Journal of Radioanalytical wzd Nuclear Che#dstry, Articles, Vol. 163, No. 2 (1992) 325-333

FIXATION AND REDtJCTION OF MOLECULAR NITROGEN BY [WH4(PC2HsPh2)4] AND [WH4(PCH3Ph2)4] IN A y-RADIATION FIELD J. O. DZI,EGIELEWSKI,* R. GIL-BORTNOWSKA, J. MRZIGOD

Institute of Chernlstsry, Silesian University, 9 Szkolna Stree~ 40-006 Katowice (Polana)

(Received March 10, 1992)

Hydride complexes of W(IV) with dpep (diphenylethylphosphine) and dpmp (diphenylmethylphosphine) were irradiated in thf + C6H12(1:1) solutions, saturated with N2 + 1-12(1:3).Radiation yields of hydrazine, ammonia and amines were evaluated. The mechanism of reduction of molecular nitrogen is discussed.

Introduction In our earlier papers 1-3 it was shown that the hydride complexes of Mo(IV) and W(IV) with bidentate phosphines (dppe and dtpe) in the coordination sphere could form, in high yields, the M(N2)2 and M-H groups in a y-radiation field. The groups are formed as a result of addition of molecular nitrogen or hydrogen or H + ions to the central ions. The coordinated nitrogen in the [M(N2)2(~L)2] complexes is reduced, according to a radical mechanisms, by M-H to N2H2, then to N2H4 and, according to an ionic mechanism, by H- ions and, to some degree, by electrons of the central atoms, to H3N. The effectiveness of the reduction depends on the ttiermodynamic stability of the hydride complexes. The influence of the solvent on the stability of the M-H groups and the yields of H4N2 and H3N was explained earlier.Z3 When the concentration of H- ions is decreased in the irradiated solution, coordinated nitrogen is reduced by radicals to alkilhydrazido groups and then to amines.3,4 In order to directly determine the influence of the W-H groups in the reduction of coordinated nitrogen, we examined the hydride complexes of W(IV) with monodentate phosphines in a thf + C6H12 mixture. Thus, we applied the same solvents as in our previous papers 3,4 to have the same donor abilities of the solvents, equal yields of radiolysis products and corresponding polarity of the environment to influence the stability of W-H groups. Similarly to dppe {bis-l,2-(diphenylphosphino)ethane} we used ethyldiphenylphosphine (dpep) and methyldiphenylphosphine (dpmp). The complexes [WH4(dpep)4] and [WH4(dpmp)4] have different charge densities on the P atom and, as a consequence, *Author for correspondence.

Elsevier Sequoia S. A., Lausanne Akad~,mialKiad6, Budapest

J. O. DZIEGIELEWSKI et al.: FIXATION AND REDUCTION

different stabilities of tlae W-P and W-H bonds. 5-9 Thus, the aim of our work was to determine the yields of HaN2, H3N and RNH 2. We applied the hydride complexes of W(IV). They are thermodynamically less stable than [WHa(dppe)z]. Their interesting property is that the W-H and W-P bonds have different stabilities. Moreover, in the course of the reaction, the central atoms of the intermediate [W(Na)2L4] have a greater ability to reduce coordinated nitrogen in comparison to that of the [W(Na)2(dppe)2 ] complex.10-12 These differences between the complexes, depending on the phosphine applied, were explained earlier. 12

Experimental DPEP and DPMP were obtained in the standard way, 13 the complexes [WH4(dpep)4] and [WH4(dpmp)4] were synthesized according to the literature, 14 [WHs(dpeP)4]HSO 4 and [WHs(dpmp)4]HSO4 were obtained as in paper.2 The solutions of the hydride complexes in a tiff + cyclohexane, or tiff + cyclohexane + H2SO4 mixture were prepared as in our previous paper2 and irradiated in a 6~ ,/-radiation source under continuous saturation with a N 2 + H 2 (1:3) mixture. The gases were passed through the washer system and then through the reaction system. The applied dose rate, calculated for a tiff + cyclohexane mixture, was 1.2.1016 eV. cm-3. s-1. Hydrazine and ammonia were determined spectrophotometrically.3,4

Results and discussion The numbers of N2H~+ and NH,~ ions and H4N2 and H3N molecules vs. dose and concentrations of the complexes are presented in Figs 1 and 2, respectively. In Table 1 the radiation yields of hydrazine and ammonia, the concentration of ammonia after absorption of 3.94.1017 eV. cm -3. s-1 and the catalytic efficiency (i.e., the number of H3N molecules per 1 tool of W complex fixing nitrogen) calculated as in our previous paper2 are shown. It is easy to notice (see Fig. 1) that the numbers of N2I~+ ions and H4N2 molecules are dependent on dose as is the case for other Mo and W complexes. The shapes of the curves were explained earlier. 12 The yields of hydrazine and ammonia (see Table 1) are much lower than those in the case of the complex with dppe. 3 The apparent maxima in Fig. 2 are evidence for decomposition of NH~ ions (and H3N molecules) by radicals (R) when a concentration ca. 10-3M is exceeded, as was shown earlier.Z15 The lack of the maximum at higher doses is due to the higher rate of formation of NH~ ions as compared with the rate of their decay, while the plateau is accompanied with comparable rates of formation and decay of NH~ ions. 326

J, O. DZI,EGIELEWSKIet al.: FIXATIONAND REDUCTION

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I

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Fig. 1. Dependenceof the numberof H4N2 moleculeson the dose for different concentrationsof [WH4L4] in thf + C6H12;A - L (upper curve) = dpmp, B - L 0over curve) - dpep; O % = 10-2'M, % = 10-2M; CI % = 10-2M, % ~ 10qM; * % - 5 910-2M,% - 10-1M.% - concentrationof complex, % - concentrationof H,zSO4 An induction period does not appear in Fig. 2, which is in contradiction with the cases of some complexes of Mo and W with dppe. That fact could be due to the dominating participation of the central W(IV) atom in the reduction of coordinated hydrazine. However, the low yields of hydrazine (Fig. 1 and Table 1) and ammonia (Fig. 2) point to a rather low rate of such a process, i.e., the reduction of a W - N 2 moiety by the central atom does not play any role. On the basis of the results presented one could assume that the central atoms take only a smaU part in the reduction of coordinated nitrogen, using the hydride complexes with monodentate phosphines. Thus, the main agent of the process seems to be the 327

J. O. DZIEGIELEWSKI et al.: FIXATION AND REDUCTION i A

,~ 6G~-

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2o z

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30

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Dose,xIOneV/cm3

Fig. 2. Dependence of the n u m b e r of H3N molecules on the dose for different concentrations of [WH4L4] in thf + C6H12;A - L (upper curve) -dpmp, B - L (lover curve) - dpep; O % = 10-21vl,% = 10-2M; [] cw = 10-2M, co = 10-~M; * cw = 5 9 10-ZM,c~, = 10-1M. cw- concentration of complex, % - concentration of H2,SO4.

hydride ions, which is supported by the yields presented in Fig. 1. Higher yields were obtained w h e n a more thermodynamically stable hydride complex [WHa(dppe)2 ] was used, viz. 400 for hydrazine and 8380 for a m m o n i a molecules per 100 eV. 3 The activity of H - ions in the reduction of coordinated nitrogen is supported by the increasing yields of H4N 2 and H3N in more acidified solutions (Table 1) since the H § ions are attached to the central atoms to increase the concentration of W - N groups. 1-3,18 The lack of a n induction period in Fig. 2 m a y be attributed to the participation of H ions from the original molecule of the hydrazide complex [WH3(N2Hz)(HSOzJL~] in the 328

J. O. D Z I E G I E L E W S K I et al.: F I X A T I O N A N D R E D U C T I O N

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J. O. DZIEGIELEWSKI et al.: FIXATION AND REDUCTION J

reduction of N2H2. The intermediate [WH(N2I-I2)(SO4)(dppe)2] was suggested earlier. 3 Hydride complexes of tungsten with monodentate phosphines exhibiting high coordination numbers are known, e.g., [WH6L3136 The lower ratio of H4N2 conversion into HaN for the W(IV) complexes with @rap and dpep is due to the greater importance of the intramolecular reduction of coordinated nitrogen (GHgq:GN2H4 = 11 or 10 at [W(IV)] = 5 9 10-2M, while for the complexes with dppe the ratio of conversion is higher GH.~:GH~2 = 18 at [W(IV)] = 5.10-2M). For some complexes with a lower number of H- ions (with bidentate phosphines as the ligands) 17 the intermolecular process is more important in the reduction of coordinated nitrogen, thus the induction period is required to reach the initial concentration of H4N2 at which the H- ions could reduce hydrazine to ammonia. 1-3 On the basis of the data of Figs 1-3, Tables 1-2 and our previous papers >4 the following mechanism of the radiation-catalytic fixation and reduction of molecular nitrogen could be assumed:

(1)

solvent/vx/,,.--, S +, es' R, H, S* H~P~e,

[WH4L4] + 2N 2 -

a

[W(N2)2L4]

"

[W(N2)2L4] + 2H2

) [H3W(N2H2)(SO4)L2] + N 2 + 2LH+ +

H'[WH6L4]SO4 R,H

(2) [WE4]

(3) b

RNH2 + [WE4] + N2

a

[H2W(N2H4)(thf)2L2]2+ + SO42-

2thf' H+ ] [H3W(N2H2)(SO4)L2] 2LH+,2H[

(4) b

~- [H2W(N2H4)L4] + H2SO4

[H2W(N2H4)L4] -'-- 2NH 3 + [WE4] [WE4] + 2N 2 ~

[W(Y92L4]

(5) (6)

The much lower yields of H3N for the hydride complexes of W(IV) with monodentate phosphines are due to their lower thermodynamic stability, whichqs also 330

J. O. DZIEGIELEWSKI et al.: F I X A T I O N A N D REDUCTION

the case for the intermediate complexes (i.e., bis-dinitrogen, hydrazide and hydrazine) compared to their analogs with dppe in the coordination sphere. 1-3 Thus, the lower stability of the complexes with dpmp and dpep resulted in the enhanced role of reactions 3a and 4a and in the decreased importance of Reaction (4b) compared to the analogous

35 o •

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40

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Fig. 3. Dependence of the number of RNH 2 molecules on the dose for different concentrations of [WH4L4] in thf+ C 6 I - I 1 2 ; # c w * 5 9 10-ZM,% - 10-1M, L ffidpep; Ocw - 10-2M, c,, - 0 , L - d p m p ; Qcw = 10-~Vl, % - 10-ZM, L - d p m p ; cw - concentration of complex, % - concentration of H,zSO4

complexes with dppe. This situation leads to lower concentrations of dinitrogen and hydride complexes. Those facts are reflected in the low catalytic activity of the complexes with monodentate phosphines (Table 1). The addition of a ligand (dpmp) to the reaction system is followed by a slight increase of the yields of H3N and by the improved catalytic activity of the complex. A similar procedure for [WH4(dppe)2] led to much better results (catalytic efficiency -- 24). 3 The lower catalytic efficiency at higher concentrations of the complexes (Table 1) is due to the low dose rate. The slightly lower yields of ammonia and hydrazine for [WH4(dpep)4] compared to [WH4(dpmp)4] could be explained by the higher Vw_H frequency for the former complex. The more strongly bound H- ions migrate with difficulty to the coordinated nitrogen. The W-H groups in the complex with dpep are, therefore, kinetically more stable. Typical dependences of the number of RNI-I2 molecules and RNH~ ions on the dose and concentration of the complex are shown in Fig. 3. The yields of amines and their catalytic efficiency are listed in Table 2. It is seen from Fig. 3 that an induction period appears in the process of amines formation, similarly as f o r ['wn4(dppe)2]. 4 The 331

J. O. DZIEGIELEWSKI et al.: FIXATION AND REDUCTION Table 2 Radiation yields of amines and catalytic effieieneies of [WH4(dpmP)4] in the reaction of their formation

Concentration of eomoplex, tool 9 -3

10-2 10-2 10-2 5- 10-2

Concentration of H2SO4.1 tool 9 din-"

0 10 -2 10 .2* 10- I

Number of RNH 2 or RNH~ per 100 eV

Concentration of RNH 2 or RNH~ after absorption of 39.4.1017 e V . cm -3 [tool. dm -3]

Number of moles of RNH 2 per 1 mole of complex after absorption of 39.4 9 1017 eV- cm -3

68.0 10.8 8.5 47.0

4 . 0 0 . 1 0 -3 4 . 2 8 . 1 0 -3 3 . 8 9 . 1 0 .3 3 . 4 5 . 1 0 -3

1.21 1.30 1.18 0.22

*dpmp was added to irradiated solution (concentration of free ligand was 10-2 mol 9 dm-3).

induction period reflects the initial reduction of coordinated nitrogen to ammonia, and then, after decreasing of the H- ion concentration, the reduction of W-N 2 by R to W(N2R2) and then to RNH 4. Therefore, the highest yields of RNH 2 are observed in a non-acidified solution (yields are inversely proportional to the concentration of sulfuric acid) as the increasing concentration of the W-H decreases the probability of W-N 2 reduction by R. The decrease of amine concentration at higher doses is due to their decomposition by R after the concentration of W-N 2 has been reduced. On comparing the results presented in Table 2 and in paper, 4 one should come to the conclusion that the catalytic efficiency of RNH 2 formation is almost the same for the complexes with monodentate and bidentate phosphines. The observations presented suggest that the concentrations of R and W-N2, at the doses when W(N2R2) are formed, are almost equal. The shorter induction period for the complex with monodentate phosphine (Fig. 3) than for the dppe complex is due to the faster depletion of H- ions in solutions of the complexes with dpmp and dpep, which is in good agreement with the lower thermodynamic stability of the complexes with those ligands, and with the data of Table 1. The low thermodynamic stability of the complexes with monodentate phosphpines is more evident after the additon of dpmp to the solution of [WHa(dpmP)4]. The increase of free ligand concentration only slightly affects the saving of W E 4 structure while the concentration of H- ions is slightly increased and the yields of RNH 2 are decreased. More detailed mechanism of the formation of amines [Reaction (3b)] was described earlier.4 In the light of the observations presented one can assume that the earlier mechanism of fixation and reduction of molecular nitrogen in a radiation field by hyride complexes 332

J. O. DZIFAGIELEWSKIet al.: FIXATION AND REDUCTION o f Mo(IV) and W ( I V ) 1-4 is generally correct for the c o m p l e x e s [W'H4L4] , provided that for c o m p l e x e s with m o n o d e n t a t e phosphines the intramolecular reduction processes could be m o r e

important.

It is the hydride ions rather than the central atoms that are the reducing agents in these intramolecular processes, which is in contradiction to c o m m o n views. 1~

Financial support from KBN (Contract No. PB 1029/83/92/02) is gratefully acknowledged.

References 1. J. O. DZIEGIELEWSKI, R. GRZYBEK, Polyhedron, 9 (1990) 645. 2. J. O. DZIEGIELEWSKI, J. MAIECKI, R. GRZYBEK, Polyhedron, 10 (1991) 1007. 3. J. O. DZIFJ3IELEWSKI, J. MAIECKI, Polyhedron, 10 (1991) 2827. 4. J. O. DZIEGIELEWSKI, J. MA,IECKI, Polyhedron, in press. 5. V. B. V()LKOVA, A. J. BORISOW, W. D. MAKHAEV, K. J. SEMENENKO, Koordinatsionnaya Khimiya, 5 (1980) 743. 6. P. MEAKIN, L. J. GUGGENBERGER, W. G. PEET, W. L. MUETTERTIES, J. P. JESSON, J. Am. Chem. Soe., 95 (1973) 1467. 7. D. S. MOORE, S. D. ROBINSON, Chem. Soc. Rev., 12 (1983) 415. 8. R. A. HENDERSON, J. C. S. Chem. Commun., 1987, p. 1670. 9. L. J. ARCHER, T. A. GEORGE, Inorg. Chem., 18 (1979) 2079. 10. J. CHATT, New Trends in the Chemistry of Nitrogen Fixation, Moskva, Mir, 1983. 11. B. B. KAUL, R. K. HAYES, T. A. GEORGE, J. Am. Chem. Sot., 112 (1990) 2002. 12. J. O. DZI,EGIELEWSKI, Polyhedron, 8 (1989) 1470. 13. V. D. BIANCO, S. DORONZO, Inorg. Synth., 16 (1976) 157. 14. F. PENELLA, Inorg. Synth., 15 (1976) 42. 15. J. O. DZI,EGIELEWSKI, R. GRZYBEK, J. Radioanal. Nucl. Chem., 132 (1989) 115. 16. D. GREGSON, J. A. K. HOWARD, J. N. NICHOLLS, J. L SPENCER, D. S. TURNER, J. C. S. Chem. Commun., 1980, 572. 17. R. PIERANTOZZI, G. L GEOFROY, Inorg. Chem., 19 (1980) 1821. 18. J. O. DZ,I ,I~IELEWSKI, R. GRZYBEK, R. GIL-BORTNOWSKA, J. Radioanal. Nuel. Chem., 140 (1990) 415.

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