The Heteropolytungstate Core {BW 13 O 46 } 11− Derived as Monomer, Dimer, and Trimer

A study of the borotungstate system has led to the characterization of new, original compounds based on the unconventional Keggin derivative [H 3 BW 13 O 46 ] 8− ion (denoted as 1 ). [H 3 BW 14 O 48 ] 6− ( 2 ) and the dimer [H 6 B 2 W 26 O 90 ] 12− ( 3 ) crystallize as mixed cesium/ammonium salts and have been characterized by single‐crystal X‐ray diffraction analysis. Anion 2 reveals an unusual arrangement, consisting of an outer {W 3 O 9 } core grafted onto the monovacant [BW 11 O 39 ] 9− Keggin moiety and exhibits an unprecedented distorted square‐pyramidal arrangement for a cis ‐{WO 2 } core. Elemental analysis, supported by bond distance analysis, is consistent with the presence of three protons distributed over the terminal oxygens of the outer {W 3 O 7 } capping fragment. The [H 6 B 2 W 26 O 90 ] 12− ion ( 3 ) is formally derived from the direct condensation of two [H 3 BW 13 O 46 ] 8− subunits. The cisoid arrangement of the two [BW 11 O 39 ] 9− subunits, coupled with the antiparallel arrangement of the two quasi‐linear OW⋅⋅⋅OWOH 2 chains within the central {W 4 O 12 } connecting group, breaks any symmetry, thereby resulting in a chiral compound. Polarography and pH‐metric titrations reveal the formation of the monomeric precursor [H 3 BW 13 O 46 ] 8− (anion 1 ) under stoichiometric conditions. 183 W NMR analysis of 2 and 3 in solution gives complex spectra, consistent with the presence of equilibria between several species. In the frame of this study, we also report on a structural re‐investigation of the [H 6 B 3 W 39 O 132 ] 15− ion ( 4 ) based on reliable results obtained in the solid state by means of single‐crystal X‐ray diffraction analysis, and in solution by means of 1D and 2D COSY 183 W NMR. X‐ray diffraction analysis revealed the presence of three attached aquo ligands on the central {W 6 O 15 } connecting core, generating three OW⋅⋅⋅OWOH 2 quasi‐linear chains, which are responsible for the chirality of the trimeric assembly. This structural arrangement accounts for the 39‐line 183 W solution spectrum. The 2D COSY spectrum permits reliable assignments of the six strongly shielded resonances (around −250 and −400 ppm) to the six central W atoms, as well as additional assignments. The origin of such strong shielding for these particular W atoms is discussed on the basis of previously published results. Infrared data for compounds 1 , 3 , and 4 are also presented.