The synthesis and reactivity of a new technetium(III) precursor. The crystal structures of [TcCl3(MeCN){P(C6H4Me-3)3}2] and [Tc(bipy)3]2+(bipy = 2,2′-bipyridine)

Abstract

The reduction of [TcCl₄(PPh₃)₂] with zinc metal in acetonitrile in the presence of triphenylphosphine gives the orange crystalline technetium(III) complex [TcCl₃(MeCN)(PPh₃)₂]1 in good yield. Suitable crystals of 1 were grown, but the crystal structure could not be determined satisfactorily due to pseudo-symmetry in the crystal lattice. To overcome this problem the crystal and molecular structure of the tri-m-tolylphosphine analogue [TcCl₃(MeCN){P(C₆H₄Me-3)₃}₂]1a was determined. Crystals of 1a are triclinic, space group P with Z= 2 in a unit cell of dimensions a= 10.157(2), b= 10.302(2), c= 22.073(2)Å, α= 87.27(2), β= 86.66(1) and γ= 66.87(1)°. The structure was refined based on 5636 reflections converging to R= 0.039. Complex 1a exhibits a distorted pseudo-octahedral geometry with two trans P(C₆H₄Me-3)₃ groups, three meridionally disposed chloride groups, and a linear terminally N-bound acetonitrile ligand. The reaction of 1 with the aromatic imines 2,2′-bipyridine (bipy), 1,10-phenanthroline (phen), and 2,2′:6′2″-terpyridine (terpy) gives the blue-black technetium(II) dicationic complexes [Tc(bipy)₃]²⁺, [Tc(phen)₃]²⁺ and [Tc(terpy)₂]²⁺ isolated as their [BPh₄]^– or [PF₆]^– salts. The crystal and molecular structure of one of these novel technetium(II) complexes, [Tc(bipy)₃][PF₆]₂2a, has been determined. Crystals of 2a are trigonal, space group Pc1 (no. 165) with Z= 2 in a unit cell of dimensions a= 10.847(2) and c= 16.299(3)Å. The structure was refined based on 414 unique reflections converging to R= 0.0525. The monomeric complex 2a exhibits exact D₃ symmetry with the Tc–N bonds [2.077(10)Å] equivalent by symmetry. The electrochemical behaviour of complexes 1, 2 and 3 has been investigated.