An exciting challenge in transition metal catalyst design is to explore whether earth-abundant base metals such as Fe, Co, and Ni can mediate two-electron reductive transformations that their precious metal counterparts (e.g., Ru, Rh, Ir, and Pd) are better known to catalyze. Organometallic metalloboranes are an interesting design concept in this regard because they can serve as organometallic frustrated Lewis pairs. To build on prior studies with nickel metalloboranes featuring the DPB and Ph DPB Mes ligands in the context of H 2 and silane activation and catalysis (DPB = bis(o-diisopropylphosphinophenyl)phenylborane, Ph DPB Mes = bis(o-diphenylphosphinophenyl)mesitylborane), we now explore the reactivity of iron, [(DPB)Fe] 2 (N 2 ), 1, and cobalt, (DPB)Co(N 2 ), 2, metalloboranes toward a series of substrates with E−H bonds (E = O, S, C, N) including phenol, thiophenol, benzo[h]quinoline, and 8-aminoquinoline. In addition to displaying high stoichiometric E−H bond activation reactivity, complexes 1 and 2 prove to be more active catalysts for the hydrosilylation of ketones and aldehydes with diphenylsilane relative to ( Ph DPB Mes )Ni. Indeed, 2 appears to be the most active homogeneous cobalt catalyst reported to date for the hydrosilylation of acetophenone under the conditions studied.

General Considerations: All operations were carried out using standard Schlenk or glovebox techniques under inert atmospheres of N2 or argon. Unless otherwise noted all solvents were deoxygenated and dried by thoroughly sparging with N2 gas followed by passage through an activated alumina column in the solvent purification system by SG Water, USA LLC and stored over 3 Å molecular sieves prior to use. Non-halogenated solvents were tested with a standard purple solution of sodium benzophenone ketyl in tetrahydrofuran in order to confirm effective oxygen and moisture removal. All reagents were purchased from commercial vendors and used without further purification unless otherwise stated. 4-bromo-2,6-diisopropylanisole, 2bromophenylphosphorusdichloride, [Na][BAr F 24], [H(OEt2)2][BAr F 24] (HBAr F 24), KC8, Cp * 2Co, and [Ph2NH2][OTf] were synthesized following literature procedures. 1,2,3,4,5,6 Deuterated solvents were purchased from Cambridge Isotope Laboratories, Inc., degassed and stored over activated 3 Å molecular sieves prior to use. Elemental analyses were performed by California Institute of Technology's Elemental Analysis facility or by Midwest Microlab, LLC, Indianapolis, IN.Nuclear Magnetic Resonance Spectroscopy: 1 H and 13 C chemical shifts are reported in ppm relative to tetramethylsilane, using residual solvent resonances as internal standards. 31 P chemical shifts are reported in ppm and referenced externally to 85% aqueous H3PO4 at 0 ppm. 19 F chemical shifts are reported in ppm and referenced externally to trifluorotoluene at -63.72 ppm. 11 B chemical shifts are reported in ppm and referenced externally to BF3•Et2O at 0 ppm. Solution phase magnetic measurements were performed by the method of Evans.Infrared Spectroscopy: Solid and thin film IR measurements were obtained on a Bruker Alpha spectrometer equipped with a diamond ATR probe. S3EPR Spectroscopy: Continuous wave X-band EPR spectra were obtained on a Bruker EMX spectrometer on 2-9 mM solutions prepared as frozen glasses in 2-MeTHF. Pulse EPR spectroscopy: All pulse X-and Q-band (9.4-9.7 and 34 GHz, respectively) EPR, electron nuclear double resonance (ENDOR), and hyperfine sublevel correlation spectroscopy (HYSCORE) experiments were acquired using a Bruker ELEXSYS E580 pulse EPR spectrometer. All Q-band experiments were performed using a Bruker D2 pulse ENDOR resonator. X-band ENDOR experiments were performed using a Bruker MD-4 X-band ENDOR resonator, and X-band HYSCORE experiments were performed using a Bruker MS-5 resonator. Temperature control was achieved using an ER 4118HV-CF5-L Flexline Cryogen-Free VT cryostat manufactured by ColdEdge equipped with an Oxford Instruments Mercury ITC temperature controller.Pulse electron spin-echo detected EPR (ESE-EPR) field-swept spectra were acquired using the 2-pulse "Hahn-echo" sequence ( /2 ---echo).Pulse ENDOR spectra were acquired using the Davies pulse sequence ( − '( − '( − '( − /2 ---echo), where '( is the delay between mw pulses and RF pulses, '( is the length of the RF pulse and the RF fr...

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.

The metallostannylene Cp*( i Pr 2 MeP)(H) 2 Fe-SnDMP (1; Cp* = η 5 -C 5 Me 5 ; DMP = 2,6-dimesitylphenyl), formed by hydrogen migration in a putative Cp*( i Pr 2 MeP)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 H 2 to generate the coordinatively unsaturated [Cp*( i Pr 2 MeP)FeSnDMP][B-(C 6 F 5 ) 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 57 Fe Mossbauer 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(κ 2 -(P,Sn)Sn(DMP)CH 2 CHMePMe i Pr) (5). Complex 5 was also accessed via thermolysis of 1, and kinetics studies of this thermolytic pathway indicate that the reductive elimination of H 2 from 1 to produce a stannylyne intermediate, Cp*( i Pr 2 MeP)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.

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.