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 (Si II ) complexes of the type (N-nacnac)SiX (where X = H, Cl, N(SiMe 3 ) 2 , P(SiMe 3 ) 2 and Si(SiMe 3 ) 3 ). A combination of experimental and quantum chemical approaches reveals (i) that in all cases rearrangement to give an aza-butadienyl Si IV 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(SiMe 3 ) 2 , but not for X = H, N(SiMe 3 ) 2 and Si(SiMe 3 ) 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 Si II centre at the β -carbon position of the N-nacnac ligand backbone.


Introduction
The enduring popularity of the β-diketiminate ("Nacnac") ligand class, [HC{(R)C(R')N} 2 ] 2][3][4] These mono-anionic ligands benefit from having two strongly electron-donating nitrogen atoms, which, when coupled with the spatial protection from bulky Nbound groups, offer an effective blueprint for the stabilization of highly reactive coordinatively-unsaturated complexes of interest in small molecule activation.In addition, the relatively robust nature of the β-diketiminate framework means that they are typically regarded as "innocent" spectator ligands in much of this chemistry.
Nonetheless, various chemical transformations have been reported involving modification of the backbone ligand sites -for example of reactive main group β-diketiminate complexes (Scheme 1).
Examples of such behaviour include (i) addition to the γ-carbon (in conjunction with an electrophilic metal centre); 6 (ii) substituent migration from the Main Group element centre to the β-carbon position; 8 and (iii) for the widely-used β-Me Nacnac ligand family, [HC{(Me)C(R')N} 2 ] -, deprotonation of the backbone methyl group, which is often acidic enough to be attacked by strong bases.
While these processes leave the six-membered chelate ring effectively intact, other studies have identified a transformation  (primarily for low-valent metal complexes) which more severely reconfigures the coordination environment.This involves C-N bond cleavage to give an aza-butadienyl N,C-chelate, often with accompanying formation of a nitrogen-heteroelement multiple bond (Scheme 2).In previous work we have been interested in developing the chemistry of a new class of electron-rich β-diketiminate ligands, featuring amino groups at the backbone β positions ("N-nacnac" ligands). 11,12During these studies we investigated the potential of Noting that -in our hands -this process seems to occur with strongly donating ancillary ligands, 12c and that previous studies on Nacnac-stabilised niobium systems had indicated that related rearrangements are usually associated with an electron-rich metal centre, 5,10d we sought to better understand the mechanism of ring contraction.With this in mind, we report here on our attempts to develop a systematic appraisal of scope/substituent effects, through experimental and quantum chemical studies of the reactivity of (1-Dipp)SiCl with amido-, phosphido-, and silylnucleophiles.

Substituent exchange reactions of (1-Dipp)SiCl with sources of (trimethylsilyl)amido-, phosphido-and silyl-anions
At the outset, a common synthetic methodology was utilised for the salt metathesis reactions of (1-Dipp)SiCl with Na[N(SiMe 3 ) 2 ], K[Si(SiMe 3 ) 3 ], and Li[P(SiMe 3 ) 2 ](dme) (dme = 1,2,-dimethoxyethane).The inorganic salt was added to the chlorosilylene at -78°C, and the reaction mixture slowly warmed to room temperature over a period of ca. 12 h.The identities of the products, isolated after recrystallization, reveal that (under these conditions) ring contraction leads to the formation of the aza-butadienyl Si     The structures of all three compounds -the ring-contracted species (2-Dipp)Si(NDipp){N(SiMe 3 ) 2 } and (2-Dipp)Si(NDipp)-{Si(SiMe 3 ) 3 }, and the simple metathesis product (1-Dipp)Si{P(SiMe 3 ) 2 } were confirmed by X-ray crystallography.For the five-membered ring systems, structural comparison with (2-Dipp)Si(NDipp)H illustrates the role of the pendant substituents on the geometry at the central silicon atom (Figure 1 and Table 1).SiMe3)3} in the solid state as determined by X-ray crystallography.Hydrogen atoms omitted and Dipp groups shown in wireframe format for clarity; thermal ellipsoids set at the 40% probability level.Key bond lengths and angles can be found in Table 1.   1.
The more sterically encumbered groups (Si(SiMe 3 ) 3 > N(SiMe 3 ) 2 >> H) are associated with (i) longer endocyclic Si─N and Si─C bonds, and (ii) a longer exocyclic Si=N bond, and a narrower (Si─N2─C ipso ) angle within the SiNDipp moiety.In addition, at the aza-butadienyl backbone, the torsion angles between the pendant NMe 2 groups and the mean (NC 3 Si) plane are wider for larger X groups.With regards to the exocyclic Si─X bonds themselves, the Si─N(SiMe 3 ) 2 distance (1.7441(18) Å) falls in the middle of the range of reported bond lengths between four-coordinate silicon and three-coordinate nitrogen (1.65 -1.85 Å), while the Si─Si(SiMe 3 ) 3 distance (2.4055(5) Å) is towards the longer end of structurally-characterised Si-Si single bonds (2.30 -2.45 Å).

14
(1-Dipp)Si{P(SiMe 3 ) 2 } represents one of the few examples of structurally characterized phosphidosilylenes reported to date (Figure 3). 13Compared to (1-Dipp)SiCl, the greater steric bulk of P(SiMe 3 ) 2 substituent gives rise to a lower symmetry structure in the solid state: a pronounced steric clash between the P(SiMe

Quantum chemical studies of the thermodynamics and kinetics of ring contraction
DFT calculations have been carried out in order to identify factors which influence the propensity for N-nacnac ligated silylene complexes to undergo ring contraction.At the outset, analyses of the frontier orbitals of the (isolable) silylene species (1-Dipp)SiX (X = Cl, P(SiMe 3 ) 2 ) were carried out.These reveal (i) that a major contribution to the HOMO comes (intuitively) from the lone pair at the silicon centre; and (ii) that the LUMO is best described as an anti-bonding π*-orbital based on the N-nacnac ligand framework (Figure 4).Please do not adjust margins Please do not adjust margins These frontier orbital characteristics are also common to the (non-isolable) silylene complexes (1-Dipp)SiX (where X = H and Si(SiMe 3 ) 3 ), the geometries of which have been optimised with no restraints.Comparing all four calculated systems (Figure 5), it is evident that the energy of the LUMO is relatively unperturbed by changes in the silicon-bound X substituent, falling in the energy range -79.3 to -101.6 kJ mol -1 . On the other hand, variation in the X group is found to significantly affect the energy of the HOMO, which ranges from -508.3 to -432.8 kJ mol -1 .This is not unexpected given the covalent bond linking X to the silicon centre -the lone pair of which contributes substantially to the HOMO.Thus, the HOMO with the highest energy (-432.8kJ mol -1 ) is associated with the strongly σ-donating Si(SiMe 3 ) 3 function, while the σ-withdrawing effect of Cl presumably explains the much lower HOMO energy in (1-Dipp)SiCl (-508.3kJ mol -1 ).

Heterocyclic ring contraction mechanisms have been proposed
for Nacnac-ligated titanium and niobium complexes, initiated by nucleophilic attack on the β-carbon of the ligand by the electron rich metal centre.
9f, 10 From an elementary molecular orbital point of view, the frontier orbital picture for the (1-Dipp)SiX systems appears to be consistent with a similar mechanistic postulate, involving interaction between the HOMO (effectively the Si-centred lone pair) and the LUMO, which has imine π* character polarized towards the β-carbon atom.Based on these observations, a mechanistic pathway has been calculated for the conversion of the six-membered heterocyclic Si II species (1-Dipp)SiX to their ringcontracted Si IV counterparts (2-Dipp)Si(NDipp)X for X = Cl, H, P(SiMe 3 ) 2 and Si(SiMe 3 ) 3 (Scheme 5 and Table 2).In each case this transformation proceeds via a single transition state, involving the formation of a bond between the Si centre and the β-carbon of the N-nacnac ligand, with accompanying C-N bond cleavage.The overall process is always exergonic (ΔG r = -81.), consistent with the experimentally observed robustness of (1-Dipp)SiCl, and the hypothesis that this is kinetic in its origin.In a broader sense, this relative inertness correlates with (1-Dipp)SiCl having the largest HOMO-LUMO gap (412.2 kJ mol -1 ; Figure 5).Consistently, access to the transition state in the case of the hypothetical silyl-silylene (1-Dipp)Si{Si(SiMe 3 ) 3 } -which has the smallest HOMO-LUMO gap (331.2 kJ mol -1 ) -is significantly more facile, with an activation energy of only 60.3 kJ mol -1 .Scheme 5: DFT-calculated free energy profiles for the ring contraction of (1-Dipp)SiX to (2-Dipp)Si(NDipp)X (energies given in each case relative to the (1-Dipp)-ligated starting complex).Interestingly, the calculated activation energy in the case of X = H (98.7 kJ mol -1 ) is lower than that determined for the P(SiMe 3 ) 2 substituted system (126.2kJ mol -1 ), despite the hypothetical hydridosilylene species (1-Dipp)SiH having a wider HOMO-LUMO gap (382.8 vs. 341.9kJ mol -1 ; Figure 5).This implies that steric factors are also important in accessing the transition state, with the bulkier phosphide function inducing greater steric repulsion with the NDipp group.
In general, the DFT-calculated free energy profiles for the ring contraction mechanism (Scheme 5 and Table 2) are in good agreement with the experimental findings, insofar as the systems with the lowest calculated activation energies (X = H and Si(SiMe 3 ) 3 ) are isolated as the ring-contracted species at room temperature, while those with higher barriers (X = P(SiMe 3 ) 2 and Cl) require significant heating in order to effect the corresponding isomerization process (vide infra).
Experimentally, the temporal profile for the six-to-fivemembered ring contraction is (given the timeframe) most conveniently monitored by in situ NMR for the Cl-substituted system.A logarithmic plot yields a straight line, consistent with the expected first-order rate law, from which the rate constant k obs is calculated to be 6.93 (±0.03) × 10 -7 s -1 , corresponding to a half-life of ca. 12 days (see ESI).

Experimental General Methods
All manipulations were carried out using standard Schlenk line or dry-box techniques under an atmosphere of argon or dinitrogen.Solvents were degassed by sparging with argon and dried by passing through a column of the appropriate drying agent.NMR spectra were measured in C 6 D 6 , which had been dried over potassium metal, distilled under reduced pressure and stored under argon in Teflon valve ampoules.NMR samples were prepared under an inert atmosphere in 5 mm Wilmad 507-PP tubes fitted with J. Young Teflon valves.

Generic method for metathesis reactions with (1-Dipp)SiCl.
A solution of (1-Dipp)SiCl (100 mg, 0.19 mmol) in toluene was treated with the reagent of choice at -78 o C, and the solution slowly warmed to ambient temperature overnight.Subsequent filtration, followed by removal of solvent in vacuo yielded the crude residue, which was then recrystallized to give the pure product (unless otherwise stated).

Crystallography
Single-crystal X-ray diffraction data were collected using an Oxford Diffraction Supernova dual-source diffractometer equipped with a 135 mm Atlas CCD area detector.Crystals were selected under Paratone-N oil, mounted on Micromount loops and quench-cooled using an Oxford Cryosystems open flow N 2 cooling device. 18Data were collected at 150 K (except in the case of (1-Dipp)Si{P(SiMe 3 ) 2 } (115 K)) using mirror monochromated Cu K α radiation (λ = 1.5418Å) or Mo K α radiation (λ = 0.71073 Å).Data collected were processed using the CrysAlisPro package, including unit cell parameter refinement and inter-frame scaling (which was carried out using SCALE3 ABSPACK within CrysAlisPro). 19Equivalent reflections were merged and diffraction patterns processed with the CrysAlisPro suite.Structures were subsequently solved using ShelXT 2014 and refined on F 2 using the ShelXL 2014 package and the graphical interface Olex2 or XSeed. 20

Quantum chemical calculations
DFT calculations were performed using Gaussian09 (Revision D.01) programme package. 21Geometry optimisations were performed with the PBE1PBE hybrid exchange-correlation functional, 22 and the Def2-TZVP basis set, 23 with an ultrafine integration grid.Structural optimisations were carried out for the full ligand systems, and were reported as Gibbs free energies in the gas phase.Full frequency calculations were performed for optimized geometries to confirm the nature of the stationary points found (minimum or saddle point).Graphics were created with the GaussView programme.

Scheme 3 :
Scheme 3: Ring contraction of the N-nacnac ligand in conjunction with substitution of the ancillary chloride ligand in (1-Dipp)SiCl by hydride or tert-butoxide.12c
This journal is © The Royal Society of Chemistry 20xx
4 to -118.4 kJ mol -1 ), reflecting established trends in the thermodynamic stability of the Si II /Si IV oxidation states.That said, the associated activation energy varies markedly.Of the systems investigated, this barrier is highest in the case of X = Cl (+193.4kJ mol -1

( 2 - 1 H} 1 H}
Dipp)Si(NDipp)X Please do not adjust margins Please do not adjust margins than the corresponding silylenes, (1-Dipp)SiX, even in the cases of X = Cl and P(SiMe 3 ) 2 .As such, it was envisaged that isomerization might be effected at elevated temperatures to form the respective five-membered ring species, (2-Dipp)Si(NDipp){P(SiMe 3 ) 2 } and (2-Dipp)Si(NDipp)Cl.To probe this hypothesis, a C 6 D 6 solution of (1-Dipp)Si{P(SiMe 3 ) 2 } was heated at 100 °C in a J. Young's NMR tube, and conversion monitored in situ by 31 P{ NMR spectroscopy (see ESI).These measurements revealed the emergence of a new resonance at δ P = -246 ppm after 1 day, which grows in intensity at the expense of the signal due to (1-Dipp)Si{P(SiMe 3 ) 2 } (δ P = -230.2ppm).Complete consumption of the phosphido-silylene is evident after 3 days, at which point the 31 P{ NMR spectrum features only one signal at -246.3 ppm.The identity of the product associated with this resonance was unambiguously confirmed by X-ray crystallography as the aza-butadienyl complex (2-Dipp)Si(NDipp)-{P(SiMe 3 ) 2 } (Figure6) thereby confirming that the expected ring contraction process has indeed occurred under forcing conditions.

Figure 6 :
Figure 6: Molecular structure of (2-Dipp)Si(NDipp){P(SiMe3)2} in the solid state as determined by X-ray crystallography.Hydrogen atoms and benzene solvate molecule omitted and Dipp groups shown in wireframe format for clarity; thermal ellipsoids set at the 40% probability level.Key bond lengths and angles can be found in Table1.

1 H} 1 J
Spectroscopically, (2-Dipp)Si(NDipp){P(SiMe 3 ) 2 } displays a diagnostic doublet signal in its 29 Si{ NMR spectrum for the central silicon atom at -75.8 ppm, in the range characteristic of the other (2-Dipp)Si(NDipp)X systems (δ Si = -60 to -90 ppm).Additionally, the associated coupling constant ( SiP = 25 Hz) is as expected for a Si IV -P III bond; those reported for Si II -P III bonds, by contrast, are typically in excess of 150 Hz.

1 H
NMR spectroscopy.After 36 days at this temperature > 90% of the starting material had been consumed (see ESI).The pattern of new 1 H NMR resonances features six Dipp-methyl signals (in integrated ratio 1:1:1:1:2:2) and three NMe 2 -methyl signals (1:1:2), and is characteristic of the (2-Dipp)Si(NDipp)X family of compounds.Furthermore, a new singlet appears at δ Si = -90.0ppm in the 29 Si{ 1 Investigation of a range of N-nacnac ligated silylene (Si II ) complexes of the type [HC{(Me 2 N)C(Dipp)N} 2 ]SiX ((1-Dipp)SiX, where X = H, Cl, N(SiMe 3 ) 2 , P(SiMe 3 ) 2 , Si(SiMe 3 ) 3 ) by a combination of experimental and quantum chemical approaches has revealed that (i) in all cases rearrangement to give an aza-butadienyl Si IV imide featuring a contracted fivemembered heterocycle is thermodynamically favourable (and experimentally viable); (ii) the kinetic lability of systems of the type (1-Dipp)SiX varies markedly as a function of X, such that compounds of this type can be isolated for X = Cl and P(SiMe 3 ) 2 , but not for X = H, N(SiMe 3 ) 2 and Si(SiMe 3 ) 3 ; and (iii) 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 Si II centre at the β-carbon position of the N-nacnac ligand backbone.
were measured on a Bruker Avance III HD nanobay 400 MHz, Bruker Avance III 500 MHz or Bruker AVII 500 MHz spectrometer at ambient temperature.
are referenced externally to tetramethylsilane and 85% H 3 PO 4 , respectively.Chemical shifts are quoted in δ (ppm) and coupling constants in Hz.Elemental analyses were carried out at London Metropolitan University.Starting materials (1-Dipp)SiCl, 12c K[Si(SiMe 3 ) 3 ] 16 and Li[P(SiMe 3 )] 2 (dme) This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins synthesized according to literature methods.Na[N(SiMe 3 ) 2 ] was used as received.

Table 2 :
Free energy changes and activation energies associated with the ring contraction process outlined in Scheme 5 as a function of X substituent.