1,2-eq,eq-[Re2(CO)8(THF)2]: A reactive Re2(CO)8 fragment that easily activates H-H and C-H bonds

Article abstract of DOI:10.1021/om9806693

The reaction of [Re₂(μ-H)₂(CO)₈] (1) with diazomethane at 193 K in THF-d₈ gives the unstable [Re₂(μ-H)(μ-CH₃)(CO)₈] derivative (2) containing a methyl group arising from the insertion of CH₂ into a Re-H-Re bond. Isotopic perturbation of the equilibria by partial deuteration demonstrated that the methyl bridges the Re-Re bond in an unsymmetrical way, with a fast exchange between one agostic and two terminal C-H bonds. At temperatures higher than 253 K, 2 decomposes, in THF solution, with CH₄ elimination to give the novel red complex 1,2-eq,eq-[Re₂(CO)₈(THF)₂] (3), which was characterized by NMR spectroscopy and X-ray analysis. In the solid form a staggered conformer of C₂ symmetry was found. ¹³C NMR analysis revealed the presence, in wet THF, of the aquo complexes [Re₂(CO)₈(THF)-(H₂O)] (4) and [Re₂(CO)₈(H₂O)₂] (5), whose formation is favored at low temperature (ΔH° for the formation of 5 from 4: -14.4(2) kJ mol^-1). In solution, due to the lability of the THF ligands, 3 behaves as a lightly stabilized Re₂(CO)₈ fragment, capable of activating different E-H bonds. Reaction with HCl in THF leads to [Re₂(μ-H)(μ-Cl)(CO)₈], while with H₂ the unsaturated starting material [Re₂(μ-H)₂(CO)₈] is obtained. In THF solution, at room temperature, reactions with phenylacetylene, styrene, and acetaldehyde give the derivatives of C-H activation [Re₂(μ-H)(μ-C≡CPh)(CO)₈], [Re₂(μ-H)(μ-CH=C(H)Ph)(CO)₈], and [Re₂(μ-H)(μ-η²-C(Me)O)(CO)₈]. Moreover, the activation of an sp³ C-H bond in ethyl acetate occurs slowly when 3 is dissolved in the reactant itself, the resultant product being [Re₂(μ-H)(μ-η²-CH₂C(O)OEt)(CO) ₈].

Full text of DOI:10.1021/om9806693

Organometallics 1999, 18, 2091-2098
2091
1,2-eq,eq-[Re2(CO)8(THF )2]: A Rea ctive Re2(CO)8
F r a gm en t Th a t Ea sily Activa tes H-H a n d C-H Bon d s
Lucia Carlucci* and Davide M. Proserpio
Dipartimento di Chimica Strutturale e Stereochimica Inorganica and Centro CNR, CSMTBO,
Via Venezian 21, 20133 Milano, Italy
Giuseppe D’Alfonso*
Dipartimento di Chimica Inorganica, Metallorganica e Analitica and Centro CNR, CSMTBO,
Via Venezian 21, 20133 Milano, Italy
Received August 4, 1998
The reaction of [Re2(µ-H)2(CO)8] (1) with diazomethane at 193 K in THF-d8 gives the
unstable [Re2(µ-H)(µ-CH3)(CO)8] derivative (2) containing a methyl group arising from the
insertion of CH2 into a Re-H-Re bond. Isotopic perturbation of the equilibria by partial
deuteration demonstrated that the methyl bridges the Re-Re bond in an unsymmetrical
way, with a fast exchange between one agostic and two terminal C-H bonds. At temperatures
higher than 253 K, 2 decomposes, in THF solution, with CH4 elimination to give the novel
red complex 1,2-eq,eq-[Re2(CO)8(THF)2] (3), which was characterized by NMR spectroscopy
and X-ray analysis. In the solid form a staggered conformer of C2 symmetry was found. 13C
NMR analysis revealed the presence, in wet THF, of the aquo complexes [Re2(CO)8(THF)-
(H2O)] (4) and [Re2(CO)8(H2O)2] (5), whose formation is favored at low temperature (∆H° for
the formation of 5 from 4: -14.4(2) kJ mol-1). In solution, due to the lability of the THF
ligands, 3 behaves as a “lightly stabilized” Re2(CO)8 fragment, capable of activating different
E-H bonds. Reaction with HCl in THF leads to [Re2(µ-H)(µ-Cl)(CO)8], while with H2 the
unsaturated starting material [Re2(µ-H)2(CO)8] is obtained. In THF solution, at room
temperature, reactions with phenylacetylene, styrene, and acetaldehyde give the derivatives
of C-H activation [Re2(µ-H)(µ-CtCPh)(CO)8], [Re2(µ-H)(µ-CHdC(H)Ph)(CO)8], and [Re2(µ-
H)(µ-η2-C(Me)O)(CO)8]. Moreover, the activation of an sp3 C-H bond in ethyl acetate occurs
slowly when 3 is dissolved in the reactant itself, the resultant product being [Re2(µ-H)(µ-
η2-CH2C(O)OEt)(CO)8].
In tr od u ction
are the [IrH2(solv)2(PPh3)2]+ (solv ) solvent)5,6 and
[Cp*Ir(CH3)(ClCH2Cl)(PMe3)]+ cations,7 both capable of
alkane activation.
Transition-metal complexes containing weakly coor-
dinated ligands are of current interest.1 In many reac-
tions such complexes appear only as a transient species,
as can be seen by the temporary coordination of the
polar cosolvent in the classical Wilkinson mechanism
of olefin hydrogenation.2 In other cases these complexes
are generated in solution as long-lived species or even
isolated in the solid state. The facile dissociation of the
labile ligands allows easy substitution and often pro-
vides the vacant site necessary for binding and activat-
ing several substrates. In organometallic chemistry this
is well-illustrated by the rich chemistry displayed by
the [Os3(CO)10L2] (L ) cyclooctene, MeCN) derivatives3
or by the [Cp2Zr(CH3)(THF)]+ cation.4 Also significant
As far as the tetrahydrofuran ligand is concerned, the
affinity of the hard donor oxygen atom for hard metallic
centers accounts for the high number of THF adducts
exhibited by transition-metal cations.8,9 However, tet-
(4) See for instance: J ordan, R. F.; Guram, A. S. Organometallics
1990, 9, 2116 and references therein.
(5) Shapley, J . R.; Schrock, R. R.; Osborn, J . A. J . Am. Chem. Soc.
1969, 91, 2816.
(6) Crabtree, R. H.; Demou, P. C.; Eden, D.; Mihelcic, J . M.; Parnell,
C. A.; Quirk, J . M.; Morris, G. E. J . Am. Chem. Soc. 1982, 104, 6994.
(7) Arndtsen, B. A.; Bergman, R. G. Science 1995, 270, 1970.
(8) See for instance: (a) Vavoulis, A.; Austin, T. E.; Tyree, S. Y.
Inorg. Synth. 1960, 6, 129. (b) Kern, R. J . J . Inorg. Nucl. Chem. 1962,
24, 1105. (c) Collman, J . P.; Kittleman, E. T. Inorg. Synth. 1966, 8,
49. (d) Dilworth, J . R.; Richards, R. L. Inorg. Synth. 1980, 20, 119. (e)
Davies, J . A.; Hartley, F. R.; Murray, S. G. J . Chem. Soc., Dalton Trans.
1980, 2246. (f) Manzer, L. E. Inorg. Synth. 1982, 21, 135. (g) J ones, P.
J .; Hale, A. L.; Levason, W.; McCullough, F. P. Inorg. Chem. 1983, 22,
2642. (h) Boudjouk, P.; So, J .-H. Inorg. Synth. 1992, 29, 108. (i)
Dilworth, J . R.; Richards, R. L. Inorg. Synth. 1990, 28, 33. (j) Dilworth,
J . R.; Zubieta, J . Inorg. Synth. 1990, 28, 36. (k) Deacon, G. B.; Tuong,
T. D.; Wilkinson, D. L. Inorg. Synth. 1990, 28, 286. (l) Deacon, G. B.;
Pain, G. N.; Tuong, T. D. Inorg. Synth. 1990, 28, 291. For a list of
crystallographically characterized examples of THF coordination in
main-group and transition metals see also the references quoted in
ref 9.
* To whom correspondence should be addressed. E-mail: L.C.,
lucia@csmtbo.mi.cnr.it; G.D., dalf@csmtbo.mi.cnr.it.
(1) See for instance the attention devoted to the theme of Weakly
Coordinating Anions at the 215th National Meeting of the American
Chemical Society (Division of Inorganic Chemistry), Dallas, TX, 1998.
(2) Collman, J . P.; Hegedus, L. S.; Norton, J . R.; Finke, R. G.
Principles and Applications of Organotransition Metal Chemistry;
University Science Books, Mill Valley, CA, 1987.
(3) Tachikawa, M.; Shapley, J . R. J . Organomet. Chem. 1977, 124,
C19.
10.1021/om9806693 CCC: $18.00 © 1999 American Chemical Society
Publication on Web 04/28/1999

2092 Organometallics, Vol. 18, No. 11, 1999
Carlucci et al.
Sch em e 1
rahydrofuran is also able to bind metals in low oxidation
states, as attested by the number of THF adducts
displayed by Re(I) carbonyl complexes.10 The only
example of a THF molecule bonded to a Re(0) center is
provided by the [Re2(CO)9(THF)] species; this is formed,
mixed with the corresponding [Re2(CO)9(H2O)] aquo
species, by protonation of [HRe2(CO)9]- or by Me3NO-
induced decarbonylation of [Re2(CO)10], in THF solu-
tion.11,12 The THF lability in this complex is much
higher than that of MeCN in the related [Re2(CO)9-
(MeCN)] derivative,13 allowing the synthesis of open-
chain tri-12 and tetranuclear11 clusters by replacement
of THF with hydridic complexes able to act as “ligands”
through their M-H σ-bond electron density.14
We report here on the synthesis and characterization
of a novel Re(0) complex containing two THF molecules,
namely 1,2-eq,eq-[Re2(CO)8(THF)2]. A related 1,2-eq,eq-
[Re2(CO)8(NCMe)2] derivative was previously obtained
by decarbonylation of [Re2(CO)10] in MeCN solution with
Me3NO.15-17 We and others15 have tried Me3NO oxida-
tion in THF solution, but no product of disubstitution
was obtained by this method. We now present a com-
pletely different approach, based on the dehydrogena-
tion of the unsaturated dinuclear species [Re2(µ-H)2-
(CO)8]18 with diazomethane. A thermally unstable
µ-hydrido µ-methyl agostic derivative was detected as
an intermediate in this reaction.
valence electrons) with diazoalkanes. Earlier studies
with diphenyldiazomethane and ethyl diazoacetate25
showed a strict parallelism between the reactivity of 1
and that of the related unsaturated triosmium cluster
[Os3(µ-H)2(CO)10].26 In both cases the reaction with ethyl
diazoacetate led to the insertion of the carbene fragment
into a M-H bond of the M(µ-H)2M moiety (Scheme 1).
We have now found that 1 reacts rapidly also with
diazomethane at 193 K, giving [Re2(µ-H)(µ-CH3)(CO)8]
(2; Scheme 1), containing a methyl group arising from
the insertion of CH2 into a Re-H-Re bond.
Resu lts a n d Discu ssion
Reaction 1 was performed directly in an NMR tube,
in THF-d8, at 193 K, and the spectra revealed two
resonances, in a 3:1 ratio, at δ -2.85 and -12.28 ppm,
attributable to the methyl group and the bridging
hydride, respectively. The unusual chemical shift of CH3
We have currently been investigating the reactivity
of the “ethylene-like” complex [Re2(µ-H)2(CO)8]19 (1; 32
(9) Luo, X.-L.; Schulte, G. K.; Crabtree, R. H. Inorg. Chem. 1990,
29, 682.
(10) (a) Vitali, D.; Calderazzo, F. Gazz. Chim. Ital. 1972, 102, 587.
(b) Calderazzo, F.; Vitali, D. Coord. Chem. Rev. 1975, 16, 13. (c)
Calderazzo, F.; Mavani, I. P.; Vitali, D.; Bernal, I.; Korp, J . D.; Atwood,
J . L. J . Organomet. Chem. 1978, 160, 207. (d) Lindner, E.; Trad, S.;
Hoehne, S.; Oetjen, H.-H. Z. Naturforsch., B 1979, 34, 1203. (e) Wong,
A. C.; Wilkinson, G.; Hussain, B.; Motevalli, M.; Hursthouse, M. B.
Polyhedron 1988, 7, 1363. (f) Angaroni, M.; Ardizzoia, G. A.; D’Alfonso,
G.; La Monica, G.; Masciocchi, M.; Moret, M. J . Chem. Soc., Dalton
Trans. 1990, 1895. (g) J oachim, J . E.; Apostolidis, C.; Kanellakopulos,
B.; Meyer, D.; Nuber, B.; Raptis, K.; Rebizant, J .; Ziegler, M. L. J .
Organomet. Chem. 1995, 492, 199.
[Re2(µ-H)2(CO)8] + CH2N2 f
[Re2(µ-H)(µ-CH3)(CO)8] + N2 (1)
is analogous to that reported by Calvert and Shapley27
for [Os3(µ-H)(µ-CH3)(CO)10] (δ -3.6 ppm), obtained by
reacting [Os3(µ-H)2(CO)10] with diazomethane. In that
case NMR experiments of isotopic perturbation of the
equilibrium by partial deuteration were elegantly used
to demonstrate that the methyl group bridged an Os-
Os bond in an unsymmetrical way, with one of the three
C-H bonds involved in an agostic interaction with an
Os center.27c The fast exchange between the hydrogen
in the agostic site and the two terminal hydrogens
accounted for the unique resonance of the CH3 group.
Following the same approach, we prepared a partially
deuterated sample of 2, by reacting 1 with deuterium-
(11) Bergamo, M.; Beringhelli, T.; D’Alfonso, G.; Mercandelli, P.;
Moret, M.; Sironi, A. Organometallics 1997, 16, 4129.
(12) Bergamo, M.; Beringhelli, T.; D’Alfonso, G.; Ciani, G.; Moret,
M.; Sironi, A. Organometallics 1996, 15, 3876.
(13) Koelle, U. J . Organomet. Chem. 1978, 155, 53.
(14) (a) Venanzi, L. M. Coord. Chem. Rev. 1982, 43, 251. (b)
Crabtree, R. H. Angew. Chem., Int. Ed. Engl. 1993, 32, 789. (c)
Bergamo, M.; Beringhelli, T.; D’Alfonso, G.; Moret, M.; Sironi, A. Inorg.
Chim. Acta 1997, 259, 291.
(15) Gard, D. R.; Brown, T. L. J . Am. Chem. Soc. 1982, 104, 6340.
(16) Top, S.; Gunn, M.; J aouen, G.; Vaissermann, J .; Daran, J .-C.;
McGlinchey, M. J . Organometallics 1992, 11, 1201.
(17) Bruce, M. I.; Low, P. J . J . Organomet. Chem. 1996, 519, 221.
(18) (a) Bennet, M. J .; Graham, W. A. G.; Hoyano, J . K.; Hutcheon,
W. L. J . Am. Chem. Soc. 1972, 94, 6232. (b) Masciocchi N.; Sironi, A.;
D’Alfonso, G. J . Am. Chem. Soc. 1990, 112, 9395.
1
enriched diazomethane.28 The H NMR spectra showed
(19) With the term “ethylene-like” we refer not only to the formal
Re-Re double bond required by the electron counting rules but also,
and chiefly, to some interesting analogies of reactivity (see for instance
refs 20 and 21; see also ref 22), that find support in the isolobal
analogy23,24 between CH2 and suitable organometallic fragments.
(20) (a) Beringhelli, T.; Ceriotti, A.; D’Alfonso, G.; Della Pergola, R.;
Ciani, G.; Moret, M.; Sironi, A. Organometallics 1990, 9, 1053. (b)
Ciani, G.; Moret, M.; Sironi, A.; Antognazza, P.; Beringhelli, T.;
D’Alfonso, G.; Della Pergola, R.; Minoja, A. J . Chem. Soc., Chem.
Commun. 1991, 1255. (c) Antognazza, P.; Beringhelli, T.; D’Alfonso,
G.; Minoja, A.; Ciani, G.; Moret, M.; Sironi, A. Organometallics 1992,
11, 1777.
(22) Bergamo, M.; Beringhelli, T.; D’Alfonso, G.; Mercandelli, P.;
Moret, M.; Sironi, A. Angew. Chem. Int. Ed. Engl. 1998, 37, 2128.
(23) Hoffmann, R. Angew. Chem., Int. Ed. Engl. 1982, 21, 711.
(24) Stone, F. G. A. Angew Chem., Int. Ed. Engl. 1984, 23, 89.
(25) Carlucci, L.; Ciani, G.; W. v. Gudenberg, D.; D′Alfonso, G. J .
Organomet. Chem. 1997, 534, 233.
(26) (a) Keister, J . B.; Shapley, J . R. J . Am. Chem. Soc. 1976, 98,
1056. (b) Burgess, K.; J ohnson, B. F. G.; Lewis, J .; Raithby, P. R. J .
Chem. Soc., Dalton Trans. 1982, 263.
(27) (a) Calvert, R. B.; Shapley, J . R. J . Am. Chem. Soc. 1977, 99,
5225. (b) Calvert, R. B.; Shapley, J . R.; Schultz, A. J .; Williams, J . M.;
Suib, S. L.; Stucky, G. D. J . Am. Chem. Soc. 1978, 100, 6240. (c)
Calvert, R. B.; Shapley, J . R. J . Am. Chem. Soc. 1978, 100, 7726.
(28) Markey, S. P.; Shaw, G. J . J . Org. Chem. 1978, 43, 3414.
(21) Bergamo, M.; Beringhelli, T.; D’Alfonso, G.; Mercandelli, P.;
Moret, M.; Sironi, A. J . Am. Chem. Soc. 1998, 120, 2971.

1,2-eq,eq-[Re2(CO)8(THF)2]
Organometallics, Vol. 18, No. 11, 1999 2093
the expected three signals for the three CH3, CH2D, and
CHD2 isotopomers of the methyl group, with ∆δ on the
order of tenths of ppm (δ(CH3) - δ(CH2D) ) 0.270, δ-
(CH2D) - δ(CHD2) ) 0.330 ppm, at 243 K), much higher
than the ∆δ value typical of secondary isotopic effects
on the nuclear shielding.29 The separation of the signals
increased on lowering the temperature (∆δ ) 0.295 and
0.366 ppm, respectively, at 223 K and 0.325 and 0.414
ppm at 203 K), in accordance with the increasing shift
of the equilibria toward the most stable forms, i.e., those
with the 1H isotope in the agostic site. Using the
approach described by Calvert and Shapley,27c we
estimated the energy difference between the D-bridged
and the H-bridged forms as 645 ( 10 J /mol (544 ( 42
J /mol for the osmium analogue)27c and the chemical
shifts of the resonances of the agostic and the terminally
bound C-H protons as δ -8.31 and -0.16 ppm, respec-
tively.
F igu r e 1. ORTEP views of the [Re2(CO)8(THF)2] complex
3 with partial labeling scheme. Thermal ellipsoids are
drawn at the 35% probability level.
Low-temperature IR monitoring provided the ν(CO)
absorptions of 2 (see Experimental Section): the pattern
as well as the frequencies of these absorptions are very
similar to those previously reported for a series of (µ-
hydrido)(µ-alkenyl)Re2(CO)8 derivatives,30 in line with
the analogous idealized Cs symmetry.
rectly from 1, according to eq 3, by performing the
reaction with diazomethane at 273 K.
[Re2(µ-H)2(CO)8] + CH2N2 + 2THF f
[Re2(CO)8(THF)2] + N2 + CH4 (3)
Reaction 1 transforms the electronically unsaturated
reactant 1 into the saturated (34 valence electrons)
derivative 2, owing to the replacement of one hydride
by the three-electron-donor (agostic) methyl ligand.
From a different point of view,24 the same reaction can
be described as the formation of a dimetallacyclopro-
pane, by addition of a carbene to the ethylene-like
molecule [Re2(µ-H)2(CO)8]. The formation of trimetal-
lacyclopropanes, by adding carbene isolobal analogues
to [Re2(µ-H)2(CO)8], has already been reported.20,24
Complex 2 is stable in solution up to ca. 253 K. Above
this temperature its resonances disappear and the
solution becomes red (λmax 485 nm). A small signal at δ
0.15 ppm and gas chromatographic analysis of the gas
present in the NMR tube revealed methane evolution.
The pattern of the ν(CO) bands of the final product
(2066 w, 2005 m, 1959 s, 1946 sh, and 1898 m cm-1, in
THF) is comparable with that of previously known 1,2-
eq,eq-[Re2(CO)8L2] derivatives (L ) MeCN,15-17 pyri-
dine,31 P donor ligands31,30),32 thus suggesting the
formation of a diequatorial substitution derivative of
[Re2(CO)10], as shown in Scheme 1 (where the eclipsed
conformer is depicted):
The 1H NMR resonances of coordinated THF were
detected in a low-temperature spectrum acquired im-
mediately after dissolving an isolated sample of 3 in
THF-d8. They shifted downfield (by 0.23 and 0.10 ppm,
for HR and Hâ, respectively) with respect to free THF,
and in a few minutes disappeared, due to the fast
substitution by the deuterated solvent.
The 1,2-diequatorial coordination of THF was further
supported by the 13C NMR data (three carbonyl reso-
nances, in the ratio 2:1:1) and was eventually confirmed
by single-crystal X-ray analysis.
Solid -Sta te Str u ctu r e of 3. The structure of 3
(Figure 1) is similar to that of [Re2(CO)10], having two
square-pyramidal ReL5 groups joined by a Re-Re single
bond. The Re-Re bond distance, 3.0224(4) Å (see Table
1), is slightly shorter than that found for [Re2(CO)10],
3.041(1) Å.33 The equatorial ligands adopt a staggered
rotational conformation between the symmetry-related
halves of the molecule (of C2 point symmetry), with the
THF ligands on the two Re centers staggered with a
torsion angle between the oxygens of 125.8(2)°. The
Re-O distance, 2.238(3) Å, is slightly longer than the
values observed in rhenium(I) compounds (range 2.13-
2.21 Å).10c-f No Re(0)-THF compounds are reported in
the database. The THF-oxygen coordination is pyrami-
dal, with the oxygen atom 0.294 Å from the Re,C(11),C-
(14) plane, suggesting that sp3 hybridization is retained
upon binding. The five-membered ring displays an
envelope conformation with C(11) 0.43(1) Å out of the
mean plane of the other atoms (maximum deviation
from planarity 0.022 Å), probably to relieve the steric
interaction with carbonyl 3′; the dihedral angle between
the O,C(11),C(12) plane and the mean plane defined by
C(12),C(13),C(14),O is 28.5(5)°. The mean plane of the
THF ligand is tilted by 23.4° with respect to the
equatorial plane defined by Re,C(2),C(3),C(4). The trans
equatorial carbonyls C(3) and C(4) are bent toward the
[Re2(µ-H)(µ-CH3)(CO)8] + 2THF f
[Re2(CO)8(THF)2] + CH4 (2)
It has already been shown that reactants similar to
2, such as the (µ-hydrido)(µ-alkenyl)dirhenium octa-
carbonyls prepared by the photochemical reactions of
[Re2(CO)10] with 1-alkenes, are excellent precursors for
1,2-diequatorial substitution derivatives.30 The 1,2-
eq,eq-[Re2(CO)8(THF)2] complex 3 can be obtained di-
(29) See for instance: Hansen, P. E. Annu. Rep. NMR Spectrosc.
1983, 15, 104 and references therein.
(30) Nubel, P. O.; Brown, T. L. J . Am. Chem. Soc. 1984, 106, 644.
(31) Lee, K.-W.; Pennington, W. T.; Cordes, A. W.; Brown, T. L.
Organometallics 1984, 3, 404.
(32) See for instance νCO 2070 w, 2015 m, 1962 s, 1909 m cm-1 for
the acetonitrile derivative.15
(33) Churchill, M. R.; Amoh, K. N.; Wasserman, H. J . Inorg. Chem.
1981, 20, 1609.

2094 Organometallics, Vol. 18, No. 11, 1999
Ta ble 1. Selected Bon d Len gth s (Å) a n d An gles (d eg) for [Re2(CO)8(THF )2] (3)
Carlucci et al.
Re-Re
Re-C(3)
3.0224(4)
1.996(6)
Re-C(1)
Re-C(4)
1.911(5)
1.982(6)
Re-C(2)
Re-O
1.883(6)
2.238(3)
Re-Re-C(2)
Re-Re-C(3)
Re-O-C(14)
90.66(2)
82.46(1)
123.4(3)
Re-Re-O
Re-Re-C(4)
Re-O-C(11)
90.42(8)
84.51(2)
120.6(3)
O-Re-Re′-O′
O-Re-Re′-C(2′)
C(14)-O-C(11)
125.8(2)
52.9(2)
106.6(4)
opposite rhenium (average 83.5°), while the THF-Re-
CO fragment is unaffected (average 90.5°). The order
of the Re-C bond distances reflects the trans influence
of the opposite ligand,34 ranging from 1.883(6) Å for C(2)
(trans to THF) to 1.911(5) Å for the axial C(1) up to 1.99
Å (average) for the two mutually trans atoms C(3) and
C(4).
The NMR data in solution indicate the equivalence
of the two Re(CO)4(THF) moieties. Most likely, this does
not arise from a true C2 symmetry of the whole
molecule, as in the solid state, but from a fast rotation
around the Re-Re bond, that averages different con-
formers and also generates equivalence in the two
mutually trans carbonyls.
kJ mol-1; C4H8O, 831 kJ mol-1 36
)
but, rather, to steric
factors that would allow the water molecule to approach
more closely to Re. In the mixed aquo-THF derivative
[IrH2(H2O)(THF)(PPh3)2]+ the Ir-O distance for water
was about 0.05 Å shorter than that of THF.9
Some insight into the factors favoring water coordina-
tion has been obtained by investigating the effects of
temperature on the above equilibria. At room temper-
1
ature, the H NMR monitoring of the stepwise addition
of water to a THF-d8 solution of 3 showed that the [5]/[4]
ratio increases linearly with the amount of water, but
the slope is much less than at 256 K (5.0(2) vs 13).
Moreover, it was clearly shown, in a variable-temper-
ature experiment, that the overall amount of 4 + 5 as
well as the [5]/[4] ratio decreased progressively when
the temperature was increased from 253 to 298 K. A
logarithmic plot of the [5]/[4] ratio vs 1/T provided an
estimate of the ∆H° value for the reaction 4 + H2O a 5
(-14.4(2) kJ mol-1). The negative ∆S° value (-34(1) J
mol-1 K-1) shows that the formation of the aquo
derivative 5 is enthalpically driven. It is reasonable to
assume that also the formation of 4 is exothermic, due
to the observed increase of the overall amount of 4 + 5
on decreasing the temperature.
Rea ctivity of [Re2(CO)8(THF )2] (3). Complex 3 can
be isolated as an orange-red solid, by precipitation from
very concentrated THF solutions at low temperature.
The solid can be manipulated in the air for short
periods.
At room temperature, in anhydrous THF and in the
absence of oxygen, the red color of 3 persists for some
hours. A t1/2 value of ca. 3 h was estimated by monitor-
ing the disappearance of the absorption band at 485 nm.
The main decomposition products, identified on the
basis of the IR and 13C NMR data, are the tetranuclear
“cubane” complex [Re4(µ3-OH)4(CO)12]35 and the di-
nuclear species [Re2(CO)9(solv)] (solv ) THF, H2O).11,12
13C and 1H NMR spectra have shown that, even in
THF solution, the presence of water produces significant
amounts of the two aquo derivatives [Re2(CO)8(THF)-
(H2O)] (4) and [Re2(CO)8(H2O)2] (5), which arise from
the partial or complete substitution of coordinated THF.
The diaquo derivative 5 shows only three carbonylic
resonances, in the ratio 2:1:1, in agreement with the
presence of two equivalent cis-Re(CO)4 moieties. The
mixed derivative 4 gives six resonances (relative ratio
2:2:1:1:1:1), as expected for two nonequivalent cis-Re-
(CO)4 groups. When water is added, the relative amount
of the aquo complexes immediately increases. An ex-
periment at 256 K showed that the [4]/[3] and [5]/[4]
ratios increase linearly with the amount of added water.
The values of the slopes provide an estimation of the
equilibrium constants for the two substitution steps (17-
(1) and 13(1), respectively, at 256 K).
When 3 was dissolved in THF-d8 distilled from Na-
benzoketyl directly into the NMR tube, the 1H NMR
spectra, at 298 K, showed the absence of 5 and only a
minor amount of 4 (about 5%). The ν(CO) IR data
reported above were acquired on dissolving some crys-
tals of 3 in freshly distilled THF; therefore, they can be
confidently attributed to true 3. The addition of water
produces little change in this region of the spectra, the
main effect being observed for the band at 1898 cm-1
,
which shifts down to 1886 cm-1 (minor shifts of about
-3 cm-1 are observed for the bands at 2066 and 2005
cm-1).
The resonances of water coordinated in 4 and 5 shift
downfield on increasing water concentration, most likely
due to increased hydrogen-bond interactions. For both
complexes the variation of δ versus the concentration
of water (M) is quite linear, the slope being higher at
low temperature (0.94(2) and 0.66(1) ppm/M at 256 K,
for 4 and 5, respectively) than at room temperature
(0.56(2) and 0.44(2) ppm/M), in agreement with the shift
of the equilibria toward H-bonded species on lowering
the temperature. This is also confirmed by the linear
downfield shift, observed on lowering the temperature
(at constant water concentration, ca. 0.1 M), of the
signals for 4 and 5 and free water (slopes -5.92(1),
-5.29(1), and -6.72(1) ppb/K, respectively).
This facile substitution had previously been observed
in the related [Re2(CO)9(THF)] complex,11,12 but the
factors favoring water coordination are not immediately
apparent. The low-frequency shift undergone by some
of the ν(CO) bands of [Re2(CO)8(solv)2] upon substitution
of water for THF (see below) suggests that water is able
to provide a higher electron density to the Re centers.
This is not attributable to an intrinsic higher σ-donor
capability of water (gas-phase proton affinity: H2O, 697
Gard and Brown previously postulated15 the aquo
complex 5 to be an intermediate in the photochemical
transformation of [Re2(CO)10] into [Re4(µ3-OH)4(CO)12],
in wet THF, but the complex was never identified. They
also suggested that 5 under their reaction conditions
(34) Appleton, T. G.; Clark, H. C.; Manzer, L. E. Coord. Chem. Rev.
1973, 10, 335.
(35) Herberhold, M.; Su¨ss, G.; Ellermannn., J .; Gabelein, H. Chem.
Ber. 1978, 111, 2931.
(36) Lias, S. G.; Liebman, J . F.; Levin, R. Y. J . Phys. Chem. Ref.
Data 1984, 13, 695.

1,2-eq,eq-[Re2(CO)8(THF)2]
Sch em e 2
Organometallics, Vol. 18, No. 11, 1999 2095
In the case of phenylacetylene the reaction was very
fast, affording, almost quantitatively, the hydridic deriv-
ative [Re2(µ-H)(µ-CtCPh)(CO)8]39,40 in the short time
needed to acquire the NMR spectrum. Also in the
presence of a strong excess of water the reaction
proceeded rapidly, even at 243 K (ca. 30 min). The
formation of [Re2(µ-H)(µ-CHdC(H)Ph)(CO)8]30,41 from
the reaction with styrene was slightly slower, reaching
completion in about 20 min at 298 K. In the case of
acetaldehyde, the reaction was still highly selective, but
much slower, requiring about 2.5 h for the complete
transformation of 3 into [Re2(µ-H)(µ-η2-C(Me)O)(CO)8].42
In contrast to this, no reaction of 3 with ethyl acetate
was observed under the above conditions. The reaction
proceeded only when ethyl acetate itself was used as
solvent: after one night at room temperature, the NMR
analysis revealed the formation of the expected [Re2(µ-
H)(µ-η2-CH2C(O)OEt)(CO)8] (previously obtained by re-
action of ethyl diazoacetate with 1).25 The selectivity,
however, was not as high as that of the above reactions
involving sp or sp2 carbon atoms (see Experimental
Section).
rapidly decomposed to a µ-hydrido µ-hydroxo intermedi-
ate, never directly observed. In the early stages of the
room-temperature decomposition of 3 in wet THF-d8 we
observed two 1H NMR resonances, of the same intensity
(δ 3.51 and -11.04 ppm), that could be attributed to
this species. Until now our attempts to isolate it have
failed, but, in the light of the reactivity patterns
described below, the formation of a such a derivative is
not unreasonable.
Con clu sion s
The [Re2(µ-H)(µ-X)(CO)8] products (X ) H, CRn)
obtained from [Re2(CO)8(THF)2] (3) had previously been
obtained by the addition of molecular hydrogen43 or
unsaturated organic molecules30,39-42 to reactive frag-
ments photochemically generated from [Re2(CO)10]. The
easy occurrence of the reactions of 3 with H2 and R-H
(room temperature or lower, in THF itself) indicates that
this novel complex does behave as a “lightly stabilized”
Re2(CO)8 fragment, capable of activating H-H or C-H
bonds under mild conditions. The reaction with ethyl
acetate has demonstrated the possibility of room-tem-
perature activation even of an sp3 C-H bond. Therefore,
the elimination of two carbonyls from the two metal
atoms of [Re2(CO)10] substantially lowers the kinetic
barrier for H-H or C-H bond activation. In connection
with this, it is interesting to recall that laser flash
photolysis studies44 of the reaction of [Re2(CO)10] with
olefins have recently provided kinetic evidence that the
C-H oxidative addition occurs in an intermediate
formed by displacement of a second carbonyl from the
Re center adjacent to that bearing the olefin (coordi-
nated to the eq-Re2(CO)9 fragment arising from the
primary photoprocess).
At room temperature, in dichloromethane solution 3
decomposes in a few minutes, giving an orange solution,
1
whose H NMR displays several hydridic resonances.37
This indicates that 3 is able to cleave E-H bonds.
Therefore, we investigated the reactivity of 3 with
different types of E-H bonds more deeply (Scheme 2).
At room temperature, gaseous HCl reacted with a
THF solution of 3 in less than 30 min, to give selectively
[Re2(µ-H)(µ-Cl)(CO)8].38
With H2, 3 reacted under atmospheric pressure, even
in THF, leading to the dihydridic parent compound 1
(Scheme 1): after about 30 min, weak ν(CO) bands
attributable to 1 were clearly recognizable in the IR
spectrum. The transformation in 1 became instanta-
neous and quantitative when THF was replaced by
dichloromethane.
To test the reactivity of 3 with C-H bonds (eq 4), we
chose four organic molecules (namely phenylacetylene,
styrene, acetaldehyde, and ethyl acetate) that (i) provide
the full hybridization range of the carbon atom (from
sp to sp3) and (ii) afford products already known in the
literature, allowing immediate recognition of the C-H
oxidative addition, if it occurred. All the reactions were
The lack of reactivity between H2 and the monosub-
stituted derivative [Re2(CO)9(solv)] (solv ) THF, H2O,
no reaction in THF at 25 °C over 2 days)15 could be
explained by similar arguments. However, the clean
addition of H2 on a single vacant site, generated by
dissociation of a ligand, has been observed for several
polynuclear carbonyls.45 The different behavior, on
[Re2(CO)8(THF)2] + R-H f
(39) Nubel, P. O.; Brown, T. L. Organometallics 1984, 3, 29.
(40) Nubel, P. O.; Brown, T. L. J . Am. Chem. Soc. 1982, 104, 4955.
(41) Franzreb, K. H.; Kreiter, C. G. J . Organomet. Chem. 1983, 246,
189.
(42) Kreiter, C. G.; Franzreb, K. H.; Sheldrick, W. S. J . Organomet.
Chem. 1984, 270, 71.
(43) Byers, B. H.; Brown, T. L. J . Am. Chem. Soc. 1975, 97, 3260.
(44) Sarakha, M.; Cozzi, M.; Ferraudi, G. Inorg. Chem. 1996, 35,
3804.
(45) Useful references and discussions on the mechanisms involved
in H2 activation by polynuclear species can be found in: (a) Garland,
M.; Pino, P. Organometallics 1990, 9, 1943. (b) Safarowic, F. J .;
Bierdeman, D. J .; Keister, J . B. J . Am. Chem. Soc. 1996, 118, 11805.
[Re2(µ-H)(µ-R)(CO)8] + 2THF (4)
performed at room temperature, by treating a THF-d8
solution of 3 with a 5-fold excess of the R-H organic
substrate in an NMR tube.
(37) The main signals are at δ -10.72, -10.97, -14.37, and -12.69
ppm. Only the last resonance is reasonably attributable to a previously
known species, namely [Re2(µ-H)(µ-Cl)(CO)8] (lit.38 δ -12.72 in CDCl3).
Attempts to identify the other species have been so far unsuccessful.
(38) Adams, R. D.; Kuhns, J . D. Polyhedron 1988, 7, 2543.

2096 Organometallics, Vol. 18, No. 11, 1999
Carlucci et al.
excess of benzoic acid. All the other reagents were used as
purchased from Aldrich. 1H and 13C NMR spectra were
collected on Bruker AC200, Bruker DRX300, and Gemini 200
spectrometers. IR spectra were recorded on a Bruker Vector
22 instrument. Low-temperature monitoring was performed
with a Graseby Specac P/N 21525 variable-temperature cell,
with CaF2 windows, equipped with a P/N 20120 temperature
controller. The UV spectrum was acquired on a Kontron
Uvikon 943 spectrophotometer. Microanalyses were performed
at the microanalytical laboratory of the University of Milan.
Rea ction of [Re2(µ-H)2(CO)8] (1) w ith CH2N2 a t 193 K.
(a ) NMR Mon itor in g. A sample of 1 (10.2 mg, 17.0 µmol)
dissolved in THF-d8 (0.7 mL) in an NMR tube at 193 K was
treated with 100 µL of a diethyl ether solution of CH2N2 (ca.
0.138 M). The 1H NMR spectrum at 193 K showed, besides
the resonance of unreacted 1, two novel signals (δ -2.85 and
-12.28 ppm, ratio 3:1) attributable to [Re2(µ-H)(µ-CH3)(CO)8]
(2). The temperature was progressively increased, and the
disappearance of the signals of 2 occurred at 263 K. In the
meantime, the growth of a signal at δ 0.15 ppm indicated CH4
evolution. When the experiment was repeated using partially
deuterated diazomethane, the 1H NMR spectra, acquired at
203 K, showed three well-separated signals for the three CH3,
CH2D, and CHD2 isotopomers (δ -2.86, -3.18, and -3.59 ppm,
respectively, at 203 K) and only one resonance for the bridging
hydride (δ -12.30) of 2. At 223 and 243 K the methyl signals
shifted to δ -2.88, -3.17, -3.54 and δ -2.90, -3.17, -3.50
ppm, respectively.
varying n from 1 to 2, of the two [Re2(CO)10-n(THF)n]
species is in contrast to what is observed for the
analogous [Os3(CO)12-n(NCCH3)n] derivatives (n ) 1, 2),
both species activating H2 under mild conditions (1 atm,
room temperature).46,47 However, the [Os3H(µ-H)(CO)10L]
products (L ) CO, MeCN) are unstable toward the
elimination of a ligand (CO or MeCN) from the adjacent
Os center, and they both eventually afford [Os3(µ-H)2-
(CO)10], the electronically unsaturated analogue of [Re2-
(µ-H)2(CO)8].48 Therefore, though not a necessary pre-
requisite for H2 oxidative addition, the presence of a
labile ligand on a metal atom adjacent to that undergo-
ing oxidative addition can be helpful in stabilizing the
dihydridic product. The thermal instability of the ex-
pected [Re2H(µ-H)(CO)9] product, precluding stabiliza-
tion by CO loss, could be responsible for the lack of H2
activation by [Re2(CO)9(solv)].50
It is noteworthy that much more drastic conditions
are required for the related complex [Re2(CO)8(NCMe)2]
to activate H2 and C-H bonds. At 80 °C the reaction
with H2 occurs over several hours,17 affording [Re3(µ-
H)3(CO)11(NCCH3)] (the unsaturated complex 1 being
known to rearrange to triangular cluster species under
these conditions).18b With phenylacetylene the reaction
requires 5 h in refluxing CH2Cl2 and moreover gives
[Re2(µ-H)(µ-CtCPh)(CO)7(NCMe)], which still contains
one coordinated MeCN ligand16 (a CO ligand is labilized
rather than the second MeCN molecule, thus confirming
the strength of the MeCN-Re(I) interaction).
(b) IR Mon itor in g. A sample of 1 (20 mg, 33.4 µmol) was
dissolved in freshly distilled THF (5 mL) in a Schlenk tube
and was treated, at 193 K, with 200 µL of a diethyl ether
solution of CH2N2 (ca. 0.26 M). The solution was briefly shaken
at room temperature, until a slight darkening was apparent,
and then an aliquot was transferred, under N2 pressure, into
the variable-temperature IR cell, cooled to 213 K, through a
previously N2-purged and -cooled stainless steel capillary tube
sealed to the cell. The IR spectra showed, besides 2, some
unreacted 1 and also some 3 (due to partial thermal decom-
position during the transfer to the cell). Spectral subtractions
gave the neat IR absorptions of 2 at 2121 (vw), 2086 (m), 2023
Finally, it is to be noted that [Re2(CO)8(THF)2] is the
key reagent in the method recently developed22 for the
synthesis of ring clusters, by condensation of complexes
containing two very labile ligands with complexes
containing two terminal hydrides and therefore able to
act as “bidentate ligands”. The reaction of 3 with [Re3H2-
(µ-H)2(CO)12]- has already allowed the synthesis of the
first examples of five-membered rings, namely [Re5(µ-
H)4(CO)20]- and [Re5(µ-H)5(CO)20].22 Studies aimed at
exploring further applications of this method are in
progress.
(vs), 2007 (m), 1983 (m), 1964 (s), and 1942 (m) cm-1
.
P r ep a r a tion of [Re2(CO)8(THF )2] (3). A sample of 1 (98
mg, 0.164 mmol) was treated with freshly distilled THF (ca. 8
mL), cooled in an ice bath and decanted to eliminate a white
insoluble residue. The clear solution was transferred via
cannula to another Schlenk and treated, at 273 K, with 1.5
mL of a solution of diazomethane in diethyl ether (ca. 0.12
M). The reaction mixture immediately acquired a bright red
color, and IR monitoring showed the formation of 3. The
solution was concentrated by evaporation under vacuum, at
273 K, until precipitate formation became evident (ca. 0.5 mL
of volume). After the mixture stood overnight at 248 K, an
orange-red precipitate of 3 was isolated by decantation from
Exp er im en ta l Section
All manipulations were carried out under a nitrogen atmo-
sphere using standard Schlenk techniques unless otherwise
stated. Solvents were distilled under nitrogen from Na/Ph2-
CO (THF, diethyl ether) or P2O5 (CH2Cl2). [Re2(µ-H)2(CO)8],51
diazomethane52 (a solution in diethyl ether), and deuterium-
enriched diazomethane28 were prepared according to literature
methods. The titer of the diazomethane solution was checked
by back-titration with NaOH after addition of a weighted
the cold solution (yield ca. 30%). Anal. Calcd for C16H16O10
-
Re2: C, 25.94; H, 2.18. Found: C, 25.0; H, 2.1. The slightly
lower content of carbon, with respect to the theoretical amount
for 3, might indicate the presence of a small amount of [Re2-
(CO)8(THF)(H2O)] (4). However, NMR analysis showed that
the samples were contaminated by [Re3(µ-H)3(CO)12] (ca.
10%),53 which is usually present in 1. The calculated analysis
for a mixture containing 90% of 3 and 10% of [Re3(µ-H)3(CO)12]
gave 25.0% C and 2.0% H, in excellent agreement with the
experimental results. Crystals suitable for X-ray analysis were
obtained from a very concentrated solution of 3 in THF at 243
(46) J ohnson, B. F. G.; Lewis, J .; Pippard, D. A. J . Chem. Soc.,
Dalton Trans. 1981, 407.
(47) Hudson, R. H.; Poe¨, A. J .; Sampson, C. N.; Siegel, A. J . Chem.
Soc., Dalton Trans. 1989, 2235.
(48) The expected [Os3H(µ-H)(CO)10(NCCH3)] was not even seen as
a
product of the reaction of the disubstituted species [Os3(CO)10-
(NCCH3)2] with H2, which gave [Os3(µ-H)2(CO)10] directly.47 Also the
product [Os3H(µ-H)(CO)11], obtained from the monosubstituted species
[Os3(CO)11(NCCH3)],46 loses a carbonyl (at room temperature, in the
absence of CO)49 to give [Os3(µ-H)2(CO)10].
(49) Poe¨, A. J .; Sampson, C. N.; Smith, R. T.; Zheng, Y. J . Am. Chem.
Soc. 1993, 115, 3174.
K.1H NMR (THF-d8, 243 K): δ 1.90 (m, 4H), 3.87 (m, 4H).13
C
NMR (THF-d8, 256 K, relative ratios in parentheses): δ 216.76
(2), 197.75 (1), 193.96 (1). IR (THF, cm-1) ν(CO) 2066 w, 2005
m, 1959 s, 1946 sh, 1898 m.
(50) The characterization of a thermally unstable [Re2H2(CO)9]
species is currently in progress: Bergamo, M.; Beringhelli, T.; D’Alfonso,
G., Mercandelli, P.; Sironi, A. Work in progress.
(51) Andrews, M. A.; Kirtley, S. W.; Kaesz, H. D. Inorg. Chem. 1977,
16, 1556.
(52) De Boer, T. H.; Backer, H. J . Organic Syntheses; Wiley: New
York, 1963, Collect. Vol. 4, p 250.
(53) The amount of this impurity has been estimated after convert-
ing 3 into [Re2(µ-H)(µ-CtCPh)(CO)8] by reaction with an excess of
phenylacetylene, directly into an NMR tube.

1,2-eq,eq-[Re2(CO)8(THF)2]
Organometallics, Vol. 18, No. 11, 1999 2097
Rea ction of 3 w ith Wa ter . (a ) 13C NMR An a lysis a t 256
K. A 13CO-enriched sample of 1 (20.7 mg, 33.4 µmol) was
dissolved in freshly distilled THF (10 mL) and treated at 273
K with 0.5 mL of a solution of CH2N2 in diethyl ether (ca. 0.1
M). After 1 h, the solution was evaporated to dryness and the
residue dissolved in THF-d8 at 193 K. The 13C NMR spectra,
at 256 K, showed three sets of signals, attributed to 3-5,
respectively, on the basis of change in intensity upon addition
of water: 3, see above; 4, δ 218.2 (2), 215.7 (2), 197.7 (1), 197.5
(1), 195.2 (1), 193.3 (1) ppm; 5, δ 217.1 (2), 197.6 (1), 194.3 (1).
The resonances of coordinated water in the 1H NMR spectrum
were identified at δ -5.60 for 4 and -5.41 ppm for 5. The
integrated intensities of these resonances and the carbonylic
ones allowed the estimation of the composition of the mix-
ture: 3, 0.46; 4, 0.34; 5, 0.20. Water was then added stepwise
to the NMR tube, causing a fast increase in the relative
amount of aquo complexes. The molar fractions of 3-5 became
at first 0.10, 0.28, and 0.62 and then 0.05, 0.22, and 0.73 after
the addition of 1.3 and 2.3 µL of water (corresponding to 2.2
and 3.8 equiv, respectively). The plots of the ratios [4]/[3] and
[5]/[4] as a function of the concentrations of added water were
satisfactorily linear, with slopes of 17(1) and 13(1), respec-
tively. Upon addition of water, the protonic resonances of water
coordinated in 4 and 5 shifted to lower field (δ 5.71 and 5.49
and δ 5.80 and 5.55 for 4 and 5, after addition of 1.3 and 2.3
µL of H2O, respectively), linearly with respect to the molar
concentration of added water, with a slope of 0.94(2) and 0.66-
(1) ppm/M, for 4 and 5, respectively.
(b) 1H NMR An a lysis a t 298 K. An isolated sample of 3
(12 mg, 20 µmol) was dissolved in 0.6 mL of THF-d8 distilled
directly into the reaction vessel from Na-benzoketyl. A 1H
NMR spectrum (298 K) showed a weak resonance at δ 5.38
(attributable to the water molecule coordinated in 4) and the
absence of the corresponding signal of 5 (expected at δ 5.20).
The amount of 4 was estimated as ca. 5%, on using as internal
standard the resonance of [Re3(µ-H)3(CO)12] (present as an
impurity, ca. 10%). Different aliquots of water were then added
and 1H NMR spectra acquired after each addition (using a
relaxation delay of 10 s to ensure reliable integration ratios).
The [5]/[4] ratio, estimated from the integrated intensities of
the respective water resonances, varied linearly vs water
concentration, with a slope of 5.0(2). The chemical shift values
of the resonances of water coordinated in 4 and 5 varied
linearly with respect to [H2O] (M), according to the equations
δ ) 5.374(4) + (0.56(2))[H2O] and δ ) 5.207(4) + (0.44(2))-
[H2O], for 4 and 5, respectively (for [H2O] up to 0.4 M). In the
meanwhile, also the resonance of free water underwent a
similar linear downfield shift, with a slope of 0.53(2).
Rea ction of 3 w ith HCl. Gaseous HCl was bubbled slowly
through a THF (6 mL) solution of 3 (prepared in situ from 1
and diazomethane. After 30 min the solution was evaporated
to dryness. IR (CH2Cl2) and NMR spectra (CDCl3) showed the
transformation of 3 into the already known complex [Re2(µ-
H)(µ-Cl)(CO)8].38 No other significant hydridic signal was
detected in the 1H NMR spectrum.
Rea ction of 3 w ith H2. A sample of 3 was prepared by
treating 1 (14.4 mg, 24.1 µmol) with 0.2 mL of a solution of
diazomethane (ca. 0.178 M) in THF (3 mL) at 273 K. H2 was
then bubbled through this solution at room temperature. The
IR spectra of the solution showed the slow increase of the ν-
(CO) band at 2022 cm-1, diagnostic of [Re2(µ-H)2(CO)8]. The
H2 bubbling was continued until the THF solvent was re-
moved; then CH2Cl2 was added, under an H2 atmosphere. The
color of the solution changed immediately from red-orange to
yellow, and the IR spectra showed the quantitative formation
of [Re2(µ-H)2(CO)8].
R ea ct ion of 3 w it h P h en yla cet ylen e. A sample of
isolated 3 (13.0 mg, 17.6 µmol) was dissolved, at 273 K, in
THF-d8 distilled from Na-benzoketyl directly into the NMR
tube and was treated with 9.8 µL (89 µmol, 5 equiv) of
1
phenylacetylene. H NMR monitoring, at room temperature,
showed the nearly quantitative formation of [Re2(µ-H)(µ-Ct
CPh)(CO)8], complete within the few minutes necessary to
acquire the spectrum. A minor unattributed resonance ap-
peared at δ -9.80, with an integrated intensity ratio of 0.05
with respect to the signal at δ -12.95, attributed to [Re2(µ-
H)(µ-CtCPh)(CO)8] (lit.39 data (CH2Cl2): -13.01). The formula-
tion of the reaction product was confirmed by its IR spectrum,
recorded after evaporation of the solvent under vacuum: ν-
(CO) (n-hexane) 2119 vw, 2094 w, 2023 s, 2002 m, 1982 ms
39
cm-1
.
No significant difference (but a decrease of the minor
byproduct at δ -9.80) was observed when the same reaction
was performed using as solvent THF-d8 (Glaser AG Basel) as
received (i.e. without anhydrification and therefore containing
3 and some 4), or THF-d8 treated with 5 µL of H2O (and
therefore containing mainly 4 and 5).
Rea ction of 3 w ith Styr en e. This reaction was performed
as described above (3, 7.4 mg, 10 µmol; styrene, 5.7 µL, 50
µmol). 1H NMR monitoring showed the quantitative formation
of [Re2(µ-H)(µ-C(H)dC(H)Ph)(CO)8], complete within 20 min.
The nature of the product was ascertained by comparison of
the NMR and IR data with literature values.40 No other
hydridic derivative was formed in significant amount.
Rea ction of 3 w ith Aceta ld eh yd e. A sample of 3 (7.2 mg,
9.7 µmol) was dissolved in THF-d8, at 273 K, in a NMR tube
and treated with 2.5 µL (44 µmol) of acetaldehyde. The
progress of the reaction was followed by 1H NMR spectroscopy,
at room temperature, showing the progressive growth of the
resonances of [Re2(µ-H)(µ-C(Me)O)(CO)8] at δ -14.0 (1) and
2.57 (3) (lit.42 data: -14.01, 2.61). No other signal of significant
intensity was observed in the hydridic region. The reaction
went to completeness after about 2.5 h. The IR spectrum,
recorded in n-hexane after evaporation of the solvent, was
found to be very similar to the literature data (apart from an
unattributed band at 2003 cm-1).
(c) Va r ia b le-Tem p er a t u r e E xp er im en t . An isolated
sample of 3 (7.7 mg, 10.4 µmol) was dissolved in 0.6 mL of
THF-d8 distilled directly into the reaction vessel from Na-
benzoketyl. A 1H NMR spectrum at 273 K showed the presence
only of the resonance of 4, at δ 5.50. Water (1 µL, 0.056 mmol)
was then added, resulting in an increase in the intensity of
the signal of 4 and in the appearance of the resonance of water
coordinated in 5 (δ 5.39, [5]/[4] ) 0.95). The temperature was
lowered to 263 K and then to 253 K and subsequently raised,
again to 273 K and then to 298 K. Several spectra were
acquired at each temperature to ensure that equilibrium had
been attained. The overall amount of 4 + 5 decreased from
73% at 253 K to 40% at 298 K (evaluated on attributing a
concentration of 10% to the “internal standard” [Re3(µ-H)3-
(CO)12], present as an impurity of 1). A logarithmic plot of the
[5]/[4] ratios at each temperature (1.55, 1.20, 0.95, and 0.55
at 253, 263, 273, and 298 K, respectively) vs 1/T was satis-
factorily linear, according to the equation ln [5]/[4] ) ln [H2O]
+ ln K ) ln [H2O] + ∆S°/R - ∆H°/RT. The values of the slope
and of the intercept (1.73(2) × 103 and -6.42(6), respectively)
allowed the estimation of the thermodynamic parameters of
the 4 a 5 equilibrium.
Rea ction of 3 w ith Eth yl Aceta te. An isolated sample of
3 (5.1 mg, 6.9 µmol) was dissolved in ethyl acetate, giving a
red solution. When the solution stood overnight at room
temperature, the red color turned to yellow. The ethyl acetate
was removed under vacuum and the residue dissolved in CD2-
1
Cl2. The H NMR spectrum showed the resonances of [Re2(µ-
H)(µ-η2-CH2C(O)OEt)(CO)8].25 An unattributed resonance at
δ -15.35 (with integrated intensity of ca. 0.66 with respect to
that at δ -14.60 of the main product) was also present,
together with other very minor signals.
Cr ysta llogr a p h y. The crystal data for 3 are listed in Table
2; the selected bond distances and angles are given in Table
1. A columnlike crystal (0.38 × 0.08 × 0.08 mm) was mounted
on a Siemens SMART CCD area-detector diffractometer, and

2098 Organometallics, Vol. 18, No. 11, 1999
Carlucci et al.
Ta ble 2. Cr ysta llogr a p h ic Da ta for
[Re2(CO)8(THF )2] (3)
independent reflections and 128 parameters (for 11 562 data
collected). Anisotropic thermal parameters were assigned to
all the non-hydrogen atoms. The final difference electron
density map showed no significant features. All calculations
were performed using SHELX-97.56 The assignment of the
absolute structure was confirmed by the statistics and the
refinement of the absolute structure parameter as imple-
mented in SHELX-97, to a value of 0.036(9).57
chem formula
C16H16O10Re2
orthorhombic
19.792(1) Å
25.910(2) Å
8.153(1) Å
fw
740.69
Fdd2 (No. 43)
20 °C
cryst syst
a
b
c
V
space group
T
Z
Fcalcd
8
2.353 g cm-3
11.618 mm-1
4180.9(6) Å3
µ
R indicesa,b (Fo > 4σ(Fo); 2026 data) R1 ) 0.0164, wR2 ) 0.0291
R indicesa,b (all data)
R1 ) 0.0220, wR2 ) 0.0294
Ack n ow led gm en t. We are indebted to Prof. Tiziana
Beringhelli and Dr. Monica Panigati for helpful discus-
sions.
a
b
2
4
R1 ) ∑||Fo| - |Fc||/∑|Fo|. wR2 ) [∑(Fo - Fc2)2/∑wFo
]
1/2; w
2
) 1/[σ2(Fo2) + (0.0109P)2], where P ) (Fo + 2Fc2)/3.
Su p p or tin g In for m a tion Ava ila ble: Complete listings
of atomic coordinates, thermal parameters and bond distances
and angles for compound 3. This material is available free of
charge via the Internet at http://pubs.acs.org.
the data collection was performed at 293 K (graphite-mono-
chromatized Mo KR λ ) 0.710 73 Å) by the ω-scan method,
within the limits 2 < θ < 28° (-25 < h < 25, -34 < k < 33,
-10 < l < 10). An empirical absorption correction was applied
(SADABS),54 maximum-minimum transmission being 0.505-
0.233. The structure was solved by direct methods (SIR97)55
OM9806693
2
(55) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.;
Burla, M. C.; Polidori, G.; Camalli, M. J . Appl. Crystallogr. 1994, 27,
435.
and refined by full-matrix least squares on Fo using all 2385
(54) Sheldrick, G. M. SADABS: Siemens Area Detector Absorption
Correction Software; University of Go¨ttingen, Go¨ttingen, Germany,
1996.
(56) Sheldrick, G. M. SHELX-97: Program for Structure Refine-
ment; University of Go¨ttingen, Go¨ttingen, Germany, 1997.
(57) Flack, H. D. Acta Crystallogr. 1983, A39, 876.