Unusual [Pt{κ2(C,N)}]+ → [Pt{κ2(N,N)}]+ Coordination Mode Flip of the Guanidinate(1−) Ligand in Cationic N,N′,N″‑Tris(3,5-xylyl)guanidinatoplatinum(II) Bis(phosphine) Complexes. Syntheses, Structural and Theoretical Studies

Separate reactions of [Pt­{κ2(C,N)}­X­(S­(O)­Me2)] (κ2(C,N) = N,N′,N″-triarylguanidinate­(1−); Ar = 2-(MeO)­C6H4 (3), 2-MeC6H4 (4), 4-MeC6H4 (5), 2,4-Me2C6H3 (6), 2,5-Me2C6H3 (7), and 3,4-Me2C6H3 (8); X = Cl (3), OC­(O)­CF3 (4–8)) with 1,1-bis­(diphenylphosphino)­methane (dppm) afforded the respective cationic complexes [Pt­{κ2(C,N)}­{κ2(dppm)}]­[X] (10–15) in high yields. In contrast, the separate reactions of [Pt­{κ2(C,N)}­(OC­(O)­CF3)­(S­(O)­Me2)] (κ2(C,N) = N,N′,N″-tris­(3,5-xylyl)­guanidinate­(1−); 9) with dppm and 1,2-bis­(diphenylphosphino)­ethane (dppe) afforded the cationic guanidinatoplatinum­(II) complexes [Pt­{κ2(N,N)}­{κ2(P,P)}]­[OC­(O)­CF3] (P,P = dppm (16), dppe (17), respectively) in high yields. Further, the separate reactions of 9 with dppe and 1,3-bis­(diphenylphosphino)­propane (dppp) afforded the cationic complexes [Pt­{κ2(C,N)}­{κ2(P,P)}]­[OC­(O)­CF3] (P,P = dppe (18), dppp (19), respectively) in high yields. The salt metathesis reaction of 19 with an excess of NH4PF6 afforded the cationic complex [Pt­{κ2(C,N)}­{κ2(dppp)}]­[PF6] (20) in good yield. The new Pt­(II) complexes were characterized by elemental analyses, mass spectrometry, IR, multinuclear (195Pt, 31P, 1H, 13C, and 19F) NMR spectroscopy, and conductivity measurements. Further, the molecular structures of 10·CHCl3, 11·CHCl3, 12·CH2Cl2, 14·1.5C7H8, 15·CH2Cl2, 16, 17, 18·0.5C7H8, 19·C4H10O, and 20·C7H8 were determined by single-crystal X-ray diffraction. To the best of our knowledge, 17 and 18·0.5C7H8 represent the first pair of crystallographically characterized metallacyclic structural isomers to be reported. The reactions of 9 with dppm and dppe carried out separately in CDCl3 at RT were monitored by variable-time 31P NMR spectroscopy, which revealed the formation of a transient species, [Pt­{κ2(C,N)}­{κ2(dppm)}]­[OC­(O)­CF3] (22), and 18 before finally forming the end products 16 and 17, respectively. The contributions of various factors in dictating the coordination flip of the guanidinate(1−) ligand from κ2(C,N) in 22 and 18 to κ2(N,N) in 16 and 17, respectively, were unraveled. The mechanism associated with the coordination flip of the guanidinate(1−) ligand was mapped with the aid of DFT calculations on a model complex, [Pt­{κ2(C,N)}­{κ2(P,P)}]­[OC­(O)­CF3] (22′), which revealed a pathway involving a Wheland intermediate, F, and further point out that the product, [Pt­{κ2(N,N)}­{κ2(P,P)}]­[OC­(O)­CF3] (16′), is a kinetically controlled product.