Introduction

The first chapter described the investigations of the structure and reactivity of a tantalum mixed amido-imido ligand system derived from a bis(trimethylsilylamino)biphenyl ligand.¹ This chapter will present the studies on a series of yttrium complexes with a related bis(silylamido)biphenyl ligand, using more sterically demanding tert-butyldimethyl-silyl substituents.² Unlike the tantalum system, where reactivity was dominated by reactions of the nitrogen-silicon bond, the yttrium complexes described here appear to possess more robust bis(silylamido) spectator ligands. Asymmetric early transition-metal complexes, usually based on substituted cyclopentadienyl ligands, have found application as olefin polymerization catalysts for control of stereoregularity of the resulting polymer.^3-8 Group 3 and lanthanide metallocene complexes have also been used as regio- and stereoselective hydrosilylation catalysts, and the effect of varying the ancillary Cp-based ligands on the reactivity and selectivity of the catalyst has been studied extensively.^9-11 Even though many alternatives to cyclopentadienyl ligands in early transition metal chemistry have been explored,^8,12-30 the study of chelating diamides as ligands has been mostly restricted to symmetric systems.

Results and Discussion

The observation of facile cleavage of the nitrogen-silicon bond, described in Chapter 1, suggested that a more sterically demanding substituent be employed. A tert-butyldimethylsilyl analogue of the biphenyl diamide ligand was obtained by silylation of the biphenyldiamine anion, as obtained via deprotonation with ⁿBuLi in THF, with ^tBuMe₂SiCl (eq 1). Lithiation of the silylamine in situ with ⁿBuLi produced the lithium salt Li₂[DADMB].2THF (1), isolated as a THF adduct in 84% yield from pentane.

Reaction of 1 with YCl₃(THF)₃ in refluxing THF yielded the yttrium complex 2 as a white crystalline solid, in 92% yield (eq 2).

Yttrium complex 2 (Figure 1) adopts a distorted, trigonal-bipyramidal coordination geometry, with the chelating diamide and chloride ligands in the equatorial plane and the two coordinated THF molecules in the axial positions. The molecule has an approximate (noncrystallographic) C₂-axis coincident with the Y-Cl bond. The ligand bite angle is 123.3(2)^o, and the average Y-N distance is 2.25 Å. The dihedral angle between the biphenyl planes is 84.8^o. The short Y-Cipso (average 2.75 Å) and Y-Cortho (average 2.87 Å) distances, and the small Y-N-Cipso angles (average 94.6^o) suggest the presence of Y-aromatic carbon bonding interactions, which are often observed in complexes of this type. ^31,32

Figure 1. ORTEP diagram of [DADMB]YCl(THF)₂ (2).

Figure 2. ORTEP diagram of [DADMB]Y(OSiMe₃)(THF)₂ (4).

The methyl derivative 3, an off-white crystalline solid, was synthesized in 60% yield by reaction of 2 with MeLi in Et₂O/THF (eq 2). The methyl group of 3 gives rise to a doublet in the ¹H NMR spectrum at -0.42 ppm, and the coupling constant ²J_YH = 1.8 Hz is slightly smaller than those reported for Cp*₂YMe(THF) (²J_YH = 2.3 Hz) and related complexes. ^14,33 The ¹J_CH = 101 Hz coupling (cf. 108.2 Hz in Cp*₂YMe(THF) ³³ and 108 Hz in Cp*Y(OC₆H₃^tBu₂)Me(THF)₂ ¹⁴) is lower than the typical 120 - 130 Hz for an sp³ CH bond. ^34-36

The synthesis of 3 was found to be very sensitive to the presence of adventitious amounts of silicone grease, such that high yields require exclusive use of hydrocarbon grease in the Schlenkware used for the synthesis. In the presence of Dow Corning silicone grease, the reaction of 2 with MeLi resulted in formation of the yttrium trimethylsiloxide derivative 4 in 50% yield (as the only isolable product). Complex 4 was characterized by X-ray crystallography (Figure 2).

The structure of 4 is very similar to that of the chloride complex 2, with an Y-N distance of 2.26 Å (average), a ligand bite angle of 118.8(1)^o, and an almost linear Y-O-Si linkage of 175.5(2)^o. The dihedral angle between the biphenyl planes is 92.0^o. The Y-Cipso and Y-Cortho bonding interactions are again present, but the corresponding distances are longer (averages of 2.82 and 3.00 Å, respectively), and the Y-N-Cipso angles are larger (average 98.2^o) than those observed for 2. The mechanism of formation of 4 is unclear, but the methyl complex 3 was found to be unreactive towards silicone grease (benzene-d₆, 24 h, room temperature), suggesting that formation of 4 proceeds via trapping by silicone grease of an intermediate in the reaction of 2 with MeLi. Analogous reactivity has been reported for the samarium hydride complex, [(C₅Me₅)₂Sm(u-H)]₂, which reacts with hexamethylcyclotrisiloxane to afford the dimeric siloxide [(C₅Me₅)₂Sm-(THF)]₂(u-OSiMe₂OSiMe₂O). ³⁷

Exposure of 2 to C₂H₄ (80 psi) in the presence of MAO cocatalyst (500 equiv) in toluene produced only 0.06 g of polyethylene. The methyl derivative 3 was also found to be inert towards olefins, since no reaction was observed between 3 and 1-hexene in benzene-d₆ (24 h, room temperature). A benzene-d₆ solution of 3 under 5 psi of C₂H₄ appears to react very slowly, as indicated by formation of a cloudy precipitate, presumed to be polyethylene, over the course of a few days. During this time, only a small decrease in the amount of C₂H₄ was observed (by ¹H NMR spectroscopy), 3 remained unchanged after 3 days, and no new organoyttrium species were detected by ¹H NMR spectroscopy.

Reasoning that the coordinated THF ligands may act to greatly diminish the reactivity of 3 toward alkenes, we sought to introduce a bulkier alkyl group which might facilitate dissociation of THF with production of a more coordinatively unsaturated complex. The bis(trimethylsilyl)methyl derivative [DADMB]Y[CH(SiMe₃)₂](THF)(OEt₂) (5) was obtained by reaction of 2 with (Me₃Si)₂CHLi in diethyl ether. The incorporation of Et₂O into 5 suggests that the more crowded steric environment in 5 can indeed favor dissociation of coordinated solvent. A similar tendency toward THF dissociation in the presence of a bulky alkyl substituent has been observed for [NON]Y[CH(SiMe₃)₂](THF) ([NON]^2- = [^tBu-d₆-N-o-C₆H₄)₂O]^2-). ³⁸ The YCH resonance in the ¹H NMR spectrum appears as a doublet at d -0.94 (²J_YH = 2.1 Hz), and the corresponding resonance in the ¹³C NMR spectrum appears at d = 39.4 (¹J_YC = 35.6 Hz). The downfield chemical shift for the alpha-carbon is much closer to that reported for [NON]Y[CH(SiMe₃)₂](THF) ³⁸ (37.9 ppm) and [PhC(NSiMe₃)₂]₂YCH(SiMe₃)₂ ¹² (43.5 ppm) than to the one for Cp*₂YCH(SiMe₃)₂ ³⁹ (25.19 ppm), and can therefore be viewed as an indication of the high electrophilicity of the metal center in 5. The small C-H coupling for the alpha C-H bond (¹J_CH = 80 Hz, cf. 88 Hz in [PhC(NSiMe₃)₂]₂YCH(SiMe₃)₂, ¹² and 84.2 Hz in Cp*₂YCH(SiMe₃)₂ ³⁹) is consistent with the presence of an alpha-agostic interaction, as observed for many group 3 and lanthanide bis(trimethylsilyl)methyl complexes. ^34,35

Reaction of 5 with C₂H₄ (5 psi, benzene-d₆) also resulted in the very slow formation of insignificant amounts of polyethylene, with no new organoyttrium species observed in the ¹H NMR spectrum after 5 days. This low olefin polymerization activity for 3 and 5 should be compared to the behavior of other group 3 and lanthanides alkyl derivatives, some of which are reported to be active polymerization catalysts. Thus, Cp*₂YMe(THF), ³³ Cp*₂ScMe, ⁴⁰ [Tp^MeYR₂(THF)] (Tp^Me = tris(3,5-dimethyl-1-pyrazolyl)borohydride), ¹⁸ and R₂Si(h⁵-C₅H₄)(h⁵-C₅Me₄)LuCH(SiMe₃)₂ ⁴¹ are reported to be active as olefin polymerization catalysts, whereas [PhC(NSiMe₃)₂]₂YCH₂Ph(THF) ⁴² is moderately active and [Me₂Si(NCMe₃)(OCMe₃)]₂YCH(SiMe₃)₂ ¹³ and Cp*₂MCH-(SiMe₃)₂ (M = La, Nd, Lu), ⁴³ and [NON]Y[CH(SiMe₃)₂](THF) ³⁸ are inactive. The lack of olefin polymerization activity in such systems is usually attributed to either steric hindrance (in the case of CH(SiMe₃)₂ derivatives), or high ionicity in the bonding which leads to more contracted vacant metal orbitals. ^12,13,42

To evaluate the activities of 3 and 5 toward sigma-bond metathesis processes involving hydrosilanes, their interactions with PhSiH₃ were investigated. Both compounds 3 and 5 were found to react with PhSiH₃ in benzene at room temperature via Si-C bond formation, to give PhMeSiH₂ and PhSiH₂CH(SiMe₃)₂,⁹ respectively. These reactions also produced a white crystalline precipitate, insoluble in pentane and benzene but soluble in THF, characterized as an yttrium hydride (eq 3). Hydride 6 was also produced by hydrogenolysis, upon exposure of 3 or 5 to H₂ gas in benzene-d₆ (5 psi, room temperature). The other products of these reactions (methane and (Me₃Si)₂CH₂, ⁴⁴ respectively) were observed to form quantitatively (by NMR spectroscopy).

The hydride resonance for 6 was observed in the ¹H NMR spectrum as a triplet at d 5.88 (¹J_YH = 28 Hz) in THF-d₈. The Y-H coupling constant is similar to that observed for other yttrium hydride dimers, ^12,13,45,46 and the chemical shift is rather downfield compared to most yttrium hydrides with bis(cyclopentadienyl) ligands (d 2.02 - 5.45).^41,45,47,48 However, the shift for 6 is not as low as those reported for bis-(benzamidinato)yttrium hydrides (d 8.28 - 8.31)¹² and {[Me₂Si(NCMe₃)(OCMe₃)]₂Y(u-H)}₂¹³ (d 6.80). This hydride signal was observed to decay slowly in intensity, and completely disappear within 4 days at room temperature, indicating exchange with deuterium from the THF-d₈ solvent. In the IR spectrum of 6, the Y-H absorption at 1240 cm^-1 was obscured by strong ligand absorptions in the same region, and could be assigned only by comparison with the spectrum of the deuterated analogue 6-d₂ (obtained by hydride exchange with THF-d₈), which exhibits a Y-D absorption at 870 cm^-1. This frequency is, however, in good agreement with those reported for other dimeric yttrium hydride species. ^44,45

X-ray quality crystals of 6 were grown by slow diffusion of PhSiH₃ (in benzene solution) layered above a benzene solution of 5, and the molecular structure (Figure 3) may be described as a centrosymmetric dimer. The hydride atom, located in the Fourier difference map, was refined isotropically. The Y-H distance is 2.25(4) Å (average), and the Y-H-Y angle is 109(2)^o.

Figure 3. ORTEP diagram of {[DADMB]YH(THF)}₂.C₆H₆ (6).

The geometry of the four membered ring is very similar to that found in other structurally characterized dimeric yttrium hydrides, [(1,3-Me₂C₅H₃)₂Y(u-H)(THF)]₂⁴⁶ (average 2.15(7) Å and 118(3)^o ), [(MeC₅H₄)₂Y(u-H)(THF)]₂⁴⁵ (2.18(8) Å and 114(3)^o), and {[PhC(NSiMe₃)₂]₂Y(u-H)}₂¹² (2.14(3) Å and 111.5(15)^o). The N-Y-N ligand bite angle in 6 (103.9(1)^o) is significantly lower than those of ca. 120^o in the 5-coordinate complexes 2 and 4, and only one of the ligand ipso carbons is in a close contact with the yttrium center (Y-C(1) = 2.702(4) Å, Y-C(6) = 3.069(5) Å, Y-N(1)-C(1) = 93.4(2)^o, whereas Y-C(8) = 3.098(5) Å, Y-C(7) = 3.409(4) Å, Y-N(2)-C(8) = 114.6(3)^o). The dihedral angle between the biphenyl planes is 81.9^o. These differences may be attributed to steric crowding in the hydride dimer relative to the monomeric complexes 2 and 4.

Compound 6 represents a rare example of an yttrium hydride that contains no cyclopentadienyl ligands. We are aware of only one other structurally characterized non-Cp yttrium hydride, the bis(benzamidinato) complex {[PhC(NSiMe₃)₂]₂Y(u-H)}₂.¹² Other examples of such hydrides are the unstable species {[Me₂Si(NCMe₃)(OCMe₃)]₂Y(u-H)}₂¹³ and a tris(pyrazolyl)borato yttrium hydride Tp^MeYH₂(THF)₄,¹⁸ for which a trimeric solution structure was suggested. All of the structurally characterized yttrium hydrides are dimers^{4,12,41,45,46} or trimers⁴⁶ in the solid state, although more complicated aggregates have also been observed in anionic or heterometallic species.⁴⁸

The reactivity of 6 appears to be restricted by its dimeric nature (preserved even in THF solution, as suggested by the ¹H NMR spectrum), and its very low solubility in nonpolar organic solvents. No polymerization activity was observed towards 1-hexene or C₂H₄ (5 psi) in benzene-d₆ at room temperature, and heating these mixtures resulted in decomposition of the hydride with precipitation of only a small amount of polyethylene. However, when dissolved in THF, compound 6 was found to rapidly insert ethylene at room temperature, to give the yttrium ethyl complex 7 (eq 4).

At longer reaction times (6-7 h), compound 7 seems to undergo a slow insertion of a second molecule of C₂H₄, but positive identification of multiple insertion products was hindered by their decomposition, and no formation of polyethylene was observed even at elevated pressure (80 psi, THF, 48 h at room temperature). Decomposition of 7 to unidentified products was also found to occur even in solid state, under N₂, over the period of a few months. The ethyl group of 7 gives rise to a broad multiplet at 0.0 ppm (YCH₂CH₃, 2 H) and a triplet of doublets at 1.67 ppm (YCH₂CH₃, 3 H) in the ¹H NMR spectrum. The ³J_YH coupling constant of 1.9 Hz for the CH₃ protons of the ethyl ligand is very similar to the related value (³J_YH = 1.4 Hz) reported for (h⁵-CH₃C₅H₄)₂Y(CH₂CH₃)-(THF),⁴⁸ and the ¹J_CH coupling of 120 Hz is typical for an sp³ C-H bond. Although beta-agostic interactions are often observed in early transition metal ethyl complexes,^{34-36,40,49-51} their existence cannot be determined based on the (averaged) ¹J_CH coupling constant only. The IR spectrum of 7 doesn't contain evidence for agostic C-H bonds (no low-energy C-H bands were detected). The relatively small ¹J_CH coupling of 70-80 Hz (multiplet broadness prevents accurate measurement) for the alpha protons, however, is consistent with the presence of an alpha-agostic interaction with the metal center,^34,35,52-54 analogous to that reported for some bis(trimethylsilyl)methyl yttrium complexes (vide supra). In the ¹³C NMR spectrum of 6, the alpha-carbon appeared as a doublet at 35.5 ppm, with a coupling of 50 Hz to yttrium.

In THF-d₈ solution, the slower insertion of 1-hexene into the Y-H bond of 6 was observed (eq 4), to give the 1-hexyl derivative 8 (by ¹H NMR spectroscopy). No reaction occurred between 6 and cyclohexene, however. Single insertion of terminal olefins to give yttrium alkyl complexes has been reported for [(h⁵-RC₅H₄)₂YH(THF)]₂ (R = H or Me).⁴⁸ The lack of olefin polymerization activity for compound 6 can be contrasted to the high catalytic activity of certain dimeric group 3 and lanthanide hydrides of the type [Cp*₂MH]₂ (M = La, Nd, Sm, Lu),⁴³ but is similar to the reported low activity for {[PhC(NSiMe₃)₂]₂-Y(u-H)}₂¹² and {[Me₂Si(NCMe₃)(OCMe₃)]₂Y(u-H)}₂.¹³ Other dimeric hydrides have been reported to undergo single, reversible olefin insertion to give mixed hydrido-alkyl complexes (e.g., [Cp*Y(OC₆H₃^tBu₂)]₂(u-H)(u-alkyl)¹⁴ and [Et₂Si(h⁵-C₅H₄)(h⁵-C₅Me₄)M]₂(u-H)(u-CH₂CH₃) for M = Lu, Y),⁴¹ but we have not observed formation of such species in the reactions of 6 with olefins.

As shown in eq 5, yttrium hydride 6 was found to react with pyridine to give a mixture of pyridine insertion products, 9 (major) and 10 (minor), in variable ratios. The structure of 9, confirmed by X-ray crystallography (Figure 4), is again based on a distorted trigonal bipyramidal geometry, with neutral donors (pyridine) occupying the axial positions.

Figure 4. ORTEP diagram of [DADMB]Y(NC₅H₆)(NC₅H₅)₂.C₅H₁₂ (9).

The ligand bite angle is 118.9^o, and the two ipso carbons are in close contact with yttrium (average distance 2.77 Å). The dihedral angle between the biphenyl planes is 92.8^o. The third pyridine unit attached to the metal center has been reduced via 1,2 insertion, as can be deduced from the changes in the C-C bond lengths and a puckering of the pyridine ring.

The identities of the 1,2 and 1,4 insertion products 9 and 10 were further confirmed by 2D NMR studies (TOCSY and HMQC), which revealed a characteristic coupling pattern for the partially hydrogenated pyridine ring,^48,55,56 and allowed assignment of the ring protons and carbons (see Experimental Section). Heating a solution containing both 9 and 10 to 80 ^oC in benzene-d₆ resulted in complete conversion of the mixture within 2 h to compound 10, as observed by ¹H NMR spectroscopy. This indicates that 10 is the thermodynamically more stable isomer, while 9 is kinetically generated. This reactivity towards pyridine has precedent in a number of previously reported reactions for yttrium hydride and alkyl complexes. Metalation of pyridine via CH activation has been reported for [Cp*₂YH]₂,⁵⁷ Cp*₂YCH(SiMe₃)₂,⁴⁴ and Cp*₂YMe-(THF),⁴⁴ while 1,2-insertion of pyridine followed by rearrangement to the 1,4 insertion product has been reported for [(h⁵-C₅H₄R)₂YH(THF)]₂ (R = H, CH₃).⁴⁸ Also, an analogous 1,2- to 1,4-isomer rearrangement has been observed for the bis(alkoxysilyl-amido)yttrium derivative [Me₂Si(NCMe₃)(OCMe₃)]₂YNC₅H₆.⁵⁵ The bis(trimethylsilyl)-benzamidinate complexes {[PhC(NSiMe₃)₂]₂YH}₂ and [PhC(NSiMe₃)₂]₂YCH₂Ph(THF) have also been reported to undergo the 1,2-insertion of pyridine,⁴² but no further isomerization was noted in these cases. The reactivity of 6 towards pyridine is thus different from that of similar pentamethylcyclopentadienyl yttrium complexes, and is more similar to that of other known yttrium hydrides and alkyls with monosubstituted Cp, or non-cyclopentadienyl, ligands.

Conclusions

A series of novel yttrium complexes with bis(silylamido)biphenyl ligands have been synthesized and characterized. The structures and reactivities of the chloride, alkyl, and hydrido derivatives have been investigated. In their reactivity, these complexes resemble more closely some benzamidinate¹² and alkoxysilylamido¹³ yttrium species, rather than the more fully investigated cyclopentadienyl yttrium derivatives.^{39,41,46,48,58} The high degree of ionicity in the yttrium-nitrogen bonding appears to result in a highly electrophilic metal center, which leads to lower reactivity towards olefin polymerization and sigma-bond metathesis, perhaps resulting from more contracted vacant metal orbitals which are less effective in substrate binding.^12,13,42 The reactivity of the hydride is also inhibited by the high stability of the bridged yttrium hydride unit, and the insolubility of the dimer in noncoordinating solvents. Very low olefin polymerization activity is exhibited by the MAO-activated chloride, and the alkyl and the hydride complexes. A single insertion of olefins into the Y-H bond, reminiscent of the reactivity of some monosubstituted-Cp yttrium hydrides,⁴⁸ results in formation of yttrium alkyl species, and the ethyl complex is sufficiently stable to be isolated and characterized. The reactivity of the yttrium hydride 6 towards pyridine also resembles that observed for related hydride derivatives containing benzamidinate and alkoxysilylamido ancillary ligands.

The yttrium derivatives studied represent another example of non-cyclopentadienyl early transition metal complexes. This study further contributes to an understanding of the factors behind the reactivity of such d⁰ complexes. As has been already suggested, in addition to the electrophilicity of the metal center, other possibly important factors include the degree of ionicity in the metal-ligand bonds, and the steric environment created by the ligand. The latter not only affects the accessibility of the metal center by the substrate, but also the ease of dimer formation, which is particularly important in the case of hydride derivatives. Although apparently not promising in terms of olefin polymerization activity, these silylamido yttrium species exhibit reactivity patterns which are distinct from those known for more traditional cyclopentadienyl derivatives.

Experimental Section

General. All reactions with air-sensitive compounds were performed under dry nitrogen, using standard Schlenk and glove box techniques. Reagents were obtained from commercial suppliers and used without further purification, unless otherwise noted. Olefin-free pentane, benzene, and toluene were prepared by pretreating with conc. H₂SO₄, 0.5 N KMnO₄ in 3 M H₂SO₄, NaHCO₃ and finally anhydrous MgSO₄. Solvents (pentane, diethyl ether, benzene, toluene, tetrahydrofuran) were distilled under nitrogen from sodium benzophenone ketyl. Benzene-d₆ was distilled from Na/K alloy. Commercial silanes were distilled and dried over molecular sieves before use. ⁿBuLi was used as a 1.6 M solution in hexanes, as supplied by Aldrich, and MeLi as a 1.6 M solution in Et₂O, as supplied by Alpha Aesar. 2,2'-Diamino-6,6'-dimethylbiphenyl,⁵⁹ YCl₃(THF)₃,³⁹ and LiCH(SiMe₃)₂⁶⁰ were prepared according to published literature procedures. NMR spectra were recorded at 300 or 500 MHz (¹H) with Bruker AMX-300 and DRX-500 spectrometers, or at 100 MHz (¹³C{¹H}) with an AMX-400 spectrometer, at ambient temperature and in benzene-d₆, unless otherwise noted. CH coupling constants were obtained from non-decoupled HMQC experiments. Signal multiplicities are reported as follows: s - singlet, d - doublet, t - triplet, q - quartet, qn - quintet, m - multiplet. Elemental analyses were performed by the Microanalytical Laboratory at UC Berkeley, E+R Microanalytical Laboratory, or Desert Analytics. Infrared spectra were recorded with a Mattson Infinity 60 MI FTIR spectrometer, as KBr pellets.

Li₂[DADMB].2THF (1). To a cold (0 ^oC) solution of 2,2'-diamino-6,6'-dimethylbiphenyl (4.00 g, 18.8 mmol) in THF (100 mL) was added dropwise 25.0 mL (40 mmol) of 1.6 M ⁿBuLi. A white precipitate formed initially, but dissolved completely after all of the ⁿBuLi had been added. The solution was allowed to warm to room temperature, and was then stirred for 3 h. A solution of ^tBuMe₂SiCl (6.22 g, 41.2 mmol) in 20 mL of THF was then added dropwise. The mixture was heated at reflux for 5 h, which resulted in the formation of a white precipitate. After cooling to room temperature, a second portion of ⁿBuLi (25.0 mL, 40 mmol) was added and the mixture was stirred overnight at room temperature. The THF was removed under vacuum to give an oily white solid. Extraction with pentane (2 x 100 mL) gave a light yellow solution, which was concentrated in vacuo until crystals appeared, and then cooled to -78 ^oC. The resulting crystalline product was recrystallized from pentane and dried to afford 9.45 g (15.8 mmol, 84% yield) of colorless crystals. ¹H NMR: d 7.06 (d, 2 H, J = 7.7 Hz), 6.95 (t, 2 H, J = 7.7 Hz), 6.48 (d, 2 H, J = 7.1 Hz biphenyl H's), 3.14 (m, 8 H, THF), 1.99 (s, 6 H, Me), 1.25 (m, 8 H, THF), 1.16 (s, 18 H, ^tBuMe₂Si), 0.49 (s, 6 H, ^tBuMe₂Si), 0.09 (s, 6 H, ^tBuMe₂Si). ¹³C{¹H} NMR: d 157.5, 140.1, 133.2, 128.0, 121.8, 116.6, 68.6 (THF), 28.7 (CMe₃), 25.4 (THF), 21.8, 21.7 (Me; CMe₃), 1.0, -0.5 (^tBuMe₂Si). IR (cm^-1): 3039 (w), 2951 (s), 2923 (s), 2879 (s), 2848 (s), 1575 (s), 1559 (s), 1460 (s), 1440 (s), 1288 (s), 1250 (s), 1049 (s), 968 (m), 899 (m), 842 (s), 824 (s), 769 (s), 668 (m), 605 (m), 457 (m), 419 (m). Anal. Calcd for C₃₄H₅₈N₂Li₂O₂Si₂: C, 68.42; H, 9.79; N, 4.69. Found: C, 68.18; H, 9.85; N, 4.56.

[DADMB]YCl(THF)₂ (2). A solution of 1 (3.00 g, 5.03 mmol) and YCl₃(THF)₃ (2.10 g, 5.10 mmol) in 100 mL of THF was heated at reflux for 4 h, resulting in formation of a white precipitate. The solvent was removed in vacuo and the resulting white powder was extracted with a mixture of 60 mL of pentane and 25 mL of THF. The filtrate was concentrated to about 15 mL and more pentane (20 mL) was added to initiate crystallization of the product as a white precipitate. After cooling to -78 ^oC, the solution was filtered and the product was then dried in vacuo to obtain 3.28 g (92% yield) of white crystalline powder. ¹H NMR: d 7.07 (d, 2 H), 6.95 (t, 2 H), 6.55 d, 2 H, aromatic H), 3.70 - 3.46 (br m, 8 H, THF), 1.88 (s, 6 H, Me), 1.24 (m, 8 H, THF), 1.11 (s, 18 H, ^tBuMe₂Si), 0.52, 0.50 (s, 6 H each, ^tBuMe₂Si). ¹³C{¹H} NMR: d 152.4, 142.4, 130.5, 129.9, 123.5, 120.1 (aromatic C), 68 (THF), 28.3 (Me₃C), 25.5 (THF), 22.3 (MeAr), 21.5 (Me₃C), 1.8, -1.6 (Me₂Si). IR (cm^-1): 3040 (w), 2949 (s), 2925 (s), 2882 (s), 2851 (s), 1578 (m), 1558 (m), 1471 (m), 1461 (m), 1441 (s), 1387 (w), 1356 (w), 1272 (s), 1248 (s), 1085 (w), 1048 (s), 1021 (m), 963 (m), 872 (m), 830 (s), 777 (s), 708 (m), 668 (m), 568 (m), 440 (m). Anal. Calcd for C₃₄H₅₈N₂O₂Si₂YCl: C, 57.73; H, 8.26; N, 3.96. Found: C, 57.61; H, 8.44; N, 3.85.

[DADMB]YMe(THF)₂ (3). Compound 2 (1.16 g, 1.64 mmol) was dissolved in 100 mL of Et₂O, and MeLi (1.10 mL, 1.76 mmol) was added at room temperature. To the reaction mixture was added a small amount of THF, to bring the remaining undissolved 2 into solution. The resulting pale yellow solution was stirred overnight, the solvents were removed in vacuo, and the oily residue was extracted with 2 x 50 mL of pentane. The product was isolated by crystallization at -78^oC in two crops (0.67 g, 60% yield) as a white crystalline powder. ¹H NMR: d 7.07 (m, 2 H), 6.96 (m, 2 H), 6.56 (m, 2 H, aromatic H), 3.43 (m, 8 H, THF), 1.89 (s, 6 H, Me), 1.24 (m, 8 H, THF), 1.14 (s, 18 H, ^tBuMe₂Si), 0.52, 0.40 (s, 6 H each, ^tBuMe₂Si), -0.42 (d, 3 H, ²J_YH = 1.8 Hz, ¹J_CH = 101 Hz, YMe). ¹³C{¹H} NMR: d 153.2, 142.1, 131.1, 129.4, 123.4, 119.5, 70.9 (THF), 28.4 (Me₃C), 25.5 (THF), 22.4 (MeAr), 21.7 (YMe), 21.7 (Me₃C), 2.0, -1.9 (Me₂Si). IR (cm^-1): 3038 (w), 3043 (m), 2951 (s), 2890 (s), 2851 (s), 1906 (w), 1577 (s), 1555 (s), 1460 (s), 1440 (s), 1398 (m), 1358 (m), 1271 (s), 1243 (s), 1085 (m), 1048 (s), 1031 (s), 981 (s), 880 (m), 830 (s), 784 (s), 769 (s), 708 (m), 673 (m), 661 (m), 607 (w), 567 (w), 454 (m), 436 (w). Anal. Calcd for C₃₅H₆₁N₂Si₂O₂Y: C, 61.20; H, 8.95; N, 4.08. Found: C, 59.74; H, 9.48; N, 3.87.

[DADMB]Y(OSiMe₃)(THF)₂ (4). Compound 2 (0.54 g, 0.76 mmol) and 0.3 g of Dow Corning Silicone Grease were dissolved in 100 mL of Et₂O, and MeLi (0.5 mL, 0.8 mmol) was added at room temperature. To the reaction mixture was added a small amount of THF, to bring the remaining undissolved 2 into solution. The resulting pale yellow solution was stirred overnight, the solvents were removed in vacuo, and the oily residue was extracted with 2 x 40 mL of pentane. The product was isolated by crystallization from pentane at -78^oC (0.29 g, 50% yield) as a white crystalline powder. ¹H NMR: d 7.04 (m, 2 H), 6.96 (m, 2 H), 6.57 (m, 2 H, aromatic H), 3.46 (m, 8 H, THF), 1.91 (s, 6 H, Me), 1.28 (m, 8 H, THF), 1.14 (s, 18 H, ^tBuMe₂Si), 0.50, 0.29 (s, 6 H each, ^tBuMe₂Si), 0.26 (s, 9 H, Me₃SiO). ¹³C{¹H} NMR: d 153.6, 142.2, 131.2, 129.4, 123.2, 119.4, 70.7 (THF), 28.5 (Me₃C), 25.4 (THF), 22.4 (MeAr), 21.6 (Me₃C), 4.0 (Me₃SiO), 1.7, -1.3 (Me₂Si). IR (cm^-1): 3078 (w), 3043 (m), 2951 (s), 2890 (s), 2851 (s), 1906 (w), 1577 (s), 1555 (s), 1460 (s), 1440 (s), 1398 (m), 1358 (m), 1271 (s), 1243 (s), 1085 (m), 1048 (s), 1031 (s), 981 (s), 880 (m), 830 (s), 784 (s), 769 (s), 708 (m), 673 (m), 661 (m), 607 (w), 567 (w), 454 (m), 436 (w). Anal. Calcd for C₃₇H₆₇N₂Si₃O₃Y: C, 58.39; H, 8.87; N, 3.68. Found: C, 58.66; H, 9.25; N, 3.58.

[DADMB]YCH(SiMe₃)₂(THF)(Et₂O) (5). A mixture of 2 (1.24 g, 1.75 mmol) and LiCH(SiMe₃)₂(Et₂O)0.1 (0.30 g, 1.76 mmol) was dissolved in 80 mL of Et₂O. The reaction mixture was kept in an ice bath for 30 min and then allowed to warm to room temperature and was stirred for 8 h. The cloudy yellow solution was filtered, the clear filtrate was concentrated to about 15 mL, and 20 mL of pentane was then added. Further concentration and cooling to -78 ^oC resulted in the formation of a white crystalline precipitate, which was isolated by filtration and dried in vacuo to give 0.59 g of product (43% yield). The product can be purified by recrystallization from pentane. ¹H NMR: d 7.23 (m, 1 H), 6.85 (m, 3 H), 6.70 (m, 1 H), 6.59 (m, 1 H, aromatic H's), 3.52 (m, broad, 4 H, THF), 3.24 (q, 6 H, Et₂O), 1.88 (s, 3 H, MeAr), 1.84 (s, 3 H, MeAr), 1.38 (m, 4 H, THF), 1.11 (t, 4 H, Et₂O), 1.10 (s, 9 H, Me₃C, overlaps with Et₂O), 1.08 (s, 9 H, Me₃C), 0.60 (s, 3 H, Me₂Si), 0.32 (s, 3 H, Me₂Si), 0.31 (s, 3 H, Me₂Si), 0.29 (s, 9 H, Me₃Si, overlaps Me₂Si), 0.26 (s, 3 H, Me₂Si), 0.25 (s, 9 H, Me₃Si), -0.94 (d, 1 H, ²J_YH = 2.1 Hz, ¹J_CH = 80 Hz, YCH). ¹³C NMR: d 150.3, 141.7, 138.6, 129.3, 120.1, 113.4, (biphenyl), 71.7, 68.5, 66.2 (THF, Et₂O), 39.4 (YCH, ¹J_YC = 35.6 Hz), 28.7, 27.9, 26.0, 25.3 (Me₃C), 26.5 (THF), 22.4 (MeAr), 21.2, 20.6 (Me₃C), 15.9 (Et₂O), 5.7, 4.8 (Me₃Si), 1.28, -0.51 (Me₂Si). IR (cm^-1): 3043 (w), 2952 (s), 2887 (s), 2851 (s), 1577 (m), 1559 (m), 1462 (s), 1443 (s), 1387 (w), 1247 (s), 1091 (w), 1044 (s), 1006 (m), 963 (m), 836 (s), 793 (m), 768 (m), 711 (w), 661 (m), 594 (w), 427 (w). Anal. Calcd for C₄₁H₇₉N₂O₂Si₂Y: C, 63.36; H, 10.25; N, 3.60. Found: C, 63.07; H, 10.45; N, 3.76.

{[DADMB]YH(THF)}₂.C₆H₆ (6). To a solution of 5 (1.05 g, 1.35 mmol) in 50 mL of benzene was added 0.6 mL (5.5 mmol) of PhSiH₃. The mixture was left undisturbed at room temperature for 5 days, resulting in formation of a highly crystalline, colorless precipitate. The product was then isolated by filtration, washed with benzene (30 mL) and pentane (30 mL), and briefly dried in vacuo (0.53 g, 61% yield). ¹H NMR (THF-d₈): d 6.99 (m, 4 H, aromatic H), 6.54 (m, 2 H, aromatic H), 5.88 (t, 1 H, J = 28 Hz, YH), 3.61 (m, THF), 1.77 (m, THF), 1.69 (s, 6 H, MeAr), 0.90 (s, 18 H, Me₃C), 0.35 (s, 6 H, Me₂Si), 0.21 (s, 6 H, Me₂Si). ¹³C{¹H} NMR (THF-d₈): d 152.7, 142.6, 130.8, 130.3, 124.0, 120.3 (aromatic C), 129.2 (C₆H₆), 68.4 (THF), 28.3 (Me₃C), 26.5 (THF), 22.2 (MeAr), 21.7 (Me₃C), 1.5 (Me₂Si), -1.7 (Me₂Si). IR (cm^-1): 3038 (m), 2949 (s), 2926 (s), 2881 (s), 2850 (s), 2703 (w), 1564 (m), 1462 (s), 1441 (s), 1388 (m), 1359 (m), 1251 (s, broad), 1087 (m), 1047 (s), 1014 (s), 966 (s), 865 (s), 834 (s, broad), 791 (s), 767 (s), 707 (m), 682 (s), 662 (s), 617 (m), 595 (m), 567 (m), 464 (m), 431 (m). A sample of the deuterated analogue (6-d₂) was obtained by allowing a THF-d₈ solution of 6 stand at room temperature for 4 days, followed by removal of the solvent in vacuo which resulted in complete exchange of the Y-hydride with deuterium. Subtraction of the IR spectrum of 6-d₂ showed the presence of a strong Y-H absorption at 1240 cm^-1, overlapping with a strong ligand absorption. Anal. Calcd for C₆₆H₁₀₈N₄O₂Si₄Y₂: C, 61.94; H, 8.51; N, 4.38. Found: C, 61.95; H, 8.55; N, 4.19.

[DADMB]YEt(THF)₂ (7). A sample of the yttrium hydride 6 (0.184 g, 0.144 mmol) was dissolved in 15 mL of THF. The reaction flask was filled with C₂H₄ at 5-10 psi, and the solution was stirred for 30 min. The solvent was removed under vacuum and the solid residue was extracted into 20 mL of hexane. The extract was filtered, and the filtrate was then concentrated to 5 mL and cooled to -35 ^oC overnight, to give 7 as a white crystalline solid (0.121 g, 60% yield). ¹H NMR: d 7.07, 6.95, 6.56 (m, 2 H each, aromatic H), 3.49 (m, 8 H, THF), 1.88 (s, 6 H, MeAr), 1.67 (dt, ³J_HH = 7.4 Hz, ³J_YH = 1.9 Hz, ¹J_CH = 70 - 80 Hz, 3 H, YCH₂CH₃), 1.25 (m, 8 H, THF), 1.13 (s, 18 H, ^tBuMe₂Si), 0.53, 0.37 (s, 6 H each, ^tBuMe₂Si), 0.0 (br m, ¹J_CH = 120 Hz, 2 H, YCH₂CH₃). ¹³C NMR: d 153.1, 141.9, 131.4, 129.3, 123.5, 119.7 (biphenyl C), 72 (THF), 35.5, (d, J = 50 Hz, YCH₂CH₃), 28.4 (Me₃C), 25.5 (THF), 22.4 (MeAr), 21.6 (Me₃C), 15.7 (YCH₂CH₃), 1.7, -1.9 (Me₂Si). IR (cm^-1): 3043 (w), 3034 (w), 2953 (s), 2851 (s), 1578 (m), 1555 (m), 1461 (m), 1440 (m), 1268 (s), 1048 (s), 964 (m), 829 (s), 785 (m), 768 (m), 662 (w). Anal. Calcd for C₃₆H₆₃N₂Si₂O₂Y: C, 61.68; H, 9.06; N, 4.00. Found: C, 61.08; H, 8.96; N, 4.05.

[DADMB]Y(n-C₆H₁₃)(THF)₂ (8). A sample of 6 (ca. 10 mg) was dissolved in THF-d₈ (0.5 mL) and ca. 0.05 mL of 1-hexene was added. After 30 min, the YH signals had nearly disappeared, and after 5 h a complete conversion to product was observed. ¹H NMR: d 6.9-7.0 (m, 4 H), 6.54 (m, 1 H), 6.47 (m, 1 H, biphenyl H), 1.65 (s, 6 H, MeAr), 1.46 (m, 2 H, YCH₂CH₂), 1.33-1.38 (m, overlaps with 1-hexene, YCH₂CH₂(CH₂)₃CH₃), 0.89 (s, 18 H, Me₃C), 0.85 (m, overlaps, Y(CH₂)₅CH₃), 0.38 (s, 6 H, Me₂Si), 0.16 (s, 6 H, Me₂Si), -0.32 (m, 2 H, YCH₂).

[DADMB]Y(NC₅H₆)(pyr)₂^.C₅H₁₂ (mixture of 9 and 10). A sample of 6 (0.36 g, 2.8 mmol) was dissolved in 20 mL of pyridine at room temperature. The dark yellow solution was stirred for 2 days, and the volatile material was then removed by vacuum transfer. The remaining solid was extracted with 2 x 75 mL pentane, and then the combined yellow extracts were concentrated to 20 mL. Cooling the solution to -35 ^oC produced bright yellow crystals (0.14 g, 30% yield). ¹H NMR: compound 9: d 8.23 (m, 4 H, pyr), 7.27 (d, 1 H, NC₅H₆ vinylic), 6.83 - 6.54 (m, pyr and aromatic H), 6.47 (t, 1 H, NC₅H₆ vinylic), 5.40 (m, 1 H, NC₅H₆ vinylic), 5.04 (m, 1 H, NC₅H₆ vinylic), 4.30 (dd, 1 H, NC₅H₆ methylene), 4.04 (dd, 1 H, NC₅H₆ methylene), 2.03 (s, 6 H, Me), 1.05 (s, 18 H, ^tBuMe₂Si), 0.23 (s, 6 H, ^tBuMe₂Si), 0.18 (s, 6 H, ^tBuMe₂Si); compound 10: 8.32 (m, 4 H, pyr), 6.92 (m, 2 H, aromatic H), 6.71 (d, 2 H, NC₅H₆ vinylic), 6.63 - 6.58 (m, pyr and aromatic H), 4.55 (m, 2 H, NC₅H₆ vinylic), 3.69 (m, 2 H, NC₅H₆ methylene), 2.02 (s, 6 H, Me), 1.04 (s, 18 H, ^tBuMe₂Si), 0.20 (s, 6 H, ^tBuMe₂Si), 0.13 (s, 6 H, ^tBuMe₂Si). ¹³C{¹H} NMR: compound 9: d 152.3, 150.4, 146.4, 141.0, 132.9, 129.3 (aromatic C), 125.8 (NC₅H₆), 124.0, 120.4 (aromatic C), 113.4 (NC₅H₆), 102.2 (NC₅H₆), 96.8 (NC₅H₆), 47.9 (NC₅H₆ methylene), 28.6 (Me₃C), 22.3 (MeAr), 21.4 (Me₃C), 0.9 (^tBuMe₂Si), -0.8 (^tBuMe₂Si); compound 10: 152.3, 150.5, 141.0 (aromatic C), 137.0 (NC₅H₆), 133.0, 129.7, 129.6, 125.8, 124.1, 120.5 (aromatic C), 95.2 (NC₅H₆), 28.6 (Me₃C), 25.5 (NC₅H₆), 21.4 (Me₃C), 0.8 (^tBuMe₂Si), -0.9 (^tBuMe₂Si). Assignment of the NMR signals is based on 2D NMR studies (TOCSY and HMQC). Heating the NMR sample (product mixture of 9 and 10) to 80 ^oC resulted in complete conversion of the mixture within 2 h to compound 10. IR (cm^-1): 3040 (m), 2952 (s), 2926 (s), 2888 (s), 2852 (s), 2795 (m), 1645 (m), 1603 (s), 1579 (s), 1557 (m), 1506 (w), 1489 (w), 1471 (m), 1461 (m), 1443 (s), 1387 (w), 1359 (w), 1304 (w), 1268 (s), 1248 (s), 1231 (m), 1152 (w), 1100 (m), 1067 (m), 1046 (s), 1038 (s), 1007 (m), 998 (m), 965 (s), 932 (w), 867 (w), 829 (s), 786 (m), 769 (m), 752 (m), 702 (s), 661 (w), 627 (m), 567 (w), 536 (w), 432 (w). Anal. Calcd for C₄₆H₇₀N₅Si₂Y: C, 65.92; H, 8.42; N, 8.36. Found: C, 65.21; H, 8.03; N, 8.31.

X-ray structure determinations. X-ray diffraction measurements were made on a Siemens SMART diffractometer with a CCD area detector, using graphite monochromated Mo-Kalpha radiation. The crystal was mounted on a glass fiber using Paratone N hydrocarbon oil. A hemisphere of data was collected using omega scans of 0.3^o. Cell constants and an orientation matrix for data collection were obtained from a least-squares refinement using the measured positions of reflections in the range 4 < 2theta < 45^o. The frame data were integrated using the program SAINT (SAX Area-Detector Integration Program; V4.024; Siemens Industrial Automation, Inc.: Madison, WI, 1995). An empirical absorption correction based on measurements of multiply redundant data was performed using the programs XPREP (Part of the SHELXTL Crystal Structure Determination Package; Siemens Industrial Automation, Inc.: Madison, WI, 1995) or SADABS. Equivalent reflections were merged. The data were corrected for Lorentz and polarization effects. A secondary extinction correction was applied if appropriate. The structures were solved using the teXsan crystallographic software package of the Molecular Structure Corporation, using direct methods, and expanded with Fourier techniques. All non-hydrogen atoms were refined anisotropically and the hydrogen atoms were included in calculated positions but not refined unless otherwise noted. The function minimized in the full-matrix least-squares refinement was Sigma w(|Fo|-|Fc|)². The weighting scheme was based on counting statistics and included a p-factor to downweight the intense reflections.

For 2: Crystals were grown from a 10:1 pentane/THF solution of 2 at -35 ^oC. The Flack parameter of the structure was 0.009(2), suggesting that the correct enantiomorph was chosen. The other enantiomorph was also tested, but this structure resulted in R = 9.64% with a Flack parameter of 0.88.

For 4: Crystals were grown from a pentane solution of 4 at -35 ^oC.

For 6: Crystals were grown by slow diffusion of PhSiH₃ (0.3 mL dissolved in 20 mL benzene) into a solution of 5 (0.16 g in 20 mL benzene), the two solutions being separated with a layer of neat benzene, at room temperature for 4 days. The non-hydrogen atoms were refined anisotropically. The hydride H was located on the Fourier difference map and refined isotropically, the rest of the hydrogen atoms were included in calculated positions.

For 9: Crystals were grown from a pentane solution of a mixture of 9 and 10 at -15 ^oC. The single crystal selected was found to be compound 9. The non-hydrogen atoms were refined anisotropically, except for those of the solvating pentane which were disordered and refined isotropically.

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