Synthese und Reaktivtät von Nickel, Cobalt und Molybdän PCP Pincer Komplexen

In Chapter 1, This chapter provides an overview of the advancements in the field of non-precious metal (Ni, Co and Mo) complexes featuring anionic PCP pincer ligands where the (CH2, O and NH) linkers between aromatic ring and phosphine moieties. While the research in nickel PCP complexes is already quite extensive, the chemistry of cobalt and molybdenum PCP complexes is comparatively sparse. In the case of nickel PCP complexes already many catalytic applications such as Suzuki-Miyaura coupling, C-S cross coupling, Michael additions, Alcoholysis of acrylonitrile, hydrosilylation of aldehyde and cyanomethylation of aldehydes were reported. Cobalt and molybdenum PCP complexes were not applied to any catalytic reactions. Surprisingly, only one molybdenum PCP complex is reported, which was capable of cleaving dinitrogen to give a nitride complex. This literature survey is motivated us to find and develop the combination of cheap and abundant metals such as nickel, cobalt and molybdenum with PCP pincer ligands may result in the development of novel, versatile, and efficient catalysts for atom-efficient catalytic reactions. In Chapter 2, We have shown here that a PCP pincer ligand based on 1,3-diaminobenzene acts as versatile supporting scaffold in cobalt chemistry. The PCP moiety provides access to a range of Co complexes in formal oxidation states +I, +II, and +III by utilizing the 15e square planar d7 complex [Co(PCPMe-iPr)Cl] as synthetic precursor. In contrast to the analogous Ni(II) complex [Ni(PCPMe-iPr)Cl], [Co(PCPMe-iPr)Cl] is able to form stable pentacoordinate square pyramidal or trigonal bipyramidal 17e complexes. For instance, [Co(PCPMe-iPr)Cl] readily adds CO and pyridine to afford the five-coordinate square-pyramidal complexes [Co(PCPMe-iPr)(CO)Cl] and [Co(PCPMe-iPr)(py)Cl], respectively, while in the presence of Ag+ and CO the cationic bipyramidal complex [Co(PCPMe-iPr)(CO)2]+ is formed. The effective magnetic moments -eff of all Co(II) complexes derived from the temperature dependence of the inverse molar magnetic susceptibility by SQUID measurements are in the range of 1.9 to 2.4-B. This is consistent with a d7 low spin configuration with a contribution from the second-order spin orbit coupling. Oxidation of [Co(PCPMe-iPr)Cl] with CuCl2 yields the Co(III) PCP complex [Co(PCPMe-iPr)Cl2], while the synthesis of the Co(I) complex [Co(PCPMe-iPr)(CO)2] was achieved by reducing [Co(PCPMe-iPr)Cl] with KC8 in the presence of CO. Complex [Co(PCPMe-iPr)Cl2] exhibits a solution magnetic moment of 3.1-B which is consistent with a d6 intermediate spin system. The tendency of Co(I), Co(II) and Ni(II) PCP complexes of the type [M(PCPMe-iPr)(CO)]n (n = +1, 0) to add CO was investigated by DFT calculations showing that the Co species readily form the five-coordinate complexes [Co(PCPMe-iPr)(CO)2]+ and [Co(PCPMe-iPr)(CO)2] which are thermodynamically favorable, while Ni(II) maintains the 16e configuration since CO addition is thermodynamically unfavorable in this case. X-ray structures of most complexes are provided and discussed. A structural feature of interest is that the CO ligand in [Co(PCPMe-iPr)(CO)Cl] deviates significantly from linearity with a Co-C-O angle of 170.0(1)°. The DFT calculated value is 172o clearly showing that this is not a packing but an electronic effect. In Chapter 3, We have shown that the 15e square-planar complexes [Co(PCPMe-iPr)Cl] and [Co(PCP-tBu)Cl], respectively, react readily with NaBH4 to afford complexes [Co- (PCPMe-iPr)(-2-BH4)] and [Co(PCP-tBu)(-2-BH4)] in high yields. The -2-bonding mode of the borohydride ligand was confirmed by IR spectroscopy and X-ray crystallography. These compounds are paramagnetic with effective magnetic moments of 2.0(1) and 2.1(1) -B consistent with a d7 low-spin system corresponding to one unpaired electron. None of these complexes react with CO2 to give formate complexes. For structural and reactivity comparisons, we prepared the analogous Ni(II) borohydride complex [Ni(PCPMe-iPr)(-2-BH4)] via two different routes. One utilizes [Ni(PCPMe-iPr)Cl] and NaBH4, the second one makes use of the hydride complex [Ni(PCPMe-iPr)H] and BH3·THF. In both cases, [Ni(PCPMe-iPr)(-2-BH4)] was obtained in high yields. While [Ni(PCPMe-iPr)(-2-BH4)] loses readily BH3 at elevated temperatures in the presence of NEt3 to form [Ni(PCPMe-iPr)H], the Co(II) complex [Co(PCPMe-iPr)(-2-BH4)] did not react with NH3 to give a hydride complex. Complexes [Ni(PCPMe-iPr)(-2-BH4)] and [Ni(PCPMe-iPr)H] react with CO2 to give the formate complex [Ni(PCPMe-iPr)(OC(C-O)H]. DFT calculations revealed that the formation of the Ni hydride is thermodynamically favorable, whereas the formation of the Co(II) hydride, in agreement with the experiment, is unfavorable. From the calculations, it is apparent that, for the Co complexes, the BH4- coordination is closer to -2, and the overall geometry can be envisaged as in between square-planar and square-pyramidal. In complex [Ni(PCPMe-iPr)(-2-BH4)], the borohydride ligand coordination is closer to -1, and the overall geometry of the molecule is closer to normal square-planar, reflecting the tendency of Ni(II) to form complexes with that geometry, as expected for a d8 metal. A very rare bidentate and tridentate ligand containing pincer complex [Co(PCPMe-iPr)(NO)(NO2)] is synthesized by the reacting substrate [Co(PCPMe-iPr)(-2-BH4)] and NaNO2. The cobalt borohydride complex [Co(PCPMe-iPr)(-2-BH4)] is readily reacted with CO and tBuNC to produce the Co(I) complexes [Co(PCPMe-iPr)(CO)2] and [Co(PCPMe-iPr)(CNtBu)2], respectively. In Chapter 4, We described the synthesis and reactivity of the first Co(I) pincer complex [Co(-3P,CH,P-P(CH)PMe-iPr)(CO)2]+ featuring an agostic -2-Caryl-H bond. This complex was obtained via protonation of the Co(I) complex [Co(PCPMe-iPr)(CO)2]. In contrast to related Rh(I) agostic pincer complexes, this complex is five coordinate and adopts a trigonal bipyramidal geometry. Since the CO ligands are not sufficiently electron rich to promote an oxidative addition of the C-H bond, a Co(III) hydride complex is not formed. A Co(III) hydride complex [Co(PCPMe-iPr)(CNtBu)2(H)]+ was obtained upon protonation of the more electron rich Co(I) complex [Co(PCPMe-iPr)(CNtBu)2]. Three ways to cleave the agostic C-H arene bond are presented. First, the agostic proton in [Co(-3P,CH,P-P(CH)PMe-iPr)(CO)2]+ is acidic and pyridine deprotonates the C-H bond to reform the starting material. Another way to cleave the agostic C-H bond is hydrogen abstraction upon exposure of [Co(-3P,CH,P-P(CH)PMe-iPr)(CO)2]+ to oxygen) or TEMPO which results in the formation of the paramagnetic Co(II) PCP complex [Co(PCPMe-iPr)(CO)2]+. This process involves oxidation of the metal center from Co(I) to Co(II). Finally, replacement of one CO ligand in [Co(-3P,CH,P-P(CH)PMe-iPr)(CO)2]+ by CNtBu promotes the oxidative addition of the agostic C-H arene bond. This yields two isomeric hydride complexes of the type [Co(PCPMe-iPr)(CNtBu)(CO)(H)]+. DFT calculations support the existence of the agostic bond in complex [Co(-3P,CH,P-P(CH)PMe-iPr)(CO)2]+ and corroborate the differences observed between the Co and the Rh species. In Chapter 5, We synthesized two diamagnetic molybdenum pincer complexes such as 18e Mo(PCPMe-iPr)(Br)(O), and 16e Mo(PCPMe-iPr)(Br)(CO)2 from the two different molybdenum precursor such as Mo(Br)3(thf)3 and [Mo(Br)2(CO)4], respectively.10

Regio- and stereoselective syntheses of piperidine derivatives via ruthenium-catalyzed coupling of propargylic amides and allylic alcohols.

International audienceIntermolecular selective coupling of linear allylic alcohols and propargylic amides occurs in the presence of a catalytic amount of the cationic ruthenium complex [Cp*Ru(NCCH(3))(3)]PF(6) followed by condensation to generate six-membered cyclic enamides or hemiaminal ethers with water as the only side product

Synthesis, Structure, and Reactivity of Co(II) and Ni(II) PCP Pincer Borohydride Complexes

The 15e square-planar complexes [Co­(PCP^Me-<i>i</i>Pr)­Cl] (<b>2a</b>) and [Co­(PCP-<i>t</i>Bu)­Cl] (<b>2b</b>), respectively, react readily with NaBH₄ to afford complexes [Co­(PCP^Me-<i>i</i>Pr)­(η²-BH₄)] (<b>4a</b>) and [Co­(PCP-<i>t</i>Bu)­(η²-BH₄)] (<b>4b</b>) in high yields, as confirmed by IR spectroscopy, X-ray crystallography, and elemental analysis. The borohydride ligand is symmetrically bound to the cobalt center in η²-fashion. These compounds are paramagnetic with effective magnetic moments of 2.0(1) and 2.1(1) μ_B consistent with a d⁷ low-spin system corresponding to one unpaired electron. None of these complexes reacted with CO₂ to give formate complexes. For structural and reactivity comparisons, we prepared the analogous Ni­(II) borohydride complex [Ni­(PCP^Me-<i>i</i>Pr)­(η²-BH₄)] (<b>5</b>) via two different synthetic routes. One utilizes [Ni­(PCP^Me-<i>i</i>Pr)­Cl] (<b>3</b>) and NaBH₄, the second one makes use of the hydride complex [Ni­(PCP^Me-<i>i</i>Pr)­H] (<b>6</b>) and BH₃·THF. In both cases, <b>5</b> is obtained in high yields. In contrast to <b>4a</b> and <b>4b</b>, the borohydride ligand is asymmetrically bound to the nickel center but still in an η²-mode. [Ni­(PCP^Me-<i>i</i>Pr)­(η²-BH₄)] (<b>5</b>) loses readily BH₃ at elevated temperatures in the presence of NEt₃ to form <b>6</b>. Complexes <b>5</b> and <b>6</b> are both diamagnetic and were characterized by a combination of ¹H, ¹³C­{¹H}, and ³¹P­{¹H} NMR, IR spectroscopy, and elemental analysis. Additionally, the structure of these compounds was established by X-ray crystallography. Complexes <b>5</b> and <b>6</b> react with CO₂ to give the formate complex [Ni­(PCP^Me-<i>i</i>Pr)­(OC­(CO)­H] (<b>7</b>). The extrusion of BH₃ from [Co­(PCP^Me-<i>i</i>Pr)­(η²-BH₄)] (<b>4a</b>) and [Ni­(PCP^Me-<i>i</i>Pr)­(η²-BH₄)] (<b>5</b>) with the aid of NH₃ to yield the respective hydride complexes [Co­(PCP^Me-<i>i</i>Pr)­H] and [Ni­(PCP^Me-<i>i</i>Pr)­H] (<b>6</b>) and BH₃NH₃ was investigated by DFT calculations showing that formation of the Ni hydride is thermodynamically favorable, whereas the formation of the Co­(II) hydride, in agreement with the experiment, is unfavorable. The electronic structures and the bonding of the borohydride ligand in [Co­(PCP^Me-<i>i</i>Pr)­(η²-BH₄)] (<b>4a</b>) and [Ni­(PCP^Me-<i>i</i>Pr)­(η²-BH₄)] (<b>5</b>) were established by DFT computations

Synthesis and Reactivity of Four- and Five-Coordinate Low-Spin Cobalt(II) PCP Pincer Complexes and Some Nickel(II) Analogues

Anhydrous CoCl₂ or [NiCl₂(DME)] reacts with the ligand PCP^Me-<i>i</i>Pr (<b>1</b>) in the presence of <i>n</i>BuLi to afford the 15<i>e</i> and 16<i>e</i> square planar complexes [Co­(PCP^Me-<i>i</i>Pr)­Cl] (<b>2</b>) and [Ni­(PCP^Me-<i>i</i>Pr)­Cl] (<b>3</b>), respectively. Complex <b>2</b> is a paramagnetic d⁷ low-spin complex, which is a useful precursor for a series of Co­(I), Co­(II), and Co­(III) PCP complexes. Complex <b>2</b> reacts readily with CO and pyridine to afford the five-coordinate square-pyramidal 17<i>e</i> complexes [Co­(PCP^Me-<i>i</i>Pr)­(CO)­Cl] (<b>4</b>) and [Co­(PCP^Me-<i>i</i>Pr)­(py)­Cl] (<b>5</b>), respectively, while in the presence of Ag⁺ and CO the cationic complex [Co­(PCP^Me-<i>i</i>Pr)­(CO)₂]⁺ (<b>6</b>) is afforded. The effective magnetic moments μ_eff of all Co­(II) complexes were derived from the temperature dependence of the inverse molar magnetic susceptibility by SQUID measurements and are in the range 1.9 to 2.4 μ_B. This is consistent with a d⁷ low-spin configuration with some degree of spin–orbit coupling. Oxidation of <b>2</b> with CuCl₂ affords the paramagnetic Co­(III) PCP complex [Co­(PCP^Me-<i>i</i>Pr)­Cl₂] (<b>7</b>), while the synthesis of the diamagnetic Co­(I) complex [Co­(PCP^Me-<i>i</i>Pr)­(CO)₂] (<b>8</b>) was achieved by stirring <b>2</b> in toluene with KC₈ in the presence of CO. Finally, the cationic 16<i>e</i> Ni­(II) PCP complex [Ni­(PCP^Me-<i>i</i>Pr)­(CO)]⁺ (<b>10</b>) was obtained by reacting complex <b>3</b> with 1 equiv of AgSbF₆ in the presence of CO. The reactivity of CO addition to Co­(I), Co­(II), and Ni­(II) PCP square planar complexes of the type [M­(PCP^Me-<i>i</i>Pr)­(CO)]^<i>n</i> (<i>n</i> = +1, 0) was investigated by DFT calculations, showing that formation of the Co species, <b>6</b> and <b>8</b>, is thermodynamically favorable, while Ni­(II) maintains the 16<i>e</i> configuration since CO addition is unfavorable in this case. X-ray structures of most complexes are provided and discussed. A structural feature of interest is that the apical CO ligand in <b>4</b> deviates significantly from linearity, with a Co–C–O angle of 170.0(1)°. The DFT-calculated value is 172°, clearly showing that this is not a packing but an electronic effect

Surface Organometallic Chemistry on Zeolites: Synthesis of Group IV Metal Alkyls and Metal Hydrides on Hierarchical Mesoporous H‑ZSM‑5

Surface organometallic chemistry (SOMC) has mainly been devoted to the reaction of organometallics with surfaces comprising highly divided and dehydroxylated oxides. The field has been extended to SOMC on metal nanoparticles. However, to the best of our knowledge, SOMC has not been extended to hierarchical fibrous zeolites, although zeolitic materials are a particular class of oxides. Zeolite catalysis is important in hydrocarbon industrial chemistry. However, having an optimum balance between the activity and selectivity of the zeolitic catalysts remains a major challenge in the field. The main difficultly is the plethora of surface sites, only some of which are catalytically active. Given that the acido–basic properties and porosity of zeolites are especially important to the refining and petrochemical industries, we decided to explore this rather unexplored area. Here, three novel well-defined single-site materials [(Np)3M@ZSM-5, M = Ti, Zr, and Hf] supported on a hierarchical mesoporous H-ZSM-5 material (1) are reported. They are prepared using the concepts and tools of SOMC. They are further converted to their corresponding metal hydride [(H)nM@ZSM-5, M = Ti, Zr, and Hf, (n = 1–2)] materials (5–7) through controlled hydrogenolysis of [(Si–O−)M(Np)3, M = Ti, Zr, and Hf] materials (2–4) under H2 (1 atm) at 150 °C for 16 h. All these surface catalysts are characterized by various spectroscopic techniques including Fourier transform infrared spectroscopy, elemental analysis, solid-state NMR spectroscopy, powder X-ray diffraction, Brunauer–Emmett–Teller surface area measurements, and scanning electron microscopy and high-resolution transmission electron microscopy analyses and are supported by density functional theory calculations. The catalytic activity of these well-defined single-site novel materials will be tested for the catalytic applications in petrochemistry for refinery processes such as hydrocracking of distillates from crude oil or intermediate refinery process streams to useful petroleum value-added products for the society