No abstract

No abstract

No abstract

Gentle thermolysis of the allyl complex, CpW(NO)(CH(2)CMe(3))(eta(3)-H(2)CCHCMe(2)) (1), at 50 degrees C in neat hydrocarbon solutions results in the loss of neopentane and the generation of transient intermediates that subsequently activate solvent C-H bonds. Thus, thermal reactions of 1 with tetramethylsilane, mesitylene, and benzene effect single C-H activations and lead to the exclusive formation of CpW(NO)(CH(2)SiMe(3))(eta(3)-H(2)CCHCMe(2)) (2), CpW(NO)(CH(2)C(6)H(3)-3,5-Me(2))(eta(3)-H(2)CCHCMe(2)) (3), and CpW(NO)(C(6)H(5))(eta(3)-H(2)CCHCMe(2)) (4), respectively. The products of reactions of 1 with other methyl-substituted arenes indicate an inherent preference of the system for the activation of stronger arene sp(2) C-H bonds. For example, C-H bond activation of p-xylene leads to the formation of CpW(NO)(CH(2)C(6)H(4)-4-Me)(eta(3)-H(2)CCHCMe(2)) (5) (26%) and CpW(NO)(C(6)H(3)-2,5-Me(2))(eta(3)-H(2)CCHCMe(2)) (6) (74%). Mechanistic and labeling studies indicate that the transient C-H-activating intermediates are the allene complex, CpW(NO)(eta(2)-H(2)C=C=CMe(2)) (A), and the eta(2)-diene complex, CpW(NO)(eta(2)-H(2)C=CHC(Me)=CH(2)) (B). Intermediates A and B react with cyclohexene to form CpW(NO)(eta(3)-CH(2)C(2-cyclohexenyl)CMe(2))(H) (18) and CpW(NO)(eta(3)-CH(2)CHC)(Me)CH(2)C(beta)H(C(4)H(8))C(alpha)H (19), respectively, and intermediate A can be isolated as its PMe(3) adduct, CpW(NO)(PMe(3))(eta(2)-H(2)C=C=CMe(2)) (20). Interestingly, thermal reaction of 1 with 2,3-dimethylbut-2-ene results in the formation of a species that undergoes eta(3) --> eta(1) isomerization of the dimethylallyl ligand following the initial C-H bond-activating step to yield CpW(NO)(eta(3)-CMe(2)CMeCH(2))(eta(1)-CH(2)CHCMe(2)) (21). Thermolyses of 1 in alkane solvents afford allyl hydride complexes resulting from three successive C-H bond-activation reactions. For instance, 1 in cyclohexane converts to CpW(NO)(eta(3)-C(6)H(9))(H) (22) with dimethylpropylcyclohexane being formed as a byproduct, and in methylcyclohexane it forms the two isomeric complexes, CpW(NO)(eta(3)-C(7)H(11))(H) (23a,b). All new complexes have been characterized by conventional spectroscopic methods, and the solid-state molecular structures of 2, 3, 4, 18, 19, 20, and 21 have been established by X-ray crystallographic analyses.

Thermal activation of CpW(NO)(CH(2)CMe(3))(2) (1) in neat hydrocarbon solutions transiently generates the neopentylidene complex, CpW(NO)(=CHCMe(3)) (A), which subsequently activates solvent C-H bonds. For example, the thermolysis of 1 in tetramethylsilane and perdeuteriotetramethylsilane results in the clean formation of CpW(NO)(CH(2)CMe(3))(CH(2)SiMe(3)) (2) and CpW(NO)(CHDCMe(3))[CD(2)Si(CD(3))(3)] (2-d(12)), respectively, in virtually quantitative yields. The neopentylidene intermediate A can be trapped by PMe(3) to obtain CpW(NO)(=CHCMe(3))(PMe(3)) in two isomeric forms (4a-b), and in benzene, 1 cleanly forms the phenyl complex CpW(NO)(CH(2)CMe(3))(C(6)H(5)) (5). Kinetic and mechanistic studies indicate that the C-H activation chemistry derived from 1 proceeds through two distinct steps, namely, (1) rate-determining intramolecular alpha-H elimination of neopentane from 1 to form A and (2) 1,2-cis addition of a substrate C-H bond across the W=C linkage in A. The thermolysis of 1 in cyclohexane in the presence of PMe(3) yields 4a-b as well as the olefin complex CpW(NO)(eta(2)-cyclohexene)(PMe(3)) (6). In contrast, methylcyclohexane and ethylcyclohexane afford principally the allyl hydride complexes CpW(NO)(eta(3)-C(7)H(11))(H) (7a-b) and CpW(NO)(eta(3)-C(8)H(13))(H) (8a-b), respectively, under identical experimental conditions. The thermolysis of 1 in toluene affords a surprisingly complex mixture of six products. The two major products are the neopentyl aryl complexes, CpW(NO)(CH(2)CMe(3))(C(6)H(4)-3-Me) (9a) and CpW(NO)(CH(2)CMe(3))(C(6)H(4)-4-Me) (9b), in approximately 47 and 33% yields. Of the other four products, one is the aryl isomer of 9a-b, namely, CpW(NO)(CH(2)CMe(3))(C(6)H(4)-2-Me) (9c) ( approximately 1%). The remaining three products all arise from the incorporation of two molecules of toluene; namely, CpW(NO)(CH(2)C(6)H(5))(C(6)H(4)-3-Me) (11a; approximately 12%), CpW(NO)(CH(2)C(6)H(5))(C(6)H(4)-4-Me) (11b; approximately 6%), and CpW(NO)(CH(2)C(6)H(5))(2) (10; approximately 1%). It has been demonstrated that the formation of complexes 10 and 11a-b involves the transient formation of CpW(NO)(CH(2)CMe(3))(CH(2)C(6)H(5)) (12), the product of toluene activation at the methyl position, which reductively eliminates neopentane to generate the C-H activating benzylidene complex CpW(NO)(=CHC(6)H(5)) (B). Consistently, the thermolysis of independently prepared 12 in benzene and benzene-d(6) affords CpW(NO)(CH(2)C(6)H(5))(C(6)H(5)) (13) and CpW(NO)(CHDC(6)H(5))(C(6)D(5)) (13-d(6)), respectively, in addition to free neopentane. Intermediate B can also be trapped by PMe(3) to obtain the adducts CpW(NO)(=CHC(6)H(5))(PMe(3)) (14a-b) in two rotameric forms. From their reactions with toluene, it can be deduced that both alkylidene intermediates A and B exhibit a preference for activating the stronger aryl sp(2) C-H bonds. The C-H activating ability of B also encompasses aliphatic substrates as well as it reacts with tetramethylsilane and cyclohexanes in a manner similar to that summarized above for A. All new com...

Gentle thermolysis of appropriate CpM(NO)(hydrocarbyl)(2) complexes (Cp = eta(5)-C(5)Me(5)) of molybdenum and tungsten results in loss of hydrocarbon and the transient formation of 16-electron CpM(NO)-containing complexes such as CpM(NO)(alkylidene), CpM(NO)(eta(2)-benzyne), CpM(NO)(eta(2)-acetylene), and CpM(NO)(eta(2)-allene) (M = Mo, W). These intermediates effect the single, double, or triple activation of hydrocarbon C-H bonds intermolecularly, the first step of these activations being the reverse of the transformations by which they were generated. This Account summarizes the various types of C-H activations that have been effected with these nitrosyl complexes and also describes the results of kinetic, mechanistic, and theoretical investigations of these processes.

CpMo(NO)(CH(2)CMe(3))(2) (1), a complex with alpha-agostic C-H.Mo interactions, evolves neopentane in neat hydrocarbon solutions at room temperature and forms the transient 16-electron alkylidene complex, CpMo(NO)(=CHCMe(3)), which subsequently activates solvent C-H bonds. Thus, it reacts with tetramethylsilane or mesitylene to form CpMo(NO)(CH(2)CMe(3))(CH(2)SiMe(3)) (2) or CpMo(NO)(CH(2)CMe(3))(eta(2)-CH(2)C(6)H(3)-3,5-Me(2)) (3), respectively, in nearly quantitative yields. Under identical conditions, 1 in p-xylene generates a mixture of sp(2) and sp(3) C-H bond activation products, namely CpMo(NO)(CH(2)CMe(3))(C(6)H(3)-2,5-Me(2)) (4, 73%) and CpMo(NO)(CH(2)CMe(3))(eta(2)-CH(2)C(6)H(4)-4-Me) (5, 27%). In benzene at room temperature, 1 transforms to a mixture of CpMo(NO)(CH(2)CMe(3))(C(6)H(5)) (6) and CpMo(NO)(C(6)H(5))(2) (7) in a sequential manner. Most interestingly, the thermal activation of 6 at ambient temperatures gives rise to two parallel modes of reactivity involving either the elimination of benzene and formation of CpMo(NO)(=CHCMe(3)) or the elimination of neopentane and formation of the benzyne complex, CpMo(NO)(eta(2)-C(6)H(4)). In pyridine, these intermediates are trapped as the isolable 18-electron adducts, CpMo(NO)(=CHCMe(3))(NC(5)H(5)) (8) and CpMo(NO)(eta(2)-C(6)H(4))(NC(5)H(5)) (9), and, in hydrocarbon solvents, they effect the intermolecular activation of aliphatic C-H bonds at room temperature to generate mixtures of neopentyl- and phenyl-containing derivatives. However, the distribution of products resulting from the hydrocarbon activations is dependent on the nature of the solvent, probably due to solvation effects and the presence of sigma- or pi-hydrocarbon complexes on the reaction coordinates of the alkylidene and the benzyne intermediates. The results of DFT calculations on these processes in the gas phase support the existence of such hydrocarbon complexes and indicate that better agreement with experimental observations is obtained when the actual neopentyl ligand rather than the simpler methyl ligand is used in the model complexes.

Sequential treatment at low temperatures of Cp*W(NO)Cl2 in THF with 1 equiv of a binary magnesium allyl reagent, followed by an excess of LiBH4, affords three new Cp*W(NO)(H)(η(3)-allyl) complexes, namely, Cp*W(NO)(H)(η(3)-CH2CHCMe2) (1), Cp*W(NO)(H)(η(3)-CH2CHCHPh) (2), and Cp*W(NO)(H)(η(3)-CH2CHCHMe) (3). Complexes 1-3 are isolable as air-stable, analytically pure yellow solids in good to moderate yields by chromatography or fractional crystallization. In solutions, complex 1 exists as two coordination isomers in an 83:17 ratio differing with respect to the endo/exo orientation of the allyl ligand. In contrast, complexes 2 and 3 each exist as four coordination isomers, all differing by the orientation of their allyl ligands which can have either an endo or an exo orientation with the phenyl or methyl groups being either proximal or distal to the nitrosyl ligand. A DFT computational analysis using the major isomer of Cp*W(NO)(H)(η(3)-CH2CHCHMe) (3a) as the model complex has revealed that its lowest-energy thermal-decomposition pathway involves the intramolecular isomerization of 3a to the 16e η(2)-alkene complex, Cp*W(NO)(η(2)-CH2═CHCH2Me). Such η(2)-alkene complexes are isolable as their 18e PMe3 adducts when compounds 1-3 are thermolyzed in neat PMe3, the other organometallic products formed during these thermolyses being Cp*W(NO)(PMe3)2 (5) and, occasionally, Cp*W(NO)(H)(η(1)-allyl)(PMe3). All new complexes have been characterized by conventional spectroscopic and analytical methods, and the solid-state molecular structures of most of them have been established by single-crystal X-ray crystallographic analyses.