Photochemical synthesis of new (η6-arene)Cr-hydrido stannyl and (η6-arene)Cr-bis-stannyl complexes. Ligand effects on the Sn-H interaction in the hydrido stannyl compounds

Article abstract of DOI:10.1016/S0022-328X(98)00917-6

Hydrido stannyl compounds containing the η²-H-SnPh₃ ligand and bis-stannyl compounds containing two SnPh₃ ligands have been obtained from the photolysis of (η⁶-arene)Cr(CO)₃ and HSnPh₃. New complexes with different arenes (mesitylene, trifluorotoluene and 1,4-dimethoxybenzene) have been obtained and characterized by X-ray diffraction: (η⁶-C₆H₄(OCH₃) ₂)Cr(CO)₂(HSnPh₃) (3a), (η⁶-C₆H₄(OCH₃) ₂)Cr(CO)₂(SnPh₃)₂ (3b). Structural data for 3a: monoclinic; space group P2₁/c (No. 14); a=17.445(3), b=9.820(3) and c=16.302(5) A; Z=4; V=2539.2(12) A³; 3b: monoclinic; space group P2₁/n (No. 14); a=13.904(3), b=20.567(5) and c=13.911(3) A; Z=4; V=3994(2) A³. ¹H-NMR as well as X-ray provided evidence for the existence of three-center two-electron bonds in the hydrido stannyl complexes. The effects of different ligands on bonding and spectroscopic parameters were studied. Arene exchange reactions of the bis-stannyl complexes, as well as reactions of some of these compounds with P(C₂H₅)₃ and CO, were also investigated.

Full text of DOI:10.1016/S0022-328X(98)00917-6

Journal of Organometallic Chemistry 572 (1999) 11–20
Photochemical synthesis of new (p6-arene)Cr–hydrido stannyl and
(p6-arene)Cr–bis-stannyl complexes. Ligand effects on the Sn–H
interaction in the hydrido stannyl compounds
Abbas Khaleel a,b, Kenneth J. Klabunde b,*, Alison Johnson a,b
a Department of Chemistry, Kansas State Uni6ersity, Manhattan, KS 66506, USA
b Department of Chemistry, St. Cloud State Uni6ersity, St. Cloud, MN 56304, USA
Received 8 December 1997; received in revised form 6 August 1998
Abstract
Hydrido stannyl compounds containing the p2-H–SnPh3 ligand and bis-stannyl compounds containing two SnPh3 ligands have
been obtained from the photolysis of (p6-arene)Cr(CO)3 and HSnPh3. New complexes with different arenes (mesitylene,
trifluorotoluene and 1,4-dimethoxybenzene) have been obtained and characterized by X-ray diffraction: (p6-
C6H4(OCH3)2)Cr(CO)2(HSnPh3) (3a), (p6-C6H4(OCH3)2)Cr(CO)2(SnPh3)2 (3b). Structural data for 3a: monoclinic; space group
3
˚
˚
P21/c (No. 14); a=17.445(3), b=9.820(3) and c=16.302(5) A; Z=4; V=2539.2(12) A ; 3b: monoclinic; space group P21/n (No.
3
1
˚
˚
14); a=13.904(3), b=20.567(5) and c=13.911(3) A; Z=4; V=3994(2) A . H-NMR as well as X-ray provided evidence for the
existence of three-center two-electron bonds in the hydrido stannyl complexes. The effects of different ligands on bonding and
spectroscopic parameters were studied. Arene exchange reactions of the bis-stannyl complexes, as well as reactions of some of
these compounds with P(C2H5)3 and CO, were also investigated. © 1999 Elsevier Science S.A. All rights reserved.
Keywords: Arene complexes; Hydrido stannyl compounds; Ligand displacement
1. Introduction
1989, Schubert et al. reported the first example of a
transition metal, hydrogen, tin three-center two-elec-
tron bond in the Mn complex (MeCp)(CO)2Mn(H)
SnPh3 [3].
Herein, this paper reports on the synthesis of addi-
tional stannyl complexes, namely (p6-arene)Cr(CO)2
(HSnR3) and (p6-arene)Cr(CO)2(SnR3)2, and X-ray
structural data for two of these. The authors also
discuss the effect of ligands at the Cr and Sn centers on
the degree of interaction between the H and Sn atoms
in the hydrido stannyl compounds; in other words, the
effect of these ligands on the nature of the addition of
the HꢀSn bonds to the Cr center.
In an earlier study, the authors reported the synthesis
of a series of (p6-arene)Cr–hydrido stannyl and (p6-
arene)Cr–bis-stannyl complexes [1]. The addition of
H–M%R3 (M%=main group metal) to a transition metal
center (M) led to M–hydride compounds, with MꢀH
and MꢀM% bonds as a result of the total oxidative
addition of the HꢀM% bond, or to compounds contain-
ing an p2 M···H···M% bonding mode. The formation of
three-center two-electron bonds between a transition
metal, H and Si has been well-established by chemical
and spectroscopic evidence after Graham et al. isolated
and characterized a number of complexes of the general
formula Cp(CO)2Mn(H)SiR3, in the early 1970s [2]. In
One of the driving forces for this research has been
the preparation of materials with high chemical reactiv-
ities, especially p6-arene lability. The lability of the
arene ring facilitates its displacement by other ligands,
which could be an initiative step in some catalytic
* Corresponding author. Tel.: +1-785-532-6665; Fax: +1-785-
532-6666.
0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(98)00917-6

12
A. Khaleel et al. / Journal of Organometallic Chemistry 572 (1999) 11–20
2. Experimental section
reactions. This phenomenon was the key factor in the
high catalytic potential of (y-arene)Ni complexes, of
the general formula (p6-arene)NiR2 (R=SiCl3, SiF3,
C6F5), in 1-butene isomerization and ethylene and
propylene dimerization as reported by Klabunde et al.
[4].
The displacement of y-arene ligands has attracted
special attention. The displacement of chromium
bound y-arene ligands by other arenes has been inves-
tigated at 170°C for (y-arene)Cr(CO)3 complexes by
Mahaffy and Pauson [5]. They found that the resis-
tance to arene exchange of (arene)tricarbonyl com-
plexes of chromium decreases in the order:
hexamethylbenzene\mesitylene\N,N-dimethylaniline
] xylene\toluene=benzene\chlorobenzeneꢀnaph-
thalene. It is clear that electronic factors govern the
reactivity of these complexes such that the more elec-
tron-rich arenes give more unreactive compounds, and
thermal stability shows similar trends.
Arene exchange processes have also been investi-
gated at temperatures between 110 and 170°C for (y-
arene)Cr(CO)2(SiCl3)2 complexes [6], analogous to the
present bis-stannyl compounds. Mesitylene displaced
fluorobenzene and difluorobenzene and again, elec-
tronic effects of the arene seemed to play a dominant
role in the reactivity of these compounds (the more
electron-rich the arenes were, the more difficult to
exchange).
2.1. General procedures
All reactions were carried out under Ar using stan-
dard Schlenk techniques [7]. Photochemical reactions
were carried out in a quartz tube equipped with a
water-cooled probe. A UV mercury lamp (450 W) was
used for irradiation. Solid transfers were accomplished
in
a
glove box. Toluene, hexanes, mesitylene,
fluorobenzene, pyridine and pentane were distilled
from Na- and K-benzophenone. Butyl ether was dis-
tilled from CaH2 under argon. HSnPh3, decalin,
P(C2H5)3, HSn(n-Bu)3, 1,4-dimethoxybenzene and
deuterated solvents were purchased from Aldrich and
used without further purification. Hexaphenyldistan-
nane, Sn2Ph6, was purchased from Gelest. Silica gel
(230–400 mesh) was purchased from Fisher. Arene-
Cr(CO)3 complexes were prepared according to litera-
ture [1] and in the case of (1,4-dimethoxy-
benzene)Cr(CO)3, butyl ether was used as the solvent.
IR spectra were obtained using a Perkin–Elmer 1330
instrument. wCrꢀH was distinguished from wCO based on
1
literature [1]. The H-NMR spectra were recorded on
1
a Varian XL-400 operating at 400 MHz. H chemical
shifts are reported relative to tetramethylsilane. Ele-
mental analyses were obtained from Galbraith Labo-
ratories.
The authors have attempted to study arene ex-
change reactions of bis-stannyl complexes with differ-
ent arenes at temperatures between 110 and 170°C.
They found that these complexes behave differently as
compared with the analogous tricarbonyl or bis-silyl
complexes discussed above. Besides the electronic ef-
fects from the arene ligand, steric effects from the
large stannyl ligands have been found to play an im-
portant role in the stability and reactivity of these
compounds. The authors have also studied the reac-
tions of some of the compounds with triethylphos-
phine and CO.
2.2. Synthesis of (p6-C6H3(CH3)3)Cr(CO)2(HSn
(n-Bu)3), 1a, and (p6-C6H3(CH3)3)Cr(CO)2(Sn
(n-Bu)3)2, 1b
(p6-C6H3(CH3)3)Cr(CO)3 (0.2 g) was dissolved in 45
ml of hexanes and 2.0 ml of HSn(n-Bu)3 were added.
The yellow solution was stirred and irradiated with
UV light at r.t. for 4 h, giving a dark red solution.
The solvent was removed under vacuum giving a yel-
low–orange oily residue, which was dissolved in 5 ml
of toluene and treated on a silica gel chromatography
column using hexanes/toluene as an eluting solvent.
Table 1
1H-NMR data (l, ppm) in C6D6 for 1a, 1b, 2, 3a and 3b
Compounda
Arene/methyls
R: butyls or phenyls
Hydride
1a
1b
2
3a
3b
4.3 (s, 3H)/1.80 (s, 9H)
4.55 (s, 3H)/1.82 (s, 9H)
3.84 (t, 1H), 5.34 (d, 2H), 5.05 (t, 2H)
4.64 (s, 4H)/2.81 (s, 6H)
5.11 (s, 4H)/2.64 (s, 6H)
1.00–1.54 (m, 27H)
1.07–1.92 (m, 54 H)
7.19–7.81 (m, 30H)
7.20–7.90 (m, 15H)
7.20–7.94 (m, 30H)
−9.86 (s, 1H)
–
–
−8.22 (s, 1H)
–
a 1a is (p6-C6H3(CH3)3)Cr(CO)2(HSn(n-But)3); 1b is (p6-C6H3(CH3)3)Cr(CO)2(Sn(n-Bu)3)2; 2 is (p6-C6H5CF3)Cr(CO)2(Sn(n-Bu)3)2; 3a is
(p6-1,4-C6H4(OCH3)2)Cr(CO)2(HSn(Ph)3); 3b is (p6-1,4-C6H4(OCH3)2)Cr(CO)2(Sn(Ph)3)2.

A. Khaleel et al. / Journal of Organometallic Chemistry 572 (1999) 11–20
13
Table 2
Solutions of unreacted starting material, hydrido stan-
nyl compound (1a) and bis-stannyl compound (1b) were
collected separately, the bis-stannyl compound eluting
first followed by the hydrido stannyl compound. Re-
moving the solvents under vacuum gave yellow–orange
crystalline solids of the products, with about 25 and
Crystal data for (p6-1,4-C6H4(OCH3)2)Cr(CO)2(HSnPh3), 3a, and
(p6-1,4-C6H4(OCH3)2)Cr(CO)2(SnPh3)2, 3b
Compound
3a
3b
Empirical formula
Formula weight
Temperature (K)
Wavelength (A)
Symmetry
SnCrC28O4H26
597.19
293
0.70930
Monoclinic
P21/c (No. 14)
17.445(3)
9.820(3)
16.302(5)
90.0
Sn2CrC46O4H40
946.19
293
0.70930
Monoclinic
P21/n (No. 14)
13.904(3)
20.567(5)
13.911(3)
90.0
1
10% yield of 1a and 1b, respectively. H-NMR data are
given in Table 1. IR data of 1a (cm−1, nujol mull): wCO
1860, 1900; wCrꢀH 1919. IR data of 1b (cm−1, nujol
mull): wCO 1832, 1880. Anal. Calc. for CrSn2O2C35H66
(1b): Cr, 6.43; C, 51.80; H, 8.23. Found: Cr, 6.59; C,
49.70; H, 7.72.
˚
Space group
˚
a (A)
˚
b (A)
˚
c (A)
h (°)
i (°)
k (°)
2.3. Synthesis of (p6-C6H5CF3)Cr(CO)2(SnPh3)2, 2
114.60(2)
90.0
91.65(2)
90.0
3
˚
V (A )
2539.2(12)
4
3994(2)
4
(p6-C6H5CF3)Cr(CO)3 (0.3 g) was dissolved in 40 ml
of hexanes and 1.0 ml of HSnPh3 was added. The
resulting yellow solution was stirred under UV light at
r.t. for 3 h giving a red solution. The solvent was
removed under vacuum giving a dark red oily residue.
Separation on the silica gel column led to the isolation
of two compounds: hexaphenyldistannane (Sn2Ph6),
which formed white crystals upon recrystallization from
toluene/pentane, and the chromium bis-stannyl com-
plex, (p6-C6H5CF3)Cr(CO)2(SnPh3)2, 2, in ca. 10%
Z
Dcalc. (g cm−3
)
1.562
1.574
Standards number
Reflections
Reflections q min/max
Refinement
2
3
4215
5447
0.00/22.43
F
0.00/22.43
F
Refinement method
Weighting scheme
Number reflections
Number parameters
R factora
Full-matrix
1/2|(F)
1856
Full-matrix
1/2|(F)
3810
182
226
0.057
0.036
Rw factorb
Goodness-of-fit
0.050
0.030
1
yield. No hydrido stannyl compound was formed. H-
2.52
2.09
NMR data of 2 are listed in Table 1. IR data of 2
(cm−1, nujol mull): wCO 1848, 1903. Anal. Calc. for
CrSn2F3O2C45H35 (2): C, 56.69; H, 3.70. Found: C,
57.81; H, 4.30.
Difference density min/max −1.18/1.93
−1.19/1.11
a R=ꢀ ꢁꢁFoꢁ−ꢁFcꢁꢁ/ꢀ ꢁFoꢁ.
b Rw=[(ꢀ w(ꢁFoꢁ−ꢁFcꢁ)2/ꢀ wF2o)]1/2
.
2.4. Synthesis of (p6-1,4-C6H4(OCH3)2)Cr(CO)2(HSn-
2.5. X-ray structural analysis
Ph3), 3a, and (p6-1,4-C6H4(OCH3)2)Cr(CO)2(SnPh3)2,
3b
Crystals of 3a and 3b were obtained by slow recrys-
tallization from toluene/pentane at low temperature.
Analysis was carried out on an Enraf–Nonius diffrac-
tometer with graphite monochromated Mo–Kh radia-
(p6-1,4-C6H4(OCH3)2)Cr(CO)3 (0.45 g) was dissolved
in 45 ml of hexanes and 5 ml of toluene. HSnPh3 (1.4
ml) was added and the yellow solution was stirred and
irradiated for 6 h at r.t. giving an orange solution.
Removing the solvents under vacuum gave an orange
oily residue. The product was dissolved in 15 ml
toluene and treated on the silica gel column using
hexanes/toluene as the eluting solvent. Solutions of
hydridostannyl, 3a, and bis-stannyl, 3b, compounds
were collected separately, the bis-stannyl eluting first.
Removing the solvent under vacuum and recrystalliza-
tion from toluene/pentane gave yellow needle crystals
of 3a (30% yield) and yellow–orange crystals of 3b
(10% yield). 1H-NMR data of both compounds are
presented in Table 1. IR data of 3a (cm−1, nujol mull):
wCO 1830, 1912; wCrꢀH 1940. Anal. Calc. for
CrSnO4C25H28 (3a): C, 56.31; H, 4.39. Found: C, 55.89;
H, 4.25. IR data of 3b (cm−1, nujol mull): wCO 1840,
1890. Anal Calc. for CrSn2O4C46 (3b): C, 58.52; H,
4.27. Found: C, 58.48; H, 4.24.
˚
tion (u=0.71073 A). The data were corrected for
Lorentz and polarization effects and for decay (see
Table 2). An empirical absorption correction based on
„ scan data was performed on both data sets; however,
application of the absorption correction to 3a obscured
the location of the bridging hydrogen atom between the
tin and chromium atoms. Thus, the absorption correc-
tion was not used in the solution and refinement of 3a.
2.6. X-ray structural analysis of (p6-1,4-C6H4(OCH3)2)
Cr(CO)2(HSnPh3), 3a
A yellow plate-like crystal having approximate di-
mensions of 0.50×0.40×0.10 mm3 was glued with
epoxy inside an argon-filled capillary tube and was
optically aligned on the diffractometer. A monoclinic
cell was obtained from a least-squares fit of 20 cen-
tered reflections. The Laue symmetry was confirmed

14
A. Khaleel et al. / Journal of Organometallic Chemistry 572 (1999) 11–20
by comparison of the intensities of symmetry equivalent
reflections. The measured intensities of two independent
octants of the reciprocal lattice were collected via 2q
scans over 4.0°52q545°. Of the 3317 data collected,
1856 with I\2.5| were utilized in the structural refine-
ment. The probable space group P21/c (No. 14) was
later corroborated by successful structural refinement.
Direct methods were employed to locate the tin and
chromium atoms. Successive Fourier syntheses coupled
with least-squares refinement yielded the positions of all
associated carbon atoms. Isotropic refinement of all
non-hydrogen atoms was followed by anisotropic refin-
ement of all non-hydrogen atoms. After several cycles
of anisotropic refinement, each phenyl ring in the
triphenyl tin ligand was constrained as a rigid group
(idealized to the well-known D6h geometry of the ben-
zene ring). The phenyl group orientation angles an-
isotropic thermal parameters were refined when
least-squares refinement was resumed. Refinement con-
verged with R=0.036, Rw=0.030. A final difference
map revealed no unusual features.
1
remaining non-hydrogen atoms. Since a H-NMR spec-
trum of this compound suggested the presence of a
metal-bonded hydrogen atom, the difference map was
carefully scrutinized. It yielded the position of a proba-
ble hydrogen atom bridging the tin and chromium
atoms in a chemically reasonable environment. Ideal-
ized coordinates for the hydrogen atoms on the phenyl
groups and the methoxy groups were calculated with
site occupations tied to the associated carbon atoms.
Isotropic refinement of all non-hydrogen atoms was
followed by anisotropic refinement of all non-hydrogen
atoms. Due to its proximity to the large tin atom, the
coordinates of the bridging hydrogen were not refined;
however, the isotropic thermal parameter was refined
(U=0.05(4)). After several cycles of anisotropic refine-
ment, each phenyl ring in the triphenyl tin ligand was
constrained as a rigid group (idealized to the well-
known D6h geometry of the benzene ring). The phenyl
group orientation angles anisotropic thermal parame-
ters were refined when least-squares refinement was
resumed. Refinement converged with R=0.057, Rw=
0.050. A final difference map revealed no unusual
features.
2.8. Reactions with CO
Reactions of (p6-C6H3(CH3)3)Cr(CO)2(HSnPh3), 4a
[1], and (p6-C6H3(CH3)3)Cr(CO)2(SnPh3)2, 4b [1], with
carbon monoxide (CO) were investigated at r.t., 60°C
and 150°C. At r.t. and 60°C, the reactions were carried
out by dissolving 0.05 g of 4a or 4b (8.6×10−5 mol 4a
or 5.3×10−5 mol 4b) in 20 ml toluene in a Schlenk
tube under argon. CO gas was allowed to bubble
through the solution at the desired temperature for 12
h. The solvent was then removed under vacuum and the
yellow precipitate obtained was studied by 1H-NMR
and IR. The reaction at 150°C was carried out in an
NMR tube by dissolving 0.015 g (1.6×10−5 mol) of 4b
in 1.0 ml of deuterated bromobenzene. CO gas was
allowed to bubble through the solution at 150°C (in a
mineral oil bath) for 6 h, during which the yellow
solution turned colorless due to the complete conver-
sion of the starting material to Cr(CO)6 and free
mesitylene.
2.9. Reaction of Sn2Ph6 with Cr(CO)6
2.7. X-ray structural analysis of (p6-1,4-C6H4
(OCH3)2)Cr(CO)2(SnPh3)2, 3b
Sn2Ph6 (1.35g; 1.9×10−3 mol) and 0.45 g (2.0×
10−3 mol) of Cr(CO)6 were dissolved in 60 ml benzene
under argon. The colorless solution was irradiated with
UV light and stirred at r.t. for 3 h giving a dark red
solution. A brown–yellow solid was obtained upon
An orange plate crystal having approximate dimen-
sions of 0.20×0.30×0.40 mm3 was glued with epoxy
inside an argon-filled capillary tube and was optically
aligned on the diffractometer. A monoclinic cell was
obtained from a least-squares fit of 24 centered reflec-
tions. The Laue symmetry was confirmed by compari-
son of the intensities of symmetry equivalent reflections.
The measured intensities of two independent octants of
the reciprocal lattice were collected via 2q scans over
4.0°52q545°. Of the 5192 data collected, 3810 with
I\2.5| were utilized in the structural refinement. The
probable space group P21/n (No. 14) was later corrobo-
rated by the successful structural refinement.
Direct methods were employed to locate the tin and
chromium atoms. Successive Fourier syntheses coupled
with least-squares refinement yielded the positions of all
remaining non-hydrogen atoms. Idealized coordinates
for the hydrogen atoms on the phenyl and methoxy
groups were calculated with site occupations tied to the
1
removing the solvent under vacuum. A H-NMR spec-
trum of the products showed a mixture of y-arene- and
stannyl-containing products that were not investigated
further. The desired bis-stannyl compound (p6-
C6H6)Cr(CO)2(SnPh3)2 was not observed.
2.10. Reaction of Sn2Ph6 with (p6-C6H6)Cr(CO)3
A solution of 0.15 g (7.0×10−4 mol) of (p6-
C6H6)Cr(CO)3 and 0.45 g (6.4×10−4 mol) of Sn2Ph6 in
25 ml benzene and 40 ml hexanes was stirred and
irradiated with UV light for 3 h at r.t. The starting
yellow solution turned dark red and removing the
solvents under vacuum resulted in a brown–yellow
solid containing the bis-stannyl compound (p6-
C6H6)Cr(CO)2(SnPh3)2 in ca. 25% yield.

A. Khaleel et al. / Journal of Organometallic Chemistry 572 (1999) 11–20
15
2.11. Reaction of (p6-C6H4(OCH3)2)Cr(CO)2(HSnPh3),
3a, with triethylphosphine
radical initiators [8], suggests that the Ph3SnꢀSnPh3
forms via reductive elimination of the two SnPh3 ligands
from
the
final
bis-stannyl
compound,
(p6-
Compound 3a (0.04 g; 6.7×10−5 mol) was dissolved
in 5 ml P(C2H5)3 in a Schlenk tube under argon. The
yellow solution obtained was stirred at r.t. for 12 h.
Removing the excess phosphine under vacuum led to an
oily brown–yellow residue, which was then dissolved in
10 ml hexanes. Upon slow evaporation of the hexanes,
a green–yellow precipitate and yellow–orange crystals
of (p6-C6H4(OCH3)2)Cr(CO)2P(C2H5)3 (5) were ob-
tained. IR data (cm−1, nujol mull): wCO 1824(s) and
1884(vs). 1H-NMR (l, ppm, in C6D6): 0.96 (d of t, 9H),
1.43 (d of q, 6H), 3.23 (s, 6H, OCH3), 4.41 (d, 4H, arene).
C6H5CF3)Cr(CO)2(SnPh3)2 (2).
3.2. Crystal structure of (p6-1,4-C6H4(OCH3)2)Cr
(CO)2)(HSnPh3), 3a
An ORTEP drawing of the structure is shown in Fig.
1. Selected bond distances and angles are given in Table
3. The ligands adopt a piano stool arrangement around
the Cr center, where the arene ring represents the seat
and the two CO ligands and the hydrido stannyl ligand
represent the legs. X-ray structural analysis as well as
1H-NMR data indicate that the triphenyltinhydride lig-
and binds in an p2 fashion to the chromium center. The
SnꢀCrꢀH angle (56°) is too small to suggest that the Sn
and H atoms occupy two coordination sites around the
Cr center; rather, the HꢀSnPh3 must be considered as
one ligand such that the SnꢀH bond occupies one
coordination site, with the H forming a delocalized,
three-center two-electron bond with the Sn and Cr.
Although the position of the H atom could not be
refined owing to its close proximity to the large Sn
2.12. Arene exchange reactions
The following general procedure was used in all
reactions of bis-stannyl compounds with free arenes. The
bis-stannyl compound (0.05 g of each) was dissolved
under argon in 10 ml decalin in a 50-ml 3-neck round-
bottom flask equipped with a condenser and an argon
inlet. A twentyfold excess of free arene was added and
the solution was stirred in an oil bath at the desired
temperature. At different reaction times, the flask was
cooled to r.t. and 0.1 ml of the solution was withdrawn
by a syringe for IR analysis. In some reactions, the
˚
electron cloud, its calculated distance of 2.23 A from the
Sn atom suggests some bonding interaction between
both atoms. Compared with the analogous mesitylene
complex, (p6-C6H3(CH3)3)Cr(CO)2(HSnPh3) (4a), the
SnꢀH interaction in this complex is weaker; in other
words, the oxidative addition is more complete. This
difference is indicated by the calculated larger SnꢀCrꢀH
1
solvent was finally removed under vacuum and a H-
NMR spectrum was recorded for the solid residue in
C6D6.
˚
angle, 56°, and the longer SnꢀH distance, 2.23 A, as
compared with 45° and 1.95 A in the analogous
˚
3. Results and discussion
3.1. Synthesis of complexes
The photochemical reaction of (p6-arene)Cr(CO)3
complexes with an excess of HSnR3 (R=n-butyl or
phenyl) in the appropriate solvent usually led to the
formation of both types of complexes, (p6-
arene)Cr(CO)2(HSnR3) and (p6-arene)Cr(CO)2(SnR3)2:
(arene)Cr(CO)3+HSnR3“hw (arene)Cr(CO)2(HSnR3)
a
+(arene)Cr(CO)2(SnR3)2
b
However, in the reaction of (p6-C6H5CF3)Cr(CO)3
with HSnPh3, no hydrido stannyl compound was iso-
lated. Besides the bis-stannyl compound, (p6-
C6H5CF3)Cr(CO)2(SnPh3)2 (2), hexaphenyldistannane
(Sn2Ph6) formed and was isolated. One might expect that
Sn2Ph6 could be generated from two SnPh3 fragments
(they may be radicals) as a result of photochemical
cleavage of the HꢀSn bond of free HSnPh3. However,
the fact that the photolytic cleavage of the HꢀSn bond
in free HSnPh3 has not been observed in other cases
where HSnPh3 existed in excess, and it has not been
observed before except by hydrogen abstraction with
Fig. 1. ORTEP drawing of (p6-1,4-C6H4(OCH3)2Cr(CO)2)(HSnPh3),
3a.

16
A. Khaleel et al. / Journal of Organometallic Chemistry 572 (1999) 11–20
Table 3
Bond
that a complete oxidative addition has taken place. This
is based on a comparison with the known osmium
compound, cis (CO)4Os(H)SnCl3, where there should be
no interaction between the hydride and SnCl3, and the
J(119SnꢀOsꢀH) and J(117SnꢀOsꢀH) were found to be 136
and 129.5 Hz, respectively [3].
˚
lengths
(A)
and
angles
(°)
in
(p6-1,4-
C6H4(OCH3)2)Cr(CO)2(HSnPh3), 3a
˚
Bond lengths (A)
Sn1ꢀCr1
Sn1ꢀC1
Sn1ꢀC1
Sn1ꢀC2
Cr1ꢀC1
Cr1ꢀC2
Cr1ꢀC3
Cr1ꢀC4
Cr1ꢀC5
Cr1ꢀC6
Cr1ꢀC9
2.688(3)
12.173(9)
72.18(1)
32.17(1)
2.24(2)
2.19(1)
2.19(2)
2.26(2)
2.22(1)
2.20(1)
1.82(2)
1.83(2)
O1ꢀC1
O1ꢀC7
O4ꢀC4
O4ꢀC8
O9ꢀC9
O10ꢀC10
C1ꢀC2
C1ꢀC6
C2ꢀC3
C3ꢀC4
C4ꢀC5
C5ꢀC6
1.37(2)
1.46(2)
1.39(2)
1.35(2)
1.14(2)
1.15(2)
1.44(3)
1.40(3)
1.35(3)
1.40(3)
1.40(3)
1.41(3)
The angle between the two CO ligands in 3a is 85.8°.
The arene ring is oriented such that the methoxy groups
on the arene are staggered with respect to the Sn atom,
presumably to avoid steric hindrance between the
methoxy groups and the triphenyltin ligand. The dis-
tance between the Cr and the center of the arene ring is
˚
˚
1.69 A, 0.04 A shorter than the corresponding distance
in the analogous bis-stannyl compound (3b). The arene
ring adopts a boat conformation, with the C(1) and C(4)
atoms bent out of the plane of the ring by 0.10 and 0.08
Cr1ꢀC10
Bond angles (°)
Cr1ꢀSn1ꢀC11
Cr1ꢀSn1ꢀC17
Cr1ꢀSn1ꢀC23
C11ꢀSn1ꢀC17
C17ꢀSn1ꢀC23
Sn1ꢀCr1ꢀC1
Sn1ꢀCr1ꢀC2
Sn1ꢀCr1ꢀC3
Sn1ꢀCr1ꢀC6
Sn1ꢀCr1ꢀC9
Sn1ꢀCr1ꢀC10
C1ꢀCr1ꢀC9
C2ꢀCr1ꢀC3
C2ꢀCr1ꢀC4
C3ꢀCr1ꢀC9
Cr1ꢀC10ꢀO10
117.3(3)
114.0(2)
112.5(3)
104.8(4)
103.7(4)
101.9(5)
136.9(6)
164.6(5)
84.3(4)
103.1(5)
74.3(6)
118.2(7)
36.0(8)
C5ꢀCr1ꢀC6
C5ꢀCr1ꢀC9
C5ꢀCr1ꢀC10
C9ꢀCr1ꢀC10
C3ꢀCr1ꢀC10
C1ꢀO1ꢀC7
C4ꢀO4ꢀC8
Cr1ꢀC1ꢀO1
O1ꢀC1ꢀC2
O1ꢀC1ꢀC6
C2ꢀC1ꢀC6
Cr1ꢀC2ꢀC1
Cr1ꢀC4ꢀO4
Cr1ꢀC4ꢀC5
O4ꢀC4ꢀC3
O4ꢀC4ꢀC5
37.1(7)
155.5(7)
91.5(7)
85.8(8)
112.4(8)
119(1)
116(1)
130(1)
122(2)
117(2)
121(2)
73.1(9)
130(1)
70.3(9)
112(2)
127(2)
˚
A, respectively. This deviation could be a result of
electronic effects, as was reported for similar
(arene)M(C6F5)2 complexes by Radonovich et al. ([9]a).
Analogously, Hunter and co-workers ([9]b) attributed
such distortion to the strong electron donating ability of
methoxy groups.
3.3. X-ray structure of (p6-1,4-C6H4(OCH3)2)Cr
(CO)2(SnPh3)2, 3b
64.8(7)
91.5(7)
175(2)
An ORTEP drawing of the structure is shown in Fig.
2. Selected bond distances and angles are given in Table
4. The ligands adopt a piano stool arrangement around
the Cr center, where the arene ring represents the seat
and the two CO ligands and two stannyl ligands repre-
sent the legs. The two CO ligands are trans to each other
with a C(9)ꢀCr(1)ꢀC(10) angle of 105.4°. The two stan-
nyl ligands are also trans to each other with a
Sn(1)ꢀCr(1)ꢀSn(2) angle of 123.47°. The CrꢀSn distances
1
mesitylene complex. This is well-supported by the H-
NMR data that shows that the J(119SnꢀCrꢀH)=212 Hz
in 3a, which is much smaller than that in the mesitylene
complex (329.2 Hz). Although the SnꢀH coupling con-
stants of 3a are much lower than those in the analogous
compounds, 1a and 4a, they are still too high to conclude
˚
are 2.707 and 2.706 A. The distance between the arene
Fig. 2. ORTEP drawing of (p6-1,4-C6H4(OCH3)2)Cr(CO)2(SnPh3)2, 3b.

A. Khaleel et al. / Journal of Organometallic Chemistry 572 (1999) 11–20
17
Table 4
to weaker Sn–H interaction, in other words, such ligands
favor oxidative addition of the SnꢀH bond to the Cr
center. Comparison of the coupling constants in these
compounds clearly shows these effects (Table 5). The
coupling J(119SnꢀCrꢀ1H)=373.6 Hz in complex 1a,
which decreases to 329.2 Hz in the mesitylene complex
(4a) upon replacing the three butyl ligands with weaker
|-electron-donating phenyl ligands. It decreases further
to 213.6 Hz in complex 3a as the methyl groups are
replaced with much stronger electron-donating groups,
namely OCH3, on the arene ligand. The weaker Sn–H
interaction in 3a is also supported by X-ray structural
Bond lengths (A) and angles (°) in (p6-1,4-C6H4(OCH3)2)Cr
(CO)2(SnPh3)2, 3b
˚
˚
Bond lengths (A)
Sn1ꢀCr1
2.707(1)
2.749(7)
2.850(7)
2.168(4)
2.161(4)
2.176(4)
2.706(1)
2.740(7)
2.879(7)
2.164(4)
2.168(4)
2.167(4)
2.284(8)
2.225(7)
2.217(7)
2.314(8)
Cr1ꢀC5
Cr1ꢀC6
Cr1ꢀC9
Cr1ꢀC10
O1ꢀC1
O1ꢀC7
O4ꢀC4
O4ꢀC8
O9ꢀC9
O10ꢀC10
C1ꢀC2
C1ꢀC6
C2ꢀC3
C3ꢀC4
C4ꢀC5
C5ꢀC6
2.228(7)
2.217(7)
1.825(8)
1.859(7)
1.37(1)
1.33(1)
1.35(1)
1.42(1)
1.17(1)
1.141(9)
1.39(1)
1.38(1)
1.41(1)
1.40(1)
1.41(1)
1.39(1)
Sn1ꢀC9
Sn1ꢀC10
Sn1ꢀC11
Sn1ꢀC17
Sn1ꢀC23
Sn2ꢀCr1
Sn2ꢀC9
Sn2ꢀC10
Sn2ꢀC29
˚
analysis, where the SnꢀH distance is 1.95 A in 4a, while
Sn2ꢀC35
˚
it is 2.23 A in 3a. These observations show that the choice
Sn2ꢀC41
Cr1ꢀC1
of the ligands is a key factor in determining the nature of
the SnꢀCrꢀH bonding. Thus, stronger electron-donating
substituents on the arene ring, and more electron-de-
manding ligands on the Sn encourage more ‘complete’
oxidative addition of the SnꢀH bond to Cr.
Cr1ꢀC2
Cr1ꢀC3
Cr1ꢀC4
Bond angles (°)
Cr1ꢀSn1ꢀC11
Cr1ꢀSn1ꢀC17
Cr1ꢀSn1ꢀC23
C11ꢀSn1ꢀC17
C11ꢀSn1ꢀC23
C17ꢀSn1ꢀC23
Cr1ꢀSn2ꢀC29
Sn1ꢀCr1ꢀC3
Sn1ꢀCr1ꢀC5
Sn1ꢀCr1ꢀC9
Sn1ꢀCr1ꢀC10
Sn2ꢀCr1ꢀC9
Sn2ꢀCr1ꢀC10
117.9(1)
C6ꢀCr1ꢀC9
C6ꢀCr1ꢀC10
C9ꢀCr1ꢀC10
C1ꢀO1ꢀC7
C4ꢀO4ꢀC8
O1ꢀC1ꢀC2
O1ꢀC1ꢀC6
O4ꢀC4ꢀC3
O4ꢀC4ꢀC5
Cr1ꢀC5ꢀC6
Cr1ꢀC5ꢀC4
Cr1ꢀC9ꢀO9
Cr1ꢀC10ꢀO10
135.7(3)
111.9(1)
114.2(1)
102.7(2)
104.3(2)
104.2(2)
114.2(1)
148.7(2)
84.3(2)
71.7(2)
74.7(2)
71.4(2)
75.7(2)
102.1(3)
105.4(3)
117.9(8)
117.6(7)
125.1(8)
114.9(8)
117.9(7)
124.1(8)
71.3(4)
3.5. Reactions with CO
The hydrido stannyl complex (p6-C6H3(CH3)3)Cr
(CO)2(HSnPh3), 4a, did not react with CO at r.t., while the
reaction did take place at 60°C, giving both the tricar-
bonyl analog and the bis-stannyl compound (p6-
C6H3(CH3)3)Cr(CO)2(SnPh3)2, 4b, in ca. a 1:2 ratio. The
bis-stannyl complex 4b showed no tendency to react with
CO at r.t. or at 60°C. At an elevated temperature (150°C),
the reaction took place as a result of reductive elimination
of the two stannyl ligands, giving hexaphenyldistannane,
Sn2Ph6, and the tricarbonyl analog, (p6-C6H3
(CH3)3)Cr(CO)3. The 1H-NMR spectrum of the reaction
solution was recorded at different periods of reaction,
indicating that the reaction took place within the first 10
min forming the tricarbonyl compound and Sn2Ph6. After
30 min, most of the starting material reacted and most of
the tricarbonyl compound produced reacted further with
CO to give free arene and Cr(CO)6. The reaction was
complete after 4 h, after which only free mesitylene and
Sn2Ph6 were observed in the 1H-NMR spectrum and the
color of the solution turned, as expected, from yellow to
colorless (Scheme 1).
75.3(4)
178.0(6)
173.8(6)
˚
˚
ring center and the Cr atom is 1.732 A, which is 0.04 A
longer than that in the analogous hydrido stannyl
complex. This longer distance is probably a result of
larger steric hindrance from the two stannyl ligands and
the methoxy groups on the arene ring. The methoxy
groups are staggered with respect to the tin atoms,
presumably to avoid steric hindrance. In this complex as
well, there is a deviation of the arene ring from planarity.
The arene ring adopts a boat conformation, with C(1) and
˚
C(4) bent out of the plane by 0.08 and 0.09 A, respectively.
3.4. Ligand effect on the SnꢀH interaction in the
hydrido stannyl compounds
3.6. Reaction of Sn2Ph6 with Cr(CO)6 and (p6-C6H6)Cr
(CO)3
The nature of the product of the addition of a SnꢀH
bond to a transition metal (M) moiety can be investigated
by studying the Sn···H interaction in the resulting com-
pound. The Sn–H NMR coupling constants,
J(119SnꢀMꢀ1H) and J(117SnꢀMꢀ1H), serve as a good tool
to measure this interaction, where higher coupling con-
stants indicate stronger residual SnꢀH bonding. As might
be expected, electronic effects of the ligands on the
transition metal and the ligands on the Sn atom greatly
affect the degree of such interaction. Indeed, the authors
have found that electron-withdrawing groups on the Sn
and electron-donating groups on the transition metal lead
In an attempt to photochemically reverse the thermal
reaction of bis-stannyl compounds with CO, the reac-
Table 5
Coupling constants (Hz) in 1a, 4a and 3a
1a
4a
3a
J(119SnꢀCrꢀ1H)
J(117SnꢀCrꢀ1H)
373.60
359.6
329.20
314.4
213.60
208.0

18
A. Khaleel et al. / Journal of Organometallic Chemistry 572 (1999) 11–20
Scheme 1.
tion of Sn2Ph6 with Cr(CO)6 in benzene was carried out
under UV light. Due to gradual photolytic decarbony-
lation of the hexacarbonyl chromium compound, sev-
eral unknown products formed as determined from the
1H-NMR spectrum of the products of the reaction. A
The reaction went to completion as no starting material
was observed. After several recrystallization attempts,
small orange crystals and a yellow–green precipitate
were obtained upon slow evaporation of hexanes from
1
a 10 ml solution of the residue. The H-NMR spectrum
(p6-
similar
photochemical
reaction
between
(p6-
of
the
crystals
showed
signals
for
C6H4(OCH3)2)Cr(CO)2P(C2H5)3, and that the CH2 and
CH3 protons are coupled to the phosphorus with
J(1Hꢀ31P)=7.6 Hz giving a doublet of quartets for
CH2, which appears as five lines due to overlapping
peaks, and a doublet of triplets for CH3, which shows
five lines due to overlapping as well. The protons of the
arene are also coupled to the phosphorous with a
coupling constant of 3.2 Hz.
The IR spectrum of the phosphine adduct showed
two sharp peaks for wCO that are shifted to lower energy
by 28 and 6 wavenumbers, as compared with the start-
ing hydrido stannyl compound 3a. This shift could be
due to an increase in the electron density at the metal
from the donor ligand, P(C2H5)3, which increases the
back-bonding to the CO ligands.
C6H6)Cr(CO)3 and Sn2Ph6 in a mixture of benzene and
hexanes led to the formation of the bis-stannyl com-
plex, (p6-C6H6)Cr(CO)2(SnPh3)2, 6. About 10% of the
starting tricarbonyl compound was converted to the
bis-stannyl complex, 6, during a 3-h reaction as a result
of photochemical decarbonylation of the tricarbonyl
compound and reaction with Sn2Ph6 (Scheme 2). (It is
possible that ‘SnPh3’ radicals were involved, since the
photochemical cleavage of Sn2Ph6 has been reported to
give two SnPh3 radicals [10].)
3.7. Reaction of (p6-C6H4(OCH3)2)Cr(CO)2(HSnPh3),
3a, with P(C2H5)3
The reaction of 3a with triethylphosphine at r.t.
resulted in a yellow oily residue containing Sn2Ph6 and
(p6-C6H4(OCH3)2)Cr(CO)2P(C2H5)3 (5) as indicated by
1H-NMR.
Similar reactions between PPh3 and CpMn(CO)2
(H)(EPh3) (E=Si, Ge) have led to the displacement of
the silane or germane by the phosphine ligand [11].

A. Khaleel et al. / Journal of Organometallic Chemistry 572 (1999) 11–20
19
Scheme 2.
In the reaction of triethylphosphine with 3a, the
HSnPh3 ligand probably was eliminated, followed by
P(C2H5)3 coordination to the metal center. The HSnPh3
obtained reacted with the excess P(C2H5)3 to form
Sn2Ph6, which is supported by the results of a separate
reaction between free HSnPh3 and excess P(C2H5)3 in
which 0.2 ml HSnPh3 and 2.0 ml P(C2H5)3 were stirred
under argon for 2 days, and Sn2Ph6 was obtained, isolated
exchange [6]. The reason that the mesitylene–bis-stannyl
compound did not favorably form in this process could be
the steric hindrance from the bulky stannyl ligands and
the methyl groups on the arene ring.
The reaction of the mesitylene–bis-stannyl complex
with free benzene was studied by IR and 1H-NMR at 110
and 150°C. At 110°C, neither decomposition nor arene
exchange took place. At 150°C, the starting compound
completely decomposed within 6 h of reaction via reduc-
tive elimination of the two stannyl ligands forming
Sn2Ph6 and (mesitylene)Cr(CO)3 (in ca. a 1: 1 ratio)
before any arene exchange could take place. This indi-
cates that the dissociation of the two SnPh3 ligands from
the bis-stannyl complex is faster when the arene ring is
mesitylene than when it is benzene. This behavior could
be due, again, to the steric hindrance from the methyl
groups and the stannyl ligands, especially since one
methyl group eclipses one of the stannyl ligands.
1
and identified by H-NMR spectroscopy.
3.8. Arene exchange reactions
The IR study of the reaction of (p6-
C6H5F)Cr(CO)2(SnPh3)2 with free mesitylene at 170°C in
decalin showed no arene exchange but instead complete
decomposition to (p6-C6H5F)Cr(CO)3 and (mesitylene)
Cr(CO)3. 1H-NMR spectra of the solid products after the
solvents were removed indicated the formation of Sn2Ph6
also. The ratio between the products changed with the
time of the reaction. The fluorobenzene tricarbonyl
compound formed first, and then it exchanged the
fluorobenzene ligand with free mesitylene giving the
mesitylene tricarbonyl derivative, (p6-C6H3(CH3)3)Cr
(CO)3. As the reaction progressed, more arene exchange
took place until the major product was the mesitylene–
tricarbonyl compound after 30 h of reaction.
(p6-1,4-C6H4(OCH3)2)Cr(CO)2(SnPh3)2, 3b, showed
the highest thermal stability among the compounds that
have been discussed in this study. A reaction between 3b
and free benzene in decalin at 170°C for 30 h showed no
decomposition or arene exchange. There are two compet-
ing factors that affect the stability of 3b: first, electronic
effects from the arene that carries two strong electron
donating methoxy groups, which in turn enhances the
arene–Cr interaction, inhibiting any arene exchange with
other less electron-rich arenes and increases the electron
density at the Cr center, strengthening the CrꢀSn bonds.
Second, steric hindrance from the two stannyl ligands and
the two methoxy groups on the arene should facilitate
reductive elimination of the stannyl ligands as well as
decrease the metal–arene interaction. Apparently, the
electronic effects in this compound dominate and en-
hance its stability. The geometry of the complex in which
the two methoxy groups are in staggered positions with
the two stannyl ligands minimizes the steric effect. Based
on this study, the following trend in stability can be
concluded:
The reaction of (p6-C6H6)Cr(CO)2(SnPh3)2, 6, with
free mesitylene was studied at 170°C by IR and 1H-NMR.
Complex 6 showed high stability at this temperature and
after 6 h of reaction, less than 10% of the starting material
decomposed giving the tricarbonyl derivative, (p6-
C6H6)Cr(CO)3 and Sn2Ph6. As the reaction progressed,
the (p6-C6H6)Cr(CO)3 exchanged the benzene ligand with
free mesitylene to give (p6-C6H3(CH3)3)Cr(CO)3. The
starting compound, 6, showed no tendency to exchange
its arene with free mesitylene, which is in contrast to what
has been reported for the analogous silyl complexes,
where benzene complexes showed a higher tendency to

20
A. Khaleel et al. / Journal of Organometallic Chemistry 572 (1999) 11–20
X-ray determined Cr–arene distances for some of these
compounds also correlate with this trend. The center of
arene ring–Cr distance in the mesitylene complex, 4b,
two stannyl ligands, which minimizes the steric hin-
drance effect.
˚
˚
was determined to be 1.79 A, as compared with 1.73 A
in the dimethoxybenzene complex, 3b.
Acknowledgements
The support of National Science Foundation is ac-
knowledged with gratitude.
4. Conclusions
A series of (p6-arene)Cr(CO)2(HSnR3) and (p6-
arene)Cr(CO)2(SnR3)2 compounds have been synthe-
sized by the photochemical addition of HSnR3 to
(p6-arene)Cr(CO)3. The authors found that the sub-
stituents on the arene ring and at the Sn atom have
dramatic effects on the nature of the SnꢀH bond addi-
tion to the Cr center. More ‘complete’ oxidative addi-
tion was obtained when stronger electron donating
substituents on the arene and electron demanding lig-
ands at the Sn were used. As a result, the following
trend was observed for the degree of remaining SnꢀH
interaction in the hydrido stannyl compounds.
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˚
˚
Substituents on the arene ring were also found to
have a great effect on the stability of the bis-stannyl
compounds. Electron demanding arenes, such as
fluorobenzene, led to weaker arene–Cr interaction, and
in turn to less stable compounds. On the other hand,
electron-rich arenes led to very stable compounds with
stronger arene–Cr interaction, as was found in com-
pound 3b, (p6-1,4-C6H4(OCH3)2)Cr(CO)2(SnPh3)2.
However, steric hindrance has a significant effect if the
geometry of the compound creates considerable steric
problems, as in compound 4b, (p6-C6H3(CH3)3)Cr
(CO)2(SnPh3)2 [1]. In compound 3b, the two methoxy
groups are in staggered positions compared with the
.