The Influence of Nucleophilic and Redox Pincer Character as well as Alkali Metals on the Capture of Oxygen Substrates: The Case of Chromium(II)

Abstract

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Examining the Generality of Metal–Ligand Cooperativity Across a Series of First-Row Transition Metals: Capture, Bond Activation, and Stabilization

“…Upon reduction, the V–Cl distances elongate by 0.15 Å (ave.) while the V–N distances (tridentate chelate) remain unchanged. Previous studies by our lab and others have shown ligands of this type, pyridine­(dipyrazole), are capable of accepting electrons. − However, comparisons of intraligand C–C and C–N bond distances between 1-V and 2-V reveal no differences, consistent with a vanadium-based reduction event to afford a V­(II) s = 3/2 complex. The isolation of octahedral 2-V in conjunction with the pentacoordinate 2-M series suggests that N 2 H 4 capture by the pair of Lewis acidic boranes in the BBN PDP t Bu ligand is general, regardless of the metal or the coordination environment at the metal .…”

Recent Developments in Reactions and Catalysis of Protic Pyrazole Complexes

“…Chemical reduction of 35 with four equiv of KC 8 suggests that the pincer-type bis(1Hpyrazol-3-yl)pyridine ligand is redox non-innocent. The reaction eventually yields the oxido-bridged dichromium complex [K 4 (thf) 10 ][Cr 2 tBu L 2 (µ 2 -O)] ( 40), possibly after reactions with adventitious water and dihydrogen evolution as postulated in the previous reactions (Scheme 12) [58]. The X-ray analysis of 40 revealed that the C-C bond distances around the 4-position of the pyridine ring (1.420(10)-1.439(10) Å) are much longer than the distances of the C2-C3 and C5-C6 bonds (1.352(9)-1.367(8) Å), indicating reduction in the pincer ligand tBu L with an unpaired electron at the 4-position of the pyridine ring.…”

“…In addition, reduction of 35 with two equiv of KC 8 as well as oxidation of 35 with ferrocenium cation is examined [58]. Recrystallization of the reaction products again results in uptake of adventitious water to produce oxido-bridged tri-and tetranuclear complexes, respectively, with ambiguous reaction stoichiometry.…”

“…The reduced species is trapped with carbon dioxide to afford a carbonato-bridged, dianionic-dinuclear complex [59]. On the other hand, the oxidation of 35 with a quinone gives rise to the formation of a bis(semiquinone) Cr(III) 2 complex [58].…”