3,3′-Bis(triphenylsilyl)biphenoxide as a sterically hindered ligand on Fe(II), Fe(III), and Cr(II)

Article abstract of DOI:10.1021/ic010909y

The structural coordination chemistry of the sterically hindered 3,3′-bis(triphenylsilyl)-1,1′-bi-2-phenoxide ligand ([^TPSLO₂]^2-), an isosteric homologue of a chiral binaphthoxide ligand used in asymmetric induction, has been investigated on Fe(II), Fe(III), and Cr(II). The ligand diol (^TPSL(OH)₂, 2) can be prepared on a multigram scale from 2,2′-dimethoxybiphenyl via a convenient, two-step ortho-metalation/silylation/deprotection sequence; the sodium salt of the ligand dianion (Na₂[^TPSLO₂], 3) can be obtained by NaH deprotonation and was crystallographically characterized with two THF ligands bound to each cation. Protonolysis of Fe[N(SiMe₃)₂]₂ or Cr[N(SiMe₃)₂]₂(THF)₂ with biphenol 2 in arene solvent gives [Fe(μ-^TPSLO₂)]₂ (4) or (^TPsLO₂)Cr(THF)₂ (9), respectively, while anion metathesis of FeCl₂ or FeCl₃/bipy with biphenoxide salt 3 in THF yields (^TPSLO₂)Fe(THF)₂ (8) or ferric monomer (^TPsLO₂)FeCl-(bipy) (10), respectively. Dimer 4 reacts with exogenous ligands to form monomeric ligand adducts of Fe(II): with py, (^TPSLO₂)Fe(py)₂ (5); with bipy, (^TPSLO₂)Fe(bipy) (6); with XyNC (Xy = 2,6-xylyl), (^TPSLO₂)Fe(CNXy)₄ (7); and with THF, 8. Complexes were characterized in solution by ¹H NMR and in the solid state by single-crystal X-ray diffraction. The 4-coordinate complexes (5, 6, 8, 9) adopt skew-distorted tetrahedral (for Fe(II)) or square planar (for Cr(II)) geometries; the 5- and 6-coordinate complexes (10 and 7) assume more typical distorted square pyramidal/trigonal bipyramidal and cis-octahedral stereochemistries, Dimer 4 possesses an unusual structure where each Fe(II) center is pseudo-4-coordinate and each biphenoxide ligand provides one terminal phenoxide donor, one bridging phenoxide, and a weak aromatic π-interaction from one of the phenyl groups of a SiPh₃ substituent. The steric influence of the hindered biphenoxide ligand within the coordination sphere is revealed structurally through distortion of coordination polyhedra in the 4-coordinate species and through conformational and deformational changes within the biphenoxide ligand itself.

Full text of DOI:10.1021/ic010909y

Inorg. Chem. 2002, 41, 321−330
3,3′-Bis(triphenylsilyl)biphenoxide as a Sterically Hindered Ligand on
Fe(II), Fe(III), and Cr(II)
Ajay Kayal and Sonny C. Lee*
Department of Chemistry, Princeton UniVersity, Princeton, New Jersey 08544
Received August 16, 2001
The structural coordination chemistry of the sterically hindered 3,3′-bis(triphenylsilyl)-1,1′-bi-2-phenoxide ligand
([TPSLO ]2-), an isosteric homologue of a chiral binaphthoxide ligand used in asymmetric induction, has been
2
investigated on Fe(II), Fe(III), and Cr(II). The ligand diol (TPSL(OH)2, 2) can be prepared on a multigram scale from
2,2′-dimethoxybiphenyl via a convenient, two-step ortho-metalation/silylation/deprotection sequence; the sodium
salt of the ligand dianion (Na2[TPSLO ], 3) can be obtained by NaH deprotonation and was crystallographically
2
characterized with two THF ligands bound to each cation. Protonolysis of Fe[N(SiMe3)2]2 or Cr[N(SiMe3)2]2(THF)2
with biphenol 2 in arene solvent gives [Fe(µ-TPSLO )]2 (4) or (TPSLO )Cr(THF)2 (9), respectively, while anion metathesis
2
2
of FeCl2 or FeCl3/bipy with biphenoxide salt 3 in THF yields (TPSLO )Fe(THF)2 (8) or ferric monomer (TPSLO )FeCl-
2
2
(bipy) (10), respectively. Dimer 4 reacts with exogenous ligands to form monomeric ligand adducts of Fe(II): with
py, (TPSLO )Fe(py)2 (5); with bipy, (TPSLO )Fe(bipy) (6); with XyNC (Xy ) 2,6-xylyl), (TPSLO )Fe(CNXy)4 (7); and
2
2
2
with THF, 8. Complexes were characterized in solution by 1H NMR and in the solid state by single-crystal X-ray
diffraction. The 4-coordinate complexes (5, 6, 8, 9) adopt skew-distorted tetrahedral (for Fe(II)) or square planar
(for Cr(II)) geometries; the 5- and 6-coordinate complexes (10 and 7) assume more typical distorted square pyramidal/
trigonal bipyramidal and cis-octahedral stereochemistries. Dimer 4 possesses an unusual structure where each
Fe(II) center is pseudo-4-coordinate and each biphenoxide ligand provides one terminal phenoxide donor, one
bridging phenoxide, and a weak aromatic π-interaction from one of the phenyl groups of a SiPh3 substituent. The
steric influence of the hindered biphenoxide ligand within the coordination sphere is revealed structurally through
distortion of coordination polyhedra in the 4-coordinate species and through conformational and deformational
changes within the biphenoxide ligand itself.
Introduction
metal coordination sphere; substituents of high steric demand
should promote stereochemically rigid environments.
Biaryloxides with very hindered 3,3′-substituents are
therefore intriguing as potential geometry constraining
ligands. Of the various substituents reported to date, the
triphenylsilyl group shows promise in this regard based on
chemistry developed from the 3,3′-bis(triphenylsilyl)-1,1′-
bi-2-naphthoxide ligand. This biaryloxide derivative has
served as a chiral auxiliary on Al(III) to mediate enantio-
selective radical cyclizations5 and catalyze asymmetric
Diels-Alder,6 ene,7 and Claisen reactions;8 the latter trans-
formations in particular give excellent diastereo- and enan-
tioselectivities under appropriate conditions.6-8 For d- and
1,1′-Bi-2-aryloxides are useful ancillary ligands for the
control of metal-based reactivity.1-3 As a bidentate, dianionic
chelate, the biaryloxy scaffold offers two intrinsic features
that can be exploited to control metal geometry. First, the
requisite torsion of the aryl-aryl linkage forces a distinctly
nonplanar 7-membered metallacycle upon coordination; this
geometry creates a pronounced C2 chiral axis of proven value
for asymmetric induction.1 Second, the ring positions (3,3′)
adjacent to the coordinating oxygen atoms allow for facile
incorporation4 of forward-directed substituents to shape the
* Author to whom correspondence should be addressed. E-mail: sclee@
princeton.edu.
(1) Pu, L. Chem. ReV. 1998, 98, 2405 and references therein.
(2) Eilerts, N. W.; Heppert, J. A. Polyhedron 1995, 14, 3255.
(3) Kayal, A.; Ducruet, A. F.; Lee, S. C. Inorg. Chem. 2000, 39, 3696
and references therein.
(4) (a) Gschwend, H. W.; Rodriguez, H. R. Org. React. (N.Y.) 1979, 26,
1. (b) Beak, P.; Snieckus, V. Acc. Chem. Res. 1982, 15, 306. (c)
Snieckus, V. Chem. ReV. 1990, 90, 879.
(5) Nishida, M.; Hayashi, H.; Nishida, A.; Kawahara, N. Chem. Commun.
1996, 579.
10.1021/ic010909y CCC: $22.00 © 2002 American Chemical Society
Published on Web 01/03/2002
Inorganic Chemistry, Vol. 41, No. 2, 2002 321

Kayal and Lee
Scheme 1
convenience, easily affording >10 g quantities of pure
product at both steps through simple recrystallizations. The
ligand dianion can be obtained as the disodium salt 3 by
treatment of biphenol 2 with excess NaH in THF (see
Scheme 2).
Coordination Chemistry and Solution Characteriza-
tion. The metal chemistry of the [TPSLO2]2- ligand presented
herein is summarized in Scheme 2. Ligand substitution
reactions involving complexes of the hindered biphenoxide
are marked by conspicuous color changes, as noted in the
scheme.
f-elements, complexes with this ancillary ligand are known
for La(III),9 Ti(IV),10 Zr(IV),10 and Ta(V).11 These high-
valent, early metal complexes show well-defined solution
chemistry, resist ligand disproportionation, and, for the
titanium and zirconium species,10 catalyze stereoselective
olefin polymerization and alkyne trimerization. All previous
studies with this chelate have emphasized metal-centered
reaction chemistry but, with one exception,11 have left its
structural chemistry largely unaddressed. To better define
the influence of this hindered ligand type on metal geometry,
we present here a comparative structural investigation of the
coordination chemistry of the biphenoxide homologue, 3,3′-
bis(triphenylsilyl)-1,1′-bi-2-phenoxide ([TPSLO2]2-), as a
ligand on Fe(II), Fe(III), and Cr(II).
16
The protonolysis of Fe[N(SiMe3)2]2 with 1 equiv of
ligand diol 2 affords the chelated ferrous dimer [Fe(µ-
TPSLO2)]2 (4) in good yield. Dimer 4 reacts smoothly with
exogenous ligands and serves as a practical entry point to
the coordination chemistry of the monomeric [(TPSLO2)Fe]
fragment. Thus, treatment of complex 4 with pyridine, 2,2′-
bipyridyl, and 2,6-dimethylphenylisonitrile (XyNC) generates
the corresponding adducts (TPSLO2)Fe(py)2 (5), (TPSLO2)Fe-
(bipy) (6), and (TPSLO2)Fe(CNXy)4 (7), respectively, while
dissolution of 4 in THF leads to the immediate formation of
the solvent adduct (TPSLO2)Fe(THF)2 (8). Complex 8 is also
available directly by treatment of FeCl2 with 1 equiv of the
biphenoxide salt 3 in THF. The bound THF ligands in
complex 8 are labile, and dissolution in poorly coordinating
solvents such as benzene or CH2Cl2 leads to the immediate
formation of dimer 4; this behavior occurs to a lesser extent
for the bis(pyridine) adduct 5, which slowly (over hours)
converts to 4 in the same solvents unless excess pyridine is
present.
All of the ferrous complexes (except THF-labile 8) present
well-defined 1H NMR spectra in CD2Cl2 or C6D6 (see
Experimental Section for NMR data), with solution molecular
symmetries that fit the solid-state structures identified by
X-ray diffraction. The spectra17 of the 4-coordinate species
(5, 6) and the dimer (4) are marked by broad signal envelopes
from the triphenylsilyl group and by a highly dispersed set
(ca. 100 ppm) of ligand biphenyl backbone resonances
consistent with the presence of high-spin Fe(II) centers;18
the spectroscopic data for pyridine and bipyridyl coligands
show similar dispersions but with one resonance missing in
each, probably from broadening of the o-H signals. In
contrast, isonitrile complex 7, which is low-spin due to its
strong-field ligand environment, displays sharp, well-resolved
signals within the diamagnetic chemical shift range for all
ligand protons.
Results
Ligand Synthesis. The 3,3′-bis(triphenylsilyl) derivatives
of biphenol and binaphthol can be obtained in good yields
by a multistep protocol where the o-silyl substituents are
incorporated via 1,3-rearrangements of metalated silyl ethers.12
We have found that direct ortho-metalation and silylation
of commercial 2,2′-dimethoxybiphenyl,13 followed by BBr3
deprotection of the resultant hindered bianisole 1, provide a
simpler alternative route to the desired 3,3′-bis(triphenylsilyl)-
1,1′-bi-2-phenol (TPSL(OH)2, 2) (Scheme 1). Although yields
are limited14 by incomplete silylation at the first stage and
competing ipso-borodesilylation15 chemistry during depro-
tection, our present synthesis has the virtue of experimental
(6) (a) Maruoka, K.; Itoh, T.; Shirasaka, T.; Yamamoto, H. J. Am. Chem.
Soc. 1988, 110, 310. (b) Maruoka, K.; Concepcion, A. B.; Yamamoto,
H. Bull. Chem. Soc. Jpn. 1992, 65, 3501. (c) Bao, J.; Wulff, W. D.;
Rheingold, A. L. J. Am. Chem. Soc. 1993, 115, 3814.
(7) Maruoka, K.; Hoshino, Y.; Shirasaka, T.; Yamamoto, H. Tetrahedron
Lett. 1988, 29, 3967.
(8) Maruoka, K.; Banno, H.; Yamamoto, H. J. Am. Chem. Soc. 1990,
112, 7791.
(9) Schaverien, C. J.; Meijboom, N.; Orpen, A. G. J. Chem. Soc., Chem.
Commun. 1992, 124.
(10) van der Linden, A.; Schaverien, C. J.; Meijboom, N.; Ganter, C.;
Orpen, A. G. J. Am. Chem. Soc. 1995, 117, 3008.
(11) Thorn, M. G.; Moses, J. E.; Fanwick, P. E.; Rothwell, I. P. J. Chem.
Soc., Dalton Trans. 2000, 2659.
(12) (a) Maruoka, K.; Itoh, T.; Araki, Y.; Shirasaka, T.; Yamamoto, H.
Bull. Chem. Soc. Jpn. 1988, 61, 2975. (b) Buisman, G. J. H.; van der
Veen, L.; Klootwijk, A.; de Lange, W. G. J.; Kamer, P. C. J.; van
Leeuwen, P. W. N. M.; Vogt, D. Organometallics 1997, 16, 2929.
(13) A similar route involving the bis(methoxymethyl) ether of binaphthol
has been reported: Cox, P. J.; Wang, W.; Snieckus, V. Tetrahedron
Lett. 1992, 33, 2253.
In order to increase the structural diversity, biphenoxide
complexes of Cr(II) and Fe(III) were also synthesized.
19
Protonolysis of Cr[N(SiMe3)2]2(THF)2 with 1 equiv of
biphenol 2 yields (TPSLO2)Cr(THF)2 (9), the chromous
(16) (a) Anderson, R. A.; Faegri, K., Jr.; Green, J. C.; Haaland, A.; Lappert,
M. F.; Leung, W.-P.; Rydpal, K. Inorg. Chem. 1988, 27, 1782. (b)
Olmstead, M. M.; Power, P. P.; Shoner, S. C. Inorg. Chem. 1991, 30,
2547.
(14) The undesired side reactions at both steps could probably be suppressed
and conversions thereby improved through the use of a more potent
directed metalation group.4,13
(15) (a) Sharp, M. J.; Cheng, W.; Snieckus, V. Tetrahedron Lett. 1987,
28, 5093. (b) Gross, U.; Kaufmann, D. Chem. Ber. 1987, 120, 901.
(c) Kaufmann, D. Chem. Ber. 1987, 120, 853.
(17) Probable assignments for the paramagnetically shifted resonances were
made by comparison of integral ratios and by correlations in chemical
shifts for signals from related complexes 5 and 6.
(18) Swift, T. J. The Paramagnetic Line Width. In NMR of Paramagnetic
Molecules; La Mar, G. N., Horrocks, W. D., Jr., Holm, R. H., Eds.;
Academic Press: New York, 1973.
322 Inorganic Chemistry, Vol. 41, No. 2, 2002

3,3′-Bis(triphenylsilyl)biphenoxide
Scheme 2
Table 1. Crystallographic Dataa for TPSL(OH)2 (2), [Na(THF)2]2(TPSLO2)‚2THF ([3](THF)4‚2THF), [Fe(µ-TPSLO2)]2‚3C6H6 (4‚3C6H6), (TPSLO2)Fe(bipy)
(6), (TPSLO2)Fe(CNXy)4‚(CH2Cl2/Et2O) (7‚(CH2Cl2/Et2O)), (TPSLO2)M(THF)2 (M ) Fe/Cr: 8/9), (TPSLO2)FeCl(bipy)‚Et2O‚(THF/Et2O)
(10‚Et2O‚(THF/Et2O))
[3](THF)4‚
2THF
7‚(CH2Cl2/
Et2O)
2
4‚3C6H6
6
8
9
10‚Et2O ‚THF
formula
C48H38O2Si2 C72H84Na2-
O6Si2
C114H90Fe2-
O4Si4
1747.92
C58H44FeN2-
O2Si2
912.98
(C84H72FeN4-
O2Si2)b
C56H52Fe-
O4Si2
901.01
C60H60Cl8Cr-
O4Si2
1236.86
C66H62ClFeN2-
O4Si2
1094.66
fw
702.96
1179.55
(1281.49)b
space group
Z
P1h (No. 2)
2
P21/n (No. 14) P21/n (No. 14) C2/c (No. 15) P21/c (No. 14) P21/n (No. 14) P21/c (No. 14) P21/n (No. 14)
4
4
2
4
4
4
4
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
temp, K
10.8073(6)
11.3587(7)
18.596(1)
75.545(2)
73.936(3)
62.598(2)
1927.4(2)
200(2)
12.4304(5)
21.3410(6)
24.439(1)
14.3710(5)
14.6800(4)
44.681(2)
13.9516(7)
15.2681(6)
21.253(1)
16.7731(6)
14.4705(3)
33.678(1)
13.8422(6)
14.6838(7)
22.3321(6)
21.783(3)
14.335(3)
20.126(3)
12.8214(6)
29.735(2)
15.7349(9)
93.278(1)
92.670(1)
96.216(2)
99.991(1)
92.939(2)
109.541(7)
99.158(3)
6472.5(4)
200(2)
22.53
1.21
0.120
99.7
9416.0(6)
200(2)
23.30
1.23
0.410
98.0
4500.6(4)
200(2)
27.53
1.35
0.436
99.5
8050.1(4)
200(2)
22.46
4533.2(3)
130(2)
25.00
1.32
0.434
99.8
5922.6(2)
200(2)
22.47
1.39
0.640
98.5
5922.3(6)
200(2)
22.48
1.29
0.390
95.2
θ
max, deg
27.35
F
calcd, g‚cm-3 1.21
(1.06)b
(0.262)b
99.9
µ, mm-1
0.131
completeness 97.3
of data, %
R (wR2),c %
5.89 (11.37) 8.55 (24.01)
1.012 1.059
5.53 (13.93)
1.019
5.22 (10.56)
1.042
5.23 (12.36)
1.059
5.12 (11.05)
1.020
8.44 (18.67)
1.021
4.97 (11.54)
1.054
Sd
a Data collected with graphite-monochromatized Mo KR radiation (λ ) 0.71703 Å) using ω scans. b Calculated with exclusion of unresolved, compositionally
2
disordered lattice solvent. c Calculated for I > 2σ(I): R ) ∑||Fo| - |Fc||/∑|Fo|, wR2 ) {∑[w(Fo2 - Fc )2]/∑[w(Fo )2]}1/2
.
d S ) goodness of fit ) [∑w(Fo
2
2
-Fc )2/(n - p)]1/2, where n is the number of reflections and p is the number of parameters refined.
2
homologue to ferrous-THF complex 8. The chromous
complex is more sensitive to oxidation than its ferrous
counterpart, but much less prone to decomposition by THF
loss; the synthesis of 9, in fact, is conducted in toluene with
only stoichiometric amounts of THF supplied by the chro-
mous amide starting material, and the product can be
recrystallized without ligand loss from arene and CH2Cl2
solvents. The reaction of FeCl3 with 1 equiv of biphenoxide
salt 3 results in an immediate color change indicative of
complex formation, although isolation attempts at this stage
have given only uncharacterized powder. Addition of 1 equiv
of 2,2′-bipyridyl to this solution, however, allows isolation
of crystalline (TPSLO2)FeCl(bipy) (10) in limited yield. The
1H NMR spectra of complexes 9 and 10 consist of very broad
and uninformative resonances that are characteristic of high-
spin Cr(II) and Fe(III).18
Structural Studies. Single-crystal X-ray diffraction analy-
ses (Table 1) were used to establish definitive compound
identities and to assess the structural influence of [TPSLO2]2-
ligation in different coordination environments. For all
(19) (a) Cr[N(SiMe3)2]2(THF)2 was prepared by modification (see Experi-
mental Section, General Considerations) of the literature procedure:
Horvath, B.; Strutz, J.; Horvath, E. G. Z. Anorg. Allg. Chem. 1979,
457, 38. (b) Bradley, D. C.; Hursthouse, M. B.; Newing, C. W.; Welch,
A. J. J. Chem. Soc., Chem. Commun. 1972, 567.
Inorganic Chemistry, Vol. 41, No. 2, 2002 323

Kayal and Lee
Table 2. Selected Interatomic Distances and Angles in
[Na(THF)2]2(TPSLO2) ([3](THF)4)
Na(1)-O(1)
Na(1)-O(2)
Na(1)-O(3)
Na(1)-O(4)
Na(1)‚‚‚C(12)
Na(1)‚‚‚Na(2)
2.334(4)
2.286(4)
2.315(4)
2.309(5)
2.775(5)
3.114(3)
Na(2)-O(1)
2.231(4)
2.313(4)
2.40(1)/2.281(9)
2.315(5)
2.680(6)
Na(2)-O(2)
Na(2)-O(5)/O(5A)a
Na(2)-O(6)
Na(2)‚‚‚C(22)
O(1)-Na(1)-O(2)
O(1)-Na(1)-O(3)
76.5(1)
143.3(2)
O(1)-Na(2)-O(2)
78.0(1)
84.1(3)/
95.6(3)
O(1)-Na(2)-O(5/5A)a
Figure 1. Structure of TPSL(OH)2 (2) with thermal ellipsoids (50%
probability level) and selected atom labels; aryl hydrogen atoms are not
shown. Selected distances (Å) and angles (deg): O(1)‚‚‚O(2), 2.751(3);
O(1)‚‚‚H(2), 1.993(4); O-H, 0.84 (fixed); O(2)-H(2)‚‚‚O(1), 149.7(5);
dihedral angle between C(11-16) and C(21-26) planes (biphenyl torsion),
51.77(9).
O(1)-Na(1)-O(4)
O(2)-Na(1)-O(3)
126.6(2)
98.7(2)
O(1)-Na(2)-O(6)
107.4(2)
101.9(4)/
123.6(3)
130.5(2)
127.5(4)/
105.1(3)
O(2)-Na(2)-O(5/5A)a
O(2)-Na(1)-O(4)
O(3)-Na(1)-O(4)
106.5(2)
90.0(2)
O(2)-Na(2)-O(6)
O(5/5A)a-Na(2)-O(6)
C(11-16)/C(21-26)b
50.2(2)
a O(5) and O(5A) are separate components of a disordered THF molecule.
b Biphenyl torsion, as defined by the dihedral angle between the two phenyl
ring planes.
unaware of any other crystallographically characterized alkali
metal/2,2′-biaryloxide structure.21
(b) 4-Coordinate Complexes. The structures of the
bipyridyl (6) and bis(THF) (8, 9) complexes are shown in
Figure 3, with selected metrical parameters listed in Table
3. Not presented here is the structure of the bis(pyridine)
adduct 5, which was solved but not refined to completion
because of extensive disorder within a large asymmetric unit
(4 independent molecules with 260 non-hydrogen atoms total,
not including disordered components).22 This structure de-
termination, nevertheless, was sufficient to verify that
compound 5 shares the same basic geometric features as the
related ferrous complexes 6 and 8.
The 4-coordinate ferrous complexes adopt distorted tet-
rahedral coordination geometries with idealized C2 point
symmetry. The metal-ligand distances are typical for high-
spin, 4-coordinate Fe(II).23 Significant departures from
regular tetrahedral geometries are evident in the dihedral
angle between the O-Fe-O and the A1-Fe-A2 planes
(53.5(1)° and 49.1(1)° for complexes 6 and 8, respectively,
vs 90° for a regular tetrahedron; A1, A2 ) bipy/THF
coligand donor atoms). These geometry distortions arise from
the frontal steric projection of the triphenylsilyl substituents,
which skews the two remaining ligand positions away from
the regular tetrahedral vertexes.
Figure 2. Structure of [Na(THF)2]2[TPSLO2] ([3](THF)4) with thermal
ellipsoids (50% probability level) and selected atom labels; ball-and-stick
representations are used for THF ligands, and hydrogen atoms are not shown.
All THF ligands are disordered over two orientations, and only the major
component for each is presented.
structures, the biphenoxy backbone is labeled consistently
as indicated in Figure 1, with the exception of dimer 4, where
the second independent ligand is designated using [O10,
C101-C106] and [O20, C201-C206] for its phenyl halves,
and complex 6, where a crystallographic 2-fold axis relates
[O1, C11-C16] to [O1A, C11A-C16A].
(a) “Free” Ligand. The solid-state structure of biphenol
2 was determined in order to provide reference metrics for
the uncomplexed ligand. The biphenol in this structure
(Figure 1) possesses one internal hydrogen bond between
phenol groups, as evidenced by the O1‚‚‚O2 separation and
the phenol hydrogen locations in the final Fourier difference
maps.20 Within the lattice, the molecule is well-isolated with
no strong intermolecular interactions.
The interaction of the ligand dianion with alkali metal
cations is illustrated in the structure of the disodium
biphenoxide salt, crystallized as the THF solvate, [Na-
(THF)2]2[TPSLO2] ((THF)4[3]) (Figure 2, Table 2). The net
molecular structure tends toward C2 symmetry. The biphe-
noxide ligand serves as a κ2-chelate to both sodium coun-
tercations, with the irregular coordination sphere of each
cation completed by two terminal THF ligands and by a
distant, weak interaction with the proximal ipso carbon of
the biphenoxide backbone. The steric protection afforded by
the triphenylsilyl groups presumably deters further aggrega-
tion to form cation-bridged oligomers or polymers. We are
A skew distortion is also obvious in the structure of
chromous complex 9 (O-Cr-O/A1-Cr-A2 dihedral: 24.4-
(2)°). As a high-spin, 4-coordinate, d4 metal ion, the Cr(II)
center should prefer a square planar geometry,24 but the
(21) Inclusion compounds of 1,1′-bi-2-naphthol with alkali metal hydroxides
have been structurally characterized; while it seems likely that the
binaphthol hydroxyls are deprotonated in these structures, the diffrac-
tion data was of limited resolution and atomic details were not
presented: Toda, F.; Tanaka, K.; Wong, M. C.; Mak, T. C. W. Chem.
Lett. 1987, 2069.
(22) Crystal parameters for (TPSLO2)Fe(py)2 (5): space group P1h (No. 2),
Z ) 8, a ) 20.3766(6) Å, b ) 22.6822(9) Å, c ) 23.9322(9) Å, R )
109.583(1)°, â ) 94.799(1)°, γ ) 112.714(1)°, V ) 9319.7(6) Å3,
temp ) 200(2) K.
(20) The phenol hydrogens were not directly refined in the structure, but
treated using an idealized riding model (see Supporting Information).
(23) Orpen, G. A.; Brammer, L.; Allen, F. H.; Kennard, O.; Watson, D.
G.; Taylor, R. J. Chem. Soc., Dalton Trans. 1989, S1.
324 Inorganic Chemistry, Vol. 41, No. 2, 2002

3,3′-Bis(triphenylsilyl)biphenoxide
Figure 3. Structures of (TPSLO2)Fe(bipy) (6, left) and (TPSLO2)M(THF)2 (M ) Fe/Cr: 8/9, center/right) with thermal ellipsoids (50% probability level) and
selected atom labels; hydrogen atoms are not shown. For 6, atoms with labels ending in “A” are generated by a crystallographic 2-fold axis. To allow
geometry comparison, all three structures are presented using the same orientation relative to their common O(1)-M-(O1A/O2) planes.
Table 3. Selected Interatomic Distances (Å) and Angles (deg) in
(
TPSLO2)Fe(bipy) (6) and (TPSLO2)M(THF)2 (M ) Fe/Cr: 8/9)a
6
8
9
M(1)-O(1)
M(1)-O(2)
M(1)-A1
M(1)-A2
1.909(2)
1.903(2)
1.915(2)
2.081(2)
2.064(2)
1.944(5)
1.933(5)
2.053(6)
2.069(5)
2.129(2)
O(1)-M(1)-O(a)
O(1)-M(1)-A1
O(1)-M(1)-A2
O(a)-M(1)-A1
O(a)-M(1)-A2
A1-M(1)-A2
102.0(1)
102.15(8)
141.32(8)
102.65(9)
150.17(8)
97.23(9)
95.24(8)
138.27(8)
84.46(9)
96.2(2)
90.4(2)
160.7(2)
163.4(2)
90.4(2)
88.3(2)
76.1(1)
biphenyl torsionb
50.07(6)
54.22(8)
52.3(2)
a For 6: A1 ) N(1), A2 ) N(1A), a ) 1a; for 8 and 9: A1 ) O(3), A2
) O(4), a ) 2. b Defined by the dihedral angle between the two phenyl
ring planes, C(11-16)/C(11A-16A) (for 6) or C(11-16)/C(21-26) (for
8 and 9).
Figure 4. Structure of [Fe(µ-TPSLO2)]2 (4) with thermal ellipsoids (50%
probability level) and selected atom labels; hydrogen atoms are not shown.
For clarity, the two independent biphenoxide ligands are differentiated using
solid and hollow bonds.
hindered biphenoxide ligand again forces a marked deforma-
tion away from the ideal. The metrical details of the
coordination sphere are otherwise unexceptional.23
Table 4. Selected Interatomic Distances and Angles in [Fe(µ-TPSLO2)]2
(4)
(c) [Fe(µ-TPSLO2)]2 Dimer. The structure of dimer 4
approaches overall C2 symmetry, with the paired iron centers
and biphenoxide ligands interrelated through an approximate
central rotational axis (Figure 4, Table 4; for clarity, a
stereoview is provided as Figure 5). The biphenoxide ligation
exhibits an unanticipated coordination mode, providing one
terminal phenoxide donor, one bridging phenoxide, and a
weak interaction from a pendant phenyl group of one silyl
substituent; the ligand thereby serves as a strongly bound
chelate to one iron (via both aryloxide donors) and a weaker
chelate to the other (via a bridging aryloxide and the remote
ring interaction). Each ferrous center adopts a distorted
pseudo-4-coordinate geometry with three Fe-OAr bonds (Ar
) aryl) and a “vacant” coordination site directed toward the
π-face of a silylphenyl ring; the iron geometry is pyrami-
dalized toward the remote phenyl ring and is best described
as a flattened tetrahedral environment. The aryl ring inter-
action is asymmetric: the Fe‚‚‚PhSi contact is closest to the
Fe(1)-O(1)
Fe(1)-O(2)
Fe(1)-O(20)
Fe(1)-C(706)
1.870(2)
2.085(2)
1.996(2)
2.468(4)
Fe(2)-O(10)
Fe(2)-O(20)
Fe(2)-O(2)
Fe(2)-C(86)
1.875(2)
2.085(2)
1.953(2)
2.419(3)
O(1)-Fe(1)-O(2)
O(1)-Fe(1)-O(20)
O(1)-Fe(1)-C(706)
O(2)-Fe(1)-O(20)
O(2)-Fe(1)-C(706)
O(20)-Fe(1)-C(706)
100.00(9) O(10)-Fe(2)-O(20)
129.8(1)
128.5(1)
78.52(9) O(20)-Fe(2)-O(2)
105.2(1)
99.1(1)
99.24(9)
145.6(1)
O(10)-Fe(2)-C(86) 105.3(1)
79.50(9)
O(10)-Fe(2)-O(2)
O(20)-Fe(2)-C(86) 119.0(1)
O(2)-Fe(2)-C(86)
94.0(1)
C(11-16)/C(21-26)a
61.8(1)
C(31-36)/C(41-46)a
59.4(1)
a Biphenyl torsion, as defined by the dihedral angle between the two
phenyl ring planes.
2-carbon, with the next nearest ring position (the 1-carbon)
ca. 0.2 Å more distant. The Fe-OAr bond lengths are
comparable to those of 4-coordinate high-spin Fe(II),23 while
the remote Fe-phenyl contacts, though longer than bonds
found in arene complexes of Fe(II) (ca. 2.01 Å),23 are much
shorter than the sum of the van der Waals radius of an aryl
carbon atom (estimated 1.7 Å)24 and the metallic radius of
iron (estimated 1.2 Å).25 Similar weak aromatic donor
(24) Hermes, A. R.; Morris, R. J.; Girolami, G. S. Organometallics 1988,
7, 2372 and examples cited therein.
Inorganic Chemistry, Vol. 41, No. 2, 2002 325

Kayal and Lee
Figure 5. Stereoview of [Fe(µ-TPSLO2)]2 (4).
Table 5. Selected Interatomic Distances (Å) and Angles (deg) in
(
TPSLO2)FeCl(bipy) (10) and (TPSLO2)Fe(CNXy)4 (7)
10
7
Fe(1)-O(1)
Fe(1)-O(2)
Fe(1)-N(1)
Fe(1)-N(2)
Fe(1)-Cl(1)
1.905(2)
1.847(2)
2.146(3)
2.144(3)
2.249(1)
Fe(1)-O(1)
Fe(1)-O(2)
Fe(1)-C(1)
Fe(1)-C(2)
Fe(1)-C(3)
Fe(1)-C(4)
1.982(2)
1.971(2)
1.858(4)
1.845(4)
1.886(4)
1.895(4)
O(1)-Fe(1)-O(2)
O(1)-Fe(1)-N(1)
O(1)-Fe(1)-N(2)
O(1)-Fe(1)-Cl(1)
O(2)-Fe(1)-N(1)
O(2)-Fe(1)-N(2)
O(2)-Fe(1)-Cl(1)
N(1)-Fe(1)-N(2)
N(1)-Fe(1)-Cl(1)
N(2)-Fe(1)-Cl(1)
96.90(9)
159.7(1)
87.1(1)
106.2(8)
88.9(1)
125.2(1)
105.93(8) O(2)-Fe(1)-C(2)
73.8(1)
O(1)-Fe(1)-O(2)
O(1)-Fe(1)-C(1)
O(1)-Fe(1)-C(2)
O(1)-Fe(1)-C(3)
O(1)-Fe(1)-C(4)
O(2)-Fe(1)-C(1)
95.64(8)
173.2(1)
89.9(1)
94.9(1)
81.1(1)
88.7(1)
174.0(1)
83.8(1)
91.5(1)
86.00(1)
90.7(1)
93.7(1)
93.5(1)
91.6(1)
173.5(1)
Figure 6. Structure of (TPSLO2)FeCl(bipy) (10) with thermal ellipsoids
(50% probability level) and selected atom labels; hydrogen atoms are not
shown.
interactions have been documented in 3d complexes of
monodentate hindered arylchalcogenide ligands with o-aryl
substituents.26,27
O(2)-Fe(1)-C(3)
O(2)-Fe(1)-C(4)
90.73(9)
125.32(8) C(1)-Fe(1)-C(2)
C(1)-Fe(1)-C(3)
(d) 5- and 6-Coordinate Complexes. Ferric complex 10
assumes a 5-coordinate geometry intermediate between
square pyramidal and trigonal bipyramidal (Figure 6, Table
5). As is the case for the ferrous bipyridyl complex 6, the
bipyridyl ligand is sandwiched within a cleft created by
flanking triphenylsilyl substituents; the fifth ligand position
occupied by chloride is exposed and directed away from this
shielded pocket. Metal-ligand bond distances are consistent
with expected values for a high-spin ferric complex.23
The 6-coordinate isonitrile complex 7 possesses a regular
cis-octahedral stereochemistry (Figure 7, Table 5). To our
knowledge, only two [M(OR)2(CNR′)4]z structures have been
reported previously.28 One, with M ) Mo(IV) and unhin-
dered, monodentate ligands, adopts a trans-octahedral
geometry.28a The other, with M ) W(II), assumes a markedly
distorted 6-coordinate cis geometry that originates from the
steric influence of hindered, conformationally constrained
monodentate aryloxide ligands.28b In contrast, for 7, the
C(1)-Fe(1)-C(4)
C(2)-Fe(1)-C(3)
C(2)-Fe(1)-C(4)
C(3)-Fe(1)-C(4)
C(11-16)/C(21-16)a 48.50(8)
C(11-16)/C(21-16)a 51.2(1)
a Biphenyl torsion, as defined by the dihedral angle between the two
phenyl ring planes.
(25) Pauling, L. The Nature of the Chemical Bond, 3rd ed.; Cornell
University Press: Ithaca, NY, 1960.
Figure 7. Structure of (TPSLO2)Fe(CNXy)4 (7) with thermal ellipsoids
(50% probability level) and selected atom labels; hydrogen atoms are not
shown. For clarity, the biphenoxide chelate is depicted with hollow bonds.
(26) (a) Ruhlandt-Senge, K.; Power, P. P. Bull. Soc. Chim. Fr. 1992, 129,
594. (b) Ellison, J. J.; Ruhlandt-Senge, K.; Power, P. P. Angew. Chem.,
Int. Ed. Engl. 1994, 33, 1178. (c) Ellison, J. J.; Power, P. P. Inorg.
Chem. 1994, 33, 4231. (d) Ellison, J. J.; Ruhlandt-Senge, K.; Hope,
H. H.; Power, P. P. Inorg. Chem. 1995, 34, 49. (e) Hauptmann, R.;
Kliss, R.; Henkel, G. Angew. Chem., Int. Ed. 1999, 38, 377.
(27) Stronger interactions with o-aryl substituents of monodentate arylthi-
olate ligands have been observed for 4d metal centers: (a) Dilworth,
J. R.; Zheng, Y.; Lu, S.; Wu, Q. Inorg. Chim. Acta 1992, 194, 99. (b)
Buyuktas, B. S.; Olmstead, M. M.; Power, P. P. Chem. Commun. 1998,
1689.
planar aryl substituents of the isonitrile ligands interleave
with each other and with the triphenylsilyl substituents to
allow the complex to attain a relatively undistorted octahedral
coordination geometry. The Fe-CNR distances are within
the expected range for Fe(II);23 the isonitriles positioned trans
to each other show somewhat longer metal-ligand separa-
tions (ca. 0.04 Å) than the two bound trans to the aryloxides,
(28) (a) Isovitsch, R. A.; Beadle, A. S.; Fronczek, F. R.; Maverick, A. W.
Inorg. Chem. 1998, 37, 4258. (b) Lockwood, M. A.; Fanwick, P. E.;
Rothwell, I. P. Organometallics 1997, 16, 3574.
326 Inorganic Chemistry, Vol. 41, No. 2, 2002

3,3′-Bis(triphenylsilyl)biphenoxide
due presumably to the mutual, opposing trans influences
(structural trans effects) for the former.29 The Fe-OAr
distances are notably longer than those found in the high-
spin iron complexes 4, 6, 8, and 10. While structurally
characterized examples of low-spin Fe(II)-aryloxide species
are rare and do not offer an adequate basis for distance
comparison,30 the long Fe-OAr separations are unusual
given that low-spin, 6-coordinate Fe(II) has an ionic radius
(0.61 Å) between that of high-spin, 4-coordinate Fe(II) (0.63
Å) and that of 5-coordinate Fe(III) (0.58 Å).31 The isonitrile
ligands trans to the aryloxides may account for some of the
bond lengthening, although isonitriles are generally held to
exert only a modest trans influence;29 the increased steric
congestion associated with a 6-coordinate metal environment
may be the other contributing factor for the longer Fe-OAr
distances in 7.
Figure 8. Plot of biaryl torsion vs chelate bite angle for biaryloxide chelate
ligands in transition metal complexes (38 unique structures with 64
observations,32 excluding 1 outlier35); biaryl torsion is defined as the dihedral
angle between aryl ring planes. Entries are subdivided between tetrahedral,
4-coordinate species (triangles) and complexes with other geometries
(circles), and between biaryloxides with backside steric hindrance (solid)
and those without (hollow); entries for complexes reported in this work
are designated by compound number (4, 6, 7, 8, 9, 10).
Discussion
3,3′-Bis(triphenylsilyl)biphenoxide as a Geometry-
Constraining Ligand. A survey31 of the Cambridge Crystal-
lographic Database33 shows biaryloxides as flexible ligands
compatible with typical 4-, 5-, and 6-coordinate transition
metal geometries.34 Biaryloxide chelates display the full range
of bite angles needed to bridge adjacent ligand sites in all
of these environments, from the ubiquitous right angles found
in octahedral, square pyramidal, trigonal bipyramidal, and
square planar complexes to the expanded angles associated
with tetrahedral metal centers (Figure 8). Backside steric
encumbrance, present in 1,1′-binaphthyl and 6,6′-dimethyl-
biphenyl ligand backbones, affects chelate structure by
increasing torsion at the biaryl linkage (Figure 8), but has
no apparent influence on bite angle. Although increases in
aryl-aryl torsion should expand chelate bite angles, the
difference between hindered and unhindered dihedral angles
for known ligands appears insufficient35 to override other
compensatory effects; indeed, the most acute bite angles
(80-88°) are confined counterintuitively to complexes of
biaryloxides with backside hindrance.
Figure 9. Space-filling representations of the [(TPSLO2)M] fragment as
viewed from the front (bottom) and rotated 90° lengthwise (top). The
relevant portion of complex 6 is used for the model; C, H, and O atoms are
depicted at full van der Waals radii (1.7, 1.2, and 1.4 Å, respectively),25
while Si and M atoms are shown with arbitrary radii (1.7 and 1.5 Å,
respectively).
(29) Coe, B. J.; Glenwright, S. J. Coord. Chem. ReV. 2000, 203, 5.
(30) We know of only three structurally characterized low-spin Fe(II)
aryloxide complexes; two of these involve κ2-2-nitrosophenoxide
chelation, which may not be metrically comparable to simple aryloxide
ligation, and all three structures are marked by limited diffraction data,
high final R factors (>9% in all cases), and large estimated standard
deviations for interatomic distances (ca. 0.01-0.03 Å for Fe-O
bonds): (a) De Sanctis, S. C.; Hodgkin, D. C. Proc. R. Soc. London
B 1973, 184, 121. (b) De Sanctis, Grdenic, D.; Taylor, N.; Hodgkin,
D. C. Proc. R. Soc. London B 1973, 184, 137. (c) Boinnard, D.;
Bousseksou, A.; Dworking, A.; Savariault, J.-M.; Varret, F.; Tuchagues,
J.-P. Inorg. Chem. 1994, 33, 271.
(31) Shannon, R. D. Acta Crystallogr. 1976, A32, 751.
(32) A complete listing of search results (structures, references, relevant
metrics) is provided as Supporting Information.
(33) (a) Cambridge Structural Database, version 5.21; Cambridge Uni-
versity: Cambridge, England, April 2001. (b) Allen, F. H.; Kennard,
O. Chem. Des. Autom. News 1993, 8, 31.
(34) The coordinative and conformational flexibility of the binaphthoxide
ligand was also noted in an earlier structural analysis: Boyle, T. J.;
Barnes, D. L.; Heppert, J. A.; Morales, L.; Takusagawa, F.; Connolly,
J. W. Organometallics 1992, 11, 1112.
Molecular modeling suggests that, for small metal ions
(3d elements or lighter) with no electronic structure prefer-
ence, the frontside steric demand of the triphenylsilyl-
substituted biaryloxide ligands will favor the formation of
mononuclear, monochelate complexes of distorted tetrahedral
metal geometry.36 The structures of the 4-coordinate ferrous
complexes demonstrate this coordination mode. Space-filling
representations of the [(TPSLO2)Fe] fragment of complex 6
illustrate the steric extent of the bound hindered biphenoxide
(Figure 9): the triphenylsilyl substituents project forward
over the metal center, bracketing the remaining coordination
sites to form a pronounced C2-symmetric binding cleft. The
prominent skew distortions away from tetrahedral and square
(35) The excluded outlier belongs to a tetrahedral Mo(VI) complex with a
biaryloxide chelate that spans a remarkable 127.0° bite angle; a
substantial biaryl torsion of 82.6° is necessary to support this wide
bite angle: Alexander, J. B.; La, D. S.; Cefalo, D. R.; Hoveyda, A.
H.; Schrock, R. R. J. Am. Chem. Soc. 1998, 120, 4041.
(36) Most5-10 of the aforementioned reactivity studies involving the
binaphthoxide homologue were conducted and interpreted with this
structural model in mind.
Inorganic Chemistry, Vol. 41, No. 2, 2002 327

Kayal and Lee
Table 6. Distortional Parametersa for the Biphenyl Rings in TPSL(OH)2 (1), (TPSLO2)Fe(bipy) (6), (TPSLO2)M(THF)2 (M ) Fe/Cr: 8/9), [Fe(µ-TPSLO2)]2
(4), (TPSLO2)FeCl(bipy) (10), and (TPSLO2)Fe(CNXy)4 (7)
4
1
6
8
9
ligand 1
ligand 2
10
0.043,
7
rms deviationb from
phenyl plane,c Å
C1′ displacement from
phenyl plane,c Å
C1′-C1‚‚‚C4 para bend,d
deg
O displacement from
phenyl plane,c Å
O-C2‚‚‚C5 para bend,d
deg
0.005,
0.004
0.001
0.019,
0.019
0.003,
0.012
0.00(1),
0.12(1)
174.8(5),
174.8(5)
0.020(9),
0.01(1)
177.6(5),
177.7(6)
0.079(9),
0.07(1)
0.029,
0.016,
0.048,
0.028
0.023
0.016
0.013
0.100(4),
0.028(4)
176.3(2),
177.3(2)
0.029(4),
0.043(4)
178.1(2),
178.2(2)
0.009(3),
0.027(4)
179.4(1),
179.2(1)
0.097(5)
173.91(9)
0.033(3)
177.3(2)
0.023(4)
178.9(2)
0.150(5),
0.244(5)
172.8(2),
170.6(2)
0.061(4),
0.063(4)
177.5(2),
177.1(2)
0.212(4),
0.068(4)
174.4(2),
178.3(2)
0.283(6),
0.170(5),
0.335(5),
0.018(5)
0.399(5),
0.265(5)
166.5(2),
169.8(2)
0.198(4),
0.058(4)
178.9(2),
175.6(2)
0.238(4),
0.176(4)
174.1(2),
175.6(2)
0.178(5)
0.186(5)
169.7(3),
172.8(3)
0.072(5),
0.128(5)
178.0(3),
176.7(2)
0.610(5),
0.209(5)
162.1(2),
174.6(2)
172.5(3),
172.4(2)
0.053(5),
0.077(4)
178.4(3),
178.0(2)
0.354(5),
0.145(4)
169.7(2),
175.4(2)
168.8(2),
173.9(3)
0.247(4),
0.129(4)
172.8(2),
174.2(2)
0.475(4),
0.165(5)
167.1(2),
175.5(2)
Si displacement from
phenyl plane,c Å
Si-C3‚‚‚C6 para bend,d
deg
177.6(4),
177.1(4)
a Positional ring notation: C1-C6 ) phenyl carbon atoms, C1′ ) carbon of the other phenyl ring bound to C1, O ) phenoxy oxygen bound to C2, Si
) silicon bound to C3. b Root-mean-square deviation for the least-squares fitted phenyl carbon atoms (C1-C6). c Perpendicular displacement from the fitted
phenyl plane. d Para bend refers to the angle formed by the substituent and the phenyl para carbon with the ipso carbon as vertex.
planar geometries noted in complexes 6, 8, and 9 provide
direct evidence of geometry-altering, nonbonded intramo-
lecular contacts between the sterically demanding triphenyl-
silyl groups and the other components of the coordination
sphere. Previous structurally characterized 4-coordinate bi-
aryloxide complexes all employ less hindered chelates and
consequently show little to no deformation of their coordina-
tion polyhedra.37
deviations in the six-carbon phenyl fragment, displacements
of the phenyl substituents relative to the plane, and bend
angles of these substituents through the ipso and para carbons
of the phenyl ring (Table 6). Free biphenol 2, whose structure
is dictated by inherent preferences, a single internal hydrogen
bond, and intramolecular packing forces, exhibits only minor
fluctuations from planarity at the backbone phenyl sites.
Relative to this “relaxed” reference structure, the 4-coordinate
mononuclear complexes display comparable, slight ring
distortions since chelate steric demands are satisfied by
twisting the coordination polyhedra. The 5- and 6-coordinate
species, however, are too crowded to relieve steric clashes
through simple adjustment of metal geometry, and so reveal
more obvious nonplanar deviations at the chelate backbone.
Although these distortions are relatively modest when
quantified as local metrical perturbations, they lead to
significant gross changes when summed across the entire
ligand framework. Superposition of the chelate structures
against the free diol framework (Figure 10) provides a
straightforward visual gauge of this overall distortion: as
strain increases, so too does the deflection of the triphenyl-
silyl substituents away from the congested metal center. The
structural overlays also reveal conformational changes at the
triphenylsilyl propeller that become very prominent as
crowding occurs; the magnitude of this response shows that
the rotational freedom associated with the Si-C bonds
significantly expands the stereochemical flexibility of the
[(TPSLO2)M] environment. All of these distortional and
conformational effects also occur in dimer 4, but arise in
this case primarily from the exceptional binding mode of
the bridging chelate.
The structures of dimer 4, 5-coordinate 10, and 6-coor-
dinate 7 establish that the [(TPSLO2)M] environment, despite
the steric demand of the triphenylsilyl substituents, also
accommodates alternate, expanded coordination geometries
with coligands of low steric demand. In comparing the
structural parameters of these species against those of the
4-coordinate complexes, we find that the biphenoxide chelate
has little effect on the higher coordinate, mononuclear metal
environments (with the possible exception of the aforemen-
tioned long Fe-OAr distances in isonitrile complex 7), but
does create an irregular metal geometry on dimer 4 due to
its unusual binding mode. The chelate itself adapts across
the entire structural series by spanning a 7° spread of bite
angles intermediate between perfect right and tetrahedral
angles; the ordering of these angles is consistent with
coordination geometry, with the sharpest angle occurring at
the octahedral metal center and the widest angles at
tetrahedral sites (see Tables 2-5 and Figure 8). The aryl-
aryl dihedral angles lie within the range expected for
torsionally unhindered biphenoxide chelates, with the excep-
tion of dimer 4 where the ligand bridging necessitates a
larger, but unremarkable, biaryl twist.
The presence of a normal metal geometry in 7 and 9 does
not imply the absence of steric conflict between the hindered
chelate and the other ligands: the strain is manifested instead
in the biphenoxide ligand itself. Structural analysis of the
phenyl units in the ligand backbones reveals a systematic
pattern of deformation from planar ideals, as measured by
Conclusion
3,3′-Bis(triphenylsilyl)-1,1′-bi-2-phenoxide is an easily
synthesized, sterically demanding dianionic bidentate ligand
of potential utility in applications requiring geometry con-
straint of reactive metal sites. As an ancillary ligand, this
biaryloxide derivative forms crystalline complexes that
undergo controlled solution chemistry even using low-
coordinate, high-spin, exchange-labile metal centers. Our
(37) Skew angles (O-M-O/A1-M-A2 dihedral) for previously charac-
terized, 4-coordinate biaryloxide complexes (9 entries, 18 observations)
range from 85.2° to 87.4° in tetrahedral geometries and 0.3° to 6.8°
for square planar centers; search results are provided as Supporting
Information.
328 Inorganic Chemistry, Vol. 41, No. 2, 2002

3,3′-Bis(triphenylsilyl)biphenoxide
Figure 10. Superposition of biphenoxide chelate structures (dashed traces) in complexes 6 (left), 9 (center), and 7 (right) against the structure of the free
diol 2 (solid traces) using least-squares fits of the biphenoxy backbones (O(1), C(11A-16A), O(1A/2), C(11A-16A)/(21-26)).
CH2Cl2 at -78 °C. After being stirred for 6 h at room temperature,
the yellow solution was treated with 500 mL of H2O and then
acidified with concentrated HCl, and more CH2Cl2 (300 mL) was
added. The biphasic mixture was partitioned and the aqueous layer
washed with CH2Cl2 (2 × 200 mL); all organic fractions were
combined, washed with H2O (500 mL), dried over Na2SO4, and
reduced to dryness in vacuo. The resulting yellowish-white residue
was redissolved in CH2Cl2 and the solution concentrated to give
colorless microcrystals of biphenol 2, which were isolated by
filtration, rinsed with n-pentane, and dried in vacuo; the mother
liquor was further concentrated and cooled to -20 °C to obtain a
structural analysis reveals that the steric influence of the
hindered chelate over the metal environment, while signifi-
cant, is also compliant and moderated by deformational and
conformational changes within the ligand itself. Although
many of the ligand distortions noted in this study are intrinsic
to the biaryloxy skeleton and thus difficult to control, the
interplay of triphenylsilyl conformation with metal geometry
and coligands suggests that further modification of this
directing group can effectively enhance the stereochemical
rigidity of the coordination sphere.
1
second crop. Combined yield: 7.2 g (50%). H NMR (500 MHz,
Experimental Section
CDCl3) δ 5.68 (s, OH, 2H), 7.06 (t, J ) 7.5 Hz, biphenyl, 2H),
7.25 (m, biphenyl, 2H), 7.36 (t, J ) 7.5 Hz, SiPh3, 12H), 7.43 (m,
biphenyl and SiPh3, 8H), 7.62 (m, SiPh3, 12H). 13C NMR (125
MHz, CDCl3): δ 121.1, 121.6, 124.5, 128.2, 129.9, 134.2, 134.21,
136.5, 138.9, 158.2. MS (EI): m/z 702 [M+, 11], 624 [(M - C6H6)+,
18], 547 [(M - C6H6 - C6H5)+, 34], 469 [(M - 2C6H6 - C6H5)+,
29], 391 [(M - 3C6H6 - C6H5)+, 39], 259 [SiPh3+, 100]. HRMS
(EI): m/z 702. 23875 [calcd M+ for C48H36O2Si2, 702.24104].
General Considerations. Synthetic operations and physical
measurements were conducted using protocols previously de-
scribed.3 Cr[N(SiMe3)2]2(THF)2 was prepared by modification of
the literature procedure:19a commercial Na[N(SiMe3)2] was used
instead of Li[N(SiMe3)2] generated in situ, and the reaction was
stirred over a period of 24 h instead of 10 days before workup;
yields for the modified synthesis were comparable to that reported
for the original preparation. Pyridine was distilled from CaH2 and
stored over 4 Å molecular sieves under a pure dinitrogen atmosphere
prior to use. Infrared spectra were recorded on a Nicolet 730 FTIR
spectrometer.
Na2(TPSLO2) (3). NaH (0.088 g, 3.67 mmol) was added in small
portions to a rapidly stirred THF (150 mL) solution of biphenol 2
(1.00 g, 1.42 mmol). Stirring was maintained for 24 h, after which
the mixture was filtered to remove excess NaH and evaporated in
vacuo to give a white solid. The crude product was recrystallized
from THF/n-pentane; rigorous drying of the microcrystals so
obtained removed all cation-bound THF to give 0.700 g (66%) of
2,2′-Dimethoxy-3,3′-bis(triphenylsilyl)-1,1′-biphenyl (1). To a
rapidly stirred, fine suspension of 2,2′-dimethoxy-1,1′-biphenyl
(20.0 g, 93.3 mmol) and TMEDA (42.3 mL, 280 mmol) in 500
mL of Et2O at -78 °C was added dropwise 22.5 mL of a 10 M
solution of n-butyllithium in hexanes (225 mmol). Stirring was
continued for 8 h at room temperature, after which the reaction
mixture was cooled to -5 °C and a solution of Ph3SiCl (60.0 g,
203 mmol) in Et2O (500 mL) introduced by cannula. The resulting
mixture was stirred for 24 h at room temperature and then filtered
to give an off-white precipitate, which was rinsed with Et2O. The
solid was redissolved in CH2Cl2 and filtered again to remove LiCl;
solvent removal in vacuo, followed by recrystallization from CH2-
Cl2/Et2O, yielded 17.5 g (26%) of pure bianisole 1 as a white
1
the unsolvated disodium salt 3. H NMR (500 MHz, CD3CN): δ
6.57 (t, J ) 7.5 Hz, biphenyl), 6.86 (dd, J ) 7.5, 1.5 Hz, biphenyl),
7.32 (m, SiPh3, 9H), 7.45 (dd, J ) 7.5, 1.5 Hz, biphenyl), 7.55 (m,
SiPh3, 6H). 13C NMR (125 MHz, CD3CN): δ 115.8, 121.9, 128.4,
129.7, 130.8, 134.1, 137.1, 137.2, 137.4, 138.2, 170.4.
[Fe(µ-TPSLO2)]2 (4). A green solution of Fe[N(SiMe3)2]216
(0.350 g, 0.93 mmol) in benzene (1 mL) was added to a solution
of biphenol 2 (0.500 g, 0.72 mmol) in benzene (5 mL), resulting
in an instantaneous color change to yellow. The solution was stirred
for 2 h and then layered with 2 vol equiv of n-pentane. The yellow
crystalline blocks that grew overnight were collected by filtration,
washed with n-pentane, and dried in vacuo to give 0.365 g (68%)
1
microcrystalline solid. H NMR (500 MHz, CDCl3): δ 2.53 (s,
OCH3, 6H), 7.08 (t, J ) 7.5 Hz, biphenyl, 2H), 7.23 (dd, J ) 7.5,
1.5 Hz, biphenyl, 2H), 7.35 (m, SiPh3, 12H), 7.40 (m, SiPh3, 6H),
7.54 (dd, J ) 7.5, 1.5 Hz, biphenyl, 2H), 7.64 (dd, J ) 7.5, 1.5
Hz, SiPh3, 12H). 13C NMR (125 MHz, CDCl3): δ 59.2, 123.5,
127.8, 128.5, 129.4, 130.7, 134.4, 135.2, 136.7, 137.7, 163.3. MS
(EI): m/z 730 [M+, 31], 653 [(M - C6H5)+, 45], 259 [SiPh3+, 100].
HRMS (EI): m/z 730.27308 [calcd M+ for C50H42O2Si2, 730.27234].
3,3′-Bis(triphenylsilyl)-1,1′-bi-2-phenol (2, TPSL(OH)2). BBr3
(3.87 mL, 41.0 mmol) was added dropwise via gastight syringe to
a stirred solution of bianisole 1 (15 g, 20.5 mmol) in 400 mL of
1
of pure 4. H NMR (500 MHz, CD2Cl2): δ -63.47 (s, biphenyl,
2H), -31.73 (s, biphenyl, 2H), -8.19 (s, biphenyl, 2H), 2-6 (v
br, SiPh3, 30 H), 24.61 (s, biphenyl, 2H), 30.03 (s, biphenyl, 2H),
30.65 (s, biphenyl, 2H). Anal. Calcd for C96H72O4Si4Fe2: C, 76.17;
H, 4.79; Fe, 7.38. Found: C, 76.06; H, 4.99; Fe, 6.99.
(
TPSLO2)Fe(py)2 (5). A stirred solution of dimer 4 (0.100 g, 0.066
mmol) in 5 mL of CH2Cl2 was treated with pyridine (0.016 g, 0.202
mmol), resulting in an instantaneous color change to dark golden
yellow. The reaction solution was stirred for 1 h and then
Inorganic Chemistry, Vol. 41, No. 2, 2002 329

Kayal and Lee
concentrated in vacuo. Vapor diffusion of several volume equiva-
lents of n-pentane into this solution yielded green crystals, which
were collected, washed with n-pentane, and dried in vacuo to give
(
TPSLO2)FeCl(bipy) (10). FeCl3 (0.045 g, 0.27 mmol) was added
to a rapidly stirred solution of disodium biphenoxide 3 (0.200 g,
0.27 mmol) in THF (10 mL), producing a dark purple-red color.
After the reaction mixture was stirred for 1 h, a solution of 2,2′-
bipyridyl (0.042 g, 0.27 mmol) in 1 mL of THF was added to it,
resulting in an opaque red-brown solution color. The mixture was
stirred for an additional 12 h and then filtered to remove NaCl and
evaporated in vacuo to afford a dark red-brown microcrystalline
residue. This material was redissolved in a minimal amount of THF
and the solution filtered again; vapor diffusion of Et2O into the
filtrate over 2 days gave very dark red (almost black) crystals of
10, which were collected, washed with Et2O, and dried in vacuo
(0.050 g, 20%). Crystallizations were always accompanied by the
deposition of limited amounts of unidentified red, insoluble powder
in addition to crystalline 10, even after successive recrystallizations;
this may account for unsatisfactory elemental analyses (multiple
attempts). 1H NMR (500 MHz, CD2Cl2): δ 6.20-8.10 (v br). Anal.
Calcd for C58H44ClN2O2Si2Fe: C, 73.45; H, 4.68; N, 2.95; Fe, 5.89.
Found: C, 75.91; H, 4.85; N, 3.22; Fe, 6.45.
1
0.085 g (70%) of complex 5. H NMR (500 MHz, CD2Cl2): δ
-27.0 (s, biphenyl, 2H), 3.20 (br, SiPh3, 12H), 4.45 (br, SiPh3,
12H), 5.80 (br, SiPh3, 6H), 10.1 (s, py, 2H), 26.0 (s, biphenyl, 2H),
29.5 (s, biphenyl, 2H), 46.4 (br, py, 4H). Anal. Calcd for
C58H46N2O2Si2Fe: C, 76.13; H, 5.07; N, 3.06; Fe, 6.10. Found:
C, 76.35; H, 5.25; N, 3.20; Fe, 6.66.
(
TPSLO2)Fe(bipy) (6). The preparative procedure for complex 5
was employed using 0.050 g (0.033 mmol) of 4, 0.011 g (0.070
mmol) of 2,2′-bipyridyl, and 5 mL of CH2Cl2. The resulting dark
red-brown solution was stirred for 2 h before concentration; vapor
diffusion of Et2O over 2 days yielded brown crystals of complex 6
(0.035 g, 62%). 1H NMR (500 MHz, CD2Cl2): δ -23.28 (s,
biphenyl, 2H), -7.74 (s, bipy, 2H), 0.5-4.0 (v br, SiPh3), 4.0-7.0
(v br, SiPh3), 23.82 (s, biphenyl, 2H), 28.79 (s, biphenyl, 2H), 62.43-
(s, bipy, 2H), 85.52 (s, bipy, 2H). Anal. Calcd for C53H44N2O2Si2-
Fe: C, 74.63; H, 5.20; N, 3.28; Fe, 6.55. Found: C, 75.19; H,
4.95; N, 3.13; Fe, 7.08.
TPSLO2)Fe(CNXy)4 (7, Xy ) 2,6-Xylyl). The preparative
X-ray Crystallography. Crystals suitable for single-crystal X-ray
diffraction analysis were grown by room temperature vapor
diffusion of benzene/n-pentane (for 2 and 4‚3C6H6, as colorless
and yellow blocks, respectively), THF/n-pentane (for [3](THF)4‚
2THF and 8, as colorless and green-blue blocks, respectively), CH2-
Cl2/Et2O (for 6, as brown blocks, and 7‚(CH2Cl2/Et2O), as orange-
red needles), and THF/Et2O (10‚Et2O‚THF, as dark red blocks),
and by vapor diffusion of CH2Cl2/Et2O at -30 °C (9, as purple
blocks).
(
procedure for complex 5 was employed using 0.050 g (0.033 mmol)
of 4, 0.035 g (0.264 mmol) of 2,6-dimethylphenylisonitrile, and 5
mL of CH2Cl2. The resulting dark orange-red solution was stirred
for 30 min before concentration; vapor diffusion of n-pentane over
1 h yielded orange-red microcrystals of complex 7 (0.073 g, 86%).
1H NMR (500 MHz, C6D6): δ 1.47 (s, Me, 12H), 2.35 (s, Me,
12H), 6.47 (d, J ) 7.5 Hz, ArH, 4 H), 6.62 (t, J ) 7.5 Hz, ArH,
2H), 6.72-6.88 (m, ArH, 20H), 6.89-7.00 (m, ArH, 8 H), 7.45
(d, J ) 7.5 Hz, ArH, 2 H), 7.85 (d, J ) 6 Hz, ArH, 12H). IR
(KBr): 2184 (w), 2145 (vs), 2127 (vs), 2105 (vs) (ν(NC)). Anal.
Calcd for C83H72N4O2Si2Fe: C, 78.73; H, 5.66; N, 4.37; Fe, 4.36.
Found: C, 79.17; H, 5.83; N, 4.18; Fe, 4.71.
General crystallographic procedures have been described else-
where;3 detailed information regarding specific structures and
refinements in this work are provided as Supporting Information.
The four cation-bound THF ligands in the structure of [3](THF)4
were each disordered over two site positions; the individual
components were resolved and refined with appropriate restraints.
Well-defined lattice-bound solvent molecules were identified and
refined in the crystal structures of 4 (2 C6H6, one of which was
constrained as a regular hexagon) and 10 (1 Et2O). In addition,
highly disordered solvent regions that could not be satisfactorily
modeled were present in the lattices of 3 (2 THF), 4 (1 C6H6), 7
(compositional disorder of CH2Cl2/Et2O), and 10 (1 THF); the
SQUEEZE-BYPASS procedure38 implemented in PLATON39 was
used to account for these residual electron densities.
(
TPSLO2)Fe(THF)2 (8). FeCl2 (0.085 g, 0.67 mmol) was added
to a rapidly stirred THF (20 mL) solution of disodium biphenoxide
salt 3 (0.500 g, 0.67 mmol), resulting in a dark blue-green solution
color. The reaction mixture was stirred for 15 min, filtered to
remove NaCl, and concentrated in vacuo to a minimal volume.
Vapor diffusion of several volume equivalents of n-pentane over 6
h at room temperature gave blue-green crystalline blocks of complex
8, which were collected, washed with n-pentane, and dried in vacuo
(0.194 g, 35%). At room temperature, complex 8 is unstable both
in solution (blue solutions decolorize within 24 h) and in the solid
state; crystalline 8 was therefore stored at -30 °C to minimize
decomposition. Dissolution of 8 in C6D6 gives a yellow solution
color and 1H NMR spectrum identical to that of 4. Anal. Calcd for
C56H52O4Si2Fe: C, 74.65; H, 5.82. Found: C, 70.57; H, 5.53.
Unsatisfactory elemental analyses (multiple attempts) reflect sample
decomposition at room temperature.
Acknowledgment. The generous support of the Arnold
and Mabel Beckman Foundation (Beckman Young Investi-
gator Award) and the NSF (CAREER CHE-9984645) is
gratefully acknowledged.
(
TPSLO2)Cr(THF)2 (9). A stirred violet solution of Cr[N(SiMe3)2]2-
(THF)219 (0.044 g, 0.085 mmol) in toluene (5 mL) was treated with
a solution of biphenol 2 (0.060 g, 0.085 mmol) in toluene (5 mL)
to give an instantaneous solution color change to dark purple and
the separation of purple microcrystals within 10 min. Stirring was
continued for an additional 30 min, after which the purple
microcrystalline precipitate was isolated by filtration, washed with
n-pentane, and dried in vacuo to give 0.061 g (79%) of complex
10. 1H NMR (500 MHz, CD2Cl2): δ -6.20 (br, biphenyl), 1.20-
9.80 (v br, SiPh3), 1.65 (br, THF), 3.85 (br, THF), 8.80 (br,
biphenyl), 15.90 (br, biphenyl). Anal. Calcd for C56H52O4Si2Cr: C,
74.97; H, 5.84; Cr, 5.80. Found: C, 75.10; H, 6.06; Cr, 5.45.
Supporting Information Available: Crystallographic data for
compounds 2, [3](THF)4‚2THF, 4‚3C6H6, 6, 7‚(CH2Cl2/Et2O), 8,
9, and 10‚Et2O‚THF (CIF) and Cambridge Crystallographic Data-
base search results (PDF). This material is available free of charge
via the Internet at http://pubs.acs.org.
IC010909Y
(38) van der Sluis, P.; Spek, A. L. Acta Crystallogr. 1990, A46, 194.
(39) (a) Spek, A. L. PLATON. A multipurpose crystallographic tool; Utrecht
University: Utrecht, The Netherlands, 1999. (b) Spek, A. L. Acta
Crystallogr. 1990, A46, C34.
330 Inorganic Chemistry, Vol. 41, No. 2, 2002