Axial Redox Tuning at a Tetragonal Cobalt Center

Abstract

Square pyramidal cobalt complexes were prepared to study their multielectron redox properties. To build a stable redox-active cobalt complex, the combination of a tridentate acriPNP (acriPNP– = 4,5-bis­(diisopropylphosphino)-2,7,9,9-tetramethyl-9H-acridin-10-ide) ligand with a bidentate ligand, such as 2,2′-bipyridine, 2-(o-phenyl)­pyridine, biphenylene, and their analogues, was employed. In a cobalt complex having a tetragonal structure, the d x 2 –y 2 orbital possesses an antibonding character and must remain empty for its structural integrity, while the d z 2 orbital acts as a redox-active frontier molecular orbital (FMO). Tuning the redox potential of the Co­(II/I) couple was successfully achieved by introducing a different axial donor. The reduction of Co­(II) to Co­(I) occurs at −2.6 V for a neutral donor but shifts to −3.4 V for an anionic donor. Since the redox-active d z 2 orbital is close in energy to other ligand-based orbitals, multielectron redox activity is also observed. Electrochemical measurements indicate three reversible redox events within a window of −3.0–0.0 V vs Fc/Fc+ in tetrahydrofuran (THF). These redox processes are fully reversible for over 100 cycles, reflecting the electrochemical stability of these cobalt complexes. Surprisingly, the oxidation potential of the acriPNP ligand varies dramatically from +0.15 to −2.4 V, which is probably due to the cobalt contribution on the amido-based molecular orbital. The electronic structure of the cobalt complexes was examined structurally, spectroscopically, and theoretically.

Polycobaltoceniumylmethylene – A Water-Soluble Polyelectrolyte Prepared by Ring-Opening Transmetalation Polymerization

“…In situ 1 H NMR spectroscopy of the reaction mixture revealed resonances at characteristically high field (δ 1 H = −45 to −61), originating from the paramagnetic 19-electron cobaltocenyl intermediate (Figure S2), which are similar to the resonances reported for paramagnetic cobaltocene (δ 1 H = −50.93) . The 1 H NMR spectrum of 1-Cl (in D 2 O) revealed two broad resonances at 5.60 and 5.66 ppm for the cyclopentadienyl (Cp) protons and a singlet at 1.54 ppm corresponding to the methyl groups attached to the bridging carbon atom (Figure S3).…”

Room-Temperature C–C σ-Bond Activation of Biphenylene Derivatives on Cu(111)

“… 1 , 7 , 9 , 11 Precious metals such as Au, 12 − 16 Pt, 17 , 18 Pd, 17 , 19 , 20 Ir, 21 − 24 and Rh 25 − 28 have been shown to cleave and insert onto the biphenylene σ-bond, generating active species for a diverse set of reactions 7 , 11 that includes dimerization, 18 formal [4+2] or [4+1] cycloadditions, 18 , 23 or coupling reactions. 19 Although this has also been observed by using earth-abundant metals such as Fe, 29 , 30 Co, 31 32 Ni, 33 − 36 and Al 37 to the best of our knowledge, 11 biphenylene C–C bond activation has not yet been observed for Cu at room temperature. Further expanding such a process to more metals would provide alternatives to rare transition metal catalysts.…”

“…The unique structure of biphenylene, with two benzene rings fused to a strained antiaromatic cyclobutadiene (Scheme ), makes it an excellent candidate for C–C activation and further incorporation of the resulting 2,2′-biphenyl moiety into more complex polycyclic aromatic systems. ,,, Precious metals such as Au, − Pt, , Pd, ,, Ir, − and Rh − have been shown to cleave and insert onto the biphenylene σ-bond, generating active species for a diverse set of reactions , that includes dimerization, formal [4+2] or [4+1] cycloadditions, , or coupling reactions . Although this has also been observed by using earth-abundant metals such as Fe, , Co, Ni, − and Al to the best of our knowledge, biphenylene C–C bond activation has not yet been observed for Cu at room temperature. Further expanding such a process to more metals would provide alternatives to rare transition metal catalysts .…”