Frustrated Lewis Pairs beyond the Main Group: Cationic Zirconocene–Phosphinoaryloxide Complexes and Their Application in Catalytic Dehydrogenation of Amine Boranes

Abstract

The cationic zirconocene-phosphinoaryloxide complexes [Cp(2)ZrOC(6)H(4)P(t-Bu)(2)][B(C(6)F(5))(4)] (3) and [Cp*(2)ZrOC(6)H(4)P(t-Bu)(2)][B(C(6)F(5))(4)] (4) were synthesized by the reaction of Cp(2)ZrMe(2) or Cp*(2)ZrMe(2) with 2-(diphenylphosphino)phenol followed by protonation with [2,6-di-tert-butylpyridinium][B(C(6)F(5))(4)]. Compound 3 exhibits a Zr-P bond, whereas the bulkier Cp* derivative 4 was isolated as a chlorobenzene adduct without this Zr-P interaction. These compounds can be described as transition-metal-containing versions of linked frustrated Lewis pairs (FLPs), and treatment of 4 with H(2) under mild conditions cleaved H(2) in a fashion analogous to that for main-group FLPs. Their reactivity in amine borane dehydrogenation also mimics that of main-group FLPs, and they dehydrogenate a range of amine borane adducts. However, in contrast to main-group FLPs, 3 and 4 achieve this transformation in a catalytic rather than stoichiometric sense, with rates superior to those for previous high-valent catalysts.

Frustrated Lewis Pairs

“…179 This situation is highly reminiscent of bifunctional transition metal catalysts. In this regard it is also important to draw attention to transition metal based FLPs in which metal centers with pendant donors activate a range of small molecule substrates, which has been explored by the groups of Wass [126][127][180][181][182][183] and Erker 128-129,184-188 among others. 130 The concept of FLPs has also been exploited to describe a variety of systems that activate small molecules.…”

Experimental and Theoretical Studies of the Potential Interconversion of the Amine–Borane iPr2NH·BH(C6F5)2 and the Aminoborane iPr2N=B(C6F5)2 Involving Hydrogen Loss and Uptake

“…Previous work has also demonstrated the use of frustrated Lewis pairs (FLPs) [76] in stoichiometric [77,78] and catalytic [25] quantities in the dehydrogenation of various amine-borane adducts. We therefore attempted the dehydrogenation of amine-borane 1 using the combination of nBu 3 SnOTf/TMP (TMP = 2,2,6,6-tetramethylpiperidine) at 20°C.…”

“…[4][5][6][7][8][9][10] More recently, the range of metal complexes reported to be capable of catalytically dehydrogenating amine-boranes has been significantly expanded. For example, homogeneous systems based on Rh, [11] Ru, [12,13] Re, [14] Ni [15,16] and Ir [17][18][19] are now known, along with those based on Group 2 and 4 metals, [20][21][22][23][24][25] and systems based on p-block metals. [26] In addition, photoactivated catalysts of the first-row transition metals have also recently been reported.…”

Cationic Group 4 Metallocene–(o‐Phosphanylaryl)oxido Complexes: Synthetic Routes to Transition‐Metal Frustrated Lewis Pairs

“…[6,7] A great deal of work has also focused on extending the range of maingroup FLPs to other main-group Lewis acids (e.g. Our initial results have established the analogy with main-group frustrated pairs, [16,17] [a] School of Chemistry, University of Bristol, Cantock's Close, Bristol, BS8 1TS, United Kingdom E-mail: duncan.wass@bristol.ac.uk the case of 2 and 3, this method leads to side reactions. Linking the two components of the FLP into a single amphoteric molecule has also led to interesting results.…”