Synthesis and Structure of Bis(π-cyclopentadienyl)vanadium(IV) 1,10-Phenanthroline and 2,2‘-Bipyridine Compounds and Their Interactions with Artificial Membranes

The reaction of in situ generated Cp2V(OTf)2 (Cp = cyclopentadienyl; OTf = O3SCF3) with excess 1,10-phenanthroline and 2,2‘-bipyridine yields the d1 vanadocene coordination compounds [Cp2V(phen)][OTf]2 (1) and [Cp2V(bpy)][OTf]2 (2), respectively. The compounds have been characterized by UV−vis and EPR spectroscopy and by cyclic voltammetry. The complexes have relatively low vanadium(IV)−vanadium(III) reduction potentials (−0.62 V vs Cp2Fe+/0 in acetonitrile). Structures of 1 and 2 have been determined by X-ray crystallography. Compound 1 crystallized in a monoclinic system, space group P21/n, with a = 10.2763(5) Å, b = 18.1646(9) Å, c = 13.5741(7) Å, β = 99.4150(10)°, and Z = 4. Refinement of its structure by full-matrix least-squares techniques gave final residuals R = 0.040 and R w = 0.096. Compound 2 crystallized in a monoclinic system, space group P21/c, with a = 10.6451(6) Å, b = 18.3863(10) Å, c = 12.6993(7) Å, β = 98.6220(10)°, and Z = 4. Refinement of its structure by full-matrix least-squares techniques gave final residuals R = 0.046 and R w = 0.101. The two nitrogen atoms and centroids of the two cyclopentadienyl rings for both compounds occupy a distorted tetrahedral geometry around the vanadium(IV) center. The chelated ring plane is inclined closer to one of the neighboring Cp rings with the tilt more evident in 1 (∼8°) than 2 (∼4°). The membrane interactions of these compounds and the titanium analogues, [Cp2Ti(phen)][OTf]2 (3) and [Cp2Ti(bpy)][OTf]2 (4), have been studied with zwitterionic unilamellar liposomes as artificial membranes. We show that the ability of metallocenes to enhance the permeability of a liposomal membrane depends on the hydrophobicity, as well as the size and planarity of the ancillary chelated ligands, but not the nature of the central metal ion. Also provided is evidence that metallocene-induced permeability changes in artificial membranes are not caused by lipid peroxidation.