Tricyclohexylphosphine derivatives of bis(2,6-difluorophenoxide)cadmium: A solution and solid-state NMR study

Article abstract of DOI:10.1021/ic010104q

The synthesis of Cd(O-2,6-F₂C₆H₃)₂ (PCy₃)₂ has been achieved by the reaction of Cd[N(SiMe₃)₂]₂ and 2,6-difluorophenol in THF in the presence of 2 equiv of tricyclohexylphosphine, and its distorted tetrahedral structure has been established by X-ray crystallography. Comparison of the solution and solid-state ¹¹³Cd NMR spectra of this derivative suggest that a facile solution equilibrium exists between it and its dimeric monophosphine analogue and free PCy₃. At ambient temperature, the equilibrium lies strongly in favor of the dimeric complexes, whereas at -80 °C, the monomeric complex is dominant.

Full text of DOI:10.1021/ic010104q

Inorg. Chem. 2001, 40, 3639-3642
3639
phine ligand, that is, [M(O-2,6-F2C6H3)2(µ-O-2,6-F2C6H3)PCy3]2
(M ) Zn or Cd) (1).11 Although the dimeric zinc derivative is
unreactive toward added PCy3, its cadmium analogue (1) readily
adds a second equivalent of PCy3 to afford the monomeric
complex, Cd(O-2,6-F2C6H3)2(PCy3)2 (2). Herein, we wish to
report the isolation and solid-state structure of 2, along with its
solution and solid-state 113Cd and 31P NMR spectra.
Tricyclohexylphosphine Derivatives of
Bis(2,6-difluorophenoxide)cadmium: A Solution
and Solid-State NMR Study
Donald J. Darensbourg,* Jacob R. Wildeson,
Jason C. Yarbrough, and Robert E. Taylor
Department of Chemistry, Texas A&M University,
P.O. Box 30012, College Station, Texas 77842
Experimental Section
Methods and Materials. Unless otherwise specified, all syntheses
and manipulations were carried out on a double manifold Schlenk
vacuum line under an atmosphere of argon or in an argon-filled
glovebox. Glassware was flamed out thoroughly prior to use. Solvents
were freshly distilled from sodium benzophenone before use. Tri-
cyclohexylphosphine and 2,6-difluorophenol were purchased from
Aldrich Chemical Co. and were sublimed and stored in a glovebox
prior to use. Cd[N(SiMe3)2]2 was prepared according to published
literature,12 stored in the glovebox, and used immediately after removal
from the box. Infrared spectra were recorded on a Mattson 6081
spectrometer with DTGS and mercury cadmium telluride (MCT)
detectors. All isotopically labeled solvents for NMR experiments were
purchased from Cambridge Isotope Laboratories. 1H and 13C NMR
spectra were recorded on Varian XL-200E, Unity +300 MHz, and VXR
300 MHz superconducting high-resolution spectrometers. 19F and 31P
data were acquired on a Unity +300 MHz superconducting NMR
spectrometer operating at 282 and 121 MHz, respectively. All 19F NMR
data are referenced to 10% CFCl3 and 1% CClH2CClF2 in acetone-d6,
whereas all 31P NMR data are referenced to H3PO4 (85% in D2O).
Solution-state 113Cd spectra were recorded on a Varian XL-400
superconducting high-resolution spectrometer operating at 88 MHz
using an external 0.1 M Cd(ClO4)2/D2O reference. Elemental analyses
were carried out by Galbraith Laboratories Inc.
ReceiVed January 25, 2001
Introduction
Recently, we have been interested in exploring the solution
and solid-state structural chemistry associated with phosphine
derivatives of bisphenoxide complexes of zinc and cadmium.
The initial impetus for investigating this chemistry was based
on our use of bisphenoxides of zinc as catalysts for the
polymerization of CO2 and cyclohexene oxide to provide
poly(cyclohexenylene carbonate).1,2 In the meantime, it has
become apparent that the coordination chemistry of the group
12 metals, zinc and cadmium, is intrinsically worthy of detailed
study.3-6 It is of particular interest to compare and contrast the
coordination chemistry of these metals since cadmium, because
of its NMR-active nuclei (113Cd and 111Cd), is routinely used
as a structural probe of zinc in enzymes and proteins.7,8
Previous investigations have illustrated significant differences
in the coordination chemistry of the bisphenoxides of zinc and
cadmium, with the latter metal derivatives being structurally
more diverse. For example, Zn(O-2,6-tBu2C6H3)2L2 (L )
tetrahydrofuran (THF) or propylene carbonate) complexes are
distorted tetrahedral in the solid-state, whereas their cadmium
analogues are square planar.2-4,9 Pertinent to a discussion of
phosphine derivatives, the larger size of cadmium accounts for
the observation that although phosphine complexes of Zn(O-
2,6-tBu2C6H3) and Zn(O-2,6-Ph2C6H3)2 are trigonal planar in
structure for basic phosphine ligands spanning a range of steric
requirements,5,10 cadmium analogues afford both trigonal planar
monophosphine complexes for the sterically encumbering PCy3
ligand and tetrahedral bisphosphine complexes for smaller
phosphines such as PMe3.6 Our most recent efforts in this area
have noted that bisphenoxides with small substituents in the
2,6-positions, such as halogens, provide dimeric metal species
that have the capacity for binding only one tricyclohexylphos-
Note! Cadmium compounds and their wastes are extremely toxic
and must be handled carefully. Cadmium waste products should be
stored in a separate, clearly marked container.
Synthesis of Cadmium(2,6-difluorophenoxide)2(PCy3)2, (2). A 10-
mL THF solution of 2,6-difluorophenol (0.120 g, 0.92 mmol) and PCy3
(0.260 g, 0.92 mmol) was added concurrently to a 5-mL THF solution
of Cd[N(SiMe3)2]2 (0.20 g, 0.46 mmol), leading to a clear solution that
was stirred at room temperature for 2 h. The solution was then
concentrated to approximately 5 mL and placed in a freezer at -20
°C. Colorless block crystals formed after several days. The supernate
was transferred off by cannula, and the crystals were dried under
vacuum to yield 0.183 g of product (76%). Anal. Calcd for C48H72O2F4-
P2Cd: C, 61.89; H, 7.81. Found: C, 60.50; H, 7.16. The disagreement
between calculated and observed C/H analysis is due to the presence
of a slight impurity of the dimeric monophosphine derivative. 1H NMR
(C6D6): δ 0.97-2.25 (m, 66H, PC6H11), 6.18 (m, 2H, 4-H), 6.73 (t,
4H, [3,5-H]). 13C {H} NMR (C5D5N): δ 27.26-32.65 (PC6H11), 108.86
(t, [4-C6H3]), 111.94 (m, [3,5-C6H3]), 148.16 (t, JC-F ) 15.6 Hz, [ipso-
C6H3]), 157.83 (dd, JC-F1 ) 235.45 Hz, JC-F2 ) 11.07 Hz, [2,6-C6H3]).
19F {H} NMR(C6D6): δ -134.74.
(1) Darensbourg, D. J.; Holtcamp, M. W. Macromolecules 1995, 28, 7577.
(2) Darensbourg, D. J.; Holtcamp, M. W.; Struck, G. E.; Zimmer, M. S.;
Niezgoda, S. A.; Rainey, P.; Robertson, J. B.; Draper, J. D.;
Reibenspies, J. H. J. Am. Chem. Soc. 1999, 121, 107.
(3) Darensbourg, D. J.; Niezgoda, S. A.; Reibenspies, J. H.; Draper, J. D.
Inorg. Chem. 1997, 36, 5686.
(4) Darensbourg, D. J.; Niezgoda, S. A.; Draper, J. D.; Reibenspies, J. H.
J. Am. Chem. Soc. 1998, 120, 4690.
(5) Darensbourg, D. J.; Zimmer, M. S.; Rainey, P.; Larkins, D. L. Inorg.
Chem. 2000, 39, 1578.
Solid-State 113Cd NMR. The solid-state NMR spectra were acquired
utilizing a Bruker MSL 300 superconducting spectrometer with a
magnet operating at 7.05 T (Larmor frequency of 66.546 MHz for
113Cd). The samples were ground and packed into zirconium oxide rotors
with Kel-F end caps for use in a 7-mm magic-angle spinning probe
from Bruker. Spinning speeds were regulated by a Bruker spin-rate
controller. All chemical shifts and tensor elements are referenced to
an external sample of 0.1 M Cd(ClO4)2 in D2O solution at 25 °C with
positive shifts denoting movement of resonances to lower shielding.
(6) Darensbourg, D. J.; Rainey, P.; Larkins, D. L.; Reibenspies, J. H. Inorg.
Chem. 2000, 39, 473.
(7) Jonsson, N. B. H.; Tibell, L. A. E.; Evelhock, J. L.; Bell, S. J.;
Sudmeier, J. L. Proc. Natl. Acad. Sci. U.S.A. 1980, 7, 3269.
(8) Bertini, I.; Gray, H. B.; Lippard, S. J.; Valentine, J. S. In Bioinorganic
Chemistry; University Science Books: Mill Valley, CA, 1994.
(9) Goel, S. C.; Chiang, M. Y.; Buhro, W. E. J. Am. Chem. Soc. 1990,
112, 6724.
(11) Darensbourg, D. J.; Wildeson, J. R.; Yarbrough, J. C.; Reibenspies,
J. H. J. Am. Chem. Soc. 2000, 122, 12435.
(10) Darensbourg, D. J.; Zimmer, M. S.; Rainey, P.; Larkins, D. L. Inorg.
Chem. 1998, 37, 2852.
(12) Burger, H.; Sawodny, W.; Wannaget, V. J. Organomet. Chem. 1965,
3, 113.
10.1021/ic010104q CCC: $20.00 © 2001 American Chemical Society
Published on Web 06/01/2001

3640 Inorganic Chemistry, Vol. 40, No. 14, 2001
Table 1. Crystallographic Data for 2
Notes
empirical
formula
fw
cryst syst
space group
V, Å3
C48H72F4O2P2Cd
931.4
monoclinic
C2/c
4509.4(7)
4
Z
a, Å
b, Å
c, Å
â, deg
T, K
28.232(3)
10.1021(9)
20.5468(8)
129.6890(10)
110(2)
1.372
0.609
d(calcd), g/cm3
abs coeff, mm-1
R, %a
7.37
17.51
Rw, %a
a R ) ∑||Fo| - |Fc||/∑Fo. Rw ) {[∑w(Fo - Fc2)2/[∑w(Fo )2]}1/2
.
2
2
Cd(NO3)2‚4H2O was used as a secondary standard relative to the
cadmium perchlorate solution in D2O to account for the cross-
polarization with proton decoupling. The recycle delay used was 15 s
with a 1H π/2 pulse width of 5.25 µs and a contact time of 15 ms for
all samples. Principle elements of the shielding tensor were extracted
utilizing the WINFIT software package from Bruker Instruments
running on an Adosea Pentium personal computer.
X-ray Crystallography. A Bausch and Lomb 10× microscope was
used to identify a suitable colorless crystal of 2 from a representative
sample of crystals of the same habit. The representative crystal was
coated in a cryogenic protectant (i.e., mineral oil and apezeon grease)
and was then fixed to a glass fiber, which in turn was fashioned to a
copper mounting pin. The mounted crystals were then placed in a cold
nitrogen stream (Oxford) maintained at 110 K on a Bruker SMART
1000 three circle goniometer.
Crystal data and details of data collection for the complexes are
provided in Table 1. The X-ray data were collected on a Bruker CCD
diffractometer and covered more than a hemisphere of reciprocal space
by a combination of three sets of exposures; each exposure set had a
different æ angle for the crystal orientation, and each exposure covered
0.3° in ω. The crystal-to-detector distance was 4.9 cm. Crystal decay
was monitored by repeating the data collection for 50 initial frames at
the end of the data set and analyzing the duplicate reflections; crystal
decay was negligible. The space group was determined based on
systematic absences and intensity statistics.13
Figure 1. Ball-and-stick representation of 1.
shown to provide the dimer 1.11 1 has been structurally
characterized in the solid state by X-ray crystallography (ball-
and-stick representation shown in Figure 1) and by cross-
polarization/magic-angle spinning(CP/MAS) 113Cd NMR spec-
troscopy. During the characterization of 1 in solution via 113Cd
and 31P NMR spectroscopy, it was noted that spectra of the
complex were very dependent on the presence of slight excesses
of PCy3, with a concomitant dependence on temperature in these
instances. That is, 31P and 113Cd NMR spectra of 1 in a THF
solution with less than 0.1 equiv of PCy3 revealed the presence
of two metal species in solution, both exhibiting 113Cd-31P
coupling at low temperature. After 6 equiv of PCy3 was added
to 1, the 31P NMR spectrum at ambient temperature displayed
two broad signals at ∼30 and ∼10 ppm, which correspond
roughly to the positions of 1 and free PCy3, respectively.
Similarly, the 113Cd NMR spectrum exhibited two broad
resonances centered at ∼220 and ∼360 ppm at ambient
temperature. As the temperature was lowered to -80 °C, both
spectra were simplified, the 31P NMR to a resonance at 22.4
ppm (J Cd-P ) 1590 Hz and J Cd-P ) 1665 Hz) and the 113Cd
111
113
113
NMR to a triplet at 395.1 ppm (J Cd-P ) 1678 Hz), see Figure
The structure was solved by direct methods. 2 crystallizes with
disorder with respect to the phenolic ligands. Full-matrix least-squares
anisotropic refinement for all non-hydrogen atoms yielded R(F) and
wR(F2) values at convergence, as indicated in Table 1. Hydrogen atoms
were placed in idealized positions with isotropic thermal parameters
fixed 1.2 or 1.5× the value of the attached atom. Neutral atom scattering
factors and anomalous scattering factors were taken from the Interna-
tional Tables for X-ray Crystallography, Vol. C.
2. This behavior was ascribed to the equilibrium reaction
depicted in eq 1, which is shifted to the right upon lowering the
temperature.
For the title compound, data reduction, SAINTPLUS (Bruker);14
program(s) used to solve the structure, SHELXS-86 (Sheldrick);15
program(s) used to refine the structure, SHELXL-97 (Sheldrick);16
program(s) used for molecular graphics, SHELXTL version 5.0
(Bruker);17 software used to prepare material for publication, SHELXTL
version 5.0 (Bruker).17
Results and Discussion
The reaction of Cd[N(SiMe3)2]2 with 2 equiv of 2,6-
difluorophenol in the presence of 1 equiv of PCy3 has been
(13) SMART 1000 CCD; Bruker Analytical X-ray Systems: Madison, WI,
1999.
(14) SAINT-Plus, version 6.02; Bruker: Madison, WI, 1999.
(15) Sheldrick, G. SHELXS-86: Program for Crystal Structure Solution;
Institut fur Anorganische Chemie der Universitat: Gottingen, Ger-
many, 1986.
(16) Sheldrick, G. SHELXS-97: Program for Crystal Structure Refinement;
Institut fur Anorganische Chemie der Universitat: Gottingen, Ger-
many, 1997.
We have synthesized 2 directly in an isolated yield of 76%
by the reaction of Cd[N(SiMe3)2]2 with 2 equiv each of 2,6-
(17) SHELXTL, version 5.0; Bruker: Madison, WI, 1999.

Notes
Inorganic Chemistry, Vol. 40, No. 14, 2001 3641
Table 2. Selected Bond Distances (Å) and Bond Angles (deg) for
2a,b
Cd(1)-O(1)
Cd(1)-P(1)
O(1)-C(1)
2.249(7)
2.6484(14)
1.329(11)
O(1)-Cd(1)-O(1AA)
O(1)-Cd(1)-P(1)
P(1)-Cd(1)-P(1A)
O(1)-Cd(1)-P(1A)
O1AA-Cd(1)-P(1A)
Cd(1)-O(1)-C(1)
108.3(3)
99.60(19)
119.79(6)
91.3(2)
103.7(2)
133.0(6)
a Estimated standard derivatives are given in parentheses. b Sym-
metry-generated atoms designated as (nA).
Figure 4. Space-filling model of 2.
Figure 2. (a) Temperature-dependent 113Cd NMR spectra of 1 in the
presence of 6 equiv of PCy3 in THF solution. (b) CP/MAS solid-state
113Cd NMR spectrum of 2. Peaks marked by asterisks are spinning
sidebands.
Figure 5. Space-filling model of 1.
containing sterically bulky phenoxides, Cd(O-2,6-tBu2C6H3)2-
PCy3,6 and the dimer 1,11 where Cd-P distances of 2.5274(12)
and 2.5426(10) Å were observed, respectively. Similarly, the
Cd-O distance of 2.249(7) Å noted in 2 is somewhat longer to
that seen for the Cd-O (terminal phenoxide) bond length in 1
of 2.124(2) Å. The shortest Cd‚‚‚F nonbonding distance in 2
was determined to be that involving Cd(1)‚‚‚F(1) of 3.043 Å.
The CP/MAS solid-state 113Cd NMR spectrum of 1 has
Figure 3. Thermal ellipsoid representation of 2. Disorder observed in
phenoxide ligands illustrated in ball-and-stick drawing in insert.
previously been reported to consist of a doublet with an isotropic
shift of 252.6 ppm with J Cd-P ) 2297 Hz.11 Correspondingly,
113
difluorophenol and PCy3 in THF solution. X-ray quality crystals
of 2 were obtained after the reaction solution was concentrated
and subsequently cooled to -20 °C. 2 crystallizes as a distorted
tetrahedron that is disordered in the unit cell with regard to the
phenoxide ligands. Figure 3 contains a thermal ellipsoid
representation of 2, along with a partial atomic numbering
scheme. Table 2 contains a selected listing of bond distances
and bond angles. The Cd-P distance of 2.6484(14) Å observed
in the bistricyclohexylphosphine derivative (2) is, as expected,
longer than that found in the trigonal monoPCy3 complex
the 113Cd NMR spectrum of 1 in THF solution is consistent
with the solid-state spectrum, exhibiting a doublet with a
113Cd-P
chemical shift of 232.2 ppm with J
) 2480 Hz at -80
°C. Similarly, the solution 113Cd NMR spectrum of 2 in THF
113
at -40 °C (δ ) 395.1 ppm and J Cd-P ) 1678 Hz, see Figure
2) correlates with that obtained in the solid state. That is, the
solid-state 113Cd NMR spectrum of 2 displays a triplet (J
113Cd-P
) 1653 Hz) with an isotropic shift (σiso) of 381.5 ppm (see
insert in Figure 3). Hence, the solid-state 113Cd NMR observa-
tions on 1 and 2 confirm that the facile equilibrium process

3642 Inorganic Chemistry, Vol. 40, No. 14, 2001
Notes
noted in solution can be ascribed to the reaction depicted in eq
1. Furthermore, the 31P NMR spectra in solution (δ ) 22.4 at
-80 °C) and in the solid state (σiso ) 21.3 ppm) of 2 are
consistent with this affirmation.18
Interestingly, as would be anticipated on the basis of the
ambient temperature 113Cd NMR spectrum illustrated in Figure
2, after a pure sample of 2 was dissolved in THF, the 113Cd
NMR spectrum revealed that the principal species in solution
at ambient temperature is 1, with 2 being favored as the
temperature is lowered. Indeed, the equilibrium is completely
shifted in the direction of 2 at -80 °C. This in turn requires
the enthalpy and entropy changes associated with eq 1 to be of
the same sign, most likely both being negative. By way of
contrast, because of the steric requirements of two very bulky
PCy3 ligands and the greater propensity of cadmium vs zinc
for phosphine ligands, the zinc analogue complex of 1 is
observed in the presence of excess PCy3 to be stable relative to
the bisphosphine monometallic complex. Certainly, this steric
crowding in the latter complex is amply demonstrated in the
space-filling model of 2 (see Figure 4). Furthermore, upon close
scrutiny of the space-filling model of 1, it would appear that
the mechanism for formation of 2 from 1 and PCy3 must be
dissociative in nature, for there is little space for an attack at
the metal center of PCy3 at the intact dimer (see Figure 5).
Acknowledgment. Financial support from the National
Science Foundation (CHE 99-10342 and CHE 98-07975 for the
purchase of X-ray equipment) and the Robert A. Welch
Foundation is greatly appreciated.
(18) Because the solution state 113Cd and 31P NMR spectra of 1 and its
monomer THF adduct, Cd(O-2,6-F2C6H3)2(PCy3)(THF), may be
similar, we cannot completely rule out that the chemical species
involved in the equilibrium described in eq 1 are not the monomeric
THF adduct and the bisphosphine complex. Studies in progress on
the chloro analogue of 1, where Cd‚‚‚F spin-spin coupling cannot
obscure the 113Cd-111Cd coupling (90-120 Hz) normally seen in
dimeric phenoxide complexes, should definitely resolve this issue.
Supporting Information Available: Complete details in CIF format
of the X-ray diffraction study of 2. This material is available free of
charge via the Internet at http://pubs.acs.org.
IC010104Q