Syntheses and Structures of the Crystalline, Highly Crowded 1,3-Bis

Dec 21, 2011 - Department of Chemistry, University of Sussex, Brighton BN1 9QJ, U.K. ... Chad T. PalumboLucy E. DaragoCory J. WindorffJoseph W...
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Syntheses and Structures of the Crystalline, Highly Crowded 1,3-Bis(trimethylsilyl)cyclopentadienyls [MCp″3] (M = Y, Er, Yb), [PbCp″2], [{YCp″2(μ-OH)}2], [(ScCp″2)2(μ-η2:η2-C2H4)], [YbCp″2Cl(μ-Cl)K(18-crown-6)], and [{KCp″}∞] Martyn P. Coles, Peter B. Hitchcock, Michael F. Lappert,* and Andrey V. Protchenko Department of Chemistry, University of Sussex, Brighton BN1 9QJ, U.K. S Supporting Information *

ABSTRACT: Yttrium and the smaller group 3 and 4f metal homoleptic M(III) cyclopentadienyls [MCp″3] (Cp″ = [η5C5H3(SiMe3)2-1,3]− and M = Y (1Y), Er (1Er), or Yb (1Yb)) were prepared from YCp″2I (2Y) or the appropriate [LnCp″2I(thf)] and KCp″ in satisfactory (1Y), very low (1Er), or low (1Yb) yield. Attempted reaction of [ScCp″2I(thf)] with KCp″ failed to produce the homoleptic Sc(III) cyclopentadienyl. Compound 1Yb was synthesized in better yield by the redox reaction between [YbCp″2(thf)] and either [PbCp″2] (3) or the in situ prepared HgCp″2 (4) analogue. Complexes 3 and 4 were obtained by a salt metathesis route, 3 was isolated as a crystalline solid, while 4 was used only as a hexane solution. The structures of crystalline 1Y, 1Yb, 3, [{YCp″2(μ-OH)}2] (5Y) [prepared by hydrolysis of 1Y], [(ScCp″2)2(μ-η2:η2-C2H4)] (6) {isolated in low yield from the reaction of ScCp″2I (2Sc) with [K(18-crown-6)]2[C6H2(SiMe3)4-1,2,4,5]}, [LaCp″2Cl(μ-Cl)K(18-crown-6)] (7La) {obtained from the foregoing potassium reagent and [{LaCp″2(μ-Cl)}2]}, [YbCp″2Cl(μ-Cl)K(18-crown-6)] (7Yb), and [{KCp″}∞] (8) were determined by X-ray diffraction.

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INTRODUCTION The [η5-C5H3(SiMe3)2-1,3]− (≡ Cp″) ligand came to the fore in 1981 in the context of the synthesis of the metal complexes [{MCp″2(μ-Cl)}2] (M = Sc, Y, La, or a 4f-metal).1 Since then, we have used Cp″ extensively, not only in the context of group 3 or 4f metal chemistry2 but also in that of Th and U3 and Zr4 (only the most recent reference in each class is cited). Significant contributions by others include the preparation and structures of [MCp″3] (M = La,5 Ce,6 Dy,5 Sm7). Among the known compounds used as precursors in the present study were [MI3(thf)3.5] (M = Sc, Y8), KCp″,9 [YbCp″2I(thf)] (A),10 [(YbCp″2)∞],11 NaCp″,11 and [K(18-crown-6)]2 [C6H2(SiMe3)4-1,2,4,5].12 The principal initial objective of this study was to provide viable routes to the hitherto unknown highly hindered homoleptic bis(1,3-trimethylsilyl)cyclopentadienyls of the smaller group 3 and 4f metal(III)s, especially of scandium, yttrium, erbium, and ytterbium. As a spin-off, mercury(II) and lead(II) cyclopentadienyls MCp″2 were sought as potential oxidative Cp″-ligand-transfer reagents for preparing [YbCp″3] from [YbCp″2(thf)].

complexes of the smaller group 3 and 4f metals [MCp″3] (M = Sc, Y, Er, Yb) were considered. Halide displacement from the appropriate MCp″2Hal and M′Cp″ [Cp″ = C5H3(SiMe3)21,3; M′ = Li, Na, K] was initially examined. Even under the forcing conditions of prolonged heating under reflux in toluene, only the iodide (rather than the chloride) MCp″2I (M = Y) with KCp″ gave a satisfactory yield of [MCp″3], Scheme 1. Scheme 1. Synthesis of New Cyclopentadienylyttrium Compounds

Unsolvated iodide derivatives (M = Er or Yb) did not react with KCp″ in toluene (the reagents dissolved upon heating and crystallized unchanged at room temperature); thus mono-thf solvates were used as more active precursors. In this case,

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RESULTS AND DISCUSSION 1. Synthesis of the Homoleptic Crystalline Yttrium, Ytterbium(III), and Erbium 1,3-Bis(trimethylsilyl)cyclopentadienyls and Failure to Obtain the Analogous ScCp″3 from ScCp″2I(thf). Several approaches to the synthesis of the homoleptic bis(trimethylsilyl)cyclopentadienylmetal(III) © 2011 American Chemical Society

Special Issue: F. Gordon A. Stone Commemorative Issue Received: October 5, 2011 Published: December 21, 2011 2682

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desolvation and metathesis occurred concurrently leading to low yields of the appropriate [MCp″3], the major product being MCp″2I, as recorded in eq 1 and Scheme 2, respectively, while Scheme 2. Synthesis of the Homoleptic Ytterbium(III) Cyclopentadienyl [YbCp″3] (1Yb)a

a

Yield of 1Yb: 50% from 3, 90% from 4, 18% from A.

[ScCp″2I(thf)] (prepared as shown in eq 2) produced the desolvation product only. KCp ″ , PhMe

[ErCp″2 I(thf)] ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ [ErCp″3 ] (1Er , 5%) reflux, 4h

(1)

Both the crystalline 1Y and 1Yb were high-melting solids, characterized by C and H microanalysis and EI-mass spectra; additionally, multinuclear NMR spectra were recorded for 1Y and were readily assigned. It is noteworthy that small but noticeable 1H−89Y and 13C−89Y couplings were observed for the cyclopentadienyl C and H atoms of [YCp″3]. Such couplings were not detected in the 1H and 13C NMR spectra of 2Y, 5Y, or (to the best of our knowledge) any other cyclopentadienylyttrium compound (cf. the absence of 13C−89Y coupling in [Y(C5Me5)3]13). This observation may be explained by the lack of Cp″ ligand rotation due to increased steric crowding. The structures of the crystalline tris[1,3-bis(trimethylsilyl)cyclopentadienyl]yttrium (1Y) and -ytterbium (1Yb) compounds in alternative views are illustrated in Figure 1a and b, respectively. Selected geometric data, together with those for the analogous gadolinium compound,5 are listed in Table 1. The metal atom M is coplanar with the midpoint (“Cent”) of each Cp″ ring, and each Cent−M−Cent′ angle is 120 ± 0.6 o. For 1Y the Y, Si1, and Si2 atoms are ca. 2.42, −0.59, and −0.64 Å out of each C5 plane. As consistent with the lanthanide contraction, the M−Cent distance increases in the sequence M = Yb < Y < Dy5 < Gd5 < Sm7 < Ce.6 2. Synthesis of the Crystalline [YbCp″3] (≡ 1Yb) by the Oxidation of [YbCp″ 2 (thf)] Using the Crystalline [PbCp″2] (3) or a Solution of HgCp″2 (4). An alternative and superior procedure was available only for the ytterbi um(III) complex, since of the four elements Sc, Y, Er, and Yb only Yb has a stable +2 as well as +3 oxidation state. Thus, using the redox system YbCp″2/M″Cp″2 [M″ = Pb (3), Hg (4)], satisfactory to excellent yields of [YbCp″3] (1Yb) were obtained from [PbCp″2] (3) or HgCp″2 (4), respectively,

Figure 1. (a) Molecular structure of [Y{C5H3(SiMe3)2-1,3}3], 1Y (30% ellipsoids, H atoms omitted). (b) Molecular structure of [Yb{C5H3(SiMe3)2-1,3}3], 1Yb (30% ellipsoids, H atoms omitted).

Scheme 2. These latter compounds were readily accessible by the metathetical routes of eqs 3 and 4, respectively. 2KCp ″ , thf

PbI2 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ [PbCp″2 ] (3, 74%) 20 ° C, 2 h

(3)

2NaCp ″ , Et 2O

HgCl 2 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ HgCp″2 (4, 95%) 20 ° C, 12 h

(4)

The low-melting [PbCp″2] (3) was purified by vacuum distillation. It was identified by C and H microanalysis, multinuclear ambient temperature solution spectra, and its EI-mass spectrum. The mercury(II) analogue 4 was obtained only as a liquid of ca. 90% purity. 2683

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angle in crystalline [Pb(η5-C5H5)2],14 [Pb(η5-C5Me5)2],15 [Pb{η5-C5(CH2Ph)5}2],16 and [Pb{η5-C5Me4(SiMe2But)}2]17 is 123°, 151°, 153°, and 180°, respectively; the corresponding Pb−Cent distance in these compounds is 2.502 and 2.820,14 2.525 and 2.497,14 2.507 and 2.500,16 and 2.460 Å,17 respectively. The lead atom in 3 is ca. 2.48 Å out of each C5 plane, and the two Si atoms are −0.16 and −0.20, or −0.8 and 0.13 Å, out of each plane. The two C5 planes deviate from parallel by ca. 12.8°. 3. Synthesis and Structure of the Crystalline [{YCp″2(μ-OH)}2] (5Y). The dimeric yttrium compound [{YCp″2(μ-OH)}2] (5Y) was initially isolated as an unexpected byproduct in the preparation of [YCp″3] (1Y), presumably due to the presence of a miniscule quantity of water. A focused high-yield synthesis of 5Y involved treatment at ambient temperature of 1Y with benzene containing a trace of water, as documented in Scheme 1; the reaction completeness was monitored by the color change from yellow to colorless. The isoleptic samarium compound had previously been prepared by the redox reaction between [SmCp″2(thf)] and water9 or by the hydrolysis of [SmCp″2][BPh4].18 The high-melting compound 5Y was characterized by C and H microanalysis, EI-MS, which showed that the dimeric structure persisted at high temperature, and the 1H NMR spectrum, which showed that the rotation of Cp″ ligands was slow on the NMR time scale and the two different orientations of the Cp″ ligand found in the solid-state structure (one with SiMe3 substituents directed toward the center of the molecule and another away) were retained in solution. The structure of the crystalline centrosymmetric [{bis[1,3bis(trimethylsilyl)cyclopentadienyl]}(μ-hydroxo)yttrium] 2 (5Y) is illustrated as an ORTEP representation in Figure 3.

Table 1. Selected Bond Distances (Å) and Angles (deg) for [MCp″3] (M = Y, 1Y; M = Yb, 1Yb; M = Gd5) 1Y (M = Y) M−Cent1 M−Cent2 M−Cent3 C1−Si1 C3−Si2 M−C1 M−C2 M−C3 M−C4 M−C5 Cent1−M−Cent2 Cent1−M−Cent3 Cent2−M−Cent3

2.424(3) 2.424(3) 2.427(3) 1.864(3) 1.864(3) 2.711(3) 2.761(3) 2.747(3) 2.666(3) 2.647(3) 120.04(8) 119.53(8) 120.43(8)

1Yb (M = Yb) 2.397(3) 2.387(3) 2.388(3) 1.867(3) 1.865(3) 2.731(3) 2.755(3) 2.705(3) 2.599(3) 2.619(3) 120.5(1) 119.5(1) 120.0(1)

M = Gd 2.465 2.459 2.465 1.856(6) 1.876(6) 2.792(5) 2.764(5) 2.671(6) 2.695(6) 2.783(6) 119.8 119.9 120.3

An ORTEP representation of the structure of crystalline bis[1,3-bis(trimethylsilyl)cyclopentadienyl]lead (3) is shown in Figure 2; selected geometrical data are listed in Table 2.

Figure 2. Molecular structure of [Pb{C5H3(SiMe3)2-1,3}2], 3 (30% ellipsoids, H atoms omitted).

Table 2. Selected Bond Distances (Å) and Angles (deg) for [PbCp″2] (3) Pb−Cent1 Pb−C1 Pb−C3 Pb−C5 C3−Si2 Cent1−Pb−Cent2 Si1−C1−C5 Si2−C3−C4 Si3−C12−C16 Si4−C14−C15

2.51(1) 2.802(10) 2.783(10) 2.787(11) 1.851(11) 171.0(3) 126.8(9) 127.9(8) 125.2(8) 126.7(9)

Pb−Cent2 Pb−C2 Pb−C4 C1−Si1 Si1−C1−C2 Si2−C3−C2 Si3−C12−C13 Si4−C14−C13

2.46(1) 2.751(10) 2.763(11) 1.850(11)

Figure 3. Molecular structure of [Y{C5H3(SiMe3)2-1,3}2(μ-OH)]2, 5Y (30% ellipsoids, H atoms except OH omitted). Symmetry transformations to generate equivalent atoms: ′ −x, −y, −z.

128.4(8) 128.2(9) 128.3(8) 126.6(9)

Selected geometrical parameters, together with those of the known isoleptic samarium compound,9 are listed in Table 3. The core for 5Y is the central YOY′O′ rhombus; the angle at each oxygen atom is wider by ca. 35° than at the yttrium atom, and the Y−O bond length is ca. 2.23 Å. The Y atom is ca. 2.4 Å out of each attached planar C5 ring; the centroid−Y−centroid angle is 128.03(9)°. The two silicon atoms are ca. 0.46 and

Particularly noteworthy is the wide Cent−Pb−Cent′ angle of 171.0(3)o, indicative of significant steric effects. The analogous 2684

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Table 3. Selected Bond Distances (Å) and Angles (deg) for [{MCp″2(μ-OH)}2] (M = Y, 5Y; M = Sm9) 5Y (M = Y) M−Cent1 M−Cent2 M−O M−O′ M−C1 M−C2 M−C3 M−C4 M−C5 M−C12 M−C13 M−C14 M−C15 M−C16 Cent1−M−Cent2 Cent1−M−O Cent2−M−O Cent1−M−O′ Cent2−M−O′ O−M−O′

2.389(3) 2.399(3) 2.233(2) 2.234(2) 2.693(3) 2.687(3) 2.683(3) 2.658(3) 2.662(3) 2.706(3) 2.707(3) 2.701(3) 2.657(3) 2.659(3) 128.03(9) 108.72(8) 112.51(6) 107.89(8) 113.68(8) 72.35(10)

Scheme 3. Attempted Reduction of Bis(cyclopentadienyl)Sc, -Y, and -Yb Compounds

M = Sm 2.46 2.44 2.40(2) 2.41(1) 2.78(2) 2.73(3) 2.76(3) 2.73(2) 2.68(2) 2.69(3) 2.70(3) 2.73(3) 2.71(3) 2.69(3) 129.5 113.0 106.6 114.6 103.9 76.6

amido- or cyclopentadienylyttrium complexes characterized by EPR spectroscopy or X-ray crystallography.21 Complex 6 is presumed to have arisen from an Sc(II)-induced cleavage of the crown ether. As for 7, a previous instance of a K(18-crown-6)/ arene complex failing to act as a reducing agent was in the conversion of [Ce(η5-C5H4SiMe2But)3] into [K(18-crown-6)(PhMe)2][{Ce(η-C5H4SiMe2But)3}2(μ-H)].22 Earlier examples of ethylene-bridged binuclear metal complexes include [{Sc(ηC5Me4SiMe2NBut)(PMe3)}(μ-C2H4)],23 [{(R8-calix-pyrrol)[ ( C H 2 C H O ) L i ] [ L i ( t h f ) ] 2 S m} 2 ( μ- C 2 H 4 ) ] , 2 4 and [{ZrCl3(PEt3)2}2(μ-C2H4)];25 the latter was the first to contain a bridging ethylene. An ORTEP representation of the structure of the crystalline centrosymmetric ethylene-bridged binuclear scandium compound [(ScCp″2)2(μ-C2H4)] (6) is illustrated in Figure 4;

ca. 0.51 Å out of one C5 ring and ca. 0.31 and ca. 0.3 Å out of the other. The M−centroid distances are ca. 0.055 Å shorter for M = Y than M = Sm. 4. Reduction of [YCp″ 3 ] (1Y), [ScCp″ 2 I] (2Sc), [{LaCp″2(μ-Cl)}2], or [{YbCp″2(μ-Cl)}2] with K/18-Crown-6 or [K(18-Crown-6)]2[C6H2(SiMe3)4-1,2,4,5]; Crystalline Complexes [(ScCp″2)2(μ-C2H4)] (6) and [MCp″2Cl(μ-Cl)K(18-crown-6)] (M = La, 7La, or Yb, 7Yb), and Evidence for Y(III) Hydride. Attempts were made to obtain lower valent bis(cyclopentadienyl)metal complexes of the smaller group 3 or 4f elements. Electrochemical reduction of 1Y in thf (0.2 M [NBu4][PF6] as a supporting electrolyte with a scan rate of 100 mV s−1 at a vitreous carbon electrode at ambient temperature vs SCE; experimental details as in ref 19) showed an irreversible reduction process at Ep = −3.0 V (cf. for [LaCp″3] reversible reduction at E1/2 = −2.24 V vs SCE20 was observed). Thus, in contrast to [LaCp″3], attempted chemical reduction of 1Y with K/18-crown-6 in benzene or thf did not produce a stable YII complex; the initial red-brown (benzene) or dark violet (thf) color disappeared in a few seconds. In benzene, the final products were tentatively assigned by 1H NMR spectroscopy as [K(18-crown-6)(Cp″)] and [{YCp″2(μ-H)}2], the latter being formed via abstraction of hydrogen from the solvent by a Y(II) transient intermediate (when the reduction was carried out in C6D6, the triplet at δ 3.25 ppm with J1(H−Y) = 32.2 Hz was absent). The dimeric yttrium hydrido compound was stable in solution upon heating to 70 °C as the triplet signal remained unchanged (broadening and partial coalescence of the Cp″ signals was consistent with unlocking the hindered ring rotation). By using the new lipophilic reducing reagent [K(18-crown6)]2[C6H2(SiMe3)4-1,2,4,5],12 as summarized in Scheme 3, Xray-characterized compounds were isolated from 2Sc or [{LaCp″2(μ-Cl)}2], albeit in minute (6) or modest (7La) yield; the ytterbium analogue 7Yb was obtained using K/18crown-6 in benzene. We infer that lower valent compounds were present in the Sc and Y systems. In support of this view there were recent observations of Y(II) species in reductions of

Figure 4. Molecular structure of [{Sc{C5H3(SiMe3)2-1,3}2}2(μ-η2:η2C2H4)], 6 (30% ellipsoids, H atoms except H2C omitted). Symmetry transformations to generate equivalent atoms: ′ −x, −y, −z.

selected geometrical parameters are listed in Table 4. The possibility of 6 containing a bridging peroxide unit was ruled out; refinement of C23 as oxygen gave abnormally large 2685

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Table 4a. Selected Bond Distances (Å) and Angles (deg) for [{ScCp″}2}2(μ-η2:η2-C2H4)] (6) Sc−Cent1 Sc−Cent2 Sc−Cent3a Sc−C23 C23−C23′ Sc−C1 Sc−C3 Cent1−Sc−Cent2 Cent1−Sc−Cent3 Cent2−Sc−Cent3 Si1−C1−C2 Si1−C1−C5 Si2−C3−C2 a

2.208(2) 2.205(2) 2.195(2) 2.314(3) 1.465(5) 2.547(2) 2.547(2) 135.89(9) 112.63(9) 111.48(9) 126.33(17) 127.34(18) 126.34(18)

Sc−C12 Sc−C14 C1−Si1 C3−Si2 C12−Si3 C14−Si4 Si2−C3−C4 Si3−C12−C13 Si3−C12−C16 Si4−C14−C13 Si4−C14−C15

2.515(2) 2.526(2) 1.872(2) 1.865(2) 1.860(3) 1.865(2) 127.73(17) 127.67(19) 124.06(19) 128.01(18) 123.86(18)

Cent 3 = midpoint of the C23···C23′ bond.

Table 4b. Additional Geometric Data for [{ScCp″2}2(μ-η2:η2C2H4)] (6) angle between C2H4 plane and C1 to C5 plane angle between C1 to C5 plane and C12 to C16 plane Sc out of C2H4 plane Sc out of C1 to C5 plane Sc out of C12 to C16 plane Si1 out of C1 to C5 plane Si2 out of C1 to C5 plane Si3 out of C12 to C16 plane Si4 out of C12 to C16 plane

65.15(1.13)° 47.19° 2.1952(8) Å 2.2047(11) Å 2.2050(11) Å 0.2820(38) Å 0.2946(38) Å 0.4731(38) Å 0.4626(37) Å

Figure 5. Molecular structure of [Yb{C 5H3(SiMe3) 2-1,3} 2Cl(μ-Cl)K(18-crown-6)], 7Yb (30% ellipsoids, H atoms omitted).

Table 5a. Selected Bond Distances (Å) and Angles (deg) for [YbCp″2Cl(μ-Cl)K(18-crown-6)] (7Yb) Yb−Cent1 Yb−Cl1 C1−Si1 C12−Si3 K−O2 K−Cl1 Cl1−Yb−Cl2 Cent1−Yb−Cent2 Cl1−Yb−Cent1 Cl1−Yb−Cent2 Cl2−Yb−Cent1 Cl2−Yb−Cent2 Si1−C1−C2 Si1−C1−C5 Si2−C3−C2

thermal ellipsoids and did not account for the two electron density peaks assigned to H23a and H23b. These latter atoms were freely refined in the final model for compound 6. The olefin plane is perpendicular to the scandium−scandium axis, with the midpoint of the ethylene coincident with the midpoint of the metal−metal vector. Each three-coordinate scandium atom is bound to the centroid of the two attached planar C5 rings; the centroid−Sc−centroid angle is 135.89(9)°. The scandium atom is ca. 2.2 Å out of each C5 plane, each of the two silicon atoms attached to one C5 ring is ca. 0.29 Å out of this plane, while the adjacent C5 ring has the Si atoms ca. 0.47 Å out of its plane. The central C−C bond length, C23−C23′, of 1.465(5) Å (consistent with the bridging ethylene being formally C2H42−) may be compared with the related 1.433(12) Å bonds in [{Sc(C5Me4SiMe2NBut)(PMe3)}2(μ-C2H4)];23 its two Sc−C distances of 2.320(9) and 2.357(9) Å are slightly longer than the Sc−C23 (≡ Sc−C23′) bond length in 6. The orange crystalline heterobinuclear compound [YbCp″2Cl(μ-Cl)K(18-crown-6)] (7Yb) was characterized by C, H, and Cl microanalysis and X-ray diffraction. Its 1H NMR spectrum in C6D6 showed paramagnetically shifted but readily identifiable bands in appropriate integral ratios. The structure of the crystalline 7Yb is shown as an ORTEP representation in Figure 5; selected geometrical parameters are listed in Table 5 (data on the La analogue 7La can be found in Table 8, with full details in the CIF file; see Supporting Information). The four-coordinate ytterbium atom is in a pseudotetrahedral environment; four of the six angles subtended at Yb are close to tetrahedral at 105.5 ± 1.7°, the other two being 96.6° and 129.5°. The Yb−Cent at ca. 2.40 Å and the Cp″ (C−Si) bond distances of 7Yb are close to those of 1Yb. The Yb−Cl1 and Yb−Cl2 bonds of 7Yb are essentially identical at ca. 2.34 Å (cf.26 2.593(1) Å in

2.339(2) 2.5619(5) 1.864(2) 1.859(2) 2.833(2) 2.9817(2) 99.64(2) 129.49(8) 107.25(5) 104.18(5) 106.38(5) 106.10(5) 126.32(15) 127.65(15) 126.11(16)

Yb−Cent2 Yb−Cl2 C3−Si2 C14−Si4 K−O4

2.342(2) 2.5789(5) 1.860(2) 1.857(2) 2.772(2)

Si2−C3−C4 Si3−C12−C13 Si3−C12−C16 Si4−C14−C13 Si4−C14−C15 O2−K−Cl1 O4−K−Cl1 K−Cl1−Yb

128.15(15) 126.26(15) 127.90(16) 123.97(16) 129.37(16) 109.31(4) 98.50(5) 166.23(3)

Table 5b. Additional Geometric Data for [YbCp″2Cl(μ-Cl)K(18-crown-6)] (7Yb) angle between C1 to C5 plane and C12 to C16 plane Yb out of C1 to C5 plane Si1 out of C1 to C5 plane Si2 out of C1 to C5 plane Yb out of C12 to C16 plane Si3 out of C12 to C16 plane Si4 out of C12 to C16 plane

51.25(8)° 2.3391(9) Å 0.2224(32) Å 0.1833(32) Å 2.3414(9) Å 0.2056(35) Å 0.3394(36) Å

[Yb(η5-C5H4SiMePh2)2(μ-Cl)2Li(OEt2)2]), as are the K−Cl1 distances in 7 and the 2.988(4) Å in [Sm(C4PMe5)3(μ-Cl)K(PhMe)]2.27 The K−Cl1−Yb angle deviates from linearity by almost 14°. Interestingly, in the Dy analogue [DyCpttt2(μ-Cl)2K(18-crown-6)] (Cpttt = [η5-C5H3But3-1,2,4]−) each of the two Cl atoms acted as a bridge.28 5. X-ray Structure of the Crystalline [{KCp″}∞] (8). The compound KCp″ has been much used as a Cp″-transfer reagent. It was first prepared in 1991 as an analytically pure white powder 2686

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metal(III) precursor failed for M = Sc and gave a minute amount of the erbium compound (1Er), a low yield of the ytterbium (1Yb) analogue, or a viable source of the yttrium (1Y) product. However, even this outcome required not only the use of the appropriate MCp″2I(thf) as the precursor (rather than the chloride) but also prolonged heating at ca. 120 °C. The bulk of the Cp″ ligand is even reflected in the structure of the crystalline [PbCp″2] (3), having a Cent−Pb−Cent′ angle approaching linearity. The use of 3 or HgCp″2 (4) as a convenient mild oxidative source of Cp″ in the high-yield preparation of 1Yb from YbCp″2 is noteworthy, and the similar use of Pb(II) or Hg(II) complexes of other bulky ligands may prove to be a valuable addition to lipophilic redox synthesis.

Table 6. Selected Bond Distances (Å) and Angles (deg) for [{KCp″}∞] (8) K1−Cent1 K2−Cent2 C3−Si2 C14−Si4 C2−C3 C4−C5 C12−C13 C14−C15 C16−C12 Cent1−K1−Cent2 K1−Cent2−K2 Si1−C1−C5 Si2−C3−C4 Si3−C12−C16 Si4−C14−C15

2.774(3) 2.760(3) 1.849(3) 1.852(3) 1.420(4) 1.383(4) 1.424(4) 1.426(4) 1.416(4) 159.83(8) 178.26(10) 126.1(2) 126.7(2) 126.8(2) 126.6(2)

K1−Cent2 C1−Si1 C12−Si3 C1−C2 C3−C4 C5−C1 C13−C14 C15−C16 Cent2−K2−Cent1′ Si1−C1−C2 Si2−C3−C2 Si3−C12−C13 Si4−C14−C13

2.765(3) 1.850(3) 1.847(3) 1.420(4) 1.432(4) 1.430(4) 1.409(4) 1.397(4) 156.12(8) 127.7(2) 127.8(2) 127.8(2) 127.8(2)

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EXPERIMENTAL SECTION

General Procedures. All manipulations were carried out in an atmosphere of dry argon using standard vacuum line and Schlenk tube techniques or under vacuum in a sealed all-glass apparatus. Solvents were dried from the appropriate drying agent, distilled, degassed, and stored over a potassium mirror. The NMR spectra were recorded on a DPX 300 or AM-500 Bruker spectrometer with residual solvent signals as internal reference (1H and 13C{1H}) or with an external reference (SiMe4, PbMe4, or YCl3 for 29Si, 207Pb, or 89Y, respectively). Electron impact mass spectra were taken from solid samples using a Kratos MS 80 RF instrument. Elemental analyses were determined by Medac Ltd., Brunel University. Compounds KCp″,9 NaCp″,11 LnI3(thf)3.5,8 [K(18crown-6)] 2 [C 6 H2 (SiMe 3) 4 -1,2,4,5], 12 [{LnCp″ 2 (μ-Cl)} 2 ], 1c and [LnCp″2I(thf)] (Ln = Er or Yb)32 were prepared according to literature procedures. Caution: Organomercury compounds are highly toxic. Neoprene gloves should be worn and an ef f icient hood must be used when handling these compounds. Residues should be disposed of according to local regulations for disposal of mercury waste. Synthesis of YCp″2I (2Y). KCp″ (2.11 g, 8.49 mmol) was added to a suspension of YI3(thf)3.5 (2.94 g, 4.07 mmol) in thf (80 mL), and the mixture was stirred for 2 h under reflux, cooled to room temperature, and filtered. The solvent was removed from the filtrate in a vacuum, and the residue was dried at 150 °C (10−3 Torr) for 1 h. The resulting solid was recrystallized from toluene, yielding colorless crystals of 2Y (2.25 g, 3.54 mmol, 87%) in two batches, mp 231−232 °C. EI-MS m/z (%, assignment): 634 (3, [M]+); 619 (7, [M − Me]+); 507 (100, [M − I]+). 1H NMR (C6D6): δ 7.24 (br s, 2 H, CaH), 7.21 (br s, 4 H, CcH), 0.41 (s, 36 H, SiMe3).

from 1,3-bis(trimethylsilyl)cylopentadiene and potassium hydride in thf.9 It has now been obtained in crystalline form from benzene. The unit cell of the crystalline 1,3-bis(trimethylsilyl)cyclopentadienylpotassium (8) is illustrated in Figure 6a; selected bond lengths and angles are listed in Table 6. The structure of the polymer, shown in Figure 6b, is composed of chains of alternating cations and anions along the 21 screw axes parallel to the unit cell c-axis; it is closely similar to that of [{K(C5H4SiMe3)}n] ≡ [{KCp′}n].29 Each K−Cent distance is ca. 2.76 ± 0.07 Å in length. The Cent1−K1−Cent2 chain deviates from linearity by ca. 20° at each K atom but only by 2σ(I) R1, wR2 [I > 2σ(I)] R1, wR2 [all data]

1Y

1Yb

3

5Y

C33H63Si6Y 717.28 monoclinic P21/c (No. 14) 17.6641(5) 13.6340(4) 19.1235(5) 90 112.724(1) 90 4248.1(2) 1.56 29 506 12 277, [0.054] 8337 0.054, 0.089 0.099, 0.103

C33H63Si6Yb 801.41 monoclinic P21/c (No. 14) 17.6367(2) 13.6170(2) 19.1352(3) 90 112.832(1) 90 4235.4(1) 2.40 49 634 7931, [0.063] 6327 0.030, 0.050 0.048, 0.055

C22H42PbSi4 626.11 monoclinic P21/n (No. 14) 11.1588(7) 21.3288(15) 12.5461(9) 90 92.927(3) 90 2982.1 5.82 19 110 4944, [0.077] 3637 0.066, 0.119 0.100, 0.129

C44H86O2Si8Y2 1049.67 monoclinic P21/n (No. 14) 12.0772(3) 15.7920(6) 15.5361(6) 90 100.045(2) 90 2917.7(2) 2.17 19 840 6907, [0.060] 5179 0.045, 0.081 0.074, 0.090

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Table 8. Crystal Data and Data Collection (at 173(2) K) and Refinement Details for 6, 7Yb, 7La, and 8 formula fw cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) μ (mm−1) reflns collected indep reflns, [Rint] reflns I > 2σ(I) R1, wR2 [I > 2σ(I)] R1, wR2 [all data]

6

7Yb

7La

8

C46H88Sc2Si8 955.80 monoclinic P21/n (No. 14) 11.9444(3) 16.1291(6) 15.2303(6) 90 98.549(2) 90 2901.55(17) 0.43 19 011 5057, [0.055] 4023 0.040, 0.082 0.058, 0.088

C34H66Cl2KO6Si4Yb 966.27 triclinic Pbar1 (No. 2) 11.4065(2) 11.8768(2) 19.0856(3) 72.988(1) 85.189(1) 70.963(1) 2337.03(7) 2.34 28 614 11 025, [0.032] 10 351 0.022, 0.051 0.025, 0.052

C34H66Cl2KLaO6Si4 932.14 monoclinic P21/c (No. 14) 19.025(5) 11.671(9) 21.525(5) 90 93.32(2) 90 4771.4(4) 1.23 6813 6598 [0.058] 4372 0.053, 0.099 0.100, 0.116

C11H21KSi2 248.56 orthorhombic P212121 (No. 19) 11.4794(5) 12.5636(5) 21.2707(9) 90 90 90 3067.7(2) 0.47 10 587 5303, [0.035] 4882 0.038, 0.111 0.042, 0.117

cool to room temperature; then it was filtered, and the filtrate was evaporated to dryness in a vacuum. The sticky yellow residue was dissolved in warm hexane (30 mL). Storing at −25 °C overnight produced bright lemon-yellow crystals of 1 contaminated with a white powder (later identified as [{YCp″2(μ-OH)}2] (5Y); see below). Sublimation at 165−170 °C (10−3 Torr) yielded 0.95 g (57%) of pure 1Y, mp 190−191 °C. Anal. Calcd for C33H63Si6Y (MW 717.27): C, 55.3; H, 8.85. Found: C, 54.0; H, 8.58. EI-MS m/z (%, assignment): 701 (3, [M − Me]+); 643 (2, [M − SiMe3]+); 507 (100, [M − Cp″]+). 1 H NMR (C6D6): δ 7.09 [triplet of doublets, 3 H, J4(1H−1H) = 1.98, J(1H−89Y) = 0.41 Hz, CaH], 6.88 [doublet of doublets, 6 H, J4(1H−1H) = 1.98, J(1H−89Y) = 0.75 Hz, CcH], 0.32 (s, 54 H, SiMe3). 13 C{1H} NMR (C6D6): δ 136.5 (s, Ca), 127.1 [d, J(13C−89Y) = 2.1 Hz, Cb], 125.1 [d, J(13C−89Y) = 1.5 Hz, Cc], 1.25 (s, SiMe3). 89Y NMR (C6D6/C6H6): δ −267.1 (s). Synthesis of [YbCp″3] (1Yb). Method A. To a solution of [YbCp″2I(thf)] (0.77 g, 0.98 mmol) in toluene (40 mL) was added KCp″ (0.32 g, 1.29 mmol), and the mixture was stirred at reflux temperature for 4 h and filtered while hot. Upon cooling the filtrate to room temperature, first unreacted KCp″ precipitated (spherical polycrystalline aggregates) and then YbCp″2I (large dark red block crystals). The solution was decanted and evaporated to dryness, and the residue was extracted with hexane. The extract was evaporated to dryness and sublimed at 165−170 °C (10−3 Torr), yielding 0.14 g (18%) of pure 1Yb, mp 197−199 °C. Anal. Calcd for C33H63Si6Yb (MW 801.41): C, 49.5; H, 7.92. Found: C, 48.4, H, 7.91. EI-MS m/z (%, assignment): 786 (4, [M − Me]+); 728 (2, [M − SiMe3]+); 592 (100, [M − Cp″]+). 1H NMR (C6D6): δ 52.5 (3 H), 30.4 (6 H), 25.9 (54 H). Method B. To a solution of [YbCp″2(thf)] (0.536 g, 0.81 mmol) in toluene (20 mL) was added a solution of HgCp″2 (4) (0.4 mmol, 1.8 mL of hexane stock solution, vide inf ra) at room temperature with stirring. After a few minutes precipitation of Hg metal was noticeable. After 5 h the mixture was evaporated to dryness and the residue was extracted with hexane. Crystallization at −27 °C yielded dark brown blocks of 1Yb (0.584 g, 90%). A similar reaction using [PbCp″2] (3) instead of 4 required heating at 80 °C. Synthesis of [ErCp″3] (1Er). Following the procedure of method A for 1Yb, from [ErCp″2I(thf)] (1.00 g, 1.27 mmol) and KCp″ (0.35 g, 1.40 mmol) in toluene (40 mL), a few yellow-orange crystals of 1Er were isolated (0.05 g, 5%), mp 188−192 °C, which were characterized by EI-MS m/z (%, assignment): 780 (5, [M − Me]+); 722 (7, [M − SiMe3]+); 586 (100, [M − Cp″]+). Synthesis of [PbCp″2] (3). PbI2 (2.32 g, 5.03 mmol) was stirred in 40 mL of thf for 2 h at room temperature, forming a pale yellow suspension of thf solvate. KCp″ (2.58 g, 10.4 mmol) was added to this suspension, producing a bright yellow solution with a white precipitate.

Figure 6. (a) Unit cell of [{K{C5H3(SiMe3)2-1,3}∞], 8 (30% ellipsoids, H atoms omitted). Symmetry transformations to generate equivalent atoms: ′ −x+1/2, −y+1, z−1/2; ″ −I+1/2, −y+1, z+1/2. (b) Alternating cations and anions in [{K{C5H3(SiMe3)2-1,3}∞], 8. Synthesis of [YCp″3] (1Y). YCp″2I (1.48 g, 2.33 mmol) and KCp″ (0.67 g, 2.70 mmol) were dissolved with heating in toluene (50 mL). The mixture was stirred at reflux temperature for 4 h and allowed to 2688

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vacuum to ∼3 mL; storing in a refrigerator overnight resulted in the formation of a significant amount of large colorless plates of C6H2(SiMe3)4-1,2,4,5 and several small, very dark brown blocks of [(ScCp″2)2(μ-C2H4)] (6) identified by X-ray crystallography. Attempted Reduction of [{LaCp″2(μ-Cl)}2]. A solution of [{LaCp″2(μ-Cl)}2] (0.438 g, 0.37 mmol) in benzene (20 mL) was added to a stirred suspension of [K(18-crown-6)]2[C6H2(SiMe3)41,2,4,5] (0.368 g, 0.38 mmol) in benzene (20 mL), producing a dark red solution. The solution was filtered, and the filtrate was concentrated in a vacuum to 10 mL; storing in a refrigerator overnight resulted in formation of dark red crystals of [LaCp″2(μ-C6H6)K(18crown-6)].33 The mother liquor was further concentrated, layered with hexane, and stored at 10 °C for three days, yielding colorless crystals of [LaCp″2Cl(μ-Cl)K(18-crown-6)] (7La) identified by X-ray crystallography. Attempted Reduction of [{YbCp″2(μ-Cl)}2]. A solution of [{YbCp″2(μ-Cl)}2] (0.264 g, 0.21 mmol) and 18-crown-6 (0.111 g, 0.42 mmol) in toluene (40 mL) was added to a Schlenk vessel containing a potassium mirror (0.017 g, 0.42 mmol). After stirring for 2 days at room temperature the potassium disappeared almost completely and a brown suspension was formed. The mixture was filtered, and the filtrate was concentrated, layered with hexane, and stored at −27 °C for 1 week, yielding orange crystals of [YbCp″2Cl(μCl)K(18-crown-6)] (7Yb) (0.175 g, 43%). 1H NMR (C6D6): δ 10.96 (s, 36 H, SiMe3), 2.56 (s, 24 H, 18-crown-6), −15.0 (br s, 4 H, CcH), −98.0 (br s, 2 H, CaH). Attempted Reduction of [YCp″3] (1Y). A solution of 1Y (0.240 g, 0.34 mmol) and 18-crown-6 (0.089 g, 0.34 mmol) in benzene (20 mL) was added to a Schlenk vessel containing a potassium mirror (0.014 g, 0.35 mmol). Immediately a red-brown color appeared in the solution, but it was decolorized quickly when the stirring was turned off (dark brown microcrystals precipitating on the K-surface). After stirring for 6 h at room temperature, potassium disappeared almost completely and a light yellow solution was formed. The solution was concentrated in a vacuum to a small volume (ca. 3 mL), resulting in formation of a colorless crystalline precipitate, which was washed with hexane and dried in a vacuum. 1H NMR (C6D6) showed the presence of KCp″(18-crown-6)32 and {YCp″2(μ-H)}2 having four nonequivalent Cp″ ligands: δ 7.64 (br t, 1 H, CaH), 7.62 (br d, 2 H, CcH), 7.41 (br t, 1 H, CaH), 7.24 (br t, 1 H, CaH), 7.10 (br t, 1 H, CaH), 7.05 (br d, 2 H, CcH), 7.03 (br d, 2 H, CcH), 6.99 (br d, 2 H, CcH), 3.25 [t, J(1H−89Y) = 32.2 Hz, 2 H, μ-H], 0.41, 0.39, 0.38, and 0.35 (four singlets, 72 H, SiMe3). X-ray Data Collection, Structure Solution, and Refinement. Diffraction data for 1Y, 1Yb, 3, 5Y, 6, 7Yb, and 8 were collected on a Nonius Kappa CCD diffractometer, and those for 7La on a Nonius CAD4 diffractometer, using monochromated Mo Kα radiation, λ 0.71073 Å at 173(2) K. Crystals were coated in oil and then directly mounted on the diffractometer under a stream of cold nitrogen gas. The structures were refined on all F2 using SHELXL-97.34 Absorption corrections were applied using MULTISCAN. Drawings were made using ORTEP-3 for Windows. Additional details below:

The solution was filtered, thf was removed from the filtrate in a vacuum, and the residue was extracted with hexane. Removing the solvent gave the yellow, sticky, crystalline product of ca. 90% purity (1H and 207Pb NMR). Vacuum distillation at 90−100 °C (10−3 Torr) gave analytically pure 3 (2.33 g, 74%), mp 52−54 °C. Anal. Calcd for C22H42PbSi4 (MW 626.11): C, 42.20; H, 6.76. Found: C, 42.16, H, 6.76. EI-MS m/z (%, assignment): 611 (34, [M − Me]+), 417 (93, [M − Cp″]+). 1H NMR (C6D6): δ 6.40 [t, J(1H−1H) = 1.8 Hz, with 207 Pb satellites, J(1H−207Pb) = 22.0 Hz, 2 H, CaH], 6.20 [d, J(1H−1H) = 1.8 Hz, with 207Pb satellites, J(207Pb−1H) = 29.3 Hz, 4 H, CcH], 0.24 [s, with 207Pb satellites, J(207Pb−1H) = 3.3 Hz, 36 H, SiMe3]. 13C{1H} NMR (C6D6): δ 126.20 [s, with 207Pb satellites, J(13C−207Pb) = 68.9 Hz, Cb], 124.40 [s, with 207Pb satellites, J(13C−207Pb) = 61.6 Hz, CaH], 120.63 [s, with 207Pb satellites, J(13C−207Pb) = 46.9 Hz, CcH], 1.07 [s, with 207Pb satellites, J(13C−207Pb) = 50.5 Hz, SiMe3]. 29Si{1H} NMR (C6D6): δ −11.2 [s, with 207Pb satellites, J(29Si−207Pb) = 13.4 Hz, SiMe3]. 207Pb{1H} NMR (C6D6): δ −4957.7 (s). X-ray quality crystals were obtained when the concentrated pentane solution of 3 was stored at −27 °C for 1 week. Synthesis of HgCp″2 (4). Solid HgCl2 (0.67 g, 2.47 mmol) was stirred with 1.27 g (5.46 mmol) of NaCp″ in Et2O (20 mL) overnight. The solvent was removed in a vacuum, and the residue was extracted with hexane. Removing the solvent gave the colorless liquid compound 4 of ca. 90% purity (1H NMR). Yield: 1.61 g (95%). 1H NMR (C6D6): δ 6.66 [d, J(1H−1H) = 1.0 Hz, 4 H, CcH], 6.36 [t, J(1H−1H) = 1.0 Hz, 2 H, CaH], 0.14 (s, 36 H, SiMe3). For the following reactions 4 was dissolved in 10 mL of hexane and stored in a refrigerator. Synthesis of [{YCp″2(μ-OH)}2] (5Y). Compound 5Y was first isolated as a byproduct in the synthesis of 1Y. Alternatively, a few drops of wet benzene (prepared by shaking pure benzene with water under argon) were added slowly to a solution of YCp″3 (0.281 g, 0.39 mmol) in benzene (10 mL) until the color changed from yellow to nearly colorless. The solution was concentrated in a vacuum to ca. 5 mL and stored at 10 °C for 2 days, yielding colorless crystals of 5Y (0.161 g, 82%), mp 261−261 °C. Anal. Calcd for C44H86O2Si8Y2 (MW 1049.65): C, 50.35; H, 8.26. Found: C, 49.75, H, 8.14. EI-MS m/z (%, assignment): 1033 (0.1, [M − Me]+); 839 (5, [M − Cp″]+); 629 (3, [M − Cp″ − Cp″H]+). 1H NMR (C6D6): δ 7.24 (br t, 2 H, CaH), 7.09 (br t, 2 H, CaH), 7.02 (br d, 4 H, CcH), 6.98 (br d, 4 H, CcH), 1.49 (s, 2 H, OH), 0.41 (s, 36 H, SiMe3), 0.38 (s, 36 H, SiMe3). Synthesis of ScCp″2I (2Sc). ScI3(thf)3 (2.00 g, 3.11 mmol) was added to a solution of NaCp″ (1.45 g, 6.24 mmol) in thf (20 mL), and the mixture was stirred at 50 °C for 3 h. Then the solvent was removed under vacuum, and the residue was dried at 100 °C for 1 h. The residue was extracted with toluene (2 × 10 mL). The extract was evaporated in a vacuum, producing the mono-thf solvate ScCp″2I(thf) as nearly colorless crystals [1H NMR δ 7.48 (t, 1.8 Hz, 2 H, CaH), 6.90 (d, 1.8 Hz, 4 H, CcH), 3.59 (m, 4 H, thf), 1.28 (m, 4 H, thf), 0.28 (s, 36 H, SiMe3); 13C NMR δ 137.8 (Ca), 129.4 (Cb), 123.8 (Cc), 71.2 (thf), 25.3 (thf), 0.66 (SiMe3)], which was transferred into a sublimation tube and heated to 125−130 °C (10−3 Torr). At this temperature it was first melted, releasing coordinated solvent, then distilled and crystallized in the cold part of the tube as solvent-free yellow crystals of 2Sc (1.523 g, 83%), mp 92−93 °C. Anal. Calcd for C22H42IScSi4 (MW 590.77): C, 44.7; H, 7.17. Found: C, 44.0, H, 7.15. EI-MS m/z (%, assignment): 590 (2, [M]+), 575 (23, [M − Me]+), 463 (100, [M − I]+). 1H NMR (C6D6): δ 7.84 [t, J(1H−1H) = 1.9 Hz, 2 H, CaH], 7.09 [d, J(1H−1H) = 1.9 Hz, 4 H, CcH], 0.22 (s, 36 H, SiMe3). 13C{1H} NMR: δ 138.8 (Ca), 130.0 (Cb), 125.5 (Cc), 1.41 (SiMe3). Attempted Reduction of ScCp″2I (2Sc). A solution of 2Sc (0.835 g, 1.41 mmol) in benzene (20 mL) was added to a stirred suspension of [K(18-crown-6)]2[C6H2(SiMe3)4-1,2,4,5] (0.686 g, 0.70 mmol) in benzene (10 mL), producing a dark brown solution. Colorless needles [tentatively assigned as KI(18-crown-6)] precipitated overnight. The solution was decanted via a cannula and evaporated in a vacuum, leaving a dark brown oil. Hexane (30 mL) was added, and the mixture was stored in a refrigerator (10 °C) for two weeks, forming a dark brown solution and a light-colored noncrystalline solid. The solution was decanted and concentrated in a

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[(ScCp″2)2(μ-C2H4)] (6): The hydrogen atoms on the bridging ligand were located on a difference map and freely refined. [LaCp″2Cl(μ-Cl)K(18-crown-6)] (7La): The diffraction from the crystal was weak and limited.

ASSOCIATED CONTENT

S Supporting Information *

CIF file containing X-ray crystal structural data for 1Y, 1Yb, 3, 5Y, 6, 7Yb, 7La, and 8. This material is available free of charge via the Internet at http://pubs.acs.org. 2689

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(22) Gun’ko, Yu. K.; Hitchcock, P. B.; Lappert, M. F. Organometallics 2000, 19, 2832. (23) Shapiro, P. J.; Cotter, W. D.; Schaefer, W. P.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 1994, 116, 4623. (24) Dubé, T.; Gambarotta, S.; Yap, G. P. A. Angew. Chem., Int. Ed. 1999, 38, 1437. (25) Cotton, F. A.; Kibala, P. A. Inorg. Chem. 1990, 29, 3192. (26) Watson, P. L.; Whitney, J. F.; Harlow, R. L. Inorg. Chem. 1981, 20, 3271. (27) Gosink, H.-J.; Nief, F.; Riant, P.; Ricard, L.; Mathey, F. Inorg. Chem. 1995, 34, 1306. (28) Jaroschik, F.; Nief, F.; Le Goff, X.-F.; Ricard, L. Organometallics 2007, 26, 1123. (29) Jutzi, P.; Leffers, W.; Hampel, B.; Pohl, S.; Saack, W. Angew. Chem., Int. Ed. 1987, 26, 583. (30) Lorberth, J.; Shin, S.-H.; Wocadlo, S.; Massa, W. Angew. Chem., Int. Ed. 1989, 28, 735. (31) Rabe, G.; Roesky, H. W.; Stalke, D.; Pauer, F.; Sheldrick, G. M. J. Organomet. Chem. 1991, 403, 11. (32) Xie, Z.; Liu, Z.; Xue, F.; Zhang, Z.; Mak, T. C.W. J. Organomet. Chem. 1997, 542, 285. (33) Cassani, M. C.; Gun’ko, Yu. K.; Hitchcock, P. B.; Lappert, M. F.; Laschi, F. Organometallics 1999, 18, 5539. (34) Sheldrick, G. M. SHELXL-97; University of Gö ttingen: Göttingen, Germany, 1997.

AUTHOR INFORMATION Corresponding Author *Tel: +44-1273-678316. Fax: +44-1273-677196. E-mail: [email protected].

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ACKNOWLEDGMENTS We thank EPSRC for the financial support and Dr. S. K. Ibrahim and Prof. C. J. Pickett (UEA) for conducting electrochemical measurements.

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DEDICATION Dedicated to the memory of Gordon Stone, a good friend of M.F.L.’s for 50 years.

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REFERENCES

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dx.doi.org/10.1021/om2009364 | Organometallics 2012, 31, 2682−2690