Systematic experimental and quantum chemical investigation into the structures, the stability, and the spectroscopic properties of alkylindium(I) compounds:: Tetrameric In4[C(SiMeRR′)3]4 versus monomeric InC(SiMeRR′)3 derivatives

The reaction of indium monobromide (InBr) with LiC(SiMeRR')(3). xTHF gives in a high yield alkylindium(I) derivatives with the indium atoms in the unusual low oxidation state of +1. The properties of these products are investigated by the systematic variation of the steric demand of the C(SiMeRR')(3) substitutents (R = Me, Et; R' = Me, Et, Bu-n, Pr-i, Ph). Tetrahedral In-4 clusters are observed in the solid state and in solution for smaller substituents such as C(SiMe3)(3), C(SiMe2Et)(3) (1), and C((SiMe2Bu)-Bu-n)(3) (2). As shown by two crystal structure determinations, those clusters exhibit undistorted tetrahedra of four indium atoms and short In-In distances of 3.00 Angstrom on average. In contrast, owing to the larger steric stress, complete dissociation into the monomeric formula units InR is observed in solution with the more voluminous C((SiMe2Pr)-Pr-i)(3) group (3), while in the solid state once more an almost undistorted tetrahedron of the tetramer is found, showing, however, much elongated In-In distances of 3.155 Angstrom on average. Single crystals could not be obtained for the compounds with C(SiMe2Ph)(3) (4) and C(SiEt2Me)(3) groups (5); 4 is monomeric in benzene, while 5 gives the formula mass of the dimer, and complete dissociation into the monomer is observed only in very dilute solutions. All compounds show unusual downfield shifts of the resonances of the carbon atoms bound to indium up to delta = 76 ppm, which are very characteristic of the alkylindium(I) compounds. Quantum chemical NMR chemical shift calculations using density functional theory indicate that the large shifts are related to the presence of low-lying magnetically allowed excited states, and they are further enhanced by unusually large spin-orbit effects. Optimized structure parameters for InCH3, In-4(CH3)(4), InH, and In4H4 are compared with the experimental results. Our best estimate for the tetramerization energy of InCH3 is cal 290 kJ/mol at the MP2 level.