Tantalum amido and imido complexes supported by tris[(2-indolyl)methyl]amine, a tetradentate trianionic ligand with reduced π-donor character

Article abstract of DOI:10.1021/ic026182a

Tris[(2-indole)methyl]amine, N(CH₂inH)₃, may be readily obtained by reaction of methyl 2-bromomethyl-1-indolecarboxylate with NH₃ followed by deprotection with NaOMe/MeOH. In its deprotonated form, [N(CH₂in)₃]^3- is an efficient tetradentate trianionic ligand for tantalum, as illustrated by the isolation and structural characterization of [η⁴-N(CH₂in)₃]Ta(NAr)(NMe₂H) (Ar = 2,6-C₆H₃Prⁱ₂), [η⁴-N(CH₂in)₃]Ta (NMe₂)₂ and [η⁴-N(CH₂in)₃]Ta(NMe₂)Cl. The [N(CH₂in)₃]^3- ligand has a structural similarity to that of [N(CH₂CH₂NR)₃]^3-, but differs electronically from the latter due to its reduced π-donor capability, a direct result of the nitrogen being a component of the aromatic π-system of the indolyl fragment.

Full text of DOI:10.1021/ic026182a

Inorg. Chem. 2003, 42, 264−266
Tantalum Amido and Imido Complexes Supported by
Tris[(2-indolyl)methyl]amine, a Tetradentate Trianionic Ligand with
Reduced π-Donor Character
Joseph M. Tanski and Gerard Parkin*
Department of Chemistry, Columbia UniVersity, New York, New York 10027
Received November 15, 2002
Tris[(2-indole)methyl]amine, N(CH inH)3, may be readily obtained
by reaction of methyl 2-bromomethyl-1-indolecarboxylate with
transition metals.6 In large part, the success of the [N(CH2C-
2
H2NR)3]3- ligand system in stabilizing high oxidation state
compounds is associated with the well-known strongly
π-donating properties of the amido functionality.7,8 Modula-
tion of both structure and reactivity is to be expected if the
π-donor properties of the ligand were to be either reduced
or eliminated. In this paper, we report the synthesis of a
tetradentate trianionic donor ligand in which the π-donor
properties of the nitrogen atom are reduced compared to that
of an alkylamido functionality.
NH followed by deprotection with NaOMe/MeOH. In its deproto-
3
nated form, [N(CH in)3]3- is an efficient tetradentate trianionic
2
ligand for tantalum, as illustrated by the isolation and structural
characterization of [η4-N(CH in)3]Ta(NAr)(NMe2H) (Ar ) 2,6-
2
i
C H Pr2), [η4-N(CH in)3]Ta(NMe2)2 and [η4-N(CH in)3]Ta(NMe2)Cl.
6
3
2
2
The [N(CH in)3]3- ligand has a structural similarity to that of
2
[N(CH CH NR)3]3-, but differs electronically from the latter due to
2
2
its reduced π-donor capability, a direct result of the nitrogen being
a component of the aromatic π-system of the indolyl fragment.
Recent studies have noted that aromatic cyclic counterparts
to R2N, such as pyrrolyl, indolyl, and carbazolyl exhibit a
reduced π-donor capability as a result of the nitrogen
π-orbital being involved in the aromatic π-system.9 There-
fore, in an effort to obtain tetradentate tripodal nitrogen
ligands for which the π-donor properties are reduced from
that of the [N(CH2CH2NR)3]3- class, we sought to synthesize
derivatives in which the nitrogen atom donors are compo-
nents of heterocyclic rings. A clear indication that delocal-
ization of the electron density on nitrogen may have a
profound effect on the electron donating properties of such
ligands is illustrated by the pKa’s of some simple deriva-
tives: thus, the pKa’s of pyrrole (17.5), indole (17.0) and
carbazole (16.5)10 are significantly lower than those of simple
amines, e.g. Pri2NH (35.7),11 (Me3Si)2NH (29.5),11 and Ph2-
NH (22.4).12 In addition to modulating the donor property
of the ligand, another reason for using heterocyclic donors
is that it should prevent so-called “cage decomposition”
reactions of [N(CH2CH2NR)3]3- which involve cleavage of
C3 symmetric tridentate tripodal ligands are a prominent
feature of modern coordination chemistry and collectively
stabilize a diverse array of metal ligand combinations. The
ability of tripodal ligands to achieve such diversity is in part
a consequence of the fact that these ligands vary from neutral
(L3) to trianionic (X3) donors.1,2 In addition to tridentate
tripodal ligands, tetradentate tripodal ligands are also known,
in which the extra donor functionality is provided by the
axial linker atom, most commonly nitrogen.3 A particularly
interesting class of such ligand is one which is trianionic
(LX3) and features amido donors, namely tris(amidoethyl)-
amine and its various derivatives, i.e. [N(CH2C-
H2NR)3]3-.4,5 For example, Schrock has extensively applied
this class of ligand to a study of the chemistry of the early
* To whom correspondence should be addressed. E-mail: parkin@
chem.columbia.edu.
(6) (a) Schrock, R. R. Acc. Chem. Res. 1997, 30, 9-16. (b) Schrock, R.
R. Pure Appl. Chem. 1997, 69, 2197-2203.
(1) Common examples of tripod nitrogen donors include neutral tris-
(pyrazolyl)methane and tris(pyridyl)methane, monoanionic tris(pyra-
zolyl)hydroborato, and trianionic tris[(amido)methyl]ethane. See, for
example: Gade, L. H. Acc. Chem. Res. 2002, 35, 575-582.
(2) For the [LlXx] classification of ligands, see: Green, M. L. H. J.
Organomet. Chem. 1995, 500, 127-148.
(7) Bradley, D. C.; Chisholm, M. H. Acc. Chem. Res. 1976, 9, 273-280.
(8) Cummins, C. C. Prog. Inorg. Chem. 1998, 47, 685-836.
(9) See, for example: (a) Riley, P. N.; Parker, J. R.; Fanwick, P. E.;
Rothwell, I. P. Organometallics 1999, 18, 3579-3583. (b) Riley, P.
N.; Fanwick, P. E.; Rothwell, I. P. J. Chem. Soc., Dalton Trans. 2001,
181-186. (c) Harris, S. A.; Ciszewski, J. T.; Odom, A. L. Inorg. Chem.
2001, 40, 1987-1988. (d) Tanski, J. M.; Parkin, G. Organometallics
2002, 21, 587-589. (e) Zhu, G.; Tanski, J. M.; Parkin, G. J. Chem.
Crystallogr. 2002, 32, 461-467.
(3) For an example of a tetradentate tripodal ligand in which B-H is the
axial donor, see: Bridgewater, B. M.; Parkin, G. Inorg. Chem.
Commun. 2000, 3, 534-536.
(4) (a) Verkade, J. G. Acc. Chem. Res. 1993, 26, 483-489. (b) Verkade,
J. G. Coord. Chem. ReV. 1994, 137, 233-295.
(10) Catalan, J.; Abboud, J. L. M.; Elguero, J. AdV. Heterocycl. Chem.
1987, 41, 187-274.
(11) Fraser, R. R.; Mansour, T. S. J. Org. Chem. 1984, 49, 3442-3443.
(12) Stewart, R.; Dolman, D. Can. J. Chem. 1967, 45, 925-928.
(5) Tetradentate tetraanionic ligands are also known. See: Kobayashi, J.;
Goto, K.; Kawashima, T.; Schmidt, M. W.; Nagase, S. J. Am. Chem.
Soc. 2002, 124, 3703-3712.
264 Inorganic Chemistry, Vol. 42, No. 2, 2003
10.1021/ic026182a CCC: $25.00 © 2003 American Chemical Society
Published on Web 12/31/2002

COMMUNICATION
Scheme 1
one of the N-CH2CH2NR bonds and the formation of a vinyl
amide fragment, CH2dCHNR.13
Figure 1. Molecular structures of [η4-N(CH2in)3]Ta(NAr)(NMe2H) (1)
and [η4-N(CH2in)3]Ta(NMe2)Cl (5).
Focusing first on the incorporation of the indolyl fragment,
we have found that tris[(2-indole)methyl]amine, N(CH2inH)3,
may be readily obtained by reaction of methyl 2-bromom-
ethyl-1-indolecarboxylate14 with NH3, followed by depro-
tection with NaOMe/MeOH (Scheme 1).15 N(CH2inH)3 has
been characterized spectroscopically and by an X-ray dif-
fraction study on the protected form, N(CH2inCO2Me)3.16
The structure of N(CH2inH)3 is closely related to that of tris-
[(2-benzimidazolyl)methyl]amine, N(CH2bimH)3,17,18 differ-
ing merely in the formal substitution of a CH group by N in
the 5-membered heterocyclic ring. Despite the structural
similarity, however, the two molecules are electronically
quite distinct because N(CH2inH)3 can only operate as a
tetradentate ligand once it has been triply deprotonated, i.e.
[N(CH2in)3]3-, whereas N(CH2bimH)3 may operate as a
tetradentate ligand in its neutral form. The ability of the latter
to act as a neutral donor is merely a consequence of the fact
that the 5-membered imidazolyl ring already has a two-
coordinate nitrogen that may function as a donor. In principle,
deprotonation of N(CH2bimH)3 would enable it to act as an
anionic ligand, but the deprotonated nitrogen would undoubt-
edly be a site of subsequent reactivity and thereby render
the ligand non-innocent; deprotonated N(CH2bimH)3 has thus
found little application as a ligand to date.19
Scheme 2
Another feature of N(CH2bimH)3 that hinders its applica-
tion to early transition metal chemistry is its insolubility in
inert solvents. Indeed, transition metal derivatives of N(CH2-
bimH)3 are mainly limited to those of the later metals20 and
are commonly obtained by reactions that employ alcohols
as solvents. In contrast, N(CH2inH)3 is soluble in benzene,
thereby facilitating the synthesis of derivatives of the early
transition metals, as specifically described here for tantalum.
The synthesis of [N(CH2in)3]-tantalum derivatives may
be achieved by the reactions of N(CH2inH)3 with dimethy-
lamido tantalum complexes, reactions that are accompanied
by elimination of Me2NH. For example, N(CH2inH)3 reacts
with (Me2N)3Ta(NAr) (Ar ) 2,6-C6H3Pri2)21 at room tem-
perature to give the arylimido complex [η4-N(CH2in)3]Ta-
(NAr)(NMe2H) (1), as illustrated in Scheme 2. The molecular
structure of [η4-N(CH2in)3]Ta(NAr)(NMe2H) has been de-
termined by X-ray diffraction (Figure 1)16 and illustrates
several important features. Firstly, the six Ta-N bond lengths
of the distorted octahedral [TaN6] entity span the sub-
stantial range of 1.78-2.38 Å, varying according to the
nature of the different donors which fall into three catego-
ries: (i) TadNimido double bond [1.778(6) Å], (ii) TasNindolyl
single bond [2.11(2) Å (av)], and (iii) TarN dative bond,
i.e. TarNR3 [2.380(5) Å] and TarNMe2H [2.344(5) Å].
Secondly, the average Ta-Nindolyl bond length of 2.11(2) Å
is distinctly longer than the Ta-Namido bond lengths in
(13) See, for example: (a) Freundlich, J. S.; Schrock, R. R.; Davies, W.
M. J. Am. Chem. Soc. 1996, 118, 3643-3655. (b) Freundlich, J. S.;
Schrock, R. R.; Davies, W. M. Organometallics 1996, 15, 2777-
2783.
(14) Nagarathnam, D. Synthesis 1992, 743-745.
(15) For a related example of the synthesis of a tris[(2-pyrrolyl)methyl]-
amine, see: Treibs, A.; Jacob, K. Liebigs Ann. Chem. 1970, 737, 176-
178.
(16) See Supporting Information.
(17) See, for example: (a) Thompson, L. K.; Ramaswamy, B. S.; Seymour,
E. A. Can. J. Chem. 1977, 55, 878-888. (b) Oki, A. R.; Bommarreddy,
P. R.; Zhang, H.; Hosmane, N. Inorg. Chim. Acta 1995, 231, 109-
114. (c) Su, C.-H.; Kang, B.-S.; Du, C.-X.; Yang, Q.-C.; Mak, T. C.
W. Inorg. Chem. 2000, 39, 4843-4849.
(18) Alkylated derivatives, N(CH2bimR)3, are also known. See, for
example: (a) Reference 17c. (b) Hendriks, H. M. J.; Birker, P.;
Verschoor, G. C.; Reedijk, J. J. Chem. Soc., Dalton Trans. 1982, 623-
631.
(19) We are aware of one example where a coordinated N(CH2bimH)3
ligand has been triply deprotonated, namely {[N(CH2bimH)3]Co-
(NCS)}+; the deprotonated nitrogen atoms are, however, readily
reprotonated, thereby indicating the non-innocent nature of the
deprotonated ligand. See: Hammes, B. S.; Kieber-Emmons, M. T.;
Sommer, R.; Rheingold, A. L. Inorg. Chem. 2002, 41, 1351-1353.
(20) For example, there are no structurally characterized N(CH2bimH)3
derivatives of the groups 4 and 5 transition metals listed in the
Cambridge Structural Database (Version 5.23).
(21) Hermann, W. A.; Baratta, W.; Herdtweck, E. J. Organomet. Chem.
1997, 541, 445-460.
Inorganic Chemistry, Vol. 42, No. 2, 2003 265

COMMUNICATION
{[N(CH2CH2NR)3]Ta} derivatives. For example, the mean
Ta-Namido bond length in structurally characterized {[N(CH2-
CH2NR)3]Ta} derivatives listed in the Cambridge Structural
Database is 2.02(3) Å, ca. 0.1 Å shorter than the average
Ta-Nindolyl bond length of [η4-N(CH2in)3]Ta(NAr)(NMe2H).
The mean Ta-Namine bond length [2.36(6) Å] for {[N(CH2-
CH2NR)3]Ta} derivatives, however, is comparable to that
for [η4-N(CH2in)3]Ta(NAr)(NMe2H) [2.380(5) Å], indicating
that the difference in Ta-Nindolyl and Ta-Namido bond lengths
is a manifestation of the different π-donor abilities of the
nitrogen donors. Thirdly, the [N(CH2in)3]3- ligand is flexible
and the local coordination geometry may deviate substantially
from C3 symmetry. For example, the N-Ta-N bond angles
between the indolyl donors are quite disparate, with values
of 86.9(2)°, 110.1(2)°, and 137.3(2)°.
Dissociation of Me2NH from [η4-N(CH2in)3]Ta(NAr)-
(NMe2H) generates 5-coordinate [η4-N(CH2in)3]Ta(NAr) (2).
However, the dissociation is reversible, such that isolation
of [N(CH2in)3]Ta(NAr) requires removal of the liberated
Me2NH, which may be achieved by treatment with MeI
(Scheme 2).22 The isolation of [η4-N(CH2in)3]Ta(NAr)-
(NMe2H) is also noteworthy in that it is stable with respect
to proton transfer from the Me2NH ligand to the arylimido
ligand that would thereby form [N(CH2in)3]Ta(HNAr)-
(NMe2). This observation is consistent with the notion that
the nitrogen lone pair interacts strongly with the electron
deficient tantalum center, thereby rendering the nitrogen
unsusceptible to protonation.23,24
Figure 2. Molecular structure of [η3-N(CH2in)2(CH2inH)]Ta(NMe2)3 (3).
Coordination of the third indolyl group may be achieved
by deprotonation with MeLi to give [η4-N(CH2in)3]Ta-
(NMe2)2 (4) (Scheme 2). The molecular structure of [η4-
N(CH2in)3]Ta(NMe2)2 has been determined by X-ray dif-
fraction, demonstrating that the structure may be described
as distorted octahedral with cis NMe2 ligands. The Ta-Nindolyl
bonds [2.13(2) Å (av)] are longer than those of the Ta-
NMe2 bonds [1.95(5) Å (av)], thereby providing a further
demonstration that indolyl nitrogen atoms are poorer donors
than the planar amide nitrogen atoms.
[η4-N(CH2in)3]Ta(NMe2)2 reacts with Me3SiCl to yield [η4-
N(CH2in)3]Ta(NMe2)Cl (5). The molecular structure of [η4-
N(CH2in)3]Ta(NMe2)Cl has been determined by X-ray
diffraction, as illustrated in Figure 1,26 and demonstrates that
the Ta-Nindolyl bond [2.08(3) Å (av)] is also longer than the
Ta-NMe2 [1.941(5) Å] bond in this complex.
In summary, [N(CH2in)3]3- has been shown to be an
efficient tetradentate trianionic ligand for tantalum. In this
regard, while the [N(CH2in)3]3- ligand has a structural
similarity to that of [N(CH2CH2NR)3]3-, it differs electroni-
cally from the latter due to its reduced π-donor capability,
which is a direct result of the nitrogen being a component
of the aromatic π-system of the indolyl fragment. Further-
more, the trianionic nature of [N(CH2in)3]3- ligand promises
access to a fundamentally different class of chemistry than
that provided by its neutral isosteric benzimidazole coun-
terparts, N(CH2bimH)3 and N(CH2bimR)3, as illustrated by
the tantalum complexes reported here.
Coordination of the tris[(2-indolyl)methyl]amine ligand to
tantalum may also be achieved by a two step sequence that
9a,25
involves the initial reaction of Ta(NMe2)5
with N(CH2-
inH)3 at 70 °C to give [η3-N(CH2in)2(CH2inH)]Ta(NMe2)3
(3) (Scheme 2). The latter complex, which has been struc-
turally characterized by X-ray diffraction (Figure 2), features
a tridentate [η3-N(CH2in)2(CH2inH)]2- ligand in which only
two of the indolyl donors coordinate to tantalum, with the
uncoordinated indole fragment remaining protonated.
Acknowledgment. We thank the U. S. Department of
Energy, Office of Basic Energy Sciences (DE-FG02-
93ER14339) for support of this research.
(22) The interconversion of [η4-N(CH2in)3]Ta(NAr)(NMe2H) and [η4-
N(CH2in)3]Ta(NAr) is also rapid on the NMR time-scale, as indicated
1
by H NMR spectroscopy.
(23) Cundari, T. R. J. Am. Chem. Soc. 1992, 114, 7879-7888.
(24) In this regard, it is also worth noting that other dimethylamine-imido
complexes are known, e.g. [Me2C(C5H4)2M(NAr)(NMe2H)]+ (M )
Nb, Ta; Ar ) 2,6-C6H3Pri2)a and (ArO)3Nb(NMe)(NMe2H) (Ar ) 2,6-
C6H3Ph2).b Furthermore, the relative inertness of many linear Tad
NR linkages has been noted.c (a) Reference 21. (b) Chesnut, R. W.;
Fanwick, P. E.; Rothwell, I. P. Inorg. Chem. 1988, 27, 752-754. (c)
Pugh, S. M.; Tro¨sch, D. J. M.; Skinner, M. E. G.; Gade, L. H.;
Mountford, P. Organometallics 2001, 20, 3531-3542.
Supporting Information Available: Experimental details (pdf)
and crystallographic CIF files. This material is available free of
charge via the Internet at http://pubs.acs.org.
IC026182A
(26) The iodide complex [η4-N(CH2in)3]Ta(NMe2)I has also been structur-
(25) Bradley, D. C.; Thomas, I. M. Can. J. Chem. 1962, 40, 1355-1360.
ally characterized.
266 Inorganic Chemistry, Vol. 42, No. 2, 2003