CHâNH Tautomerism in the Products of the Reactions of the

Article pubs.acs.org/Organometallics

CH−NH Tautomerism in the Products of the Reactions of the Methanide [HC(PPh2NSiMe3)2]− with Pnictogen and Tellurium Iodides Ramalingam Thirumoorthi,† Tristram Chivers,*,† Chris Gendy,‡ and Ignacio Vargas-Baca*,‡ †

Department of Chemistry, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada T2N 1N4 Department of Chemistry and Chemical Biology, McMaster University, 1280 Main Street West, Hamilton, Ontario, Canada L8S 4M1

‡

S Supporting Information *

ABSTRACT: The reactions of K[HC(PPh2NSiMe3)2] (K[1]) and MI3 (M = As, Sb) or TeI4 gave as the major products the complexes [{MIn}{C(PPh2NSiMe3)(PPh2NHSiMe3)}] (MIn = trans-AsI2, trans- or cis-SbI2, TeI3), which feature the ligand [C(PPh2NSiMe3)(PPh2NHSiMe3)]− (2). This anion, the NH tautomer of [HC(PPh2NSiMe3)2]−, is formally generated by a 1,3hydrogen shift to give complexes in which the ligand is N,C-chelated to the main-group-metalloid center. The M−C distances are slightly longer than the sum of the covalent radii for M and C in the case of the group 15 metalloids but significantly shorter for M = Te. The arsenic derivative [{t-AsI2}2] is monomeric in the solid state, with As−I distances that differ by ca. 0.55 Å, whereas the antimony analogues [{t-SbI2}2] and [{c-SbI2}2] are dimeric through bridging iodide ligands and the disparity in Sb−I distances of the SbI2 units is 0.10 and 0.33 Å, respectively. The tellurium derivative [{TeI3}2] is monomeric with a distortedsquare-pyramidal geometry at the Te center and Te−I distances in the narrow range 2.9142(4)−3.0337(4) Å. In contrast to the lighter pnictogens, the bismuth complex [{t-BiI2}1] is comprised of the methanide 1 coordinated in a tridentate (N,C,N) mode to a BiI2+ cation. In the case of arsenic triiodide, the metathesis is accompanied by Si−N bond cleavage to give [{AsI2}{CH2(PPh2N)(PPh2NSiMe3)}] (3), which was characterized by 31P NMR spectroscopy, and conversion to the corresponding salt [{AsI}{CH2(PPh2N)(PPh2NSiMe3)}][SbF6] (3A) by treatment with AgSbF6. The As−N distances in the sixmembered CP2N2As ring in 3A differ by 0.20 Å due to the different coordination numbers (2 and 3) of the two N atoms in the novel N,N′-chelated [CH2(PPh2N)(PPh2NSiMe3)]− anion. In contrast, reaction of [{t-SbI2}2] with AgSbF6 gives the expected salt as the dimer {[{SbI}2][SbF6]}2. The two hydrolysis products [CH2(PPh2NSiMe3)(PPh2NHSiMe3)][SbF6] (4A) and [CH2(PPh2NSiMe3) (PPh2NHSiMe3)]2[Te2I6] (4B) were also structurally characterized and shown to contain the same cation. DFT calculations indicate that the N−H tautomer 2 is stabilized by strong M−N and M−C bonding interactions which include a small degree of π character. Weaker bonds, as in the Bi complex, favor the C−H tautomer 1 as the ligand.

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INTRODUCTION

metathetical reactions between K[1] and MI2 (M = Ca. Sr, Ba) in a 1:1 molar ratio provide heteroleptic complexes of the alkaline-earth metals.9 Especially in the case of less electropositive main-group elements, metathesis between the alkalimetal reagents M[1] (M = Li, Na, K) and an appropriate metal halide is a versatile procedure that has been employed for the synthesis of heteroleptic complexes of group 13 metals (Al, Ga, and In)10 and Ge.11 The monoanion 1 is a versatile ligand as a result of the three potential donor sites (N,C,N) and, more importantly, the possibility of forming the NH tautomer [C(PPh2NSiMe3)-

Since the first report of the monolithium derivative of the methanide [HC(PPh2NSiMe3)2]− (1) in 1999,1 main-groupmetal and -metalloid complexes of this monoanion and the related ligand with a mesityl substituent on each N atom have been extensively investigated.2 The lithium derivative is prepared by metalation of the neutral ligand H 2 C(PPh2NSiMe3)2 with n-BuLi (alkane elimination) in THF. Metalation (deprotonation) is also successful for the preparation of heavier alkali-metal derivatives using Na[N(SiMe3)2]3 or MH (M = Na, K)4,5 and a magnesium complex from the Grignard reagent MeMgCl,6 as well as zinc and aluminum complexes from reactions of the neutral ligand with dimethylzinc7 and trimethylaluminum,8 respectively. The © XXXX American Chemical Society

Received: July 2, 2013

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dx.doi.org/10.1021/om400649c | Organometallics XXXX, XXX, XXX−XXX

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tridentate (N,C,N) mode to a BiI2+ cation; (c) the ionseparated complex [{SbI}{C(PPh2NSiMe3)(PPh2NHSiMe3)}][SbF6] (5A) formed by replacement of one of the I− ligands in [{t-SbI 2 }2] by [SbF 6 ] − ; (d) [{AsI}{CH 2 (PPh 2 N)(PPh2NSiMe3)}][SbF6] (3A), which incorporates the novel N,N′-chelating ligand [CH2(PPh2N)(PPh2NSiMe3)]− as a result of Si−N bond cleavage in 1; (e) the hydrolysis products [CH 2 (PPh 2 NSiMe 3 )(PPh 2 NHSiMe 3 )][SbF 6 ] (4A) and [CH2(PPh2NSiMe3)(PPh2NHSiMe3)]2[Te2I6] (4B), which contain the same cation, i.e. a diprotonated derivative of the monoanion 1. We have also carried out DFT calculations in order to provide insights into the different bonding modes in the complexes of anions 1 and 2 with the pnictogens and tellurium.

(PPh2NHSiMe3)]− (2). In the case of main-group-metal (s and p block) complexes, N,C,N coordination of the ligand to the metal center is observed for unsolvated alkali-metal and alkaline-earth-metal complexes of 1 (see Figure 1a); however, such species also feature long M···C bonds that are indicative of weak bonding interactions.3−5 This mode of coordination is also observed for heteroleptic divalent lanthanide complexes of 1.9 In contrast, in the case of THF-solvated Na+ and K+ complexes and group 12−14 complexes of 1 N,N-chelation is observed to the metal (see Figure 1b).5−9 The conformational flexibility of ligand 1 is illustrated impressively by the dimeric chromium(I) complex [1Cr(μ-Cl)]2, which coexists as two isomeric forms in the same crystal that differ by more than 0.6 Å in the Cr−C transannular interaction.12 A third mode of coordination (N,C) is observed for the rhodium(I) complex [{Rh(cod)}{HC(PPh2NSiMe3)2}].13 Although the synthesis of the neutral ligands H2C(PR2NR′)2 by the Kirsanov method produces a mixture of the bis-imino derivative and the iminoamine tautomer [HC(PR2NR′)(PR2NHR′)], both tautomers form the same methanide upon monodeprotonation;14a aryl substituents on nitrogen lead to a preference for the former, whereas alkyl substituents favor a 1,3-hydrogen shift to give the imino-amine tautomer.14b As yet, there are no reported complexes of the NH tautomeric ligand 2. Complexes of 1 with group 15 or 16 elements have not been investigated.1 In the context of the above background on the structures of main-group-metal(loid) complexes of 1, we have investigated the reactions of K[1]5 with MI3 (M = As, Sb, Bi) and TeI4. We were particularly interested in evaluating the influence of the metalloid-centered lone pair(s) on the coordination mode of the ligand and the nature of the M−C interaction (if any) in group 15 and 16 complexes.15 In this contribution, we describe the synthesis, multinuclear NMR spectra, and structures of the following: (a) the first examples of complexes of the tautomeric NH ligand 2, [{trans-MI2}{C(PPh2NSiMe3)(PPh2NHSiMe3)}] ([{t-AsI2}2], [{t-SbI2}2]) and [{TeI3}{C(PPh2NSiMe3)(PPh2NHSiMe3)}] ([{TeI3}2]); (b) [{BiI2}{HC(PPh2NSiMe3)2}] ([{t-BiI2}1]) (X-ray structure only), which comprises methanide 1 coordinated in a

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RESULTS AND DISCUSSION Synthesis of Pnictogen and Tellurium Complexes. The known reagent K[1] was obtained in ca. 70% yield by reaction of the neutral ligand CH2{PPh2NSiMe3}2 with 1 equiv of KH in THF under reflux.5 Since the 31P NMR spectrum has only been reported at room temperature, we have carried out a variabletemperature 31P NMR study of K[1] in C7D8 in order to determine whether a tautomeric equilibrium between K[1] and K[2] exists at low temperature. However, a broad singlet (δ 14.90) is observed even at −93 °C, consistent with the sole presence of the symmetrical complex K[1] (see Scheme 1). For the synthesis of group 15 and 16 complexes of 1 we chose to use iodides rather than bromides or chlorides as the metalloid source in order to minimize the likelihood of Si−N bond cleavage (Si−I bonds are substantially weaker than Si−Br or Si−Cl bonds). The reactions of K[1] with MI3 (M = As, Sb) or TeI4 in THF or toluene or benzene afforded the novel C,Ncoordinated complexes [{MI2}{C(PPh2NSiMe3)(PPh2NHSiMe3)}] (M = As ([{t-AsI2}2]), Sb ([{t-SbI2}2])) and [{TeI3}{C(PPh2NSiMe3)(PPh2NHSiMe3)}] ([{TeI3}2]), which were characterized by multinuclear NMR spectroscopy and single-crystal X-ray crystallography (vide infra). However, these reactions were not straightforward, especially in the case of AsI3, for which complex 3, a product of Si−N bond cleavage, was formed in addition to the metathesis product [{t-AsI2}2] (see Scheme 1). The reaction of K[1] with BiI3 gave a complex mixture of products (31P NMR) which proved to be intractable. However, a few crystals of the bismuth complex [{BiI2}{HC(PPh2NSiMe3)2}] ([{t-BiI2}1]) were isolated from a reaction of BiI3 with Li2[C(PPh2NSiMe3)2], and an X-ray structure was determined (vide infra). The binary selenium iodide SeI4 does

Figure 1. Examples of coordination modes of 1: (a) N,C,N (MLn = Li(THF),11 K,5 [MgI(THF)],6 [CaI(THF)2],9 [YbI(THF)2]9); (b) N,N′ (MLn = Na(THF)2,4 K(THF)2,4 AlCl2,10 GaCl2,10 InCl2,10 AlMe2,8 GeCl11); (c) N,C.13 B


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Scheme 1

not exist;16 therefore, the synthesis of the selenium analogue of [{TeI3}2] via metathesis with K[1] could not be pursued. The 31P NMR spectra in CD2Cl2 of recrystallized samples of [{t-AsI2}2], [{c-SbI2}2], and [{TeI3}2] exhibit two mutually coupled doublets with chemical shifts of δ 25.1 and 59.8 for [{tAsI2}2], δ 25.9 and 52.1 for [{c-SbI2}2], and δ 29.6 and 39.7 for [{TeI3}2]. The corresponding two-bond coupling constants 2 J(P,P) are 29.1, 24.0, and 35.1 Hz, respectively. The 31P chemical shifts of [{t-SbI2}2] in C7D8 are δ 25.1 and 49.3 with 2 J(P,P) = 25.3 Hz. The NMR spectra revealed that the SbI2+ complex of 2 exists exclusively as the trans isomer in benzene or toluene, whereas only the cis isomer is present in CD2Cl2. The 1 H NMR spectra of [{t-AsI2}2], [{c-SbI2}2], and [{TeI3}2] display doublets at δ 5.35, 4.28, and 5.21 for the N−H functionality with two-bond coupling constants 2J(H,P) of ca. 9 Hz. The relative intensity of this signal suggests the presence of one N−H proton. Two well-resolved doublets with coupling constants 2J(Si,P) that differ by ca. 1.5 Hz in the range 2.3−5.2 ppm are observed in the 29Si NMR spectra for [{t-AsI2}2], [{tSbI2}2], and [{TeI3}2], consistent with the presence of two chemically inequivalent SiMe3 groups. The 125Te NMR chemical shift of [{TeI3}2] (δ 1003) is shifted upfield in comparison with a compound having a similar coordination environment, (R)-[2-(4-ethyl-2-oxazolinyl)phenyl]tellurium(IV) triiodide (δ 1097).17 Taken together, the multinuclear NMR spectra for [{t-AsI2}2], [{c-SbI2}2], [{t-SbI2}2], and [{TeI3}2] are all consistent with an asymmetric coordination of the ligand to the metalloid center and the HN resonance strongly suggests that the ligand is the NH tautomer 2. We obtained a selectively decoupled NMR spectrum for [{cSbI2}2] in order to assign the resonances at 25.9 and 52.1 ppm for the two different phosphorus environments in the ligand 2. When the resonance at 25.9 ppm was irradiated, the 1H NMR doublet at 4.28 ppm collapsed into a singlet, showing that this resonance can be attributed to the P nuclei bonded to NH centers. The reaction of K[1] with AsI3 gave a second complex, as indicated by the 31P NMR spectrum of the crude product in

CD2Cl2, which showed the presence of two mutually coupled doublets at δ 15.2 and 30.3 with 2J(P,P) = 12.3 Hz, in addition to the resonances for [{t-AsI2}2]. We considered that this byproduct might be the result of Si−N bond cleavage to give 3 (see Scheme 1), but it could not be isolated in pure form. In order to verify its identity, we treated the compounds [{t-AsI2} 2] and 3 separately with Ag[SbF6] in CH2Cl2. As expected, the product of the former reaction exhibited a 31P NMR spectrum with parameters (δ 25.7 and 60.9, 2J(P,P) = 23.5 and 23.8 Hz) that are close to those of [{t-AsI2}2]; hence, it is tentatively identified as [{AsI}2]+, i.e. the result of the abstraction of one I− ligand in [{t-AsI2}2] by Ag+ with SbF6− as counterion (see Scheme 1). This assignment was confirmed by high-resolution ESI-MS of this product. The reaction of 3 with Ag[SbF6] in CH2Cl2 gave a product with NMR parameters (δ 15.7 and 30.8, 2J(P,P) = 11.8 Hz) that are remarkably similar to those of 3, consistent with the formation of the SbF6− species 3A. This conclusion was supported by the CHN analysis and high-resolution ESI-MS of this product and confirmed by a single-crystal X-ray diffraction analysis of colorless crystals of 3A (vide infra). In contrast to the two resonances observed for [{t-AsI2}2] in the 29Si NMR spectrum, 3A shows a single resonance at δ 19.1, consistent with cleavage of one Si−N bond in the reaction of K[1] with AsI3. For comparison, we have also made the yellow salt [{SbI} 2][SbF6] by treatment of [{t-SbI2}2] with AgSbF6 in dichloromethane. The 31P NMR spectrum of the product exhibited mutually coupled doublets (δ 27.1 and 57.1, 2J(P,P) = 17.5 Hz) consistent with the formation of [{SbI}2][SbF6] together with the byproduct 4A, which exhibits a singlet at δ 23.2. Fortuitously, crystals of [{SbI}2][SbF6] and 4A could be separated manually on the basis of their different colors, and their identities were established by X-ray crystallography (vide infra). The ion-separated salt [{SbI}2][SbF6] exists as a dimer in th e solid stat e, wh ile 4A was identified as [CH2(PPh2NSiMe3)(PPh2NHSiMe3)}][SbF6] (4A), the result of hydrolysis of [{t-SbI2}2] (see Scheme 2). The cation in 4A is C


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Scheme 2

the monoprotonated derivative of the neutral ligand H2C(PPh 2 NSiMe 3 ) 2 . The related salt [CH 2 (PPh 2 NSiMe 3 )(PPh2NHSiMe3)}]2[Te2I6] (4B), which contains the same cation, was isolated as a minor hydrolysis product from the synthesis of the tellurium(IV) complex [{TeI3}2] and identified by X-ray crystallography (see Scheme 2). The anion in 4B is the dinuclear tellurium(II) species [Te2I6]2− indicative of a redox process. The formation of [Te2I6]2− upon hydrolysis of Et3PTeI2 has previously been reported,18 but no redox process is involved in that case. Compound 4B exhibits a singlet in the 31 P NMR spectrum at δ 24.1 (cf. δ 23.2 for 4A) and a triplet for the CH2 resonance in the 13C NMR spectrum at δ 29.3 (cf. δ 37.9 for the neutral molecule H2C(PPh2NSiMe3)21). X-ray Structures of [{t-AsI2}2], [{t-SbI2}2], [{c-SbI2}2], [{SbI}2][SbF6], and [{TeI3}2]. The molecular structures of the complexes [{t-AsI2}2], [{t-SbI2}2], [{c-SbI2}2], and [{TeI3}2] are depicted in Figures 2−5. The structure determinations confirm the inference based on the multinuclear NMR data that the ligand in these complexes is the NH tautomer 2 (vide supra). In all of these cases the monoanion 2 is coordinated in a C,N fashion to the metalloid center, forming a four-membered MNPC ring. These are the first examples of complexes of 2, which is presumably formed through a 1,3-hydrogen shift from

Figure 2. Structure of [{t-AsI2}2] with 30% probability thermal ellipsoids. Solvent toluene molecule and hydrogen atoms of phenyl and methyl groups are omitted for the sake of clarity.

the methanide carbon of 1 to one of the basic imino centers. The arsenic(III) and tellurium(IV) complexes, [{t-AsI2}2] and [{TeI3}2], respectively, are monomeric in the solid state, D


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Figure 5. Structure of [{TeI3}2] with 30% probability thermal ellipsoids. Solvent benzene molecules and hydrogen atoms of phenyl and methyl groups are omitted for the sake of clarity.

around the pnictogen centers in [{t-AsI2}2] and in both [{tSbI2}2] and [{c-SbI2}2] is a distorted seesaw (if the weakest Sb···I interaction is ignored), whereas the ligands attached to tellurium in [{TeI3}2] adopt a distorted-square-pyramidal arrangement. Selected structural parameters for [{t-AsI2}2] and [{TeI3}2] are summarized in Table 1, and those for [{t-SbI2}2] and [{c-

Figure 3. Structure of [{t-SbI2}2] with 50% probability thermal ellipsoids. Solvent toluene molecule and hydrogen atoms of phenyl and methyl groups are omitted for the sake of clarity.

Table 1. Selected Bond Lengths (Å) and Angles (deg) for [{t-AsI2}2] and [{TeI3}2]

M1−C25 M1−N1 M1−I1 M1−I2 M1−I3 P1−C25 P2−C25 P1−N1 P2−N2 Si1−N1 Si2−N2 N1−M1−C25 N1−M1−I1 N1−M1−I2 N1−M1−I3 I1−M1−I2 I1−M1−I3 I3−M1−I2 I1−M1−C25 I2−M1−C25 I3−M1−C25 P1−N1−M1 P1−N1−Si1 P2−N2−Si2 P1−C25−M1 P2−C25−M1 P2−C25−P1 Si1−N1−M1 N1−P1−C25 N2−P2−C25

Figure 4. Structure of [{c-SbI2}2] with 30% probability thermal ellipsoids. Solvent dichloromethane and benzene molecules and hydrogen atoms of phenyl and methyl groups are omitted for the sake of clarity.

whereas the antimony(III) analogue is dimeric in both [{t-SbI2} 2] and [{c-SbI2}2]. These two isomers were obtained by recrystallization of the crude product of the reaction of SbI3 with K[1] from different solvent mixtures: viz. hexane−toluene and benzene−dichloromethane, respectively. The geometry E

[{t-AsI2}2]

[{TeI3}2]

1.901(4) 1.900(3) 2.7722(5) 3.3230(5)

2.073(4) 2.183(3) 3.0337(4) 2.9142(4) 2.9444(4) 1.736(4) 1.712(4) 1.618(4) 1.632(4) 1.749(4) 1.758(4) 71.47(14) 96.73(9) 88.49(9) 166.39(9) 165.162(14) 90.416(13) 87.460(13) 94.46(12) 100.37(12) 96.51(11) 96.10(16) 134.7(2) 136.4(2) 96.57(19) 131.8(2) 130.2(2) 127.89(18) 95.68(18) 110.7(2)

1.721(4) 1.699(4) 1.649(3) 1.628(4) 1.756(3) 1.767(4) 79.43(16) 98.00(10) 103.17(10) 154.74(2)

103.03(12) 94.32(12) 95.38(16) 135.9(2) 134.9(2) 93.00(19) 126.5(2) 136.3(2) 128.12(19) 92.19(18) 108.2(2)


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Significantly different P−C and P−N bond distances are observed for [{t-AsI2}2], [{t-SbI2}2], and [{TeI3}2] (d(P−C) = 1.721(4) and 1.699(4) Å for [{t-AsI2}2]; 1.717(5) and 1.691(5) Å for [{t-SbI2}2]; 1.736(4) and 1.712(4) Å for [{TeI3}2]), but in the case of [{c-SbI2}2] this disparity is smaller (1.721(5) and 1.718(5) Å). The P−N bond distances are 1.649(3) and 1.628(4) Å for [{t-AsI2}2], 1.619(5) and 1.631(5) Å for [{t-SbI2}2], 1.620(4) and 1.639(4) Å for [{cSbI2}2], and 1.618(4) and 1.632(4) Å for [{TeI3}2]. The mean P−C bond lengths in [{t-AsI2}2], [{t-SbI2}2], [{c-SbI2}2]. and [{TeI3}2] are shortened while the P−N bonds are lengthened in comparison to those in the parent molecule CH2(PPh2NSiMe3)2 (1.825(1) and 1.536(2) Å).22 The mean Si−N bond distances in these molecules (1.762(4) Å for [{tAsI2}2], 1.754(5) Å for [{c-SbI2}2], 1.745(5) Å for [{c-SbI2}2], and 1.754(4) Å for [{TeI3}2]) are longer than those in CH2(PPh2NSiMe3)2 (1.693(2) Å).22 The most important structural data concern the planarity of the carbanionic center C25: ∑ 355.8(2), 357.6(3), 357.7(3), and 358.6(2)° for [{tAsI2}2], [{t-SbI2}2], [{c-SbI2}2] and [{TeI3}2], respectively. A weak intramolecular hydrogen bond (2.823(71), 3.124(63), 2.859(65), 2.973(27) Å) was observed between the imino (NH) functionality and an iodide ligand in all cases. The molecular structure of [{SbI}2][SbF6] is illustrated in Figure 6, and bond parameters are compared with those in [{t-

SbI2}2] can be found in Table 2. The M−C bond lengths for [{t-AsI2}2], [{t-SbI2}2], [{c-SbI2}2], and [{TeI3}2] are Table 2. Selected Bond Lengths (Å) and Angles (deg) for [{t-SbI2}2], [{c-SbI2}2], and [{SbI}2][SbF6] Sb1−C25 Sb1−N1 Sb1−I1 Sb1−I2 P1−C25 P2−C25 P1−N1 P2−N2 Si1−N1 Si2−N2 N1−Sb1−C25 N1−Sb1−I1 N1−Sb1−I2 I1−Sb1−I2 I1−Sb1−C25 I2−Sb1−C25 P1−N1−Sb1 P1−N1−Si1 P2−N2−Si2 P1−C25−Sb1 P2−C25−Sb1 P2−C25−P1 Si1−N1−Sb1 N1−P1−C25 N2−P2−C25

[{t-SbI2}2]

[{c-SbI2}2]

[{SbI}2][SbF6]

2.127(5) 2.148(4) 3.0184(5) 3.1171(5) 1.717(5) 1.691(5) 1.619(5) 1.631(5) 1.749(5) 1.758(5) 72.07(18) 96.38(11) 91.12(12) 170.870(17) 93.44(14) 93.82(14) 96.1(2) 136.7(3) 136.4(4) 94.0(2) 131.1(3) 132.5(3) 126.1(2) 97.8(2) 110.4(3)

2.123(5) 2.183(4) 2.9258(6) 3.2543(6) 1.718(5) 1.721(5) 1.620(4) 1.639(4) 1.741(4) 1.749(5) 71.57(16) 95.61(10) 161.78(10) 95.954(13) 99.27(12) 92.69(12) 95.58(18) 132.3(3) 138.1(3) 94.9(2) 132.0(3) 130.8(3) 130.5(2) 97.9(2) 116.3(2)

2.095(7) 2.099(6) 2.8176(8) 1.717(8) 1.704(7) 1.635(6) 1.620(7) 1.741(6) 1.772(7) 73.6(3) 94.37(16)

101.0(2) 95.8(3) 138.7(4) 134.6(4) 93.5(3) 131.9(4) 132.9(5) 125.2(3) 97.1(3) 109.7(4)

1.901(4), 2.127(5), 2.123(5), and 2.073(4) Å, respectively. These values are slightly shorter in comparison with the related chelate structures [2-(Me2NCH2)C6H4]SbI2 (2.150(4) Å)19 and (R)-[2-(4-ethyl-2-oxazolinyl)phenyl]tellurium(IV) triiodide (2.153(2) Å)17 (cf. sum of covalent bond radii of As−C (1.89 Å), Sb−C (2.07 Å), and Te−C (2.15 Å)20). The M−N bond distances for [{t-AsI2}2], [{t-SbI2}2], [{c-SbI2}2], and [{TeI3}2] are 1.900(3), 2.148(4), 2.183(4), and 2.183(3) Å, which are marginally longer than the sum of covalent radii in the case of Sb and Te (As−N (1.89 Å); Sb−N (2.07 Å); Te−N (2.15 Å)20). In all of these complexes the M−I bond distances exhibit substantial differences, especially in the case of the monomeric arsenic(III) derivative [{t-AsI2}2] (0.5508(5) Å) (cf. 0.0987(5) Å for [{t-SbI2}2], 0.3285(6) Å for [{c-SbI2}2], and 0.1195(4), 0.0302(4) Å for [{TeI3}2]), suggesting that one of the M−I bonds in these molecules is polarized and that this effect is most pronounced for [{t-AsI2}2]. Interestingly, the disparity in Sb−I bond lengths is markedly larger in the cis isomer [{c-SbI2}2] in comparison to the trans analogue [{tSbI2}2]. The shorter M−I bond distance (2.7722(5) Å for [{tAsI2}2], 3.0184(5) Å for [{t-SbI2}2], 2.9258(6) Å for [{c-SbI2} 2], 2.9142(4) Å for [{TeI3}2]) observed in each molecule is significantly longer than the sum of the covalent radii of the metalloid and iodine atoms (As−I (2.61 Å); Sb−I (2.79 Å); Te−I (2.87 Å)20) (cf. M−I distances in the corresponding binary iodides AsI3 (2.556(4) Å),21a SbI3 (2.87(1) Å),21b TeI3+ (2.650(1) Å)21c). The dimeric molecules [{t-SbI2}2] and [{cSbI2}2] are associated via weak M···I interactions (3.3528(6) and 3.5063(7) Å, respectively).

Figure 6. Structure of [{SbI}2][SbF6] with 30% probability thermal ellipsoids. Solvent dichloromethane molecule, the counteranion SbF6−, and hydrogen atoms of phenyl and methyl groups are omitted for the sake of clarity.

SbI2}2] and [{c-SbI2}2] in Table 2. [{SbI}2][SbF6] is the product of the replacement of an iodide ion in [{t-SbI2}2] by a noncoordinating hexafluoroantimonate anion; the dimeric structure is retained. As expected, the positive charge created on the antimony center in [{SbI}2][SbF6] gives rise to Sb−C, Sb−I, and Sb−N bond lengths that are significantly shorter in comparison to those in the covalent precursor [{t-SbI2}2] (Table 2). The geometry of the antimony center changes from distorted seesaw (CN 4) in [{t-SbI2}2] to trigonal pyramidal F


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radii (2.85 Å);20 this value is comparable with that in the polymeric structure of MeBiI2 (3.086−3.128 Å).23 The Bi−N bond distance (2.437(5) Å) is shorter than those of reported compounds in a similar coordination environment ([2,6{MeN(CH2CH2)2NCH2}2C6H3]BiI2, 2.596(4) Å);24a [2,6(Me2NCH2)2C6H3]BiI2, 2.589(4) Å)24b but substantially longer than the sum of covalent radii of bismuth and nitrogen (2.16 Å).20 An intermolecular Bi···I interaction (3.6436(6) Å), which is shorter than the sum of the van der Waals radii (4.55 Å), links the two monomeric units. The Bi−C bond length (2.392(6) Å) is significantly longer than typical single-bond values (cf. PhBiI2(thf)], 2.241(16) Å;25 MeBiI2, 2.28(2) Å).23 The P−C and P−N bond lengths are shorter and longer, respectively, than those of the neutral compound CH2(PPh2NSiMe3)2 (1.825(1) and 1.536(2) Å),22 indicating delocalization of the negative charge within the monoanionic ligand 1. A similar N,C,N-coordination mode was reported previously for some uranium,26a vanadium,26b and chromium12 complexes. X-ray Structure of 3A. The molecular structure of 3A is shown in Figure 8, and the bond parameters are summarized in

(CN 3) with bond angles in the range of 73.6(3)−101.0(2)° in [{SbI}2][SbF6]. X-ray Structure of [{t-BiI2}1]. The molecular structure of [{t-BiI2}1] is depicted in Figure 7. In contrast to the structures

Figure 7. Structure of [{t-BiI2}1] with 30% probability thermal ellipsoids. Solvent toluene molecule and hydrogen atoms of phenyl, methyl, and methanide groups are omitted for sake of clarity.

of the corresponding As and Sb complexes, [{t-AsI2}2] and [{tSbI2}2], the bismuth complex is formally comprised of a BiI2+ cation coordinated to the monoanionic ligand 1 in a tridentate (N,C,N) bonding mode. The two four-membered PNCBi rings are folded along the Bi−C(H)methanide axis with C25 out of the best plane defined by the P2N2CBi system (by 0.53 Å). Selected structural parameters for [{t-BiI2}1] are summarized in Table 3. The Bi center exhibits a distorted-square-pyramidal geometry with two iodides and two nitrogen atoms that are trans to each other (I1−Bi1−I2 = 162.319(15)°, N1−Bi1−N2 = 125.33(18)°). The trans Bi−I bond length (3.0706(5) Å) in [{t-BiI2}1] is considerably longer than the sum of the covalent

Figure 8. Structure of 3A with 50% probability thermal ellipsoids. Solvent dichloromethane molecule, the counteranion SbF6−, and hydrogen atoms of phenyl and methyl groups are omitted for the sake of clarity.

Table 3. Selected Bond Lengths (Å) and Angles (deg) for [{t-BiI2}1] Bi1−N1 Bi1−N2 Bi1−I1 Bi1−I2 P1−N1 P1−C25 P1−C1 C25−Bi1−N1 C25−Bi1−N2 N1−Bi1−N2 C25−Bi1−I1 N1−Bi1−I1 N2−Bi1−I1 C25−Bi1−I2 N1−Bi1−I2 N2−Bi1−I2 I1−Bi1−I2 N1−P1−C25

2.412(5) 2.462(5) 3.0502(5) 3.0911(5) 1.596(5) 1.756(6) 1.798(7) 65.2(2) 64.6(2) 125.33(18) 113.07(16) 89.62(12) 91.00(12) 84.27(16) 95.06(12) 100.06(12) 162.319(15) 101.1(3)

P2−N2 P2−C25 P2−C13 N1−Si1 N2−Si2 Bi1−C25 N2−P2−C25 P1−N1−Si1 P1−N1−Bi1 Si1−N1−Bi1 P2−N2−Si2 P2−N2−Bi1 Si2−N2−Bi1 P1−C25−P2 P1−C25−Bi1 P2−C25−Bi1

Table 4. The X-ray structural determination confirms that Si− N bond cleavage occurred in the reaction of K[1] with AsI3, as

1.581(5) 1.763(6) 1.798(6) 1.739(6) 1.732(6) 2.392(6)

Table 4. Selected Bond Lengths (Å) and Angles (deg) for 3A As1−N1 P1−C25 P2−C25 As1−N2 Sb1−F4 As1−C25 P1−C7 P2−C19 Sb1−F1 Sb1−F5 N1−As1−N2 N2−As1−I1 N2−P2−C25 P1−N1−Si1 P2−N2−As1

101.6(3) 132.4(3) 98.3(2) 128.3(3) 133.6(3) 96.8(2) 129.0(3) 140.4(4) 94.6(3) 94.4(3)

G

1.938(5) 1.789(6) 1.820(5) 1.740(5) 1.851(4) 3.332(5) 1.778(6) 1.779(5) 1.779(8) 1.890(8) 102.6(2) 104.90(16) 110.4(3) 125.7(3) 127.0(3)

As1−I1 P1−C1 P2−C13 Sb1−F2 Sb1−F6 P1−N1 P2−N2 Si1−N1 Sb1−F3 N1−As1−I1 N1−P1−C25 P1−N1−As1 Si1−N1−As1 P2−C25−P1

2.6820(7) 1.780(6) 1.785(6) 1.860(5) 1.815(7) 1.623(5) 1.582(5) 1.798(5) 1.885(7) 95.86(14) 109.9(2) 120.9(3) 113.3(2) 112.3(3)


Organometallics

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Figure 9. Molecular structure of 4B. Hydrogen atoms of phenyl and methyl groups are omitted for the sake of clarity.

was inferred on the basis of NMR data (vide supra). Complex 3A is comprised of the novel monoanionic bidentate ligand [CH2(PPh2N)(PPh2NSiMe3)]−, which is N,N′-chelated to an AsI2+ unit to give a puckered six-membered AsN2P2C ring. Distorted-trigonal-pyramidal geometry is apparent at the threecoordinate arsenic center with bond angles in the range 95.9− 104.9°. Two distinctly different As−N bond distances are observed (1.938(5) and 1.740(5) Å) for the three- and twocoordinate nitrogen atoms, respectively. The longer distance is comparable to that of the complex [{AsI2}{{N(2,6-iPr2C6H3)C(H)}2CPh}] (1.923(4) Å).27a The As−N distance (1.740(5) Å) can be compared to those of the related cationic complex [C6H4SN(H)As][AlCl4] (1.776(4) Å)27b and the neutral compounds [{2-(6-Me)C5H3N}N(AsCl2)2] (1.808(9) Å)27c and [As(L)Cl] (L = reduced form of 2-(pyridin-2-yl)-2,3dihydrobenzo[d]thiazole; 1.846(5) Å).27d The As−I bond length in 3A is shorter than that in [{t-AsI2}2] by 0.09 Å, presumably owing to the decrease of the coordination number and the formal positive charge of the As center. The asymmetry in the monoanionic ligand is reflected in disparities in the P−N (1.582(5) and 1.623(5) Å) and P−C (1.789(6) and 1.820(5) Å) bond distances. X-ray Structures of 4A,B. The hydrolysis products 4A,B contain the same cation, i.e. [1H2]+, with the counterions [SbF6]− and [Te2I6]2−, respectively. The structure of 4B is depicted in Figure 9, and that of 4A is shown in the Supplemenatry Information; the structural parameters of 4A,B are compared in Table 5. For the purposes of comparison the cation can be considered as the monoprotonated derivative of the neutral compound CH2(PPh2NSiMe3)2 (1H).1 As expected, protonation gives rise to two distinct P−N bond distances and a similar influence is reflected in the disparities in the P−C and Si−N bond distances. A strong intramolecular hydrogen bond between the NH functionality and imino nitrogen center (2.138(66) Å for 4A and 2.109(55) Å for 4B) in the cation produces a six-membered P2N2CH ring. The bond distances in this ring indicate delocalization of the positive

Table 5. Selected Bond Lengths (Å) and Angles (deg) for 4A,B P1−C25 P2−C25 P1−N1 P2−N2 Si1−N1 Si2−N2 P1−N1−Si1 P2−N2−Si2 P2−C25−P1 N1−P1−C25 N2−P2−C25

4A

4B

1.797(6) 1.834(6) 1.610(5) 1.546(5) 1.758(5) 1.709(5) 133.7(4) 136.5(3) 115.8(3) 107.5(3) 109.8(3)

1.795(5) 1.828(5) 1.613(4) 1.547(4) 1.763(5) 1.695(4) 133.2(3) 141.9(3) 116.3(3) 107.6(2) 109.3(2)

charge over the P2N2C unit. The bond parameters for [SbF6]− in 4A28a,b and [Te2I6]2− in 4B18,29 are similar to those in known compounds. Computational Analysis. Geometry optimizations satisfactorily reproduced the internal dimensions observed in the molecules of [{t-AsI2}2], [{t-SbI2}2], [{c-SbI2}2], [{TeI3}2], and [{t-BiI2}1]. Bond length deviations were below 0.07 Å, with the exception of those of Sb−I2 and N1−Sb in [{c-SbI2}2] (0.23 and 0.18 Å, respectively); such differences are typical when molecules are associated by secondary bonding in the crystal lattice and are usually corrected when the complete aggregate is modeled. The calculations were extended to the complete series of pnictogen complexes [{t-MI2}1], [{t-MI2} 2], and [{c-MI2}2] in order to compare their stabilities. Figure 10 compares the calculated total bonding energies of [{c-MI2}2] and [{t-MI2}1] to those of [{t-MI2}2]. For all [{MI2}2] compounds, the trans isomer is more stable than the cis structure but only slightly in the case of the Sb and Bi molecules. The structure of the cis isomer features a threecenter I−M−N interaction; the corresponding M−N bond is distinctively longer than in the cis isomer (M = As, 2.2063 vs H


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tautomers and the structural changes that take place upon formation of each complex. In all three instances tautomer 1 is more stable than 2 by about 60 kJ mol−1; such a difference of energy is understandable, given the more efficient delocalization of charge in anion 1. Because of the decrease of size from Bi to As, ring strain causes an increase in the energy of both L− tautomers. Such stress can be appreciated by inspecting certain bond angles within the chelate rings (see Table 6). In both [{tMI2}1] and [{t-MI2}2] the C−P−N angles decrease by about 6° from Bi to As; in contrast, the C−P−N(H) angle of 1 is essentially constant. There is also strain in the P−C−P angle of [{t-MI2}1], which increases from Bi to As in order to bring the methanide carbon closer to M, whereas the P−C−P angle of [{t-MI2}2] is unchanged. Figure 10. Calculated (PBE, ZORA, TZ2P, ZPE-corrected) potential energy differences: E([{c-MI2}2]) − E([{t-MI2}2]) (-●-) and E([{tMI2}1]) − E([{t-MI2}2]) (-○-) (M = As, Sb, Bi).

Table 6. Selected Bond Angles (deg) Calculated for the Isomers of the Pnictogen Complexes of 1 and 2 [{t-MI2}1]

1.9402 Å; M = Sb, 2.3580 vs 2.1338 Å; M = Bi, 2.4725 vs 2.2510 Å), which indicates that these bonds are intrinsically weaker. The most interesting comparison in Figure 10 is that between the [{t-MI2}1] and [{t-MI2}2] series. In apparent agreement with the experimentally observed products, the complex of 2 is more stable when M = As and less stable than the complex of 1 when M = Bi; the latter tautomer is also more stable for M = Sb, but only by 5 kJ mol−1, according to the calculations. As both types of complexes, [{t-MI2}1] and [{tMI2}2], contain nearly linear MI2+ fragments, a comparison of their energies and electronic structures was useful in order to understand the stabilization of the N−H tautomer 2 by As and Sb. Such an analysis was performed by calculating the energy of interaction between the trans-MI2+ and ligand− (L−, tautomers 1 or 2) molecular fragments generated from the coordinates of the corresponding optimized structures. Figure 11a,b presents the calculated total bonding energy of each fragment in each complex. As calculated by ADF, this is the energy of the fragment with respect to its constitutent atoms, each in its spherically symmetric spin-restricted ground state. Therefore, the total bonding energy of the MI2+ fragment approximates a combination of the M−I bond energies and the first ionization potential of MI2; it is positive and deceases in magnitude with the mass of the pnictogen. For each M, the geometry of the MI2+ fragment changes little from [{t-MI2}1] to [{t-MI2}2]; thus, the corresponding energy values are nearly equal. The energies of the L− fragments reflect the relative stabilities of the

[{t-MI2}2]

M

P−C−P

C−P−N

P−C−P

C−P−N

C−P−N(H)

As Sb Bi

144.0 141.2 139.4

95.8 98.6 101.5

135.6 135.4 135.5

93.8 97.4 99.8

110.3 110.5 110.6

Figure 11c contains a plot of the calculated interaction energies for each MI2+−L− pair of fragments. The interaction energies are all stabilizing; their magnitudes and their difference increase in the order Bi < Sb < As. The total bonding energy, E([{t-MI2}L]), of each [{t-MI2}L] complex is given by the addition of the energies of their fragments and their interaction. Consequently, the difference of energy between the tautomeric complexes of each M corresponds to the hypothetical isomerization process [{t-MI2}1] ⇄ [{t-MI2}2] and would be calculated as shown in eq 1. ΔE = ΔE(L−) + ΔE(MI 2+) + ΔEinteraction(MI 2+−L−)

(1)

From the preceding discussion, ΔE(L−) = E(2) − E(1) ≈ 60 kJ mol−1; ΔE(MI2+) ≈ 0 kJ mol−1. Therefore, the NH tautomer 2 is specially stabilized for M = As, with ΔEinteraction(MI2+−L−) < 60 kJ mol−1. Figure 12 presents this analysis in a more rigorously quantitative manner. Under the Ziegler-Rauk30 transition-state scheme, the energy of interaction between MI2+ and L− is partitioned into Coulombic contributions (Eelectrostatic), Pauli repulsion (EPauli),

Figure 11. Calculated (PBE, ZORA, TZ2P) contributions to the potential energy of the complexes [{t-MI2}2] (-○-) and [{t-MI2}1] (-●-) (M = As, Sb, Bi): (a, b) total bonding energies of molecular fragments; (c) MI2+−L− interaction energy. I


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representation of the molecular point group, which for example is useful to distinguish σ and π interactions. However, such symmetry analysis is not applicable in the case of the pnictogen complexes of 1 or 2. Instead, the interactions between orbitals of the MI2+ and L− fragments were analyzed using the ETSNOCV31−33 method, which attributes contributions to the overall interaction energy to pairs of natural molecular orbitals for chemical valence. With this approach it was possible to distinguish and compare the contributions of M−C bonds from those of the M−N interactions in each complex. The results are presented graphically in Figure 14. For a given M, the bonding Figure 12. Comparison of differences of total fragment energy (∑ΔEfragments = ΔE(L−) + ΔE(MI2+), -○-) and MI2−ligand interaction energy (ΔEinteraction, -●-), between [{t-MI2}2] and [{tMI2}1] (M = As, Sb, Bi).

and contributions from orbital interactions (Eorbital) (eq 2). The first two terms are calculated using only the occupied orbitals of E interaction = Eelectrostatic + EPauli + Eorbital

(2)

the molecular fragments as bases;30b,c their combination accounts for the interaction before any electron transfer takes place and is termed the total steric interaction (E°; eq 3). Eorbital E° = Eelectrostatic + E Pauli

Figure 14. Components of the M−L bonding energy, M−Ctotal (-●-) and M−Ntotal (-○-), as calculated by the ETS-NOCV method for the optimized models of [{t-MI2}2] (left) and [{t-MI2}1] (right).

(3)

interactions are weaker in the complex of 1 (note that the M− N interaction energy for [{t-MI2}1] arises from two bonds). The M−C interactions are consistently the strongest. In addition, the M−C and M−N interactions decrease in magnitude as M becomes heavier, as expected on the grounds of smaller orbital overlap and lower electronegativity. The Nalewajski−Mrozek40 bond indices (see Figure 15) help to rationalize these observations; they are smaller for the complexes of 1 and larger for M−C than for M−N bonds of each complex. In the extreme case, the M−C bond order is 1.1. Inspection of the Boys−Foster41 localized molecular orbitals indicates that there is indeed a modest π character in the C−M link and that it is more important in the case of the arsenic complex, although the orbitals are strongly polarized toward carbon (Figure 16). By comparison, the corresponding π orbitals on nitrogen atoms are more localized on the electronegative element. The π M−C interaction has a hyperconjugative character that originates from the HOMO-1

is calculated by including all virtual orbitals and accounts for the reorganization of electron density that results as the fragments perturb each other. Therefore, the difference in MI2+−L− interaction energy in the complexes of 1 and 2 can be traced to the differences in each of those contributions (ΔEi = Ei([{tMI2}2]) − Ei([{t-MI2}1])). Figure 13a compares the differences of Eelectrostatic and EPauli between [{t-MI2}2] and [{t-MI2} 1]. Stronger electrostatic interactions stabilize the complexes of 2 with respect to those of 1; however, the Coulombic effect is countered by much larger Pauli repulsion contributions in all three cases. Consequently, all ΔE° values are positive, as shown in Figure 13b. Also presented in this plot are the ΔEorbital values, which are all negative and larger in magnitude than the changes of total steric interaction. The stabilization of tautomer 2 in the Sb and As complexes is therefore rooted in the orbital interaction between M and the ligand. The transition-state formalism allows further analysis of the orbital interaction by partitioning for each irreducible

Figure 13. Comparison of differences of the contributions to the total bonding energy between [{t-MI2}2] and [{t-MI2}1] (M = As, Sb, Bi): (a) ΔEPauli (-○-) and ΔEelectrostatic (-●-); (b) ΔEorbital (-○-) and ΔE° (-●-). J


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CONCLUSIONS

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EXPERIMENTAL SECTION

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In summary, the reactions of K[1] with group 15 and 16 iodides produce unprecedented complexes of the NH tautomer 2, which exhibit a C,N coordination mode toward the metalloid center. In the case of AsI3, Si−N bond cleavage is observed in addition to metathesis, to give a complex of the novel N,N′chelated anion [CH2(PPh2N)(PPh2NSiMe3)]− DFT calculations indicate that the predominant interaction between 2 and the heavy metalloids is through the carbon atoms and that this interaction has a modest π character which is more significant when M = As. The overall strength of the M−L interactions, which include some π character, stabilize the ligand as tautomer 2 as opposed to 1. Because of its weaker bonds, Bi appears to prefer tautomer 1. These investigations lead to the suggestion that tautomerism may also be observed for transition-metal complexes in some cases or for related methanide ligands with P-imino functionalities.

Figure 15. M−L Nalewajski−Mrozek bond indices for [{t-MI2}2]: (-◆-, M−C; -◇-, M−N); and [{t-MI2}1]: (-●-, M−C; -○-, M−N).

General Procedures. All reactions and manipulations were carried out under an argon atmosphere by using standard Schlenk techniques. Solvents were dried over and distilled from CaH2 (CH2Cl2) and Na/ benzophenone (hexane, benzene, toluene, Et2O, and THF). t-BuLi (1.7 M in pentane), potassium hydride, CH2(PPh2)2, azidotrimethylsilane, and arsenic(III) iodide were purchased from Aldrich Chemical Co.; antimony(III) iodide, tellurium(IV) iodide, and bismuth(III) iodide were obtained from Alfa Aesar. All commercial reagents were used without further purification. 1H, 13C, 31P, 29Si, and 125Te NMR spectra were recorded on Bruker 400 spectrometers. 1H and 13C NMR chemical shifts are reported relative to respected deuterated solvents. 31 P, 29Si, and 125Te NMR chemical shifts were referenced externally to 85% H3PO4 (0 ppm), Me4Si (0 ppm), and Ph2Te2 (+422 ppm), respectively. Elemental analyses and ESI-MS were performed by the Analytical Services Laboratory, Department of Chemistry, University of Calgary. Synthesis of K[CH{PPh2NSiMe3}2] (K[1]). K[1] was synthesized by the literature procedure;5 yield 72%. 31P NMR (400 MHz, C6D6, 25 °C): δ 13.49 (s). 31P NMR (400 MHz, C7D8, 25 °C): δ 13.76 (s). 31P NMR (400 MHz, C7D8, −93 °C): δ 14.90 (s). 1H NMR (400 MHz, C7D8, 25 °C): δ 7.92−7.87 (m, 8H, m-C6H5P), 7.15−7.07 (m, 12H, o-,p-C6H5P), 1.76 (t, 1H, CH, 2J(31P,1H) = 2.8 Hz), 0.00 (s, 18 H, SiMe3). 1H NMR (400 MHz, C6D6, 25 °C): δ 7.96−7.91 (m, 8H, mC6H5P), 7.14−7.05 (m, 12H, o-,p-C6H5P), 1.81 (t, 1H, CH, 2J(31P,1H) = 2.8 Hz), 0.04 (s, 18 H, SiMe3). General Synthesis of [{trans-MI n }{C(PPh 2 NSiMe 3 )(PPh2NHSiMe3)}] (M = As, n = 2 ([{t-AsI2}2]), M = Sb, n = 2 (3), M = Te, n = 3 ([{TeI3}2])]. A mixture of K[1] and MI3 (M = As, Sb) or TeI4 was placed in a round-bottomed flask, and then THF or toluene (5 mL) was added at −78 °C. After 15 min, the mixture was warmed to ambient temperature and then it was stirred for 8 h. Details of the subsequent workups for individual compounds are given below. [{trans-AsI2}{C(PPh2NSiMe3)(PPh2NHSiMe3)}] ([{t-AsI2}2]). The reagents used were K[1] (0.245 g, 0.367 mmol) and AsI3 (0.167 g, 0.367 mmol). When the reaction was conducted in THF, the yellow filtrate obtained after separation of insoluble potassium iodide by filtration was evaporated and dried under vacuum. When toluene was used as solvent, the insoluble solid contained [{t-AsI2}2], which was separated from potassium iodide by extraction with dichloromethane. Recrystallization of the crude product in each case gave yellow crystals of [{t-AsI2}2] (yield: 0.075 g, 23% for reaction in THF; 0.102 g, 31% for reaction in toluene). Characterization data for [{t-AsI2}2] are as follows. Anal. Calcd for C31H39P2N2Si2AsI2: C, 41.99; H, 4.44; N, 3.16. Found: C, 42.53; H, 4.85; N, 3.01. 31P{1H} NMR (400 MHz, C6D6, 25 °C): δ 24.28 (d, 2J(31P,31P) = 30.9 Hz), 57.08 (d, 2J(31P,31P) = 31.3 Hz). 31P{1H} NMR (400 MHz, C7D8, 25 °C): δ 24.14 (d, 2J(31P,31P) = 30.7 Hz), 56.84 (d, 2J(31P,31P) = 30.8 Hz). 31P{1H} NMR (400 MHz, CD2Cl2, 25 °C): δ 25.11 (d, 2J(31P,31P) = 29.1 Hz), 59.77 (d,

Figure 16. Comparison of the Boys−Foster localized molecular orbitals that depict the πM−C interactions in [{t-AsI2}2], [{t-SbI2}2], [{c-SbI2}2], and [{TeI3}2] (isosurfaces plotted at 0.033 au).

of 2, a π orbital with most prominent contribution from carbon, and the LUMO+2 of the MI2+ fragment, a combination of the σ* orbitals of the M−I bonds (see Figure 17). The acceptor

Figure 17. Nonlocalized fragment orbitals from which the πM−C interactions of [{t-MI2}2] originate: (left) LUMO+2 of MI2+; (right) HOMO-1 of 2‑ (calculated for M = Sb, isosurfaces plotted at 0.055 au).

ability of the MI2+ fragment also provides a rationale for the geometry of these fragments, bent away from the ligand in spite of the lone pair centered on M. An analogous orbital interaction was identified in the tellurium complex of the NH tautomer 2; the corresponding isosurface is displayed in Figure 16. K


Organometallics

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J(31P,31P) = 29.1 Hz). 1H NMR (400 MHz, C7D8, 25 °C): δ 8.05− 8.00 (m, 4H), 7.75−7.70 (m, 4H), 7.13−6.8 (m, 12H), 6.34 (d, 1H, NH, 2J(31P,1H) = 9.0 Hz), 0.13 (s, 9H, SiMe3), −0.16 (s, 9H, SiMe3). 1 H NMR (400 MHz, CD2Cl2, 25 °C): δ 7.92−7.87 (m, 4H), 7.58− 7.35 (m, 16H), 5.35 (d, 1H, NH, 2J(31P,1H) = 9.2 Hz), 0.04 (s, 9H, SiMe3), −0.15 (s, 9H, SiMe3). 13C{1H} NMR (101 MHz, CD2Cl2, 25 °C): δ 134.23 (d, 4J(31P,13C) = 2.9 Hz), 134.02 (d, 3J(31P,13C) = 12.5 Hz), 133.62 (d, 4J(31P,13C) = 2.9 Hz), 133.32 (d, 3J(31P,13C) = 12.0 Hz), 129.40 (d, 2J(31P,13C) = 13.2 Hz), 129.22 (d, 2J(31P,13C) = 12.8 Hz), 128.53 (d, 1J(31P,13C) = 106.2 Hz), 1.64 (d, 3J(13C,31P) = 2.0 Hz), 1.01 (d, 3J(13C,31P) = 2.1 Hz). 29Si{1H} NMR (80 MHz, CD2Cl2, 25 °C): δ 11.75 (d, 2J(31P,29Si) = 2.3 Hz), 13.95 (d, 2J(31P,29Si) = 3.8 Hz). The insoluble solid from the reaction in THF contained [{AsI2}{CH2(PPh2N)(PPh2NSiMe3)}] (3), which was separated from KI by extraction with toluene followed by diffusion of hexane into a dichloromethane solution to give 3 (yield: 0.040 g, 13%); this product contained a very small amount (>5%) of [{t-AsI2}2] (31P NMR). Characterization data for 3 are as follows. Anal. Calcd for C31H39P2N2Si2AsI2: C, 41.30; H, 3.84; N, 3.44. Found: C, 40.74; H, 4.05; N, 2.90. 31P{1H} NMR (400 MHz, CD2Cl2, 25 °C): δ 15.18 (d, 2 31 31 J( P, P) = 12.3 Hz), 30.32 (d, 2J(31P,31P) = 12.4 Hz). [{SbI 2 }{C(PPh 2 NSiMe 3 )(PPh 2 NHSiMe 3 )}]·CH 2 Cl 2 ·0.5C 6 H 6 ([{SbI2}2]·CH2Cl2·0.5C6H6). The reagents K[1] (0.300 g, 0.449 mmol) and SbI3 (0.226 g, 0.450 mmol) were combined in THF. The yellow solution was separated from potassium iodide by filtration. After removal of the solvent under vacuum, the residue was washed with hexane to give a yellow solid which was recrystallized from a benzene−dichloromethane solution to give yellow crystals of [{c-SbI2} 2]. (yield: 0.079 g, 19%). Characterization data for [{c-SbI2}2] are as follows. Anal. Calcd for C28H31P2N2SiAsI2: C, 39.92; H, 4.22; N, 3.01. Found: C, 39.56; H, 3.85; N, 2.72. 31P{1H} NMR (400 MHz, CD2Cl2, 25 °C): δ 25.94 (d, 2J(31P,31P) = 24.0 Hz), 52.08 (d, 2J(31P,31P) = 24.0 Hz). 1H NMR (400 MHz, CD2Cl2, 25 °C): δ 7.95−7.88 (m, 4H), 7.64−7.52 (m, 8H), 7.45−7.41 (m, 4H), 7.39−7.33 (m, 4H), 4.28 (d, 1H, NH, 2J(31P,1H) = 9.3 Hz), −0.08 (s, 9H, SiMe3), −0.22 (s, 9H, SiMe3). The trans isomer [{t-SbI2}2] was obtained by diffusion of hexane into a toluene solution of the crude product. 31P{1H} NMR (400 MHz, C6D6, 25 °C): δ 25.11 (d, 2J(31P,31P) = 25.3 Hz), 49.29 (d, 2 31 31 J( P, P) = 25.3 Hz). 13C{1H} NMR (101 MHz, C7D8, 25 °C): δ 133.92 (d, 1J(31P,13C) = 89.7 Hz), 133.71 (d, J(31P,13C) = 12.0 Hz), 133.26 (s), 133.21 (d, J(31P,13C) = 13.2 Hz), 129.32 (d, J(31P,13C) = 13.1 Hz), 129.00 (d, J(31P,13C) = 12.5 Hz), 1.59 (d, 3J(13C,31P) = 2.7 Hz), 1.51 (d, 3J(13C,31P) = 2.0 Hz). 29Si{1H} NMR (80 MHz, C7D8, 25 °C): δ 9.31 (d, 2J(31P,29Si) = 5.1 Hz), 10.25 (d, 2J(31P,29Si) = 3.7 Hz). Preparation of ([{AsI}{C(PPh2NSiMe3)(PPh2NHSiMe3)}][SbF6]). A mixture of [{t-AsI2}2] (0.050 g) and Ag[SbF6] (0.010 g) in CH2Cl2 (1 mL) was stirred at room temperature for 30 min. The yellow precipitate of silver iodide was separated by filtration. Removal of the solvent under vacuum gave ([{AsI}2][SbF6]) as a yellow oil. Yield: 0.029 g, 52%. Spectroscopic data for ([{AsI}2][SbF6]) are as follows. 31 1 P{ H} NMR (400 MHz, CD2Cl2, 25 °C): δ 25.73 (d, 2J(31P,31P) = 23.8 Hz), 60.88 (d, 2J(31P,31P) = 23.5 Hz). ESI-MS (positive): calcd for [{AsI}{C(PPh2NSiMe3)(PPh2NHSiMe3)}]+ m/z 759.03822, found 759.03960. Preparation of ([{AsI}{CH2(PPh2N)(PPh2NSiMe3)}][SbF6]) (3A). A mixture of 3 (0.050 g) and Ag[SbF6] (0.010 g). in CH2Cl2 (1 mL) was stirred at room temperature for 30 min. The yellow precipitate of silver iodide was separated by filtration. Removal of the solvent under vacuum gave ([{AsI}{CH2(PPh2N)(PPh2NSiMe3)}][SbF6]) (3A) as colorless crystals. Yield: 0.049 g, 87%. Characterization data for 3A are as follows. Anal. Calcd for C28H31P2N2Si1As1I1Sb1F6: C, 36.43; H, 3.39; N, 3.03. Found: C, 35.10; H, 3.59; N, 3.09. 31P{1H} NMR (400 MHz, CD2Cl2, 25 °C): δ 15.73 (d, 2J(31P,31P) = 11.8 Hz), 30.79 (d, 2 31 31 J( P, P) = 11.8 Hz). 1H NMR (400 MHz, CD2Cl2, 25 °C): δ 7.70− 7.35 (m, 20H), 4.20 (t, 2H, CH2, 2J(31P,1H) = 12.8 Hz), 0.18 (s, 9H, SiMe3). 13C{1H} NMR (101 MHz, CD2Cl2, 25 °C): δ 135.34 (s), 132.86 (d, J(31P,13C) = 12.0 Hz), 130.32 (d, J(31P,13C) = 10.8 Hz),

129.76 (d, J(31P,13C) = 13.7 Hz), 129.22 (d, J(31P,13C) = 13.4 Hz), 20.53 (dd, 1J(31P,13C) = 45.7 and 62.8 Hz), 1.69 (d, 3J(13C,31P) = 1.9 Hz). 29Si{1H} NMR (80 MHz, CD2Cl2, 25 °C): δ 19.1 (s). ESI-MS (positive): calcd for [{AsI}{CH2(PPh2N)(PPh2NSiMe3)}]+ m/z 686.99869, found 686.99589. Preparation of ([{SbI}2][SbF6]). A mixture of [{t-SbI2}2] (0.050 g) and Ag[SbF6] (0.010 g). was stirred at room temperature for 30 min. The yellow precipitate of silver iodide was separated by filtration. Removal of the solvent under vacuum gave a mixture of yellow crystals of [{SbI}2][SbF6] and colorless crystals of 4A, which were separated manually on the basis of their different colors. Characterization data for ([{SbI}2][SbF6]) are as follows. Yield: 0.013 g, 23%. Anal. Calcd for C31H39P2N2Si2I1Sb2F6: C, 35.73; H, 3.77; N, 2.69. Found: C, 36.95; H, 3.67; N, 2.37. 31P{1H} NMR (400 MHz, CH2Cl2, 25 °C): δ 27.11 (d, 2J(31P,31P) = 17.5 Hz), 57.07 (d, 2 31 31 J( P, P) = 17.5 Hz). Characterization data for 4A are as follows. Yield: 0.019 g, 45%. 31 1 P{ H} NMR (400 MHz, CD2Cl2, 25 °C): δ 23.21 (s). 1H NMR (400 MHz, CD2Cl2, 25 °C): δ 7.62−7.35 (m, 20H), 3.63 (t, 2H, CH2, 2 31 1 J( P, H) = 12.8 Hz), 0.00 (s, 18H, SiMe3). 13C{1H} NMR (101 MHz, CD2Cl2, 25 °C): δ 132.5 (t, Ph, Cmeta 3J(13C,31P) = 5.8 Hz), 127.3 (d, Ph, Cipso 1J(13C,31P) = 101 Hz), 134.4 (s, Ph, Cpara), 130.0 (t, Ph, Cortho 2J(13C, 31P) = 6.5 Hz), 29.1 (t, PCP carbon, 1J(13C,31P) = 63.5 Hz), 2.8 (s). ESI-MS (positive): calcd for [CH2(PPh2NSiMe3)(PPh2NHSiMe3)]+ m/z 559.22780, found 559.22818. ESI-MS (negative): calcd for [SbF6]− m/z 234.89479, found 234.89520. [{TeI3}{C(PPh2NSiMe3)(PPh2NHSiMe3)}] ([{TeI3}2]). Benzene (5 mL) was added to solid K[1] (0.275 g, 0.412 mmol) and TeI4 (0.261 g, 0.411 mmol), and the reaction mixture was warmed to 23 °C and then stirred for 6 h. The precipitate of potassium iodide was separated by filtration to give a red solution, which was allowed to evaporate slowly in the glovebox. After 30 days, three types of crystals with different colors were apparent (red, black and yellow (poor quality)). The red crystals (yield: 0.184 g, 42%) and black crystals (yield: 0.008 g, 2%) were separated manually and identified as [{TeI3}2] and [CH2(PPh2NSiMe3)(PPh2NHSiMe3)]2[Te2I6] (4B), respectively, by X-ray crystallography. Characterization data for [{TeI3}2] are as follows. Anal. Calcd for C31H39P2N2Si2TeI3: C, 34.84; H, 3.68; N, 2.63. Found: C, 35.15; H, 3.30; N, 2.44. 31P{1H} NMR (400 MHz, CD2Cl2, 25 °C): δ 29.58 (d, 2J(31P,31P) = 35.1 Hz), 39.75 (d, 2J(31P,31P) = 35.1 Hz). 31P{1H} NMR (400 MHz, C6D6, 25 °C): δ 28.44 (d, 2J(31P,31P) = 39.0 Hz), 36.70 (d, 2J(31P,31P) = 39.0 Hz) ppm. 1H NMR (400 MHz, C6D6, 25 °C): δ 7.80−7.75 (m, 4H), 7.58−7.52 (m, 4H), 7.03− 6.87 (m, 8H), 6.81−6.77 (m, 4H), 5.21 (d, 1H, NH, 2J(31P,1H) = 9.0 Hz), 0.06 (s, 9H, SiMe3), 0.05 (s, 9H, SiMe3). 13C{1H} NMR (101 MHz, CD2Cl2, 25 °C): δ 134.5−134.3 (m), 129.1−128.9 (m), 128.9 (s), 131.3 (d, 1J(31P,13C) = 95.2 Hz), 127.19 (d, 1J(31P,13C) = 100.9 Hz), 1.84 (d, 3J(13C,31P) = 2.1 Hz), 1.71 (d, 3J(13C, 31P) = 2.9 Hz). 29 Si{1H} NMR (80 MHz, CD2Cl2, 25 °C): δ 7.23 (d, 2J(31P,29Si) = 5.2 Hz), 11.83 (d, 2J(31P, 29Si) = 3.8 Hz). 125Te{1H} NMR (126 MHz, CD2Cl2, 25 °C): δ 1003 (s). Characterization data for 4B are as follows. Anal. Calcd for C37H47P2N2Si2I3Te: C, 38.77; H, 4.13; N, 2.44. Found: C, 38.05; H, 3.75; N, 2.29. 31P{1H} NMR (400 MHz, CD2Cl2, 25 °C): δ 24.10 (s). 1 H NMR (400 MHz, CD2Cl2, 25 °C): δ 7.83−7.45 (m, 20H), 3.65 (t, 2H, CH2, 2J(31P,1H) = 13.1 Hz), 2.43 (d, 1H, NH, 2J(31P,1H) = 13.0 Hz), 0.00 (s, 18H, SiMe3). 13C{1H} NMR (101 MHz, CD2Cl2, 25 °C): δ 132.6 (t, Ph, Cmeta 3J(13C,31P) = 6 Hz), 127.2 (d, Ph, Cipso 1 13 31 J( C, P) = 101 Hz), 134.5 (s, Ph, Cpara), 130.1 (t, Ph, Cortho 2 13 31 J( C, P) = 7 Hz), 29.3 (t, PCP carbon, 1J(13C,31P) = 63.0 Hz), 2.8 (s). X-ray Crystallography. Crystals were grown by slow diffusion of hexane into toluene ([{t-AsI2}2], [{t-SbI2}2], and [{t-BiI2}1]), dichloromethane (3A, [{SbI}2][SbF6], 4A), or a mixture of benzene and dichloromethane for the cis isomer of the Sb complex [{c-SbI2}2] at −20 °C. Good-quality crystals of [{TeI3}2] and 4B were obtained by slow evaporation of a benzene solution in the glovebox. Crystallographic data for [{t-AsI2}2], 3A, [{t-SbI2}2], [{c-SbI2}2], [{SbI}2][SbF6], [{TeI3}2], [{t-BiI2}1], and 4A,B are given in the

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Organometallics

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Supporting Information (Tables S1 and S2). Data were collected with a Nonius Kappa CCD diffractometer with use of monochromated Mo Kα radiation (λ = 0.71073 Å) at 123 or 173 K. The structures were solved by direct methods using the program SHELXS-97 and refined with SHELXL-9734 and by full-matrix least squares with anisotropic thermal parameters for the non-hydrogen atoms. One of the carbon atoms of the tert-butyl group (C26), a chlorine atom (Cl1) of dichloromethane, and carbon atoms (C33, C35) of solvent benzene molecule in [{c-SbI2}2] were disordered over two to three positions. Similarly, fluoride atoms F1, F3, F5, and F6 in the anions of [{SbI} 2][SbF6] and F2−F5 in 4A were disordered over two positions. These were fixed satisfactorily. The hydrogen atom of the N−H group was located in Fourier maps for [{t-AsI2}2], 3A, [{t-SbI2}2], [{c-SbI2}2], [{SbI}2][SbF6], [{TeI3}2], and 4A,B. Computational Methods. All calculations in this study were performed using the ADF DFT package (version 2013.01).35 The adiabatic local density approximation (ALDA) was used for the exchange-correlation kernel,36 and the differentiated static LDA expression was used with the Vosko−Wilk−Nusair parametrization.37 Preliminary calculation of model geometries was gradient-corrected with the exchange and correlation functionals proposed in 1991 by Perdew and Wang (PW91).38 Such geometry optimizations were conducted using a small double-ζ basis set with frozen cores corresponding to the configuration of the preceding noble gas and no polarization functions; the resulting structures were refined using the Perdew−Burke−Ernzerhof39 (PBE) functional, a triple-ζ allelectron basis set with one polarization function, and application of the zeroth-order regular approximation (ZORA)40 formalism with the specially adapted basis sets. Analytical frequency calculations were performed to ensure that each geometry was at an energy minimum.41 However, all the structures of [{c-MI2}2] converged with one vibrational mode of negative frequency in spite of multiple attempts to overcome the problem. This mode corresponds to a combination of librations of the aromatic rings, only without any impact on the chelate rings.

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work was made possible by the facilities of the Shared Hierarchical Academic Research Computing Network (SHARCNET: www.sharcnet.ca) and Compute/Calcul Canada.

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(1) Ong, C. M.; Stephan, D. W. J. Am. Chem. Soc. 1999, 121, 2939. (2) For a review see: Panda, T. K.; Roesky, P. W. Chem. Soc. Rev. 2009, 38, 2782. (3) Babu, R. P. K.; Aparna, K.; McDonald, R.; Cavell, R. G. Inorg. Chem. 2000, 39, 4981. (4) Babu, R. P. K.; Aparna, K.; McDonald, R.; Cavell, R. G. Organometallics 2001, 20, 1451. (5) Gamer, M. T.; Roesky, P. W. Z. Anorg. Allg. Chem. 2001, 627, 877. (6) Wei, P.; Stephan, D. W. Organometallics 2003, 22, 601. (7) Aparna, K.; McDonald, R.; Cavell, R. G. Organometallics 1999, 18, 3775. (8) Aparna, K.; McDonald, R.; Ferguson, M.; Cavell, R. G. Organometallics 1999, 18, 4241. (9) Panda, T. K.; Zulys, A.; Gamer, M. T.; Roesky, P. W. J. Organomet. Chem. 2005, 690, 5078. (10) Ong, C. M.; McKarns, P.; Stephan, D. W. Organometallics 1999, 18, 4197. (11) (a) Leung, W.-P.; So, C.-W.; Wang, Z.-X.; Wang, J.-Z.; Mak, T. C. W. Organometallics 2003, 22, 4305. (b) Leung, W.-P.; Wang, Z.-X.; Mak, T. C. W. Angew. Chem., Int. Ed. 2001, 40, 2501. (12) Wei, P.; Stephan, D. W. Organometallics 2002, 21, 1308. (13) Fang, M.; Jones, N. D.; Friesen, K.; Lin, G.; Ferguson, M. J.; McDonald, R.; Lukowski, R.; Cavell, R. G. Organometallics 2009, 28, 1652. (14) (a) For a review, see Liddle, S. T.; Mills, D. P.; Wooles, A. J. Chem. Soc. Rev. 2011, 40, 2164. (b) Demange, M.; Boubekeur, L.; Auffrant, A.; Mézailles, N.; Ricard, L.; Le Goff, X.; Le Floch, P. New. J. Chem. 2006, 30, 1745. (15) Recently, we have shown that group 15 and 16 elements of the dianion [C(PPh2S)2]2− incorporate a polar M−C single bond and a three-center−two-electron S−M−S unit: (a) Thirumoorthi, R.; Chivers, T.; Vargas-Baca, I. Organometallics 2012, 31, 627. (b) Thirumoorthi, R.; Chivers, T.; Vargas-Baca, I. Dalton Trans. 2011, 40, 8086. (16) Wang, J.; Xu, Z. In Handbook of Chalcogen Chemistry, 2nd ed.; Devillanova, F. A., Ed.; RSC Publishing: Cambridge, U.K., 2013; Vol. 1, Chapter 8.1, p 432. (17) Rakesh, P.; Singh, H. B.; Butcher, R. J. Dalton Trans. 2012, 41, 10707. (18) Konu, J.; Chivers, T. Dalton Trans. 2006, 3941. (19) Opris, L. M.; Silverstru, A.; Silvestru, C.; Breunig, H. J.; Lork, E. Dalton Trans. 2003, 4367. (20) http://www.ccdc.cam.ac.uk/products/csd/radii/table.php4. (21) (a) Trotter, J. Z. Kristallogr. 1965, 121, 81. (b) Trotter, J.; Zobel, T. Z. Kristallogr. 1966, 123, 67. (c) Krebs, B.; Buss, B.; Altena, D. Z. Anorg. Allg. Chem. 1971, 386, 257. (22) Müller, A.; Möhlen, M.; Neumüller, B.; Faza, N.; Massa, W.; Dehnicke, K. Z. Anorg. Allg. Chem. 1999, 625, 1748. (23) Wang, S.; Mitzi, D. B.; Landrum, G. A.; Genin, H.; Hoffmann, R. J. Am. Chem. Soc. 1997, 119, 724. (24) (a) Soran, A.; Breunig, H. J.; Lippolis, V.; Arca, M.; Silvestru, C. Dalton Trans. 2009, 77. (b) Soran, A. P.; Silvestru, C.; Breunig, H. J.; Balázs, G.; Green, J. C. Organometallics 2007, 26, 1196. (25) Clegg, W.; Errington, R. J.; Fisher, G. A.; Hockless, D. C. R.; Norman, N. C.; Orpen, A. G.; Stratford, S. E. J. Chem. Soc., Dalton Trans. 1992, 1967. (26) (a) Mills, D. P.; Moro, F.; McMaster, J.; van Slageren, J.; Lewis, W.; Blake, A. J.; Liddle, S. T. Nat. Chem. 2011, 3, 454. (b) Aharonian, G.; Feghali, K.; Gambarotta, S.; Yap, G. P. A. Organometallics 2001, 20, 2616.

ASSOCIATED CONTENT

S Supporting Information *

Figures showing the 31P NMR spectrum of [{AsI}2][SbF6] (S1), the 1H, 13C, and 31P NMR spectra of 4A (S2−4), the ESIMS of [{AsI}2][SbF6], 3A and 4A (S5−8), and the structure of 4A (S9), Tables S1 and S2 giving crystallographic data for [{tAsI2}2], 3A, [{t-SbI2}2], [{c-SbI2}2], [{SbI}2][SbF6], [{TeI3} 2], [{t-BiI2}1], and 4A,B, Tables S3 and S4 giving complete transition-state analysis of bonding energies for all [{MI2}L] complexes and MI2+ and L− fragment interactions, Figure S10 featuring electrostatic potential maps for tautomers 1 and 2, Tables S5 and S6 giving charges transferred upon MI2+ and L− interactions and atomic charges (Hirshfeld), Figure S11 displaying the top NOCVs of [{t-AsI2}1] and [{t-AsI2}2], and CIF files giving crystallographic data for all crystal structures. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*T.C.: e-mail, [email protected]; tel, 403-220-5741; fax, 403289-9488. *I.V.-B.: e-mail, [email protected]; tel, 905-5259140 ext. 23497; fax, 905-522-2509. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged. This M


Organometallics

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