Synthesis and Structural Studies of Ionic Monoorganopalladium(II) Complexes with Tridentate Nitrogen-Donor Ligands

Abstract

Ionic palladium(I1) complexes of the types [PdCl(N-N-N)ICl and [PdR(N-N-N)IOTf', with R = Me or aryl, N N N = tridentate nitrogen donor ligand, and OTf = trifluoromethanesulfonate (triflate), have been prepared. The tridentate nitrogen-donor ligands used are 2,6-bis[( dimethy1amino)methyllpyridine (NN'N), NJVfl-trimethylN'(2-picoly1)ethylenediamine (pico), and NJVfl,NJV-pentamethyldiethylenetriamine (pmdeta). The chloro derivatives (3-5a) were obtained as yellow ionic complexes in excellent yields (81-93%). Crystals of 4a were obtained from methanovdiethyl ether and are monoclinic, space group P21/c (No. 141, a = 15.0760(7) A, b = 7.4468(8) A, c = 13.553(2) A, #? = 115.653(6), and 2 = 4. Refinement converged at R = 0.0325 (wR2 = 0.0663). The molecular structure shows a four-coordinate palladium center surrounded by the terdentate bound pic0 ligand and a chloride anion. There is no interaction of the palladium center with the second chloride anion (Pd-C12 L 4.2617 A). The Pd-NMe bond distance (2.023(3) A) is relatively short and is accompanied by a small trans N-Pd-N bond angle (168.03(12)). The methyl derivatives (3-5b) were also obtained in good yield (79-91%) via reaction of PdIMe(tmeda) (tmeda = NjVflfl-tetramethylethylenediamine) with silver trifluoromethanesulfonate and the ligand. An alternative route, starting from PdMez(tmeda1, is reported for the synthesis of [PdMe(NN)OTf (3b). Crystals of 3b were obtained from methanovdiethyl ether and are monoclinic, space group P21/a (No. 141, a = 7.738(1) A, b = 21.280(2) A, c = 11.399(1) A, ,8 = 92.05(1), and 2 = 4. Refinement converged a t R = 0.066 (R, = 0.065). The molecular structure of 3b shows a terdentate coordination of the NN'N ligand to the metal, with a relatively short Pd-N bond distance (1.996(8) A) and small N-Pd-N bond angle (161.7(3)). Yellow crystals of [PdMe(ON)(tmeda)lOTf (8), with O N = 24hydroxymethyl)6-[(dimethylamino)methyl]pyridine (71, were accidentally obtained from the reaction of [PdMe(MeCN)(tmeda)]OTf (I) with an impure sample of the NN'N ligand, containing the O N ligand. The molecular structure of 8 shows the O N ligand monodentate coordinated to the metal via its pyridyl nitrogen donor whereas the NMez and OH functionalities are free. The triflate anion is hydrogen bonded to the hydroxymethyl group with OH-O(S02CF3) = 2.759(3) A and-0-H-0 = 173(4). Crystals of [PdMe(ONN')(tmeda)lOTf (8) are triclinic, space group P1 (No. 21, a = 9.7902(14) A, b = 10.0555(15) A, c = 12.362(2) A, a = 75.828(12), p = 81.234(12), y = 84.836(11), and 2 = 2. Refinement converged at R = 0.032 ( R , = 0.040). The first examples of simple arylpalladium(I1) cations containing tridentate ligands were obtained in moderate to high yield (35-95%). The aryl groups studied differ in both steric and electronic properties. Conformational analysis by NMR of the NCCN moieties of the pic0 and pmdeta containing complexes showed the five-membered chelate rings in the complexes to occur selectively in one of the two possible conformations. The rotational-energy barriers of the aryl groups have been studied as a function of the ligand and were shown to increase in the order NNTJ < pic0 < pmdeta. This is explained in terms of the positioning and orientation of the pyridyl and NMe2 groups around the metal center. The aryl rotation is found to be blocked in ortho-substituted aryl complexes, leading to atropisomerism in the pmdeta complex.