Strong π-interactions between tantalum and apical ligands in square pyramidal complexes supported by a rigid tetradentate spectator ligand

•A 2,2′,2″,2‴-(1,2-ethenediylidene)tetraphenoxide was coordinated to tantalum.•The ligand stabilises the formation of monomeric complexes.•X-ray diffraction analyses confirmed the square pyramidal geometry.•Structural parameters indicate strong π-interactions with the apical ligand.•Very short Ta–OEt and Ta–NMe2 bonds were observed.

The synthesis of new tantalum(V) square pyramidal complexes supported by the tert-butyl-substituted (ethenediylidene)tetraphenoxide spectator ligand (TPO) and their reactivity are herein described. The tantalum ethoxide (TPO)Ta(OCH2CH3) (1) and dimethylamide (TPO)Ta(NMe2) (2) complexes were prepared and structurally characterised, giving valuable insight into π-interactions between the metal centre and the apical ligand. The solid-state structures confirmed the geometry of both complexes, with TPO forming the base of the square pyramidal structure and the ethoxide and dimethylamide ligands occupying the apical position. Bond lengths and angles from the structures indicate significant double bond character between the metal and the apical ligand with very short metal–oxygen and metal–nitrogen bonds in 1 and 2, respectively, which results from donation of π-electron density from the heteroatom into empty metal dπ-orbitals. Reaction of (TPO)Ta(OCH2CH3) (1) and (TPO)Ta(NMe2) (2) with trimethylaluminum gave the methyl complex (TPO)Ta(CH3) (3). Complexes 1 and 2 were cleanly regenerated by addition of ethanol and dimethylamine to complex 3. Addition of strong Brønsted acids to these complexes gave mixtures of compounds that could not be characterised, possibly due to competing protonation of the spectator TPO ligand or formation of oligomeric species.

Graphical abstractTantalum(V) square pyramidal complexes supported by the tert-butyl-substituted (ethenediylidene)tetraphenoxide spectator ligand (TPO) are described. The tantalum ethoxide (1) and dimethylamide (2) complexes were prepared and both structurally characterised, showing strong π-interactions between the metal centre and the apical ligand. Reaction of 1 and 2 with trimethylaluminum gave (TPO)Ta(CH3) (3), which converts back to 1 and 2 upon proton transfer from ethanol and dimethylamine, respectively.Figure optionsDownload full-size imageDownload as PowerPoint slide