Density Functional Study on the Mechanism of Nickel-Mediated Diazo

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Density Functional Study on the Mechanism of Nickel-Mediated Diazo Carbonylation Bianka Barcs,† László Kollár,†,‡ and Tamás Kégl*,†,‡ †

Department of Inorganic Chemistry and MTA-PTE Research Group for Selective Chemical Syntheses, University of Pécs, P.O. Box 266, H-7624 Pécs, Hungary ‡ János Szentágothai Research Center, Ifjúság útja 34, H-7624 Pécs, Hungary S Supporting Information *

ABSTRACT: The mechanism of diazo activation as well as the carbonylation of the resulting carbene complexes has been investigated by means of DFT calculations at the PBE0/TZVP level of theory. The free energy profile of all elementary steps of the reaction, i.e., diazo coordination, dinitrogen extrusion, carbene−CO coupling, CO coordination, and ketene elimination, have been elucidated for diazomethane and ethyl diazoacetate as substrates and for Ni(CO)3, Ni(CO)2(PH3), and Ni(dtbpe)(CO) (dtbpe = 1,2-bis(di-tert-butylphosphino)ethane) as precursors. The reaction rate is determined by the formation of the coordinatively unsaturated precursor followed by the dinitrogen extrusion, regardless of the initial diazo compound and the catalyst precursor. For the homoleptic precursor Ni(CO)3 the free energy of activation for diazomethane and ethyl diazoacetate (EDA) is 16.9 and 22.4 kcal/mol, respectively. The replacement of one carbonyl ligand with PH3 results in a decrease in the activation barrier, modifying the energies to 15.7 and 20.5 kcal/mol, respectively. The activation free energy of the diazo extrusion step promoted by Ni(dtbpe)(CO) is 24.3 kcal/mol for diazomethane and 28.1 kcal/mol for EDA. The formation of carbene complexes is slightly endergonic starting from the homoleptic precursor and exergonic for the phosphine-substituted complex. The carbene−CO coupling, resulting in coordinatively unsaturated ketene complexes, is a fast and highly exergonic process with reaction free energies between −32.3 and −42.1 kcal/mol, depending on the substituents. Under a carbon monoxide atmosphere the ketene complexes may uptake one CO, forming coordinatively saturated ketene complexes via low barriers in exergonic reactions. The final step, i.e., the dissociation of ketene or ethoxycarbonylketene from the saturated ketene complex, regenerates the corresponding nickel−carbonyl precursor. The azine formation side reaction for the simplest case between Ni(CO)3 and diazomethane has been also examined and found to be highly exergonic; however, the large free energy barrier prevents the formation of benzophenone-azine when CO is also present. Co2(CO)7(CHCO2Et).6 After loss of one terminal carbonyl ligand, the complex Co2(CO)6(CHCO2Et) may form, which can also activate ethyl diazoacetate, thus providing the catalytically active species for the second cycle.7 In the presence of CO the ethoxycarbonyl carbene ligand of Co2(CO)7(CHCO2Et) or that of the dicarbene complex Co2(CO)6(CHCO2Et)2, formed in the second cycle, can couple with one terminal CO ligand, providing the ethoxycarbonylketene ligand, which is replaced under carbon monoxide by CO. Despite the synthetic importance of the diazo carbonylation reaction, only sporadic results have been reported regarding the reaction mechanism elucidated by theoretical methods. Grotjahn and co-workers investigated the interconversion of phosphine-substituted iridium carbene−carbonyl complexes by intramolecular CC bond cleavage/formation.8 The reaction was found to be reversible, and the equilibrium was predicted to be on the side of the carbonyl carbene system, and this was

1. INTRODUCTION Ketenes are important intermediates in synthetic organic chemistry.1 Due to their high reactivity, they undergo various cycloaddition reactions with alkenes or imines. One powerful way leading to ketene is the substitution of the diazo group in diazoalkanes by carbon monoxide. The very reactive ethoxycarbonyl ketene formed as a short living product by the carbonylation of ethyl diazoacetate (EDA) can be trapped in situ by various scavengers such as alcohols, secondary amines, and imines to obtain the corresponding malonic acid derivatives2 or β-lactams,3 respectively. Moreover, in the cobalt-catalyzed domino reaction of EDA with CO and ferrocenylimines the synthesis of unsaturated malonic acid derivatives was reported.4,5 The mechanism of carbonylation of ethyl diazoacetate was established with various cobalt carbonyl complexes as catalysts. Using Co2(CO)8 as the catalyst precursor, it was found that the formation of diethyl malonate occurs in two different cycles. In the first cycle the coordinatively unsaturated Co2(CO)7 complex serves as a repeating species which forms, after reacting with EDA, the bridging carbonyl−carbene complex © XXXX American Chemical Society

Received: March 23, 2012

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dx.doi.org/10.1021/om300243m | Organometallics XXXX, XXX, XXX−XXX

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Scheme 1. Catalytic Transformation of Diazoalkanes in the Presence of Nickel Carbonyl Catalysts

and denoted as TZVP.17 With this model chemistry a good fitting was achieved with the available results for the bond dissociation enthalpy (BDE) of nickel tetracarbonyl. According to our calculations the first dissociation enthalpy for Ni(CO)4 is 24.9 kcal/mol, which is in excellent agreement with the value of 25 ± 2 kcal/mol reported by Lineberger and co-workers based on laser photoelectron spectrometry measurements.18 Our results coincide quite well with the early calculations of Ziegler et al. employing the HFS method extended with nonlocal exchange correction as well as the correlation between electrons with different spins: 25.4 kcal/mol was reported for the first ligand dissociation energy for Ni(CO)4.19 Carbó and co-workers reported a similar value (24.8 kcal/mol) employing the BLYP/DZVP level of theory.20 Taking into account solvent effects employing the CPCM model21 with hexane (ε = 1.882), the ligand dissociation energy slightly decreased to 23.8 kcal/mol, in reasonable agreement with the experimental value of 24.3 kcal/mol.22 Nonetheless, solvation was found to have negligible effects on the course of the reactions (for instance, the free energy barriers for the N2 extrusion step in the reaction of EDA with Ni(CO)3, taking into account the solvation effects, are 22.2 and 22.5 kcal/mol for n-hexane and diethyl ether, respectively, as opposed to 22.4 kcal/mol in the gas phase); therefore, solvated energies will not be discussed within this study. The species containing the dtbpe ligand were treated with the ONIOM method developed by Morokuma and co-workers.23 The whole system was divided into two layers. For the inner layer the same functional and basis set was employed as for the simple DFT calculations, whereas the tert-butyl groups of the dtbpe ligands were treated with the PBEPBE functional24 in combination with the 3-21G* basis set.25 The calculations were carried out using Firefly 7.1.E software26 and the Gaussian 03 suite of programs.27 The stationary points were characterized by frequency calculations in order to verify that they have zero imaginary frequencies for equilibrium geometries and one imaginary frequency for transition states. Thermochemistry corrections were taken from frequency calculations at 298.15 K and 1 atm. Intrinsic reaction coordinate (IRC) analyses28 were carried out throughout the reaction pathways to confirm that the stationary points are smoothly connected to each other. For charge decomposition analyses (CDA) the AOMix software was used.29 NBO analyses30 were carried out using the NBO package included in Gaussian 03.

also corroborated experimentally.9 The analogous rhodium ketene complex, however, was found to be more stable than its carbonyl carbene isomer. For chromium cyclopentadienyl carbonyl radical complexes the η1-N-coordinated diazoalkanes were found to be much more stable than their η1-C counterparts. The carbonyl−carbene coupling pathway was excluded due to its high free energy barrier.10 Two pathways were suggested instead: a concerted reaction taking place via a direct attack of CO on the diazo carbon and a pathway via a dimer intermediate involving two CpCr(CO)2 moieties bridged by a diazoalkane. Recently, the mechanism of diazo carbonylation as well as that of the reaction of ketene with imine resulting in β-lactam was investigated by Wang and co-workers applying palladium catalysts with various ligands. For the computational studies Pd ethylene carbonyl complexes were postulated. An isomerization of the CN bond of the initially formed zwitterionic intermediate was suggested, which explains the predominant formation of trans-β-lactams.11 The first metal-mediated example for diazo carbonylation was reported by Rüchardt and Schrauzer in 1960. In the presence of a large excess of nickel tetracarbonyl, diazo compounds such as diazomethane, diphenyldiazomethane, and ethyl diazoacetate were converted into the corresponding ketenes. The carbonyl ligands of Ni(CO)4 served as a CO source instead of gaseous carbon monoxide.12 Although Ni(CO)4 exhibited remarkable activity in some carbonylation reactions, such as the carbonylation of allyl halides and alkynes13 and the Pauson−Khand reaction,14 it lost some of its popularity because of its very poisonous nature. However, recently the use of nickel(0) complexes has regained some interest, since alternative procedures have been proposed which circumvent the direct handling of Ni(CO)4. Moretó and co-workers applied Ni(COD)2, as precursor, for the Nicatalyzed cyclocarbonylations of allyl halides and alkynes,15 which were reported earlier by Chiusoli and Cassar.13 In this paper a detailed theoretical investigation of the nickelcatalyzed diazo carbonylation is presented. The first aim of this study is to propose a thorough description for the reaction mechanism and an explanation for the moderate rate of the transformation of ethyl diazoacetate into diethyl malonate in the presence of nickel tetracarbonyl by elucidating the elementary steps of the reaction. A secondary purpose of this paper is to interpret the effect of the phosphine substituent by means of electronic structure analyses when one CO ligand of the Ni(CO)3 precursor is replaced by PH3 or two carbonyls are replaced by a dtbpe ligand (dtbpe = 1,2-bis(di-tertbutylphosphino)ethane). The reaction is depicted in Scheme 1, indicating the carbene complex LnMCHR (4) as the key intermediate.

3. RESULTS AND DISCUSSION 3.1. Reaction of Nickel Carbonyls with Diazomethane and Ethyl Diazoacetate. It is generally accepted that the coordinatively unsaturated Ni(CO)3 species, formed from Ni(CO)4 by dissociation of one CO ligand, is the real active catalyst in reactions catalyzed by homoleptic nickel carbonyls.31,32 The rate of ligand substitution from CO to various ligands, such as 14CO,33 C18O,22 and PPh3,22,33 was measured by Basolo et al. It was found that the first step, which is the ratedetermining step, does not involve the reacting nucleophile. Instead, a reacting intermediate was formed which was assumed to be the coordinatively unsaturated Ni(CO)3, which reacts rapidly with the incoming nucleophile. In principle, however, a competitive associative mechanism cannot be ruled out. Therefore, the formation of diazo complexes via an SN2-like mechanism is compared with the dissociative mechanism: that is, the replacement of one CO by

2. THEORETICAL METHODS For all of the calculations the PBE0 functional was selected, i.e., the zero-parameter hybrid GGA functional by Adamo and Barone.16 For all atoms the triple-ζ basis set by Ahlrichs and co-workers was applied B


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Figure 1. Computed structures of diazomethane, ethyl diazoacetate, ketene, ethoxycarbonylketene, Ni(CO)4, and Ni(CO)3 (1), as well as the transition states describing the CO−diazo exchange via an associative mechanism for diazomethane (SNTSa) and ethyl diazoacetate (SNTSb). Selected bond lengths are given in Å. Gibbs free energy and ZPVE-corrected energy values (in parentheses) are in kcal/mol relative to Ni(CO)4 and the respective diazoalkanes.

Figure 2. Computed structures of η1-C diazo complexes 2a,b, η1-N diazo complexes 2aN and 2bN, the η2-(N,C) diazo complex 2aCNN, and the η1O diazo complex 2bO. Selected bond lengths are given in Å and bond angles in degrees. Gibbs free energy and ZPVE-corrected energy values (in parentheses) are in kcal/mol relative to Ni(CO)3 and the respective diazoalkanes separated.

Scheme 2. Coordination Modes for Diazoalkanes Bonded to a Single Transition-Metal Center

which show a 0.08 kcal/mol preference in terms of free energy.35 For both diazomethane and ethyl diazoacetate similar transition structures were found with a distorted-trigonalbipyramidal structure. The attacking diazoalkane displaces the CO ligand of Ni(CO)4 in a trans position in a concerted manner. In both SNTSa and SNTSb (see Figure 1) the nickel− carbonyl carbon bonds are contracted as the ligands around Ni are arranged in a trigonal-bipyramidal configuration. The free energy barriers for the one-step ligand exchange are 26.0 and 26.4 kcal/mol for diazomethane and EDA, respectively. The

diazomethane or ethyl diazoacetate. The computed structures of the two diazoalkanes, Ni(CO)4, Ni(CO)3, and the transition structures associated with the ligand exchange process are shown in Figure 1. The computed Ni−C and C−O bond distances for Ni(CO)4 are 1.821 and 1.134 Å, respectively, which are somewhat shorter than those reported by Hedberg et al. (1.838 Å for the Ni−C bond and 1.141 Å for the C−O bond) on the basis of electron diffraction measurements.34 For ethyl diazoacetate the s-cis conformer is very slightly favored, with a free energy difference of 0.1 kcal/mol to the s-trans structure. This is in agreement with experimental NMR data, C


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Figure 3. Computed structures of species related to the N2 extrusion and carbene−CO coupling steps. Selected bond lengths are given in Å and bond angles in degrees. Gibbs free energy and ZPVE-corrected energy values (in parentheses) are in kcal/mol relative to Ni(CO)3 and the respective diazoalkanes.

3.2. Formation of Carbene and Ketene Complexes. It is generally assumed that metal carbenoids, formed from diazoalkanes by eliminating N2, are formed via the corresponding η1-C diazo adduct,39,40 although at least one example is known when N2 elimination takes place starting from a η2(C,N) diazo complex. Milstein and co-workers studied the reaction of diazoalkanes with PCP-Rh and PCN-Rh pincer complexes systematically. They found that the η1-N diazo complexes are side intermediates instead of essential intermediates for carbene formation.38 Dinitrogen extrusion takes place via transition states 3aTS and 3bTS, leading to the carbenoids containing methylidene (4a) and ethoxycarbonyl carbene (4b) ligands, respectively (Figure 3). The characteristic imaginary frequencies are 373i cm−1 for 3aTS and 368i cm−1 for 3bTS. Both transition states are closer in nature to the corresponding carbenoids than the diazo adducts, as reflected in the significantly shorter Ni−C distances. For 3aTS the Ni−C bond is contracted from 2.135 to 1.899 Å, whereas the C−N distance is increased from 1.339 to 1.886 Å by removing the N2 group. Similar changes were computed for the transition structure 3bTS, where the Ni−C distance is shortened from 2.200 to 1.917 Å while the length of the cleaving C−N bond is stretched from 1.335 to 1.884 Å. Both metal carbenoids 4a,b formed by N2 extrusion possess a large C−Ni−C bond angle between the carbene carbon and one of the carbonyl carbons (122 and 127°, respectively). The formation of the metal carbene complexes by dinitrogen extrusion is slightly endergonic, with reaction free energies of 0.3 and 1.5 kcal/mol for 4a,b, respectively. The Gibbs free energy barrier is notably lower for the reaction of diazomethane (16.9 kcal/mol) than for that of EDA (22.4 kcal/mol) with Ni(CO)3. Thus, the carbene formation step may be considered as reversible under certain reaction conditions. This reaction profile is in contrast with earlier findings with cobalt carbonyl complexes, where the formation of the carbene complex was found to be highly exergonic and irreversible in all cases.6,7,41 It is interesting to compare our free energy barrier for N2 elimination with that obtained for other d10 transition-metal complexes. Straub studied the cyclopropanation of ethane with diazomethane and found the N2 extrusion as the ratedetermining step. The free energy barrier is 17.2 kcal/mol when the B3LYP functional is employed, starting from the

concerted displacement of one of the CO ligands to diazoalkanes leads to η1-C diazo complexes 2a,b (Figure 2). The first step of the dissociative mechanism for the diazo coordination is the dissociation of one CO ligand, affording the coordinatively unsaturated Ni(CO)3 complex, which possesses a trigonal-planar structure with D3h symmetry. It was found to be barrierless with a reaction free energy of 12.8 kcal/mol. This value is somewhat lower than the experimental result of Basolo et al. (20.1 kcal/mol), with the total exclusion of the gas phase.22 Thus, the entropy is underestimated to some extent by the in vacuo calculations. The heterocumulene structure of diazoalkanes allows several types of coordination involving a single metal center or more metal centers.36 The coordination modes for monometallic complexes are displayed in Scheme 2. Although some examples are reported for dinuclear diazo complexes of late transition metals,37 here only the mononuclear coordination will be addressed. The coordination of diazomethane and EDA to the nickel tricarbonyl moiety was systematically examined, including all the coordination types shown in Scheme 2. The diazo ligand may act as a two- or four-electron donor, having single to triple bonds to metal in the η1-N complexes (types IIa−c). The optimized structures are depicted in Figure 2. No stable minimum structures were found for types III and IV, as all attempts at geometry optimization starting from a η2-(C,N) or η2-(N,N) structure ended up in a η1-C or η1-N arrangement, respectively. A local minimum was found, however, for the η2(N,C) metallacycle structure designated as 2aCNN, but its relative energy is too high in comparison to the η1 adducts (36.8 kcal/mol as opposed to 2a). Diazoadducts similar to 2aCNN, but with higher thermodynamical stability, have been described computationally by Milstein and co-workers for PCPRh pincer systems as a reaction intermediate for the rearrangement of the diazo adduct from the η1-N to the η1-C structure.38 The η1-C (2a) and η1-N (2aN) adducts of diazomethane are almost degenerate, whereas for the EDA complexes 2b lies higher in terms of free energy as opposed to 2bN. Further discussion regarding the various diazo adducts with Ni(CO)3 is presented in the Supporting Information. D


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Scheme 3. Formation of Nickel Carbenoid 4a via the Dimer Complex 3aD Following an Associative Mechanism

Pd(0) ethene diazomethane complex.42 Salvatella and coworkers reported the Cu(I) ethylene N,N′-dimethylmalonaldimine complex as the active catalyst for the Cu(I)-catalyzed cyclopropanation, which after a ligand exchange with methyl diazoacetate undergoes N2 extrusion with a free energy barrier of 21.2 kcal/mol.43 The dimer pathway involving a nickel diazomethane complex and the coordinatively unsaturated Ni(CO)3 was also taken into consideration and is depicted in Scheme 3. Here, the formation of a dimer complex, designated as 3aD, is expected, where diazomethane bridges two Ni(CO)3 moieties. The cleavage of the C−N bond then affords the nickel carbenoid 4a as well as the Ni(CO)3(η1-N2) complex. The free energy of activation for this process exceeds that for the mononuclear mechanism by 2.8 kcal/mol. Thus, the associative mechanism can be considered to be disfavored in this case. The computed structures related to the binuclear mechanism are given in Figure S3 in the Supporting Information. The binuclear pathway was also assumed to be unlikely for the carbenoid formation of rhodium pincer complexes with diazoalkanes.38 The nickel carbene complexes 4a,b were found to be, however, highly reactive in terms of intramolecular carbene− carbonyl coupling. This elementary step is fast and highly exergonic with barriers of 7.9 and 7.5 kcal/mol and with reaction free energies of −36.0 and −33.4 kcal/mol for the methylidene complex (4a) and the ethoxycarbonyl carbene complex (4b), respectively. As a result, ketene (6a) and ethoxycarbonylketene (6b) complexes are formed via transition states 5aTS, and 5bTS, respectively. The characteristic imaginary frequency is 317i cm−1 for the former and 221i cm−1 for the latter. Since this particular step is of fundamental importance in the catalytic ketene formation from diazoalkanes, the NPA charge distribution along the reaction coordinate for the carbene−carbonyl coupling was determined and is displayed in Figure 4. The unexpectedly high negative charge on nickel in complex 4a might be responsible for the relatively low stability of the nickel carbonyl methylidene complex in comparison to the corresponding cobalt complex. Consequently, a pronounced redistribution of electron density takes place during the ketene formation, which can be attributed to the large decrease in energy. The high charge concentration on the central Ni atom is somewhat distributed to all three carbonyl groups, resulting in a change in the partial charge of the carbonyl carbons from above 0.7 to below 0.6. Carbon C2, coupling with the CH2 group, shows a quite similar change in partial charge in comparison to the other two carbonyl carbons. The most significant transfer of electron density, however, takes place from nickel toward the C1 carbon belonging to the methylidene group, as its partial negative charge starts from −0.183 in complex 4a, increases to −0.520 in the transition

Figure 4. Intrinsic reaction coordinate (IRC) of the carbene−carbonyl coupling step of the carbene complex 4a, resulting in the ketene complex 6a via transition state 5aTS (top), and the variation of the NPA charges of selected atoms along the reaction coordinate (bottom).

structure 5aTS, and increases further up to −0.750 in the ketene complex 6a. Accordingly, the NPA charge of nickel changes from −0.945 (in 4a) to −0.313 (in 6a). 3.3. Formation of Coordinatively Saturated Ketene Complexes. The coordinatively unsaturated ketene complexes 6a,b are prone to take up one CO from the carbon monoxide atmosphere, affording the corresponding saturated ketene tricarbonyl complexes. The CO-coordination step was found to be exergonic with a reaction free energy of −10.4 and −9.0 kcal/mol toward ketene complex 8a and ethoxycarbonylketene complex 8b, via transition states 7aTS and 7bTS, respectively E


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Figure 5. Computed structure of transition states complexes 7aTS and 7bTS associated with the addition of CO as well as the coordinatively saturated ketene complexes 8a,b. NPA charges are written in italics. Gibbs free energy and ZPVE-corrected energy values (in parentheses) are in kcal/mol relative to Ni(CO)3 and the respective diazoalkanes.

Figure 6. Free energy profile of the diazo carbonylation reaction catalyzed by Ni(CO)3. Energies are given in kcal/mol (the higher and lower Grel values reflect the corresponding b and a derivatives, respectively).

(Figure 5). The characteristic imaginary frequency is 10 cm−1 for 7aTS and 32 cm−1 for 7bTS. This step proceeds via a low barrier: the free energy of activation is 6.6 kcal/mol for the ketene complex and 5.6 kcal/mol for the ethoxycarbonylketene case. The very low magnitudes of the imaginary frequencies are consistent with an Ni−C distance above 3 Å for the incoming carbonyl ligand. An analogous transition state was found for the binuclear cobalt carbonyl ethoxycarbonylketene complex with a significantly shorter metal−carbon distance and much higher free energy barrier (12.0 kcal/mol).7 The free energy profile of the entire reaction (Figure 6) reveals that the slowest elementary step is predicted to be the N2 extrusion/carbene forming step. Especially for ethyl diazoacetate the rate of the reaction tends to be slower than that for the cobalt-catalyzed reactions, in accord with experimental results.7 For the overall reaction the rate is predicted to be determined by the CO dissociation from Ni(CO)4 followed by the reaction of diazoalkane with the reactive intermediate Ni(CO)3, as illustrated in Scheme 4. The competition for Ni(CO)3 between CO and the diazoalkane is expressed by the ratio k1/k−1. The resulting nickel carbene complexes, however, are expected to undergo fast intramolecular CO−carbene coupling resulting in ketene complexes, which in the presence of CO quickly extend their coordination spheres with one carbonyl ligand and release free ketenes. Relaxed potential energy surface (PES) scans revealed that the liberation of free ketene or

Scheme 4. Reversible Dissociation of CO from Ni(CO)4 Followed by the Reaction with Diazoalkane

ethoxycarbonylketene proceeds in a barrierless manner in an exergonic reaction with free energy changes of 2.5 and 4.4 kcal/ mol, respectively. Thus, in vacuo, the equilibrium is predicted to be shifted to the right-hand side of this ligand dissociation reaction. However, similarly to the CO dissociation step from nickel tetracarbonyl, the entropy term might be overestimated for the ketene dissociation step as well, changing the reaction free energy profile to slightly endergonic. The dissociation of ketene or ethoxycarbonylketene from complexes 8a,b, respectively, regenerates complex 1, which may take up one CO or diazoalkane restarting the catalytic cycle. The donor−acceptor properties of ketene complexes 6a,b as well as the coordinatively saturated 8a,b have been elucidated within the framework of charge decomposition analysis (CDA) developed by Dapprich and Frenking.44 The results are compiled in Table 1. In the 16-electron complex 6a the ketene ligand donates 0.514 e to the Ni(CO)2 fragment, which is F


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negative C1 carbon, weakening and elongating the Ni−C1 bond. The difference in the strengths of ketene coordination is also reflected in complex geometries and Wiberg bond indices (WBIs). In the coordinatively unsaturated complexes 6a,b the ketene CC bond is notably elongated: from 1.306 to 1.374 Å and 1.395 Å for 6a,b, respectively, in comparison to free ketene (Figure 1). Moreover, the C−C−O bond angle is decreased to 157° in complex 6a and 153° in complex 6b. The smaller bond angle and greater bond length in the latter complex suggests a weaker CC bond in the coordinated ethoxycarbonylketene than in the bound ketene, in line with the stronger coordination ability of the former complex, as predicted by CDA calculations. This is also reflected in the WBIs, as the bond index for the CC bond is noticeably larger for 8a (1.505, as opposed to 1.387 for 6a). Upon addition of one CO, leading to complexes 8a,b, the carbon−carbon double bond is somewhat contracted (to 1.339 Å in 8a and 1.360 Å in 8b), the C−C−O bond angle is increased to 174 and 165°, and the WBIs are decreased to 1.282 and 1.371 Å, respectively; thus, the ethoxycarbonylketene ligand is more strongly bound in the coordinatively saturated ketene complex as well. The bonding situation in nickel ketene complexes 6a and 8a, and the weaker coordination of ketene in the latter complex, can be rationalized by employing the natural bond orbital (NBO) method. The occupancy of a Lewis-type NBO is usually less than 2 e, which is a consequence of delocalization of the electron pair into adjacent non-Lewis acceptor orbitals. Natural localized molecular orbitals (NLMOs) are semilocalized orbitals that can be derived from their strictly localized parent NBOs extended with the associated “delocalization tails” in order to describe the density of full electron pairs.45 The “delocalization tails” can be accurately approximated by low-

Table 1. Charge Decomposition Analysis (CDA, Rows 1−5) and Wiberg Bond Indices (WBIs) for the Carbon−Carbon Double Bonds of the Ketene Complexes 6a,b and 8a,ba donation (d) back-donation (b) repulsive polarization (r) residual term (Δ) interaction energy WBICC a

6a

8a

6b

8b

0.514 0.348 −0.112 −0.036 −48.7 1.387

0.381 0.156 −0.109 −0.013 −21.4 1.505

0.530 0.352 −0.113 −0.044 −50.1 1.282

0.427 0.210 −0.127 −0.021 −23.4 1.371

Interaction energy values are given in kcal/mol.

exceeded by the ethoxycarbonylketene ligand in 6b donating 0.530 e. The amount of back-donation from the metal to the ketene fragment is not particularly influenced by the substituent on ketene, as it is about 0.35 e for both 6a and 6b. The interaction energy difference is 1.4 kcal/mol in favor of 6b, which is in line with the stronger donation. The stronger donating ability and hence the higher interaction energy associated with the ethoxycarbonylketene ligand compared to unsubstituted ketene can be observed in the case of the coordinatively saturated ketene complexes 8a,b. However, unlike the case for 6a,b, the amount of back-donation is also stronger for the ethoxycarbonylketene complex (0.210 e for 8b as opposed 0.156 e for 8a). Nonetheless, the interaction energy, as well as the donation and back-donation, is weaker for the saturated complexes. This might be rationalized by the competing effect of the excess carbonyl ligand with pronounced σ-donation and π-back-donation capability. The coordination of the CO increases the negative charge of Ni (see Figures 4 and 5), resulting in an electrostatic repulsion with the highly

Figure 7. Natural localized molecular orbitals (NLMOs) for the leading donating interaction (left) and NLMOs describing the electron backdonation from the Ni(CO)2 or Ni(CO)3 moiety to the ketene ligand in complex 6a or 8a, respectively. In the second and fourth rows the leading donor−acceptor interactions are shown between the parent NBOs interacting with acceptor NBOs. Second-order donor−acceptor interaction energies are given in kcal/mol. G


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carbonyl ligand is a significantly stronger interaction in comparison to that in complex 6a, with about twice as much interaction energy (15.1 kcal/mol). Thus, the excess carbonyl ligand in 8a competes successfully with the ketene ligand in terms of electron withdrawal from the appropriate lone pair of nickel, which results in a notably weaker ketene coordination in the case of the coordinatively saturated ketene complexes. 3.4. Mechanism for the Formation of Azine. The formation of azines from diazoalkanes and carbenes or metal carbenoids is a well-known side reaction which can compete with the carbonylation step of diazocarbonylation.12,47 Benzophenone azine (Ph2CNNCPh2) is the major product in some cases starting from diphenyldiazomethane, whereas usually no azine formation takes place in the case of other diazoalkanes.2,3,11,12,41 Although a detailed study regarding the azo coupling side reaction is beyond the scope of this work, this step has also been examined computationally in order to give some energetic comparison with the ketene formation step. The reaction of diazomethane with complex 4a follows a concerted pathway via the transition structure 9TS (with a characteristic single imaginary frequency of 388i cm−1), leading to the azine complex 10, which possesses a η2-(C,N) coordination (Figure 8). The formation of 10 takes place with a reaction free energy

order perturbation theory, providing useful estimates for the delocalization (charge transfer) effects associated with the energetics and composition of NLMO formation. The most important NLMOs, related to the π-donor and πacceptor interactions of the ketene ligand in 6a and 8a, are depicted in Figure 7. The NLMO, associated with the π-donor interaction of ketene in the coordinatively unsaturated 6a, resembles its parent πCC NBO with a contribution of 91%. The deviation in shape can be attributed to a strong (80.7 kcal/ mol) interaction between the filled πCC NBO and the unfilled one-center orbital of nickel (n*Ni). Interestingly, the NLMO related to the back-donation from metal to the ketene ligand reveals a strong visual difference from its parent NBO, which is a filled one-center orbital (lone pair), with a contribution of 80%, formed mainly from the dx2−y2 atomic orbital of Ni. The dissimilarity is caused by two leading donor−acceptor interactions: one with the π*CC non-Lewis orbital of the ketene ligand with a stabilization energy of 41.1 kcal/mol and one with the π*CO orbital of the carbonyl ligand adjacent to the oxygen atom of ketene with a stabilization energy of 7.8 kcal/mol. Thus, the ñNi NLMO, loosely associated with the dx2−y2 atomic orbital, represents an electron pair involved in two simultaneous back-donating interactions, increasing the bond strength between nickel and the corresponding carbonyl ligand and resulting in a noticeable contraction of the respective Ni−C bond. A minor back-donating interaction from the fragment Ni(CO)2 to the fragment OCCH2 results from the lone pair on of the carbonyl ligand adjacent to the ketene oxygen establishing a σ coordinative bonding with Ni. This parent NBO has 85% contribution from the ñC NLMO with an interesting delocalization tail caused by the nC → π*CC interaction with a 6.9 kcal/mol stabilization energy. An nC → π*CC interaction is predicted with a stabilization energy of 5.6 kcal/mol, but the overlap between the lone pair of the carbonyl group and the π*CC orbital of ketene is not sufficient to cause a notable distortion in the shape of the corresponding NLMO. The striking difference in the behavior of the σ-donating lone pairs of the two carbonyl ligands may be explained by the fundamental difference in partial charge of the two carbons of the ketene ligand. The highly electrophilic carbonyl carbon is prone to take some electron density from the adjacent ligand, thus establishing a back-donation interaction which takes place bypassing the central metal atom. A similar interaction was recently reported for the complex Ni(PH3)2(η2-CO2), where the donor pair of one of the phosphine ligands exhibited a charge transfer interaction with the highly positive carbon atom of the carbon dioxide ligand.46 An inspection of the analogous electron pairs, represented by NLMOs for the coordinatively saturated complex 8a, shows a remarkable difference for both donation and back-donation between the Ni(CO)3 and ketene fragments. The pair associated with the π-donor interaction from ketene to Ni is very reminiscent in shape of the analogous NLMO for 6a; however, the donor−acceptor stabilization energy is only 33.4 kcal/mol for the saturated case. This pair is built up mainly from the πCC NBO with a contribution of 93% and distorted by the πCC → n*Ni interaction. The resulting nNi orbital overlaps to a smaller degree with the π*CC non-Lewis NBO of ketene than does the corresponding one-center orbital in complex 6a, with an interaction energy of only 4.3 kcal/mol. On the other hand, the back-donation of the same one-center NBO to the π*CO NBO of the in-plane

Figure 8. Addition of diazomethane to the carbenoid 4a, resulting in the complex Ni(CO)3(H2CNNCH2).

of −48.6 kcal/mol, indicating that this step is even more exergonic than the intramolecular carbonyl−carbene coupling. However, the high free energy barrier (35.3 kcal/mol) makes this process strongly disfavored kinetically. This is in line with the experimental observation that azine is not formed in the reaction of diazomethane with nickel carbonyls.12 3.5. Reaction Mechanism for the Ni(CO)2(PH3) Precursor. It is well-known that nickel tricarbonyl phosphine complexes form rapidly upon addition of phosphine to a nickel tetracarbonyl solution via displacement of one CO ligand by phosphine. The contracted Ni−C bond in comparison to that in Ni(CO)4 implies carbonyl groups more strongly bound than in nickel tetracarbonyl. Indeed, the computed free energy of carbonyl ligand dissociation is 17.5 kcal/mol, which is higher by 4.7 kcal/mol than that for the Ni(CO)4 → Ni(CO)3 + CO process. The resulting complex Ni(CO)2(PH3) (11) possesses Cs symmetry with shorter Ni−P and Ni−C bonds in comparison to those in Ni(CO)3(PH3) (Figure 9). The coordination of diazomethane to complex 11 may result in either η1-C or η1-N adducts, the latter being somewhat more stable thermodynamically. It is interesting to note that all three η1-C conformers (2c1−3) are virtually degenerate in terms of free energy and the thermodynamic stability of the η1-N adducts (2cN1,2) also does not seem to depend on the conformation of the diazomethane ligand. Various EDA H


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Figure 9. Computed structures of Ni(CO)3(PH3), Ni(CO)2(PH3) (11), as well as the PH3-substituted complexes coordinating diazomethane or EDA. Gibbs free energy and ZPVE-corrected energy values (in parentheses) are in kcal/mol relative to complex 11 and the respective diazoalkanes.

Figure 10. Computed structures of PH3-substituted nickel carbenoids (4) and the transition states (3TS) describing the dinitrogen extrusion step. Selected bond lengths are given in Å, and imaginary frequencies for transition states are given in cm−1. Gibbs free energy and ZPVE-corrected energy values (in parentheses) are in kcal/mol relative to Ni(CO)2(PH3) and the respective diazoalkanes. NPA charges are given in italics.

complexes with all three coordination modes (η1-C, η1-N, and η1-O) were obtained; however, their rotamers reveal some difference in relative stability. Among the η1-C complexes (2d1,2) complex 2d2 is preferred with a 180° P−Ni−C−N dihedral angle. Among the η1-N species (2dN1,2) the coplanar arrangement of the ethoxycarbonyl group with the Ni−P bond is less favored by 0.9 kcal/mol. The η1-O adduct 2dO1 with a P−Ni−O−C dihedral angle of 180° is somewhat more stable (by 0.7 kcal/mol) than 2dO2, probably due to steric effects.

Thus, it can be concluded, regarding the diazo coordination step for the PH3-substituted complexes, that η1-N coordination is preferred, as the formation of both diazomethane and EDA complexes is slightly exergonic. This behavior is in contrast to that of the unsubstituted cases, where the η1-N EDA adduct 2bN is more stable than 2b, but the η1-C diazomethane complex 2a is almost degenerate with the η1-N complex 2aN. The computed structures of diazo adducts with various coordination modes are depicted in Figure 9. I


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Figure 11. NLMOs (a, d), their parent NBOs (b, e), and the NHOs decomposed from the parent NBOs (c, f) for the metal−carbon π bonds of methylidene complexes Ni(CO)3(CH2) (4a) and Ni(CO)2(PH3)(CH2) (4c2).

Figure 12. Computed structures of species related to the carbene−CO coupling step. Selected bond lengths are given in Å, and imaginary frequencies for transition states are given in cm−1. Gibbs free energy and ZPVE-corrected energy values (in parentheses) are in kcal/mol relative to Ni(CO)2(PH3) and the respective diazoalkanes. NPA charges are given in italics.

As a result of the dinitrogen extrusion step nickel carbenoids 4c1,2 and 4d1,2 are formed. In 4c1 and 4d1 the carbene carbon adopts a perpendicular disposition with regard to the P−Ni−Ccarbene plane, whereas the arrangement of the Ccarbene atom is parallel in complexes 4c2 and 4d2. The parallel disposition results in somewhat more stable geometries: for the methylidene complexes the free energy difference is 2.1 kcal/ mol in favor of 4c2, while 4d2 is more stable than 4d1 by 2.2 kcal/mol. A remarkable difference between pathways involving the catalyst Ni(CO)2(PH3) and those involving Ni(CO)3 is that the reaction with diazoalkanes is exergonic for the former ones whereas it is slightly endergonic for the homoleptic complex. Taking into account the preferred pathways with the more stable conformers (that is, via adducts 2c2 and 2d2) the reaction free energy is −4.4 and −3.7 kcal/mol, leading to 4c2 and 4d2, respectively. The increased stability of 4c2 as opposed to 4a was investigated by employing the NBO methodology. The resulting depictions for the π bond of these methylidene complexes are given in Figure 11. The corresponding electron pairs are represented by the NLMOs. Also given are their parent NBOs as well as the one-center hybrids (NHOs) decomposed from the NBOs. For 4a, the hybrid orbital forming the πNi−C bond from the nickel side is an almost equal composition of atomic orbitals dz2 and dyz. The overlap between the individual NHOs can be estimated in terms of the corresponding preorthogonal NHO overlap integral S = ⟨h1|h2⟩ by a Mulliken-type approximation.48 It is revealed that the overlap is stronger (S = 0.155) between the respective NHOs between Ni and C in 4c2 than those in complex 4a (S = 0.146). The stronger π bond results in a higher overall bond order for

The N2 extrusion leading to metal carbenoids takes place via transition states 3cTS1,2 for the diazomethane complex and 3dTS1,2 for the EDA complex, respectively (Figure 10). Although the species 3cTS3, with a P−Ni−C−N dihedral angle of 180°, was expected to be a transition state, describing the cleavage of the C−N bond in the adduct 2c3, vibrational analysis resulted in two imaginary frequencies. Further calculations showed that this is in fact a second-order saddle point with the two imaginary frequencies remaining (377i and 52i cm−1). In comparison to the unmodified transition structures 3aTS and 3bTS the phosphine-substituted analogues feature somewhat shorter C−N and Ni−C distances. The two remaining transition states, originating from the diazomethane adducts 2c1 and 2c2, differ mainly in the P−Ni−N−C dihedral angle. The gauche structure 3cTS2 is lower in free energy by 1.2 kcal/mol than 3cTS1, which possesses an eclipsed conformation. N2 extrusion may take place following two possible pathways for the phosphine-substituted EDA complexes as well. Unlike the diazomethane case, the transition structure 3dTS2 with the PH3 and the N2 moieties in anti positions is lower in free energy than the gauche transition state 3dTS1, although the free energy difference is only 0.6 kcal/mol. Thus, the formation of both the methylidene and ethoxycarbonyl carbene complexes proceeds via smaller activation barriers for the phosphinesubstituted catalyst 11 than for the homoleptic complex 1. It should be noted, however, that if complex 2dN2 is taken into account as a resting state, the free energy barrier for loss of N2 increases to 22.5 kcal/mol via the lower energy pathway involving transition state 3dTS2. This barrier is almost equal to that for the N2 extrusion starting from 1 and EDA via transition structure 3bTS. J


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Figure 13. Computed structures of transition states complexes 7cTS1,2 and 7dTS1,2 associated with the addition of CO as well as the coordinatively saturated ketene complexes 8c1,2 and 8d1,2. Selected bond lengths are given in Å, and imaginary frequencies for transition states are given in cm−1. Gibbs free energy and ZPVE-corrected energy values (in parentheses) are in kcal/mol relative to Ni(CO)2(PH3) and the respective diazoalkanes.

that the σ-donating effect of PH3 results in a higher negative charge on the CH2 carbon, a slightly lower positive charge on the CO carbon of the ketene ligand, and a somewhat lower charge concentration on Ni. The replacement of CO by PH3 results in a 4° decrease in the C−C−O bond angle and an increase by about 0.08 Å in the Ni−Ccarbonyl bond lengths and an increase by 0.02 Å in the Ni−CCHR distances. This suggests an increase in the coordination strength of the ketene ligands, which is also corroborated by CDA calculations (vide infra). The free energy profile of the CO uptake of the coordinatively unsaturated ketene complexes 6c1,2 and 6d1,2 resembles those of complexes 6a,b. The barrier is slightly higher for all cases: 7.2 and 7.0 kcal/mol for the pathways via 7cTS1,2, respectively, as opposed to 6.6 kcal/mol for the unsubstituted ketene complex (Figure 13). The reaction is exergonic in all cases, with reaction free energies of −10.2, −9.0, −10.0, and −7.9 kcal/mol leading to the saturated ketene complexes 8c1,2, and 8d1,2, respectively. Thus, this elementary step can also be considered as irreversible. The geometries of complexes 8c1,2 and 8d1,2 exhibit some major changes in comparison to those of 8a,b. For instance, when the analogous complexes 8a and 8c2 are compared, shorter Ni−C bond distances can be observed for the latter species, whereas the former species possesses a shorter C−C bond (1.339 Å as opposed to 1.365 Å) and larger C−C−O angle (174° as opposed to 161°), suggesting a more weakly bound ketene ligand. A significant increase in the coordination strength of the ketene ligands resulting from PH3 substitution can be confirmed by CDA calculations. The changes in the interaction energy of complexes 6c2, 8c2, 6d2, and 8d2 in comparison to 6a, 8a, 6b, and 8b are 14.4, 12.5, 15.5, and 14.3 kcal/mol, respectively, in favor of the PH3-substituted complexes. The stronger coordination is associated with stronger donation and back-donation as well, since the more negative nickel atom, accepting electron density from PH3, forms a stronger backdonating interaction with the π*CC orbital with the ketene ligand contracting the Ni−C bonds and allowing a higher overlap between the πCC and n*Ni orbitals. The change is especially noteworthy for the coordinatively saturated com-

4c2, as corroborated by the Wiberg bond indices (WBIs) calculated on the NAO basis. The WBI is thus 0.768 for 4a and 0.843 for 4c2, in line with the difference in Ni−C bond lengths, which are 1.807 and 1.793 Å for 4a and 4c2, respectively. The more effective πNi−C transmits somewhat more electron density from nickel to carbon, allowing a more even electron density distribution with a lower partial charge on nickel (−0.844 as opposed to −0.945) and a higher negative charge on the carbene carbon (−0.247 as opposed to −0.183). In addition to the higher thermodynamic stability, complex 4c2 is slightly more reactive toward intramolecular carbene− CO coupling than 4c1, as the free energy barrier via transition state 5cTS2 is 7.0 kcal/mol as opposed to 7.7 kcal/mol for 4c1 via transition state 5cTS1 (Figure 12). Both barriers, however, are somewhat smaller than that starting from the unsubstituted complex 4a: that is, the phosphine substituent with pronounced σ-donor capability increases the rate of the ketene formation step. The accelerating effect of the phosphine, however, is significantly stronger for the ethoxycarbonyl carbene complex. For the two reaction channels starting from carbenoids 4d1 and 4d2 the barriers via 5dTS1 and 5dTS2 are 4.5 and 3.4 kcal/ mol, respectively. The carbene−CO coupling step results in ketene complexes 6c1,2 and ethoxycarbonylketene complexes 6d1,2 with reaction free energies of −34.7, −33.6, −33.5, and −32.3 kcal/mol, respectively, which are somewhat lower than those for the unsubstituted ketene complexes 6a,b. For both ligands there is a clear preference for the phosphine ligand oriented in a trans position with respect to the carbonyl group in ketene or ethoxycarbonylketene. The difference in thermodynamic stability is 1.0 kcal/mol in favor of the trans isomer for both ketene ligands. The intermediate and transition structures associated with the carbene−CO coupling step for the phosphine-substituted case are depicted in Figure 12. The somewhat higher stabilities of 6c2 and 6d2 in comparison to those for 6a,b, respectively, may be explained by the stronger coordination of the ketene ligands as well as the somewhat more even electron density distribution for the phosphine-substituted complexes as interpreted by NPA charges. When 6c2 is compared to 6a, it can be concluded K


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bond and coordinatively unsaturated ketene complexes with the phosphine in a trans position. Thus, the predicted pathways for the two substrates that are the subjects of this study are 11 → 2c2 → 3cTS2 → 4c2 → 5cTS2 → 6c2 → 7cTS2 → 8c2 → 11 + ketene and 11 → 2d2 → 3dTS2 → 4d2 → 5dTS2 → 6d2 → 7dTS2 → 8d2 → 11 + ethoxycarbonylketene. 3.6. Reaction Mechanism for the (dtbpe)Ni(CO) Precursor. Several Ni(0) complexes containing the diphosphine dtbpe (dtbpe = 1,2-bis(di-tert-butylphosphino)ethane) have been prepared by Hillhouse and co-workers.49 Among them, the diazo and carbene complexes are most relevant from the diazo carbonylation point of view.50,51 With diphenyldiazomethane a stable η2-(N,N) diazo complex was prepared, which was converted to the corresponding carbene complex.51 In the presence of CO a rapid conversion of the complex (dtbpe)NiCPh2 was reported, affording the coordinatively saturated dicarbonyl complex (dtbpe)Ni(CO)2 and diphenylketene. The ketene complex (dtbpe)Ni(OCCPh2) was prepared in an independent experiment and reacted with CO in a facile reaction, giving diphenylketene. Thus, most intermediates related to the diazo carbonylation reaction were prepared; therefore, it is interesting to compare the elementary steps computationally for the Ni(0)−dtbpe system with those starting from complexes 1 or 11. Because of the bulky diphosphine ligand here the two-layer ONIOM method was employed utilizing the DFT level on both layers. The local minima and transition state structures of the preferred pathways are collected in Figure 15; the Gibbs free energy diagram is presented in Figure 16. The catalytically active species of this cycle is the coordinatively unsaturated (dtbpe)Ni(CO) (12), which is formed after CO dissociation from the saturated (dtbpe)Ni(CO)2 with a reaction free energy of 18.0 kcal/mol. Presumably due to steric reasons the η1-C diazo adducts 2e,f loosely coordinate diazomethane or EDA to the nickel center with free energy increases of 12.3 and 8.9 kcal/mol, respectively. Thus, for the determination of the barrier for N2 elimination, the free energy difference is taken between the transition states and the

plexes. The difference in donation character is up to 0.106 e on comparison of 8a and 8c2 and 0.07 e on comparison of 8b and 8d2, respectively, whereas the difference in back-donation is 0.096 e in 8a and 8c2 and 0.072 e in 8b and 8d2 (compare Tables 1 and 2). Table 2. Charge Decomposition Analysis (CDA, Rows 1−5) and Wiberg Bond Indices (WBIs) for the Carbon−Carbon Double Bonds of the Ketene Complexes 6c,d and 8c,da donation (d) back-donation (b) repulsive polarization (r) residual term (Δ) interaction energy WBICC a

6c2

8c2

6d2

8d2

0.528 0.376 −0.134 −0.038 −63.1 1.331

0.487 0.252 −0.152 −0.022 −33.9 1.390

0.549 0.385 −0.136 −0.038 −65.6 1.221

0.500 0.282 −0.165 −0.026 −37.7 1.263

Interaction energy values are given in kcal/mol.

The dissociation of the ketene and ethoxycarbonylketene ligands from the Ni(CO)2(PH3) moiety is still predicted to be exergonic, but to a smaller extent than that from the Ni(CO)3 fragment. The slope of the overall reaction profile starting from complex 11 is quite close to that originating from complex 1 (Figure 14). The slowest step is N2 extrusion, with somewhat lower barriers for the PH3-substituted cases. It is followed by the facile carbene−carbonyl coupling, which is also notably faster in the presence of the PH3 ligand. The uptake of one external CO, resulting in coordinatively saturated ketene complexes, is still very fast. The dissociation of the ketene ligands affords ketene or ethoxycarbonylketene and complex 11, which is ready to react with a new substrate molecule or with CO, resulting in Ni(CO)3(PH3). Similarly to the unsubstituted case, relaxed PES scans proved that the dissociation step proceeds in a barrierless manner. The more plausible pathways involve carbenoids with a coplanar arrangement of the alkylidene groups with the Ni−P

Figure 14. Free energy profile of the diazo carbonylation reaction catalyzed by Ni(CO)2(PH3). Energies are given in kcal/mol (the Grel values in gray and in black refer to the corresponding d and c derivatives, respectively). L


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Figure 15. Computed minimum structures and transition states associated with the diazo carbonylation promoted by (dtbpe)Ni(CO). Selected bond lengths are given in Å, and imaginary frequencies for transition states are given in cm−1. Relative Gibbs free energy and ZPVE-corrected energy values (in parentheses) are in kcal/mol relative to (dtbpe)Ni(CO) and the respective diazoalkanes.

reactants separated. The activation free energies are 24.3 and 28.1 kcal/mol for the reaction of 12 with diazomethane and EDA via transition states 3eTS and 3fTS, respectively, predicting a notably slower dinitrogen extrusion step in comparison to that for complexes 1 and 12. Interestingly, the reaction free energy shows some difference for the two substrates. The N2 extrusion step for diazomethane is endergonic by 3.9 kcal/mol, whereas it is slightly exergonic by −1.0 kcal/mol for EDA. The carbenoids 4e,f are prone to intramolecular carbene− CO coupling via 5eTS and 5fTS with very low barriers (2.8 and 0.6 kcal/mol), resulting in ketene complexes 6e,f, respectively. Presumably, the strongly basic character of the dtbpe ligand is responsible for the pronounced increase in reaction rate of this elementary step. Complexes 6e,f undergo CO uptake, resulting in the coordinatively saturated adducts 8e,f, with reaction free energies of −7.4 and −1.8 kcal/mol, respectively. The magnitudes of the free energy barriers remain very similar to

those computed for the carbonyl- and PH3-substituted complexes: namely, 7.4 kcal/mol for the ketene and 6.8 kcal/ mol for the ethoxycarbonylketene complex. Thus, the bulky substituents on the diphosphine ligand have an influence on reaction energetics only in the case of the pathway 6f → 7fTS → 8f, as the reaction free energy is noticeably less exergonic in comparison to that for the analogous pathways. The free energy and enthalpy profiles of the reaction, for all the catalysts discussed within this study, are compiled in Table 3.

4. CONCLUSION We have carried out a computational study on the carbonylation of diazomethane and ethyl diazoacetate in the presence of nickel carbonyl catalysts employing the PBE0 functional in combination with Ahlrichs’ triple-ζ basis set. As catalytically active species, complexes Ni(CO)3 (1), Ni(CO)2(PH3) (11), and (dtbpe)Ni(CO) (12) were taken into consideration. The elementary steps of the catalytic reaction, i.e., diazo M


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Figure 16. Free energy profile of the diazo carbonylation reaction catalyzed by Ni(CO)(dtbpe). Energies are given in kcal/mol (the Grel values in black and in gray refer to the corresponding e and f derivatives, respectively).

Table 3. Comparison of the Individual Steps of Diazo Carbonylation Catalyzed by Nickel−Carbonyl Complexesa N2 extrusion

a

carbene−CO coupling

ketene eliminationb

CO coordination

cat.

substrate

ΔG

ΔG⧧

ΔH

ΔH⧧

ΔG

ΔG⧧

ΔH

ΔH⧧

ΔG

ΔG⧧

ΔH

ΔH⧧

ΔG

ΔH

1 1 11 11 12 12

N2CH2 EDA N2CH2 EDA N2CH2 EDA

0.3 1.5 −4.4 −3.7 3.9 −1.0

16.9 22.4 15.7 20.5 24.3 28.1

−0.7 0.6 −3.9 −5.2 0.6 −4.6

6.7 11.7 6.6 10.6 11.3 13.3

−36.0 −33.4 −33.6 −32.4 −38.8 −41.1

7.9 7.5 7.0 3.4 2.8 0.6

−37.1 −34.6 −34.8 −31.4 −39.6 −43.1

6.3 5.6 4.8 1.2 2.4 1.4

−10.4 −9.0 −9.0 −7.8 −7.4 −1.8

6.6 5.6 7.0 6.5 7.4 6.8

−20.0 −19.0 −19.3 −18.7 −17.2 −12.4

0.5 0.6 0.5 0.5 1.2 0.8

−2.5 −4.4 −1.6 −1.4 −6.3 −1.4

8.7 7.4 8.9 9.3 7.0 11.9

Gibbs free energy and enthalpy values are given in kcal/mol. bThe ketene elimination step is barrierless in all cases.

coordination, N2 extrusion, intramolecular carbene−carbonyl coupling, CO coordination, and ketene elimination, were investigated. The formation of the catalytically active species by CO dissociation followed by N2 extrusion leading to nickel carbene complexes was predicted to be rate determining for all cases, the reaction of diazomethane with 11 being the most facile, proceeding with a free energy barrier of 15.7 kcal/mol. In principle, the reaction of EDA with 11 is also faster than that with 1; however, the thermodynamically stable η1-N complex 2cN1, as a resting state, may eliminate the difference in the Gibbs free energy of activation for the two precursors. Nonetheless, the dinitrogen extrusion step is predicted to be exergonic starting from complex 11, whereas it is slightly endergonic for the homoleptic complex 1. For (dtbpe)Ni(CO) this step is endergonic for diazomethane and exergonic for EDA. A stronger π bond between nickel and the carbene carbon accounts for the higher thermodynamic stability of the metal carbenoids containing the PH3 ligand. The intramolecular CO−carbene coupling is fast and irreversible. For pathways via transition states 5dTS2, 5eTS, and especially 5fTS particularly low barriers were detected. A significant charge redistribution, starting from metal carbenoids and ending up in ketene complexes, may explain the highly exergonic reaction profile for this step. In principle, the side reaction toward the formation of azine may compete with CO;

however, the reaction is not predicted to take place for diazomethane or EDA due to the large free energy barrier. The coordinatively unsaturated ketene complexes extend their coordination sphere with one carbonyl ligand from the CO atmosphere, resulting in coordinatively saturated ketene complexes in a practically irreversible reaction. The excess carbonyl ligand competes successfully with the ketene ligand in terms of electron withdrawal from the appropriate lone pair of nickel, resulting in a weaker coordination of the ketene ligands. Thus, the dissociation of ketene from the nickel center takes place in a barrierless reaction, regenerating the catalytically active complexes 1, 11, and 12, which might be in equilibrium with their coordinatively saturated counterparts after taking up CO. Further experimental and theoretical work is in progress in our group in order to investigate the effect of various P-donor ligands upon the rate of the reaction.

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S Supporting Information *

Figures S1−S3 giving additional structures, text giving some extension to the diazo coordination step, and tables giving the coordinates of optimized structures and their relative energies. This material is available free of charge via the Internet at http://pubs.acs.org. N


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AUTHOR INFORMATION

Corresponding Author

*Tel: +36-72-501-500/24585. Fax: +36-72-501-518. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Hungarian Academy of Sciences and the Hungarian Scientific Research Fund for financial support under Grant Nos. OTKA NK 71906 and OTKA-NKTH CK 78553 and the support of the Developing Competitiveness of Universities in the South Transdanubian Region (SROP4.2.1.B-10/2/KONV-2010-0002) project as well as the Supercomputer Center of the National Information Infrastructure Development (NIIF) Program.

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