A new methodology for the preparation of cationic organoplatinum(II) complexes with hydrogen-bonding functionality

A new methodology for the preparation, of two series of cationic organoplatinum(II) complexes with hydrogen-bonding functionality is described. The mononuclear complexes of the type trans-[Pt(sigma -aryl)L(PPh3)(2)] OTf (L = nicotinic acid, aryl = C-3-benzoic acid (9) or C-4-benzoic acid (10); L = isonicotinic acid, aryl = C-3-benzoic acid (11) or C-4-benzoic acid (12); OTf = trifluoromethanesulfonate (triflate)) constitute the first series, and the dinuclear complexes of the type trans-[Pt(sigma -aryl)(PPh3)(2)(mu -L)Pt(sigma -aryl)(PPh3)(2)](OTf)(2) (L = 1,1-bis(4-pyridyl)ethene, aryl = C-3-benzoic acid (19) or C-4-benzoic acid (20); L = 4,7-phenanthroline, aryl = C-3-benzoic acid (21) or C-4-benzoic acid (22); L = 4,4 ' -bipyridine, aryl = C-3-benzoic acid (23) or C-4-benzoic acid (24)) constitute the second series. The methodology described here involves the protection of the carboxylic acid group of a 3- or 4-iodobenzoic acid precursor as the tert-butyldiphenylsilyl ester, followed by an oxidative addition reaction of the C-I bond with the Pt(0) species Pt(PPh3)4 to yield the key (sigma -aryl)iodoplatinum(II) intermediates 3 and 4, the structures of which were determined by X-ray crystallography. Subsequent treatment of these products with AgOTf, followed by the addition of a suitable monodentate or bridging bidentate N-donor ligand and, finally, facile removal of the silyl protecting group(s) with HOW affords the target complexes with hydrogen-bonding functionality in high yield. Variable-temperature H-1 NMR experiments with the silyl-protected complexes trans[Pt(sigma -aryl)L(PPh3)(2)]OTf (aryl = C-3-tert-butyldiphenylsilyl benzoate, L = nicotinic acid (5) or isonicotinic acid (7)) confirm that a dynamic intramolecular process involving the pyridyl ligand is occurring. The rotational barrier of a pyridyl ligand in an organoplatinum(II) complex is reported for the first time, where DeltaG(double dagger) = 48.3 +/- 0.9 and 45.7 +/- 0.9 kJ mol(-1) for complexes 5 and 7, respectively.