A Bidentate Carbene Ligand Stabilizes a Low-Coordinate Iron(0

Article pubs.acs.org/Organometallics

A Bidentate Carbene Ligand Stabilizes a Low-Coordinate Iron(0) Carbonyl Complex Anne K. Hickey, Wei-Tsung Lee,† Chun-Hsing Chen, Maren Pink, and Jeremy M. Smith* Department of Chemistry, Indiana University, 800 East Kirkwood Avenue, Bloomington, Indiana 47405, United States S Supporting Information *

ABSTRACT: The bulky bis(carbene)borate ligand Ph2B(tBuIm)2− stabilizes low-coordinate and low-valent iron complexes. The fourcoordinate iron(II) complex Ph2B(tBuIm)2FeCl(THF) is a precursor to the low-spin iron(I) complex Ph2B(tBuIm)2Fe(CO)3. Despite a reversible reduction wave in the cyclic voltammogram, access to the iron(0) complex [Ph2B(tBuIm)2Fe(CO)3]− requires reduction under a CO atmosphere. Without excess CO, KC8 reduction leads to formation of the dimer K2[Ph2B(tBuIm)2Fe(CO)2] along with loss of CO. Each iron center in this complex adopts a distorted-square-planar geometry. Spectroscopic analysis of these carbonyl complexes reveals the very high strength of the bis(carbene)borate ligand.

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INTRODUCTION The photochemically generated 16-electron species Fe(CO)4 has been extensively investigated in order to understand the impact of spin state on the structure and reactivity of transitionmetal complexes.1 An extensive series of studies by multiple research groups has shown that the ground state of Fe(CO)4 is a triplet (3B2) with a C2v structure but that reactions with substrates are more facile on the singlet (1A1) surface. Thus, reactions of Fe(CO)4 require a transition from the 3B2 to the 1 A1 state.2 The transient nature of Fe(CO)4 limits investigations of its chemical properties. While other zerovalent FeL4 species have proven to be similarly elusive, their properties are also of interest, as they are implicated in important bond activation reactions: e.g., C−H insertion3 and functionalization.4 Most recently, a bulky isocyanide ligand was used as a CO mimic in the synthesis of low-valent homoleptic complexes; however, the key Fe(CNR)4 species proved to be elusive owing to ligand degradation.5 Bulky ligands are often used to stabilize low-coordinate complexes and analogues of reactive species. In this regard, suitable multidentate ligands have allowed for the synthesis of low-coordinate complexes containing iron carbonyl fragments. For example, bulky tris(pyrazolyl)borate6 and β-diketiminate7 ligands allow for the isolation of stable four-coordinate iron(I) carbonyl complexes in assorted geometries. However, to the best of our knowledge, iron(0) carbonyl complexes with these ligands, including analogues of Fe(CO)4, have not been reported. We recently reported the synthesis and properties of iron carbonyl complexes with a bulky tris(carbene)borate ligand.8 Despite the expectation that the strongly donating tripodal carbene ligand would favor the formation of high oxidation states only,9 we were able to isolate a number of low-valent iron complexes. A number of these complexes are analogues of © XXXX American Chemical Society

classical organometallic compounds but have significantly greater thermal stability. In light of these results, we were interested in extending these studies to iron complexes of a bulky bis(carbene)borate ligand with the goal of stabilizing low-valent iron carbonyl complexes, such as analogues of the elusive Fe(CO)4. This work has also allowed us to evaluate their donor properties in comparison with other anionic bidentate ligands such as β-diketiminates, guanidinates, and amidinates (Chart 1), with bis(carbene)borates being the strongest donors. Chart 1. Examples of Bulky Bidentate Anionic Ligands

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RESULTS AND DISCUSSION Reaction of the in situ prepared diphenylbis(carbene)borate ligand Ph2B(tBuIm)2− 10 with FeCl2(THF)1.5 yields the fourcoordinate complex Ph2B(tBuIm)2FeCl(THF) (1) in high yield. The molecular structure of 1, as determined by singlecrystal X-ray diffraction, reveals a trigonal-pyramidal iron center with metrical parameters typical for a high-spin iron(II) complex. The zero-field 57Fe Mössbauer spectrum provides Received: July 27, 2016

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DOI: 10.1021/acs.organomet.6b00599 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Scheme 1

parameters of 2 (δ = 0.48 mm/s, ΔEQ = 0.26 mm/s at 80 K) are similar to those of other S = 1/2 iron(I) arene complexes.16 Exposing complex 2 to an atmosphere of CO leads to formation of dark green Ph2B(tBuIm)2Fe(CO)3 (3) in high yield. The molecular structure of 3 reveals a square-pyramidal iron center (τ5 = 0.0017) with the bis(carbene)borate ligand and two carbonyl ligands forming the basal plane. The apical Fe−CO bond distance (1.868(14) Å) is greater than the basal Fe−CO bond distances (1.774(9) Å), likely as a consequence of a Jahn−Teller distortion in this low-spin (S = 1/2) iron(I) complex. The bonds between iron and the bis(carbene)borate ligand (2.024(7) Å) are longer than those in the iron(I) complex 2. Unlike the analogous allyl complex (η3-C3H3)Fe(CO)3, which is in equilibrium with its dimer in solution,18 complex 3 remains monomeric, likely due to the bulk of the bis(carbene)borate ligand. Spectroscopic methods confirm the low-spin (S = 1/2) iron(I) formulation of 3. The X-band EPR spectrum, recorded in a toluene glass, exhibits a rhombic signal having g tensors (gx = 2.19, gy = 2.13, and gz = 1.99 at 77 K) consistent with an unpaired electron residing in a metal-centered orbital (Figure 1a). The presence of a single unpaired electron is further

further evidence for the oxidation state assignment, with the isomer shift (δ = 0.65 mm/s) and quadrupole splitting (ΔEQ = 3.02 mm/s) similar to those of other four-coordinate high-spin iron(II) complexes containing two NHC donors.11 The complex has also been characterized in solution. The 1H NMR spectrum of 1 shows six paramagnetically shifted resonances with relative integrations appropriate for the bis(carbene)borate ligand. No resonances for the THF ligand are observed, likely due to the close proximity of the paramagnetic iron nucleus. The solution magnetic moment (μeff = 5.0(3) μB) is consistent with high-spin (S = 2) iron(II). Complex 1 is indefinitely stable under an inert atmosphere. The long-term thermal stability of this complex differs from that for complexes of the corresponding dihydrobis(carbene)borate ligand H2B(tBuIm)2−, which has a strong driving force for the formation of homoleptic complexes.12 For example, the three-coordinate heteroleptic iron(II) complex H 2 B(tBuIm)2Fe−N(SiMe3)2 rearranges over time to the high -spin pseudotetrahedral complex [H2B(tBuIm)2]2Fe.13 In contrast, complex 1 shows no evidence for such undesired rearrangement reactions, thus serving as an entry point to the synthesis of other coordinatively unsaturated complexes. The effect of remote substituents on the metal coordination environment has been noted for other bidentate ligands.14 One-electron reduction of 1 in toluene provides the green low-spin (S = 1/2) iron(I) complex Ph2B(tBuIm)2Fe(η6-C7H8) (2) in high yield, which has also been structurally and spectroscopically characterized (Scheme 1). The X-ray crystal structure shows that the complex adopts a piano-stool geometry, similar to that of iron(I) arene complexes of βdiketiminate7a and amidinate15 ligands. The bonds between iron and the bis(carbene)borate ligand (1.981(3) and 1.975(3) Å) are shorter than the equivalent bonds in the high-spin iron(II) precursor 1 (2.085(2), 2.100(2) Å). The average bond distance between iron and the toluene ligand carbons is 2.136 Å. The C−C bond distances within the toluene ligand (average 1.405 Å) give no evidence for bond alternation, suggesting that the toluene ligand has not lost its aromaticity and that the reduction is metal-based. This conclusion is supported by spectroscopic measurements. Specifically, the EPR spectrum of 2 in a toluene glass (77 K) reveals a rhombic signal having g tensors gx = 2.25, gy = 2.02, and gz = 1.99, consistent with lowspin (S = 1/2) iron(I). Additionally, the Mössbauer spectral

Figure 1. (a) EPR spectrum of 3 in a toluene glass at 77 K. The asterisk represents an 11% impurity. (b) Solid-state Mössbauer spectrum of 3 at 80 K.

confirmed by solution magnetometry (μeff = 2.3(3) μB). Finally, the value of the isomer shift in the zero-field 57Fe Mössbauer spectrum (δ = 0.18 mm/s, ΔEQ = 2.45 mm/s at 80 K) compares favorably with that observed for an S = 1/2 bis(imino)pyridine iron(I) carbonyl complex (Figure 1b).19 The square-pyramidal geometry is retained in solution, as characterized by IR spectroscopy (νCO 1987, 1900 cm−1). These stretching frequencies allow the donor strength of the bis(carbene)borate ligand to be evaluated by comparison with B


Article

Organometallics other fac-iron(I) tricarbonyl complexes. While there are few examples of bidentate anionic ligands (Table 1), it is clear that the bis(carbene)borate ligand is by far the most strongly donating, as evaluated by the significantly lower stretching frequencies for complex 3.

1.758(3) Å) due to the nondegenerate electron configuration of this diamagnetic complex. The solid-state structure is maintained in solution, as characterized by 1H NMR spectroscopy, while IR spectroscopy (νCO 1926, 1836, and 1800 cm−1) reveals an increase in the extent of π back-bonding upon reduction. Similarly to 3, these data show the very strong donor ability of the bis(carbene)borate ligand (Table 2). The

Table 1. Comparison of νCO for Mononuclear Iron(I) Complexes with a Facial Array of Carbonyl Ligands

a

Table 2. Comparison of νCO of Selected Mononuclear Iron(0) Tricarbonyl Complexes with a Facial Array of Carbonyl Ligands

Mössbauer spectrum shows a decrease in isomer shift on reducing complex 3 to 4 (δ = −0.10 mm/s, ΔEQ = 2.09 mm/s), likely due to the greater degree of π back-bonding in the lower valent complex.22 Similar isomer shifts have been observed for iron(0) carbonyl complexes.23 Interestingly, reduction of 3 by KC8 under an N2 atmosphere results in loss of CO, quantitatively providing the purple dimer K2[Ph2B(tBuIm)2Fe(CO)2]2 (5; Scheme 3). This complex

Bands not resolved.

Scheme 3

Complex 3 is reversibly reduced by one electron on the cyclic voltammetry (CV) time scale (−1.92 V vs Fc/Fc+), suggesting the accessibility of a lower-valent iron tricarbonyl complex. Unexpectedly, synthetic access to this reduced complex requires the reaction with KC8 to be conducted under an atmosphere of CO. With 2,2,2-cryptand as a sequestering agent for potassium, the air-stable and yellow diamagnetic complex K(crypt)[Ph2B(tBuIm)2Fe(CO)3] (4) can be isolated in high yield (Scheme 2). The molecular structure of 4, as determined by single-crystal X-ray diffraction, reveals that the complex retains the same square-pyramidal structure of 3 (τ5 = 0.017) with an apical carbonyl ligand. However, unlike the case for 3, the Fe−CO bond distances in 4 are all similar in length (1.738(3)−

crystallizes as a centrosymmetric dimer with two [Ph2B(tBuIm)2Fe(CO)2] units held together by two potassium ions (Figure 2). Each iron center adopts a distorted-square-planar geometry (τ4 = 0.24,27 sum of the angles around each iron center 358.4°), making this complex a rare example of lowvalent iron in a square-planar environment.28,29 The bridging potassium ions bind in an end-on fashion to a carbonyl ligand from one [Ph2B(tBuIm)2Fe(CO)2] unit and side-on with two carbonyl ligands from the other unit. Each potassium ion is nearly equidistant from each carbonyl ligand (K1−O1 = 2.717 Å, K1−O2 = 2.848 Å). The coordination spheres of the potassium ions are completed by one phenyl ring of the bis(carbene)borate along with two THF ligands (not shown). Due to the crystallographically imposed symmetry, a single quadrupole doublet is observed in the Mössbauer spectrum of complex 5 (Figure 3). While the isomer shift of 5 (δ = 0.05 mm

Scheme 2

C


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Organometallics

resulting metal complexes. With a low-coordinate iron(II) precursor as the starting material, a series of low-valent iron carbonyl complexes are accessible, including an isolable analogue of Fe(CO)4, albeit in the form of a binuclear complex. We anticipate that mononuclear Fe(CO)4 analogues may be accessible with bulkier ligands. The spectroscopic properties of these low-valent iron carbonyl complexes reveal the high donor strength of the bis(carbene)borate ligand. While the data set is small at the moment, it is clear that this ligand is a considerably stronger donor than analogous bulky bidentate ligands such as βdiketiminates, guanidinates, and amidinates (Tables 1 and 2). It is intriguing to consider the effect of the larger ligand field strength of the bis(carbene)borate ligand on the reactivity of its complexes.

Figure 2. X-ray crystal structure of 5: (a) structure of the full complex, with coordinated THF molecules omitted for clarity; (b) iron environment, showing the distorted-square-planar geometry. Hydrogen atoms are omitted for clarity, and thermal ellipsoids are shown at 50% probability.

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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.6b00599. Experimental methods, spectroscopic data, electrochemical data, and X-ray crystallographic collection data (PDF) X-ray crystallographic data (CIF)

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Figure 3. Solid-state Mössbauer spectrum of 5 at 80 K. The major component (red) corresponds to 5, while the minor component (blue) corresponds to 4.

AUTHOR INFORMATION

Corresponding Author

*E-mail for J.M.S.: [email protected].

s−1, ΔEQ = 1.03 mm s−1 at 80 K) is smaller than that for 3, the different coordination numbers and geometries make direct comparison of these values difficult. The Mössbauer parameters are similar to those observed for an iron dicarbonyl complex with a bis(imino)pyridine ligand (δ = 0.03 mm s−1, ΔEQ = 1.03 mm s−1), where the oxidation state assignment of iron is ambiguous.19 Two bands are observed in the IR spectrum of 5. These bands are at lower energy (νCO = 1838 and 1742 cm−1) than in 3, suggesting that the reduction event is metal-based. Indeed, these bands are at even lower energy than those in 4, although it is likely that the potassium ions play a significant role in attenuating the bond strength, similarly to our observations with tris(carbene)borate iron carbonyl complexes.8 The magnetic moment of complex 5, as determined by Evans’ method, is 3.0(2) μB for the dimer. As noted above, the four-coordinate iron centers of 5 are unusual since low-coordinate iron(0) species are usually highly reactive and cannot be isolated.30 For example, spectroscopic and chemical evidence suggests that the cyclometalated iron(II) complex Fe(PMe3)3(H)(PMe2CH2) is in equilibrium with the four-coordinate iron(0) complex Fe(PMe3)4, which is believed to be paramagnetic.31 Low-coordinate iron(0) has also been implicated in the photochemically induced C−H activation reactivity of Fe(dmpe)2H2.32

Present Address †

Department of Chemistry and Biochemistry, Loyola University Chicago, Chicago, IL 60660, USA. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funding from IU, the DOE (DE-FG02-08ER15996), and the NSF (CHE-1112299) is gratefully acknowledged. REFERENCES

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CONCLUSIONS In contrast to the previously reported dihydroborate ligand H2B(tBuIm)2−,12,13 the diphenylbis(carbene)borate analogue Ph2B(tBuIm)2−10 stabilizes low-coordinate iron complexes without evidence for the formation of undesired homoleptic species. This provides another demonstration of the effect of remote modification on the properties of ligands and the D


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A Bidentate Carbene Ligand Stabilizes a Low-Coordinate Iron(0

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