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)Tp^Me,Me, Tp^Me,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 A_Mo, the molybdenum hyperfine coupling constant, which was detected by EPR spectroscopy. In [Mo{OC₆H₄EpyMoCl}₂], the interaction was dependent on ligand conformation, |J| ≈ A_Mo when E = nothing, |J| ≫ A_Mo when E = CHCH and |J| ≪ A_Mo 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.