C O M M U N I C A T I O N S
with toluene to form the 5.6 kcal mol-1-favored [(η2-Tol)(η6-
C6H6)RudPH] as an initial adduct in the exchange of the two arene
ligands. Associative ring slippage16 via [(η4-Tol)(η4-C6H6)RudPH]
then gives [(η6-Tol)(η2-C6H6)RudPH] (∆E ) 1.4 kcal mol-1),
which requires 4.2 kcal mol-1 to lose benzene and form the product.
Implicitly, this process supports a 16-electron intermediate that
undergoes ligand exchange of aromatic molecules (8a f 8b) before
being captured by the carbene ligand to give 7b.
Catalyst tuning is generally sought via a change of ligands
because their effect is considered to be constant for a given
transition-metal complex. We have now demonstrated that the
relative σ-donor/π-acceptor ability of NHC ligands can easily be
influenced by a simple substituent-controlled conformational change.
The sterically imposed ligand rotation of the NHC fragment in 1
enhances its reactivity and thereby facilitates the synthesis of
phosphaalkene (PdC) building blocks.
-2.27 eV, E(dyz) ) -2.49 eV] are higher in energy than those of
the Ru-phosphine fragment [E(dxz) ) -2.61 eV, E(dyz) ) -2.86
eV]. RudP bond formation causes a transfer of charge from [(η6-
3
C6H6)(L)Ru] to the PPh fragment [E(px) ) -4.59 eV, E(py) )
-4.86 eV], which is largest for 1′-σ. Whereas the RudP bonds
are of similar lengths (2.216 and 2.209 Å for 1′-σ and 2′,
respectively), the polarity varies with the phosphorus atom, which
carries more charge in 1′-σ (-0.113e) than in 2′ (-0.086e).
The greater RudP bond polarity is reflected in the enhanced
reactivity of the NHC-containing phosphinidene 1 (L ) IiPr2Me2)
over that of “first-generation” 2 (L ) Ph3P) toward diiodomethane
(eq 2):11
Acknowledgment. This work was partially supported by the
Council for Chemical Sciences of The Netherlands Organization
for Scientific Research (NWO/CW). The assistance from Dr. F. J. J.
de Kanter (NMR), Dr. M. Smoluch (HR EI-MS), and J. W. H.
Peeters (HR FAB-MS; University of Amsterdam) is gratefully
acknowledged.
31P NMR monitoring of the reaction of complex 1 showed the
quantitative formation of the phosphaalkene H2CdPMes* (6, 94%
isolated yield) within 1 min at 20 °C [t1/2(0 °C, C6D6) ) 22 min;
5 equiv of CH2I2]. In contrast, the reaction of phosphine-ligated
complex 2 with CH2I2 is much slower [t1/2(20 °C, toluene) ) 60
min; t1/2(0 °C, C6D6) ) 925 min] and also less selective (6, 45%).
This difference between 1 and 2 demonstrates that like the catalytic
activity of the Grubbs catalysts, the reactivity of the isolobal
nucleophilic 18-electron phosphinidene complexes can also be
readily modified by changing the ancillary ligands. The applicability
of the illustrated reaction is underscored by the quantitative
regeneration of 1 from the transition-metal byproduct [(η6-
C6H6)(IiPr2Me2)RuI2] (4) with DBU and H2PMes*12 as determined
by 31P NMR (63% isolated yield), thereby demonstrating that
ruthenium phosphinidene complexes are viable reagents for the
synthesis of phosphaalkenes.
A final aspect to address is the presumed 16-electron phosphin-
idene intermediate 3, which could not be detected by 31P NMR
spectroscopy,13,14 suggesting that if it is indeed formed, it is readily
captured by IiPr2Me2 to yield 1. Increasing the steric bulk by using
1,3-dimesitylimidazol-2-ylidene (IMes) to slow the NHC complex-
ation enough for detection was unsuccessful, but monitoring its
ligation with less crowded [(η6-C6H6)RuCl2(PH2Mes)], which carries
a Mes instead of a Mes* substituent, did have the anticipated effect.
Besides dark-brown crystalline [(η6-C6H6)(IMes)RudPMes] (7a;
31P, 752.5 ppm; 65%), small amounts of the corresponding toluene
adduct [(η6-Tol)(IMes)RudPMes] (7b; 31P, 736.8 ppm; 3%) were
also observed (eqs 3 and 4):15
Supporting Information Available: Full experimental and com-
putational details and crystallographic data for compound 1 (CIF). This
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