Preparation and NMR Spectra of Difluoromethylated Silver(I) and Silver(III) Compounds. Structure of [PNP][Ag(CF2H)4]

Article abstract of DOI:10.1021/ic961398w

The thermally unstable [Ag(CF₂H₂] moiety is obtained in solution by reaction of silver(I) salts with Cd(CF₂H)₂ in DMF/diglyme at -80 °C. Oxidation with I₂ leads to the argentate(III) [Ag(CF₂H)₄]^-, which has been isolated as its PNP salt. According to DSC/TG analysis, the exothermic decomposition of this air-stable salt proceeds in two steps at 121 and 150 °C with formal elimination of one and three CFH units, respectively. Difluoromethylation of [Ag(CN)₂]^- in the presence of acetyl chloride leads to the argentate(I) [Ag(CF₂H)(CN)]^-, which is oxidized by bromine at low temperatures to the thermally labile [trans-Ag(CF₂H)₂(CN)₂]^- anion. Stable argentates(III) containing both difluoromethyl and trifluoromethyl groups are obtained by reaction of [Ag(CF₃)_n(CN)_4-n]^- (n = 1, 2-cis, 2-trans, and 3) with Cd(CF₂H)₂ - the cis and trans configurations of the reactants being retained in their products for n = 2. The compounds are identified and characterized by multinuclear NMR spectroscopy. Crystals of [PNP][Ag(CF₂H)₄] belong to the tetragonal space group P4₃ with a = 9.475(1) A, c = 42.159(6) A, and Z = 4. The coordination of the silver atom is approximately square-planar with an average Ag-C bond length of 2.083(14) A?. The ligands are so oriented that each C-H bond is geared between the C-F bonds of a neighboring CF₂H group-the overall symmetry being approximately C_4h.

Full text of DOI:10.1021/ic961398w

3160
Inorg. Chem. 1997, 36, 3160-3166
Preparation and NMR Spectra of Difluoromethylated Silver(I) and Silver(III) Compounds.
Structure of [PNP][Ag(CF2H)4]†
Reint Eujen,* Berthold Hoge, and David J. Brauer
Anorganische Chemie, Fachbereich 9, Universita¨t-GH Wuppertal, 42097 Wuppertal, Germany
ReceiVed NoVember 20, 1996X
The thermally unstable [Ag(CF2H)2]- moiety is obtained in solution by reaction of silver(I) salts with Cd(CF2H)2
in DMF/diglyme at -80 °C. Oxidation with I2 leads to the argentate(III) [Ag(CF2H)4]-, which has been isolated
as its PNP salt. According to DSC/TG analysis, the exothermic decomposition of this air-stable salt proceeds in
two steps at 121 and 150 °C with formal elimination of one and three CFH units, respectively. Difluoromethylation
of [Ag(CN)2]- in the presence of acetyl chloride leads to the argentate(I) [Ag(CF2H)(CN)]-, which is oxidized
by bromine at low temperatures to the thermally labile [trans-Ag(CF2H)2(CN)2]- anion. Stable argentates(III)
containing both difluoromethyl and trifluoromethyl groups are obtained by reaction of [Ag(CF3)n(CN)4-n]- (n )
1, 2-cis, 2-trans, and 3) with Cd(CF2H)2sthe cis and trans configurations of the reactants being retained in their
products for n ) 2. The compounds are identified and characterized by multinuclear NMR spectroscopy. Crystals
of [PNP][Ag(CF2H)4] belong to the tetragonal space group P43 with a ) 9.475(1) Å, c ) 42.159(6) Å, and Z )
4. The coordination of the silver atom is approximately square-planar with an average Ag-C bond length of
2.083(14) Å. The ligands are so oriented that each C-H bond is geared between the C-F bonds of a neighboring
CF2H groupsthe overall symmetry being approximately C4h.
Introduction
reactive uncoordinated species has become available only
recently.6 It has been successfully used to prepare (difluoro-
methyl)copper(I) compounds which could be oxidized to
copper(III) species. Of these, the cuprate [Cu(CF2H)4]- has
been isolated as its PNP salt. Structural characterization has
shown that the symmetry of the anion, C4h, is at variance with
the D2d arrangement found in [PNP][Cu(CF3)4],7 and the
different nature of the interligand interactions in these complexes
has been delineated. Furthermore, the mixed difluoromethyl/
trifluoromethyl cuprates(III), [Cu(CF3)n(CF2H)4-n]-, n ) 1-3,
have been identified and characterized by multinuclear NMR
spectroscopy.6
Silver in its oxidation state +3 is well stabilized by per-
fluorinated methyl groups. The surprisingly stable argentate(III)
[Ag(CF3)4]- was described first by Dukat and Naumann.1 It is
readily formed when silver(I) salts are treated with Cd(CF3)2.
The intermittently observed bis(trifluoromethyl)argentate(I) dis-
proportionates to silver metal and Ag[Ag(CF3)4]. This “binary”
salt is stable in air and water; its thermal degradation requires
a temperature of almost 100 °C,1 while the decomposition of
the PNP or PPh4 salts begins around 150 °C.2 An almost square-
planar coordination of the silver atom, which is expected for a
diamagnetic d8 complex, has been confirmed by X-ray analyses
on a tetrathiofulvene-based cation-radical salt3 as well as on its
tetraphenylphosphonium salt.2 In a preceding publication,4 we
showed that the CF3 groups may be replaced in part by functions
such as the CN or the CH3 group and halides or by donor
molecules. The stability of these silver(III) compounds de-
creases as the number of CF3 groups decreases. The tetra-
phenylphosphonium salts of [trans-Ag(CF3)2(CN)2]- and
[Ag(CF3)3(CH3)]- have been isolated as air-stable compounds
and characterized by NMR and X-ray structure analyses.
Instead of the replacement of complete CF3 groups, substitu-
tions within the CF3 groups may be useful to test the relative
stability of such silver(III) compounds, the simplest being an
exchange of one fluorine atom by a hydrogen atom. A reagent
suitable for the transfer of CF2H groups is bis(difluoromethyl)-
cadmium, Cd(CF2H)2. While the DMF complex of this
compound has been known for some time,5 the much more
In this contribution, we extend these studies to the corre-
sponding (difluoromethyl)silver(III) moieties, which, due to the
1
presence of the highly abundant spin /2 isotopes 109Ag and
107Ag, are especially suited for certain identification by NMR
techniques.
Experimental Section
(a) General Procedures and Materials. Volatile material was
handled on a vacuum line equipped with greaseless stopcocks. Air-
sensitive, nonvolatile material was manipulated under an argon or a
nitrogen atmosphere. Raman spectra were obtained on a Cary 82
spectrometer, with Kr+ excitation at 647.1 nm. Infrared spectra were
recorded with a Bruker IFS 25 spectrometer as KBr pellets for solids
and with 10 cm gas cells for gases. Combined calorimetric and
thermogravimetric analyses (DSC/TGA) were made with a simultaneous
DSC/TG instrument, NETZSCH STA 409. Elemental analyses were
performed with a Perkin-Elmer 240B microanalysis device.
Chemicals were obtained from commercial sources and used without
further purification. Bis(difluoromethyl)cadmium was prepared by
following published procedures.6
(b) NMR Spectra. NMR spectra were recorded with a Bruker ARX
400 (1H, 400.13 MHz; 13C, 100.63 MHz; 19F, 376.50 MHz) or a Bruker
AC 250 instrument (1H, 250.13 MHz; 19F, 235.36 MHz; 13C, 62.90
* Corresponding author. Phone: 049-202-439-2503. Fax: 049-202-439-
2901. E-mail: eujen@wrcd1.urz.uni-wuppertal.de.
† Dedicated to Professor Hans Bu¨rger on the occasion of his 60th
birthday.
X Abstract published in AdVance ACS Abstracts, June 15, 1997.
(1) Dukat, W.; Naumann, D. ReV. Chim. Miner. 1986, 23, 589.
(2) (a) Naumann, D. Private communication. (b) Roy, T. Dissertation,
Universita¨t Ko¨ln, 1993.
(5) Hartgraves, G. A.; Burton, D. J. J. Fluorine Chem. 1988, 39, 425.
(6) Eujen, R.; Hoge, B.; Brauer, D. J. J. Organomet. Chem. 1996, 519, 7.
(7) Naumann, D.; Roy, T.; Tebbe, K. F.; Crump, W. Angew. Chem., Int.
Ed. Engl. 1993, 32, 1482.
(3) Geiser, U.; Schlueter, J. A.; Williams, J. M.; Naumann, D.; Roy, T.
Acta Crystallogr. 1995, B51, 789.
(4) Eujen, R.; Hoge, B.; Brauer, D. J. Inorg. Chem. 1997, 36, 1464.
S0020-1669(96)01398-5 CCC: $14.00 © 1997 American Chemical Society

Difluoromethylated Silver Compounds
Inorganic Chemistry, Vol. 36, No. 14, 1997 3161
Table 1. Crystallographic Data for [PNP][Ag(CF2H)4]
MHz; 109Ag, 11.64 MHz). Spectra were referenced to external TMS
(1H, 13C), CFCl3 (19F), and 1 m AgNO3 in D2O (109Ag), corrections
being made for different lock substances. The last spectrometer was
equipped with a 19F decoupler, which used the pulse trains generated
formula
fw
cryst syst
C40H34AgF8NP2
850.5
tetragonal
F
calc (g cm-3
T (°C)
λ (Å)
)
1.493
23
1.54184
5.68
0.247-0.377
1
by the H decoupler and allowed 19F broad-band decoupling over a
space group P43
µ(Cu KR) (mm-1
)
range of more than 20 kHz after tuning the decoupler coil to the fluorine
frequency. If possible, polarization transfer from 19F by means of the
DEPT8 pulse sequence was used to record the 1D spectra of the less
sensitive nuclei 109Ag and 13C. For the latter, this was particularly
helpful because all resonances without a coupling to the fluorine nuclei
were suppressed. Two-dimensional spectra (13C-19F, 109Ag-19F) were
recorded with the pulse sequence given by Bax9 and were used for
assignments and for determining relative signs of coupling constants.
Computer simulations of high-order NMR spectra were carried out
with the program gNMR.10
a (Å)
c (Å)
V (Å3)
Z
9.475(1)
transm factor
42.159(6)
3784.7(9)
4
R(F)a (Fo > 4σ(Fo)) 0.045
Rw(F2)b (all data)
0.117
4
a R(F) ) ∑||Fo| - |Fc||/∑|Fo|. b Rw(F2) ) [∑w(Fo2 - Fc2)2/∑wFo ]1/2
.
chloride in 3 mL of diglyme was added dropwise. To remove the
precipitated CdCl2, the suspension was treated with 150 mL of CHCl3
and filtered, alnd the filtrate was repeatedly washed with water and
dried over MgSO4. Recrystallization of the crude yellow powder gave
550 mg (0.8 mmol) of [PPh4][trans-Ag(CF3)2(CF2H)2] as colorless air-
stable crystals. Anal. Calcd for C28H22AgF10P: H, 3.23; C, 48.93.
Found: H, 3.57; C, 48.83. Infrared (cm-1) (KBr pellet): 3083 vw,
3067 w, 3056 vw, 2929 w, 1587 w, 1485 m, 1439 s, 1260 vs, 1191 w,
1112 s, 1089 vs, 1022 vs, 753 m, 726 s, 690 s, 525 vs.
(c) Synthetic Reactions. (i) Solutions of Bis(difluoromethyl)-
argentate(I). To a solution of 3.9 g (8.8 mmol) of (CF2H)2Cd‚
tetraglyme in 5 mL of a 1:1 mixture of DMF and diglyme was added
a solution of 1.5 g (8.8 mmol) of AgNO3 in 5 mL of DMF/diglyme at
-80 °C with vigorous stirring. The solids contained in the black
suspension were separated from the mixture by centrifugation at -80
°C. Conversion rates and yields were monitored by integration of 19
F
Other mixed difluoromethyl/trifluoromethyl argentates(III) were
obtained similarly but were not isolated.
NMR signals after addition of a reference quantity of fluorobenzene.
(ii) Synthesis of [{(C6H5)3P}2N][Ag(CF2H)4]. A solution of iodine
(ca. 5 mmol) in diglyme was added slowly to a vigorously stirred
solution of 8.8 mmol of [Ag(CF2H)2]- in 10 mL of DMF at -50 °C
until the iodine color persisted. The solution was allowed to warm to
ambient temperature within 1 h. A measured quantity of fluorobenzene
was added as an integration standard in order to determine the amount
of the [Ag(CF2H)4]- ion being formed, and a stoichiometric quantity
of [PNP]Cl was added. The suspension was treated with 150 mL of
CHCl3 and filtered, and the filtrate was repeatedly washed with water
and dried over MgSO4. After removal of the solvent, a yellow powder
was obtained. Recrystallization from CHCl3/diethyl ether afforded 2.6
g (3.1 mmol) (35% yield) of [PNP][Ag(CF2H)4] as colorless crystals.
Anal. Calcd for C40H34AgF8NP2: H, 4.03; C 56.49; N, 1.65. Found:
H, 4.04; C, 55.73; N, 1.7. Infrared (cm-1) (KBr pellet): 3060 w, 2909
m, 1589 w, 1482 w, 1438 m, 1297 s, 1265 vs (broad), 1185 m, 1118
s, 1008 vs, 952 vs, 802 w, 741 m, 721 s, 690 s, 666 m, 618 w, 599 w,
549 s, 535/525 s, 498 s, 457 m. Raman (cm-1): 3068 s, 2917 w, 1592
m, 1250 w, 1187 w, 1166 w, 1113 m, 1031 m, 1003 vs, 667 m, 618 m,
580 sh, 567 s, 532 w, 370 w, 267 w, 252 w, 238/230 s, 107 m, 90 s,
75 s. DSC/TG analysis: exothermic decomposition peaks at 121 °C,
∆m -3.4% (calcd for elimination of CFH: ∆m -3.7%), and 150 °C,
∆m -11.1% (calcd for elimination of 3 CFH: ∆m -11.7%).
(iii) Solutions of (Difluoromethyl)cyanoargentate(I) and Bis-
(difluoromethyl)dicyanoargentate(III). A solution consisting of 2.0
g (10 mmol) of K[Ag(CN)2] and 2.2 g (5 mmol) of (CF2H)2Cd‚
tetraglyme in 10 mL of DMF was stirred for 1 h at 0 °C. After the
solution was cooled to -30 °C, 790 mg (10 mmol) of acetyl chloride
in 3 mL of diglyme was added slowly. Volatile material was removed
in vacuo at -5 °C. The amount of the argentate(I) [Ag(CF2H)(CN)]-
being formed was determined by low-temperature 19F NMR spectros-
copy with reference to a known amount of added fluorobenzene.
To a cold (-50 °C) solution of 10 mmol of [Ag(CF2H)(CN)]- in
10 mL of DMF and 3 mL of diglyme was added with vigorous stirring
2.7 g of bromine in 5 mL of diglyme. After removal of solids by
centrifugation at -50 °C, products were identified by NMR spectros-
copy. When the solution was allowed to warm above -30 °C,
decomposition and formation of CF2HCN, δ(19F) -125.1 ppm (d, 52.8
Hz), were observed.
(d) Crystal Structure Determination of [{(C6H5)3P}2N][Ag-
(CF2H)4]. Crystals were grown by isothermal distillation of diethyl
ether into a CHCl3 solution of [PNP][Ag(CF2H)4]. A truncated
tetragonal-bipyramidal specimen was glued to a glass fiber and mounted
on a Siemens P3 diffractometer which employed graphite-mono-
chromatized Cu KR radiation. The Laue group (4/m) found by
diffractometry was consistent with that revealed by a preliminary
Weissenberg examination. The lattice constants were derived from the
setting angles of 32 centered reflections by least-squares calculations.
An octant of data (5° e 2θ e 138°) was collected using the θ-2θ
scan mode. Of the 4076 reflections measured, 3595 were unique, and
3444 had Fo > 4σ(Fo). The intensities were corrected by integration
for absorptionsthe distances between the crystal faces being optimized
so as to reproduce the intensity variations found in ψ profiles of eight
strong reflections.
A large starting fragment was generated by direct methods, and the
remaining non-hydrogen atoms were found by subsequent difference
Fourier syntheses. Hydrogen atoms were placed in idealized positions
(C-H 0.95 Å). The structure was refined on F2 using all data.
Refinement of the Flack11 absolute structure parameter (x ) 0.00(1))
indicated that the crystal belongs to the space group P43 rather than to
P41, the choice found for the copper crystal.6 Crystal data and
refinement details are included in Table 1. Structure solution and
refinement and graphical display were all made with a SHELXTL12
program package.
Results and Discussion
(a) Synthetic Aspects. Silver(I) salts are readily trifluoro-
methylated at low temperatures to [Ag(CF3)2]- by means of
Cd(CF3)2 in solvents like CH3CN. At ambient temperature the
silver(I) species disproportionates to silver(III) and metallic
silver:1
Cd(CF3)2 + Ag+ glyme/DMF8 [Ag(CF3)2]- + Cd2+ (1)
30 °C
2[AgI(CF3)2]- + Ag+ f [AgIII(CF3)4]- + 2Ag0
(iv) Synthesis of [P(C6H5)4][trans-Ag(CF3)2(CF2H)2]. A solution
of 650 mg (1.0 mmol) of [PPh4][trans-Ag(CF3)2(CN)2] in 10 mL of
DMF was treated at -30 °C with 900 mg (2.1 mmol) of (CF2H)2Cd‚
tetraglyme for 10 min. A solution of 310 mg (4 mmol) of acetyl
In DMF, the disproportionation is suppressed almost completely.
When the corresponding (difluoromethyl)cadmium com-
pound, Cd(CF2H)2, is reacted with AgX (X ) NO3-, CF3COO-,
or BF4-) in CH3CN at -45 °C, there is no evidence for the
(8) Van de Ven, F. J. M. Multidimensional NMR in Liquids; VCH
Publishers, Inc.: New York, 1995; p 127 ff.
(9) (a) Bax, A.; Morris, G. A. J. Magn. Reson. 1981, 42, 501. (b) Bax,
A. J. Magn. Reson. 1983, 53, 517. (c) Wilde, J. A.; Bolten, P. H. J.
Magn. Reson. 1984, 59, 343.
(10) Budzelaar, P. H. M. gNMR Version 3.65; Cherwell Scientific Publish-
ing Ltd.: Oxford, U.K., 1996.
(11) Flack, H. D. Acta Crystallogr. 1983, A39, 876.
(12) Sheldrick, G. M. SHELXTL PC Version 5.0: An Integrated System
for SolVing, Refining and Displaying Crystal Structures from Diffrac-
tion Data; Siemens Analytical X-ray Instruments Inc.: Madison, WI,
1994.

3162 Inorganic Chemistry, Vol. 36, No. 14, 1997
Eujen et al.
Scheme 1
At first glance, the high stability of the difluoromethylated
cuprate(III) or argentate(III) is surprising because the electronic
stabilization by a CF2H group should be less than that of a CF3
group. Furthermore, steric shielding should be more favorable
in the perfluorinated species. However, relative destabilization
of the perfluorinated compound is also noted for the fluorinated
dimethylcadmium compounds Cd(CFnH3-n)2. While pure Cd-
(CF3)2 decomposes below 0 °C via CF2 elimination,13 Cd(CF2H)2
is stable below 90 °Csevolving difluoroethylenes between 90
and 100 °C and tetrafluoroethane in a vigorous reaction above
100 °C.6 The decomposition of Cd(CF3)2 is favored by both
the formation of CdF2 and the relatively high thermodynamic
stability of difluorocarbene, CF2. An elimination of the
thermodynamically less stable CFH entity from Cd(CF2H)2 is
not that favorable, and homolytic cleavage of the metal-carbon
bonds with formation of the metal becomes competitive. As a
consequence, the thermal stability of Cd(CF2H)2 is more like
that of Cd(CH3)2.14 Transfer of these observations to the Ag(III)
and Cu(III) species helps to understand the high thermal stability
of the anions [Ag(CF2H)4]- and [Cu(CF2H)4]- despite their
lower electronic stabilization. The barrier to CFH elimina-
tion seems to almost cancel the loss in electronic and steric
stability.
formation of the expected [Ag(CF2H)2]- ion. Instead, decom-
position to HF2CCF2H and metallic silver dominates, traces of
the silver(III) anion [Ag(CF2H)4]- being formed. Usage of
DMF down to its melting point (-60 °C) does not alter the
course of the reaction. Only when diglyme is added in order
to allow the even lower reaction temperature of -80 °C is the
formation of [Ag(CF2H)2]- clearly indicated by NMR spec-
troscopy (Scheme 1). Warming the reaction mixture to -50
°C results in decomposition to tetrafluoroethane and silver metal,
while disproportionation to silver(III) moieties analogous to eq
1 plays a minor role. In contrast to the case of the corresponding
trifluoromethylation reaction, an adduct such as Ag(CF2H)‚D
is not detected.
Bis(difluoromethyl)argentate(I) is less prone to oxidation than
its trifluoromethyl counterpart. No oxidation to silver(III) is
observed upon exposure to air. Application of tetraethylthiuram
disulfide, [Et2NC(S)S]2, as the oxidizing agent gives low yields
(5-10%) of the argentate(III) [Ag(CF2H)4]-. Improved conver-
sion is achieved by low-temperature oxidation with bromine or
iodine, the best yield, ca. 40%, being obtained with iodine
although some CF2HI is formed as well.
For further insight into the relative electronic stabilization
of silver(III) by CF3 and CF2H groups, it is essential to open
another decomposition channel which involves products with
better comparable thermodynamic stabilities. The cyano group
is an attractive coligand, since the anions in the trifluoromethyl
-
series [Ag(CF3)n(CN)4-n
]
have already been characterized.
Their stability is known to decrease as the number of CN groups
increases, CF3CN and argentates(I) being the decomposition
products.4
[Ag(RF)n(CN)4-n]
- 9∆8 [Ag(RF)n-1(CN)3-n]- + RFCN (3)
The conditions for the elimination of the fluorinated acetonitriles
from the [trans-Ag(RF)2(CN)2]- anions (RF ) CF2H, CF3)
should basically reflect the electronic influence of the RF group.
The preparation of bis(difluoromethyl)dicyanoargentate(III)
follows the principles developed in the synthesis of the
corresponding trifluoromethylated argentates(III).4 Silver cya-
nide, suspended in DMF, requires room temperature to react
with Cd(CF2H)2, but only decomposition products such as
CF2HCN and silver metal are observed. Usage of the more
soluble K[Ag(CN)2] at 0 °C results in an equilibrium containing
ca. 20% [Ag(CF2H)(CN)]-.
2[Ag(CF2H)2]- + I2 DMF/-50 °C8 [Ag(CF2H)4]- + AgI + I-
(2)
Addition of chloroform and cation exchange with [PNP]Cl
facilitate the removal of cadmium halides. After washing with
water, colorless crystals of [PNP][Ag(CF2H)4] are isolated in a
35% yield based on the amount of Cd(CF2H)2 consumed.
Like their CF3 or their copper analogs, crystals of [PNP][Ag-
(CF2H)4] are not sensitive to air, water, or light. Their thermal
stability is quite comparable; that is, the degradation requires a
temperature of more than 100 °C. As shown by combined DSC/
TG analysis, the decomposition of [PNP][Ag(CF2H)4] takes
place in two exothermic steps at 121 and 150 °C, respectively.
While the first is connected with a loss of mass corresponding
approximately to one CFH unit (∆m -3.4%), the second (∆m
-11.1%) is in accord with the loss of the remaining three CFH
groups. Examination of the gaseous products, however, indi-
cates that the process is somewhat more complex. In addition
to the expected cis- and trans-difluoroethylenes, 1,1,2,2-
tetrafluoroethane as well as some CF2H2 is detected, which
points to a competing homolytic cleavage of the Ag-C bonds.
Attempts to identify any fluoroargentate(III) or other silver
compounds among the solid decomposition residues have failed.
DMF/0 °C
[Ag(CN)2]- + Cd(CF2H)2 y
z
[Ag(CF2H)(CN)]- + Cd(CF2H)(CN) (4)
An almost quantitative (>95%) conversion to the
[Ag(CF2H)(CN)]- ion is achieved by in situ treatment of the
reaction mixture at -30 °C with an equimolar quantity of acetyl
chloride. Acyl cyanides can be prepared from acyl halides with
copper or silver cyanides,15 and this reaction is used here both
to eliminate ionic cyanide and to produce a highly reactive
chloro complex as an intermediate, eq 5.
(13) Eujen, R.; Hoge, B. J. Organomet. Chem. 1995, C51, 503.
(14) Price, S. J. W.; Trotman-Dickenson, A. F. Trans. Faraday Soc. 1957,
53, 939.
(15) (a) March, J. AdVanced Organic Chemistry, 4th ed.; John Wiley &
Sons: New York, 1992; p 495. (b) Hu¨nig, S.; Schaller, R. Angew.
Chem., Int. Ed. Engl. 1982, 21, 36.

Difluoromethylated Silver Compounds
Inorganic Chemistry, Vol. 36, No. 14, 1997 3163
Table 2. Selected Bond Distances (Å) and Angles (deg) for
[PNP][Ag(CF2H)4]
Ag-C(1)
Ag-C(2)
2.08(1)
2.10(2)
Ag-C(3)
Ag-C(4)
2.07(1)
2.08(2)
C(1)-Ag-C(2)
C(1)-Ag-C(3)
C(1)-Ag-C(4)
91.7(8)
177.5(8)
91.4(10)
C(2)-Ag-C(3)
C(2)-Ag-C(4)
C(3)-Ag-C(4)
88.4(8)
176.8(9)
88.4(10)
Figure 1. Perspective drawing of the [Ag(CF2H)4]- anion in [PNP]-
[Ag(CF2H)4] with 20% probability thermal ellipsoids for the non-
hydrogen atoms.
[Ag(CN)2]- +CH COCl/DMF/-30 °C8
3
-CH3COCN
+Cd(CF2H)2
{[Ag(CN)Cl]-}
8 [Ag(CF2H)(CN)]- (5)
DMF/-30 °C
As for the analogous (trifluoromethyl)cyanoargentate(I),4 the
oxidation of the (difluoromethyl)cyanoargentate(I) by bromine
at -50 °C leads to the formation of [trans-Ag(CF2H)2(CN)2]-
in 20-30% yield, the course of the reaction being monitored
by low-temperature NMR spectroscopy (see below).
2[Ag(CF2H)(CN)]- + Br2 DMF/-50 °C8
[trans-Ag(CF2H)2(CN)2]- + AgBr + Br- (6)
Minor amounts of the argentate(III) [Ag(CF2H)3(CN)]- are also
clearly detected by its characteristic NMR patterns, while no
evidence is found for the presumably less stable anions [cis-
Ag(CF2H)2(CN)2]- and [Ag(CF2H)(CN)3]-. The isolation of
[trans-Ag(CF2H)2(CN)2]- as its PNP salt is thwarted by its low
thermal stabilitysquantitative decomposition with evolution of
CF2HCN being observed above -30 °C. Furthermore, all
attempts till now to replace the CN groups have resulted in
extensive decomposition. The pronounced difference in thermal
stabilities of the CF3 and CF2H dicyanoargentates(III) with
respect to the elimination of the respective fluoromethyl
cyanides +90 °C vs -30 °Csunderscores the greater suitability
of the CF3 groups for stabilizing these highly oxidized species.
While mixed difluoromethyl/trifluoromethyl cuprates are
accessible by difluoromethylation of the (N,N-diethyldithio-
carbamato)copper(III) complex (CF3)2CuS2CNEt2, the corre-
sponding reaction of (CF3)2AgS2CNEt2 with Cd(CF2H)2 is
unspecific and leads to a large variety of unidentified products.
The members of the series [Ag(CF3)n(CF2H)4-n]- are, however,
accessed by difluoromethylation of the (trifluoromethyl)cyano-
argentates [Ag(CF3)n(CN)4-n]-, n ) 1-3. While the attempt
to replace the cyano groups directly results in quantitative
decomposition, the intermediate low-temperature chlorination
with acetyl chloride affords the new argentates specifically and
almost quantitatively for n ) 2 and 3.
Figure 2. 109Ag NMR spectrum of [PNP][Ag(CF2H)4]. The spectrum
was recorded by means of DEPT polarization transfer from 19F (14 400
scans, relaxation delay 2 s, evolution time 20 ms, variable pulse length
25°, no decoupling).
50%, all other members of the series being formed simulta-
neously in substantial quantities. Their formation may be
ascribed to a metathesis reaction of the dichlorinated inter-
mediates similar to that found for (trifluoromethyl)chloro-
argentates(III).4
(b) Description of the Crystal Structure. The crystal
structure of [PNP][Ag(CF2H)4] has been determined by X-ray
diffraction, and a drawing of the [Ag(CF2H)4]- anion is depicted
in Figure 1. Selected bond distances and angles are listed in
Table 2. The symmetry of the anion is approximately C4h.
Deviations of the atoms of the Ag, C(1), C(2), C(3), C(4)
fragment from coplanarity are less than (0.03(1) Å, and the
Ag-C bond lengths are equal to within experimental error. Their
average value, 2.083(14) Å, agrees well with that reported for
[PNP][Ag(CF3)4] (2.082(13) Å)2 and is not significantly shorter
than the Ag-CF3 distances in [PPh4][Ag(CF3)2(CN)2] and [PPh4]-
[Ag(CF3)3(CH3)], 2.105(4) and 2.119(10) Å, respectively.4
Each C-H bond of the anion lies near the coordination plane
of the metal and is geared between the C-F bonds of a
neighboring CF2H group. Such a rotamer is obviously favored
both by dipolar forces and by the minimization of F‚‚‚F
nonbonded repulsions, and the same conformation has been
found for the anion of [PNP][Cu(CF2H)4]. The latter compound
is isomorphous with the argentatesthe 0.15(2) Å longer silver-
carbon bonds provoking a 1.5% increase in the unit cell. The
large atomic displacement parameters of the fluorine atoms show
(i) CH COCl, (ii) (CF H) Cd8
3
2
2
-
[Ag(CF3)n(CN)4-n
]
-30 °C
-
[Ag(CF3)n(CF2H)4-n
]
(7)
For n )1, the yield of [Ag(CF3)(CF2H)3]- amounts to less than

3164 Inorganic Chemistry, Vol. 36, No. 14, 1997
Eujen et al.
Table 3. NMR Data for (Difluoromethyl)cyanoargentates(I) and -argentates(III)a
δ(109Ag)
δ(19F)
δ(13C)
2J(AgF)
2J(AgH)
1J(AgC)
2J(HF)c
1J(CF)
1J(CH)
AgI Compounds
[Ag(CF2H)2]- d
634.3
579.1
-113.0
-112.4
155.2
151.7
53.7
63.5
11.3
13.8
194.4
264.5
43.7
43.3
279.9
278.5
146.0
150.5
[Ag(CF2H)(CN)]- e
AgIII Compounds
trans-[Ag(CF2H)2(CN)2]- f
2110.8
-89.7
-102.9
-102.2h
132.7
15
18
35
3
b
b
42
b
b
49
49
48
334
b
b
182
b
b
[Ag(CF2H)3(CN)]- g
b
b
b
a In DMF/diglyme; internal lock C6D6. Chemical shifts are in ppm; coupling constants in Hz. References are internal 13C6D6 at 128.0 ppm,
external C19FCl3, and 1 m 109AgNO3 in D2O. Couplings involving Ag nuclei are listed for the isotope 109Ag; J(107AgE) ) J(109AgE)/1.150(5).
n
n
2
4
3
3
b Not observed or resolved. c For a trans-Ag(CF2H)2 unit, the listed value corresponds to J(HF) + J(HF). d At 213 K; J(CF) 6.3 Hz; J(CH) 1.7
Hz. e At 273 K; δ(13CN) 144.0 ppm (broad). f At 238 K; J(FF)av ca. 18 Hz; J(CH)tr 16 Hz; J(CF)tr 42 Hz; δ(13CN) 100.5 ppm; J(109AgCN) 149
4
3
3
1
Hz. g At 273 K. h Trans to CN.
Table 4. 19F NMR Data for Mixed (Difluoromethyl)(trifluoromethyl)argentates(III)a
CF3
CF2H
2J(AgF)
CF3-CF3
4J(FF)
CF3-CF2H
4J(FF)
CF2H-CF2H
4J(FF)
δ(19F)
2J(AgF)
δ(19F)
2J(HF)c
[Ag(CF2H)4]-
-111.2
24.4
48.4
7.3 (tr)
∼1.5 (cis)
10 / 8 (tr)
1.7 (cis)
1.6 (cis)
[Ag(CF3)(CF2H)3]-e
cis-[Ag(CF3)2(CF2H)2]-
-37.3
-35.8
38.2
36.8
-108.8
-106.3d
-103.6
22.5
29.3
26.4
47.9
47.9
47.6
8.6 (tr)
1.7 (cis)
9.9 (tr)
3.3 (cis)
2.7 (cis)
3.4 (cis)
4.5 (cis)
12.2 (tr)
trans-[Ag(CF3)2(CF2H)2]-
-36.7
-35.1d
-33.0
-32.7
47.8
44.5
34.7
40.7
-104.4
-98.3
20.0
22.7
48.6
47.1
b
13.3/11.3 (tr)
[Ag(CF3)3(CF2H)]-f
4.5 (cis)
[Ag(CF3)4]-g
12.8 (tr)
6.2 (cis)
a As Ph4P+ salts in CDCl3 (internal reference for 13C, 77.0 ppm) at 298 K. Otherwise, see Table 3. b Not observed. c For a trans-Ag(CF2H)2 unit,
2
4
the value corresponds to J(HF) + J(HF). d Trans to CF3. e 4J(CF3CF2H)tr 2 Hz. f 4J(CF3CF2H)tr 3.7 Hz. g As Ag[Ag(CF3)4] in CH3CN.
Table 5. 109Ag and 13C NMR Data for Mixed (Difluoromethyl)(trifluoromethyl)argentates(III)a
CF3
1J(AgC)
CF2H
1J(CH)
δ(109Ag)
2J(AgH)
δ(13C)
3J(CH)
δ(13C)
1J(AgC)
3J(CH)
[Ag(CF2H)4]- c
2041.9
1.8
140.1
88.5
171.5
8.6 (tr)
2.9 (cis)
4.3 (cis)
cis-[Ag(CF3)2(CF2H)2]-
2089.7
1.4
144.6
112.6
12.7 (tr)
5.2 (cis)
4.2 (cis)
6.0 (cis)
14.9 (tr)
133.7
91.5
183.1
trans-[Ag(CF3)2(CF2H)2]-
2098.0
2136.5
<0.5
<0.5
140.5d
139.6e
143.9
139.0
144.7
133.9
104.2
120.0
137.1
131.8
70.8
76.6
175.5
186.2
11.2 (tr)
[Ag(CF3)3(CF2H)]-
[Ag(CF3)4]- f
2233.1
a As Ph4P+ salts in CDCl3 (internal reference for 13C, 77.0 ppm) at 298 K. Otherwise, see Table 3. b Not observed. c 4J(HF)trans 1.8 Hz; ∆δ(F13C-
F12C) ) 0.101 ppm; 1J(CF) 304.5 Hz; 3J(CF)cis 9.8 Hz; 3J(CF)trans 23.0 Hz. d 1J(CF) 388 Hz; 3J(CF)trans 39.1 Hz. e Trans to CF3. f As Ag[Ag(CF3)4]
1
3
3
in CH3CN; J(CF) 393.1 Hz; J(CF)cis 8.2 Hz; J(CF)trans 45.8 Hz.
that the above-mentioned interactions between the CF2H groups
fail to provide conformational rigidity. As a result the structural
parameters determined for the CF2 fragments (C-F 1.26(5) Å,
F-C-F 109(4)°) are of low accuracy. Similar observations
have been made for [PNP][Cu(CF2H)4], for which the C-F
distances and F-C-F angles average 1.30(3) Å and 106(2)°,
respectively.
and to the central atom facilitate the unambiguous identification
and characterization of fluorinated methylsilver compounds. The
connectivity of a CF2H group to the central silver atom is
1
apparent from the couplings of the 19F, H, and 13C nuclei to
the 109Ag (48.2 %) and 107Ag (51.8 %) nuclei, which give rise
to a characteristic pair of doublets with coupling constants in a
ratio of 1.15:1. Despite their high abundance, the two silver
nuclei are usually not readily detected directly due to their low
and negative magnetogyric ratios as well as to their extremely
long relaxation times. The coupling to the sensitive 19F nuclei,
however, allows a polarization transfer to the silver nuclei by
DEPT or INEPT pulse sequences or by recourse to inverse
spectroscopy. These techniques widely compensate the deficits
of the insensitive nuclei and make the 109Ag resonance readily
detectable as long as an exchange of the CF3 groups is slow on
the NMR time scale. The 109Ag (19F-DEPT) spectrum of [PNP]-
[Ag(CF2H)4] is shown in Figure 2. Chemical shifts and coupling
constants are listed in Tables 3-5.
The conformation exhibited by these anions contrasts with
that found for the [M(CF3)4]- species in the otherwise isomor-
phous (same space group and similar cell constants) crystals of
[PNP][Cu(CF3)4]6 and [PNP][Ag(CF3)4].2 The [M(CF3)4]-
anions show D2d symmetry. Such a conformation for an
[M(CF2H)4]- entity would have, in stark contrast to observation,
each M-C-H fragment perpendicular to the coordination plane
of the metal with the hydrogen atoms alternately above and
below the plane. Such conformational differences in otherwise
isomorphous crystals seem surprising and attest to the low
specificity of the interactions of these anions with the diphos-
phazene counterion.
Difluoromethyl groups are easily recognized by their doublet
1
2
(c) NMR Spectra. The variety and abundance of spin /2
(19F) and triplet structures (1H) with J(FH) being ca. 43 Hz
nuclei (19F, 13C, 1H, 109Ag, 107Ag) with distinct couplings across
for Ag(I) and ca. 48 Hz for Ag(III) derivatives. Couplings

Difluoromethylated Silver Compounds
Inorganic Chemistry, Vol. 36, No. 14, 1997 3165
4
across the metal atom are not or barely resolvable for J(HH),
<0.5 Hz, and 4J(FH), ca. 2 Hz, respectively, whereas the 4J(FF)
couplings are considerably larger in trans Ag(CF2H)2 units.
These units constitute a [A[X]2]2 spin system similar to that
observed in other compounds with two CF2H groups.6,16,17 The
basic doublet splitting in such a system is given by the sum of
2J(FH) and 4J(FH), both being positive. Because of the
additional couplings to the two silver nuclei, the resulting two
overlapping [A[X]2]2M systems are usually not resolved in all
details.
The complex spin systems of the mixed CF3/CF2H argen-
tates(III) are greatly simplified by selective homodecoupling
experiments which also demonstrate that the relative signs of
the 2J(AgF) coupling constants of the AgCF3 and AgCF2H units
are equal. Inspection of the 19F NMR data, Table 4, shows
that both CF3 and CF2H resonances experience a high-field shift
as the number of CF2H groups increases. Whether cis or trans
relations are involved is revealed by the magnitude of the 4J(FF)
coupling constant, which is less than 4 Hz for cis but exceeds
7 Hz for trans arrangements. The size of the latter increases
markedly with increasing ionicity of the other ligands. Thus
the smallest value is found for the [Ag(CF2H)4]- ion, and
replacement of a CF2H group by the more ionic CF3 entity
increases the 4J(FF) coupling. The largest value, ca. 18 Hz, is
observed for the [trans-Ag(CF2H)2(CN)2]- ion. The same trend
has also been noted for the CF3-substituted argentates(III), the
largest value, 30 Hz, being found for the [trans-Ag(CF3)2Br2]-
ion.4
Figure 3. Correlation of 1J(AgC) vs 2J(AgF) in the mixed CF3/CF2H-
-
containing argentates [Ag(CF2H)n(CF3)4-n
]
(n ) 0-4) and trans-
[Ag(CF2H)2(CN)2]-. Closed diamonds correspond to Ag(III)-CF2H,
closed circles to Ag(III)-CF3 units, and open diamonds to the silver(I)
species [Ag(CF2H)2]- and [Ag(CF2H)(CN)]-. The solid line is the fit
line for Ag(III)-CF3 derivatives, taken from ref 4.
Essentially the same tendencies as in the 19F spectra are
observed for the 13C spectra, Table 5. Broad-band 19F decou-
pling greatly simplifies the spectra and enables the determination
of 3J(CH) couplings, which differ significantly for cis and trans
4
arrangements. In addition to the J(FF) and the much less
3
readily available J(CF) couplings, these constants are useful
in the determination of the stereochemistry of the respective
anion. A proton-decoupled 13C spectrum has been recorded for
[Ag(CF2H)4]- and analyzed as the sum of two XA2B4C2M spin
systems (X ) 13C; A, B, and C ) 19F; M ) 109Ag and 107Ag,
respectively) by computer simulation.
1
As shown in Figure 3, the good correlation of J(AgC) and
2J(AgF) couplings for the silver(III) compounds (closed squares
and circles for CF2H and CF3 groups, respectively) is not
followed by the silver(I) species (open squares). Also included
in this figure is the fit line for the various CF3AgIII moieties.4
In general, the 2J(AgF) coupling constants of the CF2H groups
are significantly smaller than those found in CF3 derivatives;
e.g., the value of 24.4 Hz for [Ag(CF2H)4]- is to be compared
with the 40.7 Hz coupling of the [Ag(CF3)4]- ion (Table 4).
The trend in 2J(AgF) couplings in the mixed CF3/CF2H
derivatives nicely follows the pattern outlined previously.4 Since
a CF2H group is less electronegative and therefore more
covalently bound than a CF3 group, the former will demand a
silver hybrid orbital which contains more 4d and less 5s
character than that demanded by a CF3 group. In accord with
this model, the 2J(AgF) coupling involving the CF2H groups is
smaller in [trans-Ag(CF3)2(CF2H)2]- than in [Ag(CF2H)4]-
whereas that of the AgCF3 unit is larger in [trans-Ag(CF3)2-
(CF2H)2]- than in [Ag(CF3)4]-.
Figure 4. 109Ag NMR spectrum of the silver(I) species [Ag(CF2H)2]-,
recorded at -60 °C.
times. Figure 4 displays the low-temperature 109Ag spectrum
of the [Ag(CF2H)2)]- ion, showing a quintet of triplet fine
structure due to couplings to the fluorines and hydrogens,
respectively. In Figure 5, the 109Ag spectrum of a 1:1 mixture
of the cis- and trans-[Ag(CF3)2(CF2H)2]- anions is shown along
with a simulation. With respect to their CF3 analogs, the
resonances of the (difluoromethyl)argentates(I) near +600 ppm
are shifted to low fields whereas the opposite effect is noticed
for the silver(III) compounds with chemical shifts around +2100
ppm.
Conclusion
Silver(I) and silver(III) species are readily distinguished by
their 109Ag chemical shifts, but polarization transfer techniques
are necessary to enable registration of these spectra in justifiable
In a preceding paper4, we showed that it is possible to replace
the CF3 groups in the argentate(III) [Ag(CF3)4]- by other ligands
such as the cyano group although the stability of these
complexes decreases with decreasing number of CF3 groups.
The amazingly high stability of the “binary” argentate(III) may
be traced to several factors. In terms of ligand field theory,
(16) Bu¨rger, H.; Eujen, R.; Moritz, P. J. Organomet. Chem. 1991, 401,
249.
(17) Eujen, R.; Jahn, N.; Thurmann, U. J. Organomet. Chem. 1994, 465,
2
2
153.
the strong field of the CF3 groups renders the dx -y orbital

3166 Inorganic Chemistry, Vol. 36, No. 14, 1997
Eujen et al.
Figure 5. 109Ag NMR spectrum of a 1:1 mixture of trans- and cis-[Ag(CF2H)2(CF3)2]-. The simulated spectrum is shown in the lower trace.
hardly accessible for donors. Steric shielding by the fluorines
which are positioned above and below the AgC4 plane might
contribute additionally. A general stabilization of orbitals with
respect to analogous methyl derivatives, denoted as the “per-
fluoromethyl effect”, is evident from the photoelectron spectra
of CF3Pt18 or CF3Ge19 compounds. In contrast to main group
chemistry, where M-CF3 bonds are rather easily cleaved
because either strong covalent M-F bonds or ionic fluorides
with high lattice energies are formed, the degradation of
Ag(III)-CF3 units to Ag(III)-F bonds is less favorable.
Reductive elimination of C2F6 does not appear to be a
thermodynamically or kinetically favored pathway. Replace-
ment of fluorines by hydrogen atoms, e.g. substitution of one
or two CF3 groups by CH3 groups, clearly destabilizes the
oxidation state +3 though the anion [Ag(CF3)3(CH3)]- is still
stable at ambient temperature.4
The comparable stabilities of the [Ag(CF3)4]- and
[Ag(CF2H)4]- anions may be an artifact arising from the lower
tendency of the latter to eliminate a carbene. When kinetically
and thermodynamically comparable decomposition pathways are
present as in case of the [trans-Ag(RF)(CN)2]- anions, the
difference in electronic stabilization becomes evident by the
respective decomposition temperatures, +90 °C for RF ) CF3
and -30 °C for RF ) CF2H.
Acknowledgment. Financial support by the Deutsche For-
schungsgemeinschaft and Fond der Chemischen Industrie is
gratefully acknowledged.
(18) Yang, D.-S.; Bancroft, G. M.; Puddephatt, R. J.; Tse, J. S. Inorg. Chem.
1990, 29, 2496.
(19) (a) Drake, J. E.; Gorzelska, K.; White, G. S.; Eujen, R. J. Electron
Spectrosc. Relat. Phenom. 1982, 26, 1. (b) Drake, J. E.; Gorzelska,
K.; Helbing, K.; Eujen, R. J. Electron Spectrosc. Relat. Phenom. 1982,
26, 19. (c) Drake, J. E.; Eujen, R.; Gorzelska, K. Inorg. Chem. 1982,
21, 1784.
Supporting Information Available: An X-ray crystallographic file,
in CIF format, for [PNP][Ag(CF2H)4] is available on the Internet only.
Access information is given on any current masthead page.
IC961398W