Synthesis and structural studies of ionic monoorganopalladium(II) complexes with tridentate nitrogen-donor ligands

Article abstract of DOI:10.1021/om00020a043

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 donor ligand, and OTf = trifluoromethane-sulfonate (triflate), have been prepared. The tridentate nitrogen-donor ligands used are 2,6-bis[(dimethylamino)methyl]pyridine (NN′N), N,N,N′-trimethyl-N′-(2-picolyl)ethylenediamine (pico), and N,N,N′,N″,N″-pentamethyldiethylenetriamine (pmdeta). The chloro derivatives (3-5a) were obtained as yellow ionic complexes in excellent yields (81-93%). Crystals of 4a were obtained from methanol/diethyl ether and are monoclinic, space group P2₁/c (No. 14), a = 15.0760(7) ?, b = 7.4468(8) ?, c = 13.553(2) ?, β = 115.653(6)°, and Z = 4. Refinement converged at R = 0.0325 (wR2 = 0.0663). The molecular structure shows a four-coordinate palladium center surrounded by the terdentate bound pico ligand and a chloride anion. There is no interaction of the palladium center with the second chloride anion (Pd-C12 ≥ 4.2617 ?). The Pd-NMe bond distance (2.023(3) ?) 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 = N,N,N′,N′-tetramethylethylenediamine) with silver trifluoromethanesulfonate and the ligand. An alternative route, starting from PdMe₂(tmeda), is reported for the synthesis of [PdMe(NN′N)OTf (3b). Crystals of 3b were obtained from methanol/diethyl ether and are monoclinic, space group P2₁/a (No. 14), a = 7.738(1) ?, b = 21.280(2) ?, c = 11.399(1) ?, β = 92.05(1)°, and Z = 4. Refinement converged at R = 0.066 (R_w = 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) ?) and small N-Pd-N bond angle (161.7(3)°). Yellow crystals of [PdMe(ONN′)(tmeda)]OTf (8), with ONN′ = 2-(hydroxymethyl)-6-[(dimethylamino)methyl]pyridine (7), were accidentally obtained from the reaction of [PdMe(MeCN)(tmeda)]OTf (I) with an impure sample of the NN′N ligand, containing the ONN′ ligand. The molecular structure of 8 shows the ONN′ ligand monodentate coordinated 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) ? and O-H-O = 173(4)°. Crystals of [PdMe(ONN′)(tmeda)]OTf (8) are triclinic, space group P1 (No. 2), a = 9.7902(14) ?, b = 10.0555(15) ?, c = 12.362(2) ?, α = 75.828(12)°, β = 81.234(12)°, γ = 84.836(11)°, and Z = 2. Refinement converged at R = 0.032 (R_w = 0.040). The first examples of simple arylpalladium(II) 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 pico 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 NN′N < pico < 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.