Cyclopentadienylphosphazene (CpPN) Complexes of Rare-Earth Metals: Synthesis, Structural Characterization, and Hydroamination Catalysis

Synthesis of the first series of rare-earth-metal constrained geometry complexes containing the P-(1-adamantylamino)-P-dimethyl-tetramethyl-cyclopentadienylidene-phosphorane ligand C(5)Me(4)PMe(2)NHAd, {(CpPN)-P-#}H, was accomplished. This monoanionic chelate ligand is isoelectronically related to the classical dianionic cyclopentadienyl-silylamine ligand C5Me4HSiMe2-NHtBu, {(CpSiN)-Si-#}H-2. The ligand stabilizes dialkyls [{(CpPN)-P-#}M(CH2SiMe3)(2)] (M = Sc, 1; Lib 2; Y, 3; Sm, 4; Nd, 5; Pr, 6; Ce, 7) over the full range of group 3 and lanthanide cation radii. Results of NMR studies of these crystalline alkyls, XRD molecular structures, and a preliminary study revealing the high catalytic activity of complexes 3-6 in the intramolecular hydroamination/cyclization are reported. The catalytic experiments reveal a trend in activity Lu < Y < Sm < Nd <= Pr resembling the trend in rare-earth-metal radii. Interestingly they reveal a distinctive substrate-dependent first-order kinetic profile for all metals investigated. The reaction of the precatalyst 3 with 1.6 equiv of the standard substrate 2,2-dimethylpenten-4-ylamine leads to a fast and selective formation of substrate complex [{(CpPN)-P-#}Y(NHCH2CMe2CH2CH-=CH2)(2)] (8). Fast cyclization was observed only after addition of more than 2 equiv of amine substrate. A noninsertive mechanism involving a six-membered transition state by a concerted C-N bond formation and N-H bond cleavage at a 3:1 substrate to complex ratio is suggested on the basis of these findings.