59th Annual Report on Research 2014

1)      We succeeded in the synthesis of the first Ge(0) compound stabilized by coordination to the non-innocent diiminopyridine (dimpyr) ligand. This result was achieved in parallel and independently from the synthesis of Ge(0) and Si(0) compounds stabilized by bis(carbene)  platforms reported last year and this year by the groups of Roesky and Driess. The Ge(0) compound dimpyrGe (1) was prepared by the reduction of the cationic Ge(II) precursor [dimpyrGeCl][GeCl₃] (dimpyr= 2,6-(ArN=CMe)_,  Ar=2,6-iPr₂C₆H₃)) by potassium graphite in benzene. Compound 1 is the first example of a zero-valent Group 14 element complex free of a carbene or carbenoid ligand. It has the singlet ground state and therefore is diamagnetic. X-ray structure study (Figure 1) suggested some electron density delocalization on the pyridine ring and also showed that the Ge atom is shifted in the plane of the pyridine fragment closer to one of the imine nitrogens. DFT calculations further revealed that the potential surface for this shift is very shallow and that there is some multiple-bond character between the Ge atom and imine nitrogens due to localization of a Ge lone pair onto the p*(C=N) orbitals (Figure 2). The second lone pair of germanium is localized in the plane of the dimpyr ligand.

Figure 1. Molecular structure of compound 1.

Figure 2. The composition of frontier MOs in compound 1.

      2) We investigated oxidative addition reactions to the Al(I) compound NacNacAl (2, NacNac = [ArNC(Me)CHC(Me)NAr]⁻ and Ar = 2,6-Prⁱ₂C₆H₃). Oxidative addition of sigma bonds to transition metals is a key step in many catalytic reactions, but such transition-metal like reactivity is poorly studied on main-group element centers. We found that compound 2 reacts with the H-X (X = H, Si, B, Al, C, N, P, O) s bonds of H₂, silanes, borane (HBpin, pin=pinacolate), allane (NacNacAlH₂), phosphine (HPPh₂), amines, alcohol (PrⁱOH), and Cp*H (Cp*= pentamethylcyclopentadiene) to give a series of hydride derivatives of the 4-coordinate derivative  NacNacAlH(X) (Scheme 1). These products of oxidative addition were characterized by spectroscopic methods (NMR and IR) and X-ray diffraction (e.g. Figure 3). In particular, this reaction allowed for the synthesis of the first boryl hydride of aluminum 3 and novel silyl hydride and phosphido hydride derivatives. Even more interesting, in the case of addition of NacNacAlH₂ to 2 to give the dimer [(NacNac)(H)Al-]₂, the reaction was found to reversible, which serves as a proof of principle for reductive elimination from the species NacNacAlH(X).            

Scheme 1. Oxidative additions to the Al(I) compound 2

      Figure 3. Molecular structure of compound NacNacAl(H)(Bpin) (3)       3) Finally, we showed that the hydride compound NacNacZnH (4) catalyzes chemoselective hydrosilylation of ketones and aldehydes under mild conditions and chemoselective reduction of nitriles to imines. The latter transformation is only the second example of such catalytic reactivity, and the first example outside the transition series. Interestingly, mechanistic studies showed that the product of nitrile insertion into the Zn-H bond of 4, the compound NacNacZn-N=C(H)(Ph) (5, Figure 4), does not react with silanes to regenerate 4 and is not a potent catalyst for this reaction. In contrast, kinetic studies for the reaction of PhCN with HSi(OEt)₃ in the presence of 5 mol% 4 suggested an unusual mechanism based on reversible coordination of silane to 4 to form a silane s-bond intermediate 6 which then reacts with the substrate (nitrile or ketone) via a cyclic transition state to give the silylated product (Scheme 2). The zinc hydride, therefore, plays a dual role in this process as a Lewis acid activator and a hydride transfer reagent.    

           
Figure 4. Molecular structure of compound 5 and its reactivity towards silanes