Electrochemical and photophysical properties of a series of group-14 metalloles

Article abstract of DOI:10.1021/ic980662d

A series of six group-14 dimethyl- or diphenyl-tetraphenylmetallacyclopentadienes were synthesized and characterized by their spectroscopic and electrochemical properties. The group-14 elements investigated were silicon, germanium, and tin. (The compounds are designated according to the heteroatom and the substituent on the heteroatom, i.e., SiMe, SiPh, SnPh.) Five of the six compounds luminesce in both the solid state and in solution. The emission maxima of SiPh, GePh, and SnPh are invariant to a change in the heteroatom, while for SiMe, GeMe, and SnMe there is a strong dependence of the emission maxima on the identity of the heteroatom. SiMe emits at a longer wavelength than GeMe, while SnMe is not luminescent. The dramatic luminescence difference between the two tin compounds was investigated. ¹³C NMR coupling to ^119/117Sn, observed in both SnMe and SnPh, was used to make ¹³C NMR resonance assignments. Qualitative results of semiempirical molecular orbital calculations support the ¹³C NMR assignments. The crystal structure data for SnPh was obtained at 20 °C: a = 10.353(2) A?, b = 16.679(2) A?, c = 9.482(1) A?, α = 99.91(1)°, β = 106.33(1)°, γ = 77.80(1)° with Z = 2 in space group P₁. It is proposed that the increased electron density at tin in SnMe is responsible for the deactivation of the emissive state. The presence of phenyl substituents in SnPh serves to stabilize the emissive state and luminescence is observed.

Full text of DOI:10.1021/ic980662d

2464
Inorg. Chem. 1999, 38, 2464-2472
Electrochemical and Photophysical Properties of a Series of Group-14 Metalloles
Justin Ferman,‡ Joseph P. Kakareka,§ Wim T. Klooster,| Jerome L. Mullin,
Joseph Quattrucci,† John S. Ricci,† Henry J. Tracy,*,† William J. Vining,‡ and Scott Wallace†
Department of Chemistry, University of Southern Maine, Portland, Maine, Department of Chemistry,
University of Massachusetts, Amherst, Massachusetts, College of Arts and Sciences, Florida Gulf Coast
University, Ft. Myers, Florida, Chemistry Department, Brookhaven National Laboratory,
Upton, New York, and Department of Life Sciences, University of New England, Biddeford, Maine
ReceiVed June 11, 1998
A series of six group-14 dimethyl- or diphenyl-tetraphenylmetallacyclopentadienes were synthesized and
characterized by their spectroscopic and electrochemical properties. The group-14 elements investigated were
silicon, germanium, and tin. (The compounds are designated according to the heteroatom and the substituent on
the heteroatom, i.e., SiMe, SiPh, ..., SnPh.) Five of the six compounds luminesce in both the solid state and in
solution. The emission maxima of SiPh, GePh, and SnPh are invariant to a change in the heteroatom, while for
SiMe, GeMe, and SnMe there is a strong dependence of the emission maxima on the identity of the heteroatom.
SiMe emits at a longer wavelength than GeMe, while SnMe is not luminescent. The dramatic luminescence
difference between the two tin compounds was investigated. 13C NMR coupling to 119/117Sn, observed in both
SnMe and SnPh, was used to make 13C NMR resonance assignments. Qualitative results of semiempirical molecular
orbital calculations support the 13C NMR assignments. The crystal structure data for SnPh was obtained at 20 °C:
a ) 10.353(2) Å, b ) 16.679(2) Å, c ) 9.482(1) Å, R ) 99.91(1)°, â ) 106.33(1)°, γ ) 77.80(1)° with Z ) 2
in space group P1h. It is proposed that the increased electron density at tin in SnMe is responsible for the deactivation
of the emissive state. The presence of phenyl substituents in SnPh serves to stabilize the emissive state and
luminescence is observed.
Introduction
Group-14 and group-15 metalloles, five-membered metalla-
cyclopentadienes (Figure 1), were first prepared by Braye and
Leavitt in the early 1960s.1-3 In a recent review of the group-
14 metalloles, Dubac has cataloged their syntheses, organic
reaction chemistry, and some of their physical and chemical
properties.4 Interest in this class of compounds has been
Figure 1. The general structure of metalloles. For group-14 metalloles,
E ) Si, Ge, Sn and R1 ) R2 ) Me or Ph. For group-15 metalloles, E
) N, P, As and R1 ) Ph and R2 is not present.
motivated by the possibility of using metalloles as monomers
for the preparation of conjugated polymers with very small band
gaps that exhibit intrinsic conductivity or semiconducting
properties.5 Our interest in this class of compounds was
stimulated by the luminescent properties of several of the group-
14 metalloles. Braye6 noted a strong blue fluorescence in
ultraviolet light with hexaphenylsilole (SiPh), and Gilman7 noted
a similar blue fluorescence with 1,1-dimethyl-2,3,4,5-tetraphen-
ylsilole (SiMe). A brief comparison of the luminescence
properties of some of the group-15 perphenylmetalloles has been
published.8 Luminescence emission for pentaphenylpyrrole,
pentaphenylphosphole, and pentaphenylarsole (E ) N, P, As,
and R1 ) Ph in Figure 1) occurred at 487 ( 5 nm (with
excitation at 320 nm). The identity of the heteroatom did not
significantly affect the observed luminescence. No similar
luminescence analyses of the group-14 metalloles have appeared.
In this paper we present a comparison of the photophysical
and electrochemical properties of the methyl- and phenyl-
substituted group-14 metalloles. For simplicity, these group-14
tetraphenylmetalloles will be referred to by their heteroatom
and heteroatom substituent: SiMe, SiPh, GeMe, GePh, SnMe,
and SnPh. We have attempted to interpret the experimental
results by examining semiempirical molecular orbital calcula-
tions of these metalloles. A high-level ab initio calculation
performed on a model compound supports the molecular orbital
description obtained at the semiempirical level.
† University of Southern Maine.
‡ University of Massachusetts.
§ Florida Gulf Coast University.
| Brookhaven National Laboratory.
University of New England.
(1) Braye, E. H.; Hubel, W.; Caplier, I. J. Am. Chem. Soc. 1961, 83, 4406.
(2) Leavitt, F. C.; Manuel, T. A.; Johnson, F. J. Am. Chem. Soc. 1959,
81, 3163.
(3) Leavitt, F. C.; Manuel, T. A.; Johnson, F.; Matternas, U.; Lehman,
D. S. J. Am. Chem. Soc. 1960, 82, 5099.
(4) Dubac, J.; Laporterie, A.; Manuel, G. Chem. ReV. 1990, 90, 215.
(5) Marynick, D. S.; Hong, S. Y. Macromolecules 1995, 28, 4991.
(6) Braye, E. H.; Hubel, W.; Caplier, I. J. Am. Chem. Soc. 1961, 83, 4406.
(7) Gilman, H.; Cottis, S. G.; Atwell, W. H. J. Am. Chem. Soc. 1964, 86,
1596.
The metalloles were prepared from the dilithium reagent
generated by the reductive coupling of diphenylacetylene in the
presence of lithium metal as pioneered by Braye.4 The improve-
ments to the original preparation noted by Eisch were em-
(8) Braye, E. H.; Raciszewski, Z. Photochem. Photobiol. 1970, 12, 429.
10.1021/ic980662d CCC: $18.00 © 1999 American Chemical Society
Published on Web 04/24/1999

Properties of a Series of Group-14 Metalloles
Inorganic Chemistry, Vol. 38, No. 10, 1999 2465
Scheme 1
Scheme 2
ployed.9 The dilithium reagent, 1,4-dilithio-2,3,4,5-tetraphen-
ylbutadiene, was then quenched with the appropriate dialkyl
(or diaryl) dichlorides of silicon, germanium, or tin (Schemes
1 and 2). In the case of GePh, 1,1-dichloro-2,3,4,5-tetraphen-
ylgermacyclopentadiene was prepared first and then phenyl-
lithium was employed to synthesize the desired GePh. The
metalloles were isolated by column chromatography on silica
and purified by recrystallization. The six metalloles studied are
yellow or pale yellow-green air-stable solids with melting points
of 172-187 °C. All display a similar absorption band at
approximately 350 nm that we attribute to the tetraphenyldiene
section of the metallole.10 It is postulated, based on simple
Hu¨ckel theory for cis-1,3-butadiene, that the shape of the highest
occupied molecular orbital (HOMO) would be of a π-type
symmetry localized at each of the formal carbon-carbon double
bonds in the metallole. The lowest unoccupied molecular orbital
(LUMO) would be predicted to have π*-type symmetry with
respect to the carbon-carbon double bonds. The planar structure
of a simplified model metallole, hydrogen-substituted silole, and
the surfaces of the HOMO and LUMO are displayed in Figure
2. Five of the six compounds are luminescent. The absorption,
emission, mass, and nuclear magnetic resonance spectra and
the cyclic voltammograms of these six metalloles were collected.
These metalloles all have planar metallacyclopentadiene
moieties. The phenyl substituents on the diene have dihedral
angles that vary from nearly 0° to a more expected 90° from
the metallacyclopentadiene plane. The X-ray crystal structures
of SiMe and GeMe have been published.11,12 Here we report
the structure of hexaphenylstannole (SnPh). In solution, a low
barrier to rotation for the phenyl substituents would be expected,
although, due to steric repulsions, no two adjacent phenyl rings
could be coplanar with the metallacyclopentadiene. The sub-
stituent-phenyl groups give the molecule a propeller-like shape.
Figure 2. Structure of a metallole (a) and the isopotential surfaces of
the HOMO (b) and LUMO (c) generated with MacSpartan at the ab
initio level of theory (3-21G* level) of nonsubstituted silole (H2SiC4H4).
B. Preparation of Compounds. In a typical reaction, 55 mmol (10
g) of diphenylacetylene, dried in vacuo overnight, was dissolved in 75
mL of diethyl ether and cannulated into a dry 500-mL three-necked
round-bottom flask attached to a Schlenk line. Under a positive pressure
of inert gas, lithium wire (55 mmol) that had been washed in hexanes
was cut into 2-5-mm pieces into the reaction flask. Within 20-30
min of stirring, the reaction mixture became dark red. Stirring was
continued for approximately 4 h, at which time the unreacted lithium
wire was removed from the reaction mixture. The reaction mixture was
stirred for an additional 1-2 h, but not longer than 16 h. (Longer
reaction times can lead to the ring closure product, 2,3,4-triphenyl-1-
naphthyllithium.7) At this point either of two procedures was fol-
lowed: THF (100 mL) was added to the mixture and stirring was
continued for several hours before aliquots of the reagent were
hydrolyzed and titrated to determine the lithium reagent concentration.
Alternatively, a cleaner lithium reagent can be obtained, if before the
addition of THF, the reaction mixture is filtered and the yellow dilithio
reagent precipitate is washed with dry hexane (3 × 20 mL) and then
dissolved in dry THF (100 mL). In either case, a stoichiometric quantity
of the (E,E)-1,2,3,4-tetraphenyl-1,3-butadien-1,4-ylidenedilithium solu-
tion was then added, by syringe, to a stirred solution of the group-14
dihalide (or, in the case of GePh, the tetrahalide) in either diethyl ether
or THF. The dimethyl dichlorides tended to react immediately, while
the diphenyl dichlorides were slower to react. In either case, the
lightening of the solution from dark blue-green (in THF solvent) to
yellow or amber indicated that the metallole had been formed. The
workup and isolation of the products followed one of two paths: Either
a hydrolytic extraction was performed, or the ethers were removed from
the reaction mixtures and the residue extracted with methylene chloride.
In general, once a crude product was obtained either it was recrystallized
or a portion of the product was subjected to column chromatography
over 60-200 mesh silica. The diphenylacetylene starting material was
eluted with petroleum ether or hexanes. The product metalloles were
then eluted with a solvent system of approximately 40% methylene
chloride and 60% petroleum ether. Further recrystallization in boiling
ethanol, 2-propanol, or hexanes yielded purified products.
Experimental Section
A. Materials. All synthetic experiments were carried out in a dry
nitrogen or argon atmosphere using standard Schlenk techniques.
Diethyl ether and tetrahydrofuran (THF) were dried over sodium/
benzophenone ketyl and distilled before use. Dimethyldichlorosilane
(Hu¨ls) and diphenyldichlorosilane (Aldrich or Hu¨ls) were distilled prior
to use and stored over magnesium chips. Germanium(IV) chloride and
diphenyltin dichloride were obtained from Alfa and used as received.
Dimethyltin dichloride was sublimed in vacuo (0.05 mmHg, 25 °C)
before use. Phenyllithium (Aldrich) and methyllithium (Fisher) were
used as received. Column chromatography was performed over 60-
200 mesh silica (Aldrich). Melting points are uncorrected.
1. Synthesis of 1,1-Dimethyl-2,3,4,5-tetraphenylsilacyclopenta-
diene (SiMe).7 The standard reaction sequence was followed as
described above using 10.1 g of diphenylacetylene (56.9 mmol) and
405 mg of lithium wire (58.4 mmol). After determining the concentra-
tion of the dilithio reagent by titration (14 mmol, 47% yield), the entire
lithium reagent solution was quenched with 2.1 mL dimethyldichlo-
rosilane (17 mmol). The solution lightened to yellow over the course
of 30 min. The solvents were removed and the residue was extracted
with methylene chloride. Concentration of the methylene chloride
solution and precipitation by the addition of 2-propanol yielded 3.52 g
(9) Eisch, J. J.; King, R. B. Organometallic Synthesis; Academic Press:
NY, 1965; Vol 2, p 98.
(10) The two colorless byproducts of the syntheses, (E,E)-1,2,3,4-tetraphen-
ylbuta-1,3-diene and (1,2,3,4-tetraphenylbuta-1,3-dienyl)dimethyltin
chloride, both lack the metallole ring.
(11) For SiMe see: Parkanyi, L. J. Organomet. Chem. 1981, 216, 9.
(12) For GeMe see: Meier-Brocks, F.; Weiss, E. J. Organomet. Chem.
1993, 453, 33.

2466 Inorganic Chemistry, Vol. 38, No. 10, 1999
Ferman et al.
(8.5 mmol) of the crude silole (62% yield based on dilithio reagent).
Recrystallization from 2-propanol yielded pure SiMe (52% yield based
on crude).
(20.4 mmol, 44% yield) was quenched with 2.7 mL of germanium-
(IV) chloride (23 mmol) dissolved in 50 mL of diethyl ether. The red
color of the dilithio reagent cleared immediately yielding a yellow
solution. After 30 min of stirring, the solvent was removed in vacuo
and the crude product was extracted with methylene chloride. Con-
centration and cooling yielded 2.76 g (5.52 mmol, 27% yield based on
dilithio reagent) of yellow-green crystals of 1,1-dichloro-2,3,4,5-
tetraphenylgermacyclopentadiene (no further purification was per-
formed). A -40 °C solution of 1,1-dichloro-2,3,4,5-tetraphenylgerma-
cyclopentadiene (247 mg, 0.494 mmol) in 10 mL of THF was quenched
with phenyllithium (0.64 mL, 1.2 mmol). The ether was removed in
vacuo yielding the crude germole (85% yield). GePh was purified by
recrystallization from boiling absolute ethanol (10% yield based on
crude).
SiMe: yellow-green solid; mp 182 °C. Elem. Anal. Calcd for
SiC30H26: C, 86.91. H, 6.32. Found: C, 86.90; H, 6.39. UV-vis (CH3-
CN, nm): 351 (3.2 × 103 M-1cm-1). IR (KBr, cm-1): 3077 (m), 3057
(m), 3022 (w), 2958 (w), 2898 (w), 1594 (m), 1572 (w) 1489 (m),
1440 (m), 1296 (m), 1244 (m), 1087 (w), 1074 (w), 1029 (m), 937
(m), 911 (m), 835 (m), 794 (s), 779 (s), 695 (s). EI-MS (70 V): major
fragments at m/z ) 105, 221, and molecular ion at m/z ) 414. 1H NMR
(300 MHz, CDCl3): δ 7.14-6.78 (m, 20H, C6H5), 0.47 (s, 6H, CH3).
13C NMR (75 MHz, CDCl3): δ 153.91, 141.77, 139.83, 138.76, 129.97,
128.81, 127.90, 127.36, 126.15, 125.48, -3.90. CV: oxidation at 1450,
1750 mV vs Ag/AgCl, reduction at -2250 mV vs Ag/AgCl. Lumi-
nescence (acetonitrile): excitation at 351 nm, emission at 480 nm.
2. Synthesis of 1,1,2,3,4,5-Hexaphenylsilacyclopentadiene (SiPh).1
The standard reaction sequence was followed as described above using
10.4 g of diphenylacetylene (58.4 mmol) and 405 mg of lithium wire
(58.4 mmol). The dilithio reagent was quenched with 6.1 mL of
diphenyldichlorosilane (29 mmol) dissolved in 100 mL of THF. The
diethyl ether was distilled off and the reaction solution was refluxed
for an additional 4.5 h. When the reaction mixture had cooled, the
solution was yellow-green. After hydrolytic workup and removal of
the solvents, the metallole was isolated by column chromatography
and purified by recrysallization from 2-propanol (3.28 g, 44% yield
based on a 50% yield of dilithio reagent).
GePh: yellow-green solid; mp 186-187 °C. Elem. Anal. Calcd for
GeC40H30: C, 82.37; H, 5.18. Found: C, 82.59; H, 5.14. UV-vis (CH3-
CN, nm): 354 (1.1 × 104 M-1cm-1). IR (KBr, cm-1): 3066 (m), 3051
(m), 3024 (m), 1594 (m), 1575 (w), 1484 (s), 1440 (m), 1431 (s), 1128
(m), 1089 (s), 1025 (m), 784 (m), 765 (m), 736 (s), 707 (s), 698 (s),
614 (m), 563 (w), 465 (m). EI-MS (70 V): major fragments at m/z )
1
105, 178, 279, 357, 538, and molecular ion at m/z ) 584. H NMR
(300 MHz, CDCl3): δ 7.61-7.34 (m, 10H, C6H5), 7.03-6.87 (m, 20H,
C6H5). 13C NMR (75 MHz, CDCl3): δ 153.62, 141.14, 139.98, 139.24,
135.17, 134.87, 130.14, 129.47, 129.19, 128.60, 127.79, 127.47, 126.22,
125.70. CV: oxidation at 1495 mV vs Ag/AgCl, reduction at -1962,
-2187, -2668 mV vs Ag/AgCl. Luminescence (acetonitrile): excita-
tion at 359 nm, emission at 486 nm.
SiPh: yellow-green solid; mp 192-193 °C. Elem. Anal. Calcd for
SiC40H30: C, 89.17; H, 5.61. Found: C, 89.09; H, 5.66. UV-vis (CH3-
CN, nm): 360 (8.5 × 103 M-1cm-1). IR (KBr, cm-1): 3062 (m), 3021
(m), 1596 (m), 1571 (w), 1483 (m), 1441 (m), 1428 (m), 1110 (m),
1075 (w), 1027 (w), 935 (m), 916 (m), 790 (s), 762 (s), 740 (s), 714
(s), 695 (s), 529 (m). EI-MS (70 V): major fragments at m/z ) 105,
1,1-Dichloro-2,3,4,5-tetraphenylgermacyclopentadiene: yellow-
green solid; mp 193 °C. UV-vis (CH3CN, nm): 199, 296, 370. IR
(KBr, cm-1): 3080 (m), 3059 (m), 3023 (m), 1595 (w), 1574 (w) 1488
(s), 1441 (s), 1074 (m), 1026 (m), 912 (w), 784 (s), 763 (m), 759 (m),
740 (w), 707 (s), 695 (s), 569 (m), 414 (s). EI-MS (70 V): major
fragments at m/z ) 178, 356, and molecular ion at m/z ) 500. 1H NMR
(300 MHz, CDCl3): 7.20-7.04 (m, 16H, C6H5), 6.88-6.83 (m, 4H,
C6H5). 13C NMR (75 MHz, CDCl3): δ 149.99, 136.61, 134.69, 132.77,
129.54, 129.52, 128.36, 128.06, 127.68, 127.47.
5. Synthesis of 1,1-Dimethyl-2,3,4,5-tetraphenylstannacyclopen-
tadiene (SnMe).3,14 The standard reaction sequence was followed as
described above to prepare the dilithio reagent using 20.1 g of
diphenylacetylene (114 mmol) and 792 mg of lithium wire (114 mmol).
A portion of the dilithio reagent solution (134 mL, 9.90 mmol) was
quenched with 11.1 g of dimethyltin dichloride (50.5 mmol) dissolved
in 60 mL of THF. Upon addition, the color of the dilithio reagent
changed to an orange-amber color. The reaction mixture was stirred at
room temperature overnight. The solvent was removed in vacuo and
the residue was washed with 3 × 60 mL of ethanol to remove lithium
chloride and unreacted tin halides, yielding 3.07 g (6.08 mmol) of SnMe
(61% yield based on dilithio reagent). The crude product was purified
by recrystallization from ethanol/methylene chloride (31% yield based
on crude).
1
181, 193, 283, and molecular ion at m/z ) 538. H NMR (300 MHz,
CDCl3): δ 7.68-7.34 (m, 10H, C6H5), 7.03-6.86 (m, 20H, C6H5).
13C NMR (75 MHz, CDCl3): δ 156.72, 139.56, 139.50, 138.76, 131.58,
130.12, 129.96, 129.20, 128.23, 127.75, 127.42, 126.36, 125.62. CV:
oxidation at 1527, 1750 mV vs Ag/AgCl, reduction at -1943, -2116
mV vs Ag/AgCl. Luminescence (acetonitrile): excitation at 360 nm,
emission at 496 nm.
3. Synthesis of 1,1-Dimethyl-2,3,4,5-tetraphenylgermacyclopen-
tadiene (GeMe).12 The standard reaction sequence was followed as
described above to prepare the dilithio reagent using 20.1 g of
diphenylacetylene (114 mmol) and 792 mg of lithium wire (114 mmol).
A portion of the dilithio reagent (96 mL, 7.1 mmol) was quenched
with 2.0 g of dimethylgermanium dichloride (12 mmol) dissolved in
30 mL of THF. An immediate color change occurred. The ethers were
removed in vacuo and the crude product was extracted with methylene
chloride. Concentration of the methylene chloride solution yielded 2.6
g (5.7 mmol, 81% based on dilithio reagent) of yellow-green crystals
of GeMe. The germole was purified by recrystallization from ethanol/
methylene chloride (51% yield based on crude).
GeMe: yellow-green solid; mp 178-180 °C. Elem. Anal. Calcd for
GeC30H26: C, 78.48; H, 5.71. Found: C, 78.82; H, 5.58. UV-vis (CH3-
CN, nm): 348 (3.8 × 103 M-1cm-1). IR (KBr, cm-1): 3075 (w), 3055
(m), 3019 (m), 2976 (w), 2906 (w), 1594 (m), 1571 (w) 1486 (m),
1440 (m), 1293 (w), 1267 (w), 1074 (w), 1028 (w), 912 (m), 830 (w),
786 (m), 763 (m), 738 (m), 706 (s), 695 (s), 562 (m). EI-MS (70 V):
major fragments at m/z ) 89, 151, 178, 267, 356, and molecular ion at
m/z ) 460. 1H NMR (300 MHz, CDCl3): 7.13-6.80 (m, 20H, C6H5),
0.67 (s, 6H, CH3). 13C NMR (75 MHz, CDCl3): δ 151.18, 143.87,
140.29, 139.40, 130.12, 128.87, 127.87, 127.40, 125.99, 125.50, -2.54.
CV: oxidation at 1496, 1827 mV vs Ag/AgCl, reduction at -2482,
-2661 mV vs Ag/AgCl. Luminescence (acetonitrile): excitation at 348
nm, emission at 466 nm.
SnMe: pale yellow solid; mp 185-187 °C. Elem. Anal. Calcd for
SnC30H26: C, 71.31; H, 5.19. Found: C, 71.11; H, 5.05. UV-vis (CH3-
CN, nm): 350 (1.1 × 104 M-1cm-1). IR (KBr, cm-1): 3073 (m), 3049
(m), 3018 (m), 2990 (w), 2921 (w), 1594 (m), 1570 (w), 1484 (m),
1438 (m), 1280 (w), 1267 (w), 1072 (m), 1027 (m), 909 (w), 783 (m),
755 (w), 729 (w), 698 (s), 556 (m), 523 (m). EI-MS (70 V): major
fragments at m/z ) 135, 178, 356, and molecular ion at m/z ) 506. 1H
NMR (300 MHz, CDCl3): 7.10-6.76 (m, 25H, C6H5), 0.62 (s, JSnH
)
)
29, 28 Hz, 6H, CH3). 13C NMR (75 MHz, CDCl3): δ 153.57 (2JSnC
41), 144.91 (1JSnC ) 209, 199 Hz), 143.02 (2JSnC ) 23 Hz), 140.76
(3JSnC ) 32 Hz), 130.25, 128.85 (3JSnC ) 10 Hz), 127.77, 127.19,
125.59, 124.99, -7.75 (1JSnC ) 166, 159 Hz). CV: oxidation at 1191,
1510 mV vs Ag/AgCl, reduction at -2197 mV vs Ag/AgCl. Lumi-
nescence (acetonitrile): excitation at 350 nm, no emission.
4. Syntheses of 1,1,2,3,4,5-Hexaphenylgermacyclopentadiene
(GePh) and 1,1-Dichloro-2,3,4,5-tetraphenylgermacyclopentadiene.13
The standard reaction sequence was followed as described above to
prepare the dilithio reagent using 16.6 g of diphenylacetylene (93.3
mmol) and 646 mg of lithium wire (93.1 mmol). The dilithio reagent
6. Synthesis of 1,1,2,3,4,5-Hexaphenylstannacyclopentadiene (Sn-
Ph).1 The standard reaction sequence was followed as described above
using 8.98 g of diphenylacetylene (50.4 mmol) and 350 mg of lithium
wire (50. mmol). The dilithio reagent was quenched with 8.47 g of
(14) Gustavson, W. A.; Principe, L. M.; Min Rhee, W.-Z.; Zuckerman, J.
(13) Curtis, M. D. J. Am. Chem. Soc. 1969, 91, 6011.
J. J. Am. Chem. Soc. 1981, 103, 4126.

Properties of a Series of Group-14 Metalloles
Inorganic Chemistry, Vol. 38, No. 10, 1999 2467
diphenyltin dichloride (24.6 mmol) dissolved in 50 mL of diethyl ether.
The reaction solution was refluxed for 0.5 h, during which time the
solution became bright yellow. The solvents were removed and the
residue was extracted with methylene chloride. A portion of the crude
stannole (5.4 g, 32 wt %) was purified by column chromatography
using petroleum ether/methylene chloride as the eluant. Further
purification by recrystallization from boiling ethanol yielded 1.22 g of
SnPh (2.0 mmol, 23% yield based on the crude stannole portion that
was purified by column chromatography).
SnPh: yellow-green solid; mp 172-173 °C. Elem. Anal. Calcd for
SnC40H30: C, 76.33; H, 4.80. Found: C, 76.40; H, 4.72. UV-vis (CH3-
CN, nm): 355 (9.5 × 103 M-1cm-1). IR (KBr, cm-1): 3075 (m), 3056
(m), 3014 (m), 1594 (m), 1577 (w), 1481 (m), 1440 (m), 1430 (s),
1071 (m), 1021 (m), 787 (w), 779 (m), 764 (s), 741 (w), 729 (s), 699
(s), 555 (m). EI-MS (70 V): major fragments at m/z ) 120, 178,
197, 356, and molecular ion at m/z ) 630. 1H NMR (300 MHz,
CDCl3): δ 7.60-7.38 (m, 10H, C6H5), 7.04-6.82 (m, 20H, C6H5).
13C NMR (75 MHz, CDCl3): δ 155.14 (2JSnC) ) 45, 43 Hz), 142.76
(1JSnC ) 223, 213 Hz), 142.31 (2JSnC ) 22 Hz), 140.56 (3JSnC ) 32
Hz), 138.04 (1JSnC ) 257, 246 Hz), 137.14 (2JSnC ) 20 Hz), 130.28,
129.43 (4JSnC ) 6 Hz), 129.30 (3JSnC ) 11 Hz), 128.95 (3JSnC ) 42
Hz), 127.88, 127.37, 125.90, 125.37. CV: oxidation at 1539 mV vs
Ag/AgCl, reduction at -2142, -2398 mV vs Ag/AgCl. Luminescence
(acetonitrile): excitation at 355 nm, emission at 494 nm.
molecular ion peak and the major fragments are tabulated in the
Experimental Section.
5. NMR Spectroscopy. H NMR spectra were recorded in CDCl3
(99.9%, Aldrich) on either a Bruker AC-P 300 or a Bruker MSL300
NMR spectrometer. Residual chloroform was used as the internal
standard (7.25 ppm). 13C NMR were similarly recorded at 75 MHz
using CDCl3 as the internal standard (77.00 ppm). 13C NMR spectra
with tin satellites were obtained by repetitive scanning (between 16 000
and 40 000 scans). Acquisition parameters are available from the
authors.
1
6. Electrochemical analyses were performed on a Bioanalytical
Systems CV-50W System operating in cyclic voltammetry mode. A
three-electrode system consisting of platinum working and auxiliary
electrodes and a Ag/AgCl reference electrode was used. Tetra-n-
butylammonium hexafluorophosphate (Aldrich) was used as a sup-
porting electrolyte in all cases. The standard scan rate was 100 mV/s.
Reduction cyclic voltammograms were recorded from 0 to -3.200 V
vs Ag/AgCl in tetrahydrofuran solutions, and oxidation voltammograms
were recorded from 0 to +2.200 V vs Ag/AgCl in methylene chloride.
Acetonitrile was not used as a solvent because prelimary experiments
indicated that it was oxidized at +2.0 V and reduced at -2.2 V. All
electrochemical measurements were performed using the following
protocol: Solutions of tetra-n-butylammonium hexafluorophosphate (0.1
M), prepared with freshly distilled tetrahydrofuran (from benzophenone
ketyl) or methylene chloride (distilled from phosphorus pentoxide), were
transferred by syringe to the dry electrochemical cell. The solution was
bubble-degassed for ca. 5 min with nitrogen and then the CV of the
electrolyte solution was recorded. Next, approximately 3-5 mg of the
metallole was dissolved in the solution, and it was degassed for an
additional 5 min before the CV was recorded. To ensure anhydrous
conditions, approximately 10 mg of activated alumina was added to
each. By scavenging any water, the alumina improved the smoothness
of the CV. The alumina was activated by heating to ∼120 °C at <
0.01 mmHg for several days.
7. Elemental analysis of each metallole was carried out using a
Perkin-Elmer 2400 Series II analyzer. No significant differences were
observed, at the 95% confidence level, between theoretical expectations
and actual compositions. Results are tabulated in the Experimental
Section.
D. Molecular orbital calculations were performed using Mac-
Spartan Plus, version 1.1.7, (Wavefunction, Inc.) operating on a 7600/
132 Power Macintosh microcomputer. The output of the semiempirical
calculations for the six metalloles, performed at the AM1 level of theory,
is available in the Supporting Information.
7. Synthesis of (1,2,3,4-Tetraphenylbuta-1,3-dienyl)dimethyltin
Chloride.14 If a hydrolytic workup is performed for the preparation of
SnMe, hydrolysis of the dimethyltin dichloride starting material can
generate HCl, which reacts with SnMe yielding the ring-opened product,
(1,2,3,4-tetraphenylbuta-1,3-dienyl)dimethyltin chloride. This byproduct
was purified by column chromatography and recrystallization from
2-propanol (34% yield).
(1,2,3,4-Tetraphenylbuta-1,3-dienyl)dimethyltin chloride: white
solid; mp 188 °C. UV-vis (CH3CN, nm): 284 (1.5 × 104 M-1cm-1).
IR (KBr, cm-1): 3079 (m), 3057 (m), 3018 (m), 2919 (w), 2796 (w),
1595 (m), 1583 (w), 1571 (w) 1487 (s), 1441 (s), 1156 (w), 1077 (m),
1027 (m), 919 (m), 786 (s), 759 (s), 730 (s), 697 (s), 659 (m), 552 (s),
1
528 (s), 512 (s). H NMR (300 MHz, CDCl3): 7.26-6.90 (m, 23H,
C6H5), 0.69 (s, JSnH ) 30, 28 Hz, 6H, CH3). 13C NMR (75 MHz,
CDCl3): δ 156.96 (JSnC ) 24 Hz), 148.11(JSnC ) 20. Hz), 147.41 (1JSnC
) 284, 272 Hz), 141.28 (JSnC ) 16 Hz), 138.32 (JSnC ) 34 Hz), 136.93,
135.18, 129.94, 129.81, 129.77, 129.72, 129.31 (JSnC ) 11 Hz), 128.29,
128.27, 128.08, 127.97, 127.58 (JSnC ) 3 Hz), 127.25, 126.74, 125.97
(JSnC ) 5 Hz), 1.56 (1JSnC ) 192, 201 Hz). CV: oxidation at 1450,
1750 mV vs Ag/AgCl, reduction at -2250 mV vs Ag/AgCl. Lumi-
nescence (acetonitrile): excitation at 284 nm, emission at 391 nm.
8. Synthesis of (E,E)-1,2,3,4-Tetraphenylbuta-1,3-diene.7 If the
dilithio reagent is hydrolyzed, (E,E)-1,2,3,4-tetraphenylbuta-1,3-diene
can be isolated by column chromatography and purified by recrystal-
lization from 2-propanol (77% yield).
E. X-ray Crystal Structure. Crystals of hexaphenylstannole (SnPh)
were obtained by recrystallization from 2-propanol. The X-ray data
were collected at room temperature using an Enraf-Nonius CAD4
diffractometer with graphite-monochromated Mo KR radiation (λ )
0.7107 Å). From the systematic absences, the space group was
determined to be P1h. The cell parameters (a ) 10.353(2) Å, b ) 16.679-
(2) Å, c ) 9.482(1) Å; R ) 99.91(1)°, â ) 106.33(1)°, γ ) 77.80(1)°)
were obtained by a least-squares fit of sin2 θ values for 25 reflections
in the range 13° < θ < 17°. Integrated intensities were measured by
ω - 2θ scans with ω -scan width (1.00 + 0.35 tan θ)°, using varying
scan speeds determined by a prescan. Three standard reflections, (-3
0 1) (0 -3 -1) (1 1 1), were monitored every 60 min. There was no
loss of intensity during the data collection. A hemisphere of data (sin
θ/λ < 0.60; 0 e h e 12, -19 e k e 19, -11 e l e 10) were collected,
yielding a total of 5571 measured reflections, excluding standard
reflections. Lorentz, polarization, and absorption corrections were
applied. The absorption coefficient, µ ) 8.73 cm-1, gave transmission
factors in the range 0.941-1.000.15 Averaging 649 symmetry related
reflections gave Rint ) 0.032 and resulted in 5143 independent
reflections, 3491 having Fo > 3σ(Fo).
(E,E)-1,2,3,4-Tetraphenylbuta-1,3-diene: white solid; mp 188 °C.
UV-vis (CH3CN, nm): 327, 316. IR (KBr, cm-1): 3099 (w), 3078
(m), 3059 (m), 3020 (m), 2996 (w), 2926 (w), 1596 (m), 1488 (s),
1441 (s), 1070 (m), 1026 (m), 927 (m), 917 (m), 867 (m), 783 (s), 763
(s), 753 (s), 702 (s), 691 (s). 1H NMR (300 MHz, CDCl3): 7.44-7.34
(m, 5H, C6H5), 7.06-7.04 (m, 3H, C6H5), 6.80-6.76 (m, 2H, C6H5),
6.35 (s, 1H, CH). 13C NMR (75 MHz, CDCl3): δ 145.58, 139.75,
137.23, 131.64, 130.38, 129.46, 128.79, 127.79, 127.33, 126.59.
C. Spectroscopic, Electrochemical, and Elemental Analyses. 1.
UV-visible absorption spectra were recorded as acetonitrile solutions
with a Perkin-Elmer Lambda 20 Spectrophotometer operating in double-
beam mode with a solvent blank as the reference cell.
2. Luminescence spectra were recorded on an SLM-Aminco AB-2
spectrofluorometer. Corrected spectra are recorded in all cases.
3. Infrared spectra were recorded in a KBr matrix on either a
Nicolet Model 205 or a BioRad FTS-7.
4. Electron impact mass spectra were obtained on a Hewlett-
Packard 5988A mass spectrometer operating at 70 V with a source
temperature of 250 °C. All samples were introduced using a direct
insertion probe. For all metalloles, the distribution and abundance of
molecular ion peaks exactly matched those predicted based on the
isotopic abundances of the constituent elements. The most abundant
A partial structure was found using direct methods.16 All remaining
non-hydrogen atoms were found in successive difference maps after
(15) Fair, C. K. MolEN. An InteractiVe Intelligent System for Crystal
Analysis; Enraf-Nonius: Delft, The Netherlands, 1990.
(16) Main, P. MULTAN78, A system of computer programmes for the
automatic solution of crystal structures from X-ray diffraction data;
Department of Physics, University of York: York, England, 1978.

2468 Inorganic Chemistry, Vol. 38, No. 10, 1999
Ferman et al.
Table 1. UV-Visible Absorption Band Maxima in nm (in
Acetonitrile Solution)
methyl
phenyl
Si
Ge
Sn
351
348
350
360
354
355
refinements by differential Fourier methods.17 The positional and
isotropic displacement parameters thus obtained were then used as input
to a full-matrix, least-squares refinement.18 From differential Fourier
maps all hydrogen atoms were located. Atomic positional, anisotropic
displacement parameters for non-hydrogen atoms and isotropic dis-
placement parameters for hydrogen atoms were varied, together with
the scale factor, and converged with ∆/σ < 0.01. The extinction
correction was omitted after the extinction coefficient failed to assume
Figure 3. Structures of two byproducts of this research: (a) (E,E)-
1,2,3,4-tetraphenylbutadiene and (b) (1,2,3,4-tetraphenylbuta-1,3-di-
enyl)dimethyltin chloride.
Table 2. Luminescence Band Maxima in nm (in Acetonitrile
Solution)a
methyl
phenyl
2
a significant value.19 The quantity ∑w||Fo| - |Fc|| was minimized with
Si
Ge
Sn
480
466
none
496
486
494
weights w ) 1/σ2(Fo) and σ2(Fo) ) σcs2 + (0.02Fo)2, where σcs2 is the
variance due to counting statistics. Atomic scattering factors were those
as calculated by Cromer and Waber.20 Anomalous dispersion corrections
were included.21 Residual maxima in the final difference Fourier map
were 0.5 e Å-3 near the Sn atom. The refinement converged with fit
indices R(Fo) ) 0.037, Rw(Fo) ) 0.038, and S ) 1.11.
a Excitations at UV-vis band maxima.
Table 3. Anodic and Cathodic Peak Potentials (mV) for the
Irreversible Oxidation and Reduction of Metalloles in Solution
oxidation
reduction
Results and Discussion
SiMe
GeMe
SnMe
SiPh
GePh
SnPh
1482
1496
1191
1527
1495
1539
-2174
-2482
-2197
-1943
-1962
-2142
A. Photophysical Spectroscopic Properties. UV-visible
absorption data obtained in acetonitrile solution are presented
in Table 1. The methyl-substituted metalloles (SiMe, GeMe,
and SnMe) all display a well-defined maximum near 350 nm,
while the phenyl-substituted metalloles display a maximum near
358 nm. The insensitivity to heteroatom substitution implies
that the absorption involves electronic transitions between
molecular orbitals localized primarily on the diene and/or on
the phenyl rings of the chromophore. In all cases, absorbance
maxima showed no significant sensitivity to changes in solvent
polarity.22 Two byproducts were isolated during the course of
this research (Figure 3); (E,E)-1,2,3,4-tetraphenylbutadiene (the
hydrolysis product of the dilithio reagent) and (1,2,3,4-tetraphen-
ylbuta-1,3-dienyl)dimethyltin chloride (the product of HCl
addition to an Sn-C1 bond), are both white solids with no
appreciable absorption at 350 nm, confirming the role of the
metallole in the chromophore. Luminescence data are presented
in Table 2. The luminescence properties of these metalloles have
been briefly noted in the literature.6,7 In contrast to the absorption
data, the luminescence of the methyl-substituted compounds
shows a strong dependence on the nature of the heteroatom
(Table 2). The SiMe compound has a featureless band with an
emission maximum at 480 nm, GeMe shows a higher energy
luminescence band at 466 nm, and SnMe fails to luminesce.
The same dependence on the heteroatom was not observed for
the corresponding phenyl-substituted compounds, which all
showed a featureless luminescence band near 490 nm.23
The simplest explanation for this heteroatom dependence
relates to the electron density associated with the heteroatom
and the ability of the substituents (not the chromophore) to
absorb electron density. When the substituents are phenyl
groups, they have a large capacity to absorb and buffer any
electron density pushed off the heteroatom.
B. Electrochemical Properties. The electrochemical proper-
ties of the six metalloles were studied in order to determine if
their redox behavior could elucidate the reason for the lack of
luminescence in SnMe. Although several of the compounds have
been known to undergo a two-electron reduction by alkali
metals, and spectroscopic properties of the reduced state of SiMe
have been reported,24,25 thorough electrochemical characteriza-
tion of the compounds has been lacking. Table 3 summarizes
the data obtained from cyclic voltammetry experiments. In cases
where multiple waves were observed, only the first potential is
tabulated. For SiMe, GeMe, and SnMe, the oxidative and
reductive cyclic voltammograms, in methylene chloride and
tetrahydrofuran, respectively, are shown in Figure 4.
Each compound exhibited at least one irreversible reduction
wave and two irreversible oxidation waves. The peak potentials
of the phenyl-substituted compounds showed little dependence
on the heteroatom. In the methyl series, two interesting
anomalies are observed. GeMe is the most difficult of the six
compounds to reduce, with a reduction potential some 300 mV
more negative than those of the other five metalloles. This is
consistent with the observation that the luminescence emission
from GeMe occurs at higher energy than that of any of the other
compounds.
(17) Sheldrick, G. M. SHELX-76, Program for Crystal Structure Deter-
mination; University of Cambridge: Cambridge, England, 1976.
(18) Craven, B. M.; Weber, H.-P.; He, X. M. The POP Least-Squares
Refinement Procedure; University of Pittsburgh: Pittsburgh, PA, 1987.
(19) Becker, P. J.; Coppens, P. Acta Crystallogr., Sect. A 1974, 30, 129-
147.
(20) Cromer, D. T.; Waber, J. T. International Tables for X-ray Crystal-
lography; Kynoch Press: Birmingham, England, 1974; Vol. IV, pp
71-147 (present distributor Reidel: Dordrecht).
An even more interesting observation is made in the case of
SnMe. Comparison of the oxidation potentials of the series
indicates that SnMe is the easiest of the compounds to oxidize.
(21) Cromer, D. T.; Waber, J. T. International Tables for X-ray Crystal-
lography; Kynoch Press: Birmingham, England, 1974; Vol. IV, pp
148-151.
(22) Toluene, methylene chloride, dimethyl formamide, acetonitrile, and
methanol were used.
(24) Janzen, E. G.; Pickett, J. B.; Atwell, W. H. J. Organomet. Chem. 1967,
(23) For all compounds studied, the luminescence band maxima are not
affected by a change in solvent. Solvents of greatly different polarity
lead to essentially identical luminescence spectra.
10, P6.
(25) Janzen, E. G.; Harrison, W. B.; DuBose, C. M., Jr. J. Organomet.
Chem. 1972, 40, 281.

Properties of a Series of Group-14 Metalloles
Inorganic Chemistry, Vol. 38, No. 10, 1999 2469
Figure 4. Cyclic voltammograms of methylmetalloles: (a) SiMe oxidation, (b) SiMe reduction, (c) GeMe oxidation, (d) GeMe reduction, (e)
SnMe oxidation, and (f) SnMe reduction.
Its oxidation peak potential is almost 300 mV less positive than
the other compounds, indicating a HOMO of significantly higher
energy than those of SiMe or GeMe. The isopotential surface
of the LUMO of SnMe has appreciable density on Sn, whereas
the LUMOs of SiMe and GeMe have no density at the metals
(the same trend is seen in the LUMOs of SiPh, GePh, and SnPh).
The relative ease of oxidation of SnMe also may be an indication
of relatively diffuse electron density on Sn, consistent with the
fact that of the three heteroatoms, Sn has the smallest elec-
tronegativity and largest size. This observation also is consistent
with the observation that the carbons of the methyl substituents
of SnMe are the most shielded of the carbons attached directly
to the heteroatom in all the metalloles. The electron density on
Sn in SnMe results in such perturbations to the chromophore
that the compound does not luminesce. In SnPh, the phenyl
substituents on the heteroatom are not shielded to the extent
that the methyl substituents are, indicating that the phenyl
substituents are able to absorb electron density from the
heteroatom more effectively than the methyl substituents,
attenuating the perturbations to the chromophore.
C. NMR Spectroscopy. The 13C NMR data of the series
provide insight into the anomalous luminescence behavior of
SnMe. The chemical shifts of the six metalloles are tabulated
in Table 4; JSnC values are included when detected.26 The spectra
indicate that the molecules possess C2V symmetry in solution
with a plane of symmetry bisecting the C2-C3 bond and
passing through the heteroatom. (See Figure 6 for the carbon
numbering scheme. For the methyl series, the two phenyl
substituents, C51-C56 and C61-C66, are replaced with two
methyl carbons, C5 and C6.)
The resonances for the C2 and C3 carbons are the most
deshielded in the 13C NMR spectra of the six metalloles, lying
significantly farther downfield than any other resonance. The
(26) The presence of two NMR-active isotopes of tin (119Sn, 8.58% and
117Sn, 7.61%) provide unequivocal assignments of the carbon reso-
nances in the 13C NMR spectra of SnMe and SnPh.

2470 Inorganic Chemistry, Vol. 38, No. 10, 1999
Ferman et al.
broad-band decoupled 13C NMR spectrum of the aromatic region
of SnPh, shown in Figure 5, is representative. Although one
might expect the C1 and C4 resonances to be the most
deshielded, this is not the case.27 The assignments of the C2
and C3 carbon resonances were made by JSnC value observa-
tions, which have been extensively documented in the literature,
and comparison of the spectra of the six metalloles (Table
4).28-31 The chemical shift of the C2 and C3 resonances of the
germoles are the most shielded with respect to the siloles and
stannoles.
In the phenyl series, the C51 and C61 resonances become
increasingly deshielded as the atomic number of the heteroatom
increases. However, in the methyl series this trend is observed
only from silicon to germanium; the C5 and C6 resonances of
SnMe are the most shielded, consistent with the anomalies noted
in the luminescence properties and the electrochemical data of
the six metalloles.
D. Infrared Spectroscopy. The infrared spectra of all the
metalloles are predictably similar. Antisymmetric and symmetric
aromatic C-H stretching absorption bands occur between 3080
and 3014 cm-1 for all the metalloles. All six metalloles display
ν(CdC) at 1595 ( 1 cm-1 and 1574 ( 4 cm-1. An out-of-
plane skeletal mode absorption band of the monosubstituted
aromatic rings at 695-700 cm-1 is present in all the spectra.32
The largest differences in the infrared spectra are the presence
of aliphatic ν(C-H) stretch bands in the methyl series. The
antisymmetric and symmetric methyl ν(C-H) stretches both
move to higher wavenumber as the mass of the metal increases;
2958 and 2898 cm-1 for SiMe, 2976 and 2906 cm-1 for GeMe,
and 2990 and 2921 cm-1 for SnMe.
E. Molecular Structure. An ORTEP diagram of the X-ray
crystal structure of the hexaphenylstannole (SnPh) is shown in
Figure 6.33 (Structural parameters are listed in Table 5. Atomic
coordinate files are available in the Supporting Information.)
This structure contains an unusual finding: one of the phenyl
groups (C11, C12, ..., C16 in Figure 6) is practically coplanar
with the metallole ring (Sn, C1, C2, C3, C4). (The torsion angle
Sn-C1-C11-C12 is 1.2(6)°.) This is most probably attribut-
able to crystal packing forces, and it certainly is not indicative
of the structure in solution, which we know to possess C2V
symmetry on average. There are no short intramolecular
distances (the shortest being 2.53 Å for H13‚‚‚H52).
F. Molecular Orbital Calculations. For all six metalloles,
equilibrium geometries were obtained at the AM1 semiempirical
level of theory using MacSpartan Plus, version 1.1.7, on a Power
Macintosh 7600/132. Starting geometries were obtained using
the SYBYL force field after assembling the molecule in the
MacSpartan graphical user interface.
(27) CNMR Predictor, a 13C NMR simulator program (Advanced Chemistry
Development Inc., Toronto, Ontario, Canada), predicted the C1 carbons
to be most downfield in the spectra of the germoles and the stannoles;
personal communication from Kathleen Gallagher, Instrumentation
Center, University of New Hampshire.
(28) See: Wrackmeyer, B. In Annual Reports on NMR Spectroscopy; Webb,
G. A., Ed.; Academic Press: London, 1985; Vol. 16, pp 73-186 and
references therein.
(29) Bullpitt, M.; Kitching, W.; Adock, W.; Doddrell, D. J. Organomet.
Chem. 1976, 116, 161.
(30) Doddrell, D.; Burfitt, I.; Kitching, W.; Bullpitt, M.; Lee, C.-H.; Mynott,
R. J.; Considine, J. L.; Kuivila, H. G.; Sarma, R. H. J. Am. Chem.
Soc. 1974, 96, 1640.
(31) Mitchell, T. N.; Podesta, J. C.; Ayala, A.; Chopa, A. B. Magn. Reson.
Chem. 1988, 26, 497.
(32) Bellamy, L. J. In The Infrared Spectra of Complex Molecules, 3rd
ed.; Chapman and Hall: London, 1975; p 87.
(33) Johnson, C. K. ORTEP-II, Report ORNL-5138, Oak Ridge National
Laboratory, TN, 1976.

Properties of a Series of Group-14 Metalloles
Inorganic Chemistry, Vol. 38, No. 10, 1999 2471
Figure 5. Aromatic region of the 13C NMR spectrum of hexaphenylstannole (SnPh). Coupling to 119/117Sn is clearly visible for the six carbon atoms
below 135 ppm. Note that the furthest downfield resonance (C2), 155.14 ppm, has a much smaller coupling than either the 142.76 ppm resonance
(C1) or the 138.04 ppm resonance (C51).
Table 5. Crystallographic Data for Hexaphenylstannole (SnPh)
formula
fw
space group
Z
SnC40H30
629.38 au
P1h
2
a, Å
b, Å
c, Å
10.353(2)
16.679(2)
9.482(1)
99.91(1)
106.33(1)
77.80(1)
1524.8(4)
25
R, deg
â, deg
γ, deg
V, Å3
T, °C
λ, Å
0.7107
1.372
20 mA, 45 kV
8.73
0.941-1.000
5571
d(calcd), g/cm3
Mo KR, mA, kV
µ, cm-1
range of transmission factors
reflections collected
Rint
Figure 6. ORTEP diagram of hexaphenylstannole (SnPh). Thermal
ellipsoids are at the 50% probability level. Hydrogens are omitted for
clarity.
0.032
independent reflections
used in refinement:
number of variables
R(Fo)a
5143
3491 (3σ)
490
0.037
0.038
1.11
Pertinent geometrical parameters are summarized in Table 6
along with internuclear distances obtained from X-ray diffraction
studies of crystal structures. For all structures, geometric
optimization was performed at the AM1 level, followed by
single energy point calculations also at the AM1 level. For each
metallole, Mulliken charges for all atoms present in the molecule
were generated. (See Supporting Information for tables of
Mulliken charges for each metallole.) In every case the charge
on C2 was considerably more positive than the charge on C1.
These results are consistent with the 13C NMR resonance
assignments, in which C2 is always downfield from C1.
Molecular orbital isopotential plots were constructed to view
qualitative changes in the electronic structure of related systems
upon substitution of the heteroatom. The HOMOs and LUMOs
for all six compounds displayed qualitatively similar surfaces.
All the HOMOs resembled the HOMO of the model compound
presented in Figure 2a with A2 symmetry under C2V. Most of
the HOMO surfaces had some density at the phenyl rings
substituted on the diene section of the metallole. The LUMOs
all resembled the model compound LUMO, Figure 2c, except
for the stannoles, where the LUMOs had significant orbital
density at the tin atoms.34 Ab initio calculations were performed
Rw(Fo)b
Sc
a R(Fo) ) ∑||Fo| - |Fc||/∑||Fo|. b Rw(Fo) ) (∑w(|Fo| - |Fc|)2/
2
∑w|Fo| )1/2
.
c S ) (∑w(|Fo| - |Fc|)2/(No - Nv))1/2 where No ) the
number of observed reflections and Nv ) the number of variables.
on the hydrogen-substituted silole and germole at the Hartree-
Fock level of theory using the Pople split-valence 3-21G* basis
set. These calculations provided qualitatively similar results as
the lower level of theory.
Conclusion
Taken together, the experimental measurements and theoreti-
cal studies allow a straightforward explanation of the activation-
deactivation cycle of these metalloles. Absorption causes an
electronic transition in the chromophore which is unaffected
by heteroatom substitution. This excited state decays to an
intermediate state that, in the case of the phenyl-substituted
compounds, undergoes radiative decay to the ground state,
emitting a photon at approximately 490 nm and showing
relatively little dependence on the heteroatom.
On the basis of the results for the methyl series, it is clear
that the heteroatom perturbs the electronic/vibrational structure
(34) The orbital information is available upon request from the authors.

2472 Inorganic Chemistry, Vol. 38, No. 10, 1999
Ferman et al.
Table 6. Comparison of Calculated and Actual Metallole Bond Distances (Å) and Bond Angles (deg)a
E-C1/C4
E-C5(1)/C6(1)
C2-C3
C1(4)dC2(3)
C1-E-C4
C5(1)-E-C6(1)
SiMe
theory
actual (ref 11)
GeMe
theory
actual (ref 12)
1.833
1.868
1.818
1.855
1.486
1.511
1.353
1.358
92.2
92.7
110.7
110.2
1.968
1.945
2.110
1.842
1.965
1.973
1.944
2.099
1.777
1.944
1.481
1.508
1.485
1.485
1.482
1.351
1.347
1.350
1.354
1.351
88.0
89.9
83.7
91.9
87.8
110.1
110.2
113.0
111.6
112.1
SnMe
SiPh
GePh
SnPh
theory
actual
2.110
2.126
2.083
2.125
1.484
1.511(6)
1.350
1.352
84.1
84.6(2)
111.6
112.3(2)
a Errors associated with the actual structures (esd’s) are reported for unique bond distances (C2-C3) and bond angles [C1-E-C4 and C5(1)-
E-C6(1)], while the average bond distances are reported for equivalent bond lengths.
of the emissive state, resulting in the higher energy emission
of GeMe and the lack of luminescence in SnMe. The perturba-
tion of the emissive state by the heteroatom cannot be attenuated
as effectively by the methyl-substituted metalloles as it appar-
ently is by the phenyl-substituted compounds. In the extreme
case of SnMe, the perturbation is so significant that a now more
efficient nonradiative decay path drains the excited state more
rapidly than does the radiative decay mechanism, effectively
quenching the luminescence.
of Southern Maine and supported by the USM Chemistry
Department and the USM Office of the Dean of the College of
Arts and Sciences. We are also grateful to the Chemistry
Department at Bowdoin College, Brunswick, ME, for the use
of their NMR and mass spectrometers.
Supporting Information Available: Tables of crystal data and
structural refinement, atomic coordinates and equivalent isotropic
displacement parameters, complete bond lengths and angles, anisotropic
displacement parameters, and hydrogen coordinates and isotropic
displacement parameters for SnPh and output of MacSpartan semiem-
pirical molecular orbitals calculations including Cartesian coordinates
and Mulliken charges for each metallole. This material is available free
of charge via the Internet at http://pubs.acs.org.
Acknowledgment. Work at Brookhaven National Laboratory
was carried out under Contract DE-AC02-98CH10886 with the
U.S. Department of Energy, Office of Basic Energy Sciences.
Special thanks to Kathleen Gallagher and Drs. David Ofer, Peter
Trumper, and Elizabeth Stemmler for thoughtful discussions and
assistance. All synthetic work was carried out at the University
IC980662D