Article pubs.acs.org/Organometallics
Computational and Experimental Study on Selective sp2/sp3 or Vinylic/Aryl Carbon−Hydrogen Bond Activation by Platinum(II): Geometries and Relative Stability of Isomeric Cycloplatinated Compounds Yumin Li,* Jeffrey Carroll, Bradley Simpkins, Deepak Ravindranathan, Christopher M. Boyd, and Shouquan Huo* Department of Chemistry, East Carolina University, Greenville, North Carolina 27858, United States S Supporting Information *
ABSTRACT: Cyclometalating ligands 6-(1-phenylethyl)-2,2′-bipyridine (L4), 6-(1-phenylvinyl)-2,2′-bipyridine (L5), and 6-(prop-1en-2-yl)-2,2′-bipyridine (L6) were synthesized by the Negishi coupling of 6-bromo-2,2′-bipyridine with the corresponding organozinc reagents. The reaction of L4 with K2PtCl4 produced only the cycloplatinated compound 4a via sp2 C−H bond activation. The reactions of L5 and L6 produced exclusively the cycloplatinated compounds 5b and 6a, respectively, via vinylic C−H bond activation. DFT calculations were performed on 12 possible cycloplatination products from the reaction of N-alkyl-N-phenyl2,2′-bipyridin-6-amine (alkyl = methyl (L1), ethyl (L2), and isopropyl (L3)) and L4−L6. The results show that compounds 1b−3b resulting from the sp3 C−H bond activation of L1− L3 are thermodynamic products, and their relative stability is attributed to the planar geometry that allows for a better conjugation. Similar reasoning also applies to the stability of products from vinylic C−H bond activation of L5 and L6. The relative stability of isomeric cycloplatinated compounds 4a and 4b may be due to the different strengths of C−Pt bonds. The steric interaction is the major cause of severe distortion from a planar coordination geometry in the cycloplatinated compounds, which leads to instability of the corresponding cyclometalated products and a higher kinetic barrier for C−H bond activation.
■
INTRODUCTION Selectivity is a fundamental issue in chemical reactions. It is common that an organic or organometallic reaction can produce multiple products through different pathways. The product distribution is determined by the thermodynamic and kinetic factors of the reaction that are often influenced by reaction conditions, and selective formation of the desired product could thus be achieved by either thermodynamic control or kinetic control. In a reaction in which two or more isomeric compounds could be formed through similar and competitive mechanisms, a selective formation of one isomer is usually challenging but the most desirable in terms of efficiency in synthesis and product isolation and purification. In the recently reported cycloplatination of N-alkyl-N-phenyl-2,2′bipyridin-6-amine (alkyl = methyl (L1), ethyl (L2), and isopropyl (L3)), the selective formation of the product from either the sp2 C−H or the sp3 C−H activation is controlled by the solvent used in the reaction (Scheme 1).1 Selective sp2/sp3 C−H bond activation has been reported in other cycloplatination reactions,2 and more examples can be found in the related cyclopalladation reactions;3 however, cases with such a degree of control in which either isomer can be prepared with excellent selectivity are rare.1,3a Experimental results suggest that the product 1b resulting from the sp3 C−H bond © XXXX American Chemical Society
Scheme 1. C−H Bond Activation of 6-(N-Alkyl-Nphenylamino)-2,2′-bipyridine by K2PtCl4
activation of N-methyl-N-phenyl-2,2′-bipyridin-6-amine (L1) in acetic acid may be more stable, and the reaction is thermodynamically controlled.1 One difference between 1a and 1b is that 1a has a fused five−six-membered metallacyle, while 1b has a fused five−five-membered ring. Although it has been suggested that a five-membered metal chelation is more Received: April 17, 2015
A
DOI: 10.1021/acs.organomet.5b00326 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics stable than a six-membered chelation,4 and in fact, sixmembered cyclometalated platinum or palladium complexes are less common, other factors could also contribute to the relative stability of isomeric sp2 and sp3 C−H bond activation products a and b. To further understand the selective C−H bond activation, it is necessary to carry out a systematic study on a series of closely related reactions. Especially, a combination of complementary experimental and theoretical studies would allow elucidation of details of the structure and relative stability of the products that could be formed in these reactions. Computational chemistry has played an increasingly important role in studying the mechanisms of organometallic C−H bond reactions.5 In particular, density functional theory (DFT) has been frequently used to investigate the reaction mechanisms of real organometallic transformations including cycloplatination reactions,6 since it allows for accurate calculations of a relatively large metal complex. Therefore, we decided to use DFT calculations to examine the relative stability of 12 isomeric cycloplatinated complexes that could be formed from cycloplatination of L1− L3 and structurally related ligands L4−L6 (Chart 1) and
phenylethyl)picolinonitrile, and the cocyclotrimerization of the nitrile with acetylene in the presence of (η5-cyclopentadienyl)cobalt-1,5-cyclooctadiene.8 We report here a much simpler and straightforward synthesis of L4. By using Negishi coupling of (1-phenylethyl)zinc bromide,9 which was generated in situ from direct insertion of zinc to (1-bromoethyl)benzene,10 with 6-bromo-2,2′-bipyridine, ligand L4 was synthesized in good yield (Scheme 2). It was found that the reaction of L4 with K2PtCl4 in either acetic acid or acetonitrile produced 4a through sp2 C−H bond activation. The 1H NMR spectrum of 4a is consistent with that reported previously for the same compound prepared under different conditions,7,11 showing the metalation at the phenyl ring. Complete assignment of the signal was accomplished by the 2D COSY experiments. Notably, H-6′ appears at 9.70 ppm and H-6″ appears at 8.10 ppm with the Pt−H-6″ coupling constant being 47 Hz. The reaction in acetonitrile is very sluggish, and no formation of 4b was detected. These results indicate that 4a could be both the kinetic and thermodynamic product, which is quite different from the reaction of L1, where the fused five−five-membered 1b is thought to be the thermodynamic product.1 It should be noted that the sp3 C−H bond activation of 6-alkyl-2,2′bipyridine by platinum was reported to preferentially form a fused five−five-membered platinacycle.12 The design of ligands L5 and L6 serves to further understand the structural effect on the selectivity of C−H bond activation. In L5, both vinylic and aryl C−H bonds are expected to have similar intrinsic reactivity, but the steric effect and the size of fused metallacycles may play a role in determining the product distribution in the cycloplatination reaction. In L6, the methyl (or allylic) C−H bonds and the vinylic C−H bonds may have different reactivity, but the products would have the same size of fused five−five-membered metallacycles. Both ligands L5 and L6 were prepared in high yields via Negishi coupling13 in the presence of the catalyst Pd(PPh3)4, as shown in Scheme 3. Interestingly, reactions of L5 and L6 with K2PtCl4 produced cycloplatinated compounds 5b and 6a, respectively, as the sole product resulting from the vinylic C−H bond activation, regardless of the solvent used in the reaction: acetic acid or acetonitrile. Compounds 5a and 6a were characterized by elemental analysis, mass spectrometry, and 1H NMR spectroscopy. The 13C NMR spectra were not recorded because of poor solubility of the complexes. The 1H NMR spectra of 5b and 6a show that the metalation occurs at the vinylic carbon, and the remaining vinylic hydrogen in 5a and 6a shifts to lower field upon metalation with a large Pt−H coupling constant, appearing at 7.95 (2JPt−H = 57 Hz) and 7.54 ppm (2JPt−H = 61 Hz), respectively. The chemical shifts of 6′ hydrogens in 5a and 6b are 8.97 and 8.96 ppm, respectively, which is
Chart 1
compare the results with those obtained experimentally to elucidate the thermodynamic control of the reaction. The cycloplatination of L4 has been previously reported,7 but we will reexamine it under different conditions. Ligands L5 and L6 are newly designed to examine the effect of structural variations on the control of selectivity in the cycloplatination reactions.
■
RESULTS AND DISCUSSION Cycloplatination of Ligands L4−L6. Ligand L4 is structurally related to L1, with the only difference in the linking atom between the bipyridyl and the phenyl groups. It would be interesting to see if the solvent-controlled selective C−H bond activation of L1 is also applicable to L4. The ligand L4 was synthesized previously by a very sophisticated synthesis involving oxidation of 2-(1-phenylethyl)pyridine to the corresponding N-oxide, sequential treatments of the N-oxide with dimethyl sulfate and potassium cyanide to give 6-(1Scheme 2. Preparation of L4 and Its Cycloplatination Reaction
B
DOI: 10.1021/acs.organomet.5b00326 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics Scheme 3. Synthesis of Ligands L5 and L6 and Their Cycloplatination Reactions
significantly smaller than that of H-6′ (9.70 ppm) in 4a. Such a difference in deshielding effect on the H-6′ due to different sizes of fused platinacycles has been noted before.12 The low yield of the reaction of L6 in acetic acid is mainly attributed to the decomposition of platinum complexes since some black precipitates were formed during the reaction. The decomposition was not observed in the reaction in acetonitrile. When the reactions of L5 and L6 were run in AcOD, no deuterium scrambling was detected at the phenyl group and the methyl group in the products 5b and 6a. These results suggest that formation of 5b and 6a might be both kinetically and thermodynamically controlled. The exclusive formation of 5b and 6a can be rationalized by considering the steric interaction between one of the vinylic H’s and the 5-H of the pyridine ring in L5 and L6 (Chart 2), which
Chart 3
the other is irreversible (III), the thermodynamic product C would be the sole product, and the difference in their stability may have to be determined with a different method if the equilibrium could not be established between B and C. The cycloplatination of L1 in acetic acid very much resembles scenario III. Although 1b was suggested to be the thermodynamic product in the reaction of L1, the formation of 1b seemed to be irreversible, as suggested by a previous deuterium-labeling experiment, because the refluxing of 1b in AcOD resulted in D scrambling only at the methylene carbon, but not at the ortho positions of the phenyl ring.1a On the other hand, the isomerization of 1a or the cycloplatination of L1 in AcOD resulted in about 90% D incorporation at both the methylene and the ortho positions of the benzene ring. Even with addition of concentrated DCl−D2O, the reflux of 1b in AcOD for 12 h did not lead to D incorporation to the ortho positions of the N-phenyl ring (Scheme 4). These D/H exchange experiments also suggest that in the reaction of L1 with K2PtCl4 in acetic acid the likely reaction pathways involve the initial coordination of L1 to the platinum salt to generate 7 and 8, cyclometalation of 7 and 8 to produce 1a and 9, respectively, and a retro “rollover” cyclometalation14 from 9 to 1a and isomerization of 1a to 1b. A “rollover” cyclometalation from 7 to 9 may not be a favorable pathway under the conditions, as the isomerization of 1a to 1b in AcOD, which proceeds likely through deuterolysis of 1a to give 7-d (deuterated at the metalated carbon of the phenyl group) as the intermediate, did not lead to a measurable D/H exchange at the 3-position of the bipyridine ring. If the equilibrium between the two isomers 1a and 1b could not be established with an experiment, theoretical calculations would be the alternative
Chart 2
occurs when L5 and L6 have to adopt a proper conformation for the aryl C−H bond activation of L5 and methyl (allylic) C− H bond activation of L6. Additional steric interaction exists between the other vinylic H and one of the ortho H’s of the phenyl ring in L5. It can be expected that the stability of 5a and 6b would also suffer from such steric hindrance. In general, for competing sp2/sp3 or vinylic/aryl C−H bond activations of compound A producing cycloplatinated compounds B and C, there are three possible scenarios, I, II, and III, as shown in Chart 3. If both reactions are irreversible, the product formation will be under kinetic control (I). On the other hand, if both reactions are reversible, the product formation will be thermodynamically controlled (II), and the ratio of the products is a reflection of their relative thermodynamic stability. If one reaction is reversible, while C
DOI: 10.1021/acs.organomet.5b00326 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics Scheme 4. D/H Exchange and “Rollover” Cyclometalation in the Reaction of L1 with K2PtCl4 in AcOH(D)
Figure 1. Optimized geometries of 1a−3a and 1b−3b.
X-ray structures of 3a and 3b have been reported,1a a systematic study on the geometry and stability of this series of platinum complexes is necessary to understand the selectivity of the reaction. In particular, a systematic change in the N-alkyl groups from methyl to isopropyl provides an excellent model to assess the steric effect of the alkyl group on the geometry and stability of these cyclometalated platinum compounds. Since the structures of 3a and 3b have been determined by X-ray crystallography, their structural parameters were used as the starting point of the optimization. Figure 1 shows optimized
tool to study the relative stability of the compounds. Moreover, compounds 4b, 5a, and 6b were not formed; thus no experimental information on the structure and stability of these compounds was available. Therefore, we decided to conduct a systematic study on 12 possible cycloplatinated compounds from the cycloplatination of L1−L6 using DFT calculations to elucidate the molecular structures and estimate the relative stability of the isomeric complexes. DFT Calculations on Compounds 1a−3a and 1b−3b. Geometric Optimization of 1a−3a and 1b−3b. Although the D
DOI: 10.1021/acs.organomet.5b00326 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics Table 1. Optimized Bond Lengths (Å), Bond Angles (deg), and Torsional Angles (deg) of 1a−3a and 1b−3b 1a Pt−Cl Pt−C(1) Pt−N2 Pt−N3 N1−C2 N1−C3 N1−C4 C(1)−Pt−N3 Cl−Pt−N3 Cl−Pt−C(1) C(1)−Pt−N2 N2−Pt−N3 N2−Pt−Cl C2−N1−C3 C3−N1−C4 C2−N1−C4
Pt−Cl Pt−C(1) Pt−N2 Pt−N3 N1−C(1) N1−C2 N1−C3 C(1)−Pt−N3 Cl−Pt−N3 Cl−Pt−C(1) C(1)−Pt−N2 N2−Pt−N3 N2−Pt−Cl C2−N1−C3 C2−N1−C(1) C(1)−N1−C3
2a
3a
gas state
in MeCN
in AcOH
gas state
in MeCN
in AcOH
gas state
in MeCN
in AcOH
X-ray1a
2.332 1.963 2.038 2.174 1.438 1.458 1.367 168.3 92.6 95.4 93.5 79.1 170.7 116.9 116.7 125.5
2.372 1.970 2.026 2.176 1.429 1.462 1.374 168.3 93.0 95.1 93.0 79.4 171.4 117.2 116.6 124.6 1b
2.362 1.967 2.030 2.178 1.431 1.461 1.372 167.9 93.0 95.2 93.1 79.3 171.1 117.1 116.7 125.8
2.330 1.962 2.041 2.178 1.438 1.470 1.368 167.0 93.2 95.4 93.1 78.9 171.2 117.9 117.8 123.8
2.371 1.968 2.026 2.179 1.431 1.472 1.375 167.2 93.6 95.1 92.7 79.2 172.0 118.3 117.6 122.9 2b
2.362 1.966 2.030 2.179 1.433 1.472 1.374 167.1 93.5 95.1 92.8 79.1 171.8 118.2 117.6 123.1
2.328 1.959 2.045 2.182 1.438 1.489 1.369 165.8 93.2 95.6 93.1 78.9 170.7 118.9 116.5 123.7
2.373 1.965 2.029 2.179 1.431 1.491 1.374 166.8 93.2 95.3 92.7 79.5 171.5 120.0 116.4 123.6
2.363 1.964 2.032 2.180 1.433 1.490 1.373 166.7 93.2 95.4 92.8 79.4 171.3 119.8 116.4 123.7
2.310 1.997 2.000 2.091 1.431 1.503 1.375 167.3 93.1 94.7 91.7 80.7 173.5 120.9 115.6 123.0
gas state
in MeCN
in AcOH
gas state
in MeCN
in AcOH
gas state
in MeCN
in AcOH
X-ray1a
2.332 1.983 1.963 2.212 1.502 1.347 1.426 161.9 99.1 99.0 84.4 77.5 176.6 119.3 121.0 119. 7
2.383 1.990 1.957 2.216 1.493 1.348 1.429 161.8 100.3 97.9 84.0 77.8 178.1 119.2 121.4 119.4
2.372 1.989 1.958 2.216 1.495 1.347 1.428 161.8 100.1 98.1 84.1 77.7 177.8 119.2 121.3 119.4
2.336 1.990 1.963 2.216 1.518 1.350 1.425 162.5 99.1 98.5 84.9 77.6 176.6 119.3 121.0 119.8
2.387 1.996 1.956 2.222 1.509 1.352 1.427 162.4 100.0 97.7 84.5 77.9 177.8 119.1 121.3 119.5
2.376 1.995 1.957 2.221 1.511 1.351 1.427 162.4 99.7 97.9 84.6 77.8 177.5 119.2 121.2 119.6
2.341 2.002 1.961 2.221 1.535 1.348 1.430 162.9 99.3 97.8 85.5 77.4 176.7 120.6 119.1 120.2
2.390 2.009 1.956 2.230 1.525 1.349 1.432 162.7 99.8 97.5 85.1 77.6 177.9 120.7 119.3 120.0
2.380 2.008 1.957 2.223 1.528 1.349 1.431 162.8 99.6 97.6 85.2 77.6 177.2 120.7 119.2 120.0
2.303 2.026 1.944 2.115 1.543 1.344 1.432 163.1 99.9 96.5 84.4 79.3 178.3 119.6 118.9 121.0
geometries of 1a−3a and 1b−3b in their gas state. The major bond parameters around the coordination center and the amino nitrogen center for the optimized geometries in both the gas state and solvated states are listed in Table 1. The bond parameters from the X-ray crystal structures of 3a and 3b are also listed for comparison. Major bond lengths and angles for optimized geometries of 3a and 3b compare favorably with those determined by X-ray crystallography. It should be noted that the optimized geometry is for the molecule in its gas phase and solvated state, while the X-ray crystallography determines the solid-state structure. Therefore, small variations on the geometry in different states are expected. Nonetheless, the results suggest that the DFT method used here is very reliable in predicting the molecular geometry of the platinum complexes. The geometries of the compounds at their solvated states display a longer Pt−Cl bond distance by 0.04 and 0.05 Å in MeCN and 0.03 and 0.04 Å in AcOH compared with those in their gas states for 1a−3a and 1b−3b, respectively. This can be reasoned by the polarity of the Pt−Cl bond, because solvation assists the dissociation of a polar bond by stabilizing the charged species. Changes to the other bonds vary and are insignificant.
3b
Complexes of platinum(II) with four ligands prefer a square planar geometry. When the four ligands are different, the square geometry may be distorted as the bond lengths between the platinum and the donor atoms of the ligands and bite angles of the ligands can be different, but a planar geometry is generally retained to maintain the efficient bonding between the metal center and the donor ligands, particularly those with extended conjugation. However, other factors such as torsional and angle strain may force the platinum coordination out of an ideal plane. Comparison between isomeric compounds a and b reveals that the coordination in complexes 1a−3a is significantly deviated from a planar geometry, while the coordination geometry in 1b−3b is nearly a perfect plane. Figure 2 displays the views of compounds 1a−3a and 1b−3b by projecting through the platinum coordination plane. In 1b−3b, the chlorine, the platinum, the carbon donor C1, the bipyridyl ring, and the linker amino nitrogen N1 are coplanar, as shown in Figure 2. The mean planes composed of the 12 atoms of the bipyridyl ring were calculated for 1a−3a and 1b−3b, and the deviations of the atoms from the plane are listed in Table 2. Also listed in Table 2 are the deviations of Pt, Cl, the amino nitrogen (N(1)), and the metalated carbon (C(1)) from the bipyridyl plane. In 3b and 1b, the E
DOI: 10.1021/acs.organomet.5b00326 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
spacious orientation of the methyl group repelling the phenyl ring and making it slightly twisted from a perpendicular orientation. The steric repulsion between the phenyl ring and the methyl group(s) can be seen clearly from small but distinct changes in angles C1−N−C3, C2−N−C3, and C1−N−C2 from 1b to 2b and to 3b. As the number of methyl groups increases from zero in 1b to two in 3b, the steric hindrance increases. As a consequence, C1−N−C3 angles increase and C2−N−C3 angles decrease, while C1−N−C2 angles stay constant. In other words, the phenyl group is pushed away from the methyl group(s). The perpendicular orientation adopted by the phenyl ring is apparently to minimize the steric strain. Another consequence of the increasing steric interaction of the methyl group(s) is the stretching of the sp3 C−N bond, which elongates gradually from 1.502 Å in 1b to 1.518 and 1.535 Å in 2b and 3b, respectively. For the same reason, the C−Pt bond increases from 1.983 Å in 1b to 2.002 Å in 3b. The amino nitrogen adopts a perfect trigonal planar geometry, and the trigonal plane is coplanar with the bipyridyl ring with dihedral angles of 0.02°, 1.74°, and 0.07° for 1b−3b, respectively. The geometry of 1a to 3a is far from coplanar, showing significant bending of the phenyl ring relative to the platinum coordination plane and even twisting and bending of the two pyridyl rings (Figure 2). The dihedral angle of two pyridyl rings is 12.78°, 12.53°, and 17.87° for 1a−3a, respectively. The dihedral angle between the phenyl ring and the bipyridyl ring is 31.19°, 35.86°, and 36.45° in 1a, 2a, and 3a, respectively. The platinum and chlorine are approximately coplanar with the bipyridyl ring with a
