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Complexes of the type (η3-allyl)Pd(L)(Cl) and (η3-indenyl)Pd(L)(Cl) are highly active precatalysts for the Suzuki–Miyaura reaction. Even though allyl and indenyl ligands are similar to cyclopentadienyl (Cp) ligands, there have been no detailed comparative studies exploring the activity of precatalysts of the type (η5-Cp)Pd(L)(Cl) for Suzuki–Miyaura reactions. Here, we compare the catalytic activity of (η5-Cp)Pd(IPr)(Cl) (IPr = 1,3-bis(2,6-diisopropylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene, Cp) with two commercially available catalysts (η3-cinnamyl)Pd(IPr)(Cl) (Cin) and (η3-1-t-Bu-indenyl)Pd(IPr)(Cl) (tBuInd). We show that Cp gives slightly better catalytic activity than Cin, but significantly inferior activity than tBuInd. This order of activity is rationalized by comparing the rates at which the precatalysts are activated to the monoligated Pd(0) active species along with the tendency of the starting precatalysts to comproportionate with monoligated Pd(0) to form inactive Pd(I) dimers. As part of this work the Cp supported Pd(I) dimer (μ-Cp)(μ-Cl)Pd2(IPr)2 (CpDim) was synthesized and crystallographically characterized. It does not readily disproportionate to form monoligated Pd(0) and consequently CpDim is a poor catalyst for the Suzuki–Miyaura reaction.

The transition-metal-mediated functionalization of CO 2is currently an area attracting considerable interest due to the potential benefits of utilizing CO 2as a C 1feedstock. Here, recent advances in CO 2activation and functionalization using monomeric Pd ? ?-allyls are summarized. Subsequently, we discuss the chemistry of unusual Pd(I)-bridging allyl dimers, which may be masks for Pd ? ?-allyl complexes, and are stable catalysts for CO 2functionalization. [ABSTRACT FROM AUTHOR]

A family of nickel allyl complexes of the type (allyl)2Ni(L) (L = PMe3(1), PEt3(2), PPh3(3), NHC (4); NHC = 1,3-bis(2,6-diisopropylphenyl)-4,5-dihydro-2H-imidazol-2-ylidene) and (2-methylallyl)2Ni(L) (L = PMe3(5), PEt3(6), PPh3(7), NHC (8)) have been prepared using literature methods. Compounds 5, 7, and 8were characterized by X-ray crystallography. Whereas compounds 5and 7are 18-electron species with two η3-allyl ligands, the NHC-supported complexes are 16-electron species with one η1- and one η3-allyl ligand. Using DFT we have identified the key factors in predicting whether complexes of the type (allyl)2M(L) (M = Ni, Pd) are 16- or 18-electron species. Compounds 1, 2, and 5–8readily decompose to give a mixture of products, while compound 4decomposes to give the unusual Ni0species (η4-1,5-hexadiene)Ni(NHC) (9), which was characterized by X-ray crystallography. The reactions of 1–8with CO2were investigated. Compounds 5–8react rapidly with CO2at low temperature to form well-defined unidentate nickel carboxylates of the type (η3-2-methylallyl)Ni(OC(O)C4H7)(L) (L = PMe3(10), PEt3(11), PPh3(12), NHC (13)). The structure of 13was elucidated using X-ray crystallography. In contrast, compounds 1–4do not react with CO2. We believe that the difference in reactivity between the 2-methylallyl-supported complexes and the allyl-supported complexes is related to the mechanism of CO2insertion. [ABSTRACT FROM AUTHOR]

The air- and water-tolerant hydroxy-bridged rhodium dimer [(COD)Rh(μ2-OH)]2cleanly activates the aliphatic C−H bond in indene to generate [(COD)Rh(η3-indenyl)]. The mechanism involves direct coordination of indene to the dimer followed by rate-determining C−H bond cleavage, in contrast to the previously reported analogous reactions of [(diimine)M(μ2-OH)]22+(M = Pd, Pt), for which the dimer must be cleaved before rate-determining displacement of solvent by indene. Another difference is observed in the reactions with indene in the presence of acid: the Rh system generates a stable η6-indene 18-electron cation, [(COD)Rh(η6-indene)]+, that is not available for Pd and Pt, which instead form the η3-indenyl C−H activation products. The crystal structure of [(COD)Rh(η6-indene)] is reported. [ABSTRACT FROM AUTHOR]

Density functional theory has been used to investigate the structures, bonding and properties of a family of hydride rich late transition metal clusters of the type [Rh6(PH3)6H12]x (x = 0, +1, +2, +3 or +4), [Rh6(PH3)6H16]x (x = +1 or +2) and [Rh6(PH3)6H14]x (x = 0, +1 or +2). The positions of the hydrogen atoms around the pseudo-octahedral Rh6 core in the optimized structures of [Rh6(PH3)6H12]x (x = 0, +1, +2, +3 or +4) varied depending on the overall charge on the cluster. The number of semi-bridging hydrides increased (semi-bridging hydrides have two different Rh–H bond distances) as the charge on the cluster increased and simultaneously the number of perfectly bridging hydrides (equidistant between two Rh centers) decreased. This distortion maximized the bonding between the hydrides and the metal centers and resulted in the stabilization of orbitals related to the 2T2g set in a perfectly octahedral cluster. In contrast, the optimized structures of the 16-hydride clusters [Rh6(PH3)6H12]x (x = +1 or +2) were similar and both clusters contained an interstitial hydride, along with one terminal hydride, ten bridging hydrides and two coordinated H2 molecules which were bound to two rhodium centers in an η2:η1-fashion. All the hydrides were on the outside of the Rh6 core in the lowest energy structures of the 14-hydride clusters [Rh6(PH3)6H14] and [Rh6(PH3)6H14]+, which both contained eleven bridging hydrides, one terminal hydride and one coordinated H2 molecule. Unfortunately, the precise structure of [Rh6(PH3)6H14]2+ could not be determined as structures both with and without an interstitial hydride were of similar energy. The reaction energetics for the uptake and release of two molecule of H2 by a cycle consisting of [Rh6(PH3)6H12]2+, [Rh6(PH3)6H16]2+, [Rh6(PH3)6H14]+, [Rh6(PH3)6H12]+ and [Rh6(PH3)6H14]2+ were modelled, and, in general, good agreement was observed between experimental and theoretical results. The electronic reasons for selected steps in the cycle were investigated. The 12-hydride cluster [Rh6(PH3)6H12]2+ readily picks up two molecules of H2 to form [Rh6(PH3)6H16]2+ because it has a small HOMO–LUMO gap (0.50 eV) and a degenerate pair of LUMO orbitals available for the uptake of four electrons (which are provided by two molecules of H2). The reverse process, the spontaneous release of a molecule of H2 from [Rh6(PH3)6H16]+ to form [Rh6(PH3)6H14]+ occurs because the energy gap between the anti-bonding SOMO and the next highest energy occupied orbital in [Rh6(PH3)6H16]+ is 0.9 eV, whereas in [Rh6(PH3)6H14]+ the energy gap between the anti-bonding SOMO and the next highest energy occupied orbital is only 0.3 eV. At this stage the factors driving the conversion of [Rh6(PH3)6H14]+ to [Rh6(PH3)6H12]2+ are still unclear. [ABSTRACT FROM AUTHOR]

A family of well-defined (η3-allyl)Pd(L)(carboxylate) (L = PR3or NHC) complexes are by far the most efficient catalysts reported to date for the catalytic carboxylation of allylstannanes into allylcarboxylates using CO2. The substrate scope of this reaction is extended to both substituted allylstannanes and allylboranes. [ABSTRACT FROM AUTHOR]

The yield of triptane from the reaction of methanol with zinc iodide is dramatically increased by addition of phosphorous or hypophosphorous acid, via transfer of hydride from a P–H bond to carbocationic intermediates. [ABSTRACT FROM AUTHOR]