The dialkyl malonate derived 1,3‐diphosphines R2C(CH2PPh2)2(R=
The production of olefins via on‐purpose dehydrogenation of alkanes allows for a more efficient, selective and lower cost alternative to processes such as steam cracking. Silica‐supported pincer‐iridium complexes of the form [(≡SiO−R4POCOP)Ir(CO)] (R4POCOP=κ3‐C6H3‐2,6‐(OPR2)2) are effective for acceptorless alkane dehydrogenation, and have been shown stable up to 300 °C. However, while solution‐phase analogues of such species have demonstrated high regioselectivity for terminal olefin production under transfer dehydrogenation conditions at or below 240 °C, in open systems at 300 °C, regioselectivity under acceptorless dehydrogenation conditions is consistently low. In this work, complexes [(≡SiO−
- Award ID(s):
- 1705746
- NSF-PAR ID:
- 10236534
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- ChemCatChem
- Volume:
- 13
- Issue:
- 1
- ISSN:
- 1867-3880
- Page Range / eLocation ID:
- p. 407-415
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
Abstract a , Me;b , Et;c ,n ‐Bu;d ,n ‐Dec;e , Bn;f ,p ‐tolCH2) are combined with (p ‐tol3P)2PtCl2ortrans ‐(p‐ tol3P)2Pt((C≡C)2H)2to give the chelatescis ‐(R2C(CH2PPh2)2)PtCl2(2 a –f , 94–69 %) orcis ‐(R2C(CH2PPh2)2)Pt((C≡C)2H)2(3 a –f , 97–54 %). Complexes3 a –d are also available from2 a –d and excess 1,3‐butadiyne in the presence of CuI (cat.) and excess HNEt2(87–65 %). Under similar conditions,2 and3 react to give the title compounds [(R2C(CH2PPh2)2)[Pt(C≡C)2]4(4 a –f ; 89–14 % (64 % avg)), from which ammonium salts such as the co‐product [H2NEt2]+Cl−are challenging to remove. Crystal structures of4 a ,b show skew rhombus as opposed to square Pt4geometries. The NMR and IR properties of4 a –f are similar to those of mono‐ or diplatinum model compounds. However, cyclic voltammetry gives only irreversible oxidations. As compared to mono‐platinum or Pt(C≡C)2Pt species, the UV‐visible spectra show much more intense and red‐shifted bands. Time dependent DFT calculations define the transitions and principal orbitals involved. Electrostatic potential surface maps reveal strongly negative Pt4C16cores that likely facilitate ammonium cation binding. Analogous electronic properties of Pt3C12and Pt5C20homologs and selected equilibria are explored computationally. -
Abstract The complex structure of the catalytic active phase, and surface‐gas reaction networks have hindered understanding of the oxidative coupling of methane (OCM) reaction mechanism by supported Na2WO4/SiO2catalysts. The present study demonstrates, with the aid of in situ Raman spectroscopy and chemical probe (H2‐TPR, TAP and steady‐state kinetics) experiments, that the long speculated crystalline Na2WO4active phase is unstable and melts under OCM reaction conditions, partially transforming to thermally stable surface Na‐WO
x sites. Kinetic analysis via temporal analysis of products (TAP) and steady‐state OCM reaction studies demonstrate that (i ) surface Na‐WOx sites are responsible for selectively activating CH4to C2Hx and over‐oxidizing CHyto CO and (ii ) molten Na2WO4phase is mainly responsible for over‐oxidation of CH4to CO2and also assists in oxidative dehydrogenation of C2H6to C2H4. These new insights reveal the nature of catalytic active sites and resolve the OCM reaction mechanism over supported Na2WO4/SiO2catalysts. -
Abstract The complex structure of the catalytic active phase, and surface‐gas reaction networks have hindered understanding of the oxidative coupling of methane (OCM) reaction mechanism by supported Na2WO4/SiO2catalysts. The present study demonstrates, with the aid of in situ Raman spectroscopy and chemical probe (H2‐TPR, TAP and steady‐state kinetics) experiments, that the long speculated crystalline Na2WO4active phase is unstable and melts under OCM reaction conditions, partially transforming to thermally stable surface Na‐WO
x sites. Kinetic analysis via temporal analysis of products (TAP) and steady‐state OCM reaction studies demonstrate that (i ) surface Na‐WOx sites are responsible for selectively activating CH4to C2Hx and over‐oxidizing CHyto CO and (ii ) molten Na2WO4phase is mainly responsible for over‐oxidation of CH4to CO2and also assists in oxidative dehydrogenation of C2H6to C2H4. These new insights reveal the nature of catalytic active sites and resolve the OCM reaction mechanism over supported Na2WO4/SiO2catalysts. -
Abstract The catalytic one‐bond isomerization (transposition) of 1‐alkenes is an emerging approach to
Z ‐2‐alkenes. Design of more selective catalysts would benefit from a mechanistic understanding of factors controllingZ selectivity. We propose here a reaction pathway forcis ‐Mo(CO)4(PCy3)(piperidine) (3 ), a precatalyst that shows highZ selectivity for transposition of alpha olefins (e. g., 1‐octene to 2‐octene, 18 : 1Z :E at 74 % conversion). Computational modeling of reaction pathways and isotopic labeling suggests the isomerization takes place via an allyl (1,3‐hydride shift) pathway, where oxidative addition offac ‐(CO)3Mo(PCy3)(η2‐alkene) is followed by hydride migration from one position (cis to allyl C3carbon) to another (cis to allyl C1carbon) via hydride/CO exchanges. Calculated barriers for the hydride migration pathway are lower than explored alternative mechanisms (e. g., change of allyl hapticity, allyl rotation). To our knowledge, this is the first study to propose such a hydride migration in alkene isomerization. -
Abstract A low‐spin and mononuclear vanadium complex, (Menacnac)V(CO)(η2‐P≡C
t Bu) (2 ) (Menacnac−=[ArNC(CH3)]2CH, Ar=2,6‐i Pr2C6H3), was prepared upon treatment of the vanadium neopentylidyne complex (Menacnac)V≡Ct Bu(OTf) (1 ) with Na(OCP)(diox)2.5(diox=1,4‐dioxane), while the isoelectronic ate‐complex [Na(15‐crown‐5)]{([ArNC(CH2)]CH[C(CH3)NAr])V(CO)(η2‐P≡Ct Bu)} (4 ), was obtained via the reaction of Na(OCP)(diox)2.5and ([ArNC(CH2)]CH[C(CH3)NAr])V≡Ct Bu(OEt2) (3 ) in the presence of crown‐ether. Computational studies suggest that the P‐atom transfer proceeds by [2+2]‐cycloaddition of the P≡C bond across the V≡Ct Bu moiety, followed by a reductive decarbonylation to form the V−C≡O linkage. The nature of the electronic ground state in diamagnetic complexes,2 and4 , was further investigated both theoretically and experimentally, using a combination of density functional theory (DFT) calculations, UV/Vis and NMR spectroscopies, cyclic voltammetry, X‐ray absorption spectroscopy (XAS) measurements, and comparison of salient bond metrics derived from X‐ray single‐crystal structural characterization. In combination, these data are consistent with a low‐valent vanadium ion in complexes2 and4 . This study represents the first example of a metathesis reaction between the P‐atom of [PCO]−and an alkylidyne ligand.