Mechanism of substitution on trinuclear incomplete cuboidal [M3X4(OH2)9]4+ ions: kinetic studies of water exchange and substitution by Cl? on [Mo3S4(OH2)9]4+

Abstract

The complex [Mo,S4(OH,),IQ+ possesses non-equivalent H, O ligands with those (d, two per Mo) which are trans to p-S undergoing water exchange 1 O5 faster than those (c, one per M o ) which are trans to p3-S.The former arises from a labilising pathway involving the monohydroxo conjugate base [Mo& (OH2),(OH)I3'

Kinetic Studies of Water Exchange and Substitution by NCS– on the Sulphur-capped Triangular Ion [Μο3(μ3-S)(μ-O)3(ΟΗ2)9]4+

“…This is not attainable by direct elution from Dowex cation-exchange columns using concentrated solutions of HCIO4, and so an alternative method reported previously [9], involving solutions initially purified by elution with HCl, was employed. Pure fractions of Mo3SC>3 + (aq) in 4M HCl obtained above were combined and evaporated to dryness on a vacuum line (10~3torr) to leave a dark reddish-purple residue of the chloroaqua salt [MogSOgCl^iOHzig-JCU-^ 0t~4).…”

“…The transverse relaxation rates, l/T2b, for both the c-and d-H20 ligands were evaluated by measurement of the linewidths [9] of their corresponding 17 0 NMR resonances, Figure 2 were added to each solution in order to relax out the intense resonance line of free H20 [22]. The rate constant for exchange at water'd' was computed from plots of ln(l/T2b) versus 1 /T(K) for both waters using a non-linear least squares program.…”

“…Those labelled'd' (two per Mo), which are approximately trans to the μ2-Χ bridging groups, undergoing exchange roughly 10 5 times faster than those labelled 'c' (one per Mo), which are trans to the μ3-Χ capping group. From detailed kinetic studies on the redox stable μ-sulphido 3 Brought to you by | Purdue University Libraries Authenticated Download Date | 6/7/15 5:58 PM species, [Μο3(μ3-5)(μ-8)3(ΟΗ2)9] 4+ [9] the faster rate of exchange in the former arises from a labilising pathway involving the monohydroxy conjugate-base species [Mo3^3-S)^-S)3(OH2)8 (OH)] 3+ (KaM = 0.18 M). The closeness in the rate constants observed for Cl~ [9] and NCS" [11] anation and that for water exchange, coupled with the relatively large ΔΗ* (83kJmol _1 ) and highly positive AS* (+107 J K _1 mol -1 ) associated with the water exchange process [9], implies a dissociative mechanism for the relevant interchange reaction on [Mo3S4(OH2)8(OH)] 3+ .…”

“…The well-characterised family of triangular incomplete-cuboidal aqua clusters [Μο 3 (μ3-Χ)(μ-X)3(OH2)9] 4+ (X=0, S or Se) [1][2][3][4][5][6][7][8][9][10][11][12][13][14] has provided interest regarding kinetic studies of the water ligand substitution process because of the presence at the molybdenum centre of non-equivalent water ligands, Figure 1, which have markedly differing rates of water exchange [6,9,10]. Those labelled'd' (two per Mo), which are approximately trans to the μ2-Χ bridging groups, undergoing exchange roughly 10 5 times faster than those labelled 'c' (one per Mo), which are trans to the μ3-Χ capping group.…”

Building Block Libraries and Structural Considerations in the Self-assembly of Polyoxometalate and Polyoxothiometalate Systems

“…The previously reported pseudocuboidal fragment [Mo3S4] 4+ tends to form low nuclearity clusters. It can react with transition metals or noble metal ions to form dimeric structures [48,49]. On the contrary, [Mo2O2S2] 2+ proved to be a lot more versatile and chemically suitable to interact constructively with a variety of organic and inorganic ligands including POM-based species [50].…”