Organometallics 1995, 14, 5209-5220
5209
Deoxygenative Reduction of Dimolybdenum Carbonyl Complexes Mo2(p5-C5&R)2(CO)4by Hydrosilane Giving Trinuclear y3-Methylidyne and y3-Ethylidyne Complexes, ~3-CR)[Mo(q6-C5H4R')(C0)213 (R, R = H, CH3), and Analysis of the Rotational Processes of the Mo(p5-C5H4R) (CO)a Moietiesf Munetaka Akita,* Kazumi Noda, Yoshiaki Takahashi, and Yoshihiko Moro-oka* Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226, Japan Received June 19, 1995@ Photolysis of the dimolybdenum carbonyl complex (r5-C6H4R)zMoz(C0)41 (a-series: R = H; b-series: R = CH3) in the presence of an excess amount of HSiMezPh produces the trinuclear ps-methylidyne complex Cu3-CH)[Mo(r5-C5%R)(C0)~132a,b and the p3-ethylidyne complex Cu3-C-CH3)[Mo(r5-C5H4R)(CO)213 3a,b along with siloxane O(SiMezPh)z 4. The structure with the tetrahedral CM03 core and fluxional properties of 2 and 3 have been characterized by means of X-ray crystallography and NMR techniques. All of the three r5C5H4R rings in the p3-methylidyne complexes 2a,b project to the same side as does the p3-CH ligand (conformer A), whereas one of the three r5-CsH4R rings in the p3-ethylidyne complexes 3a,b projects to the side distal from the p3-C-CH3 (conformer B). The C3symmetrical structure (A) of 2 is supported by the three strong semibridging interactions of CO ligands with the adjacent Mo centers, but 3 with the unsymmetrical structure B contains less strong semibridging interactions. In solutions, 2 exists as a n equilibrated mixture of two isomers A and B as revealed by variable temperature lH- and 13C-NMRmeasurements. The complicated dynamic behavior of 2 can be analyzed as a combination of rotation of one of the three Mo(q5-C5&R)(C0)~units of A leading to B (local rotation) and concerted rotation of all the three metal fragments operating for B (gearlike rotation), which are associated with cleavage and regeneration processes of the semibridging interaction. The fluxional behavior of the p3-ethylidyne complexes 3 can be explained by the gearlike rotation mechanism. Labeling experiments using l J 3 C 0 and DSiMezPh verify that all the hydrogen and carbon atoms of the p3-alkylidyne parts in 2 and 3 come from HSiMezPh and CO in 1, respectively. Therefore the present system can be viewed as a model system for a crucial step in Fischer-Tropsch mechanism, i.e. deoxygenative reduction of CO giving CH and CCH3 species. The deoxygenative reduction has been realized by using hydrosilane with high oxygenophilicity as a n equivalent for Hz.
Introduction Deoxygenative reduction of carbon monoxide giving carbide or CY, species on a heterogeneous catalyst surface has been recognized as a crucial step at an early stage of catalytic CO hydrogenation reactions (Scheme l).2,3 Although various types of model studies have been reported so far, we have been studying reduction of oxygen-containingspecies attached to a transition metal center by using hydrosilane as an equivalent for dihyd r ~ g e n . ~It?has ~ been well-established that the H-Si bond in hydrosilane shows reactivities quite similar to those of the H-H bond as typically exemplified by oxidative addition to a low valent metal center giving a hydride intermediate M(H)(X) (X: H, S ~ R S ) . This ~,~ @Abstractpublished in Advance ACS Abstracts, October 1, 1995. (1)Abbreviations used in this paper: CP: a general term for 75C5H4R ligands; Cp: +25H5; Cp': q5-C5H4CHs. (2)(a) Fischer, F.; Tropsch, H. Brennst. Chem. 1926,7, 97. (b) Brady, R. C.; Pettit, R. J.A m . Chem. SOC.1980,102, 6181. (c) Falbe, J.New Synthesis with Carbon Monoxide; Springer: Berlin, 1980. (d) Keim, W.Catalysis in C l Chemistry; D. Reidel: Dordrecht, 1983. ( e ) Anderson, R. B. The Fischer Tropsch Synthesis; Academic Press: London, 1984.
Scheme 1
reaction is involved as a key step of catalyzed addition reactions of H-X to unsaturated organic compounds, that is, catalytic hydrogenation and hydrosilylation. In addition to this feature, it is anticipated that the positively charged, highly electrophilic silyl moiety in M(H)(SiR3)6,7 resulting from the oxidative addition (i) works as a Lewis acid (ii) so as to promote subsequent hydride transfer (iii) just like the acidic point o n a heterogeneous catalyst surface (Scheme 2). Furthermore the oxygen atom may be removed as thermodynamically very stable siloxane (iv), that can be viewed as silylated water. Thus employment of hydrosilane as an equivalent for hydrogen would lead t o a model system for the deoxygenative reduction of CO. In fact, we have found catalytic hydrosilylation of acyl metal complexes MC(=O)R giving alkyl complexes MCH2R4*
0276-733319512314-5209$09.00/0 0 1995 American Chemical Society
Akita et al.
5210 Organometallics, Vol. 14,No.11, 1995
Scheme 2 our model system
catalytic system
Si-0-Si
m
and transformation of a ruthenium carbonyl complex CpzRuz(C0)r into bridging methylene complexes CpsR U ~ ( ~ - C H ~ ) , ( ~ - C O ) ~ - , (n (CO = )1, ~ 2).4c This type of reduction is hardly realized by the action of hydrogen, because the hydride intermediate M(Hh shows quite low affinity toward polar substrates. Herein we disclose the result of reduction of dimolybdenum carbonyl complexes (q5-C5H4R)2M02(C0)4 (MozMo) 1 with hydrosilane affording the trimolybdenum p3-alkylidyne complexes (u3-CR)[Mo(q5-CsH4R)(C0)zh (2: R, R = H, CH3). The molecular structure and complicated dynamic behavior of the resulting trinuclear complexes will also be discussed in detail. A preliminary communication of a part of this work already appeared.4e
Results and Discussion
[MoCp’(CO)& (2b) and p3-ethylidyne complexes 013CCH3)[MoCp’(CO)& (3b). Compared to the Cp system, the yield of 2b (18%based on CO) was lowered but a small amount of 3b (2%based on CO) was isolated from a reaction mixture. All the products 2 and 3 show complicated dynamic behavior as described below, but they are readily assigned t o the p3-alkylidyne complexes on the basis of the following NMR features (Table 1). As a typical example, the ’H-NMR spectrum of a 13C-enriched sample of (u3-*CH)[MoCp(*C0)2132a-13C(*C: ca. 30% 13C-enriched)is reproduced in Figure 1. Although the presence of two isomers (A and B) is evident, the formulation is supported by the intensity ratio [ l (CH): 15 (q5-C&)31 and the highly deshielded signals appearing in the region characteristic of p3-CH ligands id^ 11.85, 13.37; 6c 245.6, 262.3).8,9As for the ethylidyne complex, 3a is characterized by comparison of the lHNMR data with those of an authentic sample prepared by isolobal metal exchange reaction of the tricobalt p3ethylidyne complex ~ ~ - C C H ~ ) C O ~with ( C OCpzMoz)~ (CO)6reported by Vahrenkamp et al.l0 It is notable that both 2 and 3 contain several CO stretching vibrations in the range 2000-1800 cm-l indicating the presence of semibridging CO ligands. The structures of 2a,b and 3b have been determined unequivocally by X-ray crystallography (see below). The present reaction also proceeds under thermal reaction conditions (120 “C), but the reproducibility is quite poor. In addition, the photochemical reaction is
Deoxygenative Reduction of (rf-Cg&R’)&Ioz((20)s(1) Givingpa-Methylidyneand ps-Ethylidyne ( 3 ) ( a ) Masters, C. Adu. Orgunomet. Chem. 1979, 19, 63. (b) ( Cand O ) Z ] ~ Muetterties, Complexes, O ~ ~ - C H ) [ M O ( ~ ~ - C ~ K ~ R ’ )(2) E. L.; Rhodin, T. N.; Band, E.; Brucker, C. F.; Pretzer, O(~-CCHS)[MO(~~-C~)(CO)~~~ (3). Irradiation of a W. R. Chem. Rev. 1979, 79,79. (c) Roofer-DePoorter, C. K. Chem. Rev. 1981,81,447. (d) Herrmann, W. A. Angew. Chem., Int. Ed. Engl. 1982, benzene solution of the dimolybdenum carbonyl complex 21, 117. For stepwise transformations: (e) Cutler, A. R.; Hanna, P. CpzMoz(C0h l a in the presence of an excess amount of K.; Vites, J. C. Chem. Rev. 1988,88,1363 and references cited therein. HSiMezPh afforded the trinuclear p3-methylidyne comFor carbide complexes: (0 Tachikawa, M.; Muetterties, E. L. Prog. Inorg. Chem. 1981,28,203. (g) Bradley, J. S. Adu. Orgunomet. Chem. plex (us-CH)[MoCp(CO)zls2a in 26% yield (based on CO) 1983,22, 1. (h) Shriver, D. F.; Sailor, M. J . Acc. Chem. Res. 1988,21, along with the siloxane O(SiMe2Ph)z4 after chromato374. (i) Beck, W.; Niemer, B.; Wieser, M. Angew. Chem., Int. Ed. Engl. graphic separation (eq 1). Although the p3-ethylidyne 1993,32,923. Q) See also Neithamer, D. R.; LaPointe, R. E.; Wheeler,
hvl
’
2a: 26 % 2b: 18 a/a
benzene
3a: trace 3b: 2 ‘/0
+
O(SiMezPh)2 (4)
a series: CP= c p (q5-C5H5) b series: CP- Cp’ (q5-CsH4Me)
complex ( ~ ~ - C C H ~ ) [ M O C ~ (3a C Owas ) ~ I Ldtected ~ -y TLC and lH-NMR, it could not be isolated from a reaction mixture because of the low yield. Yields determined by an NMR experiment carried out in C6D6 in a sealed tube were 38% (2a;based on CO), trace (3a), and 115% (4; based on la). Photochemical reaction of the q5-methylcyclopentadienyl analogue Cp’zMoz(C0)r l b with HSiMesPh also produced a mixture of the p3-methylidyne (u3-CH)-
R. E.; Richeson, D. S.; Van Duyne, G. D.; Wolczanski, P. T. J. A m . Chem. SOC.1989,111,9056. (4) (a)Akita, M.; Mitani, 0.;Sayama, M.; Moro-oka, Y. Orgunometallics 1991, 10, 1394. (b) Akita, M.; Oku, T.; Tanaka, M.; Moro-oka, Y. Organometallics, 1991,10, 3080. (c) Akita, M.; Oku, T.; Moro-oka, Y. J . Chem. SOC.,Chem. Commun. 1992, 1031. (d)Akita, M.; Oku, T.; Hua, R.; Moro-oka, Y. J . Chem. Soc., Chem. Commun. 1993, 1670. (e) Akita, M.; Noda, K.; Moro-oka, Y. Organometallics 1994,13,4145. 8 Akita, M.; Moro-oka, Y. Stud. Surf. Sci. Cut. 1995, 92, 137. (5) Gregg, B. T.; Cutler, A. R. Organometallics 1992, 11, 4276 and references cited therein. (6) Patai, S.; Rappoport, Z. The chemistry of organic silicon compounds; John-Wiley & Sons: Chichester, 1989. (7) (a) Cundy, C. S.;Kingston, B. M.; Lappert, M. F. Adv. Orgunomet. Chem. 1973, 11, 253. (b) Aylett, B. J. Adv. Inorg. Radiochem. 1982, 25, 1. (8) For reviews: (a)(M = Co) Seyferth, D. Adu. Orgunomet. Chem. 1976,14,97. (b) Penfold, B. R.; Robinson, R. H. Acc. Chem. Res. 1973, 6 , 73. (c) (M = Ru, Os) Keister, J. B. Polyhedron 1988, 26, 1. (d) Deeming, A. J. Adv. Organomet. Chem. 1988,26, 1. (9) ( ~ u ~ - C Hcomplexes: )M~ (a) (M = Co) Seyferth, D.; Hallgren, J. E.; Hung, P. L. K. J. Organomet. Chem. 1973, 50, 265. (b) (M = Ru)
Kakigano, T.; Suzuki, H.; Igarashi, M.; Moro-oka, Y. Organometallics 1990,9, 2192. (c)(M = Os) Shapley, J. R.; Cree-Uchiyama, M. E.; St. George, G. M.; Churchill, M. R.; Bueno, C. J . Am. Chem. SOC.1983, 105,140. (d) (M = Ru) Keister, J. B.; Horling, T. L. Inorg. Chem. 1980, 19,2304. (e) (M = Fe) Vites, J . C.; Jacobsen, G.; Dutta, T. K.; Fehlner, T. P. J.A m . Chem. SOC.1985, 107, 5563. (0 Kolis, J. W.; Holt, E. M.; Shriver, D. F. J.Am. Chem. SOC.1983,105,7307. (g) (M = Os) Calvert, R. B.; Shapley, J . R. J.A m . Chem. SOC.1977,99,5225. (h) (M = Rh) Dimas, P. A,; Duesler, E. N.; Lawson, R. J.; Shapley, J. R. J.Am. Chem. SOC.1980, 102, 7787. (i) Herrmann, W. A,; Plank, J.; Riedel, D.; Weidenhammer, K.; Guggolz, E.; Balbach, B. J . Am. Chem. SOC.1981, 103, 63. (i) (MQ= Co, Cr, Mo, W, Ni) Duffy, D. N.; Kassis, M. M.; Rae, A. D. J. Organomet. Chem. 1993, 460, 97. (k) Schacht, H. T.; Vahrenkamp, H. J . Organomet. Chem. 1990,381, 261. (10) Blumhofer, R.; Fischer, K.; Vahrenkamp, H. Chem. Ber. 1986, 119, 194.
Deoxygenative Reduction of Dimolybdenum Carbonyl Complexes
Organometallics, Vol. 14, No. 11, 1995 5211
Table 1. 'H-and W-NMR data for 2a and 3aa complex
solvent (A:B)
'H-NMR
2a
CDzClz (1:1)
rt
A B
(k0.6)
-80
A
CDC13 (1:3)
rt
temp, "C
isomer
-50
B A B A
rt
A
CR 13.37 (1H) 11.85 (1H) 13.36 (1H) 11.74 (1H) 13.26 (1H) 11.78
CP 5.22 (15H) 5.16 (15H) 5.28 (15H) 5.11 (5H), 5.22 (10H) 5.14 (15H) b
12.66 (1H) 11.39 12.65 (1H) 11.35 12.41 (1H) 11.01 (1H) 3.73"(3H) 3.57 (3H)
4.73 (15H) b 4.71 (15H) b 4.56 (15H) 4.54 (lOH), 4.69 (5H) 4.73 (15H) 4.51 (lOH), 4.60 (5H)
B C6D6 (1:6)
B 2a
3a
tol-d.gd (1:6)
rt
(1:4)
-80
tol-ds
rt
-80
A B A B B
B
13C-NMR
CR
co
262.3 (156) 245.6 (162) 260.5 243' 259.7 (156) 244.3 (162) 258.2 242.4 260.4 (154) 246.1 (161) e 246.1 (162) 257.6 244.1 269.3 (5)fa
227.4,231.2 b,c 228.2, 231.2 229; 232; 234' 226.0, 230.9 b,c 226.6, 230.85 228.2, 230.79,233.9 b,c 231' b ,c 231.0 226.6,230.9 229.6,231.0,233.2 226c
CP 92.5 b ,c 92.5 b ,c 92.0 b,c 91.9 93.7 b ,c 92.0 b ,c 91.9 b ,c 90.8 91.6
0 Observed a t 400 MHz (1H) and 100 MHz (W). Chemical shifts are reported in ppm down field from TMS unless otherwise stated. Values in parentheses are for ~Jc-H in Hz. Overlapped or coalesced with the signals of the other isomer. Broad. 8~ values are referenced ~ e Too broad to be located. f 2 J c - ~ g. dc(CH3) = 54.2 (4,J = 128 Hz). ~JC-C = 32 Hz. to the residual solvent CHDz signal ( 8 2.09).
81 'JC.H
= 156 HZ
1'
i
I
7A ,co/
A
cDxL
B ' lJGH
rli
- 1 6 2 ~ ~
5!4
512
'
510
If I .
I
carbyne complexes Cp(C0)zW(WXr).l3 Seppelt reported the fluoro-analogue of 2a, (p3-CF)[MoCp(C0)2]3 6, which was formed via a curious reaction, i.e. photolysis of the difluoromalonyl dimolybdenum complex C~(CO)~MOC(=O)CF~C(=O)M~C~(CO)~.'~ The CF complex 6 is the only structurally characterized compound of this family. Molecular Structures of 2a,b and 3b. Before consideration on the behavior of the trimolybdenum p3alkylidyne complexes in solutions the molecular structures of 2a,b and 3b are investigated by X-ray crystallography. Because the isomer ratio (AB) of 2a in solutions depends on the solvent polarity, the crystals obtained from CH3CN-C&6,15 C&, and to1uenel6have been subjected to X-ray analysis. The crystallographic data and selected structural parameters are summarized in Tables 2 and 3, respectively, and the molecular structures of 2a.CH3CN, 2b, and 3b are reproduced in Figures 2-4. An ORTEP view of 2a.CsDs is included in the supporting information, because the structure is essentially the same as that of the CH3CN derivative. Although we also examined X-ray analysis Of 3a, the structure could not be refined well because of the low quality of the crystals. A sketch of its top view is reproduced in Figure 5 . The trimolybdenum p3-alkylidyne complexes 2 and 3 contain the tetrahedral CM3 core as usually observed for trimetallic p3-alkylidyne complexes.8 The Mo-Mo lengths of ca. 3.0 A and the p3-C-Mo distances of ca. 2.1 A are essentially the same irrespective of the R and T ~ ~ - C ~ (CP) H ~ Rgroups ' and are comparable t o those found in the p3-CF analogue 6.14 The most characteristic feature of 2 and 3 is the orientation of the CPMo(C0)2 moieties. In the meth-
J
L
specific to HSiMezPh. While sterically less demanding phenyl silanes such as HSiMePh2 and HzSiPhz afforded 2 in low yields together with several byproducts,ll the reaction of HSiEt3, HzSiEtz, and HSiPh3 did not produce 2 and 3 a t all but resulted in the formation of an intractable mixture of products. We also found that the HSiMezPh-reduction of Cp2MOn(CO)s,CpzW2(C0)4,and (v5-C5Me&Moz(C0)4under photochemical and thermal reaction conditions was sluggish. The reaction of Cp2Moz(CO)sresulted in Mo-Mo bond cleavage7J2to give a mixture containing CpMo(H)(C0)3as detected by the hydride resonance (lH-NMR). Although, in a reaction mixture of Cp2W2(CO)4,a couple of very weak deshielded lH-NMR signals assignable to p-CH complexes were detected, none of them was isolable. Compared to the well-established ps-alkylidyne complexes of late transition metals (Co, Fe, Ru, Os, etc.1: the family of group 6 metal derivatives is small. Green et al. reported synthesis of a series of aryl derivatives @3-CAr)[MCp(C0)2136 (M = Cr, Mo, W, mixed) via addition of dimetallic fragments to mononuclear $(11)This observation suggests a possibility that photoactivation of phenylsilane may also be involved as a key step of the transformation. (12)Jetz, W.; Graham, W. A. G. J . A m . Chem. SOC. 1967,89,2773.
(13)Green, M.; Porter, S. J.; Stone, F. G. A. J . Chem. Soc., Dalton Trans. 1983, 513. (14) Schulze, W.; Hartl, H.; Seppelt, K. J . Organomet. Chem. 1987, 319, 77. (15)Because 2a is insoluble in CHsCN, benzene was added. (16)Because the molecular structure (A-type)as well as the crystal packing of 2a.toluene is quite similar to those of 2amC6D6,the result is not included in this paper [ C ~ ~ H I ~ O ~ (toluene), MO~.~M / = 710.3, orthorhombic space group Pbca, a = 17.295(5) b = 17.449(5)A, c = 16.270(3)A, V = 4910(2)A3, 2 = 8,dcaie= 1.92 g ~ m - p~ = , 15.5 cm-*, R(R,) = 0.048(0.049) for 2935 data with I > 3dI)I. The toluene molecule sitting on a center of symmetry in a manner similar to C6D6 is included in the space where C& occupies in 2aCsD6, although the methyl carbon atom of toluene could not be refined because of the completely disordered arrangement.
1,
5212 Organometallics, Vol. 14,No. 11, 1995
Akita et al.
Table 2. Crystallographic Data for 2aCHsCN, 2a'C&, 2b, and 3b complexes
2a.CH3CN
formula formula weight crystal system space group
a/A b/A CIA
P/deg viA3
Z d c a i c d k Cm-3
p1cm-l temp/"C 28ldeg no. of data collected no. of unique data with I > 3a(I) no. of variables
R RW
orthorhombic Pbca 18.205(6) 16.684(4) 15.818(5)
-
2b
3b
C25H2206M03
C26Hz406M03
706.3 cubic Pa3 16.872(5) -
720.3 monoclinic P21/a 16.429(6) 9.682(3) 17.406(6) 117.80(2) 2449(1) 4 1.95 15.2 25 5-55 6152 3566 311 0.093 0.075
!h*C& CzzH1606MO3'1/2C6D6 (C2sHi6D306M03) 706.2 orthorhombic Pbca 17.090(6) 17.426(5) 16.111(5)
-
-
4804(4) 8 1.95 15.8 25 5-50 4714 3235
4798(2) 1.95 15.8 25 3-50 4697 3399
4807(2) 8 1.95 15.8 25 5-50 4690 821
311 0.027 0.021
307 0.052 0.049
105 0.056 0.040
8
semibridging interaction with Mo2 resulting from the ylidyne complexes 2a,b all the CP rings project t o the rotation of the Mo3 unit may induce additional semisame side as does the p3-CH ligand. The solid state bridging interaction between Mo3 and C21. The structures of the 2a part are essentially the same semibridging interactions in 3 may be weaker than irrespective of the recrystallization solvents in contrast those in 2 as judged by the slight increase in the t o the solution behavior (see below). In addition, one averaged Mo***CO distances from 2.88 8,(2a) and 2.84 of the two CO ligands attached to each Mo center 8, (2b) to 2.94 8, (3b). In this paper the conformation interacts with the adjacent Mo center with the Mo-CO found in 2 (all the CP rings projecting upward) and 3 distances ranging from 2.8 t o 2.9 8, and the bent-back and 6 (two CP rings projecting upward and the remainangles (Mo-C-0) of ca. 169". The semibridging CO ing one projecting downward) is denoted by A and B, ~ and ligands are laid nearly coplanar t o the M o plane, respectively. the remaining terminal CO ligands project downward, almost perpendicular t o the M03 plane. The distances It is notable that the structures of the CCH3 comfrom the carbon atoms of +CO ligands to the adjacent plexes 3a,b and the CF complex 6 resemble each other Mo centers (>3.36 8,)are substantially longer than the as shown in Figures 4 and 5, although the semibridging semibridging interaction. As a result, the methylidyne interaction appears random at first glance. The overall complexes 2a,b have a 3-fold symmetrical structure as geometry is quite similar t o each other and the differcan be seen from the top views. The semibridging ences in the interatomic distances are less than 0.1 8,. Mo***COinteraction in group 6 metal carbonyl comThis feature indicates that some rather strong semibridgplexes has been usually observed for sterically congested ing interactions still remain in these complexes. multinuclear complexes as reported for dinuclear The orientation of the CPMa(C0)z fragments in the C O ) ~ 1 8trimolybdenumpus-alkylidyne complexes &3-C-R)[MoCPp-alkyne complexes ~ - ~ 2 : ~ 2 - R C ~ C R ) M o z C p ~ (and homo- and heterotrimetallic p3-alkylidyne comple~es'~ (C0)zh may be correlated with the size of the substituincluding 6. ent R. As can be seen from the space-filling models of The ethylidyne complexes 3 adopt less symmetrical 2a and 2b (Figure 61, little space is left above the structure (Figures 4 and 5 ) in contrast to the methylishadowed methylidyne hydrogen atom. Therefore when dyne complexes 2. The CP ring attached to Mo3 projects a substituent bigger than hydrogen is introduced as R, downward. In addition, the two CO ligands bonded to one of the CPMo(C0)z fragments is forced to rotate so Mo3 do not interact with the adjacent metal centers, as to release the steric repulsion between the R group and instead three of the four CO ligands bonded t o and the CP rings. In accord with this consideration, Mo1,2 work as semibridging ones. Although the the CH3 groups in the upward-projecting Cp' ligands Mo2-4231 distance [3.14(2)AI slightly longer than the in 3b are oriented toward the direction distal from the Mo3-.C21 distance suggests a similar interaction, the C-CHs bridge, whereas those in 2 are located above the small bent back angle [Mo3-C31-031: 175(2)"1shows CH ligand. little bonding interaction between C31 and Mo2. When In summary, (1) the A-type structure is the most the top view of 3b is compared with those of 2a,b, the stable conformer of @3-C-R)[MoCP(C0)213in the solid structure including the Mo1,2 moieties in 2 is retained state, (2) a complex bearing a bigger R like 3 and 6 to a considerable extent. But the removal of the adopts a B-type structure owing to the steric repulsion between the R and CP groups, (3)both of the conformers (17) Estimation of the rate of process (d) may be dificult, because A and B are stabilized by a combination of semibridging a spectrum where the fluxional processes are completely frozen out interaction of CO ligands, and (4) a structure with two has not been obtained and probably the separation of the CO signals would be very small. inverted metal fragments has not been observed in our (18)Bailey, W. I., Jr.; Chisholm, M. H.; Cotton, F. A,; Rankel, L. A. complexes and 6 at least in the solid state. J . A m . Chem. Soc. 1978,100,5764. Spectroscopic Characterization and Dynamic (19) (a)Chetcuti, M. J.; Chetcuti, P. A. M.; Jeffery, J. C.; Mills, R. M.; Mitrprachachon, P.; Pickering, S. J.; Stone, F. G. A.; Woodward, Behavior of the Trimolybdenum pa-Alkylidyne P. J . Chem. SOC.,Dalton Trans. 1982,699. (b) Sutin, K. A.; Li, L.; Complexes 2 and 3. In solutions, the p3-alkylidyne Frampton, C . S.; Sayer, B. G.; McGlinchey,M. J. Organometallics 1991, 10, 2362. complexes 2 and 3 exist as an equilibrated mixture of
Organometallics, Vol.14,No. 11, 1995 5213
Deoxygenative Reduction of Dimolybdenum Carbonyl Complexes
Table 3. Interatomic Distances (A) and Bond Angles (deg) for 2a*CH&N,2a.C,98,2b, and 3b complexes
2a.CH3CN
2a.CeDe
2b
3ba
6btc
C1-Mol Cl-Mo2 Cl-Mo3 C1-C2 Mol-Mo2 MOl-MO3 MO2-MO3 Mol-C11 Mol412 M02-C21 M02-C22 M03-C31 M03-C32 Cll-011 c12-012 c21-02 1 c22-022 C31-031 C32-032 Mol-C13-17 M02-C23-27 M03-C33-37 Mol-C21 Mol-C22 M0l-C32 M02-Cl2 M02-C31 M02-C32 Mo3-C 11 Mo3-C 12 M03-C22
Interatomic Distances 2.063(5) 2.077(8) 2.09(1) 2.081(4) 2.089(9) 2.079(4) 2.074(8) 3.0624(9) 3.052(1) 3.049(1) 3.046(1) 3.056(1) 3.053(1) 3.054(1) 1.978(5) 1.97(1) 2.05(1) 1.973(5) 1.94(1) 1.93(1) 1.970(5) 1.98(1) 1.966(5) 1.95(1) 1.960(5) 2.00(1) 1.971(5) 1.96(1) 1.157(5) 1.14(1) 1.12(1) 1.140(5) 1.18(1) 1.14(1) 1.167(5) 1.16(1) 1.150(5) 1.15(1) 1.158(5) 1.13(1) 1.150(5) 1.15(1) 2.324d 2.33d 2.33d 2.329 2.34d 2.229 2.32d 2.895(5) 2.893(9) 2.84(1) 3.826(5) 3.79(1) 3.72(1) 3.447(5) 3.49(1) 3.43(1) 3.441(5) 3.41(1) 2.886(5) 2.92(1) 3.805(5) 3.75(1) 2.872(5) 2.91(1) 3.736(5) 3.73(1) 3.433(5) 3.36(1)
2.13(1) 2.11(1) 2.13(1) 1.57(2) 3.004(2) 2.985(2) 3.003(2) 1.99(2) 2.01(2) 1.95(2) 1.95(2) 1.98(2) 1.96(1) 1.14(2) 1.12(2) 1.15(2) 1.15(2) 1.13(2) 1.15(2) 2.33d 2.35d 2.34d 4.08(2) 2.86(2) 3.61(2) 4.54(2) 3.14(2) 4.35(2) 3.32(1) 2.93(2) 4.03(2)
2.08(2) 2.12(2) 2.06 1.39(2) 3.02 2.93 3.00 1.99(2) 1.99(2) 1.93(2) 1.95(2) 2.06 1.97 1.14(3) 1.16(2) 1.18(3) 1.15(3) 1.13 1.14 2.33d 2.34d 2.32d 4.10 2.90 3.57 4.54 3.18 4.36 3.31 2.86 4.05
Mol-C1-Mo2 Mol-C1-Mo3 M02-Cl-Mo3 Mo2-Mol-Mo3 Mol-MoB-Mo3 Mol-MoS-Mo2 C1-Mol-C11 Cl-Mol-Cl2 Cl-Mo2-C21 Cl-Mo2-C22 Cl-M03-C31 Cl-M03-C32 Mol-C11-011 Mol-C12-012 M02-C21-021 M02-C22-022 M03-C31-031 M03-C32-032
95.3(2) 94.7(2) 94.4(2) 59.97(2) 59.75(2) 60.28(1) 103.5(2) 119.6(2) 104.0(2) 120.8(2) 104.4(2) 120.8(2) 168.5(4) 176.4(4) 168.9(4) 175.5(5) 169.6(5) 174.8(5)
Bond Angles 94.2(3) 93.8(6) 94.8(3) 94.4(4) 60.01(3) 59.980(9) 60.06(3) 59.93(3) 104.5(4) 102.1(4) 119.8(4) 120.7(6) 105.3(3) 119.1(4) 103.7(4) 121.4(3) 169.4(9) 164(1) 177.4(9) 175(1) 168.1(9) 177.1(9) 169.4(9) 173.7(9)
90.4(6) 89.1(5) 90.2(5) 60.19(5) 59.60(5) 60.21(5) 119.7(6) 94.9(6) 117.2(6) 111.5(6) 80.2(6) 78.2(6) 174(1) 171(1) 169(1) 167(2) 175(2) 175(1)
91.7(7) 92.0(5) 91.6(6) 60.8 58.2 61.0 116.0(7) 95.7(7) 116.0(7) 111.1(7) 79.9(6) 76.3(6) 174 170 168 168 176 178
M0l-C31: 4.42(2) A; M02-Cl1: 3.60(2) A; M03-C21: 3.03(2) Parameters for one of the two independent molecules are listed. Because some of the important parameters were not reported in the original paper (ref 171, numbers without standard deviations are calculated by us. Mol-C31: 4.54 A; Mo2-C11: 3.58 A; Mo3-C21: 3.06 A. Averaged value.
A.
a
isomers which are interconverted to each other at various rates. (i) Dynamic Behavior of the Methylidyne Complex 2a via a Combination of Gearlike Rotation and Local Rotation of the MoCp(C0)2Fragments. Two isomers are detected for the methylidyne complex 2a by lH- and 13C-NMRas shown in Figure 1. The p3CH signals at 11.85 and 13.37 ppm are correlated with the p3-CH signals at 245.6 and 262.3 ppm, respectively, by means of a CH-HETCOR spectrum. The chemical values [156 (A) and 162 Hz shifts as well as the VC-H (B)1 are comparable to those for previously reported trinuclear p3-methylidyne complexes: [Cp3Rh3@3-CH)@-C0)21PF6d~ 16.2, dc 303.6;9h1i [(q5-C5Me5)3Ru3@3= 164.6 CH)@-Cl)&-H)lBF4 d~ 17.56, dc 342.2, 'Jc-H d~ 11.5, 6c 232.0, 'Jc-H = H Z ;Fe3@3-CH)b-H)3(CO)g ~~ 166 H z ; Ru~@~-CH)+-H)~(CO)~ ~~ d~ 9.75;9d0~3@3-CH)-
a
c33
26
" 0 169.6(5)"
Figure 2. Molecular structure of 2a drawn at the 30% probability level. Numbers without atom names are for CO ligands. The MeCN molecule is omitted for clarity. (a) An overview. (b) A top view. @-H)3(CO)g6~ 9.36, 6c 68.2, ~Jc-H = 171 H Z ] . ~The ~ Cp (lH) and CO signals (13C)can be divided into the ~ (Cp); dc 227.4, 231.2 (C0)I sharp signal set [ d 5.22 assignable to one isomer (A) and the rather broad signal set 5.16 (Cp); 6, -230 (CO)] assignable to the other isomer (B). The grouping is further confirmed by comparison with the spectrum observed in told8 where B is present as the predominant species. The methylidyne complex 2a gives solvent- and temperature-dependent spectra, and data enough for consistent explanation of every aspect of the dynamic behavior is not available in a single solvent. Then variable temperature NMR measurements have been carried out in various solvents. The two isomers are attributed to the two solid state structures A and B determined by X-ray crystallography on the basis of the following observations. First of all, comparison of a solution 13C-NMRspectrum with a I3C CP/MAS NMR spectrum of 2a-13C(recrystallized from toluene: A-type structure)16 leads to the assignment of the sharp signal set to the 3-fold symmetrical structure A. This assignment is further confirmed by low temperature 13C-NMR. The spectrum observed at -50 "C in CDC13 is reproduced in Figure 7, which can be explained by taking into account the structural formulae shown below the spectrum. The sharp shape of the signals of isomer A throughout the measured temperature range (rt to -80 OC) suggests that no dynamic process faster than the NMR line-broadening time scale is operating for this species. The separately observed two CO signals (a and b) and the simple spectral pattern containing only one Cp signal indicate that the semibridging interaction is considerably strong and, as
5214 Organometallics, Vol. 14, No.11,1995
Akita et al.
a
a C18
n
cl+417 C
b b
Figure 3. Molecular structure of 2b drawn at the 30% probability level. Numbers without atom names are for CO ligands. (a) An overview. (b) A top view. a result, the 3-fold symmetrical structure (A)is so rigid as to be retained even in a solution. The other isomer (B) is less symmetrical than isomer A as suggested by the increased number of the signals. The CO resonance observed as a very broad peak around 230 ppm a t room temperature [Table 1; see also Figure 1 (in CD2C12)] splits into three signals at -50 "C, the central peak of which appears as a shoulder peak of one of the signals of A [Figure 8 (in CDC13)I. Furthermore the slightly broad lH-NMR Cp signal in higher field (& 5.16 in CD2Cl2; Figure 1)also separates into two signals ( 6 5.11, ~ 5.22) in 1:2 ratio upon cooling down to -80 "C (Table 1). The number of the ancillary ligands' signals (2Cp, 3CO: 13C) and the intensity ratio of the two Cp signals (lH) at low temperature suggest a structure with apparent C,symmetry rather than the unsymmetrical structure (B) giving ten signals. The slightly broad signals of B indicate that still some dynamic process occurs, which can not be frozen out even a t -80 "C in various solvents. The spectrum can be interpreted in terms of fast switching of the semibridging CO ligands (CI and c p ) attached to the Mo atoms bearing upwardprojecting Cp rings (Figure 7). This motion (Bs B' in Figure 7) gives a spectrum consistent with a C, symmetrical structure with respect to the mirror plane passing through Mo3, CH and the midpoint of the MolMo2 bond [2Cp (2:l); Cpl = Cp2, Cp3; 3CO: al = a2, bl = b2, ~1 = CZ]. The ratio of the two species A and B present in solutions ( A B )is found to be dependent on the solvent as well as the temperature. As the solvent polarity decreases [CD2C12 (111) CDC13 (114) C6D6 (116) tol-d8(1/6)]and the temperature is raised [1/4 (-80 "C
-
-
-
Figure 4. Molecular structure of 3b drawn at the 30% probability level. Numbers without atom names are for CO ligands. (a) An overview. (b) A top view.
3a
6
Figure 5. Top views of the molecular structures of (p3CR)[MoCp(CO)& [R = CH3 (3a)and F (6)]. The roman and italicized numbers refer to the Mo-C distances (A) and Mo-C-0 angles (deg),respectively.
-
in tol-dg) 116 (rt)], the isomer B becomes dominant. In order to investigate the interconversion, attempts to get a coalesced spectrum were made. However, because the isomer B became the predominant species at higher temperature, it was impossible to differentiate coalescence of A and B from complete thermal isomerization of A to B. Then we carried out saturation transfer experiments (Figure 8). Irradiation of the p3-12CH signals of one isomer causes reduction of the intensity of the p3-12CH signals of the other isomer in a selective manner. Thus the two isomers are interconverted to each other a t a rate slower than the lH-NMR linebroadening time scale. The dynamic processes operating for 2a are summarized as follows: (i) The two isomers present in a solution are assigned to the structures A and B char-
Organometallics, Vol. 14,No.11, 1995 5215
Deoxygenative Reduction of Dimolybdenum Carbonyl Complexes top views
side views
H
Figure 6. Space-filling models of 2a,b.
1
-50°C
IU I
A
14
2jO
280
240
6 I PPm
CP
A (C3-symmetry)
240
7 2 0
B (a, b, 4
96
94
92
$0
88
1 B CP3
CP3
B
B' (C,-symmetry)
Figure 7. A low temperature 13C-NMRspectrum (at 100 MHz; in CDC13; at -50 "C) of 013-*CH)[MoCp(*CO)&2aI3C (*C: ca, 30% 13C-enriched). acterized by X-ray crystallography; (ii)The two isomers are observed separately a t room temperature, but they are interconverted to each other a t a rate slower than the NMR line-broadening time scale; (iii) The spectra of isomer A containing only one Cp signal and sharp and semibridging CO resonances are consistent with the C3-symmetrical structure; (iv) Upon lowering temperature, spectra of isomer B, suggesting an apparent C3structure at room temperature, are changed to spectra consistent with an apparent C,-structure where a COswitching process is not frozen out. The spectral features can be explained by a combination of a couple of rotational processes of the MoCp(C0)z units associated with cleavage and recombination of the semibridging interaction of CO ligands (Scheme 3). Rotation of one of the three MoCp(C0)nfragments in A gives structure B (b). The equivalent Cp rings of B at higher temperature (iv) can be explained in two ways. The first mechanism (c) involves a very fast concerted rotation of the three MoCp(C0)a fragments in the structure B (process c). As the MoCp(C0)z group projecting downward is flipped up, another MoCp(C0)z group rotates and projects downward. In this mechanism the three metal fragments rotate in a concerted manner just like a gear so as to relieve the steric repulsion around the site above the p3-C-R bridge. If the rotation of the three metal fragments is faster than the NMR line-broadening time scale, all the Cp rings
13
12
11
Figure 8. 'H-NMR saturation transfer experiments O f 013*CH)[MoCp(*C0)2]32a-13CG3-CH region; *C: ca, 30% 13Cenriched; at room temperature; at 500 MHz). Irradiated sites are indicated by arrows. become equivalent. The second mechanism involves flipping back to A followed by local rotation of another metal fragment (B A B). The latter mechanism can be eliminated on the basis of the change of the CO signals indicating that the interconversion between the two isomers A and B [via local rotation (b)] is slower than the averaging of the B s signals [via gearlike rotation (ell. Upon warming a sample from -50 "C to rt (compare Figures 1 and 7; a similar 13C-NMR spectrum was obtained a t rt in CDClS), the CO signals of isomer B changes t o a broad one without coalescence with those of isomer A, which remain rather sharp signals. Because the separation of the CO signals of the two isomers A and B at -50 "C is comparable to each other, the coalescence temperature is roughly correlated with the activation energy of a rotational process. These observations lead to the above conclusion. The concerted gearlike rotation for B is frozen out a t low temperature, but the spectrum suggesting a C,structure has been obtained. This result is explained by the fast switching of the semibridging CO ligands (d: B E? B ) as discussed above. In contrast to this process, switching of the CO ligands in A (a) is very slow as indicated by point iii. We also tried to observe coalescence of the two CO signals of A, but again our attempts were hindered by the complete thermal isomerization to B at higher temperature. Finally let us point out that a structure with two inverted metal units (C) has not been detected in the present system. The coalescence temperatures of the CO signals lead to the estimation of the rates of the dynamic processes as follows: c > b > 8.l' The relative magnitude of the rates can be explained by taking into consideration the ease of cleavage of the semibridging Mo-*.COinteractions (Scheme 3). Compared to process b where only one interaction is broken a t first, the concerted switching (a) which requires breaking of three interactions should be a higher energy process. Therefore the local rotation (b) proceeds faster than the racemization of A (a). On the other hand, the CO switching of B (d: B *
- -
Akita et al.
5216 Organometallics, Vol. 14, No. 11, 1995
Scheme 3 R
R
/
e (63 : CO)
-
CP
rates: c > b > a a: CO-switching
b: local rotation c: gear-like rotation d: CO-switching R= H (2), CH3 (3). CP= Cp (a), Cp' (b). COS are omitted for clarity.
b
C
v
c-3
CP
CP
A
B"
c P'
(?{F) CP
?
not detected
B') should be a low energy process, because the semibridging interaction with bl and b2 (Figure 8) is retained throughout the process and the structural change can be attained via least motion of the ligands. As for local rotation (c: B B),two interactions should be cleaved accompanied by rotation of two metal fragments. Because one of the interactions is a weak one (c) just mentioned and the semibridging interactions in B seem to be weaker than those in the highly symmetrical A as discussed before, the gearlike rotation ( c ) via cleavage of the rather weak interactions should proceed faster than the local rotation (b). Although we also have to consider rotational barrier of the metal units, we have no data to estimate it at present. The above discussion on the semibridging interaction apparently leads to a conclusion that isomer A should be more stable than B. Contrary to the expectation, B has proved to be the major isomer in solutions, though the ratio is dependent on the conditions. Also we don't have any data to explain the result. Some electrostatic interaction may be responsible for the ratio, because an apparent correlation between the isomer ratio and the solvent polarity has been observed. Isomer A with a larger dipole moment may be stable in a polar solvent. Thus the complicated dynamic behavior of 2a can be interpreted in terms of a combination of the rotation of the MoCp(CO)2units and switching of the semibridging CO ligands. The dynamic behavior of the dinuclear p-alkyne complexes (,M-q2:q2-RC+!R)Mo2Cp2(C0)41s that are isolobal with 2 has also been explained by the switching of the semibridging interaction. In this case, however, complete rotation of the metal fragment has not been observed. As for the aryl derivatives (5)13of 2, only one CO signal was observed at rt. Although some dynamic behavior which was frozen out a t -30 "C was noted for a mixed metal complex (M3 = Mo~W), further study was prevented by the low solubility. For the fluoride derivative 614only lgF-NMRwas recorded, and fluxional property was not mentioned at all. Recently, McGlinchey studied fluxional properties of mixed (M: Mo, W; metal clusters (q5-C5R5)MCo2(C0)8(,M3-CR) R: COOP$,AI-)~~ closely related to our system. The semibridging interaction of the Mo-CO ligands is also found in these complexes, and the dynamic behavior has
been interpreted successfully in terms of rotation of the Mo(q5-C5R5)(C0)2and cO(c0)3fragments as well as intermetallic CO transfer. Two rotamers corresponding to A and B are detected by low temperature 13C-NMR, but the structure is still considerably mobile, because the semibridging interaction available for the complexes is not so effective for making the structure rigid on the NMR line-broadening time scale as that in 2 and 3. (ii) Dynamic Behavior of @3-CH)[MoCp'(C0)2]3 2b. The spectra of 2b are much more complicated than those of 2a. As shown in Figure 9a, six p3-CH signals are detected even a t 30 "C. Because all the signals coalesce into a broad one a t 110 "C (Figure 9b), the peaks arise from a single complex. On the other hand, lowering temperature to -90 "C results in appearance of nine signals (Figure 9c). The 13C-NMR spectrum observed a t 30 "C (Figure 9d) contains three sharp signals (AI, AII, and BIII) in addition to a couple of very broad components. They are correlated with some of the lH-NMR signals by a CH-HETCOR spectrum as indicated in Figure 9. These data are consistent with the formulation of 2b as an isomeric mixture of a p3methylidyne complex. Comparison o f the &, dc, and ~Jc-H values with those of 2a [&(A) 12.5 > &(B) 11.5; dc(A) 260 > &(B) 245; 'Jc-H(A) 155 Hz; 'Jc-H(B) 160 Hz] leads to the tentative assignments of the peaks AI-AI11 and BI-BIII to A- and B-type structures, respectively. The isomers may arise from hindered rotation of the Cp' rings (see, for example, A and A' in Scheme 4)or different modes of the semibridging interactions. The dynamic behavior of 2b may be explained by Scheme 3. However, the spectra are too complicated to be analyzed sufficiently. To our surprise, saturation transfer experiments for the p3-CH signals reveals that interconversion between AI BI, AI1 BII, and AI11 c* BIII pairs takes place in a selective manner. For example, irradiation of the CH signal of AI1 results in disappearance of the CH signal of BII (Figure 9e). This means that interconversion between the two structure A and B via the local rotation (c) is
--
-
- -
-
-
-
(20) (a) D'Agostino, M. F.; McGlinchey, M. J. Polyhedron 1988, 7, 807. (b) Sutin, K. A.; Kolis, J. W.; Mlekuz, M.; Bougeard, P.; Sayer, B. G.; Quilliam, M. A.; Faggiani, R.; Lock, C. J. L.; McGlinchey, M. J.; Jaouen, G. Organometallics 1987, 6, 439.
Deoxygenative Reduction of Dimolybdenum Carbonyl Complexes
I
Organometallics, Vol. 14, No. 11, 1995 5217
*.
(c) 'H (-90%)
(a) 'H (30%)
All
iis
1i.s
12.0
ii.0
12.5
120
11.5
110
6 1 ppm
(e) irrad. at All. 4!1
410
319
318
317
316
3!5
6 / PPm
Figure 10. The p3-CCH3 region of the lH-NMR spectrum (400 MHz) of @3-*C*CH3)[MoCp'(*CO)z133b-13C (*C: ca. 30%13C-enriched)observed in C& at room temperature.
Figure 9. Variable temperature N M R spectra and saturation transfer experiment of @3-CH)[MoCp'(C0)~13 2b observed at 400 (lH) and 100 MHz (13C) in tol-ds. (a) The p3-CH region of lH-NMR spectrum observed at 30 "C. (b) At 110 "C. (c) At -90 "C. (d) The p3-CH region of 13CNMR spectrum observed at 30 "C. (e) An example of saturation transfer experiment where the p3-CH signal of AI1 being irradiated (at 30 "C). Scheme 4 Cp' rofafion
A
A'
Cp' rotation ___)
faster than the interconversion of A with another A-type structure (for example, A') via simple rotation of a Cp' ring around the Mo-Cp' centroid axis (Scheme 4). Therefore in this case the local rotation (c) (Scheme 3) proceeds while keeping the different, relative configuration of the three Cp' rings. These phenomena should originate from the hindered rotation of the Cp' ring. (iii)Dynamic Behavior ofc/rs-CC&)[lMoCp(CO)als 3a via Gearlike Rotation of the MoCp(C0)a Fragments. This complex has been characterized by comparison with an authentic samplelo as described before, and its structure is further supported by the characteristic deshielded 13C-NMR signal (& 269.3) as well as the preliminary X-ray result. The quaternary carbon signal coupled with both of the C and H atoms of the adjacent CH3 group clearly indicates the presence of an ethylidyne linkage +CCH3 [Table 1; see also the spectrum of 3b (Figure lo)].
The lH-NMR spectrum observed a t room temperature contains a single Cp resonance, which splits into two singlets of 2 (10H):l (5H) intensity ratio below -40 "C. The low temperature spectrum is consistent with the crystal structure (B), and further cooling down to -80 "C does not cause any notable change. Throughout the observed temperature range the CCH3 signal remains as a single peak and only one species has been detected by NMR analysis. The behavior of 3a can be explained by the concerted gearlike rotation ( c ) . ~ ~ (iv) Dynamic Behavior of c/rs-CCHs)[MoCp'(C0)zls 3b. A lH-NMR spectrum of 3b obtained from lb-13C0 (in C& at rt) contains two species B and D in 3:l ratio (Figure 10). The major component B exhibits spectral features close to those of 3a. Any notable change is not observed for the sharp p3-C-CH3 signal in the range of rt t o -80 "C, and the q5-C5H4CH3 resonance splits into two signals of 1:2 ratio below -50 "C. Therefore the major species has been assigned t o a B-type structure. The ps-CCH3 signal of the minor component D does not show any notable change either, but analysis of the q5-CsH4CH3 signals is hindered by broadening and overlap with the signals of B at low temperatures. Isomer D is tentatively assigned to an isomer of a B-structure with different Cp' orientation or different semibridging CO interactions. But the possibility of D being another structure C (Scheme 3) cannot be eliminated completely, because the situation around the p3-CCH3 moiety is much more sterically congested than that of the Cp analogue 3a. Consideration on the Formation Mechanism of the ps-Alkylidyne Complexes 2 and 3. At first, the source of the H and C atoms of the newly formed bridging hydrocarbyl groups Qs-CR) in 2 and 3 were investigated by labeling experiments (eqs 2-4). The reaction of la,b with DSiMe2Ph afforded the deuterated products 2 a ) b d l and 3a,b-d3 of more than 95%isotopic purity as estimated by lH-NMR (eq 2). The blank experiments (eqs 5 and 6) showed that H-D exchange reactions of the resulting 2a and 3a with DSiMe2Ph and (21)Because of the low yield of 3a variable temperature W-NMR measurements could not be examined.
5218 Organometallics, Vol. 14, No. 11, 1995
Akita et al.
the deuterated solvent (C&) did not take place a t all. Next the reaction of l-13C0(enriched with ca. 30% 13CO) with HSiMezPh gave a mixture of 2J3C and 3J3C (eq 3). As can be seen from the lH-NMR spectrum of 2aD
1a,b
DSiMezPh h v l C6H6
f5 MOhl--\MO \Mo4 3a-d3 3b-d3
'CH3 I
3a- 13c 3b- I3C
(4)
2a-dl, 13C
( l J c . ~ =166 Hz) SH
Figure 11. Spectra (lH: 400 MHz; 13C: 100 MHz) of a reaction mixture of la-WO with HSiMezPh observed at room temperature in C6&. (a) 'H-NMR ( c p region). (b) The low field region of a CH-HETCOR spectrum.
13C(Figure l),the p3-CH signals of the two isomers (A and B) which are accompanied by satellite peaks owing t o the VC-Hcoupling clearly indicate the origin of the p3-CH atom t o be CO in la. As for the ethylidyne ligand, the ps-CCH3 region of the 'H-NMR spectrum of 3b-13C is reproduced in Figure 10. The triplet-oftripletlike splitting pattern has been analyzed as a result of coupling with the two ethylidyne carbon atoms (B:'Jc-H = 128 Hz, 2 J ~=-5~Hz; D: 'Jc-H = 127 Hz, 2 J ~ =-5~H Z ) ,which ~ ~ reveals that both of the carbon atoms are enriched by 13C t o an extent comparable to the starting compound laJ3CO. The enrichment of the carbon atoms is also verified by the observation of the VC-Ccoupling (32Hz) of the C-CH3 linkage. Similar spectra have been observed for 2b and 3a. Finally, the result of the double labeling experiment (eq 4) is consistent with the above experiments, and the VC-D value for the p3-CD ligand in 2a-dl 13C has been determined to be 25 Hz. Thus the carbon atoms of the p3-C-R bridge in 2 and 3 come from CO bonded to Mo in 1, and all the hydrogen atoms are transferred from HSiMezPh that is used as an equivalent for dihydrogen. The formation of 2 from 1 should involve (1) Lewis acidic activation of CO, (2)hydride transfer t o the CO carbon atom, (3) deoxygenation, and (4) formation of the trinuclear skeleton. The first function has been studied rather extensively for Co complexes. In 1973 MacDiarmid reported that thermolysis of MesSiCo(CO)4 afforded a mixture of a ps-siloxyalkylidyne complex (443COSiMe3)CodCO)gand a C-C coupling product (MesSiO C ) ~ C O Z ( C ORecently, ) ~ . ~ ~ Do et al. reported formation (22) Marshal, J. L. Curbon-Curbon and Curbon-Proton Couplings; Verlag Chemie International: Florida, 1983. (23) (a) Ingle, W.M.; Preti, G.; MacDiarmid, A. G . J . Chem. SOC., Chem. Commun. 1973,497. (b) Baay, Y.L.; MacDiarmid,A. G. Inorg. Chem. 1969,8,986. Formation of @&SiMe3)C03(C0)9 was mentioned briefly but not at all in 25a.
of a similar cluster compound through reaction of Coz((20)s with silatrane, where a Si-Co species was assumed as an intermediate.24 The RsSi-Co species is so oxygenophilic as to activate 0-containing functional groups such as CO and >C=O groups as a Lewis acid and induce formal insertion of CO into the Si-Co bond giving a cos& group. The unique properties of silylmetal species are applied successfully to catalytic transformations using HSiR3/CO/Co2(C0)8system developed by Murai and his c o - w o r k e r ~ .But ~ ~ hydride transfer to the CO atom does not occur in previous systems.26 In addition, to our knowledge, deoxygenation has never been observed for catalytic hydrosilylation of oxygenated compounds except for the reaction of acyl ~ example 9 ~ ~ of metal complexes reported by ~ 8 . A~ rare stoichiometric deoxygenation of organic compounds was reported by Gladysz et aLZ7 Treatment of ketone (RzC=O) with the 2,5-disila-l-ferracyclopentane
-
(OC)4FeSiMez(CHz)zSiMezresults in the formation of an iron-carbene intermediate (OC)4Fe=CR2 and the cyclic disiloxane dSiMez(CHz)zSiMez. In order to investigate unstable species which could not survive during the isolation procedures, a reaction mixture of la-13C0 and HSiMezPh was monitored by NMR (Figure 11). The reaction is considerably clean as can be seen from the Cp region of the lH-NMR spectrum (Figure l l a ) , although deposition of a small amount of insoluble materials is evident. Only two major Cp peaks assignable to l a and 2a are observed in this region. In the low field region (Figure llb) a (24)Kim, M. W.; Uh,D. S.; Kim, S.; Do, Y. Inorg. Chem. 1993,32, 5883 and references cited therein. (25) Murai, S.;Seki, Y. J. Mol. Cut. 1987, 41, 197. (26)Very recently, catalytic conversion of CO and HSiRs into CH3(CHz),OSiRs and C H ~ ( C H Z ) ~ C ( S ~ R ~ ) - C Hwas O S ~reported. R~ Sisak, A.; Mark6, L.; Angyalosy, Z.; Ungvlry, F. Inorg. Chim. Acta 1994,222, 131. (27) (a) Nakazawa, H.; Johnson, D. L.; Gladysz, J. A. Orgunometullics 1983,2,1846. (b) Gladysz, J. A. Acc. Chem. Res. 1984,17, 326.
Deoxygenative Reduction of Dimolybdenum Carbonyl Complexes
Organometallics, Vol. 14,NO.11, 1995 5219
Chart 1
Scheme 5
1
Jc.c = 5 9 H z r 6~ 22.35 Y 2 9 H Z (4) CPMOZC-H-SH
(cob .
13.22
$
&
H-H
-H20
*
H-SiMe2Ph
-
I-
7
1
- (Ph2Me2Si)20
2
+
3
rather intense ‘H-NMR signal ( 6 13.22: ~ 7) is observed in addition t o 2a(A,B).28The signal is found t o be the CH ligand along with siloxane. A closely related correlated with the 13C-NMRsignal appearing in very system was reported by Fehlner et Treatment of low field (6c 313.9), and the coupling pattern (doublet, Fe(C0)5 with a borane reagent produces a mixture of ~JCH = 166 Hz) leads to the assignment to a methylidyne p3-methylidyne and p3-ethylidyne complexes, b3-CH)(CH) group. On the basis of the 6c values close to those Fe&-H)3(CO)g and b3-CCH3)Fe301-H)3(CO)g. Although of mononuclear molybdenum carbyne complexes Cp(C0hthe reaction mechanism has not been clarified so far, Mo(ECR) [6c(R)332.8 (CH~BU’),~’ 309.5 (C6&Me-2)],30 either, the Lewis acidic borane reagent should work as 7 is attributed tentatively t o the mononuclear vlan efficient deoxygenative reducing agent in a manner methylidyne complex Cp(C0)2Mo(WH) (Chart 11, alsimilar to the hydrido-silyl-intermediatein Scheme 2. value H is larger than those of the though the ~ J C In both of our and Fehlner’s systems the mechanism of previously reported methylidyne complexes [cf. W(=CH)carbon chain elongation leading to the ethylidyne Cl(PMe3)4 6c 250 ( ~ J c =H 134 H Z ) ;(But0)3W(=CH) ~~~ complexes has remained to be solved. 6c 252.4 (UCH= 150 HB(~,~-M~~Pz)~(CO)~WFinally, let us point out that interaction of dinuclear (ECH) 6c 280.6 (~JcH = 142 Hz)J3lC Isolation of 7 has carbonyl complexes with hydrosilane does not always been unsuccessful despite several attempts. While result in the metal-metal bond cleavage giving H-M another very weak CH signal is found at 8~ 15.39 (8), and/or Si-M species as established since 196O’s.l2 the corresponding 13C-NMR peak cannot be located Although the initial process may be photochemically or owing to its low intensity. In higher field a signal set thermally induced CO d i s s ~ c i a t i o nthe , ~ ~following step (9) assignable to an ethylidyne linkage is observed is dependent on several factors such as reaction condi(Chart 1). The methyl signals are located at BH 1.69 tions, metal, and structure of hydrosilane. In the and 6c 22.3 (UCH = 166 Hz), and the latter signal is present study, the reaction of the hexacarbonyl dimoalso correlated with the 6c 170.8 signal on the basis of lybdenum complex Cp2Mo2(CO)6results in Mo-Mo bond satellites due to ‘JCC coupling (59 Hz). The ratio of cleavage, whereas that of the tetracarbonyl complex Cp2these species present in the mixture are estimated to Mo2(C0)4 1 results in the deoxygenative reduction be 0.2 (2a-A):1(2a-B):0.4(7):0.5 (8):O.l(9). But unforgiving the alkylidyne complexes 2 and 3. Similarly, the tunately relationship between the isolated species 2 and consequence of the interaction of CpzRuz(C0)r with 3 and the unstable species 7-9 cannot be established. monohydrosilane is Ru-Ru bond cleavage, but the The detection of 7 in a reaction mixture suggests a reaction with di- and trihydrosilane is deoxygenative reaction pathway involving initial deoxygenative reducreduction leading t o bridging methylene tion of 1 affording 7. Subsequent addition of the dimetallic fragment 1 may give the trinuclear product Conclusions 2. The aryl derivatives 5 were actually prepared by this method starting from Cp(C0)2M(WAr) as mentioned Photochemical reaction of the dimolybdenum carbonyl before.13 As for the initial process, we assume that it complex 1 with HSiMeaPh results in deoxygenative should involve catalytic hydrosilylation as reported for reduction of a CO ligand to give a mixture of the acyl metal complexes monom metal lake tone^).^^ The elemental hydrocarbyl complexes, i.e. the trinuclear p3bridging and semibridging CO functional group in methylidyne (2) and pus-ethylidyne complexes (3). The dinuclear complexes, which can be viewed formally as present system serves as a good model system for dimetallaketone, may be susceptible to catalytic hyformation of CH and CCH3 species via Fischer-Tropsch drosilylation to give siloxymethylene group [C(H)OSiR3]. mechanism: Reduction of CO in 1 with HSiMezPh (a Although we don’t have any evidence for the catalytic H2-equivalent) produces CH (2) and CCH3 species (3) species (M in Scheme 21, the present system is a kind with elimination of siloxane (4: silylated water) (Scheme of self-catalyzed reaction effected by some unknown 5). In the actual catalytic system H2 works as an species present in a reaction mixture (M = m = Mo; reducing agent as well as a deoxygenating reagent. Scheme 2). The siloxymethylene species may be deoxyHowever, reduction of a model CO complex with H2 is genated by the action of a silyl-metal species t o form not always successful, because the hydrido-metal intermediate shows little affinity toward polar substrates (28) The Cp signal of 7 may overlap with that of 2a or 3a. The ‘Hsuch as CO as mentioned in Introduction. The present NMR signal corresponding to the rather intense signal around 6c 250 could not be located. It might be attributed to a bridging CO ligand or carbide species. Isolation of this species was unsuccessful. (29) Baker, P. K.; Baker, G. K.; Gill, D. S.; Green, M.; Orpen, A. G.; Welch, A. J. J. Chem. SOC.,Dalton Trans. 1989,1321. (30) Dossett, S. J.; Hill, A. F.; Jeffery, J . C.; Marker, F.; Shenvood, P.; Stone, F. G. A. J. Chem. SOC.,Dalton Trans. 1988,2453. (31) (a) Holms, S. J.; Clark, D. N.; Turner, H. W.; Schrock, R. R. J. Am. Chem. SOC.1982,104, 6322. (b) Chisolm, M. H.; Folting, K.; Hoffman, D. M.; Huffman, J. C. J. Am. Chem. SOC.1984,106,6794. (c) Jamison, G. M.; White, P. S.; Harris, D. L.; Templeton, J. L. in Transition Metal Carbyne Complexes; Kreissl, F. R., Ed.; Kluwer Academic: Dordrecht, 1993; p 201.
(32) (a) Wong, K. S.; Fehlner, T. P. J. Am. Chem. SOC.1981,103, 966. (b) Wong, K. S.; Haller, K. J.; Dutta, T. K.; Chipman, D. M.; Fehlner, T. P. h o g . Chem. 1982,21,3197. (c) DeKock, R. L.; Wong, K. S.; Fehlner, T. P. Inorg. Chem. 1982,21, 3203. (d) Vites, J. C.; Jacobsen, G.; Dutta, T. K.; Fehlner, T. P. J. Am. Chem. SOC.1985, 107, 5563. (e) Dutta, T. K.; Vites, J. C.; Jacobsen, G. B.; Fehlner, T. P. Organometallics 1987,6,842. (33) A pathway without CO dissociation may be possible. Alt et al.
reported that reaction of (q5-C5Me&Mo2(C0)4 with H2 produced the addition product (q5-CsMe5)2M0201-H)z(C0)4. Alt, H.; Mahmoud, K. A,; Rest, A. J . Angew. Chem., Int. Ed. Engl. 1983,22,544.
Akita et al.
5220 Organometallics, Vol. 14,No.11, 1995 transformation is realized by employment of HSiMe2Ph with the high oxygenophilic Si moiety. As a deoxygenating agent, siloxane is superior to water, when the transformation is carried out in neutral organic media. The dynamic behavior of 2 and 3 has been analyzed as a combination of the gearlike and local rotational processes of the metal fragments associated with switching of the semibridging Mo--*COinteractions. The semibridging interaction is so rigid as t o stabilize the intermediates of the rotational processes, two of which have been characterized by X-ray crystallography.
Experimental Section All manipulations were carried out under an argon atmosphere by using standard Schlenk tube techniques. Ether, THF, hexanes, benzene and toluene (Na-K alloy), and CHZClz and CH3CN (PzOs) were treated with appropriate drying agents, distilled, and stored under Ar. The dimolybdenum carbonyl complexes la and l b were prepared according to the literature p r o c e d ~ r e s .The ~ ~ 13CO-enrichedisotopomers (ca. 30% enriched) were prepared by thermal decarbonylation of I3CO-enriched CpzMOz(*C0)6and Cp'zMoz(*CO)6, which were obtained by treatment of la and l b with I3CO ('90% I3Cenriched; 2 atm) at room temperature in CHzC12, respectively. HSiMezPh was purchased from Aldrich, and DSiMezPh (-99% D-enriched) was prepared by reduction of ClSiMezPh by LiA1D4. An authentic sample of 4 was prepared by refluxing ClSiMezPh in ether/HCl aq. Column chromatography was performed on silica gel 60 [70-230 mesh ASTM (Merck Art. 7734)]. 'H- and I3C-NMR spectra were recorded on JEOL EX400 ('H: 400 MHz; I3C: 100 MHz) and GSX-500 spectrometers ('H: 500 MHz; I3C: 125 MHz). Solvents for NMR measurements containing 1%TMS were dried over molecular sieves, degassed, and distilled under reduced pressure. IR and FDMS spectra were obtained on a JASCO FT/IR 5300 spectrometer and a Hitachi M-80 mass spectrometer, respectively. As a source of UV irradiation a n Ushio UM-452 lamp was used. Deoxygenative Reduction of Cp&loz(CO)r Giving 2a and 3a. A benzene solution (300 mL) of la (3.34 g, 7.71 mmol) and HSiMezPh (4.0 mL, 26 mmol) in a Pyrex photoreaction vessel was irradiated by a high pressure mercury lamp while immersed in an ice-water bath. The irradiation was continued until most of CpzMoz(CO)4was consumed (43 h) as judged by TLC. Then the volatiles were removed under reduced pressure, and the resulting residue was subjected to column chromatography eluted by CHzClz-hexanes. The following bands were eluted in the order: (1)O(SiMe2Ph)z;(2) a mixture of Cp~Moz(C0)~ and CpzMoz(C0)6;(3) 2a (green brown band, black purple crystals; 770 mg, 1.16 mmol, 26% yield based on CO). 2a was recrystallized from CHzClz-hexanes. Although formation of 3a was evident as detected by TLC, it could not be isolated from the reaction mixture (see text). An authentic sample of 3a was prepared according to the reported method.l0 2a: NMR data (see Table 1); IR v ( C ~ 0 (MBr) ) 1974, 1899, 1834; (CHzClZ) 1989, 1939, 1927, 1871, 1852, 1810; (CHC13) 1994, 1939, 1931, 1876, 1866; (CH3CN) 1983, 1908, 1847; (C6H6) 1992,1938, 1928,1873 cm-I; FDMS: m/z 670 (M+ for the "M03 isotopomer). Anal. Calcd for C22H1606M03: C, 39.78; H, 2.43. Found: C, 39.76; H, 2.33. Labeling experiments were carried out using appropriate isotopomers. Deoxygenative Reduction of Cp'&loz(CO)4 Giving 2b and 3b. A benzene solution (30 mL) of l b (1.92 g, 4.15 mmol) and HSiMezPh (2.3 mL, 15 mmol) was irradiated for 48 h. Separation as described above gave 2b (green brown band, black purple crystals; 319 mg, 0.45 mmol, 18%yield based on CO) and 3b (yellow green band, purple brown crystals; 37 mg, 0.55 mmol, 2% yield based on CO). 2b and 3b were recrystallized from CHzClz-hexanes. 2b: 'H-NMR (tol-&: see Figure 9) 6 12.38 (AI),12.10 (MI), 11.84 (AIII),11.29 (BI), 11.21 (BII), 11.15 (BIII) (p3-CH),4.5-4.8 (v5-Ca4Me),1.84, 1.83, 1.71, 1.68 (v5-C~H4Me); I3C-NMR (tol-d8)6 269.9 ('Jc-H= 154 (34) Curtis, M. D.; Hay, M. S. Inorg. Synth. 1990,28, 150.
Hz; AIII), 267.1 ('Jc-H = 154 Hz; AII), 249.7 ('Jc-H = 161 Hz; BII) @3-CH), 4.5-4.8 (q5-C&Me), 232.2, 232.0, 231.8, 226.6 (CO), 108-90 (v5-C5H4Me),14.1, 13.6 (v5-CsH&fe);IR v(C=O) (Kl3r) 1975, 1909, 1846; (CHzClz) 1986, 1929, 1918, 1868, 1844; (CHC13) 1990 (br), 1926 (br), 1859; (C6H6) 1991, 1933, 1924, 1868, 1846 cm-l. Anal. Calcd for C25HzzO&03: C, 42.51; H, 3.13. Found: C, 42.30; H, 2.93. 3b: 'H-NMR (tol-&: see Figure 9) 6 4.5-4.8 (v5-Ca4Me), 3.82 (CH3; B), 3.79 (CH3; D), 1.89, 1.87, 1.86 (v5-C5H&fe);I3C-NMR (tol-ds) 6 269.1 ('Jc-c = 32 Hz; p&CH3), 232.6 (CO), 108-90 (v5-C5H4Me), 54.0 ('Jc-H = 127 Hz; p3-CCH3; D), 53.8 ('Jc-H = 127 Hz; p3-CCH3; B), 13.5 ('Jc-H = 127 Hz; v5-CsH&fe);IR v(CE0) ( D r ) 1982, 1942, 1914, 1864, 1841, 1783 cm-l. Anal. Calcd for C2&&&03: C, 43.35; H, 3.36. Found: C, 43.08; H, 3.22. Single Crystal X-ray Crystallography. 2aCH3CN, 2a.C6D6,2b, and 3b were recrystallized from benzene-MeCN, benzene-& from an NMR sample), hexanes-CHzClz, and THF-hexanes, respectively. Suitable crystals were mounted on glass fibers. Diffraction measurements were made on Rigaku AFC-5R (2a.CH&N, 2a.CsD6, and 3b) and AFC5 (2b) automated four-circle difiactometers by using graphite-monochromated Mo Ka radiation (1 = 0.71058 A). Unit cell was determined and refined by a least-squares method using 20 independent reflections. Data were collected with 0-28 scan technique. If dZ)/Z was more than 0.1, a scan was repeated up to three times and the results were added to the first scan. Three standard reflections were monitored at every 150 (AFC5R)and 100 (AFC5) measurements. All data processing was performed on and Micro Vax I1 (AFC5R data collection), FACOM A-70 (AFC5data collection) and IRIS Indigo computers (structure analysis) by using the teXsan structure solving program system obtained from the Rigaku Corp., Tokyo, Japan. Neutral scattering factors were obtained from the standard source.35 In the reduction of data, Lorentz and polarization corrections were made. An empirical absorption corrections (Y scan) was made for 2a*CH&N, 2a.CsD6, and 3b. In the case of 2b, an orthorhombic crystal system was established at first, and the diffraction data were collected under this condition. The refinement was converged to R = 0.066 (R, = 0.0541, and examination of the positional parameters suggested a crystal system of higher symmetry. Then the structure was refined successfully according to a cubic system (Pa.3). All the structures were solved by a combination of the direct method and Fourier synthesis (SAP191and DIRDIF). All the non-hydrogen atoms except for C13 of 3b were refined anisotropically. The positions of H(1) of 2a.CH3CN and 2b were confirmed by using isotropic thermal parameters and the remaining Cp (Cp') and solvent H atoms were fixed a t the calculated positions (C-H: 0.95 A) and not refined. Although we also attempted structure determination of 3a, the refinement was unsuccessful because of the low quality of the crystals. However, the conformation of the CpMo(C0)z moieties similar to those in 3b was confirmed by Fourier synthesis (Figure 4).
Acknowledgment. Financial support from the Ministry of Education, Science, Sports and Culture of the Japanese Government (Grants-in-Aids for Scientific Research on Priority Area, Nos. 04241105 and 05236103) is gratefully acknowledged. We wish to thank Drs. Yoshiyuki Nakamura and Masato Oshima for NMR measurements. Supporting Information Available: Tables of positional and thermal parameters and bond lengths and angles for 2a*CH3CN, 2a-CeD6, 2b, and 3b and figures showing the numbering schemes for 2aCH3CN and 2a.C6D6 (19 pages). Ordering information is given on any current masthead page.
OM9504664 ( 3 5 )International Tables for X-Ray Crystallography; Kynoch Press: Birmingham, 1975; Vol. 4.
