An unprecedented [MoIV3O4]-incorporated polyoxometalate concomitant with MoO2 nucleophilic addition

Abstract

An unprecedented [Mo(IV)3O4]-incorporated polyoxometalate (POM), [H4Mo(IV)6Mo(VI)7O36py6](2-) (1a, py = pyridine), has been successfully prepared and structurally characterized by unique, controlled oxidization of the [Mo(IV)3O4] precursor. It has a β-Keggin structure with two Mo-Mo bonded [Mo(IV)3O4] constitutional units. Unusual MoO2 nucleophilic addition of the β-Keggin structure has been observed.

Metal–metal bonds in polyoxometalate chemistry

“…Please do not adjust margins [Mo IV 6Mo VI 7O32(OH)4(py)6] 2− (py = pyridine), which featured β-Keggin isomer with two triangular {Mo3} units that are additionally capped with cationic {MoO2} unit. 109 The first polyanion was produced with the help of [Mo3O4(Hnta)3] 2− (where Hnta = nitrilotriacetic acid) precursor, 110 although in the follow-up studies [Mo3O2(O2CCH3)6(H2O)3] 2+ precursor was employed. 74 Some of the follow-up studies involve reports on [Sb III 3Mo IV 3Mo VI 15O55(OH)2(py)3] − , 111 [Mo@Mo IV 6Mo V 6Mo VI 9O54(OH)4(py)6] 4− , 112 and [Na@Mo IV 12Mo V 4Mo VI 3O43(OH)(py)12] (all in Figure 6).…”

Extended Mo3IV-Polyoxometalates: 1D-[Mo6IVMo4VIGe2ZnO31py9 (H2O)]4– and 2D-[Mo3IVMo9VIZn6P7O56py8]+ (py = Pyridine)

“…Preparation and single-crystal X-ray crystallographic chracterization of deeply reduced Mo 3 IV -POMs have been long-term challenging issues faced by POMs’ synthetic chemists for several reasons: (a) single Mo IV is extremely easily oxidized to higher oxidation states; (b) all electrochemical reduction in the solution stops at the Mo VI → Mo V stage and the further reduction to Mo IV cannot be achieved; and (c) different from W V , − d 1 -Mo V is stabilized by the single Mo V –Mo V metal–metal bond and hence is not disproportionate to Mo IV and Mo VI . With the above concerns in mind, a strategy employing low-valence oxoclusters containing the incomplete cuboidal [Mo 3 O 4 ] moiety as precursor, which are the fundamental building units of POMs, was pioneered by our group. , The triangular Mo–Mo bonded [Mo 3 IV O 4 (H 2 O) 9 ] 4+ cluster has been well known as a real Mo IV aqua ion after a long-term dispute about the nature of Mo IV in aqueous solution. , Its synthesis, structure, kinetic substitution reactions, and multiple redox properties have been the subjects of numerous studies. − In 2013, the partial oxidation aggregation of the [Mo 3 IV O 4 (Hnta) 3 ] 2– (where Hnta = nitrilotriacetic acid) precursor produced the first Mo 3 IV -POMs, namely the Mo VI O 2 -Keggin adduct [Mo 6 IV Mo 7 VI O 32 (OH) 4 py 6 ] 2– . However, the serious drawback of [Mo 3 IV O 4 ]-type precursors, namely the uncontrolled complete oxidation of all Mo IV atoms to form fully oxidized POMs of the highest oxidation state, was commonly encountered for [Mo 3 IV O 4 L 9 ] precursors under solvothermal (py/H 2 O) conditions, which led to the very low and unrepeatable yields of Mo 3 IV -POMs, and hence restrained the further development of Mo 3 IV -POM chemistry in the following five years.…”

“…The two C 2 -symmetric building units [Ge 2 Mo 6 IV Mo 4 VI O 30 py 6 ] 4– ( 1a , 1b ) have a similar structure featuring the two triangularly Mo–Mo bonded [Mo 3 IV O 4 ] incomplete cuboidal units, with the strong Mo–Mo bonds ranging from 2.503(1) to 2.515(1) Å (av 2.510 Å) within the range of the previously reported values for [Mo 3 IV O 4 ]-type clusters − and Mo 3 IV -POMs 3–17 (2.487–2.537 Å, Table ). − They are held together by the two GeO 4 tetrahedra and four-square pyramidal Mo VI O 5 to form the building block 1a / 1b . As illustrated in Figure , 1a / 1b can be viewed as derived from 16 by removing the GeO 4 @Mo 6 VI O 14 of 16 with the glycol of 1 .…”

“…Electron-rich polyoxometalates (POMs) featuring metal–metal bonding have been receiving increasing attention due to their unusual molecular/crystal/electronic structures and promising applications in sustainable batteries, supercapacitors, nanoelectronics (POMtronics), medicine, and catalysis. , Deeply reduced M 3 IV -POMs (M = Mo, − W − ) are of special interest in aspects of the peculiarities in polytropic molecular/crystal/electronic structures, − the next generation of molecular cluster batteries, , electronic storage, − memristive devices, and Lewis field catalysis − among others. The Mo VI → Mo IV super-reduction of POMs dramatically changes the molecular and electronic structures and hence physicochemical properties in the following ways: (1) the rigid peripheral Mo V/VI  O double bonds are replaced by labile Mo IV ←: L dative bonds (L = Lewis base ligand), which functionalizes POMs with Mo IV Lewis acid active sites; (2) the d 2 -Mo IV addenda atom forms strong triangular Mo–Mo bonds with the two neighboring Mo IV atoms in the resulting Mo 3 IV triad boasting six d 2 –d 2 bonding electrons; (3) the coordinating ability of the internal capping oxygen atom in the incomplete cuboidal [M 3 IV (μ 3 -O 4 )­(μ 2 -O 3 )] unit is markedly weakened by the enhanced μ 3 -O–Mo 3 IV bonding; (4) the Mo IV Lewis acidity and Lewis basicity of the bridging (O b ) or thermal (O t ) oxygen atoms are enhanced by Mo 3 IV → O x M IV y electron transfer; and (5) the much shorter Mo IV –Mo IV bonds than the Mo VI ···Mo VI separations by more than 0.6 Å lead to the contraction and lowered symmetry of POMs .…”

“…The Mo VI → Mo IV super-reduction of POMs dramatically changes the molecular and electronic structures and hence physicochemical properties in the following ways: (1) the rigid peripheral Mo V/VI  O double bonds are replaced by labile Mo IV ←: L dative bonds (L = Lewis base ligand), which functionalizes POMs with Mo IV Lewis acid active sites; (2) the d 2 -Mo IV addenda atom forms strong triangular Mo–Mo bonds with the two neighboring Mo IV atoms in the resulting Mo 3 IV triad boasting six d 2 –d 2 bonding electrons; (3) the coordinating ability of the internal capping oxygen atom in the incomplete cuboidal [M 3 IV (μ 3 -O 4 )­(μ 2 -O 3 )] unit is markedly weakened by the enhanced μ 3 -O–Mo 3 IV bonding; (4) the Mo IV Lewis acidity and Lewis basicity of the bridging (O b ) or thermal (O t ) oxygen atoms are enhanced by Mo 3 IV → O x M IV y electron transfer; and (5) the much shorter Mo IV –Mo IV bonds than the Mo VI ···Mo VI separations by more than 0.6 Å lead to the contraction and lowered symmetry of POMs . These Mo 3 IV -induced structural features empower the POMo IV–VI hybrids with the following functionalities: (1) a nanoscale Lewis catalysis field (LCF) built of an Mo 3 IV Lewis acid cluster (LAC) and O x M VI y Lewis base clusters (LBC), which are reinforced by the Mo 3 IV → O x M y electron transfer and exhibit highly efficient and stable hydrogen transfer catalysis under mild conditions; − (2) varied energy-level structures and highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO–LUMO) gaps; (3) nucleophilic addition products more often encountered for the deeply reduced Mo 3 IV -heteropolyoxometalates due to the enhanced negative charge of oxygen atoms caused by the electron transfer; ,− (4) empty Mo 3 IV -POM structures free of interstitial atoms commonly observed because of the weakened coordinating capability of the capping oxygen atoms bonded to Mo 3 IV ; − , and (5) a large number of surface d–d bonding electrons available for the design of energy materials (molecular cluster batteries, supercapacitors), memristive devices, and electron-rich modification of the material surface through the functionalized peripheral dative ligands. Further regulation of the structures and properties can be achieved by incorporating a variety of heterometals by using the synthesis strategy developed recently. , However, as indicated in Figure , all Mo 3 IV -POMs thus far reported appeared as nanoscale oxoclusters of discrete structures.…”