- Transformations of group 7 carbonyl complexes: Possible intermediates in a homogeneous syngas conversion scheme
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A variety of C-H and C-C bond forming reactions of group 7 carbonyl complexes have been studied as potential steps in a homogeneously catalyzed conversion of syngas to C2+ compounds. The metal formyl complexes M(CO)3(PPh3)2(CHO) (M = Mn, Re) are substantially stabilized by coordination of boranes BX3 (X = F, C6F5) in the form of novel boroxycarbene complexes M(CO)3- (PPh3)2(CHOBX3), but these boron-stabilized carbenes do not react with hydride sources to undergo further reduction to metal alkyls. The related manganese methoxycarbene cations [Mn(CO)5-x(PPh3)x(CHOMe)]+ (x = 1 or 2), obtained by methylation of the formyls, do react with hydrides to form methoxymethyl complexes, which undergo further migratory insertion under an atmosphere of CO. The resulting acyls, cis- and trans-Mn(PPh3)(CO) 4(C(O)CH2OMe), can be alkylated to form the cationic carbene complex [Mn(PPh3)(CO)4(C(OR)CH2OMe)] +, which undergoes a 1,2 hydride shift to form 1,2-dialkoxyethylene, which is displaced from the metal, releasing triflate or diethyl ether adducts of [Mn(PPh3)(CO)4]+. The acyl can also be protonated with HOTf to form a hydroxycarbene complex, which rearranges to Mn(PPh3)(CO)4(CH2COOMe) and is protonolyzed to yield methyl acetate and [Mn(PPh3)(CO)4]+; addition of L (L = PPh3, CO) to the manganese cation regenerates [Mn(PPh3)(CO)4(L)]+. Since the original formyl complex can be obtained by the reaction of [Mn(PPh3)(CO) 5]+ with [PtH(dmpe)2]+, which in turn can be generated from H2, this set of transformations amounts to a stoichiometric cycle for selectively converting H2 and CO into a C2 compound under mild conditions.
- Elowe, Paul R.,West, Nathan M.,Labinger, Jay A.,Bercaw, John E.
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p. 6218 - 6227
(2010/01/29)
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- Electron transfer versus nucleophilic pathways in the ion-pair annihilation of organoborate anions by carbonylmanganese(I) cations
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Substituted carbonylmanganese cations [Mn(CO)5L]+, where L = py, PPh3 and PPh2Me, readily react with various organoborate anions (tetramethylborate, methyltriphenylborate and tetraphenylborate) in THF solution to afford a mixture of dimanganese carbonyls, hydridomanganese carbonyls and alkylmanganese carbonyls. The formation of the dimanganese carbonyl dimers as well as the hydridomanganese carbonyls suggests the involvement of 19-electron carbonylmanganese radicals that stem from an initial electron transfer. On the other hand, the acetonitrile-substituted analogue [Mn(CO)5(CH3CN)]+ reacts with the same borate anions to afford the alkylated RMn(CO)5, where R = CH3 and C6H5, as the sole carbonylmanganese product. As such, this alkylative annihilation is best formulated as a direct attack on the carbonyl carbon by the borate nucleophile. The two different pathways can be understood in terms of the balance between the electrophilicity of the carbonyl ligand and the electron affinity of the carbonylmanganese cation.
- Zhu, Dunming,Kochi, Jay K.
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p. 295 - 303
(2007/10/03)
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- Formation of μ-silylene μ-hydrido manganese-platinum heterobimetallics via oxidative addition of (OC)5MnSiR2H to zerovalent platinum compounds and the structure of (OC)4Mn(μ-PPh2)(μ-H)PtPh(PPh3), a product of a solvolysis of a silylene bridge
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The complexes (OC)5MnSiR2H (R = Me, Ph, Cl) react with Pt(C2H4)(PPh3)2 or Pt(PPh3)4 via oxidative addition of the Si-H bond across Pt to give the μ-silylene μ-hydrido complexes (OC)4Mn(μ-SiR2)(μ-H)-Pt(PPh3) 2. These complexes react with PEt3 to give (OC)4Mn(μ-SiR2)(μ-H)Pt(PEt3)2, react reversibly with CO to give (OC)4Mn(μ-SiR2)(μ-H)Pt(PPh3)(CO), and react with MeOH or H2O to give (OC)4Mn(μ-PPh2)(μ-H)PtPh(PPh3) (8) (a product of P-Ph bond cleavage). The structure of 8 has been determined by single-crystal X-ray diffraction. Complex 8 is monoclinic, space group P21/c, with a = 12.929 (2) A?, b = 26.382 (5) A?, c = 11.245 (2) A?, β = 110.52 (1)°, V = 3592 A?3, and Dcalcd = 1.63 g cm-3 for 7 = 4. The structure was refined to R = 0.0348 and wR = 0.0460 for the 4763 reflections with I > 3σ(I). The structure of 8 consists of distorted pseudo-square-planar Pt and pseudooctahedral Mn centers with trans phenyl and hydride ligands on Pt. The Mn and Pt atoms are separated by 2.864 (1) A? and bridged by μ-PPh2 and μ-H ligands. The position of the μ-H was located and refined. Associated bond lengths are Pt-H = 1.64 (8) A? and Mn-H = 1.80 (8) A?; 〈PtHMn = 113 (4)°.
- Powell, John,Sawyer, Jeffery F.,Shiralian, Mahmoud
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p. 577 - 583
(2008/10/08)
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- Formation of metal-metal bonds by ion-pair annihilation. Dimanganese carbonyls from manganate(-I) anions and manganese(I) cations
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The coupling of the anionic Mn(CO)5- and the cationic Mn(CO)6+ occurs upon mixing to afford the dimeric Mn2(CO)10 in essentially quantitative yields. Dimanganese decacarbonyl is formed with equal facility from the coupling of Mn(CO)5- with Mn(CO)5(py)+ and Mn(CO)5(NCMe)+. By way of contrast, the annihilation of Mn(CO)4PPh3- with Mn(CO)6+ yields a pair of homo dimers Mn2(CO)10 and Mn2(CO)8(PPh3)2 together with the cross dimer Mn2(CO)9PPh3. Extensive scrambling of the carbonylmanganese moieties also obtains with Mn(CO)4P(OPh)3- and Mn(CO)5PPh3+, as indicated by the production of Mn2(CO)8[P(OPh)3]2, Mn2(CO)8[P(OPh)3](PPh3), and Mn2(CO)8(PPh3)2 in more or less statistical amounts. These diverse Mn-Mn couplings can be accounted for by a generalized formulation (Scheme VI), in which the carbonylmanganese anions Mn(CO)4P- and the cations Mn(CO)5L+ undergo an initial electron transfer to produce Mn(CO)4P? and Mn(CO)5L?, respectively. The behaviors of these 17- and 19-electron radicals coincide with those independently generated in a previous study of the anodic oxidation of Mn(CO)4P- and the cathodic reduction of Mn(CO)5L+, respectively. The facile associative ligand substitution of 17-electron carbonylmanganese radicals by added phosphines provides compelling evidence for the interception of Mn(CO)4P? and its interconversion with 19-electron species in the course of ion-pair annihilation. The reactivity trend for the various ion pairs qualitatively parallels the driving force for electron transfer based on the oxidation and reduction potentials of Mn(CO)4P- and Mn(CO)5L+, respectively, in accord with the radical-pair mechanism in Scheme VI.
- Lee,Kuchynka,Kochi
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p. 1886 - 1897
(2008/10/08)
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- A Radical-Chain Mechanism for Dinuclear C-H Bond Formation
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Both the formation of (Ph3PAu)2Os(CO)4 from Ph3PAuCH3 and H2Os(CO)4 and the formation of (Ph3PAu)Mn(CO)5 from Ph3PAuCH3 and HMn(CO)5 occur by radical-chain mechanisms.The chain carriers are .Mn(CO)5 and .Os(H)(CO)4, respectively, arising from hydrogen atom abstraction from the initial hydrides.Photolysis of a small amount of the appropriate dimer (Mn2(CO)10 or H2Os2(CO)8) generates the chain carrier and thus initiates the reaction.No such reaction occurs between Ph3PAuCH3 and HRe(CO)5, even in the presence of substantial amounts of .Re(CO)5.The formation of H2Os(CO)3PPh3 from Ph3P and H2Os(CO)4 also occurs by a radical-chain mechanism with .Os(H)(CO)4 as the chain carrier, and the reactions of Ph3PAuCH3 and Ph3P with H2Os(CO)4 can be simultaneously initiated.
- Edidin, Robin T.,Norton, Jack R.
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p. 948 - 953
(2007/10/02)
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- Manganese(0) radicals and the reduction of cationic carbonyl complexes: Selectivity in the ligand dissociation from 19-electron species
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Products and stoichiometry for the cathodic reduction of the series of carbonylmanganese(I) cations Mn(CO)5L+, where L - CO, MeCN, pyridine, and various phosphines, derive from 1-electron transfer to generate the 19-electron radicals Mn(CO)5L? as reactive intermediates. The CO derivative Mn(CO)6+ affords mainly the anionic Mn(CO)5- by the facile ligand dissociation of Mn(CO)6? to the 17-electron radical Mn(CO)5? followed by reduction. The acetonitrile and pyridine derivatives Mn(CO)5NCMe+ and Mn(CO)5py+ produce high yields of the dimer Mn2(CO)10 by an unusual and highly selective heterolytic coupling of Mn(CO)5- and the reactant cation. Structural factors involved in the conversion of 19-electron radicals to their 17-electron counterparts are examined in the reduction of the graded series of phosphine derivatives Mn(CO)5P+, where P = triaryl- and trialkylphosphines. The formation of the hydridomanganese complexes HMn(CO)4P is ascribed to hydrogen atom transfer to the 19-electron radicals Mn(CO)5P? followed by extrusion of CO. The lability of carbonylmanganese radicals is underscored by rapid ligand substitution to afford the bis(phosphine) byproduct HMn(CO)3P2.
- Kuchynka,Amatore,Kochi
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p. 4087 - 4097
(2008/10/08)
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