Symmetrically- and asymmetrically-substituted oxo-bridged binuclear molybdenum nitrosyls: synthetic, electrochemical, spectroscopic and structural studies

Abstract

The binuclear µ-oxo compounds [X{HB(dmpz) ₃ }(NO)MoOMo(NO){HB(dmpz) ₃ }Y] (dmpz = 3,5-dimethylpyrazol-1-yl; X = Y = OH, OMe, OEt, OCOMe or OCOPh; X = I, Y = OH, OMe, NHMe or NHEt) have been prepared, some of these occurring as two geometric isomers (X = I, Y = OH, NHMe or NHEt) which have been separated and characterised spectroscopically. The compounds where X = Y = OH, OCOPh or OMe, and X = I, Y = OH or NHMe undergo one one-electron reduction and one one-electron oxidation, established by cyclic voltammetry, the E _f values being dependent on the donor atom of X and Y. The species where X = OCOPh undergoes an irreversible second reduction and there is an indication of very strong interaction between the metal-containing redox centres. The EPR spectrum of the mixed-valence species [(PhOCO){HB(dmpz) ₃ }(NO)MoOMo(NO){HB(dmpz) ₃ }(OCOPh)] ^- (S = ½) suggests that it is probably valence-trapped at room temperature. Some of the asymmetrically-substituted species are isolated as two isomers. A single-crystal X-ray diffraction study of [C _30.9 H _47.34 B ₂ I _1.16 Mo ₂ N _14.90 O ₃ ]·0.3(C ₂ H ₆ NH), obtained in making [I{HB(dmpz) ₃ }(NO)MoOMo(NO){HB(dmpz) ₃ }(NMe ₂ )], shows that it could contain up to four molecules, the first being the main component (ca. 90%): [I{HB(dmpz) ₃ (NO)MoOMo(NO){HB(dmpz) ₃ }(NHMe)], [{Mo(NO)[HB(dmpz) ₃ ]I} ₂ O], [I{HB(dmpz) ₂ (4-I-dmpz)}(NO)MoOMo(NO){HB(dmpz) ₃ }(NHMe)], and perhaps [I{HB(dmpz) ₂ (4-I-dmpz)}(NO)MoOMo(NO){HB(dmpz) ₃ }I]. The first compound, which has S–S (or R–R) configuration at the metal centres, exhibits a slight asymmetry in the Mo–O–Mo bond system, the Mo–O–Mo bond angle being slightly bent. A geometric isomer of this compound has been isolated and characterised spectroscopically. The second compound is the enantiomer identical to the previously described [{Mo(NO)[HB(dmpz) ₃ ]I} ₂ O], and the third has an iodine atom at position 4 of one of the pyrazolyl rings. Analysis, using molecular modelling calculations, of the possible isomers of these µ-oxo-bridged species with respect to rotation about a linking Mo–O bond revealed only two significantly stable forms in which one NO and an I atom on adjacent metals are mutually cis, and the other pair are mutually trans. The methylamide [I{HB(dmpz) ₃ }(NO)MoOMo(NO){HB(dmpz) ₃ }{NHMe)] adopts one of these apparently preferentially, whereas [{Mo(NO)[HB(dmpz) ₃ ]I} ₂ O] can exist in two forms which have near-equivalent stability and which have been isolated and characterised previously.