Substituent effects on cyclometalation: N-benzylideneanilines

Chemical Physics Letters 706 (2018) 334–337

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Research paper

Substituent effects on cyclometalation: N-benzylideneanilines Igor Novak a,⇑, Leo Klasinc b, Sean P. McGlynn c a

Charles Sturt University, POB 883, Orange, NSW 2800, Australia Physical Chemistry Department, Ruder Boškovic´ Institute, HR-10002 Zagreb, Croatia c Louisiana State University, Baton Rouge, LA 70803, USA b

a r t i c l e

i n f o

Article history: Received 13 March 2018 In final form 14 June 2018 Available online 15 June 2018 Keywords: Photoelectron spectroscopy Cyclometalation N-benzylideneanilines

a b s t r a c t The electronic structures of several derivatives of N-benzylidenaniline ligands (NBA) studied previously by HeI UV photoelectron spectroscopy (UPS) have been re-analyzed with outer valence Green’s function (OVGF) and ionization-potential equation of motion coupled cluster method (IP-EOMCC) quantum chemical calculations. The calculations allowed us to clearly identify molecular orbitals with predominantly nitrogen lone pair character and correlate their ionization energies with the experimental kinetic data on the cyclopalladation of imines. We have also rationalized the stability (or the lack of it) for NBA complexes with d-block transition metals. Ó 2018 Elsevier B.V. All rights reserved.

1. Introduction N-benzylideneanilines (NBA) studied in this work are examples of molecules with general formula R3-N = CR1R2 belonging to the large family of compounds called Schiff bases [1,2]. They are in widespread use as antifungal, antibacterial, anti-inflammatory and antiviral agents. NBA are also of great importance as ligands in metal complexes which are used to activate CAH bonds in a variety of organic synthetic processes [3,4]. We shall focus in this work on the relationship between the electronic structure of NBA molecules and their chemical reactivity towards metals, especially as represented in cyclometalation reactions. The electronic structures of several NBA have been studied in the gas phase using HeI photoelectron spectroscopy [5,6]. However, in these studies low level semi-empirical quantum chemical methods like HMO or NDDO were used to analyse the spectra. The NBA compounds studied in this work are given in Fig. 1. 2. Experimental and computational methods The details of the samples compounds 1–12 studied and the conditions under which UPS measurements have been performed had been reported previously [5,6]. The experimental spectra are available in refs. 5 and 6 and have been reproduced in Supplementary Information. The quantum chemical calculations were performed with the Gaussian 09 program [7] and included full geometry optimization of neutral molecules at MP2/6-311G(d,p)

level. This model chemistry was chosen because it reproduced well the molecular geometry and conformation of 1, the only NBA molecule whose molecular structure has been studied experimentally. The structure of 1 was determined in the gas phase by electron diffraction [8]. The NBA molecules are non-planar with the ring A being twisted (by angle HN) around CAN bond vs. the rest of the molecule which is planar. The vibrational analysis for each molecule confirmed that the resulting geometry was the true minimum (no imaginary frequencies). Subsequently, the optimized geometry was used as an input into the single point calculation using the outer-valence Green’s function (OVGF) method and aug-cc-pVDZ basis set [9]. This method obviates the need for using Koopmans’ approximation and provides vertical ionization energies with typical deviation of 0.3–0.5 eV (depending on the size of the basis set). We have used aug-cc-pVDZ basis set for all OVGF calculations. The increase in basis set size e.g. by going to aug-cc-pVTZ does not result in significantly improved ionization energies while it incurs steeply rising computational costs (Supplementary information). We have also tried to use higher-order correlated methods e.g. ionization-potential equation of motion coupled cluster method (IP-EOMCC) implemented in ORCA software [10] to calculate ionization energies, but no sufficient improvement in calculated values compared to OVGF was noted to justify increased computing resources required (Supplementary Information). 3. Results and discussion 3.1. Substituent effects on electronic structure

⇑ Corresponding author. E-mail addresses: [email protected] (I. Novak), [email protected] (L. Klasinc), sean. [email protected] (S.P. McGlynn). https://doi.org/10.1016/j.cplett.2018.06.031 0009-2614/Ó 2018 Elsevier B.V. All rights reserved.

The HeI photoelectron spectra and their assignments are given in Supplementary Information. The most interesting information

I. Novak et al. / Chemical Physics Letters 706 (2018) 334–337

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Fig. 1. Structures and N-benzylideneanilines studied and ring labeling convention.

concerning the spectra is the influence of substituents on the electronic structure of these aromatic molecules. We have selected two types of orbital ionizations to analyze substituent effects: HOMO and nitrogen lone pair (nN) of the imine moiety. These two ionizations produce well resolved bands in the spectra whose ionization energies can be accurately measured in all cases. HOMO ionization comprises p-type orbital mostly delocalized over A ring with pCN contribution from C@N (imine) moiety. The substituent effects are summarized in Table 1. The fluorine substituent is strongly inductively electron withdrawing (EW) and it stabilizes (increases orbital ionization energy) both HOMO and nN to equal extent. Nitro substituent is also electron withdrawing (EW) by induction and by resonance mechanisms as can be gauged by comparing ionization energies of 1, 5 and 6. The EW effect is stronger on nN than on HOMO (Table 1). The effect of OMe group is electron donating (ED) via both induction and resonance as can be seen from comparison of HOMO and nN ionization energies in 1, 9 and 10 (Table 1). The combined effect of OMe and nitro substituents (based on comparison of 1, 11 and 12) on orbital energies leads to very small

changes in HOMO energy (EW and ED effects of the two groups approximately cancel out). However, we observe significant net EW stabilization of nN as the result of presence of the two substituent groups. Why is there a different effect on p-type HOMO and r-type nN orbitals? OMe group is ED by resonance while nitro group is EW by resonance so that the electron density reduction at nitrogen atom by nitro substituent is approximately ‘‘compensated for” by ED of the methoxy group. This results in the small net effect. On the other hand, OMe and nitro substituents are both EW by induction so the nN is stabilized by both substituents resulting in large EW effect (Table 1). The effects of methyl substitution can be gauged by comparing HOMO and nN ionization energies in two sets of molecules: 1, 3, 4 and 6, 7, 8. As expected, the methyl groups exert weak, net inductive ED effect and destabilize HOMO and nN orbitals; the destabilization is greater in dimethyl than in monomethyl derivatives (Table 1). Increased HOMO destabilization in 3 and 7 is due not only to the number of ED methyl groups present, but also to the larger HN angle compared to 4 and 8.

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Table 1 Substituent effects in NBA system evaluated by comparing ionization energies (eV) of HOMO and nitrogen lone pairs (nN)a,b. Molecule HOMO nN Molecule HOMO nN Molecule HOMO nN Molecule HOMO nN Molecule HOMO nN Molecule HOMO nN a b

1 8.23 10.02 1 8.23 10.02 1 8.23 10.02 1 8.23 10.02 1 8.23 10.02 6 8.62 10.81

2 9.21(+0.98) 11.04(+1.02) 5 8.58(+0.35) 11.08(+1.06) 9 7.96( 0.27) 9.78( 0.24) 11 8.02( 0.20) 11.00(+0.98) 3 8.00( 0.23) 9.86( 0.16) 7 8.51( 0.11) 10.50( 0.31)

Net effect EW EW Net effect EW EW Net effect ED ED Net effect ED/EW EW Net effect ED ED Net effect ED/nil ED

6 8.62(+0.39) 10.50(+0.48) 10 7.78( 0.45) 9.67( 0.35) 12 8.32(+0.09) 10.87(+0.85) 4 8.06( 0.17) 9.88(( 0.14) 8 8.66(+0.04) 10.61( 0.20)

EW = electron-withdrawing effect; ED = electron-donating effect. Numbers in brackets indicate ionization energy shift vs. parent molecule 1 or 6.

3.2. Substituent effects on cyclometalation and complex stability The NBA derivatives form five member ring derivatives involving metal (example is given in Fig. 2) in cyclometalation reactions [3–4]. These reactions lead to the activation of aromatic CAH bond and are thus of great importance in synthetic organic chemistry. Kinetic studies of reactions of NBA with palladium have been reported in acetic acid and in toluene media [12]. We compared the ionization energies of nitrogen lone pairs (nN) with rate constants for three NBA derivatives (Table 2). The comparison shows than in toluene, where the nitrogen lone pair of imine group is unprotonated, the highest nN ionization energy (in 5) corresponds to lowest reactivity (slowest reaction rate). The reaction rates in acetic acid medium are reversed because low nN ionization energy (1 and 9) leads to easier protonation of imine nitrogen and thus to slower formation of coordination complex with palladium acetate (Table 2). This observation is interesting because the formation of coordination complex is thought not to be the rate-determining step [12]. UPS determined ionization energies are also useful in

Table 2 Comparison of nitrogen lone pair ionization energies and rate constants for the cyclometalation reaction of palladium with NBA. Compound

nN/eV

k∙103/s at 298 K (acetic acid)

k∙103/s at 298 K (toluene)

1 9 5

10.02 9.78 11.08

1.1 1.5 2.3

1.4 1.2 0.58

explaining why 1 forms complexes with first row transition metals: Cr (III), Mn(II), Co (II), Ni (II), Cu(II), Zn(II) and Cd(II) while 6 does not [13] and why 6 does form complexes with second row transition element palladium. The rationalization is based on the comparison of nN orbital energies in 1 and 6, 3d orbital energies of first row transition metals and 4d orbital energy in palladium. The nN orbital energies in 6 and 1 are 10.81 eV and 10.02 eV, respectively. Based on the atomic spectral data we deduce that 4d orbital energy in Pd equals 8.34 eV while 3d orbital energies of Cr-Cd are all >15 eV [14]. Thus the poorer electron donating ability of nN in 6 precludes the formation of complexes between 6 and

Fig. 2. Structure of the complex formed in the reaction between N-benzylideneaniline and methyl pentacarbonyl manganese (ref. [11]).

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first row transition metals. On the other hand weaker nN donating ability of 6 is offset by lower 4d ionization energy in palladium with the result that the complex formation between 6 and palladium is still possible. 4. Conclusion UPS results pertaining to the electronic structures of Nbenzylidene ligands have allowed us to rationalize rates of some cyclometalation reactions in different solvents and the stability constants for some benzylidene complexes with transition metals. The ionization energy of the nitrogen lone pair appears to be the important parameter influencing not only the rates of cyclometalation reactions, but also stability and existence of Nbenzylideneaniline complexes with transition metals. Acknowledgement Authors thank the Ministry of Science, Education and Sports of the Republic of Croatia for the financial support through Project 098-0982915-2945 and Charles Sturt University for research grant (CSU ref.no. OPA 4068) (I.N.). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.cplett.2018.06.031.

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References [1] R. Hernandez-Molina. A. Mederos, Acyclic and macrocyclic Schiff base ligands, in: Comprehensive Coordination Chemistry II, 1 (2003) 411–446. [2] P.A. Vigato, S. Tamburini, The challenge of cyclic and acyclic schiff bases and related derivatives, Coord. Chem. Rev. 248 (2004) 1717–2128. [3] M. Albrecht, Cyclometalation using d-block transition metals: fundamental aspects and recent trends, Chem. Rev. 110 (2010) 576–623. [4] I. Omae, Applications of cyclometalation five-membered ring products and intermediates as catalytic agents, Mod. Res. Catal. 5 (2016) 51–74. [5] R. Akaba, K. Tokumaru, T. Kobayashi, C. Utsunomiya, Electronic structure and conformations of N-benzylidineanilines, Bull. Chem. Soc. Jpn. 53 (1980) 2002– 2006. [6] L. Klasinc, B. Rušcic, G. Heinrich, H. Güsten, Photoelectron spectroscopy of substituted N-benzylideneanilines, Z.Naturforsch. 32b (1977) 1291– 1295. [7] M.J. Frisch, et al. Gaussian09, rev. D-01; Gaussian, Inc.: Wallingford, CT, 2009. [8] M. Traetteberg, I. Hilmo, R.J. Abraham, S. Ljunggren, The molecular structure of N-benzylideneaniline, J. Mol. Struct. 48 (1978) 395–405. [9] D. Danovich, Green’s function methods for calculating ionization potentials, electron affinities, and excitation energies, WIREs Comput. Mol. Sci. 1 (2011) 377–387 (and references therein). [10] F. Neese, The ORCA program system, WIREs Comput. Mol. Sci. 2 (2012) 73–78. [11] M.I. Bruce, B.L. Goodall, M.Z. Iqbal, F.G.A. Stone, ortho-metalation of benzylideneaniline: structure of C6H5N :CH:C6H4Mn(CO)4, J. Chem. Soc. Dalton (1971) 1595–1596. [12] M. Gomez, J. Granell, M. Martinez, Solution behaviour, kinetics and mechanism of the acid-catalysed cyclopalladation of imines, J. Chem. Soc. Dalton Trans. 37–43 (1998). [13] D. Bilgiç, M. Pekin, S. Karaderi, M. Ülgen, The determination of stability constants para-substituted N-benzylideneaniline metal complexes by a potentiometric method, Rev. Anal. Chem. 23 (2004) 107–119. [14] J.E. Sansonetti, W.C. Martin, Handbook of basic atomic spectroscopic data, J. Phys. Chem. Ref. Data 34 (2005) 1559–2259.