| Autor: |
N. Sokolov, Maxim, Hernandez-Molina, Rita, R. J. Elsegood, Mark, L. Heath, Sarah, Clegg, William, Geoffrey Sykes, A. |
| Zdroj: |
Journal of the Chemical Society, Dalton Transactions; January 1997, Vol. 1997 Issue: 12 p2059-2066, 8p |
| Abstrakt: |
Using NbIVS2Cl2 as starting compound, and reaction routes involving aquation of Cl- (or NCS-) in 4 M Hpts (Hpts = p-toluenesulfonic acid, CH3C6H4SO3H), an aqua ion product was obtained and purified by cation-exchange chromatography. Yellow crystals of [Nb2(-S2)2(H2O)8 ](pts)4·4H2O 1, and orange crystals of the thiocyanato complex Cs4[Nb2(-S2)2(NCS) 8]·2H2O 2 were isolated, and the structures of both determined by X-ray diffraction. The compounds have short Nb–Nb distances (2.85–2.90 Å) consistent with metal–metal bonding, two η2:η2 disulfido bridges (S–S distance 1.99 Å), 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 H2O on [Nb2(-S2)2(H2O)8 ]4+ indicate [H+]-1-dependent rate laws, kf = k1 k2[H+]-1 for formation, and kaq = k-1 + k-2[H+] -1 for aquation. At 25 °C k1 = 0.88 M-1 s-1 and k-1 = 2.4 × 10-3 s-1, I = 2.00 M (LiClO4). The slow reaction of the low electron count di--disulfido ion [Nb2(-S2)2(H2O)8 ]4+ is an unusual feature. Neutral S-abstraction reactions from [Nb2(-S2)2(H2O)8 ]4+ with PPh3 or the water-soluble phenylsulfonate derivative (3-SO3C6H4)3P3- (PR33-) have stoichiometries of 2 moles of phosphine per dimer, and give a green product assigned as [Nb2(-S)2(H2O)8] 4+. Stopped-flow studies on the substitution of NCS- for H2O on [Nb(-S)2(H2O)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, [Nb2(-S)2(H2O)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 η2:η2 -disulfido bridges on substitution of H2O on [Nb2(-S2)2(H2O)8 ]4+ as compared with [Nb2(-S)2(H2O)8] 4+, and reference is made to a similar trend for [Mo3(3-S)(–S2)3 (H2O)6]4+ and [Mo3(3-S)(-S)3(H2O) 9]4+. Attempts to prepare the MoV2 analogue [Mo2(-S2)2(H2O)8 ]6+ gave [Mo2O2(-S)2(H2O)6 ]2+. |
| Databáze: |
Supplemental Index |
| Externí odkaz: |
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