The Role of Low Valent Transition Metal Complexes in Homogeneous Catalysis: An EPR Investigation

Low-coordinate first-row transition metal complexes in catalysis and small molecule activation

“…The combination of a six-membered ring and mesityl flanking groups was found to give the best catalytic performance, with biaryl yields of 51-83% obtained at room temperature. 12,39 A two-coordinate Kumada cross-coupling catalyst is seen in the Ni(II) bis(silylamide) complex 6a reported by Lipschutz and Tilley (Scheme 6). [48][49][50] This catalyst was more effective for electron-poor aryl halides and, notably, promoted couplings to pyridine-based heterocycles.…”

“…For example, several low-coordinate Ni(I) complexes have been investigated as catalysts for Kumada cross-coupling reactions. 12,[39][40][41][42] The Ni(0) species 2a and 2b were reacted with chlorobenzene or bromobenzene to generate the Ni(I) NHC complexes 3a-c (with elimination of biphenyl), 40,42 all of which catalyse the Kumada reaction (Scheme 3) between aryl magnesium bromides and aryl chlorides or bromides. Compounds 3b-c also catalysed the coupling of aryl boronic acids to aryl bromides (Suzuki coupling).…”

“…44 Scheme 5 Synthesis of Ni(I) NHC complexes with varying NHC ring size for Kumada cross-coupling (Anis = 2-MeOC 6 H 4 ; o-Tol = 2-MeC 6 H 4 ). 12,39 Scheme 6 Proposed reaction mechanism for Kumada cross-coupling catalysed by low-coordinate Ni(II) species 6a. 48 The reaction between proligand 10, nBuLi, and trans-[Ni(PPh 3 ) 2 (Ph)Cl] resulted in the formation of Ni(I) complex 11, with concomitant elimination of biphenyl (Scheme 8a).…”

“…109 This complex can be further reduced to the Ni(0) species 38 by reaction with KC 8 . Both complexes were able to activate carbon dioxide, undergoing reductive coupling and cleavage to generate Ni(I)CO (39), Ni(II)CO 3 (40), and Ni(II)C 2 O 4 Ni(II) (41) species (Scheme 18). 110,111 Complex 40 forms a macrocyclic structure in the solid state, consisting of six nickel and six potassium cations, which was characterised by X-ray diffraction (Fig.…”

Towards Heteroleptic Dicoordinate CuII Complexes

“…Low-coordinate (often threecoordinate) complexes have been longstanding goals for synthetic work as they usually possess high reactivity as well as interesting properties. [2][3][4][5][6][7] Notably, three-coordinate Cu II carba-zolides were proposed by Fu and Jones as reactive intermediates in photoinduced Ullmann CÀ N coupling reactions which were observed by EPR spectroscopy after irradiation of Cu I carbazolides in the presence of iodobenzene. [5] However, all attempts to isolate this Cu II carbazolide failed as it rapidly decomposed in solution.…”