[Re2(CO)9{(µ-H)ReH(CO)4}]-
Organometallics, Vol. 15, No. 18, 1996 3881
case), with formation of free [HRe(CO)5] and [M2(CO)9L]
derivatives.7-10,25
-5.77 and -16.62 for PPN+, THF, 193 K), indicating
that the interactions of the H ligands with the alkali
cation (and therefore their hydridic character) are not
particularly strong.
In line with this, strong acids did not cause significant
H2 evolution but rather clean fragmentation of the
trinuclear unit, according to eq 8. The protonation
We investigated therefore the reactivity of the anion
2 with two-electron donors. If the above reported failure
of substituting MeCN by [H2Re(CO)4]- in [Re2(CO)9-
(NCMe)] was due to thermodynamic reasons, acetoni-
trile could be a ligand able to displace [H2Re(CO)4]-.
No reaction, however, was observed, even on using
acetonitrile as solvent. In the presence of CO, on the
contrary, a slow reaction took place, leading however
not to [Re2(CO)10] and [H2Re(CO)4]- but to [HRe2(CO)9]-
and [HRe(CO)5], in equimolar ratio, according to the
reverse of eq 7. In order to clarify the origin of the [HRe-
(CO)5] fragment, the reaction was monitored using
natural abundance CO and the isotopomer of 2 selec-
tively 13CO enriched in the Re(CO)5 spike. The 13C
NMR spectra showed that both the products [HRe2-
(CO)9]- and [HRe(CO)5] contained 13CO.26 The forma-
tion of a symmetrical intermediate containing two [(µ-
H)Re(CO)5] groups cis bound to a central Re(CO)4 unit
could account for this.
[Re3H(µ-H)(CO)13]- + H+
f
[Re2(µ-H)2(CO)8] + [HRe(CO)5] (8)
likely occurs on the unbridged Re-Re interaction and
weakens the metal-metal interaction. Reaction 1 is
then reversed, [HRe(CO)5] being a worse donor than
[Re(CO)5]-.
Mixed-Metal [Mn Re(CO)9{(µ-H)ReH(CO)4}]-. The
addition of [Mn(CO)5]- to [Re2(µ-H)2(CO)8] caused the
instantaneous formation of a species that at 203 K
exhibits two hydridic resonances, of the same integrated
intensities, at δ values typical of a terminal (-5.79) and
a bridging (-16.24) H ligand (J HH ) 6.5 Hz). The
reaction product can therefore be formulated as the
addition derivative [ReMn(CO)9{(µ-H)ReH(CO)4}]- (3)
with a structure analogous to that of 2. The dynamic
behavior in solution is also quite similar to that of 2,
because, on increasing the temperature, the hydridic
resonances broaden and coalesce at room temperature,
giving an averaged signal at δ -11.0 ppm (333 K), with
∆Gq ) 55(1) kJ mol-1 at 263 K.
The reactivity with [Re(CO)5]- was also investigated,
to verify if this strong nucleophile27 was able to substi-
tute the [H2Re(CO)4]- “ligand” and/or to give mutual
exchange with the Re(CO)5 spike (a process detectable
on using 13CO-enriched isotopomers; see above). How-
ever, 1H and 13C NMR monitoring showed that no
reaction occurs between 2 and the pentacarbonylrhen-
ate. This allows also to rule out any significant protonic
polarization of the hydrido ligands, since [Re(CO)5]- is
a strong Bro¨nsted base.28
Further experiments were performed to verify if the
H ligands have some hydridic (H-) character, as is often
the case for anionic transition metals hydrido com-
plexes.29 A relationship has been previously found
between the ability of hydrido-carbonylates to serve as
hydride donors and their tendency to interact with
Lewis acids, such as alkali cations, at the metal hydride
site.30 This type of interaction can be revealed by
observing the effect of the counterion on the δ value of
the hydridic resonances: high-field shifts up to more
than 1 ppm have been observed, in THF solution, for
the alkali cations salts, with respect to the salts with
larger cations, such as Et4N+ or PPN+.30 In the present
case, the changes of chemical shifts of the hydridic
resonances are small (δ -5.82 and -16.65 for Na+, δ
Con clu sion s. Three clean synthetic routes to the
novel open cluster [Re2(CO)9{(µ-H)ReH(CO)4}]- anion
have been found (Scheme 3). All involve condensation
between a neutral and an anionic reagent and belong
to three general types of cluster growth reactions:1 (i)
addition of organometallic fragments to an unsaturated
complex;2 (ii) substitution of a labile ligand by a H-M
σ-donor anionic ligand;22 (iii) substitution of a CO ligand
by an anionic ligand (redox condensation).31 Particu-
larly interesting, within this latter class, are the clean
reactions of [HRe(CO)5] with H-M anionic σ-donors (eqs
5 and 7).
It is also worth noting that method (i) has so far
allowed the addition to [Re2(µ-H)2(CO)8] only of car-
benoid or anionic organometallic fragments, but we
failed to add neutral H-M σ-donors, such as [HRe-
(CO)5]. The fragmentation of 2 upon protonation indi-
cates that this could be due not to a failure of the
method (kinetic barrier) but to the thermodynamic
instability of the hypothetical “Re3H(µ-H)2(CO)13” prod-
uct with respect to [Re2(µ-H)2(CO)8] and [HRe(CO)5].
The anion 2 exhibits the L-shape of the previously
known 50 valence electron trinuclear clusters containing
a bridging hydrido ligand. The substitution of a car-
bonyl in the complex 4 by an H- ligand in 2 has only
minor structural effects. The staggered-staggered
conformation of 4 is maintained also in the solid-state
structure of 2. In solution, conformational freedom
around the metal-metal interactions, even if H-bridged,
has been indicated by the low-temperature 13C NMR
spectra of both the species, the number of 13C signals
being dictated only by the local symmetry.
(25) Harrill, R. W.; Kaesz, H. D. Inorg. Nucl. Chem. Lett. 1966, 2,
69.
(26) Further investigation on this point is on schedule. All the
carbonyl sites of [HRe2(CO)9]- resulted homogeneously enriched in
13CO. This could be due to some fluxional process exchanging the
carbonyls between the two metallic sites: the 13C-NMR analysis
showed indeed that, above 273 K, in THF-d8, all the resonances
broaden,24 but the reasons have still to be clarified (quadrupolar
broadening24 or/and localized or/and nonlocalized CO exchange). On
the other hand, when reaction 7 was performed with 13CO-enriched
[HRe2(CO)9]- and natural abundance [HRe(CO)5], an equilibrium
mixture was obtained in which the anion 2 and the unreacted reagents
showed the same 13CO abundance. This could be explained by the
reversibility of the reaction and an intermetallic fluxionality of the
carbonyls in [HRe2(CO)9]-, but alternative explanations are possible.
(27) (a) Dessy, R. E.; Pohl, R. L.; King, R. B. J . Am. Chem. Soc. 1966,
88, 5121. (b) Pearson, R. G.; Fidgore, P. E. J . Am. Chem. Soc. 1980,
102, 1541. (c) Lai, C. K.; Feighery, W. G.; Zhen, Y.; Atwood, J . D. Inorg.
Chem. 1989, 28, 3929.
(28) (a) Moore, E. J .; Sullivan, J . M.; Norton, J . R. J . Am. Chem.
Soc. 1986, 108, 2257. (b) Stevens Miller, A. E.; Kawamura, A. R.;
Miller, T. M. J . Am. Chem. Soc. 1990, 112, 457 and references therein.
(29) Darensbourg, M. Y.; Ash, C. E. Adv. Organomet. Chem. 1987,
27, 1.
On the contrary, the exchange of the bridging hydride
between the two Re-Re interactions, which on a struc-
(30) Kao, S. C.; Darensbourg, M. Y.; Schenk, W. Organometallics
1984, 3, 871.
(31) Chini, P. J . Organomet. Chem. 1980, 200, 37.
Bergamo, Mirka
