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

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.