Formation of Di-tert-butylurea from a Mononuclear Iron Tris

Article pubs.acs.org/Organometallics

Formation of Di-tert-butylurea from a Mononuclear Iron Tris(isocyanide) Complex Anitha S. Gowda,† Andreas Baur,‡ Carl A. Scaggs,‡ Jeffrey L. Petersen,† and Jessica M. Hoover*,† †

C. Eugene Bennett Department of Chemistry, West Virginia University, Morgantown, West Virginia 26506, United States Department of Biology, Chemistry, and Geoscience, Fairmont State University, Fairmont, West Virginia 26554, United States

‡

S Supporting Information *

ABSTRACT: We report the synthesis and characterization of the tripodal iron tris(isocyanide) complexes [TmR′Fe(CNR)3](OTf) (R = tBu, Ad; R′ = Me, Ph, Mes). These complexes generate the corresponding disubstituted ureas when treated sequentially with a reductant (KC8) and a proton source (H2O). A series of labeling experiments indicate air to be the source of the urea oxygen, while the urea carbon is derived from the isocyanide. Crossover experiments indicate a key role for H2NR in the formation of the disubstituted urea, and we propose a pathway for urea formation that involves a diaminocarbene intermediate that is formed from H2NR attack on a bound isocyanide ligand.

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INTRODUCTION Carbon monoxide (CO) is an attractive source of single carbon building blocks to generate low molecular weight hydrocarbons for use as both liquid fuels and chemical feedstocks. Although Fischer−Tropsch (FT) chemistry has been a long-practiced commercial method for converting synthesis gas (CO/H2) into hydrocarbons,1 the production of hydrocarbon mixtures and significant amounts of methane has fostered interest in identifying homogeneous alternatives to traditional heterogeneous FT catalysts.2 Remarkably, the nitrogenase enzymes are known to enable the reductive coupling of CO to generate ethane, ethylene, and propane.3 Similarly, isocyanides (CNR) are substrates for nitrogenase, undergoing reduction to form hydrocarbon products and amines.4 Isocyanides have been used to model the reductive coupling of CO by early transition metal complexes;5 however fewer examples of the reductive coupling at late metal centers have been reported. Kubiak and co-workers reported a dimeric Ir complex, Ir2(CNR)4(dmpm)2 (dmpm = Me2PCH2PMe2, R = 2,6-Me2C6H3), which upon treatment with Al2Et6 undergoes C−C coupling to form the AlEt2-chelated diaminoalkyne product Ir2[C2(NR)2AlEt2](CNR)2(dmpm)2.6a More recently, the reductive coupling of CNR has been accomplished using cubane-type tetrairon clusters to generate a bis(acetylene) cluster from reduction with LiAlH4,6b and Zanotti and coworkers have reported the hydride-promoted coupling of isocyanides with a bridging aminocarbyne ligand in a diiron complex.6c We sought to generate a monometallic Fe complex capable of the reductive coupling of CNR ligands. The Tm ligand (Tm = tris(mercaptoimidazolyl)hydroborato)7 framework was attractive because Fe complexes bearing similar tripodal ligands have been shown to activate small molecules such as N2,8 N2H4,9 and CO,2c among others (Chart 1). There are, however, © XXXX American Chemical Society

no examples of the reductive coupling of CNR at a mononuclear Fe center. Chart 1. Mononuclear Iron Complexes with Tripodal Ligand Coordination for the Activation of Small Molecules

We report here the synthesis and characterization of the [TmMeFe(CNR)3](OTf) complexes. These complexes form the corresponding disubstituted ureas, upon treatment with a reductant (KC8) and proton source (H2O). Labeling studies and mechanistic data suggest the formation of a diaminocarbene intermediate that undergoes oxidation to release the corresponding urea product.

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RESULTS AND DISCUSSION Synthesis of [TmMeFe(CNtBu)3](OTf) (1). The treatment of Fe(OTf)2 with NaTmMe (NaTmMe = sodium tris(2mercapto-1-methylimidazolyl)hydroborate) followed by the very slow addition of an equimolar amount of tertbutylisocyanide in dichloromethane affords the tris(isocyanide) iron(II) complex [TmMeFe(CNtBu)3](OTf) (1, Scheme 1). Complex 1 was isolated as an orange-red crystalline product in Received: August 29, 2016

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DOI: 10.1021/acs.organomet.6b00687 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Scheme 1. Synthesis of [TmMeFe(CNtBu)3](OTf), 1

(HCl) leads to the formation of di-tert-butylurea (1% yield, Table 1, entry 1) after exposure to air. All reaction yields in Table 1. Reaction Conditions for the Formation of Di-tertbutylurea from 1

82% yield after a slow recrystallization from dichloromethane and diethyl ether. The structure of 1 was established by singlecrystal X-ray diffraction to contain a mononuclear Fe center with a slightly distorted octahedral geometry (Figure 1). The entry 1 2 3 4 5 6 7 8 9 10 11 12 13

Fe source 1 1 1 1 1 1 1 1 1 1 1 Fe(OTf)2/ TmMe Fe(OTf)2

tBuNC

% ureaa

8 8 8 110 220 1 mLb 3 mLc 110 110 110 110 110

3 equiv 10 equiv 3 equiv

1 0 12 38 20 4 0 1 0 96 80 20

110

3 equiv

5

red.

H+ source

equiv

KC8 KC8 KC8 KC8 KC8 KC8 KC8 Cp2Co none KC8 KC8 KC8

HCl(aq) LutHCl H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O

KC8

H2O

a

All yields are reported as an average of three or more experiments and are calculated with respect to Fe.11 Yields are determined by GC integration against a tetradecane standard. Standard reaction conditions: entry 4, step 1, 1 (0.012 mmol) in THF (3 mL) with KC8 (0.09 mmol); step 2, water (1.3 mmol) and 24 h stirring at room temperature; step 3, reaction exposed to ambient air. b1 mL of H2O and 2 mL of THF (5000 equiv of H2O). c3 mL of H2O, no THF (15 000 equiv of H2O).

Figure 1. Molecular structure of the cation of 1 shown with 50% thermal ellipsoids. Hydrogen atoms are omitted for clarity.

isocyanide ligands remain fairly linear with an average Fe−C− N angle of 177.0°. Similarly, the IR spectrum of complex 1 shows CN stretches (2164 and 2118 cm−1) that are only slightly shifted from free tert-butylisocyanide (2138 cm−1). These stretches are only slightly lower than those of the related piano stool complex [CpFe(CNtBu)3](PF6), which features CN stretches of 2170 and 2137 cm−1.10 These stretches are consistent with stronger donation from the TmMe ligand than the Cp ligand. The B−H stretch (2412 cm−1) of 1 is similar to that observed in other κ3-Tm complexes7e and is consistent with a B−H bond that is not interacting with the Fe center. The 1H NMR spectrum of diamagnetic complex 1 recorded in CD3CN displays two thioimidazole proton resonances, one methyl resonance, and one tert-butyl resonance. Similarly, the 13 C NMR spectrum shows only one set of isocyanide resonances and one set of thioimidazole resonances. The equivalence of the three tert-butyl groups and the three thioimidazoles is consistent with the effective C3 symmetry of the cation seen in the crystal structure. The ESI-MS (m/z = 656, [TmMeFe(CNtBu)3]+) of 1 shows a parent ion and isotope pattern consistent with the formulation of 1. Thus, the spectroscopy shows that 1 has the same structure in solution as observed in the solid state. Formation of Di-tert-butylurea from [Tm Me Fe(CNtBu)3](OTf) (1). This new tris(isocyanide) complex is well poised to enable study of the possible reductive coupling of CNR at a single iron center. We were surprised to find that stepwise treatment of 1 with reductant (KC8) followed by acid

Table 1 are reported with respect to complex 1.11 A brief survey of the reaction conditions varying the reductant and the proton source revealed that inclusion of 110 equiv of water as the proton source generated urea in high yields (38% yield relative to complex 1, entry 4). The analogous reaction conducted with D2O (100 equiv) results in 79% deuterium incorporation, indicating water to be the source of the urea NH. The inclusion of additional isocyanide results in 96% yield of urea relative to 1 (Table 1, entry 10). Transition-metal-catalyzed reactions to form ureas from isocyanides can proceed through a variety of pathways including the oxidation of diaminocarbene complexes,12 the formation and hydrolysis of carbodiimides,13 and the reaction of isocyanate intermediates with amines,14 among others.15 To the best of our knowledge, there are no examples of the ironmediated formation of ureas from isocyanides, and in this context we were interested in understanding the pathway that this reaction follows. First, the reaction and the workup were performed under rigorously air-free conditions. Under these conditions, no urea was detected by GC analysis. When the same sample was then exposed to air, urea was formed in 18% yield. These data suggest that oxygen in the air is the source of the oxygen atom in the urea. B


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Organometallics To exclude water as the source of the urea oxygen, the reaction was conducted using H218O in place of H216O (Scheme 2). Under these conditions di-tert-butyl urea was

Scheme 3. Reactions of 1 and 2 with Amines

Scheme 2. Reaction of 1 with 18O-Labeled Water

The complementary experiment, treating the adamantylisocyanide-containing complex [TmMeFe(CNAd)3](OTf) (2) with tert-butylamine (tBuNH2), also generates significant yields of the mixed urea (32% yield with respect to 1). These data together indicate that attack on a bound CNR or CO ligand by amine is a possible pathway in the formation of urea. Amine attack on iron-bound CNR ligands is wellestablished,17 yet attack on a bound CO ligand is also possible.18 We considered the formation of CO via a hydrolysis pathway, similar to that of nitrile hydratase.19 To probe the possible intermediacy of CO, we performed the standard reaction under 1 atm of 13CO (Scheme 4). Under these

formed in 22% yield with no detectable incorporation of 18O measured by GC-MS and ESI-MS. These data exclude water as the source of the urea oxygen and confirm oxygen in the air as the most likely source. To further explore the reaction of 1 with O2, a pair of oxidation reactions was monitored by in situ IR spectroscopy. A solution of 1 was treated sequentially with KC8 and water, before it was filtered and air was introduced via balloon. Surprisingly, in situ IR measurements revealed that the oxidation proceeds rapidly after a short induction period, reaching completion (22% yield of di-tert-butylurea) within 10 min of air introduction (Figure 2). The analogous reaction

Scheme 4. Reaction of 1 with 13C-Labeled CO

conditions, di-tert-butylurea is formed in low yields (3% yield) with no detectable incorporation of 13C. These data suggest that amine attack on CO is unlikely. Furthermore, the formation of CO in our standard reactions is unlikely, as CO binding appears to inhibit the formation of urea. We next explored the possibility of amine attack on CNR. We performed the reduction of 1 followed by filtration and treatment with tBuNH2. After reaction for 24 h, no di-tertbutylurea was detected. After exposure to air, however, di-tertbutylurea was observed in 23% yield. These data are consistent with amine attack at a bound isocyanide ligand to generate a diaminocarbene intermediate, which undergoes oxidation by air to form the urea. The related acyclic diaminocarbene iron(II) complexes isolated from attack of R2NH on [CpFe(CO)[CNCH2CH2CH2PPh2-P,C]I are air-stable;17a,b,20 however, the air-sensitivity of low-valent species is not well-established. Other related systems, such as diaminocarbene copper complexes, have been shown to be highly reactive with O2 to generate ureas.12a It is generally accepted, however, that amine attack on a bound CNR ligand requires a CN stretching frequency that is about 40 cm−1 higher than that of the free isocyanide.21,18e Furthermore, we see no formation of urea when complex 1 is treated with tBuNH2 in the absence of reductant (vide inf ra). These data combined are in contrast with amine attack on a bound CNR. In the systems reported here, however, it seems likely that in the presence of the strongly basic KC8 reductant the active nucleophile may instead be an anionic amide. We are unaware of any systematic studies that explore the CN

Figure 2. Air oxidation of 1 monitored by in situ IR spectroscopy at 1571 cm−1 after treatment with KC8 and H2O followed by filtration. Air balloon was inserted at t = 0 min.

treated with a balloon of O2, in place of air, did not undergo a faster rate of oxidation, nor did the yield of urea increase (18% yield based on Fe, Figure S1). Reactions of [TmMeFe(CNtBu)3](OTf) (1) with RNH2. We detected the formation of tert-butylamine under our standard reaction conditions and wondered whether it might participate in the formation of di-tert-butylurea. To explore this possibility, we performed the reduction of complex 1 in the presence of adamantylamine (AdNH2). The reaction results in the formation of the mixed urea, N-tert-butyl-N′-adamantylurea, in moderate yields (23% yield with respect to 1, Scheme 3).16 C


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Organometallics stretching frequency associated with nucleophilic attack by anionic nucleophiles, yet we would expect such a reaction to occur readily with less activated CNR ligands like those of complex 1. Synthesis and Reactivity of [TmRFe(CNtBu)3](OTf) Complexes (3 and 4). The inclusion of the Tm ligand is necessary for the formation of significant amounts of di-tertbutylurea (Table 1, entries 12 and 13). To explore the influence of the ligand steric properties on the formation of urea, a series of Tm-substituted complexes [TmRFe(CNtBu)3](OTf) (R = Ph (3), Mes (4)) were synthesized and their reactivity was studied. Complexes 3 and 4 were synthesized from the appropriate TmR ligands in a manner analogous to complex 1 (Scheme 5), and each was characterized by 1H and 13C NMR

Employing the analogous TmPh- and TmMes-ligated complexes (3 and 4, respectively) under the same reaction conditions revealed that increasing the steric bulk of the ligand dramatically decreases the yield of di-tert-butylurea (Scheme 6).

Scheme 5. Synthesis of [TmRFe(CNtBu)3](OTf) 3 and 4

Reductive coupling reactions are often facilitated by increased steric bulk of supporting ligands.5b The opposite trend is observed in these [TmRFe(CNtBu)3](OTf) systems, consistent with intermolecular attack of an external nucleophile on a bound CNtBu. Formation of tert-Butylamine. The mechanism of tertbutylamine formation in these reactions remains unclear. Several control experiments were conducted to provide some insight. First, when complex 1 was treated with KC8 followed by filtration, no tBuNH2 was detected after exposure to air. Second, when complex 1 was treated with KC8, followed by filtration and then addition of water, no tBuNH2 was detected after exposure to air. This result indicates that hydrolysis of a reduced Fe intermediate is unlikely. Finally, the treatment of CNtBu with KC8 and water in the absence of iron shows no detectable tBuNH2 formation. From these data we conclude that the formation of tBuNH2 from CNtBu is an iron-mediated reduction requiring both KC8 (as reductant) and water (as proton source),23 although the fate of the CNtBu carbon in this reduction remains unclear. Because KC8 is needed to form tBuNH2, we wondered if this reduction is the sole role of KC8. When complex 1 is treated with amine and water in the absence of KC8, no detectable ditert-butylurea is formed. The analogous reaction performed with inclusion of 8 equiv of KC8 results in formation of 68% urea (Scheme 7). Furthermore, the use of Cp2Co as the reductant (Table 1, entry 8) is ineffective for this transformation.

Scheme 6. Influence of TmR Substituents on Urea Formation

and IR spectroscopies, ESI-MS, and elemental analysis. The phenyl-substituted complex 3 shows a 1H NMR spectrum with a single set of phenyl resonances and a single tBu signal. Similarly, the X-ray crystal structure indicates a cation with effective C3 symmetry (Figure 3).

Scheme 7. Reaction of 1 with tert-Butylamine in the Absence and Presence of KC8 Figure 3. Molecular structures of the cation of 3 shown with 50% thermal ellipsoids. Hydrogen atoms and solvent molecules are omitted for clarity.

In contrast, the 1H NMR spectrum of complex 4 displays two sets of Tm signals and two sets of tert-butyl signals, each present with an integration ratio of 1:2, suggestive of an asymmetric cation bearing a κ2-S,S-bound Tm ligand. Additionally, the asymmetry of the mesityl CH3 signals suggests hindered rotation about the N−Mes bond.22 All other characterization of 4 is consistent with the formulation of [TmMesFe(CNtBu)3](OTf). For example mass spectral analysis reveals a m/z = 968, as predicted for a complex with one Tm and three CNtBu ligands bound. Elemental analysis also confirms this formulation in the isolated solid.

Finally, when complex 1 is treated with tBuNH2 followed by exposure to air, no urea is formed. If, instead, complex 1 is reduced with KC8, followed by filtration, then treatment with amine and exposure to air, urea is formed in 23% yield (vide supra). These data combined suggest an additional role of KC8 is to generate an active reduced iron species.24 D


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Organometallics

tert-butylamine based on the formation and productive reactivity of tert-butylamine in our control reactions.

Cyclic voltammetry experiments indicate that reduction of 1 is feasible. Solutions of 1 in THF show an irreversible reduction potential at −2.37 V (Figure 4), in addition to a reversible

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CONCLUSION In summary, new mononuclear iron tris(isocyanide) complexes bearing Tm supporting ligands have been synthesized, characterized, and shown to generate disubstituted-urea products upon treatment with KC8 and water. Labeling studies and mechanistic experiments suggest a pathway involving the formation and oxidation of a low-valent iron diaminocarbene intermediate.

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EXPERIMENTAL SECTION

General Considerations. All syntheses were carried out under an inert atmosphere (N2) in a glovebox or using standard Schlenk techniques, and reaction workups were conducted on the benchtop, unless otherwise stated. Solvents were taken from a Glass Contours solvent system, in which the solvent is passed through a column of activated alumina with a pressure of argon. Commercial reagents were used without further purification unless otherwise stated. 1 H, 19F, and 13C{1H} NMR spectra were recorded on an Agilent 400 MHz spectrometer or a Varian INOVA 600 MHz spectrometer at room temperature. Chemical shifts (δ) are given in parts per million (ppm) and referenced to the residual solvent signal.28 19F NMR chemical shifts were referenced to a hexafluorobenzene internal standard (δ = −162.2 ppm).29 All coupling constants are reported in Hz. Cyclic voltammograms were recorded under argon using a PINE WaveNow portable potentiostat with a scan rate of 100 mV/s. All potentials are reported versus ferrocene/ferrocenium as internal standard. Elemental analyses were performed by Atlantic Microlab Inc., Norcross, GA, USA. High-resolution mass spectra were obtained on a Thermo Finnigan Linear Trapping Quadrupole mass spectrometer. GC analyses were performed using a SHRXI-5MS column (0.25 μm film thickness × 0.25 mm i.d. × 15 mL) installed on a Shimadzu GC-2014 with FID. GC-MS analyses were performed using a TRG-SQC column (0.25 μm film thickness × 0.25 mm i.d. × 15 mL) installed on a Thermo Scientific Trace 1310 GC with an ISQ QD single quadrupole mass spectrometer. Sodium Tris(2-mercapto-1-methylimidazole)hydroborate (NaTmMe). The title compound was prepared by a modification of the literature procedure.30 Methimazole (500 mg, 4.38 mmol) and sodium borohydride (42.0 mg, 1.11 mmol) were combined in 15 mL of xylene and heated at 170 °C for 24 h under nitrogen. The mixture was cooled, and the resulting white precipitate was filtered and washed with diethyl ether. The solid was dissolved in a minimum amount of methanol and reprecipitated using diethyl ether. The resulting white solid was collected by filtration and dried under vacuum to yield the title compound in 87% yield (0.36 g, 0.96 mmol). 1H NMR ((CD3)2SO, 400 MHz): δ 6.78 (d, J = 2.4 Hz, 1H,), 6.41 (d, J = 2.0 Hz, 1H), 3.38 (s, 3H). The spectral data are consistent with those reported in the literature.30 Sodium Tris(2-mercapto-1-phenylimidazole)hydroborate (NaTmPh). The title compound was prepared by a modification of the literature procedure.31 2-Mercapto-N-phenylimidazole32 (500 mg, 2.84 mmol) and sodium borohydride (70.0 mg, 1.85 mmol) were combined in 10 mL of toluene, and the mixture was heated at reflux

Figure 4. Cyclic voltammogram of 1 using a glassy carbon working electrode, a Pt wire counter electrode, a Ag/AgNO3 reference electrode, 0.1 M [nBu4N][PF6], and a scan rate of 100 mV/s in THF versus Cp2Fe/Cp2Fe+.

oxidation potential at 300 mV and a second irreversible oxidation at 1.1 V. Spectroscopic characterization of the reduced Fe species generated in THF-d8 from the treatment of 1 with KC8 reveals a paramagnetic product, consistent with a one-electron reduction of 1 to generate an active FeI species. From the data presented above, we propose a possible pathway for urea formation from 1 as shown in Scheme 8. First, complex 1 is reduced by KC8. Although the nature and the oxidation state of this complex are currently unclear, the electrochemical reduction and NMR experiments suggest a single reduction step. In addition, other researchers have reported the synthesis of low oxidation state iron diaminocarbene25a and N-heterocyclic carbene complexes25b,c in addition to low-valent iron complexes bearing bulky CNR ligands.26 Next, a CNtBu ligand is reduced to tBuNH2 by KC8 in the presence of water. The resulting tBuNH2 or KNHtBu (or alternatively an Fe-NHtBu species) attacks an iron-bound CNtBu, leading to the formation of a diaminocarbene species, which upon exposure to air, is oxidized by O2 to generate ditert-butylurea. The fate of the isonitrile carbon atom is unclear, although the formation of CO19 or CH44 is possible under these reaction conditions. Radical-based pathways have also been proposed for related reactions that form disubstituted ureas from isocyanides.27 Although our data do not exclude a radical-based reaction, we favor a pathway involving nucleophilic attack by Scheme 8. Proposed Pathway for the Formation of Urea from 1

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Organometallics

0.89 mmol in 1 mL of CH2Cl2) was slowly added to a suspension of iron(II) triflate (316 mg, 0.89 mmol) in 30 mL of CH2Cl2. A dark green suspension formed and was stirred for 2 h at room temperature, after which tert-butylisocyanide (110 μL, 0.97 mmol) was added over a period of 1 h. The resulting reaction solution was stirred overnight (16 h), resulting in a greenish-brown suspension. The suspension was concentrated, and the title compound was isolated from the resulting residue by sequential extraction with diethyl ether and filtration through Celite. This step removes the green (TmPh)2Fe species. The title compound was further purified by recrystallization from CH2Cl2/ pentane (slow diffusion or low-temperature recrystallization) to yield the title compound as orange-red crystals in 65% yield (0.21 g, 0.21 mmol). 1H NMR (CD3CN, 400 MHz): δ 7.59−7.47 (m, 5H), 7.25 (d, 1H, J = 2.0 Hz), 7.14 (d, 1H, J = 2.0 Hz), 1.12 (s, 9H). 13C{1H} NMR (CD3CN, 100 MHz): δ 161.58, 160.25 138.84, 130.07, 129.66, 127.71, 126.00, 122.22, 58.79, 31.04. 19F NMR (CD3CN, 400 MHz): δ −77.1 (CF3SO3−). FTIR (ATR, cm−1): 3131, 2982, 2436 (B−H), 2165 (C N), 2109 (CN), 1598, 1499, 1261, 1030, 755. HRMS (ESI, CH3CN): [M + H]+ calcd 842.27210, found 842.27365. Anal. Calcd for C43H49BF3FeN9O3S4: C, 52.07; H, 4.98; N, 12.71. Found: C, 51.51; H, 4.94; N, 12.31. Synthesis of [TmMesFe(tBuNC)3](OTf) (4). A suspension of sodium tris(2-mercapto-1-mesitylimidazole)hydroborate (NaTmMes) (0.20 g, 0.29 mmol in 10 mL of CH2Cl2) was slowly added to a suspension of iron(II) triflate (105 mg, 0.30 mmol) in 30 mL of CH2Cl2. A dark green suspension formed and was stirred for 2 h at room temperature, after which tert-butylisocyanide (33 μL, 0.29 mmol) was added over a period of 1 h. The resulting reaction solution was stirred overnight (16 h), resulting in a greenish-brown suspension. The suspension was concentrated. The title compound was isolated from the resulting residue by sequential extraction with diethyl ether and filtration. This step removes the green (TmMes)2Fe species. The title compound was further purified by recrystallization from CH2Cl2/ diethyl ether (slow diffusion recrystallization) to yield the title compound as orange-red crystals in 64% yield (0.069 g 0.062 mmol). 1 H NMR (CD3CN, 400 MHz): 7.09 (m, 2H, 2H of mesityl rings), 7.04 (m, 6H, 2H of imidazole ring and 4H of mesityl rings), 6.95 (d, J = 2.0 Hz, 2H of imidazole rings), 6.89 (d, J = 2.4 Hz, 1H of imidazole ring), 6.74 (d, J = 2.4 Hz, 1H of imidazole ring), 2.34 (s, 9H of mesityl methyl groups, 2.13 (s, 6H of mesityl methyl groups), 2.11 (s, 6H of mesityl methyl groups), 1.86 (s, 6H of mesityl methyl groups), 1.41 (s, 18H of 2 tert-butyl groups), 1.35 (s, 9H of 1 tert-butyl group). The number of peaks observed in the 1H NMR spectrum (and the 13C NMR spectrum) suggests hindered rotation about the mesityl−N bond as seen in related TmMesRe(CO)3 systems22 and a reduction in symmetry relative to the TmMeFe complexes. 13C{1H} NMR (CD3CN, 100 MHz): δ 161.34, 141.01, 140.07, 139.75, 137.27, 136.86, 136.64, 136.50, 135.88, 134.88, 133.47, 130.21, 130.09, 130.02, 129.70, 129.66, 126.74, 123.47, 121.80, 120.74, 118.68, 59.36, 59.19, 31.15, 30.87, 30.84, 30.76, 30.36, 21.13, 21.10, 21.02, 19.51, 18.39, 18.17, 18.10, 17.93. 19F NMR (CD3CN, 400 MHz): δ −77.1 (CF3SO3−). FTIR (ATR, cm−1): 3162, 3125, 2977, 2181 (CN), 2143 (CN), 1608, 1265,1031. HRMS (ESI, CH3CN): [M + H]+ calcd 968.41295, found 968.41228. Anal. Calcd for C52H67BF3FeN9O3S4: C, 55.86; H, 6.04; N, 11.28. Found: C, 55.95; H, 6.07; N, 11.29. Representative Procedure for the Reduction of 1 with KC8 and H2O (Table 1, entry 4). In a N2-filled glovebox, complex 1 (10 mg, 0.01 mmol) and KC8 (12 mg, 0.090 mmol) were combined in a Schlenk tube, and 3 mL of THF was added to the mixture. The Schlenk tube was removed from the glovebox and kept under a N2 atmosphere on a Schlenk line. H2O (23 μL, 1.3 mmol) was added all at once through a septum. The mixture was stirred at room temperature for 24 h, after which 5 μL of tetradecane internal standard was added to the reaction mixture. The mixture was exposed to ambient air and filtered through glass wool. The filtrate was analyzed by GC-FID (for product quantification) and GC-MS and 1H NMR spectroscopy (for product confirmation). Di-tert-butylurea yield = 38%.

for 3 days under nitrogen. The resulting suspension was cooled, and the solvent was removed by rotary evaporation. The crude material was dissolved in chloroform and filtered to remove unreacted sodium borohydride. The filtrate was collected, the solvent was removed, the resulting white solid was dissolved in a minimum amount of dichloromethane, and the product was precipitated using diethyl ether, filtered, and dried under vacuum to yield the title compound in 89% yield (0.53 g, 0.95 mmol). 1H NMR (CDCl3, 400 MHz): δ 7.56− 7.32 (m, 5H), 6.86 (d, J = 2.4 Hz, 1H), 6.46 (d, J = 2.0 Hz, 1H). Sodium Tris(2-mercapto-1-mesitylimidazole)hydroborate (NaTmMes). The title compound was prepared by a modification of the literature procedure.4 2-Mercapto-1-mesitylimidazole33 (500 mg, 2.29 mmol) and sodium borohydride (60.0 mg, 1.59 mmol) were combined in 10 mL of toluene and refluxed under nitrogen for 5 days. The white suspension was cooled, and the solvent was removed by rotary evaporation. The crude material was dissolved in chloroform and filtered to remove unreacted sodium borohydride. The filtrate was concentrated, and the product was precipitated using diethyl ether, filtered, and dried under vacuum to yield the title compound in 82% yield (0.52 g, 0.76 mmol). 1H NMR (CDCl3, 400 MHz): δ 6.97 (s, 2H), 6.83 (d, J = 4.0 Hz, 1H), 6.61 (d, J = 4.0 Hz, 1H), 2.32 (s, 3H), 2.06 (s, 6H). [TmMeFe(tBuNC)3](OTf) (1). A suspension of sodium tris(2mercapto-1-methylimidazole)hydroborate (NaTmMe) (500 mg, 1.34 mmol in 1 mL of CH2Cl2) was slowly added to a suspension of iron(II) triflate (473 mg, 1.34 mmol) in 30 mL of CH2Cl2. A dark green suspension formed and was stirred for 2 h at room temperature, after which tert-butylisocyanide (125 μL, 1.50 mmol) was added over a period of 1 h. The resulting reaction solution was stirred overnight (16 h), resulting in a greenish-brown suspension. The suspension was filtered through Celite, and the filtrate was concentrated. The title compound was isolated from the resulting residue by sequential extraction with diethyl ether and filtration through Celite. This step removes the green (TmMe)2Fe species. The title compound was further purified by recrystallization from CH2Cl2/diethyl ether (slow diffusion or low-temperature recrystallization) to yield the title compound as orange-red crystals in 82% yield (0.33 g, 0.41 mmol). 1 H NMR (CD3CN, 400 MHz): δ 7.04 (d, 1H, J = 2.0 Hz), 6.88 (d, 1H, J = 2.4 Hz), 3.68 (s, 3H), 1.47 (s, 9H). 13C{1H} NMR (CD3CN, 100 MHz): δ 161.24, 160.90, 124.40, 121.71, 58.97, 35.34, 31.21. 19F NMR (CD3CN, 400 MHz): δ −79.7 (CF3SO3−). FTIR (ATR, cm−1): 3132, 2980, 2412 (B−H), 2164 (CN), 2118 (CN), 1560, 1461, 1203, 1030, 748. HRMS (ESI, CH3CN): [M + H]+ calcd 656.22515, found 656.22425. Anal. Calcd for C28H43BF3FeN9O3S4: C, 41.74; H, 5.38; N, 15.65. Found: C, 41.94; H, 5.46; N, 15.38. Synthesis of [TmMeFe(AdNC)3](OTf) (2). A suspension of sodium tris(2-mercapto-1-methylimidazole)hydroborate (NaTmMe) (500 mg, 1.34 mmol in 10 mL of CH2Cl2) was slowly added to a suspension of iron(II) triflate (473 mg, 1.34 mmol) in 30 mL of CH2Cl2. A dark green suspension formed and was stirred for 2 h at room temperature, after which 1-adamantylisocyanide (220 mg, 1.36 mmol) was added over a period of 1 h. The resulting reaction solution was stirred overnight (16 h), resulting in a greenish-brown suspension. The suspension was concentrated, and the title compound was isolated from the resulting residue by sequential extraction with diethyl ether and filtration through Celite. This step removes the green (TmMe)2Fe species. The title compound was further purified by recrystallization from THF (low-temperature recrystallization) to yield the title compound as orange-red crystals in 66% yield (0.31 g, 0.30 mmol). 1 H NMR (CD3CN, 400 MHz): δ 7.04 (d, J = 2 Hz, 1H), 6.88 (d, J = 2 Hz, 1H), 3.67 (s, 3H), 2.12 (br s, 3H), 2.04 (m, 6H), 1.70 (m, 6H). 13 C{1H} NMR (CD3CN, 100 MHz): δ 161.62, 160.98, 124.44, 121.77, 68.28, 59.11, 44.87, 35.91, 35.51, 30.02, 26.25. 19F NMR (CD3CN, 400 MHz): δ −77.1 (CF3SO3−). FTIR (ATR, cm−1): 3131, 2909, 2854, 2397 (B−H), 2146 (CN), 2108 (CN), 1561, 1457, 1259, 1204, 1030, 732. HRMS (ESI): [M + H]+ calcd 890.36600, found 890.36485. Anal. Calcd for C46H61BF3FeN9O3S4: C, 53.13; H, 5.91; N, 12.12. Found: C, 53.32; H, 5.96; N, 11.85. Synthesis of [TmPhFe(tBuNC)3](OTf) (3). A suspension of sodium tris(2-mercapto-1-phenylimidazole)hydroborate (NaTmPh) (0.50 g, F


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Organometallics

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00687. Complete experimental procedures, 1H and 13C NMR spectra (PDF) Crystallographic data (CIF) (TXT)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to West Virginia University for financial support of this work. NMR spectroscopy (CHE-1228336), ReactIR (CHE-1427136), and X-ray crystallography (CHE1336071) facilities were partially supported by the NSF. We thank Prof. Steven Valentine and Gregory Donohoe for ESIMS analyses.

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REFERENCES

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Formation of Di-tert-butylurea from a Mononuclear Iron Tris

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