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NBO Orbital Interaction Analysis for the Ambiphilic Metal−Ligand Activation/Concerted Metalation Deprotonation (AMLA/CMD) Mechanism Involved in the Cyclopalladation Reaction of N,N‑Dimethylbenzylamine with Palladium Acetate M. Arif Sajjad, John A. Harrison,* and Alastair J. Nielson*

Organometallics Downloaded from pubs.acs.org by UNIV OF WINNIPEG on 10/19/18. For personal use only.

Chemistry, Institute of Natural and Mathematical Sciences, Massey University, Private Bag 102904, North Shore Mail Centre, Auckland 0632, New Zealand

Peter Schwerdtfeger Centre for Theoretical Chemistry and Physics, Institute of Advanced Studies Massey University at Auckland, Private Bag 102904, North Shore Mail Centre, Auckland 0745, New Zealand S Supporting Information *

ABSTRACT: Natural bond orbital (NBO) analysis obtained from density functional theory (DFT) calculations on the intermediates and transition states in the ambiphilic metal−ligand activation/concerted metalation deprotonation (AMLA/ CMD) mechanism for the cyclopalladation reaction of N,N-dimethylbenzylamine with palladium acetate shows the agostic, syndetic (π or σ-electron density from the ring that assists the agostic donation), and backbonding orbital overlaps involved. The analysis shows that these components are absent for the anagostic intermediate but progressively increase in going from the anagostic/agostic transition state to the agostic intermediate and then to the agostic/cyclopalladate transition state. For the allimportant agostic/cyclopalladate transition state, agostic donation is very large [NBO E(2) total, 152.3 kcal mol−1], the syndetic π-donations total over half of this at 106.3 kcal mol−1 with the overlap forming in close proximity to the carbon where palladation occurs, and there is the emergence of σ-agostic donation (3.2 kcal mol−1). In comparison to the agostic intermediate, Pd to C−Hσ* back bonding increases marginally and CO lone pair donation to this antibonding orbital increases significantly. The total back-donations have an E(2) value of 72.7 kcal mol−1, which is nearly half the magnitude of the agostic donation and provides a significant influence on the lengthening of the C−H bond.

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INTRODUCTION Cyclometalation reactions1 are now widely used in organic synthesis methodology and have contributed significantly to the development of natural product, pharmaceutical and materials chemistry.2 Cyclopalladation using palladium(II) is especially useful as post functionalization of the Pd−C bond can occur to give new C−C bonds,3 and the process can be made catalytic.4 The cyclopalladation reaction involves C−H bond activation and knowledge gained on this process can help develop efficient methodology.5 In this regard, the mechanism of © XXXX American Chemical Society

cyclopalladation has undergone scrutiny by NMR and is believed to involve an anagostic intermediate where the C−H bond lies above the palladium square plane without activation and then moves on to an agostic stage in the square plane of palladium where activation takes place.6 For palladium acetate, this sequence has been studied by computation,7a where an important lengthening of the C−H bond in close proximity to an adjacent acetato ligand oxygen was identified for the Received: May 10, 2018

A

DOI: 10.1021/acs.organomet.8b00303 Organometallics XXXX, XXX, XXX−XXX

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Organometallics ambiphilic metal−ligand activation/concerted metalation deprotonation mechanism7b−d which is now the accepted mechanistic process.7b A recent advance in the activation step for aromatic rings by an agostic interaction8 has been the recognition of syndetic donation whereby π or σ-electron density from the ring assists the agostic donation.9 In view of this discovery, we have carried out a reinvestigation of the palladium acetate computational study with a new emphasis on the orbital interactions that are involved. We report here how the agostic and syndetic donations change for the various intermediate complexes and transition states identified in the original computational study. In particular, a full orbital interaction analysis of the all-important agostic/cyclopalladate transition state is given.

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RESULTS AND DISCUSSION In the original computational communication, the mechanism for the cyclopalladation of N,N-dimethylbenzylamine with palladium acetate (Scheme 1) was elucidated and included Scheme 1

Figure 1. Calculated PBE-D3 structures for the intermediates and transition states (TS) for the reaction of N,N-dimethylbenzylamine and palladium acetate.

Table 1. Selected Structural Data (Å and o) for 1−5 1

2

3

4

5

a

bond length and intramolecular bond separation metrics as well as NBO charge analysis which indicated the cyclopalladation involved an agostic state rather than a Wheland intermediate.7a This study was carried out using the BP86 functional,10 which conforms to the uniform electron gas (UEG) limit11 but does not include dispersion which is now considered to be important in computations of weak interactions.12a,b Recently, dispersion has been shown to be a fundamental component of agostic interactions.12c In the present study, we have used the PBE-D3 functional12a which includes dispersion and is known to perform well for both agostic interactions13 as well as anagostic interactions14 and has been used in our other computational studies on agostic interactions9 as well as anagostic interactions.15 To gain complete understanding of the structural changes occurring during the reaction between N,N-dimethylbenzylamine and palladium acetate and how they relate to the different interactions, the present PBE-D3 study includes a full analysis of the structural features for the various intermediates and transition states. The structures for these species are shown in Figure 1, and the relevant metrics for them are contained in Table 1. A comparison of the pertinent bond lengths and separations that were included in the original BP86 study with the present PBE-D3 work and a present BP86 study, are included in Table S1 of the ESI and shows that in general there is good agreement between the two different functionals. A reaction profile for the present PBE-D3 study is shown in Figure S4 of the ESI, which does not differ significantly from that published previously;7a therefore, a description is not included here, and the present work concentrates on the NBO analysis. Anagostic Intermediate (1). The PBE-D3 analysis shows that in the anagostic intermediate 1, the aromatic ring lies above the coordination plane with the aromatic ring rotated so that the C(2) hydrogen (numbering for the ligand aromatic

Pd−N Pd−O(cis) Pd−O(trans) C−H

2.082 1.998 2.092 1.090

O−Pd−N-C O−Pd−O-C Ar/CP angle Pd−N−C-C N−C−C-C torsion C−H deform

135.6 178.3 74.5 70.7 84.4 0.3

Pd···OC Pd···H Pd···C1 Pd···C2 Pd···C3 CO···H

2.086 2.866 3.439 3.375 4.519 2.602

Pd−N−C Pd···H−C Pd···C−H

108.4 108.5 53.7

Bond Length (Å) 2.097 2.106 2.090 2.006 2.006 2.040 2.028 2.021 2.040 1.107 1.165 1.350 Dihedrals (o) 168.4 173.9 196.6 147.4 141.0 140.1 78.6 129.2 137.9 49.1 43.4 45.8 67.9 52.5 44.7 7.7 20.7 36.6 Separations (Å)a 2.620 3.019 2.900 2.232 1.851 1.908 3.058 2.914 2.891 2.612 2.218 2.101 3.573 3.225 3.159 2.086 2.036 1.438 Angles (o) 107.7 105.7 104.5 97.1 91.8 78.2 58.0 56.5 62.8

2.102 2.040 2.182 153.5 73.9 156.6 38.2 29.8 2.854 1.965 3.017 105.8 -

a

Values to 3 decimal places consistent with level of optimization, SCF convergence, and XC grids.

ring is shown in Scheme 1) lies over the oxygen atom of the chelated acetate ligand and away from the Pd atom (Figure 1a). With this orientation, the O···H−C separation is 2.602 Å, and the Pd···H−C separation is 2.866 Å (metrics for the intermediates and transition states in the present work are shown in Table 1). These separations arise when the O−Pd−N−C torsion angle, which is a mark of the rotation about the Pd−N bond, is relatively flat at 135.6°, and the Pd−N−CH2−C(1) and N− CH2−C(1)−C(2) torsion angles, which are a measure of how B

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Organometallics the aromatic ring sits up over the coordination plane, are 70.7 and 84.4° respectively. In this situation, the angle that the aromatic ring makes with respect to the O-cis−Pd−N coordination plane (Ar/CP angle in Table 1) is 74.5° which indicates that the ring is inclined slightly toward the Pd center. To gain information about the orbital interactions involved in the close approaches identified, we have used NBO analysis,16 which allows a quantitative estimation of the energies involved by way of the second order perturbation energy, E(2). This type of analysis is necessary as orbital interactions obtained from molecular orbitals are descriptive only16 and do not provide a meaningful quantified approach. In this regard, it should be noted that NBO energies are not comparable to other types of energies but are themselves directly comparable. For the Pd···H−C interaction in the anagostic intermediate 1, the orientation of the aromatic ring is such that the NBO analysis (Table 2) does not show any agostic donation (C−Hσ

Table 3. QTAIM Atomic Charges (e) for 1−4 q(H) q(C1) q(C2) q(C3) q(C5) q(O) q(Pd)

C−Hσ to Pd (d) C−Hσ to Pd (s) C(1)−C(2) to Pd C(1)−C(2) to Pd C(2)−C(3) to Pd C(2)−C(3) to Pd

(d) (s) (d) (s)

-

C(2)−C(3)π to Pd(d) C(2)−C(3)π to Pd(s)

-

Pd to C(2)−C(3)π* Pd to C−Hσ* CO(1) to C−H σ* CO(2) to C−H σ* total to C−H σ*

0.6 0.6

2

3

Agostic 7.0 33.6 5.1 30.2 0.5 0.6 0.2 0.1 Syndetic 4.7 12.0 Back Donations 2.6 4.0 0.6 6.4 1.7 1.6 2.7 5.9 5.0 13.9

2

3

4

0.076 −0.006 −0.062 0.000 −0.010 −1.137 0.786

0.109 0.000 −0.101 0.014 −0.006 −1.157 0.766

0.112 0.000 −0.109 0.013 −0.007 −1.157 0.776

the interaction is weakly repulsive. In addition, there is a small negative charge on C(2) [q(C) value −0.005 e] so that the carbon and palladium atoms could participate in a weakly attractive interaction. However, the C···Pd separation is 3.375 Å, so there is unlikely to be any interaction as confirmed by the NBO analysis. In our other work on anagostic interactions, we have found that electronic substituents placed on the ring for an anagostic interaction can increase the negative charge on the carbon and actually induce η1-Pd−C covalency.15a In addition, for isoquinoline (benzo[H]quinoline) complexes of Rh, in which the ligand is unable to distort much due to the aromatic ring junctions, the anagostic hydrogen can come quite close to the metal at 2.184 Å15b,c (2.866 Å in 1). As already mentioned, the C(2) hydrogen lies over one oxygen atom of the chelated acetato ligand and with the O··· H−C separation at 2.602 Å, some O to C−Hσ* back bonding might be expected, which involves the oxygen lone pairs. However, the NBO analysis indicates a negligible contribution for this back bonding component with a second-order perturbation energy, E(2), of only 0.6 kcal mol−1. As to why the C(2) hydrogen sits over the acetate ligand oxygen would appear then to be related to electrostatics. To assess the magnitude of any Coulombic effect, we have again used QTAIM atomic basin charges18 for the oxygen and palladium atoms, which indicate that the O···H−C separation is an attractive electrostatic interaction [O and H atomic charges: q(O), −1.064 e and q(H) 0.030 e] and as such indicates a weak intramolecular hydrogen bond.19 As there is little if any O to C−Hσ* back bonding shown by the NBO analysis, electrostatics appear to be the main contributing factor in the positioning of the H atom. There does not appear to be any steric influences within the structure that could contribute. Anagostic/Agostic TS (2). The transition state between the anagostic and agostic intermediates shows a move of the aromatic ring away from the above-plane situation and more into the region of the coordination plane (Figure 1b). This occurs as the O−Pd−N−C angle, which represents the rotation of the N atom about the coordination plane becomes much flatter at 168.4° (135.6° in anagostic intermediate 1) and there is an associated change in the Pd−N−CH2−C(1) torsion angle to 49.1° from 70.7° in 1. As a result of this rotation, the side of the aromatic ring facing the Pd center is able to move much closer to the metal (C···Pd separation 2.612 Å in 2, 3.375 Å in 1) with the N−CH2−C(1)−C(2) torsion angle decreasing to 67.9° (84.4° in 1) so that the C(1)−C(2) bond is almost parallel to the Pd−N bond. These ligand rotational features are the main contributors to the ability of the aromatic ring to move closer to the metal center. Another contributor could be a decrease in the N−CH2−C(1) angle which can essentially be regarded as a “push back” angle when there is close contact. However, this angle only records a small change on moving to the transition state (114.5° in 1, to 109.9° in 2)

Table 2. E(2) Values (kcal mol−1) for the Agostic, Syndetic, and Back Donations for Complexes 1−4 1

1 0.030 −0.009 −0.005 −0.009 −0.012 −1.064 0.796

4 95.9 56.4 1.5 1.7 0.5 0.3 94.0 12.3 5.6 8.1 8.3 56.3 72.7

to Pd donation), C−Cσ agostic donation (C−Cσ to Pd donation17), syndetic C−Cπ donation9 [C(1)−C(2)π and C(2)−C(3)π to Pd donation], or back-donation from Pd orbitals to the C−Hσ* orbital. In other anagostic situations involving Rh and Pd, we have found that agostic C−Hσ donation to the metal can be sufficient to indicate an emerging or preagostic condition, and there is also a small amount of Pd to C−Hσ* orbital back-donation.15 With the absence of all these orbital interactions in 1, the C−H bond length shows no change in comparison to the free ligand value (1.090 and 1.092 Å, respectively), and the anagostic C−H bond deformation from the plane of the aromatic ring, which can be regarded as a measure of steric pressure of this bond with the Pd center in particular, is negligible at 0.3°. The apparent lack of orbital interaction and change in metrics involving the C−H bond apparently arises as the nature of the N,N-dimethylbenzylamine ligand allows it to keep its distance from the metal center. In the present investigation, we have used QTAIM atomic basin charges18 to assess the electrostatic components of the anagostic intermediate 1 (Table 3) and find that the C(2)−H hydrogen has a small positive charge [q(H) value 0.030 e] and the Pd atom is also positive but larger [q(Pd) value 0.796 e] so that C

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probably meaningless and should not be over interpreted. However, there are more meaningful donations with E(2) values of 1.7 and 2.7 kcal mol−1 which arise from lone pairs on the nearby acetato ligand oxygen atom donating into the C− Hσ* orbital. In this case, it is seen that the O···H−C distance is 2.620 Å, whereas in the anagostic intermediate 1, the distance is 2.086 Å. If the Pd to C−Hσ* orbital donation E(2) value of 0.6 kcal mol−1 is added to the 4.4 kcal mol−1 involving the O··· H−C interactions, the total is then 5.0 kcal mol−1. This is important as the agostic C−H bond lengthening (i.e., 1.107 Å in 2, 1.090 Å in 1) is not only related to the depletion of the C−H bond electron density but also affected by donations into the C−Hσ* orbital.9 Thus, the incipient C−H bond lengthening seen in the transition state has a significant lengthening component provided by back-donation. The synergy between agostic interactions and back-donation in facilitating C−H bond activation has been indicated previously.7b Agostic Intermediate (3). For the agostic intermediate in the reaction, further rotation about the Pd−N bond occurs in comparison with the previous transition state which results in the aromatic ring moving downward (Figure 1c). This rotation is best illustrated by the O−Pd−N−CH3 torsion angle which changes from 46.5 to 66.8° on moving from 2 to 3. There is also a small rotation about the N−CH2 bond with the Pd−N− CH2−C(1) torsion angles changing from 49.1 to 43.4° in 2 and 3, respectively. The O−Pd−N−C angle only increases to 173.9° from 168.4°, and there is also an insignificant change in the N−CH2−C(1) angle push back angle (109.2 and 109.9° in 3 and 2, respectively). There is also a significant rotation of the aromatic ring involved as shown by the N−CH2−C(1)−C(2) torsion angle decreasing from 67.9° in 2 to 52.5° in 3. This closing-up effect can be seen in Figure 2. A further indication

so is not a major feature in the aromatic ring positioning. For comparison, this angle is 113.1° in the free ligand. The feature that is most important in allowing the transition state to become a reality is the lengthening of the Pd−O bond involving the cis-coordinated oxygen of the acetato ligand that lies trans-to the benzylamine ligand nitrogen atom. This distance increases from 2.086 Å in 1 to 2.620 Å in 2, and at the same time, the O−Pd−O−C angle (i.e., the extent to which the coordination plane is deformed by the bidentate acetato ligand transforming to monodentate) decreases from the nearly planar angle of 178.3° in 1 to 147.4°. With this deformation occurring, which would appear to be the major contributing factor allowing the rotational aspects of the ligand already mentioned, the aromatic ring section facing the metal center becomes much closer to it with the Pd···C(1) and Pd··· C(3) separations decreasing (3.439 and 4.519 Å in 1, 3.058 and 3.573 Å in 2) as does the all-important Pd···C(2) separation (3.375 and 2.612 Å in 1 and 2, respectively) which will ultimately become the Pd−C bond. It is now seen that the C(2)−H bond begins to deform with the H−C(3)−C(2)−H torsion angle increasing to 7.7° compared with 0.3° in anagostic complex 1 and the C−H bond begins to lengthen (1.107 and 1.090 Å in 2 and 1, respectively). With this deformation and elongation in place, the Pd···H(2) separation is 2.232 Å compared with 2.866 Å in 1. At this point, it is obvious that the Pd···H(2) separation results from a combination of both the C−H bond deformation and elongation. Both features are associated with agostic interactions.8b,e,20 The positional characteristics of the ligand in this transition state not only bring the C and H atoms close to the metal, but there is also an inclination of the aromatic ring exposing πorbitals on one side of the ring to the metal. The NBO analysis then shows that there is agostic donation present, which consists of two components in which there is C−Hσ to Pd d orbital donation with a second-order perturbation energy, E(2), of 7.0 kcal mol−1 and also C−Hσ to Pd s orbital donation with an E(2) value of 5.1 kcal mol−1. Our previous work on agostic interactions has shown the presence of this two-component agostic interaction where the donations are to the lowest energy orbitals that are available to receive electron density. For example, in agostic PdCl2(L) complexes, there is one strong C−Hσ bond donation to the Pd−Clσ* orbital that lies trans to the agostic C−H bond and a weaker donation to the Pd−Clσ* orbital that lies cis to it.9 In addition to the small agostic component in transition state 2, there is also the emergence of syndetic π-donation9 now that the aromatic ring is positioned with one side of the πorbital system facing the metal. In this case, there is a single donation [E(2) value 4.7 kcal mol−1] to the metal from the C(2)−C(3)π orbital to the same Pd d orbital involved in the agostic donation. Where this type of donation is more welldeveloped in agostic interactions,9 there are two donations to metal orbitals but with the syndetic donation essentially emerging in 2, only one donation is energetically significant. Along with this syndetic π-donation, there is a small amount of back-donation from the metal to the C(2)−C(3)π* orbital with an E(2) value of 2.6 kcal mol−1. A further aspect to be considered in the C(2)−H interaction with the metal is back-donation from it into the C−Hσ* orbital, and it is seen for the transition state that a small component of this arises with an E(2) value of 0.6 kcal mol−1. It should be noted that in NBO terms, such small values are

Figure 2. Structures showing how the Pd···C(2) separation closes up in going from the anagostic/agostic TS 2 to the agostic intermediate 3. Separation distances in Å.

of what is happening can be seen from the angle the aromatic ring plane makes with the coordination plane (Ar/CP angle in Table 1) which shows how the ring flattens out (78.6 and 129.2° in 2 and 3, respectively). The changes to the various torsion angles up to and including the agostic stage can be seen in Figure 3, which, in particular, shows the dramatic increase in the Ar/CP angle at the agostic stage. The movement downward and the flattening out of the angle the aromatic ring makes with the coordination plane now places the C(2)−H bond so that it faces the metal center. The C(2)−H bond is not square on to the coordination plane but lies at an acute angle of 65° for the torsion angle involving D

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Figure 3. Variation of important torsion angles for 1−4.

C(2)−H···N−Pd and with the C atom lying slightly higher above the coordination plane the H atom lies below it (Figure 4). In this situation, the Pd···C(2) separation closes up

Figure 5. Variation of the C(2)−H bond properties for 1−4. (a) bond length; (b) deformation from the plane of the aromatic ring.

Figure 4. View down the Pd−O bond lying trans to the agostic C−H bond in 3 showing the acute angle made as it faces the Pd center and the positioning of the C and H atoms with respect to it.

from the upper section of the C(2)−C(3) aromatic ring πorbital to the metal but it only involves the Pd d orbital. Accompanying this is a small amount of back-donation from Pd to the C(2)−C(3)π* orbital [E(2) value, 4.0 kcal mol−1], which is only slightly larger than that found for transition state 2 [E(2) value, 2.6 kcal mol−1]. With an E(2) value of 12.0 kcal mol−1, the syndetic π-donation is approximately one-third that of the agostic donation for the same orbital, and this is similar to that found for the ratio of agostic to syndetic donations where the ancillary ligands are Cl rather than the present oxygens of the acetato ligands.9 A comparison of how the C(2)−H σ-bond agostic donation to the Pd d and s hybrid orbitals compare in terms of E(2) values for the various reaction profile components can be seen graphically in Figure 6, which also shows how the syndetic π-donation increases in comparison. Turning to the back-donations that are present, there is now meaningful Pd to C−Hσ* electron delocalization involved

significantly to 2.218 Å in 3 compared to 2.612 Å in 2 and the Pd···H separation is now 1.851 Å (2.232 Å in 2). However, with this close separation, the C(2)−H bond is now significantly distorted away from the plane of the aromatic ring (C−H deformation angle 20.7°, cf. 7.7° in 2) This deformation is a common feature of agostic interactions and appears to prevent the Pd···H separation becoming unacceptably short. For comparison, in complexes containing terminal Pd−H bonds, the bond distances are about 1.68 Å21a but can be as low as 1.51 Å21b and in bridging palladium hydrides can be as long as 2.13 Å.21b,c With the close approach of both the Pd···H and Pd···C separations, the C(2)−H bond distance is long at 1.165 Å (1.092 in the free ligand), which is another common feature found for agostic interactions. The significant changes to the C−H bond length and deformation angle at the agostic stage in comparison to the previous mechanistic stages can be seen in Figure 5. With the agostic interaction fully developed, the NBO analysis shows that the C−Hσ to Pd agostic donations still involve the Pd d and s orbitals of transition state 2, but now the donations are significantly larger with E(2) values of 33.6 and 30.2 kcal mol−1. The actual energies related to agostic interactions are estimated to lie in the range of 1−10 kcal mol−1,8c,22 which appears to be in the range for strong, linear N−H···N hydrogen bonds.22b The closeness of the C(2) carbon to the metal also allows a small component of syndetic C(1)−C(2)σ donation (agostic C−C donation17) to the Pd d and s orbitals, but the contributions are very small (0.5 and 0.6 kcal mol−1, respectively). There is also significant syndetic π-donation

Figure 6. Variation of the E(2) values for the agostic donation and syndetic π-donation to the palladium d and s orbitals for 1−4. E

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Organometallics where the E(2) value is 6.4 kcal mol−1. In addition, there are two donations from the nearby acetato ligand CO oxygen lone pairs (CO···H−C separation, 2.036 Å) with E(2) values of 1.6 and 5.9 kcal mol−1. In total, the back-donations to the C−Hσ antibonding orbital sums to 13.9 kcal mol−1, which compares with the total agostic electron density donation at 63.8 kcal mol−1. On the basis of these values, the total NBO energy associated with the weakening of the C−H bond from these two components is 77.7 kcal mol−1 with the agostic delocalization providing 82% and the back-donations 18%. The CO lone pair component of the back bonding is thus not the major feature influencing the weakening of the C−H bond. However, the orbital connection is important as the hydrogen moves to this oxygen on formation of the Pd−C bond and provides a pathway for the hydrogen removal.7a It is also noted that the total back-donation to the C−Hσ* orbital at 13.9 kcal mol−1 is only slightly larger than the syndetic π-donation from the aromatic ring at 12.0 kcal mol−1. A comparison of how the E(2) values for the backbonding components to the C(2)− Hσ* vary along the reaction pathway can be seen graphically in Figure 7.

there was little evidence for any contribution from a Wheland intermediate. It is seen that the present PBE-D3 study shows that there is no significant buildup of positive charge at C(1) and C(3) (Table 3), which is where this charge would arise if a Wheland intermediate was involved. A small positive atomic basin charge does build up at C(3) in comparison to C(2) or at C(5) [C(5) lies para to the C(2) carbon for classic aromatic ring carbocation delocalization], but generally, overall the results are in accord with the previous study. When assessing the effect of the atomic charges leading to weak interactions, it should be noted that dipole−dipole interactions tend to align components of molecules to increase the interaction and especially in intermolecular interactions, the potential energy of the system is minimized where the angle is 180°.21 In the present case, the angles involved with the various separations are quite small (see Table 1) so that arguments based on electrostatics should be treated with caution. In passing, it is also noted that any description of the Pd··· H−C interaction in particular as a form of weak hydrogen bonding8b,19b fails to meet the criteria associated with hydrogen bonds in that there is a significant attractive dipole−dipole interaction present with associated electron density back bonded into the H−Xσ* orbital.15 The observation that both the Pd and H atoms have partial positive charges associated with them points away from this description even though Pd to C−Hσ* orbital electron density delocalization occurs. On the other hand, the CO···H−C interaction involving the acetato ligand, has partial positive and negative charges associated with it and also has electron density delocalization involved so that it is more representative of being a weak hydrogen bond. Agostic/Cyclometalation TS (4). For the transition state 4, between the agostic and cyclometalated intermediates (Figure 1d), the aromatic ring moves further downward as shown by the O−Pd−N−CH2 angle changing from173.9° in the transition state 3, to 196.6° in 4. Again, this Pd−N bond rotation is better represented by the O−Pd−N−CH3 torsion angle of 77.7°, which has increased by approximately 10° and compares with the nearly 18° change that is involved in the Pd−N bond rotation in forming the agostic intermediate. There is now a small rotation upward about the N−CH2 bond with the Pd−N−CH2−C(1) torsion angles changing from 43.4° in 3 to 45.8° in 4 and very little change in the N−CH2− C(1) push back angle (109.2 and 108.3° in 3 and 4, respectively). There is also further rotation of the aromatic ring with the N−CH2−C(1)−C(2) torsion angle decreasing from 52.5° in 3 to 44.7° in transition state 4 so that instead of the aromatic ring plane lying almost parallel to the Pd−N bond in 3, it is inclined much more toward it (compare Figures 8a and b). In this case, the angle the aromatic ring plane makes with the coordination plane flattens out but the change is not large (Ar/CP angle 129.2° in 3 and 137.9° in 4). Changes to the important torsional angles involved in moving to transition state 4 in comparison to the other mechanistic stages are shown in Figure 3. The change in the structural features in proceeding to transition state 4, brings the C(2) atom very close to the metal with the separation being 2.101 Å (the separation was 2.218 Å in the agostic intermediate 3), which is now not much longer than the Pd−C bond in the cyclopalladated intermediate (1.969 Å). This close separation is achieved by way of the now very significant deformation of the C(2)−H bond from the

Figure 7. Variation of the backbonding components to the C(2)− Hσ* orbital for 1−4.

As mentioned earlier, the C(2) carbon is quite close to the palladium center in 3 (2.218 Å) after the various donations described have taken place, but it is also seen that there are electrostatic components that could be included. The QTAIM data in Table 3 shows that there is a small negative charge developed on C(2) [q(C2) value, −0.101 e] and a more substantial positive charge on the palladium center [q(Pd) value, +0.766 e] leading to an attractive electrostatic interaction that can now operate, which was not the case for the anagostic intermediate 1. There is still a small positive charge however on the C(2)−H hydrogen, which can be expected to make a repulsive interaction with the palladium center where q(Pd) is 0.766 e. This electrostatic repulsion is not large but must still contribute to the deformation of the C(2)−H hydrogen away from the plane of the aromatic ring, which is now 20.7° (0.3 and 7.7° in the anagostic intermediate and anagostic/agostic transition state 1 and 2 respectively). With the acetato ligand oxygen now quite close to the C(2)−H hydrogen (2.036 Å), there is an attractive electrostatic situation present [q(O) and q(H) values, −1.157 and 0.109 e, respectively, that will contribute to the formation of the weak hydrogen bond involved in the O···H−C separation. In the original mechanistic study calculations,7a MacGregor and co-workers commented on the charge buildup around the aromatic ring for 3 and concluded that as the increase in positive charge on any of the ring carbons was only +0.05, F

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representations of the strong overlap for the syndetic πdonation to the Pd d hybrid are shown in the NBO 2D orbital contour plot and 3D spatial overlap images shown in Figure 9.

Figure 8. Comparison of the aromatic ring rotation. (a) agostic intermediate 3. (b) agostic/cyclopalladate transition state 4.

plane of the aromatic ring which increases to 36.6° from the already significant value of 20.7° seen in the agostic intermediate. A pictorial indication of the extent of the deformation can be seen by comparing Figure 8b with 8a. An increase in the C(2)−H bond length to 1.350 Å in the transition state also appears to assist in allowing the close Pd··· C separation. The increases in the C(2)−H bond length and deformation from the aromatic ring plane along the reaction coordinate are presented graphically in Figure 5. As in the agostic intermediate, the C(2)−H bond in 4 still faces the metal center and remains at an acute angle to the coordination plane [C(2)−H···N−Pd torsion angle 57.6° compared to 64.9° in agostic intermediate 3], but the bond has moved slightly downward, and in this situation, the Pd···H distance actually increases to 1.908 Å compared with 1.851 Å found in the agostic intermediate. Further graphical comparison of the overall change in metrics for the mechanistic intermediates and transition states are contained in Figure S2 in the SI. NBO analysis of the transition state now shows that the interaction of the C(2)−H bond electron density with the palladium center is still present and now very well-developed, with the donations from the C−Hσ to Pd d and s hybrids reaching E(2) values of 95.9 and 56.4 kcal mol−1. In the agostic intermediate, these E(2) values were 33.6 and 30.2 kcal mol−1, respectively, so it is seen that the transition state involves a much greater donation to the d and s orbitals hybrid prior to the formation of the Pd−C bond than was found for the agostic intermediate. With the aromatic ring inclined toward the palladium center and the short Pd···C(2) separation of 2.101 Å, there is now a significant increase in the syndetic π-donation to the metal in the transition state, which is again to the same Pd d and s orbitals as were the agostic donations. The increase in electron density donation to the d orbital is substantial rising to an E(2) value of 94.0 kcal mol−1 compared with the agostic intermediate syndetic π-donation of only 12.0 kcal mol−1. This large E(2) value for the syndetic π-donation compares to the agostic component of 95.9 kcal mol−1 and shows that there is now significant electron density being donated to the metal from the upper C(2)−C(3) π-system of the aromatic ring and this occurs in close vicinity to the C(2) carbon, which is where the Pd−C forms. Back donation from Pd to the C(2)−C(3)π* orbital is still present but does not increase much in comparison to the agostic complex [E(2) values, 5.6 and 4.0 kcal mol−1 for 4 and 3, respectively] and does not match the significant increase in syndetic π-donation seen. Pictorial

Figure 9. 2D contour and 3D orbital plots (left and right) for 4 showing Pd orbital proximity to C(2) in TS 4. (a) the C(2)−C(3)π to Pd d syndetic donation [E(2) value, 94 kcal mol−1]; (b) the C(2)− C(3)π to Pd s syndetic donation [E(2) value, 12.3 kcal mol−1].

Turning to the π-donation involving the Pd s hybrid, the donation is much weaker with an E(2) value of only 12.3 kcal mol−1. The syndetic π-donation in the transition state prior to the formation of the Pd−C bond is thus mainly of C(2)− C(3)π to Pd d nature, and the overlap is maximized close to the C(2) carbon. In the cyclopalladated intermediate, the Pd− C bond does not contain any of this bonding type. Changes to the E(2) values for the agostic and syndetic π-donations occurring during the mechanism are presented in graphical form in Figure 6. Now that the C(2) carbon is close to the palladium center, and the syndetic C(2)−C(3) π-donation is well-developed in the transition state, some C−C σ-bond donation to the metal arises. As already mentioned, where this is found in other complexes it has been designated as C−C σ-agostic bonding.17 However, in the present transition state, it is not well developed with C(1)−C(2) and C(2)−C(3) σ-donation which are again to the same Pd d and s orbitals as the agostic and syndetic π-donations reaching E(2) values of only 1.5 and 1.7 kcal mol−1 and 0.5 and 0.3 kcal mol−1, respectively. As in the case of the syndetic π-donations, these σ-orbital overlaps are confined to being close to the C(2) carbon [see the 2D orbital overlap plots for the C(1)−C(2) σ-donations to the Pd d and s orbitals in Figure 10], and thus, there is σ-bond association with the metal at this transition state stage. In the agostic intermediate, back-donation from the metal into the C(2)−Hσ* orbital was of significance with the E(2) value reaching 6.4 kcal mol−1. In the agostic/cyclopalladate transition state 4, although the C(2) atom is close to the palladium center, the Pd···H separation is longer than in the agostic intermediate due in large to the pronounced G

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thus strong electron delocalization around the aromatic ring system when the acetato ligand lies cis to the Pd−C bond.

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CONCLUSIONS With the inclusion of the newly discovered syndetic donation, the work presented here now gives a full orbital picture associated with the ligand structural changes involved in the cyclometalation reaction of N,N-dimethylbenzylamine with Pd(OAc)2 for which the original computational study on the reaction profile7a gave the energetics of the process and pointed to the preference of an agostic intermediate over one with Wheland characteristics. In the present work, NBO analysis shows that there is no agostic donation seen in the anagostic intermediate which forms when the ligand is initially added to Pd(OAc)2, but this progressively increases in moving through the anagostic/agostic transition state to the agostic intermediate and finally the agostic/cyclopalladate transition state. Syndetic π-donation from the aromatic ring as well as back-donation from Pd orbitals and acetato ligand oxygen lone pairs to the C(2)−Hσ* orbital also increases but the agostic donation dominates overall. In this reaction, the NBO orbital analysis shows that both the agostic and syndetic π-donations provide electron density to the same set of metal based hybrid orbitals which are mainly of d or s character. The analysis also shows that the magnitude of the Pd···H separation is not an indication of the strength of the agostic interaction. Of particular interest is the nature and strength of the orbital interactions in the agostic/cyclometallate transition state 4, where the agostic donation is very large [NBO E(2) total, 152.3 kcal mol−1], the syndetic π-donations totals well over half of this at 106.3 kcal mol−1, there is the emergence of σagostic donation (3.2 kcal mol−1), Pd to C−Hσ* backdonation increases marginally, and CO lone pair donation to this antibonding orbital increases significantly. The total backdonations have an E(2) value of 72.7 kcal mol−1 which, at nearly half of the agostic donation, can be expected to provide a significant component to the lengthening of the C−H bond. A point of interest is whether an agostic interaction can be observed during a cyclopalladation reaction. In a 400 MHz 1H NMR study of the reaction between N,N-dimethylbenzylamine and Pd(OAc)2,6b an intermediate was observed that could correspond to the agostic state based on upfield chemical shifts observed for agostic protons. However, the magnitude of this shift seen in the reaction spectra does not match that of the apparent entropically trapped agostic state seen in the 1H NMR spectra of complexes containing in-plane metal hydrogen interactions detected by X-ray crystallography. To explain this problem, we have suggested previously6b,9 that for this type of reaction in solution, rotation about the Pd−N bond can occur which results in the aromatic ring moving in and out of the coordination plane and the observed upfield shift of the C−H bond hydrogen in the 1H NMR spectrum is a timeaveraged reflection of this situation. This suggestion is in accord with recent computational studies showing diminishing influence of the equatorial current density.23 This rotational possibility is certainly a challenge to the observation of the pure agostic state and is not helped by the fact that in the NMR spectral analysis of N,N-dimethylbenzylamine with Pd(OAc)2 or PdCl4−,6b an intermediate that appears to involve the agostic interaction also includes rotation about the Ph−CH2 bond of the ligand as the resonance obtained involves both aromatic ring ortho-hydrogens. This type of rotation is removed when 1-tetralone oxime is used as

Figure 10. 2D contour orbital plots for the C(1)−C(2) σ-orbital overlaps (σ-agostic interaction) showing Pd orbital proximity to C(2) in TS 4. (a) the Pd d orbital overlap [E(2) value 1.5 kcal mol−1], (b) the Pd s orbital overlap [E(2) value, 1.7 kcal mol−1].

deformation of the C(2)−H bond with respect to the aromatic ring plane (36.6°). In this case, the Pd to C(2)−Hσ* orbital donation still increases but only to 8.1 kcal mol−1. However, with the close approach of the acetato ligand CO oxygen to the C(2)−H hydrogen (1.438 Å, CO···H−C separation, 2.036 Å in agostic intermediate 3), the two lone-pair donations increase significantly with E(2) values of 8.3 and 56.3 kcal mol−1. In all, the total back-donation to the C(2)−Hσ* orbital is 72.7 kcal mol−1, and this compares to the two agostic donations where the E(2) values of 95.9 and 56.4 kcal mol−1 total 152.3 kcal mol−1 so that the lone pair involvement is just under half the magnitude of the agostic contribution to the C(2)−H bond weakening as well as providing electron density for the ensuing O−H bond formation as is accepted by the acetato ligand oxygen. Changes in the E(2) values for these back bonding components occurring during the mechanism are presented in graphical form in Figure 7. Cyclopalladated Product (5). The lowest energy product of the published mechanism has an acetic acid ligand coordinated to palladium via the CO oxygen so that now in 5, this CO oxygen atom lies trans to the Pd−C bond (Figure 11). Relevant metrics for this product are contained in

Figure 11. Final product 5 of the published mechanism.

Table 1. In the experimentally observed product, the acetic acid ligand in 5 is not present, and an acetato ligand bridged dimer is the result.6b The NBO analysis of the donor−acceptor properties associated with the Pd−C σ bond in 5 shows that there is Pd lone-pair to C(2) hybrid orbital donation [E(2) value, 26.8 kcal mol−1] and Pd lone pair donation to the Pd−C(2)σ* orbital [E(2) value, 9.2 kcal mol−1] (see the SI). For the aromatic ring system, there is strong donation from a lone pair on C(3) (the C−H bond carbon ortho to the Pd−C bond) to the C(4)−C(5)π* orbital [E(2) value, 91.8 kcal mol−1] and also strong donation from the C(1)−C(6)π orbital into the C(2) p-hybrid orbital [E(2) value, 53.5 kcal mol−1]. There is H

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Organometallics the ligand in the reaction with PdCl42−6a but the intermediate observed with the similar type of upfield shift does not have the lowering of the1JC−H coupling constant expected for the C−H bond lengthening found in the computed agostic state.9a,b However, for an AMLA/CMD mechanism to operate, the agostic state must be reached at some stage. In this regard, it should be realized that the mechanistic calculations for the reaction of N,N-dimethylbenzylamine with palladium acetate7a show that based on the computed energy for the agostic interaction, its population at equilibrium would be negligible and with the agostic/cyclometallate transition state not much higher in energy, trapping the agostic state would be unlikely. In realizing this situation, the present computational study looks at the actual nature of the interactions involved in the computed mechanism to give further understanding of the C− H bond activation step.

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EXPERIMENTAL SECTION

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ASSOCIATED CONTENT

ORCID

Alastair J. Nielson: 0000-0003-2086-3295 Notes

The authors declare no competing financial interest.

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Computational Details. Density functional theory (DFT)-based geometry optimizations and vibrational calculations for the complexes 1−12 were performed using the dispersion corrected PBE-D313,24 functional within the Gaussian09 (G09)25 software. A triple-ζ high quality basis set (aug-cc-pVTZ-PP)26 was employed for Pd together with a scalar relativistic energy consistent Stuttgart pseusopotential; aug-cc-pVTZ27 for the attached ancillary ligands (Cl atoms), agostic hydrogen, and nitrogen (attached to Pd), and double-ζ quality basis set (aug-cc-pVDZ)27 was used for the remainder of atoms. No imaginary frequencies were found in the vibrational analysis. For the QTAIM analysis, the input files (.wfx) were obtained from G0925 and QTAIM calculations were performed with the AIMALL software28 The NBO calculations were performed with the NBO6.0 package29 and NBOView2.029 was used to visualize the contours and surfaces of donor−acceptor interactions. The same procedure was used for the ligand calculations.

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00303.

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REFERENCES

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Capped-stick structural diagrams showing the agostic C−H bond in relation to the Pd coordination plane for 1 − 5; further graphical comparison of the overall change in metrics for the mechanistic intermediates and transition states 1 − 4; NBO electron density and contour plots for the agostic and syndetic donations for 3 and 4; comparison of PBE-D3 and BP86 functionals for the metrics of 1−4; QTAIM atomic charge data for 1 − 4; second order perturbation energy E (2) values (kcal mol−1) for donor−acceptor NBOs interactions for 1−5 (PDF) Cartesian coordinates for N,N-dimethylbenzylamine, Cartesian coordinates for intermediate 1, Cartesian coordinates for transition state 2, Cartesian coordinates for intermediate 3, Cartesian coordinates for transition state 4, Cartesian coordinates for cyclopalladate 5 (ZIP (XYZ))

AUTHOR INFORMATION

Corresponding Authors

*E-mail for A.J.N.: [email protected]. Phone: 0064 09 443 9760. *E-mail for J.A.H.: [email protected]. I

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DOI: 10.1021/acs.organomet.8b00303 Organometallics XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.organomet.8b00303 Organometallics XXXX, XXX, XXX−XXX