Synthesis of Complexes with Protic NH,NH-NHC Ligands via

Article pubs.acs.org/Organometallics

Synthesis of Complexes with Protic NH,NH-NHC Ligands via Oxidative Addition of 2‑Halogenoazoles to Zero-Valent Transition Metals Rajorshi Das,† Alexander Hepp,† Constantin G. Daniliuc,‡ and F. Ekkehardt Hahn*,† †

Institut für Anorganische und Analytische Chemie and NRW Graduate School of Chemistry, Westfälische Wilhelms-Universität Münster, Corrensstrasse 30, D-48149 Münster, Germany ‡ Organisch-Chemisches Institut, Westfälische Wilhelms-Universität Münster, Corrensstrasse 40, D-48149 Münster, Germany S Supporting Information *

ABSTRACT: A versatile, one-pot synthesis for the preparation of transition metal complexes bearing protic NH,NH-NHC ligands is disclosed. The reaction of unsubstituted 2-halogenoazoles with zerovalent metal complexes of the type [M(PPh3)4] (M = Pd, Pt) in the presence of NH4BF4 proceeds by oxidative addition of the C2−X (X = Cl, Br, I) bond to the transition metal followed by protonation at the ring nitrogen atom, leading to complexes of the type trans-[MX(PPh3)2(NH,NH-NHC)] (M = PdII, PtII; NHC = benzimidazolylidene or imidazolylidene). The trans-complexes have been obtained in all cases. The time needed to complete the reaction depends on the halogen present in the azole, and the rate of the oxidative addition follows the order C2−I > C2−Br > C2−Cl. The N−H groups of the coordinated NH,NH-NHC have been deprotonated followed by reaction with MeI to give the NMe,NMe-substituted classical NHC ligands. The N−H groups of the protic NHCs are hydrogen bond donors. The formation of N−H···O hydrogen bonds has been observed via 1H NMR spectroscopy upon titration of the complexes bearing protic NHCs with DMPU.

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or by reductive desulfurization of cyclic urea derivatives.10 Additional methods for the preparation of complexes of type I like the oxidative addition of C2−H,11,12a C2−alkyl,11e,12 C2− halogeno,11e,12a,13 and C2−S14 bonds of selected azolium cations to low-valent transition metals have been described. In spite of the utility of NR,NR-NHCs, complexes bearing the less common protic NHC ligands featuring an NH,NR- (II) or NH,NH-substitution pattern (III) have recently attracted attention. Only a few methods exist for the generation of such complexes. We have described the metal template assisted cyclization of β-hydroxy- and β-amino-functionalized aryl or alkyl isocyanides leading to complexes featuring NH,O-7i,15 and NH,NH-NHC ligands (a in Scheme 1).16 Alternatively, complexes bearing protic NHCs of type II or III have been obtained by the removal of the N-protecting groups from metal-coordinated NR,NPG-NHC (PG = removable protection group) ligands (b in Scheme 1).17 The recently described oxidative addition of neutral 2halogeno-N-alkylbenzimidazoles to suitable transition metal complexes followed by N-protonation constitutes another useful method for the synthesis of complexes bearing NH,NR-NHCs of type II (Scheme 2).18 In addition, complexes of type II have been obtained by the oxidative addition of the C2−H bond of N-donor-functionalized azoles19 or by tautomerization of N-metalated azoles.20

INTRODUCTION Since the isolation of the first stable N-heterocyclic carbene (NHC) by Arduengo et al. in 1991,1 these compounds have attracted continued interest due to multiple applications in various fields of chemistry. They have been extensively employed as spectator ligands in catalytically active metal complexes,2 in organocatalysis,3 in materials science,4 and in biologically active compounds.5 The scope of applications of NHCs has been enhanced by the incorporation of NHC donor groups into poly-NHC ligands,6 leading to the two- or threedimensional metallosupramolecular architectures.7 The strong σ-donating ability and the stability of their complexes enable NHCs to function as alternative ligands to the widely used tertiary phosphines in organometallic chemistry.8 Most NHC complexes bear classical N,N′-dialkylated or N,N′-diarylated NHC (NR,NR-NHC) ligands (Figure 1, I).9 Such complexes are normally prepared by coordination of the NHC to a suitable metal center. Free NHC ligands are obtained (often in situ) by C2-deprotonation of azolium salts9

Figure 1. Complexes bearing classical (I) and protic (II and III) NHC ligands. © XXXX American Chemical Society

Received: November 6, 2014

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dx.doi.org/10.1021/om501120u | Organometallics XXXX, XXX, XXX−XXX

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= Pd, Pt) complexes, leading to the MII complexes bearing an NH,NH-NHC ligand. We report here on the general scope of this new reaction for the preparation of complexes with protic NH,NH-NHCs employing both C-halogenated (halogen = Cl, Br, I) benzimidazoles and imidazoles. In addition, we describe the N,N′-alkylation of the coordinated NH,NH-NHC ligands. The complexes with NH,NH-NHC ligands have also been studied with regard to their ability to from N−H···X hydrogen bonds with an external hydrogen bond acceptor in solution. A short preliminary account on the oxidative addition of 2chlorobenzimidazoles to Pd0 complexes has appeared.25

Scheme 1. Synthesis of Complexes Bearing Protic NH,NHNHC Ligands

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RESULTS AND DISCUSSION The treatment of 1 equiv of 2-chlorobenzimidazole 1 with 1 equiv of [Pd(PPh3)4] or 1 equiv of [Pt(PPh3)4] in the presence of an excess of NH4BF4 in refluxing toluene yielded complexes trans-[2]BF425 or trans-[3]BF4, respectively. In a simple onepot synthesis the complexes bearing an NH,NH-NHC have been obtained in excellent yields of more than 70% (Scheme 3).

Scheme 2. Oxidative Addition of Neutral 2-Chloro-NAlkylbenzimidazole to Pd0 and Pt0 Complexes

Scheme 3. Oxidative Addition of 2-Chlorobenzimidazole

The reaction proceeds via the initial oxidative addition of the C2−Cl bond to the metal(0) center followed by protonation of the ring nitrogen atom. An initial N-protonation of the neutral 2-chlorobenzimidazole by NH4BF4 followed by oxidative addition of the resulting benzimidazolium cation can be ruled out, as NH4BF4 is not acidic enough to protonate the free benzimidazole. An identical reaction has been observed for the oxidative addition of N-methyl-2-chlorobenzimidazole to Pt0 (Scheme 2).18c In principle, the oxidative addition can lead to both the cis and trans isomers of complexes [2]BF4 and [3]BF4. However, we observed only formation of the trans isomers. This observation is in accord with previous studies on square-planar NHC/PPh3 complexes of PdII and PtII where the NHC ligand normally avoids the trans position to the PPh3 ligand.26 For complexes of the type [MX2(NHC)(PR3)] (M = PdII, PtII) the transphobia of NHC and phosphine ligands consequently leads to the formation of cis complexes as the thermodynamically most stable products.27 After purification complex trans-[2]BF4 was isolated in 77% yield, whereas complex trans-[3]BF4 was obtained in 72% yield. Both complexes are colorless solids that are sensitive toward air and moisture in solution, but they are rather stable in the solid state. The platinum(II) complex trans-[3]BF4 exhibits a higher stability compared to the palladium(II) complex trans-[2]BF4. Complex trans-[2]BF4 starts to decompose in highly polar solvents such as dimethyl sulfoxide or dimethylformamide after 2 weeks, while complex trans-[3]BF4 remains unchanged in these solvents even after one month. Formation of trans-[2]BF4 and trans-[3]BF4 was confirmed by 1H, 13C{1H}, and 31P{1H} NMR spectroscopy and ESI mass spectrometry. The 1H NMR spectra of the complexes (see also

Complexes bearing protic NH,NH-NHC ligands like II and III are useful intermediates. The nitrogen atoms in these complexes are easily alkylated, ultimately leading to complexes of type I with classical NHC ligands.16a,c This alkylation has been used in a template approach to link protic NHC ligands coordinated to the same metal center to yield complexes with macrocyclic tetracarbene or heterodonor [11]ane-P2CNHC and [16]ane-P2CNHC2 ligands.6a,21 In addition, the N−H groups in complexes bearing protic NHCs can act as a recognition unit for selected substrates via the formation of N−H···substrate hydrogen bonds.19b−f,22 For catalytically active metal complexes bearing protic NHC ligands, the close proximity of the N−H group to the metal center is of advantage, and substrate recognition by this arrangement has been demonstrated.23 In most related cases involving noncovalent substrate binding and transition metal catalysis, the recognition unit is connected to the metal center by a flexible spacer, thereby causing entropic problems.24 Apart from the metal template controlled cyclization of βamino-functionalized isocyanides (a in Scheme 1), no other general method for the preparation of complexes with protic NH,NH-NHCs (type III in Figure 1) has been developed. In fact, only one complex bearing an NH,NH-NHC ligand obtained by removal of N-protection groups is known.17b However, the preparation and preparative work with functionalized isocyanides is time-consuming and cumbersome. We therefore investigated alternative methods for the preparation of complexes with protic NH,NH-NHC ligands. Encouraged by the successful oxidative addition of N-methyl-2chlorobenzimidazoles to Pd0 and Pt0 complexes with formation of NMe,NH-NHC ligands (Scheme 2),18 we studied the oxidative addition of unsubstituted 2-halogenoazoles to M0 (M B


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In the solid-state structure of complex trans-[2]BF4·C4H8O, the N−H groups are strongly involved in classical hydrogenbonding interactions with the oxygen atom of a tetrahydrofuran molecule (distance H2···O 1.829 Å) and a fluorine atom of the BF4− counterion (distance H1···F 2.003 Å). Single crystals of trans-[3]BF4 were obtained by slow evaporation of the solvents from a dichloromethane/toluene solution of the compound at ambient temperature. The X-ray diffraction analysis confirmed the conclusions drawn from the NMR spectra of the compound. The molecular structure of cation trans-[3]+ is shown in Figure 3.

Supporting Information, SI), for example, show the characteristic resonance for the strongly deshielded N−H protons at δ = 12.72 ppm (for trans-[2]BF4) and at δ = 12.38 ppm (for trans[3]BF4). These values are in good agreement with previously recorded resonances for the N−H protons of protic benzimidzolin-2-ylidene ligands.16a,b,d The 13C{1H} NMR spectrum of PdII complex trans-[2]BF4 shows a triplet for the CNHC atom at δ = 165.9 ppm (t, 2JC,P = 9.1 Hz in CD2Cl2/DMSO-d6), indicating coupling to the two chemically equivalent phosphorus atoms, which also implies the trans arrangement of the phosphine donors. Consequently, only one singlet at δ = 21.2 ppm was detected in the 31P{1H} NMR spectrum. Likewise, the 13C{1H} NMR spectrum of PtII complex trans-[3]BF4 exhibits a triplet for the CNHC atom δ = 151.8 ppm (t, 2JC,P = 9.9 Hz in CD2Cl2/DMSO-d6). A resonance at δ = 18.6 ppm (Pt satellites, 1JPt,P = 2603 Hz) was observed in the 31P{1H} NMR spectrum, confirming the trans arrangement of the phosphine ligands. The high-resolution electrospray ionization (HR-ESI) mass spectra (positive ions) featured the highest intensity peak for trans-[2]BF4 at m/z = 783.1084 (calcd 783.1090 for [2]+) and for trans-[3]BF4 at m/z = 873.1693 (calcd 873.1694 for [3]+). The composition and coordination geometry of complexes trans-[2]BF4 and trans-[3]BF4 were also established by X-ray diffraction studies. Colorless crystals of composition trans[2]BF4·C4H8O were obtained by cooling of a saturated tetrahydrofuran solution of compound trans-[2]BF4 to −20 °C. The molecular structure of trans-[2]BF4·C4H8O (Figure 2) has been previously communicated,25 but selected parameters are listed here again for comparison.

Figure 3. Molecular structure of trans-[3]+ in trans-[3]BF4. Hydrogen atoms, except for the N−H hydrogen atoms, have been omitted for clarity. Selected bond lengths (Å) and angles (deg): Pt−Cl 2.3403(10), Pt−P1 2.3169(10), Pt−P2 2.3126(10), Pt−C1 1.968(4), N1−C1 1.340(5), N2−C1 1.358(5); Cl−Pt−P1 89.03(4), Cl−Pt−P2 87.40(4), Cl−Pt−C1 178.97(11), P1−Pt−P2 168.86(4), P1−Pt−C1 91.98(10), P2−Pt−C1 91.64(10), N1−C1−N2 106.1(3).

The bond parameters observed for complex trans-[3]BF4 are consistent with those reported for related square-planar platinum(II) trans-diphosphine complexes bearing NHC donors.18a,28 The Pt−Cl (2.3403(10) Å), Pt−P1 (2.3169(10) Å), Pt−P2 (2.3126(10) Å), and Pt−C1 (1.968(4) Å) bond distances, for example, compare well with the equivalent parameters found in platinum(II) complexes containing protic NH,NR-NHC18a or NR,NR-NHC ligands.28 The bond angle Cl−Pt−C1 (178.97(11)°) deviates only slightly from linearity, while the P1−Pt−P2 angle (168.86(4)°) is further off linearity, possibly a consequence of the difference in steric demand of the chloro and NHC ligands. In the solid state, complex trans-[3]BF4 forms polymeric chains by hydrogen-bonding interactions between the complex cation and BF4− counterions (Figure 4). The N−H protons of the NH,NH-NHC ligand form classical intermolecular bridging

Figure 2. Molecular structure of trans-[2]+ in trans-[2]BF4·C4H8O. Hydrogen atoms, except for the N−H hydrogen atoms, have been omitted for clarity. Cation trans-[2]+ resides on a crystallographic mirror. Atoms Cl, and Pd and the NHC ring lie on the mirror plane. Selected bond lengths (Å) and angles (deg): Pd−Cl 2.338(5), Pd−P 2.3342(15), Pd−C1 1.963(16), N1−C1 1.360(18), N2−C1 1.306(18); Cl−Pd−P 90.19(10), Cl−Pd−C1 176.1(5), P−Pd−P* 178.95(19), P−Pd−C1 89.77(10), N1−C1−N2 106.7(13).

The coordination geometry around the palladium atom in trans-[2]+ is slightly distorted square-planar with bond angles P−Pd−P* 178.95(19)° and Cl−Pd−C1 176.1(5)°. The Pd−Cl (2.338(5) Å), Pd−P (2.3342(15) Å), and Pd−C1 (1.963(16) Å) separations fall in the range previously observed for the related palladium(II) complexes bearing protic NMe,NHNHC18a or classical NR,NR-NHC ligands.13a Apparently, the substitution of NR groups for NH groups within the NHC heterocycle has little effect on the metric parameters of the NHC palladium(II) complexes.

Figure 4. Ball-and-stick model view of the packing in trans-[3]BF4. Hydrogen atoms except those involved in hydrogen bonding have been omitted. Atom colors are identical to those of Figure 3. C


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N−H···F−BF2−F···H−N hydrogen bonds in addition to some weak interactions between the BF4− anions and protons of the phosphine phenyl groups. The classical N−H··· hydrogen bonds (F1···H1 1.952 Å, F4···H2 1.872 Å) are distinctly shorter than the nonclassical Ph−H···F hydrogen bonds (F2···H7 2.371 Å, F4···H32 2.583 Å, F4···H52 2.437 Å) in accord with previous observations for related compounds.18b This leads to indefinite zigzag chains in the solid-state structure of trans-[3]BF4 (Figure 4). In order to explore the scope of the oxidative addition of unsubstituted 2-halogenobenzimidazoles for the synthesis of complexes bearing NH,NH-NHC ligands, we next studied the reaction of 2-bromobenzimidazole with group 10 M0 metal complexes. Reaction of 2-bromobenzimidazole 4 with an equimolar amount of [Pd(PPh3)4] or [Pt(PPh3)4] in the presence of an excess of NH4BF4 in refluxing toluene led to the formation of complexes trans-[5]BF4 and trans-[6]BF4 as colorless solids in yields of 68% and 56%, respectively (Scheme 4). The weaker C2−Br bond in 4 compared to the C2−Cl bond in 1 shortens the time required for completion of the reactions from 72 h to 60 h.

Figure 5. Molecular structure of trans-[5]+ in trans-[5]BF4·CH2Cl2 (left) and trans-[6]+ in trans-[6]BF4·CH2Cl2 (right). Hydrogen atoms, except for the N−H hydrogen atoms, have been omitted for clarity. Selected bond lengths (Å) and angles (deg) for trans-[5]+ [trans-[6]+]: M−Br 2.4642(5) [2.4732(5)], M−P1 2.3457(9) [2.3263(11)], M−P2 2.3269(10) [2.3152(10)], M−C1 1.975(4) [1.978(4)], N1−C1 1.346(4) [1.346(5)], N2−C1 1.340(4) [1.329(5)]; Br−M−P1 92.35(3) [91.58(3)], Br−M−P2 87.76(3) [87.11(3)], Br−M−C1 174.05(10) [174.51(11)], P1−M−P2 167.28(4) [167.59(4)], P1−M− C1 89.88(10) [90.33(12)], P2−M−C1 91.28(10) [92.11(12)], N1− C1−N2 106.0(3) [106.4(3)].

Both complex cations trans-[5]+ and trans-[6]+ are built in a distorted square-planar fashion. The metric parameters found in trans-[5]+ fall in the range previously reported for related square-planar PdII complexes bearing NHC donors and are almost identical to equivalent parameters found for trans-[2]+. Equivalent metric parameters for platinum(II) cation trans-[6]+ are almost identical to those observed for the palladium(II) cation trans-[5]+. This similarity is based on very similar covalent radii of palladium (1.39 Å) and platinum (1.36 Å), which leads to isostructural compounds crystallizing in the same space group (P21/c) with essentially identical cell constants (see also SI). As was observed for trans-[3]BF4, cations and anions of trans-[5]BF4·CH2Cl2 and trans-[6]BF4· CH2Cl2 are engaged in a network of intermolecular N−H···F− BF2−F···H−N and Ph−H···F−BF3 hydrogen bonds. Next, the oxidative addition of the C2−I bond of 2iodobenzimidazole to group 10 M0 complexes was studied. Refluxing of equimolar amounts of 2-iodobenzimidazole 7 and [Pd(PPh3)4] in the presence of an excess of NH4BF4 in tetrahydrofuran afforded complex trans-[8]BF4 in 73% yield as a slightly yellow solid (Scheme 5). The relatively weak C2−I

Scheme 4. Oxidative Addition of 2-Bromobenzimidazole

Complexes trans-[5]BF4 and trans-[6]BF4 are more soluble in common polar solvents than the chloro complexes trans[2]BF 4 and trans-[3]BF 4 . Both complexes have been characterized by NMR spectroscopy and mass spectrometry. The 1H NMR spectra show the characteristic resonances for N−H protons at δ = 12.62 ppm (for trans-[5]BF4) and at δ = 12.29 ppm (for trans-[6]BF4), respectively. In the 13C{1H} NMR spectra, the resonances for the CNHC carbon atoms were recorded at δ = 167.3 ppm (t, 2JC,P = 8.7 Hz in CD2Cl2/ DMSO-d6) and 153.8 ppm (t, 2JC,P = 9.9 Hz in CD2Cl2/ DMSO-d6) for trans-[5]BF4 and trans-[6]BF4, respectively. These values are slightly downfield shifted compared to the resonances recorded for trans-[2]BF4 (δ = 165.9 ppm) and trans-[3]BF4 (δ = 151.8 ppm) obtained from 2-chlorobenzimidazole. The 31P{1H} NMR spectra exhibit only one singlet each at δ = 20.5 ppm and δ = 17.5 ppm, respectively, confirming the trans arrangement of the chemically equivalent PPh3 ligands. Similarly to trans-[3]BF4, the characteristic Pt−P coupling (Pt satellites, 1JPt,P = 2554 Hz) has been observed in the 31P{1H} NMR spectrum of trans-[6]BF4. In addition, complexes trans-[3]BF4 and trans-[4]BF4 were characterized by high-resolution electrospray ionization (HR-ESI) mass spectrometry (positive ions), showing the most intensive peaks with correct isotopic patterns for the molecular cations [5]+ and [6]+. Colorless crystals of compounds trans-[5]BF4·CH2Cl2 and trans-[6]BF4·CH2Cl2 were obtained by slow diffusion of diethyl ether into the concentrated dichloromethane solutions of the compounds at 4 °C. The results of the molecular structure determinations with these crystals are depicted in Figure 5.

Scheme 5. Oxidative Addition of 2-Iodobenzimidazole

bond allowed for a reduction of the time for completion of the reaction to only 12 h. This effect was even more pronounced for the reaction of 7 with [Pt(PPh3)4] in refluxing THF, giving complex trans-[9]BF4 in 46% yield after a reaction time of only 2 h (Scheme 5). However, the yield of the platinum(II) complex trans-[9]BF4 is lower than observed for the other oxidative additions, and complex trans-[9]BF4 suffers from decomposition upon increasing the reaction time and/or the reaction temperature. The reduced time for completion of the oxidative addition of 7 compared to 1 (Scheme 3) and 4 D


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iodoimidazole is completed much faster (2 h) than the related reaction of 2-iodobenzimidazole with [Pd(PPh3)4] (12 h, see Scheme 5).

(Scheme 4) might be very useful for the preparation of heterobimetallic complexes when starting from ligand precursors containing a 2-chlorobenzimidazole and a 2-iodobenzimidazole moiety. A comparison of the solubility of all PdII and PtII complexes prepared by oxidative addition of 2-halogenobenzimidazoles revealed that the iodo complexes trans-[8]BF4 and trans-[9]BF4 show the best solubility in commonly used polar solvents such as dichloromethane, acetonitrile, or dimethyl sulfoxide. Complexes trans-[8]BF4 and trans-[9]BF4 were characterized by NMR spectroscopy and mass spectrometry. The 1H NMR spectra showed the characteristic downfield resonances for the N−H protons at δ = 12.58 ppm (trans-[8]BF4) and δ = 12.45 ppm (trans-[9]BF4). The 13C{1H} NMR spectra featured triplets at δ = 169.0 ppm (t, 2JC,P = 8.5 Hz in CD2Cl2/DMSOd6) for trans-[8]BF4 and at δ = 156.1 ppm (t, 2JC,P = 9.9 Hz in CD2Cl2/DMSO-d6) for trans-[9]BF4. Only one resonance was detected for each complex in the 31P{1H} NMR spectra (δ = 18.3 ppm for trans-[8]BF4; δ = 14.7 ppm, Pt satellites, 1JPt,P = 2527 Hz for trans-[9]BF4), confirming the trans arrangement of the two phosphine donors. Both complex cations trans-[8]+ and trans-[9]+ were observed in the MALDI-TOF or highresolution ESI mass spectra, showing the correct isotope patterns. The PdII complex trans-[8]BF4 was crystallized as solvate trans-[8]BF4·CH2Cl2 by slow diffusion of diethyl ether into a solution of the compound in dichloromethane at 4 °C. The Xray diffraction analysis shows a distorted (largest distortion angle P1−Pd−P2 167.07(4)°) square-planar complex cation trans-[8]+ (Figure 6). All metric parameters in trans-[8]+ are similar to equivalent parameters found in the palladium NH,NH-NHC complexes described previously.

Scheme 6. Oxidative Addition of 2-Iodoimidazole

Treatment of [Pt(PPh3)4] with 2-iodoimidazole afforded most likely complex trans-[12]BF4, which, however, undergoes decomposition during the reaction. Even after a reaction time of only 1 h decomposition products have been detected in the reaction mixture next to some trans-[12]BF4. The isolation of trans-[12]BF4 from the product mixture obtained has so far been unsuccessful due to similar solubility properties of complex trans-[12]BF4 and the decomposition products (see also SI). Complex trans-[11]BF4 was fully characterized by NMR spectroscopy, mass spectrometry, and X-ray crystallography.25 The 1H NMR spectrum exhibits the characteristic resonance for the N−H protons at δ = 11.90 ppm. The resonance for the CNHC carbon atom was recorded in the 13C{1H} NMR spectrum at δ = 156.5 ppm (t, 2JC,P = 9.1 Hz in CD2Cl2/ DMSO-d6) as a triplet. A singlet at δ = 18.8 ppm was observed in the 31P{1H} NMR spectrum, as would be expected for a complex with phosphine ligands in trans positions. The HR-ESI mass spectrum (positive ions) shows the most intense peak at m/z = 825.0275 (calcd for trans-[11]+ 825.0291). Single crystals of trans-[11]BF4·CH2Cl2 suitable for an X-ray diffraction analysis were grown by slow diffusion of diethyl ether into a concentrated solution of trans-[11]BF4 in dichloromethane at 4 °C. The X-ray diffraction analysis confirmed the proposed geometry of trans-[11]+ as a squareplanar complex cation (Figure 7). Comparable metric parameters in the imidazolylidine/iodide complex cation trans-[11]+ are not significantly different from those observed for the benzimidazolylidene/iodide complex cation trans-[8]+ and related complex cations of type trans-[PdI(NHC)(PR3)2]+.29 An exception is the expanded P1−Pd−P2 angle

Figure 6. Molecular structure of cation trans-[8]+ in trans-[8]BF4· CH2Cl2. Hydrogen atoms, except for the N−H hydrogen atoms, have been omitted for clarity. Selected bond lengths (Å) and angles (deg): Pd−I 2.6340(4), Pd−P1 2.3305(11), Pd−P2 2.3484(11), Pd−C1 1.994(4), N1−C1 1.337(5), N2−C1 1.348(5); I−Pd−P1 87.76(3), I− Pd−P2 92.57(3), I−Pd−C1 173.44(12), P1−Pd−P2 167.07(4), P1− Pd−C1 91.25(12), P2−Pd−C1 89.85(12), N1−C1−N2 106.8(4).

Finally we attempted the oxidative addition of neutral 2halogenoimidazoles to Pd0 and Pt0 complexes. Since the 2iodobenzimidazole was the most reactive compound in the previously studied oxidative additions, we used exclusively 2iodoimidazole for our investigations. Under the previously described conditions, 2-iodoimidazole 10 reacts with an equimolar amount of [Pd(PPh3)4] in the presence of an excess of NH4BF4 in THF with formation of complex trans-[11]BF4 as a yellow solid in 77% yield (Scheme 6).25 The reaction of 2-

Figure 7. Molecular structure of trans-[11]+ in trans-[11]BF4·CH2Cl2. Hydrogen atoms, except for the N−H hydrogen atoms, have been omitted for clarity. Selected bond lengths (Å) and angles (deg): Pd−I 2.6472(5), Pd−P1 2.3325(15), Pd−P2 2.3435(15), Pd−C1 1.978(5), N1−C1 1.334(7), N2−C1 1.322(7); I−Pd−P1 90.90(4), I−Pd−P2 89.55(4), I−Pd−C1 178.67(17), P1−Pd−P2 178.18(6), P1−Pd−C1 89.83(16), P2−Pd−C1 89.76(16), N1−C1−N2 103.8(5). E


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in trans-[11]+ (178.18(6)°) compared to the much smaller P1− Pd−P2 angle in trans-[8]+ (167.07(4)°), which is most likely caused by the lesser steric demand of the imidazolylidene in trans-[11]+ compared to the benzimdazolylidene in trans-[8]+. In addition and in accord with previous reports,9 the N1−C1− N2 angle in the imidazolylidene ligand in trans-[11] + (103.8(5)°) is smaller than the equivalent angle in the benzimidazolylidene cations previously described (106−107°). Selected results regarding the oxidative addition of 2halogenoazoles to [M(PPh3)4] (M = Pd, Pt) complexes are summarized in Table 1. Both Pd0 and Pt0 complexes react in

NHC−Pt−Br (1JPt,P = 2554 Hz for trans-[6]BF4) and NHC− Pt−I (1JPt,P = 2527 Hz for trans-[9]BF4) complexes. Apart from their facile synthesis via the oxidative addition of 2-halogenoazoles, the protic NH,NH-NHC ligands are easily further modified as they contain acidic N−H protons. For NH,NH-NHC complexes with metals in the M0 oxidation state we have demonstrated the deprotonation and alkylation of the ring nitrogen atoms converting the NH,NH-NHC ligand into a classical NR,NR-NHC ligand.16a−c We have now studied this alkylation reaction for NH,NH-NHCs coordinated to PdII and PtII metal centers. The NHC ligands in complexes trans-[2]BF4 and trans[3]BF4 as well as in trans-[5]BF4 and trans-[6]BF4 can be deprotonated with KOtBu, and the subsequent addition of MeI yields the complexes bearing a doubly methylated NMe,NMeNHC ligand. Since the starting complexes for these reactions contained a chloro or bromo ligand, the initial reaction products were subsequently treated with KI to ensure the exclusive formation of the iodo complexes trans-[13]BF4 and trans-[14]BF4. This anion exchange was not necessary when the iodo complexes trans-[8]BF4 and trans-[9]BF4 were used as starting materials for the alkylation reaction (Scheme 7).

Table 1. Scope of the Oxidative Addition of 2Halogenoazoles

Scheme 7. Synthesis of Complexes Bearing NR,NR-NHC Ligands from Complexes with Protic NH,NH-NHC Ligands

a13

C NMR spectra were measured in CD2Cl2/DMSO-d6.

the oxidative addition reaction. The time for completion of the reaction is strongly dependent on the halogen present, with 2chlorobenzimidazole reacting slower than 2-bromobenzimidazole. Very fast oxidative additions have been observed with 2iodobenzimidazole (entries 3 and 7) and with 2-iodoimidazole (entry 4). These finding could be useful for the preparation of heterobimetallic complexes by the stepwise metalation of a bidentate ligand precursor featuring one 2-chloroazole and one 2-iodoazole group. In addition, Table 1 shows a significant downfield shift for the CNHC resonance when going from the chloro complexes (entries 1 and 5) via the bromo complexes (entries 2 and 6) to the iodo complexes (entries 3 and 7) from δ = 165.9 ppm to δ = 169.0 ppm for the palladium complexes and from δ = 151.8 ppm to δ = 156.1 ppm for the platinum complexes. A closer inspection reveals that such behavior can be expected and has been discussed previously by Huynh.30 The greater electron donation of the iodo ligand (compared to the chloro ligand) generates the least Lewis-acidic metal center, leading to the longest and weakest M−CNHC bond. On the other hand, the chloride complex bearing the weakest halido donor features the most Lewis-acidic metal center leading to the strongest M− CNHC and M−P bonds, which is also illustrated by the observed 1 JPt,P coupling constants, where the Pt−P coupling is larger for the more Lewis-acidic NHC−Pt−Cl metal center (1JPt,P = 2603 Hz for trans-[3]BF4) and decreases for the less Lewis-acidic

Obviously, complexes trans-[13]BF4 and trans-[14]BF4 are also directly accessible by deprotonation of the NMe,N′Mesubstituted benzimidazolium cation in the presence of a suitable palladium(II) or platinum(II) precursor.9 However, the reaction sequence presented here illustrates the facile alkylation of the coordinated NH,NH-NHC ligands and thus their synthetic utility. The 1H NMR spectra (see also SI) of complexes trans[13]BF4 and trans-[14]BF4, as expected, no longer feature any N−H resonances. Instead they exhibit new resonances at δ = 3.51 ppm and δ = 3.47 ppm for N,N′-dimethyl protons. Accordingly, the 13C{1H} NMR spectra exhibit singlets at δ = 34.6 ppm and δ = 34.4 ppm for the methyl carbon atoms of trans-[13]BF4 and trans-[14]BF4. The CNHC resonances were recorded at δ = 175.6 ppm (t, 2JC,P = 8.8 Hz in CD2Cl2) and δ = 164.2 ppm (t, 2JC,P = 9.9 Hz in CD2Cl2) for trans-[13]BF4 F


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Table 2. Selected 13C{1H} NMR Spectroscopic and Metric Parameters for PdII and PtII Complexes Bearing NH,NH-, NMe,NH-, and NMe,NMe-NHC Ligands

and trans-[14]BF4, respectively. These values are about 10 ppm downfield from the chemical shifts for the CNHC atoms in the NH,NH-NHC complexes (palladium complexes: δ = 165.9− 169.0 ppm; platinum complexes: δ = 151.8−156.1 ppm; see Table 1), confirming that the electronic situation in complexes with protic NH,NH-NHC ligands is rather similar to the situation in complexes bearing the classical NR,NR-NHC ligands. The 31P{1H} NMR spectra exhibit singlets at δ = 17.9 for complex trans-[13]BF4 and at δ = 12.6 ppm (Pt satellites, 1JPt,P = 2425 Hz) for complex trans-[14]BF4. Formation of complexes trans-[13]BF4 and trans-[14]BF4 was also confirmed by HR-ESI mass spectrometry (positive ions), showing strong peaks at m/z = 903.0735 (calcd for [13]+ 903.0763) and at m/z = 992.1346 (calcd for [14]+ 992.1363), respectively. The molecular structures of compounds trans-[13]BF4· CH2Cl2 and trans-[14]BF4·CH2Cl2 have been determined by X-ray diffraction study. The suitable crystals were obtained by slow diffusion of diethyl ether-saturated dichloromethane solutions of [13]BF4 and trans-[14]BF4, respectively. The structure analyses revealed the presence of highly symmetrical complex cations in the solid state, bisected by a crystallographic mirror (Figure 8). Most of the bond lengths

a

Spectra were measured in CD2Cl2/DMSO-d6. bSpectra were measured in CDCl3/DMSO-d6. cSpectra were measured in CD2Cl2.

about the same influence on the 13C NMR resonance for the CNHC atom (cf. entry 1 X = Cl, δ = 165.9 ppm and X = I, δ = 169.0 ppm; Δδ = 3.1 ppm) as the introduction of a methyl group to one of the nitrogen atoms of the NHC ligand (cf. entry 1, X = Cl, δ = 165.9 ppm; entry 2, X = Cl, δ = 169.7 ppm; Δδ = 3.8 ppm). From the spectroscopic and metric parameters summarized in Table 2 it can be concluded that the protic NHCs, although not stable when removed from the metal center, behave in their complexes like normal NR,NR-NHC ligands of the Arduengo type, while at the same time they offer the possibility for further modifications at the ring nitrogen atoms. With the palladium(II) and platinum(II) complexes of protic NH,NH-NHCs at hand, we now investigated their ability to recognize selected substrates via the formation of intermolecular hydrogen bonds. Such hydrogen bonds formed by complexes bearing monoprotic NR,NH-NHCs have allowed the regioselective hydrogenation of selected olefins.23 The complexes bearing NH,NH-NHC ligands with two N−H groups reported here are in principle able to form two different N−H···substrate hydrogen bonds involving both N−H groups (Scheme 8). We became interested in knowing whether these N−H groups could be activated separately for hydrogen bond formation. We studied hydrogen bond formation with the arbitrarily selected palladium complex trans-[5]BF4 and the platinum complex trans-[6]BF4 (Scheme 8). The complexes were titrated in CD2Cl2 with the strong hydrogen bond acceptor DMPU (1,3-dimethyltetrahydropyrimidin-2(1H)-one) as a model for substrates featuring hydrogen bond acceptors.

Figure 8. Molecular structure of trans-[13]+ in trans-[13]BF4·CH2Cl2 (left) and of trans-[14]+ in trans-[14]BF4·CH2Cl2 (right). Hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles (deg) for trans-[13] + [trans-[14] + ]: M−I 2.6256(4) [2.6278(5)], M−P 2.3339(7) [2.3198(12)], M−C1 1.997(4) [1.999(9)], N1−C1 1.348(5) [1.336(8)], N2−C1 1.346(5) [1.333(8)]; I−M−P 89.171(17) [88.62(3)], I−M−C1 177.87(10) [177.95(17)], P−M−P* 176.43(3) [175.73(5)], P−M−C1 90.887(17) [91.44(3)], N1−C1−N2 107.2(3) [107.7(6)].

and angles in the cations trans-[13]+ and trans-[14]+ (the compounds are isostructural) fall in the range observed for the previously discussed palladium(II) and platinum(II) complexes bearing NH,NH-NHCs. The double N,N′-alkylation has no significant influence on the metric parameters of the complexes. With the preparation of the palladium(II) and platinum(II) complexes bearing protic NH,NH-NHC and doubly alkylated NMe,NMe-NHC ligands, a complete series of compounds featuring differently N,N′-functionalized NHC ligands exists. Selected metric and spectroscopic parameters for these complexes are summarized in Table 2. The data from Table 2 illustrate that the protic NHC ligands (NH,NH-NHCs and NMe,NH-NHCs) feature spectroscopic and metric parameters very closely related to the alkylated NMe,NMe-NHCs and to classical NR,NR-NHC ligands in general. Transforming the protic NH,NH-NHCs into mono Nalkylated (NR,NH-NHCs) or N,N′-dialkylated ligands leads in general to a downfield shift of the CNHC resonance. This shift, however, is not dramatic, and the changes of the halogen in complexes of the type trans-[PdX(PPh3)2(NHC)] can have G


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Scheme 8. Titration of Complexes trans-[5]BF4 and trans[6]BF4 with DMPU

Figure 10. Comparison of N−H chemical shifts upon addition of DMPU to CD2Cl2 solutions of palladium complex trans-[5]BF4 and platinum complex trans-[6]BF4.

beginning of the titration experiment the N−H protons of palladium complex trans-[5]BF4 are more acidic than those of platinum complex trans-[6]BF4. In both cases, however, no indications for a stepwise formation of N−H···O hydrogen bonds were found.

Monitoring the titration experiment by 1H NMR spectroscopy revealed a significant downfield shift of the resonance for the N−H protons upon addition of DMPU, while the resonances for all other protons remained essentially unchanged (Figure 9). We take this as an indication for the

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CONCLUSION We present a new and facile one-pot synthesis of complexes bearing protic NH,NH-NHC ligands via the oxidative addition of 2-halogenoazoles to M0 complexes of platinum and palladium. The method is applicable to both suitably substituted imidazoles and benzimidazoles. Significant differences in the rate of the reaction have been observed depending on the halogen present at the C2 position of the azoles with the iodo derivatives exhibiting the highest reactivity. This property constitutes a potentially useful feature for the preparation of heterobimetallic complexes from diazoles bearing different halogeno substituents at their C2 positions. The new method is superior to previously reported procedures such as the cyclization of functionalized isocyanides or the removal of Nsubstituents from coordinated NHC ligands. The protic NHC ligands are still reactive, and substitution reactions at the ring nitrogen atoms are possible after N−H deprotonation. The NH groups of the protic carbenes are hydrogen bond donors, and the formation of hydrogen bonds to DMPU in solution has been observed. Upcoming studies will be directed toward utilizing the substrate recognition properties of complexes with protic NHCs for regioselective catalytic transformations. In addition, the scope of the oxidative addition will be expanded to include other C2-halogenated azoles and additional low-valent transition metals such as RhI, ReI, and RuII.

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Figure 9. 1H NMR titration of 1 mM complex trans-[5]BF4 (A) and trans-[6]BF4 (B) with DMPU in CD2Cl2 (see also SI).

EXPERIMENTAL SECTION

General Considerations. All manipulations were performed under an argon atmosphere using standard Schlenk techniques or in a glovebox. Solvents were dried by standard methods and distilled under argon prior to use. 1H, 13C{1H}, and 31P{1H} NMR spectra were measured on Bruker AVANCE I 400 or Bruker AVANCE III 400 spectrometers. Chemical shifts (δ) are expressed in ppm using the residual protonated solvent signal as an internal standard. Coupling constants are expressed in hertz. Mass spectra were obtained with an Orbitrap LTQ XL (Thermo Scientific), MicroTof (Bruker Daltonics), or Varian MAT 212 spectrometer. 2-Chlorobenzimidazole 1, [Pd(PPh3)4], and [Pt(PPh3)4] were purchased from commercial sources and were used as received. 2-Bromobenzimidazole 4,31 2-iodobenzimidazole32 7, and 2-iodoimidazole33 10 were prepared by published

formation of two hydrogen bonds as depicted in Scheme 8. After addition of 2 equiv of DMPU the N−H resonance shifts only slightly more downfield. The chemical shift difference (see also SI) after addition of 2 equiv of DMPU amounts to Δδ = 1.29 and 1.39 ppm for trans-[5]BF4 and trans-[6]BF4, respectively, while the addition of another equivalent of DMPU causes only a small additional downshift of Δδ = 0.27 ppm and Δδ = 0.39 ppm. A comparison of N−H chemical shifts of complexes trans[5]BF4 and trans-[6]BF4 upon addition of DMPU is depicted in Figure 10 (see also SI). The plot illustrates that at the H


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(v-t, 2/4JC,P = 6.3 Hz, Ph−Cortho), 132.9 (C4, C5), 130.7 (Ph−Cpara), 129.2 (v-t, 1/3JC,P = 25.3 Hz, Ph−Cipso), 127.9 (v-t, 3/5JC,P = 5.4 Hz, Ph−Cmeta), 122.1 (C7, C8), 110.8 (C6, C9). 31P{1H} NMR (162 MHz, CD2Cl2/DMSO-d6 (1:1, v/v)): δ = 20.5 (s). HRMS (ESI, positive ions): m/z = 829.0558 (calcd for [5]+ 829.0575). Synthesis of Complex trans-[6]BF4. A mixture of 2-bromobenzimidazole 4 (8 mg, 0.041 mmol), [Pt(PPh3)4] (49 mg, 0.039 mmol), and an excess of NH4BF4 (15 mg, 0.14 mmol) was suspended in toluene (10 mL). The reaction mixture was heated under reflux for 60 h and then allowed to cool to ambient temperature. Then, the solvent was removed in vacuo. The solid residue was washed with hexane (2 × 7 mL) and diethyl ether (2 × 7 mL) and subsequently suspended in dichloromethane (17 mL). Filtration gave a clear solution. After removal of the solvent, complex trans-[6]BF4 was isolated as a colorless powder. Yield: 22 mg (0.022 mmol, 56%). 1H NMR (400 MHz, CD2Cl2/DMSO-d6 (1:1, v/v)): δ = 12.29 (br, 2H, NH), 7.70− 7.60 (m, 12H, Ph−Hortho), 7.35−7.18 (m, 18H, Ph−Hmeta, Ph−Hpara), 6.95 (dd, 3JH,H = 6.1 Hz, 4JH,H = 3.1 Hz, 2H, H7, H8), 6.86 (dd, 3JH,H = 6.1 Hz, 4JH,H = 3.1 Hz, 2H, H6, H9). 13C{1H} NMR (100 MHz, CD2Cl2/DMSO-d6 (1:1, v/v)): δ = 153.8 (t, 2JC,P = 9.9 Hz, C2), 134.8 (v-t, 2/4JC,P = 6.3 Hz, Ph−Cortho), 133.1 (C4, C5), 131.3 (Ph−Cpara), 129.0 (v-t, 1/3JC,P = 29.0 Hz, Ph−Cipso), 128.4 (v-t, 3/5JC,P = 5.4 Hz, Ph−Cmeta), 122.8 (C7, C8), 111.4 (C6, C9). 31P{1H} NMR (162 MHz, CD2Cl2/DMSO-d6 (1:1, v/v)): δ = 17.5 (s, Pt satellites, 1JPt,P = 2554 Hz). HRMS (ESI, positive ions): m/z = 917.1159 (calcd for [6]+ 917.1178). Synthesis of Complex trans-[8]BF4. A mixture of 2-iodobenzimidazole 7 (10 mg, 0.041 mmol), [Pd(PPh3)4] (46 mg, 0.040 mmol), and an excess of NH4BF4 (16 mg, 0.15 mmol) was suspended in THF (10 mL). The reaction mixture was heated under reflux for 12 h and then allowed to cool to ambient temperature. Then, the solvent was removed in vacuo. The solid residue was washed with hexane (2 × 7 mL) and diethyl ether (2 × 7 mL) and subsequently suspended in dichloromethane (15 mL). Filtration gave a clear solution. After removal of the solvent, complex trans-[8]BF4 was isolated as a light yellow powder. Yield: 28 mg (0.029 mmol, 73%). 1H NMR (400 MHz, CD2Cl2/DMSO-d6 (1:1, v/v)): δ = 12.58 (br, 2H, NH), 7.59− 7.47 (m, 12H, Ph−Hortho), 7.25−7.09 (m, 18H, Ph−Hmeta, Ph−Hpara), 6.89 (dd, 3JH,H = 6.1 Hz, 4JH,H = 3.1 Hz, 2H, H7, H8), 6.79 (dd, 3JH,H = 6.1 Hz, 4JH,H = 3.1 Hz, 2H, H6, H9). 13C{1H} NMR (100 MHz, CD2Cl2/DMSO-d6 (1:1, v/v): δ = 169.0 (t, 2JC,P = 8.5 Hz, C2), 134.5 (v-t, 2/4JC,P = 6.3 Hz, Ph−Cortho), 133.1 (C4, C5), 130.8 (Ph−Cpara), 130.4 (v-t, 1/3JC,P = 25.5 Hz, Ph−Cipso), 128.0 (v-t, 3/5JC,P = 5.1 Hz, Ph−Cmeta), 122.4 (C7, C8), 111.1 (C6, C9). 31P{1H} NMR (162 MHz, CD2Cl2/DMSO-d6 (1:1, v/v)): δ = 18.3 (s). MALDI-TOF (positive ions): m/z = 875 (calcd for trans-[8]+ 875). Synthesis of Complex trans-[9]BF4. A mixture of 2-iodobenzimidazole 7 (10 mg, 0.041 mmol), [Pt(PPh3)4] (49 mg, 0.039 mmol), and an excess of NH4BF4 (15 mg, 0.14 mmol) was suspended in THF (10 mL). The reaction mixture was heated under reflux for 2 h and then allowed to cool to ambient temperature. Then, the solvent was removed in vacuo. The solid residue was washed with hexane (2 × 7 mL) and diethyl ether (2 × 7 mL) and subsequently suspended in dichloromethane (17 mL). Filtration gave a clear solution. After removal of the solvent, complex trans-[9]BF4 was isolated as a colorless powder. Analytically pure trans-[9]BF4 was obtained by recrystallization from a CH2Cl2/Et2O solvent mixture at ambient temperature. Yield: 19 mg (0.018 mmol, 46%). 1H NMR (400 MHz, CD2Cl2/DMSO-d6 (1:1, v/v)): δ = 12.45 (br, 2H, NH), 7.67−7.54 (m, 12H, Ph−Hortho), 7.29−7.11 (m, 18H, Ph−Hmeta, Ph−Hpara), 6.88 (dd, 3JH,H = 6.2 Hz, 4JH,H = 3.1 Hz, 2H, H7, H8), 6.84 (dd, 3JH,H = 6.2 Hz, 4JH,H = 3.1 Hz, 2H, H6, H9). 13C{1H} NMR (100 MHz, CD2Cl2/ DMSO-d6 (1:1, v/v)): δ = 156.1 (t, 2JC,P = 9.9 Hz, C2), 134.9 (v-t, 2/4 JC,P = 6.1 Hz, Ph−Cortho), 132.8 (C4, C5), 131.0 (Ph−Cpara), 129.6 (v-t, 1/3JC,P = 29.9 Hz, Ph−Cipso), 128.1 (v-t, 3/5JC,P = 5.5 Hz, Ph− Cmeta), 122.5 (C7, C8), 111.2 (C6, C9). 31P{1H} NMR (162 MHz, CD2Cl2/DMSO-d6 (1:1, v/v)): δ = 14.7 (s, Pt satellites, 1JPt,P = 2527 Hz). HRMS (ESI, positive ions): m/z = 964.1062 (calcd for [9]+ 964.1050).

procedures. Consistent microanalytical data for the metal complexes were difficult to obtain due to the large fluorine content (BF4− anions).34 HRMS data and a full set of NMR spectra are provided instead. For the assignment of NMR spectra see Figure 11.

Figure 11. Assignment of NMR resonances. Synthesis of Complex trans-[2]BF4. A mixture of 2-chlorobenzimidazole 1 (6 mg, 0.039 mmol), [Pd(PPh3)4] (46 mg, 0.040 mmol), and an excess of NH4BF4 (15 mg, 0.14 mmol) was suspended in toluene (10 mL). The reaction mixture was heated under reflux for 72 h and then allowed to cool to ambient temperature. Then, the solvent was removed in vacuo. The solid residue was washed with hexane (2 × 7 mL) and diethyl ether (2 × 7 mL) and subsequently suspended in dichloromethane (20 mL). Filtration gave a clear solution. After removal of the solvent, complex trans-[2]BF4 was isolated as a colorless powder. Yield: 26 mg (0.030 mmol, 77%). 1H NMR (400 MHz, CD2Cl2/DMSO-d6 (1:1, v/v)): δ = 12.72 (br, 2H, NH), 7.67− 7.59 (m, 12H, Ph−Hortho), 7.34 (t, 3JH,H = 7.2 Hz, 6H, Ph−Hpara), 7.26 (m, 12H, Ph−Hmeta), 6.99 (dd, 3JH,H = 6.1 Hz, 4JH,H = 3.1 Hz, 2H, H7, H8), 6.89 (dd, 3JH,H = 6.1 Hz, 4JH,H = 3.1 Hz, 2H, H6, H9). 13C{1H} NMR (100 MHz, CD2Cl2/DMSO-d6 (1:1, v/v)): δ = 165.9 (t, 2JC,P = 9.1 Hz, C2), 133.7 (v-t, 2/4JC,P = 6.6 Hz, Ph−Cortho), 132.7 (C4, C5), 130.4 (Ph−Cpara), 128.5 (v-t, 1/3JC,P = 26.6 Hz, Ph−Cipso), 127.7 (v-t, 3/5 JC,P = 5.1 Hz, Ph−Cmeta), 121.9 (C7, C8), 110.4 (C6, C9). 31P{1H} NMR (162 MHz, CD2Cl2/DMSO-d6 (1:1, v/v)): δ = 21.2 (s). HRMS (ESI, positive ions): m/z = 783.1084 (calcd for [2]+ 783.1090). Synthesis of Complex trans-[3]BF4. A mixture of 2-chlorobenzimidazole 1 (6 mg, 0.039 mmol), [Pt(PPh3)4] (49 mg, 0.039 mmol), and an excess of NH4BF4 (15 mg, 0.14 mmol) was suspended in toluene (10 mL). The reaction mixture was heated under reflux for 72 h and then allowed to cool to ambient temperature. Then, the solvent was removed in vacuo. The solid residue was washed with hexane (2 × 7 mL) and diethyl ether (2 × 7 mL) and subsequently suspended in dichloromethane (20 mL). Filtration gave a clear solution. After removal of the solvent, complex trans-[3]BF4 was isolated as a colorless powder. Yield: 27 mg (0.028 mmol, 72%). 1H NMR (400 MHz, CD2Cl2/DMSO-d6 (1:1, v/v)): δ = 12.38 (br, 2H, NH), 7.62− 7.50 (m, 12H, Ph−Hortho), 7.30−7.15 (m, 18H, Ph−Hmeta, Ph−Hpara), 6.88 (dd, 3JH,H = 6.1 Hz, 4JH,H = 3.1 Hz, 2H, H7, H8), 6.78 (dd, 3JH,H = 6.1 Hz, 4JH,H = 3.1 Hz, 2H, H6, H9). 13C{1H} NMR (100 MHz, CD2Cl2/DMSO-d6 (1:1, v/v)): δ = 151.8 (t, 2JC,P = 9.9 Hz, C2), 134.8 (v-t, 2/4JC,P = 6.2 Hz, Ph−Cortho), 133.0 (C4, C5), 131.2 (Ph−Cpara), 128.4 (v-t, 1/3JC,P = 29.4 Hz, Ph−Cipso), 128.3 (v-t, 3/5JC,P = 5.4 Hz, Ph−Cmeta), 122.5 (C7, C8), 111.1 (C6, C9). 31P{1H} NMR (162 MHz, CD2Cl2/DMSO-d6 (1:1, v/v)): δ = 18.6 (s, Pt satellites, 1JPt,P = 2603 Hz). HRMS (ESI, positive ions): m/z = 873.1693 (calcd for [3]+ 873.1694). Synthesis of Complex trans-[5]BF4. A mixture of 2-bromobenzimidazole 4 (8 mg, 0.041 mmol), [Pd(PPh3)4] (46 mg, 0.040 mmol), and an excess of NH4BF4 (15 mg, 0.14 mmol) was suspended in toluene (10 mL). The reaction mixture was heated under reflux for 60 h and then allowed to cool to ambient temperature. Then solvent was removed in vacuo. The solid residue was washed with hexane (2 × 7 mL) and diethyl ether (2 × 7 mL) and subsequently suspended in dichloromethane (20 mL). Filtration gave a clear solution. After removal of the solvent, complex trans-[5]BF4 was isolated as a colorless powder. Yield: 25 mg (0.027 mmol, 68%). 1H NMR (400 MHz, CD2Cl2/DMSO-d6 (1:1, v/v)): δ = 12.62 (br, 2H, NH), 7.51− 7.38 (m, 12H, Ph−Hortho), 7.19−7.04 (m, 18H, Ph−Hmeta, Ph−Hpara), 6.79 (dd, 3JH,H = 6.1 Hz, 4JH,H = 3.1 Hz, 2H, H7, H8), 6.71 (dd, 3JH,H = 6.1 Hz, 4JH,H = 3.1 Hz, 2H, H6, H9). 13C{1H} NMR (100 MHz, CD2Cl2/DMSO-d6 (1:1, v/v)): δ = 167.3 (t, 2JC,P = 8.7 Hz, C2), 134.0 I


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Synthesis of Complex trans-[11]BF4 . A mixture of 2iodoimidazole 10 (8 mg, 0.041 mmol), [Pd(PPh3)4] (46 mg, 0.040 mmol), and an excess of NH4BF4 (15 mg, 0.14 mmol) was suspended in THF (7 mL). The reaction mixture was heated under reflux for 2 h and then allowed to cool to ambient temperature. Then, the solvent was removed in vacuo. The solid residue was washed with hexane (2 × 10 mL) and diethyl ether (2 × 10 mL). The solid was suspended in dichloromethane (20 mL) and filtered to obtain a clear solution. After removal of the solvent, complex trans-[11]BF4 was isolated as a yellow solid. Yield: 28 mg (0.031 mmol, 77%). 1H NMR (400 MHz, CD2Cl2/ DMSO-d6 (1:1, v/v)): δ = 11.90 (br, 2H, NH), 7.64−7.54 (m, 12H, Ph−Hortho), 7.43 (t, 3JH,H = 7.3 Hz, 6H, Ph−Hpara), 7.38−7.31 (m, 12H, Ph−Hmeta), 6.29 (pseudo-t, 2H, H4, H5). 13C{1H} NMR (100 MHz, CD2Cl2/DMSO-d6 (1:1, v/v)): δ = 156.5 (t, 2JC,P = 9.1 Hz, C2), 134.9 (v-t, 2/4JC,P = 6.4 Hz, Ph−Cortho), 131.3 (v-t, 1/3JC,P = 25.6 Hz, Ph−Cipso), 131.05 (Ph−Cpara), 128.5 (v-t, 3/5JC,P = 5.3 Hz, Ph−Cmeta), 119.7 (C4, C5). 31P{1H} NMR (162 MHz, CD2Cl2/DMSO-d6 (1:1, v/v)): δ = 18.8 (s). HRMS (ESI, positive ions): m/z = 825.0275 (calcd for [15]+ 825.0291). Synthesis of Complex trans-[12]BF4 . A mixture of 2iodoimidazole 10 (8 mg, 0.041 mmol), [Pt(PPh3)4] (49 mg, 0.039 mmol), and an excess of NH4BF4 (15 mg, 0.14 mmol) was suspended in THF (10 mL). The reaction mixture was heated under reflux for 1 h and then allowed to cool to ambient temperature. Then, the solvent was removed in vacuo. The solid residue was washed with hexane (2 × 7 mL) and diethyl ether (2 × 7 mL) and subsequently suspended in dichloromethane (10 mL). Filtration gave a clear solution. After removal of the solvent, a beige powder was isolated. NMR spectra of this powder (in CD2Cl2) revealed the presence of multiple compounds. HRMS showed a peak at 25% intensity for [12]+ (ESI, positive ions): m/z = 914.0882 (calcd for [12]+ 914.0893). The MALDI-TOF mass spectrum, however, showed the most intense peak at m/z = 846 corresponding to [PtI(PPh3)2]+. This would indicate decomposition of complex cation [12]+. Separation of [12]BF4 from the product mixture has so far been impossible. Synthesis of Complex trans-[13]BF4. From Complex trans[2]BF4. A Schlenk flask charged with trans-[2]BF4 (13.3 mg, 0.015 mmol) in tetrahydrofuran (10 mL) was cooled in an ice/water bath. Potassium tert-butoxide (3.7 mg, 0.033 mmol) was added to the solution, and the cold reaction mixture was stirred for 30 min. Subsequently, iodomethane (5.3 mg, 0.037 mmol in 5 mL of dichloromethane) was added slowly dropwise, and the reaction mixture was stirred for 2.5 h at 0 °C and then 30 h at ambient temperature. The volume of the reaction mixture was reduced to 2 mL. Diethyl ether (15 mL) was added to precipitate a light yellow-gray solid, which was collected via filtration. To this solid were added potassium iodide (2.5 mg, 0.015 mmol), toluene (7 mL), and acetonitrile (3 mL), and the resulting suspension was heated to 90 °C for 6 h. After cooling to ambient temperature, the reaction mixture was diluted with dichloromethane (7 mL) and was filtered through a short pad of Celite to obtain a clear solution. Evaporation of the solvents in vacuo afforded trans-[13]BF4 as a yellow-green solid. Yield: 9.0 mg (0.009 mmol, 60%). From Complex trans-[5]BF4. The same procedure as described above was used for the synthesis of complex trans-[13]BF4 from trans[5]BF4 (13.7 mg, 0.015 mmol), potassium tert-butoxide (3.7 mg, 0.033 mmol), iodomethane (5.3 mg, 0.037 mmol), and potassium iodide (2.5 mg, 0.015 mmol). Yield: 8.0 mg (0.008 mmol, 53%). From Complex trans-[8]BF4. A Schlenk flask charged with trans[8]BF4 (14.4 mg, 0.015 mmol) in tetrahydrofuran (10 mL) was cooled in an ice/water bath. Potassium tert-butoxide (3.7 mg, 0.033 mmol) was added to the solution, and the cold reaction mixture was stirred for 30 min. Subsequently, iodomethane (5.3 mg, 0.037 mmol in 5 mL of dichloromethane) was added slowly dropwise, and the reaction mixture was stirred for 2.5 h at 0 °C and then 30 h at ambient temperature. The resulting suspension was filtered though a short pad of Celite, and thereafter, the volume of the solution was reduced to 2 mL. Diethyl ether (15 mL) was added to precipitate a light yellow-gray solid, which was then collected via filtration and dried in vacuo. Yield: 10.0 mg (0.010 mmol, 66%).

H NMR (400 MHz, CD2Cl2): δ = 7.60−7.51 (m, 12H, Ph−Hortho), 7.41−7.34 (m, 6H, Ph−Hpara), 7.33−7.27 (m, 12H, Ph−Hmeta), 7.25 (dd, 3JH,H = 6.2 Hz, 4JH,H = 3.1 Hz, 2H, H7, H8), 7.02 (dd, 3JH,H = 6.2 Hz, 4JH,H = 3.1 Hz, 2H, H6, H9), 3.51 (s, 6H, NCH3). 13C{1H} NMR (100 MHz, CD2Cl2): δ = 175.6 (t, 2JC,P = 8.8 Hz, C2), 134.8 (C4, C5), 134.5 (v-t, 2/4JC,P = 6.2 Hz, Ph−Cortho), 131.9 (Ph−Cpara), 130.1 (v-t, 1/3 JC,P = 26.0 Hz, Ph−Cipso), 129.0 (v-t, 3/5JC,P = 5.4 Hz, Ph−Cmeta), 124.3 (C7, C8), 110.6 (C6, C9), 34.6 (N−CH3). 31P{1H} NMR (162 MHz, CD2Cl2): δ = 17.9 (s). HRMS (ESI, positive ions): m/z = 903.0735 (calcd for [13]+ 903.0763). Synthesis of Complex trans-[14]BF4. From Complex trans[3]BF4. A Schlenk flask charged with trans-[3]BF4 (16.3 mg, 0.017 mmol) in tetrahydrofuran (10 mL) was cooled in an ice/water bath. Potassium tert-butoxide (4.2 mg, 0.037 mmol) was added to the solution, and the cold reaction mixture was stirred for 30 min. Subsequently, iodomethane (6.0 mg, 0.042 mmol in 5 mL of dichloromethane) was added slowly dropwise, and the reaction mixture was stirred for 2 h at 0 °C and then 30 h at ambient temperature. Then the volume of the reaction mixture was reduced to 2 mL. Diethyl ether (20 mL) was added to precipitate an off-white solid, which was collected by filtration. To this solid were added potassium iodide (2.7 mg, 0.016 mmol), toluene (7 mL), and acetonitrile (3 mL), and the resulting suspension was heated to 90 °C for 6 h. After cooling to ambient temperature, the reaction mixture was diluted with dichloromethane (7 mL) and was filtered through a short pad of Celite to obtain a clear solution. Evaporation of solvents in vacuo afforded trans-[14]BF4 as a white solid. Yield: 11.0 mg (0.010 mmol, 59%). From Complex trans-[6]BF4. The same procedure as described above was used for the synthesis of complex trans-[14]BF4 from trans[6]BF4 (17.0 mg, 0.017 mmol), potassium tert-butoxide (4.2 mg, 0.037 mmol), iodomethane (6.0 mg, 0.042 mmol), and potassium iodide (2.7 mg, 0.016 mmol). Yield: 13.0 mg (0.012 mmol, 71%). From Complex trans-[9]BF4. A Schlenk flask charged with trans[9]BF4 (18.0 mg, 0.017 mmol) in tetrahydrofuran (10 mL) was cooled in an ice/water bath. Potassium tert-butoxide (4.2 mg, 0.037 mmol) was added to the solution, and the cold reaction mixture was stirred for 30 min. Subsequently, iodomethane (6.0 mg, 0.042 mmol in 5 mL of dichloromethane) was added slowly dropwise, and the reaction mixture was stirred for 2 h at 0 °C and then 30 h at ambient temperature. The resulting suspension was filtered through a short pad of Celite, and thereafter, the volume of the solution was reduced to 2 mL. Diethyl ether (20 mL) was added to precipitate an off-white solid, which was collected by filtration and dried in vacuo. Yield: 13.0 mg (0.012 mmol, 71%). 1 H NMR (400 MHz, CD2Cl2): δ = 7.63−7.49 (m, 12H, Ph−Hortho), 7.45−7.35 (m, 6H, Ph−Hpara), 7.35−7.26 (m, 12H, Ph−Hmeta), 7.23 (dd, 3JH,H = 6.2 Hz, 4JH,H = 3.1 Hz, 2H, H7, H8), 6.98 (dd, 3JH,H = 6.2 Hz, 4JH,H = 3.1 Hz, 2H, H6, H9), 3.47 (s, 6H, NCH3). 13C{1H} NMR (100 MHz, CD2Cl2): δ = 164.2 (t, 2JC,P = 9.9 Hz, C2), 134.6 (v-t, 2/4 JC,P = 5.9 Hz, Ph−Cortho), 134.2 (C4, C5), 132.1 (Ph−Cpara), 129.3 (v-t, 1/3JC,P = 28.9 Hz, Ph−Cipso), 129.0 (v-t, 3/5JC,P = 5.5 Hz, Ph− Cmeta), 124.5 (C7, C8), 110.6 (C6, C9), 34.4 (N−CH3). 31P{1H} NMR (162 MHz, CD2Cl2): δ = 12.6 (s, Pt satellites, 1JPt,P = 2425 Hz). HRMS (ESI, positive ions): m/z = 992.1346 (calcd for [14]+ 992.1363). X-ray Diffraction Studies. X-ray diffraction data were collected at T = 223(2) K with a Nonius KappaCCD diffractometer equipped with a rotation anode using graphite-monochromated Mo Kα radiation (λ = 0.710 73 Å). Diffraction data were collected over the full sphere and were corrected for absorption. Programs used: data collection, COLLECT;35 data reduction, Denzo-SM;36 absorption correction, Denzo;37 structure solution, SHELXS-97;38 and structure refinement, SHELXL-97.39 Hydrogen atoms were added to the structure model on calculated positions. Exceptions and Special Features. Compounds trans-[2]BF4· C4H8O, trans-[3]BF4, trans-[5]BF4, trans-[6]BF4, trans-[8]BF4, and trans-[11]BF4 contain disordered BF4− anions. Several restraints (SADI, SAME, ISOR, and SIMU) were used in order to improve refinement stability. For compounds trans-[2]BF4·C4H8O and trans1

J


Organometallics

Article

space group P21/c, Z = 4, a = 19.6215(3) Å, b = 11.8527(1) Å, c = 18.7279(3) Å, β = 101.455(1)°, V = 4268.75(10) Å3, ρcalc = 1.630 g· cm−3, μ = 1.410 mm−1, semiempirical absorption correction (0.7657 ≤ T ≤ 0.9723), ω- and ϕ-scans, 8.2° ≤ 2θ ≤ 52.7°, 25379 measured intensities (±h, ± k, ±l), 8562 independent (Rint = 0.048) and 7631 observed (I ≥ 2σ(I)) intensities, refinement of 587 parameters against |F2| of all measured intensities with H atoms on calculated positions. R = 0.0474, wR = 0.1073, Rall = 0.0556, wRall = 0.1145. The asymmetric unit contains one molecule of the complex trans-[8]BF4 and one molecule of solvent CH2Cl2 (the CH2Cl2 molecule and the BF4− anion are disordered). trans-[11]BF4·CH2Cl2. Crystals suitable for X-diffraction study were obtained by diffusion of diethyl ether into a saturated d ichlo rometh ane s olution o f t r an s-[11 ]B F 4 at 4 ° C. C40H36N2BCl2F4IP2Pd, M = 997.66 g·mol−1, yellow crystal, 0.25 × 0.03 × 0.03 mm3, monoclinic, space group P21/c, Z = 4, a = 11.9815(2) Å, b = 18.7185(3) Å, c = 18.5124(3) Å, β = 93.913(1)°, V = 4142.20(12) Å3, ρcalc = 1.600 g·cm−3, μ = 1.449 mm−1, semiempirical absorption correction (0.7134 ≤ T ≤ 0.9578), ω- and ϕ-scans, 8.1° ≤ 2θ ≤ 52.7°, 23 247 measured intensities (±h, ±k, ±l), 8290 independent (Rint = 0.045) and 6388 observed (I ≥ 2σ(I)) intensities, refinement of 558 parameters against |F2| of all measured intensities with H atoms on calculated positions. R = 0.0555, wR = 0.0988, Rall = 0.0809, wRall = 0.1102. The asymmetric unit contains one molecule of the complex trans-[11]BF4 and one molecule of solvent CH2Cl2 (the CH2Cl2 molecule and the BF4− anion are disordered). trans-[13]BF4·CH2Cl2. Crystals suitable for X-diffraction study were obtained by diffusion of diethyl ether into a saturated d ichlo rometh ane s olution o f t r an s-[13 ]B F 4 at 4 ° C. C46H42N2BCl2F4IP2Pd, M = 1075.77 g·mol−1, yellow-green crystal, 0.25 × 0.19 × 0.17 mm3, orthorhombic, space group Pnma, Z = 4, a = 18.8451(3) Å, b = 14.9630(2) Å, c = 16.4293(3) Å, V = 4632.72(13) Å3, ρcalc = 1.542 g·cm−3, μ = 1.301 mm−1, semiempirical absorption correction (0.7368 ≤ T ≤ 0.8091), ω- and ϕ-scans, 8.1° ≤ 2θ ≤ 56.5°, 18 997 measured intensities (±h, ±k, ±l), 5851 independent (Rint = 0.029) and 5225 observed (I ≥ 2σ(I)) intensities, refinement of 309 parameters against |F2| of all measured intensities with H atoms on calculated positions. R = 0.0361, wR = 0.0940, Rall = 0.0415, wRall = 0.0992. The asymmetric unit contains 1/2 a molecule of the complex trans-[13]BF4 (residing on a crystallographic mirror plane) and 1/2 a molecule of CH2Cl2. trans-[14]BF4·CH2Cl2. Crystals suitable for X-diffraction study were obtained by diffusion of diethyl ether into a saturated d ichlo rometh ane s olution o f t r an s-[14 ]B F 4 at 4 ° C. C46H42N2BCl2F4IP2Pt, M = 1164.46 g·mol−1, colorless crystal, 0.12 × 0.10 × 0.09 mm3, orthorhombic, space group Pnma, Z = 4, a = 18.7930(2) Å, b = 14.9433(2) Å, c = 16.3633(2) Å, V = 4595.30(10) Å3, ρcalc = 1.683 g·cm−3, μ = 3.962 mm−1, semiempirical absorption correction (0.6479 ≤ T ≤ 0.7169), ω- and ϕ-scans, 8.2° ≤ 2θ ≤ 52.7°, 30 896 measured intensities (±h, ±k, ±l), 4854 independent (Rint = 0.045) and 4426 observed (I ≥ 2σ(I)) intensities, refinement of 308 parameters against |F2| of all measured intensities with H atoms on calculated positions. R = 0.0351, wR = 0.0914, Rall = 0.0395, wRall = 0.0955. The asymmetric unit contains 1/2 a molecule of the complex trans-[14]BF4 (residing on a crystallographic mirror plane) and 1/2 a molecule of CH2Cl2.

[3]BF4 an additional 1/2 of a badly disordered solvent molecule was found in the asymmetric unit. The positional parameters for these solvent molecules could not be refined satisfactorily. The program SQUEEZE40 was therefore used to remove mathematically the effect of these solvent molecules. The quoted formula and derived parameters do not include the squeezed solvent molecule. The hydrogen atoms at N1 and N2 in the compounds trans-[2]BF4 and trans-[6]BF4 were calculated and refined as riding atoms. trans-[2]BF4·C4H8O. Colorless crystals of trans-[2]BF4.C4H8O were obtained by cooling of a saturated tetrahydrofuran solution of trans-[2]BF4 to −20 °C. C47H44N2BClF4OP2Pd, M = 943.44 g·mol−1, colorless crystal, 0.12 × 0.02 × 0.02 mm3, monoclinic, space group Cm, Z = 2, a = 18.5047(6) Å, b = 15.7783(5) Å, c = 10.0906(4) Å, β = 117.359(1)°, V = 2616.63(16) Å3, ρcalc = 1.197 g·cm−3, μ = 0.513 mm−1, semiempirical absorption correction (0.9410 ≤ T ≤ 0.9898), ωand ϕ-scans, 8.6° ≤ 2θ ≤ 50.0°, 8500 measured intensities (±h, ±k, ±l), 4379 independent (Rint = 0.057) and 3629 observed (I ≥ 2σ(I)) intensities, refinement of 318 parameters against |F2| of all measured intensities with H atoms on calculated positions. R = 0.0702, wR = 0.1681, Rall = 0.0882, wRall = 0.1816. The asymmetric unit contains 1/2 a molecule of the complex trans-[2]BF4 and 1/2 a molecule of tetrahydrofuran (the BF4− anion is disordered). trans-[3]BF4. Crystals suitable for X-diffraction study were obtained by slow evaporation of a saturated dichloromethane/toluene solution of trans-[7]BF4 at ambient temperature. C43H36N2BClF4P2Pt, M = 960.03 g·mol−1, colorless crystal, 0.23 × 0.20 × 0.17 mm3, monoclinic, space group C2/c, Z = 8, a = 23.4531(4) Å, b = 17.1826(4) Å, c = 22.3726(4) Å, β = 110.774(1)°, V = 8429.70(3) Å3, ρcalc = 1.513 g· cm−3, μ = 3.518 mm−1, semiempirical absorption correction (0.4984 ≤ T ≤ 0.5862), ω- and ϕ-scans, 8.1° ≤ 2θ ≤ 52.7°, 58 022 measured intensities (±h, ±k, ±l), 8566 independent (Rint = 0.054) and 7409 observed (I ≥ 2σ(I)) intensities, refinement of 541 parameters against |F2| of all measured intensities with H atoms on calculated positions. R = 0.0312, wR = 0.0828, Rall = 0.0374, wRall = 0.0870. The asymmetric unit contains one molecule of the complex trans-[3]BF4 (the BF4− anion is disordered). trans-[5]BF4·CH2Cl2. Crystals suitable for X-diffraction study were obtained by diffusion of diethyl ether into a saturated dichloromethane solution of trans-[5]BF4 at 4 °C. C44H38N2BBrCl2F4P2Pd, M = 1000.72 g·mol−1, colorless crystal, 0.24 × 0.10 × 0.08 mm3, monoclinic, space group P21/c, Z = 4, a = 19.5161(3) Å, b = 11.8711(1) Å, c = 18.6273(2) Å, β = 102.310(1)°, V = 4216.31(9) Å3, ρcalc = 1.576 g·cm−3, μ = 1.643 mm−1, semiempirical absorption correction (0.6939 ≤ T ≤ 0.8798), ω- and ϕ-scans, 8.2° ≤ 2θ ≤ 54.2°, 33 321 measured intensities (±h, ±k, ±l), 9122 independent (Rint = 0.046) and 7435 observed (I ≥ 2σ(I)) intensities, refinement of 587 parameters against |F2| of all measured intensities with H atoms on calculated positions. R = 0.0446, wR = 0.0873, Rall = 0.0607, wRall = 0.0953. The asymmetric unit contains one molecule of the complex trans-[5]BF4 and one molecule of CH2Cl2 (the CH2Cl2 molecule and the BF4− anion are disordered). trans-[6]BF4·CH2Cl2. Crystals suitable for X-diffraction study were obtained by diffusion of diethyl ether into a saturated dichloromethane solution of trans-[6]BF4 at 4 °C. C44H38N2BBrCl2F4P2Pt, M = 1089.41 g·mol−1, colorless crystal, 0.12 × 0.10 × 0.05 mm3, monoclinic, space group P21/c, Z = 4, a = 19.5219(2) Å, b = 11.8326(1) Å, c = 18.6785(2) Å, β = 102.512(1)°, V = 4212.17(7) Å3, ρcalc = 1.718 g· cm−3, μ = 4.535 mm−1, semiempirical absorption correction (0.6121 ≤ T ≤ 0.8050), ω- and ϕ-scans, 8.1° ≤ 2θ ≤ 52.7°, 30 212 measured intensities (±h, ±k, ±l), 8530 independent (Rint = 0.039) and 7692 observed (I ≥ 2σ(I)) intensities, refinement of 579 parameters against |F2| of all measured intensities with H atoms on calculated positions. R = 0.0323, wR = 0.0794, Rall = 0.0378, wRall = 0.0836. The asymmetric unit contains one molecule of the complex trans-[6]BF4 and one molecule of CH2Cl2 (the CH2Cl2 molecule and the BF4− anion are disordered). trans-[8]BF4·CH2Cl2. Crystals suitable for X-diffraction study were obtained by diffusion of diethyl ether into a saturated dichloromethane solution of trans-[8]BF4 at 4 °C. C44H38N2BICl2F4P2Pd, M = 1047.71 g·mol−1, pale yellow crystal, 0.20 × 0.07 × 0.02 mm3, monoclinic,

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ASSOCIATED CONTENT

S Supporting Information *

NMR and mass spectra for all new compounds and crystallographic information files (CIFs) for trans-[3]BF4, trans-[5]BF 4·CH2Cl 2, trans-[6]BF4·CH2Cl2, trans-[8]BF4· CH2Cl2, trans-[13]BF4·CH2Cl2, and trans-[14]BF4·CH2Cl2 as well as data concerning the titration experiments with DMPU. Crystallographic data for trans-[2]BF4·C4H8O and trans[11]BF4·CH2Cl2 have previously been deposited with the Cambridge Crystallographic Data Centre (CCDC numbers K


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962104 and 962105).25 This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Deutsche Forschungsgemeinschaft (SFB 858) for financial support. R.D. thanks the NRW Graduate School of Chemistry, Münster, Germany, for a predoctoral grant.

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Synthesis of Complexes with Protic NH,NH-NHC Ligands via

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