Synthesis, spectroscopic characterization, and reactivity of ruthenocenes bearing pentamagnesiated cyclopentadienyl ligands

Full text of DOI:10.1021/ja960444o

5506
J. Am. Chem. Soc. 1996, 118, 5506-5507
Scheme 1
Synthesis, Spectroscopic Characterization, and
Reactivity of Ruthenocenes Bearing
Pentamagnesiated Cyclopentadienyl Ligands
Kapila N. Seneviratne, Annette Bretschneider-Hurley, and
Charles H. Winter*
Department of Chemistry, Wayne State UniVersity
Detroit, Michigan 48202
ReceiVed February 12, 1996
Grignard reagents rank among the most widely used and
studied class of organometallic compounds.1 Although numer-
ous aliphatic and aromatic Grignard reagents have been
prepared, there are very few examples of aromatic compounds
bearing two or more magnesium groups directly bonded to
aromatic carbons.2 In the case of polymagnesiated compounds
made from hexahalobenzenes, the thermal stability of higher
magnesiated species is limited by the facile elimination of
magnesium halide to afford benzynes. Polymagnesiated metal-
locenes are restricted to dimagnesiated ferrocenes.3 Recently,
we reported the synthesis of pentamethylpentalithioruthenocene
and decalithioruthenocene.4 However, subsequent work in our
laboratory has revealed that complexes containing pentalithiated
cyclopentadienyl ligands are too reactive and thermally unstable
to allow extensive use in synthesis.5 We reasoned that replace-
ment of lithium by main group metals with less reactive carbon-
metal bonds might lead to species with higher thermal stability
and more tractable reactivity patterns. Herein we report the
synthesis, characterization, and reactivity of permagnesiated
ruthenocenes based upon the ruthenocene and pentamethylru-
thenocene skeletons. To the best of our knowledge, these are
the first examples of permagnesiated aromatic compounds. The
new ruthenocenes are remarkably stable, yet readily react to
form new substituted derivatives. NMR studies indicate that
the permagnesiated ruthenocenes exist as dimers and higher
oligomers and that conversion from lower to higher oligomers
is facile.
rium content in the cyclopentadienyl ligand. Carbon-mercury
bonds in ruthenocenes are stable to water under the reaction
conditions, which supports a pentamagnesiated formulation and
rules out structures containing carbon-mercury bonds. Addi-
tion of bromine gave pentabromopentamethylruthenocene7b
(57%). Additional evidence for a magnesiated species was
obtained from reaction with methyl iodide, which gave a mixture
of methylated ruthenocenes between pentamethylruthenocene
and decamethylruthenocene (79% total yield).6 Grignard re-
agents are well known to react with alkyl iodides by electron
transfer pathways.8
Given the likelihood that a pentamagnesiated ruthenocene was
being formed, we sought to characterize this species by
spectroscopic methods. Treatment of 1 with methylmagnesium
chloride (12 equiv) in tetrahydrofuran-d8 at ambient temperature,
followed by 1H and 13C{1H} NMR analysis, revealed 85% of a
major product 2 with a 1H NMR resonance for the Cp* ligand
at δ 2.06 and 15% of at least five minor products with Cp*
resonances at δ 2.13, 2.12, 2.11, 2.10, and 2.08.9 No cyclo-
pentadienyl C-H bonds were observed in the 1H NMR
spectrum, indicating that the magnesiated ruthenocenes con-
tained e2% of hydrogen on the cyclopentadienyl ligands. The
13C{1H} NMR spectrum of 2 showed resonances due to the
Cp* ligand at 84.40 (C-CH3), and 16.32 (C-CH3) ppm.
Resonances from the magnesiated cyclopentadienyl ligand were
observed at 123.35, 121.78, and 118.67 ppm, with intensities
of approximately 2:1:2. A reasonable structure for 2 that is
consistent will all of the data is a dimer with eight terminal
chloromagnesio groups and one diruthenocenylmagnesium
unit.10 The ipso-carbons in phenyl magnesium halides11a and
Treatment of pentakis(chloromercurio)(pentamethyl)ruthe-
nocene6 (1) with methylmagnesium chloride (12 equiv) in
tetrahydrofuran at 23 °C for 1 h led to its dissolution, giving a
clear yellow-orange solution containing a pentamagnesiated
pentamethylruthenocene (Scheme 1). Hydrolysis of this solution
with H2O afforded pentamethylruthenocene7a (85%), while D2O
quench gave pentamethylruthenocene (97%) with 87% deute-
(1) For leading references, see: Bickelhaupt, F. J. Organomet. Chem.
1994, 475, 1. Walborsky, H. M.; Topolski, M. J. Am. Chem. Soc. 1992,
114, 3455. Noyori, R.; Kitamura, M. Angew. Chem., Int. Ed. Engl. 1991,
30, 49. Garst, J. F. Acc. Chem. Res. 1991, 24, 95. Walling, C. Acc. Chem.
Res. 1991, 24, 255. Walborsky, H. M. Acc. Chem. Res. 1990, 23, 286.
Erdik, E. Tetrahedron 1984, 40, 641. Lai, Y.-H. Synthesis 1981, 585. Felkin,
H.; Swierczewski, G. Tetrahedron 1975, 31, 2735.
(2) For leading references, see: Wittig, G.; Bickelhaupt, F. Chem. Ber.
1958, 91, 883. Hart, F. A.; Mann, F. G. J. Chem. Soc. 1957, 3939. Smith,
C. F.; Moore, G. J.; Tamborski, C. J. Organomet. Chem. 1971, 33, C21.
Tamborski, C.; Moore, G. J. J. Organomet. Chem. 1971, 26, 153. Haper,
R. J., Jr.; Soloski, E. J.; Tamborski, C. J. Org. Chem. 1964, 29, 2385. Ghosh,
T.; Hart, H. J. Org. Chem. 1988, 53, 3555. Harada, H.; Hart, H.; Du, C.-J.
F. J. Org. Chem. 1985, 50, 5524. Tinga, M. A. G. M.; Schat, G.; Akkerman,
O. S.; Bickelhaupt, F.; Horn, E.; Kooijman, H.; Smeets, W. J. J.; Spek, A.
L. J. Am. Chem. Soc. 1993, 115, 2808.
(3) Shechter, H.; Helling, J. F. J. Org. Chem. 1960, 26, 1043. Waka-
matsu, K. S.; Mizuta, M. J. Organomet. Chem. 1974, 78, 405. Seyferth,
D.; Hofmann, H. P.; Burton, R.; Helling, J. F. Inorg. Chem. 1962, 1, 227.
(4) Bretschneider-Hurley, A.; Winter, C. H. J. Am. Chem. Soc. 1994,
116, 6468.
(5) Bretschneider-Hurley, A.; Winter, C. H. Manuscript in preparation.
(6) Experimental procedures for the synthesis of new compounds are
given in the supporting information. Small amounts of iodinated methyl
ruthenocenes (3-11% total yields) were observed in the methylation product
mixtures. For data, see supporting information.
(7) (a) Tilley, T. D.; Grubbs, R. H.; Bercaw, J. E. Organometallics 1984,
3, 274. (b) Winter, C. H.; Han, Y.-H.; Heeg, M. J. Organometallics 1994,
13, 3009.
(8) Kharasch, M. S.; Reinmuth, O. Grignard Reactions of Nonmetallic
Substances; Prentice Hall: New York, 1954; pp 1032-1046.
(9) Also observed were dimethylmercury (1H NMR δ 0.11 (JH-Hg
)
52.0 Hz); 13C{1H} NMR 20.14 (JC-Hg ) 345 Hz) ppm), excess methyl-
magnesium chloride (1H NMR δ -1.82; 13C{1H} NMR -16.80 ppm), and
methyl chloride (1H NMR δ 3.10; 13C{1H} NMR 48.93 ppm; contaminant
in methylmagnesium chloride). Several small peaks around the Cp* methyl
and ring carbon regions could not be assigned in the 13C{1H} NMR and
are probably associated with the five minor products.
(10) (a) For a discussion of the Schlenk equilibria, see: Cotton, F. A.;
Wilkinson, G. AdVanced Inorganic Chemistry, 5th ed.; Wiley-Inter-
science: New York, 1988; pp 159-161. (b) Cyclic oligomers with 1,2,4-
or 1,2,3-MgCl groups would also give 2:1:2 intensity peaks for the
magnesiated carbons and must be considered as possible structures for 2.
However, the proposed dimer is a simpler and, we feel, more likely structure.
(11) (a) Screttas, C. G.; Micha-Screttas, M. J. Organomet. Chem. 1985,
290, 1. Allen, P. E. M.; Fisher, M. C. Eur. Polym. J. 1985, 21, 201. (b)
Jones, A. J.; Grant, D. M.; Russell, J. G.; Fraenkel, G. J. Phys. Chem. 1969,
73, 1624. (c) For 13C NMR shifts (109.1-112.1 ppm for C-Ti) of
metallocenes bearing Ti(NEt2)3 substituents, see: Bu¨rger, H.; Kluess, C. J.
Organomet. Chem. 1973, 56, 269. Z. Anorg. Chem. Allg. Chem. 1976,
423, 112.
S0002-7863(96)00444-1 CCC: $12.00 © 1996 American Chemical Society

Communications to the Editor
J. Am. Chem. Soc., Vol. 118, No. 23, 1996 5507
diphenylmagnesium11b are found to resonate about 30-35 ppm
downfield from benzene. The positions of the magnesiated
carbon resonances (34.27-38.95 ppm downfield from C-CH3)
are thus appropriate for an aromatic magnesium compound.11c
The minor compounds could not be identified due to their low
concentrations but are probably higher oligomers.
Scheme 2
The isolation of 2 was attempted. A tetrahydrofuran solution
prepared as above was treated with a large excess of hexane,
resulting in the precipitation of a yellow-ochre powder.
1
Analysis of the powder by H NMR spectroscopy in tetrahy-
drofuran-d8 revealed at least 12 broad Cp* methyl resonances
between δ 2.13-1.81, of which 2 (δ 2.06) was a minor
component.12 It was not possible to obtain a 13C{1H} NMR
spectrum with sufficient signal to noise to allow structural
assignments, due to the low concentration of each of the
components. However, we propose that the isolated material
corresponds to a mixture of oligomers10a that results from
elimination of magnesium chloride from 2. This mixture is
further evidence of the tendency of pentamagnesiated ru-
thenocenes to form oligomers. Infrared spectroscopy suggested
that tetrahydrofuran was associated with the isolated powder,
1
aromatic molecules should be extremely unfavorable due to
repulsion between the carbanionic sites. Second, NMR spec-
troscopy suggests that the permagnesiated ruthenocenes exist
preferentially as dimers or higher oligomers. The facile
formation of oligomers is in contrast to traditional Grignard
reagents, which favor monomeric formulations in ether sol-
vents.16,17 Finally, despite the unusual placement of five or ten
contiguous magnesium substituents about a metallocene skel-
eton, initial studies suggest that the permagnesiated ruthenocenes
react like typical organomagnesium reagents. Thus, the com-
plete range of reactivity associated with Grignard reagents and
diorganomagnesium compounds may be expected for permag-
nesiated cyclopentadienyl complexes and aromatic compounds.1
The above predictions are being investigated in our laboratory.18
but the exact amount could not be assigned by the H NMR
spectrum because of residual hydrogen content in the tetrahy-
drofuran-d8. The reactivity of the isolated powder was similar
to that of the compound generated in solution. Hydrolysis
afforded pentamethylruthenocene (65%), while bromination with
bromine gave pentabromopentamethylruthenocene (54%) and
tetrabromopentamethylruthenocene4 (13%).
The simple synthesis of 2 and its higher oligomers suggested
that a decamagnesiated ruthenocene should be accessible.
Accordingly, treatment of decakis(chloromercurio)ruthenocene
(3)6 with methylmagnesium chloride (ca. 22 equiv) in tetrahy-
drofuran at 23 °C gave a turbid light brown solution containing
a decamagnesiated ruthenocene 4 (Scheme 2). Attempts to
record NMR spectra of 4 in tetrahydrofuran-d8 failed due to its
low solubility. Although 4 is denoted as a monomer herein for
simplicity, its low solubility and analogy with 2 suggest a
dimeric or higher oligomeric structure. Hydrolysis of 4 afforded
ruthenocene (75%), while treatment of 4 with bromine (ca. 20
equiv) gave decabromoruthenocene13 (41%). Methylation of 4
by refluxing in neat methyl iodide gave about 15 methylated
ruthenocenes (57% total yield), as determined by GLC and GLC/
MS.6
Acknowledgment. We thank the U.S. Air Force Office of Scientific
Research (Contract No. F49620-93-1-0072) and the donors of the
Petroleum Research Fund, administered by the American Chemical
Society, for support of this work.
Supporting Information Available: Synthetic procedures and
spectroscopic and analytical data for the new compounds (16 pages).
Ordering information is given on any current masthead page.
Several significant inferences can be made from the above
results. First, and perhaps most surprising, is that ruthenocenes
bearing pentamagnesiated cyclopentadienyl ligands are stable
and can be easily isolated and manipulated at ambient temper-
ature. The complexes described herein are substantially more
robust than the perlithiated analogs we described earlier,4
implying that permagnesiated aromatic compounds and per-
metalated aromatic compounds containing other main group
metals will exhibit similar or possibly higher thermal stability.
Such stability contradicts conventional wisdom,14,15 which would
predict that placement of adjacent electropositive metals on
JA960444O
(14) For a discussion, see: Beak, P.; Meyers, A. I. Acc. Chem. Res. 1986,
19, 356.
(15) Attempts to prepare perlithiated organic aromatic compounds:
Gribble, G. W.; Saulnier, M. G. J. Org. Chem. 1983, 48, 607. Gribble, G.
W. Synlett 1991, 289. Se´vrivourt, M. C.; Robba, M. Bull. Soc. Chim. Fr.
1977, 142. Ried, W.; Bender, H. Chem. Ber. 1956, 89, 1574. Janda, M.;
Srogl, J.; Stibor, I.; Nemec, M.; Vopatrna, P. Synthesis 1972, 545. Baran,
J. C., Jr.; Hendrickson, C.; Laude, D. A., Jr.; Lagow, R. J. J. Org. Chem.
1992, 57, 3759. Shimp, L. A.; Chung, C.; Lagow, R. J. Inorg. Chim. Acta
1978, 29, 77.
(16) Smith, M. B.; Becker, W. E. Tetrahedron Lett. 1965, 3843. Smith,
M. B.; Becker, W. E. Tetrahedron 1966, 22, 3027. Smith, M. B.; Becker,
W. E. Tetrahedron 1967, 23, 4215. For a review, see: Ashby, E. C. Q.
ReV. Chem. Soc. 1967, 21, 259.
(17) For a structural discussion of di-Grignard compounds, see: Bick-
elhaupt, F. Angew. Chem., Int. Ed. Engl. 1987, 26, 990.
(18) For an initial disclosure of a hexamagnesiated benzene, see: Winter,
C. H.; Kur, S. A. Abstracts of Papers, 210th National Meeting of the
American Chemical Society, Chicago, IL, August 20-24, 1994; INOR 475.
(12) Data for isolated solid: IR (Nujol, cm-1): 1337 (m), 1308 (w),
1249 (m), 1257 (s), 1240 (m), 1172 (s), 1029 (vs, THF C-O), 949 (m),
915 (s), 881 (vs), 722 (s), 687 (s), 670 (s). Anal. Found: C, 24.68; H,
4.07; Cl, 30.06. For comparison, calcd for (C5(MgCl)5)(C5(CH3)5)Ru: C,
30.27; H, 2.54; Cl, 29.79.
(13) Winter, C. H.; Han, Y.-H.; Ostrander, R. L.; Rheingold, A. L. Angew.
Chem., Int. Ed. Engl. 1993, 32, 1161.