The sterically bulky carbene precursor 1,3-diisopropylbenzimidazolium bromide ( i Pr 2 -bimyH + Br -) (A) has been prepared by an improved method in 84% yield. Reaction of A with Pd(OAc) 2 and NaBr gave the dimeric Pd(II) benzimidazolin-2-ylidene complex [PdBr 2 ( i Pr 2 -bimy)] 2 (1), which can be easily cleaved by CH 3 CN, another equivalent of salt A, and triphenylphosphine to afford the novel benzannulated monocarbene complexes trans-[PdBr 2 (CH 3 CN)( i Pr 2 -bimy)] (2), ( i Pr 2 -bimyH)[PdBr 3 ( i Pr 2 -bimy)] (3), trans-[PdBr 2 ( i Pr 2 -bimy)(Ph 3 P)] (trans-4), and cis-[PdBr 2 ( i Pr 2 -bimy)(Ph 3 P)] (cis-4), respectively. All compounds have been fully characterized by multinuclei NMR spectroscopies and mass spectrometries (FAB, ESI). X-ray diffraction studies on single crystals of 1-3 and cis-4 revealed a square planar geometry and a fixed orientation of the N-isopropyl substituents with the C-H group pointing to the metal center to maximize C-H‚‚‚Pd interactions. The large downfield shift of the C-H protons in the 1 H NMR spectrum compared to the precursor A indicates that these C-H‚‚‚Pd interactions are retained in solution and better described as weak hydrogen bonds, rather than as agostic interactions. Furthermore, the molecular structures of especially complexes 2 and 3 clearly show a bending of the bromo ligands toward the carbene carbon atom in order to maximize intramolecular C carbene ‚‚‚Br interactions. The nature of these interactions can be attributed to a form of back-bonding to the formally vacant p-orbital of the C carbene atom with the electron density originating from the bromo ligands' lone pairs. A detailed study on the trans-cis isomerization of the mixed NHC-phosphine complexes 4 revealed that a cis arrangement in such complexes is thermodynamically favored. Furthermore, a preliminary catalytic study shows that complex 1 is highly active in the Suzuki-Miyaura coupling of aryl bromides and chlorides in pure water as environmentally benign solvent.

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A highly efficient rhodium(I) and iridium(I) catalysed dihydroalkoxylation reaction of alkyne diols is employed here for the synthesis of spiroketals and a fused bicyclic ketal. The two metal catalysts show differential selectivity and efficiency for either the cyclisation of the 5-exo or 6-endo-membered rings. For the first time, a dual metal (Rh and Ir) catalyst system is effectively utilised for the formation of the 5,6-spiroketals, more efficiently than the single metal catalysts. The two different metals create a dual activation pathway to enhance the 5- and 6-membered ring closure as compared with the equivalent single catalysts.

This work describes investigations into metal-catalyzed sequential reactions using a series of single metal and bimetallic Rh(I) and/or Ir(I) pyrazolyl complexes. Monometallic complexes with bis(1-pyrazolyl)methane (bpm) ligands [M(CO)2(bpm)]BArF 4 (1), bimetallic complexes [M2(CO)2(Lscaffold)][BArF 4]2 (2–4) where M = Rh(I) or Ir(I) bearing bitopic ligands Lscaffold = bis(1-pyrazolyl)methane-derived ligands, p-C6H4[CH(pz)2]2 (Lp), m-C6H4[CH(pz)2]2 (Lm), and anthracene-bridged 1,8-C14H8[CH(pz)2]2 (LAnt), [M2(CO)4(Lp)][BArF 4]2 (2), [M2(CO)4(Lm)][BArF 4]2 (3), and [M2(CO)4(LAnt)][BArF 4]2 (4) were used as catalysts. The efficiency of the complexes as catalysts was tested for the dihydroalkoxylation of a series of alkyne diol substrates, 2-(6-hydroxyhex-1-ynyl)benzyl alcohol (5), 1-methyl-3-heptyne-1,7-diol (6), 2-(5-hydroxypent-1-ynyl)benzyl alcohol (7), and 2-(4-hydroxybut-1-ynyl)benzyl alcohol (8), forming spiroketals. All complexes tested were highly effective catalysts for the intramolecular dihydroalkoxylation reaction. The homobimetallic complexes 2–4 showed significant enhancement in activity and selectivity relative to the single metal catalysts (1). The order of catalytic activity of the bimetallic complexes was found to be [M2(CO)4(LAnt)][BArF 4]2 > [M2(CO)4(Lm)][BArF 4]2 > [M2(CO)4(Lp)][BArF 4]2 for all substrates, and the bimetallic cooperativity index was established for each reaction.

A series of cationic rhodium(I) and iridium(I) complexes of the type [M(L[symbol: see text]L)(C2)]BAr(F)24 (where M = Rh or Ir, L[symbol: see text]L = bis(pyrazol-1-yl)methane (bpm), bis(N-methylimidazol-2-yl)methane (bim) or 1-(2-(diphenylphosphino)ethyl)-3,5-diphenylpyrazole (Ph2PyP), C2 = 1,5-cyclooctadiene (COD) or (CO)2 and BAr(F)24 = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) were synthesised in good yields. The solid-state structure of a number of complexes, including [Ir(Ph2PyP)(COD)]BAr(F)24, [Ir(bpm)(COD)]BAr(F)24 and [Ir(bim)(COD)]BAr(F)24 was determined using X-ray crystallography. The efficiency of the complexes as catalysts for the intramolecular hydroamination of 4-phenyl-3-butyn-1-amine, 4-pentyn-1-amine and 2-(2-phenylethynyl)aniline was established. The incorporation of the BAr(F)24- counter-ion in the Rh(I) and Ir(I) complexes was found to significantly improve the catalytic activity of the complexes, compared to the analogous Rh(I) and Ir(I) complexes containing BPh4- as the counter-ion. Excellent conversions were achieved for the cyclisation of 2-(2-phenylethynyl)aniline to 2-phenylindole using [Rh(bpm)(CO)2]BAr(F)24 as a catalyst. The use of a microwave reactor for enhancing the catalysed reactions was also investigated.

The synthesis of a series of Rh(I) and Ir(I) homobimetallic complexes using three different linking scaffolds is described. The cyclooctadiene (COD) complexes [M(2)(COD)(2)(L(scaffold))][BAr(F)(4)](2) (2-7) where M = Rh(I) or Ir(I), and L(scaffold) = bis(1-pyrazolyl)methane ligands, p-C(6)H(4)[CH(pz)(2)](2) (1a), m-C(6)H(4)[CH(pz)(2)](2) (1b) and the anthracene-bridged 1,8-C(14)H(8)[CH(pz)(2)](2) (1c) were synthesized. The COD co-ligands of 2-7 were replaced with the carbonyl co-ligands to form the analogous homobimetallic complexes, [M(2)(CO)(4)(L(scaffold))][BAr(F)(4)](2) (8-13). The solid-state structures of the dicationic homobimetallic complexes 2, 3, 5, 6, 9, and 10, as well as cationic monometallic complexes 15 and 22 of ligands 1b and 1c respectively, were characterized using X-ray crystallography. The solid-state XRD structures of the resulting dirhodium and diiridium complexes with the para- and meta-phenylene and anthracene scaffolds show that there are distinct differences between structures of complexes 2-10 due to the variation in the scaffold structures, in particular the relative positions of the two metal centres. Heterobimetallic RhIr complexes of the m-C(6)H(4)[CH(pz)(2)](2) ligand were also synthesized using a stepwise approach, and the observed exchange of the metal centres in the heterobimetallic complexes was found to be dependent on the nature of the coligand.

A series of novel indolyl-imine ligand precursors have been synthesized. In particular, bidentate, tridentate, and potentially tetradentate ligands were prepared, each containing both indolyl sp 2 -N and imine sp 2 -N donors. A series of neutral rhodium(I) and iridium(I) complexes of these ligands were synthesized, including monometallic and bimetallic complexes. The X-ray structures of three of the monometallic complexes with bidentate indolyl-imine ligands were determined. In each complex, the metal center was in a square planar conformation, as expected for Rh(I) and Ir(I) complexes. The fiveand six-membered metallocycles formed on complexation of the metal with the ligands were all planar, and the aromaticity of the ligands was maintained. The Rh(I) complex with the tridentate indolyl ligand (L) reacted at room temperature with CH 2 Cl 2 to form the Rh(III) chloromethyl complex [Rh(L)(CO)-(CH 2 Cl)(Cl)] via oxidative addition of the C-Cl bond of CH 2 Cl 2 . The Rh/Ir(I) indolyl-imine complexes are efficient catalysts for the intramolecular cyclization of 4-pentynoic acid to form γ-methylene-γ-butyrolactone. A dinuclear Rh(I) complex was the most active catalyst, demonstrating a significant degree of bimetallic cooperativity.

The mechanism of Ir(I)-catalyzed double hydroalkoxylation of 4-pentyn-1-ol with methanol to form cyclic acetals has been investigated with density functional theory calculations. Using a model [Ir(PyP 0 )(CO) 2 ] þ catalyst (PyP 0 = 1-[2-phosphinoethyl]pyrazole) the key steps in the first hydroalkoxylation are shown to be (i) electrophilic activation of the alkyne at the cationic Ir(I) metal center; (ii) rate-limiting C-O bond formation via intramolecular nucleophilic attack by the pendant OH group at the C4 position of the bound alkyne; and (iii) facile H þ transfer to form an Ir-bound cyclic vinyl ether intermediate. The key C-O bond forming cyclization step is greatly facilitated by the presence of an external H-bonded MeOH molecule that stabilizes the positive charge that develops at the hydroxyl proton of the bound alkyne. External MeOH also plays a key role in the H þ transfer step, for which a number of kinetically competitive pathways corresponding to either retention of the hydroxyl proton in the product or exchange with solvent were identified. The second hydroalkoxylation is initiated from the Ir-bound cyclic vinyl ether intermediate and depends on the ability of that species to access an Ir(I)-alkyl form in which the β-carbon carries a significant positive charge. Reversible C-O bond formation then occurs via nucleophilic attack of MeOH at the β-carbon and proceeds via a novel [3þ2]-addition of the O-H bond over the {Ir-C R -C β } moiety. This forms an Ir(III) hydrido-alkyl species, from which reductive elimination yields the final O,O-acetal product. This final reductive elimination is the rate-limiting step within the second hydroalkoxylation component of the cycle. The Ir(I)-alkyl intermediate can also access a MeOH-mediated C-H activation at the C γ position that leads to exchange with external MeOH. This accounts for the experimentally observed H/D exchange at that position.