Synthesis and NMR studies of new DOTP-like lanthanide(III) complexes containing a hydrophobic substituent on one phosphonate side arm

Three derivatives of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrakis(methylene phosphonic acid) (DOTP) containing a hydrophobic substituent on one side chain were prepared and their lanthanide complexes examined by NMR. The new ligands include 1-(1-octyl-methyl-phosphonic acid)-4,7,10-tris(methylene phosphonic acid)-1,4,7,10-tetraazacyclododecane (C-8-DOTP), 1-(1-undecyl-methyl-phosphonic acid)-4,7,10-tris(methylene phosphonic acid)-1,4,7,10-tetraazacyclododecane (C-11-DOTP), and 1-(1-4-nitro-phenyl-methyl-phosphonic acid)-4,7,10-tris(methylene phosphonic acid)-1,4,7,10-tetraazacyclododecane (NO2-Ph-DOTP). H-1 NMR spectra of the ytterbium(III) complexes were assigned by using a combination of COSY spectroscopy and a fitting procedure that matches experimental NMR hyperfine shifts with those estimated from a MMX-derived structure. The analysis showed that a single isomer is present in solution and that the bulky hydrophobic substituent occupies the less sterically demanding H-6 equatorial position in the YbL5- complexes. Although the YbL5- complexes have lower symmetry due to the added substituent, the average H-1 hyperfine shifts are 5-10% larger in these complexes compared to YbDOTP5-. This was magnified further in the hyperfine Na-23 NMR shifts of ion-paired sodium ions where the extracellular Na+ signal in perfused rat hearts displayed a 28% larger hyperfine shift in the presence of Tm(C-11-DOTP)(5-) than with an equivalent amount of TmDOTP5-.