The frustrated Lewis pair system consisting of 2 equiv of 2,2,6,6-tetramethylpiperidine (TMP) and tris(pentafluorophenyl)borane [B(C(6)F(5))(3)] activates carbon dioxide to form a boratocarbamate-TMPH ion pair. In the presence of triethylsilane, this species is converted to a silyl carbamate and the known ion pair [TMPH](+)[HB(C(6)F(5))(3)](-), which recently was shown to react with CO(2) via transfer of the hydride from the hydridoborate to form the formatoborate [TMPH](+)[HC(O)OB(C(6)F(5))(3)](-). In the presence of extra B(C(6)F(5))(3) (0.1-1.0 equiv) and excess triethylsilane, the formatoborate is rapidly hydrosilated to form a formatosilane and regenerate [TMPH](+)[HB(C(6)F(5))(3)](-). The formatosilane in turn is rapidly hydrosilated by the B(C(6)F(5))(3)/Et(3)SiH system to CH(4), with (Et(3)Si)(2)O as the byproduct. At low [Et(3)SiH], intermediate CO(2) reduction products are observed; addition of more CO(2)/Et(3)SiH results in resumed hydrosilylation, indicating that this is a robust, living tandem catalytic system for the deoxygenative reduction of CO(2) to CH(4).

Several diorganoscandium complexes stabilized by the β-diketiminato ligands (Ar)NC(R)-CHC(R)N(Ar) (Ar ) 2,6-iPr-C 6 H 3 ; R ) CH 3 (ligand a), R ) tBu (ligand b)) have been synthesized. Reaction of the lithium salts of the ligands with ScCl 3 ‚3THF leads to the complexes LScCl 2 (THF) n , which may be readily alkylated to form the dialkyl derivatives. Most are isolated as base-free, four-coordinate complexes. Several have been characterized via X-ray crystallography, and a detailed discussion of their structures is presented. Steric interactions between Ar and the Sc-alkyl groups force the scandium to adopt an out-ofplane bonding mode. In solution, this is manifested via a fluxional process which equilibrates the two diastereotopic alkyl groups and ligand groups as well. The barriers to this process roughly correlate with the steric bulk of the alkyl substituents. At elevated temperatures, the dialkyl derivatives LScR 2 undergo a metalation process whereby one of the alkyl groups is eliminated as RH, and a ligand iPr group is metalated in the methyl position. These reactions are first order in scandium complex, and activation parameters of ∆H q ) 19.7(6) kcal mol -1 and ∆S q ) -17(2) cal mol -1 K -1 were measured for the loss of Me 4 Si from (Ligb)-Sc(CH 2 SiMe 3 ) 2 .

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Reaction of the highly electrophilic borane B(C6F5)3 with the carbonyl Lewis bases benzaldehyde, acetophenone, ethyl benzoate, and N,N-diisopropylbenzamide led to isolation of the crystalline adducts 1-H, 1-Me, 1-OEt, and 1-NPr, respectively, in good to excellent yields (63−89%). Equilibrium measurements and exchange experiments indicated that the order of basicity (from highest to lowest) of these bases toward B(C6F5)3 follows the order N,N-diisopropylbenzamide > benzaldehyde > acetophenone > ethyl benzoate. The solution and solid-state structures were probed to rationalize these observations. In solution, the borane coordinates to the carbonyl lone pair syn to H and Me in the aldehyde and ketone adducts, as indicated by 1H/19F NOE difference experiments. The same coordination geometry was observed in the solid state upon X-ray diffraction analysis of the two adducts. The added front strain associated with the ketone adduct (C−O−B =133.6(3)° vs 126.7(5)° for the benzaldehyde complex) accounts for the observed order of basicity with these two bases. For ethyl benzoate and N,N-diisopropylbenzamide, the borane coordinates syn to the phenyl group in both solution and the solid state. In addition to the carbonyl oxygen−boron interaction, the two complexes engage in a π-stacking interaction between one of the borane C6F5 rings and the syn phenyl group. In addition to the structural proof of this interaction in the solid state, variable-temperature 19F NMR experiments suggest it is important in the solution structures of these adducts as well.

Très BN: The introduction of a BN dipole into the internal positions of pyrene (see scheme) provides a novel platform for heterocyclic fluorescent species. Comparisons of the photophysical and redox properties between the CC and BN analogues reveal both similarities and differences.