Nicholas Phillips

Stable GaX2, InX2 and TlX2 radicals

The chemistry of the Group 13 metals is dominated by the +1 and +3 oxidation states, and simple monomeric M(II) species are typically short-lived, highly reactive species. Here we report the first thermally robust monomeric MX2 radicals of gallium, indium and thallium. By making use of sterically demanding boryl substituents, compounds of the type M(II)(boryl)2 (M = Ga, In, Tl) can be synthesized. These decompose above 130 °C and are amenable to structural characterization in the solid state by X-ray crystallography. Electron paramagnetic resonance and computational studies reveal a dominant metal-centred character for all three radicals (>70% spin density at the metal). M(II) species have been invoked as key short-lived intermediates in well-known electron-transfer processes; consistently, the chemical behaviour of these novel isolated species reveals facile one-electron shuttling processes at the metal centre.

Borane to boryl hydride to borylene dihydride: explicit demonstration of boron-to-metal α-hydride migration in aminoborane activation.

The sequence of fundamental steps implicit in the conversion of a dihydroborane to a metal borylene complex have been elucidated for an [Ir(PMe(3))(3)] system. B-H oxidative addition has been applied for the first time to an aminodihydroborane, H(2)BNR(2), leading to the generation of a rare example of a primary boryl complex, L(n)(H)M{B(H)NR(2)}; subsequent conversion to a borylene dihydride proceeds via a novel B-to-M α-hydride migration. The latter step is unprecedented for group 13 ligand systems, and is remarkable in offering α- substituent migration from a Lewis acidic center as a route to a two-coordinate ligand system.

Hydrogen shuttling: synthesis and reactivity of a 14-electron iridium complex featuring a bis(alkyl) tethered N-heterocyclic carbene ligand

Solvent dependent double C-H activation in an Ir(NHC)(2) system generates an agostically stabilized 14-electron complex featuring a face-capping bis(alkyl) tethered NHC ligand [NHC = N-heterocyclic carbene]. These activation processes are reversible, and the resulting ligand-derived hydrogen shuttle can be applied to the dehydrogenation of BN-containing substrates.

Cobalt Boryl Complexes: Enabling and Exploiting Migratory Insertion in Base‐Metal‐Mediated Borylation

Cobalt boryl complexes, which have only been sporadically reported, can be accessed systematically with remarkable (but controllable) variation in the nature of the M-B bond. Complexes incorporating a very strong trans σ-donor display unparalleled inertness, reflected in retention of the M-B bond even in the presence of extremely strong acid. By contrast, the use of the strong π-acceptor CO in the trans position, results in significant Co-B elongation and to labilization of the boryl ligand via unprecedented CO migratory insertion. Such chemistry provides a pathway for the generation of coordinative unsaturation, thereby enabling ligand substitution and/or substrate assimilation. Alkene functionalization by boryl transfer, a well-known reaction for noble metals such as Rh or Pt, can thus be effected by an 18-electron base-metal complex.

Formation of sub-valent carbenoid ligands by metal-mediated dehydrogenation chemistry: coordination and activation of H2Ga{(NDippCMe)2CH}

Reactions of the β-diketiminato (‘Nacnac’) stabilized gallium dihydride H2Ga{(NDippCMe)2CH} with a range of mono- and dinuclear metal carbonyl reagents are characterized by loss of dihydrogen and formation of donor/acceptor complexes featuring the Ga(I) carbenoid ligand : Ga{(NDippCMe)2CH}. Thus, far from simply mimicking the chemistry of the corresponding alane H2Al{(NDippCMe)2CH}, which yields κ1 and κ2 Al–H σ-complexes with similar reagents, the weaker nature of Ga–H bonds leads to extensive bond activation chemistry and enables an unprecedented dehydrogenative route to Ga(I) ligand systems. By consideration of the chemistry of dinuclear systems, two alternative pathways are revealed for this chemistry, with either H2 or M–H bonds acting as the ultimate hydrogen sink.

Probing the Limits of Ligand Steric Bulk: Backbone CH Activation in a Saturated N‐Heterocyclic Carbene

The consequences of extremely high steric loading have been probed for late transition metal complexes featuring the expanded ring N-heterocyclic carbene 6-Dipp. The reluctance of this ligand to form 2:1 complexes with d-block metals (rationalised on the basis of its percentage buried volume, % Vbur , of 50.8%) leads to C-H and C-N bond activation processes driven by attack at the backbone β-CH2 unit. In the presence of Ir(I) (or indeed H(+) ) the net result is the formation of an allyl formamidine fragment, while Au(I) brings about an additional ring (re-)closure step via nucleophilic attack at the coordinated alkene. The net transformation of 6-Dipp in the presence of [(6-Dipp)Au](+) represents to our knowledge the first example of backbone C-H activation of a saturated N-heterocyclic carbene, proceeding in this case via a mechanism which involves free carbene in addition to the Au(I) centre.

Synthesis and Reactivity of Half-Sandwich Ruthenium κ2-Aminoborane Complexes

Cationic half-sandwich ruthenium complexes featuring κ2-bound aminoborane ligands can readily be accessed from 16-electron precursors via chloride abstraction in the presence of H2BNR2 (R = iPr, Cy). Complexes [Cp*Ru(L)(κ2-H2BNR2)][BArf4] (2a: R = iPr, L = PCy3; 2b: R = iPr, L = PPh3; 2c: R = iPr, L = 1,3-bis-(2,4,6-trimethylphenyl)-imidazol-2-ylidene; 3a: R = Cy, L = PCy3; Arf = C6H3(CF3)2‐3,5) were isolated in yields of ~60 %, and characterised in the solid state by X-ray crystallography (for 2a, 2c, and 3a). Low-field 11B NMR shifts for the coordinated aminoborane fragment, together with short Ru⋯B contacts (of the order of 1.97 A) imply a relatively tightly bound borane ligand, a finding which is given further credence by the results of density functional theory studies (e.g. bond dissociation energies in the range 24 kcal mol–1; 1 kcal mol–1 = 4.186 kJ mol–1). In terms of reactivity, κ2 systems of this type, while potentially offering a versatile route to asymmetric κ1 systems, in fact undergo borane extrusion even in the presence of a single equivalent of added ligand.