Tris(pyrrole‐2‐aldiminate) Complexes of Ruthenium(III): Synthesis, Structure, Formation Mechanism, and Isomer Preference

Reaction of four pyrrole‐2‐aldimines (HL‐R; R = OCH 3 , CH 3 , H, and Cl) with [Ru(PPh 3 ) 3 Cl 2 ] in 2:1 molar ratio in refluxing toluene furnished tris complexes of type [Ru III (L‐R) 3 ], instead of the expected bis complex of type [Ru II (PPh 3 ) 2 (L‐R) 2 ]. Each tris complex was isolated as facial ( fac ) and meridional ( mer ) isomers. The individual complexes are referred to as fac ‐1 and mer ‐1 ( R = OCH 3 ); fac ‐2 and mer ‐2 ( R = CH 3 ); fac ‐3 and mer ‐3 ( R = H); and fac ‐4 and mer ‐4 ( R = Cl). The formation mechanism was probed with the help of DFT calculations, which revealed that the formation sequences are thermodynamically controlled. Reaction of HL‐R with [Ru(PPh 3 ) 3 Cl 2 ] proceeds via formation of [Ru II (PPh 3 ) 2 (L‐R) 2 ], which rapidly reacts with a third equivalent of HL‐R and furnishes [Ru III (L‐R) 3 ]. The elusive nature of [Ru II (PPh 3 ) 2 (L‐R) 2 ] is indicative of the slower rate of its formation, compared to the rate of its conversion to [Ru III (L‐R) 3 ]. All the complexes were characterized by mass, IR, EPR, and UV–vis spectra. Molecular structures of mer ‐1 , mer ‐2 and fac ‐4 were determined by single crystal X‐ray diffraction analysis. Cyclic voltammetry on the complexes showed a Ru(III)–Ru(IV) oxidation within 0.88–1.11 V versus saturated calomel reference electrode (SCE) and a Ru(III)–Ru(II) reduction within −0.40 to −0.57 V versus SCE.