Can Vanadium Pyrophosphate Coordination Complexes Perform Butane Oxidation?

Doyle, Robert P.

Critical results obtained as part of the PRF project over the second year are highlighted below. We were able to make significant progress over the first year of the project and used that first year to translate into a successful second year. The grant has allowed me, along with support from Syracuse University (who funded a post-doctoral student) to establish a strong pyrophosphate catalysis group at SU. A highlight of the second year has been the development of a successful collaboration between my group and the group of Dr. Susan Hanson at Los Alamos National lab. This collaboration is due to the continued support of this project by the ACS PRF fund, cementing a strong connection between the two labs moving forward.

Over the first year we established:

1. The crystal structure of the first vanadyl-PPi complex (see Figure 2; unpublished data).

2. The complex could perform oxidative catalytic (see Figure 1).

a b

Figure 1. ¹H NMR of (a) butanol and (b) butanoic acid after conversion over vanadium pyrophosphate-bipyridine. Reaction was conducted at 80 °C over 1 hour in liquid butanol under a positive pressure of pure oxygen. Approximately 600 turnovers were calculated. Note the complete disapperance of the butanol methylene signal at 4.7 ppm.

Over the second year we have been able to develop this work to the point that we have published a paper in Dalton Trans., (in Press 2011, PMID: 21935525) and are submitting a second, with Dr. Hanson, to Angew Chemie Int. Ed. in late October 2011. Over the second year, we have completed structural investigations of the new VPO complex and conducted extensive catalytic investigations (see Figure 2 and Table 1).

Entry	Substrate	mol% 1	atm	additive	% conversion
1 2 3 4 5 6 7^[c] 8^[c,d] 9 10 11 12 13 14	benzyl alcohol	none none 1 1 1 1 1 1 1 0.5 0.25 0.1 2 5	argon air argon argon air air air air air air air air air air	Et₃N Et₃N pyr Et₃N none pyr Et₃N Et₃N Et₃N Et₃N Et₃N Et₃N Et₃N Et₃N	<1 <2 <2 <2 9 10 16 42 35 ± 4 33 ± 6 30 ± 4 10 50 46
13 14 15	pinacol	none 1 0.25	air air air	Et₃N Et₃N Et₃N	49 70 67

Table 1. Aerobic oxidation of benzyl alcohol (route 1) or pinacol (route 2) catalyzed by complex 1.^[^a,b^] [a] Reaction conditions, unless otherwise stated: 1 ml substrate, no solvent (neat); [b] % conversion determined from ¹H NMR analysis.

Figure 2. (a,b) ORTEP plots (20% probability level) of the vanadyl-pyrophosphate dimeric unit in 1 (a) and 1-pyr (b) with the atom labeling scheme (bipy and pyridine-H atoms are omitted for clarity). Vanadyl (V=O) units are evidenced with the open-type bond. (c) TEM images of 1 precipitate as fine powder by direct synthesis. (d,e) SEM images of microcrystals of 1 obtained by slow diffusion (d), and crystals of 1-pyr after washing with chloroform (e).

Basic reaction conditions established over year 2 are described below:

In a 50 ml round bottom flask, complex 1 (6.7, 17.2, 34.3, 68.6, 137.2 or 343 mg, 0.0097, 0.0241, 0.0483, 0.0965, 0.1931 or 0.4827 mmol = 0.1, 0.25, 0.50, 1, 2 or 5 mol% BA) was combined with BA (1.0 mL, 9.654 mmol) and TEA (135 μL, 0.965 mmol = 10 mol% BA). The flask was equipped with a stir bar and the mixture heated with stirring (600 rpm), under reflux, at 100 ¼C. After about 1 to 4 hrs of heating, all the catalyst dissolved, producing a bright green colored solution. Within 24 hrs, the mixture turned an orange brown color and some precipitation was observed. After 48-72 h, a larger amount of precipitate was observed, green to brown in color (see Figure 3). After 72 hrs the reaction was stopped and the flask removed from the hot oil bath. The reflux condenser was rinsed with 2-4 ml CDCl₃ (or CDCl₃/CHCl₃) and the mixture stirred for 3 to 12 hrs (overnight) at room temperature. The sample was then centrifuged, and the orange-brown supernatant (see Figure 3) was typically analyzed via ¹H, ³¹P and ⁵¹V NMR. The extent of conversion from BA to benzaldehyde was determined by integration of the BA peak around 4.6 ppm against the aldehyde peak around 9.8 ppm in the ¹H NMR spectrum. This was noted as high as 50% with 2 or 5 mol% of 1 after 72 hrs (see Table 1 in main text). No conversion to benzylic acid was observed within 72 hrs for any catalyst load. However, the reaction with 1 mol% of 1 consistently gave a 100% conversion of BA to benzaldehyde/benzylic acid after 6 days at 100 ¼C.

(a) (b)

(c)

Figure 3. Snapshots of the reaction BA (1ml) + TEA (10mol%) + 1 (1mol%): (a) 0 hrs and (b) 72 hrs. (c) 72 hrs sample after addition of CDCl₃and centrifugation. Note the dark green precipitate and the orange-brown CDCl₃ supernatant

In summary, we have shown here that a novel vanadyl-pyrophosphate coordination complex can be readily obtained as nanocrystalline powder by a one-pot synthesis at room temperature from commercially available and inexpensive reagents. To our knowledge, the compound is only the second example of molecular (non-polymeric) VPO species ever obtained in mild conditions. More importantly, this is the first ever tested as a catalyst at low temperature. While we continue to investigate the mechanism it is likely that the dimer is a pre-catalyst and the active species is formed in situ. We believe from ³¹P NMR analysis and IR of powder collected at the end of the reaction that the in situ species is still a PPi-bridged species, but is possibly a mixed valent V(IV)/V(V) species, as indicated by recent EPR studies we have conducted Prof. K. V. Lakschmi, RPI. Work on this highly intriguing and active catalyst is continuing.

www.acsprf.org

Reports: DNI348999-DNI3: Can Vanadium Pyrophosphate Coordination Complexes Perform Butane Oxidation?