A.K. Maity et al. / Journal of Organometallic Chemistry 768 (2014) 42e49
43
produces a bimetallic ‘TmeSnCl3’ motif where the transition metal
can be in low or high oxidation state. For the insertion of a SnCl2
group into a TmeCl bond, one can consider two possible mecha-
nisms. Firstly, the reaction might proceed by coordination of a SnCl2
molecule, perhaps solvated, to the transition metal, followed by an
intramolecular migration of the halide group from the transition
metal to the tin atom. Alternatively, the active species might be
SnClꢁ3 , produced under the reaction conditions, which could
displace halide ions in SN1 or SN2 reaction pathways [3].
Moreover, the above synthetic strategies are interesting in view
of the fact that they bring desirable electronic features in the
multimetallic TmeSnCl3 motif for potential application within
cooperative catalysis regime. The key features which induce cata-
lytic activity by the TmeSnCl3 motif include a high valent and
electrophilic transition metal center (Tm), an electrophilic tri-
chlorostannyl group (SnCl3) and the most importantly the bond
between these two fragments. Thus, as a whole the TmeSnCl3
moieties can act as a mild and controlled Lewis acid to trigger
electrophilic activation and indeed we have delineated success in
harnessing catalytic reactivity utilizing the concepts. For example,
quite recently we and other research groups [4] have illustrated the
unique reactivity of multimetallic catalysts in activating electro-
philes, such as benzylic alcohols [5a,5e,5g], propargylic alcohols
[5g,6a], allylic alcohols [5f,5g], ethers [5c], aldehydes [5b,5e,6b,6c],
like benzene or toluene this reaction did not proceed at all. The
multimetallic complex 1 is air and moisture stable in solid as well as
solution phase but it turned in to an oily paste after prolong
exposure to moisture. This complex is soluble in polar solvents like
DMSO, DMF, MeCN, MeOH, acetone, hot DCM, and hot DCE; and
sparingly soluble in CHCl3 and insoluble in benzene, toluene or
xylene.
Another three-legged piano-stool complex of iridium,
[Cp*Ir(PPh3)(SnCl3)Cl] 2 having one trichlorostannyl ligand and one
triphenylphosphine was synthesized via the insertion reaction of
SnCl2 across [Cp*Ir(PPh3)Cl2] in 1,2-dichloroethane at room tem-
perature (Scheme 2). After overnight stirring the color of the so-
lution changes from deep yellow to pale yellow and the product 2
was isolated as yellow powder by precipitating using n-hexane.
Unfortunately, the various attempt to purely synthesize double
insertion iridium complex, [Cp*Ir(PPh3)(SnCl3)2] with two tri-
chlorostannyl ligands and one triphenylphosphine ligand was un-
successful as the insertion of SnCl2 across Cp*Ir(PPh3)Cl2 at higher
reaction temperature always afforded the mixture of single and
double insertion complexes [8].
An interesting ionic complex of iridium, [Cp*Ir(SnCl3)3][NHt3Bu]
3 having three trichlorotannyl ligands was synthesized when the
insertion reaction of SnCl2 across [Cp*Ir(NHt2Bu)Cl2] was attempted
in 1,2-dichloroethane at refluxing condition (Scheme 3). The
complex 3 was obtained as yellow crystals upon slow diffusion of n-
hexane to the yellow solution of 1,2-dichloroethane. Our initial plan
was to synthesize the double insertion complex or iridium with an
aldimines [5i,6d],
a
,
b-unsaturated ketone [5h,6e,6f,6g],
a-methyl
substituted aryl alkene [5j], epoxide [6h,6i] and
g-hydroxy lactams
[5d] towards carbonecarbon and carboneheteroatom bond for-
mation. In view of the above facts, we demonstrate here the for-
mation of IreSnCl3 and RheSnCl3 frameworks via the insertion
reaction of SnCl2 across the half-sandwich complexes of iridium
and rhodium. It may be noted that, although examples of insertion
of SnCl2 across Group 10 metal complexes are well known, those
across Group 9 metal complexes are relatively rare [7]. In this article
we wish to report the synthesis and characterization of some three-
legged piano-stool trichlorostannyl complexes of iridium (III) and
rhodium (III) via the insertion reaction of SnCl2 and their catalytic
activity towards the alkylation reaction of aromatic aldehyde.
s-donor ligand like tert-butylamine. Instead of getting the desired
complex, we came up with an ionic complex of iridium having tert-
butylammonium as the counter cation. The complex 3 is highly
soluble in polar solvents like DMSO, DMF, MeCN, MeOH, acetone,
DCM, and DCE, CHCl3 and sparingly soluble in hydrocarbon solvents
like benzene, toluene or xylene.
An analogous double insertion rhodium complex, [Cp*Rh
(PPh3)(SnCl3)2] 4 was synthesized via the insertion reaction of
SnCl2 across [Cp*Rh(PPh3)Cl2] in 1,2-dichloroethane at refluxing
condition (Scheme 4). Interestingly, in this case we were unable to
isolate any single insertion rhodium complex even carrying out the
reaction at room temperature. Very much like the behavior of
analogous iridium complex, a mixture of single and double inser-
tion rhodium complex was always isolated.
2. Results and discussion
2.1. Synthesis of iridium and rhodium complexes having
trichlorostannyl ligands
2.2. NMR spectroscopic analysis of complex 1e4
A multimetallic three-legged piano-stool complex of iridium,
[Cp*Ir(SnCl3)2{SnCl2(H2O)2}] 1 bearing two trichlorostannyl ligands
was synthesized via insertion of SnCl2 across the IreCl bonds of
[Cp*IrCl2]2 in refluxing 1,2-dichloroethane (Scheme 1). The initially
brick-red solution slowly turned greenish yellow on completion of
the reaction. Upon slow diffusion of n-hexane to this solution, the
product 1 crystallized out as greenish yellow blocks in good yield
[5d].
All NMR spectra were recorded in deuterated acetone at room
temperature. All the NMR data are shown in Table 1. The 1H NMR
spectra of 1e4 show characteristic peaks due to C5(CH3)5 protons of
the iridium or rhodium coordinated pentamethylcyclopentadienyl
group at around 1.5e2.5 ppm. While the Cp* peaks are appeared as
singlet in the proton NMR spectra for complexes 1 and 3, for
4
complexes 2 and 4 those are observed as doublet due to the JHeP
coupling (coupling constants are in the range of 2.0e3.6 Hz). Due to
This reaction could also be done in chlorinated solvents like
dichloromethane at room temperature. But at room temperature
the reaction yield was highly compromised. In aromatic solvents
4
the long range JHeSn coupling, the satellite peaks (having the
coupling constants of 29e42 Hz) appear in the 1H NMR spectra of
Scheme 1. Insertion Reaction of SnCl2 across [Cp*IrCl2]2.
Scheme 2. Insertion reaction of SnCl2 across [Cp*Ir(PPh3)Cl2].