Synthesis and characterization of new cationic hexacoordinate silanes

Trans Element-Hydrogen Bonds: A Distinctive Difference Between Transition Metals and Main Group Elements

The change in sign of the interaction force constant between element-hydrogen stretching modes of trans-dihydrides of the d block and p block elements is analyzed for the first time. As the transition metal M approaches group 12, the higher energy symmetric trans-H-M-H vibration ν_sym approaches the energy of the antisymmetric vibration ν_asym. Crossing to group 13 elements E, the trans-H-E-H vibration ν_sym increasingly drops below ν_asym. This reversal is attributed to the d orbital that participates in the H-M-H bonding but is nonbonding in the H-E-H compounds. DFT calculations are used to probe the energetics of isoelectronic triatomic [H-M-H]ⁿ⁺ and [H-E-H]^n- to reveal this trend and also to demonstrate that the magnitude of these interactions (ν_gap) increases down groups 11, 12, and 14 but remains fairly constant for group 13. They are also used to show that this reversal is seen in the transition state for hydride transfer to CO₂ from the model compounds trans-NiH₂(porphyrin) and trans-EH₂(porphyrin), E = Si and Ge in their singlet states.

Cationic aza-macrocyclic complexes of germanium(II) and silicon(IV)

[GeCl2(dioxane)] reacts with the neutral aza-macrocyclic ligands L, L = Me3tacn (1,4,7-trimethyl-1,4,7-triazacyclononane), Me4cyclen (1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane) or Me4cyclam (1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) and two mol. equiv. of Me3SiO3SCF3 in thf solution to yield the unusual and hydrolytically very sensitive [Ge(L)][O3SCF3]2 as white solids in moderate yield. Using shorter reaction times [Ge(Me3tacn)]Cl2 and [Ge(Me3tacn)]Cl[O3SCF3] were also isolated; the preparation of [Ge(Me4cyclen)][GeCl3]2 is also described. The structures of the Me3tacn complexes show κ(3)-coordination of the macrocycle, with the anions interacting only weakly to produce very distorted five- or six-coordination at germanium. In contrast, the structure of [Ge(Me4cyclen)][O3SCF3]2 shows no anion interactions, and a distorted square planar geometry at germanium from coordination to the tetra-aza macrocycle. Crystal structures of the Si(iv) complexes, [SiCl3(Me3tacn)]Y (Y = O3SCF3, BAr(F); [B{3,5-(CF3)2C6H3}4]) and [SiHCl2(Me3tacn)][BAr(F)], obtained from reaction of SiCl4 or SiHCl3 with Me3tacn, followed by addition of either Me3SiO3SCF3 or Na[BAr(F)], contain distorted octahedral cations, with facialκ(3)-coordinated Me3tacn. The open-chain triamine, Me2NCH2CH2N(Me)CH2CH2NMe2 (pmdta), forms [SiCl3(pmdta)][BAr(F)] and [SiBr3(pmdta)][BAr(F)] under similar conditions, containing mer-octahedral cations.

Pentacoordinate silicon(IV): cationic, anionic and neutral complexes derived from the reaction of NHC→SiCl4 with highly Lewis acidic (C2F5)2SiH2

Addition of NHC→SiCl4 to the highly Lewis acidic bis(pentafluoroethyl)silane ((C2F5)2SiH2) afforded the salt [(NHC)2SiCl2H][(C2F5)2SiCl3] with pentacoordinate silicon in the cation and the anion. The anion represents the first example of a chlorosilicate structurally characterized in the solid state. In this reaction, the long sought pentacoordinate NHC-adduct of silicochloroform was identified as an intermediate and its crystal structure is presented.