2P2P-104
Comparative Theoretical Study of Reactivities of Pd (II), Pd (III), and P(IV)
Complexes towards C-C and Si-Si bond reductive elimination reactions(Fukui Institute for Fundamental Chemistry, Kyoto University) Milind Deshmukh, Shigeyoshi Sakaki[Introduction] The activation of small molecules by transition metals is of fundamental importancein recent organometallic chemistry. One of the typical examples is C-H bond activation reaction ofalkanes and aromatics. This reaction is difficult by conventional organic reagents in general. However,
the activation of C-H bond has been achieved by many transition metal complexes. During the pastfew decades, palladium (Pd) has gained much attention as a one of the most versatile and efficienttransition metal catalysts for wide range of C-C coupling and C-H functionalization. Though Pd(0)and Pd(II) chemistries have been extensively studied, much less is known about the Pd(III) complex
until the recent studies by Ritter et al.1 and Sanford et al.2 They reported that the dinuclear Pd(III)complexes play important role as a key intermediate in the oxidative functionalization of C-H bond.
Though several such dinuclear Pd(III) complexes have been structurally characterized, nomononuclear Pd(III) complex has been known till the recent report by Khusnutdinova et al.3 Theseauthors succeeded in the synthesis of mononuclear organometallic Pd(III) complexes starting fromPd(II) complex with four nitrogen containing ligands shown in Scheme 1.
In this work, we wish to report theoreticalinvestigation on the electronic structure of one of
the Pd(III) complexes (3+, where R=X=Me)
mentioned in Scheme 1, in comparison to the Pd(II)and Pd(IV) complexes. As the first step towardunderstanding the reactivity of Pd(III) complex, theC-C and Si-Si reductive elimination reactions areinvestigated. The reactivities of these Pd-complexes
for such reactions are discussed with the electronic
Scheme1: Synthesis of Pd(III) Complexes structures of these complexes.
[Computational Details] Geometry optimization was carried out with B3PW91 functional. Twokinds of basis set system (BS-I and BS-II) were employed. For H, C, N, P, and Cl, 6-31G* were usedin BS-I and cc-pVTZ basis sets were used in BS-II. The (311111/22111/411) basis set for Pd-atom
was used in both BS-I and BS-II, where its core electrons were replaced by Stuttgart-Dresden-Born(SDD) ECP. To test the suitability for DFT functionals, calculations of model systems were carriedout at various levels of theory. The reaction energies and population were calculated with basis set III(BS-III), where (311111/22111/411/11) basis sets was used for Pd atom with SDD ECP and cc-pVTZ
basis sets for H, C, N, P, and Cl atoms.
[Results and Discussion]
First, we investigated the electronic structure of the Pd(III) complex. In this complex, the dZ2
orbital is SOMO. In the other possible electron configuration, the dx2-y2 is SOMO. However, thelatter electron configuration is much less stable than the former by 53.05 kcal/mol in the DFT
calculation of the real complex and 17.85 kcal/mol in the calculations of model complex. Considering
this result, we investigated the former electron configuration hereafter.
To find out suitable DFT functional, model reactions 1 to 3 shown below were considered.
Pd(III) (NH3)4(CH3)2
Pd(II) (PH3)2(CH3)2
Pd(IV) (PH3)2(Cl)2(CH3)2Pd(I) (NH3)4 + C2H6
Pd(0) (PH3)2 + C2H6
Pd(II) (PH3)2(Cl)2 + C2H6(1)(2)(3)

The reaction energies and activation barrier in these reactions were computed with various DFT
functionals and were compared with CCSD(T) values. Only B3PW91 and M06L functionals
reproduce the trend in the reactivities obtained at CCSD(T) level. Here, we used B3PW91 and M06L
functionals for studying the reactivity in the real Pd-complexes shown in reaction 7 to 9.

(7)Pd(III)-(CH3)2

TSIII / I Pd(III)-(CH3)2Pd(I)+ C2H6(8)Pd(II)-(CH3)2

TS II / 0 Pd(II)-(CH3)2Pd(0) + C2H6(9)Pd(IV)-(CH3)2TSIV / II Pd(III)-(CH3)2Pd(II)+ C2H6Table 1: Reaction energies, activation barrier, Pd-CH3 and Pd-SiH3 bond energies for reaction 7 to 12
Level ofPd(II) → Pd(0) (8)Pd(IV) → Pd(II)(9)
Pd(III) → Pd(I) (7)theoryEa
Pd-CH3 BE
Ea
Pd-CH3 BE
Ea∆Ε∆Ε
∆Ε Pd-CH3 BE
B3PW91 28.5 -7.0
43.3
41.1 5.6
49.7
33.4 -10.4
41.7
Level ofPd(III) → Pd(I)(10)
Pd(II) → Pd(0)(11)
Pd(IV) → Pd(II)(12)
theoryEa
E
Pd-SiH3 BE
Ea
Pd-SiH3 BE∆E Pd-SiH3 BE∆Ε∆Ε
B3PW91 16.6 28.5
51.1
21.8 56.5
65.1
23.1 20.7
47.2
As can be seen in Table 1, the reductive elimination of C-C bond from the Pd(III) and Pd(IV)complexes is exothermic, whereas that for Pd(II) is slightly endothermic. The activation barrier
increases in the order Pd(III)<Pd(IV)<Pd(II) and the exothermicity decreases in the order Pd(IV) >Pd(III) > Pd(II). The exothermicity and small activation barrier suggest that the C-C bond elimination
reactions would occur easily from Pd(IV) and Pd(III) centers. Interestingly, the reactivity of Pd(III) is
close to that of Pd(IV). The Pd-CH3 bond energies are correlated to reaction energies of above
reactions, as expected. For instance, smaller Pd-CH3 bond energy in the Pd(IV) complex is inagreement with the relatively large exothermicity of the reaction 9.
[References]
1. (a) Powers, D. C.; Geibel, M. A. L.; Klein, J.; Ritter, T. J. Am. Chem. Soc. 2009, 131, 17050. (b)Powers, D. C.; Ritter, T. Nat. Chem. 2009, 1, 302.
2. Deprez, N. R.; Sanford, M. S. J. Am. Chem. Soc. 2009, 131, 11234.
3. (a) Khusnutdinova, J. R.; Rath, N. P.; Mirica, L. M. J. Am. Chem. Soc. 2010, 132, 7303. (b)Khusnutdinova, J. R.; Rath, N. P.; Mirica, L. M. J. Am. Chem. Soc. 2012, 134, 2414.