Attempted Syntheses of Transition Metal and Lanthanide (Dialkylamino)squarates. The Hydrolysis Problem

Reaction of 1-amino-2-methoxycyclobutenedione with M(NO3)2·xH2O (M = Mn, Co, Ni, Cu, Zn) in aqueous
solution results in the formation of the squarates M(C4O4)·4H2O and/or M(C4O4)·2H2O owing to the hydrolysis
of both the methoxy and amino groups on the ligand. Similarly, reaction of this ligand with M(NO3)3·6H2O (M
= La, Eu, Gd, Tb) results in the formation of the respective lanthanide squarates. Buffering the reactant solutions
at different pH's did not change the results and neither did using the alternative solvents methanol, ethanol,
propan-1-ol, and acetonitrile. In previously reported reactions between 1-amino-2-methoxycyclobutenedione and
1-(dimethylamino)-2-methoxycyclobutenedione separately with Pb(NO3)2 under similar reaction conditions, the
amino groups remained unhydrolyzed. The substituent amino groups also remain unhydrolyzed when these ligands
are left to stand in aqueous solution in the absence of transition metal ions; only the methoxy group is hydrolyzed.
1-(Dimethylamino)-2-hydroxycyclobutenedione (1), formed from such a hydrolysis of 1-(dimethylamino)-2-methoxycyclobutenedione in aqueous solution, crystallizes in space group P21/c, with a = 5.093(1) Å, b = 8.331(1) Å, c = 15.087(3) Å, β = 95.12(1)°, and Z = 4. However, attempts at complexation of the ligand with Mn(II)
ions resulted in the formation of a mixture of the Mn(II) squarate Mn(C4O4)·4H2O and the salt [Mn(H2O)6][HC4O3NH2]2·2H2O (2), in which the amino group remains intact. Compound 2 crystallizes in the tetragonal space
group P42/m with a = 7.251(3) Å, c = 15.979(8) Å, and Z = 2. If tetraethylammonium aminosquarate is used
instead of 1-amino-2-methoxycyclobutenedione, both [Co(C4O4)(H2O)4] and [Co(C4O4)(H2O)2] are formed on
reaction with Co(NO3)2·6H2O. Hydrolysis of the amino group also occurs when the higher homologue
1-(dimethylamino)-2-methoxycyclobutenedione is reacted with Co(NO3)·6H2O, but only [Co(C4O4)(H2O)2] is
formed. [Co(C4O4)(H2O)2] crystallizes in the space group Pn3̄n with a = 16.256(1) Å and Z = 24 and is
isomorphous with [Co(C4O4)(H2O)2]·0.33H2O, which has been reported previously. Evidence suggests that the
hydrolysis of the dialkylamino substituents in the 1-(dialkylamino)-2-methoxycyclobutenediones is apparently
mediated by the transition and lanthanide metals. The use of the hydrolysis of the amino group in 1-dialkylamino
derivatives of squaric acid as a convenient synthetic tool is discussed.