Triple-decker main group cations

Article abstract of DOI:10.1039/b005425i

The syntheses of the first main group triple-decker cations are described, namely, [(η⁵-C₅Me₅) Sn(μ-η⁵-C₅Me₅)Sn(η⁵ -C₅Me₅)][G a(C₆F₅)₄] and [(η⁶-C₇H₈) In(μ-η⁵-C₅Me₅) In(η⁶-C₇H₈)][(C₆ F₅)₃BO(H)B(C₆ F₅)₃], both of which have been characterized by X-ray crystallography; the former was prepared by the reaction of Sn(η⁵-C₅Me₅)₂ with Ga(C₆F₅)₃, while the latter was prepared by treatment of [In(η⁵-C₅Me₅)]₆ with an equimolar mixture of B(C₆F₅)₃ and H₂O·B(C₆F₅)₃.

Full text of DOI:10.1039/b005425i

Triple-decker main group cations†
Alan H. Cowley,* Charles L. B. Macdonald, Joel S. Silverman, John D. Gorden and Andreas Voigt
Department of Chemistry and Biochemistry, The University of Texas at Austin, Austin, Texas 78712 USA.
E-mail: cowley@mail.utexas.edu
Received (in Columbia, MO, USA) 16th June 2000, Accepted 27th November 2000
First published as an Advance Article on the web 8th January 2001
5
The syntheses of the first main group triple-decker cations
are described, namely, [(h5-C5Me5)Sn(m-h5-C5Me5)Sn(h5-
C5Me5)][Ga(C6F5)4] and [(h6-C7H8)In(m-h5-C5Me5)In(h6-
C7H8)][(C6F5)3BO(H)B(C6F5)3], both of which have been
characterized by X-ray crystallography; the former was
prepared by the reaction of Sn(h5-C5Me5)2 with Ga(C6F5)3,
while the latter was prepared by treatment of [In(h5-
C5Me5)]6 with an equimolar mixture of B(C6F5)3 and
H2O·B(C6F5)3.
type geometry while the triple decker anions [(h -C5H5)3Tl2]2
and [(h -C5H5)Cs2]2 possess transoid arrangements.1b,2
5
A second method of triple decker cation synthesis recognized
the isolobal relationship of e.g. [Sn(h -C5H5)]+ and [In(h -
5
6
C6H6)]+ thus suggesting that cations of the latter type should
add to In(h -C5Me5) units.10 Since protolytic cleavage of In(h -
5
5
C5Me5) in the presence of an arene solvent represented a
potential source of [In(arene)]+ cations, we treated In(h -
5
C5Me5) (0.1 g, 0.4 mmol) with equimolar quantities (0.195
11
mmol each) of B(C6F5)3
and the Brønsted acid
12
An elegant approach to the formation of multidecker, sandwich-
type anions of the heavier main group elements consists of the
addition of cyclopentadienide anions to neutral metallocenes in
the presence of weakly coordinating cations.1,2 However, to our
knowledge, the inverse of this approach has not been reported,
viz. the synthesis of homonuclear multidecker p-block cations
by the addition of positively charged fragments to neutral
metallocenes in the presence of appropriate anions. We report
two different but complementary reactions that demonstrate the
viability of the latter approach.
H2O·B(C6F5)3 in toluene solution at 0 °C. The reaction
mixture afforded colorless crystals (0.25 g, 70.4% yield) upon
storage at 230 °C for several days. Since the product could not
be characterized unambiguously on the basis of spectroscopic
data,6 an X-ray crystallographic study was undertaken.7 The
solid state (Fig. 2) consists of [(h -C7H8)In(m-h -C5Me5)In(h -
C7H8)]+ 2+ and [(C6F5)3BO(H)B(C6F5)3]2 ions8 with 1.5
additional toluene molecules per asymmetric unit. There are no
unusually short interionic contacts. The central core of 2+
6
5
6
5
features an h -bonded In atom on each face of the (m-C5Me5)
5
Since it is known3 that the reaction of Sn(h -C5Me5)2 with a
5
variety of acidic reagents results in salts of [Sn(h -C5Me5)]+, it
was reasoned that, in principle, this cation should be able to add
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to Sn(h -C5Me5)2 to afford the desired triple decker cation [(h -
C5Me5)Sn(m-h -C5Me5)Sn(h -C5Me5)]+ 1+. Moreover, it was
recognized that the choice of the gegenion would be important,
bearing in mind that (a) the anion should be weakly coordinat-
ing, and (b) the chance of obtaining a crystalline product would
be maximized if the anion and cation were of comparable size.
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5
5
Addition of a toluene solution of Sn(h -C5Me5)24 (0.38 g, 0.98
5
mmol) to a solution of Ga(C6F5)3 (0.60 g, 1.0 mmol) in the
same solvent at 0 °C resulted in a yellow precipitate which was
recrystallized from hot toluene solution to afford 0.51 g (74.5%
yield) of [1][Ga(C6F5)4]. The positive and negative CI mass
spectra for [1][Ga(C6F5)4] revealed the presence of 1+ and
[Ga(C6F5)4]2 ions, respectively.6 The 1H, 13C and 119Sn NMR
spectra6 evidenced only one type of C5Me5 group and a unique
Sn center thus suggesting reversible dissociation of 1+ into
Fig. 1 View of [1][Ga(C6F5)4] with hydrogen atoms omitted for clarity.
Selected bond distances (Å) and angles (°): Sn(1)–C(1n) av. 2.556(18),
Sn(1)–C(3n) av. 2.896(18), Sn(2)–C(2n) av. 2.540(14), Sn(2)–C(3n) av.
2.92(2), Sn(1)–X(1A) 2.259(18), Sn(1)–X(1C) 2.632(19), Sn(2)–X(1B)
2.232(18), Sn(2)–X(1C) 2.655(19), X(1A)–Sn(1)–X(1C) 154.6(4), Sn(1)–
X(1C)–Sn(2) 174.9(4), X(1C)–Sn(2)–X(1B) 151.8(4).
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Sn(h -C5Me5)2 and [Sn(h -C5Me5)]+. Definitive structural
information was provided by a low-temperature X-ray crystal
structure.7 As shown in Fig. 1, the solid state consists of 1+ and
[Ga(C6F5)4]2 ions8,9 and there are no unusually short interionic
contacts. The structure of 1+ is such that a pentahapto C5Me5
5
ring serves as a bridging group for two Sn(h -C5Me5) units.
Within experimental error, the two Sn atoms are located
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equidistantly from the ring centroid [X(1C)] of the m-(h -
C5Me5) group [2.644(19) Å] and the Sn(1)–X(1C)–Sn(2) angle
is close to 180° [174.9(4)°]. The average distance from the Sn
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atoms to the ring centroids of the two terminal (h -C5Me5)
5
rings, X(1A) and X(1B), is shorter than that to the bridging (h -
C5Me5) moiety [2.246(18) Å] and lies between the values
5
5
reported for Sn(h -C5Me5)2 (2.396 Å)3c and [Sn(h -C5Me5)]+
(2.157 Å).3c The X(1A)–Sn–X(1C) and X(1B)–Sn–X(1C)
angles of 154.6(7) and 151.8(7)°, respectively are very similar
Fig. 2 View of [2][(C6F5)3BO(H)B(C6F5)3]·1.5C7H8, with hydrogen atoms
and non-coordinated toluene molecules omitted for clarity. Selected bond
distances (Å) and angles (°): In(1)–C(1n) av. 2.807(3), In(1)–C(2n) av.
3.752(3), In(2)–C(1n) av. 2.722(3), In(2)–C(3n) av. 3.598(3), In(1)–X(1A)
2.528(4), In(1)–X(1B) 3.490(4), In(2)–X(1B) 2.435(4), In(2)–X(1C)
3.325(4), B–C av. 1.646(4), B–O av. 1.559(4); X(1A)–In(1)–X(1B)
124.4(4), In(1)–X(1A)–In(2) 176.0(4), X(1A)–In(2)–X(1C) 130.3(4).
to the values reported for Sn(h -C5Me5)2 (av. 154.9°).3c An
5
intriguing feature of the overall structure is that 1+ adopts a cis-
† Electronic supplementary information (ESI) available: summary of
theoretical data. See http://www.rsc.org/suppdata/cc/b0/b005425i/
DOI: 10.1039/b005425i
Chem. Commun., 2001, 175–176
This journal is © The Royal Society of Chemistry 2001
175

4 P. Jutzi and B. Hielscher, Organometallics, 1986, 5, 1201.
5 Ga(C6F5)3 was prepared by a similar procedure to that described by K.
Ludovici, W. Tyrra and D. Naumann, J. Organomet. Chem., 1992, 441,
363. We have determined the X-ray crystal structure of Ga(C6F5)3·THF;
pertinent data have been deposited at the Cambridge Crystallographic
Data Centre (file no. CCDC-137250).
group. The In–ring centroid [X(1A)] distances of 2.528(4) and
2.435(4) Å for In(1) and In(2), respectively are longer than
those reported10 for monomeric [2.288(4) Å] and hexameric
5
[2.302(4) Å] In((h -C5Me5). As in the case of 1+, the metal–X–
metal angle in 2+ is close to linear [176.0(4)°]. The triple decker
6
structure of 2+ is completed by capping h -bonded toluene
6 [1][Ga(C6F5)4] (74.5% yield, mp > 250 °C). HRMS: calc. for
molecules. The In–ring centroid [X(1B) and X(1C)] distances
of 3.490(4) and 3.325(4) Å for In(1) and In(2), respectively are
1
C30H45Sn2, m/z 641.155; found, 641.155. NMR (CD2C12): H, d 2.07
5
5
[s, 45 H, h -C5Me5, J(119Sn–1H) 27 Hz]. 13C, d 10.2 [s, h -C5(CH3)5]
considerably longer than those reported for [In(
I
)·2mesitylene]+
121.0 [s, h -C5(CH3)5]. 19F, d 2119.8 (s, o-C6F5), 2155.4 (s, p-C6F5),
5
2160.7 (s, m-C6F5). 119Sn, d 22112. Anal. calc. for C54H45F20GaSn2:
(2.83 and 2.89 Å).13 Nevertheless, it is interesting to note that,
akin to 1+, the toluene–(m-C5Me5)–toluene moieties are dis-
tinctly bent [124.4(4) and 130.3(4)° for X(1A)–In(1)–X(1B)
and X(1A)–In(2)–X(1C), respectively] and that the overall
cationic geometry is cisoid.
C,
46.95;
H,
3.26.
Found:
C,
45.61;
H,
3.25.
[2][(C6F5)3BO(H)B(C6F5)3]·1.5C7H8 (70.4% yield, mp 87 °C). HRMS:
calc. for C10H15In, m/z 364.925; found, 364.924. NMR (C6D6): 1H: d
5
1.494 (s, 15H, h -C5Me5), 2.091 (s, 6H, PhMe), 2.092 (s, 6H, PhMe),
6.9–7.0 (m, 8H, free o- and m-Tol), 7.02–7.04 (m, 2H, free p-Tol),
7.09–7.12 (m, 8H, bound o- and m-Tol), 7.12–7.13 (m, 2H, bound p-
Part of the reason for the long arene distances in 2+ may relate
to the fact that the net +1 charge is delocalized over two In
centers. However, the bonding in 2+ can be interpreted in two
different ways, namely (a) as a triple-decker sandwich cation or
(b) a base-stabilized inverse sandwich cation. Density func-
tional theory (DFT) optimization14 of the model system [In(m-
5
5
Tol), 13C, d 9.63 [s, h -C5(CH3)5], 21.36 (s, PhMe), 116.72 [s, h -
C5(CH3)5], 125.64 (s, p-Tol), 128.51 (s, m-Tol), 129.28 (s, o-Tol),
137.85 (s, ipso-Tol). 19F: d 2134.35 (d, 2JFF 17.4 Hz, p-C6F5), 2158.16
(‘t’, 2JFF 20.9 Hz, p-C6F5), (m, m-C6F5). 11B: d 29.95. Anal. Calc. for
C
70.5H44B2F30In2O1: C, 48.99; H, 2.57. Found: C, 49.66; H, 2.86%.
7 Crystal data: for [1][Ga(C6F5)4]: C54H45F20GaSn2, monoclinic, space
group Cc, yellow prisms, a = 22.147(4), b = 15.092(3), c = 17.352(4)
Å, b = 115.11(3)°, V = 5252(2) Å3, Z = 4, Dc = 1.747 g cm23, m(Mo-
Ka) = 1.561 mm21, R1 = 0.063, wR1 = 0.0823, GOF = 1.344. For
[2][(C6F5)3BO(H)B(C6F5)3]·1.5C7H8: C70.5H44B2F30In2O1, monoclinic
space group, P21/c, colorless blocks, a = 16.042(3), b = 20.771(4), c
= 21.165(4) Å, b = 107.74(3)°, V = 6717(2) Å3, Z = 4, Dc = 1.693
g cm23, m (Mo-Ka) = 0.814 mm21, R1 = 0.0411, wR1 = 0.1138, GOF
= 1.383. Both data sets were collected at 153 K on a Nonius-Kappa
CCD diffractometer. CCDC 182/1871. See http://www.rsc.org/supp-
data/cc/b0/b005425i/ for crystallographic files in .cif format.
8 The structures of the [Ga(C6F5)4]2 and [C6F5)3BO(H)B(C6F5)3]2
anions are similar to those reported in the literature: K.-F. Tebbe, T.
Gilles, F. Conrad and W. Tyrra, Acta Crystallogr., Sect. C, 1996, 52,
1663; A. A. Danopoulos, J. R. Galsworthy, M. L. H. Green, S.
Cafferkey, L. H. Doerrer and M. B. Hursthouse, Chem. Commun., 1998,
2529.
5
h -C5H5)In]+ predicts a D5h symmetric structure with a
computed In–X distance of 2.515 Å, close to the value observed
6
experimentally for 2+. Moreover, the h -coordination of two
benzene molecules to the [In(m-C5H5)In]+ moiety causes only a
slight perturbation of the core thus lending credence to model
(b). Furthermore, the benzene–In bond dissociation energy (6.6
kcal mol21) suggests a very weak interaction. In sharp contrast,
5
5
5
calculations on [(h -C5H5)Sn(m-h -C5H5)Sn(h -C5H5)]+ as a
model for 1+ predict a much more tightly bonded triple-decker
sandwich environment—the weakest bond (36.6 kcal mol21
)
5
5
5
being that between [(h -C5H5)Sn(h -C5H5)] and [Sn(h -
C5H5)]+ fragments. Thus the (C5Me5) acidolysis methodology
may be used to prepare isolobally related compounds with very
different properties. We are currently investigating the utility of
this technique for the synthesis of larger sandwich, cluster and
mixed-metal compounds.
9 Presumably, the initially formed anion is [Ga(C6F5)3(C5Me5)]2.
2
However, since [B(C6F5)nR42n
]
anions are known to undergo facile
We are grateful to the National Science Foundation and the
Robert A. Welch Foundation for financial support.
redistribution reactions, a similar process can be postulated to explain
the formation of [Ga(C6F5)4]2. See: V. K. Dioumaev and J. F. Harrod,
Organometallics, 1997, 16, 2798.
5
10 In(h -C5Me5) is a weakly-held hexamer in the solid state that undergoes
Notes and references
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14 Details available as ESI†.
176
Chem. Commun., 2001, 175–176