Treatment of Silylene–Phosphinidene with Chalcogens Resulted Exclusively in the Formation of Silicon‐Bonded Chalcogens

Abstract Chalcogen‐bonded silicon phosphinidenes LSi(E)−P−MecAAC (E=S (1); Se (2); Te (3); L=PhC(NtBu)2; MecAAC=C(CH2)(CMe2)2N‐2,6‐iPr2C6H3)) were synthesized from the reactions of silylene–phosphinidene LSi−P−MecAAC (A) with elemental chalcogens. All the compounds reported herein have been characterized by multinuclear NMR, elemental analyses, LIFDI‐MS, and single‐crystal X‐ray diffraction techniques. Furthermore, the regeneration of silylene–phosphinidene (A) was achieved from the reactions of 2–3 with L′Al (L′=HC{(CMe)(2,6‐iPr2C6H3N)}2). Theoretical studies on chalcogen‐bonded silicon phosphinidenes indicate that the Si−E (E=S, Se, Te) bond can be best represented as charge‐separated electron‐sharing σ‐bonding interaction between [LSi−P−MecAAC]+ and E−. The partial double‐bond character of Si−E is attributed to significant hyperconjugative donation from the lone pair on E− to the Si−N and Si−P σ*‐molecular orbitals.

The chemistry of compounds with double bonds between silicon and chalcogens are of great interestb ecauset hey are heavierc ongeners of ubiquitous ketones. [10] Severals table silicon-chalcogen speciesc ontaining doubleb onds have been developed using kinetic protection from the bulky ligand and/ or thermodynamic stabilization from the Lewis donor as well as from the acceptor.U tilizing kinetic and thermodynamic protection, several examples of compounds containing as iliconchalcogen double bond have been reported, by the groups of West, Kira, Driess, Filippou and others. [11] Driess and co-workers described the donor-stabilized thiosilanoic phosphane L'Si(S)PH 2 ,w hich is the only example of ac ompound with as ilicon-chalcogen double bond and ap hosphine functionality. [12] In this manuscript, we report for the first time the successful synthesis of LSi(E)ÀPÀ Me cAAC (E = S( 1); Se (2); Te ( 3)) with phosphinidene functionality.C ompounds 1-3 were characterized by single-crystal X-ray structural investigation and multinuclear NMR spectroscopy.A ne quimolar reaction of compound A with elemental sulfur and selenium at room temperature in toluene afforded compounds 1 and 2 in 68 %a nd 75 % yield, respectively (Scheme2). A1 :1:1 reaction of A with elemental tellurium in toluene at 60 8Cf or 12 hy ielded LSi(Te)ÀPÀ Me cAAC (3)i n7 9% yield (Scheme 2, for details see the Supporting Information). In catalytic processes, the regenerationo f parentmolecules are essential steps through reductive elimination. In this issue, recovering aS i II compound from its comparatively stable Si IV compound under mild condition is consider-ably challenging. With this in mind, we reacted 1 with monomeric Al I (L'Al, L' = HC{(CMe)(2,6-iPr 2 C 6 H 3 N)} 2 )a tr oom temperature as well as at elevated temperature, but no chalcogen transfer occurred.N onetheless, 2 and 3 react with L'Al at 60 8C, resultingi nt he formation of parents ilylene-phosphinidene (A,S cheme 3, for details see the Supporting Information).
Single crystalso f1-3 suitable for X-ray structurala nalysis were obtained from toluenes olution either at 0 8Co ra tr oom temperature (for details see the Supporting Information). Compounds 1-3 crystallizei nt he monoclinic space group P2 1 /c. All three structuresa re isostructural, whereas 1 and 2 are even isomorphous. Compound 3 crystallizes as apseudo-merohedral twin with two molecules in the asymmetric unit. As arepresentative for all the molecular structureo f1 is depicted in Figure1.I tr eveals the Si atom to be fourfold coordinated, adopting ad istortedt etrahedral geometry.T he amidinate ligand is bound in aN ,N' chelating fashion with two rather dif-ferentS i ÀNb ond lengths. Formation of the SiÀEb onds is accompanied by ad ecrease in the SiÀNa sw ell as SiÀPb ond length. The SiÀNb ond lengths of 1, 2,a nd 3 are about 3pm shortert han in A (Table 1) (15) ). [11i] The SiÀEb ond lengths in 1-3 are well within the range of previous reportedS i ÀEdouble bond lengths.
The calculated geometricalp arameters of 1-3 at the BP86/ def2-SVP level of theory are close to those from the experimental geometries ( Figure S2.1, Supporting Information). The SiÀEb ond lengths in 1 (2.019), 2 (2.160), and 3 (2.398 )i n turn are close to those of previously reported Si=Ed ouble bond lengths. [11] Note that the SiÀSb ond length in 1 is comparable to those in the amidinate stabilized siladithiocarboxylate (2.030 ), [15] as well as to silanethione (2.013 ). [16] The partial double-bond character in the SiÀEb onds in 1-3 is in agreement with the Wiberg bond index of SiÀEb ond( 1.48, 1.53, and 1.54, Table 2). The natural-charge analysisi ndicates that the polarity of the SiÀEb ond decreases when Ec hanges from St oT e. This is in corroboration with MESPd ata in which the global minimum of the ESP is observed near to the chalcogen ( Figure S2.3) and the corresponding ESP value decreases when Ec hanges from St oT e. The ESP values near to chalcogen are À38.9 (1), À36.1 (2), and À32.0 kcal mol À1 (3).
The complete EDA-NOCV resultsf or the best possibleb onding interaction are given in Table S2.6 in the Supporting Information. The electrostatic interaction has ag reater contribution (53.0 %i n1,5 5.3 %i n2,5 6.5 %i n3)i ns tabilizing the SiÀE bond than the orbital interaction. The magnitude of electrostatic interaction decreases from 1 (À243.6) to 2 (À233.6) to 3 (À207.0 kcal mol À1 ). This is in agreement with the natural charges( Ta ble 2) on Si and Ei n1-3.T he analysiso fc omponents of the interaction energy indicates that the electron sharing Si + + ÀE À (DE 1 ,T able S2.6) contributes 64.7-66.4 %o ft he total orbital-interaction energy.T he hyperconjugative donation from the lone pairs at E À to the SiÀNa sw ella sS i ÀP s*-MOs also contributes significantly to the orbital-interaction energy. The deformation density plots corresponding to these hyperconjugativei nteractions in 1 are depicted in Figure 2. Similar deformation-density plots are observed for compounds 2 and 3 as well ( Figure S2.4). The strength of thesestabilizing interactions decreases when the element Ec hanges from St oT e (DE 2 + +DE 3 ; À54.0 in 1, À46.9 in 2,a nd À38.4 kcal mol À1 in 3; Ta ble S2.6, Figure S2.4). Hence, the partial multiple-bond nature of SiÀEb ond is attributedt ot he hyperconjugative donation of the lone pairs at E À to the SiÀNa nd SiÀP s*-MOs.
In summary,t he reaction of silylene-phosphinidenew ith the heavierc halcogens S, Se, and Te resulted in the selectivef ormation of the first silicon-bonded chalcogenp hosphinidenes (1-3). All the compounds were fully characterized using multinuclear NMR, LIFDI-MS, X-ray crystallography,a nd theoretical calculations. The theoretical calculations confirmed the oxidation of silylene to be more favorable than that of the phosphinidene.T he parents ilylene-phosphinidene A was regenerated by the reactiono f2 and 3 with L'Al (L' = HC{(CMe)(2,6-iPr 2 C 6 H 3 N)} 2 ). Also, the dichotomy of regeneration of A by 2 and 3 only was rationalized by theoretical calculations which suggested that the SiÀSb ond is the strongesta mong the SiÀE bonds in 1-3.T he hyperconjugative donation from the lone pair on E À to the SiÀNa nd SiÀP s*-molecular orbitals induces ap artial double-bond charactert ot he SiÀEbond.