The first double oxidative addition of CH2Cl2 to a metal complex: facile
synthesis of [Ru(CH2)Cl2{P(C6H11)3}2]
Montserrat Oliva´n and Kenneth G. Caulton*
Department of Chemistry, Indiana University, Bloomington, IN 47405-4001, USA
[Ru(H)2(H2)2L2] [L = P(C6H11)3] serves as a formal source
of zerovalent ‘RuL2’, and undergoes unprecedented oxida-
tive addition of both C–Cl bonds of CH2Cl2 to a single metal
center, providing a convenient synthesis of the alkene
metathesis catalyst [Ru(CH2)Cl2L2].
This idea of competitive inhibition by H2 is supported by the
fact that, if [Ru(H)2(H2)2L2] is stirred with CH2Cl2 (1:4) at
25 °C under 1 atm H2 in pentane, there is no reaction over 3 h.
The reaction is thus not outer-sphere electron transfer from
[Ru(H)2(H2)2L2], and a 16-electron complex is the reactive
species.
Ruthenium carbenes of the form [Ru(CRRA)Cl2L2] (L = phos-
phine) play a central role in alkene metathesis methodology in
organic chemistry. The original form1 of the ruthenium catalyst
has now been simplified,2 but access to carbene complexes
remains less than rational: a-elimination from an alkyl complex
or use of a diazoalkane reagent are the primary synthetic
methodologies.
Since [RuH3ClL2]6 is also produced in < 15% yield in this
reaction, we considered that HCl {which we independently
verified could convert [Ru(H)2(H2)2L2] to [RuH3(Cl)L2]}
might participate in the reaction which forms the carbene
complex. However, when the reaction of [Ru(H)2(H2)2L2] with
CH2Cl2 is executed in the presence of NEt3 (1:4:2 mole ratio),
the carbene product and yield are unchanged, as is (qual-
itatively) the rate. No [NHEt3]Cl precipitates. This gives
support for the idea that H2 is the fate of all metal-bound H, that
[RuH3(Cl)L2] is produced in a side reaction, and that a
‘ClRuCH2Cl’ species mediates the reaction. However, any such
species must react further to give carbene product faster than it
reacts with NEt3, to quaternize the amine (giving ‘ClRuCH2-
LnM + X2CRRA ? Ln(X)2MNCRRA
(1)
gem-Dihalogeno compounds are formally attractive as a
source of a carbene ligand [eqn. (1)], but converting this idea
into reality has been elusive. Eqn. (1) makes clear that LnM
must be a 14-valence electron species, or its equivalent, and the
scarcity of such species accounts for the rarity of eqn. (1). Eqn.
(1) might be expected to proceed stepwise, with an X–M–
CRRAX intermediate, and indeed, there are numerous examples
of halogenomethyl ligands that have been formed from
CX2RRA.3 However, a double oxidation addition of an R2CX2
sp3 carbon is unprecedented. Set against this background, we
report here the first oxidative addition of both C–Cl bonds of a
gem-dihalide to a single metal center, where all constituents of
R2CX2 become attached to a single metal in the product,4 and an
example where this occurs in high yield to produce the simplest
of ruthenium alkene metathesis catalysts, [Ru(CH2)Cl2L2], with
L = P(C6H11)3.
+
NEt3 ’ and Cl2); chloromethyl ligands readily react with
nucleophiles.3a
The idea of multiple oxidative addition to [Ru(H)2(H2)2L2],
and the idea that facile multiple losses of H2 from this molecule
permits it to serve as a formal equivalent of zerovalent ‘RuL2’
deserves further exploration.
This work was supported by the NSF and by material support
from Johnson-Matthey/Aesar. M. O. thanks the Spanish
Ministerio de Educacio´n y Cultura for a postdoctoral fellow-
ship.
Footnotes and References
Reaction† of [Ru(H)2(H2)2L2] [L = P(C6H11)3] with CH2Cl2
in pentane or benzene under argon occurs over 3 h at 25 °C (1:4
mol ratio) or 15 min at 60 °C (1:1.5 mole ratio) to give the
* E-mail: caulton@indiana.edu
†
[RuCl2(NCH2){P(C6H11)3}2] (method A): To a suspension of
[RuH2(H2)2{P(C6H11)3}2]7 (100 mg, 0.15 mmol) in pentane (7 ml) was
added CH2Cl2 (38 ml, 0.60 mmol) via a syringe. The resulting suspension
was stirred at room temp. for 3 h. During this time, the suspension changed
from white to brown–red. The red solid obtained by filtration was washed
with pentane and dried in vacuo. Yield: 70 mg (63%). Alternatively (method
B), the reaction could be carried out heating at 60 °C for 15 min, starting
from [RuH2(H2)2{P(C6H11)3}2] (100 mg, 0.15 mmol) and CH2Cl2 (14.4 ml,
0.22 mmol) in pentane (5 ml). Yield: 75 mg (67%). All the spectroscopic
data are consistent with those reported previously.2b When the crude
suspension was dried in vacuo and dissolved in C6D6, 1H and 31P NMR
show the presence of [RuH3Cl{P(C6H11)3}2],6 in addition to
[RuCl2(NCH2){P(C6H11)3}2], in a yield of < 15%. This was shown
independently to be formed by the action of H2 on
[Ru(CH2)Cl2{P(C6H11)3}2].
1
known molecule [RuCl2(CH2)L2],2b characterized by H, 13C
and 31P NMR spectroscopies. The 1H NMR signal of the
carbene ligand is the most unique spectroscopic feature,
appearing at d 19.4. If the reaction is carried out with CD2Cl2,
2
[RuCl2(CD2)L2] is the only isotopomer produced (1H and H
NMR assay),‡ showing that there is no scrambling of the metal-
and carbon-derived hydrogen. This reaction is remarkable
because it involves a four-electron reduction of CH2Cl2 (to Cl2
and what is formally CH222). It thus depends upon
[Ru(H)2(H2)2L2] being a formal source of uncharged RuL2 (i.e.
zerovalent Ru), by virtue of reductive elimination of hydride
from RuII, as H2. When this reaction is repeated in a closed
NMR tube, we see (31P{1H} NMR) no growth and decay of any
intermediate. Since there is no evidence for production of
CH3Cl or CH4, this reaction is an unprecedented oxidative
addition of both C–Cl bonds of CH2Cl2 to a single metal center.
This reaction proceeds more slowly in an NMR tube under 1
atm argon, than in a well agitated, round-bottom flask with a
considerable head-space, a fact we attribute to the accumulation
of H2, which shifts eqn. (2) to the left and thus decreases the
amount of unsaturated [Ru(H)2(H2)L2], which is apparently the
necessary reaction partner for CH2Cl2.5
‡ [RuCl2(NCD2){P(C6H11)3}2]: this compound was prepared analogously as
described for [RuCl2(NCH2){P(C6H11)3}2] (method A) by starting from
[RuH2(H2)2{P(C6H11)3}2] (50 mg, 0.075 mmol) and CD2Cl2 (19 ml, 0.30
mmol). 2H NMR (61 MHz, C6H6): d 19.40 (s, RuNCD2).
1 S. D. Nguyen, L. K. Johnson, R. H. Grubbs and J. W. Ziller, J. Am. Chem.
Soc., 1992, 114, 3974.
2 (a) P. Schwab, M. B. France, J. W. Ziller and R. H. Grubbs, Angew.
Chem., Int. Ed. Engl., 1995, 34, 2039; (b) P. Schwab, R. H. Grubbs and
J. W. Ziller, J. Am. Chem. Soc., 1996, 118, 100.
3 (a) H. B. Friedrich and J. R. Moss, Adv. Organomet. Chem., 1991, 33,
235; (b) H. Werner, Angew. Chem., Int. Ed. Engl., 1983, 22, 927;
[Ru(H)2(H2)2L2] ' [Ru(H)2(H2)L2] + H2
(2)
Chem. Commun., 1997
1733
Olivan, Montserrat
