An unusual P-P double bond formed via phospha-Wittig transformation of a terminal PO complex

Article abstract of DOI:10.1021/ja903860k

(Chemical Equation Presented) The terminal phosphorus monoxide complex (OP)Mo(N[^tBu]Ar)₃, 1 (Ar = 3,5-Me₂C ₆H₃), undergoes an O-for-PSiR₃ metathesis reaction with the niobium phosphinidene

Full text of DOI:10.1021/ja903860k

Published on Web 06/08/2009
An Unusual P-P Double Bond Formed via Phospha-Wittig Transformation of
a Terminal PO Complex
Nicholas A. Piro and Christopher C. Cummins*
Department of Chemistry, 77 Massachusetts AVenue, Room 6-435, Massachusetts Institute of Technology,
Cambridge, Massachusetts 02139
Received May 12, 2009; E-mail: ccummins@mit.edu
First reported in 1997, (OP)Mo(N[tBu]Ar)3 (1, Ar ) 3,5-
Me2C6H3) is the only example of an isolable terminal phosphorus
monoxide complex.1 As a result, the chemistry of this unique
functional group has remained largely unexplored. Initial reactivity
studies indicated that 1 is electrophilic at phosphorus and nucelo-
philic at oxygen, as illustrated by its reaction with Cp2ZrMe2 to
afford Cp2MeZrOP(Me)Mo(N[tBu]Ar)3.1 Extending this concept,
we speculated that reaction of 1 with a suitable phospha-Wittig
reagent might generate a diphosphenido ligand complexed atop the
molybdenum trisanilide platform. High oxidation state, early metal
phosphinidene complexes have been shown to serve as effective
phospha-Wittig reagents in combination with a variety of reagents
including aldehydes and ketones.2-8 We have recently exploited a
versatile niobium terminal phosphide anion as a precursor to a
variety of phosphinidene complexes,9-11 and the high degree of
oxophilicity possessed by the niobium trisanilide fragment
t
Nb(N[CH2 Bu]Ar)3 represents a formidable driving force for
phosphorus transfer from niobium to main-group acceptors.11,12
i
The silylphosphinidene complex Pr3SiPNb(N[CH2tBu]Ar)3 (2) is
formed by the reaction of iPr3SiOTf (OTf ) trifluoromethanesulfonate)
and the sodium salt of [PNb(N[CH2tBu]Ar)3]-. Complex 2 is isolated
as an orange solid in 61% yield by crystallization from Et2O and
displays a broad 31P NMR resonance at 433 ppm. A single-crystal
X-ray diffraction study revealed a short Nb-P distance of 2.2454(6)
Å and a Nb-P-Si angle of 158.34(4)° (Figure 1B).13 At 22 °C,
complex 2 reacts with the purple phosphorus monoxide complex 1
over the course of several minutes to afford the oxoniobium complex
ONb(N[CH2tBu]Ar)3 (3) and one new species (4), identified by 31P,
1H, and 13C NMR spectroscopies (Figure 1A). The 31P NMR data for
Figure 1. (A) Silyldiphosphenido complex 4 and oxoniobium complex 3
are generated via an O-for-PSiR3 metathesis reaction between 1 and 2. (B
and C) ORTEP representations of complexes 2 and 4, respectively, with
50% probability ellipsoids and selected interatomic distances (Å) and angles
4 are a very broad doublet at 543 ppm and a less broad doublet at 158
ppm with a large P-P coupling constant (1JPP ) 655 Hz). These data
are consistent with the desired silyldiphosphenido product,
iPr3SiPdPMo(N[tBu]Ar)3, where the downfield resonance is attributed
to the phosphorus atom bound directly to the Mo center.14,15 A red-
orange single crystal grown from an Et2O solution of the product
mixture was subjected to an X-ray diffraction study.13 The molecular
structure of the diphosphenido complex (Figure 1C) can be described
as “singly bent” (angles at P of 158.27(3)° and 104.46(3)°) in analogy
to descriptions of diazenido complexes where the diazenido ligand
serves as a 3e- donor.16 The Mo-P (2.1439(5) Å) and P-P (2.0398(7)
Å) distances are both very short, an indication of multiple bonding
across the Mo-P-P π-system. These metrical parameters are in
contrast to the few known diphosphenido complexes,17-19 which are
best described as “doubly bent”, with their metal-phosphorus single
bonds reflected in substantially longer M-P distances. In such
complexes the R phosphorus maintains its lone pair, which has been
shown to engage in chemistry with various electrophilic reagents.19-21
Unlike nitrogen analogues of 4, such as the silyldiazenido
complex Me3SiNNMo(N[tBu]Ar)3 and the azaphosphenido complex
MesNPMo(N[tBu]Ar)3, the diphosphenido complex 4 is not stable
(deg).
in solution for extended periods of time.22,23 Over the course of
hours to days, or upon heating, complex 4 reacts to form
PMo(N[tBu]Ar)3 (5), the cyclic phosphinidene trimer (iPr3SiP)3 (6),
and the phosphinidene tetramer (iPr3Si)2P3P(SiiPr3)2 (7); the latter
two were identified by their 31P NMR spectra which were
successfully simulated (see Supporting Information).23-25 Attempts
to make analogues of 4 bearing smaller silyl groups (Me3Si, Ph3Si)
led to much more rapid formation of the corresponding phosphin-
idene trimers, such that the diphosphenido complexes were not
observed. Also, the generation of terminal phosphide complex 5
was observed to proceed more rapidly in concentrated solutions of
4 than in dilute solutions.26 Together, these observations lead us
to postulate the following mechanism: a bimolecular reaction
between 2 equiv of 4 generates 2 equiv of the terminal phosphide
complex 5 and 1 equiv of Pr3SiPdPSiiPr3 (8); this reactive
diphosphene then consumes a third equivalent of 4 to yield the
cyclic trimer 6; the tetrameric product 7 arises from an insertion
i
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8764 J. AM. CHEM. SOC. 2009, 131, 8764–8765
10.1021/ja903860k CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1a
this sense, the reaction between 4 and PPh3 serves as a model for
the bimolecular reaction that forms diphosphene 8.
Herein we have described a molybdenum diphosphenido complex
arrived at via O-for-PSiR3 metathesis involving a terminal phos-
phorus monoxide ligand. The diphosphenido ligand serves as a 3e-
donor, and its reactivity is distinct from prior examples of 1e- donor
diphosphenido complexes that are nucleophilic at the R phosphorus.
The electronic structure of diphosphenido complex 4 is unusual
and confers upon it the reactivity of a potent phosphinidene source
with terminal phosphide 5 serving as a stable leaving group.
Acknowledgment. We thank the U.S. National Science Foun-
dation for support through Grant CHE-719157 and Thermphos for
a generous donation of funds and supplies.
a 5 ) PMo(N[tBu]Ar)3.
Supporting Information Available: Full experimental details and
spectroscopic data (pdf). Details of X-ray structure determinations (cif).
This material is available free of charge via the Internet at http://
pubs.acs.org.
of the phosphinidene unit of a fourth equivalent of 4 into a P-Si
bond of trimer 6 (Scheme 1).
Having invoked intermediate diphosphene 8, we sought to engage
it in trapping reactions.27 Accordingly, complex 4 was warmed to
60 °C in a THF solution of spiro[2.4]hepta-4,6-diene, and the
product mixture was analyzed by 31P NMR spectroscopy. The [2+4]
cycloaddition product of E-diphosphene capture by the organic
diene, 9, was observed as a pair of doublets in the 31P NMR
spectrum (JPP ) 240 Hz) at -112.5 and -117.5 ppm.28 When 2,3-
dimethylbutadiene was used instead, the then C2-symmetric product,
10, displayed a single 31P resonance at -138 ppm. The observed
formation of 9 and 10 is consistent with the mechanism illustrated
in Scheme 1.
References
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2
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i
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-263.8 ppm.31 By varying the concentration of 5 and PPh3, the
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1
equilibrium constant for this reaction was measured by H NMR
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mol for the reaction PPh3 + Me3SiPdPMo(N[tBu]Ar)3
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