Attempted isolation of heavier group 14 element ketone analogues: Effect of O-H?π-Ar hydrogen bonding on geometry

Article abstract of DOI:10.1021/om010548u

The terphenyl group 14 element gem-dihydroxy (gem-diol) derivatives Ar₂M(OH)₂ (Ar = C₆H₃-2,6-Mes₂; Mes = C₆H₂-2,4,6-Me₃), M = Ge (1); Sn (2), were synthesized and characterized by X-ray crystallography, NMR, IR spectroscopy, and combustion analysis. The synthetic route involved treatment of the divalent MAr₂ compounds with N₂O or Me₃NO in hydrocarbon solution. The objective was the isolation of the heavier group 14 element ketone analogues Ar₂MO. Despite stringent precautions to exclude moisture and oxygen during the synthesis, the products 1 and 2 were isolated in ca. 30-50% yield. These results are in contrast to the recently reported stabilization of the terphenyl-protected, essentially strain-free, species (bisap)₂GeO (bisap = 2,6-di(1′-napthyl)phenyl). Seemingly,1 and 2 represent the addition of H₂O to Ar₂MO. The identity of the other products is currently unknown. Compound 1 represents the second example of a germanium gem-diol to be structurally characterized, and it features the expected distorted tetrahedral germanium environment. Compound 2 is the first instance of a monomeric gem-dihydroxy derivative of tin. Surprisingly, the C-Sn-C angle is ca. 20° wider than the corresponding angle in 1 even though the larger size of tin is expected to reduce steric congestion and so afford a narrower C-Sn-C angle. This unanticipated result was attributed to the nonclassical hydrogen-bonding interaction of the O-H groups with the mesityl ring substituents, which for geometric reasons is more favorable in the tin compound.

Full text of DOI:10.1021/om010548u

Organometallics 2001, 20, 5105-5109
5105
Attem p ted Isola tion of Hea vier Gr ou p 14 Elem en t
Keton e An a logu es: Effect of O-H‚‚‚π-Ar Hyd r ogen
Bon d in g on Geom etr y
Lihung Pu, Ned J . Hardman, and Philip P. Power*
Department of Chemistry, University of California, Davis, One Shields Avenue,
Davis, California 95616
Received J une 22, 2001
The terphenyl group 14 element gem-dihydroxy (gem-diol) derivatives Ar2M(OH)2 (Ar )
C6H3-2,6-Mes2; Mes ) C6H2-2,4,6-Me3), M ) Ge (1); Sn (2), were synthesized and character-
ized by X-ray crystallography, NMR, IR spectroscopy, and combustion analysis. The synthetic
route involved treatment of the divalent MAr2 compounds with N2O or Me3NO in hydrocarbon
solution. The objective was the isolation of the heavier group 14 element ketone analogues
Ar2MO. Despite stringent precautions to exclude moisture and oxygen during the synthesis,
the products 1 and 2 were isolated in ca. 30-50% yield. These results are in contrast to the
recently reported stabilization of the terphenyl-protected, essentially strain-free, species
(bisap)2GeO (bisap ) 2,6-di(1′-naphthyl)phenyl). Seemingly, 1 and 2 represent the addition
of H2O to Ar2MO. The identity of the other products is currently unknown. Compound 1
represents the second example of a germanium gem-diol to be structurally characterized,
and it features the expected distorted tetrahedral germanium environment. Compound 2 is
the first instance of a monomeric gem-dihydroxy derivative of tin. Surprisingly, the C-Sn-C
angle is ca. 20° wider than the corresponding angle in 1 even though the larger size of tin
is expected to reduce steric congestion and so afford a narrower C-Sn-C angle. This
unanticipated result was attributed to the nonclassical hydrogen-bonding interaction of the
O-H groups with the mesityl ring substituents, which for geometric reasons is more favorable
in the tin compound.
In tr od u ction
GeO (Mes* ) -C6H2-2,4,6-t-Bu3) led to isolation of the
germaindanol.13
Compounds of the formula R2ME (R ) organo group;
M ) Si, Ge, Sn, or Pb; E ) O, S, Se, or Te) are heavier
element analogues of ketones.1 Over the past several
years a number of stable examples of these compounds,
e.g., R2SiE (E ) S2,3 or Se3), R2GeE (E ) S,4-6 Se,4,7 and
Te6,8,9), R2SnE (E ) S6,10 or Se11), have been reported.
They were isolated through the use of large R groups
to protect the >MdE moiety. However, despite the
considerable progress that has been made, there is very
little information available for the lightest chalcogenide
derivatives in which E is oxygen. Attempts to synthesize
R2GeO (germanones) by using the bulky amide precur-
sor Ge{N(SiMe3)2}2 afforded the dimer [{(Me3Si)2N}2-
GeO]2.12 In addition, the attempted generation of Mes*2-
Nonetheless, the use of more crowded aryl ligands
enabled the germanone (Trip)(Tbt)GeO (Trip ) C6H2-
2,4,6-i-Pr3; Tbt ) C6H2-2,4,6-{CH(SiMe3)2}3) to be gen-
erated in solution.14 This compound was obtained by the
reaction of Ge(Trip)(Tbt) with (PhCH2)3NO. It rear-
ranges over a period of hours by -SiMe3 transfer to
oxygen and formation of a Ge-C bond to an ortho
substituent of the Tbt group. Ge(Trip)(Tbt) can also be
trapped by a 2 + 3 cycloaddition reaction with MeCNO.
More recently, the reaction of the ligand-protected,
strain-free, Ge(bisap)2 (bisap ) 2,6-di(1′-naphthyl)-
phenyl) with Me3NO was reported to give the ger-
manone (bisap)2GeO as a white solid, which was char-
acterized by 13C NMR spectroscopy, C, H analysis, and
mass spectrometry.15 Unfortunately, no structural de-
tails of this compound are available to confirm these
(1) Okazaki, R.; Tokitoh, N. Acc. Chem. Res. 2000, 33, 625.
(2) Arya, P.; Boyer, J .; Carre´, F.; Corriu, R.; Lanneau, G.; Lapasset,
J .; Pernot, M.; Priou, C. Angew. Chem., Int. Ed. Engl. 1989, 28, 1010.
(3) Suzuki, H.; Tokitoh, N.; Nagase, S.; Okazaki, R. J . Am. Chem.
Soc. 1994, 116, 11578.
(4) Veith, M.; Becker, S.; Huch, V. Angew. Chem., Int. Ed. Engl.
1989, 28, 1237.
(5) Tokitoh, N.; Matsumoto, T.; Manmaru, K.; Okazaki, R. J . Am.
Chem. Soc. 1993, 115, 8855.
(6) Kuchta, M. C.; Parkin, G. J . Chem. Soc., Chem. Commun. 1994,
1351.
(7) Matsumoto, T.; Tokitoh, N.; Okazaki, R. Angew. Chem., Int. Ed.
Engl. 1994, 33, 2316.
(8) Tokitoh, N.; Manmaru, K.; Okazaki, R. Organometallics 1994,
13, 167.
(9) Matsumoto, T.; Tokitoh, N.; Okazaki, R. J . Am. Chem. Soc. 1999,
121, 8811.
(10) Leung, W.-P.; Kwok, W.-H.; Low, L. T. C.; Zhou, Z. Y.; Mak, T.
C. W. Chem. Commun. 1996, 505.
(11) Tokitoh, N.; Saito, M.; Okazaki, R. J . Am. Chem. Soc. 1993,
115, 2065.
(12) Ellis, D.; Hitchcock, P. B.; Lappert, M. F. Dalton Trans. 1992,
3397.
(13) J utzi, P.; Schmidt, H.; Neumann, B.; Stammler, H.-G. Organ-
ometallics 1996, 15, 741.
(14) Tokitoh, N.; Matsumoto, T.; Okazaki, R. Chem. Lett. 1995, 1087.
10.1021/om010548u CCC: $20.00 © 2001 American Chemical Society
Publication on Web 11/02/2001

5106 Organometallics, Vol. 20, No. 24, 2001
Pu et al.
Ta ble 1. Selected Deta ils of Da ta Collection ,
Red u ction , a n d Refin em en t for 1 a n d 2
findings. The more crowded, and strained, diaryl ger-
manium and tin species M(C6H3-2,6-Mes2)2 (i.e., MAr2;
M ) Ge or Sn), which were reported some time ago,16
are plausible candidates for oxidation reactions with
either Me3NO or N2O. In this paper it is shown that
the reaction of MAr2 (M ) Ge or Sn) with Me3NO or
N2O affords the gem-dihydroxy compounds (2,6-
Mes2H3C6)2M(OH)2 as colorless crystals.
1‚0.33C6H14
2
formula
fw
C
50H56.62GeO2
C48H52SnO2
779.59
762.16
color habit
cryst dimens, mm
cryst syst
space group
a (Å)
colorless block
colorless parallelepiped
0.32 × 0.20 × 0.14 0.20 × 0.16 × 0.12
monoclinic
P21/n
monoclinic
P21/n
11.1655(7)
16.2224(14)
22.8545(15)
96.811(3)°
4110.5(5)
4
1.232
0.784
1.79-31.50
15.623(5)
16.131(5)
16.230(4)
100.44(2)°
4022(2)
4
1.287
0.672
1.66-25.01
4921
b (Å)
c (Å)
Exp er im en ta l Section
â, deg
Gen er a l P r oced u r es. All manipulations were carried out
by using modified Schlenk techniques under an atmosphere
of N2 or in a Vacuum Atmospheres HE-43 drybox. All solvents
were distilled from Na-K allow and degassed immediately
before use. The compounds Ge(C6H3-2,6-Mes2)2 and Sn(C6H3-
2,6-Mes2)2 were synthesized according to literature proce-
dures.16 Me3NO (Aldrich) was purified by sublimation prior
to use. The IR spectrum of Me3NO in a Nujol mull displayed
no observable O-H absorptions. N2O (AGG) was of the highest
available puritys99.998%. 1H, 13C, and 119Sn NMR spectra
were recorded on a Bruker 300 MHz instrument and refer-
enced to the deuterated solvent. Infrared data were recorded
as Nujol mulls on a Perkin-Elmer PE-1430 instrument.
V (Å3)
Z
d
calc (Mg/m3)
µ(mm-1
θ range
)
obsd data (I>2σ(I)) 8482
R1
wR2
0.0531
0.1574
0.0540
0.1270
Meth od 2. A purple solution of Sn{C6H3-2,6-Mes2}2 (1.62
g, 2.18 mmol) was treated with N2O gas (50 mL, ca. 2.23 mmol)
at room temperature. The solution was stirred for 16 h and
became a pale yellow color. Toluene was removed under
reduced pressure, and the residue was extracted with hexane
(60 mL). The solution was decanted from a small amount of
purple residue and was concentrated to incipient crystalliza-
tion storage in a ca. -20 °C freezer for 30 h to gave 2 as
colorless crystals. Yield: 0.90 g, 51%.
Cr ysta llogr a p h ic Stu d ies. Crystals of 1 and 2 were coated
with hydrocarbon oil, mounted on a glass fiber, and placed in
a N2 cold stream on the diffractometer.17 X-ray data for 1 were
collected on a Bruker Smart AXS 1000, and the data for 2 were
obtained on a Siemens R3m/v diffractometer. Data were
acquired with Mo ΚR radiation (λ ) 0.71073 Å) at 90(2) and
130(2) K for compounds 1 and 2, respectively. SHELXTL
programs were used for structure refinement.18 The hydrogens
attached to the oxygen atoms were found on a difference map
and then refined as an idealized OH group. The hydrogen
atoms were allowed to ride on the attached oxygen atom and
rotate around the Ge-O bond. The thermal parameter was
tied to the oxygen and allowed to increase by 20%. The solvent
molecule in 1 synthesized by method 2 could not be fully
modeled and is present at a level of at least 33%.19 An
absorption correction was applied using the program XABS2.20
All compounds were refined to convergence by using aniso-
tropic thermal parameters for all non-hydrogen atoms. Ad-
ditional experimental details for all compounds are given in
Table 1. Selected bond distances and angles are given in Table
2.
(2,6-Mes2H3C6)2Ge(OH)2, (1). A violet solution of Ge{C6H3-
2,6-Mes2}2 (0.72 g, 1.03 mmol) in Et2O (70 mL) was added to
Me3NO (0.076 g, 1.01 mmol) in Et2O (5 mL) at ca. 25 °C with
rapid stirring. The reaction mixture was stirred for 16 h and
became colorless. After filtration through Celite, the colorless
solution was concentrated under reduced pressure to incipient
crystallization (ca. 15 mL) and stored in a ca. -20 °C freezer
for 30 h to give 1 as colorless crystals. Yield: 0.30 g, 33.5%.
Mp: 253-255 °C. 1H NMR (400 MHz, 298 K, C6D6): 1.11(t,
3
6H, (CH3CH2)2O)), J HH ) 7.2 Hz, 1.96 (s, 12H, o-CH3), 2.13
(s, 6H, p-CH3), 2.42 (s, 2H, OH), 3.25 (q, 4H, (CH3CH2)2O),
3
3J HH ) 6.9 Hz, 6.68 (d, 2H, m-C6H3), J HH ) 7.8 Hz 6.76 (s,
4H, m-Mes), 7.01 (tr, 1H, p-C6H3), J HH ) 7.8 Hz. 13C{1H} NMR
(298 K, C6D6): 21.00 (p-CH3), 22.42 (o-CH3), 48.20 ((CH3-
CH2)2O), 65.87 ((CH3CH2)2O), 129.03 (m-Mes), 129.82 (p-C6H3),
130.66 (m-C6H3), 137.14 (p-Mes), 137.82 (o-Mes), 140.43 (i-
Mes), 148.37 (o-C6H3). IR (Nujol, cm-1): (O-H) 3560 (s).
Meth od 2. A violet solution of Ge{C6H3-2,6-Mes2}2 (1.35 g,
1.93 mmol) in toluene (30 mL) was treated with N2O gas (45
mL, ca. 2.0 mmol) at room temperature. The solution was
stirred for 16 h, during which time it became almost colorless.
The solvent toluene was removed under reduced pressure, and
the residue was extracted with warm hexane (50 mL). The
colorless solution was decanted to separate a small quantity
of starting violet solid. The volume of solution was reduced to
incipient crystallization and stored in a ca. -20 °C freezer for
30 h to give 1 as colorless crystals. Yield: 0.70 g, 49%.
Resu lts a n d Discu ssion
(2,6-Mes2H3C6)2Sn (OH)2, (2). Compound 2 was synthesized
in a manner similar to 1 by adding Sn{C6H3-2,6-Mes2}2 (0.91
g, 1.21 mmol) in Et2O (60 mL) to Me3NO (0.091 g, 1.21 mmol)
in Et2O (20 mL) at ca. 25 °C with rapid stirring. A similar
workup procedure afforded 2 as colorless crystals. Yield: 0 35
g, 37.2%. Mp: 243-245 °C. 1H NMR (298 K, C6D6): 1.13 (t,
Syn th esis. The reaction of MAr2 (M ) Ge or Sn)16
with Me3NO or N2O in accordance with the equation
(17) Hope, H. Prog. Inorg. Chem. 1995, 41, 1.
(18) SHELXTL version 5.03; Bruker AXS: Madison, WI, 1998.
(19) At the request of a reviewer, the SQUEEZE subroutine in
PLATON19a was used to check the solvent occupancy. The solvent void
is large enough for a whole hexane molecule (233 Å3), and the number
of electrons occupying the void is 34 (ca. two-thirds of a hexane
molecule). However, the subroutine also yielded over 30 difference map
peaks ranging from 0.50 to 3.67 e/Å3 in the void. Refinement of the
structure showed that assignment of the major residual electron
density peaks corresponding to an occupancy of ca. one-third (33%)
decreased R1, but resulted in no differences in the geometry of the
germanium bishydroxide. Therefore, the occupancy of the hexane
molecule in the structure is given as 33%, but is probably closer to
66%. Spek, A. L. Acta Crystallogr. 1990, A46, C34.
3
6H, (CH3CH2)2O)), J HH ) 7.2 Hz, 1.95 (s, 12H, o-CH3), 2.13
(s, 6H, p-CH3), 2.495 (s, 2H, OH), 3.25 (q, 4H, (CH3CH2)2O),
3
3J HH ) 6.9 Hz, 6.78 (d, 2H, m-C6H3), J HH ) 7.8 Hz 6.75 (s,
4H, m-Mes), 7.02 (tr, 1H, p-C6H3), 3J HH ) 7.2 Hz. 13C{1H} NMR
(CDCl3): 21.26 (p-CH3), 21.63 (o-CH3), 128.49 (m-Mes), 129.12
(p-C6H3), 130.00 (m-C6H3), 137.21(p-Mes), 137.44 (o-Mes),
140.10 (i-Mes), 147.75 (o-C6H3). IR (Nujol, cm-1): 3600 (s), 3580
(s). 119Sn{1H} (C6D6): -38.1.
(15) Wegner, G. L.; Berger, R. J . F.; Schier, A.; Schmidbaur, H.
Organometallics 2001, 20, 418.
(16) Simons, S. R.; Pu, L.; Olmstead, M. M.; Power, P. P. Organo-
metallics 1997, 16, 1920.
(20) SADABS is an empirical absorption correction program that
is part of the SAINT Plus NT version 5.0 package: Bruker AXS:
Madison, WI, 1998.

gem-Dihydroxy Derivatives of Ge and Sn
Organometallics, Vol. 20, No. 24, 2001 5107
Ta ble 2. Selected Bon d Len gth s (Å) a n d An gles
(d eg) for 1 a n d 2
with N2O in hexane or toluene, afforded the µ-oxo dimer
{LGa(µ-O)}2 in virtually quantitative yield.27 No hy-
droxide product was detected. In this case, the Dipp2-
nacnac ligand is apparently of insufficient size to
prevent dimerization of the reactive LGaO intermediate.
This result is similar to that already obtained in the
case of {(Me3Si)2N}2GeO, which associates to afford the
dimer [{(Me3Si)2N}2GeO]2.12 In contrast, it seems prob-
able that the putative Ar2MO intermediate would be
prevented from dimerizing by the large size of the Ar
ligands. In addition, Ar2MO is expected to be a very
reactive species as a result of the low coordination
number of germanium and the polar character of the
polar resonance form b.
parameter
1 (M ) Ge)
2 (M ) Sn)
M-O(1)
1.802(2)
1.782(2)
1.978(2)
1.976(2)
104.19(9)
105.31(8)
112.05(9)
110.95(8)
101.08(9)
122.3(1)
1.974(4)
1.977(4)
2.164(5)
2.176(5)
89.7(2)
102.4(2)
113.6(2)
110.9(2)
94.7(2)
M-O(2)
M-C(1)
M-C(25)
O(1)-M-C(1)
O(1)-M-O(2)
O(1)-M-C(25)
O(2)-M-C(1)
O(2)-M-C(25)
C(1)-M-C(25)
141.1(2)
:MAr2 +N O, +Me NO8 Ar2MO?
2
3
-N2, -NMe3
Ar ) -C6H3-2,6-Mes2; M ) Ge or Sn
did not afford the intended product Ar2MO. Instead, the
compounds Ar2M(OH)2 (M ) Ge, 1; Sn, 2) were obtained
in ca. 30-50% yield, with higher yields being obtained
for the reaction with N2O. The compounds are very rare
examples of monomeric, gem-dihydroxy (gem-diol) de-
rivatives of germanium or tin. For germanium, there is
only one structurally characterized example of such a
species: the monomer t-Bu2Ge(OH)2.21,22 A handful of
gem-dihydroxy tin derivatives have been characterized,
but these exist as associated species in which the -OH
groups bridge the tin centers, and form aggregates that
range from dimeric to polymeric.23-26 The generation
of the gem-dihydroxy products instead of the Ar2MO
compounds is most easily accounted for by the genera-
tion of an Ar2MO intermediate in the first instance. This
may then react with adventitious moisture in ac-
cordance with
The three-coordinate M+ center is expected to be a
powerful Lewis acid and the O- end of the dipole should
be a powerful nucleophile. As a result, it is possible that,
instead of dimerizing, the polar R2M+-O- species reacts
rapidly with a further equivalent of N2O or Me3NO to
generate a metal bound to two oxygens, and it is also
possible that these oxygens attack the solvent or the
ligand to generate the hydroxide moiety. It is notable
that the hydroxy isomer of H2GeO, i.e., HGeOH, has
been calculated to be 18 kcal mol-1 more stable than
the germanone H2GeO,28 which suggests a preference
for the Ge-OH moiety under certain circumstances. The
Ar2MO might also react in a manner similar to Mes*2-
GeO13 (see Introduction) to generate a cyclic product
involving a ligand mesityl group, although such a
product has not yet been isolated. The contrasting
behavior of the strain-free Ge(bisap)215 and the strained
Mes*2GeO13 and “Ar2GeO” species is therefore quite
remarkable and cannot be fully explained with currently
available data. It has been suggested that geometric
strain within these molecules could account for the
reactivity of the group 14 element center.15 Possibly, the
Ar2MO +H O8 Ar2M(OH)2
2
However, stringent measures were taken to exclude
moisture contamination of the reaction system and to
ensure reagent purity. For example, no contamination
of the SnAr2 solution was observed by 119Sn NMR
spectroscopy prior to its reaction with Me3NO or N2O.
The latter reagents were added in stoichiometric, or
slightly less than stoichiometric, amounts. The additon
of Me3NO as a solid or solution produced identical
results in each case. The reactions with N2O were
performed without adding further quantities of solvent
to the MAr2 solution. Therefore, it is doubtful that
sufficient new moisture could have been introduced to
produce the observed yields of 1 and 2 by a stoichio-
metric process. Furthermore, simultaneous experiments
under almost identical conditions, involving the reaction
of the electronically related and highly reactive species
GaL (L ) {N(C6H3-2,6-i-Pr2)CMe}CH, i.e., Dipp2nacnac)
16
wider C-Ge-C angle in the strained GeAr2 species,
114.2(2)°, cf. 102.72(9)° in Ge(bisap)2,15 results in a
smaller HOMO (lone pair)-LUMO (4p) energy gap,
which could account for increased reactivity. The exist-
ence of a smaller HOMO-LUMO gap in GeAr2 receives
support from the relatively high wavelength (528 nm)16
of the n-p absorption in its electronic spectrum. It
should be borne in mind, however, that the mass
spectrum of (bisap)2GeO, on which its monomeric for-
mulation is based, provides information only for the gas-
phase species. No data have been presented on the
degree of aggregation in solution. Ideally, molecular
weight data, and suitable ligand modification in order
to produce X-ray quality crystals, will allow important
structural parameters of the germanone to be deter-
mined.
(21) Puff, H.; Franken, S.; Schuh, W.; Schwab, W. J . Organomet.
Chem. 1983, 254, 33.
(22) Beckmann, J .; J urkschat, K.; Schu¨rmann, M. Eur. J . Inorg.
Chem. 2000, 939.
(23) Glidewell, C.; Liles, D. C. Acta Crystallogr. Sect. B 1978, 34,
129.
(24) J urkschat, K.; Kruger, C.; Meunier-Piret, J . Main Gp. Met.
Chem. 1992, 15, 61.
Str u ctu r es. The structures of 1 and 2 consist of well-
separated monomers with no short intermolecular con-
(25) Natsume, T.; Aizawa, S.; Hatano, K.; Funahashi, S. J . Chem.
Soc., Dalton Trans. 1994, 2749.
(26) Zobel, B.; Schurmann, M.; J urkschat, K.; Daktermieks, D.;
Duthie, A. Organometallics 1998, 17, 4096.
(27) Hardman, N. J .; Power, P. P. Inorg. Chem. 2001, 40, 2474.
(28) Trinquier, G.; Pellissier, M.; Saint-Roch, B.; Lavayssiere, H. J .
Organomet. Chem. 1981, 214, 169.

5108 Organometallics, Vol. 20, No. 24, 2001
Pu et al.
Ta ble 3. P a r a m eter s d (M) (Å) a n d ω (d eg) for th e
O-H‚‚‚π-a r yl In ter a ction s in 1 a n d 2
1
2
d(M) (Å)
ω (deg)
2.484, 2.374
32.5, 24.9
2.094, 2.258
7.8, 20.6
Å). The Ge-O and Sn-O distances, however, are
significantly shorter than the 1.88 and 2.06 Å predicted
by the sum of the covalent radii of germanium or tin
and oxygen (0.66 Å).30 This is probably a result of the
increased ionic character in the M-O bonds. The O(1)-
E-O(2) angles in compounds 1 (105.31(8)°) and 2 (102.5-
(2)°) differ by only 2.8° and are similar to the O-Ge-O
angle observed in t-Bu2Ge(OH)2 (102.4(1)°).22 In sharp
contrast, the C(1)-M-C(25) angles in 1 (122.26(10)°)
and 2 (141.1(2)°) differ by almost 20°. The angle in 1 is
very similar to the C-Ge-C angle (122.5(3)°) observed
in t-Bu2Ge(OH)2. The anomalously large C-Sn-C angle
in 2 is the opposite of what is expected on steric grounds
since a narrower angle could be anticipated on the basis
of the larger size of tin. The most plausible explanation
for the wide C-Sn-C angle lies in the interactions
between the O-H groups and ortho mesityl rings. These
O-H‚‚‚aryl interactions have been previously observed
in heavier group 14 element aryl hydroxy compounds,
and hydrogen bonding of this general type has been
reviewed.31
F igu r e 1. Thermal ellipsoid (30%) plot of 1. Selected bond
distances and angles are given in Table 2.
The classical hydrogen bond is generally thought of
as a three-center four-electron bond such as O-H‚‚‚O.
However, there are nonclassical hydrogen bonds that
can be described in terms of X-H interactions with
the π-electron cloud of various aryl rings. A typical
O-H‚‚‚aryl ring bond energy has been calculated to be
between -2 and -4 kcal/mol, and the strength of the
interaction depends on how close the O-H hydrogen
atom approaches the center of the phenyl ring (d(M))
and the angle (ω) between a line drawn to the center of
the phenyl ring and the C6 axis of the phenyl ring. The
strongest interactions occur with short H approaches
(d(M)) and when the X-H moiety is perpendicular to
the plane of the aryl ring (i.e., ω ) 0°). An illustration
of this interaction is provided in Figure 3, and a listing
of the values of the parameters for 1 and 2 is given in
Table 3. From these data it is clear that such interac-
tions are more favored in the tin compound 2 than in
its germanium analogue 1 and is probable that they
cause the very distorted geometry observed in 2. It
seems that the longer E-O and E-C distances in 2 as
well as the more ionic and less directional nature of
binding to electropositive tin allow the molecule to
maximize the O-H‚‚‚Ar π interactions by widening the
C(1)-M-C(25) angle. In the germanium species 1 a
weaker O-H‚‚‚Mes interaction is observed owing to the
inability of the mesityl rings to assume the optimum
geometry. This occurs as a result of the lower flexibility
of the germanium coordination angles and the shorter
distances to the oxygens and ipso carbons.
F igu r e 2. Thermal ellipsoid (30%) plot of 2. Selected bond
distances and angles are given in Table 2.
F igu r e 3. Schematic drawing of the interaction of an X-H
group with an aryl ring as defined by the parameters d(M)
(Å) and ω (deg).
tacts. The germanium and tin atoms are coordinated
in a distorted tetrahedral manner. The Ge-C distances
in 1 (Table 2) are close to the sum of the radii29 of
germanium (1.22 Å) and carbon (0.77 Å) and to the
average Ge-C distances observed in t-Bu2Ge(OH)2.22
Likewise, the Sn-C bond lengths in 2 are also close to
the sum of the covalent radii of Sn (1.4 Å) and C (0.77
The d(M) distances in 1 and 2 are at the lower end of
the distance scale.32 The shortness of the interaction
(30) Ref 28, p 498.
(31) (a) Desiraju, G. R.; Steiner, T. The Weak Hydrogen Bond; OUP,
Oxford, 1999; Chapter 3, p 122. (b) Nishio, M.; Hirota, M.; Umezawa,
Y. The CH/ π Interaction: Evidence, Nature and Consequences; Wiley-
VCH: New York, 1998. (Deals with CH/π interactions only.)
(32) Braga, D.; Grepioni, F.; Tedesco, E. Organometallics 1998, 17,
2669.
(29) Wells, A. F. Structural Inorganic Chemistry, 5th ed.; Claren-
don: Oxford, 1984; pp 913, 1280.

gem-Dihydroxy Derivatives of Ge and Sn
Organometallics, Vol. 20, No. 24, 2001 5109
may stem from the acidity of the O-H bonds, which
afford protic character to the hydrogen. This increases
the strength of H‚‚‚π-aryl interaction, which gives rise
to the short distances observed. Calculations on model
compounds to investigate the strength of these interac-
tions are in hand.
be completely discounted. The use of 18O-labeled oxi-
dants may narrow the range of possible mechanisms in
future work on this system.
Ack n ow led gm en t. We are grateful to the National
Science Foundation for financial support.
Su p p or tin g In for m a tion Ava ila ble: Tables giving full
details of the crystallographic data and data collection param-
eters, atom coordinates, bond distances, bond angles, aniso-
tropic thermal parameters, and hydrogen coordinates for 1 and
2. This material is available free of charge via the Internet at
http://pubs.acs.org.
Con clu sion s
The reaction of N2O or Me3NO with :MAr2 (M ) Ge
or Sn; Ar ) C6H3-2,6-Mes2) produced the gem-dihydrox-
ides Ar2M(OH)2. Although stringent precautions were
taken to exclude moisture during the synthesis, reaction
of the putative intermediate Ar2MO with H2O cannot
OM010548U