Organometallics 1995, 14, 4957-4959
4957
A New, Convenient Synthetic Route to Tetramethylethano-Bridged Chromocene Complexes David Ming Jin Foo and Pamela Shapiro" Department of Chemistry, University of Idaho, Moscow, Idaho 83844-2343 Received June 26, 1995@ Summary: The reaction of (CHd4Cdq5-CfldzCa with CrCl2 in the presence of 1 equiv of tert-butyl isocyanide afforcls (CHj34C6r15-Cfld~Cr(C~NtBu), offering a new and convenient synthetic entry into this ansa-chromocene system. The isocyanide complex undergoes ligand substitution with CO to form the previously reported (CHj3&6r/5-CfldzCrCO, which may also be prepared by reacting (CHj34Cdq5-CfldzCa with CrClz in the presence of a n atmosphere of carbon monoxide.
of the ~ o m p l e x . ~Encouraged by these results, we wished to explore the synthesis of additional derivatives of the tetramethylethanyl-bridged chromocene fragment. However, we sought a more facile synthetic entry into this system which would afford us with an adequate supply of starting material for pursuing further chemistry.
Introduction
In our hands, the calcium metallocene (CH3)4C2[q5C5&12Ca5 recently reported by Edelmann and coworkers is a more convenient and reliable reagent for the synthesis of the corresponding ansa-chromocene system than the di-Grignard (CH3)4Cz[y5-C5&12MgzClpTHF6 originally used by Brintzinger and co-workers. We find the calcium reagent to be more convenient since either commercially available CrClz or material prepared by reducing CrC13(THF)3 in situ with zinc dust may be used with this reagent for the preparation of compounds 1 and 2, whereas Brintzinger's method for preparing 2 calls for the use of CrC12(THF)2. We find the calcium metallocene to be more reliable since we are able t o prepare it more consistently than the di-Grignard and our yields of compounds 1 and 2 have been consistently 30% and 40%, respectively, using this reagent. Our yields of 2 using the Brintzinger method were never better than 25% and were usually much worse. Preparation of compound 1 is accomplished by simply combining the calcium metallocene with CrClz in tetrahydrofuran in the presence of an equivalent amount of tert-butyl isocyanide (eq 1). Part of the loss in our
Since the discovery of ferrocene in the early 1950's, metallocene complexes have played a major role in the development of organotransition-metal chemistry. Bentsandwich metallocene derivatives of group 3-6 metals have received the greatest amount of attention since a number of fundamentally interesting as well as synthetically useful reactions occur within the "equatorial wedge" of these electronically unsaturated bent-metallocene fragments. Conspicuously absent from this list, however, is the chemistry of bent-sandwich chromocene derivatives. Despite its electronic unsaturation, chromocene prefers a parallel ring geometry akin to that of ferrocene. It is resistant toward the coordination of ligands, such as carbon monoxide,' and readily undergoes ring loss upon incorporating additional ligands.2 The strategy we have chosen to gain entry into the chemistry of bent-sandwich chromocene complexes is to constrain the geometry about the metal to be bent by linking the rings together with a short, heteroannular bridge. In addition to enforcing a bent geometry in the chromocene complexes, due t o a chelation effect, the heteroannular bridge is expected to reduce the tendency of these complexes t o undergo ring loss. The effectiveness of using a heteroannular bridge to stabilize bent-sandwich chromocene complexes has already been demonstrated in the successful preparation of (CH3)4C2[q5-C5H412CrC0by Brintzinger and co-worke r ~ In . ~ contrast t o the parent chromocene carbonyl complex, the carbonyl adduct of this ansa-chromocene complex is quite stable. In fact, the carbon monoxide remains coordinated even upon one electron oxidation Abstract published in Advance ACS Abstracts, September 15,1995. (1) (a) Wong, K. L. T.; Brintzinger, H. H. J.Am. Chem. Soc. 1975, 97 (18),5143. (b) Brintzinger, H.H.; Lohr, L. L.; Wong, K. L. T. J. Am. Chem. Soc. 1976,97,5146.(c)Simpson, K. M.; Rettig, M. F.; Wing, R. M. Organometallics 1992,11, 4363. (2)(a) Sneeden, R. P. A. Organochromium Compounds; Academic Press: New York, 1975;p 171.(b) Davis, R.;Kane-Maguire, L. A. P. In Comprehensive Organometallic Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon: Oxford, U.K, 1982;Vol. 3,Chapter 26.2.(c) Chisholm, M. H.; Gallagher, T. D. Syn. React. Znorg. MetalOrg. Chem. 1977,7(3),279.(d) Cotton, F.A.; Rice, G. W. Znorg. Chim. Acta 1978, 27, 75. (e) Kalousova, J.; Votinsky, J.; Klikorka, J.; Nbdvornik, M. J. Organomet. Chem. 1980,184,351. (3)Schwemlein, H.; Zsolnai, L.; Huttner, G.; Brintzinger, H. H. J. Organomet. Chem. 1983,256,285. @
Results and Discussion
Cr-C=NBu
1
product yield may be attributed to the formation of an additional chromium species due to a persistent impurity in our preparations of (CH3)4C2[y5-C5H412Ca. The impurity has been identified by lH NMR to be an isopropylcyclopentadienylcalcium species which probably arises from a proton abstraction by the dimethyl(4)van Raaij, E. U.; Monkeberg, S.; Kiesele, H.; Brintzinger, H. H.
J. Organomet. Chem. 1988,356,307.
(5) Rieckhoff, M.; Pieper, U.; Stalke, D.; Edelmann, F. T. Angew. Chem., Znt. Ed. Engl. 1993,32,1079. (6)Schwemlein, H.; Brintzinger, H. H. J. Organomet. Chem. 1983, 254,69.
0276-733319512314-4957$09.00/0 0 1995 American Chemical Society
4958 Organometallics, Vol. 14, No. 10, 1995
fulvenyl radical anion intermediate. Despite the use of rigorously dried solvents and dimethylfulvene, we have not been able t o reduce this impurity below 25% of the total material as determined from the lH NMR integration. Nevertheless, the desired ansa-chromocene isonitrile complex may be isolated cleanly from the crude reaction product by fractional crystallization. The 'H NMR, 13C NMR, and IR data for the compound are consistent with structure shown in eq 1. Although we have been unable t o obtain X-ray-quality crystals of the compound, its molecular weight was determined to be 352 g/mol (theoretical: 347 @mol)by ebulliometric methods, also confirming this structure. This result also indicates that dissociation of the isocyanide ligand from the metal does not occur to any appreciable extent in solution. The only other examples of stable, bent-sandwich chromocene isocyanide complexes reported to date are of the half-open chromocenes (CdW(C5H7)Crand (C~HE.)(C~HII)C~.~ Compound 2 is prepared by reacting (CH&Cz[y5CsH412Ca with CrClz in the presence of an atmosphere of carbon monoxide. Alternatively, this complex can be formed through substitution of the isocyanide ligand by carbon monoxide. An enhancement of the 13Cresonance of the carbonyl a t 6 183.4was observed when a benzeneds solution of the ansa-chromocene carbonyl compound was placed under an atmosphere of 13C0 in a sealed NMR tube, indicating that carbonyl exchange occurs as well in this system. We suspect that the prearrangement of the ligand in a metallocene geometry about the calcium assists in the transfer of the ligand to a single chromium center as opposed t o two chromiums in these syntheses. Polymer formation, presumably due t o bridging of the ligand between chromium centers, is encountered when these ansa-chromocene syntheses are carried out in the absence of a trapping ligand and appears to be the cause of our low yields of 2 when using the di-Grignard reagent. The higher solubility of the calcium reagent in tetrahydrofuran relative to the di-Grignard probably also contributes to its better performance. Our attempts to prepare the analogous ansa-chromocene methyl isocyanide, trimethylphosphine, and trifluorophosphine complexes have been unsuccessful. The stability of the ansa-chromocene compound appears to be highly sensitive to the nature of the ligand in the equatorial wedge.
Summary In summary, we have discovered a more convenient and reliable synthetic entry into tetramethylethanylbridged metallocene complexes of Cr(I1) involving the use of the corresponding calcium ansa-metallocene as a ligand source. Besides carbon monoxide and tert-butyl isocyanide, we wish t o introduce hydride, alkyl, oxo, imido, and other formally oxidizing ligands into the equatorial wedge of this ansa-chromocene system. The fact that chromium has been able to assume +3 and f 4 oxidation states when sandwiched between cyclopentadienyl-like carborane rings8 suggests the possibility of such higher oxidation state ansa-chromocene derivatives. We are interested in these derivatives as (7) Freeman, J. W.; Hallinan, N. C.; Arif, A. M.; Gedridge, R. W.; Ernst, R. D.; Basolo, F. J.Am. Chem. SOC.1991, 113, 6509. (8)Oki, A. R.; Zhang,H.; Maguire, J. A.; Hosmane, N. S.; Ro, H.; Hatfield, W.E. Organometallics 1991,10, 2996.
Notes
potential catalysts for olefin oxidation and olefin polymerization, among other uses. We also intend to introduce other types of bridges between the cyclopentadienyl rings of the chromocene. For best results, chelated ligand transfer reagents similar t o the calcium metallocene will be employed in these synthetic efforts.
Experimental Section General Considerations. All manipulations were performed using a combination of glovebox, high-vacuum, or Schlenk techniques. All solvents were distilled under nitrogen over sodium benzophenone ketyl (THF) or CaHz (petroleum ether, hexane). The solvents were then stored in line-pots from which they were vacuum transferred from sodium benzophenone ketyl or cannulated directly. Benzene-& was dried over activated 4 A molecular sieves. Argon was purified by passage over oxy tower BASF catalyst (Aldrich) and 4 A molecular sieves. tert-Butyl isocyanide, CaClZ, and zinc dust were used as received from Aldrich. (CH3)4C2[q5-CsH412Ca6and CrCl3(THF)s9were prepared as described in the literature. NMR spectra were recorded on an IBM NR-300 (300.13MHz 'H, 74.43 MHz 13C) and an IBM NR-200 (200.13 MHz lH, 50.327 MHz 13C). All chemical shifts are reported in ppm and referenced to solvent. IR spectra were obtained on a PerkinElmer 1310 spectrophotometer. Elemental analyses were determined by Desert Analytics. tCHs)4C2[tlS-C5H412CrtC~N~Bu) (1). Method a. Tetrahydrofuran (70 mL) and tert-butyl isocyanide (0.33 mL, 2.9 mmol) were added to a flask containing (CH3)&[v5-C5H412Ca (0.72 g, 2.8 mmol) and CrClz (0.35 g, 2.8 mmol) and cooled at -78 "C. The reaction mixture was allowed to warm gradually to room temperature, and the resulting burgundy red solution was stirred for 12 h. The volatiles were removed in uacuo, and the residue was taken up in ca. 30 mL of petroleum and filtered to removed the CaClZ, which was washed repeatedly with petroleum ether t o extract the soluble reaction product. Concentrating and cooling the filtrate at -78 "C afforded the desired product as a red-brown solid (yield: 0.31 g, 30%). Method b. Tetrahydrofuran (75 mL) was added t o a flask containing the combined solids of CrC13(THF)3 (0.51 g, 1.4 mmol) and zinc dust (0.10 g, 1.5 mmol) and cooled at -78 "C. The reaction mixture was allowed t o warm slowly over 4 h to room temperature, producing a light blue slurry. The slurry was cannulated into a second flask cooled at -78 "C and containing (CH3)4Cz[q5-C&12Ca (0.35 g, 1.4 mmol). After tertbutyl isocyanide (0.20 mL, 1.8mmol) was admitted to the flask, the reaction mixture was allowed to warm slowly with constant stirring to room temperature. The reaction was stirred for another 12 h at room temperature. The resulting burgundy red solution was then dried under vacuum. The reaction residue was taken up in ca. 30 mL of petroleum and filtered to removed the CaClz and excess zinc dust, which were washed repeatedly with petroleum ether to extract the soluble reaction product. Concentration of the filtrate and cooling to -78 "C afforded 0.12 g of 1(yield: 25%). lH NMR (C&): 6 4.61,3.95 (m, 8H, CSHd, 1.21 (s, 12H, CZ(cH3)4), 1.01 (s, 9H, {CH3)3CNC). 13C NMR (CsDs): 6 206.8 ({CH3}3CNC), 79.9, 75.2 (C5H41, 31.2 (Cz(CH3)4), 27.4({CH3}3CNC). IR (KBr plates, Nujol mull, cm-'1: 1835 (v(C=N)). Anal. Calcd for CZ1HzsCrN: C, 72.59; H, 8.41; N, 4.03. Found: C, 72.61; H, 8.55; N, 3.90. (CHs)4C2[t15-C5H412Cr(C~NtBu) (2). Method a. An atmosphere of carbon monoxide was admitted to a flask containing (CH3)4C2[q5-C5H412Ca (0.50 g, 2.0 mmol) and CrClz (0.24 g, 2.0 mmol) in 50 mL of tetrahydrofuran cooled at -78 "C. The reaction mixture turned from green to red as it was allowed to warm to room temperature over 4 h, and the reaction was stirred at room temperature overnight. The (9)Collman, J. P.;Kittleman, E. T. Inorg. Synth. 1966, 8, 150.
Notes
Organometallics, Vol. 14, No. 10, 1995 4959
red powder (0.18 g, 45%). The identity of the compound was volatiles were removed in uucuo, and the residue was taken established by comparison of its benzeneds 'H NMR spectrum up in cu. 20 mL of hexane and filtered to removed the CaClz, with that of compound which was prepared as originally which was washed repeatedly with hexane to extract the reported in ref 3. soluble reaction product. Concentrating and cooling the filtrate at -78 "C afforded 2 as a red precipitate (yield: 0.23 g, 40%). The lH and 13C NMR data for the compound were Acknowledgment. Financial support from the Uniconsistent with that reported in ref 3. versity of Idaho and from the donors of the Petroleum Method b. A solution of 1 (0.50g, 1.4 "01) in petroleum Research Fund, administered by the American Chemiether (50mL) was stirred for 12 h under an atmosphere of cal Society, is gratefully acknowledged. carbon monoxide. The solution was concentrated to 20 mL OM9504967 and cooled to -78 "C to afford ( C H ~ ) ~ C Z [ ~ ~ ~ - C as ~ Ha ~ ] Z C ~O
