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Although research in the field of main‐group rings, chains, and polymers is a less mature area of chemistry than that of carbon‐based rings, chains, and polymers, it is currently very fertile, as evidenced by the significant increase in available monocyclic main‐group rings in the past two decades, including neutral, anionic, cationic, and radical homocycles and heterocycles. Furthermore, recent advances in main‐group polymer chemistry have been substantial with access to high‐molecular‐weight polyaminoboranes and polyphosphinoboranes enabled by efficient dehydropolymerization catalysts, as well as the production of advanced cross‐linked polysulfide materials. Facile preparation and stabilization of low coordinate main‐group centers, synthesis of new reactive main‐group precursors, and the development of new routes to link main‐group elements together have been keys in moving the field forward. The scope of main‐group complexes has increased drastically, with multiple routes available to access the compounds, enabling a wide range of derivatives possible. In addition, compared to the carbon‐based systems, main‐group compounds feature more bonding options due to the different electronics and sterics afforded by the noncarbon atom(s) and, therefore, modified reactivity. As a result, existing theories such as aromaticity are challenged when new main‐group cycles are created. In this article, the most common routes to prepare the different types of main‐group rings, chains, and polymers are summarized, along with selected classic examples and recent additions to the field from across the p‐block (Groups 13–16). Both homonuclear and heteronuclear systems are covered, and some relevant information on characterization, properties, and uses of the compounds is also included.
Although research in the field of main‐group rings, chains, and polymers is a less mature area of chemistry than that of carbon‐based rings, chains, and polymers, it is currently very fertile, as evidenced by the significant increase in available monocyclic main‐group rings in the past two decades, including neutral, anionic, cationic, and radical homocycles and heterocycles. Furthermore, recent advances in main‐group polymer chemistry have been substantial with access to high‐molecular‐weight polyaminoboranes and polyphosphinoboranes enabled by efficient dehydropolymerization catalysts, as well as the production of advanced cross‐linked polysulfide materials. Facile preparation and stabilization of low coordinate main‐group centers, synthesis of new reactive main‐group precursors, and the development of new routes to link main‐group elements together have been keys in moving the field forward. The scope of main‐group complexes has increased drastically, with multiple routes available to access the compounds, enabling a wide range of derivatives possible. In addition, compared to the carbon‐based systems, main‐group compounds feature more bonding options due to the different electronics and sterics afforded by the noncarbon atom(s) and, therefore, modified reactivity. As a result, existing theories such as aromaticity are challenged when new main‐group cycles are created. In this article, the most common routes to prepare the different types of main‐group rings, chains, and polymers are summarized, along with selected classic examples and recent additions to the field from across the p‐block (Groups 13–16). Both homonuclear and heteronuclear systems are covered, and some relevant information on characterization, properties, and uses of the compounds is also included.
Double deprotonation of the amidine DIPP‐N=C(tBu)‐NH2 or β‐diketimine DIPP‐N=C(Me)‐C(H)=C(Me)‐NH2 (DIPP = 2,6‐diisopropylphenyl) with strong benzylcalcium or strontium bases gave metal imido complexes with the anions DIPP‐N=C(tBu)‐N2– (Am2–) and DIPP‐N=C(Me)‐C(H)=C(Me)‐N2– (BDI2–) which crystallized as tetrameric complexes with typical Ca4N4 or Sr4N4 cubane frameworks. Crystal structures of [(Am)Ca·THF]4, [(BDI)Ca·THF]4 and the first Sr imido complex [(Am)Sr·THF]4 are presented. Calculated geometries of [(Am)Ca·THF]4 and [(BDI)Ca·THF]4 (B3PW91/6‐311++G**) are in good agreement with the crystal structures. Also complexes with monoanionic ligands (Am‐H)– and (BDI‐H)– are reported. Charge delocalization in the ligand backbone is discussed. Ligand–metal bonds are calculated to be circa 90 % ionic; the NPA charges on Ca is circa +1.8. The negative charge on the ligand is delocalized over the ligand backbone but there is still considerable electron density on the terminal N (–1.4). Despite this high negative charge, the reactivity of these complexes is generally low due to the strongly bound cubane core. Reaction of [(Am)Ca·THF]4 with phenyl cyanide gave the [(Am)Ca·PhCN]4, clearly demonstrating its low reactivity. Attempts to break the tetrameric cluster in smaller aggregates by addition of 18‐crown‐6 led to protonation and isolation of (Am‐H)2Ca·(18‐crown‐6). Reaction of [(Am)Ca·THF]4 with iPrN=C=NiPr gave after addition a complex with a unique amido‐guanidinate ligand.
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