Simon G. Bott

Two series of isomeric, redox-responsive azacrown ethers based on ortho- and para-phenylenediamine (Wurster's crowns) have been synthesized and their properties explored through ¹³C NMR spectroscopy, electrospray ionization mass spectrometry, cyclic voltammetry, and X-ray crystallography. These crowns display strong affinity for alkali metal cations while maintaining comparable selectivity profiles to the parent crown ethers from which they are derived. Like Wurster's reagent (N,N,N‘,N‘-tetramethyl-p-phenylenediamine or para-TMPD), the para-Wurster's crowns undergo two reversible one-electron oxidations. The integrity of the alkali metal ion complexes is maintained in the neutral and singly oxidized ligand states but not after removal of two electrons. In contrast, the oxidation of ortho-Wurster's crowns is scan rate dependent, occurring at potentials substantially higher than their para counterparts, with their complexes oxidizing irreversibly. X-ray crystal structures of representative complexes show, in all cases, participation of the redox-active phenylenediamine subunits in complex formation via direct bonding to the guest cation.

The ruthenium cluster Ru₃(CO)₁₂ reacts with the diphosphine ligand 3,4-bis(diphenylphosphino)-5-methoxy-2(5H)-furanone (bmf) in refluxing toluene to furnish the donor–acceptor compound Ru₂(CO)₂(bmf) as a 1 : 1 mixture of diastereomers. Photolysis of Ru₂(CO)₂(bmf) using 366 nm light leads to the oxidative cleavage of a P–C bond and formation of the phosphido-bridged complex Ru₂(CO)₆[μ-C=C(PPh₂)C(O)OCH(OMe)](μ-PPh₂). The regioselective Ph₂P–C(furanone ring) bond activation attendant upon optical excitation is traced to the phosphine group that was β to the furanone carbonyl group, as established by X-ray analysis of one of the diastereomers of Ru₂(CO)₆[μ-C=C(PPh₂)C(O)OCH(OMe)](μ-PPh₂). Both diruthenium products have been fully characterized in solution by IR and NMR (¹H and ³¹P) spectroscopies and elemental analyses. The observed regioselectivity associated with the P–C bond activation in Ru₂(CO)₂(bmf) is discussed with respect to the chemistry of other bmf-substituted compounds prepared by our groups.

Me₃NO activation of the methylidyne-bridged cluster HRu₃(CO)₁₀(μ-COMe) (1) in the presence of the unsaturated diphosphine ligand 2,3-bis(diphenylphosphino)maleic anhydride (bma) furnishes the bma-substituted cluster HRu₃(CO)₈(bma)(μ-COMe) (2) and the diphenylphosphine-substituted cluster HRu₃(CO)₈(Ph₂PH)[μ-PPh₂C=CC(O)OC(O)] (3) as the major and minor products, respectively. The ¹H and ³¹P NMR data indicate that the bma ligand in cluster 2 is chelated to one of the ruthenium atoms that is bridged by the hydride and methylidyne ligands. Cluster 3 has been fully characterized in solution by IR and NMR spectroscopies, and the solid-state structure determined by X-ray crystallography. 3 crystallizes in the monoclinic space P2₁, a = 12.1467(7) Å, b = 19.284(1) Å, c = 16.867(1) Å, β = 109.639(6)°, V = 3721.0(4) ų, Z = 4, and d_calcd = 1.774 g cm⁻³; R = 0.0325, R _w = 0.0383 for 3518 reflections with I > 3σ(I). The X-ray data confirm that one of the P–C(maleic anhydride) bonds of the bma ligand has been cleaved and that cluster 3 contains Ph₂PH and μ-PPh₂C=CC(O)OC(O) ligands, the latter which functions as a face-capping ligand to all three ruthenium atoms. Control experiments indicate that cluster 2 does not function as a precursor to cluster 3 under the employed reaction conditions.

Interest in macrocyclic lanthanide complexes such as DOTA is driven largely through interest in their use as contrast agents for MRI. The lanthanide tetraamide derivatives of DOTA have shown considerable promise as PARACEST agents, taking advantage of the slow water exchange kinetics of this class of complex. We postulated that water exchange in these tetraamide complexes could be slowed even further by introducing a group to sterically encumber the space above the water coordination site, thereby hindering the departure and approach of water molecules to the complex. The ligand 8O₂-bridged DOTAM was synthesized in a 34% yield from cyclen. It was found that the lanthanide complexes of this ligand did not possess a water molecule in the inner coordination sphere of the bound lanthanide. The crystal structure of the ytterbium complex revealed that distortions to the coordination sphere were induced by the steric constraints imposed on the complex by the bridging unit. The extent of the distortion was found to increase with increasing ionic radius of the lanthanide ion, eventually resulting in a complete loss of symmetry in the complex. Because this ligand system is bicyclic, the conformation of each ring in the system is constrained by that of the other; in consequence, inclusion of the bridging unit in the complexes means only a twisted square, antiprismatic coordination geometry is observed for lanthanide complexes of 8O₂-bridged DOTAM.