New discrete metallocycles incorporating palladium(II) and platinum(II) corners and dipyridyldibenzotetraaza[14]annulene side units

The interaction of the extended, fully-conjugated macrocycle, dipyridyldibenzotetraaza[14]annulene (1), with the square planar palladium(II) and platinum(II) complexes [M(dppp)(triflate)2]·2H2O (M = Pd, Pt) has been investigated in both solution and the solid state. In each case solid products showing a 1 : 1 ratio of metal complex : 1 were obtained. A 1 : 1 mixture of 1 and [Pd(dppp)2(triflate)2]·2H2O in dichloromethane yielded two major products as evidenced by the presence of two singlets in the 31P {1H}-NMR spectrum of the reaction solution. Similarly, two singlets were evident in the corresponding spectrum obtained on dissolving the 1 : 1 product in nitrobenzene. The temperature and concentration dependence of the spectra clearly showed that the two species present in each case were in equilibrium. From the temperature dependence, the low field signal was assigned to the smaller of the two species. Broadly parallel behaviour was observed for the corresponding platinum-containing system. The MS-ESI spectrum of the platinum derivative showed the presence of a dinuclear species corresponding to [Pt(dppp)(1)]22+ and an X-ray structure of this product confirmed that a corresponding dinuclear complex exists in the solid state. This product has a geometry in which two curved macrocyclic side units bridge two metal centres to yield an ellipse-shaped structure. Attempts to employ pulsed-field gradient spin-echo (PGSE) 31P NMR confirmed that the lower-field resonance corresponded to the smaller of the two species in solution. STM (HOPG) imaging of the palladium- and platinum-containing products revealed arrays that appear to be composed of “zipper-like” rows of dimer units, with the dimensions of the latter comparing well with those found in the X-ray structure of [Pt(dppp)(1)]22+.