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Organometallics 2010, 29, 3746–3752 DOI: 10.1021/om100241v
Synthesis of cis- and trans-Diisothiocyanato-Bis(NHC) Complexes of Nickel(II) and Applications in the Kumada-Corriu Reaction Ramasamy Jothibasu,† Kuo-Wei Huang,‡ and Han Vinh Huynh*,† †
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore, and ‡KAUST Catalysis Center and Division of Chemical & Life Sciences and Engineering, 4700 King Abdullah University of Science and Technology, Thuwal 23955-6900, Kingdom of Saudi Arabia Received March 29, 2010
Metathetical reaction of AgSCN with a series of trans-dihalido-bis(carbene) nickel(II) complexes in CH3CN readily afforded the novel diisothiocyanato-bis(carbene) complexes [Ni(NCS)2(NHC)2] (trans2a, NHC = 1,3-diisopropylbenzimidazolin-2-ylidene; trans-2b, NHC=1,3-diisobutylbenzimidazolin-2ylidene; trans-2c, NHC=1,3-dibenzylbenzimidazolin-2-ylidene; cis-2d, NHC=1,3-di(2-propenyl)benzimidazolin-2-ylidene; cis-2e, NHC=1-propyl-3-methylbenzimidazolin-2-ylidene) as greenish-yellow powders in moderate to good yields. While dihalido-bis(carbene) Ni(II) complexes exclusively form transcomplexes, a trans-cis isomerization occurs upon halido-isothiocyanato exchange with complexes bearing less bulky carbene ligands, i.e., cis-2d/e. DFT calculations indicated that this isomerization can be attributed to a reduced energy difference between trans- and cis-isomers of diisothiocyanato complexes. All complexes have been characterized by multinuclear NMR spectroscopy, ESI mass spectrometry, and X-ray diffraction analysis. A catalytic study revealed that cis-complexes generally exhibit greater activities in the Kumada-Corriu coupling reaction. Introduction Complexes of N-heterocyclic carbenes (NHCs) are now prevalent in organometallic chemistry and homogeneous catalysis.1 Although nickel carbene complexes may offer access to low-cost catalysts, they have generally received less attention in comparison to other group 10 analogues, possibly due to the lack of efficient synthetic methodologies. Recently, we have reported a general method for the preparation of dihalidobis(benzimidazolin-2-ylidene) Ni(II) complexes in liquid ammonium salts.2 In this respect, it is interesting to note that complexes of the type [NiX2(NHC)2] (X = halide) exclusively adopt a trans-configuration, whereas cis-isomers remain unknown to date.3 On the other hand, both cis- and trans-isomers are readily observed for [PdX2(NHC)2] analogues, and it has been shown that the cis-arrangement of NHC ligands translates into a faster catalyst initiation due to their strong trans-effect.4 It is possible to enforce the formation of Ni(II) cis-complexes, which are highly active in the Kumada-Corriu coupling reaction, by employing cis-chelating dicarbene ligands with a *To whom correspondence should be addressed. E-mail: chmhhv@ nus.edu.sg. (1) (a) Hahn, F. E.; Jahnke, M. C. Angew. Chem., Int. Ed. 2008, 47, 3122. (b) N-Heterocyclic Carbenes in Transition Metal Catalysis; Glorius, F. A., Ed.; Springer: Berlin, 2007. (c) N-Heterocyclic Carbenes in Synthesis; Nolan, S. P., Ed.; Wiley-VCH: Weinheim, 2006. (2) (a) Huynh, H. V.; Holtgrewe, C.; Pape, T.; Koh, L. L.; Hahn, E. Organometallics 2006, 25, 245. (b) Huynh, H. V.; Wong, L. R.; Ng, P. S. Organometallics 2008, 27, 2231. (3) (a) Herrmann, W. A.; Gerstberger, G.; Spiegler, M. Organometallics 1997, 16, 2209. (b) McGuinness, D. S.; Mueller, W.; Wasserscheid, P.; Cavell, K. J.; Skelton, B. W.; White, A. H.; Englert, U. Organometallics 2002, 21, 175. (c) Matsubara, K.; Ueno, K.; Shibata, Y. Organometallics 2006, 25, 3422. (4) Huynh, H. V.; Ho, J. H. H.; Neo, T. C.; Koh, L. L. J. Organomet. Chem. 2005, 690, 3854. pubs.acs.org/Organometallics
Published on Web 08/03/2010
propylene bridge as reported by the Bowman group and us independently.5 Although, these complexes are sufficiently stable to be employed as catalyst precursors, they show the tendency to undergo slow ligand disproportionation or autoionization to form bis(chelates).5a In relation to our research on NHC complexes bearing nonhalido anionic co-ligands,6 we have recently launched an investigation on new mixed NHC/ pseudohalido complexes, which are rare in the current literature. Toward this end, we have described a diazido-bis(carbene) nickel(II) complex trans-[Ni(N3)2(NHC)2] and its use as a template for new organometallics bearing carbodiimido, tetrazolato, and abnormal tetrazolin-5-ylidene ligands.7 As an extension, we are also interested in the synthesis of Ni(II) complexes bearing isothiocyanato ligands. The thiocyanate ion belongs to an interesting class of anionic proligands due to its ambidentate nature and variable bonding modes,8 which may lead to bimetallic9 or heterobimetallic complexes and even (5) (a) Huynh, H. V.; Jothibasu, R. Eur. J. Inorg. Chem. 2009, 1926. (b) Berding, J.; Lutz, M.; Spek, A. L.; Bouwman, E. Organometallics 2009, 28, 1845. (6) Han, Y.; Huynh, H. V. Chem. Commun. 2007, 1089. (b) Han, Y.; Huynh, H. V.; Koh, L. L. J. Organomet. Chem. 2007, 692, 3606. (c) Huynh, H. V.; Neo, T. C.; Tan, G. K. Organometallics 2006, 25, 1298. (d) Huynh, H. V.; LeVan, D.; Hahn, F. E.; Hor, T. S. A. J. Organomet. Chem. 2004, 689, 1766. (e) Huynh, H. V.; Jothibasu, R.; Koh, L. L. Organometallics 2007, 26, 6852. (7) Jothibasu, R.; Huynh, H. V. Organometallics 2009, 28, 2505. (8) (a) Clark, G. R.; Palenik, G. J. Inorg. Chem. 1970, 9, 2754. (b) Wong, Y. S.; Jacobson, S.; Ceieh, P. C.; Carty, A. J. Inorg. Chem. 1974, 13, 284. (c) Palenik, G. J.; Mathew, M.; Steffen, W. L.; Beran, G. J. Am. Chem. Soc. 1975, 97, 1059. (d) MacDougall, J. J.; Nelson, J. H.; Babich, M. W.; Fuller, C. C.; Jacobson, R. A. Inorg. Chim. Acta 1978, 27, 201. (e) MacDougall, J. J.; Verstuyft, A. W.; Cary, L. W.; Nelson, J. H. Inorg. Chem. 1980, 19, 1036. (9) (a) Edge, M.; Faulds, P.; Kelly, D. G.; McMahon, A.; Ranger, G. C.; Turner, D. Eur. Polym. J. 2001, 37, 349. (b) Kuang, S.-M.; Fanwick, P. E.; Walton, R. A. Inorg. Chem. 2001, 40, 5682. (c) Ercolani, C.; Quagliano, J. V.; Vallarino, L. M. Inorg. Chim. Acta 1973, 7, 413. r 2010 American Chemical Society
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Scheme 1. Synthesis of Diisothiocyanato-bis(carbene) Ni(II) Complexes 2a-e
coordination polymers.10 More recently, Kim and co-workers reported well-defined isothiocyanato complexes of the Ni triad supported by phosphine ligands.11 Given the current interest in NHC chemistry, it is surprising that no examples of mixed isothiocyanato/NHC complexes are known to the best of our knowledge. We herein present the synthesis and characterization of cis- and trans-configured diisothiocyanato-bis(carbene) nickel(II) complexes cis-/trans-[Ni(NCS)2(NHC)2], a theoretical study on their relative stabilities, and a comparison of their catalytic activities in the Kumada-Corriu coupling reactions.
Results and Discussion Syntheses and Characterizations. Isothiocyanato complexes can be easily obtained by metathesis reaction of metal halides with various thiocyanate salts,8a,b while a more exotic route involves the reaction of metal azido complexes with trimethylsilyl isothiocyanate.11a The reaction of AgSCN with dihalido-bis(carbene) complexes is also expected to form the desired mixed diisothiocyanato-bis(carbene) derivatives. A series of trans-dihalido-bis(carbene) nickel(II) complexes of different bulkiness (1a-e)2a,b have been chosen in order to evaluate possible steric factors in the formation of cis-/trans-isomers. It is worth noting that dihalido-bis(carbene) nickel(II) complexes are exclusively found in the trans-configuration regardless of the steric bulk. Treatment of the red complexes 1a-e with excess AgSCN in CH3CN afforded the corresponding mixed diisothiocyanato-bis(carbene) complexes (2a-e) as greenish-yellow powders in moderate to good yields (Scheme 1). With the exception of 2e, which is only sparingly soluble in DMSO, DMF, and dichloromethane, the solubility of all other complexes 2a-d has improved compared to their precursors. They are readily soluble in DMSO, DMF, and chlorinated solvents, sparingly soluble in CH3CN, THF, and ethyl acetate, and insoluble in water and nonpolar solvents. The 1H NMR (10) S anchez, G.; Momblona, F.; Sanchez, M.; Perez, J.; L opez, G.; Casab o, J.; Molins, E.; Miravitlles, C. Eur. J. Inorg. Chem. 1998, 1199. (11) (a) Kim, Y.-J.; Han, J.-T.; Kang, S.; Han, W. S.; Lee, S. W. Dalton. Trans. 2003, 3357. (b) Chang, X.; Lee, K.-E.; Jeon, S. I.; Kim, Y.-J.; Lee, H.-K.; Lee, S. W. Dalton. Trans. 2005, 3722.
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spectra of complexes 2a and 2b, with bulky carbene ligands, show a similar signal pattern to those of their precursors, suggesting the retention of the trans-arrangement. On the other hand, the 1H NMR spectrum of 2c shows the presence of transand cis-isomers. A singlet at 6.02 ppm is characteristic for the benzylic protons of the trans-isomer, whereas in the cis-isomer these protons are diastereotopic and resonate as two doublets centered at 6.30 ppm and 6.16 ppm, respectively, with a coupling constant of 2J(H,H) = 15.8 Hz. Interestingly, a slow cis to trans isomerization is observed in solution. This is corroborated by a change in the integration for the benzylic proton signals in the 1 H NMR spectrum. An initial intensity ratio of 1:0.4 for the benzylic protons of the trans- and cis-isomers changed to 1:0.1 after 12 h. However, a complete cis to trans conversion has not been reached even after 48 h. The presence of the cis-isomer upon bromido-isothiocyanato exchange is remarkable, since dihalido-bis(NHC) complexes of Ni(II) form exclusively transisomers. In the 1H NMR spectrum of 2d, a broad multiplet is observed at 6.34 ppm, characteristic for an olefinic CH group of the 2-propenyl substituent. The signal for the CH2 group overlaps with the NCH2 signal, resulting in a pseudomultiplet at 5.49 ppm. A differentiation of cis- versus trans-isomers of 2d based on 1 H NMR spectroscopy is not possible due to lack of diastereotopy of the NCH2 protons. Apparently the nonbulky N-propenyl substituents allow a free rotation around the N-C bond in both isomers, hampering its differentiation. The 1H NMR spectrum of complex 2e, bearing unsymmetrically substituted ligands, is more complicated due to the presence of rotational isomers, which gives rise to two sets of signals. Moreover, two broad signals are observed for the NCH2 groups at 5.01 and 4.53 ppm, which may be due to the diastereotopic nature of these protons. This observation supports the cis-arrangement of the carbene ligands in 2e. The major signals are tentatively assigned to the sterically more favored cis-anti-isomer. In the 13C NMR spectra of 2a-d, the carbenoid carbon atoms resonating at 173.1, 175.0, 176.9, and 175.4 ppm, respectively, are shifted to high field as compared to those in their precursors. Because of the poor solubility of complex 2e, its 13C NMR spectrum could not be obtained. The presence of the NCS ligands in all compounds is also corroborated by IR stretching bands in the range 2100-2114 cm-1 recorded in CH2Cl2 solutions, which are comparable to those of phosphine counterparts.11 Finally, the formation of the complexes 2a-e has also been confirmed by positive mode ESI mass spectrometry with peaks corresponding to [M - NCS]þ fragments. Molecular Structures. Single crystals of 2a-e suitable for X-ray diffraction analysis were obtained by slow evaporation of concentrated CH2Cl2 solutions. Selected bond lengths and angles are given in Table 1. In complexes 2a-c, the carbene ligands are oriented mutually trans to each other around the square-planar Ni center. The molecular structures of 2a and 2c are depicted in Figure 1 (upper) as representatives for trans-complexes 2a-c. The preference for the trans orientation of the carbene ligands in 2a-c may be due to the steric hindrance exerted by their bulky N-substituents. The Ni-Ccarbene bond lengths [1.927(3)-1.915(2) A˚] in 2a-c are slightly elongated as compared to those in their precursors,2b which may be due to an increase in steric bulk around the metal center. In all complexes the NCS ligands are coordinated through the N atom. This is in line with the Pearson (HSAB) concept, as Ni is a hard Lewis acid, and hence, metal-coordination involves preferably the hard N-donor. The Ni-N bond lengths [1.844(3)-1.855(3) A˚] in 2ac are in the same range as observed for phosphine analogues.11 The carbene ring planes are oriented almost perpendicularly to
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Table 1. Selected Bond Lengths and Angles for 2a-e Ni1-C1 (A˚) Ni1-N3 (A˚) C1-Ni1-C1A (deg) N1-Ni1-N1A (deg) C1-Ni1-N3 (deg) C1-Ni1-N3A (deg) Ni-N-C (deg) NiC2N2/carbene dihedral angle (deg)
2a
2b
2c
2d
2e
1.927(3) 1.844(3) 179.99(1) 179.99(1) 90.28(28) 89.72(12) 172.1(3) 73.9
1.921(3) 1.855(3) 180.0 180.0 89.98(10) 90.03(10) 172.7(3) 75.4
1.915(2) 1.848(2) 179.99(1) 179.99(1) 89.40(9) 90.60(9) 167.6(2) 78.7
1.879(2) 1.876(2) 90.61(14) 89.92(13) 90.14(10) 173.16(10) 168.4(2) 70.2
1.881(3) 1.889(3) 90.80(18) 89.22(16) 90.37(12) 173.30(13) 173.1(2) 63.7
Figure 1. Molecular structures of complexes trans-2a, trans-2c, cis-2d, and cis-2e.
the NiC2N2 coordination plane with dihedral angles of 73.978.7°. On the other hand, the carbene ligands in complexes 2d and 2e, which bear relatively less bulky N-substituents as compared to 2a-c, adopt a cis-orientation (Figure 1, lower). Compared to their trans-configured precursors,2a a shortening of the NiCcarbene bond distances is observed {2d [1.879(2) A˚] and 2e [1.881(3) A˚]}. The Ni-N bond distances of 1.876(2) A˚ in 2d and 1.889(3) A˚ in 2e, respectively, are notably elongated as compared to their trans-derivatives, owing to the stronger trans-influence of the carbene ligands. The carbene ring planes are twisted away from the NiC2N2 plane with dihedral angles of 70.2° for 2d and 63.7° for 2e, respectively. As shown in the Figure 1, the unsymmetrically substituted carbene ligands in 2e are found in the
sterically more favorable anti-arrangement. The NdCdS ligands in complexes 2a-e have an essentially linear arrangement with angles of 178.9-179.1°. The Ni-NdC angles deviate slightly from linearity, with values ranging from 167.6° to 173.1° and do not show any characteristic differences between cis- and trans-isomers. Theoretical Studies. Dihalido-bis(NHC) complexes of Pd(II) can be readily obtained as cis- and trans-isomers depending on the solvents and reaction conditions used.4 Notably, Ni(II) analogues were found to form exclusively trans-complexes regardless of the halido co-ligands present in the molecule.2,3 In this study however, a simple bromido-isothiocyanato ligand exchange can lead to an isomerization from trans- to cis-form. The formation of cis-complexes is desirable, since this configuration
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Table 2. Calculated Energies (sum of electronic and thermal free energies) for trans- and cis-Isomers of 1a, 1d, 2a, and 2d complex
trans (hartree)
cis (hartree)
ΔG at 298.15 K (kcal/mol)
dipole of cis-isomersa (Debye)
1a 1d 2a 2d 2a in MeCN 2d in MeCN
-1426.800151 -1421.932566 -2382.484457 -2377.616764 -2382.448568 -2377.597631
-1426.774747 -1421.911796 -2382.464151 -2377.598669 -2382.45836 -2377.60222
15.9 13.0 12.7 11.4 6.1 2.9
11.91 (0.0001) 13.08 (0.0000) 15.25 (0.0547) 17.38 (0.0000) 25.35 (0.0035) 25.9 (0.0023)
a
Values for the trans-isomers are shown in parentheses.
allows the NHC ligands to exhibit their strong trans-influence, which in turn may lead to better precatalysts.4 To get more insight into such isomerizations, we conducted DFT calculations to determine the energy difference between cis- and trans-isomers of bis(carbene) complexes bearing bromido and isothiocyanato co-ligands. The bromido complexes 1a and 1d and their isothiocyanato derivatives 2a and 2d have been chosen to study the effects of steric bulk and co-ligands, and their relative energies are summarized in Table 2. As one may expect, all trans-isomers were found to be more stable than their cis-isomers in the gas phase. However, the magnitude of the energy difference between the isomers varies significantly and can be influenced by the steric bulk of the NHC as well as the nature of the anionic co-ligands. trans-1a, bearing bulky 1,3-diisopropylbenzimidazolin-2-ylidene ligands, is 15.9 kcal/mol lower in energy than its fictive isomer cis-1a, but this energy difference drops to 13.0 kcal/mol for trans-1d versus cis-1d when less bulky 1,3-di-(2-propenyl)benzimidazolin-2-ylidenes are employed. Moreover, the trans/cis energy difference can be lowered further to 12.7 kcal/mol (trans-2a versus cis-2a) and 11.4 kcal/mol (trans-2d versus cis-2d), respectively, when the bromido donors are replaced with the more electronegative isothiocyanato ligands. Since the cis isomers with larger dipole moments are generally more favored in polar solvents, the solvent effect of CH3CN was taken into consideration. It was observed that the energy differences between the trans/cis-isomers of 2a and 2d become significantly smaller, at 6.1 and 2.9 kcal/mol, respectively. While the calculation results still suggest that trans-2d is slightly more stable than cis-2d in CH3CN, the dramatic decrease in the trans/cis energy difference in a polar solvent indicates that the solvent effect might overcome the steric repulsion when less bulky NHC ligands and more electronegative ligands are present. The combination of these factors would favor the formation of the cis-isomers in polar solvents as observed in our experiments. Finally, it is also worth noting that all calculations starting with Ni-S-bound isomers eventually converged to Ni-Nbound 2a/d after geometry optimization. Such Ni-S isomers cannot be located computationally as local minima and have also not been observed experimentally. Catalytic Studies. Generally, the role of Pd NHC complexes has been widely explored in homogeneous catalysis.1b,c On the other hand, nickel complexes of NHCs are not very well studied as compared to their Pd counterparts, although the former have also proven to be useful catalyst precursors in various catalytic transformations, especially in the Kumada-Corriu coupling (12) (a) B€ ohm, V. P. W.; Weskamp, T.; Gst€ ottmayr, C. W. K.; Herrmann, W. A. Angew. Chem., Int. Ed. 2000, 39, 1602. (b) B€ohm, V. P. W.; Gst€ ottmayr, C. W. K.; Weskamp, T.; Herrmann, W. A. Angew. Chem., Int. Ed. 2001, 40, 3387. (c) Matsubara, K.; Ueno, K.; Shibata, Y. Organometallics 2006, 25, 3422. (d) Wolf, J.; Labande, A.; Natella, M.; Daran, J.-C.; Poli, R. J. Mol. Catal. A 2006, 259, 205. (e) Zhou, Y.; Xi, Z.; Chen, W.; Wang, D. Organometallics 2008, 27, 5911. (f) Xi, Z.; Liu, B.; Chen, W. J. Org. Chem. 2008, 73, 3954. (g) Berding, J.; van Dijkman, T. F.; Lutz, M.; Spek, A. L.; Bouman, E. Dalton Trans. 2009, 6948.
reactions.5,12 Encouraged by the catalytic activity of a cis-chelating dicarbene nickel(II) complex in C-C bond-forming reactions,5 which we have reported recently, the mixed diisothiocyanato-bis(carbene) Ni(II) complexes (2a-e) have been tested and compared for their performance in the Kumada-Corriu coupling reaction. We were particularly interested in the comparison of cis- versus trans-isomers as well as Br- versus NCS- co-ligands and included the trans-dibromido complex 1a in this comparison as well. Initially, catalytic screening was carried out for the coupling reaction of p-tolylmagnesium bromide with (i) bromobenzene, (ii) 3,5-dimethylbromobenzene, and (iii) 4-bromoanisole at ambient temperature in THF for 24 h with 1 mol % catalyst loading as standard test conditions. The results summarized in Table 3 revealed that all six complexes (2a-e and 1a) were able to couple bromobenzene efficiently, giving high yields in all cases.13 Interestingly, the lowest yield was observed for the trans-dibromido complex 1a (entry 6) followed by the trans-diisothiocyanato complexes 2a-c (entries 1-3). Although the differences are small, the cis-complexes 2d and 2e showed the best performance, both with 93% yield (entries 4 and 5). This order of activities is even more pronounced for the coupling of deactivated 2,5-dimethylbromobenzene. Again the cis-complexes 2d and 2e outperform the rest with good yields of 87% and 85% (entries 10 and 11), respectively, while that for 1a drops to 61% (entry 12). This trend is also observed for the coupling of the deactivated substrate 4-bromoanisole, which could also reveal the superiority of 2d over 2e. It is also interesting to note that NCS complex 2a consistently performs much better than its precursor bearing bromido co-ligands 1a (entries 1, 7, 13 versus 6, 12, 18). This observation highlights the importance of co-ligand effects in the design of catalysts. While the yields for these reactions range from good to high, their TOFs are rather low, indicating that the Ni complexes are slow in their conversion. The TOFs determined after 12 h for the coupling of 4-bromoanisole demonstrate a greater initial activity for all complexes, which decreases upon product formation and with the decrease of substrate concentration. Faster reactions times have been reported for other Ni-NHC complexes, however at a higher catalyst loading of 3 mol %.5b Among the five new complexes, 2d shows the best activity, and hence, screening of further substrates was subsequently carried out with 1 mol % of 2d. The results for the substrate screening are summarized in Table 4. More than 90% yield was obtained for the coupling of other activated and neutral aryl bromides (entries 4 and 5). Heteroaryl compounds like 3-bromothiophene and 3-bromopyridine can also be coupled with p-tolylMgBr in satisfactory yields of 87% and 89%, respectively (entries 6 and 7). A moderate yield of 55% was obtained for the coupling of 2-(4-bromophenyl)benzimidazole, which has an additional basic (13) Minor amounts of byproducts from hydrolysis (toluene) and homocoupling (4,40 -dimethylbiphenyl) of the Grignard reagent were observed as well.
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Table 3. Kumada-Corriu Coupling Reactions Catalyzed by Complexes 2a-e
entrya
catalyst
R1
R2
R3
yield [%] b/TON
TOF [h-1]
1 2 3 4 5 6 7 8 9 10 11 12 13c 14c 15c 16c 17c 18c
2a 2b 2c 2d 2e 1a 2a 2b 2c 2d 2e 1a 2a 2b 2c 2d 2e 1a
H H H H H H Me Me Me Me Me Me H H H H H H
H H H H H H H H H H H H OMe OMe OMe OMe OMe OMe
H H H H H H Me Me Me Me Me Me H H H H H H
91 90 91 93 93 89 80 77 84 87 85 61 68 (47) 70 (45) 77 (49) 83 (61) 78 (55) 53 (28)
3.79 3.75 3.79 3.88 3.88 3.71 3.33 3.21 3.50 3.63 3.54 2.54 2.83 (3.92) 2.92 (3.75) 3.21 (4.08) 3.46 (5.08) 3.25 (4.58) 2.21 (2.33)
a Reaction conditions generally not optimized. b Isolated yields from the average of two runs. c Values for yield/TON and TOF in parentheses were measured after 12 h.
Table 4. Kumada-Corriu Coupling Reactions Catalyzed by Complex 2d
a
Isolated yields for an average of two runs.
N-H function (entry 8). Finally, complex 2d can also be used for the coupling of aryl and heteroaryl chlorides with p-tolylMgBr, giving yields of 82% and 49% (entries 9 and 10), respectively. The yields obtained with catalyst 2d are comparable to those with a Ni(II) complex bearing a cis-chelating dicarbene ligand,5 although longer reaction times are required. The performance of 2d is however superior to that of a reported trans-dichloridobis(carbene) Ni(II) complex bearing two common IPr carbene ligands.12c
Conclusion We have reported the straightforward synthesis of novel mixed diisothiocyanato-bis(carbene) Ni(II) complexes (2a-e)
by salt metathesis reaction of AgSCN and trans-dihalido-bis(carbene) Ni(II) complexes (1a-e). While dihalido-bis(carbene) Ni(II) complexes form exclusively trans-complexes, the cis-/trans-configuration of the complexes 2a-e can be controlled by the steric bulk of the carbene. DFT calculations indicated that bromido-isothiocyanato ligand exchange lowers the energy gap between trans- and cis-complexes, and the increased dipole moment of cis-isomers bearing SCN- ligands allows a further stabilization in polar solvents. Catalytic testing of the new complexes in the Kumada-Corriu coupling reaction revealed that cis-complexes generally perform much better than their trans-analogues, which may be due to the strong trans-influence of the carbene ligands. Among the trans-complexes, those bearing SCN- co-ligands are more active than
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their Br- analogues, highlighting the importance of co-ligands as well. Research in our laboratory is underway to develop this halido-pseudohalido exchange reaction into a selective and general methodology for the synthesis of catalytically useful cisconfigured transition metal complexes.
Experimental Section General Considerations. Unless otherwise noted, all operations were performed without taking precautions to exclude air and moisture. All solvents and chemicals were used as received without any further treatment if not noted otherwise. THF was dried over sodium/benzophenone and distilled under nitrogen prior to use. 1H and 13C spectra were recorded on a Bruker ACF 300 spectrometer, and the chemical shifts (δ) were internally referenced by the residual solvent signals relative to tetramethylsilane (1H, 13C). Mass spectra were measured using a Finnigan MAT LCQ (ESI) spectrometer. Infrared spectra were recorded with a Varian 3100 FT-IR spectrometer. Elemental analyses were performed on a Perkin-Elmer PE 2400 elemental analyzer at the Department of Chemistry, National University of Singapore. The dihalido-bis(carbene) nickel(II) complexes (1a-e) were synthesized according to literature procedures.2a,b General Procedure for the Preparation of Mixed Diisothiocyanato-bis(carbene) Ni(II) Complexes (2a-e). AgSCN (2.4 equiv) and the appropriate trans-dihalido-bis(1,3-dialkylbenzimidazolin-2-ylidene) Ni(II) complex (1 equiv) were suspended in CH3CN. The resulting mixture was stirred at 70 °C for 12 h shielded from light. The reaction mixture, initially a red suspension, turned yellow. The mixture was subsequently filtered through a sintered funnel, and the residue was washed with dichloromethane. The solvent of the filtrate was removed under reduced pressure, and the resulting residue was subsequently washed with methanol and dried under vacuum to afford complexes 2a-e as greenish-yellow powders. trans-Diisothiocyanato-bis(1,3-diisopropylbenzimidazolin-2ylidene)nickel(II) (2a). Yield: 88%. 1H NMR (300 MHz, CDCl3): δ 7.61 (dd, 4 H, Ar-H), 7.29 (dd, 4 H, Ar-H), 6.79 (m, 4 H, 3J(H,H) = 7.08 Hz, NCH), 2.04 (d, 24 H, 3J(H,H) = 7.08 Hz, CH3). 13C{1H} NMR (75.4 MHz, CDCl3): δ 173.1 (s, NCN), 142.4 (s, NCS), 134.0, 123.6, 113.3 (s, Ar-C), 54.9 (s, NCH), 22.5 (s, CH3). Anal. Calcd for C28H36N6S2Ni: C, 58.04; H, 6.26; N, 14.50. Found: C, 58.22; H, 6.10; N, 14.35. ESI (MS): m/z = 520 [M NCS]þ. FT-IR (CH2Cl2): ν~(NCS) 2106 cm-1 (m). trans-Diisothiocyanato-bis(1,3-diisobutylbenzimidazolin-2-ylidene)nickel(II) (2b). Yield: 91%. 1H NMR (300 MHz, CDCl3): δ 7.45 (dd, 4 H, Ar-H), 7.32 (dd, 4 H, Ar-H), 4.98 (d, 8 H, 3J(H, H) = 8.04 Hz, CH2), 2.95 (m, 4 H, 3J(H,H) = 6.72 Hz, CH), 1.26 (d, 24 H, 3J(H,H) = 6.72 Hz, CH3). 13C{1H} NMR (75.4 MHz, CDCl3): δ 175.0 (s, NCN), 135.1, 124.0, 112.0 (s, Ar-C), 56.0 (s, CH2), 29.7 (s, CH), 21.4 (s, CH3). Anal. Calcd for C32H44N6S2Ni: C, 60.47; H, 6.98; N, 13.22. Found: C, 60.29; H, 6.90; N, 13.55. ESI (MS): m/z = 576 [M - NCS]þ. FT-IR (CH2Cl2): ν~(NCS) 2103 cm-1 (m). trans-Diisothiocyanato-bis(1,3-dibenzylbenzimidazolin-2-ylidene)nickel(II) (2c). Yield: 89%. 1H NMR (300 MHz, CDCl3): δ 7.50 (d, 4 H, Ar-H), 7.38-7.29 (br m, 16 H, Ar-H), 7.15-7.09 (m, 8 H, Ar-H), 6.08 (s, 8 H, NCH2). 13C{1H} NMR (75.4 MHz, CDCl3): δ 176.9 (s, NCN), 135.7, 135.1, 129.9, 128.8, 127.6, 124.4, 111.9 (s, Ar-C), 52.2 (s, NCH2). Anal. Calcd for C44H36N6S2Ni: C, 68.49; H, 4.70; N, 10.89. Found: C, 68.42; H, 4.39; N, 10.35. ESI (MS): m/z = 712 [M - NCS]þ. FT-IR (CH2Cl2): ν~(NCS) 2100 cm-1 (s). cis-Diisothiocyanato-bis[1,3-bis(2-propenyl)benzimidazolin-2ylidene]nickel(II) (2d). Yield: 73%. 1H NMR (300 MHz, CDCl3): δ 7.42 (dd, 4 H, Ar-H), 7.32 (dd, 4 H, Ar-H), 6.34 (m, 4 H, CH), 5.75 (d, 4 H, 3J(H,H) = 5.25 Hz, NCH2), 5.49 (br m, 12 H, 4 H for NCH2 and 8 H for CH2). 13C{1H} NMR (75.4 MHz, CDCl3): δ 175.4 (s, NCN), 134.3 (s, Ar-C), 131.9 (s, CHdCH2), 123.1 (s, Ar-C), 119.5 (s, CHdCH2), 111.4 (s, Ar-C), 50.6 (s,
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NCH2). Anal. Calcd for C28H28N6S2Ni: C, 58.86; H, 4.94; N, 14.71. Found: C, 58.42; H, 4.80; N, 14.35. ESI (MS): m/z = 513 [M - NCS]þ. FT-IR (CH2Cl2): ν~(NCS) 2114 cm-1 (w). cis-Diisothiocyanato-bis(1-propyl-3-methylbenzimidazolin-2ylidene)nickel(II) (2e). Yield: 66%. 1H NMR (300 MHz, CDCl3): major isomer (cis-anti), δ 7.42-7.34 (br m, 8 H, ArH), 5.06-4.96 (br m, 2 H, NCH2), 4.72 (s, 6 H, NCH3), 4.53 (m, 2 H, NCH2), 1.96 (br m, 4 H, CH2), 0.93 (t, 6 H, 3J(H,H) = 6.9 Hz, CH3); minor isomer (cis-syn), δ 7.80-7.66 (m, 8 H, Ar-H), 4.44 (s, 6 H, NCH3), 4.30 (m, 4 H, NCH2), 1.53 (br m, 4 H, CH2), 1.09 (t, 6 H, 3J(H,H) = 6.9 Hz, CH3). Anal. Calcd for C24H28N6S2Ni: C, 55.08; H, 5.39; N, 16.06. Found: C, 54.91; H, 5.08; N, 16.34. ESI (MS): m/z = 464 [M - NCS]þ. FT-IR (CH2Cl2): ν~(NCS) 2102 cm-1 (w). General Procedure for the Kumada-Corriu Coupling. In a typical run, a Schlenk tube was charged with the appropriate catalyst (0.01 mmol) and aryl halide (1 mmol). A THF solution of the tolylmagnesium bromide (1.5 mL, 1 M, 1.5 mmol) was added to the reaction mixture under nitrogen. The reaction mixture was further stirred at ambient temperature for 12 or 24 h. Dichloromethane (10 mL) was added to the reaction mixture, and the organic layer was washed with water (3 10 mL) and dried over MgSO4. The solvent was removed under reduced pressure, and the product was isolated by column chromatography and analyzed by 1H NMR spectroscopy. X-ray Diffraction Studies. X-ray data for 2a-e were collected with a Bruker AXS SMART APEX diffractometer, using Mo KR radiation at 223(2) K (for 2a, 2b, 2e), 293(2) K (for 2d), and 295(2) K (for 2b) with the SMART suite of programs.14 Data were processed and corrected for Lorentz and polarization effects with SAINT15 and for absorption effect with SADABS.16 Structural solution and refinement were carried out with the SHELXTL suite of programs.17 The structure was solved by direct methods to locate the heavy atoms, followed by difference maps for the light, non-hydrogen atoms. All non-hydrogen atoms were generally given anisotropic displacement parameters in the final model. All H atoms were put at calculated positions. A summary of the most important crystallographic data is given in the Supporting Information. Computational Details. The DFT calculations were performed by using the Gaussian 09 program.18 Geometry optimizations for the trans- and cis-structures were carried out at the hybrid B3LYP level of theory.19,20 The all-electron 6-31G(d) basis sets21 were used for H, C, N, and S atoms, and LANL2DZ (14) SMART version 5.628; Bruker AXS Inc.: Madison, WI, 2001. (15) SAINTþ version 6.22a; Bruker AXS Inc.: Madison, WI, 2001. (16) Sheldrick, G. W. SADABS version 2.10; University of Gottingen, 2001. (17) SHELXTL version 6.14; Bruker AXS Inc.: Madison, WI, 2000. (18) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; 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, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision A.02; Gaussian, Inc.: Wallingford, CT, 2009. (19) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652. (20) Lee, C. T.; Yang, W. T.; Parr, R. G. Phys. Rev. B 1988, 37, 785– 789. (21) (a) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56, 2257. (b) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213. (c) Clark, T.; Chandrasekhar, J.; Spitznagel, G. W.; Schleyer, P. v. R. J. Comput. Chem. 1983, 4, 294. (d) Rassolov, V. A.; Ratner, M. A.; Pople, J. A.; Redfern, P. C.; Curtiss, L. A. J. Comput. Chem. 2001, 22, 976.
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Jothibasu et al.
ECP basis set22 for Ni and Br atoms. The default self-consistent reaction field (SCRF) method, polarizable continuum model (PCM), using the integral equation formalism variant (IEFPCM) was applied to study the solvent effects in MeCN.23 Frequency calculations were employed to determine free energies at 298.15 K for the comparison of relative stabilities. A summary of the DFT results is given in the Supporting Information.
Acknowledgment. Financial support from National University of Singapore (WBS R-143-000-327-133) to H.V.H and KAUST baseline funding to K.-W.H. is acknowledged. Technical support from staff at the CMMAC of the NUS Chemistry Department is appreciated. R.J. thanks NUS for a research scholarship.
(22) (a) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270–283. (b) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299–310. (c) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284–298. (23) (a) Miertus, S.; Scrocco, E.; Tomasi, J. Chem. Phys. 1981, 55, 117–29. (b) Miertus, S.; Tomasi, J. Chem. Phys. 1982, 65, 239–245.
Supporting Information Available: Crystallographic data, CIF files for complexes 2a-e, and summary of DFT results. This material is available free of charge via the Internet at http:// pubs.acs.org.
