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Inorganic Chemistry Article, Reduction of CO2 to CO requires formal loss of an O2. dianion In other examples of catalytic reduction stoichiometric. amounts of oxygen atom acceptors i e diboron borane. hydrosilane or anhydrides have been used to close the. catalytic cycle 13 However the formation of strong bonds with. oxygen acceptors precludes an e cient energy storing catalytic. cycle An ideal scenario would utilize protons as a. stoichiometric O atom acceptor to generate water as shown. in eq 1 However this adds the complication of competitive. proton binding at the reduced metal center and can result in. reduction of protons instead of CO2 14 Therefore we also. examined the relationship between the redox properties of the. complexes and pKa of the reduced metal centers The. thermodynamic and kinetic parameters for CO2 CO and H. binding are all important considerations for selective catalyst. design for the reaction shown in eq 1,CO2 2e 2H CO H 2O. E 0 53 V at pH 7 vs NHE 1, Figure 1 ORTEP diagram of PCNCP Co NCCH3 2 BPh4 2 1a. RESULTS AND DISCUSSION, Synthesis Characterization and Structural Studies of. Ellipsoids are shown at 50 probability Hydrogen atoms except. CH2 a molecule of CH3CN and two BPh4 ions have been. omitted for clarity,PENEP Co NCCH3 2 BF4 2 1a c The series of PENEP. Table 1 Selected Bond Distances Angles deg and 5,Co NCCH3 2 BF4 2 E C 1a N 1b O 1c complexes. for PCNCP Co NCCH3 2 BPh4 2 1a, were prepared by addition of the free ligands to Co. PNNNP Co NCCH3 2 BF4 2 1b and, NCCH3 6 BF4 2 For all three ligands addition of solid. PNNNP Co NCCH3 BF4 2b a, PENEP to an acetonitrile solution of Co NCCH3 6 BF4 2. produced a dark orange solution Removal of solvent in vacuo 1a 1b 2b. resulted in dark orange oils that precipitated the products as Co Npy 1 9632 15 1 9480 13 1 925 3. orange powders upon stirring in THF or benzene Co P1 2 2761 6 2 2623 4 2 1844 7. Two variations of the Co II PCNCP tetra uoroborate salt Co P2 2 3030 6 2 2893 4 2 1843 7. were obtained using di erent methods of recrystallization Co Leq 1 9002 16 1 8963 13 1 834 3. Di usion of pentane into a saturated dichloromethane solution Co Lax 2 0868 17 2 0720 14. produced orange crystals of the 5 coordinate cobalt complex Npy Co Leq 176 97 7 168 50 5 177 68 13. with one acetonitrile ligand in the plane of the PCNCP ligand P Co P 160 17 2 163 416 17 168 67 4. and an axial BF4 with a Co F distance of 2 3446 9 The 5 0 28 0 08. structure for this complex with the formula PCNCP Co a. Leq and Lax indicate the donor atoms of the ligands best described as. NCCH3 BF4 BF4 is shown in Figure S1 in the Supporting trans and cis respectively from the pyridine donor N 2b contains 4. Information The value of 5 is 0 18 and the sum of the angles cobalt molecules in the asymmetric unit Parameters listed above are. around the basal plane is 358 7 15 However elemental analysis for a representative molecule in the asymmetric unit cell. of crystals grown from acetonitrile toluene solution was. consistent with the formulation PCNCP Co NCCH3 2,BF 4 2 1a The dibromide PC N C P CoBr2 was also. synthesized using CoBr2 but isolated as the BPh4 salt by fragment on a PONOP Co complex An analogous nickel. treating it with 2 equiv of NaBPh4 in CH3CN forming the example of the same decomposition pathway has been. analogous complex PCNCP Co NCCH3 2 BPh4 2 1a described 16. The structure for this complex is shown in Figure 1 The Magnetic Properties of PENEP Co NCCH3 2 BF4 2. metrical parameters of the BPh4 salt which are listed in Table 1 1a c The spin states of the complexes were experimentally. are similar to those of the BF4 bound salt with the obvious determined to facilitate the computational studies EPR spectra. exception of those involving the pseudoaxial ligand at 77 K for 1a c are indicative of low spin S 1 2 systems. X ray quality crystals of 1b were grown by slow evaporation shown in Figure 3 EPR spectra 1a and 1b were taken in. of acetonitrile from an acetonitrile toluene solution The frozen EtOH solutions 1c is unstable in EtOH so its spectrum. structure of the cationic cobalt complex of 1b which is was collected in 1 1 CH3CN THF The signals for 1a c were. shown in Figure 2 is similar to that of 1a with the most centered at g 2 28 2 26 and 2 24 respectively with hyper ne. notable di erence being a bending of the apical NCCH3 ligand coupling to the cobalt center of 94 91 and 85 G respectively. away from the pyridine that is manifested in a larger Npy Co The same spin state is observed at room temperature using the. Naxial angle The 5 value for 1b is 0 08 15 The reaction between Evans method measurements for complexes 1a b in CD3CN. PONOP and Co NCCH3 2 BF4 2 1c proceeded similarly 5 C6H6 as an internal reference The magnetic moments. however attempts at growing X ray quality crystals resulted in 2 0 B contrast with the high spin S 3 2 systems observed. oils and decomposition products One attempt resulted in a for Ph PCNCP CoX2 salts X Cl Br NO3 4 3 4 7 B. crystallographically charactarized macrocyclic P2N2 Co com re ecting the stronger eld nature of the acetonitrile. plex possibly formed by nucleophilic attack of a O py O 2 ligands 6a 17. 13032 dx doi org 10 1021 ic5021725 Inorg Chem 2014 53 13031 13041. Inorganic Chemistry Article, Figure 4 Cyclic voltammograms of PENEP Co NCCH3 2 BF4 2. E C 1a top black N 1b middle blue O 1c bottom red 1 0. mM analyte in CH3CN with 0 20 M Bu4NPF6 at 100 mV s Isolation. of the rst reduction event is indicated by dotted lines Arrows indicate. scan direction and resting potential,Table 2 Redox Potentials versus Fe Cp 2 0 for. PENEP Co NCCH3 2 BF4 2 E C 1a N 1b O 1c, Figure 2 ORTEP diagram of PNNNP Co NCCH3 2 BF4 2 1b. Ellipsoids are shown at 50 probability Hydrogen atoms except N compd Epc red3 Epc red2 E1 2 red1 Epa ox. H a molecule of CH3CN and BF4 ions have been omitted for 1a 2 83 2 45 1 03 1 33. clarity 1b 2 56 0 88 1 20,1c 2 27 0 61, possibility of ligand redox noninnocence we have not formally. assigned the reduction to a metal centered Co I 0 event No. trend correlates these reduction potentials with the electronic. nature of the ligand however as the reduction is not reversible. the observed onset potential may not accurately re ect E1 2. Complexes 1a and 1b have irreversible oxidations at 1 33 and. 1 20 V respectively but 1c displays no oxidation within the. limits of the solvent window,Synthesis and Characterization of P E N E P Co. NCCH3 BF4 2a c The reversible nature of the Co II I. couples indicates that the corresponding Co I complexes. should be stable and isolable Chemical reductions were carried. out to isolate the PENEP Co complexes Treatment of. PCNCP Co NCCH3 2 BF4 2 1a with 1 equiv of KC8 in, THF CH3CN at ca 200 K produced a dark green solution. Figure 3 EPR spectra for PENEP Co NCCH3 2 BF4 2 E C 1a Filtration solvent removal and washing the resulting solid with. top black N 1b middle blue in EtOH at 77 K and PONOP Co diethyl ether gives PCNCP Co NCCH3 BF4 2a in 89. NCCH3 2 BF4 2 1c bottom red in 1 1 CH3CN THF yield Complex 2a is diamagnetic and displays a 1H NMR. spectrum in CD3CN with resonances at 7 46 ppm triplet and. Electrochemical Studies of PENEP Co NCCH3 2 BF4 2 7 08 ppm doublet corresponding to the aromatic protons. 1a c Cyclic voltammetry of 1a c shown in Figure 4 3 11 ppm singlet corresponding to the methylene groups and. reveals two reduction events The rst reduction assigned to 1 48 ppm broad singlet corresponding to the tert butyl. the Co II I couple vide inf ra is reversible E1 2 values are groups A peak for the coordinated acetonitrile ligand is not. listed in Table 2 This couple becomes more positive from 1a observed presumably due to exchange with the deuterated. 1b to 1c which is the expected trend as the pincer ligand solvent A very broad resonance at 103 0 ppm in the 31P NMR. becomes more electron withdrawing E CH2 NH O in this spectrum is assigned to the phosphorus atom in the ligand The. isostructural series The rst reductions of 1a c display infrared absorption spectrum of solid 2a reveals an absorption. di usion controlled behavior linear relationship between ip feature at 2229 cm 1 attributed to the coordinated acetonitrile. vs 1 2 scan rate for scan rates from 0 025 V s to 1 60 V s Single crystals of 2a were grown by evaporation of CH3CN. see Figure S2 in the Supporting Information from a CH3CN toluene solution and evaluated by X ray. In the same range of scan rates the second reduction for 1a crystallography The resulting structure con rmed the con. c is completely irreversible see Supporting Information Figure nectivity as 4 coordinate PCNCP Co NCCH3 BF4 and is. S3 Although reduced PCNCP cobalt complexes are known to shown in Supporting Information Figure S10 however the. exhibit ligand noninnocence 6d 18 no redox potential shifts are data was not of high enough quality for a re ned structure The. observed in the reduction of 1a c for the scan rates formulation was also supported by atmospheric pressure. investigated 25 1600 mV s indicating no chemical change chemical ionization mass spectrometry APCI MS which. on the electrochemical time scale However because of the displayed peaks at m z of 495 4 and 454 4 corresponding to. 13033 dx doi org 10 1021 ic5021725 Inorg Chem 2014 53 13031 13041. Inorganic Chemistry Article, PCNCP Co NCCH3 and PCNCP Co respectively and and PONOP Co respectively and no peak corresponding. no peak corresponding to PCNCP Co NCCH3 2 to PONOP Co NCCH3 2. PNNNP Co NCCH3 BF4 2b was obtained in a Electrochemical Reduction of PENEP Co NCCH3 2. procedure similar to that of 2a Treatment of a solution of BF4 2 1a c under 1 atm CO2 The reactivity of CO2. 1b in THF CH3CN with 1 equiv of KC8 caused a color change with the reduced forms of 1a c was examined by cyclic. from dark orange to dark purple Filtration removal of solvent voltammetry in CO2 saturated acetonitrile Figure 6 displays. and washing with diethyl ether provides 2b in 93 yield The. H NMR of 2b in CD3CN contains peaks at 7 16 ppm triplet. and 6 01 ppm doublet corresponding to the aromatic protons. and a broad singlet at 1 47 ppm corresponding to the tert butyl. group The N H protons appear as a broad singlet at 6 48. ppm The 31P NMR spectrum displays a broad peak at 121 0. ppm The IR absorption spectrum of solid 2b displays a clear. N H stretch at 3322 cm 1 and a peak at 2213 cm 1 attributed. to the coordinated acetonitrile APCI MS veri ed the. formulation of 2b exhibiting peaks at 497 4 and 456 3. corresponding to PNNNP Co NCCH3 and PNNNP, Co respectively No signi cant peak was observed corre. sponding to PNNNP Co NCCH3 2 Crystals of su cient, quality for analysis by X ray di raction were grown by cooling a Figure 6 Cyclic voltammograms of PENEP Co NCCH3 2 BF4 2. saturated CH3CN toluene solution to 238 K The structure E C 1a top black N 1b middle blue and PONOP Co. shown in Figure 5 reveals a square planar geometry around the NCCH3 BF4 2c bottom red under N2 dotted lines and CO2. solid lines 1 0 mM analyte in CH3CN with 0 20 M Bu4NBF4 at 100. mV s Arrows indicate the scan direction and the resting potential. voltammograms of 1a b and 2c under nitrogen dotted lines. and carbon dioxide solid lines Due to the instability of 1c. which slowly decomposed in solution 2c was used in its. place for electrochemical reactivity experiments For all three. compounds there is no di erence in the Co II I couple. between the voltammograms under CO2 versus N2 remaining. fully reversible at scan rates from 0 0125 to 1 6 V s shown in. Figure S4 in the Supporting Information indicating the Co I. complexes are unreactive to CO2 This was con rmed by. adding CO2 to 2a after 10 min the 1H and 31P NMR spectra. exhibited no observable change, The second reduction for 1a c on the other hand exhibits. signi cantly di erent behavior in the presence of CO2. displaying a positive shift in the reduction potential that varies. with scan rate and ligand substituent For 1a at slower scan. rates the second reductive wave under CO2 appears positive. relative to that under N2 Figure 7 left shows cyclic. Figure 5 ORTEP diagram of one of four molecules in the asymmetric voltammograms of 1a under CO2 at di erent scan rates. unit PNNNP Co NCCH3 BF4 2b Ellipsoids are shown at 50 normalized by dividing the current by 1 2 Two peaks in the. probability Hydrogen atoms except N H and a BF4 ion have been cathodic current are observed in addition to the reversible rst. omitted for clarity, reduction the rst at approximately 2 2 V and the second at. approximately 2 5 V similar to the potential under N2 At. cobalt I center with the angles around the metal center scan rates 125 mV s the former shows some reversibility. summing to 360 2 Shorter metal ligand bond distances whereas the more negative reduction is irreversible at all scan. relative to 1b are consistent with stronger bonding for the rates The shift in reduction potential relative to the reduction. more electron rich metal center Elemental analysis of these under N2 reaches a maximum of 300 mV at the slowest scan. crystals con rmed the formulation as PNNNP Co NCCH3 rate of 12 5 mV s At su ciently fast scan rates 800 mV s. BF4 no signi cant shift in potential is observed though the. Complex 2c was synthesized by stirring the free PONOP magnitude of the wave at 2 5 V is greater For 1b a similar. ligand with Co NCCH3 6 BF4 2 in THF CH3CN followed positive shift is observed in the presence of CO2 but without a. by cooling to 238 K and addition of an equivalent of KC8 The distinct maximum in ipc even at scan rates as slow as 12 5 mV s. dark purple product 2c was ltered washed with diethyl ether Figure 7 middle shows cyclic voltammograms of 1b under. and isolated in 72 yield The 1H NMR spectrum displayed CO2 at di erent scan rates normalized by 1 2 At slow. aromatic peaks at 7 67 ppm triplet and 6 55 ppm doublet scan rates 25 mV s the second reduction of 1b contains a. and a tert butyl resonance at 1 56 ppm The solid state IR feature with an onset potential of approximately 2 2 V and an. spectrum exhibits a peak at 2250 cm 1 The formulation of 2c irreversible peak at 2 7 V At 12 5 mV s a maximum shift of. was also supported by APCI MS which contained peaks at m z approximately 200 mV was observed for the new feature. of 499 3 and 458 3 corresponding to PONOP Co NCCH3 relative to the reduction under N2 At scan rates faster than 200. 13034 dx doi org 10 1021 ic5021725 Inorg Chem 2014 53 13031 13041. Inorganic Chemistry Article, Figure 7 Cyclic voltammograms of PENEP Co NCCH3 2 BF4 2 E C 1a left N 1b middle and PONOP Co NCCH3 BF4 2c right at. varying scan rates under CO2 normalized by dividing the current by 1 2 1 0 mM analyte in CH3CN with 0 20 M Bu4NBF4 Arrows indicate the scan. direction and the position of the bulk material, mV s no distinct shift was observed The cyclic voltammo spectra was measured over 12 min Figure 8 shows the C O. grams for 2c do not show any scan rate dependent reactivity double and triple bond region of the spectra over the course of. under CO2 at scan rates as low as 12 5 mV s shown in Figure. Unlike other molecular cobalt based CO2 reduction catalysts. the Co I complexes display no reactivity with CO2 11f i This. was observed electrochemically for all of the complexes and. con rmed by NMR for 2a However reactivity with CO2 is. observed at the second reduction event and was examined in. more detail using scan rate dependent cyclic voltammetry The. positive potential shift with decreasing scan rate is consistent. with a fast following reaction after the second reduction. kinetic potential shift Since the concentration of saturated. CO2 in acetonitrile19 is over 100 fold higher than the complex. in our experimental conditions the reaction was modeled using. pseudo rst order kinetics We attempted to model the. potential shift to obtain exact kinetic parameters for the. reaction with CO2 using DigiElch software However the. added complication of the irreversible electron transfer step Figure 8 Spectroelectrochemical infrared di erence spectra of. with an irreversible chemical step made extraction of exact PCNCP Co NCCH3 2 BF4 2 1a under natural abundance CO2. kinetic information di cult Using eq 220 for a rst order red lines and 13CO2 green lines 1 0 mM analyte in CH3CN with. chemical step following an electron transfer we have estimated 0 20 M Bu4NPF6. kf to be about 102 3 s 1 for the reaction of reduced 1a and 1b. with CO2 E potential shift scan rate, the experiment As the sample was electrolyzed a peak grew in. RT 1 nF at 1921 cm 1 Isotopically labeled 13CO2 was used to. E 0 780 ln, nF 2 kf RT 2 authenticate that the observed stretch is a result of CO2. reduction When the identical experiment was performed. Notably 1c with the most positive reduction potential does with 13CO2 the observed peak shifted to 1876 cm 1 This is. not give a discernible kinetic potential shift with CO2 even at consistent with the value predicted for a CO bond based on a. the slowest scan rates Prior analysis of single metal site simple harmonic oscillator. reactivity toward CO2 points toward a correlation between The CO adduct of 2a PCNCP Co CO BF4 was. reduction potential and activation of CO2 14 21 The complexes independently prepared and solution based IR measurements. support this trend since the complex with the most positive matched the stretch observed in the spectroelectrochemical. reduction potential 1c shows slow or no reaction with CO2 experiment vide inf ra The initial product for the reaction of. and the more negative reduction potentials in 1a and 1b are CO2 with the two electron reduced complex is expected to be a. concomitant with more favorable reactivity toward CO2 metal carboxylate However metal carboxylates at basic metal. Spectroelectrochemical Investigation of the CO2 centers can reductively disproportionate to CO and CO32 22. Activation Product The reaction of the reduced complex In essence a second equivalent of CO2 is acting as the oxygen. 1a with CO2 was investigated by infrared spectroelectrochem atom acceptor for the initial CO2 reduction to CO This would. istry A 2 20 V potential was applied to a CO2 saturated result in the Co II carbonyl complex which is easily reducible. acetonitrile solution of 1 0 mM 1a containing 0 20 M Bu4NPF6 under the experimental conditions to the experimentally. and 1 0 mM ferrocene and the di erence infrared absorption observed Co I carbonyl. 13035 dx doi org 10 1021 ic5021725 Inorg Chem 2014 53 13031 13041. Inorganic Chemistry Article, Reaction of PENEP Co NCCH3 BF4 2a c with CO a dichotomy where metal centers with more negative reduction. The Sabatier principle describes the importance of both potentials are more favorable toward CO2 activation but in. substrate activation and product release for catalysis This is turn also have a stronger association with the product This. particularly important as CO is a much better ligand than CO2 relationship has also been observed in heterogeneous systems. for most metal complexes but must dissociate for the catalyst by Norskov et al where the scaling relationships for CO2. to turn over To investigate the CO binding interaction the activation and CO release are directly correlated making. cobalt I complexes PENEP Co NCCH3 BF4 2a c catalyst optimization at single metal sites di cult 23. were treated with CO and the resulting products characterized Electrocatalytic Studies To test for electrocatalytic. by 1H NMR 31P NMR and IR spectroscopies In CD3CN activity 1a was titrated with proton sources and studied by. solutions of 2a c were bubbled with CO under 1 atm to cyclic voltammetry and controlled potential electrolysis Cyclic. produce new diamagnetic products Only 2a produced a single voltammetry during titration of 1a with triethylammonium. product assigned as PCNCP Co CO BF4 For 2a the 1H tetra uoroborate pKa 18 8 in CH3CN 24 resulted in a minor. NMR spectrum of the resulting orange solution indicates clean increase in current relative to the acid without complex see. conversion to a new product with aromatic peaks at 7 72 ppm Figure S5 in the Supporting Information The much weaker. triplet and 7 36 ppm doublet The methylene and tert butyl acid HDBU BF 4 was then used DBU 1 8. peaks both appear as virtual triplets at 3 86 and 1 42 ppm The diazabicyclo 5 4 0 undec 7 ene pKa 24 3 in CH3CN 24 in. P NMR spectrum displays a single peak at 106 3 ppm The an attempt to avoid protonation of the reduced cobalt. thin lm IR absorption spectrum of PCNCP Co CO BF4 complex 25 This resulted in a current enhancement under N2. displayed a carbonyl absorption at 1911 cm 1 When taken as a which increased under 1 atm of CO2 as shown in Figure 9. solution droplet with 0 2 M Bu4NBF4 in CH3CN the carbonyl. absorption was observed at 1920 cm 1 consistent with the. product in the IR spectroelectrochemical experiment described. Unlike 2a treatment of CD3CN solutions of 2b and 2c with. CO produced mixtures of two products which were observed. by 1H and 31P NMR spectroscopy Bubbling CO through a. dark purple solution of 2b in CD3CN in a septum capped. NMR tube produced a green solution The 1H NMR spectrum. of this solution appears to contain a single set of resonances. analogous to those obtained from 2a A broad triplet at 7 45. ppm and a doublet at 6 35 ppm are consistent with the. aromatic backbone protons a broad singlet at 6 99 ppm is. consistent with the N H resonance and the tert butyl Figure 9 Cyclic voltammograms of PCNCP Co NCCH3 2 BF4 2. resonance appears as a virtual triplet at 1 44 ppm The 31P 1a titrated with HDBU BF4 0 mM black 1 mM red 2 mM. green 4 mM blue under N2 dotted lines and CO2 solid lines 1 0. NMR spectrum however contains two broad peaks at 158 9. mM analyte and FeCp2 internal reference in CH3CN with 0 20 M. and 142 2 ppm suggesting the presence of two species with Bu4NPF6 at 100 mV s. overlapping 1H NMR signals No remaining 2b was detected. The infrared absorption spectrum of this solution taken as an. evaporated thin lm displays a single strong absorption in the Bulk electrolysis was performed with 1a 1 0 mM and. carbonyl region at 1923 cm 1 consistent with a cobalt I HDBU BF4 50 mM in CO2 saturated CH3CN solution. carbonyl stretch a very weak additional absorption at 1877 at 2 15 V versus Fe Cp 2 0 for 1 h During electrolysis the. cm 1 and a single broad N H stretch at 3303 cm 1 Analogous orange solution changed to dark blue green consistent with. treatment of a CD3CN solution of 2c with CO resulted in an 2a GC MS analysis of the headspace detected H2 but no. immediate color change from dark purple to green and like 2b reduced carbon containing products. the starting material was completely consumed The resulting Competitive Protonation of Reduced PENEP Co. H and 31P NMR spectra contained resonances corresponding NCCH3 2 BF4 2 1a c An external proton source is. to two di erent products In the 1H NMR spectrum two sets of necessary to complete CO2 reduction to CO and H2O. PONOP resonances are clearly represented by two broad triplets However addition of protons provides a competitive pathway. at 8 10 and 7 95 ppm overlapping doublets at 7 0 ppm and a for proton reduction to generate hydrogen The expected. distorted broad virtual triplet at 1 54 ppm The products appear pathway for the latter would involve protonation at the reduced. in a ratio of approximately 0 6 1 0 The 31P NMR spectrum metal center to give a metal hydride intermediate 26. contains broad peaks at 238 1 and 229 7 ppm Thin lm IR The basicity of cobalt I complex 2a was estimated using. absorption reveals a single strong absorption in the carbonyl cyclic voltammetry The reversibility of the cobalt II I couple. region at 1936 cm 1 and a weak but sharp signal at 1901 cm 1 ipc ipa 1 was observed as sequentially stronger acids were. On the basis of the major carbonyl stretch the Co CO bond added see Supporting Information Figure S6 The reduction. varies in strength by 2a 2b 2c The increasing electron remains reversible after the addition of 1 equiv of HDMF. withdrawing character of the ligands from CH2 to NH to O OTf DMF dimethylformamide OTf tri uoromethane. substitution is manifested in the CO stretching frequency in sulfonate pKa 6 1 in CH3CN 27 These results suggest that. addition to the potential of the cobalt II I couple The major the pKa of a protonated PCNCP CoIIIH 2 is less than 6 An. CO infrared stretch increases from E CH2 1911 cm 1 to E NMR scale experiment in which slightly less than 1 equiv of. NH 1923 cm 1 to E O 1936 cm 1 The increase in CO HBF4 OEt2 pKa 1 8 1 5 in CH3CN 28 was added to 2a in. bond strength indicates less electron density at the metal for CD3CN cleanly produced peaks consistent with a PCNCP. back bonding and a more weakly bound M CO This presents CoIIIH 2 placing a lower bound on the pKa of 3 29. 13036 dx doi org 10 1021 ic5021725 Inorg Chem 2014 53 13031 13041. Inorganic Chemistry Article, Overall the pKa of the reduced cobalt complexes was di cult basic and protonation is not possible in CH3CN These results. to experimentally measure due to complex instability There are consistent with our experimental observations. fore quantum mechanical methods B3LYP d3 including PBF At more negative potentials the Co I cations can be further. continuum solvation were employed to gauge the relative reduced as seen experimentally In both PCNCP and PONOP. energies of intermediates involved with CO2 and H reduction complexes two kinds of low energy formally Co 0 structures. pathways for the most electron donating and withdrawing were identi ed When zero one or two acetonitrile molecules. ligands PCNCP and PONOP Free energies for the reaction of are coordinated to a neutral PCNCP Co moiety through. solvento complexes with CO2 and protons can be seen in nitrogen the wavefunction consists of a triplet Co I cation. Scheme 1 For both ligands the Co I and Co II complexes antiferromagnetically coupled to a radical anion ligand with. spin distributed around the pyridine ring The PCNCP ligands. Scheme 1 Calculated pKa Values and CO2 Binding Energies can thus become redox active 6d 18 but not under conditions. for the Reduction of 1a and 1ca for neutral pH low overpotential CO2 reduction A few kcal. mol lower in free energy however lie neutral PCNCP Co 2. NCCH3 complexes incorporating a bound acetonitrile. molecule The Mulliken population on bound CH 3CN,0 75 e and C N and Co C bond lengths 1 23 and. 1 95 respectively suggest the description of a 2 bound. Co II metallacycle Preference for this mode of acetonitrile. binding has been observed on other reducing low oxidation. state metal centers 30 The existence of isomers with various. modes of solvent coordination within 8 kcal mol of this ground. state underscores the uxional coordination at the reduced. The quantum mechanical studies suggest that the neutral. complexes should favorably react with CO2 by 7 1 and 12 9. kcal mol for the PCNCP and PONOP complexes respectively. This mirrors the experimental ndings which indicate reactivity. with CO2 after reducing the Co I complex In contrast to 2a. the cyclic voltammogram of 2c did not exhibit a kinetic. potential shift with low scan rates upon reduction of 2c. indicating a slower reaction with CO2 However an increase in. current is observed under CO2 indicating the reaction does. proceed to an extent on the electrochemical time scale. With the extra electron density provided by the second. reduction the lone pair from Co is donated into the orbital. of the CO2 bending the C bound CO2 adduct This is similar. to the bonding that occurs with the bent CH3CN adduct The. competing reaction with protons is also favorable At pH. 24 3 reaction with protons is exergonic by 7 0 kcal mol for the. PCNCP complex and 10 6 kcal mol for the PONOP complex. CONCLUSION, The reduction and reactivity with CO2 CO and H were. G energies are calculated in 1 M DBU G in 1 M H both in examined using experimental and quantum mechanical. CH3CN Potentials reported with respect to FeCp2 0 methods for a series of isostructural cobalt complexes with. stepwise changes in the electronic structure These substrates. are important because the design of selective CO2 to CO and. lowest in free energy contain two acetonitrile molecules The H2O reduction catalysts involves the competitive reaction of. second solvent molecule is weakly bound by 2 2 kcal mol for CO2 over H at reduced metal centers and facile removal of the. the PONOP 2c complex It is thermodynamically unfavorable CO product. by 20 kcal mol for the Co II complexes 1a and 1c to lose Previous experimental studies have indicated that both CO2. one acetonitrile ligand and react with CO2 The high energy binding and protonation become more energetically favorable. complex loosely binds CO2 at the equatorial position through with increasing metal basicity which is also observed with these. the oxygen In the PCNCP complex the Co O bond length is complexes 22d 31 An important result is that while the free. calculated to be 2 10 and in PONOP complex the bond length energy of protonation can be tuned by adjusting the pKa of the. is 2 12 acid this has only a minimal e ect on the free energy of CO2. It is also unfavorable for Co I 2a and 2c to react with binding. CO2 and only weakly coordinated complexes form similar to In the case of these complexes the acid used in the. the Co II case Protonation of Co I by HDBU is also highly electrolysis studies HDBU BF4 pKa 24 324 in CH3CN. endergonic However protonation to form PCNCP CoIIIH was insu ciently weak to prevent protonation at the metal. NCCH3 2 2 from 2a is accessible with strong acids giving a centers A much weaker acid pKa 36 in CH3CN would be. calculated pKa of 3 5 close to the value experimentally necessary to avoid competitive hydrogen formation However. measured above However 2c is calculated to be much less metal centers with lower pKa values would allow the use of. 13037 dx doi org 10 1021 ic5021725 Inorg Chem 2014 53 13031 13041. Inorganic Chemistry Article, weaker acids to circumvent a hydrogen production pathway a The working compartment contained 1 0 mM 2a 1 0 mM ferrocene. strategy that has been demonstrated for another transition 0 20 M Bu4NPF6 and 50 mM HDBU BF4 in acetonitrile The. metal electrocatalyst 25 counter compartment contained 1 0 mM ferrocene and 0 20 M. Bu4NPF6 The headspace of the working compartment was sampled by. EXPERIMENTAL SECTION Syntheses PCNCP Co NCCH3 2 BF4 2 1a Solid PCNCP 149 4. Synthetic Methods and Materials The complexes described mg 377 7 mol was added to a solution of Co NCCH3 6 BF4 2. below are air and moisture sensitive and must be handled under an 178 3 mg 372 4 mol in 5 mL of CH3CN The dark orange solution. inert atmosphere of nitrogen using standard glovebox and Schlenk was stirred at 25 C for 15 h and then the solvent was removed in. techniques Unless otherwise noted all procedures were performed at vacuo The resulting crude product was redissolved in ca 1 mL of. ambient temperature 21 24 C All solvents were sparged with CH2Cl2 with 5 drops of CH3CN and layered with 4 mL of Et2O After. argon and dried using a solvent puri cation system Halocarbon 1 d the resulting rust colored solid was isolated by ltration and. solvents were passed through packed columns of neutral alumina and washed with 2 4 mL THF yield 244 2 mg 92 Anal Calcd. Q5 reactant Acetonitrile ethereal and halogenated solvents were Found for C27H49N3B2F8P2Co C 45 66 45 91 H 6 95 6 89. passed through two columns of neutral alumina DMF and alcohol N 5 92 6 18 eff 5 C6H6 CD3CN Evans method 298 K 2 0 B. solvents were passed through columns of activated molecular sieves EPR EtOH 77 K g1 2 28 g2 2 02 A2 94 G. The ligands PCNCP PNNNP and PONOP and the cobalt starting PNNNP Co NCCH3 2 BF4 2 1b To a stirred orange solution of. material Co NCCH3 6 BF4 were synthesized according to Co NCCH3 6 BF4 2 143 4 mg 299 5 mol in 8 mL of CH3CN. established procedures 32 Potassium graphite KC8 was synthesized was added solid PNNNP 119 4 mg 360 4 mol resulting in a dark. by heating stoichiometric amounts of potassium and graphite in a orange solution After stirring for 12 h at 25 C the solvent was. sealed evacuated Schlenk ask until a homogeneous bronze colored removed in vacuo leaving a sticky dark orange residue The residue. powder was obtained Triethylamine was freeze pump thawed three was dissolved in 10 mL of 1 1 toluene CH3CN and the vial was. times and dried over molecular sieves Graphite was dried under placed in a larger container containing ca 20 mL of toluene After 2. vacuum at 150 C All other materials including CO2 99 999 and days the resulting dark orange crystals were decanted washed with 2. CO 99 5 were purchased from commercial sources and used 2 mL THF and dried in vacuo yield 209 7 mg 98 Anal Calcd. without further puri cation Found for C27H50N6B2F8P2Co 1b CH3CN C 43 05 43 20. Physical Methods Elemental analyses EA were performed by H 6 69 6 56 N 11 16 10 90 eff 5 C6H6 CD3CN Evans. Robertson Microlit Laboratories or on a PerkinElmer 2400 Series II method 298 K 2 0 B EPR EtOH 77 K g1 2 26 g2 2 01 A2. CHNS O analyzer Electrospray ionization mass spectrometry ESI 91 G. MS and atmospheric pressure chemical ionization mass spectrometry PONOP Co NCCH3 2 BF4 2 1c To a solution of Co NCCH3 6. APCI MS were performed with a JEOL JMR 600H mass BF4 2 90 9 mg 190 mol in 7 mL of THF with 1 mL of CH3CN. spectrometer or a Waters LCT Premier mass spectrometer Gas was added solid PONOP 75 8 mg 190 mol After stirring at 25 C. chromatography GC was performed on an Agilent Technologies for 1 h the dark orange solution was evaporated in vacuo to provide. 7890A GC system with front and back TCD channels Nuclear crude 2c yield 134 7 mg 99 All attempts to recrystallize 1c. magnetic resonance NMR spectra were recorded on a Bruker resulted in partial decomposition EPR THF CH3CN 77 K g1. DRX500 spectrometer with a TCI cryoprobe 1H and 13C or a 2 24 g2 2 00 A2 85 G. DRX400 with a switchable QNP probe 1H and 31P in dry degassed PCNCP Co NCCH3 2 BPh4 2 1a A colorless solution of PCNCP. solvents 1H NMR spectra were referenced to TMS using the residual 200 5 mg 506 9 mol in 5 mL of CH2Cl2 was added slowly to a 5. proteo impurities of the solvent 13C NMR spectra were referenced to mL blue solution of CoBr2 110 8 mg 506 5 mol in CH3CN The. TMS using the natural abundance 13C of the solvent and 31P NMR solution developed a dark purple color and was stirred for 12 h The. spectra were referenced to H3PO4 using the scale with the solvent was then removed in vacuo The crude product 53 6 mg was. corresponding 1H spectra 33 All chemical shifts are reported in the dissolved in 5 mL of CH2Cl2 and 1 mL of CH3CN and treated with. standard notation in parts per million positive chemical shifts are a NaBPh4 59 9 mg 175 mol After stirring for 1 d 15 drops of. higher frequency than the reference Solution magnetic moments were CH3CN and 5 drops of toluene were added and the suspension was. determined by Evans Method using a sealed capillary containing either ltered The orange ltrate was dried in vacuo producing an orange. 5 CHCl3 CDCl3 or 5 C6H6 CD3CN as internal reference 34 powder which was recrystallized by slow evaporation of a toluene. Perpendicular mode X band electron paramagnetic resonance EPR acetonitrile solution This material was characterized by crystallog. spectra were collected using a Bruker EMX spectrometer Infrared. raphy only, IR absorption measurements were taken as thin lms or compressed. PCNCP Co NCCH3 BF4 2a A dark orange solution of 1a 123 9. solids on a Thermo Scienti c Nicolet iS5 spectrophotometer with an. mg 174 5 mol in 2 mL of THF and 2 mL of CH3CN was frozen in a. iD5 ATR attachment Spectroelectrochemical experiments were. cold well and then allowed to thaw To the just thawed solution was. performed using a 3 electrode cell with platinum working silver. added solid KC8 24 3 mg 178 mol resulting in an immediate color. reference and platinum counter electrodes 35 Background spectra were. change to dark green After stirring at room temperature for 4 h the. taken before electrolysis and di erence spectra were collected while. applying a controlled potential For each experiment potentials were solution was ltered and the solvent was removed from the ltrate. measured and applied relative to an internal ferrocene reference a ording dark green residue The sticky solid was suspended in 6 mL. Electrochemical experiments were carried out with a Biologic VSP 300 of Et2O ltered and washed with 4 mL of C6H6 and 4 mL of Et2O. potentiostat or a Pine Wavedriver 10 potentiostat Electrochemical The solid was then extracted with 2 5 mL 4 1 C6H6 CH3CN. experiments were carried out in acetonitrile solutions with 1 0 mM Removal of solvent from the ltrate provided a dark green residue. analyte and 0 20 M Bu4NPF6 or Bu4NBF4 The working electrode was Suspending in 4 mL of Et2O and removing the solvent again provided. a glassy carbon disc with a diameter of 3 mm or 1 mm the counter pure 2a as a dark green powder yield 90 3 mg 89 APCI MS. electrode was a glassy carbon rod and the reference electrode was a CH3CN m z 495 4 M BF4 454 4 M CH3CN BF4. silver wire in 0 20 M Bu4NPF6 or Bu4NBF4 in CH3CN separated from. H NMR CD3CN ppm 7 46 t 3JHH 7 7 Hz 1H aryl H 6 99, the bulk solution by a Vycor frit Potentials were referenced at 100 d 3JHH 7 7 Hz 2H aryl H 3 11 s 4H CH2 1 48 s 36H. mV s unless otherwise noted to the ferrocene ferrocenium couple. Bu 1H NMR d6 acetone ppm 7 55 t 3JHH 7 6 Hz 1H aryl. at 0 V using ferrocene as an internal reference Bulk electrolysis was H 7 08 d 3JHH 7 5 Hz 2H aryl H 3 31 s 4H CH2 1 55. carried out in a 2 compartment cell separated by a ne frit with the s 36H tBu 13C 1H NMR 126 MHz CD3CN ppm 165 0. working electrode vitreous carbon and reference electrode silver 134 1 122 0 35 8 32 7 30 0 31P 1H NMR 162 MHz CD3CN. wire in 0 2 M Bu4NPF6 in CH3CN separated by Vycor frit in one ppm 103 0 br s PtBu2 31P 1H NMR 162 MHz d6 acetone. compartment and the counter electrode nichrome wire in the other ppm 69 3 br s PtBu2 IR solid m ax cm 2229 C N. 13038 dx doi org 10 1021 ic5021725 Inorg Chem 2014 53 13031 13041. Inorganic Chemistry Article, PNNNP Co NCCH3 BF4 2b A dark orange solution of 1b 110 1 functions and 6 31G on organics 38 The single d function on. mg 155 3 mol in 4 mL of THF and 1 5 mL of CH3CN was frozen in phosphorus was replaced by two uncontracted gaussians with. a cold well and then allowed to thaw To the just thawed solution was exponents of 0 45 and 1 35 Single point energies including solvation. added solid KC8 21 2 mg 155 mol 1 equiv resulting in an were computed using the previous 3 Los Alamos basis augmented. immediate color change to dark purple After stirring at room with f functions 39 the 6 311G basis on organics 40 and 6 311. temperature for 1 5 h 2 mL of toluene was added and the solution G 3df on phosphorus 40a 41 Solvent e ects in acetonitrile were. was ltered Removal of solvent from the ltrate provided 2b yield included using the PBF implicit solvation model42 using a dielectric. 84 4 mg 93 Anal Calcd Found for C23H44N4BF4P2Co C constant of 37 5 and probe radius of 2 19 To determine accurate. 47 28 47 50 H 7 59 7 39 N 9 59 9 36 APCI MS CH3CN free energies for solvent molecules the 1 atm ideal gas free energy of. m z 497 4 M BF4 456 3 M CH3CN BF4 1H NMR acetonitrile was computed using the appropriate statistical mechanics. CD3CN ppm 7 16 t 3JHH 8 0 Hz 1H aryl H 6 48 br s 2H formulas and the empirical free energy of vaporization 1 27 kcal mol. NH 6 01 d 3JHH 8 0 Hz 2H aryl H 1 47 br vt 36H tBu derived from the vapor pressure 43 was subtracted The chemical. C 1H NMR 126 MHz CD3CN ppm 165 0 138 1 97 7 38 8 potential of H 1 M in CH3CN was the ideal gas free energy H. 28 8 31P 1H NMR 162 MHz CD3CN ppm 121 0 br s PtBu2 TS 5 2kT 298 K 26 04 e u 44 6 3 kcal mol minus the free. IR solid m 1, ax cm 3324 N H 2213 C N energy of hydration G 1 atm 1 M 264 0 kcal mol 45 plus the. P N P Co NCCH3 BF4 2c Co NCCH3 6 BF4 2 192 8 mg transfer free energy G 1 M aq 1 M CH3CN 5 7 kcal mol 46. 402 6 mol was dissolved in 2 mL of CH3CN To this solution was All calculations were completed with Jaguar 47. added PONOP 160 8 mg 402 5 mol as a 4 mL THF solution After The formally Co 0 solvent complexes are best described as high. stirring at ambient temperature for 70 min the dark orange reaction spin cationic Co I centers antiferromagnetically coupled to radical. was frozen in a cold well Immediately upon thawing solid KC8 57 8 anionic pyridine ligands The approximate projection scheme. mg 423 mol was added causing an immediate color change to dark proposed by Yamaguchi48 was applied using the large basis. purple After stirring at ambient temperature for 2 5 h the reaction was unsolvated wave functions to correct electronic energies of the. ltered separating a gray solid which was extracted with 2 1 mL of unrestricted doublets for spin contamination from the higher energy. THF Removal of the solvent in vacuo provided a sticky dark purple quartet state S2 values of the broken symmetry doublets ranged from. solid which was suspended in 4 mL of C6H6 and dried again The 1 50 to 1 65 leading to corrections of up to 3 2 kcal mol Wave. product was then isolated by suspending in 4 mL of C6H6 ltering functions for Co II and Co I states did not su er spin. contamination, and drying the solid under vacuum yield 170 2 mg 72 APCI MS. CH3CN m z 499 3 M BF4 458 3 M CH3CN BF4, H NMR CD3CN ppm 7 67 t 3JHH 8 1 Hz 1H aryl H 6 55 ASSOCIATED CONTENT. d 3JHH 8 1 Hz 2H aryl H 1 56 s 36H tBu 13C 1H NMR,S Supporting Information. 126 MHz CD3CN ppm 167 8 140 9 102 9 40 9 28 0 31P 1H CIF les CCDC deposition numbers for 1a 1022629 1a. NMR 162 MHz CD3CN ppm 214 2 br s PtBu2 IR solid, max cm 1 2250 C N 1022631 1b 1022633 2a 1022630 and 2b 1022632. Reaction of PCNCP Co NCCH3 BF4 2a with CO In an NMR Additional crystallographic data cyclic voltammograms 1H. tube capped with a septum a CD3CN solution of 2a was bubbled with NMR spectra and computational data This material is available. free of charge via the Internet at http pubs acs org. CO gas for 2 min resulting in a color change from dark green to. orange 1H NMR CD3CN ppm 7 72 t 3JHH 7 8 Hz 1H aryl. H 7 36 d 3JHH 7 8 Hz 2H aryl H 3 86 vt 4H CH2 1 42 AUTHOR INFORMATION. vt 36H tBu 13C 1H NMR 126 MHz CD3CN ppm 202 6 Corresponding Author. 162 3 139 4 122 4 t J 5 0 Hz 38 8 t J 8 3 Hz 37 8 t J 9 8. Hz 29 5 31P 1H NMR 162 MHz CD3CN ppm 106 3 br s,E mail j yang uci edu. PtBu2 IR thin lm max cm 1 1911 C O IR CH3CN solution Notes. 0 2 M Bu4NBF4 max cm 1 1920 C O The authors declare no competing nancial interest. Reaction of PNNNP Co NCCH3 BF4 2b with CO In an NMR. tube capped with a septum a CD3CN solution of 2b was bubbled with. CO gas for 2 min resulting in a color change from dark purple to. green 1H NMR CD3CN ppm 7 45 br t 1H aryl H 6 92 br s. ACKNOWLEDGMENTS, This material is based upon work performed at the Joint Center. 2H NH 6 35 d 3JHH 8 1 Hz 2H aryl H 1 44 vt 36H tBu for Arti cial Photosynthesis a DOE Energy Innovation Hub. P H NMR 162 MHz CD3CN ppm 158 9 br s 142 2 br s, 31 1 supported through the O ce of Science of the U S. IR thin lm max cm 1 3303 N H 1923 C O 1877 weak Department of Energy under Award Number DE SC0004993. Reaction of PONOP Co NCCH3 BF4 2c with CO In an NMR and additional support from the School of Physical Sciences at. tube capped with a septum a CD3CN solution of 2c was bubbled with the University of California Irvine S I J would like to. CO gas for 2 min resulting in a color change from dark purple to dark acknowledge support from the National Science Foundation. yellow green 1H NMR CD3CN ppm 8 10 br t aryl H 7 95 Graduate Research Fellowship under Grant DGE 1144469. br t aryl H 6 99 m aryl H 1 54 br tBu 31P 1H NMR 162 The authors thank C Tsay and L Henling for helpful. MHz CD3CN ppm 238 1 br s 229 7 br s IR thin lm max. discussions and assistance,cm 1 1936 C O 1901 weak. Computational Methods The free energies of organometallic. species were calculated using REFERENCES,1 a Gray H B Nat Chem 2009 1 1 7 7 b Harriman A. G298K Eelec Gsolv ZPE Hvib nkT 2 T Selec Svib Philos Trans R Soc A 2013 371 1996 1 16 c Lewis N S. 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