OXO-BRIDGED BINUCLEAR MOLYBDENUM NITROSYL HALIDES - STRUCTURAL AND REDOX STUDIES, MIXED-VALENCE BEHAVIOR, AND CHARACTERIZATION OF MONONUCLEAR HYDROXO PRECURSORS

The binuclear mu-oxo compounds [{Mo(NO)[HB(dmpz)(3)]X}(2)(mu-O)] (dmpz = 3,5-dimethylpyrazol-1-yl; X = I or Cl) have been prepared. These ca n occur as enantiomers, one of which (X = I) has been characterised pr eviously, and a single-crystal X-ray diffraction study of the other en antiomer (X = I) established that the molecule has a slightly asymmetr ic Mo-O-Mo link. The binuclear complexes undergo two one-electron redu ctions (E-f(1) and E-f(2)) and one one-electron oxidation, established by cyclic voltammetry, the separation between E-f(1) and E-f(2) being ca. 1000 mV indicating that there is extremely strong interaction bet ween the metal-containing redox centres. The EPR, IR and electronic sp ectra of the reduced monoanionic species [{Mo(NO)[HB(dmpz)(3)]Cl}(2)(m u-O)](-) (S = 1/2) are consistent with valence-trapped (Class I) behav iour which is unusual in compounds with such short bridges. Formation of [{Mo(NO)[HB(dmpz)(3)]X}(2)(mu-O)] involves prior generation of [Mo( NO){HB(dmpz)(3))X(OH)] (X = Cl or I) by hydrolysis of [Mo(NO){HB(dmpz) (3)}X-2] and reaction of the monohydroxide with [Mo(NO){HB(dmpz)(3)}X- 2]. The compound [Mo(NO){HB(dmpz)(3)}(OH)(2)] is also formed by reacti on of [Mo(NO)(HB(dmpz)(3)}I-2] with water in solution and on contact w ith silica gel in dichloromethane. Its structure has been determined c rystallographically, and it can be dehydrated on silica gel into [{Mo( NO)[HB(dmpz)(3)](mu-O)}(2)] which appears to contain asymmetrical {Mo( mu-O)(2)Mo} bridging system.