Synthesis and structures of the ytterbium(II) β-diketiminates [Yb{N(SiMe3)C(R2)C(H)C(R4)N(SiMe3)}2] (R2 = R4 = Ph, C6H4Me-4, or C6H4Ph-4;: or R2 = C6H4Me-4, R4=1-adamantyl)

Crystalline, homoleptic mononuclear ytterbium(II) beta-diketiminates [Yb{N(SiMe3)C(R-2)C(H)C(R-4)N(SiMe3)}(2)] (R-2 = R-4 = Ph 1, R-2 = R-4 = Tol 2, R-2 = R-4 = Dph 3, or R-2 = Tol and R-4 = Ad 4) (Tol = C6H4Me-4, Dph = C6H4Ph-4, Ad = 1-adamantyl) have been prepared. They have a characteristic 171 Yb{H-1} chemical shift in the region delta 2650 +/- 200 relative to [Yb(eta(5)-C5Me5)(2)(thf)], although for 2 and 3 this was only observed at low temperatures, indicative of a fast fluxional process at ambient temperature; data recorded ealier for 1 are shown to have been in error. The H-1 NMR spectra of 4 showed that two isomers were present in toluene solution, in a ratio of ca. 3 : 2, which interconverted on the spin saturation transfer scale of ca. 1 s(-1). NOE data are presented for each of 1-4; these led to (i) assignments of the two types of SiMe3 groups (adjacent to Tol or Ad) and (ii) the conclusion that the two isomers are conformers, one of which probably corresponds to that found in the crystal. The molecular structures of each of 1, 3 and 4 have the ytterbium in a distorted tetrahedral environment, the two ligand planes (1 and 3) or boats (including the Yb atom, 4) approximate to being either orthogonal (1 and 3) or parallel (4). The ligand-to-metal bonding is close to kappa(2) (1, 3) or eta(5) (4) and the ligands are both pi-delocalised (1, 3) or only one of them in 4.