Intermediates alkanes has sites available for a double bond. Second, Dr. Pines and his group have shown that terf-butylbenzene re arranges and dehydrogenates to isobutenylbenzene—a methyl carbon in sertion rearrangement—via a free-rad ical mechanism when it is passed over nonacidic chromia-alumina. In this case, the phenyl group provides the necessary unsaturation. Dr. Pines ex plains that this rearrangement is a diagnostic test for a radical mech anism, since the cationic mechanism breaks ferf-butylbenzene into benzene and isobutane. Similarly, 2-phenyl2-C 14 -butane isomerizes and dehydro genates via a radical mechanism to 1butenylbenzenes labeled at the 1-carbon and 2-carbon positions. Further support for a radical mech anism for these methyl carbon inser tion reactions over chromia catalysts is the free-radical mechanism proposed a few years ago by Dr. L. H. Slaugh of Shell Development for methyl carbon insertion reactions in the presence of gaseous iodine. Dr. Pines explains that hydrocarbons generally behave the same in the presence of iodine as in the presence of chromia catalysts. Hydrogen. For the reaction of al
kanes over nonacidic chromia-alumina, Dr. Pines suggests that the first step, the production of the alkene, proceeds via a mechanism suggested five years ago by Dr. R. L. Burwell, also of Northwestern. A chromium (II) atom and an adjacent oxygen atom of the catalyst form a pair of sites and the alkane is adsorbed at the chromium. At high temperatures, this monoadsorbed alkane can then desorb as an alkene. In the second step, according to Dr. Pines' mechanism, the chromium and oxygen sites can then remove a hydro gen from the methyl group of a methylalkene because the methyl carbonhydrogen bond is weakened by the double bond in the molecule. The monoadsorbed alkenyl group then dis sociates into a free radical which re arranges via a C ; r or a C 4 -ring inter mediate to allow insertion of the methyl carbon. Dr. Pines proposes that the C 7 -ring intermediates are produced via a freeradical mechanism in the gas phase— the free radicals being produced via the mechanism for methyl carbon in sertion. But this proposal is only tentative. The Northwestern chemist explains that more data are necessary to differentiate between cyclizations that occur on a catalyst surface and those that occur in the gas phase. However, he adds that other cycliza tions, including 1,6-ring closure, may also eventually be shown to occur by free-radical mechanisms.
Chromia catalyst first dehydrogenates a methylalkane* ·.-.-. ί
CrOa catalyst
^-CH 2 -CH-CrtfCH3
miumBaiic^
CH3-CHZ-CH-CH = CH Z + H r
&£· immmmizmmsë..
, and then removes another hydrogen from the methyl group .
CHr-CH2-CH-C*4=*CH2
.J/2H2
CH3-CH2-CH-^Cri—CH2
CM I
, to give a free radical which rearranges via insertion of the methyl carbon CHj-CHfCH
CHf
C H = C H
]
2
Rearrange» via methyl carbon insertion
Or CH*CH2 Ι 0Η^0Η Γ 0Η^ΟΗ^0Η«0Η 2
Cr C H ^ C H f Ο Η ^ Η ^ CH'CHj
Carbonyl Complexes Catalyze Hydrogénation Complexes promote reaction with olefins, acetylene in solution Several carbonyl complexes of iridium, rhodium, and osmium catalyze homogeneous hydrogénations of olefins and acetylene. Square-planar carbonyl complexes of iridium ( I ) and rhodium (I) that contain two triphenylphosphine ligands catalyze reactions of ethylene, propylene, and acetylene with molecular hydrogen in benzene or toluene solution. The reactions occur at relatively mild temperatures (40° to 60 e C ) when the reacting gases are below atmospheric pressure, according to Dr. Lauri Vaska of Clarkson College of Technology (Potsdam, N.Y.) and R. E. Rhodes of Mellon Institute (Pittsburgh, Pa.) [/. Am. Chcm. Soc. 87, 4970 (1965)]. Five-coordinated metal hydride complexes of iridium and rhodium and six-coordinated osmium complexes also catalyze the homogeneous hydrogénation of unsaturated compounds, Dr. Vaska has found [Inorg. XucL Chcm. Letters, 1, 89 (1965)]. For example, the trigonal bipyramidal hydride decomplex, IrH(CO) (Ph : ,pj ; . (Ph = phenyl), is an effective homogeneous catalyst for the reaction of molecular hydrogen with ethylene at ambient conditions (20° to 40° C. and 700 to 800 mm. Hg total pressure). In earlier work at Mellon, Dr. Vaska and J. W. DiLuzio showed that IrCl ( CO ) ( Ph,P ) L, reacts reversibly with hydrogen under normal conditions. Later, while still at Mellon, Dr. Vaska isolated both the hydrogen and oxygen adducts of the compound (C&EN, June 10, 1963, page 38). The square-planar carbonyl complexes of iridium (I) and rhodium (I) have the general formula transMX(CO) (Ph 3 P) L ,, where M is iridium or rhodium and X is a halogen. Dr. Geoffrey Wilkinson and his co-workers at Imperial College of Science and Technology (London, England) have found that the related rhodium complex, RhCl(Ph :i );> catalyzes the hydrogénation of olefins and acetylene. These British workers have also found that RhCl(CO)(Ph ; i P) 1 ( reacts very slowly with hydrogen in solution [Chem. Communications, 131 (1965)]. The composition, properties, and structures of the catalvst NOV. 8, 1965 C&EN
51
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NOV. 8, 1965
complexes are known. Their solution chemistry and infrared spectral data indicate that their trans-square configurations prevail in solution. In other words, identities of the actual catalysts in solution are known with reasonable certainty. The iridium complexes react reversibly with one mole of hydrogen per mole of complex. Volumetric measurements of hydrogen absoiption (in toluene) indicate that the reaction is first order with respect to the iridium complex. At 20° C. and a constant pressure of 700 mm. of hydrogen, the complex converts almost completely to the dihydride. At lower hydrogen pressures and the same or higher temperatures, the reverse reaction becomes appreciable, and the equilibrium shifts to formation of the original complex. Adducts. Dr. Vaska has isolated and characterized the hydrogen adducts, HoIrX(CO)(Ph 3 P) 2 . He has determined the molecular configurations of these catalytic intermediates, cis-(H)2-trans-(FhsF)2, with IR and nuclear magnetic resonance measurements. The iridium complexes react reversibly with ethylene, acetylene, and other unsaturated compounds at ambient conditions. For example, volumetric measurements of ethylene absoiption by I r I ( C O ) ( P h 3 P ) 2 in toluene at 26° C. and constant ethylene pressure of 700 mm. Hg indicate formation of a colorless adduct, probably (C 2 H 4 ) Irl (CO) (Ph 3 P) 2 . About 70° conversion is obtained. The addition compounds formed with the unsaturated compounds have not been isolated because of their rapid dissociation in the absence of excess substrate, Dr. Vaska explains. In the catalytic experiments, the reactants are added to an air-free solution of the complex, the solution is stirred, and changes in total pressure are noted. In some experiments, the gas mixture is circulated through the solution. After the reaction, gas and solution samples are analyzed chromatographically, and the metal complex is recovered and identified. At 20° to 30° C , Dr. Vaska and Mr. Rhodes obtain very little conversion after several hours. At 40° C , 12 mole % of ethane is produced in 24 hours from ethylene and hydrogen catalyzed by IrCl(CO) (Ph 3 P) L , ; at 60° C , the ethane yield is 40 mole % in 18 hours. Also at 60° C , 10 mole % of propane is obtained from
propylene, and 10 mole f,'f of ethylene and 5 mole % of ethane are obtained from acetylene in 18 hours. Metal Hydride Complexes. In the presence of the five-coordinated iridium complex, I r H ( C O ) (Ph ; .P) :{ , hydrogen and ethylene in a 1:1 mixture seem to have practically no permanent coexistence, Dr. Vaska says. In a reaction in air-free toluene at 30° C , only traces of hydrogen and ethylene remain, and over 93 mole f/f of ethane is obtained (after continuous addition of the reactant gases to the solution of the complex for five hours and then letting the system stand overnight). Most of the iridium complex is recovered unchanged. The iridium hydride complex also catalyzes the hydrogenation of acetylene, Dr. Vaska's work shows. Both ethylene and ethane are produced. This catalysis is less efficient than the ethylene conversion because the complex reacts irreversibly at the same time with acetylene. A six-coordinated osmium hydride complex, OsHCl (CO) (Ph.,P).!, also catalyzes the hydrogenation of acetylene to ethylene and ethane (in toluene at 60° C ) , Dr. Vaska finds. Hydrogen and ethylene each react reversibly with IrH(CO) (Ph,,P) ;; in toluene under normal conditions, his work indicates. The analogous rhodium hydride complex, R h H ( C O ) (Ph 3 P) ;{ , does not take up hydrogen under these conditions, but it acts as a catalyst for the hydrogenation of ethylene. The catalytic osmium hydride complex, OsHCl (CO) (Ph,P) ,„ also doesn't take up any measurable hydrogen, Dr. Vaska notes. The experiments so far suggest that the mechanism of the catalysis of the hydrogen-ethylene reaction is activation by the complex's central metal atom of either the hydrogen or both reactants. Both proposed mechanisms include formation of an active intermediate involving a higher oxidation state and a higher coordination number of the metal. Also, both proposed mechanisms provide a ready path for an irreversible reduction to the original configuration of the complex, he explains. In comparing and predicting the catalytic behavior of complexes of the same or different metals, it is important to consider the relative stabilities of their electronic and stereochemical configurations and those of the (potential) activated adducts, Dr. Vaska says.
