Preparation, structure and reactivity of the di-μ-disulfido NbIV2 aqua ion [Nb2(μ-S2)2(H2 O)8]4+, the structure of [Nb2(μ-S2)2(NCS)8 ]4−, and properties of the related (μ-S)2 aqua ion

Abstract

Using Nb IV S 2 Cl 2 as starting compound, and reaction routes involving aquation of Cl - (or NCS - ) in 4 M Hpts (Hpts = p-toluenesulfonic acid, CH 3 C 6 H 4 SO 3 H), an aqua ion product was obtained and purified by cation-exchange chromatography. Yellow crystals of [Nb 2 (µ-S 2 ) 2 (H 2 O) 8 ](pts) 4 ·4H 2 O 1, and orange crystals of the thiocyanato complex Cs 4 [Nb 2 (µ-S 2 ) 2 (NCS) 8 ]·2H 2 O 2 were isolated, and the structures of both determined by X-ray diffraction. The compounds have short Nb–Nb distances (2.85–2.90 A) consistent with metal–metal bonding, two η 2 :η 2 disulfido bridges (S–S distance 1.99 A), and 2 has N-bonded thiocyanate. Compound 2 was also shown to be diamagnetic. Conventional time range equilibration studies on the substitution of NCS - (>10-fold excess) for H 2 O on [Nb 2 (µ-S 2 ) 2 (H 2 O) 8 ] 4+ indicate [H + ] -1 -dependent rate laws, k f = k 1 k 2 [H + ] -1 for formation, and k aq = k -1 + k -2 [H + ] -1 for aquation. At 25 °C k 1 = 0.88 M -1 s -1 and k -1 = 2.4 × 10 -3 s -1 , I = 2.00 M (LiClO 4 ). The slow reaction of the low electron count di-µ-disulfido ion [Nb 2 (µ-S 2 ) 2 (H 2 O) 8 ] 4+ is an unusual feature. Neutral S-abstraction reactions from [Nb 2 (µ-S 2 ) 2 (H 2 O) 8 ] 4+ with PPh 3 or the water-soluble phenylsulfonate derivative (3-SO 3 C 6 H 4 ) 3 P 3- (PR 3 3- ) have stoichiometries of 2 moles of phosphine per dimer, and give a green product assigned as [Nb 2 (µ-S) 2 (H 2 O) 8 ] 4+ . Stopped-flow studies on the substitution of NCS - for H 2 O on [Nb(µ-S) 2 (H 2 O) 8 ] 4+ give a second-order formation rate constant k = 37 M -1 s -1 in 2.0 M Hpts solutions. On varying [H + ] in the range 1.0–2.0 M, [Nb 2 (µ-S) 2 (H 2 O) 8 ] 4+ gives a reversible green to yellow colour change with an [H + ] -1 dependent rate constant ≈4.8 × 10 -4 s -1 . An irreversible step is incident at [H + ] < 0.5 M. Comparisons are made of the inhibitory effect of η 2 :η 2 µ-disulfido bridges on substitution of H 2 O on [Nb 2 (µ-S 2 ) 2 (H 2 O) 8 ] 4+ as compared with [Nb 2 (µ-S) 2 (H 2 O) 8 ] 4+ , and reference is made to a similar trend for [Mo 3 (µ 3 -S)(µ–S 2 ) 3 (H 2 O) 6 ] 4+ and [Mo 3 (µ 3 -S)(µ-S) 3 (H 2 O) 9 ] 4+ . Attempts to prepare the Mo V 2 analogue [Mo 2 (µ-S 2 ) 2 (H 2 O) 8 ] 6+ gave [Mo 2 O 2 (µ-S) 2 (H 2 O) 6 ] 2+ .