A Facile and Efficient Method for Preparing Ring-Alkylated Vanadocenes (RCp)2V and Mono- and Dichlorides of Ring-Alkylated Vanadocenes (RCp)2VCl and (RCp)2VCl2

Given the number of applications surrounding early transition metal cyclopentadienyl sandwich complexes, the paucity of vanadocene research is truly remarkable. This is even more surprising given their potential application as potent antitumor agents,1 paramagnetic medical MRI shift reagents,2ab Ziegler-Natta catalysts,3ab and building blocks for novel magnetic materials.4 Obviously, one of the major stumbling blocks in this area of research has been the extreme reactivity of the parent vanadocene, in which the V(II) center is violently O2 sensitive. In addition, the purification of these materials by reported techniques5ab is extremely difficult due to the low volatility (as ring substitution increases) and thermal instability of the parent complexes. In this report, we wish to present a simple and efficient method for the preparation of multigram quantities of several ring-alkylated vanadocenes and their monoand dichloride derivatives with PCl3. We feel the generality of our new synthetic procedure will allow for the isolation and characterization of several, previously inaccessible vanadium cyclopentadienyl complexes. The recent synthesis of the first homobimetallic tetrathiafulvalenetetrathiolate has provided a route to numerous new building blocks that can be used in the assembly of new advanced materials. The bimetallic TTFS4 complexes are synthesized by the addition of TTFS4 to 2 equiv of various early transition metal metallocene dichlorides (Cp2MCl2). We are now focusing on examining the spin ordering and magnetic coupling in high-spin, bivanadium tetrathiafulvalene (TTF) materials. The unpaired electron on the V(IV) center affords an S * 0 ground state and the possibility of using the vanadium nuclear hyperfine (I ) /2) splitting as a spectroscopic marker were both attractive reasons for exploring this chemistry. Initially, we prepared bimetallic TTFs using the comercially available vanadocene dichloride; however, the isolated final materials were insoluble and impure. As an attempt to rectify this problem, we sought to make ring-alkylated vanadocenes in order to enhance the desired products solubility, as well as prohibit the formation of μ-S bridging dithiolenes,9 which is a major problem in the unsubstituted systems. Bis(methylcyclopentadienyl) vanadium dichloride (7), a known substituted vanadium compound, also failed to give reasonably soluble or analytically pure bimetallic tetrathiafulvalene materials.10 In an effort to synthesized other ring-alkylated vanadocenes, our work was hampered by the tendency of the dichlorides to produce analytically impure products. Also, distillation of the of the purple vanadocene (1) resulted in considerable thermal degradation. We now report the preparation of the branched ring-alkylated vanadocenes (2 and 3) and their dichlorides and new purification routes to further facilitate the scale and solubility of the desired materials.