Organometallics 1996, 15, 387-393
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Aminoboratabenzenes. An Evaluation of the Exocyclic B-N Interaction Arthur J. Ashe, III,* Jeff W. Kampf, Christian Mu¨ller, and Manuela Schneider Department of Chemistry, The University of Michigan, Ann Arbor, Michigan 48109-1055 Received August 10, 1995X
The reaction of 1-chloro-1-boracyclohexa-2,5-diene with diisopropylamine or N-benzyl-Nmethylamine followed by reaction with LDA in THF affords lithium N,N-diisopropyl-1aminoboratabenzene (9b) or lithium N-benzyl-N-methyl-1-aminoboratabenzene (9c), respectively. The reaction of 9b with FeCl2 affords bis(N,N-diisopropyl-1-aminoboratabenzene)iron (18b), while the reaction of 9b or 9c with Mn(CO)3(CH3CN)3PF6 affords tricarbonyl[N,Ndiisopropyl-1-aminoboratabenzene)manganese (19b) or tricarbonyl-1-(N-benzyl-N-methylamino) boratabenzene manganese (19c), respectively. Photochemical displacement of CO by PMe3 from 19b affords dicarbonyl(trimethylphosphine) (N-benzyl-N-methyl-1-aminoboratabenzene)manganese (20b). The rates of bond rotation about the B-N bonds of 9c, 18b, 19b,c, and 20b have been determined using variable-temperature NMR spectroscopy. The rates depend markedly on the electron-withdrawing power of the coordinating metal group. The boron atom is weakly coordinated to manganese (B-Mn ) 2.485(3) Å) and strongly bound to nitrogen (B-N ) 1.417 (3) Å). Introduction [1H]-Boratabenzene (1a) (see Chart 1) is an aromatic 6 π-electron ring which is isoelectronic with benzene (2) and pyridinium (3).1-3 However since boratabenzenes are anionic, their chemistry is more analogous to that of cyclopentadienide (4). Thus 1-methylboracyclohexadiene (5b) is even more acidic than cyclopentadiene (6) in both the gas phase4a and in solution.1b,4b Similarly 1-substituted boratabenzenes can serve as anionic ligands toward transition metals.1,3,5,6 A large number of complexes of both the sandwich (e.g. 7)3,5 and metal carbonyl (e.g. 8) types are known. These complexes are closely analogous to the corresponding cyclopentadienyl metal complexes. However boratabenzenes diverge from cyclopentadienyl in that exocyclic B-substituents may interact with boron to change the electronic character of the ring. Herberich et al. have argued that strong π-donation by the nitrogen lone pair in N,N-dimethyl-1-aminoboratabenzene (9a) saturates the boron and greatly attenuates the C-B ring delocalization.1,7 In other words cyclohexadienyl resonance structure II dominates over aromatic resonance structure I (Scheme 1). In order to probe this question we have prepared and Abstract published in Advance ACS Abstracts, December 1, 1995. (1) (a) Herberich, G. E.; Greiss, G.; Heil, H. F. Angew. Chem., Int. Ed. Engl. 1970, 9, 805. (b) Herberich, G. E.; Schmidt, B.; Englert, U. Organometallics 1995, 14, 471. (2) (a) Ashe, A. J., III; Shu, P. J. Am. Chem. Soc. 1971, 93, 1804. (b) The 11B NMR chemical shift reported for lithium 1-phenylboratabenzene in ref 2a is wrong. The correct value is δ ) 32.2. (3) (a) Herberich, G. E. In Comprehensive Organometallic Chemistry; Wilkinson, G., Stone, F. G. A, Abel, E. W., Eds.; Pergamon: Oxford, 1982; Vol. 1, p 381. (b) Herberich, G. E.; Ohst, H. Adv. Organomet. Chem. 1986, 25, 199. (4) (a) Sullivan, S. A.; Sandford, H.; Beauchamp, J. L.; Ashe, A. J., III. J. Am. Chem. Soc. 1978, 100, 3737. (b) Sandford, H. Ph.D. Dissertation, University of Michigan, 1979. (5) Ashe, A. J., III; Meyers, E.; Shu, P., Von Lehmann, T.; Bastide, J. J. Am. Chem. Soc. 1975, 97, 6865. (6) (a) Herberich, G. E.; Becker, H. J. Angew. Chem., Int. Ed. Engl. 1973, 12, 764. (b) Huttner, G.; Gartzke Chem. Ber. 1974, 107, 3786. (7) Herberich, G. E.; Schmidt, B.; Englert, U.; Wagner, T. Organometallics 1993, 12, 2891. X
0276-7333/96/2315-0387$12.00/0
Chart 1. Structures 1-8
Scheme 1. Resonance Structures of Aminoboratabenzenes
examined various metal derivatives of N,N-diisopropyl1-aminoboratabenzene (9b) and N-benzyl-N-methyl-1aminoboratabenzene (9c). The strength of exocyclic B-N π-bonding has been estimated by evaluating the barriers to rotation about these bonds. We propose that © 1996 American Chemical Society
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Organometallics, Vol. 15, No. 1, 1996 Scheme 2. Synthesis of Compounds
Ashe et al. Chart 2
boratabenzene)lithium has a similar structure (16),11 it seems highly likely that the THF solutions of 9a-c also adopt this arrangement (17). The reaction of 9b with FeCl2 affords the bis(boratabenzene)iron complex 18b isolated as beautiful deep red plates. The compound is mildly oxygen sensitive in solution but air stable in crystalline form. Similarly the reaction of 9b with Mn(CO)3(CH3CN)3 PF6 affords 19b as yellow crystals. Photochemical replacement of a carbonyl group by PMe3 affords 20b. In a similar manner 9c was converted to 19c for which we have obtained a crystal structure. Results and Discussion
the relative importance of resonance structures I and II depends markedly on the electron-withdrawing power of the coordinating metal group. Syntheses. Lithium N,N-dimethyl-1-aminoboratabenzene (9a) is easily prepared by the base-induced ring closure of bis(dimethylamino)-2,4-pentadienylborane (10a)7 (Scheme 2). However since this method fails for bulky amine substituents, it was necessary to prepare 9b and 9c by the older organotin route.2 The reaction of 1-chloroboracyclohexa-2,5-diene (12)8 with excess N,N-diisopropylamine gives the corresponding aminoborane 14b, which can be easily separated from insoluble hydrochloride of the amine. Treating 14b with a trace of LDA in THF converts it to 1:1 mixture of 14b and its conjugated isomer 13b. On the other hand, the reaction of 12 with N-benzyl-N-methylamine always afforded a mixture of the conjugated and unconjugated isomers 13c and 14c in the ratio 55:45, respectively. The rather complicated 1H NMR spectrum indicates that 13c is a mixture of E and Z isomers. No attempt was made to separate the isomers. Treating 14b with LDA in THF-d8 affords clean solutions of 9b. Similar reaction of the mixed isomers 13c and 14c with LDA gives 9c. The 1H, 11B, and 13C NMR spectra of 9b,c show typical boratabenzene patterns which closely resemble those reported for 9a. The 7Li NMR spectra show signals at -4.8 and -6.6 for 9b,c, respectively. These chemical shift values are similar to those of the LiCp dimer 159 (δ ) -8.4) (Chart 2) which has a bis-sandwich structure.10 Since (1H(8) Maier, G.; Henkelmann; Reisenauer, H. P. Angew. Chem., Int. Ed. Engl. 1985, 24, 1065.
In general the B-N bonds of aminoboranes have double-bond character.12 The barriers to rotation about B-N bonds in the range 10-24 kcal/mol have been evaluated using variable-temperature NMR spectroscopy.13 The barriers depend upon both steric and electronic effects, and in particular π-donor substituents on boron lower the barriers by weakening the B-N π bond. Earlier work has involved examination of five-14 and seven-membered15 boron heterocycles with pendant amino groups. There is no prior report on the rotational barriers of aminoboratabenzenes. We first examined 14c. The 13C NMR spectrum in CDCl3 at 25 °C shows two signals (δ 147.88 and 147.75) for the carbon atoms β to boron. Thus B-N rotation must be slow on the NMR time scale. On warming of the sample to 122 °C, these signals coalesce from which a ∆Gq value of 20.7 kcal/mol can be derived in the usual manner. This value is only slightly higher than that obtained for the corresponding dihydroborepin (∆Gq ) 19.8 kcal/mole).15 Since the six-membered rings of the boratabenzenes must be similar in size to 14c, this (9) (a) Cox, R. H.; Terry, H. W., Jr. J. Magn. Reson. 1974, 14, 317. (b) Paquette, L. A.; Bauer, W.; Sivik, M. R.; Bu¨hl, M.; Feigel, M.; Schleyer, P. v. R. J. Am. Chem. Soc. 1990, 112, 8776. (10) (a) Harder, S.; Prosenc, M. H. Angew. Chem., Int. Ed. Engl. 1994, 33, 1744. (b) Stalke, D. Angew. Chem., Int. Ed. Engl. 1994, 33, 2168. (11) Hoic, D. A.; Davis, W. M.; Fu, G. C. J. Am. Chem. Soc. 1995, 117, 8480. (12) (a) Greenwood, N. N.; Thomas, B. S. In Comprehensive Inorganic Chemistry; Bailar, J. C., Ed.; Pergamon: Oxford, U.K., 1973; pp 925-935. (b) Pelter, A.; Smith, K. In Comprehensive Organic Chemistry; Barton, D., Ollis, N. D., Eds.; Pergamon: Oxford, U.K., 1979; Vol. 3, pp 925-932. (13) (a) Dewar, M. J. S.; Rona, P. J. Am. Chem. Soc. 1969, 91, 2259. (b) Imbery, D.; Jaeschke, A.; Friebolin, H. Org. Magn. Reson. 1970, 2, 271. Friebolin, H.; Rensch, R.; Wendel, H. Org. Magn. Reson. 1976, 8, 287. (c) Brown, C.; Cragg, R. H.; Miller, T. J.; Smith, D. O’N. J. Organomet. Chem. 1983, 244, 209 and earlier papers in this series. (14) (a) Herberich, G. E.; Negele, M.; Ohst, H. Chem. Ber. 1991, 124, 25. (b) Siebert, W.; Full, R.; Edwin, J.; Kinberger, K.; Kru¨ger, C. J. Organomet. Chem. 1977, 131, 1. (c) Herberich, G. E.; Ohst, H. Chem. Ber. 1985, 118, 4303. (d) Herberich, G. E.; Englert, U.; Hostalek, M.; Laven, R. Chem. Ber. 1991, 124, 17. (15) Ashe, A. J., III; Klein, W.; Rousseau, R. J. Organomet. Chem. 1994, 468, 21.
Aminoboratabenzenes
barrier height should be an appropriate model for the B-N barrier without the electronic effects of boratabenzene. At ambient temperature THF-d8 solutions of 9c show 1H and 13C NMR spectra with single signals for CH(2)/CH(6) and for CH(3)/CH(5). Thus B-N rotation must be fast on the NMR time scale. Cooling the solution of 9c to -87 °C caused the signal for C(3)/C(5) to separate (δ 135.07, 134.85). Tc was at -72 °C from which a value of ∆Gq ) 10.1 kcal/mol can be derived. We believe that this nonequivalence is the consequence of slow B-N rotation, although the possibility that there may be slow exchange between different aggregations of the lithium salt cannot be excluded. However the observation that the N-CH3 and N-CH2 signals do not simultaneously separate makes the latter explanation less likely. There is greater than a 10 kcal/mol decrease in the B-N π bonding in the boratabenzene (9c) compared with the boracyclohexadiene (14c). This indicates that incorporation of the boron atom into the aromatic boratabenzene ring greatly diminishes its ability to form an external π-bond to nitrogen. The B-N π bonding in aminoboratabenzenes should affect the acidity of the corresponding boracyclohexadienes. Prior qualitative observations indicate that 5b is a stronger acid than cyclopentadiene in THF4b and in water.1b However there are no prior measurements of the acidity of an aminoboracyclohexadiene. We have chosen to measure the pKa of 14b in DMSO because of the large amount of comparable acidity data available in this solvent.16 Treating a DMSO-d6 solution of 14b with 1 equiv of lithium cyclopentadienide gives a complicated mixture of 4, 6, 9b, 13b, and 14b which can be analyzed by H NMR spectroscopy. On the basis of the pKa(DMSO) ) 18.0 for cyclopentadiene,16 pKa(DMSO) ) 17.7 and 17.8 for 13b and 14b, respectively. Thus 13b and 14b are nearly equivalent in acidity to cyclopentadiene. On the other hand, 5b is a much stronger acid. Treating 5b in DMSO with either 4 or 9b quantitatively converts it to 1b. Similarly treating a DMSO solution of 5b with 1 equiv of the much weaker base lithium pentaphenylcyclopentadienide converts it to 1b and 1,2,3,4,5-pentaphenylcyclopentadiene (pKa(DMSO) ) 12.5).16 The data for the acidity of 14b can be combined with the B-N rotational data for 14c and 9c in a selfconsistent manner. (It will be assumed that the B-N rotational barriers are the same for 9b,c and for 14b,c.) The structure of the transition states for B-N rotation of aminoboranes have been evaluated computationally.17 Relative to the planar aminoboranes the major changes involve a 90° rotation about the B-N bond, a lengthening of that bond, and pyramidalization of the nitrogen. Since the nitrogen lone pair is now orthogonal to the boron pz orbital, π-bonding is absent. The transition states for B-N rotation of 14b and 9b are indicated as structures 14b′ and 9b′, respectively. Scheme 3 illustrates the application of a Hess’s law cycle to the acid-base reactions of 9b, 9b′, 14b, and 14b′. It can be seen that the pKa(DMSO) of 14b′ is 10.2. Thus 14b′ is a considerably stronger acid than 14b. (16) Bordwell, F. G. Acc. Chem. Res. 1988, 21, 456. (17) (a) Dill, J. D.; Schleyer, P. v. R.; Pople, J. A. J. Am. Chem. Soc. 1975, 97, 3402. (b) Bentley, T. W. J. Org. Chem. 1982, 47, 60.
Organometallics, Vol. 15, No. 1, 1996 389 Scheme 3. Hess’s Law Cycle for B-N Rotation and Acidity of N,N-Diisopropylaminoboracyclohexadiene with ∆G Values Expressed in kcal/mol
Since 5b like 14b′ lacks π-bonding to the exocyclic substituent, its acidity should be similar. Indeed the experimental observation that 5b is a stronger acid than pentaphenylcyclopentadiene indicates that its pKa (
