Dehydrohalogenation by a germylene: Conversion of carbene ligands into carbynes at ruthenium

Article abstract of DOI:10.1021/om0508482

The dialkylgermylene [Ge(CH[SiMe₃]₂)₂] (3) converts the carbene complexes [Ru(CHR)(PCy₃)₂Cl ₂] (R = p-C₆H₄Me [2], n-C₄H ₉ [6]) into the corresponding square-planar carbyne complexes [Ru(CR)(PCy₃)₂Cl] (R = p-C₆H₄Me [1], n-C₄H₉ [7]) cleanly within minutes at 25°C in benzene; addition of HCl to a benzene solution of 1 results in quantitative re-formation of 2. Replacement of chloride in 1 affords other halide and pseudohalide complexes, [Ru[C-p-C₆H₄Me)(PCy ₃)₂X] (X = p-OC₆H₄-t-Bu, Br, F, O₃SCF₃), cleanly.

Full text of DOI:10.1021/om0508482

6074
Organometallics 2005, 24, 6074-6076
Communications
Dehydrohalogenation by a Germylene: Conversion of
Carbene Ligands into Carbynes at Ruthenium
Stephen R. Caskey, Michael H. Stewart, Yi Joon Ahn, Marc J. A. Johnson,* and
Jeff W. Kampf
Department of Chemistry, University of Michigan, 930 North University Avenue,
Ann Arbor, Michigan 48109-1055
Received October 3, 2005
Summary: The dialkylgermylene [Ge(CH[SiMe3]2)2] (3)
converts the carbene complexes [Ru(CHR)(PCy3)2Cl2] (R
) p-C6H4Me [2], n-C4H9 [6]) into the corresponding
square-planar carbyne complexes [Ru(CR)(PCy3)2Cl] (R
) p-C6H4Me [1], n-C4H9 [7]) cleanly within minutes at
25 °C in benzene; addition of HCl to a benzene solution
of 1 results in quantitative re-formation of 2. Replace-
ment of chloride in 1 affords other halide and pseudoha-
lide complexes, [Ru(C-p-C6H4Me)(PCy3)2X] (X ) p-OC6H4-
t-Bu, Br, F, O3SCF3), cleanly.
(CPh)(OPh)(PR3)2] (R ) i-Pr, Cy) by treatment of
[Ru(CHPh)(PR3)2Cl2] with excess NaOPh.7 The product
selectivity appears to be driven by steric interactions
at an intermediate stage.13 In some cases, four-coordi-
nate carbene complexes of the form [Ru(CHPh)(PR3)-
(OR)2] are formed instead by loss of phosphine rather
than alcohol.7,8 We report herein rapid and reliable
syntheses of a family of Ru-carbyne complexes of the
form [Ru(CR)(PCy3)2X] (R ) alkyl, aryl; X ) Cl, F, Br,
p-OC6H4C-t-Bu). The parent compound in this series,
[Ru(C-p-C6H4Me)(PCy3)2Cl] (1), is formed rapidly upon
treatment of the Grubbs-type olefin metathesis catalyst
[Ru(CH-p-C6H4Me)(PCy3)2Cl2] (2)14 with a bulky dialkyl-
germylene, [Ge(CH[SiMe3]2)2] (3).15 Subsequently, the
other complexes in the series are readily prepared by
substitution of the chloride in 1 (Scheme 1). For
synthesis of 1 on a multigram scale, a two-step process
involving the synthesis of the aryloxide complex [Ru-
(C-p-C6H4Me)(OC6H4-p-t-Bu)(PCy3)2] (4), followed by its
conversion into 1 using SnCl2, is more economical.
Compound 3 has been used recently to achieve C-H
activation of alkanes, ethers, and ketones;16,17 under
some conditions, net HCl addition to 3 occurs, yielding
[GeHCl(CH[SiMe3]2)2] (5).
The dehydrochlorination reaction proceeds smoothly
to completion over the course of 20 min under typical
conditions. The expected germanium byproduct, 5, was
identified in the reaction mixture by its 1H NMR
spectrum but was not isolated.17 Following removal of
volatile materials in vacuo, blue-green 1 was obtained
in 41.5% yield upon rinsing the solid residue with
hexanes. The presence of â-H atoms on the carbene
moiety does not interfere with the reaction: [Ru(CH-
n-Bu)(PCy3)2Cl2] (6)14 undergoes similarly rapid conver-
sion to [Ru(C-n-Bu)(PCy3)2Cl] (7) upon treatment with
3. In contrast, the parent methylidyne complex [Ru(CH)-
Transition-metal complexes that contain the carbyne
moiety have received considerable attention of late for
their bonding properties as well as their growing ap-
plication in organic and materials synthesis, especially
alkyne metathesis.1,2 Since Fischer reported the first
transition-metal carbyne complexes in 1973, a variety
of synthetic routes to compounds that contain a metal-
carbon triple bond have been developed.1,3-6
Notwithstanding the existence of several terminal
ruthenium-carbyne complexes,3,7-11 homogeneous alkyne
metathesis catalysis remains restricted to complexes of
Mo, W, and Re.1 In an effort directed toward the
discovery of late-metal carbyne complexes that can
mediate acetylene metathesis, we sought a general
means of obtaining carbyne complexes directly from the
multitude of carbene complexes that have been devel-
oped for olefin metathesis.12
Recently, Caulton disclosed the unanticipated forma-
tion of the square-planar Ru-carbyne complexes [Ru-
(1) Grubbs, R. H., Ed. Handbook of Metathesis; Wiley-VCH: Wein-
heim, Germany, 2003; Vols. 1-3.
(2) Bunz, U. H. F. Chem. Rev. 2000, 100, 1605.
(3) Gallop, M. A.; Roper, W. R. Adv. Organomet. Chem. 1986, 25,
121.
(4) Fischer, H.; Hofmann, P.; Kreissl, F. R.; Schrock, R. R.; Schubert,
U.; Weiss, K. Carbyne Complexes; VCH: New York, 1988.
(5) Engel, P. F.; Pfeffer, M. Chem. Rev. 1995, 95, 2281.
(6) Schrock, R. R. Chem. Rev. 2002, 102, 145.
(7) Coalter, J. N.; Bollinger, J. C.; Eisenstein, O.; Caulton, K. G.
New J. Chem. 2000, 24, 925.
(13) Conrad, J. C.; Amoroso, D.; Czechura, P.; Yap, G. P. A.; Fogg,
D. E. Organometallics 2003, 22, 3634.
(14) Schwab, P.; Grubbs, R. H.; Ziller, J. W. J. Am. Chem. Soc. 1996,
118, 100.
(15) Fjeldberg, T.; Haaland, A.; Schilling, B. E. R.; Lappert, M. F.;
Thorne, A. J. J. Chem. Soc., Dalton Trans. 1986, 1551.
(16) Miller, K. A.; Bartolin, J. M.; O’Neill, R. M.; Sweeder, R. D.;
Owens, T. M.; Kampf, J. W.; Holl, M. M. B.; Wells, N. J. J. Am. Chem.
Soc. 2003, 125, 8986.
(17) Sweeder, R. D.; Miller, K. A.; Edwards, F. A.; Wang, J.; Holl,
M. M. B.; Kampf, J. W. Organometallics 2003, 22, 5054.
(8) Sanford, M. S.; Henling, L. M.; Day, M. W.; Grubbs, R. H. Angew.
Chem., Int. Ed. 2000, 39, 3451.
(9) Gonzalez-Herrero, P.; Weberndorfer, B.; Ilg, K.; Wolf, J.; Werner,
H. Angew. Chem., Int. Ed. 2000, 39, 3266.
(10) Gonzalez-Herrero, P.; Weberndorfer, B.; Ilg, K.; Wolf, J.;
Werner, H. Organometallics 2001, 20, 3672.
(11) Amoroso, D.; Snelgrove, J. L.; Conrad, J. C.; Drouin, S. D.; Yap,
G. P. A.; Fogg, D. E. Adv. Synth. Catal. 2002, 344, 757.
(12) Trnka, T. M.; Grubbs, R. H. Acc. Chem. Res. 2001, 34, 18.
10.1021/om0508482 CCC: $30.25 © 2005 American Chemical Society
Publication on Web 11/08/2005

Communications
Scheme 1. Syntheses and Reactions of 1
Organometallics, Vol. 24, No. 25, 2005 6075
Figure 1. Thermal ellipsoid plot of 14. Ellipsoids are
drawn at the 50% probability level. Selected bond lengths
(Å) and angles (deg): Ru1-C29, 1.7178(16); Ru1-O1,
1.9938(13); Ru1-P1, 2.3875(7); Ru1-P2, 2.3964(7); Ru1-
C29-C30, 176.40(14); Ru1-O1-C8, 170.83(12); C29-
Ru1-O1, 176.42(7); C29-Ru1-P1, 91.63(6); O1-Ru1-P1,
90.36(4); C29-Ru1-P2, 90.09(6); O1-Ru1-P2, 87.67(4);
P1-Ru1-P2, 174.838(16).
the transformation of 1 into [Ru(C-p-C6H4Me)(PCy3)2F]
(12). Reaction of 1 with excess CsF in THF is slow,
proceeding to the extent of only 50% over 2 weeks at
room temperature; the rate of conversion is enhanced
by addition of 18-crown-6. Similarly, SnF2 in THF gives
slow conversion to 12. More rapid is the reaction of 1
with [S(NMe2)3][SiF2Me3] in THF. When excess CsF is
used in 3:2 DME/THF containing 1.2 equiv of 18-crown-
6, 12 is formed quantitatively over 48 h and can be
isolated in 40% yield after two recrystallizations. In
THF, 12 exhibits a distinctive doublet in the 31P NMR
spectrum at δ 47.1 (2JFP ) 37 Hz) due to coupling to
one F ligand; the 19F NMR spectrum shows a triplet at
δ -191.1 with the same coupling constant, indicating
the presence of two equivalent P nuclei. Treatment of a
blue-green solution of 1 in C6D6 with Me3SiOTf (Tf )
CF3SO2) causes precipitation of a blue-green powder
that analyzes as [Ru(C-p-C6H4Me)(PCy3)2OTf] (13). Dis-
solution of this compound in THF gives rise to a blue-
green solution that exhibits single peaks at δ 44.0 in
the 31P NMR spectrum and δ -77.5 in the 19F NMR
spectrum. This complex is unstable in THF and pyri-
dine, undergoing conversion to uncharacterized prod-
ucts. Addition of NaOAr (Ar ) p-C6H4-t-Bu) to 1 results
in clean conversion to 4, which is isolated in 59% yield.
The reverse transformation is best accomplished by
addition of 4 dissolved in THF to 0.6 equiv of SnCl2 also
in THF, resulting in an 87% isolated yield of 1. The
order of addition is important to avoid formation of a
significant amount of an uncharacterized dark purple
contaminant. Complex 4 can also be prepared directly
from 2 in 66% isolated yield by treatment with 3 equiv
of NaOAr in 4:1 toluene/THF.
(PCy3)2Cl] could not be prepared from [Ru(CH2)(PCy3)2-
Cl2] (8)14,18 in this manner. Instead, 8 underwent slow
decomposition without formation of 5. The lack of
reactivity of 8 compared to that of 2 and 6 may be due
to the substantially slower rate of phosphine dissocia-
tion from the former compared to the latter complexes.19
We note that added phosphine retards the conversion
of 2 and 6 into 1 and 7. Initial phosphine dissociation
is consistent also with the observation by 31P NMR
spectroscopy of rapid formation of a small quantity of
free PCy3 that persists throughout reactions of 2 and
6, but not of 8, with 3. Interestingly, unlike 3, the
amidogermylene [Ge(N[SiMe3]2)2] (9) fails to convert 2
into 1 in appreciable yield. The related dialkylstan-
nylene [Sn(CH[SiMe3]2)2] (10)15 also reacts with 2,
although other products are also formed, as the 1
produced in the reaction reacts further with additional
10. Thus, in the reaction of 2 with 3 equiv of 10, free
PCy3 is the only phosphorus-containing species observ-
able by 31P NMR spectroscopy after 40 min. When 2
reacts with just 1 equiv of 10 overnight, the resulting
mixture contains unreacted 2, 1, and free PCy3 in a 54:
15:31 integration ratio. Addition of 2 equiv of 10 to a
solution of 1 yields primarily free PCy3 after 3 h, as well
as minor new signals at δ 97.8 and 69.9 in the 31P NMR
spectrum; these species as well as any other Ru-
containing products have not yet been characterized.
Complex 1 is a versatile starting material for the
synthesis of other square-planar carbyne complexes,
upon substitution of the chloride ligand. Upon treatment
of 1 in 4:1 toluene:THF with a 10-fold excess of LiBr,
[Ru(C-p-C6H4Me)(PCy3)2Br] (11) is formed rapidly and
may be isolated in 71% yield. Several reagents effect
Unlike the prototypical trans-[Ru(CPh)(OPh)(P-i-
Pr3)2],7 closely related [Ru(CPh)(O-p-C6H4-t-Bu)(PCy3)2]
(14) does not suffer from disorder of the benzylidyne and
phenoxide ligands in the single-crystal X-ray structure.
Complex 14 crystallized in the space group P21/c, with
one molecule of 14 and 0.5 disordered Et2O molecules
in the asymmetric unit. A thermal ellipsoid plot of 14
is depicted in Figure 1, along with selected bond lengths
and angles. The complex adopts a square-planar geom-
etry in the solid state, with bond angles about Ru in
(18) Pietraszuk, C.; Marciniec, B.; Fischer, H. Organometallics 2000,
19, 913.
(19) Sanford, M. S.; Love, J. A.; Grubbs, R. H. J. Am. Chem. Soc.
2001, 123, 6543.

6076 Organometallics, Vol. 24, No. 25, 2005
Communications
the range 87.67(4)-91.63(6)° (angle sum 359.75°). The
RutC bond length of 1.7178(16) Å is significantly longer
than the corresponding bond length of 1.660(5) Å found
in the square-pyramidal cation [Ru(CCH2Ph)(P-i-Pr3)2-
Cl2]+ 10 but shorter than the RudC internuclear separa-
tion of 1.839(3) Å in the Grubbs-type carbene complex
[Ru(CH-p-C6H4Cl)(PCy3)2Cl2].14
In summary, dehydrohalogenation of the Ru-based
olefin metathesis catalysts 2 and 6 by the dialkylger-
mylene 3 effects their conversion into carbyne com-
plexes. Halide substitution in the latter complexes is
facile, affording access to other halide and pseudohalide
complexes. These include fluoro complex 12 as well as
aryloxide complex 14, which is square planar in the solid
state. We are currently investigating the utility of these
carbyne complexes in the synthesis of hitherto unavail-
able carbene complexes, as well as their conversion into
complexes that may be active in alkyne metathesis. We
are also examining the scope of the dehydrohalogenation
reaction of late-metal carbene complexes by germylene
3.
Acknowledgment. We thank Jeffrey M. Bartolin
and Ryan D. Sweeder for generous gifts of 3 and 9 and
for NMR spectral data for 5. Nathan W. Ockwig and
Dustyn D. Sawall are gratefully acknowledged for aid
in the crystal structure determination. This work was
supported by an award from Research Corp., by the
Camille and Henry Dreyfus New Faculty Awards Pro-
gram, and by the University of Michigan. M.H.S. is a
fellow of the NSF-sponsored IGERT program for Mo-
lecularly Designed Electronic, Photonic, and Nanostruc-
tured Materials at the University of Michigan.
Supporting Information Available: Text giving all
synthetic procedures and analytical data for new compounds
and a CIF file giving single-crystal X-ray diffraction data for
14. This material is available free of charge via the Internet
at http://pubs.acs.org.
OM0508482