Iso-specific Ziegler-Natta polymerization of α-olefins with a single-component organoyttrium catalyst

The synthesis of a sterically tailored ligand array (M)_2((C_5H_2-2-Si(CH_3)_3-4-C(CH_3)_3)S_2i(CH_3)_2]("M_2Bp") (M = Li, 16; K, 19) is described. Transmetallation of Li_2Bp with YCl_3(THF)_3 affords exclusively the C_2 symmetric product rac-[BpY(µ_2-Cl)_2Li(THF)_2], 20. A X-ray crystal structure of 20 has been determined; triclinic, P1, a= 13.110 (8), b = 17.163 (15), c = 20.623 (14) Å, α = 104.02 (7), β = 99.38 (5), γ = 100.24 (6)° , Z = 4, R = 0.056. Transmetallation of K_2Bp with YCl_3(THF)_3 affords the halide free complex rac-BpYCl, 23. The corresponding rac-BpLaCl, 28, is prepared in an anlogous manner. In all cases the achiral meso isomer is not obtained since only for the racemic isomers are the unfavorable steric interactions between the Si(CH3)_3 groups in the narrow portion of the [Cp-M'-Cp] wedge avoided. Alkylation of 20 or 23 with LiCH(Si(CH_3)_3)_2 affords rac-BpYCH(Si(CH_3)_3)_2, 26 in good yield. Alkylation of 28 with LiCH(Si(CH_3)_3)_2 affords rac-BpLaCH(Si(CH_3)_3)_2 29. Hydrogenation of 26 cleanly affords the bridging hydride species [BpY(µ_2-H)]_2, 27, as the homochiral (R,R) and (S,S) dimeric pairs. 26 is an efficient initiator for the polymerization of ethylene to high molecular weight linear polyethylene. 27 catalyzes the polymerization of propylene (25% v/v in methylcyclohexane) and neat samples of 1-butene, 1-pentene, 1-hexene to moderately high molecular weight polymers: polypropylene (M_n = 4,200, PDI 2.32, T_m 157 °C); poly-1-butene (M_n = 8,500, PDI 3.44, T_m 105 °C); poly-1-pentene (M_n = 20,000, PDI 1.99, T_m 73 °C); poly-1-hexene (M_n = 24,000, PDI 1.75, T_m < 25 °C). ^(13)C NMR spectra at the pentad analysis level indicates that the degree of isotacticity is 99% mmmm for all polymer samples. 27 is the first single component iso-specific α-olefin polymerization catalyst. The presumed origins of the high isospecificity are presented.

Investigations of the origin of stereocontrol in syndiospecific Ziegler-Natta polymerizations

In order to expand our understanding of the mechanism of stereocontrol in syndiospecific α-olefin polymerization, a family of Cs-symmetric, ansa-group 3 metallocenes was targeted as polymerization catalysts. The syntheses of new ansa-yttrocene and scandocene derivatives that employ the doubly [SiMe₂]- bridged ligand array (1,2-SiMe₂)₂{C₅H-3,5-(CHMe₂)₂} (where R = t- butyl, tBuThp; where R = i-propyl, iPrThp) are described. The structures of tBuThpY(µ-Cl)₂K(THF)₂, tBuThpSc(µ-Cl)₂K(Et₂O)₂, tBuThpYCH(SiMe₃)₂, Y₂{µ₂-(tBuThp)₂}(µ₂-H)₂, and tBuThpSc(µ-CH₃)₂ have been examined by single crystal X-ray diffraction methods. Ansa-yttrocenes and scandocenes that incorporate the singly [CPh₂]-bridged ligand array (CPh₂)(C₅H₄)(C₁₃H₈)(where C₅H₄ = Cp, cyclopentadienyl; where C₁₃H₈ = Flu, fluourenyl) have also been prepared. Select meallocene alkyl complexes are active single component catalysts for homopolymerization of propylene and 1-pentene. The scandocene tetramethylaluminate complexes generate polymers with the highes molecular weights of the series. Under all conditions examined atactic polymer microstructures are observed, suggesting a chain-end mechanism for stereocontrol.

A series of ansa-tantalocenes have been prepared as models for Ziegler-Natta polymerization catalysts. A singly bridged ansa-tantalocene trimethyl complex, Me₂Si(η⁵-C₅H₄)₂TaMe₃, has been prepared and used for the synthesis of a tantalocene ethylene-methyl complex. Addition of propylene to this ethylene-methyl adduct results in olefin exchange to give a mixture of endo and exo propylene isomers. Doubly-silylene bridged ansa-tantalocene complexes have been prepared with the tBuThp ligand; a tantalocene trimethyl complex and a tantalocene methylidene-methyl complex have been synthesized and characterized by X-ray diffraction. Thermolysis of the methylidene-methyl complex affords the corresponding ethylene-hydride complex. Addition of either propylene or styrene to this ethylene-hydride compound results in olefin exchange. In both cases, only one product isomer is observed. Studies of olefin exchange with ansa-tantalocene olefin-hydride and olefin-methyl complexes have provided information about the important steric influences for olefin coordination in Ziegler-Natta polymerization.

Ancillary ligand effects : from zirconium(IV)-catalyzed homogeneous propylene polymerization to platinum(II)-mediated C-H bond activation

A series of C_s- and C₁-symmetric doubly-linked ansa-metallocenes of the general formula {1,1'-SiMe₂-2,2'-E-('ƞ⁵-C₅H₂-4-R¹)-(ƞ⁵-C₅H-3',5'-(CHMe₂)₂)}ZrC₂ (E = SiMe₂ (1), SiPh₂ (2), SiMe₂ -SiMe₂ (3); R¹ = H, CHMe₂, C₅H₉, C₆H₁₁, C₆H₅) has been prepared. When activated by methylaluminoxane, these are active propylene polymerization catalysts. 1 and 2 produce syndiotactic polypropylenes, and 3 produces isotactic polypropylenes. Site epimerization is the major pathway for stereoerror formation for 1 and 2. In addition, the polymer chain has slightly stronger steric interaction with the diphenylsilylene linker than with the dimethylsilylene linker. This results in more frequent site epimerization and reduced syndiospecificity for 2 compared to 1.

C₁-Symmetric ansa-zirconocenes [1,1 '-SiMe₂-(C₅H₄)-(3-R-C₅H₃)]ZrCl₂ (4), [1,1 '-SiMe₂-(C₅H₄)-(2,4-R₂-C₅H₂)]ZrCl₂ (5) and [1,1 '-SiMe₂-2,2 '-(SiMe₂-SiMe₂)-(C₅H₃)-( 4-R-C₅H₂)]ZrCl₂ (6) have been prepared to probe the origin of isospecificity in 3. While 4 and 3 produce polymers with similar isospecificity, 5 and 6 give mostly hemi-isotactic-like polymers. It is proposed that the facile site epimerization via an associative pathway allows rapid equilibration of the polymer chain between the isospecific and aspecific insertion sites. This results in more frequent insertion from the isospecific site, which has a lower kinetic barrier for chain propagation. On the other hand, site epimerization for 5 and 6 is slow. This leads to mostly alternating insertion from the isospecific and aspecific sites, and consequently, a hemi-isotactic-like polymers. In comparison, site epimerization is even slower for 3, but enchainment from the aspecific site has an extremely high kinetic barrier for monomer coordination. Therefore, enchainment occurs preferentially from the isospecific site to produce isotactic polymers.

A series of cationic complexes [(ArN=CR-CR=NAr)PtMe(L)]⁺[BF₄]⁺ (Ar = aryl; R = H, CH₃; L = water, trifluoroethanol) has been prepared. They react smoothly with benzene at approximately room temperature in trifluoroethanol solvent to yield methane and the corresponding phenyl Pt(II) cations, via Pt(IV)-methyl-phenyl-hydride intermediates. The reaction products of methyl-substituted benzenes suggest an inherent reactivity preference for aromatic over benzylic C-H bond activation, which can however be overridden by steric effects. For the reaction of benzene with cationic Pt(II) complexes, in which the diimine ligands bear 3,5-disubstituted aryl groups at the nitrogen atoms, the rate-determining step is C-H bond activation. For the more sterically crowded analogs with 2,6-dimethyl-substituted aryl groups, benzene coordination becomes rate-determining. The more electron-rich the ligand, as reflected by the CO stretching frequency in the IR spectrum of the corresponding cationic carbonyl complex, the faster the rate of C-H bond activation. This finding, however, does not reflect the actual C-H bond activation process, but rather reflects only the relative ease of solvent molecules displacing water molecules to initiate the reaction. That is, the change in rates is mostly due to a ground state effect. Several lines of evidence suggest that associative substitution pathways operate to get the hydrocarbon substrate into, and out of, the coordination sphere; i.e., that benzene substitution proceeds by a solvent- (TFE-) assisted associative pathway.

新型吡咯亚胺钒烯烃聚合催化剂的研究

本文的主要工作和研究结果如下： 1. 合成与表征了一系列吡咯亚胺钒(III)配合物。在Et2AlCl的活化下，它们能高效催化乙烯聚合，活性可高达48.6 kg PE/mmolVhbar，得到高分子量且分子量分布单分散的线性聚乙烯。吡咯亚胺钒催化体系具有较好的高温耐受性，即使在70 C下聚乙烯催化活性仅比50 C时下降30-40%，并且仍然比VCl3.(THF)3活性高,且分子量分布在70 C仍能保持2.5以下，说明催化剂是单活性基的。与单配的水杨醛亚胺钒催化剂相比，吡咯亚胺钒配合物具有更高的乙烯催化活性，得到具有更窄分子量分布的聚乙烯，说明具有五元环N,N螯合的吡咯亚胺配体能更好的稳定钒活性中心，增加催化剂的活性。 2. 通过采用烷基铝预先对功能性基团进行保护的方法，我们用吡咯亚胺钒/Et2AlCl催化体系实现了乙烯与一系列功能性单体如十一烯醇、十一烯酸甲酯、烯丁醇的共聚合。与其他共聚单体相比，十一烯醇的插入率更高。在温和条件下十一烯醇的插入率可以轻松达到15.8%，活性仍能保持1.4 kg/molVh。通过控制Al/V、共聚单体浓度、聚合温度等反应参数，共聚反应的活性、功能性单体插入率、以及共聚物的分子量可在很大范围内进行调控。 3．合成并表征了一系列双吡咯亚胺钒(Ⅲ)配合物，并初步研究了其乙烯聚合行为。在Et2AlCl和三氯乙酸乙酯的存在下，这些配合物具有优异的催化乙烯聚合的能力，其聚合活性可达28.8 kg PE/mmolVh。双吡咯亚胺钒(Ⅲ)配合物比单配的吡咯亚胺钒(Ⅲ)配合物具有更好的温度耐受性，随着温度的升高，乙烯聚合活性升高，70C时活性与50C时相当或者更高。 关键词：钒催化剂，乙烯聚合，乙烯与功能性单体共聚合

非茂过渡金属烯烃聚合催化剂的研究

本论文合成、表征了一系列以镍、钦为中心离子的非茂过渡金属配合物，研究了这些配合物催化烯烃聚合的反应行为。主要工作和结论如下：1．合成、表征了一系列水杨醛亚胺中性镍配合物。在改性甲基铝氧烷（MMAO）的活化下，这些中性镍配合物可高效催化降冰片烯伽BE）的加成聚合，在优化条件下，催化活性高达7.1×107 gPNBE／molNi.h，聚降冰片烯的分子量高达1.5×106g／mol。2．合成、表征了三种新型p一二酮单亚胺中性镍配合物。X-射线分析表明，这些中性镍配合物的空间构型为扭曲的平面四边形。在Ni（CO）2的活化下，这些中性镍配合物可催化乙烯聚合生成以甲基支化为主的支化聚乙烯。在MMAO的活化下，这些中性镍配合物是降冰片烯加成聚合的高效催化剂，在优化条件下，催化活性高达4.5×107gPNBE／molNi.h，聚降冰片烯的分子量高达1.1×106g／mol。另外，这些中性镍配合物在MMAO的活化下，可催化甲基丙烯酸甲酷（MMA）聚合，生成富间规的聚甲基丙烯酸甲酷（rr一70％）。令人惊讶的是，这些中性镍配合物／MMAO体系还能催化乙烯和MMA的共聚合反应，生成乙烯与MMA的无规共聚物，极性单体的插入率可达16.7 mol％。3．合成、表征了一系列新型β-二酮单亚胺钦配合物。X-射线分析表明，这些钦配合物的立体构型为含有一个CZ轴的变形八面体。常温下，这些钦配合物在MMAO的活化下，可以高效催化乙烯活性聚合，催化活性可达1.3×l06g PE/molTi-h，生成无支链的线性聚乙烯。常温下，这些钦配合物瓜IMAO体系还能高效催化乙烯和降冰片烯的活性共聚合反应，催化活性高达3.2×106gpolymer／molTi'h，所得聚合物为乙烯与降冰片烯的交替共聚物（COC）。利用该催化体系的活性聚合性质，制备了包含半晶和无定形两种结构片段的新型A-B二嵌段共聚物（PE-b1ock-COC）。4．合成、表征了一系列新型β-二亚胺钦配合物。在MMAO的活化下，这些钦配合物可以常温催化乙烯聚合，在优化条件下，催化活性可达4.6×105 gPe／molTi-h，生成双峰分布的聚乙烯，重均分子量高达6.6×105g／mol。

过渡金属烯烃聚合催化剂的研究

本论文合成、表征了一系列以铁、钻、镍、铬为中心离子的非茂过渡金属配合物，研究了这些配合物催化烯烃聚合的反应行为。主要工作和结论如下：1，合成、表征了一系列苯环邻、对位含取代基的毗咤双亚胺铁、钻乙烯聚合催化剂。在改性甲基铝氧烷（MMAO）的活化下，这些配合物可高效催化乙烯聚合。在优化条件下，研究了邻、对位取代基电子效应对催化活性的影响。对于铁催化剂来说，不仅邻位取代基体积的大小对乙烯的插入及所得聚合物的分子量有微调作用，对位取代基的电子效应也对催化活性和聚合物分子量影响也很大。2．我们在单核毗咤双亚胺铁催化剂的基础上，设计合成了环状三核铁催化剂。这种环状三核铁催化剂的活性中心位于环状结构的内部。在有机铝的活化下，可高效催化乙烯聚合。新的催化剂展示出更长的催化剂寿命，与单核铁催化剂相比，性能上得到非常大的改善，不但能防止催化剂分子间的失活，还可有效抑制链转移反应的发生。3．在MMAO的活化下，吡啶双亚胺铁可催化（甲基）丙烯酸酷的聚合。催化MMA的聚合时，得到低分子量、窄分布的间规PMMA；催化BMA的聚合时，活性很高，得到无规、低分子量的PBMA；催化丙烯酸酷聚合时，不仅活性高，而且易得到高分子量的聚合物。催化活性、聚合物产率、立构规整度、分子量及分子量分布都会受到反应参数、催化剂结构及助催化剂的性质的影响。4．合成、表征了一系列带有不同取代基的份二亚胺镍催化剂。在MMAO的活化下，这些镍配合物可以高效催化乙烯聚合。不仅邻位取代基体积的大小对乙烯的插入及所得聚合物的分子量有微调作用，对位取代基的电子效应也对催化活性、聚合物分子量及其支化度也有很大的影响。5．合成、表征了一系列新型二苯硫醚双亚胺铬催化剂。在MMAO的活化下，这些铬配合物可常温催化乙烯聚合，生成具有宽分布的聚乙烯。配体结构以及聚合条件对催化剂的活性及所得聚合物的性质有很大影响。

Styrene polymerization catalyzed by metal porphyrin complex/MAO for in situ synthesizing polystyrene containing air stable pi cation radicals

A novel catalyst system based on nickel(II) tetraphenylporphyrin (Ni(II)TPP) and methylaluminoxane for styrene polymerization was developed. This catalyst system has a high thermal stability and show fairly good activity. The obtained polystyrene (PS) was isotactic-rich atactic polymer by C-13 NMR analysis, and its molecular weight distribution was rather narrow (M-w/M-n approximate to 1.6, by GPC analysis). ESR revealed that Ni(II)TPP pi cation radicals were formed in the polymerization and could remain in the resulting PS stably. The mechanism of the polymerization was discussed and a special coordination mechanism was proposed. The PS product containing Ni(II)TPP pi cation radicals can be used as a potential functional material.

Thiophene-NPN Ligand Supported Rare-Earth Metal Bis(alkyl) Complexes. Synthesis and Catalysis toward Highly trans-1,4 Selective Polymerization of Butadiene

A series of new rare-earth metal bis(alkyl) complexes [L(1-3)Ln(CH2SiMe3)(2)(THF)(n)] (L-1 = MeC4H2SCH2NC6H4(Ph)(2)P=NC6H2Me3-2,4,6: Ln = Sc, n = 1 (1a); Ln = Lu, n = 1 (1b); L-2 = MeC4H2SCH2NC6H4(Ph)(2)P=NC6H3Et2-2,6: Ln = Sc, n = 1 (2a); Ln = Lu, n = 1 (2b); Ln = Y, n = 1 (2c); L-3 = MeC4H2SCH2NC6H4(Ph)(2)P=(NC6H3Pr2)-Pr-i-2,6: Ln = Sc, n = 0 (3a)) and (LSc)-Sc-4(CH2SiMe3)(2()THF) (4a) (L-4 = C6H5CH2NC6H4(Ph)(2)P=NC6H3Et2-2,6) have been prepared by reaction of rare-earth metal tris(alkyl)s with the corresponding HL1-4 ligands via alkane elimination.

Highly 3,4-Selective Polymerization of Isoprene with NPN Ligand Stabilized Rare-Earth Metal Bis(alkyl)s. Structures and Performances

Deprotonation of (ArNHPPh2NAr2)-N-1 (H[NPN](n), n = 1 - 10) by Ln(CH2SiMe3)(3)(THF)(2) (Ln = Lu, Y, Sc, Er) generated a series of rare-earth metal bis(alkyl) complexes [NPN](n)Ln(CH2SiMe3)(2)(THF)(2) (1-10), which under activation with [Ph3C][B(C6F5)(4)] and AliBu(3) were tested for isoprene polymerization. The correlation between catalytic performances and molecular structures of the complexes has been investigated. Complexes 1-5 and 8, where Ar-1 is nonsubstituted or ortho-alkyl-substituted phenyl, adopt trigonal-bipyramidal geometry. The Ar-1 and Ar-2 rings are perpendicular in 1-4 and 8 but parallel in 5. When Ar-1 is pyridyl, the resultant lutetium and yttrium complexes 9a and 9b adopt tetragonal geometry with the ligand coordinating to the metal ions in a N,N,N-tridentate mode, whereas in the scandium analogue 9c, the ligand coordinates to the Sc3+ ion in a N,N-bidentate mode. These structural characteristics endow the complexes with versatile catalytic performances, With increase of the steric bulkiness of the ortho-substituents Ar-1 and Ar-2, the 3,4-selectivity increased stepwise from 81.6% for lutetium complex 1 to 96.8% for lutetium complex 6 and to 97.8% for lutetium complex 7a. However, further increase of the steric bulk of the ligand led to a slight drop of 3,4-selectivity for the attached complex 5 (95.1%).

Synthesis and characterization of the titanium complexes bearing two regioisomeric trifluoromethyl-containing enaminoketonato ligands and their behavior in ethylene polymerization

A series of new titanium complexes bearing two regioisomeric trifluoromethyl-containing enaminoketonato ligands (3a-h and 6a-h), [PhN=CRCHC(CF3)O](2)TiCl2 (3a, R = Me; 3b, R = n-C5H11; 3c, R = i-Pr; 3d, R = Cy; 3e, R = t-Bu; 3f, R = CH=CHPh; 3g, R = Et; 3h, R = n-C11H23) and [PhN=C(CF3)CHC(R)O](2)TiCl2 (6a, R = Ph; 6b, R = n-C5H11; 6c, R = i-Pr; 6d, R = Cy; 6e, R = t-Bu; 6f, R = CH=CHPh; 6g, R = CHPh2; 6h, R = CF3) have been synthesized and characterized. X-ray crystal structures analyses suggest that complexes 3c-e and 6c-d all adopt a distorted octahedral geometry around the titanium center. Complexes 3c, 3d and 6c display a cis-configuration of the two chlorine atoms around the titanium center, while complex 6d shows a trans-configuration of the two chlorine atoms. Especially, the configurational isomers (cis and trans) of complex 3e were identified both in solution and in the solid state by NMR and X-ray analyses. With modified methylaluminoxane as a cocatalyst, all the complexes are active towards ethylene polymerization, and produce high molecular weight polymers.

Influence of montmorillonite on syndiotactic polymerization behavior of styrene

The influence of montmorillonite (MMT) on the syndiotactic polymerization behavior of styrene was studied. To avoid the hydrophilic surface of the MMT coming into contact with the catalyst, which could poison it, SAN was introduced between the MMT and Cp*Ti (OCH3)(3). MMT was introduced into the catalytic system as a supporter for the Ti catalyst (supported catalytic system) or just dispersed in the polymerization solvent directly (in situ polymerization system). The polymerization results showed that surface modification of MMT dramatically affected the catalytic activity as well as the syndiotacticity of the polymers. This is mainly explained by the insulator SAN preventing the formation of the inactive/little active species Si-O-Ti and other atactic active species resulting from the reaction of the -OH on the MMT layer surface with Cp*Ti(OCH3)(3).

Pyrrolide-supported lanthanide alkyl complexes. Influence of ligands on molecular structure and catalytic activity toward isoprene polymerization

The N,N- bidentate ligands 2- {( N- 2,6- R) iminomethyl)} pyrrole ( HL1, R) dimethylphenyl; HL2, R) diisopropylphenyl) have been prepared. HL1 reacted readily with 1 equiv of lanthanide tris( alkyl)s, Ln(CH2SiMe3)(3)(THF)(2), affording lanthanide bis(alkyl) complexes L(1)Ln(CH2SiMe3)(2)(THF)(n) (1a, Ln= Lu, n = 2; 1b, Ln = Sc, n = 1) via alkane elimination. Reaction of the bulky ligand HL2 with 1 equiv of Ln(CH2SiMe3)(3)( THF)(2) gave the bis(pyrrolylaldiminato) lanthanide mono(alkyl) complexes L(2)(2)Ln- (CH2SiMe3)(THF) (2a, Ln) Lu; 2b, Ln = Sc), selectively. The N,N- bidentate ligand HL3, 2- dimethylaminomethylpyrrole, reacted with Ln( CH2SiMe3) 3( THF) 2, generating bimetallic bis( alkyl) complexes of central symmetry ( 3a, Ln = Y; 3b, Ln = Lu; 3c, Ln = Sc). Treatment of the N,N,N,N- tetradentate ligand H2L4, 2,2'-bis(2,2-dimethylpropyldiimino) methylpyrrole, with equimolar Lu(CH2SiMe3)(3)(THF)(2) afforded a C-2- symmetric binuclear complex ( 4). Complexes 3a, 3b, 3c, and 4 represent rare examples of THF- free binuclear lanthanide bis( alkyl) complexes supported by non- cyclopentadienyl ligands. All complexes have been tested as initiators for the polymerization of isoprene in the presence of AlEt3 and [ Ph3C][B(C6F5)(4)]. Complexes 1a, 1b, and 3a show activity, and 1b is the most active initiator, whereas 2a, 2b, 3b, 3c, and 4 are inert.

Pyrrolide-ligated organoyttrium complexes. Synthesis, characterization, and lactide polymerization behavior

The N,N-bidentate ligand 2-{(N-2,6-diisopropylphenyl)iminomethyl)}pyrrole (L-1) and the N,N,P-tridentate ligand 2-{(N-2-diphenylphosphinophenyl)iminomethyl)}pyrrole (L-2) have been prepared. Their reactions with homoleptic yttrium tris(alkyl) compound Y(CH2SiMe3)(3)(THF)(2) have been investigated. Treatment of Y(CH2SiMe3)(3)(THF)(2) with 1 equiv of L-1 generated a THF-solvated bimetallic (pyrrolylaldiminato)yttrium mono(alkyl) complex (1) of central symmetry. In this process, L-1 is deprotonated by metal alkyl and its imino CN group is reduced to C-N by intramolecular alkylation, generating dianionic species that bridge two yttrium alkyl units in a unique eta(5)/eta(1):kappa(1) mode. The pyrrolyl ring behaves as a heterocyclopentadienyl ligand. Reaction of Y(CH2SiMe3)(3)(THF)(2) with 2 equiv of L-1 afforded the monomeric bis(pyrrolylaldiminato)yttrium mono(alkyl) complex (2), selectively. Amination of 2 with 2,6-diisopropylaniline gave the corresponding yttrium amido complex (3). In 3 the pyrrolide ligand is monoanionic and bonds to the yttrium atom in a eta(1):kappa(1) mode. The homoleptic tris(eta(1):kappa(1)-pyrrolylaldiminato)yttrium complex (4) was isolated when the molar ratio of L-1 to Y(CH2SiMe3)(3)(THF)(2) increases to 3:1. Reaction of L-2 with equimolar Y(CH2SiMe3)(3)(THF)(2) afforded an asymmetric binuclear complex (5).