Electronic Structure and Reactivity of One-Electron-Oxidized Copper(II) Bis(phenolate)–Dipyrrin Complexes

Abstract

The sterically hindered bis(phenol)-dipyrrin ligands LH and LH were reacted with 1 equiv of copper(II) under ambient conditions to produce the copper radical complexes [Cu(L)] and [Cu(L)]. Their X-ray crystal structures show relatively short C-O bond distances (mean bond distances of 1.287 and 1.291 Å), reminiscent of mixed pyrrolyl-phenoxyl radical species. Complexes [Cu(L)] and [Cu(L)] exhibit rich electronic spectra, with an intense near-IR (NIR) band (ε > 6 mM cm) at 1346 and 1321 nm, respectively, assigned to a ligand-to-ligand charger-transfer transition. Both show a reversible oxidation wave ( E = 0.05 and 0.04 V), as well as a reversible reduction wave ( E = -0.40 and -0.56 V versus ferrocenium/ferrocene, respectively). The cations ([Cu(L)] and [Cu(L)]) and anions ([Cu(L)] and [Cu(L)]) were generated. They all display an axial ( S = /) signal with a copper hyperfine structure in their electron paramagnetic resonance spectra, consistent with ligand-centered redox processes in both reduction and oxidation. Complex [Cu(L)](SbF) was cocrystallized with [Cu(L)]. Oxidation is accompanied by a slight contraction of both the C-O bonds (mean bond distance of 1.280 Å) and the C-C bonds connecting the peripheral rings to the dipyrrin. The cations show vis-NIR bands of up to 1090 nm due to their quinoidal nature. The anions do not show a significant band above 700 nm, in agreement with their bis(phenolate)-dipyrrin character. The radical complexes efficiently catalyze the aerobic oxidation of benzyl alcohol, 1-phenylethanol, and unactivated 2-phenylethanol in basic conditions.

Ligand-Mediated Spin-State Changes in a Cobalt-Dipyrrin-Bisphenol Complex

“…The DPP ligand scaffold was first described as being redox-active in 2012 after the synthesis of cobalt- and nickel-DPP complexes. 12 Hereafter, the redox activity was further studied in Mn, 13 Pt, 14 Cu, 15 and Au 16 complexes. Catalytic applications have been reported for the Ti, Zr, Gn, and Sn complexes (copolymerization of epoxides with CO 2 ) and Cu (aerobic alcohol oxidation).…”

Gold dipyrrin-bisphenolates: a combined experimental and DFT study of metal–ligand interactions

“…The X-ray structure reveals Au-N/O distances of around 1.97Å, which are approximately 0.02-0.03Å longer than Au-N distances typically observed for Au(III) corroles. [12][13][14][15] An examination of the skeletal bond distances of Au[CF 3 DPP] led to the interesting observation that the C-C bonds in the phenolate moieties span a narrower range ($0.04Å) relative to M(DPP) derivatives ($0.08Å) that are unambiguously describable as metalloradicals, as for M ¼ Cu (CCSD: FICCEC, FICCUS) 16 and Pt (LACCUQ, LACDAX). 17 Similarly narrow C-C bond distance ranges are also observed for nonradicaloid Ge (VIVNAR, PON-GOQ, SIRFOQ), 18 Mn (UTOVEF 19 and EXOBAV 20 ), Al (NAB-FII), [21][22][23] Ga (WOMPAS), 24 and In (WOMPIA) 24 DPP complexes.…”

Synthesis and Functions of Oligomeric and Multidentate Dipyrrin Derivatives and their Complexes

“…Various original N 2 O 2 dipyrrin compounds complexed with transition metals, such as Ti, 128 Zr, 128 Mn, [131][132][133] Co, [134][135][136] Ni, 134,137 Cu, 138,139 Au, 140 Ge, 128,129 and Sn 128 , have been synthesized and structurally characterized, although the emission properties of these complexes have not been reported. They, however, exhibited various interesting properties and functions, such as catalytic activity, redox activity, chiroptical properties, and pharmaceutically relevant properties.…”

“…Some N 2 O 2 dipyrrin complexes are known to show a ligand-centered one-electron oxidation to form the radical species, because the N 2 O 2 dipyrrin ligands are electron-rich -systems. The radical complexes that contain Ni, 134,137 Co, 134,136 or Cu 139 have been isolated as a stable species, and exhibited characteristic photophysical properties and catalytic functions. The -skeleton of most of these radical complexes are protected by bulky t-Bu groups.…”