Synthesis and reactions of sterically encumbered inorganic and organometallic complexes

This work is dedicated to all the educators who have made my journey exciting and enlightening. I would like to express a special thanks to Grant Steelman for all the work he contributed to my research.

Introduction

Significantly, the same arrangements of the central element and hydrogen atoms of the ligand are present in the Group 15 complexes; For example, the hydrogen atoms bonded to the γ-carbon atoms of the [N(SiMe3)2]–ligands are oriented to maximize their distances from the metal center. A drawing of 2 is shown in Figure 14; an image of the hafnium analog 4 is in the ESI.†. This is reflected in the contraction of the Ti-O bond and the increase of the Ti-O-C angle.

Geometrically optimized structures of [BeA´3]– anions: (a) as found in crystal structure 1; (b) Related form R,R,R with one twisted ligand A´ (C1 symmetry); The R,R,S form is derived from the AlA´3 structure. Geometrically optimized structures of the K[BeA´3] complex: (a) as found in the crystal structure of 1; (b) Connected form R,R,S. The formation of the zinc species K[ZnA´3] is also similar to the formation of 1, as it arises from the reaction of 2 equivalents of K[A´].

K.; Ganguly, R.; Hirao, H.; Friščić, T.; García, F., The First Synthesis of the Sterically Encumbered Adamantoid Phosphazane P4(NtBu)6: Enabled by Mechanochemistry.

Figure 1. Distinction between attractive agostic (M…H—C) and M…Si—C interactions; only the latter is thought to be operative in M[(N,CH)(SiMe 3 ) 2 ] 3 complexes

Reaction conditions toward product formation

Bulky ligands

Amides

Allyls

Steric and electronic effects

An example of a strongly directed electronic effect can be seen in the series of mixed group 4 ligands CpxTiXy(OtBu)′4-(x+y) (X = Cl, Br). Computational studies have been used to support the existence of π-bonding character in the Ti-O bond, which results from the appropriate orbital overlap.

Methods Employed

This binding motif for the group 15 compounds mimics that of lanthanide and actinide compounds, but computational investigations have shown that the structural similarities are largely the result of interligand steric crowding, and do not arise from any peculiar electronic parallels between the p-block and f- block elements. These results are a warning not to be hasty in attributing electronic origins to structural distortions that may ultimately result from accumulation of bulky ligands.

Conclusion

This is consistent with the negligible change in M–N bond length in the calculated M[N′]3 structures in the absence of d functions on the metal. After the addition of K[OtBu], the red solution of Cp2TiCl2 in the flask turned yellow-brown and retained this color.

Introduction

Although at one time both M…(γ-H–C) and M…(β-Si–C) agostic interactions (Figure 1) were considered as possible contributors to f-element structures, ongoing investigations have shown that M …( γ-H–C) interactions do not contribute to their distinctive geometric features.43-46. Difference between attractive agostic (M…H—C) and M…Si-C interactions; only the latter is thought to be operative in the M[(N,CH)(SiMe3)2]3 complexes.

Results and Discussion

Because of the similarity between the two molecules, only the one containing P1 will be discussed here and listed in the tables. An analysis of the molecular orbitals in group 15 and f-element M[N′]3 complexes illustrates the difference in the involvement of d orbitals in their bonding.

Figure 2. Thermal ellipsoid plot of non-hydrogen atoms of P[N(SiMe) 3 ) 2 ] 3

Conclusion

In (a), the bond critical paths from the γ-carbons to the metal are outlined with dashed ellipses; neighboring critical points are circled. These results reinforce the assumption that the geometric distortions present in the Group 15 M[N′]3 complexes are not the result of M…Si–C interactions or d-orbital involvement, but are the result of severe steric crowding.

Experimental

Similar distortions are also not present in the less crowded HAs[N(SiMe3)2]2,35 although the structure is pyramidal. Assignment of charges did not improve the structure of Pu[N′]3 (eg, overpyramidalization was worse) and they were not included in the final optimization.

Introduction

This result is in contrast to a literature report,146 for which Cp2Ti(OtBu)2 was the only reported product (obtained in >90% purity); aside from possible instrumental limitations (eg, use of 90 MHz 1H NMR), we are unsure of the reason for this discrepancy. This binding motif was previously reported to be in a 1:2 ratio; i.e. the Ti–O bond order was approximately one.208.

Results and Discussion

• Nanoparticle formation
• Derivatives of P[N(SiMe 3 ) 2 ] 3

Conclusion

It does not appear at this time that the routes produce high-quality particles, and conditions would need to be highly optimized before they can be of practical use. In the case of the solution and mechanochemical approach to gold nanoparticles, it is clear that not all gold reacts, as evidenced by the yellow gold chloride-containing filtrates that remain. Although halide metathesis has been used as a method to produce other S=PNR3 complexes, it does not appear that this is a viable method to produce S=P[N′]3 and.

Future Directions

Routes to directly sulfonate or oxygenate the P[N′]3 would be theoretically possible, but are not currently being investigated.

Experimental

The volume of the yellow filtrate was reduced, and the precipitate was collected and then analyzed. The solution did not change color immediately, but darkened during the course of the reaction. Westerhausen, M., Synthesis and spectroscopic properties of bis(trimethylsilyl)amides of the alkaline-earth metals magnesium, calcium, strontium and barium.

Reaction environment and ligand liability in group 4 Cp 2 MXY (X,Y = Cl, OtBu)

Introduction

In some cases, kinetically and/or thermodynamically favored products are obtained regardless of the reaction stoichiometry.135-. The Cp´/OR complexes of the group 4 metals, Cp´2-xM(OR)2+x, are of interest for their ability to act as initiators of syndiotactic polymerization of styrene (M = Ti);139 in combination with MAO, they are highly active initiators of α-olefin polymerization.140-141 Both chiral142 and non-chiral143-144 versions of the complexes have been investigated in the polymerization of lactide monomers and ε-caprolactone.142 For M = Zr and Hf, the Cp´2M(OR)2 species are effective CVD precursors of thin-film zirconium oxide and hafnia.12, 145. The Zr analogue was prepared in a similar way (Cp2ZrCl2 + (Li or Na)[OtBu]),147 but it yields an inseparable mixture of Cp2ZrCl(OtBu) and Cp2Zr(OtBu)2, even with the use of more than 2 equiv. of the alkoxide.12 A cleaner route to Cp2Zr(OtBu)2, but with unspecified yield, was reported by alcoholysis (Cp2ZrCl2 + excess HOtBu/Et3N).12 In contrast, more predictable syntheses exist for mono(cyclopentadienyl) complexes; CpTi(OtBu)3 is produced from the reaction of CpTiCl3 with HOtBu,148 for example, and for all three metals the metathetic exchange between M(OtBu)4 and Cp2Mg in benzene produces the CpM(OtBu)3.

Results and Discussion

The formation of 1 is of particular interest, as it has not previously been reported as a member of the CpxTi(OtBu)4-x family. 1 is relatively slow, little of the bis(alkoxide) complex will be available to generate 1 by redistribution. Despite the bulk of the –OCMe3 groups, the Zr–O distance of 1.925(2) Å is slightly shorter than in related molecules (cf.

Table 8. Summary of Cp 2 MX 2 and M′[OtBu] reactions

Conclusion

All tert -butoxide complexes exhibit the hallmarks of π-bonding between the titanium ligand and the alkoxide; i.e., short Ti–O bonds and wide Ti–O–C angles. It is apparent that the calculations support an increasingly linear Ti-O-C angle in the order Me < Et < OiPr < OtBu, which is consistent with the increasing π-donating ability of the ligands. The optimized geometries of C3-symmetric K[BeA´3 found in the X-ray crystal structure of 1 and a bound R,R,S form were calculated similarly to the isolated anions and are shown in Figure 28 .

Experimental

Multicomponent mechanochemical synthesis of cyclopentadienyl titanium alkoxy halides

Introduction

The neutral framework d0 Cp´xMLyL′4-(x+y) comprises a large number of complexes in early transition metal and actinide chemistry.172 Part of the reason for this is that with four potentially different ligands on the metal center (i.e., Cp´,Cp″, L′,L″), the combination possibilities with the metallocene structure are considerable. Consider a special subset of complexes in which the cyclopentadienyl ligands are C5H5, L´ is a halide, and L″ is an alkoxide; although so restricted, 15 general compositions can satisfy the formula CpxMXy(OR)´4-(x+y) (Figure 17). Cp2MCl2 dichlorides are commonly used starting materials for bis(cyclopentadienyl) derivatives, but the reaction of Cp2TiCl2 and 2 equivalents of Li[OtBu] gives not only Cp2Ti(OtBu)2 (61%) but also CpTi(OtBu)3 (26%) and Ti (OtBu). Similarly, the reaction of Cp2ZrCl2 and (Li or Na)[OtBu] gives an inseparable mixture of Cp2ZrCl(OtBu) and Cp2Zr(OtBu)2, even using more than 2 equivalents of alkoxide.12 .

Figure 17. Combinatorial possibilities of mixed group 4 Cp x MX y (OR) 4-(x+y) complexes, starting from the homoleptic species MX 4 , M(OR) 4 , and MCp 4

Results and Discussion

Density functional theory calculations were undertaken specifically to look for evidence of π-bonding in the alkoxides with the most linear Ti–O–C bonds. Much lower in energy (MO#48) are orbitals representing Ti–O sigma bonding (Figure 23c) and the C–H bonds in the OtBu ligand. As is the case with 3, the Ti-O sigma interaction is significantly lower in energy (Figure 24c).

Figure 18. Thermal ellipsoid plot of Cp 2 TiCl(OMe)

Conclusion

Experimental

Not surprisingly, incorporation of the K + ion into the complex changes the relative stability of the species. Visualization of the extent of coordination sphere coverage (Gcomplex) of BeA´2•Et2O, BeA´2 and KBeA´3. Gottfriedsen, J.; Blaurock, S., The First Carbene Complex of a Diorganoberyllium: “Synthesis and Structural Characterization of Ph2Be(i-Pr-carbene) and Ph2Be(n-Bu2O).

Symmetric assembly of a sterically encumbered allyl complex: mechanochemical and solution

Introduction

The small size of the Be2+ cation (0.27 Å for CN = 4; cf. 0.57 Å for Mg2+)212 and its correspondingly high charge/size ratio ensure that its bonds will be significantly stronger than its heavier counterparts, even magnesium. polarized and have considerable covalent character. Not surprisingly, beryllium compounds with identical ligand sets tend to have different structures from those of other, more electropositive alkaline earth (Ae) metals. If so, beryllium allyls would join magnesium in which monodentate allyl ligands are uniformly present in ether-solvated complexes, 222 but in the absence of ethers, cation-π interactions with the metal can create “slipped-π” bonding.223.

Results and Discussion

In the solid state, 1 exhibits approximate C3 symmetry with σ-bonded A' ligands and a potassium cation that engages in cation-π interactions with the three double bonds of the allyls. The somewhat greater stability of the C3 form, possibly combined with greater ease of crystal packing, may contribute to the exclusive appearance of this form in the crystal structure. Although the coverage of the metal center in the hypothetical BeA´2 varies somewhat with the angle between the ligands, the minimum energy position depicted in Figure 29c (C2 symmetry) has only 78.7% coverage.

Table 10. Summary of K[A′] and BeCl 2 reactions; amounts of reagents given as molar ratios

Conclusion

Experimental

The nature of the stationary points was determined by analytical frequency calculations; all these optimized geometries were found to be minimal (Nimag = 0). For Group 15 complexes, ∆SiC is, in fact, either statistically indistinguishable from zero (due to the esd of the Si–C bond length) or even slightly negative; i.e., ∆SiC = –0.017 in the case of 2. E.; Maseras, F.; Etienne, M., The critical role of the correlation function in DFT descriptions of an agostic niobium complex.

M.; Parkin, G., Syntheses of the phenylchalcogenolate complexes (η5-C5Me5)2Zr(EPh)2 (E = O, S, Se, Te) and (η5-C5H5)2Zr(OPh)2: Structural comparisons within a series of complexes Contains zirconium-chalcogen single bonds. Comment206, The correlation between 9Be chemical shift is not exact and can be strongly influenced by the electronic properties of the ligands.

Figure 30. 1 H NMR of P[N(SiMe 3 ) 2 ] 3