Unprecedented Silicon ( II ) Calcium Complexes with N-Heterocyclic Silylenes

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Introduction
N-heterocyclic silylene (NHSi) complexes have been reported across the transition metals, 1 and a plethora of studies have been carried out to date, even highlighting their potential applications in catalysis. 2These efforts have been heralded by the high abundance, low toxicity and cost of silicon.Remarkably, no reports of NHSi complexes for any s-block elements exist, particularly for the alkaline-earth metals, although two seminal reports for f-block NHSi complexes have been reported. 3This is somewhat surprising since employing environmentally benign alkaline earth metals, such as calcium, might enable access to inexpensive and environmentally friendly NHSi complexes for catalysis, stoichiometric transformations, or as precursors for silicate glasses.Herein we report the first group 2 (alkaline earth metal) calcium NHSi complexes readily accessible in a quantitative fashion by simple coordination reactions with the corresponding NHSis.Preliminary reactivity studies are also reported in addition to the theoretical calculations by DFT methods elucidating the bonding situation between the Ca and Si centres.
In order to suppress this metathetical reaction, the NH-silylene 1 was functionalised with an aryloxy substituent affording The same synthetic methodology can also be employed to prepare the three coordinate NHSi complex [(η 5 -C 5 Me 5 ) 2 Ca←:Si-(N t BuCH) 2 ] (7) by reaction of the West silylene :Si(N t BuCH) 2 (8) 7 with the calcium precursor 2 in an analogous way.This affords compound 7 quantitatively as a colourless solid (Scheme 2).
Complexes 6 and 7 are remarkable examples of molecular compounds featuring Ca-Si bonds, which are exceptionally rare. 8The most diagnostic spectroscopic feature for compounds 6 and 7 is the 29 Si NMR shift in solution (6: δ = −13.7 ppm; 7: δ = 81 ppm, both in C 6 D 6 ).Strikingly, these chemical shifts are only marginally deshielded from the uncoordinated NHSis (δ = −22.8and δ = 78.3ppm 7 respectively) in both cases.This observation is in contrast to d-block NHSi complexes where a dramatic deshielding effect is observed on coordination and suggests only a very weak interaction between the Si and Ca centres in both complexes.
Dissociation in solution can be ruled out since 29 Si MASsolid state NMR spectroscopy revealed an identical chemical shift as to the solution spectrum for compound 7. Crystals of compound 6 suitable for single crystal X-ray diffraction analysis were obtained from a concentrated toluene solution with slow cooling to −30 °C.In the case of compound 7 a similar procedure also consistently afforded crystals visually of high quality, but the quality of the diffraction data was rather low (R 1 = 0.1855 for I > 2σ(I)), despite several measurements and many other crystallisation procedures.Despite this, the structural motif was unambiguously located in the structure solution (see ESI †), and the metrical parameters are in good agreement with the calculated structure (BP86-D3 functional plus DZP basis set).Fig. 2 depicts the solid state structure of compound 6 and the geometry optimised structure of 7.
Both compounds feature rather long Ca-Si bond lengths: in compound 7, bearing a three-coordinate Si atom, this distance is shorter by 18 pm compared to compound 6, indicating stronger coordination in the former compound.The observed difference in bond length might be due to sterics as compound 6 bears a substantially more sterically congested NHSi than compound 7 which would impede coordination thereby resulting in an elongated bond length.The bond lengths are somewhat longer than those reported in the calcium silyl complex Ca(thf ) 3 (Si(SiMe 3 ) 3 ) 2 at 3.042(9) and 3.086(9) Å respectively.8a Moreover, in both compounds, this bond distance is significantly larger than the sum of the single bond covalent radii proposed by Pyykkö and Atsumi 9 for Ca (1.71 Å) and Si (1.16 Å) = 2.87 Å which led us to question whether it is merely an electrostatic interaction, with no covalent character.In order to address this point we performed DFT calculations to elucidate the nature of the bonding interaction between Si and Ca in both compounds (BP86-D3 and the DZP basis).
In compound 6, the LUMO, E = −0.085eV, is located mostly on the phenyl residue while the HOMO, E = −0.146eV, mostly involves the Cp* ligands with some involvement of the Ca centre (Fig. 3).Similarly, in compound 7 the LUMO,  The most interesting MO in both compounds signifying the Si-Ca interaction is in both cases the HOMO−5 6: E = −0.207eV; 7: E = −0.230eV, which clearly shows a donoracceptor interaction between Si and Ca in both compounds (Fig. 5).Notably, in both cases some contribution arises from the Ca centre in the MO.Moreover, the calculated Wibergbond index (WBI) of compound 6 = 0.47 while in compound 7, in accord with the decreased Ca-Si bond length observed experimentally, is slightly higher at 0.53.These indices are indicative of a covalent bonding interaction between the Ca and Si centres in both compounds.
In contrast, a Bader atoms in molecules (AIM) analysis 10 of compound 7 (see ESI † and Fig. 6) revealed a (3, −1) bond critical point (BCP) between the Ca and Si centres with ρ(r) = 0.0175 a.u. and ∇ 2 ρ(r) = 0.0657 a.u. at the bond critical point.These values indicate a closed-shell (ionic) interaction between the two centres.Moreover the ratio of the eigenvalues |λ 1 |/λ 3 = 0.127 (λ 1 = −0.0115,λ 2 = −0.0130,λ 3 = 0.0902) is also indicative of a close-shell interaction.Hence the AIM analysis points to a substantial ionic character in the Si-Ca interaction; at odds with the positive WBI clearly indicating covalency in the bond.These results collectively show that the Ca-Si bonding interaction is perhaps therefore best described as a σ-donor acceptor interaction between Si and Ca which bears considerable ionic character.
The labile coordination of the NHSi in compound 7 was shown by the reaction with THF where immediate NHSi elimination results with concomitant formation of [(η 5 -C 5 Me 5 ) 2 Ca-(thf ) 2 ] (2•2thf ), observed in the 1 H NMR spectra.This result is certainly due to the highly oxophilic nature of the Ca centre.We next attempted a cycloaddition reaction of benzophenone with 7 with the goal of forming the cycloaddition product 9 (see Scheme 3).Even in this instance, the NHSi 8 is eliminated with concomitant formation of a dark purple ketone adduct   [(η 5 -C 5 Me 5 ) 2 Ca←:OvCPh 2 ] (10) (Fig. 7). 11The isolation and formation of 10 in this reaction dramatically highlights the weak coordination of the NHSi to Ca, which is even eliminated by a ketone coordination. 12Again, the oxophilic nature of the Ca centre likely drives this process.

Conclusion
The first examples of s-block N-heterocyclic silylene complexes (NHSi) have been reported for the group 2 alkaline-earth metal element calcium.7) respectively.Both compounds exhibit remarkably long Ca-Si bond lengths, but on the basis of DFT investigations these can be considered simple donoracceptor interactions from Si to Ca, with considerable ionic character.The NHSi in compound 7 is very labile to substitution, and is readily eliminated by THF and even benzophenone, affording in the latter case the ketone adduct: [(η 5 -C 5 Me 5 ) 2 Ca←:OvCPh 2 ] (10).This showcases the rather low bond energy of the Ca-Si bond.We are currently exploring NHSis that bind more strongly to the Ca centre which might enable novel Ca, Si mediated processes, without NHSi elimination.We are also currently extending this synthetic approach to other group 2 elements (Ba and Sr) and will report these finding in due course.

Experimental section
All experiments were carried out under dry oxygen-free nitrogen using standard Schlenk techniques.Solvents were dried by standard methods and freshly distilled and degassed prior to use.The NMR spectra were recorded on Bruker spectrometers (AV400 or AV200) referenced to residual solvent signals as internal standards ( 1 H NMR: C 6 D 6 , 7.15; THF-d 8 3.58 (left signal) ppm and 13 C{H} NMR: C 6 D 6 , 128.0; THF-d 8 67.6 (left signal) ppm) or with an external standard (SiMe 4 for 29 Si NMR).Concentrated solutions of samples were sealed off in a Young-type NMR tube for measurements.Signals were unambiguously assigned by a combination of H,H COSY, HSQC and HMBC correlation 2D experiments.Melting points were recorded on a "Melting point tester" device from BSGT company and are uncorrected.All the samples are sealed off in capillary under vacuum and each sample was measured in duplicate.High resolution ESI mass spectra were recorded on an Orbitrap LTQ XL of Thermo Scientific mass spectrometer and the raw data evaluated using the Xcalibur computer program.For the single crystal X-ray structure analyses the crystals were each mounted on a glass capillary in perfluorinated oil and measured in a cold N 2 flow.The data were collected on an Agilent Technologies SuperNova (single source) at 150 K (CuK α radiation, λ = 1.5418Å) and refined on F 2 with the SHELX-97 1 software package.The positions of the H atoms were calculated and considered isotropically according to a riding model.Solid state 29 Si{ 1 H} static and MAS cross polarization (CP) measurements were carried out on a Bruker Avance II spectrometer at an external magnetic field of 9.4 T (i.e. a 29 Si resonance frequency of 79.46 MHz) using a standard Bruker 4 mm double-resonance H-X MAS probe.The CP spectra were recorded with a cross polarization time of 2 ms and composite pulse 1 H decoupling was applied during the acquisition.The spectra were referenced externally to TMS (tetramethylsilane) using TKS (tetrakis(trimethylsilyl)silane) as a secondary reference.Benzophenone was purchased from Aldrich and used as received.[(η 5 -C 5 Me 5 ) 2 Ca] (2) was prepared from Ca{N(SiMe 3 ) 2 } 2 upon reaction with Cp*H and purified by sublimation as a white crystalline solid as previously reported by Tanner et al. 13 The NHSi 1 4 and 8 7a was prepared by literature procedures.

Fig. 1
Fig. 1 ORTEP representation of the molecular structure of dimer 3 in the solid state, resulting from the reaction of the chlorosilylene 1 with [(η 5 -C 5 Me 5 ) 2 Ca] (2).Thermal ellipsoids are set at the 50% probability interval and H atoms are omitted for clarity.

Fig. 6
Fig. 6 Contour map of the Laplacian in compound 7. Negative values are indicated by a dotted line (where there is charge concentration), and solid lines where the Laplacian is positive (charge depletion).