Synthesis and first structural characterisation of a homoleptic tetraorganochromate(III) salt

Article abstract of DOI:10.1039/a804473b

The homoleptic, pseudo-octahedral, d³ species [Li(thf)₄]-[Cr^III(C₆Cl₅) ₄] 1 has been obtained and characterised by analytical, spectroscopic and X-ray diffraction methods.

Full text of DOI:10.1039/a804473b

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Synthesis and first structural characterisation of a homoleptic
tetraorganochromate(iii) salt
Pablo J. Alonso, Larry R. Falvello, Juan Fornie´s,* M. Angeles Garc´ıa-Monforte, Antonio Mart´ın, Babil
Menjo´n and Gerardo Rodr´ıguez
Instituto de Ciencia de Materiales de Arago´n, Facultad de Ciencias, Universidad de Zaragoza-C.S.I.C., Pza. S. Francisco s/n,
E-50009 Zaragoza, Spain. E-mail: forniesj@posta.unizar.es
The homoleptic, pseudo-octahedral, d3 species [Li(thf)4]-
[CrIII(C6Cl5)4] 1 has been obtained and characterised by
analytical, spectroscopic and X-ray diffraction methods.
and the strain of the metallacycle formed. The overall geometry
of the [Cr(C6Cl5)4]2 anion is similar to that found in the related
heavy-metal complexes [Pt(C6Cl5)4]15 and [Rh(C6Cl5)4]2.16
The EPR spectrum of 1 was registered (Fig. 2)∑ and analysed
as a particular case** of an (S = 3/2) spin system that behaves
as an (S = 1/2) system with the following spin Hamiltonian:
The synthesis of aryl derivatives of chromium(iii) and the study
of their properties have drawn the attention of numerous
research groups since the very beginning of this century, shortly
after the synthesis of the first organomagnesium reagents. Early
reports, however, were discouraging1 or puzzling2 and it was
not until the late fifties that s-aryl complexes of chromium(iii)
were isolated and recognised as species of limited stability,
which are prone to rearrange, eventually yielding p-arene
compounds of low-valent chromium.3 This tendency to decom-
pose is more pronounced in compounds with some degree of
coordinative unsaturation,† which may be a principal reason for
the lack of homoleptic (i.e., not bearing additional stabilising
ligands) organometallic compounds of formula [CrIIIR4]2. As
far as we know, the only organometallic precedents for this
stoichiometry are probably the salts Li[Cr(C6H4OMe-
2)4]·Et2O5 and Li[Cr(C6H2Me3-2,4,6)4]·4thf;6 however, no
structural information is available for them.‡ Interestingly,
some related tetraalkylchromate(iii) salts had also been detected
as solution species but could not even be isolated because they
readily suffered oxidation giving the more stable neutral
tetraalkychromium(iv) derivatives.8
H = mBB(gxAlxSx + gyAlySy + gzAlzSz)
(1)
In the preceding equation li (i = x, y, z) are the direction cosines
of the magnetic field of magnitude B referred to the principal
axes of the g˜-tensor. Using this spin-Hamiltonian and assuming
Lorentzian line shapes, we have obtained an excellent agree-
ment between the experimental [Fig. 2(a)] and calculated [Fig.
2(b)] spectra for the following set of effective principal g-values
and band halfwidths: gxA = 4.02, gyA = 3.55, gzA = 1.946; Wx =
15.0 mT, Wy = 22.5 mT, Wz = 10.0 mT. The relationship
between the effective and true principal g-factors is a function
of the a priori unknown h-parameter which accounts for the
departure from axial symmetry.** Since the observed EPR
spectrum is practically axial it is reasonable to assume an axial
true g˜-tensor as well. Under this assumption the following true
g-values have been derived: g∑ = 1.98 and g4 = 1.90 with h =
0.08(1). It is interesting that, while the g∑-value is typical for a
Cr3+ ion, the g4-value departs from that of the free electron by
more than what is usually observed for this paramagnetic entity.
This could be attributed to an extra orbital contribution due to
the ligand as a consequence of the covalent bonding.
We have prepared the tetraorganochromate(iii) salt [Li-
(thf)4][CrIII(C6Cl5)4] 1 by arylation of [CrCl3(thf)3]9 with
10
LiC6Cl5 in 1 : 5 molar ratio.§ Complex 1 is a violet solid of
limited thermal stability; since it is also air- and moisture-
sensitive, it must be synthesised, stored and handled under an
inert atmosphere and at low temperatue (below 0 °C).
The solid-state structure of 1 has been established by single-
crystal X-ray diffraction analysis.¶ The lattice contains the
separate ions [Li(thf)4]+ and [Cr(C6Cl5)4]2 together with
interstitial solvent molecules. The cation has a tetrahedral
geometry (local at the Li+ ion) as usually found in other salts.13
The core of the anion contains four s Cr–C bonds and two
additional Cr–Cl interactions, thus resulting in a pseudo-
octahedral environment for the Cr atom (Fig. 1). Two of the
C6Cl5 groups act as monodentate s-bonded ligands, while the
other two act as chelating ligands through their ipso-C and one
of the o-Cl atoms. The small bite-angle of the chelating C6Cl5
group yields two strained four-membered metallacycles with
C–Cr–Cl angles of ca. 60°. These acute angles are responsible
for the severe deviations of the metal coordination environment
from the ideal octahedral geometry. The two Cr–C distances
involving C-trans-to-Cl atoms are virtually identical and in
good agreement with those described in other s-aryl derivatives
of CrIII (mean value 207 pm).14 The two Cr–C distances
involving C-trans-to-C atoms are slightly longer, probably due
to the higher trans influence exerted by the anionic C atom of
the aryl group. The Cr–Cl distances are clearly dissimilar and
much longer than found in standard terminal chloro-complexes
of chromium (mean value 232 pm).14 The long Cr–Cl distances
can be ascribed to both the poor donating ability of halocarbons
Fig. 1 Thermal ellipsoid diagram of the anion of 1. Selected distances (pm)
and angles (°): Cr–C(1) 213.7(11), Cr–C(7) 207.3(10), Cr–C(13) 208.1(10),
Cr–C(19) 217.3(11), Cr–Cl(2) 300.1(4), Cr–Cl(12) 281.8(4); C(1)–Cr–C(7)
111.7(4), C(1)–Cr–C(13) 94.2(4), C(1)–Cr–C(19) 141.5(4), C(1)–Cr–Cl(2)
59.5(3), C(1)–Cr–Cl(12) 83.1(3), C(7)–Cr–C(13) 103.2(4), C(7)–Cr–C(19)
90.8(4), C(7)–Cr–Cl(2) 170.1(3), C(7)–Cr–Cl(12) 63.2(3), C(13)–Cr–C(19)
111.1(4), C(13)–Cr–Cl(2) 74.2(3), C(13)–Cr–Cl(12) 163.4(3), C(19)–Cr–
Cl(2) 99.0(3), C(19)–Cr–Cl(12) 79.8(4), Cl(2)–Cr–Cl(12) 117.4(1).
Chem. Commun., 1998
1721

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in intensity; 10 103 intensity data collected, 9956 unique (Rint = 0.0887)
which were used in all calculations. The structure was solved by Patterson
and Fourier methods and refined anisotropically (except for the solvent
atoms) by full-matrix least squares on F2 (program SHELXL 93; ref. 12) to
final values of R1 = 0.0870 [for 4366 data with I > 2s(I)] and wR2
=
0.2324 (all data) for 635 parameters; GOF = 1.037, maximum D/s =
0.001, maximum Dr = 1063 e nm23 (21053 e nm23) located very near the
chromium atom. Hydrogen atoms were set geometrically. The weighting
scheme was w
= =
[s2(Fo2) + (0.0590P)2 + 115.07P]21, where P
(1/3)[max{Fo2,0} + 2Fc2]. CCDC 182/936.
∑ EPR data were taken on a Varian E-112 spectrometer working in the
X-band. The magnetic field was measured with a Bruker ER035M
gaussmeter. The diphenylpicrylhydrazyl resonance signal [g = 2.0037(2)]
was used to determine the microwave frequencies. The program WIN-EPR
SimFonia (supplied by Bruker) was used to perform the simulation. In the
calculations, the following interpolating halfwidth formula was used:
W2(lx,ly,lz) = W l + W l + W l .
2 2
2 2
2 2
x x
y y
z z
** The relevant spin Hamiltonian for the paramagnetic centre in 1 [CrIII, d3,
S = 3/2] should read:
˜
H = mBB·g˜·S + S·D·S
where mB is the Bohr magneton. The first term accounts for the electronic
Zeeman interaction and the second one for the zero field splitting. In the
˜
principal axes of the D-tensor, the latter term is given by
1
˜
S·D·S = D[{S2z 2 3S(S + 1)} + h(Sx2 2 S2y)]
where h can always be set to a value of < 1/3 by an appropriate choice of
principal axes. In the absence of any applied magnetic field, the quartet
corresponding to the (S = 3/2)-system splits into two doublets separated by
1
2
2
Do = 2D (1 + 3h ) , which can be treated independently if Do is greater than
the Zeeman energy. Moreover, for low values of the rhombic parameter, the
only transition actually observed is the one among the states of the S = ±1/2
doublet. Under these conditions, the initlal S = 3/2 problem reduces to a
quasi-axial S = 1/2 spin system, with the spin Hamiltonian given in eqn. (1).
Fig. 2 EPR spectrum of a frozen CH2Cl2 solution of 1 at liquid nitrogen
temperature (microwave frequency: 9.49 GHz): experimental (a) and
calculated (b)
˜
If the principal axes of the g˜- and D-tensors coincide the relationship
between the effective (giA) and the principal (gi) true g-factors is given by
gxA = gx (1 + cos 2b + A3 sin 2b)
gyA = gy (1 + cos 2b 2 A3 sin 2b)
gzA = gz(2cos 2b 2 1),
The homoleptic tetraorganochromate(iii) salt 1 is stable
enough to allow its isolation and characterisation. This
reasonable stability can be ascribed to the versatility of the
C6Cl5-group which is able to act as a standard monodentate
s-aryl ligand as well as a poor didentate one depending on the
electronic and steric demands of the metal centre. Further
studies aimed at exploring the chemical reactivity and redox
behaviour of the new compound 1 are in progress.
where tan 2b = (A3)h.
1 J. Sand and F. Singer, Liebigs Ann. Chem., 1903, 329, 190; G. M.
Bennett and E. E. Turner, J. Chem. Soc., 1914, 1057.
2 F. Hein, Ber. Deutsch. Chem. Ges., 1919, 52, 195. This is the first of a
series of papers dealing with so-called polyphenylchromium deriva-
tives. For an almost contemporary survey of this subject see: H. J.
Emele´us and J. S. Anderson, Modern Aspects of Inorganic Chemistry,
D. van Nostrand, New York, 1938, pp. 419–426.
3 W. Herwig and H. H. Zeiss, J. Am. Chem. Soc., 1957, 79, 6561; 1959,
81, 4798.
4 G. K. Barker, M. F. Lappert and J. A. K. Howard, J. Chem. Soc., Dalton
Trans., 1978, 734.
5 F. Hein and D. Tille, Z. Anorg. Allg. Chem., 1964, 329, 72.
6 W. Seidel and G. Kreisel, Z. Anorg. Allg. Chem., 1976, 426, 150.
7 F. Hein and K. Schmiedeknecht, J. Organomet. Chem., 1967, 8, 503.
8 W. Mowat, A. J. Shortland, N. J. Hill and G. Wilkinson, J. Chem. Soc.,
Dalton Trans., 1973, 770; W. Mowat, A. Shortland, G. Yagupsky, N. J.
Hill, M. Yagupsky and G. Wilkinson, J. Chem. Soc., Dalton Trans.,
1972, 533.
We thank the Direccio´n General de Ensen˜anza Superior
(Projects PB95-0003-C02-01 and PB95-0792) for financial
support.
Notes and References
† The use of the very bulky bis(trimethylsilyl)methyl group has enabled
Lappert and coworkers to isolate and characterise a highly unusual,
coordinatively unsaturated neutral complex of formula [CrR3] (ref. 4).
‡ Hein and Schmiedeknecht reported on the synthesis of sodium and lithium
tetraphenylchromate(iii) salts, but the authors concluded that they were, in
fact, solvent-stabilised species of formula M[CrPh4·2dme]·2dme (M = Na
or Li; dme = 1,2-dimethoxyethane). The analogous thf-stabilised salts
MCrPh4·xthf were too unstable to be isolated (ref. 7).
9 P. Boudjouk and J.-H. So, Inorg. Synth., 1992, 29, 108.
10 M. D. Rausch, F. E. Tibbets and H. B. Gordon, J. Organomet. Chem.,
1966, 5, 493.
11 R. Uso´n and J. Fornie´s, Adv. Organomet. Chem., 1988, 28, 219; E.
Maslowsky, Jr., Vibrational Spectra of Organometallic Compounds,
Wiley, New York, 1977, pp. 437–442.
12 G. M. Sheldrick, SHELXL 93, Program for the Refinement of Crystal
Structures from Diffraction Data, University of Go¨ttingen, Germany,
1993.
13 W. N. Setzer and P. von Rague´ Schleyer, Adv. Organomet. Chem., 1985,
24, 353.
§ Experimental procedure: to a solution of LiC6Cl5 (ca. 34 mmol) in Et2O
(130 cm3) at 278 °C was added [CrCl3(thf)3] (2.57 g, 6.85 mmol). The
suspension was allowed to warm to 210 °C and after about 4 h of stirring,
the by then violet solid was filtered and extracted in CH2Cl2 (60 cm3). The
solvent in the extract was replaced by thf (20 cm3) and the slow diffusion of
an n-hexane layer (60 cm3) into it at 230 °C yielded 1 as a violet solid in
57% yield. Anal. Found: C 36.21, H 2.02; C40H32Cl20CrLiO4 requires: C
35.73, H 2.40%. IR (KBr; cm21): 1321s, 1311s, 1281vs, 1219m, 1125m,
1059s [n(COC)asym], 1043vs [n(COC)asym], 915m, 887s [n(COC)sym],
822vs (C6Cl5: X-sensitive vibr.),11 667vs, 600w, 415m and 349m. MS
(FAB2): m/z 1040 [Cr(C6Cl5)4]2 and 828 [Cr(C6Cl5)3Cl]2.
¶ Crystal data for 1·0.5OEt2·0.5C6H14: C45H44Cl20CrLiO4.5, M = 1424.74,
monoclinic, a = 4151.3(6), b = 1288.25(10), c = 2387.8(3) pm, b =
117.324(12)°, U = 11.345(3) nm3, T = 150(1) K, space group C2/c (no.
15), graphite-monochromated Mo-Ka radiation, l = 71.073 pm, Z = 8, Dc
= 1.668 g cm23, F(000) = 5736, violet, approx. crystal dimensions: 0.40
3 0.25 3 0.10 mm, m(Mo-Ka) = 1.187 mm21, measured absorption
correction based on y scans, transmission factors: 0.899–0.793; Enraf-
Nonius CAD4 diffractometer, w–q scans, data collection range 4.0 < 2q <
50.0°, +h, +k, ±l, three standard reflections showed no significant variation
14 A. G. Orpen, L. Brammer, F. H. Allen, O. Kennard, D. G. Watson and
R. Taylor, J. Chem. Soc., Dalton Trans., 1989, S1.
15 J. Fornie´s, B. Menjo´n, R. M.a Sanz-Carrillo, M. Toma´s, N. G. Connelly,
J. G. Crossley and A. G. Orpen, J. Am. Chem. Soc., 1995, 117, 4295.
16 M. P. Garc´ıa, M. V. Jime´nez, A. Cuesta, C. Siurana, L. A. Oro, F. J.
Lahoz, J. A. Lo´pez, M. P. Catala´n, A. Tiripicchio and M. Lanfranchi,
Organometallics, 1997, 16, 1026.
Received in Basel, Switzerland, 15th June 1998; 8/04473B
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Chem. Commun., 1998