Preparation of a new 16-MC-4 structure type that captures Mn(II) in the central cavity

Bernhard Lippert has contributed significantly to the understanding of metallamacrocyclic complexes with low valent, late transition metals. In particular, he has pioneered the preparation of metallacalixarenes such as 12-MC-3 and 16-MC-4 structure types. In this article, we provide the preparation of a new 16-MC-4, [Mn4Br4(B(tBuM)PZG)4] [Mn(HOCO2)2], complex made with the ligand potassium N,N-bis(3-methyl-5-tert-butylpyrazolylmethyl)glycinate (KB(tBuM)PZG). In addition to being the first 16-MC-4 that is composed of manganese, this complex is the first example of a transition metal [Mn(II)] being captured in the interior cavity of the 16-MC-4. We discuss how our ligand design leads to the isolated structure due to the steric bulk that has been appended to the pyrazole ligand.

Rather than forming a discrete mononuclear species, the sterically encumbered ligand potassium N,N-bis(3-methyl-5-tert-butylpyrazolylmethyl)glycinate (KB(tBuM)PZG) reacts with Mn(II) to form what is best described as a Mn(II)(HCO3)2[16-MC-4] complex. This pentanuclear Mn(II) complex is the first 16-MC-4 to contain manganese and the only 16-MC-4 to capture a transition metal ion in the cavity. Each of the two “hinge” bicarbonates serve to sequester the central Mn(II) as well as bridge two of the four ring Mn(II) ions that form the metallacrown. This binding mode could model the interaction of bicarbonate with the photosynthetic Mn4Ca water oxidizing cluster.Figure optionsDownload as PowerPoint slide