Intramolecular nucleophilic substitution on coordinated borabenzenes: A new entry into boratabenzene complexes [5]

Full text of DOI:10.1021/ja9910661

8112
J. Am. Chem. Soc. 1999, 121, 8112-8113
Intramolecular Nucleophilic Substitution on
Coordinated Borabenzenes: A New Entry into
Boratabenzene Complexes
Markus A. Putzer, Jonathan S. Rogers, and
Guillermo C. Bazan*
Department of Chemistry, UniVersity of California
Santa Barbara, California 93106
ReceiVed April 5, 1999
Electrophilic complexes containing boratabenzene ligands1 are
finding applications in olefin polymerization and oligomerization
reactions.2 Most significant is the ability to control the reactivity
of boratabenzene catalysts by adjusting the degree of orbital
overlap between boron and its exocyclic substituent.3 Borataben-
zene catalysts have appeared with reactivities that complement
those observed with standard group 4 metallocenes.4 Furthermore,
while both cyclopentadienyl (Cp) and boratabenzene are formally
monoanionic 6π electron donors, boratabenzene is a weaker
donor.5 Isostructural complexes containing boratabenzene instead
of Cp therefore have a greater tendency for lower oxidation states
and offer altered mechanistic pathways for elementary reactions.6
One of the main difficulties in advancing a broader use of
boratabenzene complexes in industry7 and in noncatalytic reactions
useful for organic chemistry8 is the multistep synthesis of the
boratabenzene framework.9 Methods for coordination typically
make direct analogy to Cp chemistry, namely, salt metathesis by
addition of ligand salts to metal halides.10 In this paper we report
a new type of reaction that gives transition metal-boratabenzene
complexes directly from neutral borabenzene-base adducts.
As shown in Scheme 1, our approach involves the reaction of
a neutral borabenzene-base adduct11 (C5H5B-L′, 1‚L′, where
L′ ) PMe3 or Py) with a suitable early transition metal complex.
For example, addition of C5H5B-PMe3 (1‚PMe3) to Ph3Sc-
(THF)2 provides (C5H5B-Ph)ScPh2(THF) (2 in eq 1) quantita-
Figure 1. ORTEP view of 2. Thermal ellipsoids are shown at the 30%
probability level; hydrogen atoms were omitted for clarity.
Scheme 1
tively by 1H NMR spectroscopy. A single-crystal diffraction study
of 2 reveals that the molecule can be described as a three-legged
piano stool and that it contains a nearly planar boratabenzene
ligand (Figure 1). There is a slight slip-distortion of scandium
away from boron and toward the more electron-rich carbons of
the ring (in Å, average Sc-Cγ ) 2.545(3), average Sc-Câ )
2.571(3), Sc-CR ) 2.613(3)). The Sc-Cipso bond distances
(2.214(3) and 2.231(3) Å) lie within the range of the sum of their
covalent radii.12
Repeating the process by addition of 1 equiv of 1‚PMe3 to 2
in toluene affords the bis(boratabenzene)scandium complex (3
in eq 1). In THF, 3 exists as the colorless adduct (C5H5B-Ph)2-
ScPh(THF), 3(THF). Removal of THF by repeated chlorobenzene
condensation/evaporation cycles produces green [(C5H5B-Ph)2-
ScPh]2 (32). The nuclearity and composition of 32 was confirmed
by matching the isotopic distribution obtained by mass spectrom-
etry with calculated values. The reactions of 1‚PMe3 with (Me3-
13
SiCH2)3Sc(THF)2 or (PhMe2CCH2)3Sc(THF) are analogous to
those described for Ph3Sc(THF)2 by 1H NMR spectroscopy;
however, the resulting products are thermally unstable oils which
are difficult to obtain in pure form.
(1) (a) Herberich, G. E.; Holger, O. AdV. Organomet. Chem. 1986 25, 199.
(b) Herberich, G. E. Spec. Publ.sR. Soc. Chem. 1997, 201 (Advances in Boron
Chemistry), 211.
(2) (a) Bazan, G. C.; Rodriguez, G.; Ashe, A. J., III; Al-Ahmad, S.; Mu¨ller,
C. J. Am. Chem. Soc. 1996, 118, 2291. (b) Bazan, G. C.; Rodriguez, G.; Ashe,
A. J., III; Al-Ahmad, S.; Kampf, J. W. Organometallics 1997, 16, 2492.
(3) Rogers, J. S.; Bazan, G. C.; Sperry, C. K. J. Am. Chem. Soc. 1997,
119, 9305.
(4) (a) Transition Metals and Organometallics as Catalysts for Olefin
Polymerization; Kaminsky, W., Sinn, H., Eds.; Springer-Verlag: Berlin, 1988.
(b) Ziegler Catalysts; Fink, G., Mu¨lhaupt, R., Brintzinger, H.-H., Eds.;
Springer-Verlag: Berlin, 1995. (c) Brintzinger, H.-H.; Fischer, D.; Mu¨lhaupt,
R.; Rieger, B.; Waymouth, R. M. Angew. Chem., Int. Ed. Engl. 1995, 34,
1143.
(5) Sperry, C. K.; Bazan, G. C.; Cotter, W. D. J. Am. Chem. Soc. 1999,
121, 1513.
(6) Kowal, C. M.; Bazan, G. C. J. Am. Chem. Soc. 1996, 118, 10317.
(7) (a) Krishnamurti, R.; Nagy, S.; Etherton, B. P. U.S. Patent 5,554,775
(Occidental Chemical Corporation), September 10, 1996. (b) Herberich, G.
E. Ger. Patent DE 19549352, June 26, 1997.
(8) (a) ComprehensiVe Organometallic Chemistry; Abel, E. W., Stone, F.
G. A., Wilkinson, G., Eds.; Hegedus, L. S., Volume Ed.; Elsevier Science,
Ltd.: Oxford, 1995; Vol. 12. (b) Metallocenes: Synthesis, ReactiVity,
Applications; Togni, A., Halterman, R. L., Eds.; Wiley-VCH: Weinheim, 1998.
(9) (a) Ashe, A. J., III; Shu, P. J. Am. Chem. Soc. 1971, 93, 1804. (b)
Herberich, G. E.; Schmidt, B.; Englert, U.; Wagner, T. Organometallics 1993,
12, 2891. (c) Hoic, D. A.; Wolf, J. R.; Davis, W. M.; Fu, G. C. Organometallics
1996, 15, 1315. (d) Herberich, G. E.; Englert, U.; Schmidt, M. U.; Standt, R.
Organometallics 1996, 15, 2707.
Tetrabenzylzirconium and 1‚PMe3 react quickly in C6D6 to give
(C5H5B-CH2Ph)Zr(CH2Ph)3 (4 in eq 2). With group 4 complexes,
the pyridine adduct, 1‚Py, gives similar results to 1‚PMe3. The
formation of (C5H5B-CH2Ph)2Zr(CH2Ph)2 (5 in eq 2) from 4 and
1‚PMe3 is considerably slower, requiring 3 days at room tem-
perature. The molecular structure of 5 was confirmed by single-
crystal diffraction and is included in the Supporting Information.
Starting with Hf(CH2Ph)4 and 1‚PMe3 one obtains (C5H5B-CH2-
(10) Exceptions: (a) Herberich, G. E.; Greiss, G.; Heil, H. F. Angew. Chem.,
Int. Ed. Engl. 1970, 9, 805. (b) Herberich, G. E.; Englert, U.; Schmitz, A.
Organometallics 1997, 16, 3751.
(11) (a) Boese, R.; Finke, N.; Henkelmann, J.; Maier, G.; Paetzold, P.;
Reisenauer, H. P.; Schmid, G. Chem. Ber. 1985, 118, 1644. (b) Qiao, S.;
Hoic, D. A.; Fu, G. C. J. Am. Chem. Soc. 1996, 118, 6329.
(12) r(Sc) + r(C) ) 1.44 Å + 0.77 Å ) 2.21 Å, from Lange’s Handbook
of Chemistry, 13th ed.; Dean, J. A., Ed.; McGraw-Hill Book Company: New
York, 1985.
(13) Lappert, M. F.; Pearce, R. J. Chem. Soc., Chem. Commun. 1973, 126.
(14) Yang, X.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1991, 113,
3623.
(15) Chien, J. C. W.; Tsai, W.; Rausch, M. D. J. Am. Chem. Soc. 1991,
113, 8570.
10.1021/ja9910661 CCC: $18.00 © 1999 American Chemical Society
Published on Web 08/20/1999

Communications to the Editor
J. Am. Chem. Soc., Vol. 121, No. 35, 1999 8113
Ph)Hf(CH2Ph)3 (6) as yellow crystals. Figure 2 shows that 6 is a
mononuclear species and a slippage of the metal toward the carbon
atoms away from boron ring (in Å, average Hf-Cγ ) 2.519(3),
average Hf-Câ ) 2.575(3), Hf-CR ) 2.626(3). There is no
reaction between 6 and either 1‚PMe3 or 1‚Py.
Reaction of Zr(NMe2)4 with 1‚PMe3 in C6D6 at 80 °C for 12 h
gives (C5H5B-NMe2)Zr(NMe2)3 (7 in eq 3). A second equivalent
Figure 2. ORTEP view of 6. Thermal ellipsoids are shown at the 30%
probability level; hydrogen atoms were omitted for clarity.
Scheme 2
of borabenzene adduct does not react with 7, even after heating
to 80 °C for 48 h. The synthesis of 5 in high yield opens the
opportunity to examine for the first time how different activators
(i.e., MAO, B(C6F5)3,14 and [Ph3C][B(C6F5)4]15) affect the reactiv-
ity of the catalytic species. Polymer precipitation is observed for
the catalysts 5/B(C6F5)3 (76 kg PE/(mol Zr‚h), [Zr] ) 4.8 × 10-4
M) and 5/[Ph3C][B(C6F5)4] (250 kg PE/(mol Zr‚h), [Zr] ) 4.8 ×
10-4 M), while ethylene oligomers are produced by 5/MAO (440
kg OE/mol Zr‚h, [Zr] ) 4.2 × 10-4 M, 1000 Al/Zr ratio) upon
addition of 1 atm ethylene. Activators therefore control not only
the rate of ethylene consumption but also the molecular weight
of the resulting products.16
Our current thinking is that the boratabenzene ligand forms as
a result of an intramolecular nucleophilic attack on boron upon
coordination of borabenzene (Scheme 2). There is ample literature
precedent for the coordination of neutral arene rings to d0 metal
complexes.18 Since borabenzene adducts can be considered to have
a partial negative charge on the ring, such adducts should be more
favored. The tetraamide of zirconium is considerably slower than
the corresponding tetrabenzyl species, probably due to its exist-
ence as a dimer in the solid state and solution.19 Addition of THF
or Py, which compete for sites on the metal, retards the reaction.20
That the more electron-rich and smaller 1‚Py reacts faster than
1‚PMe3 suggests that ring coordination is rate-determining.
Exchange of exocyclic groups most likely occurs via a Meisen-
heimer-type21 intermediate of type A, in which borate formation
is accompanied by the binding of the metal to the back of the
ring in a pentadienyl-like fashion. Precedent also exists for attack
In view of the reactivity of Cp*2ScR complexes,17 our
expectation was that the scandium complexes would behave as
single component catalysts. However, no reaction was detected
when ethylene was added to 3(THF) or 32. It appears that the
smaller size and weaker donating abilities of boratabenzene
relative to Cp* discourage breakup of 32 into active mononuclear
species. Addition of 1 atm ethylene to a solution of 32 that has
been pretreated with H2 results in the production of 1-alkenes
(7.4 kg 1-alkenes/(mol Sc‚h); [Sc] ) 1.75 × 10-3 M).
22
by external nucleophiles on the boron atom in 1‚PMe3 and in
(1‚PMe3)Cr(CO)3.11b
In summary, we have developed a new reaction for the
synthesis of boratabenzene complexes which takes advantage of
the unique reactivity of the borabenzene ring and departs form
conventional Cp-based methodology. Scandium species as well
as alkylated zirconium and hafnium complexes are easily ac-
cessed. The availability of compound 5 allows for the comparison
of the effect of different activators on the activity of ethylene
polymerization catalysts. The high yield of reactions represented
in Scheme 1 should encourage further applications of borataben-
zene complexes.
The pathway by which neutral borabenzene converts into the
formally monoanionic boratabenzene ligand, in conjunction with
the metal ligand exchange is novel and merits examination. The
following observations have been made: (a) the reaction is first
order in 1 and in the metal complex and proceeds without
formation of detectable intermediates; (b) 1‚Py reacts considerably
faster than 1‚PMe3; and (c) the addition of bases, such as THF
or Py, retards product formation.
(16) For a role of activator in standard metallocenes, see: (a) Mashima,
K.; Nakayama, Y.; Nakamura, A. AdV. Polym. Sci. 1997, 133, 1. (b) Chien,
J. C. W.; Song, W.; Rausch, M. D. J. Polym. Sci., Part A: Polym. Chem.
1994, 32, 2387.
(17) Thompson, M. E.; Baxter, S. M.; Bulls, A. R.; Burger, B. J.; Nolan,
M. C.; Santarsiero, B. D.; Schaefer, W. P.; Bercaw, J. E. J. Am. Chem. Soc.
1987, 109, 203.
Acknowledgment. G.C.B. is an Alfred Sloan Fellow and a Henry
and Camille Dreyfus Teacher Scholar. The authors are grateful to these
agencies and to the Department of Energy and Equistar Chemicals for
financial assistance, to Dr. Rene J. Lachicotte and Dr. Xianhue Bu for
assistance in structural determinations. We also acknowledge the Deutsche
Forschungsgemeinschaft for M.A.P.’s postdoctoral fellowship.
(18) For leading references, see: (a) Gillis, D. J.; Tudoret, M.; Baird, M.
C. J. Am. Chem. Soc. 1993, 115, 2543. (b) Lancaster, S. J.; Robinson, O. B.;
Bochmann, M.; Coles, S. J.; Hursthouse, M. B. Organometallics 1995, 14,
2456. (c) Gillis, D. J.; Quyoum, R.; Tudoret, M.; Wang, Q.; Jeremic, D.;
Roszak, A. W.; Baird, M. C. Organometallics 1996, 15, 3600. For a review
of group 4 arene complexes, see: (d) Troyanov, S. J. Organomet. Chem. 1994,
475, 139. For an example of a neutral zirconium arene complex, see: (e)
Musso, F.; Solari, E.; Floriani, C.; Schenk, K. Organometallics 1997, 16, 4889.
(19) Chisholm, M. H.; Hammond, C. E.; Huffman, J. C. Polyhedron 1998,
7, 2515.
(20) (a) Felton, J. J.; Anderson, W. P. J. Organomet. Chem. 1972, 36, 87.
(b) Clarke, J. F.; Fowles, G. W. A.; Rice, D. A. J. Organomet. Chem. 1974,
74, 417. (c) Felton, J. J.; Anderson, W. P. J. Organomet. Chem. 1974, 82,
375.
Supporting Information Available: Complete details for the syn-
thesis of all compounds, the polymerization conditions, and the crystal-
lographic studies of 2, 5, and 6 (PDF). This material is available free of
charge via the Internet at http://pubs.acs.org.
JA9910661
(21) March, J. AdVanced Organic Chemistry, 4th ed.; John Wiley & Sons:
New York, 1992; Chapter 13, p 641.
(22) (a) Hoic, D. A.; Davis, W. M.; Fu, G. C. J. Am. Chem. Soc. 1995,
117, 8480. (b) Qiao, S.; Hoic, D. A.; Fu, G. C. J. Am. Chem. Soc. 1996, 118,
6329.