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The synthesis of the novel C-3-symmetric tripodal, tetradentate ligand 1, bearing only phosphorus atoms as donor groups, is described, starting from commercially available o-tolyldiphenylphosphine, and its molecular structure has been determined by X-ray crystallographic analysis. Coordination to the cationic Ir-1 precursor [Ir(COE)(2)(acetone)(2)]PF6 led to a highly unsymmetrical species (90% yield) with four inequivalent phosphorus atoms, as evidenced by P-31 NMR spectroscopy. The corresponding H-1 NMR spectrum exhibited a pseudo doublet of quartets at delta -5.9 ppm with one large trans P-H coupling ((2)J(P-H) = 115.4 Hz) and a much smaller cis coupling ((2)J(P-H) = 10.8 Hz). X-ray crystallography confirmed the formation of complex 2, [Ir(H)(kappa P-5,P,P,P,C-1)]PF6, which is a rare example of a structurally characterized mononuclear Ir hydride species bearing an ortho-metalated phosphine ligand. This species does not react with hydride sources, but addition of 1 equiv of CF3COOH resulted in facile overall formal protonation of the Ir-C bond. DFT calculations support a pathway involving initial reductive elimination, forming the highly distorted four-coordinate species 2', followed by protonation at iridium to give the dicationic monohydride species 3, with an activation barrier Delta G(dagger) of 28.2 kcal mol(-1). Deuteration experiments support this mechanism. Reductive elimination can also be induced by reaction of 2 with carbon monoxide, yielding the monocationic carbonyl complex [Ir-1(CO)(1)]PF6 as the sole product.

In natural systems, substrate activation and transformation is often achieved through metal–ligand cooperativity, and almost exclusively with earth-abundant 3d metals.[1] However, this concept of a metal and its ligand acting in concert is still strongly under-explored in synthetic chemistry with first-row transition metals, although for heavier analogues this has provided new routes to the formation and functionalization of useful building blocks.[2] Recently, a family of pincer ligands[3] was demonstrated to enable cooperative catalysis.[4] Strikingly, relatively little attention has been paid to the chemistry of Fe,[5] Co,[6, 7] and Ni[8–10] with these noninnocent ligand frameworks relative to the elegant reactivity detailed for their heavier congeners. Reports of the coinage metals Ag and Au are also scarce.[9c] Furthermore, Cu chemistry has rarely been explored with any pincer scaffold.[11]

The high-yielding synthesis and detailed characterization of two well-defined, linkage isomeric tripodal, tetradentate all-phosphorus ligands 1-3 is described. Coordination to Cu(I) resulted in formation of complexes 4-6, for which the molecular structures indicate overall tridentate coordination to the copper atom in the solid state, with one dangling peripheral phosphine. The solution studies suggest fast exchange between the three phosphine side-arms. For these new Cu(I) complexes, preliminary catalytic activity in the cyclopropanation of styrene with ethyldiazoacetate (EDA) is disclosed. The anticipated well-defined tetradentate coordination in a C(3)-symmetric fashion was achieved with Rh(I) and Ir(I), leading to the overall five-coordinated complexes 7-12. Complex 11 has the norbornadiene (nbd) ligand coordinated in an unprecedented monodentate 2,3-eta(2) mode to Rh. Furthermore, unexpected but very interesting redox-chemistry and reactivity was displayed by the Rh(Cl)-complexes 7 and 8. Oxidation resulted in the formation of stable Rh(II) metalloradicals [7]PF(6) and [8]PF(6) that were characterized by X-ray crystallography, magnetic susceptibility measurements, cyclic voltammetry, and electron paramagnetic resonance (EPR) spectroscopy. Subsequent redox-reactivity of these metalloradicals toward molecular hydrogen is described, resulting in the formation of Rh(III) hydride compounds.

The first example of an iridium(I) species bearing a terminal phosphido (PR(2)(-)) ligand is reported. This stable compound shows well-behaved reactivity toward various electrophiles, owing to its exposed phosphorus lone pair, allowing reversible protonation, selective alkylation, isolation of a phosphidoborane of iridium, and generation of a phosphido-bridged iridium(I)-gold(I) dinuclear species.

Hydrogen holds the potential to be one of the major energy carriers for the future. However, a hydrogen-based economy requires technology that allows efficient and safe storage and release of H2. In this light, the reversible storage of dihydrogen in the form of formic acid (HCOOH) provides an interesting H2 storage-release system. Formic acid can be obtained from the catalytic hydrogenation of CO2 [1a–c] or by direct hydrothermal oxidation of biomass.[1d–f] Since the early nineties, substantial developments in the production of HCOOH have stimulated interest in the reverse reaction for hydrogen release on demand.[2] Important recent breakthroughs are the highly active Fe-based dehydrogenation catalyst reported by Laurenczy, Beller, and coworkers[ 2h] and work by Fujita and co-workers on an iridium catalyst that is highly active for CO2 hydrogenation or HCOOH dehydrogenation, depending on pH.[2k] The dehydrogenation of HCOOH to H2 and CO2 is typically performed in the presence of substoichiometric amounts of base, which drastically decreases the hydrogen content (from the theoretical maximum of 4.4 wt% in pure HCOOH to 2.3 wt% based on the typical 5:2 HCOOH/ NEt3 mixture). In the context of developing efficient hydrogen- based fuel cells, HCOOH/base mixtures were recently optimized for H2 release using a ruthenium-based catalysts.[3] Ideally, catalytic dehydrogenation should be performed in the absence of base and other additives, thereby maximizing the overall hydrogen storage capacity (4.4 wt%). Besides this, hydrogen produced from HCOOH/NEt3 mixtures requires purification to remove traces of volatile amines prior to application in fuel cells.[

We describe the synthesis and characterization of compound L1H, 2-di(tert-butyl)phosphinomethyl-6- methylpyridine, and the first dimeric CuI complex 3 with this novel bidentate NPtBu ligand. We also demonstrate for the first time that this ligand scaffold exhibits noninnocent reactivity through dearomatization behavior, similar to its well-studied tridentate analog L2H, 2,6-bis((di-tert-butylphosphino)methyl)pyridine PNPtBu. The molecular structure of [Cu(CCPh)(L2H)]2 is reported, which is a rare case of a crystallographically characterized copper-acetylide dimer. We also demonstrate that copper(I) complexes with either ligand L1H or L2H or their dearomatized counterparts may act as active, cooperative catalysts for the [2+3] polar cycloaddition of azides and acetylenes. These results represent the first indications of selective Cu-based cooperative catalysis, using non-innocent lutidine-based PNP backbone and catalysts 2 and 5 could thus be termed all-inclusive systems for this reaction.

A new family of monoanionic hemilabile ligands L1H−L3H with a PNN donor set has been developed, based on Pd-catalyzed C−N bond formation and straightforward phosphorylation. For these structurally related compounds with a hybrid set of donor atoms, the coordination chemistry with both Rh and Ir has been studied. The anticipated hemilabile character of the dimethylamino group was assessed by NMR and IR competition experiments, using isopropyl isocyanide as exogenous substrate. Supporting DFT calculations were used to quantify the electronic differences between the various members of the ligand family. In effect, we have constructed a modular ligand class that exhibits tunable hemilability.

At ambient temperature, the complex [Ni(OAc)(PNPtBu)]OTf has one independent molecule in the asymmetric unit of the crystal structure with very large anisotropic displacement parameters. During cooling a fully reversible solid–solid phase transition occurs at a discrete temperature in the range 210–230 K. The low-temperature phase has six independent, well ordered molecules in the asymmetric unit. The P21/a space group symmetry of the high-temperature phase changes to P21/n for the low-temperature phase and the c-axis increases by a factor of six. The acetate ligand is shown to be coordinated in a η1-fashion through one of the oxygen atoms, with the sterically encumbered, tridentate PNPtBu ligand completing the square planar geometry around the NiII ion. The synthesis and full characterization of the complex is reported.

The synthesis and characterization of a series of cationic and neutral Ni-complexes with the non-innocent PNPtBu pincer ligand is discussed. Starting with the dicationic complex 1, [Ni(PNPtBu)(NCMe)](BF4)2, a small series of dicationic and monocationic NiII complexes has been prepared. Substitution with tert-butyl isocyanide and azide occurs readily in MeCN solution. IR spectroscopy provided a practical handle to access the formal valence state of the ligand. For the mono- and dicationic tert-butyl isocyanide species 3 and 6 the main vibrational bands in the IR spectra were reproduced quantitatively by DFT theoretical calculations, showing good agreement with the experimentally observed upon dearomatization of the PNPtBu backbone. Using a selective dearomatization-reprotonation methodology the mononuclear Ni-thiolate species 7 and 8 are cleanly generated and their structures have been determined by X-ray crystal structure determination. Alternatively, starting from the monocationic species [Ni(PNPtBu)Cl]BF4, neutral alkyl derivatives are easily available in a two-step procedure, and these species have been spectroscopically characterized.

The neutral, T-shaped complex CuI(PN−PtBu) (2), featuring a dearomatized 2,6-bis(diphosphino)pyridine (PNP)-pincer ligand, is shown to interact rapidly with electrophiles. This has enabled the synthesis of acetato complex 3. Furthermore, C−C bond formation onto the deprotonated methylene-bridgehead carbon is observed with MeOTf as the electrophile. This represents the first case of selective modification of the lutidine-based backbone of such noninnocent PNP ligands. Theoretical calculations support the formation of monomeric complex 2 and indicate the high reactivity of the methylene fragment in this CuI complex.

The versatile coordination behavior of the PNP ligands 1A (2,6- bis[(di-tert-butylphosphino)methyl]pyridine) and 1B (2,6-bis[(diphenylphosphino) methyl]pyridine) to CuI is described, whereby a hemilabile interaction of the pyridine N-donor atom to the copper center resulted in a rare T-shaped complex with 1A, while with 1B also a tetracoordinated species could be isolated. Theoretical calculations support the weak interaction of the pyridine N donor in 1A with the Cu center.

The title compound, [PtCl2(C34H31NP2)], crystallizes with four crystallographically independent molecules. All four molecules have the same R chirality, with P-Pt-P bite angles ranging from 72.14 (5) to 72.38 (5)°.

A brief historic overview of Pt/Sn-catalyzed hydroformylation as well as recent advances in the hydroformylation of internal alkenes is provided. This serves as background for the results obtained with the [Pt(Sixantphos)Cl2] system, for which the molecular structure and the spectroscopic data are described. In situ UV/Vis-spectroscopic studies have revealed rapid formation of the corresponding Pt–stannate complex upon reaction with SnCl2, whereas high-pressure in situ IR-spectroscopy showed formation of a Pt–CO species and a short-lived Pt–H species under syngas, as well as rapid evolution of aldehyde product upon addition of 1-octene to the preformed catalyst in the IR autoclave. The hydroformylation of 1-octene and the i-octenes has been performed. For the internal alkenes, selective tandem isomerization/hydroformylation towards n-nonanal is observed with this catalyst system.