摘要
本文讨论了Roussin黑盐的形成机理和“G”系模型物结构及其固氮活性成因的化学和量子化学研究结果。并由此提出了过渡金属类立方烷型簇合物合成中的“活性元件组装”设想;即过渡金属原子簇,特别是含μ2和μ3桥的类立方烷簇,可能由构成该原子簇的基本结构“元件”在一定活化条件下组装而成。考察了现有的Fe-S,Mo-S,Mo-Fe-S以及一些M-M′-X构型相类似的类立方烷簇合物,如[Fe_4S_4(SR)_4]^(2-),[MoFe_3S_4(SR)_n]^(m-),[VFe_3S_4Cl_3(DMF)3]单类立方烷簇,[Mo_2Fe_6S_9(SR)_8]^(8-),[Mo_2Fe_6S_8(SR)_9]^(3-),[Mo_2Fe_7S_8(SR)_(12)]^(3-,4-)桥联双立方烷簇以及(MeZn)_6Zn(OMe)_8孪合双立方烷簇的自兜合成反应,发现尽管它们似乎都是由简单的无机盐和有机配体“自兜”而成,然而它们都不能不经过某些“元件组装”的过程。提出了它们的可能的“元件组装”途径。此外还讨论了“元件组装在其他合成Mo、Fe、S类立方烷簇的反应中的存在。最后还进一步探讨了其他类型簇合物合成反应情况,表明“元件组装”设想是可能推广到类立方烷型簇以外的其他一些原子簇的合成的。
Chemical, structural,and quantum-chemical evidences are herein presented for the rational synthesis of black Roussinate cluster monoanion [Fe4S3(NO)7]- from the much simpler red Roussinate di- anion[Fe2S2(NO)4_]2 looked upon as a dimerjc condensation process,quite similar to the synthesis of 4Fe^4S ferredoxin from the smaller 2Fe-2S ferredoxin, presumably through a dimerie condensation re action path. It is also noted that these two Roussinate anions can be conveniently synthesized from the same starting materials Fe2+ ,S2-, NO2-, and OH' under rather similar reaction coditions in an autoclave with an atmosphere of CO as a reductant (under a pressure of ca. 20 atm. ), simply by varying the mixing ratios on accordance with straightforward stoichiometric calculations.Moreover, attempted syntheses of modelling compounds for the cubane-like 'Fuzhou string-bag models' of the active center of nitrogenase in biological nitrogen fixation, carried out in our Institute, has let to the successful isolation of a so-called 'G' series of modelling compounds of clathrate crystalstructure with rather simple dianions of the following variable constitution but essentially identical sizeTx /Xx /T' and cofiguration: ,/M( ~>M' ( (M,M =Fe, Fe or Fe, Mo; X,T,T'=S,C1) enclosed in a tri-clinic unit cell cage the corners of which are occupied by the dicationic [Mg(DMF)6]2+- Crystals of this 'G' series with a moderate Mo content of ca. 2% have been shown to exhibit the ability of reducing dinitrogen to ammonia (confirmed beyond any doubt by using isotopically labelled Nz) under standard assaying conditions. All these have led us to pander over the plausibility of our suggestion of a 'unit construction' approach to the rational syntheses of transition metal cubane-like clusters by the use of reactive fragments (activated in some way under reaction conditions) as building blocks. Our considerations are confined for the time being to the rational syntheses of Fe-S,Mo-S,Mo-Fe-S and a few other transition metal clusters. Examples are thus discussed for monocubanes such as [Fe4S, (SR),]2 ,[MoFe3S4(SR)m]n%[VFe3S4Cl3(DMF)3]- as well as for bridged dicubanes such as [Mo2Fe6 S8(SR)9]3 ,[Mo2Fe6S9(SR)8]3-,[Mo2Fe?S8(SR)12]3-'4 and even Siamese-twinned dicubane such as [ (MeZn)6Zn(OMe)8],etc. Our discussion of this 'Unit construction' approach is also further extended to some typical non-cubane-like clusters.
