I. Cycloheptatriene and Cycloheptatrienyl Tantalum Compounds II. Synthesis of Pure Boron Nanoparticles

Recently, tantalum has been increasingly studied as a diffusion barrier material for copper due to its high melting point (2669 °C) and immiscibility with Cu. Most Ta thin films are deposited by PVD, but it would be advantageous to develop halide-free precursors to produce pure Ta films by CVD. To achieve this goal, we have studied the chemistry of new organotantalum compounds that consist only of the elements of Ta, C, H, and that possess ligands likely to desorb intact from the growth surface. We chose to investigate cycloheptatrienyl tantalum compounds based on related studies of precursors for the deposition of Ti metal. The chemistry of (cycloheptatrienyl)tantalum complexes is almost completely unexplored, only the mixed sandwich compound (C5H4Me)Ta(C7H7) has been described briefly. We have investigated the possibility of preparing (η-C7H8)Ta and (η-C7H7)Ta complexes. We have synthesized the new complex (η-C7H8)TaCl2(PMe3)2 (1) by the reduction of TaCl5 in the presence of PMe3 and cycloheptatriene, and subsequently converted 1 to (η-C7H7)TaCl(PMe3)2 (2) (Figure 1a). By Mg reduction of the corresponding (C5R5)TaCl4 species in the presence of cycloheptatriene, we have also synthesized a series of mixed sandwich complexes: (C5H5)Ta(C7H7) (3), (C5H4Me)Ta(C7H7) (4) (Figure 1b), and (C5Me5)Ta(C7H7) (5). A comparison with the structures of other (C5R5)M(C7H7) compounds shows that the M-C distances to the seven membered ring are especially sensitive to the dorbital energies of the metal center and their ability to engage in δ-bonding with the ring.