Ammine and amido complexes of rhodium: Synthesis, application and contributions to analytics
Graphical abstract
Introduction
The homogenously catalyzed addition of ammonia to carbon-carbon multiple bonds [1], catalytic C-H amination with ammonia as nitrogen source [2] as well as the utilization of ammonia as a reversible hydrogen storage medium [3] are challenging goals. This stems from the high dissociation energy: 450 kJ/mol [4] of the N-H bond in ammonia, the splitting of which is required by both processes. A critical review has summarized all contributions concerning the activation of ammonia in homogenous systems before 2010 [5], while more recent achievements using transition metals are collected in Refs. [6]. According to Schneider [6n] NH3 splitting at a metal center can occur through: 1) oxidative addition, 2) deprotonation, 3) 1,2-addition, 4) bimetallic addition and 5) hydrogen atom transfer. The formation of ammine complexes, M − NH3, so-called Werner adducts [7], is usually the first step in these reactions, as isolated intermediates and DFT calculations revealed. In the only example reported so far, of catalytic hydroamination of unsaturated hydrocarbons with ammonia, a M − NH3 complex was detected as the resting state [8].
Whereas iridium is mentioned often and early as a competent metal for ammonia activation [9], the ability of related rhodium complexes was described for the first time in 2011 by Casado and Oro [6d] They found that methoxy-bridged rhodium dimers 6-L (see Scheme 1) can form amido-bridged complexes by reaction with NH3 via deprotonation. Furthermore, they showed the possibility of forming C-NH2 bonds with the coordinated alkene by addition of phosphines.
While plenty of rhodium ammine complexes Rh-NH3 have been described which bear coordinated alkenes or monodentate phosphines [10], examples with chelating diphosphines (PP) are surprisingly rare [11]. Apparently, their formation by addition of NH3 was “not readily accessible via solution techniques“ [11c]. At variance with this statement, herein we present a straight forward entry into RhI-ammine complexes via solution technique, the properties of such complexes as well as their possible applications in catalysis are described. We also introduce for the first time RhI amido complexes of chelating diphosphines via NH-bond splitting of ammonia. To the best of our knowledge, no example has been reported so far.
Cationic rhodium complexes of the type [Rh(PP)(diolefin)]+ 1-PP, are often used as stable catalyst precursors for example in hydrogenation reactions [12]. To avoid induction periods, 1-PP has to be transformed into the active species [Rh(PP)(solvent)2]+ 2-PP via prehydrogenation of the coordinated alkene. The prehydrogenation time depends on the alkene, the solvent, the temperature, H2 partial pressure and the coordinated diphosphine [13]. Recently Heller and Hapke demonstrated how ammine complexes [Rh(PP)(NH3)2]+ 3-PP can serve as easily to activate precatalysts in enantioselective cyclotrimerization reactions [11d]. Herein we extend this concept to the hydrogenation of prochiral olefins .
Section snippets
Ammine complexes: synthesis and characterization
Simple stirring of the cationic solvate complex 2-binap (an overview of all mentioned diphospine ligands is given in the supporting information) in methanol (MeOH) or tetrahydrofuran (THF) under an atmosphere of ammonia at room temperature and normal pressure immediately leads to a change of the solution color, from orange to dark red. 31P NMR measurements revealed the quantitative formation of a new species. After isolation of the complex, 1H-15N-HMQC-NMR experiments detected a correlation
Conclusions
A broad range of ammine complexes of the type [Rh(PP)(NH3)2]+ has been synthesized and characterized. The complexes could be generated by the substitution of solvent molecules, diolefins or even diphospines by ammonia. We found that the ammine complex [Rh(binap)(NH3)2]+ could be converted into a catalytic active species by protonation with the non-coordinating acid HBF4, affording the same results in terms of catalytic activity and enantioselectivity as the known solvate complex [Rh(binap)(MeOH)
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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