The P1, C5, C4, and C3 atoms of the metallacyclic ring in
2a are coplanar (mean deviation from the least-squares plane
deshielded that that in the phosphametallacyclobutene spe-
cies [Mo2Cp{m-k1,h5:k1,h1-P(C5H4) CR = CR}(CO)2L] (R =
CO2Me; dP = À24 ppm),[7f] as expected from the different
number of atoms in the metallacycle. All these resonances,
however, are dramatically shielded relative to that of the
phosphinidene complex 1 (dP = 519.0 ppm).[17]
À
À
0.010 ꢀ) and the C3 C4 and C4 C5 separations (1.518(7) ꢀ
À
and 1.343(6) ꢀ) are appropriate for single and double C C
bonds respectively, with the latter value being slightly shorter
than the corresponding one within the four-membered metal-
lacycle of the complex [Mo2Cp{m-k1,h5:k1,h1-(C5H4)PC-
1
The identification of isomers and assignment of H and
(CO2Me) C(CO2Me)}(CO)2L] (1.352(4) ꢀ),[7f] and compara-
13C NMR resonances of compounds 2 to 5 in solution was
carried out using a combination of standard NOESY, DEPT,
and HSQC experiments (see the Supporting Information).
The acyl groups in compounds 2a,b give rise to highly
deshielded 13C NMR resonances in the range d = 263–
271 ppm, a position comparable to those of mononuclear
molybdenum complexes with acyl ligands.[18] A medium-
intensity IR band in the range 1500–1600 cmÀ1 was also
observed in these compounds, which can be assigned to the
=
=
ble to that in the mononuclear complex [MoCp{Ph2PCH
CPhC(O)}(CO)2] (1.337(4) ꢀ).[13] In compound 4, however,
À
the short C4 C6 distance is somewhat longer (1.388(5) ꢀ),
which suggests some resonance contribution from the exocy-
clic NHXyl group. In line with this result, the environment
À
around the N atom is planar, and the N1 C6 distance
À
(1.377(4) ꢀ) is shorter than the N1 C7 one (1.432(4) ꢀ),
2
À
although these values are in any case consistent with C(sp )
N single bonds (ca. 1.38 ꢀ).[14] Notably, the orientation of the
xylyl and aldehyde substituents allow a close approach of the
C(O)H proton to the centroid (X) of the xylyl ring, with
geometrical parameters (H5···X 2.767 ꢀ; C-H5···X 155.98],
corresponding C O stretch. For compounds 3a,b, the imi-
À
noacyl group gives rise to a less deshielded resonance at
around d = 206 ppm, which is close to the values of d =
220 ppm recently reported for the related phosphametalla-
which are indicative of a significant intramolecular C H···p
cycles in the niobium complexes [Nb(C5H4SiMe3)2{k1,h1-
À
interaction.[15] Moreover, the syn arrangement of the C5H5
and C5H4 rings in 4 also facilitates the pair wise self-assembly
of molecules in the crystal (Figure 4) held by strong inter-
molecular p···p stacking interactions,[16] with the closest inter-
ring distances of approximately 3.29 ꢀ being slightly shorter
than the interlaminar spacing in graphite (ca. 3.35 ꢀ).
Ph2PCR CR’C(NXyl)].
[19]
=
Compounds 4 and 5 share the presence of an exocyclic
NHXyl group, but with very different conformation. Thus, the
1
À
N H H NMR resonance appears at d = 6.40 ppm for 4, but at
d = 14.77 ppm for 5. This difference is interpreted as deriving
À
from a strong intramolecular N H···O hydrogen bonding with
the ketonic oxygen in the latter case. This interaction would
À
also facilitate some exocyclic delocalization of the double C
C bond of the metallacycle.
The formation of compounds 2 to 5 can be rationalized
through the reaction pathways depicted in Scheme 3. The first
step would be the addition of the carbonyl or isocyanide
ligand to the MoCp(CO)2 fragment to give intermediate A
having a pyramidal phosphinidene ligand. This would greatly
increase the nucleophilicity of the P atom, thus allowing it to
rapidly attack the more electrophilic carbon atom of the
alkyne or alkene at room temperature. Indeed, we have
shown recently that the isolable diiron complex [Fe2Cp2(m-
PCy)(m-CO)(CO)2] with a pyramidal phosphinidene bridge is
able to react with some alkynes and even CH2CHCO2Me at
room temperature, although quite slowly in the latter case.[7e,g]
Nucleophilic attack on the alkyne would lead to a zwitterionic
intermediate B with a carbanionic center placed at the right
position to attack the adjacent metal-bound CO or CNXyl
ligands, thus completing the closure of the phosphametalla-
cycle with acyl and iminoacyl functions, respectively. In the
isocyanide reaction we must remark that the cyclization step
takes place in a highly chemoselective way, since no acyl-type
product is formed. This result suggests that the evolution of
intermediate B might be charge-controlled, because the
ligand being the less powerful p acceptor (isocyanide, rather
than carbonyl in this case) is expected to bear a more positive
charge at the metal-bound carbon atom.[20] In the reaction
with olefins, however, the cyclization step does not lead to a
stable product but to an undetected intermediate C, which
would rapidly undergo [1,3]-H shift (comparable to the
imine–enamine tautomerization), presumably favored on
the “enamine” side thanks in part to the novel intramolecular
À
Figure 4. C H···p and p···p stacking interactions in compound 4.
In solution, compounds 2a and 2b exist as mixtures of the
corresponding syn (minor) and anti (major) isomers, and the
equilibrium ratio is somewhat solvent-dependent. In contrast,
the syn isomer is the unique or very major species present in
the solutions of compounds 3 to 5, as found for 4 in the crystal.
Although we have not analyzed the syn/anti isomerism in
detail, it likely implies a transient cleavage of the P MoCp
bond, which allows the required rotation of the MoCp(CO)2
fragment.
À
The phosphametallacyclopentene moieties in compounds
2 to 5 give rise to diagnostic upfield 31P{1H} NMR resonances
in the range d = 60–80 ppm, which is substantially more
Angew. Chem. Int. Ed. 2011, 50, 6383 –6387
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
6385