Group 14 triple-decker cations

Article abstract of DOI:10.1039/b510337a

The triple-decker cations trans-[(Cp*Sn)₂(μ- η⁵:η⁵-Cp*)]⁺ and frans-[(Cp*Pb)2(u-rf:rf-Cp*)]+ have been prepared and structurally characterized as their [B(C₆F₅)₄]^- salts from the reactions of [Cp*M][B(C₆F₅) ₄] (M = Sn, Pb) with the appropriate decamethylmetallocene. Both triple-decker cations adopt a cisoid arrangement of terminal Cp* groups, whereas the two known triple-decker main-group anions possess a transoid arrangement of terminal Cp groups. The reason for this conformational difference has been probed on the basis of DFT calculations. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b510337a

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F U L L P A P E R
Group 14 triple-decker cations†
Jamie N. Jones, Jennifer A. Moore, Alan H. Cowley* and Charles L. B. Macdonald
Department of Chemistry and Biochemistry, University of Texas at Austin, Austin, TX, 78712,
USA. E-mail: cowley@mail.utexas.edu; Fax: +1 (512) 471-6822; Tel: +1 (512) 471 7484
Received 20th July 2005, Accepted 9th September 2005
First published as an Advance Article on the web 22nd September 2005
5
5
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5
The triple-decker cations trans-[(Cp*Sn)2(l-g :g -Cp*)]+ and trans-[(Cp*Pb)2(l-g :g -Cp*)]+ have been prepared and
structurally characterized as their [B(C6F5)4]− salts from the reactions of [Cp*M][B(C6F5)4] (M = Sn, Pb) with the
appropriate decamethylmetallocene. Both triple-decker cations adopt a cisoid arrangement of terminal Cp* groups,
whereas the two known triple-decker main-group anions possess a transoid arrangement of terminal Cp groups. The
reason for this conformational difference has been probed on the basis of DFT calculations.
5
5
Our isolation of trans-[(Cp*Sn)2(l-g :g -Cp*)][Ga(C6F5)4]
was somewhat surprising in the sense that the anticipated
product of the reaction of Cp*2Sn with Ga(C6F5)3 was a
classical Lewis acid–base complex. In the present contri-
bution we discuss a rational approach to the synthesis of
Group 14 triple-decker cations that involves the electrophilic
addition of [Cp*M]+ cations to the appropriate decamethyl-
metallocene. This approach has permitted the syntheses of
Introduction
Following the disclosure of the tris(cyclopentadienyl)dinickel
cation by Salzer and Werner in 1972,1 the field of triple-decker
complexes has experienced considerable growth.2 For example,
in the context of transition metal systems, it has been found
that carbocyclic ligands other than cyclopentadienyl can be
employed as the bridging moiety.2 The role of bridging ligand
can also be played by heterocyclic ligands such as phospholyl3
or boratabenzene4 as well as by homocyclic main group ring
5
5
trans-[(Cp*Sn)2(l-g :g -Cp*)][B(C6F5)4] and trans-[(Cp*Pb)2(l-
5
5
g :g -Cp*)][B(C6F5)4], both of which have been characterized
by multinuclear NMR spectroscopy and single-crystal X-ray
diffraction. We also present NMR and X-ray structural data
for the precursor complexes [Cp*M][B(C6F5)4], M = Sn, Pb.
Finally, DFT calculations have been carried out on the model
compounds exemplified by Sb5 and As5.6 Several lanthanides
5
are also capable of forming triple-decker complexes, particularly
with porphyrin and/or phthalocyanine ligands.7 However, other
ligand sets can be used as evident in e.g. the samarium complex,
trans-[CpꢀꢀSm]2(l-COT) (Cpꢀꢀ = C5Me4Et).8
5
5
triple-decker cations trans-[(CpM)2(l-g :g -Cp)] (M = Sn, Pb)
in an effort to learn why these species adopt different structures
to the dithallium and dicaesium triple-decker anions.
In sharp contrast to the foregoing, there are very few examples
of analogous triple-decker complexes of the main group ele-
ments. The first homometallic main group triple-decker complex
was published in 19959,10 and featured the dithallium anion,
trans-[(CpTl)2(l-g :g -Cp)]−. A somewhat similar dicaesium
5
5
Experimental
triple-decker anion, trans-[(CpCs)2(l-g :g -Cp)]− was disclosed
the following year.11 The structures of both anions exhibit a
pronounced bending and the terminal Cp groups are arranged
in a transoid fashion. A neutral triple-decker-like dilithium com-
5
5
General procedures
All manipulations and reactions were performed under a dry,
oxygen-free, catalyst-scrubbed argon atmosphere using a com-
bination of standard Schlenk techniques or in an M-Braun or
Vacuum Atmospheres drybox. All glassware was oven-dried and
vacuum- and argon flow-degassed before use. All solvents were
distilled over sodium benzophenone, except methylene chloride,
which was distilled over CaH2, and degassed prior to use.
The Group 14 dihalides were purchased from a commercial
source and used without further purification. The compounds
Cp*Li,17 LiB(C6F5)4,18 Cp*2Sn,19 Cp*SnCl,20 Cp*PbCl,21 and
Cp*2Pb22 were prepared according to literature procedures.
5
5
2
plex, trans-[(g -C5Bz5)Li(g -C5Bz5)Li(g -C6H6)] has also been
reported.12 The first triple-decker main-group cation, trans-
5
5
[(Cp*Sn)2(l-g :g -Cp*)]+, was isolated as its [Ga(C6F5)4]− salt
from the reaction of Cp*2Sn with Ga(C6F5)3.13 Note that the
ditin cation also possesses a bent structure; however, in this case,
the terminal rings are oriented in a cisoid manner. The latest
addition to the family of triple-decker main group complexes
is the neutral dibarium compound trans-[(4CpBa)2(l-COT)]
(4Cp = C5H(CHMe)4).14 The rings are close to parallel, with an
4
interplanar angle between the COT and the bulky Cp ligands
of 169.9◦. Some related multi-decker ferrocene/main group
Physical measurements
systems have also been disclosed.15,16
Low-resolution CI mass spectra were collected on a Finnigan
MAT TSQ-700 mass spectrometer and high-resolution mass
spectra were measured on a VG Analytical ZAB-VE sector
instrument. All MS analyses were performed on samples that
had been sealed in glass capillaries under an argon atmosphere.
Unless otherwise noted, solution-phase 13C and 1H NMR
spectra were recorded at 295 K on a GE QE-300 instrument (1H,
300 MHz; 13C, 75 MHz). Some 13C and 1H NMR spectra and all
the 11B, and 19F solution-phase NMR spectra were recorded at
295 K, unless otherwise noted, on a Varian Inova-500 spectrom-
eter (1H, 500 MHz; 11B, 160 MHz;13C, 125 MHz; 19F, 470 MHz;
27Al, 130 MHz; 119Sn, 186 MHz) instrument. NMR samples were
flame-sealed or recorded immediately following their removal
from the drybox. Deuterodichloromethane was obtained in
sealed vials from a commercial source and used without further
† Electronic supplementary information (ESI) available: Table of calcu-
lated bond lengths and angles. See http://dx.doi.org/10.1039/b510337a
3 8 4 6
D a l t o n T r a n s . , 2 0 0 5 , 3 8 4 6 – 3 8 5 1
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
©

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purification. 1H, 11B, 13C, 19F, and 119Sn spectra are referenced to
Me4Si, BF3·OEt2, Me4Si, CCl3F, and Me4Sn, respectively. Unless
otherwise noted, 13C spectra were obtained under conditions
of broadband proton decoupling. Melting points (uncorrected)
were obtained on a Fisher–Johns apparatus after flame-sealing
the samples in glass capillaries under argon.
The solvent and volatiles were reduced under reduced pressure
until the volume of the solution was approximately 10 mL, at
which point it was transferred to a narrow vessel and layered
with hexane (15 mL). A crop of bright yellow crystals of 3
(0.193 g, 74.8% yield, mp 80–84◦ decomp.) formed at the solvent
5
interface. 1H NMR (CD2Cl2 298 K): d 2.12 [s, 45 H g -C5(CH3)5];
−
1
11B NMR (CD2Cl2 298 K): d −20.23 [s, {B(C6F5)4} ]; 13C{ H}
5
5
NMR (CD2Cl2 298 K): d 10.08 [s, g -C5(CH3)5], 121.50 [s, g -
C5(CH3)5]; 19F NMR (CD2Cl2 298 K): d −133.33 [s, m-C6F5],
−163.98 [t, p-C6F5], −167.80 [s, o-C6F5]; 119Sn NMR (CD2Cl2
298 K) d −2223 (s) HRMS (CI, CH4): calcd for C30H45Sn2, m/z
645.1565; found, 645.1575.
Syntheses
Synthesis of [Cp*Sn][B(C6F5)4] (1). A pale yellow solution
of Cp*SnCl (0.505 g, 1.74 mmol) in methylene chloride (40 mL)
was treated with 40 mL of a methyl◦ene chloride solution of
LiB(C6F5)4 (1.000 g, 1.46 mmol) at 0 C. The resulting orange
solution was allowed to warm to room temperature and stirred
overnight. Lithium chloride was removed by filtration through
Synthesis of trans-[(Cp*Pb)2(l-g5:g5-Cp*)][B(C6F5)4] (4).
An orange solution of [Cp*2Pb][B(C6F5)4] (0.535 g, 0.524 mmol)
in 30 mL of methylene chloride was added to a stirred orange
solution of Cp*2Pb (0.250 g, 0.523 mmol) in 30 mL of methylene
chloride at 0 ◦C. The resulting deep red-orange solution was
allowed to reach room temperature and stirred overnight.
R
Celiteꢀ. The solvent and volatiles were removed under reduced
pressure until the volume of the solution was approximately
10 mL, at which point it was transferred to a narrow vessel
and layered with 15 mL of hexane. A crop of orange crystals
of 1 (0.698 g, 43.0% yield) formed at the solvent interface. The
spectroscopic data were consistent with the literature values.23
R
Lithium chloride was removed by filtration through Celiteꢀ.
The solvent and volatiles were removed under reduced pressure
until the volume of the solution was approximately 10 mL, at
which point it was transferred to a narrow vessel and layered
with 15 mL of hexane. A crop of ruby red crystals of 4 (∼65%
yield, mp 110–120◦ decomp.) formed at the solvent interface.
Synthesis of [(Cp*)Pb][B(C6F5)4] (2). A solution of Cp*PbCl
(0.551 g, 1.46 mmol) in 40 mL of methylene chloride was treated
with 40 mL of a methylene chloride solution of LiB(C6F5)4
(1.000 g, 1.46 mmol) at 0 ◦C. The resulting orange solution was
warmed to room temperature and stirred overnight. Lithium
5
1H NMR (CD2Cl2 298 K): d 2.29 [s, 45 H g -C5(CH3)5]; 11B
−
1
NMR (CD2Cl2 298 K): d −20.23 [(s, {B(C6F5)4} ]; 13C{ H}
R
chloride was removed by filtration through Celiteꢀ. The solvent
5
5
NMR (CD2Cl2 298 K): d 9.09 [s, g -C5(CH3)5], 125.74 [s, g -
C5(CH3)5]; 19F NMR (CD2Cl2 298 K): d −133.37 [s, m-C6F5],
−163.91 [t, p-C6F5], −167.76 [t, o-C6F5]. HRMS (CI, CH4): calcd
for C30H45Pb2, m/z 821.305; found, 821.306.
and volatiles were removed under reduced pressure until the
volume of the solution was approximately 10 mL, at which point
it was transferred to a narrow vessel and layered with 15 mL of
hexane. A crop of orange crystals of 2 (0.597 g, 40.1% yield,
◦
1
mp 88 C decomp.) formed at the solvent interface. H NMR
X-Ray crystallography
(CD2Cl2 298 K): d 2.67 [s 15H, g -C5(CH3)5]; 11B NMR (CD2Cl2
5
−
1
Suitable single crystals of 1–4 were collected directly from a
Schlenk-type flask under argon pressure and covered immedi-
ately with a perfluorinated polyether oil. The X-ray data were
collected at 153 K on a Nonius Kappa CCD diffractometer
equipped with an Oxford Cryostream liquid nitrogen cool-
ing stream and a graphite-monochromated Mo-Ka radiation
source. The total number of reflections, collection ranges, and
final R values are listed in Table 1. Corrections were applied for
Lorentz and polarization effects. All four structures were solved
by direct methods and refined by full-matrix least squares on F2
using the SHELX PLUS software package.24 All non-hydrogen
atoms were allowed anisotropic thermal motion and hydrogen
298 K): d −20.23 [s, {B(C6F5)4} ]; 13C{ H} NMR (CD2Cl2
298 K): d 8.99 [s, g -C5(CH3)5], 125.94 [s, g -C5(CH3)5]; 19F NMR
(CD2Cl2 298 K): d −133.43 [s, m-C6F5], −163.94 [t, p-C6F5],
−167.79 [s, o-C6F5]. HRMS (CI, CH4): calcd for C34H15BF20Pb,
m/z 343.0940; found, 343.0936.
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Synthesis of trans-[(Cp*Sn)2(l-g5:g5-Cp*)][B(C6F5)4] (3).
A
pale yellow solution of [Cp*Sn][B(C6F5)4] (0.463 g, 0.496 mmol)
in 30 mL of methylene chloride was added to a stirred orange
solution of Cp*2Sn (0.193 g, 0.496 mmol) in 30 mL of methylene
chloride at 0 ◦C. The resulting bright yellow solution was
warmed slowly to room temperature and stirred overnight.
R
Lithium chloride was removed by filtration through Celiteꢀ.
atoms, whichwere includedat calculated positions (C–H0.96A),
˚
Table 1 Selected crystal data, data collection and refinement parameters for 1–4
1
2
3
4
Formula
M
C34H15BF20Sn
932.96
Monoclinic
C68H30B2F40Pb2
2042.92
C54H45BF20Sn2
1322.09
Monoclinic
C2/c
22.138(5)
14.955(5)
17.097(5)
90
C54H45BF20Pb2
1499.09
Monoclinic
C2/c
22.545(5)
14.773(5)
17.087(5)
90
114.911(5)
90
5162(3)
4
1.929
2872
0.15 × 0.11 × 0.10
3.47 to 25.00
8546
Crystal system
Triclinic
¯
Space group
P21/c
P1
˚
a/A
13.442(5)
14.160(5)
17.551(5)
90
11.642(5)
11.749(5)
13.392(5)
64.003(5)
77.837(5)
85.223(5)
1609.4(11)
1
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
91.920(5)
90
3339(2)
4
1.856
1816
0.17 × 0.11 × 0.11
3.09 to 27.47
13,883
7559
0.0298
114.259(5)
90
5161(3)
3
˚
V/A
Z
4
D
calcd/g cm−3
2.108
972
0.13 × 0.12 × 0.09
3.09 to 27.39
12,396
7270
0.0350
0.0753
2.710 and −2.175
1.702
2616
0.14 × 0.12 × 0.10
3.02 to 27.44
26,112
5869
0.0424
0.0948
0.460 and −0.698
F(000)
Crystal size/mm
h range/◦
No. of reflns. collected
No. of indep reflns.
R1[I > 2r (I)]
wR2 (all data)
4518
0.0173
0.0437
0.414 and −0.878
0.0562
0.313 and −0.438
−2
˚
Peak and hole/e A
D a l t o n T r a n s . , 2 0 0 5 , 3 8 4 6 – 3 8 5 1
3 8 4 7

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were refined using a riding model and a general isotropic thermal
parameter.
CCDC reference numbers 276993–276996.
See http://dx.doi.org/10.1039/b510337a for crystallographic
data in CIF or other electronic format.
Theoretical methods
All calculations were performed at the RB3PW91 level of theory
using the Gaussian 03 suite of programs.25 For the geometry
optimizations, the 6-31 + G(d) basis set was used for carbon
and hydrogen and the Stuttgart/Dresden (SDD) ECP basis set
was employed for the Group 14 metals. Single-point calculations
were carried out at the optimized geometries using the 6–
311++G(d,p) basis set for carbon and hydrogen and the SDD
basis set for tin and lead. Stationary points were identified by the
calculation of vibrational frequencies. Graphical representations
of the calculated molecular orbitals were produced using the
Molden program26 and the POV-ray windows program.
Fig. 1 Structure of [Cp*Sn][B(C6F5)4] (1) showing the close contacts
between the tin cation and some of the fluorine atoms on the anion.
Hydrogen atoms have been omitted for clarity.
Results and discussion
As pointed out in the Introduction, the triple-decker
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5
cation, [(Cp*Sn)2(l-g :g -Cp*)]+ was isolated originally as its
[Ga(C6F5)4]− salt from the reaction of Cp*2Sn with Ga(C6F5)3.
Although the mechanism of this reaction is not known, it seems
likely that the first step is the abstraction of a [Cp*]− anion
from Cp*2Sn to form [Cp*Sn]+ and that the triple-decker cation
is formed by electrophilic attack of [Cp*Sn]+ on decamethyl-
stannocene. Indeed, an analogous process (albeit with Cp rather
than Cp* ligands) was suggested for the formation of [(CpNi)2(l-
salts followed by treatment with the appropriate neutral met-
allocene, Cp*2M. The choice of [B(C6F5)4]− as the gegenion
was based on the following considerations: (a) this is a weakly-
coordinating anion, and (b) this anion is of comparable size
to that of the target triple-decker cations, thus maximizing the
lattice energies of the resulting salts.
The salts [Cp*Sn][B(C6F5)4] (1) and [Cp*Pb][B(C6F5)4] (2)
were prepared by treatment of Cp*SnCl20 or Cp*PbCl21 with
LiB(C6F5)4 in CH2Cl2 solution. The compound [Cp*Sn]-
[B(C6F5)4] has been prepared by this method previously;23
however, no X-ray crystal data are available. The equivalence
of the 1H NMR signals for the CH3 groups and the 13C signals
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5
g :g -Cp]+, viz. electrophilic attack of [CpNi]+ on nickelocene.1
In the case of the ditin triple-decker cation, the initially-formed
anion is presumably [Ga(C6F5)3Cp*]−. Since [B(C6F5)nR4−n
]
−
anions are known to undergo facile redistribution reactions,27
a similar process can be postulated to explain the formation
of [Ga(C6F5)4]− from [Ga(C5F5)3Cp*]−. Another point worth
mentioning is that the reaction of stannocene with BF3 does not
result in triple-decker cation formation.28 In part, this might be
due to the relatively small size of the [BF4]− anion vis-a`-vis the
5
for the ring carbon suggested the g -attachment of Cp* rings
to the Group 14 element in both compounds. This suggestion
was confirmed by single-crystal X-ray diffraction. Compound
1 crystallizes in the monoclinic space group P21/c with Z =
5
5
[(CpSn)2(l-g :g -Cp)]+ cation.
4 while 2 crystallizes in the triclinic space group P1 with Z =
¯
Bearing in mind the foregoing comments, we developed the
following more rational approach to the synthesis of Group 14
triple-decker cations, namely the synthesis of [Cp*M][B(C6F5)4]
2. The tin atom of [Cp*Sn]+ is centered above the C5Me5 ring
and the Sn–Cring bond distances range from 2.428(2)–2.446(3)
˚
A (see Table 2 and Fig. 1) with an average bond distance of
◦
˚
Table 2 Selected bond lengths (A) and angles ( ) for 1–4
1
2
3
4
Sn–C(1)
Sn–C(2)
Sn–C(3)
Sn–C(4)
2.434(2)
2.428(2)
2.444(2)
2.446(2)
2.437(2)
1.427(3)
1.430(2)
1.426(3)
1.427(3)
1.431(3)
Pb–C(1)
Pb–C(2)
Pb–C(3)
Pb–C(4)
2.539(4)
2.539(4)
2.546(4)
2.557(4)
2.559(4)
1.428(6)
1.435(6)
1.428(6)
1.424(6)
1.427(6)
Sn(1)–C(1)
Sn(1)–C(2)
Sn(1)–C(3)
Sn(1)–C(4)
Sn(1)–C(5)
C(1)–C(2)
C(2)–C(3)
C(3)–C(4)
C(4)–C(5)
C(5)–C(1)
Sn(1)–C(6)
Sn(1)–C(7)
Sn(1)–C(8)
C(6)–C(7)
C(7)–C(8)
2.570(4)
2.576(4)
2.510(4)
2.507(4)
2.517(4)
1.412(6)
1.436(6)
1.424(5)
1.427(6)
1.425(6)
2.911(4)
2.846(4)
2.842(2)
1.432(5)
1.431(5)
Pb(1)–C(1)
Pb(1)–C(2)
Pb(1)–C(3)
Pb(1)–C(4)
Pb(1)–C(5)
C(1)–C(2)
C(2)–C(3)
C(3)–C(4)
C(4)–C(5)
C(5)–C(1)
Pb(1)–C(6)
Pb(1)–C(7)
Pb(1)–C(8)
C(6)–C(7)
C(7)–C(8)
2.670(3)
2.604(3)
2.592(3)
2.618(3)
2.672(3)
1.422(4)
1.422(4)
1.420(4)
1.424(4)
1.409(4)
2.953(3)
2.911(3)
2.906(2)
1.431(4)
1.423(4)
Sn–C(5)
Pb–C(5)
C(1)–C(2)
C(2)–C(3)
C(3)–C(4)
C(4)–C(5)
C(5)–C(1)
C(1)–C(2)
C(2)–C(3)
C(3)–C(4)
C(4)–C(5)
C(5)–C(1)
C(2)–C(1)–C(5)
C(1)–C(2)–C(3)
C(2)–C(3)–C(4)
C(3)–C(4)–C(5)
C(4)–C(5)–C(1)
107.7(2)
108.2(2)
107.9(2)
108.0(2)
108.1(2)
C(2)–C(1)–C(5)
C(1)–C(2)–C(3)
C(2)–C(3)–C(4)
C(3)–C(4)–C(5)
C(4)–C(5)–C(1)
108.1(3)
107.8(4)
107.8(4)
108.2(4)
108.1(4)
C(5)–C(1)–C(2)
C(1)–C(2)–C(3)
C(2)–C(3)–C(4)
C(3)–C(4)–C(5)
C(4)–C(5)–C(1)
C(6A)–C(6)–C(7)
C(6)–C(7)–C(8)
C(7)–C(8)–C(7A)
108.7(3)
107.6(3)
108.1(3)
107.8(3)
107.8(3)
108.0(2)
108.4(3)
107.2(4)
C(5)–C(1)–C(2)
C(1)–C(2)–C(3)
C(2)–C(3)–C(4)
C(3)–C(4)–C(5)
C(4)–C(5)–C(1)
C(6A)–C(6)–C(7)
C(6)–C(7)–C(8)
C(7)–C(8)–C(7A)
108.3(3)
107.8(3)
108.0(2)
107.8(2)
108.2(2)
108.1(2)
107.7(2)
108.4(3)
3 8 4 8
D a l t o n T r a n s . , 2 0 0 5 , 3 8 4 6 – 3 8 5 1

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˚
2.438(2) A. The ring-centroid to metal bond distance is 2.114(3)
correspond to the ortho, meta, and para ring carbon atoms of
the C6F5 groups. The identification of 3 was confirmed by the
11B and 19F NMR data. Overall, however, it was not possible to
assign the structure of the cation unequivocally on the basis of
NMR spectroscopy.
5
˚
A. The [g -C5Me5)] ring exhibits delocalized p-bonding as
evidenced by the Cring–Cring bond distances of 1.426(4), 1.427(4),
1.428(4), 1.432(4), and 1.432(4) A. Based on this pattern of
˚
bond distances, the tin atom can be considered to be pentahapto
bonded to the pentamethylcyclopentadienide ring. The ring
itself is planar and the methyl groups extend out of the plane and
away from the tin center. Evidence for this view is provided by
the angles between the ring plane and the least-squares planes
defined by the two adjacent ring-carbon atoms and the attached
methyl carbon atoms, which have an average value of 5.7◦.
Four of the fluorine atoms (F51, F52, F54, and F55) of the
tetrakis(pentafluorophenyl)borate anion are located at distances
which are less than the sum of the van der Waals radii for tin
5
5
In the solid state, [(Cp*Sn)2(l-g :g -Cp*)]+ exists as a cationic,
triple-decker, sandwich-type structure (Fig. 3) very similar to
5
5
that of [(Cp*Sn)2(l-g :g -Cp*)][Ga(C6F5)4] which was described
in a preliminary communication.13 A pentahapto Cp* ring
serves as a bridging group for two SnCp* units. Within
experimental error, the two tin atoms are located equidistantly
from the bridging Cp* ring-centroid (X1B), and are arranged
in a nearly linear fashion on either side of the bridging Cp*
group (Sn(1)–X(1B)–Sn(1A) = 175.5◦). The terminal Cp* rings
are positioned in a mutually cisoid arrangement with a Sn–
˚
and fluorine (van der Waals radii for Sn = 2.26 A and for F =
1.47 A).29 The close contacts between the fluorine atoms and the
ring-centroid (X1A/X1C) distance of 2.2282(5) A, which is
˚
˚
˚
tin atom range in distance from 3.172(2)–3.294(1) A. Overall, the
shorter than those to the bridging Cp* moiety (2.6135(7)
structure of [Cp*Sn]+ in 1 closely resembles that of [Cp*Sn][BF4]
A). These values lie between the value reported for Cp*2Sn
˚
reported by Jutzi et al.,30 for which the average Sn–Cring bond
(2.396 A) and that determined for [SnCp*]+ (2.114(3) A).
The X(1A)–Sn–X(1B) angle of 154.67◦(1)◦ is similar to the
values reported for Cp*2Sn (av. 154.9 ).30 As noted in the
Introduction, the cisoid arrangement of the terminal rings
30
˚
˚
˚
distance is 2.462 A.
As in the case of 1, the Cp* ring of 2 is planar and the methyl
groups extend out of the plane and away from the lead center
(See Fig. 2). Evidence for this arrangement is provided by the
values of the angles between the ring plane and the least-squares
planes defined by the two adjacent ring-carbon atoms and the
attached methyl carbon atoms, which have an average value
of 6.5◦. Three of the fluorine atoms (F23, F24, and F45) of
the neighboring tetrakis(pentafluorophenyl)borate anions are
located at distances which are less than the sum of the van der
in [(Cp*Sn)2(l-g :g -Cp*)]+ and [(Cp*Pb)2(l-g :g -Cp*)]+ (vide
5
5
5
5
infra) contrasts with the transoid arrangement of such rings
5
5
9
in the triple-decker anions trans-[(CpTl)2(l-g :g -Cp)]− and
trans-[(CpCs)2](l-g :g -Cp)]−.11 We explore the reasons for this
5
5
difference in conformational preference in a subsequent section.
Waals radii for lead and fluorine (van der Waals radii for Pb =
29
˚
˚
2.34 A and for F = 1.47 A). The close contacts between the
fluorine atoms and the lead atom range in distance from 2.944–
+
˚
3.327 A. Overall, the structure of [Cp*Pb] in 2 is similar to that
of [Cp*Pb][BF4] reported by Jutzi et al.,30 which has an average
˚
Pb–Cring bond distance of 2.565 A. In this case, there is a close
interaction of the lead atom with one fluorine atom on each of
the two adjacent tetrafluoroborate anions.
5
5
Fig. 3 Structure of the trans-[(Cp*Sn)2(l-g :g -Cp*)]+ cation. Hydro-
gen atoms have been omitted for clarity.
Overall, the solution-state NMR data are not in accord
with the solid state structure as revealed by X-ray diffraction.
Thus the 1H and 13C NMR spectra evidence only one type
of Cp* group and a unique tin center thus suggesting rapid,
5
5
reversible dissociation of [(Cp*Sn)2(l-g :g -Cp*)]+ into Cp*2Sn
and [Cp*Sn]+ (Scheme 1). In an attempt to detect the triple-
1
decker cation in solution, a low-temperature H NMR study
was undertaken; however, no perceptible spectral changes were
evident down to −100 ◦C. A proposed model for the facile
exchange process is set forth in Scheme 1.
Fig. 2 Structure of [Cp*Pb][B(C6F5)4] (2) showing the close contacts
between the lead cation and some of the fluorine atoms of the anion.
Hydrogen atoms have been omitted for clarity.
Scheme 1
The ruby red dilead triple-decker cation salt, trans-
5
5
The reaction of equimolar quantities of [Cp*Sn][B(C6F5)4]
and decamethylstannocene affords a 75% yield of bright yellow
[(Cp*Pb)2(l-g :g -Cp*)][B(C6F5)4] (4) was synthesized in 65%
yield in an analogous fashion to 3, viz the reaction of
[Cp*Pb][B(C6F5)4] with decamethylplumbocene in a 1 : 1 molar
ratio. As in the case of 3, the 1H NMR spectrum of 4 exhibited a
single resonance thus indicating the equivalence of all 45 methyl
protons. Likewise, the 13C signals for the ring and methyl carbons
were all equivalent at 293 K. The 11B and 19F NMR spectra
confirmed the presence of the [B(C6F5)4]− anion.
5
5
1
crystalline trans-[(Cp*Sn)2(l-g :g -Cp*)][B(C6F5)4] (3). The H
NMR spectrum of [(Cp*Sn)2(l-g :g -Cp*)]+ exhibits only a
5
5
single proton signal, thus indicating the equivalence of all forty-
five methyl protons on the three C5Me5 rings. Likewise, the 13
C
5
5
NMR spectrum of [(Cp*Sn)2(l-g :g -Cp*)]+ evidenced only two
signals for the carbon atoms of the C5Me5 rings, namely one for
the methyl carbons and one for the ring carbon atoms. Three
13C NMR signals were detected for the [B(C6F5)4]− anion and
5
5
The structure of lead triple-decker cation [(Cp*Pb)2(l-g :g -
Cp*)]+ (Fig. 4) is very similar to that of the tin analogue,
D a l t o n T r a n s . , 2 0 0 5 , 3 8 4 6 – 3 8 5 1
3 8 4 9

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5
5
[(Cp*Sn)2(l-g :g -Cp*)]+. Thus the Pb(1)–X(1B)–Pb(1A) angle
is close to linear [(177.2(4)◦] and the terminal Cp* rings are
positioned in a mutually cisoid arrangement with a Pb–ring-
conformations for both 5+ (∼0.6 kcal mol−1) and 6+ (∼0.2 kcal
mol−1) are very small. As noted in a DFT study of Group 14
metallocenes,31 the potential energy surfaces of the bent forms of
such compounds are extremely flat and the differences in energy
between conformers is minimal. Thus, the fact that 5+ has one
imaginary frequency and 6+ is a true minimum is not chemically
meaningful. Bearing in mind that the model calculations are
based on unsubstituted Cp ligands, the agreement between the
calculated and experimental structures is satisfactory in terms
of the overall trends in metrical parameters (Table 4). Thus,
for both 5+ and 6+, the metal–Cp distances are shorter for the
terminal Cp’s than those for the bridging Cp. Moreover, the ring
centroid–metal–ring centroid angles (i.e. metal–(l-Cp)–metal)
are larger for the bridging Cp than for those involving the
bridging and terminal rings (i.e. Cp–metal–Cp). Note that for
both the ditin and the dilead cations, the computed average
metal–carbon bond distances for the model systems exceed
those measured experimentally. Such a trend is also apparent
in e.g. the computed32 and experimental9,11 structures of
˚
centroid (X1A/X1C) distance of 2.3378(5) A. This distance is
shorter than that to the bridging Cp* moiety (2.6718(6) A) and
lies between the value reported for Cp*2Pb (av. 2.501(3) A)
˚
22
˚
+
˚
and that determined for [Cp*Pb] (2.240(3) A). The X(1A)–Pb–
X(1C) angle of 152.6◦ is similar to the value reported for Cp*2Pb
(151.3◦).22
trans-[(CpTl)2(l-g :g -Cp)]− and trans-[(CpCs)2(l-g :g -Cp)]−.
The change from a Cp to a Cp* ligand is expected to have a
greater impact on the metrical parameters for the ditin than the
dilead triple-decker cations because of the smaller metal–ring
carbon distances in the former. Accordingly, the computed and
experimental metal–ring centroid–metal angles are in better
agreement for the dilead than the ditin systems.
5
5
5
5
Fig. 4 Structure of the trans-[(Cp*Pb)2(l-g :g -Cp*)]+ cation. Hydro-
gen atoms have been omitted for clarity.
5
5
Theoretical studies
The HOMO’s and LUMO’s for the cisoid and transoid
conformations of 5+ and 6+ are depicted in Fig. 5. Like
A DFT study of the model Cp-substituted triple-decker cations
[(CpM)2(l-g :g -Cp)]+, M = Sn (5+) and M = Pb (6+) was
undertaken with a view to gaining insight into the nature of
the bonding in Group 14 triple-decker cations and also for
exploring the underlying reason that in the crystalline state
5
5
5
5
trans-[(CpTl)2(l-g :g -Cp)],− 5+ and 6+ are 22 valence-electron
systems. Accordingly, it is not surprising that the HOMO’s and
LUMO’s for 5+ and 6+ are similar.
DFT calculations have also been performed on the metal-
locenes Cp2M and the model cations [CpM]+ (M = Sn, Pb) in
order to estimate the energies of the reactions:
5
5
5
5
[(Cp*Sn)2(l-g :g -Cp*)]+ and [(Cp*Pb)2(l-g :g -Cp*)]+ adopt a
cisoid arrangement of terminal Cp* rings. A further objective
was to compute the energies of interaction of [CpM]+ with the
corresponding metallocenes, (Cp2M; M = Sn, Pb).
5
5
[CpM]+ + Cp2M → trans-[(CpM)2(l-g :g -Cp)]+
It is evident from Table 3 that for both 5+ and 6+ the
transoid geometry is of slightly lower energy than that of the
cisoid geometry. However, the energy difference between these
The energies corresponding to the geometry-optimized struc-
tures for [CpM]+ and Cp2M are listed in Table 3. Using the total
Table 3 Calculated total energies
Compound
Energy/au
Relative energy/kcal mol−1
5
5
trans-[(CpSn)2(l-g :g -Cp)]+ (5+) (cisoid)
−587.127311
−587.128352
−587.240179
−587.240568
−390.405860
−196.680375
−390.459225
−196.737728
+0.653
5
5
trans-[(CpSn)2(l-g :g -Cp)]+ (transoid)
0
5
5
trans-[(CpPb)2(l-g :g -Cp)]+ (6+) (cisoid)
+0.244
5
5
trans-[(CpPb)2(l-g :g -Cp)]+ (transoid)
0
Cp2Sn
[CpSn]+
Cp2Pb
[CpPb]+
Values refer to the single point energy calculations.
◦
a
˚
Table 4 Comparison of calculated and experimental structures; parameters for ditin and dilead triple-decker cations (distances in A; angles in
)
5
5
5
5
5
5
5
5
trans-[(CpSn)2(l-g :g -Cp)]+ trans-[(Cp*Sn)2(l-g :g -Cp*)]+ trans-[(CpPb)2(l-g :g -Cp)]+ trans-[(Cp*Pb)2(l-g :g -Cp*)]+
M(1)–C av (Ring A) 2.606
M(1)–C av (Ring B) 2.972
M(2)–C av (Ring B) 2.972
M(2)–C av (Ring C) 2.607
2.536(4)
2.866(4)
2.866(4)
2.536(4)
2.2282(5)
2.6135(7)
2.6135(7)
2.2282(5)
2.690
3.057
3.056
2.702
2.403
2.808
2.808
2.403
2.668(3)
2.923(3)
2.923(3)
2.668(3)
2.3378(5)
2.6718(6)
2.6718(6)
2.3378(5)
M(1)–X(1A)
M(1)–X(1B)
M(2)–X(1B)
M(2)–X(1C)
2.309
2.716
2.714
2.309
X(1A)–M(1)–X(1B) 149.12
M(1)–X(1B)–M(2) 166.90
X(1B)–M(2)–X(1C) 149.48
154.67(1)
175.5
154.67(1)
153.37
171.91
154.74
152.58(1)
177.2
152.58(1)
a All cations in the the cisoid conformation.
3 8 5 0
D a l t o n T r a n s . , 2 0 0 5 , 3 8 4 6 – 3 8 5 1

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Acknowledgements
We gratefully acknowledge the National Science Foundation
(Grant CHE-0240008) and the Robert A. Welch Foundation
(Grant F-135) for support of this work.
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Fig. 6 An anion–cation pair from the packing diagram of trans-
5
5
[(Cp*Pb)2(l-g :g -Cp*)][B(C6F5)4].
D a l t o n T r a n s . , 2 0 0 5 , 3 8 4 6 – 3 8 5 1
3 8 5 1