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(μ-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, [Rh2(μ-CH3COO)2(η1-CH3COO)(bpa)2]+ ([1]+), C2-[Rh2(μ-CH3COO)2(bpa)2]2+
([2a]2+), and Cs-[Rh2(μ-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(μ-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(μ-CH3COO)2(η1-CH3COO)(amp)2]+ ([4]+) and [Rh2(μ-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 ([Rh2(μ-CH3COO)4(tpy)2] (8) and [Rh2(μ-CH3COO)4(Cl-tpy)2] (9)). While the
Rh−Rh distances of these complexes and [Rh2(μ-CH3COO)2(2,2‘-bipyridine)2(py)2]2+ ([7]2+) are practically unchanged
(2.56−2.60 Å) except for 8 and 9 (2.4 Å), the Rh−Nax distances range from 2.11 to 2.35 Å. Relatively short distances
are found for the compounds with ax-eq or ax-eq-eq chelates (<2.22 Å). Longest distances (2.32−2.35 Å) 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 π*Rh-Rh → σ*Rh-Rh transition in solid-state reflectance spectra shows a red-shift as the Rh−Nax distances
becomes longer.