5-Mercaptotetrazolyl-derived metallaboratranes

The reactions of 1-tert butyl-5-thiotetrazole (HtttBu) with Na[BH4] provides Na[H2B(tttBu)2] (Na[1]) or Na[HB(tttBu)3] (Na[2]) in refluxing THF or xylene, respectively. Treating [RuCl(Ph)(CO)(PPh3)2] with Na[2] affords the triply-buttressed ruthenaboratrane [Ru(CO)(PPh3){κ4-B,S,S',S''-B(tttBu)3}] (5) whilst Na[1] with [IrCl(CO)(PPh3)2] or [RuCl(Ph)(CO)(PPh3)2] provides rare examples of doubly-buttressed metallaboratranes [IrH(CO)(PPh3){κ3-B,S,S'-BH(tttBu)2}] (7) and [Ru(CO)(PPh3)2{κ3-B,S,S'-BH(tttBu)2}] (12), the latter via the isolable thiotetrazoylborate complex [Ru(Ph)(CO)(PPh3){κ3-H,S,S'-H2B(tttBu)2}] (11).

Synthesis and ligand substitution reactions of κ4-B,S,S′,S′′-ruthenaboratranes

Ruthenaboratranes of the form [Ru(CO)L{κ⁴-B(mt)₃}] (mt = N-methimazolyl) arise via substitution of the PPh₃ ligand in [Ru(CO)(PPh₃){κ⁴-B(mt)₃}] by L (L = PMe₂Ph, PMe₃, P(OMe)₃, P(OEt)₃, P(OPh)₃) or reactions of [RuCl(R)(CO)L_n] (R = Ph, CHCHPh; n = 2, L = PCy₃; n = 3, L = P(OMe)₃, PMe₂Ph) with Na[HB(mt)₃].

Methimazolyl based diptych bicyclo-[3.3.0]-ruthenaboratranes

The reactions of [RuCl(R)(CO)(PPh₃)₂] (R = CHCHPh, Ph) with Na[H₂B(mt)₂] (mt = N-methyl-2-mercaptoimidazolyl) transiently provide [Ru(R)(CO)(PPh₃){κ³-H,S,S′-H₂B(mt)₂}] which each evolve to the ruthenaboratrane [Ru(CO)(PPh₃)₂{κ³-B,S,S′-BH(mt)₂}](Ru→B)⁸.

Functional group migrations between boron and metal centres within transition metal-borane and -boryl complexes and cleavage of H-H, E-H and E-E' bonds

This feature article examines some of the recent advances in the chemistry of Z-type transition metal-borane and X-type transition metal-boryl complexes. It focuses on the employment of these boron-based functionalities acting as stores and transfer agents for functional groups such as hydrides, alkyl groups and aryl groups which can either be abstracted or delivered to the metal centre. The review also explores the rather novel reactivity involving the cleavage of H-H, E-H and E-E' bonds (where E and E' are a range of groups) across the transition metal-boron bond in such complexes. It explores the early examples of the addition of H-H across transition metal-borane bonds and describes the new transformation in the context of other known modes of hydrogen activation including classic oxidative addition and heterolytic cleavage at transition metal centres as well as Frustrated Lewis Pair chemistry. Similar reactivity involving transition metal-boryl complexes are also described particularly those which undergo both boryl-to-borane and borane-to-borohydride transformations. The delivery of hydride to the metal centre in combination with the potential to regenerate the borohydride functional group via a recharging process is explored in the context of providing a new strategy for catalysis. Finally, a light-hearted look at the analogy of the 'stinging processes' involving Trofimenko type ligands is taken one step further to determine whether it is indeed in the nature of scorpionate ligands to repeatedly 'sting' just as the real life scorpions do.

The oxidative conversion of the N,S-bridged complexes [{RhLL'(μ-X)}2] to [(RhLL')3)(μ-X)2]+ (X = mt or taz): a comparison with the oxidation of N,N-bridged analogues

The structures of [{RhLL'(μ-X)}(2)] [LL' = cod, (CO)(2), (CO)(PPh(3)) or {P(OPh)(3)}(2); X = mt or taz], prepared from [{RhLL'(μ-Cl)}(2)] and HX in the presence of NEt(3), depend on the auxiliary ligands LL'. The head-to-tail arrangement of the two N,S-bridges is accompanied by a rhodium-eclipsed conformation for the majority but the most hindered complex, [{Rh[P(OPh)(3)](2)(μ-taz)}(2)], uniquely adopts a sulfur-eclipsed structure. The least hindered complex, [{Rh(CO)(2)(μ-mt)}(2)], shows intermolecular stacking of mt rings in the solid state. The complexes [{RhLL'(μ-X)}(2)] are chemically oxidised to trinuclear cations, [(RhLL')(3)(μ-X)(2)](+), most probably via reaction of one molecule of the dimer, in the sulfur-eclipsed form, with the fragment [RhLL'](+) formed by oxidative cleavage of a second.

Acyclic boron-containing π-ligand complexes: η2- and η3-coordination modes

Cyclic boron-containing π-ligands such as boratabenzenes and borollides are well established, in particular as supporting ligands. By contrast, the chemistry of acyclic boron-containing π-ligands has remained relatively unexplored, presumably in part due to the higher reactivity of acyclic π-ligands relative to cyclic analogues. This perspective is focused on the synthesis, structures and reactivity of isolated transition metal complexes bearing η(n)-coordinated (n = 2 or 3) acyclic boron-containing ligands. Both monometallic and multimetallic compounds are included, and are discussed with an emphasis on metal-ligand and intraligand bonding and parallels with hydrocarbon π-ligand complexes.