Activation of a CNXylyl ancillary ligand in the reaction of electron-deficient alkynes with a phosphido niobocene complex

Article abstract of DOI:10.1039/b517307h

A CNXylyl ancillary ligand was activated in the reaction of electron-deficient alkynes with a phosphido niobecene complex. The synthesis and structure of the novel heterometallacycles were obtained in a one-pot reaction from a phosphido-isocyanide niobecene and the appropriate electron-deficient alkynes. The behavior of isocyanide towards metal-carbon bonds in alkyl transition-metal complexes was documented and insertion reactions were also observed. Heterometallacyclic niobecenes were successfully synthesized through reactions between a phosphidoisocyanide derivative and electron-deficient alkynes.

Full text of DOI:10.1039/b517307h

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Activation of a CNXylyl ancillary ligand in the reaction of electron-deficient
alkynes with a phosphido niobocene complex†
Antonio Antin˜olo, Santiago Garc´ıa-Yuste, Maria Isabel Lopez-Solera, Antonio Otero,* Juan C. Pe´rez-Flores,
Rebeca Reguillo-Carmona and Elena Villasen˜or
Received 22nd November 2005, Accepted 27th January 2006
First published as an Advance Article on the web 9th February 2006
DOI: 10.1039/b517307h
5
The niobium phosphido complex [Nb(g -C5H4SiMe3)2-
(CNXylyl)(PPh2)] (2) undergoes an unusual cycloaddition
reaction with electron-deficient alkynes to give the novel
5
five-membered heteroniobacycles [Nb(g -C5H4SiMe3)2(jC-
=
=
C( N(Xylyl))C(CO2Me) C(R)PPh2-jP)] (R = H 3 and R =
Me 4).
Scheme 1 Synthesis of the complexes 3 and 4.
In the field of metallophosphines, several studies on early tran-
sition metal phosphido complexes have been reported.1 However,
examples of this kind of complex for Group 5 elements still remain
scarce.2
=
groups, respectively, and the initial band for m(C N) of the iso-
cyanide ligand in 2 disappeared. 13C NMR spectroscopy involving
G-HSQC and g-HMBC techniques allowed the assignment of the
resonances of the different carbon atoms of the heteroniobacycle
moiety, particularly the olefinic and iminoacyl carbon atoms.
The 31P NMR spectra showed one broad resonance at d ca.
80.00 ppm for the phosphorus of the phosphine group of the
heterometallacycle. This resonance was shifted downfield with
respect to that for the phosphido ligand in complex 2. This change
is consistent with the expected decrease in electron density on the
phosphorus atom. The chemical shift is in agreement with those
previously found for other related heterometallacycles.7c
We have previously studied the activation of unsaturated
molecules such as alkynes,3 alkenes,4 carbon monoxide5 and
isocyanides6 on Nb–H or Nb–C bonds of niobocene complexes.
As a continuation of this work we have found an unexpected type
of reactivity in phosphido niobocene complexes containing an
isocyanide ancillary ligand, namely CNXylyl, Xylyl = C6H3Me2-
2,6, when reacted with electron-poor alkynes in that cycloaddition
processes gave rise to novel five-membered heteroniobacycles.
Examples of heterometallacycles of late-middle transition metals
from cycloaddition processes involving alkynes are known.7
In order to establish the structure of 3 unambiguously, a
crystallographic analysis was carried out on a suitable single
crystal (Fig. 1).‡ The results of this analysis are in agreement
with the spectroscopic data in solution. Selected parameters are
summarized in Fig. 1. To the best of our knowledge, this molecular
structure is one of the few examples described for niobocene
complexes containing a heteroniobacycle moiety.9
The niobium atom is bonded to two cyclopentadienyl rings
in a g mode, the phosphorus atom from the diphenylphosphine
group and the carbon atom of the iminoacyl group in a pseudo-
tetrahedral geometry. The five-membered chelate ring is slightly
puckered, with the niobium atom 0.607(6) A out of the mean plane
We report here the synthesis and structures of the novel hetero-
5
=
metallacycles [Nb(g -C5H4SiMe3)2(jC-C( N(Xylyl))C(CO2Me)
=
C(R)PPh2-jP)] (R = H 3 and R = Me 4), which were obtained in
a one-pot reaction from a phosphido–isocyanide niobocene and
the appropriate electron-deficient alkynes.
5
The phosphido complex [Nb(g -C5H4SiMe3)2(CNXylyl)(PPh2)]
(2), a suitable precursor for 3 and 4, was prepared by a deproto-
5
nation process on [Nb(g -C5H4SiMe3)2(CNXylyl)(PHPh2)]Cl (1)8
5
using NaOH as a base.
Complexes 3 and 4 were successfully synthesized by the
[3 + 2] cycloaddition reaction of 2 with methyl propiolate
(HC≡CCO2Me) and methyl butynoate (MeC≡CCO2Me), respec-
tively, according to Scheme 1. The reaction mixture became a red
solution, the solvent was removed and the product washed with
cold hexane to afford 3 and 4 as red solids.
˚
passing through the P1, C1, C2 and C3 atoms. The dihedral angle
between planes Nb1–P1–C1 and P1–C1–C2–C3 is 18.8(1)◦. All
distances and angles are normal. The C1–C3 and C2–C3 bond
˚
lengths of 1.515(5) and 1.340(6) A, respectively, indicate that there
Complexes 3 and 4 were characterized by IR and NMR
spectroscopy in solution and 3 was further characterized by single-
crystal X-ray diffraction in the solid state. The spectroscopic data
are consistent with the formation of a heteroniobacycle moiety.
The IR spectra contain two bands at ca. 1617 and 1585 cm−1
is no charge delocalization in this ring. The Nb1–C1 distance
of 2.305(4) is slightly longer than those found in other niobium
iminoacyl derivatives.10
The formation of complexes 3 and 4 can be understood in terms
of the mechanism depicted in Scheme 2. The first step involves a
(1,2) insertion of the electron-deficient alkyne into the Nb–P bond
to give an alkenyl–phosphine intermediate. This type of complex
has been isolated in analogous reactions of electron-deficient
=
=
corresponding to m(C N) and m(C C) of iminoacyl and olefinic
Department of Inorganic Chemistry, University of Castilla-La Mancha,
Ciudad Real, Spain. E-mail: Antonio.Otero@uclm.es; Fax: +34 926
295318; Tel: +34 926 295300
5
alkynes with the complex [Nb(g -C5H4SiMe3)2(CO)(PPh2)].11 In
the second step it is proposed that a new intermediate is formed by
a (1,1) migratory insertion of the isocyanide ligand into the Nb–C
† Electronic supplementary information (ESI) available: Experimental
details. See DOI: 10.1039/b517307h
This journal is
The Royal Society of Chemistry 2006
Dalton Trans., 2006, 1495–1496 | 1495
©

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alkyne to give a charge-separated intermediate followed by an
attack on the isocyanide group.7
In summary, we have succeeded in the synthesis of heteromet-
allacyclic niobocenes through reactions between a phosphido–
isocyanide derivative and electron-deficient alkynes. These results
open a different way to prepare new types of heteroniobacycle.
We gratefully acknowledge financial support from the Di-
reccio´n General de Investigacio´n Cient´ıfica Spain (MEC Grant.
No.BQU2002-04638-CO2-02) and the Junta de Comunidades de
Castilla-La Mancha (Grant. N◦ PAC-02-003, GC-02-010, PBI05-
23 and PBI-05-029).
Notes and references
‡ Crystallographic data: C41H49NNbO2PSi2·1.5(C6H5Me), triclinic, space
¯
˚
group P1, a = 10.7939(5), b = 12.2093(6), c = 19.3334(9) A, a = 93.144(3),
◦
3
˚
b = 102.049(3), c = 108.324(3) , V = 2345.4(2) A , Z = 2, Dc
=
1.281 g cm−3, k(Mo-Ka) = 0.71073 A, l(Mo-Ka) = 0.382 mm−1, T =
100(2) K. X8 Appex II CCD diffractometer, graphite monochromator,
9067 unique reflections, R = 0.0494, Rw = 0.1338. Toluene groups
were located in disordered positions. CCDC reference number 294817.
For crystallographic data in CIF or other electronic format see DOI:
10.1039/b517307h
˚
Fig. 1 ORTEP diagram of 3 ◦with 40% probability ellipsoids. Selected
˚
bond lengths (A) and angles ( ): Nb1–C1, 2.305(4); Nb1–P1, 2.515(1);
P1–C2, 1.819(4); C1–C3, 1.515(5); C2–C3, 1.340(6); P1–Nb1–C1, 76.1(1);
Nb1–P1–C2, 103.7(1); Nb1–C1–C3, 117.0(3); P1–C2–C3, 116.7(3); C1–
C3–C2, 121.7(4). Hydrogen bond distances and angles: d(H57B · · · O1)a =
1 E. Hey-Hawkins, Chem. Rev., 1994, 94, 1661.
2.473 A; d(C57 · · · O1) = 3.117(5) A; ∠C57–H57B · · · O1 = 124.3◦.
a
a
˚
˚
2 Selected references: (a) G. Bonnet, J.-C. Leblanc and C. Moise,
New J. Chem., 1988, 12, 551; (b) G. I. Nikonov, L. G. Kuzmina, P.
Mountford and D. A. Lemenovskii, Organometallics, 1995, 14, 3588;
(c) G. I. Nikonov, D. A. Lemenovskii and J. Lorberth, Organometallics,
1994, 13, 3127.
b
d(H86 · · · O1)b = 2.453 A; d(C86 · · · O1) = 3.30(1) A; ∠C86–H86 · · ·
˚
˚
◦
O1b = 152.1 . d(H65 · · · O2)c = 2.704 A; d(C65 · · · O2) = 3.22 (1) A;
c
˚
˚
c
d
d
˚
∠C65–H65 · · · O2 = 115.9. d(H72 · · · O2) = 2.898 A; d(C72 · · · O2) =
◦
d
a
b
˚
3.75(1) A; ∠C72–H72 · · · O2 = 153.1 . Symmetry operations: x, y, z; x,
3 (a) A. Antin˜olo, I. del Hierro, M. Fajardo, S. Garcia-Yuste, A. Otero, O.
Blacque, M. M. Kubicki and J. Amaudrut, Organometallics, 1996, 15,
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Villasen˜or, Organometallics, 1996, 15, 5507.
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2002, 21, 2460.
5 A. Antin˜olo, I. del Hierro, M. Fajardo, A. Otero and Y. Mugnier,
Organometallics, 1997, 16, 4161.
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9 S. G. Bott, D. M. Hoffman and S. P. Rangarajan, J. Chem. Soc., Dalton
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Y. Mourad, Organometallics, 1993, 12, 381.
Scheme 2
bond, which subsequently undergoes ring closure by coordination
of the phosphine group.
The behaviour of isocyanide towards metal–carbon bonds in
alkyl transition-metal complexes is well documented and insertion
reactions are usually observed.12 This type of reactivity has been
described by some of us in niobocene complexes.6a Attempts to
characterize some of the proposed intermediates by NMR studies
were unsuccessful. However, other alternative proposals can not
be ruled out—e.g., through a concerted transition state or by initial
nucleophilic attack by the phosphido on the electron-deficient
11 A. Antin˜olo, S. Garc´ıa-Yuste, A. Otero and R. Reguillo, unpublished
results.
12 E. Singleton and H. E. Oosthuizen, Adv. Organomet. Chem., 1983, 22,
209.
1496 | Dalton Trans., 2006, 1495–1496
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The Royal Society of Chemistry 2006
©