Monomeric silver(I) β-diketiminate complexes

Article abstract of DOI:10.1021/om0603621

New silver(I) β-diketiminate complexes have been prepared and characterized. Monomeric complexes with ethylene (4), triphenylphosphine (5), and IMes (IMes =1,3-dimesitylimidazol-2-ylidene) (8) as well as a dinuclear acetonitrile complex (3) were characterized by ¹H NMR, ¹³C NMR, elemental analysis, and X-ray crystallography. These complexes are stable in the solid state at ambient temperature and inert atmosphere.

Full text of DOI:10.1021/om0603621

4054
Organometallics 2006, 25, 4054-4057
Communications
Monomeric Silver(I) â-Diketiminate Complexes
Hendrich A. Chiong and Olafs Daugulis*
Department of Chemistry, UniVersity of Houston, Houston, Texas 77004
ReceiVed April 26, 2006
Summary: New silVer(I) â-diketiminate complexes haVe been
prepared and characterized. Monomeric complexes with eth-
ylene (4), triphenylphosphine (5), and IMes (IMes )1,3-
dimesitylimidazol-2-ylidene) (8) as well as a dinuclear aceto-
1
nitrile complex (3) were characterized by H NMR, 13C NMR,
elemental analysis, and X-ray crystallography. These complexes
are stable in the solid state at ambient temperature and inert
atmosphere.
While monomeric â-diketiminate-copper complexes are well-
studied and a number of them have been structurally character-
ized,1 the corresponding derivatives of silver and gold remain
unknown.2 The problems preventing the synthesis of such
complexes are the sensitivity to light, the oxidizing nature of
silver(I),3 and the ease of formation of coordination polymers
and supramolecular complexes.4 Closely related fluorinated
triazapentadienyl complexes containing a [N{(C3F7)C(Dipp)N}2]-
silver(I) ligand (Figure 1, Dipp ) 2,6-diisopropylphenyl) have
recently been described.5 In this case, the triazapentadienyl group
behaves as a chelating bidentate ligand, forming monomeric
complexes with silver in the presence of triphenylphosphine.
However, it was observed that the [N{(C3F7)C(Dipp)N}2] ligand
behaves as a κ1-ligand, where the central nitrogen atom forms
linear silver complexes in the presence of acetonitrile or tert-
butylisocyanide. Shimokawa and Itoh have recently disclosed
the synthesis and structural characterization of silver complexes
Figure 1. â-Diketimine ligands.
Scheme 1. General Synthetic Route for Fluorinated Silver
â-Diketiminate Complexes
* To whom correspondence should be addressed. E-mail: olafs@uh.edu.
(1) A general review about â-diketiminate complexes: (a) Bourget-Merle,
L.; Lappert, M. F.; Severn, J. R. Chem. ReV. 2002, 102, 3031. (b) Dai, X.;
Warren, T. H. J. Am. Chem. Soc. 2004, 126, 10085. (c) Brown, E. C.;
Aboelella, N. W.; Reynolds, A. M.; Aullo´n, G.; Alvarez, S.; Tolman, W.
B. Inorg. Chem. 2004, 43, 3335. (d) Laitar, D. S.; Mathison, C. J. N.; Davis,
W. M.; Sadighi, J. P. Inorg. Chem. 2003, 42, 7354. (e) Spencer, D. J. E.;
Reynolds, A. M.; Holland, P. L.; Jazdzewski, B. A.; Duboc-Toia, C.; Le
Pape, L.; Yokota, S.; Tachi, Y.; Itoh, S.; Tolman, W. B. Inorg. Chem. 2002,
41, 6307. (f) Dai, X.; Warren, T. H. Chem. Commun. 2001, 1998. (g)
Yokota, S.; Tachi, Y.; Nishiwaki, N.; Ariga, M.; Itoh, S. Inorg. Chem. 2001,
40, 5316.
supported by â-diketiminate ligands.6 These complexes form
both macrocyclic and polymeric structures. No monomeric
complexes were observed. The silver ion was also shown to be
easily reduced to the metal with the concurrent dimerization of
the â-diketiminate ligands.
Silver complexes have found important applications in metal-
mediated organic reactions.7 Recently, several methods for
silver-mediated functionalization of C-H bonds have been
developed.8 The characterization of new monomeric silver(I)-
â-diketiminate complexes serves as an important stage in the
further study of the chemistry of these species and their eventual
applications.
(2) Recently, a gold â-diketiminate complex was used as a catalyst in
aerobic alcohol oxidation. However, the reported Au compound was not
characterized. Guan, B.; Xing, D.; Cai, G.; Wan, X.; Yu, N.; Fang, Z.;
Yang, L.; Shi, Z. J. Am. Chem. Soc. 2005, 127, 18004,
(3) Connelly, N. G.; Geiger, W. E. Chem. ReV. 1996, 96, 877.
(4) (a) Khlobystov. A. N.; Blake, A. J.; Champness, N. R.; Lemenovskii,
D. A.; Majouga, A. G.; Zyk, N. V.; Schro¨der, M. Coord. Chem. ReV. 2001,
222, 155. (b) Dong, Y.-B.; Zhao, X.; Huang, R.-Q.; Smith, M. D.; zur Loye,
H.-C. Inorg. Chem. 2004, 43, 5603. (c) Hong, M.; Su, W.; Cao, R.; Fujita,
M.; Lu, J. Chem. Eur. J. 2000, 6, 427. (d) Venkataraman, D.; Lee, S.; Moore,
J. S.; Zhang, P.; Hirsch, K. A.; Gardner, G. B.; Covey, A. C.; Prentice, C.
L. Chem. Mater. 1996, 8, 2030. (e) Xu, Z.; Kiang, Y.-H.; Lee, S.;
Lobkovsky, E. B.; Emmott, N. J. Am. Chem. Soc. 2000, 122, 8376. (f)
Min, K. S.; Suh, M. P. J. Am. Chem. Soc. 2000, 122, 6834.
(6) Shimokawa, C.; Itoh, S. Inorg. Chem. 2005, 44, 3010.
(7) (a) Cui, Y.; He, C. J. Am. Chem. Soc. 2003, 125, 16202. (b) Yao,
X.; Li, C.-J. J. Org. Chem. 2005, 70, 5752. (c) Wadamoto, M.; Yamamoto,
H. J. Am. Chem. Soc. 2005, 127, 14556. (d) Dias, H. V. R.; Browning, R.
G.; Polach, S. A.; Diyabalanage, H. V. K.; Lovely, C. J. J. Am. Chem. Soc.
2003, 125, 9270. (e) Cho, G. Y.; Bolm, C. Org. Lett. 2005, 7, 4983.
(8) (a) Urbano, J.; Belderra´ın, T. R.; Nicasio, M. C.; Trofimenko, S.;
D´ıaz-Requejo, M. M.; Pe´rez, P. J. Organometallics 2005, 24, 1528. (b)
Cui, Y.; He, C. Angew. Chem., Int. Ed. 2004, 43, 4210. (c) Dias, H. V. R.;
Browning, R. G.; Richey, S. A. Lovely, C. J. Organometallics 2004, 23,
1200.
(5) (a) Siedle, A. R.; Webb, R. J.; Brostrom, M.; Chou, S.-H.; Weil, D.
A.; Newmark, R. A.; Behr, F. E.; Young, V. G., Jr. Inorg. Chem. 2003, 42,
2596. (b) Dias, H. V. R.; Singh, S. Inorg. Chem. 2004, 43, 7396.
10.1021/om0603621 CCC: $33.50 © 2006 American Chemical Society
Publication on Web 07/13/2006

Communications
Organometallics, Vol. 25, No. 17, 2006 4055
Figure 3. ORTEP view of the molecular structure of 4. Thermal
ellipsoids are drawn to encompass 40% probability. Hydrogens are
omitted for clarity. Selected bond distances (Å) and angles (deg):
C(1)-C(2) 1.392(19), Ag(1)-C(1) 2.20(2), Ag(1)-C(2) 2.24(2),
Ag(1)-N(1) 2.198(6), Ag(1)-N(2) 2.193(6), N(1)-Ag(1)-N(2)
88.9(2).
Figure 2. ORTEP view of the molecular structure of 3. Thermal
ellipsoids are drawn to encompass 40% probability. Hydrogens and
fluorines are omitted for clarity. Selected bond distances (Å) and
angles (deg): Ag(1)-N(1) 2.246(4), Ag(1)-N(2) 2.624(4), Ag-
(1)-N(3) 2.272(4), Ag(1)-N(4) 2.203(4), N(1)-Ag(1)-N(4)
149.88(16), N(1)-Ag(1)-N(3) 119.56(15), N(3)-Ag(1)-N(4)
88.03(16).
the reaction mixture indicated that replacement of the acetonitrile
ligand in 3 by the sterically demanding alkyne did not occur.
On the other hand, facile complexation of the less sterically
demanding ethylene with silver, producing complex 4, is
noteworthy. Although several silver-ethylene complexes are
known, only a few have been structurally characterized.9 The
yellow solid is stable at -35 °C under argon or at room
temperature if stored under slight ethylene pressure. The
ethylene ligand is quite labile when 4 is dissolved in dichlo-
romethane or benzene. Although the complex survives NMR
characterization, the solution of 4 will eventually precipitate
silver metal on standing. Decomposition is avoided if an excess
We describe here the first preparation and isolation of stable
monomeric silver(I) complexes with â-diketiminate ligands. Two
methods for stabilizing silver(I) complexes have been used: (1)
the introduction of electron-withdrawing groups in the â-diketim-
inate ligands (1, Figure 1) and (2) the use of an N-heterocyclic
carbene (NHC) as a supporting ligand for silver. This allows
the isolation of an unfluorinated â-diketiminate silver-IMes
complex. The use of fluorinated â-diketiminate ligands permits
the isolation of acetonitrile, ethylene, and triphenylphosphine
silver complexes.
1
of ethylene is added to the solution of the complex. The H
NMR spectrum of 4 in C6D6 shows a resonance at 3.78 ppm
that is assigned to the signal of the ethylene ligand protons.
The coordinated ethylene in 4 is kinetically labile in solution.
Saturating the C6D6 solution of 4 with ethylene at room
temperature causes the coalescence of the peak at 3.78 ppm
and that of the unbound ethylene. This observation is indicative
of a fast dynamic exchange between bound and free ethylene
at room temperature.
Synthesis of Highly Fluorinated â-Diketiminate Silver
Complexes. Compound 1 can be deprotonated by a slight excess
of Ag2O in warm acetonitrile (Scheme 1). Removal of the
solvent under reduced pressure yields the acetonitrile complex
3 as a yellow-brown solid. Both the solid and its solutions in
organic solvents are stable in the presence of light but
decompose slowly when exposed to air for prolonged periods.
The solid does not lose coordinated acetonitrile when stored
under reduced pressure for 2 days at 50 °C. The 1H NMR spectra
of 3 taken in C6D6 display the resonance due to bound
acetonitrile at 0.49 ppm. Interestingly, crystallographic examina-
tion of the yellow crystals of 3 grown from pentane-CH2Cl2
at -35 °C shows a well-defined dinuclear silver complex
consistent with the stoichiometry implied by elemental analysis
(Figure 2). The structure shows that two acetonitrile molecules
are bound to one of the two silver ions in 3. The Ag-N(nitrile)
distances are 2.404 and 2.146 Å.5 Analysis of the structure of
3 shows that two silver ions are arranged to assume a distorted
T-shaped geometry. This complex is a convenient starting
material for the synthesis of a variety of new silver complexes
Single crystals of 4 suitable for X-ray structure determination
were grown from ethylene-saturated benzene solution layered
with pentane at room temperature. The ORTEP diagram of
complex 4 is shown in Figure 3. The solid-state structure of 4
shows an ethylene ligand coordinated to silver in a typical η2-
fashion. Ethylene is coplanar with the â-diketiminate ligand,
and the ligand chelates the silver ion with a bite angle of 88.9-
(2)°. The planar arrangement of ethylene ligands has been
reported also for another Ag-ethylene complex.9b The ethylene
CdC distance is 1.392(19) Å, which is slightly lengthened
compared to the CdC distance of free ethylene (1.337 Å).10
The Ag-C(ethylene) distances for 4 are 2.20(2) and 2.24(2)
Å.
The preparation of 5 was accomplished by stirring 3 with 1
equiv of triphenylphosphine in benzene. The solutions of this
complex in benzene or dichloromethane are stable at room
temperature for several days. Complex 5 can be recrystallized
from hot heptane, and the solution yields yellow crystals upon
cooling. The 31P NMR of 5 in CD2Cl2 at -50 °C shows an
overlapping doublet centered at δ 16.48 (J(Ag-P) ) 632.05 Hz)
and δ 16.49 (J(Ag-P) ) 720.29 Hz) corresponding to 107Ag-
1
(vide infra). Interestingly, the H spectrum is consistent with
either a mononuclear solution structure or a fluxional process
1
exchanging the ligands. No change in H NMR (CD2Cl2) was
observed upon cooling the sample to -80 °C.
Compound 3 undergoes ligand substitution with ethylene and
PPh3 in benzene solution to yield the new silver complexes 4
and 5, respectively (Scheme 1). These complexes were char-
acterized by 1H and 13C NMR spectroscopy, FT-IR, and
elemental analysis (vide infra). An attempted preparation of the
carbon monoxide complex by reacting a solution of 3 with
gaseous CO led to the immediate formation of a silver mirror.
No reaction was observed if 3 was combined with diphenyl-
1
31P and 109Ag-31P resonances. The J(109Ag-31P)/1J(107Ag-
(9) (a) Dias, H. V. R.; Wang, X. J. Chem. Soc., Dalton Trans. 2005,
2985. (b) Krossing, I.; Reisinger, A. Angew. Chem., Int. Ed. 2003, 42, 5725.
(c) Dias, H. V. R.; Wang, Z.; Jin, W. Inorg. Chem. 1997, 36, 6205.
(10) Tables of Interatomic Distances and Configuration in Molecules
and Ions; Sutton, L. E., Ed.; The Chemical Society: London, 1958.
1
acetylene in CD2Cl2. The examination of H NMR spectra of

4056 Organometallics, Vol. 25, No. 17, 2006
Communications
Figure 5. ORTEP view of the molecular structure of 8. Thermal
ellipsoids are drawn to encompass 40% probability. Selected bond
distances (Å) and angles (deg): Ag-C(26) 2.076(5), Ag-N(1)
2.234(5), Ag-N(2) 2.246(4), C(27)-C(28) 1.340(8), C(1)-C(2)-
C(3) 131.9(5), N(3)-C(26)-N(4) 103.4(4).
Figure 4. ORTEP view of the molecular structure of 5. Thermal
ellipsoids are drawn to encompass 40% probability. Hydrogens are
omitted for clarity. Selected bond distances (Å) and angles (deg):
Ag-P(1) 2.3401(11), Ag-N(1) 2.240(3), Ag-N(2) 2.232(3),
N(1)-Ag-N(2) 85.03(13), C(1)-C(2)-C(3) 128.5(4).
Scheme 3. Synthesis of 8
Scheme 2. Synthesis of 2 and 6
coordinated with a neutral ligand, followed by metathesis
reaction with the â-diketiminate salt 6. Silver salts such as PPh3-
AgNO3, PPh3AgCl, Me2SAgNO3, COD-AgNO3, (Pyr)AgNO3,
and (IMes)AgCl (7)14 were explored as potential sources of
stabilized silver. These salts were reacted with either the sodium
or potassium salt of the â-diketiminate ligand using different
conditions. Only one reaction yielded promising results. The
(NHC)AgCl salt proved effective in forming stable unfluorinated
â-diketiminate silver complexes that could be isolated in good
yields. Reaction of (IMes)AgCl with 6 in dichloromethane or
toluene at 0 °C yielded the silver complex 8 as a pale yellow
solid in 61% yield (Scheme 3). Using THF instead of dichlo-
romethane led to precipitation of silver metal. Complex 8 is
partially soluble in hydrocarbon solvents but may be washed
free of undesired contaminants with cold pentane and can be
recrystallized from a benzene-heptane mixture. Slow decom-
position of 8 in solution was observed if manipulations were
performed at ambient temperature. The dichloromethane solution
of the unfluorinated complex is stable only below 0 °C. At room
temperature, silver mirror is formed within minutes. In contrast,
the benzene solutions are stable at room temperature for several
days.
31P) ratio is found to be 1.14, which is close to the characteristic
ratio of 1.15 reported by Muetterties.11
A single crystal of complex 5 grown from warm heptane was
subjected to X-ray structure determination, and the ORTEP
diagram of the monomeric structure is shown in Figure 4.
Complex 5 possesses a Y-shaped trigonal planar geometry. The
Ag-P distance of 2.34 Å is typical for several structurally
characterized Ag(I)-PPh3 complexes.5,12
Synthesis of an Unfluorinated â-Diketiminate Silver
Complex. By adapting a literature procedure, ligand 2 can be
obtained by refluxing a mixture of 1,4-pentanedione with 2.2
equiv of 4-tert-butylaniline in benzene with catalytic amounts
of p-toluenesulfonic acid (Scheme 2).13 Following the success
in the synthesis of a number of silver complexes based on
fluorinated â-diketiminate ligands, attempts were made to
prepare the unfluorinated analogues. Initial reactions to form
silver complexes from the â-diketiminate 6 and silver salts
(AgOTf, AgNO3, AgCl) resulted in immediate precipitation of
silver metal. A strategy was used where a silver salt was first
Single crystals of 8 grown from dichloromethane solution
were analyzed by X-ray crystallography (Figure 5). Inspection
of the ORTEP diagram indicates that 8 is monomeric in the
solid state. The Ag-C(carbene) distance of 2.076(5) Å is slightly
longer than the Ag-C(carbene) distance (2.056(7) Å) reported
for 7.14 The resonance of the C(carbene) could not be located
in the 13C NMR spectrum of 8. This is characteristic also for
other Ag(NHC) complexes.14,15
(11) Muetterties, E. L.; Alegranti, C. W. J. Am. Chem. Soc. 1972, 94,
6386.
(12) (a) Burgos, M.; Crespo, O.; M. Gimeno, C.; Jones, P. G.; Laguna,
A. Eur. J. Inorg. Chem. 2003, 11, 2170. (b) Zhang, F., Bolte, M., Lerner,
H.-W., Wagner, M. Organometallics 2004, 23, 5075. (c) Effendy; Lobbia,
G. G.; Pettinari, C.; Santini, C.; Skelton, B. W.; White, A. H. J. Chem.
Soc., Dalton Trans. 1998, 2739. (d) Effendy; Lobbia, G. G.; Pellei, M.;
Pettinari, C.; Santini, C.; Skelton, B. W.; White, A. H. J. Chem. Soc., Dalton
Trans. 2000, 2123.
(13) Feldman, J.; McLain, S. J.; Parthasarathy, A.; Marshall, W. J.;
Calabrese, J. C.; Arthur, S. D. Organometallics 1997, 16, 1514.
(14) Ramnial, T.; Abernethy, C. D.; Spicer, M. D.; McKenzie, I. D.;
Gay, I. D.; Clyburne, J. A. C. Inorg. Chem. 2003, 42, 1391.

Communications
Organometallics, Vol. 25, No. 17, 2006 4057
In conclusion, new silver(I) â-diketiminate complexes have
been prepared and characterized. The structures of a silver(I)-
ethylene, -acetonitrile, and -triphenylphosphine complexes
supported by fluorinated â-diketiminate ligands and silver(I)-
IMes supported by an unfluorinated â-diketiminate ligand were
determined by X-ray crystallography. At present, investigations
of the reactivity of these complexes are underway and will be
reported later.
Acknowledgment. This research was supported by the UH
(GEAR). We thank Dr. James Korp for collecting and solving
the X-ray structures.
Supporting Information Available: Experimental procedures
and X-ray crystallographic data for 3, 4, 5, and 8. This material is
available free of charge via the Internet at http://pubs.acs.org.
(15) (a) de Fremont, P.; Scott, N. M.; Stevens, E. D.; Ramnial, T.;
Lightbody, O. C.; Macdonald, C. L. B.; Clyburne, J. A. C.; Abernethy, C.
D.; Nolan, S. P. Organometallics 2005, 24, 6301. (b) Garrison, J. C.;
Youngs, W. J. Chem. ReV. 2005, 105, 3978. (c) Arduengo, A. J., III; Dias,
H. V. R.; Calabrese, J. C.; Davidson, F. Organometallics 1993, 12, 3405.
OM0603621