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Article

Orbital Interaction and Electron Density Transfer in PdII([9]aneB2A)L2 Complexes: Theoretical Approaches

by
Ock Keum Kwak
1,
Mahreen Arooj
1,
Yong-Jin Yoon
2,
Euh Duck Jeong
3 and
Jong Keun Park
1,*
1
Department of Chemistry Education and Research Institute of Natural Science, Educational Research Institute Teachers College, Gyeongsang National University, Jinju 660-701, Korea
2
Department of Chemistry and Research Institute of Natural Science, Gyeongsang National University, Jinju 660-701, Korea
3
Division of High-Technology Materials Research, Korea Basic Science Institute, Busan 618-230, Korea
*
Author to whom correspondence should be addressed.
Molecules 2013, 18(10), 12687-12706; https://doi.org/10.3390/molecules181012687
Submission received: 29 July 2013 / Revised: 13 September 2013 / Accepted: 29 September 2013 / Published: 14 October 2013
(This article belongs to the Special Issue Macrocyclic Chemistry)
Browse Figures
Versions Notes

Abstract

:
The geometric structures of Pd-complexes {Pd([9]aneB2A)L2 and Pd([9]aneBAB)L2 where A = P, S; B = N; L = PH3, P(CH3)3, Cl−}, their selective orbital interaction towards equatorial or axial (soft A…Pd) coordination of macrocyclic [9]aneB2A tridentate to PdL2, and electron density transfer from the electron-rich trans L-ligand to the low-lying unfilled a1g(5s)-orbital of PdL2 were investigated using B3P86/lanl2DZ for Pd and 6-311+G** for other atoms. The pentacoordinate endo-[Pd([9]aneB2A)(L-donor)2]2+ complex with an axial (soft A--Pd) quasi-bond was optimized for stability. The fifth (soft A--Pd) quasi-bond between the σ-donor of soft A and the partially unfilled a1g(5s)-orbital of PdL2 was formed. The pentacoordinate endo-Pd([9]aneB2A)(L-donor)2]2+ complex has been found to be more stable than the corresponding tetracoordinate endo-Pd complexes. Except for the endo-Pd pentacoordinates, the tetracoordinate Pd([9]aneBAB)L2 complex with one equatorial (soft A-Pd) bond is found to be more stable than the Pd([9]aneB2A)L2 isomer without the equatorial (A-Pd) bond. In particular, the geometric configuration of endo-[Pd([9]anePNP)(L-donor)2]2+ could not be optimized.

1. Introduction

Carbon-carbon/carbon-heteroatom (e.g., C–C, C–N, C–O, C–S) bond formation using palladium-mediated cross-coupling reactions has been extensively studied during the last few decades [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36]. In this context, the achievements in development of novel and efficient synthetic methodologies for these types of reactions have been acknowledged with the 2010 Nobel Prize in Chemistry. Much attention has been paid to the design and development of powerful Pd-catalysts/precursors [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,37,38,39,40,41,42,43] that contain bulky and electron-rich ligands (such as phosphines and N-heterocyclic carbenes, NHCs) and to their potential synthetic applications [18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36] in the Pd-catalyzed cross-couplings of various substrates (such as allyl, aryl, and vinyl halides and olefins). Catalytic activity enhancement of Pd(0)(L)n-precursors is greatly influenced by the electronic and steric properties of electron-rich and bulky L-ligands [1,2,3,4,5,6,7,8,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42]. Due to the electronic effect of the trans L-donor, the oxidation state of Pd in neutral and anionic Pd(0)LnX catalysts is zero. The geometric structure of bulky L-ligand affects the oxidative addition reaction of Ar-X to the Pd(0)(L)n-precursor [6,7,8,25,26,27,28,29,30,31,32].
The geometric structures of the active RPd(L)nX intermediates produced in various steps of Pd-mediated cross-coupling reactions (e.g., oxidative addition, transmetalation, reductive elimination) have been investigated both theoretically [9,10,11,12,13,14,15,16,17] and experimentally [18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42]. The oxidative addition of substrates (R-X) to PdLn-precursor resulted in penta-, hexa-, and octacoordinate geometries [33,34,35,36] of the RPdII(L)nX intermediates with relative stability and a life time of 30 s [3]. The binding atoms in the penta-, hexa-, and octacoordinate Pd intermediates are not located on the x, y, and z-axis of trigonal bipyramidal, octahedral, and cubic structures. In particular, the geometric conformation of RPd(L)nX intermediates is altered by the cis-trans isomerization of L-ligand. The isomerization has been exclusively explained by the energy relationship between the isomers and low potential barrier (or binding energy of Pd-L) [3,4,5,6,7,8,44,45]. However, Goossen et al. [14,15] provided no evidence for mechanistic steps involving stable pentacoordinate PdII intermediates in Pd-mediated cross-coupling reactions. To the best of our knowledge, the relative stabilities of various RPd(L)nX intermediates produced by oxidative addition of the substrate (R-X) to PdLn and the geometric changes in intramolecular interaction of RPdII(L)nX have not been investigated.
In previous studies using hemilabile multidentate ligands [37,38,39,40,41,42,43,44,45,46,47], exceptional oxidation state and specific coordination selectivity have been observed for the Pd([9]aneB2A)L2 complexes with mixed soft A and hard B tridentates. The uncommon geometric structures of Pd([9]aneB2A)L2 complexes with an axial (A…Pd) interaction were mainly formed under the following restrictive conditions: (1) coordination bonds of P- and N-functionalized derivatives are present [18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39] and (2) polymeric side chain interactions exist [40,41,42]. In the Pd complex with an apical (hard N…Pd) interaction [42], the apical interaction was explained by an antibonding interaction between the lone pair of the apical N site and the dz2-orbital of the PdII species. In the ligand exchange reaction of square-planar Pd complexes, a vertical L…Pd interaction also has been optimized [43,44,45,46,47]. The mechanism of hydration exchange processes in the five-coordinate PdII intermediates suggested two models for the axial interaction: σ-donor…dz2-orbital and HOH…dz2-orbital. The apical (A…Pd) orbital interaction of Pd([9]aneB2A)L2 complex in mixed A and B sites has not been explained very well.
Origins of the unusual coordination structures for the Pd complexes and the configurational changes in the ArPdLnX intermediates of Pd-mediated cross-coupling reaction have not been explored yet. To investigate these properties, the present study proposed the following geometric structures and the relative stabilities of macrocyclic Pd([9]aneB2A)L2 complexes within the frameworks of its orbital interaction and electronic effect. For the Pd-mediated cross-coupling reactions, we suggest a two-step mechanism for the electron density transfer: the abundant electron density of the trans L-donor may transfer to a low-lying unoccupied a1g(5s)-orbital of PdII and then the partially unfilled a1g(5s)-orbital can interact with the Lewis base substrate (σ-donor) as shown in Scheme 1.
Molecules 18 12687 g007 550
Scheme 1. Two-step mechanism for the electron density transfer from an equatorial trans L-donor {PH3, P(CH3)3} and a σ-donor of substrate to a low-lying unoccupied a1g(5s)-orbital of the PdII center.
Scheme 1. Two-step mechanism for the electron density transfer from an equatorial trans L-donor {PH3, P(CH3)3} and a σ-donor of substrate to a low-lying unoccupied a1g(5s)-orbital of the PdII center.
Molecules 18 12687 g007
To justify the intermediate step for the additional reaction of [9]aneB2A to PdL2, the relative stabilities (Δ exo-endo, ΔBAB-B2A) and the apical and equatorial orbital interactions [σ-donor…a1g(5s), σ-donor…4dx2-y2] shown in Scheme 2 are suggested in this research exertion. The relative affinity of Pd towards the soft A (or hard B) in PdL2, the electronic characteristics owing to the donating (or withdrawing) property of the trans L-ligand, the axial σ-donor…a1g(5s) interaction between the σ-donor of the A site and the low-lying unfilled a1g(5s)-orbital of PdIIL2, and the relative stability of the uncommon Pd complexes were examined in detail.
Molecules 18 12687 g008 550
Scheme 2. A plausible orbital interaction of an equatorial or axial coordination bond of [9]aneB2A (or [9]aneBAB) with PdL2. (a): exo-type with an apical site pointing to outside. (b): endo-type with an apical site pointing to the Pd center. (c): endo-Pd complex with an apical (A…Pd) attraction. (d): endo-Pd complex without any axial interaction. (e): endo-Pd complex with an apical (A…Pd) repulsion.
Scheme 2. A plausible orbital interaction of an equatorial or axial coordination bond of [9]aneB2A (or [9]aneBAB) with PdL2. (a): exo-type with an apical site pointing to outside. (b): endo-type with an apical site pointing to the Pd center. (c): endo-Pd complex with an apical (A…Pd) attraction. (d): endo-Pd complex without any axial interaction. (e): endo-Pd complex with an apical (A…Pd) repulsion.
Molecules 18 12687 g008

2. Computational Methods

To explore the geometric structure and the relative stability of the Pd complexes, we selected the Pd([9]aneB2A)L2 [A = P, S; B = N; L = donor {PH3, P(CH3)3}, acceptor (Cl−)] complex as a model with [9]aneB2A, PH3, P(CH3)3, and Cl− groups as ligands. The equilibrium structure of tetracoordinate Pd complexes was fully optimized with the B3P86/6-311+G** (lanl2DZ for Pd) level using Gaussian 03 [48]. The macrocyclic [9]aneN2P and [9]aneN2S ligands are 1,4-diaza-7-phospacyclononane and 1,4-diaza-7-thiacyclononane, respectively (as shown in Scheme 2). The hybrid B3P86 density functional utilizes the exchange function of Becke [49,50] in conjunction with the Perdew 1986 correlation function [51] and yields good structural and energetic information, even for relatively large chemical systems such as transition metal complexes. For the optimized equilibrium PdII complexes, the atomic charges obtained by the CHelpG method were analyzed using atomic radii (H = 0.53 Å, C = 0.67 Å, N = 0.56 Å, O = 0.48 Å, P = 0.98 Å, S = 0.88 Å, Cl− = 1.67 Å, PdII = 0.78 Å) [52]. The relative energies of the PdII complexes were compared. To confirm the existence of stable structures, the harmonic vibration frequencies of the species were analyzed at the B3P86 level. In addition, the geometric structures of endo-Pd([9]aneA2B)L2 {A = P, S; B = N; L = PH3, P(CH3)3, Cl−} were optimized at the B3P86/6-311+G** (lanl2DZ for Pd) level. The optimized structures including HOMO and the geometric parameters are described in Supplementary Information Figure S1 and Table S1, respectively. To check the rationality of our results at the effective core potential level for Pd, the geometric structures of the equilibrium Pd complexes were also optimized at the CAM-B3LYP//6-311+G** (lanl2DZ for Pd) and B3P86/6-311+G** (3-21g* for Pd) levels using Gaussian 09. Further, optimized structures and parameters are given in Supplementary Information Figures S2 and S3 and Tables S2 and S3.

3. Results and Discussion

Optimized geometric structures of Pd([9]aneB2A)L2 and Pd([9]aneBAB)L2 types are represented in Figure 1, and their parameters are listed in Table 1. The values of optimized parameters in this study were compared to the values reported previously [17,18,23,37,40,41,42,44,53,54]. In the Pd complexes, the tetracoordinate structures of both the endo- and exo-types of structures were optimized. PdII locates at the center of the tetracoordinate mean plane that is coordinated with a [9]aneB2A (or [9]aneBAB) tridentate and two monodentate ligands. In the case of the endo-[Pd([9]aneB2A)(L-donor)2]2+ with trans L-donor, endo-[Pd([9]aneB2A)(L-donor)2]2+ pentacoordinates with an axial (A--Pd) quasi-bond were stably optimized, and the PdII center lies slightly above the mean plane. The (A--Pd) quasi-bond lengths (RPd…A = 2.782 ~ 2.935 Å) between the axial A site and PdII center are much shorter than the corresponding bonds in the other endo- and exo-Pd tetracoordinates. The optimized structure of endo-Pd([9]aneB2A)Cl2 complex with a trans Cl-acceptor has a tetracoordinate geometry. The distances between the axial A site and PdII center are long (RPd…A = 3.033~3.190 Å). Our values are in agreement with the geometric structures and the apical (soft A…Pd) distances (R(Pd…A) = 3.087~3.293 Å) seen in the previous crystal studies [37,40,41,42].
Molecules 18 12687 g001 550
Figure 1. The geometric structures of the Pd complexes [Pd([9]aneB2A)L2 and Pd([9]aneBAB)L2 (A = P, S; B = N; L = PH3, P(CH3)3, Cl−] were optimized at the B3P86/6-311+G** (lanl2DZ for Pd) level. All structures are viewed from the side.
Figure 1. The geometric structures of the Pd complexes [Pd([9]aneB2A)L2 and Pd([9]aneBAB)L2 (A = P, S; B = N; L = PH3, P(CH3)3, Cl−] were optimized at the B3P86/6-311+G** (lanl2DZ for Pd) level. All structures are viewed from the side.
Molecules 18 12687 g001
The (soft A--Pd) quasi-bond in pentacoordinate endo-[Pd([9]aneB2A)(L-donor)2]2+ complex causes an isomer of that complex to be more stable than tetracoordinate endo-[Pd([9]aneBAB)(L-donor)2]2+ complex {ΔEBAB-B2A = 0.10 ~ 0.37 eV for endo-[Pd([9]aneN2P)(L-donor)2]2+, ΔEBAB-B2A = 0.08 ~ 0.35 eV for endo-[Pd([9]aneN2S) (L-donor)2]2+}. The relative energy of endo-[Pd([9]aneB2A)(L-donor)2]2+ pentacoordinate (ΔEexo-endo = 0.23~0.70 eV) is lower than the corresponding exo-type tetracoordinate. Except for these four endo-Pd pentacoordinate complexes, the relative energy of exo-[Pd([9]aneBAB) (L-donor)2]2+ tetracoordinate is lower than that of exo-[Pd([9]aneB2A)(L-donor)2]2+ tetracoordinate {ΔEBAB-B2A = −0.70 ~ −0.20 eV for endo-Pd([9]aneBAB)(L-donor)2]2+}. The relative energy of tetracoordinate [Pd([9]aneBAB)Cl2 complex is also lower than that of [Pd([9]aneB2A)Cl2 {ΔEBAB-B2A = −0.35 ~ −0.18 eV for Pd([9]aneBAB)L2}. Therefore, the axial (soft A--Pd) quasi-bond contributes largely to their relative stability. The geometric parameters listed in Table 1 are very similar to the corresponding parameters given in Supplementary Information Tables S2 and S3.
Table
Table 1. Optimized average bond distances (Å), average atomic charges (CHelpG, au), and relative energies (eV) of the equilibrium structures of {Pd([9]aneB2A)L2 and Pd([9]aneBAB)L2} at the B3P86/6-311+G** (lanl2DZ for Pd) level.
Table 1. Optimized average bond distances (Å), average atomic charges (CHelpG, au), and relative energies (eV) of the equilibrium structures of {Pd([9]aneB2A)L2 and Pd([9]aneBAB)L2} at the B3P86/6-311+G** (lanl2DZ for Pd) level.
CompoundAverage distanceAverage atomic chargeRelative energy
RaPd-NRaPd-PRbPd…NRbPd…PQcPdQcPH3QdNQdPΔEeH-LΔEfBAB-B2AΔEgexo-endo
endo-[Pd([9]aneNPN)(PH3)2]2+2.1952.2892.848 0.3480.368−0.151 3.450.370.00
exptl 2.3214h2.298i2.3207i
endo-[Pd([9]aneN2P)(PH3)2]2+2.142 2.7820.2250.412 −0.2892.680.000.00
theo 2.340j 0.598k0.150k
exptl 2.3337i
exo-[Pd([9]aneNPN)(PH3)2]2+2.1542.3633.504 0.4050.380−0.152 3.39−0.70−0.37
exo-[Pd([9]aneN2P)(PH3)2]2+2.129 3.9630.2630.449 −0.0182.740.000.70
RaPd-NRaPd-SRbPd…NRbPd…SQcPdQcPH3QdNQdS
endo-Pd([9]aneNSN)(PH3)2]2+2.1552.3592.720 0.3320.377−0.159 3.400.100.00
endo-[Pd([9]aneN2S)(PH3)2]2+2.138 2.8770.2940.447 −0.2692.670.000.00
exo-[Pd([9]aneNSN)(PH3)2]2+2.1352.3823.602 0.4350.366−0.112 2.87−0.25−0.03
exo-[Pd([9]aneN2S)(PH3)2]2+2.127 3.9540.3430.444 −0.0332.420.000.32
RaPd-NRaPd-PRbPd…NRbPd…PQcPdQcP(CH3)3QdNQdPΔEeH-LΔEfBAB-B2AΔEgexo-endo
endo-[Pd([9]aneNPN){P(CH3)3}2]2+2.2142.3243.076 0.4640.425−0.133 4.010.350.00
endo-[Pd([9]aneN2P){P(CH3)3}2]2+2.195 2.8430.3800.389 −0.4163.510.000.00
exo-[Pd([9]aneNPN){P(CH3)3}2]2+2.1882.3683.678 0.5750.4390.048 3.85−0.51−0.28
exo-[Pd([9]aneN2P){P(CH3)3}2]2+2.178 4.0490.4140.372 −0.0613.520.000.58
RaPd-NRaPd-SRbPd…NRbPd…SQcPdQcP(CH3)3QdNQdS
endo-Pd([9]aneNSN){P(CH3)3}2]2+2.1852.4022.920 0.409-0.392−0.118 3.880.080.00
endo-[Pd([9]aneN2S){P(CH3)3}2]2+2.194 2.9350.353-0.396 −0.3003.500.000.00
exo-[Pd([9]aneNSN){P(CH3)3}2]2+2.1792.4203.705 0.434-0.3520.129 3.55−0.20−0.05
exo-[Pd([9]aneN2S){P(CH3)3}2]2+2.176 4.0370.458-0.412 −0.1073.190.000.23
RaPd-NRaPd-PRbPd…NRbPd…PQcPdQcClQdNQdPΔEeH-LΔEfBAB-B2AΔEgexo-endo
endo-Pd([9]aneNPN)Cl22.1132.2253.040 0.801−0.746−0.077 4.04−0.280.00
exptl 2.115l2.3337i
2.117l
endo-Pd([9]aneN2P)Cl22.088 3.0330.720−0.740 −0.3573.840.000.00
theo 2.101k 0.628k−0.516k
2.049k
exptl 2.0410i
2.0354i
exo-Pd([9]aneNPN)Cl22.1062.3463.471 0.920−0.762−0.086 4.08−0.35−0.13
exo-Pd([9]aneN2P)Cl22.095 3.8680.695−0.708 −0.1524.030.00−0.06
RaPd-NRaPd-SRbPd…NRbPd…SQcPdQcClQdNQdS
endo-Pd([9]aneNSN)Cl22.0942.3242.923 0.771−0.747−0.073 3.73-0.180.00
exptl 2.124m2.2492m2.638m
2.332n
endo-Pd([9]aneN2S)Cl22.091 3.1900.609−0.722 −0.3043.770.000.00
exptl 2.077m 3.0865m
3.293o
2.117o
2.144o
exo-Pd([9]aneNSN)Cl22.0942.3293.465 0.787−0.7270.076 3.85−0.21−0.13
exptl 2.068p2.2685m3.499p
2.050p2.317q
exo-Pd([9]aneN2S)Cl22.094 3.8540.774−0.724 −0.1784.040.00−0.09
aBond length between the PdII center and equatorial binding site of the tridentate ligand; bBond length between the PdII center and axial binding site of the tridentate; cAtomic charges of the PdII center and the binding atom of trans L-donor; dAtomic charge of the apical binding atom; eEnergy gap between HOMO and LUMO; fRelative energy gap between Pd([9]aneBAB)L2 and Pd([9]aneB2A)L2; gEnergy gap between exo- and endo-type structures; hRef. [23]; iRef. [42]; jRef. [17]; kRef. [44]; lRef. [18]; mRef. [40]; nRef. [53]; oRef. [41]; pRef. [37]; qRef. [54].
In previous studies [18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36], unusual penta-, hexa-, and octacoordinate Pd complexes with some apical (A-Pd) bonds were synthesized under some restrictions such as electronic effect of the strong σ-donor in an electron-rich trans L-ligand and steric effect of the bulky ligand. In the Pd-mediated cross-coupling reactions of an allylic group, the geometry of the (allyl)PdL2 crystals was observed to be distorted penta- and hexacoordinate structures [18,19,20,21,22,23,24]. Three carbons of an allylic group (η3-C3H5) are axially coordinated to the Pd center of the Pd(0)Ln precursor. The average distances of the apical (allyl-Pd) bond are short (RC-Pd = 2.101~2.253 Å) [18,19,22,23]. Interestingly, in the aza-macrocyclic Pd(0)(C10H16N2Me2) complexes [33,34,35] and macrocyclic PdL complexes where L = 1,4a,5,8a-tetra-hydronaphthalene-2,6-dione [36], the six and eight coordination bonds of the 3~4 olefin units with Pd were formed by the steric effect of the bulky ligand. Due to the steric hindrance, none of these three carbons of the olefin units was positioned at the edge of the formal geometry (i.e., the trigonal bipyramid or square pyramid) [18,19,20,21,22,23,24,25,26,27,28,29]. Furthermore, in the (olefin)PdL2 complex with an apical olefin-Pd bond, various (olefin)PdL2 intermediates were observed during the Pd-mediated cross-coupling reactions [20,25,26,30]. However, in [Pd([9]aneN3)2]2+ and [Pd([9]aneN3)2(H[9]aneN3)2]3+ complexes without geometric constraints, the structures with an apical (soft A…Pd) interaction were not observed [40].
As shown in Figure 2, when the soft A and hard B binding sites simultaneously coordinate to the 4dx2-y2 orbital, the large σ-donor of the A site overlaps more with the 4dx2-y2 orbital than the small σ-donor of B. With increasing atomic size (P:107 pm ≅ S:105 pm >> N:71 pm) [52], the larger σ-orbital of the soft A site in [9]aneBAB first interacts with the 4dx2-y2 orbital on the square-planar plane. In endo-Pd([9]aneBAB)L2 of (f) of Figure 2, due to its atomic size, the bond length (rPd-P = 2.324 ~ 2.225 Å and rPd-S = 2.402 ~ 2.324 Å) of the equatorial (A-Pd) bond is longer than the corresponding (N-Pd) bond (rPd-N = 2.214 ~ 2.094 Å). Two equatorial bond lengths (rPd-A, rPd-B) are unsymmetrical in nature. The axial distance between the axial N site and PdII center is long (rPd-N = 2.720 ~ 3.076 Å). As shown in (g) of Figure 2, in the endo-[Pd([9]aneB2A)(L-donor)2]2+ complex, the average distance between the equatorial N site and PdII center is short (RPd-N = 2.138 ~ 2.195 Å). Because both the equatorial (N-Pd) bonds are short, the large σ-donor of the apical A site closely approaches to the Pd center. Thus, the large σ-donor of the soft A site can interact with the low-lying unoccupied a1g(5s)-orbital of PdL2 complex.
The ratio of products in Pd-catalyzed cross-coupling reactions was experimentally determined by the strength of the σ-donor ligand (e.g., n-Bu3N or an acetate anion) [6,30,31]. As the softness (P ≅ S > N) [47] and basicity (N > P > S) [6,26] of the binding site increases, the coordination bond of the soft A site to the 4dx2-y2-orbital was more preferred than that of the hard B site. That is, the σ-donor of the soft A site demonstrates a stronger overlap on the 4dx2-y2 orbital than the hard B site. As shown in Table 1, exo-[Pd([9]aneBAB)(L-donor)2]2+ and Pd([9]aneBAB)Cl2 complexes with an equatorial (A-Pd) bond are more stable than the corresponding exo-[Pd([9]aneB2A)(L-donor)2]2+ and Pd([9]aneB2A)Cl2 isomers without the equatorial (A-Pd) bond, respectively. As described in Supplementary Information Figure S1 and Table S1, endo-[Pd([9]anePNP)(L-donor)2]2+ could not be optimized. By strong softness and basicity of the binding A site, the relative energy of tetracoordinate endo-[Pd([9]aneA2B)(L-donor)2]2+ complex with two equatorial (A-Pd) bonds is lower than that of pentacoordinate endo-[Pd([9]aneABA)(L-donor)2]2+ complex with an axial (A-Pd) bond {ΔEBAB-B2A = −0.01 ~ −0.02 eV for endo-Pd([9]aneS2N)(L-donor)2]2+}. However, in the endo-[Pd([9]aneB2A)(L-donor)2]2+ complex with an axial fifth (A--Pd) quasi-bond, the pentacoordinate Pd complex is more stable than the tetracoordinate endo-[Pd([9]aneBAB)(L-donor)2]2+ complexes. Therefore, in case of interaction of [9]aneB2A to PdL2, the relative stability of the Pd complexes and the selective formation of the coordination bond may depend on the strength of the hardness/softness and basicity of A (or B) and the position of the equatorial (or apical) binding atoms, and the existence of an axial (A--Pd) quasi-bond [6,26,30,31,47,55].
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Figure 2. The equatorial coordination bonds of the A (or B) site to the 4dx2-y2-orbital and the axial (A…Pd) interaction in the endo-Pd([9]aneB2A)L2 complexes.
Figure 2. The equatorial coordination bonds of the A (or B) site to the 4dx2-y2-orbital and the axial (A…Pd) interaction in the endo-Pd([9]aneB2A)L2 complexes.
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In some PdII complexes with terminal amino derivatives [37,38,39,40,41,42], both apical soft S…Pd and hard N…Pd interactions were observed under the strong strain of a polymeric side chain. In particular, the apical (hard N…Pd) distances (rPd…N = 2.523 ~ 2.638 Å) [40] are slightly shorter as explained by an axial σ*…dz2 interaction between an antibonding orbital of the apical N site and the filled dz2-orbital [42]. Meanwhile, in the ligand exchange processes of the Pd complexes [46,47], an axial water…Pd interaction was also observed. The mechanism of the hydration reactions is described by two models: axial H2O…dz2-orbital and OH2…dz2-orbital interactions with the former being rarely formed due to electrostatic repulsions. In studies by Kozelka et al. [56], both the axial O2H…Pt and H2O…Pt interactions in the Pt complex were explained by an electrostatic attraction between the dispersion components. Until now, the axial L…d8-metal interaction between the axial L-ligand and d8-metal have not been explained in detail using an orbital interaction.
The 3a1g(5s)-orbital [(h), (j), (l)] and the 2a1g-orbital [(i), (k), (m)] of endo-Pd complexes are illustrated in Figure 3. As shown in (h), (j), and (l) of Figure 3, the orbital shape of the 3a1g(5s)-orbital of the endo-Pd([9]aneB2A)L2 complexes is largely varied by the donating (or withdrawing) property of the trans L-ligand. Due to the electron donating property of trans L-donor, its electron density moves to the PdII center, and then the increased electron density at the PdII center is transferred to the low-lying unoccupied 3a1g(5s)-orbital. The orbital lobe of the 3a1g(5s)-orbital around the PdII center is huge [as shown in tetracoordinate endo-[Pd([9]aneBAB)(L-donor)2]2+ complex of (h) and (j) of Figure 3]. This huge lobe of the 3a1g(5s)-orbital can interact with the σ-donor of the soft A site [or filled π-donor orbital of substrates]. In the endo-[Pd([9]aneB2A)(L-donor)2]2+ complex with an apical (A--Pd) quasi-bond [(h) and (j) of Figure 3], the shape of the 3a1g(5s)-orbital is not symmetric. The partially unfilled and lowered a1g(5s)-orbital strongly interacts with the large σ-donor of A to make the apical (A--Pd) quasi-bond. The upper part of the a1g(5s)-orbital lobe is used for the apical (A…Pd) interaction. Meanwhile, in the endo-[Pd([9]aneBAB)(L-donor)2]2+ complex without the apical (A--Pd) quasi-bond [(h) and (j) of Figure 3], the orbital lobe of the 3a1g(5s)-orbital is huge and symmetric. The 3a1g(5s)-orbital of endo-[Pd([9]aneBAB)(L-donor)2]2+ complex is quite different from that of endo-[Pd([9]aneB2A)(L-donor)2]2+. As shown in (l) of Figure 3 with trans Cl-acceptor, the lobe size of the 3a1g(5s)-orbital is very small. No interaction between the σ-donor of the soft A site and the 3a1g(5s)-orbital is formed. The orbital shapes of the 3a1g(5s)-orbital and the 2a1g-orbital are very similar to those given in Supplementary Information Figures S2 and S3. As listed in Table 1, the atomic charge of Pd in [Pd([9]aneB2A)(L-donor)2]2+ complex is more negative than that of Pd in Pd([9]aneB2A)Cl2 complex.
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Figure 3. The 3a1g(5s)-orbital [(h), (j), (l)] and 2a1g-orbital [(i), (k), (m)] of endo-Pd([9]aneB2A)L2 and endo-Pd([9]aneBAB)L2 complexes calculated at the B3P86/6-311+G** (lanl2DZ for Pd) level, respectively.
Figure 3. The 3a1g(5s)-orbital [(h), (j), (l)] and 2a1g-orbital [(i), (k), (m)] of endo-Pd([9]aneB2A)L2 and endo-Pd([9]aneBAB)L2 complexes calculated at the B3P86/6-311+G** (lanl2DZ for Pd) level, respectively.
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Based on Figure 3, orbital interaction between the σ-donor of the apical soft A (or hard B) site and the partially unfilled a1g(5s)-orbital of PdII center as well as the direction for the electron transfer of trans L-ligand (electron-donor or acceptor) are schematically depicted in Figure 4. In endo-[Pd([9]aneB2A)(L-donor)2]2+ and endo-[Pd([9]aneBAB)(L-donor)2]2+ complexes, the electron density of the unfilled a1g(5s)-orbital of PdII center is greatly increased by the strong electron-donating property of the trans L-donor. The spatial distribution of electron density of the a1g(5s)-orbital is larger than that of the a1g(4dz2)-orbital, thus positioning the lobe of a1g(5s)-orbital at the outer space of the a1g(4dz2)-orbital. As represented in (n) of Figure 4, the soft σ-donor of the axial A site (or substrate of Lewis base) first interacts with the partially unoccupied a1g(5s)-orbital (or Pd-complex of Lewis acid). A fifth apical (soft A--Pd) quasi-bond is formed as shown in Figure 1. In (o) of Figure 4, the size of σ-donor in an axial B site is small [covalent radius of the N atom: 71 pm [52]]. In the regular Pd tetracoordinate, the axially hard N atom cannot easily interact with the Pd center. Meanwhile, in endo-Pd([9]aneB2A)Cl2 and endo-Pd([9]aneBAB)Cl2 complexes with a trans Cl-acceptor, the filled a1g(4dz2)-orbital of PdII center lies spatially outside than that of the a1g(5s)-orbital. There is no interaction between the soft σ-donor of A and the filled a1g(4dz2)-orbital.
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Figure 4. The axial orbital interaction between the σ-donor of the soft A site (or hard B site) and the partially unfilled a1g(5s)-orbital is influenced by the electronic property of the trans L-ligand and the size of the σ-donor of A (or B).
Figure 4. The axial orbital interaction between the σ-donor of the soft A site (or hard B site) and the partially unfilled a1g(5s)-orbital is influenced by the electronic property of the trans L-ligand and the size of the σ-donor of A (or B).
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Experiments to analyze the electronic effect of a donating (or withdrawing) Z group were performed by some research groups [30,31]. The results of these experiments indicated that the rate of catalytic activity of the neutral Pd0(dba-n,n'-Z)2 precursor is greatly dependent on the electronic property (donor or acceptor) of the bulky dba-n,n’-Z ligand. Owing to the increase in the strength of electron donating dba-n,n’-Z ligand, the overall rate of the oxidative addition of phenyl iodide to Pd0(dba-n,n’-Z)2 precusor is faster than that of the Pd0 complex with an electron withdrawing Z group. In particular, the rate of oxidative addition of aryl halide to Pd0Ln depends on the concentration of the active Pd0Ln precursor with a strong electron-donating trans L-ligand. Scrivanti et al. [38,39] found that the rate of oxidative addition of an aryl halide to the (iminophosphine)Pd0(η2-olefin) complex also increased. The increase in reaction rate is explained by the catalyst stability of the moderate π-accepting ligand. These experimental results [31,39] showing an increased rate in the catalytic activity may have originated from the electronic property of a strong electron-donating trans L-ligand in Pd0Ln. Based on Figure 3 and Figure 4, in the oxidative addition of ArX to Pd0Ln, the donating π-orbital of the aryl halide can interact with the low-lying unfilled a2u(5p)-orbital of Pd. Thus, an a2u(5p)…π-orbital interaction between the low-lying unfilled a2u(5p)-orbital of Pd and the filled π-orbital of ArX can take place. These experimental results can be understood with the help of results shown in Figure 3.
The orbital energy levels for the orbital interaction associated with the coordination bond of [9]aneB2A to PdL2 are drawn in Figure 5. As shown in (r) of Figure 5, there is a large gap in the energy level between the A site of [9]aneB2A and the a1g(5s)-orbital of PdL2. Therefore, the σ-orbital of A cannot easily interact with the unoccupied a1g(5s)-orbital. Meanwhile, as shown in (s) of Figure 5, the energy gap between the axial A site and a1g(5s)-orbital is largely reduced. In the endo-[Pd([9]aneBAB)(L-donor)2]2+ complex as shown in (h) and (j) of Figure 3, the unoccupied 3a1g(5s)-orbital is partially filled by the electron density transfer from trans L-donor and the level of the partially filled 3a1g(5s)-orbital decreases. In endo-[Pd([9]aneB2A)(L-donor)2]2+ complex as shown in (i) and (k) of Figure 3, the huge σ-orbital of the soft A site can overlap with the partially filled 3a1g(5s)-orbital and then the energy level of the A site is increased by the [σ-donor ↔ 3a1g(5s)] overlap. Therefore, the electron density of increased energy level in σ-orbital of A can share with that of the decreased energy level of 3a1g(5s)-orbital. The unfilled 3a1g-molecular orbital is occupied and the energy level is largely lowered as HOMO. The energy difference between the HOMO and LUMO is also reduced {ΔE2H-L = 2.67~3.51 eV for endo-[Pd([9]aneB2A)(L-donor)2]2+}. As shown in (t) of Figure 5, the orbital energy levels are similar to that in (r) of Figure 5. The σ-orbital cannot interact with the 3a1g(5s)-orbital and the energy gap between the HOMO and LUMO is large {ΔE1H-L = 3.77 and 3.84 eV for endo-Pd([9]aneB2A)Cl2}.
Similar to the interaction suggested above [σ-donor…unfilled a1g(5s)], the fifth [sixth, eighth] axial olefin-Pd coordination bond in the oxidative addition of olefin to Pd(0)Ln can be formed by an axial π-donor…unfilled a2u(5px,y) interaction between the π-donor of olefin and an unfilled a2u(5px,y)-orbital of Pd. The electron density transfer from the trans L-donor to the unfilled a2u(5px,y)-orbital makes it partially occupied, and thus the energy level of the partially unfilled a2u(5px,y)-orbital is lowered. Consequently, the π-donor electron-rich substrates such as olefin can interact with the partially unfilled and lowered a2u(5px,y)-orbital of Pd. In the Pd-mediated cross-coupling reactions, the results of this study can describe the mechanism for the formation of the apical σ-donor…unfilled a1g(5s) and π-donor…unfilled a2u(5px,y) interactions in ArPdLnX. Furthermore, in d9-electron systems such as [Cu(NH3)4(H2O)2]2+ [57,58], a distorted octahedron with two water molecules at a longer distance than four ammonias is formed via the long range interactions. The vertical Cu-OH2 bond length (RCu-O = 2.204 Å) is longer than that (RCu-N = 1.933 Å) of the equatorial Cu-NH3 bond [55]. The two longer Cu-OH2 bonds along the z-axis are explained by the interaction between the half unoccupied 3dz2-orbital of CuII and the filled σ-orbital of the oxygen atom in water.
The relative energies through the structural variation from endo-Pd([9]aneB2A)L2 complex to endo-Pd([9]aneBAB)L2 are represented in Figure 6. In the (u) reaction path, the relative energy level of endo-[Pd([9]aneB2A)(L-donor)2]2+ complex with the fifth (A--Pd) quasi-bond is lower than that of endo-[Pd([9]aneBAB)(L-donor)2]2+ {ΔEBAB-B2A = −0.37, −0.10, −0.35, and −0.08 eV for endo-[Pd([9]aneB2A)(L-donor)2]2+}. The axial (A--Pd) quasi-bond greatly contributes to the relative stability of the pentacoordinate endo-[Pd([9]aneB2A)(L-donor)2]2+ complexes. In the (v) reaction path, the relative energy level of endo-Pd([9]aneBAB)Cl2 {endo-Pd([9]aneA2B)Cl2} complex is more stable than that of endo-Pd([9]aneB2A)Cl2 {endo-Pd([9]aneABA)Cl2} complex (ΔE = −0.18~−0.28 eV for endo-Pd([9]aneBAB)Cl2). Due to the relative stability between the axial and equatorial (A-Pd) bonds, the Pd([9]aneABA)L2 and Pd([9]aneA2B)L2 structures with one or two equatorial (A-Pd) bonds are more stable than the Pd([9]aneB2A)L2 and Pd([9]aneBAB)L2 structures, respectively (as shown in Table 1 and Supplementary Information Table S1). In particular, in the (w) reaction path, the geometric configuration of endo-[Pd([9]anePNP)(L-donor)2]2+ automatically optimizes to the endo-[Pd([9]aneP2N)(L-donor)2]2+ structure without the transition energy barrier. {endo-[Pd([9]anePNP)(L-donor)2]2+ could not be optimized.} Therefore, the relative stability and configurational and conformational changes (such as axial-equatorial coordination and cis-trans isomerization) of the RPdII(L)nX intermediates are largely influenced by the axial (soft A…Pd) interaction, relative Pd affinity of the soft A atom, and low potential energy barrier.
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Figure 5. The variation in the orbital energy levels of endo-Pd([9]aneB2A)L2 complex, showing the coordination bonds of [9]aneB2A to [Pd(L-donor)2]2+ and [PdCl2]. In endo-[Pd([9]aneB2A)(L-donor)2]2+ complex (r), the separated configuration of two electron spins filled in the 3a1g-orbital indicates the (A--Pd) quasi-bond.
Figure 5. The variation in the orbital energy levels of endo-Pd([9]aneB2A)L2 complex, showing the coordination bonds of [9]aneB2A to [Pd(L-donor)2]2+ and [PdCl2]. In endo-[Pd([9]aneB2A)(L-donor)2]2+ complex (r), the separated configuration of two electron spins filled in the 3a1g-orbital indicates the (A--Pd) quasi-bond.
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Figure 6. The variation of the structural configuration from endo-Pd([9]aneB2A)L2 {or endo-Pd([9]aneABA)L2} complex to endo-Pd([9]aneBAB)L2 {or endo-Pd([9]aneA2B)L2} complex, including the relative energy level (A:P, S; B:N). (u) is a reaction path from the pentacoordinate endo-[Pd([9]aneB2A)(L-donor)2]2+ complex with an axial (soft A--Pd) quasi-bond to endo-[Pd([9]aneBAB)(L-donor)2]2+. (v) is a reaction path from the tetracoordinate endo-Pd([9]aneB2A)(L-acceptor)2 complex with a trans L-acceptor to endo-Pd([9]aneBAB)(L-acceptor)2. (w) is a reaction path from the endo-Pd([9]anePNP)(L-donor)2 complex to endo-Pd([9]aneP2N)(L-donor)2 without the transition energy barrier.
Figure 6. The variation of the structural configuration from endo-Pd([9]aneB2A)L2 {or endo-Pd([9]aneABA)L2} complex to endo-Pd([9]aneBAB)L2 {or endo-Pd([9]aneA2B)L2} complex, including the relative energy level (A:P, S; B:N). (u) is a reaction path from the pentacoordinate endo-[Pd([9]aneB2A)(L-donor)2]2+ complex with an axial (soft A--Pd) quasi-bond to endo-[Pd([9]aneBAB)(L-donor)2]2+. (v) is a reaction path from the tetracoordinate endo-Pd([9]aneB2A)(L-acceptor)2 complex with a trans L-acceptor to endo-Pd([9]aneBAB)(L-acceptor)2. (w) is a reaction path from the endo-Pd([9]anePNP)(L-donor)2 complex to endo-Pd([9]aneP2N)(L-donor)2 without the transition energy barrier.
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4. Conclusions

We investigated the geometric structures and relative stabilities of Pd([9]aneB2A)L2 complexes {Pd([9]aneBAB)L2, endo-Pd([9]aneA2B)L2, endo-Pd([9]aneABA)L2}, the selective orbital interaction for an axial or equatorial (A…Pd) coordination of [9]aneB2A with PdL2, and the electronic effects of the soft A/hard B donors and the donating/withdrawing trans L-ligand. The endo-[Pd([9]aneB2A)(L-donor)2]2+ complex with an axial (soft A--Pd) quasi-bond was optimized as a pentacoordinate geometry, while the other Pd-complexes were optimized as tetracoordinate structures. The relative energy of the pentacoordinate Pd complexes is lower than that of the corresponding endo-Pd tetracoordinates. Among the Pd tetracoordinated species, the Pd([9]aneBAB)L2 structure with an equatorial (soft A-Pd) bond is more stable than the Pd([9]aneB2A)L2 type with both the equatorial (hard B-Pd) bonds. Furthermore, the endo-[Pd([9]aneA2B)(L-donor)2]2+ tetracoordinated with two equatorial (A-Pd) bonds is more stable than pentacoordinated endo-[Pd([9]aneABA)(L-donor)2]2+. And the endo-[Pd([9]anePNP)(L-donor)2]2+ type of structures is automatically optimized to the stable endo-[Pd([9]aneP2N)(L-donor)2]2+ type of structures. The relative stability of the Pd-complexes and the equatorial or axial selectivity of the soft A/hard B donors to PdII center are dependent upon its softness, basicity, and electronic effects.
In the Pd-catalyzed coupling reactions, the PdLn intermediates are activated by an electron-rich trans L-donor possessing a strong Lewis base character. The strong Lewis base character of the trans L-donor allows its electron density to transfer to the low-lying unoccupied orbitals [a1g(5s) or a2u(5p)] of Pd and the energy level of the partially unfilled a1g(5s)-orbital of Pd is decreased. Therefore, an axially fifth substrate…PdL2 interaction between the σ(or π)-donor of substrates (such as hemilabile multidentates, olefinic halides, and solvents) and the PdL2 precursor should occur energetically, resulting in the electronic and steric effects of the bulky and electron-rich ligand. Because of the low transition barrier and an axial substrate…PdL2 interaction, the configurational and conformational changes of RPdII(L)nX can easily take place (e.g., axial-equatorial and cis-trans exchanges and interconversions in RPdII(L)nX intermediates). Consequently, the geometric structure and relative stability of the Pd complexes are influenced by the relative strength of the axial or equatorial (soft A…Pd) interaction, donating electron property of the trans L-ligand, and relative Pd affinity of the A and B donors. The mechanisms for orbital interaction and electron density transfer proposed in this study can be considerably valuable in the design of useful Pd-catalyst and synthetic applications for the Pd-catalyzed cross-couplings of substrates.

Supplementary Materials

Supplementary materials can be accessed at: https://www.mdpi.com/1420-3049/18/10/12687/s1.

Acknowledgments

We gratefully acknowledge the support of Chemistry Research Fund of Gyeongsang National University. This work was supported by the fund of Research Promotion Program (RPP-2010-018), Gyeongsang National University.

Conflicts of Interest

The authors declare no conflict of interest.

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  • Sample Availability: Samples of the compounds are available from the authors.

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MDPI and ACS Style

Kwak, O.K.; Arooj, M.; Yoon, Y.-J.; Jeong, E.D.; Park, J.K. Orbital Interaction and Electron Density Transfer in PdII([9]aneB2A)L2 Complexes: Theoretical Approaches. Molecules 2013, 18, 12687-12706. https://doi.org/10.3390/molecules181012687

AMA Style

Kwak OK, Arooj M, Yoon Y-J, Jeong ED, Park JK. Orbital Interaction and Electron Density Transfer in PdII([9]aneB2A)L2 Complexes: Theoretical Approaches. Molecules. 2013; 18(10):12687-12706. https://doi.org/10.3390/molecules181012687

Chicago/Turabian Style

Kwak, Ock Keum, Mahreen Arooj, Yong-Jin Yoon, Euh Duck Jeong, and Jong Keun Park. 2013. "Orbital Interaction and Electron Density Transfer in PdII([9]aneB2A)L2 Complexes: Theoretical Approaches" Molecules 18, no. 10: 12687-12706. https://doi.org/10.3390/molecules181012687

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