Mechanical Activation of Platinum–Acetylide Complex for Olefin Hydrosilylation
Authors: Xiaojun Wu
Zongchun Gao
Huarong Liu
Hangxun Xu
Kai Wei
Feng Wang
Journal: ACS Macro Letters (2017)
Discipline: CHEMISTRY
Keywords: mechanical forces; latent catalysts; polymer mechanochemistry; platinum-based catalysts; mechanophore
Affiliations: Department
of Polymer Science and Engineering, CAS Key Laboratory of Soft Matter
Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, China, Department
of Materials Science and Engineering, CAS Key Laboratory of Materials
for Energy Conversion, Hefei National Laboratory of Physical Sciences
at the Microscale, and CAS Center for Excellence in Nanoscience, University of Science and Technology of China, Hefei, Anhui 230026, China

Abstract:  Harnessing mechanical forces to activate latent catalysts has emerged as a novel approach to control the catalytic reactions in organic syntheses and polymerization processes. However, using polymer mechanochemistry to activate platinum-based catalysts, a class of important organometallic catalysts in industry, has not been demonstrated so far. Here we show that the platinum-acetylide complex is mechanoresponsive and can be incorporated into a polymer backbone to form a new mechanophore. The mechanically induced chain scission was demonstrated to be able to release catalytically active platinum species which could catalyze the olefin hydrosilylation process. Various control experiments were conducted to confirm that the chain scission and catalytic reaction were originated from the ultrasound-induced dissociation of platinum-acetylide complex. This work further exemplifies the utilization of organometallic complexes in design and synthesis of latent catalysts for mechanocatalysis and development of self-healing materials based on silicone polymers. 

Retracted: No

PubPeer comments:
 In the letter by Wei, K. et al., the authors propose the development and demonstration of a new platinum-acetylide mechanophore for use in hydrosilylation chemistry. At first glance, the paper appears to be sound, however, upon critical review I have serious concerns about this letter. These concerns are outlined below. 1) Lack of support for the desired mechanochemical activation of the mechanophore. a)tThe spectroscopic evidence for mechanophore activation suggests the mechanophore has been altered in some manner, yet cleavage of the intended Pt-alkyne bond is unclear. No comparison to any experimental or theoretical results is provided to support that a blue shift in the UV-vis or a loss in PL intensity suggests that the desired bond is cleaved. b)tThe lack of UV-vis signal in the 326 nm GPC trace for the un-sonicated polymer seems unlikely given the spectra shown in Figure S9 (the blue shift only leads to a mild increase in intensity at this wavelength). c)tThe polymer cleaving near the mid-point does not support the desired bond breaking. All polymers cleave near the mid-point (even without a mechanophore).[ref 1] The increase in degradation rate does not also necessarily support the desired bond cleavage. Numerous new bonds were introduced with the Pt-acetylide bonds (i.e. the ATRP initiators). Control polymers could rule out the significance of the other bonds. d)tThe 1H NMR for the sonicated polymers is absent. The aromatic region could show change in the mechanophore aryl peaks for the smaller molecular weight polymers. Additionally 31P NMR might provide better support for change in the electronics of the Pt center. e)tThe theoretical calculations in the SI show an activation barrier of ca. 374 kJ mol (yet the authors write 274 kJ mol). This is generally stronger than C-C bonds and does not support this being a weak bond. Additionally, the theoretically calculated structures after bond cleavage are not shown. 2) Unprecedented and or unclear hydrosilylation mechanism a)tAccording to Figure 2A the polymer should be fully degraded after 24h. This is inconsistent with the hydrosilylation reaction continuing well after this point. Were these experiments conducted with the same sonication conditions? An inadequate description of the experimental protocol is provided. b)tIf the desired Pt(II)-acetylide bond is cleaved heterolytically, the result is a Pt(II) cationic species. If the bond breaks homolytically, the result is a Pt(I) species. To my knowledge, neither of these have been shown to promote the hydrosilylation reaction (which is typically achieved with Pt(0) species[ref 2]). How the mechanophore would further be oxidized is unclear. This topic is ignored in the manuscript. c)tA mechanism for the reaction of the chain-end functionalized polymer should be presented (4% conversion is 15% of the value obtained with the mechano-active polymer and is not an insignificant amount). d)tThe chain-end Pt species has different ligands than the chain-centered Pt. This would undoubtedly lead to different reactivity of the catalysts and is a poor control. A better control would be an acetylide ligand that simply does not have polymer attached to it. e)tThere is no explanation for the sudden rate increase in the hydrosilylation reaction near 40 h (this is also observed for the controls). What happens with longer sonication periods? It seems unlikely that the experiment would be stopped after a dramatic increase in conversion. f)tThe authors claim that this has potential applications in self-healing materials, however, there is no demonstration of this reactivity in the solid-state or gels (where it would be relevant to the proposed application). 3) Inconsistencies with other reports of PMA sonication a)tIt is unclear why the degradation of the polymers is so slow. Similar PMA polymers of equivalent molecular weight (with similar sonication conditions to those reported in this letter) undergo complete degradation within a few hours, not a full day or more.[ref 3] An explanation for this difference is needed and or the experimental protocols may need to be updated to explain the decreased efficiency of sonication. b)tSimilarly, 70 kDa is significantly above the limiting molecular weight for PMA using typical sonication conditions. 50 kDa PMA (for instance) has been shown to readily degrade.[ref 3] An explanation is needed for this difference. c)tIn Figures S15 and S18 the concentration of high molecular weight species has increased based on the RI traces. This suggests there may be issues with the GPC (similarly, the baselines of many of the GPC traces have issues) or that some other reactivity may be taking place. This is not generally observed in well-behaved degradation experiments. d)tIt is unclear why the sonication experiment with a 1:1 mixture of PMA with mechanophore and without mechanophore was completed. A better explanation is needed as this is not a common mechanochemistry control experiment and it is unclear what the authors are trying to probe. References: 1) Caruso, M. M. et al. Chem. Rev. 2009, 109, 5755. 2) Nakajima, Y.; Shimada, S. RSC Adv. 2015, 5, 20603. 3) May, P. A. et al. ACS Macro Lett. 2016, 5, 177. ,