Structural considerations of terdentate ligands: crystal structures of 2,2′ : 6′,2″-terpyridine and 2,6-bis(pyrazol-1-yl)pyridine

Abstract

The crystal structures of the terdentate ligands 2,2′ : 6′,2″-terpyridine (terpy) and 2,6-bis(pyrazol-1-yl)pyridine (bppy) were determined by single-crystal diffraction studies. The compound terpy crystallizes in the non-centrosymmetric orthorhombic space group P2₁2₁2₁ with a= 3.947(1), b= 16.577(7) and c= 17.840(6)Å; the structure was refined to R= 0.0745 for all 1087 independent data and R= 0.0470 for those 609 data with F > 6σ(F). The compound bppy crystallizes in the centrosymmetric orthorhombic space group Pnma with a= 11.929(3), b= 21.320(3) and c= 3.889(0)Å; this structure was refined to R= 0.0409 for all 896 independent data and R= 0.0300 for those 723 reflections with F > 6σ(F). The structures of the free terpy and bppy ligands were compared directly with the structures of the co-ordinated terpy and bppy ligands in the [RuL(NO₂)(PMe₃)₂][ClO₄] complexes (L = terpy 1 or bppy 2) in order to determine if any ligand structural changes occur upon co-ordination to ruthenium. To act as terdentate ligands, it was observed that both terpy and bppy must adopt the cis,cis ligand configuration as opposed to the trans,trans configuration found in the solid state and as the equilibrium configuration in solution. Both terpy and bppy distort upon co-ordination to ruthenium. The greatest distortions for terpy occur primarily at the central pyridine ring. Large distortions were observed for bppy at both in the central pyridine ring and in the terminal pyrazole rings.