Synergism in molybdenum iron sulphur cluster compounds. Synthetic, structural, magnetic, cyclic voltammetric evidence and the reaction of dicubane clusters containing [MoFe3S4] unit

Dicubane cluster compounds (Et₄N)₄[Mo₂Fe₇S₈(SR)₁₂] (2A) (R=Ph,a;o-tolyl,b;m-tolyl,c;p-tolyl,d) were made by reaction of (Et₄N)₂[Fe₄(SR)₁₀] (1) with (Et₄N)₂MoS₄ in MeCN at room temperature. The structure,¹Hn.m.r.,⁵⁷Fe Mössbauer spectrum, magnetic susceptibility and cyclic voltammogram are described. (Et₄N)₃[Mo₂Fe₆S₈Cl₆(SR)₃] (3) (R=Ph,a;m-tolyl,b) was obtained from the reaction of (2Aa) or (2Ac) with acetyl chloride in MeCN. This is the first time that compound of structural type (2) is transformed into that of structural type (3) by chemical conversion. Compound (2Aa) crystallizes in the triclinic space group P <img src="/fulltext-image.asp?format=htmlnonpaginated&src=T1282PPQ104K24R6_html\11243_2005_Article_BF01092584_TeX2GIFIE1.gif" border="0" alt=" $$\bar 1$$ " /> with Z=1 and unit cell dimensionsa=12.775(4),b=13.076(3), andc=20.576(4) Å; the structure was refined to R=7.7% using 4031 unique data with I>3ϖ(I_o). Compound (2Ac) 2THF crystallizes in the monoclinic space group P2₁/n with Z=2 and unit cell dimensionsa=18.022(2),b=18.375(2) andc=22.254(3) Å; the structure was refined to R=6.4% using 5173 unique data with I>3ϖ(I_o). Compound (3b) crystallizes in the hexagonal space groupP6₃/m with Z=2 and unit cell dimensionsa=b=16.827(3) andc=15.951(16) Å; the structure was refined to R=4.9% using 1296 unique data with I>3ϖ(I_o). Its characteristics are discussed and compared with those of known compounds. The ratios of core volumes S₄/M₄ are within the 2.34–2.40 range for core oxidation level [MoFe₃S₄]³⁺ indicating that distortion of the cubane core is a general phenomenon. Different thiolato ligands induce significant changes of structural parameters only at the Fe(SR)₆ region for compound (2A) while terminal chlorides induce changes over the whole molecule of (3b) with the latter structure more comparable to [Mo₂Fe₆S₈(SPh)₉]⁵⁻ (3f) with [MoFe₃S₄]²⁺ core than to [Mo₂Fe₆S₈(SPh)₉]³⁻ (3d). The isotropic shifts of (2A) originate mainly from π-contact interaction. Both¹H n.m.r. spectra and magnetic susceptibility measurements indicate practically no magnetic interaction among the three magnetic centres,i.e. a Fe(SR)₆ bridge and two [MoFe₃S₄(SR)₃] units. CV studies showed that the reduction of cubane unit having aromatic thiolates is easier than that having aliphatic thiolates as the aliphatic group is electron-donating. In addition, the very similar differences of E_p,c for first and second cubane units in compounds (2A) and in (3d) and (3e) imply that the effect of the first reduced unit [MoFe₃S₄]²⁺ upon the second unit [MoFe₃S₄]³⁺ is very similar in the two types of dicubane cluster compounds. Synergism in Mo−Fe−S cluster compounds is thus proposed to play an important role in their structural correlation with reactivities and must function in nitrogenase.