Synthesis of the Five-Coordinate Ruthenium (II) Complexes [(PCP)Ru(CO)(L)][BAr'4] {PCP = 2,6-(CH2PtBu2)2 C6H3, BAr'4 = 3,5-(CF3)2C6H3, L= ɳ1-CICH2CI, ɳ 1-N2, or μ-Cl-Ru(PCP)(CO)}: Reactions with Phenyldiazomethane and Phenylacetylene Page: 8,387
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Synthesis of Five-Coordinate Ru(II) Complexes
Jia et al. and Gusev et al. have reported similar coupling
reactions of terminal acetylenes with ruthenium pincer
complexes.77-79 However, reaction of terminal acetylenes
with osmium derivatives resulted in isolable vinylidene
complexes.79,80 For the reaction of (PCP-Ph)Ru(PPh3)(Cl)
(PCP-Ph = 2,6-(CH2PPh2)2C6H3) with PhC--CH, Jia et al.
discussed the mechanism as proceeding via alkyne coordina-
tion, transformation to a vinylidene complex followed by
the C-C coupling step to form the product. Neither the
proposed alkyne-coordinated intermediate nor the vinylidene
complex has been observed.77'78 In contrast, an intermediate
Ru complex is observed for the reaction of 3 with PhC=CH
to form 8, and likely identities of this system are the 2-
alkyne complex [(PCP)Ru(CO)(Q2-PhCCH)] [BAr'4] or the
vinylidene complex [(PCP)(CO)Ru=C=CHPh] [BAr'4]. The
observed intermediate is unlikely a Ru hydride-alkynyl
complex, since the anticipated upfield resonance due to a
hydrido ligand is not observed. No resonance downfield of
200 ppm, except a triplet at 208.5 ppm due to CO, is
observed in the 13C NMR spectrum, which provides evidence
against the presence of a vinylidene intermediate; however,
due to likely C-P coupling and relaxation, the intensity of
this resonance is anticipated to be weak and difficult to
observe. Thus, the absence of an assignable vinylidene
carbon resonance is not sufficient evidence to conclusively
assign the identity of the intermediate. On the basis of DFT
calculations (see below), we suggest that the intermediate is
the vinylidene complex [(PCP)(CO)Ru=C=CHPh] [BAr'4];
however, the inability to cleanly isolate and grow crystals
of this system precludes a definitive conclusion based solely
on experimental data.
The reactions of phenylacetylene with five-coordinate
complexes 1 or 6 were studied to determine if the weakly
bound CH2C12 of 3 is necessary to observe the formation of
8. Reaction of triflate complex 6 with PhC=CH yields
similar results as observed for complex 3, including the
formation of an intermediate and the eventual conversion to
the final coupling product, 8, albeit the total reaction time is
-10 days at room temperature compared to -3 days starting
from complex 3. In contrast, there is no observable reaction
between chloride complex 1 and PhC=CH at room temper-
ature after 7 days.
DFT Calculations on the Formation of Complexes 7
and 8. The formation of complexes 7 and 8 was probed using
similar computational methodologies to those described
above. Given the difficulties in isolating appropriate transition
states for carbene (and vinylidene) insertion, the correspond-
ing DFT calculations on truncated PCP' models (vide supra)
were also performed. Calculations support the experimental
inference about the intermediacy of terminal [(PCP)(CO)-
Ru=(C)o,1=CHPh] complexes, as both species are found
to be stable minima. Furthermore, the insertion reactions are
(77) Lee, H. M.; Yao, J.; Jia, G. Organometallics 1997, 16, 3927.
(78) Jia, G.; Lee, H. M.; Xia, H. P.; Williams, I. D. Organometallics 1996,
15, 5453.
(79) Gusev, D. G.; Maxwell, T.; Dolgushin, F. M.; Lyssenko, M.; Lough,A. J. Organometallics 2002, 21, 1095-1100.
(80) Wen, T. B.; Cheung, Y. K.; Yao, J.; Wong, W.-T.; Zhou, Z. Y.; Jia,
G. Organometallics 2000, 19, 3803-3809.P P
SCO P co
Ru , Ph -8.3 kcal/mol Ru
IP C_,Ph c Ph
P i
H p
(7) H
P CO 7+ P
I -26.6 kcal/mol I co
Ru Ru
S C, HC
Ph -H
(8) phFigure 7. Calculated ONIOM reaction energies in kcal/mol.
T-
Figure 8. Calculated structure of the product from insertion of the alkyne
into the PCP ligand starting from [(PCP)(CO)Ru(r2-HC-CPh)] . Some
atoms of the PCP ligand are shown in wire frame for clarity.
found to be thermodynamically feasible: AE = -8 kcal/
mol for =C(H)Ph insertion to form 7 and -27 kcal/mol for
=C=C(H)Ph insertion to form 8 (Figure 7). The correspond-
ing enthalpy values for truncated PCP' models are -17 kcal/
mol (carbene insertion) and -22 kcal/mol (vinylidene
insertion). These QM energetics on small models, combined
with the analysis of the QM portion of the QM/MM
extrapolated energies, suggest that the difference in the
energetics of carbene and vinylidene has both electronic and
steric components. Interestingly, the carbene insertion is
retarded (-4-5 kcal/mol) by steric factors, while the
vinylidene insertion is facilitated by a comparable amount
by steric factors. Presumably, this is a reflection of the extra
carbon atom of the vinylidene minimizing steric hindrance
between the phenyl substituent and the phosphine tert-butyl
groups of complex 8 as compared with complex 7.
A terminal alkyne complex as a possible intermediate in
the formation of 8 was probed through QM/MM calculations.
Construction of [(PCP)(CO)Ru(Q2-HC=CPh)]+ (alkyne in
the equatorial plane) followed by geometry optimization
yielded a high energy intermediate (ca. 13 kcal/mol above
8) corresponding to the insertion of the alkyne triple bond
into the Ru-Cipso of the PCP ligand (Figure 8). Model
calculations on [(PCP')(CO)Ru(Q2-HC=CH)]+ yielded a
stationary point, but this species was a transition state with
the imaginary frequency corresponding to the rotation of the
acetylene ligand to a conformation with the C=C bond
perpendicular to the equatorial plane, which is 23 kcal/molInorganic Chemistry, Vol. 44, No. 23, 2005 8387
-I-
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Zhang, Jubo; Barakat, Khaldoon A.; Cundari, Thomas R., 1964-; Gunnoe, T. Brent; Boyle, Paul D.; Petersen, Jeffrey L. et al. Synthesis of the Five-Coordinate Ruthenium (II) Complexes [(PCP)Ru(CO)(L)][BAr'4] {PCP = 2,6-(CH2PtBu2)2 C6H3, BAr'4 = 3,5-(CF3)2C6H3, L= ɳ1-CICH2CI, ɳ 1-N2, or μ-Cl-Ru(PCP)(CO)}: Reactions with Phenyldiazomethane and Phenylacetylene, article, October 6, 2005; [Washington, D.C.]. (https://digital.library.unt.edu/ark:/67531/metadc75424/m1/9/: accessed May 26, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT College of Arts and Sciences.