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(OC 6 HPh 4 -2,3,5,6) 2 Cl 4 ] in the presence of pyridine or 4- tert -butylpyridine leads to the W(IV) compounds [W(OC 6 HPh 4 -2,3,5,6) 2 Cl 2 (py) 2 ] ( 1 ) and [W(OC 6 HPh 4 -2,3,5,6) 2 Cl 2 (4- tert -butylpyridine) 2 ] ( 2 ) in good yields. The ambient-temperature 1 H 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(OC 6 HPh 3 -η 6 -C 6 H 5 )(OC 6 HPh 4 -2,3,5,6)(PEt 3 )] produced [W(OC 6 HPh 4 -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 d 0 -W(VI) is unclear. The addition of 4- tert -butylpyridine to [W(OC 6 HPh 3 -η 6 -C 6 H 5 )(OC 6 HPh 4 -2,3,5,6)(PEt 3 )] was found ( 1 H NMR) to lead only to partial substitution of the phosphine ligand to produce [W(OC 6 HPh 3 -η 6 -C 6 H 5 )(OC 6 HPh 4 -2,3,5,6)(py-Bu t -4)] ( 4 ). The sodium amalgam reduction (4 Na per W) of hydrocarbon solutions of [W(OC 6 HPh 4 -2,3,5,6) 2 Cl 4 ] in the presence of 4- tert -butylpyridine produced a mixture found to contain the expected compound 4 along with a new salt compound, [W(OC 6 HPh 3 -η 5 -C 6 H 6 )(OC 6 HPh 3 -η 1 -C 6 H 4 )(OC 6 HPh 4 -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 η 5 -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.