A New Target for Synthesis of Triply Bonded Plumbacetylene (RC

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A New Target for Synthesis of Triply Bonded Plumbacetylene (RCtPbR): A Theoretical Design Po-Chao Wu and Ming-Der Su* Department of Applied Chemistry, National Chiayi University, Chiayi 60004, Taiwan

bS Supporting Information ABSTRACT: The effect of substitution on the potential energy surfaces of RCtPbR (R = F, H, OH, CH3, SiH3, Tbt, Ar*, SiMe(SitBu3)2, SiiPrDis2) was investigated using density functional theory (B3LYP/LANL2DZdp and B3PW91/Def2-QZVP) and the CCSD/LANL2DZdp method. Our theoretical results suggest that all the triply bonded RCtPbR species prefer to adopt a trans-bent geometry, which agrees well with the theoretical model (mode II). In addition, we show that the stabilities of the RCtPbR species bearing smaller substituents (R = F, H, OH, CH3, SiH3) decrease in the order R2CdPb: > RCtPbR >:CdPbR2. Nevertheless, the triply bonded RCtPbR molecules with bulkier substituents (R = Tbt, Ar*, SiMe(SitBu3)2, SiiPrDis2) were found to possess the global minimum on the singlet potential energy surface and are both kinetically and thermodynamically stable. In other words, both electronic and steric effects of bulkier substituent groups play a crucial role in making triply bonded plumbacetylene (RCtPbR) synthetically accessible and isolable in a stable form.

I. INTRODUCTION The synthesis and isolation of molecules with triple bonds containing heavier main-group elements have attracted widespread interest due to their unique bonding and structures, which differ tremendously from those of their corresponding carbon compounds.1 Thanks to Kira, Power, Sekiguchi, Tokitoh, Wiberg, and many co-workers, the stable homonuclear alkyne analogues of all the heavier group 14 elements have now been isolated and characterized.25 Nevertheless, triple bonds to lead are still unknown in a stable form, despite many synthetic attempts. For instance, there is only one synthetic example for the lead analogue of an alkyne (Ar*PbPbAr*; Ar* = C6H3-2,6-Trip2) reported by Power and co-workers in 2000.5 However, from a purely geometrical perspective, the leadlead bond in Ar*PbPbAr* appears to be best characterized as a single bond: i.e., more consistent with a diplumbylene form, rather than a diplumbyne species. That is to say, Ar*PbPbAr* carries a lone pair at each lead and empty p orbitals that lie orthogonal to the CPbPbC plane, as shown in 1. Therefore, triple bonds to lead are currently a considerable challenge in main-group chemistry.

In addition to these, the synthesis and characterization of heteronuclear alkyne analogues, compounds in which one carbon atom of a acetylene has been replaced by a heavier element of group 14, have received comparatively less attention. To the best r 2011 American Chemical Society

of our knowledge, for example, several molecules with a triple bond between carbon and silicon have been identified by spectroscopy or by various other methods.6 It was reported that the formation of transient Me2SiCtGeAr has been suggested.7 However, conclusive evidence for the existence of such molecules containing a CtGe triple bond has as yet neither been reported nor been clearly characterized even by trapping experiments. To date, only stable transition-metal complexes involving a GetM8 as well as a PbtM9 bond have been prepared and structurally characterized by several experimental laboratories. In particular, for compounds with a CtPb triple bond, such molecules (plumbacetylenes; RCtPbR) have not been examined and synthesized either experimentally or theoretically. Presumably, the reason for this could be due to the facts that the electronegativity difference (x(C) x(Pb) = 0.9)10 is quite large and the size differences between the valence s and p orbitals of carbon and lead are rather large.11 These would cause the carbonlead π-system to be rather polarized and, in turn, their p orbitals could not be easily hybridized between them. As a result, this would readily lead to dimerization or polymerization. Therefore, the syntheses of stable triply bonded compounds containing carbon and lead would be of particular interest to synthetic chemists. One thus wonders whether the RCtPbR triply bonded molecules can be synthetically accessible and isolable as stable compounds. Indeed, it is astonishing how little is known about the stability and molecular properties of plumbacetylenes, considering the importance of lead, which remains an important industrial commodity,12 and the extensive research activity related to the corresponding acetylene and silaacetylene species.6 According to many experimental experiences for synthesizing compounds with multiple bonds in group 1416 chemistry,1,13,14 Received: January 12, 2011 Published: May 20, 2011 3293

dx.doi.org/10.1021/om2000234 | Organometallics 2011, 30, 3293–3301

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Scheme 1

it is important to consider the possibility of stabilizing this moiety by utilizing suitable substituents. As far as we are aware, however, no attention has been paid to their substituent effects. For this purpose, we have thus performed the theoretical calculations of RCtPbR using both smaller ligands such as organic groups (i.e., R = F, H, OH, CH3, SiH3) and larger ligands containing bulky aryl and silyl groups (i.e., R = Tbt, Ar*, SiMe(SitBu3)2, SiiPrDis2; Dis = CH(SiMe3)2) (see Scheme 1). As a result, the effect of substituents on carbonlead triple bonds has been systematically investigated by utilizing density functional theory (DFT) and ab initio molecular orbital calculations. It is thus hoped that the present theoretical interpretations of substituent effects can be used as preparing the fruitful precursors of plumbacetylenes.

II. THEORETICAL METHODOLOGY Geometries were fully optimized with hybrid density functional theory at both the B3LYP and B3PW91 levels using the Gaussian 03 program package.15 In both the B3LYP and B3PW91 calculations, Becke’s three-parameter nonlocal exchange functional (B3)16 was used together with the exact (HartreeFock) exchange functional in conjunction with the nonlocal correlation functional of Lee, Yang, and Parr (LYP)17 and Perdew and Wang (PW91).18 Thus, the geometries of all the stationary points were fully optimized at the B3LYP and B3PW91 levels of theory. These B3LYP calculations were carried out with pseudorelativistic effective core potentials on group 14 elements modeled using the double-ζ (DZ) basis sets19 augmented by a set of d-type polarization functions.20 The DZ basis set for the hydrogen element was augmented by a set of p-type polarization functions (p exponents 0.356). The d exponents used for C, Si, Ge, Sn, and Pb were 0.587, 0.296, 0.246, 0.186, and 0.179, respectively. Accordingly, we denote our B3LYP calculations by B3LYP/LANL2DZdp. Moreover, the geometries and energetics of the stationary points on the potential energy surface were also calculated using the B3PW91 method in conjunction with the Def2QZVP basis set.21 Consequently, we denote our B3PW91 calculations by B3PW91/Def2-QZVP). The spin-unrestricted (UB3LYP and UB3PW91) formalisms were used for the open-shell (doublet and quartet) species. The S2 expectation values of the doublet and quartet states for the calculated species all showed an ideal value (0.75 and 3.75, respectively) after spin annihilation, so that their geometries and energetics are reliable for this study. Frequency calculations were performed on all structures to confirm that

Figure 1. Two interaction models, I and II, in forming triply bonded group 14 RCtPbR species. the reactants and products had no imaginary frequencies and that the transition states possessed only one imaginary frequency. The relative energies were thus corrected for vibrational zero-point energies (ZPE, not scaled). As suggested by one reviewer, for better energetics, fully optimized calculations were carried out at a higher level of theory: coupled cluster theory with single and double excitations (CCSD)22 using the standard LANL2DZdp basis sets. Although it would be desirable to carry out this using even higher level calculations (such as CCSD(T), including a perturbative triples correlation),22c these were prevented by the constraints of available CPU time and disk space. On the other hand, sequential conformation analyses were carried out for each stationary point with bulky ligands (R = Tbt, Ar*, SiMe(SitBu3)2, SiiPrDis2). They were first performed by HartreeFock calculations (RHF/3-21G*). Thus, the model reactants have 998 (648 electrons), 882 (568 electrons), 830 (536 electrons), and 714 (472 electrons) basis functions for TbtCtPbTbt, Ar*CtPbAr*, SiMe(SitBu3)2CtPbSiMe(SitBu3)2, and SiiPrDis2CtPbSiiPrDis2, respectively. It is well-known that the HartreeFock level of theory is insufficient for even a qualitative description of the chemical potential energy surface. Thus, these stationary points were then further calculated at the B3LYP/LANL2DZdp level using the opt=readfc keyword with a tight convergence option (maximum gradient convergence tolerance 5.0 105 hartree/bohr). Due to the limitation of both CPU time and memory size available, frequencies were not calculated for the triply bonded RCtPbR systems with bulky ligands at the B3LYP/LANL2DZdp level of theory. As a result, the ZPE of B3LYP/ LANL2DZdp could not be applied to such systems in the present work.

III. RESULTS AND DISCUSSION (1). Theoretical Models for RCtPbR. In order to gain an understanding of the molecular structures of plumbacetylenes, we use the valence bond theory to describe bonding patterns of such molecular species. The formation of RCtPbR species is viewed as consisting of one CR and one PbR moiety. As a result, two interaction models (I and II) between a CR and a PbR moiety are illustrated schematically in Figure 1. In model I, the triple bonding between carbon and lead elements with a linear 3294

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Figure 2. Potential energy surfaces for RCtPbR (R = F, H, OH, CH3, SiH3). Energies are given in kcal/mol, calculated at B3LYP/ LANL2DZdp and B3PW91/Def2-QZVP levels of theory. For details, see the text and Table 1.

geometry can be viewed as a 3-fold interaction between two quartet fragments, as represented in Figure 1(I). Therefore, this bond can be considered a triple bond because it has one pσ bond and two pπ bonds. Also, model I plays a decisive role in determining the linear structure of RCtPbR. On the other hand, since the doubletquartet separation can be estimated considerably larger for the much heavier PbR case than for the corresponding CR one (vide infra), model II becomes dominant for the RCtPbR molecule, as given in Figure 1(II). Nevertheless, such an initial linear structure arrangement will undergo severe repulsion between the lone pairs on carbon and lead elements. As a result, not only is the CPb distance elongated to avoid the repulsion but also its structure adopts the trans-bent geometry to gain stabilization by means of electron transfer. As a result, the CPb bonding can be regarded as consisting of two donoracceptor bonds (indicated by arrows) plus a pπ bond due to coupling between the unpaired electron in the p orbital on each element (denoted by the dashed line between the p orbitals

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in an end-on view). We shall use the above models to explain the geometrical structures of triply bonded RCtPbR species in the following section. (2). Small Ligands on Substituted RCtPbR. For purposes of comparison, the structures of RCtPbR studied for R = F, H, OH, CH3, SiH3 have been optimized at both the B3LYP/ LANL2DZdp and B3PW91/Def2-QZVP levels of theory in this work. The calculated potential energy surfaces for these species at both hybrid density functional theory levels are summarized in Figure 2. The selected geometrical parameters, doubletquartet energy splitting (ΔEDQ = Equartet Edoublet), natural charge densities (QC and QPb), and binding energies (BE) are collected in Table 1. Their Cartesian coordinates calculated for the stationary points at the B3LYP/LANL2DZdp, B3PW91/Def2QZVP, and B3LYP/LANL2DZdp levels of theory are available as Supporting Information. As can be seen from Table 1, the geometrical parameters as well as the relative energies of the RCPbR multiply bonded isomers are quite similar at both DFT levels employed. In particular, one of the interesting structural features of the RCPbR species is the CtPb bond length. In the parent HCtPbH molecule, the carbonlead distances are 1.993 and 1.955 Å, respectively, at the B3LYP and B3PW91 levels of theory. The data in Table 1 show that the CtPb bond lengths, which fall in the ranges of 1.9842.278 and 1.9482.363 Å, respectively, change quite substantially upon substitution. To the best of our knowledge, no CtPb triple-bond distances have been reported so far, either experimentally or theoretically.2326 It is worth noting that, on the basis of the present study, highly electronegative substituents at carbon and lead elements lengthen the CtPb bond relative to HCtPbH, while electropositive substituents result in a shortened CtPb bond distance. The effect of the substituents on the CtPb triple-bond length can be understood in terms of bond polarity. The CtPb bond in RCtPbR is polarized, as mentioned in the Introduction,10 so that the lead atom is positively charged and the carbon is negatively charged: i.e., RCδtPbδþR. Substituents that increase this polarization and thus the degree of ionicity of the CtPb bond are expected to shorten this bond, as indicated by the natural charge densities on the central carbon and lead elements (QC and QPb). Further, our both DFT calculations indicate that both RC and RPb moieties have a doublet ground state, which are more stable than their corresponding quartet state for the small substituents (R = F, H, OH, CH3, SiH3), except that the quartet state of H3SiC is lying at an energy lower than that of its corresponding doublet state by only 4.4 kcal/mol at the B3PW91/Def2-QZVP level of theory. Our two DFT computational results thus show that all the sum of doubletquartet energy differences (ΔEDQ) for RC and RPb units are at least þ63 kcal/mol, except for the case of the SiH3 group. Accordingly, these ΔEDQ values strongly imply that mode II (Figure 1) prevails for the RCtPbR molecule, in which a trans-bent geometry is preferred. This model prediction was confirmed by our present DFT computations, as already demonstrated in Table 1. Additionally, as seen in Table 1, the CtPb bond distance in RCtPbR is strongly correlated with the doubletquartet energy difference (ΔEDQ) of the CR and the PbR moieties. For instance, our B3LYP computations show that the sum of the ΔEDQ (kcal/mol) values decreases in the order F (189) > OH (166) > CH3 (77.8) > H (73.9) > SiH3 (37.2), which follow the same order as their corresponding triple-bond lengths (Å): F (2.278) > OH (2.255) > CH3 (2.057) > H (1.993) > SiH3 (1.984). It has to be emphasized 3295


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Table 1. Selected Geometrical Parameters, Doublet Quartet Energy Splitting (ΔEDQ), Natural Charge Densities (QC and QPb), and Binding Energies (BE) of RCtPbR at the B3LYP/LANL2DZdp, B3PW91/Def2QZVP, and CCSD/LANL2DZdp Levels of Theory R F

H

OH

CH3

SiH3

B3LYP/LANL2DZdp CtPb (Å)

2.278

1.993

2.255

2.057

1.984

— RCPb (deg) — CPbR (deg)

141.0 87.90

142.1 115.8

131.4 88.71

167.5 97.33

156.5 111.6

— RCPbR (deg) 88.71

180.0

86.66

180.0

179.5

QCa

0.2114 0.5174 0.3290 0.4979 0.5427

QPbb

þ0.7724 þ0.5216 þ0.6244 þ0.3709 þ0.2766

ΔEDQ for C

82.08

20.51

86.09

35.46

1.490

ΔEDQ for Pb

106.9

53.36

80.24

42.29

35.67

(kcal mol1)d BE (kcal mol1)e

23.25

57.84

24.78

48.72

70.22

CtPb (Å)

2.363

1.955

2.282

2.065

1.948


126.4 85.85

128.2 138.7

131.0 87.70

159.1 105.3

141.3 143.2


180.0

83.68

179.9

179.6

(kcal mol1)c

B3PW91/Def2-QZVP

QCa

0.1647 0.4474 0.3031 0.3113 0.3889

QPbb

þ0.7537 þ0.4360 þ0.6129 þ0.3942 þ0.3574

ΔEDQ for C

76.81

13.45

81.47

30.30

4.354

ΔEDQ for Pb

106.2

49.98

73.74

39.48

31.50


20.81

58.23

23.33

46.38

70.96

(kcal mol1)c

CCSD/LANL2DZdp CtPb (Å)

2.376

1.985

2.307

2.060

2.007


125.5 83.35

134.3 123.1

121.5 86.83

160.3 101.5

157.6 106.0


179.9

86.49

179.5

179.0

QCa

0.1904 0.5357 0.3123 0.5362 0.6870

QPbb

þ0.9872 þ0.6980 þ0.8431 þ0.6017 þ0.4032

ΔEDQ for C

74.76

12.89

77.96

25.09

6.864

ΔEDQ for Pb

100.5

48.56

71.16

37.09

30.18


16.41

52.79

22.19

44.80

66.82

(kcal mol1)c

a

The natural charge density on the central carbon atom. b The natural charge density on the central lead atom. c ΔEDQ = E(quartet state of RC) E(doublet state of RC). d ΔEDQ = E(quartet state of RPb) E(doublet state of RPb). e BE = E(doublet state of RC) þ E(doublet state of RPb) E(RCtPbR).

that the smaller ΔEDQ values for H and SiH3 are due to the electropositive character that helps to decrease the size difference between the valence s and p orbitals on the central carbon and lead atoms.11 Moreover, the BE values which can break the central CtPb bond to result in one RC and one RPb fragment in the doublet ground state are summarized in Table 1. Both B3LYP

and B3PW91 calculations demonstrate that the BE values are in the ranges of 2370 and 2071 kcal/mol for R = F, H, OH, CH3, SiH3, respectively. These values confirm that the central C and Pb atoms in the substituted RCtPbR molecules are strongly bonded. Again, it is noted that the above two parameters (triplebond length and ΔEDQ) are strongly dependent on their BE of RCtPbR. For example, the B3LYP calculations show that the BE (kcal/mol) was calculated to increase in the order F (23) < OH (25) < CH3 (49) < H (58) < SiH3 (70). The same situation can also be applied to the RCtPbR systems using the B3PW91 level of theory, as already given in Table 1. As a consequence, our theoretical investigations demonstrate that the more electropositive the attached ligand R, the smaller the sum of the ΔEDQ values of the CR and PbR components, the easier hybridization between the central carbon and lead elements, the larger the BE of the RCtPbR molecule, the shorter the CtPb triple bond length is, and the stronger the prevailing central CtPb triple bond becomes. After this paper was submitted, one reviewer has suggested the CGMT theory,27 which is greatly related to the models as shown in Figure 1. According to this theory, the relative RCtPbR bond dissociation energy is attributed to the doublet-to-quartet excitation energies (ΔEDQ) of the RC and RPb fragments. Thus, both RC and RPb units exist as doublets in the ground state and the ΔEDQ is required to convert each doublet state to a quartet: i.e., 2ΔEDQ for the whole molecule. The quartet units can be coupled to give a linear RCtPbR species that has a triple bond. Thus, the energy of this molecule is Eσþ2π. Moreover, the energy difference between the linear and trans-bent geometries is E2π. As a result, it can be predicted that if the magnitude of E2π exceeds 2ΔEDQ, a linear structure should be observed, which is in accordance with our theoretical observations as already shown in Table 1. Accordingly, the substituent effects of these components play a major role in determining the RCtPbR triple-bond dissociation energy. According to the previous discussion, the more electropositive (and/or the more bulky, see below) the substituents, the smaller the ΔEDQ values of the CR and PbR fragments and, in turn, the larger the PbtC bond dissociation energy.28 Further supporting evidence comes from the fact that using the more sophisticated ab initio molecular orbital method can give the same trend. That is to say, the series of substituted plumbacetylenes RCtPbR (R = F, H, OH, CH3, SiH3) were fully optimized using the CCSD/LANL2DZdp level of theory.22 For instance, from Table 1, the calculated ΔEDQ values (kcal/ mol) decrease in the order OH (77.96) > F (74.76) > CH3 (25.09) > H (12.89) > SiH3 (6.864) and F (100.5) > OH (71.16) > H (48.56) > CH3 (37.09) > SiH3 (30.18) for the RC and RPb moieties, respectively. Also, the BE values (kcal/mol) for the RCtPbR species calculated at the CCSD theory decrease in the order SiH3 (66.82) > H (52.79) > CH3 (44.80) > OH (22.19) > F (16.41). These trends are quite consistent with both B3LYP and B3PW91 methods, as already given in Table 1. Incidentally, the computational molecular parameters for the RCtPbR species bearing small substituents are quite similar for both DFT and CCSD methods, as demonstrated in Table 1. These successful results strongly suggest that the DFT can provide reliable information for the discussion of the triply bonded RCtPbR species, for which experimental data are still not available. With regard to the stability of plumbacetylene, we then describe the results of theoretical calculations on the energy surface of the model RCtPbR (R = F, H, OH, CH3, SiH3) 3296


Organometallics system. This system exhibits a number of stationary points, including local minima corresponding to RCtPbR, R2CdPb:, :CdPbR2, and the saddle points connecting them. The transition structures separating the three stable molecular forms involve the successive unimolecular 1,2-shift TS1 (from RCtPbR to :CdPbR2) and the 1,2-shift TS2 (from RCtPbR to R2CdPb:). The potential energy surfaces calculated at both B3LYP/ LANL2DZdp and B3PW91/Def2-QZVP levels with zero-point energy correction for the RCtPbR (R = F, H, OH, CH3, SiH3) model compounds are collected in Figure 2. The optimized geometries of RCtPbR, :CdPbR2, and R2CdPb: and of the two transition structures (TS1 and TS2) separating them are given in the Supporting Information. The theoretical results based on two DFT levels have denoted that all the RCtPbR species are local minima on the singlet potential energy surface; they are, however, neither kinetically nor thermodynamically stable. It appears that the effects of smaller substituents (R) on the relative stabilities of RCtPbR and :CdPbR2 and of RCtPbR and R2CdPb: are small. Our calculations show that two planar doubly bonded CdPb structures with Cs symmetry exist as minima on the potential energy surface. That is to say, one of these is :CdPbR2, which has a lone electron pair residing on the carbon, and the other is R2CdPb:, which has a lone pair residing on the lead. It is noteworthy that the former structure (:CdPbR2) possesses the highest energy of all the minima on the singlet RCtPbR surface, whereas the latter structure (R2CdPb:) is predicted to be the most stable RCtPbR structure at the computational levels employed in this work. According to the B3LYP and B3PW91 results as given in Figure 2, the energy difference between:CdPbR2 and R2CdPb: is at least 72 and 85 kcal/mol, respectively. These observations reflect the fact that the singlet states of the CdPb species prefer to have the nonbonding electrons residing on lead rather than on carbon. Moreover, from a bonding point of view, the thermodynamic stability of R2CdPb: relative to :CdPbR2 is attributed to the ability of lead’s diffuse electron cloud to accommodate a lone electron pair more easily than that of carbon.11 In addition, in the portions of the singlet energy surface explored, the stabilities of the three local minima decrease in the order R2CdPb: > RCtPbR > :CdPbR2. When the reaction is viewed as starting from :CdPbR2, the successive conversions of :CdPbR2 to RCtPbR and RCtPbR to R2CdPb: are estimated to be about 7251 (8057) and 64 to 15 (63 to 20) kcal/mol, respectively, at the B3LYP (B3PW91) levels of theory. Namely, the triply bonded structure RCtPbR seems to be unstable on the singlet energy surface and undergoes unimolecular rearrangement to the doubly bonded isomer R2CdPb:. Due to the fact that it proceeds in an endothermic direction, the nondissociative rearrangement R2CdPb: f RCtPbR f :CdPbR2 is much more difficult to achieve, with energy barriers of about 6619 (6723) and 7756 (8359) kcal/mol, respectively, at the B3LYP (B3PW91) levels of theory. Accordingly, our theoretical investigations again demonstrate that the singlet RCtPbR with smaller substituents (R) is neither kinetically nor thermodynamically stable with respect to isomerization reactions. In other words, the prospects of observing singlet RCtPbR (R = F, H, OH, CH3, SiH3) species in a matrix or even as transient intermediates seem to be impossible. Before going further, let us compare the difference of substituent effects on the thermodynamic and kinetic stabilities between HCtSiR (silynes) and RCtPbR (plumbacetylenes). It was reported that substituent effects on the potential energy

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surface of HCtSiR (R = F, H, OH, CH3, SiH3) were investigated by ab initio molecular orbital methods.29 The unimolecular and kinetic stability of triply bonded HCtSiR is strongly dependent on the substituent (R). It was theoretically found that electronegative groups (such as R = F, OH) have a remarkable effect on the stability of the HCtSiR species, making them viable candidates for experimental observation.29 In other words, the perturbative electronic effects of the substituents play a decisive role in determining both kinetic and thermodynamic stabilization of the HCtSiR molecule. In contrast, however, our present theoretical study indicates that the electronic effects of small substituents comparatively play a minor role in making triply bonded RCtPbR synthetically accessible. The reason for such different conclusions concerning the effects of small substituents on the stabilization of HCtSiR and RCtPbR is still uncertain. Presumably, the difference of the electronegativities of carbon and lead atoms (x(C) x(Pb) = 0.9)10 is greater than that of carbon and silicon atoms (x(C) x(Si) = 0.8).10 Moreover, it is well-known that the size difference between the valence s and p orbitals of carbon and lead atoms is much greater than that between carbon and silicon elements.11 Consequently, the electronic effects of small substituents cannot greatly influence the stability of the triply bonded RCtPbR compound. (3). Large Ligands on Substituted RCtPbR. With reference to the above conclusions concerning the small substituents on the RCtPbR species, bulky substituents are therefore expected to destabilize R2CdPb: and :C=PbR2 relative to RCtPbR due to the steric overcrowding effect. In addition, the presence of very bulky substituents at both ends of the RCtPbR species can also protect its triple bond from intermolecular reactions such as dimerization. To test the effect of bulky substituents, the structures of RCtPbR optimized for R = Tbt, Ar*, SiMe(SitBu3)2, SiiPrDis2 (Scheme 1)30 at the B3LYP/LANL2DZdp level are shown in Figure 3. Also, selected geometrical parameters, doubletquartet energy splitting (ΔEDQ), natural charge densities (Q), and binding energies (BE) are collected in Table 2. Their Cartesian coordinates calculated for the stationary points at the B3LYP/LANL2DZdp level are available in the Supporting Information. It was experimentally reported that the CPb single-bond length is in the range of 2.12(2)2.397(6) Å,24 while the CdPb double-bond distance is so far theoretically found to be about 2.045 Å.25 These reported bond lengths are longer than those in our present computational results (2.0321.937 Å), as given in Table 2. This strongly implies that the central C and Pb elements in the RCtPbR compounds with the bulky groups (R = Tbt, Ar*, SiMe(SitBu3)2, SiiPrDis2) are triply bonded. Additionally, the central CtPb triple bond lengths of 2.032 and 2.017 Å calculated for R = Tbt, Ar* are 0.040.08 Å longer than those of 1.992 and 1.937 Å calculated for R = SiMe(SitBu3)2, SiiPrDis2. All these results support that the Tbt and Ar* groups are electronegative, whereas the SiMe(SitBu3)2 and SiiPrDis2 ligands are electropositive. The supporting evidence comes from the natural charge densities on the central carbon (QC) and lead (QPb) elements, as represented in Table 2. That is to say, the CtPb lengths of 1.992 and 1.937 Å for SiMe(SitBu3)2 and SiiPrDis2 (Table 2) are evidently shorter than those of 2.278, 2.255, 1.993, and 2.057 Å for R = F, OH, H, CH3 (Table 1), respectively, despite the steric congestion of the SiMe(SitBu3)2 and SiiPrDis2 ligands. Again, the reason for this is due to the fact that SiMe(SitBu3)2 and SiiPrDis2 are more electropositive than the small substituent ligands. 3297


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Figure 3. Optimized structures of RCtPbR (R = Tbt, Ar*, SiMe(SitBu3)2, SiiPrDis2) at the B3LYP/LANL2DZdp level of theory. For details see the text and Table 2.

Our present theoretical results demonstrate that the RCtPbR molecules containing the bulky substituents (R) all adopt a transbent geometry. In fact, the B3LYP calculations indicate that all the RC and RPb units exhibit the ground doublet state, except for the SiiPrDis2C unit, which has the ground quartet state but with only a doubletquartet energy difference (ΔEDQ) of about 1.4 kcal/mol. Accordingly, these RC and RPb moieties make the interaction model II in Figure 1 preferable, which can result in the trans-bent structure of RCtPbR. Again, our B3LYP computations revealed that a linear correlation exists between the CtPb triple-bond distance and the sum of ΔEDQ for the RC and RPb components as well as the BE energies of the RCtPbR compounds. In other words, the smaller the sum of ΔEDQ for the RC and RPb fragments, the shorter the CtPb triple-bond length and the stronger the CtPb bond strength. It should be noted that, as pointed out in Table 2, the ΔEDQ values of 82 and 74 kcal/mol calculated for R = Tbt, Ar* are much larger than those of 45 and 24 kcal/mol for R = SiMe(SitBu3)2 and SiiPrDis2. As mentioned in the section (2), the reason for the smaller ΔEDQ values for the SiMe(SitBu3)2 and SiiPrDis2 units is ascribed to the fact that the electropositive properties of these fragments can greatly reduce the size difference between the valence s and p orbitals on the central carbon and lead elements.30 However, it could be argued that the RCtPbR compound has a tendency to dissociate in solution as the substituent R becomes bulkier. The energy necessary to cleave the central CtPb bond (i.e., the BE), leaving two ground doublet state fragments, is

represented in Table 2. The BE values were calculated to be 26, 33, 39, and 56 kcal/mol for R = Tbt, Ar*, SiMe(SitBu3)2, SiiPrDis2, respectively. Again, the large BE values for Ar* and SiMe(SitBu3)2 substituents strongly imply that the central carbon and lead elements are strongly bonded and that RCtPbR compounds containing bulkier substituents will not dissociate in solution. That is to say, the larger the binding energy (BE) of the CtPb bond, the shorter and stronger the CtPb triple bond. Further, it was traditionally anticipated that bulky substituents destabilize both R2CdPb: and :C=PbR2 species relative to RCtPbR because they crowd around one end of the central carbonlead bond. Consequently, the bulky substituents (R) can prevent the 1,2-R shifted isomerization of RCtPbR compounds, as shown in Scheme 2 and Table 2. Our DFT results indicate that the RCtPbR molecules with Tbt, Ar*, SiMe(SitBu3)2, and SiiPrDis2 groups (ΔH1 and ΔH2) are 118 and 71.0, 94.6 and 63.4, 63.5 and 36.8, and 77.2 and 47.1 kcal/mol more stable than the :C=PbR2 and R2CdPb: doubly bonded isomers, respectively. These theoretical results strongly suggest that these 1,2-R shifted isomers with bulky substituents are both kinetically and thermodynamically unstable and thus rearrange spontaneously to the global minimum RCtPbR triply bonded molecules. Another major difficulty in preparing the RCtPbR triply bonded species is ascribed to the facile dimerization that leads to the four-membered-ring products, as represented in Scheme 3. Our B3LYP calculations in Table 2 demonstrate that the 3298


Organometallics

ARTICLE

Table 2. Geometrical Parameters, DoubletQuartet Energy Splitting (ΔEDQ), Nature Charge Densities (QC and QPb), Binding Energies (BE), and Relative Energies for the Isomerization (ΔH1 and ΔH2) and Dimerization (ΔHa and ΔHb) Reactions of RCtPbR at the B3LYP/LANL2DZdp Level R

Scheme 3

SiMe(SitBu3)2 SiiPrDis2

Tbt

Ar*

CtPb (Å)

2.032

2.017

1.992

— RCPb (deg)

141.7

141.3

124.0

141.3

— CPbR (deg)

117.9

126.8

151.6

137.8

— RCPbR (deg) QCa

178.0 176.7 1.254 0.8371

122.5 1.416

130.6 1.474

QPbb

0.6735

0.3436

0.9999

0.5871

ΔEDQ for C (kcal mol1)c

31.15

30.39

10.72

1.428

ΔEDQ for Pb (kcal mol1)d 51.09

44.03

34.20

25.86

BE (kcal mol1)e

25.66

33.31

39.42

56.12

ΔH1 (kcal mol1)f

118.0

94.59

63.51

77.23

ΔH2 (kcal mol1)g

70.98

63.41

36.82

47.05

ΔHa (kcal mol1)h ΔHb (kcal mol1)i

112.8 84.76

102.2 76.41

82.38 59.18

71.37 47.90

1.937

a

The natural charge density on the central carbon atom. b The natural charge density on the central lead atom. c ΔEDQ = E(quartet state of RC) E(doublet state of RC). d ΔEDQ = E(quartet state of RPb) E(doublet state of RPb). e BE = E(doublet state of RC) þ E(doublet state of RPb) E(RCtPbR). f ΔH1 = E(:C=PbR2) E(RCtPbR); see Scheme 2. g ΔH2 = E(R2CdPb:) E(RCtPbR); see Scheme 2. h ΔHa = E(headtail dimer) 2E(RCtPbR); see Scheme 3. i ΔHb = E(headhead dimer) 2E(RCtPbR); see Scheme 3.

Scheme 2

IV. CONCLUSION In this work, we have studied the substituent effects of plumbacetylenes using both smaller groups and bulkier substituents by density functional theory and the CCSD method. Our theoretical findings strongly suggest that both the electronic and steric effects of substituents are crucial in making plumbacetylenes synthetically accessible. However, according to our present theoretical study, the smaller substituents (such as R = F, H, OH, CH3, SiH3) neither kinetically nor thermodynamically stabilize the triply bonded RCtPbR species. Nevertheless, our theoretical findings demonstrate that these triply bonded derivatives are generally stabilized due to large steric congestion, which prevents their 1,2-R shifted isomerization, dimerization, or oligomerization. We thus concluded that the triply bonded RCtPbR molecules bearing suitable aryl substituents are intriguing synthetic targets, which is worthy of experimental testing to open a new academic area of lead chemistry. We eagerly await experimental results to confirm our predictions. ’ ASSOCIATED CONTENT

bS

Supporting Information. Tables of Cartesian coordinates. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

dimerization reaction enthalpies (ΔHa and ΔHb) are 113 and 84.8, 102 and 76.4, 82.4 and 59.2, and 71.4 and 47.9 kcal/mol endothermic for R = Tbt, Ar*, SiMe(SitBu3)2, SiiPrDis2, respectively, owing to the steric hindrance of the four bulky substituent groups. These energy differences are large enough that such RCtPbR species are unlikely to undergo polymolecular reactions such as dimerizations. Consequently, these theoretical findings indicate that it is important to carefully prepare bulky substituent groups in order to make plumbacetylene structures stable kinetically and thermodynamically.

’ ACKNOWLEDGMENT We are grateful to the National Center for High-Performance Computing of Taiwan for generous amounts of computing time. We also thank the National Science Council of Taiwan for financial support. Special thanks are also due to reviewers 1 and 2 for very helpful suggestions and comments. ’ REFERENCES (1) For recent reviews, see:(a) Power, P. P. Chem. Rev. 1999, 99, 3463. (b) Jutzi, P. Angew. Chem., Int. Ed. 2000, 39, 3797. (c) Weidenbruch, M. J. Organomet. Chem. 2002, 646, 39. (d) Power, P. P. Chem. Commun. 2003, 2091. (e) Weidenbruch, M. Angew. Chem., Int. Ed. 2004, 43, 2. (f) Power, P. P. Appl. Organomet. Chem. 2005, 19, 488. (g) 3299


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(28) Indeed, it was already reported that both structures and singlettriplet energy splitting (ΔEST = Etriplet Esinglet) of the substituted disilenes with the SidSi double bond are strongly dependent on the substituents’ electronegativities. For instance, the bond dissociation energies of substituted disilenes correlate linearly with the sum of ΔEST. These conclusions can be well explained by the CGMT model. Also see: (a) Reference 27. (b) Liang, C.; Allen, L. C. J. Am. Chem. Soc. 1990, 112, 1039. (c) Su, M.-D. Inorg. Chem. 2004, 43, 4846. (d) Su, M.-D. J. Phys. Chem. A 2004, 108, 823. (e) Karni, M.; Apeloig, Y. J. Am. Chem. Soc. 1990, 112, 8589. (29) Apeloig, Y.; Karni, M. Organometallics 1997, 16, 310. (30) For instance, see: (a) Kobayashi, K.; Nagase, S. Organometallics 1997, 16, 2489. (b) Nagase, S.; Kobayashi, K.; Takagi J. Organomet. Chem. 2000, 611, 264. (c) Kobayashi, K.; Takagi, N.; Nagase, S. Organometallics 2001, 20, 234. (d) Takagi, N.; Nagase, S. Organometallics 2001, 20, 5498. (e) Takagi, N.; Nagase, S. Chem. Lett. 2001, 966. (f) Takagi, N.; Nagase, S. Eur. J. Inorg. Chem. 2002, 2775.

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A New Target for Synthesis of Triply Bonded Plumbacetylene (RC

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