Oxo-bridged binuclear molybdenum nitrosyl halides: structural and redox studies, mixed-valence behaviour, and characterisation of mononuclear hydroxo precursors
Abstract
The binuclear µ-oxo compounds
[{Mo(NO)[HB(dmpz)
3
]X}
2
(µ-O)]
(dmpz = 3,5-dimethylpyrazol-1-yl; X = I or
Cl) have been prepared. These can occur as enantiomers, one of which
(X = I) has been characterised previously, and a
single-crystal X-ray diffraction study of the other enantiomer
(X = I) established that the molecule has a slightly
asymmetric Mo–O–Mo link. The binuclear complexes undergo
two one-electron reductions (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 between the metal-containing
redox centres. The EPR, IR and electronic spectra of the reduced
monoanionic species
[{Mo(NO)[HB(dmpz)
3
]Cl}
2
(µ-O)]
-
(S = ½) are consistent with
valence-trapped (Class I) behaviour which is unusual in compounds with
such short bridges. Formation of
[{Mo(NO)[HB(dmpz)
3
]X}
2
(µ-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 reaction of [Mo(NO){HB(dmpz)
3
}I
2
]
with water in solution and on contact with silica gel in
dichloromethane. Its structure has been determined crystallographically,
and it can be dehydrated on silica gel into
[{Mo(NO)[HB(dmpz)
3
](µ-O)}
2
] which
appears to contain a symmetrical {Mo(µ-O)
2
Mo}
bridging system.
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