The "diindene" ArInInAr (Ar = C6H3-2,6-Dipp2, Dipp = C6H3-2,6-Pri2). Dimeric versus monomeric In(I) aryls: para-substituent effects in terphenyl ligands

Article abstract of DOI:10.1021/ja026285s

The synthesis and structure of a "diindene" with significant metal-metal bonding are described. It has an In-In distance of 2.9786(5) A, an In-In-C angle of 121.23(6)°, and an In-In bond order that is probably less than unity. Copyright

Full text of DOI:10.1021/ja026285s

Published on Web 06/28/2002
The “Diindene” ArInInAr (Ar ) C6H3-2,6-Dipp2, Dipp ) C6H3-2,6-Pri2). Dimeric
versus Monomeric In(I) Aryls: para-Substituent Effects in Terphenyl Ligands
Robert J. Wright, Andrew D. Phillips, Ned J. Hardman, and Philip P. Power*
Department of Chemistry, UniVersity of California, DaVis, One Shields AVenue, DaVis, California 95616
Received March 22, 2002
Metal-metal bonded clusters of the heavier group 13 elements
of formula (MR)n (M ) Al, Ga, In, or Tl; R ) organo group) are
an unusual compound class that has expanded rapidly over the past
decade.1 They were originally isolated as cyclopentadienide deriva-
tives which crystallize as tetramers (e.g., {Al(η5-C5Me5)}4})2,
loosely bonded dimers (e.g., [M{η5-C5(CH2Ph)5}]2, M ) In3a or
Tl3b), or hexamers {M(η5-C5Me5)}6 (M ) Ga4 or In5).6 Monodentate
ligands, for example, C(SiMe3)3 or Si(SiMe3)3, also yielded
tetramers which dissociate to monomers in solution or the vapor
phase.7 However, use of a bulky terphenyl ligand afforded the
monomers MC6H3-2,6-Trip2 (M ) In8a or Tl;8b Trip ) C6H2-2,4,6-
Pri3), which have a one-coordinate metal in the solid state. The
absence of stable, neutral dimeric or trimeric clusters with
significant9 M-M bonding is notable.10 Dimers of formula RMMR
(M ) B-Tl, R ) organic group) are of high current interest since
they are formally double-bonded. Calculations have predicted a
linear triplet structure for the diborene11 and a planar trans-bent
Figure 1. Selected bond lengths (Å) and angles (deg) for 1. H atoms are
structure with no unpaired electrons for the heavier congeners.12
Here, it is shown that use of a modified terphenyl ligand -C6H3-
2,6-Dipp2 (Dipp ) C6H3-2,6-Pri2) enables the isolation of a stable
dimeric group 13 “dimetallene” species (1) ArInInAr (Ar ) C6H3-
2,6-Dipp2).
not shown. In(1)-In(1A) ) 2.9786(5), In(1)-C(1) ) 2.256(2), C(1)-C(2)
) 1.406(3), C(1)-C(6) ) 1.405(3), In(1)-In(1A)-C(1A) ) 121.23(6),
C(2)-C(1)-C(6) ) 118.6(2), In(1)-C(1)-C(2) ) 119.95(15), In(1)-
C(1)-C(6) ) 120.47(16)°.
Compound 1 was isolated13 as dark red crystals by treatment of
InCl with LiAr14 in PhMe. It was characterized by 1H and 13C NMR,
UV/vis spectroscopy, and X-ray crystallography.15 The structure
(Figure 1) shows that 1 exists as centrosymmetric dimers with a
trans-bent C(1)-In(1)-In(1A)-C(1A) array, In-In and In-C
distances of 2.9786(5) and 2.256(2) Å, as well as a In-In-C angle
of 121.23(6)°. The angle between the In2C2 core and the central
aryl ring is 39.3°. The In lies 0.36 Å from the plane of the C(1)
ring, and there is an angle of 9.3° between In(1)-C(1) and the
C(1)-C(4) vectors. The dimeric 1 may be compared to the
monomeric InC6H3-2,6-Trip2,8a which differs from 1 only in that it
has para-Pri groups on the flanking aryl rings. Their absence permits
dimerization through In-In bonding in 1. In cyclohexane solution,
however, 1 is extensively dissociated (cryoscopy), indicating that
the In-In bond is weak. The In-In distance is shorter than the
3.329 Å calculated for trans-bent HInInH (cf. 2.545 Å predicted
for the triplet form),16a but is closer to the 3.0596 Å obtained in
more recent calculations.16b It lies above the currently known range
(2.696(2)-2.938(1) Å) for In-In single bonds17,18 and resembles
the ca. 3.0 Å in electron-deficient (InR)4 tetramers (R ) C(SiMe3)3,
C(SiMe2Et)3, or C(SiMeEt2)3).19 The In-C distance in 1, 2.256(2)
Å, is indistinguishable from the 2.260(7) Å observed for the
monomer InC6H3-2,6-Trip2,8a which indicates that dimerization has
little effect on the In-C bond length.
Figure 2. Selected bond lengths (Å) and angles (deg) for one of the
molecules of 2. H atoms are not shown. The numbers in braces refer to 3.
In(1)-C(1) ) 2.139(2) {2.152(2)}, In-B(1) ) 2.299(2) {2.322(2)},
In(1)‚‚‚F(1) ) 2.535(2) {2.587(2)}, C(1)-In(1)-B(1) ) 176.21(9)
{176.27(6)}, In(1)-C(1)-C(2) ) 110.3(2) {117.0(1)}, In(1)-C(1)-C(6)
) 128.5(2) {121.7(1)}, av In(1)-B(1)-C(ipso) ) 105.7(3)° {106.1(1.5)}.
occurred to give 2,6-Trip2H3C6InB(C6F5)3 (3).13 The structure of 2
(Figure 2)15 features two independent molecules with average In-B
and In-C distances of 2.298(2) and 2.139(2) Å and a C-In-B
angle of 176.4(2)°. For 3, the In-B (2.322(2) Å) and In-C
(2.152(2) Å) bonds are marginally longer than those in 2, whereas
the C-In-B angle (176.27(6)°) is unchanged. The longer In-B
and In-C bonds in 3 are probably due to the steric effects of the
para-Pri groups. The In-B bond lengths in 2 and 3 are in fair
Weak In-In bonding is also supported by the rapid reaction of
1 with B(C6F5)3 in PhMe at ca. 25 °C to afford 2,6-Dipp2H3C6-
InB(C6F5)3 (2).13 Reaction of InC6H3-2,6-Trip2 with B(C6F5)3 also
* To whom correspondence should be addressed. E-mail: pppower@ucdavis.edu.
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10.1021/ja026285s CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
agreement with the sum (2.2 Å)20 of the covalent radii of In and B
and fall within the 2.25-2.52 Å range observed in In-B cages.21
The In atoms are two-coordinate, although there are close contacts
(In‚‚‚F ) 2.54-2.72 Å) between In and one or two of the six ortho-
fluorines. The interacting C-F bonds (e.g., F(1)-C(32) ) 1.372(3)
Å and F(6)-C(38) ) 1.375(2) Å) in 1 are ca. 0.03 Å longer than
the average (ca. 1.34 Å) for the remaining C-F bonds in the
molecule. The amount of geometrical change from planarity to
tetrahedral in B(C6F5)3 has been viewed as an indicator of the
strength of donor acceptor interactions.22 The sums of the C-B-C
angles in 2 (338.89 and 339.76°) and 3 (337.79°) may be compared
to 339.8(2), 342.2(2), or 333.5° reported for η5-C5Me5AlB(C6F5)3,23
η5-C5Me5GaB(C6F5)3,24,25 or HC{(Me)C(Dipp)N}2GaB(C6F5)3,24
suggesting that 1 and InC6H3-2,6-Trip2 are similar to these in donor
strength. The C-B-C angular sum in 2 is slightly greater than
that in 3 despite the shorter In-B bond. The greater pyramidal
distortion in the B(C6F5)3 moiety in 3 further supports the important
steric effects of the para-Pri substituents. A feature of the structures
of 2 and 3 is that their In-C bonds are (ca. 0.1 Å) shorter than
those in 1 or InC6H3-2,6-Trip2. The decrease in the ligand group
13 bond distance upon complexation is common in donors of this
type.1d It is probably due to an increase in Inδ+-Cδ- polarity and
bond strength upon removal of electron density from In through
donor action. The In-C shortening in 2 and 3 contrasts with its
absence upon formation of the dimer 1, a fact which underlines
the weakness of the In-In association in this molecule.
calculations on dimers with alkyl or aryl substituents, see: (a) Allen, T.
L.; Fink, W. H.; Power, P. P. Dalton (2000) 2000, 407. (b) Takagi, N.;
Schmidt, M. N.; Nagase, S. Organometallics 2001, 20, 1646.
(13) Under anaerobic and anhydrous conditions, a solution of LiAr14 (2.5 g,
6.2 mmol) in PhMe (80 mL) was added dropwise to a rapidly stirred
slurry of InCl (0.93 g, 6.2 mmol) in PhMe (20 mL) at -78 °C. The PhMe
mixture was allowed to warm to ca. 25 °C overnight. The precipitates
were allowed to settle over ca. 5 h to afford a dark red solution which
was filtered. The volume was reduced to ca. 20 mL under reduced pressure.
Dark red crystals of (InAr)2, 1, were obtained upon overnight storage in
1
a ca. -20 °C freezer. Yield: 1.15 g, 36.2%. Mp 201-203 °C. H NMR
3
(400 MHz, C6D6, 25 °C): δ 1.10 (d, JHH ) 6.9 Hz, 12H, o-CH(CH3)2),
3
3
1.15 (d, JHH ) 6.9 Hz, 12H, o-CH(CH3)2), 3.03 (sept, JHH ) 6.9 Hz,
4H, CH(CH3)2), 7.14 (m, 6H, m-C6H3 m-Dipp), 7.22 (m, 3H, p-C6H3
p-Dipp). 13C{1H} NMR (C6D6, 100.6 MHz, 25 °C): δ 24.64 (CH(CH3)2),
25.36 (CH(CH3)2), 30.70 (CH(CH3)2), 119.77 (m-Dipp), 127.00 (p-C6H3),
141.12 (i-Dipp), 144.79 (o-C6H3), 147.32 (o-Dipp), 204.02 (i-C6H3),
m-C6H3 p-Dipp obscured by C6D6. UV/vis (hexanes): λmax nm (ꢀ mol
L-1 cm-1): 335 (17 800), 441 (18 070). 2: 1 (0.45 g, 0.44 mmol) and
B(C6F5)3 (0.45 g, 0.88 mmol) were combined in a Schlenk tube. Toluene
(40 mL) was added, and the colorless solution was stirred (ca. 1 h). The
solvent was evaporated, and the residue was redissolved in hexane (60
mL). The volume was reduced to ca. 20 mL, and the precipitate was
redissolved by warming. Cooling to ca. 25 °C over a period of ca. 3 h
afforded the product as colorless crystals. Yield: 0.48 g, 53.3%. Mp 195-
3
197 °C. 1H NMR (400 MHz, C6D6, 25 °C): δ 0.90 (d, JHH ) 6.9 Hz,
12H, o-CH(CH3)2), 1.06 (d, 3JHH ) 6.9 Hz, 12H, o-CH(CH3)2), 2.77 (sept,
3
3JHH ) 6.9 Hz, 4H, CH(CH3)2), 6.97 (d, JHH ) 8.0 Hz, 4H, m-Dipp),
7.10 (d, 2H, m-C6H3), 7.17 (m, 3JHH ) 8.0 Hz, 3H, p-C6H3 and p-Dipp).
13C{1H} NMR (C6D6, 100.6 MHz, 25 °C): δ 23.64 (CH(CH3)2), 25.90
(CH(CH3)2), 31.27 (CH(CH3)2), 124.53 (m-Dipp), 129.43 (p-Dipp), 130.75
3
(p-C6H3), 130.81 (m-C6H3), 136.84 (br, JCF ) 251 Hz, C6F5), 139.33
3
3
(br, JCF ) 251 Hz, C6F5), 139.89 (i-Dipp), 141.83 (br, JCF ) 251 Hz,
3
C6F5), 146.72 (o-C6H3), 146.93 (br, JCF ) 234 Hz, C6F5), 147.85 (o-
Dipp), 149.27 (br, 3JCF ) 234 Hz, C6F5), 170.19 (i-C6H3). 11B NMR (128
MHz, C6D6, 25 °C): δ -14.09. 19F{1H} NMR (376 MHz, C6D6, 25 °C):
3
3
δ -128.13 (d, JFF ) 25.0 Hz, o-C6F5), -154.50 (t, JFF ) 25.0 Hz,
p-C6F5), -159.55 (d, 3JFF ) 25.0 Hz, m-C6F5). 3 was obtained as colorless
crystals from InC6H3-2,6-Trip28a (0.60 g, 1.00 mmol) and B(C6F5)3 (0.51
g, 1.00 mmol). Yield: 0.45 g, 41%. Mp 130-132 °C. 1H (400 MHz,
In summary, the In-In bond in 1 is weak and corresponds to a
bond order less than unity. The compounds 1-3 illustrate the
important effects of flanking ring para-substituents of terphenyl
ligands on the structures of weakly bonded species.
3
3
C7D8, 25 °C): δ 0.94 (d, JHH ) 6.8 Hz, 12H, CH(CH3)2), 1.04 (d, JHH
) 6.8 Hz, 24H, CH(CH3)2), 2.75 (sept, 3JHH ) 6.8 Hz, 2H, p-CH(CH3)2),
3
2.84 (sept, JHH ) 6.8 Hz, 4H, o-CH(CH3)2), 7.02-7.20 (m, aromatic).
13C{1H} NMR (100 MHz, C7D8, 25 °C): δ 22.95 (CH(CH3)2), 23.52
(CH(CH3)2), 25.50 (CH(CH3)2), 30.92 (o-CH(CH3)2), 34.19 (p-CH(CH3)2),
122.26 (m-Trip), 136.27 (br, 3JCF ) 245 Hz, C6F5), 137.22 (i-Trip), 138.73
(br, 3JCF ) 245 Hz, C6F5), 141.22 (br, 3JCF ) 245 Hz, C6F5), 146.28 (br,
3JCF ) 237 Hz, C6F5), 146.69 (p-Trip), 147.07 (o-C6H3), 148.65 (br, 3JCF
) 237 Hz, C6F5), 150.52 (o-Trip), 169.53 (i-C6H3), p-C6H3 and m-C6H3
obscured by solvent. 11B NMR (128 MHz, C7D8, 25 °C): δ -13.67.
19F{1H} NMR (376 MHz, C7D8, 25 °C): δ -126.75 (d, 3JFF ) 19.9 Hz,
Acknowledgment. We are grateful to the National Science
Foundation (CHE-0096913) for financial support and the Albemarle
Corporation for a gift of B(C6F5)3.
Supporting Information Available: X-ray data (PDF and CIF)
for 1-3. This material is available free of charge via the Internet at
http://pubs.acs.org.
3
3
o-C6F5), -154.90 (t, JFF ) 19.9 Hz, p-C6F5), -159.42 (m, JFF ) 19.9
Hz, m-C6F5).
(14) Schiemenz, B.; Power, P. P. Angew. Chem., Int. Ed. Engl. 1996, 35, 2150.
(15) Crystal data for compounds 1, 2, and 3 at 90 K with Mo KR (λ ) 0.71073
Å) radiation: (1) a ) 20.648(4) Å, b ) 15.622(2) Å, c ) 16.294(3) Å,
orthorhombic, space group Pccn, Z ) 4, R1 ) 0.0463 for 6650 (I > 2σ(I))
data, wR2 ) 0.1251 for all (8503) data; (2) a ) 17.2344(9) Å, b )
23.2126(12) Å, c ) 22.2382(12) Å, â ) 102.738(4)°, monoclinic, space
group P21/c, Z ) 4, R1 ) 0.0401 for 20 506 (I > 2σ(I)) data, wR2 )
0.0965 for all (28 170) data; (3) a ) 12.7252(11) Å, b ) 20.684(2) Å, c
) 21.7198(18) Å, â ) 106.729(3)°, monoclinic, space group P21/c, Z )
4, R1 ) 0.0338 for 13 843 (I > 2σ(I)) data, wR2 ) 0.0984 for all (17 154)
data.
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Organometallics 2001, 20, 2854.
JA026285S
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J. AM. CHEM. SOC. VOL. 124, NO. 29, 2002 8539