The tridentate preligand HSiMe(o-C6H4SMe)2 (L2) was used as a silane substrate for the
dehydrogenative coupling reaction. Initially, Wilkinson’s complex [RhCl(PPh3)3] was used as the
catalyst for this process. The tertiary silane L2 was dissolved in 1 mL of toluene at 110 °C with a catalytic amount of [RhCl(PPh3)3] (5 mol %). The formation of the dehydrocoupling product, (o-
C6H4SMe)2MeSi-SiMe(o-C6H4SMe)2, was quantitatively determined by NMR spectroscopy after
24 hours of reaction (Scheme 32).
Scheme 32. Formation of the disilane by dehydrogenative coupling catalysed by [RhCl(PPh3)3].
The formation of the disilane can be confirmed by 1H NMR. The spectroscopic data show the signals for the two SiMe groups and four SMe moieties at 1.29 and 2.51 ppm respectively. In the 19Si NMR spectrum, the disilane exhibits a multiplet at -31.0 ppm (J
H-Si = 7 Hz), in a similar
position to that showed by the L2 monosilane (δ = -31.1 ppm, JH-Si = 202 Hz,JH-Si = 7 Hz). The
lack of a JH-Si coupling costant in the 19Si NMR spectrum for the disilane product indicates the
absence of a Si-H bond (Figure 31). The ESI-MS of the disilane shows a molecular ion at 579.16 m/z, corresponding to the [Disilane + -H+]± fragment. The IR spectrum of the product excludes
84 the formation of the disiloxane product (o-C6H4SMe)2MeSi-O-SiMe(o-C6H4SMe)2, as the
characteristic ν(SiO) band around 1000 cm-1 is not observed. Observation of disiloxanes as
products or side products in dehydrogenative coupling of silanes is relatively common.97
Figure 31. Comparation of 19Si NMR spectra of L2 and the disilane product.
Monitoring of the catalytic reaction by NMR spectroscopy allowed the identification of intermediate organometallic species. The reaction was stopped after 16 hours in solution, and the organic phase containing the disilane product was separated by extraction with pentane. The 1H NMR spectrum of the organic phase showed a mixture of the L2 substrate and the disilane product in a 75 % of conversion. The organometallic phase revealed the presence of the neutral [RhClH(SiMe(o-C6H4SMe)2)(PPh3)] (5) complex, already described in this work,
along with other unidentified species in a minor proportion (Figure 32). It is known that complex 5 is synthesised by oxidative addition of the tertiary L2 silane to the rhodium(I) centre of the [RhCl(PPh3)3] complex. This fact suggests the involvement of a Si-H activation step in the
reaction mechanism. 29Si{1H} of L2 29Si of L2 29Si of the disilane product (P) 1J H-Si= 202 Hz 2J H-Si= 7 Hz no 1J H-Si 2J H-Si= 7 Hz
85
Figure 32.1H NMR spectra of theorganic (top) and organometallic (bottom) phases after 16 hours of catalysis with [RhCl(PPh3)3].
Therefore, we decided to evaluate this silyl-hydride rhodium(III) complex 5 as a catalyst for the dehydrocoupling of tertiary silane L2. The performance of the unsaturated rhodium(III) compound [RhH(SiMe(o-C6H4SMe)2)(PPh3)]BArF4 (6) was also tested, with the purpose of
studying the effect of a coordinative vacancy in the catalytic activity of the complex. The results obtained with these two rhodium(III) complexes were compared to the activity exhibited by Wilkinson’s catalyst under the same catalytic conditions. Table 1 shows the collection of these results. As a control reaction, a solution of L2 in toluene was heated at 110 °C in the absence of a catalyst, and silane coupling products were not observed (Table 1, entry 1).
86 When using Wilkinson’s catalyst (Table 1, entry 2), a 95 % of conversion was reached. The use of neutral compound 5 as a catalyst for the dehydrogenative coupling of L2 afforded similar results, with a conversion of >99 % (Table 1, entry 3). With cationic compound 6, however, a decrease in the catalytic activity was observed, reaching only a 60 % of conversion (Table 1, entry 4). This reduction could indicate a deactivation of the catalyst when the rhodium complex is unsaturated. To give insight into the possible deactivation mechanism, rhodium(III) complexes 5 and 6 were treated with L2 in an equimolecular reaction.
Unsaturated compound 6 reacts with an equivalent of L2 in toluene at 110 °C for 2 hours, to yield the stable rhodium(III) compound [Rh(SiMe(o-C6H4SMe)2)2]BArF4 (21), with two units of
the tertiary silane derived from L2 coordinated as a tridentate ligand, with the loss of dihydrogen and PPh3 (Scheme 33). The disilyl-rhodium(III) species 21 showed no activity as a
catalyst for the dehydrocoupling of L2 (Table 1, entry 5). The formation of this inactive species would consequently be the cause of the decrease in the catalytic activity of compound 6. The reaction of neutral complex 5 with an extra equivalent of L2 failed to afford 21.
Scheme 33. Formation of compound 21.
Cationic complex 21 was characterised in solution by NMR spectroscopy and ESI-MS. The two [SiMe(o-C6H4SMe)2]– units are equivalent in solution, and showed three singlet signals in the 1H
NMR spectrum at 0.27, 2.46 and 2.90 ppm, with a relative integral of 6H, corresponding to the two SiMe and the four SMe groups. The presence of a molecular ion at m/z = 681.01 in the ESI- MS corresponds to the [Rh(SiMe(o-C6H4SMe)2)2]+ fragment.
Isolation of single crystals of 21 allowed the determination of the solid state structure by X-Ray diffraction. Although the low quality of the data did not permit the discussion of bond distances and angles, the disposition of the ligands and the geometry adopted around the metal centre can be analysed and confirm the proposed structure (Supporting Information). The geometry around the Rh(III) centre is pseudo octahedral, with the two terdentate ligand
87 units coordinated through the silicon and sulphur atoms, each with a facial disposition. The two silicon atoms are located mutually cis.
To prove the importance of the additional donor functions exhibited by the L2 hydrosilane in the catalysis, an alkyl tertiary hydrosilane without an additional functional group was also tested as substrate under the same reaction conditions. Entries 6 and 7 in Table 1 show treatment of Et3SiH with [RhCl(PPh3)3] and complex 5 respectively, which resulted in no
conversion to the disilane product.
Table 1. Rhodium catalysed dehydrocoupling of silanes.
Entry [Rh] Catalyst Substrate[a] Conversion to disilane [%][b]
1 - L2 0 2 [RhCl(PPh3)3] L2 95 3 5 L2 >99 4 6 L2 60 5 21 L2 0 6 [RhCl(PPh3)3] Et3SiH 0 7 5 Et3SiH 0
[a] Catalytic conditions: Silane substrate (0.1 mmol), [Rh] catalyst (0.005 mmol) in 1 mL of toluene at 110 °C for 24 h. [b] Conversions were determined by 1H RMN spectroscopy.
On the basis of these results, a simplified mechanism and a deactivation process for the rhodium catalysed dehydrocoupling of L2 has been proposed. Addition of the L2 substrate to [RhCl(PPh3)3] allows the Si-H activation of the silane, leading to the formation of silyl-hydride
compound 5 by chelate assisted oxidative addition. Compound 5 can be considered to be the active species in the catalysis. In a second step, we propose a σ-bond metathesis route after the formation of 5, with the involvement of a four-centre transition state that would allow the formation of the Si-Si bond.98 The presence of the chloride ligand in the saturated complex would prevent formation of the inactive species 21 via a second Si-H activation and the
88 subsequent inhibition of the disilane product formation by reductive elimination under these catalytic conditions.99 Finally, the reaction of a new molecule of L2 with the dihydride-Rh(III)
compound resulting in the previous step would result in the regeneration of the catalytic species 5. Unsaturated compound 6 would follow the same σ-bond metathesis pathway for the formation of the disilane product, but the reaction of 6 with a second L2 molecule would result into the inactive species 21 (Scheme 34).
Scheme 34. Proposed mechanism for the disilane product formation and deactivation process.
II. 1.3.3. Tandem isomerisation-hydrosilylation of alkenes