Bimetallic Complexes for Enhancing Catalyst Efficiency: Probing the Relationship between Activity and Intermetallic Distance

Indium-catalyzed synthesis of benzannulated spiroketals by intramolecular double hydroalkoxylation of ortho-(hydroxyalkynyl)benzyl alcohols

The novel indium-catalyzed synthesis of benzannulated spiroketals by a double intramolecular hydroalkoxylation reaction of o-(hydroxyalkynyl)benzyl alcohols is reported.

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 versatile heteroditopic ligand scaffolds for accommodating group 8, 9 & 11 heterobimetallic complexes

Two highly versatile xanthene scaffolds containing pairs of heteroditopic ligands were found to be capable of accommodating a range of transition metal ions, including Au(i), Ir(i), Ir(iii), Rh(i), and Ru(ii) to generate an array of heterobimetallic complexes. The metal complexes were fully characterised and proved to be stable in the solid and solution state, with no observed metal-metal scrambling. Heterobimetallic complexes containing the Rh(i)/Ir(i) combinations were tested as catalysts for the two-step dihydroalkoxylation reaction of alkynediols and sequential hydroamination/hydrosilylation reaction of alkynamines.

Gold(III) NHC Complexes for Catalyzing Dihydroalkoxylation and Hydroamination Reactions

A gold(III) complex of an N-heterocyclic carbene based hemilabile ligand with two pendant pyrazole arms (1,3-bis((1H-pyrazol-3-yl)methyl)-2,3-dihydro-1H-imidazole, LH) was synthesized. Complex [LAu(III)Cl₃] is an excellent catalyst for promoting dihydroalkoxylation at room temperature, even catalyzing this reaction at 0 °C. [LAu(III)Cl₃] is one of the most efficient catalysts reported to date for the spirocyclization of alkynyl diols. Furthermore, [LAu(III)Cl₃] catalyzed intra- and intermolecular hydroamination reactions, achieving good to excellent conversions. [LAu(III)Cl₃] is a more efficient catalyst than a gold(I) analogue, [LAu(I)Cl]. The dependence of the quantity of weakly coordinating anion [BAr^F₄]^- ((3,5-trifluoromethyl)phenyl borate) present on catalysis efficiency was probed for the dihydroalkoxylation reaction. X-ray diffraction analysis of single crystals demonstrated the solid-state structure of gold complexes [LAu(III)Cl₃] and [LAu(I)Cl], which displayed the expected square-planar and linear coordination geometries, respectively.

Enhancements in catalytic reactivity and selectivity of homobimetallic complexes containing heteroditopic ligands

Rh(i) and Ir(i) homobimetallic complexes were synthesised using a heteroditopic ligand system on a xanthene scaffold containing a monodentate N-heterocyclic carbene ligand and a bidentate bis(pyrazol-1-yl)methane ligand. The complexes were tested as catalysts for the two-step dihydroalkoxylation and two-step hydroamination/hydrosilylation reactions. This is the first known report of an organometallic group 9 complex, Ir(i) bimetallic complex, 13, to selectively favour the opposite spirocyclisation product from that reported in the literature, 14cvs.14b. The Ir(i) homobimetallic complex catalyses the intramolecular hydroamination reaction of alkynamines efficiently and proved to be a highly active catalyst for promoting the subsequent hydrosilylation of the pyrrolines; completing the hydrosilylation reactions in less than 40 seconds. A chloro-bridged bimetallic species was observed in the solid state, revealing that the COD co-ligands present underwent an oxidation.

Bimetallic aluminum complexes with cyclic β-ketiminato ligands: the cooperative effect improves their capability in polymerization of lactide and ε-caprolactone

Novel binuclear aluminum complexes bearing β-ketiminato ligands were synthesized and characterized. The cooperative effect around the metal centers played a key role in improving catalytic activity.

Bi- and tri-metallic Rh and Ir complexes containing click derived bis- and tris-(pyrazolyl-1,2,3-triazolyl) N-N' donor ligands and their application as catalysts for the dihydroalkoxylation of alkynes

A series of bi-topic and tri-topic pyrazolyl-1,2,3-triazolyl donor ligands (; = 1,X-bis((4-((1H-pyrazol-1-yl)methyl)-1H-1,2,3-triazol-1-yl)methyl)benzene (X = 2, 3 and 4; o-C6H4(PyT)2, m-C6H4(PyT)2 and p-C6H4(PyT)2) and = 1,3,5-tris((4-((1H-pyrazol-1-yl)methyl)-1H-1,2,3-triazol-1-yl)methyl)benzene, 1,3,5-C6H3(PyT)3) were conveniently synthesised in 'one pot' reactions using the Cu(i) catalysed 'click' reaction. Rh(i), Ir(i), Rh(iii) and Ir(iii) complexes with ligands of the general formulae C6H6-n[(PyT)M(CO)2]n[BAr]n (M = Rh, Ir; n = 2, 3; ; ) and C6H6-n[(PyT)MCp*Cl]n[BAr]n (M = Rh, Ir; n = 2, 3; ; ) were synthesised and fully characterised. In solution each of the bi- or tri-metallic complexes and exists as a mixture of two (, ) or three ( and ) diastereomers due to the presence of a chiral centre at each metal centre in these complexes. The solid state structures of complexes and were determined by single crystal X-ray crystallography and showed that each bidentate arm of these multitopic ligands coordinates to the Rh or Ir centre in a bidentate fashion via the pyrazolyl-N2 and 1,2,3-triazolyl N3' donors. The intermetallic distances in these solid state structures vary from 8.66 Å to 15.17 Å. These complexes were assessed as catalysts for the dihydroalkoxylation of alkynes using the cyclisation of 2-(5-hydroxypent-1-ynyl)benzyl alcohol, , to a mixture of two spiroketals, 2,3,4,5-tetrahydro-spirol[furan-2,3'-isochroman], , and 3',4',5',6'-tetrahydro-spiro[isobenzofuran-1(3H),2'(2H)pyran], , as the model reaction. The Rh(i) complexes (), with the highest TOF of 2052 h(-1) for complex , were the most active catalysts when compared with the other complexes under investigation here. The Ir(i) complexes () were moderately active as catalysts for the same transformation. No significant enhancement in catalytic reactivity was observed with the Rh(i) series bi- and trimetallic complexes () when compared with their monometallic analogues. The bi- and trimetallic Ir(i) complexes () were much more efficient as catalysts for this transformation than their monometallic analogues, suggesting some intermetallic cooperativity. Rh(iii), , and Ir(iii), , complexes were not active as catalysts for this transformation.