Thiolate ligands, sterically hindered

Complexes of Sterically Hindered Thiolate Ligands J. R. Dilworth and J. Hu... 

Reviews on mercury—thiolate complexes and their bio-relevance333 and complexes with steric-ally hindered thiolate ligands have been published.334... 

Transition metal complexes of sterically hindered thiolate ligands have been reviewed. " These ligands give rise to unusual geometries and oxidation states, and low coordination numbers in their complexes. 

More recently three other mononuclear [Fe(SR)4]- complexes containing monodentate thiolate have been structurally defined. In these SR = SPh-,219 SEt-219 and 2,3,5,6-tetramethylben-zenethiolate.220 Sterically hindered thiolate ligands appear to stabilize the [Fe(SR)4] coordination unit and thus provide a means for the preparation by ligand exchange of the previously elusive (non-hindered) monodentate tetrathiolato iron(III) complexes. 

In a number of cases with sterically hindered ligands trigonal-planar coordination is observed, for example, in dialkylamides, Co2(/a-NR2)2(NR2)2, with large R. The magnetic moments of these compounds vary considerably from = 4.83 (R = SiMe3) to 1.72 (R = Ph) BM. The bulky thiolate Co2(ju,-SAr")2(SAr")2 is also three-coordinate,12 while other thiolates, e.g. [Co(SPh)4]2- and [Co2(/r,-SPri)3(SPri)2]-, are tetrahedral.13... 

A systematic investigation of the reactions of [MBr2(CO)4l (M = Mo, W) with sterically hindered thiolate ligands led to the series of five-coordinate 14-electron anions [M(CO)2(SR)3] (SR = TIPT, DIPT, TMT, PFTP) (26). This contrasts with the analogous reaction with thiphenol, which gives unstable polymeric species. An X-ray crystal... 

The electrochemical reversibility of the M(VI)/M(V) couple for the complexes with sterically hindered ligands contrasts with the reported behavior of the [MoO(SPh)4] complex, which exhibited electrochemical irreversibility for the Mo(V)/Mo(VI) step but a reversible Mo(IV)/ Mo(V) couple. The sterically hindered aromatic substituent groups stabilize the molybdenum(VI) complex and decrease relative to the thiophenol derivative. The molybdenum(VI) species can also be isolated by chemical oxidation. [MoO(PFTP)4] was prepared by chemical redu-tion of [MoO(PFTP)4] . The presence of the electron-withdrawing substituents on the aromatic thiolate increases E ei relative to the thio-phenolate derivative. Evidently the properties of these last complexes are influenced primarily by the electron-withdrawing characteristics of the fluorine substituents rather than by steric factors (33). 

The homoleptic sterically hindered thiolate complex [Mo2(TMT)g] containing Mo— Mo triple bonds was first prepared by a multistep reaction (35). The X-ray crystal structure showed a staggered configuration for the thiolate ligands with an Mo— Mo distance of 2.228(1) A (35). They were synthesized subsequently by a simple one-step synthesis involving the reaction of M0CI4 with the TIPT anion in 1,2-dimeth-oxyethane and structurally characterized. A study of their chemistry showed them to be rather unreactive compared with their alkoxide analogues (36). 

An interesting series of molybdenum and tungsten hydride complexes with sterically hindered thiolate ligands has recently been established due to the interest in their possible relevance to hydrodesulfurization catalysis and to the active site of the enzyme nitrogenase. The complexes [MH(SAr)3(PMe2Ph)2l (M = Mo, W SAr = TIPT, TMT) were prepared as summarized in Eqs. (8) and (9). 

However, as observed elsewhere, the slightly less steric hindered thiolate TEMT ligands display rather different chemistry in analogous reactions. The resultant complex is a dimer, [ Co(TEMT)2(bipy) 2], with two five-coordinate cobalts linked by two asymmetric thiolate bridges, as shown in Fig. 16 (73). 

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