Chemists in general, and inorganic chemists in particular, have long been interested in the possibility of r electron de- localization through elements other than carbon ( I ) . Con- sideration has been given to bothps and drorhitals as routes of delocalization. Of the former type, many examples can be found in the chemistry of boron compounds. Traditionally, much of this discussion has been involved with boron-nitrogen compounds (2). Recently, however, there has been consider- able interest in the interaction of organic centers of relectron density with boron acceptor centers (3).

One of the most interesting and significant of these orga- nohoron derivatives is the borabenzene anion (I).

Christopher W. Allen and Donna E. Palmer

University of Vermont Burlington. Vermont 05401

The replacement of a BH- fragment in polyhedral borohy- dride anions by the isoelectronic C-H fragment has led to the rapidly expanding area of carborane chemistry (4). In the system under consideration in this paper, the replacement of a C-H fragment in benzene with a RR- fragment leads to an organoboron anion which is isoelectronic with benzene. In this Dauer, we will exdore the svnthesis. structure. and reactivity

The Borabenzene Anion and Its Transition Metal Complexes \,

bf ihe.borahenzebe anion and its metal complexes in order probe the ability of the boron atom to support conjugative electron delocalization. We can briefly examine the expecta- tions for this t w e of bonding throurh qualitative comparison of the r mole&lar orhitals in benzeneand the borabenzene anion as is shown in Figure 1. The lowering of symmetry at- tendant with 1 he inrlu& of a bornn atom into the ring r&lts in the removnl i#fdtgeneracyd the highest filled (el.) henz~nr molecular orbitals.-A second effectof incorporahg a less electronegative boron 2p orbital into the r system would he the destabilization of those molecular orbitals which contain contributions from the boron 2p orbital i.e., the a, and a2

molecular orbitals. As is the case for many novel organic species containing s

electrons (e.~.. cvclobutadiene (5)). the borabenzene anion was . -. - . ... first obtained as a ligand in a transition metal complex, spe- cifically, a cobaltocene derivative.

(11)

Cobaltocene bas one electron over the stable 18 electron

configuration and this extra electron imparts chemical be- havior reminiscent of free radicals (6) particularly in reactions with halogenated hydrocarbons (7). Aqueous solvolysis of compound (11) leads to a ring expanded cyclobexadienyl complex, (111) (8).

The solvolvsis reaction can be envisioned as ~roceedine through a transition state with considerable carbonium ion character a t the methylene carbon atom. If this is the case, then replacement of the methylene carbon atom by an electron deficient boron atom should provide a low energy pathway to insertion of a boron atom into a carbocyclic ring. Herberich and coworkers carried out this experiment and developed the following synthesis of a borabenzene complex of cobalt (9).

Hydrolysis of the borabenzene cationic complex (N) leads to the mono and bis borabenzene complexes (V) and (VI) (10).

(CsHdCo(CsH$R)+X- (CaH5)Co(C5H$R) + (V) . .

(CsH$R)Lh (VI)

Soon after Herberich's original discovery, an elegant syn- thesis of an alkali metal salt of the borabenzene anion was presented by Asbe (11). The general route involves hydro- stannation of 1,4-pentadiyne to yield the organotin hetero- cycle (VII).

Figure 1. s ~olecular &~tals fw benzene and the borabenzene anion

Volume 55. Number 8. August 1978 1 497

A redistribution reaction with phenylhoron dibromide leads to the organohoron heterocycle from which the horahenzene anion may be ohtained hy deprotonation. I t is worth noting that a similar set of synthetic transformations can be em- ployed to produce six-membered carhocyclic rings with a group V element in the ring (12). Alternatively, alkali metal salts of the borahenzene anion can be prepared by the inter- action of alkali metal cyanides with the bisborabenzene cobalt complex, (VI) (13).

MCN --C

(CSHSBR)~CO CKSCN C~HSBR-M+ (VI) M = K.Na

The hishorahenzene cobalt complex and the alkali metal salts of the borabenzene anion have served as starting mate- rials for a variety of other horabenzene complexes. A summary of twical svnthetic transformations of horabenzene com~lexes is f&d in-~igure 2.

The strone drivine force to the formation of metal com- plexes of t h i horagnzene anion is demonstrated in the thennolvsis of the neutral dienes in the presence of metal

There are two other chemical ohse~a t ions which relate to the nature of the horahenzene anion which should he men- tioned. The reaction of (IX) with acetic acid-d, yields cis- 1,3-pentadiene with the incorporation of three deuterium atoms; thus the negative charge is not localized at one carbon atom (11); furthermore the FriedelLCrafts acylation of bis(1-methy1borabenzene)iron (X) results in a monoacetylated product indicating some susceptibility to electrophilic suh- stitution (14). Ferrocene will undergo mono and diacyla- tion.

Recent studies have been extended to synthesis of five- and seven-membered carhocyclic species with one boron atom in the ring (20.21).

Figure 2. Synmeses of bansltlon metal complexes of the barabenlene mion.

Although the structure of the uncoordinated horahenzene anion, unfbrtunately, has not been determined, the crystal and molecular structures of some transition metal com~lexes of the horabenzene ion have been ohtained (22,23). he struc- ture of a bis(borabenzene)cohaIt(II) complex (22) is shown in Figure 3. The geometry of this, and related molecules is such that the horon atoms in the nearly parallel planar rings adopt a transoid arrangement. The cohalt atom is displaced away from the horon atoms towards the opposite end of the ring.

It is particularly instructive to examine structural param- eters which relate to the question of T electron delocalization within the horabenzene ring. The planarity of the ring implies s p 2 hybridization on each ring atom. The change in ring conformation is dramatic when one of the ring atoms exhibits sp3 hybridization e.g., in (XI).

The silicon atom is considerably displaced from the plane of the other five tine. atoms (241 'I'he presence of rinr planarity does not, of course, establish delocaiization through the boron atum.

The interatomic distnnces in the ccn,rdinated horabenaene ion show carhon-carhon hond lencths which are in the ranre of those observed in aromatic compounds (benzene = 1.39 A). The observation that the shorter carhon-carbon bonds are adjacent to the horon-carbon bonds suggests that the polarity of the horon-carbon a system results in greater localization of a charge density on the adjacent (as opposed to the distant) carhon-carbon bonds. The electronic effects which are man- ifested in the horon-carbon hond lengths are somewhat dif- ficult to evaluate. The sum of the covalent radii for horon and carbon is 1.55 A. which isloneer than the observed hond l e n d of 1.514 A. This difference &ld have its origins in a variSty ofeffens. not the least of which is the nossibilitv of n electron delocalization. The variation in s character in boron-carbon bonds will also play a significant role in establishing the boron-carbon distance, so i t is difficult to separate the a and r contributions to the observed bond lendh (25). Finallv. it h a heen proposed that in rertatn systems (e.g.. phorphonium vlides) short hond lengths may reflect increased coulomhic ittraction rather than-increased orbital overlap (26). One point in favor of the a delocalization proposal is that the horon-phenyl hond length in (XII) is 1.577 h (23).

In this complex the phenyl ring is twisted 14' with respect to the borobenzene ring, so this distance could he used to ap- proximate asp2 horon-spzcarhon hond length in the absence of a-conjunction.

498 I Journal of Chsmioal Education

Selected NMR Parameters for Borabenzene Derivatives

Coupling Canstantso Chemical Shinsd Refer-

Compound H3,S ti2,. " E d J ~ s J24 Jsl ence

r tn ppm. d ppm from BFrO(CzH&.

Half of an AH dnuhlet in with phenyl protons. I Hailof an AR doublet. , t,

8 Cp = r - cyeiopentadienyi In Hz.

The fact that the metal ion in these comvlexes is not located symmetrically in the center of the ring, but rather is displaced awav from the horon atom. has been attrihuted to the repul- sion"of two elertroposn~ve rentrrs (22) One could also biew this effect as demonstratinr the nreferenre of the metal ion for the electron rich part oflthe molecule.

A variety of spectroscopic studies have been carried out on horabenzene anions and related transition metal complexes. Typical nmr data are shown in the table. On going from the neutral diene (VIII) to the borahenzene anion (IX), the chemical shift of the ring proton para to the boron atom (HI) shifts dramatically downfield. All the horabenzene ring pro- tons are a t low field compared to the protons in the cyclo- pentadienide anion Thr ring protons i i (IX) are thus in the range expected for aromatic prutons. The decrease in Jp3 and inrrease in . I , . on eoing from IVIIII to 41x1 is consistent with 0 - e . . . . a change in hybridization (and hence in % s character) a t carbon 4. The ring proton chemical shift of (IX) shows some dependence on the nature of the IA cation (13) suggesting some anionlcation association. However, the concentration dependence of these shifts has not been explored so any con- clusions are tentative. The laree uvfield shift of the "B chemical shift of (IX) comparedio (CIII) is characteristic of a situation where neeative charee densitv is transferred to the horon atom (11,27);The 'H n& ~ a t t e i n of (XIII) with well separated chemical shifts is typical of a system without cyclic delocalization. The borahenzene complex, (IV), exhibits a spectrum with all ring protons shifts encom~assina anarrow range at relatively I& field thus s~~p.~esri& a delomlized system. The "H rhemical shift in (IVI is alaoconsistent with considerable charge density being transferred to the horon atom.

The electronic environments about the iron atom in ferro- cene and his(1-phenglhorahenzene)iron (X) have been com- . ~

pared using Mossbauer spectroscopy (14). The isomer shifts are identical in the two compounds; hence the s electron density on iron is unchanged on going from the cyclopenta- dienide to the l-phenylborahenzene ligand. The quadrupole s~l i t t ine in (XI is lower than in ferrocene. A similar reduction n , . of the quadrupole splitting is observed in ferrocenes with strong electron withdrawing substituents, thus suggesting that the l-phenylhorabenzene anion is a better acceptor (and hence weaker donor) than the cyclopentadienide anion.

The observation of significantly greater (0.8 to 1.0 eV)

C,H; (C.H,l,Co Co CO(CSH,BR)~ C , ~ B R -

Figure 4. Qualitative molecular abital energy level diagram fweDbaitocene ard bis (borabenzene) cobalt (11) (29).

ionization votentials for the bis-borabenzene iron (14) and . . - cobalt (28) complexes compared to the corresponding me- tallocenes is consistent with the model of imoroved acceotor and reduced donor abilities for the boraienzene jigand. Similarly, the ohservation that the CO stretching frequencies in l-phenylborahenzene manganese tricarbonyl (XII) are 19 cm-' higher than those for C5HsMn(C0)3 also supports this model (17).

For a more so~histicated treatment of the ionization no- tential data and bonding, one must consider some features of theq~~alitative molecular orhital (MOI descriotion (2.91 of the cyclopentadienide and horabenzene complixes which have been derived from electron spin resonance (esr) studies (see Fig. 4). The analysis of the experimentalg tensorsshows that, as opposed tocohaltocene, the unpaired electron in (VI) is in a MO which has significant d,z character. The reason for the difference in order of levels in cobaltocene and (VI) is that the removal of degeneracy in the highest filled MO of the bora- benzene anion results in differential amounts of interaction with the metal d,, and d,, orbitals and a lowering of the d,, with respect to the d,, orbital; i.e. the asymmetrical distri- bution of 7 electron density in the l i e a d ;s reflected in the differential interactions with the d acceptor orhitals on the metal. This change in the order of levels allows the unpaired electron to be placed in a lower energy orbital (compared to the unpaired electron in cohalhcene) and hence one ohsewes a larger ionization potential.

The quadmpole coupling data from the esr spectrum of (VI) gives evidence for considerable covalency in these types of molecules (29) The bonding is very similar to that found in ferrocene with ligand electrons donated to metal d,z, d,,, and d,, orhitals and back bonding from metal d,z-,z and d,, or- bital to ligand antihonding acceptor orhitals. The acceptor ahility of the borabenzene anion is calculated to be slightly less than the ligand in ferrocene. Therefore, there is a conflict I)etwren the interpr~tntinn ol'the Miiishausr and esr W 6 l l h

a3 to the dmor,acceptor ahility of the horahenzene anion with respect to the cyclopentadienide anion but all the data support the concept of the horahenzene anion exhibiting significant

donor and acceptor roles in these complexes. In conclusion, it appears that the horahenzene anion is a

system which is best described as a delocalized r system perturbed by the difference in horon and carbon orbital electronegativities. The resulting molecular orhitals have an asymmetrical distribution of ?r electron density with the car-

Volume 55, Number 8. August 1978 1 499

bocyclic portion of the ring being somewhat more electron rich than the boron center. The horabenzene anion acts as a good r donor and acceptor and consequently has a rich and varied r complex chemistry with the transition metals.

Literature Cited

(1) AUmck,H.R.,"Heteroatom~n~Sys~msandPol~em,"AeadcmiePmrs.NewYork. 1967: Mitchell, K.A. R., Chem. Reu.. 69.167 (1972).

(2) Niedenzu. K.. and Dawron. J. W.. "Boron~Nilroeen Chemistrv Comnounds? Springsr-Verlag, Berlin. 19% Nath. H., in "ltognss in Boron Chemirry,"3(~ditora: Brotherton. R. J.. and Steinhem. H.1 Pergaman. New York, 1970. ~ p . 211-312.

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(26) Whangho, M-H., Wolfe, %,and Bernardi, F., Can. J. Chsm., 53,3M0 (19751. (271 Davis, F. A,, Mwar, M. J. %and D a x s , R., J Amer Chem Soc.. SO, 706 (1881: &ton,

G. a.. and Lipammh. W. N.. "Nmr Studie. of Bomn Hvdrides and Relstrd Com- pounds," W. A. Benjamin, New York, 1969, pp. 437-616,562.

(28) Hcrherich.G. E..Greisr,G.. Hei1.H. F., Chrm Commun.. 1328 (1971). (29) Herherich, G. E.,Lund,T., and Raynor, J. R., J C. S. Doltan, 985 (19751.

500 / Journal of Chemical Education