η6-coordination of arsenine to titanium, vanadium, and chromium

Article abstract of DOI:10.1021/om9810091

By means of metal-ligand vapor co-condensation techniques the homoleptic arsenine sandwich complexes (η⁶-C₅H₅As)₂Ti (2), (η⁶-C₅H₅As)₂V (3), and (η⁶-C₅H₅As)₂Cr (4) have been prepared. 2 is the first example of an unsubstituted group 15 heteroarene sandwich complex which yielded to full structural characterization by X-ray diffraction. The most remarkable feature of the structure of 2 in the crystal is the synperiplanar conformation and the short intramolecular interannular As?As distances which imply secondary bonding. The latter may also contribute to the packing since interannular As?As distances, which fall short of the sum of the van der Waals radii, are detected. The result of a competition experiment, in which benzene and arsenine are offered as ligands to chromium, illustrate the pronounced preference of chromium for arsenine as an η⁶ ligand. The mixed-ligand complexes (η⁶-C₅H₅As)(η⁶-C ₆H₆)Cr (6) and (η⁶-C₅H₅As)Cr(CO)₃ (8) have also been prepared and studied spectroscopically in order to underpin the notion that arsenine, compared to benzene, is the superior η⁶ ligand. The study is rounded off by an investigation of the redox behavior of 2, 3, 4, and 6 (cyclic voltammetry) and by EPR measurements on 3^? and 4^?+. The latter confirm the pronounced π-acceptor character of η⁶-arsenine. This conclusion is based on the increase of the hyperfine coupling constant α(⁵¹V) upon going from bis(benzene)vanadium to the arsenine counterpart which is thought to reflect V(d_z2) orbital contraction in 3^?, caused by a slight increase of positive partial charge on vanadium.

Full text of DOI:10.1021/om9810091

Organometallics 1999, 18, 1495-1503
1495
η6-Coor d in a tion of Ar sen in e to Tita n iu m , Va n a d iu m , a n d
Ch r om iu m †,1
Christoph Elschenbroich,* J o¨rg Kroker, Mathias Nowotny, Andreas Behrendt,
Bernhard Metz, and Klaus Harms
Fachbereich Chemie der Philipps-Universita¨t, D-35032 Marburg, Germany
Received December 10, 1998
By means of metal-ligand vapor co-condensation techniques the homoleptic arsenine
sandwich complexes (η6-C5H5As)2Ti (2), (η6-C5H5As)2V (3), and (η6-C5H5As)2Cr (4) have been
prepared. 2 is the first example of an unsubstituted group 15 heteroarene sandwich complex
which yielded to full structural characterization by X-ray diffraction. The most remarkable
feature of the structure of 2 in the crystal is the synperiplanar conformation and the short
intramolecular interannular As‚‚‚As distances which imply secondary bonding. The latter
may also contribute to the packing since interannular As‚‚‚As distances, which fall short of
the sum of the van der Waals radii, are detected. The result of a competition experiment, in
which benzene and arsenine are offered as ligands to chromium, illustrate the pronounced
preference of chromium for arsenine as an η6 ligand. The mixed-ligand complexes (η6-C5H5-
As)(η6-C6H6)Cr (6) and (η6-C5H5As)Cr(CO)3 (8) have also been prepared and studied
spectroscopically in order to underpin the notion that arsenine, compared to benzene, is the
superior η6 ligand. The study is rounded off by an investigation of the redox behavior of 2,
3, 4, and 6 (cyclic voltammetry) and by EPR measurements on 3• and 4•+. The latter confirm
the pronounced π-acceptor character of η6-arsenine. This conclusion is based on the increase
of the hyperfine coupling constant a(51V) upon going from bis(benzene)vanadium to the
•
2
arsenine counterpart which is thought to reflect V(dz ) orbital contraction in 3 , caused by a
slight increase of positive partial charge on vanadium.
In tr od u ction
Zenneck.3 While these authors have employed peripher-
ally substituted heteroarenes, we usually aim at coor-
dination of the unsubstituted species in order to collect
information, relating to the unperturbed parent com-
plexes.
The ambidentate nature of the group 15 heteroarenes
C5H5E (E ) N, P, As, Sb, Bi) raises fundamental
questions concerning the preferred coordination modes
η1 via the lone pair at E or η6 via the aromatic π
systemsand the dependence of this predilection on the
nature of the heteroatom E and the metal atom M.
Furthermore, the question of what extent the het-
eroarenes C5H5E (which, except for pyridine, are highly
labile under ambient conditions) may be stabilized by
metal coordination is worth exploring. To support
conjectures given previously,2 we now describe the
synthesis and characterization of homoleptic complexes
of arsenine with the early transition metals titanium,
vanadium, and chromium. We also discuss a competition
experiment in which benzene and arsenine are offered
to chromium atoms with the intent of putting the
complexing abilities of the homo- and the heteroarenes
in perspective. In addition to our own efforts, to which
papers cited in ref 2 provide access, the ligating proper-
ties of group 15 heteroarenes have been studied exten-
sively by the groups of Green, Cloke, Nixon, and
Resu lts a n d Discu ssion
Bis(η6-arsenine)titanium (2), bis(η6-arsenine)vana-
dium (3), and bis(η6-arsenine)chromium (4)4 are acces-
sible by means of metal-ligand vapor co-condensation
employing an electron beam heated metal evaporation
source.3l (Scheme 1). While the arsenine complex 2
resembles bis(η6-benzene)titanium (5)5 in terms of
(3) (a) Ashmore, J .; Green, J . C.; Green, M. L. H.; Smith, M. L.;
Mehnert, C.; Wucherer, E. J . J . Chem. Soc., Dalton Trans. 1995, 1873.
(b) Mehnert, C. P.; Chernega, A. N.; Green, M. L. H. J . Organomet.
Chem. 1996, 513, 247. (c) Arnold, P. L.; Cloke, F. G. N.; Khan, K.; Scott,
P. J . Organomet. Chem. 1997, 528, 77. (d) Le Floch, P.; Knoch, F.;
Kremer, F.; Mathey, F.; Scholz, J .; Scholz, W.; Thiele, K.-H.; Zenneck,
U. Eur. J . Inorg. Chem. 1998, 119. (e) Knoch, F.; Kremer, F.; Schmidt,
U.; Zenneck, U.; Le Floch, P.; Mathey, F. Organometallics 1996, 15,
2713. (f) Bo¨hm, D.; Knoch, F.; Kummer, S.; Schmidt, U.; Zenneck, U.
Angew. Chem., Int. Ed. Engl. 1995, 34, 198. (g) Binger, P.; Leininger,
S.; Stannek, J .; Gabor, B.; Mynott, R.; Bruckmann, J .; Kru¨ger, C.
Angew. Chem., Int. Ed. Engl. 1995, 34, 2227. (h) Arnold, P. L.; Cloke,
F. G. N.; Hitchcock, P. B.; Nixon, J . F. J . Am. Chem. Soc. 1996, 118,
7630. (i) Mathey, F.; Le Floch, P. Chem. Ber. 1996, 129, 263. (j) Le
Floch, P.; Mathey, F. Coord. Chem. Rev., in press. (k) Breit, B. J . Chem.
Soc., Chem. Commun. 1996, 2071. Breit, B.; Winde, R.; Harms, K. J .
Chem. Soc., Perkin Trans. 1997, 2681. (l) Cloke, F. G. N.; Green, M.
L. H. J . Chem. Soc., Dalton Trans. 1981, 1938.
† Dedicated to Professor Helmut Werner on the occasion of his
65th birthday.
(1) Metal Complexes of Heteroarenes. 11. Part 10: Elschenbroich,
C.; Voss, S.; Harms, K. Z. Naturforsch. 1999, 54B, 209. Taken in part
from the Ph.D. Thesis of M. Nowotny, Marburg, Germany, 1993.
(2) (a) Elschenbroich, C.; Voss, S.; Schiemann, O.; Lippek, A.; Harms,
K. Organometallics 1998, 17, 4417. (b) Elschenbroich, C.; Nowotny,
M.; Behrendt, A:; Harms, K.; Wocadlo, S.; Pebler, J . J . Am. Chem. Soc.
1994, 116, 6217.
(4) We have described the synthesis of 4 previously: Elschenbroich,
C.; Kroker, J .; Massa, W.; Wu¨nsch, M.; Ashe, A. J . Angew. Chem., Int.
Ed. Engl. 1986, 25, 571.
10.1021/om9810091 CCC: $18.00 © 1999 American Chemical Society
Publication on Web 03/23/1999

1496 Organometallics, Vol. 18, No. 8, 1999
Elschenbroich et al.
Ta ble 1. Selected Bon d Len gth s (p m ) a n d Bon d
An gles (d eg) for (C5H5As)2Ti (2)a
Sch em e 1
Ti1-C2
Ti1-C6
Ti1-C8
Ti1-C12
Ti1-C3
Ti1-C5
Ti1-C9
Ti1-C11
Ti1-C4
Ti1-C10
Ti1-As1
Ti1-As7
As1‚‚‚As7
225.4(14)
225.8(3)
As1-C2
C2-C3
192.0(12)
138.7(19)
147.0(20)
141.2(17)
142.5(14)
188.3(2)
189.7(12)
145.8(19)
142.0(20)
139.4(17)
146.4(18)
184.5(16)
231.8(14)
227.1(12)
227.2(12)
227.6(13)
228.1(13)
226.0(14)
227.7(13)
227.6(13)
258.4(3)
C3-C4
C4-C5
C5-C6
As1-C6
As7-C8
C8-C9
C9-C10
C10-C11
C11-C12
As7-C12
limited thermal stability, decomposing at 120 °C, 3 may
be sublimed and is stable up to 268 °C. Correspondingly,
the molecular ion 2+ in the mass spectrum shows an
abundance of 38%, whereas 3+ constitutes the base
peak.
259.1(3)
336.6(2)
b
C2-As1-C6
94.2(5)
C8-As7-C12
96.4(6)
C5As(centroid)-Ti1-C5As(centroid)
C5As(centroid)-Ti3-C5As(centroid)
C5As(centroid)-Ti4-C5As(centroid)
175.6(5)
176.1(5)
171.0(4)
An aspect of considerable interest in the class of bis-
(η6-heteroarene)metal complexes is the question of
whether interannular “secondary bonding”6 between the
heavier group 15 elements could stabilize the syn-
periplanar conformation. The As‚‚‚As interaction in 2
is expected to be small though, since in the related class
of arsametallocenes conformational preference is con-
trolled by the extent of ring substitution by methyl
groups; 2,2′,5,5′-tetramethyl-1,1′-diarsaferrocene in the
crystal adopts the eclipsed synperiplanar conformation
(C2v),7 whereas 2,2′,3,3′,4,4′,5,5′-octamethyl-1,1′-diarsa-
ferrocene takes the staggered antiperiplanar form (C2h).8
Our previous attempts to derive structural information
for unsubstituted bis(heteroarene)metal complexes were
frustrated by disorder problems.4,9,10 Similar problems
were initially encountered with crystals of 2, a specimen
of which at 223 K proved to be monoclinic with two
molecules in the unit cell, titanium residing at a
crystallographic center of symmetry. Severe rotational
disorder prevented unequivocal refinement, however.
When the temperature was lowered to 123 K, a transi-
tion into a twinned triclinic phase takes place which
yielded to X-ray diffraction and refinement. Three of the
four molecules in the asymmetric unit are now ordered;
the fourth molecule adopts the same conformation as
the former with 83% occupancy, while the As atom is
distributed over the different ring positions at 17%
occupancy. The structure of 2 is depicted in Figure 1;
selected bond lengths and angles are collected in Table
1. The most important features are the eclipsed syn-
periplanar conformation of 2 and the intramolecular
interannular As‚‚‚As distances (322, 337, 338 pm),
which are considerably smaller than the sum of the van
der Waals radii (385 pm11a-40011b pm) and fall short
of the As‚‚‚As distance of 362 pm in 2,2′,5,5′-tetram-
ethyl-1,1′-diarsaferrocene.7 The juxtaposition of the As
a
As a representative example the dimensions of the species
containing Ti1 are given here. For the remaining data see the
Supporting Information. Mean values for the species Ti1, Ti3, and
Ti4: As-Cortho, 189 pm; Cortho-Cmeta, 141 pm; Cmeta-Cpara, 143 pm;
Cortho- As-Cortho, 94.5°. Species Ti2 has been excluded because of
rotational disorder of one ring. Data for the free ligand C5H5As
(1): As-Cortho, 185 pm; Cortho-Cmeta, 139 pm; Cmeta-Cpara, 140 pm;
b
Cortho-As-Cortho, 97°. Intermolecular As‚‚‚As distances are given
in Figure 1b.
atoms in 2 and their short interatomic distance consti-
tute another manifestation of secondary bonding be-
tween heavier group 15 elements. This phenomenon is
most obvious in the solid-state structures of the com-
pounds R2E-ER2 (E ) As, Sb, Bi), which exhibit short
intermolecular contacts.12 This has also been observed
for distiba- and dibismaferrocenes13a and for dibis-
moles.13b In the present case, adoption of the syn-
periplanar conformation is facilitated by the absence of
ring substituents which would render an eclipsed rota-
mer less favorable. The sandwich structure of bis-
(arsenine)titanium is slightly tilted; deviations of the
ligand planes from a parallel disposition amounting to
4.4° (Ti1), 3.9° (Ti3), and 9.0° (Ti4), respectively. The
Ti2-containing unit is not considered for reasons of
disorder. The direction of tilt brings the As-containing
corners of the rings into closer proximity. As is apparent
from Figure 1, the ligands are folded, however, along
the axis connecting the ortho carbon atoms. The tilt of
the two C5 planes is probably derived from the peculiar
shape of the C5H5As ring (the transannular ortho carbon
distance exceeds the meta carbon distance) since, in this
way, constancy of the Ti-C distances may be reached.
This is counterbalanced by ring folding which prevents
the As atoms from approaching each other too closely.
η6 coordination of arsenine to titanium effects significant
changes in ring geometry: in comparison to the free
ligand 1, in the complex 2 the angle C-As-C is
decreased by 3° and the bond lengths As-Cortho (+4 pm),
Cortho-Cmeta (+2 pm), and Cmeta-Cpara (+5 pm) are
increased (mean values for the units 1, 3, and 4). This
is the normal behavior encountered for bis(arene)metal
(5) (a) Benfield, F. W. S.; Green, M. L. H.; Ogden, J . S.; Young, D.
J . Chem. Soc., Chem. Commun. 1973, 866. (b) Anthony, M. T.; Green,
M. L. H.; Young, D. J . Chem. Soc., Dalton Trans. 1975, 1419. (c) Bandy,
J . A.; Berry, A.; Green, M. L. H.; Perutz, R. N.; Pront, K.; Verpeaux,
J .-N. J . Chem. Soc., Chem. Commun. 1984, 729.
(6) (a) Alcock, N. W. Adv. Inorg. Chem. Radiochem. 1972, 15, 1. (b)
Pyykko¨, P. Chem. Rev. 1997, 97, 597.
(7) Chiche, E.; Galy, J .; Thiollet, G.; Mathey, F. Acta Crystallogr.
1980, B36, 1344.
(8) Ashe, A. J ., III; Kampf, J . W.; Pilotek, S.; Rousseau, R. Orga-
nometallics 1994, 13, 4067.
(9) Elschenbroich, C.; Koch, J .; Kroker, J .; Wu¨nsch, M.; Massa, W.;
Baum, G.; Stork, G. Chem. Ber. 1988, 121, 1983.
(12) (a) Mundt, O.; Riffel, H.; Becker, G.; Simon, A. Z. Naturforsch.
1988, 43B, 952 and literature cited therein. (b) Ashe, A. J ., III. Adv.
Organomet. Chem. 1990, 30, 77.
(10) Elschenbroich, C.; Nowotny, M.; Metz, B.; Massa, W.; Graulich,
J .; Biehler, K.; Sauer, W. Angew. Chem., Int. Ed. Engl. 1991, 30, 547.
(11) (a) Bondi, A. J . Phys. Chem. 1964, 68, 441. (b) Pauling, L. The
Nature of the Chemical Bond, 3rd ed.; Cornell University Press: Ithaca,
NY, 1960.
(13) (a) Ashe, A. J ., III; Diephouse, T. R.; Kampf, J . W.; Al-Taweel,
S. M. Organometallics 1991, 10, 2068. Ashe, A. J ., III; Kampf, J . W.;
Pilotek, S.; Rousseau, R. Organometallics 1994, 13, 4067. (b) Ashe, A.
J ., III; Kampf, J . W., Puranik, D. B.; Al-Taweel, S. M. Organometallics
1992, 11, 2743.

η6-Coordination of Arsenine to Ti, V, and Cr
Organometallics, Vol. 18, No. 8, 1999 1497
F igu r e 1. Molecular structure of bis(arsenine)titanium (2) in the crystal. (a, top) Stereodrawing of the unit cell. (b, bottom
left) Part of the molecular packing indicating intra- and intermolecular As‚‚‚As distances (pm): As1‚‚‚As7, 337;
As13‚‚‚As19, 339; As25‚‚‚As31, 338; As37‚‚‚As43, 322; As7‚‚‚As43, 384; As19‚‚‚As43, 387; As13‚‚‚As43, 357; As31‚‚‚As43,
353; As31‚‚‚As37, 371; As25‚‚‚As37, 366. (c, bottom right) Side views of the units 1, 3, and 4 of 2 demonstrating the tilting
and ring puckering in the sandwich structures. Unit 2 is excluded because of rotational disorder of one η6-arsenine ligand.
coordination; it contrasts with the structural data
derived for the species (C5H5P)6M (M ) Cr, Mo, W),
which revealed that η1 coordination leaves the geometry
of phosphinine virtually unaffected.1 Not unexpectedly,
η6 coordination represents a more severe perturbation
of the π-electronic structure of hetereoarenes than η1
coordination.
Inspection of molecular packing in the crystal shows
that, in addition to intramolecular As‚‚‚As interaction,
slightly reduced intermolecular As‚‚‚As distances also
prevail. Rather than discussing the numerical values
in detail, attention should be drawn to the fact that the
packing is such that, instead of avoiding each other, the
As atoms of neighboring sandwich units point toward
each other (Figure 1). This may also be regarded as an
outcome of secondary As‚‚‚As bonding which overides
an arrangement governed by molecular polarity only.
NMR Sp ectr a . Bis(η6-arsenine)titanium (2), like bis-
(η6-benzene)titanium (5), is diamagnetic; high-resolution
NMR spectra may thus be obtained. The former prop-
erty is not self-evident, since another axially symmetric
metal (d4) sandwich complex, bis(η5-cyclopentadienyl)-
chromium, possesses a E2 ground state. Obviously, the
3
superior (δ) acceptor character of C5H5As and C6H6,
compared to that of C5H5-, increases the e2g/a1g gap by
stabilizing the e2g orbital, thereby favoring the low-spin
4
3
configuration e2g over the high-spin case e2g a1g1. The
1H NMR spectrum of 2 is depicted in Figure 2; the
pertinent data are collected in Table 2. 2 and 4 share
the change in the extent of ortho, meta, and para proton
shielding and the reduction of scalar proton-proton
coupling, effected by metal coordination. The change in
gradation of the coordination shifts ∆δ(1H) for 2 and 4
is governed by ∆δ(1Ho) being about twice as large as
∆δ(1Hm,p). Thus, deshielding contributions which place
δ(1Ho) for the free ligand 1 at the remarkably low value
of 9.79 ppm are largely quenched upon complex forma-
tion. The conspicuously low field position of the ortho-H
resonance for 1 has been rationalized previously.14
(14) Ashe, A. J ., III; Sharp, R. R.; Tolan, J . W. J . Am. Chem. Soc.
1976, 98, 5451. Ashe, A. J ., III. Top. Curr. Chem. 1982, 105, 125.

1498 Organometallics, Vol. 18, No. 8, 1999
Elschenbroich et al.
Ta ble 2. NMR Da ta for Ar sen in e (1), Bis(a r sen in e)tita n iu m (2), Bis(a r sen in e)ch r om iu m (4),
(Ar sen in e)(ben zen e)ch r om iu m (6), a n d (Ar sen in e)tr ica r bon ylch r om iu m (8)a
1
2
4
6
8
δ(H) (∆δ)
7.15b
9.79
11.1
7.90
8.4
7.60
4.96 (-2.19)c
4.69 (-5.10)
4.34 (-2.81)d
4.25 (-5.54)f
7.0
4.54 (-2.61)e
4.62 (-5.17)
δ(H-2,6) (∆δ)
3J (H-2,H-3)
δ(H-3,5) (∆δ)
3J (H-3,H-4)
δ(H-4) (∆δ)
δ(C) (∆δ)
6.00 (-1.15)
5.55 (-1.60)
5.99 (-1.16)
5.42 (-2.48)
4.97 (-2.63)
5.14 (-2.76)
5.07 (-2.83)
6.2
4.92 (-2.76)f
75.1 (-52.9)d
165.0d
4.85 (-2.88)
77.5 (-50.5)e
169.8e
89.9 (-78.5)
162.2
81.8 (-52.0)
163.3
74.2 (-54.8)
162.1
128.0b
158.4b
168.3
160.6
133.8
157.0
129.0
159.2
234.1g
1J (13C,1H)
δ(C-2,6) (∆δ)
95.3 (-73.0)
165.7
90.8 (-43.0)
168.6
79.9 (-49.1)
168.5
86.8 (-81.6)
164.8
84.8 (-49.0)
163.2
75.7 (-53.3)
164.5
114.9h
(167.9)
95.6h
(170.3)
91.1
1J (13C,1H)
δ(C-3,5) (∆δ)
1J (13C,1H)
δ(C-4) (∆δ)
1J (13C,1H)
171.6
a
b
d
In THF-d8 at 30 °C: δ in ppm; J in Hz; ∆δ ) δ(complex) - δ(ligand), coordination shift. C6H6. c (C6H6)2Ti. (C6H6)2Cr. e η6-C6H6.
f The chemical shifts of 4 are markedly temperature dependent: δ(H-2,6) 4.18 (0 °C), 4.15 (-10 °C), 4.07 (-40 °C), 4.01 (-70 °C). CO.
Assignment uncertain.
g
h
Sch em e 2
Reduction of intraligand bond order effected by metal
coordination is an outcome of the depopulation of
bonding π (ligand) orbitals (donation) and the population
of antibonding π* (ligand) orbitals (back-donation).
Therefore, from the observed trend in the J (H,H) data
one is led to conclude that metal-arene bonding is
weaker in bis(η6-arsabenzene)titanium (2) compared to
its chromium cousin 4. The widely differing thermal
stabilities attest to this notion: 2 (dec pt 120 °C), 4 (dec
pt 253 °C). Note, however, that metal-η6-benzene bond
energies determined by ion beam tandem MS tech-
niques and backed by quantum-chemical calculations
suggest an opposite ranking.15
The comparison of titanium and chromium as central
metal atoms is complemented by an assessment of
arsenine and benzene as ligands. We therefore per-
formed the three-component co-condensation Cr + C5H5-
As + C6H6 with the aim of deriving conclusions as to
the propensity of arsenine and benzene to bind to
transition metals by checking the product ratio. Fur-
thermore, examination of the mixed-ligand complex (η6-
arsenine)(η6-benzene)chromium (6) should furnish spec-
troscopic evidence reflecting the donor/acceptor character-
istics of the two ligands simultaneously present.
In the metal vapor synthesis (Scheme 2) a ligand ratio
1
C6H6:C5H5As of 25:1 was employed. H NMR analysis
of the product mixture before separation revealed the
F igu r e 2. 1H NMR spectra (400 MHz, THF-d8) of η6-
proportion 4:6:7 ) 6:3:1 (Figure 2). Thus, although
benzene was present in large excess, formation of η6-
arsenine complexes of Ti and Cr: (a) 2, at 30 °C; (b)
individual component spectra of 4, 6, and 7, at 30 °C; (c)
mixture of 4, 6, and 7 obtained from a co-condensation of
coordination of arsenine to chromium atoms via the lone
Cr atoms with C6H6 + C5H5As (25:1), at -60 °C.
arsenine complexes is strongly favored. Possibly, pre-
pair at As offers an additional path to the final η6
complex which is inaccessible to benzene.
Interestingly, the proton coupling constants decrease
according to J (1H,1H;1) > J (1H,1H;2) > J (1H,1H;4). Since
scalar proton coupling correlates with intraligand π
bond order, the latter is smaller in 4 as compared to 2.
(15) Meyer, F.; Farooq, A. K.; Armentrout, P. B. J . Am. Chem. Soc.
1995, 117, 9740. Bauschlicher, C. W.; Partridge, H.; Langhoff, S. R. J .
Chem. Phys. 1992, 96, 3273.

η6-Coordination of Arsenine to Ti, V, and Cr
Organometallics, Vol. 18, No. 8, 1999 1499
Sch em e 3
Inspection of Figure 2 reveals features which deserve
to be addressed briefly: resolution in the spectrum of
the titanium complex 2 is far superior to that for the
chromium complexes 4 and 6. This may be traced to
electron self-exchange line broadening in the couples
40/+ and 60/+, traces of the respective radical cations
being adventitiously present. The effect is absent in the
case of 2, where the corresponding radical cation 2•+ is
unstable (vide infra). Another phenomenon is the con-
spicuously large temperature dependence of δ(H-2,6;4),
which may be related to the self-exchange process as
well.
1917 cm-1) clearly demonstrate that arsenine accepts
a larger share of π back-bonding electron density than
does benzene. On the basis of the criterion of the
carbonyl stretching frequencies in half-sandwich com-
plexes, η6-arsenine matches fluorobenzene ((C6H5F)Cr-
(CO)3 (10), νCO ) 1996, 1930 cm-1 16).
η6-Coordinated arsenine holds an additional coordina-
tion site in that the arsenic atom can function as a two-
electron donor to carbonylmetal fragments. The µ-η6:η1
bridging mode of phosphinine was first realized in the
complexes 11.17 We therefore treated 8 with the Cr(CO)5
transferring agent (η2-cyclooctene)Cr(CO)5 (12). The
reaction has to be performed in a hydrocarbon solvent,
where the product [(η6-C5H5As)Cr(CO)5]Cr(CO)3 (13)
precipitates upon formation. Tetrahydrofuran displaces
8 from the coordination sphere of Cr(CO)5, as shown in
Scheme 3. This substitution demonstrates the lability
of η1 coordination of arsenine to chromium, which seems
to be enhanced by simultaneous η6 coordination. Note
that, conversely, free arsenine displaces pyridine from
(C5H5N)Mo(CO)5; the η1 coordination mode appears to
be disfavored for arsenine, though, since (η1-C5H5As)-
Mo(CO)5 decomposes at 120 °C, yielding inter alia (η6-
C5H5As)Mo(CO)3 (14).18 A pronounced effect of Cr(CO)5
coordination to the As atom of 8 is the decreased
Attempts to completely separate the product mixture
in order to prepare analytically pure 6 were unsuccess-
ful. Chromatography under mild conditions failed to
effect separation; harder stationary phases such as
Al2O3 caused decomposition. Partial separation could be
achieved by means of slow fractional sublimation (tem-
perature gradient 50° > T > 20 °C), which in the central
section yielded 6 contaminated by 20% of 7. The
assignment of the signals in Figure 2 is, of course,
unequivocal for intensity reasons, since the singlet at δ
4.54 (η6-C6H6 of 6) may serve as a gauge. As suggested
above, the mixed-ligand complex 6 could provide hints
to the capacity of benzene and arsenine, respectively,
to act as a π acid/π base ligand. To the extent that
intraligand π bond order reflects metal-ligand bond
strength, a comparison of the coupling constants
J (1H,1H;arsenine) in 4 and 6 should reveal whether
benzene and arsenine strongly differ in their bonding
characteristics. However, no significant difference in the
3J (1H,1H;arsenine) values is observed for 4 and 6.
Therefore, on the basis of this criterion, arsenine and
benzene are of comparable bonding ability. This state-
ment does not necessarily contradict the product dis-
tribution obtained in the competition complexation,
since the latter is kinetically controlled. Another obser-
vation, relevant in the present context, concerns the
relative abundance of the half-sandwich fragments in
the mass spectrum of 6, which amounts to 46% for C5H5-
AsCr+ and 11% for C6H6Cr+, substantiating the superior
π bonding capacity of arsenine.
1
chemical shift dispersion in the H NMR spectrum of
13 (see Figure 1b). Attempts to ligate Cr(CO)5 fragments
to bis(η6-arsenine)chromium (4) did not lead to the
isolation of pure products. As in the case of 8, reaction
of 4 with 12 in hydrocarbon solvents afforded a black
precipitate which, according to mass spectrometry,
contained 4‚Cr(CO)5 from which Cr(CO)5 was cleaved
off instantaneously upon dissolving in THF.
Red ox P r op er ties. The redox properties of the
homoleptic arsenine complexes 2-4 were probed by
cyclic voltammetry (CV); electrochemical traces are
depicted in Figure 3. A rationalization of the CV data
compiled in Table 3 must be based on the electronic
configurations of the neutral species (η6-arene)2M, which
4
0
in the frontier orbital region are e2g a1g (M ) Ti),
e2g a1g (M ) V), and e2g a1g (M ) Cr).19 Since the
4
1
4
2
IR Sp ectr a . An evaluation of the particularity of
arsenine as an η6 ligand can also be derived from a
comparison of the IR spectra of (arsenine)tricarbonyl-
chromium 8 and its benzene counterpart 9. The stretch-
ing frequencies νCO for 8 (1995, 1933 cm-1) and 9 (1987,
(16) O¨ fele, K. Chem. Ber. 1966, 99, 1732.
(17) Nainan, K. C.; Sears, C. T. J . Organomet. Chem. 1978, 148,
C31.
(18) Ashe, A. J ., III; Colburn, J . C. J . Am. Chem. Soc. 1977, 99, 8099.
(19) Cloke, F. G. N.; Dix, A. N.; Green, J . C.; Perutz, R. N.; Seddon,
E. A. Organometallics 1983, 2, 1150.

1500 Organometallics, Vol. 18, No. 8, 1999
Elschenbroich et al.
Ta ble 3. Cyclic Volta m m etr ic Da ta for th e Liga n d Ar sen in e (1), th e Ben zen e Com p lexes 5, 15, a n d 7, a n d
th e Ar sen in e Com p lexes 2, 3, 6, a n d 4a
1b
5
2
15
3
7
6
4
E
1/2(0/-) (V)c
Epa (V)
pc (V)
-2.08d
-1.98
-2.18
200
-2.34
-2.31
-2.38
70
-1.82d
-1.77
-1.85
80
-2.71e
-2.67
-2.74
74
-2.02d,f
-1.97f
-2.06f
92
<-3.1
-1.80d
-1.78
-1.85
70
-1.36d
-1.32
-1.40
80
E
∆Ep (mV)g
rg
1.14
0.97
0.93
0.93
0.8
0.8
0.6
E
1/2(0/+) (V)8
Epa (V)
pc (V)
-0.35h
-0.32
-0.38
66
0.01h
0.05
-0.02
62
-0.72
-0.61
-0.56
-0.65
90
-0.52
-0.46
-0.57
110
(-1.15)b
-0.65
-0.222i
E
∆Ep (mV)g
87
0.95
rg
1
1
1
0.92
E
pa(+/2+) (V)
1.15i
0.24i
0.87i
0.97
a
0/-
In DME/(n-Bu4)NClO4 (0.1 M) at glassy carbon vs SCE: T ) -45 °C, v ) 100 mV s-1. For the couple C6H6 the value E1/2 ) 3.29
V has been derived.23 The low-temperature trace signalizes a quasi-reversible process. At room temperature and faster scan rates (v g
b
300 mV s-1) an additional anodic wave at E ) -1.15 V is observed which points to an ECE mechanism. Yet, under these conditions, the
trace for 10/- approaches reversibility: E1/2 ) 2.097 V, ∆Ep ) 62 mV, r ) 0.96. E1/2 ) (Epa + Epc)/2. Quasi-reversible. e Reversible at
c
d
-48 < T < 25 °C. f Reduction reversible at 25 °C. ∆Ep ) Epa - Epc, r ) ipa/ipc
.
Reversible at -48 °C, irreversible at 25 °C. Irreversible.
g
h
i
absent. A similar gradation emerges from a comparison
of the potentials for the bis(arsenine)metal complexes,
2
0/-, 30/- (cathodic shift -0.20 V) and 3+/0, 4+/0 (cathodic
shift -0.51 V); the magnitudes of the shifts differ,
however.
2
The a1g orbital is almost exclusively of M 3dz char-
acter;21,22 therefore, modifications of the coordinated π
perimeter will only have an indirect effect on the
reduction of 2, reduction and oxidation of 3, and oxida-
tion of 4 and 6. Furthermore, the essentially nonbonding
nature of the a1g orbital leads to the expectation that
changes in its occupation should not greatly affect
complex stability. Thus, the reversible character of the
processes 20/-, 3+/0, 30/-, 4+/0, 6+/0, and 7+/0 does not come
as a surprise. Conversely, the chemically irreversible
nature of the process 2+/0 is a consequence of the
bonding character of the e2g orbital: the metal-ligand
bond is weakened through its depopulation. This is
underscored by the behavior of bis(η6-benzene)titanium
(5), which loses the aromatic ligand upon oxidation.22
All redox processes which involve changes in a1g oc-
cupation display anodic shifts upon substitution of CH
units by As atoms, as had already been observed for
arsaferrocenes.22b We contend that this effect reflects
the superior π-acceptor ability of arsenine, relative to
benzene, which increases the partial positive charge on
the central metal atom, thereby lowering the energy of
2
the M(3dz ) redox orbital. As inspection of the data in
Table 3 reveals, a universally applicable increment
anodic shift (mV/As atom) cannot be derived, however.
Reduction of 4, 6, and 7 is a different case because here,
conceivably, it is the LUMO e1g* which forms the redox
orbital. This function is composed of metal and ligand
contributions,16 and therefore, it is plausible that the
CH/As replacement effects the reduction potentials of
the complexes more extensively. For the free ligands,
the difference E1/2(arsenine0/-) - E1/2(benzene0/-) amounts
F igu r e 3. Cyclic voltammograms for arsenine (1), bis-
(arsenine)titanium (3), and bis(benzene)titanium (5) in
DME/n-Bu4NClO4 at -45 °C νs SCE (100 mV/s).
electrochemistry of bis(benzene)titanium was also part
of this study, an interesting relation in the series
(C6H6)2M, M ) Ti (5), V (15), Cr (7) should be pointed
out. From the data in Table 3 it is apparent that the
couples 50/- and 150/- differ by a cathodic shift of 0.37
V, as do the couples 15+/0 and 7+/0. It is tempting to
(20) Prins, R.; Reinders, F. J . Chem. Phys. Lett. 1969, 3, 45.
(21) Burrow, P. D.; Modelli, A.; Guerra, M.; J ordan, K. D. Chem.
Phys. Lett. 1985, 118, 328.
(22) (a) Stable bis(arene)titanium(I) species may be isolated, how-
ever, if the arene bears sterically shielding substituents and the
counterion is innocent: Calderazzo, F.; Ferri, I.; Pampaloni, G.;
Englert, U.; Green, M. L. H. Organometallics 1997, 16, 3100. (b) Ashe,
A. J ., III; Mahmoud, S.; Elschenbroich, C.; Wu¨nsch, M. Angew. Chem.,
Int. Ed. Engl. 1987, 26, 229. Ashe, A. J ., III; Al-Ahmed, S.; Pilotek, S.,
Puranik, D. B.; Elschenbroich, C.; Behrendt, A. Organometallics 1995,
14, 2689.
interpret this coincidence in terms of the pairing energy
1
which has to be invested in the reduction of 15 (a1g
f
a1g2) and is released in the oxidation of 7 (a1g f a1g1),
2
whereas in the reduction of 5 and the oxidation of 15
1
0
(both a1g f a1g processes) pairing energy effects are

η6-Coordination of Arsenine to Ti, V, and Cr
Organometallics, Vol. 18, No. 8, 1999 1501
to (-2.08) - (-3.29)23 ) +1.21 V. Quantitative assess-
ment of the heteroatom shifts δE1/2[60/-,70/-] and
δE1/2[40/-,70/-] is impossible because E1/2[70/-] is experi-
mentally inaccessible. As a lower limit for the former
heteroatom shift a value of +1.30 V can be given which,
surprisingly, exceeds the heteroatom shift of the reduc-
tion potentials of the free ligands benzene and arsenine.
Apparently, reduction of the 18-valence-electron com-
plexes 4, 6, and 7 is largely ligand-centered.24 Interest-
ingly, the secondary heteroatom shift is much smaller,
δE1/2[40/-,60/-] amounting to +0.44 V only. This con-
trasts with oxidation, δE1/2[6+/0,7+/0] versus δE1/2[4+/0,6+/0],
where the effects of introducing As atoms into the η6-
arene are additive, amounting to 100 mV per As atom.
The disparate behavior may be traced to the fact that
in 6•- the additional electron is localized in the single
arsenine ligand, leading to better solvation as compared
to 4•-, where the charge may be delocalized over two
ligands. This reasoning does not apply to the oxidation
processes which involve a change in occupation of the
single, central metal dominated molecular orbital.
EP R Stu d ies. The complexes (η6-C5H5As)2M, in
principle, could give rise to the paramagnetic species
F igu r e 4. EPR spectra (X-band) of the (η6-arsenine)metal
(d5) species 3• (M ) V) and 4•+ (M ) Cr) in fluid (a) and
2
•+, 2•-, 3•, 4•+, and 4•-, not all of which are accessible
rigid (b) solution (ST ) Fre´my salt,
g
) 2.0057). 3•
for EPR study, however. We will treat the isoelectronic
radicals 3• and 4•+ here and first discuss 3•, which in
its EPR spectral characteristics closely resembles its
group homologue (η6-C5H5P)2V (16).10 The spectra of 3•
in fluid and rigid solution are presented in Figure 4;
the parameters are given in the caption. The complexes
(C6H6)2V (15), (C5H5P)2V (16), and (C5H5As)2V (3)
feature a gradual increase in the isotropic hyperfine
coupling constant a(51V) (-6.29, -7.03, -7.34 mT) at
the expense of ring proton coupling a(1H) (0.40, 0.35,
0.31 mT). Again, as in the discussion of the IR and CV
results, a gain in the π-acceptor character C6H6 < C5H5P
< C5H5As can explain the observed trend in that
increasing positive partial charge on the central metal
(toluene, 300-120 K): g ) 1.9897, gz ) 2.002, gx ) 1.9906,
gy ) 1.9765, a(51V) ) -7.34, Az ) -2.06, Ax ) -9.29, Ay )
-10.65 mT, a(1H) ) 0.31 mT. 4•+ (DMF/CHCl3 (1:1),
298-140 K):
g ) 1.9895, g| ) 1.9730, g ) 1.9977,
a(53Cr) ) 1.93 mT. The superhyperfine structure defies
analysis.
data, however. The isoelectronic radical cation 4•+
,
which can be generated by air oxidation or electrochemi-
cal means, rapidly decomposes in all common solvents.
This accords with the high acceptor character of the
ligand arsenine, since we have noted previously that η6
ligands of high electron affinity such as aryl ketones
facilitate nucleophilic attack at the central metal leading
to metal-ligand bond cleavage for bis(arene)chromium
radical cations.26 Thus, the limited quality and resolu-
tion of the EPR spectrum of 4•+ warrants only the
following inferences. (1) The number and spacing of
hyperfine components in the isotropic spectrum of 4•+
point to inequality of the ring proton coupling constants
and/or the contribution of splitting from the 75As nuclei
(I ) 3/2, 100%). (2) The value a(53Cr) ) 1.93 mT exceeds
that of the parent 7•+ (1.80 mT). A similar gradation
2
atom causes contraction of the V 3dz orbital with
attendant decrease in metal f ligand spin delocalization
2
and more effective 3dz /ns spin polarization. Analysis
of the rigid solution EPR spectrum of 3• suggests that,
in comparison to its phosphinine counterpart, for 3• the
rhombicity of the g tensors is somewhat larger and that
of the A(51V) tensor is smaller. While from this observa-
tion the intuitively reasonable conclusion emerges that
the perturbation by As as a ring member exceeds that
of P, a more detailed discussion would be presumptuous
in view of the uncertainty as to the coaxiality of the two
tensors.25 With regard to the conformation 3• adopts in
rigid solution, the observation of three g values pre-
cludes exclusive realization of a rotamer, which pos-
sesses a 90° torsional angle. All other conformers,
including the synperiplanar form chosen by 2 in the
single crystal, are compatible with the spectroscopic
was found for the couple 3•, 15•. (3) The relation g
gfs > g| is unusual for an (arene)2M(d5) complex.27
≈
Exp er im en ta l Section
Chemical manipulations and physical measurements were
carried out using techniques and instruments specified
previously.1a Arsenine (1) was prepared by a literature method,28
as was bis(η6-benzene)titanium.29 For NMR, EPR, and CV data
see text.
(23) Gerson, F.; Ohya-Nishiguchi, H.; Wydler, C. Angew. Chem., Int.
Ed. Engl. 1976, 15, 552.
(24) Electron transmission spectroscopy21 of bis(benzene)chromium
has ascribed the lowest lying anion state to electron capture into the
e1g* molecular orbital which is composed of about equal contributions
from chromium and ligand orbitals. This is followed by a first excited
anion state of e2u symmetry and pure ligand π* character. Since the
energy difference between e1g* and e2u* is small, it is conceivable that
in the case of arsenine as a ligand, the two levels are inverted in
ordering. This would explain the large heteroatom shift observed for
(26) Elschenbroich, C.; Bilger, E.; Heck, J .; Stohler, F.; Heinzer, J .
Chem. Ber. 1984, 117, 23.
(27) The “perpendicular” component of the rigid solution spectrum
of 4•+ indicates a shoulder, implying nonequivalence of gx and gy. We
will investigate this detail in a future study, utilizing high-frequency
(Q-band, W-band) EPR.
(28) Ashe, A. J ., III; Chang, W.-T. J . Org. Chem. 1979, 44, 1409.
(29) Anthony, M. T.; Green, M. L. H.; Young, D. J . Chem. Soc.,
Dalton Trans. 1975, 1419.
the redox couples 70/- and 50/-
(25) Rieger, P. H. Coord. Chem. Rev. 1994, 135/ 136, 203.
.

1502 Organometallics, Vol. 18, No. 8, 1999
Elschenbroich et al.
Bis(η6-a r sen in e)tita n iu m (2). A vapor co-condensation
reactor equipped with an electron beam heated metal vapor-
ization source3l is cooled to 77 K and the wall is first coated
with a layer of methylcyclohexane (50 mL). Subsequently, after
the vacuum is 5 × 10-6 mbar or better, within 1.5 h at a power
of 600 W 0.28 g (5.8 mmol) of titanium and 9.8 g (70 mmol) of
arsenine are co-condensed. After it is flooded with dinitrogen
and warmed to ambient temperature, the reaction mixture is
filtered through a 2 cm layer of Celite and the filtrate is taken
to dryness in vacuo. Recrystallization from THF at -25 °C
affords 870 mg (2.65 mmol, 46% yield relative to titanium
evaporated) of 2 as dark brown rhombi, mp 120 °C dec. EI-
MS (70 eV): m/z (%) 328 (38) [M+], 302 (8) [M+ - C2H2], 250
(23) [M+ - 3 C2H2], 188 (6) [M+ - L], 140 (100) [L+], 114 (28)
[L+ - C2H2], 113 (56) [M+ - L - As], 48 (11) [Ti+]. Anal. Calcd
for C10H10As2Ti (327.91): C, 36.63; H, 3.07. Found: C, 37.00;
H, 3.31.
Ta ble 4. Cr ysta llogr a p h ic Da ta a n d Refin em en t
P a r a m eter s for (C5H5As)2Ti (2)
Crystal Data
habit, color
cryst size
cryst syst
irregular, dark
0.30 × 0.20 × 0.20 mm
triclinic
space group
unit cell dimens
P1h, Z ) 8
a ) 1226.1(2) pm
b ) 1288.4(2) pm
c ) 1326.5(2) pm
R ) 102.190(16)°
â ) 103.921(17)°
γ ) 91.472(17)°
1981.6(5) × 10-30 m3
5000 rflns
V
cell determination
chem formula
fw
C
10H10As2Ti
327.92
F(000)
density (calcd)
abs coeff
1264
2.198 Mg/m3
7.437 mm-1
Bis(η6-a r sen in e)va n a d iu m (3). In analogy to the prepara-
tion of 2, 0.73 g (14.3 mmol) of vanadium is condensed with
7.2 g (51.4 mmol) of arsenine 1 during a period of 2 h at 480
W electron beam power. After warming to room temperature,
filtration of the brown co-condensate over Celite, evaporation
to dryness, and sublimation at 100 °C/10-3 mbar, 55 mg (0.17
mmol, 1.2%) of 3 is isolated in microcrystalline form. The black
brown rhombi obtained from crystallization (toluene/petroleum
ether) proved to be X-ray amorphous. Dec pt: 268 °C. EI-MS
(70 eV): m/z (%) 331 (100) [M+], 253 (13) [M+ - 3 C2H2], 191
(76) [M+ - L], 178 (28) [M+ - 3 C2H2 - As], 165 (11) [M+ - L
- C2H2], 140 (8) [L+], 116 (22) [M+ - L - As], 51 (7) [V+].
Data Collection
Stoe IPDS
diffractometer type
wavelength
temp
Mo KR (71.073 pm)
123(2) K
θ range for data collecn
2.02-25.80°
-14 e h e 14, -15 e k e 15,
-16 e l e 16
0-200, 2°
index ranges
φ range/increment
expose time
4 min
data collecn software
cell refinement software
data reduction software
Stoe Expose
Stoe Cell
Stoe Integrate
Anal. Calcd for
C10H10As2V (330.97): C, 36.29; H, 3.05.
Solution and Refinement
Found: C, 36.53; H, 2.96.
Bis(η6-a r sen in e)ch r om iu m (4). A 3 g (21 mol) amount of
arsenine (1) and 300 mg (6 mmol) of chromium are co-
condensed in a reactor employing a resistively heated metal
evaporation source over a period of 1.5 h, the wall being kept
at 77 K. Workup as described for 2 and 3 and recrystallization
from toluene at -20 °C yielded 310 mg (0.9 mmol, 15%) of 4
as reddish black, rod-shaped crystals which may be sublimed
at 100 °C/10-3 mbar with some loss of material. Dec pt: 253
°C. EI-MS (70 eV): m/z (%) 332 (26) [M+], 192 (39) [M+ - L],
140 (58) [L+], 65 (7) [C5H5+], 52 (100) [Cr+]. Anal Calcd for
no. of rflns collected
no. of indep rflns
no. of obsd rflns
11 380
11 380 (Rint ) 0.0000)
5242 (I > 2σ(I))
11 380
no. of rflns used
for refinement
extinction cor formula
Fc* ) Fck[(1 + 0.001Fc2λ3)/
(sin 2θ)]-1/4 a
none
abs cor
max and min
transmissn
largest diff peak and
hole
0.3178 and 0.2138
1.156 and -1.297 × 1030 e/m-3
C
10H10As2Cr (332.03): C, 36.17; H, 3.04. Found: C, 36.08; H,
3.02.
Note: If the solvent is removed carefully by means of trap-
soln
direct methods/difference Fourier
full-matrix refinement at F2
calcd positions, fixed isotropic U’s
refinement
treatment of hydrogen
atoms
to-trap condensation, a large portion of unreacted arsenine
may be recovered.
programs used
SHELX-97 (Sheldrick, 1990)
SHELX-97 (Sheldrick, 1997)
SHELXTL, STOE IPDS software,
TWINXL (Massa, 1998)
11380/224/501
(η6-Ar sen in e)(η6-ben zen e)ch r om iu m (6). A 300 mg (6
mmol) amount of chromium was co-condensed with 1.5 g (11
mmol) of arsenine and 20 g (256 mmol) of benzene over 1.5 h,
as described for 4. After it is warmed to ambient temperature,
the dark red suspension is filtered through Celite and the
filtrate is taken to dryness in vacuo. The product mixture (250
no. of data/restraints/
params
weighting scheme
w ) 1/[σ2(Fo2) + (0.0872P)2 +
0.0000P]; P) (Fo + 2Fc2)/3
2
1
mg) according to H NMR consists of (η6-C5H5As)2Cr (4), (η6-
goodness of fit on F2
R index (all data)
R index conventional
(I >2σ(I))
0.878
wR2 ) 0.1835
R ) 0.0691
C5H5As2)(η6-C6H6)Cr (6), and (η6-C6H6)2Cr (7) in the molar ratio
6:3:1. Slow fractional sublimation at 50 °C/10-3 mbar/3 days
effected partial separation into pure 7 (upper zone), 6 con-
taminated with 20% of 7 (central zone), and 7, 6 and 4 (5:2:1
lower zone). Analytically pure 6 could not be obtained in this
or other ways. EI-MS (70 eV): m/z (%) central zone 270 (21)
[M+(6)], 192 (46) [M+(6) - C6H6], 208 (7) [M+(7)], 140 (13)
[C5H5As+], 130 (11) [M+(6) - C5H5As], 78 (18) [C6H6+], 52 (100)
[Cr+].
a
k ) overall scale factor.
(100) [Cr+]. IR (hexane): νCO 1995, 1933 cm-1. Anal. Calcd for
C8H5O3AsCr (276.03): C, 34.81; H, 1.83. Found: C, 34.56; H,
1.76.
[µ-η1:η6-Ar sen in e][pen tacar bon ylch r om iu m ][tr icar bon -
ylch r om iu m ] (13). A 94 mg (0.34 mmol) amount of 8 and 120
mg (0.4 mmol) of (cis-η2-cyclooctene)pentacarbonylchromium
(12) are dissolved simultaneously in 30 mL of cis-cyclooctene/
hexane (5:1) with vigorous stirring. Over 15 min an orange
solid separates. The supernatant is decanted, and the product
is washed with four 25 mL portions of cold (-10 °C) hexane.
A 130 mg amount (81% yield) of microcrystalline 13 is
obtained, which may be sublimed at 80 °C/10-3 mbar with
partial decomposition. EI-MS (70 eV): m/z (%) 468 (7) [M+],
(η6-Ar sen in e)(tr ica r bon yl)ch r om iu m (8). A solution of
100 mg (0.7 mmol) of arsenine (2) and 330 mg (1.5 mmol) of
Cr(CO)6 in 50 mL of n-Bu2O/THF (4:1) are refluxed at 100 °C
for 60 h. The orange-red solution is cooled to 0 °C and filtered
to separate solid Cr(CO)6, and the filtrate is taken to dryness.
The residue is sublimed at 50 °C/10-3 mbar to generate 125
mg (76.5% yield) of red air-stable 8. Mp (uncorr): 141 °C. EI-
MS (70 eV): m/z (%) 276 (16.5) [M+], 248 (5) [M+ - CO], 220
(11) [M+ - 2 CO], 195 (56) [M+ - 3 CO], 140 (4) [C5H5As+], 52

η6-Coordination of Arsenine to Ti, V, and Cr
Organometallics, Vol. 18, No. 8, 1999 1503
356 (1) [M+ - 4 CO], 328 (4) [M+ - 5 CO], 276 (17) [8+],
248 (3) [8+ - CO], 220 (12) [8+ - 2 CO], 192 (37) [8+ - 3 CO],
140 (2) [C5H5As+], 52 (68) [Cr+], 28 (100) [CO+]. IR (hexane):
permitted an anisotropic refinement with a number of re-
straints for the geometrical parameters and the temperature
factors. Three of the four independent molecules are ordered
and have similar conformations. There is disorder of one As
position in the fourth molecule (87% exhibit the same confor-
mation as the other molecules; 17% adopt other ring orienta-
tions). Further details of the crystal structure determination
may be found in Table 4.
ν
CO 2079, 1995, 1963, 1940 cm-1. Anal. Calcd for C13H5O8AsCr2
(468.11): C, 33.35; H, 1.08. Found: C, 33.33; H, 0.93.
X-r a y Cr ysta llogr a p h ic Stu d y of 2 (Ta ble 4). The data
collection has been performed on a STOE IPDS area detector
at two different temperatures. T ) 223 K: monoclinic, space
group P21/n, a ) 655.5(2) pm, b ) 798.4(2) pm, c ) 985.9(3)
pm, â ) 97.15(4)°, V ) 512 Å3, Z ) 2, wR2 ) 0.0873, R )
0.0343. The Ti atom is located at a center of inversion, the
ring atoms are disordered, and no discrete As position can be
derived. T ) 123 K: the crystal has changed to twinned
triclinic, space group P1h, V ) 1981.6(5) Å3, Z ) 8. The two
orientation matrixes of the twin species could be determined
using the program RECIPEl.30 The integration has been
performed with the matrix of one species. The resulting data
set has been treated with the program TWINXL31 so that a
twin refinement by means of SHELXL-9732 using the “HKLF
5” instruction could be carried out. Due to the pronounced
mosaicity after the phase change, the quality of the data only
Ack n ow led gm en t. This work has been supported
by the Deutsche Forschungsgemeinschaft and the Fonds
der Chemischen Industrie. Support by the NATO Sci-
entific Affairs Division is also gratefully acknowledged.
Su p p or tin g In for m a tion Ava ila ble: Tables giving crys-
tal data and details of the structure determination, positional
and thermal parameters at collection temperatures of 123 and
223 K, and all bond distances and angles for 2. This material
is available free of charge via the Internet at http://pubs.acs.org.
OM9810091
(30) Stoe IPDS Program System (Version 2.03, 1997).
(31) SHELX-97, Program for the Refinement of Crystal Structures;
University of Go¨ttingen, Go¨ttingen, Germany, 1997.
(32) Hahn, F.; Massa, W. Program for Handling Diffraction Data of
Twinned Crystals; Marburg, Germany, 1997.