Back-Bonding Important for a-Bound Aldehyde and Ketone Complexes?

Sep 15, 1995 - Ronald L. Cicero and John D. Protasiewicz". Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106-7078...
0 downloads 0 Views 923KB Size
Download PDF
Organometallics 1995, 14, 4792-4798

4792

Is %-Back-Bonding Important for a-Bound Aldehyde and Ketone Complexes? Synthesis and Structural Characterization of Aromatic Aldehyde Complexes of the [CpFe(CO)z] Cation +

Ronald L. Cicero and John D. Protasiewicz" Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106-7078 Received May 30, 1995@ The synthesis and characterization of a series of organometallic complexes, [CpFe(CO)z(O=C(H)h)]PF6 (Ar = Ph (11,p-MeCsH4 (2), p-NMezC6H4 (31,p-OMeC6H4 (4,p-CF&& (5), p-ClC6Hc (6), p-(dimethy1amino)cinnamaldehyde (7);Cp = q5-cyclopentadienyl) are presented. Oxidation of [CpFe(CO)& with AgPFs or [CpzFeIPFs in the presence of a 5-fold excess of aromatic aldehyde led to the formation of 1-7 in good to excellent yields (especially with the latter oxidant). Compounds 1-7 were analyzed by a combination of lH NMR (solution) and FTIR spectroscopic (solid and solution state) methods. Single-crystal X-ray ( 8 )were also performed. diffraction studies on 6 and [CpFe(C0)2(0=C(H)-p-oMeC6H4)1SbF6 Binding of these aldehydes to [CpFe(CO)z]+in solution and the solid state occurs exclusively through oxygen o-donor interactions. Structural evidence for z-back-bonding in o-bound aldehyde complexes is presented. These solid-state effects, however, do not manifest themselves in the solution equilibrium binding constants for this class of substrates.

Introduction Addition reactions promoted by the binding of an organic carbonyl group to a transition-metal complex are central to many catalytic and stoichiometric react i o n ~ . ~A- particularly ~ attractive class of substrates for nucleophilic activation are ketones and aldehyde^.^,^ Chiral organometallic Lewis acids such as [CpRe(NO)(PPh3)1+,[Cp*Re(NO)(PPh3)1+,and [TpW(CO)(PhC=CMe)l+ (Tp' = hydrotris(3,5-dimethylpyrazolyl)borate) can promote enantioselective nucleophile additions to aldehydes8-11and ketones.12-ls Nucleophile addition t o these substrates can potentially be greatly affected Abstract published in Advance ACS Abstracts, September 15,1995. (1)Shambayati, S.;Schreiber, S. L. In Comprehensive Organic Synthesis; Trost, B. M., Ed.; Pergamon Press: New York, 1991;Vol. 1,p 283. (2)Yamaguchi, M.In comprehensive Organic Synthesis; Trost, B. M. Ed.; Pergamon Press: New York, 1991;Vol. 1,p 325. (3)Rosenblum, M. Acc. Chem. Res. 1974,7, 122. (4)Reger, D. L. Acc. Chem. Res. 1988,21,229. (5)Fatiadi, A. J. J. Res. Natl. Inst. Stand. Technol. 1991,96,1. (6)Huang, Y.-H.; Gladysz, J. A. J . Chem. Educ. 1988,65, 298. Gunderson, L.-L. Tetrahedron Lett. 1993,34,2275. (7)Faller, J.W.; ( 8 )Fernandez, J. M.; Emerson, K.; Larsen, R. H.; Gladysz, J. A. J . A m . Chem. SOC.1986,108,8268. (9)Garner, C. M.; Fernandez, J. M.; Gladysz, J. A. Tetrahedron Lett. 1989,30,3931. (10)Garner, C. M.; Mendez, N. Q.; Kowalczyk, J. J.; Fernandez, J. M.; Emerson, K.; Larsen, R. D.; Gladysz, J. A. J. A m . Chem. SOC.1990, 112,5146. (ll)Agbossou, F.; Ramsden, J. A,; Huang, Y.-H.; Arif, A. M.; Gladysz, J. A. Organometallics 1992,11, 693. (12)Fernandez, J. M.; Emerson, K.; Larsen, R. D.; Gladysz, J. A. J . Chem. SOC.,Chem. Commun. 1988,37. (13)Dalton, D. M.;Gladysz, J. A. J. Organomet. Chem. 1989,370, C17. (14)Dalton, D. M.; Gladysz, J. A. J. Chem. SOC.,Dalton Trans. 1991, 2741. (15)Dalton, D. M.;Fernandez, J. M.; Emerson, K.; Larsen, R. D.; Arif, A. M.; Gladysz, J. A. J. Am. Chem. SOC.1990,112,9198. (16)Dalton, D. M.; Garner, C.M.; Fernandez, J. M.; Gladysz, J. A. J. Org. Chem. 1991,56,6823. (17)Klein, D. P.; Dalton, D. M.; Mendez, N. Q.; Arif, A. M.; Gladysz, J. A. J . Organomet. Chem. 1991,412,C7. (18)Caldarelli, J. L.; Wagner, L. E.; White, P. S.; Templeton, J. L. J . A m . Chem. SOC.1994,116,2878. @

by the binding mode of the ketone or aldehyde t o the metal center. Adducts of [CpRe(NO)(PPhdI+ with aromatic aldehydes contain equilibrium mixtures of a-bound (rl)and nbound (q2)isomers:

o-bound

n-bound

The a to JZ ratios correlate with the electron-donating or -withdrawing ability of the substituents on the aromatic nucleus, hence the presence of electronwithdrawing groups results in an increase of the n-bound isomer, while electron-donatinggroups shiR the equilibrium to favor the a-bound i s o m e r ~ . l ~ - One ~l aspect of the reactivity studies that intrigued us was the proposition that the a-bound aldehydes are more reactive toward nucleophile addition than their n-bound counterpart^.^^ Reduction can promote an isomerization from a- to rc-bound complexation of aldehydes and ketones in [OS(NH~)E~O=CR~R~)]~+.~~ An investigation of the electronic factors governing this equilibrium has suggested that aldehyde complexes of the organometallic Lewis acid [CpFe(C0)21f should exist exclusively as a-bound isomers.24 The synthesis of [CpFe(CO)z(aisobutyraldehyde)lPF6and [CpFe(CO)z(a-benzaldehyde)lPF6 have been reported, and the latter has been structurally chara~terized.1>~5 Benzaldehyde and cin(19)Mendez, N. Q.;Arif, A. M.; Gladysz, J. A. Angew. Chem., Int. Ed. Engl. 1990,29,1473. (20)Mendez, N. Q.; Mayne, C.L.; Gladysz, J.A. Angew. Chem., Int. Ed. Engl. 1990,29,1475. (21)Mendez, N.Q.; Seyler, J. W.; Arif, A. M.; Gladysz, J. A. J. Am. Chem. SOC.1993,115,2323. (22)Klein, D. P.; Gladysz, J. A. J . A m . Chem. SOC.1992,114,8710. (23)Harman, W.D.; Sekine, M.; Taube, H. J. Am. Chem. SOC.1988, 110,2439. (24)Delbecq, F.; Sautet, P. J . A m . Chem. SOC.1992,114,2446. (25)Foxman, B.M.; Klemarczyk, P. T.; Liptrot, R. E.; Rosenblum, M. J . Organomet. Chem. 1980,187,253.

0276-7333/95/2314-4792$09.00/0. 0 1995 American Chemical Society

Aromatic Aldehyde Complexes of [CpFe(CO)d+

namaldehyde complexes of [CpRu(PPh3)(CO)l+have been prepared and shown to exist as a-adducts.26 On the other hand, benzaldehyde binds to [Fe(PEt&(C0)21 in a mode.^^ Lewis acid catalysis of reactions by [CpFeLL’l+ is becoming increasingly important, and an understanding of the factors which dictate the binding of organic carbonyl functionalities to these iron centers will be required for the rational design of new and enantioselective system^.^^-^^ We have thus undertaken the synthesis of a wide range of aromatic aldehyde complexes of the form [CpFe(CO)2(O=C(H)Ar)l+,in order to ascertain whether or not such species might display the “amphichelic”behavior observed for aromatic aldehyde complexes of [CpRe(NO)(PPh3)1+and to begin explorations of their reactivity.l~~~ Previous work has established that preparation of the ketone and aliphatic aldehyde complexes [CpFe(C0)2(0=CR1R2)1+could be effected by oxidation of [CpFe(CO)nl:! by Ag+ in the presence of O=CR1R2.25234We have found that this simple route, by modification of the oxidant utilized and the method used for product isolation, provides. a general route t o analogous aromatic aldehyde complexes. We furthermore provide detailed evidence that these aromatic aldehyde complexes exist entirely as a-bound isomers in both the solid and solution phases and supply evidence for the importance of n-backbonding in a-bound aldehydes.

Experimental Section General Methods. All reactions and manipulations were carried out under a nitrogen atmosphere by using standard Schlenk techniques or a Vacuum Atmospheres drybox. Solvents were distilled under nitrogen from sodium benzophenone ketyl. Deuterated solvents were dried by passage over a column of alumina and stored under nitrogen. Benzaldehyde, p-tolualdehyde, p-anisaldehyde, and p(trifluoromethy1)benzaldehyde were purchased from Aldrich Chemical Co. and were used as purchased after degassing. [CpFe(CO)z]z, silver hexafluorophosphate, p-(dimethylamino)benzaldehyde, p-chlorobenzaldehyde, and p-(dimethy1amino)cinnamaldehydewere also purchased from Aldrich Chemical Co. and were dried under vacuum. Ferrocenium hexafluorophosphate was prepared by reported methods.35 Proton chemical shifts were referenced to residual solvent peaks. ‘H NMR spectra were recorded on Varian XL200 and Varian Gemini 300 instruments using CDzClz as the solvent. Both liquid and solid-state IR spectra were recorded on a Midac PRS FTIR spectrometer using freshly distilled dichloromethane as the solvent for IR samples and Nujol mulls for solid-state samples. Elemental analysis was performed by Oneida Research Services Inc., Whitesboro, NY. [CpFe(C0)2(0=C(H)CeHa)]PFa(1). Method A. To a Schlenk flask charged with 13 mL of CHzClz were added 0.500 (26) Faller, J. W.; Ma, Y.; Smart, C. J.;DiVerdi, M. J. J. Organomet. Chem. 1991,420,237. (27)Birk, R.; Berke, H.; Hund, H.-U.; Evertz, K.; Huttner, G.; Zsolnai, L.J. Orgunomet. Chem. 1988,342,67. (28) Kiindig, E. P.; Bourdin, B.; Bernardinelli, G . Angew. Chem., Int. Ed. Engl. 1994,33,1856. (29) Bonnesen, P. V.; Puckett, C. L.; Honeychuck, R. V.; Hersh, W. H. J . A m . Chem. SOC.1989,111, 6070. (30)Bach, T.; Fox, D. N. A.; Reetz, M. T. J. Chem. SOC.,Chem. Commun. 1992,1634. (31)Colombo, L.; Ulgheri, F.; Prati, L. Tetrahedron Lett. 1989,30, 6435. (32) Olson, A. S.; Seitz, W. J.; Hossain, M. M. Tetrahedron Lett. 1991,32,5299. (33) Shambayati, S.; Crowe, W. E.; Schreiber, S. L. Angew. Chem., Int. Ed. Engl. 1990,29,256. (34) Boudjouk, P.; Woell, J. B.; Radonovich, L. J.; Eying, M. W. Organometallics 1982,1 , 582. (35) Duggan, D. M.; Hendrickson, D. N. Inorg. Chem. 1975,14,955.

Organometallics, Vol. 14, No. 10, 1995 4793 g of [CpFe(CO)zIz(1.4 mmol) and 1.502 g of benzaldehyde (14.2 mmol). The mixture was stirred for 10 min, after which time 0.924 g of CpzFePF6 (2.8 mmol) was rapidly added. The initial dark red mixture turned dark red-brown after 5 min and was stirred for an additional 2 h. The resulting solution was filtered. To the red solution was added 20 mL of diethyl ether in 0.5 mL increments over the course of 45 min, effecting precipitation of red crystals. The red solid was collected by filtration, washed with three 2 mL portions of diethyl ether, and dried under vacuum for 1 h, yielding 0.941 g (77.8%) of dark red microcrystalline 1. Method B. In this method, 0.262 g of [CpFe(CO)& (0.7 mmol) and 0.617 g of benzaldehyde (5.8 mmol) were added to a Schlenk flask charged with 6 mL of CH2C12. The resulting mixture was stirred and chilled in an ice bath for 10 min. To the vigorously stirred chilled solution was rapidly added 0.359 g of AgPFe (1.4 mmol), effecting a n immediate color change from red to black with a noticeable precipitate forming. The resulting solution was stirred at 0 “C for 20 min, after which time the mixture was warmed to room temperature. The mixture was filtered, and the filtrate was worked up as in method A using 10 mL of diethyl ether. This method yielded 0.195 g (30.7%)of red 1. FTIR (Nujol, cm-l): 3121 (w), 2072 (s, YCO), 2036 (9, YCO), 2021 (s, YCO), 1634 (s, YO-CHR), 1599 (m), 1579 (m), 1321 (w), 1225 (w), 1177 (w), 838 (s), 760 (w), 722 (w), 684 (w). FTIR (CH2C12, cm-l): 2076 and 2031 (YCO),1626 (YO-CHR). ‘H NMR: 6 9.48 (1H, s), 7.79 (2 H, m), 7.56 (2 H, m), 5.43 (5 H, s). Anal. Calcd for C14H1103FePFs: C, 39.28; H, 2.59. Found: C, 39.19; H, 2.41. [CpFe(CO)a(O=C(H)-g-C&C€&)lPF~ (2). Method A. A 0.501 g amount of [CpFe(CO)zlz (1.4 mmol), 1.704 g of p tolualdehyde (14.2 mmol), and 0.939 g of Cp2FePF6 (2.8 mmol) yielded 1.008 g (80.5%) of dark red microcrystalline 2. Method B. A 0.255 g amount of [CpFe(CO)& (0.7 mmol), 0.282 g of p-tolualdehyde (2.3 mmol), and 0.355 g of AgPF6 (1.4 mmol) yielded 0.262 g (41.1%)of the red powder 2. FTIR (Nujol, cm-l): 3123 (m), 2070 (s, YCO), 2012 (s, VCO), 1637 (m), 1620 (m), 1599 (s), 1568 (s), 1395 (m), 1317 (w), 1235 (w), 1178 (s), 1125 (w), 1019 (w), 878 (s), 844 (s), 825 (s), 772 (m), 740 (w), 722 (w), 703 (w). FTIR (CH2C12, cm-’): 2075 and 2030 (YCO),1596 (YO-CHR). ‘H NMR b 9.35 (1H, s), 7.69 (2 H, d ( J = 8.2 Hz)), 7.36 (2 H, d ( J = 8.2 Hz)), 5.41 (5 H, s), 2.46 (3 H, s). Anal Calcd for C15H1303FePF~:C, 40.75; H, 2.96. Found: C, 40.61; H, 2.76. [C~F~(CO)Z(O=C(H)-~C~H~N(CHS)Z)IPF~ (3). Method A. A 0.502 g amount of [CpFe(CO)& (1.4 mmol), 1.796 g of p-(dimethy1amino)benzaldehyde (12.0 mmol), and 0.940 g of Cp2FePF6 (2.8 mmol) yielded 1.216 g (90.8%) of dark red microcrystalline 3. Method B. A 0.251 g amount of [CpFe(CO)zlz(0.7 mmol), 0.327 g of p-(dimethy1amino)benzaldehyde (2.2 mmol), and 0.357 g of AgPF6 (1.4 mmol) yielded 0.526 g (79.0%) of red 3. FTIR (Nujol, cm-’1: 3118 (w), 2067 (s, YCO), 2008 (s, YCO), 1621 (w), 1572 (s), 1533 (s), 1327 (w), 1270 (w), 1183 (SI, 843 (s), 737 (w), 722 (w). FTIR (CH2C12, cm-l): 2070 and 2024 (YCO), 1619 (YO-CHR). ’H NMR 6 8.58 (1H, s), 7.55 (2 H, d ( J = 9.2 Hz)), 6.67 (2 H, d ( J = 9.2 Hz)), 5.34 (5 H, s), 3.16 (6 H, s). Anal. Calcd for C16H1603FeNPF6: c, 40.79; H, 3.42; N, 2.97. Found: C, 40.88; H, 3.22; N, 3.05. [CpFe(CO)~(O=C(H)-p-CeH4OCHdlPFe (4). Method A. A 0.503 g amount of [CpFe(CO)zlz (1.4 mmol), 1.907 g of p-anisaldehyde (14.0 mmol), and 0.942 g of Cp2FePF6 (2.8 mmol) yielded 1.112 g (85.4%) of red microcrystalline 4. Method B. A 0.251 g amount of [CpFe(CO)& (0.7 mmol), 0.966 g ofp-anisaldehyde (7.1 mmol), and 0.356 g of AgPF6 (1.4 mmol) yielded 0.349 g (53.7%) of red microcrystalline 4. FTIR (Nujol, cm-’): 3123 (w), 2062 (s, YCO),2022 (s, vco), 1629 (w), 1599 (m), 1568 (m), 1514 (w), 1339 (w), 1307 (w), 1267 (m), 1246 (w), 1167 (m), 1022 (w),842 (~1,722(w). F”T (CH2Clz, cm-l): 2074 and 2029 (YCO),1594, 1586 (sh) (YO=CHR). ‘H NMR 6 9.15 (1H, s), 7.77 (2 H, d ( J = 8.9 Hz)), 7.00 (2 H, d ( J = 8.9 Hz)), 5.39 (5 H, s), 3.92 (3 H, SI. Anal. Calcd for C1&304FePF6: C, 39.33; H, 2.86. Found: C, 39.20; H, 2.67.

4794 Organometallics, Vol. 14,No. 10, 1995

compd ~~~~

1 2

3

4 5 6 7

Cicero and Protasiewicz

Table 1. Infrared Data for [C~Fe(CO)~(aldehyde)lPF~ FTIR (CHzClz) FTIR (Nqjol) VC-0 yo-CHAr aldehyde vc-0 YO-CHAI 1626 2075,2031 2072,2021 1634 benzaldehyde 2075,2030 1596 2070,2012 1637 p-tolualdehyde 1619 2070,2024 2067,2008 1621 p-(dimethy1amino)benzaldehyde 1594, 1586 (sh) 2074,2029 2062,2022 1629 p-anisaldehyde 1637 2077,2033 2077,2026 1650 p(trifluoromethy1)benzaldehyde 1626 2075,2031 2072,2015 1631 p-chlorobenzaldehyde 1546 2070,2024 2066,2030 1563 p-(dimethy1amino)cinnamaldehyde

[CpFe(C0)2(0=C(H)-p-CeH4CF3)1PFs (5). Method A. A 0.500 g amount of [CpFe(CO)zln (1.4 mmol), 2.457 g of p (trifluoromethy1)benzaldehyde(14.0 mmol), and 0.944 g of CpzFePF6 (2.8 mmol) yielded 0.296 g (21.1%) of the red powder 5. Method B. A 0.510 g amount of [CpFe(CO)zlz(1.4 mmol), 2.413 g of p-(trifluoromethy1)benzaldehyde(13.8 mmol), and 0.712 g ofAgPF6 (2.8 mmol) yielded 0.487 g of (68.1%) of bright red crystalline 5. FTIR (Nujol, cm-l): 2077 (s, YCO), 2026 (s, V C O ) , 1650 (m, YO=CHR), 1582 (w), 1514 (w), 1324 (s), 1314 (s), 1221 (w), 1176 (m), 1133 (m), 1113 (w), 1065 (m), 1017 (w), 829 (s), 722 (w). FTIR (CH2C12, cm-'): 2077 and 2033 (YCO), 1637 (YO=CHR). 'H NMR: 9.67 (1 H, s), 7.96 (2 H, d ( J = 8.2 Hz)), 7.81 (2 H, d ( J = 8.2 Hz)), 5.44 (5 H, s). Anal. Calcd for C ~ ~ H ~ ~ O ~ FC, ~ 36.32; F~PF H,S2.03. : Found: C, 36.30; H, 1.83. [CpFe(CO)z(O=C(H)-p-CfiCl)lPFe (6). Method A. A 0.500 g amount of [CpFe(CO)zlz (1.4 mmol), 2.102 g of p chlorobenzaldehyde (15.0 mmol), and 0.944 g of CpzFePFs (2.8 mmol) yielded 0.605 g (46.3%) of red microcrystalline 6. Method B. A 0.250 g amount of [CpFe(CO)zlz(0.7 mmol), 1.013 g of p-chlorobenzaldehyde (7.2 mmol), and 0.357 g of AgPFs (1.4 mmol) yielded 0.240 g (36.7%) of the red powder 6. FTIR (Nujol, cm-I): 3125 (w), 2072 (s, Y C O ) , 2015 (s, Y C O ) , 1631 (s, YO-CHR), 1594 (m), 1567 (m), 1416 (w), 1305 (w), 1228 (w), 1172 (w), 1088 (w), 1016 (w), 875 (m),847 (SI, 823 (s), 721 (w). FTIR (CH2C12, cm-'): 2076 and 2031 ( V C O ) , 1626 (YO-CHR). 'H NMR: 6 9.48 (1H, s), 7.77 (2 H, d ( J = 8.6 Hz)), 7.54 (2 H, d ( J = 8.6 Hz)), 5.42 (5 H, 9). Anal. Calcd for C14H1003ClFePF6: C, 36.36; H, 2.18. Found: C, 36.47; H, 1.92.

[CpFe(C0)2(p-(dimethylamino)cinnamaldehyde)lPFe (7). Method A. A 0.501 g amount of [CpFe(CO)zIz (1.4 mmol), 2.100 g of p-(dimethy1amino)cinnamaldehyde (12.0 mmol), and 0.944 g of CpzFePFs (2.8 mmol) yielded 0.690 g (49.0%)of orange microcrystalline 7. FTIR (Nujol, cm-'): 3124 (w), 2066 (s, Y C O ) , 2030 (s, V C O ) , 1563 (s, YO=CHR), 1338 (m), 1168 (SI, 1067 (w), 1025 (w), 974 (w), 943 (w), 842 (SI, 728 (m). FTIR (CH2C12, cm-l): 2070 and 2024 (YCO), 1546 (YO-CHR). 'H NMR: 6 8 . 5 6 ( 1 H , d ( J = 8 . 9 H z ) ) , 7 . 6 1 ( 1 H , d ( J = 1 4 . 9 H z ) ) , 7.54 (2 H, d (J = 9.0 Hz)), 6.72 (2 H, d ( J = 9.0 Hz)), 6.47 (1 H, dd (J= 14.9 Hz, J = 8.9 Hz)), 5.32 (5 H, s), 3.13 (6 H, s). Anal. Calcd for C&lgO3FeNPF6: C, 43.49; H, 3.65; N, 2.82. Found: C, 43.59; H, 3.48; N, 2.72. X-ray Crystallography for [CpFe(CO)Z(O=C(H)-pCeH&1)]PFs (6). Data Collection and Reduction. Data were collected with a Siemens P4 instrument (Mo Ka radiation (1 = 0.710 73 A)). An irregularly shaped red crystal of dimensions 0.12 x 0.36 x 0.48 mm, grown by vapor diffision of Et20 into a concentrated solution of 6 in CHZCIZ,was glued onto the tip of a glass fiber. The crystal was judged to be acceptable on the basis of w scans and rotation photography. A random search located reflections t o generate the reduced primitive cell, cell lengths being corroborated by axial photography. Additional reflections with 28 values between 24.8 and 25" were appended to the reflection array and yielded the refined cell constants. A monoclinic cell was confirmed by examination on the diffractometer. The final cell constants obtained are presented in Table 2. Data were collected as presented in Table 2 and were corrected for absorption (empirical scan). Determination and Refinement of the Structure. The crystal was found to have 2/m Laue symmetry, and the systematic absences were in agreement with the space group P21/c. Direct methods (Siemens SHELXTL PLUS, PC Version

Table 2. Crystal Data and Structure Refinement Details for 6 and 8 ~O~P Empirical formula C ~ ~ H I O C ~ F ~ F ClsH1309eSbFs 462.49 548.85 fw 298(2) 298(2) temp K 0.710 73 0.710 73 wavelength (A) orthorhombic monoclinic cryst syst space group P21lc P212121 unit cell dimens 7.8844(4) 7.0509(6) a (A) 12.5920(5) 15.065(2) b (A) 18.8630(12) 16.7850(13) c tii, 100.907(7) 1872.7(2) 1750.7(3) 4 4 Z 1.947 1.755 density (calcd) 2.292 abs coeff (mm-l) 1.177 1064 920 F(OO0) 0.08 x 0.08 x 0.20 0.12 x 0.36 x 0.48 cryst size (mm) 1.94-24.00 2.47-25.00 0 range for data collection (deg) -1 < h < 8, -1 < k < -1 < h < 9 , - 1 < k < index ranges 14, -1 < 1 < 2 1 17, -19 < 1 < 19 4179 2280 no. of rflns co11ected no. of indep Ans 3071 (Rint = 0.0253) 2104 (Rint = 0.0237) full-matrix leastfull-matrix leastrefinement squares on F squares on F method 3070/0/236 2103/0/245 no. of datal restraints lparams 1.291 1.000 goodness of fit on F Final R indices R1 = 0.0484. R1 = 0.0449. wR2 = 0.1064 (1'20(1)1 wR2 = 0.1268 R1 = 0.0625, R1 = 0.0727, R indices wR2 = 0.1183 wR2 = 0.1425 (all data) O.OOOO(4) 0.0016(8) extinction coeff NA 0.04(7) absolute structure param 0.776 and -0.591 0.580 and -0.448 largest diff peak and hole (e/A3) 5.018) revealed all of the non-hydrogen atoms. All nonhydrogen atoms were refined anisotropically. Hydrogen atoms were generated at idealized positions with common isotropic thermal parameters. The final least-squares refinement converged a t the R factors reported in Table 2. Atomic coordinates and isotropic displacements are given in Table 3. Table 4 provides selected bond lengths and bond angles. X-ray Crystallography for [CpFe(CO)z(O=C(H)-pOMeC&)]SbFe (8). Data Collection and Reduction. A small red needle-shaped crystal (dimensions 0.08 x 0.08 x 0.20 mm) was cut from a longer specimen (grown by vapor diffusion of Et20 into a concentrated solution of 8 in CHzClZ; FTIR (Nujol, cm-') YCO 2075,2029) and glued onto the tip of a glass fiber. Data were collected as described for 6 and in Table 2. Determination and Refinement of the Structure. The crystal was found to have mmm Laue symmetry, and the systematic absences were in agreement with P212121. Direct methods (Siemens SHEIXTL PLUS, PC Version 5.01p) revealed all of the non-hydrogen atoms. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were generated at idealized positions with common isotropic thermal parameters. The final least-squares refinement converged a t the R factors reported in Table 2. Atomic coordinates and isotropic dis-

Aromatic Aldehyde Complexes of [CpFe(CO)$

Organometallics, Vol. 14, No. 10, 1995 4795

Table 3. Atomic Coordinates ( x lo4) and Equivalent Isotropic Displacement Parameters (A2x 10s) for N

X

1287(1) 2684(2) 4717(2) 5283(5) -290(5) 1717(4) 3756(7) 353(6) 2160(6) 2337(5) 2651(7) 2742(7) 2560(6) 2275(7) 2155(7) 1586(8)

858(8) -900(7) -1282(7) 235(9) 4547(7) 6901(5) 2502(5) 4339(9) 4976(7) a

6393(1) 11986(1) 6589(1) 5862(3) 5988(2) 7662(2) 6074(3) 6157(3) 8065(3) 9027(2) 9444(3) 10355(3) 10836(3) 10445(3) 9531(3) 6099(4) 5349(3) 5585(3) 6463(3) 6771(3) 7580(2) 6626(3) 6516(3) 6227(3) 6954(3)

2

Table 5. Atomic Coordinates ( x lo4)and Equivalent Isotropic Displacement Parameters (Aa x lo3) for 8a

U(ed

X

3860(1) 4925(1) 7024(1) 4457(2) 5314(2) 4079(2) 4256(3) 477x3) 4719(3) 4760(2) 5501(3) 5555(3) 4868(3) 4116(3) 4053(3) 2679(3) 3052(3) 3232(3) 3005(3) 2650(3) 6744(2) 7080(4) 7031(3) 6146(2) 7916(2)

-4301(2) -652(13) -4136(16) -4246( 11) -3241(14) -2078(18) -4167(20) -3871(14) -3758(15) -3309( 19) -3108(21) -3418( 17) -3889(20) -4051(20) -3338(24) -5291( 18) -4634(19) -5507(18) -6741(18) -6618(17) 920(1) - 1289(14) 3116(15) 1236(23) 637(22) 445(20) 1429(14)

U(eq) is defined as one-third of the trace of the orthogonalized

U, tensor.

Table 4. Selected Bond Lengths (A)and Angles (deg) for 6 Fe-C(l) Fe -O(3) 0(2)-C(2) Fe-C(11) Fe-C(12) Fe-C(14) C(l)-Fe-C(2) C(2)-Fe-0(3) O(l)-C(l)-Fe 0(3)-C(3)-C(4)

Bond Lengths 1.807(5) Fe-C(2) 1.960(3) O(l)-C(l) 1.115(5) 0(3)-C(3) 2.062(4) Fe-C(10) 2.087(4) Fe-C(13) 2.104(4) Cl-C(7) Bond Angles 96.6(2) C(l)-Fe-0(3) 95.7(2) C(3)-0(3)-Fe 176.0(4) 0(2)-C(2)-Fe 122.8(4)

1.818(5) 1.112(5) 1.221(5) 2.080(4) 2.090(4) 1.737(4) 94.9(2) 130.9(3) 176.6(4)

placements a r e given in Table 5. Table 6 lists selected bond lengths a n d bond angles.

Y

2

3088(1) 3379(8) 2573(9) 1623(5) -3217(6) 3254( 10) 2746(10) 805(9) -223(8) -1 108(9) -2098(10) -2212(9) -1360(8) -383(9) -3413(13) 3815(12) 4582(11) 4532(11) 3727(11) 3314(12) 804(1) 937(17) 675(15) 2217(7) -612(7) 994(8) 621(8)

1199(1) 1030(6) 2715(5) 821(4) -67(4) 1102(7) 2132(6) 1124(6) 795(6) 1205(7) 886(7) 176(6) -240(6) 66(6) -828(8) 292(7) 766(8) 1399(8) 1337(8) 669(8) 2384( 1) 2630(9) 2172(7) 2500(6) 2279(5) 1447(4) 3337(4)

Ueq)

U(eq) is defined as one-third of the trace of the orthogonalized

Uu tensor.

Table 6. Selected Bond Lengths (A)and Angles (deg) for 8 Fe-C(l) Fe -O(3) 0(2)-C(2) Fe-C(13) Fe-C(11) Fe-C(15)

Bond Lengths 1.774(14) Fe-C(2) 1.978(7) 0(1)-C(1) 1.121(13) 0(3)-C(3) 2.086(13) Fe-C(14) 2.091(13) Fe-C(12) 2.102(13) 0(4)-C(7) Bond Angles 94.0(7) C(l)-Fe-0(3) 97.2(5) C(3)-0(3)-Fe 178.5(12) 0(2)-C(2)-Fe 125.1(10)

1.815(13) 1.14(2) 1.214(13) 2.102(14) 2.067(12) 1.352(13) 92.9(5) 128.9(7) 176.6( 14)

Results and Discussion Ketone complexes of [CpFe(CO)z]+ have been described. The first such complex prepared, [CpFe(CO)zbasic enough, displacement of isobutylene from [CpFe(acetone)]C104,was obtained by reaction of [CpFe(CO)zlz (CO)z(isobutylene)l+yields ketone complexes (diphenylwith ferric perchlorate in Since then, several cyclopropenone and phenalenone; eq 3).34 general synthetic routes have been d e v e l ~ p e d . ~ ~ ~ ~ ~ ~ ~ ~ Employing AgPF6 in place of ferric perchlorate as the O-CRlRZ (excess) [CpFe(CO)z(isobutylene)lBF4 -H2C-cMe2 oxidant allowed isolation of a wider range of ketone adducts (acetone, cyclohexenone, cyclohexanone, 3-methylcyclohexenone;eq 1).3,37For some substrates, better

[CpFe(CO),(O=CR1R2)IPF6 (1) yields were obtained if [CpFe(CO)&] (X = Br, I) replaced [CpFe(CO)& (isobutyraldehyde, methyl isopropyl ketone, diisopropyl ketone, 7-norbornanone, acrolein, and crotonaldehyde; eq 2).29334 If the ketone is (36) Johnson, E. C.; Meyer, T. J.; Winterton, N. Inorg. Chem. 1971, 10, 1673.

(37) Williams, W. E.; Lalor, F. J. J.Chem. SOC.,Dalton Trans. 1973, 1329.

The first method (eq 1) was developed for the synthesis of aromatic aldehyde complexes, as this method potentially offered a simple one-pot procedure. Reaction of [CpFe(CO)& with AgPF6 in CHzClz at 0 "C in the presence of excess benzaldehyde led immediately to a deep red solution and a gray precipitate. Removal of the precipitate of Ag(0) followed by addition of excess Et20 to precipitate the product as previously described afforded red oils which could not be purified further.25 The red sticky solids which could be obtained in some cases displayed some of the desired infrared spectral properties.

Cicero and Protasiewicz

4796 Organometallics, Vol. 14,No. 10,1995

Scheme 1

presence of a a-bound aldehyde.21 Compound 3,however, displayed broad resonances for protons of the aromatic ring. Variable-temperature lH NMR studies of 3 revealed the signals (protons Ha, Ha, and Hb, Hw) to undergo a decolascence phenomenon upon cooling (while all other signals remain sharp down to -80 "C), suggesting that rotation about the aryl-formyl bond is hindered owing to contributions from resonance form D. The spectra are reminiscent of that reported for the

Yield (%)

B AgPF,

A [Cp&IPF,

30.7

77.8

41.1

80.5

79.0

90.8

54.7

85.4

21.1

68.1

36.7

46.3

-

49.0

Slow addition of excess Et20 (over a '/z h period) to CHzClz solutions containing [CpFe(C0)2(0=C(H)PhlPF6 (11, however, produced crystalline 1in good yield (30.7%) and in analytical purity after washing with Et20 and drying under vacuum. In this manner we were successful in obtaining a series of aromatic aldehyde complexes (Scheme 1, method B). Compounds 1-7 are all highly crystalline red solids and are air stable in the solid state. These complexes have limited solubility in CHzC12 solution and show signs of decomposition over time in this solvent. Although a ready method, the yields were not optimal. Variation of the reaction conditions (temperature and concentrations) did not substantially alter the yields for 1. Reactions conducted in chlorobenzene also did not improve yields. A dramatic improvement in the yield was realized (77.8%), however, when AgPF6 was replaced by [CpzFeIPF6 as the oxidant (Scheme 1;method A). This substitution not only resulted in substantially higher yields for 1-7, but also resulted in easier workups, as the byproduct ferrocene is soluble in Et20 and stays in solution; only a small amount of unreacted [CpzFeIPFs need be removed prior to crystallization of the product with EtzO. Yields of isolated aldehyde complexes obtained by both methods varied as a function of the basic nature of the aldehyde; hence, the highest isolated yields (method A)were realized for p-(dimethy1amino)benzaldehyde (90.8%)and p-anisaldehyde (85.4%),while the lowest yield was obtained for p-chlorobenzaldehyde (46.3%). Attempts a t isolation of adducts of [CpFe(CO)zl+with either p-nitrobenzaldehyde or pentafluorobenzaldehyde failed, presumably owing to the exceedingly weak donor ability of the carbonyl groups in these electron-deficient aldehydes. lH NMR (CDzClz) spectral characterization of 1, 2, and 4-7 reveals a single set of resonances for each species, consistent with a single isomer existing in solution or rapid equilibration of isomeric complexes. A resonance for the aldehyde proton in each complex is observed in the region 8.5-9.7 ppm, consistent with the

D

p-anisaldehyde adduct of [HC(py)3M(N0)zl2+ (M = Mo, W).38 Binding of anisaldehyde to the molybdenum and tungsten Lewis acids resulted in an increase of about 4 kcdmol in AG*298 for rotation about the aryl-formyl bond. The [CpFe(CO)zl+cation has been shown to be a weaker Lewis acid than [HC(py)3M(N0)~1~+;~~ thus, this effect is not observed for 4 but becomes apparent in 3 owing to the presence of the more donating NMez versus the OMe group. A value of AG*z98 = 15 kcaVmol for 3 was obtained, which is 4.5 kcdmol greater than for free p4dimethylamino)ben~aldehyde.~'This is in contrast with the observed reduced activation barrier for rotation about the aryl-formyl bond determined for [Cr(C0)3(y6-p-(dimethylamino)benzaldehyde)l.39 The carbonyl stretching frequencies for the [CpFe(CO)zl+fragment in 1-7 (Table 1)are somewhat invariant except for the solid-state spectrum of 4, perhaps for reasons indicated above. Only one set of vclo signals in the FTIR spectra of 1-7 was observed in both solidstate measurements (Nujol mull) and solution-phase studies (CHZCW. The carbonyl stretch for the aldehyde species was identified in the region of 1550-1650 cm-l, consistent with a digating mode for these aldehydes and paralleling previously characterized aldehyde and ketone adducts of [CpFe(CO)zI+. The carbonyl stretching frequency of 1555 cm-l (CHzClz) for [CpRe(NO)(PPh3)(a-p-anisaldehyde)lPFsis somewhat lower than the values of 1594 and 1586 (sh) cm-l obtained for 4, consistent with the expected higher Lewis acidity for the rhenium species. The presence of two vc=o in the solution FTIR spectrum of 4 might arise from the existence of two isomers with differing orientations of the OMe group with respect to the formyl group. Complete establishment of the solid-state structure of 6 was achieved through a single-crystal X-ray diffraction study (Figure 1). Table 3 lists atomic coordinates and isotropic displacement parameters. Table 4 lists selected bond distances and angles. As indicated by the infrared studies, the aldehyde is bound to the (38) Faller, J. W.; Ma, Y. J.Am. Chem. SOC.1991, 113, 1579. (39) Roques, B. P. J. Organomet. Chem. 1977, 136, 33.

Aromatic Aldehyde Complexes of [CpFeCCO)J+

Organometallics, Vol. 14,No.10,1995 4797

0121 CI

Figure 1. ORTEP diagram of the cation of [CpFe(CO)z(p-chlorobenzaldehyde)lPF6(6). Figure 3. ORTEP diagram of the cation of [CpFe(CO)z(p-anisaldehyde)lSbFs(8).

II Figure 2. Packing diagram of the cations in [CpFe(CO)z(p-chlorobenzaldehyde)lPFs(6). Table 7. Important Structural Features of Ketone and Aldehyde Comtdexes of ICDF~(COM+ ketone or aldehyde p-chlorobenzaldehyde p-anisaldehyde 4-methoxy-3-butenoate 3-methylcyclohexenone tropone

Fe-0

(A)

1.960(3) 1.978(7) 1.962 1.980(6) 1.983(5)

0-C

(A)

1.22l(5) 1.214(13) 1.256(10) 1.238(8)

Fe-O-C (dea)

ref

130.9(3) 128.9(7) 133.9 133.4(5) 136.9(4)

47 47 40 25 34

organometallic Lewis acid solely through the oxygen atom of the carbonyl group. Four complexes of [CpFe(CO)zl+ with ketones have been crystallographically characterized to date: [CpFe(CO)z(tr~pone)IPFs,~~ [CpFe(C0)2(3-methylcyclohexenone)lPF~,25[CpFe(C0)2(4methoxy-3-b~tenoate)IPF6,~~~~~ and [CpFe(CO)z(cinnamaldehyde)lPF6.33s40These structures compare favorably with the structure of the aldehyde complex 6, as can be seen in Table 7. The most notable difference between 6 and the other structures is the decreased Fe-0 bond length of 1.960(3) A. A shorter C-0 bond length can be expected on the basis of the greater inductive effect of the p-chlorophenyl A shorter Fe-0 bond is somewhat counterintuitive, however, as aldehydes are known to bind more weakly than ketones.25 Analysis of the packing of the cations in 6 reveals extensive n-stacking of the aromatic residues in the crystal lattice (Figure 2). It is not clear if such effects would increase, rather than decrease, the iron-oxygen distances. To pursue this point further, we have conducted crystallographic studies of [CpFe(CO)2(p-anisaldehyde)lPF6 (4) and [CpFe(CO)z(benzaldehyde)lPFs(l).42 These studies clearly revealed the a-binding mode for each aldehyde (and a reasonable Fe-0 bond length of 1.965(40)Shambayati, S.; Schreiber, S. L. Personal communication. (41) Berthier, G.; Serre, J. In The Chemistry of the Functional Groups; Patai, S., Ed.; Interscience: New York, 1966; Vol. 1,p 1. (42) The unpublished structure of [CpFe(CO)z(ql-benzaldehyde)]SbFs is mentioned in ref 26.

(10)A for l),but unfortunately the crystals in each case were of insufficient quality and suffered from disorder problems, thus preventing high-quality structural details. For this reason we prepared [CpFe(CO)z(panisa1dehyde)lSbFs(8)by replacing AgPF6 with AgSbF6 (eq 1) as an oxidant to introduce the SbFs- anion. Figure 3 shows the resulting ORTEP diagram of the cation of 8. Table 5 lists atomic coordinates and isotropic displacement parameters. Table 6 lists selected bond distances and angles. The more electronrich anisaldehyde complex contains a Fe-0 distance of 1.987(7) A, which is significantly longer than the corresponding distance in 6 but shorter than that in all but one of the other structurally characterized ketone complexes listed in Table 7. No stacking of the aromatic residues of 8 is observed in the crystal lattice. The p-anisaldehyde ligand is essentially coplanar in the structures of 8 , [CpRe(NO)(PPhs)(p-anisaldehyde)IPF6, and [Zn(SeC6HztBu3)~(P-anisaldehyde)12.43 The corresponding C=O distances are 1.214(13), 1.271(8), and 1.242(10)A respectively. Shortened Fe-0 bond lengths for 6 and 8 might be justified if there exists a significant n-back-bonding component to the bonding in the Fe-0 bond, especially in light of the shorter Fe-0 bond length for 6 relative to 8. Such effects are clearly manifested in the Re-0 and Re-C distances for z-bound aldehydes of [CpRe(NO)(PPh3)1+.44Structurally characterized a-bound aldehyde and ketone complexes of [CpRe(NO)(PPh3)1+ suggest a similar trend (albeit smaller).21 Thus, [CpRe(NO)(PPh3)(p-anisaldehyde)lPF6,[CpRe(NO)(PPhs)(acetophenone)]PFs,and [CpRe(NO)(PPh3)(acetone)lPF6 display Re-0 bond distances of 2.080(5), 2.080(5),and 2.099(5) A, respectively. Observation of an elongated C=O bond length (1.271(8)A) for [CpRe(NO)(PPh3)(panisa1dehyde)lPFs and comparisons to other a-bound aldehyde complexes led Gladysz and co-workers to first point out the structural consequences of n-back-bonding in these complexes. Arguments for z-back-bonding in [CpFe(CO)s(acrolein)lPFs have also been forwarded to explain the reduced lability of the Fe-0 bond relative to stronger organometallicLewis acids used for catalysis of the Diels-Alder reaction between acrolein and iso~ r e n e .Yields ~ ~ for compounds 1-7, however, do not reflect the added stability which should occur for the electron-deficient aldehydes. (43) Bochmann, M.; Webb, K. J.; Hursthouse, M. B.; Mazid, M. J. Chem. Soc., Chem. Commun. 1991,1735. (44) Boone, B. J.; Klein, D. P.; MBndez, N. Q.; Seyler, J. W.; Arif, A. M.; Gladysz, J. A. J. Chem. Soc., Chem. Commun. 1996,279.

4798 Organometallics, Vol. 14,No. 10,1995 We thus decided to determine if the existence of n-back-bonding in these complexes would necessarily result in increased stability in the resultant aldehyde complex. The competitive binding of several of the aldehydes to the organometallic Lewis acid [CpFe(CO)zl+ was investigated by lH NMR. Addition of p chlorobenzaldehydeto a CDzClZ solution of [CpFe(CO)s(p-tolua1dehyde)lPFsleads to partial formation of [CpFe(CO)~@-chlorobenzaldehyde)lPF~and free p-tolualdehyde. The equilibrium (eq 4) was rapidly established,

&+

0

and NMR integration of the characteristic aldehyde protons revealed the relative amounts of the species present. Addition ofp-tolualdehyde to a CDzClz solution of [CpFe(C0)~@-chlorobenzaldehyde)lPF~(the complementary experiment) leads to the same equilibrium distribution of products, and an average Kes value of 0.26 was obtained. This value corresponds to a difference in bond strengths (if differential solvation effects are ignored) of about 1 kcallmol. We have also determined Keq= 0.075 for the corresponding set of competition experiments between p-chlorobenzaldehyde and p-anisaldehyde (eq 5). Both of these values are consis-

&+

Q

Cicero and Protasiewicz

aldehyde adducts have the shorter Fe-0 bond lengths. Thus, binding of aldehydes and ketones to [CpFe(CO)zl+ follows the same behavior as PR3 adducts of [Ti(2,4C7Hii)l. A possible explanation for the counterintuitive Fe-0 bond lengths may lie in repulsive (2c-4e) interactions between the Fp-centered HOMO and the filled n-orbital of the carbonyl group. Fenske-Hall calculations on the isoelectronic [CpFe(PH3)~(vinyl)lsystem have shown that such repulsive interactions are more important than attractive n-backbonding interaction^.^^ The current complexes may be even more sensitive to such interactions, owing t o the lower energy of the [CpFe(CO)zl+HOMO's compared to HOMO's of [CpFe(PH3)21+ and to the polarization of the carbonyl n-bond (causing it t o be 0-centered). The longer Fe-0 bonds observed for the Fp+ ketone (and p-anisaldehyde) adducts compared to those for the p-chlorobenzaldehyde complex could be justified by an increase in the energy of the n-orbital of the carbonyl group and increased repulsive interactions. We cannot conclude at this time, however, how the presence of shortened bond lengths or n-backbonding effects may influence the rates of substitution in [CpFe(CO)2(aldehyde)IPFsor how the reactivity of the bound aldehydes in such complexes may be influenced. These effects may be most important for comparisons among different Lewis acids.29

Conclusions Syntheses of a series of aromatic aldehyde complexes of the organometallic Lewis acid [CpFe(CO)# have revealed that even with n-acidic aromatic aldehydes the organic carbonyl groups display only a-binding modes. No evidence for the alternative side-bound, or n-bound, isomers was detected by lH NMR and FTIR studies. We have further obtained the single-crystalX-ray structures of two of these species, [CpFe(CO)z(p-chlorobenzaldehyde)lPFs and [CpFe(C0)2@-anisaldehyde)lSbFs, which clearly demonstrate that binding occurs exclusively through the oxygen atom of the carbonyl group. These structures reveal shortened Fe-0 bond distances that could be rationalized by the presence of n-backbonding between the iron complex and the aldehydes. Shortened Fe-0 bond distances in these complexes do not appear to result in increased Fe-0 bond strengths, however, and suggest that repulsive (2c-4e) interactions between the filled iron-centered HOMO and the n-orbital of the carbonyl group may be equally or more influential than n-backbonding in dictating Fe-0 distances.

tent with expectations that the more electron-rich carbonyls should serve as better ligands for the organometallic Lewis acid. Recently the commonly held belief that shorter bonds always reflect stronger bonds has been ~ h a l l e n g e d . ~ ~ Acknowledgment. We gratefully acknowledge supErnst and co-workershave discovered that P& (X = Me, port for this work by a CWRU Research Initiation OMe, F) adducts of the open titanocene Ti(2,4-C,HlJ Grant, crystallographic assistance by Rebecca Zaniew(C7Hll = dimethylpentadienyl) display titaniumski, and an insightful reviewer for bringing ref 46 to phosphorus bond lengths that decrease across this our attention. series, while at the same time the titanium-phosphorus bond strengths decrease. nBack-bonding would be Supporting Information Available: Structural diagrams expected to lead to shorter bonds, but for these comand tables of all bond distances and angles, anisotropic plexes the differing electronegativities of the ligand thermal parameters, and H atom positional parameters for 6 substituents apparently lead to differing orbital contriand 8 (10pages). Ordering information is given on any current butions or extensions for the Ti-P bond. The data in masthead page. Table 7 clearly indicate that the more strongly held OM9503959 ketones have the longer Fe-0 bonds, while the weaker (45) Ernst, R. D.; Freeman, J. W.; Stahl, L.; Wilson, D. R.; k i f , A. M.; Nuber, B.; Ziegler, M. L. J. Am. Chem. SOC.1995,117, 5075.

(46)Kostic, N. M.; Fenske, R. F. Organometallics 1982, I , 974. (47)This work.