Synthesis and characterization of the first example of a gallocenium cation [1]

Full text of DOI:10.1021/ja002888x

J. Am. Chem. Soc. 2000, 122, 11725-11726
11725
Communications to the Editor
Synthesis and Characterization of the First Example
of a Gallocenium Cation
Charles L. B. Macdonald, John D. Gorden,
Andreas Voigt, and Alan H. Cowley*
Department of Chemistry and Biochemistry
The UniVersity of Texas at Austin, Austin, Texas 78712
ReceiVed August 4, 2000
The past decade has witnessed an upsurge of interest in the
chemistry of main group metallocenes.1 Apart from the fact that
much less is known about s- and p-block metallocenes than their
d- and f-block counterparts, interest in the main group metal-
locenes has been stimulated by structure and bonding consider-
ations,1 their utility as reagents and chemical vapor deposition
sources,2 and the possibility that cationic species might serve as
useful catalysts for, e.g., alkene polymerization.3 In the context
of group 13 metallocenes, we are aware of only two structurally
authenticated cations, namely the decamethylborocenium cation
(1+)4,5 and the decamethylaluminocenium cation (2+).6 Cation 1+
features an unprecedented “tightly squeezed” η5/η1 structure, while
2+ possesses a ferrocene-like structure. Herein, we report for the
first time (i) various synthetic approaches to the decamethylgal-
locenium cation (3+), (ii) the unique structure of 3+ as determined
by X-ray crystallography, and (iii) insights gained from DFT
calculations on the parent gallocenium cation, [Cp2Ga]+.
Figure 1. Molecular structure of [(η1-C5Me5)(η3-C5Me5)Ga][BF4] ([3]-
[BF4]) showing the atom numbering scheme. Important parameters:
Ga(1)-C(11) 2.011(5) Å, Ga(1)-C(12) 2.359(5) Å, Ga(1)-C(15)
2.360(5) Å, Ga(1)-C(21) 1.974(5) Å, Ga(1)-F(21) 2.176(3) Å, Ga(1)-
F(11) 2.184(3) Å, B(1)-F(11) 1.422(6) Å, B(1)-F(12) 1.358(6) Å, B(2)-
F(21) 1.422(6) Å, B(2)-F(22) 1.361(5) Å, C(11)-C(12) 1.471(8) Å,
C(12)-C(13) 1.409(9) Å, C(11)-C(15) 1.458(7) Å, C(13)-C(14)
1.416(9) Å, C(14)-C(15) 1.391(8) Å, C(21)-C(22) 1.515(7) Å, C(21)-
C(25) 1.496(7) Å, C(22)-C(23) 1.355(8) Å, C(23)-C(24) 1.473(8) Å,
C(24)-C(25) 1.357(8) Å, C(21)-Ga(1)-C(11) 157.6(2)°, C(21)-Ga(1)-
F(21) 100.2(2)°, C(11)-Ga(1)-F(21) 98.0(2)°, C(21)-Ga(1)-F(11)
100.1(2)°, C(11)-Ga(1)-F(11) 95.7(2)°, F(21)-Ga(1)-F(11) 80.2(2)°,
C(21)-Ga(1)-C(12) 123.7(2)°, C(11)-Ga(1)-C(12) 38.3(2)°, F(21)-
Ga(1)-C(12) 135.8(2)°, F(11)-Ga(1)-C(12) 95.1(2)°, C(21)-Ga(1)-
C(15) 128.5(2)°, C(11)-Ga(1)-C(15) 37.9(2)°, F(21)-Ga(1)-C(15)
91.0(2)°, F(11)-Ga(1)-C(15) 131.3(2)°, C(12)-Ga(1)-C(15) 59.4(2)°,
F(12)-B(1)-F(12)#1 114.1(7)°, F(12)-B(1)-F(11) 109.5(2)°, F(12)-
#1-B(1)-F(11) 108.4(2)°, F(12)-B(1)-F(11)#1 108.4(2)°, F(12)#1-
B(1)-F(11)#1 109.5(2)°, F(11)-B(1)-F(11)#1 106.8(6)°, B(1)-F(11)-
Ga(1) 169.1(3)°, F(22)#1-B(2)-F(22) 114.4(6)°, F(22)#1-B(2)-
F(21)#1 108.4(2)°, F(22)-B(2)-F(21)#1 109.5(2)°, F(22)#1-B(2)-F(21)
109.5(2)°, F(22)-B(2)-F(21) 108.4(2)°, F(21)#1-B(2)-F(21) 106.3(6)°,
B(2)-F(21)-Ga(1) 167.2(2)°.
Of the various synthetic methods employed, the most successful
one for the isolation of crystalline material was the protonolysis
of Cp*3Ga with HBF4 in CH2Cl2 solution, which resulted in a
56% yield of pale yellow crystalline [3][BF4].7 The HRMS data
for [3][BF4]7 were in satisfactory accord with the calculated m/e
(1) For recent reviews, see: (a) Jutzi, P.; Burford, N. Chem. ReV. 1999,
99, 969. (b) Shapiro, P. J. Coord. Chem. ReV. 1999, 189, 1.
values for both 3+ and the BF4 anion, and the presence of the
-
(2) See, for example: Wojtczak, W. A.; Fleig, P. F.; Hampden-Smith, M.
J. AdV. Organomet. Chem. 1996, 40, 215.
latter in CD2Cl2 solution was established on the basis of 11B and
(3) (a) Bochmann, M.; Dawson, D. M. Angew. Chem., Int. Ed. Engl. 1996,
35, 2226. (b) Shapiro, P. J.; Burns, C. T. Abstracts of Papers, 218th ACS
National Meeting, New Orleans, LA, August 22-26, 1999; American
Chemical Society: Washington, DC, 1999; INOR 329.
1
19F NMR spectroscopy.7 The H and 13C NMR spectra for 3+
exhibited only singlet resonances for the CH3 groups and ring
carbon atoms down to -70 °C.7 However, given the possibility
of fluxional behavior, an X-ray crystal structure was desirable.9
The solid-state structure of [3][BF4] comprises pairs of deca-
methylgallocenium cations that are connected by two bridging
(4) Voigt, A.; Filipponi, S.; Macdonald, C. L. B.; Gorden, J. D.; Cowley,
A. H. Chem. Commun. 2000, 911.
(5) The salt [1][BCl4] was prepared first by Jutzi et al.; however, the X-ray
crystal structure was not determined. See: Jutzi, P.; Seufert, A. J. Organomet.
Chem. 1978, 161, C5.
(6) (a) Dohmeier, C.; Schno¨ckel, H.; Robl, C.; Schneider, U.; Ahlrichs, R.
Angew. Chem., Int. Ed. Engl. 1993, 32, 1655. (b) U¨ ffing, C.; Ecker, A.; Baum,
E.; Schno¨ckel, H. Z. Anorg. Allg. Chem. 1999, 625, 1354. (c) Dohmeier, C.;
Baum, E.; Ecker, A.; Koppe, R.; Schno¨ckel, H. Organometallics 1996, 15,
4702.
-
BF4 anions such that the symmetry of each dimeric unit is C2
(Figure 1). In contrast to the decamethylborocenium cation, the
Cp* rings of 3+ are almost parallel (1.8° angle between the
normals to the least-squares planes). One ring is attached to
(7) For the preparation of [3][BF4], a solution of HBF4 (54% solution in
Et2O, 1.1 mmol) in CH2Cl2 (20 mL) was added to a stirred pale yellow solution
of Cp*3Ga8 (0.377 g, 0.79 mmol) in CH2Cl2 (20 mL) at room temperature.
After the dark yellow reaction mixture was stirred for 48 h, the volatiles were
removed under reduced pressure, and the resulting pale yellow powder was
recrystallized from CH2Cl2 to give pale yellow plates of [3][BF4] (0.19 g,
56%). Mp 170-171 °C. HRMS (CI, CH4): calcd for C20H30Ga, 339.1603;
found, 339.1597; calcd for BF4, 87.0029; found 87.0029. 1H NMR (300 MHz,
CD2Cl2): δ 1.80 (s, C5Me5). 13C NMR (75.48 MHz, CD2Cl2): δ 11.9 (s, C5-
(CH3)5), 120.4 (s, η5-C5(CH3)5). 11B NMR (96.28 MHz, CD2Cl2): δ 0.8 (s,
sharp).19F NMR (282 MHz, CD2Cl2): δ -104.2. For the preparation of 4, a
solution of B(C6F5)3 (0.325 g, 0.92 mmol) in Et2O (20 mL) was added to a
stirred pale yellow solution of Cp*2GaMe (0.468 g, 0.91 mmol) in Et2O (20
mL) at room temperature. After the pale yellow reaction mixture was stirred
for 48 h, the solution was concentrated to a volume of 5 mL and cooled to
-20 °C to afford a crop of colorless needles of 4 (0.4 g, 78%). Mp 145 °C
dec. CI HRMS: calcd for C26H31F5Ga, 507.1602; found, 507.1627. 1H NMR
(300 MHz, C6D6): δ 1.73 (s, C5Me5). 13C NMR (75.48 MHz, C6D6): δ 14.3
(s, C5(CH3)5), 121.0 (s, η5-C5(CH3)5). 19F NMR (282 MHz, C6D6): δ -92.5
(s, 2F, o-Ar-F), -110.8 (s, 1F, p-Ar-F), -123.7 (s, 2F, m-Ar-F).
(8) Schumann, H.; Nickel, S.; Wiemann, R. J. Organomet. Chem. 1994,
468, 43.
(9) Crystal data for [3][BF4]: C20H30BF4Ga, orthorhombic, Pbcn, a )
11.972(2), b ) 16.481(3), and c ) 20.449(4) Å, V ) 4.034.7(14) Å3, Z ) 8,
Dcalcd ) 1.406 g cm-3, µ(Mo KR) ) 1.40 mm-1. Crystal data for 4: C26H30F5-
Ga, monoclinic, P21/n, a ) 9.7661(2), b ) 16.3514(4), and c ) 15.2095(4)
Å, â ) 95.384(1)°, V ) 2418.1(1) Å3, Z ) 4, Dcalcd ) 1.393 g cm-3. Suitable
single crystals of [3][BF4] and 4 were covered with perfluorinated polyether
oil and mounted on a Nonius-Kappa CCD diffractometer at 133 K. Totals of
4539 and 5518 unique reflections were collected in the ranges 6.32° < 2θ <
54.94° and 4.92° < 2θ < 54.96° for [3][BF4] and 4, respectively, using Mo
KR radiation (λ ) 0.71073 Å). Both structures were solved by direct methods
and refined by full-matrix least squares on F2 using the Siemens SHELX PLUS
5.0 (PC) software package.10 Final R values: 3 [BF4], R1 ) 0.0478, wR2 )
0.0836; 4, R1 ) 0.0396, wR2 ) 0.0721. In the case of 4, the η2-bonded Cp*
ring was modeled in two positions; only the major contributor is shown in
Figure 2.
(10) Sheldrick, G. M., SHELXTL PC Version 5.0, Siemens Analytical
X-ray Instruments, Inc., 1994.
10.1021/ja002888x CCC: $19.00 © 2000 American Chemical Society
Published on Web 11/08/2000

11726 J. Am. Chem. Soc., Vol. 122, No. 47, 2000
Communications to the Editor
gallium in an η1 fashion, as indicated by the localized bonding
(average CR-Câ ) 1.348(4) Å; Câ-Câ ) 1.475(4) Å) and by
the protrusion of the ipso methyl carbon (C(211)) above the least-
squares plane of the C5 ring (Ga(1)-C(21)-C(211) ) 109.5(2)°).
Furthermore, the Ga-C bond length of 1.974(5) Å is similar to
that reported8 for (η1-C5Me5)3Ga (2.038(4) Å). The other ring is
bonded in an η3 fashion, as evidenced by the fact that three of
the Ga-C distances (2.001(3) Å to C(11), 2.352(3) Å to C(12),
and 2.353(3) Å to C(15)) are considerably shorter than the other
two (2.735(3) Å to C(13) and 2.741(3) Å to C(14). The bonding
in the η3 ring is much more delocalized (average CR-Câ
)
1.381(4) Å; Câ-Câ ) 1.415(4) Å); however, the ipso methyl
carbon atom (C(111)) lies 0.43 Å out of the least-squares plane
of the C5 ring. The interactions between the decamethylgalloce-
Figure 2. Molecular structure of (η1-C5Me5)(η2-C5Me5)Ga(C6F5) (4)
showing the atom numbering scheme. Important parameters: Ga-C(11)
2.183(5) Å, Ga-C(12) 2.17(2) Å, Ga-C(31) 1.985(2) Å, Ga-C(21)
2.043(2) Å, C(11)-C(12) 1.50(2) Å, C(11)-C(15) 1.451(13) Å, C(12)-
C(13) 1.406(10) Å, C(13)-C(14) 1.403(12) Å, C(14)-C(15) 1.38(2) Å,
C(21)-C(25) 1.470(3) Å, C(21)-C(22) 1.473(3) Å, C(22)-C(23) 1.371-
(3) Å, C(23)-C(24) 1.432(4) Å, C(24)-C(25) 1.373(3) Å, C(31)-Ga-
C(12) 117.6(3)°, C(21)-Ga-C(12) 119.3(3)°, C(31)-Ga-C(11) 116.7(2)°,
C(21)-Ga-C(11) 123.4(2)°, C(31)-Ga-C(21) 117.14(9)°.
-
nium cations and BF4 anions are manifested by the Ga‚‚‚F
contacts (Ga(1)-F(11) ) 2.186(2); Ga(1)-F(21) ) 2.174(2) Å)
and disparities in the B-F bond distances (e.g., B(1)-F(12) )
1.358(3); B(1)-F(11) ) 1.419(3) Å).
The structure of 3+ is clearly different from those of the cognate
boron and aluminum cations. We have already explained the
structure of 1+ on the basis of repulsions between the π clouds
of the Cp* rings.4 Our expectation was that such repulsions would
be negligible in the case of 3+ because of the larger ionic radius
of Ga3+ and that consequently, like 2+, it would possess a
ferrocene-like structure. We have gained some insight into this
structural question by means of DFT calculations11 on the model
cations [Cp2Ga]+ and [Cp2Al]+. The global minimum for the
gallocenium cation corresponds to the [(η1-Cp)(η5-Cp)Ga]+
geometry with Cs symmetry. The [(η5-Cp)2Ga]+ structure is higher
in energy by 8.81 kcal mol-1 (cf. [Cp2B]+, where the energy
difference is 45 kcal/mol),4 and there is only 0.09 kcal mol-1
difference in energy between the staggered (D5d) and eclipsed
(D5h) conformations. It is important to note that the D5d and D5h
structures are not minima on the potential energy surface (Nimag
) 2 for each). Each pair of doubly degenerate imaginary
frequencies corresponds to a ring slippage that lowers the
symmetry to Cs. In contrast, the global minimum for [Cp2Al]+ is
the D5d [(η5-Cp)2Al]+ structure, and there is no stationary point
corresponding to the Cs geometry. A haptotropic search across
the π-bonded Cp ring reveals a very shallow potential with respect
to deformation of the π-Cp-Ga bond (see the Supporting
Information for full details). In summary, the unique bonding
mode observed in gallocenium cation 3+ appears to be a
consequence of three factors: (i) the π-cloud repulsion effect is
insignificant compared to that in the tightly bonded borocenium
cation, 1+; (ii) the ionic character of the Cp*-metal bonding in
3+ is less than that in the aluminocenium cation 2+ due to the
lower electronegativity of aluminum; and (iii) the π-Cp*-Ga
bonding has a shallow potential which allows for perturbation of
the cation structure by the accompanying anions.
[MeB(C6F5)3]. The initial formation of the desired product in CH2-
Cl2 solution was evident from the identification of [Cp*2Ga]+
and [MeB(C6F5)3]- ions by multinuclear NMR experiments.
However, upon attempted isolation of this salt, it was converted
into Cp*2GaC6F5 (4) via a putative C6F5 back-transfer reaction.14
Interestingly, if the reaction of Cp*2GaMe with B(C6F5)3 is carried
out in Et2O solution, there is no evidence for the intermediacy of
[Cp*2Ga][MeB(C6F5)3]. In view of the recent concern with the
molecular structures of bis(cyclopentadienyl) compounds of
aluminum,3b,15 we decided to investigate that of 4. The solid-
state structure (Figure 2) consists of monomeric units with an
unprecedented (for gallium) geometry in which the Cp* rings are
η1- and η2-bonded to the group 13 element. As in the aluminum
derivatives, (η2-C5R5)2AlMe (R ) H,15 Me3b), the faces of the
cyclopentadienyl rings are located above and below the metal
center. In the case of 4, the η2-attached Cp* ring is close to planar,
and the bonding in the η1-attached Cp* ring is somewhat less
localized than that in 3+.
In summary, we have prepared [(η1-C5Me5)(η3-C5Me5)Ga]-
[BF4], the first example of a structurally characterized gallocenium
cation. The structure of this cation is quite different from that of
the analogous boron and aluminum cations and furthermore
exhibits only marginal stability, particularly with respect to back-
transfer reactions.
Acknowledgment. Funding of this work by the National Science
Foundation and the Robert A. Welch Foundation is gratefully acknowl-
edged.
Since methyl abstraction by B(C6F5)3 represents a viable route
to d-block metallocene cations,12 we investigated the reaction of
Cp*2GaMe13 with B(C6F5)3 in the hope of preparing [Cp*2Ga]-
Supporting Information Available: X-ray experimental details with
positional parameters and full bond distances and angles for [3][BF4]
and 4 and a summary of theoretical results (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
(11) (a) Becke, A. D. Phys. ReV. A 1988, 38, 3098. (b) Perdew, J. P. Phys.
ReV. B 1986, 33, 8822. All DFT calculations were performed using the
Gaussian 94 (Revision B2) suite of programs. All-electron basis sets were
used for C, H (6-31 G(d)), and the group 13 elements (6-31+G(d)).
(12) See, for example: Piers, W. E.; Chivers, T. Chem. Soc. ReV. 1997,
26, 345.
JA002888X
(14) For a discussion of C6F5 transfer reactions, see, e.g.: Dioumaev, V.
K.; Harrod, J. F. Organometallics 1997, 16, 2798.
(15) Fisher, J. D.; Wei, M.-Y.; Willett, R.; Shapiro, P. J. Organometallics
1994, 13, 3324.
(13) Prepared by treatment of MeGaCl2 with 2 equiv of Cp*Li. Details
will be published in a forthcoming full paper.