Mixed valence η6-arene cobalt(i) and cobalt(ii) compound

Ramachandran Azhakar, Rajendra S. Ghadwal, Herbert W. Roesky, Jakob Hey, Lennard Krause, Dietmar Stalke
2013 Dalton Transactions  
The first carbonyl free mixed valence cobalt(I)/cobalt(II) compound [2{L 2 Co(I)(η 6 -C 7 H 8 )}] 2+ [Co(II) 2 Cl 6 ] 2− (1) [L = PhC(N t Bu) 2 SiCl] was obtained by the reaction of four equivalents of anhydrous CoCl 2 with five equivalents of N-heterocyclic chlorosilylene L. In contrast, the reaction of L with CoBr 2 yielded [L 2 CoBr 2 ] (2). Compound 1 was formed by the cleavage of Co-Cl bonds, the reduction of Co(II) to Co(I) and by the coordination of a toluene molecule. The chlorosilylene
more » ... (L) functions as a reducing agent as well as a neutral σ-donor ligand. The toluene molecule coordinates to the Co(I) atom in an η 6 -fashion. Transition metal-arene complexes have attracted considerable attention because of their potential useful applications in organic synthesis and catalysis. 1 Activation of an arene molecule on coordination to a transition metal significantly modifies the reactivity of the arene molecule in several distinct ways. 2 One important consequence is the increased acidity of the arene protons, thereby facilitating the functionalization of rather inert C-H bonds. 1b,3 High yield access to stable chlorosilylenes L [PhC(N t Bu) 2 SiCl] 4 and NHC·SiCl 2 [NHC = 1,3-bis(2,6diisopropylphenyl)imidazol-2-ylidene (IPr) or 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene] 5 by convenient synthetic routes permitted us to explore their chemistry. [6] [7][8][9] In particular, their application as reducing agents 6 as well as neutral σ-donor ligands 7 provides a new area of investigation in transition metal chemistry. A striking example is the isolation of the mixed valence cobalt(I/II) compound [(IPr·SiCl 2 ) 2 Co(CO) 3 ] + -[CoCl 3 (THF)] − on reaction of Co 2 (CO) 8 with IPr·SiCl 2 . 7a Carbonyl free complexes with low-valent 3d transition metals are scarce but show interesting applications in the activation of small molecules and as enzyme mimics. 2 Among the first row transition metal complexes, some carbonyl free cobalt(I) complexes have been prepared using sterically crowded monodentate or chelating di-, tri-, or poly-dentate ligands. Such complexes have been prepared by the reduction of Co(II) halides using alkali metals (e.g., KC 8 ) as reducing agents. 10 In continuation of our ongoing research work to develop safer and more convenient synthetic methods for compounds with low-valent elements and their further applications, we became interested in establishing facile methods for transition metal complexes with low-valent main group elements. Herein, we report on a carbonyl-free mixed valence cobalt compound of composition [2{L 2 Co(I)(η 6 -C 7 H 8 )}] 2+ [Co(II) 2 Cl 6 ] 2− (1) using anhydrous CoCl 2 and a chlorosilylene (L) and [L 2 CoBr 2 ] (2) which was obtained by the reaction of L and CoBr 2 . Interestingly, protocols for preparation of mixed valence cobalt(I)/ cobalt(II) compounds are extremely rare. 7a,11 To the best of our knowledge this is the first report of a carbonyl free mixed valence cobalt(I)/cobalt(II) compound.
doi:10.1039/c3dt50939g pmid:23732516 fatcat:bzrxfh242jaxhjf2uocb7hz3vi