SYNTHESIS AND STRUCTURAL STUDIES OF IONIC MONOORGANOPALLADIUM(II) COMPLEXES WITH TRIDENTATE NITROGEN-DONOR LIGANDS

Ionic palladium(II) complexes of the types [PdCl(N-N'-N'')]Cl and [PdR (N-N'-N'')]OTf, with R = Me or aryl, N-N'-N'' = tridentate nitrogen do nor ligand, and OTf = trifluoromethane-sulfonate (triflate), have been prepared. The tridentate nitrogen-donor ligands used are 2,6-bis[(dim ethylamino)methyl]pyridine (NNN), N,N,N'-trimethyl-N'-(2-picolyl)ethyl enediamine (pico), and N,N,N',N'',N''-pentamethyldiethylenetriamine (p mdeta). The chloro derivatives (3-5a) were obtained as yellow ionic co mplexes in excellent yields (81-93%). Crystals of 4a were obtained fro m methanol/diethyl ether and are monoclinic, space group P2(1)/c (No. 14), a = 15.0760(7) angstrom, b = 7.4468(8) angstrom, c = 13.553(2) an gstrom, beta = 115.653(6)-degrees, and Z = 4. Refinement converged at R = 0.0325 (wR2 = 0.0663). The molecular structure shows a four-coordi nate palladium center surrounded by the terdentate bound pico ligand a nd a chloride anion. There is no interaction of the palladium center w ith the second chloride anion (Pd-Cl2 greater-than-or-equal-to 4.2617 angstrom). The Pd-NMe bond distance (2.023(3) angstrom) is relatively short and is accompanied by a small trans N-Pd-N bond angle (168.03(12 )-degrees). The methyl derivatives (3-5b) were also obtained in good y ield (79-91%) via reaction of PdIMe(tmeda) (tmeda = N,N,N',N'-tetramet hylethylenediamine) with silver trifluoromethanesulfonate and the liga nd. An alternative route, starting from PdMe2(tmeda), is reported for the synthesis of [PdMe(NNN)OTf (3b). Crystals of 3b were obtained from methanol/diethyl ether and are monoclinic, space group P2(1)/a (No. 1 4), a = 7.738(l) angstrom, b = 21.280(2) angstrom, c = 11.399(1) angst rom, beta = 92.05(1)-degrees, and Z = 4. Refinement converged at R = 0 .066 (R(w) = 0.065). The molecular structure of Sb shows a terdentate coordination of the NNN ligand to the metal, with a relatively short P d-N' bond distance (1.996(8) angstrom) and small N-Pd-N bond angle (16 1.7(3)-degrees). Yellow crystals of [PdMe(ONN')(tmeda)]OTf (8), with O NN' = -(hydroxymethyl)-6-[(dimethylamino)methyl]pyridine (7), were acc identally obtained from the reaction of [PdMe(MeCN)(tmeda)]OTf (I) wit h an impure sample of the NN'N ligand, containing the ONN' ligand. The molecular structure of 8 shows the ONN' ligand monodentate coordinate d to the metal via its pyridyl nitrogen donor whereas the NMe2 and OH functionalities are free. The triflate anion is hydrogen bonded to the hydroxymethyl group with OH=O-(SO2CF3) = 2.759(3) angstrom and O-H-O = 173(4)-degrees. Crystals of [PdMe(ONN')(tmeda)]OTf (8) are triclinic , space group P1BAR (No. 2), a = 9.7902(14) angstrom, b = 10.0555(15) angstrom, c = 12.362(2) angstrom, alpha = 75.828(12)-degrees, beta = 8 1.234(12)-degrees, gamma = 84.836(11)-degrees, and Z = 2. Refinement c onverged at R = 0.032 (R(w) = 0.040). The first examples of simple ary lpalladium(II) cations containing tridentate ligands were obtained m m oderate to high yield (35-95%). The aryl groups studied differ in both steric and electronic properties. Conformational analysis by NMR of t he NCCN moieties of the pico and pmdeta containing complexes showed th e 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 NNN < pico < pmdeta. This is expla ined in terms' of the positioning and orientation of the pyridyl and N Me2 groups around the metal center. The aryl rotation is found to be b locked in ortho-substituted aryl complexes, leading to atropisomerism in the pmdeta complex.