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. 2020 Jul 13;25(14):3189.
doi: 10.3390/molecules25143189.

4,5-Diazafluorene and 9,9'-Dimethyl-4,5-Diazafluorene as Ligands Supporting Redox-Active Mn and Ru Complexes

Wade C Henke  1 , Julie A Hopkins  1 , Micah L Anderson  1 , Jonah P Stiel  1 , Victor W Day  1 , James D Blakemore  1
Affiliations

4,5-Diazafluorene and 9,9'-Dimethyl-4,5-Diazafluorene as Ligands Supporting Redox-Active Mn and Ru Complexes

Wade C Henke et al. Molecules. .

Abstract

4,5-diazafluorene (daf) and 9,9'-dimethyl-4,5-diazafluorene (Me2daf) are structurally similar to the important ligand 2,2'-bipyridine (bpy), but significantly less is known about the redox and spectroscopic properties of metal complexes containing Me2daf as a ligand than those containing bpy. New complexes Mn(CO)3Br(daf) (2), Mn(CO)3Br(Me2daf) (3), and [Ru(Me2daf)3](PF6)2 (5) have been prepared and fully characterized to understand the influence of the Me2daf framework on their chemical and electrochemical properties. Structural data for 2, 3, and 5 from single-crystal X-ray diffraction analysis reveal a distinctive widening of the daf and Me2daf chelate angles in comparison to the analogous Mn(CO)3(bpy)Br (1) and [Ru(bpy)3]2+ (4) complexes. Electronic absorption data for these complexes confirm the electronic similarity of daf, Me2daf, and bpy, as spectra are dominated in each case by metal-to-ligand charge transfer bands in the visible region. However, the electrochemical properties of 2, 3, and 5 reveal that the redox-active Me2daf framework in 3 and 5 undergoes reduction at a slightly more negative potential than that of bpy in 1 and 4. Taken together, the results indicate that Me2daf could be useful for preparation of a variety of new redox-active compounds, as it retains the useful redox-active nature of bpy but lacks the acidic, benzylic C-H bonds that can induce secondary reactivity in complexes bearing daf.

Keywords: 4,5-diazafluorene; 9,9’-dimethyl-4,5-diazafluorene; electrochemistry; manganese tricarbonyl; ruthenium.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Chart 1
Chart 1
Manganese tricarbonyl and ruthenium complexes supported by bpy, daf, and Me2daf discussed in this study.
Scheme 1
Scheme 1
The synthetic pathway for the generation of daf and Me2daf. (a) 1. KOH, KMNO4; H2O, 16 h, 100 °C (b) NH2NH2·H2O; diethylene glycol, 170 °C, (c) 1. tBuOK 2. MeI; THF, −10 °C to rt.
Figure 1
Figure 1
Partial 1H NMR spectra of 2 (bottom), 3 (middle), and 5 (top) in CD3CN. Peak integrations are given beneath each resonance or multiplet in colored text.
Figure 2
Figure 2
Electronic absorption spectra for 2 (left panel) and 3 (right panel) in MeCN.
Figure 3
Figure 3
FTIR spectra of 1–3 in THF solution.
Figure 4
Figure 4
Solid-state structures of 2 (left), 3 (middle), and 5 (right, from structure q36k). Displacement ellipsoids are shown at 50% probability level. Hydrogen atoms (except H14A and H14B for 2) and outer sphere hexafluorophosphate counteranions and disordered co-crystallized solvent (for 5, from structure q36k) are omitted for clarity.
Figure 5
Figure 5
Cyclic voltammetry of 4 (orange) and 5 (purple) in MeCN solution with 0.1 M TBAPF6 supporting electrolyte (working electrode: highly oriented pyrolytic graphite; pseudo-reference electrode: Ag+/0; counter electrode: Pt wire). Ferrocene was used as an internal potential reference.
Figure 6
Figure 6
Cyclic voltammogram of complexes 1 (black), 2 (red), and 3 (blue) in MeCN solution with 0.1 M TBAPF6 electrolyte (WE: HOPG, Psuedo Ref: Ag+/0, CE: Pt, internal Ref: Fc+/0).
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