Homoleptic transition metal complexes of the 7-azaindolide ligand featuring κ(1)-N1 coordination

Abstract

Homoleptic complexes of the anion of 7-azaindole (AzaIn) were synthesized and characterized for a series of 3d transition metals. For Mn(II), Fe(II), and Co(II), complexes of formula Na2[M(AzaIn)4]·2L (L = tetrahydrofuran (THF), 2-MeTHF, toluene, or benzene) were isolated by treatment of the corresponding metal chloride salts with 7-azaindole in the presence of sodium hexamethyldisilazide. The complexes adopt tetrahedral geometries with exclusive coordination to the transition metal ion through the pyrrolic N1 nitrogen atoms of the AzaIn ligands. Solid-state structures of the complexes demonstrate that the sodium cations remain tightly associated with the coordination entities through interaction with both the pyrrolic and pyridine nitrogen atoms of the azaindolide ligands. For Fe(II), replacement of the sodium cations by other alkali metal ions (Li or K) generates new complexes that demonstrate similar coordination geometries to the sodium salts. As a means of comparison, the Fe(II) complex of 4-azaindolide was also investigated. Na2[Fe(4-AzaIn)4]·2L adopts a similar solution structure to the 7-azaindolide complexes as judged by NMR spectroscopy and cyclic voltammetry. Density functional theory calculations were performed to investigate the bonding in the 7-azaindolide complexes. Results demonstrate that 7-azaindolide-κ(1)-N1 is a nearly pure sigma donor ligand that features a high degree of ionic character in its bonding to mid 3d transition metal ions.

Figure 1

Thermal ellipsoid drawings (50%) of 3·thf and 3·tol. Hydrogen atoms are omitted for clarity. Selected average bond distances and angles for compounds 1-3 can be found in Table 1.

Figure 2

Thermal ellipsoid rendering (50%) of the solid-state structure of K₂[Fe(AzaIn)₄], 4. The solid-state network is truncated to display a single coordination entity. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): Fe(1)-N(1) = 2.056(2); Fe(1)-N(3) = 2.047(2); Fe(1)-K(1) = 3.6249(11); Fe(1)-K(2) = 3.8683(10); K(1)-N(4) = 2.785(3); K(2)-N(2A) = 2.819(3); N(1)-Fe(1)-N(1A) = 122.40(13); N(3)-Fe(1)-N(3A) = 122.04(14); N(1)-Fe(1)-N(3) = 103.45(9); N(1A)-Fe(1)-N(3) = 103.54(10).

Figure 3

Thermal ellipsoid rendering (50%) of the solid-state structure of Li₂[Fe(AzaIn)₄]·2THF·C₆H₆, 5·thf. Hydrogen atoms and co-crystallized benzene molecule are omitted for clarity. Selected bond distances (Å) and angles (deg): Fe(1)-N(1) = 2.0175(11); Fe(1)-N(3) = 2.0674(11); Fe(1)-Li(1) = 3.021(2); Li(1)-N(2) = 2.034(2); Li(1)-N(4) = 2.015(2); Li(1)-N(3A) = 2.234(2); N(1)-Fe(1)-N(1A) = 112.48(6); N(3)-Fe(1)-N(3A) = 106.91(6); N(1)-Fe(1)-N(3) = 114.03(4); N(1)-Fe(1)-N(3A) = 104.74(4).

Figure 4

Cyclic voltammogram of 2·mthf in THF at a platinum electrode. Scan rate is 50 mV/s and the supporting electrolyte is 0.1 M Bu₄NPF₆.

Figure 5

Region of the electronic absorption spectrum for 3 in toluene showing the ligand field transitions. Band energies are labeled on the spectrum. Computed frontier molecular orbital diagrams for complexes 1-3.

Figure 5

Region of the electronic absorption spectrum for 3 in toluene showing the ligand field transitions. Band energies are labeled on the spectrum. Computed frontier molecular orbital diagrams for complexes 1-3.

Figure 6

Selected frontier molecular orbitals for 2·thf.

Chart 1

The azaindole family of molecules.

Chart 2

Coordination modes of 7-azaindole.