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 [Rh-2(mu-CH3COO)(4)(CH3OH)(2)] with a tridentate ligand bis(2-pyridylmethyl)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, [Rh-2(mu-CH3COO)(2)(eta(1)-CH3COO)(bpa)(2)] ([1](+)), C-2-[Rh-2(mu-CH3COO)(2)(bpa)(2)](2+) ([2a](2+)), and C-s-[Rh-2(mu-CH3COO)(2)(bpa)(2)](2+) ([2b](2+)), where C-2 and C-s 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-Rh-2-unit bridging coordination of boa 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 [Rh-2(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 [Rh-2(mu-CH3COO)(2)(eta(1)-CH3COO)(amP)(2)](+) ([4](+)) and [Rh-2(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 2,2':6',2"-terpyridine (tpy) and 4'-chloro-2,2':6',2"-terpyridine (Cl-tpy) have been introduced at ax sites in a monodentate mode ([Rh-2(mu-CH3COO)(4)(tpy)(2)] (8) and [Rh-2(mu-CH3COO)(4)(Cl-tpy)(2)] (9)). While the Rh-Rh distances of these complexes and [Rh-2(mu-CH3COO)(2)(2,2'-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-N-ax 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 (2.32-2.35 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)(*) --> sigma(Rh-Rh)(*) transition in solid-state reflectance spectra shows a red-shift as the Rh-N-ax distances becomes longer.