Substitution chemistry of sterically demanding boryl ligands

Transition-Metal Complexes of Boron—New Insights and Novel Coordination Modes

Since the publication of the last review in 1998, the transition-metal chemistry of boron has continued to raise unceasing interest. Boryl complexes, representing the most extensive subclass, have remained a focus of intense research, particularly for their implication in the metal-mediated functionalization of organic substrates. Absolute novelties such as borane complexes and terminal borylene complexes have been structurally authenticated. Upon further elaboration of these compounds, the known coordination modes of boron-based ligands have grown considerably. Combined structural and theoretical investigations have contributed to elucidate the fundamental electronic characteristics of the transition-metal-boron bond and are leading to applications of these compounds. The most useful synthetic strategies for the generation of transition-metal-boron bonds are highlighted here, and the most recent and intriguing compounds that have been reported are outlined and discussed.

Synthetic and reaction chemistry of heteroatom stabilized boryl and cationic borylene complexes

The synthesis, spectroscopic and structural characterization of the aryloxy and amino functionalized chloroboryl complexes (eta(5)-C(5)R(5))Fe(CO)(2)B(OMes)Cl (R = H, 2a ; R = Me, 3a) and (eta(5)-C(5)H(5))Fe(CO)(2)B(N(i)Pr(2))Cl (7a) are reported. Compound 2ais shown to be a versatile substrate for further boron-centred substitution chemistry leading to the asymmetric boryl complexes (eta(5)-C(5)H(5))Fe(CO)(2)B(OMes)ER(n) [ER(n) = OC(6)H(4)(t)Bu-4, 2c; ER(n) = SPh, 2d] with retention of the metal-boron bond. The reactivities of 2a, 3a and 7a towards the halide abstraction agent Na[BAr(f)(4)] have also been examined, in order to investigate the potential for the generation of cationic heteroatom-stabilized terminal borylene complexes. The application of this methodology to the mesityloxy derivatives and gives rise to B-F containing products, presumably via fluoride abstraction from the [BAr(f)(4)](-) counter-ion. By contrast, amino-functionalized complex 7a is more amenable to this approach, and the thermally robust terminal aminoborylene complex [(eta(5)-C(5)H(5))Fe(CO)(2)B(N(i)Pr(2))][BAr(f)(4)] (9) can be isolated in ca. 50% yield. The reactivity of 9towards a range of nucleophilic and/or unsaturated reagents has been examined, with examples of addition, protonolysis and metathesis chemistries having been established.

Carbonyl analogues? Analysis of Fe–E (E = B, Al, Ga) bonding in cationic terminal diyl complexes by density functional theory

A series of DFT calculations has been carried out with the aim of characterizing the metal-group 13 element interaction in the novel cationic borylene complex [(eta5-C2Me5)Fe(CO)2(BMes)]+ (1) and related species of the type [eta5-C5R5)M(L)2(EX)]n+. In addition, comparisons have been made with charge neutral borylene complexes and with related group 14 based ligand systems (e.g. cationic metal carbonyls, carbenes and vinylidenes) for which models of bonding have previously been established. In this regard particular attention has been focused on the interpretation of (i) molecular orbital composition; (ii) bond dissociation energies (BDEs) and the ratio of ionic to covalent contributions (DeltaEelstat/DeltaEorb); and (iii) sigma and pi symmetry covalent contributions. The molecular orbital compositions for the prototype borylene complex 1 and for related cationic and neutral systems [e.g.[(eta5-C5H5)Fe(PMe3)2(BMes)]+ and (eta5-C5H5)Mn(CO2(BMes)]] are consistent with the presence of bonding interactions between metal and borylene fragments of both sigma and pi symmetry. Furthermore, on the basis of BDEs, DeltaEorb values and sigma/pi covalent ratios, the bonding in cationic terminal borylene complexes such as 1 appears to have as much right to be termed a M=E double bond as does that in archetypal Fischer carbene and related complexes such as [(eta5-C5R5)Fe(CO)2(CCMe2)]+ and [(eta5-C5R5)Fe(CO)2(CH2)]+.