Crystallographically Confirmed Metal-POSS Complexes

Polyhedral oligomeric silsesquioxanes (POSS) have long served as more than a hybrid organic-inorganic filler for polymers. Their partially condensed silanol forms present a rigid, oxygen-rich pocket that closely resembles a fragment of amorphous silica, and this pocket is an excellent ligand for metal ions of nearly every part of the periodic table. Single-crystal X-ray diffraction has been the decisive tool for confirming what these metal-POSS assemblies actually look like, since spectroscopic and computational evidence alone can rarely distinguish between competing cage topologies. This review surveys recent crystallographically confirmed metal-POSS complexes, organized by the metal center, and closes with a synthesis of the structural trends that emerge once the diffraction data are compared side by side.

POSS as a Ligand Platform for Metal Ions

The starting point for nearly every metal-POSS complex discussed below is an incompletely condensed silsesquioxane, most often a trisilanol of general formula R7Si7O9(OH)3, in which R is a bulky organic substituent such as cyclohexyl, isobutyl, or phenyl.¹ Deprotonation of the three remaining hydroxyl groups generates a trianionic, oxygen-donor pocket that reacts with a metal-organic precursor MR'n to cap the open corner of the cage, expelling the R' groups as volatile byproducts and leaving the metal bound in a fashion reminiscent of a tripodal ligand (Figure 1). Depending on the metal and its coordination requirements, this corner-capped unit can be a stable end point in its own right, as is typical for many main-group and alkaline-earth derivatives, or it can undergo further self-assembly into larger cage structures through bridging oxo, hydroxo, or alkoxo groups, as is the case for most of the transition metal and lanthanide complexes discussed below. Because these frameworks so closely mimic isolated surface sites on silica, they have functioned for decades as molecular models for silica-supported catalysts, and single-crystal X-ray diffraction has been essential for relating a given coordination geometry to catalytic behavior observed in solution or in the heterogeneous phase.

Figure 1. General corner-capping strategy for forming a closed-cage metallasilsesquioxane from an open-cage trisilanol and a metal-organic precursor MR'n, where M is a metal from group 1, 2, or 13 of the periodic table, R is an unreactive alkyl or aryl substituent, and R' is an alkyl group or hydrogen.

Nickel, Manganese, Iron, and Vanadium Cages

Beyond copper, the same cage-forming chemistry has been extended to several other transition metals, each contributing distinctive structural motifs. A hexanuclear nickel(II) phenylsilsesquioxane, isolated as a dioxane-benzonitrile-water solvate and studied by X-ray and topological analysis, adopts an unusual cylinder-like architecture in which benzonitrile coordinates directly to the metallasilsesquioxane core, a rare mode of complexation for this ligand class.¹⁴ This complex was also tested as a catalyst for oxidations with peroxides: tert-butyl hydroperoxide converted 1-phenylethanol to acetophenone in 90 percent yield after 24 hours, whereas meta-chloroperoxybenzoic acid oxidized cyclohexane to a cyclohexanone/cyclohexanol mixture in a more modest 24 percent yield, with methylcyclohexane giving a broader spread of isomeric ketones and alcohols in comparable overall yield.¹⁴

Figure 2. X-ray crystal structure of the cylinder-like Ni6 architecture of Bilyachenko et al., Molecules 2016, 21, 665 (ref. 14), rendered directly from the deposited crystal structure. Top view (left) shows the Ni1, Ni2, and Ni3 pairs forming the Ni6O6 ring sandwiched between the two 12-membered siloxane cycles, with the encapsulated chloride anion at the crystallographic center of inversion. Side view (right) shows the resulting cylinder shape, alongside a detail of the distorted octahedral coordination environment at one nickel center.

Manganese chemistry has produced an especially structurally diverse series, beginning with the first heterobimetallic Mn/Na and Mn/Li derivatives reported by Lorenz, Blaurock, and Edelmann in 2008, both confirmed by single-crystal X-ray diffraction with manganese-oxygen bond lengths of 1.985(2) to 2.146(2) angstroms in the Mn/Na cage and a narrower 2.046(3) to 2.048(3) angstrom range in the Mn/Li cage. This early series was purely structural in purpose and was reported without any catalytic testing.¹⁵ More recent manganese cages incorporating 1,10-phenanthroline or bathophenanthroline ligands have revealed a still more unusual feature, the coexistence of sodium, manganese(II), and manganese(III) within a single cage, a consequence of spontaneous oxidation during self-assembly that only diffraction analysis could unambiguously confirm, with the manganese(III)-oxygen bonds consistently 0.2 to 0.3 angstroms shorter than the corresponding manganese(II)-oxygen bonds in the same cage.^16,17 The phenanthroline series was synthesized specifically to evaluate manganese silsesquioxanes as precatalysts for the oxidative amidation of alcohols and aldehydes with tert-butyl hydroperoxide, and at 5 mol percent manganese loading the two simplest cages in the series gave the benzamide product in 84 and 79 percent yield, outperforming a range of simple manganese salts and oxides tested for comparison.¹⁶ The bathophenanthroline series was instead developed as a catalyst for the solvent-free cycloaddition of CO2 to epoxides, reaching a 90 percent yield of cyclic carbonate under standard conditions and up to 99 percent yield after optimizing the reaction temperature and pressure, roughly double the yield obtained with simple manganese chloride under the same conditions, and the same complexes additionally showed antifungal activity against several phytopathogenic Fusarium and related fungal species that exceeded that of the commercial fungicide triadimefon at equal concentration.¹⁷ Iron(III)-based phenylsilsesquioxane/acetylacetonate complexes reported in 2024 show a comparable sensitivity to the identity of the alkali metal template, with lithium, sodium, and potassium each directing the assembly toward a distinct cage size and silsesquioxane ring size, including the first observation of a trimeric silsesquioxane ligand within a metallasilsesquioxane cage.¹⁸ Catalytically, the Fe4Na4 congener stood out among the three, reaching a record 55 percent yield of oxygenates in the peroxidative oxidation of cyclohexane, and the series as a whole also promoted the cycloaddition of CO2 to epoxides, giving cyclic carbonates in 58 to 96 percent yield depending on the substrate.¹⁸

Figure 3 (Fe4Na4 and Fe3K3 renders pending). X-ray crystal structure of the Fe2Li2 cage from the alkali-metal-dependent series of Bilyachenko et al., Inorg. Chem. 2024, 63, 1909-1918 (ref. 18), rendered directly from the deposited crystallographic coordinates (CCDC 2263801). The Fe4Na4 (CCDC 2267402) and Fe3K3 (CCDC 2280386) congeners from the same series will be added as companion panels once rendered.

Vanadium chemistry has taken a different direction altogether, yielding polyoxovanadate clusters protected by silsesquioxane ligands rather than classical cage metallasilsesquioxanes. A fourteen-vanadium cluster represents the highest nuclearity yet reported for this combination, displaying vanadium-oxygen distances of 1.933 to 2.071 angstroms to the silsesquioxane ligands and considerably shorter terminal vanadium-oxo bonds of 1.519 to 1.695 angstroms, while a six-vanadium congener adopts a more regular planar hexagonal arrangement with vanadium-oxygen bonds of 1.987 to 2.006 angstroms and terminal vanadium-oxo bonds of 1.562 to 1.601 angstroms.¹⁹ The hexavanadium cluster was developed specifically as a heterogeneous catalyst for the synthesis of quinazolinones from 2-aminobenzamide and aldehydes, reaching essentially complete conversion with selectivity typically between 90 and 99 percent across a broad substrate scope of electron-rich, electron-poor, heterocyclic, and aliphatic aldehydes, and it clearly outperformed the higher-nuclearity fourteen-vanadium cluster under comparable conditions. The catalyst could be recycled for at least six successive runs with only a negligible loss of conversion and a slight decrease in selectivity, and inductively coupled plasma mass spectrometry confirmed that no more than 0.015 percent of the vanadium content leached into solution during the reaction, supporting its heterogeneous mode of action.

Zinc-POSS Complexes: Structure and Applications in Catalysis

Zinc occupies a smaller but catalytically significant niche within this literature. Di Iulio, Jones, Mahon, and Apperley reported in 2010 the first structurally characterized zinc(II) silsesquioxane complexes, describing their solid-state structures as unprecedented at the time and evaluating the resulting compounds as initiators for the ring-opening polymerization of rac-lactide.²⁰ A structurally distinct approach was taken in our own work on an imine-functionalized POSS ligand, in which the silsesquioxane core and the steric bulk of the imine side arms direct the formation of a tetrazinc complex, denoted Zn4@POSS-1, whose solid-state structure was established by X-ray crystallography alongside an extensive spectroscopic characterization in solution.²¹ The four zinc centers adopt a distorted tetrahedral geometry, two with a delta and two with a lambda configuration, with Zn-O bond lengths of 1.914(5) to 1.924(4) angstroms and Zn-N bond lengths of 1.979(6) to 2.016(5) angstroms, both ranges comparable to typical Zn(II) Schiff-base complexes, while the silicon-oxygen-silicon angles within the POSS core widen to between 139.6(3) and 167.3(3) degrees at the corners capped by the zinc centers. Zn4@POSS-1 is also structurally distinctive within the broader body of work surveyed in this review. In most of the crystallographically confirmed complexes discussed above, from the copper, manganese, and iron cages to the nickel cylinder and the lanthanide and actinide clusters, the metal centers are encapsulated inside a closed cage, sandwiched between two silsesquioxane ligand planes or embedded within a bridging oxo core sitting at the heart of the assembly. Zn4@POSS-1 departs from this pattern: the four zinc centers are held on the outside of an open, partially condensed POSS scaffold, each capped by one arm of the imine-functionalized ligand rather than being enclosed within a fully closed double-decker or sandwich core. This places Zn4@POSS-1 closer in spirit to the corner-capping assembly mode described for the main-group and alkaline-earth derivatives later in this review, in which a metal center caps an open corner of the cage from the outside, than to the encapsulated, internally templated architectures that dominate the transition-metal and lanthanide chemistry surveyed elsewhere in this article. This complex proved effective as a multisite catalyst for the cycloaddition of CO2 with epoxides to form cyclic carbonates under low CO2 pressure, a transformation of considerable interest for carbon utilization chemistry, and it illustrates how the same coordination chemistry that produces striking copper and manganese cages can be redirected toward small-molecule catalysis when a redox-inactive metal such as zinc is used instead. With styrene oxide as the model substrate, tetrabutylammonium iodide as co-catalyst, and 1 atmosphere of CO2 at 100 degrees Celsius, the catalyst alone was inactive, but the Zn4@POSS-1/TBAI pair reached 96 percent conversion to styrene carbonate at a turnover frequency of 25 per hour, rising to 98 percent conversion at 130 degrees Celsius, and it retained a useful 95 percent conversion even at a ten-fold lower catalyst loading when the reaction time was extended.²¹

Figure 4. X-ray crystal structure of Zn4@POSS-1 from Janeta, Lis, Szafert, Chem. Eur. J. 2020, 26, 13686-13697 (ref. 21), rendered directly from the deposited crystal structure (CCDC 1504223). The imine-functionalized POSS core is capped by four Zn(II) centers, each chelated by an N,N-imine side arm in the meso-(delta,delta,lambda,lambda) configuration described in the text.

Lanthanide and Actinide Metallasilsesquioxanes: Magnetism and Luminescence

Lanthanide-based metallasilsesquioxanes form a comparatively young but fast-moving subfield, motivated by the combination of structural novelty with magnetic and luminescent function. The first CeIV metallasilsesquioxane complex, reported by Gun'ko, Reilly, Edelmann, and Schmidt in 2001, established that even a large, high-valent lanthanide ion could be accommodated within a silsesquioxane coordination environment.²² Lorenz and coworkers subsequently reported an erbium complex in which two silsesquioxane cages become coupled through a bridging siloxy unit, a transformation that had not previously been observed for a lanthanide-templated system.²³ Since 2020, a series of tetranuclear cage-like lanthanide silsesquioxanes built around a prism-like Ln4 core has produced some of the most functionally interesting examples in the entire field. Terbium and europium derivatives display the first luminescence reported for a cage-like lanthanide silsesquioxane, and one terbium-based cage additionally exhibits a magnetic spin-flip transition.²⁴

Figure 5. X-ray crystal structure of the mixed-lanthanide cage anion of (Et₄N)₂[(PhSiO_1.5)₈(Y_0.75Dy_0.25O_1.5)₄(O)(NO₃)₆(EtOH)₂(MeCN)₂], compound 1 of Kulakova et al., Eur. J. Inorg. Chem. 2021, 2696-2701 (ref. 26), CCDC 2062487, rendered directly from the deposited crystallographic coordinates. Two parallel tetraphenylcyclotetrasiloxanolate macrocycles sandwich a distorted-square (Y_0.75Dy_0.25O_1.5)₄ core (left). The tetraethylammonium counter-cation is shown alongside the cage in the side view (right). This is the compound that displays field-induced single-molecule magnet behavior in addition to the characteristic dysprosium green-yellow emission.

Figure 6. X-ray crystal structure of the homometallic cage anion of (Ph₄P)₂[(PhSiO_1.5)₈(DyO_1.5)₄(O)(NO₃)₆(EtOH)₂(MeCN)₂], compound 3 of Kulakova et al., Eur. J. Inorg. Chem. 2021, 2696-2701 (ref. 26), CCDC 2002355, rendered directly from the deposited crystallographic coordinates. The same prism-like topology as in Figure 5 is retained with four Dy(III) centers in the core (left). The tetraphenylphosphonium counter-cation is shown alongside the cage (right). Unlike the mixed Y/Dy analogue, this all-dysprosium cage and its tetraethylammonium salt (compound 2, CCDC 2002354, not shown) do not display slow relaxation of the magnetization, a difference attributed to magnetic interactions between the four Dy(III) centers.

Related dysprosium and yttrium/dysprosium cages combine field-induced single-molecule magnet behavior with green-yellow emission, yielding the first bifunctional magneto-luminescent silsesquioxane reported in the literature, while mixed terbium/europium cages have been developed specifically as luminescent thermometers operating between 41 and 100 degrees Celsius through a temperature-dependent energy transfer between the two lanthanide centers.^25,26 A further mixed-lanthanide series combining dysprosium, europium, terbium, and yttrium in various ratios extends this dual functionality: the dysprosium/europium member behaves as a field-induced single-molecule magnet, with an effective energy barrier of 11.3 wavenumbers under an applied direct-current field, while simultaneously operating as a europium-based emissive thermometer across the range of 293 to 373 kelvin, reaching a maximum relative thermal sensitivity of 1.15 percent per kelvin at 293 kelvin, whereas the terbium- and yttrium-containing congeners of the same series show neither slow magnetic relaxation nor temperature-dependent emission.²⁷ At still higher nuclearity, a tridecanuclear gadolinium(III) cluster with an unprecedented body-centered cuboctahedron core represents the highest nuclearity yet reported for a lanthanide silsesquioxane and was synthesized specifically to explore cryogenic magnetic refrigeration, reaching a maximum magnetic entropy change of 20 joules per kilogram per kelvin at 2 kelvin under an applied field of 7.0 tesla.²⁸ Actinide chemistry entered this field only recently, with Tricoire, Mazzanti, and coworkers reporting in 2024 the first trinuclear uranium(III) metallasilsesquioxane, a complex that exhibits magnetic exchange between the three uranium centers and promotes the reduction of dinitrogen in the presence of a reducing agent, with U-O bond lengths ranging from 2.125(16) to 2.568(17) angstroms and an average value of 2.36(2) angstroms.²⁹

Figure 7. X-ray crystal structure of the first trinuclear U(III) metallasilsesquioxane, [U3(iBuPOSS)3], of Tricoire et al., Chem. Commun. 2024, 60, 55-58 (ref. 29), rendered directly from the deposited crystallographic coordinates. The three uranium centers (U1, U2, U3) are arranged in a triangular U3(mu-O)6 core, each capped by one iBuPOSS ligand and linked to its neighbors by bridging siloxide oxygens, with U-O distances ranging from 2.125(16) to 2.568(17) angstroms and an average value of 2.36(2) angstroms, consistent with previously reported U(III) siloxide complexes.

Main-Group and Alkaline-Earth Derivatives

A smaller number of structurally confirmed metal-POSS complexes involve main-group or alkaline-earth elements rather than transition metals or f-block ions. Duchateau and coworkers prepared a family of tin-containing polyhedral oligometallasilsesquioxanes in 2004 starting from cyclopentyl-substituted silsesquioxane trisilanols. Protonolysis with a tin(II) amide gave three-coordinate Sn(II) dimers of the type [(cyclopentyl)7Si7O11(OX)Sn]2 (compound 2 shown below), which proved hydrolytically unstable and feature a planar, strained (SnO)2 four-membered ring with an O-Sn-O angle of 75.39(13) degrees and an Sn-O-Sn angle of 104.61(13) degrees, the bridging tin-oxygen bonds of 2.177(3) and 2.172(3) angstroms being noticeably longer than the terminal, sigma-bonded tin-oxygen bond of 1.995(3) angstroms, whereas reaction with Cl2Sn(acac)2 gave hydrolytically robust, octahedrally coordinated Sn(IV) mono-tin complexes. Slow hydrolysis of the corresponding tin(IV) chloride cluster instead produced an unprecedented anionic trimeric cluster (compound 6) in which three octahedral Sn(IV) centers are held together by three silsesquioxane cages and four bridging hydroxyl groups, three of them bridging two tin atoms and one bridging all three, with a triethylammonium counterion occupying the remaining corner of the resulting incomplete cube. In this cluster the average tin-oxygen-tin angle at the doubly bridging hydroxyls, 112.3(3) degrees, is noticeably wider than the corresponding angle at the triply bridging hydroxyl, 100.9(3) degrees, while the tin atoms are linked to the silsesquioxane cages by tin-oxygen-silicon bonds averaging 1.974(4) angstroms in length, with a tin-oxygen-silicon angle of about 140 degrees at the monodentate silsesquioxane linkages and about 134 degrees at the bidentate ones. Despite this structural sophistication, none of the tin(IV) complexes showed any catalytic activity when screened in the Baeyer-Villiger oxidation of cyclohexanone, the Oppenauer oxidation of isopropanol, or the epoxidation of cyclooctene, a result the authors attributed to the coordinative saturation and reduced Lewis acidity of the octahedral, and in one case anionic, tin centers.³⁰

Figure 8. Left: the anionic trinuclear Sn(IV) cluster {[(cyclopentyl)7Si7O12Sn]3(mu2-OH)3(mu3-OH)}-{HNEt3}+ (compound 6, CCDC 238581). Right: the three-coordinate Sn(II) dimer [(cyclopentyl)7Si7O11(OSiMe3)Sn]2 (compound 2, CCDC 238582), with a center of inversion in the planar (SnO)2 ring. Both structures from Duchateau et al., Dalton Trans. 2004, 2677-2682 (ref. 30), rendered directly from the deposited crystallographic coordinates.

Lorenz, Blaurock, and Edelmann reported in the same period the first metallasilsesquioxane derivative of a heavier alkaline-earth metal, a dinuclear calcium complex in which two anionic silsesquioxane cages bridge a pair of unsymmetrically solvated calcium centers.³¹ Although these examples remain fewer in number than their transition metal and lanthanide counterparts, they demonstrate that the silsesquioxane cage can stabilize an unusually broad span of ionic radii and electronic configurations, from small trivalent tin to large, weakly coordinating alkaline-earth cations.

Copper Cage Silsesquioxanes: The Dominant Family

Copper accounts for the largest and most rapidly growing body of crystallographically confirmed metal-POSS chemistry, driven principally by the systematic work of Bilyachenko and coworkers. Across this family the recurring synthetic purpose was catalytic rather than purely structural, since nearly every reported cage was subsequently tested as a precatalyst for the peroxidative oxidation of light alkanes, cyclic hydrocarbons, and secondary alcohols with hydrogen peroxide or tert-butyl hydroperoxide, and crystallographic confirmation of the cage architecture was pursued specifically because catalytic performance was found to depend on the precise nuclearity and connectivity of the metal-oxo core rather than on solution composition alone.^{2,3,4,6,7,8,9,10,11,13}

The binuclear "cooling tower" complex [(PhSiO1.5)10(CuO)2(NaO0.5)2] reported in 2013 established the basic architecture of a copper-silsesquioxane cage, in which cyclic phenylsilsesquioxane ligands sandwich a small metal-oxo core, and its structure was confirmed by single-crystal X-ray diffraction, with copper-oxygen bond lengths of 1.906(3) to 1.956(4) angstroms and a silicon-oxygen-silicon bridging angle as wide as 166.2(3) degrees.² The complex was synthesized specifically to probe alkane and alcohol oxidation catalysis, and in benzene oxidation with hydrogen peroxide and nitric acid as promoter it reached a turnover number of 550 and a product yield of up to 41 percent, while oxidation of 1-phenylethanol to acetophenone with tert-butyl hydroperoxide proceeded in a yield approaching 100 percent after four hours.

Nonanuclear methylsilsesquioxane cages appeared in 2017, alongside trinuclear and hexanuclear congeners obtained from the same reaction system, each nuclearity and connectivity again established by diffraction, with mean copper-copper and copper-oxygen siloxanolate distances of 2.869(5) and 1.935(9) angstroms in the trinuclear cage.³ These complexes were likewise developed as alkane and alcohol oxidation catalysts, and the trinuclear cage converted 1-phenylethanol to acetophenone in 96 percent yield and cyclohexanol to cyclohexanone in 77 percent yield, while cyclohexane oxidation with hydrogen peroxide reached a total yield of 20 percent and a turnover number of 184, several times higher than the yield obtained with simple copper nitrate under the same conditions.

A distinct nonanuclear phenylsilsesquioxane cage of Cu9Na6 composition, formed with the assistance of a phenanthroline ligand that does not itself enter the final structure, followed in 2018, together with an ionic Cu9Na4 congener obtained under related conditions.⁴ Both complexes were prepared as catalysts for the oxidative amidation of benzyl alcohol, a reaction in which the Cu9Na6 cage required only 100 parts per million of copper and reached a turnover number of 7700 and a turnover frequency of 325 per hour, substantially outperforming copper oxide alone under the same conditions, and it additionally converted cyclohexanone or acetophenone in 63 or 98 percent yield in alcohol oxidation with tert-butyl hydroperoxide.

The coordinating role of the alkali metal cation proved to be a recurring structural variable rather than an incidental detail. Sandwich-like Cu4M2 complexes built around lithium, sodium, and potassium templates, distinguished crystallographically by intramolecular alkali metal to alkali metal separations ranging from 10.35 to 11.45 angstroms, were shown to direct the assembly toward distinct architectures even when the silsesquioxane and transition metal components were otherwise unchanged, although specific catalytic yields for this particular series were not reported in the accessible source text.⁵ The corresponding rubidium and cesium templated tetracopper cages, resolved crystallographically with rubidium to rubidium separations as long as 9.39 angstroms, were tested extensively as alkane oxidation and carboxylation catalysts, reaching a 35 percent yield and a turnover number of 320 in cyclohexane oxidation with hydrogen peroxide and a 50 percent total yield with a turnover number of 277 in the oxidative carboxylation of n-butane to a mixture of pentanoic acids, in addition to alcohol oxidations that converted 1-phenylethanol to acetophenone in 98 percent yield.⁶

Heptanuclear cages bridged additionally by benzoate ligands were reported between 2019 and 2021, with the three linearly disposed copper centers in the cage subtending an angle of 178.04(2) degrees.⁷ The heptanuclear complex was again evaluated as an alkane and alcohol oxidation catalyst, converting cyclohexane to a combined yield of 32 percent of alcohol and ketone with a turnover number of 290 within two hours, a result that compared favorably with a copper-sodium silsesquioxane analog tested under the same conditions, and it oxidized the acetophenone precursor 1-phenylethanol with tert-butyl hydroperoxide in 90 percent yield and a turnover number of 900.

Tetranuclear complexes obtained through the oxidative cleavage of a bis(phosphine) ligand, in this case 1,1-bis(diphenylphosphino)methane, were reported in 2019 as a palanquin-like Cu4Na4 cage in which the shortest distance between opposing silicon atoms of the silsesquioxane ligand measured 5.114 angstroms.⁸ This complex converted cyclohexane to cyclohexanol in 26 percent yield with a turnover number of 240 and an activation energy of 13.6 kilocalories per mole, and it oxidized 1-phenylethanol to acetophenone in essentially quantitative yield.

Heterometallic Cu4/M4 architectures further diversified the structural landscape reported between 2019 and 2023. A series of eight cesium and rubidium templated Cu4M4 coordination polymers, in which the centers of neighboring cages were separated by 13.97 angstroms in the solid state, were developed for the hydrocarboxylation of cycloalkanes, converting cyclohexane to cyclohexanecarboxylic acid in up to 37.8 percent yield and cyclopentane to the corresponding acid in 26.4 percent yield, in addition to reaching a 45 percent yield in direct cyclohexane oxidation with hydrogen peroxide.⁹ A related cage-like Cu5Cs4 phenylsilsesquioxane, with cesium to cesium separations of 9.54 and 11.62 angstroms, was instead developed for the Baeyer-Villiger oxidation of cyclohexanone to epsilon-caprolactone with meta-chloroperoxybenzoic acid, reaching a quantitative yield and a turnover number of 250 under microwave irradiation, and it additionally supported a tandem route directly from cyclohexane to the lactone in 17 percent yield, together with alcohol oxidations reaching 90 percent yield in the conversion of 1-phenylethanol to acetophenone.¹⁰

The record for nuclearity in this family has continued to rise, from a Cu12 cage decorated with additional Cu(I) phosphine moieties in 2024, whose copper-copper-copper trimer units are significantly distorted with angles of 140.70 and 141.77 degrees, to a Cu13Na2 cage assembled with the assistance of acetate ligands in 2025, the latter structure solved using synchrotron radiation.^11,12 The Cu12 complex proved to be an active catalyst for both the peroxidative oxidation and the oxidative carboxylation of C2 to C4 alkanes, reaching a turnover number of up to 534 in propane oxidation and a 50 percent combined yield of pivalic and isovaleric acids in the carboxylation of isobutane, and it oxidized 1-phenylethanol to acetophenone in 96 percent yield with a turnover number of 960. The Cu13Na2 cage itself was tested as a precatalyst in the Baeyer-Villiger oxidation of cyclohexanone, reaching essentially quantitative yields of epsilon-caprolactone with meta-chloroperoxybenzoic acid as oxidant, and it also supported a direct tandem route from cyclohexane to the same lactone in 25 to 26 percent yield, a modest but meaningful improvement over the heptanuclear Cu7-benzoate congener from the same group on this more demanding substrate.¹²

An octanuclear Cu8 complex bearing 3-phenyl-5-(2-pyridyl)pyrazolate ligands, in which two zigzag Cu4 tetramers are sandwiched by two cyclic pentameric silsesquioxane ligands with a longest intramolecular copper to copper separation of 11.08 angstroms, was likewise synthesized as an alkane oxidation catalyst, converting n-heptane to a 14 percent yield of oxidation products and methylcyclohexane to a 15 percent yield under comparable conditions with hydrogen peroxide and nitric acid as promoter.¹³

Figure 9. X-ray crystal structure of the record-nuclearity Cu13Na2 cage of Bilyachenko et al., Chem. Eur. J. 2025, 31, e202403604 (ref. 12), rendered directly from the deposited crystallographic coordinates. The left view shows the fused copper-siloxanolate core, with the crystallographically independent Cu1 and Cu2 centers and their symmetry-related counterparts (Cu3, Cu4 and primed equivalents) bridged by phenylsiloxanolate oxygens. The right view, rotated to look down the pseudo-macrocyclic axis, shows how the copper centers are threaded through the large siloxanolate ring formed by the ligand framework. Acetate ligands and the two bridging sodium centers that complete the Cu13Na2 formulation lie above and below this copper-rich core and are omitted here for clarity.

What Crystallography Reveals: Emerging Structural Trends

Several patterns recur across this body of crystallographic work regardless of which metal is involved. The alkali metal cation present during self-assembly, whether lithium, sodium, potassium, rubidium, or cesium, is rarely a passive spectator. Its ionic radius consistently governs whether the silsesquioxane ligand adopts a cyclic or acyclic conformation and whether the resulting cage is neutral, cationic, or anionic, a relationship documented independently for copper, manganese, and iron systems. A second recurring theme is the steady increase in reported nuclearity over time within the copper family in particular, from the two-copper cooling tower of 2013 to cages containing twelve or thirteen copper centers within the past two years, a trend that appears to reflect refinements in self-assembly conditions and the increasing use of synchrotron radiation to solve structures that would otherwise diffract poorly. Finally, the correlation between confirmed cage topology and function, whether catalytic activity in oxidation chemistry, single-molecule magnet behavior, or luminescence thermometry, illustrates why single-crystal X-ray diffraction remains indispensable in this field rather than a merely confirmatory afterthought. Spectroscopic and computational methods can propose a plausible structure, but only diffraction data have consistently distinguished between the closely related cage topologies that give rise to markedly different physical and chemical properties.

Conclusions and Outlook

The metal-POSS complexes surveyed here span nearly the entire periodic table, from copper cages of steadily increasing nuclearity to the first actinide example reported only in 2024, and in every case single-crystal X-ray diffraction has been the method that transformed a plausible formula into a confirmed molecular structure. The copper family in particular continues to grow at a pace that suggests further records in nuclearity are likely within the next few years, while the lanthanide and actinide subfields remain comparatively open for structural discovery. For a research program centered on zinc Schiff base and imine-functionalized POSS chemistry, the comparative view offered by this survey suggests that the alkali metal template effect documented so thoroughly for copper, manganese, and iron systems may offer an underexplored strategy for directing the nuclearity and topology of future zinc-based cages as well.

References

1. Feher, F. J.; Budzichowski, T. A. Silasesquioxanes as ligands in inorganic and organometallic chemistry. Polyhedron 1995, 14, 3239-3253. https://doi.org/10.1016/0277-5387(95)85009-0
2. Bilyachenko, A. N.; Dronova, M. S.; Yalymov, A. I.; Korlyukov, A. A.; Shul'pina, L. S.; Arkhipov, D. E.; Shubina, E. S.; Levitsky, M. M.; Kirilin, A. D.; Shul'pin, G. B. Binuclear cage-like copper(II) silsesquioxane ("cooling tower"), its high catalytic activity in the oxidation of benzene and alcohols. Eur. J. Inorg. Chem. 2013, 5240-5246. https://doi.org/10.1002/ejic.201300878
3. Bilyachenko, A. N.; Kulakova, A. N.; Levitsky, M. M.; Petrov, A. A.; Korlyukov, A. A.; Shul'pina, L. S.; Khrustalev, V. N.; Dorovatovskii, P. V.; Vologzhanina, A. V.; Tsareva, U. S.; Golub, I. E.; Gulyaeva, E. S.; Shubina, E. S.; Shul'pin, G. B. Unusual tri-, hexa-, and nonanuclear Cu(II) cage methylsilsesquioxanes. Inorg. Chem. 2017, 56, 4093-4103. https://doi.org/10.1021/acs.inorgchem.7b00061
4. Astakhov, G. S.; Bilyachenko, A. N.; Korlyukov, A. A.; Levitsky, M. M.; Shul'pina, L. S.; Bantreil, X.; Lamaty, F.; Vologzhanina, A. V.; Shubina, E. S.; Dorovatovskii, P. V.; Nesterov, D. S.; Pombeiro, A. J. L.; Shul'pin, G. B. High-cluster (Cu9) cage silsesquioxanes. Inorg. Chem. 2018, 57, 11524-11529. https://doi.org/10.1021/acs.inorgchem.8b01496
5. Bilyachenko, A. N.; Khrustalev, V. N.; Zueva, A. Y.; Titova, E. M.; Astakhov, G. S.; Zubavichus, Y. V.; Dorovatovskii, P. V.; Korlyukov, A. A.; Shul'pina, L. S.; Shubina, E. S.; Kozlov, Y. N.; Ikonnikov, N. S.; Gelman, D.; Shul'pin, G. B. A novel family of cage-like (CuLi, CuNa, CuK)-phenylsilsesquioxane complexes with 8-hydroxyquinoline ligands. Molecules 2022, 27, 6205. https://doi.org/10.3390/molecules27196205
6. Bilyachenko, A. N.; Khrustalev, V. N.; Astakhov, G. S.; Zueva, A. Y.; Arteev, I. S.; Zubavichus, Y. V.; Korlyukov, A. A.; Dorovatovskii, P. V.; Shul'pina, L. S.; Ikonnikov, N. S.; Kirillova, M. V.; Shubina, E. S.; Kozlov, Y. N.; Kirillov, A. M.; Shul'pin, G. B. Cagelike Rb2-, K2-, and Na2-tetracopper(II) silsesquioxanes with quaternary ammonium cations. Organometallics 2023, 42, 2577-2589. https://doi.org/10.1021/acs.organomet.2c00649
7. Bilyachenko, A. N.; Khrustalev, V. N.; Gutsul, E. I.; Zueva, A. Y.; Korlyukov, A. A.; Shul'pina, L. S.; Ikonnikov, N. S.; Dorovatovskii, P. V.; Gelman, D.; Shubina, E. S.; Shul'pin, G. B. Hybrid silsesquioxane/benzoate Cu7-complexes. Molecules 2022, 27, 8505. https://doi.org/10.3390/molecules27238505
8. Kulakova, A. N.; Khrustalev, V. N.; Zubavichus, Y. V.; Shul'pina, L. S.; Shubina, E. S.; Levitsky, M. M.; Ikonnikov, N. S.; Bilyachenko, A. N.; Kozlov, Y. N.; Shul'pin, G. B. Palanquin-like Cu4Na4 silsesquioxane. Catalysts 2019, 9, 154. https://doi.org/10.3390/catal9020154
9. Bilyachenko, A. N.; Astakhov, G. S.; Kulakova, A. N.; Korlyukov, A. A.; Zubavichus, Y. V.; Dorovatovskii, P. V.; Shul'pina, L. S.; Shubina, E. S.; Ikonnikov, N. S.; Kirillova, M. V.; Zueva, A. Y.; Kirillov, A. M.; Shul'pin, G. B. Exploring cagelike silsesquioxane building blocks for the design of heterometallic Cu4/M4 architectures. Cryst. Growth Des. 2022, 22, 2146-2157. https://doi.org/10.1021/acs.cgd.1c01225
10. Bilyachenko, A. N.; Arteev, I. S.; Khrustalev, V. N.; Shul'pina, L. S.; Korlyukov, A. A.; Ikonnikov, N. S.; Shubina, E. S.; Kozlov, Y. N.; Reis Conceicao, N.; Guedes Da Silva, M. F. C.; Mahmudov, K. T.; Pombeiro, A. J. L. Cage-like Cu5Cs4-phenylsilsesquioxanes. Inorg. Chem. 2023, 62, 13573-13586. https://doi.org/10.1021/acs.inorgchem.3c01989
11. Bilyachenko, A. N.; Khrustalev, V. N.; Arteev, I. S.; Shul'pina, L. S.; Ikonnikov, N. S.; Kirillova, M. V.; Shubina, E. S.; Kirillov, A. M.; Kozlov, Y. N.; Lobanov, N. N.; Ragimov, K. G.; Sun, D. Cu12-methylsilsesquioxane cage decorated with Cu(dppe)2 moieties for mild oxidative functionalization of alkanes. Inorg. Chem. 2024, 63, 20404-20414. https://doi.org/10.1021/acs.inorgchem.4c02806
12. Bilyachenko, A. N.; Khrustalev, V. N.; Huang, Z.; Dorovatovskii, P. V.; Shubina, E. S.; Lobanov, N. N.; Wang, Z.; Ragimov, K.; Reis Conceicao, N.; Mahmoud, A. G.; Pombeiro, A. J. L. Combination of phenylsilsesquioxane and acetate ligands as an approach to a record high nuclear Cu13Na2-cage. Chem. Eur. J. 2025, 31, e202403604. https://doi.org/10.1002/chem.202403604
13. Bilyachenko, A. N.; Khrustalev, V. N.; Huang, Z.; Shul'pina, L. S.; Dorovatovskii, P. V.; Shubina, E. S.; Ikonnikov, N. S.; Lobanov, N. N.; Ragimov, K.; Sun, D. An octanuclear 3-phenyl-5-(2-pyridyl)pyrazolate/phenylsilsesquioxane complex. Dalton Trans. 2024, 53, 19102-19106. https://doi.org/10.1039/D4DT02690J
14. Bilyachenko, A.; Yalymov, A.; Shul'pina, L.; Mandelli, D.; Korlyukov, A.; Vologzhanina, A.; Es'kova, M.; Shubina, E.; Levitsky, M.; Shul'pin, G. Novel cage-like hexanuclear nickel(II) silsesquioxane. Molecules 2016, 21, 665. https://doi.org/10.3390/molecules21050665
15. Lorenz, V.; Blaurock, S.; Edelmann, F. T. The first heterobimetallic metallasilsesquioxane derivatives of manganese. Z. Anorg. Allg. Chem. 2008, 634, 2819-2824. https://doi.org/10.1002/zaac.200800292
16. Astakhov, G. S.; Khrustalev, V. N.; Dronova, M. S.; Gutsul, E. I.; Korlyukov, A. A.; Gelman, D.; Zubavichus, Y. V.; Novichkov, D. A.; Trigub, A. L.; Shubina, E. S.; Bilyachenko, A. N. Cage-like manganesesilsesquioxanes. Inorg. Chem. Front. 2022, 9, 4525-4537. https://doi.org/10.1039/D2QI01054B
17. Bilyachenko, A. N.; Gutsul, E. I.; Khrustalev, V. N.; Chusova, O.; Dorovatovskii, P. V.; Aliyeva, V. A.; Paninho, A. B.; Nunes, A. V. M.; Mahmudov, K. T.; Shubina, E. S.; Pombeiro, A. J. L. A family of cagelike Mn-silsesquioxane/bathophenanthroline complexes. Inorg. Chem. 2023, 62, 15537-15549. https://doi.org/10.1021/acs.inorgchem.3c02040
18. Bilyachenko, A. N.; Khrustalev, V. N.; Dorovatovskii, P. V.; Shul'pina, L. S.; Ikonnikov, N. S.; Shubina, E. S.; Lobanov, N. N.; Aliyeva, V. A.; Nunes, A. V. M.; Mahmudov, K. T.; Kozlov, Y. N.; Pombeiro, A. J. L. Fe(III)-based phenylsilsesquioxane/acetylacetonate complexes. Inorg. Chem. 2024, 63, 1909-1918. https://doi.org/10.1021/acs.inorgchem.3c03587
19. Shen, Q.; Sheng, K.; Gao, Z.-Y.; Bilyachenko, A.; Huang, X.-Q.; Azam, M.; Tung, C.-H.; Sun, D. Vanadium-silsesquioxane nanocages as heterogeneous catalysts for synthesis of quinazolinones. Inorg. Chem. 2024, 63, 13022-13030. https://doi.org/10.1021/acs.inorgchem.4c01748
20. Di Iulio, C.; Jones, M. D.; Mahon, M. F.; Apperley, D. C. Zinc(II) silsesquioxane complexes and their application for the ring-opening polymerization of rac-lactide. Inorg. Chem. 2010, 49, 10232-10234. https://doi.org/10.1021/ic101809r
21. Janeta, M.; Lis, T.; Szafert, S. Zinc imine polyhedral oligomeric silsesquioxane as a quattro-site catalyst for the synthesis of cyclic carbonates from epoxides and low-pressure CO2. Chem. Eur. J. 2020, 26, 13686-13697. https://doi.org/10.1002/chem.202002996
22. Gun'ko, Y. K.; Reilly, R.; Edelmann, F. T.; Schmidt, H.-G. The first CeIV metallasilsesquioxane complex. Angew. Chem. Int. Ed. 2001, 40, 1279-1281. https://doi.org/10.1002/1521-3773(20010401)40:7<1279::AID-ANIE1279>3.0.CO;2-2
23. Lorenz, V.; Blaurock, S.; Hrib, C. G.; Edelmann, F. T. Coupling of silsesquioxane cages in the coordination sphere of erbium. Eur. J. Inorg. Chem. 2010, 2605-2608. https://doi.org/10.1002/ejic.201000186
24. Kulakova, A. N.; Bilyachenko, A. N.; Levitsky, M. M.; Khrustalev, V. N.; Shubina, E. S.; Felix, G.; Mamontova, E.; Long, J.; Guari, Y.; Larionova, J. New luminescent tetranuclear lanthanide-based silsesquioxane cage-like architectures. Chem. Eur. J. 2020, 26, 16594-16598. https://doi.org/10.1002/chem.202003351
25. Kulakova, A. N.; Nigoghossian, K.; Felix, G.; Khrustalev, V. N.; Shubina, E. S.; Long, J.; Guari, Y.; Carlos, L. D.; Bilyachenko, A. N.; Larionova, J. New magnetic and luminescent Dy(III) and Dy(III)/Y(III) based tetranuclear silsesquioxane cages. Eur. J. Inorg. Chem. 2021, 2696-2701. https://doi.org/10.1002/ejic.202100308; Nigoghossian, K.; Kulakova, A. N.; Felix, G.; Khrustalev, V. N.; Shubina, E. S.; Long, J.; Guari, Y.; Sene, S.; Carlos, L. D.; Bilyachenko, A. N.; Larionova, J. Temperature sensing in Tb3+/Eu3+-based tetranuclear silsesquioxane cages with tunable emission. RSC Adv. 2021, 11, 34735-34741. https://doi.org/10.1039/D1RA06755A
26. Felix, G.; Sene, S.; Kulakova, A.; Bilyachenko, A. N.; Khrustalev, V. N.; Shubina, E. S.; Guari, Y.; Larionova, J. Tetranuclear lanthanide-based silsesquioxanes, towards a combination of a slow relaxation of the magnetization and a luminescent thermometry. RSC Adv. 2023, 13, 26302-26312. https://doi.org/10.1039/D3RA04901A
27. Sheng, K.; Wang, R.; Bilyachenko, A.; Khrustalev, V.; Jagodic, M.; Jaglicic, Z.; Li, Z.; Wang, L.; Tung, C.; Sun, D. Tridecanuclear Gd(III)-silsesquioxane, synthesis, structure, and magnetic property. Chemphysmater 2022, 1, 247-251. https://doi.org/10.1016/j.chphma.2022.04.008
28. Tricoire, M.; Jori, N.; Fadaei Tirani, F.; Scopelliti, R.; Zivkovic, I.; Natrajan, L. S.; Mazzanti, M. A trinuclear metallasilsesquioxane of uranium(III). Chem. Commun. 2024, 60, 55-58. https://doi.org/10.1039/D3CC05390C
29. Duchateau, R.; Dijkstra, T. W.; Severn, J. R.; Van Santen, R. A.; Korobkov, I. V. Synthesis and characterization of tin containing polyhedral oligometallasilsesquioxanes (POMSS). Dalton Trans. 2004, 2677. https://doi.org/10.1039/b407471h
30. Lorenz, V.; Blaurock, S.; Edelmann, F. T. The first metallasilsesquioxane derivative of a heavier alkaline earth metal. Z. Anorg. Allg. Chem. 2008, 634, 441-444. https://doi.org/10.1002/zaac.200700370