Investigating the Microstructure of Poly(cyclosilane) by ²⁹Si Solid-State NMR Spectroscopy and DFT Calculations

Abstract

Using high-resolution magic-angle spinning (MAS) solid-state NMR spectroscopy and density-functional theory (DFT) calculations, we determine the microstructure of the silicon-based functional polymer poly(1,4Si6) arising from the dehydrocoupling polymerization of cyclosilane 1,4Si6. 1H-29Si refocused-INEPT experiments allow the unambiguous determination of the number of attached protons to a silicon atom for each 29Si signal in 1,4Si6 and poly(1,4Si6). One-dimensional 1H→29Si cross-polarization MAS (CPMAS) spectra of poly(1,4Si6) show the development of SiH resonances upon polymerization and peak integration indicates an average degree of polymerization of 20. The 1H→29Si CPMAS spectrum of poly(1,4Si6) also shows two sets of isotropic signals, suggesting the presence of at least two distinct species. Two-dimensional 29Si dipolar double-quantum-single-quantum and single-quantum-singlequantum homonuclear correlation spectra reveal similar connectivity in the two species, pointing to stereochemical and/or conformational heterogeneity. DFT calculations on trimer models predict that chair or twist-boat conformations and with cis or trans diastereomers are all energetic minima. 29Si chemical shift calculations of the lowest energy structures show that conformers and stereoisomers are expected to give rise to distinct 29Si NMR peaks and likely explain the appearance of multiple sets of 29Si NMR signals. The strategy outlined here is expected to be widely useful for the microstructural elucidation of silicon-based functional polymers. Disciplines Disciplines Materials Chemistry Comments Comments

Highly selective addition of cyclosilanes to alkynes enabling new conjugated materials

“…Our group is interested in the synthesis and properties of (macro)molecular mimics of crystalline silicon and we have described new approaches to the selective synthesis of materials based on cyclohexasilanes. We reported the ve-step synthesis and chemoselective polymerization of the cyclohexasilane building blocks 1,4Si 6 and 1,3Si 6 , [16][17][18][19][20][21] as well as the seven-step stereocontrolled synthesis of cis-and trans-Si 10 H 4 (Fig. 1).…”

Optimisation of 1H PMLG homonuclear decoupling at 60 kHz MAS to enable 15N–1H through-bond heteronuclear correlation solid-state NMR spectroscopy

“…Analogously to the case of 29 Si– 1 H J -couplings in SiH n moieties, 82–84 there is a different dependence on the first spin-echo duration, τ 1 , for a NH 3 moiety:(NH 3 ) sin(2π J IS τ 2 )[sin(2π J IS τ 1 ) + sin(6π J IS τ 1 )]As discussed below, a consequence of this is that different signal build-up with respect to τ 1 for a NH and a NH 3 moiety (and also for a NH 2 which has a sin(2π J IS τ 2 )sin(4π J IS τ 1 ) dependence.…”

“…1b was recorded with t 2 (1.0 ms) shorter than t 1 (2.1 ms); as discussed further below, note that 1 H homonuclear decoupling scales the J coupling. [79][80][81] Analogously to the case of 29 Si-1 H J-couplings in SiH n moieties, [82][83][84] there is a different dependence on the first spin-echo duration, t 1 , for a NH 3 moiety:…”

“…Such an experiment could hence be used to distinguish different NH x moieties, as for example has been demonstrated analogously for SiH x groups. [82][83][84] Furthermore, windowed PMLG5 xx mm was employed to record a 2D 15 N-1 H CP-refocused INEPT spectrum of the pharmaceutical cimetidine at natural abundance (Fig. 9d), for which 1 H, 15 N CPMAS and 14 15 N-1 H heteronuclear spin echo with 1 H homonuclear decoupling to demonstrate spectral editing.)…”

Isolation of a Cyclopentasilane from Magnesium Reduction of a Linear Hexasilane

“…[15] Towards the goal of uncovering new nanostructured materials that combine these compelling properties with the processability and tunability of organic conjugated polymers, we reported the synthesis of organosilicon polymer mimics of the crystalline silicon lattice such as polycyclosilane poly(1,4Si 6 ) (Scheme 1a). [16] Key to this work is the synthesis of multifunctional cyclosilanes [17,18] amenable to dehydrocoupling [19][20][21][22][23] or reductive polymerization. [24] The rational design and synthesis of new building blocks could further expand the structural and functional diversity of polycyclosilanes and other nanostructured silanes.…”