Benjamin E. Kucera

Properties, solution state behavior, and crystal structures of chelates of DOTMA.

The chemistry of polyamino carboxylates and their use as ligands for Ln(3+) ions is of considerable interest from the point of view of the development of new imaging agents. Of particular interest is the chemistry of the macrocyclic ligand 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) and its derivatives. Herein we report that the tetramethylated DOTA derivative, DOTMA, possess several properties that, from an imaging agent development point of view, are more advantageous than those of the parent DOTA. In particular, the Ln(3+) chelates of DOTMA exhibit a marked preference for the monocapped twisted square antiprismatic coordination isomer which imparts more rapid water exchange kinetics on the chelates; τ(M)(298) was determined to be 85 ns for GdDOTMA. Differential analysis of the (17)O R(2ρ) temperature profiles of both GdDOTA and GdDOTMA afforded the τ(M)(298) values for the square (SAP) and twisted square antiprismatic (TSAP) isomers of each chelate that were almost identical: 365 ns (SAP) and 52 ns (TSAP). The origin of this accelerated water exchange in the TSAP isomer appears to be the slightly longer Gd-OH(2) bond distance (2.50 Å) that is observed in the crystal structure of GdDOTMA which crystallizes in the P(2) space group as a TSAP isomer. The Ln(3+) chelates of DOTMA also exhibit high thermodynamic stabilities ranging from log K(ML) = 20.5 for CeDOTMA, 23.5 for EuDOTMA and YbDOTMA comparable to, but a shade lower than, those of DOTA.

Analysis of the conformational behavior and stability of the SAP and TSAP isomers of lanthanide(III) NB-DOTA-type chelates

Controlling the water exchange kinetics of macrocyclic Gd(3+) chelates, a key parameter in the design of improved magnetic resonance imaging (MRI) contrast media, may be facilitated by selecting the coordination geometry of the chelate. The water exchange kinetics of the mono- capped twisted square antiprism (TSAP) being much closer to optimal than those of the mono capped square antiprism (SAP) render the TSAP isomer more desirable for high relaxivity applications. Two systems have been developed that allow for selection of the TSAP coordination geometry in 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)-type Gd(3+) chelates, both based upon the macrocycle nitrobenzyl cyclen. In this paper we report investigations into the stability and formation of these chelates. Particular focus is given to the production of two regioisomeric chelates during the chelation reaction. These regioisomers are distinguished by having the nitrobenzyl substituent either on a corner or on the side of the macrocyclic ring. The origin of these two regioisomers appears to stem from a conformation of the ligand in solution in which it is hypothesized that pendant arms lie both above and below the plane of the macrocycle. The conformational changes that then result during the formation of the intermediate H(2)GdL(+) chelate give rise to differing positions of the nitrobenzyl substituent depending upon from which face of the macrocycle the Ln(3+) approaches the ligand.

Bis(pyrene)metal complexes of vanadium, niobium and titanium: isolable homoleptic pyrene complexes of transition metals

Reduction of VCl3(THF)3 (THF is tetrahydrofuran) and NbCl4(THF)2 by alkali metal pyrene radical anion salts in THF affords the paramagnetic sandwich complexes bis[(1,2,3,3a,10a,10b-η)-pyrene]vanadium(0), [V(C16H10)2], and bis[(1,2,3,3a,10a,10b-η)-pyrene]niobium(0), [Nb(C16H10)2]. Treatment of tris(naphthalene)titanate(2-) with pyrene provides the isoelectronic titanium species, isolated as an (18-crown-6)potassium salt, namely catena-poly[[(18-crown-6)potassium]-μ-[(1,2-η:1,2,3,3a,10a,10b-η)-pyrene]-titanate(-I)-μ-[(1,2,3,3a,10a,10b-η:6,7-η)-pyrene]], {[K(C12H24O6)][Ti(C16H10)2]}n. The first two compounds have very similar packing, with neighboring molecules arranged orthogonally to one another, such that aromatic donor-acceptor interactions are likely responsible for the specific arrangement. The asymmetric unit contains a half-occupancy metal center η(6)-coordinated to one pyrene ligand, with the full M(pyrene)2 molecule generated by a crystallographic inversion center. In the titanium compound, the cations and anions are in alternating contact throughout the crystal structure, in one-dimensional chains along the [101] direction. As in the other two compounds, the asymmetric unit contains a half-occupancy Ti atom η(6)-coordinated to one pyrene ligand. Additionally, the asymmetric unit contains one half of an (18-crown-6)potassium cation, located on a crystallographic inversion center coincident with the K atom. The full formula units are generated by those inversion centers. In all three structures, the pyrene ligands are eclipsed and sandwich the metals in one of two inversion-related sites. These species are of interest as the first isolable homoleptic pyrene transition metal complexes to be described in the scientific literature.