Macrocyclic dinuclear complexes self-assembled from (en)Pd(NO3)(2) and pyridine-based bridging ligands

Treatment of (en)Pd(NO3)(2) (1, en denotes ethylenediamine) with pyridine-based bridging ligands, Py-X-Py (2, Py = 4-pyridyl, X = a: CH2, b: C(=O), c: C(=CH2)) gives rise to the self-assembly of macrocyclic dinuclear Pd(II) complexes [(en)Pd(mu-2a-c)](2)(NO3)(4) (3a-c, respectively). The structures of 3a and 3b were confirmed by NMR and FAB-MS (for 3a) or an X-ray study (for 3b). NMR and X-ray diffraction studies showed that the macrocycle 3b existed as hydrated form 3b' (X = C(OH)(2)) in both solution and solid, respectively. The crystal data of 3b' (ClO4 salt): triclinic, space group P (1) over bar, a = 10.064 (2), b = 10.070 (2), c = 12.163 (2) Angstrom; alpha = 96.82 (2), beta = 108.73 (2), gamma = 108.11 (2)degrees, Z = 1. The unfavorable hydration implies that the reduction of an sp(2) angle to an sp(3) angle at the carbonyl carbon is essential to remove the ring strain from the macrocyclic structure. In fact, ligand 2c whose Py-X-Py angle is fixed at an sp(2) angle gave a mixture of 39 and a trinuclear macrocycle [(en)Pd(mu-2c)](3)(NO3)(6).