Synthesis of aryl-substituted triamidoamine ligands and molybdenum(IV) complexes that contain them

Article abstract of DOI:10.1021/om980764b

Compounds of the type [(ArNHCH₂CH₂)₃N] (Ar = phenyl (H₃[1a]), 4-fluorophenyl (H₃[1b]), 4-tertbutylphenyl (H₃[1c]), 3,5-dimethylphenyl (H₃[1d]), 2-methylphenyl (H₃[1e]), and mesityl (H₃[1f])) have been synthesized by the Pd-catalyzed coupling of (H₂NCH₂CH₂)₃N with aryl bromides. Molybdenum methyl complexes containing [1a-d]^3- can be prepared by adding 4 equiv of methyllithium to a mixture of the ligand and MoCl₄(THF)₂; the corresponding chloride complexes are obtained upon addition of 2,6-lutidinium chloride to the methyl complexes. . Copyright 1998 American Chemical Society.

Full text of DOI:10.1021/om980764b

© Copyright 1998
Volume 17, Number 26, December 21, 1998
American Chemical Society
Communications
Syn th esis of Ar yl-Su bstitu ted Tr ia m id oa m in e Liga n d s
a n d Molybd en u m (IV) Com p lexes th a t Con ta in Th em
George E. Greco, Alexandru I. Popa, and Richard R. Schrock*
Department of Chemistry, Massachusetts Institute of Technology,
Cambridge, Massachusetts 02139
Received September 11, 1998
Summary: Compounds of the type [(ArNHCH2CH2)3N]
(Ar ) phenyl (H3[1a ]), 4-fluorophenyl (H3[1b]), 4-tert-
butylphenyl (H3[1c]), 3,5-dimethylphenyl (H3[1d]), 2-meth-
ylphenyl (H3[1e]), and mesityl (H3[1f])) have been syn-
thesized by the Pd-catalyzed coupling of (H2NCH2-
CH2)3N with aryl bromides. Molybdenum methyl com-
plexes containing [1a -d ]3- can be prepared by adding
4 equiv of methyllithium to a mixture of the ligand and
MoCl4(THF)2; the corresponding chloride complexes are
obtained upon addition of 2,6-lutidinium chloride to the
methyl complexes.
decompose via R-elimination by as much as 6 orders of
magnitude faster than via â-elimination5,6). We have
been particularly interested in the ability of triami-
doamine molybdenum complexes to bind and activate
dinitrogen.2,7-10 Unfortunately, both [(Me3SiNCH2-
CH2)3N]3- and [(C6F5NCH2CH2)3N]3- suffer from some
disadvantages in certain circumstances. For example,
we have begun to encounter chemistry at the amido
nitrogen in [(Me3SiNCH2CH2)3N]3- complexes, e.g.,
exchange of the trimethylsilyl group for a methyl group.8
The perfluorophenyl group, on the other hand, we
believe tends to be attacked by strong nucleophiles.
Therefore we have been looking for a method of prepar-
ing ligands that contain ordinary aryl substituents. We
report here a potentially general approach to such
ligands using a palladium-catalyzed coupling reaction
between an aryl halide and an amine.11-17
In the past several years we have prepared a large
variety of transition metal complexes that contain
triamidoamine ligands, [(RNCH2CH2)3N]3- (R ) SiMe3
1
or C6F52), and explored their chemistry.3 Ligands of this
type have proven useful for stabilizing trigonal bipyra-
midal species in which the apical position is sterically
protected against bimolecular decomposition reactions
and have given rise to a number of unusual species (e.g.,
molybdenum or tungsten terminal phosphido and ars-
enido complexes4) or reactions (e.g., a demonstration
that certain molybdenum and tungsten alkyl complexes
(5) Schrock, R. R.; Seidel, S. W.; Mo¨sch-Zanetti, N. C.; Shih, K.-Y.;
O’Donoghue, M. B.; Davis, W. M.; Reiff, W. M. J . Am. Chem. Soc. 1997,
119, 11876.
(6) Schrock, R. R.; Seidel, S. W.; Mo¨sch-Zanetti, N. C.; Dobbs, D.
A.; Shih, K.-Y.; Davis, W. M. Organometallics 1997, 16, 5195.
(7) Shih, K.-Y.; Schrock, R. R.; Kempe, R. J . Am. Chem. Soc. 1994,
116, 8804.
(1) Verkade, J . G. Acc. Chem. Res. 1993, 26, 483.
(2) Kol, M.; Schrock, R. R.; Kempe, R.; Davis, W. M. J . Am. Chem.
Soc. 1994, 116, 4382.
(3) Schrock, R. R. Acc. Chem. Res. 1997, 30, 9.
(4) Mo¨sch-Zanetti, N. C.; Schrock, R. R.; Davis, W. M.; Wanninger,
K.; Seidel, S. W.; O’Donoghue, M. B. J . Am. Chem. Soc. 1997, 119,
11037.
(8) O’Donoghue, M. B.; Davis, W. M.; Schrock, R. R. Inorg. Chem.
1998, 37, 5149.
(9) O’Donoghue, M. B.; Zanetti, N. C.; Schrock, R. R.; Davis, W. M.
J . Am. Chem. Soc. 1997, 119, 2753.
(10) O’Donoghue, M. B.; Davis, W. M.; Schrock, R. R.; Reiff, W. M.
Inorg. Chem., in press.
(11) Driver, M. S.; Hartwig, J . F. J . Am. Chem. Soc. 1996, 118, 7217.
10.1021/om980764b CCC: $15.00 © 1998 American Chemical Society
Publication on Web 11/25/1998

5592 Organometallics, Vol. 17, No. 26, 1998
Communications
The generic form of the reaction is shown in eq 1.
Pd2(dba)3, rac-BINAP
N(CH2CH2NH2)3 + 3ArBr NaO-t-Bu, toluene, heat8
N(CH2CH2NHAr)3 (1)
Under the reaction conditions reported by Buchwald,15
an excess of amine is used and the catalyst loading is
0.5 mol %. We use 3 equiv of ArBr and a catalyst loading
of 3% per mole of N(CH2CH2NH2)3. The (ArNHCH2-
CH2)3N species that have been prepared on a scale of
∼5-15 g include those in which the Ar group is phenyl
itself (H3[1a ]; 57% yield),18 4-fluorophenyl (H3[1b]; 27%
yield), 4-tert-butylphenyl (H3[1c]; 49% yield), 3,5-dim-
ethylphenyl (H3[1d ]; 58% yield), 2-methylphenyl (H3-
[1e]; 90% yield), and 2,4,6-trimethylphenyl (H3[1f]; 79%
yield). The major byproduct when the Ar group is
relatively small is that in which one of the arms is
doubly arylated; for example, (PhNHCH2CH2)2N(CH2-
CH2NPh2) usually comprises ∼15% of the total product
mass. The desired (ArNHCH2CH2)3N product can be
separated from (ArNHCH2CH2)2N(CH2CH2NAr2) by
column chromatography on silica gel. The product
mixture in the case of H3[1b] contains numerous other
unidentified sideproducts besides (ArNHCH2CH2)2N(CH2-
CH2NAr2). Diarylation is not observed in the case of H3-
[1e] and H3[1f], even though higher temperatures are
required (100 °C), presumably as a consequence of the
greater steric demands of the 2-methylphenyl and
mesityl substituents. The white products can be crystal-
lized from mixtures of ether and pentane at -35 °C.
They all dissolve readily in ether and, in the case of H3-
[1c], also in pentane. During the course of this work
the arylation of mono- and diamines (e.g., diethylen-
etriamine) under conditions similar to those reported
here appeared in the literature.19
(NMe2)4 requires 2 days at 70 °C. [1b]Mo(NMe2) (2b;
93% yield) and [1d ]Mo(NMe2) (2d ; 64% yield)21 are
obtained as purple-black diamagnetic crystalline solids
(cf. [(C6F5NCH2CH2)3N]Mo(NMe2)2). They each react
with 2,6-lutidinium chloride in THF to yield the respec-
tive chloride derivatives, [1b]MoCl (4b) and [1d ]MoCl
(4d ) (see below). Compounds 4b and 4d are paramag-
netic, red crystalline species with proton and (in the case
of 4b) fluorine NMR resonances that are shifted (gener-
ally to high field) and broadened. Nevertheless, NMR
spectra are useful for diagnostic purposes. (See Sup-
porting Information for details.)
We felt that it should be possible to prepare [(ArNCH2-
CH2)3N]MoCl derivatives from the free ligand, MoCl4-
(THF)2, and triethylamine, as is the case for [(C6F5-
NCH2CH2)3N]MoCl.2 However, using NMR spectra of
4b and 4d as analytical probes, only low yields of the
monochlorides were observed, and yields were not
improved using DBU, KH, or proton sponge as the base.
The direct reaction between MoCl4(THF)2 in THF or
ether with Li3[(ArNCH2CH2)3N] (prepared from the
parent ligand and 3 equiv of an alkyllithium reagent)
also did not yield significant quantities of the chloride
derivatives. However, when the free ligands (H3[1a ], H3-
[1b], H3[1c], or H3[1d ]) were added to MoCl4(THF)2 in
THF followed by 3 equiv of LiMe, the paramagnetic
monochloride derivatives (4a -4d ) were found to be
formed in good yields after a period of 24 h. However, a
second product was present in comparable amounts in
each case after 1 h that was slowly converted in the
reaction mixture to the monochloride over time. The
second product was formed virtually exclusively after
∼1 h in each case when 4 equiv of LiMe was employed.
Analytical data are consistent with compounds 4 being
the monochloride complexes, and with the products
formed upon addition of 4 equiv of LiMe being the
monomethyl derivatives, 3a -3d (equation 3).22 A more
We have found that the most certain method of
placing a [(ArNCH2CH2)3N]3- ligand on Mo is the
20
reaction between the free ligand and Mo(NMe2)4 (eq
2). The reaction between H3[1b ] or H3[1d ] and Mo-
(12) Guram, A. S.; Rennels, R. A.; Buchwald, S. L. Angew. Chem.,
Intl. Ed. Engl. 1995, 34, 1348.
(13) Paul, F.; Patt, J .; Hartwig, J . F. J . Am. Chem. Soc. 1994, 116,
5969.
(14) Driver, M. S.; Hartwig, J . F. J . Am. Chem. Soc. 1995, 117, 4708.
(15) Wolfe, J . P.; Wagaw, S.; Buchwald, S. L. J . Am. Chem. Soc.
1996, 118, 7215.
(21) A 250 mL Schlenk flask was charged with Mo(NMe2)4 (1.77 g,
6.5 mmol), H3[1d ] (2.29 g, 5.0 mmol), and toluene (125 mL). The
reaction was heated to 70 °C for 2 days on the Schlenk line. Toluene
was removed on the Rotovap, and the residue was washed with pentane
until the pentane washings were colorless. The resulting solid was
collected and recrystallized from ether/pentane to yield 1.9 g (3.18
mmol, 64%) of dark purple crystals.
(22) The synthesis of 3a will be used as the example. A 100 mL
round-bottom flask was charged with MoCl4(THF)2 (1.07 g, 2.8 mmol),
H3[1a ] (1.05 g, 2.8 mmol), and THF (30 mL). The mixture was stirred
at room temperature for 15 min, during which time it turned dark
red. The mixture was cooled to -40 °C, methyllithium (8 mL of a 1.4
M solution in ether) was added dropwise, and the reaction was allowed
to warm to room temperature over a period of 1 h, while being stirred.
The THF was removed, and the residue was extracted with 50 mL of
toluene. Gentle heating was required to dissolve all of the complex.
LiCl and some purple insoluble material were removed by filtration.
Toluene was removed in vacuo, and the residue was extracted with
ether. The insoluble product was collected, washed with ether and
pentane, and dried in vacuo to yield 689 mg (1.43 mmol, 51%) of dark
orange powder.
(16) Wagaw, S.; Buchwald, S. L. J . Org. Chem. 1996, 61, 7240.
(17) Wolfe, J . P.; Buchwald, S. L. J . Org. Chem. 1996, 61, 1133.
(18) A 1 L Schlenk flask was charged with N(CH2CH2NH2)3 (8.77
g, 60 mmol), bromobenzene (28.26 g, 180 mmol), Pd2(dba)3 (0.82 g, 0.9
mmol), rac-BINAP (1.49 g, 2.4 mmol), sodium tert-butoxide (19.97 g,
208 mmol), and toluene (500 mL). The reaction mixture was heated to
80 °C under dinitrogen for 20 h. It was then cooled to room temper-
ature, and NaBr was removed by filtration. Toluene was removed in
vacuo, and the resulting brown oil was purified by column chroma-
tography on silica gel. The column was eluted with a 3:1 mixture of
hexane and ethyl acetate to which was added 3% (by volume) of a
saturated solution of NH3(g) in methanol. A yellow oil was isolated
from the column, which solidified when exposed to high vacuum. White
crystals (12.82 g, 34.2 mmol, 57%) were obtained upon recrystallization
from ether/pentane at -40 °C.
(19) Hong, Y.; Senanayake, C. H.; Xiang, T.; Vandenbossche, C. P.;
Tanoury, G. J .; Bakale, R. P.; Wald, S. A. Tetrahedron Lett. 1998, 39,
3121.
(20) Bradley, D. C.; Chisholm, M. H. J . Chem. Soc. (A) 1971, 2741.

Communications
Organometallics, Vol. 17, No. 26, 1998 5593
of [1b]MoCl is added dropwise to a solution of sodium
naphthalenide at -40 °C in THF under dinitrogen
followed by addition of 1 equiv of Me3SiCl, diamagnetic
[1b]Mo-NdNSiMe3 (5b) can be isolated in 56% yield
(eq 5).25 We presume that an intermediate in this
[1b]MoCl 1. 2e, N , THF8 [1b]Mo-NdNSiMe3 (5)
2
2. Me3SiCl
reaction is the sodium salt of {[1b]Mo-NdN}- on the
basis of literature precedent in the related reductions
of compounds containing the [(C6F5NCH2CH2)3N]3- and
[(Me3SiCH2CH2)3N]3- ligands.2,9 A stretch at 1674 cm-1
in the infrared spectrum is characteristic of the diaz-
enido functionality in complexes of this general type,
and proton, carbon, and fluorine NMR are all consistent
with the formulation of [1b]Mo-NdNSiMe3.
We conclude that (ArNHCH2CH2)3N compounds are
readily prepared using the palladium-catalyzed amine
arylation reaction and that several analogues of known
molybdenum triamidoamine complexes can be prepared
under the right circumstances. “Direct” routes to alkyl
complexes analogous to the methods disclosed here (eq
3) may prove to be a useful and short method of
accessing other early transition metal alkyl complexes
that contain a multidentate amido ligand. We will be
exploring the utility of [(ArNCH2CH2)3N]3- ligands in
triamidoamine molybdenum chemistry, in particular
with respect to binding and reducing dinitrogen. We also
expect that these new ligands will be useful for a variety
of other reactions involving triamidoamine transition
metal complexes, and we will be especially interested
in the extent to which the steric and electronic proper-
ties of the aryl substituents can be manipulated in order
to control the chemistry at the apical site in pseudo-
trigonal bipyramidal species.
reliable synthesis of 4a -4d consists of addition of 2,6-
lutidinium chloride to 3a -3d in THF (eq 4).23
A
resonance cannot be observed for the methyl group in
compounds 4, as was also the case for related Mo(IV)
triamidoamine methyl and alkyl complexes.5,24 Addition
of methyllithium to compounds 4 gives compounds 3
immediately.
The sequence of reactions that leads to 3 is not clear.
A red solution is formed upon addition of the free
ligands to MoCl4(THF)2, which suggests that some
adduct is formed initially. We assume that at least
partial binding of the potentially tetradentate ligands
to Mo then allows smooth deprotonation of the amine
nitrogens and alkylation of the metal by methyllithium
without destructive reduction of the metal. At this stage
it is not known how compounds 3 are converted into
compounds 4 under the reaction conditions with time.
So far, we have not been able to prepare compounds
of type 3 or 4 that contain the bulkier aryl-substituted
ligands, [1e]3- or [1f]3-, using methods analogous to
those described above for compounds that contain [1a -
d ]3-. When H3[1e] or H3[1f] is added to MoCl4(THF)2
in THF, a red “adduct” is not formed readily at room
temperature. We propose that in the case of H3[1e] and
H3[1f] steric inaccessibility of the aryl-substituted amines
prevents formation of the required intermediate that
reacts smoothly with methyllithium, and therefore
addition of methyllithium to what amounts to a mixture
of free ligand and MoCl4(THF)2 simply leads to decom-
position.
Ack n ow led gm en t. R.R.S. is grateful to the National
Institutes of Health (GM 31978) for research support,
and A.I.P. thanks the Beckman Foundation for under-
graduate research support.
Su p p or tin g In for m a tion Ava ila ble: Experimental pro-
cedures and spectroscopic information for the reported com-
pounds (8 pages). Ordering information is given on any current
masthead page.
OM980764B
Since we are especially interested in dinitrogen
activation, we were pleased to find that when a solution
(25) A solution of sodium naphthalenide (7.5 mL of a 0.17 M
solution) was diluted to 20 mL and cooled to -40 °C. A separate
solution of 4b (278 mg, 0.5 mmol) was also dissolved in 10 mL of THF
and cooled to -40 °C. The solution of 4b was added dropwise to the
naphthalenide solution, causing a color change from green to red. The
reaction was allowed to warm to room temperature with stirring for
30 min, and then it was cooled back down to -40 °C and added slowly
to a solution of Me3SiCl (87 mg, 0.8 mmol) in THF (5 mL). The reaction
mixture was allowed to warm to room temperature with stirring for
30 min, and then the THF was removed in vacuo. The residue was
extracted with ether and filtered. The dark yellow filtrate was
concentrated to dryness, and the resulting residue was washed with
pentane and dried in vacuo; yield 175 mg (0.28 mmol, 56%).
(23) Synthesis of 4a . A solution of 3a (724 mg, 1.5 mmol) in THF
(50 mL) was cooled to -40 °C, 2,6-lutidinium chloride (215 mg, 1.5
mmol) was added as a solid, and the reaction was allowed to warm to
room temperature while being stirred over a period of 1 h. Some red
product began to precipitate out of solution after 1 h. The solvent was
removed in vacuo, and the residue was extracted with ether. The
insoluble product was collected, washed with ether and pentane, and
dried in vacuo to give 609 mg (1.21 mmol, 81%) of orange solid.
(24) Seidel, S. W.; Schrock, R. R.; Davis, W. M. Organometallics
1998, 17, 1058.