Low-Valent Tungsten Aryloxide Compounds with Pyridine Ancillary Ligation:  Intramolecular Reduction of Heterocyclic and Aromatic Rings Including Formation of an η<sup>5</sup>-Cyclohexadienyl Group

The sodium amalgam reduction (2 Na per W) of hydrocarbon solutions of [W(OC<sub>6</sub>HPh<sub>4</sub>-2,3,5,6)<sub>2</sub>Cl<sub>4</sub>] in the presence of pyridine or 4-<i>tert</i>-butylpyridine leads to the W(IV) compounds [W(OC<sub>6</sub>HPh<sub>4</sub>-2,3,5,6)<sub>2</sub>Cl<sub>2</sub>(py)<sub>2</sub>] (<b>1</b>) and [W(OC<sub>6</sub>HPh<sub>4</sub>-2,3,5,6)<sub>2</sub>Cl<sub>2</sub>(4-<i>tert</i>-butylpyridine)<sub>2</sub>] (<b>2</b>) in good yields. The ambient-temperature <sup>1</sup>H NMR spectra of both <b>1 </b>and<b> 2</b> 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<sub>6</sub>HPh<sub>3</sub>-η<sup>6</sup>-C<sub>6</sub>H<sub>5</sub>)(OC<sub>6</sub>HPh<sub>4</sub>-2,3,5,6)(PEt<sub>3</sub>)] produced [W(OC<sub>6</sub>HPh<sub>4</sub>-2,3,5,6)<sub>2</sub>(bipy)<sub>2</sub>] (<b>3</b>), containing two bipy rings and two terminal aryloxide ligands coordinated mutually <i>cis</i>. The W−N(bipy) distances of 2.092(2), 2.093(2), 2.096(2), and 2.115(2) Å in <b>3</b> are among the shortest so far reported for bipy ligands bound to either molybdenum or tungsten. The solid state and solution data for <b>3</b> indicate that reduction of both bipy ligands has occurred, but whether this involves four-electron reduction to yield d<sup>0</sup>-W(VI) is unclear. The addition of 4-<i>tert</i>-butylpyridine to [W(OC<sub>6</sub>HPh<sub>3</sub>-η<sup>6</sup>-C<sub>6</sub>H<sub>5</sub>)(OC<sub>6</sub>HPh<sub>4</sub>-2,3,5,6)(PEt<sub>3</sub>)] was found (<sup>1</sup>H NMR) to lead only to partial substitution of the phosphine ligand to produce [W(OC<sub>6</sub>HPh<sub>3</sub>-η<sup>6</sup>-C<sub>6</sub>H<sub>5</sub>)(OC<sub>6</sub>HPh<sub>4</sub>-2,3,5,6)(py-Bu<sup>t</sup>-4)] (<b>4</b>). The sodium amalgam reduction (4 Na per W) of hydrocarbon solutions of [W(OC<sub>6</sub>HPh<sub>4</sub>-2,3,5,6)<sub>2</sub>Cl<sub>4</sub>] in the presence of 4-<i>tert</i>-butylpyridine produced a mixture found to contain the expected compound <b>4</b> along with a new salt compound, [W(OC<sub>6</sub>HPh<sub>3</sub>-η<sup>5</sup>-C<sub>6</sub>H<sub>6</sub>)(OC<sub>6</sub>HPh<sub>3</sub>-η<sup>1</sup>-C<sub>6</sub>H<sub>4</sub>)(OC<sub>6</sub>HPh<sub>4</sub>-2,3,5,6)][Cl] (<b>5</b>), containing a total of three aryloxide ligands. Crystals were obtained containing a 1:1 ratio of <b>4</b>/<b>5</b>. One of the aryloxides in <b>5</b> is terminally bound, the second is cyclometalated, while the third chelates to the metal center via an η<sup>5</sup>-interaction with an ortho-cyclohexadienyl ring. The formal oxidation state of the metal center in <b>5</b> is (VI), and the unit cell contains one chloride ion to balance the positive charge on the tungsten. In compound <b>5</b> 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 <b>4</b> and <b>5</b> and their relevance to catalytic arene hydrogenation are discussed.