Low-Valent Tungsten Aryloxide Compounds with Pyridine Ancillary Ligation: Intramolecular Reduction of Heterocyclic and Aromatic Rings Including Formation of an η5-Cyclohexadienyl Group
2003-05-01T00:00:00Z (GMT) by
The sodium amalgam reduction (2 Na per W) of hydrocarbon solutions of [W(OC6HPh4-2,3,5,6)2Cl4] in the presence of pyridine or 4-tert-butylpyridine leads to the W(IV) compounds [W(OC6HPh4-2,3,5,6)2Cl2(py)2] (1) and [W(OC6HPh4-2,3,5,6)2Cl2(4-tert-butylpyridine)2] (2) in good yields. The ambient-temperature 1H NMR spectra of both 1 and 2 were found to be sharp and to exhibit extensive contact shifts due to the paramagnetic tungsten center. The addition of 2,2‘-bipyridine to an emerald green solution of [W(OC6HPh3-η6-C6H5)(OC6HPh4-2,3,5,6)(PEt3)] produced [W(OC6HPh4-2,3,5,6)2(bipy)2] (3), containing two bipy rings and two terminal aryloxide ligands coordinated mutually cis. The W−N(bipy) distances of 2.092(2), 2.093(2), 2.096(2), and 2.115(2) Å in 3 are among the shortest so far reported for bipy ligands bound to either molybdenum or tungsten. The solid state and solution data for 3 indicate that reduction of both bipy ligands has occurred, but whether this involves four-electron reduction to yield d0-W(VI) is unclear. The addition of 4-tert-butylpyridine to [W(OC6HPh3-η6-C6H5)(OC6HPh4-2,3,5,6)(PEt3)] was found (1H NMR) to lead only to partial substitution of the phosphine ligand to produce [W(OC6HPh3-η6-C6H5)(OC6HPh4-2,3,5,6)(py-But-4)] (4). The sodium amalgam reduction (4 Na per W) of hydrocarbon solutions of [W(OC6HPh4-2,3,5,6)2Cl4] in the presence of 4-tert-butylpyridine produced a mixture found to contain the expected compound 4 along with a new salt compound, [W(OC6HPh3-η5-C6H6)(OC6HPh3-η1-C6H4)(OC6HPh4-2,3,5,6)][Cl] (5), containing a total of three aryloxide ligands. Crystals were obtained containing a 1:1 ratio of 4/5. One of the aryloxides in 5 is terminally bound, the second is cyclometalated, while the third chelates to the metal center via an η5-interaction with an ortho-cyclohexadienyl ring. The formal oxidation state of the metal center in 5 is (VI), and the unit cell contains one chloride ion to balance the positive charge on the tungsten. In compound 5 the hydride ligand formed via the cyclometalation has been transferred to the ortho-position of a phenyl ring. The structural parameters for the π-bound rings in 4 and 5 and their relevance to catalytic arene hydrogenation are discussed.
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