Zinc Imine-POSS Quattro-Site Catalyst for CO2 Cycloaddition

The publication titled “Zinc Imine Polyhedral Oligomeric Silsesquioxane as a Quattro-Site Catalyst for the Synthesis of Cyclic Carbonates from Epoxides and Low-Pressure CO₂” by Mateusz Janeta et al., published in Chemistry – A European Journal in 2020, describes the first structurally characterized polyhedral oligomeric silsesquioxane (POSS) that anchors more than one metal cation through its organic side arms. The tetranuclear zinc imine-POSS complex Zn₄@POSS-1 converts styrene oxide into styrene carbonate with 96% conversion in four hours at only 1 atm of CO₂, eliminating the high carbon dioxide pressures that normally accompany this transformation. The four zinc centers act cooperatively rather than independently, and the catalyst tolerates five consecutive runs with conversion falling only from 99% to 90%.

Polyhedral oligomeric silsesquioxanes are cage-like organosilicon compounds of general formula (RSiO_3/2)_n whose diameters of 1 to 3 nm make them the smallest existing silica nanoparticles. Metal complexes of silsesquioxanes have attracted sustained interest as soluble models for heterogeneous catalysis and as homogeneous catalysts in their own right, but until this work every structurally characterized example fell into one of two categories: a metallated T7 fragment of the type R₇Si₇O₁₂M, or a T8 cage carrying seven inert organic substituents and a single metallated site on one outside arm. No well-defined T8-POSS coordinating more than one metal cation through its side arms had been reported. That gap is what this study closes.

Fig. 1. Previously reported classes of catalytically active metal silsesquioxane complexes. (a) T7 systems in which the metal binds directly to the silsesquioxane core, illustrated for iron and ruthenium. (b, c) T8 cages carrying a single metallated site, shown for aluminum and palladium. In every case only one metal center is present. Reproduced from Janeta et al., Chem. Eur. J. 2020, 26, 13686 (Wiley-VCH).

The synthesis proceeds in two steps under mild conditions. Condensation of octa(3-aminopropyl)silsesquioxane hydrochloride with 3,5-di-tert-butyl-2-hydroxybenzaldehyde and triethylamine in a dichloromethane and methanol mixture at room temperature affords the octa-imino-functionalized ligand POSS-1 in 91% yield. In solution the ligand possesses O_h symmetry, evidenced by a single imine proton resonance at δ = 8.33 ppm, a phenol O–H signal at 13.95 ppm, and a single ²⁹Si NMR resonance at δ = −66.6 ppm. High-resolution electrospray mass spectrometry gave m/z 2610.4979 for [M + H]⁺ against a calculated 2610.4984, confirming the intact closed cage bearing eight terminal alkyl-imine arms. The ligand belongs to the same family of imine-functionalized POSS nanobuilding blocks previously shown to assemble into three-dimensional supramolecular networks.

Metalation of POSS-1 with zinc(II) acetate, or alternatively with diethylzinc in toluene at −78 °C under dinitrogen, delivers the tetranuclear complex Zn₄@POSS-1 in high yield. The stoichiometry is strikingly rigid: reacting the ligand with five to eight equivalents of diethylzinc still yields exclusively the tetrazinc species, never a more highly metallated product. Coordination through the imine nitrogen and phenoxo oxygen was confirmed by the disappearance of the broad hydroxyl resonance and the shift of the CH=N signals to 8.17 and 8.10 ppm, and in the DRIFT spectrum by new bands at 1531 and 1434 cm⁻¹ assigned to the coordinated ν_C–O mode together with Zn–O and Zn–N vibrations at 539 and 517 cm⁻¹. Both solution and CP-MAS ²⁹Si NMR gave a single symmetric resonance at −66.8 ppm, establishing that the T8 cage survives complexation without rearrangement.

Fig. 2. Single-crystal X-ray structure of Zn₄@POSS-1. Each zinc center is ligated by two imine nitrogen atoms and two phenolate oxygen atoms drawn from two neighboring iminophenolate side arms, each arm acting as a bidentate (κ²) ligand. Two zinc atoms adopt the Δ configuration and two the Λ configuration, giving the overall meso-(Δ,Δ,Λ,Λ) arrangement and D_4h molecular symmetry. The inset at right details the distorted tetrahedral coordination sphere. Reproduced from Janeta et al., Chem. Eur. J. 2020, 26, 13686 (Wiley-VCH).

Single crystals suitable for diffraction were grown by slow evaporation of a chloroform solution. Zn₄@POSS-1 crystallizes in the monoclinic system, space group C2/c, as a neutral four-coordinate tetranuclear complex. Zn–O bond distances span 1.914(5) to 1.924(4) Å and Zn–N distances 1.979(6) to 2.016(5) Å, both typical of zinc(II) Schiff base complexes. The Si–O distances of 1.611(5) to 1.633(5) Å and O–Si–O angles of 107.8(2) to 109.9(2)° remain within the standard T8 range, although the Si–O–Si angles widen considerably to 139.6(3) to 167.3(3)° under the strain imposed by the cuboidal geometry. Steric bulk from the tert-butyl groups blocks rotation about the zinc atoms and locks the complex against isomerization, which is why a single diastereomer is obtained.

Fig. 3. Crystal structure of Zn₄@POSS-1⋅10CHCl₃. (a) Chloroform molecules trapped in the cavity formed by the side arms, held by C–H⋅⋅⋅π and C–H⋅⋅⋅O interactions. (b) Space-filling representation. (c) Surface plot of the interconnected torus-like channels running along the b axis, shown for a 1.5 × 1.5 × 1.5 array of unit cells. The chloroform contact surface occupies 32.3% of the unit cell volume (19 683 ų), with a solvent-accessible pore volume of 7.6% (1 489 ų). Reproduced from Janeta et al., Chem. Eur. J. 2020, 26, 13686 (Wiley-VCH).

The complex co-crystallizes with ten chloroform molecules trapped between the side arms and bound through C–H⋅⋅⋅π and C–H⋅⋅⋅O interactions, forming an interconnected torus-like aggregate. Weak cooperative contacts including CH₂⋅⋅⋅H₃C at 2.146 Å and Si⋅⋅⋅HC at 3.149 Å extend the assembly into a three-dimensional network. The resulting cavities measure roughly 12 Å in diameter, and the separation between adjacent zinc atoms is 11 Å, close enough for the two centers to act on a single substrate molecule. This architectural detail turns out to govern both the catalytic mechanism and the substrate selectivity described below.

Catalytic testing used styrene oxide, industrially the most demanding substrate for cyclic carbonate synthesis on account of its low reactivity and selectivity. Under 1 atm CO₂ at 100 °C with 1 mol% catalyst and tetrabutylammonium iodide as co-catalyst, conversion reached 96% in four hours with a turnover frequency of 25 h⁻¹. Neither component alone shows activity: the complex without co-catalyst gives no conversion, and the co-catalyst without the complex gives 1%. The halide identity proves decisive, with conversions of 6% for fluoride, 10% for chloride, 40% for bromide, and 96% for iodide, tracking the nucleophilicity and leaving group ability that govern the ring-opening and ring-closing steps. Raising the temperature to 130 °C lifted conversion to 98%, while reducing catalyst loading to 0.1 mol% still delivered 95% conversion when the reaction ran for twelve hours.

The question of whether the four zinc centers operate independently or in concert was settled by varying the co-catalyst loading. Independent sites would each require a full equivalent of tetrabutylammonium iodide, whereas cooperative sites need less. Halving the loading to 0.5 equivalents per zinc retained 90% conversion and quartering it to 0.25 equivalents still gave 85%, pointing clearly to cooperative action in which one zinc center activates the epoxide as a Lewis acid while a neighboring center binds the carbamate formed in situ from carbon dioxide. The comparison with a mononuclear analogue is decisive: under identical conditions the bis(salicylaldimine) zinc(II) reference complex converted only 60% of styrene oxide, rising to just 70% at 130 °C. The same intramolecular cooperative principle was later exploited in difluoroboron-functionalized POSS photocatalysts for the aerobic oxidation of sulfides.

Across a panel of terminal epoxides including 1-chloro-2,3-epoxypropane, 1,2-epoxybutane, 1,2-epoxyhexane, and 1,2-epoxy-3-phenoxypropane, isolated yields of 85 to 99% were obtained under the same mild conditions, with the catalyst tolerating halide and ether functionality and producing no polycarbonate or hydrolysis byproducts. The internal epoxide cyclohexene oxide, by contrast, gave only 5% yield. That sharp discrimination is a direct consequence of the crystallographically observed architecture: the bulky silsesquioxane core sits above the active site region and admits substrates only through a deep, narrow channel, which favors small unhindered epoxides and suggests the reaction occurs inside the cavities. Thermogravimetric analysis places the 5% weight loss temperature of desolvated Zn₄@POSS-1 at 346 °C under nitrogen and 334 °C in an oxidative atmosphere, consistent with the thermal robustness expected of a rigid siloxane cage.

Fig. 4. Overall transformation catalyzed by Zn₄@POSS-1. The four zinc centers, held in a fixed three-dimensional arrangement by the rigid Si₈O₁₂ core and the sterically demanding tert-butyl-substituted iminophenolate arms, cooperatively convert terminal epoxides and carbon dioxide into the corresponding cyclic carbonates at 1 atm pressure. Reproduced from Janeta et al., Chem. Eur. J. 2020, 26, 13686 (Wiley-VCH).

Recyclability and structural integrity were confirmed over five consecutive runs, with conversion declining only from 99% to 90%. NMR and mass spectrometry recorded after the fifth cycle matched the spectra of the fresh complex, and inductively coupled plasma optical emission spectroscopy of the filtered reaction mixture detected below 8 ppm of zinc, indicating negligible leaching. Set against amino- and iminophenolate complexes of zinc, aluminum, manganese, iron, cobalt, and chromium reported previously, which typically demand carbon dioxide pressures up to 30 bar and reaction times reaching 48 hours, the atmospheric-pressure four-hour operation of this system represents a substantial practical improvement. For reference, a mononuclear zinc salphen catalyst reaches 66% conversion only after 18 hours.

This work establishes that an octa-imino-functionalized silsesquioxane cage can serve as a rigid multidentate platform holding four catalytically active metal centers in a defined three-dimensional geometry, and that the resulting proximity translates into genuine cooperative catalysis rather than merely additive activity. The combination of atmospheric-pressure operation, short reaction times, a relatively nontoxic metal, solvent-free conditions, and demonstrated recyclability aligns the system with green chemistry objectives for carbon dioxide valorization. More broadly, the study shows that the POSS scaffold functions not only as an inert support but as an active element of catalyst design, shaping substrate access through its own steric bulk, a design principle that continues to be developed in work on octa-functional POSS monomers and their polymer networks.

Full Citation Mateusz Janeta, Tadeusz Lis, and Sławomir Szafert. “Zinc Imine Polyhedral Oligomeric Silsesquioxane as a Quattro-Site Catalyst for the Synthesis of Cyclic Carbonates from Epoxides and Low-Pressure CO₂.” Chemistry – A European Journal, 2020, 26, 13686–13697.
DOI: 10.1002/chem.202002996
Full text: Chemistry – A European Journal → Publisher