Trinuclear 9-membered rings

A quantitative evidence for the cr-antiaromatic nature of saturated tetranuclear metal-carbonyl clusters [M4(CO)i2] (M = Fe, Ru, Os), which are similar metal-carbonyl analogs of cyclobutane was recently provided by Corminboeuf et al. [124], The isotropic NICS of the C2v [M4(CO)i2] (M = Fe, Ru, Os) clusters are about one-third less negative than those of their trinuclear congeners. Some differences in the magnetic response between cyclobutane and transition metal four-membered rings were identified and analyzed in detail using CMO-NICS (Fig. 37). 

As a general trend, six-membered mononuclear N-heteroaromatics such as pyridine and derivatives are much less prone to undergo hydrogenation than bi-and trinuclear N-ring compounds (e.g., quinolines, benzoquinolines, acridines) due to their higher resonance stabilization energy. 

The synthesis of 11 (an analogue of 7 in which its 12-membered cyclen had been replaced by 14-membered cyclam rings) was reported in 1998 using the tri-N-tosylamide derivative of cyclam and l,3,5-tris(bromomethyl)benzene as precursors [24], The trinuclear Hg(II) derivative was shown to selectively recognise and bind a tripodal tris(histidine) ligand in which the three (S)-histidines are 12 A apart. The system was seen as a model for peptide recognition. 

Structural studies on the trimer (4) showed that the conformation of the six-membered Pt303 ring is dependent on the counter-ion present with the cation [ 14,15]. On this basis it was suggested that the ring does not have a rigid conformation in solution. No studies with the trinuclear complexes and DNA have been reported. 

---