Are organotin reagents derived from bis(trimethylsilyl)picoline suitable precursors for the preparation of cyclometallated complexes?

Article abstract of DOI:10.1002/zaac.200800339

The organotin reagents [2-PyC(SiMe₃)₂SnR₃] (R = Me, ⁿBu) were prepared in good yields from the reaction between the lithium salt of 2-bis(trimethylsilyl)picoline and the corresponding trialkyltin chlorides. Reac

Full text of DOI:10.1002/zaac.200800339

ARTICLE
DOI: 10.1002/zaac.200800339
Are Organotin Reagents Derived from Bis(trimethylsilyl)picoline Suitable
Precursors for the Preparation of Cyclometallated Complexes?
Anja Molter[a] and Fabian Mohr*[a]
Keywords: Cyclometallation; Organotin reagents; Palladium; Platinum; DFT calculations
Abstract. The organotin reagents [2-PyC(SiMe3)2SnR3] (R ϭ Me,
nBu) were prepared in good yields from the reaction between the
lithium salt of 2-bis(trimethylsilyl)picoline and the corresponding
trialkyltin chlorides. Reactions of these organotin reagents were
carried out with various Pd and Pt complexes including
[MCl2(cod)] and [MCl2(PhCN)2] (M ϭ Pd, Pt). The results show
that a Me group is transferred to the metal atom, rather than the
2-bis(trimethylsilyl)picolyl group. The mechanism for this reaction
is discussed and reasons why Me group transfer occurs based on
DFT computed structural data are given.
preparation of organotin compounds as well as their sta-
bility towards air and moisture (compared to organolithium
reagents), we were interested to discover if organotin species
derived from 2-picoline could be used as reagents for the
preparation of cyclometallated complexes containing four-
membered chelate rings. Cyclometallated bis(chelate) com-
plexes containing picoline derivatives, in particular 2-bis(tri-
methylsilyl)picoline, are known for a range of transition me-
tals including Cu, Ag, Au (eight-membered ring dimers) as
well as Hg, Zn, Cd, Co, Fe, Ni and Mn (four-membered
ring chelate complexes) as shown in Figure 1 [11Ϫ20].
Introduction
The activation of C-H bonds has been a topic of con-
siderable interest and research activity over the past decades
[1]. In particular, cyclometallation i.e. intramolecular C-H
activation of compounds containing donor atoms such as
sulfur, oxygen, nitrogen and phosphorus to give cyclic metal
compounds has been investigated in some detail [2]. Various
methods of forming cyclometallated complexes are known,
however, the most commonly used are direct metallation of
an aromatic compound (known for Pd and Pt), thermal or
photochemical C-H activation of a coordinated ligand and
transmetallation using organolithium or organomercury re-
agents. All these methods suffer from various drawbacks,
including poor yields, difficulty of reaction control as well
as the dangers associated with handling the highly toxic or-
ganomercury reagents as well as the disposal of metallic
mercury which forms as side-product. Furthermore, organ-
olithium reagents, being one-electron reducing agents, often
lead to reduction of the metal precursors rather than to
cyclometallated products. Thus, there is an interest in the
development of alternative, non-reducing organometallic
reagents that can be used as transfer reagents in the syn-
thesis of cyclometallated compounds under mild conditions.
Organotin compounds of the type ArSnR3 (Ar ϭ various
aromatic groups, Rϭ nBu, Me, iPr) have been used to trans-
fer organic groups to transition metal atoms such as Pt, Pd,
and Au under mild conditions [3Ϫ10]. Given the ease of
Figure 1. Examples of cyclometallated complexes containing
bridging and chelating 2-bis(trimethylsilyl)picoline ligands.
Somewhat surprisingly, even though some Ni chelate
complexes with this ligand have been reported, no Pd or
Pt complexes are known. We therefore attempted to utilise
organotin reagents derived from 2-bis(trimethylsilyl)pico-
line to prepare cyclometallated chelate complexes of Pd and
Pt. The results of this investigation are presented herein.
Experimental Section
* Prof. Dr. F. Mohr
Fax: ϩ49-202-439-3053
E-Mail: fmohr@uni-wuppertal.de
[a] Fachbereich C Ϫ Anorganische Chemie
Bergische Universität Wuppertal
Gaußstr. 20
General
1H, 13C and 119Sn NMR spectra were recorded on a 400 MHz
Bruker ARX or a 250 MHz Bruker Avance spectrometer. Chemical
shifts are quoted relative to external TMS (1H, 13C) and Me4Sn
42119 Wuppertal, Germany
Supporting information for this article is available on the
WWW under http://www.wiley-vch.de/home/zaac or from the
author
(
119Sn) in ppm. Mass spectra were measured on a Finnigan MAT
8200 spectrometer. IR spectra were run as KBr pellets on a Bruker
134
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Z. Anorg. Allg. Chem. 2009, 635, 134Ϫ138

Are Organotin Reagents Suitable Precursors
Tensor 27 instrument. Elemental analyses were performed by staff
of the microanalytical laboratory of the University of Wuppertal.
All reactions were carried out under dried dinitrogen using stand-
ard Schlenk techniques. Chemicals and solvents (anhydrous
quality) were sourced commercially and used as received. 2-Bis-
(trimethylsilyl)picoline, [MCl2(PhCN)2], [MCl2(SMe2)2] and
[MCl2(PPh3)2] (M ϭ Pt, Pd) were prepared as described in the
literature [11, 21]. Theoretical calculations were performed using
the DFT method, specifically functional PBE [22], incorporated
in the program package Priroda [23, 24]. In the PBE calculations
relativistic Stevens-Basch-Krauss (SBK) effective core potentials
(ECP) optimized for DFT-calculations have been used [25Ϫ27].
The basis set was 311-split for main group elements with one ad-
ditional polarization p-function for hydrogen and two additional
polarization d-functions for the heavier elements. Full geometry
optimization was performed without constraints on symmetry. All
minima were checked for the absence of imaginary frequencies.
Reactions of Organotin Reagents with Pt and Pd Complexes
To a solution of organotin reagent (20 mg, 0.05 mmol) in CDCl3
(0.4 mL) in an NMR tube with Young valve was added 0.5 equiva-
1
lents of the appropriate metal complex. H NMR spectra were re-
corded at various intervals after mixing the two components. Once
no further changes were observed, the mixture was passed through
Celite and the filtrate taken to dryness. The solid residue was then
analysed by NMR spectroscopy and mass spectrometry.
Chlorodimethyl-2(bis-trimethylsilyl)-picolyltin (1a)
This was prepared as described for compound 1 from bis(trimethyl-
silyl)picoline (2.00 g, 2 mmol) and Me2SnCl2 (1.85 g, 8 mmol).
Yield: 2.38 g (67 %) yellow solid. Elemental analysis: calculated for
C14H28ClNSi2Sn (420.7): C 39.97 %, H 6.71 %, N 3.33 %; found
C 39.91 %, H 6.98 %, N 3.33 %.
2
1H NMR (CDCl3): δ ϭ 0.20 (s, 18 H, SiMe3), 0.78 (s, 6 H, J1H-119Sn
ϭ
2
65.1 Hz, J1H-117Sn ϭ 62.1 Hz, SnMe3), 7.11 (m, 1 H, Py H-5), 7.16 (dt, J ϭ
8.1, 1.0 Hz, 1 H, Py H-4), 7.68 (dt, J ϭ 7.1, 1.5 Hz, 1 H, Py H-3), 8.04 (dd,
Caution: Organotin compounds are highly toxic and should there-
fore be handled with utmost care (hood and neoprene gloves).
J ϭ 5.1, 1.0 Hz, 1 H, Py H-6). 13C NMR (CDCl3): δ ϭ 1.92 (3J13C-Sn
ϭ
52 Hz, SiMe3), 6.02 (1J13C-119Sn ϭ 487.3 Hz, 1J13C-117Sn ϭ 465.7 Hz, SnMe3),
119.92 (Py C-5), 123.70 (3J13C-119Sn ϭ 59.8 Hz, 3J13C-117Sn ϭ 57.7 Hz, Py C-3),
138.02 (Py C-4), 145.08 (Py C-6) 166.51 (Py C-2). 119Sn{1H} NMR (CDCl3):
δ ϭ Ϫ46.05 ppm. MS (m/z): 421 (3 %) [M]ϩ, 406 (10 %) [M-Me]ϩ, 386
(100 %) [M-Cl]ϩ.
Trimethyl-2(bis-trimethylsilyl)-picolyltin (1)
To an ice cooled solution of 2-bis(trimethylsilyl)picoline (4.75 g,
20 mmol) in Et2O (30 mL) containing TMEDA (2.88 mL) was ad-
n
ded BuLi (8 mL, 20 mmol). The red solution was then cooled to
Results and Discussion
Ϫ65 °C and Me3SnCl (3.99 g, 20 mmol) was added in one portion.
After stirring the mixture over night, the yellow solution was fil-
tered through Celite. The filtrate was evaporated to dryness and
the residue was extracted into hexane. The hexane extracts were
passed through Celite and evaporated to dryness. The resulting oil
was treated with MeOH to give a colourless solid, which was iso-
lated by filtration, washed with a small amount of MeOH and
dried. Yield: 5.91 g (74 %) colourless solid. Elemental analysis: cal-
culated for C15H31NSi2Sn (400.3): C 45.01, H 7.8, N 3.50; found
C 44.98, H 7.92, N 3.34 %.
The organotin reagents [2-PyC(SiMe3)2SnR3] (R ϭ Me 1,
nBu 2) are readily prepared in good yields from the reaction
between the lithium salt of 2-bis(trimethylsilyl)picoline and
the corresponding trialkyltin chlorides (Scheme 1).
1H NMR (CDCl3): δ ϭ 0.08 (s, 18 H, SiMe3), 0.23 (s, 9 H, 2J1H-Sn ϭ 50.9 Hz,
SnMe3), 6.92 (m, 1 H, Py H-5), 7.13 (d, J ϭ 8.1 Hz, 1 H, Py H-3), 7.48 (dt,
J ϭ 7.7, 1.8 Hz, 1 H, Py H-4), 8.21 (d, J ϭ 4.1 Hz, 1 H, Py H-6). 13C NMR
(CDCl3): δ ϭ Ϫ0.61 (1J13C-119Sn ϭ 346.4 Hz, 1J13C-117Sn ϭ 320.9 Hz, SnMe3),
2.57 (SiMe3), 118.78 (Py C-5), 123.97 (3J13C-Sn ϭ 42.5 Hz, Py C-3), 135.58
(Py C-4), 147.34 (Py C-6) 165.41 (Py C-2). 119Sn{1H} NMR (CDCl3): δ ϭ
Ϫ38.19 ppm. MS (m/z): 401 (10 %) [M]ϩ, 386 (100 %) [M-Me]ϩ.
Scheme 1.
Both compounds were fully characterized by spectro-
scopic methods, the data of which are fully consistent with
the proposed structures (see Experimental Section). Com-
pound 1 shows, in addition to the pyridyl and SiMe3 sig-
nals, a singlet with Sn satellites at 0.23 ppm due to the
SnMe3 group in the 1H NMR spectrum. Similarly, com-
Tri-n-butyl-2(bis-trimethylsilyl)-picolyltin (2)
This was prepared as described above from 2-bis(trimethylsilyl)- pound 2 shows four resonances due to the nBu chains,
n
picoline (2.00 g, 8 mmol) and Bu3SnCl (2.28 mL, 8 mmol). Yield:
4.43 g (100 %) pale yellow oil. Although the compound appeared
pure by 1H NMR spectroscopy and GC-MS, it constantly gave bad
elemental analysis results probably due to incomplete combustion
and/or decomposition.
which could be unambiguously assigned by 2D NMR
experiments. In the 13C{1H} NMR spectra of compounds
1 and 2 (and also 1a) the signal of the carbon atom bound
to both Si and Sn could not be detected. The mass spectra
show weak molecular ion peaks for compounds 1 and 2 as
well as peaks arising from the loss of one Me or nBu group,
respectively. In both cases, these fragment peaks are the
most intense signals in the spectrum.
We initially examined the reaction of organotin reagent
1 with [PdCl2(PhCN)2] on NMR scale, since the proton
chemical shift as well as the Sn-H coupling constants of
the SnMe3 group signal should serve as a convenient probe
to monitor the reaction. Within minutes of addition of the
1H NMR (CDCl3): δ ϭ 0.07 (s, 18 H, SiMe3), 0.95 (t, J ϭ 7.1 Hz, 9 H,
nBu CH3), 1.04 (m, 6 H, SnCH2), 1.40 (m, 6 H, nBu CH2-γ), 1.62 (m, 6 H,
nBu CH2-β), 6.90 (m, 1 H, Py H-5), 7.12 (d, J ϭ 8.1 Hz, 1 H, Py H-3), 7.46
(dt, J ϭ 7.7 Hz, 1.9 Hz, 1 H, Py H-4), 8.23 (d, J ϭ 4.9 Hz, 1 H, Py H-6).
13C NMR (CDCl3): δ ϭ 2.66 (3J13C-Sn ϭ 51.2 Hz, SiMe3), 13.70 (nBu C-δ),
2
15.87 (2J13C-119Sn ϭ 335.1 Hz, J13C-117Sn ϭ 320.2 Hz, nBu C-α), 27.78
3
(3J13C-119Sn ϭ 69.4 Hz, J13C-117Sn ϭ 66.6 Hz, nBu C-γ), 29.71 (4J13C-Sn
ϭ
17.6 Hz, nBu C-β), 118.34 (Py C-5), 123.91 (3J13C-Sn ϭ 138.1 Hz, Py C-3),
135.26 (Py C-4), 147.07 (Py C-6), 165.54 (2J13C-Sn ϭ 125.9 Hz, Py C-2).
119Sn{1H} NMR (CDCl3): δ ϭ Ϫ44.45 ppm. MS (m/z): 527 (10 %) [M]ϩ, 470
(100 %) [M-nBu]ϩ.
Z. Anorg. Allg. Chem. 2009, 134Ϫ138
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135

A. Molter, F. Mohr
ARTICLE
Pd complex to a solution of the organotin reagent, a black
precipitate formed and evolution of a gas could be ob-
served. The 1H NMR spectrum of the mixture recorded
immediately after mixing, showed two signals with Sn satel-
lites at δ ϭ 0.23 and 0.78 ppm, respectively, the former due
to the SnMe3 group of compound 1; after ca. 1 h the signal
due to compound 1 had completely disappeared. The prod-
uct was isolated by filtration of the reaction mixture
through Celite and evaporation of the solvent. The yellow
1
solid thus obtained was characterised by H NMR spec-
troscopy and mass spectrometry. The spectral data indi-
cated that the product contained no palladium, instead, the
mass spectrum showed a molecular ion peak at m/z 420
with an isotope pattern consistent with the presence of
chlorine and tin. Combined with proton NMR data, we
could identify the yellow product as 2-PyC(SiMe3)2-
SnClMe2 (1a). The same compound could be prepared in
almost quantitative yield on a larger scale by the reaction
of 1 with 0.5 equivalents of [PdCl2(PhCN)2]. Furthermore,
we could independently prepare 1a from the reaction of the
lithium salt of 2-bis(trimethylsilyl)picoline with Me2SnCl2.
The spectral data of the products prepared by the two dif-
ferent methods are identical (Scheme 2).
Scheme 3.
cluding [PdCl2(MeCN)2], [PdCl2(SMe2)2], [PdCl2(cod)],
[PdCl2(PPh3)2] and [PdCl2(AsPh3)2]. For the first three
complexes we again observed clean transfer of the Me
group to the Pd atom giving 1a accompanied by formation
of ethane and Pd metal. In the case of [PdCl2(PPh3)2] and
[PdCl2(AsPh3)2] we observed no reaction at all; even after
prolonged reaction times (several days) only starting mate-
1
rials were observed in the H NMR spectra of the reaction
mixture. Use of more forcing conditions (heating in benzene
or toluene) only led to decomposition as judged by the in-
tractable product mixture observed by 1H NMR spec-
troscopy. These results led us to conclude that the organotin
compound 1 is not a suitable reagent to transfer the 2-bis-
(trimethylsilyl)picolyl group to a Pd atom. Since RSnnBu3
derivatives are preferentially used over RSnMe3 species in
Scheme 2.
The obvious question now is how the Me3Sn com- Stille coupling reactions to prevent Me group transfer, we
n
pound 1 is transformed into the SnClMe2 compound 1a by examined analogous reactions using the Bu derivative 2.
the palladium complex. Scheme 3 illustrates the mechanism Unfortunately, here too, no transfer of the 2-bis(trimethylsi-
we propose for this reaction: In the first step, the organotin lyl)picolyl group was observed. The reaction takes a similar
reagent transfers one methyl group to the Pd centre to give course i.e. deposition of Pd0, but the fate of the organic
1a and [PdClMe(PhCN)2] via a cyclic intermediate. Since groups is unclear, since the 1H NMR spectra of the reaction
there are two equivalents of 1 per Pd, the same reaction i.e. mixtures and the isolated products give much more complex
n
transfer of a Me group from Sn to Pd occurs a second time, spectra due to overlapping signals of the Bu groups.
yielding a further molecule of 1a and [PdMe2(PhCN)2].
Given the detailed studies on the use of RSnMe3 species
This dimethyl palladium complex is unstable and readily to transfer the organic group to a variety of Pt complexes
undergoes reductive elimination giving ethane (the gas ob- by Eaborn and Pidcock, we decided to also examine the
served during the reaction) and Pd0, which precipitates out reaction of organotin reagent 1 with some Pt precursors.
of the reaction. Thus, overall two molecules of 1 are con- Consistent with the observations by Eaborn and Pidcock,
verted into two molecules of 1a by one molecule of the reaction of 1 with [PtCl2(cod)] was much slower than
[PdCl2(PhCN)2], accompanied by formation of one mol- that with the Pd species. In fact, at room temperature no
ecule of ethane and palladium metal as by products.
reaction was observed as judged by 1H NMR spectroscopy,
It appears that in this case, the organotin reagent trans- which showed on signals of the starting materials. Upon
fers a methyl group to the palladium centre, rather than the heating the sample to ca. 60 °C for a few hours the mixture
2-bis(trimethylsilyl)picolyl unit as was hoped for. Methyl darkened and eventually deposited a metallic mirror. The
group transfer from Me4Sn to Pd is known, in fact 1H NMR spectrum of the product isolated after heating the
[PdClMe(cod)] (cod ϭ 1,5-cyclooctadiene) is conveniently reaction mixture was identical to that of compound 1a. It
prepared by the reaction of Me4Sn with [PdCl2(cod)] [28]. seems that here too, the organotin reagent transfers a
However, the transfer of Me to Pd from substituted methyl group to Pt rather than the 2-bis(trimethylsilyl)pico-
RSnMe3 species, especially those containing aryl groups or lyl unit. This finding is surprising in the context of the ob-
donor functionalities have, to the best of our knowledge, servations of Eaborn and Pidcock. They reported, that the
never been reported. In further attempts to produce the de- reaction of (Me3Si)3CSnMe3 and (4-MeC6H4)CH2SnMe3
sired chelate complexes by this route, we examined the reac- with [PtCl2(cod)] under forcing conditions (> 10 h, 100 °C)
tion of compound 1 with various other Pd complexes in- gives only minor amounts (15Ϫ30 %) of [PtMeCl(cod)] de-
136
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Z. Anorg. Allg. Chem. 2009, 134Ϫ138

Are Organotin Reagents Suitable Precursors
Figure 3. Molecular structure of compound 1a as computed by
DFT methods. Spheres with no labels represent carbon atoms. Hy-
drogen atoms have been omitted for clarity.
Figure 2. Molecular structure of compound 1 as computed by DFT
methods. Spheres with no labels represent carbon atoms. Hydrogen
atoms have been omitted for clarity.
been observed in the crystal structures of 2-
(Me2NCH2)C6H4SnXRRЈ (X ϭ Cl, Br, OMe; R and RЈ ϭ
Me, Ph) [31]. Here too, the non-halogenated tetraorganotin
derivative shows tetrahedral geometry with weak N-Sn in-
rived from Me transfer from Sn to Pt; the main products
of the reaction are [PtCl{C(SiMe3)3}(cod)] and
[PtCl(CH2C6H4Me)(cod)], respectively [4]. They deter-
mined that the order of reactivity of RSnMe3 species with
Pt complexes follows the pattern aryl >> CH2Ph > Me i.e.
aryl groups are easiest to transfer, followed by benzylic
groups and Me transfer occurs only under forcing con-
ditions. Which group is transferred to Pt is also dependent
on the Pt precursor used. For example, the reaction of
[Pt(CO)Cl(PPh3)2]ClO4 and [Pt(O2CCF3)(PMe2Ph)2] with
various RSnMe3 species gives in some cases the product
resulting from Me transfer and, in other cases the product
resulting from R group transfer [29, 30]. We attempted
further reactions with organotin reagent 1 and other Pt
complexes such as [PtCl2(PhCN)2] and [PtCl2(PPh3)2] but
again either Me transfer occurs or, in the latter case, no
reaction occurs at all. We could therefore conclude that the
2-bis(trimethylsilyl)picolyl group cannot be transferred to
either Pt or Pd centres using the organotin species. An
interesting question now arises as to why this is so. We were
unable to obtain X-ray quality crystals of compound 1 in
order to examine the structure in the solid state. However,
using DFT calculations we were able to compute the gas-
phase structure of compound 1 (Figure 2).
˚
teractions [d(N-Sn) ϭ 3.46 A], whilst in the halogen ana-
logues the tin atom adopts a trigonal bipyramidal coordi-
˚
nation with short (2.49 A) N-Sn contacts. What is apparent
upon inspection of the structure of compound 1, is that the
SiMe3 substituted carbon atom is sterically well shielded.
For cyclometallation to occur, this C atom has to get close
enough to the metal atom for the M-C bond formation to
occur. We therefore suggest, based on the observed reactiv-
ity of compound 1 and the computed structure, that cyclo-
metallation with Pd or Pt complexes does not occur due to
steric reasons. The new C-M bond cannot form as the two
atoms cannot approach each other well enough. The SnMe3
groups on the other hand, lie on the periphery of the mol-
ecule and can thus readily react with the incoming metal
complexes. For this reason Me group transfer is favoured,
leading to the observed reactivity of this compound. We are
currently developing less sterically crowded derivatives in
the hope that these may undergo cyclometallation with Pd
and Pt complexes.
Supporting Information: Final energies and atomic coordinates of
the structures of compound 1 and 1a as computed by DFT meth-
ods in PDF file format.
The coordination at the Sn atom can be described as
tetrahedral, with angels in the range of 105Ϫ113°, as ex-
pected for a tetra-substituted organotin compound. In ad-
dition, there appears to be an interaction between the pyri-
dyl nitrogen atom and the Sn atom with a bond distance of
Acknowledgement
˚
We wish to thank the Fonds der Chemischen Industrie and the
University of Wuppertal for generous financial support of this
work.
2.79 A, which may be the cause of the slight distortion of
the tetrahedral coordination about the tin atom. This
N-Sn interaction seems to stabilize the structure, since DFT
calculations comparing two isomers with and without intra-
molecular N-Sn interactions show that the former is lower
in energy by ca. 2.7 kcal/mol. The DFT computed structure
of the chloro-derivative 1a (Figure 3), is considerably differ-
ent. Here, the N-Sn interaction is much stronger (N-Sn dis-
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Z. Anorg. Allg. Chem. 2009, 134Ϫ138
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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137

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ARTICLE
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Received: June 11, 2008
Accepted: July 29, 2008
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