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

Abstract

Reaction of 1-amino-2-methoxycyclobutenedione with M(NO(3))(2).xH(2)O (M = Mn, Co, Ni, Cu, Zn) in aqueous solution results in the formation of the squarates M(C(4)O(4)).4H(2)O and/or M(C(4)O(4)).2H(2)O owing to the hydrolysis of both the methoxy and amino groups on the ligand. Similarly, reaction of this ligand with M(NO(3))(3).6H(2)O (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(NO(3))(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 P2(1)/c, with a = 5.093(1) Å, b = 8.331(1) Å, c = 15.087(3) Å, beta = 95.12(1) degrees, 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(C(4)O(4)).4H(2)O and the salt [Mn(H(2)O)(6)][HC(4)O(3)NH(2)](2).2H(2)O (2), in which the amino group remains intact. Compound 2 crystallizes in the tetragonal space group P4(2)/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(C(4)O(4))(H(2)O)(4)] and [Co(C(4)O(4))(H(2)O)(2)] are formed on reaction with Co(NO(3))(2).6H(2)O. Hydrolysis of the amino group also occurs when the higher homologue 1-(dimethylamino)-2-methoxycyclobutenedione is reacted with Co(NO(3)).6H(2)O, but only [Co(C(4)O(4))(H(2)O)(2)] is formed. [Co(C(4)O(4))(H(2)O)(2)] crystallizes in the space group Pn&thremacr;n with a = 16.256(1) Å and Z = 24 and is isomorphous with [Co(C(4)O(4))(H(2)O)(2)].0.33H(2)O, 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.