Octasubstituted biphenylenes: is there a favoured conformation?

Abstract

Octakis(pyrazol-1-ylmethyl)biphenylene ethanol solvate, C 44 H 40 N 16 ·C 2 H 6 O, (1) has two independent centrosymmetric molecules, one of which is hydrogen bonded to the solvate molecule. One adopts an arrangement with three arms up and one down in each benzene ring, whilst the other has a conformation with two adjacent arms on the same side of the ring.In neither case is the expected fully alternating form observed. CommentWe have long been involved in the synthesis and study of new N-heterocyclic ligands for use in coordination and metallosupramolecular chemistry (Steel, 2005). In particular we have prepared a large library of ligands that contain a central arene core to which are appended various heterocycles via flexible linker units (McMorran & Steel, 2002;McMorran et al., 2004). In the course of designing new ligands, we have employed the concept of "pre-organization" (Hennrich & Anslyn, 2002). This relies on the principle that six bulky substituents attached to a benzene ring will tend to arrange themselves on alternating faces of the ring in an ababab fashion (a and b being above and below the plane of the ring, as defined by MacNicol et al., 1985). For example, hexakis(pyrazol-1-ylmethyl)benzene adopts this conformation (Hartshorn & Steel, 1995). A common extension of this approach is to differentiate the two faces of the ring with differing 1,3,5-and 2,4,6-substituents, as for example in 1,3,5-triethyl-2,4,6-tris(pyrazol-1-ylmethyl)benzene (Hartshorn & Steel, 1997). We were interested to know whether this design concept could be extended to larger aromatic systems, such as biphenylenes. The X-ray crystal structure of octaethylbiphenylene revealed that this compound adopts an ababbaba conformation in the solid state rather than the fully alternating abababab conformation that was calculated to be the most stable (Taha et al., 2000;Marks et al., 2003). However, we previously prepared the new eight armed ligand octakis(2-pyridylmethylsulfanyl)biphenylene and were encouraged to find that in the solid state it was preorganized into the fully alternating abababab conformation (McMorran & Steel, 2003). We now report the synthesis of octakis(pyrazol-1-ylmethyl)biphenylene and the X-ray crystal structure of its ethanol solvate (1).The new ligand octakis(pyrazol-1-ylmethyl)biphenylene was prepared by an eightfold phase-transfer catalysed alkylation of octakis(bromomethyl)biphenylene with pyrazole. It was purified by chromatography followed by recrystallization and was characterized by microanalysis, 1 H NMR spectroscopy and electrospray mass spectrometry. In order to ascertain the conformation of this compound we sought to determine its X-ray structure: the analysis of (1) reveals that the crystals are an ethanol solvate which has two independent half molecules of the ligand in the asymmetric unit. The two independent