Synthesis and structure of bis(pi-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 d( 1) vanadocene coordination compounds [Cp2V(phen)][OTf](2) (1) and [Cp2V(bpy )][OTf](2) (2), respectively. The compounds have been characterized by W-vi s and EPR spectroscopy and by cyclic voltammetry. The complexes have relati vely 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 cr ystallography. Compound 1 crystallized in a monoclinic system, space group P2(1)/n, with a = 10.2763(5) Angstrom, b = 18.1646(9) Angstrom, c = 13.5741 (7) Angstrom, beta = 99.4150(10)degrees, and Z = 4. Refinement of its struc ture 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 gro up P2(1)/c, with a = 10.6451(6) Angstrom, b = 18.3863(10) Angstrom, c = 12. 6993(7) Angstrom, beta = 98.6220(10)degrees, and Z = 4. Refinement of its s tructure 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 cyclo pentadienyl rings for both compounds occupy a distorted tetrahedral geometr y around the vanadium(IV) center. The chelated ring plane is inclined close r to one of the neighboring Cp rings with the tilt more evident in 1 (simil ar to 8 degrees) than 2 (similar to 4 degrees). The membrane interactions o f 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 hydroph obicity, as well as the size and planarity of the ancillary chelated ligand s, but not the nature of the central metal ion. Also provided is evidence t hat metallocene-induced permeability changes in artificial membranes are no t caused by lipid peroxidation.