Structure and Reactivity of Pd Complexes in Various Oxidation States

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Structure and Reactivity of Pd Complexes in Various Oxidation States in Identical Ligand Environments with Reference to C−C and C−Cl Coupling Reactions: Insights from Density Functional Theory Rajangam Jagadeesan,†,# Gopal Sabapathi,†,# Jaccob Madhavan,‡ and Ponnambalam Venuvanalingam*,† †

Theoretical and Computational Chemistry Laboratory, School of Chemistry, Bharathidasan University, Tiruchirappalli 620024, India Department of Chemistry, Loyola College, Chennai 600034, India

‡

S Supporting Information *

ABSTRACT: Bonding and reactivity of [(RN4)PdnCH3X](n−2)+ complexes have been investigated at the M06/BS2//B3LYP/BS1 level. Feasible mechanisms for the unselective formation of ethane and methyl chloride from mono-methyl PdIII complexes and selective formation of ethane or methyl chloride from PdIV complexes are reported here. Density functional theory (DFT) results indicate that PdIV is more reactive than PdIII and Pd in different oxidation states that follow different mechanisms. PdIII complexes react in three steps: (i) conformational change, (ii) transmetalation, and (iii) reductive elimination. In the first step a fivecoordinate PdIII intermediate is formed by the cleavage of one Pd−Nax bond, and in the second step one methyl group is transferred from the PdIII complex to the above intermediate via transmetalation, and subsequently a six-coordinate PdIV intermediate is formed by disproportion. In this step, transmetalation can occur on both singlet and triplet surfaces, and the singlet surface is lying lower. Transmetalation can also occur between the above intermediate and [(RN4)PdII(CH3)(CH3CN) ]+, but this not a feasible path. In the third step this PdIV intermediate undergoes reductive elimination of ethane and methyl chloride unselectively, and there are three possible routes for this step. Here axial−equatorial elimination is more facile than equatorial−equatorial elimination. PdIV complexes react in two steps, a conformational change followed by reductive elimination, selectively forming ethane or methyl chloride. Thus, PdIII complex reacts through a six-coordinate PdIV intermediate that has competing C−C and C−Cl bond formation, and PdIV complex reacts through a five-coordinate PdIV intermediate that has selective C−C and C−Cl bond formation. Free energy barriers indicate that iPr, in comparison to the methyl substituent in the RN4 ligand, activates the cleaving of the Pd−Nax bond through electronic and steric interactions. Overall, reductive elimination leading to C−C bond formation is easier than the formation of a C−Cl bond.

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planes. The two pyridyl rings are roughly parallel to each other.8 This tetradentate macrocyclic ligand could adopt stable boat− boat conformations in metal complexes.8,9 The one-electron and two-electron oxidation of PdII leads to the formation of PdIII and PdIV complexes, respectively. Palladium(III) intermediates are reported to be formed in the reaction of dimethyl PdII complex with O2.10 The palladium(III) intermediate is also formed by the reaction of dimethyl PdII complex with the peroxy radical.11 The first report on the isolation and X-ray structure of the mononuclear PdIII complex and their aerobic oxidation and C−C/C-heteroatom bond formation was done by Mirica et al.6,7b PdIII and PdIV species have been proposed as reactive intermediates,12 yet they have been less commonly isolated and characterized.13 Further, it has been reported that the tridentate ligand N′,N″-trimethyltriazacyclononane (Me3tacn) could also

INTRODUCTION C−C and C−X bond formation is a key step in organic synthesis, and they find wider applications in the synthesis of pharmaceuticals, fine chemicals, materials, and polymer building blocks.1 Transition metal catalysts are used in such cross coupling reactions1e and particularly Pd among the transition metals has been used for C−C coupling reactions.1c,f Palladium catalyzed Suzuki coupling,2 Stille coupling,3 Heck coupling,4 and Sonogashira coupling5 are efficient methods to form C−C bonds. Oxidation of organometallic PdII species generates high-valent PdIII or PdIV intermediates that undergo facile reductive elimination and C−C bond formation.6 On many occasions, Pd catalysis involves highvalent palladium centers. The PdIII and PdIV species have been reported as active intermediates for several chemical and stoichiometric aerobic transformations.7 The free R N4N,N-di-alkyl-2,11-diaza[3,3](2,6)pyridinophane has a pseudo C2v symmetry with a C2 axis passing through the center of a 12-membered ring and two pseudo-mirror © XXXX American Chemical Society

Received: January 30, 2018

A

DOI: 10.1021/acs.inorgchem.8b00239 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. Structure of the complexes with RN4 ligand and Pd in different (+2, +3, and +4) oxidation states.

stabilize the halogen-bridged dinuclear PdIII complex.14 These high-valent PdIII and PdIV centers are stabilized with a multidentate flexible ligand such as RN4 = N,N′-di-alkyl-2,11-diaza[3,3](2,6)pyridinophane,15 and these complexes undergo thermal or photo-induced reductive elimination forming ethane and methyl chloride.16 The aerobic oxidations of organometallic PdII complexes also directly yield ethane via an elimination reaction.7a During catalysis, thermal or light-induced C−C and C−X reductive eliminations formation of PdIII and PdIV intermediates have been observed.16 Mirica and co-workers have done lot of experimental work to understand the mechanisms of these reactions, but some critical issues remain unclear. Therefore, we model these reactions here using DFT with a focus on the following points: (i) Structure and reactivity of Pd complexes in various oxidation states in C−C and C−Cl coupling reactions. (ii) Mechanism and selectivity of these reactions toward ethane or methyl chloride formation. (iii) Reductive elimination via equatorial−equatorial or equatorial−axial paths. (iv) Nature of bonding between ligand and Pd that influences the reactivity: σ-donation vs. π-back-donation. (v) The role of R in R N4 (R = Me, iPr) in C−C and C−Cl bond formation.

complexes are taken up. In each section the effect of substituent R in RN4 ligand on the reaction is analyzed. Finally the synergy of computed data with experiments is presented. [(RN4)PdnCH3X](n−2)+ (n = 2, 3, and 4) Complexes. In an identical ligand environment with RN4 as ligand, PdII, PdIII, and PdIV centers assumes square planar, distorted octahedral, and octahedral geometry, respectively. Particularly RN4 ligand stabilizes the complexes with Pd in higher oxidation states. Structures of the complexes with Pd in different (+2, +3, and +4) oxidation states are shown in Figure 1. In the complexes there are no PdII−Nax bonds, and the PdIII−Nax bond is longer than the PdIV−Nax bond. Pd−Neq bond lengths in all the complexes are found to lie in the range 2.127−2.287 Å, and the decrease of Pd−Nax is due to increasing electrophilicity of the Pd in various oxidation states.17 The Kohn−Sham energy level diagram is presented in Figure 2, and d-based molecular orbital pictures are given in Figure S1. The electronic configuration and orbital ordering in PdIII in these complexes is found to be as follows: dxz2 < dyz2 < dxy2 < dz21 < dx2−y2. With increasing oxidation state from PdII to PdIV, d-orbitals progressively stabilize (Figure 2). NPA charges reveal that the negative charge on the Nax atom (N3 and N4) decreases from PdII to PdIV complex, and this is because the σ-donation of the RN4 ligand to Pd increases with the oxidation state, while back-donation decreases in the reverse order. When the oxidation state increases, the Pd−Neq and Pd−Nax bond lengths decrease as σ-donation increases (Table S1). Overall, high-valent oxidation states are largely stabilized by an increase in the σ-donation of the RN4 (R = Me, iPr) ligand to Pd center. It plays a vital role in the reactivity of these complexes especially in C−C and C−Cl reductive elimination.18 Reactivity of PdIII Complexes. PdIII complexes are formed by one-electron oxidation of [(RN4)PdIICH3X]. These complexes are reported to be active catalytic intermediates in aerobic oxidative photo-induced C−C bond formation by PdII complexes.7a PdIII centers are paramagnetic d7 systems and have Jahn−Teller distorted octahedral geometry. Mirica et al. stabilized the PdIII center with the RN4 ligand.6,7,16a Geometry optimization of PdIII complexes (1+−4+) have led to distorted octahedral geometry in agreement with the X-ray structure. Selected bond parameters of the complexes are listed in Table S2 along with X-ray data.16a Notably the Pd−Neq bonds are smaller than the

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RESULT AND DISCUSSION The structure of the complexes and intermediates with Pd in +2, +3, and +4 oxidation states along with atom numbering are given in Figure 1. R in the RN4 ligand denotes methyl (1, 3) or iPr (2, 4) and X represents Cl (1, 2) or methyl (3, 4) ligand in the complexes. While PdII complexes are neutral (1−4), PdIII and PdIV complexes are cations (1+−4+) and dications (12+−42+), respectively. Correspondingly, the intermediates and transition states are denoted as INT1 or INT2 or INT3 and TS1 or TS2 or TS3 or TS4 or TS5 respectively prefixing reactant names. TSs in singlet and triplet surfaces (TS2) are denoted as 1TS2 and 3TS2 respectively. First, bonding in the above complexes and specifically bonds originating from Pd, are discussed to understand how Pd in different oxidation states reorganizes the bonds around it and activates certain bonds. N1 and N2 form equatorial bonds with Pd, and N3 and N4 form axial bonds with Pd. Therefore, N1 and N2 atoms are also denoted as Neq, and N3 and N4 atoms are marked as Nax in the discussion. Next, mechanisms of the reaction of PdIII complexes are discussed, and then those of PdIV B

DOI: 10.1021/acs.inorgchem.8b00239 Inorg. Chem. XXXX, XXX, XXX−XXX

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equatorial Cl atom develop a bond (1+-TS4B and 2+-TS4B) leading to formation of methyl chloride. From 3+-TS4B and 4+-TS4B ethane is formed. In path C, the equatorial methyl group and the equatorial Cl atom interact (1+-TS4C and 2+-TS4C) and form methyl chloride again. From 3+-TS4C and 4+-TS4C ethane is formed. In the C−C coupling reaction via path A, [(RN4)PdII(sol)Cl]+ and ethane are products and in the C−Cl coupling reaction via path B and path C [(RN4)PdII(sol)CH3]+ and methyl chloride (with mono-methyl complexes 1+ and 2+) and ethane (with dimethyl complexes 3+ and 4+) are formed as products. The structure of the intermediates and transition states of the reactions of PdIII complexes are presented in Figure 3. The free energy profiles for the reaction of PdIII complex are shown in Figure 4a−d. Optimized geometries of all the species on the reaction path for the reaction of 3+ are given in Figure 5 and the same for the reaction of 1+, 2+, and 4+ are compiled in Figure S3a−c. Selected bond lengths and bond angles of the optimized transition states (1+-1TS2−4+-1TS2, 1+-TS3−4+-TS3, and 1+TS4A−4+-TS4A, 1+-TS4B−2+-TS4B, and 1+-TS4C−4+-TS4C) of the reactions are listed in Table 2 and Table 3 respectively. Percentages (%) of bond cleavage (BCj) and bond formation (BFi) in the transmetalation (1+-1TS2−4+-1TS2), (1+-TS3−4+TS3) and reductive elimination (1+-TS4A−4+-TS4A, 1+-TS4B− 2+-TS4B, and 1+-TS4C−4+-TS4C) of the PdIII complexes are listed in Table 4, panels a−c, respectively. Conformational Change. Mirica and co-workers15 have reported that tetradentate pyridinophane ligands RN4 can stabilize PdII, PdIII, and PdIV oxidation states because of their conformational flexibility and the ability to adopt various coordination modes. In the bidentate mode, the two pyridyl N atoms bind with the Pd center, and the two axial amine N atoms point away from the metal center, and this is syn chair−chair conformation. In the tridentate coordination mode besides the two pyridyl N atoms one axial N atom also binds with the Pd center, and this is syn boat−chair conformation. In the tetradentate mode, all the N atoms bind with the Pd center, and this is syn boat−boat conformation. Reactant PdIII complex (1+−4+) exists in syn boat−boat conformation and under thermolysis or photolysis complex undergoes conformational change to syn boat−chair form cleaving one of the Pd−Nax bonds forming a five-coordinate intermediate(1+-INT1−4+-INT1) through a conformational transition state TS1. This can occur in the doublet or quartet surface. We found the quartet surface is high lying compared to doublet surface (see Figure S2) and therefore (1+-TS1−4+-TS1) are discussed here. These are conformational TSs on the doublet surface. Pd−Nax bond is weaker and therefore the free energy of activation for this conformational change lie in the range 20−25 kcal/mol and is an endergonic process (Figure 4a−d). Wiberg bond indices indicate that it is a “late” TS (Table S3). Transmetalation. The oxidative transmetalation of two mono-methyl PdII complexes to form a dimethyl PdIV species has been studied theoretically by Sanford et al.19 This intermediate reacts with one molecule of reactant (1+−4+) and the reactant complex transfers a methyl group to the intermediate via transmetalation. The possibility of the methyl transfer via free radical path is not considered here as the homolytic secession of C−C bond is highly energy demanding and as indicated by Mirica that it may be a less competitive or minor path. The methyl group migrates from the equatorial position of the complex to the axial position of intermediate. Transmetalation of methyl group between the complex (1+−4+) and intermediate (1+-INT1−4+-INT1) pass through an “early” transition state in

Figure 2. Kohn−Sham energy levels (eV) of the PdII, PdIII, and PdIV complexes (3, 3+, 32+).

Pd−Nax bonds in all the complexes, and this indicates that the latter bonds are activated and are ready to participate in the reaction. PdIII complexes undergo thermal- or photo-induced ethane or methyl chloride elimination, and based on the experimental observations reported12,16,19 a mechanism is proposed (Scheme1). This reaction can be considered to take place in three steps; (i) conformational change (ii) transmetalation and followed by disproportion (iii) reductive elimination. In the first step Reactant (1+−4+) breaks one of the Pd−Nax bonds and forms a fivecoordinate PdIII complex as an intermediate (1+-INT1−4+-INT1) via the transition state (1+-TS1−4+-TS1). This is a conformational change from the syn boat−boat form (Reactant) to the syn boat− chair form (INT1). Next is transmetalation; this can occur in two ways (Scheme 1). First, INT1 reacts with one molecule of Reactant and transfers one methyl group from the Reactant to 1+-INT1−4+-INT1 via transmetalation (TS2). Here there are two surfaces possible here, “singlet and triplet” and both TSs, 1TS2 and 3TS2, have been located and characterized. This 1,3TS2 undergoes disproportion and forms a sixcoordinate PdIV complex (1+-INT2−4+-INT2) and PdII complex ([(RN4)PdII(sol)X]+). Second, there is another possibility that the transmetalation can occur between INT1 and [(RN4)PdII(sol)CH3]+, but this possibility is less probable as this PdII complex was reported to be unstable by Mirica and coworkers.16 However, this possibility is also being considered here (Scheme 1). The TSs on the doublet surface (1+-TS3−4+-TS3) have been located and characterized and are discussed here. INT2 formation has been confirmed by Mirica and coworkers through crossover experiments. This intermediate (1+-INT2−4+-INT2) undergoes reductive elimination, and for this step three possible routes are proposed. In path A, the methyl groups in the equatorial and axial positions develop a bond (1+-TS4A−4+-TS4A) leading to the formation of ethane. In path B, the axial methyl group (CH3ax) and the C

DOI: 10.1021/acs.inorgchem.8b00239 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 1. Mechanism for the Ethane or Methylchloride Elimination from PdIII Complexesa

a

(i) Conformational change, (ii) transmetalation, (iii) reductive elimination.

which the migrating methyl group is closer to the initial PdIII complex (1+ −4+) than the approaching PdIII intermediate (1+-INT1− 4+-INT1) as revealed by the Pd−CH3 distance (Table 2). The geometry around the carbon of the transferring methyl group in the transition states is slightly pyramidal with all the three protons pointing toward the incoming PdIII intermediate (1+-INT1−4+-INT1), and the methyl group undergoes umbrella motion on the reaction coordinate. The animation of the single imaginary frequency of the transition structure clearly reveals this. The reaction can take place on singlet (1TS2) or triplet (3TS2) surfaces, and the spin state energetics show that the singlet is low lying with an energy gap of 2−3 kcal/mol for 1+-1TS2 and 2+-1TS2 and 10−14 kcal/mol for 3+-1TS2 and 4+-1TS2 complexes. The difference in the 1,3ΔG‡ can be attributed to presence of Cl in (1+-1TS2 and 2+-1TS2) and CH3 in the 3+-1TS2 and 4+-1TS2. Therefore, in the following discussion the singlet surface alone will be considered. Bond lengths presented in Table 2 show that Pdint−N1, Pdint−N2, and Pdint−N3 (1+-INT1−4+-INT1) are slightly lengthened in the 1+-1TS2− 4+-1TS2 to accommodate incoming methyl group. Pdint−C5 and Pdint−X6 bonds show very little changes in the 1+-1TS2− 4+-1TS2. Significant changes in bond length and angles could be observed around forming and breaking bonds in the transmetalation process. Wiberg bond indices for selected bonds in 1 TS2 at B3LYP/BS1 are given in Table S4a. In the 1+-1TS2− 4+-1TS2, Pdint−CH3 (1+-INT1−4+-INT1) is a forming bond and Pdcpx−CH3 (1+−4+) is a cleaving bond, and these two vital bond parameters are monitored. Here Pdint and Pdcpx indicate

palladium in the intermediate (1+-INT1−4+-INT1) and in the reactant (1+−4+) complex. While the forming bond matures in the range 7.3−29.2%, the cleaving bonds cleave around 7.4− 30.1% (Table 4a). BFCave values reveal that the 1+-1TS2−4+-1TS2 is an “early” TS or reactant like TS. Specifically Pdint−CH3 and Pdcpx−CH3 are longer and comparison of their lengths reveals that 1+-1TS2−4+-1TS2 is “early” TS with Pdcpx−CH3(1+−4+) shorter and Pdint−CH3 (1+-INT1−4+-INT1) is longer. Pdint− CH3−Pdcpx bond angle (162−167°) indicates that the methyl group is transferred from equatorial position of complex to axial position of the intermediate. Computed spin density value on Pd in the 1+-1TS2−4+-1TS2 (Pdint = 0.48−0.55; Pdcpx = 0.59−0.78) reveal that both palladium atoms in the 1+-1TS2−4+-1TS2 exist as PdIII and they undergo disproportion only in the “post TS phase”. In the transmetalation step the bulky iPr group repels the iPr group of the incoming reactant PdIII complex (1+-INT1− 4+-INT1 and 1+−4+) and therefore increases the barrier. This repulsion is less felt with the methyl substituent in the RN4 ligand. Therefore, the iPr group increases the barriers and makes the step more endothermic. Transmetalation of the methyl group can also occur via the reaction between INT1 and [(RN4)PdII(CH3)(CH3CN)]+ that comes out as a product at the end of the reaction (Scheme 1). Mirica and co-workers have reported that this PdII complex is very unstable, yet we have considered this possibility. The transition state corresponding to this step (1+-TS3−4+-TS3) has been located on the doublet surface and their optimized geometries are shown in Figure 5 and Figure S3a−c. Free energy profiles show D

DOI: 10.1021/acs.inorgchem.8b00239 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. (a) Structure of the intermediates and transition states of the reaction of PdIII complexes. (b) Dinuclear PdIII complexes with a bridging halide ligand.

Reductive Elimination. The six-coordinate PdIV intermediate (1+-INT2−4+-INT2) formed in step (ii) can undergo reductive elimination concertedly in three possible ways. In path A one axial methyl group and another equatorial methyl group break bonds with the PdIV center and simultaneously form a C−C bond between them leading to ethane elimination. In path B, the Pd−Cl bond in 1+-INT2 and 2+-INT2 and Pd−CH3ax cleave and a CH3−Cl bond forms. In 3+-INT2 and 4+-INT2 there are no chlorine atoms and therefore in path B ethane is eliminated instead of CH3−Cl. In path C, Pd−X partially breaks and bonds with the equatorial methyl group leading to elimination of CH3−X. It may be noted that CH3−X is methyl chloride in the

that TS3 is systematically higher than 1TS2, pointing to a less favorable pathway. We have also considered transmetalation of chlorine atom between the palladium centers. Calculations reveal that optimization leads to a complex with a bridging halide located at the middle between the palladium centers (Figures 3b and 5b), and the structure corresponds to a minimum on the reaction surface (no imaginary frequency). This is exactly a similar observation reported by Mirica et al. on dinuclear PdIII complexes with a single unsupported bridging halide ligand.14 Reaction of this species has not been considered further here as elimination of Cl2 in this reaction has not been observed. E

DOI: 10.1021/acs.inorgchem.8b00239 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. (a) Relative free energy profiles for transmetalation and reductive elimination of 1+ complex (R = CH3), (b) 2+ complex (R = iPr), (c) 3+ complex (R = CH3), (d) 4+ complex (R = iPr) computed at the M06/BS2//B3LYP/BS1 level.

reaction of 1+-INT2 and 2+-INT2 and ethane in the reaction of 3+-INT2 and 4+-INT2. One molecule of solvent occupies the vacant site created in the complex due to elimination, and this results the formation of either [(RN4)PdII(sol)Cl]+ (path A, 1+-P1−4+-P1) or [(RN4)PdII(sol)CH3]+ (path B and C, 1+-P2−4+-P2). Selected geometry parameters of transition states (1+-TS4A− + 4 -TS4A, 1+-TS4B−2+-TS4B, and 1+-TS4C−4+-TS4C) presented in Table 3 show some interesting trends. In the transition states (1+-TS4A−4+-TS4A and 1+-TS4C−4+-TS4C), the equatorial bonds Pd−N1 and Pd−N2 shorten and axial bonds Pd−N3 and Pd−C7 lengthen and the bond angle N3−Pd−C7 decreases. All the changes indicate that the geometry around Pd reorganizes from distorted octahedral PdIV to square planar PdII. The bond angles C5−Pd−C7, X6−Pd−C7, and X6−Pd−C5 fall in the range of 54−55.9°, and it is interesting to note the lowest lying TS has the highest bond angle. Wiberg bond indices presented in Table S3 show that when going from 1+-INT2−4+-INT2 to 1+-TS4A−4+-TS4A and 1+-INT2−2+-INT2 to 1+-TS4B−2+-TS4B, Pd−N1/N2 bonds are strengthened and Pd−N3 is weakened. From 1+-INT2− 4+-INT2 to 1+-TS4A−4+-TS4A, Pd−C5 and Pd−C7 bonds break and the C5−C7 bond develops. This indicates that the axial and equatorial methyl groups couple to eliminate ethane. From 1+-INT2−2+-INT2 to 1+-TS4B−2+-TS4B, the Pd−N1 bond strengthens and Pd−N2/N3 bonds weaken, Pd−X6 and Pd−C7 bonds cleave, and the X6−C7 bond develops. This shows that C−X coupling takes place eliminating methyl chloride or ethane respectively in mono- and dimethyl complexes.

From 1+-INT2−4+-INT2 to 1+-TS4C−4+-TS4C, the Pd−N1 bond weakens, while Pd−N2/N3 bonds undergo little changes as revealed by the bond order. The weakening of the Pd−N1 bond is due to the weakening of the equatorial Pd−X and Pd−CH3 bonds. The Pd−C5 and Pd−X6 bonds break and simultaneously the C5−X6 bond forms. This leads to elimination of methyl chloride or ethane as the case may be. The Pd−N3 bond progressively weakens and finally breaks in the product. The Pd−N3 bond weakens in the order TS4A > TS4B > TS4C, and this reflects in the respective barrier heights. That is, path C is systematically high lying than path B and path A. BCj, BFi, and BFCave values presented in Table 4c show that bonds form and break almost 40−50% and TS is “middle” TS, middle in the sense that it is neither “reactant” nor “product” like. The free energy profile of the reactions shown in Figure 4a,b reveals that the barrier increases in the order path A < path B < path C and exothermicity decreases in the order path A > path B > path C. This clearly shows that path A is highly favored over path B and path C. Selectively ethane is eliminated. It also shows between paths B and C, B (axial−equatorial elimination) is more favored than C (equatorial−equatorial elimination). The ΔG‡ values for path A (1+-TS4A−4+- TS4A) shows that this path for 3+-TS4A has the lowest barrier and highest exothermicity among the four. The free energy data further reveal that the iPr group on the RN4 ligand (2+ and 4+) (Figure 4b,d) have led to the lowest barriers than the methyl group on RN4 ligand in (1+ and 3+) complexes (Figure 4a,c). Here in the elimination step substituents (Me/iPr) do not suffer any steric repulsion, but the iPr group pushes more electron density toward PdIV than the methyl group, F

DOI: 10.1021/acs.inorgchem.8b00239 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 5. Optimized geometries of the (a) 3+ complex, intermediates (3+-INT1, 3+-INT2), and transition states (3+-TS1, 3+-1TS2, 3+-TS3, 3+-TS4A, 3+-TS4B, and 3+-TS4C). (b) Halide bridged complexes.

Table 1. Selected Bond Lengths (Å) and Bond Angles (deg) of the Optimized Transition States (TS1) at B3LYP/BS1a species +

1 -TS1 2+-TS1 3+-TS1 4+-TS1 a

Pd−N1

Pd−N2

Pd−N3

Pd−C5

Pd-X6

Pd−N4

N3−Pd−N4

2.255 2.250 2.279 2.269

2.141 2.135 2.278 2.272

2.309 2.389 2.351 2.423

2.053 2.055 2.056 2.057

2.329 2.329 2.056 2.053

3.344 3.382 3.390 3.423

137.7 137.3 134.2 134.2

For atom numbering, see Figure 1.

and this facilitates cleaving of the PdIV−CH3ax bond (path 2A and 2B) and PdIV−CH3eq bond (path 2C). This favors reductive elimination and makes the process more exothermic. CDA has been used to measure the electron donation from R N4 ligand to Pd and back-donation from Pd to RN4 ligand

during the reaction. The charge donation (σ) from RN4 to Pd decreases from Reactant to INT1 and then increases from INT1 to INT2, and this is due to changing mode of coordination of R N4 ligand to palladium. Very small changes are observed in π-back-donation from palladium to RN4 ligand (Table S5). G

DOI: 10.1021/acs.inorgchem.8b00239 Inorg. Chem. XXXX, XXX, XXX−XXX

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Table 2. Selected Bond Lengths (Å) and Bond Angles (deg) of the Optimized Transition States (1TS2 and TS3) at B3LYP/BS1 species

Pdint−N1

Pdint−N2

Pdint−N3

Pdint−C5

Pdint−X6

Pdint−CH3

Pdcpx−CH3

Pdcpx−CH3−Pdint

angle around the carbon (CH3)

1+-1TS2 2+-1TS2 3+-1TS2 4+-1TS2 species

2.351 2.356 2.360 2.323 Pdint−N1

2.195 2.177 2.350 2.363 Pdint−N2

2.384 2.471 2.399 2.479 Pdint−N3

2.058 2.063 2.061 2.064 Pdint−C5

2.392 2.386 2.060 2.062 Pdint−X6

2.635 2.603 2.822 2.777 Pdint−CH3

2.359 2.355 2.201 2.217 Pdsol−CH3

162.2 163.6 167.1 165.5 Pdsol−CH3−Pdint

359.5 359.9 351.9 354.1 angle around the carbon (CH3)

1+-TS3 2+-TS3 3+-TS3 1+-TS3

2.370 2.371 2.379 2.394

2.218 2.206 2.427 2.360

2.331 2.447 2.368 2.458

2.059 2.058 2.063 2.063

2.385 2.376 2.060 2.063

2.359 2.381 2.496 2.510

2.490 2.511 2.365 2.384

165.7 167.3 175.7 174.6

355.7 355.5 359.9 359.9

Table 3. Selected Bond Lengths (Å) and Bond Angles (deg) of the Optimized Transition States (TS4A, TS4B, and TS4C) at B3LYP/BS1a

a

species

Pd−N1

Pd−N2

Pd−N3

Pd−C5

Pd−X6

Pd−C7

C5−X6

C5−C7

X6−C7

C5−Pd−X6

C5−Pd−C7

N3−Pd−C7

X6−Pd−C7

1+-TS4A 2+-TS4A 3+-TS4A 4+-TS4A 1+-TS4B 2+-TS4B 1+-TS4C 2+-TS4C 3+-TS4C 4+-TS4C

2.210 2.203 2.329 2.252 2.289 2.294 2.811 2.766 3.166 3.225

2.132 2.109 2.308 2.319 2.183 2.191 2.235 2.295 2.198 2.174

2.549 2.722 2.538 2.708 2.478 2.552 2.306 2.333 2.349 2.442

2.233 2.220 2.224 2.198 2.048 2.049 2.541 2.563 2.190 2.195

2.332 2.336 2.039 2.045 2.366 2.373 2.353 2.360 2.212 2.206

2.283 2.261 2.276 2.270 2.625 2.614 2.063 2.073 2.058 2.060

3.262 3.166 2.967 3.116 3.178 3.218 2.291 2.307 2.062 2.067

2.073 2.091 2.001 2.040 3.063 2.995 3.059 2.926 2.892 2.873

3.105 3.146 2.969 2.853 2.309 2.329 3.030 3.015 2.992 2.986

91.2 87.9 88.1 94.4 91.8 93.1 55.7 55.7 55.9 56.0

54.6 55.6 52.8 54.3 80.9 78.9 82.6 77.5 85.7 84.8

145.8 143.3 152.1 145.8 153.3 157.5 176.1 176.6 165.9 162.2

84.5 86.4 86.7 82.6 54.8 55.4 86.4 85.5 88.9 88.7

For atom numbering see Figure. 1

Table 4. (a) Percentages (%) of Bond Cleavage (BCj) and Bond Formation (BFi) in the Transmetalation (1TS2) Involving PdIII complexes (1+−4+), (b) Percentages (%) of Bond Cleavage (BCj) and Bond Formation (BFi) in the Transmetalation (TS3) Involving PdIII and PdII Complexes (1+ −4+), and (c) Percentages (%) of Bond Cleavage (BCj) and Bond Formation (BFi) in the Reductive Elimination (TS4) Involving PdIII Complexes (1+ −4+) (a)

complex

BCj

BFi

Pd−CH3cpx

Pd−CH3int

BFCAVe

30.1 7.4 13.2 14.4

28.2 7.3 14.2 15.8

29.2 7.3 13.7 15.1

+ 1

1 - TS2 2+-1TS2 3+-1TS2 4+-1TS2

(b) BCj

BFi

complex

Pd−CH3sol

Pd−CH3int

BFCAVe

1+-TS3 2+-TS3 3+-TS3 4+-TS3

63.7 65.8 51.9 53.2

58.7 58.7 50.2 50.0

61.2 62.3 51.0 51.6

(c) TS4A

complex +

1 2+ 3+ 4+

TS4B BFi

BCj

BCj

TS4C BFi

BCj

BFi

Pd−C5

Pd−C7

C5−C7

BFCAVe

Pd−X6

Pd−C7

C7−X6

BFCAVe

Pd−C5

Pd−X6

C5−X6

BFCAVe

46.0 44.6 50.7 46.6

46.2 42.5 49.7 47.9

51.1 48.3 57.3 53.7

48.6 45.9 53.7 50.4

20.9 20.5

50.1 47.2

46.4 43.6

40.9 38.7

53.5 53.3 45.4 46.0

22.1 22.7 48.8 47.9

52.8 51.1 54.4 54.8

45.3 44.6 50.8 50.9

Reactivity of PdIV Complexes. The PdIV complexes are formed by two-electron oxidation of [(RN4)PdII(CH3)(X)]. The structure of the intermediates and transition states with Pd in PdIV complexes is given in Figure 6. The optimized structure of the PdIV complexes (12+−42+) has octahedral geometry in agreement

with the X-ray structure. Selected bond length and bond angles of the PdIV complexes are listed in Table S6. PdIV complexes undergo photo- or thermally induced selective reductive elimination of ethane and methyl chloride, and the mechanism for the reaction is proposed here (Scheme 2). There are two H

DOI: 10.1021/acs.inorgchem.8b00239 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 6. Structure of the intermediates and transition states of PdIV complexes.

Scheme 2. Mechanism for Ethane or Methylchloride Elimination from PdIV Complexesa

a

(i) Conformational change, (ii) reductive elimination.

Figure 7. Optimized geometries of the 32+ and 42+ complex, intermediates (32+-INT3 and 42+-INT3) and transition states (32+-TS5, 32+-TS6, 42+-TS5, and 42+-TS6).

(i) Formation of the five-coordinate Pd(IV) complex (INT3) via conformational transition state TS5. (ii) Reductive elimination via concerted transition state (TS6). Optimized geometries of all the species for the reaction of 32+ and 42+ are given in Figure 7, and those of 12+ and 22+ are given in Figure S4. Selected bond parameters of the optimized PdIV reactants (12+−42+) and intermediates (12+-INT3−42+-INT3) are listed in Table S6.

possibilities for transmetalation here. The first is the transmetalation between INT3 and Reactant. Experimentally, Mirica et al.16 have shown that there were no crossover products in this reaction, and this rules out transmetalation between INT3 and Reactant. Second is the transmetalation between INT3 and [(RN4)PdII(sol)2]2+ (P3). Here the P3 has no methyl group to transfer. Therefore, PdIV complexes reacts through two steps I

DOI: 10.1021/acs.inorgchem.8b00239 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 8. Relative free energy profiles for reductive elimination of (a) 12+ complex (R = CH3), (b) 22+ complex (R = iPr), (c) 32+ complex (R = CH3), and (d) 42+ complex (R = iPr) computed at the M06/BS2//B3LYP/BS1 level.

Table 5. Selected Bond Lengths (Å) and Bond Angles (deg) of the Optimized Transition States (TS5) at B3LYP/BS1a species 2+

a

Pd−N1

Pd−N2

Pd−N3

1 -TS5 22+-TS5 32+-TS5 42+-TS5 species

2.230 2.243 2.228 2.230 Pd−N1

2.104 2.098 2.228 2.228 Pd−N2

2.160 2.232 2.137 2.203 Pd−N3

12+-TS6 22+-TS6 32+-TS6 42+-TS6

2.380 2.287 2.596 2.511

2.026 2.041 2.056 2.056

2.222 2.280 2.207 2.334

Pd−C5 2.074 2.072 2.061 2.056 Pd−C5 2.487 2.440 2.203 2.194

Pd−X6

Pd−N4

2.294 2.296 2.061 2.059 Pd−X6

N3−Pd−N4

3.534 3.574 3.606 3.646 Pd−N4

137.8 137.0 136.1 135.4 C5−X6

2.326 2.332 2.218 2.213

2.428 2.464 2.155 2.156

For atom numbering, see Figure 6.

Percentages of bond cleavage (BCj) and bond formation (BFi) in the reductive elimination (12+-TS5−42+-TS5) of PdIV complexes are listed in Table 6. Free energy of activation (ΔG‡, kcal mol−1) for the conformational change and reductive elimination of PdIV complexes computed with M06 functional are presented in Figure 8a−d. The CDA of these complexes and intermediates are listed in Table S5. In the complex (12+−42+) the Pd−Nax bond (Pd−N3/N4) is longer than the Pd−Neq bond (Pd−N1/N2) in all the PdIV complexes, and this shows that the former bond is weaker and therefore could be easily cleaved than the latter bond. Conformational Change. Reactant PdIV complex (12+−42+) exists in syn boat−boat conformation, and these complexes undergo, under thermolysis or photolysis, conformational change

Table 6. Percentages (%) of Bond Cleavage (BCj) and Bond Formation (BFi) in the Reductive Elimination (TS6) Involving PdIV Complexes (12+−42+) BCj

BFi

complex

Pd−C5

Pd−X6

C5−X6

BFCAVe

12+-TS6 22+-TS6 32+-TS6 42+-TS6

51.1 48.7 45.0 44.8

18.4 15.1 41.8 40.4

40.7 37.7 48.9 48.8

37.7 34.8 46.2 45.7

to syn boat−chair form cleaving one of the Pd−Nax bonds forming a five-coordinate intermediate (12+-INT3−42+-INT3) through a conformational transition state TS5. (1+-TS5−4+-TS5) have J

DOI: 10.1021/acs.inorgchem.8b00239 Inorg. Chem. XXXX, XXX, XXX−XXX

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Synergy with Experiments. Experimentally Mirica et al. observed that high-valent PdIV complexes undergo facile and selective ethane formation, whereas PdIII complexes undergo non-selective ethane formation.9 The effect of methyl versus isopropyl substitution on the RN4 axial ligand has a bearing on the observed yield. On the basis of the computed free energy values, we found that the formation of ethane is selective with PdIV, while PdIII complexes produce ethane and methyl chloride in competing yields. Further, the role of alkyl substitution on the R N4 axial ligand is also well established through charge decomposition analysis and computed free energy values. Finally MO and charge analysis clearly justify the origin of the striking differences in the reactivity of two different high-valent PdIII and PdIV complexes. This is in good agreement with experimentally reported results. Further, the conformational flexibility of RN4 axial ligand in stabilizing high-valent oxidation states of palladium complexes and their role in ethane/CH3−Cl elimination is clearly explained through computations.

been located and characterized, and they show that the free energy of activation for this conformational change lies in the range 37−46 kcal/mol and is an endergonic process (Figure 8a−d). Compared to the PdIII complex, the barrier to the conformational change here is much higher. Pd−N4 bond length in the PdIV complex is shorter than that in PdIII complex, and this shows that it is a stronger bond in the former than in the latter and this explains the higher barrier for conformational change in the PdIV complexes. The Wiberg bond indices further indicate that the conformational TS is a “late” TS (Table S7). Reductive Elimination. The five-coordinate PdIV intermediate (12+-INT3−42+-INT3) undergoes thermal reductive elimination concertedly and selectively forming a single product, methyl chloride or ethane, while 12+ and 22+ form methyl chloride, and 32+ and 42+ form ethane. Bond lengths presented in Table 5 and Table S6 show that the Pd−N1/N2/N3 distances undergo smaller changes during the reaction, while breaking Pd−CH3 and Pd−Cl bonds undergo greater changes and CH3−Cl and CH3−CH3 bonds grow significantly. Wiberg bond indices listed in Table S7 reveal the progress of the reaction in quantitative detail. Pd−N1/N2/N3 bonds undergo little changes. Between Pd−N1 and Pd−N2 the former is weaker, and the latter is stronger in 12+-TS5−22+-TS5. The Pd−N4 bond is almost broken in 12+-INT3−42+-INT3, while the Pd−C5 bond cleaves to a greater degree and Pd−X6 undergoes little change in 12+-TS6−42+-TS6. Simultaneously the C−C bond grows significantly. In the product Pd−N3, Pd−C5, and Pd−X6 bonds totally break and C5−X6 forms completely. Pd−N1 and Pd−N2 bonds become stronger leading to the formation of PdII complexes (12+-P3−42+-P3) and CH3Cl/ CH3−CH3. In the reductive elimination Pd−C5 and Pd−X6 bonds cleave and the C5−X6 bond forms. Bond cleavage indices (Table 6) very clearly show Pd−C bonds are breaking >25% and Pd−Cl bonds break