Ligand-assisted heterolytic activation of hydrogen and silanes mediated by nitrosyl rhenium complexes

Article abstract of DOI:10.1021/om010409c

Abstraction of the hydride in the dinitrosyl rhenium complexes [Re(H)(NO)₂(PR₃)₂] (R = ⁱPr (1a), Cy (1b)) affords the dinuclear saturated compounds [Re₂(H)(μ,η²-NO)(NO)₃(PR₃) ₄][BAr₄′] (R = ⁱPr, 2a; Cy, 2b) or the mononuclear unsaturated 16-electron derivatives [Re(NO)₂(PR₃)₂][BAr₄′] (R = ⁱPr, 3a; Cy, 3b). In complexes 2a,b the metal centers are connected via a nitrosyl-isonitrosyl linkage. Complexes 2 and 3 react with classical two-electron donor ligands such as CH₃CN, CO, C₆H₅CHO, and THF, but also with H₂ and HSiEt₃. In the two latter cases heterolytic rupture of the hydrogen-hydrogen or hydrogen-silicon bonds is achieved. Complexes 2a,b and 3a,b are active in the catalytic scrambling of H₂/D₂ to give HD under very mild conditions. In the presence of tetramethylpiperidine as a base the reaction of these complexes with H₂ produces [Re(H)(NO){NOH₂NC₅H₆(CH₃) ₄}(PR₃)₂][BAr₄′] (R = ⁱPr, 9a; Cy, 9b). Heterolysis of HSiEt₃ mediated by 2a,b and 3a,b results in the formation of [Re(H)(NO)(NOSiEt₃) (PR₃)₂][BAr₄′] (R = ⁱPr, 8a; Cy, 8b). Complexes 8a and 9b have been characterized by X-ray diffraction studies.

Full text of DOI:10.1021/om010409c

Organometallics 2001, 20, 5277-5288
5277
Liga n d -Assisted Heter olytic Activa tion of Hyd r ogen a n d
Sila n es Med ia ted by Nitr osyl Rh en iu m Com p lexes
Angela Llamazares, Helmut W. Schmalle, and Heinz Berke*
Anorganisch-chemisches Institut Universita¨t Zu¨rich, Winterthurerstrasse 190,
CH-8057 Zu¨rich, Switzerland
Received May 17, 2001
Abstraction of the hydride in the dinitrosyl rhenium complexes [Re(H)(NO)2(PR3)2] (R )
iPr (1a ), Cy (1b)) affords the dinuclear saturated compounds [Re2(H)(µ,η2-NO)(NO)3(PR3)4]-
i
[BAr4′] (R ) Pr, 2a ; Cy, 2b) or the mononuclear unsaturated 16-electron derivatives [Re-
(NO)2(PR3)2][BAr4′] (R ) iPr, 3a ; Cy, 3b). In complexes 2a ,b the metal centers are connected
via a nitrosyl-isonitrosyl linkage. Complexes 2 and 3 react with classical two-electron donor
ligands such as CH3CN, CO, C6H5CHO, and THF, but also with H2 and HSiEt3. In the two
latter cases heterolytic rupture of the hydrogen-hydrogen or hydrogen-silicon bonds is
achieved. Complexes 2a ,b and 3a ,b are active in the catalytic scrambling of H2/D2 to give
HD under very mild conditions. In the presence of tetramethylpiperidine as a base the
reaction of these complexes with H2 produces [Re(H)(NO){NOH2NC5H6(CH3)4}(PR3)2][BAr4′]
i
(R ) Pr, 9a ; Cy, 9b). Heterolysis of HSiEt3 mediated by 2a ,b and 3a ,b results in the
i
formation of [Re(H)(NO)(NOSiEt3)(PR3)2][BAr4′] (R ) Pr, 8a ; Cy, 8b). Complexes 8a and
9b have been characterized by X-ray diffraction studies.
In tr od u ction
enhance the acidity of these units.2a,c,e,f,j,3 On the other
hand, much less is yet known about the ligand influence
of other good π-acidic groups such as the strong π-ac-
ceptor NO.4 Although most of the features of the NO
bound to a metal center can be qualitatively described
similarly to CO bonding,5 the presence of nitrosyl groups
in the coordination sphere should confer special proper-
ties. The NO ligand is normally regarded as a stronger
π-aceptor than CO, and in addition it possesses an
oxygen atom that shows a certain Lewis basic behavior
forming adducts with Lewis acids.5,6 This “intramolecu-
lar base” could therefore be exploited in heterolytic
activation pathways of HX substrates. In addition, NO
Activation of HX molecules (X ) H, SiR3) by transition
metal complexes is one of the key steps in very impor-
tant catalytic and stoichoimetric reactions. Cleavage of
the HX bond requires the intermediacy of some hydride
or dihydrogen species, either as precursors or interme-
diates in the reaction, and may be achieved via ho-
molytic or heterolytic pathways.1 Homolytic cleavage
occurs normally in low-valent, electron-rich metal com-
plexes, in which the predominant interaction after
coordination of the HX substrate is the π-back-donation
from metal d orbitals to the σ* orbital of the substrate.
Heterolytic cleavage, on the other hand, usually requires
complexes containing metals in medium or high oxida-
tion states in which the predominant interaction is σ
donation from the HX molecule to a d orbital of the
metal. Heterolysis of the HX bond should be assisted
by a Bro¨nsted base, which can be externally added or
“built in” in the complex. In this latter case, one of the
ligands must possess a basic site easily accessible to the
X+ fragment.
(2) For recent examples of heterolytic activation of H2 and silanes
see: (a) Fang, X.; Scott, B. L.; J ohn, K. D.; Kubas, G. J . Organometallics
2000, 19, 4141. (b) Toner, A. J .; Gru¨ndemann, S.; Clot, E.; Limbach,
H. H.; Donnadieu, B.; Sabo-Etienne, S.; Chaudret, B. J . Am. Chem.
Soc. 2000, 122, 6777. (c) Fang, X.; Huhmann-Vincent, J .; Scott, B. L.;
Kubas, G. J . J . Organomet. Chem. 2000, 609, 95. (d) Ayllon, J . A.;
Sayers, S. F.; Sabo-Etienne, S.; Donnadieu, B.; Chaudret, B.; Clot, E.
Organometallics 1999, 18, 3981. (e) King, A. W.; Scott, B. L.; Eckert,
J .; Kubas, G. J . Inorg. Chem. 1999, 38, 1069. (f) Huhmann-Vincent,
J .; Scott, B. L.; Kubas, G. J . Inorg. Chim. Acta 1999, 294, 240. (g) Fong,
T. P.; Forde, C. E.; Lough, A. J .; Morris, R. H.; Rigo, P.; Rocchini, E.;
Stephan, T. J . Chem. Soc., Dalton Trans. 1999, 4475. (h) Sweeney, Z.
K.; Polse, J . L.; Bergman, R. G.; Andersen, R. A. Organometallics 1999,
18, 5502. (i) Sellmann, D.; Gottschalk-Gaudig, T.; Heinemann, F. W.
Inorg. Chem. 1998, 37, 3982. (j) Huhmann-Vincent, J .; Scott, B. L.;
Kubas, G. J . J . Am. Chem. Soc. 1998, 120, 6808.
The transition metal mediated heterolytic activation
of small molecules such as H2 and silanes is a topic of
continuously growing interest.1,2 A great number of
studies have been carried out on octahedral compounds,
and among them, special attention has been paid to
those containing good π-acceptor ligands such as CO
coordinated trans to HX, which is expected to greatly
(3) (a) Heinekey, D. M.; Radzewich, C. E.; Voges, M. H.; Schomber,
B. J . Am. Chem. Soc. 1997, 119, 4172. (b) Rocchini, E.; Mezzetti, A.;
Ru¨egger, H.; Burckhardt, U.; Gramlich, V.; Del Zotto, A.; Martinuzzi,
P.; Rigo, P. Inorg. Chem. 1997, 36, 711.
(4) Yaundolov, D. V.; Steib, W. E.; Caulton, K. G. Inorg. Chim. Acta
1998, 280, 125.
* To whom correspondence should be addressed. E-mail: hberke@
aci.unizh.ch. Fax: int +41-1-6356802.
(5) Richter-Addo, G. B.; Legzdins, P. Metal Nytrosyls; Oxford
University Press: New York, 1992.
(1) For reviews see: (a) J ia, G.; Lau, C.-P. Coord. Chem. Rev. 1999,
190-192, 108. (b) Corey, J . Y.; Braddock-Wilking, J . Chem. Rev. 1999,
99, 175. (c) Esteruelas, M. A.; Oro, L. Chem. Rev. 1998, 98, 577. (d)
Schneider, J . J . Angew. Chem., Int. Ed. Engl. 1996, 35, 1068. (e)
Heinekey, D. M.; Oldham, J . W., J r. Chem. Rev. 1993, 93, 913. (f)
Crabtree, R. H. Angew. Chem., Int. Ed. Engl. 1992, 32, 789. (g) J essop,
P. G.; Morris, R. H. Coord. Chem. Rev. 1992, 92, 289.
(6) (a) Bo¨hmer, J .; Haselhorst, G.; Wieghart, K.; Nuber, B. Angew.
Chem., Int. Ed. Engl. 1995, 14, 1473. (b) Gusev, D.; Llamazares, A.;
Artus, G.; J acobsen, H.; Berke, H. Organometallics 1999, 18, 75. (c)
J acobsen, H.; Heinze, K.; Llamazares, A.; Schmalle, H. W.; Artus, G.;
Berke, H. J . Chem. Soc., Dalton Trans. 1999, 1717. (d) Llamazares,
A.; Artus, G.; Berke, H. Manuscript in preparation. (e) Legzdins, P.;
Sayers, S. F. Chem. Eur. J . 1997, 3, 1579.
10.1021/om010409c CCC: $20.00 © 2001 American Chemical Society
Publication on Web 11/10/2001

5278 Organometallics, Vol. 20, No. 25, 2001
Llamazares et al.
Sch em e 1
is known to be a noninnocent ligand able to act as a
three- (linear) or one-electron donor (bent).5 Bending a
NO group at a coordinatively saturated metal center
creates a vacancy for HX coordination without the
requirement of previous ligand dissociation. Bending
also causes a formal oxidation of the metal center, a fact
that will have great consequences for the subsequent
activation process. The possibility of a reversible linear
to bent conversion might be of special relevance since
it allows a modification of the electronic properties of
the metal without the need of externally added reagents.
The rhenium(-I) hydride complexes [Re(H)(NO)2-
(PR3)2] (R ) iPr, 1a ; Cy, 1b), which have been recently
prepared in our group,6b are expected to display an
enhanced hydridic reactivity mainly attributed to the
influence of the NO groups. The “nitrosyl effect” in
transition metal hydride complexes produces a strong
polarization of the M-H bond which results in a higher
partial negative charge at the hydride group.7 This is
also reflected in an increased propensity toward inser-
tions of polar unsaturated organic substrates into the
metal-hydrogen bond.8 Taking advantage of this prop-
erty, we were interested in the possibility of abstracting
the hydride from 1a or 1b to generate 16-electron
cationic species. These in turn were expected to coor-
dinate H2 or silanes. Indeed we have found that this is
possible. Our current work has also revealed an unex-
pected nitrosyl reactivity caused by the greatly en-
hanced basicity of the oxygen atom of the NO group.
Thus, the possibility of coordination of 1 or 2 equiv of
the Lewis acid BF3 to one NO ligand of 1a or 1b was
discovered taking place in a reversible way.6b,d In these
molecules therefore, two preferred reactive sites are
available: the hydride and the nitrosyl groups. Conse-
quences of this particular feature on the reactivity of
such complexes will be discussed in this paper.
F igu r e 1. Molecular structure of complex 2a .6c Displace-
ment ellipsoids are shown at the 20% level. Hydrogen
atoms are omitted for clarity, except for the hydride ligand,
which is displayed as a sphere with arbitrary size. Not
shown is the counterion. Selected bond distances (Å) and
angles (deg): Re(1)-N(1) ) 1.78(1); Re(1)-N(2) ) 1.80(2);
Re(1)-O(3) ) 2.199(9); P(1)-Re(1)-P(2) ) 162.8(1); Re-
(1)-N(1)-O(1) ) 172(1); Re(1)-N(2)-O(2) ) 158(1).
hydridic character of the rhenium-bonded hydrogen in
complexes 1a ,b facilitates hydride abstraction upon
treatment with [Ph3C][BAr′4] (Ar′ ) [3,5-(CF3)2C6H3]).9
The products that are obtained depend on the nature
of the phosphine substituent as well as on the [Ph3C]-
[BAr′4]/complex ratio employed in the reaction. The
geometrical and structural details concerning the X-ray
diffraction study of some of these compounds have
already been published earlier.6c The present paper
mainly refers to the reacitivity of the former and other
species and in this context also to structural parameters
crucial to it.
Reaction of 1a ,b with half an equivalent of [Ph3C]-
[BAr4′] produces the binuclear derivatives [Re2(H)(µ,η2-
Resu lts a n d Discu ssion
i
NO)(NO)3(PR3)4][BAr4′] (R ) Pr, 2a ; Cy, 2b) (Scheme
R ea ct ivit y of 1a ,b t ow a r d [P h 3C][BAr ′4] a n d
H(OEt2)2[BAr 4′]. As already anticipated by theoretical
calculations and measurements of bond ionicities,6b,c the
1). An X-ray diffraction study carried out on the triiso-
propylphosphine derivative 2a 6c reveals an isonitrosyl
linkage connecting the two rhenium centers, and the
compound may be described as composed of the neutral
Re(H)(NO)2(PiPr3)2 molecule and a cationic Re(NO)2-
(7) Ledzgins, P.; Martin, D. T. Inorg. Chem. 1979, 18, 1250. (b)
Bursten, B. E.; Gatter, M. G. J . Am. Chem. Soc. 1984, 106, 2554. (c)
Bursten, B. E.; Gatter, M. G.; Goldberg, K. I. Polyhedron 1990, 16,
279. (d) J acobsen, H.; Berke, H. Hydridicity of Transition Metals and
its Implications for Reactivity. In Recent Advances in Hydride Chem-
istry; Poli, R., Peruzzi, M., Eds.; Elsevier: New York, 2001; in press.
(8) (a) Ho¨ck, J .; J acobsen, H.; Schmalle, H. W, Artus, G. R.; Fox, T.;
Amor, J . I.; Ba¨th, F.; Berke, H. Organometallics 2001, 20, 1533. (b)
Liang, F.; J acobsen, H.; Schmalle, H. W.; Fox, T.; Berke, H. Organo-
metallics 2001, 19, 1950. (c) Nietlischpach, D.; Bosch, H. W.; Berke,
H. Chem. Ber. 1994, 127, 2403. (d) Van der Zeijden, A. A. H.; Bosch,
H. W.; Berke, H. Organometallics 1992, 11, 2051. (e) Van der Zeijden,
A. A. H.; Bosch, H. W.; Berke, H. Inorg. Chim. Acta 1992, 11, 563. (f)
Van der Zeijden, A. A. H.; Bosch, H. W.; Berke, H. Organometallics
1992, 11, 563.
(PiPr3)2 fragment (Figure 1). The coordination geom-
+
etry around the rhenium atoms are trigonal pyramidal
and tetragonal pyramidal, respectively. The latter unit
shows a nitrosyl ligand occupying the apical position
with a comparatively strong bend (Re-N(5)-O(5) )
158(1)°).
Reaction of 1b with 1 equiv of [Ph3C][BAr4′] affords
the unsaturated rhenium complex [Re(NO)2(PCy3)2]-
(9) Bahr, S. R.; Boudjouk, P. J . Org. Chem. 1992, 57, 5545.

Ligand-Assisted Heterolytic Activation
Organometallics, Vol. 20, No. 25, 2001 5279
F igu r e 2. Molecular structure of complex 3b.6c Displacement ellipsoids are shown at the 30% level. Not shown is the
counterion and solvate molecules. Selected distances (Å) and angles (deg): Re-N(1) ) 1.735(10); Re-N(2) ) 1.766(8);
P(1)-Re-P(2) ) 159.93 (8); N(1)-Re-N(2) ) 115.9(4); Re-N(1)-O(1) ) 166.9(9); Re-N(2)-O(2) ) 165.7(9).
Sch em e 2
[BAr4′] (3b), which constitutes the first example of a 16-
electron dinitrosyl compound (Scheme 1). The X-ray
diffraction study6c reveals a butterfly-like C2v geometry
for the compound (Figure 2). This structure bears a
great resemblance with that of the isoelectronic ruthe-
nium complex [Ru(NO)(CO)(PMetBu2)2][BAr4′].10 At least
in the solid state, 3b displays agostic interactions that
take place between the transition metal and two carbon-
hydrogen bonds belonging to different tricyclohexyl-
phosphine groups (distances of 2.637 and 2.886 Å have
complex containing PiPr3 than for the one with PCy3.
been determined between the rhenium and the agostic
At -40 °C complex 2a shows in C6D5Cl a 31P{1H} NMR
hydrogen atoms). In solution, however, confirmation of
1
spectrum consisting of one singlet and two doublets in
a 2:1:1 ratio, which correspond to an A2MX spin system
(Figure 3). Upon warming, the two doublets start to
broaden until they finally collapse at higher tempera-
tures giving a singlet resonance.
these interactions using variable-temperature (VT) H
NMR failed. Spectra recorded in C6D5Cl showed no
changes for the cyclohexyl protons in the temperature
range between -40 and +60 °C. Deuterated solvents
that would allow a study at lower temperatures could
not be employed, since the compound decomposes in
CD2Cl2 and is not soluble in nonpolar solvents such as
Tol-d8. Furthermore it reacts with donor molecules such
as THF-d8.
Applying the same conditions as for 1b, the reaction
of 1a with 1 equiv of [Ph3C][BAr4′] does not produce
mononuclear compounds analogous to 3b, instead it
yields the binuclear complex 2a plus unreacted [Ph3C]-
[BAr4′]. Nevertheless the 16-electron triisopropylphos-
phine derivative [Re(NO)2(PiPr3)2][BAr4′] (3a ) could
finally be isolated from the reaction of 1a with H(OEt2)2-
[BAr4′].11 This process takes place instantaneously in
solution and is accompanied by hydrogen evolution
(Scheme 2).
Although in the solid state the molecule does not have
symmetry properties that could account for the equiva-
lence of any of the four phosphorus nuclei, it is very
likely that in solution a geometry of higher symmetry
like the one shown in Figure 4 could be adopted. This
would explain the observed chemical equivalence of two
of the phosphorus atoms through a equatorial mirror
plane. Whether the actual geometry of the NO-Re-
(NO)2(PiPr3)2 moiety resembles more a tetragonal pyra-
mid or a trigonal bipyramid could not be derived from
the spectra. A situation, like the one shown in Figure
4, in which the phosphines of both metal fragments are
staggered to minimize steric repulsions, could therefore
Some closer insight into the different reactivities of
compounds 1a ,b toward [Ph3C][BAr4′] is provided by the
VT NMR studies of the binuclear complexes 2a ,b, which
revealed that the isonitrosyl linkage is stronger for the
(10) Ogasawara, M.; Huang, D.; Streib, W. E.; Huffman, J . C.;
Gallego-Planas, N.; Maseras, F.; Eisenstein, O.; Caulton, K. G. J . Am.
Chem. Soc. 1997, 119, 8642.
(11) Brookhart, M.; Grant, B.; Volpe, F. Organometallics 1992, 11,
3920.

5280 Organometallics, Vol. 20, No. 25, 2001
Llamazares et al.
The given picture of dissociation of 2b against non-
dissociation of 2a explains also why 1a does not react
with [Ph3C][BAr4′] to afford 3a . The nondissociating 2a
cannot release reactive 1a , nor does it allow any further
attack of the metal-hydride bond. The rhenium- and
the metal-bound hydrogen atom in 2a have a reduced
basicity caused by coordination of one of the nitrosyl
ligands. This prevents the second hydride abstraction
by the trityl cation. On the other hand, the analogous
binuclear derivative with PCy3 2b with a less robust
isonitrosyl linkage allows dissociation of the “free”
hydride 1b subsequently available for further reaction
with [Ph3C][BAr4′]. Curiously and despite the known
capability of nitrosyl ligands to interact with Lewis acids
via the oxygen atom,5,6 only two other examples of
isonitrosyl linkages between transition metal moieties
have structurally been characterized in the solid state;
both of them are clusters containing molybdenum/
cobalt12a in one case and rhenium in the other.12b
Rea ctivity of 3a ,b tow a r d Tw o-Electr on Don or s.
Due to their coordinative unsaturation, compounds 3a ,b
react readily in solution with two electron donor ligands
to yield the corresponding addition derivatives [Re-
(NO)2(PR3)2L][BAr4′] (R ) iPr, L ) CH3CN, 4a ; CO, 5a ;
C6H5CHO, 6a ; THF, 7a ) (R ) Cy, L ) CH3CN, 4b; CO,
5b; C6H5CHO, 6b; THF, 7b) (Scheme 4), which can be
isolated as pure solids. Except for THF, all these ligands
appear to be strongly bound to the metal. No decompo-
sition takes place after long periods of treatment under
vacuum, and there is no indication of significant dis-
sociation in solution. Structural details concerning the
X-ray diffraction studies of the carbonyl and benzalde-
hyde complexes 5b and 6b have been reported.6c Com-
pound 5b displays a rather regular trigonal bipyramidal
geometry around the rhenium center, while 6b can be
better described as a tetragonal pyramid (Figure 5). As
in the case of 2a , the NO group of complex 6b is lo-
cated in the apical position and shows a significant bend
(Re-N-O ) 150.9(3)°). The geometry changes exhibited
by compounds 5b and 6b confirm that the fragment
[Re(NO)2(PR3)2]+ is extremely flexible in adjusting itself
to the particular stereoelectronic demands of the incom-
ing ligands.
Addition of the above-mentioned ligands to solutions
of the binuclear complexes 2a ,b affords 1:1 mixtures of
1a ,b and the corresponding [Re(NO)2(PiPr3)2L][BAr4′]
species. There is however one exception to this series.
When 2a is dissolved in THF, an equilibrium is estab-
lished showing predominantly starting material and
minor amounts of the dissociated species. If a large
excess of THF is not present in the solution, dissociation
is not observed at all. This can be taken as an additional
proof that the isonitrosyl bridge is stronger in 2a than
in 2b.
Coordination of THF is in any case a reversible
temperature-dependent process for both 7a and 7b, as
demonstrated by VT NMR experiments. Dissociation of
THF does not occur in the pure yellow solids, which are
stable for hours under vacuum and give rise to satisfac-
tory elemental analyses, indicating that the ligand is
not easily released. However, in solution unless an
F igu r e 3. Variable-temperature 31P{1H} NMR spectra
(C6D5Cl) of complex [Re2(H)(µ,η2-NO)(NO)3(PiPr3)4]BAr′4
(2a ) recorded on a Bruker DRX 500.
F igu r e 4. Proposed structure in solution for complex [Re2-
(H)(µ,η2-NO)(NO)3(PiPr3)4]BAr′4 (2a ).
very likely be adopted in solution. Broadening and final
collapse of the two doublets into a singlet upon warming
can be explained on the basis of progressively free
rotation of the Re(NO)2(PiPr3)2 fragment around the
+
NO-Re dative bond.
A similar temperature dependence of the 31P{1H}
NMR spectra is displayed by compound 2b except for
the difference that in this case two very broad singlets
and two doublets are observed in C6D5Cl (-40 °C). This
indicates that the structure is one in which none of the
phosphorus nuclei are related by symmetry. As an
additional phenomenon, dissociation of 2b into 1b and
3b was recognized upon warming of the solution (Scheme
3). At 90 °C the signals of the 1H and 31P{1H} NMR
spectra correspond in fact to those of the independent
+
Re(H)(NO)2(PCy3)2 and Re(NO)2(PCy3)2 species. In
contrast to this, the isonitrosyl linkage of 2a remains
under the same conditions. This observation may be
explained on the basis of steric arguments due to the
larger cone angle of PCy3 or, otherwise, by the fact of
the agostic interactions in 3b providing extra stabiliza-
tion for the 16-electron species.
(12) (a) Kyba, E. P.; Kerby, M. C.; Kashyap, R. P.; Mountzouris, J .
A.; Davis, R. E. J . Am. Chem. Soc. 1990, 112, 905. (b) Beringhelli, T.;
Ciani, G.; D’Alfonso, G.; Molinari, H.; Sironi, A.; Freni, M. J . Chem.
Soc., Chem. Commun. 1984, 1327.

Ligand-Assisted Heterolytic Activation
Organometallics, Vol. 20, No. 25, 2001 5281
Sch em e 3
Sch em e 4
excess of the ligand is present, an equilibrium between
the 16- and 18-electron species can be traced. As
expected from entropy considerations, the unsaturated
species 3a ,b are preferred at higher temperatures, while
at low temperatures association of a THF molecule
prevails to give the 18-electron complex. Thus, the
31P{1H} NMR spectrum in C6D5Cl of a freshly prepared
solution of complex 7b presents at 70 °C a signal at 46
ppm (δ ) 47 ppm for 3b under the same conditions). At
20 °C a very broad signal at 42 ppm is detected, while
at -40 °C the signal sharpens and shifts to 35 ppm. This
latter chemical shift is also obtained when 3b is dis-
solved in pure THF. The described process is ac-
companied by a color change of the solution, which is
yellow at low temperatures and becomes dark red at
temperatures higher than 60 °C, reflecting the colors
of the THF and the unsaturated complexes, respectively.
A similar behavior is observed for complex 7a except
that in this case the affinity for THF is greater and the
temperature required to favor unsaturated 3a over the
saturated species 7a is higher. At +30 °C the 31P{1H}
chemical shift of a solution of 7a in C6D5Cl is 47 ppm,
still closer to the signal at 44 ppm corresponding to the
THF compound 7a than to the signal of the 16-electron
species 3b, which appears at 57 ppm. In the given range
significant shift of the THF resonances. The signals
corresponding to free and coordinated THF can however
be distinguished in CD2Cl2 at -80 °C when a slight
excess of this ligand is present. Further evidence for the
THF lability comes from the fact that when solutions
of 7a ,b in C6H5Cl are placed under vacuum, the color
of them gradually darkens apparently due to loss of THF
and when the process is repeated several times the
unsaturated complexes 3a ,b can be recovered.
Rea ctivity of 3a ,b tow a r d HSiEt3 a n d H2. In the
molecules 3a and 3b a free basic site provided by the
oxygen atom of a nitrosyl group and a free acidic site
provided by the transition metal center coexist and
allow the possibility of ligand-mediated heterolytic
cleavage of nonpolar substrates such as hydrogen or
organosilanes. With this in mind, we set out to inves-
tigate the reactions of compounds 3a and 3b with
HSiEt3 and H2. Coordination of these molecules to the
metal center might be followed by heterolytic splitting
assisted by the built-in NO Bro¨nsted base (Scheme 5).
At room temperature addition of an excess of HSiEt3
to a solution of 3a ,b results in clean formation of
[Re(H)(NO)(NOSiEt3)(PR3)2][BAr4′] (R ) iPr, 8a ; Cy, 8b)
(Scheme 6). If only stoichoimetric amounts of HSiEt3
are employed, heating of the reaction mixture is re-
quired for the process to be completed. The reaction also
1
of temperatures the H NMR spectra (C6D5Cl) show no

5282 Organometallics, Vol. 20, No. 25, 2001
Llamazares et al.
F igu r e 5. Molecular structure of complexes 5b (left) and 6b (right).6c Displacement ellipsoids are shown at the 20% level.
Hydrogen atoms are omitted for clarity. Not shown is the counterion. Selected distances (Å) and angles (deg): For 5b:
(Re(1)-N(1) ) 1.790(7); Re(1)-N(2) ) 1.825(5); Re(1)-C(1) ) 1.979(6); P(1)-Re(1)-P(2) ) 169.62(5); Re(1)-N(1)-O(1)
174.0(6); Re(1)-N(2)-O(3) ) 176.3(6). For 6b: (Re(1)-N(1) ) 1.758(4); Re(1)-N(2) ) 181.1(4); Re(1)-O(3) ) 218.8(3);
P(1)-Re(1)-P(2) ) 158.40(4); Re(1)-N(1)-O(1) ) 175.9(4); Re(1)-N(2) ) 150.9(3).
Sch em e 5
phosphines strongly bent toward the position of the
hydride. However, the most remarkable observations
are as follows: (i) coordination of the SiEt3 moiety to
the nitrosyl produces a very significant lengthening of
the respective N-O bond, (ii) only a slight shortening
of the corresponding Re-N distance is observed, (iii) a
N-O-Si angle close to 120° supports an sp2 hybridiza-
tion at this oxygen atom, (iv) the average C-Si-C angle
is 113.3°, which is closer to a tetrahedral silicon species
than to a trigonal planar three-coordinated Si cation,
although given this geometry, a certain degree of
remaining silylium character has to be assumed.13
The lengthening of the N-O bond is interpreted in
terms of increased electronic donation from the metal
center to the NO unit initiated by the attachment of the
Et3Si+ fragment. Donation of electronic density from the
metal to the NO is further enforced by a more pro-
nounced phosphine bending away from the nitrosyls in
the direction of the hydride ligand. This is a process that
has a low energetic barrier as has been analyzed by DFT
calculations on related model systems.6c Complex 8a
shows a resonance at 42.1 ppm (C6D5Cl, TMS) in the
29Si NMR spectrum, a chemical shift that compares well
with other oxygen-coordinated silylium species reported
in the literature.14 Remarkably and contrary to what is
observed for silylium-ether adducts, which are only
stable at low temperatures, complexes 8a and 8b
remain unchanged in solution at room temperature for
long periods of time, confirming the considerable basic-
ity of the [Re(H)(NO)(NO)(PR3)2] unit.
Sch em e 6
occurs with the binuclear complexes 2a ,b, but as
expected in this case, it is accompanied by formation of
the corresponding [Re(H)(NO)2(PR3)2]. The 1H NMR
spectra of 8a and 8b show a resonance for a metal-
bonded hydride which appears as a triplet due to the
coupling with the phosphorus nuclei. In addition, signals
for a triethylsilyl fragment can be detected, in which
the coupling between the CH2 groups and the terminal
H-Si protons is no longer observable. These two facts
together seem to indicate that cleavage of the H-Si
bond has occurred. On the basis of the known marked
oxophilicity of silicon and the observed basicity of the
oxygen NO atom, the cationic Et3Si+ fragment is
anticipated to be now bonded to this oxygen atom, while
the hydrogen atom binds the transition metal center.
This has been confirmed by an X-ray diffraction study
carried out on derivative 8a (Figure 6). Suitable crystals
were obtained from a mixture of C6H5Cl and HSiEt3.
The structure displays a highly distorted trigonal
pyramidal coordination geometry with the two “trans”
It is reasonable to assume that the first step in the
reactions of 3a ,b with HSiEt3 should be the coordination
of the silane to the vacant rhenium site. However,
extensive VT NMR studies have failed to detect any
intermediates on the way to 8a or 8b. This seems to
(13) Reed, C. A. Acc. Chem. Res. 1998, 31, 325.
(14) (a) Kira, M.; Hino, T.; Sakurai, H. J . Am. Chem. Soc. 1992,
114, 6697. (b) MacLachlan, M. J .; Bourke; S. C.; Lough, A. J .; Manners,
I. J . Am. Chem. Soc. 2000, 122, 2126.

Ligand-Assisted Heterolytic Activation
Organometallics, Vol. 20, No. 25, 2001 5283
Sch em e 7
converted further in less controlled ways by reactions
with the solvent, adventitious water, counteranions, or
the complex itself leading usually to mixtures of silicon-
containing products. The apparent contradiction of an
electron-rich metal center performing heterolytic activa-
tion might be reconciled taking into account the versa-
tility of the [Re(NO)2(PR3)2L]+ cations in adjusting
themselves to different electronic and steric situations.
Following coordination of HSiEt3 and due to steric
repulsions, one might envisage bending of one NO
ligand induced perhaps by the proximity of the ethyl
groups (Scheme 7). There is precedence for this to
happen, since a similar steric response has already been
observed in the cases of complexes 2a and 5a upon
coordination of benzaldehyde or [Re(H)(NO)2(PR3)2],
respectively. But nitrosyl bending will also provoke a
change in the formal oxidation state of the metal center
from -1 to +1 and for this reason will create a new
coordinative vacancy.17 Note as well that like in com-
pounds 2a and 5a , the geometry after NO bending
should be closer to a tetragonal pyramid than to a
trigonal bipyramid, since the former one is favored for
a 16-electron complex. This would leave the linear NO
as very good π-aceptor in an approximate trans position
to HSiEt3. It is reasonable to assume that the acidity
of the resulting intermediate should be now greatly
enhanced, facilitating the heterolytic rupture of the
silane. It is also reasonable to propose that the Et3Si+
fragment interacts first with the nitrogen atom of the
NO, since as a consequence of the anticlinal-type
bending, the oxygen atom would slide away from the
silicon. Subsequently the silicon residue could then
migrate via a 1,2-shift to the ONO atom to yield the
observed 18-electron final products, in which in turn the
rhenium possesses the oxidation state -1.
F igu r e 6. Molecular structure of 8a . Displacement elip-
soids in the PLATON24 plot are shown at the 30% prob-
ability level. Not shown is the counterion. Hydrogen atoms
are omitted for clarity, except for the hydride ligand H1,
which is displayed as a sphere with arbitrary size. Selected
distances (Å) and angles (deg): Re(1)-P(1) ) 2.4224(11),
Re(1)-P(2) ) 2.4346 (12), Re(1)-N(1) ) 1.790(4), Re(1)-
N(2) ) 1.765(4), Re(1)-H(1) ) 1.75(6), N(1)-O(1) ) 1.194-
(5), N(2)-O(2) ) 1.321(5), O(2)-Si(1) ) 1.744(4), Si(1)-
C(1) ) 1.857(7), Si(1)-C(3) ) 1.845(7), Si(1)-C(5) )
1.856(7), P(1)-Re(1)-P(2) ) 141.50(4), N(2)-Re(1)-N(1)
) 128.1(2), O(1)-N(1)-Re(1) ) 167.7(4), O(2)-N(2)-Re-
(1) ) 174.0(4), N(2)-O(2)-Si(1) ) 119.3(3), C(3)-Si(1)-
C(5) ) 113.6(3), C(3)-Si(1)-C(1) ) 114.0(4), C(5)-Si(1)-
C(1) ) 112.4(4).
indicate that coordination of the silane to the metal is
the rate-limiting step in the process and that heterolytic
cleavage of the Si-H bond takes place very fast after
that. Further support for this hypothesis is provided by
the fact that the reaction rate increases considerably
with the concentration of HSiEt3, indicating that the
initial adduct formation significantly contributes to the
rate law.
The reactions of the [Re(NO)2(PR3)2]+ fragments with
HSiEt3 are unique in various ways. First, it should be
mentioned that there are only few examples of transi-
tion metal mediated heterolytic cleavage of silanes
reported, and remarkably they are accomplished by
electron-deficient, poor π-donating systems which greatly
increase the acidity of the HSiR3 molecule upon
coordination.2a,c,f,h,15,16 Furthermore, in complexes con-
taining electron-rich metals, homolytic cleavage is
preferred. However, in the case of 3a ,b heterolysis of
HSiEt3 is achieved by an electron-rich species formally
containing a d8 rhenium(-I) center, which pressumably
can act as a π-donor, as corroborated by the fact that it
forms stable adducts with a strong π-aceptor ligand like
CO (complexes 5a ,b). In the compounds 8a and 8b the
extreme electrophilicity of the silylium ion Et3Si+
enables it to attack one of the nitrosyl groups that acts
as a trap for this fragment. With very few exceptions,2h,16
in the cases of other systems the silylium species are
We have also investigated the reactivity of complexes
2a ,b and 3a ,b toward dihydrogen and dideuterium.
1
Under 1 atm of these gases, the H and 31P{1H} NMR
spectra of these compounds in C6D5Cl solutions show
no significant changes after a period of time varying
from several hours to several days depending on the
compound. Finally decomposition of the starting materi-
(15) (a) Luo, X.-L.; Crabtree, R. H. J . Am. Chem. Soc. 1989, 11, 2527.
(b) Scharrer, E.; Chang, E.; Brookhart, M. Organometallics 1995, 14,
5686.
(16) Fryzuk, M. D.; Love, J . B.; Retting, S. J .; Young, V. G. Science
1997, 275, 1445.
(17) Thorsteinson, E. M.; Basolo, F. J . Am. Chem. Soc. 1966, 88,
3929.

5284 Organometallics, Vol. 20, No. 25, 2001
Llamazares et al.
Sch em e 8
als to give mixtures of unidentified products occurs.
Samples of 2a , 2b, and 3b decompose slowly at room
temperature over a period of several days, while com-
plex 3a deteriorates relatively fast under H2 atmosphere
(1-2 h). Although no apparent reaction seems to take
place with the complex, when identical solutions of the
former compounds are exposed to 1 atm of an equimolar
mixture of H2/D2, catalytic H/D scrambling is observed
at room temperature by 1H NMR. The rate of the
scrambling reaction depends on the solvent, occurring
very fast in C6D5Cl or toluene-d8, which are considered
noncoordinating molecules, and being considerably slower
in donor solvents such as THF-d8. When complexes 3a ,b
are employed, the equilibration in toluene or chloroben-
zene is finished after a few minutes. This is a high rate,
especially in view of the poor solubility of these cationic
derivatives in a solvent like toluene. In THF the reaction
requires several hours to be completed, reflecting the
lower availability of the coordinative vacancy for H2/D2
binding. If the binuclear derivative 2a is used instead,
equilibration in C6D5Cl at room temperature takes
approximately 30-40 min. In addition to the scrambling
processes, it has been observed that when complexes
2a ,b are placed under an atmosphere of D2, replacement
of the hydride ligand by deuterium is achieved to give
the corresponding [Re2(D)(µ,η2-NO)(NO)3(PR3)4][BAr4′]
i
(R ) Pr, 2a D; Cy, 2bD). In contrast exchange of the
hydride ligands by deuterium does not occur under the
same conditions for the mononuclear compounds 1a ,b
after several days, suggesting that a mechanism with
dissociation of the binuclear unit is operative for 2a D
and 2bD.
Sch em e 9
The described experiments confirm that coordination
of H2 and D2 must indeed occur to some extent in
solution, although it could not be traced by NMR
spectroscopy. Derivatives analogous to those obtained
in the reaction with HSiEt3, namely, [Re(H)(NO)(NOH)-
(PR3)2][BAr4′], are not detectable, but could very well
be responsible for the observed scrambling process via
a mechanism like the one proposed in Scheme 8. As in
the case of the silane activation, it would involve
heterolytic rather than homolytic splitting. A homolytic
pathway cannot be completely excluded, but we believe
it is very unlikely, since it would require simultaneous
coordination of H2 and D2 to the same metal center.
Also, although water was rigorously excluded from the
reactions, the presence of trace amounts able to catalyze
the exchange of protons cannot be completely ruled out.
The observed final decomposition of the complexes
might arise from the known instability of the BAr4′
anion over a long period of time under strong acidic
conditions.11
4.1 ppm together with another resonance assigned to a
NH2 fragment. The reaction is clean and almost
+
instantaneous, but instead of the expected neutral
hydrides [Re(H)(NO)2(PR3)2] plus the corresponding
isolated ammonium salt [H2NC5H6(CH3)4][BAr4′], the
new hydrogen-bonded adduct complexes [Re(H)(NO)-
i
{NOH2NC5H6(CH3)4}(PR3)2][BAr4′] (R ) Pr (9a ), Cy
(9b)) are isolated (Scheme 9). The source of the hydride
and the proton is without doubt molecular hydrogen.
This point has been confirmed by experiments carried
out with D2 in a non-deuterated solvent, which showed
the formation of the corresponding isotopomers [Re(D)-
(NO){NOHDNC5H6(CH3)4}(PR3)2][BAr4′].
Suitable crystals of the tricyclohexylphosphine de-
rivative 9b were obtained in a C6H5Cl/pentane mixture,
and an X-ray diffraction study was carried out (Figure
7). From Figure 7 it can be seen that the environment
around the rhenium atom is essentially the same as the
one found in the neutral hydride [Re(H)(NO)2(PiPr3)2]
(1a ) and the distorted trigonal bipyramidal geometry
is maintained. The structural study shows that a strong
interaction between the acidic substrate, in this case
the ammonium protons, and the oxygen atoms of the
nitrosyl ligands takes place. In the solid state, the
ammonium cation is placed in a bridging position
between two NO groups connecting adjacent metal
Further support for the possible involvement of the
nitrosyl ligands in the heterolytic cleavage of H2 comes
from the reaction of compounds 3a ,b with H2 in the
presence of a stoichiometric amount of the amine
1,1′,6,6′-tetramethylpiperidine. This amine was chosen
because it is too bulky to be able to interact significantly
with the unsaturated rhenium center but should be
basic enough to abstract a proton following H2 coordina-
tion. The 1H NMR spectra of the resulting solutions
(C6D5Cl) indicate clearly that activation of one molecule
of H2 has taken place. A signal corresponding to a metal-
bonded hydride appears as a triplet at approximately

Ligand-Assisted Heterolytic Activation
Organometallics, Vol. 20, No. 25, 2001 5285
of hydrogen can take place; when this position is already
occupied by another ligand, the exchange may or may
not occur depending on whether the ligand can be
displaced by H2. The base then must assist in the
splitting of the bonded hydrogen molecule. If its affinity
for protons is strong, the proton cannot be released
anymore and the exchange between H2 and D2 is
suppressed, but if the base is weak, as it seems to be
the case with the NO groups, reversible protonation of
the metal hydride as well as exchange between H+ and
D+ can account for the observed H2/D2 equilibration.
Con clu d in g Rem a r k s
The enhanced hydridicity of the Re-H bond in the
compounds [Re(H)(NO)2(PR3)2] makes it possible to use
them as precursors for the synthesis of unsaturated
rhenium dinitrosyl derivatives [Re(NO)2(PR3)2][BAr4′].
In these complexes the phosphine seems to play a very
important role in the stabilization of the molecule. Thus,
the affinity of the tricyclohexylphosphine derivatives for
σ-donor ligands is much lower than the affinity of the
species containing triisopropylphosphine, even despite
the fact that both phosphines have similar basic proper-
ties. In the earlier case, the presence of agostic interac-
tions between the metal and two C-H bonds of the
cyclohexyl groups might be responsible for this behavior.
Despite the electronic richness of the rhenium center,
the unsaturated complexes 3a and 3b are able to
perform heterolytic activation of H2 and HSiEt3 under
very mild conditions. In these reactions the oxygen atom
of one nitrosyl ligand appears to provide the basic site
which is necessary for the process. This has been
demonstrated in the reactions with HSiEt3 where the
driving force is presumably coming from the extreme
oxophilicity of the silicon atom. Although analogous
derivatives coming from the reaction with H2 have not
been isolated, the observed catalytic equilibration of
H2/D2 mixtures could be achieved via a related mech-
anism. From this work it is clear that noninnocent
ligands such as NO take an active part in the transfor-
mations of small molecular substrates and cannot be
regarded as mere spectator groups.
F igu r e 7. Molecular structure of 9b. Displacement elip-
soids in the PLATON24 are shown at the 20% probability
level. Not shown is the counterion. Hydrogen atoms are
omitted for clarity, except for the hydride ligand, H66, and
ammonium hydrogens, which are shown as a spheres of
arbitrary size. Selected distances (Å) and angles (deg):
Re-P(1) ) 2.4252(8), Re-P(2) ) 2.4191(9), Re-H(66) )
1.4789, Re-N(1) ) 1.782(4), Re-N(2) ) 1.781(3), N(1)-
O(1) ) 1.213(5), N(2)-O(2) ) 1.237(4), P(2)-Re-P(1) )
148.38(3), N(2)-Re-N(1) ) 126.60(16), O(1)-N(1)-Re )
174.1(4), O(2)-N(2)-Re ) 175.7(3).
units. Both of the ammonium hydrogen atoms are
involved in hydrogen bonding in an asymmetric fashion
with nitrosyl substituents which belong to two crystal-
lographically different molecules related by the opera-
tion 1/2+x,-y,z (NO- -HN distances of 1.908 and 1.987
Å, respectively). Figure 8 shows the overall result of the
solid-state hydrogen bonding, which occurs in parallel
zigzag chains progressing in the a-direction of the
crystal.
The great strength of the hydrogen-oxygen interac-
tion is manifested by the fact that the ammonium salt
[H2NC5H6(CH3)4][BAr4′] cannot be separated from the
complex. Even in quite polar solvents such as dichlo-
romethane it is possible to detect the existence of this
linkage, because the chemical shift of the hydride in
1a ,b is a very sensitive probe to detect any interaction
of an acidic substrate with the NO groups.6b,d,18 Indeed
a shift of about +0.3 ppm for the hydride signal is
observed when a stoichiometric amount of [H2NC5H6-
(CH3)4][BAr4′] is added to CD2Cl2 solutions of 1a ,b.
Evolution of hydrogen gas as the result of reversal of
the heterolytic splitting is not observed when solutions
of 9a ,b are heated or placed under vacuum. Exchange
with D2 does not take place even after several hours at
70 °C in C6D5Cl, demonstrating that in the presence of
a strong base, the activation of hydrogen is an irrevers-
ible process. This result contrasts with the observed
H2/D2 scrambling that occurs with the 16-electron
complexes 3a ,b and is very likely the consequence of
the proton being now trapped between the amine and
nitrosyl ligands. It shows that both a coordinative
vacancy on the metal center and a weak base are
necessary for the catalytic activation of hydrogen. The
vacant position is required so that primary coordination
Exp er im en ta l Section
All operations were carried out under nitrogen atmosphere
using a M. Braun 150 G-B glovebox. The solvents were dried
over sodium diphenylketyl (THF, Et2O, hydrocarbons) or P2O5
(CH2Cl2, CH3CN, C6H5Cl) and distilled under N2 prior to use.
The deuterated solvents used in the NMR experiments were
dried over sodium diphenylketyl (C6D6, toluene-d8, THF-d8)
or P2O5 (CD2Cl2, C6D5Cl) and vacuum transferred for storage
in Schlenk flasks fitted with Teflon valves.
NMR experiments were carried out on a Varian Gemini 300,
Varian Mercury 200, or Bruker DRX 500 spectrometer using
5 mm diameter NMR tubes equipped with Teflon valves, which
allow degassing and further introduction of gases into the
probe. Chemical shifts are given in ppm. 1H and 13C{1H} NMR
spectra were referenced to the residual proton or 13C reso-
nances of the deuterated solvent. 31P chemical shifts are
relative to 85% H3PO4. Microanalyses were performed at the
Anorganisch-chemisches Institut of the University of Zu¨rich.
IR spectra were recorded on a Bio-Rad FTS-45 spectrometer.
The following reagents were purchased from Aldrich, de-
gassed, and used without further purification: HSiEt3,
PhCHO, and 2,2,6,6-tetramethylpiperidine.

5286 Organometallics, Vol. 20, No. 25, 2001
Llamazares et al.
F igu r e 8. PLATON24 drawing showing hydrogen bonding 9b, which is continuing at O(2) and O(3). Selected distances
(Å): O(1)-H(3C) ) 1.908, O(2)-H(3D) ) 1.987.
Compounds 1a , 1b,6b 2a , 3b, 5b, 6b,6c [Ph3C][BAr4′],9 and
H(OEt2)2BAr4′11 have been prepared according to the reported
procedures.
9b had to be refined with split atomic positions using the
PART and EADP instruction of SHELXL-97.22 The two hy-
drogens of the ammonium cation have been calculated, and
therefore the O‚‚H distances of the N-H‚‚O hydrogen bonds
are indicated without standard uncertainties.
X-r a y Diffr a ction Stu d ies of Com p lexes 8a a n d 9b.
Crystallographic and experimental parameters are summa-
rized in Table 1. Single crystals of 8a and 9b were mounted
on top of a glass fiber using perfluoropolyether oil and
immediately transferred to the diffractometer, where they
were cooled at 192(2) K using an Oxford cryogenic system.
Determination of the unit cell parameters and the collection
of intensity data were performed with an image plate detector
system (Stoe IPDS diffractometer) using the Stoe IPDS
software.19 Due to the large diffracting volume of the crystals,
a colimator of 0.8 mm diameter was used. Lorentz and
polarization corrections were done with INTEGRATE, and
numerical absortion corrections20 were performed with XRED,
using 9 (for 8a ) and 12 (for 9b) measured and indexed crystal
faces with a video camera installed at the IPDS diffractometer.
Both structures were solved with direct methods using
SHELXS-97.21 The refinements were performed with SHELXL-
97.22
In complex 8a , the hydride H1 was found by difference
electron density calculations. Its coordinates and the isotropic
displacement parameters could be refined. For complex 9b,
the hydride H66 was also found in a difference Fourier map;
however its positional parameters had to be restrained at a
distance of 1.700(0.025) Å during the refinement and resulted
in a relatively short Re-H distance of 1.47 Å. The displace-
ment parameters were refined isotropically. Flack’s x-param-
eter was -0.014(3), confirming the absolute configuration of
the structure 9b.23 One of the disordered cyclohexyl rings in
Other crystallographic data for the structures reported in
this paper are included as Supporting Information.
P r ep a r a tion of th e Com p lexes. [Re2(H)(µ,η2-NO)(NO)3-
(P Cy3)4][BAr 4′] (2b). Rou te A. A mixture of 50 mg of 1b
(0.0619 mmol) and 31 mg of [Ph3C][BAr4′] (0.0280 mmol) in 5
mL of benzene was stirred for 1 h. The solvent was removed
under vacuum, and then the residue was washed once with
pentane at -35 °C to facilitate the transformation of the oil
into a solid. After that, the yellow-orange solid was washed
with pentane at room temperature (2 × 10 mL) and dried
several hours in vacuo to give 50 mg of 2b 90% pure by NMR.
Recrystallization of this material did not afford analytically
pure material, and therefore route B is recommended for this
synthesis.
Rou te B. A mixture of 40 mg of 3b (0.0239 mmol) and 19.3
mg of 1b (0.239 mmol) was dissolved in 0.75 mL of chloroben-
zene. Pentane was layered over this solution and the mixture
placed in the freezer at -35 °C during 5 days. A crop of orange
crystals was collected, washed with cold pentane (2 × 4 mL,
-35 °C), and dried under vacuum to give 40 mg of 2b (67.4%).
Anal. Calcd for C104H145N4O4P4BF24Re2: H, 5.90; C, 50.40; N,
2.26. Found: H, 5.70; C, 50.66; N, 2.26. IR (Nujol, νNO): 1688
(m), 1640(s), 1594(s). 1H NMR (C6D5Cl, 300 MHz), T ) -40
°C: 8.42 (br, m, 8H, BAr4′), 7.58 (br, m, 4H, BAr4′), 1.00-3.00
(m, 132H, PCy3) (the hydride signal was not observed under
these conditions); T ) 0 °C: 8.33 (br, m, 8H, BAr4′), 7.61 (br,
m, 4H, BAr4′), 3.35 (t, br, J H-P ) 46.2 Hz), 1.05-2.60 (m, 132H,
PCy3); T ) 80 °C: 8.15 (br, m, 8H, BAr4′), 7.62 (br, m, 4H,
BAr4′), 4.23 (t, J H-P ) 36.9 Hz), 1.05-2.60 (m, 132H, PCy3).
31P{1H} NMR (C6D5Cl, 121.5 MHz), T ) -40 °C: 42.6 (s, br,
1P), 37.2 (s, br, 1P), 33.5 (2P, d, J P-P ) 162 Hz), 28.5 (d, 2P);
T ) 0 °C: 39.8 (s, br, 2P), 34.7 (s, br, 2P); T ) +70 °C: 46.0
(s, 2P), 38.5 (s, 2P).
(18) Compare also the shift of the hydride signal of complexes 1a ,b
with respect to complexes 8a ,b, in which
a
+SiEt3 fragment is
coordinated.
(19) Stoe IPDS sofware for data collection, cell refinement and data
reduction, Version 2.90; Stoe&Cie: Darmstadt, Germany, 1998.
(20) Coppens, P.; Leiserowitz, L.; Rabinovich, D. Acta Crystallogr.
1965, 18, 1035.
(21) Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467.
(22) Sheldrick, G. M. SHELXL 97; University of Go¨ttingen: Ger-
many, 1997.
(23) Flack, H. D. Acta Crystallogr. 1983, A39, 876. Bernardinelli,
G.; Flack, H. D. Acta Crystallogr. 1985, A41, 500.
(24) Spek, A. L. Acta Crystallogr. 1990, A46, C-34.
[Re(NO)2(P iP r 3)2][BAr 4′] (3a ). A 190 mg sample of 1a
(0.335 mmol) and 339 mg of H(OEt2)2BAr4′ (0.335 mmol) were
dissolved in 5 mL of C6H5Cl. Immediate evolution of H2 takes
place, and a dark red solution is obtained. The solvent is
removed under vacuum and the oily residue washed with

Ligand-Assisted Heterolytic Activation
Organometallics, Vol. 20, No. 25, 2001 5287
Ta ble 1. Cr ysta l Da ta a n d Str u ctu r e Refin em en t
P a r a m eter s for 8a a n d 9b
[Re(NO)2(CH3CN)(P Cy3)2][BAr 4′] (4b). A heterogeneous
mixture containing 30 mg of 3b (0.0180 mmol) in C6H6 (1 mL)
was treated with one drop of CH3CN. A yellow solution was
immediately obtained, and pentane was layered over it. After
some hours yellow crystals were formed. The crystals were
washed with pentane (2 × 5 mL) and dried under vacuum to
8a
9b
formula
Mr
C56H70BF24N2O2P2ReSi C77H99BF24N3O2P2Re
1546.18
1813.54
cryst habit
cryst size
cryst syst
space group
a (Å)
yellow prism
0.67 × 0.65 × 0.34
monoclinic
C2/c
12.6986(7)
15.8124(6)
34.375(2)
90.00
105.217(7)
90.00
6660.4(6)
1.542
yellow prism
0.76 × 0.31 × 0.27
monoclinic
Iaa
16.1372(10)
26.0691(19)
20.8115(14)
90.00
109.410(7)
90.00
8257.4(10)
1.459
give 20 mg of 4b (65.1%). Anal. Calcd for C70H81N3O2P2BF24
-
Re: H, 4.77; C, 49.13; N, 2.46. Found: H, 4.73; C, 49.04; N,
1
2.45. IR (Nujol): 2279 (w, νCN), 1663 (s, νNO), 1625 (s, νNO). H
NMR (CD2Cl2): 7.73 (br, m, 8H, BAr4′), 7.57 (br, m, 4H, BAr4′),
2.79 (s, 3H, CH3CN), 2.40-1.05 (m, 66H, PCy3). 31P{1H} NMR
(CD2Cl2): 27.2 (s). 13C{1H} NMR (CD2Cl2): 4.3 (s, CH3CN),
140.2 (s, CH3CN).
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
[Re(NO)2(CO)(P iP r 3)2][BAr 4′] (5a ). F r om 3a . A 50 mg
sample of 3a (0.0350 mmol) in 3 mL of C6H5Cl was placed in
a 25 mL flask equipped with a Teflon valve. The solution was
placed under 1 atm of CO. Immediately a yellow-brown
solution was formed. By addition of 15 mL of pentane a yellow
solid precipitated. The solid was washed with pentane (3 ×
10 mL) and dried in vacuo to give 40 mg of 5a (78.5%). Anal.
Calcd for C51H54N2O3P2BF24Re: H, 3.73; C, 42.02; N, 1.92.
Found: H, 3.81; C, 42.00; N, 1.87. IR (Nujol): 2029 (s, νCO),
1726 (s, νNO), 1682 (s, νNO). 1H NMR (CD2Cl2): 7.73 (br, m,
8H, BAr4′), 7.57 (br, m, 4H, BAr4′), 2.65 (m, 6H, P(CHMe2)3),
1.32 (m, 36H, P(CHMe2)3). 31P{1H} NMR (CD2Cl2): 31.6 (s).
13C{1H} NMR (CD2Cl2): 202.8 (t, 1C, CO, J P-C ) 9.8 Hz).
F r om 2a . A 70 mg sample of 2a (0.035 mmol) was placed
in a 25 mL flask equipped with a Teflon valve and then 3 mL
of C6H6 added. The mixture was placed under 1 atm of CO
and shaken until all the starting material dissolves. The
solvent was removed under vacuum and the oily residue
washed with pentane (2 × 10 mL), leaving a brown solid, which
was recrystallized from C6H5Cl/pentane. After 24 h brown
crystals were collected, washed with pentane (2 × 5 mL), and
dried in vacuo to give 35 mg of 5a (68.5%).
F
calc (g cm-3
)
Z
4
4
F(000)
temp (K)
µ (mm-1)
abs corr
3104
193(2)
1.998
numerical
0.3479/0.5499
2.79 < θ < 30.46
-18 < h < 17
-22 < k < 22
-48 < l < 48
57 177
3696
193(2)
1.610
numerical
0.3742/0.6704
2.56 < θ < 28.95
-21 < h < 21
-35 < k < 35
-28 < l < 28
32 429
T
min/Tmax
θ range (deg)
hkl range
data measd
unique data
data obs
19 791
14 829
19 635
17 560
(I g 2σ(I))
parameters
806
985
max. shift/s.u. 0.003
0.003
resd d (e Å-3
R1 (obs)
wR2 (obs)
GooF
)
+1.799, -2.050
0.0571
+0.864, -1.253
0.0293
0.0710
0.994
0.1507
1.093
a
Baur and Kassner (1992)25 and Marsh (1997)26 have shown
that many structures published in the non-centrosymmetric space
group Cc were in fact misidentified and should be better described
in a higher true symmetry, e.g., C2/c. Compound 9b with the space
group Ia, which is another setting for the space group Cc, was
[Re(NO)2(P h CHO)(P iP r 3)2][BAr 4′] (6a ). F r om 3a . A 50
mg sample of 3a (0.0350 mmol) in C6H6 (3 mL) was treated
with 50 µL of PhCHO. Immediately a yellow solution and a
brown oil formed. A 10 mL portion of pentane was added, and
the liquid was discharged. The residue was washed with more
pentane (2 × 10 mL) until a yellow-orange solid was obtained
and dried in vacuo to give 46 mg of 6a (85.7%). Anal. Calcd
for C57H60N2O3P2BF24Re: H, 3.94; C, 44.57; N, 1.82. Found:
H, 4.09; C, 44.95; N, 1.82. IR (Nujol): 1674 (s) 1618 (s), 1593
therefore checked for
a higher symmetry with the program
MISSYM, but no additional symmetry elements were found.24
pentane (3 × 20 mL) to give a dark red solid, which was dried
under vacuum, leaving 440 mg of 3a (91.8%). Anal. Calcd for
C
50H54N2O2P2BF24Re: H, 3.81; C, 42.03; N, 1.96. Found: H,
1
(s), 1573 (s). H NMR (CD2Cl2): 9.95 (s, 1H, C6H5CHO), 7.99
1
4.11; C, 41.94; N, 1.95. IR (Nujol, νNO): 1712 (s), 1654 (s). H
NMR (C6D5Cl): 8.23 (br, m, 8H, BAr4′), 7.62 (br, m, 4H, BAr4′),
2.22 (m, 6H, P(CHMe2)3), 0.92 (m, 36H, P(CHMe2)3). 31P{1H}
NMR (C6D5Cl): 58.2 (s, br).
(m, 3H, C6H5CHO), 7.72 (br, m, 10H, BAr4′ + C6H5CHO), 7.56
(br, m, 4H, BAr4′), 2.45 (m, 6H, P(CHMe2)3), 1.31 (m, 36H,
P(CHMe2)3). 31P{1H} NMR (CD2Cl2): 42.3 (s). 13C{1H} NMR
(CD2Cl2): 207.0 (br, s, 1C, C6H5CHO).
F r om 2a . A 70 mg sample of 2a (0.0350 mmol) in C6H6 (3
mL) was treated with an excess of PhCHO (1 drop added with
a pipet). The mixture was shaken until the starting material
dissolved and a brown solution with some brown oil was
obtained. The solution was filtered and pentane layered over
it. After 24 h brown crystals were collected, washed with
pentane (2 × 5 mL), and dried in vacuo to give 30 mg of 6a
(55.8%).
[Re(NO)2(CH3CN)(P iP r 3)2][BAr 4′] (4a ). F r om 3a . A 50
mg sample of 3a (0.0350 mmol) was dissolved in 0.75 mL of
CH3CN. The yellow solution was evaporated and the residue
washed with pentane (3 × 10 mL) to give a yellow-orange solid,
which was dried in vacuo, leaving 47 mg of 4a (91.4%). Anal.
Calcd for C52H57N3O2P2BF24Re: H, 3.91; C, 42.46; N, 2.86.
Found: H, 3.66; C, 42.26; N, 2.82. IR (Nujol): 2273 (w, νCN),
1675 (s, νNO), 1634 (s, νNO). 1H NMR (CD2Cl2): 7.72 (br, m,
8H, BAr4′), 7.57 (br, m, 4H, BAr4′), 2.76 (s, 3H, CH3CN), 2.58
(m, 6H, P(CHMe2)3), 1.32 (m, 36H, P(CHMe2)3). 31P{1H} NMR
(CD2Cl2): 34.0 (s). 13C{1H} NMR (THF-d8): 3.2 (s, CH3CN),
146.7 (s, CH3CN).
[Re(NO)2(THF )(P iP r 3)2][BAr 4′] (7a ). A 75 mg sample of
3a (0.0524 mmol) was dissolved in 0.75 mL of THF and then
15 mL of pentane added. The light yellow flakes that precipi-
tated were collected, washed with pentane, and dried 2 h in
vacuo to give 70 mg of 7a (88.8%). Anal. Calcd for C54H62N2O3P2-
BF24Re: H, 4.16; C, 43.18; N, 1.87. Found: H, 4.45; 43.37; N,
F r om 2a . A 44 mg sample of 2a (0.022) mmol in C6H6 (2
mL) was treated with an excess of CH3CN (3 drops added with
a Pasteur pipet). The resulting orange solution was dried
under vacuum and the oily residue washed with pentane (3 ×
10 mL) to give a yellow-orange solid. The solid was recrystal-
lized in C6H5Cl/pentane, leaving a fraction of orange crystals,
which were washed with pentane (2 × 5 mL) and dried in
vacuo to afford 25 mg of 4a (77.2%).
1
1.74. IR (Nujol, νNO): 1678 (s), 1630 (s). H NMR (CD2Cl2, 25
°C, 3 equiv of THF): 7.73 (br, m, 8H, BAr4′), 7.57 (br, m, 4H,
BAr4′), 3.83 (br, m, 12H, THF), 2.59 (m, 6H, P(CHMe2)3),
1
1.91 (br, m, 12H, THF), 1.35 (m, 36H, P(CHMe2)3). H NMR
(CD2Cl2, -80 °C, 3 equiv of THF): 7.73 (br, m, 8H, BAr4′),
7.55 (br, m, 4H, BAr4′), 4.13 (br, m, 4H, coord. THF), 3.65 (br,
m, 8H, noncoord. THF), 2.49 (m, 6H, P(CHMe2)3), 2.06 (br, m,
4H, coord. THF), 1.78 (br, m, 8H, noncoord. THF), 1.35 (m,
(25) Baur, W. H.; Kassner, D. Acta Crystallogr. 1992, B48, 356.
(26) Marsh, R. E. Acta Crystallogr. 1997, B53, 317.

5288 Organometallics, Vol. 20, No. 25, 2001
Llamazares et al.
36H, P(CHMe2)3). 31P{1H} NMR (CD2Cl2, 25 °C): 45.9 (br, s).
31P{1H} NMR (CD2Cl2, -80 °C): 44.2 (s). 31P{1H} NMR (C6D5-
Cl, 80 °C): 56.3 (s, br).
[Re(NO)2(THF)(P Cy3)2][BAr 4′] (7b). A heterogeneous mix-
ture containing 30 mg of 3b (0.0180 mmol) in C6H6 (1 mL)
was treated with an excess of THF (4 drops added with a
pipet). A yellow solution was obtained, and pentane was
layered over it. Small yellow crystals were collected after 12
h, washed with pentane (5 mL), and dried 15 min in vacuo to
(s, 12H, H2NC5H6(CH3)4), 1.26 (m, 36H, P(CHMe2)3). 31P{1H}
NMR (CD2Cl2): 51.3 (s).
F r om 2a . A solution of 80 mg of 2a (0.0400 mmol) in 2 mL
of C6H5Cl was placed in a 100 mL flask equipped with a Teflon
valve. A 7 µL portion of 2,2,6,6-tetramethylpiperidine (0.0403
mmol) was then added and the solution placed under 950 mbar
of H2. After some minutes the solution was concentrated under
vacuum until 1 mL of C6H5Cl was left and then pentane
layered. A microcrystalline yellow solid was collected in 24 h,
washed with pentane (2 × 10 mL), and dried in vacuo to give
50 mg of 9a (79.1%).
[Re(H)(NO){NOH2NC5H6(CH3)4}(P Cy3)2][BAr 4′] (9b). A
heterogeneous mixture containing 54 mg of 3b (0.0323 mmol)
and 5.7 µL of 2,2,6,6-tetramethylpiperidine (0.0328 mmol) in
5 mL of C6H6 was placed in a 100 mL flask equipped with a
Teflon valve and placed under 900 mbar of H2. Upon stirring
for 20 min the starting material dissolved and an oily residue
precipitated. The solvent was removed under vacuum and the
residue crystallized at -35 °C in CH2Cl2/pentane. After 3 days
yellow crystals were collected, washed with pentane (2 × 10
mL), and dried in vacuo to give 35 mg of 9b (60.6%). The
crystals used for the X-ray diffraction study were obtained by
slow vapor diffusion of pentane into a C6H5Cl solution of the
complex. Anal. Calcd for C77H99N3O2P2BF24Re: H, 5.50; C,
give 28 mg of 7b (89.5%). Anal. Calcd for C72H86N2O3P2BF24
-
Re: H, 4.97; C, 49.63; N, 1.61. Found: H, 5.06; C, 49.68; N,
1
1.47. IR (Nujol, νNO): 1664 (s), 1585(s). H NMR (CD2Cl2, 25
°C, 1.5 equiv THF): 7.73 (br, m, 8H, BAr4′), 7.57 (br, m, 4H,
BAr4′), 3.77 (br, m, 6H, THF), 2.50-1.05 (m, 72H, PCy3
+
THF). 1H NMR (CD2Cl2, -80 °C, 1.5 equiv THF): 7.73 (br, m,
8H, BAr4′), 7.57 (br, m, 4H, BAr4′), 4.10 (br, m, 4H, coord.
THF), 3.66 (br, m, 2H, noncoord. THF), 2.60-1.05 (br, m,
72H, PCy3 + THF). 31P{1H} NMR (THF, 25 °C): 36.2 (br, s).
31P{1H} NMR (CD2Cl2, 25 °C): 44.4 (br, s). 31P{1H} NMR
(CD2Cl2, -80 °C): 35.3 (br, s). 31P{1H} NMR (C6D5Cl, 70 °C):
46.0 (s, br).
[Re(H)(NO)(NOSiEt3)(P iP r 3)2][BAr 4′] (8a ). F r om 3a . A
75 mg sample of 3a (0.0524 mmol) was dissolved in 2 mL of
C6H5Cl and then 2 mL of HSiEt3 added. After 15 min the
yellow solution was filtered, and 10 mL of pentane was layered
over it. The mixture was kept at -35 °C for 24 h. After this
time, a yellow crystalline solid was collected, washed with
pentane (3 × 10 mL), and dried in vacuo to give 70 mg of 8a
(86.3%). Anal. Calcd for C56H70N2O2SiP2BF24Re: H, 4.56; C,
43.50; N, 1.81. Found: H, 4.39; C, 43.70; N, 1.86. IR (Nujol,
51.00; N, 2.32. Found: H, 5.50; C, 51.09; N, 2.40. IR (Nujol,
1
ν
NO): 1556 (m), 1506 (s). H NMR (CD2Cl2): 7.72 (br, m, 8H,
BAr4′), 7.57 (br, m, 4H, BAr4′), 5.51 (br, s, 2H, H2NC5H6(CH3)4),
4.20 (t, 1H, ReH, J H-P ) 39.6 Hz), 2.20-1.10 (m, 66H, PCy3),
1.48 (s, 12H, H2NC5H6(CH3)4). 31P{1H} NMR (CD2Cl2, 25 °C):
41.2 (br, s).
ν
NO): 1667 (s). 1H NMR (C6D5Cl): 8.23 (br, m, 8H, BAr4′), 7.62
H2/D2 Equ ilibr a tion Exp er im en ts. The equilibration was
assumed to be complete when the ratio between the integrals
of the H2 and HD molecules in the 1H NMR spectra of the
solution remained constant. The equilibration times should be
considered only as approximations, since accurate integral
values of such low intensity signals are difficult to determine.
Usin g 2a . In two separate experiments 30 mg (0.0150
mmol) and 20 mg (0.0100 mmol) of 2a in C6D5Cl or toluene-
d8, respectively (0.5 mL), were placed in NMR tubes which
were degassed and filled with 1000 mbar of a 1:1 mixture of
(br, m, 4H, BAr4′), 4.53 (t, 1H, ReH, J H-P ) 41.0 Hz), 2.14 (m,
6H, P(CHMe2)3), 0.98 (m, 36H, P(CHMe2)3), 0.90 (t, 9H, Si-
(CH2CH3)2), 0.70 (q, 6H, Si(CH2CH3)2). 31P{1H} NMR (C6D5-
Cl): 57.8 (s). 29Si NMR (C6D5Cl, TMS): 42.1 (m, br).
F r om 2a . A 85 mg sample of 2a (0.0425 mmol) was
dissolved in 1 mL of C6H5Cl and then 1 mL of HSiEt3 added.
The brown solution turns slowly red and finally yellow-brown.
After 1 day yellow crystals were collected, washed with
pentane (3 × 5 mL), and dried under vacuum, leaving 61 mg
of 8a (92.7%).
1
H2 and D2. At room temperature the H NMR spectra showed
[Re(H)(NO)(NOSiEt3)(P Cy3)2][BAr 4′] (8b). A 50 mg sample
of 3b (0.0299 mmol) was dissolved in 0.5 mL of C6H5Cl and
then 0.5 mL of HSiEt3 added. The dark red solution became
even darker, then gradually lighter and finally yellow. The
solution was placed at -35 °C, and after 3 days yellow crystalls
were collected, washed with pentane (3 × 5 mL), and dried in
vacuo, leaving 45 mg of 8b (84.25%). Anal. Calcd for C74H93N2O2-
SiP2BF24Re: H, 5.25; C, 49.78; N, 1.57. Found: H, 5.02; C,
49.48; N, 1.52. IR (Nujol, νNO): 1677 (s). 1H NMR (CD2Cl2):
7.72 (br, m, 8H, BAr4′), 7.57 (br, m, 4H, BAr4′), 4.74 (t, 1H,
ReH, J H-P ) 41.8 Hz), 2.37-1.10 (m, 66H, PCy3), 1.06 (t,
9H, Si(CH2CH3)2), 0.94 (q, 6H, Si(CH2CH3)2). 31P{1H} NMR
(CD2Cl2, 25 °C): 46.8 (br, s). 13C{1H} NMR (C6D5Cl): 3.9 (s,
Si(CH2CH3)3, 6.2 (s, Si(CH2CH3)3).
that the equilibration was finished after 30 min in C6D5Cl and
10 min in toluene-d8.
Usin g 3a . In an NMR tube 20 mg of 3a (0.0140 mmol)
dissolved in C6D5Cl (0.5 mL) was degassed and placed under
1000 mbar of a 1:1 mixture of H2 and D2. At room temperature
the 1H NMR spectrum showed that the equilibration was
completed after 10 min.
Usin g 3b. In three separate experiments 20 mg of 3b
(0.0120 mmol) in toluene-d8, C6D5Cl, or THF-d8 (0.5 mL) was
placed in NMR tubes which were degassed and filled with ca.
950 mbar of a 1:1 mixture of H2 and D2. At room temperature
the 1H NMR spectra showed that the equilibration was
finished after 10 min in toluene and chlorobenzene. In the case
of THF-d8 only after 5 h formation of small amounts of HD
was detected.
[R e(H )(NO){NOH 2NC5H 6(CH 3)4}(P iP r 3)2][BAr 4′] (9a ).
F r om 3a . A solution of 50 mg of 3a (0.0350 mmol) in 2 mL of
C6H5Cl was placed in a 100 mL flask equipped with a Teflon
valve. A 6 µL portion of 2,2,6,6-tetramethylpiperidine (0.0355
mmol) was then added and the solution placed under 950 mbar
of H2. The red solution turned brown immediately. The solvent
was removed under vacuum and the oily residue recrystallized
in CH2Cl2/pentane at -35 °C. After 2 days yellow crystals were
collected, washed with pentane (3 × 10 mL), and dried in vacuo
to give 45 mg of 9a (70.9%). Anal. Calcd for C59H79N3O2P2-
BF24Re: H, 5.05; C, 44.93; N, 2.66. Found: H, 4.90; C, 44.65;
N, 2.82. IR (Nujol, νNO): 1561 (m), 1503 (m). 1H NMR
(CD2Cl2): 7.72 (br, m, 8H, BAr4′), 7.57 (br, m, 4H, BAr4′), 5.75
(br, s, 2H, H2NC5H6(CH3)4), 4.30 (t, 1H, ReH, J H-P ) 37.5 Hz),
2.43 (m, 6H, P(CHMe2)3), 1.80-1.62 (m, 5H H2NC5H6(CH3)4),
1.46
Ack n ow led gm en t. This work has been supported
by the Swiss National Science Foundation (SNSF). We
would like to thank Dr. Thomas Fox for his very
valuable help in recording some of the NMR spectra
included in this publication.
Su p p or tin g In for m a tion Ava ila ble: Tables of crystal
and data collection parameters, atomic coordinates, bond
lengths, bond angles, and thermal displacement parameters
for 8a and 9b in CIF format. This material is available free of
charge via the Internet at http://pubs.acs.org.
OM010409C