Rare Carbon-Bridged Bimetallic Lanthanide (Nd or Sm) and Tl(I) Geminal Carbon Derivatives of a Bis(iminophosphorano)methanediide

The bimetallic carbon-bridged bis­(iminophosphorano)­methanediide complexes [CpNd­(Tl)­{C­(Ph₂PNSiMe₃)₂}­Cl₂Li­(THF)₂]·Et₂O (2) and [SmCl₃Tl­{C­(Ph₂PNSiMe₃)₂}­Li­(THF)₂]₂ (3) were prepared from reactions of the dilithiated bis­(iminophosphorano)­methanediide [Li₂L] (1; L = C^2–(Ph₂PNSiMe₃)₂) with Nd and Sm chlorides and with CpTl and TlCl, respectively, in THF. Both final products contain coordinated Li linked with Cl bridges. The complexes have been structurally characterized and show that the bridging methanediide backbone carbon of the ligand is substituted with one Tl­(I) and one rare-earth atom and has a tetrahedral geometry. The ³¹P­{¹H} NMR spectrum (THF-d₈) of 2 shows a broad signal at 13.75 ppm with a two-bond spin–spin coupling (²J_Tl–P = 410 Hz), indicating that the C–Tl bond is maintained in solution. The ³¹P­{¹H} NMR spectrum of 3 was less informative because the complex is more paramagnetic, and thus only a very broad single peak was observed. DFT calculations on the model compounds [CpNd­(Tl)­{C­(PhMePNSiH₃)₂}­Cl₂Li­(THF)₂] (2a) and [SmTl­{C­(PhMePNSiH₃)₂Cl₂]₂ (3a), whose crystal structures are based on those for 2 and 3, were performed; the results clearly demonstrate the bonding characteristics among the metal centers (Nd­(III), Sm­(III), Tl­(I)) and bridged carbon. In contrast to most Tl­(I) compounds, 2 and 3 are reasonably stable in air; this is attributed to protection of the Tl­(I) by some of the phenyl substituents on the phosphorus through π-bonding interactions. X-ray and DFT data indicate that, for 2, Tl­(I) is a five-coordinate center through one strong carbene carbon bond and four weak phenyl carbon bonds, while for 3, Tl­(I) is six-coordinate through strong carbene carbon and chloride bonds and four weak phenyl carbon bonds. These complexes are the first examples of a single carbon or carbene forming a bridge between the main-group heavy metal Tl­(I) and another rare-earth metal.