A New Mode of Activation of CO2 by Metal–Ligand Cooperation with Reversible C-C and M-O Bond Formation at Ambient Temperature

R. Shanmugam29.06.2013

ReferenceMatthias Vogt, M oti Gargir, Mark A. Iron, Yael Diskin-Posner, Yehoshoa Ben- David, David Milstein, Chem. Eur. J. 2012, 18, 9194 – 9197

Efficient conversion of the abundant gaseous C1 feedstock to a liquid fuel is an attractive goal.

Metal–ligand cooperation (MLC), in which both the metal and the ligand undergo bond cleavage and formation in the process of substrate activation, plays an important role in metalloenzyme catalysed reactions.

In recent years, MLC has become a key direction for the development of efficient chemical transformations. Pincer-type complexes that exhibit MLC activation of CH, NH, HH, and OH bonds through aromatization/dearomatization.

Recently, they reported the mild hydrogenation of carbonates, carbamates, and methyl formate catalyzed by a dearomatized ruthenium-pincer complex, thus providing a mild approach for a two-step hydrogenation of CO2 to methanol .

Combination of this complex with another ruthenium catalyst has led to the cascade formation of methanol from CO 2.

Introduction

Of these processes, we are exploring the possibility of CO2 activation by MLC. Although CO2 binding to metal centers commonly involves π coordination to a C=O double bond (Scheme 1, A ), σ bonds to the carbon (Scheme 1, B ), or more seldom to the oxygen atom (Scheme 1, C ) of the CO2 molecule.

The hydrogenation of CO2 to formate salts catalysed by highly active PNP pincer IrIII complexes has been described by Nozak and, recently, by our group by using an iron-pincer complex under very mild conditions.

Herein , report a new mode of CO2 coordination promoted by an aromatization/dear omatization process, which proceeds through a reversible addition forming both M-O and a C-C bonds to the exo-cyclic methine carbon of the pincer “arm”(Schem e 1, D).

As previously reported, the pincer complex [Ru-(PNPtBu)(Cl)(CO)( H)] ( 1,PNPtBu = 2,6-bis-(di-tert-butyl-phosphinomethyl)pyridine) is readily deprotonated by one equivalent of potassium hexamethyldisilazide (KHMDS) giving the dearomatized species [Ru(PNPtBu*)(CO)( H)] (2 ; the dearomatized ligand is denoted by an asterisk). Remarkably, the latter complex rapidly reacts with CO2 (1 bar) at ambient temperature to form the [1,3]-addition product [Ru(PNP t Bu-COO )(CO)(H)] ( 3 , Scheme 2). The addition is reversible, and 2 can be regenerated from a solution of 3 inbenzene upon subjection to reduced pressure - 3 does not readily loose CO2 in the solid state. During the reaction of 2 with CO2 to form 3 , the 31P{1H} NMR resonances shifted to higher frequencies, indicating the re-aromatization of 3 .

The IR spectrum of 3 exhibits a characteristic band for a C=O carboxylate moiety at ʋ = 1628 cm-1, in addition to the absorption at ʋ =1912 cm -1 assigned to the CO ligand.

Single crystals of 3 suitable for X-ray diffraction analysis were obtained from a concentrated solution of 3 in toluene. The molecular structure of 3 (ORTEP diagram shown in Figure 1 ) reveals a distorted octahedral Ru II complex withthe CO ligand located trans to the pyridine moiety.

The new C-C bond between CO2 and the arm (C1-C25) is elongated and is approximately 0.06Å longer than the C6-C7 single bond. The Ru1-O2 bond is in the range of other Hru-OR bonds (H trans to O).

When a sample of 2 is reacted with 13 CO2 at ambient temperature in C6D6/CD2Cl2 (10 :1), the labeled complex [Ru(PNP t Bu13COO) (CO)(H)] ( 3 ’ ) is obtained.

This C-C bond-formation process is reversible : when an excess of nonlabeled CO2 is added to the solution of 3 ’ , the non labeled complex 3 is obtained. The reaction can be monitored by 13C{1H} NMR spectroscopy. Figure 2 displays the region of the 13C{1H} NMR spectrum associated with the benzylic carbon (δ = 62.5 ppm,C H13COO).

Initially, compound 3 ’ exhibits a doublet of dou-blets of doublets (ddd) with a large vicinal C-C couplingconstant (1JCC = 40.1 Hz) and two smaller C-P coupling constants ( 1JCP=5.3 and 3JCP=1.6Hz ; Figure 2, spectrum I).After adding one equivalent of CO2 , signals for both labeled( 3 ’ ) and nonlabeled ( 3 ) complexes appear (Figure 2 , spectrum II). When an excess of CO2 is added, only the nonlabeled product is observed (Figure 2, spectrum III). This demonstrates that [1, 3]-addition of CO2 is reversible, and rapid CO2 exchange occurs at ambient temperature.

Allowing for the necessary acquisition time of the 13C{1H} NMR spectra, the resonance for the non-labeled species 3 could be detected 15 min after the addition of one equivalent of CO2 to 3’ . The activation parameters for the reaction of 2 with CO2

were determined from variable temperature spin-saturation transfer (VT SST ; 320–350 K) NMR experiments. Intermolecular magnetization transfer was observed between 2 and 3 .

A solution of 2 and 3 (1 : 1) i n C6D6 under N2 was subjected to a selective saturation of the hydride resonance of 2 (VT, d = 25.85 ppm), and the magnetization transfer wasdetected and quantified in the corresponding hydridic resonance of 3 (dd, δ = 16.38 ppm). The magnitude of the transferred magnetization increases with rising temperature.

The rate constants were calculated according to the Forsn–Hoffman equation. The Eyring plot gives activation parameters of ∆G ǂ = 23.1± 0 .5 kcal mol-1 , ∆Sǂ = 8.1± 1.4 cal mol-1K, and ∆G ǂ

298K = 20.7 ±0.9 kcal mol-1 for the conversion of 32 +CO2 . The positive entropy of activation is as expected for a dissociative process and suggests no involvement of the solvent in the transition state (TS).

DFT calculation s provided additional insight into this reaction. Four coordination modes of CO2 to 2 were considered η1-coordination of oxygen to ruthenium (4a ), η2-coordination of the C=O bond ( 4b), [1,3]-addition forming C-C and Ru-O bonds ( 3 ), and the inverse [1,3]-addition forming C-O and Ru-C bonds (5 ; Figure 3). In agreement with the experimental findings, 3 was found to be by far the most stable complex (Table 1). The TS ( 2 – 3 ) was located for the reaction 2 + CO2 3 and resulted in a low barrier (Table 1).

TS ( 2 – 3 ) involves the concerted addition of CO2 to the unsaturated complex 2 through the exo -cyclic methine carbon C1 and the Ru center. The C-Cand Ru-O bonds are barely formed in this early TS. Table 2 summarizes the structural parameters of the calculated complexes 2 , 3 ,TS ( 2 – 3 ), and, for comparison , the experimental data from the X-ray structure of 3 .

The calculated structural parameters for 3 can be well compared with the experimental values.

The elongated C1arm-C25 CO 2 and Ru1-O2 bonds (1.602 and 2.275 , respectively) are also observed in the DFT structure of 3 .

Calculations by using solvents of different polarities (i.e. , THF and C6H6, Table 1) did not result in any significant differences in the relative energies, which corroborates a concerted mechanism and the absence of charge separation (e.g. , a zwitterionic Ru + -COO- intermediate).

This is strikingly different from the proposed mechanisms for the decarboxylation of 4-pyridine acetic acid or prominent examples of decarboxylase enzymes, in which cases it has been indicated that decarboxylation proceed s through a zwitterionic intermediate, resulting in a large increase of decarboxylation rate in non-polarmedia compared with aqueous solutions.

It should also be noted that the relative energy of TS (2 – 3 ) is lower than the energies of 4a , 4b, and 5 . Thus, in both solvents the [1,3]-addition proceeds without prior coordination of the CO2 to the metal center. The reaction barrier height from the DFT calculations for the reaction 3 2 + CO2 (∆G ǂ

298K = 15.4 kcal mol-1 in THF) is in reasonable agreement with the experimental activation parameters obtained from the VT SST NMR measurements (∆G ǂ

298K = 20.7 ±0.9 kcal mol-1).

A new mode of activation of CO2 was observed. It involves reversible C-C and Ru-O bond formation promoted by MLC, with aromatization/dearomatization of a pincer ligand. The cooperative [1,3]-addition of CO2 to the unsaturated PNP-pincer complex [Ru-(PNPtBu*)(H)(CO)]was investigated in a combined experimental and computational study.

The addition of CO2 through MLC prevails over π and σ coordination to the RuII center. Experiments by using isotopically labelled 13CO2 demonstrated the rapid exchange of CO2 in 3 even at ambient temperature. VT SST NMR measurements yielded the activation parameters (∆G ǂ

298K = 20.7 ±0.9 kcal mol-1) for this CO2 dissociation.

DFT calculations suggested a concerted CO2 addition by the Ru center and the exo -cyclic methine carbon of the dearomatized complex 2 involving an early TS.

Conclusion

An early report by Braunste in et al. describes a reversible bifunctional activation of CO2

in a PdII system, in which the CO2 carbon reacts with the electrophilic α-carbon in the [Ph2P-CH=C(O)OEt ]- ligand.Piers and co-workers observed spectroscopically C-C bond formation between CO2 and a b-diket iminato ligand in a scandi-um alkyl complex, [25]which eventually resulted in the dis-placem ent of the b -diketiminato liga nd. Very recently, Anni-bale and Song rep orted the revers ible binding of CO 2 intoaCH bon d of a diaz afluorenide RuIIcomplex witho ut ob-served involvemen t of the metal cente r.[26]

The two doublets observed at δ = 107.0 and 114.0 ppm ( 2JPP =251.0 Hz) indicate that the two phosphorus atoms are chemically inequivalent (AB spin system). The 13C{1H} NMR spectrum of 3 shows a doublet at δ = 172.6 ppm ( 2J CP =9.2 Hz), characteristic of a carboxylate carbon resonance ; this signal shares a strong cross peak in the two-dimensional 13C– 1H heteronuclear multiple-bond coherence(HMBC) NMR spectrum with the benzylic arm proton (1H NMR CHCOO d, δ = 4.76 ppm, 2J PH = 5.7 Hz). The 1H NMR spectrum of 3 has a typical hydridic resonance at δ = 16.38 ppm coupled to two inequivalent phosphorus nuclei (dd, 2JHP = 21.8 and 11 .6 Hz).