The Trans Influence in Unsymmetrical Pincer Palladacycles : An Experimental and Computational Study

A library of unsymmetrical SCN pincer palladacycles, [ClPd{2-pyr-6-(RSCH2)C6H3}], R = Et, Pr, Ph, p-MePh, and p-MeOPh, pyr = pyridine, has been synthesized via C–H bond activation, and used, along with PCN and N’CN unsymmetrical pincer palladacycles previously synthesized by the authors, to determine the extent to which the trans influence is exhibited in unsymmetrical pincer palladacycles. The trans influence is quantified by analysis of structural changes in the X-ray crystal and density functional theory (DFT) optimized structures and a topological analysis of the electron density using quantum theory of atoms in molecules (QTAIM) to determine the strength of the Pd-donor atom interaction. It is found that the trans influence is controlled by the nature of the donor atom and although the substituents on the donor-ligand affect the Pd-donor atom interaction through the varied electronic and steric constraints, they do not influence the bonding of the ligand trans to it. The data indicate that the strength of the trans influence is P > S > N. Furthermore, the synthetic route to the family of SCN pincer palladacycles presented demonstrates the potential of late stage derivitization for the effective synthesis of ligands towards unsymmetrical pincer palladacycles.

The pincer palladacycle structures are stabilized by an intramolecular coordination to the metal of the two donor atoms in the side arms.Their reactivity and other properties are influenced by the donor group around the metal [3].The attractive feature of pincer palladacycles is the possibility for fine-tuning the catalytic activity by varying the two side arms to modify the palladium environment, by changing the donor atoms and their substituents, providing the opportunity to alter hard/soft acid base properties, or by changing the ring size, giving rise to varying steric hindrance [12].These factors provide the potential for hemilabile coordination of the donor ligand arms with the metal center, an important consideration in the design of pincer palladacycles [15][16][17].This can lead to different physical and chemical properties of the donor ligand arms, resulting in preferential decoordination of one of the ligand arms, providing the opportunity to fine tune the catalytic activity of unsymmetrical pincer palladacycles [18][19][20][21].An excellent example by Wendt and co-workers reported the hemilabile nature of nitrogen and phosphorus donor atoms by reacting with a strong nucleophile (MeLi) [22].The results showed that the nitrogen donor atom arm decoordinated from the Pd center, while the phosphorus donor atom arm remained coordinated to the Pd center (Scheme 1).It is clear that the different properties of the side arms result in hemilability due to the changing strength and/or nature of interaction between donor atoms and the Pd center.Scheme 1. Reaction of unsymmetrical pincer palladacycles with MeLi showing the hemilability of the nitrogen donor atom arm by Wendt et al. [22].
Pincer palladacycles exist in a square planar configuration at the Pd(II) center, and a key factor determining the strength of the interaction between Pd and the donor atoms is the trans influence, potentially affecting hemilability of donor atom arms.The trans influence is defined by Pidcock [23] as "the tendency of a ligand to weaken the bond trans to itself".The "trans influence" affects the structure in the ground, or thermodynamic state.Therefore, sometimes, it is called the thermodynamic trans effect, while the "trans effect" is related to the kinetic rate of reaction, depending on substitution of the bond trans to itself.The trans influence has been used to explain the stability of square planar complexes [24], while the trans effect has been used to study reaction pathways [25].There are many experimental studies into the trans influence, generally using spectroscopic or X-ray crystallographic methods [26][27][28].Additionally, density functional theory (DFT) structure optimization and molecular orbital analysis have been employed in the study of the Unsymmetrical pincer palladacylces, SCN (1a) [13], PCN (2a-2b), and N'CN (3a-3c) [14].
The pincer palladacycle structures are stabilized by an intramolecular coordination to the metal of the two donor atoms in the side arms.Their reactivity and other properties are influenced by the donor group around the metal [3].The attractive feature of pincer palladacycles is the possibility for fine-tuning the catalytic activity by varying the two side arms to modify the palladium environment, by changing the donor atoms and their substituents, providing the opportunity to alter hard/soft acid base properties, or by changing the ring size, giving rise to varying steric hindrance [12].These factors provide the potential for hemilabile coordination of the donor ligand arms with the metal center, an important consideration in the design of pincer palladacycles [15][16][17].This can lead to different physical and chemical properties of the donor ligand arms, resulting in preferential decoordination of one of the ligand arms, providing the opportunity to fine tune the catalytic activity of unsymmetrical pincer palladacycles [18][19][20][21].An excellent example by Wendt and co-workers reported the hemilabile nature of nitrogen and phosphorus donor atoms by reacting with a strong nucleophile (MeLi) [22].The results showed that the nitrogen donor atom arm decoordinated from the Pd center, while the phosphorus donor atom arm remained coordinated to the Pd center (Scheme 1).It is clear that the different properties of the side arms result in hemilability due to the changing strength and/or nature of interaction between donor atoms and the Pd center.
The pincer palladacycle structures are stabilized by an intramolecular coordination to the metal of the two donor atoms in the side arms.Their reactivity and other properties are influenced by the donor group around the metal [3].The attractive feature of pincer palladacycles is the possibility for fine-tuning the catalytic activity by varying the two side arms to modify the palladium environment, by changing the donor atoms and their substituents, providing the opportunity to alter hard/soft acid base properties, or by changing the ring size, giving rise to varying steric hindrance [12].These factors provide the potential for hemilabile coordination of the donor ligand arms with the metal center, an important consideration in the design of pincer palladacycles [15][16][17].This can lead to different physical and chemical properties of the donor ligand arms, resulting in preferential decoordination of one of the ligand arms, providing the opportunity to fine tune the catalytic activity of unsymmetrical pincer palladacycles [18][19][20][21].An excellent example by Wendt and co-workers reported the hemilabile nature of nitrogen and phosphorus donor atoms by reacting with a strong nucleophile (MeLi) [22].The results showed that the nitrogen donor atom arm decoordinated from the Pd center, while the phosphorus donor atom arm remained coordinated to the Pd center (Scheme 1).It is clear that the different properties of the side arms result in hemilability due to the changing strength and/or nature of interaction between donor atoms and the Pd center.Scheme 1. Reaction of unsymmetrical pincer palladacycles with MeLi showing the hemilability of the nitrogen donor atom arm by Wendt et al. [22].
Pincer palladacycles exist in a square planar configuration at the Pd(II) center, and a key factor determining the strength of the interaction between Pd and the donor atoms is the trans influence, potentially affecting hemilability of donor atom arms.The trans influence is defined by Pidcock [23] as "the tendency of a ligand to weaken the bond trans to itself".The "trans influence" affects the structure in the ground, or thermodynamic state.Therefore, sometimes, it is called the thermodynamic trans effect, while the "trans effect" is related to the kinetic rate of reaction, depending on substitution of the bond trans to itself.The trans influence has been used to explain the stability of square planar complexes [24], while the trans effect has been used to study reaction pathways [25].There are many experimental studies into the trans influence, generally using spectroscopic or X-ray crystallographic methods [26][27][28].Additionally, density functional theory (DFT) structure optimization and molecular orbital analysis have been employed in the study of the Scheme 1. Reaction of unsymmetrical pincer palladacycles with MeLi showing the hemilability of the nitrogen donor atom arm by Wendt et al. [22].
Pincer palladacycles exist in a square planar configuration at the Pd(II) center, and a key factor determining the strength of the interaction between Pd and the donor atoms is the trans influence, potentially affecting hemilability of donor atom arms.The trans influence is defined by Pidcock [23] as "the tendency of a ligand to weaken the bond trans to itself".The "trans influence" affects the structure in the ground, or thermodynamic state.Therefore, sometimes, it is called the thermodynamic trans effect, while the "trans effect" is related to the kinetic rate of reaction, depending on substitution of the bond trans to itself.The trans influence has been used to explain the stability of square planar complexes [24], while the trans effect has been used to study reaction pathways [25].There are many experimental studies into the trans influence, generally using spectroscopic or X-ray crystallographic methods [26][27][28].Additionally, density functional theory (DFT) structure optimization and molecular orbital analysis have been employed in the study of the trans influence and give a good explanation of the trans influence in organometallic complexes [29][30][31][32][33].
In this study, we have investigated the trans influence in both model and experimentally-characterized unsymmetrical pincer palladacycles, using DFT calculations and quantum theory of atoms in molecules (QTAIM) analysis.Additionally, in order to determine the effect of varying the substituents on the donor atom, we have synthesized a library of unsymmetrical SCN pincer palladacycles, providing the opportunity to vary the steric and electronic properties on the sulfur atom.We have then used these palladacycles to further investigate the trans influence using DFT.

SCN Pincer Palladacycle Synthesis
Our previous work has demonstrated a novel synthetic route to an unsymmetrical SCN pincer palladacycle, via a key biaryl benzyl bromide intermediate 5 (Scheme 2) [13].By changing the sulfur nucleophile in step c (Scheme 2), the ability to synthesize a library of SCN pincer ligands is possible.This provides the opportunity to vary the thioether substituent to tune the steric and electronic properties of the sulfur atom, which will be bound to the palladium atom in the resulting palladacycle.The SCN ligands then undergo C-H bond activation with in situ-generated Pd(MeCN) 4 (BF 4 ) [13,34] synthesizing a library of SCN pincer palladacycles 1b-1f (Scheme 2, Table 1).
In this study, we have investigated the trans influence in both model and experimentallycharacterized unsymmetrical pincer palladacycles, using DFT calculations and quantum theory of atoms in molecules (QTAIM) analysis.Additionally, in order to determine the effect of varying the substituents on the donor atom, we have synthesized a library of unsymmetrical SCN pincer palladacycles, providing the opportunity to vary the steric and electronic properties on the sulfur atom.We have then used these palladacycles to further investigate the trans influence using DFT.

SCN Pincer Palladacycle Synthesis
Our previous work has demonstrated a novel synthetic route to an unsymmetrical SCN pincer palladacycle, via a key biaryl benzyl bromide intermediate 5 (Scheme 2) [13].By changing the sulfur nucleophile in step c (Scheme 2), the ability to synthesize a library of SCN pincer ligands is possible.This provides the opportunity to vary the thioether substituent to tune the steric and electronic properties of the sulfur atom, which will be bound to the palladium atom in the resulting palladacycle.The SCN ligands then undergo C-H bond activation with in situ-generated Pd(MeCN)4(BF4) [13,34] synthesizing a library of SCN pincer palladacycles 1b-1f (Scheme 2, Table 1).The SCN ligand syntheses presented in Table 1 reveal excellent to moderate yields, followed by C-H bond activation, also in moderate to excellent yield.Single crystals of palladacycles 1b-d and 1f were obtained by slow evaporation of dichloromethane (DCM) from a saturated solution, and were characterized by X-ray crystallography (Figure 2).The CCDC numbers for the structures are 1486787 for 1b, 1486788 for 1c, 1486789 for 1d and 1486790 for 1f.The data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/structures.The SCN ligand syntheses presented in Table 1 reveal excellent to moderate yields, followed by C-H bond activation, also in moderate to excellent yield.Single crystals of palladacycles 1b-d and 1f were obtained by slow evaporation of dichloromethane (DCM) from a saturated solution, and were characterized by X-ray crystallography (Figure 2).The CCDC numbers for the structures are 1486787 for 1b, 1486788 for 1c, 1486789 for 1d and 1486790 for 1f.The data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/structures.

Investigaing the Trans Influence
To determine the accuracy of the Perdew-Burke-Ernzerhof exchange-correlation functional (PBE) for optimizing the YCY' pincer complexes, we have analyzed the mean signed errors (MSE), which is the average of the deviation between calculation and experiment, and the mean unsigned errors (MUE), which is the average of the absolute deviation between calculated and experimental Pd-L bond lengths (L = Y, Y', C and Cl).The MSE for 1a-1d, 1f is 0.001 Å , for 2a-2b is 0.012 Å and for 3a-3c is 0.001 Å.The MUE for 1a-1d, 1f is 0.017 Å , for 2a-2b is 0.012 Å and for 3a-3c is 0.001 Å .
A topological analysis of the electron density was performed using QTAIM.The bond path is a single line of locally-maximum density linking the nuclei of any two bonded atoms in a molecule [35].The minimum along this path is defined as the bond critical point (BCP) and the magnitude of the electron density ρ(r) at this point can be used to determine the strength of the interaction.
Several AIM parameters determined at the Pd-Y and Pd-Y' BCPs are provided in the Supplementary Information (electron density ρ(r), Laplacian of the density  2 ρ(r), delocalization index, ellipticity, bond degree parameter, etc.) which can be used to determine the nature and strength of the interaction; the conclusions regarding the nature of the bonding are in complete agreement with our previous work on the nature of the bonding in symmetrical pincer palladacycles and, so, are not presented again here [36].In the present work, the focus is on the trans influence and,

Investigaing the Trans Influence
To determine the accuracy of the Perdew-Burke-Ernzerhof exchange-correlation functional (PBE) for optimizing the YCY' pincer complexes, we have analyzed the mean signed errors (MSE), which is the average of the deviation between calculation and experiment, and the mean unsigned errors (MUE), which is the average of the absolute deviation between calculated and experimental Pd-L bond lengths (L = Y, Y', C and Cl).The MSE for 1a-1d, 1f is 0.001 Å, for 2a-2b is 0.012 Å and for 3a-3c is 0.001 Å.The MUE for 1a-1d, 1f is 0.017 Å, for 2a-2b is 0.012 Å and for 3a-3c is 0.001 Å.
A topological analysis of the electron density was performed using QTAIM.The bond path is a single line of locally-maximum density linking the nuclei of any two bonded atoms in a molecule [35].The minimum along this path is defined as the bond critical point (BCP) and the magnitude of the electron density ρ(r) at this point can be used to determine the strength of the interaction.
Several AIM parameters determined at the Pd-Y and Pd-Y' BCPs are provided in the Supplementary Information (electron density ρ(r), Laplacian of the density ∇ 2 ρ(r), delocalization index, ellipticity, bond degree parameter, etc.) which can be used to determine the nature and strength of the interaction; the conclusions regarding the nature of the bonding are in complete agreement with our previous work on the nature of the bonding in symmetrical pincer palladacycles and, so, are not presented again here [36].In the present work, the focus is on the trans influence and, so, the magnitude of the electron density ρ(r) at the BCP is used to determine the increase or decrease in the strength of the Pd-Y interaction when the ligand trans to it is varied.

Trans Influence in Model Palladacycles I-III
Normally, the trans influence has been studied in systems with four monodentate ligands coordinated to the metal center to form a square planar complex.Furthermore, a trans Pt-Cl bond length, situated trans to the donor atom arm, of a square planar complex is normally used to consider the trans influence [37][38][39][40].From the unsymmetrical SCN pincer palladacycle structures, we do not have a Cl atom for monitoring the strength of the trans influence in this way.Therefore, first, simple model palladacycles I-III (Figure 3) have been studied using DFT to evaluate the trans influence in CY palladacycles before studying the unsymmetrical YCY' pincer palladacycles.The models I-III contain a Cl ligand trans to a donor atom group (NMe 2 , SMe 2 , and PMe 2 , respectively) to monitor the strength of the trans influence.A topological analysis of the electron density was performed using QTAIM and the magnitude of the electron density ρ(r) at the bond critical point (BCP), the minimum along the bond path between interacting atoms, was used to determine the strength of the Pd-Cl interaction.A larger ρ(r) value corresponds to a stronger interaction between atoms [41] and, therefore, can be used to study the trans influence in palladacycles I-III.When ρ(r) at the BCP of Pd-Cl bond has a high value (strong interaction), it indicates that the donor atom trans to Cl has a weak trans influence, whereas a low ρ(r) value (weak interaction) indicates that the donor atom trans to the Pd-Cl bond has a strong trans influence.A relative change in bond length is a physical manifestation that indicates the strength of the trans influence.When the Pd-Cl bond is situated trans to a donor atom that exhibits a strong trans influence, the Pd-Cl bond lengthens compared to when the Pd-Cl bond is situated trans to a weak trans influence donor atom.The data provided in Table 2 show that the ρ(r) value of the Pd-Cl bond of III is smaller than that in II, which is smaller than in I, indicating that the trans influence of PMe 2 is greater than that of SMe which is greater than NMe 2 .The ρ(r) data is supported by the bond lengths, with I having the shortest Pd-Cl bond length and III a significantly longer Pd-Cl bond length than in I and II, again demonstrating the stronger PMe 2 trans influence.Based on this analysis the ordering of the trans influence series is PMe 2 > SMe > NMe 2 .so, the magnitude of the electron density ρ(r) at the BCP is used to determine the increase or decrease in the strength of the Pd-Y interaction when the ligand trans to it is varied.

Trans Influence in Model Palladacycles I-III
Normally, the trans influence has been studied in systems with four monodentate ligands coordinated to the metal center to form a square planar complex.Furthermore, a trans Pt-Cl bond length, situated trans to the donor atom arm, of a square planar complex is normally used to consider the trans influence [37][38][39][40].From the unsymmetrical SCN pincer palladacycle structures, we do not have a Cl atom for monitoring the strength of the trans influence in this way.Therefore, first, simple model palladacycles I-III (Figure 3) have been studied using DFT to evaluate the trans influence in CY palladacycles before studying the unsymmetrical YCY' pincer palladacycles.The models I-III contain a Cl ligand trans to a donor atom group (NMe2, SMe2, and PMe2, respectively) to monitor the strength of the trans influence.A topological analysis of the electron density was performed using QTAIM and the magnitude of the electron density ρ(r) at the bond critical point (BCP), the minimum along the bond path between interacting atoms, was used to determine the strength of the Pd-Cl interaction.A larger ρ(r) value corresponds to a stronger interaction between atoms [41] and, therefore, can be used to study the trans influence in palladacycles I-III.When ρ(r) at the BCP of Pd-Cl bond has a high value (strong interaction), it indicates that the donor atom trans to Cl has a weak trans influence, whereas a low ρ(r) value (weak interaction) indicates that the donor atom trans to the Pd-Cl bond has a strong trans influence.A relative change in bond length is a physical manifestation that indicates the strength of the trans influence.When the Pd-Cl bond is situated trans to a donor atom that exhibits a strong trans influence, the Pd-Cl bond lengthens compared to when the Pd-Cl bond is situated trans to a weak trans influence donor atom.The data provided in Table 2 show that the ρ(r) value of the Pd-Cl bond of III is smaller than that in II, which is smaller than in I, indicating that the trans influence of PMe2 is greater than that of SMe which is greater than NMe2.The ρ(r) data is supported by the bond lengths, with I having the shortest Pd-Cl bond length and III a significantly longer Pd-Cl bond length than in I and II, again demonstrating the stronger PMe2 trans influence.Based on this analysis the ordering of the trans influence series is PMe2 > SMe > NMe2.In order to extend our investigation of the trans influence to unsymmetrical pincer palladacycles, the palladacycles IV-VI have been studied using DFT and QTAIM, and their bond strengths and bond lengths compared to previous results found for symmetrical pincer palladacycles PdNCN, PdSCS and PdPCP [36] (Figure 4).Considering the ρ(r) value at the BCP of the Pd-Y bond in IV-VI, the ρ(r) value of the Pd-P bond of V (0.110 a.u) and VI (0.114 a.u.) are greater compared to the ρ(r)  In order to extend our investigation of the trans influence to unsymmetrical pincer palladacycles, the palladacycles IV-VI have been studied using DFT and QTAIM, and their bond strengths and bond lengths compared to previous results found for symmetrical pincer palladacycles PdNCN, PdSCS and PdPCP [36] (Figure 4).Considering the ρ(r) value at the BCP of the Pd-Y bond in IV-VI, the ρ(r) value of the Pd-P bond of V (0.110 a.u) and VI (0.114 a.u.) are greater compared to the ρ(r) values for the Pd-P bond in the PdPCP (0.101 a.u.) [36].This is due to the weaker trans influence of N and S, compared to P, leading to stronger Pd-P bonds in V and VI (Table 3).The ρ(r) value of the Pd-S bond of IV (0.097 a.u.) increases, whereas the ρ(r) value of the Pd-S bond of V (0.082 a.u.) decreases, compared to the ρ(r) value of the Pd-S bond in the PdSCS (0.091 a.u.), therefore showing that S has a moderate trans influence.Furthermore, the ρ(r) values of the Pd-N bond of IV (0.083 a.u.) and VI (0.075 a.u.) decrease compared to the ρ(r) value of the Pd-N bond in the PdNCN (0.086 a.u.) [36] indicating that P and S exhibit a stronger trans influence than N.
Inorganics 2016, 4, 25 7 of 14 to the Pd center, this stronger bond does not exert a trans influence on the amine donor ligand.Thus, it would appear the nature of the donor atom is the sole driver for the trans influence.to the Pd center, this stronger bond does not exert a trans influence on the amine donor ligand.Thus, it would appear the nature of the donor atom is the sole driver for the trans influence.Supporting the ρ(r) value results, the bond lengths of Pd-Y and Pd-Y' are reported in Table 4.When the donor ligand Y has a trans influence the Pd-Y' bond distance increases (and the ρ(r) value decreases) indicating a weakened interaction.By comparing with the symmetrical YCY pincer palladacycles it can be seen that the P donor ligand has a trans influence on the S donor ligand and the N donor ligand, and that the S donor ligand has a trans influence on the N donor ligand.For example, in VI, the PCN palladacycle, the P donor ligand has a strong influence on the N donor ligand trans to it, which manifests as an increased Pd-N (2.203 Å) bond distance compared to the Pd-N bond in PdNCN (2.140 Å), and a commensurate decrease in the Pd-P bond distance (2.222 Å) compared to the Pd-P bond length in PdPCP (2.287 Å) (Table 4).The results confirm the conclusion from the model systems with Cl as a reference, that P exhibits the greatest trans influence and N the least.To determine whether the trans influence is induced when the substituents on the donor atom are varied, thereby introducing subtle electronic effects, the library of SCN pincer palladacycles synthesized in the present work (1b-1f), along with 1a, have been investigated computationally to determine the influence of the thioether group on the coordinated pyridine trans to it.The Pd-N bond distances (experimental and calculated) show very little change when the substituent on the S atom is changed (bond distance differences <0.005 Å , with the exception of the experimental Pd-N bond length for 1a) (Table 4).Similarly, the ρ(r) values at the Pd-N BCP in the SCN pincer palladacycles are unaffected by changing substitution on donor atom.
Furthermore, when the substituent is changed on the P (2a and 2b) (which both incorporate the phosphinite donor group) or the N (3a-3c), Figure 1, it does not alter the trans influence on the Pdpyr interaction within the PCN or N'CN pincer palladacycles.The ρ(r) values for the Pd-pyr bond trans to the Pd-N' bond is independent of the nature of the N' ligand, and although the interactions (ρ(r) and bond length) due to the Pd-P ligands exhibit a slight difference (0.002 a.u. and 0.015 Å ) they are extremely small.Based on the ρ(r) values and Pd-Y bond lengths, the ordering of the trans influence series is PMe 2 > SMe > NMe 2 .This is in good agreement with that of Kapoor and Kakkar's study [40] into the square planar Pt complexes using DFT calculations.Their results showed a trans influence series in order of P > S > N.Moreover, Sajith and Suresh [42] studied the correlation between ρ(r) and trans influence in a square planar Pd complex, showing good linear relation between ρ(r) and trans influence, with a trans influence series of PMe 3 > SMe 2 > NH 3 .

Trans Influence in Experimentally-Characterized Unsymmetrical YCY' Pincer Palladacycles
In this section DFT and the QTAIM method is used to study the trans influence in 1a (PdSCN), 2a (PdPCN), and 3a (PdN'CN).By comparing the Pd-N bond length in the structures 1a, 2a, and 3a (optimized and experimental), the Pd-N bond is longest in 2a and shortest in 3a (Table 4).In addition, the smallest ρ(r) values for the Pd-N bond is in 2a (0.087 a.u.), while the largest is found in 3a (0.102 a.u.) with 1a (0.098 a.u.) intermediate (Table 3).The different Pd-N bond lengths and strengths demonstrate the difference in trans influence due to the nature of the donor atom of the Pd-Y bond.These results further confirm that the P donor ligand exhibits the strongest trans influence, while the N donor ligand has the weakest trans influence and that the trans influence series for the unsymmetrical pincer palladacycles considered is P > S > N.
The N donor ligand in the experimentally-characterized unsymmetrical SCN pincer palladacycle (Figure 1) is a pyridine rather than the amine considered in the previous section (IV).The change in electronic and steric effects when replacing NMe 2 (IV) with pyridine (1a) in a SCN pincer palladacyle is reflected in the bond strength: ρ(r) value of the Pd-NMe 2 bond is 0.083 a.u. in IV whereas the Pd-pyr is 0.098 a.u. in 1a, and the Pd-NMe 2 bond length is 2.156 Å in IV and the Pd-pyr bond length is 2.074 Å in 1a, demonstrating the stronger Pd-pyridine bond (Tables 3 and 4).However, this does not appear to effect the trans influence exerted on the SMe ligand when trans to these N donor ligands.The ρ(r) value of the Pd-S bond is 0.091 a.u. in PdSCS and increases to 0.097 a.u. in IV and 0.096 a.u. in 1a, and the bond length in PdSCS is 2.313 Å and shortens to 2.285 Å in IV and 2.288 Å in 1a.Thus, in both IV and 1a the Pd-S bond is strengthened relative to the symmetric PdSCS analog and, thus, can only be attributed to the effect of the N-donor ligand trans to it.
Furthermore, by comparing PdNCN where N = NMe 2 , to PdNCN' (3a), where one of the amine ligands has been replaced by pyridine, we can assess the trans influence in an unsymmetrical pincer palladacycle where the donor atom is the same (N) for distinctly different donor ligands (NMe 2 and pyr).In 3a the ρ(r) value of the Pd-NMe 2 bond has not changed and the bond length has increased insignificantly (0.005 Å) from that in the symmetric PdNCN palladacycle.Therefore, we can conclude that, although the electronic and steric effects of the pyridine result in a considerably stronger bond to the Pd center, this stronger bond does not exert a trans influence on the amine donor ligand.Thus, it would appear the nature of the donor atom is the sole driver for the trans influence.

Trans Influence on Unsymmetrical Pincer Palladacycles: Donor Atom Substituent Effects
To determine whether the trans influence is induced when the substituents on the donor atom are varied, thereby introducing subtle electronic effects, the library of SCN pincer palladacycles synthesized in the present work (1b-1f), along with 1a, have been investigated computationally to determine the influence of the thioether group on the coordinated pyridine trans to it.The Pd-N bond distances (experimental and calculated) show very little change when the substituent on the S atom is changed (bond distance differences <0.005 Å, with the exception of the experimental Pd-N bond length for 1a) (Table 4).Similarly, the ρ(r) values at the Pd-N BCP in the SCN pincer palladacycles are unaffected by changing substitution on donor atom.
Furthermore, when the substituent is changed on the P (2a and 2b) (which both incorporate the phosphinite donor group) or the N (3a-3c), Figure 1, it does not alter the trans influence on the Pd-pyr interaction within the PCN or N'CN pincer palladacycles.The ρ(r) values for the Pd-pyr bond trans to the Pd-N' bond is independent of the nature of the N' ligand, and although the interactions (ρ(r) and bond length) due to the Pd-P ligands exhibit a slight difference (0.002 a.u. and 0.015 Å) they are extremely small.

General Details
Solvents and chemicals were purchased from Sigma-Aldrich (Merck KGaA, Damstadt, Germany), VWR International (VWR, Radnor, PA, USA), Fisher Scientific (Fisher Scientific UK Ltd., Loughborough, UK) and Fluorochem (Fluorochem Ltd., Hadfield, UK) and used without further purification, with reactions taking place open to atmosphere and moisture.

Instrumentation
1 H and 13 C spectra were recorded on either a Varian 400 or 500 MHz spectrometer (Agilent Technologies, Yarnton, UK).High resolution mass spectrometry (HRMS) data were obtained on an electrospray ionization (ESI) mass spectrometer using a Bruker Daltonics Apex III (Brucker, Billerica, MA, USA), with source Apollo ESI, using methanol as the spray.Flash chromatography was performed on an automated ISCO RF75 (Teledyne ISCO Inc., Licoln, NE, USA).
Gas chromatography (GC) measurements were obtained using a Perkin Elmer Autosystem XL Gas Chromatograph (PerkinElmer Inc., Waltham, MA, USA), utilizing a flame ionization detector, and a Supelco MDN-5S 30 m × 0.25 mm × 0.25 µm column, with a He mobile phase.Elemental analyses were run by the London Metropolitan University Elemental Analysis Service (ThermoFisher Scientific, Waltham, MA, USA).Crystal structures were obtained by the UK National Crystallography Service at the University of Southampton [43].

Computational Section
Geometry optimization calculations were performed using Gaussian09 [44], in the gas-phase.The minimized structures were confirmed by the absence of any imaginary modes of vibration using frequency analysis.All structures were optimized using the generalized gradient approximation (GGA) PBE density functional [45,46].The SDD ECP basis set was used for Pd, and the 6-31+G(d,p) basis set was used for all other atoms (PBE/6-31+G (d,p)[SDD]).This methodology has been validated in our previous study into the structures of symmetrical pincer palladacycles [36].The topological analysis using quantum theory of atoms in molecules (QTAIM) was performed using the Multiwfn program [47].The ωB97XD [48]/6-311+G(2df,2p)[DGDZVP] model chemistry was used for these calculations.The all-electron relativistic DGDZVP basis set was used to treat Pd [49] as the bond path cannot be traced when treated using ECP.

Conclusions
It has been shown that the trans influence plays a key role in the stability of unsymmetrical pincer palladacycles, with the bond strength, and the bond length of the Pd-donor atom interaction affected significantly when trans to a ligand exhibiting a strong trans influence.The topological analysis of the electron density at the bond critical point, and the structure determination, show that the strength of the trans influence is in the order P > S > N.This is in agreement with previous work [40,42].
A library of SCN pincer palladacycles were synthesized via C-H bond activation and characterized using X-ray crystallography, demonstrating the utility of late stage derivitization.These SCN palladacycles, along with PCN and N'CN previously synthesized by the authors, were used to investigate the driving force for the trans influence.It was shown, by investigating the electron density at the bond critical point and changes in the Pd-donor ligand bond length, that it is the donor atom that is responsible for the trans influence.The electronic and steric factors of the ligand do not influence significantly the bond strength of the ligand trans to it.This demonstrates the important role of unsymmetrical pincer palladacycles, with different donor atoms, in the search for harnessing and exploiting hemilability in the design of effective new palladacycle catalysts.

Figure 3 .
Figure 3. Model palladacycles I-III studied to investigate the trans influence.

Figure 3 .
Figure 3. Model palladacycles I-III studied to investigate the trans influence.

Table 2 .
The electron density ρ(r) and Pd-Cl bond lengths.

Table 2 .
The electron density ρ(r) and Pd-Cl bond lengths.

Table 3 .
Electron density ρ(r) in symmetrical and unsymmetrical pincer palladacycles (values are in atomic units).The donor atom is shown in bold for each side arm, Y and Y'.

Table 3 .
Electron density ρ(r) in symmetrical and unsymmetrical pincer palladacycles (values are in atomic units).The donor atom is shown in bold for each side arm, Y and Y'.

Table 3 .
Electron density ρ(r) in symmetrical and unsymmetrical pincer palladacycles (values are in atomic units).The donor atom is shown in bold for each side arm, Y and Y'.

Table 4 .
Calculated and experimental Pd-Y and Pd-Y' bond distances in symmetrical and unsymmetrical pincer palladacycles (bond distances are in Å).The donor atom is shown in bold for each side arm, Y and Y'.

Table 4 .
Calculated and experimental Pd-Y and Pd-Y' bond distances in symmetrical and unsymmetrical pincer palladacycles (bond distances are in Å ).The donor atom is shown in bold for each side arm, Y and Y'.