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Inorganica Chim Acta. Author manuscript; available in PMC 2010 September 15.
Published in final edited form as:
Inorganica Chim Acta. 2009 September 15; 362(12): 4609–4615.
doi:  10.1016/j.ica.2009.05.025
PMCID: PMC2777662
NIHMSID: NIHMS145161

2-Mercapto-1-t-butylimidazolyl as a Bridging Ligand: Synthesis and Structural Characterization of Nickel and Palladium Paddlewheel Complexes

Abstract

Nickel and palladium paddlewheel complexes that feature 2-mercapto-1-t-butylimidazolyl (mimBut) bridging ligands, namely Ni2[mim But]4 and Pd2[mim But]4, have been synthesized and structurally characterized by X–ray diffraction. Since the mim But ligand bridges in an asymmetric manner via a sulfur and nitrogen donor, paddlewheel compounds of the type M2[mim But]4 may exist as isomers that are distinguished by the relative orientations of the ligands. In this regard, the (4,0)-Ni2[mim But]4 and trans-(2,2)-Ni2[mim But]4 isomers have been isolated for the nickel system, while the (4,0)-Pd2[mim But]4 and (3,1)-Pd2[mim But]4 isomers have been isolated for the palladium system.

INTRODUCTION

Metal-thiolate complexes are of considerable interest due to the significance of M–S bonds in biological systems.1 For example, there is presently interest in nickel-thiolate chemistry due to its relevance to hydrogenase chemistry.2 In many cases, biological thiols are derived from cysteine. However, there is also a class that is related to histidine, as illustrated by ergothioneine,3 a derivative of 2–mercaptoimidazole that exists primarily as the resonance stabilized thione tautomer (Figure 1). Since the coordination chemistry of ergothioneine is not well established, useful information may be obtained by investigating simpler 1-R-2-mercaptoimidazole/1-R-imidazole-2-thione systems (Figure 2), [HmimR],[1]4 that are also of interest due to the discovery that the methyl derivative, methimazole, is an antithyroid drug.5 In this paper, we report the synthesis of nickel complexes derived from 2-mercapto-1-t-butylimidazole [Hmim But], including dinuclear paddlewheel complexes.

Figure 1
Structure of ergothioneine.
Figure 2
Thiol/thione tautomerism for 1-R-2-mercaptoimidazoles.

RESULTS AND DISCUSSION

The tetrakis(1-But-imidazole-2-thione) nickel complex {Ni[Hmim But]4}I2 is readily obtained by the reaction of Hmim But with NiI2 in acetonitrile (Scheme 1). The molecular structure of {Ni[Hmim But]4}I2 has been determined by X–ray diffraction, thereby demonstrating that it possesses a square-planar geometry (Figure 3), with each iodide counterion bridging two Hmim But ligands via hydrogen bonding interactions.[2]6 Interestingly, the coordinated Hmim But groups may be deprotonated by treatment with KH to generate incipient {Ni[mim But]4}2− which, in the presence of NiI2, yields dinuclear Ni2[mim But]4 (Scheme 1). The latter compound may also be isolated from the reaction of [κ4-B(mim But)3]NiCl7 with KOH (Scheme 1).

Figure 3
Molecular structure of {Ni[Hmim But]4}I2.

X–ray diffraction studies demonstrate that Ni2[mim But]4 exists as two crystalline modifications, each of which adopts a paddlewheel (or lantern) structure with two square-planar nickel centers (Figures 4 and and5).5). The two square planar metal centers are not, however, eclipsed, as illustrated by the view down the Ni···Ni vector shown in Figure 6. For example, the N–Ni–Ni–S torsion angle for the P4/n modification is 25°.

Figure 4
Molecular structure of (4,0)-Ni2[mim But]4 (P4/n).
Figure 5
Molecular structure of (4,0)-Ni2[mim But]4 (C2/c).
Figure 6
View of (4,0)-Ni2[mim But]4 (C2/c) down Ni···Ni axis.

While paddlewheel compounds that feature four bridging bidentate ligands are a common feature in coordination chemistry, the bridging ligands are frequently symmetric with respect to the interaction, such that only one isomer is possible.8 However, if the bridging ligand (A–B–C) is asymmetric with respect to its coordination to the two metals, as with [mim But], four different isomeric arrangements are possible (Figure 7).8 The different isomers may be designated by the prefixes (4,0), (3,1), cis-(2,2) and trans-(2,2), in which the (x,y) values indicate the number of ligands with each orientation relative to the metal···metal vector.

Figure 7
Classification of isomers of paddlewheel complexes with asymmetric bridging ligands.

With respect to this classification, the structures shown in Figures 4 and and55 may be described as (4,0)-Ni2[mim But]4. In this regard, while paddlewheel complexes of nickel are known,[3]9 we are only aware of two that possess the (4,0) motif, and both of these also possess axial ligands.10 Thus, (4,0)-Ni2[mim But]4 is unique by being devoid of axial ligation.

The two nickel centers of (4,0)-Ni2[mim But]4 are electronically distinct, with one nickel center having a {Ni[N4]} coordination environment while the other has a {Ni[S4]} coordination environment. The formation of this (4,0)-isomer, rather than a more symmetric (2,2) variant with a {Ni[N2S2]Ni[N2S2]} core and equivalent nickel centers, is presumably a consequence of the method of synthesis. Specifically, if the four sulfur atoms attached to {Ni[Hmim But]4}2+ remain coordinated following deprotonation, then only the nitrogen atoms are available for coordinating the second nickel, thereby resulting in a polar isomer of Ni2[mim But]4.[4]

In this regard, a nonpolar isomer that features two identical nickel centers, namely trans-(2,2)-Ni2[mim But]4 with a {Ni[N2S2]Ni[N2S2]} core, has been isolated via degradation of trans-Ni(PMe3)21-N-mim But]211 by [PhC(O)O]2 (Scheme 2) and has been structurally characterized by X–ray diffraction (Figure 8). The formation of a more symmetric isomer may be conceptually rationalized by considering that dissociation of PMe3 would generate incipient trans-{Ni[κ1-N-mim But]2} and subsequent dimerization of this species would generate a dinuclear {Ni[N2S2]Ni[N2S2]} core (Scheme 2).[4]

Figure 8
Molecular structure of trans-(2,2)-Ni2[mim But]4.

The Ni–S and Ni–N bond lengths in the two isomers of Ni2[mim But]4 are very similar, as are the Ni···Ni separations (Table 1). A variety of paddlewheel complexes of the type Ni2[(LX)]4, where LX is a bidentate bridging ligand with a three-atom linker (e.g. RNCHNR, RNNNR, RC(O)S, and RCS2) have been structurally characterized10,12 and have Ni···Ni distances that are in the range 2.38 Å12b – 2.65 Å.10b Since these distances are comparable to the Ni–Ni bond length in the elemental form (2.49 Å),13 a degree of Ni–Ni bonding was originally proposed,12b,c but calculations on hypothetical Ni2(HNNNH)4 indicate that the bond order is formally zero.12a Specifically, with a configuration of σ2π4δ2δ*2π*4σ*2 for the interaction between the two d8 metal centers, all Ni–Ni bonding and antibonding orbitals are filled and so there is no net bonding interaction.[5]14

Table 1
M···M distances in paddlewheel complexes.

Paddlewheel complexes of palladium are not as common as those of nickel,[3],[5],15 and so it is noteworthy that we have also structurally characterized two palladium derivatives in which [mim But] serves as the bridging ligand. For example, (4,0)-Pd2[mim But]4 has been isolated from the reaction of {[μ–κ1, κ3–B(mim But)3]Pd}216 with 1-methylimidazole (HimMe) in acetonitrile in the presence of air (Scheme 3).[6]17 Furthermore, isomeric (3,1)-Pd2[mim But]4 has been isolated from the reaction of {Ni[Hmim But]4}I2 with KH and Pd(OAc)2; in the absence of KH, the simple substitution product {Pd[Hmim But]4}I2 is obtained (Scheme 4). The molecular structures of (4,0)-Pd2[mim But]4, (3,1)-Pd2[mim But]4 and {Pd[Hmim But]4}I2 have been determined by X–ray diffraction, as illustrated in Figures 911. Of particular note, the Pd···Pd distances in (4,0)-Pd2[mim But]4 [2.728(1) Å] and (3,1)-Pd2[mim But]4 [2.734(2) Å] are considerably shorter than that in [(μ-N,S-mimMe)PdCl(PMe3)]2 [3.104(2) Å], which possesses only two bridging ligands.18

Figure 9
Molecular structure of (4,0)-Pd2[mim But]4.
Figure 11
Molecular structure of {Pd[Hmim But]4}I2.

CONCLUSIONS

In summary, a series of nickel and palladium paddlewheel complexes that feature 2-mercapto-1-t-butylimidazolyl bridging ligands has been synthesized. As a result of the fact that the 2-mercapto-1-t-butylimidazolyl ligand bridges asymmetrically, coordinating to each metal via either a nitrogen or a sulfur donor, the M2[mim But]4 paddlewheel complexes may exist as isomers that are differentiated by the relative orientations of the bridging [mim But] ligands. X–ray diffraction studies demonstrate that Ni2[mim But]4 exists as both (4,0)-Ni2[mim But]4 and trans-(2,2)-Ni2[mim But]4 isomers. The two nickel centers of the former isomer are inequivalent, with one nickel having a {Ni[S4]} coordination environment, while the other has a {Ni[N4]} coordination environment. In contrast, the two nickel centers of the latter isomer are equivalent, with both having a {Ni[N2S2]} coordination environment. For the palladium system, (4,0)-Pd2[mim But]4 and (3,1)-Pd2[mim But]4 isomers, have been isolated, with the latter being characterized by palladium centers that have {Pd[NS3]} and {Pd[N3S]} coordination environments.

EXPERIMENTAL SECTION

General Considerations

All manipulations were performed using a combination of glovebox, high vacuum, and Schlenk techniques under a nitrogen atmosphere unless otherwise specified.19 Solvents were purified and degassed by standard procedures. 1H NMR spectra were measured on Bruker 300 DRX and Bruker 400 DRX spectrometers. 1H chemical shifts are reported in ppm relative to SiMe4 (δ= 0) and were referenced internally with respect to the protio solvent impurity (δ1.94 for CD3CN).20 Coupling constants are given in hertz. Infrared spectra were recorded on Nicolet Avatar 370 DTGS spectrometer and are reported in cm−1. Hmim But,21 {[μ-κ1, κ3-B(mim But)3]Pd}2,164-B(mim But)3]NiCl7 were prepared by the literature methods.

X-ray structure determinations

X-ray diffraction data were collected on either a Bruker Apex II diffractometer or a Bruker P4 diffractometer equipped with a SMART CCD detector. Crystal data, data collection and refinement parameters are summarized in Table 2. The structures were solved using direct methods and standard difference map techniques, and were refined by full-matrix least-squares procedures on F2 with SHELXTL (Version 6.10).22 The crystallographic data for (4,0)-Ni2[mim But]4(CCDC #724098 and #724100), trans-(2,2)-Ni2[mim But]4 (CCDC #724097), (4,0)-Pd2[mim But]4 (CCDC #724101), (3,1)-Pd2[mim But]4 (CCDC #724102),{Ni[Hmim But]4}I2 (CCDC #724096), and {Pd[Hmim But]4}I2 (CCDC #724099) have been deposited with the Cambridge Crystallographic Data Centre.

Table 2
Crystal, intensity collection and refinement data.

Synthesis of {Ni[Hmim But]4}I2

A mixture of Hmim But (1.0 g, 6.4 mmol) and NiI2 (0.50 g, 1.6 mmol) in CH3CN (30 mL) was stirred for 14 hours. The resulting brown precipitate was isolated by filtration, washed with pentane (20 mL) and then dried in vacuo (1.32 g, 88%). Crystals of composition {Ni[Hmim But]4}I2·CH2Cl2 suitable for X-ray diffraction were grown by slow evaporation of a dichloromethane solution. 1H NMR (CD3CN): 2.24 (s, 36H of But), 6.42 (s, 4H of CH mim But), 8.94 (s, 4H of CH mim But). Anal. calcd. for C28H48I2N8NiS4: C, 35.9%; H, 5.2%; N, 12.0%. Found: C, 35.6%; H, 5.6%; N, 12.1%. IR Data (KBr pellet, cm−1): 3176 (m), 3130 (m), 2980 (m), 2882 (w), 1569 (m), 1450 (s), 1418 (w), 1363 (w), 1325 (w), 1315 (w), 1247 (w), 1235 (s), 1219 (m), 1132 (w), 1117 (w), 1099 (w), 735 (m), 692 (w), 683 (w).

Synthesis of Ni2[mim But]4

  1. A suspension of [κ4-B(mim But)3]NiCl (100 mg, 0.18 mmol) in MeOH (7 mL) was treated with a solution of KOH (10 mg, 0.18 mmol) in CH3OH (3 mL). The resulting mixture was stirred for 10 hours and filtered. The green filtrate was allowed to stand at room temperature for 1 week, thereby depositing dark green needle-shaped crystals of (4,0)-Ni2[mim But]4 suitable for X-ray diffraction analysis. The crystals were isolated, washed with acetone (2 × 2 mL) and hexanes (2 × 2 mL) and then dried in vacuo (37 mg, 57%). 1H NMR (CD3CN): 1.33 (s, 36H of But), 6.67 (s, 4H of CH mim But), 7.06 (s, 4H of CH mim But). Anal. calcd. for C28H44N8Ni2S4: C, 45.6%; H, 6.0%; N, 15.2%. Found: C, 45.3%; H, 5.8%; N, 15.0%. IR Data (KBr pellet, cm−1): 3113 (w), 2979 (m), 2930 (w), 1414 (m), 1394 (m), 1367 (m), 1348 (s), 1253 (s), 1187 (w), 1150 (m), 1134 (m), 1050 (w), 1021 (w), 712 (w), 685 (s).
  2. A mixture of {Ni[Hmim But]4}I2 (25 mg, 0.027 mmol) and KH (5 mg, 0.13 mmol) in CH3CN (0.5 mL) was stirred for 5 minutes, after which period NiI2 (8 mg, 0.026 mmol) was added. The resulting mixture was stirred for 5 hours and filtered. The filtrate was allowed to stand at room temperature, thereby depositing dark green needle-shaped crystals within 2 weeks, along with some colorless blocks (presumably KI). The green crystals were identified as (4,0)-Ni2[mim But]4 by X–ray diffraction.
  3. Trans-(2,2)-Ni2[mim But]4 was obtained by treating a solution of trans-Ni(PMe3)21-N-mim But]2 (15 mg) in benzene (ca. 0.7 mL) with [PhC(O)O]2 (7 mg) and crystals were obtained by slow evaporation.

Synthesis of Pd2[mim But]4

  1. A mixture of {[μ-κ1, κ3-B(mim But)3]Pd}2 (20 mg, 0.017 mmol) and HimMe (10 μL, 0.13 mmol) in CD3CN (1 mL) in an NMR tube equipped with a J-Young valve was exposed to air then heated at 100°C for 2 days. The resulting solution was allowed to cool down to room temperature and stand overnight, thereby depositing orange needle-shaped crystals of (4,0)-Pd2[mim But]4 suitable for X-ray diffraction analysis. An additional crop of orange crystals were obtained by adding EtOH (95%, ca. 10 mL) to the mother solution. The crystals were separated by decanting, washed with EtOH (0.5 mL) and Et2O, and then dried in vacuo (8 mg). 1H NMR (CD3CN): 1.75 (s, 36H of But), 6.69 (d, 2JH-H = 2, 4H of CH mim But), 6.94 (d, 2JH-H = 2, 4H of CH mim But). Anal. calcd. for C28H44N8Pd2S4·CH3CN: C, 41.2%; H, 5.4%; N, 14.4%. Found: C, 41.5%; H, 5.3%; N, 13.6%. IR Data (KBr pellet, cm−1): 2978 (m), 2910 (w), 1418 (w), 1396 (m), 1384 (s), 1367 (m), 1347 (s), 1266 (m), 1252 (m), 1190 (m), 1163 (m), 1155 (m), 1060 (w), 823 (w), 794 (m), 771 (w), 745 (w), 712 (w), 706 (w), 670 (w).
  2. A suspension of {Ni[Hmim But]4}I2 (50 mg, 0.053 mmol) in CD3CN (0.5 mL) was treated with KH (9 mg, 0.22 mmol) in small portions. The resulting mixture was stirred for 15 minutes, after which period Pd(OAc)2 (12 mg, 0.053 mmol) was added. The mixture was stirred for 2 hours and filtered. The red filtrate was placed at room temperature, thereby depositing red crystals over a period of 2 weeks. The crystals were isolated and demonstrated to be (3,1)-Pd2[mim But]4·CH3CN by X–ray diffraction.

Synthesis of {Pd[Hmim But]4}I2

A suspension of {Ni[Hmim But]4}I2 (30 mg, 0.032 mmol) in CH3CN was treated with a solution of Pd(OAc)2 (7 mg, 0.031 mmol) in CH3CN. The resulting mixture was stirred for 1 hour and filtered. The red filtrate was placed at room temperature, thereby depositing crystals over a period of 2 weeks. The crystals were isolated and demonstrated to be {Pd[Hmim But]4}I2·CH3CN by X–ray diffraction.

Figure 10
Molecular structure of (3,1)-Pd2[mim But]4.

Supplementary Material

Acknowledgments

We thank the National Institutes of Health (GM046502) for support of this research. The National Science Foundation (CHE-0619638) is thanked for the acquisition of an X–ray diffractometer.

Footnotes

Dedicated to the memory of Swiatoslaw (“Jerry”) Trofimenko.

1For representative 2–mercaptoimidazole/imidazole–2–thione compounds, see reference 4.

2For {Ni[(mimMe)H]4}X2 derivatives, see reference 6.

3For a review of Pt, Pd and Ni paddlewheel compounds, see reference 9.

4It is, of course, possible that other isomers could be present in the reaction mixture.

5For calculations on related dipalladium compounds, see reference 14.

6For a heterobimetallic counterpart that features 6-coordinate Pd, see reference 17.

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