The gas-phase reactions of metal porphyrins with diazoacetate esters

International Journal of Mass Spectrometry 316–318 (2012) 68–75

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The gas-phase reactions of metal porphyrins with diazoacetate esters Jamal T. Aldajaei, Scott Gronert ∗ Department of Chemistry, Virginia Commonwealth University, Richmond, VA 23229, USA

a r t i c l e

i n f o

Article history: Received 27 October 2011 Received in revised form 19 December 2011 Accepted 21 December 2011 Available online 27 December 2011 Keywords: Gas-phase reaction Porphyrin Carbene Catalyst Quadrupole ion trap

a b s t r a c t The Mn(III), Fe(III) and Co(III) complexes of tetraphenylporphyrin were allowed to react with ethyl and t-butyl diazoacetate in an ion trap mass spectrometer. The manganese system produces only adducts, but the iron and cobalt systems give addition with loss of N2 to produce carbene-like species. All the reactions are fast and approach the collision-controlled limit. Fragmentation of the iron and cobalt carbene species follow three major pathways: (a) alkene loss from the ester to give a carboxylic acid (which can subsequently decarboxylate to give CH2 complexed to the metal porphyrin), (b) homolytic cleavage of the ester O R bond with loss of CO2 and an alkyl radical to produce CH complexed to the metal porphyrin, and (c) alcohol loss to give C C O complexed to the metal porphyrin. Computational data from density functional theory (B3LYP) are consistent with the observed reactivity trends and indicate that all the carbene complexes prefer a M N insertion structure where the carbene carbon bonds to the metal and one of the porphyrin nitrogens (metal nitrogen bond is lost). The M N insertion structures are generally more than 25 kcal/mol more stable than the conventional metal carbene structures, M C, at the B3LYP level and should dominate their reactivity. © 2012 Elsevier B.V. All rights reserved.

1. Introduction The gas phase has proven to be a useful medium for probing catalytic systems, particularly those that involve transition metal centers. An advantage of the gas phase is that highly reactive intermediates can be isolated in the inert atmosphere of a mass spectrometer and their properties can be probed. This has been demonstrated in many systems in recent years [1–10]. In the present contribution, we report a detailed study of the reactions of a series of metal porphyrins with diazoacetate esters. This is a common protocol for the formation of metal carbene species that have been widely used in synthesis as agents for cyclopropanations and C H insertion reactions (Scheme 1) [11–25]. A two-step mechanism for the formation of the metal carbene is shown in Scheme 2. Depending on the metal, the first or second step has been reported as being rate-determining [2,26]. Although other ligand systems are often used in these synthetic applications, the same chemistry is seen in metal porphyrin systems and the simple, rigid ligand is useful for model studies. Here, Mn(III), Fe(III) and Co(III) complexes of tetraphenylporphyrin are allowed to react with ethyl and t-butyl diazoacetate in a quadrupole ion trap mass spectrometer (Scheme 3). This is an extension of an earlier study focused on the reactions of cobalt salens with these same diazo reagents [2]. Rate constants have been measured as well as product distributions. The

∗ Corresponding author. Tel.: +1 804 828 8551; fax: +1 804 828 8599. E-mail address: [email protected] (S. Gronert). 1387-3806/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.ijms.2011.12.012

products of the reactions have been subjected to collision-induced dissociation and fragmentation pathways have been characterized. In addition, density functional theory has been applied to the analogous reactions of a more tractable model system, diazoacetic acid reacting with an unsubstituted metal porphyrin. The results provide insights into the impact of the metal on the reaction mechanism as well as its impact on the fragmentation pathways of the reaction products. Finally, the computational work reveals that the carbenes derived from the metal porphyrins can collapse to give more stable complexes where the carbene carbon bridges between the metal center and a nitrogen on the porphyrin ligand. This has been seen in the past in the carbenes of metal porphyrins [27–32] and there also have been more recent reports in porphyrins [14] and related systems [2]. 2. Results 2.1. Gas-phase experiments 2.1.1. Reactions with diazoacetates In Fig. 1, representative spectra are given for the reactions of the Mn(III), Fe(III) and Co(III) tetraphenylporphyrins with ethyl diazoacetate. With manganese (Fig. 1a), the reaction leads to an adduct between the diazoacetate and the metal complex – there is no evidence of N2 loss leading to the formation of a metal carbene. In contrast, the iron porphyrin (Fig. 1b) readily reacts with ethyl diazoacetate by addition with loss of N2 to nominally give a carbene product. In addition, there is some adduct formation.

J.T. Aldajaei, S. Gronert / International Journal of Mass Spectrometry 316–318 (2012) 68–75

N2

O

Ph

+n O M CHC OR

N2CHC OR

Ph N

N2CHCO2R

N

R = CH2CH3, C(CH3)3

R'–H

or H CO2R C

Reagents

N M(III) N +n M

69

Ph

Ph H

M = Mn, Fe, Co

R' CHCO2R Scheme 3. Reaction systems. Scheme 1. Catalytic cyclopropanations and C H insertions with diazoacetates.

Table 1 Rate constants for the reactions of metal tetraphenylporphyrins with diazoacetates.a Metal

Mn Fe Co

2.0E+06 1.8E+06

k

(a)

1.6E+06

Ethyl diazoacetate

t-Butyl diazoacetate

1.4E+06

9.87 × 10−10 ± 0.4% 1.39 × 10−9 ± 5.0% 2.14 × 10−9 ± 2.9%

6.41 × 10−10 ± 3.7% 1.18 × 10−9 ± 2.6% 1.49 × 10−9 ± 2.9%

1.2E+06

Rate constants in cm3 mol−1 s−1 . Uncertainties are a single standard deviation and only reflect precision. Systematic factors lead to uncertainties of approximately ±15% in the absolute rates. ADO collision-rate estimate is 1.1 × 10−9 cm−3 mol−1 s−1 for all the reactions [33]. a

1.0E+06 8.0E+05 6.0E+05 4.0E+05 2.0E+05 0.0E+00 600 620 640 660 680 700 720 740 760 780 800 820 840 860 880 900

m/z

Finally, a secondary reaction is evident in the spectrum and involves a second addition of the diazoacetate with loss of N2 . Finally, the cobalt species (Fig. 1c) cleanly produces the nominal metal carbene with little impact from secondary processes and no evidence of an adduct. It should be noted that Dzik et al. apparently produced the same species by electrospray ionization (ESI) of a solution containing ethyl diazoacetate and Co(II) tetraphenylporphyrin (i.e., condensed-phase reaction followed by oxidation via ESI) [14]. The corresponding reactions of the t-butyl diazoacetates produce analogous products (spectra shown in Supporting information). The only significant difference is a larger contribution from the adduct in the iron porphyrin system. Rates constants for the reactions with the diazoacetates are given in Table 1. All of the reactions are fast and are near the collision-controlled limit. In fact, the rate of the cobalt porphyrin with ethyl diazoacetate is well above the rate estimated by ADO theory [33]. The ions used in this study are unusual in that they are extremely large and polarizable. This may explain the poor fit to ADO theory in these systems. The slowest reactions are in the manganese system where only adducts are formed. The rates are clearly below the collision-controlled limit, but only modestly. The barrier to N2 expulsion is apparently too high to form a carbene here. The iron and cobalt reactions lead to carbenes and are roughly twice as fast as the manganese porphyrin reactions. In each case, the reactions with the ethyl diazoacetate are faster. In the ADO calculations, the rate advantage from the added polarizability of the t-butyl ester is almost perfectly cancelled by its greater mass, so both reagents give nearly identical, predicted collision rates. The higher rates for the ethyl systems are most likely related to steric effects (see below). +n

M

2.8E+05 2.3E+05 2.0E+05 1.8E+05 1.5E+05 1.3E+05 1.0E+05 7.5E+04 5.0E+04 2.5E+04 0.0E+00 600 620 640 660 680 700 720 740 760 780 800 820 840 860 880 900

m/z 3.5E+06

N N CH COR nucleophilic addition

(c)

3.0E+06 2.5E+06 2.0E+06 1.5E+06 1.0E+06 5.0E+05 0.0E+00 600 620 640 660 680 700 720 740 760 780 800 820 840 860 880 900

m/z Fig. 1. Spectra for the reactions of metal tetraphenylporphyrins, P(M)+ , with ethyl diazoacetate: (a) manganese, (b) iron, (c) cobalt. The small unlabeled peak at m/z 698 in the iron spectrum is an adduct with trace methanol in the ion trap buffer gas.

+n

O

(b)

2.5E+05

+n

O

M CH COR N

O

M CH COR back-bonding

N Scheme 2. Two-step mechanism of metal carbene formation.

N N

70

J.T. Aldajaei, S. Gronert / International Journal of Mass Spectrometry 316–318 (2012) 68–75 1.4E+05

4.5E+04

+

[P(Fe) + CH] (5)

(a)

1.2E+05

+

(b)

4.0E+04

[P(Fe) + CHCO2H] (4)

3.5E+04

1.0E+05

3.0E+04 8.0E+04

2.5E+04 6.0E+04

2.0E+04

4.0E+04

[P(Fe) + CHCO2Et] [P(Fe) + C=C=O] (6)

2.0E+04

1.5E+04

+

0.0E+00 650

670

690

710

730

750

770

[P(Fe) + CH] (5) + [P(Fe) + CH2] (4)

0.0E+00 650

790

670

690

710

730

750

770

790

m/z

+

(c)

+

+

5.0E+03

m/z 3.5E+05

[P(Fe) + CHCO2tBu]

1.0E+04

+

[P(Co) + CH] (5)

4.5E+05

3.0E+05

4.0E+05 2.5E+05

+

[P(Co) + CHCO2H] (4)

(d)

3.5E+05 3.0E+05

2.0E+05

2.5E+05 1.5E+05

[P(Co) + CHCO2Et]

+

2.0E+05

+

[P(Co) + CH] (5) + [P(Co) + CH2] (4)

1.5E+05

1.0E+05

[P(Co) + CHCO2tBu]

+

1.0E+05 5.0E+04 0.0E+00 650

5.0E+04 670

690

710

730

750

770

790

0.0E+00 650

670

690

710

730

750

770

790

m/z

m/z

Fig. 2. Fragmentation spectra of CHCO2 R complexes of metal porphyrins (P represents the tetraphenylporphyrin and the parenthetic number following the structure identifies the reaction mechanism scheme): (a) Complex from reaction of iron tetraphenylporphyrin with ethyl diazoacetate. The unlabeled peak at m/z 726 is the ethene loss product. (b) Complex from reaction of iron tetraphenylporphyrin with t-butyl diazoacetate. (c) Complex from reaction of cobalt tetraphenylporphyrin with ethyl diazoacetate. The unlabeled peaks at m/z 729 and 711 are ethene and ethanol loss products. (d) Complex from reaction of cobalt tetraphenylporphyrin with t-butyl diazoacetate.

H

+

CH2 + C R M CHC O R O

OH

+

– CO2

M CH2

M CHC O +

R = H or CH3

CH2 C R R

Scheme 4. Alkene elimination mechanism.

2.1.2. Fragmentation of reaction products In Fig. 2, CID spectra are given for the nominal carbene products from the iron and cobalt porphyrins. Although not seen in this mass window, there are additional peaks associated with secondary reactions of the carbenes or their fragmentation products (second addition/elimination with diazoacetate in the ion trap). These secondary reactions appear to be fast and lead to substantial product yields in the CID spectra. The spectra show that the metal has only a modest effect on the fragmentation pathway, whereas the ester group has a large one. There appear to be three general breakdown pathways (Schemes 4–6). The first accounts for fragmentation products bearing a CHCO2 H or CH2 group on the metal porphyrin (loss of 28 or 56 and loss of 72 or 100, respectively, for the ethyl and t-butyl esters). The former is the well-known thermal elimination reaction of esters to give carboxylic acids [34]. As with literature analogies, this elimination is more favorable

with a t-butyl than an ethyl group. The latter is a decarboxylation product of the resulting carboxylic acid. This has been confirmed by isolation and subsequent CID of metal porphyrins bearing the CHCO2 H group. This nominally leads to the parent metal carbene, M CH2 . It will be shown below that the likely structure is an M N insertion product rather than a conventional metal carbene. This is also the case for the initial reaction product, which was drawn as M CHCO2 R in Scheme 2. The second breakdown pathway accounts for metal porphyrin fragments bearing a CH group (loss of 73 or 101, respectively, from the ethyl and t-butyl esters). These products are

+

O

+

+2

M CHC OR

M C C O H

M C C O

+ RO–

+ ROH

R = CH2CH3 or C(CH3)3 or

+

O

+

M CHC O R R = CH2CH3 or C(CH3)3

O

M CHC O + R

Scheme 5. Homolytic cleavage mechanism.

+ M CH + CO2

O

+ M C C H

+ M C C O

OR + ROH

Scheme 6. Alcohol elimination mechanism.

J.T. Aldajaei, S. Gronert / International Journal of Mass Spectrometry 316–318 (2012) 68–75

N N C H I

CO2R

N N C H II

CO2R

Scheme 7. Diazo resonance forms.

not derived from the carboxylic acid intermediate and appear to be direct fragmentation products from the ester. Scheme 5 gives a plausible mechanism that is initiated by a homolytic cleavage of the O C bond of the ester. The product is shown in the scheme as a metal carbyne (M CH), but as with the carbenes, it is more likely an M N insertion product (see Section 3, Fig. 5). The last pathway is the least common. It involves elimination of an alcohol from the ester to give a fragment with a mass that is consistent with a cumulene product. A possible mechanism is given in Scheme 6. It is drawn as a metal oxidation path with loss of an alkoxide anion that then undergoes a proton transfer to give the cumulene. Alternatively, it could be a concerted 1,2-elimination via a 4-center transition state. The present data cannot distinguish between these possibilities. Comparing the iron to the cobalt systems, only subtle differences can be seen. The iron system gives a larger signal for the product from the alcohol elimination mechanism (Scheme 6). This is particularly evident in the ethyl ester. It also appears that the cobalt system is more prone to breakdown to the simple CH2 and CH complexes via the paths in Schemes 4 and 5, respectively.

2.2. Computational modeling As noted above, more tractable systems are used in the modeling, diazoacetic acid reacting with an unsubstituted metal porphyrin. These metal systems offer a suite of spin states with d4 , d5 , and d6 configurations, respectively for Mn(III), Fe(III), and Co(III). In each case, three states need to be considered and there is previous evidence of mixing of states in these systems [37]. For Mn(III) and Co(III), singlet, triplet, and quintet states are viable, whereas Fe(III) could be a doublet, quartet, or sextet. Previous work indicates that the unsubstituted metal porphyrins prefer a quintet for Mn(III) [35,36], a quartet for Fe(III) [37], and a triplet for Co(III) [37]. This ordering is reproduced in B3LYP computations. It is difficult to predict the accuracy of DFT methods in the reaction products associated with this study and we will limit ourselves to the preferred spin states of the parent porphyrins – we do not consider spin crossing or mixing. As a result, the present calculations are only meant to provide a qualitative aid in probing the reactivity and mechanisms in these systems.

2.2.1. Diazo adduct with metal porphyrins Using diazoacetic acid as the model, the initial complexes of the metals with the diazo compound were explored. We have focused on the expected reaction pathway, which involves an interaction between metal and the carbon of the diazo group’s ␲-system, consistent with resonance form II in Scheme 7 (an enolate zwitterion). That is, the plane of the diazoacetic framework is aligned with the plane of the metal porphyrin framework. This alignment can lead to steric effects between the porphyrin and the alkyl group of the diazoacetate ester. The C M bond lengths are long in all of the ˚ and the interaction energies are relatively weak complexes (>2.4 A) (Fig. 3, Table 2). The diffuse nature of the cation and the energy cost to rehybridizing the carbene center are likely causes of the weak interaction. As a result, the initial complexation of the diazo reagent with the metal porphyrin provides little energy to drive the subsequent loss of N2 .

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Table 2 Relative energies of metal porphyrin reaction products with diazoacetic acid.a Metal (spin)

Metal porphyrin

Adduct

Carbene

M N insertion product

Mn (quintet) Fe (quartet) Co (triplet)

0 0 0

−4.5 −7.5 −2.3

15.5 9.3 −0.4

−13.6 −17.5 −27.5

a Values in kcal/mol. B3LYP with a general basis set. See text for computational details. Model system is reaction of diazoacetic acid with unsubstituted metal porphyrin.

2.2.2. Metal carbene species The calculated structures of the metal carbene species exhibit a marked shift in the orientation of the carbene unit. Unlike the complex with the diazoacetic acid, the carbene framework is orthogonal to the plane of the metal porphyrin – a clear shift from a ␲-type complex to a ␴-type complex. There is also a dramatic reduction in the C M bond length, with values now in the 1.9–2.0 A˚ range (Fig. 3). In the carbene systems, the Co C bond is the shortest of this set and Mn C bond is the longest. Using the spin states outlined above, the binding energies follow this trend with formation of the cobalt carbene being the most favorable of the group. Formation of the cobalt carbene is nearly thermoneutral in the B3LYP calculations, but formation of manganese and iron carbenes is significantly endothermic (15.5 kcal/mol for manganese and 9.3 kcal/mol for iron). For manganese, this is consistent with the experimental result, where only an adduct with the diazo acetate was observed – no carbene formation. The loss of N2 could provide an entropic driving force to tip the balance in the reaction of the cobalt porphyrin, but it is more difficult to rationalize the rapid addition/N2 elimination in the iron porphyrin given the computed carbene energy; however, the observation of a small amount of adduct in the iron reaction is consistent with some barrier to carbene formation, and this level of theory is expected to be only semi-quantitative.

2.2.3. Bridged metal carbene species There is growing evidence that the carbene complexes of transition metals can prefer forming bridged structures by introducing an interaction with an anionic coordination site on the metal’s polydentate ligand (these have been referred to as bridged carbenes in the literature, but they are more accurately called insertion products – that descriptor will be used throughout the following text). This has been observed crystallographically [27–32] and we have proposed such structures on the basis of DFT calculations in related metal salen species [2]. Very recently, extensive DFT calculations have also identified them in neutral cobalt(II) porphyrin systems [14]. For this reason, calculations were also completed on the M N insertion products. For each of the metals, the insertion product is over 25 kcal/mol more stable than the conventional structure. As a result, the overall reaction of the metal porphyrin with the diazoacetic acid is favorable for all the metals if the M N insertion product is formed (by at least 13 kcal/mol). In these structures (Fig. 4), a porphyrin nitrogen bonds to the carbene center at the expense of its interaction with the metal – in the insertion process, the metal goes from 5-coordinate to 4-coordinate. The insertion also distorts the porphyrin framework and creates a significant bend near the nitrogen. A noteworthy aspect of the C N insertion is that the metal carbon bond length is hardly affected, despite the conversion to a four-coordinate carbon with no possibility of ␲bonding. The new bond to nitrogen appears to be strong and its length is typical for a M N bond. The C N insertion leads to a distorted tetrahedron with a M C N angle of approximately 95◦ at the carbene.

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Fig. 3. Computed structures of adducts (left) and carbene complexes (right).

3. Discussion 3.1. Reaction rates Not unexpectedly, the metal has a powerful impact on the reactivity of the porphyrin species with the diazoacetate. The d4 system, manganese, is the least reactive and does not proceed past the formation of an adduct with the diazoacetate. This is consistent with the DFT calculations, which indicate that formation of the manganese carbene species, Mn C, is unfavorable and unlikely under gas phase conditions. In the d5 iron system, the reaction rapidly proceeds through addition with loss of N2 to give a product with a mass consistent with the formation of a metal carbene, Fe C. The computations indicate that this product is still considerably less stable than the reactants, but can collapse to give a very favorable insertion species where the carbene carbon is bonded to the iron and one of the nitrogens of the porphyrin. The M N insertion provides approximately 27 kcal/mol of added stability to the addition elimination product. M N insertion is also possible in the manganese system and provides sufficient stabilization to make the overall process exothermic. This result suggests that the path to the M N insertion species leads through the metal carbene (M C) and the high energy of this species in the manganese system prevents nitrogen loss and stops the reaction at the adduct. The d6 cobalt system reacts very rapidly with the diazoacetate to give an addition/N2 loss product. Computations indicate the cobalt system gives the most favorable formation of a metal carbene (M C) and that these species can gain much added stabilization from forming an M N insertion product. The higher stability of the metal carbene in the cobalt system is consistent with the higher rate of the addition/elimination process. Overall, the rate constants follow a periodic trend with the rate increasing

with the valence electron density on the metal. Given that the mechanism for the addition/elimination process requires electron donation (back-bonding) from the metal, this result can be rationalized. It is also consistent with our earlier work in substituted cobalt salens where we showed that electron-donating groups on the salen ligand enhanced the rate of the addition/elimination process [2]. 3.2. Carbene properties Although they are not the final products in the addition/elimination process, the M C species are of substantial theoretical interest. A key issue is the nature of the bonding and the distribution of charge and spin into the CHCO2 H unit used in the model calculations. To this end, natural bond orbital (NBO) [38] calculations were completed on the carbenes in the targeted spin states (here a full B3LYP/6-311+G* calculation was employed). The data are summarized in Table 3. The results indicate that there is limited charge transfer to the carbene unit, but extensive spin transfer. This is reasonable for these spin states and suggests significant delocalization via back-bonding of a metal’s unpaired d-electron into an enolate-type orbital on the CHCO2 H unit. The fact that much of the spin in the CHCO2 H unit is on the alpha carbon and the carbonyl oxygen is consistent with this interpretation. The NBO Table 3 Natural bond orbital analysis of metal carbenes at the B3LYP/6-311+G* level. Metal (spin) Mn (quintet) Fe (quartet) Co (triplet)

Charge (CHCO2 H) −0.15 −0.21 −0.16

Spin (CHCO2 H) 0.89 0.87 0.98

J.T. Aldajaei, S. Gronert / International Journal of Mass Spectrometry 316–318 (2012) 68–75

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Fig. 5. Computed structure of bridged iron porphyrin complex of CH (triplet).

Fig. 4. Computed structures of bridged porphyrin complexes: (a) manganese (quintet) with diazoacetic acid carbene, (b) iron (quartet) with diazoacetic acid carbene, and (c) cobalt (triplet) with diazoacetic acid carbene.

analysis identifies only one bond between the metal and the carbene carbon in these systems, suggesting that any ␲-interaction is weak. This is consistent with the fact that the C M bond is roughly the same length in the conventional carbene, where ␲-bonding is possible, and in the M N insertion structure, where it is not. The picture that emerges of the conventional carbene is a structure with a strong sigma interaction between the carbon and metal, but the ␲-interaction is better characterized as a single electron transfer to the ␲-framework of the carbene leading to significant radical character and limited C M ␲-bonding. This analysis is completely consistent with one recently proposed for the carbenes of neutral Co(II) porphyrins [14]. 3.3. M N insertion products A novel aspect of the computational work is the strong preference for M N insertion in the carbene species. Although there is precedence for M N insertion in the carbene complexes of metal porphyrins, the strong preference for insertion and the prevalence of M N insertion structures is striking in these systems. In no case is a conventional M C type carbene preferred over an M N insertion species. The energy differences are large and typically in the range of 25–30 kcal/mol. As a result, all low energy processes involving the carbene species will have to be initiated from an M N insertion complex and the chemistry of these carbenes should be dominated by the properties of the M N insertion species rather

than the conventional M C bonded species. This is very different than the results of a recent DFT study of a neutral Co(II) porphyrin system where little energy difference between the conventional and M N insertion structures was seen [14]. We have completed single-point calculations on B3LYP geometries using the functional from that study (BP86) and find that the large preferences for M N insertion persist in the charged systems (19.1 and 22.3 kcal/mol in the iron and cobalt systems, respectively). Dzik et al. [14] provide evidence that the M N insertion structure is a catalytic dead end, so the strong preference for insertion in the cationic manganese, iron and cobalt systems suggests that they would have limited catalytic capacity. It is true that competence has been seen in cationic iron porphyrins in diazoacetate cyclopropanations, but there is also evidence that an axial ligand may play a role in condensed-phase reactions of these species, which could alter the balance between M N insertion and conventional carbene structures [11]. One question is whether the M N insertion species is formed directly in the reaction with the diazoacetate or results from the collapse of the conventional carbene to the insertion structure. The direct formation of the M N insertion species requires an awkward transition state in which the diazoacetate inserts into the metal nitrogen bond of the porphyrin while expelling N2 . We computationally sought such transition states, but could not locate one. This may be a result of the complexity of the process or it may indicate that the reaction is truly stepwise. The calculated energies of the conventional carbenes are roughly consistent with the observed behavior of the metals, where manganese fails to reach a carbene and cobalt does so readily. If a direct path to the M N insertion structure existed, one might anticipate that the manganese system could proceed because formation of the M N insertion product is exothermic. Although it is not possible to rule out a concerted path to the insertion products, it seems more likely to be a stepwise mechanism, and if not, the transition state for the concerted path likely resembles the conventional carbene to some extent. It should be noted that Dzik et al. found a modest barrier to the rearrangement of the conventional to the insertion products in their Co(II) system [14]. The M N insertion also allows for the formation of some highly novel species in terms of the organic unit bound to the metal porphyrin. For example, the carbene species can be converted under CID conditions to give what appear to be metal carbynes, M C, and metal cumulenes, M C C O (Chevrier et al. have crystallographic evidence of M N insertion in a vinylidene [31]). One might expect them to be highly reactive and unstable, but the ability to insert into the M N bond allows for an increase in the coordination state of the carbon and results in stabilization. For example, the insertion complex of CH with the iron porphyrin (Fig. 5) gains 30.4 kcal/mol of stabilization relative to the conventional structure ˚ In this despite having a much longer Fe C bond (1.80 A˚ vs. 1.66 A). case, the added group, CH, carries an unpaired electron and the calculations assumed spin-pairing to give a triplet from the quartet iron porphyrin. The structure exhibits significantly shorter bonds to the inserted carbon than the carbenes in Fig. 4, a much wider N C Fe angle, and a pyramidal geometry for the CH group.

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3.4. Fragmentation processes The iron and cobalt systems exhibit very similar fragmentation patterns for their addition/elimination products (CHCO2 R complexes). For the t-butyl systems, the alkene elimination path dominates, presumably leading to a carboxylic acid product (CHCO2 H complex). This is a common path for t-butyl esters in general, and it is plausible that the fragmentation occurs from the M N insertion product, which may act much like a normal ester. In addition, two other pathways are observed. They produce complexes of either CH2 or CH with the metal porphyrin. The former can also be formed by fragmentation of the CHCO2 H metal complex by decarboxylation. It is a reasonable to assume that the metal CH2 complex is the result of a secondary fragmentation process that passes through the CHCO2 H metal complex (i.e., is not directly formed from the fragmentation of the ester). The formation of the CH metal complex is more unusual and the loss of a CO2 R radical unit from simple esters is not common. In this case, it seems more likely, that the fragmentation process involves breakdown of the M N insertion product and the intermediary of the true metal carbene species, M CHCO2 R. As noted above, the metal carbene species has unusual properties and these could facilitate the fragmentation process. The most important factor is probably the radical character that is present in the ␲-system of the CHCO2 R unit, which could aid in the radical cleavage that eventually leads to the metal CH species. The homolytic cleavage of this mechanism (Scheme 5) is consistent with the absence of this channel in the fragmentation of the metal complex with CHCO2 H, where a hydrogen atom would be a required product based on this mechanism. The fragmentation patterns with the ethyl diazoacetates are similar, but in this case, the alkene elimination process is less important. This is a typical reactivity trend in the eliminations of esters [34]. As a consequence, the formation of the metal CH2 complex is also suppressed in the ethyl esters. The net effect is to shift the fragmentation path mainly to the formation of the metal CH product. 4. Conclusions Metal porphyrins react readily with diazoacetates, producing either adducts (manganese) or addition with loss of N2 products (iron and cobalt). The addition/elimination products are formally metal carbenes, but DFT calculations suggest limited ␲-bonding and considerable radical character in the conventional metal carbene structures, M CHCO2 R. DFT calculations also indicate that the complexes strongly prefer M N insertion structures, though it is likely that they are initially formed via a conventional metal carbene species, M C. Under CID conditions, the addition/elimination products fragment in a variety of ways and give unusual ions in some cases. The key fragmentation products are metal porphyrins complexed with CHCO2 H, CH2 , CH, and C C O units. Future work will focus on the gas-phase reaction chemistry of the bridged carbene species and their fragmentation products. 5. Methods 5.1. Mass spectrometry All experiments were completed in modified ThermoFinnigan Deca quadrupole ion trap mass spectrometers equipped with electrospray ionization (ESI). The metal porphyrins and diazoacetates were purchased commercially (Sigma-Aldrich, St. Louis, MO) and used without purification. In this study, we began with chloride salts of the Mn(III) and Fe(III) porphyrins. For cobalt, the ESI source oxidized the Co(II) porphyrin to Co(III). The metal porphyrins were

dissolved in HPLC-grade methanol (10−4 to 10−5 M) and pumped through the electrospray interface at flow rates from 3 to 10 ␮l/min. Typical ESI conditions involved needle potentials from 3.5 to 4.5 kV and heated capillary temperatures from 125 to 200 ◦ C. A notched waveform was applied to isolate the metal porphyrin in the ion trap. Once a steady signal was obtained, the neutral reagent was introduced into the helium system via a custom gas-handling system. The system has been described previously [39–41], but a brief overview is given here. The liquid reagent was delivered to a measured flow of helium (1–2 l/min) by a syringe pump. With reagent flows of 30–300 ␮l/h, rapid evaporation occurs at the syringe needle to give mixing ratios of ∼102 –104 (He/reagent). For less volatile substrates, the mixing region was heated to 50–75 ◦ C. In same cases, the reagent was diluted in cyclohexane. Most of the gas mixture is diverted to an exhaust system and ∼0.1 ml/min is carried through the LCQ’s restriction capillary to the ion trap to establish a helium pressure of about 1 mTorr. At these pressures, the mean free path of the molecules is considerably longer than the dimensions of the trap and the loss of gases out of the end cap holes of the trap can be treated as an effusion process. The lighter helium atoms effuse more quickly than the reagent molecules and the mixing ratio must be corrected for differential effusion (square root of the mass ratio correction). Typical reagent pressures were between 10−5 and 10−7 Torr. The system is routinely calibrated against the known rate of reaction of Br− with iodomethane [42]. Once an appropriate flow of the neutral reagent was established, the system was given several minutes for the reagent pressure to equilibrate to a steady state. Kinetic measurements were completed by varying the time delay between ion isolation and the expulsion of all ions to obtain a mass spectrum. In most cases, 9 different time delays were used in each run. Time delays and reagent flows were adjusted to obtain plots that covered 2–3 half-lives of the reactant ion. Reported rates are the average of at least 6 kinetic runs using at least 3 different reagent flow rates and were obtained on at least two different days. Kinetic plots showed good linearity and gave correlation coefficients (r2 ) greater than 0.98. Other work from our laboratory indicates that the ion trap provides an environment that is effectively at ambient temperature (∼300 K) [43].

5.2. Computational Calculations were completed with the GAUSSIAN03 quantum mechanical package [44]. Geometries were optimized at the B3LYP level using a general basis set. The basis set used 6-311+G* on nitrogen, oxygen, and the metal. Carbon and hydrogen were given a 6-31G* basis set. For many species, calculations were completed with a full 6-311+G* basis set and the results changed very little in these cases (the larger basis set led to more troublesome problems with SCF convergence and therefore, the smaller, general basis was more attractive for this study). For example in the manganese system, the full 6-311+G* basis set gave reaction energies of −4.1, +16.2, and −12.5 kcal/mol for forming the adduct, conventional carbene, and bridged carbene, respectively (−4.5, +15.5, and −13.6 kcal/mol with the general basis set). Given the complexity of the species and the anticipation of semi-quantitative results at this level of theory, no corrections were made for zero-point energies.

Acknowledgement Support from the National Science Foundation (CHE-1011771) is gratefully acknowledged.

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