Alkane activation by silica supported Group VB metal hydrides. A quantum-chemical study

Михайлов, М. Н.; Kustov, L. М.

doi:10.1007/s11172-005-0252-1

Cited by 16 publications

(20 citation statements)

References 21 publications

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“…The tantalum atom is covalently bonded to two vicinal oxygen surface atoms leading to a non-constrained 6-membered ring [{(l-O)[(HO) 2 SiO] 2 }Ta(NH)(NH 2 )], 2 q, whose notation will be simplified to [(-SiO) 2 Ta(NH)(NH 2 )]. This model is very similar or equal to those used in previous studies for modeling silica-supported transition metal hydrides [63][64][65][66][67][68]. The reliability of the representation of 2T (Si 2 O 7 H 4 )Ta has been verified by comparing the structures and energetics of all minima obtained with the cluster model with those obtained with the periodic calculations, using (C (100) ) as model for silica.…”

Section: Dft Calculationssupporting

confidence: 52%

H/D Exchange on Silica-Grafted Tantalum(V) Imido Amido [(≡SiO)2Ta(V)(NH)(NH2)] Synthesized from Either Ammonia or Dinitrogen: IR and DFT Evidence for Heterolytic Splitting of D2

et al. 2009

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The silica-grafted Ta (V) imido amido complex [(:SiO) 2 Ta(NH)(NH 2 )], 2, obtained from the reaction of either ammonia or dinitrogen plus hydrogen with the silicagrafted hydrides [(:SiO) 2 Ta (III) H], 1a, and [(:SiO) 2-Ta (V) H 3 ], 1b, undergoes H/D exchange with D 2 . In situ IR spectroscopy shows that the fully labelled compound [(:SiO) 2 Ta(ND)(ND 2 )], 2-d, can be obtained by moderate heating (60°C, 3 h) under D 2 atmosphere (550 torr, 300 eq. with respect to Ta), and that the exchange is reversible. The observed stretching and bending frequencies of 2-d are in agreement with the expected isotopic shift upon H/D replacement with respect to literature values reported for 2 and have been corroborated by the independent synthesis of 2-d by reaction of deuterated 1a and 1b with N 2 and D 2 . Density functional theory (DFT) calculations, performed using a periodic or a cluster model, explored the structures and energetics of all minima involved in the reaction with H 2 and showed that among the explored pathways the energetically preferred mechanisms for H 2 reaction with [{(l-O)[(HO) 2 SiO] 2 }Ta (V) (NH) (NH 2 )], 2q, is the heterolytic cleavage of either the imido Ta=N or the amido Ta-N bonds, to yield respectively [{(l-O)[(HO) 2 SiO] 2 }TaH(NH 2 ) 2 ], 3q (DE = -9.5 kcal mol -1 and DG 298K = ?2.6 kcal mol -1 with respect to 2q) and [{(l-O)[(HO) 2 SiO] 2 }Ta(NH) (NH 3 )], 4q (DE = -6.0 kcal mol -1 and DG 298K = ?7.9 kcal mol -1 with respect to 2q). All activation barriers are moderate (between 17.7 and 30.2 kcal mol -1 ) in agreement with the observed mild heating conditions necessary for the reaction to occur.

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Section: Dft Calculationssupporting

confidence: 52%

H/D Exchange on Silica-Grafted Tantalum(V) Imido Amido [(≡SiO)2Ta(V)(NH)(NH2)] Synthesized from Either Ammonia or Dinitrogen: IR and DFT Evidence for Heterolytic Splitting of D2

et al. 2009

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“…[9,11,12] Silica-supported tantalum hydrides also catalyze the conversion of alkanes into their direct lower and higher homologues. This reaction, named alkane metathesis, [13] involves carbene and metallacyclobutane intermediates, [14,15] and it explains the selective formation of the directly lower and higher linear homologues of the starting alkane through the transfer of one carbon atom at a time, thereby preventing the formation of long-chain alkanes.…”

mentioning

confidence: 99%

Homologation of Propane Catalyzed by Oxide‐Supported Zirconium Dihydride and Dialkyl Complexes

et al. 2007

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Zirconium hydrides supported on oxide materials catalyze several classical reactions, such as hydrogenation, reductive cyclization, cyclotrimerization, and olefin polymerization. [1][2][3][4][5] Notably, they also catalyze the hydrogenolysis of polyolefins and alkanes, except ethane, and this reaction corresponds to the reverse of the polymerization of olefins. [6][7][8][9] Under similar conditions, alkanes, including ethane, are transformed into their lower homologues in the presence of a catalytic amount of silica-supported tantalum hydride.[10] Cleavage of the carbon-carbon bond is different for both catalysts (Group 4 vs. Group 5): b-alkyl transfer for d 0 zirconium catalysts versus a-alkyl transfer for d 2 tantalum catalysts so that hydrogenolysis of a C À C bond in the skeleton of an alkane occurs by removal of one carbon atom at a time for tantalum catalysts and of at least two carbon atoms for zirconium catalysts, hence the difference in the selectivity of the final product (Scheme 1). [9,11,12] Silica-supported tantalum hydrides also catalyze the conversion of alkanes into their direct lower and higher homologues. This reaction, named alkane metathesis, [13] involves carbene and metallacyclobutane intermediates, [14,15] and it explains the selective formation of the directly lower and higher linear homologues of the starting alkane through the transfer of one carbon atom at a time, thereby preventing the formation of long-chain alkanes. We have exploited the specific properties of Zr, which is able to 1) activate C À H bonds of alkanes through s-bond metathesis, 2) carry out hydrogenolysis (the transfer of at least two carbon atoms through b-alkyl transfer), and 3) polymerize olefins by insertion. [16,17] Herein, we report on the transformation of propane into longer-chain higher homologues (up to C 10 ) at moderate temperatures.In an autoclave, zirconium hydride supported on silicaalumina was brought into contact with 500 equivalents of propane under supercritical conditions (43 bar, d = 510 Kg m À3 ). After 48 h at 200 8C, 12.6 % of the propane was converted into lower (39.1 %) and higher homologues (60.9 %). Notably, the major product was 2-methylpropane (20.7 %), and overall branched higher homologues up to C 10 are favored, and particularly the iso homologues, except for pentanes (see the Supporting Information, Table S1 for detailed selectivities). This selectivity is in contrast with that reported with tantalum hydride or tungsten hydride supported on oxide supports: [13,18] 1) linear higher homologues are the major products; 2) higher homologues are mainly butanes and pentanes along with trace amounts of hexanes.The reaction was studied in a flow reactor to identify primary products by investigating the effect of inverse space velocity (contact time) on product selectivity.First, using a constant flow of propane (600 kPa, 3 N mL) at 216 8C, we observed the formation of H 2 , methane, and ethane, which resulted from the C À H bond activation of propane, and its subsequent hydrogenolysis (Supportin...

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“…While zirconium and group 4 metals cleave the CeC bond of hydrocarbons by b-alkyl transfer, tantalum cleave the CeC bond of alkane (including ethane) one carbon at a time [113,114]. Thus, it was proposed that a-alkyl transfer [105] was the key for the CeC bond activation, so that alkylidenes were again involved as key intermediates in this process [115].…”

Section: Hydrogenolysis Of Alkanes: Formation Of Alkylidene Intermedimentioning

confidence: 99%

Alkylidene and alkylidyne surface complexes: Precursors and intermediates in alkane conversion processes on supported single-site catalysts

Rascón¹,

Copéret²

2011

Journal of Organometallic Chemistry

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Alkane activation by silica supported Group VB metal hydrides. A quantum-chemical study

Cited by 16 publications

References 21 publications

H/D Exchange on Silica-Grafted Tantalum(V) Imido Amido [(≡SiO)2Ta(V)(NH)(NH2)] Synthesized from Either Ammonia or Dinitrogen: IR and DFT Evidence for Heterolytic Splitting of D2

H/D Exchange on Silica-Grafted Tantalum(V) Imido Amido [(≡SiO)2Ta(V)(NH)(NH2)] Synthesized from Either Ammonia or Dinitrogen: IR and DFT Evidence for Heterolytic Splitting of D2

Homologation of Propane Catalyzed by Oxide‐Supported Zirconium Dihydride and Dialkyl Complexes

Alkylidene and alkylidyne surface complexes: Precursors and intermediates in alkane conversion processes on supported single-site catalysts

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