Tropocoronand ligand

Introduction

The tropocoronand family of ligands(H₂TC-m,n) have proved valuable for studying how changes in macrocycle ring size and flexibility can tune the physical and chemical properties of transition metal ions. Although the size of the binding cavity in most N₄ macrocycles, including porphyrins, can vary to only a small degree, a series of tropocoronand ligands (TC-m,n) ²⁻ having the same set of four aromatic nitrogen donor atoms but with a range of macrocycle sizes can be prepared. The hole size can be systematically adjusted by controlling the number of methylene bridges, m and n, in the arms connecting the two aminotroponiminate rings. Moreover, the flexibility of the linker chains allows the seven-membered rings to twist relative to each other as the macrocyclic ring size expands. In mononuclear complexes with large m and n, the rings will rotate to reduce torsional strain within the linker arms. These distortions induce substantial alterations in the energies of the molecular orbitals at the metal center, thus allowing its properties to be tuned. This features of the tropocoronand ligand could be investigated mostly with Group 4 metal centers. They represent a new class of molecules with potential to be modified with a chiral moiety and applied to enantioselective reactions.

Synthesis of the Tropocoronand Ligand

Tropocoronands were first synthesized by Nakanishi, Lippard, and Nozoe in 1983. Symmetrical tropocoronands where m = n = 2 – 6 have been made in a four-step synthesis (Scheme 1) from tropolone.

Either 2-tosyloxytropone (1) or 2-chlorotropone can be made from tropolone and then reacted with the appropriate diamine to give diaminodiketones 2. These diketones then give dialkoxydiimines 3 by treatment with dimethyl sulfate in refluxing toluene, or with triethyloxonium tetrafluoroborate in refluxing chloroform/hexamethylphosphoramide. The resulting dialkoxides can then undergo amine displacement and ring closure at 25°C to form tropocoronands 4. Reported yields in the cyclization are generally in the range of 20 – 40%, but only 2% when m = n = 2. In most of the cases decreasing the straight-chain linkages size results in lower yields however Nozoe has demonstrated yields of 55– 65% for m = n = 3 using methylfluorosulfate in dichloromethane as the alkylating agent.

Asymmetrical tropocoronands where the number of carbons of the simple straight-chain linkages is different (m ≠ n, but variability in the m,n lengths tends to be only by 1 methylene group) as well as chiral tropocoronands can be synthesized using the method outlined in Scheme 1 with the modification of the last step where the straight-chain diamine can be substituted to the diamine with a different length of chain (m ≠ n) or to the chiral one. An alternative synthesis, that of placing the chiral group in the sequence first to form a diaminoketone, followed by cyclization with a straight chain diamine in the last step, reported to be not successful.

Synthesis of Tropocoronand Complexes

Tropocoronand complexes with transition metals (Cu, Zn, Ni, Cd, Co, Rh, Fe, etc.) are synthesized by reaction of tropocoronands with a variety of metal salts in the presence of base to form colored solutions. Obtained complexes can be characterized with ¹H NMR, UV-vis spectroscopy as well as X-ray crystallography.

Different Transition Metal Centers

Various metals (Zn(II), Cd(II), Co(II), Ni(II), Cu(II), etc.) have been studied in order to determine how the differences in macrocycle ring size, metal ionic radius, and electronic structure can affect the dihedral angle of the tropocoronand ligand. Structural analysis is indicative of a correlation between metal ion size and properties; for example, the relatively large Zn²⁺ ion prohibits the formation of a four-coordinate metal center with 14-membered tropocoronand complex [Zn(TC-3,3)]. Indeed, the tropocoronands with the larger zinc metal center exhibit significantly larger dihedral angles relative to their smaller copper congeners. However, the dihedral angles of the cadmium tropocoronand complexes are smaller despite the larger metal ion radius. This may be attributed to the increase in M—N distance with the larger metal. Studies of divalent Co²⁺ and Ni²⁺ complexes showed that the electronic structure of the transition metal ion affects the dihedral angle: the TC-4,5 ligand adopts a larger dihedral angle for the Co²⁺ ion, likely resulting due to its greater preference for tetrahedral over square-planar geometry as well as differences in ligand field stabilization energies (LFSE).

Variable Reactivity of TC Complexes with Nitric Oxide

Studies have been performed looking at the reactivity of tropocoronand complexes with biologically relevant nitric oxide (NO). The electronic structure, metal ion center, length of the polymethylene chain linker, and other variables all influence the reactivity of a tropocoronand complex with nitric oxide. In varying the aforementioned elements, the mononitrosyl, nitrito, and dinitrosyl complexes have been formed in various conditions. For example, [Mn(THF)(TC-5,5)] favors the mononitrosyl complex after adding stoichiometric additions of NO whereas the nitrito complex is the favored tropocoronand product when in the presence of excess NO respectively. [Fe(TC-5,5)] also promotes NO disproportionation when in the presence of excess NO; however, in contrast to Mn, the final product is [Fe(NO)(TC-5,5-NO₂)], where the iron retains the nitrosyl and the nitrite becomes bound to the ligand.

Similarly, the length of the polymethylene linker chain has been shown to significantly influence the NO reactivity of tropocoronand complexes, as shown below: