Heavy‐Element Impact on Hidden Frustrated‐Lewis‐Pair Catalysis
Daniel N. Heuer, Peter Meyer, Jan‐Hendrik Lamm, Hans‐Georg Stammler, Beate Neumann, Andreas Mix, Yury V. Vishnevskiy, Norbert W. MitzelThe hidden iminophosphorane‐frustrated Lewis pair (FLP) systems of the type (2‐[LA]‐C 6 H 4 )Ph 2 PN(SiMe 3 ) with Sn(C 2 F 5 ) 3 , Al t Bu 2 , Ga t Bu 2 , or In t Bu 2 as the Lewis acid (LA) functions were synthesized from the ortho ‐lithiated iminophosphorane [ o ‐C 6 H 4 ‐(PPh 2 NSiMe 3 )Li] 2 ·Et 2 O. All compounds were completely characterized by elemental analysis, single‐crystal X‐ray diffraction, and NMR spectroscopy. The calculated ring‐opening energies (N–LA) were largest for the hard (according to the HSAB principle) aluminum compound and lowest for the softer indium compound. All systems were employed as catalysts in the reduction of CO 2 , using the hydroboranes HBPin (to MeOBPin) and BH 3 ·SMe 2 (to MeOBO) 3 ) as reducing agents. With BH 3 ·SMe 2 , all FLPs were catalytically active, with turnover frequencies (TOFs) reflecting the trends of the ring‐opening energies In > Ga > Al. The same trend was observed using HBPin as a reducing agent, with the indium‐based compound achieving the highest TOFs and the aluminum‐based system being practically inactive. For the softer indium‐ and tin‐based FLP, the first reduction step of CO 2 towards H(O)COBPin was the fastest, and this reaction afforded not only the product MeOBPin, but also the mono‐reduced intermediates.