Neutral zinc(II) and molybdenum(0) complexes with chiral guanidine ligands: synthesis, characterisation and applications

The synthesis and properties of three bidentate zinc(II) complexes with several chiral bidentate and one molybdenum(0) with analogous tridendate neutral chiral guanidine ligands are reported. N′,N″-Diisopropyl-(S)-2-(N,N-dialkylaminomethyl) pyrrolidine-1-carboximid-amides 1–3 reacted with anhydrous zinc(II) chloride to form mono-ligand complexes 4–6, which exist as chain structures in solid state with an NH⋯Cl interaction between the monomer units. The X-ray crystal structures of 4–6 show the zinc(II) ion in a tetrahedral environment in which the C2 side chain nitrogen atom and the imine group coordinate with the metal centre. Chiral guanidine 2 was treated with (nbd)Mo(CO)4 to yield the corresponding molybdenum complex 7. The molecular structure of 7 has been determined by X-ray crystallography. In contrast to 5, guanidine 2 acts as a tridentate ligand in 7 and the Mo(0) centre is in an octahedral coordination environment. NMR spectroscopy was performed to determine solution structures which were further supported by X-ray analysis. The chiral zinc guanidine complexes 4–6 were prepared in an effort to design chiral guanidine catalysts for asymmetric synthesis.

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Dichloro{N,N′-diisopropyl-(S)-2-(pyrrolidin-1-yl-methyl)-pyrrolidine-1-carboximid-amide}zinc(II)C16H32N4Cl2Zn[α]D20 = −121.1 (c 2.8, CH3CN)Source of chirality: 1-(pyrrolidin-(S)-2-ylmethyl)pyrrolidineAbsolute configuration: 2S

Dichloro{N,N′-diisopropyl-(S)-2-(morpholin-4-yl-methyl)-pyrrolidine-1-carboximid-amide}zinc(II)C16H32N4OCl2Zn[α]D20 = −83.5 (c 2.3, CH3CN)Source of chirality: 4-(pyrrolidin-(S)-2-ylmethyl)morpholineAbsolute configuration: 2S

Dichloro{N,N′-diisopropyl-(S)-2-(piperidin-1-yl-methyl)-pyrrolidine-1-carboximid-amide}zinc(II)C17H34N4Cl2Zn[α]D20 = −90.6 (c 1.7, CH3CN)Source of chirality: 1-(pyrrolidin-(S)-2-ylmethyl)piperidineAbsolute configuration: 2S

[N,N′-Diisopropyl-(S)-2-(morpholin-4-yl-methyl)-pyrrolidine-1-carboximid-amide]Mo(CO)3C19H32N4O4Mo[α]D20 = −50.5 (c 1.6, CH3CN)Source of chirality: 4-(pyrrolidin-(S)-2-ylmethyl)morpholineAbsolute configuration: 2S