Nitride Formation by Thermolysis of a Kinetically Stable Niobium Dinitrogen Complex Michael D. Fryzuk,* Christopher M. Kozak, Michael R. Bowdridge, Brian O. Patrick, † and Steven J. Rettig †,‡ Contribution from the Department of Chemistry, UniVersity of British Columbia, 2036 Main Mall, VancouVer, British Columbia, V6T 1Z1, Canada Received February 21, 2002 Abstract: The reduction of [P2N2]NbCl (where [P2N2] ) PhP(CH2SiMe2NSiMe2CH2)2PPh) with KC8 under a dinitrogen atmosphere generates the paramagnetic dinuclear dinitrogen complex ([P2N2]Nb)2(µ-N2)(2). Complex 2 has been characterized crystallographically and by EPR spectroscopy. Variable-temperature magnetic susceptibility measurements indicate that 2 displays antiferromagnetic coupling between two Nb(IV) (d 1 ) centers. A density functional theory calculation on the model complex [(PH3)2(NH2)2Nb]2(µ-N2) was performed. Thermolysis of ([P2N2]Nb)2(µ-N2) in toluene generates the paramagnetic bridging nitride species where one N atom of the dinitrogen ligand inserts into the macrocycle backbone to form [P2N2]- Nb(µ-N)Nb[PN3](3) (where [PN3] ) PhPMe(CHSiMe2NSiMe2CH2P(Ph)CH2SiMe2NSiMe2N)). Complex 3 has been characterized in the solid state as well as by variable-temperature magnetic susceptibility measurements. The reaction of ([P 2N2]Nb)2(µ-N2) with phenylacetylene displaces the dinitrogen fragment to generate a paramagnetic η 2 -alkyne complex, [P2N2]Nb(η 2 -HCCPh) (4). Introduction The literature concerning the synthesis and characterization of dinitrogen complexes is experiencing a resurgence. This has been fueled by a number of separate events that include the X-ray characterization of the active site of a nitrogenase enzyme, 1 the observation of new bonding modes for the dinitrogen ligand in coordination chemistry, 2,3 the reports of new transformations of coordinated dinitrogen that involve reaction with H 2 and silanes, 4 and the formation of nitrides from coordinated N 2 . 5-9 Taken together these results suggest that this area is undergoing a renaissance. 10 The cleavage of coordinated dinitrogen to generate metal nitride derivatives is considered to be a fundamental process in nitrogen fixation both industrially and biologically. 3,11-13 The recent discovery that bulky molybdenum(III) tris(amide) com- plexes cleave N 2 to form Mo(VI) nitrides was the first of a series of important studies of nitride formation from coordinated dinitrogen. 5-7,14,15 Many of these systems involve the formation of a dinuclear dinitrogen complex as an observed intermediate. 6,8 Irrespective of observable intermediates, a total of six electrons must be formally added to the dinitrogen unit, and so far this has been best accomplished by using two metal centers. In this paper we present our attempts to design a system that can activate dinitrogen and furthermore induce N-N bond cleavage to generate a nitride complex. Our success with the formation of the side-on bound dinuclear zirconium dinitrogen complex, 4 ([P 2 N 2 ]Zr) 2 (µ-η 2 :η 2 -N 2 ) (where [P 2 N 2 ] ) PhP(CH 2 - SiMe 2 NSiMe 2 CH 2 ) 2 PPh), provided a starting point since incor- poration of the same macrocyclic ancillary ligand system onto a group 5 metal (i.e., V, Nb, or Ta) would formally provide one extra electron per metal over that found for the aforemen- tioned zirconium analogue. If one assumes that the formation of ([P 2 N 2 ]Zr) 2 (µ-η 2 :η 2 -N 2 ) corresponds to a formal addition of 4 electrons to neutral N 2 , then the availability of an additional two electrons per dinuclear group 5 complex would provide the necessary six electrons required for cleavage of dinitrogen. We recently reported the synthesis of niobium(III) complexes of the [P 2 N 2 ] ligand system. 16,17 In this work, we examine the preparation and characterization of a dinuclear dinitrogen complex of niobium, its reactivity with protons and alkynes, and its thermolysis to form a dinuclear nitride species. * Corresponding author. E-mail: fryzuk@chem.ubc.ca. † UBC X-ray Structural Chemistry Laboratory. ‡ Deceased October 27, 1998. (1) Kim, J.; Rees, D. C. Nature 1992, 360, 563. (2) Fryzuk, M. D.; Johnson, S. A.; Patrick, B. O.; Albinati, A.; Mason, S. A.; Koetzle, T. K. J. Am. Chem. Soc. 2001, 123, 3960. (3) Fryzuk, M. D.; Johnson, S. A. Coord. Chem. ReV. 2000, 200-202, 379. (4) Fryzuk, M. D.; Love, J. B.; Rettig, S. J.; Young, V. G. Science 1997, 275, 1445. (5) Laplaza, C. E.; Cummins, C. C. Science 1995, 268, 861. (6) Laplaza, C. E.; Johnson, M. J. A.; Peters, J. C.; Odom, A. L.; Kim, E.; Cummins, C. C. J. Am. Chem. Soc. 1996, 118, 8623. (7) Laplaza, C. E.; Johnson, A. R.; Cummins, C. 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Published on Web 06/20/2002 10.1021/ja025997f CCC: $22.00 © 2002 American Chemical Society J. AM. CHEM. SOC. 2002, 124, 8389-8397 9 8389