Cooperative 1,3-P,N-ligands and non-innocent NHCs

The reactivity and selectivity of transition metal catalysts can be synergistically enhanced by interaction with either a second metal atom or a cooperative ligand. Exemplary are redox non-innocent ligands and hybrid ligands, when its second donor atom is subject to hemilabile coordination, thereby providing, for instance, desirable Lewis or Brønsted basicity. Hybrid 1,3-P,N-ligands are valuable because of their diverse binding modes by which they can provide cooperative interactions in mononuclear complexes and facilitate metal-metal interactions to access multinuclear species. Chapter 1 reviews their mononuclear transition metal complexes, in which their versatile bonding nature is linked to a number of applications, including coordination chemistry, homo- and heterogeneous catalysis, and bio-inorganic purposes. Chapter 2 reviews the interesting coordination chemistry of multinuclear homo and hetero metallic complexes, and their use in metal-activation and (cooperative) catalysis. Historically, 2-pyridyl- and 2-imidazolylphosphane ligands are the most widely applied 1,3-P,N-ligands, but novel synthetic methods based on nitrilium ion synthons provide access to iminophosphanes that can be independently substituted on the phosphorus, carbon, and nitrogen positions to allow electronic and steric variation. Chapter 3 explores their applicability in coordination chemistry and catalysis. Six novel iminophosphanes were synthesized from nitrilium salts and secondary phosphanes. The ligands were obtained as a (dynamic) mixture of E/Z isomers, but coordinate readily in the desired Z-conformation to [RhCp*Cl2]2 to provide κ1/κ2-complexes, in which the hapticity equilibrium depends on the properties of the ligand. Silver triflate enforces κ2 chelation. The potential for catalysis was explored using Ru(II)-catalyzed benzonitrile hydration which selectively provided benzamide in yields of up to 96%. The substitution pattern of the iminophosphanes affected the performance of the catalysts, which was attributed to the electronic properties of the ligands. Cyclised iminophosphanes with a ‘locked’ C,N-conformation are explored in Chapter 4 and were obtained from the nitrilium intermediate of the Beckmann rearrangement of cyclohexanone that was captured by benzotriazole. The benzotriazole group could be quantitatively replaced by a secondary phosphane under triflic acid promotion. Whereas the obtained ligands are ‘non-isomerizable’, their backbone remains flexible. The ligands have a far higher N-basicity than previously reported 1,3-P,N-ligands, which was also demonstrated using their W(carbonyl) and hemilabile Rh(III) complexes. Coordination to [RhCp*Cl2]2 gave a dynamic mixture of competing P/N-κ1- and P,N-κ2-bonding modes; treatment with silver triflate allowed full conversion to the κ2-complexes. Only P-κ1-complexes were obtained with [Ru(p-cym)Cl2]2. These were effective catalysts for the hydration of benzonitrile and for the transfer hydrogenation of cyclohexanone, that could likewise be conducted with Ir(I) catalysts. An asymmetric cycloiminophosphane could be synthesized from the readily available natural precursor L-menthoneand was used to access P,N-κ2-Rh(III)- and P-κ1-Ru(II)-complexes. As no asymmetric 1,3-P,N-ligand-based catalysts have been reported, this is a valuable preamble to asymmetric catalysis. NHCs are less explored as cooperative ligands. Protic NHCs became synthetically accessible only recently and have reactive NH sites that can be exploited for cooperative interactions. Chapter 5 describes the reactivity of NHC(H)-iridium complexes, which were synthesized by reducing azido-phenylene-isocyanide ligands. The reactive NHC(H) N−H bond seemingly interacts with the metal center under reductive conditions. For instance, acetonitrile could be inserted to provide κ2-NHC-imidoyl ligand-based complexes, but the corresponding deprotonated analogue was obtained under reductive conditions and in the absence of a base. When zinc was used as reductor, the NHC(H)-iridium chloride complex was shown to rearrange to a highly unusual Cs-symmetric dinuclear iridium hydrido complex. Reduction of the complexes enhances the reactivity of the NHC(H) N−H, which suggests that the observed transformations are facilitated by metallophilic interactions (β-H activation). This non-innocent activation contrasts with more common reactions induced by an external base.