Organometallics 1995, 14, 4668-4675
4668
Organotransition-MetalMetallacarboranes. 41. Synthesis and Structure of B-B- and Cp*-Cp*-Linked Cobaltacarborane Xiaotai Wang, Michal Sabat, and Russell N. Grimes* Department of Chemistry, University of Virginia, Charlottesville, Virginia 22901 Received May 12, 1995@ In an application of Wurtz-type coupling reactions to metallacarborane chemistry, (X = C1, Br, treatment of the B(5l-halogenated closo complexes Cp*Co(2,3-EtzCzB4H3-5-X) I; Cp* = q5-C5Me5)with sodium metal in THF gave a single red-orange, air-stable product in 20-34% isolated yield. This species was characterized via NMR, W-visible, and mass spectroscopy and X-ray crystallography as a B-B-linked dimer, 5,5’-[Cp*Co(EtzC~B4H3)]~ (2). Reactions of the same halogenated monomers, as well as the parent complex (X = H), with alkyllithium reagents in THF generated in 28-56% yield the orange, air-stable dimeric (3a-d; X = H, C1, Br, I), which are linked via products [(-CH~C~M~~)C~(E~ZC~B~H~-~-X)IZ Cp*-Cp* connections. This geometry, apparently novel to metallacarborane chemistry, was established by an X-ray diffraction study on 3c and supported by spectroscopic data for the four species. Decapitation of these dimers in wet TMEDA gave in high yield the (4a-d; X = H, C1, Br, I) corresponding nido complexes [(-CHZC~M~~)CO(E~ZCZB~H~-~-X)IZ as air-stable yellow solids. In contrast, 2 was unaffected by similar treatment. The reaction of nido-Cp*Co(EtzCzB3&-5-C1) with sodium in THF gave the yellow air-stable dimeric species (8) together with the unsubstituted monomer Cp*Co(EtzCzB3H5), a [Cp*Co(EtzCzB3H4)1~ previously characterized complex. An X-ray crystal structure determination on 8 disclosed that the two CoCzB3 units are connected via a B-B-B three-center bond such that the two CzB3 rings are almost mutually perpendicular.
Introduction Organometallic complexes containing two or more transition-metal centers command widespread interest for a variety of reasons, among which are their potential for effecting cooperative multinuclear catalytic action: their synthetic utility as building blocks in constructing electroactive or magnetoactive polymeric materials: and their value in studies of metal-metal electronic comm u n i ~ a t i o n . ~For - ~ all such purposes, it is essential that the complexes be reasonably robust (preferably airstable) and not fragment into monometallic species in solution, be soluble in common organic solvents, and be synthetically accessible. These conditions are satisfied by most metallacarboranes, and when one also takes into account the enormous variety of such complexes,6 it is clear that polymetal-centered carborane complexes Abstract published in Advance ACS Abstracts, September 1,1995. (1)(a) Part 40: Stockman K.; Garrett, D.; Grimes, R. N. Organometallics 1995,14, 4661. (b) Part 39: Greiwe, P.; Sabat, M.; Grimes, R. N. Organometallics 1995,14,3683. (2)Based in part on the Ph.D. dissertation of X.W., University of Virginia, 1995. Presented in part at the Third Boron U.S.A. Workshop, Washington State University, Pullman, WA, July 1992;Abstract 52. (3)(a)Marks, T.J . Acc. Chem. Res. 1992,25,57.(b) Suss-Fink,G.; Meister, G. Adu. Organomet. Chem. 1993,35, 41 and references @
therein. (4)Recent examples: (a) Foxman, B. M.; Rosenbloom, M.; Sokolov, V.; Khrushchova, N. Organometallics 1993,12,4805.(b)Atzkern, H.; Bergerat, P.; Beruda, H.; Fritz, M.; Hiermeier, J.;Hudeczek, P.; Kahn, 0.; Kohler, F. H.; Paul, M.; Weber, B. J . Am. Chem. SOC.1995, 117, 997. (5)Recent examples: (a) Tilset, M.; Vollhardt, K. P. C.; Boese, R. Organometallics 1994, 13, 3146. (b) Pipal, J. R.; Grimes, R. N. Organometallics 1993,12,4452,4459. (c) Merkert, J.; Davis, J. H., Jr.; Geiger, W.; Grimes, R. N. J . Am. Chem. SOC.1992,114,9846.(d) Chin, T.T.; Lovelace, S. R.; Geiger, W. E.; Davis, C. M.; Grimes, R. N. J . A m . Chem. SOC.1994,116,9359.
offer a particularly fertile area of investigation. Many examples of dimetallic or polymetallic metallacarborane clusters of 6-14 vertices have been reported,6 some of which were obtained by serendipitous means and others by directed syntheses. In the small metallacarborane category, methods for assembling and tailoring CZB3bridged multidecker sandwich complexes via stacking of small building-block units have been developed in our l a b ~ r a t o r y . In ~ species of this class (Chart lA), the metal centers are in close proximity t o each other (typically ca. 3.2 A) and unpaired electrons are often (although not always8) extensively d e l o ~ a l i z e d . ~ ~ ~ ~ ~ ~ ~ Other classes of polymetallic small metallacarboranes consist of linked clusters such as those depicted in Chart lB, in which the metals reside in discrete polyhedral units that are linked via ligand-ligand, multicenter B-B-B, or other bonding modes.g One can also combine the stacking and linked-cluster motifs in the same system, as in the type C specieslO in Chart 1. Although (6)Grimes, R. N. In Comprehensive organometallic Chemistry II; Abel, E., Stone, F. G. A,, Wilkinson, G., Eds.; Pergamon Press: Oxford, England, 1995;Vol. 1, Chapter 9,and references therein. (7) (a) Grimes, R. N. Chem. Reu. 1992,92,251. (b) Piepgrass, K. W.; Meng, X.; Holscher, M.; Sabat, M.; Grimes, R. N. I r w g . Chem. 1992,31,5202. (c) Wang, X.; Sabat, M.; Grimes, R. N. J.Am. Chem. SOC.1994,116,2687.(d) Benvenuto, M.A.; Sabat, M.; Grimes, R. N.
Inorg. Chem. 1992,31,3904. (8)Stephan, M.; Muller, P.; Zenneck, U.; Pritzkow, H.; Siebert, W.; Grimes, R. N. Inorg. Chem. 1995,34,2058. (9) (a) Piepgrass, K. W.; Curtis, M. A.; Wang, X.; Meng, X.; Sabat, M.; Grimes, R. N. Inorg. Chem. 1993,32,2156.(b) Stephan, M.; Davis, J. H., Jr.; Meng, X.; Chase, K. P.; Hauss, J.; Zenneck, U.; Pritzkow, H.; Siebert, W.; Grimes, R. N. J . Am. Chem. SOC.1992,114,5214. (c) Finster, D. C.; Grimes, R. N. Inorg. Chem. 1981,20,863. (10)Meng, X.;Sabat, M.; Grimes, R. N. J . Am. Chem. SOC.1993, 115,6143.
0276-7333l95/2314-4668$09.QQI0 0 1995 American Chemical Society
Organotransition-Metal Metallacarboranes
Organometallics, Vol. 14, No. 10,1995 4669
Chart 1
9 = CsHS. C,MeS, Orenes M = Fe, Co. Rh, M'
-
Ru
Fa, Co. Ni. Ru. Rh,
Os, Ir, P t
Fe I
C = Ni,
we have unexpectedly isolated and characterized several boron-linked dimers in the course of attempted metal stacking reactions (see we sought in the present study to develop systematic routes t o linked cobaltacarborane clusters via Wurtz reactions and similar approaches drawn from organic chemistry.
Results and Discussion Synthesisand Characterizationof Linked CbsoCobaltacarborane Dimers. The halogenated complexes Cp*Co(2,3-Et~CzB4H3-5-X) (lb-d), in which X = C1, Br, I and Cp* = q5-C5Me5,were readily obtained in high-yield, regiospecific reactions via treatment of the parent complexll l a with N-halosuccinimides.l* As shown in Scheme 1, each of these halo derivatives reacted with sodium in THF to afford the linked product 5,5'-[Cp*Co(EtzCzB4H3)12(2) in isolated yields of 20,42, (11)Davis, J. H., Jr.; Sinn, E.; Grimes, R. N. J. Am. Chem. SOC. 1989,111, 4776.
CO
and 34% for X = C1, Br, I, respectively. To our knowledge, these are the first applications of Wurtztype reactions to the directed syntheses of B-B-linked metallaboron cluster dimers. Pretreatment of the sodium with methanol was found to increase both the rate of formation and the yields of the products, probably owing to activation of the sodium surface as well as t o the formation of sodium methoxide which may promote heterolytic cleavage of the B-X bond. Orange air-stable crystals of 2 were isolated following column chromatography on silica in air, and the compound was characterized by 'H and llB NMR, Wvisible, and mass spectroscopy and elemental analysis. High symmetry is apparent in both the lH and llB N M R spectra (Table 1); in the boron spectrum, only two signals in a 3:l area ratio are observed, owing to the superposition of area 1 and area 2 peaks (as is also the case in the unsubstituted monomerll CpCo(EtzCzB4Hd, although the spectrum of Cp*Co(EtCzB4H4)exhibits a resolved 1:2:1pattern1'). Since these data do not con-
Wang et al.
4670 Organometallics, Vol. 14, No. 10, 1995
(B5&)213 (1.660(8)A), and C2BsHii-C2BioH1ii4 (1.681(2) A strong covalent link between the polyhedral units in 2 is thereby indicated, probably involving some transfer of bonding electron density from the two cage frameworks to the intercluster bond as the hydrogens on B(5) and B(5’) are replaced by a B-B interaction. This may account for the very slight deshielding of the groton NMR resonances one observes in 2 relative to the monomer la. A related observation is that 2 appears unreactive toward wet tetramethylethylenediamine (TMEDA), in contrast to la, which readily undergoes decapitation (loss of the apical BH) with this reagent. As part of an exploration of possible alternative routes to cobaltacarborane dimers, the closo complexes la-d were treated with alkyllithium reagents in THF. Other than nido-Cp*Co(Et2C2B3H5)(a“decapitation”byproduct of the reaction of l a (X = H)) the only isolated products, obtained in 28-56% yield on treatment with n-butyllithium or methyllithium, were the dimers 3a-d, whose Cp*-Cp* linkages represent a geometry not previously encountered in metallacarborane chemistry (Scheme 1). The spectroscopic characterization of these orange airstable species was straightforward, and the geometry shown was established via an X-ray diffraction study on 3c. Although the mechanistic details of these reactions are unclear, it is probable that the initial step is deprotonation of the Cp* ligand by the alkyllithium reagent. Such reactions are relatively rare, as the Cp* hydrogens in v5-Cp*metal complexes are usually inert to bases; in contrast, methyl groups on rf-arenes are easily alkylated.15J6 Precedents for Cp* activation are apparently limited to the studies on Cp*IrL, species by Maitlis et al.15and a report by Gloaguen and Astruc on the polyalkylation of Cp*CoCp+by t-BuOK and MeI.16 In our work, we infer that the strongly electronwithdrawing ( E ~ Z C ~ B ~ ligands, H ~ X ) which ~ ~ stabilize high metal oxidation states, enhance the polarity of the Cp* C-H bonds and hence promote their acidity. In contrast, carborane B-H bonds in general are far less polar and are not susceptible to base attack,17 so that the Cp* ligand is the preferred (and very likely the only possible) site for such reactions in la-d. The molecular structure of the B(5),B(5’)-dibromo derivative 3c is presented in Figure 2, with crystallographic information listed in Table 2 and bond distances and angles in Table 4. As in 2, the molecule is centrosymmetric in the solid state, and the C O C ~ B ~ cluster units exhibit normal seven-vertexcloso geometry with typical bond lengths and bond angles. The cobalt lies approximately above the centroid of the C2B3 ring, although slightly closer to the cluster carbons than to the boron atoms. The central C-C (C(lR8)-C(lR8’) distance of 1.546(7)A and the C(lR3)-C(lRS)-C(lRS’)
A).
b
07
C2E
Figure 1. Molecular structure of 5,5‘-[Cp*Co(EtzCzB4H3)]~ (2) with hydrogen atoms omitted.
Scheme 1
l o X Ib X IC X l d X
2
= H = Cl =
Br
= I L & .
co
1
-
BuLi/THF
X
I
H.CI.Br.l
do
TMEDA
1 co
clusively distinguish between the B(5)-B(5’)-linked and B(7)-B(7’) (apically) linked isomers, an X-ray crystal structure determination was conducted on 2. Tables 2 and 3 list the crystallographic parameters and bond distances and angles, respectively. As depicted in Figure 1 , 2 was identified as the B(5)B(5’) isomer with a centrosymmetric transoid conformation in the solid state. The two C2B3 rings are constrained by the inversion center to be mutually coplanar in the crystal, and the Co atoms are nearly centered over their respective carborane rings, the Co-C distances being slightly shorter than the co-B bonds. The intercluster B-B distance in 2 is virtually identical with the neighboring B(5)-B(4) and B(5)-B(6) bonds (all ca. 1.69 A) and is in the range observed in other boroncoupled species, e g . , l,l’-(B5H8)2,12 (1.74(6) &, 1,2‘~~
(12)Gemes, R.;Wang, F. E.; Lewin, R.; Lipscomb, W. N. Pmc. Natl. Acad. Scz. U S A . 1961,47,996.
113)Briguglio, J. J.;Carroll, P. J.; Corcoran, E. J., Jr.; Sneddon, L. G. Inorg. Chem. 1986,25,4618.Single-bond B-B distances for other related structures are also cited in this paper. (14)Subrtova, V.; Linek, A.; Hasek, J. Acta Crystallogr. 1982,B38, 3147. (15)(a) Miguel-Garcia, J. A.; Adams, H.; Bailey, N. A,; Maitlis, P. M. J. Organomet. Chem. 1991,413, 427. (b) Miguel-Garcia, J. A.; Adams, H.; Bailey, N. A,; Maitlis, P. M. J. Chem. SOC.,Dalton Trans. 1992,131. (16)Gloaguen, B.; Astruc, D. J. A m . Chem. SOC.1990,112,4607. (17)For example, deprotonation of the icosahedral (C2BloH12)car-
boranes via organolithium reagents takes place exclusively at the CH groups. See: Grimes, R. N. Carboranes; Academic Press: New York, 1970,and references therein.
Organotransition-Metal Metallacarboranes
Organometallics, Vol. 14, No. 10, 1995 4671
Table 1. llB and 'H FT NMR Data 115.8 MHz llB NMR Data compd
,p,b
12.1 (134),4.3 (162),2.6 (185 18.8,2.7, 1.1(157) 18.8, 2.7 17.3; 1.7 (154), 0.2 (160) 20.2,4.2 2.6, -6.2 23.0, 0.8 18.5,2.9 3.5 8.0,4.7 9.4, -12.0 12.0, -0.6 -7.4, -12.8 10.0, 6.7, 4.5, 1.2 7.5 (146), 3.9 (134)
re1 areas 1:2:1 1:2:1 1:3f 1:2:1 1:3f 1:3f 1:3f 1:3f
f
1:2 1:2 1:2 1:2
f
1:2
300 MHz lH NMR Data compd
lad lbe le Ide 2e
3a 3b 3c 3d 4a 4b 4c 4d 8
9d
&sh
2.51 m (ethyl CH2), 2.28 m (ethyl CH2), 1.77 s (CsMes), 1.19 t (ethyl CH3) 2.54 dq (ethyl CH2), 2.30 dq (ethyl CH2), 1.80 s (CsMes), 1.21 t (ethyl CH3) 2.47 dq (ethyl CH2), 2.29 dq (ethyl CH2), 1.80 s (CsMes), 1.16 t (ethyl CH3) 2.50 dq (ethyl CH2), 2.30 dq (ethyl CH2), 1.77 s (CsMes), 1.16 t (ethyl CH3) 2.57 dq (ethyl CH2), 2.39 dq (ethyl CHz), 1.80 s (CsMed, 1.20 t (ethyl CH3) 2.45 m (ethyl CH2), 2.29 s (CHzCHz), 2.24 m (ethyl CH2), 1.75 s (C5Me5),1.66 s (CsMes), 1.15 t (ethyl CH3) 2.43 m (ethyl CHz), 2.36 s (CH~CHZ), 2.26 m (ethyl CH2), 1.78 s (CsMes), 1.73 s (CsMes), 1.13 t (ethyl CH3) 2.43 m (ethyl CH2), 2.36 s (CH2CH21, 2.27 m (ethyl CHd, 1.77 s (CsMes), 1.73 s (CsMes), 1.13 t (ethyl CH3) 2.28 m (ethyl CH2), 1.76 s (CsMes), 1.73 s (CsMes), 1.13 t (ethyl CH3) 2.46 m (ethyl CH2), 2.33 s (CHZCH~), 2.02 m (ethyl CH2), 1.83 m (ethyl CH2), 1.74 s (CsMes), 1.64 s (CsMes), 1.06 t (ethyl CH3), -6.0 br (BHB) 2.25 s (CH~CHZ), 2.28 s (CHzCHz), 2.06 m (ethyl CH2), 1.80 m (ethyl CH2), 1.74 s (CsMes), 1.65 s (CsMes), 1.05 t (ethyl CH3), -4.0 br (BHB) 2.30 s (CHZCH~), 2.08 m (ethyl CH2), 1.82 m (ethyl CH2), 1.74 s (CsMes), 1.65 s (CsMes), 1.05 t (ethyl CH3), -4.3 br (BHB) 2.11 m (ethyl CH2), 1.84 m (ethyl CH2), 1.73 s (CsMes), 1.65 s (CsMes), 1.06 t (ethyl CH3), -4.6 br (BHB) 2.30 s (CHZCH~), 2.15 m (ethyl CH2), 2.00 m (ethyl CH2), 1.90 m (ethyl CH2), 1.81s (CsMes), 1.64 s (CsMes), 1.24 t (ethyl CHd, 1.16 t (ethyl CH3), 1.01 t (ethyl CH3), 0.87 t (ethyl CH3), -5.1 br (BHB), -6.5 br (BHB) 2.09 m (ethyl CHz), 1.89 m (ethyl CH2), 1.76 s (CsMes), 1.10 t (ethyl CH3), -5.5 br (BHB)
a Shifts relative to BF3OEt2, positive values downfield. H-B coupling (in Hz) is given in parentheses when resolved. Dichloromethane solution except where otherwise indicated. n-Hexane solution. Reference 11.e Reference la. f Overlappedhperimposed peaks. g CDCb solution; shifts relative to (CH&Si. Integrated peak areas in all cases are consistent with the assignments given. Legend: m = multiplet, s = singlet, sb = broad singlet, d = doublet, t = triplet, q = quartet. B-Hkminal resonances are broad quartets and are mostly obscured by other signals.
Table 2. Experimental X-ray Diffraction Parameters and Crystal Data 2
empirical formula
fw crvst color, habit cryst dimens, mm space group a,A b, A
8, a, deg
C,
B. dee
C3E
8
C O Z C ~ Z B ~CHO~ Z~ B ~ Z C ~ Z CBO~ ZHC~~~Z B ~ H ~ S
645.1 red plate 0.4f x 0.37 x 0.14 Pi 8.998(3) 12.277(5) 8.925(3) 105.1413) 116.32(3) 85.81(3) 852
802.9 orange plate 0.46; 0.33 x 0.21 Pi 8.892(3) 13.346(2) 8.367(4) 94.48(3) 111.88(2) 86.35(2) 918
1
1
9.92 transmission factors 0.80-1.00 D(calcd),g ~ m - ~ 1.257 50.0 2&,, deg no. of reflns measd 3227 no. of reflns obsd 3016 (1 '3 d n ) R 0.030 0.041 RW largest peak i final 0.33 diff map, e l l 3
p(Mo Ka),cm-l
3c
625.5 red prism 0.48 x 0.42 x 0.28 P21ln 8.678(2) 32.560(4) 12.604(3) 103.34(1)
30.80 0.84-1.00 1.452 50.0 2858 2630
3465 4 9.75 0.94- 1.00 1.199 50.0 6661 6231
0.028 0.039 0.31
0.031 0.054 0.29
angle of 109.2' describe a normal -CH2CH2- bridge linking the Cp* units. One-expects that the chemistry of 3a-d will closely parallel that of the corresponding monomers, and the proton NMR data for these complexes are very similar
Figure 2. Molecular structure of [(-CH&5Me4)Co(EtzCzB4H3-5-Br)I~(Sc).
to those of the monomersla,ll except for the complexity introduced by replacing CsMes with C5Me4CH2 ligands. The llB NMR spectra of the dimers exhibit unusually
Wang et al.
4672 Organometallics, Vol. 14, No. 10,1995 Scheme 2
Table 3. Bond Distances and Selected Bond Angles for 5,5'-[Cp*Co(EtzCzB4H)lz(2) Co-C(2) co-c(3) Co-C(lR1) Co-C(lR2) Co-C(lR3) CO-C(lR4) Co-C(lR5) Co-B(4) CO-B(5) Co-B(6) C(2)-C(2M) C(2)-C(3) C(2)-B(6) C(2)-B(7) C(2M)-C(2E) C(3M)-C(3) C(3M)-C(3E) C(3)-B(4)
Bond Distances (A) 2.011(3) C(3)-B(7) 2.014(3) C(lRl)-C(lRB) 2.062(3) C(lRl)-C(lR5) 2.057(3) C(lRl)-C(lRG) 2.039(3) C(lR2)-C(lR3) 2.039(3) C(lR2)-C(lR7) 2.047(3) C(lR3)-C(lR4) 2.100(3) C(lR3)-C(lR8) 2.184(3) C(lR4)-C(lR5) 2.092(3) C(lR4)-C(lR9) 1.505(4) C(1R5)-C( 1R10) 1.472(4) B(4)-B(5) 1.570(4) B(4)-B(7) 1.771(4) B(5)-B(5') 1.519(4) B(5)-B(6) 1.508(4) B(5)-B(7) 1.523(4) B(6)-B(7) 1.571(4)
1.777(4) 1.425(4) 1.422(4) 1.503(4) 1.426(4) 1.499(4) 1.432(4) 1.496(4) 1.427(4) 1.491(4) 1.500(4) 1.695(4) 1.778(4) 1.690(6) 1.695(4) 1.762(4) 1.770(4)
137.9(3) 134.3(2) 123.3(2) 112.0(2) 124.3(2) 134.7(2) 106.6(2) 113.0(2) 112.4(2)
k Y * & ? co
A
/
+
Cp*Co(Et,C,B,H,)COCp*
+
IC~*CO(E~~C~EJH,)I,CO
(multidecker sandwiches)
Table 4. Bond Distances and Selected Bond Angles for [ ( - C H Z C ~ ~ ~ ) C O ( E ~ Z C Z B(3c) ~-~-B~)I~ Bond Distances (A) 1.963(5) C(3)-B(4) 2.037(4) C(3)-B(7) 2.033(4) C(lRl)-C(lR2) 2.072(4) C(lRl)-C(lR5) 2.045(4) C(lRl)-C(lR6) 2.016(3) C(lR2)-C(lR3) 2.036(4) C(lR2)-C(lR7) 2.059(4) C(lR3)-C(lR4) 2.099(4) C(lR3)-C(lR8) 2.107(4) C(lR4)-C(lR5) 2.110(4) C(lR4)-C(lR9) 1.518(5) C(lRS)-C(lRlO) 1.520(6) C(lRS)-C(lR8') 1.470(5) B(4)-B(5) 1.558(6) B(4)-B(7) 1.776(6) B(5)-B(6) 1.509(5) B(5)-B(7) 1.537(6) B(6)-B(7)
Br-B(5)-B(6) Br-B(5)-B(7) B(4)-B(5)-B(6) C(2)-B(6)-B(5)
Bond Angles (deg) 134.7(3) Co-C(3)-C(3M) 120.1(3) C(2)-C(3)-C(3M) 127.0(3) C(2)-C(3)-B(4) 133.5(3) C(3M)-C(3)-B(4) 112.4(3) C(3M)-C(3)-B(7) 103.9(3) C(3)-C(3M)-C(3E) 135.6(2) C(2)-C(2M)-C(2E) 125.5(3) Co-C(lR3)-C(lR81 127.9(3) C(lR3)-C(lR8)-CilR8') 134.3(3) C(lR2)-C(lR3)-C(lR8) 106.4(3) C(lR4)-C( lR3)-C(lR8) 104.3(3)
1.562(6) 1.770(5) 1.437(5) 1.423(6) 1.496(6) 1.425(5) 1.494(5) 1.434(5) 1.508(5) 1.435(5) 1.492(5) 1.503(5) 1.546(7) 1.667(6) 1.802(6) 1.671(6) 1.748(6) 1.792(6) 135.3(3) 123.2(3) 112.9(3) 123.5(3) 132.8(3) 111.8(3) 115.8(3) 128.0(2) 109.2(4) 126.4(3) 125.0(3)
broad, overlapping signals that obscure the anticipated 1:2:1pattern (which is clearly present in the spectra of the monomers la,ll 1b,laand 1d,lathough not in Wa). The llB shifts of 3a and 3d differ significantly from those of their monomeric counterparts la,d (Table l), demonstrating that the replacement of a Cp* hydrogen atom with a -(CHzCsMe4)CoEtzCzB4H4) unit has measurable electronic consequences. Whether this will lead
8
9
to major differences in reactivity remains to be seen, but in at least one important respect these dimers are similar t o the monomers: unlike the B-B-linked complex 2, they are readily decapitated in wet TMEDA, generating the linked nido-cobaltacarborane species 4a-d as yellow air-stable solids (Scheme 1). These double open-ended complexes furnish a new family of synthons that in principle can be used to construct oligomeric or polymeric -[(CH~C~M~~)C~(E~C~B~H~X)M(E~CZB CO(CF,M~~CHZ)-I,linked-quadruple-decker chains via deprotonation of the B-H-B bridges and face-coordination to Mq+ metal ions.1° Synthesisand Characterizationof Linked nidoCobaltacarborane Dimers. Metallacarborane substrates having open CzB3 faces are highly versatile, reactive species and can be bound together in a variety of modes, including two-center B-B or three-center B-B-B interactions or combinations thereof. Additionally, in the presence of metal ions, they can form tripleor quadruple-decker sandwich complexes.' In recent workgawe isolated the dimers [Cp*Co(EtzCzB3H3)1~ (5, 7) and [Cp*Co(EtzCzB3H4)1z (6) as shown in Scheme 2. Compounds 5 and 7 were structurally characterized via X-ray crystallography and are isomers whose CoCzB3 units in both cases are joined via a pair of B-B-B threecenter bonds (Chart 2A); in 6, which has two more hydrogens than 5 and 7, the linkage is a two-electron B-B single bond.
Organotransition-Metal Metallacarboranes
P
d3E
C5E
Organometallics, Vol. 14,No. 10, 1995 4673
Table 5. Bond Distances and Selected Bond Angles for [Cp*Co(EtzCzB$h)Iz (8) 1.15(3) 1.522(4) 1.513(4) 1.43l(4) 1.519(4) 1.528(4) 1.512(4) 1.542(4) 1.521(4) 1.432(4) 1.426(4) 1.499(4) 1.427(3) 1.492(4) 1.426(3) 1.500(3) 1.422(3) 1.493(4) 1.493(4) 1.430(4) 1.414(4) 1.499(4) 1.419(4) 1.504(4) 1.426(4) 1.504(4) 1.432(4) 1.25(4) 1.22(3) 1.27(3) 1.11(2) 1.09(3) 1.27(3) 1.18(3) 1.27(3)
ClR7
Figure 3. Molecular structure of [Cp*Co(EtzC2B3H4)12(8). Chart 2
A
C'
I
B
I
H
,by
I /H I B, H
122.0(2) 111.6(2) 128.6(2) 119.6(2) 54.7(1) 165.6(2) 112.5(2) 104.3(2) 114.7(2) 129.6(2) 125.5(2) 117.7(2) 106.7(2) 60.8(1) 127.4(2) 64.5(1) 117.0(2) 98.5(2) 104.7(2)
C' B
B'H
I /C\B/H I
H
In the present study, our synthesis of 2 via Wurtztype reactions of halogenated closo clusters, described above, led us t o attempt an analogous linkage of nidocobaltacarborane units, recognizing that the B-H-B bridging protons on the open face could lead t o more complex interactions. This is in fact the case: as depicted in Scheme 2 (bottom),the treatment of neutral Cp*Co(Et~CzB3H4-5-C1) with sodium in THF afforded two major products that were identified as the dimeric species [Cp*Co(Et2CzB3H4)1~(B), isolated as yellow crystals, and the unsubstituted nido monomer 911in yields of 15 and 42%, respectively. Since 9 can be quantitatively converted to the B(5)-chloro starting material, the net conversion of the latter species to 8 is ca. 25%. From its lH and llB NMR spectra (Table 1) it is evident that 8 is asymmetric, and an X-ray diffraction analysis revealed yet another mode of intercage binding that differs from those found in 5-7. The molecular structure of 8 is presented in Figure 3, while Tables 2 and 5 present the relevant crystal structure information, bond distances, and selected bond angles. This structure is "dimeric" only in a general sense, consisting of two dissimilar Cp*Co(EtzCzB3H4)units, one of which contains two B-H-B and two terminal hydrogens while the other has one B-H-B bridge and three terminal H atoms. The C2B3 ring planes are almost exactly per-
pendicular, with a dihedral angle of 90.6'. The intercluster binding is unusual, with B(5) on one CZB3 ring coordinated t o B(7) and B(6) on the other ring, a t distances of 1.873(4) and 1.937(4) A, respectively. Together with the intracage B(6)-B(7) interaction (1.751(4) A), this triangular array suggests a threecenter B-B-B bond as depicted in Chart 2B; in effect, B(5) can be viewed as replacing the "missing" bridge hydrogen on the other Et2C2B3H4 ring. While relatively long, the B(5)-B(6) and B(5)-B(7) distances are within bonding range and are similar to those found in a cobaltacarborane-carborane-coupled species isolated earlier by Sneddon et a1.18 (18)Borelli, A. J.;Plotkin, J. S.; Sneddon, L. G. Inorg. Chem. 1982, 21, 1328.
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Organometallics, Vol. 14,No. 10,1995
Compound 8 probably forms from an initially produced symmetrical B(Ei),B(5')-coupled isomer that undergoes a rearrangement whose net effect is the replacement of one of the original B-H-B bridging hydrogens (that involving B(6) and B(7))with a B-B-B linkage. However, the accompanying formation of neutral 9 in substantial yield indicates that the sodiumpromoted disproportionation of the chloro complex is a major competitive process in this reaction. Electronic Spectra. The visible-untraviolet absorption bands of the dimeric complexes (Experimental Section) appear at nearly the same wavelengths as their monomeric counterparts, and their molar extinction coefficients are approximately twice those of the monomers (reflecting,of course, the doubled molar concentration of metallacarborane cluster units in solutions of the dimers). For example, A,,, values (nm) for la and the B-B-linked species 2 are respectively 300 ( E = 11718 M-l cm-l) and 294 ( E = 24469 M-l cm-l), while the corresponding values for the nido monomer Cp*Co(EtzCzB3H4Cl) and its Cp*-Cp*-linked dimer 4b are 286 ( E = 20 332 M-l cm-l) and 292 ( E = 37 526 M-l cm-l). From these observations, it appears that the metalligand interactions in the B-B-linked and Cp*-Cp*linked clusters are not greatly different from those in the corresponding monomeric complexes. The high extinction coefficientsin both monomers and dimers are strongly suggestive of charge-transfer excitations, probably involving filled bonding MOs on the carborane ligands and empty antibonding MOs on the metal centers. Similar electronic behavior is seen in related metallacarborane complexes, e.g., in a number of our recently reported triple- and quadruple-decker sandwi~hes.~~ summary
It is apparent from this work and our earlier studies on coupling of small metallacarboranesgathat reactions of closo derivatives such as la-d can be synthetically useful, allowing the preparation of specific target dimers in good yield with minimal side reactions as in Scheme 1. On the other hand, nido complexes such as those depicted in Scheme 2 generate a much richer and less easily controlled chemistry that leads to structurally varied dimers featuring a range of intercage binding modes. Of the new products reported here, the dimethylene-linked species 4a-d, prepared via a reasonably efficient route, are potentially useful building blocks in the construction of polysandwich systems as noted earlier. If conditions can be found for decapitation of 2 to generate 6 directly, this would provide an additional synthetic tool that may permit the controlled assembly of directly B-B-coupled multidecker sandwich complexes. While 2 itself appears inert to TMEDA attack, as reported above, it may be possible to decap suitably tailored (e.g., halogenated) derivatives of 2, and this strategy is among those to be explored in future studies.
Experimental Section General operations, procedures, and instrumentation were as described in the accompanying paper.la Visible-ultraviolet spectra were recorded on a Hewlett-Packard 8452A diode array with a H P Vectra computer interface. Synthesis of 5,5'-[Cp*Co(EtzCaB4Hs.s)12(2). THF (30 mL) was distilled under vacuum into a two-neck round-bottom flask
Wang et al. charged with 170 mg (0.42 mmol) of lcla to give a n orange solution. An excess of pretreated sodium m e t a P was added from an attached side tube a t room temperature. The mixture was stirred for 1.5 h, during which time the solution turned deep red, and the flask was opened to the air. The solution was transferred via pipet to a flask, and the unreacted sodium was destroyed with 2-propanol. The solution was evaporated to dryness and flash-chromatographed in CHzClz through 2 cm of silica gel. The solvent was removed via rotary evaporation to afford a red solid, which was dissolved in a minumum volume of CHzClz and hexane, placed on a silica column, and eluted with 3:l hexane-CHzClz. Two major orange bands were collected, the first of which was 2 (30 mg, 0.046 mmol, 42% based on ICconsumed) and the second was recovered IC (80 mg, 0.20 mmol). MS: mlz 646 (molecular ion envelope). UV-visible absorptions (nm): 294 (100%; E = 24469 M-l cm-'),354 (64%),466 (10%). Anal. Calcd for C O Z C ~ ~ BC, ~H~~: 59.58; H, 8.75. Found: C, 59.86; H, 9.22. Compound 2 was also prepared from lb,d via a n identical procedure, except that the reaction times employed were 6 h and 40 min, respectively. The isolated yields of 2 were 20% (from lb) and 34% (from Id). Synthesis of [ ( - C H ~ C ~ M ~ ~ ) C O ( E(Sa). ~ ~ By C ~the )I~ above procedure, a solution of 410 mg (1.26 mmol) of la1' in 30 mL of THF was prepared and the solution was cooled in a dry ice-ethanol bath. n-Butyllithium (0.79 mL of 1.6 M solution in hexanes) was added via syringe, producing a color change from yellow-orange t o red. Following 3 h of stirring a t room temperature, the deep red solution was opened to the air and the solvent was removed to give a red solid that was worked up as in the synthesis of 2. Elution from a silica column using hexane gave one yellow band, which was nidoCp*Co(EtzCzB3H5)11(54 mg, 0.17 mmol). The eluent was switched to 2:l hexane-CHzClz, producing two major orange bands, the first of which was l a (127 mg, 0.40 mmol) and the second was 3a (80 mg, 0.12 mmol, 28%). MS: mlz 646 (molecular ion envelope). UV-visible absorptions (nm): 306 (loo%), 426 (4%). Anal. Calcd for C O Z C ~ Z B ~C,H59.58; ~ ~ : H, 8.75. Found: C, 58.92; H, 8.62. Synthesis of [(-CH~C~M~~)CO(E~~C~B~H~.S-S-C (3b). An identical procedure was followed employing 80 mg (0.22 mmol) of 1b.la Column chromatography on silica in hexaneCHzClz with a ratio ranging from 2:l to 1:l gave two major orange bands that were identified as l b (37 mg, 0.10 mmol) and 3b (24 mg, 0.034 mmol, 56%). MS: mlz 716 (molecular ion envelope). UV-visible absorption (nm): 300. Anal. Calcd for Co&12C3&H54: C, 53.83; H, 7.62. Found: C, 53.95; H, 8.26.
Synthesis of [ ( - C H ~ C ~ M ~ ~ ) C O ( E ~ ~ C (34. ~~~-S-B~)IZ The preceding synthesis and workup were repeated employing 110 mg (0.27 mmol) of lc,laof which 27 mg (0.067 mmol) was recovered, affording 30 mg (0.037 mmol, 36%) or orange solid 3c. MS: mlz 804 (molecular ion envelope), 646 (-2Br). UVvisible absorption (nm): 232 (27%),298 (100%). Anal. Calcd for CozBrzC32B~H54: C, 47.87; H, 6.78. Found: C, 47.59; H, 6.79.
Synthesis of [(-CHZC~M~~)CO(E~~C~~H~.S-S-I)I~ (3d). The preceding synthesis and workup were repeated employing 135 mg (0.30 mmol) of 1d,laof which 82 mg (0.18 mmol) was recovered, affording 23 mg (0.026 mmol, 43%) of orange solid 3c. MS: mlz 898 (molecular ion envelope), 771 (-1). UVvisible absorption (nm): 306. Anal. Calcd for C O Z I ~ C ~ ~ B ~ H ~ ~ : C, 42.85; H, 6.07. Found: C, 42.40; H, 6.58. Synthesis of [(-CHzCaMe4)Co(EtzCzBsHs)la(4a). A twoneck round-bottom flask was charged with 25 mg (0.039 mmol) of 3a and 4 mL of TMEDA, and 3 drops of water were added. THF was added via pipet until all of the solid was dissolved, and the solution was stirred for 3 h under a flow of Nz. (19)Sodium pieces were allowed to react with absolute methanol for several seconds under a flow of Nz. The methanol was syringed out and the metal was washed with dry THF and stored under vacuum in a side tube on the reaction flask.
Organotransition-Metal Metallacarboranes
Organometallics, Vol. 14, No. 10,1995 4675
Volatiles were removed by evaporation on a Schlenk line, giving a red solid which was dissolved in a minimum volume of CHzCl2 and hexane and placed on a silica column. Elution with hexane gave one yellow band, which was 4a (10mg, 41%). MS: mlz 626 (molecular ion envelope). UV-visible absorptions (nm): 290 (loo%), 386 (8%). Anal. Calcd for COZc32B6H58: C, 61.44;H, 9.35. Found: C, 62.41;H, 9.53.
yellow-orange 9ll (242mg, 0.77 mmol) and yellow crystalline 8 (83 mg, 0.13 mmol). Allowing for quantitative conversion of recovered 9 to the starting material Cp*Co(Et2C2B3H4-5Cl), the net yield of 8 was 25%. MS: mlz 626 (molecular ion envelope), 324 (Cp*CoEt&zB4H4+). UV-visible absorptions (nm): 300 (loo%),422 (26%). Anal. Calcd for Co2C32B6H58: C, 61.44;H, 9.35. Found: C, 62.11;H, 9.85. Synthesis of [(-CH~C~M~~)CO(E~~C~B~)~-S-C~)I~ (4b). X-ray Structure Determinations. Diffraction data were collected on a Rigaku AFC6S diffractometer a t -120 "C using The preceding procedure was employed using 50 mg of 3b and 4:1 hexane-CH2Clz as the eluting solvent, which afforded 40 Mo Ka radiation. Details of the data collections and structure determinations are listed in Table 1. For each crystal, the mg (ca. 100%) of 4b. Further elution of the column with 2:l hexane-CHzC12 gave 8 mg of recovered 3b. MS: mlz 696 intensities of three standard reflections were monitored, showing no significant variation. Empirical absorption cor(molecular ion envelope). UV-visible absorptions (nm): 232 rections were applied following ly scanning of several reflec(44%), 248 (38%), 292 (loo%, E = 37 526 M-lcm-'), 390 (5%). tions (transmission factors are reported in Table 1). All Anal. Calcd for C0&12C32B6&6: c, 55.35;H, 8.13. Found C, 54.23;H, 8.65. calculations were performed on a VAXstation 3520 computer employing the TEXSAN 5.0 crystallographic software packSynthesis of [(-CH~C~M~*)CO(E~~C~B~H.~.B~)I~ (4c). age.20 The structures were solved by direct methods in The procedure described for 4a was employed using 50 mg of SIR88.21 Full-matrix least-squares refinement with anisotro3c, which gave 39 mg (80%)of 4c as the only major band. pic thermal displacement parameters was carried out for each MS: mlz 784 (molecular ion envelope), 704 (-Br). W-visible structure, with the results summarized in Table 2. The final absorptions (nm): 292 (loo%), 392 (5%). Anal. Calc for difference Fourier maps were featureless. CozBrzC&~H58: C, 49.07;H, 7.21. Found: C, 49.83;H, 7.58.
Synthesis of [(-CHzCaMe4)Co(EtzCaBs)4-5-1)12 (4d). The procedure described for 4a was employed using 250 mg Acknowledgment. This work was supported by the of 3d, except that the reaction time was 12 h and a 4:l U.S.Army Research Office and the National Science hexane-CHzCl2 mixture was employed to elute the product. Foundation (Grant No. CHE 9322490). We thank A total of 150 mg (61%) of 4d was obtained. MS: mlz 878 Professor John Gladysz for helpful discussions on the (molecular ion envelope). UV-visible absorptions (nm): 234 activation of Cp* ligands. (48%), 294 (loo%),388 (5%). Anal. Calcd for C O ~ I Z C ~ ~ B ~ H ~ ~ : C, 43.81;H, 6.43. Found: C, 43.42;H, 7.15. Supporting Information Available: Tables of atomic Synthesis of [Cp*Co(Et2C&&)l2 (8). THF (30mL) was coordinates (including observed and calculated hydrogen atom distilled under vacuum into a two-neck round-bottom flask positions for 8), isotropic and anisotropic displacement pacharged with 636 mg (1.83mmol) of Cp*Co(EtzC~B3H4-5-C1). rameters, and calculated mean planes for 2, 3c, and 8 (8 An excess of sodium was added under a flow of nitrogen, and pages). Ordering information is given on any current mastthe flask was attached t o a condenser and heated until the head page. THF commenced boiling. Reflux with stirring was maintained OM950344B for 3 h, during which time the solution turned deep red-black. The solution was opened to the air, evaporated to dryness, and (20) TEXSAN 5.0: Single Crystal Structure Analysis Software flash-chromatographed in hexane through 2 cm of silica gel (19891, Molecular Structure Corp., The Woodlands, TX 77381. to give a red solution, from which the volatiles were removed (21) SIR88: Burla, M. C.; Camalli, M.; Cascarano, G.; Giacovazzo, to leave a red oily solid. The solid residue was dissolved in C.; Polidori, G.;Spagna, R.; Viterbo, D. J . Appl. Crystallogr. 1989,22, 389. pentane and eluted in that solvent on a silica column, affording ~~
~
