Towards transuranic polyoxometalate coordination chemistr

The Keggin-derived mono-vacant lacunary heteropolymolybdate anion, [PMo[11]O[39]][7-] reacts with f-block elements Ce(IV) and Th(IV) to form the sandwich complexes [Ce(PMo[11]O[39])[2]][10-] (1) and [Th(PMo[11] O[39])[2]][10-] (2) in solution, and as caesium and ammonium salts in the solid phase. Both complexes have been characterised structurally and spectroscopically. Electronic absorption spectroscopy also indicates that an analogous Np(IV) complex can be prepared in solution. The reaction of the d-block metal Hf(IV) with phosphomolybdate has yielded the complex anion [Hf(PMo[11]O[39])(PMo[12]O[20])][6-] (3), which contains a Keggin anion and a lacunary anion. Crystallisation of 3 as an ammonium salt and subsequent structural characterisation reveals that the Hf(IV) cation does not coordinate through the four unsaturated lacunary site oxygen atoms as might be expected, but instead via four bridging oxygen atoms, as observed for Keggin anion coordination. A solution speciation study of tetravalent metal phosphomolybdate complexation as a function of pH and ligand:metal ratio reveals the ability of the d-block metal cations Hf(IV) and Zr(IV) to stabilise both Keggin and lacunary anions over a range of conditions, with Keggin complexation observed over the lower pH range. However, the tetravalent f-block elements Ce(IV) and Th(IV) are unable to stabilise Keggin anions on increasing pH and form solely the di-lacunary sandwich complex. The seven coordinate, pentagonal bipyramidal uranyl complexes in Na[14][(UO[2])[2](H[2]O)[2](B-alpha-AsW[9]O[33])].32H[2]O (4), (NH[4])[14][(UO[2])[2](H[2]O)[2](B-alpha-BiW[9]O[33])[2]].38H[2]O (5a), Na[14][Na[2](UO[2])[2](A-a-GeW[9]O[34])[2]].25H[2]O (6) and (NH[4])[15][NH[4](UO[2])[2](A-alpha-GeW[9]O[34])[2]]-32H[2]O (7) have been synthesised from the reactions of {UO[2]}[2+] with the respective heteropolytungstate anion. Their syntheses, structural and spectroscopic characterisations are reported, with solid and solution Raman vibrational spectroscopy revealing a weakening of the bonding in the {O=U=O}[2+] moiety on complexation of polyoxoanion in the equatorial plane. An electronic absorption spectroscopic study of {NpO[2]}[+] and {NpO[2]}[2+] complexation by tri-vacant heteropolytungstate ligands is presented and reveals the coordination of the polyoxoanion on elevation of pH, evident by significant shifts in Vis/nIR transitions and a concomitant colour change in solution. Both the Np(V) and Np(VI) heteropolytungstate compounds, (NH[4])[16][Na[2](NpVO[2])[2](A-alpha-GeW[9]O[34])[2]].27H2O (8a) and Na[14][Na[2](Np[v]IO[2])[2](A-alpha-GeW[9]O[34])[2]].36H[2]O (9) have been crystallised from solution and characterised structurally and spectroscopically. Each complex is isostructural to the reported uranyl(VI) equivalent, with the {NpO[2]}+ complex exhibiting a longer equatorial bond distance to polyoxoanion ligand as a result of the lower charge. The tri-neptunium compounds Na[4](NH[4])[14][(Np[V]O[2])[3](H[2]O)[3](BiW[9]O[33])[3](W[4]O[9])].62H[2]O (10) and Na[3](NH[4])[15][(Np[V]O[2])[3](H[2]O)[3](SbW[9]O[33])[3](W[4]O[9])].27H[2]O (11) have been crystallised from solution and exhibit pentagonal bipyramidal geometry at neptunyl. The two isostructural complexes contain a centrally coordinated W(VI) addenda atom that is coordinated to the three neptunyl(V) moieties via axial oxygen bonds i.e. W-O=Np linkages. Raman spectroscopy suggests that the tungsten-neptunyl cation-cation interaction further weakens the neptunyl moiety. The B-type ligand [B-alpha-AsW[9]O[33]][9-] reacts with {NpO[2]}[2+] to form the sandwich complex in (NH[4])[12]Na[2][(Np[VI]O[2])[2](H[2]O)[2](B-alpha-AsW[9]O[33])[2]].26H[2]O (12), which is analogous to the {UO[2]}[2+] equivalent and has been characterised structurally and by electronic absorption spectroscopy in solution. {PuO[2]}[2+] complexation by tri-vacant heteropolytungstate anions has been studied by electronic absorption spectroscopy with distinct spectral changes accompanied by a colour change in solution being observed on elevation of pH, confirming the coordination of polyoxoanion. The shift to lower energy of the major f-f transition on complexation is comparable to the shift of the analogous band in isoelectronic {NpO[2]}[+] on complexation. The complexes in solution and solid phase are predicted to be isostructural with the {UO[2]}[2+] and {NpO[2]}[2+] derivatives. The proposed sodium salt of [(PuO[2])[2](H[2]O)[2](SbW[9]O[33])[2]][14-] has been crystallised.