Combinatorial screening of an in situ generated library of tungsten oxyhalide and imido complexes for olefin metathesis.

A series of substituted tungsten(VI) halides with general formula WECl4 (E = O or -NR (imido)) were screened via a high throughput study to identify potential new olefin metathesis catalysts. The tungsten species were treated with a series of aluminum alkyl activators and modifier ligands to generate active catalyst species in situ. Ring-opening metathesis polymerization (ROMP) of cyclooctene was used as a primary screen to identify potential metathesis catalysts and active catalysts were subjected to a secondary screen to evaluate tolerance toward polar functional groups. Several combinations from the high throughput campaign yielded active metathesis catalysts for the ROMP of cyclooctene. However, none of the catalysts examined in this study exhibited any evidence of significant polar functional group tolerance as determined by the results of the secondary cyclooctene/butyl acetate screen.

(η(4)-Cyclo-octa-tetra-ene)(η(8)-cyclo-octa-tetra-ene)iodido-tantalum(V).

The title complex, [Ta(η(4)-C8H8)(η(8)-C8H8)I], lies across a crystallographic mirror plane that includes the Ta(V) atom and the iodide ligand. One cyclo-octa-tetra-ene (cot) ring is η(4)-coordinating and is bis-ected by the mirror plane. The fold angle between the plane of the coordinating butadiene portion and the middle plane of the ring is 27.4 (4)°. An additional minor fold angle of 9.3 (7)° exists between the final plane in the ring and the middle plane. The other cot ring is η(8)-coordinating and is also cut by the mirror plane. In this case, the ring is disordered over the mirror plane, and one position is modeled with appropriate restraints and constraints with respect to distances, angles and displacement parameters (the second position is generated by symmetry). This ring is nearly planar, with an r.m.s. deviation of only 0.05 Šwhen all eight C atoms are included in the calculation. Pairs of inter-molecular η(8)-cot rings are parallel stacked and slightly off center, with a centroid-centroid distance of 3.652 Å. No other significant inter-molecular inter-actions are observed. The compound is of inter-est as the first structurally characterized mixed halogen-cot complex of the group 5 metals and contains the longest terminal Ta-I distance [3.0107 (5) Å] reported to date.

A total loss of innocence: double ortho-metallation of bis(triphenylphosphano)iminium cation, [N(PPh(3))(2)](+), by tris(eta-naphthalene)tantalate(1-).

For the first time [N(PPh(3))(2)](+), or [PPN](+), has been shown to undergo an irreversible reaction with a transition metal complex under ambient conditions and affords a product containing a unique structural motif in which two phenyl groups on one PPh(3) substituent of [PPN](+) are ortho-metallated, while the third phenyl ring is hydrogenated to provide a tantalum bound 1,3-cyclohexadiene group.

Homoleptic isocyanidemetalates of 4d- and 5d-transition metals: [Nb(CNXyl)(6)](-), [Ta(CNXyl)(6)](-), and derivatives thereof.

Treatment of [M(CO)(6)](-), M = Nb, Ta, with Ag(+), I(2) or NO(+) in the presence of CNXyl provided [M(CNXyl)(7)](+), M(CNXyl)(6)I, or cis-[M(CNXyl)(4)(NO)(2)](+), which are isocyanide analogues of the unknown carbonyl complexes [M(CO)(7)](+), M(CO)(6)I, or cis-[M(CO)(4)(NO)(2)](+), respectively. Reduction of M(CNXyl)(6)I by cesium graphite gave the respective Cs[M(CNXyl)(6)], which have been structurally characterized and represent the first isolable homoleptic isocyanidemetalates for second or third row transition metals. Nitrosylation of [Ta(CNXyl)(6)](-) affords a rare example of a mononitrosyl tantalum complex, Ta(CNXyl)(5)NO, which is an isocyanide analogue of the unknown Ta(CO)(5)NO. This study emphasizes, inter alia, the remarkable versatility of the CNXyl ligand compared to CO in stabilizing various electronic environments at heavier group 5 metal centers.

Tris(eta4-naphthalene)- and tris(1-4-eta4-anthracene)tantalate(1-): first homoleptic arene complexes of anionic tantalum.

Reduction of tantalum pentachloride by 6 equiv of sodium naphthalene in 1,2-dimethoxyethane provided, after recrystallization from tetrahydrofuran, 50-55% yields of yellow, pyrophoric [Na(THF)][Ta(C10H8)3]. The product was shown by 1H and 13C NMR spectra and an X-ray study (on the corresponding [Na(crypt 2.2.2)]salt) to be tris(eta4-naphthalene)tantalate(1-), 1, the first homoleptic naphthalene complex of a third row (5d) transition metal. Salts of 1 react under mild conditions with excess CO (1 atm pressure, -60 degrees to +20 degrees C) and 3 equiv of anthracene, C14H10 (20 degrees C), to give 99 and 52% yields of yellow [Ta(CO)6]- and orange [Ta(C14H10)3]-, (2), respectively. The latter is the first homoleptic anthracene complex of a group 5 element and only the third one known, the others being Cr(eta6-C14H10)2 and [Co(eta4-C14H10)2]-. NMR spectra and X-ray structural characterization, as the [Na(crypt 2.2.2)] salt, established 2 to be [Ta(1-4-eta4-C14H10)3]- and is very similar to 1 in solution and in the solid state. Salts of 2 also undergo facile ligand substitution reactions. For example, it reacts with 1,3,5,7-cyclooctatetraene, COT, at 20 degrees in THF to give high yields of the previously known [Ta(COT)3]-, which was structurally characterized as the Na(crypt 2.2.2)salt. One particularly important feature concerning 1 and 2 is that they are the first available synthons for "naked" atomic Ta- and promise to be useful reagents for the general exploration of low-valent tantalum chemistry. Also, 1 and 2 represent the first homoleptic arene tantalum complexes to have been prepared by conventional syntheses. The only previously known substance of this class is the neutral bis(benzene)tantalum(0), which was accessed by the co-condensation of atomic tantalum and benzene vapor in a sophisticated (electron-gun furnace equipped) metal atom reactor.