Synthesis and characterization of technetium(V) oxo-complexes with quadridentate Schiff-base ligands: X-ray structures of µ-oxo-bis-{oxo[N,N′-propane-1,3-diylbis(salicylideneiminato)]technetium(V)} and chloro-oxo[N,N′-propane-1,3-diylbis(salicylideneiminato)]-technetium(V)

Abstract

The reactions of quadridentate Schiff-base ligands derived from salicylaldehyde and diamines with [TcOCl₄]^– have been investigated. The complexes [NBu₄][TcOCl₃(Hsalpd)], [NBu₄][TcOCl₃(Hsalbd)], [{TcO(salpd)}₂O], [TcOCl(salpd)], and [TcOCl(salbd)][salpd =N,N′-propane-1,3-diylbis-(salicylideneiminate), salbd =N,N′-butane-1,4-diylbis(salicylideneiminate)] were isolated from the reaction mixture in ethanol. The complexes have been characterized by elemental analysis and by i.r. spectroscopy. The crystal structures of [{TcO(salpd)}₂]O (1) and [TcOCl(salpd)](2) have been determined from three-dimensional X-ray data: (1), monoclinic, space group P2₁/c, a= 15.041 (2), b= 12.630(3), c= 16.522(4)Å, β= 95.35(3)° and Z= 4; (2), orthorhombic, space group Pn2₁a, a= 12.010(4), b= 11.702(4), c= 11.625(6)Å, and Z= 4. The structures have been solved by Patterson and Fourier methods and refined by least-squares methods to R 0.059 for (1) and 0.076 for (2). The ‘dimeric’ structure of (1) consists of two crystallographically independent and chemically well separated TcO(salpd) moieties bridged by an oxygen atom with the bridging Tc–O–Tc angle symmetrically imposed at 180°. A novel feature is the presence of the almost linear OTc–O–TcO group. The technetium atoms have octahedral co-ordination, with the equatorial plane formed by the N₂O₂ donor set, and the chelate ligand, as a whole, is very nearly planar. The structure of (2) consists of discrete molecules of [TcOCl(salpd)] with a pseudo-octahedral co-ordination around technetium. The salpd ligand occupies the four equatorial positions, whereas the Cl and the oxygen are trans to each other in axial positions. The two salicylaldimine groups are bent in an ‘umbrella shape’. Bond distances (Å) in the ‘inner core’: (1), TcO 1.68, Tc–O(bridging) 1.90, Tc–O 2.01, and Tc–N 2.12 Å; (2), TcO 1.66, Tc–O 1.98, Tc–N 2.12, Tc–Cl 2.44 Å, estimated standard deviations being 0.01 Å for (1) and 0.02 Å for (2).