Quinquedentate Coordination of 10-Methyl-1,4,8,12-tetraazacyclopentadecan-10-amine (L) to Rhodium(III). Crystal Structure of cis-[Rh(L)Cl][ClO₄]₂.H₂O And Comparative Activation Volumes for Base Hydrolysis of [M(L)Cl]₂₊ (M = Rh, Co, Cr)

Abstract

The pendant-arm macrocycle 10-methyl-1,4,8,12-tetraazacyclopentadecan-10-amine (1) reacts slowly in hot water with rhodium(III) chloride to yield, following cation-exchange chromatography, exclusively cis -[ Rh (l) Cl ]²⁺. The cis -complex was crystallized readily as a perchlorate monohydrate salt, in the orthorhombic space group Pn2₁a, a 16.854(3), b 13.341(3), c 9.985(1) Å , Z = 4, isomorphous with its cobalt(III) counterpart; a single-crystal X-ray structure determination was refined to R 0.027 for 1626 'observed' reflections. The pendant primary arnine and two adjacent secondary amines necessarily occupy an octahedral face, with the chloro ligand cis to the primary amine, and secondary amines adopt R,R,S,S stereochemistries. The Rh -N distances [2.056(6) Å (pendant primary amine), average 2.08 Å (secondary amines)], are at the short end of the range for Rh-N bonds. The Rh-Cl distance is 2.347(2) Å. Activation volumes for chloride base hydrolysis were determined for cis -[ Rh (l) Cl ]²⁺as +19.5( ± 1.2), for cis -[Co(l) Cl ]²⁺ as +27.1(±0.4), and for trans-[Cr(l) Cl ]²⁺ as +31.1( ±0.1) cm3 mol^-l, and are consistent with a conjugate base mechanism in each case; variations with metal ion are discussed.