Chapter 5: The Effect of Ligand and Reaction Conditions on Fe Mediated N 2
5.2 Results and Discussion
5.2.4 Comparison of Catalytic Conditions
This inflexibility strongly contrasts with the TPB system, where large variations in the Fe-B distance take place to accommodate geometric and electronic changes at the Fe center.17 The fact that complex 5.4 is not a competent catalyst for N2 reduction while 4.1 is further implies that the flexible linkage in the TPB system may be important for
catalysis.
Similarly, the tris-phosphino borate ligated complex 5.5 produces a sub-
stoichiometric amount of NH3, which is again consistent with the empirical observation of a need for a hemi-labile interaction between the Fe and the apical B. Interestingly, in the tetraphosphine complex 5.6 there exists, in principle, the possibility of a hemi-labile interaction with the apical P atom of the ligand scaffold. The low yields of NH3 for this system when compared with the other entries in Table 5.2 suggest that if any flexibility of the Fe-P bond exists, it does not enable catalysis in this system. Finally, complex 5.7 was subjected to the standard catalytic conditions to test a different tetra-phosphine complex but also did not exhibit catalytic NH3 formation. Similar Fe phosphine complexes had been tested for NH3 formation previously and had shown only ~ 10% of an equivalent of NH3 per Fe.18 While it is difficult to draw definite comparisons, the higher yields observed here may be reflective of the conditions used for catalysis in this study.
ethereal solvents. Unsurprisingly, iPr2O serves equally well as Et2O in the reaction, but moving to a more polar solvent in DME results in a drop in NH3 yield. Furthermore, the presence of some ethereal solvent is critical, likely due to solubility of the HBArF4 · 2 Et2O acid, as is evidenced by the low yields of NH3 in toluene (entry 4) compared with much higher yields when even a small amount of Et2O is included in the reaction mixture (entry 5). The lower yield of NH3 in Bu2O is likely similarly explained by lowered solubility of HBArF4 · 2 Et2O. The requirement for an ethereal solvent likely stems from the need for good solubility of the acid and any BArF4- ions. Additionally, it is not clear whether KC8 will be thermodynamically or kinetically suitable to form 4.1 in less polar solvents such as toluene. Conversely, the lowered activities in more polar ethers likely result from accelerated H2 formation.
Entry Solvent NH3 eq./Fe
1 iPr2O 6.53
2 DME 3.37
3 Bu2O 3.16
4 Toluene 0.78
5 1:6 Et2O:Toluene 3.12
Table 5.3. Effect of different solvents on the catalytic reduction of N2 to NH3 by complex 4.1. Note that all reported values are an average of at least 2 runs (See Appendix 4).
Aside from solvent, the reaction is quite sensitive to the choice of reductant (Table 5.4). The use of weaker reducing agents such as Cp*2Co or Cp*2Cr results in very low yields of NH3. It is unclear whether this is a reflection of the need for a reducing agent strong enough to generate [(TPB)Fe(N2)]- or whether the potentially higher solubility of these reagents increases side reactions with the acid. The higher yields obtained with NaC10H8 (entry 3), which should be a strong and soluble reductant, suggest
that the potentials of the reductants in entries 1 and 2 do hinder the reactivity. The use of K metal or MgC14H10 also leads to low yields of NH3. Finally, although other evidence supports the presence of a molecular catalyst, some sort of graphite bound complex as the catalytically active species cannot be ruled out. Entry 6 illustrates that Na/Hg is also competent for catalysis, albeit only nominally, suggesting that graphite is not an essential component of the reaction mixture and further supporting the homogeneity of the
catalyst.
Entry Reductant NH3 eq./Fe
1 Cp*2Co 0.6
2 Cp*2Cr <0.2
3 NaC10H8 1.0
4 K metal 0.4
5 MgC14H10 · 3 THF 0.3
6 Na/Hg 2.1
Table 5.4. Effect of different reductants on the catalytic reduction of N2 to NH3 by complex 4.1. Note that all reported values are an average of at least 2 runs (See Appendix 4). Note that the Na/Hg was 10% Na by weight.
A number of different acids were tested in the catalysis with the results shown in Table 5.5. The use of HBArF4 · 2 Et2O in the standard catalytic runs was chosen due to its high acidity and the poor coordinating ability of the BArF4- counterion. Other strong acids, with potentially better coordinating counterions in HOTf and HCl, showed very low yields of NH3 (entries 1 and 2). The use of a weaker acid in [Lutidinium][BArF4] also showed very low yields of NH3. Interestingly, the use of a slightly stronger acid in [2,6-dimethylanilinium][OTf] or [2,6-dimethylanilinium][BArF4] resulted in catalytic turnover. This result is somewhat surprising, as the pKa of [Lutidinium] is 6.77 versus 3.95 for [2,6-dimethylanilinium].19,20 The cause for the disparity in activity between
these two acids is unclear, but could potentially arise from different redox properties between anilinium and pyridinium acids. Alternately, the relatively narrow pKa range between these acids could bracket the acidity required for one of the catalytic steps. The last observation of note is that the two anilinium acids in entries 4 and 5 show similar activity despite having different counterions, which seems to suggest that the difference between [OTf]- and [BArF4]- is not significant for catalysis.
Entry Acid NH3 eq./Fe
1 HOTf 0.4
2 HCl <0.1
3 [Lutidinium][BArF4] <0.1
4 [2,6-dimethylanilinium][OTf] 2.1
5 [2,6-dimethylanilinium][BArF4] 2.9
Table 5.5. Effect of different acids on the catalytic reduction of N2 to NH3 by complex 4.1. Note that all reported values are an average of at least 2 runs (See Appendix 4).
Note that [2,6-dimethylanilinium][BArF4] was obtained as a 1.5 Et2O adduct.
Finally, while the initial reaction screening was carried out at -78°C, it was desirable to determine whether low temperature was a prerequisite for NH3 formation, or whether higher temperatures could still be viable for N2 reduction. In this context, the catalysis was tested at -110 and 25 °C. The reduced yield of 1.33 equivalents of NH3 per Fe at RT illustrates that NH3 yields are drastically reduced at higher temperatures and that cooling of the reaction is necessary. Interestingly, the catalysis is still viable at
temperatures as low as -110 °C with a yield of 5.40 equivalents of NH3 per Fe. Although it will be difficult to exclude the possibility that reaction only occurs upon warming the solution, the conversion of the bronze color of KC8 to black graphite in the cold reaction mixture suggests reaction at these temperatures may be occurring. Furthermore, attempts
to quench the catalysis at low temperature with either [TBA][CN] or with t-BuNC have resulted in diminished, but still substantial yields of NH3, suggesting that catalysis is occurring at low temperature.