Studies of rheniumtricarbonyl complexes of tripodal pyridyl-based ligands

The reaction of N-benzoyl and N-acetyl tris(pyridin-2-yl)methylamine 1b and 1c (LH = tpmbaH and tpmaaH) with [Re(CO)5Br] has been investigated and shown to proceed via the initial formation of a cationic rheniumtricarbonyl complex [(LH)Re(CO)3]Br in which coordination of the ligand occurs via the three pyridine rings. For tpmbaH 1b, but not tpmaaH 1c, this initial complex 2b readily undergoes the loss of HBr to give a neutral octahedral complex 4b [(L)Re(CO)3] where coordination occurs via two of the pyridine rings and the deprotonated amide nitrogen. The 1H NMR spectrum of the latter complex 4b is very unusual in that at room temperature the signals for the 3-H protons on the coordinated pyridine rings are not visible due to extreme broadening of these resonances. Comparison with the analogous complex 7 from N-benzoyl bis(pyridin-2-yl)methylamine 6b (bpmbaH) confirms that this is due to rotation of the uncoordinated pyridine ring. The structure of the cationic complex 3d [(LH)Re(CO)3]Br formed from N-benzyl tris(pyridin-2-yl)methylamine 1d (bz-tpmaH) is also discussed. The crystal structures of complexes [(tpmba)Re(CO)3] 4b, [(bz-tpmaH)Re(CO)3]Br 3d and [(bpmba)Re(CO)3] 7 have been determined. In all complexes the coordination geometry around Re is distorted octahedral with a fac-{Re(CO)3}+ core.