Open-Shell Lanthanide(II+) or -(III+) Complexes Bearing σ-Silyl and Silylene Ligands: Synthesis, Structure, and Bonding Analysis

Complexes featuring lanthanide (Ln)–Si bonds represent a highly neglected research area. Herein, we report a series of open-shell LnII+ and LnIII+ complexes bearing σ-bonded silyl and base-stabilized N-heterocyclic silylene (NHSi) ligands. The reactions of the LnIII+ complexes Cp3Ln (Ln = Tm, Ho, Tb, Gd; Cp = cyclopentadienide) with the 18-crown-6 (18-cr-6)-stabilized 1,4-oligosilanyl dianion [(18-cr-6)KSi(SiMe3)2SiMe2SiMe2Si(SiMe3)2K(18-cr-6)] (1) selectively afford the corresponding metallacyclopentasilane salts [Cp2Ln({Si(SiMe3)2SiMe2}2)]−[K2(18-cr-6)2Cp]+ [Ln = Tm (2a), Ho (2b), Tb (2c), Gd (2d)]. Complexes 2a–2d represent the first examples of structurally characterized Tm, Ho, Tb, and Gd complexes featuring Ln–Si bonds. Strikingly, the analogous reaction of 1 with the lighter element analogue Cp3Ce affords the acyclic product [Cp3CeSi(SiMe3)2SiMe2SiMe2Si(SiMe3)2-Cp3Ce]2–2[K(18-cr-6)]+ (3) as the first example of a complex featuring a Ce–Si bond. In an alternative synthetic approach, the aryloxy-functionalized benzamidinato NHSi ligand Si(OC6H4-2-tBu){(NtBu)2CPh} (4a) and the alkoxy analogue Si(OtBu){(NtBu)2CPh} (4b) were reacted with Cp*2Sm(OEt2), affording, by OEt2 elimination, the corresponding silylene complexes, both featuring SmII+ centers: Cp*2Sm ← :Si(O–C6H4-2-tBu){(NtBu)2CPh} (6) and Cp*2Sm ← :Si(OtBu){(NtBu)2CPh} (5). Complexes 5 and 6 are the first four-coordinate silylene complexes of any f-block element to date. All complexes were fully characterized by spectroscopic means and by single-crystal X-ray diffraction analysis. In the series 2a–2d, a linear correlation was observed between the Ln–Si bond lengths and the covalent radii of the corresponding Ln metals. Moreover, in complexes 5 and 6, notably long Sm–Si bonds are observed, in accordance with a donor–acceptor interaction between Si and Sm [5, 3.4396(15) Å; 6, 3.3142(18) Å]. Density functional theory calculations were carried out for complexes 2a–2d, 5, and 6 to elucidate the bonding situation between the LnII+ or LnIII+ centers and Si. In particular, a decrease in the Mayer bond order (MBO) of the Ln–Si bond is observed in the series 2a–2d in moving from the lighter to the heavier lanthanides (Tm = 0.53, Ho = 0.62, Tb = 0.65, and Gd = 0.75), which might indicate decreasing covalency in the Ln–Si bond. In accordance with the long bond lengths observed experimentally in complexes 5 and 6, comparatively low MBOs were determined for both silylene complexes (5, 0.24; 6, 0.25) .


■ INTRODUCTION
Compounds featuring lanthanide (Ln)−Si bonds are somewhat rare in contemporary literature, 1−3 further highlighted by the fact that, for the elements Ce, Tb, and also Pm, no examples exist. Sm and the late lanthanide metals Yb and Lu are the beststudied elements for this class of compounds, with several reported examples. Arguably, the most straightforward synthesis of lanthanide silyl compounds was described in the seminal work of Bochkarev and co-workers, 4 who obtained neutral (Ph 3 Si) 2 Yb II (THF) 4 directly from elemental Yb and Ph 3 SiCl in tetrahydrofuran (THF; eq 1). (1) Utilizing a common method for the preparation of earlytransition-metal complexes, Schumann and co-workers carried out salt elimination reactions of rare-earth halide complexes with Me 3 SiLi. Reactions with Cp 2 Ln(μ-Cl) 2 Na in 1,2dimethoxyethane (DME) led to the respective ate complexes of the type [Li(DME) 3 ][Cp 2 Ln(SiMe 3 ) 2 ] for Ln = Sm, 5,6 Lu, 5−7 Dy, 7 Ho, 7 Er, 7 and Tm 7 (eq 2). More recently, Sgro and Piers reported the synthesis of gadolinium silyl complexes by reacting a potassium silanide with GdI 3 In close analogy, Tilley and co-workers obtained the neutral Sc complexes Cp 2 Sc(SiR 3 )(THF) from [Cp 2 ScCl] 2 and a series of larger lithium silanides. 9 In a related reaction, Lawless and co-workers found that conversion of [Cp* 2 Yb(OEt 2 )] with LiSi(SiMe 3 ) 3 in THF afforded the ytterbium silyl [Cp*Yb II (Si-(SiMe 3 ) 3 )(THF) 2 ] with concomitant elimination of LiCp*. 10 Other examples of Cp 2 Ln II silyl complexes were obtained, with the likely involvement of silyl anions from the reaction of Cp* 2 Ln(THF) 2 (Ln = Sm, Eu, Yb) with PhSiH 3 /KH, which yielded K[Cp* 2 Ln II (SiH 3 )(THF)]. 11 In the latter complex, the K ion coordinates to two Cp* ligands of different molecules, one THF moiety, and even to the SiH 3 ligand, forming a twodimensional-layered structure in the solid state. 12 The Cp*K unit thus serves as a neutral stabilization ligand for Ln II complexes otherwise intractable and difficult to isolate.
Alternatively, a number of rare-earth silyl compounds have been obtained by σ-bond metathesis routes in reactions of rareearth alkyl complexes with hydrosilanes. Reactions of Cp* 2 LnCH(SiMe 3 ) 2 with neat H 2 Si(SiMe 3 ) 2 gave neutral Cp* 2 LnSiH(SiMe 3 ) 2 (Ln = Sm, Nd, Y), 13−15 described as extremely air-and moisture-sensitive but thermally stable in solution and in the solid state. The use of a high excess of silane was found necessary for suppression of thermal decomposition of Cp* 2 LnCH(SiMe 3 ) 2 . When Cp* 2 SmCH(SiMe 3 ) 2 was treated with PhSiH 3 instead of a bulky hydrosilane (in benzene), a number of interesting cluster compounds were obtained, all of which included three Cp* 2 Sm units and different silyl ligands (bridging SiH 2 and SiH 3 groups) bearing no phenyl groups. 15,16 The same reaction in pentane led to trimeric Cp* 2 SmSiH 3 , which was found to be an intermediate in the cluster formation. 17,18 Reaction of o-MeOC 6 H 4 SiH 3 with [Cp*Lu(μ-H)] 2 led to the neutral lutetium silyl complex Cp*LuSiH 2 (o-MeOC 6 H 4 ). 19 Reactions of the thermally stable N-heterocyclic silylene (NHSi) Si[{N(CH 2 tBu)} 2 C 6 H 4 ] (Si[NN]) and Cp 3 Ln (Ln = Y, Yb) led to the first group 3 (Y) and lanthanide (Yb) metal silylene complexes Cp 3 LnSi[NN] 20 (eq 3). Under similar reaction conditions, Cp 3 La turned out to be unreactive. Evans and co-workers reported the reaction of Cp* 2 Sm with West's stable silylene, and a Sm II −Si II complex was obtained. 21 That the nature of the interaction between Sm and Si is likely to be of the donor−acceptor type was shown by the fact that the silylene ligand could easily be displaced by THF, but no further studies were carried out in this direction, nor was any detailed bonding analysis carried out to date. Inspired by this seminal work and as an extension of our recent studies on the chemistry of disilylated group 4 metallocenes 22,23 with the oxidation state 3+, we decided to explore this neglected area of Ln chemistry, with a view of systematically studying the nature of the La−Si bond for both Ln II+ and Ln III+ complexes. Herein we report a novel and facile synthetic route to a range of σ-bonded lanthanide silyl complexes in the III+ oxidation state, as well as novel samarium(II+) silylene complexes. A rigorous investigation into their spectroscopic properties and structures by X-ray diffraction (XRD) studies and the first systematic density functional theory (DFT) investigation for this class of complexes are reported.

■ RESULTS AND DISCUSSION
Synthesis. Reactions of the readily accessible metallocenes (Cp 3 Ln) of the elements Gd, Tb, Ho, and Tm with the 1,4oligosilanyl dianion 1 24,25 afford cleanly the corresponding metallacyclopentasilane ate complexes (2a−2d) with the Ln metals in the oxidation state 3+ together with [{K(18-cr-6)} 2 Cp] + (where 18-cr-6 = 18-crown-6) as the counterion (Scheme 1). The ease of access to these complexes prompted us to explore the reaction with (Cp 3 Ce), a lighter analogue of the heavier lanthanides. Contrary to the other used metallocenes (Cp 3 Ln), Cp 3 Ce is not commercially available but can readily be synthesized. 26 Reaction of dianion 1 with Cp 3 Ce did not lead to a five-membered ring but to the acyclic compound 3 (Scheme 1), which to the best of our knowledge is the first example of a compound featuring a Si−Ce bond. Each Ce III atom retains all of its three Cp ligands while forming an additional Ce−Si bond. One of the Cp ligands on each Ce coordinates to the cationic moiety K(18-cr-6)·THF. All complexes 2a−2d and 3 are thermally stable but extremely sensitive to moisture and air, as expected for organometallic Ln complexes.
In addition to these lanthanide silyl complexes, we are also interested in the synthesis of base-stabilized silylene lanthanide metal complexes because instead of featuring a σ-bonded silyl group they would rather exhibit coordinative (donor−acceptor) bonds between Si and the Ln element, which would be of interest as a comparison. As a starting point, we decided to investigate the reactivity of Cp* 2 Sm(OEt 2 ), which is readily available, 27 toward NHSi ligands. Given the robust coordination of Roesky's chlorosilylene Si(Cl)[(NtBu) 2 CPh] (4) toward a range of transition metals, 28 we first explored its reaction with Cp* 2 Sm(OEt 2 ). Reaction of Cp* 2 Sm(OEt 2 ) with silylene 4, 29,30 in fact, produced an inseparable mixture of the "interconnected" silylene(I) dimer LSi−SiL 31 and the Sm III complex Cp* 2 SmCl rather than the desired coordination complex, based on multinuclear NMR spectroscopy and comparison to authentic samples of both complexes. Clearly, redox chemistry dominates this reaction and precludes the isolation of the desired NHSi complex. To circumvent this problem, 4 was functionalized by replacement of the Cl atom at Si by an aryloxy (4a), 32 or alkoxy group (4b), 33 reasoning that the increased bond strength of Si−O versus Si−Cl could suppress this redox chemistry and thus facilitate formation of the desired NHSi coordination complex. This was indeed the case, and the silylene complexes 5 and 6 could be isolated in moderate-to-good yields (Scheme 2). Complexes 5 and 6 represent the first examples of isolated four-coordinate silylene complexes of any f-block element. The dinuclear mixed-valent complex 7 (Scheme 3), featuring Sm II and Sm III bridged by a μ-iodide ligand, was coincidentally isolated as a minor byproduct of the supernatant solution from the reaction mixture of complex 5. Presumably, its formation stems from trace amounts of Cp* 2 SmI, present in the starting material Cp* 2 Sm(OEt 2 ), upon reaction with 5. Unfortunately, only a few crystals were obtained, which precluded further spectroscopic characterization, but nevertheless it is an interesting complex from a structural point of view.

Inorganic Chemistry
Complexes 5 and 6 are paramagnetic, with magnetic moments μ eff = 2.7 μ B (5) and 2.6 μ B (6) (Evans' method). 34 This is dramatically reduced upon comparison to the Sm II starting material Cp* 2 Sm(OEt 2 ) (μ eff = 3.6 μ B ) possibly because of the NHSi ligands affecting spin−orbit coupling in the Sm center. This suggests that the NHSi ligand influences the magnetic properties of the emerging Sm II complexes considerably, which is likely attributable to the different nature of the Si: → coordination versus the O: → coordination of Et 2 O in Cp* 2 Sm(OEt 2 ).
NMR Spectroscopy. NMR spectroscopic characterization of open-shell Ln complexes is somewhat challenging, particularly because of spin−orbit coupling phenomena. These properties have been exploited in Ln shift reagents. 35,36 Diamagnetic complexes, however, also exist and possess either f 0 (La III and Ce IV ) or f 14 (Lu III and Yb II ) electron configurations.
Nevertheless, even for the respective lanthanide metallocene complexes, investigations started early on, 37 and currently chemical shifts for a substantial number of lanthanide metallocene complexes are known. It is important to note that while there are certainly trends governing the chemical shift behavior, their interpretation is not straightforward. The chemical shifts of protons attached directly to cyclopentadienyl ligands are frequently dramatically different from the shifts observed for methyl groups on cyclopentadienyl ligands (see below).
A short survey on the existing NMR characterization data for Cp 2  Cl) 2 Na·DME (Ln = Sm, Gd, Tb, Dy, Ho, Er, Tm, Lu) versus [Li·DME n ]Cp 2 Ln(SiMe 3 ) 2 (Ln = Sm, Dy, Ho, Er, Tm, Lu). 7 Inspection of the chemical shifts in this series reveals that, in particular, the Cp−H resonances of Tb III and Ho III are strongly downfield-shifted to values close to 160 ppm, whereas the respective signals of Er III and Tm III are considerably upfieldshifted to values of around −60 ppm. Extremely shifted resonances of the SiMe 3 groups were found for Ho III and Er III . The sign of the magnetically induced shift of the SiMe 3 resonance is, however, reversed compared to that of the Cp resonance. 7 For compounds 2a−2d, we recorded 1 H NMR spectra (Table 1). It is important to consider the fact that not only are NMR spectra of Ln compounds paramagnetically shifted and generally exhibit line-broadening, but chemical shifts are frequently also concentration-and temperature-dependent. 51 Nuclei with reduced natural abundance are therefore frequently difficult to detect. Nevertheless, for compounds 2b−2d, it was possible to measure two-dimensional 1 H− 13 C and 1 H− 29 Si correlation NMR spectra at room temperature, which was indispensable for the assignments of the signals.
It is instructive to compare the 1 H chemical shifts recorded for 2a−2d to those observed for the complexes of Schumann.  2 Ho-(SiMe 3 ) 2 ] − was described at very low field (164.2 ppm), we observed the analogous resonance of 2b at −22.4 ppm. It should, however, be noted that the number of NMR spectroscopically characterized Cp n Ho III compounds is rather limited, 7,37,45 and therefore it is difficult to assess the observed chemical shift values. Compared to the Cp resonances, the SiMe signals are found to exhibit comparably normal chemical shifts. Considering the fact that the shift caused by the paramagnetic atoms decreases with 1/r 3 , it can be expected that more remote atoms experience a much smaller shifting effect. Only for complex 2a, which also exhibits a very strong effect on the CpH resonances (−48.3 ppm), also the SiMe 3 resonances are severely shifted to 16.7 ppm. With respect to the 29 Si NMR data, we were able to obtain values for compounds 2b−2d ( Table 2). The values of 2b and 2c are quite similar, which is consistent with Schumann's proton NMR study where Ho and Tb showed similar shift behavior. The values for the SiMe 3 resonance of the Gd compound 2d are also in line with those observed for 2b and 2c. The resonance detected for the SiMe 2 units is, however, clearly different.
Strikingly, for the silylene complexes 5 and 6 both featuring Sm II+ centers, in the respective 1 H NMR spectra, most of the signals are observed in the diamagnetic spectral window, at odds with the complexes discussed above. Noteworthy, however, are dramatic shifts of the aryl protons in both complexes, to both high-and low-field regions, because of the paramagnetic influence of the Sm II center (Figure 1). Some of the signals are also somewhat line-broadened. The 13 C NMR spectra for both complexes also reveal most of the signals in the expected diamagnetic spectral range, some again somewhat line-broaded, although the resonance signals corresponding to the ring atoms of the coordinated Cp* ligand could not be observed in both complexes, akin to the Cp complexes described above.
Two-dimensional HSQC and HMBC correlation experiments were also carried out for complexes 5 and 6 for an unambiguous assignment of the 13 C NMR signals. Because of the paramagnetic nature of the complexes, although cross peaks were observed in alignment with the signals in the 1 H NMR spectrum, these did not correspond to any observable signals in the 13 C NMR spectra, in both cases. This suggests that the routine pulse sequence for these traditional two-dimensional correlation experiments is not appropriate for the Sm II complexes, although with the Ln III complexes described above, it was possible. Moreover, despite extremely long (ca. 72 h) acquisition periods, at high concentrations, no signals could be observed in the 29 Si NMR spectra because of the paramagnetic influence of the Sm II center.
In all four structures, the ion pairs are separated. The countercation (18-cr-6)K-Cp-K(18-cr-6) shows disorder in one crown ether for 2b and in both crown ethers for 2a and 2d, a fact that is reflected by the number of restraints used. The Ce complex 3 (Figure 3) crystallized in the triclinic space group P1̅ with an additional toluene molecule in the asymmetric unit.   Because 2a−2d and 3 are the first structurally characterized complexes bearing a Ce−Si, Tm−Si, Ho−Si, Tb−Si, or Gd−Si bond, no data for comparison are available. The most striking features of these molecular structures are the values for the lanthanide−silyl bond lengths, which are in good agreement with the sum of the covalent radii (Figure 4). 62 The distances between Ln and Si decrease incrementally with decreasing metal size, as expected. This indicates that the Ln−Si bond is rather polar but might possess at least some covalent character. 63 The bond angle Si−Ln−Si varies only slightly from 94.15°in 2a (Tm) to 94.68°in 2b (Ho) to 95.47°in 2c (Tb) to 94.84°in 2d (Gd).
DFT Calculations. The electronic structures of synthesized complexes (2a−2d, 5, and 6) at the B3PW91/Basis1 level of theory were calculated. According to these calculations, the Tm−Si bond distances (2.958 and 2.971 Å) in the optimized geometry of 2a show perfect agreement with the experimentally characterized distances (2.966 and 2.980 Å, respectively). Natural bond orbital (NBO) analysis suggests that the negative charge of 2a is mainly localized on the Si atoms (−0.25) connected to the Tm center (Table 3), whereas other Si atoms show large positive natural population between 0.3+ and 0.55+.
In addition, NBO analysis shows lone pairs on the Si atoms. Interestingly, the shape of the highest occupied molecular orbital (HOMO) and HOMO−1 exhibits some covalent and ionic character bonding as well. The HOMO (−0.54 eV) depicted in Figure 8 shows a lone pair on the Si center, which can be interpreted in light of the negative charge of 2a as a bond with strongly polarized or even ionic character where the Si bears the two electrons. In contrast to that, HOMO−1 (−0.86 eV) denotes more covalent character because the orbital is extended toward the Tm center as well. This also agrees with the Mayer bond order (MBOs) that supports the picture of weak, very polarized bonds (0.53).
The analogous Gd complex, 2d, features a half-filled f shell (S = 7 / 2 ), and the Gd−Si bond distances (3.025 and 3.030 Å) in     (Table  3) are between the discussed theoretical results of 2a and 2d; therefore, the calculated parameters follow the experimentally characterized series of bond lengths, providing further evidence of the partial covalent character of the Ln−Si bonds as HOMO and HOMO−1 also suggest (Figures 9−11). The calculated data in the series of 2a−2d might also indicate, in agreement with the previously suggested relationship of bond lengths, some covalency in the bonding character ( Figure 4). 39 Samarium silylene complexes (5 and 6) have triplet ground states in accordance with the Evans magnetometric measurements. The calculated structures are in moderate agreement with the experimentally characterized geometries; the calculated Sm−Si distances are somewhat shorter (3.411 and 3.278 Å, respectively; Table 3) than the experimental values (3.440 and 3.315 Å, respectively). In spite of the shorter Sm−Si distance, none of the applied methods suggests considerable covalent character of the Sm−Si bond, which is in good agreement with the relatively large bond distances. NBO analysis displays a lone pair on the Si centers because the HOMOs of 5 and 6 exhibit the same (Figure 12 ■ EXPERIMENTAL SECTION General Remarks. All reactions involving air-sensitive compounds were carried out under an atmosphere of dry nitrogen or argon using either Schlenk techniques or a glovebox. All solvents were dried using a column-based solvent purification system. 67 Chemicals were obtained from different suppliers and used without further purification. Silyl dianion 1, 24,25 Cp 3 Ce, 26 Cp* 2 Sm(OEt 2 ), 27 and ligands 4a 32 and 4b 33 were prepared following reported procedures.
1 H (300 MHz), 13 C (75.4 MHz), and 29 Si (59.3 MHz) NMR spectra were recorded on a Varian INOVA 300 spectrometer or on Bruker spectrometers (AV400 or AV200). To compensate for the low isotopic abundance of 29 Si, the INEPT pulse sequence was used where possible for amplification of the signal. 68,69 To obtain reliable 1 H, 13 C, and 29 Si NMR shifts, samples with tetramethylsilane (TMS) added were used to obtain a reference point. 1 H Cp signals could only be observed with more concentrated samples that were referenced using the shifts obtained from the dilute spectra. gHMBC 1 H− 29 Si experiments (without TMS added) were carried out to determine all 29 Si NMR shifts. To rule out measurement of the most plausible side product 1,1,2,2-tetrakis(trimethylsilyl)tetramethylcyclotetrasilane, which might resonate at different chemical shifts because of paramagnetic interaction, several of the gHMBC 1 H− 29 Si experiments were repeated with the deliberate addition of 1,1,2,2-tetrakis-(trimethylsilyl)tetramethylcyclotetrasilane, exhibiting the expected additional set of signals.
High-resolution (HR) electrospray ionization mass spectrometry (ESI-MS) spectra were recorded on an Orbitrap LTQ XL of a Thermo Scientific mass spectrometer and the raw data evaluated using the Xcalibur computer program. UV spectra were measured on a PerkinElmer Lambda 35 spectrometer using spectroscopic grade pentane as the solvent.
X-ray Structure Determination. For X-ray structure analyses, the crystals were mounted onto the tip of glass fibers. Data collection was performed with a Bruker-AXS SMART APEX CCD diffractometer using graphite-monochromated Mo Kα radiation (0.71073 Å) for 2a− 2d and 3 and on an Agilent Technologies SuperNova (single source, Cu Kα radiation, λ = 1.5418 Å) for 5−7. The data were reduced to F o 2 and corrected for absorption effects with SAINT 70 and SADABS, 71,72 respectively. The structures were solved by direct methods and refined   by a full-matrix least-squares method (SHELXL97). 73 If not noted otherwise, all non-H atoms were refined with anisotropic displacement parameters. All H atoms were located in calculated positions to correspond to standard bond lengths and angles. All diagrams were drawn with 30% probability thermal ellipsoids, and all H atoms were omitted for clarity. Crystallographic data (excluding structure factors) for the structures of compounds 2a−2d, 3, and 5−7 reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publications CCDC 1024428 (2a), 1024431 (2b), 1024432 (2c), 1024430 (2d), 1024429 (3), 1024433 (5), 1024434 (6), and 1024435 (7). Copies of these data can be obtained free of charge at http://www.ccdc.cam.ac.uk/products/csd/request/.
Cp* 2 Sm ← :Si(OtBu){(NtBu) 2 CPh} (5). A Schlenk tube was charged with Cp* 2 Sm(OEt 2 ) (0.173 g, 0.351 mmol) and ligand 4b (0.117 g, 0.351 mmol). A total of 10 mL of degassed toluene was recondensed onto this solid mixture by submersion thereof in a liquid-nitrogen cold trap, under a static vacuum. After the recondensation was complete, the solvent was degassed once more upon thawing and allowed to slowly warm to room temperature with rapid stirring. Upon warming to room temperature, the reaction solution had a dark-green appearance. After stirring for 3 h at room temperature, an in situ 1 H NMR spectrum of the reaction solution revealed the selective formation of a new product. The reaction solution was concentrated in vacuo under reduced pressure to ca. 2 mL and cooled to −30°C for 1 h to complete the precipitation of the product. The light-green supernatant was removed by cannula filtration at −50°C, and the remaining emerald-green solid was dried in vacuo for 1 h at a pressure of 3 × 10 −2 mbar. (Crystals of the dinuclear complex 7 were obtained from the light-green supernatant solution as a trace byproduct and structurally characterized by single-crystal XRD analysis.) The product was afforded as 5·toluene (0.189 g, 71%  13 (6). A procedure similar to that in the preparation of complex 7 was employed: A Schlenk tube was charged with Cp* 2 Sm(OEt 2 ) (0.181 g, 0.367 mmol) and ligand 4a (0.150 g, 0.367 mmol). A total of 10 mL of degassed toluene was recondensed onto this solid mixture by submersion thereof in a liquid-nitrogen cold trap, under static vacuum. After the recondensation was complete, the solvent was degassed once more upon thawing and allowed to slowly warm to room temperature with rapid stirring. Upon warming to room temperature, the reaction solution had a dark-green appearance. After stirring for 3 h at room temperature, an in situ 1 H NMR spectrum of the reaction solution revealed the selective formation of a new product. The reaction solution was concentrated in vacuo under reduced pressure to ca. 2 mL and cooled to −30°C for 1 h to complete the precipitation of the product. A light-brown supernatant was removed by cannula filtration at −50°C, and the remaining emerald-green solid was dried in vacuo for 1 h at a pressure of 3 × 10 −2 mbar. The product was afforded as 6 (0.177 g, 55%). Mp: 164−166°C. HR ESI-MS. Calcd for C 45  ■ ASSOCIATED CONTENT * S Supporting Information X-ray crystallographic data in CIF format, details of the XRD studies, EPR, ESI-MS, and UV/vis spectra, tables of Cartesian coordinates, and absolute energies for all of the complexes studied. This material is available free of charge via the Internet at http://pubs.acs.org.