Iminoacylberyllium compounds derived from octamethyl- and decamethylberyllocene and 2,6-dimethylphenyl isocyanide. Evidence for the existence of η5/η1 isomers of beryllocenes

Article abstract of DOI:10.1021/om010051k

The existence in solution of η⁵/η¹ isomers of the beryllocenes Be(C₅Me₅)₂ and Be(C₅Me₄H)₂ is inferred from the results of their reaction with CNXyl to give iminoacyl products. The process is reversible and involves coupling of a Be-η¹-Cp′ group with the isocyanide.

Full text of DOI:10.1021/om010051k

2434
Organometallics 2001, 20, 2434-2436
Im in oa cylber ylliu m Com p ou n d s Der ived fr om
Octa m eth yl- a n d Deca m eth ylber yllocen e a n d
2,6-Dim eth ylp h en yl Isocya n id e. Evid en ce for th e
Existen ce of η5/η1 Isom er s of Ber yllocen es
M. del Mar Conejo, Rafael Ferna´ndez, and Ernesto Carmona*
Instituto de Investigaciones Quı´micassDepartamento de Quı´mica Inorga´nica, Universidad de
SevillasConsejo Superior de Investigaciones Cientı´ficas, Avda. Ame´rico Vespucio s/ n,
41092 Sevilla, Spain
Enrique Gutie´rrez-Puebla and AÄ ngeles Monge
Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Cient´ıficas,
Campus de Cantoblanco, 28049 Madrid, Spain
Received J anuary 23, 2001
Sch em e 1. η5/η1 Isom er s of Be(C5Me4H)2
Summary: The existence in solution of η5/ η1 isomers of
the beryllocenes Be(C5Me5)2 and Be(C5Me4H)2 is inferred
from the results of their reaction with CNXyl to give
iminoacyl products. The process is reversible and in-
volves coupling of a Be-η1-Cp′ group with the isocya-
nide.
Since its preparation in 1959,1 beryllocene, Be(C5H5)2,
has attracted considerable attention due to the unusual,
nonsymmetric η5/η1 structure it possesses in the solid
state.2,3 Recently, we reported the synthesis and solid-
state structure of two new beryllocenes, Be(C5Me5)2 (1)
and Be(C5Me4H)2 (2). In the solid state, molecules of the
latter also exhibit an η5/η1 structure, whereas those of
the former contain two η5-C5Me5 rings.4
Sch em e 2. Isom er ic Str u ctu r es for Be(C5Me4H)2
Beryllocene is a highly fluxional molecule,5 whose
dynamic behavior has been explained as involving two
very facile processes, namely ring exchange and a
sigmatropic shift around the η1 ring.5,6 For octamethyl-
beryllocene (2) ring exchange also causes the two C5-
Me4H rings to be indistinguishable in the 1H and
13C{1H} NMR spectra, down to -90 °C.4 However, while
in Be(C5H5)2 the sigmatropic shift exchanges degenerate
structures, in Be(C5Me4H)2 this dynamic process would
interconvert the isomeric structures A-C, depicted in
Scheme 1, which are not degenerate.
With regard to Be(C5Me5)2, a 13C{1H} resonance at
110.5 ppm due to the ring carbons was observed at 20
°C, and this resonance shifted slightly to higher field
(δ 109.7) upon cooling to -90 °C. Known compounds
with a Be(η5-C5Me5) unit7,8 give the corresponding signal
in the very narrow range δ 110-108; hence, it is
tempting to propose that the solution structure of 1 is
represented by the η5/η5 structure found for the mol-
ecules of this compound in the solid state. Nevertheless,
as Be(C5H5)2 and Be(C5Me4H)2 have η5/η1 structures and
molecular dynamic calculations for the former point to
a very small energy difference of 11 kJ mol-1 between
the ground-state η5/η1 and the higher energy η5/η5 D5d
structures,6 it is reasonable to suggest a fast equilibrium
between the isomeric structures D and E, represented
in Scheme 2, exists in solutions of 1. The NMR data do
not allow such a distinction to be made.4 Accordingly,
we have gathered corroborating chemical evidence, and
this is presented herein.
Even though formal insertions of carbon monoxide
and organic isocyanides into transition-metal-η1-Cp′
bonds are not common transformations, examples exist
for both CO9 and CNR.10 Neither of the beryllocenes 1
and 2 appear to give an adduct or an acyl product upon
reaction with CO,11 even under forcing conditions (100
atm in the case of 2). Nevertheless, the addition of 1
mol equiv of CNXyl (Xyl ) C6H3-2,6-Me2) to solutions
* To whom correspondence should be addressed. E-mail:
guzman@cica.es.
(1) Fischer, E. O.; Hofmann, H. P. Chem. Ber. 1959, 92, 482.
(2) (a) Nugent, K. W.; Beattie, J . K.; Hambley, T. W. Aust. J . Chem.
1984, 37, 1601. (b) Beattie, J . K.; Nugent, K. W. Inorg. Chim. Acta
1992, 198-200, 309.
(3) (a) J utzi, P.; Burford, N. Chem. Rev. 1999, 99, 969. (b) J utzi, P.;
Burford, N. Metallocenes; Wiley-VCH: Weinheim, Germany, 1998; Vol.
1, Chapter 2. (c) Bell, N. A. Comprehensive Organometallic Chemistry
II; Pergamon: Oxford, U.K., 1995; Vol. 1, Chapter 2.
(4) Conejo, M. M.; Ferna´ndez, R.; Gutie´rrez-Puebla, E.; Monge, A.;
Ruiz, C.; Carmona, E. Angew. Chem., Int. Ed. 2000, 39, 1949.
(5) (a) Nugent, K. W.; Beattie, J . K.; Field, L. D. J . Phys. Chem.
1989, 93, 5371. (b) Nugent, K. W.; Beattie, J . K. J . Chem. Soc., Chem.
Commun. 1986, 186.
(7) Burns, C. J .; Andersen, R. A. J . Organomet. Chem. 1987, 325, 31.
(8) Pratten, S. J .; Cooper, M. K.; Aroney, M. J . J . Organomet. Chem.
1990, 381, 147.
(9) Casey, C. P.; O’Connor, J . M.; Haller, K. J . J . Am. Chem. Soc.
1985, 107, 3172.
(10) Al´ıas, F. M.; Belderra´ın, T. R.; Paneque, M.; Poveda, M. L.;
Carmona, E. Organometallics 1997, 17, 55620.
(6) Margl, P.; Schwarz, V.; Blo¨chl, P. J . Chem. Phys. 1995, 103, 683.
10.1021/om010051k CCC: $20.00 © 2001 American Chemical Society
Publication on Web 05/15/2001

Communications
Organometallics, Vol. 20, No. 12, 2001 2435
of 1 at room temperature (eq 1) produces the iminoacyl
F igu r e 1. van’t Hoff plot for the dissociation process of 3.
complex 3 in 70% isolated yield (at -78 °C the reaction
seems to be very slow). The C-C coupling could involve
the η5/η1 isomer of 1, i.e., E, but as already noted there
is no experimental indication for its existence in the
solutions of Be(C5Me5)2. However, compelling evidence
in favor of this proposal derives from studies of a closely
related metallocene. Thus, since the isovalent Zn(C5-
Me5)2 has a slip-sandwich η5/η1 structure,12 we studied
its interaction with CNXyl and obtained a related
iminoacyl, Zn(η5-C5Me5)(C(NXyl)C5Me5), with spectro-
scopic properties similar to those of 3.13
F igu r e 2. Molecular structure of compound 3.
IR and NMR data for 3 are in accord with the
formulation proposed. The Be-η1-iminoacyl linkage is
characterized by a sharp IR absorption at 1570 cm-1
and by a low-field 13C{1H} resonance at δ 155.7 due to
the beryllium-bound iminoacyl carbon. Room-tempera-
Sch em e 3. Rea ction of 2 w ith CNXyl
1
ture H and 13C{1H} NMR studies give no indication of
reversibility, but at higher temperatures partial dis-
sociation of 3 into 1 and CNXyl is observed. Figure 1 is
a van’t Hoff plot for the dissociation process in the
temperature range from 80 to 115 °C.
Compound 3 has been characterized additionally by
low-temperature (-130 °C) X-ray studies, the results
of which are summarized in Figure 2.14 As expected, the
Be-Cp′ centroid distance in 3 (1.472 Å) is appreciably
shorter than in the parent compound 1 (1.655 Å),
doubtless as a reflection of a stronger Be-C5Me5
electronic interaction and of a reduced steric strain.
The analogous reaction of 2 and CNXyl gives the
of octamethylberyllocene. At variance with this room-
temperature reaction and isolation result, conducting
the reaction at -78 °C (Scheme 3b) generates cleanly
the iminoacyl isomer 4b in 68% isolated yield. Once
again, 1H and 13C{1H} NMR data (see the experimental
data below) for isomer 4b are in accord with the pro-
posed formulation and indicate it is the iminoacyl
derived from isomer C of Be(C5Me4H)2: i.e., the product
of the coupling of the isocyanide carbon with one of the
C(Me) carbons in a position â with respect to the CH
unit.
As an isolated solid, compound 4b is stable at room
temperature. However, when its solutions are prepared
at -78 °C and warmed to room temperature, spectro-
scopic monitoring reveals a clean conversion to 4a . At
-30 °C, small amounts of another product are detected,
since an olefinic 1H resonance can be discerned at δ 5.69
(the corresponding resonance in 4b appears at δ 5.78).
While it is tempting to propose that this signal corre-
sponds to the missing iminoacyl isomer (i.e., that
1
iminoacyl compound 4a (Scheme 3a). H and 13C{1H}
NMR data are in complete agreement with the proposed
structure (see the experimental data below). In addition,
the iminoacyl ligand is characterized by a ν(CdN) value
of 1560 cm-1. Obviously, complex 4a derives from the
coupling of CNXyl with the Be-Cσ bound of isomer A
(11) For the analogous reaction of Ca(C5Me5)2 and CO see: Selg,
P.; Brintzinger, H. H.; Andersen, R. A.; Horvath, I. T. Angew. Chem.,
Int. Ed. Engl. 1995, 34, 791.
(12) (a) Blom, R.; Boersma, J .; Budzelaar, P. H. M.; Fischer, B.;
Haaland, A.; Volden, H. V.; Weidlein, J . Acta Chem. Scand. 1986, A40,
113. (b) Fischer, B.; Wijkens, P.; Boersma, J .; van Koten, G.; Smeets,
W. J . J .; Spek, A. L.; Budzelaar, P. H. M. J . Organomet. Chem. 1989,
376, 223.
(13) Resa, I.; Ferna´ndez, R.; Carmona, E. To be submitted for
publication.
(14) Crystal data for C29H39BeN (3). The compound crystallizes in
the P1h space group, with cell parameters a ) 8.6728(7) Å, b ) 16.751(2)
Å, c ) 18.357(2) Å, R ) 99.5050(10)°, â ) 103.4820(10)°, and γ )
90.4370(10)°.

2436 Organometallics, Vol. 20, No. 12, 2001
Communications
irritating to the respiratory system, and present the danger
of very serious irreversible effects.
resulting from the coupling to CNXyl of one of the two
R-CMe atoms), the concentration of the complex remains
too low (<5%) to allow its unequivocal characterization.
The fast formation of 4b at -78 °C implies that
structure C is sufficiently populated at this tempera-
ture. It also indicates that the Be-C(Me) bond of
structure C undergoes coupling with CNXyl at a faster
rate than the Be-C(H) bond of A, perhaps as a reflec-
tion of the difference in the strength of the two Be-C
bonds. It is actually well-established that the stability
of alkyl ligands decreases in the order primary >
secondary . tertiary, to the point that observable
tertiary complexes are rare.15
The reaction of beryllocenes 1 and 2 with CNXyl to
form the iminoacyls 3 and 4a ,b resembles closely the
analogous addition of organolithium and organomag-
nesium reagents to isonitriles to give the corresponding
metalloimines, which are useful synthetic reagents.16
Present data suggest that the reaction may proceed by
direct attack of CNXyl onto the polar, relatively weak
Be-η1-Cp′ bond, with formation of a cyclic three-center
Be-C(NXyl)-Cp′ transition state. The traditional mech-
anism, a formal migratory insertion, cannot be com-
pletely ruled out. However, as beryllocenes 1 and 2 are
sterically congested molecules and furthermore the
metal has only the 2s and 2p valence orbitals available,
the coordination of the isocyanide prior to migration of
the η1-Cp′ group would require a change in the hapticity
of the η5 ring.17 Since stronger nucleophiles such as
1,3,4,5-tetramethylimidazol-2-ylidene and pyridine do
not react with 1 (80 °C), and moreover Be(η5-C5Me5)-
(CH3)18 is recovered unaltered after treatment with
CNXyl (80 °C, 24 h), such a coordination change seems
unlikely. Kinetic and mechanistic studies aimed at
clarifying these questions are in progress. Nevertheless,
and regardless of the precise mechanistic details, the
isolation of iminoacyls 3 and 4a ,b provide direct experi-
mental evidence for the existence of the η5/η1 beryllocene
isomers represented in Schemes 1 and 2.
3. Be(C5Me5)2 (1; 0.279 g, 1 mmol) and CNXyl (Xyl ) C6H3-
2,6-Me2; 0.131 g, 1 mmol) were dissolved in petroleum ether
(30 mL), and the mixture was stirred overnight at room
temperature. Removal of the solvent in vacuo and extraction
with petroleum ether afforded crystals of 3 after cooling at -30
°C, in 73% isolated yield. Selected data are as follows. 1H NMR
(500 MHz, [D6]benzene, 25 °C, TMS): δ 1.40 (s, 3 H; C(NXyl)-
C5Me5), 1.44 (s, 15 H; η5-C5Me5), 1.73 (s, 6 H, C(NXyl)C5Me5),
1.83 (s, 6 H, C(NXyl)C5Me5). 13C{1H} NMR (125 MHz, [D6]-
benzene, 25 °C, TMS): δ 8.7 (s, η5-C5Me5), 11.1 (s, C(NXyl)-
C5Me5), 11.6 (s, C(NXyl)C5Me5), 18.8 (s, C(NXyl)C5Me5), 71.7
(s, C(NXyl)C5Me5), 108.8 (s, η5-C5Me5), 135.2 (s, C(NXyl)C5-
Me5), 140.7 (s, C(NXyl)C5Me5), 155.7 (s, CdN). IR (Nujol): ν
1571 (CdN), 1200, 1090, 1065, 826, 758 cm-1. Anal. Calcd for
C
29H39NBe: C, 84.9; H, 9.5; N, 3.4. Found: C, 84.8; H, 9.6; N,
3.4.
4a . The synthesis is similar to that of compound 3. 4a was
obtained in 68% isolated yield. Selected data are as follows.
1H NMR (400 MHz, [D6]benzene, 25 °C, TMS): δ 1.36 (s, 6 H,
η5-C5Me4H), 1.55 (s, 6 H, η5-C5Me4H), 1.77 (s, 6 H, C(NXyl)-
C5Me4H), 1.86 (s, 6 H, C(NXyl)C5Me4H), 3.96 (s, 1 H, C(NXyl)-
C5Me4H), 4.69 (s, 1 H, η5-C5Me4H). 13C{1H} NMR (125 MHz,
[D6]benzene, 25 °C, TMS): δ 8.8 (s, η5-C5Me4H), 10.9 (s, η5-
C5Me4H), 11.6 (s, C(NXyl)C5Me4H), 13.3 (s, C(NXyl)C5Me4H),
77.2 (s, C(NXyl)C5Me4H), 100.4 (s, η5-C5Me4H), 110.2 (s, η5-
C5Me4H), 111.5 (s, η5-C5Me4H), 134.6 (s, C(NXyl)C5Me4H),
137.3 (s, C(NXyl)C5Me4H), 156.7 (s, CdN). IR (Nujol): ν 2726,
1643, 1560 (CdN), 1200, 1087, 1033, 884, 774 cm-1. Anal.
Calcd for C27H35NBe: C, 84.8; H, 9.2; N, 3.7. Found: C, 85.0;
H, 9.2; N, 3.7.
4b. The synthesis is similar to that of compounds 3 and 4a ,
but the reaction is conducted at -78 °C for 30 min, the solvent
removed at ca. -20 °C, and the residue extracted with
petroleum ether and crystallized at the same temperature. 4b
was obtained in 55% isolated yield. Selected data are as
follows. 1H NMR (400 MHz, [D8]toluene, -78 °C, TMS): δ 1.24
(s, 3 H, η5-C5Me4H), 1.25 (s, 3 H, η5-C5Me4H), 1.53 (s, 3 H, η5-
C5Me4H), 1.55 (s, 3 H, η5-C5Me4H), 1.57 (s, 3 H, C(NXyl)-
C5Me4H), 1.67 (s, 3 H, C(NXyl)C5Me4H), 1.74 (s, 3 H, C(NXyl)-
C5Me4H), 1.80 (s, 3 H, C(NXyl)C5Me4H), 4.65 (s, 1 H, η5-
C5Me4H), 5.78 (s, 1 H, C(NXyl)C5Me4H). 13C{1H} NMR (125
MHz, [D8]toluene, -78 °C, TMS): δ 8.9 (s, η5-C5Me4H), 11.1
(s, η5-C5Me4H), 11.3 (s, C(NXyl)C5Me4H), 13.4 (s, C(NXyl)-
C5Me4H), 14.2 (s, C(NXyl)C5Me4H), 16.7 (s, C(NXyl)C5Me4H),
71.8 (s, C(NXyl)C5Me4H), 101.5 (s, η5-C5Me4H), 109.5 (s, η5-
C5Me4H), 109.7 (s, η5-C5Me4H), 111.2 (s, η5-C5Me4H), 111.5 (s,
η5-C5Me4H), 131.4 (s, C(NXyl)C5Me4H), 134.1 (s, C(NXyl)C5-
Me4H), 141.3 (s, C(NXyl)C5Me4H), 148.6 (s, C(NXyl)C5Me4H),
156.5 (s, CdN). IR(Nujol): ν 2725, 1652, 1560 (CdN), 1200,
1089, 1032, 886, 757 cm-1. Anal. Calcd for C27H35NBe: C, 84.8;
H, 9.2; N, 3.7. Found: C, 84.7; H, 9.8; N, 3.7.
Exp er im en ta l Section . Caution! Beryllium compounds are
very toxic by inhalation and in contact with the skin, are
(15) The crystal, with dimensions (in mm) 0.20 × 0.20 × 0.25, was
mounted on a Brucker-Siemens Smart CCD diffractometer equipped
with a low-temperature device and a normal-focus, 2.4 kW sealed-tube
X-ray source (molybdenum radiation, λ ) 0.710 67 Å) operating at 50
kV and 20 mA. Data were collected at 143(2) K using ω scans over the
range 2.68 < θ < 20.81°. The total number of reflections measured
was 7641 of which 5049 were considered independent. The structure
was solved by direct methods (Sheldrick, G. M. SHELX-86, Program
for Crystal Structure Determination; University of Cambridge, 1992).
Hydrogen atoms were located in difference Fourier maps. Refinements
were by full-matrix least-squares analysis with anisotropic thermal
parameters for all non-hydrogen atoms and isotropic parameters for
Ackn owledgm en t. We thank Professor R. A. Ander-
sen for helpful discussions and for a sample of Be(η5-
C5Me5)Me. Financial support from the DGESIC (to E.C.;
Project PB97-0733), the Ministerio de Educacio´n y
Ciencia (grants to M.d.M.C. and R.F.), and the J unta
de Andaluc´ıa is gratefully acknowledged.
hydrogen atoms in both cases. The final
R value is R1 ) 0.1.
Crystallographic data for the structure reported in this paper have
been deposited with the Cambridge Crystallographic Data Center as
supplementary publication no. CCDC-151965.
(16) 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.
Su ppor tin g In for m ation Available: Tables giving atomic
and thermal parameters and all bond distances and angles
for 3. This material is available free of charge via the Internet
at http://pubs.acs.org.
(17) (a) Fujiwara, S.; Maeda, H.; Matsuya, T.; Shin-ike, T.; Kambe,
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Ronman, P. J . Org. Chem. 1978, 43, 731.
(18) Arduengo, A. J .; III; Dias, H. V. R.; Davidson, F.; Harlow, R. L.
J . Organomet. Chem. 1993, 462, 13.
(19) Burns, C. J .; Andersen, R. A. J . Am. Chem. Soc. 1987, 109, 5853.
OM010051K