DOI: 10.1002/chem.201100780 Factors That Regulate the Conformation of m-Benziporphodimethene Complexes: Agostic Metal–Arene Interaction, Hydrogen Bonding, and h 2 ,p Coordination Gao-Fong Chang, [b, c] Chun-Hung Wang, [a] Hung-Chieh Lu, [a] Lou-Sing Kan, [a] Ito Chao, [a] Wei Hao Chen, [a] Anil Kumar, [a] Liyang Lo, [a] Mira Anne C. dela Rosa, [a] and Chen-Hsiung Hung* [a] Introduction The formation of a complex in which the metal ion and C À H bond interact is the most crucial step in C À H bond activa- tion processes. [1] This type of interaction, caused by an at- tractive force between the C À H bond and the metal ion, can effectively stabilize metal complexes. [2–4] Pincer-type ligands have been demonstrated to have a high tendency to form agostic C ÀH or C ÀC bond interaction with metal ions and result in bond activation. [5–7] Recently, carbaporphyrinoids— for example, benziporphyrins (Scheme 1A) or N-confused porphyrins—have emerged as new scaffolds for the explora- tion of the metal-assisted C À H bond activation. [8–10] Facilitat- ed by macrocyclic constraint, the complexes of carbapor- phyrinoids can readily form agostic intramolecular interac- tions, [9, 11–13] which may result in the formation of a metal– carbon bond through C ÀH bond activation. [9, 14–17] Weak metal–arene interactions in cadmium and nickel complexes of m-benziporphyrin, with distances shorter than the sum of van der Waals radii, have been observed through X-ray structure analyses. The unusually large intramolecular through-space scalar couplings between the cadmium nu- cleus and the arene protons in m-benziporphyrin Cd II com- plexes as well as the downfield chemical shifts as a result of spin transfer from nickel center to an agostic proton in the paramagnetic m-benziporphyrin Ni II complexes provide solid supporting evidence for the presence of the metal– arene interaction. [8] Interestingly, the strength of the metal– arene interaction is highly dependent on the conformation of the arene moiety. It has been demonstrated that the inter- action in chloronickel(II) p-benziporphyrin is not as strong Abstract: Group 12 and silver(I) tetra- methyl-m-benziporphodimethene (TMBPDM) complexes with phenyl, methylbenzoate, or nitrophenyl groups as meso substituents were synthesized and fully characterized. The dimeric sil- ver(I) complex displays an unusual h 2 ,p coordination from the b-pyrrolic C =C bond to the silver ion. All of the com- plexes displayed a close contact be- tween the metal ion and the inner C(22) ÀH(22) on the m-phenylene ring. The downfield chemical shifts of H(22) and large coupling constants between Cd II and H(22) strongly support the presence of an agostic interaction be- tween the metal ion and inner C(22)– H(22). Crystal structures revealed that the syn form is the predominant con- formation for TMBPDM complexes. This is distinctively different from the exclusive anti conformation observed in m-benziporphyrin and tetraphenyl- m-benziporphodimethene (TPBPDM) complexes. Evidently, intramolecular hydrogen-bonding interactions between axial chloride and methyl groups stabi- lize syn conformations. Unlike the merely syn conformation observed in the solid-state structures of TMBPDM complexes that contain an axial chlo- ride, in solution these complexes dis- play highly solvent- and temperature- dependent syn/anti ratio changes. The observation of dynamic 1 H NMR spec- troscopic scrambling between syn and anti conformations from the titration of chloride ion into the solution of the TMBPDM complex suggests that axial ligand exchange is a likely pathway for the conversion between syn and anti forms. Theoretical calculations re- vealed that intermolecular hydrogen- bonding interactions between the axial chloride and CHCl 3 stabilizes the anti conformation, which explains the in- creased ratio for the anti form when di- chloromethane or chloroform was used as the solvent. Keywords: agostic interactions · arenes · C À H activation · conforma- tion analysis · porphyrinoids [a] C.-H. Wang, H.-C. Lu, Dr. L.-S. Kan, Dr. I. Chao, W.H. Chen, Dr. A. Kumar, Dr. L. Lo, M. A. C. dela Rosa, Dr. C.-H. Hung Institute of Chemistry, Academia Sinica Nankang Taipei 115 (Taiwan) Fax: (+ 886) 2-27831237 E-mail : chhung@chem.sinica.edu.tw [b] G.-F. Chang Department of Chemistry National Tsing Hua University, Hsinchu 30013 (Taiwan) [c] G.-F. Chang Molecular Science and Technology Program Taiwan International Graduate Program Academia Sinica, Taipei 115 (Taiwan) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201100780.  2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Eur. J. 2011, 17, 11332 – 11343 11332