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Cooperative Metal−Ligand Reactivity and Catalysis in Low-Spin Ferrous Alkoxides Wan-Yi Chu, Xiaoyuan Zhou, and Thomas B. Rauchfuss* School of Chemical Sciences, University of Illinois, Urbana, Illinois 61801, United States S Supporting Information *

ABSTRACT: This report describes examples of combined Fe- and O-centered reactivity of Fe(P2O2)(CO)2 (1), where P2O2 is the diphosphinoglycolate (Ph2PC6H4CHO)22−. This 18e low-spin ferrous dialkoxide undergoes substitution of CO to give the labile monosubstituted derivatives Fe(P2O2)(CO)(L) (L = PMe3, pyridine, MeCN). Treatment of Fe(P2O2)(CO)2 with Brønsted acids results in stepwise O-protonation, affording rare examples of low-spin Fe(II) complexes containing alcohol ligands. Substitution reactions with amides (RC(O)NH2) proceeds with binding of the carbonyl and formation of an intramolecular hydrogen bond between NH and the neighboring alkoxo ligand. This two-site binding was confirmed with crystallographic characterization of the thiourea-substituted derivative. Fe(P2O2)(CO)2 reacts with Ph2SiH2 to give the O-silylated hydrido complex, which is inactive for hydrosilylation. The monocarbonyl derivatives Fe(P2O2)(CO)(L) (L = NCMe, PMe3, acetamide) are precursors to catalysts for the hydrosilylation of benzaldehyde, acetophenone, and styrene.

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INTRODUCTION Monomeric alkoxoiron(II) complexes are rare.1 The oxygen centers in such complexes are expected to be highly basic owing to electrostatic interactions between the metal and ligand, although the Fe−O bonds themselves remain strong.2 The enhanced basicity of the alkoxide ligand in such complexes usually results in multinuclear complexes with bridging alkoxo ligands. For these reasons, studies on the reactivity of ferrous alkoxides are few. We recently described a one-pot synthesis of an iron(II) diphosphine-dialkoxide dicarbonyl complex Fe(P2O2)(CO)2 (1; P2O2 = (Ph2PC6H4CHO)22−). This complex arises by the coupling of 2 equiv of 2-diphenylphosphinobenzaldehyde (PCHO)3 in the presence of iron(0) carbonyl reagents (eq 1; bda = benzylideneacetone).4

the high basicity of alkoxo ligands and an unprecedented activation mode of Si−H bonds by alkoxo-iron functionalities to afford a series of new hydrido-iron complexes. We also found that the diphosphino-dialkoxo iron species with labile ligands (e.g., MeCN, MeC(O)NH2) are active catalysts for the hydrosilylation of aldehydes, ketones, and even alkenes. The spectroscopic features of all newly synthesized compounds are summarized in Table 1.

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Substitution of Fe(P2O2)(CO)2 by Lewis Bases. The CO ligands of 1 are labile. In CH2Cl2 solutions, 1 exchanges with 1 atm of 13CO over the course of several hours (see the Supporting Information). The lability of the CO ligands was further demonstrated by the synthesis of the acetonitrile derivative Fe(P2O2)(CO)(NCMe) (2), generated by warming an acetonitrile solution of 1 for 24 h. Substitution desymmetrizes the complex, as manifested by the appearance of two doublets in the 31P NMR spectrum. The large value of JP−P (289 Hz) was consistent with mutually trans phosphines. The structure of 2 was confirmed by X-ray crystallography (Figure 1). The structures of 1 and 2 differ only slightly. The MeCN ligand shows a slight contraction of the NC bond (1.135(6) Å) in comparison to that of free acetonitrile (1.157 Å). In the absence of added MeCN, CH2Cl2 solutions of 2 decomposed at room temperature over the course of a few hours to give 1, free MeCN, and unidentified NMR-silent products.

Similar condensations had been observed using PCHO with low-valent tungsten and technetium.5 Our iron system is attractive because the conversion is highly efficient and the reagents are inexpensive. As a dialkoxo iron(II) carbonyl, 1 is unique. In the previous report, 1 was shown to form stable adducts with a variety of Lewis acids, including BF3, which bind to the alkoxide sites. In this paper, we provide new perspectives on the O-centered reactivity of 1. These results demonstrate © XXXX American Chemical Society

RESULTS

Received: November 16, 2014

A

DOI: 10.1021/om501152h Organometallics XXXX, XXX, XXX−XXX

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Organometallics Table 1. Spectroscopic Properties (CH2Cl2 Solution) for Iron Complexes Containing the P2O2 Ligand compound Fe(P2O2)(CO)2 (1)4 Fe[P2O(OH)](CO)2 ([H1]OTf) [Fe[P2(OH)2](CO)2] (OTf)2 ([H21](OTf)2) Fe(P2O2)(CO)(MeCN) (2) Fe(P2O2)(CO)(PMe3) (3)

νCO (cm‑1) 1965, 2025 2003, 2052 2034, 2074 1938a

δ(1H) (JP−H, Hz)

21.1

4.49 (OCH)

26.1 (224)

5.08, 4.63 (OCH) 3.95 (OCH)

29.9 28.2, 25.5 (289)

Fe(P2O2)(CO)(py) (4)

1917

24.4, 21.3, 19.7 (44.1, 44.3, 278) 29.6, 25.6 (283)

Fe(P2O2)(CO)(MeC(O) NH2) (5) Fe(P2O2)(CO) (SC(NH2)2) (6)b Fe(PCy2O2)(CO)2 (7)

1933

32.4, 31.7 (261)

1928

23.2, 21.1 (287) 25.6

HFe[P2O(OSiHPh2)] (CO)2 (8)

1995, 1930 2004, 1939

HFe(P2O2SiEt3)(CO)2 (9)

2000

70.2, 52.7 (197)

HFe[PCy2O(OSiHPh2)] (CO)2 (10)

1974

50.9, 74.0 (179)

a

1918

δ(31P) (JP−P, Hz)

71.2, 52.8 (195)

Scheme 1. Stepwise Protonation of 1

4.67, 3.86 (OCH) 4.56, 4.01 (OCH) 4.83, 3.93 (OCH) 4.99, 3.78 (OCH) 4.64, 3.42 (OCH) 3.95 (FeH)

confirmed coordination of PMe3 in the equatorial position, whereas the diphosphine ligands remained mutually trans. The solid-state structure of 3 showed elongated Fe(1)−P distances and a smaller P(1)−Fe(1)−P(2) angle in comparison to that in 1, reflecting substitution of a π-acidic CO with a bulkier, more electron donating phosphine ligand. The pyridine complex Fe(P2O2)(CO)(py) (4) was synthesized by dissolving 1 in pyridine. Unlike 2, complexes 3 and 4 proved stable as CH2Cl2 solutions in room temperature over prolonged periods. Reactions of Fe(P2O2)(CO)2 with Brønsted Acids. Previous work revealed that 1 forms 1:1 and 2:1 adducts with Lewis acids, which bond to the oxygen centers of the alkoxide. This work explored the reactivity of 1 with Brønsted acids, with the expectation that the alkoxides will react to form labile neutral ligands, enabling binding of substrates to the iron center. Titration of a CH2Cl2 solution of 1 with HOTf resulted in stepwise protonation of the alkoxo ligands to give [H1]+ and [H21]2+. The first protonation occurs independently of the second, suggesting very different basicities for 1 and [H1]+ (Scheme 1). At ≤1 equiv of HOTf, two broad peaks were observed in the 31P NMR spectrum at δ 21.1 (unreacted 1) and δ 26.1, assigned to [H1]+. Broadening of the signals results from intermolecular transfer of protons between 1 and [H1]+. At 1 equiv of acid, the signal for [H1]+ sharpened into an AB

5.07, 4.82 (OCH) −2.97 (76, 45) (FeH) 4.88, 5.16 (OCH) −2.92 (75, 46) (FeH) 4.90, 5.06 (OCH) −3.19 (63, 52) (FeH)

Recorded in MeCN. bRecorded in THF.

An analogous PMe3 complex, Fe(P2O2)(CO)(PMe3) (3), was obtained in good yield by treating a THF/CH2Cl2 solution of 1 with excess PMe3. No evidence for disubstitution was observed. The 31P NMR spectrum of 3 revealed an ABX spin system with JP−P = 44, 44, 280 Hz. X-ray crystallography of 3

Figure 1. Structure (50% probability thermal ellipsoids) of 2 (left) and 3 (right). Selected hydrogen atoms have been omitted for clarity. Selected distances (Å) and angles (deg): 2, Fe(1)−N(1) 1.905(4), Fe(1)−P(1) 2.239(1) Fe(1)−P(2) 2.245(1), Fe(1)−O(2) 1.944(3), Fe(1)− O(3)1.949(4), Fe(1)−C(1) 1.752(6), P(1)−Fe(1)−P(2) 168.27; 3, Fe(1)−P(1) 2.244(1), Fe(1)−P(2) 2.283(1), Fe(1)−P(3) 2.273(1), Fe(1)− O(2) 1.973(2), Fe(1)−O(3) 1.970(2), Fe(1)−C(1) 1.737(4), P(2)−Fe(1)−P(3) 160.15(4). B

DOI: 10.1021/om501152h Organometallics XXXX, XXX, XXX−XXX

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Figure 2. Titration of 1 with HOTf monitored by 31P NMR (left) and IR spectroscopy (right).

quartet (JP−P = 235 Hz). Addition of HOTf to [H1]+ gave rise to a broad 31P NMR signal at δ 29.9, corresponding to the doubly protonated species [H21]2+. This signal sharpened to a singlet for ≥2 equiv of HOTf. This stepwise protonation process was also monitored by IR spectroscopy (Figure 2). Cations [H1]+ and [H21]2+ are rare examples of low-spin ferrous complexes containing alcohol ligands.6 To confirm the assignment, X-ray crystallographic analysis of [H21]OTf2 was carried out. The cation retains the idealized C2 symmetry of 1 (Figure 3), consistent with its simple 31P NMR spectrum. In

Titration of 1 with H(OEt2)2BArF4 gave similar results by NMR spectroscopy in terms of chemical shifts and number of species (BArF4 = B[3,5-(CF3)2C6H3]4−). Differences in the NMR spectra between protonation with HOTf and H(OEt2)2BArF4 are attributed to the weaker hydrogen bonding of H+ with Et2O in comparison to that with OTf−. Thus, faster proton exchange between [H1]BArF4 and 1 is evident. For example, addition of 0.5 equiv of H(OEt2)2(BArF4) to 1 gave a broadened signal observed in the 31P NMR spectrum at the average of the chemical shifts of [H1]+ and 1. Under the same conditions but with HOTf, separate signals are observed for [H1]+ and 1. Substitution of Fe(P2O2)(CO)2 by H-Bonding Ligands. The 16e entity Fe(P2O2)(CO) should feature adjacent basic alkoxide and Lewis acidic iron sites. Such a species is suited for binding with ligands that could function both as Lewis bases and H-bond donors. Indeed, under prolonged heating with excess benzamide or acetamide, 1 converts to such a complex, as evidenced by NMR spectroscopy. The 1H NMR spectrum of Fe(P2O2)(CO)(MeC(O)NH2) (5) was characterized by a strong singlet at δ 18, assigned to the NH group that is hydrogen-bonded to the adjacent alkoxide. Curiously, the 31P NMR spectrum of 5 consists of a singlet in the presence of excess acetamide, suggesting fast exchange between the coordinated and free acetamide ligands. This symmetrization is proposed to proceed via an associative mechanism (Scheme 2). This type of symmetrization was not observed in the 31P NMR spectrum of 2 upon addition of MeCN to a CH2Cl2 solution. After removal of excess acetamide by multiple recrystallizations, the 31P NMR spectrum of 5 exhibits an AB quartet, as expected for monosubstituted compounds of 1 (see the Supporting Information).7 Complex 5 decomposed in a CH2Cl2 solution in the absence of excess acetamide to give 1 and other NMR-silent species. To obtain more stable amide adducts, thioureas were examined. Thioureas are good hydrogen-bond donors and are softer Lewis bases than ureas.8 Treatment of 1 with 1 equiv of thiourea in CH2Cl2 gave the bright green compound Fe(P2O2)(CO)(SC(NH2)2 (6; νCO 1928 cm−1 in THF solution). Its 31P NMR spectrum revealed a characteristic AB quartet (JP−P = 287 Hz) indicative of a monosubstituted derivative of 1. A signal at δ 12.4 in the 1H NMR spectrum is assigned to the hydrogen-bonded NH (in thiourea δ(NH) 6.58).

Figure 3. Structure (50% probability thermal ellipsoids) of [H21](OTf)2. Selected hydrogen atoms, solvent, and anions have been omitted for clarity. Selected distances (Å) and angles (deg): Fe(1)− O(3) 1.993(3), Fe(1)−O(4) 1.982(2), O(3)−H(3) 0.80(4), O(4)− H(4) 0.81(3), Fe(1)−P(1) 2.293(1), Fe(1)−P(2) 2.295(1), Fe(1)− C(1) 1.784(4), Fe(1)−C(2) 1.783(4), P(1)−Fe(1)−P(2) 171.30(4).

comparison to the structure of 1, the Fe−O and Fe−P bonds are elongated by 0.03 and 0.05 Å, respectively. The O−H centers were located in the electron difference map with O− Havg 0.81 Å. A hydrogen-bonding interaction was also observed for the alcohol protons and one oxygen of each triflate anion, with H- - -Oavg 1.71 Å. C

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Organometallics Scheme 2. Mechanism Proposed for Amide Exchange in 5a

a

The P2O2 ligand is simplified for clarity.

Scheme 3. Hydrosilylation of Fe(P2O2)(CO)2 (1) and Fe(PCy2O2)(CO)2 (7) with Ph2SiH2 and Et3SiH

Figure 4. Structure (50% probability thermal ellipsoids) of 6. Selected hydrogen atoms have been omitted for clarity. Selected distances (Å) and angles (deg): Fe(1)−P(1) 2.2601(7), Fe(1)−P(2) 2.2563(6), Fe(1)−S(1) 2.3122(5), Fe(1)−O(2) 1.971(1), Fe(1)−O(3) 1.976(1), Fe(1)−C(1) 1.736(2), S(1)−C(2) 1.719(2), O(2)−H(1)A 1.81(3), O(2)−N(1) 2.661(2), N(1)−C(2) 1.318(3), P(1)−Fe(1)−P(2) 168.20(3), O(2)−H(1)A−N(1) 156(3).

characteristic of mutually trans phosphine ligands. The 1H NMR spectrum features a doublet of doublets at δ −2.97 with JP−H = 44.6 and 76.4 Hz, consistent with a metal hydride coupled to inequivalent phosphine groups. Complex 8 was stable toward excess Ph2SiH2, indicating that the second alkoxo group resisted silylation. Similarly, the Si−H bond in 8 was unreactive toward another 1 equiv of 1. Crystallographic analysis of 8 confirmed the expected endo stereochemistry of the siloxy group, consistent with retention of stereochemistry at carbon (and probably Fe; Figure 5). The structure exhibited a distorted-octahedral geometry with the trans-phosphines canted away from the cis-dicarbonyl groups and a P−Fe−P angle of 157.68(2)°. This bending was more prominent than that in the parent complex 1 (167.43(4)°). The Fe(1)−C(1) distance is 1.820(2) Å, longer than the Fe-CO bond trans to the alkoxo ligand, reflecting the stronger trans influence of the hydride ligand in comparison to that of the alkoxo group. Although alkoxo iron hydrides are rare, the phenolato hydride HFe(Ph2PC6H4O)(PMe3)3 and related derivatives have been characterized.11 Compound 1 also reacted with Et3SiH, initially affording HFe(P2 O2SiEt3 )(CO)2 (9), albeit at a slower rate in comparison to the Ph2SiH2 reaction. As seen for 8, the 31P NMR spectrum of 9 showed doublets at δ 70.2 and 52.7 with a large JP−P value of 197 Hz, indicative of mutually trans phosphines. A doublet of doublets at δ −2.95 (JP−H = 74.9, 46.5 Hz) in the 1H NMR spectrum was again consistent with a hydrido iron(II) complex coupled to inequivalent phosphine ligands. Upon standing in solution for several hours at room temperature, 9 partially isomerized to a new hydrido species (9′) of unknown structure (see the Supporting Information).

X-ray crystallographic analysis confirmed that 6 is the expected complex of thiourea (Figure 4). All four hydrogen centers on the thiourea ligand were located in the electron difference map, and hydrogen-bonding interaction between one alkoxo ligand and one amino group on the thiourea ligand is indicated. The O(2)···H(1)A distance of 1.81(3) Å and the O(2)−H(1)A−N(1) angle of 156(3)° indicate a moderatestrength H-bonding interaction.9 Silane Derivatives. In view of the strength of O−H bonds, it was reasoned that 1 would be susceptible to hydrogenation to give the hydride complex with a pendant alcohol group: i.e., the complex HFe[P2O(OH)](CO)2. Solutions of 1, however, proved unreactive toward H2 even in the presence of a catalytic amount of Pd/C and acids. Similar nonreactivity was seen for the more electron rich dicyclohexyl analogue of 1, abbreviated Fe(PCy2O2)(CO)2 (7), which was synthesized in a manner similar to that for 1 from Cy2P-2-C6H4CHO (Cy = C6H11).10 Complex 1 and its substituted derivatives do react with silanes to give hydride complexes. For example, a CH2Cl2 solution of 1 and Ph2SiH2 gave the hydride HFe[P2O(OSiHPh2)](CO)2 (8) (Scheme 3). Spectroscopic measurements are consistent with formation of a metal hydride species with an O-silylated diphosphine. The 31P NMR spectrum consists of doublets at δ 71.2 and 52.8 (JP−P = 195 Hz), D

DOI: 10.1021/om501152h Organometallics XXXX, XXX, XXX−XXX

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Organometallics

kobs vs [Ph2SiH2] (Figure 6), was found to be [1.14(6)] × 10−3 M−1 s−1. rate = k[Ph 2SiH 2][1]

(2)

Mechanism of Silane Addition to 1. Prior to this report, reactions of alkoxoiron(II) complexes with organosilanes have not been well studied. However, hydridonickel(II) complexes have been prepared by similar routes.12 For the formation of 8 from 1 and Ph2SiH2, the Lewis basic alkoxide ligand may attack the weakly Lewis acidic silicon center, weakening the Fe−O bond (see [H21](OTf)2), followed by transfer of hydride to iron. A related mechanism has been proposed for the addition of silanes to (PPh3)2Re(O)2I.13 A pathway involving the dissociation of CO is unlikely, since CO does not affect the rate of hydride formation. Hydrosilylation Studies. Neither 1 nor its silylated derivative 8 catalyzed the hydrosilylation of benzaldehyde, acetophenone, or styrene. We then turned to derivatives that contained labile ligands for catalytic hydrosilylation studies. Using 2 (1 mol %) as a catalyst, PhCHO was quantitatively hydrosilylated with Ph2SiH2 and PhSiH3 at room temperature within 5 h (Table 2). No catalysis was observed using the tertiary silanes Et3SiH and PhMe2SiH. Acetophenone also underwent hydrosilylation, although more slowly than PhCHO. Similar to the case for 2, complexes 3 (entry 4) and 4 (entry 5) are also active catalysts. Interestingly, 2 was active as a catalyst for the hydrosilylation of styrene, although this reaction required more forcing conditions and lower TONs were achieved (entry 9). The MeC(O)NH2 complex 5, which was proposed to be more labile than the MeCN derivative 2, was twice as active for hydrosilylation of styrene. Comments on Mechanism of Catalytic Hydrosilylation. Catalytic hydrosilylation of ketones and aldehydes has been well studied in the past decade, including several basemetal catalysts that are active under very mild conditions and with low loading.14 Illustrative high-activity catalysts feature diiminopyridine, N(SiMe3)2−, iminophosphines, and N-phosphinoamidinate as ligands.15 The mechanism of hydrosilylation using 2 is not known. When the hydrosilylation of acetophenone by Ph2SiH2 was

Figure 5. Structure (50% probability thermal ellipsoids) of 8. Selected hydrogen atoms have been omitted for clarity. Selected distances (Å) and angles (deg): Fe(1)−H(2) 1.42(2), Fe(1)−P(1) 2.1863(5), Fe(1)−P(2) 2.2445(5), Fe(1)−O(3) 1.983(1), Fe(1)−C(1) 1.820(2), Fe(1)−C(2) 1.745(2), Si(1)−O(4) 1.640(1), P(1)−Fe(1)−P(2) 157.68(2).

Complex 7 reacted with Ph2SiH2 in a similar fashion to give HFe[PCy2O(OSiHPh2)](CO)2 (10), with spectroscopic features closely resembling those of 8 and 9 (Table 1). Interestingly, complex 7 was unreactive toward Et3SiH even at elevated temperatures (ca. 50 °C in CH2Cl2). Given that Et3SiH also had a lower reactivity with 1 in comparison to Ph2SiH2, the nonreactivity of 7 toward Et3SiH is attributed to steric crowding due to the large cyclohexyl groups. In order to clarify aspects of the hydride-forming process, the rate of formation of 8 from 1 was monitored by 1H NMR spectroscopy. At 10 °C in the presence of >10 equiv of Ph2SiH2, the conversion was first order in iron. The rate was also found to depend linearly on the concentration of silane (Figure 6). The rate was unaffected by the presence of CO (1 atm). These results are consistent with the rate law in eq 2. The second-order rate constant (k), determined from the plot of

Figure 6. (left) Representative kinetic plot for the reaction of 1 with Ph2SiH2. Conditions: [1] = 0.015 M, [Ph2SiH2] = 0.15 M, 10 °C in CD2Cl2. Disappearance of the signal at 4.52 ppm was monitored by 1H NMR. (right) Plot of kobs versus the concentration of Ph2SiH2. Conditions: [Ph2SiH2] = 0.15−0.47 M, [1] = 0.015 M, 10 °C in CD2Cl2. E

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Organometallics Table 2. Results of Hydrosilylation Catalyzed by Various Fe Complexesa entry

[Fe]

substrate

silane

time (h)

product(s)b

TON

1 2 3 4 5 5 6 7 8 9 10c

2 2 2 3 4 2 2 2 2 5 5

PhCHO PhCHO PhCHO PhCHO PhCHO PhCHO PhCHO PhC(O)Me PhC(O)Me PhCHCH2 PhCHCH2

Ph2SiH2 Ph2SiH2 PhSiH3 Ph2SiH2 Ph2SiH2 Et3SiH PhMe2SiH Ph2SiH2 Ph2SiH2 Ph2SiH2 Ph2SiH2

3 5 3 24 24 3 3 5 24 15 15

PhCH2OSiHPh2, (PhCH2O)2SiPh2 (69:20) PhCH2OSiHPh2, (PhCH2O)2SiPh2 (69:27) PhCH2OSiH2Ph, (PhCH2O)2SiHPh (77:18) PhCH2OSiH2Ph, (PhCH2O)2SiHPh (47:42) PhCH2OSiH2Ph, (PhCH2O)2SiHPh (18:16) 0 0 PhCH(OSiHPh2)Me PhCH(OSiHPh2)Me PhCH(SiHPh2)Me PhCH(SiHPh2)Me

89 96 95 89 34

26 48 18 48

a Reaction conditions (unless specified otherwise): substrate (1.0 mmol), silane (1.1 mmol), [Fe] (10 μmol), CH2Cl2 (2 mL), 25 °C. bThe yield was determined by 1H NMR spectroscopy with an internal standard. cT = 60 °C, no solvent.

functionality is crucial in this transformation, as the strong Si−O bond (∼108 kcal/mol) is thermodynamically favored. The addition of silanes is proposed to proceed via alkoxideassisted activation of the Si−H bond. Due to the strong coordination of the other ligands, the resulting hydrido complexes were not catalytically active. Substituting one CO in 1 with labile ligands gives Fe(P2O2)(CO)(L) (L = NCMe (2), MeC(O)NH2), which are precatalysts with good to excellent activity for catalytic hydrosilylation of aldehydes, ketones, and styrene with catalyst loadings of ≤1 mol %. Recently, Li et al. reported hydrosilylation of benzaldehyde and ketones using hydridoiron(II) complexes bearing labile PMe3 ligands.19 Guan et al. also described hydridoiron(II) precatalysts for hydrosilylation.20 In contrast to Li’s reports, the hydrido ligand in Guan’s precatalyst is a spectator ligand. These and other studies point to intermediates in the catalytic cycle that could not be observed by NMR spectroscopy,21 which possibly involves high-spin iron(II) complexes.

monitored in situ by NMR spectroscopy, only the O-silyl hydride 8 was detected by 31P NMR spectroscopy in ∼50% yield. As mentioned above, purified 8 is catalytically inactive, reflecting the inertness of its two carbonyl ligands. The activity of catalysts of the type Fe(P2O2)(CO)L correlated qualitatively with the lability of L. This correlation suggests that an important step in the catalytic cycle is the dissociation of L to allow for substrate binding. In view of the rapidity of the silylation of 1, the active catalyst is proposed to arise via the sequence of reactions in eqs 3 and 4, consisting of formation of an O-silylated precatalyst followed by its activation following transfer of CO. Fe(P2O2 )(CO)L + H 2SiPh 2 → HFe[P2O(OSiPh 2H)](CO)L

(3)

L = PMe3, MeCN, MeC(O)NH 2

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2HFe[P2O(OSiPh 2H)](CO)L → HFe[P2O(OSiPh 2H)](CO)2

Materials and Methods. Unless otherwise indicated, reactions were conducted using standard Schlenk techniques under an N2 or argon atmosphere at room temperature with stirring. Fe(bda)(CO)3,22 PCyCHO,10 and FeP2O2(CO)24 were synthesized according to literature preparations. Silanes, thiourea, acetamide, benzaldehyde, styrene (Sigma-Aldrich), and PMe3 (Strem) were used as received. 1H, 13 C, and 31P NMR spectra were acquired on Varian UNITY INOVA 500NB and UNITY 500 NB instruments. Chemical shifts are reported relative to the peak of residual solvent.23 FT-IR spectra were recorded on a PerkinElmer 100 FT-IR spectrometer. Elemental analyses were conducted in the School of Chemical Sciences Microanalysis Laboratory utilizing a Model CE 440 CHN analyzer. Fe(P2O2)(CO)(NCMe) (2). A suspension of 1 (200 mg, 0.25 mmol) in 500 mL of acetonitrile was heated in a 80 °C oil bath for 24 h. After the reaction mixture was cooled to room temperature, solvent was removed under reduced pressure. The resulting brown solids were washed with pentane (∼15 mL) to afford an analytically pure brown powder. Yield: 145 mg (80%). Brown crystals suitable for X-ray diffraction were obtained by slow evaporation of the CH2Cl2 solution. 1 H NMR (500 MHz, CD2Cl2): δ 8.01−7.13 (m, 28H, phenyl-H), 4.67 (dd, J = 2.4, 5.5 Hz, 1H, OCH), 3.86 (d, J = 5.9 Hz, 1H, OCH), 1.01 (s, 3H, NCCH3). 13C NMR (126 MHz, CD2Cl2/CD3CN): δ 218.7 (CO), 157.8, 156.6, 137.7, 137.3, 136.8, 135.4, 134.5, 134.2 134.1, 133.9, 131.0, 130.9, 130.4, 130.2, 129.8, 129.2, 129.0, 128.7, 128.1, 127.9, 127.4, 127.3, 126.5, 126.2, 88.54 (OCH), 87.66 (OCH). 31P NMR (202 MHz, CD2Cl2): δ 28.2, 25.5 (AB quartet, JP−P = 289 Hz). Anal. Calcd (found) for C41H33NO3P2Fe: C, 69.8 (69.61); H, 4.71 (4.73); N, 1.99 (2.1) IR (MeCN): νCO 1938 cm−1.

8 (inactive)

+ “HFe[P2O(OSiPh 2H)]Lx” (active)

EXPERIMENTAL SECTION

(4)

Catalyst activation thus entails accumulating the CO ligands in the thermodynamic sink 8. The remaining iron complex, being CO-free, is possibly high spin, which would explain the absence of detectable 31P NMR signals.16 Consistent with this scenario, when 2 was treated with 1 equiv of Ph2SiH2, a hydridoiron(II) species was observed after 10 min. This species was characterized by an AB quartet in its 31P NMR spectrum and a triplet in the high-field 1H NMR spectrum (see the Supporting Information), consistent with its assignment as HFe[P2O(OSiPh2H)](CO)(NCMe).

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CONCLUSIONS The ferrous dialkoxodiphosphine complex 1 displays rich reactivity involving CO substitution, reactions at the alkoxide centers, and the cooperative behavior of these sites. This project established ligand−substrate cooperativity.17 Although 1 was anticipated to activate H2, the strengths of the O−H and Fe−H bonds are apparently insufficient to overcome the H−H bond energy of 104 kcal/mol.18 Owing to a much weaker Si−H bond (∼75 kcal/mol), silanes react with 1 to give hydridoiron(II) complexes. Certainly, the alkoxo F

DOI: 10.1021/om501152h Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics Fe(P2O2)(CO)(PMe3) (3). A solution of 1 (200 mg, 0.25 mmol) in 150 mL of a THF/CH2Cl2 mixture (4/1) was added to PMe3 (3 mL, 29 mmol), and the mixture was stirred for 3 days. Evaporation of the reaction solution yielded an orange solid. The residue was recrystallized from a minimum amount of CH2Cl2 by addition of pentane, giving an analytically pure orange powder. Yield: 149 mg (78%). Red crystals suitable for X-ray diffraction were obtained by vapor diffusion of pentane into a CH2Cl2 solution. 1H NMR (500 MHz, CD2Cl2): δ 8.40 (m, 2H, phenyl-H), 7.73 (m, 2H, phenyl-H), 7.52−6.85 (m, 26H, phenyl-H), 4.54 (m, 1H, OCH), 3.99 (d, J = 5.3 Hz, 1H, OCH), 0.77 (d, 9H, J = 8.8 Hz, PCH3). 13C NMR (126 MHz, CD2Cl2): δ 215.9 (CO), 158, 155.2, 141.8, 140.2, 137.9, 136.3, 134.5, 133.2, 132.7, 132.6, 129.9, 129.6, 129.1, 128.7, 128.4, 128.2, 127.5, 126.5, 126.4, 126.1, 86.4 (OCH), 86.3 (OCH). 31P NMR (202 MHz, CD2Cl2): δ 24.4, 21.3, 19.7 (ABX spin system, JP−P = 278, 44, 44 Hz). Anal. Calcd (found) for C42H39O3P3Fe: C, 68.12 (68.03); H, 5.31 (5.36). IR (CH2Cl2): νCO 1918 cm−1. Fe(P2O2)(CO)(py) (4). A solution of 1 (140 mg, 0.18 mmol) in pyridine (20 mL) was stirred in room temperature for 72 h to afford a greenish brown solution. The solution was evaporated, and the resulting solid was washed with pentane (∼15 mL) to give an analytically pure product as a greenish brown powder. Yield: 121 mg (83%). 1H NMR (500 MHz, CD2Cl2): δ 8.47 (d, J = 5.9 Hz, 1H, pyH), 8.22 (d, J = 5.8 Hz, 1H, py-H), 8.03−7.98 (m, 2H, phenyl-H), 7.79−7.74 (m, 2H, phenyl-H), 7.55−6.70 (m, 24H, py-H, phenyl-H), 6.43−6.39 (m, 1H, py-H), 5.87−5.78 (m, 1H, py-H), 4.83 (dd, J = 5.8, 2.4 Hz, 1H, OCH), 3.93 (d, J = 5.9 Hz, 1H, OCH). 31P NMR (202 MHz, CD2Cl2): δ 29.6, 25.6 (AB quartet, JP−P = 283 Hz). IR (CH2Cl2): νCO 1917 cm−1. Fe(P2O2)(CO)(MeC(O)NH2) (5). A solution of 1 (70 mg, 0.1 mmol) in CH2Cl2 (6 mL) was treated with a solution of acetamide (0.36 g, 6.0 mmol) in THF (6 mL). After this solution was stirred for 36 h, solvent was removed under reduced pressure. The green residue was mixed with toluene (5 mL) and filtered. The filtrate was maintained at −30 °C overnight and filtered, and the filtrate was diluted with pentane (∼15 mL) to precipitate the product as a green solid. Yield: 27 mg (38%). 1H NMR (CD2Cl2): δ 17.87 (s, 1H, NH), 4.99 (s, 1H, OCH), 3.78 (d, J = 3.9 Hz, 1H, OCH), 2.86 (s, 1H, NH), 0.65 (s, 3H, CH3). 31P NMR (202 MHz, CD2Cl2): δ 32.4, 31.7 (AB quartet, JP−P = 261 Hz). IR (CH2Cl2): νCO = 1933 cm−1. Fe(P2O2)(CO)(SC(NH2)2) (6). A solution of 1 (61 mg, 0.08 mmol) and thiourea (5.4 mg, 0.07 mmol) was prepared in CH2Cl2 (3 mL). Over the course of 20 h, the yellow solution turned green and a bright green solid precipitated. The suspension was concentrated to 1.5 mL and filtered to afford a fine green powder, which was washed with CH2Cl2 (3 × 0.5 mL) to give an analytically pure bright green powder. Yield: 53 mg (91%). 1H NMR (500 MHz, THF-d8): δ 12.3 (br, 1H, NH), 7.84 (m, 2H, phenyl-H), 7.70 (m, 2H, phenyl-H), 7.38 (m, 1H, phenyl-H), 7.21−6.76 (m, 23H, phenyl-H), 5.49 (br, 3H, NH), 4.64 (m, 1H, OCH), 3.42 (m, 1H OCH). 13C NMR (126 MHz, THF-d8): 186.5 (SC), 157.5, 157.4, 137.8, 137.4, 135.6, 135.0, 134.7, 134.1, 130.2, 129.9, 129.8, 129.54, 128.9, 128.6, 128.5, 128.3, 127.9, 127.5, 127.4, 126.7, 126.5, 87.4 (OCH), 87.2 (OCH). The absence of the FeCO signal is attributed to poor solubility. 31P NMR (202 MHz, THF-d8): 23.2, 21.1 (AB quartet, JP−P = 287). Anal. Calcd (found) for C40H34FeN2O3P2S·THF: C, 65.03 (64.93); H, 5.21 (5.31), N, 3.45 (3.67). IR (THF): νCO 1928 cm−1. FePCy2O2(CO)2 (7). A solution of Fe(bda)(CO)3 (472 mg, 1.65 mmol) and PCyCHO (998 mg, 3.30 mmol) in THF (30 mL) was stirred in a 55 °C oil bath for 4 h. After the reaction mixture was cooled to room temperature, solvents were removed under reduced pressure. The resulting yellow solid was washed with pentane to afford an analytically pure orange powder. Yield: 782 mg (66%). 1H NMR (500 MHz, CD2Cl2): δ 7.56 (m, 2H, phenyl-H), 7.35−7.24 (m, 4H, phenyl-H), 7.04 (m, 2H, phenyl-H), 3.94 (s, OCH), 2.64−0.66 (m, 44H, Cy-H). 31P NMR (202 MHz, CD2Cl2): δ 25.8 (s). IR (CH2Cl2): νCO 1995, 1930 cm−1. HFe[P2O(OSiHPh2)](CO)2 (8). A solution of Fe(P2O2)(CO)2 (832 mg, 1.07 mmol) and Ph2SiH2 (0.21 mL) in CH2Cl2 (60 mL) was stirred for 8 h. Solvent was removed under reduced pressure, and the

resulting yellow solid was washed with pentane to give an analytically pure product. Yield: 845 mg (90%). Yellow crystals suitable for X-ray diffraction were obtained by vapor diffusion of pentane into a CH2Cl2 solution. 1H NMR (500 MHz, CD2Cl2): δ 7.97−6.73 (m, 38H, phenyl-H), 5.06 (d, JHH = 3.52 Hz, 1H, OCH), 4.81 (dd, JHH = 3.52 Hz, JPH = 7.50 Hz, 1H, OCH), 3.74 (s, 1H, SiH), −2.98 (dd, JPH = 76.3, 44.7 Hz, 1H, FeH). 13C NMR (126 MHz, CD2Cl2): δ 214.9 (CO), 208.8 (CO), 150.4, 147.5, 142.0, 136.9, 136.8, 136.4, 135.6, 135.1, 134.8, 134.6, 134.4, 133.8, 133.5, 132.8, 132.7, 132.2, 131.9, 130.9, 130.8, 130.7, 130.6, 130.5, 130.3, 130.1, 130.0, 129.7, 129.0, 128.9, 128.2, 128.1, 128.0, 128.0, 127.9, 127.8, 127.6, 125.9, 84.9 (OCH), 77.8 (OCH). 31P NMR (202 MHz, CD2Cl2): δ 71.2 (d, JP−P = 194 Hz), 52.7 (d, JP−P = 194 Hz). Anal. Calcd (found) for C52H42FeO4P2Si·0.5CH2Cl2: C, 68.60 (68.58); H, 4.71 (4.69). IR (CH2Cl2): νCO 2025, 1965 cm−1. Reaction of Et3SiH with 1 To Give HFe[P2O(OSiEt3)](CO)2 (9 and 9′). An NMR tube was charged with a solution of Fe(P2O2)(CO)2 (3 mg, 0.4 μmol) in CD2Cl2 (0.5 mL). To the solution was added Et3SiH (1.2 μL, 0.8 μmol), and the reaction was monitored over time by 1H NMR and 31P NMR spectroscopy. After 48 h, the ratio of 9 and the isomer 9′ is 79:21. Data for 9 are as follows. 1H NMR (500 MHz, CD2Cl2): δ 8.01−7.88 (m, 4H, phenyl-H), 7.13−7.03 (m, 1H, phenyl-H), 6.86 (dd, J = 11, 7.9 Hz, 1H, phenyl-H), 5.21 (d, J = 3.4 Hz, 1H, OCH), 4.93 (dd, J = 7.5, 3.2 Hz, 1H, OCH), 0.37 (t, J = 7.9 Hz, 9H, CH3), 0.04 (dq, J = 16, 7.9 Hz, 3H, CH2), −0.26 (dq, J = 16, 7.9 Hz, 3H, CH2), −2.88 (dd, J = 75, 46 Hz, 1H, FeH). 31P NMR (202 MHz, CD2Cl2): δ 72.1 (d, JP−P = 197 Hz), 54.6 (d, JP−P = 197 Hz). Data for 9′ are as follows. 1H NMR (500 MHz, CD2Cl2): 8.15−8.12 (m, 4H, phenyl-H), 7.01−6.92 (m, 1H, phenyl-H), 6.58 (t, J = 8.7 Hz, 1H, phenyl-H) 4.90 (d, J = 3.1 Hz, 1H, OCH), 4.87−4.82 (m, 1H, OCH), 0.73 (t, J = 7.9 Hz, 9H, CH3), 0.278−0.132 (m, 6H, CH2), −4.21 (dd, J = 74, 66 Hz, 1H, FeH). 31P NMR (202 MHz, CD2Cl2): δ 53.7, 53.2 (AB quartet, JP−P = 188 Hz). IR of mixture (CH2Cl2): νCO 2000, 1939 cm−1. HFe[PCy2O(OSiHPh2)](CO)2 (10). A solution of Fe(PCy2O2)(CO)2 (197 mg, 0.27 mmol) in THF was added to Ph2SiH2 (55 μL, 0.27 mmol). The resulting mixture was heated in a 55 °C oil bath for 1 h. After the mixture was cooled to room temperature, solvents were removed under reduced pressure. The resulting yellow-green solid was washed with pentane to afford an analytically pure brown-green powder. Yield: 157 mg (63%). 1H NMR (500 MHz, CD2Cl2): δ 7.78− 6.88 (m, 18H, phenyl-H), 5.10 (d, J = 3.6 Hz, 1H, OCH), 4.93 (s, 1H, SiH), 4.61 (dd, J = 5.7, 3.7 Hz, 1H, OCH), 2.99−2.75 (m, 2H, cyclohexyl-H), 2.58−0.35 (m, 42H, cyclohexyl-H), −3.14 (dd, J = 64, 50 Hz, 1H, FeH). 31P NMR (202 MHz, CD2Cl2): δ 74.0 (d, JP−P = 179 Hz), 50.9 (d, JP−P = 179 Hz). IR (CH2Cl2): νCO 1974, 1919 cm−1. Exchange of 13CO on 1. A J. Young NMR tube was charged with a CH2Cl2 solution of 1 (∼5 μM). The solution was freeze−pump− thawed three times to remove N2 and then opened to 13CO (1 atm). After 20 h at room temperature, the solution was analyzed. IR (CH2Cl2): νCO 2024, 1976, 1965, 1923 cm−1, assigned to singly and doubly labeled product. Kinetic Study. An NMR tube was charged with 1 (10 μmol), hexamethylbenzene (internal standard), and CD2Cl2 (0.5 mL). After addition of Ph2SiH2, the tube was shaken vigorously, and the reaction was monitored over time by 1H NMR spectroscopy at 10 °C. Concentrations were determined by integration of the methine resonances of 1 and 8. Catalytic Hydrosilylation. A vial was charged with the substrate (1.0 mmol), silane (1.1 mmol), and CH2Cl2 (2 mL), followed by the addition of catalyst (10 μmol). The solution was stirred at room temperature for the desired time period. Mesitylene (0.14 mmol, 20 μL) was added as the internal standard. An aliquot was taken and mixed with C6 D6 . The yield was determined by 1H NMR spectroscopy. G

DOI: 10.1021/om501152h Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

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(19) Zuo, Z.; Sun, H.; Wang, L.; Li, X. Dalton Trans. 2014, 43, 11716. Zhao, H.; Sun, H.; Li, X. Organometallics 2014, 33, 3535. (20) Bhattacharya, P.; Krause, J. A.; Guan, H. Organometallics 2011, 30, 4720. Bhattacharya, P.; Krause, J. A.; Guan, H. Organometallics 2014, 33, 6113. (21) Wu, J. Y.; Moreau, B.; Ritter, T. J. Am. Chem. Soc. 2009, 131, 12915. (22) Alcock, N. W.; Richards, C. J.; Thomas, S. E. Organometallics 1991, 10, 231. (23) Fulmer, G. R.; Miller, A. J. M.; Sherden, N. H.; Gottlieb, H. E.; Nudelman, A.; Stoltz, B. M.; Bercaw, J. E.; Goldberg, K. I. Organometallics 2010, 29, 2176.

ASSOCIATED CONTENT

S Supporting Information *

Figures, a table, and CIF files giving 1H NMR and 31P NMR spectra for new compounds and X-ray crystallographic data for 2, 3, [H21](OTf)2, 6 and 8. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for T.B.R.: [email protected].

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Notes

The authors declare no competing financial interest.

NOTE ADDED AFTER ASAP PUBLICATION This paper was published on the Web on February 6, 2015, with an error in Scheme 3. The corrected version was reposted on February 13, 2015.

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ACKNOWLEDGMENTS This research was conducted under Contract DEFG0290ER14146 with the U.S. Department of Energy by its Division of Chemical Sciences, Office of Basic Energy Sciences. We thank Dr. Hao Lei for preliminary studies on protonation of 1 and Danielle Gray and Jeffrey Bertke for assistance with the X-ray crystallography.

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REFERENCES

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DOI: 10.1021/om501152h Organometallics XXXX, XXX, XXX−XXX