Spectroscopic and magnetic investigation of the K-5[PMo2VW9O40]center dot 24H(2)O heteropolyoxometalate and its monoprotonated form

The K-5[PMo2VW9O40]. 24H(2)O heteropolyoxometalate complex was synthesized and investigated by means of spectroscopic and magnetic susceptibility meas urements. The co-ordination of the vanadium(IV) ion to the monovacant alpha -[PMo2W9O39](7-) heteropolyanion results in the shift of the bicentric nu ( asym)(M=O-a) and nu (asym)(M=O-d) and tricentric nu (asym)(M-O-b,O-c-M) vib rations (M=W or Mo) in the 700-1100 cm(-1) region of the IR spectrum of the complex compared to those of the ligand. Two low wavenumber bands in VIS s pectrum were assigned to B-2(2)--> E-2 (11310 cm(-1)) and B-2(2)--> B-2(1) (13710 cm(-1)) transitions for V-IV in an axial C-4v environment. The two s houlders at 16150 and 19220 cm(-1) correspond to V-IV--> Mo-VI,W-VI and Mo- V--> Mo-VI,W-VI transitions, respectively. The UV transitions (38480 and 47 680 cm(-1)) correspond to charge transfer bands. The simulation of the powd er EPR spectrum of the sample obtained at room temperature indicates the pr esence, in the same amounts, of one-electron-reduced species (g parallel to =1.922, g(perpendicular to)=1.972, A(parallel to)=181 G, A(perpendicular t o)=63 G) and a monoprotonated two-electron-reduced species with mixed valen ce V-IV, Mo-V and Mo-VI (g(iso)=1.972, DeltaB(iso)(p-p)=450 G). The magneti c parameters (mu (eff)=2.11 mu (B), theta=-59 K) of the sample and the effe ctive spin S'=0.84 estimated for the two-electron-reduced species suggest t he presence of one antiferromagnetic coupling between the V-IV and the Mo-V -Mo-VI pair.