Determination of Co–C bond dissociation energies for organocobalt complexes related to coenzyme B₁₂ using photoacoustic calorimetry

Abstract

Photolysis of organocobalt complexes with Schiff base equatorial ligand, bis(salicylidene)ethylenediamine (salen), has been investigated by pulsed, time-resolved photoacoustic calorimetry in methanol at ambient temperature. The observed enthalpy changes and measured cobalt–carbon bond dissociation energies for the above compounds lie within the range 80–170 kJ mol⁻¹. The influence of the steric and electronic factors of both the alkyl group and trans base ligand on Co–C bond strength is discussed. It is interesting to note that we obtain the highest Co–CH₃ bond dissociation energy (BDE) value of 168 kJ mol⁻¹ for CH₃Co(salen)H₂O compared with those for coenzyme B₁₂ and its model compounds.

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Introduction

Transition-metal alkyl complexes represent an interesting class of organometallic compounds that have potential applications in the areas of both transition-metal catalysis and bioinorganic chemistry. Study of the transition-metal–alkyl bond dissociation energies is crucial for predicting new reactions, explaining reactivities, or understanding chemical reaction mechanisms, which often involve the cleavage and the formation of these bonds. Organocobalt complexes often function as intermediates in the radical polymerization of alkenes and organic substrate transformations etc.¹ Furthermore, among them there are a type of widely studied coenzyme B₁₂ model compounds containing a σ-type cobalt–alkyl bond. Coenzyme B₁₂ (5′-deoxyadenosylcobalamin) is one of the few stable organometallic complexes found in nature. It serves as a cofactor for dozens of enzymatic reactions in which 1,2-intramolecular rearrangements of substrates occur. The first step in these reactions is the facile homolytic cleavage of the cobalt–carbon bond of coenzyme B₁₂ to generate a 5′-deoxyadenosyl radical and a Co(II) corrinoid, which is considered to play an essential role in the catalytic cycle.^1–4 Accordingly, a knowledge of the Co–C bond dissociation energy (BDE) of coenzyme B₁₂ and related organocobalt complexes, and of the factors that influence such bond dissociation, is important to an insight into the coenzyme's role. In the past few decades, extensive investigations determining the strength of the cobalt–carbon bond and analyzing the factors which influence it have been carried out on both biologically active cobalamin and organocobalt model compounds.^5–10

The groups of Halpern,⁵ Finke,⁶ and others,^7,8 have employed a kinetic method for the determination of the Co–C BDEs of coenzyme itself and related model compounds using appropriate radical traps. This approach entails measuring the activation enthalpy (ΔH₁^≠) of the homolytic dissociation reaction of thermolysis for the alkylcobalamin and then deducting the activation enthalpy for viscous flow of solvent (ΔH_η^≠). Halpern and co-workers⁹ have also utilized an equilibrium thermodynamic method to determine the Co–C BDEs of a series of alkylcobaloxime complexes, which is independent of the details of the reaction or nature of solvent. Toscano et al.¹⁰ have reported the calorimetric titration measurement of Co–C BDEs for RCo(DH)₂py (where DH = dimethylglyoxime, py = pyridine) in conjunction with thermochemical cycles.

The above works quantitatively give rise to Co–C BDE values for coenzyme B₁₂ and its model compounds. Their values fall within the range 80–130 kJ mol⁻¹, and depend on the axial and equatorial ligand of those organocobalt complexes.^5–9 Notwithstanding this, it is necessary to extend this work to other organocobalt complexes and to gain a deeper understanding of the relationship between the Co–C bond and structure.

With the development of time-resolved photoacoustic calorimetry (PAC) in the 1980s, it is now feasible to determine the enthalpies that accompany ligand dissociation and/or molecular movements in photoinduced chemical reactions.^11–15 In the PAC experiment, acoustic waves, which are generated from the resulting thermal expansion of the solution after laser excitation, can be detected by the transducer, thus enabling investigation of the energetics of fast photochemical processes. Provided that the mechanism and the quantum yield for the photochemical reaction are well defined, the enthalpy changes accompanying assigned chemical processes can be evaluated. In many cases, this overall enthalpy change may be attributed to the cleavage and/or formation of specific bonds.¹⁴ The utility of the PAC method is that it allows one to determine the metal–carbon bond dissociation energy, even for very high enthalpic values, at room temperature. By using this method we have already determined the Co–C bond dissociation energies in neutral and acidic aqueous solutions for coenzyme B₁₂ and methylcobalamin.^15b The results are in good agreement with the literature values obtained by kinetic methods.

In the present paper we employed the PAC technique to assess enthalpy changes during the photolysis of alkylcobalt complexes of Schiff bases, RCo(salen)L, in which the equatorial ligand is bis(salicylidene)ethylenediamine (salen). Accordingly, their cobalt–carbon bond dissociation energies were determined. The influence of the axial ligand on the cobalt–carbon bond dissociation energy is discussed both in terms of electronic and steric factors.

Experimental

Materials

Solvent, methanol (AR), was distilled once before use. Ferrocene (AR) was used as calorimetric reference and sublimed once before use. RCo(salen)L were prepared according to the literature¹⁶ and were recrystallized from a methanol–water mixture. Co, C, H, N analyses (%) of the above organocobalt derivatives are reported in Table 1. TEMPO (2,2,6,6-tetramethylpiperidinyl-1-oxy) was obtained from Aldrich and sublimed once before use.

Table 1 Elemental analyses (%) of RCo(salen)L complexes^a

Compound	Formula	C	H	N	Co
a Calculated values in parentheses. b im = imidazole. c bzim = benzimidazole.
CH3Co(Salen)H2O	C17H19CoN2O3	56.55(56.98)	5.17(5.35)	7.86(7.82)	16.58(16.45)
CH3Co(Salen)py	C22H22CoN3O2	62.81(63.01)	5.42(5.29)	10.07(10.02)	14.35(14.05)
CH3Co(Salen)im^b	C20H21CoN4O2	58.62(58.83)	5.16(5.18)	13.90(13.72)	14.52(14.43)
CH3Co(Salen)bzim^c	C24H23CoN4O2	62.94(62.88)	5.21(5.07)	12.07(12.22)	12.91(12.86)
C2H5Co(Salen)H2O	C18H21CoN2O3	61.12(61.01)	5.47(5.40)	7.73(7.90)	16.72(16.63)
i-C3H7Co(Salen)H2O	C19H23CoN2O3	58.90(59.07)	5.97(6.00)	7.31(7.25)	15.25(15.25)
n-C4H9Co(Salen)H2O	C20H25CoN2O3	60.44(59.99)	6.24(6.28)	6.61(6.99)	14.72(14.72)
i-C4H9Co(Salen)H2O	C20H25CoN2O3	59.71(59.99)	5.80(6.28)	7.04(6.99)	14.70(14.72)

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Quantum yield measurements

Samples of RCo(salen)L (≈10⁻⁴ M) in methanol with excess TEMPO (≈5 mM), were introduced, under anaerobic conditions, into a 10 × 10 mm quartz cuvette and photochemical studies were carried out at 355 nm. The solution was prepared in this way to ensure that enough TEMPO was present to capture each radical formed and to prevent underestimation of the quantum yield due to diffusion-controlled recombination of Co(II) species and radical. Reactions were monitored at the maximum UV-Vis absorbance change of RCo(salen)L after the laser pulse. The quantum yields, Φ_d, for photolysis of RCo(salen)L were determined with the use of potassium trioxalatoferrate actinometry.¹⁷Here, ΔA_s/E_a and ΔA_a/E_a are the energy-normalized absorbance changes at wavelength λ for the sample solution after the 355 nm laser pulse and for the actinometer at 510 nm, respectively, Δε_s is the difference in the molar extinction coefficients at the monitoring wavelength λ between RCo(salen)L and Co(salen)L, ε_a is the molar extinction coefficient of the actinometer at 510 nm, and Φ_a is the quantum yield of the potassium ferrioxalate actinometer. The quantum yields were recorded as the average values from three measurements. In the quantum yield calculations corrections were made for absorption of TEMPO and the Co(II) products.

PAC experiments

The experimental photoacoustic apparatus has been described previously.^11–15 In our experiment a Q-switched Nd:YAG laser (Continuum NP70) operating at 10 Hz, 355 nm, with a pulse width of 8 ns, was used as excitation source. The laser pulse energy was attenuated by means of a set of neutral density filters which allowed the pulse energy to be varied from 5–30 μJ. The laser beam diameter was fixed by a 0.9 mm pinhole, which could determine the time resolution to be equal to the travel time of the acoustic wave through the laser beam diameter (≈600 ns). The acoustic wave induced in solution was detected by a 1.5 MHz PZT piezoelectric transducer. The signal was then amplified by a HP-8847F instrument and recorded on a digitizing oscilloscope (HP-54510B). The data were transferred to a personal computer where each acoustic wave was normalized to the laser pulse energy measured by a transient radiometer (Digi Rad R-752 and P-444). Sample absorbance in methanol ranged from 0.15 to 0.25 at 355 nm (1 cm path length) and was matched to that of a calorimetric reference, ferrocene. Linearity of the signal versus excitation energy was observed over the whole range. The waveforms obtained for the sample and reference were unchanged as shown in Fig. 1. The sample solutions were degassed by bubbling argon for 15 min and maintained under an argon atmosphere during the experiment.

Fig. 1 Energy normalized photoacoustic signals from ferrocene (solid) and CH₃Co(salen)H₂O (dotted) (λ_exc = 355 nm) at 22 °C in methanol.

Results

Quantum yields

In order to calculate Co–C bond dissociation energies, quantum yield data for photo-dissociation, Φ_d, of a series of RCo(salen)L complexes is necessary [see eqns. (4) and (5)]. It is now generally accepted that TEMPO is an efficient scavenger which traps the alkyl radical during the photolysis of alkyl cobalt(III) complexes.^6a,b,18–20 Accordingly, photolysis of RCo^III(salen)L forms Co^II(salen)L and an alkyl radical. In the presence of radical scavenger (e.g., TEPMO) the latter is effectively trapped with a rate constant²¹ of ≈10⁷–10⁹ M⁻¹ s⁻¹.

At present, the quantum yields, Φ_d, for photolysis of RCo(salen)L are determined in methanol under anaerobic conditions at 355 nm, with an added excess of TEMPO trapping agent, by following the disappearance in absorption of RCo(salen)L at about 345 nm. The isosbestic points are found at about 310 and 415 nm, respectively, and the final spectrum is consistent with that of an authentic Co^II(salen)L species (Fig. 2). It indicates that during photolysis the RCo^III(salen)L totally converts into Co^II(salen)L, and no side reactions occur. Since the absorbance for R-TEMPO in the visible region is very low, the spectrum in the presence of TEMPO is qualitatively similar to that in its absence. Using the values of Φ_a = 1.22 and ε_a(510 nm) = 1.11 × 10⁴ M⁻¹ cm⁻¹, along with eqn. (1), the quantum yields, Φ_d, for photo-dissociation of RCo(salen)L are obtained (Table 2).

Table 2 Cobalt–carbon bond dissociation energies for alkyl cobalt complexes

R	Chelate	L	Φ d ^c	Co–C BDE^a	Ref.
a In kJ mol^−1, error margins ±8 kJ mol^−1. b This work, measured at 22 °C in methanol under an argon atmosphere. c Error margins ± 0.02. d saloph = N,N′-bis(salicylidene)-o-phenylenediamine. e DH = dimethylglyoxime.
CH3	salen	H2O	0.32	168	This work^b
CH3	salen	imidazole	0.31	152	This work
CH3	salen	benzimidazole	0.32	136	This work
CH3	salen	pyridine	0.29	127	This work
C2H5	salen	H2O	0.31	125	This work
n-C4H9	salen	H2O	0.30	107	This work
n-C4H9	salen	—		108	8
i-C4H9	salen	H2O	0.30	90	This work
i-C3H7	salen	H2O	0.29	81	This work
i-C3H7	saloph^d	pyridine		84	5a
i-C3H7	(DH)2^e	pyridine		89	10

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Fig. 2 UV-Vis spectra obtained during the ambient temperature photolysis of CH₃Co(salen)H₂O in methanol in the presence of the radical trap TEMPO.

It is interesting to note that the quantum yield values we obtained are in the range of 0.29∼0.32. The results indicate that the quantum yields change little while varying the alkyl group in this system. Our value for n-C₄H₉Co(salen)H₂O of 0.30 is in good agreement with the literature value (0.29 in toluene).²²

Co–C bond dissociation energy

According to previous studies,^{18–20,22–24} the mechanism for anaerobic photolysis of organocobalt complexes involves homolytic cleavage of the cobalt–carbon bond to form a geminate pair [Co(II):R] very rapidly. The caged geminate pair can either recombine to form the reactant or diffuse into the solvent to yield free radicals. This means that the Co(II) species and free radical are the final products for our experimental nanosecond to microsecond time window. During the anaerobic photolysis without TEMPO there are some radical side reactions, including hydrogen abstraction by carbon radical from solvent, dimerization, and diffusion recombination, etc. For the sample concentrations of about 10⁻⁵ M, the concentration of free radicals should be much less than 10⁻⁵ M. Our photo-dissociation of the cobalt–carbon bond is not contaminated by artificial side reactions and the apparent enthalpy change is therefore a net enthalpy change from an excited state to form a Co(II) species and a free radical.

In the case of CH₃Co(salen)L, there might be an exchange equilibrium between the axial ligand and solvent (methanol or water). However, the nitrogen donor coordinated form, RCo(salen)L, is predominant,^23,24 since nitrogen donors (e.g., pyridine, imidazole, and benzimidazole) have a much stronger coordination ability for transition-metals than do oxygen donors (e.g., methanol, water). Moreover, it is assumed that only cleavage of the Co–C bond occurs during photolysis, and the axial base ligand still remains on the Co(II) macrocyclic complexes. Therefore, this study only considers the signal contribution from the base ligand coordinated species.

From the photoacoustic waveforms, for example Fig. 1, we have found that RCo(salen)L and ferrocene give identically shaped acoustic waves, furthermore, within the time window of the experiment, ca. 600 ns, the sample acoustic wave does not shift in time relative to the ferrocene wave form. This indicates that all of the observed kinetic processes, which correspond to the homolytic decomposition products of the Co–C bond within our experimental time scale, occur in less than 600 ns and there are no measurable kinetic processes on the time scale of 600 ns to 10 μs. Therefore, the enthalpy changes for photolysis of RCo(salen)L may be attributed to cleavage of the Co–C bond. A plot of the photoacoustic response for CH₃Co(salen)H₂O in methanol under an argon atmosphere versus pulsed laser intensity is shown in Fig. 3. The corresponding plot for the ferrocene reference is included for comparison. The observed reaction enthalpy is related to the term α (defined as the ratio of slopes of the sample and the reference)¹⁴ by eqn. (4), where, E_hν is the molar energy of the pulsed laser at 355 nm (336 kJ mol⁻¹) and Φ_d is the quantum yield for the observed photochemical process. Since RCo(salen)L complexes are relatively rigid, the reaction volume changes during their photolysis are so small that they can be neglected, especially in organic solvents.^12a In this case, only the Co–C bond cleavage may contribute to the overall enthalpy change, so the Co–C BDE in RCo(salen)L can be estimated by using eqn. (5). The Co–C bond dissociation energies for photolysis of a series of RCo(salen)L measured by PAC are given in Table 2. Most of the error in the measurement is derived from the quantum yield. For comparison, the data of related organocobalt complexes from thermolysis kinetic and equilibrium measurements are also listed. The Co–C BDE value for n-C₄H₉Co(salen)H₂O obtained in this work is in excellent agreement with the literature value (108 kJ mol⁻¹) in acetone derived from previous kinetic methods on thermolysis.⁸

ΔH_R = E_hν (1 − α)/Φ_d (4)

BDE(Co–C) ≊ ΔH_R (5)

Fig. 3 The relationship between signal amplitude and excitation energy of ferrocene (■) and CH₃Co(salen)H₂O (●) at 22 °C in methanol.

Discussion

Previous studies have established that the structural, spectroscopic, thermodynamic and kinetic properties of coenzyme B₁₂ model compounds are strongly related to the steric and electronic properties of the axial ligands R and L.^25–28 Electronic effects induced by the trans axial ligands, and steric interaction between bulky trans axial ligands and equatorial macrocycles are thought to contribute significantly to the Co–C bond strength in organocobalt complexes. For example, with the increase in bulk of R on going from methyl to CH(CH₃)Ph the Co–C BDEs in alkylcobaloxime are decreased by 50 kJ mol⁻¹.^9,10 Furthermore, when R varied from electron-withdrawing ligands, such as CH(CN)Cl, to electron-donating ones, such as i-Pr, the Co–C bond distances lengthen up to about 0.2 Å and logK for L displacement increases by several orders of magnitude.²⁵ The alkyl cobalt Schiff base complexes appear to have properties closer to cobalamins than do cobaloximes,^29a however, due to the difficulty in preparing and studying these compounds, they have been less well investigated. The very few available X-ray structural data on RCo(salen)L also show the steric influence of the bulky alkyl ligands on the Co–C and trans Co–L bond lengths, as well as the bond angles around the coordinated C atom.²⁹

Inspection of the Co–C BDE values of RCo(salen)L complexes measured by photoacoustic calorimetry (see Table 2), reveals the influence of the alkyl group directly bonded to cobalt on Co–C BDE. For a given trans ligand, H₂O, there is a general trend for weakening of the Co–C BDE in the order CH₃ > C₂H₅ > n-C₄H₉ > i-C₄H₉ > i-C₃H₇ which qualitatively follows the trend of increasing steric bulk of the alkyl ligand and accords with our previous Co–C bond order calculations³⁰ as well as experimental data³¹ for Co–C bond cleavage of RCo(salen)H₂O complexes. In fact, increasing the size of the alkyl group induces the cobalt–carbon bond lengthening or Co–C–C bond angles widening due to conformational distortion of the equatorial ligand away from the alkyl group and towards the trans ligand, as a result of Co–C bond weakening.^25,32,33 Thus the large (≈87 kJ mol⁻¹) difference between Co–C BDE of CH₃Co(salen)H₂O and i-C₃H₇Co(salen)H₂O presumably is mainly due to steric effects. It is noteworthy that the same quantitative decrease in Co–C BDE between methyl and iso-propyl for cobalamin analogues are found.³⁴ More interestingly, there is an excellent parallel between our order of BDEs for RCo(salen)H₂O and the orders both of rates of Co–C fission and pKs for displacement of DMBz in alkylcobalamin.³⁴ Generally, the inductive effect component, I,³⁵ and the Taft polar substituent constant, σ*,³⁶ could qualitatively reflect the electronic effect, as well as both electrostatic and steric effects of the alkyl groups, respectively. In order to provide insight into how steric and electronic factors of alkyl groups affect the Co–C bond dissociation energy of alkylcobalt complexes, it is necessary to assess the influences of the above two constants (Table 3). Plots of Co–C BDE for RCo(salen)H₂O versus the corresponding I and σ* values, depicted in Fig. 4, are essentially linear. In the sequence CH₃, C₂H₅, n-C₄H₉, i-C₄H₉, i-C₃H₇, the capability of electronic donation of the alkyl group increases (increasing negative I), so that Co(III) in RCo(salen)L is easily reduced to form Co^II(salen)L, and hence, the cobalt–carbon bond dissociation energy decreases. Furthermore, when negative σ* increases, the steric effect of the alkyl group increases and the stability of the Co–C bond decreases, and hence, the cobalt–carbon bond dissociation energy is decreased.

Table 3 Correlation between Co–C BDE and inductive effect exponent, Taft constant for alkyl Co(salen) complexes

R	Co–R BDE/kJ mol^−1	10^3I	σ*
CH3	168	−9.00	0.00
C2H5	125	−12.37	−0.100
i-C3H7	81	−15.72	−0.190
n-C4H9	107	−14.07	−0.130
i-C4H9	90	−14.85	−0.125

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Fig. 4 Relationship between homolytic cobalt–carbon bond dissociation energies and the Taft polar substituent constant, σ* (●), and the inductive effect component, I (■).

It has been proposed that both steric and electronic effects of the trans base ligand influence the strength of the Co–C bond. In the case of phosphorus donors steric factors would dominate and in the case of nitrogen donors electronic factors would dominate.²⁷ Measurements of the Co–C BDE for a series of alkylcobaloximes showed that an increase in the basicity of the trans nitrogen ligand resulted in an increase in the Co–C BDE.⁹ It seems somewhat artificial to specify an exclusive electronic effect for nitrogen donors, however. Although there is no simple linear correlation for different kinds of base ligand of CH₃Co(salen)L, where L (pK_a³⁷) is pyridine (5.25), imidazole (6.95), or benzimidazole (5.54), their Co–C BDE values increase following the order of increasing pK_a of the trans axial ligand, i.e. CH₃Co(salen)pyridine < CH₃Co(salen)benzimidazole < CH₃Co(salen)imidazole. This trend can probably be explained in terms of the Co–C bond dissociation involving a decrease in oxidation state of cobalt (i.e. Co^III → Co^II). Thus, more basic ligands are expected to stabilize the higher oxidation state, and accordingly, to increase the barrier to homolytic cleavage.^2,9,38 Among these base ligands, imidazole is thought to have the greater capacity to stabilize higher positive charge by dissipation of its charge over the imidazole ring. Our experimental result is in agreement with the report of Stynes et al.,³⁹ in which they found imidazole stabilized the Co(III)-superoxide porphyrin more than pyridines. It is suggested that the values of pK_a for trans base ligands might reflect, at least qualitatively, the corresponding trends of the cobalt–carbon bond dissociation energies. In this case, the present cobalt–carbon BDE for CH₃Co(salen)H₂O is the largest among CH₃Co(salen)L complexes. One possible explanation is that displacement of the trans base group by water would make the cobalt atom more electrophilic and, as a result, the alkyl group would be more tightly held.⁴⁰ Apparently, any ligand that is able to replace water would decrease the electrophilic character of cobalt and weaken the cobalt–carbon bond. However, to completely elucidate the dependence of dissociation energy on the nature of axial ligand requires further investigation.

Although some Co–CH₃ BDEs have been reported in the literature, for example, that of the base-on methylcobalamin was found to be 155 kJ mol⁻¹ from thermal kinetics by Finke and Martin^6g and 163 kJ mol⁻¹ from photoacoustic calorimetry by us, the values of Co–C BDE for the base-off methylcobalamin^15b and CH₃Co(DH)₂H₂O⁴¹ are still uncertain but are estimated to be ≥176 kJ mol⁻¹ and ≈196 kJ mol⁻¹, respectively. It is interesting to note that the Co–C BDE value of 168 kJ mol⁻¹ in CH₃Co(salen)H₂O represents, to the best of our knowledge, the accurate determination of the highest cobalt–carbon bond strength in a coenzyme B₁₂ model compound.

Conclusion

In this report, we have quantitatively determined Co–C bond dissociation energies for a series of organocobalt(III) Schiff base complexes by using photoacoustic calorimetry, values for which fall in the range of literature values for B₁₂ model compounds obtained by kinetic and thermodynamic equilibrium measurements. We have also obtained the highest value of Co–C BDE of CH₃Co(salen)H₂O for coenzyme B₁₂ model compounds and discussed the influence of the steric and electronic effects of the axial R and L ligands on the Co–C bond strength. The application of photoacoustic methods to the measurement of cobalt–carbon bond strengths in B₁₂ model compounds under mild conditions has been demonstrated successfully. This approach should provide a method complementary to the thermal kinetic or equilibrium methods for determining transition-metal–alkyl bond dissociation energies, and appears to have wide applications.^12b

On the basis of our results, the organocobalt(III) Schiff base complexes probably represent analogues of the corresponding B₁₂ derivatives. Our results also reinforce the view that steric and electronic factors may play an important role in promoting Co–C bond homolysis.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (No. 20071017, 29823001 and 19774031) and by the Research Fund for the Doctoral Program of Higher Education, China (200028401). The authors thank Prof. J. M. Pratt for helpful discussion and comments on the draft of the manuscript.

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