock, R. R.; Davis, W. M. Angew. Chem., Int.
Ed. Engl. 1993, 32, 756. (f) Arney, D. S. J.; Schnabel, R. C.; Scott, B.
C.; Burns, C. J. J. Am. Chem. Soc. 1996, 118, 6780.
(3) (a) Ehlers, A. W.; Baerends, E. J.; Lammertsma, K. J. Am. Chem. Soc.
2002, 124, 2831. (b) Grigoleit, S.; Alijah, A.; Rozhenko, A. B.; Streubel,
R.; Schoeller, W. W. J. Organomet. Chem. 2002, 643-644, 223. (c) Creve,
S.; Pierloot, K.; Nguyen, M. T.; Vanquickenborne, L. G. Eur. J. Inorg.
Chem. 1999, 107. (d) Ehlers, A. W.; Lammertsma, K.; Baerends, E. J.
Organometallics 1998, 17, 2738. (e) Frison, G.; Mathey, F.; Sevin, A. J.
Organomet. Chem. 1998, 570, 225.
to phosphorus from the metal or the phosphorus substituent,
electrophilic terminal phosphinidene complexes, even of first row
metals, are not fundamentally unstable.
(4) Dillon, K. B.; Mathey, F.; Nixon, J. F. Phosphorus: The Carbon Copy;
John Wiley & Sons, Ltd.: Chichester, 1998; pp 19-32.
(5) (a) Sava, X.; Marinetti, A.; Ricard, L.; Mathey, F. Eur. J. Inorg. Chem.
2002, 2002, 1657. (b) Tran Huy, N. H.; Compain, C.; Ricard, L.; Mathey,
F. J. Organomet. Chem. 2002, 650, 57. (c) Vlaar, M. J. M.; Valkier, P.;
Schakel, M.; Ehlers, A. W.; Lutz, M.; Spek, A. L.; Lammertsma, K. Eur.
J. Org. Chem. 2002, 1797. (d) Vlaar, M. J. M.; van Assema, S. G. A.; de
Kanter, F. J. J.; Schakel, M.; Spek, A. L.; Lutz, M.; Lammertsma, K.
Chem.-Eur. J. 2002, 8, 58. (e) Lammertsma, K.; Vlaar, M. J. M. Eur. J.
Org. Chem. 2002, 1127. (f) Mathey, F.; Tran Huy, N. H.; Marinetti, A.
HelV. Chim. Acta 2001, 84, 2938.
(6) Sterenberg, B. T.; Udachin, K. A.; Carty, A. J. Organometallics 2001,
20, 4463.
(7) Sterenberg, B. T.; Udachin, K. A.; Carty, A. J. Organometallics 2001,
20, 2657.
The electrophilicity of complex 2 has been demonstrated by its
reaction with diphenyl acetylene to form the phosphirene complex
[Co(CO)3(PPh3){P(NiPr2)C(Ph)C(Ph)}][AlCl4] (3).15 This reactivity
toward alkynes thus parallels the behavior of transient electrophilic
phosphinidene complexes.4,16,17 An ORTEP diagram of the cation
of 3 (Figure 1) again displays trigonal bipyramidal geometry at
cobalt with the two phosphorus ligands occupying the axial
positions. The alkyne has added to the phosphinidene phosphorus,
forming a three-membered ring. The Co-P(1) distance of 2.2235-
(3) Å is very close to the cobalt-triphenylphosphine distance of
2.2320(3) Å. The P-C distances within the ring are 1.7635(6) and
1.7561(6) Å, and the CdC distance is 1.3396(9) Å. The angles
within the ring are 67.34(3)°, 67.92(3)°, and 44.74(3)°, respectively,
at C(11), C(12), and P(1). The diisopropyl-amino group is oriented
similar to that in 2, and the P-N bond distance of 1.659(2) Å is
now consistent with a single bond, indicating that N to P donation
is no longer necessary to stabilize the phosphorus center.
(8) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles and
Applications of Organotransition Metal Chemistry; University Science
Books: Mill Valley, 1987; pp 120-129.
(9) Melenkivitz, R.; Mindiola, D. J.; Hillhouse, G. L. J. Am. Chem. Soc. 2002,
124, 3846.
(10) Termaten, A. T.; Nijbacker, T.; Schakel, M.; Lutz, M.; Spek, A. L.;
Lammertsma, K. Organometallics 2002, 21, 3196.
(11) Data for [Co(CO)3(PPh3){P(Cl)NiPr2}] (1): 1H NMR (CDCl3) δ 7.4-7.7
(m, 15H, PPh3), 4.1 (b, 2H, CH), 1.43 (d, 6H, 3J(HH) ) 6.7 Hz, CH3),
1.26 (d, 6H, 3J(HH) ) 6.7, CH3). 31P NMR (CDCl3) δ 282.8 (d, P(Cl)-
NiPr2), 54.1 (d, PPh3), 2J(PP) ) 24 Hz. IR (νCO) 2000m, 1977s, 1967s
cm-1. Anal. Calcd for C28H31NO3P2Cl3Co: C, 51.20; H, 4.76; N, 2.13.
Found: C, 51.58; H, 5.31; N, 2.21. X-ray data: monoclinic, P2(1)/n, a
) 11.8269(8), b ) 19.988(1), c ) 14.214(1), â ) 112.229(1), V ) 3110.5-
(4), Z ) 4, Dcalc ) 1.402, T ) 173(2) K, 8041 independent reflections
[R(int) ) 0.0343], R ) 0.0393, wR2 ) 0.0974.
The 31P NMR spectrum of 3 shows two doublets with a peak at
δ -76.6 showing the characteristic high-field shift of the phos-
phorus nucleus in a phosphirene ring.18 The coupling constant is
140 Hz, typical of a trans arrangement of the phosphorus ligands.
(12) Data for [Co(CO)3(PPh3)(PNiPr2)][AlCl4] (2): 1H NMR (CDCl3) δ 7.4-
7.7 (m, 15H, PPh3), 5.66 (sept., 1H, 3J(HH) ) 6.3 Hz, CH), 4.75 (b, 1H,
CH), 1.67 (d, 6H, 3J(HH) ) 6.3, CH3), 1.58 (d, 6H, 3J(HH) ) 6.5 Hz).
31P NMR (CDCl3) δ 861.2 (s, PNiPr2), 47.6 (PPh3). IR (νCO) 2029s, 2017s
cm-1. Anal. Calcd for C27H29NO3P2AlCl4Co: C, 45.99; H, 4.14; N, 1.99.
Found: C, 45.53; H, 4.25; N, 2.06. X-ray data: orthorhombic, P2(1)2-
(1)2(1), a ) 12.5260, b ) 12.8291(6), c ) 20.424(1), V ) 3282.1(3), Z
) 4, Dcalc ) 1.427, T ) 173(2) K, 8516 independent reflections [R(int)
) 0.0392], R ) 0.0379, wR2 ) 0.0819.
1
The H NMR spectrum shows single resonances for the CH and
CH3 hydrogen atoms of the isopropyl groups, indicating free rotation
about the PN bond, again consistent with a P-N single bond.
In summary, the first terminal cobalt phosphinidene complex has
been isolated. The complex is stable despite a lack of steric
protection, and its structural and spectroscopic parameters clearly
show that the phosphinidene ligand is stabilized by a donor
interaction with the amino substituent. The electrophilicity of the
phosphinidene complex has been demonstrated in its reaction with
diphenylacetylene.
(13) (a) Lang, H.; Eberle, U.; Leise, M.; Zsolnai, L. J. Organomet. Chem.
1996, 519, 137. (b) Lang, H.; Orama, O. J. Organomet. Chem. 1989, 371,
C48.
(14) Orpen, A. G.; Brammer, L.; Allen, F. H.; Kennard, O.; Watson, D. G.;
Taylor, R. J. Chem. Soc., Dalton Trans. 1989, S1.
(15) Data for [Co(CO)3(PPh3){P(NiPr2)C(Ph)dC(Ph)}][AlCl4]‚CH2Cl2 (3): 1H
NMR (CDCl3) 7.3-7.9 (m, 25H, Ph), 3.80 (d septets, 2H, 3J(HH) ) 6.7
Hz, 3J(HP) ) 6.2, CH), 1.25 (d, 12H, 3J(HH) ) 6.7 Hz). 31P NMR (CDCl3)
δ 54.4 (d, PPh3, 2J(PP) ) 140 Hz), -76.6 (d, PC2, 2J(PP) ) 140 Hz). IR
(νCO) 2019s, 2001s cm-1. Anal. Calcd for C42H41O3P2NAlCl6Co: C,
52.09; H, 4.27; N, 1.45. Found: C, 51.26; H, 4.66; N, 1.55. X-ray data:
triclinic, P-1, a ) 12.3668(5), b ) 13.0461(6), c ) 15.9147(7), R )
Acknowledgment. This work was supported by the National
Research Council of Canada and grants from the Natural Sciences
and Engineering Research Council of Canada (to A.J.C.) and an
NRC-NSERC Canadian Government Laboratories Visiting Fel-
lowship (to B.T.S.).
88.193(1), â ) 75.292(1), γ ) 73.836(1), V ) 2383.2(2), Z ) 2, Dcalc
)
1.290, T ) 173(2) K, 12 275 independent reflections [R(int) ) 0.0284],
R ) 0.0487, wR2 ) 0.1650.
(16) Marinetti, A.; Mathey, F. J. Am. Chem. Soc. 1982, 104, 4484.
(17) Although we initially favored a (2 + 2) cycloaddition of the alkyne to
the cationic phosphinidene complex [Cp*Mo(CO)3(η1-PNiPr2)]+ on the
basis of spectroscopic data alone, further work has shown that the product
of this reaction is in fact a phosphirene rather than a metallocycle. See:
Sterenberg, B. T.; Carty, A. J. J. Organomet. Chem. 2001, 617-618, 696.
Supporting Information Available: Synthetic procedures, analyti-
cal, spectroscopic, and crystallographic data for compounds 1-3 (PDF
and CIF). This material is available free of charge via the Internet at
http://pubs.acs.org.
References
(18) Mathey, F. Chem. ReV. 1990, 90, 997.
(1) Hitchcock, P. B.; Lappert, M. F.; Leung, W.-P. J. Chem. Soc., Chem.
Commun. 1987, 1282.
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