Reactions of Low-Valent Titanocene(II) Fragments with trans-4,4′-Azobispyridine (RN═NR, R = C5H4N): Formation of Tetranuclear Molecular Squares by trans−cis Isomerization

Abstract

The reactions of the low-valent titanocene sources [Cp*2Ti(η2-C2(TMS)2)] (2) and [tBuCp2Ti(η2-C2(TMS)2)] (3) with trans-4,4′-azobispyridine (8) leads to novel supramolecular squares [(Cp*2Ti)4(μ2-N,N′;η2-N,N′-C10H8N4)2] (10) and [(tBuCp2Ti)4(μ2-N,N′;η2-N,N′-C10H8N4)2] (11). These complexes consist of four bent-titanocene corner units and two azo ligands 8. Within this self-assembly process the azo ligands experience a conformational rearrangement from trans to cis. The titanocene moieties are embedded with two different N-donor environments, provided by the pyridyl rings and the azo functionality of 8. Single-crystal X-ray analyses of the tetranuclear compounds 10 and 11 revealed the structural features of these molecular polygons. Exclusively cis-configurated azopyridine units are detected in 10 and 11. Comparison of bond lengths and angles in coordinated and free ligands shows a reduced state of the bridging ligands in the low-valent titanium complexes 10 and 11. In such a way, the Nazo−Nazo distances are elongated from 1.251(2) Å in 8 to av 1.405(3) Å (10) and 1.434(3) Å (11), respectively. The syntheses and attributes of these novel compounds are discussed. From the reaction of [Cp2Ti(η2-C2(TMS)2)] (1) with 8, 1,2-bis(4-pyridyl)hydrazine (13) was isolated and fully characterized as a subsequent product.