Rosenthal Joel, Young Elizabeth R, Nocera Daniel G
Department of Chemistry, 6-335, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139-4307, USA.
Inorg Chem. 2007 Oct 15;46(21):8668-75. doi: 10.1021/ic700838s. Epub 2007 Sep 14.
Alkynylamidinium groups have been introduced at the beta and meso positions of a nickel(II) porphyrin (PNi(II)) framework. The modification permits the distance between the amidinium-amidine acid-base group and porphyrin to be increased while effectively maintaining pi conjugation between the porphyrin macrocycle and the acid-base functionality. Use of an ethynyl spacer as a linker (i) extends the amidinium functionality away from the sterically bulky mesityl groups of the porphyrin, allowing it to be nearly planar with respect to the porphyrin ring, and (ii) draws the pi-orbital character of the porphyrin out toward the amidinium functionality, thereby engendering sensitivity of the electronic properties of the porphyrin macrocycle to the protonation state of the amidinium. The barrier for rotation of the amidinium group, as calculated by time-dependent density functional theory (TDDFT), is approximately 8.5 kT (5 kcal/mol) for both porphyrins. Analysis of UV-visible absorption profiles for the beta- and meso-alkynylamidinium PNi(II) upon deprotonation enables accurate determination of the amidinium acidity constants for the ground state (pK(a)(beta) = 7.03 +/- 0.1, pK(a)(meso) = 7.74 +/- 0.1 in CH(3)CN) and excited state (pK(a)(beta) = 6.89 +/- 0.1, pK(a)(meso) = 8.37 +/- 0.1 in CH(3)CN) porphyrins. Whereas pK(a)* < pK(a) for the beta-alkynylamidinium porphyrin, pK(a)* > pK(a) for the meso-alkynylamidinium porphyrin, indicating that beta-alkynylamidinium PNi(II) is a photoacid and meso-alkynylamidinium PNi(II) is a photobase. These divergent behaviors are supported by analysis of the frontier molecular orbitals of the homologous pair with TDDFT.
炔基脒鎓基团已被引入到镍（II）卟啉（PNi（II））骨架的β位和中位。这种修饰使得脒鎓 - 脒酸碱基团与卟啉之间的距离得以增加，同时有效地保持了卟啉大环与酸碱官能团之间的π共轭。使用乙炔基间隔基作为连接体：（i）将脒鎓官能团从卟啉空间位阻较大的均三甲苯基基团处延伸开，使其相对于卟啉环近乎呈平面状；（ii）将卟啉的π轨道特征拉向脒鎓官能团，从而使卟啉大环的电子性质对脒鎓的质子化状态产生敏感性。通过含时密度泛函理论（TDDFT）计算得出，两种卟啉的脒鎓基团旋转势垒约为8.5 kT（5千卡/摩尔）。对β - 和中位 - 炔基脒鎓PNi（II）去质子化后的紫外 - 可见吸收光谱进行分析，能够准确测定基态（在CH₃CN中，pKₐ(β) = 7.03 ± 0.1，pKₐ(meso) = 7.74 ± 0.1）和激发态（在CH₃CN中，pKₐ*(β) = 6.89 ± 0.1，pKₐ*(meso) = 8.37 ± 0.1）卟啉的脒鎓酸度常数。对于β - 炔基脒鎓卟啉，pKₐ* < pKₐ；而对于中位 - 炔基脒鎓卟啉，pKₐ* > pKₐ，这表明β - 炔基脒鎓PNi（II）是一种光酸，中位 - 炔基脒鎓PNi（II）是一种光碱。这些不同的行为通过用TDDFT对同源对的前沿分子轨道进行分析得到了支持。
