CH2 PMe2 Research Articles

Electrochemical and Theoretical Investigations of the Oxidatively Induced Reactivity of the Complex [Fe2 (CO)4 (κ2 -dmpe)(μ-adtBn )] Related to the Active Site of [FeFe] Hydrogenases.

Electrochemical oxidation of the complex [Fe2 (CO)4 (κ2 -dmpe)(μ-adtBn )] (adtBn =(SCH2 )2 NCH2 C6 H5 , dmpe=Me2 PCH2 CH2 PMe2 ) (1) has been studied by cyclic voltammetry (CV) in acetonitrile and in dichloromethane in the presence of various substrates L (L=MeCN, trimethylphosphite, isocyanide). The oxidized species, [1-MeCN](PF6 )2 , [1-(P(OMe)3 )2 ](PF6 )2 and [1-(RNC)4 ](PF6 )2 (R=tert-butyl, xylyl), have been prepared and characterized by IR and NMR spectroscopies and, except [1-MeCN](PF6 )2 , by X-ray diffraction analysis. The crystallographic structures of the new FeII FeII complexes reveal that the association of one additional ligand (P(OMe)3 or RNC) occurs and, according to the nature of the substrates, further substitutions of one or three carbonyl groups, by P(OMe)3 or RNC, respectively, arise. Density functional theory (DFT) calculations have been performed to elucidate and discriminate, in each case, the mechanisms leading to the corresponding oxidized species. Moreover, the different degree of ligand substitution in the diiron core has been theoretically rationalized.

Steric and Electronic Effects of Bidentate Phosphine Ligands on Ruthenium(II)-Catalyzed Hydrogenation of Carbon Dioxide.

The reactivity difference between the hydrogenation of CO2 catalyzed by various ruthenium bidentate phosphine complexes was explored by DFT. In addition to the ligand dmpe (Me2 PCH2 CH2 PMe2 ), which was studied experimentally previously, a more bulky diphosphine ligand, dmpp (Me2 PCH2 CH2 CH2 PMe2 ), together with a more electron-withdrawing diphosphine ligand, PN(Me) P (Me2 PCH2 N(Me) CH2 PMe2 ), have been studied theoretically to analyze the steric and electronic effects on these catalyzed reactions. Results show that all of the most favorable pathways for the hydrogenation of CO2 catalyzed by bidentate phosphine ruthenium dihydride complexes undergo three major steps: cis-trans isomerization of ruthenium dihydride complex, CO2 insertion into the Ru-H bond, and H2 insertion into the ruthenium formate ion. Of these steps, CO2 insertion into the Ru-H bond has the lowest barrier compared with the other two steps in each preferred pathway. For the hydrogenation of CO2 catalyzed by ruthenium complexes of dmpe and dmpp, cis-trans isomerization of ruthenium dihydride complex has a similar barrier to that of H2 insertion into the ruthenium formate ion. However, in the reaction catalyzed by the PN(Me) PRu complex, cis-trans isomerization of the ruthenium dihydride complex has a lower barrier than H2 insertion into the ruthenium formate ion. These results suggest that the steric effect caused by the change of the outer sphere of the diphosphine ligand on the reaction is not clear, although the electronic effect is significant to cis-trans isomerization and H2 insertion. This finding refreshes understanding of the mechanism and provides necessary insights for ligand design in transition-metal-catalyzed CO2 transformation.

Fluorenylidene-Functionalized Lithium Phosphonium Di- and Triylides

The 9-fluorenylidene phosphorane Me3P═Flu (2) is easily accessible by deprotonation of [Me3P+–FluH]Br– (1). Its selective deprotonation in diethyl ether yielded the phosphonium di- and triylides [(Et2O)Li{CH2–PMe2–Flu}]2 (3) and [(Et2O)Li2{(CH2)2PMe–Flu}]2 (4), respectively. Both complexes crystallize as dimers, exhibiting bridging ylidic methylene fragments. Moreover, the fluorenylidene moieties also act as soft ligands in the vicinity of lithium.

Bond Activation in Iron(II)‐Coordinated Polypodal Phosphane Ligands

AbstractA mononuclear iron(II) complex is readily formed when combining equimolar amounts of iron(II) tetrafluoroborate hexahydrate, the pyridine‐derived triphosphane C5H3N{2‐[CMe(CH2PMe2)2]}{6‐[CMe2(CH2PMe2)]} (2) and diethylphosphane (Et2PH) in methanol at room temperature. The chelate ligand is in fact pentadentate, as one of the methyl groups of the neopentyl‐like sidearm engages in a C–H‐bonded contact (agostic interaction) with the metal centre, in addition to the expected NP3 coordination. The remaining site of what is a distorted coordination octahedron is occupied by monodentate Et2PH. The autoclave reaction of this purple complex with CO (10 bar, ethanol solvent, 65 °C) yields a yellow microcrystalline precipitate, whose analysis reveals a mixture of two products, in an approximate ratio of 40:1. One of the products is the cis‐dicarbonyl complex[Fe(2)(CO)2](BF4)2 (4), in which the chelate ligand acts as an NP3 donor, and the other is the monocarbonyl complex of the tetrapodal pentadentate NP4 ligand 1. The latter ligand is formed from 2 by incorporation of an additional PMe2 donor, in what, in effect, is a metal‐mediated phosphenium group transfer. A mechanism is suggested for this reaction. Other species in the reaction mixture have been identified on the basis of mass spectra, and the full NMR spectroscopic assignment of complex 4 (1H, 31P, 13C) is reported.

Bond Activation in Iron(II) and Nickel(II) Complexes of Polypodal Phosphanes

A pyridine-derived tetraphosphane ligand (donor set: NP4) has been found to undergo remarkably specific C-P bond cleavage reactions, thereby producing a ligand with an NP3 donor set. The reaction may be reversed under suitable conditions, with regeneration of the original NP4 ligand. In order to investigate the mechanism of this reaction, the NP3 donor ligand C5H3N[CMe(CH2PMe2)2][CMe2(CH2PMe2)] (11) was prepared, and its iron(II) complex 4 generated from Fe(BF4)2 ・6H2O, with methyl diethylphosphinite (7) as an additional monodentate ligand. Ligand 11 has, in addition to the NP3 donor set, one methyl group in close contact with the iron center, reminiscent of an agostic M・ ・ ・H-C interaction. Depending on the stoichiometric amount of iron(II) salt, a side product 15 is formed, which has a diethylphosphane ligand instead of the phosphinite 7 coordinated to iron(II). While attempts to deprotonate the metal-coordinated methyl group in 4 were unsuccessful, the reaction was shown to occur in an alternative complex (18), which is similar to 4 but has a trimethylphosphane ligand instead of the phosphinite 7. The reaction of complex 15 with CO gave two different products, which were both characterized by single-crystal X-ray diffraction. One (19) is the dicarbonyl iron(II) complex of the triphosphane ligand 11, the other (3) is the carbonyl iron(II) complex of the tetraphosphane C5H3N[CMe(CH2PMe2)2]2 (1). This suggests an intermolecular mechanism for the C-P bond formation in question.

Tetra‐ and Triphosphane Pyridine Podands and Their Cobalt(II) and Nickel(II) Complexes

AbstractThe reaction of the 2,6‐diethylpyridine‐derived tetraphosphane ligand C5H3N[CMe(CH2PMe2)2]2 (1Me) with nickel(II) or cobalt(II) perchlorate or tetrafluoroborate in methanol produces complexes of approximate square‐pyramidal geometry (NP4 coordination). The nickel complexes are diamagnetic, with NMR spectral features (31P) that reflect a tetrahedral distortion of diametrically opposite phosphorus donors; this is also found in the solid state. In terms of bond lengthsand angles, all complexes differ significantly from theiranalogues containing the phenyl‐substituted ligand C5H3N[CMe(CH2PPh2)2]2. The ligand 1Me is readily and completely oxidised to the corresponding tetrakis(phosphane oxide) upon reaction with NO. Slow oxidation of the cobalt(II) tetrafluoroborate complex of 1Me with aerobic oxygen in acetonitrile produces a trinuclear cobalt complex containing two equivalents of partially oxidised ligand (C2‐symmetrical; donor set: [PMe2]2[P(O)Me2]2). Four phosphane oxide oxygen atoms coordinate the central cobalt(II) ion in tetrahedral fashion, whereas the lateral cobalt(II) ions are in a square‐pyramidal environment provided by two PMe2 donors and three acetonitrile ligands in each case. The reaction of the cobalt(II) perchlorate complex of 1Me with carbonmonoxide gives an octahedral, 19‐valence‐electron dicarbonyl complex in which one of the dimethylphosphanyl groups is uncoordinated. A structural relative of 1Me, C5H3N[CMe(CH2PMe2)2][CMe2(CH2PMe2)] (2), which contains one fewer dimethylphosphanyl donor, can be prepared in a straightforward manner from 2‐ethyl‐6‐isopropylpyridine in a four‐stage process. In a series of mononuclear nickel(II) complexes, 2 employs only its three phosphane donors which, together with a monodentate ligand (acetonitrile, acetamide, or bromide), provide a tetragonal coordination environment for the metal ion which is intermediate between square planar and tetrahedral. The acetamide complex is generated from the acetonitrile complex by slow hydrolysis of the coordinated ligand. Full spectroscopic details for the complexes, as well as X‐ray structure analyses, are reported. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Solvent dependent reactivity: solvent activation vs. solvent coordination in alkylphosphane iron complexes

The pyridine-derived tetrapodal tetraphosphane C5H3N[CMe(CH2PMe2)2]2 is susceptible to selective protonolysis of a phosphorus-carbon bond in the presence of iron(II) salts. Water produces dimethylphosphinic acid, Me2POH, and protonates the anionic remainder of the tetraphosphane. The resulting iron(II) complexes and (tetrafluoroborate and perchlorate salts, respectively) contain the residual chelate ligand in which a methyl group, derived from the ligand skeleton, is in agostic interaction with the metal centre, and in which Me2POH, unavailable in the free state owing to rapid tautomerisation, is metal-coordinated and thus stabilised. Full NMR details are presented, including 31P simulations. The reactivity towards alcohols is similar (compounds), and has been studied using deuterium labels (NMR). P-C bond cleavage may be suppressed only if all protic agents are rigorously excluded, as in the reaction of with Fe(SO3CF3)2.2CH3CN in acetonitrile solution, which produces the complex [Fe(NCMe)](SO3CF3)2. In it, the ligand acts as an NP4 coordination cap but is severely distorted from square-pyramidal geometry. The reaction of with anhydrous ferrous bromide, FeBr2, in methanol again produces a dimethylphosphinic acid ester ligand, but the complex now contains ferric iron coordinated by a carbanionic residual chelate ligand, implicating H+ as the oxidising agent under these conditions. Full spectroscopic and X-ray structural details are presented for all compounds.