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DOI: 10.1039/B902524C (Communication) Dalton Trans., 2009, 4420-4422

Intermediates in the reduction of N2 to NH3: synthesis of iron η2 hydrazido(1−) and diazene complexes†

Justin L. Crossland , Chantal G. Balesdent and David R. Tyler *
Department of Chemistry, University of Oregon, Eugene, Oregon 97403, US. E-mail: dtyler@uoregon.edu; Fax: +1 541 346 0487; Tel: +1 541 346 4649

Received 6th February 2009 , Accepted 1st April 2009

First published on 14th April 2009

Abstract

Iron complexes containing hydrazido(1−) and diazene ligands were investigated as potential intermediates in the reduction of N2 to NH3. Generation of cis-[Fe(DMeOPrPE)2(η2-N2H3)]+ and cis-Fe(DMeOPrPE)2(η2-N2H2) (DMeOPrPE = 1,2-bis(dimethoxypropylphosphino)ethane) was achieved by addition of base to cis-[Fe(DMeOPrPE)2(N2H4)]2+. The hydrazine, hydrazido(1−), and diazene complexes can be interconverted by protonation/deprotonation reactions.

Iron is common to both the industrial (Haber–Bosch) and biological (nitrogenase) production of ammonia.1,2 Research has accordingly focused on studying the chemistry of iron with dinitrogen and other dinitrogen derivatives.3–9 Some iron dinitrogen complexes have shown the ability to produce ammonia and/or hydrazine in the presence of acid, although with limited success.10–13 Whereas the mechanism of ammonia production in these synthetic systems is unknown, growing data for nitrogenase supports a mechanism that proceeds through diazene and hydrazine intermediates in route to ammonia formation.14,15 In an earlier paper, we reported the synthesis and characterization of the first η2-hydrazine complex of iron, cis-[Fe(DMeOPrPE)2(N2H4)]2+ (DMeOPrPE = 1,2-bis(dimethoxypropylphosphino)ethane).16 Recently, Field and co-workers described the synthesis of a diazene complex, cis-Fe(DMPE)2(N2H2), formed by deprotonation of the corresponding hydrazine complex, cis-[Fe(DMPE)2(N2H4)]2+.17 In this communication, we extend our prior work and that of Field by describing the generation of the η2-hydrazido(1−) complex cis-[Fe(DMeOPrPE)2(N2H3)]+ and the synthesis of cis-Fe(DMeOPrPE)2(N2H2). As discussed below, these results support the hypothesis that hydrazine, hydrazido(1−), and diazene complexes are intermediates in the formation of ammonia from the protonation of Fe(PP)2N2-type complexes (PP = a bidentate phosphine).

Synthesis of cis-Fe(DMeOPrPE)2(N2H2) (III) was achieved by the addition of three equivalents of KtBuO to a THF solution of cis-[Fe(DMeOPrPE)2(N2H4)]2+ (I) (Scheme 1).‡ The reaction was complete within minutes as evidenced by NMR spectroscopy and a colour change from orange to yellow. The 31P{1H} NMR spectrum of III shows two triplet resonances at δ 75.8 and 71.2 ppm (2JPP = 38 Hz) (Fig. 1). The two triplets are characteristic of two bidentate phosphine ligands and two equivalent cis ligands, suggesting an η2 coordination mode of the diazene ligand.§ The N–H protons of III were obscured by the DMeOPrPE resonances in the 1H NMR spectrum, but were assigned as a broad singlet at 2.1 ppm using a 1H-15N HMQC experiment. In order to confirm the coordination geometry of the diazene moiety, the 15N isotopologue was synthesized. The 15N{1H} NMR spectrum revealed a single resonance at −315.2 ppm, consistent with a symmetric coordination mode (Fig. 1). Upon turning the proton decoupler off, the 15N resonance split into a doublet (1JN–H = 52 Hz), confirming a single proton bound to each nitrogen atom. The 15N resonance of III is ∼80 ppm upfield from the resonance in the corresponding hydrazine complex (I).


NMR spectra in THF-d8 at 298K for complex III.
Fig. 1 NMR spectra in THF-d8 at 298K for complex III.

Synthetic interconversion of iron hydrazine, hydrazido(1−), and diazene complexes.
Scheme 1 Synthetic interconversion of iron hydrazine, hydrazido(1−), and diazene complexes.

The deprotonation reaction I→III represents the microscopic reverse of the actual reaction that is proposed to take place in the reduction of N2 to NH3 (Scheme 2).18 The protonation of III to regenerate I was therefore investigated. The reprotonation could be achieved by addition of 2 equiv. of 1 M trifluoromethanesulfonic acid (TfOH), as indicated by a colour change from yellow to orange and by 31P{1H} and 15N NMR spectroscopy, which showed the reappearance of I.16||

 
ugraphic, filename = b902524c-u1.gif(1)


Possible mechanism for reduction of N2 to NH3via the protonation of an iron(0) phosphine complex.
Scheme 2 Possible mechanism for reduction of N2 to NH3via the protonation of an iron(0) phosphine complex.

Complex III can also be generated via an alternative route (eqn (1)). Addition of N2H4 to a THF–Et2O solution of Fe(DMeOPrPE)2N2 resulted in the formation of III and cis-Fe(DMeOPrPE)2(H)2.** Similar reactivity was also observed with the analogous Fe(DMPE)2 scaffold.19 The mechanism of this transformation is unclear; however, it likely involves substitution of hydrazine for dinitrogen followed by dehydrogenation of the hydrazine to form III and H2. The H2 then reacts with remaining Fe(DMeOPrPE)2N2 to form cis-Fe(DMeOPrPE)2(H)2 (see the ESI for details).†

The iron(II) hydrazido(1−) complex cis-[Fe(DMeOPrPE)2(N2H3)]+ (II), which is the intermediate species between complexes I and III, could also be synthesized from Ivia a deprotonation reaction using a slighty weaker base (Scheme 1). Thus, addition of two equivalents of DBU (DBU = 1,8-diazabicyclo{5.4.0}undec-7-ene) to I resulted in the disappearance of the two resonances of I and the appearance of four broad resonances in the 31P{1H} NMR spectrum. The broadness of the spectrum was not surprising as the hydrazido protons of II are likely undergoing rapid intramolecular exchange. Upon cooling to 193K, two overlapping ABMX splitting patterns are observed in the 31P{1H} spectrum (Fig. 2). The four-spin system is consistent with two inequivalent cis ligands. The two sets of ABMX patterns arise from two isomers frozen out at low temperatures, where the lone pair on the deprotonated nitrogen atom is either pointing parallel or perpendicular to the ethane bridges of the DMeOPrPE ligands (Fig. 3).


NMR spectra in THF-d8 at 193 K for complex II. The peak labels correspond to the atomic labels given in Fig. 3. The spectrum labelled 1H represents the 1H-15N HMQC (15N decoupled) experiment. The red and blue labels distinguish between the two isomers.
Fig. 2 NMR spectra in THF-d8 at 193 K for complex II. The peak labels correspond to the atomic labels given in Fig. 3. The spectrum labelled 1H represents the 1H-15N HMQC (15N decoupled) experiment. The red and blue labels distinguish between the two isomers.

Isomers present at 193 K for complex II. The methoxypropyl groups have been omitted for clarity.
Fig. 3 Isomers present at 193 K for complex II. The methoxypropyl groups have been omitted for clarity.

The 15N isotopologue was generated to confirm the coordination mode and protonation state of the N2H3 moiety. The 15N{1H} NMR spectrum of complex II at room temperature shows a single broad resonance centered at −375 ppm. Upon cooling the sample to 193 K, the 15N{1H} spectrum showed three resonances (Fig. 2). The resonances at −367.6 and −369.9 ppm are assigned as the two isomers of the deprotonated nitrogen atom (Nα). The -NH2nitrogen atoms (Nβ) of the two isomers overlap and are observed as a single resonance at −377.4 ppm. The 15N NMR spectrum shows that the -NH (Nα) resonances are split into doublets (1JNH = 50 Hz) and the -NH2 (Nβ) resonance is split into a triplet (1JNH = 80 Hz), confirming the η2 coordination geometry of the hydrazido(1−) ligand.

The assignment of the hydrazido protons of II in the 1H NMR spectrum was ambiguous, as the resonances were obscured by the DMeOPrPE resonances. However, a 1H-15N HMQC experiment was able to locate the hydrazido protons. The 1H HMQC (15N decoupled) spectrum revealed six resonances (Fig. 2). Again this is explained as two isomers with three inequivalent protons each. The hydrazido proton (Hα) resonance is upfield from the hydrazine proton (Hβ and Hγ) resonances as expected and the three proton resonances for each isomer are present in a 1 : 1 : 1 ratio. Although complex II could not be isolated, the combination of 31P, 15N, and 1H-15N HMQC NMR data strongly suggests that complex II contains an η2-hydrazido(1−) ligand.

To probe the ability of these three species to interconvert, a series of acid/base experiments was performed and monitored by 31P{1H} and 15N NMR spectroscopy (see the ESI for details).† As with complex III, complex II was reprotonated with 1 M TfOH to yield I. Complex II was also further deprotonated by addition of KtBuO, yielding complex III. The last reaction to complete Scheme 1 involves reprotonating complex III in a stepwise manner, first to complex II, then to complex I. This was achieved in a two-step reaction, as monitored by 31P{1H} NMR spectroscopy. First, [HDBU][OTf] was used as the acid to convert III to II. Then, as before, complex II could be converted back to complex I by addition of 1 M triflic acid.

In summary, the hydrazine, hydrazido(1−), and diazene complexes based on the Fe(DMeOPrPE)2 scaffold have now been synthesized and characterized, and the three complexes were shown to interconvert in protonation/deprotonation reactions. The successful identification of this trio of molecules may have implications regarding the mechanism by which Fe-coordinated N2 is converted to NH3. Specifically, DFT calculations suggest that these three molecules are key intermediates in the protonation reactions of Fe(PP)2N2-type complexes that yield NH3 (Scheme 2).18 (Note that in a previous paper we showed that, upon protonation, the cis-[Fe(DMeOPrPE)2(N2H4)]2+ complex produces ammoniavia a disproportionation mechanism.16) The conversion III→II→I under acidic conditions is consistent with the pathway in Scheme 2, and is in contrast to the “asymmetric” protonation mechanism (Chatt/Schrock mechanism)20,21 observed in Mo systems. It is tempting to extend the mechanism in Scheme 2 to other systems containing Fe, including nitrogenase, but further investigations will be required before this generalization is possible.

Acknowledgements

Acknowledgement is made to the US National Science Foundation for the support of this work (CHE-0809393).

Notes and references

  1. M. Hidai and Y. Mizobe, Chem. Rev., 1995, 95, 1115 CrossRef CAS.
  2. B. A. MacKay and M. D. Fryzuk, Chem. Rev., 2004, 104, 385 CrossRef.
  3. J. M. Smith, R. J. Lachicotte, K. A. Pittard, T. R. Cundari, G. Lukat-Rodgers, K. R. Rodgers and P. L. Holland, J. Am. Chem. Soc., 2001, 123, 9222 CrossRef CAS.
  4. J. M. Smith, A. R. Sadique, T. R. Cundari, K. R. Rodgers, G. Lukat-Rodgers, R. J. Lachicotte, C. J. Flaschenriem, J. Vela and P. L. Holland, J. Am. Chem. Soc., 2006, 128, 756 CrossRef CAS.
  5. T. A. Betley and J. C. Peters, J. Am. Chem. Soc., 2003, 125, 10782 CrossRef CAS.
  6. T. A. Betley and J. C. Peters, J. Am. Chem. Soc., 2004, 126, 6252 CrossRef CAS.
  7. M. P. Hendrich, W. Gunderson, R. K. Behan, M. T. Green, M. P. Mehn, T. A. Betley, C. C. Lu and J. C. Peters, Proc. Natl. Acad. Sci. U. S. A., 2006, 103, 17107 CrossRef CAS.
  8. D. Sellmann, W. Soglowek, F. Knoch, G. Ritter and J. Dengled, Inorg. Chem., 1992, 31, 3711 CrossRef CAS.
  9. D. Sellmann and A. Hennige, Angew. Chem., Int. Ed., 1997, 36, 277.
  10. J. D. Gilbertson, N. K. Szymczak and D. R. Tyler, J. Am. Chem. Soc., 2005, 127, 10184 CrossRef CAS.
  11. T. A. George, D. J. Rose, Y. Chang, Q. Chen and J. Zubieta, Inorg. Chem., 1995, 34, 1295 CrossRef CAS.
  12. G. J. Leigh, Acc. Chem. Res., 1992, 25, 177 CrossRef CAS.
  13. N. P. Mankad, M. T. Whited and J. C. Peters, Angew. Chem., Int. Ed., 2007, 46, 5768 CrossRef CAS.
  14. B. M. Barney, D. Lukoyanov, T. Yang, D. R. Dean, B. M. Hoffman and L. C. Seefeldt, Proc. Natl. Acad. Sci. U. S. A., 2006, 103, 17113 CrossRef CAS.
  15. B. M. Barney, H. Lee, P. C. Dos Santos, B. M. Hoffman, D. R. Dean and L. C. Seefeldt, Dalton Trans., 2006, 2277 RSC.
  16. J. L. Crossland, L. N. Zakharov and D. R. Tyler, Inorg. Chem., 2007, 46, 10476 CrossRef CAS.
  17. L. D. Field, H. L. Li and A. M. Magill, Inorg. Chem., 2009, 48, 5 CrossRef CAS.
  18. R. B. Yelle, J. L. Crossland, N. K. Szymczak and D. R. Tyler, Inorg. Chem., 2009, 48, 861 CrossRef CAS.
  19. L. D. Field, H. L. Li, S. J. Dalgarno and P. Turner, Chem. Commun., 2008, 1680 RSC.
  20. J. Chatt, A. J. Pearman and R. L. Richards, J. Chem. Soc., Dalton Trans., 1977, 1852 RSC.
  21. R. R. Schrock, Angew. Chem., Int. Ed., 2008, 47, 5512 CrossRef CAS.
  22. M. Hirano, M. Akita, T. Morikita, H. Kubo, A. Fukuoka and S. Komiya, J. Chem. Soc., Dalton Trans., 1997, 3453 RSC.

Footnotes

† Electronic supplementary information (ESI) available: Experimental details of syntheses, and relevant spectroscopic data. See DOI: 10.1039/b902524c
‡ These reactions were performed with varying amounts of base (0.5–3 equiv.). It was found that a slight excess of base was required to achieve clean conversion. See the ESI for the 31P{1H} NMR spectra of the base titrations.†
§ The 31P{1H} NMR data alone rule out the possibility of η1 coordination of diazene, as five-coordinate complexes containing bidentate phosphine ligands generally appear as singlets in 31P{1H} NMR spectra.10,22
The 15N resonance for cis-[Fe(DMeOPrPE)2(N2H4)]2+ was incorrectly referenced in our prior paper (140.5 ppm).16 The correct value is −395.9 ppm (nitromethane scale).
|| Addition of excess 1 M TfOH to I results in the disproportionation of N2H4 to NH3 and N2.16
** The identity of cis-Fe(H)2(DMeOPrPE)2 was confirmed by two independent syntheses, see the ESI for details.†
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