Dehalogenation and Dehydrogenation Mediated by NHC-Ni Complexes

For related hydrogenation reactions, the reader is referred to Chapter 13. 

Two years later, Schaub and Radius showed that dimeric complex [(IiPr2)4Ni2(COD)] also exhibited an interesting reactivity towards C-F bonds. This catalyst activated the carbon-fluorine bond of hexa-fluorobenzene producing a new complex result of oxidative addition (Equation (10.3)). However, this reaction only worked with strongly activated substrates i.e. hexafluorobenzene, octafluoronaphtalene). 

By a combined experimental and theoretical study on C F bond activation by in situ generated [(IiPr)2Ni], Radius and co-workers also showed that the reaction was very fast, regio- and chemoselective. For example, the activation of octafluorotoluene was only observed at the para position to afford a trans-[(IiPr2)2Ni(F)(4-(CF3)C6F4)] complex. Mechanistic studies on this activation led to the detection in solution of intermediates with the aromatic system coordinated in a t -fashion. For the C-F activation step, the authors suggested a pre-coordination of the aromatic system prior to a concerted oxidative 

In the last few years, ammonia-borane (AB) has attracted increasing attention as a very promising chemical hydrogen storage material. AB is solid at room temperature and presents a potential capacity of 19.6 wt % 

In 2007, Baker et al. reported the first example of a homogeneous NHC-Ni catalyst active in dehydrogenation of AB. Enders NHC, TPT, exhibited higher activity than IPr or IMes and the dehydrogenation then produced 18wt % H2. The TPT/Ni system consumed AB twice faster than Ru and four times faster than Rh systems. The authors postulated a mechanistic pathway involving initial formation of a a-complex, followed by B-H bond activation and p-H elimination from the N-H (Seheme 10.1). 

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