A highly reactive rhodium(I)-boryl complex as a useful tool for C-H bond activation and catalytic C-F bond borylation

Chemical Equation Presentation C-F bond borylation: A 16-electron rhodium (1)-boryl complex was synthesized by borylation of a rhodium (1)-fluorine complex. The former reacts with benzene or 2,3,5,6-tetrafluoropyridine by C-H activation. A catalytic C-F borylation reaction of pentafluoropyridine was also developed, which uses [Rh(Bpin) (PEt3)3] as a catalyst and Me3SiSiMe3 as a solvent. pin = pinacol.</p

Stoichiometric analysis of nutrient availability (N, P, K) within soils of polygonal tundra

Plant growth in arctic tundra is known to be commonly limited by nitrogen. However, biogeochemical interactions between soil, vegetation and microbial biomass in arctic ecosystems are still insufficiently understood. In this study, we investigated different compartments of the soil-vegetation system of polygonal lowland tundra: bulk soil, inorganic nutrients, microbial biomass and vegetation biomass were analyzed for their contents of carbon, nitrogen, phosphorus and potassium. Samples were taken in August 2011 in the Indigirka lowlands (NE Siberia, Russia) in a detailed grid (4 m × 5 m) in one single ice-wedge polygon. We used a stoichiometric approach, based on the N/P ratios in the vegetation biomass and the investigated soil fractions, to analyze limitation relations in the soil-vegetation system. Plant growth in the investigated polygonal tundra appears to be co-limited by nitrogen and phosphorus or in some cases only limited by nitrogen whereas potassium is not limiting plant growth. However, as the N/P ratios of the microbial biomass in the uppermost soil horizons were more than twice as high as previously reported for arctic ecosystems, nitrogen mineralization and fixation may be limited at present by phosphorus. We found that only 5 % of the total nitrogen is already cycling in the biologically active fractions. On the other hand, up to 40 % of the total phosphorus was found in the biologically active fractions. Thus, there is less potential for increased phosphorus mineralization than for increased nitrogen mineralization in response to climate warming, and strict phosphorus limitation might be possible in the long-term

Rhodium(I) silyl complexes for C-F bond activation reactions of aromatic compounds:Experimental and computational studies

The rhodium(I) silyl complexes [Rh{Si(OEt)3}(PEt 3)3] (2a) and [Rh{Si(OMe)3}(PEt 3)3] (2b) were synthesized by treatment of [Rh(CH 3)(PEt3)3] (1) with the corresponding silanes HSi(OEt)3 and HSi(OMe)3 at low temperature. The intermediate oxidative addition products fac-[Rh(H)(CH3){Si(OR) 3}(PEt3)3] (R = Et, 6a; R = Me, 6b) were observed by low-temperature NMR spectroscopy. A reaction of 2a with CO afforded trans-[Rh(CO){Si(OEt)3}(PEt3)2] (7) by the replacement of the phosphine ligand in the position trans to the silyl group. Treatment of 2a,b with pentafluoropyridine led to C-F activation reactions at the 2-position, yielding [Rh(2-C5F4N)(PEt 3)3] (11). The silyl complexes [Rh{Si(OR) 3}(PEt3)3] (2a,b) gave with 2,3,5,6-tetrafluoropyridine the C-F activation product [Rh(2-C5F 3HN)(PEt3)3] (10), whereas complex 7 reacted by C-H activation to furnish trans-[Rh(CO)(4-C5F4N)(PEt 3)2] (12). The C-F activation of pentafluoropyridine at 2b was studied with density functional theory calculations using a [Rh{Si(OMe)3}(PMe3)3] model complex (2′). The calculations indicate that a silyl-assisted C-F activation mechanism, analogous to related ligand-assisted processes at metal-phosphine and metal-boryl bonds, is more accessible than a C-F oxidative addition/Si-F reductive elimination pathway. The silyl-assisted process also proceeds with a kinetic preference for activation at the 2-position, as the transition state in this case derives extra stabilization through a Rh⋯N interaction. The C-F oxidative addition transition states show a significant degree of phosphine-assisted character and are not only higher in energy than the silyl-assisted process but also favor activation at the 4-position.</p

Rhodium(I) silyl complexes for C-F bond activation reactions of aromatic compounds: experimental and computational studies

The rhodium(I) silyl complexes [Rh{Si(OEt)3}(PEt 3)3] (2a) and [Rh{Si(OMe)3}(PEt 3)3] (2b) were synthesized by treatment of [Rh(CH 3)(PEt3)3] (1) with the corresponding silanes HSi(OEt)3 and HSi(OMe)3 at low temperature. The intermediate oxidative addition products fac-[Rh(H)(CH3){Si(OR) 3}(PEt3)3] (R = Et, 6a; R = Me, 6b) were observed by low-temperature NMR spectroscopy. A reaction of 2a with CO afforded trans-[Rh(CO){Si(OEt)3}(PEt3)2] (7) by the replacement of the phosphine ligand in the position trans to the silyl group. Treatment of 2a,b with pentafluoropyridine led to C-F activation reactions at the 2-position, yielding [Rh(2-C5F4N)(PEt 3)3] (11). The silyl complexes [Rh{Si(OR) 3}(PEt3)3] (2a,b) gave with 2,3,5,6-tetrafluoropyridine the C-F activation product [Rh(2-C5F 3HN)(PEt3)3] (10), whereas complex 7 reacted by C-H activation to furnish trans-[Rh(CO)(4-C5F4N)(PEt 3)2] (12). The C-F activation of pentafluoropyridine at 2b was studied with density functional theory calculations using a [Rh{Si(OMe)3}(PMe3)3] model complex (2′). The calculations indicate that a silyl-assisted C-F activation mechanism, analogous to related ligand-assisted processes at metal-phosphine and metal-boryl bonds, is more accessible than a C-F oxidative addition/Si-F reductive elimination pathway. The silyl-assisted process also proceeds with a kinetic preference for activation at the 2-position, as the transition state in this case derives extra stabilization through a Rh⋯N interaction. The C-F oxidative addition transition states show a significant degree of phosphine-assisted character and are not only higher in energy than the silyl-assisted process but also favor activation at the 4-position.</p

Rhodium(I) Silyl Complexes for C–F Bond Activation Reactions of Aromatic Compounds: Experimental and Computational Studies

The rhodium­(I) silyl complexes [Rh­{Si­(OEt)₃}­(PEt₃)₃] (<b>2a</b>) and [Rh­{Si­(OMe)₃}­(PEt₃)₃] (<b>2b</b>) were synthesized by treatment of [Rh­(CH₃)­(PEt₃)₃] (<b>1</b>) with the corresponding silanes HSi­(OEt)₃ and HSi­(OMe)₃ at low temperature. The intermediate oxidative addition products <i>fac</i>-[Rh­(H)­(CH₃)­{Si­(OR)₃}­(PEt₃)₃] (R = Et, <b>6a</b>; R = Me, <b>6b</b>) were observed by low-temperature NMR spectroscopy. A reaction of <b>2a</b> with CO afforded <i>trans</i>-[Rh­(CO)­{Si­(OEt)₃}­(PEt₃)₂] (<b>7</b>) by the replacement of the phosphine ligand in the position <i>trans</i> to the silyl group. Treatment of <b>2a</b>,<b>b</b> with pentafluoropyridine led to C–F activation reactions at the 2-position, yielding [Rh­(2-C₅F₄N)­(PEt₃)₃] (<b>11</b>). The silyl complexes [Rh­{Si­(OR)₃}­(PEt₃)₃] (<b>2a</b>,<b>b</b>) gave with 2,3,5,6-tetrafluoropyridine the C–F activation product [Rh­(2-C₅F₃HN)­(PEt₃)₃] (<b>10</b>), whereas complex <b>7</b> reacted by C–H activation to furnish <i>trans</i>-[Rh­(CO)­(4-C₅F₄N)­(PEt₃)₂] (<b>12</b>). The C–F activation of pentafluoropyridine at <b>2b</b> was studied with density functional theory calculations using a [Rh­{Si­(OMe)₃}­(PMe₃)₃] model complex (<b>2′</b>). The calculations indicate that a silyl-assisted C–F activation mechanism, analogous to related ligand-assisted processes at metal–phosphine and metal–boryl bonds, is more accessible than a C–F oxidative addition/Si–F reductive elimination pathway. The silyl-assisted process also proceeds with a kinetic preference for activation at the 2-position, as the transition state in this case derives extra stabilization through a Rh···N interaction. The C–F oxidative addition transition states show a significant degree of phosphine-assisted character and are not only higher in energy than the silyl-assisted process but also favor activation at the 4-position