Unique trans-bis(heteroleptic)Ru(II) complexes with flat and bent bidentate ligands: Syntheses, crystal structures, and substitution reactions of axial sites

• trans -[RuLL′(N-N)(N′-N′)] 2+ is directly synthesized without photoisomerization. • Flat bipyridine and bent di-2-pyridylamine interlock to stabilize trans geometry. • Two axial sites with differing environments result from bent di-2-pyridylamine. • Two axial aqua ligands undergo selective substitution in DMSO. • NMR, UV-Vis, and DFT results reveal single-pathway substitution selectivity. trans -Bis(polypyridyl)Ru(II) complexes are important in the field of stereochemistry; however, they have been less extensively studied than their cis counterparts. trans -[RuLL′(N–N) 2 ] 2+ complexes (N–N = 2,2′-bipyridine (bpy) or di-2-pyridylamine (Hdpa); L and L′ = monodentate ligands) have been obtained by photoisomerization of their cis isomers. However, trans -[RuCl(dmso- S )(bpy)(Hdpa)] + has not. Therefore, in this project, a trans -[RuLL′(bpy)(Hdpa)] 2+ complex was constructed by reacting mono(Hdpa)Ru(II) with bpy. The mono(Hdpa)Ru(II) complex, trans (Cl)-[RuCl 2 (dmso- S )(OH 2 )(Hdpa)]·2H 2 O ( 1 ·2H 2 O), was reacted with bpy in ethanol at room temperature to produce trans -[RuCl 2 (bpy)(Hdpa)]·3H 2 O ( 2 ·3H 2 O). Reacting 2 ·3H 2 O in dimethyl sulfoxide (DMSO) at 373 K for 2 h afforded trans -[RuCl(dmso- S )(bpy)(Hdpa)]Cl·H 2 O ( 3 ·Cl·H 2 O). Hydrolysis of 3 + released a coordinated Cl − ligand to form trans -[Ru(dmso- S )(OH 2 )(bpy)(Hdpa)] 2+ ( 4 2+ ). Dissolving 4 ·(OTf) 2 (OTf − = CF 3 SO 3 − ) in acetonitrile (CH 3 CN) or DMSO yielded trans -[Ru(dmso- S )(CH 3 CN)(bpy)(Hdpa)](OTf) 2 ( 5 ·(OTf) 2 ) and trans -[Ru(dmso- S )(dmso- O )(bpy)(Hdpa)](OTf) 2 ( 6 ·(OTf) 2 ), respectively. The thermally inert dmso- S ligand in 4 ·(OTf) 2 was released upon photoirradiation in acidic water to form trans -[Ru(OH 2 ) 2 (bpy)(Hdpa)](OTf) 2 ( 7 ·(OTf) 2 ). Crystallographic analysis of 2, 3 ·Cl·H 2 O, 3 ·(OTf), 4 ·(OTf) 2 , 5 ·(OTf) 2 , 6 ·(OTf) 2 , and 7 ·(OTf) 2 revealed an interlocking interaction between flat bpy and bent Hdpa in the trans configuration of all complexes, resulting in thermal stability of the trans -[Ru(bpy)(Hdpa)] unit. Additionally, the NH group of Hdpa was close to the dmso- S ligand in 3 + , 4 2+ , 5 2+ , and 6 2+ . 1 H NMR and absorption spectroscopy of 7 ·(OTf) 2 in DMSO, along with density-functional theory (DFT) and time-dependent DFT calculations, indicated that the reaction of 7 ·(OTf) 2 in DMSO produced 6 ·(OTf) 2 via only one of four possible trans -[Ru(dmso)(OH 2 )(bpy)(Hdpa)] 2+ isomers. Our results provide insight for stereochemistry and biochemistry research.