Synthesis and reactivity of dimeric Ar′TlTlAr′ and trimeric (Ar″Tl)3 (Ar′, Ar″ = bulky terphenyl group) thallium(I) derivatives: Tl(I)-Tl(I) bonding in species ligated by monodentate ligands

Article abstract of DOI:10.1021/ja0432259

The synthesis and characterization of three new organothallium(I) compounds are reported. Reaction of (Ar′Li)₂ (Ar′ = C ₆H₃-2,6-(C₆H₃-2,6-Pr ⁱ₂)₂) and Ar″Li (Ar″ = C ₆H₃-2,6-(C₆H₃-2,6-Me ₂)₂) with TlCl in Et₂O afforded (Ar′Tl)₂ (1) and (Ar″Tl)₃ (2). The dithallene 1 is the heaviest group 13 dimetallene and features a planar, trans-bent structure with Ar″Tl-Tl = 119.74(14)° and Tl-Tl = 3.0936(8) A. Compound 2 is the first structurally characterized neutral, three-membered ring species of formula c-(MR)₃ (M = Al-Tl; R = organo group). The Tl₃ ring has Tl-Tl distances in the range ca. 3.21-3.37 A as well as pyramidal Tl geometries. The Tl-Tl bonds in 1 and 2 are outside the range (2.88-2.97 A) of Tl-Tl single bonds in R ₂TlTlR₂ compounds. The weak Tl-Tl bonding in 1 and 2 leads to their dissociation into Ar′Tl and Ar″Tl monomers in hexane. The Ar′Tl monomer behaves as a Lewis base and readily forms a 1:1 donor-acceptor complex with B(C₆F₅)₃ to give Ar′TlB(C₆F₅)₃, 3. Adduct 3 features an almost linear thallium C(ipso)-Tl-B angle of 174.358(7)° and a Tl-B distance of 2.311 (2) A, which indicates strong association. Treatment of 1 with a variety of reagents resulted in no reactions. The lower reactivity of 1 is in accord with the reluctance of Tl(I) to undergo oxidation to Tl(III) due to the unreactive character of the 6s² electrons.

Full text of DOI:10.1021/ja0432259

Published on Web 03/11/2005
Synthesis and Reactivity of Dimeric Ar′TlTlAr′ and Trimeric
(Ar′′Tl)3 (Ar′, Ar′′ ) Bulky Terphenyl Group) Thallium(I)
Derivatives: Tl(I)-Tl(I) Bonding in Species Ligated by
Monodentate Ligands
Robert J. Wright, Andrew D. Phillips, Shirley Hino, and Philip P. Power*
Contribution from the Department of Chemistry, One Shields AVenue, UniVersity of California,
DaVis, California 95616
Received November 10, 2004; E-mail: pppower@ucdavis.edu
Abstract: The synthesis and characterization of three new organothallium(I) compounds are reported.
Reaction of (Ar′Li)2 (Ar′ ) C6H3-2,6-(C6H3-2,6-Pri2)2) and Ar′′Li (Ar′′ ) C6H3-2,6-(C6H3-2,6-Me2)2) with TlCl
in Et2O afforded (Ar′Tl)2 (1) and (Ar′′Tl)3 (2). The “dithallene” 1 is the heaviest group 13 dimetallene and
features a planar, trans-bent structure with Ar′Tl-Tl ) 119.74(14)° and Tl-Tl ) 3.0936(8) Å. Compound
2 is the first structurally characterized neutral, three-membered ring species of formula c-(MR)3 (M ) Al-
Tl; R ) organo group). The Tl3 ring has Tl-Tl distances in the range ca. 3.21-3.37 Å as well as pyramidal
Tl geometries. The Tl-Tl bonds in 1 and 2 are outside the range (2.88-2.97 Å) of Tl-Tl single bonds in
R2TlTlR2 compounds. The weak Tl-Tl bonding in 1 and 2 leads to their dissociation into Ar′Tl and Ar′′Tl
monomers in hexane. The Ar′Tl monomer behaves as a Lewis base and readily forms a 1:1 donor-acceptor
complex with B(C6F5)3 to give Ar′TlB(C6F5)3, 3. Adduct 3 features an almost linear thallium C(ipso)-Tl-B
angle of 174.358(7)° and a Tl-B distance of 2.311(2) Å, which indicates strong association. Treatment of
1 with a variety of reagents resulted in no reactions. The lower reactivity of 1 is in accord with the reluctance
of Tl(I) to undergo oxidation to Tl(III) due to the unreactive character of the 6s2 electrons.
interactions, in particular those in thallium(I) centers.10 Al-
though, stable thallium(I) clusters with as many as six11 or
Introduction
Metal-metal σ and π bonding in neutral, heavier group 13
metal (Al-Tl; i.e., heavier triels) clusters is an area that has
received considerable attention over the past decade.1-9 The
M-M bonded clusters, in which the triel metal has a +1
oxidation state and weakly interacting ns2 lone pairs, are of
particular interest due to the nature of the bonding and the
unusual optical properties that can be produced by M-M
eight12 thallium atoms have been isolated, the bonding in the
simplest dimeric species of formula RTlTlR has generated the
most theoretical interest. Several calculations on various thallium
model species have been carried out to obtain a clearer picture
of the metal bonding.13-17 The computational work was
prompted initially by the synthesis of [Tl{η5-C5(CH2Ph)5}]2 and
its indium analogue by Schumann and co-workers.7 These
compounds crystallized as trans-bent dimers which had almost
identical In-In and Tl-Tl distances near 3.63 Å. Later
experimental work demonstrated that several other molecular
compounds have “close” Tl(I)-Tl(I) interactions.18 These
(1) (a) Uhl, W. Coord. Chem. ReV. 1997, 163, 1. (b) Janiak, C. Coord. Chem.
ReV. 1997, 163, 107. (c) Uhl, W. ReV. Inorg. Chem. 1998, 18, 239. (d)
Linti, G.; Schno¨ckel, H. Coord. Chem. ReV. 2000, 206-207, 285. (e)
Schno¨ckel, H.; Schnepf, A. AdV. Organomet. Chem. 2001, 47, 235. (f)
Power, P. P. Struct. Bonding 2002, 103, 57.
(2) Loos, D.; Baum, E.; Ecker, H.; Schno¨ckel, H.; Downs, A. J. Angew. Chem.,
Int. Ed. Engl. 1997, 36, 860.
(3) Beachley, O. T.; Churchill, M. R.; Fettinger, J. L.; Pazik, J. C.; Victoriano,
L. J. Am. Chem. Soc. 1986, 108, 4666.
(10) Ferna´ndez, E. J.; Lo´pez-de-Luzuriaga, J. M.; Monge, M.; Olmos, M. E.;
Pe´rez, J.; Laguna, A. J. Am. Chem. Soc. 2002, 124, 5942.
(11) (a) Hellmann, K. W.; Gade, L. H.; Fleischer, R.; Kottke. Chem. Eur. J.
1997, 3, 1801. (b) Galka, C. H.; Gade, L. H. Inorg. Chem. 1999, 28, 1038.
See also: (c) Wiberg, N.; Blank T.; Lerner, H. W.; Fenske, D.; Linti G.
Angew. Chem., Int. Ed. 2001, 40, 1232, for the structures of R*6Tl6Cl2 and
R4*Tl3Cl (R* ) SiBut3) in which the Tl oxidation state is slightly > 1.
(12) Boesveld, W. M.; Hitchcock, P. B.; Lappert, M. F.; No¨th, H. Angew. Chem.,
Int. Ed. 2000, 39, 222.
(13) Janiak, C.; Hoffmann, R. J. Am. Chem. Soc. 1990, 112, 5924.
(14) (a) Schwerdtfeger, P. Inorg. Chem. 1991, 30, 1660. (b) Schwerdtfeger, P.;
Heath, G. A.; Dolg, M.; Bennett, M. A. J. Am. Chem. Soc. 1992, 114,
7518.
(15) Treboux, G.; Barthelat, J. C. J. Am. Chem. Soc. 1993, 115, 4870.
(16) Pyykko¨, P.; Straka, M.; Tamm, T. Phys. Chem. Chem. Phys. 1999, 1, 3441.
(17) Han, Y.-K.; Bae, C.; Lee, Y. S.; Lee, S. Y. J. Comput. Chem. 1998, 19,
1526.
(18) See: Ghosh, P.; Rheingold, A. L.; Parkin, G. Inorg. Chem. 1999, 28, 5464,
for a useful summary of the structures of species with Tl‚‚‚Tl interactions.
Tl(I) amides have been reviewed in: Gade, L. H. J. Chem. Soc., Dalton
Trans. 2003, 267.
(4) Dohmeier, C.; Robl, C.; Tacke, M.; Schno¨ckel, H. Angew. Chem., Int. Ed.
Engl. 1991, 30, 564.
(5) (a) Uhl, W.; Hiller, M.; Layh, M.; Schwarz, W. Angew. Chem., Int. Ed.
Engl. 1992, 31, 1364. (b) Schluter, R. O.; Cowley, A. H.; Atwood, D. A.;
Jones, R. A.; Atwood, J. L. J. Coord. Chem. 1993, 30, 25. (c) Uhl, W.;
Graupner, R.; Layh, M.; Schu¨tz, U. J. Organomet. Chem. 1995, 493, C1.
(d) Schultz, S.; Roesky, H. W.; Koch, H. J.; Sheldrick, G. M.; Stalke, D.;
Kuln, A. Angew. Chem., Int. Ed. Engl. 1993, 32, 1729. (e) Schnitter, C.;
Roesky, H. W.; Ro¨pken, C.; Herbst-Irmer, R.; Schmidt, H.-G.; Noltemeyer,
M. Angew. Chem., Int. Ed. 1998, 37, 1952. (e) Brothers, P. J.; Power, P.
P. AdV. Organomet. Chem. 1996, 39 1.
(6) Schumann, H.; Janiak, C.; Go¨rlitz, F.; Loebel, J.; Dietrich, A. J. Organomet.
Chem. 1989, 363, 243.
(7) Schumann, H.; Janiak, C.; Pickhardt, J.; Bo¨rner, U. Angew. Chem., Int.
Ed. Engl. 1987, 26, 789.
(8) Beachley, O. T.; Noble, M. J.; Allendoerfer, R. D. J. Organomet. Chem.
1999, 582, 32.
(9) Uhl, W.; Cuypers, L.; Harms, K.; Kaim, W.; Wanner, M.; Winter, R.; Koch,
R.; Saak, W. Angew. Chem., Int. Ed. 2001, 40, 566.
9
4794
J. AM. CHEM. SOC. 2005, 127, 4794-4799
10.1021/ja0432259 CCC: $30.25 © 2005 American Chemical Society

Tl(I)−Tl(I) Bonding in Monodentate-Ligated Species
A R T I C L E S
include dimers Tl2B2H9C2Me2 (Tl-Tl ) 3.67 Å),19 Tl2(η5:η5-
But4C10H4)2 (Tl-Tl ) 3.76 Å),20 {{Tl(Tpp-tol)}2 (Tpp-tol
was dried at 100 °C under reduced pressure (0.01 mmHg) for 15 min
prior to use. Tris(pentafluorophenyl)borane (Albemarle Corp.) was used
)
1
as received. H, 11B, 13C, and 19F NMR were recorded on Varian 300
hydridotris(3-p-tolyl)pyrazolyl borate; Tl-Tl ) 3.86 Å),21 {Tl-
(Bp}2 (Bp ) hydridobis(pyrazolyl) borate; Tl-Tl ) 3.70 Å),18
{MeSi[N(Tl)But]3}2 (Tl-Tl ) 3.15 Å),22 and the tetramers [Tl-
{N(SiMe3)C6H3-2,6-Pri2}]4 (Tl-Tl ) 4.06 Å),23 {TlC(SiMe3)3}4
(Tl -Tl ) 3.32-3.64 Å),24 and {Tl(Tpcpr)}4; (Tpcpr hydrotris-
(3-cyclopropylpyrazol-1-yl) borate) (Tl - Tl ) 3.64 Å).25 These
intermetallic distances may be compared to the 3.46 Å observed
for the pure metal,26 but they are much longer than the single
Tl-Tl bonds observed in R2Tl-TlR2 (R ) Si(SiMe3)3, 2.914
Å;27 SiBut3, 2.97 Å; and SiBut2Ph, 2.881(2) Å),28 which result
from overlap of formally sp2 orbitals. Despite the interest in
Tl-Tl bonded compounds, there has been no structural char-
acterization of a stable dimeric Tl(I)-Tl(I) bonded species in
which thallium is bound only to a monodentate ligand. In
parallel work it has been demonstrated that terphenyl ligands
were effective in the isolation of a variety of low coordinate
group 13 element compounds.29 Moreover, it has been shown
by Niemeyer and Power that, if the terphenyl ligand was
sufficiently large, the monomer TlAr* (Ar* ) C6H3-2,6(C6H2-
2,4,6-Pri2)2), with one-coordinate thallium, could be isolated.30
Recently we reported that terphenyl ligands with slightly less
crowding aryl rings permitted the isolation of the first neutral,
metal-metal bonded gallium31 or indium32a dimers; i.e., Ar-
′GaGaAr′ and Ar′InInAr′ (Ar′ ) C6H3-2,6-(C6H3-2,6-Pri2)2). We
now show that the first “dithallene” analogue of these, Ar′TlT-
lAr′ (1), can be isolated. In addition, it is shown that the
introduction of the less crowded ligand Ar′′ (Ar′′ ) C6H3-2,6-
(C6H3-2,6-Me2)2) results in the synthesis of the first neutral
trimeric group 13 metal ring derivative (TlAr′′)3, 2. Furthermore,
the lower reactivity of 1, in comparison to the lighter element
dimetallenes, is demonstrated.
and 400 spectrometers and referenced to known standards. UV/vis data
were recorded on a Hitachi-1200, and the melting points were recorded
using a Meltemp apparatus and were not corrected.
(TlAr′)2 (1). With the exclusion of as much light as possible, (LiAr′)2
(2.01 g, 2.50 mmol) in Et2O (25 mL) was added dropwise to a rapidly
stirred -10 °C slurry of TlCl (1.20 g, 5.0 mmol) in Et2O (75 mL). The
resulting mixture was stirred for ca. 4 h during which the cooling bath
naturally warmed to 10 °C. The stirring was discontinued, and the flask
was immediately placed in the refrigerator (ca. 4 °C) for overnight
storage. The pale red mother liquor was removed from the precipitate
(LiCl) and decanted into a precooled (-10 °C) aluminum foil wrapped
Schlenk tube. The solution was concentrated to ca. 30 mL and placed
in the freezer (ca. -25 °C) overnight, which afforded dark red, almost
black, X-ray quality crystals of the product. Yield: 1.20 g, 40%; mp
1
173-174 °C. H NMR (400 MHz, C6D6, 25 °C): δ 1.115 (d, 12H,
3
3
o-CH(CH3)2, JHH ) 6.9 Hz), 1.181 (d, 12H, o-CH(CH3)2, JHH ) 6.9
Hz), 3.160 (sept, 4H, CH(CH3)2, 3JHH ) 6.9 Hz), 7.228 (s, 4H, m-Dipp),
7.20-7.32 (m, 7H, p-C6H3, m-Dipp, and p-Dipp), 7.931 (d, 2H,
m-C6H3). 13C {1H}NMR (C6D6, 100.6 MHz, 25 °C): δ 24.21 (CH-
(CH3)2), 25.17 (CH(CH3)2), 30.41 (CH(CH3)2), 123.02 (m-Dipp), 125.05
(p-C6H3), 140.30 (i-Dipp), 145.10 (o-C6H3), 147.50 (o-Dipp); m-C6H3
and p-Dipp are likely obscured by the C6D6 signal. UV/vis (hexanes;
λ
max, nm (ꢀ, mol-1 L cm-1)): 364 (2900), 492 (1400).
(TlAr′′)3 (2). The trimer 2 was synthesized in a manner analogous
to that of 1.Yield: 0.86 g, 35%; mp 153-154 °C. 1H NMR (300 MHz,
C6D6, 25 °C): δ 2.107 (s, 12H, o-CH3), 7.066 (s, 6H, m-Xyl and p-Xyl),
3
3
7.310 (t, 1H, p-C6H3, JHH ) 7.5 Hz), 7.739 (d, 2H, m-C6H3, JHH
)
7.5 Hz). 13C {1H}NMR (C6D6, 75.46 MHz, 25 °C): δ 21.34 (o-CH3),
125.88 (p-Xyl), 126.98 (m-Xyl), 127.45 (p-C6H3), 127.98 (m-C6H3),
136.32 (o-Xyl), 143.86(i-Xyl), 147.62 (o-C6H3). UV/vis (hexanes; λmax
,
nm (ꢀ, mol-1 L cm-1)): 368 (2600), 492 (1100).
Ar′TlB(C6F5)3 (3). (TlC6H3-2,6-Dipp2)2 (0.40 g, 0.33 mmol) and
B(C6F5)3 (0.425 g, 0.83 mmol) were combined in a foil wrapped Schlenk
tube. Toluene (60 mL) cooled to -78 °C was added, and the precipitate
was stirred for ca. 12 h. The stirring was discontinued, and the solution
was allowed to settle. The pale yellow mother liquor was transferred
from a small amount of precipitate (TlOH) into another vessel. The
volume was concentrated to ca. 15 mL, and overnight storage in a
freezer (ca. -20 °C) afforded large colorless X-ray quality crystals.
Experimental Section
General Procedures. All manipulations were carried out using
modified Schlenk techniques under an atmosphere of N2 or in a Vacuum
Atmospheres HE-43 drybox. All solvents were distilled from molten
Na/K alloy and degassed three times prior to use. (LiAr′)2 was prepared
according to the literature procedure.33 Ar′′I and Ar′′Li was synthesized
in a manner anlagous to that of Ar′I and (LiAr′)2.34 TlCl (99%; Acros)
1
Yield: 0.26 g, 35 %; mp 165-167 °C, turns deep red at 175 °C. H
NMR (300 MHz, C6D6, 25 °C): δ 0.957 (d, 12H, o-CH(CH3)2, 3JHH
)
6.9 Hz), 1.097 (d, 12H, o-CH(CH3)2, 3JHH ) 6.9 Hz), 2.803 (sept, 4H,
3
3
(19) Jutzi, P.; Wegner, D.; Hursthouse, M. B. Chem. Ber. 1991, 124, 295.
(20) Jutzi, P.; Schnittger, J.; Hursthouse, M. B. Chem. Ber. 1991, 124, 1693.
(21) Ferguson, G.; Jennings, M. C.; Lalor, F. J. Starahan, C. Acta Crystallogr.
1999, C47, 2079.
CH(CH3)2, JHH ) 6.9 Hz), 7.035 (d, 4H, m-Dipp, JHH ) 7.5 Hz),
7.16-7.23 (m, 3H, p-C6H3 and p-Dipp, 3JHH ) 7.5 Hz), 7.465 (d, 2H,
m-C6H3). 13C {1H}NMR (C6D6, 75 MHz, 25 °C): δ 23.29 (CH(CH3)2),
25.49 (CH(CH3)2), 30.85 (CH(CH3)2), 123.78 (m-Dipp), 129.34 (p-
Dipp), 130.16 (p-C6H3), 132.64 (m-C6H3), 135.88 (br, C6F5), 139.18
(br, C6F5), 137.93 (i-Dipp), 142.23 (br, C6F5), 145.50 (br, C6F5), 146.87
(o-C6H3), 147.85 (o-Dipp), 149.43 (br, C6F5). 11B NMR (128 MHz,
C6D6, 25 °C): -11.3. 19F {1H} NMR (376 MHz, C6D6, 25 °C): δ
(22) Veith, M.; Spaniol, A.; Po¨hlmann, J.; Gross, F.; Huch, V. Chem. Ber. 1993,
126, 2625.
(23) Waezsada, S. D.; Belgardt, T.; Noltemeyer, M.; Roesky, H. W. Angew.
Chem., Int. Ed. Engl. 1994, 33, 1351.
(24) Uhl, W.; Keimling, S. U.; Klinkhammer, K. W.; Schwarz, W. Angew.
Chem., Int. Ed. Engl. 1997, 36, 64.
(25) Rheingold, A. L.; Liable-Sands, L. M.; Trofimenko, S. Chem. Commun.
1997, 1691.
3
-128.44 (broad, 6F, o-C6F5), -153.59 (t, 3F, p-C6F5, J ) 25.0 Hz),
(26) Wells, A. F. Structural Inorganic Chemistry, 5th ed.; Clarendon Press:
Oxford, U.K., 1984; p 1279.
3
-159.57 (d, 6F, m-C6F5, J ) 25.0 Hz).
(27) Henkel, S.; Klinkhammer K W.; Schwarz, W. Angew. Chem., Int. Ed. Engl.
1994, 33, 681.
X-ray Crystallographic Studies. Sample preparation consisted of
removing the crystal from the Schlenk tube under a rapid flow of argon
and immediately submerging it in hydrocarbon oil. A suitable crystal
was selected, mounted on a glass fiber attached to a copper pin, and
rapidly placed in a cold stream of N2 (-183 °C) of the diffractometer
for data collection.35 Data were collected on a Bruker SMART 1000
diffractometer with use of Mo KR (λ ) 0.710 73 Å) radiation and a
CCD area detector. Data collection and processing were performed
(28) Wiberg, N.; Blank T.; Amelunxen K.; No¨th, H.; Schno¨ckel, H.; Baum, H.;
Purath, A.; Fenske, D. Eur. J. Inorg. Chem. 2002, 341.
(29) (a) Twamley, B.; Haubrich, S. T.; Power, P. P. AdV. Organomet. Chem.
1999, 44, 1. (b) Clyburne, J. A. C.; McMullen, N. Coord. Chem. ReV. 2000,
210, 73.
(30) Niemeyer, M.; Power, P. P. Angew. Chem., Int. Ed. 1998, 37, 1277.
(31) (a) Hardman, N. J.; Wright, R. J.; Phillips, A. D.; Power, P. P. J. Am.
Chem. Soc. 2003, 125, 2667. (b) Hardman, N. J.; Wright, R. J.; Phillips,
A. D.; Power, P. P. Angew. Chem., Int. Ed. 2002, 41, 2842.
(32) (a) Wright, R. J.; Phillips, A. D.; Hardman, N. J.; Power, P. P. J. Am.
Chem. Soc. 2002, 124, 8538. (b) Haubrich, S. T.; Power, P. P. J. Am. Chem.
Soc. 1998, 120, 2202.
(34) Schiemenz, B.; Power, P. P. Organometallics 1996, 15, 958.
(35) Hope, H. Prog. Inorg. Chem. 1995, 41, 1.
(33) Schiemenz, B.; Power, P. P. Angew. Chem., Int. Ed. Engl. 1996, 35, 2150.
9
J. AM. CHEM. SOC. VOL. 127, NO. 13, 2005 4795

A R T I C L E S
Wright et al.
Table 1. Selected Crystallographic Data for Compounds 1-3
Ar′
TlTlAr
′
(1)
(Ar′′Tl)3 (2)
Ar′TlB(C6F5)3 (3)
formula
fw
color, habit
cryst syst
space group
a, Å
C60H74Tl2
1203.93
dark red, block
orthorhombic
Pccn
20.648(5)
15.547(4)
16.395(5)
90
90
90
5263(2)
2
C66H66Tl3
C55H45BF15Tl
1206.09
colorless, block
monoclinic
P21/n
13.0842(7)
23.2719(13)
16.1097(9)
90
94.782(3)
90
4888.2(5)
4
1469.37
dark red, block
monoclinic
P21/n
21.2335(14)
14.0989(10)
20.1848(13)
90
117.601(3)
90
5355.0(6)
4
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
Z
cryst dimens, mm
0.62 × 0.43 × 0.26
0.32 × 0.25 × 0.14
0.32 × 0.17 × 0.11
d
calc, g cm-3
1.519
1.822
1.639
µ, mm-1
6.151
5168
4378
0.0493
0.1314
9.046
10 515
9434
0.0232
3.399
15 390
13 147
0.0222
no. of reflns
no. of obsd reflns
R1, obsd reflnsa
wR2, allb
0.0593
0.0548
2
2
2
a R1 ) ∑||Fo - Fc||/∑|Fo|. b wR2 ) [∑[w(Fo - Fc )2]/∑[w(Fo )2]]1/2
.
using the programs SMART36a and SAINT36b provided by Bruker AXS.
Empirical absorption corrections were applied to all samples using
SADABS.36c The crystal structures were solved using either direct
methods or the Patterson option in SHELXS37 and refined by the full-
matrix least-squares procedure in the SHELXL37 program. All non-
hydrogen atoms were refined anisotropically, while hydrogen atoms
were placed at calculated positions and included in refinement using a
riding model. Selected crystal data and data collection parameters are
provided in Table 1. Further details are given in the Supplementary
Information.
observable at low temperature even though the Tl-Tl bond is
very weak. In fact, recent experiments by Andrews and Wang
showed by infrared spectroscopy that HTlTlH could be formed
along with other thallium hydrides and trapped in a solid neon,
argon, or hydrogen matrix.38 In addition, theoretical studies by
Treboux and Barthelat using a double-ú-plus polarization basis
set, and QCI energies at Hartree-Fock geometries, treated all
the triel hydrides HMMH (M ) B-Tl) in terms of the
interactions between the two MH fragments and showed that,
for the heavier elements, the hydrogen-bridged isomer M(µ-
H)2M was the most stable in all cases with the vinylidene isomer
H2MM lying next highest in energy.15 The trans-bent isomers
were even higher in energy (by ca. 7-9 kcal mol-1), but a linear
HMMH configuration (a triplet) was much less stable. Nonethe-
less, no minimum on the potential energy surface was found to
exist for the trans-bent H-Tl-Tl-H. More recent theoretical
studies of the trans-bent HMMH and (η5-C5H5)MM(η5-C5H5)
model compounds by Pyykko¨ et al.,16 with use of ab initio
calculations incorporating MP2 and CCSD(I) methods, afforded
results that were in broad agreement with those of Schwerdtfeger
and which yielded dimerization energies of ca. 4.8 kcal mol-1
for the HTlTlH species (Tl-Tl ) 3.217 Å) versus ca. 7.1 kcal
mol-1 for HInInH (In-In ) 3.02 Å).14 Calculations for (η5-
C5H5)MM(η5-C5H5) yielded M-M distances of (In) 3.96 Å and
(Tl) 3.819 Å, which were longer than those experimentally
observed (3.63 Å) for the M2{η5-C5(CH2Ph)5}2 species. The
calculated M-M-(centroid-η5-C5H5) angles of (In) 102.8° and
(Tl) 94.5° were narrower than the 136.5° and 131.8° found
experimentally. As suggested by the longer In-In distance, the
interaction energy is less than that predicted for the thallium
compounds: 2.96 versus 3.78 kcal mol-1. The longer calculated
M-M bonds in comparison to those measured and the narrower
M-M-(η5-C5H5) angles were reasonably attributed to crystal
packing effects, although no plausible explanation suggested
itself for the longer bonds and weaker interactions of the indium
species. As pointed out in the Introduction, the use of terphenyl
ligands has allowed the lighter gallium and indium derivatives
Ar′GaGaAr′ and Ar′InInAr′ to be synthesized. The use of the
Results and Discussion
The extensive theoretical interest in Tl(I)-Tl(I) bonded
species, as well as their lighter M(I)-M(I) (M ) Al, Ga, or In)
homologues, prompted us to investigate the isolation and
characterization of unbridged thallium(I) aggregates with low
coordination numbers. In particular we were interested in
compounds in which the thallium substituent was η1-bound in
order to maximize the metal-metal interaction. Calculations
(extended Hu¨ckel) by Janiak and Hoffmann on the simplest
possible model of this type, i.e., the planar, trans-bent hydride
H-Tl-Tl-H, led to the conclusion that there was “strong
bonding” when the bending angle was 120°.13 Later work by
Schwerdtfeger involving configurational interactions, relativistic
pseudopotentials, and extensive basis sets at various levels of
approximation also showed that a maximum interaction between
the Tl-H units occurred at a similar trans-bending angle near
115°.14 But the minimum on the potential plot of the interaction
energy versus Tl-Tl distance occurred at only about 3.25 kcal
mol-1 below the dissociation limit and at a Tl-Tl distance of
3.28 Å. Further analysis showed that the stretching force
constant for the bond is below 1 mdyn/Å.14 Moreover, the
difference in the total zero point vibrational energies for the
dissociation of Tl2H2 f 2TlH was ca. 1.2 kcal mol-1. It was
predicted that the trans-bent H-Tl-Tl-H species should be
(36) (a) SMART: Area-Detector Software Package; Bruker AXS, Inc.: Madison,
WI, 1993. (b) SAINT: Area-Detector Integration Software; Bruker AXS,
Inc.: Madison, WI, 1995. (c) SADABS: Area-Detector Absorption Cor-
rection; Bruker AXS, Inc.: Madison, WI, 1996.
(37) (a) SHELXL PC, version 5.03; Bruker AXS, Inc.: Madison, WI, 1994. (b)
Cooper, R. I.; Gould, R. O.; Parsons, S.; Watkin, D. J. J. Appl. Crystallogr.
2002, 35, 168.
(38) Wang, X.; Andrews, L. J. Phys. Chem. A 2004, 108, 3396.
9
4796 J. AM. CHEM. SOC. VOL. 127, NO. 13, 2005

Tl(I)−Tl(I) Bonding in Monodentate-Ligated Species
A R T I C L E S
Table 2. UV/Vis Spectroscopic Data (nm) (ꢀ, mol-1 L cm-1) for
TlAr*, 1, and 2 in Hexane Solution
TlAr*
TlAr′
TlAr′′
367 (1200)
364 (2900)
368 (2600)
492 (640)
492 (1400)
492 (1100)
Ar′ ligand for thallium raised the possibility that the corre-
sponding dimeric species could also be obtained, thereby
affording a homologous series of this type for M ) Ga, In, or
Tl and permitting the M-M distances to be compared.
Synthesis. The thallium(I) aryls 1 and 2 were synthesized
under rigorously anaerobic and anhydrous conditions by the
treatment of thallium(I) chloride with 1 equiv of the lithium
aryl in Et2O at ca. -78 °C in accordance with the equation.
1
2
1
3
Figure 1. Thermal ellipsoid (30%) drawing of 1. Hydrogen atoms are not
shown. Selected bond lengths (Å) and bond angles (deg) are as follows:
Tl(1)-Tl(1A) ) 3.0936(8); Tl(1)-C(1) ) 2.313(5); C(1)-Tl(1)-Tl(1A)
) 119.74(14); C(2)-C(1)-Tl(1) ) 117.6(4); C(6)-C(1)-Tl(1) ) 122.9-
(4).
(TlAr′)
(TlAr′′)3
LiAr′ or LiAr′′ + TlCl f
2 or
+ LiCl
1
2
The addition, which was carried under conditions of low
illumination, resulted in red solutions upon warming to 0 °C.
Separation of the precipitated LiCl and concentration of the
filtrate under reduced pressure afforded upon storage at ca. -20
°C dark red, almost black, crystals of 1 or 2 in ca. 40% yield.
The crystals of the products are soluble in hydrocarbons such
as benzene or toluene, but these solutions are prone to
decomposition at room temperature to afford a deposit of
thallium and a colorless supernatant solution. Analysis of these
solutions by 1H and 13C NMR spectroscopy showed the presence
of free arene (Ar′H or Ar′′H), which is probably a result of
hydrogen abstraction from the hydrocarbon solvent. However,
storage of 1 and 2 as solids for several months at ca. - 20 °C
in the absence of light afforded no evidence of decomposition.
The compounds were characterized by 1H and 13C NMR
spectroscopy. Attempts to obtain 205Tl NMR data were unsuc-
cessful. This is probably due to broadening of the signal by the
large anisotropy of the chemical shift tensor, which shortens
the relaxation times.39 A similar problem was encountered in
NMR studies of the monomer TlAr*.25 The electronic spectra
of 1 and 2, along with those of TlAr*, are presented in Table
2. Each compound displays two absorbances at ca. 366 and 492
nm. Each absorbance has a relatively large ꢀ value, which is
consistent with orbitally allowed transitions. It is probable that
n-p (6s-6p) and σ-σ* (Tl-C) transitions are responsible for
these absorptions. The electronic spectra of 1 and 2 are almost
identical to that of TlAr*, which has already been shown to be
monomeric in both the solid state and in solution. Attempts to
measure the molecular weight of 1 by cryoscopy were hampered
by the decomposition of the solution with the deposition of Tl
metal.
yielded a very pale yellow oil that was prone to decomposition,
which severely hindered attempts to analyze the compound
spectroscopically. Attempts to crystallize the oil from common
solvents were unsuccessful.
Structure, Bonding, and Reactivity. The structure of 1 is
illustrated in Figure 1. It is a centrosymmetric dimer with a
planar Tl2C(ipso)2 core. The Tl-Tl distance, 3.0936(8) Å, is
the shortest that has been observed in a molecular Tl(I) species.
The only Tl(I)-Tl(I) separation in a molecular species that
approaches this value is the 3.15 Å observed in the structure of
{MeSi[N(Tl)But]3}2. The trans-bending angle for the Tl(1A)-
Tl(1)-C(1) array is 119.74(14)°. This is in good agreement with
the 120, 115, and 119.5° angles calculated for HTlTlH.13-15
The calculated distances, however, are longer (3.28 and 3.217
Å)13-15 than the experimentally observed 3.0936(8) Å in 1.
Calculations for the dimerization energies of (TlH)2 have
afforded values of 3.35 and 4.78 kcal mol-1. The dissociated
structures of 1 and 2 in solution underline the weakness of the
Tl(I)-Tl(I) bonding and are consistent with the calculated and
experimentally observed bond lengths as well as the calculated
low association energies. It is also notable that the Tl(I)-Tl(I)
distance in 1, which contains two-coordinate thallium, is longer
than the 2.88-2.97 Å range observed for Tl-Tl single bonds
in R2TlTlR2 species. This also is indicative of weak bonding27,28
and suggests that the Tl-Tl bond in such dimers could be
shortened by packing forces to values less than the calculated
distances which concern gas phase species only.
The Tl-C bond length in 1 is 2.313(5) Å, which is essentially
indistinguishable from the 2.34(1) Å seen in TlAr*.30 This
suggests that the electronic arrangement in the valence orbitals
surrounding thallium seems little affected by association. This
is, of course, also consistent with weak Tl-Tl bonding. In
contrast, the Tl-C distance in 1 and TlAr* are ca. 0.09-0.14
Å longer than the Tl(III)-C bond lengths in the compounds
TlMes3 (2.169-2.224 Å)40 and TlMe3 (2.196-2.207 Å).41
Presumably the smaller size of the Tl3+ in comparison to Tl+
accounts for some of this difference.
The monomeric character of 1 and 2 in solution is further
supported by their immediate reaction with B(C6F5)3. The
reaction between 1 and B(C6F5)3 occurs instantly upon mixing
toluene solutions of the reactants and afforded colorless crystals
of the 1:1 donor-acceptor complex 3. Compound 3 displayed
three 19F resonances (o-F, m-F, p-F) and a 11B NMR signal at
-11.3 ppm, which are similar to those of the analogous lighter
element complexes Ar′MfB(C6F5)3. (M ) Ga, δ ) -17.73;
In, δ ) -14.1). However, the reaction between 2 and B(C6F5)3
(40) Blumel, J.; Werner, B.; Krauter, T.; Neumu¨ller, B. Z. Anorg. Allg. Chem.
1997, 623, 309.
(41) Boese, R. Downs, A. J.; Greene, T. M.; Hall, A. W.; Morrison, C. A.;
Parsons, S. Organometallics 2003, 22, 2450.
(39) Poole, C. P.; Farach, H. A. Relaxation in Magnetic Resonance; Academic
Press: New York, 1971; p 75.
9
J. AM. CHEM. SOC. VOL. 127, NO. 13, 2005 4797

A R T I C L E S
Wright et al.
Figure 2. Thermal ellipsoid (30%) drawing of 2. Hydrogen atoms, as well
as the flanking 2,6-dimethylphenyl substituents of the terphenyl groups,
are not shown. Selected bond lengths (Å) and bond angles (deg) are as
follows: Tl(1)-Tl(2) ) 3.2144(3); Tl(1)-Tl(3) ) 3.3782(3); Tl(2)-Tl(3)
) 3.3590(3); Tl(2)-Tl(1)-Tl(3) ) 61.203(6); Tl(1)-Tl(2)-Tl(3) ) 61.805-
(5); Tl(2)-Tl(3)-Tl(1) ) 56.992(6); Tl-C(av.) ) 2.331(4); C(1)-Tl(1)-
Tl(2) ) 150.02(10); C(1)-Tl(1)-Tl(3) ) 110.93(9); C(23)-Tl(2)-Tl(1)
) 96.96(10); C(23)-Tl(2)-Tl(3) ) 131.61(9); C(45)-Tl(3)-Tl(2) )
111.9(1); C(45)-Tl(3)-Tl(1) ) 168.3(1).
Figure 3. Thermal ellipsoid (30%) drawing of 3. Hydrogen atoms are not
shown. Selected bond lengths (Å) and bond angles (deg) are as follows:
Tl(1)-B(1) ) 2.31(2); Tl(1)-C(1) ) 2.1647(17); Tl(1)‚‚‚F(1) ) 2.6689-
(13); C(1)-Tl(1)-B(1) ) 174.358(7).
other (ca. 3.21 Å). This leads to a narrower, 56.992(6)°, angle
opposite the shortest Tl-Tl bond. The planarity of the Tl3 array
does not extend to their attached ipso carbon atoms. The C(1),
C(23), and C(45) carbons lie -1.1479, 1.6734, and -0.1508 Å
from the Tl3 plane, the C(23) carbon being on the opposite side
of the plane from C(1) and C(45). Each thallium is pyramidally
coordinated with the sum of the angles (Σ°) as follows: Tl(1)
) 323.15(10)°, Tl(2) ) 290.38(10)°, Tl(3) ) 337.22(10)°, which
is indicative of substantial lone pair character at each metal.
This is consistent with weakened Tl-Tl bonding, and it is
notable that the Tl-Tl distances in 2 are 0.1-0.25 Å longer
than those in 1. In contrast, the Tl-C bonds, average 2.331(8)
Å, are essentially unaltered from those in 1 and TlAr*. As
discussed above, the unchanged Tl-C bond length is consistent
with weak bonding of the thallium centers and is in agreement
with the monomeric character of 2 in solution.
The main reason for the weakness of the metal-metal
bonding in 1 and 2 is the large energy difference between the
lone pair orbital, mostly 6s in character, and empty valence 6p
orbitals in the TlAr′ and TlAr′′ moieties. The Tl-H singlet-
triplet energy difference (which is somewhat less than the 6s2-
6p energy difference) has been calculated to be 51.9 kcal mol-1
by Treboux and Barthelat.15 Thus, formation of strong covalent
bonds between the thalliums require very high preparation
energy to induce the s-electrons into combination. This cannot
be compensated for by the low inherent strength of the metal-
metal bonds that are formed.14
The energy difference between the 6s2 and 6p levels does
not prevent the TlAr moiety from behaving as a good Lewis
base if a suitable acceptor is available. Thus, when it is treated
with B(C6F5)3, the thermally robust complex 3 is formed readily.
Complex 3 features an almost linear thallium coordination and
a Tl(1)-C(1) distance of 2.1647(17) Å (Figure 3), which is ca.
0.15 Å less than the C(1)-Tl(1) bond in 1.
The Tl-B distance is 2.311(2) Å, which is in good agreement
with the sum of the covalent radii of thallium and boron.45
Compounds having Tl-B bonds are scarce and limited to
various thallium boron cage species, which feature a wide range
The three-membered ring arrangement of 2 (Figure 2) is the
first observation of a species of formula c-(MR)3 (M ) Al-Tl;
R ) organo group) for group 13 metals.42 Cryoscopic measure-
ments of molecular weights of some Ga(I) alkyls have afforded
values that correspond to (GaR)3 species.1c,5a The data, however,
were interpreted in terms of an equilibrium mixture of monomers
and tetramers rather than trimers. The observation of dimeric
and trimeric structures in our work suggests that such species
might also exist in equilibrium alongside the tetramers and
monomers in Ga(I) or In(I) alkyl or aryl solutions, even though
no trimeric organogallium(I) or organoindium(I) species cor-
responding to 2 have been isolated to date. The reaction of Ar′′Li
with InCl did not afford a trimeric indium cluster analogous to
2, but instead a mixed In(I)-In(II) compound with bridging
chlorides of formula Ar′′4In4Cl2 was isolated.43 Moreover,
reaction of the mesityl-substituted m-terphenyl, ArylLi, (Aryl
) C6H3-2,6-(C6H2-2,4,6-Me3)2), a reagent that differs from Ar′′
only by the addition of a p-Me group on the flanking aryl, with
InCl did not generate a trimer like 2; instead a In8Aryl4 cluster
was isolated.44 In addition, reaction of ArylLi with TlCl
generated highly unstable thallium products that readily de-
composed above -30 °C.30
The three Tl atoms in 2 are arranged as an isosceles triangle
in which two Tl-Tl distances (ca. 3.37 Å) are longer than the
(42) The alkali metal-gallum clusters M2(GaC6H3-2, 6-Mes2)3 (M ) Na or K)
have a Ga3 arrangement which can be regarded to have an aromatic
(GaC6H3-2,6-Mes2)2- unit: Li, X.-W.; Pennington, W. T.; Robinson, G.
H. J. Am. Chem. Soc. 1995, 117, 7578. Li, X.-W.; Xie, Y.; Schreiner, P.
R.; Gripper, K. D.; Crittendon, R. C.; Campana, C. F.; Schaefer, H. F.;
Robinson, G. H. Organometallics 1996, 15, 3798. In addition, free radical
aluminum and gallum ring species of formula M3(SiBut3)4 (M ) Al or
Ga) have been synthesized. Wiberg, N.; Blank, T.; Kaim, W.; Schwerdeski,
B.; Linti, G. Eur. J. Inorg. Chem. 2000, 1475. Wiberg, N.; Blank, T.;
Amelunxen, K.; No¨th, H.; Knizek, J.; Habereder, T.; Kaim, W.; Wanner,
M. Eur. J. Inorg. Chem. 2001, 1719.
(43) Fox, A. F.; Wright, R. J.; Richards, A. F.; Power, P. P. Unpublished work,
2004.
(44) Eichler, B. E.; Hardman, N. J. Power, P. P. Angew. Chem., Int. Ed. 2000,
39, 383. For a recent review of heavier group 14 clusters see: Wiberg, N.;
Power, P. P. Mol. Clusters Main Group Elem.; Driess, M., No¨th, H., Eds.;
WCH-Wiley: Weinheim, 2004; p 188.
(45) Pauling, L. The Nature of the Chemical Bond, 3rd ed.; Cornell: Ithaca,
NY, 1960; p 246.
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4798 J. AM. CHEM. SOC. VOL. 127, NO. 13, 2005

Tl(I)−Tl(I) Bonding in Monodentate-Ligated Species
A R T I C L E S
of Tl-B distances (2.063-2.772 Å).19,46 These data suggest that
the thallium boron bond in 3 is relatively strong. This is borne
out by other structural parameters such as the degree of
geometrical change from planarity to tetrahedral in B(C6F5)3,
which has been used to gauge the strength of donor-acceptor
complexes.47 The sum of the C-B-C angles in 3 is 341.03-
(15)°, which is just slightly greater than the corresponding
angular sums at B(C6F5)3 in Ar′MB(C6F5)3 (M ) Ga, 337.5-
(3)° 31 and M ) In, 339.33 (1)°).32a One major fluorine contact
Tl(1)‚‚‚F(1) with a distance of 2.6689(13) Å is present in the
solid state. The distance is at least 0.29 Å longer than known
Tl(III)-F single bonds that span the range 2.38-2.44 Å.48
However, the Tl-F interaction lengthens the C(32)-F(1)
distance to 1.370(2) Å, which is longer than the 1.347(2) Å
average for the 14 remaining ligand C-F bonds. The Tl-F
interaction is also preserved in solution as indicated by the broad
ortho-chemical shift in the 19F NMR spectra. This is in contrast
to the lighter congeners Ar′MB(C6F5)3 (M ) Ga, In), where
three sharp signals are observed for the ortho, meta, and para
fluorines in the 19F NMR.31,32a
1 in comparison to TlAr*, and the ready formation of 3 at low
temperatures, provides further evidence for the weakness of the
Tl-Tl bond in 1.
The diindene (Ar′InInAr′),32 digallene (Ar′GaGaAr′),31 and
dialuminene (Ar′AlAlAr′)49 analogues of 1 have been shown
to be highly reactive. For example, the diindene and digallene
readily react with bulky azides with the elimination of N2 to
form the imides Ar′MNAr that contain M-N bonds with
multiple character.50 The dialuminene, Ar′AlAlAr′ is so reactive
that it forms a cycloaddition product with toluene at room
temperature.49 The diindene and digallene also react with N2O
to form compounds with bridging oxides of formula (Ar′MO)2.51
However, reaction of Ar′TlTlAr′ with N3SiMe3, N3Ar, N2O or
W(CO)5THF, which yielded isolable and characterizable prod-
ucts with the lighter dimetallenes, afforded no reaction with 1
under the same conditions. The inert nature of the dithallene
toward oxidation is in accord with the so-called inert pair
effect.52
Conclusions
The heaviest group 13 dimetallene of formula Ar′TlTlAr′ has
been synthesized. It displays weak Tl-Tl bonding which is
manifested by dissociation of Ar′TlTlAr′ into two monomeric
Ar′Tl units in hydrocarbon solvent. In solution, the Ar′Tl
monomer displays Lewis base character but a lower tendency
to form oxidized products in comparison to its Al(I), Ga(I), or
In(I) dimetallene analogues. This is in accord with the greater
stability of Tl(I) in comparison to Tl(III). Use of the smaller
terphenyl ligand Ar′′ gave the first example of a neutral group
13 element three-membered ring compound (Ar′′Tl)3. The trimer
also dissociates to monomeric Ar′′Tl units in hydrocarbon
solution as shown by its UV/vis spectrum which is very similar
to that of the known monomer Ar*Tl.
The Tl-C bond in 3 is ca. 6% shorter than that in 1, despite
the increase in coordination number at thallium. The shortening
may be rationalized in a number of ways. One involves an
increase ionic contribution to the Tl-C bond strength as a result
of the donation of the lone pair to boron, which increases the
δ+ at thallium. Alternatively, the removal of charge from
thallium by lone pair donation effectively decreases its ionic
radius so that shorter bonds are observed. Similar arguments
have been used to account for the metal-carbon bond shortening
in complexes of gallium(I) and indium(I) aryls which display a
Ga-C and In-C bond shortening of at least 0.1 Å upon reaction
with acceptors such as B(C6F5)3 or metal carbonyls.31,32 In a
similar vein, it can be argued that the absence of shortening in
Acknowledgment. We are grateful to the National Science
Foundation for financial support and the Albemarle Corporation
for a generous gift of B(C6F5)3.
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Supporting Information Available: Crystallographic data
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