Co(i) complexes with a tetradentate phenanthroline-based PNNP ligand as a potent new metal–ligand cooperation platform†
A series of low spin cobalt(I) complexes bearing a tetradentate phenanthroline-based PNNP ligand (2,9-bis((diphenylphosphanyl)methyl)-1,10-phenanthroline), [CoCl(PNNP)] (1), [CoMe(PNNP)] (2) and [Co(CH2SiMe3)(PNNP)] (3), were synthesized and structurally identified. Complex 3 underwent a structural rearrangement of the PNNP skeleton upon heating to form [Co(PNNP′)] (4), which is supported by an asymmetrical PNNP′ ligand with a dearomatized phenanthroline backbone. Mechanistic studies supported that the transformation from 3 to 4 was initiated by the homolysis of either a Co–CH2SiMe3 bond or a benzylic C–H bond. Complex 4 achieved H–H bond cleavage of H2 (1 atm) at ambient temperature, to form [Co(PNNP′′)] (6), in which two H atoms were incorporated into the endocyclic double bond of the PNNP′′ ligand backbone.