Metal alkoxides: models for metal oxides. 16. Synthesis and

Malcolm H. Chisholm , Judith C. Gallucci , Carl B. Hollandsworth ... Malcolm H. Chisholm , Victor Johnston , James D. Martin , Emil Lobkovsky , John C...
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J . Am. Chem. SOC.1989, 111, 5300-5312

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Metal Alkoxides: Models for Metal Oxides. 16.' Synthesis and Characterization of a New Class of Homoleptic 12-Electron Molybdenum and Tungsten Alkoxide Clusters of Formula M4(OR)12: Structural Characterization of M o ~O( C H ~ - C - B U )H~o~c*H( 2 - c - B ~ ) M. H. Chisholm,**+K. Folting,' C. E. Hammond,+M. J. Hampden-Smith,+and K. G . Moodleyf Contribution from the Department of Chemistry a n d Moiecular S t r u c t u r e Center, Indiana University, Bloomington, Indiana 47405, a n d Department of Chemistry, Unicersity of Durban- Westuille, P/B X54001,Durban, S o u t h Africa. Received May 26, 1988

Abstract: Addition of primary alcohols, R C H 2 0 H (>6 equiv), to hydrocarbon solutions of M , ( O - ~ - B Ucompounds )~ has led to the isolation of the first homoleptic series of cluster alkoxides M4(OCH2R)12for both M = Mo and W where R = i-Pr, Cy (Cy = cyclohexyl), Cp (Cp = cyclopentyl), and c-Bu (c-Bu = cyclobutyl). The N M R data (IH and "C) indicate a common structural unit for this class of compounds containing a molecular mirror plane. For each M4(OCH2R)12compound there are eight types of alkoxide ligands in the integral intensity 2:2:2:2:1: 1: 1: I . Those having integral intensity two possess diastereotopic methylene protons, indicating that they do not lie on the mirror plane, whereas the other four alkoxides (1:l:l:l) are contained within the mirror plane. Though these compounds yield crystals from 1,2-dimethoxyethane, they failed to diffract beyond IOo indicating a molecular disorder within hexagonal packing of M4 units. However, the cyclobutylmethoxide derivatives and the molecular structure of the crystallize with an additional molecule of alcohol, M4(OCH2-c-Bu),2(HOCHz-c-B~), molybdenum compound was determined by an X-ray study. Crystal data for MO~(OCH~-C-B~)~~(HOCH~-C-B~) at -1 56 OC: a = b = 19.952 ( 7 ) A, c = 34.755 (16) A, Z = 8, dcalcd= 1.43 g and space group P41. The unit cell contains two crystallographically independent molecules that are essentially equivalent with respect to M-M and M-0 distances and angles. In each there is a central M o butterfly ~ with a dihedral angle between the Mo3 triangles of 132.5'. One wingtip metal atom is octahedrally coordinated by oxygen atoms of which three are terminal, two edge bridging, and one triply bridging. The backbone Mo atoms are evidently strongly bonded, Mo-Mo 2.47 A, and are coordinated to two edge bridging alkoxides and one terminal alkoxide. The other wingtip Mo atom is similarly coordinated, and the unique molecule of alcohol is bound to one of the backbone Mo atoms, Mo-0 (HOCH2-c-Bu) = 2.34 ( I ) A. In solution (benzene-d6 and toluene-d8) the N M R spectra are similar to those obtained for the other members of this class of compounds, M4(OCH2R),,, with additional signals due to free HOCH2R. This implies that the cyclobutylmethanol is weakly bonded and readily dissociates in solution. The present findings are compared with other I2-electron alkoxide supported clusters of molybdenum and tungsten.

Molybdenum and tungsten alkoxide clusters provide templates

for organometallic chemistry.2 The essential features that should be present in these alkoxide clusters include both coordinative unsaturation and redox flexibility a t the metal center^.^ We have recently demonstrated that w2(0-&Pr)6 (I) exists in reversible equilibrium with its 12-electron cluster, W4(O-i-Pr),2 (II), in solution at room temperature, eq 1: whereas Mo2(O-i-Pr), is indefinitely stable with respect to cluster f ~ r m a t i o n . ~Even by 0

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reducing the steric demands a t the molybdenum center we were unable to induce the formation of new metal-metal bonds in [MoZ(O-i-Pr),(OMe)j2 (111), which instead retains isolated, unbridged Mo-Mo triple bonds5 We report here the isolation and characterization of members of a new class of 12-electron tetranuclear alkoxide clusters, IV-XI, together with the single-crystal X-ray diffraction structure of one derivative, [ Mo4(OCH2-cB U ) ~ ~ ( H O C H ~ - C - B(XI) U ) ] (where c-Bu = cyclobutyl). This represents the first group of structurally related homoleptic alkoxide clusters for molybdenum and tungsten in oxidation state (+3). A preliminary aspect of this work has been published.6 The new compounds are defined as follows: (IV), W4(OCH2Cy),,; Indiana University. *University of Durban-Westville

0002-7863/89/151 I-5300$01.50/0

(VI, WdOCH2-i-Pr)12; (VI), W4(OCHzCp)12; (VII), W4(OCH,-C-BU)~~.HOCH~-C-BU; (VIII), M04(0CH2Cy)12; (IX), Mo4(OCH2-i-Pr)12; (X), Mo4(OCH2Cp),,; (XI), Mo4(OCH2-cBu),,.HOCH2-c-Bu; where C y = cyclohexyl; i-Pr = isopropyl; C p = cyclopentyl, and c-Bu = cyclobutyl. Experimental Section General Procedures. All preparations and manipulations were carried out under an atmosphere of dry nitrogen using standard Schlenk techniques together with a Vacuum Atmosphere's Company Dri-Lab System. Aromatic and aliphatic hydrocarbon solvents, diethyl ether, and I ,2-dimethoxyethane were dried and distilled from sodium benzophenone ketyl and stored over 4 A molecular sieves prior to use. Alcohols and BH3. Me2S were purchased from Aldrich Chemical Company, and NMep was obtained from Matheson. Isobutyl alcohol was dried and distilled from magnesium and stored over 4 A molecular sieves prior to use; other alcohols were dried over sieves but not distilled prior to use. 13C02(99 mol %) was purchased from MSD Isotopes. M,(O-t-Bu), and M2(1) For Part 15 see: Chisholm, M. H.; Huffman, J. C.; Hampden-Smith, M. J. J . A m . Chem. SOC.,previous paper in this issue. (2) Chisholm, M . H.; Folting, K.; Eichhorn, B. W.; Huffman, J. C. J . Am. Chem. SOC.1987, 109, 3146 and references therein. (3) Reduced metal alkoxides of Mo and W may provide a complementary reaction chemistry with the heteropolyoxoanions. The latter may provide a good class of homogeneous oxidation catalysts: Pope, M. T. In Heteropoly and Isopo/y Oxometa/lates; Springer: New Ycrk, 1983. Day, V. W.; Klemperer, W. G.; Schwartz, C.; Wang, R. C. N A T O Workshop Report, in press. Faraji, M.; Hill, C. L. J . Chem. SOC.,Chem. Commun. 1987, 1487. (4) Chisholm, M.H.; Clark, D. L.; Folting, K.; Huffman, J. C. Angew. Chem.,Int. Ed. Engl. 1986, 25, 1014. Chisholm, M. H.; Clark, D. L.; Folting, K.; Huffman, J. C.; Hampden-Smith, M . J. J . Am. Chem. SOC.1987, 109,

7750. (5) Chisholm, M. H.; Hammond, C. E.; Hampden-Smith, M. J.; Huffman, J. C.; Van Der Sluys, W. G. Angew. Chem., Ini. E d . Engl. 1987, 26, 905.

(6) Chisholm, M. H.; Folting, K.; Hammond, C. E.; Hampden-Smith, M. J . J . A m . Chem. Soe. 1988, 110, 3314.

0 1989 American Chemical Society

12- Electron Mo and W Alkoxide Clusters

J . A m . Chem. SOC.,Vol. 1 1 1 , No. 14, 1989

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H, Ja-,, = 6.9 Hz, Ja+= 10.3 Hz; Hi,, 4.23 ppm, d X d, 1 H, Ja-b = 6.3 (NMe2)6 (M = Mo and W) were prepared by the literature routes.' Hz; H:, 4.48 ppm, d X d, 1 H, JB-8t= 9.7 Hz, Ja-b = 6.6 Hz; H:,, 4.28 Spectroscopic Instrumentation and Interpretation. 'H and "C NMR ppm, d X d, 1 H, Ja-b = 5.9 Hz; H: = 4.15 ppm, d X d, 1 H, Ja-,, = were obtained on Varian XL300 and Bruker AM500 NMR spectrome10.0 Hz,Ja-b = 5.4 Hz; H:,, 3.18 ppm, d X d, 1 H, Ja+= 10.0 HZ; Hi, ters. 'H and "C assignments were made with the aid of a variety of 3.88 ppm, d, 1 H, Ja-b = 6.9 Hz; H:, 3.65 ppm, d, 1 H, Ja-b = 8.3 Hz; two-dimensional NMR experiments on the Bruker instrument. Typical methines, CH, HI, 3.29 ppm, m, 1 H; Hi, 2.43 ppm, m, 2 H ; H', 2 07 acquisition parameters were as follows, COSY [SI2 W (4 scans) X 1281; B 2.25 ' ppm, m, 1 H ; H qb, 1 9 9 ppm, m, 2 H; HS,1.97 ppm, m, 2 H; Hb, XHCORR [ I K (84 scans) X 2561; JRES, [ I K (16 scans) X 1281, using 9 1.76 ' ppm, m, 1 H; Ha, I 1.68 ppm, m, 1 H; methyls, ppm, m, 2 H; Ha, solutions containing about 70 mg of sample. The 'H and "C NMR data CH,, HL, 1.16 ppm, d, 6 H, J-b = 6.7 Hz; Hi, 1.12 ppm, d, 6 H, Jc-b = are reported in detail because of their importance in interpretation of 6.2 Hz; HZ,, 1.12 ppm, d, 6 H, J-b = 6.0 Hz; H:, 1.24 ppm, d, 6 H, Jc-b molecular structure of these molecules in solution. In future papers we = 6.7 Hz; Hb, 1.11 ppm, d, 6 H, Jc-b = 5.7 Hz; H$, 1.09 ppm, d, 6 H, will describe reactions involving this class of M,(OR),, cluster and small Jcb = 6.2 Hz; Ht, 1.16 ppm, d, 6 H, Jcb = 6.3 Hz; Ht,, 1.24 ppm, d, 6 molecules such as CO for which the spectroscopic data are of primary H, Jcb = 6.7 Hz; H:, 1.01 ppm, d, 6 H, Jcb = 6.5 Hz; H,: 0.96 ppm, importance in determining the reaction mechanism and stereochemistry d, 6 H, Jcb= 6.8 Hz; HS, 0.86 ppm, d, 6 H, Jc-b = 6.5 Hz; HJ, 0.86 ppm, of products. The present data for the parent molecules will serve as a d, 6 H, J a - b = 6.4 Hz; I3C NMR (23 OC, 125.76 MHZ, C6D6) methydata base. The methylene and methine 'H and "C NMR resonances of lenes, CH,,Ci, 80.0 ppm, 1C; C:, 79.9 ppm, 2C; C:, 72.4 ppm, IC; C:, these species are diagnostic for this type of structure. For the case of the 78.2 ppm, 2C; C:, 74.7 ppm, 2C; Ct, 75.3 ppm, 2C; C' 82.5 ppm, 1C; isobutoxide clusters, M,(OCH,-i-Pr),, (M = Mo, W), the connectivity 'i' 32.9 ppm, 2C; c:, 77.2 ppm, IC; methines, a:Cb, 28.6 ppm, IC; cb, of the 'H and "C NMR resonances within all the isobutoxide ligands can Cl, 34.1 ppm, 1C; C,; 33.2 ppm, 2C; Ci, 32.7 ppm, 2C; Ci, 31.0 ppm, be assigned. For the other derivatives, only the methylene and methine 2C; Cl, 32.2 ppm, IC; C:, 32.9 ppm, 1C; methyls, CH,,Cr, 20.4 ppm, protons will be identified for simplicity. The NMR data are labeled as 1C; Cz, 19.8 ppm, IC; Cf,, 19.9 ppm, IC; C:, 19.7 ppm, IC; C:, 21.9 follows: (i) the eight types of alkoxide ligands will be labeled with ppm, 1C; C$, 20.4 ppm, IC; C:, 19.5 ppm, IC; C:,. 20.7 ppm, 1C; Cg, superscripts 1-8; (ii) subscripts a-c will designate methylene (a), methine 19.5 ppm, 1C; C$, 19.2 ppm; Cl, 19.2 ppm, IC; CJ, 19.3 ppm, IC; IR (b), or methyl (c) (in the case of isobutoxide only) 'H or ')C resonances; Y (cm-I) 1366 s, 1354 sh, 1341 sh, 1291 w, 1261 w, 1069 s, 1028 s, 977 (iii) a prime over the subscript indicates one of a pair of diastereotopically m, 960 m, 944 m, 822 w, 661 m, 635 m, 467 w, 399 w. Elemental Anal. related methylene or methyl resonances ( e g , methylene proton H$, is Calcd for W4012C48H108: C, 35.73; H, 6.75. Found: C, 35.49; H, 6.57. bonded to the same carbon atom as H: of the alkoxide ligand arbitrarily W,(OCH,Cp),, (VI). IR u (cm-I) 1366 s, 1337 w sh, 1260 w, 1169 number 4). Using this labeling scheme the NMR data are presented in w, 1068 vs, 1030 s, 1006 s, 963 m, 9402, 898 w, 749 w, 643 w, 585 w. this text for W,(OCH2-i-Pr)12(V). The data for other compounds are Elemental Anal. Calcd for W4OI2C7,Hl3,:C, 44.93; H, 6.86. Found: listed in the Supplementary Material. C, 44.87; H, 6.89. Ultrahigh resolution I3C NMR spectra were obtained in collaboration with Allerhand et al. on a Nicolet NT200 NMR instrument using M o , ( O C H , - ~ - P ~ )(IX). , ~ 95MoNMR (23 'C, 23.59 MHz, toluenetechniques that have been previously described.s Infrared spectra were d,) 2628 ppm, A q i 2 = 488 Hz; IR Y (cm-') 1364 s, 1352 m, 1104 m sh, recorded on a Perkin Elmer 283 infrared spectrometer as Nujol mulls. 1064 s, 1042 s, 982 m, 957 w, 943 w, 820 w, 660 m, 664 m, 607 m, 479 95M0NMR spectra were recorded on a Nicolet NT360 NMR specw, 404 w. Elemental Anal. Calcd for Mo4012C48H108: C, 45.71; H, 8.63; trometer at 23.59 MHz, in 10-mm NMR tubes, containing approxiN , 0.00. Found: C, 45.48; H, 8.46; N , 0.05. mately 1 g of material dissolved in 3 mL of toluene-ds. Chemical shifts Mo4(0CH,Cy),, (VIII). IR Y (cm-I) 1305 w, 1285 w, 1270 w, 1260 are reported relative to an external standard of 2 M Na2[Mo04]solution w,1216w,1180w,1147w,1100s,1099s,1079s,1057s,1045vs,1024 in D 2 0 of pH 1 1 . Typically IO5 transients were acquired by adding s, 950 m, 913 w, 890 m, 840 m, 788 w, 777 w, 690 w, 655 m sh, 635 s together the FID's of individual experiments each consisting of 3000 br d, 595 m sh, 550 m, 475 w, 429 w, 295 w. Elemental Anal. Calcd transients to avoid memory overflow. for M04012C84H1s6: C, 57.92; H, 9.02. Found: C, 57.62; H, 9.02. Molecular Weights. The molecular weight of M o , ( O C H ~ - ~ - Pwas ~),~ Mo,(OCH,Cp),, (X). IR Y (cm-') 1364 s, 1335 m, 1310 w, 1170 w, measured by the freezing point depression method in benzene at two 1055 s, 1000 sh, 970 m, 939 m, 897 m, 800 w, 650 m, 590 m, 470 w, 464 different concentrations. The values obtained were 1140 g at a concenw. Elemental Anal. Calcd for M O , O ~ , C ~ ~ HC, ~ ,54.96; ~: H, 8.46. tration of 1.46 X lo-) M and 1360 g at a concentration of 3.41 X lo-, Found: C, 54.70; H, 8.32. M. These values were obtained from three measurements at each conW,(OCH~-C-BU)I~(HOCH~-C-BU) (VII). W,(O-l-BU)6 (1.000 g, 1.24 centration. The calculated molecular weight of M O ~ ( O C H , - ~ -isP ~ ) ~ ~ mmol) was placed in a Schlenk flask and dissolved in 5 mL of hexane. 1263 g. The instrument was calibrated using biphenyl solutions of the Cyclobutylmethanol was added (1.5 mL), dropwise, while the solution two different concentrations. was vigorously stirred at room temperature. The solution immediately Syntheses. M4(OCH2R)12.All the compounds of molecular formula changed color from deep red to deep purple. After stirring for 2 h, the M4(OCH2R)12where M = Mo, W; R = cyclohexyl, isopropyl, or cyvolatile components were removed in vacuo leaving an oily purple residue. clopentyl were prepared in a virtually identical manner. A representative The residue was dissolved in 2 mL of 1,2-dimethoxyethane and cooled example of this procedure is given below, followed by the spectroscopic to -20 OC for 16 h. During this time a dark purple solid precipitated and data for all the new compounds. was isolated by filtering the solution at 0 OC. The filtrate was concenW4(0CH2-i-Pr)12(V). W,(O-r-Bu), (0.750 g, 0.93 mmol) was placed trated to about a quarter of its original volume, and then a second crop in a 30-mL Schlenk flask and dissolved in 3 mL of hexane at room was isolated by cooling to -20 OC and working up as above. The comtemperature. An excess of isobutyl alcohol (1 mL) was added dropwise bined mass of material isolated was 1.030 g (5.6 mmol), a yield of 90.2% to the solution which rapidly turned deep purple. The mixture was stirred based on tungsten. The 'H and 13CNMR and IR spectroscopic data are for 2 h , and then all the volatile components were removed in vacuo to reported below. On dissolution of a sample of the purple solid obtained give on some occasions a black solid or an oily black residue. 'H NMR in this way, 1 equiv of uncoordinatedfree cyclobutylmethanol per W4spectra of the crude material showed it to be essentially pure W4(OCH2-c-butyl)12was observed by IH NMR spectroscopy. The reso(OCH2-i-Pr)12.This compound is extremely soluble in most hydrocarbon nances attributed to free HOCH2-c-Bu were identified by their 'H and solvents and attempts to crystallize it from any of pentane, hexane, 13Cchemical shifts. Furthermore, the addition of 20 pL of an authentic toluene, diethyl ether, or mixtures of these solvents were unfruitful. sample of the alcohol to a solution of 32 mg of VI1 dissolved in 0.5 mL However, by redissolving the residue in a small amount of 1,2-dimethof benzene-d6 merely resulted in an increase in intensity of the signals oxyethane (- 1.5 mL) a green solution was obtained and by cooling to attributed to dissociated HOCH2-c-Bu. -20 OC over 12 h crystals of V were obtained. The solution was filtered W,(OCH~-C-BU)~~(HOCH~-C-BU) (VII). ' H NMR (23 O C , 500.13 at 0 "C to give 0.536 g of dark green crystalline material. The filtrate MHz, C6D6) methylenes, CH2, HA, 5.13 ppm, d X d, 1 H, J,,. = 10.4 was concentrated to half its volume, and more crystalline material was Hz, J a - b = 7.3 Hz; C,: 5.00 ppm, d X d, 1 H, Ja-b = 7.0 Hz; Hi, 5.05 isolated as above to yield a second crop, 0.112 g. The combined yield ppm, d, 1 H, Ja+,= 6.1 Hz; HZ, 4.70 ppm, d, 1 H, Ja-b = 6.4 Hz; H:, of V was 0.648 g, 86.4% yield based on tungsten. 4.57 ppm, d X d, 1 H, Ja-ar= 11.0 Hz, Ja-b= 6.8 Hz; Hi,, 4.35 ppm, d W,(OCH,-i-Pr),, (V). 'H NMR (23 OC, 500.13 MHZ, C6D6) meX d, 1 H, Ja-b = 6.5 Hz; H:, 4.48 ppm, d X d, 1 H, Ja+ = 11.0 Hz, Ja-b thylenes, CHI, HA, 5.08 ppm, d, 1 H, J a - b = 6.1 Hz; H:, 4.86 ppm, d X = 6.8 Hz; Hit, 2.34 ppm, d X d, 1 H, Ja-b= 6.0 Hz; H:, 4.34 ppm d X d, I H, J,-,. = 10.0 Hz, Ja+ = 7.2 Hz; Hf,, 4.63 ppm, d X d, 1 H, Ja-b d, 1 H, Ja..a,= 10.7 Hz, Ja-b = 6.8 Hz; Hi, 4.09 ppm, d, 1 H, Ja-b = 6.8 = 5.8 Hz; H:, 4.53 ppm, d, 1 H, Ja-b = 6.1 Hz; H:, 4.50 ppm, d X d, 1 Hz; HZ, 3.90 ppm, d, 1 H, Ja-b = 6.7 Hz; H:, 3.69 ppm, d, 1 H, Ja+ = 6.6 Hz; methines, CH, HA, 3.17 ppm, m, 2 H; Hi, 3.78 ppm, m, 1 H; Hi, 2.91 ppm, m, 1 H; Hi, 2.71 ppm, m, 2 H; Hi,2.69 ppm, m, 2 H; Hf. 2.94 (7) Chisholm, M. H.; Haitko, D. A.; Murillo, C. A. Inorgunic Syntheses; ppm, m, 2 H; HL, 2.50 ppm, m, 1 H; Hi, 2.52 ppm, m, 1 H; Hi, 2.60, Fackler, J . R., Ed.; John Wiley and Sons: New York, 1982; Vol. XXI,pp m, 1 H. Other resonances overlapping between 2.30 and 1.60 ppm. 13H 5 1-57, and references therein. NMR (23 OC, 125.76 MHz, C6D6)methylenes, CH,,Cl, 82.8 ppm, 2C; (X) Allerhand, A.; Maple, S. R. Anal. Chem. 1987, 59, 441A. Allerhand, A.; Addleman, R. E.; Osman, D.J . Am. Chem. SOC.1985, 107, 5809. Ct 81.5 ppm. 1C; Ci, 75.0 ppm, 1C; Ct, 78.3 ppm, 2C; C:, 76.9 ppm, 2C;

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Chisholm et al.

Ct, 77.2 ppm, 2C; Ci, 81.2 ppm, IC; C:, 78.7 ppm, IC; Cz, 67.0, IC; stirred for 18 h to ensure complete reaction. The products were hydromethines, CH,Cf 39.2 ppm, 2C; Ci, 36.1 ppm, IC; Ci, 41.6 ppm, IC; lyzed by adding a small excess of distilled water and finally a small C4, 39.5 ppm 2C; Ci, 40.4 ppm, 2C; Ct, 38.3 ppm, 2C; Ci, 38.3 ppm, 1C; amount (0.5 mL) of dilute nitric acid. The organic layer was separated, t 39.4 ppm, IC. Remaining resonances observed as sharp singlets cb, and the aqueous layer was extracted with three 15-mL portions of ether; between 28.2 and 25.0 ppm and from 19.4 to 18.7 ppm. IR u (an-') 1362 the extracts were combined and dried. The diethyl ether was removed s, 1319 m, 1333 m, 1150 w, 1100 s, 1060 s, I010 s, 972 m, 940 m, 912 by distillation at atmospheric pressure, and 'H and I3C NMR spectra of w, 782 w, 650 m, 605 m, 589 m, 440 w. Elemental anal. Calcd for the crude product (3.2 g) showed that it contained mainly [13C,]isobutyl W4013C65H118: C, 42.34; H, 6.45. Found: C, 42.15; H, 6.32. alcohol together with some diethyl ether and T H F impurities. The crude product was distilled at atmospheric pressure, and the fraction that boiled Mo4(0CH2-c-Bu)12(HOCH2-c-Bu). Mo2(0-r-Bu), (0.805 g) was dissolved in 20 mL of pentane at 0 'C in a solvent-seal flask. Cyclobetween 98-105 OC (lit. value = 108 OC, unlabeled) was collected. The distilled product was spectroscopically pure (IH and ''C NMR), and 1.5 butylmethanol (1 mL) was added dropwise to the vigorously stirred g of [13Cl]isobutylalcohol was isolated, a yield of 49.3% based on I3CO2. solution which immediately turned dark green. The solution was stirred Thls product was stored over 4 A molecular sieves. The undistilled for 2.5 h after which the volatile components were removed in vacuo. A residue gave some evidence of decomposition (by 'H and "C NMR) but 'H NMR spectrum of the crude material showed the reaction to be still mainly contained ['3Cl]isobutyl alcohol. incomplete. The solid was redissolved in 20 mL of pentane, and another portion of cyclobutylmethanol (1 mL) was added. The reaction was HO*CH2CHMe2. 'H NMR methylene, *CH2,3.25 ppm, d X d, 2 H, stirred for 12 h, and then the volatile components were again removed JH 'i = 143 Hz, IJH-"= 6 Hz; methine, CH, 1.62 ppm, m, 1 H; methyl, in vacuo. The residue was redissolved in the minimum amount of 1,2CHMe,, 0.81 ppm, t. 6 H, 'JH-H = 7 Hz; OH, 1.75 ppm, br, - 1 H; "C dimethoxyethane and cooled to -10 OC. After 1 month 0.150 g of large NMR methylene, *CH2,69.4 ppm, s, Jc-H= 140 Hz; methyl, CH, 31 .O crystals were isolated by filtration at 0 OC. 'H and I3C NMR specppm, d, J c =~35 Hz; methyl, CHMe,, 19.0 ppm. troscopy of this sample showed it to contain the cluster Mo4(0CH2-c(ii) Preparation of W4(O*CH2-i-Pr)12. W,(NMe2), (1.000 g, 1.58 B U ) ] ~together with 1 equiv of uncoordinated cyclobutylmethanol in mmol) was suspended in 5 mL of toluene, and a solution of I .5 g (an benzene-d, solution. A sample of these crystals were submitted for a excess) of the ['3Cl]isobutylalcohol in 8 mL of toluene was added over single-crystal X-ray diffraction study. The filtrate was concentrated, and about 10 min. The solution turned deep red-purple as the amide dissolved a second crop of crystals was obtained and isolated by the procedure and the solution was stirred for 2 h at room temperature with the flask outlined above. The 'H and 13CNMR and IR spectroscopic data for this open to a nitrogen vent. The volatile components were vacuum distilled compound are reported below. into a trap (to recover unreacted alcohol) and a red-purple oil obtained. I3C NMR spectra of a sample of the crude oil were consistent with Mo~(OCH~-C-BU)I~(HOCH~-C-BU) (XI). 'H NMR (23 'C, 500.13 formation of W2(0*CH2-i-Pr),(NMe2H),.The oil was redissolved in 8 MHz, C6D,) methylenes, CH2, Hi, 5.26 ppm, d, 1 H, Ja-b = 6.1 Hz; Hi, mL of toluene and stirred for 10 h at room temperature. During this time 5.19 ppm, d X d, 1 H, J,-,, = 10.6 Hz, Ja-b = 7.2 Hz; Hi,, 5.09 ppm, d the solution turned black, and when the volatile components were reX d, 1 H, Ja-b = 7.1 Hz; Hi, 4.71 ppm, d, 1 H, Ja+= 6.5 Hz; Hi, 4.56 moved, an oily black solid was obtained. The IH and "C NMR spectra ppm, d X d, 1 H, Ja-at= 10.6 Hz, Ja-b = 6.5 Hz; Hi,, 4.33 ppm, d X d, of a sample of this crude material showed it to be W4(O*CH2-i-Pr)12. 1 H, Je-b= 6.0 Hz; H:, 4.50 ppm, d X d, 1 H, Je-,,= 11.4 Hz, Ja-b= The crude product was taken up in 2 mL of 1,2-dimethoxyethane and 6.4 Hz; Hi,, 4.38 ppm, d X d, 1 H, Je-b = 6.5 Hz; H:, 4.46 ppm, d X d, 1 H , J e ~ , ~ = 1 0 . 7 H z , J a - ~ = 7 . 1 H ~ ; H ~ ~ , 3 . 9 3 p p m , d X d , I H , Jstored a - ~ =at -20 OC for 16 h during which time black crystals were formed. The solution was filtered at 0 OC, the volume of the filtrate was reduced 7.1 Hz; H:, 4.39 ppm, d, 1 H, Ja-b = 6.8 Hz; H:, 4.06 ppm, d, 1 H, Ja-b by half, and the flask was replaced at -20 OC to yield a second crop of = 6.8 Hz; H:, 3.52 ppm, d, 1 H, Ja+= 6.8 Hz; methines, CH, HI, 3.77 ppm, m, 1 H; Ht, 3.16 ppm, m, 2 H; Hi, 2.94 ppm, m, 1 H ; Hb, B 2.69 black crystals. W4(O*CH2-i-Pr),,(0.827 g) was isolated, a yield of 64.6% based on tungsten. The volatile components were removed from ppm, m, 2 H; 2.69 ppm, m, 2 H; Ht, 2.88 ppm, m, 2 H; Hi, 2.49 ppm, the filtrate, and the residue (which appears spectroscopically pure) was m, 1 H; Hi, 2.52 ppm, m, 1 H; H:, 2.48 ppm, m, 1 H. Remaining saved. The crystallized material was characterized by 'H and 13CNMR resonances, broad overlapping multiplets between 2.32 and 1.97 ppm. I3C spectroscopy, IR spectroscopy, and by elemental analysis. This material N M R (23 OC, 125.76 MHz, C6D6) methylenes, CHI, C:, 83.0 ppm, IC; was used for the ultrahigh resolution I3C NMR experiment in an attempt Ct, 82.0 ppm, 2C; Ci, 76.3 ppm, IC; C:, 77.3 ppm, 2C; Ci, 80.2 ppm, to observe two bond couplings between the 99 mol % I3C-labeled isobu2C; Ct, 77.6 ppm, 2C; Ca, 81.3 ppm, IC; Ct, 79.3 ppm, IC; C9, 66.8 toxide resonances and lE3W. ppm, IC; methines, CH, Ci, 36.9 ppm, IC; Ci, 39.9 ppm, 2C; Ci, 42.0 W2(O*CH2-i-Pr)6(NMe2H)2."C NMR methylenes, CH,, 79.6 ppm, ppm, 1C; C:, 40.7 ppm, 2C; Cb, 39.6 ppm, 2C; Cg, 38.8 ppm, 2C; Cl 2C; 74.4 ppm, IC. 39.8 ppm, 1C; Ct, 39.2 ppm, IC; C:, 38.2 ppm, IC. Remaining cycloW4(O*CH2-i-Pr),, (Where *C = 99 mol % "C Labeled). "C NMR butyl resonances sharp singlets from 28.0-25.0 and 18.5-19.5 ppm. IR Y (cm-I) 1362 s, 1313 w, 1235 w, 1095 sh, 1065 s, 1035 s, 995 sh, 980 (23 OC, 125 MHz, C6D6) methylene carbon chemical shifts (C:) same sh, 955 m, 940 m, 915 w, 781 w, 645 m, 595 m, 580 m, 467 w. Elemental as above. Methines, CH, Cb, 28.6 ppm, IC, Jh = 37 Hz; Ct, 32.9 ppm, Anal. Calcd for M04013C65Hll8: C, 52.34; H, 7.97. Found: C, 52.64; 2C, Jbe= 37 Hz;C:, 34.3 ppm, IC, J, = 36 Hz; Ct. 33.2 ppm, 2C, H, 7.61; N, 0.0 no. of F?3.0u(F) R(F) W F )

goodness of fit for the last cycle maximum S/u for last cycle

-156 19.952 (7) 34.755 (16) 8 138 35.24 1.432 0.71069 1491.40 7.467 22.5 23.5 0.25 6.0 1.3 5 3.0 2 4.0 6-45 11293 9263 8759 7977 0.0570 0.0575 1.105 0.05

Table 11. Selected Bond Distances (A) for the Two Crystallographically Independent Molecules A and B,

Mod(OCH,-c-Bu)i,(HOCH,-c-Bu) bond distances in A B Mo(1)A Mo(2)A Mo(1)A Mo(4)A Mo(1)A O(5)A U Mo(1)A O(1l)A Figure 2. A ball-and-stick drawing of the M O ~ ( O C H ~ - C - B U ) ~ ~ - Mo(1)A O(29)A Mo(I)A O(35)A (HOCH2-c-Bu) molecule emphasizing the octahedral Moo6 unit at Mo(1)A O(41)A Mo(l). Mo(1)A O(47)A Mo(2)A Mo(3)A O(2)A Mo(4)A Mo(2)A O(5)A Mo(2)A O(l1)B Mo(2)A O(17)A Mo(2)A O(53)A Mo(2)A O(59)A Mo(3)A Mo(4)A Mo(3)A O(17)A Mo(3)A O(23)A Mo(3)A O(65)A Mo(3)A O(71)A Mo(4)A O(5)A Mo(4)A O(23)A Mo(4)A O(29)A Mo(4)A O(77)A

molecule A distance 2.6637 (16) 2.6810 (16) 2.111 (9) 2.048 (9) 2.080 (9) 1.967 (9) 1.913 (10) 2.011 (9) 2.5071 (18) 2.4751 (18) 2.078 (9) 2.053 (9) 2.105 (9) 1.956 (9) 2.342 ( I O ) 2.4788 (19) 2.018 (9) 2.057 (9) 1.937 (10) 1.876 (9) 2.163 (9) 2.105 (9) 2.026 (9) 1.923 ( I O )

bond distances in A B Mo(l)B Mo(2)B Mo(l)B Mo(4)B Mo(l)B O(5)B Mo(1)B O(1l)B Mo(l)B O(29)B Mo(1)B O(35)B Mo(l)B O(41)B Mo(l)B O(47)B Mo(2)B Mo(3)B Mo(2)B Mo(4)B Mo(2)B O(5)B Mo(2)B O(l1)B Mo(2)B O(17)B Mo(2)B O(53)B Mo(2)B O(59)B Mo(3)B Mo(4)B Mo(3)B O(17)B Mo(3)B O(23)B Mo(3)B O(65)B Mo(3)B O(71)B Mo(4)B O(5)B Mo(4)B O(23)B Mo(4)B O(29)B Mo(4)B O(77)B

molecule B distance 2.6757 (23) 2.6901 (23) 2.108 (9) 2.041 (9) 2.099 ( I O ) 1.968 (IO) 1.907 (10) 2.004 (IO) 2.5128 (23) 2.4614 (23) 2.091 (9) 2.061 (9) 2.114 (10) 1.930 ( I O ) 2.349 (10) 2.4730 (24) 2.009 (10) 2.073 (9) 1.915 (10) 1.880 (10) 2.110 (9) 2.105 ( I O ) 2.026 ( I O ) 1.920 (IO)

I and the pertinent bond distances and angles in Tables I1 and 111, respectively. Atomic positional parameters are given in Table IV. The molecule consists of a butterfly arrangement of metal atoms

Figure 3. A ball-and-stick drawing of the Mo4(OCH2-c-Bu)12(HOCH2-c-Bu) molecule emphasizing the central M04 butterfly. The dihedral angle between the two Mol triangles is 132.5O.

in Figure 1. Figures 2 and 3 demonstrate the butterfly geometry of metal atoms as well a s the orientations of the cyclobutylmethoxide ligands. T h e crystallographic data are given in Table

where Mo(1) and Mo(3) are defined as the wingtip metal atoms and M o ( 2 ) and Mo(4) as the backbone metal atoms, with a dihedral angle between the triangular faces of 132.5’. O n e triangular face is capped with a triply bridging alkoxide ligand, and each backbone-to-wingtip metal atom edge is bridged by alkoxide ligands. Mo(2) possesses one terminal alkoxide ligand and is also coordinated to another oxygen atom a t a distance of 2.34 A, in the range we have previously observed for metal-toalcohol oxygen distances.’2 Mo( 1) has coordination number 6 with respect to alkoxide ligands such that the Mo( 1)-6(0) moiety

12- Electron M o a n d W Alkoxide Clusters

is a distorted octahedron, Mo(3) and Mo(4) are four-coordinate with respect to oxygen atoms. Mo(2) is five-coordinate with respect to oxygen atoms. T h e coordinated alcohol ligand takes up the axial position of the square-based pyramid that is missing for the Mo(4)-4(0) moiety. The molybdenum-molybdenum distances within the cluster are quite remarkable and require some comment. T h e relevant distances are given in Table 11. T h e distances within the triangle defined by Mo(2), Mo(3), and Mo(4) are markedly shorter than those of the triangle defined by Mo( I ) , Mo(2), and Mo(4) Typical bond lengths for Mo-Mo single, double, and triple bonds are of the order 2.73 8, (e.g., in M o , C ~ ~ ( O - ~ - P2.52 ~ ) ~8, ) , (e.g., in U)~). M ~ ~ ( o - i - P r )and ~ ) , 2.22 8, (e.g., in M o ~ ( O C H ~ - ~ - B respectively (note that the first two examples contain alkoxide bridged metal-metal bonds).', The metal-to-metal distances for the atoms within the triangle Mo(4), Mo(3), and Mo(2) are consistent with some multiple bond character and can be compared to the metal-metal distances of 2.53 A within the cluster Mo3(l3-0)(w3OR)(p,-OR),(OR),, although the latter is no doubt constrained by the presence of both triply bridging oxo and alkoxo ligands. T h e distances Mo( I)-Mo(4) and M o ( 1)-Mo(2) are closer to single bond separations, although, as always, the presence of bridging ligands complicates the simple assignment of bond order. T h e metal-oxygen bond distances all lie within the expected ranges, 1.88-2.01 A for terminal alkoxides and 2.03-2.16 8, for the doubly and triply bridging alkoxides.'2 T h e Mo-oxygen distance between Mo(2) and O(59) of 2.34 A, the ligand believed to be coordinated cyclobutylmethanol, is notably longer than any other M o - 0 distance in the molecule. T h e presence of one molecule of coordinated alcohol for each cluster unit in the solid state is also consistent with the solution N M R results discussed below where it was observed that 1 equiv of free alcohol was liberated when the crystalline materials VI1 and XI were dissolved in benzene-d6. The reasons why only one molecule of alcohol and not two should be coordinated (the other to Mo(4)) remains unclear at this stage. The observation that the molecular symmetry of the cluster formed on dissolution of a sample of the crystals of XI used for the structure determination is identical with that of all the other complexes of this class supports the idea that the solid-state structure is related to the species characterized spectroscopically in solution. B. Solution Structure of Compunds IV-XI. In solution, the ' H and 13CN M R spectroscopic data for all the complexes IV-XI are consistent with a common, stereochemically rigid cluster framework. T h e methylene, and to a lesser extent the methine, IH and I3CN M R resonances of the alkoxide ligands are sensitive probes of the cluster geometry, and their assignments and connectivity have been examined in detail (see Experimental Section). In the ' H N M R spectrum, there is evidence for eight types of alkoxide ligands from the methylenic proton resonances which occur in the relative ratios of 1:1:1:1:2:2:2:2. T h e methylenic resonances of relative intensity one occur as doublets coupled to the single methine proton, while the resonances due to the alkoxide ligands of relative intensity two arise from diastereotopic methylene protons and can easily be identified by their additional geminal coupling constants. In most cases the individual proton chemical shifts could be distinguished using the chemical shift dispersion of a 500 M H z instrument. Figure 4 shows a two-dimensional 'H N M R chemical shift correlated contour plot of Mo,(OCH,-i-Pr),, obtained at 500.13 M H z . The four types of diastereotopic alkoxide ligands of relative intensity two consisting of eight individual methylenic proton resonances can be identified by the four offdiagonal contours (marked Hi,H:, H:, He6 in Figure 4) within the methylene region. This assignment is confirmed by noting that the pairwise related methylenic protons are coupled to the s a m e methyne protons, respectively, e.g., between the pair Hi, Hi,and H i (see Experimental Section for definition of terms). The nondiastereotopic methylene protons exhibit single crosspeaks to the methine region. Therefore, there are a total of 12 offdiagonal elements between methylene and methyne protons, and, together with the equivalent relative intensities of methylene resonances, a minimum cluster nuclearity of tetranuclear can be

J . Am. Chem. SOC.,Vol. 111, No. 14, 1989

5305

M04 lOCH2-IPRI 12

I::: 1

1

k

2.0

3.5

5.0

L PPI,

Figure 4. A 2D 'H NMR chemical shift correlated contour plot (COSY) for Mo,(OCH,-i-Pr),, recorded in benzene-& at 500.13 MHz and 23 ' C .

set, i.e., the molecular formula in solution must be [M4-

(OCH,R),,I,. The 13CN M R spectra of these compounds were also consistent with these observations. Eight types of methylenic carbons were observed in the approximate ratio 1: 1:I :I :2:2:2:2. Two-dimensional heteronuclear I3C-IH chemical shift correlated experiments clearly confirm not only the presence of eight different types of alkoxide ligands but also the assignment that the resonances of relative intensity two arise from diastereotopic alkoxide ligands. Figure 5 shows a n expansion of the two-dimensional I3C-lH chemical shift correlated contour plot for M o ~ ( O C H , - C - B U )to, ~show the methylenic region. This experiment was optimized to determine correlations between 13Cnuclei and their directly attached protons. The I3C resonances labeled Ci, C:, Cp, and Ct are of approximate relative intensity two, and each possess two chemically inequivalent directly attached protons, while I3C resonances Ci, Ci, C:, and Ci, of approximate relative intensity one, are attached to one type of methylenic proton. Similar arguments were used to determine the connectivity between the methine 'H and I3C resonances of all the new compounds, and in the case of M,(OCH,-i-Pr),, molecules the connectivity of all the ' H and I3C nuclei within every alkoxide ligand could be determined. In cases where any a m biguity of assignment existed, two-dimensional J-resolved IH N M R experiments were used to separate chemical shifts from homonuclear coupling constants. Complexes VI1 and XI showed a slightly different behavior. On dissolving the isolated crystalline material in benzene-& 'H NMR resonances were obtained that were consistent with the same molecular geometry for the cluster in solution, but in addition IH N M R resonances similar to those of free cyclobutylmethanol were observed in each case. On addition of a n authentic sample of cyclobutylmethanol to a benzene-d6 solution of isolated VII, the resonances originally attributed to the dissociated alcohol increased in intensity, confirming their assignment to uncoordinated alcohol.

5306

J . Am. Chem. Soc., Vol. 11 1, No, 14, 1989

Chisholm et al.

Table 111. Selected Bond Angles (deg) for the Two Crystallographically Independent Molecules A and B, MO~(OCH~-C-B~)~~(HOCH~-C-BU) bond angles for molecule A bond angles for molecule B

Mo(2)A Mo(2)A Mo(2)A Mo(2)A Mo(2)A Mo(2)A Mo(2)A MO(4)A MO(4)A MO(4)A MO(4)A MO(4)A MO(4)A O(5)A 0 ( 5 ~ 0(5)~ O( 1 l)A O( 1 l)A 0 ( 1 l)A O(l1)A O(29)A O(29)A O(29)A O(35)A O(35)A O(41)A Mo(1)A Mo(1)A Mo(l)A Mo(1)A Mo(1)A Mo(1)A Mo( I)A MO(3)A MO(3)A MO(3)A Mo(3)A MO(3)A MO(3)A Mo(4)A MO(4)A MO(4)A Mo(4)A Mo(4)A 0 ( 5 ~ 0(5)~ 0 ( 1 l)A O(l1)A O( 1 l)A O( 17)A O(17)A O(53)A Mo(2)A Mo(2)A Mo(2)A Mo(2)A Mo(2)A MO(4)A MO(4)A Mo(4)A MO(4)A O(17)A O(17)A O( 17)A O(23)A O(23)A O(65)A Mo(1)A Mo(l)A Mo(l)A Mo(l)A

Mo( l)A Mo(1)A Mo(1)A Mo(1)A Mo(1)A Mo(1)A Mo(l)A Mo(1)A Mo(1)A Mo(l)A Mo(1)A Mo(1)A Mo(1)A Mo(1)A Mo(l)A Mo(1)A Mo(l)A Mo( l)A Mo( l)A Mo(l)A Mo(1)A Mo(1)A Mo(1)A Mo(1)A Mo(1)A Mo(l)A Mo( l)A Mo( l)A Mo(2)A Mo(2)A Mo(2)A Mo(2)A Mo(2)A Mo(2)A Mo(2)A Mo(2)A Mo(2)A Mo(2)A Mo(2)A Mo(2)A Mo(2)A Mo(2)A Mo(2)A Mo(2)A Mo(2)A Mo(2)A Mo(2)A Mo(2)A Mo(2)A Mo(2)A Mo(2)A Mo(2)A Mo(2)A Mo(2)A Mo(2)A Mo(2)A MO(3)A MO(3)A MO(3)A MO(3)A MO(3)A MO(3)A MO(3)A MO(3)A MO(3)A MO(3)A MO(3)A MO(3)A MO(3)A MO(3)A MO(3)A MO(4)A MO(4)A MO(4)A MO(4)A

MO(4)A 0 ( 5 ~ O(l1)A O(29)A O(35)A O(41)A O(47)A 0 ( 5 ~ O(1l)A O(29)A O(35)A O(41)A O(47)A O(l1)A O(29)A O(35)A O(41)A O(47)A O(29)A O(35)A O(41)A O(47)A O(35)A O(41)A O(47)A O(41)A O(47)A O(47)A MO(3)A MO(4)A 0 ( 1 I)A O( 17)A O(53)A O(59)A MO(4)A

o w

O(l1)A O(17)A O(53)A O(59)A O(l1)A O(17)A O(53)A O(59)A O(l1)A O(17)A O(53)A O(59)A O(17)A O(53)A O(59)A O(53)A O(59)A O(59)A MO(4)A O(17)A O(23)A O(65)A O(71)A O(17)A O(23)A O(65)A O(71)A O(23)A O(65)A O(71)A O(65)A O(71)A O(71)A Mo(2)A MO(3)A O(5)A O(23)A

55.17 (4) 49.98 (25) 49.58 (25) 90.23 (24) 87.73 (25) 139.5 (3) 130.62 (28) 52.03 (25) 87.71 (25) 48.35 (25) 132.41 (25) 96.5 (3) 134.79 (28) 99.2 (3) 99.6 (3) 82.0 (3) 90.3 (4) 173.1 (4) 87.3 (3) 88.5 (4) 170.3 (4) 81.0 (4) 175.6 (4) 89.0 (4) 87.3 (4) 95.1 (4) 81.1 (4) 89.8 (4) 106.75 (5) 62.77 (4) 51.06 (25) 49.40 (25) 134.81 (25) 134.3 (3) 86.70 (24) 59.67 ( 5 ) 114.56 (25) 93.74 (25) 50.97 (24) 102.40 (28) 151.53 (26) 55.92 (25) 93.38 (24) 110.61 (24) 105.86 (27) 145.78 (25) 100.1 (3) 163.7 (3) 85.0 (4) 93.5 (4) 89.3 (3) 159.3 (4) 75.8 (3) 90.9 (4) 101.8 (3) 83.9 (4) 59.53 (5) 54.16 (27) 113.83 (26) 109.2 (3) 109.5 (3) 113.65 (27) 54.35 (26) 110.8 (3) 106.7 (3) 167.4 (4) 89.3 (4) 96.5 (4) 91.8 (4) 91.5 (4) 135.8 (4) 62.06 (4) 107.05 ( 5 ) 50.28 (24) 135.22 (25)

Mo(2)B Mo(2)B Mo(2)B Mo(2)B Mo(2)B Mo(2)B Mo(2)B MO(4)B MO(4)B MO(4)B Mo(4)B MO(4)B MO(4)B o(5)~ 0(5)~ 0(5)~ 0 ( 1 l)B 0 ( 1 l)B O( 1 l)B 0 ( 1 l)B O(29)B O(29)B O(29)B O(35)B O(35)B O(41)B Mo(1)B Mo(1)B Mo(1)B Mo(1)B Mo(1)B Mo(1)B ,Mo(l)B MO(3)B MO(3)B MO(3)B MO(3)B MO(3)B MO(3)B MO(4)B MO(4)B MO(4)B MO(4)B MO(4)B 0(5)~ 0 ( 5 ~ O( 1 l)B O(l1)B O( 1 l)B O(17)B O(17)B O(53)B Mo(2)B Mo(2)B Mo(2)B Mo(2)B Mo(2)B MO(4)B MO(4)B MO(4)B MO(4)B O(17)B O(17)B O(17)B O(23)B O(23)B O(65)B Mo(1)B Mo(1)B Mo(l)B Mo(1)B

Mo(1)B MO(~)B Mo(1)B Mo(1)B Mo( l)B Mo(1)B Mo(1)B Mo(1)B Mo(l)B Mo(1)B Mo(1)B Mo(1)B Mo(l)B Mo(1)B Mo(1)B Mo(1)B Mo(1)B Mo(1)B Mo(1)B Mo(1)B Mo(1)B Mo(1)B Mo(1)B Mo(1)B Mo(1)B Mo(1)B Mo(1)B Mo(1)B Mo(2)B Mo(2)B Mo(2)B Mo(2)B Mon2)B Mo(2)B Mo(2)B Mo(2)B Mo(2)B Mo(2)B Mo(2)B Mo(2)B Mo(2)B Mo(2)B Mo(2)B Mo(2)B Mo(2)B Mo(2)B Mo(2)B Mo(2)B Mo(2)B Mo(2)B Mo(2)B Mo(2)B Mo(2)B Mo(2)B Mo(2)B Mo(2)B MO(3)B MO(3)B MO(3)B MO(3)B MO(3)B MO(3)B MO(3)B MO(3)B MO(3)B MO(3)B MO(3)B MO(3)B MO(3)B MO(3)B MO(3)B MO(4)B MO(4)B MO(4)B MO(4)B

MO(4)B o(5j~ 0 ( 1 l)B O(29)B O(35)B O(41)B O(47)B O(5)B O(l1)B O(29)B O(35)B O(41)B O(47)B O(l1)B O(29)B O(35)B O(41)B O(47)B O(29)B O(35)B O(41)B O(47)B O(35)B O(41)B O(47)B O(41)B O(47)B O(47)B MO(3)B MO(4)B O(5)B O( 1 l)B O(17)B O(53)B O(59)B MO(4)B 0(5)~ 0(1l)B O(17)B O(53)B O(59)B 0 ( 5 ~ O( 1 l)B O(17)B O(53)B O(59)B O( 1 l)B O(17)B O(53)B O(59)B O(17)B O(53)B O(59)B O(53)B O(59)B O(59)B MO(4)B O(17)B O(23)B O(65)B O(71)B O(17)B O(23)B O(65)B O(71)B O(23)B O(65)B O(71)B O(65)B O(71)B O(71)B Mo(2)B MO(3)B 0(5)~ O(23)B

54.61 (4) 50.13 (24) 49.60 (26) 89.33 (26) 90.5 (3) 138.8 (3) 130.1 (3) 50.40 (25) 87.37 (26) 48.13 (26) 132.3 (3) 93.3 (3) 134.8 (3) 99.5 (4) 97.8 (4) 82.8 (4) 90.3 (4) 174.6 (4) 86.7 (4) 92.7 (4) 167.8 (4) 80.5 (4) 179.2 (4) 84.8 (4) 87.6 (4) 95.8 (4) 91.9 (4) 90.5 (4) 106.85 (6) 62.99 ( 5 ) 50.69 (24) 48.97 (26) 135.50 (27) 133.8 (3) 84.98 (24) 59.62 (6) 113.16 (25) 94.02 (27) 50.59 (26) 103.5 (3) 153.91 (26) 54.49 (24) 93.36 (26) 110.20 (27) 106.5 (3) 143.60 (25) 99.4 (4) 161.9 (4) 85.6 (4) 92.4 (3) 90.6 (4) 158.3 (4) 76.0 (3) 90.7 (4) 104.7 (4) 82.7 (4) 59.16 (7) 54.4 (3) 113.48 (27) 110.3 (3) 109.8 (3) 113.5 (3) 54.32 (27) 108.9 (3) 107.3 (3) 167.8 (4) 92.9 (4) 95.4 (4) 91.0 (4) 89.9 (4) 135.7 (4) 62.40 ( 5 ) 107.58 (6) 50.33 (24) 137.34 (26)

J . Am. Chem. SOC.,Vol. 11 1. No. 14, 1989 5301

12- Electron Mo a n d W Alkoxide Clusters Table 111 (Continued)

bond angles for molecule B

bond angles for molecule A .

Mo( 1 )A Mo(2)A Mo(2)A Mo(2)A Mo(2)A Mo(2)A Mo(3)A MO(3)A MO(3)A Mo(3)A 0 ( 5 ~ o(5)~ O(23)A O(23)A O(29)A Mo(1)A Mo(1)A Mo(l)A Mo(2)A Mo(2)A Mo(4)A Mo( I)A Mo(l)A Mo(2)A Mo(2)A Mo(2)A MO(3)A MO(3)A Mo(3)A Mo(4)A Mo(l)A Mo(l)A MO(4)A Mo(l)A Mo(l)A Mo(1)A Mo(2)A Mo(2)A Mo(3)A Mo(3)A MO(4)A

Mo(4)A Mo(4)A MO(4)A MO(4)A MO(4)A MO(4)A MO(4)A MO(4)A MO(4)A MO(4)A MO(4)A MO(4)A MO(4)A MO(4)A MO(4)A MO(4)A o(51.4 WA O(5)A O(5)A O(5)A O(5)A O(l1)A O( 1 l)A O( 1 l)A O( 17)A O( 17)A O( 17)A O(23)A O(23)A O(23)A O(29)A O(29)A O(29)A O(35)A O(41)A O(47)A O(53)A O(59)A O(65)A O(71)A O(77)A

MO(3)A O(5)A O(23)A O(29)A O(77)A MO(5)A O(23)A O(29)A O(77)A O(23)A O(29)A O(77)A O(29)A O(77)A O(77)A Mo(2)A MO(4)A C(6)A Mo(4)A C(6)A C(6)A Mo(2)A C( 12)A C( 12)A MO(3)A C(18)A C(18)A Mo(4)A C(24)A C(24)A MO(4)A C(30)A C(30)A C(36)A C(42)A C(48)A C(54)A C(60)A C(66)A C(2)A C(78)A

.

129.7 (3) 60.81 ( 5 ) 52.72 (24) 113.32 (25) 97.10 (25) 118.3 (3) 112.54 (24) 52.56 (25) 98.20 (26) 115.5 (3) 163.6 (3) 99.7 (4) 88.1 (4) 90.1 (4) 93.1 (4) 139.4 (4) 79.0 (3) 77.7 (3) 126.1 (8) 71.4 (3) 126.2 (8) 150.1 (8) 81.0 (3) 134.6 (8) 141.7 (8) 74.9 (3) 145.4 (8) 132.8 (9) 73.1 (3) 136.7 (9) 146.3 (9) 81.5 (3) 132.9 (8) 144.9 (8) 122.4 (8) 134.6 (9) 125.3 (8) 128.6 (9) 130.0 (8) 157.2 (9) 141.3 (9) 133.0 (9)

.

Mo(l)B Mo(2)B Mo(2)B Mo(2)B Mo(2)B Mo(2)B MO(3)B MO(3)B MO(3)B MO(3)B O(5)B O(5)B O(23)B O(23)B O(29)B Mo(1)B Mo(1)B Mo(1)B Mo(2)B Mo(2)B MO(4)B Mo(1)B Mo(1)B Mo(2)B Mo(2)B Mo(2)B MO(3)B MO(3)B MO(3)B MO(4)B Mo(l)B Mo(l)B MO(4)B Mo(l)B Mo(l)B Mo(l)B Mo(2)B Mo(2)B MO(3)B MO(3)B MO(4)B

MO(3)B 0(5)~ O(23)B O(29)B O(77)B O(5)B O(23)B O(29)B O(77)B O(23)B O(29)B O(77)B O(29)B O(77)B O(77)B Mo(2)B MO(4)B C(6)B MO(4)B C(6)B C(6)B Mo(2)B C( 12)B C( 12)B MO(3)B C( 18)B C(18)B MO(4)B C(24)B C(24)B MO(4)B C(30)B C(30)B C(36)B C(42)B C(48)B C(54)B C(60)B C(66)B C(72)B C(78)B

.

128.0 (3) 61.22 (5) 53.77 (24) 114.34 (26) 97.32 (27) 115.2 (3) 114.04 (25) 53.12 (26) 97.7 (3) 115.0 (3) 164.4 (4) 100.0 (4) 84.8 (4) 91.2 (4) 93.1 (4) 141.7 (4) 79.2 (3) 79.3 (3) 132.7 (8) 71.7 (3) 119.3 (8) 146.0 (8) 81.4 (3) 136.7 (9) 140.1 (9) 75.1 (3) 147.6 (9) 135.1 (9) 72.6 (3) 131.4 (9) 142.2 (9) 81.4 (4) 130.6 (8) 147.1 (9) 122.8 (8) 128.5 ( I O ) 128.3 (8) 133.9 (9) 126.0 (8) 168.5 (9) 141.6 (9) 129.6 (9)

of VI1 or X I in benzene-d6. T h e result is consistent with the elemental analysis data for the isolated solids VI1 and X I (see Experimental Section) and led to the formulation for the isolated materials as M4(OCH2-~-B~)12~HOCH2-~-B~. W e were fortunate in obtaining a single-crystal X - r a y diffraction structure of MO~(OCH~-C-B~),~(HOCH~-C-BU), XI, which confirmed this assignment. O n cooling toluene-d, solutions of crystalline VI1 and X I to -90 O C , broadening of many of the methine resonances was observed in the ' H N M R spectra as well as a downfield shift of the alcohol proton from 3.72 to 7.28 ppm. However, no clear evidence was obtained for a species consistent with the X-ray structural data in which the mirror plane of symmetry was lost. Similarly, a toluene-d8 solution of VI1 containing 5 equiv of PMe3 showed no clear-cut evidence for formation of W4(OCH2-c-Bu),,(PMe3) at low temperatures (-90 "C) although some methylene resonances were broadened. It is also noteworthy that these cluster compounds remain intact in the presence of PMe3, which provides evidence that they do not exist in reversible equilibrium with their dinuclear counterparts as observed for I1 (see eq 1). It is clear from the solution spectroscopic data that the cluster molecules IV-XI, [M4(OCH2R),,],, contain a mirror plane of symmetry and eight types of alkoxide ligands. T h e four types of Figure 5. A portion of the "C-'H 2D heteronuclear chemical shift relative intensity one lie in the mirror plane, and the four types correlated contour plot (XHCORR) for W,(OCH2-c-Bu),2(HOCH2-c-of relative intensity two are related by the mirror plane. Molecular Bu) in benzene-d6 at 125.76 MHz and 23 OC. weight determinations using I X confirm that the clusters a r e It was determined, by ' H N M R integration, that 1 equiv of free tetranuclear, Le., M4(OCH,R) 12 in benzene solution. The solution alcohol per M4(0CH2-c-Bu)12cluster was liberated on dissolution structures can be related to the solid-state structure of XI by

I

1

i

Chisholm et al.

5308 J . Am. Chem. SOC..Vol. 1 1 1 . No. 14, 1989 Table IV. Fractional Coordinates and Isotropic Thermal Parameters for

atom Mo(l)A Mo(2)A MO(3)A MO(4)A 0 ( 5 ~ C(6)A C(7)A C(8)A C(9)A C(I0)A O(l1)A C( 12)A C( 13)A C(14)A C(15)A C( 16)A O( 17)A C(18)A C(19)A C(20)A C(21)A C(22)A O(23)A C(24)A C(25)A C(26)A C(27)A C(28)A O(29)A C(30)A C(31)A C(32)A C(33)A C(34)A O(35)A C(36)A C(37)A C(38)A C(39)A C(40)A O(41)A C(42)A C(43)A C(44)A C(45)A C(46)A O(47)A C(48)A C(49)A C(50)A C(51)A C(52)A O(53)A C(54)A C(55)A C(56)A C(57)A C(58)A O(59)A C(60)A C(61)A C(62)A C(63)A C(64)A O(65)A C(66)A C(67)A C(68)A C(69)A C(70)A O(71)A C(72)A C(73)A C(74)A C(75)A C(76)A O(77)A

1 0 4 ~

6943 (1) 7664 (1) 7048 (1) 6432 (1) 7095 (4) 7330 (7) 7085 (7) 6398 (8) 6763 (8) 7449 (8) 7602 (4) 7739 (7) 8457 (7) 8607 (7) 9020 (8) 8665 (8) 8042 (4) 8542 (8) 8491 (8) 8968 (9) 9028 '0) 8758 (1 1) 6026 (5) 5457 (7) 5500 (8) 4977 (8) 5553 (9) 6054 (8) 6187 (4) 5814 (7) 5142 (8) 4540 (8) 4073 (9) 4695 (8) 7703 (4) 7783 (8) 8316 (9) 8421 (9) 8456 (10) 8131 (10) 6270 (5) 5897 (8) 5181 (8) 4738 (8) 4597 (9) 4839 (8) 6886 (5) 6427 (7) 6477 (9) 7120 (11) 6677 (11) 6080 (1 1) 8040 (5) 8485 (8) 8582 (10) 7942 (9) 8259 (10) 8749 (10) 8595 (5) 9120 (7) 9770 (7) 10046 (8) 10765 (9) 10447 (8) 6968 (5) 6650 (7) 6989 (8) 6709 (9) 7426 (9) 7677 (8) 7110 (5) 6901 (8) 7283 (8) 7100 (9) 6978 ( 12) 6998 (9)' 6072 (5)

I0 4 ~ 3419 (1) 2335 ( i j 1731 (1) 2199 (1) 2529 (4) 2485 (7) 1882 (7) 1871 (8) 1516 (8) 1796 (8) 3248 (4) 3569 (7) 3839 (7) 446 (7) 4730 (8) 4289 (8) 1916 (4) 1971 (8) 1363 (8) 1332 (9) 559 (10) 687 (11) 1710 (4) 1619 (7) 938 (8) 774 (8) 434 (10) 830 (8) 3098 (4) 3425 (7) 3661 (7) 3156 (8) 3772 (9) 4128 (8) 3725 (4) 4425 (8) 4452 (9) 5131 (9) 4731 (11) 41 16 (10) 3697 (5) 3371 (8) 3370 (8) 3212 (8) 3974 (9) 4094 (8) 4315 (5) 4841 (7) 5379 (9) 5806 (1 1) 6439 (1 1) 6059 (1 1) 1597 (5) 1096 (8) 544 (10) 196 (9) 224 (10) 801 (9) 2928 (5) 2718 (7) 2725 (7) 3407 (8) 3184 (9) 2658 (8) 2314 (5) 2636 (7) 2462 (8) 2836 (9) 2879 (9) 2775 (8) 803 (5) 290 (8) -343 (8) -713 (9) -1351 (12) -956 (9) 1550 (5)

1042 4378* 4549 (1) 5073.8 (5) 4515.9 (5) 4061 (3) 3654 (4) 3447 (5) 3236 (5) 2897 (5) 3048 (5) 4819 (3) 5178 (4) 5166 (4) 4909 (5) 5246 (5) 5525 (5) 5061 (3) 5358 (5) 5624 (5) 5967 (6) 5889 (6) 5480 (7) 4999 (3) 5234 (4) 5442 (5) 5753 (5) 5995 (6) 5749 (5) 4747 (3) 5040 (4) 4880 (5) 4864 (5) 4975 (5) 5135 (5) 4063 (3) 3945 (5) 3649 (5) 3437 (6) 3056 (7) 3251 (6) 4020 (3) 3722 (5) 3827 (5) 3461 (5) 3412 (5) 3835 (5) 4641 (3) 4542 (5) 4843 (5) 4876 (7) 4944 (7) 4757 (7) 4248 (3) 4367 (5) 4063 (6) 3893 (5) 3510 (6) 3657 (6) 4320 (3) 4064 (4) 4272 (4) 4432 (5) 4318 (5) 4026 (5) 5517 (3) 5830 (4) 6200 (5) 6567 (5) 6713 (6) 6297 (5) 4994 (3) 4731 (5) 4817 (5) 5200 (5) 4970 (8) 4584 (6) 4163 (3)

lOBi,

Mo~(OCH,-C-B~)~~(HOCH,~-C-B~) atom C(78)A C(79)A C(80)A C(81)A C(82)A Mo(1)B Mo(2)B MO(3)B MO(4)B 0(5)~ C(6)B C(7)B C(8)B C(9)B C(1O)B O( 1 l)B C(12)B C(13)B C(14)B C(15)B C(16)B O(17)B C(18)B C(19)B C(20)B C(21)B C(22)B O(23)B C(24)B C(25)B C(26)B C(27)B C(28)B O(29)B C(30)B C(31)B C(32)B C(33)B C(34)B O(35)B C(36)B C(37)B C(38)B C(39)B C(40)B O(41)B C(42)B C(43)B C(44)B C(45)B C(46)B O(47)B C(48)B C(49)B C(50)B C(51)B C(52)B O(53)B C(54)B C(55)B C(56)B C(57)B C(58)B O(59)B C(60)B C(61)B C(62)B C(63)B C(64)B O(65)B C(66)B C(67)B C(68)B C(69)B C(70)B O(17)B C(72)B

104~ 5644 (7) 5001 ( 7 j 4494 (8) 3927 (8) 4484 (8) 8099 (1) 7283 (1) 7832 (1) 8492 (1) 7904 (4) 7634 (8) 8121 (8) 8651 (9) 8342 (10) 7780 (8) 7419 (4) 7269 (7) 6552 (8) 6371 (9) 5939 (9) 6347 (9) 6859 (5) 6312 (8) 6276 (8) 5702 (10) 5644 (1 1) 5993 (10) 8852 (5) 9386 (8) 9420 (7) 9937 (9) 9368 (11) 8860 (9) 8811 (5) 9175 (7) 9877 (8) 10480 (8) 10933 (9) 10295 (8) 7425 (5) 7400 (7) 6971 (8) 6995 (9) 7580 (9) 7313 (8) 8852 (5) 9353 (8) 9844 (10) 9485 (13) 10030 (12) 10162 (10) 8218 (5) 8731 (7) 8711 (8) 8108 (10) 8631 (10) 9205 (8) 6848 (5) 6486 (7) 6522 (8) 7739 (8) 6944 (8) 6304 (8) 6437 (5) 5897 (8) 5327 (9) 4922 (9) 4295 (10) 4635 (11) 7999 (5) 8227 (8) 7690 (9) 7916 (11) 7415 (10) 7444 (11) 7960 (5) 7942 (7)

104~ 992 (7) 1104 (7) 495 (8) 1035 (8) 1579 (8) 1642 (1) 2618 (1) 3143 (1) 2863 (1) 2593 (4) 2805 (8) 2712 (8) 3294 (9) 3418 (10) 2903 (8) 1653 (4) 1247 (8) 1000 (8) 480 (9) 129 (9) 458 (9) 2897 (5) 2826 (8) 3453 (8) 3462 (1 0) 4213 (10) 4101 (10) 3299 (4) 3269 (8) 3922 (7) 3896 (9) 4218 (11) 4025 (9) 1946 ( 5 ) 1573 (7) 1368 (8) 1844 (8) 1195 (9) 832 (8) 1352 (5) 681 (7) 685 (8) -8 (9) 311 (9) 1007 (8) 1537 (5) 2011 (8) 1751 (10) 1490 (13) 935 (13) 1021 (10) 705 (5) 252 (7) -329 (8) -821 (10) -1410 (10) -937 (9) 3365 (5) 3922 (7) 4481 (8) 4715 (8) 4733 (8) 4328 (8) 2005 ( 5 ) 2279 () 2408 (9) 1777 (9) 2228 (10) 2651 (11) 2480 (5) 2075 (8) 1980 (9) 1629 (1 1) 2114 (10) 2571 (11) 4074 (5) 4680 (7)

1042 4210 (4) 3979 (4) 4036 (5) 4121 (5) 4151 (5) 9281.4 (4) 9524.9 (5) 10101 (1) 9521 (1) 9041 (3) 8665 (5) 8337 (5) 8264 (5) 7873 (6) 7936 (5) 9720 (3) 10042 (5) 10024 (5) 9700 (6) 9999 (5) 10319 (5) 10059 (3) 10327 (5) 10571 (5) 10879 (6) 10801 (6) 10398 (6) 10032 (3) 10294 (5) 10518 (5) 10873 (5) 11114 (6) 10827 (5) 9690 (3) 9958 (4) 9801 (4) 9839 (5) 9900 (6) 10019 ( 5 ) 8903 (3) 8752 (4) 8395 (5) 8180 (5) 7914 (6) 8043 (5) 8948 (3) 8854 (5) 8555 (6) 8191 (8) 8177 (8) 8611 (6) 9478 (3) 9358 (4) 9634 (5) 9628 (6) 9671 (6) 9568 (5) 9275 (3) 9409 (4) 9121 (5) 8963 (5) 8552 (5) 8690 (5) 9214 (3) 8991 (5) 9235 (5) 9417 (5) 9384 (6) 9051 (7) 10488 (3) 10796 (5) 11096 (6) 11474 (6) 11689 (6) 11324 (7) 10096 (3) 9930 (4)

1OBina 20 (3)

J. Am. Chem. SOC.,Vol. 111, No. 14, 1989 5309

12- Electron Mo a n d W Alkoxide Clusters Table IV (Continued) atom 104x C(73)B C(74)B C(75)B C(76)B O(77)B

7471 (8) 7364 (9) 7944 (9) 7843 (9) 8753 (5)

1 0 4 ~

5247 (8) 5367 (9) 5915 (9) 5943 (9) 3618 (5)

1042

10030 (5) 10484 (5) 10463 (5) 10028 (5) 9212 (3)

1OEi, 27 35 36 36 23

(3) (4) (4) (4) (2)

removing the molecule of coordinated alcohol to form a higher symmetry species (see diagram for XI1 below) which is consistent with the N M R spectroscopic data obtained in solution.

XI1

The butterfly cluster now has three types of molybdenum atoms, wingtip, MoA and Moc, and backbone, MoB. A mirror plane passes through the following atoms: 01,MOA, 04,Moc, O, and Os. The mirror plane then relates the remaining alkoxide ligands to form four pairs of which each methylene group would be diastereotopic. This explanation is consistent with the observation of eight types of alkoxide ligands in solution in the ratio 1 : 1:1 :1 :2:2:2:2 with the alkoxide ligands of relative intensity two containing diastereotopic groups. In order to obtain more information about the structure of these clusters in solution we prepared the isotopically labeled species W4(O*CH2-i-Pr)12 where *C = 99% 13C to examine the ultrahigh resolution 13C N M R spectrum of the methylene carbons. We have previously observedi0 that terminal alkoxide ligand a-carbon nuclei exhibit a relatively large two-bond 1e3W-O-i3C coupling, e.g., 5.9 H z in W , ( O - ~ - B U ) whereas ~, bridging alkoxide ligand a-carbons exhibit two much smaller two-bond 183W-0-13C couplings in the range -2-3 Hz. In the case of W4(O*CH2-i-Pr)12there are some marked differences in the magnitudes of the two-bond 183W-0-13C J values for the different alkoxide ligands. Of the nondiastereotopic ligands, the former 2 J ~ s 3 w G ~ 3 c values are 4.93, -4 (slightly obscured by coincident diastereotopic I3C resonances), 6.93, and 1.93 Hz. This would be consistent with three terminal alkoxide ligands and one bridging alkoxide ligand. Of the diastereotopic alkoxide ligands, the methylene resonances with 183W-O-13C couplings of 6.43 and -4 H z are consistent with terminal alkoxide ligands and those with couplings of 3.84 and C3.4 H z are consistent with bridging alkoxide ligand. These data are not unambiguous, and an absolute distinction between bridging and terminal alkoxide ligands is not possible. However, it seems that the assignments made above are consistent with the structure XII. For example, the nondiastereotopic methylene with the two-bond 183W-O-13C coupling of 1.93 H z may be assigned to the triply bridging alkoxide ligand. This methylene I3C resonance had la3W satellites that have relatively large intensity (Le., > 14% by comparison with other resonances), although it was not possible to integrate them accurately due to overlap with the central resonance. The 95MoN M R spectrum of Mo4(OCH2-i-Pr),, exhibited only a single broad resonance a t 2628.4 ppm, downfield of N a 2 M o 0 4 . Although the spectrum was measured on two separate occasions, it was not possible to resolve any fine structure. This chemical shift is in the region similar to that previously observed for Mo2(OR), compounds.11 N o other resonances were observed over (9) Chisholm, M. H.; Folting, K.; Huffman, J. C.; Kirkpatrick, C. C. Inorg. Chem. 1984, 23. (10) Allerhand, A.; Maple, S. R.; Chisholm, M. H.; Hampden-Smith, M. J., results to be published.

atom C(78)B C(79)B C(80)B C(81)E C(82)B

1 0 4 ~

9162 (8) 9839 (8) 10338 (8) 10747 (15) 10286 (9)

1 0 4 ~

4181 4132 4654 4043 3533

(8) (8) (8) (15) (9)

1042 9311 ( 5 ) 9105 (5) 9277 (5) 9417 (9) 9251 (6)

1OEi, 27 (3) 26 (3) 30 (3) 86 (7) 37 (4)

Scheme I n

a chemical shift range of 3700-2200 ppm. Comments of Cluster Bonding. This new class of M 4 ( 0 C H 2 R ) , , compounds are members of a growing series of 12e- tetranuclear metal clusters. Although only two structurally different examples of homoleptic metal alkoxide tetranuclear 12e- clusters have been unequivocally characterized thus far$s there exist examples that contain heteroligands, especially halides.13 These include a distorted M4 rhombus in W4(pO-i-Pr)4(O-i-Pr)8,11, a square M4 moiety in M o , ( ~ - O - ~ - P ~ ) ~XIII, C ~ , , and a symmetrical C2, M 4 butterfly in M0,(~~-0-i-Pr)~(pO.i.Pr)~(O-i-Pr)~X~, where X = Br and I, shown in XIV below. X

K

X'

X

I Br

XI11

XIV

Perhaps the most remarkable feature of the present cluster system, XII, is the arrangement of the alkoxide ligands to form a cluster in which one metal atom is octahedrally coordinated a t the expense of the coordination environment of the other metal atoms. The thermodynamic driving force for a n octahedral environment is well-known in reduced molybdenum and tungsten oxides,I4 polyoxoanion ~ y s t e m s ,and ~ in the structures of metal alkoxides.ls There are a variety of ways the cluster bonding electrons may be distributed about the butterfly cluster. By simply assuming each metal center of XI1 has retained a +3 oxidation could lead to metal-metal bonding configuration similar to that of 11, with alternating single and double metal-metal bonds and a small interaction between the backbone metal atoms. This is clearly not the case. In contrast to all previously characterized 12-electron alkoxides of M o or W , the present class shows a distinct asymmetry with respect to metal-alkoxide and metalmetal bonding. A mixed-valence structure is represented in XV below that is consistent with the idea that Mo(A) is in the +4 oxidation state and forms only metal-metal single bonds to the (1 1 ) Young, C. G.; Kober, E. M.; Enemark, J. H. Polyhedron 1987,6,255. Minelli, M.; Enemark, J. H.; Brownlee, R. T. C.; O'Connor, M. J.; Wedd, A. G. Coord. Chem. Rev. 1985, 68, 169. (12) Chisholm, M. H. Polyhedron 1983, 2, 681 and references therein. (13) Chisholm, M. H.; Errington, R. J.; Folting, K.; Huffman, J. C. J . Am. Chem. SOC.1982, 104, 4389. Chisholm, M. H.; Clark, D. L.; Folting, K.; Errington, R. J.; Huffman, J. C. Inorg. Chem. 1988, 27, 2071. (14) McCarley, R. E. ACS Symp. Ser. 1983, 211, 273. (15) Bradley, D. C.; Mehrotra, R. C.; Gaur, P. D. In Metal Alkoxides; Academic Press: 1978.

5310

J . Am. Chem. SOC.,Vol. 1 1 1 , No. 14, 1989 0

Chisholm et al.

Table V. Summary of Bond Length-Bond Strength Calculations and the Assignment of Oxidation States in Mo~(OCH,-C-BU) ~~(HOCH~-C-BU) valency units s(Mo-0) bond molecule A molecule B Mo( 1)-O(5) 0.502 (13) 0.506 (13) M0(1)-0(11) 0.602 (16) 0.615 (16) Mo( 1)-O(29) 0.549 (14) 0.520 (15) Mo( 1)-O(35) 0.767 (22) 0.765 (24) xv Mo(l)-0(41) 0.901 (35) 0.924 (29) backbone molybdenum atoms. The remaining eight cluster Mo( 1)-O(47) 0.672 (18) 0.686 (21) electrons are shared, or delocalized, over the other triangle to form Zs[Mo(l)-O] = 3.99 (2) 4.02 (2) M-M bonds of bond order one and one-third. This is the first Mo(2)-0(5) 0.552 (14) 0.532 (14) time we have seen an intramolecular valence disproportionation Mo(2)-O(11) 0.593 (16) 0.580 (15) in an alkoxide cluster of molybdenum and tungsten although a M0(2)-0( 17) 0.511 (13) 0.498 (14) number of mixed valent dinuclear alkoxides are known, e.g., Mo(2)-O(53) 0.793 (23) 0.860 (2) M~~(O-i-Pr)~(dmpe),.'~ Zs[M0(2)-0] = 2.45 (2) 2.47 (2) The structure depicted by XI1 can be related to the rhomboidal Mo(3)-0(17) 0.658 (18) 0.676 (20) W4(O-i-Pr)lZstructure, 11. Indeed, the local geometry about Moc Mo(3)-0(23) 0.587 (16) 0.560 (15) in XI1 (Mo(3) in Figure 1) is the same as that of the wingtip Mo(3)-O(65) 0.841 (27) 0.901 (29) tungsten atoms in 11. T h e M4Ol2 unit of I1 can easily be Mo(3)-0(7 1) 1.019 (3) 1.006 (33) transformed to that of XI1 by the opening and closing of various Zs[M0(3)-0] = 3.10 (2) 3.14 (2) bridges as shown in Scheme I. T h e 12 electrons in M 4 alkoxide clusters are accommodated Mo(4)-0(5) 0.434 (11) 0.504 (13) in M-M bonds in a variety of geometries. In the square (D4,,) Mo(4)-O(23) 0.511 (13) 0.511 (15) M O ~ ( ~ - O - ~ - P molecule ~ ) ~ C I ~the four M o - M o distances of 2.38 M0(4)-0(29) 0.643 (17) 0.563 (19) Mo(4)-O(770) 0.879 (28) 0.887 (28) 8, are indicative of some M-M multiple bond order, while in the C , M o butterfly ~ M0,(~~-0-i-Pr)~(p-O-i-Pr)~(O-i-Pr)~Br~ there Zs[M0(4)-0] = 2.47 (2) 2.54 (2) are four Mo-Mo distances of 2.51 8, (av) and one slightly shorter distance between the backbone M o atoms, 2.48 A. This is conformal oxidation for Mo(1) is determined to be +4.0 for both sistent with the slightly greater M-M bonding expected for molecules, while the other wing-tip M o atom, Mo(3), that is backbonebackbone interactions based on M O calculation^.'^ It coordinated to four oxygen atoms, has an oxidation state of is also worth noting that a number of tetranuclear halide clusters +3.1(1). T h e backbone M o atoms Mo(2) and Mo(4) a r e efof molybdenum have been prepared by McCarley and co-workers fectively +2.5 each, although when the M o - 0 bond of distance with varying cluster electron counts ranging from 12 to 16.18 2.342 (10) to the alcohol ligand is included the oxidation state However, in none of these clusters is there such asymmetry with of Mo(2) is increased by +0.25 valence units. respect to ligand-to-metal bonding. Therefore, using the existing bond length-bond strength relaApplication of Bond Length-Bond Order Relationships. Nontionships for molybdenum and oxygen, we find a numerical method linear bond strength-bond length relationships have been proposed for establishing the nature of the charge delocalization in the for element-oxygen bonds and find use as independent checks on clusters of structural type XII. The formal oxidation states +4, certain solid-state structural preferences, as a means of (i) de+3, and two +2.5 give a central M4I2+core, a 12-electron cluster tecting secondary bonding effects and (ii) determining the valency in which the oxidation states of the wingtip metal atoms are greater distribution in mixed-valence compound^.'^ T h e bond than those of the backbone. strength-bond length relationships that have been developed for It is interesting to apply the same type of calculation to the molybdenum and oxygen find profitable use for both coordination rhomboidal W4(O-i-Pr)lZcluster of structural type 11, and when compounds and solid-state oxides with and without metal-metal we make the assumption that W - 0 and M o - 0 bond distances bonds.20 Specifically, these have been used by McCarleyZ1 in are equivalent, which is good to within one or two hundredths of studies on lower valent molybdenum oxides such as NaMO406 and an A, we find this gives +3.5 and +2.5 for the wingtip and Ca5.45M018032. backbone W atoms, respectively. The equation relating M o - 0 bond lengths and valency of M o McCarley2' has used the empirical formula is n,(M-M) = Zn(M-M) S( Mo-0) = [ d (M o - 0 ) / 1.8821-6.0 to determine the number of electrons used by a given metal in where S = bond strength in valence units and d ( M o - 0 ) = oba cluster to form M-M bonds. A single bond radius d(Mo-Mo) served bond distance in 8,. The value 1.882 8, is taken to represent was taken to be 2.614 A. By using this procedure the electrons a standard single bond distance between molybdenum and oxygen. used in M-M bonding in Mo~(OCH~-~-BU)~~(HOCH~-Z-BU) are The sum of the values of S ( M o - 0 ) about each metal represent for Mo(1) 1.76 and Mo(3) 2.66 and by the backbone M o atoms, the formal oxidation state of that molybdenum atom. For the Mo(2) 3.57 and Mo(4) 3.62. Again this formalism of counting structurally characterized compound M O ~ ( O C H ~ - C - B U ) ~ ~ electrons provides a complementary picture of M-M bonding (HOCH2-t-Bu), there a r e two crystallographically similar but within the 12-electron M04I2+ core. Using the same single bond independent molecules in the unit cell. As is seen in Table V the distance for a W-W bond gives 2.78 and 2.13 electrons available for M-M bonding by the backbone and wingtip W atoms, re(16) Chisholm, M. H.; Huffman, J. C.; Van Der Sluys, W. G. J . Am. spectively, in W4(O-i-Pr)lZ. Chem. SOC.1987, 109, 2514. Mechanism of Cluster Formation. We have recently described (17) Bursten, B. E.; Clark, D. L.; Chisholm, M. H. Inorg. Chem. 1988, the base-free alcoholysis reactions of W , ( O - ? - B U ) ~that lead to 27, 2084. the formation of new W 2 ( O R ) 6 species (where R = CH2-t-Bu, (18) Aufdembrink, B. A.; McCarley, R. E. J . Am. Chem. SOC.1986, 108, 2474. Stensvad, S . ; Helland, B. J.; Babich, M. W.; Jacobson, R. A.; c y , i-pr).22 Whereas solutions of W,(O-i-Pr)6 exist in equilibrium McCarley, R. E. J . Am. Chem. SOC.1978, 100, 6257. (19) Brown, I. D.; Shannon, R. D. Acta Crysfalogr. 1973, A29, 266. (22) Chisholm, M. H.; Hampden-Smith, M. J. National Meeting of the (20) Bart, J. C. J.; Ragaini, V. Inorg. Chim. Acta. 1979, 36, 261. Brown, American Chemical Society, New Orleans, LA, September 1987; Abst. Inorg. I. D.; Wu, K. K. Acta Crstallogr. 1976, 832, 5105. 44. Chisholm, M. H.; Huffman, J. C.; Hampden-Smith, M. J. J . Am. Chem. (21) McCarley, R. E. Polyhedron 1986,5, 51. See, also: Cheetham, A. SOC.submitted for publication. K., et al. J . Am. Chem. SOC.1988, 110, 3295.

J . Am. Chem. SOC., Vol. I l l , NO. 14, 1989 5311

12- Electron M o a n d W Alkoxide Clusters with the corresponding cluster, W4(O-i-Pr)lz (see eq I), w 2 ( o c y ) 6 was found to be completely inert with respect to cluster formation. This has been attributed to the large steric demand of the cyclohexoxide ligand compared to isopropoxide. By further reducing the steric demands of the alkoxide ligands we had expected to obtain either higher aggregates of MI1' alkoxide clusters (hexanuclear or octanuclear for example) or examples of new structural types of existing nuclearity. Oxidative addition of alcohol to form higher oxidation state metal alkoxide clusters is unlikely because these processes are base promoted.23 Since no evidence for new dinuclear species M 2 ( 0 C H , R ) 6 was obtained in the alcoholysis reactions according to eq 2, we sought to determine if dinuclear species were present in such solutions. Isobutyl alcohol ( H O C H 2 - i - P r ) was added to a hexane solution of W2(O-t-Bu), containing a large excess of PMe,. T h e solution immediately turned dark red/brown, the typical color of W2(OR)6L2species, in contrast to the deep purple color of W 4 ( 0 C H 2 - i - P r ) , 2 . O n working up the solution, W2(OCH2-i-Pr)6(PMe3)2was isolated, and no evidence was obtained for a n y cluster formation. In a n attempt to isolate an example of a Lewis base-free dinuclear species M 2 ( 0 C H 2 R ) , , the target molecule M o , ( O C H , C ~ ) was ~ chosen for two reasons: (i) molybdenum(II1) alkoxides are more inert with respect to cluster formation than their tungsten analogues and (ii) the rate of cluster formation is reduced as the steric demands of the alkoxide ligands increase. Attempts to isolate pure M o ~ ( O C H ~ Cby~ performing )~ the alcoholysis reaction, eq 3, a t low temperature (-78 "C) were unfruitful because of the very high solubility of these complexes and the low volatility of the alcohols involved. T h e yellow solutions, probably containing the dinuclear species, generated at low temperature turned green on workup, and only the cluster species were isolated. A second approach was taken employing the initial formation of a Lewis base-stabilized dinuclear complex. W e have previously used this approach to prepare W2(0-i-Pr)6,4where a suitable Lewis base was N M e 3 . Treatment of a solution of Mo,(NMe,), with cyclohexylmethanol a t 0 OC in the presence of a large excess of N M e 3 resulted in a deep red color and after concentrating the solution and cooling overnight, the red-brown crystals of M O ~ ( ~ C H ~ C ~ ) were ~ ( Nobtained. M ~ ~ ) O~n dissolving these crystals in benzene-d6 solution a t room temperature, the IH N M R spectrum (500 M H z , 25 "C) obtained was consistent with a dinuclear species with coordinated N M e 3 and rapid rotation about the metal-metal triple bond. However, on standing for a period of about 4 h, the compound completely reacted to form the M o 4 ( 0 C H 2 C y ) , , cluster together with free N M e 3 , as shown by 'HN M R spectroscopy. It appears that in the absence of an excess of Lewis base, the Lewis base free dinuclear compound is long-lived enough for successful dimerization to occur, see eq 3. M O ~ ( ~ C H ~ C ~ ) == ~ ( NMo2(0CH2Cy)6 M ~ ~ ) ~

+

2NMe3

I Mo~(OCH~CY)~~

(3)

A crystalline sample (32 mg) of M o , ( O C H ~ C ~ ) ~ ( N M ~was ,), placed in a reaction vessel which was evacuated and heated in a n oil bath a t 70 OC over 5 h under a dynamic vacuum in order to remove any liberated NMe,. [In the solid state, the rate of cluster formation is likely to be a lot slower than in solution.] During this time the dark solid lightened considerably, and when it had cooled it was dissolved in toluene-d8 a t -78 O C and sealed under nitrogen. A ' H N M R spectrum taken a t -40 OC showed only the presence of the M o 4 ( 0 C H 2 C y ) , , cluster, and there was no evidence of a n y resonance due to dissociated N M e , . These trapping experiments using Lewis base ligands provide convincing evidence that M 2 ( O C H 2 R ) , species are first formed in the alcoholysis of M,(O-t-Bu), compounds as depicted below. It has already been noted that the cluster species formed from the dimerization of W,(O-GPr), has a different structure from those in the present study. The question therefore arises, are the

+

M ~ [ O - ~ - B U ) ~6HOCHzR

-

[M2(OCH2R),I

+

6HO-t-BU

i '5 [M d(0CH2R ) 12 1

Conclusions T h e work describes a new class of 12e- tetranuclear metal alkoxide clusters M4(OCH2R),, that have been prepared in high yield by simple alcoholysis reactions between M , ( O - ~ - B U )and ~ the appropriate primary alcohols ( H O C H 2 R ) . T h e cluster compounds are the first examples of homoleptic alkoxide clusters that exist for both molybdenum and tungsten. The solid-state structure of Mo4(OCH2-c-Bu),,(HOCH2-c-Bu) reveals that an interesting ligand redistribution has taken place on formation of these compounds. O n e metal atom has achieved a n octahedral geometry a t the expense of the coordination environment of the remaining metal atoms. This, we hope, will allow us a unique opportunity to examine the mechanism of activation of small molecules by tetranuclear metal alkoxide clusters that may serve as models for fragments of reduced metal oxides. Note that three of the metal atoms in M 4 ( O R ) , , compounds of type XI1 are coordinatively unsaturated, and the ability of the cyclobutylmethoxide cluster to coordinate a single molecule of cyclobutylmethanol testifies to the metal atoms desire to bind donor ligands. T h e presence of the M-M bonding electrons should allow for ?r-acid ligands to bind, and the presence of the tetranuclear metal center provides for template or ensemble effects.24 I t will be particularly interesting to compare the reactivity of these M 4 ( O C H 2 R ) , , compounds with their dinuclear M,(OR), counterparts toward activation of carbon monoxide and alkynes. Finally we note that the structures of the homoleptic alkoxides of molybdenum and tungsten of empirical formula M(OR), differ from those seen for M = AI and Cr,I5 in that in addition to the

(23) Chisholm, M . H.; Huffman, J. C.; Smith, C. A. J . Am. Chem. Soc. 1986, 108, 202.

(4)

two clusters I1 and XI1 formed mutually exclusively or are they directly related (Le., one formed from the other). Since no other ' H N M R resonances were observed in the conversion of Mo2(OCH2Cy)6(NMe3)2to Mo4(0CH2Cy),,, it can be assumed that if an intermediate species is formed, it is (a) formed in concentrations below the sensitivity of N M R spectroscopy and/or (b) is rapidly converted to the cluster of geometry XII. In order to examine the possibility that upon the coupling of two M2( O C H 2 R ) , molecules the first formed cluster species might be analogous to 11, we conducted an independent alcoholysis reaction involving W4(0-i-Pr),, (11). A 50:50 toluene-isobutyl alcohol mixture was added to pure w 4 ( o - i - P r ) l z ,( w 2 ( o + P r ) 6 free) a t 0 O C such that a solution was formed that was identical in concentration [W4(O-i-Pr)12]with those previously used to measure the kinetic data for cluster formation and d i ~ s o c i a t i o n . After ~ stirring the reactants for only 0.5 h a t 0 OC, the volatile components were removed, and the whole of the dry residue dissolved in benzene-d,. At room temperature IH N M R spectrum indicated only the presence of W4(OCH2-i-Pr)12. Since, under these conditions the extent of dissociation of W4(0-i-Pr),, to 2W,(O+Pr), is negligible, the new butterfly tetranuclear cluster XI1 has been formed directly from the previously known rhomboidal cluster unit 11. Thus, if the dimerization of M o 2 ( 0 C H 2 R ) , compounds proceeds first via a planar rhomboidal cluster analogous to I1 it would subsequently rapidly rearrange to the new butterfly cluster type XI1 as shown in eq 5 below. However, we cannot establish that a rhomboidal cluster of type I1 is actually a precursor to XI1 in a reaction starting from two M 2 ( O C H 2 R ) 6 ( M G M ) molecules.

(24) Shriver, D. F.; Sailor, M. J. Acc. Chem. Res. 1988, 21, 374

J . Am. Chem. Soc. 1989, 1 1 1 , 5312-5320

5312

desire to maximize M - O bonding, the metal atoms ( M = M o and W ) also want to maximize M-M bonding. Hence for [M(OR),],, where n = 2, the ethane-like 0 , M = M 0 3 geometry, I, with the M-M bonding configuration uZp4,is preferred over the [AI(OR),], structure involving two fused MO, tetrahedra. For n = 3, there is no known example for M = M o or W-perhaps not surprisingly since n = 3 yields nine electrons available for M-M bonding which would necessarily lead to paramagnetism and would not maximize M-M bonding. For n = 4,there are 12 cluster electrons that can be accommodated in a variety of structures as seen for W4(Oi-Pr)12,11, M O ~ C I ~ ( O - ~ -XIII, P ~ ) ~and , M ~ ~ B r ~ ( o - i - PXIV, r)~, and in the new homoleptic class M z ( O C H 2 R ) , 2 , XII. T h e preference for XI1 over structures 11, XIII, and XIV appears to represent a situation wherein one metal atom achieves the maximum number of M - 0 bonds, namely six for M = W and Mo, while the other three metal atoms maximize their M-M bonding a t the expense of M - 0 bonding.

Acknowledgment. W e thank the National Science Foundation and the Department of Energy, Basic Sciences, Chemistry Division for support. K.G.M. thanks the South African C S I R for financial support and the University of Durban-Westville for sabbatical

leave. M . H . 4 . thanks Dr. Jotham W. Coe and Alan D. Palkowitz for helpful discussions. W e thank Stephen Maple for conducting the ultrahigh resolution I3C N M R experiments. Registry No. V, 114130-29-9; VI, 114130-30-2; VII, 120853-44-3; VIII, 114130-32-4; IX, 114130-33-5;X, 114130-34-6;XI, 114094-48-3; W,(O-t-Bu),, 57125-20-9; W,(O*CH2-i-Pr)12, 120853-45-4; W2(NMe2)6,54935-70-5; HO*CH2-i-Pr, 93667-77-7; i-PrMgBr, 920-39-8; ' ) C 0 2 , 11 11-72-4; W2(0CH2-i-Pr),(PMe3)2, 120829-90-5; Mo2(OCH2cy),(NMe3),, 120829-87-0;M0~(0CH~-i-Pr)~(py),, 120829-88-1; W2(O*CH2-i-Pr),(NMe2H),,120829-89-2; Mo2(NMe&, 5 1956-20-8; W4(O-i-Pr)12,104911-26-4.

Supplementary Material Available: Tables of anisotropic thermal parameters and bond distances and angles, VERSORT drawings and stereoviews of the molecules giving the atom numbering scheme, and N M R data for compounds M 4 ( O C H 2 R ) 1 2 , where M = W , R = cyclohexyl (IV), R = cyclopentyl (VI) and M = Mo, R = i-Pr (IX), R = cyclohexyl (VIII), and M4(OCH2-c-Bu)4(HOCH2-c-Bu)where M = M o (XI) and M = W (VII) (29 pages); table of observed and calculated structure factors for Mo~(OCH~-C-BU),~(HOCH~-C-BU) (21 pages). Ordering information is given on any current masthead page.

+

Competitive Intramolecular [4 21 Cycloaddition and Tandem [ 2 21 Cycloaddition/[ 3,3]-Sigmatropic Rearrangement Sequence of Allenyl 3-Vinyl-2-cyclohexenyl Ethers: Evidence for Switching of the Reaction Pathway by the Substituent Effectst

+

Kenji Hayakawa,IBKazuyoshi Aso,la Motoo Shiro,lband Ken Kanematsu*'" Contribution from the Institute of Synthetic Organic Chemistry, Faculty of Pharmaceutical Sciences, Kyushu University 62, Fukuoka 81 2, Japan, and Shionogi Research Laboratories, Shionogi & Co., Ltd., Fukushima, Osaka 553, Japan. Received November 28, 1988

Abstract: The base-catalyzed intramolecular cycloaddition reactions (t-BuOK,t-BuOH, 83 "C) of variously substituted propargyl 3-vinyl-2-cyclohexenyl ethers have been investigated. The reaction proceeded smoothly via the initial isomerization to the corresponding allenyl ethers followed by the intramolecular Diels-Alder ([4 + 21) reaction and/or tandem [2 + 21 cycloaddition, [3,3]-sigmatropic rearrangement ([2 21 [3,3]) depending upon the substitution pattern. The C(2) substituent showed a remarkable switching effect. While the compounds bearing no substituents at C(2) (la,b, 21, 22) underwent selectively the [4 21 cycloaddition, the 2-substituted compounds (lc,d) performed exclusively the tandem [2 + 21, [3,3] reactions. On the other hand, the C(6) substituents also influenced the reaction pathway in a unique manner. While the reaction of the 1,6-trans compounds (15, 20) gave exclusively the [2 + 21 + [3,3] products, the similar reaction of the 1,6-cis isomers (14, 19) resulted in a concomitant formation of [4 + 21 and [2 21 + [3,3] products. In the latter case, the allene intermediates (36, 37) could be isolated. The structure-reactivity relationship in the intramolecular cycloadditions of the allenyl ethers was discussed in detail.

+ +

+

+

The ability of allenes to undergo either inter- or intramolecular cycloaddition reactions with various unsaturated functionalities provides a convenient route for the construction of complex ring systems.2 T h e intramolecular Diels-Alder reaction utilizing appropriate allenic dienophiles has proved to be an extraordinarily useful synthetic tool because of unusual facilitation of cycloaddition as well as a high degree of stereochemical control due to the unique geometry of aIIene m o I e c u I e ~ . ~ - ' ~ Recently, we have reported a remarkable substituent effect in the intramolecular cycloaddition reaction of allenyl 3-vinyl-2cyclohexenyl ethers (2) prepared in situ by the base-catalyzed +Thispaper is dedicated to Professor Haruaki Yajima on the occasion of his retirement from Kyoto University in March, 1989. 0002-7863/89/ 15 11-5312$0l.50/0

rearrangement of the corresponding propargyl ethers 1 (Scheme I).' T h e key feature of the substituent effect is switching of the ~~

~

(1) (a) Kyushu University. (b) Shionogi & Co., Ltd. (2) Schuster, H. F.; Coppola, G. M . AIlenes in Organic Synthesis; Wiley, New York, 1984. (3) Hayakawa, K.; Yodo, M.; Ohsuki, S.; Kanematsu, K. J . Am. Chem. SOC.1984, 106, 6135. (4) (a) Hayakawa, K.; Yamaguchi, Y.; Kanematsu, K. Tetrahedron Lett. 1985, 26, 2689. (b) Yamaguchi, Y.; Hayakawa, K. J . Chem. SOC.,Chem. Commun 1987, 515. (c) Yamaguchi, Y.; Yamada, H.; Hayakawa, K.; Kanematsu, K. J . Urg. Chem. 1987, 52, 2040. (5) (a) Hayakawa, K.; Yasukouchi, T.; Kanematsu, K. Tetrahedron Lett. 1986, 27, 1837. ( b ) Ibid. 1987, 28, 5895. (6) Hayakawa, K.; Ohsuki, S.; Kanematsu, K. Tetrahedron Lett. 1986,27,

947.

0 1989 American Chemical Society