Low-Valent Tungsten Aryloxide Compounds with Pyridine Ancillary Ligation:  Intramolecular Reduction of Heterocyclic and Aromatic Rings Including Formation of an η⁵-Cyclohexadienyl Group

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-Bu^t-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.