Thermal rearrangement of a 1,2-bis(silylene): Synthesis and crystal structure of a silyl-silylene

West’s 1981 synthesis of the first stable disilene, Mes₂Si=SiMes₂ (Mes = mesityl), via dimerization of a transient silylene, :SiMes₂, is considered by many as one of the pivotal results in the development of low oxidation state main group element chemistry (West et al., 1981). It was not until 1994 that the same group reported the first example of an isolable silylene, the two-coordinate silicon center of which was stabilized by incorporation within an N-heterocyclic framework (Denk et al., 1994). Since that time numerous heterocyclic and Lewis base stabilized silylenes have been reported, and their chemistry extensively explored (Asay et al., 2011). In 2012 the first acyclic two-coordinate silylenes were reported in simultaneous papers, one of which described the enhanced reactivity of silylene 1a (Figure 1) towards H-H and C-H bond activations (Protchenko et al., 2012; Rekken et al., 2012). Several two-coordinate acyclic silylenes have followed, including the highly reactive silyl-silylene 1b (Protchenko et al., 2013).

Figure 1

Examples of previously reported acyclic two-coordinate silylenes.

Our interest in low oxidation state silicon chemistry is not restricted to silylenes such as 1, but also includes the stabilization of formally silicon(I) species. In 2011 we were able to prepare such a species, viz. the 1,2-bis(silylene) 2 (Scheme 1), which was stabilized by incorporation of a bulky amidinate ligand developed in the group (Jones et al., 2011). It is noteworthy that Roesky and co-workers had prepared a closely related, but less bulky, amidinate substituted 1,2-bis(silylene), [{PhC(NBut)₂}Si−]₂ two years earlier (Sen et al., 2009). During the exploration of the further chemistry of 2, we have found that the compound slowly thermally isomerizes to yield a rare example of an amidinate substituted silyl-silylene, the X-ray crystal structure of which we report herein.

The deep blue 1,2-bis(silylene) 2 was dissolved in toluene, and subsequently heated at reflux for 4 days. During this time the initially deep blue solution lightened in color, eventually becoming yellow. Layering of this yellow solution with pentane led to the deposition of yellow crystals of the silyl-silylene 3 in an isolated yield of 34% (Scheme 1). It is of note that attempts to thermally isomerize the 1,2-bis(germylene) and 1,2-bis(stannylene) analogues of 2, led only to deposition of elemental germanium or tin, respectively, and generation of the protonated amidine, (4-ButC₆H₄)C(NDip){N(H)Dip} (Dip = 2,6-diisopropylphenyl).

With regard to a mechanism for the formation of 3, it seems reasonable that there is an initial de-complexation of one arm of one amidinate ligand from a silicon center to give the transient bis(silylene) intermediate, 4. Rotation of the ligand backbone about its N-C single bond could then bring the tert-butyl phenyl substituent into close proximity with the acyclic two-coordinate silylene center of 4. The phenyl then undergoes an intramolecular C-H activation at one of its ortho-positions to give 3. Such a process is not dissimilar to intramolecular C−H activation processes previously reported for silylenes 1 (Protchenko et al., 2012; Protchenko et al., 2013).

As compound 3 has limited solubility in normal non-coordinating deuterated solvents once crystallized, only its ¹H NMR spectrum could be obtained. That spectrum is consistent with the compound retaining its unsymmetrical solid state structure in solution, in that it exhibits two sets of ligand resonances. In addition, a broad signal at δ 6.39 ppm was assigned to the hydride ligand, close to where the hydride resonance for the only other example of an amidinato substituted hydridosilyl-silylene, {PhC(NBut)₂}Si−Si(H) {(ButN)₂C(H)Ph}, occurs, viz. δ 6.70 ppm (Zhang et al., 2010). It is of note that this silylene was not prepared by thermal rearrangement of a 1,2-bis(silylene), but by reduction of the silicon(IV) precursor compound, {PhC(NBut)₂}Si(H)Cl₂.

The X-ray crystal structure of compound 3 was obtained, and its molecular structure is depicted in Figure 2 (see also Table 1). This shows it to possess a three-coordinate pyramidal silylene center, Si(2), that is chelated by an amidinate ligand with an electronically delocalised NCN backbone. Further ligation comes from a hydridosilyl fragment that results from the aforementioned intramolecular thermal isomerization. That fragment is a five-membered heterocycle, which encompasses a distorted tetrahedral silicon center, Si(1). The backbone of the five-membered ring is substituted at planar C(1) by an exo-cyclic imino moiety. To the best of our knowledge, this structural motif is unprecedented in silyl-silylene chemistry. The Si−Si distance in 3, 2.394(1) Å, lies in the known range for such interactions, and is comparable to Si−Si distances in related silyl-silylenes, e.g. 2.386(1) Å in 1b, and 2.377(5) Å in {PhC(NBut)₂}Si-Si(H){(ButN)₂C(H)Ph} (Protchenko et al., 2013; Rodriguez et al., 2015; Yamaguchi and Sekiguchi, 2011; Zhang et al. 2010).

Scheme 1

Synthesis of silyl-silylene 3.

Figure 2

Thermal ellipsoid plot (25% probability surface) of the molecular structure of 3 (hydrogen atoms, except the hydride, omitted). Selected bond lengths (Å) and angles (°): Si(1)-N(1) 1.849(10), Si(1)-C(3) 1.875(2), Si(1)-Si(2) 2.394(1), Si(1)-H(1) 1.43(2), N(1)-C(1) 1.335(11), C(1)-N(2) 1.275(3), C(1)-C(2) 1.494(3), Si(2)-N(3) 1.8662(18), Si(2)-N(4) 1.8836(16), C(2)-C(3) 1.404(3), N(1)-Si(1)-C(3) 89.8(3), Si(2)-Si(1)-H(1) 121.1(9), N(3)-Si(2)-N(4) 69.39(7), N(3)-Si(2)-Si(1) 96.97(6), N(4)-Si(2)-Si(1) 106.08(6), C(1)-N(2)-C(24) 124.56(18).

In summary, an unusual amidinate coordinated silylsilylene has been prepared by thermal rearrangement of a 1,2-bis(silylene). The mechanism of the rearrangement is thought to include a transient intermediate that possesses a reactive acyclic two-coordinate silylene center. This undergoes an intramolecular C-H activation of one of its phenyl substituents to ultimately yield the isolated silylsilylene. In the solid state this is shown to incorporate a five-membered heterocyclic silyl fragment that is substituted by an imine moiety. We continue to explore the stabilization and reactivity of low oxidation state silicon compounds in our laboratory.