Cadmium(II) and Nickel(II) Complexes of Benziporphyrins. A Study of Weak Intramolecular Metal−Arene Interactions

Weak metal−arene interactions have been investigated in Zn, Cd, Hg, and Ni complexes of
meso-tetraaryl m- and p-benziporphyrin (1 and 2) and of the new compound, m-benziporphodimethene
(3). Compounds 1−3 incorporate the phenylene moiety into a macrocyclic structure so as to facilitate the
interaction between the arene and coordinated metal ion. X-ray studies performed on Cd(II) and Ni(II)
complexes show that the arene fragment approaches the ion at a distance much shorter than the sum of
van der Waals radii. In chloronickel(II) m-benziporphyrin, a weak agostic bond is actually formed. In the
NMR spectra of the Cd(II) and Hg(II) species, unusual 1H−M and 13C−M scalar couplings have been
observed that are transmitted directly between the metal and the arene. DFT calculations performed for
two Cd(II) species and subsequent AIM analysis show that the accumulation of electron density between
the metal and arene necessary to induce these couplings is fairly small and the interaction is steric in
nature. In the paramagnetic Ni(II) complexes of 1 and 3, the agostic proton of the m-phenylene exhibits
large downfield 1H NMR shifts (386 and 208 ppm at 298 K, respectively). An agostic mechanism of spin
density transfer is proposed to explain these shifts as resulting from electron donation from the CH bond
to the metal. In chloronickel(II) p-benziporphyrin, the inner protons of the p-phenylene have a contrastingly
small shift (0.0 ppm at 298 K), indicating that in this case the agostic interaction is inefficient, in agreement
with the X-ray data.