Synthesis and Reactivity of Low Valent Main Group Element Complexes

Abstract

The β-diketiminate aluminum(I) complex NacNacAl (III-1) was shown to activate a range of substrates containing robust single and double bonds. Compound III-1 oxidatively adds a variety of H–X bonds (X = H, B, Al, C, Si, N, P, O) to give a series of four-coordinate aluminum hydride derivatives including the first example of an aluminum boryl hydride. In the case of Al–H addition, the reaction was shown to be in equilibrium and reversible. Furthermore, cleavage of aryl and alkyl C–F bonds, the latter a rare reaction with only a handful of examples in the literature, was observed with III-1. Robust C–O and C–S bonds were also activated by III-1 along with RS–SR and R2P–PR2 bonds. All novel aluminum complexes were characterized by spectroscopic methods and X-ray diffraction analysis for the majority of them. Activation of the C=S or P=S bonds in a thiourea or phosphine sulfide, respectively, was accomplished by III-1 to give the first examples of Lewis base-stabilized monomeric terminal aluminum sulfides. The nature of the Al=S bond was examined computationally as well as experimentally. Related reaction with a urea derivative gave an unexpected aluminum hydride while reaction of III-1 with phosphine oxides gave a putative aluminum oxide as a result of P=O bond cleavage. However, the aluminum oxo promptly deprotonates a neighbouring molecule to furnish an aluminum hydroxide as the isolated product. Reduction of the cationic germanium(II) complex IV-1 affords the formally zero valent germanium complex IV-4 stabilized by the bis(imino)pyridine platform. Compound IV-4 was fully characterized by spectroscopic methods and X-ray diffraction analysis. The molecule has a singlet ground state and DFT studies revealed partial delocalization of one of the germanium lone pairs into the ligand framework. Complex IV-4 was unreactive towards H–X bond activation, the lack of reactivity ascribed to the large singlet-triplet energy gap calculated. The same bis(imino)pyridine ligand was also used to prepare reduced zinc complexes. Monoreduction of the zinc dichloride precursor gave the formally Zn(I) compound IV-6. Further reduction of IV-6 in the presence of DMAP gave the formally zero valent zinc complex IV-9. Both compounds were fully characterized by spectroscopic methods, DFT calculations, and X-ray diffraction analysis which revealed that both zinc atoms are four-coordinate and adopt unusual square planar and see-saw geometry, respectively.