Tris(pyrrolyl-α-methyl)amines that sterically protect a trigonal metal site

Article abstract of DOI:10.1021/ic701472y

Three substituted tris(pyrrolyl-α-methyl)amines (H ₃[Aryl₃TPA]) (Aryl = 2,4,6-C₆H ₂Me₃, 2,4,6-C₆H₂(i-Pr)₃ (Trip), or 3,5-C₆H₃(CF₃)₂) have been prepared. An X-ray study of [Trip₃TPA]MoCl shows it to be a distorted trigonal bipyramidal species in which the 2,4,6-triisopropylphenyl substituents surround and protect the apical chloride. Attempts to prepare other Mo, Zr, and Hf complexes yielded species in which one pyrrole-containing arm remained free (Mo) or dimethylamine remained in the coordination sphere of [Aryl₃TPA]^3- complexes (Zr, Hf).

Full text of DOI:10.1021/ic701472y

Inorg. Chem. 2007, 46, 8463−8465
Tris(pyrrolyl-
Metal Site
r-methyl)amines that Sterically Protect a Trigonal
Keith M. Wampler and Richard R. Schrock*
Department of Chemistry 6-331, Massachusetts Institute of Technology,
Cambridge, Massachusetts 02139
Received July 24, 2007
Three substituted tris(pyrrolyl-
2,4,6-C6H2Me3, 2,4,6-C6H2(i-Pr)3 (Trip), or 3,5-C6H3(CF3)2) have
been prepared. An X-ray study of [Trip3TPA]MoCl shows it to be
distorted trigonal bipyramidal species in which the 2,4,6-
R
-methyl)amines (H3[Aryl3TPA]) (Aryl
and aryl halides reported by Sadighi.7 The H3[Aryl3TPA]
species where Aryl ) 2,4,6-C6H2Me3 (Mes, 1a),8 2,4,6-C6H2-
(i-Pr)3 (Trip, 1b), or 3,5-C6H3(CF3)2 (ArF, 1c) can then be
prepared through a triple Mannich reaction in either ethanol/
water or isopropanol/water at 55 °C (eq 1).6 As one would
)
a
triisopropylphenyl substituents surround and protect the apical
chloride. Attempts to prepare other Mo, Zr, and Hf complexes
yielded species in which one pyrrole-containing arm remained free
(Mo) or dimethylamine remained in the coordination sphere of
-
[Aryl3TPA]3 complexes (Zr, Hf).
We have been exploring complexes of the earlier
metals that contain a substituted triamidoamine ligand
([(RNCH2CH2)3N]3-),1 especially versions in which R is an
aryl group,2 and in particular molybdenum complexes in
which R is 3,5-(2,4,6-i-Pr3C6H2)2C6H3 or HIPT (HexaIso-
PropylTerphenyl).3-5 The main focus is reduction of di-
nitrogen, i.e., complexes such as [HIPTN3N]Mo(N2)
([HIPTN3N]3- ) [(HIPTNCH2CH2)3N]3-) will catalyze the
reduction of dinitrogen with protons and electrons at room
temperature and pressure.4 A trianionic ligand that sterically
protects a trigonal coordination site is a key feature of these
catalysts. An interest in other trianionic four-coordinate
ligands that might enforce a trigonal sterically protected
binding site drew us to substituted variations of tris(pyrrolyl-
R-methyl)amine (H3TPA) introduced by Odom.6
expect, Mannich reactions that produce 1a, 1b, and 1c require
both higher temperatures and longer reaction times than
reactions involving pyrrole itself.
Attempted syntheses of [Aryl3TPA]MoCl species followed
the protocol established for synthesizing [HIPTN3N]MoCl.3
Addition of either 1a or 1c to MoCl4(THF)2 in THF followed
by 3 equiv of LiN(TMS)2 led to intractable mixtures.
However, the analogous reaction with 1b gave [Trip3TPA]-
MoCl (2) in low isolated yield (14%) after crystallization
from pentane at -30 °C. (Compound 2 is highly soluble,
and crystallization requires weeks.) The 1H NMR spectrum
of 2 at 23 °C (benzene-d6) is broad and featureless, consistent
with a five-coordinate Mo(IV) complex of this general type
in which S ) 1.3 The measured µeff (Evans’ method9) of
2.34 µB is consistent with this proposal.
A single-crystal X-ray diffraction study of 2 revealed it
to be a pseudotrigonal bipyramidal molecule in which the
three pyrrolyl nitrogens occupy approximately equatorial
positions and chloride and N(4) occupy axial positions
(Figure 1). The Mo(1)-Cl(1) and Mo(1)-N(4) bond lengths
are similar to the analogous distances in crystallographically
The desired substituted pyrroles can be prepared through
Pd-catalyzed cross-coupling between pyrrolylzinc chloride
* To whom correspondence should be addressed. E-mail: rrs@mit.edu.
(1) Schrock, R. R. Acc. Chem. Res. 1997, 30, 9.
(2) (a) Greco, G. E.; Schrock, R. R. Inorg. Chem. 2001, 40, 3850. (b)
Greco, G. E.; Schrock, R. R. Inorg. Chem. 2001, 40, 3860.
(3) (a) Yandulov, D. V.; Schrock, R. R. J. Am. Chem. Soc. 2002, 124,
6252. (b) Yandulov, D. V.; Schrock, R. R.; Rheingold, A. L.;
Ceccarelli, C.; Davis, W. M. Inorg. Chem. 2003, 42, 796.
(4) (a) Yandulov, D. V.; Schrock, R. R. Science 2003, 301, 76. (b)
Yandulov, D.; Schrock, R. R. Inorg. Chem. 2005, 44, 1103. (c)
Schrock, R. R. Acc. Chem. Res. 2005, 38, 955.
(5) (a) Ritleng, V.; Yandulov, D. V.; Weare, W. W.; Schrock, R. R.; Hock,
A. R.; Davis, W. M. J. Am. Chem. Soc. 2004, 126, 6150. (b) Weare,
W. W.; Schrock, R. R.; Hock, A. S.; Mu¨ller, P. Inorg. Chem. 2006,
45, 9185. (c) Weare, W. W.; Dai, C.; Byrnes, M. J.; Chin, J.; Schrock,
R. R. Proc. Nat. Acad. Sci. 2006, 103, 17099.
(7) Rieth, R. D.; Mankad, N. P.; Calimano, E.; Sadighi, J. P. Org. Lett.
2004, 6, 3981.
(8) The mesityl-substituted ligand has also been reported elsewhere; see
Betley, T. A.; Surendranath, Y.; Childress, M. V.; Alliger, G. E.; Fu,
R.; Cummins, C. C.; Nocera, D. G. Philos. Trans. R. Soc., Ser. B
Submitted for publication.
(9) (a) Evans, D. F. J. Chem. Soc. 1959, 2003. (b) Schubert, E. M. J.
Chem. Educ. 1992, 69, 62. (c) Grant, D. H. J. Chem. Educ. 1995, 72,
39.
(6) Shi, Y.; Cao, C.; Odom, A. L. Inorg. Chem. 2004, 43, 275.
10.1021/ic701472y CCC: $37.00
Published on Web 09/14/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 21, 2007 8463

COMMUNICATION
Figure 1. POV-Ray rendering of 2 with thermal ellipsoids at 50%
probability. Hydrogen atoms and co-crystallization pentane omitted; (Å)
Mo-N(1) ) 2.004(3), Mo-N(2) ) 2.009(3), Mo-N(3) ) 2.009(3),
Mo-N(4) ) 2.213(3), Mo-Cl(1) ) 2.3095(13).
Figure 2. POV-Ray rendering of 3 with thermal ellipsoids at 50% prob-
ability. Hydrogen atoms, co-crystallized dichloromethane, and minor compo-
nent of disorder are omitted; (Å) Mo-N(1) ) 1.657(2), Mo-N(2) )
2.330(2), Mo-N(3) ) 2.108(2), Mo-N(4) ) 2.096(2), Mo-N(6) )
1.926(2).
Scheme 1
Figure 3. POV-Ray rendering of one of the two independent molecules
in the asymmetric unit of 5 with thermal ellipsoids at 50% probability.
Hydrogen atoms and co-crystallized dichloromethane omitted; (Å) Zr-N(1)
) 2.346(2), Zr-N(2) ) 2.199(2), Zr-N(3) ) 2.220(2), Zr-N(4) )
2.221(2), Zr-N(5) ) 2.040(3), Zr-N(6) ) 2.441(3).
characterized substituted TREN analogues5a,10 and the aver-
age Mo(1)-NPyrrolyl bond length (2.007 Å) is similar to what
has been observed in recently prepared Mo(η1-Pyrrolyl)
complexes.11 The Trip substituents interdigitate to form a
cavity 6.2 Å deep, which is ∼0.9 Å shallower than that found
in [(3,5-(2,4,6-t-Bu3C6H2)2C6H3NCH2CH2)3N]MoCl.5a Com-
pound 2 appears to be thermally unstable at 22 °C under
dinitrogen over a period of days.
nitrogen, and a dimethylamide comprise the basal coordina-
tion set. Close inspection of the structure reveals what
appears to be a hydrogen bond between the proton of the
remaining pyrrole and the nitride, judging from the fact that
the distance between N(1) and N(5) (2.893(3) Å) is shorter
than the sum of their Van der Waals radii (3.10 Å). Attempts
to form a trispyrrolyl complex through addition of bases
(LiN(TMS)2, NEt3, n-BuLi, etc.) or acids (2,6-lutidinium
chloride or N,N-dimethylanilinium chloride), as well as
attempts at substituting the dimethylamido ligand with a
halogen using TMSCl or TMSI, have led either to no reaction
or to decomposition.
Dimethylamido complexes were also employed as precur-
sors (Scheme 1). Reactions of either 1a or 1b with Mo(N)-
(NMe2)4 or M(NMe2)4 (M ) Mo, Zr, or Hf) were incomplete
in the temperature range studied (23-100 °C; 12-48 h). In
contrast, a reaction between 1c and Mo(N)(NMe2)3 in
dichloromethane at 22 °C (Scheme 1) yielded the highly
crystalline complex, Mo(N)(NMe2)(H[ArF TPA]) (3). Solu-
3
tion 1H and 19F NMR at 23 °C (CD2Cl2) confirmed that 3 is
a Cs symmetric species containing one unreacted (protonated)
“arm”.
Reaction of 1c with Mo(NMe2)4 in pentane at -30 °C
(Scheme 1) readily yields the highly crystalline five-
coordinate complex Mo(NMe2)2(H[ArF TPA]) (4) in which
3
The proposed structure of 3 was confirmed through a
single-crystal X-ray diffraction study (Figure 2). Complex
3 has a distorted square-pyramidal geometry in which the
nitride occupies the axial position. Two pyrrolyls, the donor
again only two pyrrolyls are attached to the metal. Reso-
1
nances in the H and 19F NMR spectra of 4 at 23 °C (in
CD2Cl2) were slightly paramagnetically shifted. A single-
crystal X-ray study revealed that 4 is a square-based
pyramidal geometry similar to 3 with a dimethylamide
occupying the axial position rather that a nitride. (see
Supporting Information.) The “free arm” of the ligand in 4
is pointed “down” instead of “up”, as it is in 3.
(10) (a) Kol, M.; Schrock, R. R.; Kempe, R.; Davis, W. M. J. Am. Chem.
Soc. 1994, 116, 4382. (b) Duan, Z.; Verkade, J. G. Inorg. Chem. 1995,
34, 1576.
(11) (a) Katayev, E.; Li, Y.; Odom, A. L. Chem. Commun. 2002, 838. (b)
Al Obaidi, N.; Brown, K. P.; Edwards, A. J.; Hollins, S. A.; Jones, C.
J.; McCleverty, J. A.; Neaves, B. D. Chem. Commun. 1987, 1733. (c)
Hock, A.; Schrock, R. R.; Hoveyda, A. H. J. Am. Chem. Soc. 2006,
128, 16373.
In contrast to the results obtained with molybdenum,
reactions between 1c and either Zr(NMe2)4 or Hf(NMe2)4 in
8464 Inorganic Chemistry, Vol. 46, No. 21, 2007

COMMUNICATION
dichloromethane yielded Zr(NMe2)(NHMe2)[ArF TPA] (5)
complexes. Whether [Ar3TPA]Mo complexes have the subtle
balance of properties required for catalytic dinitrogen reduc-
tion remains to be determined. However, complexes that
contain protective [Aryl3TPA]3- ligands could prove useful
for other purposes.8
3
or Hf(NMe2)(NHMe2)[ArF TPA] (6) in good yield (87%
3
1
and 85%, respectively). H and 19F NMR spectra at 23 °C
(CD2Cl2) show that 5 and 6 have C3 symmetry on the NMR
time scale. At this stage, we believe that C3 symmetry results
from dissociation of dimethylamine. A single-crystal X-ray
study of 5 shows that it adopts a highly distorted octahedral
geometry in the solid state (Figure 3) in which N(6) is the
remaining dimethylamine nitrogen.
In conclusion, three new H3[Aryl3TPA] species have been
prepared that serve as precursors to Mo, Zr, and Hf
complexes, several of which contain [Aryl3TPA]3- ligands.
Issues remain in terms of obtaining the desired five-
coordinate trispyrrolyl complexes, perhaps in part because
of the poor π-bonding ability of the pyrrolyl groups compared
to the amido groups in aryl-substituted triamidoamine
Acknowledgment. R.R.S. is grateful to the National
Institutes of Health (GM 31978) for research support. We
thank Professors Joseph Sadighi, Dan Nocera, and Ted Betley
for discussions and Dr. Peter Mu¨ller for assistance with X-ray
structural studies.
Supporting Information Available: Experimental details for
the synthesis of all compounds and the X-ray structural studies.
This material is available free of charge via the Internet at
http://pubs.acs.org.
IC701472Y
Inorganic Chemistry, Vol. 46, No. 21, 2007 8465