Ligand exchange reactions within the coordination sphere of a molybdenum η2(4e)-alkyne complex: The formation of an indole in a cascadereaction involving an alkyne and isonitrile ligands

Article abstract of DOI:10.1039/b208867c

The exchange of phosphorus-containing ligands within the coordination sphere of molybdenum alkyne complexes is shown to be facile, a similar reaction with CNxyl (xyl = C₆H₃Me₂-2,5) results in a remarkable coupling between the alkyne and three isonitrile ligands and C-H bond activation.

Full text of DOI:10.1039/b208867c

Ligand exchange reactions within the coordination sphere of a
molybdenum h2(4e)-alkyne complex: the formation of an indole in a
cascade reaction involving an alkyne and isonitrile ligands
John C. Jeffery, Michael Green and Jason M. Lynam†*
School of Chemistry, University of Bristol, Bristol, UK BS8 1TS. E-mail: jason.lynam@bris.ac.uk;
Fax: +44(0)117 9251295; Tel: +44(0)117 9546325
Received (in Cambridge, UK) 11th September 2002, Accepted 28th October 2002
First published as an Advance Article on the web 18th November 2002
The exchange of phosphorus-containing ligands within the
coordination sphere of molybdenum alkyne complexes is
shown to be facile, a similar reaction with CNxyl (xyl =
C6H3Me2-2,5) results in a remarkable coupling between the
alkyne and three isonitrile ligands and C–H bond activa-
tion.
were surprised to observe the molybdenum-mediated formation
of a substituted indole. Treatment of a CH2Cl2 solution of 1 with
10 equivalents of CNxyl resulted in an instantaneous colour
change to deep red. Slow diffusion of Et2O into this solution
afforded crystals of 4 in 62% yield. Although the NMR spectra¶
5
of a CD2Cl2 solution of 4 confirmed the presence of a h -C5H5
ligand the nature of the remaining ligands could not be deduced
and therefore the structure of 4 was unambiguously determined
by a single crystal X-ray diffraction study.§ The unit cell of 4
contained two crystallographically-independent cations, one of
which is shown in Fig. 2.6 The resulting structure demonstrated
Although our earlier studies of the reactions of nucleophilic
2
reagents with mononuclear h (4e)-bonded alkyne complexes
2
5
[Mo{h (4e)-alkyne}{P(OMe)3}2(h -C5H5)][BF4] led to the
discovery of interesting new chemistry including the formation
of h (3e)-vinyl complexes,1 there was little evidence to suggest
that as well as being coordinated to a h -C5H5 ligand the
2
5
that the trimethylphosphite ligands in these complexes were
labile. Recently, however, 31P NMR magnetisation transfer
experiments have shown that facile exchange occurs between
free P(OMe)3 and the coordinated P(OMe)3 in the complex
molybdenum was also bonded to two terminally-bound CNxyl
ligands. The remaining coordination sites at the metal were
2
5
[Mo{h (4e)-PhC2Ph}{P(OMe)3}2(h -C5H5)][BF4] 1.2 This led
us to explore the reaction of these complexes with other ligands
and, in expanding our investigation to isonitriles, we have
observed the surprising formation of a coordinated substituted
indole.
Our preliminary studies with phosphorus ligands indicated
that, as expected, ligand exchange at molybdenum was facile,
but reversible. Therefore, in order to drive the equilibrium
process to completion, an excess of reacting ligand was used.
For example, treatment of a CH2Cl2 solution (room tem-
perature) of 1 with 10 equivalents of PMe2Ph resulted in an
instantaneous colour change from deep purple to blue. Slow
diffusion of an Et2O layer into this solution afforded blue
crystals of the product 2 in 86% yield. The NMR spectra‡ of a
CD2Cl2 solution of the crystals indicated that a selective
reaction had occurred and that only one P(OMe)3 ligand had
been exchanged by a PMe2Ph. Therefore the product was
Fig. 1 The molecular structure of the cation of complex 2. Hydrogen atoms
removed for clarity and thermal ellipsoids are shown at the 50% probability
level. O(22) is disordered between the two sites O(22A) (80%) and O(22B)
(20%).
2
assigned the formula [Mo{h (4e)-PhC2Ph}{P(OMe)3}(P-
5
Me2Ph)(h -C5H5)][BF4].
The structure of 2 was confirmed by a single crystal X-ray
diffraction study,§ (Fig. 1), and showed that the alkyne ligand
was lying parallel to the Mo–P(OMe)3 vector, consistent with
more effective interaction with the p*∑ -orbital of the coordinated
alkyne.3 The short Mo–C(alkyne) bond lengths [2.0006(26) and
2.0309(25) Å] and the appearance of a low field resonance (d
222.5) for the contact-carbons of the alkyne ligand in the 13C
NMR spectrum of 2 are consistent with the proposed four-
electron bonding mode of the alkyne.4 In a similar experiment,
treatment of a CH2Cl2 solution of 1 with dppe {dppe =
1,2-bis(diphenylphosphinoethane)} afforded the known com-
2
5
plex [Mo{h (4e)-PhC2Ph}(dppe)(h -C5H5)][BF4], 3, in 85%
yield. In this case, however, only one equivalent of dppe was
required to drive the reaction to completion: presumably the
chelate effect prevents any appreciable reaction between 3 and
P(OMe)3 to yield 1. Previously this complex had been obtained
5
in 60% yield by reaction of PhC2Ph with [Mo(dppe)2(h -
C5H5)][BF4].5
In seeking to extend our studies to the preparation of the
compound [Mo{h (4e)-PhC2Ph}(CNxyl)2(h -C5H5)][BF4], we
2
5
Fig. 2 The molecular structure of one of the independent cations of complex
4. Only the ipso carbons of the xyl rings are shown: hydrogen atoms (except
H120) have been removed for clarity. Thermal ellipsoids are shown at the
50% probability level.
† Current address: Department of Chemistry, University of York, Hesling-
ton, York, YO10 5DD. jml12@york.ac.uk; Tel: +44(0)1904432534.
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CHEM. COMMUN., 2002, 3056–3057
This journal is © The Royal Society of Chemistry 2002

taken up by a single tridentate ligand, formed by the coupling of
three isonitrile ligands and the alkyne. Remarkably, one of the
phenyl rings which was previously part of the diphenylacety-
lene ligand had undergone an ortho-C–H activation and coupled
with an isonitrile to give an indole group. The remainder of the
ligand consisted of the carbon and phenyl group of the alkyne
and two isonitrile residues forming a C(Ph)–C–N(xyl)–C–
NH(xyl) backbone. It is presumed that the proton attached to
N(120) was previously attached to the phenyl ring of the alkyne
and migrated during the C–H activation process. The three
molybdenum–carbon bond lengths [Mo(1)–C(119) 2.209(4),
Mo(1)–C(129) 2.189(3), Mo(1)–C(159) 2.254(4), Mo(2)–
C(219) 2.203(4), Mo(2)–C(229) 2.195(4), Mo(2)–C(259)
2.271(4) Å] are consistent with metal–carbon single bonds: the
lack of low-field resonances in the 13C NMR spectrum of 4 also
demonstrates the absence of metal–carbon multiple bonding.
This would imply that the molybdenum atom in 4 has
undergone a formal oxidation from Mo(II) (in complex 1) to
Mo(IV). On the basis of the bond lengths within the ligand
framework, the bonding within the complex is best described by
canonical form A, (Scheme 1), although a contribution to the
bonding from form B cannot be excluded. The ligand backbone
is planar (maximum torsion angle 7.2°), indicating a significant
amount of delocalisation across the skeleton.
Although the transition metal centred coupling of an alkyne
and one or two isonitriles has previously been observed7–9 the
formation of compounds like 4 is without precedent. This
cascade process apparently involves ortho-C–H activation and
the selective linking of three isonitriles with an alkyne. There is
currently interest in the metal-mediated formation of indoles10
and we are exploring the scope of this reaction and also methods
for the removal of the indole from the molybdenum centre.
We would like to thank the Ramsay Memorial Fellowships
Trust and the University of Bristol for a fellowship to J. M. L.
and the Leverhulme Trust for an Emeritus Fellowship to M. G.
The authors would also like thank Professor N. G. Connelly and
Dr C. J. Adams for helpful discussions and Dr A. P. Leedham,
Mr T. Riis-Johansen and Mr H. Hadimov for assistance with X-
ray crystallography.
Notes and references
‡ Selected NMR data for complex 2: 1H (CD2Cl2) d 5.37 (dd, 5 H, 3JPH 1.8,
3JPH 0.7 Hz, C5H5), 3.60 (d, 9 H, 3JPH 11.1 Hz, P{OMe}3), 1.80 (d, 3 H, 2JPH
10.2 Hz, PMe2Ph), 1.48 (d, 3 H, JPH 10.1 Hz, PMe2Ph); 31P (CD2Cl2) d
2
2
2
175.8 (d, JPP 53.0 Hz, P{OMe}3), 30.2 (d, JPP 53.0 Hz, PMe2Ph); 13C
1
(CD2Cl2) d 221.5 (br, PhC2Ph), 139.6 (br, PhC2Ph), 138.9 (dd, JPC 48.1,
3JPC 2.0 Hz, PMe2Ph C1), 130.2 (d, JPC 2.9 Hz, PMe2Ph C4), 129.3 (br,
4
PhC2Ph), 129.0 (d, 3JPC 9.2 Hz PMe2Ph C3), 128.5 (d, 2JPC 9.8 Hz PMe2Ph
C2), 128.5 (s, PhC2Ph), 127.3 (br, PhC2Ph), 95.1 (s, C5H5), 53.6 (d, 2JPC 8.1
Hz, P{OMe}3), 20.4 (d, 1JPC 30.5 Hz, PMe2Ph), 18.5 (dd, 1JPC 33.4, 3JPC 1.7
Hz, PMe2Ph). All the resonances in the 13C NMR spectrum due to the
alkyne ligand in this complex show significant broadening which, coupled
with appearance of only one contact-carbon resonance, is consistent with
restricted rotation of the alkyne ligand. We are currently investigating this
behaviour using variable temperature NMR studies. Elemental analysis:
calc.: C, 52.35; H, 5.13; found: C, 52.31; H, 5.34%.
§
Crystal data: for complex 2: C30H35BO3F4P2Mo, M = 688.3,
monoclinic, space group P21/c, a = 12.0856(13), b = 21.494(2), c =
11.8962(13) Å, b = 97.473(2)°, V = 3064.1(6) Å3, T = 173 K, Z = 4, m
= 0.59 mm21, l¯(Mo-Ka) = 0.71073 Å. 19836 reflections measured, 6992
unique (Rint = 0.0228) which were used in all calculations. The final
wR(F2) was 0.1006 (all data).
For complex 4: C64H60BF4MoN5, M = 1081.95, monoclinic, space
group P21/c, a = 18.992(2), b = 36.245(4), c = 16.5204(18) Å, b =
93.070(2)°, V = 11356(2) Å3, T = 173 K, Z = 8, m = 0.290 mm21, l¯(Mo-
Ka)
= 0.71073 Å.74235 reflections measured, 26054 unique (Rint =
0.0574) which were used in all calculations. The final wR(F2) was 0.1904
(all data). The crystal structure determination of 4 indicated the presence of
some disordered solvent molecules which could not be succesfully
modelled, this probably accounts for the discrepancy in analytical data.
CCDC reference numbers 193466 (2) and 193467 (4). See http:/
/www.rsc.org/suppdata/cc/b2/b208867c/ for crystallographic data in CIF or
other electronic format.
¶ Selected NMR data for complex 4: 1H (CD2Cl2) d 7.48 (s, NH), 4.79 (s, 5
H, C5H5), 2.44 (s, 6 H, CNC6H3Me2), 2.20 (s, 6 H, CNC6H3Me2), 2.18 (s,
12 H, terminal CNC6H3Me2), 1.81 (s, 6 H, CNC6H3Me2); 13C (CD2Cl2), all
resonances are singlets; d 201.9, 171.7, 167.6, 145.6, 142.0, 140.4, 137.3,
136.6, 135.8, 135.5, 135.3, 134.8 (terminal C6H3Me2), 134.2, 130.2, 129.9,
129.3, 129.1, 129.0, 128.8, 128.5 (terminal C6H3Me2), 128.4, 127.3, 126.7,
126.1 (terminal C6H3Me2), 118.7, 118.4, 116.8, 108.5, 90.4 (C5H5), 19.4
(C6H3Me2), 18.5 (terminal C6H3Me2), 18.1 (C6H3Me2), 17.4 (C6H3Me2).
The NH proton was assigned on the basis of a D2O shake coupled with 1H–
1H COSY, 1H–13C HETCOR and 1H–13C COLOC experiments. IR
(CH2Cl2): 2129 (terminal CNxyl), 1605 and 1507 cm21 (ligand backbone).
Elemental analysis: calc.: C, 71.04; H, 5.56; N, 6.47; found: C, 69.66; H,
6.35; N, 6.63%.
1 See D. S. Frohnapfel and J. L. Templeton, Coord. Chem. Rev., 2000,
206, 234 and references therein.
2 A. D. Burrows, N. Carr, M. Green, J. M. Lynam, M. F. Mahon, M.
Murray, B. Kiran, M. T. Nguyen and C. Jones, Organometallics, 2002,
21, 3076.
3 B. E. R. Schilling, R. Hoffmann and J. W. Faller, J. Am. Chem. Soc.,
1979, 101, 592.
4 J. L. Templeton, Adv. Organomet. Chem., 1989, 29, 1.
5 M. L. H. Green, J. Knight and J. A. Segal, J. Chem. Soc., Dalton Trans.,
1977, 2189. This complex may also be prepared by a modification of the
2
5
procedure for the synthesis of [Mo{h (4e)-MeC2Me}(dppe)(h -
C5H5)][BF4]: S. R. Allen, P. K. Baker, S. G. Barnes, M. Green, L.
Trollope, L. Manojlovic-Muir and K. W. Muir, J. Chem. Soc., Dalton
Trans., 1981, 873.
6 The topologies of the two cations are essentially identical, with the
majority of the bond lengths and angles within the two molecules being
the same within experimental error. The cyclopentadienyl ligands
within the two independent molecules show slightly different conforma-
tions and in one of the molecule there is a short contact between H(120)
and a BF4 anion (2.137 Å). The largest discrepancy between the two
cations is in the angle between the two isonitrile ligands, {C(169)–
Mo(1)–C(179) 144.71(14)°; C(269)–Mo(2)–C(279) 147.13(14)°}.
7 J. L. Davidson, M. Green, J. Z. Nyathi, F. G. A. Stone and A. J. Welch,
J. Chem. Soc., Dalton Trans., 1977, 2246.
8 J. L. Davidson, M. Green, J. A. K. Howard and F. G. A. Stone, J. Chem.
Soc., Dalton Trans., 1977, 1779.
9 C. J. Adams, K. M. Anderson, I. M. Bartlett, N. G. Connelly, A. G.
Orpen, T. J. Paget, H. Phetmung and D. W. Smith, J. Chem. Soc., Dalton
Trans., 2001, 1284.
Scheme 1 R = xyl, X2 = BF4, L = P(OMe)3, LA = PMe2Ph, LL = dppe,
LB
= CNxyl. (i) +PMe2Ph, 2P(OMe)3; (ii) +dppe, 2P(OMe)3; (iii)
+CNxyl, 2P(OMe)3.
10 A. Penoi and K. M. Nicholas, Chem. Commun., 2002, 484.
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