Nitrogen atom exchange between molybdenum, tungsten and carbon. A convenient method for N-15 labeling

Abstract

The compound (Bu^tO)₃W≡N serves to exchange the nitrogen atoms between nitriles (MeC≡N and PhC≡N), itself and (Bu^tO)₃Mo≡N in solution at room temperature.

---

Modern computational methods aid in identifying new modes of reactivity based upon their predictions of thermodynamic preference and reaction pathways. We are currently examining the reactions shown in eqn. (1) by experimental procedures and

L₃M≡ML₃ + X≡Y ⇌ L₆M₂XY (1a)

L₆M₂XY ⇌ L₃M≡X + L₃M≡Y (1b)

computational methods based on model compounds (M = Mo or W; L = alkoxide, thiolate, amide, alkyl; X = Y = N or CR, X = N, Y = CR).^1–5

The reaction 1b involves the reversible formation and rupture of X–Y and M–M bonds and was the subject of a recent communication in this journal dealing with the coupling of benzylidine ligands when M = W and L = 2-MeC₆H₄S.⁵ During this work we noted that the calculated barrier to X–Y and M–M bond formation was higher than that for X, Y group transfer via L₃M(μ-X)(μ-Y)ML₃ intermediates. This prompted us to probe for the existence of such reactions. We describe herein the isotopically labeled reactions shown in eqn. (2), (3) and (4), where L = OBu^t that confirm the predictions based on the computations.⁶

L₃W≡¹³CMe + L₃Mo≡CPrⁱ ⇌ L₃W≡CPrⁱ + L₃Mo≡¹³CMe (2)

L₃W≡¹³CMe + L₃Mo≡N ⇌ L₃W≡N + L₃Mo≡¹³CMe (3)

L₃W≡¹⁵N + L₃Mo≡N ⇌ L₃W≡N + L₃Mo≡¹⁵N (4)

Evidence for X, Y group exchange in reactions (2)–(4) was seen by NMR spectroscopy⁶ (¹H, ¹³C or ¹⁵N) and by mass spectrometry. The calculations based on model compounds L = OH or CH₃, X = Y = N or CH predict that the thermodynamic products involving the reactants present in eqn. (4) are N≡N and L₃Mo≡WL₃ but these are not observed. However, L₃Mo≡WL₃ (where L = OBu^t) is seen in reaction (5) although the L₃Mo≡WL₃ compound reacts further with L₃Mo≡N⁷ to give Mo₂L₆ and L₃W≡N.^6,8

L₃Mo≡N + L₃W≡WL₃ → L₃W≡N + L₃Mo≡WL₃ (5)

The facility of nitrogen atom exchange in reactions (3)–(5) led us to examine the potential for nitrogen atom exchange between molybdenum, tungsten and carbon.⁹ The compound (Bu^tO)₃W≡N⁸ was shown to exchange its nitrogen atom with that of acetonitrile, eqn. (6) and furthermore catalyze ¹⁵N

(Bu^tO)₃W≡¹⁵N + MeC≡N ⇌ (Bu^tO)₃W≡N + MeC≡¹⁵N (6)

scrambling with benzonitrile, eqn. (7). See Fig. 1.

Fig. 1 ¹⁵N NMR spectrum of the reaction between labelled MeC≡¹⁵N and PhC≡N (natural abundance) in the presence of (Bu^tO)₃W≡N recorded in d₈-THF at 298 K, 50.6 MHz, showing the ¹⁵N for ¹⁴N atom exchange.

The ¹⁵N/¹⁴N scrambling in reactions (6) and (7) has been monitored by ¹⁵N NMR spectroscopy and mass spectrometry.⁹ Rather interestingly the closely related compound (Bu^tO)₃Mo≡N⁷ does not exhibit similar nitrogen atom exchange with MeC≡¹⁵N or catalyze scrambling with PhC≡N, at room temperature in d₈-THF. However, upon addition of a trace amount of (Bu^tO)₃W≡N, scrambling of nitrogen atoms occurs leading to formation of PhC≡¹⁵N and (Bu^tO)₃Mo≡¹⁵N.

In order to establish that the ¹⁵N labeling exchange observed in reaction 7 occurs exclusively as a result of nitrogen atom exchange and not cyano group exchange, we followed the reaction between PhC≡N, Me¹³C≡N and MeC≡¹⁵N in d₈-THF in the presence of (Bu^tO)₃W≡N. By NMR spectroscopy the appearance of Me¹³C≡¹⁵N and PhC≡¹⁵N could be detected but no ¹³C enrichment of the benzonitrile cyano group carbon was detected. See Fig. 2.

Fig. 2 ¹⁵N NMR spectrum of the reaction mixture between Me¹³C≡N, MeC≡¹⁵N and PhC≡N in the presence of a trace of (Bu^tO)₃W≡N recorded in d₈-THF at 298 K, 50.6 MHz. The PhC≡¹⁵N signal shows enhancement due to ¹⁵N atom exchange and appears as a singlet due to lack of coupling to ¹H or ¹³C whereas the MeC≡N ¹⁵N signal shows coupling to ¹H, ³J¹H–¹⁵N = 1.7 Hz and for Me¹³C≡¹⁵N coupling to ¹³C, ¹J¹³C–¹⁵N = 17 Hz. The signal thus appears as a central 1∶3∶3∶1 quartet flanked by ¹³C satellites. The unsymmetrical nature of the ¹³C satellites arises from ¹²C/¹³C isotopic chemical shift perturbation.

The seemingly most plausible pathway leading to nitrogen atom exchange involves the formation of a 2 + 2 cycloaddition reactive intermediate as represented schematically by eqn. (8).

This bears analogy to the reaction pathway of alkyne metathesis by (Bu^tO)₃W≡CR complexes.^10–12 Calculations employing density functional theory¹³ on the model reactants (HO)₃W≡N and MeC≡N predict that formation of the five-coordinate intermediate shown in Fig. 3 to be enthalpically higher in energy than the starting materials by 18 kcal mol⁻¹. In contrast, formation of the related molybdenum 2 + 2 cycloaddition product is enthalpically disfavored by an additional 10 kcal mol⁻¹.

Fig. 3 A drawing of the calculated geometry for the minimum energy of a hypothetical reactive intermediate (HO)₃W(η²-N₂CMe) showing the asymmetric nature of the metallacycle.

Based on these preliminary results we believe that these and related nitrogen atom exchange reactions hold considerable promise for ¹⁵N isotope labeling studies in a wide variety of chemical systems. Furthermore, it should be possible to link nitrogen atom exchange reactions to dinitrogen cleavage via a reactive mononuclear L₃M fragment of the type pioneered by Cummins.^14–16

We thank the National Science Foundation for support.

Notes and references

1. R. R. Schrock, M. L. Listemann and L. G. Sturgeoff, J. Am. Chem. Soc., 1982, 104, 4291.
2. H. Strutz and R. R. Schrock, Organometallics, 1984, 3, 1600.
3. M. H. Chisholm, Chem. Record, 2001, 1, 12.
4. M. H. Chisholm, E. R. Davidson, J. C. Huffman and K. B. Quinlan, J. Am. Chem. Soc., 2001, 123, 9652.
5. M. H. Chisholm, E. R. Davidson, M. Pink and K. B. Qunilan, Chem. Commun., 2002, 2770.
6. These reactions were followed by ¹H, ¹³C{H} and ¹⁵N NMR spectroscopy and the labeled compounds L₃W≡¹³CMe and L₃W≡¹⁵N were prepared by the ‘chop–chop’ reaction¹ between L₃W≡WL₃ and Me¹³C≡N and MeC≡¹⁵N respectively (L = OBu^t). All spectra were aquired at room temperature over a 12–16 h period after which time a statistical isotopic distribution of ¹⁵N was observed. Selected ¹³C NMR data (d₆-benzene, 125.8 MHz): ¹³C{H}: L₃W≡¹³CMe, δ 254.1, J(¹⁸³W–¹³C) 306.5 Hz; L₃Mo≡¹³CMe, δ 279.6; L₃W≡¹³CPrⁱ, δ 268.3; L₃Mo≡¹³CPrⁱ, δ 292.7. ¹⁵N NMR data (d₈-THF, 50.6 MHz): L₃W≡¹⁵N, δ 731.8, J(¹⁸³W–¹⁵N) 54; L₃Mo≡¹⁵N, δ 828.8 (relative to NH₃). The compound L₃W≡MoL₃ shows two singlets in the ¹H NMR spectrum (d₆-benzene, 298 K, 400 MHz), δ 1.58 and 1.60 and a molecular ion with the anticipated isotope pattern for MoWL₆⁺ in the mass spectrum..
7. D. M. T. Chan, M. H. Chisholm, K. Folting, J. C. Huffman and N. S. Marchant, Inorg. Chem., 1986, 25, 4170.
8. M. H. Chisholm, D. M. Hoffman and J. C. Huffman, Inorg. Chem., 1983, 22, 2903.
9. All ¹⁵NMR experiments where performed at room temperature over a 12–16 h period using a 5 mm J. Young NMR tube with dried and degassed d₈-THF. Organic nitriles were used as received and in some cases stored over activated molecular sieves. Reagents were carefully weighed out in the drybox and dissolved in d₈-THF. All ¹⁵N data were collected using a Bruker 500 MHz spectrometer operating at 50.6 MHz.
10. J. H. Wengrovius, J. Sancho and R. R. Schrock, J. Am. Chem. Soc., 1981, 103, 3932.
11. J. H. Freudenberger, R. R. Schrock, M. R. Churchill, A. L. Rheingold and J. W. Ziller, Organometallics, 1984, 3, 1563.
12. R. R. Schrock, J. Chem. Soc., Dalton Trans., 2001, 2541.
13. J. A. Pople, et al., Gaussian Inc., Pittsburgh, PA. 1998 Gaussian 98, Revision A.6.
14. C. C. Cummins, Chem. Commun., 1998, 1777.
15. C. E. Laplaza, M. J. A. Johnson, J. Peters, A. L. Odom, E. Kim, C. C. Cummins, G. N. George and I. J. Pickering, J. Am. Chem. Soc., 1996, 118, 8623.
16. C. E. Laplaza, A. R. Johnson and C. C. Cummins, J. Am. Chem. Soc., 1996, 118, 709.

---

Footnote

† Dedicated to Professor Dr. G. Huttner on the occasion of his 65th birthday.

---