Assigning hapticity to cyclopentadienyl derivatives of antimony and bismuth

Article abstract of DOI:10.1021/om400434f

The chelating diamide [Me₂Si{NAr}₂]^2- (Ar = 2,6-i-Pr₂C₆H₃) has been used as an ancillary ligand in antimony and bismuth chemistry. In contrast to the monomeric antimony chloride compound Sb(Me₂Si{NAr}₂)Cl (1a), the bismuth derivative (1b) exists as a trimer in the solid state, linked by Bi···(arene) π-bonding and Bi···Cl interactions. Compounds 1a,b are used as an entry point into rare antimony and bismuth compounds incorporating Cp and Cp* ligands. Reactions of the chlorides with LiCp and KCp* afford the organometallic species E(Me ₂Si{NAr}₂)(C₅R₅) (2a, R = H, E = Sb; 2b, R = H, E = Bi; 3a, R = Me, E = Sb; 3b, R = Me, E = Bi). Variable-temperature NMR experiments indicate different fluxional processes occur in solution involving facile haptotropic shifts of the [C ₅R₅]^- ring. Structural characterization of 2b and 3a,b reveals monomeric compounds in which the cyclopentadienyl-derived ligand adopts a low hapticity in the solid state.

Full text of DOI:10.1021/om400434f

Article
pubs.acs.org/Organometallics
Assigning Hapticity to Cyclopentadienyl Derivatives of Antimony
and Bismuth
Benjamin M. Day and Martyn P. Coles*,†
Department of Chemistry, University of Sussex, Falmer, Brighton BN1 9QJ, U.K.
S
* Supporting Information
ABSTRACT: The chelating diamide [Me2Si{NAr}2]2− (Ar =
2,6-i-Pr2C6H3) has been used as an ancillary ligand in
antimony and bismuth chemistry. In contrast to the
monomeric antimony chloride compound Sb(Me2Si{NAr}2)
Cl (1a), the bismuth derivative (1b) exists as a trimer in the
solid state, linked by Bi···(arene) π-bonding and Bi···Cl
interactions. Compounds 1a,b are used as an entry point
into rare antimony and bismuth compounds incorporating Cp
and Cp* ligands. Reactions of the chlorides with LiCp and
KCp* afford the organometallic species E(Me2Si{NAr}2)-
(C5R5) (2a, R = H, E = Sb; 2b, R = H, E = Bi; 3a, R = Me, E =
Sb; 3b, R = Me, E = Bi). Variable-temperature NMR experiments indicate different fluxional processes occur in solution involving
facile haptotropic shifts of the [C5R5]− ring. Structural characterization of 2b and 3a,b reveals monomeric compounds in which
the cyclopentadienyl-derived ligand adopts a low hapticity in the solid state.
INTRODUCTION
■
The cyclopentadienyl anion and its derivatives, [C5R5]−, are
ubiquitous in organometallic chemistry and have been used as
ligands to elements from across the periodic table. In contrast
to compounds of the transition elements in which π bonding of
the C5 ring predominates, those of the main-group elements
contain a range of hapticities varying from symmetrical η5
through the “slipped” ηx structures (x = 3−1) to purely σ-
bound ligands.1 In addition to the structural diversity for these
Figure 1. Schematic of the extended chain structure of E(Me2Si{Nt-
Bu}2)Cl (E = Sb, Bi).
compounds, their ability to undergo fluxional processes in
solution is well known.2 Despite their widespread application in
examples of transition-metal (Mn,5 Zr6), lanthanide (Y, Nd,
main-group chemistry, the use of cyclopentadienyl anions as
ligands for the heavier group 15 elements antinomy and
Sm, Yb),7 and main-group (Li,6 Mg,8 Ca,9 Sr,9 Ba,9 Zn,5,10 Cd,5
B,11 Ge,12 Sn,12,13 Pb12) compounds. We report herein the use
bismuth is relatively underexplored.
of this ligand as a stable platform at antimony and bismuth for
To prevent complications arising from multiple metal···-
cyclopentadienyl interactions, we targeted compounds incor-
porating a single Cp-derived group. We therefore chose a
chelating, dianionic ligand to provide a stable ancillary
the study of isolated metal···[C5R5]− (R = H, Me) interactions.
During the preparation of this paper, the synthesis and crystal
structure of the antimony derivative Sb(Me2Si{NAr}2)Cl (1a)
was reported by the group of Ma and Roesky.14
framework that satisfies the remaining valencies of the E(III)
element. Work from Veith and co-workers indicated that the
EXPERIMENTAL SECTION
chelating diamide [Me2Si{Nt-Bu}2]2− may be suitable. The
■
General Considerations. All manipulations were carried out
chlorides E(Me2Si{Nt-Bu}2)Cl (E = As, Sb (A), Bi (B)) have
under dry nitrogen using standard Schlenk-line and cannula
techniques or in a conventional nitrogen-filled glovebox. Solvents
been reported,3 and their chemistry has been briefly explored.4
The solid-state structures of A and B, however, show
intermolecular E···Cl interactions that linked the molecules
into 1-D chains (Figure 1), suggesting access to the metal is still
possible, which may give rise to intermolecular interactions in
the proposed Cp derivatives. The N-aryl analogue [Me2Si-
{NAr}2]2− (Ar = 2,6-i-Pr2C6H3) is a bulky alternative to Veith’s
diamide that has demonstrated favorable properties as a
chelating, dianionic ligand, with structurally characterized
were dried over appropriate drying agents and degassed prior to use.
NMR spectra were recorded using a Bruker Avance DPX 300 MHz
spectrometer at 300.1 (1H) and 75.4 MHz (13C{1H}) or a Varian
VNMRS 500 MHz spectrometer at 500.1 (1H) and 125.4 MHz
(13C{1H}). Proton and carbon chemical shifts were referenced
Received: May 16, 2013
© XXXX American Chemical Society
A
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internally to residual solvent resonances. S. Boyer at London
Metropolitan University performed elemental analyses. Sb(Me2Si-
{NAr}2)Cl (1a) was made by a modified literature procedure,14 using
isolated [Me2Si{N(Ar)Li}2].6 The high air and moisture sensitivity
and possible photolability of bismuth compounds 2b and 3b prevented
accurate elemental analyses from being obtained.
(2b, 3a,b). Data were collected at 173(2) K using Mo Kα radiation at
0.71073 Å. The structures were refined with SHELXL-97.15
RESULTS
■
The lithium diamide [Me2Si{N(Ar)Li}2]6 reacts with ECl3 to
afford E(Me2Si{NAr}2)Cl (1a, E = Sb;14 1b, E = Bi) as
colorless (1a) or orange (1b) crystals (Scheme 1). This
Preparation of Bi(Me2Si{NAr}2)Cl (1b). A THF solution of
[Me2Si{N(Ar)Li}2] (750 mg, 1.8 mmol) was added dropwise to a
THF solution of BiCl3 (568 mg, 1.8 mmol) at −78 °C with the
exclusion of light. The resultant solution was left to stirred for 3 h
while it was warmed to ambient temperature. Removal of the volatiles
in vacuo afforded an orange solid, from which the product was
extracted into toluene. Concentration and storage of this solution at
−20 °C for 24 h afforded orange crystals of 1a. Yield: 0.800 g, 68%.
Anal. Calcd for C26H40N2BiClSi (653.1): C, 47.81; H, 6.17; N, 4.29.
Scheme 1. Synthesis of E(Me2Si{NAr}2)Cl (1) and
Conversion to Cyclopentadienyl (2) and
Pentamethylcyclopentadienyl (3) Derivatives,
E(Me2Si{NAr}2)(C5R5) (a, E = Sb; b, E = Bi)
1
Found: C, 47.69; H, 6.27; N, 4.17. H NMR: δ 7.23 (br, 4H, m-CH),
6.86 (t, 3JHH = 7.6 Hz, 2H, p-CH), 4.41 (m br, 2H, CHMe2), 3.79 (m
br, 2H, CHMe2), 1.65−0.90 (br, 24H, CHMe2), 0.60, 0.07 (s, 3H,
SiMe). 13C NMR: δ 137.3 (i-C), 125.9 (p-CH), 122.8 (br, m-CH), 26.9
(CHMe2), 25.0 (br, CHMe2), 11.8, 6.7 (SiMe); o-C not observed.
Formation of Sb(Me2Si{NAr}2)Cp (2a). A solution of 1a (112
mg, 0.20 mmol) in 0.5 mL of C6D6 was added to a suspension of LiCp
(14 mg, 0.20 mmol) in d6-benzene (0.1 mL). The resultant solution
1
was stirred for 5 h before being filtered. Analysis of the filtrate by H
1
NMR spectroscopy indicated complete consumption of 1a with H
1
and 13C resonances consistent with the formation of 2a. H NMR: δ
3
7.17 (partially obscured by solvent peak, 4H, m-CH), 7.08 (t, JHH
=
method is similar to that used for the synthesis of the N-tert-
butyl derivatives A and B3 and the recently reported synthesis
of the antimony compound 1a,14 although we found it
convenient to isolate the dilithio salt.6 Initially, low yields
(∼30%) of the bismuth compound 1b were obtained, with
formation of an insoluble black solid observed as the reaction
mixture warmed to room temperature. Repeating the reaction
with the exclusion of light increased the yield to 68%. Given
that, once isolated, compound 1b is stable to ambient light
under inert conditions in both the solid state and solution, this
may indicate the formation of light-sensitive intermediates
during its formation.
7.5 Hz, 2H, p-CH), 6.26 (s, 5H, C5H5), 3.93 (sept, 3JHH = 6.9 Hz, 4H,
3
CHMe2), 1.32, 1.29 (d, JHH = 6.9 Hz, 12H, CHMe2), 0.30 (s, 6H,
SiMe2). 13C NMR: δ 146.4 (o-C), 138.8 (i-C), 124.3 (p-CH), 123.7
(m-CH), 114.3 (C5H5), 27.7 (CHMe2), 25.8, 25.0 (CHMe2); SiMe2
not observed.
Preparation of Bi(Me2Si{NAr}2)Cp (2b). Compound 2b was
prepared in a manner analogous to that described for 2a, using 150 mg
of 1b (0.23 mmol) and 17 mg of LiCp (0.23 mmol). The reaction
mixture was filtered and the solvent allowed to slowly evaporate,
1
affording 2b as a red crystalline solid. Yield: 94 mg, 60%. H NMR:
7.28 (d, 3JHH = 7.7 Hz, 4H, m-CH), 6.85 (t, 3JHH = 7.7 Hz, 2H, p-CH),
3
i
6.26 (s, 5H, C5H5), 3.94 (sept, JHH = 6.9 Hz, 4H, Pr-CH), 1.29 (br,
24H, Pr-CH3), 0.24 (s, 6H, SiCH3). 13C NMR (C6D6, 100 MHz):
i
The room-temperature 1H NMR spectra of 1b shows
inequivalent SiMe2 groups (δH 0.60, 0.07) consistent with C1
symmetry and inequivalent faces of the metallacycle caused by a
pyramidal metal geometry; this is consistent with data reported
for 1a.14 In contrast to the single broad resonance observed for
the CHMe2 proton in 1a, however, the peaks corresponding to
the methine protons are resolved into separate broad
resonances for the bismuth derivative, prompting a study by
variable-temperature NMR. At high temperature (383 K), a
symmetrical environment is observed for the diamide ligand
consisting of one septet and one doublet resonance for the i-Pr
substituents and a single resonance for the SiMe2 groups. In
contrast, at low temperature (193 K) two septets and four
doublets are noted for the i-Pr groups, with distinct silicon−
methyl resonances.
Assuming the diamide remains bidentate in solution, the
single SiMe2 environment is consistent with averaged C2h
symmetry for the metallacycle, which can be attained through
a number of possible pathways. Rapid pyramidal inversion of
the bismuth atom would result in a single averaged silicon
methyl resonance in the NMR spectrum (Figure 2, pathway i).
A similar inversion has been postulated in the chiral
organobismuth chloride Bi[2-(Me2 NCH2 )C6 H4 ]-
[(Me3Si)2CH]Cl (C),16 although this process required the
use a strongly coordinating solvent (DMSO) that promoted
“edge” inversion17 via a T-shaped transition state. Inversion of
C was not observed in C6D6, suggesting a different mechanism
148.1 (o-C), 138.2 (i-C), 125.3 (p-CH), 123.0 (m-CH), 113.6 (C5H5),
27.5 (CHMe2), 27.2 (CHMe2), 25.3 (CHMe2), 9.4 (SiCH3).
Preparation of Sb(Me2Si{NAr}2)Cp* (3a). Compound 3a was
prepared in a manner analogous to that described for 2a, using 150 mg
of 1a (0.27 mmol) and 46 mg of KCp* (0.28 mmol). The solvent was
allowed to slowly evaporate from the filtrate in a glovebox, resulting in
the isolation of 3a as colorless crystals. Yield: 135 mg, 77%. Anal.
Calcd for C36H55N2SiSb (665.7): C, 65.00; H, 8.33; N, 4.21. Found: C,
1
3
64.92; H, 8.48; N, 4.12. H NMR: δ 7.15 (d, JHH = 7.5 Hz, 4H, m-
3
CH), 7.05 (t, JHH = 7.5 Hz, 2H, p-CH), 4.05 (br, 4H, CHMe2), 1.80
(s, 15H, C5Me5), 1.41, 1.29 (d, 3JHH = 6.6 Hz, 12H, CHMe2), 0.41 (br,
6H, SiMe2). 13C NMR: δ 145.7 (o-C), 141.9 (i-C), 124.3 (p-CH),
123.7 (m-CH), 121.3 (C5Me5), 28.3 (CHMe2), 26.8, 25.8 (CHMe2),
10.7 (C5Me5); SiMe2 not observed.
Preparation of Bi(Me2Si{NAr}2)Cp* (3b). Compound 3b was
prepared in a manner analogous to that described for 2a, using 180 mg
of 1b (0.28 mmol) and 48 mg of KCp* (0.28 mmol). The reaction
mixture was filtered and the solvent allowed to slowly evaporate,
1
affording 2b as a red crystalline solid. Yield: 145 mg, 70%. H NMR:
7.25 (d, 3JHH3 = 7.6 Hz, 4H, m-CH), 6.86 (t, 3JHH = 7.6 Hz, 2H, p-CH),
4.02 (sept, JHH = 6.8 Hz, 4H, CHMe2), 2.08 (s, 15H, C5Me5), 1.36,
1.30 (d, 3JHH = 6.8 Hz, 12H, CHMe2), 0.35 (s, 6H, SiMe2). 13C NMR:
147.0 (o-C), 141.1 (i-C), 123.9 (p-CH), 123.3 (m-CH), 121.1
(C5Me5), 27.5 (CHMe2), 26.8, 26.7 (CHMe2), 10.6 (SiMe2), 10.3
(C5Me5).
Crystallographic Data Collection and Refinement Proce-
dures. Crystals were covered in inert oil, and suitable single crystals
were selected under a microscope and mounted on a Bruker AXS
diffractometer (1b) or an Enraf Nonius Kappa CCD diffractometer
B
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Figure 3. Thermal ellipsoid plot (30%) of one of the independent
molecules of Bi(Me2Si{NAr}2)Cl (1b). Hydrogen atoms and hexane
solvate are omitted.
Table 1. Selected Bond Lengths (Å) and Angles (deg) for
Bi(Me2Si{NAr}2)Cl (1b)
Bi1−N1
Bi2−N3
Bi2−Cl2
Si1−N2
N1−Bi1−N2
N2−Bi1−Cl1
N3−Bi2−N3′
N3−Si2−N3′
2.132(3)
2.181(3)
2.4857(16)
1.733(3)
71.78(11)
96.34(9)
70.74(16)
93.3(2)
Bi1−N2
Bi1−Cl1
Si1−N1
Si2−N3
N1−Bi1−Cl1
N1−Si1−N2
N3−Bi2−Cl2
2.144(3)
2.5560(10)
1.735(3)
1.736(3)
97.89(9)
92.58(15)
96.37(9)
Figure 2. Solution-state processes affording equivalent SiMe2
1
resonances observed in the high-temperature H NMR spectrum of
1b.
occurs in 1b. Ionization of compound 1b can potentially occur
to afford the corresponding C2h cation [Bi(Me2Si{NAr}2)]+.
The related [Bi(Me2Si{Nt-Bu}2)]+ cation derived from B has
been isolated and characterized as the [AlCl4]− salt, showing a
single resonance for the SiMe2 protons in the 1H NMR
spectrum.4a However, spontaneous chloride dissociation has
not been reported in B and the cation is only produced in the
presence of the group 13 chlorides MCl3 (M = Al, Ga, In).
Finally an intramolecular chloride transfer process to the
opposite face of the metallacycle also explains the coalescence
of the SiMe2 resonances (pathway iii). Although long-range
Bi···Cl interactions that would facilitate chloride transfer have
been observed in the solid state (see Figure 1 and discussion
below), their retention in solution has not been documented.
We also note that while pathways i−iii explain coalescence of
the silicon methyl singlets at high temperature, additional
rotation of the aryl substituents must take place to account for
the observed changes to the i-Pr resonances.
The molecular structure of 1b is shown in Figure 3; selected
bond lengths and angles are collected in Table 1. The
compound crystallizes with one and a half molecules in the
unit cell, with the latter component on a mirror plane; the
independent molecules are essentially identical. Each molecule
consists of a three-coordinate bismuth atom with a chelating
diamide and terminal chloride. The geometry at the metal
atoms is pyramidal with bond angles in the range 71.78(11)−
97.89(9) and 70.74(16)−96.37(9)° for Bi1 and Bi2,
respectively. The most acute angle is described by the bite of
the ligand and is comparable to that in B (71.8(4)°). The
smaller N−Bi−N value at Bi2 reflects the increased Bi−N
distances attributed to multiple intermolecular Bi···Cl inter-
actions (vide infra). Bismuth atoms Bi1 and Bi2 are displaced
0.294(6) and 0.229(8) Å “below” the N2Si least-squares plane
(defined as being on the same side as the third ligand), which
generates fold angles across the nitrogen···nitrogen vector of
−170.2(2) and −172.6(3)°, respectively (where the negative
value indicates a fold below the N2Bi plane). By analogy with a
recent definition applied to pyramidal, three-coordinate group
14 metal(II) centers with a ligating β-diketiminate ligand,18 this
corresponds to an endo configuration.
The crystal structure of 1b shows two types of long-range
interactions involving bismuth, forming trimeric [1b]3 units
(Figure 4a). Bi1 is involved in bismuth···arene π bonding to the
aryl substituent from an adjacent molecule, generating a pseudo
piano-stool geometry (Figure 4b). The metal is 3.194(2) Å
from the C6 plane with an angle of 7.08(9)° between the ring
normal and the metal to ring center vector corresponding to a
ring slippage of 0.40 Å. Comparing these data to a series of
crystal structures [BiCl3·(C6H6‑nMen)] (n = 0,19 n = 2,20 n = 3
(1,2,3-21 and 1,3,5-22,23) and n = 622,24), we note that the
distortion from η6 bonding in 1b is less than in 1,2,3-
21
Me3C6H3 (ring slippage 0.75 Å), despite having the same
substitution pattern about the aromatic ring. This is attributed
to a comparatively large distance between bismuth and the C6
plane in 1b (Bi···C distances 3.350(4)−3.674(4) Å) in
comparison with [BiCl3·(1,2,3-Me3C6H3)] (Bi···C distances
3.168(7) to 3.751(8) Å), reflecting the bulk of the amide
substituents.
There are two intermolecular Bi···Cl interactions between
Bi2 and Cl1/Cl1′ (3.199(1) Å; cf. Bi−Cl bonds of 2.556(1)
and 2.4857(16) Å), generating a square-based-pyramidal
geometry (Figure 4b). Compound B shows a similar
intermolecular Bi···Cl interaction that generates a pseudo-
four-coordinate metal (Figure 1), which was described as an
acid−base interaction between an sp3d-hybridized Bi acceptor
and a chloride donor from a neighboring molecule.3 Extending
this acid−base model to 1b, the bonding is consistent with
sp3d2 hybridization of the bismuth, with two chloride donors
and a lone-pair of electron trans to the Bi−Cl bond. The bond
order (BO) can be calculated using eq 1,25 where r is the
experimental distance between the bismuth and chlorine atoms,
C
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Figure 5. Schematic representations of haptotropic shifts.
hence this pathway accounts for both observations described
above.
Figure 4. (a) Association of three Bi(Me2Si{NAr}2)Cl units into
[1b]3, showing Bi···π-aryl and Bi···Cl interactions. (b) Local
environment at Bi1 and Bi2 in [1b]3.
The NMR spectra of Cp* derivatives 3a,b suggest fluxional
processes involving the [C5Me5]− ring similar to those noted
for the Cp derivatives. Only one environment is observed for
the ring methyl groups for both compounds (3a, δH 1.80 ppm;
3b, δH 2.08 ppm), although in contrast to the data for 2a,b, the
SiMe2 resonance for 3a was broad at room temperature.
Cooling a sample to 193 K resolved this peak into two singlets,
with no apparent broadening of the C5Me5 signal (Figure 6). It
was not possible to resolve the peaks in the corresponding
bismuth compound at this temperature, although broadening
⎡
⎤
(r0 − r)
BO = exp
⎢
⎣
⎥
⎦
B
(1)
and using values of r0 = 2.423 Å26 and B = 0.39.27 These data
give a BO value of 0.14 for the intramolecular Bi···Cl
interaction in 1b, notably less than the corresponding
interaction in B (BO = 0.20).
The cyclopentadienyl derivatives E(Me2Si{NAr}2)(C5R5) (a,
E = Sb; b, E = Bi; 2, R = H; 3, R = Me) were synthesized from
the reaction of the metal chlorides 1a,b with LiCp or KCp*
(Scheme 1). Compound 2a was characterized in solution by 1H
and 13C NMR; the remaining derivatives were isolated as
colorless (3a) or red (2b and 3b) crystalline solids. The high
sensitivity of these compounds to air/moisture (and possibly
light) prevented accurate elemental analyses from being
obtained. This was particularly evident for the bismuth
derivatives, for which sealed NMR samples were observed to
deposit black solid material on exposure to ambient light
conditions.
The 1H and 13C NMR spectra of cyclopentadienyl derivatives
2a,b show a single proton and carbon resonance for the [Cp]−
ligand. Both compounds also show one singlet for the SiMe2
groups in the 1H NMR spectrum (2a, δH 0.30 ppm; 2b, δH 0.24
ppm). Assuming the ligands remain coordinated to the metal in
solution and only considering intramolecular processes, these
results can be explained by haptotropic shifts of the C5 ring,
ubiquitous in main-group cyclopentadienyl compounds (Figure
5).2,28 For example, if a high-hapticity intermediate is adopted
in which the metal passes through a C2h-symmetric
intermediate, shown in pathway iv, the metal can undergo
sigmatropic shifts scrambling the Cp positions. In this case, the
average structure will also have equivalent SiMe2 groups and
1
Figure 6. VT H NMR spectra of Sb(Me2Si{NAr}2)(Cp*) (3a): (a)
CHMe2; (b) C5Me5; (c) CHMe2; (d) SiMe2; (*) d8-toluene.
D
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was observed, suggesting a similar process was occurring, albeit
with a lower energy barrier. These data are consistent with a
[1,n]-Sb/Bi sigmatropic shift in which the C5 ligand remains
bonded to the same face of the metallacycle, illustrated for a
[1,2]-Sb/Bi shift in pathway v. This process accounts for the
equivalent Cp* resonances and would result in inequivalent
SiMe2 groups through retention of the C1 symmetry of the
complex. The i-Pr resonances of the aromatic rings are also
resolved at low temperature, consistent with multiple fluxional
processes occurring in solution (vide supra).
The molecular structures of 2b and 3a,b are presented in
Figures 7−9, with the primary metal−carbon interaction to the
Figure 9. Thermal ellipsoid plot (30%) of Bi(Me2Si{NAr}2)(Cp*)
(3b). Hydrogen atoms are omitted.
Table 2. Selected Bond Lengths (Å) and Angles (deg) for
Bi(Me2Si{NAr}2)Cp (2b)
Bi−N1
Si−N1
Bi−C27
Bi···C29
Bi···C31
N1−Bi−N2
Bi−N1−Si
N1−Bi−C27
2.180(2)
1.728(2)
2.468(8)
3.121(3)
2.708(3)
70.55(8)
97.74(10)
110.23(11)
Bi−N2
Si−N2
Bi···C28
2.176(2)
1.729(2)
2.784(3)
3.073(3)
Bi···C30
N1−Si−N2
Bi−N2−Si
N2−Bi−C27
93.37(11)
97.85(9)
99.89(11)
Figure 7. Thermal ellipsoid plot (30%) of Bi(Me2Si{NAr}2)(Cp)
(2b). Hydrogen atoms are omitted.
Table 3. Selected Bond Lengths (Å) and Angles (deg) for
Sb(Me2Si{NAr}2)(Cp*) (3a) and Bi(Me2Si{NAr}2)(Cp*)
(3b)
Complex 3a
Sb−N
2.096(2)
2.281(3)
3.115(2)
73.36(9)
97.03(7)
Si−N
Sb···C16
1.735(2)
2.691(2)
Sb−C15
Sb···C17
N−Sb−N′
Sb−N−Si
N−Si−N′
N−Sb−C15
92.39(11)
103.22(7)
Complex 3b
Bi−N1
Si−N1
Bi−C27
Bi···C29
2.204(4)
1.729(5)
2.397(6)
3.052(6)
2.707(6)
70.11(17)
97.1(2)
Bi−N2
Si−N2
Bi···C28
2.214(4)
1.733(5)
2.702(6)
3.083(6)
Bi···C30
Bi···C31
N1−Bi−N2
Bi−N1−Si
N1−Bi−C27
N1−Si−N2
Bi−N2−Si
N2−Bi−C27
94.3(2)
96.7(2)
Figure 8. Thermal ellipsoid plot (30%) of Sb(Me2Si{NAr}2)(Cp*)
(3a). Hydrogen atoms and benzene solvate are omitted.
102.9(2)
105.9(2)
C5 ring shown in each case. Selected bond lengths and angles
are collected in Tables 2 and 3. The basic molecular unit is
common to all structures and consists of a pyramidal metal
center bonded to a chelating diamide and a [C5R5]− anion. The
combination of bulky ligands is sufficient to shield the metal
from long-range interactions, and each compound is mono-
meric in the solid state.
In each case the metallacycle is essentially planar, with the
metal atoms located above the least-squares plane defined by
N1−Si−N2 (2b, 0.218(4) Å; 3a, 0.129(4) Å; 3b, 0.425(8) Å),
corresponding to an exo configuration18 with positive fold
angles of +173.0(2), +175.62(6), and +166.4(4)°, respectively.
The dihedral angle between the metallacycle and the plane
described by the C5 ring appears to be dependent on the metal,
with the values for the bismuth derivatives 2b (31.9(1)°) and
3b (31.6(3)°) being considerably greater than that in the
antimony compound 3a (23.25(4)°). This is partially explained
by the greater distance between the bismuth atom and the C5
plane (2b, 2.446(2) Å; 3b 2.382(4) Å) in comparison with the
corresponding distance in the antimony derivative (2.302(2)
Å). There is also an apparent relationship between the
orientation of the aromatic rings with respect to the
metallacycle, based on whether the organometallic ligand is
Cp or Cp*. Large dihedral angles are found in 2b (75.2(1) and
70.5(1)°) and smaller values in 3a (66.2(1)°) and 3b (65.4(2)
and 63.5(2)°). These parameters are also influenced by the
overall hapticity of the C5 ring at the metal center, discussed in
detail below.
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bridging chlorides (Figure 11a).27 The cyclopentadienyl ligand
was disordered by pseudorotation, and the bonding was
assigned as a mixture of η2 and η3 coordination. The crystal
structure of the red crystalline modification of BiCp3 shows η1
coordination of each cyclopentadienyl ligand, with weak
intermolecular binding of one Cp group to a neighboring
bismuth (Figure 11b).32
DISCUSSION
■
The ηx hapticity of cyclopentadienyl ligands at main-group-
metal centers has been examined using many criteria derived
from crystallographic data.1a,29 The area was comprehensively
reviewed by Jutzi and Burford in 19991c and more recently by
Budzelaar and co-workers1a and continues to elicit interest as
new applications for the Cp ligands are developed.29 The key
parameters used for comparison in this section have been
summarized in Figure 10,30 with the relevant values for
compounds 2b and 3a and 3b collected in Tables 4−6, along
with data taken from examples of related structures.
The primary metal−carbon interaction in 2b (Bi−C(α),
2.468(3) Å) is at the long end of the range observed for η1-
coordinated Cp ligands in BiCp332 and is more closely matched
to the metal−carbon distances in Bi(η2-Cp)Cl2 (Table 4).27
These distances will, however, also be influenced by the relative
bulk of the other ligands at the metal center. Thus in 2b, where
a bulky diamide ligand is present, longer metal−carbon
distances are predicted. Examining the remaining metal−
carbon distances, we note that the [Cp]− anion is not
symmetrically bonded to the metal center. The Bi···C31
distance in 2b (2.708(3) Å) is considerably shorter than the
Bi···C28 distance (2.784(3) Å), tilting the C5 plane of the
organometallic ligand relative to the SiN2Bi metallacycle
(Figure 12a). The smaller distance is in the 3σ range attributed
to a bond in the η3-haptotropic isomer of BiCpCl2 (longest Bi−
C bond 2.64(3) Å) but is a distance not considered as
contributing to the overall hapticity in the η2-haptotropic
isomer (shortest nonbonding Bi···C interaction 2.76(4) Å).27
The assignment of the Cp ligand in 2b as η1 or η2 is therefore
ambiguous if it is based only on metal−carbon distances.
Sitzmann et al. examined the hapticity of cyclopentadienyl-
substituted bismuth dihalides, examining the position of the
metal atom relative to the C(α) atom and the pattern of
metal−carbon distances, rather than their absolute values.33
Using these criteria, a bismuth atom located above the C(α)···Z
vector (Figure 10) with three shorter and two longer Bi···C
distances corresponds to η3 coordination. In contrast, the metal
shifted “to one side” with two shorter and three longer Bi···C
distances is designated as η2 coordination.34 Extending this
classification to η1 coordination, a regular increase in the Bi···C
distances with C(α) < C(β/β′) < C(γ/γ′) would be anticipated.
The Bi···C distances in 2b are consistent with this pattern,
suggesting an η1 description of the bonding is appropriate. This
criterion does not, however, distinguish between η1(π) or η1(σ)
(Figure 13), and previous work has shown the distinction
between the two can be small.35
Figure 10. Parameters used to analyze the hapticity of the metal−C5
interactions: (a) assignment of hapticity according to displacement of
the metal along the C(α)···Z vector, with values of d, according to
Budzelaar;1a (b) X = C5 ring centroid, N = normal to C5 least-squares
plane (lsp), a, b, c = C−C bond lengths, d = ring slippage, θ = angle
between C5 lsp and metal···centroid, ϕ = angle between C5 lsp and
C(α) methyl substituent in Cp* derivatives.
Bismuth Cyclopentadienyl Derivatives. Cp−bismuth
compounds date back to studies conducted by Fischer in
1960;31 compound 2b is, however, only the third Cp derivative
of bismuth to be structurally characterized. The first example,
BiCpCl2, forms polymeric chains in the solid state linked by
According to Budzelaar et al. the classification of hapticity
can be quantified according to the ring slippage d of the metal
Table 4. Geometric Parameters for the Bi−Cp Bonding in Bi(Me2Si{NAr}2)(Cp) (2b) and Related Bismuth−Cp Structures
b
b
bc
,
2b
2.468(3)
Bi(η1-Cp)3 (i)
Bi(η1-Cp)3 (ii)
Bi(η1-Cp)3 (iii)
Bi(η2-Cp)Cl2
Bi(η3-Cp)Cl2
E−C(α)/Å
E···C(β/β′)/Å
E···C(γ/γ′)/Å
E···X/Å
2.34(2)
2.37(2)
2.41(2)
2.47(3)
2.38(3)
2.708(3), 2.784(3)
3.073(3), 3.121(3)
2.584(2)
2.86(2), 2.86(2)
3.39(2), 3.37(2)
2.738(8)
3.04(2), 3.14(2)
3.79(2), 3.89(3)
3.065(9)
2.94(2), 3.05(2)
3.66(2), 3.68(2)
2.952(7)
2.48(3), 2.81(4)
2.76(4), 2.90(4)
2.40(2)
2.61(3),, 2.64(3)
2.91(3), 2.92(3)
2.39(2)
E···C5(lsp)/Å
d/Å
2.446(2)
2.39(1)
2.21(3)
2.36(2)
2.34(2)
2.31(3)
0.83
1.34
2.12
1.78
0.53
0.60
θ/deg
108.8(1)
119.3
133.7
127.1
102.7
104.5
a, a′/Å
b, b′/Å
c/Å
1.412(5), 1.428(6)
1.351(5), 1.371(6)
1.385(6)
1.46(2), 1.40(3)
1.40(3), 1.34(3)
1.43(2)
1.47(3), 1.43(3)
1.40(2), 1.33(2)
1.41(2)
1.44(3), 1.39(3)
1.40(2), 1.36(2)
1.43(3)
1.36(4), 1.38(5)
1.47(4), 1.47(4)
1.43(5)
1.50(5), 1.50(4)
1.44(5), 1.44(4)
1.50(4)
a
ΔC−C/Å
0.08
0.12
0.14
0.08
0.11
0.06
a
b
c
Maximum difference in C−C bond lengths in the C5 ring. (i), (ii), and (iii) represent the three Cp groups in BiCp3. Cp ring involved in long-
range intermolecular π-bonding to a second Bi atom (see Figure 11a).
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Table 5. Geometric Parameters for the Sb−Cp* Bonding in Sb(Me2Si{NAr}2)(Cp*) (3a) and Related Structures
a
a
3a
[Sb(η1-Cp*)]4
Sb(η2-Cp*){Cr(CO)5}2
Sb(η3-Cp*)Cl2
(η3-Cp*)2SbCl (i)
(η3-Cp*)2SbCl (ii)
E−C(α)/Å
E···C(β/β′)/Å
E···C(γ/γ′)/Å
E···X/Å
E···C5(lsp)/Å
d/Å
2.281(3)
2.691(2)
3.115(2)
2.520(1)
2.302(2)
1.03
2.260(4)
2.343(3)
2.254(9)
2.507(4)
2.527(4)
3.121(5), 2.921(5)
3.835(6) 3.740(4)
2.990(3)
3.058(3), 2.394(3)
3.446(3), 3.105(3)
2.636(2)
2.595(10),\ 2.620(11)
2.988(12), 2.951(11)
2.402(5)
2.630(4), 2.671(4)
2.810(4), 2.846(6)
2.409(3)
2.633(5), 2.625(4)
2.813(4), 2.787(4)
2.396(2)
2.087(5)
2.198(3)
2.249(6)
2.377(3)
2.371(2)
2.14
1.46
0.84
0.39
0.34
θ/deg
114.0(6)
1.463(3)
1.381(3)
1.424(4)
0.08
135.7
123.5
110.6
99.3
98.2
a, a′/Å
b, b′/Å
c/Å
1.501(6), 1.493(7)
1.361(7), 1.344(7)
1.441(8)
1.487(4), 1.428(4)
1.412(4), 1.397(5)
1.412(4)
1.468(14), 1.496(14)
1.414(15), 1.404(15)
1.405(17)
1.428(7), 1.429(7)
1.409(6), 1.418(6)
1.427(7)
1.419(8), 1.408(7)
1.419(8), 1.411(6)
1.394(7)
b
ΔC−C/Å
0.16
0.09
0.09
0.02
0.03
ϕ/deg
17.6
38.5
14.8
8.0
9.0
a
b
(i) and (ii) represent the two Cp* groups in SbCp*2Cl. Maximum difference in C−C bond lengths in the C5 ring.
Table 6. Geometric Parameters for the Bi−Cp* Bonding in Bi(Me2Si{NAr}2)(Cp*) (3b) and Related Structures
a
a
3b
Bi(η2-Cp*)I2
Bi(η3-Cp*)Br2
Bi(η5-Cp*)2Cl (i)
Bi(η5-Cp*)2Cl (ii)
E−C(α)/Å
E···C(β), E···C(β′)/Å
E···C(γ), E···C(γ′)/Å
E···X/Å
2.397(6)
2.531(11)
2.466(8)
2.625(12)
2.709(12), 2.704(12)
2.831(12), 2.798(12)
2.449(7)
2.638(12)
2.707(6), 2.702(6)
3.083(6), 3.052(6)
2.528(3)
2.541(4), 2.668(5)
2.686(4), 2.742(3)
2.349(6)
2.584(8), 2.560(8)
2.754(8), 2.771(8)
2.334(4)
2.677(12), 2.719(13)
2.880(13), 2.828(12)
2.452(6)
E···C5(lsp)/Å
2.382(4)
2.335(7)
2.303(4)
2.438(7)
2.471(5)
a, a′/Å
b, b′/Å
c/Å
1.445(9), 1.454(10)
1.370(9), 1.403(11)
1.395(9)
1.39(2), 1.42(2)
1.41(2), 1.38(2)
1.42(2)
1.444(11), 1.459(11)
1.405(12), 1.418(12)
1.398(12)
1.42(2), 1.434(19)
1.43(2), 1.426(16)
1.43(2)
1.43(2), 1.43(2)
1.423(18), 1.398(17)
1.44(2)
0.30
97.0
0.04
7.0
d/Å
0.84
0.26
0.38
0.23
θ/deg
109.6(2)
96.5
99.3
95.4
b
Δ
C−C/Å
0.08
0.04
0.06
0.01
ϕ/deg
13.9
6.2
9.4
6.1
a
b
(i) and (ii) represent the two Cp*-groups in BiCp*2Cl. Maximum difference in C−C bond lengths in the C5 ring.
Figure 11. Structurally characterized bismuth−Cp compounds (see
text for references).
atom along the C(α)···Z vector;1a such haptotrophic shifts were
first examined for lighter organometallic fragments by Hoffman
and co-workers.36 The extent of this slippage will influence the
angle θ. Thus, an η5 coordination would have a value of d = 0 Å
(θ = 90°), with increasing d values attributed to lower
hapticities (Figure 11). The slippage for σ-bonded ligands will
depend on the radius of the metal, and the overall distance of
the metal from the Cp ligand will dictate the magnitude of the
angle θ. Both d and θ values in 2b are much less than noted in
the η1(σ)-bonded ligands in BiCp3 (Table 4), indicative of a π
component to the bonding. The ring slippage (d = 0.83 Å)
matches that predicted by Budzelaar for η3 coordination but is
greater than that observed in Bi(η3-Cp)Cl2 (0.60 Å). Viewing
the structure along the normal (N) to the C5 plane (Figure
12b), we note the bismuth atom deviates from the C(α)···Z
vector toward C(31), indicating bias toward η2 coordination.
Finally, the C−C bond distances of the C5 ring are
considered. In 2b they alternate around the C5 ring, with a
range of 1.351(5)−1.428(6) Å corresponding to a ΔC−C value
of 0.08 Å. This parameter has been used to indicate the relative
delocalization of π-electron density in arsenic and antimony
Figure 12. (a) View along the Bi···Si vector of 3a showing the twisting
of the Cp ring relative to the N2SiBi metallacycle. (b) View
perpendicular to the C5 ring.
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between the C(α) methyl group and the plane of the C5 ring is
inconsistent with a σ bond between the metal and the carbon
atom, for which a much higher value would be expected (e.g. ϕ
= 38.5° for the η1(σ) bonding in [SbCp*]4). We therefore
conclude that the Cp* ligand in 3a is bonded to the antimony
in a η1(π) bonding mode.
Figure 13. Different geometries of classical η1(π)- and η1(σ)-bonded
Cp rings.
There have been three previous reports of structurally
characterized compounds containing the Bi−Cp* frag-
ment;38f,40 related neutral and cationic bismuth compounds
containing other alkyl-substituted cyclopentadienyl derivatives
are known.33,41 Most pertinent to this discussion are BiCp*Br2
and BiCp*I2,40b assigned as containing η3 and η2 coordination
of the Cp* rings, respectively, and BiCp*2Cl,38f in which both
rings are η5 coordinated. Data derived from these structures are
included in Table 6 alongside those for 3b.
As for 3a, the pattern of metal···carbon distances in 3b that
matches that expected for η1 or η3 bonding. Comparing data
with Bi(η3-Cp*)Br2, we note that the Bi···C(β) distances are
considerably longer in 3b, with a greater ring slippage (d = 0.84
Å); these data suggest a η1 coordination. The low ΔC−C (0.08
Å) and ϕ (13.9°) values indicate π delocalization in the Cp*
ring, consistent with η1(π) bonding. The view in Figure 14,
however, shows that there is a more pronounced bias toward η3
bonding in comparison with the analogous antimony
compound.
compounds of the tetramethylcyclopentdienyl ligand.37 The
ΔC−C value for 2b is less than for the η1(σ)-bonded rings of
BiCp3 (except the ligand involved in π bonding to a second Bi
atom of a neighboring molecule, Figure 11a), indicating greater
delocalization around the ring, consistent with a π component
to the metal−Cp bond.
It is apparent from these data that it is not appropriate to
assign a specific hapticity to the bonding of the cyclo-
pentadienyl ligand in 2b, with data indicating an intermediate
between η3 coordination and η1(π) and with a bias toward η2
coordination of the Cp ligand. This likely reflects the flat
potential energy surface for the rotation of the Cp ring and the
influence that minor packing forces can have on the bonding of
the Cp rings.
Antimony and Bismuth Pentamethylcyclopentadienyl
Derivatives:1b Derivatives of the cyclopentadienyl anion have
been used previously in the organometallic chemistry of
antimony, and structurally characterized examples of neu-
tral35,37,38 and cationic38f,39 compounds have been reported,
showing a range of hapticities for the [C5R5]− ring. Most
relevant to this discussion are the cyclotetrastibine [SbCp*]4,38c
the trimetallic Sb/Cr compound Sb(Cp*){Cr(CO)5}2,38e and
CONCLUSIONS
■
In conclusion, we have isolated a series of antinomy and
bismuth compounds supported by the chelating diamide
[Me2Si{NAr}2]2−. NMR data for the chlorides E(Me2Si-
{NAr}2)Cl (E = Sb, Bi) are consistent with a pyramidal
metal at room temperature, although at high temperature
fluxional processes are occurring that render both faces of the
metallacycle equivalent on the NMR time scale. The bismuth
chloride associates into trimers in the solid state through a
combination of bismuth···arene π bonding and long-range
Bi···Cl interactions. Conversion to the Cp and Cp* derivatives
was facile, although the resultant organometallic compounds
are highly sensitive. VT NMR data are consistent with a
number of fluxional processes taking place in solution, most
likely involving haptotropic shifts of the C5 ring. The molecular
structures of both Cp and Cp* derivatives show a tendency
toward low-hapticity rings in the solid state. Detailed structural
analysis has shown that assigning a specific hapticity to the ring
is not straightforward, due to structures that are intermediate
between “classical” ηx-bonding situations.
35
the chloro species SbCp*Cl2 and SbCp*2Cl;38f geometric
data for these compounds are presented alongside that of 3a in
Table 5.
Compound 3a crystallizes in the Pnma space group with the
molecular unit on a mirror plane that passes through the metal
and the SiMe2 group and bisects the Cp* ligand. This enforces
a symmetric bonding of the Cp* ligand with respect to the
C(α)···Z vector, with a pattern of Sb···C distances in 3a which
matches that for η1 or η3 bonding. The ring slippage (d = 1.03
Å) is significantly less than that for the η1(σ)-bonded ligand in
[SbCp*]4 (d = 2.14 Å),38c approaching the value of 1.2 Å
defined by Budzelaar for a η1(π)-ligated ring. A projection
perpendicular to the plane of the C5 ring shows the Sb atom
directly above the C(α) position consistent with η1
coordination of the Cp* ligand (Figure 14). The relatively
small ΔC−C value (0.08 Å) suggests delocalization of electron
density in the C5 ring consistent with a π component to the
bonding. In addition, the relatively acute angle ϕ of 17.6°
ASSOCIATED CONTENT
* Supporting Information
■
S
CIF files giving crystallographic data for 1b, 2b, and 3a,b and a
table giving crystal structure and refinement data for 1b, 2b,
and 3a,b. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail for M.P.C.: martyn.coles@vuw.ac.nz.
■
Present Address
†School of Chemical and Physical Sciences, Victoria University
of Wellington, P.O. Box 600, Wellington 6140, New Zealand.
Notes
Figure 14. Views perpendicular to the C5 ring in 3a (left) and 3b
(right).
The authors declare no competing financial interest.
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R. J.; Howard, J. A. K.; Puschmann, H. J. Appl. Crystallogr. 2009, 42,
339−341.
ACKNOWLEDGMENTS
■
We thank the EPSRC UK National Crystallographic Service at
the University of Southampton for the collection of the
crystallographic data for 1b and Dr. S. Mark Roe (University of
Sussex) for the collection of the crystallographic data for 2b.
(31) Fischer, E. O.; Schreiner, S. Chem. Ber. 1960, 93, 1417−1424.
(32) Lorberth, J.; Massa, W.; Wocadlo, S.; Sarraje, I.; Shin, S.-H.; Li,
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(34) This definition of η2 coordination, with the metal atom located
above the midpoint of the C(α)−C(β) bond, corresponds to a 144°
anticlockwise rotation of the η2 coordination illustrated in Figure 10,
with a renaming of the carbon atoms such that α → γ′, β → β, β′ →
γ,γ → α, and γ′ → β.
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dx.doi.org/10.1021/om400434f | Organometallics XXXX, XXX, XXX−XXX