Oxidation reactions of the phosphinidene oxide ligand

Article abstract of DOI:10.1021/ja054941t

The (H-DBU)+ salt of the anionic phosphinidene oxide complex [MoCp(CO)₂{P(O)R*}]^- (1) (DBU = 1,8-diazabicyclo[5.4.0]undec-7-ene; R* = 2,4,6-C₆H₂^tBu₃) reacts with different oxidizing agents, displaying a multisite activity located at the Mo and P atoms or at the Mo=P bond. Thus, reaction of 1 with [FeCp₂]BF₄ gives the dimer [Mo₂Cp₂(CO)₄{P(O)R*}₂], and reaction with bromine gives the phosphinous acid complex [MoBrCp{P(OH)(CH₂CMe₂C₆H₂^tBu₂}(CO)₂], the latter arising from an unprecedented C-H bond addition to the oxide P=O moiety. In contrast, reaction of 1 with p-benzoquinone occurs at the P site to give the P,O-bound phosphonite complex [MoCp{κ²-OP(OC₆H₄OH)R*}(CO)₂]. Finally, oxygen or sulfur atoms are added to the Mo=P bond by reaction of 1 with Me₂CO₂ and S₈ to give the novel dioxophosphorane or thiooxophosphorane complexes [MoCp(CO)₂{κ²-EP(O)R*}]^- (E = O, S). The thiooxophosphorane anion is a good nucleophile and is methylated at either the S or O positions depending on the electrophile used (MeI or (Me₃O)BF₄) to give the isomers [MoCp{κ²-(MeS)P(O)R*}(CO)₂] and [MoCp{κ²-SP(OMe)R*}(CO)₂], both having novel organophosphorus ligands. Copyright

Full text of DOI:10.1021/ja054941t

Published on Web 10/11/2005
Oxidation Reactions of the Phosphinidene Oxide Ligand
Mar´ıa Alonso,‡ M. Angeles Alvarez,‡ M. Esther Garc´ıa,‡ Miguel A. Ruiz,*,‡ Hayrullo Hamidov,† and
John C. Jeffery†
Departamento de Qu´ımica Orga´nica e Inorga´nica/IUQOEM, UniVersidad de OViedo, 33071 OViedo, Spain, and
School of Chemistry, UniVersity of Bristol, Bristol BS8 1TS, U.K.
Received July 22, 2005; E-mail: mara@fq.uniovi.es
Phosphinidene oxides (R-PdO) are unstable molecules thought
to be generated in the thermal or photochemical decomposition of
several precursors, such as phospholene, phosphirane, or phospha-
norbornadiene oxides, and others. These transient species are very
reactive toward different organic molecules and have been thus used
as efficient synthons for new organophosphorus derivatives.1
Stabilization of R-PdO species can be, in principle, achieved
through coordination to metal centers, a strategy that could allow
for a more detailed study of the chemical behavior of these
unsaturated molecules. However, only a few phosphinidene oxide
complexes have been reported so far,2 and their reactivity has not
been explored.
Figure 1. Molecular structure of compounds 3 (left) and 4 (right). Hydrogen
atoms and methyl groups are omitted for clarity.
Scheme 1
Recently, we reported the preparation of the (H-DBU)+ salt of
the anionic phosphinidene oxide complex [MoCp(CO)2{P(O)R*}]-
(1) (R* ) 2,4,6-C6H2 Bu3; Cp ) η5-C5H5, DBU ) 1,8-diazabicyclo-
t
[5.4.0]undec-7-ene) and a preliminary study on its reactivity toward
different electrophiles.3 From this, we concluded that anion 1
exhibits unique acid-base properties, with three different nucleo-
philic sites located at the O, P, and Mo atoms. In this paper, we
report a preliminary study on the reactions of anion 1 with different
oxidizing reagents, which again reveals a multisite reactivity, now
located at the P and Mo atoms or at the ModP bond. This in turn
allows for the synthesis of complexes having novel organophos-
phorus ligands, displaying new coordination modes or experiencing
unexpected bond activation processes under mild conditions
(Scheme 1).
Anion 1 reacts with an innocent oxidant such as [FeCp2]BF4 to
cleanly give the binuclear compound [Mo2Cp2(CO)4{P(O)R*}2] (2),
which displays two terminally bound phosphinidene oxide ligands.4
The formation of 2 is interpreted as a result of the dimerization of
a metal-centered radical [MoCp(CO)2{P(O)R*}] arising from the
one-electron oxidation of anion 1. From this, however, it cannot
be concluded that other oxidants would preferentially attack the
metal position in the anion. In fact, the site of attack turns out to
be strongly dependent on the oxidizing agent used.
The mild oxidant p-benzoquinone attacks the anion 1 at the
phosphorus position to give, after spontaneous H+ transfer from
the (H-DBU)+ counterion, the neutral compound [MoCp{κ2-
OP(OC6H4OH)R*}(CO)2] (3), which is the first complex reported
to have a P,O-bound phosphonite ligand.5 The binding of the
phosphonite ligand in 3 (Figure 1) is similar to that of the
phosphinite complex [MoCp{κ2-OP(C3H5)R*}(CO)2],3 with the
internuclear P-O(3) (1.549(3) Å) and P-Mo (2.354(1) Å) separa-
tions being indicative of the retention of significant double bond
character in the corresponding bonds.6
t
CMe2C6H2 Bu2}] (4) is formed (Figure 1).7,8 Compound 4 is
generated in solution as a single diastereoisomer, possibly favored
by the presence of substantial O-H‚‚‚Br hydrogen bonding. This
is clear in the solid state, with the value of the relevant normalized
length9 (H(3)‚‚‚Br ) 0.80) pointing to an interaction of medium
strength. The formation of 4 requires the intramolecular addition
of a C-H bond to the PdO bond in the phosphinidene oxide ligand,
which is unprecedented and highly remarkable due to the mild
reaction conditions used. Only transient dioxophosphoranes (RPO2)
have been previously proposed to experience related reactions, this
occurring in the gas phase at temperatures in the range of 900-
1100 K.10 Interestingly, the transient R*-PdO molecule, generated
in the gas phase at 573 K, has been proposed to experience the
oxidative addition of a C-H bond to its phosphorus atom to yield
the corresponding phosphinic acid.1b
Bromine attacks the anion 1 at the metal position, but does not
give the expected neutral phosphinidene oxide complex. Instead,
the phosphinous acid complex cis-[MoBrCp(CO)2{P(OH)(CH2-
Compound 4 experiences hydrolysis readily to give, in the
presence of bases, the anionic dioxophosphorane complex [MoCp-
‡ Universidad de Oviedo.
† University of Bristol.
9
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10.1021/ja054941t CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
reaction at the Mo and P positions, or at the ModP bond. New
organophosphorus ligands or new coordination modes of the latter
can be thus revealed, these including P,O-bound phosphonite and
dioxophosphorane, as well as thiooxophosphorane, phosphonothio-
late, and thiolophosphinide ligands. Further work to expand the
synthetic potential of anionic phosphinidene oxide or dioxide and
thiooxophosphorane complexes is now in progress.
Acknowledgment. We thank the MCYT/MECD of Spain for
a grant (to M.A.) and financial support (BQU2003-05471).
Figure 2. Molecular structure of compounds 7 (left) and 8 (right). Hydrogen
atoms and methyl groups are omitted for clarity.
Scheme 2
Supporting Information Available: Experimental procedures and
spectroscopic data for new compounds (PDF); crystallographic data
for compounds 3, 4, 7, and 8 (CIF). This material is available free of
charge via the Internet at http://pubs.acs.org.
References
(1) (a) Gaspar, P. P.; Qian, H.; Beatty, A. M.; d’Avignon, D. A.; Kao, J. L.
F.; Watt, J. C.; Rath, N. P. Tetrahedron 2000, 56, 105-119 and references
therein. (b) Cowley, A. H.; Gabba¨ı, F. P.; Corbelin, S.; Decken, A. Inorg.
Chem. 1995, 34, 5931-5932. (c) Wang, K.; Emge, T. J.; Goldman, A. S.
Organometallics 1994, 13, 2135-2137. (d) Niecke, E.; Zorn, H.; Krebs,
B.; Henkel, G. Angew. Chem., Int. Ed. Engl. 1980, 19, 709-710.
(2) (a) Niecke, E.; Engelmann, M.; Zorn, H.; Krebs, B.; Henkel, G. Angew.
Chem., Int. Ed. Engl. 1980, 19, 710-712. (b) Hitchcock, P. B.; Johnson,
J. A.; Lemos, M. A. N. D. A.; Meidine, M. F.; Nixon, J. F.; Pombeiro, A.
J. L. J. Chem. Soc., Chem. Commum. 1992, 645-646. (c) Johnson, M. J.
A.; Odom, A. L.; Cummins, C. C. Chem. Commum. 1997, 1523-1524.
(d) Kourkine, V.; Glueck, D. S. Inorg. Chem. 1997, 36, 5160-5164. (e)
Schmitt, G.; Ullrich, D.; Wolmersha¨user, G.; Regitz, M.; Scherer, O. J.
Z. Anorg. Allg. Chem. 1999, 625, 702-704. (f) Buchholz, D.; Huttner,
G.; Imhof, W. J. Organomet. Chem. 1990, 388, 307-320.
(CO)2{κ2-OP(O)R*}]- (5), a transformation implying an easy and
unprecedented reversal of the former C-H bond addition to the
PdO moiety, which thus appears to be triggered by subtle changes
in the coordination sphere around the metal atom. The anion 5 (as
its (H-DBU)+ salt)11 can be more conveniently prepared through
direct oxidation of the phosphinidene oxide complex 1 using a mild
oxygenating agent such as Me2CO2. Reaction of 1 with elemental
sulfur proceeds analogously by incorporation of a S atom to the
ModP bond to give the S,P-bound thiooxophosphorane complex
[MoCp(CO)2{κ2-SP(O)R*}]- (6).12
As it is the case of the phosphinidene oxides, dioxophosphoranes
(phosphinidene dioxides, RPO2) and thiooxophosphoranes (RPOS)
are unstable molecules thought to be generated in the thermolysis
of suitable organophosphorus precursors. These transient species
are strongly electrophilic at the phosphorus atom and have thus
found use as efficient phosphorylating agents.13 Surprisingly, the
coordination chemistry of these molecules is virtually unknown;
no complexes having thiooxophosphorane ligands appear to have
been ever prepared, and there is just a single example of a
dioxophosphorane complex.14 The anions 5 and 6 thus provide the
opportunity to explore the chemistry of these unsaturated ligands.
Initial experiments on 6 reveal that the electrophilic nature of the
uncoordinated RPOS ligand is reversed by the negative charge of
the complex, which then allows reactions with electrophiles, at
either the O or the S positions depending on the reagent used
(Scheme 2).
Thus, selective methylation at sulfur is achieved by MeI to yield
the S,P-bound thiolophosphinide complex [MoCp{κ2-(MeS)P(O)-
R*}(CO)2] (7),15 whereas reaction with (Me3O)BF4 gives a mixture
of the latter and the isomeric phosphonothiolate complex [MoCp-
{κ2-SP(OMe)R*}(CO)2] (8),16 as confirmed crystallographically
(Figure 2).17,18 It should be noted that no complexes containing
phosphonothiolate or thiolophosphinide ligands appear to have been
previously described, thus further stressing the synthetic potential
of anion 1. In the case of 8, the coordination geometry is very
similar to that of the phosphonite complex 3, as expected. In
contrast, the isomeric complex 7 exhibits internuclear separations
within the Mo-P-S ring closer to the values expected for the
corresponding single bonds.
(3) Alonso, M.; Garc´ıa, M. E.; Ruiz, M. A.; Hamidov, H.; Jeffery, J. C. J.
Am. Chem. Soc. 2004, 126, 13610-13611.
(4) The structure of 2 has been confirmed through an X-ray study on its
ditungsten analogue (unpublished results from the authors; see also
Supporting Information). Selected spectroscopic data for 2: νCO (CH2-
Cl2) 1918 (m, sh), 1901 (s) cm-1 31P{1H} NMR (CD2Cl2) δ 463.5 ppm.
;
(5) Selected spectroscopic data for 3: νCO (CH2Cl2) 1954 (s), 1865 (s) cm-1
;
31P{1H} NMR (CD2Cl2) δ 74.5 ppm; 1H NMR (CD2Cl2) δ 5.14 (s, 1H,
OH), 5.01 (s, 5H, Cp) ppm; 13C{1H} NMR (CD2Cl2) δ 252.7 (d, JCP
)
30 Hz, CO), 245.6 (s, CO) ppm.
(6) X-ray data for 3‚CH2Cl2: red crystals, monoclinic (P21/c), a ) 15.229(2),
b ) 10.935(2), c ) 20.058(3) Å, â ) 103.39(1)°, V ) 3249.6(8) Å3, T )
293 K, Z ) 4, R1 ) 0.0386 (observed data with I > 2σ(I)), GOF ) 1.041.
(7) Selected spectroscopic data for 4: νCO (CH2Cl2) 1972 (vs), 1894 (s) cm-1
;
31P{1H} NMR (CD2Cl2) δ 132.7 ppm; 1H NMR (CD2Cl2) δ 6.86 (s, 1H,
OH), 2.66, 2.34 (2 × m, 2 × 1H, PCH2) ppm.
(8) X-ray data for 4: red crystals, triclinic (P1h), a ) 10.903(2), b ) 11.395(2),
c ) 12.157(2) Å, R ) 104.57(3), â ) 108.57(3), γ ) 103.77(3)°, V )
1299.8(5) Å3, T ) 298 K, Z ) 2, R1 ) 0.0367 (observed data with I >
2σ(I)), GOF ) 1.031.
(9) Brammer, L.; Bruton, E. A.; Sherwood, P. Cryst. Growth Des. 2001, 1,
277-290.
(10) Cadogan, J. I. G.; Cowley, A. H.; Gosney, I.; Pakulski, M.; Wright, P.
M.; Yaslak, S. J. Chem. Soc., Chem. Commun. 1986, 1685-1686 and
references therein.
(11) Selected spectroscopic data for 5: νCO (CH2Cl2) 1904 (vs), 1803 (s) cm-1
31P{1H} NMR (CD2Cl2) δ 39.6 ppm.
;
(12) Selected spectroscopic data for 6: νCO (CH2Cl2) 1911 (vs), 1816 (s) cm-1
;
31P{1H} NMR (CD2Cl2) δ 83.8 ppm; 13C{1H} NMR (CD2Cl2) δ 254.6
(d, JCP ) 29 Hz, CO), 247.1 (s, CO) ppm.
(13) Quin, L. D. Coord. Chem. ReV. 1994, 137, 525-559.
(14) Menye-Biyogo, R.; Delpech, F.; Castel, A.; Gornitzka, H.; Rivie`re, P.
Angew. Chem., Int. Ed. 2003, 42, 5610-5612.
(15) Selected spectroscopic data for 7: νCO (CH2Cl2) 1963 (vs), 1887 (s) cm-1
;
1
31P{1H} NMR (CD2Cl2) δ 102.0 ppm; H NMR (CD2Cl2) δ 2.09 (d, JHP
) 3 Hz, 3H, SMe) ppm.
(16) Selected spectroscopic data for 8: νCO (CH2Cl2) 1947 (vs), 1862 (s) cm-1
;
31P{1H} NMR (CD2Cl2) δ 122.0 ppm; H NMR (CD2Cl2) δ 3.18 (d, JHP
1
) 16 Hz, 3H, OMe) ppm.
(17) X-ray data for 7: yellow crystals, monoclinic (P21/n), a ) 8.900(2), b )
23.129(5), c ) 13.248(3) Å, â ) 94.72(3)°, V ) 2717.8(9) Å3, T ) 100
K, Z ) 4, R1 ) 0.0214 (observed data with I > 2σ(I)), GOF ) 1.060.
(18) X-ray data for 8: orange crystals, monoclinic (C2/c), a ) 30.656(10), b
) 9.859(3), c ) 20.754(7) Å, â ) 122.01(1)°, V ) 5319(3) Å3, T ) 293
K, Z ) 8, R1 ) 0.0539 (observed data with I > 2σ(I)), GOF ) 1.018.
In summary, we have shown that anion 1 reacts with innocent
and noninnocent oxidizing reagents to give products resulting from
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