Part I: Experimental Data
2. Part II: Photochemical Reactivity of Nickel Pincer Complexes
2.3 Nickel Pincer Complex Mediated Reverse Water-Gas Shift Reactivity
2.3.1 Thermal and Photochemical CO 2 Activation by [NiH( tBu P=N=P)] (12)
The ability of the tBuP=N=P pincer scaffold to participate in electronic transitions is shown in Chapter 1.3.2 and motivates investigation of photochemical reactivity of tBuP=N=P nickel complexes. Hydrogen transfer from transition metal hydride complexes is pivotal as elementary reaction in many catalytic chemical and biochemical transformations. Therefore, photochemical excitation of NiII hydride 12 is an attractive approach to substrate activation.
Initially, the thermal reactivity of 12 with CO2 was investigated. When a THF solution of 12 is exposed to 1 atm of CO2, formation of a new species [Ni(O2CH)(tBuP=N=P)] (15) can be observed spectroscopically by NMR. 31P{1H} NMR spectra of complex 15 shows a resonance at δ = 56.1 ppm.
Scheme 34: Thermal reactivity of 12 with CO2.
Figure 55: 31P{1H} NMR spectra of the reaction of 12 with CO2 (top) at p(CO2) = 1 atm in THF, (middle) at p(CO2)
= 10 atm in THF and (bottom) at p(CO2) = 1 atm in MeCN. All spectra are recorded after 1 day of reaction time.
The conversion of 12 to 15 is slow in THF but can be accelerated by working at higher CO2 pressures as shown in Figure 55. Still, after two weeks under 10 atm CO2 pressure, residual 12 can be detected.
Continuous conversion of 12 to 15 was monitored NMR spectroscopically over this time, suggesting a slow
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reaction rather than an equilibrium between both species. While compound 15 was not isolated, it can be readily assigned as a formate complex by its indicative NMR spectroscopic features including a downfield shifted triplet resonance in the 1H NMR spectrum centered at δ = 7.25 ppm with a coupling constant of 4JHP
= 3.4 Hz (Figure 56). Furthermore, by 1H,13C HSQC NMR spectroscopy the cross peak to a triplet resonance in the 13C{1H} NMR spectrum at δ = 168.3 ppm with 3JCP = 1.1 Hz can be detected. Both signals are in good agreement with reported pincer NiII formate complexes.[223]
Figure 56: (a) 1H and (b) 1H,13C HSQC NMR spectra of in situ formed 15 by stirring 12 at p(CO2) = 10 atm in THF for 14 days (*denotes THF-d8).
Since both, the hydricity ΔGHT of the formate anion and a transition metal hydride complex can be strongly depending on the solvent, the equilibrium constant of CO2 insertion into a transition metal hydride complexes can be influenced by changing the solvent.[2] In addition, Hazari and Bernskoetter recently have shown that the rate of CO2 insertion into transition metal hydrides giving formate complexes is solvent dependent for both inner- and outersphere processes. Here, a faster reaction is observed upon moving to solvents with a higher acceptor number.[245] In agreement with this, the reaction of 12 with CO2 to 15 is accelerated upon changing the solvent from THF to MeCN as shown in Figure 55.
A change in selectivity can be observed if a solution of 12 in THF under CO2 (p(CO2) = 1 atm) atmosphere is photolyzed with a 150 W Xe arc lamp using a white glass filter cutting off light of λexc. < 305 nm.
Formation of a mixture of hydroxycarbonyl [Ni(CO2H)(tBuP=N=P)] (16) and hydrocarbonate [Ni(OCO2H)(tBuP=N=P)] (17) is observed (Scheme 35). The formation of 17 is attributed to follow-up photochemical decarbonylation and subsequent CO2 insertion of initial product 16 as will be discussed in detail in Chapter 2.4.1. In 1H NMR spectroscopy a set of resonances corresponding to a C2v symmetric
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pincer ligand can be observed for both compounds. A single low field shifted broad resonance is observed, indicating chemical exchange of the protic hydrogen atoms of the bicarbonate and carboxylate moiety.
Scheme 35: Photochemical reactivity of 12 with CO2.
The rate of the reaction drops significantly upon irradiation with λexc. > 420 nm, proving that UV light is crucial (Figure 57b). UV-vis measurement of 12 shows three transitions between = 200 nm and = 400 nm with the highest oszillator strength at = 233 nm ( = 4.1∙104M-1cm-1) (Figure 57c). The transitions with an absorption maxima at = 305 nm ( = 1.2∙104M-1cm-1) and = 334 nm ( = 2.1∙104M-1cm-1) are excited upon photolyzing with exc. > 305 nm, but the conversion of 12 by photolysis with exc > 420 nm indicates the red shifted transition which tails off to > 400 nm to be responsible for the observed reactivity.
The magnitude of extinction of all three transitions suggests spin and symmetry allowed charge transfer or LC (ligand centered) transitions, excluding metal centered excitation.[152]
Figure 57: 1H NMR spectra of a solution of 12 in THF-d8 after 4 h of photolysis at p(CO2) = 1 atm with (a) exc > 305 nm and (b) exc > 420 nm (*denotes THF-d8; †denotes TMS2O). (c) UV-vis spectra of hydride 12 (black)
and hydroxycarbonyl 16 (red) in THF.
Since the photochemical conversion of 12 to 16 and 17 occurs within hours, only traces of formate 15 are formed over the course of the reaction. Compound 16 can be isolated from the obtained mixture by precipitation with unpolar solvents like benzene and n-pentane. Preparative synthesis of 16 is performed in benzene which results in precipitation of the desired product from the reaction mixture in an isolated yield of 73%.
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Figure 58: 1H NMR spectrum of 16 in THF-d8.
NMR spectroscopic analysis of isolated 16 shows a singlet in the 31P{1H} NMR spectrum at δ = 66.2 ppm.
In the 1H NMR spectrum the carboxylate gives rise to a broad downfield shifted resonance at δ = 9.51 ppm.
A triplet at δ = 203.6 ppm can be observed in the 13C{1H} NMR spectrum. The coupling constant of
2JCP = 31.1 Hz is larger than in formate 15 as expected for a shorter distance between the coupling nuclei.
The broad resonance of the carboxylic acid proton in 16 indicates chemical exchange which is commonly observed in carboxylic acids due to oligomerization in solution. [290]
Figure 59: Solid state structure of 16 determined by X-ray diffraction. Thermal ellipsoids are drawn at the 50%
probability level. Selected hydrogen atoms are omitted for clarity. (a) Asymmetric unit of the solid-state structure of 16 and (b) dimeric structure of 16 by hydrogen bonding.
Analysis of 16 by X-ray diffraction reveals a dimeric structure in the solid state which is formed due to hydrogen bonding between two carboxylic acid functions with d(O2-H2A) = 0.80 Å and d(O1’-H2A) = 1.85 Å (Figure 59). Further bond metrics of the carboxylic acid moiety in 16 agree well with reports of related structures in the literature (d(C21-O1) = 1.274(3) Å; d(C21-O2) = 1.299(3) Å;
d(Ni1-C21) = 1.854(2) Å; α(O1-C21-O2) = 119.09(16)°).[223,225]
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Figure 60: Infrared spectra of 16 in nujol (red) and THF solution (black) (*denotes absorption of nujol).
Infrared spectroscopy of 16 in nujol shows three strong bands in the range of ῦ = 1500–1600 cm-1 which is the typical observed region for carbonyl stretching vibrations of PNP ligated metallacarboxylates and the C-C double bond stretch of the pincer backbone in 16. Further, the O-H stretch is visible as a broad absorption at ῦ = 2645 cm-1 which agrees with related PNP pincer Ni(II) hydroxycarbonyl compounds.[223,225] Upon measuring IR spectra of a THF solution of 16 the carbonyl region simplifies to two strong bands. Measuring the sample in nujol or THF solution is expected to have a strong effect on hydrogen bonding of the carboxylic acid moiety and accordingly a bigger influence on the carbonyl stretch is expected compared to the C-C double bond. The absorption at ῦ = 1620 cm-1 can be assigned as the carbonyl stretching vibration since it shifts strongly with respect to the spectrum in nujol. Accordingly, the absorption at ῦ = 1527.4 cm-1 resonates close to the absorption in nujol and can be identified as the C-C double bond stretching vibration.
Hydrocarbonate 17 cannot be isolated from the mixture of products obtained by photolysis of 12 under CO2
since it forms hydroxide [Ni(OH)(tBuP=N=P)] (18) by decarboxylation upon evaporation of the solvent.
Accordingly, an alternative synthetic route to 17 was established starting from 18 which can be synthesized from NiII bromide 3 using potassium hydroxide. One equivalent 15-crown-5 is necessary to provide solubility of the KOH in THF and is separated from the reaction mixture by filtration over Celite®.
Scheme 36: Synthesis of hydroxide 18 and reversible CO2 insertion to hydrocarbonate 17.
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Figure 61: (a) 1H and (b) 31P{1H} NMR spectra of 18 in THF-d8 under Ar atmosphere and (a) 1H and (b) 31P{1H}
NMR spectra of 17 in THF-d8 obtained by applying p(CO2) = 1 atm to a solution of 18 in THF-d8.
In 1H NMR spectroscopy, the hydroxide ligand in 18 gives rise to a triplet resonance at δ = -4.88 ppm with
3JHP = 5.6 Hz, similar to what is observed for Mindiola’s related pincer NiII system (Figure 61).[291] IR spectroscopy supports the presence of a hydroxide ligand by showing a vibration at ῦ = 3643.8 cm-1.
Figure 62: (a) Solid state structure of 18 determined by X-ray diffraction. Thermal ellipsoids are drawn at the 50%
probability level. Selected hydrogen atoms are omitted for clarity. H1A and H1A’ are populated with 50%
probability, each.
X-ray diffraction of 18 confirms the structural assignment of a hydroxide complex with a nickel oxygen bond length of d(Ni1-O1) = 1.845(3) Å, marking the lower limit in reported Ni hydroxide complexes.[291–
294]
Upon applying CO2 to a THF solution of 18 the intense orange solution immediate brightens and 31P{1H}
NMR spectroscopy shows clean conversion of 18 (δ = 50.3 ppm) to 17 (δ = 54.6 ppm) (Figure 61). The carbonate moiety is clearly identified by 31C{1H} NMR spectroscopy showing a singlet resonance at δ =
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158.9 ppm. Hydrocarbonate 17 shows a broad singlet resonance at δ = 9.28 ppm in the 1H NMR spectrum, indicating aggregation in solution as is observed for hydroxycarbonyl 16. X-ray crystallographic analysis shows a dimeric structure of 17 in the solid state similar to 16. While the hydrogen bond has similar bond metrics (d(O3-H3) = 0.98(4) Å, d(O2’-H3) = 1.61(4) Å) compared to 16, the geometry of the carbonate ligand in 17 results in parallel orientation of the planes defined by the {Ni(tBuP=N=P)} scaffold of the monomers in the dimeric structure, whereas the monomers in the dimeric structure of 16 are sharing the same plane.
Figure 63: Solid state structure of 17 determined by X-ray diffraction. Thermal ellipsoids are drawn at the 50%
probability level. Selected hydrogen atoms are omitted for clarity. (a) Asymmetric unit of the solid-state structure of 17 and (b) dimer structure of 17 by hydrogen bonding.
2.3.2 (De-)Protonation of [Ni(CO2H)(tBuP=N=P)] (16)
Hydroxycarbonyl complexes play an important role in transition metal mediated waster-gas shift (WGS) reactivity (Chapter 2.1.3). The WGS reaction is highly sensitive to pH and therefore the pKa of metallacarboxylic acids is of great interest. While pKa determination of these species is scarce, reports on [Co(CO2H)(H2O)(en)2] (pKaaq = 2.5) and [Ru(CO2H)(CO)(bpy)2] (pKaaq = 9.6) show decent proton donating ability.[295–297]
The reaction of 16 with 1 eq NaHMDS (HMDS: hexamethyldisilazane) in THF gives clean conversion to a new species. In the 31P{1H} NMR spectrum a singlet resonance at = 57.5 ppm is visible. 1H NMR spectroscopy only shows the spectroscopic signature of the pincer ligand, in agreement with deprotonation of the carboxylic acid to carboxylate Na[Ni(CO2)(tBuP=N=P)] (19Na). While carboxylate 19– can also be regarded as a Ni0 CO2 complex, detailed investigation by Lee and coworkers on related PNP nickel complexes suggests formulation as NiII carboxylates.[223,225] Titration of 16 with 1 eq of different bases results in a shift of the singlet resonance in 31P{1H} NMR spectroscopy towards 19Na as can be seen in Figure 64. Addition of DBU (1,8-diazabycyclo[5.4.0]undec-7-ene) results in formation of a fast exchange equilibrium between 16 and 19–, while the stronger base TBD gives a resonance close to what is observed
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for 19Na (Figure 64). The different in the chemical shift Δ observed in 31P{1H} NMR spectroscopy of 16 in the presence of NaHMDS and TBD (1,5,7-triazabicyclo[4.4.0]dec-5-ene) is assumed to result from differences in aggregation due to variation of the cation.
Figure 64: 31P{1H} NMR spectra of the reaction of 16 (top) without addition of base, (upper middle) with addition of 1 eq DBU, (lower middle) with addition of 1 eq TBD and (bottom) 19Na prepared by reaction of 16 with 1 eq
NaHMDS in THF-d8.
Determination of pKα(16) can be performed assuming coupled equilibria in THF, as described for pKα(10)BArF (Chapter 1.4.2). Monomer-dimer equilibria of 16 are probably also involved as discussed earlier. These equilibria influence the experimental determined Keq, are however neglected in the following discussion.
Scheme 37: Acid base equilibrium for deprotonation of 16.
The molar fractions χ of 16 and 19– are available from the observed chemical shift upon titration of 16 with 1 eq DBU (δ(16+DBU) = 65.7 ppm) and the chemical shift the conjugate acid and base measured at the identical concentration (δ(16) = 66.2 ppm, δ(19Na) = 57.4 ppm). Following eq. (34), eq. (46) results, giving χ(16) = 0.949 and χ(19–) = 0.051.
δ(16+DBU) = χ(16)δ(16) + χ(19–)δ(19–) (46)
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The equilibrium constant Kip for proton transfer giving the acid-base ion pair is available as Keq from the experiment following eq. (47).
Keq = c(16)2/c(19–)2 = χ(16)2/ χ(19–)2 = 357 (47) Deprotonation of 16 by DBU converts two neutral reactants to charged products. Accordingly, the influence of contact ion-pair formation is expected to be more severe as compared to the reaction of 10BArF with NEt3. The equilibrium constant Kd is calculated according to Fuoss equation eq. (32) using an ionic radius of r= 4.0 Å determined from the X-ray structure of 16 and an ionic radius of r = 2.5 Å for protonated DBU taken from literature.[298–300] Due to a net charge built-up, the influence of the dissociation of the contact ion-pair plays a much greater role in the determination of pKα(16)THF (ΔpKd = 4.90) than in the titration of 10BArF with NEt3 (ΔpKd = 0.43).
Comparison of pKα(16)THF with reported acidities of metallacarboxylic acids shows, that 16 shows remarkably low acidity and is only deprotonated by strong bases like TBD.
Deprotonation of hydroxycarbonyl complexes results in formation of carboxylates which may undergo CO2
liberation under formation of a reduced metal center. Accordingly, WGS catalysis is usually observed under basic conditions. Working under acidic conditions results in proton induced dehydration of the hydroxycarbonyl, a microscopic reverse of the Hieber base reaction, giving a metal carbonyl and reverse water-gas shift (rWGS) reactivity. Experimentally determined pKα(16)THF suggests that strongly basic conditions are necessary to drive WGS reactivity. Further, no signs for decarboxylation of 19– were observed rendering this {Ni(tBuP=N=P)} platform unsuitable for performing WGS catalysis. Interested in performing rWGS reactivity, examination of dehydration of 16 in acidic media was investigated.
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If 16 is reacted with strong acids like [H(OEt2)2]BArF or [H(OEt2)]BF4, clean conversion to NiII carbonyl [Ni(CO)(tBuP=N=P)]X (20X, X = BF4, BArF) is observed. The reaction of 16 with 1 eq [H(OEt2)2]BArF proceeds in 97% spectroscopic yield determined by integration against an internal standard in 1H NMR spectroscopy (Figure 65). Trap-to-trap condensation of the volatiles confirms formation of water in 77%
yield with respect to 20BArF.
Figure 65: 1H NMR spectra of the reaction of 16 with 1 eq of [H(OEt2)2]BArF in THF-d8, showing the recorded spectra (a) before and (b) after addition of [H(OEt2)2]BArF and (c) after trap-to-trap condensation of the volatiles
(*denotes THF-d8, †denotes TMS2O and ‡denotes Et2O).
NMR spectra of compound 20X show the signature of diamagnetic C2v symmetric {Ni(tBuP=N=P)}
complexes in 1H NMR spectroscopy with a downfield shifted resonance in 31P{1H} NMR spectroscopy at
= 114.7 ppm. The carbonyl ligand can be clearly identified by a triplet resonance at = 191.3 ppm (2JCP = 22.5 ppm) in the 31C{1H} NMR spectrum of 20BF4(Figure 71a). Accordingly, a strong signal at ῦ = 2062.4 cm-1 for the CO stretching vibration can be observed by ATR-IR spectroscopy.
Interested in the pKa constrains of dehydration, 16 was titrated with different protonated nitrogen bases as can be seen in Figure 66. While [H(NEt3)]BArF and [H(piperidine)]BArF yield full conversion to 20BArF, addition of 1 eq [H(TMG)]BArF (TMG = 1,1,3,3-tetramethylguanidine) results in formation of an equilibrium between 16 and 20BArF. Determination of the pKα for this reaction is complicated by coupling
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of the acid base equilibrium to the dehydration reaction. Therefore, the titration experiments serve as determination of an upper limit of pKα(H(TMG)+)THF = 15.3 for the conversion of 16 to 20X.[300]
Scheme 38: Acid induced dehydration of hydroxycarbonyl 16.
Figure 66: 31P{1H} NMR spectra of 16 in THF in the presence of (top) 1 eq [H(NEt3)]BArF, (mid) 1 eq [H(piperidine)]BArF, and (bottom) 1 eq [H(TMG)]BArF.
Investigation of the deprotonation of hydroxycarbonyl 16 to 19M and the acid induced dehydration of 16 to 20X shows a difference in pKαTHF of approximately 9 for the corresponding reactions. While 16 is a stable compound and not signs for dehydration to 19M and 20X are observed experimentally, formation of 19M and 20X is feasible in unpolar solvents like THF and Et2O considering a significant gain in driving force by ion pair formation.
Scheme 39: Formation of hydrocarbonate 17 from carbonyl 20X in the presence of lewic acid and CO2.
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The ability of 16 to efficiently liberate water upon addition of weak Brønsted acids renders this an attractive platform to perform rWGS reactivity.Aside from Brønsted acids, Lewis acids can be used to convert hydroxycarbonyl 16 to NiII carbonyl 20X by hydroxide abstraction. Interestingly, dissolving a mixture of 16 and NaBArF in THF results in no apparent reaction overnight at room temperature. If the argon atmosphere is exchanged for carbon dioxide, within 30 minutes formation of 20X is observable by NMR spectroscopy, indicating formation of hydrocarbonate as anion rather than hydroxide. Accordingly, hydrocarbonate 17 can be observed in barely detectable amounts in the mixture, indicatingg substitution of the carbonyl ligand in 20X by bicarbonate (Figure 67).
Figure 67: 31P{1H} NMR spectra of photolysis (λ > 305 nm) of a solution of 16 in THF-d8 in the presence of 1 eq NaBArF (top) under argon atmosphere and (bottom) under carbon dioxide atmosphere.
2.3.3 Conversion of [Ni(CO)(tBuP=N=P)]X (20X) to [NiH(tBuP=N=P)] (12) by addition of Li[HBEt3]