Lewis acid activation of molybdenum nitrides for alkyne metathesis

Article abstract of DOI:10.1039/c0cc03113e

The substantial kinetic barrier to molybdenum nitride-alkyne metathesis is facilitated by precomplexation of the borane Lewis acid B(C₆F ₅)₃, providing convenient access to metathesis-active molybdenum alkylidynes. Spectroscopic and X-ray structural analysis suggest MoN bond weakening upon borane complexation.

Full text of DOI:10.1039/c0cc03113e

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Lewis acid activation of molybdenum nitrides for alkyne metathesisw
Aaron D. Finke and Jeffrey S. Moore*
Received 7th August 2010, Accepted 1st September 2010
DOI: 10.1039/c0cc03113e
The substantial kinetic barrier to molybdenum nitride–alkyne
metathesis is facilitated by precomplexation of the borane
6 5 3
Lewis acid B(C F ) , providing convenient access to metathesis-
active molybdenum alkylidynes. Spectroscopic and X-ray
structural analysis suggest MoRN bond weakening upon borane
complexation.
Alkyne metathesis, once the underutilized cousin of olefin
1
metathesis, has undergone a resurgence in the past ten years.
One major advance in this field has been the development
of well-defined alkyne metathesis precatalysts based on
2
molybdenum alkylidynes. The preparation of Mo alkylidynes
is not straightforward; highly air- and moisture-sensitive
precursors and difficult multistep syntheses complicate facile
3
preparation. The molybdenum nitrides, by contrast, are much
4
easier to prepare from commercially-available precursors. In
6 5 3
Scheme 2 Reactions of various Mo-nitrides with B(C F ) .
a seminal report, Gdula and Johnson reported that molybdenum
nitrides could metathesize with aliphatic alkynes to generate
5
the Mo-alkylidyne and an alkylnitrile irreversibly. The kinetic
overcome by precomplexation with a Lewis acid (Scheme 1),
weakening the MoRN bond and making it more susceptible
barrier for this reaction is high, as with many N-transfer
reactions, and occurs slowly at elevated temperatures. Never-
theless, recently-reported precatalysts based on Mo-nitrides
have demonstrated excellent activity for ring-closing metathesis
6 5 3
to metathesis. The borane B(C F ) was particularly attractive
due to its strong Lewis acidity and relative inertness compared
8
to other boranes.
Our initial studies focused on pyridine complex 1, developed
2b,4b
by Fu
catalyst and is now commercially available. However, upon
reaction of 1 with 1 equiv. B(C , we observed the forma-
tion of B(C
ꢀpyridine only (Scheme 2); 1-B was generated
with 2 equiv. B(C . Catalytic amounts of 1-B reacted with
-phenyl-1-butyne in toluene at 90 1C overnight to generate
¨
rstner as a stable, convenient alkyne metathesis pre-
of aliphatic alkynes.
4b
In this communication, we describe the activation of
Mo-nitrides toward metathesis with alkynes via N-ligation of
6 5 3
F )
9
6 5 3
F )
x
a Lewis acid. Activation of MoO precatalysts with oxophilic
6
trialkylaluminiums for alkyne metathesis is well-known;
however, the effects of Lewis acid addition to Mo-nitrides in
alkyne metathesis have not been reported. The nitrogen atom
in Mo-nitrides has sufficient nucleophilic character to display
6 5 3
F )
1
diphenylacetylene and 3-hexyne.
Encouraged by these results, we then turned our focus to
7
a
7b,c
¨
Mo-nitride 2, the precursor to 1, which Furstner reported to be
active in alkyne metathesis upon in situ substitution with
predictable reactivity with Brønsted acids, Lewis acids,
7
d
and organic electrophiles. We hypothesized that the typically
high activation barrier to nitride–alkyne metathesis could be
4b
appropriate silanol ligands. Reaction of 2 with B(C F ) in
6 5 3
DCM solvent generated 2-B in quantitative yield as a white
solid. Crystals suitable for X-ray analysis were formed from
slow diffusion of pentane into a DCM solution of 2-B at
ꢁ
30 1C.z Analysis of the structure of 2-B displayed some
10a
unusual attributes (Fig. 1a). First, the MoRN bond length
˚
of 1.696(3) A is elongated compared to terminal nitrido
1
0b
complexes of Mo.
Furthermore, the Mo–N–B bond angle
Scheme 1 Promotion of molybdenum nitride–alkyne metathesis by
is 159.31, significantly distorted from the expected linearity.
Neither 2 nor 2-B were metathesis-active with 3-hexyne or
precomplexation with borane.
1
-phenyl-1-butyne.
Department of Chemistry, University of Illinois, Urbana-Champaign,
To directly compare the structural effects of Lewis acid
11
addition to Mo-nitrides, the well-studied Mo-nitride 3 was
600 S. Mathews, Urbana, IL 61801, USA.
E-mail: jsmoore@illinois.edu; Fax: +1 217-244-8024;
Tel: +1 217-244-5289
6 5 3
prepared and complexed with B(C F ) to generate 3-B
w Electronic supplementary information (ESI) available: Synthesis
procedures, compound characterization, crystal structures in CIF
format for 2-B and 3-B. CCDC 788129 and 788130. For ESI and
crystallographic data in CIF or other electronic format see DOI:
(Scheme 2). Crystals suitable for X-ray analysis were grown
from slow diffusion of pentane into a DCM solution of 3 and
˚
B(C
6
F
5
)
3
(Fig. 1b).z The MoRN bond length in 3-B (1.692(5) A)
1
1,12
˚
10.1039/c0cc03113e
is significantly longer (>3s) than in 3 (1.661(4) A).
This journal is c The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 7939–7941 7939

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Fig. 1 Thermal ellipsoid plots (35% probability) of (a) 2-B and (b) 3-B. Hydrogens omitted for clarity.z
5
coordinating solvents. The insoluble solid, as well as the
adducts with coordinating solvents, resisted attempts at crystalli-
zation, both from solution and via sublimation. However,
reaction of 2 and 2-B with 3 equiv. 2-trifluoromethylphenol
generated a complex soluble in non-coordinating solvents.
Attempts to crystallize these complexes were unsuccessful.
The metathesis activity of 2 versus 2-B with 2-trifluoro-
methylphenol was tested for the effect of borane addition. The
results are summarized in Table 1. It is clear that the presence
of borane accelerates metathesis with alkynes. Notably,
metathesis of the diarylalkyne 4-methoxydiphenylacetylene
(
entries 5 and 6), which has not previously been reported to
6 5 3
occur with Mo-nitrides, is rapid in the presence of B(C F ) ,
1
1
Fig. 2 ATR-IR spectra of 3, 3-B, and (C
6 5 3
F ) . The reported (for 3)
establishing equilibrium at 90 1C in 20 minutes. By contrast,
metathesis in the absence of borane led to less than 10%
conversion after 12 h at 90 1C. Lewis acid activation of the
nitride thus appears to be necessary to efficiently metathesize
diarylalkynes.
and proposed (for 3-B) MoRN stretches are labelled.
Furthermore, a weakening of the MoRN bond is observed by
IR (Fig. 2). This is in contrast to the MoRN bond length in
MoN[(t-Bu)Ar]
3
ꢀBF
3
, which does not deviate significantly
7
b
Cross-metathesis of diphenylacetylene and 3-hexyne to
generate 1-phenyl-1-butyne is also accelerated by the addition
of borane (Table 1, entries 3 and 4). The reaction of 10 equiv.
from the free nitride. As with 2, neither 3 nor 3-B were
metathesis-active with 3-hexyne or 1-phenyl-1-butyne.
From these data, it became clear that a different strategy
for nitride/alkyne metathesis was necessary, and the choice
of ligand may be critical. We previously reported that the
3
-hexyne with 2 or 2-B and 2-trifluoromethylphenol at 90 1C
Table 1 Effect of borane addition on alkyne metathesis
Mo-alkylidyne Mo(CEt)(Nt-BuAr)
3
in conjunction with an
electron-poor phenol was extremely active toward alkyne
2
a
metathesis. With this in mind, we envisaged that Lewis acid
complexation of Mo-nitrides, followed by ligand displacement
with electron-poor phenols, may lead to enhanced rates of
Mo-alkylidyne formation. Due to its ease of preparation, we
decided on 2 as the Mo-nitride precursor of choice for use in
catalytic reactions.
Convn
(%) 1 h
a
b
Entry
R
1
R
2
R
3
R
4
Borane
1
2
3
4
5
6
Ph
Ph
Ph
Ph
Ph
Ph
Et
Et
Ph
Ph
4-OMePh
4-OMePh
Ph
Ph
Et
Et
Ph
Ph
Et
Et
Et
Et
2-B
2
2-B
2
2-B
2
64
29
55
3
51
8
Reaction of 2 or 2-B in toluene with 3 equiv. 4-nitrophenol
led to the precipitation of a red solid which was insoluble in
4
non-coordinating solvents such as CCl and toluene but very
4-OMePh
4-OMePh
soluble in THF and MeCN. Attempts to metathesize this
complex in any solvent were unsuccessful, possibly due to
its insolubility in non-coordinating solvents and the known
unreactivity of Mo-nitrides toward alkyne metathesis in
a
2
6 5 3
-B was made in situ from 10 mol% of 2 and 20 mol% B(C F ) .
b
1 2
Percent conversion of starting material (R –R–R ) by GC after 1 h.
7
940 Chem. Commun., 2010, 46, 7939–7941
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led exclusively to alkyne polymerization and no alkylidyne was
3 K. Dehnicke and J. Stra
¨
955–978.
hle, Angew. Chem., Int. Ed. Engl., 1992, 31,
1
observed by C NMR.
3
4
5
(a) H.-T. Chiu, Y.-P. Chen, S.-H. Chuang, J.-S. Jen, G.-H. Lee and
S.-M. Peng, Chem. Commun., 1996, 139–140; (b) M. Bindl,
R. Stade, E. K. Heilmann, A. Picot, R. Goddard and
In summary, the activation of Mo-nitrides with the Lewis
acid B(C F ) significantly enhances the rate of metathesis of
5 3
6
alkynes. The elongation of the MoRN bond upon complexa-
tion with borane has been directly established crystallo-
graphically and spectroscopically. Future work will elaborate
on the mechanistic effects of Lewis acid addition on alkyne
metathesis of Mo-nitrides.
A. Fu
(a) R. L. Gdula and M. J. A. Johnson, J. Am. Chem. Soc.,
006, 128, 9614–9615; (b) In the 1980’s, Schrock demon-
¨
rstner, J. Am. Chem. Soc., 2009, 131, 9468–9470.
2
strated the reverse reaction for tungsten alkylidynes: J. H.
Freudenberger and R. R. Schrock, Organometallics, 1986, 5,
398–400.
6
(a) A. Bencheick, M. Petit, A. Mortreux and F. Petit, J. Mol.
Catal., 1982, 15, 93; (b) R. K. Bly, K. M. Dyke and U. H. F. Bunz,
J. Organomet. Chem., 2005, 690, 825–829.
We thank the National Science Foundation (CHE-1010680)
for support; Danielle L. Gray, School of Chemical Sciences,
University of Illinois Urbana-Champaign, for assistance with
X-ray crystallographic analysis; and Mark R. Ringenberg
and Thomas B. Rauchfuss, University of Illinois, Urbana-
Champaign, for helpful discussions.
7 (a) T. Yoshida, T. Adachi, N. Yabunouchi, T. Ueda and
S. Okamoto, J. Chem. Soc., Chem. Commun., 1994, 151–152;
(
b) E. L. Sceats, J. S. Figueroa, C. C. Cummins, N. M.
Loening, P. V. d. Wel and R. G. Griffin, Polyhedron, 2004, 23,
751–2768; (c) L. H. Doerrer, A. J. Graham and M. L. H. Green,
2
J. Chem. Soc., Dalton Trans., 1998, 3941–3946; (d) S. Sarkar,
K. A. Abboud and A. S. Veige, J. Am. Chem. Soc., 2008, 130,
Notes and references
16128–16129.
G. Erker, Dalton Trans., 2005, 1883–1890.
J. Vela, G. R. Lief, Z. Shen and R. F. Jordan, Organometallics,
z Crystal data for 2-B: C30
2 2 4
H36BF15MoN O Si , M = 960.70, mono-
8
9
˚
/n, a = 14.367(5) A, b = 17.170(6) A,
˚
clinic, space group P2
1
3
˚
˚
c = 16.896(6) A, b = 92.256(9)1, V = 4165(2) A , Z = 4, D
1
c
=
2
007, 26, 6624–6635.
0 (a) 2 is a liquid at room temperature and has resisted attempts
at X-ray structure determination. similar complex,
MoN(OSiMe [N(SiMe ](pyr), has an MoRN bond length
ꢁ3
ꢁ1
.532 g cm , m(Mo-K) = 0.527 mm , F(000) = 1936, T = 193 K.
(I > 2s) = 0.0541, wR (all data) = 0.1559 for 7582 independent
reflections with a goodness-of-fit of 1.032. Crystal data for 3-B:
27BF15MoNO , M = 841.27, orthorhombic, space group Fdd2,
1
R
1
2
A
3
)
2
3 2
)
C
30
H
3
˚
˚
of 1.640 A. See: H. T. Chiu, S. H. Chuang, G. H. Lee and
˚
˚
a = 21.3488(18) A, b = 66.889(5) A, c = 10.0299(10) A, V =
3
ꢁ3
–1
S. M. Peng, Adv. Mater., 1998, 10, 1475–1479; (b) The
average MoRN bond length for terminal nitrido species is
˚
1
4 323(2) A , Z = 16, D
F(000) = 6720, T = 193 K. R
.0866 for 6055 independent reflections with a goodness-of-fit of 0.962.
Further information can be found in the ESI.w
c
= 1.561 g cm , m(Mo-K) = 0.476 mm
,
1
(I > 2s) = 0.0472, wR (all data) =
2
˚
.654 A, based on Cambridge Structural Database information:
1
0
F. H. Allen, Acta Crystallogr., Sect. B: Struct. Sci., 2002, 58,
80–388.
3
1
(a) A. Fu
¨
rstner, in Handbook of Metathesis, ed. R. H. Grubbs,
11 D. M.-T. Chan, M. H. Chisholm, K. Felting, J. C. Huffman and
N. S. Marchant, Inorg. Chem., 1986, 25, 4170–4174.
Wiley-VCH, Weinheim, 2003, vol. 2, pp. 432–462; (b) U. H. F.
Bunz, Acc. Chem. Res., 2001, 34, 998–1010.
(a) W. Zhang, S. Kraft and J. S. Moore, J. Am. Chem. Soc., 2004,
12 3 undergoes a phase change in the solid state between ꢁ90 1C and
˚
2
ꢁ160 1C. The MoRN bond length at ꢁ90 1C is 1.661(4) A; at
˚
126, 329–335; (b) J. Heppekausen, R. Stade, R. Goddard and
A. Furstner, J. Am. Chem. Soc., 2010, 132, 11045–11057.
ꢁ160 1C, the bond length is 1.673(5) A. See ref. 11. Structural data
for 3-B was collected at ꢁ80 1C.
¨
This journal is c The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 7939–7941 7941