Facile insertion of terminal acetylenes into the RuII-NR 2 bond of a 14-valence-electron complex

Article abstract of DOI:10.1021/om049435b

The reaction of alkynes RC≡CH (R = H, Ph) with (PNP)RuCl, where PNP is (^tBu₂PCH₂SiMe₂)₂N, occurs rapidly below 23°C to give first an η²-alkyne adduct and then a final product with a vinylidene group, C=CHR, inserted into the N-Ru bond. Characterization included X-ray diffraction (R = Ph) and DFT calculations to probe mechanistic aspects of the reaction.

Full text of DOI:10.1021/om049435b

4814
Organometallics 2004, 23, 4814-4816
F a cile In ser tion of Ter m in a l Acetylen es in to th e
Ru II-NR2 Bon d of a 14-Va len ce-Electr on Com p lex
Amy N. Walstrom, Lori A. Watson, Maren Pink, and Kenneth G. Caulton*
Department of Chemistry and Molecular Structure Center, Indiana University,
Bloomington, Indiana 47405
Received J uly 23, 2004
Summary: The reaction of alkynes RCtCH (R ) H, Ph)
with (PNP)RuCl, where PNP is (tBu2PCH2SiMe2)2N,
occurs rapidly below 23 °C to give first an η2-alkyne
adduct and then a final product with a vinylidene group,
CdCHR, inserted into the N-Ru bond. Characterization
included X-ray diffraction (R ) Ph) and DFT calcula-
tions to probe mechanistic aspects of the reaction.
amide N provides a viable intermediate for vinylidene
formation. The formation of an N-C(vinyl) bond here
is contrasted with earlier stoichiometric and catalytic
examples, the latter involving electron-poor metal cen-
ters.
When yellow (PNP)RuCl in d8-toluene is combined
1
with excess HCCH at -196 °C and H and 31P{1H} NMR
The molecule [(tBu2PCH2SiMe2)2N]RuCl, (PNP)RuCl,
is of interest because of its low coordination number (4),
its planar structure, and its triplet ground state for the
d6 configuration of Ru(II).1 Its high degree of unsatura-
tion, a 14-valence-electron configuration, could give it
special chemical reactivity, since it would appear to be
suited for simple addition of 4-electron-donor ligands.
Among those, we have chosen to explore terminal
alkynes as a compact way to deliver 4 electrons (A). This
spectra are recorded in 20 °C increments beginning at
-60 °C, the spectra can be summarized by eq 1 (showing
δ(31P)). Conversion to 1 is complete at -60 °C and is
fast
(PNP)RuCl + HCtCH y K z 1 (26.0 ppm) slower8
2 (50.0 ppm) (1)
visible by a color change to deep purple-red. The signals
of 2 grow only above -40 °C, but at -20 °C, some
reversion from 1 to (PNP)RuCl is evident, due to the
changing equilibrium constant K. At +20 °C, all (PNP)-
RuCl has been consumed and 2 is the dominant product,
forming a green solution. Both species 1 and 2 have only
mirror symmetry: two equivalent P atoms, two types
of tBu groups, and two types of SiMe groups. Species 2
has inequivalent acetylene-derived hydrogens (5.1 and
1
4.7 ppm). Species 1 has a two-proton H NMR signal at
-40 °C, but this broadens in apparent decoalescence at
-60 °C. Species 2 shows 13C{1H} NMR signals at 168.3
(CR) and 99.4 (Câ) ppm for the acetylene-derived carbons,
neither of which is sufficiently positive to establish an
RudCdCH2 structure.5,6
offers as additional possible products the vinylidene B
and, given the reducing power of the metal due to the
π-donor amide ligand in PNP, the oxidative addition
product C. The pioneering studies of the Fryzuk group2
and others3 have shown the potential for H migration
from a metal to the lone pair present on the amide
nitrogen, which thus makes D another product for
consideration. Overlaid on all of the above is the
question4 of possible “spin forbiddenness” of the reac-
tion: will the change from triplet reactant to singlet
product detectably retard the rate of reaction? Here we
show (1) facile (time of mixing at -60 °C) binding of
RCCH by (PNP)RuCl, (2) facile rearrangement to a
diamagnetic product of H migration to form a vinylidene
group, (3) near-thermoneutral insertion of the vi-
nylidene into the Ru-amide bond vs retention of a
terminal vinylidene on Ru, and (4) DFT calculations
which reveal that proton transfer from acetylene to
Equimolar (PNP)RuCl and PhCtCH react7 in ben-
zene at 22 °C with an immediate color change to red
and complete consumption of (PNP)RuCl to form 2P h
.
A strong 31P{1H} NMR singlet at 50.8 ppm (cf. eq 1) is
accompanied by a weaker peak (4:1 intensity ratio) at
1
46.4 ppm. The H NMR of the more populated species
(2) (a) Fryzuk, M. D.; Montgomery, C. D.; Rettig, S. J . Organome-
tallics 1991, 10, 467. (b) Fryzuk, M. D.; Huang, L.; McManus, N. T.;
Paglia, P.; Rettig, S. J .; White, G. S. Organometallics 1992, 11, 2979.
(c) Fryzuk, M. D.; Gao, X.; J oshi, K.; MacNeil, P. A.; Massey, R. L. J .
Am. Chem. Soc. 1993, 115, 10581. (d) Fryzuk, M. D.; Gao, X.; Rettig,
S. J . J . Am. Chem. Soc. 1995, 117, 3106. (e) Fryzuk, M. D.; MacNeil,
P. A. Organometallics 1983, 2, 682. (f) Fryzuk, M. D.; MacNeil, P. A.;
Rettig, S. J . Organometallics 1985, 4, 1145.
(3) (a) Watson, L. A.; Coalter, J . N.; Ozerov, O. V.; Pink, M.;
Huffman, J . C.; Caulton, K. G. New J . Chem. 2003, 27, 263. (b) Watson,
L. A.; Caulton, K. G. Mol. Phys. 2002, 100, 385. (c) Ozerov, O. V.;
Huffman, J . C.; Watson, L. A.; Caulton, K. G. Organometallics 2003,
22, 2539.
(4) (a) Harvey, J . N.; Poli, R. Dalton 2003, 4100. (b) Harvey, J . N.;
Poli, R.; Smith, K. M. Coord. Chem. Rev. 2003, 238-239, 347. (c)
J ensen, V. R.; Poli, R. J . Phys. Chem. A 2003, 107, 1424.
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* To whom correspondence should be addressed. E-mail:
caulton@indiana.edu.
(1) Watson, L. A.; Ozerov, O. V.; Pink, M.; Caulton, K. G. J . Am.
Chem. Soc. 2003, 125, 8426.
(6) Bianchini, C.; Casares, J . A.; Peruzzini, M.; Romerosa, A.;
Zanobini, F. J . Am. Chem. Soc. 1996, 118, 4585.
10.1021/om049435b CCC: $27.50 © 2004 American Chemical Society
Publication on Web 09/15/2004

Communications
Organometallics, Vol. 23, No. 21, 2004 4815
F igu r e 2. DFT(B3PW91) optimized geometries and ener-
gies (E + ZPE, kcal/mol) of isomers of (H2PCH2SiH2)2-
NRuClC2H2.
F igu r e 1. ORTEP view (50% probability ellipsoids) of the
non-hydrogen atoms of [(tBu2PCH2SiMe2)2NCCHPh]RuCl,
showing selected atom labeling. Unlabeled atoms are
carbon. Notable structural parameters (distances in Å and
angles in deg): Ru1-C23, 1.903(8); Ru1-N1, 2.270(6);
Ru1-Cl1, 2.379(2); C23-C24, 1.347(10); N1-Ru1-Cl1,
168.88(18); C23-C24-C25, 126.7(7); N1-C23-Ru1, 83.9(4).
tertiary amine nitrogen is essentially planar, with its
lone pair very poorly oriented to bind to Ru, but the
Ru-N distance, 2.270(6) Å, is bonding.
The results of DFT(B3PW91) calculations8 of the
model [(H2PCH2SiH2)2N]RuClC2H2 shown in Figure 2
reveal that the vinylidene isomer B is more stable than
both the η2-acetylene complex A (by 21 kcal/mol) and
the C-H oxidative addition product C (by 40.1 kcal/
mol). The (observed) insertion product is calculated to
be only 1.6 kcal/mol more stable than the vinylidene and
has an Ru-N distance of 2.26 Å. The structure of the
η2-acetylene product A has HCCH lying in the mirror
plane, not perpendicular to it; this product conformation
also minimizes steric repulsion between alkyne and the
tBu groups during adduct formation. We propose this
as the identity of 1. The proton-transfer intermediate
D is indeed a minimum, and the structure shows signs
of hydrogen bonding from NH to the alkyne π density,12
which could be a mechanistic step in the facile conver-
sion (see above) to inserted vinylidene; it would give the
observed stereoisomer of phenyl anti to N in 2P h . In
contrast, the complex Ru(NH2)H(Me2PC2H4PMe2)2 re-
acts13 with PhCtCH by proton transfer to an evidently
very Brønsted basic amide nitrogen, to form Ru(CCPh)H-
(Me2PC2H4PMe2)2 and release NH3.
t
has, like 2 above, two Bu and two SiMe signals. The
13C{1H} NMR spectrum of 2P h shows signals at 170.4
(CR) and 113.6 (Câ) ppm for the acetylene-derived
carbons. A crystal of 2P h , grown from benzene, was
shown by X-ray diffraction8 to have the structure in
Figure 1. The structure has an aminovinyl ligand η2
coordinated to Ru via N and one carbon. This is a
product of C-N bond formation, and the C-C π bond
in the vinyl group does not donate to Ru, leaving the
metal still unsaturated (16 electrons). The hydrogen
migration needed to convert an alkyne to a vinylidene
has been shown, both experimentally9 and computa-
tionally,10 to often be a high activation energy process,
and the rate observed here at -20 °C is unusually fast.
t
The Bu groups anti to C23 do not have agostic interac-
tions with Ru, nor does the ortho C26-H.11 The three
nonmetal atom angles at N1 total 358.9°; therefore, this
(7) Rea ction of (P NP tBu )Ru Cl w ith p h en yla cetylen e: to 10.7
mg (0.171 mmol) of (PNPtBu)RuCl in C6D6 was added 1.88 µL of
PhCCH (C8H6, 1 equiv). Upon addition of phenylacetylene at 22 °C
there was a rapid, distinct color change of the solution from yellow to
deep reddish brown. 1H NMR (400 MHz, C6D6): δ 7.4-7.3 (m, PhCCH),
7.2-7.0 (m, PhCCH), 6.9 (m, PhHCCRuN), 6.27 (s, 1H, PhHCCRuN),
2.71 (s, 1H, free PhCCH), 1.48 (t, J P-H ) 8 Hz, >18H, PCMe3), 1.37 (t,
J P-H ) 8 Hz, >18H, PCMe3), 0.89 (t, J P-H ) 3.7 Hz, 2H, SiCH2P of
major isomer), 0.82 (t, J P-H ) 3.7 Hz, 2H, SiCH2P of major isomer),
Vinylidene insertion into the M-amide (i.e. pyrrole)
bond of FeII(CCHR)(porphyrin)14 occurs only after one-
electron oxidation and coordination of a nucleophile to
MIII in the insertion product obtained. Moreover, since
the nitrogen lone pair in the product is involved with
the porphyrin π-system, there is no NfM bond. In
summary, these precedents illustrate C-N bond forma-
tion when the metal is electrophilic. The analogous
vinylidene insertion into a metal-acetylide bond is
thekey C-C bond-forming event in dimerization of
0.49 (t, J P-H ) 2.5 Hz, <2H, SiCH2P of minor isomer), 0.42 (t, J P-H
)
2.5 Hz, 2H, SiCH2P of minor isomer), 0.40 (s, <6H, SiMe, minor
isomer), 0.33 (s, <6H, SiMe, minor isomer), 0.27 (s, 6H, SiMe, major
isomer), 0.031 (s, 6H, SiMe, major isomer). 31P{1H} NMR (162 MHz,
C6D6): δ 50.8 (s, major isomer), 46.1 (s, minor isomer). 13C{1H} NMR
(101 MHz, C6D6): δ 170.4 (t, J C-P ) 9.2 Hz, PhHCCRuN), 139.4 (s,
PhHCCRuN), 126.9 (s, PhHCCRuN), 125.2 (s, PhHCCRuN), 124.1 (s,
PhHCCRuN), 123.6 (s, PhHCCRuN), 118.3 (s, PhHCCRuN), 113.6 (t,
J C-P ) 5.3 Hz, PhHCCRuN), 37.3 (t, J C-P ) 6.8 Hz, PCMe3, minor
isomer), 36.5 (t, J C-P ) 6.9 Hz, PCMe3, minor isomer), 36.1 (t, J C-P
)
5.5 Hz, PCMe3, major isomer), 35.0 (t, J C-P ) 5.5 Hz, PCMe3, major
isomer), 31.5 (t, J C-P ) 2.8 Hz, PCMe3), 30.3 (t, J C-P ) 2.7 Hz, PCMe3),
29.7 (s), 26.4 (br s), 22.5 (s), 11.0 (s, SiMe, minor isomer), 8.8 (s, SiMe,
minor isomer), 7.1 (s, SiMe, major isomer), 5.2 (s, SiMe, major isomer),
2.53 (t, J C-P ) 4 Hz, SiCH2P).
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(8) See the Supporting Information.
(9) (a) Ipaktschi, J .; Mohsseni-ala, J .; Uhlig, S. Eur. J . Chem. 2003,
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4816 Organometallics, Vol. 23, No. 21, 2004
Communications
acetylenes to enynes (eq 2).5,15 Examples of a vinylidene
unactivated alkyne couple in the slow step (eq 4); there,
a vinylidene intermediate is not involved.
group inserting into an M-phenyl bond of a pincer
ligand (eq 3) have been established,16 and certain
Finally, any rate reduction due to the “spin forbidden”
character of this reaction is not relevant, on the time
scale of τ1/2 > 1 min at -60 °C.
Ack n ow led gm en t. This work was supported by the
National Science Foundation. L.A.W. is holder of the
Indiana University Wells Fellowship.
examples show some unconventional interaction of the
ipso carbon with the metal. The difference is that the
nitrogen lone pair in a metal amide can confer extra
stability to the product. Especially interesting in the
aminovinyl isomer is the presence of an N-Ru interac-
tion (E), because an M-N bond is absent in these
Su p p or t in g In for m a t ion Ava ila b le: Full crystallo-
graphic details (CIF file) and tables giving details of the
ground-state geometries from DFT calculations. This material
is available free of charge via the Internet at http://pubs.acs.org.
OM049435B
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