Heterolytic Cleavage of Dihydrogen by Frustrated Lewis Pairs Derived from α-(Dimesitylphosphino)ferrocenes and B(C6F5)3

Exploring the Reactivity of B-Connected Carboranylphosphines in Frustrated Lewis Pair Chemistry: A New Frame for a Classic System

The primary phosphines MesPH2 and tBuPH2 react with 9-iodo-m-carborane yielding B9-connected secondary carboranylphosphines 1,7-H2 C2 B10 H9 -9-PHR (R=2,4,6-Me3 C6 H2 (Mes; 1 a), tBu (1 b)). Addition of tris(pentafluorophenyl)borane (BCF) to 1 a, b resulted in the zwitterionic compounds 1,7-H2 C2 B10 H9 -9-PHR(p-C6 F4 )BF(C6 F5 )2 (2 a, b) through nucleophilic para substitution of a C6 F5 ring followed by fluoride transfer to boron. Further reaction with Me2 SiHCl prompted a H-F exchange yielding the zwitterionic compounds 1,7-H2 C2 B10 H9 -9-PHR(p-C6 F4 )BH(C6 F5 )2 (3 a, b). The reaction of 2 a, b with one equivalent of R'MgBr (R'=Me, Ph) gave the extremely water-sensitive frustrated Lewis pairs 1,7-H2 C2 B10 H9 -9-PR(p-C6 F4 )B(C6 F5 )2 (4 a, b). Hydrolysis of the B-C6 F4 bond in 4 a, b gave the first tertiary B-carboranyl phosphines with three distinct substituents, 1,7-H2 C2 B10 H9 -9-PR(p-C6 F4 H) (5 a, b). Deprotonation of the zwitterionic compounds 2 a, b and 3 a, b formed anionic phosphines [1,7-H2 C2 B10 H9 -9-PR(p-C6 F4 )BX(C6 F5 )2 ]- [DMSOH]+ (R=Mes, X=F (6 a), R=tBu, X=F (6 b); R=Mes, X=H (7 a), R=tBu, X=H (7 b)). Reaction of 2 a, b with an excess of Grignard reagents resulted in the addition of R' at the boron atom yielding the anions [1,7-H2 C2 B10 H9 -9-PR(p-C6 F4 )BR'(C6 F5 )2 ]- (R=Mes, R'=Me (8 a), R=tBu, R'=Me (8 b); R=Mes, R'=Ph (9 a), R=tBu, R'=Ph (9 b)) with [MgBr(Et2 O)n ]+ as counterion. The ability of the zwitterionic compounds 3 a, b to hydrogenate imines as well as the Brønsted acidity of 3 a were investigated.

Geometrical influence on the non-biomimetic heterolytic splitting of H2 by bio-inspired [FeFe]-hydrogenase complexes: a rare example of inverted frustrated Lewis pair based reactivity

Despite the high levels of interest in the synthesis of bio-inspired [FeFe]-hydrogenase complexes, H2 oxidation, which is one specific aspect of hydrogenase enzymatic activity, is not observed for most reported complexes. To attempt H-H bond cleavage, two disubstituted diiron dithiolate complexes in the form of [Fe2(μ-pdt)L2(CO)4] (L: PMe3, dmpe) have been used to play the non-biomimetic role of a Lewis base, with frustrated Lewis pairs (FLPs) formed in the presence of B(C6F5)3 Lewis acid. These unprecedented FLPs, based on the bimetallic Lewis base partner, allow the heterolytic splitting of the H2 molecule, forming a protonated diiron cation and hydrido-borate anion. The substitution, symmetrical or asymmetrical, of two phosphine ligands at the diiron dithiolate core induces a strong difference in the H2 bond cleavage abilities, with the FLP based on the first complex being more efficient than the second. DFT investigations examined the different mechanistic pathways involving each accessible isomer and rationalized the experimental findings. One of the main DFT results highlights that the iron site acting as a Lewis base for the asymmetrical complex is the {Fe(CO)3} subunit, which is less electron-rich than the {FeL(CO)2} site of the symmetrical complex, diminishing the reactivity towards H2. Calculations relating to the different mechanistic pathways revealed the presence of a terminal hydride intermediate at the apical site of a rotated {Fe(CO)3} site, which is experimentally observed, and a semi-bridging hydride intermediate from H2 activation at the Fe-Fe site; these are responsible for a favourable back-reaction, reducing the conversion yield observed in the case of the asymmetrical complex. The use of two equivalents of Lewis acid allows for more complete and faster H2 bond cleavage due to the encapsulation of the hydrido-borate species by a second borane, favouring the reactivity of each FLP, in agreement with DFT calculations.

Heterolytic Splitting of Molecular Hydrogen by Frustrated and Classical Lewis Pairs: A Unified Reactivity Concept

Using a set of state-of-the-art quantum chemical techniques we scrutinized the characteristically different reactivity of frustrated and classical Lewis pairs towards molecular hydrogen. The mechanisms and reaction profiles computed for the H₂ splitting reaction of various Lewis pairs are in good agreement with the experimentally observed feasibility of H₂ activation. More importantly, the analysis of activation parameters unambiguously revealed the existence of two reaction pathways through a low-energy and a high-energy transition state. An exhaustive scrutiny of these transition states, including their stability, geometry and electronic structure, reflects that the electronic rearrangement in low-energy transition states is fundamentally different from that of high-energy transition states. Our findings reveal that the widespread consensus mechanism of H₂ splitting characterizes activation processes corresponding to high-energy transition states and, accordingly, is not operative for H₂-activating systems. One of the criteria of H₂-activation, actually, is the availability of a low-energy transition state that represents a different H₂ splitting mechanism, in which the electrostatic field generated in the cavity of Lewis pair plays a critical role: to induce a strong polarization of H₂ that facilities an efficient end-on acid-H₂ interaction and to stabilize the charge separated "H⁺-H^-" moiety in the transition state.

Frustrated Lewis pair chemistry of chiral (+)-camphor-based aminoboranes

Dimethylamino-(+)-camphorenamine reacted with an equimolar amount of Piers' borane, HB(C6F5)2, to give the corresponding iminium-hydroborate zwitterionic salt. Being in equilibrium with the parent enamine-HB(C6F5)2 N-B pair, this salt was able to split hydrogen heterolytically, hydrogenating the iminium group in the molecule. Detailed studies revealed that the hydrogen splitting in this reaction proceeded through an intermolecular pathway leading to a bornylamine-HB(C6F5)2 adduct. When the starting enamine is present in excess over HB(C6F5)2, the produced bornylamine-HB(C6F5)2 adduct breaks up, eliminating free bornylamine and forming the initial camphorenamine-HB(C6F5)2 pair. This results in hydrogenation of the camphorenamine framework in a catalytic fashion.

N,N-addition of frustrated Lewis pairs to nitric oxide: an easy entry to a unique family of aminoxyl radicals

The intramolecular cyclohexylene-bridged P/B frustrated Lewis pair [Mes(2)P-C(6)H(10)-B(C(6)F(5))(2)] 1b reacts rapidly with NO to give the persistent FLP-NO aminoxyl radical 2b formed by P/B addition to the nitrogen atom of NO. This species was fully characterized by X-ray diffraction, EPR and UV/vis spectroscopies, C,H,N elemental analysis, and DFT calculations. The reactive oxygen-centered radical 2b undergoes a H-atom abstraction (HAA) reaction with 1,4-cyclohexadiene to give the diamagnetic FLP-NOH product 3b. FLP-NO 2b reacts with toluene at 70 °C in an HAA/radical capture sequence to give a 1:1 mixture of FLP-NOH 3b and FLP-NO-CH(2)Ph 4b, both characterized by X-ray diffraction. Structurally related FLPs [Mes(2)P-CHR(1)-CHR(2)-B(C(6)F(5))(2)] 1c, 1d, and 1e react analogously with NO to give the respective persistent FLP-NO radicals 2c, 2d, and 2e, respectively, which show similar HAA and O-functionalization reactions. The FLP-NO-CHMePh 6b derived from 1-bromoethylbenzene undergoes NO-C bond cleavage at 120 °C with an activation energy of E(a) = 35(2) kcal/mol. Species 6b induces the controlled nitroxide-mediated radical polymerization (NMP) of styrene at 130 °C to give polystyrene with a polydispersity index of 1.3. The FLP-NO systems represent a new family of aminoxyl radicals that are easily available by N,N-cycloaddition of C(2)-bridged intramolecular P/B frustrated Lewis pairs to nitric oxide.

Synthesis of [3]ferrocenophane-bridged Cp-amido zirconium complexes and ansa-zirconocene complexes and their use in catalytic polymerisation reactions

Starting from 1,1'-diacetylferrocene the ortho-amino,α-dimethylamino[3]ferrocenophane system 3 was prepared. This was converted to the o-imino,α-Cp-derivative 6. Subsequent treatment with e.g. an excess of methyllithium followed by ZrCl(2)(NMe(2))(2)(thf)(2) and Me(2)SiCl(2) eventually gave the [3]ferrocenophane based Cp-amido zirconium complex 11b. In a similar way the [3]ferrocenophane derived fluorenyl-Cp ansa-zirconocene dichloride complex 20 was obtained. The 20-methylalumoxane (MAO) system is an active homogeneous Ziegler-Natta catalyst for ethylene polymerization. The 11b-MAO system reacts efficiently with an ethene-1-octene mixture to yield a mixture of linear polyethylene plus the ethene-1-octene copolymer. The compounds 5, 8, 10b, 11b, 13, 14, and 20 were characterized by X-ray diffraction.

Organometallic frustrated Lewis pair chemistry

Frustrated Lewis pairs are playing an increasingly important role in organometallic chemistry. Examples are presented and discussed where organometallic systems themselves serve as the Lewis base or Lewis acid components in frustrated Lewis pair chemistry, mostly through their attached functional groups. Activation of dihydrogen takes place easily in many of these systems. This may lead to the generation of novel catalyst systems but also in many cases to the occurrence of specific reactions at the periphery of the organometallic frameworks. Increasingly, FLP reactions are used to carry out functional group conversions in organometallic systems under mild reaction conditions. The limits of typical FLP reactivity are explored with selected organometallic examples, a discussion that points toward new developments, such as the discovery of facile new 1,1-carboboration reactions. Learning more and more about the broad spectrum of frustrated Lewis pair chemistry helps us to find novel reactions and applications.