Sodium Aminodiboranates Na(H3BNR2BH3): Structural and Spectroscopic Studies of Steric and Electronic Substituent Effects.

We describe the syntheses of a series of sodium aminodiboranate salts, Na(H3B-NR2-BH3), with different substituents on nitrogen, including sodium salts of the unsubstituted aminodiboranate, H3B-NH2-BH3-, and of the N-substituted anions H3B-NRR'-BH3-, where NRR' = NHMe, NHEt, NH(SiMe3), NEt2, N(i-Pr)2, N(SiMe3)2, NMe(i-Pr), NMe(t-Bu), NMe(SiMe3), and the pyrrolidide and piperidide derivatives NC4H8, NC5H10, and NC5H8-cis-2,6-Me2. The compounds have been characterized by 1H and 11B NMR spectroscopy and IR spectroscopy; crystallographic studies have been carried out for the unsolvated N,N-dimethylaminodiboranate salt Na(H3B-NMe2-BH3) and several sodium aminodiboranate salts in which the sodium ions are solvated with ethers (dioxane, diglyme, tetrahydrofuran, and 12-crown-4) or amines (N,N,N',N'-tetramethylethylenediamine). One of the structures contains a rare example of an ether ligand in which one oxygen atom bridges between two metal ions. General structural and spectroscopic trends as a function of the substituents on nitrogen are discussed.

Versatile Fe-Sn Bonding Interactions in a Metallostannylene System: Multiple Bonding and C-H Bond Activation.

The metallostannylene Cp*(iPr2MeP)(H)2Fe-SnDMP (1; Cp* = η5-C5Me5; DMP = 2,6-dimesitylphenyl), formed by hydrogen migration in a putative Cp*(iPr2MeP)HFe[Sn(H)DMP] intermediate, serves as a robust platform for exploration of transition-metal main-group element bonding and reactivity. Upon one-electron oxidation, 1 expels H2 to generate the coordinatively unsaturated [Cp*(iPr2MeP)Fe═SnDMP][B(C6F5)4] (3), which possesses a highly polarized Fe-Sn multiple bond that involves interaction of the tin lone pair with iron. Evidence from EPR and 57Fe Mössbauer spectroscopy, along with DFT studies, shows that 3 is primarily an iron-based radical with charge localization at tin. Upon reduction of 3, C-H bond activation of the phosphine ligand was observed to produce Cp*HFe(κ2-(P,Sn)═Sn(DMP)CH2CHMePMeiPr) (5). Complex 5 was also accessed via thermolysis of 1, and kinetics studies of this thermolytic pathway indicate that the reductive elimination of H2 from 1 to produce a stannylyne intermediate, Cp*(iPr2MeP)Fe[SnDMP] (A), is likely rate-determining. Evidence indicates that the production of 5 proceeds through a concerted C-H bond activation. DFT investigations suggest that the transition state for this transformation involves C-H cleavage across the Fe-Sn bond and that a related transition state where C-H bond activation occurs exclusively at the tin center is disfavored, illustrating an effect of iron-tin cooperativity in this system.

Menaquinone Biosynthesis: New Strategies to Trap Radical Intermediates in the MqnE-Catalyzed Reaction.

Aminofutalosine synthase (MqnE) is a radical SAM enzyme that catalyzes the conversion of 3-((1-carboxyvinyl)oxy)benzoic acid to aminofutalosine during the futalosine-dependent menaquinone biosynthesis. In this Communication, we report the trapping of a radical intermediate in the MqnE-catalyzed reaction using sodium dithionite, molecular oxygen, or 5,5-dimethyl-1-pyrroline-N-oxide. These radical trapping strategies are potentially of general utility in the study of other radical SAM enzymes.

Characterization of the Earliest Intermediate of Fe-N2 Protonation: CW and Pulse EPR Detection of an Fe-NNH Species and Its Evolution to Fe-NNH2.

Iron diazenido species (Fe(NNH)) have been proposed as the earliest intermediates of catalytic N2-to-NH3 conversion (N2RR) mediated by synthetic iron complexes and relatedly as intermediates of N2RR by nitrogenase enzymes. However, direct identification of such iron species, either during or independent of catalysis, has proven challenging owing to their high degree of instability. The isolation of more stable silylated diazenido analogues, Fe(NNSiR3), and also of further downstream intermediates (e.g., Fe(NNH2)), nonetheless points to Fe(NNH) as the key first intermediate of protonation in synthetic systems. Herein we show that low-temperature protonation of a terminally bound Fe-N2- species, supported by a bulky trisphosphinoborane ligand (ArP3B), generates an S = 1/2 terminal Fe(NNH) species that can be detected and characterized by continuous-wave (CW) and pulse EPR techniques. The 1H-hyperfine for ArP3BFe(NNH) derived from the presented ENDOR studies is diagnostic for the distally bound H atom ( aiso = 16.5 MHz). The Fe(NNH) species evolves further to cationic [Fe(NNH2)]+ in the presence of additional acid, the latter being related to a previously characterized [Fe(NNH2)]+ intermediate of N2RR mediated by a far less encumbered iron tris(phosphine)borane catalyst. While catalysis is suppressed in the present sterically very crowded system, N2-to-NH3 conversion can nevertheless be demonstrated. These observations in sum add support to the idea that Fe(NNH) plays a central role as the earliest intermediate of Fe-mediated N2RR in a synthetic system.

Di-µ-bromido-bis-[benz-yl(diethyl ether)magnesium].

The title benzyl Grignard reagent, [Mg(2)Br(2)(C(7)H(7))(2)(C(4)H(10)O)(2)], was obtained by reaction of benzyl bromide with magnesium in diethyl ether, followed by crystallization from toluene. The asymmetric unit comprises one half-mol-ecule, the structural dimeric unit being generated by inversion symmetry with an Mg⋯Mg distance of 3.469 (2) Å. The Mg(II) atom exhibits a distorted tetrahedral coordination geometry. The crystal packing is defined by van der Waals inter-actions only.

Synthesis and structural diversity of barium (N,N-dimethylamino)diboranates.

The reaction of a slurry of BaBr(2) in a minimal amount of tetrahydrofuran (THF) with 2 equiv of Na(H(3)BNMe(2)BH(3)) in diethyl ether followed by crystallization from diethyl ether at -20 °C yields crystals of Ba(H(3)BNMe(2)BH(3))(2)(Et(2)O)(2) (1). Drying 1 at room temperature under vacuum gives the partially desolvated analogue Ba(H(3)BNMe(2)BH(3))(2)(Et(2)O)(x) (1') as a free-flowing white solid, where the value of x varies from <0.1 to about 0.4 depending on whether desolvation is carried out with or without heating. The reaction of 1 or 1' with Lewis bases that bind more strongly to barium than diethyl ether results in the formation of new complexes Ba(H(3)BNMe(2)BH(3))(2)(L), where L = 1,2-dimethoxyethane (2), N,N,N',N'-tetramethylethylenediamine (3), 12-crown-4 (4), 18-crown-6 (5), N,N,N',N'-tetraethylethylenediamine (6), and N,N,N',N",N"-pentamethylethylenetriamine (7). Recrystallization of 4 and 5 from THF affords the related compounds Ba(H(3)BNMe(2)BH(3))(2)(12-crown-4)(THF)·THF (4') and Ba(H(3)BNMe(2)BH(3))(2)(18-crown-6)·2THF (5'). In addition, the reaction of BaBr(2) with 2 equiv of Na(H(3)BNMe(2)BH(3)) in the presence of diglyme yields Ba(H(3)BNMe(2)BH(3))(2)(diglyme)(2) (8), and the reaction of 1 with 15-crown-5 affords the diadduct [Ba(15-crown-5)(2)][H(3)BNMe(2)BH(3)](2) (9). Finally, the reaction of BaBr(2) with Na(H(3)BNMe(2)BH(3)) in THF, followed by the addition of 12-crown-4, affords the unusual salt [Na(12-crown-4)(2)][Ba(H(3)BNMe(2)BH(3))(3)(THF)(2)] (10). All of these complexes have been characterized by IR and (1)H and (11)B NMR spectroscopy, and the structures of compounds 1-3, 4', 5', and 6-10 have been determined by single-crystal X-ray diffraction. As the steric demand of the Lewis bases increases, the structure changes from polymers to dimers to monomers and then to charge-separated species. Despite the fact that several of the barium complexes are monomeric in the solid state, none is appreciably volatile up to 200 °C at 10(-2) Torr.