4.2 Results and Discussion
4.2.2 Catalytic N 2 Reduction to NH 3
Scheme 4.4. A synthetic cycle for the formation of NH3 mediated by (TPB)Fe.
Studies with HBArF4 · 2 Et2O and KC8 at -78 °C showed unusually high yields of NH3, and the use of Et2O as solvent resulted in the formation of an average of 7.0 ± 1 equivalents of NH3 per Fe center, effectively demonstrating the efficacy of 4.1 as a pre- catalyst for N2 reduction to NH3 (Figure 4.1, Entry 1 Table 4.1) . This reaction has been performed sixteen times, with some single runs producing as much as 8.5 equivalents of NH3 per Fe. In a typical reaction, complex 4.1 is suspended in Et2O at -78 °C and 48 equivalents of HBArF4 · 2 Et2O similarly dissolved in Et2O and cooled were added, resulting in a homogenous yellow solution. Addition of 48 equivalents of KC8 to the reaction vessel as a suspension in cold Et2O is then immediately followed by sealing of the reaction vessel. Quantification of NH3 was then performed as described above after allowing the reaction to stir for 40 minutes at -78 °C.
Figure 4.1. UV-Visible spectrum of the indophenol dye generated from NH3. This particular absorbance spectrum corresponds to 7.5 equivalents of NH3 generated per Fe
center.
Entry Fe precursor NH3 equiv/Fe 1 [(TPB)Fe(N2)][Na(12-crown-4)2] 7.0 ± 1
2 [(TPB)Fe][BArF4] 6.2
3 [(TPB)Fe(NH3)][BArF4] 5.7
4 [(TPB)Fe(N2H4)][BArF4] 5.9
5 (TPB)Fe(N2) 2.0
6 FeCl2·1.5 THF <0.1
7 FeCl3 <0.1
8 Cp2Fe <0.2
9 Fe(CO)5 <0.1
10 none <0.1
Table 4.1. Catalytic runs using the standard conditions described in the text with any changes noted in the experimental section. All numbers shown are an average of a minimum of four runs. All individual runs can be found in Appendix 3.
In addition to complex 4.1, several other (TPB)Fe complexes have been tested (entries 2-5). In all cases, catalysis was observed with similar yields of NH3 as those observed when 4.1 was used as the pre-catalyst. Most notably, species that could be potential intermediates during the catalytic cycle, such as 3.2 (entry 2), 3.3 (entry 3), and 3.4 (entry 4), all function as capable pre-catalysts. The exception to these observations is (TPB)Fe(N2) (entry 5), which shows substantially decreased activity towards catalysis when compared with the other (TPB)Fe species studied. Reversal of the order of reagent addition, with reductant being added first followed by acid, resulted in an improved yield of 4.8 equivalents of NH3 per Fe center. Possible explanations for the lowered activity of (TPB)Fe(N2) under the standard conditions will be discussed below. As control
experiments, several simple Fe(II) and Fe(III) salts were also tested as pre-catalysts, but did not show any catalytic activity. The absence of any Fe species in the catalytic mixture also did not produce significant quantities of NH3. Taken together, these results implicate the intermediacy of a molecular (TPB)Fe catalyst for N2 reduction, especially when considered alongside the NMR studies discussed in 4.2.1.
Although there are no exogenous N atoms in the (TPB)Fe scaffold, the acid, or the reductant employed, it was still valuable to verify that the NH3 being produced was coming from N2. In this context, the catalytic reaction was run under 14N2 or isotopically labeled 15N2 and the resulting solids were analyzed by 1H NMR (Figure 4.2). Under 14N2
the triplet for [NH4][Cl] with 1JN-H = 51 Hz was clearly observed and this signal became a doublet with 1JN-H = 71 Hz typical of [15NH4][Cl] when 15N2 was used instead, with only trace [14NH4][Cl] being observed, likely arising from the residual 14N2 bound in 4.1.
These results verify that gaseous N2 is the source of the resulting NH3.
Figure 4.2. 1H NMR spectra of [14NH4][Cl] and [15NH4][Cl] in DMSO-d6 obtained from correspondingly labeled N2 gas via catalytic reduction with complex 4.1 using the
standard catalytic protocol described in the text.
It is somewhat noteworthy that the catalysis appears to be occurring at -78 °C.
The disappearance of the bronze color of KC8 at -78 °C seems to suggest that
consumption of reductant occurs at low temperature as opposed to only reacting upon warming to RT. Furthermore, the fact that N-H bond formation is occurs faster than H2
formation under these conditions is somewhat surprising, as KC8 and HBArF4 do react in the absence of any Fe complex under the reaction conditions to generate > 75% of the expected H2 within 40 minutes. Formation of H2 is nevertheless an issue, as ~ 30% of the hydrogen atom equivalents on average, as determined by GC analysis of the reaction head space, end up as H2 during the course of catalytic runs (See Appendix 3). The formation of the N-H bonds during catalysis is likely aided by the heterogeneity of the reaction conditions, slowing H2 formation enough to allow for effective delivery of hydrogen atom equivalents to N2.
It is not straightforward to compare this Fe-based system to the Mo-based systems of Shrock and Nishibayashi. The turnovers for Fe (7) are quite similar to those observed for Schrock (7.5) and Nishibayashi (12.2), but the Fe system benefits from a substantially stronger reductant. It is not yet clear whether the redox potential required to form 4.1 (- 2.19 V vs. Fc/Fc+) mandates a stronger reductant, or whether weaker reductants only capable of accessing species such as (TPB)Fe(N2) could participate in catalysis. While a stronger reductant is required at this point, the Fe system does appear to operate at substantially lower temperatures than the Mo systems, potentially highlighting higher reactivity at the Fe based system.