*2.1. Ti/Zr-Substituted Monomeric POMs*

It is well known that titanium/zirconium-based compounds (e.g., TiO2, ZrO2) have been widely used in the fields of energy conversion, catalysis, and environmental treatment. As interesting molecular models of TiO2 and ZrO2 structures, the syntheses of titanium-/zirconium-substituted POMs could be dated back to the 1980s. In 1983, Knoth et al. reported the first case of Ti-substituted Keggin-type monomeric [TiW11PO40] 5− polyoxoanion cluster (Figure 1a), which was prepared by the reaction of monovacant (Bu4N)4H3W11PO39 with titanium tetrachloride in dichloroethane solution [24]. In 2000, Kholdeeva and co-workers also reported two similar cases of [PTiW11O40] 5− and [PTiW11O41] <sup>7</sup><sup>−</sup> polyoxoanions [25]. Qu et al. reported Ti-substituted Dawson-type monomeric α2-[P2W17(TiO2)O61] <sup>8</sup><sup>−</sup> polyoxoanion (Figure 1b), which was synthesized from vacant heteropolytungstate precursors α2-[P2W17O61] <sup>10</sup><sup>−</sup> and Ti(SO4)2 using an aqueous solution-based synthetic approach [26]. Successively, Ti3-substituted monomeric POM has also been reported with multi-lacunary POMs α-1,2,3-[P2W15O56] <sup>12</sup><sup>−</sup> as precursors. For instance, Nomiya et. al. successfully reported a tris-[peroxotitanium(IV)]-substituted α-Dawson monomeric [*α*-1,2,3-P2W15(TiO2)3O56(OH)3] <sup>9</sup><sup>−</sup> polyoxoanion (**1,** Figure 1c), which are derived from {[*α*-1,2,3-P2W15Ti3O59(OH)3]4[*μ*3-Ti(OH)3]4Cl}<sup>33</sup><sup>−</sup> (**2a**) in 30% aqueous hydrogen peroxide solution. Thereinto, the four bridging Ti octahedral groups of **2a** were considered as the crucial roles for the synthesis of polyoxoanion **1 [27]**. Subsequently, they prepared [[{Ti(OH)(ox)}2(*μ*-O)](*α*-PW11O39)]<sup>7</sup><sup>−</sup> (Figure 1d) by using the trilacunary species of [A-PW9O34] <sup>9</sup><sup>−</sup> and the anionic titanium(IV) complex as precursors with the molar ratio of 1:2 under acidic conditions. The molecular structure can be recognized as a hybrid containing one mono-lacunary POM ligand and two octahedral Ti-oxo moieties [28]. Then, a tetra-Ti-substituted di-lacunary α-Keggin monomeric [[{Ti(ox)(H2O)}4(*μ*-O)3](*α*-PW10O37)]<sup>7</sup><sup>−</sup> polyoxoanion (Figure 1e) was also prepared, which was constructed by using the dimeric dititanium(IV)-substituted POM [(*α*-1,2-PW10Ti2O39)2] <sup>10</sup><sup>−</sup> as precursor under acidic conditions [29]. Additionally, [[{Ti(H2O)3}2{Ti(H2O)2}2(*μ*-O)3(SO4)](PW10O37)]<sup>−</sup> polyoxoanion (Figure 1f) was synthesized through the reaction of Ti(SO4)2 with [(*α*-1,2-PW10Ti2O38)2O2] 10− and [(*α*-1,2,3-PW9Ti3O37)2O3] <sup>12</sup><sup>−</sup> under strongly acidic conditions, where the tetra-titanium(IV) oxide cluster was anchored onto the binding sites of lacunary Keggin POM [30]. In 2018, An et al. reported two organic-inorganic hybrid POMs, [(H2O)4(3-Hpic)2Ln][(H2O)5(3-Hpic)2Ln][PW10Ti2O40] <sup>−</sup> (Ln= Ce, Nd, Sm), where binuclear Ti-substituted Keggin-type [PW10Ti2O40] <sup>7</sup><sup>−</sup> polyoxoanion was further modified by four Ln-3-Hpic coordinating groups [31]. Recently, Poblet et al. successfully prepared the other organic-inorganic hybrid POM, [B-*α*-SbW9O33( *t* BuSiO)3Ti(O<sup>i</sup> Pr)]3−, by anchoring Ti(O<sup>i</sup> Pr) moiety on the silanol-functionalized an antimony-containing trilacunary POM ligand. The resulting complex has been utilized as a catalyst for the catalytic epoxidation of alkenes [32].

Compared to Ti-substituted POMs, the exploration of Zr-containing POMs has seldom been studied. In 1985, Chauveau et al. reported a compound of Zr-containing POM, [ZrW5O19H2] <sup>2</sup>−, which was considered as the Lindqvist-type structure. However, it is still doubted about the exact structure given the presented low signal-to-noise ratio and incorrect intensity ratio of 183W NMR data [33]. Subsequently, Villanneau et al. has been unambiguously determined the structure as [W5O18Zr(H2O)3] <sup>2</sup><sup>−</sup> by using EXFAS data [34,35]. Meanwhile, the same group also reported a similar compound with the structural formula of [{W5O18Zr(*μ*-OH)}2] <sup>6</sup><sup>−</sup> (Figure 2a). In 2009, Sokolov et al. reported two mono-Zr-substituted Dawson-type monomeric polyoxoanion clusters [{(H2O)2ZrP2W17O61}]6<sup>−</sup> and [Zr(L-OOCCH(OH)CH2COO)P2W17O61] <sup>8</sup><sup>−</sup> (Figure 2b) through the reaction of monovacant α2-[P2W17O61] <sup>10</sup><sup>−</sup> ligand and Zr salt under aqueous solution conditions [36]. The latter complex exhibited chirality due to the presence of chiral L-malic acid ligand. Later on, other organic ligand-modified Zr-POM, (tpp)-Zr-(PW11O39)[TBA]5 (tpp referring to ternary porphyrin) (Figure 2c) [37] and (Pc)-Zr-(PW11O39)[TBA]5 (Pc referring to Phthalocyanine) [38], has been reported by Drain et al. in 2009 and 2013, respectively.

**Figure 1.** The ball-and-stick and polyhedral illustration of (**a**) [TiW11PO40] <sup>5</sup><sup>−</sup> polyoxoanion, (**b**) *α*2-[P2W17(TiO2)O61] <sup>8</sup><sup>−</sup> polyoxoanion, (**c**) [*α*-1,2,3-P2W15(TiO2)3O56(OH)3] <sup>9</sup><sup>−</sup> polyoxoanion, (**d**) [[{Ti(OH)(ox)}2(*μ*-O)](*α*-PW11O39)]<sup>7</sup><sup>−</sup> polyoxoanion; (**e**) [[{Ti(ox)(H2O)}4(*μ*-O)3](*α*-PW10O37)]7<sup>−</sup> polyoxoanion, and (**f**) [[{Ti(H2O)3}2{Ti(H2O)2}2(*μ*-O)3(SO4)](PW10O37)]<sup>−</sup> polyoxoanion. Color codes: WO6, red; PO4, yellow; Ti, green; S, dark blue; C, gray; O, red.

**Figure 2.** The ball-and-stick and polyhedral illustration of (**a**) [W5O18Zr(*μ*-OH)2] <sup>6</sup><sup>−</sup> polyanion, (**b**) [Zr(L-OOCCH(OH)CH2COO)P2W17O61] <sup>8</sup><sup>−</sup> polyanion and (**c**) (tpp)Zr(PW11O39)[TBA]5. Color codes: WO6, red; PO4, yellow; Zr, bright green; N, blue; C, gray; O, red.

#### *2.2. Ti/Zr-Substituted Dimeric POMs*

In addition to the monomeric POMs, some presentative Ti/Zr-substituted dimeric POMs have also been reviewed herein in detail. In 1993, Finke et al. reported the first hexa-Tisubstituted sandwich-type dimeric silicotungstate, [A-*β*-Si2W18Ti6O77] 14− (Figure 3a), which was prepared by the reaction of [A-*β*-HSiW9O34] <sup>9</sup><sup>−</sup> with Ti(O)(C2O4)2 2− or Ti(O)SO4 in a regulated pH environment. The structural formulation has been lately corrected as [A-*β*-(SiW9O37)2(Ti-O-Ti)3] <sup>14</sup>−, implying the dimerization of two hypothetical

"[A-*β*-SiW9(TiOH)3O37] <sup>7</sup>−" Keggin units through the linkage of Ti-O-Ti bridges [39]. In 2000, Kholdeeva and co-workers reported a Ti2-substituted dimeric POMs [(PTiW11O39)2OH]7<sup>−</sup> which was prepared using [PTiW11O40] <sup>5</sup><sup>−</sup> subunit as reaction materials [25]. Then, Nomiya and co-workers prepared a similar hexa-Ti-substituted sandwich-type dimeric POMs except for the replacement of {A-SiW9O34} with {A-PW9O34} [40]. Subsequently, Cronin and coworkers reported a hexa-Ti-substituted tungstoarsenate, K6[Ti4(H2O)10(AsTiW8O33)2]·30H2O, where two {AsTiW8O33}fragments were used to encapsulate a {Ti4(H2O)10} 16+ moiety [41]. Additionally, they also reported the first mono-Ti-substituted tungstoantimonate [TiO(SbW9O33)2] <sup>16</sup>−, which was two B*-α-*{SbIIIW9O33} fragments linked by five sodium cations and an unprecedented square pyramidal Ti(O)O4 group with a terminal Ti = O bond. In 2013, Kortz et al. reported two Ti-substituted phosphotungstates, [Ti8(C2O4)8P2W18O76(H2O)4] <sup>18</sup><sup>−</sup> (Figure 3b) and [Ti6(C2O4)4P4W32O124] <sup>20</sup><sup>−</sup> (Figure 3c). The former is the Ti8-substituted Keggin-type phosphotungstates, consisting of two {PW9} units encapsulating eight titanium centers bridged with two Ti–O–Ti bonds. The latter represents the first Ti6-substituted Dawson-type phosphotungstates, which are constructed by two di-Ti-substituted {P2W16} units connected via two Ti(C2O4) moietes [42]. In 2015, Nomiya reported the first hexa-Ti-substituted Well–Dawson phosphotungstates [{*α*-P2W15Ti3O60(OH)2}2(Cp\*Rh)2] <sup>16</sup><sup>−</sup> (Figure 3d). The polyoxoanion was constructed with two tri-Ti-substituted protonated Wells−Dawson subunits "[P2W15Ti3O60(OH)2] <sup>10</sup>−" bridged by the two organometallic Cp\*Rh2+ groups [43]. Additionally, they also reported the first tetra-TiIV-1,2-substituted *α*-Keggin polyoxotungstate in aqueous solution, [*α,α*-P2W20Ti4O78] 10− (Figure 3e). The polyoxoanion consisted of a dimeric anhydride form of two [α-1,2-PW10Ti2O40] <sup>7</sup><sup>−</sup> Keggin units linked with two Ti–O–Ti bonds [44]. Similar structures have also been reported by Mizuno and Wang's groups, respectively [45,46]. With continuous research, more di-Ti-substituted POMs have been further developed. In 2007, Kortz et al. reported a special di-Ti-substituted tungstodiarsenate (III) [Ti2(OH)2As2W19O67(H2O)]8<sup>−</sup> (Figure 3f), prepared by the reaction of TiOSO4 and K14[As2W19O67(H2O)] in a 2:1 molar ratio in acidic media (pH 2). The polyoxoanion was a sandwich-type structure with nominal *C2v* symmetry, which was constructed with two (B*-α-*AsIIIW9O33) Keggin moieties linked by an octahedral {WO5(H2O)} fragment and two unprecedented square-pyramidal {TiO4(OH)} groups [47]. Nomiya et al. further reported the synthesis of a novel molecular solid Brønsted acid based on the Dawson-type sandwich POM [Ti2{P2W15O54(OH2)2}2] 8− (Figure 3g) [48]. Subsequently, they synthesized a similar structure using mono-lacunary Dawson precursor K10[*α*2-P2W17O61]·23H2O [49]. In 2015, Li's group synthesized two isomorphic di-Ti-substituted Keggin-type phosphotungstate ([(Ti2O)(PW11O39)2] <sup>8</sup>−, Figure 3h) containing dissimilar copper under hydrothermal condition. The resulting organic–inorganic hybrid assemblies contained a rare corner-sharing double-Keggin type POM architecture in the Ti-POM species, which was further connected with the butterfly-type [CuIILo] units to form a 1-D chain and a square plane, respectively [50].

In contrast to the diverse structures of Ti-substituted POMs, Zr-substituted dimeric POMs have been far less reported. Some presentative examples are summarized below. In 2003, May et al. reported an example of a dimeric structure of mono-Zr-substituted Keggingtype POM, [Zr(PMo12O40)(PMo11O39)]6<sup>−</sup> (Figure 4a), which also represented the first crystallographic determination of the [PMo11O39] <sup>7</sup><sup>−</sup> anion [51]. The similar Keggin-type chiral phosphotungstate and borotungstate were also reported by Liu and Xue's groups in 2009, respectively [52,53]. In 2006, Nomiya et al. also reported the first Zr-substituted Well-Dawson phosphotungstate [Zr(*α*2-P2W17O61)2] <sup>16</sup><sup>−</sup> (Figure 4b), deriving from mono-lacunary precursor [*α*2-P2W17O61] <sup>10</sup><sup>−</sup> [54]. Similar mono-Zr-substituted Well-Dawson phosphotungstate POMs ([{P2W15O54(H2O)2}2Zr]12<sup>−</sup>, and [{P2W15O54(H2O)2}Zr{P2W17O61}]14<sup>−</sup>) have been further reported by Hill et al. in 2007 [55]. To increase the nuclearity of Zr centers, Kholdeeva et al. prepared three Zr2-substituted Keggin-type phosphotungstate ([{PW11O39Zr(*μ*-OH)}2] <sup>8</sup>−, [{PW11O39Zr(*μ*-OH)}2] <sup>8</sup>−, and [{PW11O39Zr}2(*μ*-OH)(*μ*-O)]9−) [56]. Then, Mizuno et al. synthesized a di-Zr-substituted Keggin-type silicotungstate [(*γ*-SiW10O36)2Zr2 (*μ*-OH)2] <sup>10</sup><sup>−</sup> (Figure 4c) [57], and Sokolov et al. reported di-Zr-substituted Dawson-type

phosphotungstate [{(H2O)Zr(*μ*2-OH)(P2W17O61)}2] <sup>14</sup><sup>−</sup> [58]. In 2011, Villanneau et al. reported two Zr2-containing POMs derivatives [{PW9O34{PO(R)}2}2{Zr(H2O)(*μ*-OH)}2] 4− and [{PW9O34{PO(R)}2}2{Zr(DMF)(*μ*-OH)}2] <sup>4</sup><sup>−</sup> (R = Ph, *<sup>t</sup>* Bu), which were obtained by using [(*<sup>n</sup>*Bu4N)3Na2[PW9O34{PO(R)}2] and ZrOCl2·8H2O [59]. Subsequently, the same group also reported a similar structure in 2013 [60]. In 2005, Hill et al. reported the first chiral tri-Zr-substituted Dawson-type phosphotungstate {[*α*-P2W15O55(H2O)]Zr3(*μ*3-O)(H2O)(LtartH)[*α*-P2W16O59]}15<sup>−</sup> (Figure 4d) and {[*α*-P2W15O55(H2O)]Zr3(*μ*3-O)(H2O)(D-tartH)[*α*-P2W16O59]}<sup>15</sup><sup>−</sup> [61]. After that, Cadot et. al. reported a tri-Zr(IV)-substituted sandwich-type Keggin POM [Zr3O(OH)2(SiW9O34)2] <sup>12</sup>−, which consists of a [Zr3O(OH)2] triangular central cluster closely embedded between two A-α-[SiW9O34] <sup>10</sup><sup>−</sup> subunits [62]. Subsequently, three cases of isomorphic compounds were also reported by Xue, Nomiya and Yang's groups, respectively [63–65]. Among these tri-Zr-substituted POMs, it is worth mentioning that Yang's group reported the first tri-ZrIV-substituted POM [Zr3(*μ*2-OH)2(*μ*2-O)(A-*α*-GeW9O34)(1,4,9-*α*-P2W15O56)]14<sup>−</sup> (Figure 4e), where the tri-Zr centers were stabilized by mixed types of tri-lacunary POM ligands including Keggin-type [A-*α*-GeW9O34] <sup>10</sup><sup>−</sup> and Dawson-type [1,4,9-*α*-P2W15O56] <sup>12</sup><sup>−</sup> units [66]. In addition, a number of tetra-Zr substituted POMs have also been prepared. For instance, Pope et al. reported a Zr4-substituted phosphotungstate, [Zr4(*μ*3-O)2(*μ*2-OH)2(H2O)4(P2W16O59)2] <sup>14</sup><sup>−</sup> (Figure 4f). Therein, the divacant lacunary {P2W16O59} ligands were derived from the plenary Wells–Dawson (*α*-P2W18O62) polyoxoanion [67]. Then, similar structures were reported by Hill and Li's groups in 2005 and 2013, respectively [68,69]. Subsequently, tetra-Zr substituted Keggin-type silicotungstates [(*γ*-SiW10O36)2Zr4(*μ*4-O)(*μ*-OH)6] <sup>8</sup><sup>−</sup> (Figure 4g) and five other similar structures were successively reported [70–77]. The nuclearity of Zr substitution has been further improved to six by Kortz and co-workers. they reported the first hexa-Zr-substituted dimeric tungstoarsenates [Zr6O4(OH)4(H2O)2(CH3COO)5(AsW9O33)2] 11− (Figure 4h). In the polyoxoanion, the unprecedented hexa-Zr unit is perfectly located at the cavity formed by two (B-*α*-AsW9O33) fragments lying at an angle of about 74◦ with respect to each other [78].

**Figure 3.** The ball-and-stick and polyhedral illustration of (**a**) [A-*β*-Si2W18Ti6O77] <sup>14</sup><sup>−</sup> polyoxoanion, (**b**) [Ti8(C2O4)8P2W18O76(H2O)4] <sup>18</sup><sup>−</sup> polyoxoanion, (**c**) [Ti6(C2O4)4P4W32O124] <sup>20</sup><sup>−</sup> polyoxoanion, (**d**) P2W15Ti3O60(OH)2}2(Cp\*Rh)2] <sup>16</sup><sup>−</sup> polyoxoanion; (**e**) [*α,α*-P2W20Ti4O78] <sup>10</sup><sup>−</sup> polyoxoanion; (**f**) [Ti2(OH)2As2W19O67(H2O)]8<sup>−</sup> polyoxoanion; (**g**) [Ti2{P2W15O54(OH2)2}2] <sup>8</sup><sup>−</sup> polyoxoanion and (**h**) [(Ti2O)(PW11O39)2] <sup>8</sup><sup>−</sup> polyoxoanion. Color codes: WO6, red; PO4, yellow; SiO4, light blue; Ti, green; As, lavender; C, gray; O, red; H, black.

**Figure 4.** The ball-and-stick and polyhedral illustration of (**a**) [Zr(PMo12O40)(PMo11O39)]6<sup>−</sup> polyoxoanion, (**b**) [Zr(*α*2-P2W17O61)2] <sup>16</sup><sup>−</sup> polyoxoanion, (**c**) [(*γ*-SiW10O36)2Zr2(*μ*-OH)2] <sup>10</sup><sup>−</sup> polyoxoanion, (**d**) {[*α*-P2W15O55(H2O)]Zr3(*μ*3-O)(H2O)(L-tartH)[*α*-P2W16O59]}15<sup>−</sup> polyoxoanion; (**e**) [Zr3(*μ*2-OH)2(*μ*2-O)(A-*α*-GeW9O34)(1,4,9-*α*-P2W15O56)]14<sup>−</sup> polyoxoanion; (**f**) [Zr4(*μ*3-O)2(*μ*2- OH)2(H2O)4(P2W16O59)2] <sup>14</sup><sup>−</sup> polyoxoanion; (**g**) [(*γ*-SiW10O36)2Zr4(*μ*4-O)(*μ*-OH)6] <sup>8</sup><sup>−</sup> polyoxoanion and (**h**) [Zr6O4(OH)4(H2O)2(CH3COO)5(AsW9O33)2] <sup>11</sup><sup>−</sup> polyoxoanion. Color codes: W/MoO6, red; PO4, yellow; SiO4, light blue; GeO4, turquoise; Zr, bright green; As, lavender; C, gray; O, red; H, black.

#### *2.3. Ti/Zr-Substituted Trimeric POMs*

In contrast to the dimeric POM structures, the syntheses of Ti/Zr-substituted trimeric POMs have rarely been reported. The early reported Zr-substituted trimeric POM is Zr6O2(OH)4(H2O)3(*β*-SiW10O37)3] <sup>14</sup><sup>−</sup> (Figure 5a), which was synthesized using an aqueous solution-based method by Kortz et al. in 2006. This polyoxoanion consists of three *β*23- SiW10O37 units linked by an unprecedented Zr6O2(OH)4(H2O)3 cluster with *C1* point group symmetry [79]. The similar polyoxoanion [Zr6(*μ*3-O)3(OH)3(OAc)(H2O)(*β*-GeW10O37)3] 16− was also synthesized via hydrothermal method by Yang's group in 2019, except that {*β*-SiW10O37} was replaced by {*β*-GeW10O37} [80]. Late on, Kortz et al. reported another Zr6-substituted silicotungstate [Zr6(O2)6(OH)6(*γ*-SiW10O36)3] <sup>18</sup><sup>−</sup> (Figure 5b) in 2008, where 6-Peroxo-6-Zr Crown embedded in a triangular polyoxoanion. This polyoxoanion is composed of three [*γ*-SiW10O36] <sup>8</sup><sup>−</sup> units encapsulating the unprecedented [Zr6(O2)6(OH)6] 6+ wheel, while it can also be considered as a cyclic assembly of three fused {Zr2(O2)2(OH)2(*γ*-SiW10O36)} monomers. This work belongs to the first structurally characterized Zr-peroxo POM with side-on, bridging peroxo units [81]. Additionally, Kortz et al. reported the first examples of Ti-containing trimeric polytungstates, two cyclic Ti9-containing trimeric POMs, [(*α*-Ti3PW9O38)3(PO4)]18<sup>−</sup> (Figure 5c) and [(*α*-Ti3SiW9O37OH)3(TiO3(OH2)3)]<sup>17</sup><sup>−</sup> (Figure 5d) using the solution-based synthetic method. Both compounds were composed of three (Ti3XW9O37) units (X = P or Si) bridged with three Ti-O-Ti bonds and a capping group (tetrahedral PO4 or octahedral TiO6) [82]. Liu's group reported two similar trimeric nine-TiIV contained tungstogermanates {K⊂[(Ge(OH)O3)(GeW9Ti3O38H2)3]}<sup>14</sup><sup>−</sup> (Figure 5e) and {K⊂[(SO4)(GeW9Ti3O38H3)3] <sup>10</sup>−. The two compounds were obtained from the reactions between K8[*γ*-GeW10O36] [83] and TiO(SO4) under different pH conditions. The former polyoxoanion consisted of three tri-TiIV-substituted Keggin fragments [GeW9Ti3O38] and a GeO4 tetrahedral linker bridged with both Ti–O–Ti and Ti–O–Ge bonds, and the structure of the latter one is similar except for the replacement of SO4 with GeO4 [84]. Recently, a novel trimer compound [{Ca6(CO3)(*μ*3-OH)(OH2)18}(P2W15Ti3O61)3Ca(OH2)3] <sup>19</sup><sup>−</sup> (Figure 5f) was reported that contains a hexacalcium cluster cation, one carbonate anion, and one calcium cation assembled on a trimeric tri-Ti-substituted Wells–Dawson polyoxometalates. This complex was obtained through the reaction of calcium chloride with the monomeric trititanium(IV)-substituted Wells−Dawson POM species "[P2W15Ti3O59(OH)3] <sup>9</sup>−". During the synthesis, the [Ca6(CO3)(*μ*3-OH)(OH2)18] 9+ cluster cation, composed of six calcium

cations linked by one *μ6*-carbonato anion and one *μ3*-OH- anion, assembled with one calcium ion, a trimeric "[P2W15Ti3O59(OH)3] <sup>9</sup>−" species to form the target product. The compound is an unprecedented POM species containing an alkaline-earth-metal cluster cation, and it is also the first example of alkaline-earth-metal ions clustered around a Ti-substituted POM [85].

**Figure 5.** The ball-and-stick and polyhedral illustration of (**a**) Zr6O2(OH)4(H2O)3(*β*-SiW10O37)3] 14− polyoxoanion, (**b**) [Zr6(O2)6(OH)6(*γ*-SiW10O36)3] <sup>18</sup><sup>−</sup> polyoxoanion, (**c**) [(*α*-Ti3PW9O38) 3(PO4)]18<sup>−</sup> polyoxoanion, (**d**) [(*α*-Ti3SiW9O37OH)3(TiO3(OH2)3)]<sup>17</sup><sup>−</sup> polyoxoanion, (**e**) {K⊂[(Ge (OH)O3)(GeW9Ti3O38H2)3]}<sup>14</sup><sup>−</sup> polyoxoanion and (**f**) [{Ca6(CO3)(*μ*3-OH)(OH2)18}(P2W15Ti3 O61)3Ca(OH2)3] <sup>19</sup><sup>−</sup> polyoxoanion. Color codes: WO6, red; PO4, yellow; SiO4, light blue; GeO4, turquoise; Ti, green; Zr, bright green; Ca, orange; C, gray; O, red.

## *2.4. Ti/Zr-Substituted Tetrameric POMs*

In this section, a number of Ti/Zr-substituted tetrameric POMs will be briefly introduced. The early example is a dodeca-Ti-substituted Dawson-type tetrameric, [{Ti3P2W15O57.5(OH)3}4] <sup>24</sup><sup>−</sup> (Figure 6a), representing a supramolecular phosphotungstate reported by Kortz's group in 2003 [86]. The polyoxoanion was composed of four lacunary [P2W15O56] <sup>12</sup><sup>−</sup> Well–Dawson building blocks linked with terminal Ti-O bonds, resulting in a structure with *Td* symmetry. The {Ti12O46} core of the polyoxoanion is composed of four groups of three edge-shared, corner-linked TiO6 octahedra. Such a rare arrangement resembles one set of the four corner-shared faces of an octahedron, described as a "reversed Keggin structure", which is very similar to the [As4Mo12O50] <sup>8</sup><sup>−</sup> geometry reported by Sasaki and Nishikawa [87]. Apart from this Ti12 cluster, they also discovered another deca-Ti-substituted tetrameric species [{Ti3P2W15O57.5(OH)3}2{Ti2P2W16O60(OH)}2] <sup>26</sup><sup>−</sup> containing two {Ti3P2W15} and two {Ti2P2W16} fragments, therefore resulting in a structure with *C2v* symmetry. Meanwhile, Nomiya et al. also reported two multi-Ti-substituted tetrameric POMs. The first one is a giant "tetrapod"-shaped dodeca-Ti-substituted Dawson-type tetrameric phosphotungstate, [(*α*-1,2,3-P2W15Ti3O60.5)4Cl]37<sup>−</sup> (Figure 6b), which contains the four Wells-Dawson units fused together through Ti–O–Ti bonds. The structure exhibits an approximately *Td* symmetry, where the four Ti3O6 facets of "P2W15Ti3" occupied four alternate facets of an octahedron, and the one Cl- ion was encapsulated in the central octahedral cavity [88]. It is noted that a similar structure [(P2W15Ti3O60.5)4(NH4)]35<sup>−</sup> was also reported in 2011, except that Cl- was replaced by NH4 <sup>+</sup> [89]. The other example belongs to a "tetrapod"-shaped Ti16-substituted Dawson-type tetrameric phosphotungstate [(*α*-1,2,3-P2W15Ti3O62)4{*μ*3-Ti(OH)3}4Cl]45<sup>−</sup> (Figure 6c). The polyoxoanion, prepared by the reaction of [P2W15O56] <sup>12</sup><sup>−</sup> with an excess amount of TiCl4 in aqueous solution, was composed of four tri-TiIV-1,2,3-substituted α-Dawson substructures, four Ti(OH)3 bridging groups, and one encapsulated chloride ion [90]. Subsequently, Nomiya et al. also reported three similar Ti16-substituted POMs, [(*α*-P2W15Ti3O59(OH)3)4{*μ*3-Ti(H2O)3}4X]21<sup>−</sup>

(X = Br−, I−, and NO3 <sup>−</sup>), except that halogen atoms and some oxygen atoms are protonated [91]. In 2004, Kortz et al. synthesized a unique cyclic octa-Ti- substituted tetrameric tungstosilicate [{*β*-Ti2SiW10O39}4] <sup>24</sup><sup>−</sup> (Figure 6d) assembly under mild, one-pot reaction conditions. The polyoxoanion is composed of four {*β*-Ti2SiW10O39} Keggin fragments bridged with Ti–O–Ti bonds, leading to a cyclic assembly. The successful preparation of this compound provides future possibilities for preparing even larger wheel-shaped polyoxotungstates and other discrete nanomolecular objects of similar size, structure, and function as those made with polyoxometalates [92]. Then, Kortz's group also successfully prepared a hepta-Ti substituted arsenotungstate [Ti6(TiO6)(AsW9O33)4] <sup>20</sup><sup>−</sup> (Figure 6e) in 2014 using a simple one-pot procedure. The polyoxoanion contains a novel Ti7-core consisting of a central TiO6 octahedron surrounded by six TiO5 square pyramids, which was further capped by four trilacunary {AsIIIW9} fragments, leading to an assembly with *Td* point-group symmetry [93]. In 2019, Yang's group also reported a similar structure [Ti7O6(SbW9O33)4] <sup>20</sup><sup>−</sup> by replacing {AsW9O33} with {SbW9O33} [94]. In 2022, Yang's group further synthesized a ring-shaped 12-Ti-substituted poly(polyoxometalate) [{K2Na(H2O)3}@{(Ti2O)2(Ti4O4)2(A*α*-1,3,5-GeW9O36)2(A-*α*-2,3,4-GeW9O36)2}]25<sup>−</sup> (Figure 6f) under hydrothermal conditions, which represents the highest number of Ti centers in Keggin-type poly(POM) family to date. In this structure, two types of novel chiral trivacant [GeW9O36] <sup>14</sup><sup>−</sup> (A-*α*-1,3,5-GeW9O36 and A-*α*-2,3,4-GeW9O36) fragments have been first discovered in POM chemistry, and four [GeW9O36] <sup>14</sup><sup>−</sup> fragments are alternately connected by two Ti2O and two Ti4O4 cores to form a ring-shaped poly(POM) [95].

In addition to these Ti-substituted POMs, some Zr-substituted tetrameric POMs have also been actively investigated. For instance, a Zr4-substituted tungstoselenites, [(*α*-SeW9O34) {Zr(H2O)}{WO(H2O)}(WO2)(SeO3){*α*-SeW8O31Zr(H2O)}]2 <sup>12</sup><sup>−</sup> (Figure 7a), has been firstly constructed using {*α*-SeW9} building blocks. Such a tetrameric structure can be divided into two same subunits, each containing a dimer sandwich-type structure that consists of a well-known trivacant Keggin-type {*α*-SeW9O34} building blocks [96]. Then, Yang's group continuously reported eight tetrameric Zr-substituted POMs. The first example represents a new tetra-Zr-substituted tungstophosphate, {Zr2[SbP2W4(OH)2O21][*α*2-PW10O38]}2 20− (Figure 7b) through the hydrothermal reaction of the [B-*α*-SbW9O33] <sup>9</sup><sup>−</sup> building block with Zr4+ cations and PO4 <sup>3</sup><sup>−</sup> anions in the presence of dimethylamine hydrochloride in NaOAc-HOAc buffer solution. The compound exhibits a toroidal structure formed by two divacant [*α*2-PW10O38] <sup>11</sup><sup>−</sup> units and two [SbP2W4(OH)2O21] <sup>7</sup><sup>−</sup> fragments linked by four Zr4+ cations. It is noted that the triangular pyramidal SbO3 in the [B-α-SbW9O33] <sup>9</sup><sup>−</sup> precursor was replaced by tetrahedral PO4 unit in the final compound, and the pendant SbO3 derives from the dissociation of the [B-*α*-SbW9O33] <sup>9</sup><sup>−</sup> precursor [97]. Then, the same group also reported a Zr(IV) substituted tetramer polyoxotungstate, [Zr4(*β*-GeW10O38)2(A-*α*-PW9O34)2] <sup>26</sup><sup>−</sup> (Figure 7c) [98], which was obtained in a one-pot reaction of the hexalacunary polyanion [P2W12O48] 12− and trilacunary [GeW9O34] <sup>10</sup><sup>−</sup> polyanion precursors with Zr4+ in a slightly alkali aqueous solution in the presence of borates. Subsequently, a Zr9-substituted tetrameric germanotungstate, [{Zr5(*μ*3-OH)4(OH)2}@{Zr2(OAc)2(*α*-GeW10O38)2}2] <sup>22</sup><sup>−</sup> (Figure 7d) was constructed by two novel sandwich-type dimers [Zr2(OAc)2(*α*2-GeW10O38)2] <sup>18</sup><sup>−</sup> and one unique [Zr5(*μ*3-OH)4(OH)2] 14+ core in an approximately orthogonal fashion, showing a staggering tetrahedral polyoxoanion [80]. In addition, Yang et al. also reported a series of ring-shaped Zr8 substituted silicotungstates, [{Zr2(OH)2(*α*-SiW10O38)}2{Zr2(OH)2(*β*-SiW10O38)}2] 24− (Figure 7e) (dap = 1,3-diamino-propane), [(Zr2(OH)2)2(Zr2BO(OH)4)2(*β*-SiW10O38)4] 26− (Figure 7f) and [(Zr2BO(OH)4)2(Zr2B2O2(OH)5)2(*β*-SiW10O38)4] <sup>28</sup><sup>−</sup> [99]. The latter two compounds first provided the possibility of introducing Zr–B–O linkage to the lacunary sites. Very recently, they also reported a di-Zr-substituted polyoxotungstate [ZrSb4(OH)O2(A-*α*-PW8O32)(A-*α*-PW9O34)]2 <sup>18</sup><sup>−</sup> (Figure 7g) [100] and hepta-Zr-incorporated polyoxometalate [SbZr7O6(OH)4(B-*α*-GeW9O34)2(B-*α*-GeW11O39)2] <sup>21</sup><sup>−</sup> (Figure 7h) [101], these works greatly enriched the family of Zr-substituted POMs.

**Figure 6.** The ball-and-stick and polyhedral illustration of (**a**) [{Ti3P2W15O57.5(OH)3}4] 24− polyoxoanion, (**b**) [(*α*-1,2,3-P2W15Ti3O60.5)4Cl]37<sup>−</sup> polyoxoanion, (**c**) [(*α*-1,2,3-P2W15Ti3O62)4 {*μ*3-Ti(OH)3}4Cl]<sup>45</sup><sup>−</sup> polyoxoanion, (**d**) [{*β*-Ti2SiW10O39}4] <sup>24</sup><sup>−</sup> polyoxoanion, (**e**) [Ti6(TiO6) (AsW9O33)4] <sup>20</sup><sup>−</sup> polyoxoanion and (**f**) [{K2Na(H2O)3}@{(Ti2O)2-(Ti4O4)2(A-*α*-1,3,5-GeW9O36)2(A-*α*-2,3,4-GeW9O36)2}]<sup>25</sup><sup>−</sup> polyoxoanion. Color codes: WO6, red; PO4, yellow; SiO4, light blue; GeO4, turquoise; Ti, green; As, lavender; O, red.

**Figure 7.** The ball-and-stick and polyhedral illustration of (**a**) [(*α*-SeW9O34){Zr(H2O)}{WO(H2O)} (WO2)(SeO3){*α*-SeW8O31Zr(H2O)}]2 <sup>12</sup><sup>−</sup> polyoxoanion, (**b**) {Zr2[SbP2W4(OH)2O21][*α*2-PW10 O38]}2 <sup>20</sup><sup>−</sup> polyoxoanion, (**c**) [Zr4(*β*-GeW10O38)2(A-*α*-PW9O34)2] <sup>26</sup><sup>−</sup> polyoxoanion; (**d**) [{Zr5(*μ*3-OH)4 (OH)2}@{Zr2(OAc)2(*α*-GeW10O38)2}2] <sup>22</sup><sup>−</sup> polyoxoanion, (**e**) [{Zr2(OH)2(*α*-SiW10O38)}2{Zr2(OH)2 (*β*-SiW10O38)}2] <sup>24</sup><sup>−</sup> polyoxoanion, (**f**) [(Zr2(OH)2)2(Zr2BO(OH)4)2(*β*-SiW10O38)4] <sup>26</sup><sup>−</sup> polyoxoanion, (**g**) [ZrSb4(OH)O2(A-*α*-PW8O32)(A-*α*-PW9O34)]2 <sup>18</sup><sup>−</sup> polyanion and (**h**) [SbZr7O6(OH)4(B-*α*-GeW9O34)2(B-*α*-GeW11O39)2] <sup>21</sup><sup>−</sup> polyoxoanion. Color codes: WO6, red; PO4, yellow; SiO4, light blue; GeO4, turquoise; Se, pale blue; Sb, violet; Zr, bright green; B, rose; O, red.

#### *2.5. Ti/Zr-Substituted Multimeric POMs*

Compared to those mono-, di-, tri- and tetrameric POM structures, there are very few reports on the preparation of multimeric Ti/Zr-substituted POMs. To date, only two related compounds have been reported. The first example belongs to an octa-Tisubstituted Dawson-type supramolecular polyoxoanion reported by Kortz's group in 2003 [86]. However, the polyoxoanion was described by the preliminary and incomplete formula "Ti8P12W84" or "(Ti2P2W15)2(Ti2P2W16)2(P2W11)2" (Figure 8a) due to the poor quality

of the crystallographic data. The second representative example is the gigantic Zr24-clustersubstituted Keggin-type germanotungstates [Zr24O22(OH)10(H2O)2(W2O10H)2(GeW9O34)4 (GeW8O31)2] <sup>32</sup><sup>−</sup> (Figure 8b) reported by Yang's group in 2014 [102]. The polyoxoanion was successfully synthesized under hydrothermal conditions, which contains the largest [Zr24O22(OH)10(H2O)2] cluster among all reported Zr-based poly(polyoxometalate)s to date. Detailed structural analyses of this complex showed that the centrosymmetric Zr24-clusterbased hexamer contained two symmetry-related [Zr12O11(OH)5(H2O)(W2O10H)(GeW9O34)2 (GeW8O31)]<sup>16</sup><sup>−</sup> trimers linked via six *μ*3-oxo bridges, which were further encapsulated by different POM fragments including B-*α*-GeW9O34, B-*α*-GeW8O31, and W2O10. Catalytic experiments also showed that this compound worked as a good catalyst for the oxygenation of thioethers to sulfoxides/sulfones in the presence of H2O2, which could be attributed to the unique redox property of oxygen-enriched polyoxotungstate fragments as well as the Lewis acidity of the Zr24 cluster. Although the synthesis of multimeric POMs is rather difficult, these pioneering works provide some insights and future direction for the exploration of this specific research area.

**Figure 8.** The ball-and-stick and polyhedral illustration of (**a**) (Ti2P2W15)2(Ti2P2W16)2(P2W11)2 polyoxoanion, (**b**) [Zr24O22(OH)10(H2O)2(W2O10H)2(GeW9O34)4(GeW8O31)2] <sup>32</sup><sup>−</sup> polyoxoanion. Color codes: WO6, red; GeO4, turquoise; Zr, bright green; O, red. Adapted with permission from ref. [86], copyright 2003 John Wiley and Sons.

#### **3. The Applications of Representative Ti/Zr-Substituted POMs**

It is well known that transition-metal clusters are a unique area in inorganic chemistry, considering their vital contribution to the blossom of modern chemistry as well as their potential application as structural models for various industrial and biological catalytic processes [103–106]. To date, the Ti/Zr-substituted POMs have been widely investigated as catalysts for the oxidation of organic substrates. For example, Poblet et al. investigated the oxidation of alkenes by H2O2 catalyzed using Ti(IV)-containing POMs, which were models of Ti single-site catalysts at the DFT computational level. The catalytic mechanism of the C2H4 epoxidation with H2O2 mediated by [PTi(OH)W11O39] <sup>4</sup><sup>−</sup> and [Ti2(OH)2As2W19O67(H2O)]8<sup>−</sup> can be processed by following two main steps (Scheme 1): (i) H2O2 was activated to form the titanium-peroxo or -hydroperxo intermediate, and (ii) the reactive intermediate further attack alkene to form the epoxide and water [107]. Kholdeeva and co-workers also reported several cases of various organic catalytic studies with Ti-containing POMs. For example, they investigated the mechanism of thioether oxidation of (Bu4N)7{[PW11O39Ti]2OH} dimeric heteropolytungstate in 2000 [108]. In 2004, they also reported a protonated titanium peroxo complex [Bu4N]4[HPTi(O2)W11O39] and found that protonated titanium peroxo complex has a higher redox potential, which can improve the catalytic performance [109]. In the same year, Ti(IV)-monosubstituted Keggin-type POMs were reported to exhibit excellent catalytic oxidation properties with H2O2 [110–112]. Subsequently, Kholdeeva et al. also reported the epoxidation of a range of alkenes easily proceeds with aqueous H2O2 as oxidant and the dititanium-containing 19-tungstodiarsenate (III) as catalyst [113]. In 2012, the same group also published two works on alkene oxidation by Ti-containing POMs. They claimed that the energy barrier for

the heterolytic oxygen transfer from the reactive Ti hydroperoxo intermediate was significantly reduced by the protonated Ti-containing POM, as revealed by the kinetic and DFT studies, thereby greatly enhancing the activity and selectivity of alkene oxidation [114,115]. Subsequently, Poblet and Guillemot reported the alkene epoxidation catalyzed by the hybrid [B-*α*-SbW9O33( *t* BuSiO)3Ti(O*<sup>i</sup>* Pr)]3−, [PW9O34( *t* BuSiO)3Ti(O*<sup>i</sup>* Pr)]3<sup>−</sup>, and Ti-complexe of silanol functionalized POMs [SbW9O33(RSiO)3Ti(O*<sup>i</sup>* Pr)]3−, respectively [32,116,117]. Based on the research of alkene epoxidation catalysis, Kholdeeva et al. also revealed the mechanism of thioether oxidation of Ti-substituted POMs by kinetic modeling and DFT calculations. Two possible models regarding the active group were proposed: (1) the active group is the terminal Ti−OH group for the mononuclear, and (2) the active group is the bridging Ti2(*μ*-OH) moiety for the multinuclear [118]. Subsequently, Yang's group also reported two cases of the catalytic oxidation of thioethers using Ti7- and Ti12-substituted POMs, respectively, which both exhibited good catalytic properties [94,95]. In addition, Li's group investigated the photocatalytic degradation of MB with Ti2-substituted POM units under UV irradiation, which proved that Ti-substituted Keggin-type POMs showed better photocatalytic activities than that of typical Keggin-type POMs [50]. Some Ti-substituted POMs have also been reported with good electrocatalytic properties [119].

**Scheme 1.** Schematic diagram of the proposed mechanistic processes.

Compared with the applications of Ti-substituted POMs, a part of Zr-substituted POMs also exhibited good catalytic oxidation of thioether, electrocatalytic and nonlinear optical properties, etc. For example, Yang's groups reported a few works on the oxidation of sulfide using Zr2-, Zr4-, Zr7- and Zr8-substituted POMs, respectively [76,99–101]. These compounds showed remarkable heterogeneous catalysts for the catalytic oxidation of sulfides into the corresponding sulfones with H2O2. The same groups also reported several Chiral Zr-substituted POMs which have excellent nonlinear optical properties [75,77]. In addition, transition-metal-substituted POMs are air- and water-stable Lewis acids that were often used in organic reactions [120], and in the last two decades, most Zr-substituted POMs also were reported to be used as optimal Lewis acid catalysts to hydrolyze the O = C-NH- bonds in proteins or peptides (Scheme 2), leading to the formation of amino acids [121–126]. These works implied the potential applications of Zr-substituted POMs in biological systems.

**Scheme 2.** Schematic diagram of hydrolysis of GG in the Presence of POMs.
