Organometallics 1995, 14,4043-4045
4043
Zircona[ l]metallocyclophanes: Synthesis, Properties, and Structure of ( t B u - ; r ~ ~ - C ~ H ~ ) ~ Z r ( ; r ~ ~ - l , ~ ~ - l ’ ) ( ; r ~ ~ - c ~ and Its Chromium Analog1? Christoph Elschenbroich,* Eckhardt Schmidt, Bernhard Metz, and Klaus Harms Fachbereich Chemie der Philipps-Universitat, 0-35032 Marburg, Germany Received March 30, 1995@ Summary: The tilt caused by the one-atom interannular bridge in a zirconocene unit in 7 imposed at his($benzene)vanadium (3’) is very small. As judged from EPR spectroscopy, the electronic structure of 3’ is virtually unaffected by zircona[llvanadocyclophane formation. The novel binuclear complexes are very robust thermally but quite labile chemically. The latter property precludes a thorough electrochemical study of metal-metal interaction in 7’. Organometallic chemistry abounds with structures featuring bent metallocene units.2 In the class of bis(@-arene)metal complexes, however, precedent for bent sandwich units is much more limited. Typically, compounds of the composition (q6-C&)2ML, (L = halogen, ~ diverH, alkyl, CO, phosphane, etc.) are ~ c a r c e .This gence is even more pronounced in the class of sandwich complexes without ancillary ligands: numerous hetera[llmetallocenophanes containing Si? Ge,5 Zr,6 P,5,7or As7 as interannulary bridging atoms are parallelled by just two reports dealing with the homo[3.3lmetallocyclophane (118and the hetera[llmetallocyclophane (2),9 respectively.
q 1
2
The example to be presented in this communication features CpzZrc as a bridging unit at bidbenzene)vanadium and -chromium. This choice derives from the following reasons: (1)The small but well defined tilting + Dedicated to Professor Henri Brunner on the occasion of his 60th birthday. @Abstractpublished in Advance ACS Abstracts, Junex 15,1995. (1)Metal z Complexes of Benzene Derivatives. 47.Part 4 6 Elschenbroich, Ch.; Isenburg, T.; Behrendt, A. Inorg. Chem. 1996,34, 0000. (2)Lauher, W. J.; Hoffmann, R. J . Am. Chem. SOC. 1976,98,1729. (3)(a) Cloke, F. G. N.; Green, M. L. H.; Morris, G. E. J.Chem. SOC., Chem. Commun. 1978,72.(b) Cloke, F. N. G.; Green, M. L. H. J . Chem. SOC.,Chem. Commun. 1979, 127. (c) Green, M. L. H.; O’Hare, D.; Watkin, J. D. J . Chem. SOC.,Chem. Commun. 1989,698.(d) Cloke, F. G. N.; Cox, P. K.; Green, M. L. H.; Bashkin, J.; Prout, K. J . Chem. Soc., Chem. Commun. 1981,117. (4)Stoeckli-Evans, H.; Osborne, A. G.; Whiteley, R. H. Helu. Chim. Acta 1976.59. 2402. (5)Stoeckli:Evans, H.; Osborn, A. G.; Whiteley, R. H. J . Organomet. Chem. 1980,193,345;1980,194,91. (6)Broussier, R.; Da Rold, A.; Gautheron, B.; Dromzee, Y.; Jeannin, Y.Inorg. Chem. 1990,29, 1817. Broussier, R.; Da Rold, A,; Kubicki, M.; Gautheron, B. Bull. Soc. Chim. Fr. 1994,131,951. (7)Seyferth, D.; Withers, H. P., Jr. organometallics 1982,1, 1275. (8) Elschenbroich, Ch.; Schneider, J.; Prinzbach, H.; Fessner, W.D. Organometallics 1986,5,2091. (9)Elschenbroich, Ch.; Hurley, J.; Metz, B.; Massa, W.; Baum, G. Organometallics 1990,9,889.
angle t o be expected from the large Zr atom serving as an interannular link is of interest with regard to the response of metal-ligand spin delocalization t o structural distortion. (2) The reduction Zrrv Zr”’ in the target molecule 7 could possibly lead to an organometallic biradical composed of two paramagnetic sandwich units a t close proximity and in an orthogonal disposition. Obviously, the extent of spin-spin interaction would be the significant question here. Reaction of the zirconocene dichlorides 5 and 6 with lithiated bis(@benzene)vanadium (3) or -chromium (4) affords zircona[llmetallocyclophanes as highly airsensitive black (7, 9) or green (8) crystalslO which are virtually insoluble in hydrocarbons and sparingly soluble in diethyl ether (Scheme 1). The compounds 7-9 are surprisingly stable against thermal degradation. Whereas diphenylzirconocene (10) in the solid state decomposes at 140 O C , l l exchanges its phenyl substituents for aryl groups from the solvent a t 70 “C, and upon photolysis at ambient temperature eliminates biphenyl,12 the complexes 7-9 are thermally very robust: 7 may be sublimed at 240 “C/10-3 mbar, and
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(10)Procedures are as follows. [(rlG-CsH5)~V]Zr(tBu-rl5-C5H~)2 (7):To 1.14g (5.5mmol) of bis(y6-benzene)vanadium(31,dissolved in 150 mL of cyclohexane, are added 8.25 mol of an 1.6 M solution of nbutyllithium (13.2 mmol) in hexane and 2.0 mL (13.28mmol) of Nfl,”,N”’-tetramethylethylenediamine. After being refluxed under NZ for 1.5 h, the mixture is cooled to room temperature and decanted, and the residue is washed with petroleum ether and suspended in 120 mL of diethyl ether. To this is added at -40 “C during 1 h under vigorous stirring a solution of 2.2 g (2.5mmol) of (tBu-v5-C5H&ZrClz (6) in 700 mL of diethyl ether. After 2 h of stirring at room temperature and filtration, the solid product is extracted with boiling toluene ( ~ 2 5 0 mL) until the filtrate is colorless. From this solution at -25 “C 7 is obtained as black sparingly soluble crystals. Yield: 410 mg (0.76mmol, 14%). The product may be sublimed at 240 0C/10-3 mbar. Anal. Calcd for C30H36VZr (538.78):C, 66.88;H, 6.73.Found C, 66.54;H, 6.74. MS (EI, 70 eV): m l z 537 (M+, loo%), 51 (V+, 34).EPR data: see text. [(r16-CsH5)2Cr]Zr(tBu-~6-C~H4)z (8)was prepared from bis(@benzene)chromium (4)(2g, 0.7 mmol), n-butyllithium (23.2mmol), N,N,”,”tetramethylethylenediamine (3.44 mL, 23.2 mmol), and (tBu-$C5H4)2ZrC12(6)(3.9g, 9.7mmol) in 1.3 L of diethyl ether following the directions given for 7.8 precipitates at -25 “C as a green microcrystalline material. Yield 650 mg (1.2mmol, 12%). Anal. Calcd for CrZr (539.84):C, 66.75;H, 6.72.Found: C, 66.79;H, 6.65.MS (EI, 70 eV): m l z 538 (M+, loo%), 52 (Cr+,63). ‘H NMR (500MHz, C&): 6 1.32(~,9H;tBu),3.70(t,J=5.70Hz,4H,metaAr),4.69(d,J=5.30 Hz, 4 H, ortho Ar),4.81 (t, J = 5.75,2 H, para Ar), 5.48 (t, J = 2.65 Hz, 4 H, ortho Cp), 5.98(t, J = 2.65 Hz, 4 H, meta Cp). W{’H}NMR (125MHz, biphenyl-dlz, 120 “C): 6 30.7 (ipso, tBu), 34.1 (CH3, tBu), 75.5(meta,Ar), 78.2(para, Ar), 93.3(ortho, Ar) 106.3(meta, Cp), 107.3 (ortho,Ar),139.5(ipso, Cp), 174.4(ipso, Ar). [(rl6-CsH5)zCr1Zr(v5-CgHg)2 (9)was synthesized from 3.54 g (17 mmol) of 4, lithiation, and subsequent reaction with (q5-C5H&ZrC15(5) (5g, 17 mmol) according to the procedure for 8 as a black, amorphous material which is highly insoluble in all common (and uncommon) solvents. Yield 840 mg (11%). Anal. Calcd for CzzHzoCrZr (427.66):C 61.79;H, 4.71.Found: C, 61.48;H, 5.13.MS (EI, 70 eV): m l z 426 (M+, 100%), 348 (M’ C&, 30.6),296 (M+ - CsH&r, 93.3).220 (M+ - (Cs&)zCr, 84.0),52 ( C P , 77.9).NMR data are unavailable for lack of solubility. (11)Samuel, E.; Rausch, M. D. J . A m . Chem. SOC.1973,95,6263. (12)(a)Erker, G. J . Organomet. Chem. 1977,134,189.(b) Cardin, D. J.; Lappert, M. F.; Raston, C. L. Chemistry of Organo-Zirconium and -Hafnium Compounds; Ellis Horwood: Chichester, U.K., 1986; Chapter 9.
0276-7333/95/2314-4043$09.00/0 0 1995 American Chemical Society
Communications
4044 Organometallics, Vol. 14, No. 9, 1995 Scheme 1
3 Liz
4 Li2
M
=V
5
R=H
7
M = V
Cr
6
r-Bu
8
Cr
R-r-Bu t-Bu
Cr H the mass spectra exhibit the M+ peak at 100%intensity. Furthermore, the lack of solubility required 13C NMR spectra of 8 to be recorded a t 120 "C, biphenyl-dlo serving as a solvent; after 3 h of signal accumulation at this temperature about 25% only of 8 had been cleaved as inferred from the parent bidbenzenekhromium. Since the degree of tilting imposed by the CpzZr< bridge is the aspect of principal interest here, compound 7 was subjected t o X-ray diffraction analysis;13 a plot of the structure is presented in Figure 1,where important bond distances and bond angles are also given. Structural details of the zirconocene unit in 7 may be assessed by comparing them with the published data for dichlorobis(tert-butyl-y5-cyclopentadienyl)zirconium(6)14and (1,l'-ferrocenediyl)di-krt-butylzirconocene11.6Our prime interest is, however, focused on the bis(@-arene)vanadium moiety. Concording with the larger atomic radius of Zr, the distortion in 7 is less severe than in the case of the PhzSi< bridged complex 2. Thus, the angle centroid-V-centroid of 176.3" signals very small bending of the sandwich axis. The deformation manifests itself more clearly if the y6-areneligands are considered; they are folded along the axes C(2)-C(6) and C(2')C(6'), respectively (5.1"),and the two planes C(2-6) and C(2'-6') exhibit a dihedral angle of 8". The deviation of the y6-arene ligand plane and the C(ipso)-Zr bond from coplanarity is considerable (56.6"). The tilting of the two arenes causes the interannular distances of corresponding pairs of carbon atoms to differ: C(1> C(1')= 3.076, C(2).*C(2')= 3.295, C(3).**C(3')= 3.465, and C(4) **C(4')= 3.507 A. This gradation is reflected in the considerable spread of the chemical shifts in the NMR spectra of the chromium complexes 8 and 9.1° A spectroscopic parameter which is very sensitive to bending of the sandwich structure is hyperfine coupling 9
(13) Crystal data for 7: C ~ O H ~ ~MV, Z =~538.75, , monoclinic, space group C2/c, a = 16.404(1) A, b = 19.308(1) A,c = 7 . 5 3 0 ~A, ) p = 98.00(I)', V = 2361.8(4)As, 2 = 4, D, = 1.515 g cm-3, p = 7.032 mm-l, and F(000) = 1116. Data were collected on an Enraf-Nonius CAD4 diffractometer at 193(2 K using graphite-monochromated Cu Ka radiation (1= 1.541 78 3737 measured reflections, 3.5" 5 0 5 60°, h(-18/18), k(-21/21j, l(0/18), w/20 scans, and 3 intensity control reflections every 1 h. After the Lp correction and merging of equivalent reflections there were 1751 unique reflections (R,,t = 0.11). The structure was solved by direct methods,21the non-hydrogen atoms were refined2*anisotropically on F with all unique data, and the hydrogen atoms were located and refined with common isotropic temperature factors for different groups. The extinction coefficientz1was 0.00005(8), and the parameters for the weighting scheme were 0.0755 and 0.21The refinement of 178 parameters converged to wR2 = 0.1255 for all reflections (R1 = 0.050 for 1439 reflections with I > 2dI)). The data were corrected with DIFAF3SZ3 (141 Howie, R. A.; McQuillan, G. P.; Thompson, D. W.; Lock, G. A. J. Organomet. Chem. 1986, 303, 213. (15) In the presence of ancillary ligands paramagnetic bent metallocenes in rigid solution often display three components of the g tensor, for example, (CH3-q5-C5H&VC12: Peterson, J. L.;Dahl, L. F. J.Am. Chem. SOC.1975,97,6422.
A),
Figure 1. Molecular structure of 7 with atomic labeling scheme. Selected bond lengths (A) and angles (deg): Zr(l)-C(l) = 2.317(5), V(l)-C(l) = 2.162(5),V(l)-C(2) = 2.179(5),V(l)-C(3) = 2.212(5),V(l)-C(4) = 2.230(5),V(1)C(5)= 2.217(5),V(l)-C(6) = 2.174(5),Zr(l)-C(7) = 2.546(51, Zr(l)-C(8) = 2.518(5), Zr(l)-C(9) = 2.565(5), Zr(1)C(10) = 2.645(5), Zr(l)-C(11) = 2.625(5), C(l)-C(2) = 1.437(8),C(2)-C(3) = 1.418(8),C(3)-C(4) = 1.404(8),C(4)C(5) 1.405(8),C(5)-C(6) = 1.422(8),C(l)-C(6) = 1.429(71, C(l)-Zr(I)-C(l') = 83.1, C5(centroid)-Zr(l)-C5(centroid) = 123.1(2),C(1)Zr(1)C(l')-C5(centroid)Zr(l)C~'(centroid) = 98.9(2).For additional parameters, see text. present in the EPR spectra of bis(y6-arene)metal(d5) complexes, ligand as well as central metal magnetic nuclei being affected. In the case of the radical cation 1+*(tilting angle = 18"),compared to undistorted (y6CsH&Cr+', a reduction of the coupling constant a(53Cr) and inequivalence of the ortho- and meta-protons was observed.8 For the neutral radical 2'(tilting angle 14.4"), where proton hyperfine structure is only barely resolved, the reduction of a(51V)from 6.35 mT [(y6C6H6)2v, 3'1 to 5.63 mT (2') is character is ti^.^ The EPR spectrum of 7'in fluid and rigid solution is presented V) in Figure 2. The parameters (g), gll, gl, and u ( ~ ~ for 7'differ only marginally from those of the undistorted reference molecule 3 0 . ~There is, however, a strong dependence of line width on the nuclear spin quantum number m ~ ( ~ l V This ) . effect, caused by restricted molecular tumbling motion, proves that in fact the spectrum of the bulky radical 7' rather than that of a dezirconation product is recorded. The almost identical hyperfine couplings u ( ~ ~ for V )3' and 7'accord with the very small tilt present in 7'. They also strongly suggest that bending distortion with the attendant changes in metal ligand overlap rather than an electronic effect caused by peripheral substitution is responsible for the ligand spin delocalization. modulation of metal Bending deformation of a sandwich complex lowers its symmetry from axial to orthorhombic. Therefore, one would expect the rigid solution EPR spectrum to exhibit three values g,, g,, and g,. However, as yet, resolution of gl into the components g, and g, has not been achieved for a tilted sandwich structure void of ancillary ligands. This also applies to 7'. Interestingly, three g
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Communications
Organometallics, Vol. 14, No. 9, 1995 4045 FS
J.
zi
/v
n"
10 mT
Figure 2. EPR spectra (X-band) of complex 7'in toluene (FS = Fremy salt,g = 2.0059, a(14N)= 1.309mT): (a)Fluid solution, 300 K, (g) = 1.9863, C Z ( ~ ~=V6.23 ) mT; (b) rigid solution, 130 K, gll= 2.022, gl = 1.969,A I I ( ~ ~=V0,)A L ( ~ ~ V ) = 9.34 mT. values could be extracted from the spectra of half-open and open ferrocenium ion@ and of (q6-heteroarene)zM(d5) complexes" possessing a linear axis centroidmetal-centroid. Thus for bis(q6-arene)metal(d5)complexes bending distortions have a pronounced influence on the hyperfine coupling pattern whereas electronic perturbations like the insertion of a heteroatom into the n-perimeter or even the elimination of a ring atom to form the "open" congeners strongly affect the g tensor. Since X-ray diffraction failed to disclose significant sandwich slippage18 in the bis(q6-arene)vanadium moiety of 7, we set out to generate the biradical anion 7-" in order to search for exchange-coupling J which would point t o V(d5). *Zr(d') interaction. To this end, cyclovoltammetry was first performed on 7'l9 (Figure 3). In the potential range -3.0 < E < 0 V four reversible waves are observed, two of which change in intensity with time: whereas, initially, the peak current of wave 3 exceeds that of wave 4,the former diminishes and has disappeared after 6 h. Wave 1 is caused by the couple (lGjElschenbroich, Ch.; Bilger, E., Ernst, R. D.; Wilson, D. R.; Kralik, M. S. Organometallics 1985, 4 , 2068. (17)(a) Elschenbroich, Ch.; Nowotny, M.; Metz, B.; Massa, W.; Graulich, J. Angew. Chem., Znt. Ed. Engl. 1991,30, 547. (b) Elschenbroich, Ch.; Bar, F.; Bilger, E.; Mahrwald, D.; Nowotny, M.; Metz, B. Organometallics 1993,12, 3373. ( c ) Nowotny, M.; Elschenbroich, Ch.; Behrendt, A,; Massa, W.; Wocadlo, S.; 2.Naturforsch. 1993,48b, 1581. (18)Sandwich slippage is defined as the distance between ring centroid and the perpendicular projection of the central metal atom on the ring plane. (19)Lappert, M. F.; Pickett, C. J.;Riley, P. J.; Yarrow, P. J. W. J . Chem. SOC.,Dalton Trans. 1981, 805. (20) Elschenbroich, Ch.; Bilger, E.; Metz, B. Organometallics 1991, 10, 2823. (21) SHELXS-86: Sheldrick, G. M. Univ. of Gottingen, 1986. (22) SHELXS-93: Sheldrick, G. M. Univ. of Gottingen, 1993. (23) Walker, N.; Stuart, D. Acta Crystallogr. 1983, A39, 158.
V
r
-3
rV
I
-1
-1
0
1
Figure 3. Cyclovoltammetryof complex 7 in DMEIn-Bu4NC104 (0.1 M) (-45 "C, at glassy carbon versus SCE, 200 mV s-l): (a) -3 < E < +0.5 V; (b) expanded scale, limited range -1 E < 0 V, immediately after sample preparation; (c) -1 < E < 0 V, after 6 h at -45 "C. Key: (1)E,, = -2.73 V; (2)El12= -1.952 V, AE, = 92 mV; (3): El12 = -0.573 V, AE, = 54 mV, (4) Ell2 = -0.368 V, AE, = 64 mV. V0'-' of the bis($-arene)vanadium unit,20 wave 2 corresponds to the couple ZrrviII1 in (tBu-q5-C5H4)2(q1C6HdzZr (12) as proved by a n independent measurement, and wave 4 is identical with that of the couple 2+'0.20 Since 12 does not feature a redox process a t potentials E > -1.8 V, wave 3 must represent the couple 7+'O, oxidation occurring at the his($-arenelvanadium moiety. The observation that this wave eventually vanishes t o be replaced by wave 4 suggests cleavage of the single atom interannular bridge in 7. Solvolytic lability of hetera[llmetallocyclophanes has been noted p r e v i o ~ s l y . In ~ the present case it precludes electrochemical generation of the biradical7-" for EPR study.
Acknowledgment. The authors thank the Deutsche Forschungsgemeinschafb and the Fonds der Chemischen Industrie for support of this work. E.S. is indebted to the "Graduiertenkolleg Metallorganische Chemie" for the award of a scholarship. Supporting Information Available: Additional crystallographic data for 7 including tables of atomic coordinates and equivalent isotropic displacement parameters (A2) (Table Sl), bond lengths and angles (Table S2), torsion angles (Table S3), anisotropic displacement parameters (TableS4), and hydrogen (Table coordinates and isotropic displacement parameters (A2) S5) (11pages). Ordering information is given on any current masthead page. OM9502345
