Solid State and Solution Dynamics of Pyridine Based Tetraaza-Macrocyclic Lanthanide Chelates Possessing Phosphonate Ligating Functionality (Ln-PCTMB): Effect on Relaxometry and Optical Properties

Abstract

The macrocyclic ligand 3,6,9-tris(methylenebutyl phosphonic acid)-3,6,9-15-tetraazabicyclo [9.3.1] pentadeca-1(15),11,13-triene (PCTMB) was synthesized and complexes of Eu 3+ , Tb 3+ , and Gd 3+ studied by X-ray crystallography, luminescence, and relaxometry. In the crystal these complexes are dimeric and possess 8-coordinate Ln 3+ centers that are linked by bridging phosphonates. The rigidity introduced by the pyridyl nucleus forces the EuPCTMB and TbPCTMB to adopt a twisted snub disphenoid (TSD) coordination geometry. Examination of the 5 D 0 → 7 F 0 luminescent transition of EuPCTMB in the solid state confirmed the existence of a single distinct Eu 3+ coordination environment, whereas two Eu 3+ coordination environments were observed in aqueous solution. Lifetime analysis of aqueous TbPCTMB solutions determined that q = 0.1 and q = 1.0 for the two coordination environments and Stern-Volmer quenching constants (K SV τ = 1101 M −1 , K SV Φ = 40780 M −1 ) support the presence of a monomer/dimer equilibrium. Relaxivity studies of GdPCTMB in H 2 O/CH 3 OH exhibited a concentration dependency (0.02 mM -10.00 mM) ranging from r 1 = 7.0 mM −1 s −1 to 4.0 mM −1 s −1 consistent with the trend observed by luminescence.

Expanding the Scope of Pyclen-Picolinate Lanthanide Chelates to Po-Tential Theranostic Applications

“…As discussed previously, 16(a),(b) for LuL3 and TbL3 this results in a nine-fold coordination around the metal ion with a very distorted and irregular 1:5:3 coordination polyhedron containing three vertexes related by a 2-fold pseudosymmetry axis (O(5), O(3) and N(6), Figure 1), five vertexes related by a 5-fold pseudosymmetry axis (N(1), N(2), N(3), O(1) and N(5)) and the remaining vertex (N(4)) sitting on the latter axis. 18 The coordination polyhedron in LuL4 and TbL4 can be also described by a similar 1:5:3 polyhedron, but also as a severely distorted tricapped trigonal prism (Figure 2), where the upper triangular face is defined by O(1), O(3) and O(5), and the lower triangular face is described by N(1), N (2) 20 As a result, the four five-membered chelate rings formed by the coordination of the pyclen moiety adopt a (δλδλ) conformation, which is characterized by the presence of a mirror plane that makes pyclen achiral. 20 Thus, the only source of chirality in these complexes was related to the layout of the pendant arms, which generate two enantiomers that can be denoted as Δ and Λ.…”

“…18 The coordination polyhedron in LuL4 and TbL4 can be also described by a similar 1:5:3 polyhedron, but also as a severely distorted tricapped trigonal prism (Figure 2), where the upper triangular face is defined by O(1), O(3) and O(5), and the lower triangular face is described by N(1), N (2) 20 As a result, the four five-membered chelate rings formed by the coordination of the pyclen moiety adopt a (δλδλ) conformation, which is characterized by the presence of a mirror plane that makes pyclen achiral. 20 Thus, the only source of chirality in these complexes was related to the layout of the pendant arms, which generate two enantiomers that can be denoted as Δ and Λ. 21 As observed previously, 16(b),(d) the two enantiomers are present in the crystal lattice related by an inversion center.…”

Synthesis and Characterization of Ligands and their Gadolinium(III) Complexes

“…In the case of the symmetrical substitution, the "naked" macrocycle is prepared in the first step and the pendants are introduced in the second step. This strategy was used for preparation of tri(substituted) ligands with acetate (H 3 PCTA) [340,350], glutarate [351], propionate [352], and acetamide pendant arms [353] as well as for phosphonate derivative (H 6 PCTP) [354,355]. A different strategy can be used for ligands with mixed pendant arms.…”

“…From the family of pyclene derivatives, two crystal structures of Ln(III) complexes of tris(monobutyl) ester of H 6 PCTP have recently been reported [355]. The lanthanide(III) ion is octacoordinated and adopts distorted twisted dodecahedral (disphenoidal) coordination sphere; seven atoms comes from the ligand molecule (N 4 O 3 ), and the last site is occupied by phosphonate oxygen atom from centrosymmetrically related neighboring complex unit (Figure 3.4).…”

“…Contrary to the cyclen-based derivatives, the macrocycle ring adopts δλδλ configuration, leading to rectangular (2,4,2,4)-B ring conformation [433] with nitrogen atoms in center of each edge. It results in loss of one chiral element and, thus, no diastereoisomerism is possible in the case of pyclene derivatives [355].…”