A monophosphonate analogue of H4dota, 1,4,7,10-tetraazacyclododecane-4,7,10-tris(carboxymethyl)-1-methylphosphonic acid (H5do3aP), and its complexes with lanthanides were synthesized. Multinuclear NMR studies reveal that, in aqueous solution, lanthanide(III) complexes of the ligand exhibit structures analogous to those of H4dota complexes. Thus, the central ion is nine-coordinate, surrounded by four nitrogen atoms, three acetate and one phosphonate oxygen atoms, and one water molecule in an apical position. For complexes of H5do3aP with Ln(III) ions in the middle of the series, the abundance of the desired twisted square-antiprismatic (TSAP) isomer is higher than for the corresponding H4dota complexes. The TSAP/square-antiprismatic (SAP) isomer ratio is highly sensitive to protonation of the phosphonate group: a higher abundance of the TSAP isomer was found in acidic solutions. The microscopic protonation constants of the TSAP isomers are higher than those of the SAP isomers. The presence of one water molecule in the first coordination sphere of the complexes in the pH region studied (pH 2.5-7.0) is confirmed by 17O NMR spectroscopy. The results of a simultaneous fit of variable-temperature 17O NMR relaxation data and 1H NMRD profiles show that the residence time of water (tauM) in the Gd(III) complex is much smaller than for [Gd(dota)(H2O)]-. The exchange rate appears to be dependent on the pH of the solution. The values of tauM are 37, 40, and 14 ns at pH 2.5, 4.7, and 7.0, respectively. These observations can be explained by an extensive second-sphere hydrogen-bonding network that varies with the state of protonation of the phosphonate moiety. Upon protonation of the complex, the second-sphere hydration probably becomes more ordered, which may result in a decrease in penetrability and an increase in tauM. The relaxivity of the Gd(III) complex is almost independent of the pH and is equal to 4.7 s(-1) mM(-1) (20 MHz, pH 7 and 37 degrees C). The solid-state structure was determined for the Nd(III) complex. It crystallizes as the TSAP isomer and the unit cell contains two independent molecules of the complex with different Nd-O(water) bond lengths of 2.499 and 2.591 A.

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A series of lanthanide(III) complexes formulated as M[Ln(Hdo3ap)].xH(2)O (M = Li or H and Ln = Tb, Dy, Er, Lu, and Y) with the monophosphonate analogue of H(4)dota, 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic-10-methylphosphonic acid (H(5)do3ap), was prepared in the solid state and studied using X-ray crystallography. All of the structures show that the (Hdo3ap)(4-) anion is octadentate coordinated to a lanthanide(III) ion similarly to the other H(4)dota-like ligands, i.e., forming O(4) and N(4) planes that are parallel and have mutual angle smaller than 3 degrees . The lanthanide(III) ions lie between these planes, closer to the O(4) base than to the N(4) plane. All of the structures present the lanthanide(III) complexes in their twisted-square-antiprismatic (TSA) configuration. Twist angles of the pendants vary in the range between -24 and -30 degrees, and for each complex, they lie in a very narrow region of 1 degree. The coordinated phosphonate oxygen is located slightly above (0.02-0.19 Angstroms) the O(3) plane formed with the coordinated acetates. A water molecule was found to be coordinated only in the terbium(III) and neodymium(III) complexes. The bond distance Tb-O(w) is unusually long (2.678 Angstroms). The O-Ln-O angles decrease from 140 degrees [Nd(III)] to 121 degrees [Lu(III)], thus confirming the increasing steric crowding around the water binding site. A comparison of a number of structures of Ln(III) complexes with DOTA-like ligands shows that the TSA arrangement is flexible. On the other hand, the SA arrangement is rigid, and the derived structural parameters are almost identical for different ligands and lanthanide(III) ions.

Cyclam derivatives with methylphosphonic acid arms in position 1,8 and substituent R = H, Me, CH2Ph in positions 4 and 11 are synthesised by Mannich reaction of an appropriate cyclam derivative, formaldehyde and phosphonic acid/diethyl phosphite followed by removal of protecting benzyl groups from nitrogen atoms. Mono(methylphosphonic acid) derivative of cyclam can be obtained by a similar route. Crystal structures of four phosphonic acid derivatives show the same ring conformation and orientation pendants due to strong intramolecular hydrogen bonds between phosphonate oxygen atoms and protonated nitrogen atoms adjacent over ethylene chains. The hydrogen bonds are stable even in aqueous solution. Activation parameters for destabilisation of the conformation are estimated from temperature-dependent NMR measurement. The protonation constants determined confirm the expected high basicity of the compounds and its dependence on the nitrogen atom substituents. The enhanced basicity of the nitrogen atoms non-bonded to methylenephosphonic acid moiety, is explained by the presence of the strong hydrogen bonds.

Helicenes have attracted attention as unique inherently chiral three-dimensional aromatic compounds for several decades. Despite significant recent progress in the synthesis and applications of carbohelicenes and thiaheterohelicenes, [1] the potential of the aza analogues with a pyridine unit (pyridohelicenes) have not been explored. [2,3] There are only scattered examples of the preparation of pyridohelicenes, [4] but with no general synthetic methodology, since the photochemical approach can fail with pyridohelicenes [4b,c,e] while non-photochemical alternatives can be difficult to adapt to the synthesis of N-heteroaromatic compounds. The properties and chemical behavior of pyridohelicenes are practically unknown apart from their basicities [4e,g] and the self-assembly [5] of certain derivatives. Nevertheless, promising applications of pyridohelicenes in various branches of chemistry and material science might be envisaged and, therefore, further research in the field is required.Recently, we observed remarkably high proton affinities of pyridohelicene derivatives, as measured by mass spectrometric techniques.[6] In the gas phase, helically chiral 1-aza[6]helicene (2, Scheme 1), for example, exhibits a comparable proton affinity (1000 kJ mol À1 ) to 4-(dimethylamino)-pyridine (DMAP, 997 kJ mol À1 ). Herein, we report the practical syntheses of 1,14-diaza[5]helicene (1), [7] 1-aza[6]helicene (2), [8] and 2-aza[6]helicene (3).[9] Moreover, we have succeeded in resolving racemates of 2 and 3 into their enantiomers, assigning their absolute configuration, determining the energy barriers to racemization, and obtaining Xray structures of their corresponding silver complexes. The strategy for the preparation of 1-3 relies on a [2+2+2] cyclotrimerization of an aromatic triyne in the presence of a Co I catalyst to build the helical scaffold. We have already proven that such a methodology is robust and reliable for the preparation of helicenes and their analogues.[10] Thus, the straightforward synthesis of 1 started from the readily accessible bromopyridine 4, [11] which was treated with lithiated 1-(triisopropylsilyl)-1-propyne to yield 5 which contained an attached alkyne side arm (Scheme 2). Sonoga-