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Cyclooctatetraene Complexes of Niobium and Tantalum and the Structure of R. R. Schrock,* L. J. Guggenberger, and A. D. English Contribution No. 2259 from the Central Research and Development Department, Experimental Station, E . I . du Pont de Nemours and Company, Wilmington, Delaware 19898. Received April 18, 1975 Abstract: Cyclooctatetraene complexes of the type M(CsH&R (M = Nb, Ta; R = C6Hs, CH3) and Ta(C&s)(CH3)3 have been prepared employing [K+]2[C8H8l2- as the reducing agent and source of C8H8. In the former the q4 and q8 rings interconvert on the N M R time scale with AG& = 15.7-17.5kcal mol-’ depending on M and R. Except where R = C6H5 both H ~ ) ~R. = C6H5, forcing conditions give comform adducts with some donor ligands such as ( C H ~ ) ~ P C H Z C H ~ P ( CWhen plexes containing the 2,3,4,5,6-q-endo-8-phenylbicyclo[5.l.0]octadienylligand, e&, Nb(q4-CsHs)[qS-CsHs(c6H~)][ ( C H ~ ) ~ A S C ~ H ~ A S ( Cthe H ~structure ) ~ ] , of which was determined by x-ray methods. The cyclopropyl ring is exo to the metal and the phenyl substituent endo, the first instance where the geometry of this bicyclic ligand has been established. The q4-C8Hs and diars geometries are normal. Similar complexes are obtained when electrophiles (E = H+, D+,and H J facility )~. of the q8-CsH8 C(CsHs),+) attack [M(q4-C8H8)(q3-CsHs)2]- in the presence of ( C H ~ ) ~ P C H ~ C H ~ P ( CThe (e.g.) q4-CsH8 process suggests that complexes containing q8-C8H8 are potentially valuable catalysts (or catalyst precursors) for reactions taking place at the metal center
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Cyclooctatetraene complexes are known for most transition metals’ as well as for several lanthanides and actinides2 Cyclooctatetraene is a versatile ligand which adapts to the metal’s requirements by increasing or decreasing the number of formal olefin-metal bonds, extremes being planar q8 bonding, which predominates for group 4 and 5 metals, lanthanides, and actinides, and q2 bonding, which is rare and known only for late metals like C U .The ~ predominant mode is q4 or q6 in which only the bonded carbons lie in a plane.4 Planar q8-CgH8 may be viewed either as a dianion or as a neutral tetraene. Each is reasonable, the first because one would otherwise have to postulate unusually low formal metal oxidation states like Ti(0) in Ti(q4-CgHg)(q8-C8H8),S the second because, formally at least, q8-C8Hg can become q6-CgH8 or q4-CgH8, or be displaced entirely. Overall, q8-CgH8could be called a “protecting” ligand, since each of the steps, q8 q6 q4, creates a coordination site at a potentially reactive metal in a low formal oxidation state, or, alternatively, simply as a “stabilizing” ligand analogous to v’-C~H~.~ Several C8Hg complexes of niobium and tantalum have been prepared in order to determine the predominant CgHg bonding modes and to study the relationship of bonding to reactivity with simple donor ligands, reactions which may be models for more general reactions at the metal center. A crystal structure of one product shows that cyclooctatetraene can play a more active role by accepting a ligand transferred from the metal.
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Experimental Section Preparations. All manipulations were done in a nitrogen-filled Vacuum Atmospheres drybox. Solutions of [K+]2[C8Hs12- were prepared by stirring the appropriate quantities of C8Hs (redistilled under nitrogen) and potassium pieces in T H F overnight. Niobium and tantalum pentahalides were used as received. Solvents were passed through 4A molecular sieve columns and degassed before use. Compounds were analyzed at the Central Research and Development Department. The most air and moisture sensitive consistently analyzed low in carbon (up to ca. l%), even though drybox procedures were employed and all reasonable precautions taken to ensure sample purity. Less sensitive derivatives usually analyzed satisfactorily.
* Department of Chemistry, 6-33 I,Massachusetts Institute of Technology, Cambridge, Massachusetts 021 39.
1. Preparation of Nb(CsHs)&Hs). NbCls (50 g) in 600 ml of toluene at -78 OC was treated with 800 ml of 0.46 M [K+I2[C8H8lZ- in T H F with stirring over a period of 2 hr. After an additional 1 hr, 98 ml of 17.82% LiC6Hs in diethyl ether-benzene (23:77)was added over 1 hr and the final mixture was warmed to room temperature. Potassium chloride was separated by centrifugation in gas-tight polypropylene bottles, homogenized with 800 ml of THF, and again separated by centrifugation. The decanted solutions were combined and filtered and the solvent was removed in vacuo to a volume of -600 ml. Solids were filtered off and the volume further reduced to -100 ml. Solids from a second filtration combined with the first were dried in vacuo; yield 74 g. The crude product was stirred in 500 ml of acetonitrile, filtered off, washed with 200 ml of diethyl ether, and stirred into 500 ml of dichloromethane. Filtering and reducing the filtrate volume to -100 ml gave 27 g of Nb(C8H&(C&) which could be recrystallized from T H F (35 g in 600 ml at 60 OC) or CH2C12 as red-brown plates. The acetonitrile wash above contained 18 g of [Li(CH3CH),]+[Nb(CsH8)3]which could be recrystallized from T H F as the [Li(THF)4]+ salt. Anal. Calcd for NbC22H21: C, 69.85; H, 5.60; Nb, 24.56. Found: C, 68.87;H, 5.60;Nb, 24.44.‘H NMR ( 7 , CD2C12, 0 “C): 2.56 (2,dd, J = 7.5 and 1.5 Hz, ortho), 3.09 (3,m, meta and para), 4.10(8,s, C ~ H E )4.57 , (8,S, CEHE). 2. Preparation of Nb(CsH&(CH3). Nb(CH3)2C13 (8.52g, 37.2 mmol) and NbClJ (9.5g, 35.2 mmol) were stirred in toluene at room temperature for 3 hr. After cooling to -78 OC, a solution of [K+]2[CsH812- (0.47 M in THF, 300 ml, 140 mmol) was added over 45 min and the mixture was warmed to room temperature and stood overnight. The mixture was centrifuged to remove KCI and the volume was reduced to near dryness, filtering periodically to give 8.6g of crude Nb(CeH&(CH3). The product was recrystallized from hot T H F by cooling to -30 OC overnight. Anal. Calcd for NbC17H19: C, 64.57;H , 6.05.Found: C, 63.25; H, 6.00.‘ H N M R ( T , CD2C12, 0 “C): 4.14 (8, s, Cg.H8), 4.65 (8,s, CsHs), 9.83 (3, S, CH3). 3. Preparation of Ta(CsHs)&Hs). TaCls (10.0 g) in 600 ml of toluene was stirred for ca. 15 min until all of it nearly dissolved. Zn(CsH5)2 (6.08g) was added as a solid and gelatinous ZnCI2 filtered from the yellow solution. TaClS (10.0 g) was added and the mixture was stirred for ca. 15 min then cooled to -78 OC.A 0.47 M solution of [K+]2[C8H812- in tetrahydrofuran (240 mi) was added dropwise over 1 hr. The mixture was warmed to room temperature and centrifuged to remove KCI (filtrate A). The KCI was homogenized in 750 ml of T H F and again removed by centrifugation (filtrate B). The volume of filtrate B was reduced in vacuo and a total of 6.0 g of Ta(C*H&(C&Is) filtered off periodically (filtrate C). Filtrate A gave 7.5 g of brown solid (by a similar isolation procedure) which was extracted with 250 ml of dichlorometh-
Schrock, Guggenberger, English
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Cyclooctatetraene Complexes of N b and Ta
904 Table 1. A G Values for the n8-C.H.
t
q4-C.H. Process
a Accuracy limited by peak broadening. b Accuracy limited by peak broadening and decomposition.
I '
Figure 1. The variable temperature 100-MHz 'H NMR spectrum of in o-dichlorobenthe CsHs ligands in Ta(rl8-CsHs)(rl4-CsHs)(CH3) zene. ane. The extract was filtered and the filtrate combined with C, reduced to -50 ml in vacuo, and stood at -30 OC overnight to give 3.5 g of Ta(csH&(C6H5); total yield 9.5 g (37%). Anal. Calcd for TaC22H21: C, 56.65; H, 4.55. Found: C, 56.31; H, 4.61. ' H N M R ( T , CDzCI2,O "C): 2.45 (2, dd, J = 1.5 and 7.5 Hz, ortho), 3.05 (3, m, meta and para), 4.08 (8, s, C ~ H S )4.61 , (8, S, CsHs). 4. Preparation of Ta(CsHs)z(CH3).Ta(CH3)3C12 (5.0 g) and TaCls (12.1 g) were stirred in toluene overnight. The solution of Ta(CH3)C14 was cooled to -78 OC and 213 ml of 0.47 M [K+]2[CsHsI2- was added over 1 hr. The mixture was warmed to room temperature, centrifuged to remove KCI, reduced in volume to ca. 150 ml, and filtered to give 8.0 g of red-brown Ta(CsHs)2(CH3). A 7.5-g sample was recrystallized from 150 ml of dichloromethane by volume reduction to give 5.0 g of Ta(CsHs)2(CH3) as glittering red-brown, almost black, plates. Anal. Calcd for TaC17Hlg: C, 50.38; H, 4.73. Found: C, 49.68; H, 4.72. ' H N M R ( T , CD2C12, 0 "C): 4.09 (8, S, CsHs), 4.64 (8, S, CsHs), 9.95 (3, s, CH3); see Figure 1 for variable temperature spectra. 5. Preparation of Ta(CsHs)(CH3)3.Ta(CH3)3C12 (40 g) in toluene ( 1 I.) was cooled to -78 OC and 300 ml of 0.47 M [ K + ] ~ [ C ~ H Swas ] ~ - added over 2 hr. The solution was warmed to room temperature and stood for 2 hr. KCI was removed by centrifugation and the volume was reduced to ca. 25 ml, filtering periodically, to give 35.4 g of crude Ta(CsHs)(CH3)3. The product may be purified by recrystallization in high yield from hot toluene by cooling to -30 OC; Ta(CsH&(CH,) is the impurity. It also sublimes without decomposition (100 OC, 1 p ) . Anal. Calcd for TaClIH17: C, 40.01; H, 5.19; Ta, 54.80. Found:
Journal of the American Chemical Society
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c , 39.83; H, 5.23: Ta, 54.80. ' H NMR ( T , C6D6): 4.03 (8, S, CsHs), 9.17 (9, s, CH3). Mass spectrum: 330.0810 (calcd 330.0787). Cryoscopic molecular weight in benzene: 35 1 (calcd 330). 6. Preparation of Ta(CsHs)(CH~)j(dmpe)(dmpe = (CH3)zPCHzCHzP(CH3)z).Ta(CsHs)(CH3)3 (2.0 g) in toluene (40 ml) was treated with 0.95 g of dmpe. Crystals formed immediately. After 4 hr at -40 OC the product was filtered off, washed with pentane, and vacuum dried; yield 2.5 g (85%) of green-black crystals. Anal. Calcd for Tac17H33P2: C, 42.51; H, 6.92; P, 12.90. Found: c , 42.53; H, 6.99; P, 13.31. ' H N M R ( T , C6D6): 4.27 (8, t, J = 1.1 Hz, CsHs), 7.5-8.4 (4, m, dmpe CH2's), 8.73 (6, d, J = 8 Hz, dmpe CH3 set), 8.87 (6, d, J = 8 Hz, dmpe CH3 set), 9.61 (3, dd, J = 3 and 13 Hz, single CH,), 11.05 (6, dd, J = 9.5 and 11.5 Hz, CH3 pair). At 220 MHz the dmpe CH3 sets are well separated. 3 i P { ' H )NMR (CH2C12): 7.81 and 18.97 ppm downfield from 85% H3P04 (doublets, J = 11 Hz). 7. Preparation of Ta(CsHs)(CHj)3(diars)(diars = (CH3)zAsC&AS(CHJ)Z). Hexane (30 ml) was added to a solution of 1.Og of Ta(CsHs)(CH3)3 and 0.87 g of diars in 20 mi of toluene and the solution stood at -30 OC overnight to give deep olive-green plates which were filtered off, washed with pentane, and dried; yield 0.80 g. Anal. Calcd for TaC21H33As~:C, 40.93; H, 5.49; As, 24.32. Found: C, 40.70; H, 5.40; As, 23.70. ' H N M R ( T , CD2C12, -20 OC, phenyl protons omitted): 4.22 (8, s, CsHg), 8.66 and 8.79 (each 6, s, diars CH3 set), 9.52 (3, s, CH3), 11.05 (6, s, CH3 set). 8. Preparation of Ta(CsHs)(CH3)dP(CH3)3].Ta(CsHs)(CH3)3 (330 mg) in 5 ml of toluene was treated with 0.5 ml of PMe3. Pentane (5 mi) was added, the sample stood for I hr at -30 "C, and 270 mg of red-brown crystals was filtered off. Anal. Calcd for TaCldH26P: C, 41.39; H, 6.44. Found: C, 40.56; H, 6.29. ' H NMR ( T , CD2C12, -70 "C): 4.66 (8, d, J = 1.4 Hz, CsHs), 8.79 (9, d, J = 7 Hz, P(CH&), ca. 8.9 (3, d (partly obscured at 100 MHz), CH,), 10.02 (6, d, J = 13 Hz, CH3 set). 9. Preparation of Ta(csHs)z(CH~XP(cH3)3].PMe3 (50 PI) was added to 0.25 g of Ta(CsH&CH3 in ca. 5 ml of CH2C12. Removal of all volatiles in vacuo left dark olive-green T(CsHs)2(CH3)[P(CH3)3] which was characterized by ' H N M R only. ' H N M R (7,CD2C12, -30 "C): 5.04 (16. d, J 5 Hz, CsHs), 8.70 (9, d, J = 8 Hz, P(CH3)3), 10.09 (3, d, J = 11 Hz. CH3). 10. Observation of M(CsHs)z(CH~)(dmpe) (M = Nb, Ta). These adducts can be observed at -30 OC by 'H N M R but not isolated at room temperature. ' H NMR ( T , CDzC12, -30 "C): for M = Ta, 4.37 (16, poor dd, J = 1.8 and 2.0 Hz,CsHs), 7.73 (4, AB quartet, dmpe CHz's), 8.52 (6, d, J = 7 Hz, dmpe CH3 set), 8.87 (6, d, J = 8, dmpe CH3 set), 14.07 (3, dd, J = 8 and 12 Hz, CH3); for M = Nb, 4.43 (16. poor dd, J = 1.8 and 2.0 Hz, CsHs). 7.80 (4, AB quartet, dmpe CHz's), 8.51 (6, d, J = 7 Hz, dmpe CH3 set), 8.91 (6, d. J = 8 Hz, dmpe CH3 set), 14.24 (3, dd, J = 6 and 12 Hz, CH3). 11. Preparation of N~(C~HSXCSH~(C~H~)X~~~~) Nb(CsHs)2(C&5) (3.0 g) and 1.4 g of dmpe were dissolved in 100 ml of toluene and the solution was heated to reflux for 5 hr. The solution was cooled to room temperature and filtered leaving fluffy brown crystals. This product was dissolved in 100 ml of boiling toluene and the solution was filtered leaving some dark brown solid behind. Cooling gave 1.5 g of Nb(C&Is)[CsHs(C6Hs)](dmpe) which was recrystallized from T H F with diethyl ether. Anal. Calcd for CzsH37P~Nb:C, 63.64; H, 7.06; P, 11.72; Nb. 17.58. Found: C, 63.57; H, 7.21; P, 11.16; Nb, 16.98. See Figure 2 for the 220-MHz IH N M R spectrum and Table 11. 12. Preparation of Nb(CsH~)[CsH8(CaHs)Xdiars). Nb(CsHs)2(C6Hs) (2.0 g) and 1.7 g of diars were placed in 50 ml of T H F and the solution was refluxed for 3 hr, cooled to room temperature, filtered, reduced to a volume of ca. 20 ml, and let stand at -30 OC overnight to give 1.65 g of red crystals. Nb(CsHs)[CsH~(C6H5)](diars) may be recrystallized from T H F with hexane. This sample retained ca. 1 mol of T H F according to 'H NMR. Anal. Calcd for C36H450AszNb: C, 58.87; H, 5.90; As, 20.40; Nb, 12.65. Found: C, 59.00; H, 6.10; As, 19.71; Nb, 13.08. See Table I1 for 'H N M R data. 13. Preparation of Ta(CsHaMC~Hs(CsHs)Kdmper Ta(CsH8)2(C6Hs) (2.0 g) and dmpe (0.75 g) were heated in re-
February 18,1976
905
_ri *
It
Lo: 464
569
6 03
894’ l-92
* TMS
Table 11. Tabulation of ‘H Data for “M(C,H,),(C,H,)(L-L)” Complexes0
Static C,H,C
L-Le Dynamic C.H.f
4.70 (t) 5.81 (4) 6.95 (m)
4.53 5.58 6.75 8.75 9.25 (m) 9.52 10.89 (dd) 10.68
1 {
7.99 8.27 8.67 8.79 4.90
(d) (d) (d) (d) (t)
7.96 8.06 8.50 8.66 4.85
Nb(dmpe)
Nb(diars)
(t) 4.66 5.73 (t) (dd) 6.86 (t) 8.94 (m) 9.31 (t) 10.95 (s) (s) (s) (s)
(s)
T MS
J1,
A 10.95
Figure 2. The 220-MHz ‘ H NMR spectrum of Nb(q4-C8Hg)[q5C8H8(CsH5)](dmpe) in THF-dg (q4-C8H8 scale is 500 Hz; remainder is on the 2500-Hz scale). (*) THF-d, (x < 8). The upfield peak partially hides one dmpe methyl doublet.
Ta(dmpe)b Ta(diars)
*
(t) 4.50 (t) (4) 5.55 (dd) (m) 6.60 (dd) (m) 8.86 (m) (m) 9.58 (m) (t)d 10.89 (poor t) 7.94 (d) 7.97 (s) 8.31 (d) 8.09 (s) 8.65 (d) 8.52 (s) 8.83 (d) 8.64 (s) 4.93 (t) 4.89 (s)
in CD,Cl, at 220 MHz unless otherwise stated (phenyl rescnances omitted). Note sample in Figure 2 run in THFd,. b At 100 MHz. C A U H-H couplings are 7-10 Hz. dIn THF-d, (see Figure 2). e J ~ ~ =, 6-8 p Hz. f J ~ , p p= 1.5-2.0 Hz. 07
fluxing THF for 18 h. Cooling to -30 OC for several hours gave 1.7 g of Ta(C~Hs)[cgHg(CaHs)](dmpe) containing ca. 1 mol of T H F of crystallization. Recrystallization from CH2C12 gave orange crystals containing ca. 1 mol of CH2C12. Anal. Calcd for TaC29H39P2Clz: C. 49.65; H, 5.62; C1, 10.1 1. Found: C, 50.71; H, 5.75; C1, 9.48. See Table I1 for ‘H N M R data. 14. Preparation of T P ( C ~ H ~ W C ~ H ~ ( C ~ H The , ) Xprepara~~~~). tion is the same as the preparation of Nb(CgHg)[CsHs(C&)](diars) from 1.5 g of Ta(CgH&(C6Hs) and 0.92 g of diars except the solution was refluxed for 8 h; yield 1.0 g. Shorter reaction times gave mixtures of product and starting material. Anal. Calcd for TaC32H37As2: C, 51.08; H, 4.95; As, 19.91. Found: C, 50.85; H, 4.90; As, 19.15. The ‘H N M R spectrum is similar to that shown in Figure 2. See Table I1 for chemical shifts. 15. Preparation of Nb(CaHaXCaH9Xdmpe). [Li(THF)4]+[Nb(CgH&]- (10.0 g) and dmpe (2.6 g) in 100 ml of T H F (wellstirred and cooled to -78OC) were treated dropwise over ,’$ h with a solution of 1.3 ml of CF3C02H in 50 ml of THF. The mixture was warmed to room temperature and stripped to an orange syrup which was extracted with ca. 250 ml of toluene with stirring for 3 h. The mixture was filtered and the filtrate stripped to a syrup in which red-orange granular crystals formed slowly. After several hours the product was filtered, washed briefly with toluene, then diethyl ether, and dried in vacuo; yield 0.70 g (1 1%). A small sample was recrystallized from T H F for analysis. Anal. Calcd for NbC22H33P2: C, 58.41; H, 7.35. Found: C, 58.46; H, 7.38.
The IH N M R spectrum is similar to that in Figure 3 with minor variations. 16. Preparation of Nb(CaHs)(CaHaD)(dmpe). The procedure was
(10.28
&
Figure 3. The 220-MHz ‘H NMR spectrum of Ta(q4-CgHg)(q5C~Hs)(dmpe)in THF-ds (q4-CgH8 scale is 500 Hz; remainder is on the 2500-Hz scale). (*) THF-d, ( x < 8). identical with that for Nb(CgHg)(CgH9)(dmpe) employing CF3C02D; yield 1.20 g (19%). The stripped solvent was saved and analyzed for Cs hydrocarbons by GC/mass spectrometry. Hydrocarbon C,H, 0 9 7 % ) C,H,D (
