An all-vanadium(III) hexametalate Lindqvist structure and its chromium and iron analogues.

The hexametalate Lindqvist ion {M6O19} n , comprising a superoctahedron of condensed {MO6} fragments, is one of the classic structures in polyoxometalate chemistry and is known for Mo, W, Nb and Ta. Although the naked vanadium clusters are not known, the all-vanadium(V) form has been stabilised by capping with organometallic fragments. Replacement of some or all of the twelve m-oxides with malkoxides, thus reducing the charge of the core (a common route to stabilising polyoxovanadates), can stabilise reduced V forms and Zubieta and co-workers isolated the fully reduced, all-V hexametalate by using tris-alkoxides in [(VIV6O)6OACHTUNGTRENNUNG{EtC ACHTUNGTRENNUNG(CH2O)3}4]2 (1 , Figure 1). Here we report the isolation of the six-electron further reduced (i.e., the all-vanadium ACHTUNGTRENNUNG(III)) hexavanadate by formally replacing {VO} with the {VCl} fragment. Furthermore, we can extend this chemistry to the analogous Cr and Fe species. This series of hexametalate structures is attractive for magnetic studies because 1) families of isostructural high-nuclearity clusters for several different metal ions (here spanning d, d, d and d) are very rare and 2) the cluster anions approximate very closely to Td symmetry and this is a rare example in which an analytical solution exists to the spin Hamiltonian for a large cluster. Solvothermal reaction of [VCl3 ACHTUNGTRENNUNG(thf)3] with pyridine (1:2) in MeCN at 100 8C, followed by addition of the triol MeCACHTUNGTRENNUNG(CH2OH)3 (2 equiv) and further heating at 150 8C produced crystalline [(VCl)6O ACHTUNGTRENNUNG{MeC ACHTUNGTRENNUNG(CH2O)3}4] ACHTUNGTRENNUNG(Hpy)2 (2). Complex 2 crystallises in the cubic space group P4̄3n. The [(VCl)6OACHTUNGTRENNUNG{MeC ACHTUNGTRENNUNG(CH2O)3}4]2 (2 ) anion has crystallographic C3v point symmetry (two unique metal ions; Figure 1 and Table 1), but approximates very closely to Td (0.1% deviation, from comparison of edge lengths of the tetrahedron defined by the four methyl groups). The anion structure is identical to that found by Zubieta et al. , 1 , but with replacement of all {VO} with {VCl} (replacement of MeCACHTUNGTRENNUNG(CH2OH)3 by EtC ACHTUNGTRENNUNG(CH2OH)3 in the synthesis gave the directly analogous [(VCl)6O ACHTUNGTRENNUNG{EtCACHTUNGTRENNUNG(CH2O)3}4] ACHTUNGTRENNUNG(Hpy)2 (3)). A superoctahedron of six V ions (each with a {VO5Cl} coordination sphere) is centred on a m6-oxide (O1), with alternate faces of the octahedron capped by the four m,m,m-MeC ACHTUNGTRENNUNG(CH2O)33 ligands (defining a tetrahedron). Hence, each RC ACHTUNGTRENNUNG(CH2O)33 replaces three moxides from the parent Lindqvist structure. All of the vanadium ions have a terminal chloride and are V as confirmed by the charge balance (there are two pyridinium cations), bond valence sum calculations (bond valence sum= 3.12 and 3.10) and magnetic properties (see below). In comparison with the V structure 1 (and related V structures) the {V6O} core in 2 2 has contracted in size despite reduction of the metal ions (V (m6)O=2.315(2) and 2.191(4) for 1 and 2 (mean value), respectively). The longer V O1 bond length in 1 presumably reflects the stronger trans influence of terminal oxide (compared with [a] L. J. Batchelor, Dr. R. Shaw, S. J. Markey, Dr. M. Helliwell, Prof. E. J. L. McInnes School of Chemistry, The University of Manchester Manchester M13 9PL (UK) Fax: (+44) 161-275-4616 E-mail : eric.mcinnes@manchester.ac.uk Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201000222. Table 1. Selected bond lengths and angles for 2, 4 and 5.