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Journal of the American Chemical Society
and J. I. Brauman, Trans. Faraday Soc., 65, 464 (1969). Zeev 9. Alfassi, Sidney W. Benson, and David M. Golden, J. Am. Chem.
Soc., 95, 4784 (1973). (7) (a)A. s. Gordon and s. R. Smith, J. them. mYS., 34,331 (1961); (b) J. phys. Chem., 66, 521 (1962). (8) S. E. Stein and 6. S. Rabinovitch. J. Phys. Chem., 79, 191 (1975). (9) (a)D.M. Goldenand S. W. Benson, Chem. Rev.. 69, 125(1969); (b)J. D. COX and G. Pilcher, “Thermochemistry of Organic and Organometallic Compounds”, Academic Press, New York. 1970.
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(a)R. J. Ellis and H. M. Fray, J. Chem. Soc. A, 553 (1966); (b) S. W. Benson and R. Shaw, Trans. Faraday Soc., 63, 985 (1967). (1 1) D. C. Tardy, J. Am. Chem. SOC.,97, 5695 (1975). (12) D. C. Tardy, A. S. Gordon and W. P. Norris, J. Phys. Chem., 60, 1398
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(13) John E. Baldwin, Tetrahedron Lett., 2953 (1966). (14) K. D. King, Int. J. Chem. Kinet., 10, 117 (1978). (15) (a)M. L. Boyd, T. M. Wu, and M. H. Back, Can. J. Chem., 46, 2415 (1968); (b) M. L. Boyd and M. H. Back, ibid., 46, 2427 (1968).
Synthesis and Reactivity of Coordinated Imines Derived from 2-Keto Acids J. MacB. Harrowfield and A. M. Sargeson” Contribution from the Research School of Chemistry, The Australian National UniGersity, Canberra, A.C.T., Australia 2600. Received May 26, I978
Abstract: The synthesis of a series of [Co(NH3)5OCOCORI2+ ions ( R = H, CH3, C6Hj, C O O H , -CHzCsHs, C(CH3)3, CH2CH2C02-) and a number of derivatives is described. In aqueous base most of these ions undergo cyclization to tetraammine iminocarboxylato chelates which undergo a variety of reactions: alkylation of the deprotonated imine N center, addition of nucleophiles at the imine C center, reduction of the imine to give an amino acid, and intramolecular condensations after the addition of the nucleophile. The compounds illustrate the potential of the metal ion to protect and activate organic molecules as well as to organize such intramolecular condensations. The kinetics of the imine cyclization are also discussed.
Introduction The imine functional group, >C=N-, plays a near-ubiquitous role in synthetic organic, biological, and coordination chemistry. Nonetheless, while there is a vast literature concerning the effects of Schiff base ligands on metal ion properties, only for complexes of Schiff bases derived from pyridoxal has there been any systematic and thorough investigation of the properties of imine containing ligands as affected by a metal ion.’ The reactivity of simple coordinated imines is essentially unknown. Further, while an extremely large number of so-called “template” reactions2 involve imine formation, no detailed study of the kinetics of genuine intramolecular condensations is available. It might well be anticipated from the behavior of related systems3 that the (carbinolamine) addition intermediate involved in imine formation would be stabilized by metal ion coordination and that the kinetics of its formation and decomposition would therefore be amenable to investigation. It is frequently difficult to detect and therefore to understand factors controlling the stability of carbinolamine intermediates in simple organic system^.^ In preliminary publications5 we have outlined the synthesis and some aspects of the chemistry of simple cobalt(lI1) imine complexes and in related work detailed studies of the kinetics of genuine intramolecular imine formation with cobalt( 111) have been made.6 The present work describes the synthesis, formation kinetics and basic physical and chemical properties of a variety of cyiminocarboxylate complexes of cobalt(ll1). Experimental Section pH measurements were made under nitrogen at 25 OC using a Radiometer PHM26 meter with G202B glass electrode. Infrared spectra of KBr disk, Nujol mull (NaCI plates), and solution (DzO, CaF2 cell) samples were recorded with a Perkin-Elmer Model 337 instrument. Visible spectra and slower reaction rates were measured with a Cary-Varian 118C UV-visible spectrophotometer while rcx t i o n rates corresponding to half-lives of C=O), + I 736 (COzH, hydrate form), +I576 (C02H. keto form), +394 (>C(OH)*), -597 (CH3, keto), -619 Hz (CH3, hydrate) (assigninents tentative). Visible spectrum ,,A,(, e,,,,, in 1 M HCI): 500 nm, 71.5 M-I cm-1. Anal. Calcd for [Co(NH3)50COCOC6H5](clo4)2: c o , 11.98: c, 19.53; H, 4.10; N , 14.23; CI, 14.41. Found: Co, 12.0; C , 19.6; H, 4.2; N, l 4 , I ; C l , 14.2. I3C N M R : +2880 (>C=O), +2751 (COzH, approximately twice the intensity of the 2880-Hz resonance), +2148, 2087, 2068, 20.53 M DCI. Hz (CbH5 group) relative to dioxane in in 1 M HCI) 498 nm, 77.1 M - ’ Visible spectrum (A.,, cm-1. Anal. Calcd for [CO(NH~)~OCOCOC(CH~)~](CIO~)~: Co, 12.48;
0 1979 American Chemical Society
Harrowfield, Sargeson
/ Coordinated Imines Derived from 2-Keto Acids
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C, 15.26;H,5.12;N, 14.83;CI, 15.02.Found:Co, 12.4;C, 15.3;H, of the whole initial reaction mixture provided no evidence for more than one isomer in the product. 5.2; N, 14.9; CI. 14.9. Anal. Calcd for C O C ~ H ~ ~ N ~ O ~ C Co, LH 14.53; ~ O :C, 14.81; H, tmax in 1 M HCI) 502 nm, 73.7 M-' Visible spectrum ,,A,(, 5.22;N, 17.27;C1,8.74.Found:Co, 14.3;C, 14.6;H,5.1;N, 16.8;C1, cm-I. 9.2. Preparations of other [(NH3)5Co02C.CORJ2+complexes involved Visible spectrum (Amax, tmax):500 nm, 69.2 M-' cm-I. sufficient modifications of this procedure to justify more detailed 2. Formation of Imine Chelate Tetraamminecobalt(II1) Complexes. description. Conversion of the ketocarboxylatopentaammine complexes to the [(NH~)&oO~CC(OH)~CO~]CI. The principal difficulty associated imine chelate tetraammine species was generally achieved by reacting with the preparation of this complex was its isolation from the prefor 30swithaslight excess (-1.I:l molar ratio) ofdilute parative mixture. The protonated form, [ ( N H ~ ) ~ C O O ~ C . C ( O H ) ~ thecomplex . aqueous NaOH. Isolation of the products presented various minor C 0 2 H I 2 + , is extremely soluble in water (as CI- or ClO4- salts), problems except for the phenylglyoxylate and trimethylpyruvate dewhereas the deprotonated form, though readily precipitated with CIrivatives, where quenching of the reaction mixture with concentrated or Clod-, is present at a pH where excess sodium ketomalonate is also HC104 led to ready precipitation of perchlorate salts, and for the readily deposited. I n addition, several minor complex ion species ketomalonate derivative, where quenching with concentrated HCI contaminated the desired product if isolated by direct precipitation provided the slightly soluble CI- salt. (Note that this last imine chelate and hence it was simplest to isolate it by cation exchange chromais so acidic that it is essential to add a t least a 10% excess of N a O H tography. Sodium ketomalonate (25 g) was added in portions to a hot to ensure complete cyclization of the pentaammine precursor within (80 "C) solution of [(NH3)5CoOH2](C104)3(32 g) and HC104 (70%, 30 s. This is not so crucial a requirement for the other species, which 12.5 mL) in H2O (300 mL), the mixture being well stirred to ensure also cyclize more rapidly.) rapid dissolution. The final solution was heated on a steam bath for Anal. Calcd for [Co(N H ~ ) ~ ( N H = ~ ( ~ ~ ~Co,~ ) ~ ~ ~ ) ] 2 h and cooled and a small amount of red precipitate (apparently 12.43; C, 20.27; H, 3.83; N, 14.77; CI, 14.9. Found: Co, 12.4; C , 20.6; [ ( N H ~ ) ~ C O C ~ O ~ H ] ( Cfiltered I O ~ )off. ~ ) The filtrate was diluted to H, 4.2; N , 14.9; CI, 14.7. -3 L and absorbed on a large column (approximately 30 X I2 cm ' H NMR:62.82(3 H,NH3),3.26(6H,2"3),3.86(3H,NH3), diameter) of Li+ form S P Sephadex C25 resin. The column was -7.46; 7.90 (5 H, multiplets, C6H5), 12.65 (1 H, =NH) in Me2SO-d6 washed well with water and eluted with 0.2 M LiCl to reveal at least acidified with a trace of D2SO4, Me4Si reference. seven components. The major (red) band, which was preceded by two IR: UC=N 1680 cm-' (N-deuterated complex). very weak, pink bands, was collected and the eluate volume reduced Visible spectrum ,,A,(, emax, in 1 M HCI): 480 nm, 94.0 M-' to -50 mL on a rotary evaporator a t water pump pressure. Pyridine cm-1. was then added dropwise until the pH of the solution rose to apAnal. Calcd for [Co(N H3)4N H=C(C(CH3)3)COO](ClO4)2: Co, proximately 5, at which stage rapid deposition of red crystals began. 12.98;C, 15.87;H,4.88;N,15.42;CI, 15.61. Found:Co, 12.8;C, 15.9; After the mixture was cooled for 30 min on ice, the crystals were filH, 4.8; N, 15.2; CI, 15.6. tered off and washed with methanol and ether (yield 10 g). The ' H NMR: 6 2.80 (3 H, NH3), 3.07 (6 H, 2 NH3), 3.67 (3 H, NH3), complex was recrystallized by adding concentrated HCI to its aqueous 11.37 ( I H, =NH), 1.19 (9 H, C(CH3)l) in MezSO-d6 D2SO4, slurry ( 5 mL/g) until dissolution just occurred, filtering, raising the Me& reference. (NB. = N H resonance is rather sensitive to the pH to 5 with pyridine, and cooling the solution on ice. The perchlorate amount of D2S04 in the Me2SO-d6. Apparent differences between salt was obtained by metathesis with NaC104 i n aqueous solution. complexes may therefore be due largely to this effect). Anal. Calcd for C O C ~ H ~ ~ N ~ O ~ CCo, I - 18.79; ~ H ~CO, 11.49; : H, I R: UC=N 1680 cm-'. 5.46; N, 22.33; CI, 11.31. Found: Co, 18.3; C , 11.3; H, 5.9; N. 21.3; t,,, in 1 M HCI): 483 nm, 80.5 M-I Visible spectrum ,,A,(, CI. 11.8. cm-'. Visible spectrum ,,A,(, f m a x in I M HCI): 501 nm, 70.3 M-' Anal. Calcd for C O C ~ H ~ ~ N ~ C I OC, ~ -11.40; ~ H ~H, O :5.46; N , cm-1. 22.33. Found: C , 11.7; H, 5.8; N , 22.3. H N MR: Ammine group proton resonances in dilute DCI at 8 2.9 ' H NMR:62.94(3 H,NH3),3.38(6H,2"3),3.98(3 H,NH3), ( 3 H, NH3 trans to 0) and 3.9 (12 H, 4NH3 cis to 0) typify the 13.80 ( I H, =NH) in Me2SO-d3 + D2S04, Me4Si reference. present complexes and are as expectedI3 for carboxylatopentaamIR: UC=N 1680 cm-'. minecobalt(lll) species. c,,,~,): 484 nm, 92.9 M-' cm-l ( 1 M HCI); Visible spectrum,,,A(, [ ( N H ~ ) ~ C O O ~ C C O . C H ~ C ~ H ~A] (slurry C I O of ~ ) C6H ~ . 5C H 2CO. 486 nm, 85. I M-' cm-I (H20). COzH (9 g) and [(NH3)5CoOH](C104)2 (7 g) in water (100 mL) was The pyruvate imine complex could also be directly precipitated from heated on a steam bath for 90 min. NaC104 ( I O g) was dissolved in the quenched reaction mixture as its clod- salt, but in the concenthe hot solution, which was then cooled on ice for 30 min. The pink trated media required for efficient precipitation the base cyclization precipitate formed was filtered off, sucked well dry in air, and twice step led to production of considerable amounts of dimerized product extracted with a mixture of ether (100 mL) and ethanol ( I O mL) to as a contaminant. It was therefore preferable to use very dilute base free the complex of coprecipitated phenylpyruvic acid. The complex (
