NEW BI-OXO-CAPPED TRIANGULAR TRINUCLEAR CLUSTER COMPOUNDS OF NIOBIUM

Article abstract of DOI:10.1021/ja00324a021

The compounds (1) and (2) have been prepared by reactions of the sodium carboxylates with Nb2Cl6(Me2S)3.Crystals of 1 were unstable (solvent loss?) and diffracted poorly.The structure was solved and refined sufficiently to define the trinuclear cation and the anion unambiguously, but not accurately.Compound 2 formed excellent crystals which allowed a complete and accurate structure determination.The trinuclear cation has no crystallographically imposed symmetry but in its essentials approximates to the D_3h symmetry found for M3O2(O2CR)6L3 species formed by Mo and W; it is clearly closely related to the previously known (5-) ion.The mean values of important structural parameters are the following: Nb-Nb, 2.842 <3> Angstroem; Nb-O(cap), 2.035 <6> Angstroem; Nb-O(carbox), 2.150 <4> Angstroem; Nb-O(THF), 2.25 <1> Angstroem; and Nb-O(cap)-Nb, 88.5 <3> deg.Compound 2 crystallizes in space group Pbca with a=27.033 (5) Angstroem, b=26.251 (4) Angstroem, c=20.609 (3) Angstroem, V=14.625 (7) Angstroem³, and Z=8.The 1H NMR spectrum of 2 is consistent with the composition and structure established crystallographically, and together with an Evans method measurement it shows the compound to be diamagnetic in solution.The solid is also diamagnetic, but when a correction is made for the diamagnetic contributions of all the atoms, it is concluded that the cation has μ_eff=1.3μ_B.No epr signal was detected at liquid nitrogen temperature.All data are consistent with a spin triplet ground state and a Nb-Nb bond order of 2/3 (i.e., ground configuration of a²e² or e³a).