REACTIVITY OF ALIPHATIC-AMINES TOWARD PLATINUM(II) COMPLEXES

The rates of displacement of 5-aminoquinoline (5-Aq) and diethyl sulfi de (SEt2) from cis-[PtPh2(CO)(L)] (L = 5-Aq or SEt2) by an extended se ries of aliphatic amines of comparable basicity (pK(a) = 10.6 +/- 0.4) and of widely different steric properties have been measured in dichl oromethane solution. The reaction products cis-[PtPh2-(CO)(am)] (1-15) were characterized either as solids or in solution by their IR and H- 1 and C-13 NMR spectra. Crystals of cis-diphenyl(pyridine)carbonyl pla tinum(II) (12) belong to the monoclinic space group P2(1)/n with latti ce constants a = 11.756(2) angstrom, b = 9.853(2) angstrom, c = 13.552 (2) angstrom, beta = 100.27(2)-degrees, and Z = 4. cis-diphenyl-(diiso propylamine)carbonyl platinum(II) 13 crystallizes in the monoclinic sp ace group P2(1)/n) with lattice constants a = 8.926(2) angstrom, b = 1 4.439(2) angstrom, c = 14.468(2) angstrom, beta = 92.88(2)-degrees, an d Z = 4. Least-squares refinement of the structures led to R factors o f 1.93% and 3.08%, respectively. The substitution reactions take place by way of a direct bimolecular attack of the ligand on the substrate. The sulfide group is 100 times more labile than 5-Aq, but both comple xes exhibit the same discrimination ability toward the entering amines . The sequence of reactivity observed is NH2(n)Pr almost-equal-to NH2( n)Pent almost-equal-to NH2(n)Hex > NH2Cy almost-equal-to NH2(n)BU > NH 2(s)Bu > NH2Bz almost-equal-to pip almost-equal-to py > NH2(t)Bu > NHE t2 almost-equal-to NH(n)Bu2 > NH(i)Pr2, and it is determined by the nu mber and the encumbrance of the substituents on the nitrogen atom. The difference of reactivity between the first and the last members of th e series spans 2 orders of magnitude. For the most sterically hindered amines, such as NHCy2 or NEt3, the bimolecular attack is prevented an d the reaction proceeds only by way of a nucleophile independent pathw ay, which most likely involves dissociation of a ligand (5-Aq or SEt2) from the coordination sphere of the metal. Different shapes of steric profiles for the two reactions were obtained by correlating the react ivity data with different sets of steric parameters such as the amine cone angles (theta) of space-filling CPK molecular models or the van d er Waals steric repulsion (E(R)) as derived from molecular mechanics c alculations. Limits and significance of such relationships are discuss ed in order to account for apparently conflicting interpretations.