Multiple α-agostic interactions in a metal-methyl complex: The neutron structure of [Mo(NC6H3Pri2-2,6) 2Me2]

Article abstract of DOI:10.1039/a803221a

The neutron structure of [Mo(NC₆H₃Prⁱ₂)-2,6) ₂Me₂] reveals two highly distorted methyl ligands; this distortion is due to the presence of multiple C-H...M α-agostic interactions.

Full text of DOI:10.1039/a803221a

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Multiple a-agostic interactions in a metal–methyl complex: the neutron
i
structure of [Mo(NC H Pr -2,6) Me ]
6
3
2
2
2
Jacqueline M. Cole*a,b† Vernon C. Gibson, Judith A. K. Howard, Garry J. McIntyre and Gary L. P.
c
a
b
Walkerc
a
Department of Chemistry, University of Durham, South Road, Durham, UK DH1 3LE
Institut Laue Langevin, B.P. 156, 38042 Grenoble Cedex 9, France
Department of Chemistry, Imperial College, South Kensington, London, UK SW7 2AY
b
c
The neutron structure of [Mo(NC
6
H
3
Pri
2
)-2,6)
2
Me
2
] reveals
A large amount of librational motion therefore exists, thus
disguising the precise position of the hydrogen atoms. However,
despite the librational effects, the average geometry gives
conclusive results. The coordination of each methyl group is
markedly distorted from regular tetrahedral geometry and of
special note are the particularly small angles Mo(1)–
C(13)–H(13A) [103.7(4)°] and Mo(1)–C(13)–H(13B)
[104.7(3)°] which are consistent with the hydrogen atoms being
drawn towards the metal centre. Consequently, the angles,
Mo(1)···H(13A)–C(13) (52.6°) and Mo(1)···H(13B)–C(13)
(51.8°) are noticeably larger than those corresponding to the
hydrogen atoms H(13C) and H(13A) (43.0 and 43.9° re-
spectively).
two highly distorted methyl ligands; this distortion is due to
the presence of multiple C–H···M a-agostic interactions.
Since the formalisation of the three-centre C–H···M ‘agostic’
interaction by Brookhart and Green in the mid 1980s, these
1
interactions have been found to occur in many coordinatively
unsaturated alkyl, aryl and polyhapto hydrocarbon metal
2
complexes. They are relatively readily observed and detected
in alkyl derivatives possessing b-hydrogens, either in solution
for rapidly averaging agostic hydrogens via the effect of
isotopic perturbation on C–H coupling constants and chemical
shift positions, or in the solid state by X-ray crystallography.‡
Agostic interactions in metal–methyl species have proved more
difficult to detect because (1) any isotopic perturbation of the
resonance effect is diluted over a greater number of hydrogen
environments and (2) in X-ray structure determinations, there
are no obvious ligand distortions of the type seen in longer chain
alkyls and it is inherently difficult with X-rays to locate the
methyl hydrogen atom positions with any accuracy. The only
reliable method of establishing the presence of agostic inter-
actions in metal–methyl complexes is via diffraction.
The Mo(1)···H(13A) and Mo(1)···H(13B) distances [2.585(8)
and 2.598(5) Å, respectively] are also significantly shorter than
Mo(1)···H(13C) and Mo(1)···H(13A) [2.755(5) and 2.76(1) Å]
and the Mo(1)–C(13) bond length of 2.112(2) Å is very short,
To date, the only metal methyl complex to have been
characterised by neutron diffraction that shows clear evidence
3
for an a-agostic methyl C–H···M interaction is [TiMeCl -
3
(
Me
2 2 2 2
PCH CH PMe ). Here, we describe the neutron struc-
ture of the coordinatively unsaturated molybdenum a-agostic
i
dimethyl complex [Mo(NC
pared from the reaction of [Mo(NC
equiv. of methylmagnesium chloride. Full synthetic details
6
H
3
Pr
2
-2,6)
2
Me
2
] which was pre-
i
H
6 3
Pr
2
-2,6) Cl (dme)] with
2
2
2
4
will be reported elsewhere. A crystal of dimensions 6.0 3 2.0
3
2
1.0 mm was grown from a saturated pentane solution at
i
6 3 2 2 2
35 °C. The molecular structure of [Mo(NC H Pr -2,6) Me ]
is shown in Fig. 1, along with selected bond lengths and
angles.
The geometry about the metal centre is best described as
distorted tetrahedral with inter-ligand angles in the range
107–114°. The bond distances for the Mo–N and N–C(ipso)
bonds [1.756(2) and 1.382(2) Å, respectively] in combination
with Mo–N–C angles [159.6(1)°] are consistent with linear
5
imido ligands. The two methyl groups are crystallographically
equivalent so the following discussion concentrates on only one
methyl group. Each methyl group shows a partial positional
disorder amongst the hydrogen atom sites, which can be
described as resulting from a slight rocking motion about C(13),
the pivot atom for a fluxional rotation which, averaged over all
possible sites in all unit cells, gives the ‘static average’ picture
shown. H(13A) can be resolved into two positions [(H13A) and
Pri
6 3 2 2 2
Fig. 1 The molecular structure of [Mo(NC H -2,6) Me ] with 50%
probability thermal ellipsoids shown on the methyl hydrogen atoms, thus
indicating the extent of libration. Selected bond dimensions (Å and °):
Mo(1)–N(1) 1.756(2), Mo(1)–C(13) 2.112(2), Mo(1)···H(13A) 2.585(8),
Mo(1)···H(13A) 2.76(1), Mo(1)–H(13B) 2.598(5), Mo(1)···H(13C) 2.755(5),
N(1)–C(1) 1.382(2), C(13)–H(13A) 1.07(1), C(13)–H(13A) 1.09(1),
C(13)–H(13B) 1.068(7), C(13)–H(13C) 1.049(8); N(1)–Mo(1)–N(1A)
(
H13A)] with occupancies in the ratio 2:1 and these refine
1
12.5(1), N(1)–Mo(1)–C(13) 107.86(7), C(13)–Mo(1)–C(13A) 114.1(2),
independently both isotropically and anisotropically. The latter
tends to a distortion and an overlap in the two density
distributions, but with derived molecular parameters very close
to those reported herein, which result from an isotropic
description of the disordered sites.
C(13)–Mo(1)···H(13A) 23.7(2), 21.0(3),
C(13)–Mo(1)···H(13A)
C(13)–Mo(1)···H(13B) 23.4(2), C(13)–Mo(1)···H(13C) 19.8(2), Mo(1)–
N(1)–C(1) 159.6(1), Mo(1)–C(13)–H(13A) 103.7(4), Mo(1)–C(13)–C(13A)
115.1(6),
117.2(3).
Mo(1)–C(13)–H(13B)
104.7(3),
Mo(1)–C(13)–H013C)
Chem. Commun., 1998
1829

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comparing with literature values of 2.554 Å (mean) and 2.189 Å
Institut Laue Langevin (ILL), Grenoble, France in a beam of wavelength
0.8405(2) Å obtained by reflection from a Cu(220) monochromator. Half-
wavelength contamination was removed with an erbium filter. The sample
was mounted on the cold head of an Air Products closed-cycle refridgerator.
Background corrections following Lehmann and Larsen7 and Lorentz
6
(lower quartile). This short Mo–C distance is also likely to be
a consequence of the agostic interactions. All C–H distances are
identical within experimental error. Of the two a-agostic
hydrogen atoms per methyl group (therefore, four agostic
interactions per molecule) both appear strong and of the same
strength, judging by their relative geometries.
corrections were applied. Cryostat shield and absorption corrections were
made using the local program ABSCAN and the DATAP program8
respectively (transmission range: 0.6477–0.8045). The structure was
More than one agostic interaction in [Mo(NC
6
H
3
Pri
2
-
refined by full-matrix least-squares refinement using SHELXL-93 against
9
2
,6) Me ] is not unreasonable since this molecule is quite
2
2
2100 reflections. Refinement of 312 positional and anisotropic displace-
severely coordinatively unsaturated. It is not possible to place a
precise electron count on the complex since, for symmetry
reasons, the two cis imido ligands are in direct competition for
ment parameters for all atoms [except for isotropic H(13A) and H(13A)]
2
converged to R1 [I > 2s(I)] = 0.0467 and wR [I > 2s(I)] = 0.0462
2
2
[
o
w = 1/s (F ) ] with S = 1.431.
p
one of the metal d orbitals. This means that the electron count
must be below 16 electrons (the count derived from assuming
that both imido ligands can form two p bonds) and formally
could be as low as 14 electrons (i.e. the count arising from three
p-bonds between the molybdenum and the two imido groups).
In general, the results described here underscore the notion that
agostic interactions will occur whenever a metal centre
possesses a suitably oriented and energetically accessible
vacant orbital, and that multiple interactions of this kind may
occur at a single metal centre. The neutron diffraction study
1
2
M. Brookhart and M. L. H. Green, J. Organomet. Chem., 1983, 250,
95.
3
See, for example: A. D. Poole, D. N. Williams, A. M. Kenwright,
V. C. Gibson, W. Clegg, D. C. R. Hockless and P. A. O’Neil,
Organometallics, 1993, 12, 2549; J. M. Bondcella, M. L. Cajigal and
K. A. Abboud, Organometallics, 1996, 15, 1905; R. H. Grubbs and
G. W. Coates, Acc. Chem. Res., 1996, 29, 85; D. Braga, F. Grepioni,
K. Biradha and G. R. Desiraju, J. Chem. Soc., Dalton Trans., 1996, 3925;
M. Etienne, R. Mathieu and B. Donnadieu, J. Am. Chem. Soc., 1997, 119,
3
218; W. Trakarnpruk, I. HylaKryspin, A. M. Arif, R. Gleiter and
described here shows clear evidence for four a-agostic methyl
R. D. Ernst, Inorg. Chim. Acta, 1997, 259, 197.
Z. Dawoodi, M. L. H. Green, V. S. B. Mtetwa, K. Prout, A. J. Schultz,
J. M. Williams and T. F. Koetzle, J. Chem. Soc., Dalton Trans., 1986,
1629.
i
C–H···M interactions within [Mo(NC
6
H
3
Pr
2
-2,6)
2
Me
2
]. It
3
represents the second metal–methyl complex showing
a-agostic interactions and the first one to show multiple agostic
interactions.
The authors wish to thank the Institut Laue Langevin,
Grenoble, France (J. M. C.) and the EPSRC (J. M. C. and
G. L. P. W.) for financial support.
4 V. C. Gibson, C. Redshaw, G. L. P. Walker, J. M. Cole, J. A. K. Howard,
V. J. Hoy and L. Kuzmina, J. Chem. Soc., Dalton Trans. in
preparation.
5
D. E. Wigley, Prog. Inorg. Chem., 1994, 42, 239; W. A. Nugent and
J. M. Mayer, Metal–Ligand Multiple Bonds, Wiley Interscience, New
York, 1988; A. Bell, W. Clegg, P. W. Dyer, M. R. J. Elsegood,
V. C. Gibson and E. L. Marshall, J. Chem. Soc., Chem. Commun., 1994,
Notes and References
2
247.
†
Present address: School of Physical Sciences, University of Kent at
6 F. H. Allen, L. Brammer, O. Kennard, A. G. Orpen, R. Taylor and
D. G. Watson, J. Chem. Soc., Dalton Trans., 1989, S1.
7 M. S. Lehmann and F. K. Larsen, Acta Crystallogr. Sect. A, 1974, 30,
580.
8 P. Coppens, in The Evaluation of Absorption and Extinction in Single-
Crystal Structure Analysis in Crystallographic Computing, ed. F. R.
Ahmed, Munksgaard, 1970.
Canterbury, Canterbury, Kent, UK CT2 7NR. E-mail: J.M.Cole@ukc.
ac.uk
‡
It should be noted that the evidence for b-agostic interactions is often
derived from other structural parameters such as a low M–C–C bond angle
rather than direct location of the agostic hydrogen atom position.
i
§
Crystal data for [Mo(NC
H
6 3
Pr
2 2
-2,6) Me
2
]:C26
H40MoN
2
, M
r
= 475.94,
deep orange–red rectangular crystal (6.0 3 1.8 3 1.0 nm), monoclinic,
9 G. M. Sheldrick, SHELXL-93. Program for the refinement of crystal
structures, University of G o¨ ttingen, Germany, 1993.
space group C2/c, a = 20.240(4), b = 6.550(1), c = 19.910(4) Å,
3
b = 103.99(3)°, V = 2561.2(8) Å , Z = 4, T = 150.0(2) K, D
c
= 1.234
23
21
g cm , F(000) = 348, m = 0.233 mm , 2110 unique reflections (6.56 @
q @ 60.02°) were measured on the D9 four circle diffractometer at the
2
Received in Cambridge, UK, 29th April 1998; 8/03221A
1830
Chem. Commun., 1998