Ian M. Riddlestone

Cooperative Bond Activation and Catalytic Reduction of Carbon Dioxide at a Group 13 Metal Center

A single-component ambiphilic system capable of the cooperative activation of protic, hydridic and apolar HX bonds across a Group 13 metal/activated β-diketiminato (Nacnac) ligand framework is reported. The hydride complex derived from the activation of H2 is shown to be a competent catalyst for the highly selective reduction of CO2 to a methanol derivative. To our knowledge, this process represents the first example of a reduction process of this type catalyzed by a molecular gallium complex.

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.

A Comparison of the Stability and Reactivity of Diamido- and Diaminocarbene Copper Alkoxide and Hydride Complexes

The mononuclear N-heterocyclic carbene (NHC) copper alkoxide complexes [(6-NHC)CuOtBu] (6-NHC = 6-MesDAC (1), 6-Mes (2)) have been prepared by addition of the free carbenes to the tetrameric tert-butoxide precursor [Cu(OtBu)]4, or by protonolysis of [(6-NHC)CuMes] (6-NHC = 6-MesDAC (3), 6-Mes (4)) with tBuOH. In contrast to the relatively stable diaminocarbene complex 2, the diamidocarbene derivative 1 proved susceptible to both thermal and hydrolytic ring-opening reactions, the latter affording [(6-MesDAC)Cu(OC(O)CMe2C(O)N(H)Mes)(CNMes)] (6). The intermediacy of [(6-MesDAC)Cu(OH)] in this reaction was supported by the generation of Cu2O as an additional product. Attempts to generate an isolable copper hydride complex of the type [(6-MesDAC)CuH] by reaction of 1 with Et3SiH resulted instead in migratory insertion to generate [(6-MesDAC-H)Cu(P(p-tolyl)3)] (9) upon trapping by P(p-tolyl)3. Migratory insertion was also observed during attempts to prepare [(6-Mes)CuH], with [(6-Mes-H)Cu(6-Mes)] (10) isolated, following a reaction that was significantly slower than in the 6-MesDAC case. The longer lifetime of [(6-Mes)CuH] allowed it to be trapped stoichiometrically by alkyne, and also employed in the catalytic semi-reduction of alkynes and hydrosilylation of ketones.

Activation of H2 over the Ru-Zn Bond in the Transition Metal-Lewis Acid Heterobimetallic Species [Ru(IPr)2(CO)ZnEt]+

Reaction of [Ru(IPr)2(CO)H]BAr(F)4 with ZnEt2 forms the heterobimetallic species [Ru(IPr)2(CO)ZnEt]BAr(F)4 (2), which features an unsupported Ru-Zn bond. 2 reacts with H2 to give [Ru(IPr)2(CO)(η(2)-H2)(H)2ZnEt]BAr(F)4 (3) and [Ru(IPr)2(CO)(H)2ZnEt]BAr(F)4 (4). DFT calculations indicate that H2 activation at 2 proceeds via oxidative cleavage at Ru with concomitant hydride transfer to Zn. 2 can also activate hydridic E-H bonds (E = B, Si), and computed mechanisms for the facile H/H exchange processes observed in 3 and 4 are presented.

Extending the Chain: Synthetic, Structural, and Reaction Chemistry of a BN Allenylidene Analogue†

α versus γ: [CpFe(CO)(PCy3)(BNCMes2)]+, synthesized by halide abstraction, represents the first example of a BN allenylidene analogue, and features an unsaturated MBNC π system. Although DFT calculations show significant LUMO amplitude at the γ (carbon) position, primary reactivity towards nucleophiles occurs at the sterically less hindered α (boron) center.

Unexpected Migratory Insertion Reactions of M(alkyl)2 (M=Zn, Cd) and Diamidocarbenes

The electrophilic character of free diamidocarbenes (DACs) allows them to activate inert bonds in small molecules, such as NH3 and P4 . Herein, we report that metal coordinated DACs also exhibit electrophilic reactivity, undergoing attack by Zn and Cd dialkyl precursors to afford the migratory insertion products [(6-MesDAC-R)MR] (M=Zn, Cd; R=Et, Me; Mes=mesityl). These species were formed via the spectroscopically characterised intermediates [(6-MesDAC)MR2 ], exhibiting barriers to migratory insertion which increase in the order MR2 = ZnEt2 < ZnMe2 < CdMe2 . Compound [(6-MesDAC-Me)CdMe] showed limited stability, undergoing deposition of Cd metal, by an apparent β-H elimination pathway. These results raise doubts about the suitability of diamidocarbenes as ligands in catalytic reactions involving metal species bearing nucleophilic ligands (M-R, M-H).

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.

Well-Defined Heterobimetallic Reactivity at Unsupported Ruthenium-Indium Bonds

The hydride complex [Ru(IPr)2 (CO)H][BArF4 ], 1, reacts with InMe3 with loss of CH4 to form [Ru(IPr)2 (CO)(InMe)(Me)][BArF4 ], 4, featuring an unsupported Ru-In bond with unsaturated Ru and In centres. 4 reacts with H2 to give [Ru(IPr)2 (CO)(η2 -H2 )(InMe)(H)][BArF4 ], 5, while CO induces formation of the indyl complex [Ru(IPr)2 (CO)3 (InMe2 )][BArF4 ], 7. These observations highlight the ability of Me to shuttle between Ru and In centres and are supported by DFT calculations on the mechanism of formation of 4 and its reactions with H2 and CO. An analysis of Ru-In bonding in these species is also presented. Reaction of 1 with GaMe3 also involves CH4 loss but, in contrast to its In congener, sees IPr transfer from Ru to Ga to give a gallyl complex featuring an η6 interaction of one aryl substituent with Ru.