Low-Valent Iron Carbonyl Complexes with a Tripodal Carbene Ligand

Article pubs.acs.org/Organometallics

Low-Valent Iron Carbonyl Complexes with a Tripodal Carbene Ligand Anne K. Hickey, Chun-Hsing Chen, Maren Pink, and Jeremy M. Smith* Department of Chemistry, Indiana University, 800 East Kirkwood Avenue, Bloomington, Indiana 47405, United States

Downloaded by SUNY UPSTATE MEDICAL UNIV on September 6, 2015 | http://pubs.acs.org Publication Date (Web): September 4, 2015 | doi: 10.1021/acs.organomet.5b00646

S Supporting Information *

ABSTRACT: A bulky tris(carbene)borate ligand allows several lowvalent iron carbonyl complexes to be isolated. One-electron reduction of the cationic iron(II) complex [PhB(MesIm)3Fe(CO)3][B(C6F5)4] (1) ([PhB(MesIm3)]− = phenyltris(1-mesitylimidazol-2-ylidene)borate) yields the low-spin (S = 1/2) iron(I) complex PhB(MesIm)3Fe(CO)2 (2), as determined by structural and spectroscopic methods. This complex can in turn be reduced to provide the anionic dicarbonyl complex [K][PhB(MesIm)3Fe(CO)2] (3), which crystallizes as a dimer in which the potassium cation coordinates in a side-on fashion to one CO ligand. Protonation of 3 yields the weakly acidic iron hydride PhB(MesIm)3Fe(CO)2H (4), which can also be isolated by treating the κ3-coordinated alkylborohydrido complex PhB(MesIm)3Fe(κ3-BH(CH2CH3)3) (5) with CO. The strong donor ability of the tris(carbene)borate ligand results in significant reduction of the CO bonds, as measured by IR spectroscopy.

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INTRODUCTION Iron carbonyls are well-established members in the pantheon of classical organometallic complexes, with decades of investigation. A prominent example is the dimer [(η5-C5H5)Fe(CO)2]2, one of the oldest organometallic complexes,1 which serves as a precursor to synthetically useful low-valent iron carbonyl complexes. For example, photolysis of the complex provides the transient species (η5-C5H5)Fe(CO)2 (Fp),2 while reduction affords anionic [(η5-C5H5)Fe(CO)2]− (Fp−). The high nucleophilicility of the latter complex, which readily substitutes alkyl halides and pseudohalides, has led to numerous applications in both organometallic and organic chemistry.3 Low-valent iron carbonyl complexes can be prepared with other supporting ligands, although the chemistry of these complexes has not been as extensively explored as for Fp. In recent years, a number of bulky ligands have been shown to stabilize low-valent iron carbonyl complexes with low coordination numbers. For example, bulky β-diketiminate ligands allow for the isolation of both five- and four-coordinate iron(I) complexes, depending on the nature of the groups on the ligand backbone.4 Similarly, the redox-active ligand iPrPDI (iPrPDI = 2,6-(2,6-iPr2-C6H3NCMe)2C5H3N) allows for the isolation of low-coordinate iron carbonyl complexes, although the noninnocence of this ligand complicates oxidation-state assignments.5 Tripodal scorpionate ligands are more closely related to cyclopentadienyls in that they are generally six-electron-donor, face-capping ligands, albeit 3-fold rather than 5-fold symmetric. The modular nature of the scorpionate ligand design allows the extent of back-bonding to the CO ligands to be modified © XXXX American Chemical Society

through changes to the donor groups. Thus, for example, for low-spin (S = 1/2) iron(I) complexes, higher energy CO stretching bands are observed for thioether donors in PhTttBuFe(CO)2 (νCO = 1984 and 1911 cm−1)6 than for phosphine donors in FFe(iPrNPPh2)3Fe(CO)2 (νCO = 1982 and 1921 cm−1)7 and PhB(CH2PPh2)3Fe(CO)2 (νCO = 1979 and 1914 cm−1).8 The latter complex can be further reduced, causing these bands to decrease in energy to 1870 and 1781 cm−1.9 Similarly, chemical reduction of the five-coordinate monocarbonyl complex [Si(o-C6H4PiPr2)3]Fe(CO) causes νCO to shift from 1848 to 1717 cm−1. A remarkable feature of the latter complex is that treatment with an electrophile gives a terminal carbyne species.10 Our group has previously investigated the chemistry of tripodal tris(carbene)borate ligands.11 An outstanding characteristic of these ligands is their strong donor ability, manifested in their ability to stabilize high oxidation states12,13 and lowspin, low-coordinate iron(II) complexes.14 We have also observed that these ligands have been shown to dramatically reduce the NO bond in four-coordinate {NiNO}10 complexes, in some cases decreasing νNO by over 100 cm−1 in comparison to related tripodal ligands. 15 On the basis of these considerations, we hypothesized that strongly electron donating tris(carbene)borate ligands will induce substantial reduction of iron-bound CO. Herein we report the synthesis, characterization, and preliminary reactivity of low-valent iron carbonyl complexes Received: July 28, 2015

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DOI: 10.1021/acs.organomet.5b00646 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics containing the tris(carbene)borate scaffold PhB(MesIm)3− (PhB(MesIm3)− = phenyltris(1-mesitylimidazol-2-ylidene)borate). The strong electron-donating ability of the tris(carbene)borate ligand leads to CO ligands that are significantly reduced when compared with analogous Fp complexes. Additionally, the bulky nature of this ligand allows for the isolation of species that are not stable with cyclopentadienyl ligands. More generally, these results further demonstrate the flexibility of tris(carbene)borate ligands in stabilizing a wide variety of oxidation states.

observed (Ep,a = −0.88 V) (Figure 2). While the first reduction does not show linear dependence on the square root of the scan


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RESULTS AND DISCUSSION The thermal reaction of PhB(MesIm)3FeCl with K[B(C6F5)4] in the presence of CO leads to formation of a pale green slurry that leads to isolation of pale green [PhB(MesIm)3Fe(CO)3] [B(C6F5)4] (1) in excellent yield. The molecular structure of 1 shows three carbonyl ligands bound facially to the iron(II) center, giving an overall pseudo-octahedral geometry at iron (Figure 1). The bond lengths between iron and the carbon

Figure 2. Cyclic voltammogram of [PhB(MesIm)3Fe(CO)3][B(C6F5)4] (1) (100 mV/s, 0.4 M [n-Bu4N][PF6], THF).

rate, the second reduction does for both the cathodic and anodic directions (Figure S4, see the Supporting Information). These results suggest that the initial reduction of 1 leads to the formation of a new complex and that this complex can subsequently be reversibly reduced. This unusual electrochemical behavior prompted us to investigate the species formed upon reduction of 1. Treating complex 1 with 1 equiv of KC8 in room-temperature THF gives a dark green solution within minutes, which allows the green 17-electron species PhB(MesIm)3Fe(CO)2 (2) to be isolated and structurally characterized (Figure 3). Thus, the irreversible electrochemical behavior is related to CO ligand loss that occurs upon reduction of 1 to 2.

Figure 1. Molecular structure of [PhB(MesIm)3Fe(CO)3][B(C6F5)4] (1). Color scheme: orange = iron, red = oxygen, blue = nitrogen, pink = boron, green = fluorine, black = carbon. Thermal ellipsoids are shown at 50% probability. One cocrystallized pentane molecule and hydrogen atoms have been omitted for clarity. Selected bond distances (Å) and angles (deg): Fe1−C1 1.989(2); Fe1−C13 2.020(2); Fe1− C25 2.022(2); Fe1−C43 1.822(3); Fe1−C44 1.821(3); Fe1−C45 1.820(3); C43−O1 1.132(3); C44−O2 1.132(3); C45−O3 1.136(3); C1−Fe1−C43 176.13(10); C13−Fe1−C44 179.08(10); C25−Fe1− C45 176.86(10); Fe1−C43−O1 176.5(2); Fe1−C44−O2 173.4(2); Fe1−C45−O3 175.1(2).

atoms of the tris(carbene)borate ligand (average = 2.010(2) Å) are statistically indistinguishable and are similar to the iron− carbene distances in the related complex [PhB(MesIm)3Fe(CNtBu)3][B(C6F5)4].16 The 1H NMR spectrum (THF-d8, 25 °C) of complex 1 shows resonances between 0 and 10 ppm, where the tris(carbene)borate ligand is identified based on chemical shift and integration values that match the 3-fold symmetry expected for the complex, in agreement with the solid-state structure.16 Two bands at 2100 and 2040 cm−1 in the infrared spectrum (KBr) are assigned to C−O stretching frequencies, consistent with idealized C3v symmetry observed for complex 1. This range of stretching frequencies is consistent with other five- and six-coordinate iron(II) carbonyl complexes.17 Interesting electrochemical behavior is observed in the cyclic voltammogram of complex 1. Sweeping cathodically, an irreversible reduction is observed (Ep,c = −2.07 V versus Fc/ Fc+ in THF), followed by a reversible reduction (E1/2 = −2.52 V). Lastly, an irreversible oxidation of the reduced complex is

Figure 3. Molecular structure of PhB(MesIm)3Fe(CO)2 (2). Thermal ellipsoids are shown at 50% probability. Four cocrystallized THF molecules and hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): Fe1−C1 1.978(5), Fe1−C13 2.007(5), Fe1−C25 1.999(5), Fe1−C43 1.791(5), Fe1−C44 1.771(5), C43−O1 1.139(6), C44−O2 1.154(6); C1−Fe1−C43 170.7(2), C25−Fe1−C44 171.4(2).

Single-crystal X-ray diffraction reveals the molecular structure of 2 to have a square pyramidal iron(I) center (τ = 0.01)18 bound to the tris(carbene)borate ligand and two carbonyl ligands. The bond distances from iron to the tris(carbene)borate ligand are slightly shorter than in 1 (average 1.992(5) Å), possibly due to the lower coordination number in 2. Interestingly, these bond lengths are significantly shorter than in the iron(I) complex PhB(MesIm)3Fe(COE) (COE = ciscyclooctene), where the corresponding bond lengths average 2.065(3) Å. This difference is attributed to the different spin B


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Organometallics


Scheme 1. Synthesis of Complexes 1−5

states of 2 and PhB(MesIm)3Fe(COE) (see below).12a The iron carbonyl distances in 2 (1.771(5) and 1.791(5) Å) are significantly shorter than in 1, likely due to an increase in πback-bonding upon reduction. One sharp signal (giso = 2.052) is observed in the roomtemperature EPR spectrum of 2 (toluene), which becomes rhombic in frozen solution (77 K), with gx = 2.084, gy = 2.064, and gz = 2.002. Together with the room-temperature magnetic moment measured by the Evans’ method (μeff = 1.4(1) μB), the spectroscopic data are indicative of a single, metal-based electron (Figure S8, see the Supporting Information). Thus, the spin state differs from high-spin (S = 3/2) PhB(MesIm)3Fe(COE),12 but is consistent with similar iron(I) dicarbonyl complexes supported by 3-fold symmetric ligands.7,8 As suggested by the structural data, the IR spectrum of 2 confirms a substantial increase in π-back-bonding to the CO ligands upon reduction (νCO = 1958, 1886 cm−1). These stretching frequencies are significantly lower in energy than observed for complex 1 and are also lower in frequency than previously characterized five-coordinate carbonyl complexes (Tables 1 and 2). The enhanced reducing ability of iron in 2 likely arises from the strong σ-donor ability of the tris(carbene)borate ligand.

Table 2. Comparison of νCO and C−O Bond Distances of Low-Valent Iron Carbonyl Complexes complex iPr

( PDI)Fe(CO)2 PhB(CH2PPh2)3Fe(CO)2 FFe(iPrNPPh2)3Fe(CO)2 PhTttBuFe(CO)2 (η5-C5H5)2Fe2(CO)4 (η5-C5H5)Fe(CO)2 [K][(η5-C5H5)Fe(CO)2] [K(18-crown-6)][(η5-C5H5)Fe(CO)2] (η5-C5H5)Fe(CO)2H

νCO (cm−1) 1979, 1979, 1982, 1984, 1954 2004, 1861, 1874, 2014,

1914 1914 1921 1911 1938 1785 1727 1951

ref 5 8 7 6 19, 20 17b 17b 25 17b

Complex 3 crystallizes as a solvent-free dimer (Figure 4). The two K cations form bridging interactions between the four carbonyl ligands. There are no interactions between the K ions

Table 1. Summary of Infrared Spectroscopic Data for New Tris(carbene)borate Iron Carbonyl Complexesa complex 1 2 3 3-cryptand 4 a

νCO (cm−1) 2100, 1956, 1812, 1843, 1980,

2040 1886 1728 1770 1922

IR spectra measured as KBr pellets. Figure 4. (a) Molecular structure of 3. Hydrogens are omitted for clarity. Thermal ellipsoids are shown at 50% probability. Selected bond distances (Å) and angles (deg): Fe1−C1 1.997(3), Fe1−C13 1.963(3), Fe1−C25 1.955(3), Fe1−C43 1.722(3), Fe1−C44 1.735(3), C43−O1 1.208(4), C44−O2 1.184(4); C1−Fe1−C43 174.39(2), C25−Fe1−C44 151.22(15). (b) Close-up depiction of the interactions between the potassium cations and the carbonyl ligands.

In accordance with the cyclic voltammogram (Figure 2), complex 2 can be reversibly reduced by one electron (E1/2 = −2.4 V). Thus, treating complex 2 with 1 equiv of KC8 in THF at room temperature gives a dark red slurry, and after workup the reduced complex [K][PhB(MesIm)3Fe(CO)2] (3) can be isolated as dark red crystals in moderate yield. C


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Organometallics and the aryl rings of the tris(carbene)borate ligand, which are over 3.4 Å away. The geometry about the iron center is square pyramidal, although more distorted from an ideal geometry than is complex 2 (τ = 0.38). One-electron reduction of 2 results in a decrease in the bond distances from iron to the tris(carbene)borate ligand (average 1.995(3) Å in 2 to 1.972(3) Å in 3) as well as the carbonyl ligands (average 1.781(6) Å in 2 to 1.729(4) Å in 3). The short Fe−Ccarbene bond distances (average 1.972(3) Å) suggest that complex 3 is low spin in nature, similarly to complex 2. Notably, the average C−O bond distance of 1.196(4) Å in 3 is longer than comparable examples in the literature.4−8,21,22 An unusual feature of complex 3 is the side-on interaction between the K cations and the CO ligands (C−K = 3.028(4) Å, O−K = 2.879(3) Å). While not unprecedented, such a side-on interaction is rare, and we are only aware of two previous examples. In the complex [K(tmeda)][(C5H5)Co(CO){Ga(NArCH) 2 }] (tmeda = tetramethylethylenediamine, (CH3)2NCH2CH2N(CH3)2; Ar = 2,6-iPr2-C6H3), the K cation is at a distance of 3.168 Å from the C atom and 3.121 Å from the O atom of a cobalt-bound carbonyl ligand.21 An even closer interaction is observed in pyrazolate-based reduced manganese complex [K][N2C3{CH2(C5H5)Mn(CO)2}], where the C−K and O−K distances are 3.016 and 2.915 Å, respectively (Chart 1).22

Figure 5. Molecular structure of 3-cryptand. Three cocrystallized THF molecules and hydrogens are omitted for clarity. Ellipsoids are shown at 50% probability. Selected bond distances (Å) and angles (deg): Fe1−C1 1.954(3), Fe1−C13 1.944(3), Fe1−C25 1.966(3), Fe1−C43 1.739(4), Fe1−C44 1.752(4), C43−O1 1.176(4), C44−O2 1.180(4); C13−Fe1−C44 160.10(16), C25−Fe1−C43 169.12(16).

shorter than the same distances in complexes 2 and 3. Interestingly, the C−O stretching frequencies measured for 3cryptand (1851, 1773 cm−1) are considerably higher in energy than in complex 3 (1812, 1728 cm−1), suggesting that the interaction between potassium and the π orbitals of the CO ligand plays a role in weakening the C−O bond, similar to observations with iron dinitrogen complexes.4,23 Additionally, the UV−vis spectrum of 3-cryptand shows an absorbance at 600 nm (ε = 960 M−1 cm−1, Figure S14). This feature was not present in the UV−vis spectrum of 3, so the two complexes are not isostructural in solution. Comparison of complexes 3 and 3-cryptand with isoelectronic Fp− is instructive. The average C−O bond distances in [K][(η5-C5H5)Fe(CO)2] (1.177 Å)24 and [K(18-crown6)][(η5-C5H5)Fe(CO)2] (1.182 Å)25 are similar to those observed in 3-cryptand; however both of these complexes show interactions between potassium and a CO ligand in the solid state. The IR data more clearly show the difference in donor strength between the cyclopentadienyl (Cp) and tris(carbene)borate ligands, with C−O stretching frequencies of 1874 and 1727 cm−1 in [K(18-crown-6)][(η5-C5H5)Fe(CO)2] and 1851 and 1773 cm−1 in 3-cryptand.25 When potassium interaction is present such as in complex 3, the C−O stretching frequenices are 1812 and 1728 cm−1, lower than those observed for [K][(n5-C5H5)Fe(CO)2],17b demonstrating the tris(carbene)borate ligand is a stronger donor to iron than Cp. The low stretching frequencies of complex 3-cryptand are comparable to those of [(η5-C5H5)Fe(CO)2)]− (1862, 1786 cm−1)17b and lower in energy than those of the complex [Na(THF)5][PhB(CH2PPh2)3Fe(CO)2], also supported by a 3-fold symmetric ligand (1870, 1781 cm−1).26 Despite its dimeric nature, complex 3 is a synthon for monomeric iron complexes, reacting with ethereal HCl to give the iron(II) complex PhB(MesIm)3Fe(CO)2H (4) (Figure 6). An X-ray diffraction study of this complex reveals the hydride ligand, which was located in the Fourier difference map (Fe−H = 1.69(4) Å). This bond is considerably longer than has been observed for other low-spin, six-coordinate iron(II) hydride complexes, where the Fe−H bond length is generally between 1.4 and 1.5 Å.27All other bond lengths to iron are indicative of a low-spin iron(II) complex. The 1H NMR spectrum of complex 4 is in agreement with the solid-state structure. The 1H NMR spectrum of 4 reveals 11

Chart 1. Side-on Coordination of Potassium to Carbonyl Ligands Bound to Transition Metalsa

a

See refs 21 and 22.

Reduction of 2 to 3 significantly increases the extent of πback-donation to the CO ligands, as revealed by IR spectroscopy. Thus, both νCO are shifted to lower energy by over 100 cm−1 from the stretching frequency observed in complex 2 (Table 1). The 1H NMR spectrum of 3 (THF-d8, 25 °C) shows resonances between 0 and 10 ppm, which are consistent with a diamagnetic, 3-fold symmetric species in solution. This observed symmetry is one indication that the dimer remains intact in solution. While some of these resonances are broadened at room temperature, no changes in the spectrum occur upon cooling a THF-d8 solution to −60 °C (Figure S11). In addition, well-resolved resonances are observed in the 13 C{1H} NMR spectrum (THF-d8, 25 °C) of 3, confirming its diamagnetic nature. The UV−vis spectrum of 3 has one shoulder at 410 nm (ε = 2950 M−1 cm−1). The absorbance at this wavelength increases linearly with concentration, further indicating that the structure is dimeric both in the solid state and in solution (Figure S12). The potassium cation in complex 3 can be encapsulated using 2.2.2.-cryptand, giving the ion-separated complex [K(2.2.2.-cryptand)][PhB(MesIm)3Fe(CO)2] (3-cryptand) as a brown, microcrystalline powder (Figure 5). The coordination geometry of iron in this monomeric complex is closer to that of an ideal square pyramid (τ = 0.15). The iron tris(carbene)borate ligand distances (average 1.955(3) Å) are considerably D


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Organometallics


in pKa’s is offset by similarly large differences in redox potentials (E0 ≈ −1.235 V for Fp•/Fp−).33 We attempted to test the calculated Fe−H BDFE by reactions with compounds having known X−H bond energies. As expected, complex 4 cannot be prepared by reaction of 2 with dihydroanthracene (BDFE = 76.0 kcal/mol), 2,4,6-tBu3C6H2OH (BDFE = 77.1 kcal/mol), or TEMPO-H (BDFE = 66.5 kcal/mol). However, in contrast to the thermodynamic predictions, complex 4 does not react with TEMPO to provide 2, which we attribute to the sterically imposing tris(carbene)borate ligand hindering access to the hydride ligand. The hydricity of metal hydrogen bonds can be calculated by a different thermodynamic cycle:34 ΔG 0 H− = 1.37pK a + 46.1E2e 0 + 79.6

Figure 6. Molecular structure of PhB(MesIm)3Fe(CO)2H (4). Three cocrystallized THF molecules and carbon-bonded hydrogen atoms are omitted for clarity. Thermal ellipsoids are shown at 50% probability. Selected bond distances (Å) and angles (deg): Fe1−C1 1.997(3), Fe1−C13 1.963(3), Fe1−C25 1.955(3), Fe1−C43 1.722(3), Fe1− C44 1.735(3), C43−O1 1.208(4), C44−O2 1.184(4); C1−Fe−C44 172.8(2), C13−Fe1−C43 171.8(2), C25−Fe−H1 176.7(13).

where E2e is the two-electron potential of the conjugate base. Since the oxidation of 2 is irreversible (Ep,a = −1.14 V vs Fc/ Fc+, Figure S5), we can calculate only an upper bound for the hydricity of 4, ΔG0H− = 58 kcal/mol, which is less than that observed for FpH (62 kcal/mol).17b Since a smaller ΔG0H− indicates that the complex is a better hydride donor, this result indicates that the tris(carbene)borate ligand increases the hydridicity of the Fe−H bond. Complex 4 can be independently prepared by the reaction of PhB(MesIm)3FeCl with NaBEt3H under an atmosphere of CO. Interestingly, when the reaction is conducted in the absence of CO, the iron borohydride complex PhB(MesIm)3Fe(κ3-BH(CH2CH3)3) (5) can be isolated. The molecular structure of dark purple 5 reveals an unusual bonding mode for the triethylborohydride ligand. Specifically, the ligand binds in a κ3mode (Figure 7) composed of a Fe−H−B bond and agostic C−H interactions with two of the triethylborohydride ethyl groups. While the hydrogens of the ethyl groups were placed geometrically, the bridging hydride ligand was located in the Fourier difference map (Fe−H = 1.61(5) Å). The average iron−tris(carbene)borate bond distance (1.953(3) Å) is similar to that observed in complexes 1 and 4 and consistent with a low-spin (S = 0) spin state. As mentioned, the triethylborohydride bonding mode in 4 is unusual. The majority of crystallographically characterized alkylborohydride coordination complexes contain lanthanide,35 alkali,36 and alkaline earth metals.37 Transition metal-containing alkylborohydride complexes are limited to the bidentate alkyl borohydride [BH2(CH2CH3)2]−, which is known to coordinate to iron,38 rhenium,39 and nickel40 via the hydride atoms. Coordination of borohydride (BH4−) to iron is known to occur in κ1-,41 κ2-,42 and κ3-fashion.43 To our knowledge this is the first example of κ3-[BH(CH2CH3)3]− coordination to a transition metal that has been characterized by X-ray diffraction. Spectroscopic characterization provides evidence that the interaction between the iron center and the [BH(CH2CH3)3]− ligand is agostic in nature, as opposed to purely electrostatic. The 1H NMR spectrum of complex 5 contains resonances between 0 and 10 ppm, with a quadrupole-broadened resonance for the hydride ligand observed at −7.6 ppm, consistent with the low-spin (S = 0) state suggested by the crystallographic data. While the resonances of the tris(carbene)borate ligand are easily assigned by chemical shift and integration, the ethyl group proton signals of [BH(CH2CH3)3]− are unresolved at room temperature. However, on cooling to −60 °C, a 1:1:1 ratio of three new resonances for the methylene groups resolve at 0.46, −3.44, and −7.63 ppm.

resonances between 0 and 10 ppm from the tris(carbene)borate ligand, consistent with the Cs symmetric structure determined by X-ray crystallography.16 An additional resonance at −10.0 ppm is assigned to the hydride. Reaction with DCl confirms the hydride ligand assignment, with a single resonance at −10.0 ppm observed in the 2H NMR spectrum. No evidence of deuterium incorporation into the tris(carbene)borate ligand is observed by 2H NMR spectroscopy (Figure S18). Two C−O stretching bands are observed in the IR spectrum, but at considerably lower wavelength (1980 and 1922 cm−1), consistent with the strong donor ability of the tris(carbene)borate ligand. In contrast to FpH, which is a liquid that slowly decomposes to Fp2 and H2,28 4 is an indefinitely stable solid when stored at −35 °C in an inert atmosphere. This difference in stability is likely related to the bulk of the tris(carbene)borate ligand, which prevents bimolecular decomposition reactions. Given the ability of FpH to act as a catalyst for H+ reduction,29 we were interested in evaluating the effect of the tris(carbene)borate ligand on the thermodynamic propensity of the hydride ligand to participate in hydrogen atom and hydride transfer reactions. The bond dissociation free energy (BDFE, kcal/mol) of the Fe−H bond can be calculated according to a thermodynamic cycle in acetonitrile:30 BDFE = 1.37pK a + 23.06E 0 + 54.9

(2)

0

(1) 0

where pKa is the acidity of the hydride ligand and E is the standard potential for the anion. Remarkably, complex 3-cryptand was found to deprotonate toluene in THF solution, allowing us to estimate that the pKa = 44.4 for 4 in THF (pKa = 43.1 in MeCN).31 Not only is the hydride ligand in 4 considerably less acidic than that in FpH (pKa = 27.1), but it is among the least acidic hydride ligands to be reported.32 The standard potential for 2 (E1/2 = −2.34 V in MeCN) allows the Fe−H BDFE of 4 to be calculated as 60(1) kcal/mol. Despite the significant difference in hydride acidities, the Fe−H bond in 4 is only 3 kcal/mol weaker than the corresponding bond in FpH by (for calculation details, see the Supporting Information).17b This is because the large difference E



Organometallics

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CONCLUSIONS In summary, several new carbonyl complexes of iron supported by a tris(carbene)borate ligand have been prepared. The cationic iron complex [PhB(MesIm)3Fe(CO)3][B(C6F5)4] (1) can be chemically reduced by one or two electrons, giving the isolable neutral iron radical complex PhB(MesIm)3Fe(CO)2 (2) and the anionic iron dicarbonyl complex [K][PhB(MesIm)3Fe(CO)2] (3), which crystallizes as a dimer with an unusual side-on interaction between the potassium cation and a CO ligand. Similarly to our observations for {NiNO} 10 complexes,15 the strong donor ability of the tris(carbene)borate ligand results in significant π-back-donation to the CO ligands. Interestingly, the interaction with potassium helps to further reduce the CO bond order. Thermodynamic measurements of the Fe−H bond in PhB(MesIm)3Fe(CO)2H provide additional insights into the impact of the tris(carbene)borate ligand. The strong donor ability of the tripodal ligand significantly attenuates the acidity of the hydride, and thus its conjugate base is able to deprotonate toluene. Nonetheless, despite this very low acidity, the Fe−H BDFE is only slightly smaller than that observed in FpH. While similar measurements on related L2XFe(CO)2H complexes are required, this result suggests that it may be possible to tune the basicity of the hydride ligand without significantly affecting the homolytic bond strength. Such flexibility in tailoring basicity could be important for the design of new molecular catalysts.44 Finally, it is remarkable that with the isolation of these lowvalent complexes we have shown that tris(carbene)borate ligands are able to stabilize complexes in six oxidation states, ranging from iron(0) to iron(V).12,13a

Figure 7. (a) Molecular structure of PhB(MesIm)3Fe(κ3-BH(CH2CH3)3) (5). Two cocrystallized THF molecules and hydrogen atoms not associated with the [BH(CH2CH3)3]− ligand are omitted for clarity. Thermal ellipsoids are shown at 50% probability. Selected bond distances (Å) and angles (deg): Fe1−C1 1.961(3), Fe1−C13 1.975(3), Fe1−C25 1.923(3), Fe1−H1 1.61(5), Fe1−B2 2.124(4), B2−H1 1.36(5), Fe1−C45 2.378(3), Fe1−C47 2.328(4); B2−H1− Fe1 91(3), C1−Fe1−H1 95.0(16), C13−Fe1−H1 177.4(17), C25− Fe1−H1 92.6(16). (b) Close-up depiction of the iron−alkylborate interaction.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00646. Experimental methods, spectroscopic data, and electrochemical data (PDF) X-ray crystallographic collection data in CIF format (CIF)

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In addition, decoalescence of the room-temperature [BH(CH2CH3)3]− methyl resonance occurs to provide two resonances in a 2:1 ratio at −60 °C. One of these resonances is assigned to two equivalent methyl groups and one unique methyl group, consistent with the crystallographically determined molecular structure (Figure S19, see Supporting Information). Thus, each of the Fe−CH2 bonds is resolved by 1H NMR spectroscopy at −60 °C. At higher temperatures, the triethylborohydride ligand is fluxional and the three ethyl arms scramble on the NMR time scale, with line shape analysis providing ΔG⧧ = 12 kcal/mol at 298 K (Figures S22 and S23). Since the rate-determining step for intramolecular rotation in complex 5 likely involves cleavage of the agostic interaction, the ΔH⧧ value of 7.7 kcal/mol is an upper bound for the energy of the C−H agostic bond in this complex. The observed agostic interactions in complex 5 are sufficiently strong that the geometry is six-coordinate, giving a low-spin (S = 0) iron(II) complex, rather than a four-coordinate, high-spin (S = 2) hydride complex analogous to PhB(MesIm)3FeCl.

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*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funding from IU and the NSF (CHE-1112299) is gratefully acknowledged. REFERENCES

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Organometallics

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