Catalytic B-N dehydrogenation using frustrated Lewis pairs: evidence for a chain-growth coupling mechanism

The catalytic dehydrogenation of ammonia- and amine-boranes by a dimethylxanthene-derived frustrated Lewis pair is described. Turnover is facilitated on a thermodynamic basis by the ready release of H2 from the weakly basic PPh2-containing system. In situ NMR studies and the isolation of intermediates from stoichiometric reactions support a mechanism initiated by B-H activation, followed by end-growth BN coupling involving the terminal NH bond of the bound BN fragment and a BH bond of the incoming borane monomer

N-nacnac stabilized tetrylenes: access to silicon hydride systems via migration processes

The use of an amino-functionalized β-diketiminate (′N-nacnac′) ligand in low-valent silicon chemistry is investigated. In particular, the use of (LDipp)SiCl (LDipp=HC{(Me2N)CNDipp}2) to generate silicon-containing products via metathesis chemistry is explored, in light of previously reported complications arising from heterocycle ring contraction. In the case of Na[C5H5], chloride metathesis is accompanied not by rearrangement of the N-nacnac ligand, but by a C-to-Si hydrogen migration process, generating the hydridosilicon(IV) species (LDipp)Si(H)(C5H4), which features a silafulvene core. The potential intermediate arising from initial chloride/cyclopentadienide substitution can be modelled by the chemistry of the corresponding Ge(II) and Sn(II) systems, which generate (LDipp)E(η1−C5H5) (E=Ge, Sn) via straightforward metathesis chemistry. A Si(II) hydride species can be generated from (LDipp)SiCl via metathesis by making use of a d-block reagent which can act as both hydride source and coordinative trap for (LDipp)SiH. Thus, the reaction of (LDipp)SiCl with K[(η5−C5H4Me)Mn(CO)2H] leads to the formation of (η5−C5H4Me)Mn(CO)2{Si(H)LDipp} – the first silylene complex containing this half-sandwich manganese fragment

N‐nacnac stabilized tetrelenes: formation of an N,P‐Heterocyclic germylene via C–C bond Insertion

The use of an amino‐functionalized β‐diketiminate (“N‐nacnac”) ligand in low‐valent germanium chemistry is reported, with a view to comparison with “conventional” nacnac systems. Transmetallation of the N‐nacnac ligand from lithium allows access to a versatile chlorogermylene system, and subsequent substituent exchange processes are used to generate related hydrido‐, and phosphaketenyl‐germylenes. The latter undergoes photolytically‐induced cleavage of the P–CO bond to yield an unusual imine‐coordinated N,P‐heterocyclic germylene. On the basis of DFT calculations this transformation is proposed to occur via concerted attack by the electron‐rich carbon–carbon bond of the N‐nacnac backbone accompanying CO loss, rather than via the generation of a free phosphinidene

Reversible borohydride formation from aluminium hydrides and {H(9-BBN)}2: structural, thermodynamic and reactivity studies

A series of novel β-diketiminate stabilised aluminium borohydrides of the type (Nacnac)Al(R){H2(9-BBN)} has been synthesised offering variation in both the auxiliary R substituent and in the Nacnac backbone itself. A number of these complexes show unusual dissociation of the borane from the aluminium hydride in solution under ambient conditions. The lability of the borane is shown (by variable temperature NMR analyses) to be influenced by the electronic character of both the aluminium-bound R substituent and the Nacnac ligand itself, such that electron-withdrawing substituents lead to greater dissociation of the borane. Comparison of these complexes with related systems featuring the tetrahydroborate [BH4]− ligand illustrates the impact of the boron-bound substituents on the ability of the borane fragment to dissociate from the aluminium hydride. This dissociative behaviour is shown to be highly influential on the ability of the borohydride complexes to reduce carbon dioxide in a stoichiometric manner

N‐nacnac stabilized tetrelenes: formation of an N,P‐Heterocyclic germylene via C–C bond Insertion

The use of an amino‐functionalized β‐diketiminate (“N‐nacnac”) ligand in low‐valent germanium chemistry is reported, with a view to comparison with “conventional” nacnac systems. Transmetallation of the N‐nacnac ligand from lithium allows access to a versatile chlorogermylene system, and subsequent substituent exchange processes are used to generate related hydrido‐, and phosphaketenyl‐germylenes. The latter undergoes photolytically‐induced cleavage of the P–CO bond to yield an unusual imine‐coordinated N,P‐heterocyclic germylene. On the basis of DFT calculations this transformation is proposed to occur via concerted attack by the electron‐rich carbon–carbon bond of the N‐nacnac backbone accompanying CO loss, rather than via the generation of a free phosphinidene

Reversible borohydride formation from aluminium hydrides and {H(9-BBN)}2: structural, thermodynamic and reactivity studies

A series of novel β-diketiminate stabilised aluminium borohydrides of the type (Nacnac)Al(R){H2(9-BBN)} has been synthesised offering variation in both the auxiliary R substituent and in the Nacnac backbone itself. A number of these complexes show unusual dissociation of the borane from the aluminium hydride in solution under ambient conditions. The lability of the borane is shown (by variable temperature NMR analyses) to be influenced by the electronic character of both the aluminium-bound R substituent and the Nacnac ligand itself, such that electron-withdrawing substituents lead to greater dissociation of the borane. Comparison of these complexes with related systems featuring the tetrahydroborate [BH4]− ligand illustrates the impact of the boron-bound substituents on the ability of the borane fragment to dissociate from the aluminium hydride. This dissociative behaviour is shown to be highly influential on the ability of the borohydride complexes to reduce carbon dioxide in a stoichiometric manner

On the viability of catalytic turnover via Al-O/B-H metathesis: The reactivity of β-diketiminate aluminium hydrides towards CO2 and boranes

A series of β‐diketiminate stabilized aluminium hydrides of the type (Nacnac)Al(R)H has been synthesized offering variation in the auxiliary R substituent and in the Nacnac backbone. These show significant variation in the nature of the Al−H bond: electron‐donating R groups give rise to weaker (and presumably more hydridic) Al−H bonds, leading to enhanced rates of reactivity towards CO2. The resulting κ1‐formate complexes (Nacnac)Al(R){OC(O)H} have been isolated and their reactivity towards B−H‐containing reductants probed. In the case of HBpin no reaction is observed (even under forcing conditions), while the more reactive boranes HBcat and {H(9‐BBN)}2 ultimately yield boryloxy complexes of the type (Nacnac)Al(R)(OBX2) (X2=cat, 9‐BBN). However, no hint of Al−O/B−H metathesis is observed even under forcing conditions. With BH3⋅SMe2 the major product is a related boryloxy system, although (uniquely) in this case a minor product is observed which contains an Al−H bond. We hypothesize that (Nacnac)Al(R)(κ2‐BH4) is formed (despite the unfavourable thermodynamics of Al−O/B−H metathesis) due to the additional driving force provided by coordination of the strongly Lewis acidic BH3 unit to the Al−H bond. That said, we find that (unlike the analogous gallium systems) no catalytic turnover can be achieved in the reduction of CO2 by boranes mediated by these aluminium hydrides

A β-diketiminate-stabilized sila-acyl chloride: systematic access to base-stabilized silicon analogues of classical carbonyl compounds

An oxidation/substitution strategy for the synthesis of silicon analogues of classical organic carbonyl compounds is reported, by making use of a novel β-diketiminate-supported sila-acyl chloride-the first example of such a compound isolated without the use of a stabilizing Lewis acid. Nucleophilic substitution at the SiIV center allows direct access to the corresponding sila-aldehyde and sila-ester. An alternative approach utilizing the reverse order of synthetic steps is thwarted by the facile rearrangement of the corresponding SiII systems featuring either H or OR substituents. As such, the isolation of (N-nacnac)Si(O)Cl represents a key step forward in enabling the synthesis of sila-carbonyl compounds by a synthetic approach ubiquitous in organic chemistry

A β-diketiminate-stabilized sila-acyl chloride: systematic access to base-stabilized silicon analogues of classical carbonyl compounds

An oxidation/substitution strategy for the synthesis of silicon analogues of classical organic carbonyl compounds is reported, by making use of a novel β-diketiminate-supported sila-acyl chloride-the first example of such a compound isolated without the use of a stabilizing Lewis acid. Nucleophilic substitution at the SiIV center allows direct access to the corresponding sila-aldehyde and sila-ester. An alternative approach utilizing the reverse order of synthetic steps is thwarted by the facile rearrangement of the corresponding SiII systems featuring either H or OR substituents. As such, the isolation of (N-nacnac)Si(O)Cl represents a key step forward in enabling the synthesis of sila-carbonyl compounds by a synthetic approach ubiquitous in organic chemistry

Borylated N-heterocyclic carbenes: rearrangement and chemical trapping

This study details attempts to access N-heterocyclic carbenes (NHCs) featuring the diazaborolyl group, {(HCNDipp)2 B}, as one or both of the N-bound substituents, to generate strongly electron-donating and sterically imposing new carbene ligands. Attempts to isolate N-heterocyclic carbenes based around imidazolylidene or related heterocycles, are characterized by facile N-to-C migration of the boryl substituent. In the cases of imidazolium precursors bearing one N-bound diazaborolyl group and one methyl substituent, deprotonation leads to the generation of the target carbenes, which can be characterized in situ by NMR measurements, and trapped by reactions with metal fragments and elemental selenium. The half-lives of the free carbenes at room temperature range from 4-50 h (depending on the pattern of ancillary substituents) with N-to-C2 migration of the boryl function being shown to be the predominant rearrangement pathway. Kinetic studies show this to be a first-order process that occurs with an entropy of activation close to zero. DFT calculations imply that an intramolecular 1,2-shift is mechanistically feasible, with calculated activation energies of the order of 90-100 kJ mol-1 , reflecting the retention of significant aromatic character in the imidazole ring in the transition state. Trapping of the carbene allows for evaluation of steric and electronic properties through systems of the type LAuCl, LRh(CO)2 Cl, and LSe. A highly unsymmetrical (but nonetheless bulky) steric profile and moderately enhanced σ-donor capabilities (compared with IMes) are revealed