Redox-mediation of electron–electron spin–spin exchange interactions, |J|, in paramagnetic trinuclear molybdenum complexes: an example of a ‘J switch’

Abstract

A series of trinuclear molybdenum nitrosyl complexes has been prepared using the dinucleating ligands 4-(4-hydroxyphenyl)pyridine (HL1), 1-(4-pyridyl)-2-(4-hydroxyphenyl)-ethene (HL2) and -ethane (HL3), and the trinucleating ligands 3,5-bis(4-hydroxyphenyl)pyridine (H2L4) and 2,6-bis(4-ethenylpyridyl)-4-hydroxytoluene (HL5). These complexes are of the type [Mo{OC6H4EpyMoCl}2] (Mo = Mo(NO)TpMe,Me, TpMe,Me = tris(3,5-dimethylpyrazolyl)borate; E = nothing, CHCH and CH2CH2; py = C5H4N or C5H3N; from HL1, HL2 and HL3), [{ClMo(OC6H4)}2pyMoCl] (from H2L4), and [ClMo{OC6H3Me[CHCHpyMoCl]2}] (from HL5). The species [Mo{OC6H4EpyMoCl}2] contains one 16 valence electron (ve) ([Mo{OC6H4–}2]) and two 17 ve centres ([(–py)MoCl]), [{ClMo(OC6H4)}2pyMoCl] has two 16 ([ClMo{OC6H4–}]) and one 17 ([(–py)MoCl]) ve centres and [ClMo{OC6H3Me[CHCHpyMoCl]2}] one 16 and two 17 ve centres. Reduction of these species by cobaltocene in tetrahydrofuran/dichloromethane mixtures affords complexes having three 17 ve centres with one unpaired electron per metal centre. The interaction between these unpaired electrons in solution is determined by the relationship between |J|, the electron spin–spin exchange interaction, and AMo, the molybdenum hyperfine coupling constant, which was detected by EPR spectroscopy. In [Mo{OC6H4EpyMoCl}2], the interaction was dependent on ligand conformation, |J| ≈ AMo when E = nothing, |J| ≫ AMo when E = CHCH and |J| ≪ AMo when E = CH2CH2. Reduction of [Mo{OC6H4EpyMoCl}2] to [Mo{OC6H4EpyMoCl}2]− resulted in exchange between all three spins irrespective of ligand conformation, and the EPR spectra of [{ClMo(OC6H4)}2pyMoCl]2− and [ClMo{OC6H3Me[CHCHpyMoCl]2}]− were similar to that of [Mo{OC6H4EpyMoCl}2]−. Oxidation reconstitutes the original EPR spectra of [Mo{OC6H4EpyMoCl}2], [{ClMo(OC6H4)}2pyMoCl] and [ClMo{OC6H3Me[CHCHpyMoCl]2}]. This behaviour is consistent with full three centre interaction being ‘switched on’ when the 17∶16∶17 or 16∶17∶16 ve systems are reduced to a 17∶17∶17 ve system, and ‘switched off’ on reoxidation.