Manganese(II) and Gadolinium(III) Complexes with New 13- and 16-Membered Dioxopolyazacycloalkanes Having Pendant Hydroxy and Carboxymethyl Groups

New 13-membered and 16-membered functionalized macrocycles, dioxopolyazacycloalkanes with pendant hydroxy and carboxymethyl groups, have been synthesized: the 13-membered macrocycle is 12-hydroxy-2,9-dioxo-1,4,7,-10-tetraaza-4,7-cyclotridecanediacetic acid [abbreviated as (13edtapnOH) H₂] and the 16-membered macrocycle is 15-hydroxy-2,12-dioxo-1,4,7,10,13-pentaaza-4,7,10-cyclohexadecanetriacetic acid [(16dtpapnOH) H₃]. The resulting macrocycles have been characterized by ¹H NMR at different pD values, and the protonation sites and the proton populations have been determined for each protonation step. Nonionic Mn²⁺ and Gd³⁺ complexes with these ligands, [Mn(13edtapnOH) (H₂O)]-3H₂O and [Gd(16dtpapnOH) (H₂O)]-3H₂O, have been characterized by single-crystal X-ray analyses. The Mn²⁺ complex crystallized in the triclinic space group P1 with a = 11.453-(1) Å,b= 12.432(1) Å, c = 14.432(2) Å, α = 77.39(1) °, β = 76.24(1) °, γ = 88.50(1) °, and Z = 4. The unit cell contains two types of metal chelate molecules: one has a six-coordination geometry described by a quasi-trigonal prism and the other has a seven-coordination geometry described by a distorted capped trigonal prism. The formation of the two types of Mn²⁺ chelate molecules arises from interaction between an OH oxygen and an amide nitrogen in a coordinated ligand molecule. The Gd³⁺ complex crystallized in the monoclinic space group Pc with a = 8.405(2) Å, b = 9.688(3) Å, c = 16.392(4) Å, β = 109.117(3) °, and Z = 2. The coordination geometry of the Gd³⁺ complex is a tricapped trigonal prism. The electron paramagnetic resonance hyperfine structure of the Mn²⁺ complex in a glass matrix showed the so-called forbidden-transition (∆m_I = ±1) lines at intermediate fields between the allowed-transition (∆m_I = 0) lines. The NMR T₁ and T₂ relaxivities of the Gd³⁺ complex in 90% D₂O were determined to be r₁ = 3.45 s⁻¹ mM⁻¹ and r₂ = 4.4 s⁻¹ mM⁻¹ at 250 MHz. The ¹H NMR spectra of the free ligands show that the introduction of an OH group increases the rigidity of the ring systems, owing to interaction between the hydroxy and amide groups. This interaction is responsible in part for the structural and solution properties of the metal chelates.