@article {1572,
	title = {Synthesis, reactivity, and DFT studies of tantalum complexes incorporating diamido-N-heterocyclic carbene ligands. Facile endocyclic C-H bond activation},
	journal = {Journal of the American Chemical Society},
	volume = {128},
	number = {38},
	year = {2006},
	note = {ISI Document Delivery No.: 085OFTimes Cited: 25Cited Reference Count: 73Spencer, Liam P. Beddie, Chad Hall, Michael B. Fryzuk, Michael D.},
	month = {Sep},
	pages = {12531-12543},
	type = {Article},
	abstract = {The syntheses of tantalum derivatives with the potentially tridentate diamido-N-heterocyclic carbene (NHC) ligand are described. Aminolysis and alkane elimination reactions with the diamine-NHC ligands, (Ar)[NCN]H-2 (where (Ar)[NCN]H-2 = (ArNHCH2CH2)(2)(C3N2); Ar = Mes, p-Tol), provided complexes with a bidentate amide-amine donor configuration. Attempts to promote coordination of the remaining pendent amine donor were unsuccessful. Metathesis reactions with the dilithiated diamido-NHC ligand ((Ar)[NCN]-Li-2) and various ClxTa(NR{\textquoteright}(2))(5-x) precursors were successful and generated the desired octahedral (Ar)[NCN]TaClx(NR{\textquoteright}(2))(3-x) complexes. Attempts to prepare trialkyl tantalum complexes by this methodology resulted in the formation of an unusual metallaaziridine derivative. DFT calculations on model complexes show that the strained metallaaziridine ring forms because it allows the remaining substituents to adopt preferable bonding positions. The calculations predict that the lowest energy pathway involves a tantalum alkylidene intermediate, which undergoes C-H bond activation R to the amido to form the metallaaziridine moiety. This mechanism was confirmed by examining the distribution of deuterium atoms in an experiment between (Mes)[NCN]Li-2 and Cl2Ta(CD2Ph)(3). The single-crystal X-ray structures of (p-Tol)[NCNH]Ta(NMe2)(4) (3), (Mes)[NCNH]Ta=CHPh(CH2Ph)(2) (4), (p-Tol)[NCN]Ta(NMe2)(3) (7), (Mes)[NCCN]Ta((CH2Bu)-Bu-t)(2) (11), and (Mes)[NCCN]-TaCl((CH2Bu)-Bu-t) (14) are included.},
	keywords = {COORDINATION CHEMISTRY, CRYSTAL-STRUCTURE, EFFECTIVE CORE POTENTIALS, GAUSSIAN-TYPE BASIS, METAL-CARBON BONDS, METHODS, MOLECULAR-ORBITAL, ORGANIC-MOLECULES, SET MODEL CHEMISTRY, STABLE CARBENES, STRUCTURAL-CHARACTERIZATION},
	isbn = {0002-7863},
	url = {<Go to ISI>://000240612700055},
	author = {Spencer, L. P. and Beddie, C. and Hall, M. B. and Fryzuk,Michael D.}
}
@article {5076,
	title = {Reaction of [P2N2]Ta=CH2(Me) with ethylene: Synthesis of [P2N2]Ta(C2H4)Et, a neutral species with a beta-agostic ethyl group in equilibrium with an alpha-agostic ethyl group ([P2N2] = PhP(CH2SiMe2NSiMe2CH2)(2)PPh)},
	journal = {Journal of the American Chemical Society},
	volume = {123},
	number = {8},
	year = {2001},
	note = {ISI Document Delivery No.: 405LXTimes Cited: 26Cited Reference Count: 65},
	month = {Feb},
	pages = {1602-1612},
	type = {Article},
	abstract = {The photolysis of [P2N2]TaMe3 ([P2N2] = PhP(CH2SiMe2NSiMe2CH2)(2)PPh) produces [P2N2]Ta=CH2(Me) as the major product. The thermally unstable methylidene complex decomposes in solution in the absence of trapping agents to unidentified products. However, in the presence of ethylene [P2N2]Ta=CH2(Me) is slowly converted to [P2N2]Ta(C2H4)Et, with [P2N2]Ta(C2H4)Me observed as a minor product. A mechanistic study suggests that the formation of [P2N2]Ta(C2H4)Et results from the trapping of [P2N2]TaEt, formed by the migratory insertion of the methylene moiety into the tantalum-methyl bond. The minor product, [P2N2]Ta(C2H4)Me, forms from the decomposition of a tantalacyclobutane resulting from the addition of ethylene to [P2N2]Ta=CH2(Me) and is accompanied by the production of an equivalent of propylene. Pure [P2N2]Ta(C2H4)Et can be synthesized by hydrogenation of [P2N2]TaMe3 in the presence of PMe3, followed by the reaction bf ethylene with the resulting trihydride. Crystallographic and NMR data indicate the presence of a beta -agostic interaction between the ethyl group and tantalum center in [P2N2]Ta(C2H4)Et. Partially deuterated analogues of [P2N2]Ta(C2H4)Et show a large isotopic perturbation of resonance for both the beta -protons and the alpha -protons of the ethyl group, indicative of an equilibrium between a beta -agostic and an a-agostic interaction for the ethyl group in solution. An EXSY spectrum demonstrates that an additional fluxional process occurs that exchanges all of the H-1 environments of the ethyl and ethylene ligands. The mechanism of this exchange is believed to involve the direct transfer of the beta -agostic hydrogen atom from the ethyl group to the ethylene ligand, via the so-called beta -hydrogen transfer process.},
	keywords = {ALKYL ALKENE, C-H, CHAIN PROPAGATION, COMPLEXES, METAL-CARBON BONDS, MIGRATORY INSERTION, MOLECULAR-STRUCTURES, NUCLEAR-MAGNETIC-RESONANCE, OLEFIN, POLYMERIZATION, RAY CRYSTAL-STRUCTURE, TRANSITION-METALS},
	isbn = {0002-7863},
	url = {<Go to ISI>://000167162100008},
	author = {Fryzuk,Michael D. and Johnson, S. A. and Rettig, S. J.}
}
@article {4543,
	title = {Mechanistic studies of the formation of zirconium alkylidene complexes [eta(5)-C5H3-1,3-(SiMe2CH2PPr2i)(2)]Zr=CHR(Cl) (R = Ph, SiMe3)},
	journal = {Journal of the American Chemical Society},
	volume = {121},
	number = {11},
	year = {1999},
	note = {ISI Document Delivery No.: 180AUTimes Cited: 33Cited Reference Count: 32},
	month = {Mar},
	pages = {2478-2487},
	type = {Article},
	abstract = {The reaction of [P2Cp]ZrCl3 (1) with 2 equiv of KCH2Ph generates an equilibrium mixture of alkyl complexes consisting of [P2Cp]ZrCl2(CH2Ph) (2), [P2Cp]ZrCl(CH2Ph)(2) (3), and [P2Cp]Zr(CH2Ph)(3) (4). Thermolysis of this mixture yields the alkylidene complex [P2Cp]Zr=CHPh(Cl) (5) in 85\% overall yield. Kinetic studies reveal a composite mechanism that incorporates the above preequilibrium, followed by an intramolecular alpha-abstraction reaction of dibenzyl 3 which follows a first-order rate, with the rate parameters Delta H double dagger = 19(1) kcal mol(-1) and Delta S double dagger = -22(5) cal(-1) mol K-1. A kinetic isotope effect of 3.0(5) was measured at 70 degrees C for the perdeuterated analogue [P2Cp]ZrCl(CD2C6D5)2. The reaction of 1 with 2 equiv of LiCH2-EMe3 (E = C, Si) produces a similar equilibrium mixture as observed for the benzyl analogues, consisting of [P2Cp]ZrCl2(CH2EMe3) (7), [P2Cp]Zr(CH2EMe3)(3) (8), and [P2Cp]ZrCl(CH2EMe3)(2) (9). Thermolysis of this mixture yields [P2Cp]Zr=CHEMe3(Cl) (6). A kinetic analysis conducted on 9 (E = Si) indicated a first-order reaction from which the activation parameters Delta H double dagger = 6(1) kcal mol(-1) and Delta S double dagger = -62(5) cal mol(-1) K-1 were obtained. The results indicate that reaction rates follow the order CH2Ph > CH2SiMe3 > CH2CMe3, an exact reversal of the trend for the homoleptic Ta systems Ta(CH2R)(5). The role of phosphine coordination is discussed to account for this trend. A crystal structure determination obtained for 6b reveals an alpha-agnostic interaction and a structure analogous to that of 5.},
	keywords = {ALPHA-HYDROGEN ABSTRACTION, CHEMISTRY, LIGAND, METAL-CARBON BONDS, NIOBIUM, OPENING METATHESIS POLYMERIZATION, tantalum, THERMOLYSIS, TITANIUM ALKYLIDENES, TITANOCENE},
	isbn = {0002-7863},
	url = {<Go to ISI>://000079363300019},
	author = {Fryzuk,Michael D. and Duval, P. B. and Mao, Sssh and Zaworotko, M. J. and Macgillivray, L. R.}
}
@article {4546,
	title = {Synthesis and structure of the tantalum trimethyl complex [P2N2]TaMe3 and its conversion to the tantalum methylidene species [P2N2]Ta = CH2(Me) ([P2N2] = PhP(CH2SiMe2NSiMe2CH2)(2)PPh)},
	journal = {Organometallics},
	volume = {18},
	number = {20},
	year = {1999},
	note = {ISI Document Delivery No.: 244CDTimes Cited: 25Cited Reference Count: 42},
	month = {Sep},
	pages = {4059-4067},
	type = {Article},
	abstract = {A tantalum(V) trimethyl complex and its photolysis product, a relatively stable tantalum(V) methyl methylidene, were prepared using the macrocyclic bis(amido-phosphine) PhP(CH2SiMe2NSiMe2CH2)(2)PPh, abbreviated [P2N2], as an ancillary ligand. The methylidene precursor [P2N2]TaMe3 is generated by the reaction of [P2N2]Li-2. C4H8O2 (C4H8O2 = 1,4-dioxane) with TaMe3Cl2. The solid-state X-ray molecular structure demonstrates that the geometry of [P2N2]TaMe3 is a seven-coordinate capped trigonal prism, and NMR studies confirm this is also true in solution at low temperature. Photolysis with a UV source generates [P2N2]Ta=CH2(Me), with the elimination of methane. The X-ray solid-state molecular structure of [P2N2]Ta=CH2(Me) is intermediate between octahedral and trigonal-prismatic geometry. Variable-temperature NMR studies demonstrate that the double-bonded methylidene moiety rotates with a barrier of 33.5 +/- 0.6 kJ/mol, in contrast to the larger barrier estimated for the previously characterized 18-electron complex (eta(5)-C5H5)(2)Ta=CH2(Me). Reaction of [P2N2]TaMe3 with Ph3C+BF4- or PhNMe2H+B(C6F5)(4)(-) produced {[P2N2]TaMe2}+BF4- and {[P2N2]TaMe2}B+(C6F5)(4)(-), respectively. Attempts to deprotonate {[P2N2]TaMe2}(+) did not provide a chemical route to [P2N2]Ta=CH2(Me) and instead generated either [P2N2]TaMe2F or the product of deprotonation of the [P2N2] ligand methylene backbone, Ta(Me-2)[(CHSiMe2NSiMe2CH2P(Ph)CH2SiMe2NSiMe2CH2PPh)], depending on the base and counteranion used.},
	keywords = {ABSTRACTION, CATALYSIS, COORDINATION, CRYSTAL-STRUCTURES, DECOMPOSITION, DERIVATIVES, imido, METAL-CARBON BONDS, METHYLENE COMPLEX, REACTIVITY},
	isbn = {0276-7333},
	url = {<Go to ISI>://000083033500019},
	author = {Fryzuk,Michael D. and Johnson, S. A. and Rettig, S. J.}
}
