Quantification of Thermodynamic Hydridicity of Hydride Complexes

The molecular electrostatic potential (MESP) at the hydride nucleus, VH, is proposed as a powerful measure of the hydride donor ability (hydridicity) ...
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Quantification of Thermodynamic Hydridicity of Hydride Complexes of Mn, Re, Mo, and W Using the Molecular Electrostatic Potential K. S. Sandhya and Cherumuttathu H. Suresh* Inorganic and Theoretical Chemistry Section, CSTD, CSIR-National Institute for Interdisciplinary Science and Technology, Trivandrum 695 019, India S Supporting Information *

ABSTRACT: The molecular electrostatic potential (MESP) at the hydride nucleus, VH, is proposed as a powerful measure of the hydride donor ability (hydridicity) of metal hydride complexes. VH values have been determined for several group VI and group VII octahedral metal hydride complexes of Mo, W, Mn, and Re at the B3LYP level of DFT. Further, the hydridicity, defined by the thermodynamic parameter ΔG°H− is determined for all of these complexes using a thermodynamic cycle that describes hydride abstraction reactions by H3O+ ions. The ΔG°H− of most of the W and Mo complexes corresponding to the reaction with H3O+ are lower than 20 kcal/ mol, whereas a majority of other complexes showed ΔG°H− in the range of 20−60 kcal/mol. In all cases, a lower value of ΔG°H− is correlated to a higher negative VH value. The increase in the negative character of VH indicated higher hydridicity of the complex and easy elimination of the hydride ion. Thus, the MESP approach provided a simple yet accurate prediction of the hydride donor ability of the metal hydride complex compared to a more tedious and demanding calculation to obtain the thermodynamic parameter. This approach and its applicability are validated by correlating VH with experimentally known ΔG°H− values of W and Mo hydride complexes.

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hydride complexes using a specific thermodynamic cycle.24 Later, Papai and Kovacs expressed hydridicities in terms of the bond dissociation Gibbs free energy and without considering a thermodynamic cycle.25 This method is computationally less expensive and very attractive for finding hydridicities of metal hydride complexes. Moreover, hypothetical isodesmic reactions have also contributed to the theoretical prediction of hydridicities of metal hydride complexes in terms of the Gibbs free energy.26 Though many metal hydride complexes used in catalytic water splitting reactions are neutral, most of the complexes studied for their hydridicity are cationic metal hydride species.23,27−29 In this context, we propose a method based on the molecular electrostatic potential (MESP) for evaluating the hydridicity of metal hydride complexes. Earlier, MESP-based ideas were used to predict the nucleophilic site, substituent effects, molecular reactivity, intermolecular interactions, and a number of physiochemical phenomena.30−37 The advantage of the MESP over the thermodynamic cycle is its simplicity in finding the potential at a hydride nucleus using popular ab initio programs. For instance, to calculate the hydridicity from a thermodynamic cycle, one has to calculate a reaction sequence to obtain the required thermodynamic data. These data must be very

INTRODUCTION Mechanistic understanding of hydride/proton transfer processes involving transition metal complexes is of immense importance in many biological and catalytic processes such as conversion of NADP+ to NADH for CO2 reduction, nitrogen fixation, hydrosilyation dehydrogenation of formic acid and alcohol, enzymatic H2 cleavage reactions, water splitting, and so forth.1−13 The pKa, ΔG°H−, and bond dissociation energy (BDE) are important thermodynamic parameters used for analysis of the cleavage an M−H bond (M = metal) to form H+, H− and H•.14,15 Further, hydridicity and acidity are two terms representing the energy requirement for the heterolytic dissociation of an M−H bond yielding H − and H + , respectively.16 The idea of hydridicity or hydride donor ability arises from knowledge that negative charge accumulates at hydrogen in M−H bonds. Molecular design strategies to obtain M−H bonds with a highly electron rich H center are attractive for water splitting reactions promoted by transition metal hydride complexes.17−20 Experimental determination of hydridicity is a challenging task.21 DuBois et al. studied the hydridicities of Ni, Pt, and Pd hydride complexes by thermodynamic cycle.22,23 Owing to the high solvation energy of the hydride ion and the charge imbalance in the M−H bond dissociation reaction (two ionic species at the product side), erroneous results are expected in computational calculation. To circumvent this problem, DuBois et al. proposed a method to characterize hydride donor abilities in a series of Co octahedral © XXXX American Chemical Society

Received: December 6, 2016 Revised: March 23, 2017 Published: March 23, 2017 A

DOI: 10.1021/acs.jpca.6b12271 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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Table 1. Thermodynamic Parameters and VH Valuesa

accurate to get a reliable prediction that demands high-accuracy calculation, and hence, computationally the procedure becomes expensive. Herein we will show that the MESP at the hydride nucleus, VH, is reliable to make a good prediction on the hydride-donating tendency of the M−H bond. Hence, VH will be used as a measure of the thermodynamic hydridicity to screen several metal hydride complexes.

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COMPUTATIONAL METHODS All of the metal hydride complexes were optimized at B3LYP38,39 level density functional theory using Gaussian09 software.40 The Hay and Wadts effective core potential (ECP) was used for transition metals to replace the core electrons41,42 along with LanL2DZ basis set and additional f polarization functions. Other atoms were described using the 6-31G(d,p) basis set, and the combined basis set is named Gen1.43 All of the structures were confirmed as minima by locating only positive vibrational modes in the frequency calculation. For more accurate energy parameters, single-point calculations were done with LanL2TZ43 with the Hay and Wadts ECP for transition metals and 6-311++G(d,p) for all other atoms (Gen2 basis set). The free energy values were obtained by summation of the Gibbs free energy correction at B3LYP/Gen1 and the single-point SCF energy at B3LYP/Gen2. The solvent effects were incorporated through the SMD-UAHF method as implemented in Gaussian09.44 The selected solvent was acetonitrile because the experimental dissociation energy as well as pKa values for most of the metal complexes and organic acids were conducted in acetonitrile media.45 The MESP at nuclei, VA, is given by46 N

VA =

∑ A≠B

ZB − |R B − R A |

∫

ρ(r′) d3r′ |r − r′|

(1)

where ρ(r′) is the electronic charge density and N is the total number of nuclei in the molecule. ZB is the charge on the particular nuclei at RB. VH is evaluated for optimized structures. All phosphine ligands are modeled with PH3 to reduce the computational cost. In fact, real models do not significantly change the VH values or their trends.47 However, to assess the usefulness of VH to predict pKa, four Mo and three W complexes with experimentally known pKa values have been studied.

complexes

ΔGg

ΔGs

ΔG°H−

VH

Mn(CO)5H iso-Mn(CO)4(PH3)H-a iso-Mn(CO)4(PH3)H-b iso-Mn(CO)3(PH3)2H-a iso-Mn(CO)3(PH3)2H-b iso-Mn(CO)3(PH3)2H-c iso-Mn(CO)2(PH3)3H-a iso-Mn(CO)2(PH3)3H-b iso-Mn(CO)2(PH3)3H-c iso-Mn(CO)(PH3)4H-a iso-Mn(CO)(PH3)4H-b Mn(PH3)5H Mn(CO)2(dpp)H Mn(CO)(dpp)2H MnN2(dpp)2H MnN2(PH3)4H Re(CO)5H iso-Re(CO)3(PH3)2H-a iso-Re(CO)3(PH3)2H-b Re(CO)(PH3)4H Re(CO)2(PH3)3H Mo(CO)2(PH3)2(NO)H Mo(CO)2(dpe)(NO)H Mo(CO)(PH3)3(NO)H Mo(NO)(dpe)2H Mo(PH3)4(NO)H Mo(NO)(dpcp)2H W(CO)3(PH3)(NO)H W(CO)2(PH3)2(NO)H W(PH3)4(NO)H W(CO)(PH3)3(NO)H W(NO)(dpe)2H W(CH)(dpe)2H

−27.15 −35.84 −37.80 −54.15 −52.00 −54.53 −63.56 −66.99 −64.27 −78.27 −77.82 −84.35 −62.33 −82.46 −83.18 −79.60 −20.49 −46.73 −46.07 −68.18 −57.90 −61.54 −64.42 −71.25 −85.32 −80.27 −89.18 −41.30 −52.48 −70.45 −62.18 −76.95 −93.03

8.37 7.35 13.48 16.52 17.16 15.54 21.08 18.80 26.53 21.65 25.60 25.67 21.68 26.55 27.72 22.98 −9.56 2.40 2.29 11.56 6.79 5.00 8.83 9.59 19.17 13.71 22.24 −5.40 0.70 9.73 4.83 15.17 24.30

59.61 49.89 54.07 40.75 43.55 39.40 35.90 30.19 40.65 21.77 26.16 19.71 37.74 22.47 22.92 21.77 48.34 34.06 34.61 21.76 27.28 21.85 22.79 16.73 12.23 11.82 11.44 31.68 26.60 17.66 21.04 16.61 9.66

−680.64 −696.19 −694.39 −716.06 −708.62 −712.67 −722.01 −725.63 −723.25 −736.47 −734.84 −743.66 −721.07 −741.15 −744.83 −744.98 −695.87 −721.52 −722.42 −740.40 −732.50 −726.65 −729.60 −736.68 −747.73 −743.48 −752.03 −718.64 −729.38 −746.11 −739.47 −749.85 −752.58

a

All values are in kcal/mol. Abbreviations: dpp = 1,2-diphosphinepropane, dpe = 1,2-diphosphine-ethane, and dpcp = 1,2-diphosphinecyclopentane. Some structures are isomers (iso) due to the various arrangements of ligands around the metal center, and the corresponding structures are shown in the Supporting Information.

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Mn(PH3)5H shows a higher negative VH value of −743.66 kcal/ mol than its lower analogous complexes. These analogous complexes (including isomers), such as Mn(CO)4(PH3)H, Mn(CO)3(PH3)2H, and Mn(CO)2(PH3)3H, show VH values in the range of −696.19 to −734.84 kcal/mol. The nature of the metal influences the electron-rich character of the hydride ligand. However, occurrence of exceptional cases is due to the influence of electron-withdrawing ligands, which overshadow the effect of the metal center. In our previous paper, we demonstrated on the basis of the most negative electrostatic potential (Vmin) at the hydride ligand that W and Mo complexes are more effective for the water splitting reaction than other metal complexes.47 In these metals, the d orbital is fully occupied and, hence, they back-donate electrons to the hydride ligand. A similar case was applicable to Re and Mn complexes. Groups like NO and CH present at the trans position with respect to the hydride ligand increased its electron-rich character, and such an effect was attributed to the trans influence.55 The best example displaying this feature is W(CH) (dpe)2H, where VH is −752.58 kcal/mol.

RESULTS AND DISCUSSION The general combination of ligands with metal centers Mo, W, Mn, and Re are given in Scheme 1 (total of 33 complexes). Scheme 1. Metal Hydride Complexes Considered for This Study

Most of them with monodendate and bidentate phoshine ligands are reported in the literature.48−54 In Table 1, VH values at the hydride nucleus are given for all of the complexes. All VH values are in the range of −680.64 to −752.58 kcal/mol (Table 1). The most negative values of VH are observed for those complexes having more phosphine ligands. For illustration, B

DOI: 10.1021/acs.jpca.6b12271 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A To study the hydride donor character of the M−H bond in a complex, heterolytic cleavage of the M−H bond to M+ and H− is considered (eq 2). Equation 2 can be written as the sum of eqs 3−5. Hence, eqs 3−5 constitute a thermodynamic cycle for the reaction given in eq 2. L5M−H → L5M+ + H−

(2)

L5M−H + H3O+ → L5M+ + H 2 + H 2O

(3)

H 2O + H+ → H3O+

(4)

H 2 → H + + H−

(5) Figure 1. Correlation between VH and ΔG°H− for the hydride abstraction reaction of the metal hydride complexes with H3O+. Red and green symbols indicate group IV and VII complexes, respectively.

If ΔG1, ΔG2, and ΔG3 are the free energy change in solution for eqs 3−5, respectively, the hydridicity ΔG°H− for eq 2 can be written as ΔG°H− = ΔG1 + ΔG2 + ΔG3

(6)

coefficient becomes 0.980, 0.996, 0.957, and 0.971, respectively. It may be noted that ΔG°H− values are usually studied with base Et3NH+ in many types of complexes. The present results show that H3O+ is more favorable for reaction with metal hydride complexes. Further, the use of a water medium instead of acetonitrile solvent is attractive because water provides H3O+ ions (Supporting Information), and hence, establishing the hydridicities of metal hydride complexes in aqueous media is very promising.16 The correlation equation in Figure 1 suggest that ΔG°H− values of metal hydride complexes can be expressed in terms of VH. Therefore, VH could be used as an electronic descriptor for assessing the hydride donor ability of metal hydride complexes. We have also tested the applicability of the relationship between VH and ΔG°H− to more realistic examples reported in the literature. The square-pyramidal Mo and W hydride complexes reported by Sarker and Bruno in ref 60 come in very handy here (Figure 2).60 They have reported the free energy for hydride donation in acetonitrile using infrared spectroscopy. We have computed the VH values of these complexes using B3LYP/Gen1. The M−H distances of these complexes are in the range of 1.697−1.724 Å, which reflects the large variation in the ligand environment. The experimentally reported ΔG°H− values of these complexes lie in the range of 79.4−88.6 kcal/mol, meaning that the M−H bond is very strong and difficult to cleave, which can be attributed to the five-coordinate configuration of the metal center and the lack of a trans influencing ligand for the M−H bond.61 The computed VH values of these complexes are in the range of −699.67 to −675.82 kcal/mol. These values fall in the lower range of VH values of octahedral complexes. The VH values showed good linear correlation with the experimental ΔG°H− values (correlation coefficient of 0.934; Figure 3). This correlation strongly supports the sensitive nature of VH to describe the hydride donor ability of metal hydride complexes. Further, VH values strongly depend on the overall shape of the complexes, which is clearly seen in the difference in the linearity trends and slopes for both sets (Figures 1 and 3).

According to the DuBois and Papai method, ΔG2 = −1.37pKa(H3O+) kcal/mol and ΔG3 = 76.0 kcal/mol.25 Hence, thermodynamic hydridicity ΔG°H− = ΔG1 − 1.37pK a(H3O+) + 76 kcal/mol

(7)

where ΔG1 values of all complexes are calculated from the sum of the gas-phase free energy (ΔGg) and solvation free energy of the reaction (ΔGs). The pKa value of H3O+ is −1.74. The ΔGg values of all reactions show exothermic (−20 to −93 kcal/mol) character. Except for two cases, ΔGs values show positive values in the range of 1−28 kcal/mol (Table 1). The ΔG°H− values are in the range of 60−10 kcal/mol. The high withdrawing power of H3O+ to abstract the H− ion is very clear in the low ΔG°H− values. Among all of the complexes, the lowest ΔG°H− of 9.66 kcal/mol is observed for W(CH)(dpe)2H. Most of the group VI complexes show ΔG°H− values below 25 kcal/mol, whereas the majority of the group VII complexes show ΔG°H− well above 25 kcal/mol. It is also important to note that the ligand environment influences ΔG°H−. Electron-donating ligands, especially phosphine ligands, always decrease the ΔG°H−. From Table 1, it is clear that those complexes with lower ΔG°H− values contain more phosphine ligands. For example, Mo(CO)2(PH3)2(NO)H shows high ΔG°H− values (21.85 kcal/mol) compared to Mo(CO)(PH3)3(NO)H (16.73 kcal/mol), which in turn shows high ΔG°H− values compared to Mo(PH3)4(NO)H (11.82 kcal/ mol). Also ΔG°H− decreases with an increase in the atomic number of the metal center in each group. This feature is obvious in the case of different metal centers with the same ligand environment. For example, the ΔG°H− values of Mn(CO)5H and Re(CO)5H are 59.61 and 48.34 kcal/mol, respectively. One example for comparison is the ΔG°H− values of Mo(PH3)4(NO)H (11.82 kcal/mol) and W(PH3)4(NO)H (17.66 kcal/mol). In general, a relatively small ΔG°H− value correlates to a high negative VH value, meaning that hydride removal is easier for the metal complex having an electron-rich hydride ligand. The strong linear correlations between ΔG°H− and VH shown in Figure 1 support the above argument. The hydride dissociation in the cases of Mo and W complexes is easier than that for Mn and Re complexes, and hence, the former systems are attractive for many catalytic applications.56,57 In fact, some metal hydride complexes react with water under thermal conditions, affording H2 formation.7,58,59 If the correlation line is drawn for each set of Mn, Re, Mo, and W complexes separately, the correlation

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CONCLUSIONS The hydridicity ΔG°H− of various transition metal hydride complexes (metal = Mn, Re, Mo and W) has been studied at the B3LYP/Gen1//B3LYP/Gen2 level of theory, employing a thermodynamic cycle based on the base, namely, H2O in acetonitrile solvent. The MESP at the hydride nucleus, VH, of the metal hydride complex showed strong linear correlation C

DOI: 10.1021/acs.jpca.6b12271 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

Figure 2. Optimized geometries of five coordinated Mo and W hydride complexes. Cp = cyclopentadiene, *Cp = 1,2,3,4,5-pentamethylcyclopentadiene.

ORCID

Cherumuttathu H. Suresh: 0000-0001-7237-6638 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is supported by the CSC0129 project of CSIR, Govt. of India.

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(1) Boddien, A.; Loges, B.; Gartner, F.; Torborg, C.; Fumino, K.; Junge, H.; Ludwig, R.; Beller, M. Iron-Catalyzed Hydrogen Production from Formic Acid. J. Am. Chem. Soc. 2010, 132, 8924−8934. (2) Belkova, N. V.; Revin, P. O.; Epstein, L. M.; Vorontsov, E. V.; Bakhmutov, V. I.; Shubina, E. S.; Collange, E.; Poli, R. Kinetics and Mechanism of the Proton Transfer to Cp*Fe(dppe)H: Absence of a Direct Protonation at the Metal Site. J. Am. Chem. Soc. 2003, 125, 11106−11115. (3) Belkova, N. V.; Besora, M.; Epstein, L. M.; Lledos, A.; Maseras, F.; Shubina, E. S. Influence of Media and Homoconjugate Pairing on Transition Metal Hydride Protonation. An IR and DFT Study on Proton Transfer to CpRuH(CO) (PCy3). J. Am. Chem. Soc. 2003, 125, 7715−7725. (4) Cheng, T. Y.; Brunschwig, B. S.; Bullock, M. R. Hydride Transfer Reactions of Transition Metal Hydrides: Kinetic Hydricity of Metal Carbonyl Hydrides. J. Am. Chem. Soc. 1998, 120, 13121−13137. (5) Muckerman, J. T.; et al. Calculation of Thermodynamic Hydricities and the Design of Hydride Donors for CO2 Reduction. Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 15657−15662. (6) Boddien, A.; Mellmann, D.; Gartner, F.; Jackstell, R.; Junge, H.; Dyson, P. J.; Laurenczy, G.; Ludwig, R.; Beller, M. Efficient Dehydrogenation of Formic Acid Using an Iron Catalyst. Science 2011, 333, 1733−1736. (7) Kohl, S. W.; Weiner, L.; Schwartsburd, L.; Konstantinovski, L.; Shimon, L. J. W.; Ben-David, Y.; Iron, M. A.; Milstein, D. Consecutive Thermal H2 and Light-Induced O2 Evolution from Water Promoted by a Metal Complex. Science 2009, 324, 74−77. (8) Dixon, R.; Kahn, D. Genetic Regulation of Biological Nitrogen Fixation. Nat. Rev. Microbiol. 2004, 2, 621−631.

Figure 3. Correlation between experimental ΔG°H− values (kcal/mol) and VH (kcal/mol) for square-pyramidal W and Mo hydride complexes.

with ΔG°H−. VH is proposed as a measure of the hydride donor ability of the complexes. The usefulness of this theoretical approach is proved by correlating VH with experimentally determined ΔG°H− values for W and Mo hydride complexes. A higher negative VH indicated lower ΔG°H− values. Several Mo and W complexes showed small ΔG°H− values in the range of 10−30 kcal/mol, indicating their potential use in developing water splitting reactions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.6b12271. Thermodynamic values and Cartesian coordinates of all of the systems (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +91-471-2535506. D

DOI: 10.1021/acs.jpca.6b12271 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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