FAPRI 2002 World Agricultural Outlook

Crop Production/Industries, Livestock Production/Industries,

Molecular and Electronic Structures of Mononuclear and Dinuclear Titanium Complexes Containing π-Radical Anions of 2,2'-Bipyridine and 1,10-Phenanthroline: An Experimental and DFT Computational Study

Whereas reaction of [(η5-Cp*)TiIVCl3]0 (1) with 2 equiv of neutral 2,2′-bipyridine (bpy) and 1.5 equiv of magnesium in tetrahydrofuran affords the mononuclear complex [(η5-Cp*)TiIII(bpy•)2]0 (2), performing the same reaction with only 1 equiv each of magnesium and bpy provides the dinuclear complex [{(η5-Cp*)Ti(μ-Cl)(bpy•)}2]0 (3). Conducting the latter reaction using 1,10-phenanthroline (phen) in place of bpy resulted in formation of dinuclear [{(η5-Cp*)Ti(μ-Cl)(phen•)}2]0 (4). The structures of 2, 3, and 4 have all been determined by high-resolution X-ray crystallography at 153 K; the Cpy–Cpy distances of 1.420(3) and 1.431(4) Å in the N,N′-coordinated bpy ligands of 2 and 3, respectively, are indicative of the presence of (bpy•)1– ligands, rather than neutral (bpy0). The electronic spectra (300–1600 nm) of these two complexes are similar in form, and contain intense π → π* transitions associated with the (bpy•)1– radical anion. Temperature dependent magnetic susceptibility measurements (4–300 K) show that mononuclear 2 possesses a temperature independent magnetic moment of 1.73 μB, which is indicative of an S = 1/2 ground state. Broken symmetry density functional theory (BS-DFT) calculations yield a picture consistent with the experimental findings, in which the central Ti atom possesses a +3 oxidation state and is coordinated by a η5-Cp* ligand and two (bpy•)1–. Strong intramolecular antiferromagnetic coupling of these three unpaired spins, one each on the TiIII center and on the two (bpy•)1– ligands, affords the experimentally observed doublet ground state. The magnetic susceptibility measurements for dinuclear 3 and 4 display weak but significant ferromagnetic coupling, and indicate that these complexes possess S = 1 ground states. The mechanism of the spin coupling phenomenon that yields the observed behavior was analyzed using BS-DFT calculations, and it was discovered that the tight π-stacking of the N,N′-coordinated (bpy•)1–/(phen•)1– ligands in these two complexes results from direct overlap of their SOMOs and formation of a two-electron multicentered bond. This yields a diamagnetic {(bpy)2}2–/{(phen)2}2– bridging unit whose doubly occupied HOMO is spread equally over both ligands. The two remaining unpaired electrons, one at each TiIII center, couple weakly in a ferromagnetic fashion to yield the experimentally observed S = 1 ground states

Controlling the Rate of Shuttling Motions in [2]Rotaxanes by Electrostatic Interactions: A Cation as Solvent-Tunable Brake

A series of rotaxanes, with phenolic axle centerpieces and tetralactam macrocycles as the wheels, has been prepared in good yields. The threaded rotaxane structure is confirmed in the gas phase by tandem mass spectrometric experiments through a detailed fragmentation pattern analysis, in solution by NMR spectroscopy, and in the solid state through X-ray crystallography. A close inspection of the 1H,1H NOESY and 1H,1H ROESY NMR data reveals the wheel to travel along the axle between two degenerate diamide stations close to the two stoppers. By deprotonation of a phenolic OH group in the axle centerpiece with Schwesinger's P1 base, surprisingly no additional shuttling station is generated at the axle center, although the wheel could form rather strong hydrogen bonds with the phenolate. Instead, the wheel continues to travel between the two diamide stations. Experimental data from 1H,1H NOESY spectra, together with theoretical calculations, show that strong electrostatic interactions between the phenolate moiety and the P1 cation displace the wheel from the phenolate station. The cation acts as a brake for the shuttling movement. Instead of suppressing the shuttling motion completely, as observed in other rotaxanes, our rotaxane is the first system in which electrostatic interactions modulate the speed of the mechanical motion between a fast and a slow motion state as a response to a reversible external stimulus. By tuning these electrostatic interactions through solvent effects, the rate of movement can be influenced significantly, when for example different amounts of DMSO are added to dichloromethane. Besides the shuttling motion, circumrotation of the wheel around the axle is observed and analyzed by variable temperature NMR spectroscopy. Force field and AM1 calculations are in good agreement with the experimental findings