Aryl–X Bond-Forming Reductive Elimination from High-Valent Mn–Aryl Complexes

Abstract

C−X bond reductive elimination and oxidative addition are key steps in many catalytic cycles for C−H functionalization catalyzed by precious metals; however, engaging first row transition metals in these overall 2e − processes remains a challenge. Although high-valent Mn aryl species have been implicated in Mn-catalyzed C−H functionalization, the nature and reactivity of such species remain unelucidated. In this work, we report rare examples of stable, cyclometalated monoaryl Mn III complexes obtained through clean oxidative addition of Ar−Br to Mn I (CO) 5 Br. These isolated Mn III −Ar complexes undergo unprecedented 2e − reductive elimination of the Ar−X (X = Br, I, and CN) bond and Mn II induced by 1e − oxidation, presumably via transient reactive Mn IV species. Mechanistic studies suggest a nonradical pathway.

Synthesis and Reactivity of Manganese Complexes Bearing Anionic PNP- and PCP-Type Pincer Ligands toward Nitrogen Fixation

“…Because this value is close to the theoretical value for an S = 2 high-spin state (4.9 μ B ), 3 is assignable as an S = 2 state. Reported Mn(III)-aryl complexes exhibited effective magnetic moments between 4.8–5.3 μ B [ 57 , 58 , 59 ].…”

Manganese‐Catalyzed Cross‐Coupling Processes

“…It should however be noted that such a direct oxidative addition has never been observed for a Mn(II) complex. Very recently, the first example of an oxidative addition involving a Mn(I) complex was reported [23]. The role of Cu in the bimetallic system would be to accelerate the formation of a reactive organometallic species J from the stannane via transmetallation (complex I), which would then transmetallate with MnX2 to afford complex K. Scheme 22: Proposed mechanism for Mn/Cu-catalyzed Stille coupling [22].…”

Well-Defined Aryl-FeII Complexes in Cross-Coupling and C–H Activation Processes

“…An alternative strategy to get access to well-defined aryl-Fe II species consists of the use of macrocyclic aryl-X and aryl-H model substrates capable of stabilizing otherwise very reactive species. These size-tunable macrocyclic model substrates have been used by our group and others to stabilize square-planar aryl-Cu III , aryl-Ag III , and aryl-Ni II , as well as octahedral aryl-Co III and aryl-Mn III species . Following this strategy, herein we report the reactivity of well-defined octahedral aryl-Fe II species and their C–C cross-coupling reactivity with ArMgX reagents (Scheme c).…”

“…These size-tunable macrocyclic model substrates have been used by our group and others to stabilize square-planar aryl-Cu III , 44 aryl-Ag III , 45 and aryl-Ni II , 46 as well as octahedral aryl-Co III 47 and aryl-Mn III species. 48 Following this strategy, herein we report the reactivity of welldefined octahedral aryl-Fe II species and their C−C crosscoupling reactivity with ArMgX reagents (Scheme 1c). The model arene substrate Me L H was exposed to FeCl 2 in CH 3 CN to obtain the coordination complex [Fe II (Cl) 2 ( Me L H )] (1•Cl 2 ) in 86% yield, which was isolated as a yellowish crystalline solid (Figure 1a).…”