ARTICLE pubs.acs.org/JPCA
Theoretical Study of Pt(PR3)2(AlCl3) (R = H, Me, Ph, or Cy) Including an Unsupported Bond between Transition Metal and Non-transition Metal Elements: Geometry, Bond Strength, and Prediction Shinya Tsukamoto and Shigeyoshi Sakaki* Fukui Institute for Fundamental Chemistry, Kyoto University, Nishihiraki-cho 34-4, Takano, Sakyo-ku, Kyoto 606-8103, Japan
bS Supporting Information ABSTRACT: The molecular structure and the binding energy of Pt(PR3)2(AlCl3) (R = H, Me, Ph, or Cy) were investigated by DFT, MP2 to MP4(SDTQ), and CCSD(T) methods. The optimized structure of Pt(PCy3)2(AlCl3) (Cy = cyclohexyl) by the DFT method with M06-2X and LC-BLYP functionals agrees well with the experimental one. The MP4(SDTQ) and CCSD(T) methods present similar binding energies (BE) of Pt(PH3)2(AlCl3), indicating that these methods provide reliable BE value. The DFT(M06-2X)-calculated BE value is close to the MP4(SDTQ) and CCSD(T)-calculated values, while the other functionals present BE values considerably different from the MP4(SDTQ) and CCSD(T)-calculated values. All computational methods employed here indicate that the BE values of Pt(PMe3)2(AlCl3) and Pt(PPh3)2(AlCl3) are considerably larger than those of the ethylene analogues. The coordinate bond of AlCl3 with Pt(PR3)2 is characterized to be the σ charge transfer (CT) from Pt to AlCl3. This complex has a T-shaped structure unlike the well-known Y-shaped structure of Pt(PMe3)2(C2H4), although both are three-coordinate Pt(0) complex. This T-shaped structure results from important participation of the Pt dσ orbital in the σ-CT; because the Pt dσ orbital energy becomes lower as the PPtP angle decreases, the T-shaped structure is more favorable for the σ-CT than is the Y-shaped structure. [Co(alcn)2(AlCl3)] (alcn = acetylacetoneiminate) is theoretically predicted here as a good candidate for the metal complex, which has an unsupported MAl bond because its binding energy is calculated to be much larger than that of Pt(PCy3)2(AlCl3).
1. INTRODUCTION Examples of unsupported direct bond between transition metal and nontransition metal elements have been limited so far.1,2 Synthesis of the complex bearing the unsupported direct bond is challenging for chemists, and the basic understanding of the unsupported bond is of considerable interest in inorganic, organometallic, and physical chemistry. In one pioneering work, [Cp(CO)2Fe(AlPh3)] was synthesized by Burlitch et al.3 Later, platinum(0) complexes Pt(dope)(ECp*)2 (dope = bis(dicyclohexyl-phosphino)ethane; E = Al, Ga; Cp* = pentamethylcyclopentadienyl) and Pt(PCy3)2(AlCl3) (Cy = cyclohexyl), which contain an unsupported PtAl or PtGa bond, were synthesized by Weiss et al.4 and Braunschweig et al.,5 respectively. Theoretical study of M(PMe3)2(EX3) (M = Ni, Pd, or Pt; E = B, Al, Ga, In, or Tl; X = H, F, Cl, Br, or I) was also carried out by Goedecke et al.6 They disclosed that the ME bond is formed by σ-type charge transfer (CT) interaction from the Pt center to EX3. However, there remain important issues to be theoretically investigated, as follows: Pt(PCy3)2(AlCl3) has a T-shaped structure, interestingly. This T-shaped structure is considerably different from the well-known Y-shaped structure of the similar three-coordinate platinum(0) complex of alkene, M(PR3)2L (L = alkene); the PPtP angle is 111.6 for Pt(PPh3)2(C2H4)7,8 r 2011 American Chemical Society
and 162.07 for Pt(PCy3)2(AlCl3).5 The T-shaped structure of Pt(PR3)2(AlCl3) is expected to relate to the electronic structure and fundamental features of the direct bond between transition metal and nontransition metal elements, because similar Pt(0) three-coordinate complexes Pt(PR3)2(L) (L = alkene, alkyne, etc.) usually take Y-shaped structure unlike Pt(PR3)2(AlCl3). This type of T-shaped structure is not often observed in the Pt(0) complexes. Thus, it is worthy investigating theoretically the T-shaped structure and the bonding nature of Pt(PR3)2(AlCl3). Also, the binding energy of AlCl3 with Pt(PR3)2 has not been well investigated yet, although it is one of the important properties to be evaluated theoretically. It is also interesting to theoretically predict a new compound, which contains an unsupported direct bond between transition metal and nontransition metal elements, because such compounds have been still limited so far. In this work, we investigated Pt(PR3)2(AlCl3) (R = H, Me, Ph, or Cy) by post HartreeFock and DFT methods. Our main purposes here are to evaluate the binding energy of AlCl3 with Pt(PR3)2, to make a comparison between Pt(PR3)2(AlCl3) and Received: March 26, 2011 Revised: June 25, 2011 Published: June 28, 2011 8520
dx.doi.org/10.1021/jp202829w | J. Phys. Chem. A 2011, 115, 8520–8527
The Journal of Physical Chemistry A
ARTICLE
Table 1. Basis Set Effects on the Important Geometrical Parameters and Binding Energy of Pt(PMe3)2(AlCl3)a Pt
Table 2. Functional Effects on the Several Important Geometrical Parameters of Pt(PMe3)2(AlCl3)a AlClavb
PRavb
PtAl
PtPav
AlClav
PRav
B.E.
functional
HW-dzp
6-31G(d)
2.425
2.304
2.179
1.837
26.0
B3LYP
2.441
2.320
2.182
1.845
HW-dzp HW-dzp
6-311G(d) cc-pVDZ
2.420 2.438
2.307 2.303
2.173 2.187
1.831 1.838
21.3 41.3
BLYP B3PW91
2.465 2.422
2.333 2.300
2.201 2.174
1.863 1.837
HW-dzp
cc-pVTZ
2.434
2.299
2.164
1.830
39.5
PW91
2.429
2.301
2.185
1.846
HW-tzp
cc-pVTZ
2.434
2.298
2.165
1.830
39.4
PBEPBE
2.431
2.301
2.187
1.847
LANL08(f)
cc-pVTZ
2.433
2.299
2.165
1.831
39.8
M06-L
2.455
2.310
2.142
1.833
SDD
6-31G(d)
2.421
2.308
2.180
1.837
25.8
M06
2.465
2.324
2.147
1.839
SDD
6-311G(d)
2.418
2.310
2.175
1.832
29.0
M06-2X
2.382
2.291
2.155
1.840
SDD
cc-pVDZ
2.443
2.312
2.183
1.839
40.9
M06-HF
2.329
2.249
2.169
1.848
SDD cc-pVDZ-pp
cc-pVTZ 6-31G(d)
2.437 2.422
2.307 2.306
2.164 2.181
1.831 1.837
38.8 26.2
LC-BLYP LC-BVWN
2.399 2.422
2.293 2.306
2.144 2.150
1.831 1.838
cc-pVTZ-pp
6-311G(d)
2.414
2.306
2.175
1.832
29.6
expl.
2.386
2.306
2.155
1.844
cc-pVDZ-pp
cc-pVDZ
2.441
2.310
2.184
1.839
41.3
cc-pVTZ-pp
cc-pVTZ
39.6
expl.
2.435
2.302
2.165
1.831
2.386
2.306
2.155
1.844
PtAl
PtPavb
other atoms
a
Bond distances are in angstroms. The HW-dzp and 6-31G(d) basis sets were employed for Pt and other atoms, respectively. b Bond distances are in angstroms. These are averaged values.
a
Bond distances are in angstroms, and binding energies are in kcal/mol. A diffuse function was added to Cl, whereas f and g functions were omitted in cc-pVTZ and aug-cc-pVTZ. The DFT method was employed here with B3PW91 functional.
Pt(PR3)2(C2H4), to elucidate the reason Pt(PR3)2(AlCl3) takes the T-shaped structure unlike the Y-shaped structure of Pt(PR3)2(C2H4), and to theoretically predict what transition metal forms an unsupported direct bond with AlCl3.
2. COMPUTATIONAL METHOD Because theoretical studies of the system including an unsupported bond between transition metal and nontransition metal elements have been limited so far,1,2,5 basis set effects on geometry and binding energy were first examined with the DFT method, where the B3PW91 functional was employed. For Pt, we employed three basis sets, (541/541/111/1),911 (5311/5311/ 111/1),911 and (11111/11111/111/1),12 with relativistic effective core potentials (RECPs) proposed by Hay and Wadt.9 They are named HW-dzp, HW-tzp, and LANL08(f),12 respectively, hereafter. Also, we employed the (311111/22111/411/11) basis set with relativistic small core RECPs,13,14 and (7771/6661/551/1) and (99991/8881/7771/1) basis sets with energy consistent RECPs.15 They are called SDD, cc-pVDZ-pp, and cc-pVTZ-pp, respectively, hereafter. For other atoms, we employed 6-31G(d), 6-311G(d), cc-pVDZ, and cc-pVTZ basis sets, where a diffuse function was added to Cl, but f and g polarization functions in cc-pVTZ and aug-cc-pVTZ were deleted to save CPU time. To examine effects of functional on geometry, we employed BLYP,16,17 B3LYP,1619 B3PW91,20 PW91,21,22 PBEPBE,23 M06-L,24 M06,25 M06-2X,25 M06-HF,25 LC-BLYP,26 and LCBVWN26 functionals. Finally, we carried out the geometry optimization by the DFT method with M06-2X functional,25 where HW-dzp and 6-31G(d) were used for Pt and other atoms, respectively. NBO analysis was carried out on the basis of the DFT(M06-2X) calculations, using LANL08(f) for Pt and ccpVDZ for other atoms. For Co whose complexes were also investigated here, we employed a (541/541/311/1)911 basis set in the geometry optimization and a (11111/11111/11111/1)12 basis set in population analysis, where RECPs of Hay and Wadt9 were employed.
The binding energy of model complex Pt(PH3)2(AlCl3) was evaluated by MP2 to MP4(SDTQ), CCSD, CCSD(T), and DFT methods, where LANL08(f) and cc-pVDZ were employed for Pt and other atoms, respectively. The binding energy of more realistic compound Pt(PR3)2(AlCl3) (R = Me, Ph, or Cy) was evaluated by the DFT method with B3PW91, M06-2X, and LCBLYP functionals. All calculations were carried out with GAMESSUS,27 Gaussian 03,28 and Gaussian 0929 program packages.
3. RESULTS AND DISCUSSION Effects of Basis Set and DFT Functional on Structure and Binding Energy of Pt(PMe3)2(AlCl3). First, we employed Pt-
(PMe3)2(AlCl3) as a realistic model to examine effects of basis set and functional. Although the PtP distance agrees well with the experimental value, as seen in Table 1, the PtAl and AlCl bonds are moderately longer than the experimental values by 0.030.05 Å and 0.010.03 Å, respectively. The PtAl and the PtP distances little depend on the basis sets employed. Only the AlCl distance becomes slightly closer to the experimental value by about 0.02 Å when going to cc-pVTZ from cc-pVDZ. The optimized structure significantly depends on the functional, as shown in Table 2. Both of the hybrid functionals such as B3LYP and B3PW91 and pure functionals such as BLYP, PW91, and PBEPBE provide a somewhat longer PtAl bond distance than the experimental result. On the other hand, M06-2X and LC-BLYP functionals reproduce well the experimental PtAl and AlCl bond distances. The optimized PtAl distance becomes shorter in the order M06L > M06 > M06-2X and closer to the experimental value in this order. Also, the LC-BLYP presents the better Pt-Al distance than the BLYP. These results indicate that the inclusion of HartreeFock exchange interaction is crucial to reproduce the PtAl distance in the present system, which relates to the electronic structure of Pt(PR3)2(AlCl3), as will be discussed below. The AlCl distance is also reproduced well by M06-2X and LC-BLYP functionals. These results indicate that the M06-2X functional presents a good geometry of this type of compound. Actually, the DFT(M06-2X)-optimized structure of Pt(PCy3)2(AlCl3) agrees well with the experimental one, as shown in Figure 1 and Table 3. 8521
dx.doi.org/10.1021/jp202829w |J. Phys. Chem. A 2011, 115, 8520–8527
The Journal of Physical Chemistry A
ARTICLE
Figure 1. Optimized structures of Pt(PR3)2(AlCl3) (R = H, Me, Ph, or Cy). Bond distances are in angstroms, and angles are in degrees. Experimental values of Pt(PCy3)2(AlCl3) are in parentheses.
Table 3. Several Important Geometrical Parameters of Pt(PR3)2(AlCl3)a — PtAlCl1
— PtAlCl2
— PtAlCl3
— P1PtP2
PR3
PtAl
PtPav
AlClav
PH3 PMe3
2.420 2.382
2.275 2.291
2.141 2.157
102 103
109 112
109 112
178 169
PPh3
2.386
2.285
2.169
105
109
114
166
PCy3
2.385
2.310
2.165
104
113
115
161
exp.
2.386
2.306
2.155
105.35
114.95
116.50
162.07
a
Units in angstroms and degrees. HW-dzp basis set and 6-31G(d) basis set were employed for Pt and for other atoms, respectively, where a diffuse function was added to Cl.
Figure 2. Binding energy (in kcal/mol) of Pt(PH3)2(AlCl3) calculated by various methods. LANL08(f) and cc-pVTZ basis sets were employed for Pt and other atoms, respectively, where one diffuse function was added to Cl.
To investigate what method reliably provides the binding energy of AlCl3 with Pt(PR3)2, the various computational methods and basis sets were applied to the evaluation of the binding energy. As shown in Table 1, the binding energy little depends on the basis set of Pt. However, the binding energy significantly depends on the basis sets of ligand atoms; cc-pVDZ and cc-pVTZ present larger binding energy than 6-31G(d) and 6-311G(d) basis sets by about 10 kcal/mol. The binding energy
somewhat increases when going to 6-311G(d) from 6-31G(d), but little changes when going to cc-pVTZ from cc-pVDZ. These results suggest that the use of cc-pVDZ for ligand atoms is a reasonable choice; see also Table S1 in the Supporting Information for detailed results. Hereafter, we will employ LANL08(f) and cc-pVDZ for Pt and ligand atoms, respectively. As shown in Figure 2, the binding energy of Pt(PH3)2(AlCl3) moderately decreases when going to MP3 from MP2 but converges to 44 kcal/mol when going from MP3 to CCSD(T). It is noted that the binding energies evaluated by SCS-MP230 and MP4(SDTQ) are close to that by CCSD(T), suggesting that the SCS-MP2 and MP4(SDTQ) and CCSD(T) present reliable binding energy of this compound. In contrast, the DFT method provides considerably smaller binding energy by about 30 kcal/mol than the MP4(SDTQ) and CCSD(T) methods except for DFT calculations with M06-2X, M06-HF, and LC-BLYP functionals. Although the M06-HF considerably overestimates the binding energy as compared to the CCSD(T)-calculated value, M06-2X functional provides the binding energy close to the CCSD(T)-calculated value, and the LC-BLYP presents moderately smaller binding energy; the difference between LC-BLYPand CCSD(T)-calculated values is considerably smaller than those of other functionals. From these results, it is concluded that the SCS-MP2, MP4(SDTQ), CCSD(T), and DFT with M06-2X and LC-BLYP functionals can be applied to the evaluation of the 8522
dx.doi.org/10.1021/jp202829w |J. Phys. Chem. A 2011, 115, 8520–8527
The Journal of Physical Chemistry A
ARTICLE
Table 4. Binding Energy (in kcal/mol)a of Pt(PR3)2(AlCl3), Pt(PMe3)2(C2H4), and [Co(alcn)2(AlCl3)] Pt(PR3)2 (AlCl3) Pt(PR3)2
[Co(alcn)2
(C2H4)
(AlCl3)]
54.2 48.4
40.6 31.0
79.3 76.0
41.2
50.5
20.4
73.8
41.5
50.9
16.1
71.6
MP4(SDQ)
42.6
52.1
27.7
67.9
MP4(SDTQ)
45.4
55.3
35.2
69.2
B3PW91
33.6
36.4
29.3
40.3
18.8
64.0
M06-2X
43.0
50.5
49.7
62.5
20.4
63.9
LC-BLYP expl.32,33
39.7
46.8
41.4
54.6
methods
PH3
PMe3
MP2 SCS-MP2
47.6 44.2
MP3 MP4(DQ)
PPh3
PCy3
36.3
a
LANL08(f) basis sets were employed for Pt and Co. For other atoms, cc-pVDZ basis sets were employed, where one diffuse function was added to Cl.
binding energy here. Note that LC-BLYP functional is constructed by adding only a small modification for long-range HartreeFock exchange to BLYP functional, while M06type functionals are constructed carefully by fitting several parameters. Characteristic Features of Structures of Pt(PR3)2(AlCl3) (R = H, Me, Ph, or Cy). Several important geometrical parameters of Pt(PR3)2(AlCl3) are shown in Figure 1 and Table 3. It is noted that the PtAl distance of Pt(PMe3)2(AlCl3) is shorter than that of Pt(PH3)2(AlCl3) but similar to those of the PPh3 and PCy3 analogues. The PtP distance moderately increases in the order PH3 < PMe3 < PPh3 < PCy3. The PPtP angle is almost 180 in the PH3 complex but moderately decreases in the order PH3 > PMe3 > PPh3 > PCy3. It should be noted that Pt(PH3)2(AlCl3) takes a pure T-shaped structure. This structure is much different from the well-known Y-shaped structure of Pt(PR3)2(C2H4) in which the PPtP angle was reported to be 111.67 for Pt(PPh3)(C2H4).8 Even in Pt(PCy3)2(AlCl3), the PPtP angle (162.08 for exp7 and 161 for calcd here) is much larger than that of Pt(PR3)2(C2H4) and different very much from 120. These results indicate that the T-shaped structure is one of the important characteristic features of Pt(PR3)2(AlCl3). Tolman discussed the electron-donating ability and steric effects of phosphine ligand based on the CO stretching frequency (νco) of Ni(CO)3(PR3) and the cone angle of phosphine.31 The decreasing order of the PPtP angle agrees with the increasing order of Tolman’s cone angle (θ),31 PH3 (87) < PMe3 (118) < PPh3 (145) < PCy3 (170), suggesting that the PPtP angle decreases by the steric repulsion between PR3 and AlCl3. Summarizing the above results, it is likely concluded that Pt(PR3)2(AlCl3) takes a T-shaped structure for small PR3 from electronic factor and a pseudo T-shaped structure for bulky PR3 from the combination of electronic factor and steric factor. Another important geometrical feature is that the AlCl3 moiety takes a pyramidal structure, although it is planar in free AlCl3; the average of PtAlCl angles moderately increases in the order PH3 (109) < PMe3 (112) ≈ PPh3 (112) < PCy3 (114), where the PtAlCl angle is in parentheses; see also
Table 3. Although this increasing order suggests that the steric repulsion between AlCl3 and PR3 is one of the factors inducing the pyramidal distortion, it is likely that the electronic factor mainly contributes to this pyramidal structure because the pyramidal structure is found in the smallest PH3 complex. In summary, characteristic features of this type of compounds are the pseudo T-shaped structure of the PtAlP2 flame and the pyramidal structure of the AlCl3 moiety. These geometrical features will be discussed below in detail. Ligand Effects on Binding Energy and Bonding Nature of Pt(PR3)2(AlCl3) (R = H, Me, Ph, or Cy). Considering that the M06-2X provides binding energy closer to the MP4(SDTQ)calculated values of Pt(PH3)2(AlCl3) and Pt(PMe3)2(AlCl3) (Table 4), we employed M06-2X here to evaluate the binding energy of the series of Pt(PR3)2(AlCl3). The DFT(M06-2X)calculated binding energy increases in the order PH3 , PPh3 < PMe3 < PCy3. This increasing order is consistent with the Tolman’s donating ability,31 which will be discussed below in detail. Previous energy decomposition analyses of Pt(PR3)2(AlX3)6 and Pt(PR3)2(BeX2)32 clearly reported the importance of σ-type CT in the PtAl and PtBe bonds. However, we need to know what molecular orbitals participate in the PtAl bond to understand the molecular structure of Pt(PR3)2(AlX3). Here, we wish to briefly discuss participations of several important orbitals in the population changes and bonding interaction. As shown in Table 5, the electron population of AlCl3 considerably increases by the coordination of AlCl3 with Pt(PR3)2, which mainly arises from the considerably large increase of the 3pz orbital population of Al. On the other hand, the Pt atomic population somewhat decreases by the coordination of AlCl3, which mainly arises from the decreases of the σ-type 5dy2z2, 5dx2, and 6s orbital populations. These population changes are consistent with previous results of energy decomposition analysis6,32 in which the importance of σ-type CT was discussed in detail. We wish to mention here that the electron population of PR3 considerably decreases in Pt(PR3)2(AlCl3) despite the absence of the direct interaction between PR3 and AlCl3. This is because the CT occurs from PR3 to Pt to compensate the decrease in Pt atomic population, which is induced by the CT from the 5dy2z2, 5dx2, and 6s orbitals of Pt to the 3pz orbital of Al in Pt(PR3)2(AlCl3). The HOMO of Pt(PCy3)2 mainly consists of Pt 5dx2 orbital, into which the lone pair orbital of PCy3 mixes in an antibonding way with the Pt 5dx2 because the lone pair of PCy3 exists at lower energy than the 5dx2, as shown in Scheme 1. The Pt 6s orbital mixes into this 5dx2 orbital in a bonding way with the lone pair orbital of PCy3 because the 6s orbital exists at higher energy than the lone pair of PCy3; see also Scheme 1. This is the reason the 6s orbital somewhat participates in the HOMO. Thus, we denote this HOMO as 5dx2 + λ6s hereafter, where λ represents a small LCAO coefficient. The HOMO2 consists of the Pt 5dy2z2 orbital; see Supporting Information Figure S1. Because these HOMO and HOMO2 overlap well with the empty 3pz orbital of Al, these orbitals participate in the CT, which leads to the decrease in the 5dy2z2, 5dx2, and 6s orbital populations and the increase in the 3pz orbital population of Al, as seen in Table 5. These 5dx2 + λ6s and 5dy2z2 orbital energies become higher in the order PH3 (7.64, 8.81) < PPh3 (6.99, 8.30) < PMe3 (6.82, 8.05) < PCy3 (6.71, 7.92) eV, where in parentheses are the HartreeFock orbital energies; see Supporting Information Table S4 for LC-BLYP-calculated orbital energies. 8523
dx.doi.org/10.1021/jp202829w |J. Phys. Chem. A 2011, 115, 8520–8527
The Journal of Physical Chemistry A
ARTICLE
Table 5. NBO Population Changesa Induced by Coordination of AlCl3 or C2H4 with Pt(PR3)2 and [Co(alcn)2] Pt(PR3)2(AlCl3) PH3
PPh3
PCy3
PMe3
[Co(alcn)2(AlCl3)]
0.040
0.102
0.020
0.029
0.054
0.135
s
0.038
0.089
0.037
0.031
0.059
0.024
dxy
0.006
0.006
0.002
0.001
0.000
0.097
dxz
0.023
0.020
0.027
0.027
0.326
0.004
dyz
0.028
0.027
0.030
0.027
0.013
0.111
dy2z2
0.082
0.076
0.068
0.065
0.019
0.003
dx2
0.063
0.057
0.083
0.096
0.025
0.267
0.259 0.299
0.247 0.350
0.323 0.344
0.317 0.346
0.331 0.278
0.488 0.352
Pt or Co
PR3 or alcn AlCl3 or C2H4 Al or Cav s px Clav or Hav
PMe3
Pt(PR3)2(C2H4)
0.176
0.200
0.198
0.197
0.171
0.068
0.026
0.028
0.035
0.041
0.058
0.041
0.206
0.177
0.198
0.199
0.172
0.114
0.041
0.050
0.049
0.050
0.016
0.095
a
A positive value means increase in population, and vice versa. (11111/11111/111/1) and (11111/11111/11111/1) basis set were employed for Pt and Co with RECPs by Hay and Wadt, respectively, and cc-pVDZ basis sets were employed for other atoms, where a diffuse function was added to Cl.
Scheme 1. HOMO of Pt(PCy3)2 with Linear Structure
This increasing order agrees with the increasing order of Tolman’s donating ability. These results indicate that the donating PR3 raises the 5dx2 + λ6s and 5dy2z2 orbital energies, as expected, which induces stronger CT from Pt to AlCl3. As a result, the binding energy increases in the order PH3 < PPh3 < PMe3 < PCy3. Here, we wish to mention the reason the inclusion of HartreeFock exchange in the functional is crucial in these complexes. The strong CT interaction plays a key role to form the PtAl bond. As well-known, the long-range exchange interaction is important to represent well such a highly polarized bond. This is the reason the LC-BLYP as well as M06-2X is successfully applied to this type of PtAl complex. As mentioned above, the pyramidal structure of the AlCl3 moiety is one of the characteristic features of Pt(PR3)2(AlCl3). Because the 3pz orbital of Al participates in the CT from Pt, we examined the unoccupied 3pz orbital of AlCl3. As shown in Figure 3, the 3pz orbital energy becomes lower as the pyramidal distortion increases. This energy stabilization of the 3pz enhances the CT from Pt to AlCl3, leading to the large binding energy of Pt(PR3)2(AlCl3). Actually, the electron populations of the AlCl3 moiety and the Al 3pz orbital increase as the bent angle increases;
Figure 3. The 3pz orbital energy and electron populations of AlCl3 against bent angle θ. aThe orbital energy was calculated by Hartree Fock calculation. The population was calculated by DFT (M06-2X) method. For Al and Cl, cc-pVDZ and aug-cc-pVDZ basis sets were employed, respectively. The planar structure of AlCl3 was taken as a standard (charge = 0).
see also Figure 3. As compared to the 3pz orbital of the planar AlCl3 (Scheme 2A), the LUMO of the pyramidal AlCl3 expands toward the Pt atom, as shown in Scheme 2B. This feature is also favorable for the orbital overlap with the 5dy2z2, 5dx2, and 6s orbitals of Pt. The LUMO expansion and its energy lowering are explained, as follows: In the planar AlCl3, the LUMO of AlCl3 mainly consists of the 3s of Al into which the 3pσ of Cl mixes in an antibonding way and the LUMO+1 mainly consists of the 3pz of Al into which the 3pz of Cl mixes in an antibonding way, as shown in Scheme 2A. In the pyramidal AlCl3, the pzpz antibonding overlap between Al and Cl becomes small, and hence this MO energy becomes lower. On the other hand, the 3s of Al keeps well antibonding overlaps with the px and py orbitals of Cl because these are σ-type overlap. Thus, this MO energy little changes in the pyramidal distortion. As a result, the Al 3pz becomes LUMO and the Al 3s becomes LUMO+1 in the pyramidal AlCl3. Moreover, the Al 3s orbital mixes into the 3pz orbital of Al due to symmetry lowering, so as to decrease the pzpz antibonding 8524
dx.doi.org/10.1021/jp202829w |J. Phys. Chem. A 2011, 115, 8520–8527
The Journal of Physical Chemistry A
ARTICLE
Scheme 2. Frontier Orbital of AlCl3
Figure 4. Optimized geometries of Pt(PMe3)2(C2H4) and [Co(alcn)2(AlCl3)]. Bond distances are in angstroms, and angles are in degrees. Experimental values of Pt(PPh3)2(C2H4) are in parentheses.
overlap. This mixing leads to the LUMO expanding toward the Pt center. All of these are favorable for the CT from Pt to AlCl3. T-Shaped Structure of Pt(PMe3)2(AlCl3) and Comparison between Pt(PMe3)2(AlCl3) and Pt(PMe3)2(C2H4). In this section, we wish to focus on the differences in geometry and binding energy between Pt(PR3)2(AlCl3) and Pt(PR3)2(C2H4). To save computational costs, Pt(PMe3)2(AlCl3) and Pt(PMe3)2(C2H4) were employed here. The MP4(SDTQ)-calculated binding energy of Pt(PMe3)2(C2H4) agrees well with the experimental value of Pt(PPh3)2(C2H4),33,34 as compared in Table 4. It is noted here that the binding energy of Pt(PMe3)2(AlCl3) is considerably larger than that of the ethylene analogue, independent of the computational method. Because the CTs from Pt(PR3)2 to AlCl3 and C2H4 are important in these complexes, we examined here the orbital energies of 3pz of AlCl3 and π* of C2H4. The 3pz orbital energy of AlCl3 in both the planar structure (1.80 eV) and the pyramidal one (1.10 eV) is much lower than the π* orbital energy of C2H4 in both the equilibrium structure (4.61 eV) and the distorted one taken in the Pt(PMe3)2(C2H4) (3.34 eV), where in parentheses are Hartree Fock orbital energies. The lower 3pz orbital energy of AlCl3 yields the stronger CT and the larger binding energy of Pt(PMe3)2(AlCl3) than those of Pt(PMe3)2(C2H4). The optimized geometrical parameters of Pt(PMe3)2(C2H4) are close to the experimental values,7,8 as shown in Figure 4. As mentioned above, it takes a Y-shaped structure, where its PPtP angle is 112. To find the reason Pt(PMe3)2(AlCl3) takes a T-shaped structure unlike the Y-shaped structure of Pt(PMe3)2(C2H4), several important orbital energies of Pt(PMe3)2 are plotted against the PPtP angle in Figure 5; see Supporting Information Figure S2 for LC-BLYP-calculated orbital energies. Apparently, the 5dx2 + λ6s orbital energy becomes
Figure 5. Orbital energies (in eV) of Pt(PMe3)2 against the PPtP angle. HartreeFock calculation was carried out with LANL08(f) and cc-pVDZ basis sets for Pt and other atoms, respectively.
lower as the PPtP angle decreases. This is because the antibonding overlap between the 5dx2 orbital of Pt and the lone pair orbitals of PR3 decreases as the PPtP angle decreases. The 5dy2z2 orbital energy moderately decreases as the PPtP angle decreases, although this is a nonbonding orbital; the reason the nonbonding orbital energy changes is not clear at this moment. Because 5dx2 and 5dy2z2 orbitals participate in the CT from Pt to AlCl3, the linear structure of Pt(PMe3)2 is more favorable for the σ-CT than the bent structure. This is the reason Pt(PR3)2(AlCl3) takes a T-shaped structure. The Y-shaped structure of Pt(PMe3)2(C2H4) was previously discussed well.3437 Here, we wish to briefly discuss its geometry. The dxz orbital energy becomes higher with the decrease in the PPtP angle because the antibonding overlap between the dxz and the lone pair of PR3 increases with the decrease in the PPtP angle; see Scheme S1. This dxz orbital interacts with the π* orbital of ethylene to form the CT in Pt(PR3)2(C2H4), which is evidenced by the decrease in the dxz orbital population by coordination of C2H4, as shown in Table 5. Thus, the Y-shaped structure becomes stable in Pt(PR3)2(C2H4). Prediction of New Transition Metal Complex Bearing the Unsupported Direct Bond between Transition Metal and AlCl3. One of the key factors to form a transition metal complex with AlCl3 is the presence of the doubly occupied dσ orbital at high energy. This situation is essentially the same as that of the η1-coordinated CO2 complex. Three examples of η1-CO2 complex have been experimentally reported, so far. They are K[Co(pr-salan)(η1-CO2)],38,39 [RhCl(diars)2(η1-CO2)] (diars = o-phenylenbis(dimethylarsine)),40 and [Ru(bpy)2(CO)(η1-CO2)] (bpy = 2,20 -bipyridyl).41 Theoretical studies reported that the CT 8525
dx.doi.org/10.1021/jp202829w |J. Phys. Chem. A 2011, 115, 8520–8527
The Journal of Physical Chemistry A from the metal dσ orbital to the π* orbital of CO2 plays important roles to form these η1-CO2 complexes.4244 This bonding feature of the η1-CO2 complex is essentially the same as that of Pt(PR3)2(AlCl3), suggesting that the metal moieties of these η1-CO2 complexes can form an unsupported MAl bond with AlCl3. Herein, we examined whether [Co(alcn)2] (alcn = acetylacetoneiminate) can form a stable complex with AlCl3 or not, where Co(alcn)2 was employed as a model of Co(pr-salen). The optimized geometry of [Co(alcn)2(AlCl3)] is shown in Figure 4. The CoAl distance of 2.333 Å is somewhat shorter than the PtAl distance (2.385 Å) of Pt(PCy3)2(AlCl3). The AlCl3 moiety is not planar but pseudotetrahedral, similar to that in Pt(PCy3)(AlCl3); the average angle of CoAlCl is 110. The binding energy moderately decreases when going from MP2 to MP4(SDQ) but then moderately increases when going to MP4(SDTQ), as shown in Table 4. Because the energy difference between MP4(SDQ) and MP4(SDTQ) is small, it is concluded that the binding energy calculated by these methods is reliable. The DFT calculations with B3PW91 and M06-2X provide moderately smaller binding energy, but the calculation with LC-BLYP presents moderately larger binding energy than does the MP4(SDTQ) method. The most important result to be noted here is that the binding energy of [Co(alcn)(AlCl3)] is much larger than that of Pt(PMe3)2(AlCl3) in all computational results here, indicating that [Co(alcn)(AlCl3)] is a good target for the synthesis of the complex bearing an unsupported bond between Co and Al. The HOMO of [Co(alcn)2] is the dz2 orbital of Co. This HOMO interacts with the pz of Al to form the σ-CT interaction; see Supporting Information Figure S3. As seen in Table 5, the electron population of the [Co(alcn)2] moiety considerably decreases by the formation of [Co(alcn)(AlCl3)], which mainly arises from the decrease in the Co atomic population. It is noted that the Co dz2 orbital population considerably decreases and the electron population of AlCl3 considerably increases. In particular, the 3pz orbital population of Al considerably increases. There population changes clearly indicate that the CT occurs from the Co dz2 orbital to AlCl3 like that in Pt(PR3)2(AlCl3). On the basis of these results, we wish to propose here that the metal complex that forms the η1-CO2 complex can be applied to the synthesis of the transition metal complex bearing the unsupported MAl bond with AlCl3.
ARTICLE
Pt(PR3)2(AlCl3) takes a T-shaped structure unlike the wellknown Y-shaped structure of Pt(PR3)2(C2H4). This is interpreted in terms of the 5dx2 + λ6s and 5dy2z2 orbital energies of Pt(PR3)2 because these orbitals participate in the σ-CT to AlCl3. Their orbital energies become lower as the PPtP angle decreases from 180. Thus, the PPtP angle of 180 is favorable for the σ-CT to AlCl3. We wish to present the theoretical prediction that [Co(alcn)2(AlCl3)] is a good candidate for a new cobalt(I) complex bearing the unsupported CoAl bond. This complex contains a stronger CoAl bond than the PtAl bond of Pt(PR3)2(AlCl3).
’ ASSOCIATED CONTENT
bS
Supporting Information. Full references of Gaussian 03 and Gaussian 09, basis sets and DFT functional effects on binding energy of Pt(PH3)2(AlCl3) (Table S1), all of the structural parameters of Pt(PR3)2(AlCl3) (Table S2), the NBO populations of d orbitals of Pt(PR3)2(AlCl3), Pt(PMe3)2(C2H4), and [Co(alcn)2(AlCl3)] (Table S3), the LC-BLYP-calcualted Al 3pz orbital energy of AlCl3 and LC-BLYP-frontier orbital energies of Pt(PR3)2 (Table S4), dπ-π* back-donation of Pt(PR3)2(C2H4) (Scheme S1), frontier orbitals of Pt(PCy3)2 (Figure S1), HartreeFock, LC-BLYP, and M06-2X-calculated d orbital energies changes along the PPtP angle (Figure S2), HOMO of [Co(alcn)2] and HOMO1 of [Co(alcn)2(AlCl3)] (Figure S3), and Cartesian coordinate of optimized structures of Pt(PR3)2(AlCl3) and Pt(PR3)2. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work is financially supported by Grant-in-Aids from the Ministry of Education, Culture, Science, Sport, and Technology through Grant-in-Aids of Specially Promoted Science and Technology (No. 22000009) and Grand Challenge Project (IMS). We are also thankful to the computational facility at the Institute of Molecular Science, Okazaki, Japan. ’ REFERENCES
4. CONCLUSIONS The molecular structure and binding energy of Pt(PR3)2(AlCl3) (R = H, Me, Ph, or Cy) were theoretically investigated. The optimized structures of Pt(PCy3)2(AlCl3) by M06-2X and LC-BLYP are in good agreement with its X-ray structure, while other functionals such as B3LYP provide the longer PtAl distance than the experimental value. The binding energies evaluated by M06-2X and LC-BLYP are similar to that evaluated by MP4(SDTQ). Population analysis suggests that the CT occurs from the 5dx2 + λ6s and 5dy2z2 orbitals of Pt to the 3pz orbital Al. The binding energy increases in the order PH3 < PPh3 < PMe3 < PCy3. This trend is the same as the increasing order of Tolman’s donating ability. It is noted that the binding energies of Pt(PPh3)2(AlCl3) and Pt(PMe3)(AlCl3) are considerably larger than those of the corresponding ethylene analogues.
(1) Braunschweig, H.; Colling, M. Coord. Chem. Rev. 2001, 223, 1. (2) Aldridge, S.; Coombs, D. L. Chem. Rev. 2004, 248, 535. (3) Burlitch, J. M.; Leonowicz, M. E.; Petersen, R. B.; Hughes, R. E. Inorg. Chem. 1978, 18, 1097. (4) Weiss, D.; Steinke, T.; Winter, M.; Fischer, R. A.; Fr€ohlich, N.; Uddin, J.; Frenking, G. Organometallics 2000, 19, 4583. (5) Braunshweig, H.; Gruss, K.; Radacki, K. Angew. Chem., Int. Ed . 2007, 46, 7782. (6) Goedecke, C.; Hillebrecht, P.; Uhlemann, T.; Haunschild, R.; Frenking, G. Can. J. Chem. 2009, 87, 1470. (7) Cheng, P.-T.; Cook, C. D.; Nyburg, S. C.; Wan, K. Y. Inorg. Chem. 1971, 20, 2210. (8) Cheng, P.-T.; Nyburg, S. C. Can. J. Chem. 1972, 50, 912. (9) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299. (10) Couty, M.; Hall, M. B. J. Comput. Chem. 1996, 17, 1359. (11) Ehlers, A. W.; B€ohme, M.; Dapprich, S.; Gobbi, A.; H€ollwarth, A.; Jonas, V.; K€ohler, K. F.; Stegmann, R.; Veldkamp, A.; Frenking, G. Chem. Phys. Lett. 1993, 208, 111. 8526
dx.doi.org/10.1021/jp202829w |J. Phys. Chem. A 2011, 115, 8520–8527
The Journal of Physical Chemistry A
ARTICLE
(12) Lindsay, E. R.; Hay, P. J.; Martin, R. L. J. Chem. Theory Comput. 2008, 4, 1029. (13) Andrae, D.; Haeussermann, U.; Dolg, M.; Stoll, H.; Preuss, H. J. Theor. Chim. Acta 1990, 77, 123. (14) Martin, J. M. L.; Sundemann, A. J. Chem. Phys. 2001, 114, 3408. (15) Figgen, D.; Peterson, K. A.; Dolg, M.; Stoll, H. J. Chem. Phys. 2009, 103, 164108. (16) Beche, A. D. J. Chem. Phys. 1993, 98, 5648. (17) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (18) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1998, 58, 1200. (19) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. J. Phys. Chem. 1994, 98, 11623. (20) Perdew, J. P.; Wang, Y. Phys. Rev. B 1992, 45, 13244. (21) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Phys. Rev. B 1992, 46, 6671. 1993, 48, 4978(E). (22) Perdew, J. P.; Burke, K.; Wang, Y. Phys. Rev. B 1996, 54, 16533. 1998, 57, 14999(E). (23) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865. (24) Zhao, Y.; Truhlar, D. G. J. Chem. Phys. 2006, 125, 194101. (25) Zhao, Y.; Truhlar, D. G. Theor. Chem. Acc. 2008, 120, 215. (26) Iikura, H.; Tsuneda, T.; Yanai, T.; Hirao, K. J. Chem. Phys. 2001, 115, 3540. (27) Schmidt, M. W.; Baldridge, K. K.; Boatz, J. A.; Elbert, S. T.; Gordon, M. S.; Jensen, J. H.; Koseki, S.; Matsunaga, N.; Nguyen, K. A.; Windus, S.; Su, T. L.; Dupuis, M.; Montgomery, J. A., Jr. J. Comput. Chem. 1993, 14, 1347. (28) Frisch, M. J.; et al. Gaussian 03, revision D.02; Gaussian, Inc.: Wallingford, CT, 2003. (29) Frisch, M. J.; et al. Gaussian 09, revision A.02; Gaussian, Inc.: Wallingford, CT, 2009. (30) Grimme, S. J. Chem. Phys. 2003, 118, 9095. (31) Tolman, C. Chem. Rev. 1977, 77, 313. (32) Parameswaran, P.; Frenking, G. J. Phys. Chem. A 2010, 114, 8529. (33) Mortimer, C. T. Rev. Inorg. Chem. 1984, 6, 233. (34) Sim~oes, J. A. M.; Beauchamp, J. L. Chem. Rev. 1990, 90, 629. (35) Albright, T. A.; Hoffmann, R.; Thibeault, J. C.; Thorn, D. L. J. Am. Chem. Soc. 1979, 101, 3801. (36) Sakaki, S.; Mizoe, N.; Musashi, Y.; Biswas, B.; Sugimoto, M. J. Phys. Chem. A 1998, 102, 8027. (37) Ohnishi, Y.; Nakao, Y.; Sato, H.; Sakaki, S. J. Phys. Chem. A 2008, 112, 1946. (38) Fachinetti, G.; Floriani, C.; Zanazzi, P. F. J. Am. Chem. Soc. 1978, 100, 7405. (39) Gambarotta, S.; Francesco, A.; Floriani, C.; Zanazzis, P. F. J. Am. Chem. Soc. 1982, 104, 50. (40) Calabrese, J. C.; Herskovitz, T.; Kinney, J. B. J. Am. Chem. Soc. 1983, 105, 5914. (41) Tanaka, H.; Tzeng, B.-C.; Nagao, H.; Peng, S.-M.; Tanaka, K. Inorg. Chem. 1983, 32, 1508. (42) (a) Sakaki, S.; Dedieu, A. J. Organomet. Chem. 1986, 314, C63. (b) Sakaki, S.; Dedieu, A. Inorg. Chem. 1987, 26, 3278. (43) Sakaki, S.; Aizawa, T.; Koga, N.; Morokuma, K.; Ohkubo, K. Inorg. Chem. 1989, 28, 103. (44) (a) Sakaki, S. J. Am. Chem. Soc. 1990, 112, 7813. (b) Sakaki, S. J. Am. Chem. Soc. 1992, 114, 205.
8527
dx.doi.org/10.1021/jp202829w |J. Phys. Chem. A 2011, 115, 8520–8527
