Phosphorus monoxide as a terminal ligand

Article abstract of DOI:10.1039/a703105j

Dimethyldioxirane oxidation of phosphide PMo[N(R)Ar]₃ [R = C(CD₃)₂Me, Ar = C₆H₃Me₂-3,5] efficiently provides (OP)Mo[N(R)Ar]₃, a complex shown by X-ray crystallography to contain a terminal phosphorus monoxide ligand, and which reacts with dimethylzirconocene to give addition of a Zr-Me bond across the phosphoryl moiety.

Full text of DOI:10.1039/a703105j

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Phosphorus monoxide as a terminal ligand
Marc J. A. Johnson, Aaron L. Odom and Christopher C. Cummins*†
Department of Chemistry, 2-227, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139-4307,
USA
Dimethyldioxirane oxidation of phosphide PMo[N(R)Ar]3
[R = C(CD3)2Me, Ar = C6H3Me2-3,5] efficiently provides
(OP)Mo[N(R)Ar]3, a complex shown by X-ray crystallo-
graphy to contain a terminal phosphorus monoxide ligand,
and which reacts with dimethylzirconocene to give addition
of a Zr–Me bond across the phosphoryl moiety.
two-coordinate phosphorus compounds.19 Proton NMR data for
2 are indicative of a single environment for the N(R)Ar ligands,
consistent with the threefold (propellor) symmetry typical of
complexes containing three N(R)Ar ligands.20–25
Single crystals of 2 suitable for an X-ray diffraction study§
were obtained by slow cooling to 235 °C of a diisopropyl ether
solution. Molecular 2 evinces pseudo-threefold symmetry about
the near-linear [177.6(8)°] Mo–P–O moiety, which is flanked
by the three tert-butyl substituents (Fig. 1). An intriguing
feature of the structure is the molybdenum–phosphorus distance
d(MoP) = 2.079(5) Å, which is shorter by 0.04 Å than d(MoP)
for terminal phosphide 1.11 Contraction of the molybdenum–
phosphorus distance in this manner is explicable in terms of
rehybridization at P upon addition of a terminal substituent.26
The phosphorus–oxygen distance [1.49(2) Å] in 2 is compara-
ble to that measured by IR diode laser spectroscopy for gaseous
PO [1.476370(15) Å)],27,28 and to d(PO) values for m3-PO
complexes: 1.509(3),3 1.462(9) and 1.480(10).1 Phosphoryl
halide species such as OPCl3 contrastingly exhibit longer
phosphorus–oxygen distances, on the order of 1.54–1.58 Å,29
while d(PO) for OP(C6H11)3 is 1.490(2) Å.30 Other metrical
parameters pertaining to the structure of 2 are unexceptional.
Previously reported (SP)Mo[N(R)Ar]3 311 was shown re-
cently by X-ray crystallography to resemble 2 in terms of its
monomeric nature and gross structural characteristics.31 The
rough similarity of 2 and 3 was used to advantage to obtain
assignment of the nPO (1232 cm21) and nPS (782 cm21) IR
stretching frequencies for the two complexes, by digital
subtraction of solution FTIR spectra. The nPO value for 2 is
similar to that reported for gaseous PO (1220.25 cm21), and
also is comparable to values reported for m3-PO complexes
(1260, 1169 cm21).
Known phosphorus monoxide complexes1–3 are those in which
PO4–10 serves as a triply bridging ligand. These have been
prepared by oxo transfer to a P2Ni2W cluster,1 and by
hydrolysis of a m3-P(NPri2) functional group.3 Recent reports of
terminal phosphide (P32) complexes11–15 hint at the possibility
of preparing terminal PO complexes by oxygen-atom transfer to
the terminal P ligand. Accordingly, dimethyldioxirane oxida-
tion16,17 has been found to effect the desired conversion of
PMo[N(R)Ar]3 1 to (OP)Mo[N(R)Ar]3 2. Synthetic, structural,
and initial reactivity data pertaining to 2 are detailed herein.
Treatment of a 278 °C gold-colored solution of 1 (1.0009 g,
1.4853 mmol) in dichloromethane (50 ml) with dimethyldioxi-
rane (20 ml of a 0.075 m solution in acetone18) elicited a rapid
color change to purple, the color of 2.‡ Workup consisted of
cold-filtration, precipitation of 2 by addition of acetonitrile (75
ml, pre-chilled to just above its melting point), and collection of
solid 2 (73.8% yield) by filtration. Once thus obtained in pure
form, solid 2 can be manipulated and maintained at 25 °C for at
least ca. 5 days without noticeable decomposition. When
dissolved in various solvents (e.g. benzene, diethyl ether,
heptane and dichloromethane) 2 is more sensitive, undergoing
gradual reversion to 1 according to 1H and 31P NMR
spectroscopic monitoring.
Diamagnetic 2 exhibits a single 31P NMR signal at d 269.8,
shifted dramatically upfield relative to 1 but in a range typical of
Initial reactivity studies indicate that the phosphoryl phos-
phorus in 2 is susceptible to nucleophilic attack in a manner
reminiscent of the Fischer carbene synthesis.32 Accordingly,
treatment of 2 (282.0 mg, 0.4088 mmol) with dimethylzirco-
nocene33 (103.3 mg, 0.4108 mmol) in benzene (7.5 ml) led to
O
formation of [(R1)(R2O)P]Mo[N(R)Ar]3 [4, R1
=
Me,
P
R2 = Zr(Me)(h-C5H5)2], which was isolated by crystallization
Mo
O
N(3)
N(2)
O
P
P
N(1)
R
R
O
Mo
Mo
R
R
N
N
R
R
CH2Cl2–acetone
–78 °C
N
N
N
N
Ar
Ar
Ar
Ar
74 %
Ar
Ar
2
1
benzene
28 °C, 2 h
75 %
ZrMe2(η-C5H5)2
Me
Zr
O
Me
P
Ar
Fig. 1 ORTEP drawing of the molecular structure of 2 with 35% probability
ellipsoids. Selected bond lengths (Å) and angles (°): Mo–P 2.079(5), Mo–
N(1) 1.95(2), Mo–N(2) 1.94(2), Mo–N(3) 1.994(13), P–O 1.49(2); Mo–
P–O 177.6(8), P–Mo–N(1) 101.7(4), P–Mo–N(2) 101.4(5), P–Mo–N(3)
101.6(5), N(1)–Mo–N(2) 113.1(6), N(1)–Mo–N(3) 117.2(6), N(2)–Mo–
N(3) 118.0(6).
Mo
R
N
R
N
N
R
Ar
Ar
4
Scheme 1
Chem. Commun., 1997
1523

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parameter ratio = 7.9, R1 = 0.1207, wR2 = 0.2683, GOF 1.050, residuals
based on I > 2s(I).
For 4: orthorhombic, space group Pbca, a = 11.3996(2), b = 23.2075(4),
c = 39.2443(7) Å, U = 10382.3(3) Å3, Z = 8, 7456 unique reflections,
data/parameter ratio 15, R1 = 0.0669, wR2 = 0.2023, GOF = 1.269,
residuals based on I > 2s(I). CCDC 182/527.
C(2)
Zr
C(1)
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Fig. 2 ORTEP drawing of the molecular structure of 4 with 35% probability
ellipsoids. Selected bond lengths (Å) and angles (°): Mo–P 2.169(2), Mo–
N(1) 1.995(5), Mo–N(2) 2.011(5), Mo–N(3) 1.993(5), P–O 1.613(5),
P–C(1) 1.832(8), Zr–O 2.000(4), Zr–C(2) 2.253(11); Mo–P–O 127.7(2),
Mo–P–C(1) 133.9(3), O–P–C(1) 97.6(3), Zr–O–P 163.6(3), O–Zr–C(2)
97.1(4), P–Mo–N(1) 101.6(2), P–Mo–N(2) 103.2(2), P–Mo–N(3) 101.7(2),
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from a 1:1 diethyl ether–heptane mixture as orange–brown
plates in 75.0% yield (Scheme 1). Diamagnetic 4 exhibits a
single 31P NMR signal at
d 284.4 {cf. d 312 for
Mo[P(C6H11)2]434}. A reasonable oxidation state assignment
for the Mo centre in 4 is +4. Proton NMR data for 4 are
indicative of a single N(R)Ar ligand environment on the NMR
timescale, although the solid-state structure of 4 reveals a low-
symmetry conformation. A doublet (2JPH 14 Hz) at d 1.535 is
attributed to the PMe moiety.
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The three-coordinate phosphorus in 4 was shown by X-ray
diffraction§
to
be
essentially
planar
[sum
of
angles = 359.2(3)°], consistent with Mo–P p bonding in the
complex (Fig. 2). The observed d(MoP) of 2.169(2) Å is also
consistent with a molybdenum–phosphorus multiple bond {cf.
d(MoP) of 2.265(2) Å for Mo[P(C6H11)2]434}. The conforma-
tion of 4 is noteworthy in that one of the N(R)Ar ligands is
oriented with its tert-butyl group distal from phosphorus,
presenting an Ar p cloud to the methyl substituent on
phosphorus. The Zr–O–P(Me)–Mo core in 4 is reminiscent of
Zr–O–C(RA)–M cores in zirconoxy carbenes prepared by
addition of Zr–RA (RA = H, alkyl) bonds across the CO unit in
metal carbonyl complexes.35
We thank Monsanto Company for support of this work, and
Michael K. Stern for helpful discussions. M. J. A. J. thanks
NSERC for a predoctoral fellowship.
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31 M. J. A. Johnson, A. L. Odom and C. C. Cummins, manuscript in
preparation.
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Scientific, Singapore, 1993, p. 105.
Footnotes and References
* E-mail: ccummins@mit.edu
† Alfred P. Sloan Fellow, 1997–2000.
‡ All manipulations were carried out under an atmosphere of dry nitrogen
using solvents purified by standard methods. The new compounds 2 and 4
gave satisfactory elemental analyses (C, H and N).
§ Crystallographic data: For 2: triclinic, space group P1, a = 10.7615(8),
b = 11.4942(9), c = 15.1899(11) Å, a = 89.865(2), b = 80.838(2),
g = 89.4900(10)°, U = 1854.9(2) Å3, Z = 2, 2988 unique reflections, data/
33 E. Samuel and M. D. Rausch, J. Am. Chem. Soc., 1973, 95, 6263.
34 R. T. Baker, P. J. Krusic, T. H. Tulip, J. C. Calabrese and S. S. Wreford,
J. Am. Chem. Soc., 1983, 105, 6763.
¯
35 P. T. Wolczanski and J. E. Bercaw, Acc. Chem. Res., 1980, 13, 121.
Received in Bloomington, IN, USA, 7th May 1997; 7/03105J
1524
Chem. Commun., 1997