Insights into the Mechanistic Role of Diphenylphosphine Selenide

Article pubs.acs.org/JPCA

Insights into the Mechanistic Role of Diphenylphosphine Selenide, Diphenylphosphine, and Primary Amines in the Formation of CdSe Monomers Ting Qi,† Hua-Qing Yang,*,† Dennis M. Whitfield,§ Kui Yu,*,§ and Chang-Wei Hu‡ †

College of Chemical Engineering, ‡Key Laboratory of Green Chemistry and Technology, Ministry of Education, College of Chemistry, and §College of Physics, Sichuan University, Chengdu, Sichuan 610064, People’s Republic of China S Supporting Information *

ABSTRACT: The formation mechanism of CdSe monomers from the reaction of cadmium oleate (Cd(OA)2) and SePPh2H in the presence of HPPh2 and RNH2 was studied systematically at the M06//B3LYP/6-31++G(d,p),SDD level in 1-octadecene solution. Herein, SePPh2H, HPPh2, and RNH2 act as hydrogen/proton donors with a decreased capacity, leading to the release of oleic acid (RCOOH). The longer the radius of the coordinated atom is, the larger the size of the cyclic transition state is, which lowers the activation strain and the Gibbs free energy of activation for the release of RCOOH. From the resulting RCOOCdSe− PPh2, for the formation of Ph2P−CdSe−PPh2 (G), SePPh2H acts as a catalyst, in which the turnover frequency determining transition state (TDTS) is characteristic of the Se−P bond cleavage. For the formation of RHN−CdSe−PPh2 (H), SePPh2H also serves as a catalyst, in which the TDTS is representative of the N−H bond cleavage. For the formation of Ph2PSe− CdSe−NHR (I), HPPh2 behaves as a catalyst, in which the TDTS is typical of the Se−P and N−H bond cleavage. The rate constants increase as kI < kH < kG, which is in good agreement with our previous experimental observations reported. The present study brings insight into the use of additives such as HPPh2 and RNH2 to synthesize colloidal quantum dots.

1. INTRODUCTION Over the past years, semiconductor nanocrystalline quantum dots (QDs) have attracted considerable interest because of their potentially groundbreaking applications, including light-emitting diodes,1,2 lasers,3,4 photovoltaic cells,5,6 and biological labeling.7−9 The interest in the QD potential has led to advances in QD preparation methods.10−14 Hereinto, group II−VI semiconductor QDs, especially CdSe, have been extensively researched due to their ready synthesis to tune their emission in the visible range. A large number of recipes have been developed to synthesize colloidal QDs;15−17 however, the molecular mechanism involved in the monomer formation has been documented little. Only until recently, some studies have aimed at understanding the chemical mechanism of their formation mechanisms,18−24 and particularly the mechanism by which the monomers are generated.25−33 In order to realize the QD potential, it is urgently necessary to advance our mechanistic understanding of the formation of monomers to enhance synthetic reproducibility and yield at low temperature, with the use of a secondary phosphine such as diphenylphosphine (HPPh2). Generally, the reaction of M(OOCR)n + EPR3 → ME QDs is a representative approach to binary colloidal metal chalcogenide (ME) semiconductor QDs, where M(OOCR)n represents metal carboxylates, such as cadmium oleate (Cd(OOCC17H33)2 or Ca(OA)2), and EPR3 symbolizes tertiary phosphine chalcogenides, such as tri-n-octylphosphine selenide (SeP(C8H17)3). Primary alkylamines (RNH2) have been used as one beneficial additive and were proposed to function as surface © 2016 American Chemical Society

ligands via coordination that stabilizes the QDs and affects their optical properties17,30,34−37 and/or as a component in the reaction medium to shape nucleation/growth.33,38−41 However, with regard to the molecular role of amine affecting reactions, there is considerable controversy. Some studies claimed that amine “activates” precursors and/or increases the rate of nanocrystal growth, while others demonstrated exactly the opposite effect, claiming that amine decreases the activity of precursors and/or the growth rate of nanocrystals.38,42−47 Recently, Liu and co-worker commented that alkylamines slow down the reaction of Cd(OA)2 + SePR3 by binding to cadmium and preventing the activation of SePR3 through its binding to Cd. In this model, the cleavage of the PSe bond is through nucleophilic attack by carboxylate instead of alkylamine.30 In our reaction of Cd(OA)2 + SePPh2H + RNH2, the amine participates the cleavage of the PSe bond during the monomer formation.33 Furthermore, commercial dialkylphosphines such as HPPh2 are further beneficial additives, which improve the lowtemperature nucleation/growth of colloidal QDs with high particle yield and reproducibility.23,25,27,31,48−51 For the reaction of M(OOCR)n + EPR3 + HPPh2 → ME QDs, because the Se exchange of SeP(C8H17)3 + HPPh2 ⇔ P(C8H17)3 + SePPh2H takes place readily, diphenylphosphine selenide (SePPh2H) rather than trin-octylphosphine selenide (SeP(C8H17)3) has Received: November 1, 2015 Revised: December 21, 2015 Published: January 8, 2016 918

DOI: 10.1021/acs.jpca.5b10675 J. Phys. Chem. A 2016, 120, 918−931

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The Journal of Physical Chemistry A

found.59,60 Computed ⟨S2⟩ values suggested that only small spin contamination is included in the calculations. Systematic frequency calculations were performed to characterize stationary points obtained and to take corrections of zero-point energy (ZPE) into account. For the reaction pathway analysis, we ensured that every transition structure has only one imaginary frequency, and the connections between transition states and corresponding intermediates were verified by means of intrinsic reaction coordinate (IRC) calculations.61,62 A polarized continuum model (PCM-SMD)63,64 with dielectric constant ε = 2.0 was utilized to simulate the solvent effect of 1-octadecene (ODE), via a hybrid M06 functional method65 with the same basis sets as mentioned earlier, by performing single-point calculations on the optimized structures at the B3LYP/6-31+ +G(d,p),SDD level, namely, M06//B3LYP/6-31++G(d,p),SDD. The dominant occupancies of natural bond orbitals and dominant stabilization energies E(2) between donors and acceptors for some species have been analyzed with the help of the natural bond orbital (NBO) analysis.66,67 Unless otherwise mentioned, the Gibbs free energy of formation (ΔG) is relative to the initial reactants including ZPE correction obtained at the M06//B3LYP/6-31+ +G(d,p),SDD level in ODE solution under atmospheric pressure and room temperature (1 atm and 298.15 K). Besides, to evaluate the present computational method, for the reaction pathways of Cd(OA)2 with SePPh2H, full geometry optimizations were run to attain all of the stationary points and transition states, via a hybrid M06 functional method65 with the SDD basis set55,56 and the corresponding effective core potential (ECP) for the Cd and Se atoms, and the 6-31++G(d,p) basis set57,58 for other atoms of C, H, O, and P, in ODE solvent with dielectric constant ε = 2.0 using PCM-SMD model,63,64 namely, M06/6-31++G(d,p),SDD. The geometric structures and the schematic energy diagrams are depicted in Figure S6 of the Supporting Information (SI). As shown in Figure S6 in the SI, both the geometric structures and the relative energies of the species are almost identical between using the M06/6-31+ +G(d,p),SDD level and using the M06//B3LYP/6-31++G(d,p),SDD level. Thereupon, to save computational time, the method of the M06//B3LYP/6-31++G(d,p),SDD level is preferred in the present study. The TOF of the catalytic cycle determines the efficiency of the catalyst. Based on the transition state theory,68,69 TOF can be calculated by eqs i and ii,70−72 in which δE (the energetic span73) is defined as the energy difference between the summit and trough of the catalytic cycle. GTDTS and GTDI are the Gibbs free energies of the TOF determining transition state (TDTS) and the TOF determining intermediate (TDI), and ΔGr is the global free energy of the whole cycle.

been suggested to be the Se precursor.27,31 Concerning the mechanistic understanding, one general mechanism postulates that trialkylphosphine selenide PSe bond cleavage starts by nucleophilic attack of carboxylate on Cd2+-activated phosphine selenide, followed by proton transfer from carboxylic acid to Se and Cd−Se bond formation. The rate-limiting step lies at or before formation of an acyloxytrialkylphosphonium ion, whose existence was detected by trapping with alcohols.26,29,30 The second general reaction mechanism that has been postulated starts with the addition of HPPh2, which results in a reduction of the metal (Pb) carboxylate.25 The HPPh2 coordinates to available sites on the metal center before reducing it by removal of the carboxylates. Reaction of the metal M0 with the phosphine chalcogenide produces QD monomer and PR3.25 Subsequently, Krauss et al. supported the idea that secondary and not tertiary phosphines are the reactive species in QD formation.27,52 We have suggested that it is SePPh2H rather than SePR3 that reacts with Cd(OA)2 at low temperature, leading to the formation of the CdSe QDs.31,32 Moreover, a third possible chemical mechanism has been postulated: the release of acid C17H33COOH from (C17H33COO)2Cd(SePPh2H)2 followed by the release of HPPh2 from C17H33COOCd(SePPh2)(Se PPh2H) through the SeP bond cleavage of the SePPh2H arm.31,32 One of the possible reasons that contributed to the inconsistencies in the literature is the lack of complete mechanistic understanding of the roles of HPPh2, RNH2, and SePPh2H in the formation of CdSe monomers. In this study, we will investigate the target reaction 1 mechanism Cd(OA)2 + SeP(C8H17)3 + C18H35NH 2 + HPPh 2 → CdSe QDs

(1)

in 1-octadecene (ODE), using density functional theory (DFT). The goals are as follows: (a) to provide reliable structures and vibration frequencies of the reactants, intermediates (IMs), transition states (TSs), and products as well as their chemically accurate energetics, (b) to elucidate the coordination stability of PR3, SePR3, HPPh2, SePPh2H, and RNH2 toward Cd(OA)2, (c) to shed light on the role of HPPh2, RNH2, and SePPh2H in the CdSe monomers formation, (d) to obtain a deep understanding of the preferred reaction pathway over the preparative temperature range, and (e) to gain an insight into SePPh2H and HPPh2 as catalyts in various catalytic cycles by the determining transition state (TDTS) and the determining intermediate (TDI) of the turnover frequency (TOF); see eqs i and ii.

2. COMPUTATIONAL DETAILS All DFT calculations were performed with the programs of Gaussian 09.53 In the present calculation for the target reaction system, ethyl groups (C2H5) were applied to represent the alkyl group (R) of C17H33COOH and C18H35NH2, while no simplification was applied for the phenyl group of HPPh2. Full geometry optimizations were carried out to locate all of the stationary points and transition states, via a hybrid B3LYP functional method54 with the SDD basis set55,56 and the corresponding effective core potential (ECP) for the Cd and Se atoms, and the 6-31++G(d,p) basis set57,58 for other atoms of C, H, O, N, and P, namely, B3LYP/6-31++G(d,p),SDD. Meanwhile, the stability of the wave function of the auxiliary Kohn−Sham determinant in density function theory (DFT) was tested.59,60 If an instability was found, the wave function was reoptimized with appropriate reduction in constraints, and the stability tests and reoptimizations were repeated until a stable wave function was

TOF =

kBT −δE / RT e h

(i)

⎛GTDTS − GTDI if TDTS appears after TDI ⎞ ⎟⎟ δE = ⎜⎜ ⎝GTDTS − GTDI + ΔGr if TDTS appears before TDI ⎠ (ii)

where kB is the Boltzmann constant, T is the absolute temperature, and h is the Planck constant.

3. RESULTS AND DISCUSSION In this work, we will mainly discuss the following reaction formulas: (1) Se exchange between SePR3 and HPPh2, (2) release of the first RCOOH in the reaction of Cd(OA)2 with 919


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apparent that the PR3 coordination is stronger than HPPh2 coordination toward Cd(OA)2, which is in agreement with the theoretical result at the TPSSh/def2-TZVPP level.31 The Se exchange between SePR3 and HPPh2 can be expressed by the following chemical reaction:

SePPh2H, HPPh2, and RNH2, (3) the reaction of resultant RCOOCdSe−PPh2 (C) with SePPh2H, leading to Ph2PSe−Cd− SePPh2 (E) or RCOOCdSe2PPh2 (F), (4) formation of Ph2P− CdSe−PPh2 (G) from C, E, or F, (5) formation of RHN− CdSe−PPh2 (H) from C, E, F, and G, and (6) formation of Ph2PSe−CdSe−NHR (I) from F, RHN−CdSe2PPh2(I-i), Ph2P−CdSe2PPh2 (E-i), and H. 3.1. Coordination of PR3, SePR3, HPPh2, SePPh2H, and RNH2 toward Cd(OA)2. For the target reaction stage, the reactants are Cd(OA)2, SePR3, C18H35NH2, HPPh2, and SePPh2H. First of all, we will investigate the coordination of PR3, SePR3, C18H35NH2 (RNH2), HPPh2, and SePPh2H toward Cd(OA)2. The geometric structures and the relative energies relative to the discrete reactants are depicted in Figure S1a of the SI. For Cd(OA)2, NBO results show that there is hyperconjugation interaction in each four-membered cycle Cd−O− C−O. That is to say, Cd(OA)2 behaves as a four-coordination complex. As shown in Figure S1a of the SI, when one ligand (L) coordinates to Cd(OA)2, the five-coordination complex should be formed, with the stabilization energies of 37.2, 22.6, 17.9, 11.9, and −33.4 kJ mol−1 for SePR3, RNH2, SePPh2H, PR3, and HPPh2, respectively. Particularly, for the binding energy of SePR3 toward Cd(OA)2, the present calculated value of 37.2 kJ mol−1 in ODE solution is higher than the experimental value of 7.4 ± 0.4 kJ mol−1 and the calculated value of (6.3 ± 0.8) kJ mol−1 in CDCl3 solution.74 Alternatively, when two ligands coordinate to Cd(OA)2, the six-coordination complex should be generated, with the stabilization energies of 51.1, 6.0, −8.9, −1.5, and −79.0 kJ mol−1 for SePR3, RNH2, SePPh2H, PR3, and HPPh2, respectively. Thereupon, both for the five- and six-coordination complexes, the coordination stability of ligands to metal centers of Cd(OA)2 decreases as SePR3 > RNH2 > SePPh2H > PR3 > HPPh2. That is to say, the coordination of ligand to the Cd center in Cd(OA)2 through Se and N atoms is stronger than through P atom. The NBO analysis shows that the NBO charges of Se in SePR3, N in RNH2, and Se in SePPh2H are negative by −0.522, −0.916, and −0.460, respectively, while the NBO charges of P in PR3 and HPPh2 are positive by 0.824 and 0.617, respectively. Furthermore, the NBO charge of Cd in Cd(OA)2 is positive by 1.359. It is indicated that the coordination stability mainly stems from the electrostatic interaction between the ligand and the central Cd atom in Cd(OA)2. Exceptionally, as mentioned earlier, although the NBO charges of N in RNH2 are more negative than those of Se in SePR3, the stabilization energies of SePR3 are larger than those of RNH2 toward Cd(OA)2 for both the five- and six-coordination complexes. This can be ascribed to the steric hindrance, in which the Se atom in the linear Se−P of SePR3 coordinates more readily than the N atom in the quadrihedron of RNH2, toward Cd(OA)2. Thereupon, both electrostatic interaction and steric hindrance play crucial roles in the coordination stability. On the other hand, for SePR3, the ΔG values of the sixcoordination complex are lower than that of the fivecoordination complex. It is indicated that the six-coordination complex (SePR3)2Cd(OA)2 is more stable than the five-coordination complex (SePR3)Cd(OA)2. However, for SePPh2H, RNH2, and PR3, the ΔG values of the five-coordination complex are lower than that of the six-coordination complex, indicating that the fivecoordination complex is more stable than the corresponding complex. Exceptionally, for HPPh2, both the five- and sixcoordination complexes are unstable, in view of their positive ΔG values. This coordination instability may be ascribed to the decrease of entropy and steric hindrance. Moreover, it is

SePR3 + HPPh 2 → SePPh 2H + PR3

(2)

The geometric structures and the schematic energy diagrams for reaction 2, SePR3 + HPPh2 → SePPh2H + PR3, are depicted in Figure S1b of the SI. As shown in Figure S1b of the SI, reaction 2 is calculated to be endergonic by 51.8 kJ mol−1, with the energy barrier of 139.4 kJ mol−1 via a linear TS1-1 through electrophilic attack of HPPh2 toward Se atom of SePR3. This result is in agreement with both the theoretical and experimental investigations.31 3.2. Release of the First RCOOH in the Reaction of Cd(OA)2 with SePPh2H, HPPh2, and RNH2. In the reaction of Cd(OA)2 with SePPh2H, HPPh2, and RNH2, the release of the first RCOOH for the formation of intermediates RCOOCdSe− PPh2 (C), RCOOCd−PPh2 (C′), and RCOOCd−NHR (C″), respectively, can be represented by the following reaction formulas: Cd(OA)2 + SePPh 2H → RCOOCdSe− PPh 2 + RCOOH

(3)

Cd(OA)2 + HPPh 2 → RCOOCd − PPh 2 + RCOOH

(4)

Cd(OA)2 + RNH 2 → RCOOCd −NHR + RCOOH

(5)

The geometric structures and the schematic energy diagrams for reaction 3 are depicted in Figure 1a,b, respectively; for reactions 3, 4, and 5, in Figure S2a,b of the SI. As shown in Figure 1a,b, for the reaction of Cd(OA)2 with SePPh2H, initially, the five-coordination complex (SePPh2H)Cd(OA)2 (C-IM1) is formed with the stabilization energy of 17.9 kJ mol−1. From C-IM1, H-shift (formally a proton shift) from SePPh2H to carboxylate takes place via a seven-membered ring C-TS1, leading to the carboxylic acid molecular complex C-IM2, with the energy barrier of 22.1 kJ mol−1 and the energy height of the highest point (EHHP) of 4.2 kJ mol−1 at C-TS1. After that, C-IM2 releases the carboxylic acid, leaving transRCOOCdSe-PPh2 (C-i) behind, with the exoergicity of 18.9 kJ mol−1. Last, C-i isomerizes to C, with the exoergicity of 6.4 kJ mol−1. For complex C, the occupancies of Cd−Se are 1.985 e, indicating a complete single bond in Cd−Se. Furthermore, NBO results show that there is a hyperconjugation interaction in the four-membered cycle Cd−O−C−O. In other words, complex C includes a three-coordinated Cd center. As indicated in Table S7 and Figure S7 in the SI, when RNH2 or SePPh2H coordinates to C, a more stable four-coordinated complex can be formed. When HPPh2 or RCOOH coordinates to C, the corresponding fourcoordinated complex is less stable. This decrease of coordination number stems from the formation of Cd−Se single bond. With regard to the release of carboxylic acid from Cd(OA)2, the reaction of Cd(OA)2 with SePPh2H is exoergic by 56.4 kJ mol−1 while the reaction of Cd(OA)2 with HPPh2 or RNH2 is endergic by 34.0 or 86.3 kJ mol−1, as seen in Figure S2a,b of SI. Thereupon, the carboxylate release from Cd(OA)2 decreases thermodynamically as SePPh2H > HPPh2 > RNH2. For the reaction of Cd(OA)2 with SePPh2H, HPPh2, and RNH2, both the energy barrier and the EHHP increase as SePPh2H < HPPh2 < RNH2. One can see that the kinetics of carboxylic acid release should decrease as SePPh2H > HPPh2 > RNH2. Thus, the capacity of providing hydrogen/proton decreases as SePPh2H > HPPh2 > RNH2, both thermodynamically and kinetically. That is to say, for 920


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Figure 1. Geometric structures of various species (a) and the schematic energy diagrams (b) for reaction 3, Cd(OA)2 + SePPh2H → C + RCOOH. Bond lengths are reported in angstroms. Relative Gibbs free energies (Gr, kJ mol−1) for the corresponding species relative to Cd(OA)2 + SePPh2H at the M06//B3LYP/6-31++G(d,p),SDD level are shown.

As shown in Figure 2a,b, for the reaction C with SePPh2H, initially, the five-coordination complex C/E-IM1 is formed with the stabilization energy of 9.6 kJ mol−1. From C/E-IM1, H-shift from SePPh2H to carboxylate occurs via a seven-membered ring C/E-TS1, resulting in a carboxylic acid molecular complex C/E-IM2, with the energy barrier of 23.2 kJ mol−1 and the EHHP of −42.8 kJ mol−1 at C/E-TS1. Next, C/E-IM2 releases the free carboxylic acid, leaving E behind, with the exoergicity of 29.2 kJ mol−1. For the reaction C-i with SePPh2H, first, the five-coordination complex C-i/F-IM1 is generated with the stabilization energy of 24.2 kJ mol−1. From C-i/F-IM1, there are two reaction pathways, one for F and another for trans-Ph2PSe−Cd−SePPh2 (E-ii trans with respect to the Se−P bond). On the one hand, from C-i/F-IM1, the SeP bond cleavage takes place via fivemembered ring C-i/F-TS1 through electrophilic attack of the -PPh2 group toward the Se atom of SePPh2H, leading to the HPPh2 molecular complex C-i/F-IM2, with the energy barrier of 40.6 kJ mol−1 and the EHHP of −33.6 kJ mol−1 at C-i/F-TS1. After that, C-i/F-IM2 set the HPPh2 molecule free, leaving F behind, with the exoergicity of 36.6 kJ mol−1. For complex F, NBO results show that there is a hyperconjugation interaction both in Cd−O−C−O and Cd−Se−P−Se four-membered cycles. It is indicated that complex F includes a four-coordinated Cd center. As indicated in Table S7 and Figure S7 in the SI, when RNH2 or SePPh2H coordinates to F, a more stable

the carboxylic acid release from Cd(OA)2 and SePPh2H is the most favorable among these three reactants. Therefore, intermediate C is the most preferred, both thermodynamically and kinetically. From the reaction mechanisms of SePPh2H, HPPh2, and RNH2 with Cd(OA)2, one can see that SePPh2H, HPPh2, and RNH2 can be regarded as the hydrogen/proton donors in the precursor conversion for the first RCOOH release. Next, we will discuss further reaction of C with SePPh2H, vide infra. 3.3. Reaction of RCOOCdSe−PPh2 with SePPh2H. For the formation of intermediates Ph2PSe−Cd−SePPh2 (E) and RCOOCdSe2PPh2 (F), the reaction of RCOOCdSe−PPh2 (C) with SePPh2H can be referred to by the following two reaction formulas: RCOOCdSe− PPh 2 + SePPh 2H → Ph 2PSe −Cd −SePPh 2 + RCOOH

(6)

RCOOCdSe− PPh 2 + SePPh 2H → RCOOCdSe2PPh 2 + HPPh 2

(7)

The geometric structures and the schematic energy diagrams for reactions 6 and 7 are depicted in Figure 2a,b, respectively. As mentioned earlier, C may readily isomerize to transRCOOCdSe−PPh2 (C-i) with a low energy barrier of 6.4 kJ mol−1. Herein, we will discuss the reaction of C-i with SePPh2H. 921


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Figure 2. Geometric structures of various species (a) and the schematic energy diagrams (b) for the reaction Cd(OA)2 + 2SePPh2H → E + 2RCOOH and Cd(OA)2 + 2SePPh2H → F + RCOOH + HPPh2. Bond lengths are reported in angstroms. Relative Gibbs free energies (Gr, kJ mol−1) for the corresponding species plus RCOOH relative to Cd(OA)2 + 2SePPh2H at the M06//B3LYP/6-31++G(d,p),SDD level are shown. Green and blue lines represent reaction 6, C + SePPh2H → E + RCOOH. The red line represent reaction 7, C + SePPh2H → F + HPPh2.

five-coordinated complex can be formed. When HPPh2 or RCOOH coordinates to F, the corresponding five-coordinated complex is less stable. On the other hand, from C-i/F-IM1, an H-shift from SePPh2H to carboxylate happens via a seven-membered ring C-i/E-ii-TS1, producing a carboxylic acid molecular complex C-i/E-ii-IM2, with the energy barrier of 37.6 kJ mol−1 and the EHHP of −36.6 kJ mol−1 at C-i/E-ii-TS1. Next, C-i/E-ii-IM2 liberates the carboxylic acid, leaving trans-E-ii behind, with the exoergicity of 23.1 kJ mol−1. Last, E-ii can readily isomerize to E, with the exoergicity of 14.5 kJ mol−1. E has the two PPh2’s syn with

respect to the Se−Cd−Se moiety. For complex E, the occupancies of two Cd−Se are 1.869 and 1.871 e, respectively, indicating a single bond in each Cd−Se. As indicated in Table S7 and Figure S7 in the SI, when RNH2 or SePPh2H coordinates to E, a weak three-coordinated complex can be formed. When HPPh2 or RCOOH coordinates to E, the corresponding threecoordinated complex is less stable. This decrease of coordination number stems from the formation of two Cd−Se single bonds. From the schematic energy diagrams for the reaction of C with SePPh2H, one can conclude that the formation of F is the 922


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Figure 3. Geometric structures of various species (a) and the schematic energy diagrams (b) for the reaction Cd(OA)2 + SePPh2H + HPPh2 → G + 2RCOOH (F + 2HPPh2 → G + RCOOH + SePPh2H) calculated at the M06//B3LYP/6-31++G(d,p),SDD level. Bond lengths are reported in angstroms. Relative Gibbs free energies (Gr, kJ mol−1) for the corresponding species plus HPPh2 + RCOOH−SePPh2H from F + HPPh2 to E-i, and for the corresponding species plus 2RCOOH−SePPh2H from E-i to G are shown relative to Cd(OA)2 + SePPh2H + HPPh2.

thermodynamically most preferable because of its lowest energy level. Alternatively, the formation of E is the kinetically most favorable owing to its lowest energy barrier and lowest EHHP. That is to say, both E and F should be the crucial intermediates in the reaction of Cd(OA)2 with SePPh2H. From the reaction mechanisms of SePPh2H with C, one can conclude that SePPh2H functions as not only a hydrogen/proton donor but also acts as Se source via cleavage of the SeP bond through the electrophilic attack in the precursor conversion, for the RCOOH and HPPh2 formation, respectively. Next, we will discuss the further reactions of C, E, and F with HPPh2 and RNH2 vide infra, respectively. 3.4. Formation of Ph2P−CdSe−PPh2 Monomer in the Presence of HPPh2. For the Ph2P−CdSe−PPh2 (G) monomer formation, the reactions of RCOOCdSe−PPh2 (C), Ph2PSe− Cd−SePPh2 (E), and RCOOCdSe2PPh2 (F) with HPPh2 can be formulated upon the following three reaction formulas:

Ph 2PSe −Cd− SePPh 2 + HPPh 2 → Ph 2P− CdSe − PPh 2 + SePPh 2H RCOOCdSe2PPh 2 + 2HPPh 2 → Ph 2P −CdSe −PPh 2 + SePPh 2H + RCOOH

(10)

The geometric structures and the schematic energy diagrams for reaction 10 are depicted in Figure 3a,b, respectively; for reactions 8, 9, and 10, in Figure S3a,b of the SI. As shown in Figure 3a,b, for the reaction of RCOOCdSe2PPh2 (F) with HPPh2, there are two reaction formulas: RCOOCdSe2PPh 2 + HPPh 2 → Ph 2P −CdSe2PPh 2 + RCOOH

(11)

Ph 2P −CdSe2PPh 2 + HPPh 2 → Ph 2P− CdSe − PPh 2 + SePPh 2H

RCOOCdSe− PPh 2 + HPPh 2 → Ph 2P −CdSe −PPh 2 + RCOOH

(9)

(12)

As mentioned earlier, when the first HPPh2 molecule coordinates to F, the five-coordination complex C-i/F-IM2 is

(8) 923


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The Journal of Physical Chemistry A formed. From C-i/F-IM2, there are two reaction pathways for the formation of Ph2P−CdSe2PPh2 (E-i). On the one hand, H-shift from HPPh2 to carboxylate occurs via a six-membered ring F/E-i-TS1a, leading to a carboxylic acid molecular complex F/E-i-IM2a. On the other hand, H-shift from HPPh2 to carboxylate occurs via a four-membered ring F/E-i-TS1b, resulting in a carboxylic acid molecular complex F/E-i-IM2b. Both F/E-i-IM2a and F/E-i-IM2b release the carboxylic acid, leaving E-i behind. Because F/E-i-TS1a locates 68.5 kJ mol−1 below F/E-i-TS1b, the reaction pathway via F/E-i-TS1a is kinetically more favored than that via F/E-i-TS1b. This may be ascribed to be the fact that the strain of six-membered ring F/E-i-TS1a is less than that of four-membered ring F/E-i-TS1b. That is to say, the optimal reaction pathway should prefer the sixmembered F/E-i-TS1a to the four-membered F/E-i-TS1b. For the complex E-i, the occupancies of Cd−P are 1.921 e, indicating a complete single-bond in Cd−P. Furthermore, NBO results show that there is a hyperconjugation interaction in the Cd−Se− P−Se four-membered cycle. It is indicated that complex E-i includes a three-coordinated Cd center. As indicated in Table S7 and Figure S7 in the SI, when RNH2 or SePPh2H coordinates to E-i, a more stable four-coordinated complex can be formed. When HPPh2 or RCOOH coordinates to E-i, the corresponding four-coordinated complex is less stable. This decrease of coordination number orignates from the formation of a Cd−P single bond. When the second HPPh2 molecule interacts with E-i, a fourcoordination complex E-i/G-IM1 is generated. Next, from E-i/G-IM1, Se−P bond cleavage takes place via a five-membered ring E-i/G-TS1, through the electrophilic attack of HPPh2 toward Se atom of the -Se2PPh2 group, producing a SePPh2H molecular complex E-i/G-IM2. Then, E-i/G-IM2 liberates the SePPh2H molecule free, leaving trans-Ph2P−CdSe−PPh2 (G-i). Finally, G-i can readily isomerize to G. For complex G, NBO results show that there is a complete single bond both in Cd−P and Cd−Se. As indicated in Table S7 and Figure S7 in the SI, when RNH2, SePPh2H, HPPh2, or RCOOH coordinates to G, the corresponding three-coordinated complex is less stable. This decrease of coordination number can be ascribed to the formation both of Cd−P and Cd−Se single bonds. For the reaction of F with HPPh2, the minimal energy reaction pathway (MERP) should involve the HEB of 56.2 kJ mol−1 at the C-i/F-IM2 →F/E-i-TS1a reaction step, the EHHP of 5.9 kJ mol−1 at E-i/G-TS1, and the endoergicity of 113.0 kJ mol−1. For the G formation, in view of the three reactions of C, E, and F with HPPh2, the reaction of F with HPPh2 is kinetically the most favorable because of the lowest HEB and lowest EHHP, while the reaction of C with HPPh2 is thermodynamically the most preferable due to the lowest endoergicity, as shown in Figure 3a,b in the SI. In other words, G should be the crucial intermediate in the target reaction. Next, we will discuss the reactions of C, E, F, and G with RNH2, respectively, both thermodynamically and kinetically. From the reaction mechanisms of HPPh2 with C, E, and F, one can see that HPPh2 not only acts as a hydrogen/proton donor but also cleaves the Se−P bond through the electrophilic attack in the precursor conversion, for the RCOOH and SePPh2H formation, respectively. Together with the formation of E and F from C, for the formation of G, four reaction pathways (C → G, C → E → G, C → F → E → G, and C → F → E-i → G) are depicted in Scheme 1, denoted as RP-CG, RP-CEG, RP-CFEG, and RP-CFE-iG, respectively. As shown in Scheme 1, compared with RP-CG, the

Scheme 1. Reaction Pathways for the Ph2P−CdSe−PPh2 (G) Monomer Formation

three reaction pathways (RP-CEG, RP-CFEG, and RP-CFE-iG) are the catalytic processes, where SePPh2H acts as the catalyst. As shown in Figure S3a,b of the SI, for the four reaction pathways RP-CG, RP-CEG, RP-CFEG, and RP-CFE-iG, the EHHPs are 55.9 kJ mol−1 at C/G-TS1, 25.1 kJ mol−1 at E/G-TS1, and 5.9 kJ mol−1 at E-i/G-TS1, respectively. It is apparent that the EHHP has been evidently lowered owing to SePPh2H. Because the EHHP of RP-CFE-iG is the lowest, the reaction pathway RP-CFE-iG is kinetically most favorable. For RP-CFE-iG, the TDI and TDTS are E-i + HPPh2 and E-i/G-TS1, respectively, using the TOF analysis. Then, the rate constant of E-i + HPPh2 → E-i/G-TS1 (kG) is characteristic of the rate constant of RP-CFE-iG (kG). Over the 220−460 K temperature range, the rate constant kG can be adapted by the following expression (s−1 mol−1 dm3): kG = 5.554 × 102 exp( −44.741/RT )

(iii)

From the rate constant kG, one can see that the apparent activation energy is about 45 kJ mol−1 in the presence of SePPh2H. Such low apparent activation energy makes easily the formation of G. This can be qualitatively explained by the experimental phenomena, in which the corresponding species Ph2P−PPh2 of G can be observed in the presence of SePPh2H at very low temperature of 218 K.31 3.5. Formation of RHN−CdSe−PPh2 Monomer in the Presence of both HPPh2 and RNH2. For the RHN−CdSe− PPh2 (H) monomer formation, the reactions of RCOOCdSe− PPh2 (C), Ph2PSe−Cd−SePPh2 (E), RCOOCdSe2PPh2 (F), and Ph2P−CdSe−PPh2 (G) with RNH2 and/or HPPh2 can be dealt with throught the following four reaction formulas: RCOOCdSe− PPh 2 + RNH 2 → RHN− CdSe− PPh 2 + RCOOH

(13)

Ph 2PSe −Cd − SePPh 2 + RNH 2 → RHN−CdSe −PPh 2 + SePPh 2H

(14)

RCOOCdSe2PPh 2 + RNH 2 + HPPh 2 → RHN−CdSe −PPh 2 + SePPh 2H + RCOOH

(15)

Ph 2P −CdSe− PPh 2 + RNH 2 → RHN−CdSe −PPh 2 + HPPh 2

(16)

The geometric structures and the schematic energy diagrams for reactions 14 and 15 are depicted in Figure 4a,b, respectively; for reactions 13, 14, 15, and 16, in Figure S4a,b of the SI. As depicted in Figure 4a,b, for the reaction of E with RNH2, to begin, when the N-end of RNH2 interacts with the central Cd 924


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Figure 4. Geometric structures of various species (a) and the schematic energy diagrams (b) for the reaction Cd(OA)2 + SePPh2H + RNH2 → H + 2RCOOH calculated at the M06//B3LYP/6-31++G(d,p),SDD level. Bond lengths are reported in angstroms. Relative Gibbs free energies (Gr, kJ mol−1) for the corresponding species plus HPPh2 + RCOOH−SePPh2H from F + RNH2 to I-i and for the corresponding species plus 2RCOOH−SePPh2H from I-i and E to H are shown relative to Cd(OA)2 + SePPh2H + RNH2. Red and blue lines represent reactions 14, E + RNH2 → H + SePPh2H, and , F + RNH2 + HPPh2 → H + RCOOH + SePPh2H, respectively.

occurs via a five-membered ring E/H-TS1, leading to a SePPh2H molecular complex E/H-IM2. Next, E/H-IM2 releases the free

atom of E, a three-coordination complex E/H-IM1 is formed. Then, from E/H-IM1, H-shift from RNH2 to the -PPh2 group 925


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The Journal of Physical Chemistry A SePPh2H, leaving H monomer behind. This reaction pathway comprises the HEB of 148.6 kJ mol−1 at the reaction step of E/H-IM1 → E/H-TS1, the EHHP of 37.6 kJ mol−1 at E/H-TS1, and the endoergicity of 141.5 kJ mol−1. As shown in Figure 4a,b, there are two reaction formulas for the formation H from F,

Scheme 2. Reaction Pathways for the RHN−CdSe−PPh2 (H) Monomer Formation

RCOOCdSe2PPh 2 + RNH 2 → RHN− CdSe2PPh 2 + RCOOH

(17)

RHN−CdSe2PPh 2 + HPPh 2 → RHN−CdSe −PPh 2 + SePPh 2H

(18)

In the beginning, when the N-end of RNH2 molecule coordinates to F, the five-coordination complex F/I-i-IM1 is generated. From F/I-i-IM1, H-shift from RNH2 to carboxylate occurs via a fourmembered ring F/I-i-TS1, producing a carboxylic acid molecular complex F/I-i-IM2. After that, F/I-i-IM2 liberates the carboxylic acid, leaving I-i behind. For the complex I-i, NBO results show that there is a very strong hyperconjugation interaction in N−Cd−Se− P−Se. Complex I-i includes a three-coordinated Cd center. As indicated in Table S7 and Figure S7 in the SI, when SePPh2H coordinates to I-i, a more stable four-coordinated complex can be formed. When RNH2, HPPh2, or RCOOH coordinates to I-i, the corresponding four-coordinated complex is less stable. This decrease of coordination number stems from the very strong hyperconjugation interaction in N−Cd−Se−P−Se. Next, when the P-end of HPPh2 molecule interacts with I-i, a four-coordination complex I-i/H-IM1 is formed. From I-i/H-IM1, Se−P bond cleavage occurs via a five-membered ring I-i/H-TS1, through the electrophilic attack of HPPh2 toward Se atom of the -Se2PR2 group, yielding a SePPh2H molecular complex I-i/H-IM2. Next, I-i/H-IM2 sets the SePPh2H molecule free, leaving trans-RHN−dSe−PPh2 (H-ii). Lastly, H-ii can readily isomerize to H. For complex H, the occupancies of Cd−Se are 1.982 e, indicating a complete single bond in Cd−Se. NBO results show that there is a very strong hyperconjugation interaction in Cd−N. As indicated in Table S7 and Figure S7 in the SI, when SePPh2H coordinates to H, a more stable three-coordinated complex can be formed. When RNH2, HPPh2, or RCOOH coordinates to I-i, the corresponding threecoordinated complex is less stable. This decrease of coordination number stems from the formation of Cd−Se single bond and a very strong hyperconjugation interaction in Cd−N. This reaction pathway involves the HEB of 138.6 kJ mol−1 at the F/I-i-IM1 →F/I-i-TS1 reaction step, the EHHP of 67.1 kJ mol−1 at I-i/H-TS1, and the endoergicity of 164.0 kJ mol−1. With regard to the four reaction pathways for the H formation from C, E, F, and G, it is difficult to judge which reaction pathway is kinetically the most favorable only from their HEBs and EHHPs, as shown in Figure S4a,b of SI. It is necessary to analyze the global cycle using the energetic span model vide infra. From the reaction mechanisms of RNH2 with C, E, F, and G, one can conclude that RNH2 serves as a hydrogen/proton donor in the precursor conversion, for the RCOOH, HPPh2, and SePPh2H formations, respectively. In conjunction with the formations of E and F from C, for the formation of H, five reaction pathways (C → H, C → E → H, C → F → E → H, C → F → I-i → G, and C → F → E-i → G → H) are depicted in Scheme 2, denoted as RP-CH, RP-CEH, RPCFEH, RP-CFI-iH, and RP-CFE-iGH, respectively. As shown in Scheme 2, compared with RP-CH, the four reaction pathways

(RP-CEH, RP-CFEH, RP-CFI-iH, and RP-CFE-iGH) are the catalytic processes, with SePPh2H as the catalyst. As discussed earlier, for the five reaction pathways RP-CH, RP-CEH, RP-CFEH, RP-CFI-iH, and RP-CFE-iGH, the EHHPs are 57.4 kJ mol−1 at C/H-TS1, 37.6 kJ mol−1 at E/H-TS1, 67.1 kJ mol−1 at I-i/H-TS1, and 152.1 kJ mol−1 at G/H-TS1, respectively. Since the EHHPs of RP-CEH and RP-CFEH are the lowest, the reaction pathways RP-CEH and RP-CFEH are the kinetically most favorable. Compared with RP-CH, the EHHPs for RPCEH and RP-CFEH have been apparently reduced by SePPh2H catalyst. The corresponding species Ph2P−NHR of H have been observed at room temperature of 298 K in the presence of SePPh2H, but at high temperature of 393 K in the absence of SePPh2H.33 These results embody the catalytic role of SePPh2H. The EHHP for RP-CFI-iH has been slightly raised in the presence of SePPh2H additive. However, the EHHP for RP-CFE-iGH has been evidently raised albeit with SePPh2H additive. It is indicated that the formation of H proceeds via E or F other than via G. In other words, once G is formed, it is impossible to produce H. For both RP-CEH and RP-CFEH, the TDI and TDTS are E + RNH2 and E/H-TS1, respectively, by the TOF analysis. Thereupon, the rate constant of E + RNH2 → E/H-TS1 (kH) is representative of the rate constants of both RP-CEH and RP-CFEH (kH). Over the 220−460 K temperature range, the rate constant kH can be fitted by the following expression (s−1 mol−1 dm3): k H = 1.459 × 105 exp( −93.682/RT )

(iv)

From the rate constant kH, one can see that the apparent activation energy is about 94 kJ mol−1 in the presence of SePPh2H. 3.6. Formation of Ph2PSe−CdSe−NHR(I) Monomers in the Presence of Both HPPh2and RNH2. For the Ph2PSe− CdSe−NHR (I) monomer formation, the reactions of Ph2PSe− Cd−SePPh2 (E) and RCOOCdSe2PPh2 (F) with RNH2 and/or HPPh2 can be referred to the following two reactions 19 and 20 and three reaction formulas 21, 22, and 23: Ph 2PSe −Cd − SePPh 2 + RNH 2 → Ph 2PSe −CdSe − NHR + HPPh 2

(19)

RCOOCdSe2PPh 2 + RNH 2 → Ph 2PSe −CdSe −NHR + RCOOH

(20)

RHN − CdSe −PPh 2 + SePPh 2H → Ph 2PSe −CdSe − NHR + HPPh 2 926

(21)


Article


Figure 5. Geometric structures of various species and the schematic energy diagrams for the reaction Cd(OA)2 + 2SePPh2H + RNH2 → I + HPPh2 + 2RCOOH (E-i + RNH2 → I + HPPh2). Bond lengths are reported in angstroms. Relative Gibbs free energies (Gr, kJ mol−1) for the corresponding species plus 2RCOOH relative to Cd(OA)2 + 2SePPh2H + RNH2 at the M06//B3LYP/6-31++G(d,p),SDD level are shown.

RHN−CdSe2PPh 2 → Ph 2PSe −CdSe −NHR

less stable. This decrease of coordination number comes from the formation of two Cd−Se single bonds. For the I formation from F, this reaction pathway RP-CFE-iI comprises the HEB of 174.9 kJ mol−1 at the reaction step of E-i/ I-IM1 → E-i/I-TS1, the EHHP of 71.0 kJ mol−1 at E-i/I-TS1, and the endoergicity of 116.0 kJ mol−1, together with reaction 11 of F + HPPh2 → E-i + RCOOH. As shown in Figure S5a,b of the SI, comparing with RP-CFI-iI, RP-CFE-iI possesses a lower EHHP (71.0 vs 107.7 kJ mol−1). This embodies the catalytic performance of HPPh2 toward reaction 20 of F + RNH2 → I + RCOOH. This result is in good agreement with the experimental observation.33 For the formation of I from C, three reaction pathways RPCEHI, RP-CFI-iI, and RP-CFE-iI are depicted in Scheme 3. As shown in Scheme 3, for the three reaction pathways RP-CEHI, RP-CFI-iI, and RP-CFE-iI, the EHHPs is 84.0 kJ mol−1 at H/ITS1, 107.7 kJ mol−1 at I-i/I-TS1, and 71.0 kJ mol−1 at E-i/I-TS1, respectively. Because the EHHP of RP-CFE-iI is the lowest, the reaction pathway RP-CFE-iI is the kinetically most favorable, due to the catalytic performance of HPPh2. The corresponding species Ph2P(Se)−NHR of I have been observed at room temperature of 298 K in the presence of HPPh2, but at high temperature of 393 K in the absence of HPPh2.33 These results also reflect the catalytic role of HPPh2. For RP-CFE-iI, the TDI and TDTS are E-i + RNH2 and E-i/I-TS1, respectively, using the TOF analysis. Thereby, the rate constant of E-i + RNH2 → E-i/I-TS1 (kI) is typical of the rate constant of RP-CFE-iI (kI). Over the 220−460 K temperature range, the rate constant kI can be expressed by the following expression (s−1 mol−1 dm3):

(22)

Ph 2P −CdSe2PPh 2 + RNH 2 → Ph 2PSe −CdSe − NHR + HPPh 2

(23)

Reaction 19 is composed of reactions 14 and 21, denoted as RP-CEHI. For reaction 20, there are two kinds of combinations, one of reactions 17 and 22 and another of reactions 11 and 23, denoted as RP-CFI-iI and RP-CFE-iI, respectively. Reaction formulas 11, 14, and 17 have been discussed earlier. Herein, we will mainly discuss the thermodynamics and kinetics of reactions 21, 22, and 23. The geometric structures and the schematic energy diagrams for reaction 23 are depicted in Figure 5; for reactions 21, 22, and 23, in Figure S5a,b of the SI, respectively. As depicted in Figure 5, for the reaction of Ph2P−CdSe2PPh2 (E-i) with RNH2, in the beginning, when the N-end of RNH2 coordinates to the central Cd atom of E-i, a four-coordination complex E-i/I-IM1 is formed. Then, from E-i/I-IM1, Se exchange happens through the simultaneous Se−P bond cleavage of the Se2PPh2 moiety, Se−N bond formation, and [1,4]-H-shift occurs via a seven-membered twist-chair-like E-i/I-TS1, leading to a HPPh2 molecular complex E-i/I-IM2. After that, E-i/I-IM2 liberates the free HPPh2, leaving the trans-Ph2PSe−CdSe−NHR (I-ii) monomer behind. Finally, I-ii can readily isomerize to I. It is apparent that RNH2 both acts as a hydrogen/proton donor and promotes the Se−P bond cleavage of the Se2PPh2 moiety for the precursor formation. For complex I, the occupancies of two Cd−Se are 1.880 and 1885 e, respectively, indicating a single bond in each Cd−Se. As indicated in Table S7 and Figure S7 in the SI, when RNH2 or SePPh2H coordinates to I, a more stable threecoordinated complex can be formed. When HPPh2 or RCOOH coordinates to I, the corresponding three-coordinated complex is

k I = 5.334 × 105 exp( −125.250/RT )

(v)

From the rate constant kI, one can see that the apparent activation energy is about 125 kJ mol−1 in the presence of HPPh2. 927


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activation strain is related to the percentage-wise extent of bond stretching in the TS. Typical strengths of the X−H bonds are the following: 300.4 kJ mol−1 (P−H in SePPh2H) ≈ 300.2 kJ mol−1 (P−H in HPPh2) < 400.4 kJ mol−1 (N−H in RNH2). Thereby, both the bond strength and the percentage-wise bond elongation in the TS increase as SePPh2H < HPPh2 < RNH2. This nearly correlates with ΔE⧧strain. It is indicated that the activation strain favors the P−H cleavage over the N−H cleavage. That is to say, ΔE⧧strain changes in the same order as ΔE⧧, for the P−H and N−H cleavages. On the other hand, the stabilizing TS interaction ΔE⧧int increases as −116.3 kJ mol−1 (HPPh2) < −176.4 kJ mol−1 (SePPh2H) < −199.9 kJ mol−1 (RNH2). It is indicated that the stabilizing interaction with HPPh2 is the least. This may originate from the repulsion of the positive P in HPPh2 and the positive Cd in Cd(OA)2, whereas the electrostatic interaction exists between the negative Se in SePPh2H or the negative N in RNH2 and the positive Cd in Cd(OA)2. Furthermore, ΔE⧧int increases from HPPh2 to RNH2, which follows in the reverse order with ΔE⧧. It is indicated that ΔE⧧strain plays an important role for the release of RCOOH from Cd(OA)2. Thereupon, for the release of RCOOH from Cd(OA)2, SePPh2H meets the following demands, the lowering of the activation strain of substrate and the strengthening of the stabilizing TS interaction, which lower the activation energy. Overall, interacting with Cd(OA)2 for the release of RCOOH, RNH2 is the most difficult because of the strong activation strain of N−H bond activation. On the other hand, SePPh2H is the easiest both due to the weak activation strain of P−H bond activation and the strong stabilizing TS interaction, whereas HPPh2 is in the middle owing to the weak activation strain of P−H bond activation and the weak stabilizing TS interaction. Alternatively, for the Se−P bond cleavage, ΔE⧧ increases from 43.0 kJ mol−1 (HPPh2) to 125.2 kJ mol−1 (RNH2). ΔE⧧strain increases from 107.4 kJ mol−1 (HPPh2) to 385.8 kJ mol−1 (RNH2). ΔE⧧strain[L] increases from 7.6 kJ mol−1 (HPPh2) to 102.4 kJ mol−1 (RNH 2), whereas the substrate E-i term ΔE⧧strain[E-i] increases from 99.8 kJ mol−1 (HPPh2) to 283.4 kJ mol−1 (RNH2). It is apparent that the activation strain is related to the percentage-wise extent of Se−P bond stretching in the TS. It is indicated that the activation strain favors the Se−P bond cleavage in the presence of HPPh2 over that in the presence of RNH2. This result can be ascribed to the fact that only Se−P bond cleavage is required in the presence of HPPh2, whereas not only Se−P bond cleavage but also N−H bond cleavage are involved in the presence of RNH2. In other words, for the Se−P cleavage, ΔE⧧strain varies in the same order as ΔE⧧, in the presence of HPPh2/RNH2. On the other hand, ΔE⧧int increases from −64.4 kJ mol−1 (HPPh2) to −260.6 kJ mol−1 (RNH2), which obeys in the reverse order with ΔE⧧. It is indicated that the stabilizing interaction prefers RNH2 to HPPh2. Consequently, for the Se−P bond cleavage, ΔE⧧strain play a central role, in which ΔE⧧strain favors HPPh2 over RNH2.

Scheme 3. Reaction Pathways for the Ph2PSe−CdSe−NHR (I) Monomer Formation

Over the 220−460 K temperature range, the rate constants for the G, H, and I formation increase as kI < kH < kG. The rate constant kG is 9 to 3 orders of magnitude greater than the rate constant kH, and the rate constant kH is 6 to 3 orders of magnitude greater than the rate constant kI. This result can qualitatively explain the experimental phenomenon that the corresponding product Ph2P−PPh2 species of G have been observed at 218 K.31 As well, compounds Ph2P−NHR and Ph2P(Se)−NHR which could be formed from H and I, respectively, have been observed in the presence of HPPh2 at room temperature of 298 K.33 3.7. Origin of Reactivity Difference of SePPh2H, HPPh2, and RNH2. To shed some light on the reactivity difference of additives (SePPh2H, HPPh2, and RNH2), we will analyze the activation strain of the X−H (X = P and N) and Se−P bond cleavage. And, then, we will discuss the reason, which causes the reactivity difference for the SePPh2H, HPPh2, and RNH2 additives toward the CdSe monomers formation. 3.7.1. Activation Strain Analysis of the X−H (X = P and N) and Se−P Bond Activation. To gain insight into how ligands (SePPh2H, HPPh2, and RNH2) affect the formation of RCOOH or Se−P bond cleavage, the trends in reactivity are analyzed using the activation strain model of chemical reactivity.75,76 In this model, activation energies ΔE⧧ of the TS are divided into the activation strain ΔE⧧strain and the stabilizing TS interaction ΔE⧧int, i.e., ΔE⧧ = ΔE⧧strain + ΔE‡int. The activation strain ΔE⧧strain is the strain energy associated with deforming the reactants from their equilibrium geometry to the geometry they adopt in the TS. The stabilizing TS interaction ΔE⧧int is the actual interaction energy between the deformed reactants in the TS.75,76 The results of the activation strain analysis are listed in Table 1. As shown in Table 1, for the X−H (X = P and N) bond cleavage, ΔE⧧ increases as −44.7 kJ mol−1 (SePPh2H) < 52.4 kJ mol−1 (HPPh2) < 79.4 kJ mol−1 (RNH2). ΔE⧧strain increases as 131.7 kJ mol−1 (SePPh2H) < 168.7 kJ mol−1 (HPPh2) < 279.3 kJ mol−1 (RNH2). The substrate ligand term ΔE⧧strain[L] increases as 30.9 kJ mol−1 (SePPh2H) < 72.0 kJ mol−1 (HPPh2) < 113.4 kJ mol−1 (RNH2), whereas the substrate Cd(OA)2 term ΔE⧧strain[Cd(OA)2] increases as 96.7 kJ mol−1 (HPPh2) ≈ 100.8 kJ mol−1 (SePPh2H) < 165.9 kJ mol−1 (RNH2). It is apparent that the

Table 1. Activation Strain Analysis of Transition State (kJ mol−1) and Typical Lengths (Å) for the P−H and N−H Bond Cleavage in Reaction of Cd(OA)2 with Ligand (L) and the Se−P Bond Cleavage in the Reaction of Ph2P−CdSe2PPh2 (E-i) with L L

bond

TS

length in L or E-i

length in TS

stretching in TS

Stretching in TS (%)

ΔE⧧strain (L)

SePPh2H HPPh2 RNH2 HPPh2 RNH2

P−H P−H N−H Se−P Se−P

C-TS1 C′-TS1 C″-TS1 E-i/G-TS1 E-i/I-TS1

1.415 1.423 1.017 2.246 2.246

1.549 1.596 1.424 2.712 4.492

0.134 0.173 0.407 0.466 2.246

9.5 12.2 40.0 20.7 100

30.9 72.0 113.4 7.6 102.4

928

ΔE⧧strain [Cd(OA)2 or E-i] ΔE⧧strain 100.8 96.7 165.9 99.8 283.4

131.7 168.7 279.3 107.4 385.8

ΔE⧧int

ΔE⧧

−176.4 −116.3 −199.9 −64.4 −260.6

−44.7 52.4 79.4 43.0 125.2


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For the formation of G, SePPh2H acts as the catalyst, in which the TDI and TDTS are E-i + HPPh2 and E-i/G-TS1 with the Se−P bond cleavage of E-i, respectively. For the formation of H, SePPh2H also serves as the catalyst, in which the TDI and TDTS are E + RNH2 and E/H-TS1 with the N−H bond cleavage, respectively. For the formation of I, HPPh2 acts as the catalyst, in which the TDI and TDTS are E-i + RNH2 and E-i/I-TS1 with the Se−P and N−H bond cleavages, respectively. Over the 220−460 K temperature range, the rate constants for the G, H, and I formations increase as kI < kH < kG. This result is in good agreement with our previous experimental observations reported. For the release of RCOOH from Cd(OA)2, RNH2 is the most difficult because of the strong activation strain of N−H bond activation, whereas SePPh2H is the easiest both due to the weak activation strain of P−H bond activation and the strong stabilizing TS interaction. Further HPPh2 is in the middle owing to the weak activation strain of P−H bond activation and the weak stabilizing TS interaction. For the release of RCOOH from Cd(OA)2, the longer the radius of the coordinated atom (Se vs P vs N) is, the larger the size of the cyclic transition state is, which lowers the activation strain and the Gibbs free energy of activation. For the Se−P bond cleavage of E-i in the CdSe monomers formation, the activation strain ΔE⧧strain plays a central role, in which HPPh2 is more beneficial than RNH2. The present study brings the insight into the use of additives such as HPPh2 and RNH2 to synthesize colloidal QDs.

Summarily, both for the release of RCOOH and Se−P bond cleavage, the reactivity difference of SePPh2H, HPPh2, and RNH2 may mainly come from the corresponding discrepancy of ΔE⧧strain. Herein, we will investigate the origin of the activation strain difference of SePPh2H, HPPh2, and RNH2, by analyzing the release of RCOOH from Cd(OA)2. 3.7.2. Origin of Activation Strain Difference of SePPh2H, HPPh2, and RNH2. As mentioned earlier, the discrepancy of activation strain causes the reactivity difference of SePPh2H, HPPh2, and RNH2. Then, for the release of RCOOH from Cd(OA)2, we will analyze the origin of activation strain discrepancy vide infra. Table 2 lists the radii and natural bond Table 2. Radius (Å) of the Coordinated Atom (X = Se, P, and N) in Ligand, Cd−X Distance (Å) in (L)Cd(OA)2, Number of the TS Cyclic Member, and Relative Activation Gibbs Free Energy (kJ mol−1) of TS for SePPh2H, HPPh2, and RNH2 toward the Release of the RCOOH from Cd(OA)2

a

item

SePPh2H

HPPh2

RNH2

radius of X Cd−X distance no. of TS cyclic members ΔG of TS

1.17a 2.765 7 4.2

1.10a 2.668 6 103.6

0.70a 2.365 4 125.0

From ref 77.

orbital charges of the coordinated atom (X = Se, P, and N) in ligand and Cd−X distance in (L)Cd(OA)2, and the relative activation Gibbs free energy (kJ mol−1) of TS for SePPh2H, HPPh2, and RNH2. As shown in Table 2, the radius of the coordinated atom decreases as 1.17 Å (SePPh2H) > 1.10 Å (HPPh2) > 0.70 Å (RNH2), and the Cd−X distance in (L)Cd(OA)2 shortens in the same order as 2.765 Å (SePPh2H) > 2.668 Å (HPPh2) > 2.365 Å (RNH2). The number of TS cyclic members lessens in the same order as 7 (SePPh2H) > 6 (HPPh2) > 4 (RNH2), and the activation Gibbs free energies follow in the reverse order as 4.2 kJ mol−1 (SePPh2H) < 103.6 kJ mol−1 (HPPh2) < 125.0 kJ mol−1 (RNH2). Overall, the longer the radius of the coordinated atom is, the longer the Cd−X distance is, and the more the number of TS cyclic members is; then the lower the activation Gibbs free energies is. As mentioned earlier, the less the activation strain is, the lower the activation energy is and the lower the activation Gibbs free energy is. Overall, the longer radius of the coordinated atom makes the size of TS cyclic member larger, which makes the activation strain less, and thereby makes the activation Gibbs free energy lower.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.5b10675. Geometric structures and schematic energy diagrams, zero-point energies (ZPEs, hartree), thermal correction to Gibbs free energy (G0, hartree) of various species calculated at the B3LYP/6-31++G(d,p),SDD level in the gas phase under atmospheric pressure and room temperature (298.15 K and 1 atm), polarizable continuum model correction energies (PCM-E, hartrees), total energies (Ec, hartrees) corrected by ZPE, relative energies (Er, kJ mol−1) of various species with respect to the reactants, sum of electronic and thermal free energies (Gc, hartrees) corrected by G0, and relative Gibbs free energies (Gr, kJ mol−1) of various species with respect to the reactants calculated at the M06//B3LYP/ 6-31++G(d,p),SDD level in 1-octadecene solution under atmospheric pressure and room temperature (298.15 K and 1 atm), standard orientations of various species calculated at the B3LYP/6-31++G(d,p),SDD level, and Arrhenius plots of calculated rate constants for the crucial reaction step (PDF)

4. CONCLUSIONS The roles of HPPh2, RNH2, and SePPh2H in the formation of CdSe monomers from Cd(OA)2 have been systematically investigated. The following conclusions can be drawn from the present calculations. When the ligand coordinates to Cd(OA)2, the coordination stability of ligand to metal centers of Cd(OA)2 decreases as SePR3 > RNH2 > SePPh2H > PR3 > HPPh2. The coordination of ligand to the Cd center in Cd(OA)2 through Se and N atoms is stronger than through P atom. For the reaction SePPh2H, HPPh2, and RNH2 with Cd(OA)2, these molecules can be regarded as the hydrogen/proton donors in the precursor conversion for the first RCOOH release. The capacity of providing hydrogen/proton decreases as SePPh2H > HPPh2 > RNH2, both thermodynamically and kinetically.

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AUTHOR INFORMATION

Corresponding Authors

*(H.-Q.Y.) E-mail: [email protected]. Fax: 02885415608. Tel.: 028-85415608. *(K.Y.) E-mail: [email protected]. Fax: 028-85415608. Tel.: 028-85415608. Author Contributions

The manuscript was written through contributions of all authors. T. Qi is responsible for main computation, H.Q. Yang for part computation, analysis, and writing, D.M. Whitfield for analysis and writing, K. Yu and C.W. Hu for design and revision. All 929


Article

The Journal of Physical Chemistry A authors have given approval to the final version of the manuscript.

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for financial support by the National Natural Science Foundation of China (Grant Nos. 21573154 and 21573155) and the Applied Foundation Research of Sichuan Province (Grant No. 2014JY0218).

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