Development of tethered dual catalysts: synergy between photo- and transition metal catalysts for enhanced catalysis.

While dual photocatalysis-transition metal catalysis strategies are extensively reported, the majority of systems feature two separate catalysts, limiting the potential for synergistic interactions between the catalytic centres. In this work we synthesised a series of tethered dual catalysts allowing us to investigate this underexplored area of dual catalysis. In particular, Ir(i) or Ir(iii) complexes were tethered to a BODIPY photocatalyst through different tethering modes. Extensive characterisation, including transient absorption spectroscopy, cyclic voltammetry and X-ray absorption spectroscopy, suggest that there are synergistic interactions between the catalysts. The tethered dual catalysts were more effective at promoting photocatalytic oxidation and Ir-catalysed dihydroalkoxylation, relative to the un-tethered species, highlighting that increases in both photocatalysis and Ir catalysis can be achieved. The potential of these catalysts was further demonstrated through novel sequential reactivity, and through switchable reactivity that is controlled by external stimuli (heat or light).

Controlling the selectivity and efficiency of the hydrogen borrowing reaction by switching between rhodium and iridium catalysts.

The catalytic alkylation of ketones with alcohols via the hydrogen borrowing methodology (HB) has the potential to be a highly efficient approach for forming new carbon-carbon bonds. However, this transformation can result in more than one product being formed. The work reported here utilises bidentate triazole-carbene ligated iridium and rhodium complexes as catalysts for the selective formation of alkylated ketone or alcohol products. Switching from an iridium centre to a rhodium centre in the complex resulted in significant changes in product selectivity. Other factors - base, base loading, solvent and reaction temperature - were also investigated to tune the selectivity further. The optimised conditions were used to demonstrate the scope of the reaction across 17 ketones and 14 alcohols containing a variety of functional groups. A series of mechanistic investigations were performed to probe the reasons behind the product selectivity, including kinetic and deuterium studies.

Simple and reactive Ir(i) N-heterocyclic carbene complexes for alkyne activation.

Two simple unsymmetrical monometallic Ir(i) complexes with an N-heterocyclic carbene ligand and an analogous bimetallic Ir(i) complex were synthesised. These complexes were found to be extremely active catalysts for a range of C-X (X = N or O) and Si-N bond forming reactions involving alkyne and imine activation for dihydroalkoxylation, hydroamination and hydrosilylation reactions. These catalysts exhibited reaction rates far exceeding those of other Rh(i) and Ir(i) complexes previously reported. In addition, a small change to the ligand design (phenyl vs. mesityl) substantially affected both the reactivity and product selectivity of the catalyst. The Ir(i) complex bearing a mesitylene wingtip provided unprecedented regioselectivity in the dihydroalkoxylation reaction and a new kinetic product from the typical hydrosilylation protocol of 2-benyzlpyrroline to produce an N-silylaminoalkene. Our mechanistic studies indicated that this transformation proceeded via a dehydrogenative coupling mechanism.

Highly Efficient Rh(I) Homo- and Heterogeneous Catalysts for C-N Couplings via Hydrogen Borrowing.

Rhodium(I) complexes were explored as catalysts for the hydrogen borrowing reactions of amines and alcohols. Bidentate carbene-triazole ligands were readily synthesized via "click" reactions which allowed a diversity of ligand backbones to be accessed. The catalytic transformations are highly efficient, able to reach completion in under 6 h, and promote C-N bond formation across a range of primary alcohol and amine substrates. Moreover, site-selective catalysis can be achieved using substrates with more than one reactive site. A rhodium(I) complex covalently attached to a carbon black surface was also deployed in the hydrogen borrowing coupling reaction of aniline with benzyl alcohol. This represents the first report of a heterogeneous rhodium catalyst used for hydrogen borrowing.

Highly Electron-Rich ß-Diketiminato Systems: Synthesis and Coordination Chemistry of Amino-Functionalized "N-nacnac" Ligands.

The synthesis of a class of electron-rich amino-functionalized ß-diketiminato (N-nacnac) ligands is reported, with two synthetic methodologies having been developed for systems bearing backbone NMe2 or NEt2 groups and a range of N-bound aryl substituents. In contrast to their (Nacnac)H counterparts, the structures of the protio-ligands feature the bis(imine) tautomer and a backbone CH2 group. Direct metalation with lithium, magnesium, or aluminium alkyls allows access to the respective metal complexes through deprotonation of the methylene function; in each case X-ray structures are consistent with a delocalized imino-amide ligand description. Transmetalation using lithium N-nacnac complexes is then exploited to access p- and f-block metal complexes, which allow for like-for-like benchmarking of the N-nacnac ligand family against their more familiar Nacnac counterparts. In the case of SnII , the degree of electronic perturbation effected by introduction of the backbone NR2 groups appears to be constrained by the inability of the amino group to achieve effective conjugation with the N2 C3 heterocycle. More obvious divergence from established structural norms is observed for complexes of the harder YbII ion, with azaallyl/imino and even azaallyl/NMe2 coordination modes being demonstrated by X-ray crystallography.

Magnesium(I) Dimers Bearing Tripodal Diimine-Enolate Ligands: Proficient Reagents for the Controlled Reductive Activation of CO2 and SO2.

The first examples of magnesium(I) dimers bearing tripodal ligands, [(Mg{κ(3) -N,N',O-(ArNCMe)2 (OCCPh2 )CH})2 ] [Ar=2,6-iPr2 C6 H3 (Dip) 7, 2,6-Et2 C6 H3 (Dep) 8, or mesityl (Mes) 9] have been prepared by post-synthetic modification of the ß-diketiminato ligands of previously reported magnesium(I) systems, using diphenylketene, OCCPh2 . In contrast, related reactions between ß-diketiminato magnesium(I) dimers and the isoelectronic ketenimine, MesNCCPh2 , resulted in reductive insertion of the substrate into the MgMg bond of the magnesium(I) reactant, and formation of [{(Nacnac)Mg}2 {µ-κ(2) -N,C-(Mes)NCCPh2 }] (Nacnac=[(ArNCMe)2 CH](-) ; Ar=Dep 10 or Mes 11). Reactions of the four-coordinate magnesium(I) dimer 8 with excess CO2 are readily controlled, and cleanly give carbonate [(LMg)2 (µ-κ(2) :κ(2) -CO3 )] 12 (L=[κ(3) -N,N',O-(DepNCMe)2 (OCCPh2 )CH](-) ; thermodynamic product), or oxalate [(LMg)2 (µ-κ(2) :κ(2) -C2 O4 )] 13 (kinetic product), depending on the reaction temperature. Compound 12 and CO are formed by reductive disproportionation of CO2 , whereas 13 results from reductive coupling of two molecules of the gas. Treatment of 8 with an excess of N2 O cleanly gives the µ-oxo complex [(LMg)2 (µ-O)] 14, which reacts facilely with CO2 to give 12. This result presents the possibility that 14 is an intermediate in the formation of 12 from the reaction of 8 and CO2 . In contrast to its reactions with CO2 , 8 reacts with SO2 over a wide temperature range to give only one product; the first example of a magnesium dithionite complex, [(LMg)2 (µ-κ(2) :κ(2) -S2 O4 )] 16, which is formed by reductive coupling of two molecules of SO2 , and is closely related to f-block metal dithionite complexes derived from similar SO2 reductive coupling processes. On the whole, this study strengthens previously proposed analogies between the reactivities of magnesium(I) systems and low-valent f-block metal complexes, especially with respect to small molecule activations.

Bis(phosphine)boronium salts. Synthesis, structures and coordination chemistry.

The synthesis of a range of bis(phosphine)boronium salts is reported [(R2HP)2BH2][X] (R = Ph, (t)Bu, Cy) in which the counter anion is also varied (X(-) = Br(-), [OTf](-), [BAr(F)4](-), Ar(F) = 3,5-(CF3)2C6H3). Characterization in the solid-state by X-ray diffraction suggests there are weak hydrogen bonds between the PH units of the boronium cation and the anion (X(-) = Br(-), [OTf](-)), while solution NMR spectroscopy also reveals hydrogen bonding occurs in the order [BAr(F)4](-) < [OTf](-) < Br(-). [(Ph2HP)2BH2][BAr(F)4] reacts with RhH(PPh3)3, by elimination of H2, forming [Rh(κ(1),η-PPh2BH2·PPh2H)(PPh3)2][BAr(F)4] which shows a ß-B-agostic interaction from the resulting base stabilised phosphino-borane ligand. Alternatively such ligands can be assembled directly on the metal centre by reaction of in situ generated {Rh(PPh3)3}(+) and Ph2HP·BH3 to afford [Rh(κ(1),η-PPh2BH2·PPh3)(PPh3)2][BAr(F)4], which was characterised by X-ray crystallography. Addition of H3B·PPh2H to the well-defined 16-electron "T-shaped" complex [Rh(P(i)Bu3)2(PPh3)][BAr(F)4] (characterised by X-ray crystallography) formed of a mixture of base-stabilised phosphino borane ligated complexes [Rh(κ(1),η-PR2BH2·PR3)(PR3)2][BAr(F)4] (R = (i)Bu or Ph). These last observations may lend clues to the formation of bis(phosphine)boronium salts in the catalytic dehydrocoupling reaction of phosphine boranes as mediated by Rh(I) compounds.