Organometallics 1995, 14, 3746-3750
3746
Bimetallic Complexes with Chiral Molybdenum Centers and Bis(g'z-cyclopentadienyl)Bridges: Interchange between Legs in Three-Legged Piano Stool Complexes Mijail V. Galakhov, Alicia Gil, Ernest0 de Jesus, and Pascual Royo" Departamento de Quimica Inorganica, Universidad de Alcala de Henares, Campus Universitario, 28871 Alcala de Henares, Madrid, Spain Received February 28, 1 9 9 P
Reactions of [{Mo(C0)3Cl}z@-CpCp)l(CpCp = (q5-C5H&SiMez (la)or (q5-C5H3)2(SiMed2 (lb))with AgBF4 and with 2-butyne in THF (THF = tetrahydrofuran) give [{Mo(C0)(q2MeCCMe)~}z@-CpCp)l[BF412(2a,b). Addition of PPh3 or P P b C l to 2a or 2b results in the substitution of one 2-butyne ligand at each Mo center by PPh3 or C1-, giving ionic [{Mo(CO>(qz-MeCCMe)(PPh3)}~@-CpCp)l[BF41~ (3a,b)or neutral [{Mo(CO)(q2-MeCCMe)Cl}2@CpCp)] (5), respectively. Addition of dmpe (dmpe = dimethylphosphineethane)to 2a gives the free carbonyl complex [{Mo(q2-MeCCMe)(dmpe)}~@-CpCp)1[BF41~ (4a). Complexes 3 and 5 are obtained as a ea. 1:l mixture of the RS isomer and the RR,SS racemate. The RR,SS racemate of 3a can be obtained >90% pure by slow crystallization of the 3a diastereomeric product mixture. In the absence of free ligands, conversion of RR,SS-3a into RS-3a is a first-order reaction with K = (8 f 1) x s-l and AG*= 94.6 f 0.2 k J mol-l at 293 K, and the basic mechanism is likely to be intramolecular. Addition of PPh3 t o 2a or 2b results in the substitution of a single 2-butyne ligand at each molybdenum Recently, we have reported the use of bridged cyclocenter, giving 3a or 3b. These are the only complexes pentadienyl ligands such as (CsH4)zSiMez' and (C5H3)2obtained even when an excess of PPh3 is added. These (SiMez)z2as anchoring ligands for dinuclear molybderesults are similar to those reported for the mononuclear num complexes. In the first part of this article we analog^,^ for which steric effects have been invoked. In report the synthesis of the new complexes 2-6 which contrast, dmpe (dmpe = dimethylphosphineethane) contain one or more 2-butyne ligands. In the second replaces one 2-butyne and one carbonyl ligand at each molybdenum center in 2a, giving 4a, even when a part we study by 1H NMR the kinetics of the ligand deficiency of dmpe is used. interchange in the three-legged piano stool complex 3a. The neutral complex Sa is obtained as a green solid Observation by NMR of the interconversion between cis by reaction of 2a with PbPC1 in THF. The BF4- anion and trans isomers of the type CpMX3Y or CpMXzYz has formed an important part of their kinetic ~ t u d i e s . ~ precipitates as [PbPI[BF4], and one 2-butyne ligand per molybdenum atom is replaced by a chloride ligand. However, ligand interchange in mononuclear threeComplexes 2-4 have molar conductivities between 160 legged piano stool complexes of the type CpMXYZ and 200 ohm-l cm2 mol-l, consistent with the values interconverts R and S enantiomers, and cannot be expected for 1:2 electrolyte^.^ monitored by NMR. In contrast, ligand interchange in Structural Study. Spectroscopic data for all comdinuclear three-legged piano stool complexes such as 3a plexes are given in the Experimental Section. The more interconverts diastereoisomers that give distinguishable valuable information is given by the cyclopentadienyl NMR resonances. and the Me-Si resonances in the lH NMR spectra, that appear in the range 4.23-7.98 and -0.91 t o 0.67 ppm, Results and Discussion respectively. In 2a and 4a, the Cp protons appear as an AA'BB' spin system and the Me resonances appear Preparative Methods. Complex 2a is synthesized as a singlet. These data indicate the existence of two by reaction of la with AgBF4, in THF, and subsequent planes of symmetry (planes Me-Si-Me and cp-Si-cp; addition of excess 2-butyne (Scheme 1). Irradiation of cp = centroid of the cyclopentadienyl ring) either in the the mixture with W light increases the rate and yield structure of a rigid molecule or in the average structure of the reaction. Complex 2b is synthesized by a similar of a fluxional molecule (Chart 11.l These planes of method but is produced only in poor yields. Complexes symmetry are also reflected in the l3CC1H} NMR 2a and 2b are obtained as yellow microcrystalline solids, spectrum of 2a, in which only three resonances are stable for weeks in an inert atmosphere. observed for the ring carbons, and in the 31P{lH} NMR spectrum of 4a, in which a single resonance is observed @Abstractpublished in Advance ACS Abstracts, July 1, 1995. for the four phosphorus atoms of the two dmpe ligands (1)mmez-Sal, P.;de Jesus, E.; Perez, A. I.; Royo, P. Organometallics in each molecule. An analogous situation is observed 1993,12,4633.
Introduction
(2)Amor, F.; G6mez-Sa1, P.; de Jesus, E.; Royo, P.; de Miguel, A. V. Organometallics 1994,13,4322. (3) See for example: McLain, S. J.; Wood, C. D.; Schrock, R. R. J. Am. Chem. SOC.1979,101,4558.Faller, J. W.; Anderson, A. S. J.Am. Chem. SOC.1970,92,5852.
(4) Allen, S. R.; Baker, P. K.; Barnes, S. G.; Green, M.; Trollope, L.; Manojlovic-Muir, L.; Muir, K. W. J . Chem. SOC.,Dalton Trans. 1981, 873. (5) Geary, W. J. Coord. Chem. Rev. 1971,7 , 81.
0276-7333/95/2314-3746$09.00/0 0 1995 American Chemical Society
Bimetallic Complexes with Chiral Mo Centers
Organometallics, Vol. 14,No. 8,1995 3747
Scheme 1
2 BF4-
I
(1) AgBF4, MeCpMe
THF, hv, 6 h
(3)
1 2 +
I
I
I
I
I
I
1
/
2 BF42 dmoe
-
I
I
+ 2 BF4-
I
MeC-CMe
'up a
I
2
Me2PCH2CH2PMe2(dmpe)
in 2b,in which the cyclopentadienyl ring protons appear as an ABB' spin system and the Me-Si resonances appear as two singlets, one for the methyl groups above the ring plane and one for the methyl groups below the plane. Molecules 3 and 5 each have two chiral molybdenum atoms and three stereoisomers: two are enantiomers t o each other (RR and SS) and diastereoisomers with respect to the third (RS). R S and SR are the same isomer because the two Mo atoms are equivalent. The Me-Si groups of 3a or 5a appear in the lH and 13C{'H} NMR spectra as two singlets assigned t o the R S isomer which integrate as 1:l and as an intense singlet assigned to the RR,SS racemate: the two Me-Si groups of the molecule are equivalent in the RR and SS isomers but not equivalent in the R S isomer (Chart 2). The Me-Si groups of 3b appear in the 'H NMR spectrum as two resonances for the RR,SS racemate and
as four resonances for the R S isomer. In each case, the R S isomer and the RR,SS racemate are in a ea. 1:l molar ratio. At room temperature, 2a and 2b show separate resonances in the lH NMR spectrum for the endo and ex0 methyl groups of the 2-butyne ligand, indicating that the ligand turns slowly through its bond to the metal. The 2-butyne of 3a or 5a appear as two resonances a t low temperature which coalesce to a single resonance at 42 "C (hGS315K= 57 f 5 kJ mol-') for 3a, or 20 "C = 56 f 5 k J mol-') for 5a. Analysis of the line (AG*z~~K shape for 3a gives AlP = 73 f 5 k J mol-l and AS* = 43 A= 14 J K-' mol-l. Transformation of RR,SS-Sa in RS-Sa. Isomer RR,SS-3a is isolated >90% pure (lH NMR evidence) as prismatic purple crystals when 3a is crystallized slowly in CHzCldether at -40 "C. RR,SS-3a transforms into RS-3a in a process that reaches equilibrium after
Galakhov et al.
3748 Organometallics, Vol. 14, No. 8, 1995
excluded, at least in part, as the latter is the main mechanism when free ligands as CO or PPh3 are added.
Chart 1 \
0
-
e..
[Plane Me-Si-Me -
/
.._..._ 'lane cp-Si-Cp
-...A..@. .-.....
-7 '
A'
0'
several hours in CDCl3. The transfonnation was monitored by 'H NMR for 10-3-10-2 M solutions of RR,SS3a a t 20 "C. In determination of the rate law (see Experimental Section for details), we assumed that the forward and reverse processes RR,SS-3a == RS-3a follow identical mechanisms and have the same rate constant k, in agreement with the measured equilibrium constant of ca. 1. The observed reaction rate corresponds to the contribution of both forward and reverse reactions, since the concentrations of RR,SS-3a and RS-3a during the measurements were close to the equilibrium concentrations. Under these conditions, the integrated rate law for a first order reaction is6 [RR,SSl = ((1 e-2kt)/2}[RRISSIo or ln([RR,SSl - [RSI) = -2Kt ln[RR,SSIo in its logarithmic form. We have observed a linear dependence between ln([RR,SSl - [RS]) and time, and calculated a rate constant of (8 f 1) x s-l for the first order rate law (AGf293K= 94.6 f 0.2 kJ mol-l). The process is accelerated by the addition of free PPh3 or CO but not by the addition of 2-butyne, equilibrium being reached in a matter of hours when no ligands are added, in minutes when CO is bubbled at 1 atm (k w 2 x 10-3 s-l), and in seconds when PPh3 (%loT2M) is added. The mechanism is likely to be associative in the presence of free PPh3 or CO (Scheme 2). In the absence of free PPh3 or CO an associative mechanism could be induced by traces of free ligand derived from impurities or decomposition. However, an associative mechanism and the observed first-order rate would only be compatible if we made the unlikely assumption that the trace concentration of free ligand was comparable in all the experiments regardless of changes in sample concentration o r source. We therefore do not consider that this process makes a significant contribution to the overall rate. The observed first-order rate law is consistent with either an intramolecular or a dissociative mechanism. However, the addition of the efficient CO or PPh3 trap [Pt(C2H4)2(PPh3)1did not affect the reaction rate, making associative or dissociative pathways that require the presence of free ligands unlikely. The intramolecular route is therefore the more plausible mechanism for RR,SS- to RS-3a interconversion, although the dissociative or associative routes are not
+
+
(6)Atkins, P. W. Physical Chemistry, 4th ed.; Oxford University Press: Oxford, 1990; p 789.
Experimental Section
Reagents and General Techniques. All reactions were carried out under an inert atmosphere (argon or nitrogen) using Schlenk and high-vacuum techniques. Solvents were dried and distilled under nitrogen: diethyl ether and tetrahydrofuran from sodium benzophenone ketyl; hexane from sodium; CHzClz over P 4 0 1 0 . Unless otherwise stated, reagents were obtained from commercial sources and used as received. IR spectra were recorded in Nujol mulls over the range 4000200 cm-1 on a Perkin-Elmer 583 spectrophotometer. IR data are given in cm-l. The 'H, 31P,and 13C NMR spectra were recorded at 299.95, 121.42, and 75.43 MHz, respectively, on a Varian Unity 300 spectrometer; chemical shifts, in ppm, are positive downfield relative to external SiMe4 for 'H and 13C in H2O for 31P. C, H, and N and to external 85% analyses were performed with a Perkin-Elmer 240-B instrument. Conductivity measurements were carried out in a WTW M. L42 instrument from acetone solutions ca. 5 x Syntheses. Complexes la1and lb2were prepared according to reported methods. Preparation of I(M0(CO)(tl~-MeCCMe)2}201-(t1~-CsH4)2SiMez)][BF4]2(2a). AgBF4(0.29 g, 1.5 mmol) and a solution 1.42 M of 2-butyne in THF (4.0 mL, 5.7 mmol) were added at 0 "C to a solution of l a (0.48 g, 0.77 mmol) in THF (50 mL). The mixture was irradiated with a UV lamp (Philips, HPK 125 W) over 6 h. The mixture was then filtered through Celite, and the solid extracted in a Soxhlet apparatus with CHzClz (100 mL) until colorless (ca. 8 h). The solution was concentrated i n vacuo until a solid began to precipitate. Then diethyl ether (30 mL) was added, the mixture was filtered, and a yellow microcrystalline solid was obtained (0.35 g, 55961, which was washed with hexane (3 x 30 mL). Anal. Calcd for C ~ O H ~ ~ B ~ O Z F EC, S ~43.7; M OH, Z : 4.6. Found: C, 43.5; H, 4.6. Conductivity (acetone): AM225 ohm-l cm2 mol-'. IR (Nujol): v(C0) 2031 vs. lH NMR (acetone-&): 6 6.76, 6.28 (M' and BB' parts of an AA'BB' spin system, 4 H, C a 4 ) , 3.11 (s, 6 H, C&fe2),2.81 (s, 6 H, C&fe2),0.28 (s, 3 H, SiMez). 13C(lH}NMR (acetone-&): d 223.2 (s, CO), 164.7 (s, C2Me21, 145.2 (s, CZMe2), 111.4 (s, C5H4), 108.1 (s, C a 4 ) , 98.1 (s, C a d , C ipso), 20.0 (s, C&feZ), 15.7 (s, C&fe2), -2.1 (s, SiMez). Preparation of [ ( M o ( C O ) ( ~ ~ ~ - M ~ C C M ~ ) ~ } Z ~ ~ - ( ~ ~ (SiMez)~)][BF4]2 (2b). This complex was prepared (0.25 g, 38%) by reaction of l b (0.50 g, 0.75 mmol), AgBF4 (0.29 g, 1.5 mmol) and a 1.42 M solution of 2-butyne in THF (4.0 mL, 5.7 S ~43.7; Z M H, O ~4.8. : mmol). Anal. Calcd for C ~ ~ H ~ ~ B Z O Z F EC, Found: C, 43.9; H, 4.7. IR (Nujol): v(C0) 2028 vs. 'H NMR (acetone-&): d 7.98 (A part of an ABB' spin system, 1 H, C a s ) , 6.42 (BB' part of an ABB' spin system, 2 H, C a 3 ) , 3.18 (s, 6 H, C&fe2), 2.90 (s, 6 H, C&e2), 0.52 (s, 3 H, SiMez),0.29 (s, 3 H, SiMez). 13C(lH) NMR (acetone-&): d 225.3 (s, CO), 165.5 (s, C2Me2), 143.8 (s, C2Me2), 123.6 (s, CbHd), 108.1 (s, C&4), 105.2 (s, C a 4 , C ipso), 19.9 (s, C&e2), 15.4 (s, C&fe2), 5.1 (s, SiMez), -0.3 (s, SiMez). Preparation of [(M0(CO)(rl~-MeCCMe)(PPhs))z01-(11~C&)2SiMez)l[BF412 (3a). PPh3 (0.15 g, 0.56 mmol) was added to a suspension of 2a (0.23 g, 0.28 mmol) in CHzClz (30 mL). The solution became progressively purple. Stirring was continued for 4 h followed by filtration. Solvent was removed i n vacuo to yield a purple solid (0.27 g, 7881, which was washed with hexane (3 x 30 mL) and dried i n vacuo. Anal. Calcd for C ~ ~ H ~ ~ B ~ O Z F ~ C,S56.1; ~ P ~H,M4.6. O ~ Found: : C, 55.8; H, 4.5. Conductivity (acetone): AM200 ohm-l cm2mol-l. IR (Nujol): v(C0) 1955 s, br. IH NMR (CDC13) for RS-3a: 6 7.5-7.2 (30 H, PPh3), 6.23 (br, 2 H, C a d ) , 6.18 (br, 2 H, C a d ) , 6.02 (br, 2 H, C a d ) , 5.12 (br, 2 H, C a J , 3.15 (br, 6 H, C a e z ) , 2.60 (br, 6 H, C&fe2), -0.61 (s, 3 H, SiMez), 0.25 (s, 3 H, SiMez); for RR,SS-3a: 6 7.49 (m, 18 H, PPh& 7.26 (m, 12 H, PPh3), 6.27 (br, 2 H, C a d ) , 6.02 (br, 4 H, C a d , 5.12 (br, 2 H, C a d ) ,
Organometallics, Vol. 14, No. 8, 1995 3749
Bimetallic Complexes with Chiral Mo Centers
Chart 2
(SS-3a)
(RS-3a)
Me,
zI
Me
(SS-3b)
(RS-3b)
Scheme 2
3.15 (br, 6 H, C&e2), 2.60 (br, 6 H, C&fez), -0.11 (s, 6 H, SiMe2). I3C{lH} NMR (CDC13)for the 1:l mixture of RR,SS and RS diastereoisomers: 6 230.8 (5, CO), 230.6 (s, CO), 128135 (PPh3 and C2Me2), 111.6 (s, C5H4), 111.4 (s, C&), 109.1 (s,C5H4), 108.8 (s,C5H4), 102.3 (s,C5H4), 102.2 (s,C5H4), 101.4 (s, C5H4),95.5 (6, C5H4), 95.3 (s, C5H41, 23.5 (br, C&ez), 20.1 (br, C&fe2), -0.8 (s, SiMez, RS isomer), -2.4 (s, SiMe2, RR,SS isomer), -3.9 (s, SiMez, RS isomer). 31P{1H}NMR (CDC13): 6 52.3 (s).
isomer). 13C{'H} NMR (CDC13) for the 1:l mixture of RR,SS and RS diastereoisomers: 6 223.7 (s, CO), 223.6 (s, CO), 128135 (PPh3 and CzMez), 117.9 (s, C5H3), 116.8 (s, C5H3), 111.1 (s,C5H3), 111.0 (s,C5H3), 110.5 (s,C5H3), 109.9 (9, C5H3), 108.4 (s,C5H3), 106.8 (s,C5H3), 106.5 (s,C5H3), 105.6 (s,C5H3), 25.6 (s, C&fe2), 20.2 (s, C&feZ), 4.4 (s, SiMez, RS isomer), 2.9 (s, SiMez, RS isomer), 2.3 (s, SiMe2, RR,SS isomer), 0.8 (s, SiMe2, RR,SS isomer), 0.4 (s, SiMez, RS isomer), -1.4 (s, SiMe2, RS isomer). 31P{1H}NMR (CDC13): 6 52.3 (s).
Preparationof [{Mo(r12-MeCCMe)(dmp)}~~-(rls-CBH4)2Preparation of [{MO(CO)(~~~-M~CCM~)(PP~S)}~~-(~~SiMe2)][BF4I2 (4a). DMPE (0.26 mL, 0.56 mmol) was added C~H~)~(S~M~~)~)I[BFII~ (3b). This complex was prepared to a suspension of 2a (0.23 g, 0.28 mmol) in CHZC12 (30 mL). (0.14 g, 77%) by reaction of 2b (0.12 g, 0.14 mmol) with PPh3 (0.073 g, 0.28 mmol). Anal. Calcd for C ~ O H ~ O B ~ O Z F E S ~ ~ PThe ~ Myellow O ~ : solution became blue, and the solid dissolved. Stirring was continued for 1 h. Solvent was removed in C, 55.6; H, 4.7. Found: C, 55.4; H, 4.7. IR (Nujol): v ( C 0 ) uucuo, yielding a blue solid (0.20 g, 74%) which was washed 1951 s, br. 'H NMR (CDC13)for the 1:l mixture ofRR,SS and with hexane (3 x 15 mL) and dried in uucuo. Anal. Calcd RS diastereoisomers: 6 7.5-7.2 (60 H, PPh31, 6.01 (A part of for C ~ Z H ~ ~ B ~ F E S ~C,P 40.1; ~ M OH,Z 6.1. : Found: C, 40.5; H, a n ABB' spin system, 2 H, C&), 5.85 (BB' part of a n ABB' 6.2. Conductivity (acetone): AM 209 ohm-' cm2 mol-'. 'H spin system, 4 H, C&), 4.62 (BB' part of a n ABB spin system, NMR (acetone-&): 6 5.92, 5.42 (AA' and BB' parts of a n 4 H, C5H3),4.23 (A part of a n ABB' spin system, 2 H, C5H31, AA'BB' spin system, 4 H, C a d ) , 2.93 (s, 6 H, C&fed, 2.05 (m, 3.40 (br, 12 H, C&e2), 2.57 (br, 12 H, C&fe2), 0.67 (s, 3 H, p 9.3), 1.45 (d, 6 H, PMe2, 4 H, PCHz), 1.48 (d, 6 H, PMe2, 2 J ~ = SiMe2, RS isomer), 0.62 ( s , 3 H, SiMez, RS isomer), 0.42 (5, 6 VHP = 9.9), 0.21 (s, 3 H, SiMez). 31P(1H}NMR (acetone-&): H, SiMep, RR,SS isomer), 0.39 (s, 3 H, SiMez, RS isomer), 6 51.7 (s). -0.02 (s, 6 H, SiMe2, RR,SS isomer), -0.91 (s, 3 H, SiMe2, RS
3750 Organometallics, Vol. 14, No. 8, 1995
Galakhov et al.
Preparation of [ { M O ( C ~ ) ( ~ ~ - M ~ C C M ~ ) C ~ } ~ ~ - (trations ~ ~ - C of ~H ~ ) ~ - in the range (2.53-8.43)x RS,SS-3a M. The relative concentration of RS,SS-3a and RS-3a was calculated (SiMe2)z)l (5a). PPh4Cl (0.18g, 0.48mmol) was added t o a by integration of the lH NMR Me-Si resonances. For each suspension of 2 a (0.20g, 0.24mmol) in THF (30mL). The yellow solution became green. The solution was filtered, and sample, the concentrations were checked a t least seven times the solvent was removed in uucuo. The green solid obtained in the 2-250 min interval, and ln([RR,SSl - [RSI) was plotted against time. In all the cases, fitting of data to a linear (0.08 g, 54%) was washed with hexane (3 x 10 mL) and dried in uacuo. Anal. Calcd for C22H2602ClzSiMo2: C, 43.1;H, 4.3. function by least-squares analysis gives R > 0.99. The rate Found: C, 43.4;H, 4.2.IR (Nujol): v(C0) 1927 s, br. 'H NMR constants k were obtained from the slopes of the lines, (CDC13) for the 1:l mixture of RR,SS and RS diastereoisoaccording to the first-order rate law ln([RR,SSl - [RSI) = -2kt mers: 6 6.14(br, 4 H, C5H4), 5.77(br, 4 H, C&), 5.70(br, 4 In[RR,SS]o (see Results and Discussion). The values H, C a d ) , 5.04(br, 4 H, C a d ) , 3.19(br, 24 H, C&e2), 0.35(s, s-l, with 8.4 x obtained fork were in the range (7-10)x 3 H, SiMe2, RS isomer), 0.10(s, 6 H, SiMez, RR,SS isomer), s-l as mean value and 0.6 x s-l as standard error (five measurements). The free energy of activation (AG* = 94.6 -0.10 (s, 3 H, SiMe2, RS isomer). l3C(lH} NMR (CDC13) for the 1:l mixture of RR,SS and RS diastereoisomers: 6 232.3 i 0.2kJ mol-l) was estimated from the Eyring e q ~ a t i o n . ~ (s, CO), 200.9(s, CzMez), 194.4(s, C2Me2), 114.7(5, C5H4), 114.2 Acknowledgment. We gratefully acknowledge fi(s, C a 4 ) , 109.0(s, C5H4), 108.3(s,C5H41, 101.6(s, Cad), 101.4 nancial support from the Comisih Asesora de Investi(s, C a d ) , 99.3(s, C&), 22.3 (br, C&e2), 18.6(br, C d e z ) , -1.0 gaci6n Cientifica y TBcnica (ref PB92/0178-C). (s, SiMez, RS isomer), -1.2 (s, SiMez, RR,SS isomer), -1.4 (s, SiMen, RS isomer). OM950162H Kinetic S t u d y of the Transformation of RR,SS-3a in RS-Sa. All measurements were monitored a t 20 "C in CDC13 (7) Gunther, H. NMR Spectroscopy, 1st ed.; John Wiley & Sons: as a solvent. Five solutions were prepared a t initial concenChichester, 1990; p 241.
+
