Evidence for Non‐Innocence of a β‐Diketonate Ligand

Abstract

β‐Diketonates, such as acetylacetonate, are amongst the most common bidentate ligands towards elements across the entire periodic table and are considered wholly redox‐inactive in their complexes. Herein we show that complexation of 1,1,1,5,5,5‐hexafluoroacetylacetonate (hfac−) to CrII spontaneously affords CrIII and a reduced β‐diketonate radical ligand scaffold, as evidenced by crystallographic analysis, magnetic measurements, optical spectroscopy, reactivity studies, and DFT calculations. The possibility of harnessing β‐diketonates as electron reservoirs opens up possibilities for new metal–ligand concerted reactivity in the ubiquitous β‐diketonate coordination chemistry.

Non‐Spectator Feature of α‐Diimine Mimicked Di/tetrahydro‐bisisoquinoline and Biimidazopyridine on {Ru(acac)2} Platform

“…The involvement of trans‐annular interaction of hydrogen atoms at C7 and C14 in the imidazopyridine rings of L3 (as noted before for the crystals of L3 in Scheme and [ 4 ]ClO 4 in Figure ) facilitated the formation of short‐lived κ 1 ‐coordinated C in Scheme A. Nevertheless, the transformation of L3 to L4 in 5 also elucidates the rare example of non‐spectator feature of coordinated acac in the specific direction of C‐C cross coupling reaction , …”

Ligand field-actuated redox-activity of acetylacetonate

“…This large, negative potential is close to that found for the cathodic peak potential in the irreversible reduction of Hacac, 23 the reduction potential of the related 1,3diphenylpropane-1,3-dione, 24 and the oxidation potential in [Cr III (hfac) 2 c 3À (pyrazine) 2 ]. 20 The oxidation is thus assigned as ligand-centered and corresponding to the formation of trans-[Cr III (acac) 2 (pyridine) 2 ] + (3 + , Fig. S7, ESI †).…”

“…19 We recently demonstrated that six-coordinate trans-[Cr(hfac) 2 L 2 ] (hfac ¼ 1,1,1,5,5,5-hexauoroacetylacetonate; L ¼ pyrazine, or tetrahydrofuran) is most adequately described as a Cr(III) complex with a single unpaired electron delocalized over two hfacligands. 20 In contrast, the related square-planar complex [Cr(acac) 2 ], reported in 1958, 21 is known to possess an S ¼ 2 spin ground state, which is indicative of a high-spin Cr(II) center. 21,22 Herein, we show that introduction of modest s donors in the vacant, axial positions of [Cr(acac) 2 ] leads to reduction of the acac ligand scaffold (Scheme 1), providing a novel route to chemically control the electronic delocalization and spin-density in a metal-ion complex.…”

Ligand Field-Actuated Non-Innocence of Acetylacetonate

“…The formulation of {Cr III -(acac)2 •3-} implies a concurrent occupation of the antibonding LUMO of the acac ligands, which, is expected to predominantly elongate the C-O bonds. 20 To corroborate this hypothesis, two additional [Cr(acac)2L2] complexes with the axial ligands containing an electronpoor (L = 4-(trifluoromethyl)pyridine = CF3py; 2) and an electron-rich (L = 4-(dimethylamino)pyridine = DMAP; 4) pyridine ring were synthesized and structurally characterized (Fig. 1).…”

“…We recently demonstrated that six-coordinate trans-[Cr(hfac)2L2] (hfac = 1,1,1,5,5,5-hexafluoroacetylacetonate; L = pyrazine, or tetrahydrofuran) is most adequately described as a Cr(III) complex with a single unpaired electron delocalized over two hfacligands. 20 In contrast, the related square-planar complex [Cr(acac)2], reported in 1958, 21 is known to possess an S = 2 spin ground state, which is indicative of a high-spin Cr(II) center. 21,22 Herein, we show that introduction of modest s donors in the vacant, axial positions of [Cr(acac)2] leads to loss of…”