5230
Inorg. Chem. 1994,33, 5230-5237
Amino Acid Derivatives of Tungsten Carbonyl. Structure and Reactivity Investigations of Zerovalent Tungsten Glycine Derivatives Donald J. Darensbourg; Earl V. Atnip, Kevin K. Klausmeyer, and Joseph H. Reibenspies Department of Chemistry, Texas A&M University, College Station, Texas 77843
Received May 26, I994@
The amino acid derivatives of tungsten(0) [ E W I [W(CO)~(O~CCH~NHZ)I (11, [ E a ][W(C0)4(02CCH2NHMe)l (2), and [Ea][W(C0)4(02CCH2NMe2)] (3) have been synthesized from the reaction of W(C0)sTHF with the tetraethylammonium salt of the corresponding a-amino acid in a tetrahydrofuran solution. The glycine derivative, complex 1, is highly soluble in water. The complexes have been characterized in solution by infrared and NMR spectroscopies and in the solid state by X-ray crystallography. Complex 1 crystallized in the orthorhombic space group E12121 with a = 10.408(14) A, b = 13.155(11) A, c = 13.657(10) A, V = 1870(3) A3, and dcdc = 1.777 g ~ m - for ~ , Z = 4, from h e x a n e m . Complex 2 crystallized in the monoclinic system, space group P2&, with a = 14.012(5) A, b = 10.335(2) A, c = 14.728(4) A, = 112.28(2)", V = 1973.5(8) A3, and dcdc = 1.731 g ~ m - for ~ , Z = 4, from hexane/THF. Complex 3 crystallized in the triclinic space group Pi with a = 11.586(5) A, b = 11.711(6) A, and c = 15.068(7) A, a = 90.01(4)",3! , = 96.28(4)", y = 90.27(4)', V = 2032(2) A3, and dcdc = 1.727 g ~ m - for ~ , Z = 4, from a hexane-layered THF solution of 3. The geometry of the anion in each case is that of a slightly distorted octahedral arrangement of four carbonyl ligands and a puckered five-membered glycinate chelate ring. Complexes 1 and 2 exihibit intermolecular hydrogen-bonding interactions with N. * .O distances of 2.746 and 2.826 A, respectively. Weaker hydrogen-bonding interactions in solution with THF and CH3CN are noted as well. Although the coordination environments about the tungsten centers of the three glycinate derivatives are essentially the same, the CO lability is greatly enhanced in complexes 1 and 2 as compared to complex 3. This CO labilization in complexes 1 and 2 is attributed to a solvent-assisted removal of a proton from the amine ligand leading to a substitutionally labile amide transient species. Deprotonation of the amine ligand is aided by the n-donation of the thus formed amide ligand to the coordinatively unsaturated metal center in the reactive intermediate.
This study represents part of a systematic examination of the interactions and reactions of zerovalent group 6 metal carboxylates.' Specifically, we are interested in carboxylate ligands which possess additional functional groups for metal binding. These investigations are directed by a desire to better understand transition-metal catalyzed decarboxylation processes? For example, we recently demonstrated that the electrophilic character of the metal center greatly influences the rate of C-C bond cleavage in the decarboxylation of the cyanoacetate ligand when bound via the cyano m ~ i e t y . ~The , ~ skeletal reaction is depicted in Scheme 1 for the decarboxylation of cyanoacetic acid catalyzed in the presence of W(C0)502CCH2CN-, where the site for acid binding is created by the labilizing ability of
Scheme 1 r
L
1-
oII
1
L
II
@Abstractpublished in Advance ACS Abstracts, October 1, 1994. (1) (a) Cotton, F. A.; Darensbourg, D. J.; Kolthammer, B. W. S. J. Am. Chem. SOC. 1981,103, 398. (b) Darensbourg, D. J.; Rokicki, A. J. Am. Chem. SOC. 1982,104, 349. (c) Cotton, F. A.; Darensbourg, D. J.; Kolthammer,B. W. S.; Kudaroski, R. Inorg. Chem. 1982,2I,1656. (d) Darensbourg, D. J.; Kudaroski, R.; Bauch, C.G.; Pala, M.; Simmons, D.; White, J. N. J. Am. Chem. SOC. 1985,107, 7463. (e) Tooley, P. A,; Ovalles, C.; Kao, S. C.; Darensbourg, D. J.; Darensbourg, M.Y.J. Am. Chem. Sac. 1986,108,5465. (f)Darensbourg, D. J.; Wiegreffe, H. P. Inorg. Chem. 1990,29, 592. (g) Darensbourg, D. J.; Joyce, J. A.; Bischoff, C. J.; Reibenspies,J. H. Inorg. Chem. 1991, 30, 1137. (h) Darensbourg, D. J.; Joyce, J.A.; Rheingold, A. Organometallics 1991,10, 3407. (i) Darensbourg, D. J.; Chojnacki, J. A,; Reibenspies, J. H. Inorg. Chem. 1992,31, 3951. (2) Deacon, G. B.; Faulks, S. J.; Pain, G. N. Adv. Organomet. Chem. 1986,25, 237. (3) Darensbourg, D. J.; Longridge, E. M.; Atnip, E. V.; Reibenspies, J. H. Inorg. Chem. 1992,31, 3951. (4) Darensbourg, D. J.: Lonmidge, E. M.: Holtcamu. M. W.; Klausmever, . K. K.; Reib;nspies, J. 6 J.-Am. Chem. SOC. i993, 115, 8839.
0
the bound carboxylate ligand.5 Importantly, Cu(I), being a better electrophile than tungsten(O), is more effective at catalyzing the decarboxylation of cyanoacetic acid using both monomeric and dimeric Cu(1) catalyst precursors. A logical and significant extension of these efforts would be analogous studies involving carboxylate ligands derived from amino acids. Indeed, the process of cleavage of the carboncarbon bond in a-amino acids conforms to the requirement which facilitates the decarboxylation reaction, Le., that the substrate has the ability to stabilize the developing carbanion transition state as the reaction proceeds to products (eq I). Some (5) Darensbourg, D. J.; Chojnacki, J. A.; Atnip, E. V. J. Am. Chem. SOC. 1993,115, 4675.
0020- 166919411333-5230$04.50/0 0 1994 American Chemical Society
Inorganic Chemistry, Vol. 33, No. 23, 1994 5231
Amino Acid Derivatives of Tungsten Carbonyl
- I RTH---=021 r
R-CHCOI
I
AH3+
\
Table 1. Crystallographic Data for Complexes 1-3 1 2
space group
v,A3
z of the amines thus formed by these enzymatic reactions have important physiological effects. For example, Dopa decarboxylase decarboxylates 3,4-dihydroxyphenylalanineto form dopamine, a substance which is an intermediate in the formation of adrenaline.6 The electron-releasing effect of NH2CH2 is greatly diminished upon protonization, as evidenced by the corresponding Taft u* parameter being much more positive in the latter instance, 0.50 vs 2.24.7 Metal binding of the amine group is expected to have a similar electron-withdrawing effect. Although the literature is replete with examples of amino acid derivatives of transition metals in positive oxidation states,* zerovalent or low-valent transition metal derivatives of amino acids, in particular glycine, are extremely rare.g Most structurally characterized examples encompass complexes of R(I1) and Ag(I).l0 In this paper we describe the synthesis of a series of glycine derivatives of tungsten carbonyl. The structures of these derivatives have been fully delineated both in solution via infrared and 13C/lH NMR spectroscopies and in the solid state by X-ray crystallography. Furthermore, the reactivity of these derivatives toward carbon monoxide dissociation is examined as a function of the nature of the glycinate ligand.
Experimental Section Methods and Materials. All manipulations were performed on a double-manifold Schlenk vacuum line under an atmosphere of argon or in an argon-filled glovebox. Solvents were dried and deoxygenated by distillation from the appropriate reagent under a nitrogen atmosphere. Photolysis experiments were performed using a mercury arc 450-W W immersion lamp purchased from Ace Glass Co. Infrared spectra were recorded on a Mattson 6021 spectrometer with DTGS and MCT detectors. Routine infrared spectra were collected using a 0.10-mm CaFz cell. I3C NMR spectra were obtained on a Varian XL-200 spectrometer. 13C0was purchased from Cambridge Isotopes and used as received. W(CO)6 was purchased from Strem Chemicals, Inc., and used without further purification. Microanalyses were performed by Galbraith Laboratories, Inc., Knoxville, TN. Synthesis of [E@J][O*CCHfiRR’] Salts. The synthesis of [Et4N][OzCCHzNRR’] was accomplished by the reaction of the corresponding a-amino acid, NRR’CCHzCOOH with one equivalent of E m O H in methanol, where R = H or Me and R’ = H or Me. The mixture was stirred for 60 min, and the methanol was removed under vacuum, leaving a white solid product. ( 6 ) Meister, A. Biochemistry of the Amino Acids, 2nd ed.;New York,
Academic Press: 1965. (7) (a) Taft, R. J. Am. Chem. SOC.1952, 74,3120. (b) Values taken from: Lunge’s Handbook, 12th ed.; Dean, J. A., Ed.; McGraw-Hill: New York, 1979. (8) Laurie, S. H. In Comprehensive Coordination Chemistry; Wilkinson, G.. Gillard. R. D.. McClevertv. J. A.. EMS.:. Pereamon: Oxford. England, 1987; Vol. 2, Chaptei20. (9) (a) Petri, W.: Beck, W. Chem. Eer. 1984, I 1 7, 3265. (b) Meder, H.J.; Beck, W. Z. Natulforsch. 1986, 418, 1247. (c) Grotjahn, D. B.; Gray, T. L. J . Am. Chem. SOC.1994, 116, 6969. (10) (a) Baidima, I. A.; Pcdberezskaya, N. V.; Krylova, L. F.; Borisov, S. V.; Bakakin, V. V. Zh. Strukt. Khim. 1980, 21, 106. (b) Pozhidaev, A. I.; Simonov, M.A.; Kruglik, A. I.; Shestakova,N. A.; Mal’chikov, G. D. Zh. Strukt. Khim. 1975,16, 1080. (c) Freeman, H. C.; Golomb, M. L. Acta Crystallogr., Sect. E 1969, 25, 1203. (d) Iakovidis, A.; Hadjiliadis, N.; Schollhom,H.; Thewalt, U.; Trotscher, G. Inorg. Chim. Actu 1989,164,221.(e) Erickson, L.E.; Jones, G . S.;Blanchard, J.L.; Ahmed, K. J. Inorg. Chem. 1991,30,3147. (f)Aclard, C. B.; Freeman, H. C. J. Chem. SOC.D.1971, 1016. (g) Rao, J.K.M.; Viswamitra, M. A. Acta Crystallogr., Sect. E 1972, 28, 1484.
dcalc,dcm3 a , 8,
b, 8, C,
A
a , deg B. deg Y deg T, K p(Mo Ka), mm-I wavelength, 8, RF,” % R~F,% ‘
P212121 1870(3) 4 1.777 10.408(14) 13.155(11) 13.657(10)
P21Ic
1973.5(8) 4 1.731 14.012(5) 10.335(2) 14.728(4) 112.28(2)
I
293.0 11.711 (Cu Ka) 1.541 78 7.09 18.73b
193 6.007 0.710 73 4.06 3.83
3
pi 2032(2) 4 1.727 11.586(5) 11.71l(6) 15.068(7) 90.01(4) 96.28(4) 90.27(4) 193 5.715 0.710 73 5.21 12.34b
‘ R F = zlF0 - FcIEFo and R w p = {[zw(F0- Fc)zl/(zwFo2)~In. = ([Cw(Fo2- Fc2)2]/(~wFo2)}in.
RWF
Synthesis of [Et&‘][W(CO)~(02CCHfiRR’)]Derivatives. The synthesis of [ E m ][W(C0)4(02CCH$RR’)] was accomplished on a 1.5 mmol scale in greater than 90% yield by the reaction of w(co)5THF (prepared by photolysis of W(CO)6in THF) with 1 equiv of [E@l][O2CCH2NRR’] at ambient temperature, where R = H or Me and R’ = H or Me. The THF was removed from the reaction mixture by vacuum, and the remaining light yellow solid was washed with hexane three times to afford a bright yellow powder. Anal. Calcd for [Et.+N][W(C0)4(02CCHzNHz)] (Ci4Ha206W): C, 33.62; H, 4.84. Found: C, 32.83; H, 4.60. Anal. Calcd for [E~N][W(CO)~(OZCCHZNHMe)] ( C I ~ H ~ ~ N ~ OC,~ 35.04; W ) : H, 5.10. Found: C, 35.48; H, 5.51. Calcd for [W(C0)4(0zCCHzNMez)l (C16Hz8Nz06W): c , 36.38; H, 5.34. Found: C, 35.74; H, 5.49. X-ray Crystallography of [Et4N][W(C0)4(0*CCHfiRR’)] Derivatives. Crystal data and details of data collection are given in Table 1. A light yellow-green needle crystal of 1 was mounted in a glass capillary at room temperature. A yellow parallelpiped crystal of 2 and an orange block crystal of 3 were mounted on a glass fiber with epoxy cement at room temperature and cooled to 193 K in a Nz cold stream. Preliminary examination and data collection were performed on a Rigaku AFC5R X-ray diffractometer (CuKa, d = 1.54178 A radiation) for 1 and a Nicolet R 3 d v X-ray diffractometer (Mo Ka, 1 = 0.710 73 8, radiation) for 2 and 3. Cell parameters were calculated from the least-squares fitting of the setting angles for 24 reflections. w scans for several intense reflections indicated acceptable crystal quality. Data were collected for 4.0” I 28 I 50.0”. Three control reflections, collected every 97 reflections, showed no significant trends. Background measurements by stationary-crystal and stationary-counter techniques were taken at the beginning and end of each scan for half of the total scan time. Lorentz and polarization corrections were applied to 2186 reflections for 1, 3852 for 2, and 7452 for 3. An empirical absorption correction was applied. A total of 2030 unique reflections for 1, 2822 for 2, and 4994 for 3, with 111 2 2.Oa(I), were used in further calculations. All three structures were solved by direct methods [SHELXS, SHELXTL-PLUS program package, Sheldrick (1988)l. Fullmatrix least-squares anisotropic refinement for all non-hydrogen atoms yielded R = 0.071, R, = 0.187, and S = 1.175 at convergence for 1, R = 0.041, R, = 0.038, and S = 2.86 for 2, and R = 0.051, R, = 0.123, and S = 1.018 for 3. Hydrogen atoms were placed in idealized positions with isotropic thermal parameters fixed at 0.08. Neutralatom scattering factors and anomalous scattering correction terms were taken from Intemational Tables for X-ray Crystallography. Kinetic Measurements. The rates of 13C0 exchange with complexes 1 and 2 were determined by placing an acetonitrile solution of the correspondingcomplex (0.10 mmol in 20 mL) under an atmosphere of 13C0(’90% in 13C). The solution was situated in a thennostated water bath at 40.0 ‘C (& 0.1’) and shaken periodically, and the exchange process was monitored by observing the decrease in absorbance of the highest frequency v(C0) band of the starting complex as a function of time.
5232 Inorganic Chemistry, Vol. 33, No. 23, 1994
Darensbourg et al.
Table 2. Carbonyl Stretching Frequencies of W(C0)4,5(NRR'CH~C02)-Derivatives complex" W(CO)5(NHzCHzCOz)W(CO)s(NHMeCHZC02)W(C0)5(02CCH2NHMe)W(CO)~(O~CCHZNM~~)W(C0)4(OzCCHzNHz)W(C0)4(OzCCH2NHMe)W(C0)4(02CCHzNMe2)-
2066 (w) 2068 (w) 2060 (w) 2057 (w) 1996 (w) 1996 (w) 1997 (w)
1921 (vs) 1923 (vs) 1910 (vs) 1908 (vs) 1859 (vs) 1859 (vs) 1859 (vs)
1860 (m)b,c 1868 (m)b,c 1852 (mPd 1852 (m)c,d 1849 (sh) 1847 (sh) 1846 (sh)
1795 (my 1798 (my 1801 (my
1587
1457
1640 1642 1642 1638 1644
1362 1367 1364
As the tetraethylammonium salt. Nitrogen-bound complex. Spectra determined in THF. Oxygen-bound complex. e Spectra determined in CH3CN.
Table 3. I3C NMR Data for the Carbonyl and Carboxylate Ligands in the W(C0)4(0zCCHzNRR')- Anions" I3C resonance, ppmc complexb CO(axia1) CO(trans to N) CO(trans to 0 ) 216.0 216.8 W(C0)4(0zCCHzNHz)205.5 W(C0)4(OzCCH2NHMe)205.3, 206.4 216.6 216.8 W(CO)~(OZCCHZNM~~)206.3 216.9 217.0
coz180.5 179.1 177.9
Spectra determined in acetonitriled3. As the tetraethylammonium salt. The assignments of the I3Csignals for CO groups trans to N vs those trans to 0 are based on the general observation that the I3C resonance for a CO group trans to N is upfield relative to a CO group trans to 0 in W(CO)5L derivatives. (See e.g.: Todd, L. J.; Wilkinson, J. R. J. Organomet. Chem. 1974, 77, 1. Reference If.) Nevertheless, in this instance the signals are quite close in value and an absolute distinction is not necessary at this time.
Scheme 2 RR'NCH2COOH+ EtpOH W(CO), + THF
L
A W(C0l5THF
W(C0)4aminate)argon SW2e.J
co
W(CO)&aminate)A similar kinetic run involving complex 1 was carried out exactly as described above in the presence of 50 p L of distilled, deionized, degassed H20. The rate of CO ligand exchange was only slightly enhanced in this instance (
