Reversible Intramolecular Base Stabilization in Silylene (Silanediyl) Complexes: Surprising Reactivity for Silylene Coordination Compounds with a Dynamic N⋯Si⋯N Bond

The silane [2-(Me₂NCH₂)C₆H₄]₂SiCl₂ (7) shows a new dynamic N⋯Si⋯N “flip-flop” coordination mode below T_c = 233 K (ΔG^ǂ = 46.5 (±0.5) kJ mol⁻1) with both amine donors displacing each other. 7 is pentacoordinated in the solid state; crystal data: orthorhombiC., P_bca, a = 13.802(1) Å, b = 17.908(1) Å, c = 15.544(2) Å, Z = 8. Reaction of the silanesC₆H₅[2-(Me₂NCH₂)C₆H₄]SiCl₂ (6), 7, and [2-(Me₂NCH₂)-5-(t-C₄H₉)C₆H₃]₂SiCl₂ (8) with chromiumpentacarbonylmetallate yields the silanediyl complexes [2-(Me₂NCH₂)C₆H₄]C₆H₆SiCr(CO)₅ (11), [2-(Me₂NCH₂)C₆H₄]₂SiCr(CO)₅ (12), and [2-(Me₂NCH₂)-5-(t-C₄H₉)C₆H₃]₂SiCr(CO)₅ (13); [2-(Me₂NCH₂)C₆H₄]HSiCr(CO)₅ (14) was obtained by photolytic methods. Silanediyl complexes with one chelating donor show a rigid coordination of the donor to silicon which can be lifted at higher temperatures (95 °C for 11, ΔG^ǂ = 80.4 (±0.5) kJ mol⁻1). A single crystal X-ray structure determination of 11 reveals a CrSi bond distance of 2.409(1) Å and a N→Si bond length of 1.991(2) Å. Crystal data: tricliniC., P1̅, a = 9.401(1) Å, b = 10.207(1) Å, c = 11.586(1) Å, Z = 2. Silanediyl complexes with two intramolecular donor functions feature a dynamic “flip-flop” coordination of the amine groups to silicon. Both (dimethylamino)phenyl groups in 12 and 13 can be detached from silicon under liberation of a three-coordinate silicon atom at T_c = 58 °C (VT-¹H-NMR), ΔG^ǂ = 67.1 (±0.5) kJ mol⁻¹ for 12 and T_c = 61 °C., ΔG^ǂ = 70.1 (±0.5) kJ mol⁻¹ for 13. A single crystal X-ray structure determination of 12 gives 2.408(1) Å for the SiCr bond length and 2.046(2) Å for the N1Si dative bond (the second contact N2Si has a nonbonding distance of 3.309 Å; the sum of bond angles at silicon amounts to 351.3°). Crystal data: tricliniC., P1̅, a = 9.531(1) Å, b = 10.339(1) Å, c = 11.676(1) Å, Z = 2. The donor in 12 has been functionalized at the nitrogen atom by protonation or complexation with BF₃. Photolysis of 12 and 13 leads to a 1,2-amine shift of one donor from silicon to the metal with loss of CO. The product [formula omitted] (20) has been characterized by a single crystal X-ray structure determination and has a bond distance CrSi 2.3610(4) Å and N1Si 1.981(1) Å. Crystal data: monocliniC., P2₁/C., a = 10.344(5) Å, b = 11.761(3) Å, c = 18.96(1) Å, Z = 4. Furthermore, the silanediyl complex 12 has been immobilized on silica gel. IR and UV spectroscopy and ¹³C CPMAS NMR provide evidence for the fixation of (silicaO)[2-(HNMe₂CH₂)C₆H₄][2-(Me₂NCH₂)C₆H₄]SiCr(CO)₅ (22) to the surface via the silicon atom. Reaction of 13 with H₂O leads to (HO)[2-(HNMe₂CH₂)C₆H₄][2-(Me₂NCH₂)C₆H₄]SiCr(CO)₆ (23) which has a structure similar to 22 with a CrSi bond length of 2.469(2) µ. The dimethylamino or dimethylammonium substituent is pointing away from the silicon atom forming hydrogen bridges between N1⋯HO⋯HN2; crystal data: monocliniC., P2₁/n, a = 13.198(2) Å, b = 17.017(2) Å, c = 17.066(1) Å, Z = 4.