Article Cite This: Organometallics XXXX, XXX, XXX-XXX
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Interplay of Steric and Electronic Effects on the Bonding Components in Aromatic Ring Agostic Interactions M. Arif Sajjad, John A. Harrison,* and Alastair J. Nielson* Chemistry, Institute of Natural and Mathematical Sciences, Massey University, Private Bag 102904, North Shore Mail Centre, Auckland 1603, New Zealand
Peter Schwerdtfeger Centre for Theoretical Chemistry and Physics, Institute of Advanced Studies, Massey University at Auckland, Private bag 102904, North Shore Mail Centre, Auckland 1603, New Zealand S Supporting Information *
ABSTRACT: Density functional theory (DFT) calculations on the effect of steric size adjacent to an agostic interaction in ligand assisted Pd−C bond formation involving aromatic rings gives insight into why the synthetic reaction can fail as the size of an alkyl group is increased. In [PdCl2(1-tetralone oxime)] agostic complexes, changing the C(7)-substituent on the ligand through the series H, Me, CHMe2, and CMe3 progressively reduces agostic and syndetic donations. For (N)−CMe3 imine complexes, CMe3 steric pressure at C(7) switches off agostic donation and increases syndetic donation significantly, especially where the aromatic ring can rotate. Electron withdrawal from the aromatic ring in this type of system has little effect, but electron donation into the ring invokes η1covalency, especially with strong π-donation. This covalency can be switched off by further π-donation and the syndetic donation restored. These steric effects can be expected to impact the success of C−C bond formation chemistry derived from Pd−C cyclometalation reactions involving agostic and syndetic donations.
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INTRODUCTION We have recently reported that agostic interactions1 involving aromatic ring systems in transition metal complexes are much more complicated than previously recognized as they can be assisted by electron donation to the metal from ring orbitals adjacent to the agostic C−H bond. This assistance was given the term “syndetic donation” to distinguish it from the agostic component.2 With the increasing sophistication of aromatic substrates being used in site-selective cyclometalation reactions that produce reactive palladium−carbon bonds for postfunctionalization,3 it is important to understand factors that might influence this process. In early synthetic work on cyclopalladation reactions that was directed toward the functionalization of natural products, we found that a steric effect operated that prevented the formation of palladocycles when the size of the group adjacent to the expected Pd−C bond was varied.4 In the present work, we show that steric effects can have significant impact on the formation of agostic and syndetic donations and thus have the potential to ultimately affect Pd−C bond formation.
aromatic rings in steroidal precursors, we used the 1-tetralone framework (Chart 1) as a model system to study the Chart 1. 1-Tetralone Framework
cyclopalladation reaction.4 In this case, it was found that the reaction of 7-methyl-1-tetralone oxime with PdCl42− in the presence of NaOAc carried out in MeOH gave a cylopalladated product, whereas the reaction failed to take place when the size of the C(7)-substituent was increased to CHMe2 or CMe3. Subsequent 1H NMR analysis of these two mixtures showed that the reactions did not proceed past an anagostic stage,4,5 where there were NMR spectral characteristics of an above-
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RESULTS AND DISCUSSION Steric Effects Adjacent to the Agostic Interaction. In our early work directed toward the functionalization of © XXXX American Chemical Society
Received: August 29, 2017
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DOI: 10.1021/acs.organomet.7b00656 Organometallics XXXX, XXX, XXX−XXX
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Organometallics Chart 2. Complexes 1−12
plane orientation of the C(8) hydrogen. No further information about these reactions could be obtained as stable intermediates could not be isolated from the reaction mixtures. More recently, we have engaged in computational studies of both anagostic5 and agostic2 interactions to broaden the understanding of the C−H bond activation process and have now used this approach to gather information about the steric consequences of the earlier synthetic and NMR studies. The calculated structures of the 1-tetralone oxime palladium complexes 1−4 were obtained by dispersion corrected density finctional theory6 (DFT-D) whereby the C(7)-group size was increased in the order H, Me, CHMe2, and CMe3 (Chart 2). Chloro ligands were utilized to prevent the agostic C−H bond lengthening observed with acetato ligands,7 and an oxime donor nitrogen atom was found to be beneficial as (N)−OH···· Cl hydrogen bonding locks Pd−N bond rotation [the Cl(2)− Pd−CN torsion angles vary by only 5.8° in 1−4], giving more rigidity.2 An Me substituent at C(7) in 2 results in a hydrogen of the group making a close approach to the Cl(2) ligand (2.706 Å) (important intramolecular distances for complexes 1−4 are shown on the structures in Figure 1) but this has little effect on the N−C−C(8a)−C(8) torsion angles and angles between the benzene ring and coordination planes (Ar/CP angle) in comparison to the unsubstituted complex 1 (18.2 and 35.5° and 15.1 and 34.5° in 2 and 1, respectively. Selected structural data for all the complexes in this work are contained in Table 1. However, with the C(7)-Me group present, the lower section of the aromatic ring is pushed back significantly [Pd····C(7) distances 3.387 and 3.283 Å in 2 and 1, respectively; Pd····C(8) distances 2.321 and 2.240 Å]. Further, the Pd····H separation increases markedly (1.894 c.f. 1.819 Å), with the overall effect being that the ligand has moved down marginally (Figure 1.2 of the Supporting Information). The C−H bond length is now shorter (1.138 and 1.152 Å in 2 and 1, respectively), but the C−H bond deformation away from the aromatic ring plane8,9 is slightly larger (torsion angles −32 and −27.2°, respectively). It is also noted that the distance from H(8) to the cis-Cl ligand is longer in 2 than in 1 (3.044 and 2.865 Å, respectively), which suggests less interaction between these two atoms which would end up as HCl after the metalation process. Second-order perturbation energy E(2) values from an NBO analysis10 for the agostic and syndetic donations for the complexes are shown in Table 2. The result of adding an Me group at C(7) is that the C−Hσ agostic donation to the Pd−
Figure 1. Calculated structures of complexes 1−4 showing relevant intra-atomic distances (Å).
Clσ*(trans) orbital in 2 is significantly smaller than in 1 [E(2) values 44.5 and 58.6 kcal mol−1, respectively], whereas the C− Hσ to Pd−Clσ*(cis) orbital donation does not change much [E(2) value 8.7 and 10.0 kcal mol−1, respectively]. This is also the case for the syndetic C(7)−C(8)π donations, which also involve the Pd−Clσ*(trans) and Pd−Clσ*(cis) orbitals [E(2) values 18.3 and 20.3 kcal mol−1, 5.9 and 6.5 kcal mol−1, respectively]. In complex 3, the C(7)-isopropyl group orients with the methine hydrogen pointing toward Cl(2) (see structure 3 in Figure 1), and the H····Cl(2) separation is only slightly shorter than in C(7)-methyl complex 2 (distances 2.600 and 2.706 Å). In this case, the Pd····C(7) and Pd····C(8) separations lengthen slightly, but overall, the metrics are very similar to 2 and the agostic and syndetic C(7)−C(8)π donations do not change much (compare the various values for 3 and 2 in Table 2). For C(7)-tert-butyl complex 4, two of the methyl groups are now aligned toward Cl(1) (see structure 4 in Figure 1). and two of the hydrogens make close approaches to it [H····Cl(1) separations, 2.731 and 2.613 Å]. The Pd····C(7) and Pd····C(8) B
DOI: 10.1021/acs.organomet.7b00656 Organometallics XXXX, XXX, XXX−XXX
a
C
116.7 93.2 54.2 15.1 35.5 18.2 −32.3
13.1 34.5 15.1 −27.2
1.894 3.387 2.321 2.961 3.044
2.011 2.268 2.291 1.138 1.081
2
119.9 95.2 54.0
1.819 3.283 2.240 2.939 2.865
2.015 2.272 2.293 1.152 1.081
1
15.5 35.8 18.4 −30.4
112.1 88.3 53.3
1.899 3.411 2.345 2.970 3.062
2.011 2.272 2.289 1.138 1.081
3
18.9 46.4 26.8 −20.3
114.3 94.4 59.8
2.128 3.511 2.457 2.931 3.147
2.006 2.267 2.284 1.111 1.080
4
51.4 68.9 20.7 −23.4
112.1 88.3 59.9
1.884 3.189 2.177 2.814 2.989
2.090 2.288 2.292 1.148 1.090
5
52.8 66.1 18.4 −24.3
112.1 89.0 59.0
1.863 3.194 2.174 2.821 3.011
2.075 2.287 2.291 1.151 1.090
6
30.5 75.4 43.8 −13.9
96.5 63.3 91.2
2.534 3.206 2.264 2.282 3.949
2.095 2.288 2.281 1.091 1.088
7
32.6 84.1 51.1 −12.0
97.9 61.0 94.4
2.605 3.206 2.284 2.259 4.032
2.087 2.295 2.294 1.091 1.086
8
28.9 76.4 45.8 −13.0
95.8 61.2 93.8
2.575 3.182 2.262 2.243 4.027
2.101 2.287 2.283 1.091 1.089
9
35.9 73.5 38.8 −16.2
100.4 65.9 87.8
2.460 3.231 2.247 2.429 3.815
2.082 2.298 2.330 1.093 1.089
10
41.6 73.2 31.9 −22.6
104.0 63.0 90.4
2.446 3.194 2.179 2.554 3.813
2.094 2.311 2.376 1.097 1.087
11
54.2 74.6 16.9 −20.5
100.6 73.6 79.4
2.361 3.377 2.304 2.648 3.805
2.082 2.339 2.436 1.094 1.086
12
Values to three decimal places consistent with level of optimization, SCF convergence and XC grids. Values in bold are for the free ligand. bAngle shown is 180° minus the actual dihedral angle.
Separations (Å) Pd····H Pd····C(7) Pd····C(8) Pd····C(8a) Cl····H(8) Angles (deg) Pd−N−C Pd····H−C Pd····C−H Dihedrals (deg) Cl(2)−Pd−NCb Ar/CP angle N−C−C−C torsion C−H deformation
Bond Length (Å) Pd−N Pd−Cl(cis) Pd−Cl(trans) C−H
Table 1. Selected Calculated Structural Data (Å and deg) for 1−12a
Organometallics Article
DOI: 10.1021/acs.organomet.7b00656 Organometallics XXXX, XXX, XXX−XXX
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Organometallics Table 2. NBO Agostic, Syndetic, and Back-Donation Donation E(2) Values (kcal mol−1) for Complexes 1−12 Agostic Donation C−Hσ to Pd−Clσ*(trans) Pd−Clσ*(cis) C(8)−C(8a)σ to Pd−Clσ*(trans) Pd−Clσ*(cis) Syndetic Donation C(7)−C(8)π to Pd−Clσ*(trans) Pd−Clσ*(cis) C(8)−C(8a)π to Pd−Clσ*(trans) Pd−Clσ*(cis) Back Donation Pd(1) to C−Hσ* Pd(2) to C−Hσ* a
1
2
3
4
5
6
7
8
9
10
11
12a
58.6 10.0
44.5 8.7
41.5 8.1
19.8 5.1
57.1 11.6
54.7 9.8
7.5 3.7
6.4 3.3
6.3 3.4
10.6 4.3
12.8 4.3
15.5
10.0 1.2
10.3 1.4
10.6 1.3
8.8 1.3
11.0 2.7
5.4
20.3 6.5
18.3 5.9
17.8 5.9
15.0 4.3
20 6.6
19.5 6.2
15.6
59.6 15.7 2.9 4.9
3.0 2.7
3.3 2.4
1.0 0.5
0.7 5.5
91.7 33.3
57.9 13.8
2.8 0.7
4.6 3.2
6.1
Donations are to NBO Pd lone vacancy type (LV) orbitals rather than the Pd−Clσ*(trans) and Pd−Clσ*(cis) orbitals.
and 4.3 kcal mol−1 and 17.8 and 5.9 kcal mol−1 in 4 and 3, respectively]. In a further study of steric effects, we have removed the (N)−OH····Cl(2) “lock” in 1 and used an (N)−CMe3 group in 5 (Chart 2) as steric pressure at the coordinating atom is reported to be beneficial in cyclometalation reactions.11 This change causes the ligand to roll upward so that the Pd−Cl(2) bond now points almost directly at the agostic hydrogen (Figure 3a). The Ar/CP plane angle increases markedly (68.9°
separations are now very long at 3.511 and 2.457 Å [3.411 and 2.345 Å in C(7)-isopropyl complex 3], and the N−C−C(8a)− C(8) and Ar/CP angles increase significantly (26.8 and 46.4° in 4; 18.4 and 35.8° in 3). The overall result is that the aromatic ring has moved downward, and a line drawn along the Pd− Cl(2) bond points directly at the C(7)−C(8) bonding system and well away from the C−H bond (Figure 2a). In this case, the agostic donations decrease dramatically [C−Hσ to Pd− Clσ*(trans) and Pd−Clσ*(cis) orbital E(2) values, 19.8 and 5.1 kcal mol−1 and 41.5 and 8.1 kcal mol−1 in 4 and 3, respectively], but the syndetic donations do not [C(7)−C(8)π to Pd−Clσ*(trans) and Pd−Clσ*(cis) orbital E(2) values, 15.0
Figure 3. Effect of changing the (N)−R substituent on the geometry of the agostic interaction. (a) R = CMe3, complex 5. (b) R = OH, complex 1. The orange lines trace a path along from the Pd−Cl(2) bond.
in 5, 34.5° in 1) and the Pd····C(7) and Pd····C(8) separations become shorter (3.189 and 2.177 Å in 5; 3.283 and 2.240 Å in 1), and the Pd····H separation increases to 1.884 Å (1.819 Å in 1). The C−H bond length and deformation angles are similar in both complexes as are the agostic and C(7)−C(8)π syndetic donations (compare donations for 5 and 1 in Table 2). However, with the ligand rolled upward in 5, the back-donation situation from the metal changes a little with the Pd to C−Hσ* E(2) values, becoming 0.7 and 5.5 kcal mol−1 compared to 2.9 and 4.9 kcal mol−1 in 1. Changing to the [PdCl2(acetophenone imine) analogue 6 where there is no alicyclic ring and the aromatic ring could rotate, the metrics and donations are very similar to those in 5 which shows that the agostic and syndetic
Figure 2. Calculated structures of [PdCl2(1-tetralone oxime)] complexes showing the downward shift of the ligand aromatic ring. (a) Complex 4, CMe3 group at C(7). (b) Complex 3, CHMe2 group at C(7). The orange line traces a path along from the Pd−Cl(2) bond. D
DOI: 10.1021/acs.organomet.7b00656 Organometallics XXXX, XXX, XXX−XXX
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Organometallics components are not dictated by any restricting influence from the alicyclic ring. On the basis of the result for 4, where a C(7)−CMe3 group significantly lowered the agostic donation but not the syndetic C(7)−C(8)π donation, this group was added in addition to an (N)−CMe3 group, to give 7. The result is dramatic with two of the (N)−CMe3 methyl groups now straddling Cl(2) and the Pd−Cl(2) bond pointing toward the C(8)−C(8a) section of the aromatic ring and completely away from the C−H bond (Figure 4). In this case, the agostic donation is almost
Figure 4. Calculated structure of [PdCl2(1-tetralone imine)] complex 7. The orange line traces a path along from the Pd−Cl(2) bond.
Figure 5. Contour and orbital plots for 7. (a) C−Hσ to Pd− Clσ*(trans) agostic donation. (b) C(8)−C(8a)π Pd−Clσ*(trans) syndetic donation.
completely turned off [C−Hσ to Pd−Clσ*(trans) and Pd− Clσ*(cis) donations, 7.5 and 3.7 kcal mol−1; C−H bond length 1.091 Å], and there is now strong syndetic donation from the C(8)−C(8a)π orbital which is the “inner” π orbital system of the aromatic ring [E(2) values for the Pd−Clσ*(trans) and Pd−Clσ*(cis) donations are 59.6 and 15.7 kcal mol−1]. This donation is accompanied by π-back bonding from the metal [Pd to C(7)−C(8a)π* orbital E(2) value, 18.9 kcal mol−1]. Also, there is C−Cσ agostic donation12 to the Pd−Clσ*(trans) orbital [E(2) value 10.0 kcal mol−1] but very little to the Pd− Clσ*(cis) orbital [E(2) value 1.2 kcal mol−1] (see Table 1 of the Supporting Information). Turning to a rotatable aromatic ring analogue in [PdCl2(acetophenone imine)] complex 8 [CMe3 groups at N and at both meta-positions of the aromatic ring (Chart 2, R1, R2 = CMe3)], without the constraint of the alicyclic ring, the aromatic ring straightens up a little (N−C−C−C torsion angle 51.1° in 8, 43.8 in 7; Ar/CP angles, 84.1 and 75.4°, respectively) to remove the clash of the two methyl groups [the H····Cl(2) separations are now 3.898 and 2.898 Å] and in this situation the Pd····C(8) separation becomes slightly longer at 2.284 Å (2.264 Å in 7) and the Pd····C(8a) separation a little shorter at 2.259 Å (2.282 Å in 7). There is now very little change in the minimal agostic donation seen in 7, but there is a dramatic increase in the C(8)−C(8a)π donation to the metal [E(2) values 91.7 and 33.3 kcal mol−1 in 8, 59.6 and 15.7 kcal mol−1 in 7]. This indicates that with a rotatable ligand, there is some flexibility which allows the π-donation to the metal from the aromatic ring to increase significantly. Contour and orbital plot diagrams for the major agostic and C(8)−C(8a)π donations for 7 which are to the Pd−Clσ*(trans) orbital, are shown in Figure 5. Along with the donations identified, there is a small increase in the Pd to C(8)−C(8a)π* back-bonding component [E(2) values 22.3 and 18.9 kcal mol−1 for 8 and 7, respectively, Table 1 of the Supporting Information], but the C−Cσ agostic bonding component does not change significantly [E(2) values for the C(8)−C(8a)σ to Pd−Clσ*(trans) and Pd−Clσ*(cis) donations are 10.3 and 1.4 kcal mol−1 for 8 and 10.0 and 1.2
kcal mol−1 for 7, Table 1 of the Supporting Information]. It should be noted at this stage that for both 7 and 8, with the drastic reduction in the agostic C−Hσ donation, the dominant bonding situation is that involving the aromatic ring C(8)− C(8a)π and π* orbitals so that the complex is possibly better characterized as involving aromatic ring η2 π-donation with Lewis base character13,14 based on the C(8)−C(8a)π donation to the metal being the major contributor but with a small agostic component. In this case, the term syndetic2 may then not apply to this orbital contribution. For the syndetic donation in complex 1, this donation is concentrated much more in the vicinity of the C(8)−carbon2 (see Figure 2 of the Supporting Information) than the symmetrical distribution shown in Figure 5b. Electronic Perturbation of the Steric Condition in 7. Given the steric situation in complex 7, it was of interest to ascertain if electronic effects could challenge the bonding components that were dictated by the steric effects acting on the complex. Replacing the C(5)−H atom in 7 with the strong σ-withdrawing SO2Cl group (F value 1.16; R value −0.0514) complex 9, has very little effect on the metrics and the agostic and syndectic or π-donations are similar to those found for 7 (Table 2). The strong π-withdrawing NNPO(OCH2CH3)2 substituent (F value −0.05; R value 0.7914) was not used in this study as it produces a conformational change to the alicyclic ring section of the ligand due to its size.5 However, replacing the C(5)−H atom with B(OH)3− (complex 10), which is the strongest pure σ-donating group available (F value −0.42; R value −0.0214), does not change the N−C−C(8a)−C(8), Ar/ CP or alicyclic ring torsion angles significantly, but the Pd···· C(8) separation decreases a little (2.247 and 2.264 Å in 10 and 7, respectively) and the Pd····C(8a) separation increases significantly (2.429 and 2.282 Å, respectively). Effectively, the ligand has shifted upward (Figure 1.10 of the Supporting Information), and in this case the agostic donation increases a little [C−Hσ to Pd−Clσ*(trans) and Pd−Clσ*(cis) E(2) values, 10.6 and 4.3 kcal mol−1, 7.5 and 3.7 kcal mol−1 in 10 and 7, respectively] and the syndetic C(8)−C(8a)π E
DOI: 10.1021/acs.organomet.7b00656 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
syndetic donation increases dramatically. In attempting to influence the agostic and syndetic bonding components in tetralone imine complex 7 by electronic means, electron withdrawal from the aromatic ring by SO2Cl at C(5) in 9 has little effect but with σ donation at C(5) by B(OH)3− in 10, the syndetic component switches almost completely off and Pd−C σ-bond covalency is invoked which can be made stronger by πdonation at C(5) by S− in 11. In complex 12 [S− at C(5) and C(6)], this covalency is switched off and syndetic donation is reinstated. Overall, there is an interplay between steric and electronic influences which can reduce agostic activation, increase syndetic donation, and invoke η1-covalency. Such features can then be expected to have a significant impact on the overall success of C−C bond formation chemistry, as the ability of Pd−C bond formation to arise via agostic and syndetic donations may be influenced by steric effects and these in turn may be influenced by electronic effects.
donation is switched almost completely off [E(2) values, 2.8 and 0.7 kcal mol−1 in 10, 59.6 and 15.7 kcal mol−1 in 7]. The C−Cσ agostic donation does not decrease that much [C(8)− C(8a)σ to Pd−Clσ*(trans) and Pd−Clσ*(cis) E(2) values, 8.8 and 1.3 kcal mol−1, 10.0 and 1.2 kcal mol−1 in 10 and 7, respectively], but there is now Pd−C bond covalency invoked [C(8)σ to Pd−Clσ*(trans) and Pd−Clσ*(cis) E(2) values, 66.6 and 19.2 kcal mol−1, see Table 1 of the Supporting Information]. Introduction of the strong π-donating S− substituent (F value −0.03; R value −1.2414) at C(5) in 11 shifts the ligand further upward so that the Pd−Cl(2) bond points almost directly at the C(8)−carbon (Figure 6), and with this change the Pd····
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EXPERIMENTAL SECTION
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ASSOCIATED CONTENT
General Information. The synthetic experimental details relating to the work presented here can be found in ref 4. Computational Details. Density functional theory (DFT) based geometry optimizations and vibrational calculations for the complexes 1−12 were performed using the dispersion corrected PBE-D316 functional within the Gaussian09 (G09)17 software. A triple-ζ high quality basis set (aug-cc-pVTZ-PP)18 was employed for Pd together with a scalar relativistic energy consistent Stuttgart pseusopotential; aug-cc-pVTZ19 for the attached ancillary ligands (Cl atoms), agostic hydrogen and nitrogen (attached to Pd), and double-ζ quality basis set (aug-cc-pVDZ)19 were used for the remainder of atoms. No imaginary frequencies were found in the vibrational analysis. For the QTAIM analysis, the input files (.wfx) were obtained from G09,17 and QTAIM calculations were performed with the AIMALL software20 The NBO calculations were performed with the NBO6.0 package,21 and NBOView2.021 was used to visualize the contours and surfaces of donor−acceptor interactions. The same procedure was used for the ligand calculations.
Figure 6. View down the Cl(2)−Pd bond of complex 11 containing the C(5)−S− substituent.
C(8) separation decreases to 2.179 Å (2.264 Å in 7), which is in the range of palladium η1- complexes.15 The various donations are slightly larger than those in 10, but there is now significant σ-bond Pd−C covalency present [C(8)σ to Pd−Clσ*(trans) and Pd−Clσ*(cis) E(2) values, 100.2 and 28.4 kcal mol−1, Table 1 of the Supporting Information]. Placing π-donating S− substituents at both C(5) and C(7) positions of the aromatic ring as in 12 shifts the ligand again, and the Pd−Cl(2) bond now points slightly below C(8) and toward the C(7)−C(8) bond (Figure 1.12 of the Supporting Information). As a result, there is no Pd−C bond covalency, the C−H bond agostic donation becomes slightly larger than in 11 [donation E(2) values, 15.5 and 12.8 kcal mol−1] and the syndetic π-donation is on again but is from the C(7)−C(8)π orbital [E(2) value 15.6 kcal mol−1].
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00656. Structures 1−12; structural diagrams showing the agostic C−H bond in relation to the Pd coordination plane for complexes 1−12; NBO electron density and contour plots for the agostic and syndetic donations (complexes 1 and 7); combined metrics and NBO donation table; second-order perturbation energy E(2) values (kcal mol−1) for donor−acceptor NBOs interactions for complexes 1−12; overlap matrix for complexes 1−12; Occupancy (n) and energies (a.u.) of NBOs involved donor−acceptor interactions for complexes 1−12 (PDF) Cartesian coordinates for complexes 1 (XYZ) Cartesian coordinates for complexes 2 (XYZ) Cartesian coordinates for complexes 3 (XYZ) Cartesian coordinates for complexes 4 (XYZ) Cartesian coordinates for complexes 5 (XYZ) Cartesian coordinates for complexes 6 (XYZ) Cartesian coordinates for complexes 7 (XYZ) Cartesian coordinates for complexes 8 (XYZ) Cartesian coordinates for complexes 9 (XYZ) Cartesian coordinates for complexes 10 (XYZ) Cartesian coordinates for complexes 11 (XYZ)
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CONCLUSIONS The results of this work show that steric effects adjacent to an agostic C−H bond in aromatic ring systems that are apparent in synthetic work can have significant impact on the ability to form a strong agostic interaction. In 1-tetralone oxime complexes 1−4, an increase in size of the C(7)-substituent through the series H, Me, CHMe2, and CMe3 has the effect of turning down the agostic component much more than the syndetic component. A CMe3 group attached to a coordinating N atom as in complex 5 has little effect on these donations, and this is also found when there is a potentially rotatable aromatic ring system as in acetophenone imine complex 6. However, when a CMe3 substituent is added to the ring position adjacent to the agostic C−H bond in 7, agostic donation becomes very small, syndetic π-donation changes to an inner π-orbital set of the ring, and C−Cσ agostic donation12 arises. This is also the case with a potentially rotatable aromatic ring system as in acetophenone imine complex 8, but this allows much better overlap of the inner π-orbital set with the metal and the F
DOI: 10.1021/acs.organomet.7b00656 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
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Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö .; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, revision D.01; Gaussian, Inc.: Wallingford CT, 2009. (18) Peterson, K. A.; Figgen, D.; Dolg, M.; Stoll, H. J. Chem. Phys. 2007, 126, 124101−124112. (19) Dunning, T. H., Jr. J. Chem. Phys. 1989, 90, 1007−1023. (20) AIMAll, version 16.08.17; Keith, T. A.; TK Gristmill Software: Overland Park KS, USA, 2016; http://aim.tkgristmill.com. (21) Glendening, E. D.; Badenhoop, J. K.; Reed, A. E.; Carpenter, J. E.; Bohmann, J. A.; Morales, C. M.; Landis, C. R.; Weinhold, F. NBO, version 6.0; Theoretical Chemistry Institute, University of Wisconsin: Madison, WI, 2013; http://nbo6.chem.wisc.edu/.
Cartesian coordinates for complexes 12 (XYZ)
AUTHOR INFORMATION
Corresponding Authors
*For A.J.N.: Phone: 0064 09 443 9760; E-mail, a.j.nielson@ massey.ac.nz. *For J.A.H.: E-mail,
[email protected]. ORCID
Alastair J. Nielson: 0000-0003-2086-3295 Notes
The authors declare no competing financial interest.
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REFERENCES
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DOI: 10.1021/acs.organomet.7b00656 Organometallics XXXX, XXX, XXX−XXX
