1718
and suggests bonding of an analogous natures2l s Z 2 Assuming this to be true, the metal ion should be in a tripositive oxidation state and should supply electrons into conduction bands. In this phase, each neodymium ion could presumably donate one-half electron to the conduction band, assuming all carbon units exist as C4- ions and the stoichiometry is Nd403C. The silver-gray color of the phase is also indicative of (21) N. N. Greenwood and A. J. Osborn, J . Chem. Soc., 1775 (1961). (22) R. C. Vickery, R. Sedlacek, and A. Ruben, ibid., 498 (1959).
conduction band occupation, but further conductivity and magnetic studies are needed before definite statements can be made about the bonding. Acknowledgment. The partial support of the U. S. Atomic Energy Commission (COO-716-030) is gratefully acknowledged. The help of D. Cromer, Los Alamos Scientific Laboratory (program, Antifact), Dow Chemical Co., Midland, Mich. (X-ray diffractometer time), and J. A. Steward (undergraduate assistant) are appreciated.
Crystal and Molecular Structures of Methoxytetraphenylantimony and Dimethoxytriphenylantimony Kei-wei Shen,'" William E. McEwen,'" Sam J. La Placa,Ib Walter C. Hamilton,'b and Alfred P. Wolflb
Contribution from the Chemistry Departments, University of Massachusetts, Amherst, Massachusetts 01002, and Brookhaven National Laboratory, Upton, New York 11973. Received September 12, 1967. Abstract: The structures of methoxytetraphenylantimony, (C6H&SbOCH3,and dimethoxytriphenylantimony,
(C6HS)3Sb(OCH3)2, have been determined from three-dimensional single crystal X-ray diffraction counter data. Both compounds are monomeric and contain pentacoordinated antimony atoms. Methoxytetraphenylantimony crystallizes with eight molecules in spaFe group Pbca .Of the orthorhombic system with cell dimensions of a = 14.81 (2), b = 16.95 (3), c = 16.74 (2) A ; V = 4202 A3. The conformation of the organic ligands around the antimony atom is that of a trigonal bipy;amid with the methoxyl group occupying one 04 the apical positions. The Sb-0 bond distance is 2.061 (7) A. The apical Sb-C bond distance qf 2.199 (14) A is significantly longe: than the equatorial Sb-C distances of 2.131 ( l l ) , 2.128 (13), and 2.097 (15) A. The Sb atom is displaced 0.10 A out of the equatorial plane toward the apical phenyl group. Dimethoxytriphenylantimony crystallizes with four molecules inospacegroup P24c of the monocljnic system with cell dimensions of a = 11.51 (2), b = 9.40 (2), c = 17.30 (3) A ; p = 101.75 (1)'; V = 1832 A3. The trigonal bipyramidal conformation around the antimony atom has a carbon atom from each of the three benzene rings in the equatorial plane with Sb-C distances of 2.119 (lo), 2.121 ( l l ) , and 2.119 (12) A. The methoxyl groups occupy both apical positions with Sb-0 bond distances of 2.039 (8) and 2.027 (8) A. The conventional crystallographic R factors are respectively 2.8 and 3 . 4 z . The nmr spectrum of methoxytetra-p-tolylantimony has been taken and a single sharp peak was observed for the p-methyl protons. This result suggests that this compound exists in solution either as a rapidly established equilibrium mixture of trigonal-bipyramidal and square-pyramidal configurations or as the latter configuration only.
M
ethoxytetraphenylantimony and dimethoxytriphenylantimony have been obtained by Briles and McEwen2 by reaction of tetraphenylstibonium bromide with sodium methoxide in methanol solution. The preparation of the latter compound by this method is of particular interest in that it represents a clear case of a nucleophilic substitution reaction at antimony (C6Hs)Sb+,Br-
+ NaOCH, +NaBr + CHBOH
(CBH~~S~O ---f CH~
(C6H&Sb(OCH&
+ CsHe
Although the subject of pentacoordinate species has, in recent years, received considerable attention, relatively few s t r ~ c t u r e s ~ -of' ~ organic antimony(V) com(1) (a) University of Massachusetts; (b) Brookhaven National Laboratory. (2) (a) G. H. Briles and W. E. McEwen, Tetrahedron Letters, 5191 (1966); ( b ) G. H. Briles, Ph.D. Thesis, University of Massachusetts, 1965. (3) E. L. Muetterties and I,SbOCH3)has been synthesized, and the nmr spectrum of this compound exhibits only a single sharp peak at 6 2.35 ppm for the p-methyl protons even at -60”. This result suggests that this compound exists in solution either as a rapidly established equilibrium mixture of trigonal-bipyramidal and squarepyramidal configurations or as the latter configuration only.z7 Tracer studies and dipole moment measurements presently under investigation hopefully will shed further light on the stereochemistry of these and other organoantimony(V) compounds in solution. Dimethoxytriphenylantimony. The crystal structure described by the space group, the parameters of Table IV, and cell parameters indicates the packing of monomeric molecules containing pentacoordinated Sb atoms (Figure 3). The molecular structure, shown in Figure 4, is a trigonal bipyramid with two oxygen atoms occupying the apical positions and with the benzene ring carbon atoms in the basal plane. The intramolecular bond distances and angles are given in Table VI. The (26) D. Hellwinkel [Angew. Chem. Intern. Ed. Engl., 5 , 725 (1966)] has suggested that pentaphenylantimony in solution might undergo rapid transformation between the two states of pentaphenylantimony. (27) There is no evidence presently available to indicate whether the methyl hydrogen atoms of a p-tolyl group located in a basal position of a trigonal bipyramidal molecule will have a different chemical shift in the nmr spectrum from the methyl hydrogens of a p-tolyl group located in an apical position. Experiments designed to clarify this point are presently in progress.
/ 90:7 / March 27, 1968
1723 Table VI. Intramolecular Angles and Distances for (CsHs)aSb(OCHa)r
.
Intramolecular angle, deg 01-sb-02 c3-sb-c9 c3-sb-c15 C9-Sb-Cl5 01-Sb-C3 01-Sb-C9 01-Sb-C15 0 2 --Sb-C3 02-Sb-C9 02-SbC15 sb-01-c1 sb-02-c2
175.3 (3) sb-c3-c4 124.0 ( 5 ) Sb-C3-C8 121.4 ( 5 ) C4C3-C8 114.6 (4) c3-c4-c5 85.5 (4) C4-C5-C6 93.4 (4) c5-c6-c7 92.2 (4) C6-C7-C8 9 1 . 5 (4) C7-C8-C3 8 5 . 6 ( 4 ) Sb-C9-C10 92.1 (4) Sb-C9-C14 122.5 (6) c 16 c 9 - C l 4 120.7 (6) c9-Clo-c 11
121.5(1.1) 119.7 (9) 118.8 (1.0) 118.9 (1.2) 122.0 (1.1) 119.4 (1.2) 119.1 (1.1) 121.7 (1.0) 119.8 (1.0) 121.6 ( 1 .O) 118.5(1.0) 118.7(1.1)
Clo-Cll-Cl2 C1 l-Cl2-Cl3 C12-Cl3-Cl4 C13-Cl4C9 Sb-Cl S C 1 6 Sb-C 15-C20 c16 C 15-c20 C15-Cl6-Cl7 C16C17-Cl8 C17-C 18-C19 C18-C19-C20 C19-C2O-C15
Table VII. Phenyl Group Least-Squares Planes" Carbon
atom nos. in
plane
A
C
D
1.527 16.132 -10.538 13.647
3.873 1.750 -0.250 0.250
B
Compound I 2-7 8-1 3 1419 2625
7.514 2.837 10.760 6.256
3-8 9-14 15-20
8.408 5.448 7.204
14.437 3.158 -4.643 -6.683
Compound I1 -6.145 7.478 -0.924
a Of the form A x f By + Cz - D fractional coordinates of the atoms.
-5.919 -8.078 -15.312
-4.382 -1.382 -4.982
= 0 where x, y , and z are the
Sb, Cs, C9, and Clg atoms are coplanar; the best leastsquares plane through these four atoms has the equation 11.251X 1.731Y - 1.7012 2.709 = 0, and
+
+
120.9 (8) 120.1 (1.3) 121.8(1.2) 120.1 (1.1) 119.4 (9) 121.4 (9) 119.2 (1.0) 120.6(1.0) 121.8 (1.1) 117.3 (1.2) 122.1 (1.1) 119.1 (1.1)
distance, A-
--Intramolecular Sb-01 sb-02 01-01 02-c2 sb-c3 sb-c9 Sb-CIS c3-c4 c4c5 C5-C6 C6C7 C7-C8 C8-C3
2.039 (8) 2.027 (8) 1.429 (12) 1.411 (13) 2.119 (10) 2.121 (11) 2.119 (12) 1.366 (14) 1.371 (16) 1.363 (17) 1.335 (16) 1.395 (15) 1.353 114)
C9-C10 Clo-c11 C11-Cl2 C12-Cl3 C13-Cl4 C 14-C9 C15-Cl6 C16-Cl7 C17-Cl8 C18-C19 C19-C20 C26C15
1.388 (14) 1.390 (18) 1.344 (16) 1.327 (16) 1.355 (16) 1.380 (14) 1.395 (14) 1.376(15) 1.356 (16) 1.399 (17) 1.394 (17) 1.366 (14)
the oatomic displacements [rom this plane are 0.0001 (7) A for Sb and -0.01 (1) A for Cp, Ca, and CIS.The slight distortion of the 01-Sb-02 angle to 175.3 (3)" may plausibly be the result of packing requirements; i.e., the six oxygen nearest o-phenyl hydrogen sontacts are at distances which range from 2.45 to 2.67 A. The coefficients of the least-squares plane through the carbon atoms for each phenyl ring of both compounds are listed in Table VII. The maximum displacement from these planes is 0.018 (18) .$, with the average being about half this value. The nmr spectrum2 of this compound is in agreement with the molecular structure determined here. Acknowledgment. This research was supported in part by the U. S . Atomic Energy Commission and in part by the National Science Foundation. One of us (K. S . ) also acknowledges support in the form of a University Fellowship from the University of Massachusetts.
Stereoselectivity in the Metal-Complex-Catalyzed Hydrolysis of Amino Acid Esters'" James E. Hix, Jr.,lb and Mark M. Jones
Contributionfrom the Department of Chemistry, Vanderbilt University, Nashuille, Tennessee 37203. Received September 18, 1967 Abstract: The hydrolysis of (I?)-( -)- and (S)-(+)-histidine methyl esters in the presence of the catalytically active complexes Ni((R)-(-)-histidinate)+andNi((S)-( +)-histidinate)+ has been examined in detail. The rate of hydrolysis of the (I?)-( -) ester is greater in the presence of the Ni((S)-(+)-histidinate)+ ion than in the presence of Ni((I?)-(+)histidinate)+, and these rate differences are mirrored in the behavior of the (S)-(+)ester. The system with opposite ester-histidine configurations has an observed rate constant approximately 40 greater than that found with identical ester-histidine configurations. A more detailed evaluation of the experimental data indicates that the stereoselectivity results from differences in the specific rates of hydrolysis for the coordinated ligands, arising from differences in the degree of interaction between the ester carbonyl group and the coordination center. Differences in the stability constants of the species involved are not believed to play an important role in the stereoselectivity.
I
In these instances, coordination of the ester to a metal ion makes it more susceptible to attack by the nucleo-
(1) (a) Abstracted from the thesis submitted by J. E. Hix, Jr., to Vanderbilt University in partial fulfillment of the requirements for the
degree of Doc tor of Philosophy. (b) This investigation was supported by National Institutes of Health Predoctoral Fellowship No. 5-F1GM-20,537 for which J. E. Hix, Jr., wishes to express sincere thanks.
t has previously been shown in various studies that several metal ions and their complexes are effective catalysts for the hydrolysis of amino acid esters.2-6
Hix, Jones
1 Hydrolysis of Amino Acid Esters
