Coordinatively unsaturated Cp*Ru alkoxo complexes. 16. Addition products Cp*Ru(acac)L and solution equilibria

Article abstract of DOI:10.1021/om960718g

With [Cp*Ru(OMe)]₂ (1) as starting material, a number of 2,4-pentanedionato complexes Cp*Ru(2,4-pentanedionate) were prepared. The solid-state structure of Cp*Ru(1-phenyl-2,4-pentanedionate) (3) was elucidated by X-ray crystallography, and the complex was found to be a dimer in the solid state, as previously established for Cp*Ru(2,4-pentanedionate) (2). The molecular weight of 3 in benzene indicates a dimer (3-3) in solution as well. Low-temperature ¹H NMR spectroscopy reveals a process involving dissociation of the dimer into monomers associated with an inversion of the pentanedionate ligand at Ru. The activation energy (E_a = 43.5 ± 0.7 kJ/mol, toluene-d₈) reflects the enthalpy of dissociation of the dimer. All dimers are easily cleaved with two-electron ligands L, resulting in the mononuclear complexes Cp*Ru(2,4-pentanedionate)L. Besides isolable complexes with L = phosphine, phosphite, and CO, labile adducts of the same composition with σ-S and N donor ligands (L = methyl p-tolyl sulfoxide, ethyl methyl sulfide, tetrahydrothiophene, diaza[2.2.2]-bicyclooctane, 3-cyanopyridine) were detected in solution by the influence of these ligands on the observed inversion barrier. With chiral or enantiotopic ligands diastereoselective adduct formation was observed.

Full text of DOI:10.1021/om960718g

Organometallics 1997, 16, 3273-3281
3273
Coor d in a tively Un sa tu r a ted Cp *Ru Alk oxo Com p lexes.
16.1 Ad d ition P r od u cts Cp *Ru (a ca c)L a n d Solu tion
Equ ilibr ia 1
U. Koelle,* C. Rietmann, and G. Raabe
Institute for Inorganic Chemistry, Aachen Technical University, D-52056 Aachen, Germany
Received August 20, 1996X
With [Cp*Ru(OMe)]2 (1) as starting material, a number of 2,4-pentanedionato complexes
Cp*Ru(2,4-pentanedionate) were prepared. The solid-state structure of Cp*Ru(1-phenyl-
2,4-pentanedionate) (3) was elucidated by X-ray crystallography, and the complex was found
to be a dimer in the solid state, as previously established for Cp*Ru(2,4-pentanedionate)
(2). The molecular weight of 3 in benzene indicates a dimer (3-3) in solution as well. Low-
temperature 1H NMR spectroscopy reveals a process involving dissociation of the dimer into
monomers associated with an inversion of the pentanedionate ligand at Ru. The activation
energy (Ea ) 43.5 ( 0.7 kJ /mol, toluene-d8) reflects the enthalpy of dissociation of the dimer.
All dimers are easily cleaved with two-electron ligands L, resulting in the mononuclear
complexes Cp*Ru(2,4-pentanedionate)L. Besides isolable complexes with L ) phosphine,
phosphite, and CO, labile adducts of the same composition with σ-S and N donor ligands (L
) methyl p-tolyl sulfoxide, ethyl methyl sulfide, tetrahydrothiophene, diaza[2.2.2]-
bicyclooctane, 3-cyanopyridine) were detected in solution by the influence of these ligands
on the observed inversion barrier. With chiral or enantiotopic ligands diastereoselective
adduct formation was observed.
Sch em e 1. P r ep a r a tion of Cp *Ru (a ca c) Com p lexes
In tr od u ction
Coordinatively unsaturated transition-metal com-
plexes are of principal interest to coordination chemis-
try, since those species are crucial to many substitution
processes as well as being involved in the elementary
steps in catalytic cycles. A particular class of such
coordinatively unsaturated d6-5 complexes are the Cp*
and arene Ru dimers bridged by OR, SR, NR, and NR2
groups, which, apart from the Cp ligand, are devoid of
additional stabilizing π-acceptor ligands. Only the
chemistry of [Cp*RuOR]22 (1) and [Cp*RuSR]23 has been
studied in detail.
The complex Cp*Ru(acac) (2) was first synthesized in
the hope of generating a monomeric analogue to 14 but
was later recognized to be a dimer in the solid state5
(Scheme 1). The monomers are connected by a long
(2.43 Å) bond between Ru and the central carbon of the
acac ligand of a second Cp*Ru(acac) fragment, in the
instantaneously and could not be followed kinetically
at ambient temperature. To further elucidate the
concept, we have synthesized a number of pentane-
dionate derivatives and studied their reactions with two-
electron-donor ligands. In particular the benzyl deriva-
tive Cp*Ru(OC(CH2Ph)CHC(Me)O) (3), featuring dia-
stereotopic protons in the dimer, allowed a detailed
study of solution equilibria by NMR.
same way as has been found previously for [Cp*Rh-
2+ 6
(acac)]2
.
Chemically, however, 2 behaved as an
unsaturated molecule being rapidly cleaved by e.g.
phosphines to give the monomeric Cp*Ru(acac)PR3. In
fact, these addition and cleavage reactions occur almost
X Abstract published in Advance ACS Abstracts, J une 15, 1997.
(1) Part 15: Bu¨cken, K.; Koelle, U.; Pasch, R.; Ganter, B. Organo-
metallics 1996, 15, 3095.
Resu lts
Der iva tives, Exch a n ge, a n d Dia ster eoselectivity
in Isola ted Ad d u cts. In addition to generation of 2
from 1 by exchange of the methoxo for the acac ligand,4
we found a direct route to 2 from the dichloride
[Cp*RuCl2]2. The reaction is analogous to the prepara-
tion of 1 from the same material2c employing 2-propanol
instead of methanol for reduction of RuIII. Since 2-pro-
panol does not act as a bridging ligand or is easily
displaced by a pentanedionate, the reaction, carried out
with a stoichiometric amount of acacH in the presence
(2) (a) Koelle, U.; Kossakowski, J . J . Chem. Soc., Chem. Commun.
1988, 549. (b) Loren, S. D.; Campion, B. K.; Heyn, R. H.; Tilley, T. D.;
Bursten, B. E.; Luth, K. W. J . Am. Chem. Soc. 1989, 111, 4712. (c)
Koelle, U.; Kossakowski, J . Inorg. Synth. 1992, 29, 225.
(3) (a) Koelle, U.; Rietmann, C.; Englert, U. J . Organomet. Chem.
1992, 423, C20. (b) Takahashi, A.; Mizobe, Y.; Matsuzaka, H.; Dev,
S.; Hidai, M. J . Organomet. Chem. 1993, 456, 243.
(4) Koelle, U.; Kossakowski, J .; Raabe, G., Angew. Chem., Int. Ed.
Engl. 1990, 29, 773.
(5) Smith, M. E.; Hollander, F. J .; Andersen, R. A. Angew. Chem.
1993, 32, 1294.
(6) Rigby, W.; Lee, H. B.; Bailey, P. M.; McCleverty, J . A.; Maitlis,
P. M. J . Chem. Soc., Dalton Trans. 1979, 387.
S0276-7333(96)00718-2 CCC: $14.00 © 1997 American Chemical Society

3274 Organometallics, Vol. 16, No. 15, 1997
Koelle et al.
Ch a r t 1. Dia ster eom er ic P h osp h in e Ad d u cts of
Cp *Ru (P F BC)
of a large excess of 2-propanol, gave the acac complex 2
in high yield. However, the method was less successful
in the case of other pentanedionates, which were better
prepared by methanol substitution from 1. In this way
Cp*Ru complexes (in some cases also C5Me4EtRu ana-
logues) of 1-phenyl-2,4-pentanedionate (Phacac; 3), ben-
zoylacetonate (4), acetoacetic ester (5), diethyl malonate
(6), 1,1,1-trifluoro-2,4-pentanedionate (7) and a 3-(per-
fluorobutyryl)camphor derivative (8) were obtained. The
compounds are dull red, similar to 2, and are air-
sensitive in solution.
Two complexes, 6a and 6b, in the ratio 2:1 with
slightly differing NMR shifts for all groups, were formed
from 1 and diethyl malonate, which were both of
composition [Cp*Ru(diethyl malonate)]n according to
NMR and chemical analysis. The major isomer 6a was
easily cleaved with PPh3 to give the mono(phosphine)
adduct and is therefore believed to be a dimer with a
structure analogous to 2, whereas air-stable 6b with-
stands cleavage and is most probably an oligomer where
OEt groups are engaged in binding. The latter com-
pound appears to be the thermodynamically more stable
one, since all of the material was transformed into 6b
on recrystallization.
With phosphines, phosphites, and CO stable adducts
of composition Cp*Ru(acac)L are formed with nearly all
2,4-pentanedionate ligands. An exception was found in
the case of 2,2′-bipyridine, which cleaved one Ru-O
bond in 2, resulting in Cp*Ru(η1-OC(Me)CHC(Me)O)-
(η2-bpy). Similarly, 5 reacted with P(OMe)3 to yield
Cp*Ru(η1-OC(Me)CHCOOEt)(P(OMe)3)2. NMR data for
these adducts are given in Table 3.
When 1 equiv of PMe3 was added to 3‚PPh3, an
instantaneous reaction was indicated by a deepening
of the color. The NMR spectrum showed the formation
of the PMe3 adduct and the presence of 1 equiv of free
PPh3. Equilibrium 1 is thus shifted far to the right.
Nevertheless, 3‚PMe3 shows, different from 3‚PPh3, a
singlet for the benzylic protons at ambient temperature,
indicating rapid dissociation in solution.
F igu r e 1. Schakal representation of the dimer 3-3.
Distances (Å) and angles (deg) are as follows: Ru-O2,
2.116(2); Ru-O1, 2.116(2); Ru-CCp(av), 2.127(3); Ru-C7′,
2.408; O1-Ru-O2, 85.32(6); Ru-O1-C6, 128.8(2); Ru-
O2-C8, 128.4(2); O1-C6-C16, 115.9(2); O2-C8-C17,
126.1(2).
stereomers in the ratio 1:0.12. Though we have no
direct proof, it seems reasonable to assign structure 8a
to the major isomer.
Com p osition a n d Str u ctu r e of 3. The Phacac
derivative 3, due to its particular NMR spectroscopic
properties, plays a key role in the present investigation
and was therefore subjected to closer investigation.
Osmometric molecular mass determination in benzene
resulted in a value of 845 Da, consistent with a dimer
(M ) 822 Da). Determination of the solid-state struc-
ture revealed a molecular geometry (Figure 1) which
largely conforms to the one found for 2, i.e. a dimer
formed from two centrosymmetrically related Cp*Ru-
(Phacac) units, connected by a Ru-C7 bond of length
2.408(3) Å. The O-Ru-O plane and the Ru-Cp*
normal vector enclose an angle of δ ) 36.5° (2, 37.4°).
The plane O2-C6-C8-O3 is bent toward Ru by 5.5°.
The same is true for carbon atom C7, which lies 0.2 Å
below the plane O2-C6-C8-O3 and is shifted toward
the metal atom of the opposing subunit. The Me group
and the C8-C17 bond of the PhCH2 group at the acac
ring are also bent outwardly with respect to the acac
plane and thus away from the metal. Bending of the
acac ligand with respect to the Ru-Cp* normal vector
is most likely due to the Ru-C7 interaction. To confirm
this assumption, we performed nonempirical quantum-
chemical calculations with the density functional method7
on a model compound where the phenyl substituent at
acac and the methyl groups of the Cp* system were
replaced by hydrogen atoms. The calculations were
carried out in the local approximation, employing a
numerical basis set of approximately 6-31G** quality.
Complete geometry optimization starting from the
experimentally determined value of δ resulted in a final
angle of about 6.5°. In the solid state the phenyl group
within each subunit is bent toward the Cp* ligand with
3‚PPh3 + PMe3 u + PPh3 + 3‚PMe3
3‚PMe3 U 3 + PMe3
(1)
Using an acac-like ligand featuring diastereofaces
such as D-(+)-3-(perfluorbutyryl)camphor (PFBC, whose
EuIII complex is used as a NMR shift reagent), two
diastereomeric adducts Cp*Ru(PFBC)L are possible. In
this particular case the primary reaction product Cp*Ru-
(PFBC) (8), obtained from 1 and PFBCH, was not easily
separated from excess ligand. The PPh3 adduct, how-
ever, could be purified, and integration of the Cp*
doublets (4J PH ) 1.5 Hz) indicated a mixture of dia-
(7) (a) Biosym Dmol2.2 User Guide; Bisoym Technologies: San
Diego, CA, 1992. (b) Moruzzi, V. L.; J anak, J . F.; Williams A. R. In
Calculated Electronic Properties of Metals; Pergamon Press: New York,
1978. (c) Hedin, L.; Lundqvist, B. I. J . Phys. Chem. 1971, 4, 2064.

Coordinatively Unsaturated Cp*Ru Alkoxo Complexes
Organometallics, Vol. 16, No. 15, 1997 3275
Sch em e 2. En a n tiom er s a n d Dia ster eom er s of Cp *Ru (OC(CH2P h )CHC(Me)O)
an O2-C8-C17-C18 dihedral angle of 18.5°. This
conformation of the phenyl group is probably enforced
by the opposite subunit or by packing forces of the
lattice, because EHT calculations on the (bent) monomer
result in energy minima of about -60 and 80° respec-
tively.
Solven t -Dep en d en t Va r ia b le-Tem p er a t u r e 1H
NMR Sp ectr a of 3. The dimer 3-3 can exist in four
isomeric forms, depicted in Scheme 2. They consist of
two pairs of enantiomers arranged horizontally and two
pairs of diastereoisomers arranged vertically. In the
latter PhCH2 groups are disposed cis and trans to each
other. Interconversion of enantiomers A/B and C/D
interchanges diastereotopic protons HA/HB and HC/HD
and can thus be monitored by NMR. In addition, two
separate signal sets due to diastereoisomers are to be
expected in the slow-interconversion limit. Both hori-
zontal and vertical processes require dissociation into
monomers and should therefore show the same activa-
tion parameters.
Variable-temperature 1H NMR spectra were recorded
in CD2Cl2, in toluene-d8 and in THF-d8 at 400 and 500
MHz proton frequency. At ambient temperature ben-
zylic protons appear as a singlet in all solvents. In
CD2Cl2 the low-temperature spectrum (400 MHz) ex-
hibits a sharp AB quartet for the benzylic protons which
persists up to -10 °C. Above this temperature the
complex starts to react irreversibly with the solvent to
give unstable Cp*RuIII(OC(CH2Ph)CHC(Me)O)Cl as one
of the products.
1
F igu r e 2. Part of the H NMR spectrum of 3 in toluene-
d8 at -85 °C showing the benzylic proton signal pattern
as two overlapping AB quartets of unequal intensity,
labeled A and B.
has emerged as a seven-line pattern at -90 °C. This
can be interpreted as two overlapping AB quartets of
unequal intensity (labeled A and B in Figure 2).
Concomitant with the splitting of the benzylic protons
appears a splitting of the methyl group at C3 and also
of the protons of the phenyl ring (fortunately well
separated from residual solvent signals at low temper-
ature). This was most noticeable for the ortho protons,
which, at -90 °C, appear as two doublets instead of one.
The overall pattern is attributed to the two dynamic
processes of Scheme 2, which, at sufficiently low tem-
perature, generate diastereomeric pairs with four dia-
The singlet of the benzylic protons in toluene (δ 3.25)
at room temperature starts broadening at -40 °C and

3276 Organometallics, Vol. 16, No. 15, 1997
Koelle et al.
Sch em e 3. Ad d u ct F or m a tion a n d In ver sion of 3
F igu r e 3. Arrhenius plot from the temperature-dependent
line width of benzylic protons of 3.
stereotopic benzylic protons HA, HB, HC, and HD in the
form of two AB quartets with an intensity ratio of about
55:45.
Three interconversion barriers can be determined
independently from VT NMR spectra by monitoring the
coalescence of the AB quartets, the methyl singlets, and
the ortho proton doublets. Fitting the line shape to the
coalescing AB quartets by varying the rate constant k
as well as the ratio of diastereoisomers led to the
Arrhenius plot given in Figure 3 with an apparent value
of Ea ) 43 kJ /mol. The activation energy obtained for
the interconversion of ortho phenyl protons is close
enough to this value to consider the processes respon-
sible for the two spectral changes as being concerted,
that is identical, and reflect opening and recombination
of the dimer. To investigate the adduct equilibria
discussed below, in any particular case the most con-
venient of these spectral changes was chosen to follow
inversion of the acac ligand.
(or 2, respectively), the above interconversion process
was modulated by addition of a variety of ligands L
ranging from π-acceptor to weak σ-donor. Isolable
adducts are formed with CO and PPh3; adduct formation
with the other bases was detected by a shift of NMR
signals and by the influence on NMR barriers.
In THF only a slight broadening without splitting into
separate signals was observed down to the lowest
accessible temperature of about -100 °C.
The largely differing inversion barriers (interconver-
sion of enantiomers corresponds to an inversion at Ru)
in the three solvents must be interpreted as different
stabilization of the monomer. From a beginning line
broadening at -10 °C in dichloromethane the barrier
g60 kJ /mol (∆Gq) is estimated, at least 30 kJ /mol higher
than in toluene (Table 2). In contrast, a considerably
lower barrier (if any) follows from LT NMR in THF. A
specific stabilization of the monomer by addition of
CH2Cl2, the same as has been found for the isoelectronic
CpRe(NO)(PPh3)+,8 is the logical explanation for the
persisting AB quartet in this solvent. Stabilization of
the monomer by toluene seems to be less pronounced,
and therefore, the complex in this solvent is dimeric,
as it is in the solid state. A possible explanation for
the rapid interconversion in THF is a solvent-stabilized
monomer in this solvent as well. However, dissociation
must be much faster in the THF than in the dichloro-
methane adduct.
The dissociation-association steps involved in the
interconversion of enantiomers of 3‚L are shown in
Scheme 3. In the presence of a ligand L the (vertical)
inversion equilibrium is coupled to a (horizontal) dis-
sociation equilibrium. In order to conceptually distin-
guish both processes, inversion is depicted as a separate
step, even though in reality it may be indefinitely fast,
that is concomitant with the dissociation. In general
the observable inversion barrier with rate constant
inv
kobsd will be influenced by the presence of L, since
inversion can only occur in the unsaturated monomer.
Dissociation-association of L can be detected in the
NMR either by changes in the ligand spectrum on
complexation to the metal or, in case the ligand is
present in excess, by separate signals for free and
complexed ligand in the regime of slow exchange. Both
criteria have been used to evaluate the dissociation-
inversion kinetics of the adducts. An equilibrium
constant Kass. > 1 for adduct formation under conditions
of rapid exchange at ambient temperature, which is the
case for most of the σ-donor ligands, is evident from
shifts of the NMR signals of dimers 2-2 and 3-3 on
ligand addition. Since there is no free ligand signal for
the 1:1 Ru-L mixture at low temperature (when slow
exchange is obvious from split ligand signals), Kass.
(kass./kdiss.) appears to be .1 and is not easily determined
Ad d u ct F or m a tion Equ ilibr ia . In order to gain
more insight into the adduct-forming capabilities of 3
(8) (a) Ferna´ndez, J . M.; Gladysz, J . A. Organometallics 1989, 8,
207. (b) Dewey, M. A.; Knight, D. A.; Klein, D. P.; Arif, A. M.; Gladysz,
J . A. Inorg. Chem. 1991, 30, 4995. (c) Knight, D. A.; Arif, A. M.;
Gladysz, J . A. Z. Naturforsch., B 1992, 47, 1184. (d) Richter-Addo, G.
B.; Knight, D. A.; Dewey, M. A.; Arif, A. M.; Gladysz, J . A. J . Am. Chem.
Soc. 1993, 115, 11863.

Coordinatively Unsaturated Cp*Ru Alkoxo Complexes
Ta ble 1. Ch em ica l Sh iftsa of Ad d u cts of 3
Bz CH o-phenyl m-phenyl p-phenyl
5.20 7.25 7.16 7.07
Organometallics, Vol. 16, No. 15, 1997 3277
ligand
Cp*
1.597
Me
1.92
2.04/2.02b 3.53/3.54
other
none (298 K)
none (183 K)
PPh3
3.52
1.38
1.15c
1.305
1.58
1.62
1.53
1.46
5.30 7.52/7.60 7.34/7.31 7.21/7.19
1.54
1.66
1.35
1.85
1.81
1.95
3.03/3.16
3.18
3.01/2.97
4.83 7.01
5.07 7.12
5.13 7.24
4.05
7.59
CO
THT‚3
7.14
7.05
2.47 (R)/2.29 (â)
THT‚2
MeSEt
3.28d
3.35f
5.10 7.00/6.95 6.81/6.82 6.72
1.22 (s), 1.0 (t) 1.98 (q)
2.28, 2.46/2.58e
7.88/7.93, 6.88/6.84, 2.13/2.23
DABCO
5.25 7.25
7.13
6.92
7.05
6.84
p-Me-C6H4S(O)Me 1.255/1.275 1.79
3.183/3.150 5.09 7.02
3.185/3.120
3-cyanopyridine
1.48
1.47
1.82
3.354/3.078 4.83 7.26/7.19 7.23
7.14
8.71/8.0, 8.16/7.98 (d), 5.96/6.18 (d),
5.62/5.80 (t)
HCtCSiMe3
2.10/1.98 3.80/3.52f
5.23 7.56/7.50 7.31/7.28 7.16
a
b
In toluene-d8; reference is δ(CD2H-C6D5) 2.09. Pairs of chemical shifts refer to low-temperature spectra and are given in the order
c 4
d
major/minor diastereoisomer.
J
) 1.45 Hz. Not resolved at low temperature; see text. e CH2 of bisadduct and monoadduct, respectively.
PH
f Mean chemical shifts of two AB quartets.
Eyring plots to 273 K. ∆Sq values obtained from the
temperature dependence of ∆Gq were small ((2-5
J /(mol K)) and were subject to greater uncertainty.
They cannot be used as a basis for a mechanistic
discussion.
Ta ble 2. Activa tion P a r a m eter s for Liga n d
Exch a n ge a n d a ca c In ver sion in Ad d u cts of 3
Ea kJ /mol
ligand
exch
∆Gq kJ /mol
273
Tc,
K
ligand
inversion exch
ligand added to 3
inversion
The activation parameters are derived from first-order
rate constants as inverse NMR lifetimes. To convert
these into chemical rate constants as defined by Schemes
2 and 3, they have to be multiplied by a factor of 2, since
on average every second dissociation event will inter-
change benzylic protons. This difference, however, does
not influence Ea and consequently ∆Gq273, which depend
only on the temperature dependence of k and not on its
absolute value.
none
PPh3
THT
DABCO
MeSEt
p-Me-C6H4S(O)Me 273 55.5a
3-cyanopyridine
195 43.5 ( 0.7
350 111.3
233 55.2
233 (44.2)a
228 27.7b
33.2
116.4
95.5
48.1 ( 0.8
32.7
60.5b,c
46.4
76.0
41.8
47.2
82.6
42.1
45.8
47.1
56.3c
49.5
56.2
48.9
233 50.8
a
b
From line shape of Cp* singlets. From line shape of phenyl
o protons. c Subject to larger uncertainty due to change in popula-
tion and chemical shift over the temperature range where line
shape changes.
Sym m et r ica l Ba ses. P P h 3. PPh3 forms a stable
adduct with 3 similar to the P(OMe)3 adduct of 2, which
was characterized by X-ray structure analysis.4 In the
NMR benzylic protons appear as a sharp AB quartet at
ambient temperature. Heating of a toluene solution of
this adduct shows coalescence of the AB quartet at 80
°C. A virtual inversion barrier of 80.6 kJ /mol (∆Gq) at
the coalescence temperature was found from the 1H
NMR line shape. A similar value (Table 2) was obtained
from coalescing 31P signals in a sample containing 1 mol
of PPh3 in excess. Since in that case phosphine ex-
change (k ≈ 70 s-1 at 80 °C11) is 2 orders of magnitude
slower than inversion (extrapolated as ∼7000 s-1 at 80
°C), the coalescence only measures the dissociation of
the phosphine. The same conclusion is drawn from the
similar values for ∆Gq and Ea for the two processes. The
energy of activation can thus be equated to the enthalpy
of phosphine dissociation, which roughly represents the
binding energy of the phosphine to 3.
by integration. In case the fraction of complexed L or
3 respectively changes appreciably with temperature,
a sizeable shift of the corresponding NMR signals would
result (the observed shift is the sum of δixi), which was
not observed. For Kass. . 1, however, changes in K
would no longer be obvious from NMR shifts. In this
case a direct and independent determination of kdiss. and
kass. is the only means for determining Kass.
.
The case of a rapid equilibrium, apparently slowed
down by diminishing the fraction of the equilibrating
species, has been treated in detail for the inversion of
amines protonated to a large extent by adjusting the
pH.9,10 The observed inversion rate constant is related
to the true rate constant kinv simply as
inv
kobsd ) kinv/Kass.
(2)
inv
10
a relation which holds as long as kobsd . kdiss., kass.
.
CO. The CO adduct of 3 shows a single line for
benzylic hydrogens at ambient temperature. Lowering
the temperature causes this line to split into an AB
quartet not by coalescence but by a temperature-
dependent shift difference ∆νAB that steadily increases
with lower temperature, reaching a value of 70 Hz (at
500 MHz) at -100 °C. The AB coupling is not much
different from that of the dimer. This peculiar behavior
is only compatible with a fast, but strongly temperature-
dependent, equilibrium. If this were a dissociation-
association equilibrium, then from the apparent AB
shift difference and under the assumption of complete
This seems to be the case for most of the ligands dealt
with below.
A further feature to be noted in Scheme 3 is the
monomer-dimer equilibrium, which competes with the
association equilibrium. Dimers should be formed as
soon as the monomer concentration, as determined by
the association equilibrium, exceeds that given by the
monomer-dimer equilibrium. As a consequence, inver-
sion barriers observed in the presence of L should never
be lower than those found in pure toluene.
Table 2 collects activation parameters as Ea and as
∆Gq values. The former were obtained from Arrhe-
273
nius plots, whereas the latter are extrapolations of
(11) The rate of chemical exchange kex is twice that value, since only
every second dissociation event causes magnetization transfer from
bound to free phosphine and vice versa; cf.: Green, M. L. H.; Wong,
L.-L.; Sella, A. Organometallics 1992, 11, 2660.
(9) Saunders, M.; Yamada, F. J . Am. Chem. Soc. 1963, 85, 1822.
(10) Delpuech, J .-J . Org. Magn. Reson. 1970, 2, 91.

3278 Organometallics, Vol. 16, No. 15, 1997
Koelle et al.
which is split into separate signals for complexed and
free ligands at low temperature. This means that
different signals for diastereotopic protons in the adduct
are seen only if the exchange between free and coordi-
nated ligand has become slow enough. The same
reasoning is valid for the unsymmetrical sulfide MeSEt
discussed below.
Simulation of the line shape for the R protons gave,
for example, kdiss. ) 450 s-1 at -50 °C. At this same
inv
≈ 50 s-1 M-1 14
,
which means,
F igu r e 4. Preferred conformation of the tetrahydro-
thiophene ligand in 3‚THT, obtained through minimization
of torsional energies.
temperature kobsd
apparently, that inversion is still slower than associa-
tion-dissociation. Applying eq 2 and inserting kinv
)
)
inv
7017 s-1 as a lower limit, gives Kass. ) kinv/kobsd
adduct formation at -100 °C one would obtain only 15%
adduct at -20 °C and 43% at -60 °C: that is, essentially
complete dissociation at room temperature. (Values
were simulated by interchanging the AB pattern rapidly
with a singlet, i.e. a single chemical shift for both lines
but no interchange of individual spins.) The conclusion
is incompatible with the chemical behavior of the CO
adduct, which can be recrystallized without decomposi-
tion. Moreover, the mean chemical shift (νA + νB)/2 is
nearly temperature-independent but differs by 0.35 ppm
from that of 3 in toluene (Table 1), which is incompatible
with substantial dissociation. An alternative explana-
tion would be a conformational equilibrium of the
phenyl group with strongly temperature dependent site
populations. An exchange constant of about 2000 s-1
at -40 °C is required to simulate the observed line
width, and such a fast process is best accounted for by
some internal motion. As a consequence, however, no
dissociation or interconversion barrier could be deter-
mined because the AB shift difference has vanished at
a temperature where onset of dissociation is to be
expected. A relatively stable adduct can be inferred
from sharp lines for the AB system up to 0 °C.
Tetr a h yd r oth iop h en e (THT). Adduct formation of
3 with a ligand of at least Cs symmetry is indicated by
an NMR pattern of lower symmetry in the limit of slow
exchange. Thus, 1:1.05 mixture of 3 and THT (by NMR
intergation) shows sharp signals for THT protons as
well as protons of 3 at ambient temperature. Line
broadening of R (δ 2.44) and â (δ 1.24) THT protons
starts at -20 °C, and the signal finally splits into a 2:1:1
pattern for R and a broad, similar multiplet for â protons
at -80 °C. Figure 4 shows a conformation of the adduct
where the geometry of 3 from the X-ray structure was
combined with a minimum conformation of THT from
a modeling program.12 In this structure the THT ligand
and the phenyl group of 3 are rotated into minimum
energy conformations, respectively. Assignment to the
observed low-temperature multiplet would thus be as
HA + HD:HB:HC. Note that, due to the pyramidal
configuration at sulfur, exchange of HA with of HC and
of HB with HD is only achieved by dissociation of THT,
not by rotation around the Ru-S bond. The assignment
is corroborated by a 1:1 splitting of the R-protons in the
THT adduct of 2 at low temperature. In contrast to the
case for sulfide adducts [CpRe(NO)(PPh3)SR2]+,13 we
measure dissociation of sulfide rather than rotation and
inversion at the sulfur atom. This is because in all cases
where the ligand was added in excess, a single signal
for the ligand groups occurs at ambient temperature,
7017/50 ) 140 M-1, that is, an association equilibrium
far toward the adduct.
Dia za [2.2.2]bicycloocta n e (DABCO). In a toluene
solution containing a 1:1.7 mixture of 3 and DABCO line
broadening of the DABCO singlet starts at -20 °C. At
-80 °C the signal has split into a sharp singlet for
uncomplexed amine (δ 2.34) and three broad signals of
unequal intensity for complexed amine assigned to the
monoadduct (δ 2.57, 2.47) and the symmetrical bis-
adduct 3‚DABCO‚3 (δ 2.28). Concomitant with these
splittings the Cp*, 3-Me, and all three phenyl multiplets
are doubled in the same ratio (1:1.65) given by
3‚DABCO‚3:3‚DABCO. Benzylic protons appear as a
single AB quartet, and also the methine proton of 3 is
one singlet. Both seem to have accidentally identical
shifts in both adducts. Chemical shifts δA and δB
different from those of 3-3 in toluene again show that 3
is entirely incorporated into adducts with the amine. It
is difficult in this case to determine the exchange rate
of the amine due to two different, possibly temperature-
dependent, equilibria involved. A qualitative estimate
shows, however, that this exchange is rather fast and
is in any case faster than the apparent inversion, which
is considerably slowed down by amine complexation
(Table 2).
Un sym m etr ica l Ba ses: Dia ster eoselective Ad -
d u ct F or m a tion . When the ligand added to 3 is
prochiral, two new chiral centers at Ru and at the
ligating atom are generated on adduct formation, giving
rise to two diastereomeric pairs. Diastereoselectivity
in adduct formation was evaluated with four unsym-
metrical (prochiral) ligands: MeSEt, p-Me-C6H4S(Me)O,
3-cyanopyridine, and an alkyne, Me3SiCtCH. Dia-
stereoisomers are visible in the NMR in the first two
cases but do not show up in the cyanopyridine adduct
and are not clearly distinguishable for the acetylene.
MeSEt. Due to the prochiral nature of the unsym-
metrical sulfide, in principle four diastereoisomeric
adducts can be formed. LT NMR spectra containing 3
and S(Me)Et in the ratio 1:1.14 show two diastereo-
isomers in the ratio 1:0.7, visible from, e.g., the two
doublets of the phenylic ortho protons (Table 1) or the
two methyl triplets of the S-Et group. This is to be
expected, since only in one of the two possible orienta-
tions of the sulfide is the lone pair positioned for adduct
formation.
Benzylic protons start line broadening at -20 °C and
appear as a partially resolved AB quartet with a very
small shift difference (∆νAB = 0.03 ppm) at -50 °C.
(12) Ho¨veler, U. Moby; University of Mu¨nster, Springer 1992.
(13) Me´ndez, N. Q.; Arif, A. M.; Gladysz, J . A. Organometallics 1991,
10, 2199.
(14) kobsdinv is a second-order rate constant; thus, the inverse lifetime
as determined from NMR has to be divided through the ligand
inv
concentration (about 0.1 M) to convert it to kobsd
.

Coordinatively Unsaturated Cp*Ru Alkoxo Complexes
Organometallics, Vol. 16, No. 15, 1997 3279
Below -60 °C the signals broaden again and at -80 °C
appear as two AB quartets. Chemical shifts at ambient
and low temperature are different from those of 3-3 in
toluene. Thus, most of the Ru complex is present as
the sulfide adduct. Simulating line broadening of the
o-protons and of the Me triplets yields activation ener-
gies around 30 kJ /mol. Though these figures are not
very precise in this case, they nevertheless prove an
association equilibrium faster than the inversion with-
out extra ligand present. It appears that, notwithstand-
ing a more mobile association-dissociation equilibrium,
the virtual inversion can still be slower than in the
dimer. The peculiar temperature-dependent behavior
of the benzylic protons points to the existence of two
processes with different activation energies (not resolved
explicitly in the experiment). One slows down inversion
and the second one, operative at still lower temperature,
causes splitting into diastereoisomers. The point is
further discussed below.
one another in the temperature interval of slow to
medium exchange.
Me3SiCtCH. In the case of this unsymmetrical
acetylene, adduct formation is less easy to prove. When
the acetylene is added in about 3-fold excess, the Me3Si
singlet at δ 0.28 starts to broaden from -20 °C but
exchange is slow only at -50 °C, at which point separate
signals for complexed and free acetylene have developed.
Since the chemical shift of the free acetylene Me3Si
group has undergone only a minor change between
ambient and low temperature (and in the wrong direc-
tion, i.e. downfield), it must be concluded that associa-
tion equilibrium at ambient temperature is not only fast
but also to a larger extent is shifted toward the free
complex: that is, 3-3. Consequently, line broadening
of the benzylic singlet starts at the same temperature
as in neat toluene but much lower temperature is
needed for individual lines to reappear. The spectrum
at -90 °C, though not yet sharp in the benzyl region,
shows a pattern similar to that of the diastereoisomeric
pair of dimers of 3 in toluene, but with additional peaks
for an acetylene adduct. Two methyl signals of unequal
intensity for complexed acetylene around δ 0.44 at this
temperature can be interpreted as originating from two
orientations of the acetylene with respect to the benzyl
group in the adduct. From line broadening of the ortho
protons the activation energy Ea ) 18 kJ /mol can be
estimated for the inversion of the acac ligand, which
is much lower than the value found for the dimer in
toluene. In this case the adduct seems to be rather
weak and to compete with the dimer.
p-Me-C6H4S(Me)O. The sulfoxide was added to the
toluene solution of 3 as the pure S enantiomer to obtain
a 3:p-Me-C6H4S(Me)O ratio of 1:1.4. Signal broadening
is already obvious for some signals at ambient temper-
ature. All signals broaden below 20 °C and are sharp
again at -45 °C. At this temperature the S(O)Me group
and, most noticeably, the ortho protons from the sulf-
oxide arene around δ 7.9 have separated into three sets,
assigned to the two diastereoisomeric adducts RRu and
SRu and the signal for the uncomplexed ligand. In the
same manner were Cp* and Me singlets from 3 split
into two signals, each of intensity ratio 1:0.45 (at -45
°C). Benzylic protons, at -45 °C, appear as two sharp
AB quartets of unequal intensity with chemical shifts
and intensity ratios distinctly different from those of 3
in toluene. Temperature-dependent line shapes of the
different groups were simulated, and the resulting k
values were found close enough to ascertain the same
process as being responsible for all line shape changes.
The value for Ea given in Table 2 has been obtained by
simulating the line shape of the Cp* signal. It also
became obvious during the simulation that intensity
ratios had to be changed for different temperatures,
being closer to 1 at higher temperature. The ratio
change in the temperature interval where separate
signals are seen for e.g. the Cp* group (-45 to -15 °C)
is, however, too small to allow thermodynamic param-
eters for the equilibrium of diastereoisomers to be
determined.
Discu ssion
Addition of two-electron-donor ligands L to the solu-
tions of the dimers 2-2 and 3-3 in all cases cleaves them
into monomeric adducts (2,3)‚L. This occurs with
π-acceptor as well as with pure σ-donor ligands. The
adducts are, however, of widely differing stability,
ranging from isolable compounds for phosphines, phos-
phites, and CO to solution species, whose formation is
evident from the influence of added ligand on the
inversion rate of the dimer.
Due to a number of shortcomings, such as chemical
shifts and equilibrium populations varying with tem-
perature, which cannot be evaluated above coalescence
temperature, some of the activation parameters Ea and
∆Gq in Table 2 pertaining to ligand association can
273
bear a greater uncertainty which is not reflected in the
standard deviation of the Arrhenius plots and are
believed to be realistic within 5-10%. Bearing in mind
these uncertainties, it still follows from Table 2 un-
ambiguously that for the ligand MeSEt not only the
association equilibrium but also the inversion, measured
in that case from line broadening of phenylic ortho
protons, is faster than for the pure dimer (Ea is below
the value of the dimer in pure toluene). The same is
true for the Me3SiCtCH adduct, where Ea has been
found from ortho proton line broadening as low as about
18 kJ /mol. The situation arises if, in spite of an
equilibrium shifted toward the adduct 3‚L, the latter is
kinetically still very fast. This fact arises because the
observed NMR lifetime is determined by the faster of
the two competing equilibria, adduct formation-dis-
sociation or dimerization. This will hold as long as the
inversion in the monomer is much faster than either
3-Cya n op yr id in e. The LT NMR of a solution con-
taining 3 and 3-cyanopyridine in the ratio 1:1.23 shows
two sets of pyridine signals in a corresponding ratio with
relatively large displacements, assigned to complexed
and free pyridine. Benzylic protons are split into a
single AB quartet; aromatic protons from the Phacac
ligand are not subject to any temperature-dependent
line broadening in this case. Moreover, Me and Cp*
signals remain singlets down to low temperature. This
means that the ligand gives rise to one single dia-
stereoisomer only, or more realistic, to two diasteromers
still rapidly interconverting at low temperature through
rotation around the Ru-N bond. The existence of a
sharp AB quartet, on the other hand, shows that the
process occurs without Ru-N bond cleavage. The first-
order pyridine exchange rate and the rate of acac
inversion have been found to be not much different from

3280 Organometallics, Vol. 16, No. 15, 1997
Koelle et al.
Ta ble 3. NMR Sh ifts of Com p lexes [Cp *Ru (η2-OC(R1)CR3C(R2)O)]n a n d Cp *Ru (η2-OC(R1)CHC(R2)O)L
R1, R2, R3
Cp*
CH (R3)
R1 R2
OEt, OEt, H (6a )a
OEt, OEt, H (6b)a
Me, Me, Bzb
1.62
1.67
1.66
1.61
1.68
5.15
3.03
4.09 (q), 1.18(t)
3.88 (q), 0.87(t)
1.97 (s)
2.01 (s), 4.17 (q), 1.17 (t)
2.10 (s)
3.38 (s), 7.01 (m), 7.16 (m)
Me, OEt, Ha
5.07
5.85
5.65
Me, Ph, Hb
Me, CF3, Ha
1.58, 2.15 (q), 0.87 (t)c
Cp*
2.01
R1 R2 R3
L
CH (R3)
R1, R2
4
Me, OEt, H,d P(OMe)3
OEt, OEt, H, PPh3
1.64 (t, J PH ) 2.6 Hz)
4.96
4.65
5.60
5.83
5.92
5.10
3.46 (P(OMe)3), 2.66 (Me), 4.36, 1.26 (OEt)
3.90, 0.94 (OEt)
1.73
1.86
1.38 (d) (4J PH ) 1.45 Hz)
1.42 (d) (4J PH ) 0.6 Hz)
1.56 (d) (4J PH ) 1.6 Hz)
1.54
b
Me, Ph, H, PPh3
b
Me, Ph, H, PMe3
Me, Ph, H, COb
1.92
e
Me, Bz, H, PMe3
1.44 (d) (4J PH ) 1.7 Hz)
3.25 (CH2), 1.73 (CMe), 1.11 (PMe)
a
b
d
In C6D12
CD3COCD3.
.
In C6D6. c Cp ligand is C5Me4Et. According to NMR the complex is Cp*Ru(η1-OC(Me)CHCOOEt)(P(OMe)3)2. e In
Ta ble 4. Cr ysta llogr a p h ic Da ta , Collection
P a r a m eter s, a n d Refin em en t Resu lts for 3
one of these processes. The existence of a thermody-
namically favored (over dimer formation) but kinetically
labile adduct explains the benzyl singlet in 3‚PMe3 at
ambient temperature as well as the rapid inversion of
3-3 at low temperature found in THF. In the latter case
the adduct 3‚THF, though thermodynamically favored,
must be kinetically particularly labile in order to
counterbalance the high concentration of solvent (about
10 M) with respect to complex.
empirical formula
fw
(C21H26O2Ru)2
823.02
diffractometer
cryst color
cryst shape
cryst dimens (mm)
cryst syst
space group
a (Å)
Enraf-Nonius CAD4
dark red
irregular
ca. 0.3 × 0.3 × 0.3
triclinic
P1h (No. 2)
9.580(1)
10.336(1)
11.003(1)
100.63(1)
110.71(1)
103.176(9)
949.28
b (Å)
c (Å)
We finally turn to the two consecutive processes
observed in the case of MeSEt as ligand, when the
temperature was lowered. A single, partially developed
AB quartet at -60 °C means slow inversion at this
temperature. Since the chemical shift of this AB
quartet is quite different from that observed for the
dimers 3-3 (Table 1), it seems clear that the splitting
at this temperature refers to the adduct 3‚S(Me)Et. At
this temperature, however, dissociation of the adduct
must still be rapid, since separate patterns for indi-
vidual diastereoisomers, such as splitting of the ligand
signals, occur only at still lower temperature. The AB
pattern at -60 °C is reproduced with an inversion rate
constant of about 10 s-1, whereas from the difference
of chemical shifts of the two AB quartets at -90 °C a
rate constant k > 50 s-1 is calculated as being necessary
for coalescence of these quartets. In other words, a
sulfide molecule must have a chance to dissociate and
reassociate with Me and Et groups interchanged with-
out inversion of the acac ligand occurring. This in turn
means a finite lifetime of the coordinatively unsaturated
pyramidal species as depicted in Scheme 3.
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
2.5
1.440
424
8.2
ω/2θ
298
Dcalcd (g/cm3)
F000
µ(Mo KR) (cm-1
scan type
)
temp (K)
no. of unique rflns
4321
no. of obsd rflns (I > 2σ(I))
no. of params in refinement
4113
218
R
0.026
Rw
0.028
16
(acac)2 (98.8 ppm) or 3‚CO (98.78 ppm) (C6D6 for all).
The small energy of dimerization ensures that 2-2 and
3-3 are convenient sources of the coordinatively unsat-
urated monomers for all practical preparative purposes.
Exp er im en ta l Section
Our quantum-chemical calculations on the model
compound CpRu(acac) revealed a very shallow mini-
mum for the pyramidal Cs as compared to the straight
C2v conformation. In fact, the stabilization of the 16-
VE pyramidal species calculated for CpMn(CO)2 by
Hofmann15 and verified experimentally for CpRe(NO)-
PPh3+ by Gladysz and co-workers8 is counteracted if the
ligands are σ-donors and in particular π-donors instead
of acceptors, a fact already obvious at the EHT level.
Preparation of acac complexes was done with anhydrous,
nitrogen-saturated solvents under nitrogen using conventional
Schlenk techniques. NMR spectra were recorded at 500 MHz
proton frequency on a Varian Unity 500 instrument. Tem-
perature calibration was done with methanol. Line shape
simulations were performed with the GEM-NMR17 or the
DNMR518 program. Line shapes of the two overlapping AB
quartets of 3 were kindly simulated by Dr. H. Brussaard at
the University of Nijmegen.
P r ep a r a tion of 2 fr om [Cp *Ru Cl2]2. The heterogeneous
mixture of 0.224 g (0.37 mmol) of [Cp*RuCl2]2, 1 g of anhydrous
K2CO3, and 78 mg (0.74 mmol) of acacH in 20 mL of 2-propanol
The relatively weak intermolecular binding within the
dimers is reflected not only in the long Ru-C7 distance
as discussed above but also in the very small coordina-
tion shift of C7 in the 13C NMR. Thus, in 2-2 C7
resonates at 95.4 ppm, which can be compared to
monomeric acac complexes of RuII such as (COD)Ru-
(16) Koelle, U.; Flunkert, G.; Go¨rissen, R.; Schmidt, M. U.; Englert,
U. Angew. Chem., Int. Ed. Engl. 1992, 31, 440.
(17) A version of GEM NMR by U. Seimet, University of Kaisers-
lautern, was kindly provided by Dr. C. G. Kreiter, University of
Kaiserslautern.
(18) A PC version of DNMR 5 was kindly provided by Dr. H.
Wadepol, University of Heidelberg.
(15) Hofmann, P. Angew. Chem., Int. Ed. Engl. 1977, 16, 536.

Coordinatively Unsaturated Cp*Ru Alkoxo Complexes
Organometallics, Vol. 16, No. 15, 1997 3281
was stirred overnight at ambient temperature, whence the
color changes from brown to red. 2-Propanol was stripped in
vacuo; the dry residue was extracted with several 10 mL
portions of pentane, giving a red solution. When it was cooled
to -78 °C, the concentrated pentane solution gave 0.23 g (94%)
of red needles. 1H NMR (C6D6): δ 1.64 (s, 15 H), 1.95 (s, 6 H),
5.11 (s, 1 H).
Complexes 3-8 were prepared by adding a stoichiometric
amount of the pentanedionate to a solution of 1 in pentane.
After 1 h at ambient temperature the solutions were concen-
trated to a small volume. When they were cooled to -78 °C,
the products crystallized as red needles or plates. Yields
depend on the solubility and are nearly quantitative for the
less soluble derivatives. Complexes were characterized by
NMR, MS, and in a few cases by elemental analysis. For NMR
data, see Table 3.
48.98 (CH), 47.66, 48.9, 50.1, 57.6, 57.9 (bridge and bridgehead
C), 114.9, 203.1 (OC), 161.3, 117.2, 111.7 (CF); 31P NMR δ
41.11/42.01.
X-r a y Str u ctu r e Deter m in a tion of 3. A single crystal
of suitable quality was grown from pentane. The crystal was
sealed in a glass capillary under a protecting argon atmo-
sphere. Data were collected at room temperature. Details of
the X-ray structure determination are listed in Table 4. The
solid-state structure of 3 is shown in Figure 1, and selected
structural parameters are listed in the figure caption. The
structure was solved by the heavy-atom method, employing
the Fourier routine of the XTAL3.219 package of crystal-
lographic programs.
Su p p or tin g In for m a tion Ava ila ble: Tables giving X-ray
experimental details, positional and thermal parameters, bond
distances and angles, and dihedral angles for 3 (15 pages).
Ordering information is given on any current masthead page.
3: Anal. Calcd for C21H26O2Ru (Mr 411.5): C, 61.29; H, 6.37.
Found: C, 61.07; H, 6.31. 6a /6b: Anal. Calcd for C17H26O4-
Ru (Mr 395.4): C, 51.63; H, 6.63. Found: C, 51.33; H, 6.67.
OM960718G
4
8a /8b: 1H NMR (C6D6) δ 1.03/0.97 (d, J PH ) 1.6 Hz, Cp*),
0.31, 0.33, 0.564 (s, Me), 2.39/2.48 (m, CH), 1.38 (m, CH2), 0.85
(m, CH2), 7.42/7.07 (m, m phenyl), 6.8 (m, o,p phenyl); 13C NMR
(C6D6) δ 9.24 (Cp*), 8.45, 17.88, 20.01 (Me), 27.99, 29.76 (CH2),
(19) Hall, S. R.; Flack, H. D.; Stewart, J . M. XTAL3.2 Reference
Manual, Universities of Western Australia, Geneva, and Maryland,
1992.