COMMUNICATION
Scheme 1
Figure 1. Molecular structure of 2a. Hydrogen atoms are omitted for
clarity. Bond distances (Å) and angles (deg): Mo-Cl(1), 2.4452(6);
Mo-Cl(2), 2.447(1); Mo-P(1), 2.4335(8); Mo-P(2), 2.4128(7); Mo-P(3),
2.3792(7); Mo-P(4), 2.4499(5); Cl(1)-Mo-Cl(2), 81.14(2); Cl(1)-Mo-
P(1), 86.02(2); Cl(1)-Mo-P(2), 88.46(2); Cl(2)-Mo-P(3), 88.00(2);
Cl(2)-Mo-P(4), 80.54(2); P(1)-Mo-P(2), 73.16(2); P(2)-Mo-P(3),
73.30(2); P(3)-Mo-P(4), 74.32(2); P(1)-Mo-P(4), 90.20(2).
also to be noted that these trans complexes are paramagnetic
due to the high-spin d4 electron configuration of such high-
symmetry structures. The presence of three consecutive
strained chelate rings is the likely reason why complexes of
structure type 2 disfavor an octahedral geometry of trans or
even cis configuration. In the solid-state structure of 2a, the
P-Mo-P angles in the three five-membered chelate rings
fall in the range 73-75°, and the sum of these values at
∼221° is much smaller than 270° required to locate all four
P atoms at the equatorial sites in the idealized octahedron.
Even the meridional arrangement of two adjacent chelate
rings results in a distortion between the mutually trans P
atoms, as observed in the octahedral complex 1 (see the
artwork in eq 1), in which the trans P-M-P angles within
P4 are at 152.63(9)° and 152.55(7)° for M ) Mo and W,
respectively.4 Therefore, it is likely that the tetradentate P4
ligand determines the geometry of 2 not electronically but
by its intrinsic steric constraints.
By X-ray crystallography, the structure of 2a has been
determined unequivocally as shown in Figure 1. The Mo
center in 2a has a trigonal-prismatic geometry, in which one
of the rectangular faces is capped by the tetradentate P4
ligand. The three edges of the prism are defined by the pairs
of the outer P atoms (P(1) and P(4)), the inner P atoms (P(2)
and P(3)), and the chloride ligands. The twist angles φ are
only 2°, 4°, and 4° with respect to these edges, respectively,
where the values of φ have been defined previously by Stiefel
and co-worker with the limiting values being 0° and 60° for
trigonal-prismatic and octahedral structures.7 The dihedral
angle between two basal triangles is 12°. Well-resolved NMR
signals indicate that all complexes of 2 are diamagnetic, and
the 31P{1H} NMR spectra of these complexes, which exhibit
two signals each assignable to the outer and the inner P
nuclei, confirm the Cs symmetry.
Most reactions of trans-[MX2(tP)4] are proposed to be
initiated by dissociation of phosphine or, less commonly,
the X- ligands,2 and therefore, the complexes with bidentate
phosphine ligands such as trans-[MX2(dppe)2] are less
reactive than monophosphine complexes. In the case of 2,
the 16-electron metal center is not as sterically protected and
directly accessible, since the P4 ligand covers only one
hemisphere of the metal. Thus, reactions of 2 with ethyl
diazoacetate rapidly occurred at room temperature to form
unstable complexes, which spontaneously decomposed to
give a mixture of diethyl maleate and diethyl fumarate. When
a 10-fold molar amount of ethyl diazoacetate was treated
with 2a, a total of ∼2.8 mol (per mol of 2a) of these
carbenoid-coupling products was detected in a molar ratio
of ca. 2:3. To our knowledge, Mo complexes that can
catalyze the carbenoid coupling of diazo compounds are quite
rare.10 Indeed, trans-[MoCl2(dppe)2] did not react with ethyl
diazoacetate. Since neither the detection of any intermediary
It is noteworthy that trigonal-prismatic structure is uncom-
mon for six-coordinate Mo(II) and W(II) complexes.8 All
the other structurally determined complexes of the type
[MX2(tP)4] (M ) Mo, W; X ) halogens; tP ) tertiary
phosphine ligand) adopt a trans octahedral geometry.9 It is
(6) The following method was used to synthesize 2a. To a red suspension
of 1a (2.51 g, 1.94 mmol) in benzene (230 mL) was added PhCH2Cl
(0.70 mL, 6.1 mmol). After the mixture had been stirred in the dark
at room temperature for 48 h, deposited green microcrystals of 2a
were filterd off, washed with benzene and diethyl ether, and dried in
vacuo to give 1.25 g (73% yield). 1H NMR (CD2Cl2): δ 2.2-2.5 (m,
4H, PCH2), 2.8-3.0, 3.45-3.65 (m, 2H each, PCH2), 6.15-7.75 (m,
34H, aromatic). 31P{1H} NMR (CD2Cl2): δ 101.8, 141.9 (AA′XX′
pattern: JAX + JAX′ ) 56 Hz). Complexes 2a′ and 2b are prepared
from the cooresponding 1 and PhCH2Br in an analogous manner. 2a′:
82% yield. 1H NMR (CD2Cl2): δ 2.2-2.4, 2.55-2.75, 3.0-3.2, 4.05-
4.25 (m, 2H each, PCH2), 6.0-7.75 (m, 34H, aromatic). 31P{1H} NMR
(CD2Cl2): δ 98.7, 142.0 (AA′XX′ pattern: JAX + JAX′ ) 53 Hz). 2b:
42% yield. 1H NMR (CD2Cl2): δ 1.9-2.1, 2.25-2.5, 2.55-2.8, 3.25-
3.45 (m, 2H each, PCH2), 6.3-7.8 (m, 34H, aromatic). 31P{1H} NMR
(CD2Cl2): δ 55.6 (br with 183W satellites, JPW ) 246 Hz), 105.1 (br
with 183W satellites, JPW ) 258 Hz).
(7) Stiefel, E. I.; Brown, G. F. Inorg. Chem. 1972, 11, 434.
(8) (a) Templeton, J. L.; Ward, B. C. J. Am. Chem. Soc. 1980, 102, 6568.
(b) Burrow, T. E.; Hughes, D. L.; Lough, A. J.; Maguire, M. J.; Morris,
R. H.; Richards, R. L. J. Chem. Soc., Dalton Trans. 1995, 1315. (c)
Baker, P. K.; Drew, M. G. B.; Parker, E. E.; Robertson, N.; Underhill,
A. E. J. Chem. Soc., Dalton Trans. 1997, 1429. (d) Fomitchev, D. V.;
Lim, B. S.; Holm, R. H. Inorg. Chem. 2001, 40, 645.
(9) (a) Carmona, E.; Mar´ın, J. M.; Poveda, M. L.; Atwood, J. L.; Rogers,
R. D. Polyhedron 1983, 2, 185. (b) Bakir, M.; Cotton, F. A.; Cudahy,
M. M.; Simpson, C. Q.; Smith, T. J.; Vogel, E. F.; Walton, R. A.
Inorg. Chem. 1988, 27, 2608. (c) Pietsch, B.; Dahlenburg, L. Inorg.
Chim. Acta 1988, 145, 195. (d) Filippou, A. C.; Schnakenburg, G.;
Philippopoulos, A. I. Acta Crystallogr. 2003, E59, m602.
Inorganic Chemistry, Vol. 46, No. 12, 2007 4785