Pyridine Degradation Intermediates as Models for Hydrodenitrogenation Catalysis:  Preparation and Properties of a Metallapyridine Complex

We report the preparation, structure, and reactions of a stable metallapyridine complex of tantalum prepared in the course of model studies of hydrodenitrogenation (HDN) reactions. <ETIEBAR ID="om970749ra10001">Monomeric T<TIEBAR NUM="om970749r00000">a(&dbd;NC^tBu&dbd;CHC^tBu&dbd;</TIEBAR>CH)(OAr)<INF>2</INF>(THF) (<BO>5</BO>·THF)</ETIEBAR> is isolated upon thermolyzing the η²(N,C)-pyridine complex [η²(N,C)-2,4,6-NC<INF>5</INF>^tBu<INF>3</INF>H<INF>2</INF>]Ta(OAr)<INF>2</INF>Me (<BO>2</BO>) in the presence of THF, while the metallapyridine <ETIEBAR ID="om970749ra10002">dimer [T<TIEBAR NUM="om970749r00000">a(μ-NC^tBu&dbd;CHC^tBu&dbd;</TIEBAR>CH)(OAr)<INF>2</INF>]<INF>2</INF> (<BO>6</BO>)</ETIEBAR> is isolated when this reaction is carried out in benzene. Complete NMR characterization of <BO>5</BO>·THF is described, along with its conversion into <BO>6</BO>. The bis(pyridine) <ETIEBAR ID="om970749ra10003">adduct T<TIEBAR NUM="om970749r00000">a(&dbd;NC^tBu&dbd;CHC^tBu&dbd;</TIEBAR>CH)(OAr)<INF>2</INF>(py)<INF>2</INF> (<BO>5</BO>·py)</ETIEBAR> is also described. <ETIEBAR ID="om970749ra10004">T<TIEBAR NUM="om970749r00000">a(&dbd;NC^tBu&dbd;CHC^tBu&dbd;</TIEBAR>CH)(OAr)<INF>2</INF>(THF) (<BO>5</BO>·THF)</ETIEBAR> is shown to react with ^tBuNCO and ⁱPrNCNⁱPr to afford the σ (η¹) and π (η³) insertion products <ETIEBAR ID="om970749ra10005">respectively T<TIEBAR NUM="om970749r00000">a[&dbd;NC^tBu&dbd;CHC^tBu&dbd;CHC(&dbd;N^tBu)</TIEBAR>O](OAr)<INF>2</INF> (<BO>7</BO>)</ETIEBAR> and Ta[&dbd;NC^tBu&dbd;CHC^tBu&dbd;CH(η³-C(&dbd;NⁱPr)<INF>2</INF>)](OAr)<INF>2</INF> (<BO>8</BO>). The molecular structure of the <ETIEBAR ID="om970749ra10006">metallapyridine T<TIEBAR NUM="om970749r00000">a(&dbd;NC^tBu&dbd;CHC^tBu&dbd;</TIEBAR>CH)(OAr)<INF>2</INF>(THF) (<BO>5</BO>·THF)</ETIEBAR> was detemined by X-ray crystallography and shown to adopt a trigonal-bipyramidal configuration with aryl oxide oxygens and the metallacyclic carbon occupying equatorial positions. The TaNC<INF>4</INF> metallacycle is very nearly planar, and discrete single and double bonds are evident around the ring. This π localization clearly favors the imido <ETIEBAR ID="om970749ra10007">form T<TIEBAR NUM="om970749r00000">a(&dbd;NC^tBu&dbd;CHC^tBu&dbd;</TIEBAR>CH)(OAr)<INF>2</INF>(THF) rather</ETIEBAR> than a carbene structure. The relevance of these compounds to hydrodenitrogenation catalysis is described.