Divalent cobalt, nickel and zinc halide complexes with multimodal ligands based on the cyclotriphosphazene platform: A structural study

Abstract

The reaction of hexakis(2-pyridyloxy)cyclotriphosphazene (L) and hexakis(4-methyl-2-pyridyloxy)cyclotriphosphazene (MeL) with the dihalides of cobalt(II), nickel(II) and zinc(II) afford the complexes [CoLX 2], [Co(MeL)X 2], [(CoLX)CoX 3], [{Co(MeL)X}CoX 3], [CoLX]PF 6 (X = Cl or Br), [NiLCl]PF 6, [NiLCl 2] (a green and a red isomer) and [(ZnCl 2) 2L]. Single crystal X-ray structures on a selection of the complexes show that L and MeL are versatile multimodal ligands, which exhibit a variety of coordination modes. [Co(MeL)Br 2] and the red isomer of [NiLCl 2] have the metal atom in a distorted trigonal bipyramidal environment ( τ = 0.83 and 0.78, respectively) with the ligand acting as a κ 3 N donor coordinating via the nitrogen atoms from two non-geminal pyridyloxy pendant arms, a nitrogen atom in the phosphazene ring and two halide ions. For the cobalt complex the pyridyloxy groups are cisoid and for the nickel complex they are transoid. The dimetallic complexes, [{Co(MeL)X}CoX 3] (X = Cl or Br) are zwitterionic with one cobalt atom in a rhombic coordination sphere with the ligand acting as κ 5 N donor binding to the metal through the nitrogen atoms of four pyridyloxy pendant arms, phosphazene nitrogen atom and a halide ion. The second cobalt atom is in a distorted tetrahedral site with the ligand acting as a κ 1 N donor via one pyridyloxy nitrogen, the remaining ligands being three halide ions. The dimetallic zinc complex, [(ZnCl 2) 2L], contains two ZnCl 2 units in distorted trigonal bipyramidal ( τ = 0.82) and tetrahedral coordination environments. In the five-coordinate site, L acts in a κ 3 N non-geminal transoid fashion as observed for the red nickel complex [NiLCl 2]. In the other site, L acts as a κ 2 N geminal bidentate via two pyridyloxy arms, with two chloride ions completing the coordination sphere. The cationic cobalt complexes, [MLBr]PF 6 (M = Co or Ni) both contain the metal atom in a rhombic environment similar to that found in the dimetallic [{Co(MeL)X}CoX 3] species. The green isomer of [NiLCl 2] contains a six coordinate ‘NiN 4Cl 2’ distorted octahedral coordination sphere with L behaving as a κ 4 N donor via the nitrogen atoms from three pyridyloxy pendant arms and one nitrogen in the cyclophosphazene ring; two halide ions complete the coordination sphere. In all the complexes the phosphazene ring atoms remain virtually coplanar as a consequence of the flexibility of the phenoxy-hinge, which links the pyridine pendant donors to the cyclophosphazene platform. The formation of the resulting six-membered chelate rings enhances the binding of the phosphazene nitrogen to metal bond which, in each of the five- or six-coordinate metal binding sites, is always shorter than the pyridyloxy nitrogen to metal bonds [2.023(6)–2.1075(18) cf. 2.157(4)–2.397(3) Å].