Structural Studies on the Stepwise Chelating Processes of Bidentate 2-(Aminomethyl)pyridine and Tridentate Bis(2-pyridylmethyl)amine toward an Acetate-Bridged Dirhodium(II) Center

Several intermediates and final products of the reactions of [Rh2((mu)-CH3COO)4(CH3OH)2] with a tridentate ligand bis(2pyridylmethyl)amine (bpa) and bidentate 2-(aminomethyl)pyridine (amp) have been isolated, and the chelation processes of these ligands to the dirhodium(II) center are discussed. The reaction of a 2 equiv amount of bpa in chloroform afforded three products, [Rh2 ((mu)-CH3COO)2( 1-CH3COO)(bpa)2]+ ([1]+), C2-[Rh2((mu)-CH3COO)2(bpa)2]2+ ([2a]2+), and Cs-[Rh2((mu)-CH3COO)2(bpa)2]2+ ([2b] 2+), where C2 and Cs denote the molecular symmetry of the two geometrical isomers. X-ray crystallography revealed that [1]+ contains ax-eq chelated bidentate and ax-eq-eq tridentate bpa and that [2a]2+ and [2b]2+ have two ax-eq-eq tridentate bpa ligands (ax denotes the site trans to the Rh-Rh bond, and eq, the site perpendicular to it). The reaction is initiated by almost instantaneous monodentate or inter-Rh2-unit bridging coordination of bpa at the ax sites, which is followed by very slow ax-eq chelate formation and then ultimate ax -eq-eq tridentate coordination. The reaction of [Rh2((mu)-CH3COO)4(CH3OH)2] with amp in 1:2 ratio in chloroform initially gives an insoluble polymer in which amp interconnects the ax sites of the dirhodium(II) units. Further reactions afforded [Rh2((mu)-CH3COO)2 ((eta)1-CH3COO)(amp)2]+ ([4]+) and [Rh2((mu)-CH3COO)2(amp)2]2+ ([5]2+). The X-ray structural studies show that [4]+ has one ax-eq and one eq-eq chelate and [5]2+ two eq-eq chelates. More rigid tridentate ligands 2,2ʹ:6ʹ,2"-terpyridine (tpy) and 4ʹ-chloro-2,2ʹ:6ʹ,2"terpyridine (Cl-tpy) have been introduced at ax sites in a monodentate mode ([Rh2((mu)-CH3COO)4(tpy)2] (8) and [Rh2((mu)-CH3COO)4 (Cl-tpy)2] (9)). While the Rh-Rh distances of these complexes and [Rh2((mu)-CH3COO)2(2,2ʹ-bipyridine)2(py)2]2+ ([7]2+) are practically unchanged (2.56-2.60 (angstrom)) except for 8 and 9 (2.4 (angstrom)), the Rh-Nax distances range from 2.11 to 2.35 (angstrom). Relatively short distances are found for the compounds with ax-eq or ax-eq-eq chelates (<2.22 (angstrom)). Longest distances (2.322.35 (angstrom)) found for 8 and 9 may be due to the steric effect. The distances of other complexes fall in the normal region. The visible band of the (pi)*Rh-Rh (right arrow)(sigma) *Rh-Rh transition in solid-state reflectance spectra shows a red-shift as the Rh-Nax distances becomes longer.