Novel porphyrinoid metal complexes
Funding: DFG BR2010/11-1 (Normalverfahren)
New corrinoid ligands
Currently, there is an increasing need for porphyrinoid metal complexes with new property profiles. Demand of such compounds is found e.g. in biohybrid and bioinorganic model compounds. Porphyrinoid-based quantum dots on surfaces as well as functional polymers and π-stack polymers for nanoscience and energy research are also approaching the spotlight of interdisciplinary interest. In particular corroles that differ from porphyrins by the absence of one methine bridge have now been incorporated into these technologies. New variations are now required to satisfy the desire for targeted optimization studies within these research approaches . One well-known general method for the preparation of porphyrins is the oxidative cyclization of linear tetrapyrroles. In most cases, the ring closure is induced by a templating metal ion, particularly copper(II). In addition, some very successful electrocyclic processes like the nickel(II)- and cobalt(II)-supported corrin syntheses are known. With alternative methods also corroles, corrin analogues and other non-natural porphyrinoids are accessible through template-assisted ring closure reactions. These options are systematically explored.
Overview: Cyclization reactions of 2,2'-bidipyrrins
Metal complexes with unusual electronic structures
Some complexes of the middle and late 3d transition metals with corrole and similar non-innocent ligands show unusual electronic structures. Therefore, applications in catalysis and material synthesis is of interest. Special attention deserves the occurrence of open-shell singlet structures in copper corroles. Copper corroles are Cu(II) complexes of radical corrole dianion ligands and show a strong antiferromagnetic coupling between the copper atom and the ligand-based single electrons (see chart). The compounds are not planar but distorted towards a saddle-shaped. Within the inversion vibrations a (nearly) planar excited state is accessible. In this state, the magnetic coupling is ferromagnetic, so that an effective paramagnetic moment forms as the temperature rises.
SOMOs of a copper(II)corrole with α and β spin densities
Molecular structure of an i.s. (S = 3/2) iron(III)-bidipyrrin
The formation of iron(III) derivatives in the intermediate spin (S = 3/2) state is observed in open-chain and helical 2,2 '-bidipyrrin complexes (see chart). The phenomenon occurs only in the crystalline solid and can be detected i.a. by the crystallographically shorter Fe-N bonds. SQUID magnetometry shows a continuous transition to the high spin (S = 5/2) state at elevated temperatures, which is also found in solution. Apparently, a thermal helix inversion process leads to an effective extension of the Fe-N bonds and thus to the spin crossover.
Other studies include NO complexes and compounds in (formally) high valent states.
Catalytically active metal corroles
The chemistry of metal corroles has taken a dynamic development in recent years. One of the driving forces behind this development is the finding that iron, manganese and rhodium corroles are feasible to catalyze atom transfer reactions. In particular, the well-known processes of metalloporphyrins in catalytic oxygenation, aziridination, and cyclopropanylation, offer several fields of application. With a series of superstructured corroles, the so-called bis-picket-fence-corroles, we introduce chemo-and stereoselectivity in these processes. Most of the precatalysts in this study are from the formally high-valent chromium, manganese and iron class.
Schematic formation of bis-picket-fence-corroles
Steric encumbrance within a nitrosyl iron-bis-picket-fence-corrole
Model complexes for biological corrinoids
Rhodium complexes of porphyrins and corroles have been studied in the past due to their structural analogy to cobalamin (vitamin B12). If open-chain tetrapyrroles are employed in the metallation with rhodium(I) precursors, an oxidized, ring-closed product is obtained, in addition to the expected mono- and dinuclear rhodium(I) complexes. The macrocyclic ligand of this product has no aromatic conjugation and is thus akin to the corrins and other hydroporphyrinoids. This reaction occurs also in the presence of iridium ions, and with further transition metal ions under different conditions. The products show interesting electrochemical properties and are studied as model complexes for biological corrin and hydroporphyrin cofactors.
Formation of a B12 analog from Rh(I) complexes and 2,2'-bidipyrrins
Strained palladium complexes for CH activation studies
The chemistry of palladium complexes with sterically restricted tripyrrin and BAI ligands is an unusually versatile and fertile field. Kinetically stabilized reaction intermediates have been observed occasionally among palladium tripyrrin complexes. The more flexible BAI ligands on the other hand give rise to a different behavior. By positioning a suitable C-H bond in close proximity to the palladium atom, C(sp2)-H and C(sp3)-H activation processes can be initiated here. While the former process is mediated by a second equivalent of palladium acetate, the less frequently observed C(sp3)-H activation occurs spontaneously with tert-butyl groups as well as with benzylic CH bonds. These reactions are used for the preparation of doubly activated CCNN complexes and for the synthesis of pincer-like precatalysts. Detailed experimental and theoretical studies on the mechanism of palladium-induced CH activation reactions of optimized substrates are currently in progress.
C(sp2)-H- and C(sp3)-H activated palladium-BAI-complexes