Synthesis, characterization and comparison of group 14 pyrrolides and indolides Ph3MX (M = Si, Ge, Sn; X = C4H4N, C8H6N)

Article abstract of DOI:10.1016/j.jorganchem.2010.07.029

The biologically important heterocycles pyrrole, C₄H ₄N, and indole, C₈H₆N, ought to be useful as reagents in organic synthesis. Unfortunately, working with them has proved to be difficult because they tend to self-polymerize in solution, especially in the presence of acid catalysts. When the self-polymerization can be controlled, however, the pyrrole and indole units should provide an important route to selective N-metal binding, particularly when these ligands are activated by alkyl-lithium reagents. Using this approach, a general synthesis of the group 14 pyrrolides and indolides, Ph₃MX (M = Si, Ge, Sn; X = C ₄H₄N, C₈H₆N), has been developed and the results are reported here. The compounds are formed as high-melting, white crystalline solids and have been characterized by ¹³C-, ²⁹Si- and ¹¹⁹Sn-NMR, Raman and electron-impact mass spectroscopy as well as elemental analysis. A single-crystal X-ray study of Ph₃Si(C₄H₄N) has shown that the compound is disordered in the tetragonal lattice, even at low temperature (100 K).

Full text of DOI:10.1016/j.jorganchem.2010.07.029

Journal of Organometallic Chemistry 695 (2010) 2557e2561
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthesis, characterization and comparison of group 14 pyrrolides and indolides
Ph3MX (M ¼ Si, Ge, Sn; X ¼ C4H4N, C8H6N)q
Joël Poisson, Ivor Wharf, D. Scott Bohle, Mirela M. Barsan, Yuxuan Gu, Ian S. Butler
*
Department of Chemistry, McGill University, 801 Sherbrooke Street West, Montreal, Quebec, Canada H3A 2K6
a r t i c l e i n f o
a b s t r a c t
Article history:
The biologically important heterocycles pyrrole, C4H4N, and indole, C8H6N, ought to be useful as reagents in
organic synthesis. Unfortunately, working with them has proved to be difficult because they tend to self-
polymerize in solution, especially in the presence of acid catalysts. When the self-polymerization can be
controlled, however, the pyrrole and indole units should provide an important route to selective N-metal
binding, particularly when these ligands are activated by alkyl-lithium reagents. Using this approach,
a general synthesis of the group 14 pyrrolides and indolides, Ph3MX (M ¼ Si, Ge, Sn; X ¼C4H4N, C8H6N), has
been developed and the results are reported here. The compounds are formed as high-melting, white
crystalline solids and have been characterized by 13C-, 29Si- and 119Sn-NMR, Raman and electron-impact mass
spectroscopy as well as elemental analysis. A single-crystal X-ray study of Ph3Si(C4H4N) has shown that the
compound is disordered in the tetragonal lattice, even at low temperature (100 K).
Received 17 May 2010
Received in revised form
28 July 2010
Accepted 29 July 2010
Available online 6 August 2010
Keywords:
Pyrrolides
Indolides
Multinuclear NMR
X-Ray diffraction
Raman spectroscopy
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
both cases by using an alkyl-lithium or alkyl-sodium reagent, and it
has already proven possible to attach various metal atoms to them,
The chemistry of transition metals coupled to organic moieties
has been explored for many years. This major research area is still
continuing to develop today, as evidenced by the ever-increasing
number of journals dedicated solely to transition metal-organic
chemistry [1]. One class of main group compounds, which is of
considerable practical interest, is that of organotins [2]. These
compounds have many important uses, most notably as biocides
and in the development of novel treatments for various diseases
[3]. There are only a few compounds known in which a tin atom is
bonded to the hetero-atom of a heterocycle. Two heterocycles of
particular biological significance are pyrrole, C4H4N, and indole,
C8H6N, the synthetic organic chemistry of which has been studied
since the early 20th century [4,5]. These heterocycles play impor-
tant roles in nature, e.g., in chlorophyll, the green pigment of plant
leaves and algae, and in tryptophan, (C8H6N)CH2CH(NH2)(COOH),
the essential amino acid that is released from proteins by proteo-
lytic cleavage during digestion. Linkage of a metal centre to the
heterocycles can be achieved exclusively at the nitrogen atoms in
usually with the formation of only one major product [6].
There are several instances of syntheses of pyrrolides and indo-
lides using the group 14 elements in the literature [7e11]. These
studies have usually focused on the reactivity of only one particular
group 14 metal with pyrrole and indole. The number of comparative
experiments employing several of the group 14 elements is,
however, limited [12,13]. There is also no instance of a comparative
study contrasting the unsubstituted group 14 pyrrolides with the
corresponding indolides. The group 14 triphenylchlorides, Ph3MCl
(M ¼ Si, Ge, Sn), are ideal precursors for the controlled coupling of
pyrrole and indole to main group metal centres to facilitate such
a comparison. While the synthesis of triphenylgermanium(IV) pyr-
rolide [14,15],and triphenyltin(IV) pyrrolide [16,17], and the use of
triphenyllead(IV) pyrrole and indole derivatives [18] in mass spec-
trometric studies havebeen reportedpreviously, the synthesis of the
analogous pyrrolides and indolides of the other group 14 elements
have apparently not been attempted. It is possible that the best
synthetic conditions for these compounds had not been found
previously because of the facile self-polymerization of the hetero-
cyclic reagents, which would hinder this type of product formation
[19] by competing with chloride substitution on the metal centre.
We report here the first successful synthesis and characterization of
the complete series of group 14 triphenylmetal(IV) pyrrolides and
indolides (Fig. 1), Ph3MX (M ¼ Si, Ge, Sn; X ¼C4H4N, C8H6N).
Abbreviations: Ind, C8H6N; Pyr, C4H4N.
Presented, in part, at the 92nd Canadian Chemistry Conference and Exhibition,
q
Hamilton, Ontario, Canada, MayeJune 2009.
* Corresponding author.
E-mail address: ian.butler@mcgill.ca (I.S. Butler).
0022-328X/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2010.07.029

2558
J. Poisson et al. / Journal of Organometallic Chemistry 695 (2010) 2557e2561
conditions followed by stirring for 24 h afforded the lithiated indole
as a white precipitate. Triphenylgermanium(IV) chloride (31 mmol)
in toluene (50 mL) was added dropwise to the solution and a pink
colour was observed to develop. The mixture was stirred overnight
at room temperature. Filtering over silica yielded the crude reaction
product, which was then crystallized from toluene. Yield: 6.32 g,
54.7% (based on Ph3GeCl). M.p. 145e147 ꢂC. Anal. Calcd for
C
26H21NGe: C, 74.34%; H, 5.04%; N, 3.01%. Found: C, 74.35%; H,
4.99%; N, 3.26%. 13C-NMR (ppm): 141.3 (C-9), 134.8 (C-o), 133.2 (C-i),
131.8 (C-2), 131.2 (C-p), 130.4 (C-m), 128.8 (C-8), 121.0 (C-5), 120.5
(C-4), 119.4 (C-6), 113.8 (C-7), 103.9 (C-3). Raman (cmꢁ1): 130w,
n
(GeeInd); 171w,
n
[GeePh(x)]; 203vw,
(CeN).
d[GeePh(m)]; 226mw,
n
[d
GeePh(t0)]; 1337m,
n
2.1.2. Ph3Si(C8H6N) (2)
Compound 2 was prepared as a white crystalline solid using the
same procedure as for compound 1: C8H7N (21.6 mmol), n-C4H9Li
(24.0 mmol) and Ph3SiCl (22.3 mmol). Yield: 3.04 g, 37.5%, based on
Ph3SiCl. M.p. 153.3e153.6 ꢂC. Anal. Calcd for C26H21NSi: C, 83.15%;
H, 5.64%; N, 3.73%. Found: C, 82.76%; H, 5.57%; N, 3.58%. 29Si-NMR
(ppm): ꢁ14.6. 13C-NMR (ppm), [nJ(29Sie13C), Hz]: 141.0 (C-9), 135.9
(C-o), [2J, 11], 132.2 (C-i), 131.8 (C-2), 131.7 (C-p), 130.6 (C-m), 128.2
(C-8), 121.5 (C-5), 120.6 (C-4), 120.3 (C-6), 114.7 (C-7), 105.5 (C-3).
Fig. 1. General structure of indolides and pyrrolides.
2. Experimental
Raman (cmꢁ1): 146w,
SiePh( )]; 237w, n
SiePh(t0)]; 1334m,
n(SieInd); 172vw,
n[SiePh(x)]; 212vw, n
[d
m
n
[d
(CeN).
2.1. Materials and methods
2.1.3. Ph3Sn(C8H6N) (3)
Triphenyltin(IV) chloride, pyrrole and n-butyllithium were
obtained from Aldrich Chemical Co., while triphenylsilicon(IV)
chloride and indole were purchased from Alfa Aesar. Triphe-
nylgermanium(IV) chloride was provided by Strem Chemical Co. All
solvents were dried over sodium ribbon prior to use. Solid reagents
were used as received and pyrrole was distilled under vacuum and
stored under N2 prior to use.
Compound 3 was prepared by the same method as compound 1,
using C8H7N (24 mmol), n-C4H9Li (24 mmol) and Ph3SnCl (25
mmol) to give white crystals. Yield: 4.22 g, 38.0%, based on C4H9Li.
M.p. 144.7e145.1 ꢂC. 119Sn-NMR (ppm): ꢁ109.9. 13C-NMR (ppm), [nJ
(
119Sne13C), Hz]: 143.0 (C-9), 137.0 (C-o), [2J, 45], 135.9 (C-i), [1J,
627], 133.2 (C-2), 130.6 (C-p), [4J, 13], 129.5 (C-m), [3J, 60], 121.0 (C-
5), 120.6 (C-4), 119.3 (C-6), 113.6 (C-7), 104.2 (C-3), [3J, 18]. Raman
Raman spectra (bandposition accuracy, ꢀ1 cmꢁ1) were measured
on a Renishaw InVia spectrometer using an argon-ion (514.5-nm)
laser. The proprietry Renishaw WiRE 2.0 software was used for data
acquisitionandprocessing. NMRspectra(CDCl3)wereobtainedusing
JEOL 270-MHz or Varian 300-MHz [13C (SiMe4, int.), 1H (SiMe4, int.)]
and Unity 500-MHz [119Sn (SnMe4, ext.), 29Si, (SiMe4, int.)] spec-
trometers. Electron-impact (70 eV) spectra were recorded on a Kra-
tos MS25RFA instrument. Elemental analyses were performed using
a Costech ECS4010 combustion system.
(cmꢁ1): 124vw,
n
(SneInd); 152vw,
[SnePh(t0)]; 263vw,
[SnePh(t)]; 277vw,
(CeN).
n
[SnePh(x)]; 206 mw,
d[SnePh
(m)]; 215
d
n
n
[SnePh(t)];
1294m,
n
2.1.4. Ph3Ge(C4H4N) (4)
Compound 4 was prepared by the same method as compound 1,
using 33 mmol C4H5N, 36.0 mmol n-C4H9Li and 31.0 mmol Ph3GeCl
to give white crystals. Yield: 3.40 g, 20.9%, based on Ph3GeCl. M.p.
203e204 ꢂC. Anal. Calcd for C22H19NGe: C, 71.42%; H, 5.18%; N,
3.79%. Found: C, 71.0%; H, 5.16%; N, 3.67%. 1H-NMR (ppm): 6.7 (H-2,
H-5), 6.3 (H-3, H-4). 13C-NMR (ppm): 134.7 (C-o), 133.0 (C-i), 130.4
(C-p), 128.6 (C-m), 124.9 (C-2, C-5), 110.1 (C-3, C-4). Raman (cmꢁ1):
The single-crystal X-ray diffraction study of Ph3Si(C4H4N) was
undertaken at 100 K on a BRUKER SMART CCD diffractometer, using
ꢀ
graphite-monochromated Mo K
a
radiation (
l
¼ 0.71073 A). The
structure was solved by direct methods (using SHELX 97 software)
on an absorption-corrected model generated by SADABS (SAINT-V7
software). Refinement was achieved by a full-matrix least-squares
procedure based on F2. The phenyl ring was modeled with a site
occupancy factor of 0.75 with the meta and para carbon atoms
being modeled anisotropically, while the ipso and ortho carbons
could only be modeled isotropically because of the overlap of
electron density with the pyrrole ring. The pyrrole ring was
modeled with a site occupancy of 0.25 with isotropic carbons. The
hydrogen atoms were located at calculated positions.
162sh,
n
(GeePyr); 174mw,
d
[GePh(x)]; 215vw,
d
[GePh(x)]; 234m,
n
[GeePh(t0)]; 247vw, GeePh(t); 263vw,
n
(GeeN); 1385m, (CeN).
n
2.1.5. Ph3Si(C4H4N) (5)
Compound 5 was prepared by the same method as compound 1,
using 29.0 mmol C4H5N, 32.0 mmol n-C4H9Li and 26.0 mmol Ph3SiCl
to give white crystals. Yield: 1.15 g, 13.6% based on Ph3SiCl. M.p.
202e203 ꢂC. Anal. Calcd for C22H19NSiꢃnH2O; n ¼ 0: C, 81.18%;
H, 5.88%; N, 4.30%; n ¼ 1: C, 76.93%; H, 6.16%; N, 4.08%. Found: C,
77.02%; H, 5.64%; N, 4.05%.29Si-NMR (ppm): ꢁ21.6. 1H-NMR (ppm):
6.7 (H-2, H-5), 6.3 (H-3, H-4). 13C-NMR (ppm): 135.9 (C-o), 132.1 (C-
i), 130.6 (C-p), 128.2 (C-m), 125.8 (C-2, C-5), 111.5 (C-3, C-4). Raman
The X-ray data for compound 5 are: C22H19NSi, colourless,
plates, tetragonal, space group P421/c, a ¼ b ¼ 11.331(2), c ¼ 6.561
3
(2) A,
a
¼
b
¼
g
¼ 90.00 , V ¼ 842.3(4) A , T ¼ 100 K, Z ¼ 2, 963
(cmꢁ1): 176mw,
247w, [SiPh(t)]; 263vw,
d
[SiePh(
m
)]; 227vw,
(SieN), 1385 n
n
[SiPh(t0)]; 244m,
(CeN).
n
[SiePh(t0)];
ꢂ
ꢀ
ꢀ
reflections measured, 753 observed (I > 2
s
(I)), parameters ¼ 62, R
n
n
(obs) ¼ 0.0822, R(total) ¼ 0.1098
2.1.6. Ph3Sn(C4H4N) (6)
2.1.1. Ph3Ge(C8H6N) (1)
Dropwise addition of n-butyllithium (36 mmol,1.6 M solution in
hexane) to indole (33 mmol) in dried hexane (100 mL) under inert
Compound 6 was prepared by the same method as compound 1,
using 38 mmol C4H5N, 36.0 mmol n-C4H9Li and 33.0 mmol Ph3SnCl
to give white crystals. Yield: 1.15 g, 8.2% based on Ph3SnCl. M.p.

J. Poisson et al. / Journal of Organometallic Chemistry 695 (2010) 2557e2561
2559
Fig. 2. Mass spectrum decomposition of group 14 pyrrolides and indolides.

2560
J. Poisson et al. / Journal of Organometallic Chemistry 695 (2010) 2557e2561
204e205 ꢂC. Anal. Calcd for C22H19NSn: C, 63.51%; H, 4.60%; N,
3.37% Found: C, 63.18%; H, 4.68%; N, 3.08%. 119Sn-NMR (ppm):
ꢁ104.8. 1H-NMR (ppm): 6.7 (H-2, H-5), 6.3 (H-3, H-4). 13C-NMR
(ppm), [nJ(119Sne13C), Hz]: 136.9 (C-o), [2J, 44], 135.9 (C-i), 130.6 (C-
p), [4J, 13], 129.3 (C-m), [3J, 60] 126.1 (C-2, C-5), [2J, 15] 110.1 (C-3, C-
3.3. Mass spectra
From the mass spectral data, it was evident that the parent
molecular ions were present in each case and several reasonable
fragmentation pathways can be postulated (Fig. 2). The fragments
containing group 14 elements were readily identified from their
isotope abundance patterns [23]. Mass peaks corresponding to the
formation of biphenyl-silicon fragments, for which there is prece-
dent in the literature [24], were observed. Similar behavior was
noted for the germanium species, but not for the tin compounds.
This is contrary to what has been observed previously for triphe-
nylgermanium(IV) pyrrolide. The isotopic patterns obtained
experimentally for the molecular ions were in excellent agreement
with the theoretically calculated values (see Table S1 in the elec-
tronic supplementary information).
4), [3J, 21]. Raman (cmꢁ1): 158mw,
218s, [SnPh(t)]; 295w,
[SnePh(t0)]; 270w,
(CeN).
d[SnPh(x)]; 188vw,
d[SnPh(x)];
n
n
n
(SneN); 1383mw, n
3. Results and discussion
3.1. Synthesis of compounds 1e6
The reactions of the group 14 triphenylchlorides, Ph3MCl (M ¼
Si, Ge, Sn), with lithiated pyrrole and indole were all successful
and resulted in the formation of a single product in each case.
Most products gave the appropriate microanalyses but for
compound 5, carbon results were consistently low (w77%) for
several samples, even after drying. These data are consistent with
5 being a monohydrate at room temperature. In contrast, 1H-
NMR (CDCl3) suggests that 5 has about 1/2H2O per pyrrolyl unit.
This presumed hydrate is now under further investigation. All
white crystalline solids obtained are stable over a period of
several days in air and light but eventually darken due to
hydrolysis of the heterocyclic moiety. Relatively low tempera-
tures had to be used in the syntheses in order to minimize the
possibility of self-polymerization of the heterocycles and,
presumably for this reason, the product yields were quite low,
with the best yields being obtained for the germanium
compounds in both cases: Ph3M(C4H4N) [Sn (8.2%); Si (13.6%); Ge
(20.9%)]; Ph3M(C8H6N): [Si (37.5%); Sn (38.0%); Ge (54.7%)]. The
melting points for the pyrrole compounds (4e6) range between
202 and 205 ꢂC and are significantly higher than are those for the
corresponding indole compounds (1e3) (144.7e153.6 ꢂC). This
trend parallels that observed earlier for the related tetraphenyl
derivatives, Ph4M, which upon replacement of one of the phenyl
groups with another substituent results in a lowering of the
overall spherical geometry of the molecules. For example, tetra-
phenyltin(IV) (m.p. ¼ 225 ꢂC) melts at a much higher tempera-
ture than does triphenyltin(IV) acetate (m.p. ¼122e124 ꢂC). The
similarity in the shapes of the pyrrole and phenyl groups allows
an energetically favorable arrangement of substituents around
the metal centre. The more bulky nature of the indole group,
however, hinders the packing in the crystal lattices and, conse-
quently, the indolides melt at lower temperatures than do the
analogous pyrrolides. The sizes of the central atoms can also be
correlated with the stabilities in this series of complexes. Upon
going from Si to Ge and then to Sn, the melting points of the
triphenylmetal(IV) pyrrolides increase. Conversely, the melting
points decrease for the same central atom replacement in the
indolides. This difference can be attributed to lengthening of the
N-metal linkage as the main group element becomes larger.
3.4. Raman spectra
Raman spectroscopy provided key information in support of the
formation of the metal indolides and pyrrolides. Two linkages of
interest, which are broken and formed, respectively, are the NeH
bond of the free heterocycle and the N-metal bond of the resulting
complex. There are a number of weak vibrational modes, associated
with the metal-indole and metal-pyrrole motions, observed in the
500e100 cmꢁ1 region. In addition, a peak at approx. 3480 cmꢁ1
,
which is attributed to the NeH stretching motion in free indole and
pyrrole, disappears upon reaction with the triphenylmetal(IV)
chlorides. These observations provide further evidence that the
heterocycles are attached to the metal centres via the N atom in
these reactions. Moreover, the Raman spectra of the indolides and
pyrrolides reveal the presence of both the triphenylmetal(IV) and
the heterocycles, and are quite similar to those of the tetraphe-
nylmetal(IV) derivatives, Ph4M (M ¼ Si, Ge, Sn). There are charac-
teristic stretching and bending modes appearing at low
wavenumbers accompanied by the well-known CeN aromatic
stretch in the 1290e1390 cmꢁ1 region.
3.5. Single-crystal X-ray diffraction study of compound 5
In addition to the spectroscopic measurements for 5, single-
crystal X-ray diffraction data were obtained. While a satisfactory
solution at ambient temperature was not possible, the low
temperature data set (100 K) allowed for a good, albeit disordered,
model for the structure. Fig. 3 depicts the asymmetric unit for
3.2. NMR spectra
The 13C-NMR chemical shifts for the four new group 14 metal
compounds (1, 2, 3 and 5) are consistent with the literature values
for the trimethyltin(IV) indolide and triphenyltin(IV) pyrrolide (4),
and the observed chemical shifts are similar. The deshielding effect
on the ortho, para and ipso aromatic carbons in both the pyrrolides
and indolides follows the order Ge < Si < Sn. This inversion of Ge
and Si from the expected periodic trend may be one more example
of the tendency of germanium to resemble carbon more than
silicon [20]. NMR data are not available for Ph3C(Pyr) [21] or Ph3C
(Ind) [22].
Fig. 3. Asymmetric unit for Ph3Si(C4H4N).

J. Poisson et al. / Journal of Organometallic Chemistry 695 (2010) 2557e2561
2561
Table 1
Comparison of arylsilane unit cell dimensions.
3
ꢀ
ꢀ
ꢀ
Compound
T (K)
a, b (A)
c (A)
V (A )
dcalc (g/mL)
dexp (g/mL)
Reference
SiPh4
5
Si(pyr)4
100 (2)
153
11.331 (2)
10.918 (2)
6.561 (2)
6.046 (1)
842.3 (4)
720.7 (2)
1.283
1.348
This work.
[13]
SiPh4
5
Si(pyr)4
293 (10)
295 (1)
293 (10)
11.440 (2)
11.334 (6)
10.924 (6)
7.060 (4)
6.882 (8)
6.238 (4)
924 (1)
883 (1)
744.4 (7)
1.207
1.223(2)
1.30
[25]
This work.
[7]
1.23(2)
compound 5, which is otherwise isostructural with both tetra-
phenylsilane [25] and tetrapyrrol-1-ylsilane [7] and all three crys-
tallize in the same P421/c space group. The pyrrole ring is
disordered among the four coordination sites of the central Si atom
with the other sites being occupied by phenyl rings. The disorder
increases the uncertainty of specific molecular dimensions and the
geometry. Consequently, we will not discuss specific metric
parameters. However, the values for the crystal unit cell of 5 can be
reliably compared with those of tetraphenylsilane and tetrapyrrol-
1-ylsilane (Table 1). Systematic differences in unit cell dimensions,
and densities, correlate with a single phenyl/Pyr substitution.
Moreover, the experimental density determined for 5 mirrors the
calculated density and independently confirms the structure as
having only one phenyl/Pyr substitution (Table 1).
Appendix A. Supplementary material
Full details of the structure determination have been deposited
with the Cambridge Crystallographic Data Centre as CCDC 775301.
Copies of this information may be obtained free of charge from The
Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: þ44
1223 336 033; www: http://www.ccdc.cam.ac.uk).
Supplementary data associated with this article can be found in
the online version, at doi:10.1016/j.jorganchem.2010.07.029.
References
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The coupling of lithiated N-heterocycles to the group 14
elements (Si, Ge, Sn) under inert atmosphere conditions is an
efficient method for the synthesis of novel compounds. While the
yields are relatively low, the reactions seem unquestionably to
work when completed over long periods under moderate reaction
conditions. These conditions provide adequate samples to
perform spectroscopic studies and are a convenient way to
expand the knowledge base in this relatively unexplored field of
chemistry. Gentle heating may be useful in obtaining even better
yields, however, great care must be taken to avoid any self-
polymerization of the heterocycles. In future work, it is planned
to examine the possibility of coupling the triphenylmetal(IV)
pyrrolides and indolides to a Cr(CO)3 fragment. These reactions
could lead to new heterocyclic organometallic complexes and
these bimetallic compounds may well have some interesting
biocidal properties. The thermal lability of the N-metal bond
might also be worth investigating, leading to novel catalytic
agents.
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The research was generously supported by a Discovery grant
from the NSERC (Canada). The authors would like to thank the
Laboratoire d’analyse élémentaire de l’Université de Montréal for the
elemental analyses.
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