Synthesis of trans-Mono(silyl)palladium(II) Bromide Complexes

The stoichiometric reaction of cis-[Pd(ITMe)2(SiR3)2], where (SiR3 = SiMe3 and SiMe2Ph and ITMe = 1,3,4,5-tetramethylimidazol-2-ylidene) with allyl bromide affords the corresponding allylsilanes along with complexes of the type trans-[Pd(ITMe)2(SiR3)(Br)]. The structure of trans-[Pd(ITMe)2(SiMe2Ph)Br] 2b has been determined in the solid state and displays a slightly distorted square-planar geometry with the two N-heterocyclic carbene ligands in a trans-configuration.


Results and Discussion
The bis(silyl)palladium complexes, cis-[Pd(ITMe)2(SiR3)2] (1a: SiR3 = SiMe3 and 1b: SiMe2Ph [13,14] were reacted with excess allylbromide at room temperature under an nitrogen atmosphere to yield trans-[Pd(ITMe)2(SiMe3)(Br)] 2a and trans-[Pd(ITMe)2(SiMe2Ph)Br] 2b in 92 and 93% yield, respectively (Scheme 1). Reaction progress was monitored by 1 H NMR spectroscopy. Characteristic resonances corresponding to silanes 3a and 3b were observed (in a 1:1 stoichiometry with 2a/2b, respectively, upon examination of the crude mixtures).   In order to further characterize the organometallic complexes, single crystals of 2b suitable for X-ray analysis were grown by slow evaporation of a saturated deuterated benzene solution at room temperature. X-ray analysis revealed that 2b displays a marginally distorted square-planar geometry with the two NHCs in a trans-configuration and orthogonal to the Br-Pd-Si plane ( Figure 1, Table 1). In order to further characterize the organometallic complexes, single crystals of 2b suitable for X-ray analysis were grown by slow evaporation of a saturated deuterated benzene solution at room temperature. X-ray analysis revealed that 2b displays a marginally distorted square-planar geometry with the two NHCs in a trans-configuration and orthogonal to the Br-Pd-Si plane ( Figure 1, Table 1). Based on these data, the intensity of the trans-influence in these two structures follows the sequence: Br < ITMe < SiMe2Ph. Thus, the preference for the trans-configuration observed in 2b may be attributed to the high trans-influence of SiMe2Ph and the large steric size of Br.
A possible mechanism for the formation of 2 includes either a σ-bond metathesis between a Pd-Si, in cis-[Pd(ITMe)2(SiR3)2], and Br-C bond, in allylbromide, or an SN2/SN2′ by the nucleophilic Pd-Si bond at the electrophilic sites in the allyl halide, leading to a trans complex. As we have previously suggested using computational studies on related bis-ITMe complexes, an NHC would then dissociate from the palladium center followed by a cis to trans isomerization of the Br and Si moieties (Scheme 2) [11]. Finally, the dissociated NHC would re-coordinate, constrained by the bulk of the other ligands, in a cis-configuration [16,17].    (8) Å] infers a stronger Pd-Si bond in 2b and demonstrates the weak trans-influence of Br. Based on these data, the intensity of the trans-influence in these two structures follows the sequence: Br < ITMe < SiMe 2 Ph. Thus, the preference for the trans-configuration observed in 2b may be attributed to the high trans-influence of SiMe 2 Ph and the large steric size of Br.
A possible mechanism for the formation of 2 includes either a σ-bond metathesis between a Pd-Si, in cis-[Pd(ITMe) 2 (SiR 3 ) 2 ], and Br-C bond, in allylbromide, or an S N 2/S N 2 by the nucleophilic Pd-Si bond at the electrophilic sites in the allyl halide, leading to a trans complex. As we have previously suggested using computational studies on related bis-ITMe complexes, an NHC would then dissociate from the palladium center followed by a cis to trans isomerization of the Br and Si moieties (Scheme 2) [11]. Finally, the dissociated NHC would re-coordinate, constrained by the bulk of the other ligands, in a cis-configuration [16,17].

Experimental
The handling of air-sensitive compounds and their spectroscopic measu were undertaken using standard Schlenk line techniques using pre-dried Ar BASF R3-11(G) catalyst and 4 Å molecular sieves), or in a MBraun glovebox unde < 10.0ppm). All glassware was dried in a 160 °C oven prior to use. Celite was pre a 200 °C oven and then dried with a heat gun under a dynamic vacuum prior to u cannulae equipped with microfiber filters were dried in an oven at 160 °C prio Scheme 2. Possible mechanistic routes for the formation of 2.

Experimental
The handling of air-sensitive compounds and their spectroscopic measurements were undertaken using standard Schlenk line techniques using pre-dried Ar (using a BASF R3-11(G) catalyst and 4 Å molecular sieves), or in a MBraun glovebox under N 2 (O 2 < 10.0 ppm). All glassware was dried in a 160 • C oven prior to use. Celite was predried in a 200 • C oven and then dried with a heat gun under a dynamic vacuum prior to use. Filter cannulae equipped with microfiber filters were dried in an oven at 160 • C prior to use. Solvents employed in air-sensitive reactions were dried using vacuum distillation, followed by distillation over potassium or stored over activated 4 Å molecular sieves under an Ar atmosphere. NMR spectra were recorded on a Varian VNMRS 400 (Palo Alto, CA, USA) ( 1 H 399.5 MHz; 13 [10].

Synthesis of trans-[Pd(ITMe) 2 (SiMe 3 )(Br)] (2a) and Allyltrimethylsilane (3a)
Allylbromide (0.032 g, 0.26 mmol) was added to a solution of cis-[Pd(ITMe) 2 (SiMe 3 ) 2 ] (0.043 g, 0.09 mmol) in C 6 D 6 or toluene (3.0 mL) and the resulting reaction mixture was stirred at room temperature for 1.5 h. At this stage, the volatiles were removed in vacuo and the off-white powder was washed with hexane (3 × 4.0 mL). Crude 1 H NMR data are consistent with the formation of allyldimethylphenylsilane (3b) as a product of this reaction. However, this was not isolated in this instance [18].

Conclusions
Under mild conditions, non-pincer bis(NHC)(silyl)palladium halide complexes of the type trans-[Pd(ITMe) 2 (SiR 3 )(Br)] (SiR 3 = SiMe 2 Ph (2a), and SiMe 3 (2b)) were synthesized, by the reaction of allylbromide with the corresponding complexes cis-[Pd(ITMe) 2 (SiR 3 ) 2 ], 1a or 1b, respectively. A possible mechanistic route for the formation of 2 involves either a σ-bond metathesis or an S N 2/S N 2 reaction between allybromide and 1. This would necessitate a cis-trans isomerization via dissociation of an NHC ligand- [19]. The reactivity of trans-[Pd(ITMe) 2 (SiR 3 )(Br)] is unexplored but will soon be carried out. The facile formation and apparent stability of trans-2 may indeed hinder the catalytic silylation of ally halides mediated by ITMe 2 Pd-based complexes since the adoption of a cis-configuration is a prerequisite for reductive elimination and involvement in a catalytic cycle. Solutions to these unexplored questions are currently being sought, e.g., the potential for halide abstraction, and will be reported in due course.