[6,5] CNN Palladium(II) Pincer Complexes Containing <i>N</i>-Substituted Monoanionic and Dianionic Guanidinate Ligands: Syntheses, Structural Aspects, and Their Utility in Suzuki–Miyaura Coupling Reactions

Abstract

The reactions of guanidines [(ArNH)(ArN═)CN(H)(C(O)Py-2)] (Ar = 2-RC6H4; R = Me (1) and CF3 (2)) with Pd(OC(O)R′)2 (R′ = Me, tBu and CF3) were studied with a view aimed at understanding the influence of substituent effects upon the nature of the resulting products. The reactions of 1 with Pd(OC(O)R′)2 (R′ = Me and CF3) in toluene at 65 °C afforded [Pd{κ3(C,N,N)}(OC(O)R′)] (R′ = Me (3) and CF3 (4)), while the analogous reaction involving 1 and Pd(OC(O)tBu)2 afforded [Pd{κ3(C,N,N)}(μ-OC(O)tBu)Pd{κ3(C,N,N)}] (5). The analogous reactions of 2 with Pd(OC(O)R′)2 (R′ = Me, tBu and CF3) afforded [Pd{κ3(C,N,N)}(μ-OC(O)R′)Pd{κ3(C,N,N)}] (R′ = Me (6) and tBu (7)) and [Pd{κ3(C,N,N)}(OC(O)CF3)] (8), respectively. Complexes 5–7 were treated with 2 equiv of Lewis bases, L at RT to afford [Pd{κ3(C,N,N)}(L)] [L = PPh3(9), 4-aminopyridine (10 and 11), and MeCN (12)]. The mechanisms of formation of new complexes were proposed. Complex 3 was treated with 12 in the presence of excess of NaOC(O)tBu in CHCl3 under reflux condition to afford a mixed Pd(II) pincer, [Pd{κ3(C,N,N)}(μ-OC(O)tBu)Pd{κ3(C,N,N)}] (15), and this observation indirectly supports the mechanism proposed for the formation of 5–7. The mechanisms of formation of 9–12 were verified by in situ reaction involving 7 and PPh3 (1 and 2 equiv) through 19F NMR spectroscopy. The new compounds were fully characterized, and molecular structures of 1, 3·C7H8, 4–6, 8·C7H8, 9, and 10·H2O were determined by SCXRD. The CNN tridentate guanidinate ligands were present in monoanionic (3·C7H8, 4 and 8·C7H8), mono- and dianionic (5 and 6), and dianionic (9 and 10·H2O) forms, and hence the structures of 5 and 6 are unprecedented. Complex 3·C7H8 was used as an efficient catalyst in Suzuki–Miyaura reactions involving sterically hindered aryl bromides and aryl boronic acids.

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