Article pubs.acs.org/Organometallics
Synthesis, Structures, and Reactivities of Guanidinatozinc Complexes and Their Catalytic Behavior in the Tishchenko Reaction Jie Li,† Jingchao Shi,† Hongfei Han,† Zhiqiang Guo,‡ Hongbo Tong,‡ Xuehong Wei,*,†,‡ Diansheng Liu,‡ and Michael F. Lappert*,§ †
School of Chemistry and Chemical Engneering, Shanxi University, Taiyuan 030006, People’s Republic of China Institute of Applied Chemistry, Shanxi University, Taiyuan 030006, People’s Republic of China § Department of Chemistry, University of Sussex, Brighton, BN1 9QJ, U.K. ‡
S Supporting Information *
ABSTRACT: Treatment of a secondary amine (pipH, Bz2NH, Et2NH) with sequentially diethylzinc and N,N′-dicyclohexylcarbodiimide in hexane has afforded in good yield the new crystalline guanidinatozinc complexes. Each has been X-ray and solution NMR spectrally characterized. Three are dimeric (mono)guandinatozinc alkyls, two are dinuclear (tris)guanidinatozinc amides, and one is a homoleptic zinc bis(guanidinate). The reactions of dinuclear (tris)guanidinatozinc amides with diethylzinc and N,N′-dicyclohexylcarbodiimide in the molar ratios of 1:2:1 led to the dimeric (mono)guandinatozinc alkyls; homoleptic zinc bis(guanidinate) with an equimolar portion of diethylzinc also yielded dimeric (mono)guandinatozinc alkyls. Each of the complexes exhibited good to excellent catalytic activity for the solvent-free Tishchenko reaction under mild conditions.
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INTRODUCTION The coordination and applied chemistry of the guanidinato ([RNC(NR′2)NR]−, R, R′ = alkyl, aryl, silyl, etc.) ligands is extensive and has received much attention; there have been several excellent and comprehensive reviews.1 Since the preparation of the first transition metal guanidinate by Lappert’s group in 1970,2 complexes involving metals from across the periodic table have been synthesized and applied in many areas, including synthesis,1 stabilization of complexes of low oxidation state metals,1a materials science,3 and catalysis.4 The monoanionic chelating guanidinato ligands are related to those of amidinates, triazinates, and carboxylates, each of which can delocalize the charge and hence increase its potential for electron donation to the metal; several coordination modes have been observed, but by far the two most common are as N,N′-chelating or -bridging ligands (Figure 1).1a Although the chemistry of numerous metal guanidinates has been much investigated, zinc guanidinates have been largely neglected. Only a few papers have been published, by Coles’5 or Jones’6 group. In their studies, the zinc guanidinate compounds were synthesized by (i) the insertion of a carbodiimide into a compound containing a Zn−NRR′ bond, (ii) the deprotonation of a guanidine by dimethylzinc, or (iii) a salt-metathesis reaction between a zinc halide and an alkali metal guanidinate. As recently noted,6a the chemistry of zinc guanidinates (or their related amidinates) is poorly developed. An early (2004) paper reported on their use as catalysts in the ring-opening polymerization of lactide.5a Guanidinatozinc complexes related to the present study have been 1 (R = Pri or Cy),5a 2,5a and 3 (R = Pri, R′ = OAr;5a © 2013 American Chemical Society
Figure 1. General formula of the guanidinate ligand, the two most common ligand-to-metal(s) bonding modes, and resonance structures of the guanidinate anions.
R = Ar, R′ = I6b). Compound 1 was obtained from Zn[N(SiMe3)]2 + CyNCNCy,5a 2 from ZnMe2 + Me2NC(NPri)[N(H)Pri],5a and 3 from 2 and 2ArOH.5a From ZnI2 + K[N(Ar)C(NPri2)NAr], the product was 4.6b Amidinato analogues of 1, 2, and 3 lately reported are Zn[N(Ar)C(But)NAr]2 and Pri2N[Zn(l)N(Ar)C(H)N(Ar)Znl], prepared from respectively ZnBr2/K[N(Ar)C(But)NAr] and ZnI2/K[N(Ar)C(NPri2)NAr].6b A further zinc amidinate skeletally related to 2 and to one of our new zinc guanidinates is MeZn{N(Pei)C(Me)N(Ar)}Zn(Me){N(Pri)C(Me)NPri}, isolated from the reaction of ZnMe2 + Zn[N(Pri)C(Me)NPri]2; Received: April 20, 2013 Published: June 20, 2013 3721
dx.doi.org/10.1021/om400345f | Organometallics 2013, 32, 3721−3727
Organometallics
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was added to the above reactant mixture, which was stirred for 8 h to afford a cloudy solution, which was filtered. The filtrate was concentrated to ca. 25 mL in vacuo and stored at −10 °C, yielding colorless crystals of 6 (1.06 g, 65%) (found: C, 65.20; H, 9.75; N, 12.83; C59H106N10Zn2 requires C, 65.23; H, 9.84; N, 12.89), mp 156−158 °C. 1 H NMR (C6D6): δ (ppm) 0.86−0.88 (m, 12 H, C6H11), 0.91−0.95 (m, 2 H, C5H10N), 1.19 (m, 6 H, C5H10N), 1.48−1.67 (m, 24 H, C6H11), 1.77−1.97 (m, 4 H, C5H10N), 2.18−2.35 (m, 12 H, C5H10N), 2.53−2.69 (m, 24 H, C6H11), 3.18−3.32 (m, 12 H, C6H11), 3.65 (m, 6 H, C6H11), 4.13−4.15 (m, 4 H, C5H10N). 13C NMR (C6D6): δ (ppm) 24.62, 24.89, 25.18, 25.82, 26.26, 35.66, 36.39, 36.78, 48.53, 66.08 (C6H11), 31.85, 33.10, 34.27, 34.76, 48.19, 51.33, 52.04, 53.63, 54.11, 57.52 (C5H10N), 167.49 (NCN). [{(PhCH2)2NC(NC6H11)2}ZnEt]2 (7). Method 1. To a stirred solution of dibenzylamine (0.38 mL, 2.0 mmol) in hexane (40 mL) was added diethylzinc (2.00 mL of a 1.0 M solution in hexane, 2.0 mmol). The mixture was heated to 60 °C for 12 h and cooled to room temperature, and N,N′-dicyclohexylcarbodiimide (0.41 g, 2.0 mmol) was added. The mixture was stirred for 8 h to afford a cloudy solution, which was filtered. The filtrate was concentrated to ca. 20 mL in vacuo and stored at −10 °C, yielding colorless crystals of 7 (0.76 g, 76%) (found: C, 70.10; H, 8.28; N, 8.37; C58H82N6Zn2 requires C,70.08; H, 8.31; N, 8.45), mp 167− 169 °C. 1H NMR (C6D6): δ (ppm) 0.70−0.76 (m, 4 H, ZnCH2CH3), 0.97 (m, 6 H, ZnCH2CH3), 1.17−1.25 (m, 8 H, C6H11), 1.42−1.55 (m, 16 H, C6H11), 1.67−1.73 (m, 16 H, C6H11), 3.31 (m, 4 H, C6H11), 3.99 (s, 8 H, PhCH2), 6.91−7.07 (m, 20 H, C6H5). 13C NMR (C6D6): δ (ppm) 3.22 (ZnCH2CH3), 19.42 (ZnCH2CH3), 19.42, 30.94, 42.47, 57.89 (C6H11), 59.36 (C6H5CH2), 127.48, 140.06, 144.58, 154.49 (C6H5) 173.43 (NCN). Method 2. A stirred suspension of compound 8 (0.76 g, 1.17 mmol), N,N′-dicyclohexylcarbodiimide (0.24 g, 1.17 mmol) in hexane (50 mL), and diethylzinc (2.34 mL of a 1.0 M solution in hexane, 2.34 mmol) was stirred at room temperature for 8 h to afford a cloudy mixture, which was filtered. The filtrate was concentrated to ca. 15 mL in vacuo and stored at −15 °C, yielding colorless crystals of 7 (1.51 g, 65%). [{(PhCH 2 ) 2 NC(NC 6 H 11 ) 2 Zn} 2 (μ-{(PhCH 2 ) 2 NC(NC 6 H 11 ) 2 } 2 )(μ-N(PhCH2)2)] (8). To a stirred solution of dibenzylamine (0.38 mL, 2.0 mmol) in hexane (40 mL) was added diethylzinc (1.00 mL of a 1.0 M solution in hexane, 1.0 mmol), and the mixture was heated to 60 °C for 12 h, then cooled to room temperature. N,N′-Dicyclohexylcarbodiimide (0.31 g, 1.5 mmol) was added, and the mixture was stirred for 8 h to afford a cloudy solution, which was filtered. The volatiles were removed from the filtrate in vacuo, and the residue was extracted with diethyl ether; the extract was filtered. The filtrate was concentrated and stored at −10 °C, yielding colorless cubic crystals of compound 8 (0.60 g, 78%) (found: C, 74.42; H, 8.25; N, 9.07; C95H122N10Zn2 requires C,74.34; H, 8.01; N, 9.13), mp 155−156 °C. 1H NMR (C6D6): δ (ppm) 0.89−0.91 (m, 12 H, C6H11), 1.48−1.66 (m, 24 H, C6H11), 1.83−1.87 (m, 24 H, C6H11), 3.38−3.40 (m, 6 H, C6H11), 4.12 (s, 16 H,PhCH2), 7.08−7.11 (m, 16 H, ArH), 7.18−7.21 (m, 8 H, ArH), 7.25−7.28 (m, 16 H, ArH). 13 C NMR (C6D6): δ (ppm) 26.62, 26.74, 35.36, 36.30, 52.33, 54.94, 57.97, 60.29 (C6H11), 67.07 (PhCH2), 128.50, 129.55, 140.59, 155.77 (C6H5), 164.86 (NCN). [{(C2H5)2NC(NC6H11)2}ZnEt]2 (9). Method 1. To a stirred solution of diethylamine (0.20 mL, 2.0 mmol) in hexane (40 mL) was added diethylzinc (2.00 mL of a 1.0 M solution in hexane, 2.0 mmol), and the mixture was heated to 60 °C for 12 h, then cooled to room temperature. N,N′-Dicyclohexylcarbodiimide (0.41 g, 2.0 mmol) was added. The mixture was stirred for 10 h, then filtered. The filtrate was concentrated to ca. 20 mL in vacuo and stored at −10 °C, yielding colorless crystals of 9 (0.6g, 80%) (found: C, 61.25; H, 9.95; N, 11.32; C38H74N6Zn2 requires C,61.20; H, 10.00; N, 11.27), mp 148−150 °C. 1H NMR (C5D5N): δ (ppm) 0.97 (q, J = 7.9, 4 H, ZnCH2CH3), 1.09 (t, J = 6.9, 6 H, ZnCH2CH3), 1.15−1.23 (m, 8 H, C6H11), 1.28−1.58 (m, 16 H, C6H11), 1.70−1.73 (m, 16 H, C6H11), 1.87−1.89 (m, 12 H, N(CH2CH3)2), 3.15−3.17 (m, 8 H, N(CH2CH3)2), 3.22 (m, 4 H, C6H11). 13C NMR (C5D5N): δ (ppm) 6.69 (ZnCH2CH3), 22.37 (ZnCH2CH3), 22.97 (N(CH2CH3)2), 62.88 (N(CH2CH3)2), 22.37, 34.62, 45.86, 51.92 (C6H11), 176.91 (NCN).
the latter zinc amidinate was the product of the reaction of ZnI2 and 2 Li[N(Pri)C(Me)NPri].7a Herein, we report (i) six new zinc guanidinates synthesized from one-pot reactions of a secondary amine, diethylzinc, and N,N′-dicyclohexylcarbodiimide and (ii) their use as catalysts for the homodimerization of an aromatic aldehyde to the corresponding ester.
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EXPERIMENTAL SECTION
General Remarks. All manipulations were carried out under dry nitrogen using standard Schlenk and cannula techniques. Solvents were dried with appropriate drying agents, degassed, and stored over a potassium mirror or activated molecular sieves prior to use. CyNC NCy (Alfa Aesar) and ZnEt2 (1.0 M solution in hexane; Alfa Aesar) were obtained commercially and used as received. Piperidine, (PhCH2)2NH, and Et2NH (Aldrich) were dried over KOH and redistilled before use. 1 H NMR (300 MHz) and 13C NMR (75 MHz) spectra of the compounds were recorded on a Bruker DRX 300 instrument and referenced internally to the residual solvent resonances (chemical shift data in δ). All NMR spectra (1H, 13C) were measured in C6D6 at 298 K. Elemental analyses were performed on a Vario EL-III instrument. Melting points were taken on an electrothermal apparatus and are uncorrected. Preparations. [{C5H10NC(NC6H11)2}ZnEt]2 (5). Method 1. To a stirred solution of piperidine (0.60 mL, 6.0 mmol) in hexane (60 mL) was added diethylzinc (6.00 mL of a 1.0 M solution in hexane, 6.0 mmol). The mixture was heated to 60 °C for 12 h and then cooled to room temperature. N,N′-Dicyclohexylcarbodiimide (1.24 g, 6.0 mmol) was added. The mixture was stirred for 8 h to afford a cloudy solution, which was filtered. The filtrate was concentrated to ca. 20 mL in vacuo and stored at −10 °C, yielding colorless crystals of 5 (1.85 g, 80%) (found: C, 62.34; H, 9.71; N, 10.86; C40H74N6Zn2 requires C, 62.41; H, 9.69; N, 10.92), mp 194 °C (dec). 1H NMR (C6D6): δ (ppm): 0.84 (q, J = 8.4, 4 H, ZnCH2CH3), 1.14 (t, J = 11.4, 6 H, ZnCH2CH3), 1.29− 1.34 (m, 8 H, C6H11), 1.38−1.46 (m, 4 H, C5H10N), 1.56−1.60 (m, 16 H, C6H11), 1.72−1.76 (m, 16 H, C6H11), 1.92−1.94 (m, 8 H, C5H10N), 3.20 (m, 8 H, C5H10N), 3.30 (m, 4 H, C6H11). 13C NMR (C6D6): δ (ppm) −9.72 (ZnCH2CH3), 6.59 (ZnCH2CH3), 17.09, 18.12, 29.27, 46.31 (C6H11), 18.39, 18.53, 41.89 (C5H10N), 160.72 (NCN). Method 2. A stirred suspension of compound 6 (0.84 g, 0.77 mmol), N,N′-dicyclohexylcarbodiimide (0.16 g, 0.77 mmol) in hexane (40 mL), and diethylzinc (1.54 mL of a 1.0 M solution in hexane, 1.54 mmol) was stirred at room temperature for 8 h to afford a cloudy mixture, which was filtered. The filtrate was concentrated to ca. 15 mL in vacuo and stored at −10 °C, yielding colorless crystals of 5 (0.89 g, 75%). [{C5H10NC(NC6H11)2Zn}2(μ-{C5H10NC(NC6H11)2}2)(μ-NC5H10)] (6). To a stirred solution of piperidine (0.60 mL, 6.0 mmol) in hexane (60 mL) was added diethylzinc (3.00 mL of a 1.0 M solution in hexane, 3.0 mmol), and the solution was heated to 60 °C for 12 h, then cooled to room temperature. N,N′-Dicyclohexylcarbodiimide (0.93 g, 4.5 mmol) 3722
dx.doi.org/10.1021/om400345f | Organometallics 2013, 32, 3721−3727
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Method 2. A stirred suspension of compound 10 (0.62 g, 1.0 mmol) in hexane (40 mL) and diethylzinc (1.0 mL of a 1.0 M solution in hexane, 1.0 mmol) was stirred at room temperature for 8 h to afford a cloudy mixture, which was filtered. The filtrate was concentrated to ca. 15 mL in vacuo and stored at −15 °C, yielding colorless crystals of 9 (0.66 g, 89%). [Zn{Et2NC(NCy)2}2] (10). To a stirred solution of diethylamine (0.2 mL, 2.0 mmol) in hexane (40 mL) was added diethylzinc (1.00 mL of a 1.0 M solution in hexane, 1.0 mmol), and the mixture was heated to 60 °C for 12 h and then cooled to room temperature. N,N′Dicyclohexylcarbodiimide (0.41 g, 2.0 mmol) was added. The mixture was stirred for 10 h, then filtered. The filtrate was concentrated to 20 mL in vacuo and stored at −10 °C, yielding colorless crystals of 10 (0.43 g, 69%) (found: C, 65.7; H, 10.42; N, 13.43; C34H64N6Zn requires C, 65.62; H, 10.37; N, 13.51), mp 128−130 °C. 1H NMR (C5D5N): δ (ppm) 1.05 (t, J = 6.9, 12 H, N(CH2CH3)2), 1.25−1.29 (m, 8 H, C6H11), 1.44−1.65 (m, 16 H, C6H11), 2.12−2.16 (m, 16 H, C6H11), 3.18 (q, J = 6.9, 8 H, N(CH2CH3)2), 3.26−3.29 (m, 16 H, C6H11). 13C NMR (C5D5N): δ (ppm) 1.89 (N(CH2CH3)2), 14.12, 27.00, 37.90, 43.78 (C6H11), 55.30 (N(CH2CH3)2), 170.39 (NCN). Typical Procedure Employed for the Tishchenko Reaction. The appropriate zinc complex (0.5 mmol) was placed in a dry Schlenk flask with a stirring bar under an argon atmosphere, and freshly distilled aldehyde (10 mmol) was introduced. The reaction mixture was heated at 80 °C for 12 h. The reaction was monitored by TLC until complete consumption of aldehyde. The product was isolated by column chromatography on silica gel (hexane/ether = 1:15 as eluent). X-ray Crystallography. Crystallographic measurements were performed with Mo Kα radiation (λ = 0.71073 Å) on a Bruker Smart Apex CCD diffractometer. Crystals were coated in oil and then directly mounted on the diffractometer under a stream of cold nitrogen gas. A total of N reflections was collected using the ω scan mode. Corrections were applied for Lorentz and polarization effects as well as absorption using multiscans (SADABS).8 Each structure was solved by direct methods and refined on F2 by full matrix least-squares (SHELX97)9 using all unique data. Then the remaining non-hydrogen atoms were obtained from the successive difference Fourier map. All non-hydrogen atoms were refined with anisotropic displacement parameters, whereas the hydrogen atoms were constrained to parent sites, using a riding mode (SHELXTL).10 Details of the modeling of disorder in the crystals can be found in their CIF files.
Scheme 1. Synthetic Routes to Compounds 5−10
was prepared as colorless crystals in moderate yield by changing the molar ratios of N,N′-dicyclohexylcarbodiimide, diethylamine, and diethylzinc in hexane to 2:2:1. Each of 5, 6, 7, 8, 9, and 10 was easily purified by crystallization from hexane at −10 °C and was characterized by satisfactory C, H, and N microanalysis, 1H and 13 C{1H} NMR spectra in C6D6 at ambient temperature, and single-crystal X-ray structural data. Reactivities of 6, 8, or 10 with Diethylzinc and N,N′Dicyclohexylcarbodiimide or Diethylzinc. Consistent with the chemical formula of the (mono)guanidinatozinc alkyl complex 5 or 7, the dinuclear (tris)guanidinatozinc amide 6 or 8 was smoothly transformed to 5 or 7 by treatment of 6 or 8 with the carbodiimide and diethylzinc in relative molar proportions of 1:1:2 in hexane. Treatment of 10 with an equivalent portion of diethylzinc in hexane yielded 9 in 89% yield. X-ray Single-Crystal Structures of 5−10. The molecular structure of the fused tricyclic boat-shaped crystalline compound 5, having a 2-fold rotation axis through the midpoint of the central slightly puckered Zn1N3Zn1′N3′ ring, is illustrated as an ORTEP diagram in Figure 2 and is closely similar (except for the NR2 groups) to that of 2. Selected geometrical parameters are listed in Table 1. The two guanidinate ligands with Zn1 and Zn1′ adopt a κ1N−κ1,2-N′ bonding mode with a central Zn2N2 ring and a tricyclic structure as the previously reported 2.5a The Zn1−N2 (2.029 Å) and Zn1−N3′ (2.068 Å) bond distances are shorter than those in 2 (2.063, 2.101 Å), respectively, while the bond distance of Zn1−N3 (2.314 Å) is much longer than that of 2 (2.201 Å). The molecular structure of the crystalline compound 6 is shown in an ORTEP representation in Figure 3. At its core is the Zn1N4C19N6Zn2N10 six-membered ring, in which relative to the N4C19N6 plane the atoms Zn1, Zn2, and N10 are 0.921 Å above, 0.766 Å below, and 0.238 Å above, respectively; dihedral angles between adjacent planes are as follows: N4C19N6/ N1C1N2Zn1, 80.39°; N4C19N6/N7C37N9Zn2, 89.22°; N1C1N2Zn1/N7C37N9Zn2, 40.13°; N4C19N6/Zn1N10Zn2, 33.83°; N1C1N2Zn1/Zn1N10Zn2, 81.31°; N7C37N9Zn2/ Zn1N10Zn2, 69.44°. Selected bond lengths and angles are listed in Table 2. The above ring with its pendant C19−N4 and C19−N6 bonds is similar to the corresponding feature in 3,5a but the two structures differ in their substituents and the zinc coordination number (four in 5 but three in 3).
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RESULTS AND DISCUSSION Synthesis of Zinc Guanidinate Complexes 5−10. A onepot reaction route has been employed in the synthesis of zinc guanidinate complexes. As shown in Scheme 1, the stoichiometric reaction of a secondary amine, diethylzinc, and N,N′dicyclohexylcarbodiimide in hexane was first investigated. Hence, when piperidine, dibenzylamine, or diethylamine was employed as the secondary amine, the (mono)guanidinatozinc alkyl complex was obtained in good yield (80% for 5, 65% for 7, and 80% for 9), respectively. It was evident that the insertion of a molecule of carbodiimide into the Zn−N bond is easier than that of the Zn−C bond. Interestingly, the minor dinuclear (tris)guanidinatozinc amide [{C 5 H 10 NC(NC 6 H 11 ) 2 Zn} 2 (μ{C5H10NC(NC6H11)2}2)(μ-NC5H10)] (6) was formed when piperidine was used. This spurred us to explore the reaction in different molar ratios of starting materials. The results show that the reaction products depend extremely on the molar ratios of the amine, diethylzinc, and carbodiimide. Thus, the colorless crystals of the complex 6 or 8 were obtained in moderate or good yield from diethylzinc and successive portions of piperidine or dibenzylamine and N,N′-dicyclohexylcarbodiimide in relative molar ratios of 1:2:1.5, respectively. Attempts to prepare the corresponding dinuclear (tris)guanidinatozinc amide based on diethylamine under the same condition were unsuccessful. However, the homoleptic, bis(guanidinato)zinc complex (10) 3723
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Figure 3. Molecular structure of [{C5H10NC(NC6H11)2Zn}2(μ{C5H10NC(NC6H11)2}2)(μ-NC5H10)] (6) with thermal ellipsoids drawn at the 25% probability level; hydrogen atoms are omitted for clarity. Figure 2. Molecular structure of [{C5H10NC(NC6H11)2}ZnEt]2 (5) with thermal ellipsoids drawn at the 25% probability level; hydrogen atoms are omitted for clarity.
Table 2. Selected Bond Lengths [Å] and Angles [deg] for 6 Zn1−N1 Zn1−N4 Zn2−N7 Zn2−N6 N4−C19 N1−C1 N7−C37 N4−Zn1−N10 N6−Zn2−N10 Zn1−N10−Zn2 C19−N4−Zn1 C1−N1−Zn1 N2−C1−N1 C37−N9−Zn2
a
Table 1. Selected Bond Lengths [Å] and Angles [deg] for 5 Zn1−C19 Zn1−N3′ Zn1−C1 N1−C1 N1−C2 N2−C7 N3−C13 C19−Zn1−N2 N2−Zn1−N3′ N2−Zn1−N3 C19−Zn1−C1 N3′−Zn1-C1 C1−N1−C6 C6−N1−C2 C1−N2−Zn1 C1−N3−C13 C13−N3−Zn1′ C13−N3−Zn1 N2−C1−N3 N3−C1−N1 N3−C1−Zn1 N1−C2−C3 N2−Zn1−Zn1′ N3−Zn1−Zn1′ a
1.979(4) 2.068(2) 2.577(3) 1.384(3) 1.471(4) 1.464(3) 1.484(3) 120.25(15) 111.13(9) 62.23(8) 127.11(15) 107.36(9) 121.5(2) 113.0(2) 98.60(17) 119.3(2) 114.02(16) 126.17(17) 113.7(2) 122.4(2) 63.34(14) 111.8(2) 84.56(6) 43.19(6)
Zn1−N2 Zn1−N3 Zn1−Zn1′ N1−C6 N2−C1 N3−C1 N3−Zn1′ C19−Zn1−N3′ C19−Zn1−N3 N3′−Zn1-N3 N2−Zn1−C1 N3−Zn1−C1 C1−N1−C2 C1−N2−C7 C7−N2−Zn1 C1−N3−Zn1′ C1−N3−Zn1 Zn1′−N3−Zn1 N2−C1−N1 N2−C1−Zn1 N1−C1−Zn1 C19−Zn1−Zn1′ N3′−Zn1−Zn1′ C1−Zn1−Zn1′
2.029(2) 2.314(2) 3.0171(9) 1.459(4) 1.314(3) 1.382(3) 2.068(2) 124.74(15) 127.26(17) 93.16(9) 30.27(9) 32.24(8) 121.9(2) 125.1(2) 136.21(19) 119.24(16) 84.42(15) 86.82(9) 123.8(2) 51.13(13) 171.67(19) 147.66(17) 49.99(7) 64.20
2.124(2) 2.008(2) 2.061(2) 1.998(2) 1.344(4) 1.338(4) 1.318(4) 110.92(10) 110.19(9) 100.72(10) 121.3(2) 89.10(17) 113.8(3) 88.86(18)
Zn1−N2 Zn1−N10 Zn2−N9 Zn2−N10 N6−C19 N2−C1 N9−C37 N2−Zn1−N1 N7−Zn2−N9 N6−C19−N4 C19−N6−Zn2 C1−N2−Zn1 C37−N7−Zn2 N7−C37−N9
2.056(2) 2.013(2) 2.116(2) 2.020(2) 1.338(4) 1.332(4) 1.333(4) 64.69(9) 64.65(9) 120.0(3) 120.40(19) 92.21(18) 91.66(18) 114.8(3)
Symmetry elements for 5: ′ −x+3/2, −y+1/2, z.
The molecular structure of the crystalline compound 7 is shown as an ORTEP diagram in Figure 4, with selected bond lengths and angles in Table 3. The two guanidinate ligands with Zn and Zn′ present a κ1N−κ2-N bonding mode to form an eightmembered-ring core structure. The transannular Zn−Zn′ distance is 2.857 Å (compared with 3.0171 Å in 5), and Zn or Zn′ is 0.713 or 0.785 Å out of the N2C1N1 plane (compared with 0.355 or 1.738 Å out of the N2C1N3 plane of 5). The molecular structure of the crystalline compound 8 is shown as an ORTEP diagram in Figure 5. Apart from the exocyclic substituents at the carbon atoms C1, C28, and C55 (corresponding to C1, C19, and C37 in 6) and nitrogen atoms
Figure 4. Molecular structure of [{Bz 2NC(NC6H11)2}ZnEt]2 (Bz = PhCH2) (7) with thermal ellipsoids drawn at the 25% probability level; hydrogen atoms are omitted for clarity.
N3 and N4 (corresponding to N4 and N6 in 6), its geometry is closely similar to that of 6. Selected bond lengths and angles are listed in Table 4. The molecular structure of the crystalline compound 9 is shown in Figure 6 with selected data in Table 5. The core of the centrosymmetric structure of 9 is the puckered eight-membered 3724
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Table 3. Selected Bond Lengths [Å] and Angles [deg] for 7 Zn−N2′ N1−C1 N2−C(8) N3−C23 Zn−Zn′ N2−Zn′ N2′−Zn−N1 N1−Zn−C14 C1−N1−Zn C1−N2−C8 C8−N2−Zn′ C1−N3−C16 N1−C1−N2 N2−C1−N3 N2′−Zn−Zn′ N1−C2−C3
1.996(2) 1.320(3) 1.482(4) 1.455(4) 2.8570(12) 1.996(2) 111.74(10) 119.20(12) 112.58(17) 119.6(2) 112.98(17) 122.0(2) 118.6(2) 120.3(2) 69.86(8) 109.4(2)
Zn−N1 N1−C2 N3−C1 N3−C16 Zn−C14 N2−C1 N2′−Zn−C14 C1−N1−C2 C2−N1−Zn C1−N2−Zn′ C1−N3−C23 C23−N3−C16 N1−C1−N3 N1−C2−C7 N1−Zn−Zn′ C14−Zn−Zn′
1.985(2) 1.464(3) 1.397(3) 1.468(4) 1.990(3) 1.357(4) 129.01(12) 124.7(2) 122.64(17) 127.33(19) 120.9(2) 116.3(2) 121.1(2) 111.2(2) 83.93(7) 116.45(11)
Figure 6. Molecular structure of [{C5H10NC(NC6H11)2}ZnEt]2 (9) with thermal ellipsoids drawn at the 25% probability level; hydrogen atoms are omitted for clarity.
Table 5. Selected Bond Lengths [Å] and Angles [deg] for 9a Zn1−C18 Zn1−N1 N1−C7 N2−C7′ N3−C7 N3−C16 C18−Zn1−N2 N2−Zn1−N1 N2−Zn1−Zn1′ C7−N1−C1 C1−N1−Zn1 C7′−N2−Zn1 C7−N3−C14 C14−N3−C16 N1−C1−C6 a
1.949(3) 1.964(3) 1.321(3) 1.299(3) 1.383(3) 1.435(4) 122.02(12) 108.71(10) 83.60(6) 120.5(2) 113.75(16) 116.26(17) 121.5(2) 117.6(2) 110.4(2)
Zn1−N2 Zn1−Zn1′ N1−C1 N2−C8 N3−C14
1.954(2) 2.742(2) 1.456(3) 1.452(3) 1.434(4)
C18−Zn1−N1 C18−Zn1−Zn1′ N1−Zn1−Zn1′ C7−N1−Zn1 C′−N2−C8 C8−N2−Zn1 C7−N3−C16 N1−C1−C2
129.26(10) 110.08(9) 74.34(7) 125.73(15) 123.8(2) 119.97(15) 119.9(2) 110.3(2)
Symmetry elements for 9: ′ −x+2, y, −z+3/2.
Figure 5. Molecular structure of [{Bz2NC(NC6H11)2Zn}2(μ-{Bz2NC(NC6H11)2}2)(μ-NBz2)] (Bz = PhCH2) (8) with thermal ellipsoids drawn at the 25% probability level; hydrogen atoms are omitted for clarity.
Table 4. Selected Bond Lengths [Å] and Angles [deg] for 8 Zn1−N1 Zn1−N3 Zn2−N5 Zn2−N4 N4−C28 N1−C1 N5−C5 N3−Zn1−N10 N4−Zn2−N10 Zn1−N10−Zn2 C28−N3−Zn1 C1−N1−Zn1 N2−C1−N1 C55−N6−Zn2
2.106(3) 2.008(3) 2.129(3) 1.995(3) 1.333(5) 1.334(5) 1.331(5) 110.85(14) 106.94(13) 102.39(17) 123.3(2) 89.1(2) 114.4(3) 92.3(2)
Zn1−N2 Zn1−N10 Zn2−N6 Zn2−N10 N3−28 N2−C1 N6−C55 N2−Zn1−N1 N5−Zn2−N6 N3−C28−N4 C28−N4−Zn2 C1−N2−Zn1 C55−N5−Zn2 N5−C55−N6
2.057(3) 2.007(3) 2.033(3) 2.021(4) 1.343(5) 1.328(5) 1.332(5) 65.02(12) 65.08(12) 119.6(3) 121.9(3) 91.5(2) 88.1(2) 114.5(3)
Figure 7. Molecular structure of [Zn{Et2NC(NCy)2}2] (10) with thermal ellipsoids drawn at the 25% probability level; hydrogen atoms are omitted for clarity.
complex 10, the zinc atom is in a distorted tetrahedral environment and is the spiro junction of the two almost isostructural planar fourmembered rings ZnN1C7N2 and ZnN4C24N5. The endocyclic angles at each of these rings decrease in the sequence N1C7N2 [112.5(2)°] > ZnN1C7 ≈ ZnN2C7 > N1ZnN2 [66.89(8)°].
Zn1N1C7N2′Zn1′C7N2 ring, each constituent atom of which is in a distorted trigonal planar environment, which is closely similar to that of 7. The molecular structure of the crystalline compound 10 is shown in Figure 7, with selected data in Table 6. In the mononuclear 3725
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conditions (Table 7, entries 1−6). The activity of the complex 6 or 8 was much lower than that of the complex 5, 7, 9, or 10. This is attributed to the highly crowded environment of the zinc atom in complex 6 or 8, which decreased the attack probability of benzaldehyde. Complex 9 at 2.5 mol % or 1 mol % loading gave a 93% or 80% yield, respectively, of benzyl benzoate under the same conditions (Table 7, entries 7, 8). [The dimeric compound names (5, 7, and 9) discussed here were still used, but their solution states should be monomeric, and the 13CNMR data are consistent with the monomeric solution structure.] In view of the above results, a variety of aromatic and heteroaromatic aldehydes were examined, using 9 as a catalyst at 5 mol % loading (Table 7, entries 9−15). Except for 2-pyridylaldehyde and furfural (entries 14 and 15), complex 9 catalyzed the conversion of the chosen aryl aldehyde to its corresponding ester with good to excellent activities (Table 7, entries 9−13).
Table 6. Selected Bond Lengths [Å] and Angles [deg] for 10 Zn1−N1 Zn1−N4 C7−N1 C24−N4 N1−Zn1−N2 Zn1−N1−C7 Zn1−N4−C24 N1−C7−N2
2.0168(19) 2.0336(19) 1.338(3) 1.338(3) 66.89(8) 90.23(14) 89.27(15) 112.5(2)
Zn−N2 Zn1−N5 C7−N2 C24−N5 N4−Zn1−N5 Zn1−N2−C7 Zn1−N5−C24 N4−C24−N5
2.0170(19) 2.011(2) 1.335(3) 1.323(3) 66.72(8) 90.30(14) 90.66(15) 113.4(2)
The structure of 10 is similar to that of 15a except for the substituents at the nitrogen atoms [NC6H11 and NEt2 in 10 and NPri or NCy and N(SiMe3)2 in 15a]. Tishchenko Reaction of Aromatic Aldehydes Catalyzed by Guanidinatozinc Complexes. The Tishchenko reaction (eq 1), which has been known for more than a century, is
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CONCLUSION The synthesis and structures of six new crystalline guanidinatozinc complexes, 5−10, is described. Each was obtained from a secondary amine (A), diethylzinc (B), and N,N′-dicyclohexylcarbodiimide (C). The relative proportions of A:B:C were 1:1:1 (5, 7, and 9), 4:2:3 (for 6 and 8), or 2:1:2 (10). Treatment of 6 or 8 with diethylzinc and N,N′-dicyclohexylcarbodiimide in a molar ratio of 1:2:1 also afforded 5 or 7, respectively. Compound 9 was also obtained from 10 and an equivalent portion of diethylzinc. Whereas 10 is mononuclear, the others contain two zinc atoms. Compounds 6, 8, and 10 have two terminal guanidinato ligands, while the corresponding bridging ligands are found in 6 and 8 (one only) and 5, 7, and 9 (two each). The coordination number of zinc is four, but compounds 5 and 7 are three-coordinate. Compounds 6 and 8 are tricylic, with a central six-membered ring; the core of 5, 7, and 9 is a boat-shaped octacycle. All of the complexes exhibited good (6, 8) to excellent (5, 7, 9, and 10) catalytic activities for the solvent-free Tishchenko reaction under mild conditions.
an atom-efficient method for the synthesis of symmetric esters from the dimerization of the corresponding aldehydes. Since then, the Tishchenko reaction has been used as an efficient method for the preparation of dimeric esters in industry, but development of efficient catalysts is crucial. A number of catalysts for the Tishchenko reaction have been reported, including metal, metalloid, and nonmetal compounds, such as Na, Mg, Al, Ca, Ba, Sr, B, Fe, Ru, Os, Hf, Zr, La, Y, and Sc.11 Recently, a magnesium guanidinate compound,12 Th(IV) compounds,13 Ni(0)/NHC,14 including an organic compound thiolate,15 and selenide ions16 have been reported as effective catalysts in the Tishchenko reaction. But, to the best of our knowledge, there have been no reports on zinc compounds used as the catalyst of the Tishchenko reaction. Benzaldehyde was selected as a model substrate, and the guanidinatozinc complexes 5−10 were screened as potential catalysts for the Tishchenko reaction (eq 1); the results are summarized in Table 7. Good to excellent yields (78−98%) were obtained when
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Table 7. Tischenko Reaction Catalyzed by Zinc Complexes 5−10a entry
catalyst
loading (mol %)
substrate
yield (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
5 6 7 8 9 10 9 9 9 9 9 9 9 9 9
5 5 5 5 5 5 2.5 1 5 5 5 5 5 5 5
benzaldehyde benzaldehyde benzaldehyde benzaldehyde benzaldehyde benzaldehyde benzaldehyde benzaldehyde 4-Br-benzaldehyde 4-Cl-benzaldehyde 2-Cl-benzaldehyde 4-Me-benzaldehyde 4-MeO-benzaldehyde 2-pyridylaldehyde furfuraldehyde
95 80 92 78 98 96 93 80 98 98 96 88 75 54 35
a
ASSOCIATED CONTENT
S Supporting Information *
NMR data (S1), crystal data details of data collection and refinements (S2), and crystallographic information files (CIF) for 5−10 (CCDC 838523−838528) (S3). This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected];
[email protected]. Tel: 0086-15536566161. Fax: 0086-351-7011688. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the NSFC (No. 20572065), SXNSFC (2008011021; 2008012013-2; 2011021011-1), and Research Project Supported by Shanxi Scholarship Council of China for financial support.
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Reaction conditions: 80 °C, 12 h, solvent-free.
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