Organometallics 1995, 14, 2159-2161
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Formation of a Metallaaziridine Ring and Dinitrogen Fixation Promoted by a Niobium Amide Complex Pietro Berno and Sandro Gambarotta" Department of Chemistry, University of Ottawa, Ottawa, Ontario K I N 6N5, Canada Received February 27, 1995@ Summary: Reaction of NbC14THF)z with 3 equiv of Cy2NLi i n toluene formed ( C y f l ) f l b ( C y N C a & Y (11, where one cyclohexyl ring was metalated to form a niobaaziridine ring. Further reaction of 1 with NaHBEt3 in toluene led to the formation of the end-on dinitrogen complex [(Cyfl)flbIkNd, which was isolated as pale yellow crystals in good yield. The interest in the chemistry of medium-valent niobium and tantalum has been stimulated by the variety of transformations performed by these species which range from alkyl chain dehydrogenation,l to oxidative addition reactions,2 formation of stable COZ3and Nz4 adducts, metal-promoted organic ~ y n t h e s i sreductive ,~ couplings,6and formation of p~lyhydrides.~ In contrast, medium-valent niobium and tantalum complexes are very rare* and, besides the polynuclear halidesga and have been poorly charthe cyclopentadienyl acterized or prepared in situ. We thus became interested in the preparation and characterization of lowAbstract published in Advance ACS Abstracts, April 1, 1995. (1)(a) Yu, J. S.; Fanwick, P. E.; Rothwell, I. P. J . Am. Chem. SOC. 1990,112, 8171. (b) Bishop, P. T.; Dilworth, J. R.; Nicholson, T.; Zubieta, J. A. J . Chem. Soc., Chem. Commun. 1986,1123. ( c ) Steffey, B. D.; Chamberlain, L. R.; Chesnut, R. W.; Chebi, D. E.; Fanwick, P. E.; Rothwell, I. P. Organometallics 1989,8, 1419. (d) Chamberlain, L. R.; Kerschner, J. L.; Rothwell, A. P.; Rothwell, I. P.; Huffman, J. C. J . Am. Chem. SOC.1987,109,6471. (2)(a) Curtis, M. D.; Real, J.; Kwon, D. Organometallics 1989,8, 1644. (b) Arnold, J.; Tilley, T. D.; Rheingold, A. L.; Geib, S. J. Organometallics 1987,6 , 473. ( c ) Neithamer, D. R.; LaPointe, R. E.; Wheeler, R. A.; Richeson, D. R.; Van Duyne, G. D.; Wolczanski, P. T. J . Am. Chem. SOC.1989,111,9056. (d) Neithamer, D.R.; Parkanyi, L.; Mitchell, J. F.; Wolczanski, P. T. J . Am. Chem.SOC.1988,110,4421. (3) Fu, P. F.; Khan, M. A.; Nicholas, K. M. J . Am. Chem. SOC.1992, 114,6579. (4)(a) Dilworth, J. R.; Henderson, R. A.; Hills, A,; Hughes, D. L.; McDonald, C.; Stephen, A. N.; Walton, D. R. M. J . Chem. SOC.,Dalton Trans. 1990,1077. (b) Rocklage, S. M.; Schrock, R. R. J . Am. Chem. SOC.1982,104,3077. ( c ) Green, D. W.; Hodges, R. V.; Gruen, D. M. Inorg. Chem. 1976, 15, 970. (d) Turner, H. W.; Fellmann, J. D.; Rocklage, S. M.; Schrock, R. R.; Churchill, M. R.; Wassermann, H. J. J . Am. Chem. SOC.1980,102, 7809. (e) Rocklage, S. M.; Turner, H. W.; Fellman, J. D.; Schrock, R. R. Organometallics 1982,I , 703. (0 Schrock, R. R.; Wesolek, M.; Liu, A. H.; Wallace, K. C.; Dewan, J. C. Inorg. Chem. 1988,27,2050. (5)(a) Roskamp, E. J.; Pedersen, S. F. J . Am. Chem. SOC.1987,109, 6551. (b) Roskamp, E.J.; Dragovich, P. S.; Hartung, J. B.; Redersen, S. F. J . Org. Chem. 1989,54,4736. (6)(a)Etienne, M.; White, P. S.; Templeton, J. L. Organometallics 1993,12,4010. (b) Bianconi, P.A.; Williams, I. D.; Engeler, M. P.; Lippard, S. J. J . Am. Chem. SOC.1986,108, 311. ( c ) Cotton, F. A,; Hall, W. T. Inorg. Chem. 1978,17,3525.(d) Bruck, M. A,; Copenhaver, A. S.; Wigley, D. E. J . Am. Chem. SOC.1987,109,6525. (7)(a) Profilet, R. D.; Rothwell, A. P.; Rothwell, I. P. J.Chem. Soc., Chem. Commun. 1993,42.(b) Steffey, B.D.; Rothwell, I. P. J . Chem. Soc., Chem. Commun. 1990,213.(c) Yu, S.J.; Rothwell, I. P. J . Chem. Soc., Chem. Commun. 1992,632. (d) Steffey, B. D.; Chesnut, R. W.; Kerschner, J. L.; Pellechia, P. J.; Fanwick, P. E.; Rothwell, I. P. J . Am. Chem. SOC.1989,111, 378. (e) Tebbe, F. N.; Parshall, G. W. J . Am. Chem. SOC.1971,93,3793.(0 Foust, D. F.; Rogers, R. D.; Rausch, M. D.; Atwood, J. L. J . Am. Chem. SOC.1982,104,5646. (8)Coffindaffer, T. W.; Steffy, B. D.; Rothwell, I. P.; Folting, K.; Huffman, J. C.; Streib, W. J. J . Am. Chem. SOC.1989,111, 4742. (9)(a) Cotton, F. A.; Walton, R. R. Multiple Bonds Between Metal Atoms; Clarendon Press: Oxford, U.K., 1993. (b) Wilkinson, G. Comprehensive Organometallic Chemistry Update; Pergamon Press: Oxford, U.K., in press. @
0276-733319512314-2159$09.00/0
and medium-valent niobium complexes supported by anionic organic amides. The employment of these anions as supporting ligands was suggested by their well-known characteristics, which include the ability to stabilize low oxidation stateslo of the metal, their electronic flexibility, the possibility of fine-tuning the steric hindrance, and their ability to engage in C-H o-bond metathesis reactions t o form highly reactive cyclometallaazabutane rings.ll In addition, organic amide ligands have recently been proven to be involved in H-transfer reactions12and to sufficiently enhance the reactivity of the metal center to support coordination of dinitrogen.13 In this paper we describe the reaction of NbC14(THF)z with CyzNLi to form a niobaaziridine ring and its further reaction with hydride to form a dinitrogen complex. The reaction of NbCld(THF)z with 3 equiv of CyzNLi in toluene gave a brown oil which, upon crystallization from hexane, yielded orange crystals of (CyzN)zNb(CyNC&lo)Cl (1; 43%).14 The formulation was indicated by consistent combustion analysis data and the correct NbIC1 ratio as determined by X-ray fluorescence. The complex is diamagnetic, which implies that the metal center has undergone oxidation to the pentavalent state. The lH NMR spectrum of 1 showed the presence of two different cyclohexyl groups, as indicated by two triplets of triplets at 3.60 and 3.41 ppm which are integrated in the ratio 1:4. In addition, the 13C NMR spectrum showed the presence of a quaternary carbon atom at 82.4 ppm, thus indicating that partial dehydrogenation of one cyclohexyl ring had occurred. Ac(10) Duchateau, R.; Beydoun, N.; Gambarotta, S.; Bensimon, C. J . Am. Chem. SOC.1991,113,8986. (11)(a) Berno, P.; Hao, S.; Minhas, R.; Gambarotta, S. Organometallics 1994,13, 1052. (b) Berno, P.; Gambarotta, S. J . Chem. SOC., Chem. Commun., in press. (12)(a)Khulman, R.; Folting, K.; Caulton, K. G. J . Am. Chem. SOC. 1993, 115, 5813. (b) Planalp, P. R.; Andersen, R. A.; Zalkin, A. Organometallics 1983,2, 16. (c) Simpson, S. J.; Andersen, R. A. J . Am. Chem. SOC.1981,103,4063. (d) Sonnenberger, D. C.; Mintz, E. A,; Marks, T. J. J . Am. Chem. SOC.1984,106, 3484. (e) Cummins, C. C.; Schrock, R. R.; Davis, W. M. Organometallics 1992,11, 1452. (0 Berno, P.; Gambarotta, S. Organometallics 1994,13,2569. (13)Song, J.; Berno, P.; Gambarotta, S. J . Am. Chem. SOC.1994, 116, 6927. (14)The addition of LiNCyz (10.1g, 53.8 mmol) to a suspension of NbCl4(THF)z (6.8g, 17.9mmol) in toluene (100mL) a t room temperature turned the color orange-brown. The suspension was stirred a t 60 "C for 3 h and then filtered. The clear solution was evaporated to dryness, and the brown oily residue was heated overnight at 44 "C in vacuo. The resulting brown solid was crystallized from hexane (120 mL), affording orange crystalline 1 (5.1g, 43%). IR (Nujol mull, cm-l): Y 2802 (sp), 1660 (w), 1353 (m), 1265 (br), 1209 (w), 1182 (w), 1159 (w), 1145 (sp), 1113 (br), 1054 (w), 1025 (sp), 981 (sP), 947 (SI, 894 (sp), 842 (sp), 807 (m), 781 (sp), 687 (SI, 620 (w), 588 (w), 513 (w), 498 (w), 451 (w). 'H NMR (&De, 500 MHz, 25.4"C): d 3.60(tt, lH), 3.41 (tt,4H), 2.7-0.95(m, 60H). l3C NMR (C&, 75 MHz, 25.4"c): 6 82.40, 64.49,59.00,39.68, 36.32,36.13,35.11, 27.46,27.15,27.12,26.63,26.06, 25.99,25.93. Anal. Calcd (found) for C36HssN3NbC1: c, 64.70(64.66); H, 9.80(9.61); N, 6.29(6.21).
0 1995 American Chemical Society
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2160 Organometallics, Vol. 14, No. 5, 1995
Scheme 1"
LIU
2
C4
a Selected values of bond distances (8) and angles (deg) are as follows. Complex 1: Nbl-N1 = 1.977(5), Nbl-N2 = 1.956(5), Nbl-N3 = 1.984(5), Nbl-C11 = 2.415(2), Nbl-C24 = 2.196(6), N2-C24 = 1.432(7), N2-Cl3 = 1.466(7), N1-C1 = 1.483(7); N2-Nbl-C24 = 39.8(2), N1-Nbl-C11 = 102.1(1), C11-Nbl-N3 = 101.2(1),Nbl-Nl-C7 = 108.9(3), Nbl-N1-C1 = 137.6(4), Nbl-N2-C13 = 150.8(4); Nbl...H64 = 2.46, Nbl-.-H65 = 2.54. Complex 2: Nbl-N1 = 1.812(6),N1-Nla = 1.34(1), Nbl-N2 = 2.020(3), N2-C1 = 1.477(5); N1-Nbl-N2 = 108.96(9), Nbl-N1-Nla = 180.00, C1-N2-Nb1 = 132.7(3), C7-N2-Nbl = 115.0(2), Cl-N2-C7 = 112.3(3),N2-Nbl-N2a = 109.98(9).
cordingly, hydrogen gas was found in the atmosphere of the reaction mixture (0.47 equiv/Nb). Conversely, chemical degradation of 1 carried out with gaseous HC1 did not produce H2, thus ruling out the possibility that 1 might be a hydride species. The salient feature of the molecular structure of 1, as elucidated by an X-ray crystallographic study,15 is the metalation of one cyclohexyl ring to form a niobaaziridine ring. The structure of 1 may be regarded to be composed of a (Cy2N)zNbClmoiety coordinated side-on to a CyN=C&o imine (Scheme 11, although the Nb-C and Nb-N bond distances are significantly different. The steric interaction between cyclohexyl rings is responsible for the significant distortion of the trigonalplanar geometry around each nitrogen atom, with one of the two rings significantly bent toward the less crowded chlorine. This bending causes the methyne hydrogen atom of one cyclohexyl ring of each of the two amide groups to form short Nb-H contacts. A reasonable pathway for the formation of 1 from NbC14(THF)2 involves the intermediate formation of (Cy2NI3NbC1 followed by elimination of hydrogen, and formation of the Nb-C bond. There are only a very few examples of this type of transformation described in the , ~ ~ $ being ~J~ literature of niobium and t a n t a l ~ m l ~ which, promoted by in situ generated trivalent Nb and Ta complexes, involve two-electron-transfer processes. The (15)Crystal data are as follows: 1, C ~ ~ H C ~ N ~ fw N ~668.29, CI, triclinic, P i , a = 10.067(1) A, b = 18.7588(6) 8, c = 9.7431(8) A,a = 93.89(1)",/3 = 105.26(1)",y = 88.579(9)", V = 1707.9(6)k ,2 = 2, deale = 1.253 g ~ m - p~ = , 4.27 cm-', F(OO0) = 720, R = 0.057 (R,= 0.062) for 519 parameters and 4624 unique reflections. Non-hydrogen atom positions were refined anisotropically. All the hydrogen atom positions were located in different Fourier maps. However, isotropic refinement was possible for only some of the H atoms due to the unfavorable parameter/observation ratio.
formation of 1 provides the first case where such a transformation is performed by a Nb(IV) metal center. Attempts t o reduce 1 by reaction with NaHBEt3 in toluene yielded a yellowish red solution from which pale yellow crystals of the diamagnetic dinitrogen complex [(CyzN)3Nblz01-771:11-N2) (2) were isolated in reasonable yield (57%).17 Both the lH- and 13C NMR spectra showed the presence of only one type of cyclohexyl ring, while the presence of the dinitrogen moiety was demonstrated by the formation of Na (25%) during degradation experiments carried out with anhydrous HC1 in a closed vessel connected to a Toepler pump. The molecular structure of 2, as clarified by X-ray diffraction methods, consists of two pyramidal [Cy2Nl3Nb moieties connected by a bridging end-on dinitrogen molecule.18 The very short Nb-N distance formed by the Nz moiety suggests a significant contribution of Nb-N multiple-bond character, while the elongated (16) (a)Ballard, K. R.; Gardiner, I. M.; Wigley, D. E. J.Am. Chen. SOC.1989,111,2159. (b)Shickler, J. R.; Wexler, P. A.; Wigley, D. E. Organometallics 1988, 7, 2067. (17) An orange suspension of 1(2.6 g, 3.9 mmol) in toluene (50 mL) was treated with a solution of NaHBEt3 (4 mL, 1 M)in toluene. The mixture was stirred overnight at room temperature under Nz. The resulting reddish orange suspension was filtered and evaporated to dryness. The yellow residue was crystallized from hot hexane, yielding pale yellow crystals of 2 (1.45 g, 57%). IR (Nujol mull, cm-'1: Y 1344 (sp), 1242 (br), 1155 (sp), 1144 (sp), 1093 (s), 1030 (br, s), 949 (vs), 888 (sp), 849 (s), 835 (sh), 801 (w), 782 (w), 681 (s), 584 (br), 506 (sp), 484 (sp), 447 (sp). 'H NMR (C&, 500 MHz, 25.4 "C): 6 2.58 (tt, lH), 1.830.86 (m, 10H).13CNMR (C&, 75 MHz, 25.4 "C): 6 53.13,34.86,26.72, 25.45. Anal. Calcd (found) for C3&66N4Nbb: C, 66.74 (66.69); H, 10.27 (10.19); N, 8.65 (8.58). (18)Crystal data: C36H6 NdNb, fw 647.85, hexagonal, R%h), a = 18.998(1) A,c = 16.652(1) Z = 6, dealc= 1.240 g cm-3, fi = 3.59 cm-1, R = 0.043 (R, = 0.045) for 128 parameters and 1410 unique reflections. Non-hydrogen atom positions were refined anisotropically. All the hydrogen atom positions were located but not refined.
1,
Communications
N-N distance (Nl-N1* = 1.34(1) A; the longest ever found for an end-on complex) suggests a significant extent of dinitrogen reduction. Complex 2 is exceedingly robust, as may be expected on the basis of previous observations carried out on other niobium dinitrogen complexes formed by the reaction of niobium halides with hydrazines or Schiff bases.4 Similar to the case of the isostructural vanadium amide dinitrogen complex,13dinitrogen could not be displaced by coordinating ligands such as pyridine or phosphine. In addition, the cyclic voltammogram of 2 in THF-(TBA)BF4 solution ((TBA)BF4= n-tetrabutylammonium tetrafluoborate; E,, - E,, = 71 mV vs Fc/Fc+, room temperature, scan rate 200 mV s-l) did not show any oxidation wave up to 1.2 V, possibly indicating that, in agreement with the structural data, a significant extent of dinitrogen reduction occurred in this complex.
Organometallics, Vol. 14, No. 5, 1995 2161 Although we do not know the mechanism of formation of complex 2, it is reasonable to expect that the reaction proceeds via a two-electron reduction operated by one hydride, forming an intermediate (CyzN)sNb"' complex. Although there are a few examples of dinitrogen complexes of niobium in the literature: complex 2 provides the first example where the Nz moiety is actually derived from elemental nitrogen.
Acknowledgment. This work was supported by the Natural Sciences and Engineering Research Council of Canada (operating and strategic grants). Supplementary Material Available: Tables giving details on the structure determination, atomic positional parameters, anisotropic thermal parameters, and bond distances and angles (24 pages). Ordering information is given on any current masthead page. OM9501541
