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Dihydrogen Formation in a Trihydride Metallocene and Its Elimination Both Assisted by Lewis Acids The [Cp2NbH3]+BH3 System.

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[3 I] D. E. Bergbreiter, D. A. Weatherford, J1 Chem. SOC.Chem. Commun. 1989,
883. Recently the palladium complex catalyzed reaction of allylic alcohols
with CH-active methylene compounds under CO, pressure was described:
M. Sakamoto, I.Shimidzu, A. Yamamoto, Bull. Chem. SOC.Jpn. 1996, 69,
1065.
[I21 I-Alkene complexes are more stable than complexes with substituted double
bonds; see R. Cramer, J Am. Chem. SOC.1967,89, 4623.
[I31 A. Van Rooy, E. N. Oriji, P. C. J. Kramer, P. W. N. M. Van Leeuwen,
Organometallics 1995, 14, 34.
[14] B. iikermdrk, S. Hansson. B. Krakenberger, A. Vitagliano, K. Zetterberg,
Organometallics 1984, 3, 679.
[IS] B. M. Trost, T. R. Verhoeven, ,
I
Am. Chem. SOC.1980, fU2,4730.
[I61 A mechanism with net double retention of configuration cannot be ruled out;
see J. W. Faller. D. Linebarrier, Orgunometullics 1988, 7, 1670.
Dihydrogen Formation in a Trihydride
Metallocene and Its Elimination,
Both Assisted by Lewis Acids:
The [Cp,NbH,] BH3 System**
+
Santiago Camanyes, Feliu Maseras, Miquel Moreno,
Agusti Lledos," JosC M. Lluch," and Juan Bertran
Before 1984 hydrogen coordination to a transition metal was
assumed to involve breaking of the H - H bond to produce two
separate hydride ligands.['] Nowadays the existence of coordinated molecular hydrogen in certain transition metal complexes
is well established.['] The number of new dihydrogen complexes
discovered has not ceased to grow, and the reinvestigation of
polyhydride systems is opening new routes to dihydrogen complexes. These two structures are indeed closely related.l3] It has
been shown that trihydride metallocene complexes possess a
thermally accessible dihydrogen state.[41The presence of such a
state has been used to explain the abnormally large and temperature-dependent NMR JHH
coupling constants, a phenomenon
associated with the quantum mechanical exchange of a pair of
h y d r i d e ~ .51~Recent
~,
studies have revealed that the replacement
of one hydride by a n-acceptor ligand in niobocene and tantalocene trihydride complexes enables the observation and characterization of the first stable dihydrogen complexes of
Group 5.16]It has been suggested that the formation of an adduct between the trihydride complex and a Lewis acid could
similarly stabilize the dihydrogen species by decreasing the electron density around the metal.[71These arguments have been
used to explain the exchange couplings observed in Lewis acid
adducts of niobocene trihydride~.~~.
The most common reaction exhibited by these trihydride species is the loss of molecular hydrogen on heating.['] It is interesting to note that addition of a HBR, Lewis acid clearly lowers the
temperature for hydrogen evolution.["] In this communication
we present, for the first time. ab initio DFT calculations indicating that the formation of an adduct between a trihydride com[*] Prof. Dr. A. Lledos, Prof. Dr. J. M. Lluch, Dip1.-Chem. S. Camanyes,
Dr. M. Moreno, Prof. Dr. J. Bertrin
Unitat de Quimica Fisica, Departament de Quimica
Universitat Autonoma de Barcelona
E-08193 Bellaterra, Barcelona (Spain)
Fax: Int. code +(3)581 2920
Dr. F. Maseras
Laboratoire de Structure et Dynamique des Systemes Moleculaires et Solides
Universite de Montpellier I1 (France)
[**I
Financial support is acknowledged from the Spanish Direccion General de
Ensefianza Superior (DGES) under projects PB95-0637 and PB95-0639.
Angew. Chem. In 1. Ed. Engl. 1991, 36, N o . 3
plex and a Lewis acid converts the dihydrogen structure into a stable complex
and, in addition, so assists
the loss of molecular hydrogen that it becomes quite
easy. To this end we have
performed an ab initio density functional theory['']
(DFT) study on the
[Cp,NbH,] + BH,
SYStem.['2-171 BH, has been
chosen as the simplest modFigure I . Energies [kcalrnol-'1 of the
srationary points located. Compounds
el of a Lewis acid.
1,2a, Zb,and 4 correspond to minimum
f?igure1 the energies
energy structures; 3 is a transition state.
of all the stationary points
(that is, points of zero gradient) are presented schematically. The corresponding geometries are sketched in Figure 2. Structure l is for the
[Cp,NbH,] trihydride.
BH, can interact with any of the
three hydrides of [Cp,NbH,]. This
interaction leads to two different
minimum energy adducts (2a and
2b). Geometrical analysis of 2a
shows a strong interaction between
the BH, and the central hydride that
lengthens the distance between Nb
3
4
Figure 2. Geometries of the stationary points 1-4. Bond distances are given in A.
and the central H by 0.08 A with respect to that in 1. Furthermore, the BH, fragment, which is planar when isolated, has
already adopted a nonplanar geometry that begins to resemble
the tetrahedral BH, anion. This affinity of the Lewis acids for
the hydride ligands of metallocene complexes had already been
observed experimentally for [Cp,MH,] (M = Nb, Ta)"sdl and
[Cp,Nb(CO)H] .[18b1 Adduct 2a involves a strong interaction
between the two fragments as indicated by the large stabilization of this complex with respect to the separated partners
(14.6 kcalmol- ', see Figure 1). However, this interaction is
weaker than that between NH, and BH,, which is
32.4 kcal mol- ' at our level of calculation. For comparison, an
0 VCH ~rlugsgesells~huft
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265
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ab initio MP2 calculation with a triple-<, double-polarized basis
set gives a value of 30.7 kcalmol-'[191for the dissociation energy of the NH,-BH, complex, whereas the approximate expcrimental value is 31.1 kcalmol~'.r201
Very interestingly, the interaction of BH, with a lateral hydride leads to the formation of dihydrogen complex 2b. The
energy of this structure is only 3.3 kcalmol-' above that of the
trihydride 2a. The formation of this stable y2-H, complex can be
attributed to the lower electron density at the niobium metal
caused by the presence of the Lewis acid attached to one of its
hydride ligands. When two of the hydrogen atoms are fixed in
the same positions as dihydrogen in 2b and geometry of 1 is
optimized, the encrgy is 6.7 kcalmol-' higher than the trihydride minimum. Since attempts of optimization starting from
this constrained structure always revert to the original trihydride complex 1, the stabilizing role of the Lewis acid on the
dihydrogen complex is confirmed.
The structures of 2a and 2b are similar to those of the recently
isolated [Cp,NbH,L]+ (L = P(OEt), , PMe,Ph) and [Cp,TaH,(CO)]' species in which both a transoid dihydride and a cisoid
dihydrogen isomer have been shown to exist.[']
The existence of the dihydrogen structure 2b led us to postulate that addition of a Lewis acid could facilitate the elimination
of molecular hydrogen from the original trihydride complex. We
indeed succeded in locating a transition state 3, which corresponds to the elimination of H, from the dihydrogen complex
2b. This transition state is 11.7 kcalmol- ' above the dihydrogen
minimum 2b and 15.0 kcalmol-' above the most stable trihydride complex 2a. However, as seen in Figure 1, this transition state is only 0.4 kcalmol-' above the separated [Cp,NbH,]
and BH,. This result is consistent with the experimental fact
that a Lewis acid lowers the temperature at which molecular
hydrogen is eliminated.["] In comparison, direct molecular
hydrogen elimination from the trihydride complex 1 to give
the coordinatively unsaturated [Cp,NbH] complex implies a
monotonous energy increase of 30.8 kcalmol- . The geometry
of transition state 3 is also depicted in Figure 2. Analysis of this
geometry explains why the molecular hydrogen elimination is
facilitated by the presence of the BH, fragment. The H, molecule being eliminated is almost completely formed in this transition state (the H-H distance is 0.754 A, close to the value of the
free hydrogen molecule), and it is already far away from the rest
of the molecule (the Nb-H distances are 2.410 and 2.660 A). To
compensate for this loss of hydrogen, a new bond between the
metal and one of the hydrogen atoms of the BH, molecule (H4)
is forming (the Nb-H4 distance is 2.713 A). The main components of the transition vector also point to this mechanism in
which elimination of the dihydrogen molecule is coupled to
motion of the HI -BH, fragment which reorganizes in order to
form a tetrahydroborate ligand coordinated in a y 2 way to the
niobium.
The final product of this reaction (4) confirms the formation
of the y2-BH, complex. Its geometry (Figure 2 ) reproduces the
coordination pattern observed in an X-ray study of this complex
performed several years ago.[211It is also very close to the X-ray
structure of [Cp,Nb{BH,(C,H,,)}] prepared by addition of 9borabicyclenonane to [Cp,NbH,] .["I The presence of a HBR,
Lewis acid allows the final product to remain coordinatively
saturated; consequently, the global reaction is now highly
exothermic.
To summarize, we have shown that interaction between a
Lewis acid and a trihydride metallocene complex stabilizes the
dihydrogen structure and facilitates molecular hydrogen elimination. This result could be very interesting for the study of the
storage and elimination of H, in transition metal complexes.
266
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VCH V e i l a g ~ ~ g e ~ r l l ~ ~mhH
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Finally although the BH, molecule is a very crude model of a
Lewis acid, we believe that these conclusions can be generalized.
Additional theoretical work with a set of more realistic Lewis
acids is presently in progress in our laboratory.
Received: July 18, 1996 [Z9345IE]
German version: Angew. Chem. 1997. i09,259-261
-
Keywords: ab initio calculations dihydrogen complexes
hydrido complexes * Lewis acids niobium
-
*
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[12] Calculations have becn carried out with the GAUSSlAN 94 series of programs
[I31 using the DFT methodology with the nonlocal B3LYP functional [14].
This computational method has been proved to yield satisfactory results in the
case of [Cp,MH,] complexes (M = Nb, Ta) [4c]. The geometry optimization of
the Cp rings was always restricted to a local C,, symmetry. The stationary
points were located with the Schlegel gradient minimization algorithm [15]. If
the approximate derivative matrix is forced to contain zero or a single negative
eigenvalue, the optimization converges to a minimum or a transition state,
respectively.
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electrons of niobium [ I 7a]. The basis set used for the metal was that associated
with the pseudopotential [17a] with a standard wlence double-< LANL2DZ
contraction [13]. The 6-31G(d,p) basis set was used for the boron and hydrogen
atoms of the BH, group, and for the hydrogen atoms directly attached to the
metal [17b,c,d]. The double-i; 6-31G valence basis set was used for the C and
H atoms of the Cp ligands [17e].
b) R. Ditchfield, W J.
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