The first stable scandocene: Synthesis and characterisation of bis(η-2,4,5-tri-tert-butyl-1,3-diphosphacyclopentadienyl)scandium(II)

Article abstract of DOI:10.1039/a800089a

The first isolable example of a molecular scandium(II) complex, the dark purple scandocene [Sc(η⁵-P₂C₃Bu₃^t)] 1, has been isolated from the cocondensation of scandium vapour with tert-butylphosphaalkyne, Bu^tCP at 77 K.

Full text of DOI:10.1039/a800089a

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The first stable scandocene: synthesis and characterisation of
bis(h-2,4,5-tri-tert-butyl-1,3-diphosphacyclopentadienyl)scandium(ii)
Polly L. Arnold, F. Geoffrey N. Cloke*† and John F. Nixon*
School of Chemistry, Physics and Environmental Sciences, University of Sussex, Brighton, UK BN1 9QJ
The first isolable example of a molecular scandium(II)
P3C2But2)], can be extracted from the sublimant and has been
5
complex, the dark purple scandocene [Sc(h -P2C3But3)] 1,
the subject of a recent communication.8
has been isolated from the cocondensation of scandium
The 1,2,4-triphospha-3,5-di-tert-butylcyclopentadienyl ring
is regularly formed in conjunction with the 1,3-diphospha-
2,4,5-di-tert-butylcyclopentadienyl ring in cyclisation reactions
at a number of both transition elements and main group metal
centres.5b Since the yield of 1 is approximately the same as that
vapour with tert-butylphosphaalkyne, ButCP at 77 K.
To date the only stable divalent lanthanoid metallocenes are
those of the elements Sm, Eu and Yb for reasons which have
been well documented.1 These metallocenes are all highly
reactive, showing strong reducing properties and consequent
unusual reaction chemistry.2 Very recently, two lanthanum
species that are intermediates in the potassium reduction of
[La(CpB)3] {CpB = h-C5H3(SiMe3)2} have been observed and
characterised, by EPR spectroscopy, as containing a LaII centre;
one of the latter has been shown to be a dme solvated
metallocene, La(CpB)2(dme).3
5
of the scandium(i) triple-decker complex [(h -P3C2But2)Sc(m-
6
6
5
h :h -P3C3But3)Sc(h -P3C2But2)], which contains a greater
proportion of ligated phosphorus, the number of phosphaalkyne
monomers is preserved between the two complexes. This leads
us to believe, at least in this case, that the fusion of a number of
ButCP molecules around the metal does not simply involve one
metal centre.
Complex 1 is extremely air- and moisture-sensitive and is
soluble in pentane and other non-polar solvents; a solution in thf
decomposes immediately giving no tractable products, preclud-
ing electrochemical studies. Unfortunately, single crystals of 1
suitable for X-ray structural determination could not be
obtained owing to its extreme solubility in solvents with which
it does not react. The intense purple colour of 1, due to an
absorption maximum observed at 571 nm in the UV–VIS
spectrum, is characteristic of a charge transfer band and appears
to be a common feature in low- and zero-valent group 3 and
lanthanide complexes.4a
Magnetic and EPR studies have been employed to study the
d1 electronic configuration of 1. The EPR spectrum of 1 in
toluene at 295 K is broad and shows coupling only to the 45Sc
nucleus, presumably due to rapid relaxation at this temperature,
characterised by giso 1.9823 and Aiso(45Sc) 3.757 mT (I = 7/2,
100%). Upon cooling the sample to a glass at 120 K, well
In recent years metal vapour synthetic techniques have
afforded a variety of novel and reactive low valent organome-
tallic complexes of group 3 and the lanthanides directly from the
elements; the technique remains the only route to zero-valent
molecular lanthanide complexes.4 Cyclisation reactions under-
gone by 2,2-dimethylpropylidynephosphine (tert-butylphos-
phaalkyne, ButCP) at metal atoms during cocondensation
reactions are of considerable current interest since the choice of
metal clearly dictates the nature of ring or rings formed.5 The
most common cyclisation observed to date is the combination of
five phosphaalkyne units resulting in the isolation of sandwich
5
5
complexes such as [(V(h -P3C2But2)(h -P2C3But3)].6 Here, we
report the synthesis of [Sc(h -P2C3But3)2] 1, which represents
5
only the second reported scandium(ii) complex. The first
example, also described by our laboratory, resulted from ligand
metallation during the cocondensation of scandium vapour with
tri-tert-butylbenzene; this product was unfortunately not sepa-
rable from the admixture with [Sc(h-C6H3But3-1,3,5)2].7 The
present work is thus the first instance in which we have been
able to isolate, and thus fully characterise, a complex of
scandium in this oxidation state.
resolved coupling to scandium is observed, accompanied by
1
further hyperfine coupling to the four phosphorus nuclei (I = ,
2
100%) which are equivalent on the EPR timescale at this
temperature, Fig. 1. The resonance is characterised by g4
2.0098 with a calculated value of g∑ 1.9273 and is well
simulated using values of A4(45Sc) 2.99 mT, A∑(45Sc) 5.29 mT
and A(31P) 0.72 mT. The solution magnetic moment is found to
be 1.70 mB at room temperature,9 showing no evidence for any
orbital contribution to the spin only value (meff = 1.73 mB).
Cocondensation of electron beam vaporised scandium with
an excess of ButCP at 77 K affords a dark brown matrix which
is soluble in hexanes. Two products are separable from the dark
solution by flash sublimation (220 °C, 1025 mbar), Scheme 1.‡
5
The dark purple microcrystalline [Sc(h -P2C3But3)2] 1 may be
obtained from the sublimate by repeated sublimation and
removal of by-product volatile oils. The forest-green triple-
100
50
5
6
6
5
decker complex [(h -P3C2But2)Sc(m-h :h -P3C3But3)Sc(h -
But
P
But
P
But
But
0
P
P
P
But
Sc
But
P
P
–196 °C
But
–50
–100
Sc(at) + ButC
P
+
P
Sc
But
But
But
Sc
P
P
But
P
But
P
P
But
3200
3250
3300
3350
B / G
3400
3450
3500
1
Scheme 1 Synthesis of 1
Fig. 1 EPR spectrum of 1 (toluene glass, 120 K)
Chem. Commun., 1998
797

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Warren has suggested,10 in a review of the ligand field theory
for pseudo-axially symmetric metallocenes that a distortion and
subsequent lowering to C2v symmetry occurs since the
degenerate ground state is necessarily Jahn–Teller unstable. The
majority of d1 metallocene systems extant in the literature are
bent from pseudo-axial symmetry and the resulting quenching
of any orbital contribution results in isotropic g values close to
resultant solid yielded a purple oil. From this, 1 is obtained pure in 5–10%
yield, ca. 30 mg (based on scandium) after resublimation. UV–VIS for 1:
lmax/nm (pentane) 571 (e/dm3 mol21 cm21 15000). MS (EI): m/z 583
(90%, M+), 269 (43, P2C3But3+); HRMS: m/z found 583.273922.
C30P4H54Sc requires 583.273519. Solution state magnetism (Evans method,
[2H8]toluene): meff = 1.70 mB.
2.00 and near spin-only values of the magnetic moment.11
A
1 W. J. Evans, Polyhedron, 1987, 6, 803.
similar argument is appropriate here since 1 is not modelled
well as axial, owing to the unsymmetric substitution of the
cyclopentadienyl rings.12 The phosphorus atom in the diphos-
phatri-tert-butylcyclopentadienyl ligands provides a second
magnetic nucleus, thus the resolution of coupling to the ligand
nuclei by the electron, particularly at lower temperatures when
the relaxation processes are slowed, strongly suggests that the
2 W. J. Evans, J. W. Grate, L. A. Hughes, H. Zhang and J. L. Atwood,
J. Am. Chem. Soc., 1985, 107, 3728; P. B. Hitchcock, J. A. K. Howard,
M. F. Lappert and S. Prashar, J. Organomet. Chem., 1992, 437, 177;
W. J. Evans, L. A. Hughes and T. P. Hanusa, Organometallics, 1987, 5,
1285; W. J. Evans, T. A. Ulibarri and J. W. Ziller, J. Am. Chem. Soc.,
1988, 110, 6877; C. J. Burns and R. A. Andersen, J. Am. Chem. Soc.,
1987, 109, 5853.
3 M. C. Cassani, M. F. Lappert and F. Laschi, Chem. Commun., 1997,
1563.
2
2
single electron resides in the degenerate dxy or dx 2y orbitals
2
(rather than the dz orbital), as predicted.
4 (a) F. G. N. Cloke, Chem. Soc. Rev., 1993, 17; (b) P. L. Arnold,
F. G. N. Cloke and P. B. Hitchcock, Chem. Commun., 1997, 481.
5 J. F. Nixon, A. G. Avent, F. G. N. CLoke, K. R. Flower, P. B. Hitchcock
and D. M. Vickers, Angew. Chem., 1994, 106, 2406; Angew. Chem., Int.
Ed. Engl., 1994, 33, 2330; J. F. Nixon, Chem. Soc. Rev., 1995, 319 and
references therein; D. M. Vickers, D.Phil. Thesis, University of Sussex,
1997.
6 F. G. N. Cloke, K. R. Flower, P. B. Hitchcock and J. F. Nixon, J. Chem.
Soc., Chem. Commun., 1995, 1659.
7 F. G. N. Cloke, K. Khan and R. N. Perutz, J. Chem. Soc., Chem.
Commun., 1991, 1372.
8 P. L. Arnold, F. G. N. Cloke, P. B. Hitchcock and J. F. Nixon, J. Am.
Chem. Soc., 1996, 118, 7630.
9 NMR method; [1H8]toluene solution, 190–320 K.
10 K. D. Warren, J. Phys. Chem., 1973, 77, 1681.
11 K. D. Warren, Struct. Bonding (Berlin), 1976, 27, 45; K. D. Warren,
Inorg. Chem., 1974, 13, 1317; B. A. Goodman and J. B. Raynor, Adv.
Inorg. Radiochem., 1970, 218; R. Couttes and P. C. Wailes, Inorg. Nucl.
Chem. Lett., 1967, 3, 1.
The existence and stability of 1, which thus completes the
series of divalent metallocenes for the 3d elements, presumably
result from a combination of steric shielding of the diphospha-
tri-tert-butylcyclopentadienyl rings and the increased
p-electron accepting properties of the rings induced by
phosphorus incorporation.
The authors are grateful to Dr A. Abdul-Sada for determina-
tion of the mass spectrum of 1, to Mr C. Dadswell for recording
of the EPR spectrum of 1 and to the EPSRC (ROPA) for
funding.
Notes and References
† E-mail: f.g.cloke@sussex.ac.uk
‡ Scandium (0.30 g, 0.66 mol) was vaporised (power input 200 W) and
cocondensed with ButCP (10 g, 0.1 mol, M:L ratio 1:10) at 77 K.1 The dark
brown matrix formed over a period of 2.5 h persisted on warming to room
temperature under an inert atmosphere. The product was washed from the
reactor with hexanes (2 l), filtered through a bed of Teflon powder (20 mm),
and evaporated to dryness. Flash sublimation (by introduction of the
sublimer into a tube furnace pre-heated to 220 °C) at 1025 mbar of the
12 A. H. Maki and T. E. Berri, J. Am. Chem. Soc., 1965, 87, 4437.
Received in Basel, Switzerland, 5th January 1998; 8/00089A
798
Chem. Commun., 1998