| Autor: |
Lastowski RJ; School of Chemical Sciences, University of Illinois at Urbana-Champaign, 600 South Mathews Avenue, Urbana, Illinois 61801, United States., Yarranton JT; Department of Chemistry, Michigan State University, East Lansing, Michigan 48824, United States., Zhu L; School of Chemical Sciences, University of Illinois at Urbana-Champaign, 600 South Mathews Avenue, Urbana, Illinois 61801, United States., Vogiatzis KD; Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, United States., Girolami GS; School of Chemical Sciences, University of Illinois at Urbana-Champaign, 600 South Mathews Avenue, Urbana, Illinois 61801, United States. |
| Abstrakt: |
We describe new compounds of stoichiometry M(CH 2 NMe 2 BH 3 ) 3 (M = Ti, Cr, and Co), each of which contains three chelating boranatodimethylaminomethyl (BDAM) ligands. In all three compounds, the BDAM anion, which is isoelectronic and isostructural with the neopentyl group, is bound to the metal center at one end by a metal-carbon σ bond and at the other by one three-center M-H-B interaction. The crystal structures show that the d 1 titanium(III) compound is trigonal prismatic (or eight-coordinate, if two longer-ranged M···H interactions with the BH 3 groups are included), whereas the d 3 chromium(III) compound and the d 6 cobalt(III) compounds are both fac -octahedral. The Cr and Co compounds exhibit two rapid dynamic processes in solution: exchange between the Δ and Λ enantiomers and exchange of the terminal and bridging hydrogen atoms on boron. For the Co complex, the barrier for Δ/Λ exchange (Δ G ⧧ 298 = 10.1 kcal mol -1 ) is significantly smaller than those seen in other octahedral cobalt(III) compounds; DFT calculations suggest that Bailar twist and dissociative pathways for Δ/Λ exchange are both possible mechanisms. The UV-vis absorption spectra of the cobalt(III) and chromium(III) species show that the ligand field splittings Δ o caused by the M-H-B interactions are unexpectedly large, thus placing them high on the spectrochemical series (near ammonia and alkyl groups); their nephelauxetic effect is also large. The DFT calculations suggest that these properties of M-H-B interactions are in part a consequence of their three-center nature, which delocalizes electron density away from the metal center and reduces electron-electron repulsions. |
