Ruthenium-Stabilized Low-Coordinate Phosphorus Atoms: Structural Evidence for Monomeric Metaphosphate

Article abstract of DOI:10.1002/anie.200352306

Monomeric metaphosphonate is generated and stabilized within the coordination sphere of a ruthenium atom. The π system of a PO moiety coordinates to the transition metal center and this allows thermodynamic stabilization to give the first structural evidence for monomeric dioxophosphorane.

Full text of DOI:10.1002/anie.200352306

Communications
Phosphorus Coordination Chemistry
Ruthenium-Stabilized Low-Coordinate Phospho-
rus Atoms: Structural Evidence for Monomeric
Metaphosphonate**
Richard Menye-Biyogo, Fabien Delpech,* Annie Castel,
Heinz Gornitzka, and Pierre Rivi›re*
The metaphosphate anion has attracted steady interest since
1955[1] in connection with wide applications in phosphoryla-
tion reactions.[2] It has relevance in biochemistry in particular
with respect to ATP hydrolysis.[3] However, despite consid-
erable efforts, monomeric metaphosphates and other dioxo-
phosphoranes are only known as transient species in solution
and their existence can be inferred from trapping experi-
ments.[2–4] Their high reactivity arises from the powerful
electrophilic character at the phosphorus center. Several
approaches have been tested to stabilize these highly reactive
species. Kinetic stabilization by steric protection[5] proved to
be ineffective for dioxophosphorane and led to products that
resulted from the insertion of the PO2 moiety in a neighboring
group.[6] The electrophilicity and the tendency to expand the
valence shell were exploited for their stabilization as Lewis
salts. This strategy allowed only spectroscopic characteriza-
tion by 31P NMR in a few cases.[7] Thus, the chemistry of
dioxophosphorane suffers from a paucity of information and
new approaches to stabilize and enable deeper investigation
are highly desirable. This work describes the first isolation
and full characterization, including X-ray structural analysis,
of stabilized metaphosphonate.
We have developed a novel and successful route to
ruthenium-stabilized monomeric metaphosphonate by oxida-
tion of a nucleophilic terminal phosphinidene complex of
ruthenium. As recently described by Lammerstma and co-
workers, the terminal phosphinidene ruthenium complexes
[(h6-p-cymene)Ru(PR3)(PMes*)] (3a: R = Cy, 3b: R = Ph[8])
are formed as dark green solids by the reaction of the primary
phosphane complex [(h6-p-cymene)RuCl2(PH2Mes*)](1; p-
cymene = 4-methyl-iso-propylbenzene; Mes* = 2,4,6-tri-tert-
butylphenyl) with two equivalents of 1,8-diazabicyclo[5-4-
0]undec-7-ene (DBU) in the presence of PR3 as a stabilizing
ligand (R = Ph, Cy).[8] Alternatively, treatment of [(h6-p-
cymene)RuCl2(PR3)] (2a: R = Cy, 2b: R = Ph) and PH2Mes*
with DBU affords 3 in high yields (Scheme 1).
[*] Dr. F. Delpech, P. Rivi›re, R. Menye-Biyogo, Dr. A. Castel,
Dr. H. Gornitzka
Laboratoire d'HØtØrochimie Fondamentale et AppliquØe, UMR 5069
UniversitØ Paul Sabatier
118, Route de Narbonne, 31062 Toulouse CØdex 04 (France)
Fax: (+33)5–6155–8204
E-mail: delpech@chimie.ups-tlse.fr
riviere@chimie.ups-tlse.fr
[**] This work was supported by the Minist›re de la Jeunesse, de
L'Education Nationale et de la Recherche and by the CNRS. R.M.B.
is grateful to the Gabonese government for a fellowship.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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DOI: 10.1002/anie.200352306
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Angewandte
Chemie
proximity between p-cymene and Mes* moieties results in
À
restricted rotation about the P C(Mes*) bond as evidenced
by the presence of two Mes* aryl signals in addition to two o-
Me resonances. No change is observed in the NMR spectra
over the temperature range of 208C to 1008C. The molecular
geometry of 4a was determined by X-ray crystallographic
analysis (Figure 1).[15]
Scheme 1. Synthesis of the complexes 3, 4: a) PR3, DBU, C7H8;
b) PH2Mes*, DBU, C7H8; c) O2, Et2O.
The 31P NMR spectra show a high-field resonance (d =
811 ppm for 3a and d = 835 ppm for 3b) in the typical region
for terminal phosphinidene complexes[8,9] as well as a signal
which corresponds to the PR3 ligand.[10] X-ray diffraction
study ascertains the structure of 3a, which compares well with
those of [(h6-p-cymene)(PPh3)Os(PMes*)] and [(h6-benz-
ene)(PPh3)Ru(PMes*)].[8] The nucleophilic behavior of the
phosphinidene ligand in these complexes arises from a
substantial back-donation from the metal. Thus, complexes
3 offer an attractive opportunity to stabilize strong electro-
philic phosphorus species such as dioxophosphorane.
Figure 1. Molecular structure of 4a (50% thermal ellipsoids; hydrogen
atoms and noncoordinated ether molecules are omitted for clarity).
Selected bond lengths (Š) and angles (8): Ru1-O2 2.130(4), Ru1-P1
2.2963(17), Ru1-P2 2.3830(17), P1-O1 1.491(5), P1-O2 1.569(4), P1-
Ru1-P2 89.91(6), P2-Ru1-O2 87.09(11), O1-P1-O2 120.1(2), O1-P1-C1
115.2(3), O2-P1-C1 104.2(3).
Using similar oxidation route to that reported to generate
metaphosphonate from related s2-l3 phosphorus in diphos-
phene,[11] we obtained the first monomeric stabilized-dioxo-
phosphorane. Indeed, upon oxidation with O2, the dark green
solution of 3 turned instantly yellow and yielded [(h6-p-
cymene)(PR3)Ru(h2-OPOMes*)] (4a: R = Cy, 4b: R = Ph) in
high yields (Scheme 1). Interestingly, 4 are mildly air-sensitive
and the high electrophilicity of dioxophosphorane is thor-
oughly quenched since no reaction occurs in the presence of
any of the typical trapping agents (CH3OH, PhNH2). The
formulation of 4 was confirmed by spectroscopic studies.
Conversion of 3 into 4 was followed by IR spectroscopy and
the spectra (nujol) show appearance of two strong and
The environment around the ruthenium center can be
=
considered as a two-legged piano stool with a P O bond
occupying one of the coordination sites. Complex 4a features
a side-on h2-coordinated metaphosphonate, which is unpre-
À
cedented (Figure 1). The metal-coordinated P(1) O(2) bond
=
(1.569(4) Š) is longer than the P(1) O(1) double bond
(1.491(5) Š) which is similar to that found in phosphinidene
oxide complexes.[16] However, P(1) O(2) bond is still shorter
À
À
than a single P O bond (ꢀ 1.63Š) and indicates significant
=
residual double-bond character. The lengthening of the P O
characteristic absorptions (at 1168 cmÀ1 and 907 cmÀ1
;
bond and the planarity of the p-cymene ligand (deviations out
1153cm À1 and 925 cmÀ1 for 4a and 4b respectively) assigned
to asymmetric and symmetric OPO stretches. Although these
bands cannot be considered as pure modes (coupling with
the plane from 0.001 to 0.005 Š) in 4 are strong signs of
[17,18]
=
substantial Ru (dp) to P O (p*) back-donation.
This
bonding mode induces pyramidalization at the P atom with a
sum of bond angles of 3398 and loss of the planarity for
metaphosphonate as observed upon coordination of olefin[19]
or of related bis(imino)- and amino(imino) thiophosphor-
ane[20] toward a transition metal. Interestingly, the acute P(2)-
À
À
Ru P and Ru O must be present), they are clearly shifted to
low frequencies by comparison to the band observed in RPO2
(1448 cmÀ1 and 1143cm À1
)
and indicate a significant
[12]
=
weakening of the P O bond consistent with ligation to
ruthenium. Analogous behavior has been reported in the
well-documented coordination chemistry of the somewhat
related CO2 ligand.[13] 31P NMR resonances are in the
expected region for four-coordinate phosphorus atoms (d =
40.8 ppm for PO2Mes* and d = 27.1 ppm for PCy3 in 4a and
d = 38.5 ppm for PO2Mes* and d = 25.7 ppm for PPh3 in 4b).
1H NMR spectroscopic data provide additional structural
information. 1H NMR spectra show for the aryl protons of the
p-cymene ligand three resonances in a 2:1:1 ratio, which are
significantly shifted upfield relative to those of 1–3. Such a
behavior had been previously observed for half-sandwich
complexes exposed to magnetic anisotropy cone of phenyl
substituent (known as “b-phenyl effect”).[14] Consistently, the
À
Ru-A angle (A is the projection of the metal Ru on the P(1)
À
O(2) bond, O(2) A = 0.551 Š) of 88.02 8 does not vary
significantly than P-M-P (M = Ru, Os) angle in related
terminal phosphinidene complexes,[8] despite the increased
steric congestion between the Mes* group and p-cymene
ligand. This unexpected arrangement has its origin in orbital
factors. Ab initio calculations performed on monomeric
dioxophosphorane have shown that OPO moiety may be
described as a four-electron, three-center p system, in which
HOMO is mainly localized at the oxygen positions while the
LUMO corresponds to P = O (p*) antibonding orbitals.[21]
Examination of the bonding situation for the metaphospho-
nate ligand in 4 by using the Dewar–Chatt–Duncanson model
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ꢀ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Communications
(by analogy with olefin complexes) provides pertinent
Keywords: coordination modes · P ligands · phosphanes ·
pi interactions · ruthenium
.
information. Both donation and back-donation involving
the metaphosphonate ligand implicate orbitals that are
perpendicular to the plane of the free dioxophosphorane
and impose such a face-on orientation for the Mes* group.
In conclusion, the accessibility of monomeric meta-
phosphonate relies on its complexation to electron-rich
species which reduce electrophilicity. Complexes 4, in which
dioxophosphorane Mes*PO2 has been generated and stabi-
lized within the coordination sphere of a transition metal,
represent the first direct observation of fully characterized
monomeric metaphosphonate.
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Experimental Section
3a: DBU (0.29 mL; 1.97 mmol) was added to a red slurry of [(h6-p-
cymene)(PCy3)RuCl2] (0.602 g; 1.03mmol) and Mes*PH 2 (0.274 g;
0.98 mmol) in toluene (10 mL). The mixture was stirred for 1 h at
room temperature to afford a dark green solution. The toluene was
removed in vacuo and the dark green solid then extracted with
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bubbled with oxygen for 1 min. Removal of the ether to a minimal
volume and cooling to À258C gave 0.150 g of a yellow crystalline solid
in 87% yield. m.p.: 1768C.
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Atom labeling used in the NMR assignments of 4a is given
below.
[10] For synthesis, spectroscopic data, and elemental analysis see
Supporting Information.
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2
31P NMR (81 MHz, C6D6): d = 27.1 (d, J(P,P) = 45.8 Hz, PCy3),
[15] CCDC 213806 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cam-
bridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB21EZ, UK; fax: (+ 44)1223-336-033; or deposit@
ccdc.cam.ac.uk). Information about crystal data and structure
measurements and refinements are summarized in the Support-
ing Information.
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40.8 ppm (d, 2J(P,P) = 45.8 Hz, PO2Mes*). 1H NMR (200 MHz,
C6D6): d = 1.05 (d, 3J(H,H) = 6.8 Hz, 3H, CH(CH3)2 p-cymene),
1.20 (d, J(H,H) = 6.6 Hz, 3H, CH(CH3)2 p-cymene), 1.29 (m, 12H,
3
CcH2), 1.41 (s, 9H, p-C(CH3)3), 1.56 (m, 6H, CdH2), 1.81 (s, 3H, CH3
p-cymene), 1.88 (s, 9H, o-C(CH3)3), 1.91 (m, 12H, CbH2), 2.14 (s, 9H,
o-C(CH3)3), 2.53(m, 4H, C aH and CH(CH3)2 p-cymene), 2.95 (s, 1H,
C2H p-cymene), 4.17 (s, 1H, C3H p-cymene), 5.40 (s, 2H, C2H and
C3H p-cymene), 7.43(s, 1H, m-Mes*), 7.55 ppm (s, 1H, m-Mes*).
13C{1H} NMR (50 MHz, C6D6): d = 19.1 (s, CH3 p-cymene), 20.5 (s,
CH(CH3)2 p-cymene), 25.3(s, CH( CH3)2 p-cymene), 27.3(s, Cd), 28.4
(d, 3J(C,P) = 10.2 Hz, Cc), 30.6 (d, 2J(C,P) = 33.3 Hz, Cb), 31.1 (s,
CH(CH3)2 p-cymene), 31.6 (s, p-C(CH3)3), 33.7 (s, o-C(CH3)3), 33.8 (s,
o-C(CH3)3), 34.9 (s, p-C(CH3)3), 37.3 (d, 3J(C,P) = 8.3Hz, Ca), 39.8 (d,
3J(C,P) = 2.8 Hz, o-C(CH3)3), 40.7 (d, 3J(C,P) = 2.8 Hz, o-C(CH3)3),
2
73.7 (s, C4 p-cymene), 75.2 (d, J(C,P) = 11.1 Hz, C2 p-cymene), 84.6
(d, 2J(C,P) = 9.2 Hz, C2 p-cymene), 86.3(s, C3 p-cymene), 93.9 (s, C3 p-
cymene), 99.4 (s, C1 p-cymene), 119.1 (d, 3J(C,P) = 12.9 Hz, m-Mes*),
121.7 (d, 3J(C,P) = 10.2 Hz, m-Mes*), 137.9 (d, 1J(C,P) = 65.7 Hz,
ipso-Mes*), 147.7 (d, 4J(C,P) = 3.7 Hz, p-Mes*), 153.6 (s, o-Mes*),
153.9 ppm (s, o-Mes*); IR (nujol): 1168, 907 cmÀ1 (asym and sym
OPO); elemental analysis (%) calcd. for 4a·2Et2O C54H96O4P2Ru: C
66.70, H 9.95; found: C 66.33, H 9.46.
The derivatives 3b and 4b were prepared by an analogous
[21] W. W. Schoeller, T. Bush, Chem. Ber. 1992, 125, 1319.
method.[10]
Received: July 4, 2003[Z52306]
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Angew. Chem. Int. Ed. 2003, 42, 5610 –5612