ARTICLE pubs.acs.org/Organometallics
Synthesis and Characterization of Silver(I), Gold(I), and Gold(III) Complexes Bearing Amino-Functionalized N-Heterocyclic Carbenes Christoph Topf,† Christa Hirtenlehner,† Manfred Zabel,‡ Manuela List,§ Michel Fleck,|| and Uwe Monkowius*,† †
Institut f€ur Anorganische Chemie, Johannes Kepler Universit€at Linz, Altenbergerstrasse 69, A-4040 Linz, Austria Zentrale Analytik der Universit€at Regensburg, R€ontgenstrukturanalyse, Universit€atsstrasse 31, D-93053 Regensburg, Germany § Institut f€ur Chemische Technologie Organischer Stoffe, Johannes Kepler Universit€at Linz, Altenbergerstrasse 69, A-4040 Linz, Austria Institut f€ur Mineralogie und Kristallographie, Geozentrum-Universit€at Wien, 1090 Wien, Austria
)
‡
bS Supporting Information ABSTRACT: 1-[2-(Dialkylamino)ethyl]-3-methylimidazolium salts (alkyl = Me (1a), i-Pr (1b)) have been prepared and used as precursors for the synthesis of the corresponding [(NHC)2Ag][AgCl2] complexes (NHC = N-heterocyclic carbene, Me (2a), i-Pr (2b)). Upon treatment of 2a with HBF4, crystals of the unprecedented, NHC-stabilized silver cluster [(NHC)4Ag10Cl10] (5) were obtained and characterized by X-ray diffraction. The crystal structure reveals that the carbene carbon atom exists in the rare μ2-coordination pattern, bridging two Ag(I) atoms with further stabilization of the cluster by numerous argentophilic interactions and a coordination of the amino nitrogen donor to one of the silver atoms. Transmetalation of 2a,b with (R2S)AuCl leads to the respective Au(I) complexes 3a,b, which are further oxidized with Br2 to (NHC)AuBr2Cl (4a,b). In red crystals of 4a the gold atom is coordinated in the unusual square-pyramidal geometry with the amine nitrogen atom in the axial position. Upon dissolution in wet organic solvents the amino group is protonated and the color changes to yellow. In square-planar Au(III) halide complexes electronic absorption spectra are dominated by LMCT absorption bands, but in the case of a square-pyramidal coordination sphere the dz2 orbital is destabilized, becoming the HOMO and causing a low-energy dd absorption. This interpretation is supported by DFT calculations.
’ INTRODUCTION In the past decade silver(I) and gold(I) complexes bearing N-heterocyclic carbenes (NHC) have been thoroughly investigated, whereas corresponding gold(III) complexes have not attracted the same attention.1 The interest in NHCAg(I) complexes has grown significantly since Wang and Lin proved their capability as versatile carbene transfer agents.2 Owing to their easy preparation, a plethora of compounds have been reported. Solid-state structures of Ag(I) compounds have revealed multifarious coordination patterns which are hard to predict. Depending on the substituents at the 1- and 3-positions of the imidazolium heterocycle, the anion, reaction conditions, and solvents used, but typically irrespective of the actual stoichiometry of the starting materials, neutral as well as ionic complexes are formed. For 1:1 complexes, frequently found coordination patterns in the solid state are (1) NHCAgX with either isolated, linear coordinate Ag(I) atoms or (2) aggregated via bridging X anions and (3) an ionic form of the type [(NHC)2Ag][AgX2], sometimes associated via argentophilic interactions. PF6 or BF4 salts typically contain the complex cation [(NHC)2Ag]þ with linearly coordinated Ag(I). Neutral, trigonal-planar complexes of the type (NHC)2AgX are rare, and in the case of X = I silver(I) iodide cluster anions are occasionally formed.14 r 2011 American Chemical Society
NHCAu complexes have been widely studied for numerous applications. In particular, their catalytic activity for unique CC, CO, and CN bond-forming reactions5,6 and their pharmaceutical potential as anticancer, antiarthritis, and antibacterial agents have stimulated intensive research activities.7,8 Still, the chemistry of gold is dominated by the oxidation state þ1 and the linearly coordinated complexes of the type NHC AuIX or [(NHC)2Au]X, sometimes aggregated via aurophilic interactions. They are conveniently synthesized by the reaction of R2SAuX (R2S = Me2S, tetrahydrothiophene, X = halide) with a NHCAg complex or the free carbene. Reports about Au(III) complexes are less common and are usually limited to simple, nonfunctionalized NHCs.6,8,9 Mostly, they are synthesized by the oxidation of the corresponding Au(I) complexes with halogens (Cl2, Br2, I2). The direct synthesis starting from an Au(III) precursor (e.g., KAuCl4) only works with a special functionality on the NHC ligand.10 In general, only a few studies dealing with Au(III) complexes bearing functionalized NHCs have been published.11 The majority of NHCAu(III) complexes are of the type (NHC)AuX3 or [(NHC)2AuX2]X. The exchange of the halides Received: February 16, 2011 Published: April 26, 2011 2755
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Scheme 1
by other anionic (or neutral) ligands seems to be critical and frequently leads to reduction to Au(I) or decomposition. One of the rare examples are the just recently reported complexes of the type (NHC)Au(Ar)Cl2 (Ar = C6H5, C6F5).6 Therefore, synthesis of Au(III) complexes bearing NHC ligands functionalized with donor groups might provide insights into the binding capabilities of these donors toward Au(III). Furthermore, potential hemilabile, mixed donor carbene ligands could prove useful in stabilizing a catalytic gold atom or opening a free coordination site for biomolecules.
’ RESULTS AND DISCUSSION Synthesis and Characterization. The imidazolium salts 1a,b are readily prepared by the reactions of R2NCH2CH2Cl 3 HCl (R = Me, i-Pr) with 1-methylimidazole in acetonitrile solution under reflux conditions as white, hygroscopic powders. Reactions with an excess of Ag2O in DCM yield the Agcarbenes 2a,b in good yields and high purity as white powders (Scheme 1). The stoichiometric reactions with (R2S)AuCl in DCM give the Au(I) complexes 3a,b, which can be oxidized by Br2 at 40 °C to the corresponding (NHC)AuBr2Cl complexes 4a,b. Complexes 3a,b are highly hygroscopic and unstable at ambient temperature. They decompose both in solution and in the solid state within hours and have to be stored at low temperature. All compounds furnish the expected 1H NMR spectra. Upon silver complex formation the signals for the C2H imidazolium protons vanish (see the Supporting Information). As described later, the neutral complexes 4a,b are partially protonated by solvent moisture. Therefore, the 1H NMR spectra of these complexes in DMSO furnishes broad and unresolved signals of both the protonated and deprotonated species (Figures S1 and S2, Supporting Information). Upon addition of solid KOH, spectra with sharp peaks of the neutral, nonprotonated forms result. Under these conditions the complexes decompose quickly and no 13C NMR spectra of the pure nonprotonated 4b could be recorded. In the 13C NMR spectra of the silver or gold complexes the chemical shift for the carbene carbon atom is found between
Table 1. 13C NMR Chemical Shift for the C2 Carbon Atom in Compounds 14 R = Me
a
R = i-Pr
imidazolium
138.1
Ag(I)
182.0
138.4 178.5
Au(I) Au(III)
a 174.3
172.1 172.9
Decomposition during measurement.
172 and 182 ppm, which is approximately 3540 ppm downfield of that for the C2 of the imidazolum cation (Table 1). Due to the low stability (3a,b) and the unknown degree of protonation and halide ligand scrambling reactions (4a,b) no reliable elemental analysis could be given for the gold complexes. Complexes 2a,b tend to crystallize in the form of thin platelets which are of low quality and are not suitable for X-ray diffraction. Suitable prismatic crystals are obtained by slow gas-phase diffusion of diethyl ether into a dilute DCM solution under anaerobic conditions. Both compounds crystallize in monoclinic space groups, 2a in P21/c (Z = 2), 2b in C2/c (Z = 4), in an ionic form with linearly coordinated [(NHC)2Ag]þ cations and [AgCl2] anions aggregated to infinite [þ ] chains by short silversilver contacts (Figures 13, Table 2). The [(NHC)2Ag]þ and the [AgCl2] ions are almost perpendicular to each other. For 2a the imidazolyl ring planes are exactly coplanar, whereas for 2b they deviate from coplanarity (N2C1C10 N20 = 172.0(5)°). The amine nitrogen atoms are not involved in any coordinative bonds. There are subtle effects defining the aggregation of NHCAg complexes in the solid state: A recent publication reported that the competition between the neutral form (NHC)AgCl and the ionic form [(NHC)2Ag][AgCl2] is caused by ligand scrambling reactions and determined by the polarity of the solvent.4b Consequently, the ionic form is favored in the solid state by using polar solvents during the crystallization process. In general, the majority of halogenoargentates are oligo- or polymeric anions; isolated [AgX2] anions (X = halogenide) are scarcely found in 2756
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Figure 1. Molecular structure of 2a (ORTEP; displacement ellipsoids at the 50% probability level, H atoms omitted for clarity). Figure 3. Molecular structure of 2b.
Table 2. Selected Bond Lengths (Å), Bond Angles (deg), and Torsion Angles (deg) in 2a,b 2a
2b
Ag1C1
2.083(5)
2.079(5)
Ag1Cl1
2.330(1)
2.329(2)
Ag1Ag2 C1Ag1C1i
3.376(1) 180
3.198(1) 179.7(2)
Cl1Ag2Cl1i
180
179.2(1)
C1Ag1Ag2Cl1
76.8(4)
92.5(1)
N1C1C1iN1i
180
172.0(5)
Figure 2. Excerpt of a cell plot of the crystalline phase of compound 2a depicting the linear chain formed by close argentophilic interactions.
crystal structures of silver compounds.12 Interestingly, a CSD query revealed that the majority of crystal structures containing an isolated [AgX2] anion are complexes of the type [(NHC)2Ag][AgX2], and among these structures,3,14a,13 only one does not exhibit close Ag 3 3 3 Ag distances, presumably because of steric reasons.4b Also frequently formed is the dimeric [Ag2X4]2 anion bridging two adjacent [(NHC)2Ag]þ cations by short Ag 3 3 3 Ag contacts.1,4,14 The ionic aggregation pattern is governed synergetically by both attractive closed-shell d10d10 as well as Coulombic interactions, but the energetic differences between the different aggregation types seem to be small and the aggregation is very sensitive to small changes in crystallization conditions and alterations of the substitution of the NHC igand. It should be noted that several attempts to slowly crystallize 2a, b and 3a,b under aerobic conditions led to partial decomposition and the formation of the salt [(NHC)2M]þA with a highly disordered and therefore not clearly identifiable anion A. Due to the presence of the basic amino groups, this anion might be a
Figure 4. Asymmetric unit in the cluster compound 5.
carbonate formed from gaseous CO2 captured from laboratory air. Because of the uncertainty of the identity of the anion and the degree of protonation these results are not included in this report. Due to the absence of a crystal structure it is not clear if the complexes 3a,b exist in the ionic form [(NHC)2Au][AuCl2] or the neutral form (NHC)AuCl.15 In order to obtain protonated forms of the complexes 2 and 3, several attempts using different acids (HCl, AcH, picric acid) resulted in the formation of elemental silver or gold. An exception is 2a, where in one reaction batch the treatment of a DCM solution with aqueous HBF4 yields a few crystals of the unprecedented cluster compound 5 (P21/n, Z = 2). The asymmetric 2757
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Figure 6. Neutral complex in crystals of 4a. The relation Br3/Cl1 is 55/ 45. Figure 5. Structure of the silver cluster 5.
Table 3. Selected Bond Lengths (Å) in the Cluster Compound 5 Ag1C1
2.156(6)
Ag3C9
2.304(5)
Ag2C1
2.398(6)
Ag4C9
2.194(7)
Ag2N3 Ag1Ag2
2.460(4) 2.870(1)
Ag3N6 Ag3Ag4
2.481(5) 2.826(1)
Ag2Ag5
3.183(1)
Ag3Ag5
3.044(1)
Ag1Ag5
3.280(1)
Ag4Ag5
3.335(1)
Ag1Cl2
2.558(2)
Ag4Ag10
3.274(1)
Ag1Cl5
2.615(2)
Ag2Cl1
2.743(2)
Ag5Cl1
2.558(2)
Ag2Cl3
2.577(2)
Ag5Cl2
2.683(2)
Ag3Cl1
2.573(2)
Ag5Cl4 Ag5Cl40
2.650(2) 2.664(2)
Ag3Cl3 Ag4Cl20
2.630(2) 2.771(2)
Ag4Cl50
2.756(2)
Ag4Cl40
2.602(2)
unit of 5 contains two NHC ligands, five silver atoms, and five chlorine atoms (Figure 4). Each carbene carbon atom binds in a slightly asymmetric μ2 fashion to two silver atoms (C1Ag1 = 2.16(1), C1Ag2 = 2.40(1), C15Ag3 = 2.30(1), C15Ag4 = 2.19(1) Å). The chloride ions bridge either in a μ2 or in a μ3 mode to the silver ions (Figures 4 and 5 and Table 3). Within the [AgCl]10 cluster short Ag 3 3 3 Ag interactions are present. Additional stabilization is obtained by the coordination of the amine nitrogen atoms N3 and N6 to the Ag2 and Ag3 atoms, respectively (Ag2N3 = 2.460(4), Ag3N6 = 2.481(5) Å). Between the clusters, no further interactions are present. The μ2 coordination pattern of carbenes is unusual and has only been reported for a few complexes bearing pyridine-substituted NHC ligands.16 The cluster has a dimension of approximately 7 12 14 Å and a diagonal of about 17 Å and might be regarded as a snapshot of a growing, carbene-stabilized AgCl nanoparticle. Just recently, the synthesis of NHC-stabilized gold nanoparticles has been reported, but reports about NHC-stabilized nanoparticles are still rare.17 Complexes 4a,b are red, and complexes 4a 3 HBr and 4b 3 HCl are yellow. Upon dissolution of 4a,b in various solvents the color changes from red to yellow within a few minutes. Upon slow
Figure 7. Structure of the cation of the protonated form 4b 3 HCl.
crystallization from MeCN/Et2O both yellow and a small fraction of red crystals are formed. The crystal structures revealed that the red complexes are not protonated at the amine nitrogen atom, whereas the yellow complexes are. 4a crystallizes in the monoclinic space group P21/n (Z = 4) and 4b 3 HCl in C2/c (Z = 8). In the measured crystal of 4a the trans position is occupied by Cl and Br in a ratio of 45:55. The angles between the imidazolyl ring plane and the gold coordination plane approach 90°. Taking into account the N3Au1 bond length of 2.835(2) Å, which is shorter than the sum of the van der Waals radii (3.21 Å), the Au atom in compound 4a is coordinated in a square-pyramidal geometry (Figure 6). This coordination geometry is rare and has been reported for diimine-type ligands such as 2,20 -bipyridine and 1,8-phenanthroline in complexes of the form (N∩N)AuCl3, featuring AuN bond lengths between 2.58 and 2.76 Å.18,19 4a is the first example of a square-planarpyramidal coordination geometry of Au(III) which is not enforced by a rigid aromatic ligand skeleton. Because of the low quality of the crystals of 4a 3 HBr, the structure could not be refined to an acceptable quality.20 Therefore, no bond lengths and angles can be discussed. Nevertheless, the connectivity and coordination geometry are unambiguous and are similar to those 2758
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for the structure of 4b 3 HCl: upon protonation of the nitrogen atom the amine function is turned away from the gold atom. The Au(III) atoms exhibit a distorted-square-planar environment defined by the carbene C, the Cl atom, and two Br atoms with the Cl atom strictly in the trans position (Figure 7 and Figure S3 (Supporting Information)). In 4b 3 HCl, the interatomic distances AuCl and AuBr are comparable (2.396(2) vs 2.41(1)/ 2.424(2) Å) (Table 4). With the smaller radius of the chloride ligand this distance reflects the elongation of the bond due to the trans effect of the carbene ligand. Both 4a and 4b 3 HCl feature practically identical AuCcarbene distances of 2.00(1) and 2.02(1) Å, respectively. Comparable geometrical parameters are known for other NHCAu(III) halide complexes.6,9,11 It should be noted that Au(III) complexes bearing different halide ligands are rare and so far only the crystal structure of the complex Ph3PAuBr2Cl have been reported.21 It is not obvious where the free halide anions in the compounds 4a 3 HBr and 4b 3 HCl might stem from. Sometimes, the CX activation of solvents such as CH2Cl2 or CHCl3 by NHC complexes has been reported.22 Other halide sources might be a marginal decomposition of the DCM during the bromination reaction,23 a contamination of the used solvents with chloride, or a partial decomposition of the gold complexes, liberating its halide ions. Table 4. Selected Bond Lengths (Å), Bond Angles (deg), and Torsion Angles (deg) 4a
4b 3 HCl
C1M1 Au1Cl1/Br3
2.00(1)
2.02(1) 2.396(2)
Au1Br1
2.400(2)
2.41(1)
Au1Br2
2.410(2)
2.424(2)
Au1N3
2.835(2)
5.220(2)
C1Au1Br1
86.9(3)
86.2(3)
C1Au1Br2
88.5(3)
88.7(3)
Cl1Au1Br1
91.9(1)
92.6(6)
Cl1Au1Br2 N1C1Au1Br1
92.6(1) 102(1)
92.8(6) 96(0)
All attempts to remove one halide ligand from the gold(III) atom of 4a,b with AgBF4 to facilitate the κN coordination of the amine atom failed and resulted in the decomposition of the complex, yielding elemental gold. Electronic Spectra and Quantum-Chemical Calculations. The color change from red to yellow upon dissolution is caused by the protonation of the amine group by moisture of the solvent. After the addition of a base, the solution turned red, the lowenergy band at 330 nm vanished, and the high-energy absorbance decreased slightly in comparison to the neutral form (spectrum b f c in Figure 8). Subsequent addition of acid resulted in a yellow solution, again. Due to some decomposition of the complex this cycle is not completely reversible. Unfortunately, under standard spectroscopic concentration no long-wavelength absorption responsible for the red color could be detected. These nondetectable absorptions were already reported for the squarepyramidal complexes of the type (N∩N)AuCl3.18 As can be seen clearly in the UV/vis spectrum d in Figure 8, a long-tailing absorption above 400 nm could be detected at an unusually high concentration of 4a 3 HBr upon addition of DABCO to an acetonitrile solution. Upon in situ protonation of the amine group with gaseous HCl the absorption band above 400 nm vanishes and a spectrum similar to spectrum b with peaks at 227 and 330 nm and a shoulder at ∼250 nm results. The spectrum of the pure complex 4a (spectrum a in Figure 8) shows absorption bands of both the neutral and the protonated species, indicating an equilibrium between the two forms. The differences in the spectra can be explained by comparing the frontier molecular orbitals of a square-planar and a squarepyramidal complex. Derived from simple ligand field theory arguments the orbital energy order for a square-planar complex is dxz, dyz < dz2 < dyz , dx2y2. Upon axial coordination the dz2 orbital is energetically destabilized, whereas the other d orbitals remain almost unaffected.24 For comparison the thoroughly investigated square-planar d8 anion AuX4 (X = Cl, Br) can be used to assign the electronic transitions in 4a:25 e.g., a solution of K[AuBr4] in MeCN features signals at 394 nm and a shoulder at ∼460 nm, which can be assigned to n(Br) f 5dx2y2(Au) and π(Br) f 5dx2y2(Au) ligand-to-metal charge-transfer (LMCT)
Figure 8. UV/vis spectra of the Au(III) complex 4a: (a) ethanolic solution of 4a after an equilibration time of 5 min (c = 1 104 mol L1); (b) ethanolic solution of 4a 3 HBr (c = 5 105 mol L1); (c) the same solution as in (b) after the addition of solid KOH; (d) a highly concentrated solution of 4a 3 HBr after addition of DABCO in MeCN (scale factor 2; c = 3 103 mol L1). 2759
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Figure 9. Calculated energy levels and contour plots of the frontier orbitals of the square-planar and square-pyramidal coordination geometries of (NHC)AuBr2Cl (DFT calculations at the B3LYP/6-31G(d0 ,p0 ) level).
states, respectively. The high-energy band at 258 nm originates from a σ(Br) f 5dx2y2(Au) LMCT state (Figure S4, Supporting Information). This means that in square-planar Au(III) halide complexes the HOMO is not constituted by 5d orbitals of the gold atom but by low-lying orbitals of the halide ligands. These low-lying ligand orbitals facilitate a LMCT to the 5dx2y2 orbital of the strongly oxidizing gold atom upon light absorption. In complexes of the type LAuIIIX3 (e.g., L = phosphine, HNC; X = Cl, Br) the 5dx2y2 orbital is further destabilized due to coordination of the σ-donating ligand L, leading to a hypsochromic shift of the LMCT. In the analogous phosphine complex Ph3PAuBr3 the LMCT absorption is found at 346 nm,26 compared to 330 nm for the square-planar NHC complex 4a 3 HBr, indicative of the
higher σ-donating character of the carbene ligand (spectrum b of Figure 8). On comparison of spectrum b with the UV/vis spectrum of the imidazolium salt (Figure S5, Supporting Information), the shoulder at ∼250 nm might tentatively be assigned to a LMCT. The higher energy bands below 250 nm originate from ππ* transition states of the imidazolyl moiety, which might cover further LMCT transitions. In the squarepyramidal geometry the dz2 orbital is shifted to an energy which lies above the energy of the halide orbitals, thus decreasing the HOMOLUMO gap. The resulting dz2 f 5dx2y2 transition is at lower energy and, because of the selection rule Δl = (1, of very low intensity. This explains the difficulties in the detection of a low-energy absorption band in the UV/vis spectra despite the 2760
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Table 5. Crystal Data and Data Collection and Structure Refinement Details for Compounds 2a,b, 4a, 4b 3 HCl, and 5 2a
2b
4a
4b 3 HCl
5 C32H60Ag10Cl10N12
formula
C16H30Ag2N6Cl2
C24H46Ag2C2lN6
C8H15AuBr2.55Cl0.45N3
C12H24AuBr2Cl2N3
mol wt
593.10
705.31
569.90
638.01
2046.12
cryst size (mm)
0.10 0.45 0.53
0.50 0.50 0.17
0.40 0.40 0.60
0.42 0.16 0.08
1.00 0.70 0.50
cryst syst
monoclinic
monoclinic
monoclinic
monoclinic
monoclinic
space group
P21/c
C2/c
P21/n
C2/c
P21/n
a (Å)
10.5959(18)
15.473(3)
8.150(1)
10.6554 (11)
8.834(1)
b (Å)
16.134(3)
6.572(1)
13.855(2)
26.780 (3)
12.506(2)
c (Å) R (deg)
6.7508(10) 90
31.521(6) 90.0
13.078(2) 90.0
13.8517 (17) 90.0
25.395(3) 90.0
β (deg)
93.88(1)
103.77(1)
94.17(1)
98.07(1)
91.11(1)
γ (deg)
90
90.0
90.0
90.0
90.0
V (Å3)
1151.8(3)
3113.3(10)
1472.7(4)
3913.5(8)
2805.0(6)
Fcalcd (g cm3)
1.710
1.394
2.570
2.166
2.423
Z
2
4
4
8
2
μ (mm1)
1.944
1.505
16.966
11.87
3.923
T (K) θ range (deg)
300 3.223.5
300 2.725.1
300 2.825.0
297 2.126.9
300 2.323.4
no. of rflns collected
9756
9240
13 855
18 322
15 503
no. of unique rflns
1702
2738
2567
4 126
4060
no. of obsd rflns (I > 2σ(I))
1144
2251
2132
2 794
2920
no. of params refined/restraints
125/0
161/0
131/1
185/1
295/0
abs cor
multiscan
multiscan
multiscan
analytical
multiscan
Tmin, Tmax
0.430, 0.830
0.530, 0.790
0.040, 0.060
0.074, 0.274
0.110, 0.240
max/min σfin (e Å3) R1 (I g 2σ(I))
0.32/0.30 0.0318
1.41/2.19 0.046
2.34/1.34 0.0435
2.80/0.82 0.048
0.55/0.68 0.035
wR2
0.0872
0.111
0.1079
0.135
0.061
CCDC no.
809585
809586
809587
809588
809589
clearly observable red color of the solution in our case as well as in the case of Hudson et al.18 The DFT calculations at the B3LYP/6-31G(d0 ,p0 ) level of the square-planar and square-pyramidal geometries supports this interpretation. The results of the ground-state geometry optimizations are summarized in Figure 9: The orbital profiles and the energies of the three lowest LUMOs of both coordination geometries are very similar. The orbital shapes of the three highest HOMOs of the square-planar and HOMO-1 and HOMO-2 of the square-pyramidal geometry indicates halide-based molecular orbitals without a considerable contribution of the metal atom. In contrast, the HOMO of the square-pyramidal compound is located at higher energy and has a high metallic character due to the significant contribution of the dz2 orbital of the gold atom.
’ SUMMARY Amino-functionalized imidazolium salts have been synthesized by the reaction of 1-methylimidazole and ClCH2CH2NR2 3 HCl (R = Me, i-Pr) and used as precursors for the preparation of the corresponding NHCAg(I), Au(I), and Au(III) complexes. The Ag(I) complexes are readily available by standard reactions. They crystallize in an ionic form as [(NHC)2Ag][AgCl2], associated via short Ag 3 3 3 Ag contacts to infinite [þ ] chains. Upon treatment of the silver salt (R = Me) with HBF4 an unprecedented Ag10Cl10 cluster which is stabilized by four NHC ligands has been obtained and characterized by single-crystal diffraction analysis. The carbene carbon atom exists in the rare μ2 coordination mode bridging two Ag(I) atoms. Further stabilization
of the cluster is gained by the coordination of the amine nitrogen atom to a silver atom. This cluster compound could be regarded as a snapshot of a growing NHC-stabilized AgCl nanoparticle. Therefore, the synthesis of NHC-stabilized nanoparticles bearing nitrogen donor atoms may be envisaged. Transmetalation reactions using these NHCAg complexes and [(Me2S)AuCl] yield (NHC)AuICl complexes, which are quite unstable under ambient conditions. The corresponding Au(III) complexes are obtained by oxidation with Br2. The highly basic amine group gives rise to a protonation/deprotonation reaction leading to an equilibrium between a square-planar and square-pyramidal coordination environment around the gold atom which is accompanied by a color change from yellow to red. All attempts to facilitate a κN coordination of the amine nitrogen atom by removing one halide ligand as AgX failed and resulted in the decomposition of the compounds and formation of elemental gold.
’ EXPERIMENTAL SECTION General Considerations. All reactions and manipulations of airand/or moisture-sensitive compounds were carried out under an atmosphere of dry nitrogen using standard Schlenk techniques. Dichloromethane was dried and distilled over a Na/Pb alloy. All solvents and other reagents were commercially obtained and used as received. NMR spectra were recorded either on a Bruker Digital Avance DPX 200 (200 MHz) or on an Avance DRX 500 (500 MHz) spectrometer, and 1H and 13C shifts are reported in ppm relative to Si(CH3)4 and were 2761
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Organometallics referenced internally with respect to the residual signal of the deuterated solvent. UVvis spectra were recordered on a Cary 300 Bio photometer. Single-crystal structure analysis of the compounds 2b, 4a, and 5 were carried out on a Bruker Smart X2S diffractometer, whereas the structure of the salt 4b 3 HCl was determined on a STOE-IPDS device27 with graphite-monochromated Mo KR radiation (λ = 0.710 73 Å). For 4a 3 HBr, a Nonius Kappa diffractometer with a CCD area detector was used, employing the program suite COLLECT.28,29 Further crystallographic and refinement data can be found in Table 5. The structures were solved by direct methods (SIR-97 and SHELXS-97)30,31 and refined by full-matrix least squares on F2 (SHELXL-97).32 The H atoms were calculated geometrically, and a riding model was applied during the refinement process. Crystals of 4a 3 HBr are of low quality, and the structure could not be refined to an acceptable level. Therefore, the tentative crystallographic data have not been deposited in the CSD but included in the Supporting Information. Computational Details. The Gaussian03 program was used in the calculations.33 Initial coordinates were taken from the corresponding X-ray molecular structure. All quantum-chemical calculations were carried out using a density functional theory (DFT) based method with the hybrid B3LYP34 functional. The 6-31G(d0 ,p0 ) basis set35 was used through the calculations, whereas for the complexed metal a LanL2DZ basis set36 was applied. The obtained geometries were verified to correspond to a real minimum by establishing an absence of imaginary IR frequencies.
Procedure for the Synthesis of 1-[2-(Dialkylamino)ethyl]3-methylimidazolium Salts. 1-[2-(Dimethylamino)ethyl]-3-methylimidazolium Chloride Hydrochloride (1a). The imidazolium salts were prepared by a procedure analogous to that reported.37 A Pyrex tube was charged with solid 2-(dimethylamino)ethyl chloride hydrochloride (2.00 g, 13.89 mmol) and 50 mL of ACN. The mixture was heated to 80 °C and stirred until the solid had dissolved completely. To the solution was slowly added 1-methylimidazole (1.14 g, 13.88 mmol) by syringe. After further stirring for 48 h at 150 °C the formed precipitate was filtered off, washed with diethyl ether, and dried in vacuo. The procedure afforded the product as a highly hygroscopic white powder in 80% yield (2.51 g). 1H NMR (200 MHz, DMSO-d6, 30 °C): δ 11.38 (s, br, 1H, NH), 9.38 (s, 1H, C2-H), 7.85 (s, 1H, ImHH), 7.70 (s, 1H, ImHH), 4.65 (t, 3JHH = 6.05 Hz, 2H, NImH-CH2), 3.80 (s, 3H, NImH-CH3), 3.60 (t, 3JHH = 6.07 Hz, 2H, NImHCH2CH2), 2.74 (s, 6H, NH(CH3)2). 13C NMR (125.8 MHz, DMSO-d6, 30 °C): δ 36.28, 42.73, 43.79, 55.42, 122.57, 124.28, 138.14. MS (ESI pos): m/z 154 [M]þ. Due to the high hygroscopicity no reliable elemental analysis could be obtained. 1-[2-(Diisopropylamino)ethyl]-3-methylimidazolium Chloride Hydrochloride (1b). Starting materials: 2-(diisopropylamino)ethyl chloride hydrochloride (2.00 g, 9.99 mmol), 1-methylimidazole (0.82 g, 9.99 mmol). Yield: 74% (2.09 g, highly hygroscopic white powder). 1H NMR (200 MHz, DMSO-d6, 30 °C): δ 11.05 (s, br, 1H, NH), 9.53 (s, 1H, C2H), 8.06 (s, 1H, ImHH), 7.74 (s, 1H, ImHH), 4.73 (t, 3JHH = 7.16 Hz, 2H, NImH-CH2), 3.83 (s, 3H, NImH-CH3), 3.663.58 (m, 4H, NHCH(Me)2/NImH-CH2CH2), 1.33 (d, 3JHH = 6.94 Hz, 6H, NHCH(CH3)2), 1.28 (d, 3JHH = 6.78 Hz, 6H, NHCH(CH3)2). 13C NMR (125.8 MHz, DMSO-d6, 30 °C): δ 16.79, 18.24, 36.20, 45.52, 45.87, 55.18, 123.05, 123.80, 138.41. MS (ESI pos): m/z 210 [M]þ. Due to the high hygroscopicity no reliable elemental analysis could be obtained.
Procedure for the Synthesis of {1-[2-(Dialkylamino)ethyl]3-methylimidazol-2-ylidene}silver(I) Carbene Complexes
Chloro{1-[2-(dimethylamino)ethyl]-3-methylimidazol-2-ylidene}silver(I) (2a). A 100 mL round-bottom flask was covered with aluminum foil and charged with 1a (1.00 g, 4.42 mmol) and 20 mL of DCM. To the resulting suspension was added dry Ag2O (1.03 g, 4.44 mmol) with stirring, whereupon a precipitate of AgCl was formed immediately. After an overall stirring time of 3 h at ambient temperature the reaction mixture was filtered over Celite. The solvent was removed in vacuo, and
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recrystallization of the oily residue from DCM/diethyl ether afforded the product as colorless, light-sensitive platelets. Yield: 37% (0.49 g). 1H NMR (200 MHz, DMSO-d6, 30 °C): δ 7.43 (d, 3JHH = 1.71 Hz, 1H, ImHH), 7.36 (d, 3JHH = 1.71 Hz, 1H, ImHH), 4.14 (t, 3JHH = 6.22 Hz, 2H, NImH-CH2), 3.75 (s, 3H, NImH-CH3), 2.58 (t, 3JHH = 6.24 Hz, 2H, NImH-CH2CH2), 2.14 (s, 6H, N(CH3)2). 13C NMR (125.8 MHz, DMSO-d6, 30 °C): δ 41.48, 48.60, 51.99, 63.03, 125.54, 125.91, 182.01. MS (ESI pos): m/z 415 [L2Ag]þ, 260 [LAg]þ. Anal. Calcd for C8H15N3AgCl (296.55): C, 32.40; H, 5.10; N, 14.17. Found: C, 32.76; H, 5.19; N, 14.36. Chloro{1-[2-(diisopropylamino)ethyl]-3-methylimidazol-2-ylidene}silver(I) (2b). Starting materials: 1b (1.00 g, 3.54 mmol), dry Ag2O (0.82 g, 3.54 mmol). Recrystallization from diethyl ether/n-pentane afforded colorless, light-sensitive platelets. Yield: 55% (0.69 g). 1H NMR (200 MHz, DMSO-d6, 30 °C): δ 7.42 (d, 3JHH = 1.50 Hz, 1H, ImHH), 7.35 (d, 3JHH = 1.48 Hz, 1H, ImHH), 3.97 (t, 3JHH = 6.42 Hz, 2H, NImHCH2), 3.73 (s, 3H, NImH-CH3), 2.92 (sept, 3JHH = 6.53 Hz, 2H, NCH), 2.70 (t, 3JHH = 6.24 Hz, 2H, NImH-CH2CH2), 0.86 (s, 12H, N(CH(CH3)2)). 13C NMR (125.8 MHz, DMSO-d6, 30 °C): δ 20.67, 38.03, 46.19, 47.97, 51.92, 122.39, 122.45, 178.46. MS (ESI pos): m/z 525 [L2Ag]þ. Anal. Calcd for C12H23N3AgCl (252.66): C, 40.87; H, 6.57; N, 11.92. Found: C, 41.15; H, 6.73; N, 12.09.
Procedure for the Synthesis of {1-[2-(Dialkylamino)ethyl]3-methylimidazol-2-ylidene}gold(I) Carbene Complexes. Chloro{1-[2-(dimethylamino)ethyl]-3-methylimidazol-2-ylidene}gold(I) (3a). A 100 mL Schlenk vessel was charged with 2a (0.20 g, 0.67 mmol) and 20 mL of dry DCM. The contents were stirred thoroughly, and to the solution was added solid [AuCl(tht)] (0.21 g, 0.66 mmol), whereupon a precipitate of AgCl was formed immediately. After further stirring for 30 min at ambient temperature the AgCl was filtered off. The solvent and volatile materials were removed in vacuo, and the remaining oil was washed with n-pentane. The Au(I)carbene complex was obtained as a highly hygroscopic oily substance. Yield: 77% (0.20 g). 1H NMR (200 MHz, DMSO-d6, 30 °C): δ 7.52 (d, 3JHH = 1.76 Hz, 1H, ImHH), 7.44 (d, 3JHH = 1.68 Hz, 1H, ImHH), 4.24 (t, 3JHH = 6.28 Hz, 2H, NImH-CH2), 3.81 (s, 3H, NImH-CH3), 2.67 (t, 3JHH = 6.36 Hz, 2H, NImH-CH2CH2), 2.16 (s, 6H, N(CH3)2). 13C NMR (125.8 MHz, DMSO-d6, 30 °C): decomposition during measurement. Due to the hygroscopicity and low stability of the complex no reliable elemental analysis could be obtained. Chloro{1-[2-(diisopropylamino)ethyl]-3-methylimidazol-2-ylidene}gold(I) (3b). Starting materials: 2b (0.20 g, 0.57 mmol), [AuCl(dms)] (0.17 g, 0.58 mmol). Yield: 71% (0.19 g, highly hygroscopic oily substance). 1H NMR (200 MHz, DMSO-d6, 30 °C): δ 7.41 (d, 3JHH = 1.67 Hz, 1H, ImHH), 7.35 (d, 3JHH = 1.70 Hz, 1H, ImHH), 4.103.88 (m, 2H, NImH-CH2), 3.082.83 (m, 2H, NCH), 2.832.62 (m, 3H, NImHCH2CH2), 0.86 (s, 12H, N(CH(CH3)2)). 13C NMR (125.8 MHz, DMSO-d6, 30 °C): δ 23.89, 49.10, 51.28, 58.02, 125.22, 125.54, 172.13. MS (ESI pos): m/z 441 [M]þ. Due to the hygroscopicity and low stability of the complex no reliable elemental analysis could be obtained.
Procedure for the Synthesis of {1-[2-(Dialkylamino)ethyl]3-methylimidazol-2-ylidene}gold(III) Carbene Complexes Dibromochloro{1-[2-(dimethylamino)ethyl]-3-methylimidazol-2ylidene}gold(III) (4a). In a 100 mL Schlenk vessel 3a (0.12 g, 0.31 mmol) was dissolved in 20 mL of dry DCM and cooled to 40 °C (isopropyl alcohol/liquid nitrogen). To the well-stirred reaction mixture was added a solution of Br2 (0.05 g, 0.31 mmol) in 5 mL of DCM slowly by syringe. The cooling bath was removed, and stirring was continued for 5 h at ambient temperature. Volatile materials were removed in vacuo, and the pasty residue was recrystallized from MeCN/Et2O. This afforded a mixture of red (4a) and yellow (4a 3 HBr) crystals. Yield: 77% (0.20 g). 1H NMR (200 MHz, DMSO-d6, 30 °C, presence of KOH): δ 7.45 (d, 3JHH = 1.68 Hz, 1H, ImHH), 7.38 (d, 3JHH = 1.72 Hz, 1H, ImHH), 4.14 (t, 3JHH = 6.40 Hz, 2762
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Organometallics 2H, NImH-CH2), 3.70 (s, 3H, NImH-CH3), 2.62 (t, 3JHH = 6.67 Hz, 2H, NImH-CH2CH2), 2.14 (s, 6H, N(CH3)2). 13C NMR (125.8 MHz, DMSOd6, 30 °C, presence of KOH): δ 39.86, 46.43, 49.11, 59.95, 123.82, 125.02, 174.30. MS (ESI pos, M is related to the neutral form of 4a): m/z 432 [M Br Cl þ H]þ, 503 [L2Au]þ. UVvis for 4a 3 HBr (EtOH): λ (log ε) 227 nm (4.21), 247 nm (sh, 3.91), 330 nm (3.06). UVvis for 4a (EtOH): λ (log ε) 250 nm (3.85), 235 nm (sh, 3.97). The protonated form is soluble in water. Due to the unknown degree of protonation no elemental analysis could be obtained. Dibromochloro{1-[2-(diisopropylamino)ethyl]-3-methylimidazol2-ylidene}gold(III) (4b). Starting materials: 3b (0.10 g, 0.23 mmol), Br2 (0.04 g, 0.25 mmol) in 5 mL of DCM. Yield: 72% (0.10 g, yellowish, crystalline powder, mixture of 4b and 4b 3 HCl). 1H NMR (DMSO-d6, 30 °C, presence of KOH): δ 7.42 (d, 3JHH = 1.57 Hz, 1H, ImHH), 7.36 (d, 3JHH = 1.56 Hz, 1H, ImHH), 3.98 (t, 3JHH = 6.60 Hz, 2H, NImHCH2), 3.69 (s, 3H, NImH-CH3), 2.94 (sept, 3JHH = 6.53 Hz, 2H, NCH), 2.74 (t, 3JHH = 6.52 Hz, 2H, NImH-CH2CH2), 0.88 (s, 12H, N(CH(CH3)2)). 13C NMR (125.8 MHz, DMSO-d6, 30 °C, protonated form): δ 17.04, 18.68, 38.13, 46.38, 47.13, 55.47, 122.59, 123.37, 172.87 MS (ESI pos, M is related to the neutral form of 4b): m/z 486 [M Br Cl]þ. Due to the unknown degree of protonation no elemental analysis could be obtained. Synthesis of the Silver Cluster 5. A 5 mL portion of DCM and 5 mL of aqueous HBF4 solution were mixed thoroughly. The resulting mixture was treated with solid Na2SO4 in order to remove the water. After filtration, compound 2a (50 mg, 0.17 mmol) was added to the clear filtrate and dissolved. To this solution was added n-pentane until it became turbid. When the liquid was stored at 18 °C for 2 weeks, compound 5 was afforded as colorless crystals. Anal. Calcd for C32H60Ag10N12Cl10 (1025.08): C, 18.78; H, 2.96; N, 8.21. Found: C, 18.95; H, 3.06; N, 8.36.
’ ASSOCIATED CONTENT
bS
Supporting Information. Figures, spectra, and tables giving additional analytical and computational details and CIF files giving crystal structure data. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
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