Scheiner Steve
Department of Chemistry and Biochemistry , Utah State University , Logan , Utah 84322-0300 , United States.
J Phys Chem A. 2018 Mar 8;122(9):2550-2562. doi: 10.1021/acs.jpca.7b12357. Epub 2018 Feb 27.
The tetravalent character of tetrel atoms leaves only limited room for an incoming nucleophile to approach and engage in a noncovalent bond with a tetrel atom. Any such approach can only occur at the expense of internal geometric distortions. The balance between attractive forces and repulsive steric crowding was studied for a series of Lewis acids of the type FTR (T = Si, Ge, Sn, Pb) which were allowed to interact with various bases. The strength and other properties of the tetrel bond are examined as the R groups are made progressively larger, varying from H and CH to isopropyl and tert-butyl, which induce steric crowding with the incoming base. The effects of crowding which impede the bond can be offset by enlarging the T atom, by adding electron-withdrawing substituents to the Lewis acid, or by considering stronger bases such as anions. The tetrel bond energies reach up to 10 kcal/mol for a pair of neutral molecules with no electron-withdrawing substituents on the Lewis acid. Adding -CF substituents grows the interaction energy to as high as 35 kcal/mol, and a further increment occurs for an anionic base, taking the maximum up to 54 kcal/mol.
四面体原子的四价特性使得亲核试剂接近并与四面体原子形成非共价键的空间非常有限。任何这样的接近都只能以内部几何畸变作为代价。研究了一系列FTR型（T = Si、Ge、Sn、Pb）路易斯酸与各种碱相互作用时吸引力和排斥性空间拥挤之间的平衡。随着R基团逐渐增大，从H和CH到异丙基和叔丁基，它们会与进入的碱产生空间拥挤，从而研究了四元键的强度和其他性质。阻碍键形成的拥挤效应可以通过增大T原子、在路易斯酸上添加吸电子取代基或考虑更强的碱（如阴离子）来抵消。对于路易斯酸上没有吸电子取代基的一对中性分子，四元键能高达10千卡/摩尔。添加-CF取代基会使相互作用能高达35千卡/摩尔，对于阴离子碱会进一步增加，最高可达54千卡/摩尔。
