Organometall-Synthesen mit Diazoalkanen

Group 14 hydrides with low valent elements for activation of small molecules

Transition metal compounds are well known as activators of small molecules, and they serve as efficient catalysts for a variety of homogeneous and heterogeneous transformations. In contrast, there is a general feeling that main group compounds cannot act as efficient catalysts because of their inability to activate small molecules. Traditionally, the activation of small molecules is considered one of the key steps during a catalytic cycle with transition metals. As a consequence, researchers have long neglected the full range of possibilities in harnessing main group elements for the design of efficient catalysts. Recent developments, however, have made it possible to synthesize main group compounds with low-valent elements capable of activating small molecules. In particular, the judicious use of sterically appropriate ligands has been successful in preparing and stabilizing a variety of Group 14 hydrides with low-valent elements. In this Account, we discuss recent advances in the synthesis of Group 14 hydrides with low-valent elements and assess their potential as small-molecule activators. Group 14, which comprises the nonmetal C, the semimetals Si and Ge, and the metals Sn and Pb, was for years a source of hydrides with the Group 14 element almost exclusively in tetravalent form. Synthetic difficulties and the low stability of Group 14 hydrides in lower oxidation states were difficult to overcome. But in 2000, a divalent Sn(II) hydride was prepared as a stable compound through the incorporation of sterically encumbered aromatic ligands. More recently, the stabilization of GeH(2) and SnH(2) complexes using an N-heterocyclic carbene (NHC) as a donor and BH(3) or a metal carbonyl complex as an acceptor was reported. A similar strategy was also employed to synthesize the Si(II) hydride. This class of hydrides may be considered coordinatively saturated, with the lone pair of electrons on the Group 14 elements taking part in coordination. We discuss the large-scale synthesis of hydrides of the form LMH (where M is Ge or Sn, L is CH(N(Ar)(CMe))(2), and Ar is 2,6-iPr(2)C(6)H(3)), which has made it possible to test their reactivity in the activation of small molecules. Unlike the tetravalent Group 14 hydrides, the Ge(II) and Sn(II) hydrides were found to be able to activate a number of small molecules in the absence of any externally added catalyst. For example, the Ge(II) hydride and Sn(II) hydride can activate CO(2), and the reaction results in the formation of Ge(II) and Sn(II) esters of formic acid. This product represents a prototype of a new class of compounds of Group 14 elements. Moreover, we examined the activation of carbonyl compounds, alkynes, diazo and azo compounds, azides, and compounds containing the C═N bond. These Group 14 hydrides with low-valent elements are shown to be able to activate a number of important small molecules with C≡C, C═O, N═N, and C═N bonds. The activation of small molecules is an important step forward in the realization of main group catalyst development. Although it is not yet customary to assay the potential of newly synthesized main group compounds for small-molecule activation, our results offer good reason to do so.

Metal Nitrosyl Reactivity: Acetonitrile-Promoted Insertion of an Alkylidene into a Nitrosyl Ligand with Fission of the NO Bond

Treatment of the complexes [Re(NO)2(PR3)2][BAr(F)4] (R = Cy, 1 a; R = iPr, 1 b) with phenyldiazomethane gave the cationic benzylidene species [Re{CH(C6H5)}(NO)2(PR3)2][BAr(F)4] (2 a and 2 b) in good yields. Upon reaction of 2 a and 2 b with acetonitrile, the consecutive formation of [Re(N[triple bond]CCH3)(N[triple bond]CPh)(NO)(OC(CH3)=NH)(PR3)][BAr(F)4] (3 a and 3 b) and [Re(NCCH3)(OC{CH3}NH{C6H5})(NO)(PR3)2][BAr(F)4] (4 a and 4 b) was observed. The proposed reaction sequence involves the coupling of coordinated NO, carbene and acetonitrile molecules to yield the (1Z)-N-[imino(phenyl)methyl]ethanimidate ligand. The coupling of the nitrosyl and the benzylidene is anticipated to occur first, forming an oximate species. The subsequent acetonitrile addition can be envisaged as a heteroene reaction of the oximate and the acetonitrile ligand yielding 3 a and 3 b, which in turn can cyclise and undergo a prototropic shift initiated by an internal attack of the ethaneimidate ligand on the benzonitrile moiety to afford 4 a and 4 b.

The (Calix[4]arene)chloromolybdate(IV) Anion [MoCl(Calix)]-: A Convenient Entry into Molybdenum Calix[4]arene Chemistry

The complex (HNEt(3))[MoCl(NCMe)(Calix)] (1), prepared from the reaction of [MoCl(4)(NCMe)(2)] with p-tert-butylcalix[4]arene, H(4)Calix, in the presence of triethylamine, has been used as a source of the d(2)-[Mo(NCMe)(Calix)] fragment. Complex 1 is readily oxidized with PhICl(2) to afford the molybdenum(VI) dichloro complex [MoCl(2)(Calix)] (2). Both complexes are a convenient entry point into molybdenum(VI) and molybdenum(IV) calixarene chemistry. The reaction of 1 with trimethylphosphine and pyridine in the presence of catalytic amounts [Ag(OTf)] led to the formation of neutral d(2) complexes [Mo(PMe(3))(NCMe)(Calix)] (3) and [Mo(NC(5)H(5))(NCMe)(Calix)] (4). The role of the silver salt in the reaction mixture is presumably the oxidation of the chloromolybdate anion of 1 to give a reactive molybdenum(V) species. The same reactions can also be initiated with ferrocenium cations such as [Cp(2)Fe](BF(4)). Without the presence of coordinating ligands, the dimeric complex [[Mo(NCMe)(Calix)](2)] (5) was isolated. The reaction of 1 with Ph(2)CN(2) led to the formation of a metallahydrazone complex [Mo(N(2)CPh(2))(NCMe)(Calix)] (6), in which the diphenyldiazomethane has been formally reduced by two electrons. Molybdenum(VI) complexes were also obtained from reaction of 1 with azobenzene and sodium azide in the presence of catalytic amounts of silver salt. The reaction with azobenzene led under cleavage of the nitrogen nitrogen bond to an imido complex [Mo(NPh)(NCMe)(Calix)] (7), whereas the reaction with sodium azide afforded the mononuclear molybdenum(VI) nitrido complex (HNEt(3))[MoN(Calix)] (8).