Three step vs one pot synthesis and X-ray crystallographic investigation of heptadentate triamide cyclen (1,4,7,10-tetraazacyclododecane) based ligands and some of their lanthanide ion complexes

Donor Radii in Rare-Earth Complexes

We present a set of donor radii for the rare-earth cations obtained from the analysis of structural data available in the Cambridge Structural Database (CSD). Theoretical calculations using density functional theory (DFT) and wave function approaches (NEVPT2) demonstrate that the Ln-donor distances can be broken down into contributions of the cation and the donor atom, with the minimum in electron density (ρ) that defines the position of (3,-1) critical points corresponding well with Shannon's crystal radii (CR). Subsequent linear fits of the experimental bond distances for all rare earth cations (except Pm3+) afforded donor radii (rD) that allow for the prediction of Ln-donor distances regardless of the nature of the rare-earth cation and its oxidation state. This set of donor radii can be used to rationalize structural data and identify particularly weak or strong interactions, which has important implications in the understanding of the stability and reactivity of complexes of these metal ions. A few cases of incorrect atom assignments in X-ray structures were also identified using the derived rD values.

Anion Control of Lanthanoenediyne Cyclization

A suite of lanthanoenediyne complexes of the form Ln(macrocycle)X₃ (Ln = La³⁺, Ce³⁺, Eu³⁺, Gd³⁺, Tb³⁺, Lu³⁺; X = NO₃^-, Cl^-, OTf^-) was prepared by utilizing an enediyne-containing [2 + 2] hexaaza-macrocycle (2). The solid-state Bergman cyclization temperatures, measured via DSC, decrease with the denticity of X (bidentate NO₃^-, T = 267-292 °C; monodentate Cl^-, T = 238-262 °C; noncoordinating OTf^-, T = 170-183 °C). ¹³C NMR characterization shows that the chemical shifts of the acetylenic carbon atoms also rely on the anion identity. The alkyne carbon closest to the metal binding site, C_A, exhibits a Δδ > 3 ppm downfield shift, while the more distal alkyne carbon, C_B, displays a concomitant Δδ ≤ 2.5 ppm upfield shift, reflecting a depolarization of the alkyne on metal inclusion. For all metals studied, the degree of perturbation follows the trend 2 < NO₃^- < Cl^- < OTf^-. This belies a greater degree of electronic rearrangement in the coordinated macrocycle as the denticity of X and its accompanying shielding of the metal's Lewis acidity decrease. Computationally modeled structures of LnX₃ show a systematic increase in the lanthanide-2 coordination number (CN_La-mc = 2 (NO₃^-), 4 (Cl^-), 5 (H₂O, model for OTf^-)) and a decrease in the mean Ln-N bond length (La-N_average = 2.91 Å (NO₃^-), 2.78 Å (Cl^-), 2.68 Å (H₂O)), further suggesting that a decrease in the anion coordination number correlates with an increase in the metal-macrocycle interaction. Taken together, these data illustrate a Bergman cyclization landscape that is influenced by the bonding of metal to an enediyne ligand but whose reaction barrier is ultimately dominated by the coordinating ability of the accompanying anion.

Energy Transfer in Dendritic Systems Having Pyrene Peripheral Groups as Donors and Different Acceptor Groups

In this feature article, a specific overview of resonance energy transfer (FRET) in dendritic molecules was performed. We focused mainly on constructs bearing peripheral pyrene groups as donor moieties using different acceptor groups, such as porphyrin, fullerene C60, ruthenium-bipyridine complexes, and cyclen-core. We have studied the effect of all the different donor-acceptor pairs in the energy transfer efficiency (FRET). In all cases, high FRET efficiency values were observed.

Glycosylated lanthanide cyclen complexes as luminescent probes for monitoring glycosidase enzyme activity

The development of synthetic chemical probes for the detection of enzymes is extremely important for biological, medicinal, and industrial applications. Here we report the synthesis of an array of novel glycosylated Tb(iii) complexes, their photophysical properties in solution, and their ability to function as luminescent probes for observing glycosidase enzyme activity in real time. Our initial studies into the application of these complexes for the detection of the Concanavalin A (ConA) lectin is also reported, highlighting the broad scope of these novel chemical probes.

Complexes possessing rare "tertiary" sulfonamide nitrogen-to-metal bonds of normal length: fac-[Re(CO)3(N(SO2R)dien)]PF6 complexes with hydrophilic sulfonamide ligands

Tertiary sulfonamide nitrogen-to-metal bonds of normal length are very rare. We recently discovered such a bond in one class of fac-[Re(CO)3(N(SO2R)(CH2Z)2)](n) complexes (Z = 2-pyridyl) with N(SO2R)dpa ligands derived from di-(2-picolyl)amine (N(H)dpa). fac-[M(CO)3(N(SO2R)(CH2Z)2)](n) agents (M = (186/188)Re, (99m)Tc) could find use as radiopharmaceutical bioconjugates when R is a targeting moiety. However, the planar, electron-withdrawing 2-pyridyl groups of N(SO2R)dpa destabilize the ligand to base and create relatively rigid chelate rings, raising the possibility that the rare M-N(sulfonamide) bond is an artifact of a restricted geometry. Also, the hydrophobic 2-pyridyl groups could cause undesirable accumulation in the liver, limiting future use in radiopharmaceuticals. Our goal is to identify a robust, hydrophilic, and flexible N(CH2Z)2 chelate framework. New C2-symmetric ligands, N(SO2R)(CH2Z)2 with (Z = CH2NH2; R = Me, dmb, or tol), were prepared by treating N(H)dien(Boc)2, a protected diethylenetriamine (N(H)dien) derivative, with methanesulfonyl chloride (MeSO2Cl), 3,5-dimethylbenzenesulfonyl chloride (dmbSO2Cl), and 4-methylbenzenesulfonyl chloride (tolSO2Cl). Treatment of fac-[Re(CO)3(H2O)3](+) with these ligands, designated as N(SO2R)dien, afforded new fac-[Re(CO)3(N(SO2R)dien)]PF6 complexes. Comparing the fac-[Re(CO)3(N(SO2Me)dien)]PF6 and fac-[Re(CO)3(N(SO2Me)dpa)]PF6 complexes, we find that the Re(I)-N(sulfonamide) bonds are normal in length and statistically identical and that the methyl (13)C NMR signal has an unusually upfield shift compared to that in the free ligand. We attribute this unusual upfield shift to the fact that the sulfonamide N undergoes an sp(2)-to-sp(3) rehybridization upon coordination to Re(I) in both complexes. Thus, the sulfonamide N of N(SO2R)dien ligands is a good donor, even though the chelate rings are conformationally flexible. Addition of the strongly basic and potentially monodentate ligand, 4-dimethylaminopyridine, did not affect the fac-[Re(CO)3(N(SO2tol)dien)]PF6 complex, even after several weeks. This complex is also stable to heat in aqueous solution. These results indicate that N(SO2R)dien ligands form fac-[Re(CO)3(N(SO2R)dien)]PF6 complexes sufficiently robust to be utilized for radiopharmaceutical development.

Formation of a metal-to-nitrogen bond of normal length by a neutral sufonamide group within a tridentate ligand. A new approach to radiopharmaceutical bioconjugation

We demonstrate that a tertiary sulfonamide group, N(SO2R)R'2, can rehybridize to form a M-N bond of normal length even when the group is in a linear tridentate ligand, such as in the new tridentate N(SO2R)dpa ligands derived from di-(2-picolyl)amine (N(H)dpa). N(SO2R)dpa ligands were used to prepare fac-[Re(CO)3(N(SO2R)dpa)](PF6 or BF4) complexes. Structural characterization of the new complexes established that the tertiary sulfonamide nitrogen atom binds to Re with concomitant sp(2)-to-sp(3) rehybridization, facilitating facial coordination. The new fac-[Re(CO)3(N(SO2R)dpa)]X structures provide the only examples for any metal with the sulfonamide as part of a noncyclic linear tridentate ligand and with a normal metal-to-nitrogen(tertiary sulfonamide) bond length. Rare previous examples of such normal M-N bonds have been found only in more constrained situations, such as with tripodal tetradentate ligands. Our long-term objectives for the new tridentate N(SO2R)dpa ligands are to develop the fundamental chemistry relevant to the eventual use of the fac-[M(I)(CO)3](+) core (M = (99m)Tc, (186/188)Re) in imaging and therapy. The sulfonamide group uniquely contributes to two of our goals: expanding ways to conjugate the fac-[M(I)(CO)3](+) core to biological molecules and also developing new symmetrical tridentate ligands that can coordinate facially to this core. Tests of our conjugation method, conducted by linking the fac-[Re(I)(CO)3](+) core to a new tetraarylporphyrin (T(N(SO2C6H4)dpa)P) as well as to a dansyl (5-(dimethylamino)naphthalene-1-sulfonyl) group, demonstrate that large molecular fragments can be tethered via a coordinated tertiary sulfonamide linkage to this core.

Helical self-assembly of 2-(1,4,7,10-tetraazacyclododecan-1-yl)cyclohexan-1-ol (cycyclen)

The title macrocyclic amino alcohol compound, C(14)H(30)N(4)O, is investigated as a solid-state synthon for the design of a self-assembled tubular structure. It crystallizes in a helical column constructed by stereospecific O-H···N and N-H···N interactions. The hydrogen-bonding interactions, dependent upon macrocyclic ring helicity and molecular conformation, link R,R and S,S enantiomers in a head-to-tail fashion, forming a continuous hydrophilic inner core.

A new synthesis of cystamine modified Eu3+ DOTAM-Gly-Phe-OH: a conjugation ready temperature sensitive MRI contrast agent

Several approaches towards asymmetrically derivatized peptide-decorated cyclens that yield lanthanide metal chelators, in which three of the nitrogen atoms of cyclen share a common substituent and the fourth nitrogen atom is differentially substituted, have been evaluated. The most effective route consisted of selective monoalkylation followed by peralkylation with a second different electrophile. The unique substituent also possessed a masked sulfanyl group that was suitable for subsequent chemoselective conjugation chemistry.

Poly[tris-(μ(4)-5-amino-isophthalato)diaqua-dilanthanum(III)]

The title compound, [La₂(C₈H₅NO₄)₃(H₂O)₂]_n, is a three-dimensional network coordination polymer in which each La^III ion is nine-coordinated by eight carboxyl­ate O atoms from six 5-amino­isophthalate ligands and one O atom from a water mol­ecule. One organic ligand lies on a twofold rotation axis. O—H⋯O, O—H⋯N and N—H⋯O hydrogen bonds are observed in the crystal structure.

Gadolinium(III) complexes as MRI contrast agents: ligand design and properties of the complexes

Magnetic resonance imaging is a commonly used diagnostic method in medicinal practice as well as in biological and preclinical research. Contrast agents (CAs), which are often applied are mostly based on Gd(III) complexes. In this paper, the ligand types and structures of their complexes on one side and a set of the physico-chemical parameters governing properties of the CAs on the other side are discussed. The solid-state structures of lanthanide(III) complexes of open-chain and macrocyclic ligands and their structural features are compared. Examples of tuning of ligand structures to alter the relaxometric properties of gadolinium(III) complexes as a number of coordinated water molecules, their residence time (exchange rate) or reorientation time of the complexes are given. Influence of the structural changes of the ligands on thermodynamic stability and kinetic inertness/lability of their lanthanide(III) complexes is discussed.

Synthesis and characterisation of bis-cyclen based dinuclear lanthanide complexes

The design and synthesis of several bis-macrocyclic cyclen (1,4,7,10-tetraazacyclododecane) ligands and their corresponding lanthanum or europium complexes is described; these dinuclear lanthanide systems were made by connecting two macrocyclic cyclen moieties through a rigid, covalent, p-xylylenediamide bridge or a flexible aliphatic hexane bridge. These ligands were subsequently functionalised with six acetamide pendant arms (CONR1R2: R1 = R2 = H or CH3, or R1 = H, R2 = CH3). The corresponding lanthanide bis-complexes were then formed by reaction with La(III) and Eu(III) triflates, yielding overall cationic (+VI charged) complexes.

Luminescent Eu(III) and Tb(III) Complexes: Developing Lanthanide Luminescent-Based Devices

This mini review gives some highlights of the work recently carried out in our research group in Dublin on the developments of lanthanide luminescent devices, where the future goal is to produce devices that can operate as sensors. A few examples demonstrate our design principles for targeting both anion and cations that are of biological or pharmaceutical relevance, where the recognition occurs in aqueous competitive media. We also discuss the possibility of developing mixed f-d metal complexes and conjugates that can be employed as novel supramolecular architectures.

Lanthanide luminescent switches: modulation of the luminescence of bis-macrocyclic based Tb(iii) conjugates in water by H+, Na+ and K+

The synthesis of four bis-macrocyclic conjugates made from the coupling of either diaza-15-crown-5 ethers (1 and 3) and diaza-18-crown-6 ethers (2 and 4) to either amide or carboxylate functionalized cyclen (1,4,7,10-tetraazacyclododecane), and their corresponding cationic Tb(III) complexes, Tb-1, Tb-2, and neutral complexes Tb-3 and Tb-4 are described. The effect on the ground, singlet excited states and the Tb(III) emission, was investigated either as a function of pH or the concentration of several Group I and II cations, upon excitation at 300 nm. The ground state and singlet excited states of the Tb(III) complexes were found to be modulated by ions such as H+, Na+ or K+, signifying the recognition of these ions by the crown ether receptors. In acidic media, below pH 4, the Tb(III) emission was highly pH sensitive, gradually increasing with large orders of magnitude of luminescence enhancements. For Tb-1 and Tb-2 complexes, the Tb(III) emission was also "switched on" in alkaline media above pH 8. At pH 7.4, the recognition of Na+ or K+ also gave rise to a significant change in the Tb(III) emission due to the modulation of the antenna-receptor moieties by these ions. For Tb-1 and Tb-3 the largest changes were seen for Na+, whereas for Tb-2 and Tb-4 the largest changes were seen for K+.

Eu(iii)–cyclen–phen conjugate as a luminescent copper sensor: the formation of mixed polymetallic macrocyclic complexes in water

The cationic cyclen based Eu(III)-phen conjugated 1.Eu was synthesised as a chemosensor for Cu(II), where the recognition in water at pH 7.4 gave rise to quenching of the Eu(III) luminescence and the formation of tetranuclear polymetallic Cu(II)-Eu(III) macrocyclic complexes in solution where Cu(II) was bound by three 1.Eu conjugates.