Applications of Bis(1-R-imidazol-2-yl)disulfides and Diselenides as Ligands for Main Group and Transition Metals: κ²-[N,N] Coordination, S-S Bond Cleavage and S-S/E-E (E = S, Se) Bond Metathesis Reactions

Abstract

Bis(1-R-imidazol-2-yl)disulfides, (mim^R)₂ (R = Ph, Bu^t), and diselenides, (seim^Mes)₂, serve as bidentate [N,N]-donor ligands for main group and transition metals. For example, [κ²-(mim^But)₂]MCl₂ (M = Fe, Co, Ni, Zn), [κ²-(mim^Ph)₂]MCl₂ (M = Co, Zn), [κ²-(mim^But)₂]CuX (X = Cl, I) and [κ²-(seim^Mes)₂]MCl₂ (M = Fe, Co, Ni) are obtained by treatment of (mim^But)₂ or (seim^Mes)₂ with the respective metal halide, and have been structurally characterized by X-ray diffraction. On the other hand, the zerovalent nickel complex Ni(PMe₃)₄ effects cleavage of the disulfide bond of (mim^But)₂ to give square planar trans-Ni(PMe₃)₂(mim^But)₂ in which the (mim^But) ligands coordinate via nitrogen rather than sulfur, a most uncommon coordination mode for this class of ligands. Although [κ²-(mim^R)₂]MCl₂ (M = Fe, Co, Ni, Zn) are not subject to homolytic cleavage of the S-S bond because the tetravalent state is not readily accessible, the observation that [κ²-(mim^Ph)₂]CoCl₂ and [κ²-(mim^But)₂]CoCl₂ form an equilibrium mixture with the asymmetric disulfide [κ²-(mim^Ph)(mim^But)]CoCl₂ indicates that S-S bond cleavage via another mechanism is possible. Likewise, metathesis between disulfide and diselenide ligands is observed in the formation of [κ²-(mim^But)(seim^Mes)]CoCl₂ upon treatment of [κ²-(mim^But)₂]CoCl₂ with [κ²-(seim^Mes)₂]CoCl₂.

INTRODUCTION

Current interest in metal-thiolate chemistry derives to a large extent from the roles that such motifs, most commonly involving cysteine residues, play in biological systems.¹ In this regard, it is important to emphasize that the function of a biological thiolate is not merely to serve as a ligand, because the sulfur itself is also a site of critical reactivity. For example, cysteine ligands are subject to attack by electrophiles (as exemplified by the Ada DNA repair protein) and are also redox active.² With respect to the redox activity of cysteine ligands, a common transformation involves the reversible formation of a disulfide bond, a conversion that plays a role in protein folding and cell signaling;^2,3 furthermore, the disulfide linkage may also serve as a ligand for metals in proteins.^4,5 While cysteine is also a common component of intracellular thiols (e.g. glutathione, tropanothione and mycothiol), there is also a class of aromatic thiols that are derived from histidine, of which ergothioneine and ovothiols are representative (Figure 1).^2b,c,6 The disulfides of ergothioneine and ovothiols⁷ are known but their chemistry has not been thoroughly investigated and their ligating properties are unknown. Therefore, we describe herein the applications of compounds that are structurally related to ergothioneine disulfide, namely bis(1-R-imidazol-2-yl)disulfides, to the chemistry of main group and transition metals.

Figure 1.

Structure of ergothioneine and ovothiols.

RESULTS AND DISCUSSION

1. Bis(1-R-imidazol-2-yl)disulfides as Models for Ergothioneine Disulfide

Ergothioneine is a derivative of 2-mercaptoimidazole and exists primarily as the resonance stabilized thione tautomer (Figure 1).⁶ As such, essential features of its chemistry may be modeled by investigating simpler 1-R-2-mercaptoimidazole/1-R-imidazole-2-thione systems which are also subject to such tautomerization (Figure 2) and primarily exist in the resonance stabilized thione form. With respect to ergothioneine disulfide, it is pertinent to note that although analogues, bis(1-R-imidazol-2-yl)disulfides (R = H, Me, Ph, Bu^t), herein abbreviated as (mim^R)₂, are long known,⁸ their application as ligands is virtually nonexistent. In fact, we are aware of only a single metal compound that contains the (mim^R)₂ ligand, namely the zinc derivative [κ²-(mim^Me)₂]ZnCl₂.⁹ However, this complex was not obtained by using the disulfide (mim^Me)₂ as a reagent; rather, [κ²-(mim^Me)₂]ZnCl₂ was only isolated as a side-product in 4 % yield resulting from adventitious oxidation of methimazole (mim^Me)H during the reaction with ZnCl₂ over a period of several months at 0°C.⁹ It is, therefore, evident that a deliberate investigation of the reactivity of the disulfides (mim^R)₂ towards metal compounds is warranted.

Figure 2.

Thiol/thione tautomerism for 1-R-2-mercaptoimidazoles; the resonance stabilized thione form is more stable.

While a variety of (mim^R)₂ derivatives are available,⁸ the synthetic method used to prepare the t-butyl derivative (mim^But)₂ is not described in the original report of this compound.^8b Access to the yellow-orange disulfide (mim^But)₂ is, nevertheless, readily achieved by oxidation of 1-tert-butylimidazole-2-thione, (mim^But)H,¹⁰ with I₂ (0.5 equiv) in the presence Et₃N (Scheme 1).¹¹ The phenyl derivative (mim^Ph)₂ is likewise obtained from the corresponding reaction of (mim^Ph)H¹² with I₂ and Et₃N. The molecular structures of (mim^But)₂ and (mim^Ph)₂ have been determined by X-ray diffraction, as illustrated in Figures 3 and 4. Selected metrical data are listed in Table 1, but noteworthy features are that (i) the S-S and S-C bonds are typical of single bonds,¹³ (ii) the two C_ipso-N bond lengths in each (mim^R) fragment are different and are in accord with a resonance structure in which the C=N double bond is more localized between the ipso carbon atom and the unsubstituted nitrogen atom (Scheme 1), and (iii) the C-S-S-C torsion angles are approximately 90°, in accord with an orientation that minimizes repulsions between the lone pairs on the two sulfur atoms.⁵

Scheme 1.

Figure 3.

Molecular structure of (mim^But)₂.

Figure 4.

Molecular structure of (mim^Ph)₂.

Table 1.

Selected metrical data for (mim^But)₂ and (mim^Ph)₂

	(mimBut)2	(mimPh)2
S(1)-S(2)/Å	2.0807(9)	2.0699(4)
S(1)-C(11)/ Å	1.747(2)	1.749(1)
S(2)-C(21)/ Å	1.748(2)	1.743(1)
C(11)-N(11)/Å	1.319(3)	1.321(1)
C(11)-N(12)/Å	1.381(3)	1.373(2)
N(11)-C(12)/Å	1.362(3)	1.373(2)
C(12)-C(13)/Å	1.355(3)	1.365(2)
C(13)-N(12)/Å	1.364(3)	1.380(1)
C(21)-N(21)/Å	1.318(3)	1.330(1)
C(21)-N(22)/Å	1.379(3)	1.376(1)
N(22)-C(23)/Å	1.365(3)	1.375(1)
C(22)-C(23)/Å	1.356(4)	1.379(1)
C(23)-N(22)/Å	1.365(3)	1.375(1)
C(11)-S(1)-S(2)-C(21)/°	93.1	83.7

The disulfides (mim^R)₂ are effective bidentate ligands for a variety of metals, binding through two of the imidazolyl nitrogen atoms. For example, (mim^R)₂ coordinates to a series of metal dichlorides, namely FeCl₂, CoCl₂, NiCl₂, and ZnCl₂, giving rise to four-coordinate compounds of the type [κ²-(mim^R)₂]MCl₂ (Scheme 2). The molecular structures of [κ²-(mim^But)₂]MCl₂ (M = Fe, Co, Ni, Zn) and [κ²-(mim^Ph)₂]CoCl₂ have been determined by X-ray diffraction, as illustrated in Figures 5-9, with selected metrical data presented in Tables 2 and 3. Evaluation of the bond angles and the τ₄ four-coordinate geometry index¹⁴ indicates that distortions from idealized tetrahedral geometry (with τ₄ = 1) are greatest for the nickel and iron complexes, [κ²-(mim^But)₂]NiCl₂ (τ₄ = 0.87) and [κ²-(mim^But)₂]FeCl₂ (τ₄ = 0.90). It is also pertinent to note that, in each complex, the [κ²-(mim^R)₂] ligands adopt a conformation in which the C-S-S-C torsion angles are approximately 90°, similar to that observed for the uncoordinated ligands.

Scheme 2.

Figure 5.

Molecular structure of [κ²-(mim^But)₂]FeCl₂.

Figure 9.

Molecular structure of [κ²-(mim^Ph)₂]CoCl₂.

Table 2.

Selected bond lengths for (κ²-(mim^R)₂]MCl₂ and [κ²-(seim^Mes)₂]MCl₂

	M-N(11)/Å	M-N(21)/Å	M-Cl(1)/Å	M-Cl(2)/Å	E(1)-E(2)/Å	E1-C(11)/Å	E2-C(21)/Å
[κ2-(mimBut)2]FeCl2	2.060(4)	2.060(4)	2.262(2)	2.263(2)	2.076(3)	1.761(5)	1.749(5)
[κ2-(mimBut)2]CoCl2	2.031(3)	2.031(3)	2.282(1)	2.252(1)	2.056(1)	1.764(3)	1.757(3)
[κ2-(mimPh)2]CoCl2	2.028(8)	2.030(8)	2.255(3)	2.254(2)	2.081(3)	1.753(9)	1.748(9)
[κ2-(mimBut)2]NiCl2	1.987(3)	1.973(3)	2.241(1)	2.242(1)	2.063(2)	1.760(4)	1.760(4)
[κ2-(mimBut)2]ZnCl2	2.039(2)	2.007(2)	2.252(1)	2.250(1)	2.055(1)	1.766(3)	1.750(3)
[κ2-(seimMes)2]FeCl2	2.109(3)	2.088(3)	2.267(1)	2.255(1)	2.312(1)	1.904(4)	1.891(4)
[κ2-(seimMes)2]CoCl2	2.025(2)	2.025(2)	2.253(1)	2.253(1)	2.3284(6)	1.907(3)	1.907(3)
[κ2-(seimMes)2]NiCl2	2.014(4)	1.993(4)	2.246(1)	2.223(2)	2.3134(8)	1.905(5)	1.894(5)

Table 3.

Selected bond angles for [κ²-(mim^R)₂]MCl₂ and [κ²-(seim^Mem)₂]MCl₂

	N(11)-M-N(21)	Cl(1)-M-Cl(2)	N(11)-M-Cl(1)	N(21)-M-Cl(2)	N(11)-M-Cl(2)	N(21)-M-Cl(1)	τ4
[κ2-(mimBut)2]FeCl2	108.0(2)	120.42(7)	104.0(1)	103.2(1)	108.3(1)	112.5(1)	0.90
[κ2-(mimBut)2]CoCl2	107.5(1)	111.44(4)	107.81(8)	112.37(8)	112.37(8)	106.74(8)	0.96
[κ2-(mimPh)2]CoCl2	112.0(3)	111.4(1)	103.8(2)	107.3(2)	111.6(2)	110.8(2)	0.97
[κ2-(mimBut)2]NiCl2	108.3(1)	127.66(4)	102.8(1)	100.9(1)	106.2(1)	110.0(1)	0.87
[κ2-(mimBut)2]ZnCl2	106.56(9)	112.23(3)	107.49(6)	110.55(7)	111.69(7)	108.07(7)	0.97
[κ2-(seimMes)2]FeCl2	107.5(1)	120.72(5)	103.8(1)	108.1(1)	110.5(1)	105.5(1)	0.91
[κ2-(seimMes)CoCl2(a)	112.4(1)	114.43(5)	112.94(7)	112.94(7)	102.29(7)	102.29(7)	0.94
[κ2-(seimMes)2]NiCl2	105.7(2)	124.72(6)	100.3(1)	109.8(1)	111.4(1)	103.1(1)	0.89

^(a)[κ²-(seim^Mes)₂]CoCl₂ resides on a crystallographic two fold site and so Cl(1)/Cl(2) and N(11)/N(21) are symmetry equivalent pairs in this case.

In addition to four coordinate [κ²-(mim^R)₂]MCl₂ complexes, three-coordinate copper(I) compounds [κ²-(mim^But)₂]CuX (X = Cl, I) have also been isolated (Scheme 2) and structurally characterized by X-ray diffraction (Figures 10 and 11), with selected bond lengths and angles being listed in Tables 4 and 5. Copper(I) complexes with trigonal planar geometries are less prevalent than tetrahedral derivatives;¹⁵ the observation that [κ²-(mim^But)₂]CuX exists as a three-coordinate monomer rather than a halide-bridge dimer is, therefore, noteworthy, especially so because (mim^But)₂ is not a sterically demanding ligand.

Figure 10.

Molecular structure of [κ²-(mim^But)₂]CuCl.

Figure 11.

Molecular structure of [κ²-(mim^But)₂]CuI.

Table 4.

Selected bond lengths for [κ²-(mim^But)₂]CuX (C = Cl, I)

	Cu-N(11)/Å	Cu-N(21)/Å	Cu-X/Å	S(1)-S(2)/Å	S(1)-C(11)/Å	S(2)-C(21)/Å
[κ2-(mimBut)2]CuCl	1.950(2)	2.038(2)	2.2390(7)	2.0708(8)	1.749(2)	1.754(2)
[κ2-(mimBut)2]CuI	2.014(14)	1.970(14)	2.533(2)	2.070(6)	1.755(15)	1.765(17)

Table 5.

Selected bond angles for [κ²-(mim^But)₂]CuX (C = Cl, I)

	N(11)-Cu-N(21)/°	N(11)-Cu-X/°	N(21)-Cu-X/°
[κ2-(mimBut)2]CuCl	120.91(8)	131.39(6)	107.60(6)
[κ2-(mimBut)2]CuI	120.9(5)	121.9(4)	116.1(4)

An interesting aspect of ergothioneine disulfide is that it is less readily accessible via oxidation than are the corresponding disulfides derived from ovothiols.⁶ A corollary of this observation is that the S-S bond of ergothioneine disulfide and (mim^R)₂ should be subject to facile cleavage. In this regard, a common reaction of organodisulfides, R₂S₂, is cleavage of the S-S bond by oxidative addition to a low valent metal center.¹⁶ Such cleavage is not, however, observed for the above [κ²-(mim^R)₂]MCl₂ and [κ²-(mim^But)₂]CuX complexes because access to M(IV) (M = Fe, Co, Ni) and Cu(III) is not typically a facile process in the absence of strongly electron donating ancillary ligands. Therefore, in an endeavor to effect cleavage of the S-S bond, the reactivity of (mim^But)₂ towards the electron rich Ni(0) complex Ni(PMe₃)₄¹⁷ was investigated.¹⁸ Ni(PMe₃)₄ is indeed reactive towards (mim^But)₂ and yields the square planar trans-Ni(PMe₃)₂(mim^But)₂ complex at room temperature (Scheme 3).

Scheme 3.

The most interesting aspect of trans-Ni(PMe₃)₂(mim^But)₂ is concerned with the fact that the (mim^But) ligands coordinate only via their nitrogen atoms (Figure 12). This is a most uncommon coordination mode for mercaptoimidazolyl ligands, which typically coordinate via either sulfur alone or via both sulfur and nitrogen.¹⁹ We are aware of only one complex that features unidentate nitrogen coordination of a mercaptoimidazolyl ligand, namely the tin complex SnPh₂[C₆H₃(CH₂OBu^t)₂](mim^Me).²⁰

Figure 12.

Molecular structure of trans-Ni(PMe₃)₂(mim^But)₂.

The observation that the (mim^But) ligand coordinates via the nitrogen atom indicates that trans-Ni(PMe₃)₂(mim^But)₂ is not simply formed by oxidative addition of the S-S bond since such a mechanism would yield the S-coordinated isomer; an additional step, therefore, involving rearrangement of the (mim^But) ligand would be required to give the observed product in which the nitrogen becomes the ligating atom. Density functional theory calculations indicate that the N-coordinated isomer is 8.63 kcal mol^-1 more stable than the S-coordinated isomer, and so it is feasible that the S-coordinated isomer could be the kinetic product which rearranges to the N-coordinated thermodynamic product.²¹

An alternative mechanistic possibility, however, which is suggested by the fact that (mim^But)₂ coordinates to MX₂ via the nitrogen atoms, is that formation of trans-Ni(PMe₃)₂(κ¹-mim^But)₂ proceeds via a tetrahedral [κ²-(mim^But)₂]Ni(PMe₃)₂ intermediate in which (mim^But)₂ coordinates initially by two of its nitrogen atoms and that S-S bond scission results from subsequent electron transfer from the nickel center through the ligand framework (Scheme 4).²² In this regard, the disulfide (mim^But)₂ may be viewed as a non-innocent ligand and the pair of {[κ²-(mim^But)₂]M} and {[κ¹-(mim^But)]₂M} moieties may be viewed as redox isomers of each other.²³ Thus, it is possible that cleavage of the S-S bond could occur without requiring a direct Ni-S interaction.

Scheme 4.

2. Bis(1-Mesitylimidazol-2-yl)diselenide Complexes of Fe, Co, and Ni

In view of the above success of applying (mim^R)₂ as a bidentate ligand, we were prompted to evaluate the potential of the diselenide counterparts, and specifically the bis(1-mesitylimidazol-2-yl)diselenide derivative, (seim^Mes)₂, that we have recently reported.²⁴ Indeed, the diselenide complexes [κ²-(seim^Mes)₂]MCl₂ (M = Fe, Co, Ni) may be readily obtained via treatment of (seim^Mes)₂ with the corresponding metal dichloride (Scheme 5).²⁵ The molecular structures of [κ²-(seim^Mes)₂]MCl₂ have been determined by X-ray diffraction, as illustrated in Figures 13-15. Selected metrical data are listed in Tables 2 and 3 and the overall features are similar to the corresponding [κ²-(mim^R)₂]MCl₂ derivatives, with the nickel complex (τ₄ = 0.89) showing the greatest deviation from tetrahedral geometry.

Scheme 5.

Figure 13.

Molecular structure of [κ²-(seim^Mes)₂]FeCl₂.

Figure 15.

Molecular structure of [κ²-(seim^Mes)₂]NiCl₂.

3. S-S/E-E (E = S, Se) Bond Metathesis Reactions of Bis(1-R-imidazol-2-yl)disulfide and Bis(1-Mes-imidazol-2-yl)diselenide Ligands

Each of the {[κ²-(mim^R)₂]M} and {[κ²-(seim^Mes)₂]M} complexes described above contain ligands with symmetric disulfide and diselenide bridges. In order to increase the diversity of this class of molecules, we sought to synthesize variants with asymmetric bridges. It is, therefore, significant that a solution of [κ²-(mim^Ph)₂]CoCl₂ and [κ²-(mim^But)₂]CoCl₂ in CDCl₃ results in the rapid formation of an equilibrium mixture with the asymmetric disulfide [κ²-(mim^Ph)(mim^But)]CoCl₂ (Scheme 6). Similarly, the zinc complexes, [κ²-(mim^Ph)₂]ZnCl₂ and [κ²-(mim^But)₂]ZnCl₂, also undergo redistribution of the (mim^R) groups to form an equilibrium mixture with the asymmetric disulfide [κ²-(mim^Ph)(mim^But)]ZnCl₂. The temperature dependence of the equilibrium constant for the cobalt system (Table 6) indicates that the reaction is almost thermoneutral, with ΔH = -0.3(1) kcal mol^-1 and ΔS = 1.3(2) e.u.. If the enthalpy of the reaction is considered to be mainly associated with differences in the S-S bond enthalpies of the [κ²-(mim^Ph)₂], [κ²-(mim^But)₂] and [κ²-(mim^Ph)(mim^But)] ligands, the derived value of ΔH indicates that the S-S bond enthalpy of the mixed ligand [κ²-(mim^Ph)(mim^But)] is only ca. 0.15 kcal mol^-1 more than the average of the S-S bond enthalpies of the [κ²-(mim^Ph)₂] and [κ²-(mim^But)₂] ligands.

Scheme 6.

Table 6.

Equilibrium constant data for S-S and S-Se metathesis (M = CoCl₂)

T/°C	K for {[κ2-(mimPh)2]M} + {[κ2-(mimBut)2]M} ⇄ 2 {[κ2-(mimPh)(mimBut)]M}	K for {[κ2-(mimBut)2]M} + {[κ2-(seimMes)2]M} ⇄ 2 {[κ2-(mimBut)(seimMes)]M}
0	6.61(47)	0.98(3)
30	6.46(15)	1.06(4)
40	6.14(12)	1.09(4)
50	-	1.13(4)
55	5.97(42)	-

In addition to metathesis involving two disulfide ligands, metathesis between disulfide and diselenide ligands has also been observed. Specifically, [κ²-(mim^But)₂]CoCl₂ and [κ²-(seim^Mes)₂]CoCl₂ undergo facile exchange to generate an equilibrium mixture with [κ²-(mim^But)(seim^Mes)]CoCl₂ (Scheme 7). The temperature dependence of the equilibrium constant (Table 6) affords ΔH = 0.4(1) kcal mol^-1 and ΔS = 1.6(1) e.u.. Assuming that the enthalpy of the reaction is largely determined by differences in S-S, Se-Se, and S-Se bond enthalpies, the S-Se bond enthalpy is 0.2 kcal mol^-1 less than the average of the corresponding S-S and Se-Se bond energies. For comparison purposes, there is relatively little information pertaining to experimental E-E bond enthalpies, although the PhE-EPh bond enthalpies have recently been reported to be 46 (E = S), 41 (E = Se), and 33 (E = Te) kcal mol^-1.²⁶

Scheme 7.

While the mechanism of the metathesis reactions are unknown, the observation that the zinc complexes also undergo facile redistribution indicates that the exchange does not proceed via a process in which the S-S and Se-Se bonds are cleaved due to a redox process involving the metal. A variety of different possible mechanisms exist and it is possible that trace quantities of (mim^R)H or (seim^R)H serve as a catalyst. In this regard, we have recently demonstrated that the (seim^R) groups of the selone (seim^R)H and diselenide (seim^R)₂ undergo exchange that is facile on the NMR time-scale.²⁴ Furthermore, thiol/disulfide exchange,²⁷ thiol/diselenide exchange,²⁸ and selenol/diselenide exchange²⁹ is well precedented. Evidence that trace quantities of (mim^R)H could serve as a catalyst for the exchange is provided by the observation that treatment of [κ²-(mim^But)₂]CoCl₂ with (mim^Ph)H rapidly gives an equilibrium mixture composed of [κ²-(mim^But)₂]CoCl₂, [κ²-(mim^Ph)₂]CoCl₂, [κ²-(mim^But)(mim^Ph)]CoCl₂, (mim^Ph)H and (mim^But)H.

CONCLUSIONS

Bis(1-R-imidazol-2-yl)disulfides and diselenides are useful bidentate [κ²-N,N]-donor ligands for main group and transition metals, as illustrated by the formation of [κ²-(mim^But)₂]MCl₂ (M = Fe, Co, Ni, Zn), [κ²-(mim^Ph)₂]CoCl₂, [κ²-(mim^But)₂]CuX (X = Cl, I) and [κ²-(seim^Mes)₂]MCl₂ (M = Fe, Co, Ni) upon treatment of (mim^R)₂ or (seim^Mes)₂ with the respective metal halide. However, if the metal center is capable of accessing a higher valence state, a redox transformation involving cleavage of the E-E bond becomes possible. Thus, the zerovalent nickel complex Ni(PMe₃)₄ effects cleavage of the disulfide bond of (mim^But)₂ to give the square planar complex trans-Ni(PMe₃)₂(mim^But)₂. X-ray diffraction studies demonstrate that the (mim^But) ligands of trans-Ni(PMe₃)₂(mim^But)₂ coordinate via nitrogen rather than sulfur; as such, the complex is not simply the result of oxidative addition of the S-S bond to the nickel center. While the observed isomer could be a result of ligand rearrangement, another possibility is that the formation of trans-Ni(PMe₃)₂(κ¹-mim^But)₂ proceeds via a tetrahedral [κ²-(mim^But)₂]Ni(PMe₃)₂ intermediate in which (mim^But)₂ coordinates initially by two of its nitrogen atoms and that S-S bond scission results from subsequent electron transfer from the nickel center through the ligand framework. Although [κ²-(mim^R)₂]MCl₂ are not subject to homolytic cleavage of the S-S bond because the tetravalent state is not readily accessible, the observation that [κ²-(mim^Ph)₂]CoCl₂ and [κ²-(mim^But)₂]CoCl₂ form an equilibrium mixture with the asymmetric disulfide, [κ²-(mim^Ph)(mim^But)]CoCl₂, indicates that S-S cleavage via another mechanism is possible. Likewise, metathesis between disulfide and diselenide ligands is observed in the formation of [κ²-(mim^But)(seim^Mes)]CoCl₂ upon treatment of [κ²-(mim^But)₂]CoCl₂ with [κ²-(seim^Mes)₂]CoCl₂.

EXPERIMENTAL SECTION

General Considerations

All manipulations were performed using a combination of glovebox, high vacuum, and Schlenk techniques under an argon atmosphere unless otherwise specified.³⁰ Solvents were purified and degassed by standard procedures. ¹H NMR spectra were measured on Bruker 300 DRX, Bruker 400 DRX, and Bruker Avance 500 DMX spectrometers. ¹H chemical shifts are reported in ppm relative to SiMe₄ (δ = 0) and were referenced internally with respect to the protio solvent impurity (δ 7.16 for C₆D₅H;³¹ 7.26 for CHCl₃). ³¹P chemical shifts are reported in ppm relative to 85% H₃PO₄ (δ = 0) and were referenced using P(OMe)₃ (δ = 141.0) as external standard. Coupling constants are given in hertz. Mass spectra were obtained on a Micromass Quadrupole-Time-of-Flight mass spectrometer using fast atom bombardment (FAB). Combustion analyses were carried out by Robertson Microlit Laboratories, Madison, NJ USA. (mim^But)H,¹⁰ (mim^Ph)H,^12,32 (seim^Mes) ²⁴₂ and Ni(PMe₃) ¹⁷₄ were prepared by literature methods.

Synthesis of (mim^But)₂

I₂ (1.62 g, 6.4 mmol) was added in portions to a solution of (mim^But)H (2.00 g, 12.8 mmol) and NEt₃ (1.36 g, 13.4 mmol) in CH₂Cl₂ (100 mL). The resulting orange mixture was allowed to stir for an additional 30 min and then treated with H₂O (100 mL). The two layers were separated and the CH₂Cl₂ layer was washed with H₂O (2 × 50 mL). The volatile components were removed in vacuo to give (mim^But)₂ as an orange powder (1.79 g, 90 %). ¹H NMR (CDCl₃): δ 7.10 [d, 2H, J = 1.5 Hz, Im], 7.06 [d, 2H, J = 1.5 Hz, Im], 1.52 [s, 18 H, Bu^t]. IR Data (KBr, cm^-1): 3118(vs), 3102(vs), 2992(vs), 2970(vs), 2714(w), 2614(m), 2581(w), 2538(w), 2482(w), 2444(w), 2349(w), 2295(w), 1742(m), 1696(m), 1652(w), 1608(m), 1574(w), 1522(m), 1497(s), 1475(vs), 1420(vs), 1397(s), 1371(vs), 1365(vs), 1324(vs), 1292(m), 1249(vs), 1229(s), 1214(vs), 1193(s), 1155(s), 1138(s), 1120(vs), 1048(m), 1028(s), 932(w), 915(s), 849(w), 820(m), 787(vs), 763(vs), 693(vs), 593(s), 528(vs), 465(s), 447(m). Crystals suitable for X-ray diffraction were obtained from Et₂O.

Synthesis of (mim^Ph)₂

I₂ (2.18 g, 8.60 mmol) was added in portions to a solution of (mim^Ph)H (3.05 g, 17.3 mmol) and NEt₃ (1.80 g, 17.7 mmol) in CH₂Cl₂ (100 mL). The resulting orange mixture was allowed to stir for an additional 30 min and then treated with H₂O (100 mL). The two layers were separated and the CH₂Cl₂ layer was washed with H₂O (2 × 50 mL). The volatile components were removed in vacuo to give (mim^Ph)₂ as an orange powder (2.54 g, 84 %). ¹H NMR (CDCl₃): δ 7.40 [m, 6H, Ph], 7.22 [s, 2H, Im], 7.14 [m, 6H, Ph & Im]. IR Data (KBr, cm^-1): 3132(m), 3115(m), 1716(w), 1652(w), 1595(m), 1558(w), 1539(vw), 1500(vs), 1487(s), 1455(m), 1426(vs), 1338(w), 1307(m), 1299(s), 1263(w), 1173(w), 1159(m), 1123(s), 1092(m), 1085(m), 1074(m), 1030(w), 1003(w), 980(w), 908(m), 870(w), 831(w), 782(s), 771(vs), 764(vs), 756(vs), 693(vs), 668(m), 639(w), 561(m), 493(s), 482(m). Crystals suitable for X-ray diffraction were obtained from CH₂Cl₂.

Synthesis of [κ²-(mim^But)₂]FeCl₂

A solution of (mim^But)₂ (0.600 g, 1.93 mmol) in CH₂Cl₂ (5 mL) was added to a slurry of FeCl₂ (0.275 g, 2.17 mmol) in CH₂Cl₂ (1 mL). The mixture was allowed to stir for 18 h and then filtered. The volatile components were removed from the filtrate in vacuo and the resulting residue was extracted with CH₂Cl₂ (3 mL). The solution was placed at -35°C, thereby depositing [κ²-(mim^But)₂]FeCl₂ as orange crystals over a period of 3 days, which were isolated and dried in vacuo (0.264 g, 31 %). ¹H NMR (CDCl₃, 300 MHz, 25 °C): δ 64.0 (s, 2H, Im), 47.5 (s, 2H, Im), 1.51 (s, 18 H, Bu^t). μ_eff (Evans Method) = 5.2 μ_B. Anal. Calcd. for [κ²-(mim^But)₂]FeCl₂: C, 38.5 %; H, 5.1 %; N, 12.8 %. Found: C, 38.2 %; H, 4.9 %; N, 12.6 %. Crystals suitable for X-ray diffraction were obtained from CH₂Cl₂.

Synthesis of [κ²-(mim^But)₂]CoCl₂

A solution of (mim^But)₂ (0.500 g, 1.61 mmol) in CH₂Cl₂ (5 mL) was added to a slurry of CoCl₂ (0.234 g, 1.80 mmol) in CH₂Cl₂ (1 mL). The mixture was allowed to stir for 18 h and then filtered. The volatile components were removed from the filtrate in vacuo and the resulting residue was extracted with CH₂Cl₂ (3 mL). The solution was placed at -35°C, thereby depositing [κ²-(mim^But)₂]CoCl₂ as blue crystals over a period of 3 days, which were isolated and dried in vacuo (0.313 g, 44%). ¹H NMR (CDCl₃): δ 71.9 [s, 2H, Im], 49.6 [s, 2H, Im], 2.92 [s, 18 H, Bu^t]. μ_eff (Evans Method) = 4.4 μ_B. Anal. Calcd. for [κ²-(mim^But)₂]CoCl₂: C, 38.2 %; H, 5.0 %; N, 12.7 %. Found: C, 38.1 %; H, 5.0 %; N, 12.7 %. Crystals suitable for X-ray diffraction were obtained from CH₂Cl₂.

Synthesis of [κ²-(mim^But)₂]NiCl₂

A solution of (mim^But)₂ (0.250 g, 0.80 mmol) in CH₂Cl₂ (3 mL) was added to a slurry of NiCl₂ (0.156 g, 1.20 mmol) in CH₂Cl₂ (1 mL). The mixture was allowed to stir for 18 h and then filtered. The volatile components were removed from the filtrate in vacuo and the resulting residue was extracted with CH₂Cl₂ (1.5 mL). The solution was placed at - 35 °C, thereby depositing [κ²-(mim^But)₂]NiCl₂ as a purple precipitate over a period of 1 day, which was isolated and dried in vacuo (0.108 g, 31 %). ¹H NMR (CDCl₃): δ 99.3 [s, 2H, Im], 61.7 [s, 2H, Im], 6.17 [s, 18 H, Bu^t]. μ_eff (Evans Method) = 2.9 μ_B. Mass spectrum: m/z = 403.15 {M - Cl}⁺. Crystals suitable for X-ray diffraction were obtained from CHCl₃.

Synthesis of [κ²-(mim^But)₂]ZnCl₂

A solution of (mim^But)₂ (0.300 g, 0.97 mmol) in CH₂Cl₂ (3 mL) was added to a slurry of ZnCl₂ (0.144 g, 1.06 mmol) in CH₂Cl₂ (1 mL). The mixture was allowed to stir for 18 h and then filtered. The volatile components were removed from the filtrate in vacuo and the resulting residue was extracted with CH₂Cl₂ (2 mL). The solution was placed at -35°C, thereby depositing [κ²-(mim^But)₂]ZnCl₂ as pale yellow crystals over a period of 3 days, which were isolated and dried in vacuo (0.086 g, 20 %). ¹H NMR (CDCl₃): δ 7.55 [d, 2H, J = 1.5 Hz, Im], 7.27 [d, 2H, J = 1.5 Hz, Im], 1.72 [s, 18 H, Bu^t]. Anal. Calcd. for [κ²-(mim^But)₂]ZnCl₂: C, 37.6 %; H, 5.0 %; N, 12.5 %. Found: C, 37.7 %; H, 4.7 %; N, 12.4 %. Crystals suitable for X-ray diffraction were obtained from CH₂Cl₂.

Synthesis of [κ²-(mim^Ph)₂]CoCl₂

A solution of (mim^Ph)₂ (0.200 g, 0.57 mmol) in CH₂Cl₂ (3 mL) was added to a slurry of CoCl₂ (0.081 g, 0.62 mmol) in CH₂Cl₂ (1 mL). The mixture was allowed to stir for 18 h and then filtered. The volatile components were removed from the filtrate in vacuo and the resulting residue was extracted with CH₂Cl₂ (3 mL). The solution was placed at -35 °C, thereby depositing [κ²-(mim^Ph)₂]CoCl₂ as blue crystals over a period of 2 days, which were isolated and dried in vacuo (0.198 g, 72%). ¹H NMR (CDCl₃): δ 63.7 [s, 2H, Im], 45.7 [s, 2H, Im], 8.11 [s, 4H, Ph], 6.18 [s, 2H, Ph], 5.95 [s, 4H, Ph]. Anal. Calcd for [κ²-(mim^Ph)₂]CoCl₂·(CH₂Cl₂): C, 40.4 %; H, 2.9 %; N, 9.9 %. Found: C, 39.9 %; H, 2.4 %; N, 9.7 %. Crystals suitable for X-ray diffraction were obtained from CH₂Cl₂.

Synthesis of [κ²-(mim^Ph)₂]ZnCl₂

A solution of (mim^Ph)₂ (0.300 g, 0.85 mmol) in CH₂Cl₂ (3 mL) was added to a slurry of ZnCl₂ (0.127 g, 0.94 mmol) in CH₂Cl₂ (1 mL). The mixture was allowed to stir for 18 h and then filtered. The volatile components were removed from the filtrate in vacuo and the resulting residue was extracted with CH₂Cl₂ (2 mL). The solution was placed at -35 °C, thereby depositing [κ²-(mim^Ph)₂]ZnCl₂ as pale yellow crystals over a period of 6 days, which were isolated and dried in vacuo (0.089 g, 22 %). ¹H NMR (CDCl₃): δ 7.72 [s, 2H, Ph], 7.54 [s, 6H, Im & Ph], 7.35 [s, 2H, Im], 7.31 [s, 4H, Ph]. Anal. Calcd. for [κ²-(mim^Ph)₂]ZnCl₂·(CH₂Cl₂): C, 39.9 %; H, 2.8 %; N, 9.8 %. Found: C, 38.7 %; H, 2.8 %; N, 9.3 %.

Synthesis of [κ²-(seim^Mes)₂]FeCl₂

A solution of (seim^Mes)₂ (0.200 g, 0.38 mmol) in CH₂Cl₂ (5 mL) was added to a slurry of FeCl₂ (0.048 g, 0.38 mmol) in CH₂Cl₂ (1 mL). The mixture was allowed to stir for 18 h and then filtered. The volatile components were removed from the filtrate in vacuo and the resulting residue was extracted with CH₂Cl₂ (2 mL). The solution was placed at -35 °C, thereby depositing [κ²-(seim^Mes)₂]FeCl₂ as orange crystals over a period of 2 days, which were isolated and dried in vacuo (0.153 g, 62 %). ¹H NMR (CDCl₃): δ 65.9 [s, 2 H, Im], 42.9 [s, 2H, Im], 7.37 [s, 4H, m-Mes], 2.10 [s, 12 H, p-CH₃-Mes], 1.12 [s, 6H, o-CH₃-Mes]. Anal. Calcd. for [κ²-(seim^Mes)₂]FeCl₂: C, 44.0 %; H, 4.0 %; N, 8.6 %. Found: C, 43.0 %; H, 3.8 %; N, 8.3 %. Mass spectrum: m/z = 621.0 {M - Cl}⁺. Crystals suitable for X-ray diffraction were obtained from CH₂Cl₂.

Synthesis of [κ²-(seim^Mes)₂]CoCl₂

A solution of (seim^Mes)₂ (0.085 g, 0.16 mmol) in CH₂Cl₂ (3 mL) was added to a slurry of CoCl₂ (0.031 g, 0.24 mmol) in CH₂Cl₂ (1 mL). The mixture was allowed to stir for 18 h and then filtered. The volatile components were removed from the filtrate in vacuo and the resulting residue was extracted with CH₂Cl₂ (2 mL). The solution was placed at -35 °C, thereby depositing [κ²-(seim^Mes)₂]CoCl₂ as green crystals over a period of 2 days, which were isolated and dried in vacuo (0.049 g, 47 %). ¹H NMR (CDCl₃): δ 69.8 [s, 2 H, Im], 44.6 [s, 2H, Im], 7.83 [s, 4H, m-Mes], 2.72 [s, 6 H, p-CH₃-Mes], 1.26 [bs, 12H, o-CH₃-Mes]. Mass spectrum: m/z = 622.2 {M - Cl}⁺. Crystals suitable for X-ray diffraction were obtained from CH₂Cl₂.

Synthesis of [κ²-(seim^Mes)₂]NiCl₂

A solution of (seim^Mes)₂ (0.090 g, 0.17 mmol) in CH₂Cl₂ (5 mL) was added to a slurry of NiCl₂ (0.029 g, 0.22 mmol) in CH₂Cl₂ (1 mL). The mixture was allowed to stir for 30 h and then filtered. The volatile components were removed from the filtrate in vacuo and the resulting residue was extracted with CH₂Cl₂ (2 mL). The solution was placed at -35 °C, thereby depositing [κ²-(seim^Mes)₂]NiCl₂ as green crystals over a period of 2 days, which were isolated and dried in vacuo (0.033 g, 30 %). ¹H NMR (CDCl₃): δ 88.2 [s, 2 H, Im], 54.4 [s, 2H, Im], 8.66 [s, 4H, m-Mes], 3.60 [s, 6 H, p-CH₃-Mes], 3.47 [s, 12H, o-CH₃-Mes]. Mass spectrum: m/z = 623.1 {M - Cl}⁺. Crystals suitable for X-ray diffraction were obtained from CH₂Cl₂.

Synthesis of [κ²-(mim^But)₂]CuCl

A solution of (mim^But)₂ (0.300 g, 0.96 mmol) in CH₂Cl₂ (3 mL) was added to a slurry of CuCl (0.100 g, 1.01 mmol) in CH₂Cl₂ (1 mL). The mixture was allowed to stir for 18 h and then filtered. The volatile components were removed from the filtrate in vacuo and the resulting residue was extracted with CH₂Cl₂ (2 mL). The solution was placed at -35 °C, thereby depositing [κ²-(mim^But)₂]CuCl as orange crystals over a period of 3 days, which were isolated and dried in vacuo (0.238 g, 60 %). ¹H NMR (CDCl₃): δ 7.51 [s, 2 H, Im], 7.34 [s, 2 H, Im], 1.72 [s, 18 H, Bu^t]. Anal. Calcd. for [κ²-(mim^But)₂]CuCl·(CH₂Cl₂): C, 36.4 %; H, 4.9 %; N, 11.3 %. Found: C, 36.5 %; H, 4.8 %; N, 11.3 %. Crystals suitable for X-ray diffraction were obtained from CH₂Cl₂.

Synthesis of [κ²-(mim^But)₂]CuI

A solution of (mim^But)₂ (0.100 g, 0.32 mmol) in CH₂Cl₂ (2 mL) was added to a slurry of CuI (0.064 g, 0.34 mmol) in CH₂Cl₂ (1 mL). The mixture was allowed to stir for 18 h and then filtered. The volatile components were removed from the filtrate in vacuo and the resulting residue was extracted with CH₂Cl₂ (2 mL). The solution was placed at -35°C, thereby depositing [κ²-(mim^But)₂]CuI as orange crystals over a period of 2 days, which were isolated and dried in vacuo (0.030 g, 19 %). ¹H NMR (CDCl₃): δ 7.44 [s, 2 H, Im], 7.17 [s, 2 H, Im], 1.70 [s, 18 H, Bu^t]. Mass spectrum: m/z = 373.1 {M - I}⁺. Crystals suitable for X-ray diffraction were obtained from CH₂Cl₂.

Synthesis of trans-Ni(mim^But)₂(PMe₃)₂

A solution of Ni(PMe₃)₄ (0.461 g, 1.27 mmol) in C₆H₆ (3 mL) was added to a solution of (mim^But)₂ (0.395 g, 1.27 mmol) in C₆H₆ (3 mL) and the mixture was stirred for 45 min at room temperature. After this period, the volatile components were removed in vacuo and the resulting orange residue was extracted into toluene (5 mL) and filtered. The filtrate was placed at -15 °C, thereby depositing orange crystals of trans-Ni(mim^But)₂(PMe₃)₂, over a period of 2 days, which were isolated and dried in vacuo (0.190 g, 29 %). ¹H NMR (C₆D₆): δ 6.84 [s, 2H, Im], 6.47 [s, 2H, Im], 1.70 [s, 18 H, Bu^t], 1.29 [s, 9H, PMe₃]. ³¹P{¹H} NMR (C₆D₆): δ 32.7 ppm. Anal. Calcd. for Ni(mim^But)₂(PMe₃)₂: C, 46.1 %; H, 7.7 %; N, 10.8 %. Found: C, 48.2 %; H, 8.2 %; N, 10.1 %. Crystals suitable for X-ray diffraction were obtained from toluene.

S-S Bond metathesis between [κ²-(mim^But)₂]CoCl₂ and [κ²-(mim^Ph)₂]CoCl₂

A solution of [κ²-(mim^But)₂]CoCl₂ (0.014 g, 0.032 mmol) in CDCl₃ (0.6 mL) containing Cp₂Fe (0.004 g, 0.022 mmol) as an internal standard was added to a solution of [κ²-(mim^Ph)₂]CoCl₂ (0.016 g, 0.032 mmol) in CDCl₃ (0.5 mL). The mixture was transferred to a Teflon-capped NMR tube (J-Young) and monitored by ¹H NMR spectroscopy, thereby demonstrating that an equilibrium mixture with [κ²-(mim^Ph)(mim^But)]CoCl₂ was reached over a period of ca. 24 h at room temperature. ¹H NMR data for [κ²-(mim^Ph)(mim^But)]CoCl₂ (CDCl₃): δ 68.0 [s, 2H, Im], 55.1 [s, 1H, Im], 41.6 [s, 1H, Im], 8.19 [s, 2H, Ph], 6.32 [s, 3H, Ph], 2.81 [s, 9H, Bu^t]. The equilibrium constant for formation of [κ²-(mim^Ph)(mim^But)]CoCl₂ was measured as a function of temperature (Table 6) from which values of ΔH and ΔS were obtained from a van’t Hoff plot.

S-S Bond metathesis between [κ²-(mim^But)₂]ZnCl₂ and [κ²-(mim^Ph)₂]ZnCl₂

A solution of [κ²-(mim^But)₂]ZnCl₂ (0.020 g, 0.045 mmol) in CH₂Cl₂ (0.5 mL) was added to a solution of [κ²-(mim^Ph)₂]ZnCl₂ (0.022 g, 0.045 mmol) in CH₂Cl₂ (0.5 mL). The mixture was transferred to a Teflon-capped NMR tube (J-Young) and monitored by ¹H NMR spectroscopy, thereby demonstrating that an equilibrium mixture with [κ²-(mim^Ph)(mim^But)]ZnCl₂ was reached over a period of ca. 1 day at room temperature, although it was not possible to obtain a reliable value for the equilibrium constant because of overlapping of signals for the different species. A mass spectrum obtained from the reaction mixture in CH₂Cl₂ revealed the formation of three molecular ions, m/z = 409.2 {[κ²-(mim^But)₂]ZnCl₂ - Cl}⁺, 429.1 {[κ²-(mim^Ph)(mim^But)]ZnCl₂ - Cl}⁺ and 451.1 {[κ²-(mim^Ph)₂]ZnCl₂ - Cl}⁺, consistent with the proposed metathesis.

S-Se Bond metathesis between [κ²-(mim^But)₂]CoCl₂ and [κ²-(seim^Mes)₂]CoCl₂

A solution of [κ²-(mim^But)₂]CoCl₂ (0.014 g, 0.032 mmol) in CDCl₃ (0.7 mL) containing Cp₂Fe (0.004 g, 0.022 mmol) as an internal standard was added to a solution of [κ²-(seim^Mes)₂]CoCl₂ (0.021 g, 0.032 mmol) in CDCl₃ (0.7 mL). The mixture was transferrred to a Teflon-capped NMR tube (J-Young) and monitored by ¹H NMR spectroscopy, thereby demonstrating that an equilibrium mixture with [κ²-(mim^But)(seim^Mes)]CoCl₂ was reached over a period of ca. 24 h at room temperature. ¹H NMR data for [κ²-(mim^But)(seim^Mes)]CoCl₂ (CDCl₃): δ 73.6 [s, 2H, Im], 56.8 [s, 1H, Im], 39.5 [s, 1H, Im], 8.30 [s, 2H, m-Mes], 2.83 [s, 9H, Bu^t], 2.55 [s, 3H, p-CH₃-Mes], 1.26 [bs, 6H, o-CH₃-Mes]. The equilibrium constant for formation of [κ²-(mim^But)(seim^Mes)]CoCl₂ was measured as a function of temperature (Table 6) from which values of ΔH and ΔS were obtained from a van’t Hoff plot.

S-S Bond metathesis between [κ²-(mim^But)₂]CoCl₂ and (mim^Ph)H

A solution of [κ²-(mim^But)₂]CoCl₂ (0.020 g, 0.045 mmol) in CDCl₃ (0.5 mL) was added to a solution of (mim^Ph)H (0.008 g, 0.045 mmol) in CDCl₃ (0.5 mL). The mixture was analyzed by ¹H NMR spectroscopy after 20 minutes, thereby revealing signals indicative of a mixture containing [κ²-(mim^But)₂]CoCl₂, [κ²-(mim^Ph)₂]CoCl₂, [κ²-(mim^Ph)(mim^But)]CoCl₂, (mim^Ph)H and (mim^But)H.

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 7. 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 (Versions 5.10 and 6.1).³³

Table 7.

Crystal, intensity collection and refinement data

Computational Details

All calculations were carried out using DFT as implemented in the Jaguar 6.5 suite of ab initio quantum chemistry programs.³⁴ Geometry optimizations were performed with the B3LYP density functional³⁵ and the 6-31G** (C, H, N, P and S) and LACVP (Ni)³⁶ basis sets. Cartesian coordinates for geometry optimized structures are listed in the Supporting Information.

Figure 6.

Molecular structure of [κ²-(mim^But)₂]CoCl₂.

Figure 7.

Molecular structure of [κ²-(mim^But)₂]NiCl₂.

Figure 8.

Molecular structure of [κ²-(mim^But)₂]ZnCl₂.

Figure 14.

Molecular structure of [κ²-(seim^Mes)₂]CoCl₂.