Kinetic Destabilization of Metal–Metal Single Bonds: Isolation of a Pentacoordinate Manganese(0) Monoradical

Abstract

For decades,t ransiently generated 17e À monoradicals have occupied ac entral role in the development of atom-transfer and radical-mediated processes in organometallic chemistry. [1][2][3] Ap rototypical example of this class is the zerovalent manganese pentacarbonyl radical [Mn(CO) 5 ]which has been extensively investigated for its role in radical-type olefin hydrogenation, [4,5] [7][8][9][10] or at higher temperatures by using either ultrafast spectroscopic [11] or nitrosoarene spin-trapping techniques. [12][13][14] Importantly,efforts to prepare stabilized variants of [Mn(CO) 5 ]bysubstitution of one or more CO ligands with more encumbering ligands such as phosphines (PR 3 )h ave been shown to extend the lifetime of [MnL 5 ]-type radicals to the order of hours in solution. [15][16][17][18] However,such derivatized [Mn(CO) 5Àn L n ]c omplexes have remained susceptible to facile dimerization/redistributionp rocesses when concentrated and isolable examples remain unknown. Additionally, investigations into the one-electron activation chemistry of [Mn(CO) 5Àn L n ]s pecies with small molecule substrates are limited. [16,17] Given the challenges faced in exploring the properties of [Mn (CO) ) 2 ]( Ar Dipp2 = 2,6-(2,6-(iPr) 2 C 6 H 3 ) 2 C 6 H 3 )f eaturing two m-terphenyl isocyanide ligands. [22,23] Thesterically encumbering nature of these isocyanidel igands effectively prevents the formation of MnÀMn single bonds,thus rendering this 17eÀ radical both isolable in the solid state and persistent in solution. Additionally,asaresult of the isolobal analogy between carbon monoxide (CO) and organoisocyanides (C NR), [22,24] ) 2 ]delivers 1 most cleanly and in the highest yield after isolation (92 %; Figure 1A). Upon isolation, 1 is stable for days in solution at elevated temperatures (C 6 D 6 ,808 8C) and shows no sign of dimerization and/or ligand redistribution when concentrated to asolid.In both solution and the solid state,c omplex 1 possesses several spectroscopic and structural features consistent with ad 7 monoradical formulation. The 1 HNMR spectrum of 1 in C 6 D 6 features broad, shifted resonance signals characteristic of ap aramagnetic species,w hereas determination of the

Associative Ligand Exchange and Substrate Activation Reactions by a Zero-Valent Cobalt Tetraisocyanide Complex

“…In contrast, the isolable S = 1 / 2 metalloradical Mn(CO) 3 (CNAr Dipp2 ) 2 reacts upon mixing with both HSnBu 3 and HCCl 3 to cleanly produce products indicative of hydrogen-or chlorine-atom abstraction, respectively, as the major species. 58 To date, we have not identified a substrate class in which Co(CNAr Mes2 ) 4 functions in its reaction chemistry as a simple metalloradical capable of X• abstraction. In fact, even when treated with stable radical TEMPO, Co(CNAr Mes2 ) 4 undergoes substantial electronic reorganization and the loss of two ligands to form diamagnetic, square-planar complex (η 2 -O,N-TEMPO)Co(CNAr Mes2 ) 2 (8; Scheme 3).…”

“…59 As an additional probe of 1e − metalloradical behavior from Co(CNAr Mes2 ) 4 , its reactivity toward elemental phosphorus was surveyed because this reagent can react via 1e − pathways with sterically constrained metal centers. 58,60 As shown in Scheme 3, Co(CNAr Mes2 ) 4 readily reacts with P 4 in benzene solution. However, rather than producing a product indicative of a 1e − reduction by Co(CNAr Mes2 ) 4 , this reaction generates the diamagnetic cyclo-P 3 complex (η 3 -P 3 )Co(CNAr Mes2 ) 3 (9) as the only cobalt-containing product.…”

The Stabilization of Three-Coordinate Formal Mn(0) Complex with NHC and Alkene Ligation

“…Synthesis and Characterization of the Three-Coordinate Formal Mn(0) Complexes Low-valent manganese complexes are mostly known for five-and six-coordinate Mn(ÀI), Mn(0), and Mn(I) complexes with CO, isocyanide, cyclopentadienyl, and phosphine ligands, [50][51][52][53][55][56][57][58][59] and low-valent manganese complexes with the lower coordination numbers of 2-4 are rare. 16,17,46,47,[60][61][62][63] Noting this and also the success of a combined ligand set of NHC with the bis(alkene) ligand dvtms in stabilizing three-coordinate Pt(0), 64 Pd(0), 65 Ni(0), 31,32 Co(0), [35][36][37][38] and Fe(0) 39,41 complexes reported by Markó , Beller, Hazari, Johnson and co-workers, and us, we were curious whether this ligand set could allow stabilization of three-coordinate formal Mn(0) species or not.…”

“…[48][49][50][51] A recent study by Figueroa and co-workers showed that the use of sterically encumbering isocyanide ligands allows preparation and isolation of 17e À species [Mn(CO) 3 (CNAr*) 2 ] (Ar* = 2,6-(2 0 ,6 0 -diisopropylphenyl)phenyl, Figure 1). 52,53 The metalloradical species displays intriguing reactivity of one-electron redox reactions to convert to Mn(I) and Mn(ÀI) complexes. Relevant to this, Chirik and co-workers reported a five-coordinate complex, [( iPr PDI)Mn(THF) 2 ] ( iPr PDI = 2,6-(2,6-i Pr 2 -C 6 H 3 N=CMe) 2 C 5 H 3 N).…”

Low-Coordinate Manganese Complexes Perform Unusual Bond-Formation Reactions

“…Radical recombination can be thwarted by the use of sterically encumbering ligands that preserve the precise electronic nature of Mn(CO) 5 , thereby allowing for the isolation and structural verification of thermally stable analogs of transient Mn(CO) 5 . 8 These five-coordinate Mn complexes have proven to be potent one-electron, metal-based reductants and abstraction and transfer agents for hydrogen atoms, consistent with their 17-electron, metalloradical nature. However, there remain no known examples of authenticated zero-valent Mn complexes surrounded by fewer than five ligands.…”