Organometallic Benzene Complexes Edward Maslowsky, Jr. Loras College, Dubuque, IA52001 Students in themost basic ofchemistry courses associate the molecular formula C6Hswith the planar benzene molecule of D6h symmetry in which the carbon atoms form a delocalized ring and a hydrogen atom is attached to each carbon (1).This is probably one of the most recognizable molecular structures in chemistry
In a recent article in this Journal (I), however, it was noted that the planar structure is only one of~evenvalence Isomers that have been proposed for C6H6and that all I but possibly one) of these isom& have been prepared. Modern organometallic chemistry owes its birth in part to the preparation and structural characterization of compounds formed by reacting metals and cyclic, aromatic hydrocarbons. Although the most attention often is given to sandwich compounds such as ferrocene, (q5-C5H&Fe,that contain the cyclopentadienyl ring, there are many analogous compounds such a s dibenzene chromium, (q6C6H6I2Cr,that contain benzene. The interaction of metal atoms and benzene can squeeze and distort the benzene ring to produce a rich variety of structural variations in addition to the seven valence isomers previously noted. The following summarizes the several different types of metal-benzene complexes that have been prepared and illustrates some of the structural changes produced in the benzene ring on complex formation. Although the focus of this article is on complexes of bens benzene com~lexes zene. when s~ecificstructural t v ~ e of have'not bee; studied in detail; are unknown, ap&opriate arene com~lexeshave been used as examoles. For the sake of simplicity and brevity, however, such examples have been held to a minimum. I ~ ~ - cRings ~ H ~ The first structural variation is found among the most common benzene complexes in which benzene rings are bonded in a terminal q6-manner to a single metal atom. Although this bonding mode has been proposed for dibenzene complexes of several transition elements, the structures of only (q6-CeH6hCr(2) and (q"CsH&RuZt (3) have been studied in detail. These studies show that although all of the carbon atoms in each ring are coplanar and equidistant from each other, the hydrogen atoms are not co~lanarwith the carbon rine. Rather. thev are nushed slightiy out of the ring plane &ard the m e k atom (2). This structure also has been found for the benzene ring in u sinfile crystal X-ray study ofthe half-sandw~chcompo~nd
L - ~ . I -~ ~ - - C ~- H ~ -- J K U ( H ~ O ~ I ' - ~ ~ , .
Benzene complexes of the main group metals are not as common as those of the transition elements. Single crystal
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X-ray studies of Ga(1) (51,Sn(I1)(6-9)and Pb(I1) (10) complexes show the presence of symmetric, delocalized n6rings. As the result of g a ~ - ~ h aexperiments se and theoretical studies, a similar structure has been proposed for two unstable benzene soecies of Si (11). . . For all of these complexes, however, the positions of the ringH atoms were not determined. The q6-ring structure also has been found in a matrix study of the condensation products of Na atoms and benzene (12). In this study, the complex (q6-CsH6)Nawas characterized as having a weak Na atom-benzene ring interaction and the Na atom was located on the C8 ring axis. The condensation of Li atoms and benzene in a low temperature matrix has produced hoth mono- and dibenzene complcxcs(131.Although the structuresofthese complexes show that the Li atoms-are located on the C6 ring axk, the benzene molecules appear to be distorted from their sixfold symmetry producing, in the case of (CsHs)Li, a molecule with C3" or Czu symmetry, and, in the case of (CGH&Li,a symmetry of at least D2h(3).This distortion was attributed to the presence of a small Jahn-Teller effect.
The boat (4) and inverted boat (5) distortions that both produce a nonplanar ring of carbon atoms have been reported for complexes that have been formallv characterized as containingq6-type rings. In the boat structure an opposite (para) pair of ring carbon atoms (C(1)and C(4))is closer to the metal atom than the other four-carbon atoms. In addition to the distortion, there is a disruption of the aromatic character of the ring that produces a localization of electrons. The extreme of this distortion would produce a 1,Cdiene ring and a structure that would be better characterized as of the q2(1,4)type. The distinction between a q6- and q2-type ring depends on how many of the metal-carbon bond lengths are equal to or less than the sum of the metal and carbon atom covalent radii. If they are, bonding can be assumed, while bond
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The possibility of a q4-type structure containing an inverted boat arene ring has been discussed above in the section on q6 rings. . q 3 - ~ s ~ Rings s No such bonding mode has been confirmed to exist in any reported complexes of benzene, although it has been found in an X-ray study of an arene complex of Pt (7)(33).
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lengths greater than this would imply that bonding is not present. A small distortion and localization of ring electrons has been observed for the benzene ring of (q6C6H6)Ru(1,5-COD)(14). while much larger distortions and or electron localization have been observed in several hexamethylbenzene complexes of Ta and Nb (15,161. In the inverted boat structure, two para carbon atoms from the ring are not pointed toward the metal atom as found in the boat structure but rather away from it. Again, a t the extreme, this type of complex would be more appropriately characterized as containing a q4-ring.Although no such complexes have been reported of benzene, several arene complexes with an inverted boat-type ring have been characterized (17-21).
q 2 - ~ 6 Rings ~ 6
Single crystal X-ray studies of (C6&)MAICl4(M = Cu(1) (341, AgU) (35)) show that in both the metal ion is within
bonding distance of only one ad'acent pair of carbon atoms in the benzene ring to give a q s' (1,Z)-typeof metal-ring interaction (8). Also, the ring was noted to be planar in the
q 4 - c s ~ sRings
For the complexes (C6H&MZI(M = Fe, Ru; Os), in which the metal atoms formally have 18 valence shell electrons, both benzene rings are bonded in the q6-manner. In the series (C&&M (M = Fe, Ru, Os), however, the metal atoms would have 20 valence shell electrons if both benzene rings were of the q6-type. Not surprisingly, therefore, in all three of these neutral compounds (22-24) one ring is q6 while the other ring is q4 in nature. An analysis of spectral data for these compounds as well as a single crystal X-ray study of the anal(25) have ogous compound bi~(hexamethylbenzene)~Ru shown that theq4-ringisnonplanar and bent slightly away from the metal atom (6). In addition, the C(5)-C(6)bond
length is that expected for a carbon-carbon double bond, while the part of the ring from C(1) to C(4)roughly resembles that found for a metal-complexed buta-1,3-diene molecule. In a more recent single crystal X-ray study, a similar type of q4-bondinghas been found for the benzene ring in the BF4- salt of [(q4-C6H6)Ir(triphos)l+ (26). Although they could not be isolated, the isoelectmnic q4benzene complexes [(q4-CsH6)Cr(CO)J2(27-29) and (q4C6H6)Mn(CO)31-(30, 31) have been reported to form as unstable intermediates. The q4 structure also has been nronosed for benzene and other arene rinrrs of intermediates formed in catalytic reactions of several q6-arene complexes of Ni (32).
A
&(I) compound. Although the Ag(1) complex showed an apparent alternation in the carbon-carbon bond lengths expected for a cyclohexatriene structure, it was warned that the data were insufficient to come to this conclusion. This type of bonding also has been proposed for the benzene ring in [(q2-C&)Os(NH3)512+on the basis of variable temperature NMR spectral data (36), as well as for the intermediate formed in the reaction pathway for benzene activation by the fragment [(C5Me5)Rh(PMe3)l(37).The existence of the q2-benzene-Rh species more recently has been confirmed for this intermediate in transient laser flash studies of t h e interaction of benzene with [(CsMe.dRh(PMe3)1and the analogous [(C5H5)Rh(PMe3)1 fragment (38). In addition to the q2(1,2)-typeof metal-ring interaction discussed above in which the metal atom is bonded to two adjacent (ortho) ring carbon atoms, there is also the possibility of the q41,4)-type of bonding (4) in which the metal atom is bonded to two opposite carbon atoms. This produces a boat structure, as was noted previously for several transition metal complexes (17-21) in the section on q6-typerings. Although the q2(1,2)-typestructure was proposed originally on a basis of an ESR study of (C6H6)Alformed in a low-temperature matrix (391, a more recent matrix ESR (40) as well as a theoretical study (41) of this complex support the existence of the q2(1,4)-typestructure. Although not yet prepared, theoretical calculations appear to sunnort the nossibilitv of the n2(1.4)-bondine mode ... for one or both of the benzene rings in (C6H6hBeas well as for the benzene rings in other related Be compounds (42). A
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rl'-CsHs Rings
Examples of metal complexes with ql-type rings are rare, with all but one of the examples reported containing arene rather than benzene rings. The one example of a q1benzene ring complex is of Ag(1). A single crystal X-ray study of this complex shows the benzene ring to be planar with all of the carbon-carbon bond lengths equal (91, thus indicating that the ring carbon atoms maintain their formal sp2hybridization (43).
In the ql- arene complexes that have been reported, both planar rings (441, such as that found for the above mentioned Ag(1) complex, and a ring with a slight boat-type of distortion (10) (45) have been observed. In the latter case,
the nonplanarity of the ring indicates that the carbon atoms might have lost some of their sp2hybrid character. The presence of 11'-type bonding also has been proposed as theoretically possible for one of the benzene rings in (C6H6hBe,although, as mentioned above, it was proposed that the calculations also supported the possibility that both rings are of the q2-typein this hypothetical compound (42). r12:r12:q2-~s~s Rings
Although delocalized q6-benzene rings were found for (C6H&Cr (2) as well as for (C6H6)Cr(C0)3(46, 47) in mom temperature X-ray crystallographic studies, an X-ray diffraction (48. 49) and a neutron diffraction studv (49) of (CRIIGlCr(CO), at llqwd nitrogen temperatures as well as an X-rav diffraction studv of a related (areneJCr(C01.complex (56) reveal that th; rings are distorted to give the Kekul-type structure in which there is an alternation in the carbon-carbon bond lengths (11).A similar distortion H H. ,C=C
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has been reported in structural studies of (CsH6)Mo(CO)3 (51)and [(C6H6)Ru(CH3CN)312+(52). The structural studies of these compounds also show the H atoms of the benzene ring to be bent slightly out of the ring plane toward the metal atom. pm6:116-Ck~sRings
The retention by benzene of the delocalized, planar structure ofDsh symmetry after complexation with metals is uncommon. One crystalline com lex in which this struc2.q6-C6H61(53) in which ture is found is [q5-C~HsWz[pz-q the benzene ring bridges two V atoms (12). 982
Journal of Chemical Education
X-ray crystallographic data indicate that the carbon and hydrogen atoms in the benzene ring are coplanar and that the carbon-carbon bond lengths are essentially equal. For the compound Hf2L[P(CH8)2C6HS)I[~2-q6:q6-C&l, in which the benzene ring again bridges two metal atoms, the carbon atoms also are fairly equidistant from each other. However, the benzene ring carbon atoms are not coplanar. Rather, the ring was found to be slightly puckered (13) (54).
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Benzene rings also are able to bridge two metal atoms in such a way that each metal atom interacts with only a portion of the ring. This example of this type of bonding appears to be a possibility in the Ta complex (55, 56) discussed in the following section on p2-q2:q2-C& rings. Other than for possibly this complex, however, no others with this bonding mode have been reported. p m 2 : r 1 2 - ~ 6 Rings ~6
Bridging of metal atoms by portions of the benzene ring with this type of bonding can be accomplished in several ways, depending on which parts of the ring the metal atoms attach themselves to as well as on whether the metal atoms are on the same or opposite faces of the original ring plane. A sin le crystal study of [(qS-C5Mes)Re(CO)212[p2q2(1,2):q$(3,4)-C&l (57) shows that the Re atoms are on opposite faces of the ring, and that they are attached in an asymmetric manner to carbon atoms C(1) & (2)and C(3) & C(4),respectively (14).Although the ring is essentially planar, the carbon-carbon bond lengths within the ring are unequal. Therefore, bond length C(5)-C(6)is consistent with that of an isolated carbon-carbon double bond, bond
lengths C(1)-C(2)and C(3>C(4),to which the Re atoms are attached, are in the range expected for q2-olefmbonds coordinated to transition metal atoms, and the remaining ~(4),-~~,4f carbon-carbon bonds lengths are slightly shorter than : those found for the central bond of the buta-1,3-diene mol+,) ecule. This carbon-carbon bond length pattern is some,s C(6) 8II, what similar to those discussed previously for complexes with q4-rings (22-26). A similar structure has been pro' f i posed for [(N~)~Osl~[p~q~(l,2)~(3,4)-C~H~l~+ (58) and 16 the mixed-metal complex [(NH3)50sl[(NH~)&ultp~q2(1,2)q2(3,4)-C6H6)14+ (59) which is produced from it by replacing one (NH3)sOs fragment with (NHakRu. It also (61).The structures of these Fe complexes differ from those has been reported in a preliminary study of a polymeric of the V complex, however, in that the carbon-carbonbond Cu(1) complex (60) that consists of lemths in the ring are not all eaual. ~ & l e crystal x-ray studies oftwo interesting bimetallic ...Benzene...Cu+...Benzene...Cu+... Pd sandwich complexes of benzene (64)illustrate one last chains. The data available could not be used to determine type of bis q2-bonding.In the structure determined for the if there were any deviations in the carbon-xrbon bond most easily characterized of these two compounds, lengths from the values found for free benzene. [(C&s)Pd(AlC14)12(17),four carbon atoms of each ring are A different structure is found for [(siloxXTal~[u~q2(1,2)q2(4,5)-C&l(55,56). Although both ~a atoms bond to the opposite faces of the benzene ring, they -C C\ are located at the opposite ends of the ring, being attached to bonds C(l)-C(2)and C(4!-C(5), respectively, j to produce a symmetrically bridged structure (15). ; ,, ,\ II
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The data, therefore, appeared most consistent with the bisq2 type of structure, although the possibility of a bis-q3 structure could not be completely eliminated. In addition, no mention was made of the relative carbon-carbon bond lengths or of the degree of planarity of the ring. In a single crystal X-ray study of a related Co complex of p-xylene (61), however, in which two Co atoms from two larger fragments are bonded to opposite faces and at opposite ends of the arene ring, it was noted that the ring was nonplanar and that the carbon-carbon bond lengths were unequal. A single crystal X-ray study of (C6H6)AgC10r(62) also reveals the presence of p2-q2(1,2)$(4,5)-CsHs rings. The structure of this oolvmeric comolex consists of . ...Benzene...Ag+...Benzene...Ag*... chains. The Ag(1)ions again lie on the opposite faces of the benzene rines. and the two carbon-carbon bonds from each ring.that ;a k t h i n bonding distance of the Ag(1) ions are loneer than the four non-interactinebonds. This structure co&asts with that of the polyme& Cu(1)-benzene comolex mentioned a b v e (60) in which, although the benzene molecule bridges two CuU) ions that are on opposite ring faces. the two CutI) ions are bonded to carbon atoms 1 & 2 and 3 and 4 rather than to 1and 2 and 4 and 5. Another type of bis-q2 structure is illustrated by [q4 CpVHI2[pz-q2(1,2):$(4,5)-C6m(63). Because the two V atoms in this complex are bonded directly to one another, they are both located over the same face of the benzene ring (16). In addition, the ring was observed to be nonplanar and folded in the middle with all ofthe carbon-carbon bond distances of equal length. A similar structure with a nonplanar, arene ring that is folded in the middle and that bridges two metal atoms that are on the same face of the ring is found for [q5-C5Hge12[C&l(R=Me, Et)
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bonded in a diene fashion to two Pd atoms that are over the same face of each ring. The nonbonded ring fragments are bent out of the ring planes toward the Pd atoms. This is unusual in that in all other complexes with nonplanar carbon rings the bending is away from the metal atoms. The nonplanarity of the rings implies that they have lost some of their aromatic character. More recently, the same type of structure has been found for the benzene ring in the half-sandwich bimetallic compound [(q5-C5Me5)Ru12[(b-PPh2)(p~.H)(p~~12(1,2)q2(3,4)CeHd (65). ~
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These com~oundsall have a cluster of three metal atoms arranged in ^a triangular pattern over one of the faces of a benzene rim. In most of them. each metal atom bonds to one of the c&bon-carbon doubie bonds to produce a Kekullike rather than a delocalized ring structure (18).It also has been concluded from single crystal X-ray studies of some of these complexes (51, 66-69) that the hydrogen
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atoms are not coplanar with the ring, but that they point away from the carbon-ring plane. This contrasts with the structures discussed previously for complexes in which a single metal atom is located over one face of either a delocalized ring or localized Kekule-type ring in that the hydrogen atoms, although not coplanar with the ring, point toward rather than away fiom the metal atom. This type of bonding is not universal for all arene molecules bonded to a threefold metal site. Indeed, no alternation was obsewed in the carbon-carbon bond lengths of the p3-q6-arene complex in which a triangular array of Co atoms from a (q5-C&Co)3 fragment is located above a trans-+-methylstyrene ring (70).
20. Silverthorn, W. E.; Couldwell, C.; Pmut, K J Cham. Soe.,Cham. Commun 1976, ,M e,. ". ,,. . . .. .
21. Atwmd, J. L.; Hunter, W. E.: Row,R. D.: Carmana-Gueman, E.; Wilkinaon, G. J. Cham. Soc, Dalfon h n s . 1979,15191521. 22. Parker, S. F; Peden, C . H. F J. O g o m m e f d . Cham. 1984,272,411416. 23. T i m e , P L.; King, R. B. J Chem. Soc,Chem. Commun. 1976,898499. 24. Bandy. J. A,: Green,M. L. H.; GHare, D. J C k m . Soc., Dolton h n a 1985,247725. Hut& G;Lange, S.Aeto Crytollogr 1971,B28,204%2060. 26. Bianchini, C: Caulton, K G.; Chardon, C.;Ekmstein, 0;Foldng, R Johnson, T. J.; Me1i.A.: Pemezini. M.:Rauacher. . D. J.:,Streib.. W. E.:Viza. . . F J h . Cham. Soe m i ,li3,512~129. 27. Leon.V.S.:Coo~%% N. J. J A m . Chem. Sae.1988.110.264PZM6. 2% C L i.~ 11 .I' A . i & r N. J . J Am. Cl-m So?. 1890.112.2832-2834 2v We). 11
