Although reversible covalent activation of molecular hydrogen (H2) is a common reaction at transition metal centers, it has proven elusive in compounds of the lighter elements. We report that the compound (C6H2Me3)2PH(C6F4)BH(C6F5)2 (Me, methyl), which we derived through an unusual reaction involving dimesitylphosphine substitution at a para carbon of tris(pentafluorophenyl) borane, cleanly loses H2 at temperatures above 100 degrees C. Preliminary kinetic studies reveal this process to be first order. Remarkably, the dehydrogenated product (C6H2Me3)2P(C6F4)B(C6F5)2 is stable and reacts with 1 atmosphere of H2 at 25 degrees C to reform the starting complex. Deuteration studies were also carried out to probe the mechanism.

The concept of "frustrated Lewis pairs" involves donor and acceptor sites in which steric congestion precludes Lewis acid-base adduct formation. In the case of sterically demanding phosphines and boranes, this lack of self-quenching prompts nucleophilic attack at a carbon para to B followed by fluoride transfer affording zwitterionic phosphonium borates [R(3)P(C(6)F(4))BF(C(6)F(5))(2)] and [R(2)PH(C(6)F(4))BF(C(6)F(5))(2)]. These can be easily transformed into the cationic phosphonium-boranes [R(3)P(C(6)F(4))B(C(6)F(5))(2)](+) and [R(2)PH(C(6)F(4))B(C(6)F(5))(2)](+) or into the neutral phosphino-boranes R(2)P(C(6)F(4))B(C(6)F(5))(2). This new reactivity provides a modular route to a family of boranes in which the steric features about the Lewis acidic center remains constant and yet the variation in substitution provides a facile avenue for the tuning of the Lewis acidity. Employing the Gutmann-Beckett and Childs methods for determining Lewis acid strength, it is demonstrated that the cationic boranes are much more Lewis acidic than B(C(6)F(5))(3), while the acidity of the phosphine-boranes is diminished.

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Just like transition metal complexes, N-heterocyclic carbenes (NHCs) promote the aggregation of white phosphorus (P4) as shown by the high yield synthesis of a P12 cluster capped by two NHCs. The nuclearity observed is equal to that of the largest phosphorus cluster prepared using transition metals, but the architecture of the P12 core is entirely novel.

For many years, it was believed that only transition-metal centers could activate small molecules and enthalpically strong bonds. However, it has recently been shown that several nonmetallic systems are capable of some of these tasks. [1,2] For example, stable singlet carbenes can activate CO, [3a] H 2 , [3b] and P 4 .[3c-e] Such reactions have long been known for transition metals. [4,5] However, stable singlet carbenes can also activate NH 3 ;[3b] a much more difficult task for transition metals. [6,7] The oxidative addition of hydrosilanes, hydroboranes, and hydrophosphines at vacant coordination sites of transition metals are well-exemplified and are considered as key steps in the transition-metal-catalyzed hydrosilylation, hydroboration, and hydrophosphination of multiple bonds.[8]Herein, we report the first examples of the activation of E À H bonds (E = Si, B, P) at a single nonmetal center.On the basis of our successful results with H 2 , [3b] we began our study with the activation of SiÀH bonds. Indeed, silanes are similar to H 2 in that they lack both nonbonding electron pairs and p electrons. They can bind to various metal centers to form stable Si À H s complexes, which undergo subsequent oxidative addition.[4] To test the possible activation of Si À H bonds with carbenes, we treated the cyclic (alkyl)-(amino)carbenes (CAACs) 1 a and 1 b [9] with primary, secondary, and tertiary silanes.The addition of phenylsilane to 1 a and 1 b occurred readily at room temperature, and the corresponding adducts 2 a,b were isolated in 91 and 83 % yield, respectively (Scheme 1). As expected, in the case of the enantiomerically pure CAAC 1 a, two diastereomers 2 a,a' were formed (in a 2:1 ratio), as shown by two singlets at d = À36.4 and À29.3 ppm in the 29 Si NMR spectrum. The 13 C NMR spectrum revealed the loss of the carbene signal and a new C À H peak at d = 63.2 (2 a) and 65.5 ppm (2 b). The 1 H NMR spectrum of the major isomer 2 a revealed a pseudotriplet at d = 4.78 ppm (SiCH) and two doublets at d = 4.29 and 4.21 ppm corresponding to the diastereotopic hydrogen atoms of the SiH 2 fragment. The structure of 2 a was confirmed by X-ray crystallography [10] (Figure 1, top), whereas the presence of a triplet at d = 4.53 ppm and a doublet at d = 4.08 ppm in the 1 H NMR spectrum confirmed the identity of adduct 2 b.CAACs 1 a,b also reacted with (EtO) 3 SiH to afford 3 a (d.r. 3:1) and 3 b in 64 and 73 % yield, respectively. However, when Ph 2 SiH 2 was used, only the less bulky carbene 1 b underwent insertion into the Si À H bond (to give 4 b in 65 % yield), and a reaction time of 16 hours at 80 8C was necessary for the reaction to reach completion. Surprisingly, although it has been shown that, in contrast to CAACs, N-heterocyclic carbenes (NHCs) do not react with H 2 , [11] we found that imidazolidin-2-ylidene 5[12] also reacted at room temperature with phenylsilane to afford the Si À H insertion product 6 in 88 % yield (Figure 1, bottom).