Coupling of carbon monoxide to isocyanides and enones induced by alkyl Zr(IV) and Hf(IV) tropocoronand complexes

Full text of DOI:10.1021/ja9640861

J. Am. Chem. Soc. 1997, 119, 3411-3412
Coupling of Carbon Monoxide to Isocyanides and
3411
Enones Induced by Alkyl Zr(IV) and Hf(IV)
Tropocoronand Complexes
Michael J. Scott and Stephen J. Lippard*
Department of Chemistry, Massachusetts Institute of
Technology, Cambridge, Massachusetts 02139
ReceiVed NoVember 26, 1996
Although numerous group 4 organometallic compounds have
been prepared with tetraazamacrocyclic ancillary ligands includ-
ing porphyrins,1-5 few of these molecules undergo the C-C
bond-forming reactions for which their metallocene analogs are
celebrated. Despite similarities between the {Cp2M} and
{(N4-macrocycle)M} fragment frontier molecular orbitals,4 the
propensity for alkyl migration to the highly electrophilic imino
carbon atoms of many tetraazamacrocycles4,5 has severely
limited the ability of their metal complexes to promote reductive
coupling of ligated groups with small extrinsic substrates.
Multiple insertion and coupling reactions, in particular, are quite
uncommon owing to this decomposition pathway. Previous
work in this laboratory has demonstrated the tetraazamacrocyclic
tropocoronand, (TC-n,m)2-, to be a robust ligand. Moreover,
Figure 1. ORTEP diagrams with 50% thermal ellipsoids of [Hf(TC-
3,5)(CH2Ph)2] (1) and its CO insertion product, [Hf(TC-3,5)(η2-
OC(CH2Ph)2)] (2). Selected bond distances (Å) are as follows: for 1,
Hf(1)-C(23), 2.355(4); Hf(1)-C(30), 2.331(5); for 2, Hf(1)-O(1),
1.993(5); Hf(1)-C(13), 2.217(8); O(1)-C(13), 1.448(9). Primed and
unprimed atoms are related by a crystallographic mirror plane.
[4]arenes16 or by ligand exchange reactions.17-20 With group
4 macrocycles, an η2-ketone species similar to 2 was suggested
to form transiently upon exposure of a tetraazaannulene complex
to carbon monoxide, but this highly reactive fragment rapidly
migrated to the ligand, inducing its decomposition.4 In the
present case, an acyl group formed after initial insertion of CO
into the metal-alkyl bond in group 4 macrocyclic complexes
may have considerable carbenoid character, which would favor
migration of the second alkyl substituent and formation of the
η2-ketone moiety. Recent work with [Zr(TC-3,3)Ph2] and [Zr-
(TC-3,3)Me2] has confirmed the generality of this transformation
with tropocoronand complexes.21
The metrical parameters of the three-membered metalloxirane
ring in 2 indicate significant reduction in the bond order of the
C-O moiety and a short Hf-O bond of 1.993(5) Å (Figure 1).
The Hf-C bond distance of 2.217(8) Å is 0.10-0.15 Å smaller
than that in [Cp2Zr(η2-OCPh2)]2,22 [Cp2Zr(η2-OCPh2)(THF)]
(2.301(5) Å),19 or 1 (2.355(4) Å, 2.331(5) Å), further emphasiz-
ing the single-bond character of the C-O unit (1.448(9) Å).
Analogous group IV complexes of smaller tropocoronands such
as [Zr(TC-3,3)(η2-OC(CH2Ph)2)] have similar metalloxirane ring
geometric features.21
simple alterations in the size of the tropocoronand binding cavity
afforded control of the coordination geometry and reactivity at
the metal center in complexes with a variety of different
transition metals.6-10 With these properties in mind, we decided
to prepare group IV tropocoronand organometallic complexes,
[M(TC-n,m)R2], and explore their reactivity with substrates
including carbon monoxide, isocyanides, and enones, the first
results of which are described here.
At ambient temperature and pressure, [Hf(TC-3,5)(CH2Ph)2]
(1)11 inserts carbon monoxide into the metal-carbon bonds
affording [Hf(TC-3,5)(η2-OC(CH2Ph)2)] (2) in 64% yield.12
Crystallographic chemical analysis (CCA) revealed 2 to be the
first group 4 macrocyclic η2-ketone complex (Figure 1). By
contrast, group 4 metallocenes typically insert one CO into a
single M-C bond, forming acyl complexes13 or, in some
instances, enolates.14 Group 4 metallocene complexes contain-
ing η2-ketones and aldehydes can be obtained under forcing
conditions,15 through the use of ancillary ligands such as calix-
Metallocenes containing η2-ketone ligands such as [Cp2Zr-
(η2-OCPh2)]2 are very reactive, inserting substrates such as R,â-
unsaturated esters, phosphorus ylides, and aromatic hydrocar-
bons.22 Compound 2 is similarly susceptible to nucleophilic
attack; over the course of several hours, the Hf-C bond reacts
with dichloromethane affording [Hf(TC-3,5)(OCH(CH2Ph)2)Cl]
(3, Figure 2). When isocyanides are present, however, dichlo-
romethane solutions of 2 preferentially react with these ligands
to afford the ketenimine complex 4 in good yield (Figure 2).12
By comparison, a metallocene analogue, [(Cp2ZrCl)2(µ:η1(O),η2-
(C,O)CH2O)],17,18,23 containing a bound formaldehyde ligand
will insert only one isocyanide into the metal-carbon bond.
The resulting four-membered metallacyclic (η1-iminoacyl)-
zirconocene species suggests that 4 may arise through an
(1) Floriani, C.; Ciurli, S.; Chiesi-Villa, A.; Guastini, C. Angew. Chem.,
Int. Ed. Engl. 1987, 26, 70-72.
(2) Giannini, L.; Solari, E.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C.
Angew. Chem., Int. Ed. Engl. 1994, 33, 2204-2206.
(3) Brand, H.; Arnold, J. Organometallics 1993, 12, 3655-3665.
(4) Giannini, L.; Solari, E.; De Angelis, S.; Ward, T. R.; Floriani, C.;
Chiesi-Villa, A.; Rizzoli, C. J. Am. Chem. Soc. 1995, 117, 5801-5811.
(5) Black, D. G.; Swenson, D. C.; Jordan, R. F.; Rogers, R. D.
Organometallics 1995, 14, 3539-3550.
(14) Choukroun, R.; Douziech, B.; Soleil, F. J. Chem. Soc., Chem.
Commun. 1995, 2017-2018.
(15) Erker, G.; Dorf, U.; Czisch, P.; Petersen, J. L. Organometallics 1986,
5, 668-676.
(6) Villacorta, G. M.; Lippard, S. J. Pure Appl. Chem. 1986, 58, 1477-
1484.
(16) Giannini, L.; Solari, E.; Zanotti-Gerosa, A.; Floriani, C.; Chiesi-
Villa, A.; Rizzoli, C. Angew. Chem., Int. Ed. Engl. 1996, 35, 85-87.
(17) Schmuck, S.; Erker, G.; Kotila, S. J. Organomet. Chem. 1995, 502,
75-86.
(7) Davis, W. M.; Zask, A.; Nakanishi, K.; Lippard, S. J. J. Am. Chem.
Soc. 1985, 107, 3864-3870.
(8) Davis, W. M.; Zask, A.; Nakanishi, K.; Lippard, S. J. Inorg. Chem.
1985, 24, 3737-3743.
(18) Bendix, M.; Grehl, M.; Fro¨lich, R.; Erker, G. Organometallics 1994,
13, 3366-3369.
(9) Jaynes, B. S.; Ren, T.; Liu, S.; Lippard, S. J. J. Am. Chem. Soc.
1992, 114, 9670-9671.
(19) Peulecke, N.; Ohff, A.; Tillack, A.; Baumann, W.; Kempe, R.;
Burlakov, V. V.; Rosenthal, U. Organometallics 1996, 15, 1340-1344.
(20) Askham, F. R.; Carroll, K. M.; Alexander, S. J.; Rheingold, A. L.;
Haggerty, B. S. Organometallics 1993, 12, 4810-4815.
(21) Scott, M. J.; Lippard, S. J. To be submitted for publication.
(22) Erker, G.; Czisch, P.; Schlund, R.; Angermund, K.; Kru¨ger, C.
Angew. Chem., Int. Ed. Engl. 1986, 25, 364-365.
(23) Ro¨ttger, D.; Schmuck, S.; Erker, G. J. Organomet. Chem. 1996,
508, 263-265.
(10) Jaynes, B. S.; Doerrer, L. H.; Liu, S.; Lippard, S. J. Inorg. Chem.
1995, 34, 5735-5744.
(11) Scott, M. J.; Lippard, S. J. Submitted for publication.
(12) Complexes were isolated in yields of 31-64% and characterized
by crystallographic chemical analysis (CCA), elemental analysis, and NMR
and IR spectroscopy. Complete experimental details are provided as
Supporting Information.
(13) Erker, G. Acc. Chem. Res. 1984, 17, 103-109.
S0002-7863(96)04086-3 CCC: $14.00 © 1997 American Chemical Society

3412 J. Am. Chem. Soc., Vol. 119, No. 14, 1997
Communications to the Editor
Figure 3. Representation of the hydrolysis of 4 to afford the hydroxide
complex 8. Diagrams of 4 and 8 were generated from crystallographic
coordinates. Selected bond lengths (Å) are as follows: for 4, Hf(1)-
O(1), 2.012(3); Hf(1)-N(6), 2.155(4); O(1)-C(23), 1.402(6); for 8:
Hf(1)-O(1), 2.028(6); Hf(1)-OH, 2.075(6), Hf(1)-Nimine, 2.716(7);
O(1)-C(23), 1.390(10).
Figure 2. Schematic representation of the reaction of the η2-ketone
tropocoronand complex 2 with isocyanides, enones, and dichlo-
romethane. All diagrams were generated from crystallographic coor-
dinates and represent accurate structural perspectives of the complexes.
ORTEP diagrams and full crystallographic details for 3-8 are supplied
as Supporting Information.
Activated ketones also react with the metalloxirane unit in 2
in a manner analogous to pinacol coupling. One equiv of an
enone such as cyclohexenone (6)12 or trans-4-phenyl-3-buten-
2-one (7)12 (Figure 2) inserts into the Hf-C bond, generating a
tropocoronand diolato complex. We have also investigated
aspects of the hydrolysis of the newly formed C-C coupled
products obtained in this chemistry since, as with the reductive
coupling chemistry of CO previously studied in our laboratory,
unprecedented organic compounds might be accessed in this
manner.29 An initial product in the hydrolysis of 4 is the
hydroxide-ligated hafnium tropocoronand complex 8, obtained
following addition of 1 equiv of water to a dichloromethane
solution (Figure 3). From the nature of the product, identified
by CCA, the first step in the hydrolysis reaction would appear
to be coordination of water followed by proton transfer to the
bound nitrogen atom and rearrangement to form 8. In 8 the
metal-imine distance is very long, 2.716(7) Å, and the metal-
alkoxide and hydroxide bond distances are quite similar,
2.028(6) and 2.075(6) Å, respectively.
In conclusion, organometallic group 4 tropocoronand com-
plexes couple carbon monoxide to a variety of substrates
generating complexes incorporating ketenimine, diolate, and
cyanide ligands. In contrast to similar tetraazamacrocycles, the
TC-n,m ligand is relatively robust and not as readily attacked
by reactive intermediates in the coupling chemistry. This
potential has been further exploited in a series of novel chemical
transformations with this system, as will be reported shortly.
analogous intermediate. Although the η2-acyl complex Cp*2-
ThCl(η2-OCCH2Ph) reacts similarly with isocyanides,24 the
reductive coupling of CO + 2RCN to form first the η2-ketone
and then ketenimines is unprecedented. Even the multiple C-C
coupling of isocyanides to generate ketenimine moieties is an
unusual transformation with group 4 metallocenes and may
require high pressure.25 In contrast, a broad range of Zr and
Hf tropocoronand species with varying macrocycle ring size
and aryl or alkyl ligands will induce this transformation at room
temperature and pressure. Treatment of [Zr(TC-3,3)(η2-OCPh2)]
with 2 equiv of 2,6-dimethylphenyl isocyanide, for example,
affords a ketenimine product analogous to 4.21
The only alkyl or aryl isocyanide which afforded a different
product in the ketenimine synthesis was benzyl isocyanide.
Addition of 2 equiv of PhCH2NC to a dichloromethane solution
of 2 yielded 5, a rare example of group 4 complex containing
a terminal cyanide ligand,26 νCN ) 2067 cm-1 (Figure 2).
During the formation of 5, the first equiv of isocyanide most
likely inserts into the metalloxirane Hf-C bond, producing a
transient η1-iminoacyl complex. Reaction of this species with
an equiv of benzyl isocyanide might induce the cleavage of the
CN-benzyl bond, but the role of the metal center in facilitating
this reaction remains a matter to be investigated. Decyanation
reactions of benzyl and other isocyanides to afford cyanide
ligands have been previously noted.27,28
(24) Moloy, K. G.; Fagan, P. J.; Manriquez, J. M.; Marks, T. J. J. Am.
Chem. Soc. 1986, 108, 56-67.
(25) Valero, C.; Grehl, M.; Wingbermu¨hle, D.; Kloppenburg, L.;
Carpenetti, D.; Erker, G.; Petersen, J. L. Organometallics 1994, 13, 415-
417.
Acknowledgment. This work was supported by a grant from the
National Science Foundation. M.J.S. is grateful to the National Institute
of General Medical Science for a postdoctoral fellowship.
(26) Klouras, N.; Nastopoulos, V.; Tzavellas, N. Z. Anorg. Allg. Chem.
1995, 621, 1767-1770.
Supporting Information Available: Experimental procedures and
characterization data including X-ray structural details for all com-
pounds (75 pages). See any current masthead page for ordering and
Internet access instructions.
(27) Dewan, J. C.; Giandomenico, C. M.; Lippard, S. J. Inorg. Chem.
1981, 20, 4069-4074.
(28) Bell, A.; Lippard, S. J.; Roberts, M.; Walton, R. A. Organometallics
1983, 2, 1562-1572.
(29) Carnahan, E. M.; Protasiewicz, J. D.; Lippard, S. J. Acc. Chem.
Res. 1993, 26, 90-97.
JA9640861