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

Abstract

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.

1. Introduction

Mono(silyl)palladium(II) halide species are purported intermediates in a number of catalytic routes towards allylsilanes [1,2]. Palladium pincer chemistry accounts of such complexes are rather numerous, although examples of their isolation in this catalytic cycle are rare [3,4,5,6,7]. Trans-[PdCl(SiF₂Ph)(L)₂] (L = PMe₃, PMe₂Ph or PMePh₂) and allyl bromide were shown to react to afford trans-[Pd(L)₂(SiF₂Ph)(Br)] and the corresponding allylsilane [1], and [(^tBuPAr₂)Pd(SiMe₃)(I)] (Ar = 3,5-Me₂-4-OMe-C₆H₂) was synthesized from stoichiometric quantities of [(cod)Pd(CH₂SiMe₃)₂] (cod = 1,5-cyclooctadiene), ^tBuPAr₂ and Me₃SiI [2]. Analogues have been used in silyl-Negishi couplings [8]. We wish to report here our preliminary findings on the reaction of (ITMe)₂Pd(silyl)₂ complexes with allyl bromide (ITMe = 1,3,4,5-tetramethylimidazol-2-ylidene) [9,10,11,12].

2. 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 ¹H 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. Crystal data and structure refinement for 2b.

Empirical formula	C22H35BrN4PdSi
Formula weight	569.94
Temperature/K	173
Crystal system	orthorhombic
Space group	P212121
a/Å	10.5467(4)
b/Å	14.3455(3)
c/Å	16.7301(4)
α/°	90
β/°	90
γ/°	90
Volume/Å3	2531.23(13)
Z	4
ρcalcg/cm3	1.496
μ/mm−1	2.374
F(000)	1160.0
Crystal size/mm3	0.22 × 0.2 × 0.15
Radiation	MoKα (λ = 0.71073)
2θ range for data collection/°	6.836 to 52.744
Index ranges	−13 ≤ h ≤ 8, −17 ≤ k ≤ 11, −14 ≤ l ≤ 20
Reflections collected	7612
Independent reflections	4799 [Rint = 0.0320, Rsigma = 0.0568]
Data/restraints/parameters	4799/0/272
Goodness-of-fit on F2	1.018
Final R indexes [I >= 2σ (I)]	R1 = 0.0322, wR2 = 0.0614
Final R indexes [all data]	R1 = 0.0376, wR2 = 0.0640
Largest diff. peak/hole/e Å−3	0.51/−0.34
Flack parameter	0.004(8)
CCDC deposition number	2076437

).

The carbenic carbon-Pd bond lengths in 2b [2.028(5) and 2.025(5) Å] are significantly shorter than in cis-[Pd(ITMe)₂(SiMe₂Ph)₂] [2.105(3) and 2.123(3) Å], suggesting SiMe₂Ph exhibits a stronger trans-influence than ITMe [15]. The decreased length of the Pd-Si bond in 2b [2.2948(18) Å] versus cis-[Pd(ITMe)₂(SiMe₂Ph)₂] [2.3445(8) and 2.3346(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 < 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 S_N2/S_N2′ 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].

3. 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 N₂ (O₂ < 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) (¹H 399.5 MHz; ¹³C{1H} 100.5 MHz; ¹¹B{1H} 128.2 MHz; ¹⁹F 375.9 MHz; ²⁹Si{¹H} 79.4 MHz), or 500 (¹H 499.9 MHz; ¹³C{¹H} 125.7 MHz). Chemical shifts are reported in ppm. All other experimental details are outlined elsewhere [10].

Synthesis of trans-[Pd(ITMe)₂(SiMe₃)(Br)] (2a) and Allyltrimethylsilane (3a)

Allylbromide (0.032 g, 0.26 mmol) was added to a solution of cis-[Pd(ITMe)₂(SiMe₃)₂] (0.043 g, 0.09 mmol) in C₆D₆ 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).

2a, Yield: 0.040 g, 92%. ¹H NMR (399.5 MHz, C₆D₆): δ_H = 3.68 [s, 12H, N(1,3)-CH₃], 1.42 [s, 12H, C(4,5)-CH₃], 0.12 [s, 9H, SiMe₃]. ¹³C{¹H} NMR (100.5 MHz, C₆D₆): δ_C = 184.9 [NCN], 124.0 [C(4,5)-CH₃], 35.1 [N(1,3)-CH₃], 8.5 [C(4,5)-CH₃], 6.9 [SiMe₃]. ²⁹Si{¹H} NMR (79.4 MHz, C₆D₆): δ_Si = 7.68. Elem. Anal. Calcd. for C₁₇H₃₃N₄SiBrPd: C, 40.20%; H, 6.55%; N, 11.03%. Found: C, 40.15%; H, 6.54%; N, 10.95%. 3a (from crude reaction solution), ¹H NMR (399.5 MHz, C₆D₆): δ_H = 5.77 [m, 1H, CH=], 4.92 [m, 1H, CH=], 4.89 [m, 1H, CH=], 1.44 [m, 2H, CH₂], −0.03 [s, 9H, SiMe₃]. [Agrees with an independently taken ¹H NMR sample of commercially available allyltrimethylsilane].

Synthesis of trans-[Pd(ITMe)2(SiMe2Ph)(Br)] (2b)

Allybromide (6.0 µL, 0.07 mmol) and cis-[Pd(ITMe)₂(SiMe₂Ph)] (0.021 g, 0.03 mmol) were dissolved in C₆D₆ or toluene (1.0 mL). The resulting reaction mixture was stirred at room temperature for 2 h under an N₂ atmosphere. At this stage, all volatiles were removed in vacuo and the resulting white solid was washed with hexane (3 × 2.0 mL). Yield: 0.018 g, 93%. ¹H NMR (399.5 MHz, C₆D₆): δ_H = 7.20 [m, 2H, SiMe₂Ph], 7.07 [m, 3H, SiMe₂Ph], 3.51 [s, 12H, N(1,3)-CH₃], 1.42 [s, 12H, C(4,5)-CH₃], 0.31 [s, 6H, SiMe₂Ph]. ¹³C{¹H} NMR (100.5 MHz, C₆D₆): δ_C = 183.4 [NCN], 149.6 [SiMe₂i-Ph], 133.1 [SiMe₂Ph], 127.0 [SiMe₂Ph], 126.5 [SiMe₂p-Ph], 124.2 [C(4,5)-CH₃], 34.9 [N(1,3)-CH₃], 8.5 [C(4,5)-CH₃], 4.2 [SiMe₂Ph]. ²⁹Si{¹H} NMR (79.4 MHz, C₆D₆): δ_Si = 2.44. (It was not possible to obtain elemental analysis for 2b– every attempt resulted in numbers that were inconsistent with calculated values. A possible reason for this is decomposition of 2b by exposure to air or moisture on transit to data collection).

Crystal data for 2b: C₂₂H₃₅N₄SiBrPd, M_r = 569.94 g mol⁻¹, orthorhombic, space group P2 = 2₁2₁, a = 10.5467(4) Å, b = 14.3455(3) Å, c = 16.7301(4) Å, α = 90°, β = 90°, γ = 90°, V = 2531.23(13) ų, Z = 4, T = 173 K, λMo(Kα) = 0.71073, R₁ [I > 2σ(I)] = 0.0345, wR₂ (all data) = 0.0677, GooF = 1.011.

Crude ¹H 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].

4. Conclusions

Under mild conditions, non-pincer bis(NHC)(silyl)palladium halide complexes of the type trans-[Pd(ITMe)₂(SiR₃)(Br)] (SiR₃ = SiMe₂Ph (2a), and SiMe₃ (2b)) were synthesized, by the reaction of allylbromide with the corresponding complexes cis-[Pd(ITMe)₂(SiR₃)₂], 1a or 1b, respectively. A possible mechanistic route for the formation of 2 involves either a σ-bond metathesis or an S_N2/S_N2′ 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(SiR₃)(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 ITMe₂Pd-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.