Amino-functionalised β-diketiminate ligands (N-nacnacs): Synthesis and coordination chemistry with Main Group elements

This thesis examines the synthesis of a series of novel amino-functionalised β-diketiminate protio-ligands ("N-nacnacH"), and the subsequent application of these N-nacnac ligands in the coordination chemistry of Main Group elements, with the main focus being on the heavier Group 14 elements (Si, Ge, Sn). Comparison between the novel N-nacnac complexes and their classical β-diketiminate "Nacnac" analogues are drawn wherever possible, in order to highlight the differences in molecular properties brought about by the introduction of the σ-electron withdrawing/π-electron donating NMe2 functions at the ligand backbone. Chapter 3 introduces an optimised synthetic protocol for the preparation of N-nacnacH protio-ligands of the form (1-Ar)H (Ar = Ph, o-Xyl, Dipp). Attempts to synthesise (1-tBu)H and (1-Mes)H are frustrated by the formation of the rearranged products 2-tBu and 3, while modifying the work-up method eventually allows access to (1-tBu)H, albeit in low yield. The isolated protio-ligands (1-Ar)H exist in the diimine configuration in both solid- and solution-state, in contrast to the imino-enamine form typical of most NacnacH systems. Direct metallation of (1-Ar)H by nBuLi or AlMe3 generates the corresponding lithiated and aluminated complexes, respectively; the former are of synthetic significance as useful reagents for the synthesis of more exotic N-nacnac Main Group species via transmetallation. Spectroscopically, the donor capability of the NMe2 functions is demonstrated by 1H NMR resonances of the backbone γ-CH protons in the metallated compounds, which are shifted significantly upfield compared to their Nacnac counterparts. In chapter 4, the formation of N-nacnac-ligated monochlorotetrelenes of the generic form (1-Ar)ECl (E = Si, Ge, Sn) is discussed. The establishment of a library of germylenes and stannylenes bearing different N-bound aromatic substituents with varying degrees of steric bulk allows for a systematic study of the steric effects of the flanking aryl groups on the overall molecular metrics of these complexes. Most importantly, (1-Dipp)SiCl represents one of the first examples of a β-diketiminate-stabilised chlorosilylene. Preliminary reactivity studies have then been carried out with the bulky chlorotetrelene systems (1-Dipp)ECl (E = Si, Ge, Sn): chloride abstraction leads to the isolation of the corresponding cationic species [(1-Dipp)E][Al{OC(CF3)3}4]. Chapter 5 reports on further investigations into the reactivities of (1-Dipp)ECl (E = Ge, Sn). The syntheses and characterisation of the hydrido- ((1-Dipp)EH) and phosphaketenyl-tetrelenes ((1-Dipp)E(PCO)) via salt metathesis are reported, and the photochemistry of (1-Dipp)Ge(PCO) subsequently explored. UV-induced P─C bond cleavage generates the bicyclic germylene 4; the formation of which is proposed by DFT calculations to proceed via either a concerted mechanism (attack of the backbone C─C bond on phosphorus with accompanied loss of CO), or a step-wise pathway involving a germaphosphinidene intermediate. Chapter 6 presents the chemistry of the N-nacnac-stabilised chlorosilylene. Mild oxidation of (1-Dipp)SiCl affords the first example of a Lewis-acid-free sila-acyl chloride (1-Dipp)Si(O)Cl, which undergoes nucleophilic sila-acyl substitution to yield the sila-aldehyde (1-Dipp)Si[O(BEt3)]H and sila-ester (1-Dipp)Si(O)(OtBu). Notably, this substitution chemistry – while well known in carbonyl chemistry – is demonstrated for the first time to be compatible with heavier Group 14 carbonyl analogues. Crystallographic studies reveal very short Si=O bonds in the Lewis-acid-free siliconyl systems, which hint at marked multiple bond character of the Si=O function. On the other hand, metathesis reactions of (1-Dipp)SiCl with sources of H- or tBuO- are found to result in a ring contraction process, generating five-membered aza-butadienyl SiIV-complexes of the form (5-Dipp)Si(NDipp)R (R = H, tBuO). Intrigued by this mode of intramolecular rearrangement, and based on related examples with Nacnac-stabilised low-valent metal complexes, the relationship between the σ-donor capability of the R substituent and the tendency for compounds of the form (1-Dipp)SiR to undergo ring contraction has been probed, both experimentally and computationally. The proposed mechanism suggests that in most cases, strong σ-donating groups favour the N,C-chelate aza-butadienyl configuration under ambient conditions. This is broadly in line with the experimental findings, with the exception of the R = P(SiMe3)2 system which, together with the σ-withdrawing R = Cl system, exists in the N-nacnac-chelate form at room temperature.</p

Amino-functionalised β-diketiminate ligands (N-nacnacs): Synthesis and coordination chemistry with Main Group elements

This thesis examines the synthesis of a series of novel amino-functionalised β-diketiminate protio-ligands ("N-nacnacH"), and the subsequent application of these N-nacnac ligands in the coordination chemistry of Main Group elements, with the main focus being on the heavier Group 14 elements (Si, Ge, Sn). Comparison between the novel N-nacnac complexes and their classical β-diketiminate "Nacnac" analogues are drawn wherever possible, in order to highlight the differences in molecular properties brought about by the introduction of the Ï-electron withdrawing/Ï-electron donating NMe2 functions at the ligand backbone. Chapter 3 introduces an optimised synthetic protocol for the preparation of N-nacnacH protio-ligands of the form (1-Ar)H (Ar = Ph, o-Xyl, Dipp). Attempts to synthesise (1-tBu)H and (1-Mes)H are frustrated by the formation of the rearranged products 2-tBu and 3, while modifying the work-up method eventually allows access to (1-tBu)H, albeit in low yield. The isolated protio-ligands (1-Ar)H exist in the diimine configuration in both solid- and solution-state, in contrast to the imino-enamine form typical of most NacnacH systems. Direct metallation of (1-Ar)H by nBuLi or AlMe3 generates the corresponding lithiated and aluminated complexes, respectively; the former are of synthetic significance as useful reagents for the synthesis of more exotic N-nacnac Main Group species via transmetallation. Spectroscopically, the donor capability of the NMe2 functions is demonstrated by 1H NMR resonances of the backbone γ-CH protons in the metallated compounds, which are shifted significantly upfield compared to their Nacnac counterparts. In chapter 4, the formation of N-nacnac-ligated monochlorotetrelenes of the generic form (1-Ar)ECl (E = Si, Ge, Sn) is discussed. The establishment of a library of germylenes and stannylenes bearing different N-bound aromatic substituents with varying degrees of steric bulk allows for a systematic study of the steric effects of the flanking aryl groups on the overall molecular metrics of these complexes. Most importantly, (1-Dipp)SiCl represents one of the first examples of a β-diketiminate-stabilised chlorosilylene. Preliminary reactivity studies have then been carried out with the bulky chlorotetrelene systems (1-Dipp)ECl (E = Si, Ge, Sn): chloride abstraction leads to the isolation of the corresponding cationic species [(1-Dipp)E][Al{OC(CF3)3}4]. Chapter 5 reports on further investigations into the reactivities of (1-Dipp)ECl (E = Ge, Sn). The syntheses and characterisation of the hydrido- ((1-Dipp)EH) and phosphaketenyl-tetrelenes ((1-Dipp)E(PCO)) via salt metathesis are reported, and the photochemistry of (1-Dipp)Ge(PCO) subsequently explored. UV-induced PâC bond cleavage generates the bicyclic germylene 4; the formation of which is proposed by DFT calculations to proceed via either a concerted mechanism (attack of the backbone CâC bond on phosphorus with accompanied loss of CO), or a step-wise pathway involving a germaphosphinidene intermediate. Chapter 6 presents the chemistry of the N-nacnac-stabilised chlorosilylene. Mild oxidation of (1-Dipp)SiCl affords the first example of a Lewis-acid-free sila-acyl chloride (1-Dipp)Si(O)Cl, which undergoes nucleophilic sila-acyl substitution to yield the sila-aldehyde (1-Dipp)Si[O(BEt3)]H and sila-ester (1-Dipp)Si(O)(OtBu). Notably, this substitution chemistry â while well known in carbonyl chemistry â is demonstrated for the first time to be compatible with heavier Group 14 carbonyl analogues. Crystallographic studies reveal very short Si=O bonds in the Lewis-acid-free siliconyl systems, which hint at marked multiple bond character of the Si=O function. On the other hand, metathesis reactions of (1-Dipp)SiCl with sources of H- or tBuO- are found to result in a ring contraction process, generating five-membered aza-butadienyl SiIV-complexes of the form (5-Dipp)Si(NDipp)R (R = H, tBuO). Intrigued by this mode of intramolecular rearrangement, and based on related examples with Nacnac-stabilised low-valent metal complexes, the relationship between the Ï-donor capability of the R substituent and the tendency for compounds of the form (1-Dipp)SiR to undergo ring contraction has been probed, both experimentally and computationally. The proposed mechanism suggests that in most cases, strong Ï-donating groups favour the N,C-chelate aza-butadienyl configuration under ambient conditions. This is broadly in line with the experimental findings, with the exception of the R = P(SiMe3)2 system which, together with the Ï-withdrawing R = Cl system, exists in the N-nacnac-chelate form at room temperature.</p

A flat carborane with multiple aromaticity beyond Wade–Mingos’ rules

The polyhedral skeletal electron pair theory (PESPT), also known as Wade-Mingos’ rules, defines a relationship between skeletal bonding electron pairs and structure of clusters. Here the authors report the synthesis, structure and computational studies of planar C2B4R4 carboranes that do not adhere to PESPT

Probing the non-innocent nature of an amino-functionalised β-diketiminate ligand in silylene/iminosilane systems

Electron-rich β-diketiminate ligands{,} featuring amino groups at the backbone β positions (“N-nacnac” ligands) have been employed in the synthesis of a range of silylene (SiII) complexes of the type (N-nacnac)SiX (where X = H{,} Cl{,} N(SiMe3)2{,} P(SiMe3)2 and Si(SiMe3)3). A combination of experimental and quantum chemical approaches reveals (i) that in all cases rearrangement to give an aza-butadienyl SiIV imide featuring a contracted five-membered heterocycle is thermodynamically favourable (and experimentally viable); (ii) that the kinetic lability of systems of the type (N-nacnac)SiX varies markedly as a function of X{,} such that compounds of this type can be isolated under ambient conditions for X = Cl and P(SiMe3)2{,} but not for X = H{,} N(SiMe3)2 and Si(SiMe3)3; and (iii) that the ring contraction process is most facile for systems bearing strongly electron-donating and sterically less encumbered X groups{,} since these allow most ready access to a transition state accessed via intramolecular nucleophilic attack by the SiII centre at the β-carbon position of the N-nacnac ligand backbone.peerReviewe

Synthesis of N‑Heterocyclic Carbene Stabilized Catecholatoborenium Cations by Ligand Substitution

Ligand substitution is not a common procedure for the preparation of different borenium cations. This work demonstrates that the chloride ligands of several NHC-stabilized dichloroborenium cations [NHC·BCl₂]­[X] (NHC = (R′C)₂(NR)₂C; <b>1</b>, R = ⁱPr, R′ = H; <b>2</b>, R = ⁱPr, R′ = Me; <b>3</b>, R = ^tBu, R′ = H; X = AlCl₄, B­(3,5-Cl₂-C₆H₃)₄) could be replaced with a catecholato moiety to produce [NHC·Bcat]­[X]. According to single-crystal X-ray analyses this particular ligand exchange enhanced the Lewis acidity of the target borocations with respect to the dichloro precursors

Counterion Dependence on the Synthetic Viability of NHC-stabilized Dichloroborenium Cations

The synthetic viability of several N-heterocyclic carbene stabilized dichloroborenium cations [NHC·BCl₂]⁺ (NHC = (RC)₂(NR′)₂C; <b>1</b>, R = R′ = Me; <b>2</b>, R = H, R′ = ⁱPr; <b>3</b>, R = Me, R′ = ⁱPr; <b>4</b>, R = H, R′ = ^tBu; <b>5</b>, R = H, R′ = 2,6-ⁱPr₂-C₆H₃) in the presence of Cl^–, AlCl₄^–, OTf^– (Tf = O₂SCF₃), NTf₂^–, and [BAr^Cl₄]⁻ (Ar^Cl = 3,5-Cl₂-C₆H₃) was investigated. None of the target borocations could be synthesized in the presence of Cl^–, as only neutral NHC·BCl₃ compounds were observed. On the other hand, it was not surprising that all targeted cations were synthetically viable in the presence of AlCl₄^– but a different degree of interion interaction was evident from ¹¹B NMR experiments. This was confirmed by X-ray analyses of [<b>1·BCl</b>_<b>2</b>]⁺, [<b>2·BCl</b>_<b>2</b>]⁺, and [<b>3·BCl</b>_<b>2</b>]⁺ in the presence of AlCl₄^– counterions, as the degree of cation–anion interaction was dependent on the steric encumbrance of the corresponding NHCs. Apart from [<b>4·BCl</b>_<b>2</b>]⁺, no borocation was synthetically viable when OTf^– and NTf₂^– were used as the counterions. Finally, we were able to show that only [<b>4·BCl</b>_<b>2</b>]⁺ could be synthetically viable without the counterion stabilization effect(s) as the preparation of [<b>4·BCl</b>_<b>2</b>]­[BAr^Cl₄] was achieved. Even though the presence of [<b>3·BCl</b>_<b>2</b>]­[BAr^Cl₄] was detected, this compound appeared not to be thermally stable, as it decomposed in solution after 48 h. The thermal stability of [<b>4·BCl</b>_<b>2</b>]⁺ and instability of [<b>3·BCl</b>_<b>2</b>]⁺ in the presence of [BAr^Cl₄]⁻ was attributed to the presence and absence, respectively, of very weak intraion (agostic) interactions in these two borocations

Highly electron rich β-diketiminato systems:synthesis and coordination chemistry of amino functionalized "<i>N</i>-nacnac" ligands

The synthesis of a class of electron-rich amino-functionalized β-diketiminato (N-nacnac) ligands is reported, with two synthetic methodologies having been developed for systems bearing backbone NMe2 or NEt2 groups and a range of N-bound aryl substituents. In contrast to their (Nacnac)H counterparts, the structures of the protio-ligands feature the bis(imine) tautomer and a backbone CH2 group. Direct metallation with lithium, magnesium or aluminium alkyls allows access to the respective metal complexes via deprotonation of the methylene function; in each case X-ray structures are consistent with a delocalized imino-amide ligand description. Trans-metallation using lithium N-nacnac complexes has then been exploited to access p and f-block metal complexes which allow for like-for-like benchmarking of the N-nacnac ligand family against their more familiar Nacnac counterparts. In the case of SnII the degree of electronic perturbation effected by introduction of the backbone NR2 groups appears to be constrained by the inability of the amino group to achieve effective conjugation with the N2C3 heterocycle. More obvious divergence from established structural norms are observed for complexes of the larger, harder YbII ion, with azaallyl/imino and even azaallyl/NMe2 coordination modes being demonstrated by X-ray crystallography

Counterion dependence on the synthetic viability of NHC-stabilized dichloroborenium cations

The synthetic viability of several N-heterocyclic carbene stabilized dichloroborenium cations [NHC·BCl2]+ (NHC = (RC)2(NR′)2C; 1, R = R′ = Me; 2, R = H, R′ = iPr; 3, R = Me, R′ = iPr; 4, R = H, R′ = tBu; 5, R = H, R′ = 2,6-iPr2-C6H3) in the presence of Cl–, AlCl4–, OTf– (Tf = O2SCF3), NTf2–, and [BArCl4]− (ArCl = 3,5-Cl2-C6H3) was investigated. None of the target borocations could be synthesized in the presence of Cl–, as only neutral NHC·BCl3 compounds were observed. On the other hand, it was not surprising that all targeted cations were synthetically viable in the presence of AlCl4– but a different degree of interion interaction was evident from 11B NMR experiments. This was confirmed by X-ray analyses of [1·BCl2]+, [2·BCl2]+, and [3·BCl2]+ in the presence of AlCl4– counterions, as the degree of cation–anion interaction was dependent on the steric encumbrance of the corresponding NHCs. Apart from [4·BCl2]+, no borocation was synthetically viable when OTf– and NTf2– were used as the counterions. Finally, we were able to show that only [4·BCl2]+ could be synthetically viable without the counterion stabilization effect(s) as the preparation of [4·BCl2][BArCl4] was achieved. Even though the presence of [3·BCl2][BArCl4] was detected, this compound appeared not to be thermally stable, as it decomposed in solution after 48 h. The thermal stability of [4·BCl2]+ and instability of [3·BCl2]+ in the presence of [BArCl4]− was attributed to the presence and absence, respectively, of very weak intraion (agostic) interactions in these two borocations.Accepted versio

Synthesis of N-Heterocyclic Carbene Stabilized Catecholatoborenium Cations by Ligand Substitution

Ligand substitution is not a common procedure for the preparation of different borenium cations. This work demonstrates that the chloride ligands of several NHC-stabilized dichloroborenium cations [NHC·BCl2][X] (NHC = (R′C)2(NR)2C; 1, R = iPr, R′ = H; 2, R = iPr, R′ = Me; 3, R = tBu, R′ = H; X = AlCl4, B(3,5-Cl2-C6H3)4) could be replaced with a catecholato moiety to produce [NHC·Bcat][X]. According to single-crystal X-ray analyses this particular ligand exchange enhanced the Lewis acidity of the target borocations with respect to the dichloro precursors.ASTAR (Agency for Sci., Tech. and Research, S’pore)Accepted versio