The coordination chemistry of dioxygen (O 2 ) is of interest in the context of bioinorganic chemistry and oxidation catalysis. Catalytic transformations utilizing O 2 as the oxidant are considered environmentally benign; however, they typically require ™activation∫ of the dioxygen by a metal center. Whereas the chemistry of chromium-a common ingredient in oxidation reagents-is replete with high-valent oxo (O 2À ) or peroxo (O 2 2À ) compounds, there exist few superoxo complexes of chromium resulting from the binding of gaseous O 2 , and none that have been structurally characterized. [1] In a recent report on the insertion of O 2 into the chromium ± carbon bond of [Tp tBu,Me -Cr-Ph] (Tp tBu,Me hydrotris(3-tert-butyl-5-methylpyrazolyl)borate) we provided spectroscopic evidence for a reactive Cr III superoxide intermediate (A, Scheme 1), and we proposed a ™side-on∫ bonding mode for the superoxo ligand. [2] Herein we describe the synthesis and structural characterization of a stable Cr III superoxide complex that supports our earlier assignment.Key to our investigation was the synthesis of a coordinatively unsaturated Cr II precursor that would not suffer insertion of a coordinated O 2 , for example, into a chromium ± carbon bond. Thus we prepared [Tp tBu,Me Cr(pz×H)]-BARF (1, pz×H 3-tert-butyl-5-methylpyrazole, BARF tetrakis(3,5-bis(trifluoromethyl)phenyl)borate) by reaction of [Tp tBu,Me Cr(pz×H)Cl] with NaBARF (see Scheme 1). Complex 1 featured the characteristic cis-divacant octahedral structure of four-coordinate [Tp tBu,Me CrX] derivatives; [3] hence it should be able to coordinate O 2 . Indeed, exposure of a diethyl ether solution of 1 at À 78 8C to excess O 2 caused a rapid color change from blue to red. Warming to room temperature followed by standard work-up of the reaction mixture yielded [Tp tBu,Me Cr(pz×H)(O 2 )]BARF (2) as a dark red solid in high yield. The solid-state IR spectrum of 2 showed an OÀO stretching vibration at 1072 cm À1
The Cr(III) hydroxide [TptBu,MeCr(OH)(pz'H)] BARF (1) is produced by reaction of [TptBu,MeCr(pz'H)]BARF with [TptBu,MeCr(O2)(pz'H)]BARF or oxygen atom donors ONMe3 or PhIO in Et2O. However, reaction of [TptBu,MeCr(pz'H)]BARF with PhIO in pure CH2Cl2 yields the Cr(IV) oxo complex [TptBu,MeCr(O)(pz'H)]BARF (2). 2 abstracts hydrogen atoms from organic molecules with weak C-H bonds to form 1. Both 1 and 2 have been structurally characterized.
The coordination chemistry of dioxygen (O 2 ) is of interest in the context of bioinorganic chemistry and oxidation catalysis. Catalytic transformations utilizing O 2 as the oxidant are considered environmentally benign; however, they typically require ™activation∫ of the dioxygen by a metal center. Whereas the chemistry of chromium-a common ingredient in oxidation reagents-is replete with high-valent oxo (O 2À ) or peroxo (O 2 2À ) compounds, there exist few superoxo complexes of chromium resulting from the binding of gaseous O 2 , and none that have been structurally characterized. [1] In a recent report on the insertion of O 2 into the chromium ± carbon bond of [Tp tBu,Me -Cr-Ph] (Tp tBu,Me hydrotris(3-tert-butyl-5-methylpyrazolyl)borate) we provided spectroscopic evidence for a reactive Cr III superoxide intermediate (A, Scheme 1), and we proposed a ™side-on∫ bonding mode for the superoxo ligand. [2] Herein we describe the synthesis and structural characterization of a stable Cr III superoxide complex that supports our earlier assignment.Key to our investigation was the synthesis of a coordinatively unsaturated Cr II precursor that would not suffer insertion of a coordinated O 2 , for example, into a chromium ± carbon bond. Thus we prepared [Tp tBu,Me Cr(pz×H)]-BARF (1, pz×H 3-tert-butyl-5-methylpyrazole, BARF tetrakis(3,5-bis(trifluoromethyl)phenyl)borate) by reaction of [Tp tBu,Me Cr(pz×H)Cl] with NaBARF (see Scheme 1). Complex 1 featured the characteristic cis-divacant octahedral structure of four-coordinate [Tp tBu,Me CrX] derivatives; [3] hence it should be able to coordinate O 2 . Indeed, exposure of a diethyl ether solution of 1 at À 78 8C to excess O 2 caused a rapid color change from blue to red. Warming to room temperature followed by standard work-up of the reaction mixture yielded [Tp tBu,Me Cr(pz×H)(O 2 )]BARF (2) as a dark red solid in high yield. The solid-state IR spectrum of 2 showed an OÀO stretching vibration at 1072 cm À1
C15H15N1O5, triclinic, P 1 ‾ $P\overline{1}$ (no. 2), a = 5.075(4) Å, b = 11.147(8) Å, c = 13.319(9) Å, α = 106.064(10)°, β = 96.091(11)°, γ = 98.189(10)°, V = 708.2(9) Å3, Z = 2, R gt (F) = 0.0444, wR ref (F 2) = 0.1581, T = 296 K.
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