Mechanism of Deoxygenation and Cracking of Fatty Acids by Gas-Phase Cationic Complexes of Ni, Pd, and Pt

reactions

Article Mechanism of Deoxygenation and Cracking of Fatty Acids by Gas-Phase Cationic Complexes of Ni, Pd, and Pt

Kevin Parker 1 , Victoria Pho 1, Richard A. J. O’Hair 2 and Victor Ryzhov 1,*

1 Department of Chemistry and Biochemistry, Northern Illinois University, DeKalb, IL 60115, USA; [email protected] (K.P.); [email protected] (V.P.) 2 School of Chemistry, Bio21 Institute of Molecular Science and Biotechnology, The University of Melbourne, Melbourne, VIC 3010, Australia; [email protected] * Correspondence: [email protected]; Tel.: +1-(815)-753-6871

Abstract: Deoxygenation and subsequent cracking of fatty acids are key steps in production of biodiesel fuels from renewable plant sources. Despite the fact that multiple catalysts, including those containing group 10 metals (Ni, Pd, and Pt), are employed for these purposes, little is known about the mechanisms by which they operate. In this work, we utilized tandem mass spectrometry experiments (MSn) to show that multiple types of fatty acids (saturated, mono-, and poly-unsaturated) can be catalytically deoxygenated and converted to smaller hydrocarbons using the ternary metal + complexes [(phen)M(O2CR)] ], where phen = 1,10-phenanthroline and M = Ni, Pd, and Pt. The mechanistic description of deoxygenation/cracking processes builds on our recent works describing simple model systems for deoxygenation and cracking, where the latter comes from the ability of group 10 metal ions to undergo chain-walking with very low activation barriers. This article extends our previous work to a number of fatty acids commonly found in renewable plant sources. We found that in many unsaturated acids cracking can occur prior to deoxygenation and show that mechanisms Citation: Parker, K.; Pho, V.; O’Hair, involving group 10 metals differ from long-known charge-remote fragmentation reactions. R.A.J.; Ryzhov, V. Mechanism of Deoxygenation and Cracking of Fatty Keywords: biomass; gas phase; mass spectrometry; cracking; hydrocarbons Acids by Gas-Phase Cationic Complexes of Ni, Pd, and Pt. Reactions 2021, 2, 102–114. https:// doi.org/10.3390/reactions2020009 1. Introduction Interest in biomass-derived hydrocarbons has grown due to socio-economic and Academic Editor: Dmitry Yu. Murzin environmental pressures to transition from fossil fuel to renewable resources [1–3]. Pro- duction of parafﬁnic fuel and biodiesel have drawbacks, with poor storage stability of the Received: 20 April 2021 fuels and unfavorable cold-ﬂow properties that can be overcome with the new-generation Accepted: 11 May 2021 Published: 15 May 2021 methods of biomass conversion [1]. One of the main components of biomass suitable for biodiesel production is triglycerides containing fatty acids of varying lengths and degrees

Publisher’s Note: MDPI stays neutral of unsaturation [4]. While thermal cracking of fatty acids was shown to produce hydro- with regard to jurisdictional claims in carbons as early as the beginning of the 20th century by Sabatier [2], oxygen-containing published maps and institutional afﬁl- impurities pose a problem of compatibility with modern diesel engines [3]. Through de- iations. oxygenation, access to the pure hydrocarbon chain is granted and, by cracking chemistry, diesel-like hydrocarbons are produced [5]. These “drop-in” fuels can be used with little to no modiﬁcation to combustion engines on the road today. Characterization of the hydrocarbons released by deoxygenation and cracking of fatty acids has been studied extensively in condensed-phase experiments [5–9]. Metal Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. catalysts greatly reduce the energy (or temperature) requirements needed to afford the This article is an open access article desired hydrocarbons, with Ni, Pd, and Pt showing the greatest potential [5]. The methods distributed under the terms and of deoxygenation have been studied in multiple experiments with saturated fatty acids conditions of the Creative Commons such as stearic acid [5,6,10]. In these studies, three deoxygenation pathways have been Attribution (CC BY) license (https:// observed: (i) decarbonylation paired with dehydration, (ii) decarboxylation, or (iii) a creativecommons.org/licenses/by/ combination of both. While saturated fatty acids only exhibit cracking of the hydrocarbon 4.0/). chain after deoxygenation [10], it has been shown that, for unsaturated hydrocarbons like

Reactions 2021, 2, 102–114. https://doi.org/10.3390/reactions2020009 https://www.mdpi.com/journal/reactions Reactions 2021,, 2,, FORFOR PEERPEER REVIEWREVIEW 2

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chain after deoxygenation [10], it has been shown that, for unsaturated hydrocarbons like oleicoleic acid,acid, deoxygenation deoxygenation is is not not required required for for cracking cracking [11–13]. [11–13 Variou]. Various-lengths-length alkanes, alkanes, ter- terminalminal alkenes, alkenes, and and internal internal alkenes alkenes were were alll observedobserved all observed throughthrough through Gas-chromatographyGas-chromatography Gas-chromatography (GC)(GC) (GC)after aftervarying varying temperature temperature and time and timein a heated in a heated reaction reaction chamber chamber [4]. [4]. Gas-phaseGas-phase studies, studies, often often carried carried out out via via mass mass spectrometry spectrometry (MS) (MS) experiments, experiments, provide pro- avide different a different angle forangle looking for looking at deoxygenation/cracking at deoxygenation/cracking chemistry. chemistry. For example, For example, charge- remotecharge-remote fragmentation fragmentation of unsaturated of unsaturated fatty acids fatty has acids been shownhas been to produceshown to hydrocarbons produce hy- viadrocarbons fatty acid via chain fatty cracking acid chain without cracking deoxygenation without deoxygenation [14,15]. These experiments [14,15]. These have experi- long beenments used have to long determine been used the number to determine and position the number of double and bondsposition in theof double fatty acid bonds chain in [the5]. Thefatty main acid advantage chain [5]. ofThe studying main advantage these reactions of studying in the gas these phase reactions is that, ininconjunction the gas phase with is theoreticalthat,that, inin conjunctionconjunction calculations, withwith they theoreticaltheoretical can be used calculatiocalculatio to determinens, they detailed can be used mechanisms to determine and pathway detailed energetics,mechanisms thus and allowing pathway for energetics, intelligent thus catalyst allowi designng for intelligent [16,17]. Given catalyst the design prevalence [16,17]. of unsaturatedGiven the prevalence fatty acids of found unsaturated in biomass, fatty it isacids important found toin understand biomass, it theis important mechanism to and un- chemistryderstand the involved mechanism in production and chemistry of hydrocarbons involved fromin production their cracking of hydrocarbons reactions. Recent from worktheirtheir fromcrackingcracking our reactions. groupreactions. has RecentRecent shown workwork such details fromfrom our for group the catalytic has shown deoxygenation such details and for cracking the cat- ofalytic stearic deoxygenation acid by Ni and and Pd cracking complexes of stearic studied acid by MS.by Ni The and deoxygenation Pd complexes mechanism studied by was MS. calculatedThe deoxygenation for propionic mechanism acid used was as calculated the simple for gas-phase propionic model acid for used larger as the fatty simple acids, gas- as shownphase model in Scheme for larger1[10]. fatty acids, as shown in Scheme 1 [10].

SchemeScheme 1.1. CatalyticCatalytic deoxygenation deoxygenation and and dehydrogenation dehydrogenation of propionic of propionic acid by acid ternary by ternary 1,10-phe- 1,10- phenanthrolinenanthroline and and metal metal (Ni (Ni and and Pd) Pd) complexes. complexes.

OnceOnce deoxygenationdeoxygenation has has occurred, occurred, the the resulting resulting organometallic organometallic ion ion can can undergo undergo facile fac- “chainileile “chain“chain walking” walking”walking” (when (when(when the the metalthe metalmetal is Ni isis or NiNi Pd). oror Pd).Pd). The TheThe isomerization isomerizationisomerization barriers barriersbarriers were werewere shown shownshown to betoto be lowbe lowlow enough enoughenough to toto produce produceproduce a mixtureaa mixturemixture of ofof organometallic organometallicorganometallic isomers isomersisomers prior priorprior to toto fragmentation fragmentationfragmentation (Scheme(Scheme2 )[2) 18[18].].

SchemeScheme 2.2. IsomerizationIsomerization ofof of alkanealkane alkane fromfrom from alphaalpha alpha toto to betabeta beta andand and betabeta beta toto togammagamma gamma isis reversiblereversible is reversible andand and lowlow low enough in energy to be observed in simple gas-phase experiments. enoughenough inin energyenergy toto bebe observedobserved inin simplesimplegas-phase gas-phaseexperiments. experiments.

TheThe subsequentsubsequent “cracking”“cracking” allowedallowed forfor anyany lengthlength ofof neutralneutral alkenealkene toto bebe extruded extruded fromfrom thethe hydrocarbonhydrocarbon chain chain [ [19–24].19–24]. ThisThis mechanismmechanism was was modeled modeled with with a a hexyl hexyl chain chain in in ourour recentrecent workwork andand cancan bebe usedused toto describedescribe thethethecracking crackingcrackingchemistry chemistrychemistry observed observedobserved in inin the thethefatty fattyfatty acidacid chemistrychemistry describeddescribed inin thisthis workwork (Scheme(Scheme2 )[2) 18[18].]. The The compositions compositions of of unsaturated unsaturated fattyfatty acidsacids in in most most vegetable vegetable oils oils and and fats fats are are over over 90% 90% oleic oleic acid acid(18:1), (18:1), linoleic linoleic acid (18:2), acid (18:2), and linolenic acid (18:3) [6]. The main goal of this study was to see if this approach Reactions 2021, 2 104

could be applied to multiple saturated and unsaturated fatty acids for the production of hydrocarbons via mass spectrometry. We also extended our previous studies to include and compare Ni, Pd, and Pt catalysts.

2. Materials and Methods 2.1. Materials Palladium acetate, nickel acetate, acetic acid, propionic acid, butyric acid, and stearic acid were all purchased from Sigma-Aldrich (St. Louis, MO, USA) and used as received. Decanoic acid, linoleic acid, linolenic acid, oleic acid, and elaidic acid were all purchased from Cayman Chemicals (Ann Arbor, MI, USA) and used without any further puriﬁcation. K2[PtCl4] and Ag(OAc) were purchased from Sigma-Aldrich. DMSO, THF, dichloromethane, ethanol, and diethyl ether were all used in the synthesis of [(phen)Pt (OAc)2] and were purchased from Sigma-Aldrich [7]. Solutions were prepared by mixing 100 µL (1 mg/mL) of ligand, metal acetate, and fatty acid dissolved in MeOH and diluting them with 700 µL of methanol to 0.1 mg/mL. Samples were allowed to react for ﬁve minutes to allow for the complexation of the metal, ligand, and fatty acid. Solutions were then introduced into the mass spectrometer via electrospray ionization (ESI).

2.2. Mass Spectrometry The primary instrument for the gas-phase studies was a ThermoFisher LTQ (Ther- moFisher Scientific, San Jose, CA, USA) which was modified to allow for ion–molecule reactions; the modification is described in other sources [8]. Samples were introduced into the ion trap through ESI. Source parameters were set to 5 kV needle voltage, 275 ◦C inlet temperature, and 3 a.u. for nitrogen gas flow rate. The precursor carboxylate complexes were subjected to activation via collisions with the helium bath gas as a 15% NCE (normalized collision energy) for the decarboxylation to occur. An isolation window of 1 m/z was used when isolating the resulting organometallic ion. Further collision-induced dissociation (CID) MS4 experiments were done between 20–22% NCE, ensuring maximum product intensity while still preserving some of the parent ion. High resolution mass spectrometry was carried out using a Bruker Maxis II Plus (Bruker Daltonics, Billerica, MA, USA). Samples were prepared in the same manner as above and introduced into the gas phase through ESI. The ESI source was set to a nebulizing gas pressure of 40 psi and a voltage of 4.5 kV. Ions were selected with a quadrupole mass filter and, in the collision cell, CID was used to probe the neutral losses from deoxygenation and cracking. The collision cell was set to 2.5 kV.

3. Results 3.1. Metal (Ni, Pd, Pt) Comparison in Fragmentation of Oleic Acid Complexes Previous studies and work recently published by our group have shown that the forma- + tion of fatty acid metal complexes ([(phen)M(O2CR)] ], where phen = 1,10-phenanthroline and M = Ni, Pd, and Pt) can be easily achieved via ESI [8]. Investigation of the different mechanisms and pathways in the deoxygenation and cracking of fatty acids can be done through CID. For Ni and Pd complexes, stearic acid was shown to deoxygenate through neutral loss of CO2 and H2 with subsequent cracking of the hydrocarbon chain resulting in a catalytic cycle for the formation of various hydrocarbons varying in length from C2–C15 [8]. Here, we expanded the metal comparison to include Pt and used oleic acid (mono-unsaturated fatty acid) to see if similar chemistry operates in the presence of a double bond in the chain. Saturated hydrocarbons like stearic acid undergo decarboxylation with dehydrogenation (Equation (1)); however, unsaturated fatty acid deoxygenation occurs in relatively high abundance without dehydrogenation (purple peaks in Figure1). Figure1 shows the + comparison of MS/MS spectra for ternary metal complexes ([(phen)M(OOCC16H31)] ) with oleic acid (M = Ni, Pd, and Pt). The deoxygenation of oleic acid, as well as other Reactions 2021, 2, FOR PEER REVIEW 4

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([(phen)M(OOCC16H31)]+) with oleic acid (M = Ni, Pd, and Pt). The deoxygenation of oleic acid,unsaturated as well as fattyother acids, unsaturated like linoleic fatty (18-2),acids, like linolenic linoleic (18-3), (18-2), and linolenic undecanoic (18-3), (10-1) and un- acids decanoic(Figure 2(10-1)/Figure acids S1), (Figure occurs 2/Figure via decarboxylation S1), occurs via (Equation decarboxylation (2)). (Equation (2)). + + [(phen)M(OOCC16H33)]+  [(phen)M(C16H33-2n)] + CO2 + nH2 (1) [(phen)M(OOCC16H33)] → [(phen)M(C16H33-2n)] + CO2 + nH2 (1) M = Ni (Figure 2A), Pd (Figure S1A), Pt (Figure S1F) M = Ni (Figure 2A), Pd (Figure S1A), Pt (Figure S1F)(2) (2) [(phen)M(OOCC16Hy)]++  [(phen)M(C16Hy)]+ ++ CO2 [(phen)M(OOCC16Hy)] → [(phen)M(C16Hy)] + CO2 MM = Ni,= Ni, Pd, Pd, Pt Pt y =y 31= 31 (oleic), (oleic), 29 29 (linoleic), (linoleic), 27 27 (linolenic) (linolenic)

+ + FigureFigure 1. 1.CIDCID of of[(phen)M(OOCC [(phen)M(OOCC17H1733H)]33 for)] Mfor = MNi = (A Ni), (PdA), (B Pd), (andB), andPt (C Pt). (RedC). Redpeaks peaks correspond correspond to dehydrogenation to dehydrogenation of theof parent the parent peak peakimmediately immediately to the toright. the Blue right. peaks Blue are peaks the areresult the of result cracking of cracking of the hydrocarbon of the hydrocarbon chain without chain deoxy- without genationdeoxygenation (number (number of neutral of neutral carbons carbons lost in lost neutral in neutral molecu molecule).le). Purple Purple represents represents deoxygenation, deoxygenation, while while green green indicates indicates crackingcracking of ofthe the hydrocarbon hydrocarbon chain chain after after deoxygenati deoxygenation.on. * indicates * indicates the the mass-s mass-selectedelected precursor precursor ion. ion.

WhileWhile dehydrogenation dehydrogenation does does occur occur (red (red peaks peaks in inFigure Figure 1)1 )in in high high abundance, abundance, the the occurrenceoccurrence of ofthe the loss loss of 44 of m/z 44 m/z (-CO(-CO2) is2 a) significant is a significant difference difference in the in pathway the pathway of deoxy- of de- genationoxygenation for coordinated for coordinated unsaturated unsaturated fatty ac fattyids.acids. After Afterdeoxygenation, deoxygenation, cracking cracking of theof hydrocarbonthe hydrocarbon chain chaincan be can observed, be observed, similar similar to that to of that stearic of stearic acid [8] acid (green [8] (greenpeaks peaksin Fig- in ureFigure 1). In1 ).addition In addition to the to differing the differing deoxygena deoxygenationtion observed, observed, a significant a significant variance variance of un- of saturatedunsaturated vs. saturated vs. saturated fatty acids fatty acidsis the appear is the appearanceance of cracking of cracking peaks that peaks come that from come non- from deoxygenatednon-deoxygenated species species (blue peaks (blue in peaks Figure in Figure1). Confirmed1). Confirmed via high via resolution high resolution mass spec- mass trometry,spectrometry, the hydrocarbons the hydrocarbons neutrally neutrally lost from lost the from fatty the acid fatty before acid beforedeoxygenation deoxygenation vary Reactions 2021, 2 106

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vary in length, and this loss is observed mostly for Pd and, to a much smaller extent, for Ni and Pt.

+ FigureFigure 2. CID2. CID of [(phen)Ni(OOCR)]of [(phen)Ni(OOCR)]for+ for R =R C=16 CH1633H33(A (A),), C C1616HH2929 ((BB),), andand CC1616H2727 ((CC)) (A (A = = stearate, stearate, B B = = linoleate, linoleate, C C = = lino- linole- nate).lenate). Red Red peaks peaks correspond correspond to dehydrogenationto dehydrogenation of theof the parent parent peak peak immediately immediately to theto the right. right. Blue Blue peaks peaks are are the the result result of crackingof cracking of the of the hydrocarbon hydrocarbon chain chain without without deoxygenation deoxygenation (number (number of neutralof neutral carbons carbons lost lost in in neutral neutral molecule). molecule). Purple Pur- representsple represents deoxygenation, deoxygenation, while while green green indicates indicates cracking cracking of the of hydrocarbonthe hydrocarbon chain chain after after deoxygenation. deoxygenation. * indicates * indicates the mass-selected precursor ion. the mass-selected precursor ion.

OneSimilar of thecomparisons major differences can be made between for Pd the and metals Pt complexes for CID ofof oleateunsaturated species fatty occurs acids. in theLike Pd with and oleic Ni spectrum acid (Figure (Figure 1B),1 B)the around CID sp theectra neutral of other loss unsaturated of C 13H26. Thisfatty phenomenonacid Pd com- canplexes be yield observed a lone with peak higher around intensity the loss in of the C13 PdH26 spectrum for both linoleic compared acid with (Figure Ni, S1C) while and Pt showslinolenic more aciduniformity (Figure 1C). across The increased the spectrum level ofof neutraldehydrogenation hydrocarbon for Pt losses. with oleic Another acid obvious(Figure 1C) difference is mirrored between in the the spectrum metals of is Pt the with high linoleic level ofacid dehydrogenation (Figure S1F). High seen resolu- with thetion Pt mass complex. spectrometry This increased identified dehydrogenation the neutral loss of of platinum C13H26 within complexes an 8 ppm is in error agreement (Table withS1). investigations that our group is currently working on involving DFT calculations demonstratingWe also compared the low activationfragmentation energy of cis required versus fortrans Pt isomers dehydrogenation of unsaturated compared fatty ac- to Niids. and Fragmentation Pd. (Work describing of oleic acid the (cis) difference for Ni, betweenPd, and Pt (Figure and Pd 1) dehydrogenation was compared with is being the preparedtrans isomer for submission.of the fatty acid, DFT calculationselaidic acid. have(Figure shown S2). No that discernable dehydrogenation differences by ternary could [(phen)M(cyclohexyl)]+be seen in fragmentation complexes of cis and where trans Misomers. = Pd are This ~38 shows kJ/mol the less effectiveness favorable than of deoxy- when Mgenation = Pt.) A and similar cracking stabilization towards has a wide been range described of fatty in theacids dehydrogenation found in nature of or methane converted by + bareduring metal thermal ions. isomerization The difference [10]. in bond strength, 155 kJ/mol, between Ni-CH2 (310 kcal −1 + −1 mol Metal) and ion Pt-CH complexes2 (464 of kcal fatty mol acids) madehave long Pt dehydrogenate been used for CHdetermination4 much more of the easily, lo- whichcation wasof double explained bonds by a strongin unsaturated relativistic acids effect [11]. in the Recent Pt complex work [9 ].by Randolph and McLuckey [5,12] showed that ternary Mg complexes of unsaturated fatty acids formed by ion–ion reactions demonstrate a double bond location with depressions in the cracking region surrounding the double bond. We formed similar Mg complexes in solution, and Reactions 2021, 2 107

3.2. CID for Various Ternary Nickel Complexes with Fatty Acids In addition to the oleic acid fragmentation shown in Figure1, we investigated several other saturated (stearic acid) and unsaturated (linoleic and linolenic) fatty acids. Com- parison of deoxygenation and cracking for these fatty acids in Ni complexes is shown in Figure2 (and Figure1A). In the unsaturated fatty acid complex fragmentation spectrum (Figure2A ), cracking after deoxygenation is observed (green peaks). While deoxygenation in the saturated fatty acids like stearic acid usually corresponds to a loss of 46 m/z (-CO2, -H2), the unsaturated fatty acids all have deoxygenation without dehydrogenation (purple peak, Figures1A and2B,C). The largest difference between the fatty acids is when the hydrocarbon chain is unsaturated. Stearic acid only shows cracking chemistry after deoxygenation (Figure1A). Blue peaks, indicating cracking without deoxygenation, only appear in the unsaturated fatty acid spectra. The CID of ternary Ni complexes with unsaturated fatty acids yields similar spectra for oleic acid (Figure1A), linoleic acid (Figure2B), and linolenic acid (Figure2C), with a similar level of cracking with and without deoxygenation across all fatty acids. Similar comparisons can be made for Pd and Pt complexes of unsaturated fatty acids. Like with oleic acid (Figure1B), the CID spectra of other unsaturated fatty acid Pd complexes yield a lone peak around the loss of C13H26 for both linoleic acid (Figure S1C) and linolenic acid (Figure1C). The increased level of dehydrogenation for Pt with oleic acid (Figure1C) is mirrored in the spectrum of Pt with linoleic acid (Figure S1F). High resolution mass spectrometry identiﬁed the neutral loss of C13H26 within an 8 ppm error (Table S1). We also compared fragmentation of cis versus trans isomers of unsaturated fatty acids. Fragmentation of oleic acid (cis) for Ni, Pd, and Pt (Figure1) was compared with the trans isomer of the fatty acid, elaidic acid. (Figure S2). No discernable differences could be seen in fragmentation of cis and trans isomers. This shows the effectiveness of deoxygenation and cracking towards a wide range of fatty acids found in nature or converted during thermal isomerization [10]. Metal ion complexes of fatty acids have long been used for determination of the location of double bonds in unsaturated acids [11]. Recent work by Randolph and McLuckey [5,12] showed that ternary Mg complexes of unsaturated fatty acids formed by ion–ion reactions demonstrate a double bond location with depressions in the cracking region surrounding the double bond. We formed similar Mg complexes in solution, and they showed identical results (Figure S3). In contrast to Mg, none of group 10 metals (Ni, Pd, and Pt) show similar depressions in the intensity of the cracking peaks in their CID spectra (Figure1, Figure2B,C, and Figure S1). The differences in mechanisms between fragmentation of Mg and group 10 metal complexes are discussed in the Section4 below. Multi-stage tandem mass spectrometry (MSn) experiments provide continuous collisional “heating” of the activated ions, leading to an increased extent of fragmentation. Previously, we showed that saturated fatty acids complexes of Ni and Pd subjected to MSn eventually lead to the formation of the hydride and methyl species [13]. For example, MS3 on the deoxygenated ion in the stearate complex (purple peak in Figure2A) leads to the enhanced production of “cracking” ions (green peaks) (Figure S4). Further MS4 on one of the green peaks leads to lower m/z green peaks, and so on, until the formation of + + [(phen)M(H)] or [(phen)M(CH3)] , which are obviously incapable of further cracking. Similarly, unsaturated fatty acids complexes of Ni, Pd, and Pt display sequential fragmentation leading to the formation of the hydride and methyl species, as shown in Figure S3. The observed cracking without deoxygenation (red peaks in Figure2B, for example) does not change the result—these peaks eventually lose carbon dioxide (i.e., deoxygenate) and fragment via cracking (Figure S5B). Reactions 2021, 2, FOR PEER REVIEW 7

they showed identical results (Figure S3). In contrast to Mg, none of group 10 metals (Ni, Pd, and Pt) show similar depressions in the intensity of the cracking peaks in their CID spectra (Figure 1, Figure 2B,C, and Figure S1). The differences in mechanisms between fragmentation of Mg and group 10 metal complexes are discussed in the Discussion section below. Multi-stage tandem mass spectrometry (MSn) experiments provide continuous collisional “heating” of the activated ions, leading to an increased extent of fragmentation. Previously, we showed that saturated fatty acids complexes of Ni and Pd subjected to MSn eventually lead to the formation of the hydride and methyl species [13]. For example, MS3 on the deoxygenated ion in the stearate complex (purple peak in Figure 2A) leads to the enhanced production of “cracking” ions (green peaks) (Figure S4). Further MS4 on one of the green peaks leads to lower m/z green peaks, and so on, until the formation of [(phen)M(H)]+ or [(phen)M(CH3)]+, which are obviously incapable of further cracking. Similarly, unsaturated fatty acids complexes of Ni, Pd, and Pt display sequential frag- Reactions 2021, 2 mentation leading to the formation of the hydride and methyl species, as shown in Figure 108 S3. The observed cracking without deoxygenation (red peaks in Figure 2B, for example) does not change the result—these peaks eventually lose carbon dioxide (i.e., deoxygenate) and fragment via cracking (Figure S5B). As previously described in our recent work, [(phen)M(H)]+, [(phen)M(CH )]+, and + 3 + 3 As previously described in+ our recent work, [(phen)M(H)] , [(phen)M(CH )] , and [(phen)M(C2H3)] are all capable of reacting with volatile carboxylic acids (acetic, propionic, [(phen)M(C2H3)]+ are all capable of reacting with volatile carboxylic acids (acetic, propi- and butyric acids) and regenerating a ternary carboxylate ion (Equation (3)) [13]. onic, and butyric acids) and regenerating a ternary carboxylate ion (Equation (3)) [13]. + + [(phen)M(X)][(phen)M(X)]+ + HOOCR  +[(phen)M(OOCR)] HOOCR → [(phen)M(OOCR)]+ + HX + HX (3) (3)

(X = H, CH3, C(X2H =3 H,) CH3,C2H3)

(R = CH3, C2H(R5, =C CH3H7)3 ,C2H5,C3H7) This chemistryThis allowed chemistry us to allowedclose the uscatalytic to close cycle the catalyticfor the deoxygenation cycle for the deoxygenationand and cracking of propioniccracking acid of propionic(Scheme 1). acid (Scheme1).

4. Discussion 4. Discussion 4.1. Mechanism 4.1.for Decarboxylation Mechanism for Decarboxylation + The mechanismsThe for mechanisms decarboxylation for decarboxylation of ternary cationic of ternary complexes cationic [LM(OOCR)] complexes+ [LM(OOCR)] have been studiedhave in been detail studied by our in detailgroup by [13–17] our group and [others13–17] [19–21] and others and [ 19are–21 well] and estab- are well established. lished. The firstThe step ﬁrst always step alwaysinvolves involves the dissociation the dissociation of one of onethe M-O of the bonds M-O bondsof the ofsym- the symmetrically 1 1a metrically boundbound carboxylate carboxylate 1 formingforming the intermediate the intermediate 1a (Scheme(Scheme 3): 3):

Scheme 3. Decarboxylation mechanism. Scheme 3. Decarboxylation mechanism. The step 1  1a is highly endothermic. For example, for dissociation in [(phen)Zn(OOCH)]+The it stephas 1 been→ 1a calculatedis highly endothermic. to be 128 ForkJ/mol example, [16], forwhile dissociation in in [(phen) + + [(phen)Pd(OOCCZn(OOCH)]2H5)]+, it is it174 has kJ/mol been calculated[13]. The species to be 128 1a kJ/mol has a coordinatively [16], while in [(phen)Pd(OOCC unsatu- 2H5)] , rated metal ionit that is 174 can kJ/mol get stabilized [13]. The by species forming1a has a metal–carbon a coordinatively bond unsaturated to the α-carbon, metalion that can get eventually expellingstabilized carbon by dioxide forming (purple a metal–carbon peaks) and bond forming to the theα-carbon, organometallic, eventually spe- expelling carbon + cies 2, [(L)M(R)]dioxide+, which (purple is often peaks) stabilized and via forming β-H agostic the organometallic, interaction with species the metal2, [(L)M(R)] ion, , which is especially whenoften the metal stabilized is Ni, via Pd,β -Hor agosticPt [22]. interaction with the metal ion, especially when the metal is Ni, Pd, or Pt [22].

4.2. Cracking and Dehydrogenation of the Deoxygenated Hydrocarbon Chain + The organometallic ternary metal complex [(phen)M(CH2CH2R)] can undergo further fragmentation when additional energy is supplied through MSn experiments (Figure S5C). The initial structure of species 2 immediately after the loss of carbon dioxide has a terminal carbon bound to the metal center (Scheme3). This species 2 can either de- + compose via the loss of a 1-alkene RCH=CH2, leaving the hydride species 3, [(phen)M(H)] , or via the loss of hydrogen, forming an unsaturated organometallic species (red peaks). Both of these channels proceed via a common intermediate 2a (Scheme4): Reactions 2021, 2, FOR PEER REVIEW 9

Reactions 2021, 2, FOR PEER REVIEW 9

4.2. Cracking and Dehydrogenation of the Deoxygenated Hydrocarbon Chain 4.2. Cracking and Dehydrogenation of the Deoxygenated Hydrocarbon Chain The organometallic ternary metal complex [(phen)M(CH2CH2R)]+ can undergo further Thefragmentation organometallic when ternary additional metal energy complex is supplied [(phen)M(CH through2CH MS2R)]n experiments+ can undergo (Figure fur- therS5C). fragmentation The initial structure when additional of species energy 2 immediately is supplied after through the loss MS ofn carbonexperiments dioxide (Figure has a S5C).terminal The carbon initial boundstructure to the of speciesmetal center 2 immediately (Scheme 3). after This the species loss of 2 cancarbon either dioxide decompose has a terminalvia the loss carbon of a bound1-alkene to theRCH=CH metal center2, leaving (Scheme the hydride 3). This species species 3 2, can[(phen)M(H)] either decompose+, or via Reactions 2021, 2 viathe theloss loss of hydrogen, of a 1-alkene forming RCH=CH an unsaturated2, leaving the or ganometallichydride species species 3, [(phen)M(H)] (red peaks).+ ,Both or via109 of thethese loss channels of hydrogen, proceed forming via a common an unsaturated intermediate organometallic 2a (Scheme species 4): (red peaks). Both of these channels proceed via a common intermediate 2a (Scheme 4):

Scheme 4. Mechanism of alkene loss or dehydrogenation of organometallic species 2. SchemeScheme 4. 4. MechanismMechanism of of alkene alkene loss loss or or dehydrogenation dehydrogenation of organometallic species 2. We showed that these two competitive channels have similar energetics. For example, forWeWe theshowed C2 chain, thatthat thesethesethe loss twotwo of competitivecompetitive alkene C2H channels cha4 fromnnels the have have Pd similar complexsimilar energetics. energetics. requires For 162 For example, vs.exam- 149 ple,kJ/molfor thefor Cforthe2 chain, theC2 chain,loss the of loss theH2 ofloss[17]. alkene ofThe alkeneC respective2H4 Cfrom2H4 thefromnumbers Pd the complex Pdfor complexthe requires C6 chain requires 162 are vs. 182162 149 and kJ/molvs. 149140 kJ/molfor the lossfor[15]. the of Hloss2 [17 of]. H The2 [17]. respective The respective numbers fornumbers the C6 forchain the are C6 182 chain and are 140 182 kJ/mol and [14015]. kJ/molThis [15]. β-hydride -hydride shift shift mechanism mechanism has has been been well well studied in alkene polymerization and depolymerizationThis β-hydride [23,24]. [23 shift,24]. mechanism We We showed has theoreticall theoretically been welly studied and experimentally in alkene polymerization that it operates and in depolymerizationthethe decomposition [23,24]. ofof aa CC6 6We chainchain showed in in a a recent recent theoreticall work work [15y [15]. and]. The Theexperimentally common common intermediate intermediate that it operates2a 2acan can bein thebeformed formed decomposition independently independently of a C by6 by chain a reactiona reaction in a recent of of the the hydridework hydride [15]. species species The common3 3and anda a terminal intermediateterminal alkene.alkene. 2a canWe bestudied formed this this independently reaction reaction and and showedby showed a reaction that that the theof the decomposition decomposition hydride species products products 3 and aof of terminal 2a2a areare very very alkene. similar, similar, We studiedindependentindependent this reaction of the way and this showed intermediate that the decomposition was formed (Figure products S6).S6). of 2a are very similar, independentAs mentioned mentioned of the before,way before, this species intermediate species 2 can2 undergocan was undergo formed isomerization (Figure isomerization S6). via chain via walking chain (Scheme walking 5).(Scheme As mentioned5). before, species 2 can undergo isomerization via chain walking (Scheme 5).

− − . SchemeScheme 5. 5.Chain-walking Chain-walking isomerizationisomerization fromfromα α−β and β−γγ.This This type type of of mechanism mechanism has has been been described described before before [23,24]. [23,24]. Scheme 5. Chain-walking isomerization from α−β and β−γ. This type of mechanism has been described before [23,24]. As we showed earlier, the barriers for sequentialsequential chain walkingwalking viavia ββ-hydride shift are very veryAs we small. small. showed For For instance, earlier, instance, the barriers barriers barriers for for forα−αβ sequential− andβ and β−γβ isomerization −chainγ isomerization walking of via the ofβ Pd-hexyl-hydride the Pd-hexyl com-shift + + − − areplexcomplex very [(phen)Pd(C small. [(phen)Pd(C For6H instance,13)]6H are13)] 19 barriers areand 19 42 for and kJ/mol, α 42β kJ/mol,and respectively, β γrespectively, isomerization which are which of much the are Pd-hexyl lower much than lowercom- is plextypicallythan [(phen)Pd(C is typically required6 requiredH for13)] +fragmentation. are for 19 fragmentation.and 42 kJ/mol,Thus, in respectively, Thus, a saturated in a saturated which alkyl arechain alkyl much R chain (like lower Rin (likethanstearic inis typicallyacid),stearic the acid), requiredmetal the can metal for “travel” fragmentation. can “travel” along the along Thus,whole the in chain whole a saturated with chain very withalkyl small very chain energy small R (like input. energy in Ifstearic input.there acid),isIf a there double the is metal abond double can in the“travel” bond fatty in alongacid the fattychain the whole acid(e.g., chain oleicchain acid), (e.g., with oleicthevery metal acid),small can energy the “chain metal input. walk” can If “chain thereup to isthewalk” a doubleα-carbon up to bond thenext αin -carbonto the the fatty double next acid to bond,chain the double (e.g.,forming oleic bond, an acid), allylic forming the intermediate. metal an allylic can “chain intermediate. Our walk”calculations up Our to theshowcalculations α-carbon that this show next type thatto of the structure this double type ofis bond, structurethe most forming isstable the an mostby allylic as stable much intermediate. by as as 20 much kJ/mol asOur [25]. 20 calculations kJ/mol The chain [25]. showwalkingThe chain that can this walking be type observed canof structure be observedexperimentally is the experimentally most thro stableugh by throughthe as muchappearance the as appearance 20 kJ/molof peaks [25]. of peaksin The the chain inmass the walkingspectrum,mass spectrum, can indicating be indicatingobserved an almost experimentally an almost equal equal distribution thro distributionugh theof hydrocarbon ofappearance hydrocarbon oflosses peaks losses from in from theC2 C-C mass2-C14 in14 spectrum,stearicin stearic acid acidindicating complexes complexes an (Figure almost (Figure 2A equal2 Aand and diFigurestribution Figure S1A,E). S1A,E). of hydrocarbon losses from C2-C14 in stearicThe acid isomeric complexes species (Figure 2c formed 2A and by Figure this chain-walking S1A,E). mechanism can then undergo Reactions 2021, 2, FOR PEER REVIEW 10 a β-alkide-alkideThe isomeric shift, shift, leading leading species to to 2c the the formed loss of by a this smaller chain-walking alkene and mechanism leaving behind can thethen shortenedshortened undergo achain β-alkide (Scheme shift, 66). ).leading to the loss of a smaller alkene and leaving behind the shortened chain (Scheme 6).

Scheme 6. MechanismScheme of 6. crackingMechanism of organometallic of cracking of species organometallic 2c. species 2c.

Obviously, alkene CH2=CH-R2 can also be formed (scheme not shown). The losses of alkenes after deoxygenation correspond to the green ions in the mass spectra (Figure 1, Figure 2 and Figure S1). This process, combined with chain walking, can occur multiple times if sufficient energy is supplied, as it is here via multi-stage CID (Figure S5). Fragmentation via alkene extrusion continues until either a hydride or methyl complex is formed. If the chain contains double bonds, some of the extruded hydrocarbons will contain two double bonds or a triple bond (Figure S5C).

4.3. Cracking and Dehydrogenation of the Hydrocarbon Chain without Deoxygenation There is a series of peaks in the mass spectra (Figure 1 and Figure 2, shown in blue) that corresponds to losses of neutral alkenes without losing carbon dioxide first. These losses were confirmed by high resolution MS (Figure S7). By inspecting Figure 1, it is obvious that this channel is more pronounced in the case of Pd and Pt (Figure 1B,C), while it is almost absent in Ni complexes (Figure 1A). This channel is only observed in the complexes of unsaturated fatty acids (Figure 1, Figure 2 and Figure S1). To explain the origin of these peaks, we need to consider decomposition of activated carboxylate species 1a alternative to Scheme 3. Subsequent collisional heating of intermediate 1a can also lead to its coordinatively unsaturated metal ion, which undergoes oxidative addition (OA) via insertion into a C-C bond of the fatty acid chain forming 1c (Scheme 7). This lactone-type intermediate has been found in some Ni(II) complexes [26].

Scheme 7. Mechanism for cracking of hydrocarbon chain without deoxygenation. This channel is only observed when R is unsaturated.

The subsequent activation of this intermediate 1c leads to alkene extrusion via reductive elimination (RE). It can proceed via abstraction of a β-H atom by one of the metal- bound carbons. If the carbon is part of the chain containing the R group then the lactone portion is converted to an unsaturated carboxylate, as shown in Scheme 7. Alternatively, the lactone carbon bound to the metal can abstract hydrogen from the free R-containing chain, forming a saturated carboxylate complex and extruding R-CH=CH2, as shown in Scheme S1. The initial size of the cyclic lactone shown by n in Scheme 7 explains the series of “cracking” peaks shown in blue (Figure 1 and Figure 2). The length of the fatty acid hydrocarbon chain is a factor here, as the chain cannot be too short for this mechanism to Reactions 2021, 2, FOR PEER REVIEW 10

Reactions 2021, 2 110 Scheme 6. Mechanism of cracking of organometallic species 2c.

Obviously, alkene CH2=CH-R2 can also be formed (scheme not shown). The losses of Obviously, alkene CH2=CH-R2 can also be formed (scheme not shown). The losses alkenes after ofdeoxygenation alkenes after deoxygenationcorrespond to the correspond green ions to thein the green mass ions spectra in the (Figure mass spectra 1, (Figure1, Figure 2 and Figure 2S1). and Figure S1). This process, Thiscombined process, with combined chain walking, with chaincan occur walking, multiple can times occur if multiple sufficient times en- if sufﬁcient ergy is supplied,energy as it is is supplied, here via as multi-st it is hereage via CID multi-stage (Figure S5). CID Fragmentation (Figure S5). Fragmentation via alkene via alkene extrusion continuesextrusion until continues either a hydride until either or methyl a hydride complex or methyl is formed. complex If the ischain formed. con- If the chain tains double bonds,contains some double of the bonds, extruded some hy ofdrocarbons the extruded will hydrocarbons contain two willdouble contain bonds two or double bonds a triple bond or(Figure a triple S5C). bond (Figure S5C).

4.3. Cracking and4.3. Dehydrogenation Cracking and Dehydrogenation of the Hydrocarbon of the Chain Hydrocarbon without ChainDeoxygenation without Deoxygenation There is a seriesThere of peaks is a series in the of mass peaks spec in thetra (Figure mass spectra 1 and (FiguresFigure 2,1 shownand2, shownin blue) in blue) that that correspondscorresponds to losses to of losses neutral of neutralalkenes alkeneswithout withoutlosing carbonlosing carbondioxide dioxide first. These ﬁrst. These losses losses were confirmedwere conﬁrmed by high by resolution high resolution MS (Figure MS (FigureS7). By S7).inspecting By inspecting Figure 1, Figure it is ob-1, it is obvious vious that thisthat channel this channel is more ispronounced more pronounced in the case in theof Pd case and of Pt Pd (Figure and Pt 1B,C), (Figure while1B,C), while it is it is almost absentalmost in absentNi complexes in Ni complexes (Figure 1A). (Figure This1A). channel This channelis only observed is only observed in the com- in the complexes plexes of unsaturatedof unsaturated fatty acids fatty (Figure acids (Figure 1, Figure1, Figure 2 and2 andFigure Figure S1). S1).To explain To explain the origin the origin of these of these peaks,peaks, we need we needto consider to consider decomposition decomposition of activated of activated carboxylate carboxylate species species 1a al-1a alternative ternative to Schemeto Scheme 3. Subsequent3. Subsequent collisional collisional heating heating of of intermediate intermediate 1a1a cancan also also lead lead to to its coordina- its coordinativelytively unsaturated unsaturated metal metal ion, whichwhich undergoesundergoes oxidative oxidative addition addition (OA) (OA) via via insertion into a insertion intoC-C a C-C bond bond of theof the fatty fatty acid acid chain chain forming forming1c (Scheme 1c (Scheme7). This 7). This lactone-type lactone-type intermediate has intermediate beenhas been found found in some in some Ni(II) Ni(II) complexes complexes [26]. [26].

Scheme 7. MechanismScheme 7. forMechanism cracking offor hydrocarbon cracking of hydrocarbon chain without chain deoxygenation. without deoxygenation. This channel This is only channel observed is when R is unsaturated.only observed when R is unsaturated.

The subsequentThe activation subsequent of this activation intermediate of this 1c intermediate leads to alkene1c extrusionleads to alkene via reduc- extrusion via re- tive eliminationductive (RE). eliminationIt can proceed (RE). via It abstraction can proceed of viaa β-H abstraction atom by one of a ofβ -Hthe atommetal- by one of the bound carbons.metal-bound If the carbon carbons. is part Ifof thethe carbonchain containing is part of thethe chainR group containing then the thelactone R group then the portion is convertedlactone to portion an unsaturated is converted carboxyl to anate, unsaturated as shown carboxylate,in Scheme 7. asAlternatively, shown in Scheme 7. Al- the lactone carbonternatively, bound the to the lactone metal carbon can abstract bound hydrogen to the metal from can the abstract free R-containing hydrogen from the free chain, formingR-containing a saturated chain, carboxylate forming complex a saturated and carboxylate extruding R-CH=CH complex and2, as extruding shown in R-CH=CH 2, as Scheme S1. Theshown initial in size Scheme of the S1. cyclic The lactone initial size shown of the by cyclic n in Scheme lactone 7 shown explains by then in series Scheme 7 explains of “cracking”the peaks series shown of “cracking” in blue (Figure peaks shown 1 and inFigure blue (Figures2). The length1 and2 ).of The the lengthfatty acid of the fatty acid hydrocarbon hydrocarbonchain is a factor chain here, is as a factorthe ch here,ain cannot as the be chain too short cannot for be this too mechanism short for this to mechanism to operate. Indeed, our data for shorter fatty acids (C2–C4) show no cracking without deoxygenation (Figure S8). The dominant loss of C13H26 from the oleic acid complexes of Pd and Pt (Figure1) corresponds to the six-membered cyclic lactone in Scheme1. This type of OA–RE mechanism is known for the group 10 metals (Ni, Pd, and Pt). It involves transition between M(II) and M(IV) and, thus, is more prevalent for Pd and, especially, Pt [16,26]. However, it has been postulated as a viable pathway in Ni systems as well [27]. There is a stark difference between this OA–RE mechanism and charge-remote fragmentation that operates in gas-phase fragmentation of fatty-acid/alkali or alkaline–earth Reactions 2021, 2Reactions, FOR PEER 2021 REVIEW, 2, FOR PEER REVIEW 11 11

operate. Indeed,operate. our Indeed,data for ourshorter data fatty for shorter acids (C fatty2–C4 acids) show (C no2–C cracking4) show nowithout cracking deoxy- without deoxygenation (Figuregenation S8). The(Figure dominant S8). The loss dominant of C13H 26loss from of Cthe13H oleic26 from acid the complexes oleic acid of complexes Pd and of Pd and Pt (Figure 1)Pt corresponds (Figure 1) corresponds to the six-membered to the six-membered cyclic lactone cyclic in Scheme lactone 1. in This Scheme type 1.of This type of OA–RE mechanismOA–RE mechanismis known for is the known group for 10 the metals group (Ni, 10 Pd,metals and (Ni, Pt). Pd,It involves and Pt). transi- It involves transi- Reactions 2021, 2 tion betweention M(II) between and M(IV) M(II) and,and M(IV)thus, isand, more thus, prevalent is more for prevalent Pd and, forespecially, Pd and, Ptespecially, 111 Pt [16,26]. However,[16,26]. it However,has been postulated it has been as postulated a viable pathway as a viable in Nipathway systems in asNi well systems [27]. as well [27]. There is a starkThere difference is a stark between difference this between OA–RE thismechanism OA–RE andmechanism charge-remote and charge-remote frag- frag- mentation thatmentation operates that in gas-phaseoperates in fragmentation gas-phase fragmentation of fatty-acid/alkali of fatty-acid/alkali or alkaline–earth or alkaline–earth metal complexesmetalmetal (Scheme complexescomplexes 8). (SchemeThe(Scheme charge-remote8 8).). The The charge-remote charge-remote mechanism, mechanism, mechanism, as the name as assuggests, the the name name occurs suggests, suggests, occurs occurs away from theawayaway charge, fromfrom which thethe charge,charge, in this whichwhich case inisin the thisthis metal casecase isiscation. thethe metalmetal cation.cation.

Scheme 8. ChargeSchemeScheme = remote 8.8. ChargeCharge fragmentation == remote fragmentation of fatty-acid/alkaline–earth of fatty-acid/alkaline–earthfatty-acid/alkaline–earth metal ternary metal cationic metal ternary ternary com- cationic cationic com- com- plexes. M = Mg,plexes.plexes. Ca, Ba. MM = = Mg, Ca, Ba.

There has beenThereThere extensive hashas beenbeen work extensiveextensive done on workwork identi donedonefication onon identiﬁcationidenti of fattyfication acids ofof by fattyfatty MS acidsvia charge- by MS via charge- remote fragmentationremoteremote fragmentationfragmentation [5,12]. The key [[5,12].5,12 feature]. The keyof this feature mechanism ofof thisthis mechanismmechanismis that the breakage isis thatthat thethe is breakagebreakage isis minimized orminimizedminimized even absent oror evenevenat the absentabsent position atat the theof thepositionposition double ofof bond(s),thethe doubledouble leading bond(s),bond(s), to “dips” leadingleading in to tothe “dips”“dips” inin thethe peaks correspondingpeakspeaks correspondingcorresponding to neutral alkene toto neutralneutral extrusion. alkenealkene For extrusion.extrusion. example, ForFor fragmentation example,example, fragmentationfragmentation of the oleic ofof thethe oleicoleic + + acid complex,acidacid [(phen)Mg(OOC(CH complex,complex, [(phen)Mg(OOC(CH2)7CH=CH(CH2))77CH=CH(CH2CH=CH(CH)7CH3)]+, shown2)27)CH7CH in3)] 3Figure)], shown, shown S3, in shows inFigure Figure no S3, S3, shows shows no peaks correspondingnopeaks peaks corresponding correspondingto the extrusion to the to of theextrusion C9 extrusion hydrocarbon, of C9 of C9hydrocarbon, where hydrocarbon, the double where where bondthe the double is double lo- bond bond is lo- is cated. Our located.resultscated. Ourare Our consistentresults results are are with consistent consistent the recently with with the thepublished recently recently wo publishedrk from workwoMcLuckey’srk fromfrom McLuckey’sMcLuckey’s group usinggroupgroup the same usingusing complexes thethe samesame but complexescomplexes prepared butbut in prepared prepareda different inin way aa differentdifferent (via gas-phase wayway (via(via ion–ion gas-phasegas-phase ion–ionion–ion reactions) [12].reactions)reactions) In contrast, [[12].12]. IntheIn contrast,contrast, fragmentation thethe fragmentationfragmentation of the Pd complex ofof thethe Pd Pd(Figure complexcomplex 1B) (Figuredisplays(Figure1 1B)B) no displays displays no no “dips” in the“dips”“dips” blue peaks inin thethe (extrusion blueblue peakspeaks of (extrusion(extrusion alkenes), of indicatingof alkenes),alkenes), that indicatingindicating Pd complexes thatthat PdPd fragment complexescomplexes via fragmentfragment viavia a different mechanism.aa differentdifferent mechanism. mechanism. There is alsoThereThere a series isis also of peaks a series in theof peaks mass inspectra the the mass mass (Figure spectra spectra 1 and (Figure (Figures Figure 1 1 2,and and shown Figure2, shown in 2, shown in red) in red) spaced spacedred)by 2 spacedm/ byz immediately 2 mby/ z2 immediatelym/z immediately to the left to of the tothe leftthe activated ofleft the of activatedthe precursor activated precursor that precursor correspond that that correspond tocorrespond to the to the loss of onelossthe (or loss of more) one of one (or molecules more)(or more) molecules of molecules hydrogen of hydrogen of without hydrogen decarboxylation.without withoutdecarboxylation. decarboxylation. The intermediate The The intermediate intermediate 1a 1a can also undergocan1a can also also undergoanother undergo variation another another variation of variationOA–RE of process OA–RE of OA–RE via process insertionprocess via insertionvia into insertion a C-H into bond into a C-H aof C-H bond bond of the of the fatty acidfattythe chain fatty acid (Scheme acid chain chain (Scheme 9), forming(Scheme9), forming 1d9),. forming 1d. 1d.

Scheme 9. Mechanism for loss of H2 or protonated ligand from common intermediate. SchemeScheme 9. Mechanism 9. Mechanism for forloss loss of H of2 Hor2 protonatedor protonated ligand ligand from from common common intermediate. intermediate.

The activationTheThe of activation this intermediate of of this this intermediate intermediate1d can lead to1d1d decomposition cancan lead lead to todecompositiondecomposition via RE, forming via viaRE, mo- RE,forming forming molecular hydrogenmolecularlecular and hydrogen hydrogenthe unsaturated and and the the unsaturated carboxyl unsaturatedate carboxylcomplex. carboxylateate The complex. complex. process Thecan The processbe process repeated, can can be repeated, leading to theleadingleading losses toto of thethe additional losseslosses ofof H additionaladditional2 molecules. HH2 2The molecules.molecules. fact that TheThe the factfact OA–RE thatthat thethemechanism OA–REOA–RE mechanism mechanismis isis much more muchlikelymuch moretomore occur likelylikely with toto Pt occuroccur and withwithPd than PtPt andand with PdPd Ni thanthan [26] with withis consistent NiNi [[2266]] isis with consistentconsistent our results. withwith ourour results.results. It should beItIt noted shouldshould that bebe hydrogen notednoted that that losses hydrogen hydrogen are more losses losses prevalent are are more more prevalentfor prevalent oleic acid for for oleic(monoun- oleic acid acid (monounsat- (monoun- saturated) thanurated)saturated) for thanstearic than for acid stearicfor (saturated),stearic acid acid (saturated), (saturated),as can be as seen can as becanby seen comparingbe seen by comparing by thecomparing intensity the intensity the of intensity of “red’ of “red’ peakspeaks “red’in Figure inpeaks Figure 1, inFigure 1Figure, Figure 2, 1,and2, Figure and Figure Figure 2, S1.and S1. This Figure This can can S1. be be Thisexplained explained can be by explained the factfact that thatby abstractingthe fact that a hydrogen atom from an allylic position demands much less energy (bond dissociation energy of a saturated C-H is as much as 58 kJ/mol higher than those for allylic C-H [27]. The ease of hydrogen abstraction seems to increase even more when going from oleic to linoleic (doubly unsaturated) acid complexes (compare Figures1 and2 for Ni or 1 and Figure S1 for Pd and Pt). However, complexes of linoleic (triply unsaturated) acid display less dehydrogenation because at this point the number of hydrogen atoms on the fatty acid chain available for removal is diminished. Reactions 2021, 2, FOR PEER REVIEW 12

abstracting a hydrogen atom from an allylic position demands much less energy (bond dissociation energy of a saturated C-H is as much as 58 kJ/mol higher than those for allylic C-H [27]. The ease of hydrogen abstraction seems to increase even more when going from oleic to linoleic (doubly unsaturated) acid complexes (compare Figure 1 and Figure 2 for Reactions 2021, 2 Ni or 1 and Figure S1 for Pd and Pt). However, complexes of linoleic (triply unsaturated)112 acid display less dehydrogenation because at this point the number of hydrogen atoms on the fatty acid chain available for removal is diminished. The length of the hydrocarbon chain in the fatty acids also plays a role in the ability to performThe length cracking/dehydrogenation of the hydrocarbon chain chemis in thetry fatty without acids also deoxygenation. plays a role in Shorter the ability chains to performlike butyric cracking/dehydrogenation acid and heptanoic acidchemistry show no dehydrogenation without deoxygenation. or cracking Shorter without chains deox- like butyric acid and heptanoic acid show no dehydrogenation or cracking without deoxygena- ygenation [13]. Chains longer than C10 begin to exhibit these pathways as they are long tion [13]. Chains longer than C begin to exhibit these pathways as they are long enough enough to interact with the metal.10 to interact with the metal. While the hydrocarbon products afforded by this cracking chemistry are in the form While the hydrocarbon products afforded by this cracking chemistry are in the form of olefins, with a high enough pressure of H2 gas in the reaction chamber, alkane formation of oleﬁns, with a high enough pressure of H gas in the reaction chamber, alkane formation could be favored [28]. It is known that bidentate2 N ligands, similar to the phenanthroline could be favored [28]. It is known that bidentate N ligands, similar to the phenanthroline used in this work, when bound to Ni catalyze hydrogenation of olefins [29]. Ni catalysts used in this work, when bound to Ni catalyze hydrogenation of oleﬁns [29]. Ni catalysts have also been used in partial hydrogenation of oils [30,31]. The unique capacity of group have also been used in partial hydrogenation of oils [30,31]. The unique capacity of group 10 metals to catalyze both cracking (i.e., depolymerization) and hydrogenation thus shows 10 metals to catalyze both cracking (i.e., depolymerization) and hydrogenation thus shows their advantage in producing biofuels from renewable sources. their advantage in producing biofuels from renewable sources.

4.4.4.4. ClosingClosing aa CatalyticCatalytic CycleCycle forfor DeoxygenationDeoxygenation andand CrackingCracking IndustrialIndustrial scalescale processesprocesses forfor thethe productionproduction ofof hydrocarbonshydrocarbonsfrom fromfatty fattyacids acidswould would bebe bolsteredbolstered byby thethe abilityability toto makemake thesethese reactionsreactionscatalytic. catalytic. ClosingClosing thethe catalyticcatalytic cyclecycle inin thethe gasgas phasephase cancan bebe easilyeasily achievedachieved throughthrough ion–moleculeion–molecule reactionsreactions (IMRs)(IMRs) ofof neutralneutral carboxyliccarboxylic acidsacids withwith ionsions generatedgenerated fromfrom CIDCID experiments,experiments, asas describeddescribed inin ourour previousprevious workwork detailingdetailing stearicstearic acidacid deoxygenationdeoxygenation andand cracking.cracking. OneOne ofof thethe keykey ﬁndingsfindings inin thatthat workwork waswas thatthat the agostic interactio interactionsns of of hydrocarbons hydrocarbons longer longer than than C C2 2werewere too too great great to toovercome overcome for for gas-phase gas-phase IMRs IMRs (an (an issue issue that, that, most most likely, likely, will will not notbe as be important as important for con- for condensed-phasedensed-phase reactions) reactions) but, but, through through multiple multiple stages stages of CID, of CID, metal metal hydride, hydride, methyl, methyl, and andethene ethene can canbe produced, be produced, which which all all react react with with carboxylic carboxylic acids acids to to reform aa carboxylatecarboxylate metalmetal complexcomplex (Scheme(Scheme 10 10).).

+ + SchemeScheme 10.10.Catalytic Catalytic deoxygenation deoxygenation and and cracking cracking of of stearic stearic acid acid by by [(phen)M(OOCC [(phen)M(OOCC17H1735H)]35)](M(M = Ni,= Pd,Ni, andPd, and Pt). Pt) .

5.5. ConclusionsConclusions ThisThis workwork expandedexpanded upon our recent stud studyy of of deoxygenation deoxygenation and and cracking cracking of of satu- sat- uratedrated fatty fatty acid acid complexes complexes of of Ni Ni and and Pd. Pd. We We showed showed that that all three all three group group 10 metals 10 metals (Ni, (Ni,Pd, Pd,and andPt) in Pt) the in the+2 oxidation +2 oxidation state, state, when when complexed complexed with with 1,10-phenanthroline, 1,10-phenanthroline, can can in- induceduce decarboxylation decarboxylation of ofmono- mono- and and polyun polyunsaturatedsaturated fatty fatty acids. While thethe detaileddetailed mechanismsmechanisms ofof decarboxylationdecarboxylation andand fattyfatty acidacid chainchain crackingcracking havehave beenbeen describeddescribed byby usus previouslypreviously for for modelmodel systems,systems, thisthis studystudy expandedexpanded thethe scopescope ofof thethe proposedproposed mechanismsmechanisms andand summarizedsummarized multiplemultiple processesprocesses occurringoccurring uponupon collisionalcollisional activationactivation of of thesethese cationiccationic complexescomplexes inin thethe gasgas phase.phase. A common theme for all systems under study is that multiple stages of collisional A common themen for all systems under study is that multiple stages of collisional activation (via MS ) lead to decarboxylation of fatty acid (via loss of CO2) followed by activation (via MSn) lead to decarboxylation of fatty acid (via loss of CO2) followed by “cracking” of the carbon chain, which occurs via “chain walking” of the coordinatively unsaturated metal ion. In addition to these two processes, dehydrogenation is also observed, especially for Pt. A unique feature of unsaturated fatty acid complexes is that they can undergo cracking before deoxygenation, thus requiring a different mechanism. While we did not investigate this process computationally, the most likely scenario is that this occurs via an oxidative addition/reductive elimination mechanism, consistent with the fact that this channel is much more pronounced for Pd and Pt than for Ni. Reactions 2021, 2 113

The order of the deoxygenation and cracking notwithstanding, the ultimate products of MSn are the hydride and methide complexes, which are reactive towards carboxylic acids (or esters), thus completing a catalytic cycle. These processes are common for all three metals and any type of fatty acid. Thinking about the relevance to the industrial deoxygenation/cracking processes that employ the metals Ni, Pd, or Pt on carbon, the metal surface often terminates in M-OH or M-H [32,33] (in the presence of molecular hydrogen) groups that would react with neutral fatty acids by forming carboxylates. Then, mechanisms similar to those described in this study can operate. The main difference is that the gas-phase mechanism for cracking produces alkenes, while industrial biodiesel production demands alkanes, but the latter are formed from alkenes in hydrogen atmosphere. It is advantageous that group 10 metals are effective catalysts for both cracking and hydrogenation. Lastly, while the OA–RE mechanism proposed in this work requires initial oxidation of +2 metals to the +4 oxidation state, under the conditions of heterogeneous catalysis, the carboxylate binding and C-C (or C-H) bond insertion can occur at different metal atoms, thus requiring a more feasible M0 to M2+ transition.

Supplementary Materials: The following are available online at https://www.mdpi.com/article/10 .3390/reactions2020009/s1. Figure S1: Spectra for CID of ternary metal complexes, Table S1: High resolution mass spectral neutral losses. Author Contributions: Conceptualization, V.R. and K.P.; methodology, K.P.; data curation, V.P., K.P.; writing—original draft preparation, K.P., V.R.; writing—review and editing, R.A.J.O., K.P., and V.R.; funding acquisition, V.R. All authors have read and agreed to the published version of the manuscript. Funding: We acknowledge The Department of Chemistry and Biochemistry of Northern Illinois University and the NIU Office of VP for Research and Innovation Partnership and the NSF MRI program (award #CHE:1726931 to VR) for the purchase of the high-resolution mass spectrometer. Conflicts of Interest: The authors declare no conflict of interest.

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