Non‐Aqueous Microwave‐Assisted Syntheses of Deca‐ and Hexa‐Molybdovanadates

Abstract

Abstract:We report an ew approach for the synthesis of heterohexa-and heterodecametalates via the use of nonaqueous,m icrowave-assisted reaction conditions.T he two novel molybdovanadates have been isolated and characterized in the solid and solution states using single-crystal X-ray diffraction, FT-IR, UV/Vis,m ultinuclear NMR spectroscopy, and ESI-MS.T he relative stabilities of the possible structural isomers were probed using dispersion-corrected DFT calculations for both polyoxometalate systems.Polyoxometalates (POMs) are molecular early-transitionmetal oxides that can be isolated through the condensation of simple oxyanions to yield oligomeric structures with somewhat predictable structural features and nuclearities.[1] Control of the synthetic conditions within well understood pH and concentration domains can be exploited to yield molecules suitable for further derivatization by incorporation of heteroatoms,o rganometallic complexes,o rg rafting of organic ligands to the surface of the molecular metal oxide.[2] Given this compositional and structural versatility,POMs have been utilized for catalytic, biological, and molecular electronic applications. [3][4][5][6][7][8][9][10][11] Historically the compositional diversification of POMs has been achieved via:reaction of stoichiometric mixtures of the desired constituents in pH controlled media;the addition of electrophiles to preformed plenary or lacunary POMs;o r the covalent functionalization of the POM surface by organic moieties. [12][13][14][15][16][17][18][19][20][21][22][23][24] Often the precise atomic ratios of early transition metals in the products can be controlled, however, occasionally the solution chemistry is sufficiently complex that separation and structural characterization of all products is not possible.[ 3À species". Furthermore,t he resulting arrangement of heteroatoms in POMs bearing oxo ligands of varying basicity and hence propensities for protonation has been utilized over many years.T his enables the direct functionalization of the molecular metal oxide surface by av ariety of alkoxo species resulting in inorganic-organic hybrids with ahost of applications. [27,28] Herein, we build upon our recent efforts towards the microwave-assisted synthesis of POMs [29,30] and report the characterization of two elusive molybdovanadates as their tetrabutylammonium salts (TBA )] (2;F igure 1). Each of the compounds has been characterized in the solid state by X-ray diffraction and FT-IR, and with solution measurements including 1 H-, 13 C-, and 51 V-NMR spectroscopy,E SI-MS and UV/Vis.Q uantumchemically optimized structural isomers,o btained with reliable and modern dispersion-corrected density [31] functional theory (DFT) approximations complement experimental observations and provide insight towards the future rationale preparation of POMs under non-aqueous conditions.Compound 1 was prepared by as imple one-pot microwave-assisted reaction whereby the tetrabutylammonium

Configurational Isomerism in Polyoxovanadates

“…50 and 12 kJ mol À1 higher in energy than the previously isolated a and b isomers,respectively.W ed iscuss this unexpected reaction in light of supramolecular SbÀO···V and SbÀO···Sb contacts manifested in {V 14 Sb 8 O 42 } 2 dimers detected in the solid state. This isomer is characterized by the inverted orientation (that is,pointing inwards) of one of its edge-sharing VO 5 pyramids,reducing the molecular symmetry to C 2v .Compound 2 features the first hetero-POV displaying configurational isomerism, and as the isomerization of the vanadyl moieties can occur at different positions, this Sb-POVisfurthermore subject to positional isomerism, [6] an aspect that may complicate synthesis and isolation of atargeted isomer.Thestructures of all hitherto known Sb-POVcluster shells can be derived from the {V IV 18 O 42 }a rchetype by substitution of 2, 3, or 4{ VO 5 }s quare pyramids with dumbbell-like {Sb III 2 O 5 }u nits.O fa ll possible [V 14 Sb 8 O 42 (H 2 O)] 4À isomers (see Supporting Information, Figure S1), the D 2d -symmetric a [7] and the D 2h -symmetric b [8][9][10][11][12][13] isomers have so far been observed. [2] As such, the syntheses of semimetalfunctionalized POVs often are entirely serendipitous.M oreover, within the most widely explored class of antimonato polyoxovanadates, [3] the obtained cluster compounds usually are nearly insoluble,l imiting the study of consecutive reactions.V ery recently,w ed iscovered aw ater-soluble antimonato polyoxovanadate (Sb-POV), {Ni-(en) 3 } 3 [V IV 15 Sb III 6 O 42 (H 2 O) x ]·ca.15 H 2 O( 1; x = 0, 1; en = ethylenediamine), representing as uitable starting material for the in situ generation of novel Sb-POVc ompounds.…”

“…[4,5] The addition of cyclen (1,4,7,10- This isomer is characterized by the inverted orientation (that is,pointing inwards) of one of its edge-sharing VO 5 pyramids,reducing the molecular symmetry to C 2v .Compound 2 features the first hetero-POV displaying configurational isomerism, and as the isomerization of the vanadyl moieties can occur at different positions, this Sb-POVisfurthermore subject to positional isomerism, [6] an aspect that may complicate synthesis and isolation of atargeted isomer. [4,5] The addition of cyclen (1,4,7,10- This isomer is characterized by the inverted orientation (that is,pointing inwards) of one of its edge-sharing VO 5 pyramids,reducing the molecular symmetry to C 2v .Compound 2 features the first hetero-POV displaying configurational isomerism, and as the isomerization of the vanadyl moieties can occur at different positions, this Sb-POVisfurthermore subject to positional isomerism, [6] an aspect that may complicate synthesis and isolation of atargeted isomer.…”

“…Thef our niches formed between the two V 3 arcs and the V 8 ring are then occupied by Sb 2 Os ubunits,w hereby each Sb atom binds to one Oo ft he V 8 ring and one Oo faV 3 arc. [6][7][8][9][10][11][12] An inter-cluster V7ÀO4 bond (2.348 (2) )g enerates an octahedral environment for V7 and increases its BVS from 3.71 to 3.92 ( Figure 2). Within the cluster shell of the a 1 *-{V 14 Sb 8 O 42 }isomer present in 2,t he central vanadyl group of one V 3 arc points to the center of the cluster sphere (V7ÀO42;F igure 1).…”

“…[4,5] The addition of cyclen (1,4,7,10-tetraazacyclododecane) to an aqueous solution of this precursor results in the rapid formation of {Ni(cyclen)(en)} 2 [V 14 Sb 8 O 42 (H 2 O)]·ca.10 H 2 O (2), as the a 1 *-{V 14 Sb 8 O 42 }i somer. This isomer is characterized by the inverted orientation (that is,pointing inwards) of one of its edge-sharing VO 5 pyramids,reducing the molecular symmetry to C 2v .Compound 2 features the first hetero-POV displaying configurational isomerism, and as the isomerization of the vanadyl moieties can occur at different positions, this Sb-POVisfurthermore subject to positional isomerism, [6] an aspect that may complicate synthesis and isolation of atargeted isomer.Thestructures of all hitherto known Sb-POVcluster shells can be derived from the {V IV 18 O 42 }a rchetype by substitution of 2, 3, or 4{ VO 5 }s quare pyramids with dumbbell-like {Sb III 2 O 5 }u nits.O fa ll possible [V 14 Sb 8 O 42 (H 2 O)] 4À isomers (see Supporting Information, Figure S1), the D 2d -symmetric a [7] and the D 2h -symmetric b [8][9][10][11][12][13] isomers have so far been observed. All isomers comprise one eight-membered ring of edge-sharing VO 5 pyramids that is capped from above and from below by arcs consisting of three edge-sharing VO 5 pyramids.The relative orientation of these arcs differentiates the a (908 8), the b (08 8), and the (hypothetical) g (458 8)i somer.…”

“…[4,5] The addition of cyclen (1,4,7,10-tetraazacyclododecane) to an aqueous solution of this precursor results in the rapid formation of {Ni(cyclen)(en)} 2 [V 14 Sb 8 O 42 (H 2 O)]·ca.10 H 2 O (2), as the a 1 *-{V 14 Sb 8 O 42 }i somer. This isomer is characterized by the inverted orientation (that is,pointing inwards) of one of its edge-sharing VO 5 pyramids,reducing the molecular symmetry to C 2v .Compound 2 features the first hetero-POV displaying configurational isomerism, and as the isomerization of the vanadyl moieties can occur at different positions, this Sb-POVisfurthermore subject to positional isomerism, [6] an aspect that may complicate synthesis and isolation of atargeted isomer.…”