Complexes of Aminosquarate Ligands with First-Row Transition Metals and Lanthanides: New Insights into Their Hydrolysis

Attempts at synthesizing first-row transition-metal complexes of the 3-hydroxy-4-[(1‘S,2‘R)-(2-hydroxy-1‘,2‘-diphenylethyl)amino]-3-cyclobutene-1,2-dione ligand in alcoholic solutions resulted in the formation of the monomers
[M(NH2C4O3)2(H2O)4] [M = Mn (1), Co (2), Ni (3), Cu (4), Zn (5)] instead, as a result of the hydrolysis of the ligand.
1, 2, and 3 are isomorphous (C2/c), with the metal atoms octahedrally coordinated to four aqua and two cis
aminosquarate ligands. The copper and zinc complexes (4 and 5) have the same molecular formula as 1−3 but
belong to the C2/m and P2(1)/c space groups respectively. 4 has square-pyramidal geometry with trans-oriented
aminosquarate ligands in the basal plane; aqua ligands complete the coordination sphere. 5 has octahedral geometry,
with four aqua and two trans-oriented aminosquarate ligands. Reaction of aqueous solutions of the anilinosquarate
ligand with Ln(NO3)3·xH2O produced the eight-coordinate complexes {Sm(μ-C6H5NHC4O3)3(H2O)4·3H2O}n (6),
{[M(μ2-C4O4)(H2O)6][C6H5NHC4O3]·4H2O}n [M = Er (7), Yb (8)], {Sm(C6H5NHC4O3) (μ3-C4O4)(H2O)4·H2O}n (9),
and {[{(C6H5NHC4O3)2(H2O)5Yb}2(μ-C4O4)]·4H2O}n (10). 7 and 8 are isomorphous with the previously reported
analogues Eu, Gd, and Tb ionic polymers. The presence of the squarate ligand in 7−10 is indicative of some form
of hydrolysis of the anilinosquarate ligand during their syntheses. However, hydrolysis was not evident in the
synthesis of 6. The mechanism for the hydrolysis in the syntheses of 1−5 is apparently different from that for 7−10.