Transition Metal Diazoalkane Complexes. Synthesis, Structure, and Photochemistry of Rh[C(N_2)SiMe_3](PEt_3)_3

Abstract

Reaction of RhCl(PR_3)_n (R = Me (n = 4), Et (n = 3)) and RhCl(CO)(PEt_3)_2 with (trimethylsilyl)diazomethyl lithium at −78 °C in ether yields the three complexes Rh[C(N_2)SiMe_3](PEt_3)_3 (1), Rh[C(N_2)SiMe_3](PMe_3)_4 (2), and Rh[C(N_2)SiMe_3](CO)(PEt_3)_2 (3). 2 could not be isolated as a solid at room temperature but 1 was precipitated as red crystals that were stable enough to be handled under argon. X-ray work on 1 reveals a tetrahedrally distorted square-planar geometry with the planar (trimethylsilyl)diazomethyl ligand roughly perpendicular to the P3RhC coordination plane. This distortion makes the PEt3 ligands nonequivalent in the crystal and produces an ABB‘X pattern in the solid-state ^(31)P NMR spectrum. Photolysis of Rh[C(N_2)SiMe_3](PEt_3)_3 leads quantitatively to the dimer [Rh{C(SiMe_3)(PEt_3)}(PEt_3)_2]_2 (4). The presence of the two ylide bridges and terminal phosphines is deduced from the COSY ^(31)P−^(31)P NMR spectrum. This photochemical reactivity suggests that the transient carbene (PEt_3)_3RhC̈(SiMe_3) is electrophilic, which is typical of a singlet carbene. We believe the singlet state is stabilized by the presence of the electron-rich low-spin Rh(PEt3)3 fragment. Reaction with ^nBuNC and ^tBuNC leads to stereo- and regioselective formation of a triazole that is σ bonded to the rhodium center. The X-ray structure of the ^tBuNC derivative Rh[CC(SiMe_3)N_2N^tBu](^tBuNC)_2(PEt_3) (5) shows a distorted square-planar geometry around Rh with the planar triazolato ligand roughly orthogonal to this plane. The probable reaction mechanism involves addition and substitution reactions of isocyanides at Rh followed by insertion into the Rh−C bond.