60th Annual Report on Research 2015

Since CO₂ coordination can be a prerequisite for CO₂ activation, the preparation and study of isolable CO₂ complexes is of great interest. Side-on (η²) coordination is often observed for monomeric complexes of CO₂ which are most frequently prepared by substitution of a labile ligand. Our work in this area began with the report that η²-CO₂ complexes of molybdenum and tungsten can be prepared via thermal displacement of a substitutionally labile aromatic ligand under slightly elevated pressures of CO₂. Figure 1 depicts the synthesis of TpMo(NO)(1-methylimidazole)(η²-CO₂) (Tp = tris(pyrazolyl)borate) by displacement of furan from TpMo(NO)(1-methylimidazole)(η²-furan).

As we endeavored to prepare η²-CO₂ complexes of this type featuring σ-donors (L_d) beyond 1-methylimidazole, several features of this methodology hindered our progress. First the preparation of the η²-furan complex from TpMo(NO)(1-methylimidazole)Br requires reduction with sodium dispersion and typically results in low yields (~25%). More importantly, when the complex TpMo(NO)(L_d)Br features a σ-donor less donating than 1-methylimidazole, the complex can become too electron deficient for an η²-furan complex to be isolable. Consequently, a new synthetic route to η²-CO₂ complexes was pursued.

Dihapto carbon-dioxide complexes can sometimes be prepared by the oxidation of a carbonyl ligand. Thus we pursued a simplified synthetic strategy (Figure 2). A carbonyl ligand can be thermally liberated from Tp^RMo(NO)(CO)₂  (Tp^R = Tp or Tp*, tris(3,5-dimethylpyrazolyl)borate) in the presence of a σ-donor ligand to give Tp^RMo(NO)(L_d)(CO). Using this method, fifteen complexes featuring L_d = P(OMe)₃, PPh₃, P(p-tolyl)₃, PMe₃, 3-fluoropyridine (F-Py), 3-chloropyridine (Cl-py), pyridine (Py), 4-dimethylaminopyridine (DMAP), 1-methylimidazole (1-MeIm), 1,3-dimethylimidazol-2-ylidene (NHC),  have been prepared and purified by column chromatography. Several of these complexes are air sensitive. In some cases, IR spectroscopy indicates that small quantities of η²-CO₂ complex are formed during heating if oxygen is not rigorously excluded. Consequently, the best results were obtained by performing thermolysis in a sealed pressure tube. The analogous tungsten complexes cannot be prepared via this method due to the reticence of Tp^RW(NO)(CO)₂ to release CO upon heating. However, we are currently pursuing photochemical methods to address this limitation.

The range Tp^RMo(NO)(L_d)(CO) is constrained by the steric and electronic properties of L_d. For Tp, complexes featuring alkyl phosphines larger than PMe₃ could not be isolated, including PEt₃ and P(n-Bu)₃. For Tp* the more sterically demanding ligand platform means that a handful of complexes isolable for Tp were not isolated, including PPh₃, NHC, and Cl-Py. On the other hand, the added electron-donating ability of the Tp* chelate allowed a 3-fluoropyridine (F-Py) complex to be isolated for Tp*, while the analogous Tp version could not be isolated.

Oxidation of these carbonyl complexes to give Tp^RMo(NO)(L_d)(η²-CO₂) is accomplished using a range of mild oxidants including cumene hydroperoxide, ^tBuOOH, or oxygen gas. Most commonly, a hydroperoxide gives the best yields of η²-CO₂ complex, usually giving isolated yields between 40 and 80%. In the case of L_d = DMAP, the hydroperoxide oxidants produces low yields of η²-CO₂ complex. However, oxygen proves to be the best oxidant giving Tp*Mo(NO)(DMAP)(η²-CO₂) in 44% and TpMo(NO)(DMAP)(η²-CO₂) in 38% isolated yield after chromatography.

Dihapto-CO₂ complexes are isolable for a range of complexes featuring σ-donors that are phosphines, pyridines, imidazoles, and N-heterocyclic carbenes.  Carbon dioxide complexes could not be isolated for the more electron-deficient L_d (PPh₃, P(OMe)₃, P(p-tolyl)₃). For these complexes, cumene hydroperoxide did not oxidize the requisite complex. Stronger oxidants such as PhIO did react, but CO₂ complexes could not be isolated or definitively identified in situ via spectroscopic methods. Since oxidation is a proven decarbonylation method via formation of CO₂, it seems likely that these complexes are simply too electron deficient to maintain coordination of CO₂. Heretofore, the least electron rich η²-CO₂ complex isolated is TpMo(NO)(Cl-Py)(η²-CO₂) based on IR spectroscopy and oxidation potential as measured by cyclic voltammetry.

Unsurprisingly, carbonyl and nitrosyl stretching frequencies are excellent indicators of the electron density of Tp^RMo(NO)(L_d)(CO) complexes. Complexes featuring ν_CO > 1893 cm^-1 and ν_NO > 1607 cm^-1 did not result in isolated η²-CO₂ complexes upon oxidation. However, all isolated Tp^RMo(NO)(L_d)(CO) complexes featuring ν_CO and ν_NO lower than these have led to isolable η²-CO₂ complexes upon oxidation (Figure 3).

Dihapto-CO₂ complexes feature nitrosyl stretching frequencies from 1611-1633 cm^-1. In contrast to the CO complexes, the nitrosyl stretch of the η²-CO₂ complex appears to be insensitive to Tp^R substitution, nor is an obvious trend based on L_d clear. Certainly the electron density at the metal is more governed by the presence of the η²-CO₂ ligand than the σ-donor.

The formation of Tp^RMo(NO)(L_d)(CO) complexes from η²-CO₂ complexes can occur under certain reducing conditions, usually in a few hours at room temperature. Activated magnesium gives CO complexes at room temperature in 50-75% isolated yield after column chromatography, but requires the addition of P(NMe₂)₃, presumably as an oxygen acceptor since O=P(NMe₂)₃ is detected by ³¹P NMR spectroscopy. The hydrides LiAlH₄ and LiBH₄ also give good yields of the CO complexes. For example, TpMo(NO)(PMe₃)(η²-CO₂) can be reduced to the CO complex in 75% yield by LiBH₄ at room temperature. Having synthesized eleven molybdenum η²-CO₂ complexes with a range of steric and electronic features, we are currently working to explore the reactivity of these complexes, especially the coupling of η²-CO₂ complexes with alkenes to give acrylates.

During the first fiscal year of this grant (January 2014-August 2015), seven undergraduates have been engaged in research in my laboratory. Two of these students were supported by the grant during the fiscal year, one during both summers. During this period, students have presented posters at the ACS National Meeting in Dallas (March, 2014) and Denver (March, 2015) as well at the Mid-Atlantic Seaboard Inorganic Symposium in Philadelphia (July, 2014), and the Philadelphia Inorganic Colloquium (February 2015). Three of these students have graduated with a B.S. in Chemistry and two of those are now attending Ph.D. programs in Chemistry at Duke University and University of Georgia.