Iron-catalyzed dehydropolymerization: a convenient route to poly(phosphinoboranes) with molecular-weight control.

The catalyst loading is the key to control the molecular weight of the polymer in the iron-catalyzed dehydropolymerization of phosphine-borane adducts. Studies showed that the reaction proceeds through a chain-growth coordination-insertion mechanism.

Iron-catalyzed dehydrocoupling/dehydrogenation of amine-boranes.

The readily available iron carbonyl complexes, [CpFe(CO)2]2 (1) and CpFe(CO)2I (2) (Cp = η-C5H5), were found to be efficient precatalysts for the dehydrocoupling/dehydrogenation of the amine-borane Me2NH·BH3 (3) to afford the cyclodiborazane [Me2N-BH2]2 (4), upon UV photoirradiation at ambient temperature. In situ analysis of the reaction mixtures by (11)B NMR spectroscopy indicated that different two-step mechanisms operate in each case. Thus, precatalyst 1 dehydrocoupled 3 via the aminoborane Me2N═BH2 (5) which then cyclodimerized to give 4 via an off-metal process. In contrast, the reaction with precatalyst 2 proceeded via Me2NH-BH2-NMe2-BH3 (6) as the key intermediate, affording 4 as the final product after a second metal-mediated step. The related complex Cp2Fe2(CO)3(MeCN) (7), formed by photoirradiation of 1 in MeCN, was found to be a substantially more active dehydrocoupling catalyst and not to require photoactivation, but otherwise operated via a two-step mechanism analogous to that for 1. Significantly, detailed mechanistic studies indicated that the active catalyst generated from precatalyst 7 was heterogeneous in nature and consisted of small iron nanoparticles (≤10 nm). Although more difficult to study, a similar process is highly likely to operate for precatalyst 1 under photoirradiation conditions. In contrast to the cases of 7 and 1, analogous experimental studies for the case of photoactivated Fe precatalyst 2 suggested that the active catalyst formed in this case was homogeneous. Experimental and computational DFT studies were used to explore the catalytic cycle which appears to involve amine-borane ligated [CpFe(CO)](+) as a key intermediate.

(Acetonitrile-κN)[2-(diphenylphosphan-yl)ethanamine-κN,P][(1,2,3,4,5-η)-1,2,3,4,5-pentamethylcyclopentadienyl]iron(II) hexafluoridophosphate tetrahydrofuran monosolvate.

In the title cationic Cp(*)Fe(II) complex, [Fe(C(10)H(15))(CH(3)CN)(C(14)H(16)NP)]PF(6)·C(4)H(8)O, the metal ion is coordinated by the η(5)-Cp* ring as well as the P and N atoms of the chelating 2-(diphenyl-phosphino)ethyl-amine ligand and an additional acetonitrile mol-ecule in a piano-chair conformation. The PF(6) (-) anion is disordered over two sets of sites with occupancies of 0.779 (7) and 0.221 (7).

[1,2-Bis(dimethyl-phosphino)ethane]carbon-yl(η-cyclo-penta-dien-yl)iron(II) diphenyl-phosphinoylborate.

In the title compound, [Fe(C(5)H(5))(C(6)H(16)P(2))(CO)](C(12)H(13)BOP), the Fe(II) ion adopts a three-legged piano-stool geometry, with Fe⋯Cg = 1.721 (5)Š(Cg = the centroid defined by the C atoms of the cyclo-penta-dienyl ring). The 1,2-bis-(dimethyl-phosphino)ethane (dmpe) ligand chelates to form a five-membered C(2)P(2)Fe ring which is in a pseudo-half-chair conformation. In the crystal structure, associations of one cation and two anions are formed via weak inter-molecular C-H⋯O hydrogen bonds, giving rise to R(4) (2)(9) rings.

Synthesis and dehydrocoupling reactivity of iron and ruthenium phosphine-borane complexes.

The Fe and Ru phosphine-borane complexes CpM(CO)2PPh2 x BH3 (1, M = Fe, 4, M = Ru) were synthesized utilizing the reaction of the phosphine-borane anion Li[PPh2 x BH3] with the iodo complexes CpM(CO)2I. The Fe complex 1 reacted with PMe3 to yield CpFe(CO)(PMe3)(PPh2 x BH3) (2) and CpFe(PMe3)2(PPh2 x BH3) (3) whereas the Ru species 4 gave only CpRu(CO)(PMe3)(PPh2 x BH3) (5). The complexes 1-5 were characterized by 1H, 11B, 13C and 31P NMR spectroscopy, MS, IR and X-ray crystallography for 1 to 4, and EA for 1, 2 and 4. The reactivity of 1 and 4 towards PPh2H x BH3 was explored. Although no stoichiometric reactions were detected under mild conditions, both 1 and 4 were found to function as dehydrocoupling catalysts to afford Ph2PH x BH2 x PPh2 x BH3 in the melt at elevated temperature (120 degrees C). The carbonyl Fe2(CO)9 also functioned as a dehydrocoupling catalyst under similar conditions. Complex 1 and Fe2(CO)9 represent the first reported active Fe complexes for the catalytic dehydrocoupling of phosphine-borane adducts.

Transition-metal-catalyzed dehydrocoupling: a convenient route to bonds between main-group elements.

The development of transition-metal-catalyzed dehydrocoupling reactions as a synthetic method for the formation of main-group element-element bonds provides an increasingly attractive and convenient alternative to traditional routes such as salt metathesis/elimination-type reactions. Since the first reported examples in the early 1980s, there has been a rapid expansion of this field, with extensions to a wide variety of metal-mediated homonuclear and heteronuclear bond-forming processes. Applications of this new chemistry in molecular and polymer synthesis, materials science, hydrogen storage and the transfer hydrogenation of organic substrates are attracting growing attention. An overview of this emerging area is presented in this Concepts article with a focus on recent results.