Heterobimetallic Complexes Comprised of Nb and Fe: Isolation of a

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Heterobimetallic Complexes Comprised of Nb and Fe: Isolation of a Coordinatively Unsaturated NbIII/Fe0 Bimetallic Complex Featuring a NbFe Triple Bond Gursu Culcu,† Diana A. Iovan,‡ Jeremy P. Krogman,† Matthew J. T. Wilding,‡ Mark W. Bezpalko,† Bruce M. Foxman,† and Christine M. Thomas*,† †

Department of Chemistry, Brandeis University, 415 South Street, Waltham, Massachusetts 02454, United States Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138, United States

‡

S Supporting Information *

ABSTRACT: Heterometallic multiple bonds between niobium and other transition metals have not been reported to date, likely owing to the highly reactive nature of low-valent niobium centers. Herein, a C3-symmetric tris(phosphinoamide) ligand framework is used to construct a Nb/Fe heterobimetallic complex Cl−Nb(iPrNPPh2)3Fe−Br (2), which features a Fe→Nb dative bond with a metal−metal distance of 2.4269(4) Å. Reduction of 2 in the presence of PMe3 affords Nb(iPrNPPh2)3Fe−PMe3 (6), a compound with an unusual trigonal pyramidal geometry at a NbIII center, a NbFe triple bond, and the shortest bond distance (2.1446(8) Å) ever reported between Nb and any other transition metal. Complex 6 is thermally unstable and degrades via P−N bond cleavage to form a NbVNR imide complex, iPrNNb(iPrNPPh2)3Fe−PMe3 (9). The heterobimetallic complexes iPrNNb(iPrNPPh2)3Fe−Br (8) and 9 are independently synthesized, revealing that the strongly π-bonding imido functionality prevents significant metal−metal interactions. The 57Fe Mössbauer spectra of 2, 6, 8, and 9 show a clear trend in isomer shift (δ), with a decrease in δ as metal−metal interactions become stronger and the Fe center is reduced. The electronic structure and metal−metal bonding of 2, 6, 8, and 9 are explored through computational studies, and cyclic voltammetry is used to better understand the effect of metal−metal interaction in early/late heterobimetallic complexes on the redox properties of the two metals involved.

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been subsequently reported.6 Multiple bonds between Nb and other transition metals have not been reported. Even Nb−M single bonds are scarce, and in all cases the Nb−M intermetallic distances are longer than the sum of the single bond atomic radii of the two metals and may be at least somewhat dictated by the constraints of the bridging ligands (Chart 1). One reason for studying early/late heterobimetallic complexes and their metal−metal bonding is their relevance to strong metal−support interactions (SMSIs), which were identified as a primary factor in the efficiency of heterogeneous catalysts featuring late transition metals on early metal oxide supports.10 Although much of the early work on SMSIs focused on titania, SMSIs were also documented when niobia (Nb2O5) was used as a reducible metal support in heterogeneous catalysis.11 In one particular Fe/Nb2O5 system, the SMSI effect was shown to lead to a greater selectivity for higher molecular weight hydrocarbons in the Fischer−Tropsch reaction,12 but the origin of this phenomenon was poorly understood.

INTRODUCTION Since Cotton’s discovery of the first metal−metal quadruple bond in [Re2Cl8]2− over 50 years ago,1 metal−metal multiple bonding has been uncovered between a variety of different transition metals.2 Although this research field may, superficially, seem well-understood, there are still many underexplored areas of metal−metal bonding, including multiple bonds between early and late transition metals in heterobimetallic systems. A seminal contribution to this research area was the Ti≡Rh triple bond in the C3-symmetric complex Ti(μ-OCMe2CH2PPh2)3Rh, reported by Slaughter and Wolczanski.3 The Thomas, Lu, and Tonks groups have recently reported the first examples of multiple bonds between a variety of early/late transition metal combinations including pairs of first row metals (Ti/M, V/M, and Cr/M) and the mixed period combination of Zr and Co.4 However, remarkably few examples of metal−metal multiple bonding have been described for certain transition metals, even in homobimetallic species. For example, the paddlewheel diniobium complex featuring a NbNb triple bond, Nb2(hpp)4 (hpp = 1,3,4,6,7,8-hexahydropyrimido[1,2-a]pyrimidinate), was first described by Cotton in 1997 (Chart 1),5 and only a few instances of Nb−Nb multiple bonds have © 2017 American Chemical Society

Received: April 27, 2017 Published: June 14, 2017 9627

DOI: 10.1021/jacs.7b04151 J. Am. Chem. Soc. 2017, 139, 9627−9636

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Journal of the American Chemical Society

isolation of analogous Nb/Fe complexes that display minimal metal−metal interactions permitted the examination of throughligand communication between the two appended metal centers.

Chart 1. Selected Examples of Previously Reported Complexes Featuring NbNb,5 Nb−Rh,7 and Nb−Fe8 Bondsa

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RESULTS AND DISCUSSION Synthesis. At the outset of this study, a tris(phosphinoamide) niobium precursor was targeted. The NbIV complex Cl− Nb(iPrNPPh2)3 (1) was prepared by the reaction of 3 equiv of Li(iPrNPPh2)(OEt2) with NbCl4(THF)2 in diethyl ether (Scheme 1). The 1H NMR spectrum of gray-green 1 contains Scheme 1

a

Metal−metal distances are reported along with formal shortness ratio (FSR) values, which assist in the comparison of metal−metal distances among metals with different radii (FSR = the ratio of the metal−metal distance to the sum of the Pauling R1 radii9 of the two metals).2

Inspired by the pioneering work of Nagashima and coworkers,13 our group has demonstrated that tris(phosphinoamide) ligand frameworks are remarkably versatile and can accommodate metal−metal distances ranging from 2.02 to 3.18 Å.4c,f,14 The mix of hard and soft donors in phosphinoamide ligand platforms renders them particularly well-suited for linking early/late heterobimetallic combinations.15 The Thomas group’s most well-studied early/late heterobimetallic combination to date is Zr/Co, and using this system, we have uncovered both unusual metal−metal multiple bonding and reactivity that involves both metals acting cooperatively.4c,14,16 For example, the reduced Zr/Co bimetallic complex (THF)−Zr(MesNPiPr2)3Co−N2 was shown to oxidatively add the CO bonds of CO2 or ketone substrates to generate μ-oxo complexes with terminal Co carbonyl or carbene ligands, respectively.16a,c We hypothesized that the tris(phosphinoamide) ligand scaffold would be ideal for the incorporation of Nb centers to target Nb−Fe multiple bonds and for the exploration of the reaction profile of Nb/M bimetallics for comparison to their isoelectronic Zr/Co counterparts. Nb provides a distinct advantage over Zr in that it has a wider range of accessible oxidation states and can therefore bind substrates and participate in redox processes. Our initial attempts to target Nb/M complexes via metalation of the phosphinoamide ligand [iPrNPPh2]− were hampered by inadvertent imide formation (NbVNiPr). The propensity for P−N bond cleavage is, perhaps, not surprising given the many examples of C−N bond cleavage by niobium complexes upon transmetalation with lithium amides.17 Nonetheless, our ability to isolate RN Nb(iPrNPPh2)3 precursors prompted us to explore [tBuN Nb(iPrNPPh2)3Co-L]n complexes and their metal−metal interactions (L = I, n = 0; L = N2, n = −1).18 However, the strong π-electron-donating imido functionality on Nb prevented significant Nb/Co interactions while also blocking a potential substrate binding site at the Nb center. Herein, we report the successful synthesis of an imido-free tris(phosphinoamide) NbIV metalloligand precursor and its successful coordination to Fe to afford metal−metal bonded complexes, including a reduced NbIII/Fe0 complex featuring a NbFe triple bond with the shortest niobium−metal distance reported to date. Although the NbIII/Fe0 complex can be isolated, it does degrade over time via P−N bond cleavage to afford a bimetallic NbV/imide complex without metal−metal bonds. The

four broad paramagnetically shifted peaks, as expected for a symmetric NbIV complex. The solution magnetic moment of 1 is 1.61 μB, slightly lower than the spin-only value of 1.73 μB expected for an S = 1/2 ground state. The X-band EPR spectrum of 1 in frozen toluene is also in agreement with the S = 1/2 ground state assignment (Figure S2). Complex 1 was combined with 1 equiv of FeBr2 in THF at room temperature to produce a dark blue solution of the heterobimetallic Nb/Fe complex Cl−Nb(iPrNPPh2)3Fe−Br (2). The 1H NMR spectrum of 2 exhibited four new broad paramagnetically shifted signals, as expected for a C3-symmetric complex. The room temperature solution magnetic moment of 2 is consistent with an S = 1 ground state (μeff = 3.04 μB). However, the isolated yield of 2 was relatively low (39%), and as formation of 2 involves the reduction of FeII to FeI, efforts were made to improve the yield by adding external reductants (e.g., Zn, KC8) to the reaction mixture; unfortunately, these efforts were unsuccessful. Whereas this seemingly straightforward two-step synthesis of complex 2 was nominally successful, the thermal instability of 1 warranted the development of an alternative synthetic route to 2. Inspired by literature precedent for the transformation of Nb O fragments into triflate or other X-type ligand functionalities, 17d,19 a synthetic pathway starting from a ONb V tris(phosphinoamide) precursor was developed (Scheme 1). The NbV oxo complex ONb(iPrNPPh2)3 (3) was obtained by anion metathesis of Nb(O)Cl3(THF)2 with 3 equiv of Li( i PrNPPh 2 )(OEt 2 ). Treatment of 3 with 1 equiv of trifluoromethanesulfonic anhydride at −35 °C resulted in the instant precipitation of a highly insoluble yellow powder, presumably a bis(triflate) complex of the general form (TfO)2Nb(iPrNPPh2)3. In situ addition of LiCl in a 4:3 CH2Cl2/THF solution converted the yellow suspension into a homogeneous red solution. From the reaction mixture, [Cl− Nb(iPrNPPh2)3][OTf] (5) was isolated as colorless crystals in 45% yield and was characterized by a broad 31P{1H} NMR signal at −35 ppm.20 The 19F NMR spectrum of 5 shows a single 9628


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Journal of the American Chemical Society resonance at −77.7 ppm indicating the presence of an outersphere triflate anion, which was confirmed crystallographically (vide infra). Given the instability of 1, we sought to directly convert 5 into our targeted Nb/Fe complex 2. To this end, a one-pot route to Nb/Fe complex 2 was devised using in situ reduction of complex 5. A frozen THF solution of 5 was treated with 1.1 equiv of KC8 in thawing THF. The brown suspension instantly became a dark homogeneous solution, at which point FeBr2 was added to the reaction mixture to afford 2. With a reliable synthesis for complex 2 in hand, its reduction was explored. Upon addition of a cold solution of 2 and PMe3 in THF to a thawing suspension of KC8 in THF, a color change from blue to red was observed (Scheme 2). The 31P{1H} NMR

Scheme 3

Scheme 2

Structural Characterization. The connectivity of monometallic tris(phosphinoamide) Nb complexes 1, 5, and 7 was confirmed by single crystal X-ray diffraction (see Table 1 and Figures S20, S22, and S24). In the solid-state, NbIV complex 1 exhibits an asymmetric environment around the niobium center with one of the three Nb−P distances significantly elongated as compared to the other two (2.7847(7) Å vs 2.5554(7) Å and 2.5891(8) Å). In contrast, complex 5 is C3-symmetric with three nearly identical Nb−P bond lengths, which is likely the result of the more electrophilic nature of the cationic NbV center. Owing to the strongly π-donating NR-group, the NbV center in complex 7 is less electrophilic and adopts an asymmetric geometry in the solid state with only one phosphine ligand bound to Nb. The solid state structure of Nb/Fe dihalide complex 2 reveals a C3-symmetric geometry with a Nb−Fe distance of 2.4269(4) Å (Figure 1). This distance is 0.4 Å shorter than the shortest known Nb−Fe distance reported (2.8156(6) Å in Cp′2Nb(μ-CO)2Fe(CO)Cp, where Cp′ = η5-C5H4SiMe3, Chart 1).8 To compare metal−metal distances among a series of different bimetallic combinations, Cotton’s “formal shortness ratio” (FSR) is used to normalize interatomic distances by representing the ratio of the metal−metal distance to the sum of the single bond atomic radii (R1, Pauling)9 of the two metal ions.2 The FSR for 2 was calculated to be 0.97, suggesting a Nb/Fe interaction stronger than the V/Fe interaction in F−V(iPrNPPh2)3Fe−I (2.4571(5) Å, FSR = 1.02)21 and stronger than the Zr/Co interaction in Cl− Zr(iPrNPPh2)3Co−I (2.7315(5) Å, FSR = 1.04).22 The geometry about the Nb center is trigonal bipyramidal (∑N−Nb−N = 360.0°), whereas the Fe center adopts a pseudotetrahedral geometry and sits 0.75 Å above the plane of the three phosphorus atoms. The average Fe−P distance in 2 is 2.29 Å, and is comparable to the Fe−P distances in V( i PrNPPh 2 ) 3 Fe−I, F−V( i PrNPPh 2 ) 3 Fe−I, and Cr(iPrNPPh2)3Fe−I (2.29 Å, 2.31 Å, and 2.27 Å, respectively), where the Fe center in each case was assigned as FeI.21,23 The solid-state structure of complex 6 reveals that twoelectron reduction results in a significant contraction of the Nb− Fe distance to 2.1446(8) Å (FSR = 0.85, Figure 1). This distance is the shortest Nb−M distance ever reported and is even shorter than the Nb−Nb distance observed for the triply bonded homobimetallic complex Nb2(hpp)4 2.2035(9) Å (FSR = 0.82).5 Corrected for differences in radii, the metal−metal interaction in complex 6 is comparable to the metal−metal triple bond in

spectrum of the isolated red powder revealed the formation of a diamagnetic product, Nb(iPrNPPh2)3Fe−PMe3 (6), with two resonances at 32.9 ppm and −1.4 ppm in a 3:1 integral ratio. These signals can be attributed to the ligand phosphines in a C3symmetric environment and a terminal PMe3 ligand. Complex 6 decomposes over time when stirred at room temperature in solution via P−N bond cleavage to form imido complex iPrN Nb(iPrNPPh2)3Fe-PMe3, which was confirmed via independent synthesis and comparison of 1H NMR and 57Fe Mössbauer spectra (vide infra). In a previous study, we reported a series of Nb/Co complexes with terminal imido functionalities on the Nb center and attributed the absence of strong metal−metal interactions to the strongly π-donating NbNR functionality.18 However, we were unable to verify this hypothesis by synthesizing Nb/Co complexes in the absence of an imido functionality. Thus, complexes 2 and 6 potentially provide a unique opportunity to systematically explore the effects of the presence/absence of a Nb-bound imido on metal−metal interactions. For this purpose, a synthetic route to imido-bound Nb/Fe complexes was devised (Scheme 3). The addition of 1 equiv of solid iPrNNbCl3(py)2 (py = pyridine) to a diethyl ether solution of Li(iPrNPPh2)(OEt2) at −35 °C produced analytically pure colorless crystals characterized as iPrNNb(iPrNPPh2)3 (7). When a solution of 7 in THF was added to a combination of solid FeBr2 and Zn dust, a red solution was observed over the course of several hours. After 16 h, iPrNNb(iPrNPPh2)3Fe−Br (8) was isolated as a bright yellow powder, and its 1H NMR spectrum showed seven paramagnetically shifted resonances. The solution magnetic moment of 8 is 4.56 μB, indicative of an S = 3/2 ground state. Complex 8 was reduced with Na/Hg amalgam in the presence of PMe3 to generate iPrNNb(iPrNPPh2)3Fe−PMe3 (9). The 1 H NMR spectrum of 9 is paramagnetically shifted and broader than that of 8 with only five discernible resonances. The solution magnetic moment of 9 is 3.07 μB, consistent with one-electron reduction to an S = 1 spin state. 9629


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Journal of the American Chemical Society Table 1. Selected Interatomic Distances and Angles for Complexes 1, 2, 5-9, and Selected Nb/Co and V/Fe Analogues M1−M2 (Å)

FSRd

M1−N (Å)

M2−P (Å)

N−M1−N (°, avg)

P−M2−P (°, avg)

Cl−Nb(iPrNPPh2)3 (1)

−

−

2.086(2), 2.073(2), 2.0258(19)

2.5891(7), 2.5554(7), 2.7847(7)

116.3b

−

[Cl−Nb(iPrNPPh2)3][OTf] (5)

−

−

2.046(3), 2.060(4), 2.045(4)

119.6

−

i

−

−

113.9c

−

Cl−Nb(iPrNPPh2)3Fe−Br (2) Nb(iPrNPPh2)3Fe−PMe3 (6) i PrNNb(iPrNPPh2)3Fe−Br (8)

2.4269(4) 2.1446(8) 3.0851(2)

0.97 0.85 1.22

2.2913(7), 2.2930(8), 2.2965(8) 2.2069(7) 2.3418(4), 2.3187(4), 2.3393(4)

120.0 119.9 115.4

109.8 114.8 103.7

i

3.0010(4) 2.9887(4) 3.0319(4) 2.8492(4) 2.0745(5) 2.0637(5) 2.0495(5) 2.4571(5) 2.4377(5)

1.20 1.19 1.21 1.14 0.87 0.86 0.86 1.02

2.0466(12), 2.0867(12), 2.0718(12) 2.049(2), 2.044(2), 2.054(2) 2.042(2) 2.0529(11), 2.0506(11), 2.0450(11) 2.049(2), 2.054(2), 2.049(2) 2.053(2), 2.059(2), 2.052(2) 2.050(2), 2.052(2), 2.053(2) 2.055(2), 2.057(3), 2.057(3) 1.9254(19), 1.922(2), 1.917(2) 1.929(2), 1.908(2), 1.936(2) 1.9123(12) 1.939(2), 1.937(2), 1.9205(19) 1.9438(19), 1.926(2), 1.941(2)

2.4871(10), 2.4689(11), 2.4957(12)a 2.9683(5), 2.6460(4), 2.9626(4)

2.2362(7), 2.2262(7), 2.2282(7) 2.2300(7), 2.2267(7), 2.2370(7) 2.2458(7), 2.2524(7), 2.2519(8) 2.1231(8), 2.1231(8), 2.1355(8) 2.2708(7), 2.2844(7), 2.3233(7) 2.2839(7), 2.3238(7), 2.2682(7) 2.2172(4) 2.3118(7), 2.3061(7), 2.3222(6) 2.3274(6), 2.3103(7), 2.3141(7)

115.6 115.8 114.8 115.4 119.1 119.1 119.3 120.0 120.0

105.1 105.1 105.4 109.2 112.9 113.3 113.7 108.5 108.9

complex

PrNNb(iPrNPPh2)3(7)

PrNNb(iPrNPPh2)3Fe−PMe3 (9)

BuNNb(iPrNPPh2)3Co−I18 [tBuNNb(iPrNPPh2)3Co−N2]−18 V(iPrNPPh2)3Fe−I21

t

V(iPrNPPh2)3Fe−PMe321 F−V(iPrNPPh2)3Fe−I21

a

a

a The Nb−P distances are reported for monometallic complexes. bIn complex 1, the asymmetric geometry leads to a large deviation in N−Nb−N angles: 93.71(8)°, 122.78(8)°, and 132.52(8)°. cIn complex 7, the asymmetric geometry leads to a large deviation in N−Nb−N angles: 99.95(5)°, 110.71(5)°, and 131.13(5)°. dFSR = ratio of the metal−metal distance to the sum of the single bond atomic radii (R1, Pauling)9 of the two metal ions.2

structural changes at the Nb center makes the assessment of redox changes at Nb more ambiguous from structural data alone. Similar to the previously reported tBuNNb/Co complexes,18 the solid-state structures of niobium(V) imido complexes 8 and 9 reveal the absence of significant Nb/Fe interactions with Nb−Fe distances of 3.0851(2) Å and 2.9886(4) Å, respectively (FSR = 1.22 and 1.19, Figure 2, Table 1). The π-

Figure 1. Displacement ellipsoid (50%) representations of 2 (left) and 6 (right). Hydrogen atoms and solvate molecules have been omitted for clarity.

V( i PrNPPh 2 ) 3 Fe−PMe 3 (FSR = 0.86). 21 Similar to V(iPrNPPh2)3Fe−PMe3, there is an open coordination site at the rigorously trigonal monopyramidal Nb center (∑N−Nb−N = 359.8°). In the case of Nb, this geometry is quite unusual, and, to our knowledge, is the first example of a trigonal monopyramidal Nb center. Even though the Nb−N distances remain essentially unchanged upon reduction, the Fe−P distances contract by nearly 0.1 Å to 2.2069(7) Å in 6. To accommodate the shorter Fe−Nb distance, the pseudotetrahedral geometry at the Fe center flattens and the Fe center is only 0.51 Å above the plane of the three phosphorus atoms in 6. For comparison, the Fe center is 0.57 Å above the P3 plane in V(iPrNPPh2)3Fe−PMe3,21 consistent with the slightly stronger metal−metal interaction in 6 as predicted by the lower FSR. The significant structural changes at the Fe center upon reduction of 2 to 6 are all indicative of some degree of iron-based reduction, although the absence of

Figure 2. Displacement ellipsoid (50%) representations of 8 and 9. Hydrogen atoms and solvate molecules have been omitted for clarity. Only one of the two individual molecules in the asymmetric unit of 9 is shown.

donating terminal imido functionality reduces the Lewis acidity of the NbV center, disfavoring donor/acceptor interactions with the electron-rich low-valent Fe centers in 8 and 9. In contrast to complexes 2 and 6, the geometry about the Nb center in 8 and 9 is closer to tetrahedral, with the Nb center pulled 0.43−0.44 Å out of the plane of the three amide nitrogen atoms. The Fe center in complexes 8 and 9 also adopts a tetrahedral geometry which, in the absence of a Nb−Fe bond, results in more acute P−Fe−P angles (Table 1). The Fe center in bromide complex 8 sits 0.98 Å above the plane formed by the three phosphorus atoms. Upon 9630


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Journal of the American Chemical Society reduction, the geometry about Fe flattens slightly and the Fe moves to a distance 0.89 Å above the plane of the phosphines. This ∼0.09 Å geometric adjustment is coincident with the ∼0.09 Å contraction of the Nb−Fe distance. Reduction of 8 to 9 is also accompanied by a 0.1 Å contraction of the Fe−P distances from 2.33 Å (avg) to 2.23 Å (avg), which is consistent with an Fecentered reduction. 57 Fe Mö ssbauer Spectroscopy. Zero-field 57Fe Mössbauer spectroscopy is a useful tool for probing the electronic structure and the bonding in Fe-containing heterobimetallic complexes. Our group and others have found that strong metal−metal interactions lead to a decrease in isomer shift (δ) and that metal− metal bonding often has a more profound effect on this parameter than metal oxidation-state changes.4a,f,21,23 The electronic structures of 2, 6, 8, and 9 were therefore probed using Mössbauer spectroscopy, and their parameters were compared with closely related complexes (Table 2). The Table 2. Mössbauer Parameters for Complexes 2, 6, 8, 9, and a Selection of Closely Related V/Fe,4b,21 Ti/Fe,4a,f and Cr/Fe23 Complexesa

a

complex

δ (mm/s)

|ΔEQ| (mm/s)

FSR

Cl−Nb(iPrNPPh2)3Fe−Br (2) Nb(iPrNPPh2)3Fe−PMe3 (6) i PrNNb(iPrNPPh2)3Fe−Br (8) i PrNNb(iPrNPPh2)3Fe−PMe3 (9) V(iPrNPPh2)3Fe−I V(iPrNPPh2)3Fe−PMe3 Cr(iPrNPPh2)3Fe−PMe3 Ti(XylNPiPr2)3Fe−Br FeTi(N(o-(NCH2PiPr2)C6H4)3) FeV(N(o-(NCH2PiPr2)C6H4)3) [FeV(N(o-(NCH2PiPr2)C6H4)3)]+

0.37 0.15 0.62 0.41 0.33 0.19 0.25 0.51 0.35 0.25 0.25

0.48 1.37 0.81 1.05 2.01 1.85 0.31 0.17 2.01 5.97 4.04

0.97 0.85 1.23 1.19 0.87 0.86 1.01 0.89 0.83 0.79 0.83

Figure 3. Zero-field 57Fe Mössbauer spectra of complexes 2 (A), 6 (B), 8 (C), and 9 (D) in the solid state at 90 K, with data shown in black and fit shown in red. Complex 6 is unstable and decomposes to imide complex 9 over time. The Mössbauer spectrum shown in B could be fit satisfactorily as two quadrupole doublets, one corresponding to 9 (green, 39% δ = 0.41 mm/s, |ΔEQ| = 0.96 mm/s) and the larger signal assigned to 6 (blue, 61% δ = 0.15 mm/s, |ΔEQ| = 1.37 mm/s).

mm/s, respectively. The same trend holds among M/FeI bimetallic complexes, as the isomer shift consistently decreases with FSR when comparing 8, 2, and V(iPrNPPh2)3Fe−I.21 The Mössbauer parameters of Lu’s VIII/FeI, VIII/Fe0, and TiIII/ Fe0 complexes, [FeV(N(o-(NCH2PiPr2)C6H4)3)]+, FeV(N(o(NCH 2 P i Pr 2 )C 6 H 4 ) 3 ), 4b and FeTi(N(o-(NCH 2 P i Pr 2 )C6H4)3),4a are also included in Table 2 for comparison. Although these compounds have stronger metal−metal bonds as indicated by smaller FSRs, their isomer shifts are greater than that of NbIII/ Fe0 complex 6 as a result of the more electron-rich PiPr2 ligand substituents as compared to the PPh2 donors in 6. For the same reason, the TiIII/Fe complex Ti(XylNPiPr2)3Fe−Br also has a higher isomer shift than might be predicted on the basis of the trends in FSR alone.4f Even though the trends in isomer shift were apparent, no obvious trends in |ΔEQ| as a function of geometry, d-electron count, or electron distribution (vide infra) emerged from the Mössbauer data collected for 2, 6, 8, and 9 and compiled in Table 2. However, it is worth noting that all of the heterobimetallic M/ Fe complexes supported by the tris(phosphinoamide) ligand framework have |ΔEQ| < 2 mm/s, which is in stark contrast to Lu’s early/late heterobimetallic complexes supported by the heptadentate “double-decker” ligands with similar amide and phosphine donors, which typically have very large |ΔEQ| parameters (4−6 mm/s).4a,24 This difference can be attributed to the different coordination geometry at the Fe centers, as the additional methylene linker in the heptadentate ligand allows the Fe center to adopt a trigonal pyramidal geometry without an axial ligand. Redox Behavior. The redox behavior of both the monometallic Nb and bimetallic Nb/Fe complexes were explored by cyclic voltammetry to evaluate the effect of metal− metal interactions on the redox potentials of each metal. The cyclic voltammogram (CV) of NbIV complex 1 contains one reversible oxidation and two irreversible reductive features at

FSR values are provided for comparison.

Mössbauer spectra of 2, 8, and 9 feature clean quadrupole doublets centered at δ = 0.37 mm/s, 0.62 mm/s, and 0.41, respectively (Figure 3, Table 2). The reduced complex 6 is unstable and decomposes to imide complex 9 over time, and repeated attempts to obtain a clean Mössbauer spectrum of 6 resulted in spectra with varying degrees of contamination by 9. Nonetheless, the Mössbauer spectrum obtained for 6 could be fit satisfactorily as a two-component mixture using the Mössbauer parameters of 9 for the minor component (39%), with the majority of the spectrum (61%) attributed to a quadrupole doublet centered at δ = 0.15 mm/s (|ΔEQ| = 1.37 mm/s). Regardless of the metal−metal interactions, reduction of the Fe center from FeI to Fe0 results in a decrease in isomer shift, which is in agreement with previous observations for the analogous V/Fe complexes.21 Complexes 2 and 6, which have stronger metal−metal interactions and lower FSRs, have lower isomer shifts than the NbV imido complexes 8 and 9 in which metal−metal bonding is negligible. This phenomenon is in line with previously reported trends and is attributed to increased selectron density in the presence of metal−metal interactions, as the Fe d-electrons involved in metal−metal bonding have diminished shielding capabilities.4f,21 The direct correlation between isomer shift and metal−metal distance is evident when a series of M/Fe0 bimetallic complexes with varying FSR is compared: As FSR decreases in the order 9 > Cr(iPrNPPh2)3Fe−PMe323 > V(iPrNPPh2)3Fe−PMe321 > 6, δ decreases from 0.41 mm/s to 0.25 mm/s to 0.19 mm/s to 0.15 9631


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Journal of the American Chemical Society −2.55 V and −3.15 V, which are assigned to reductions to NbIII and NbII, respectively (Figure 4, Table 3). The CV of cationic

complex 1 complicates this comparison. A 0.3 V anodic shift in the VIV/III redox potential was also observed upon addition of Fe to V(iPrNPPh2)3.21 The FeI/0 reduction potential of 2 (−1.71 V) is shifted anodically compared to the FeI/0 potentials reported for both V(iPrNPPh2)3Fe−I and Cr(iPrNPPh2)3Fe−I (−2.0 V and −2.03 V).21,23 Although the metal−metal interaction in V(iPrNPPh2)3Fe−I is stronger than that of 2 based on metal− metal distance, the NbIV center in 2 is anticipated to be a stronger Lewis acid than either VIII or CrIII, serving to diminish the electron density at the Fe center in the Nb/Fe case and promote reduction to Fe0. A similar phenomenon was observed when comparing the reduction potentials of Cl−Zr(iPrNPPh2)3Co−I to its monometallic analogue (iPrNHPPh2)3Co−I.22 The NbV imido complex 8 provides the unique opportunity to evaluate the contribution of through-ligand metal−metal communication, as there is no direct metal−metal interaction in this complex. The CV of complex 8 features two reversible redox waves assigned as reduction from FeI to Fe0 at E1/2 = −1.84 V and oxidation from FeI to FeII at E1/2 = −0.52 V. Both redox processes can be assigned as Fe-based given the absence of any metal-based redox events in the CV of monometallic NbV imide precursor 7 (see Figure S18). A comparison of FeI complexes 2 and 8 reveals that the metal−metal interaction in 2 leads to a 130 mV anodic shift in the FeI/0 reduction potential; however, this shift is not as large as might be expected as compared to a monometallic tris(phosphine)FeI complex, and the FeI/0 couple of 8 is still more positive than that of V(iPrNPPh2)3Fe−I or Cr(iPrNPPh2)3Fe−I, suggesting that through-ligand interactions contribute substantially to the shifted redox potentials of heterobimetallic tris(phosphinoamide) complexes. An FeII/I couple is not observed for any of the bimetallic M/Fe complexes that have metal−metal bonds (M = Nb, Cr, V),21,23 so the accessibility of the FeII/I oxidation of complex 8 can be attributed to the absence of direct metal−metal interactions. Compared to the oxidation potential of a monometallic tris(phosphine) complex, PhB(CH2PPh2)3FeCl (reversible FeII/I wave at −0.82 V vs Fc),27,28 the FeII/I couple of heterobimetallic complex 8 is shifted positively by 300 mV as a result of through-ligand interactions with the pendent Lewis acidic NbV center, although variation arising from the inherent differences in charge between the complexes in question cannot be excluded. Computational Analysis. The electronic structure and bonding in heterobimetallic complexes 2, 6, 8, and 9 was also investigated computationally using density functional theory (DFT). The computed structural parameters, including the metal−metal distances, are in excellent agreement with those determined by X-ray crystallography, suggesting an appropriate choice of functional and basis set (Table S3). The qualitative frontier molecular orbital diagrams of 2 and 6 are shown in Figure 5 along with pictorial representations of selected Kohn− Sham orbitals relevant to the metal−metal bonding in these molecules (see Figures S27 and S28 for a complete collection of molecular orbital pictures). Examination of the orbitals calculated for 2 reveals overlap between the dz2 orbitals of Nb and Fe which is in line with the existence of a σ bond between the two metal centers (Figure 5). However, most of the σ bonding orbital’s electron density is located on the dz2 orbital of Fe, suggesting a dative Fe→Nb interaction, similar to that reported for the isoelectronic complexes Cl−Zr(MesNPiPr2)3Co−I and F−V(iPrNPPh2)3Fe−I.21,22 Although the Fe dxz and dyz orbitals are occupied and are of the right symmetry, no significant π-overlap is observed between Fe and Nb. Nonetheless, both the calculated

Figure 4. Cyclic voltammograms of 1, 2, and 8 in 0.4 M [nBu4N][PF6] in THF (scan rate = 100 mV/s). All of the potentials are referenced to the ferrocene (Fc)/ferrocenium (Fc+) redox couple. Arrows are used to indicate the scan direction and the initial potential, which was chosen as the most positive potential rather than the open-circuit potential in all cases.

Table 3. Redox Potentials (V vs Fc/Fc+) Measured for Nb Complexes 1 and 5 and Nb/Fe Complexes 2 and 8c complex

MV/IV

MIV/III

FeI/0

Cl−Nb(iPrNPPh2)3 (1) [Cl−Nb(iPrNPPh2)3][OTf] (5)a Cl−Nb(iPrNPPh2)3Fe−Br (2) i PrNNb(iPrNPPh2)3Fe−Br (8) V(iPrNPPh2)3Fe−I Cr(iPrNPPh2)3Fe−I

−1.65 −1.79b − − − −

−2.55 − −2.17b − −0.83 −0.58

− − −1.71b −1.84 −2.0b −2.03b

b

a

CV was recorded in CH2Cl2, as dictated by solubility. bThis redox process was irreversible, so the potential reported is Epc. cAnalogous V/Fe21 and Cr/I23 complexes provided for comparison (in 0.4 M [nBu4N][PF6] in THF at a scan rate of 100 mV/s, unless otherwise noted). All reported potentials are E1/2 values unless otherwise noted.

NbV complex 5 reveals an irreversible reduction at Epc = −1.79 V (Figure S11), supporting the assignment of the reversible oxidative wave at E1/2 = −1.65 V in the CV of 1 as the NbV/IV redox couple.25 Redox events for the Nb/Fe bimetallic species 2 and 8 were assigned on the basis of comparisons with the monometallic Nb precursors and with other reported M/Fe-halide heterobimetallic complexes such as V( i PrNPPh 2 ) 3 Fe−I and Cr(iPrNPPh2)3Fe−I.21,23 The CV of 2 exhibits two quasi-reversible reduction waves at −1.71 V and −2.17 V attributed to the reduction of FeI to Fe0 and the reduction of NbIV to NbIII, respectively (Figure 4, Table 3). The irreversibility of these two waves can likely be attributed to halide dissociation under reductive conditions, particularly as the isolated product of twoelectron reduction, 6, is halide-free (vide supra). A third reduction is observed at −2.37 V, and the assignment of this redox event to either Nb or Fe remains ambiguous at this stage.26 Nonetheless, it can be observed that the NbIV/III potential of bimetallic complex 2 is 0.38 V more positive than in monometallic complex 1, suggesting that the proximity of Fe to the Nb center facilitates further reduction; however, the coordination of two phosphine ligands to Nb in monometallic 9632



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CONCLUSIONS A Nb/Fe complex with a very short metal−metal distance and a triple bond between the two metals has been synthesized and fully characterized. Although second and third row transition metals are more suitable for multiple bonding than their first row congeners, multiple bonds involving Nb have proven difficult to stabilize owing to the high reactivity of niobium in its lower oxidation states.2,5 The NbFe triple bond in complex 6 represents the first example of multiple bonding between Nb and another transition metal. Prior to this study, even heterobimetallic Nb−M single bonds with FSRs < 1 had not been structurally characterized. Furthermore, the unique geometry of 6 suggests that Nb−Fe multiple bonding serves to stabilize an unusual coordinatively unsaturated low-valent Nb center. The heterobimetallic Nb/Fe complexes 2 and 6 join a growing family of structurally similar Ti/M, V/M, Cr/M, and Zr/M complexes.4c,e,f,21,23,29 The isoelectronic nature of complex 6 with its highly reactive Zr/Co analogues suggests that its reactivity toward small molecule and bond activation is a ripe area for future study. Furthermore, given the versatility of the tris(phosphinoamide) platform, expansion of the heterobimetallic Nb/M series to other first row transition metals is currently underway.

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EXPERIMENTAL SECTION

General Considerations. All air- and moisture-sensitive manipulations were performed using standard Schlenk techniques or in an MBraun inert atmosphere drybox with an atmosphere of purified nitrogen. The MBraun drybox was equipped with a cold well designed for freezing samples in liquid nitrogen as well as a −35 °C freezer for cooling samples and crystallizations. Solvents for air- and moisturesensitive manipulations were dried and deoxygenated with ultrahigh purity argon using a Glass Contours solvent system following literature procedures.30 All solvents were stored over 3-Å molecular sieves. Deuterated benzene and dichloromethane were purchased from Cambridge Isotope Laboratories, Inc., degassed via repeated freeze− pump−thaw cycles, and dried over 3-Å molecular sieves. iPrNHPPh2,31 NbCl4(THF)2,32 and (O)NbCl3(THF)233 were prepared by literature methods. All other chemicals were purchased from Aldrich, Strem, or Alfa Aesar and used without further purification. Li(iPrNPPh2)(OEt2). A modified literature procedure was followed.34 A solution of iPrNHPPh2 (2.06 g, 8.47 mmol) in 10 mL Et2O was frozen. A solution of nBuLi (5.8 mL, 1.6 M in hexanes) was slowly added via a 10 mL syringe over 3 min. The mixture was warmed to room temperature and stirred for 1 h. The solvent was evaporated in vacuo, and the remaining white solids were washed with pentane (5 mL). The solids were dried in vacuo to obtain Li(iPrNPPh2)(OEt2) as a white crystalline powder (1.58 g, 58% yield). The spectroscopic features of the product were identical to those reported for the compound in the literature.34 i PrNNbCl3(py)2. A modified literature procedure was followed.35 A chilled solution of iPrNH2 (0.950 mL, 11.7 mmol) in 2 mL CH2Cl2 was added to NbCl5 (1.05 g, 3.88 mmol) in 10 mL CH2Cl2 at −35 °C. The yellow solution was stirred for 2.5 h at room temperature. The solution was then cooled to −35 °C, and pyridine (0.626 mL, 7.76 mmol) was added. After being stirred for 14 h at room temperature, the solution was filtered through Celite on a medium porosity frit, and the solvent was evaporated from the filtrate in vacuo. The remaining solids were washed with 10 mL Et2O and 10 mL pentane, and the product was dried under vacuum to give a yellow powder (350 mg, 22%). The spectroscopic features of the product were identical to those reported for the compound in the literature.36 Cl−Nb(iPrNPPh2)3 (1). A 20 mL scintillation vial and a pipette plugged with a piece of glass microfiber filter paper along with a 1-in. plug of Celite were cooled and kept at −35 °C. A solution of Li(iPrNPPh2)(OEt2) (128 mg, 0.39 mmol) in Et2O (3 mL) was frozen. Solid NbCl4(THF)2 (50 mg, 0.13 mmol) was added to this thawing solution in one portion. The solution was stirred for 3 min until the color

Figure 5. Computed frontier molecular orbital diagrams for 2 and 6, along with graphical representations of the molecular orbitals relevant to metal−metal bonding in 6 (top) and 2 (left).

Mayer bond order (1.64) and Wiberg bond index (1.10) suggest a small degree of multiple bonding between the metal atoms in 2 (Table S8). Upon reduction to 6, the σ bond between Nb and Fe becomes more covalent in nature and two additional π bonds are formed, leading to an overall formal bond order of three. The Nb−Fe σ bond in 6 has substantially more electron density on Nb than the σ bond in 2, leading to a stronger and more covalent bond between the two metals. In addition, the orbitals shown in Figure 5 reveal good π-overlap between the dxz and dyz orbitals of the two metals. The calculated Mayer bond order (2.69) is relatively consistent with the assignment of a NbFe triple bond, and natural bond orbital (NBO) analysis also finds three Fe−Nb NBOs corresponding to one σ and two π bonds (Figure S32). For comparison, the frontier molecular orbitals of iPrNNbV imide complexes 8 and 9 were also examined but revealed no orbitals with significant metal−metal overlap (Figure S30 and S31). The Mayer bond orders and Wiberg bond indices of these molecules also indicate negligible metal−metal interactions (Table S8). As previously surmised for the tBuNNb(iPrNPPh2)3Co−I and [tBuNNb(iPrNPPh2)3Co−N2]− complexes,18 strong π-donation from the imide to the NbV center alleviates the Lewis acidity of the Nb center and prevents significant Nb−Fe interactions. 9633


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through Celite on a medium porosity filter frit after 16 h. The filtrate was evaporated to dryness, and the remaining powder was extracted into benzene (100 mL) and filtered through Celite on a medium porosity filter frit. Compound 2 (271 mg, 30%) was isolated as described above. Nb(iPrNPPh2)3Fe−PMe3 (6). A suspension of KC8 (72 mg, 0.53 mmol) in THF (2 mL) was frozen. To a chilled solution of 2 (211 mg, 0.21 mmol), PMe3 (32 μL, 0.30 mmol) was added and the resulting blue solution was added to the frozen KC8. Upon thawing, the solution instantly became red and was stirred vigorously for 2 h at room temperature. The red solution was then filtered through Celite, and the solvent was evaporated from the filtrate in vacuo. The remaining red powder was extracted into Et2O (30 mL) and filtered through Celite. The solvent was evaporated from the filtrate in vacuo to give 6 (140 mg, 69%). X-ray quality crystals were grown via slow diffusion of pentane into a concentrated Et2O solution of 6 at room temperature. 1H NMR (400 MHz, C6D6): δ 7.52−7.58 (m, 12H, m-Ph), 6.96−7.02 (m, 12H, oPh), 7.04−7.16 (m, overlapping with benzene peaks, 6H, p-Ph), 4.18 (m, 3H, (CH3)2CH), 1.08 (d, 18H, JH−H = 4.8 Hz, (CH3)2CH), 0.58 (d, 9H, JH−H = 4.8 Hz, PMe3). 13C{1H} NMR (100.6 MHz, C6D6): δ 22.2 (m, PMe3), 30.0 (s, (CH3)2CH), 48.2 (d, JC−P = 9.2 Hz, (CH3)2CH), pPh signal overlaps with C6D6 resonance, 133.2 (d, JC−P = 14.1 Hz, mPh), 134.4 (dd, JC−P = 12.9 Hz, o-Ph), 141.8 (d, JC−P = 17.5 Hz, ipso-Ph). 31 1 P{ H} NMR (161.8 MHz, C6D6): δ 32.9 (s), −1.4 (s). UV−vis (THF, λmax, nm (ε, L mol−1 cm−1)): 425 (8600). Elemental analysis data could not be obtained owing to the thermal instability and the air/moisture sensitivity of compound 6. i PrNNb(iPrNPPh2)3 (7). A solution of Li(iPrNPPh2)(OEt2) (468 mg, 1.44 mmol) in Et2O (10 mL) was cooled to −35 °C, and solid i PrNNbCl3(py)2 (200 mg, 0.49 mmol) was added in one portion. The solution was then allowed to warm to room temperature and stirred for 2 h. The resulting solution was filtered through Celite, concentrated to half its volume, and left at −35 °C for 12 h to obtain analytically pure colorless crystals of 7 (319 mg, 74%). X-ray quality crystals were grown from a concentrated solution of 7 in Et2O at −35 °C. 1H NMR (400 MHz, CD2Cl2): δ 7.34−7.42 (br m, 12H, o-Ph), 7.18−7.23 (m, 6H, pPh), 7.10−7−15 (m, 12H, m-Ph), 4.50 (m, 1H, ((CH3)2CH)NNb), 3.80 (m, 3H, (CH3)2CH), 1.38 (d, JH−H = 6.5 Hz, 6H, ((CH3)2CH)N Nb), 1.20 (d, JH−H = 6.5 Hz, 18H, (CH3)2CH). 13C{1H} NMR (100.6 MHz, CD2Cl2): δ 26.2 (s, ((CH3)2CH)NNb), 27.4 (s, (CH3)2CH), 54.4 (s, (CH3)2CH), (CH3)2CH)NNb resonance overlaps with CD2Cl2 resonance, 127.3 (m, m-Ph), 128.0 (s, p-Ph), 133.6 (m, o-Ph), 138.8 (m, ipso-Ph). 31P{1H} NMR (161.8 MHz, CD2Cl2): δ 13.0 (s). Anal. Calcd for C48H58N4NbP3: C, 65.75; H, 6.67; N, 6.39. Found: C, 65.49; H, 6.61; N, 6.35. i PrNNb(iPrNPPh2)3Fe-Br (8). Zinc dust (4 mg, 0.06 mmol) and FeBr2 (27 mg, 0.12 mmol) were combined as solids in a 20 mL vial equipped with a stir bar. A solution of 7 (110 mg, 0.12 mmol) in THF (5 mL) was added to the solids, and the solution was stirred for 16 h at room temperature. The red solution was filtered through Celite, and the solvent was evaporated from the filtrate in vacuo. The remaining solids were extracted into CH2Cl2 (10 mL) and filtered through Celite. The solvent was evaporated from the filtrate to give a bright yellow powder which was washed with Et2O (2 mL) and pentane (2 mL) and dried under vacuum. Compound 8 was obtained as a yellow crystalline powder (100 mg, 68%). Crystals suitable for X-ray diffraction were grown via slow diffusion of pentane into a THF solution of 8 at room temperature. 1H NMR (400 MHz, C6D6): δ 24.8 (s), 12.7 (s), 9.3 (s), 5.5 (s), 4.6 (s), −10.0 (s), −21.2 (s). UV−vis (THF, λmax, nm (ε, L mol−1 cm−1)): 311 (sh), 362 (5220). μeff (C6D6): 4.56 μB. Multiple attempts to obtain satisfactory elemental analysis were unsuccessful. i PrNNb(iPrNPPh2)Fe-PMe3 (9). To a chilled solution of 8 (50 mg, 0.049 mmol) and PMe3 (7.5 μL, 0.074 mmol) in THF (2 mL), a thawing THF (2 mL) solution of KC8 (10 mg, 0.074 mmol) was added. The resulting solution was allowed to stir and warm slowly to room temperature. After 1.5 h, the red solution was filtered through Celite, and the solvent was evaporated from the filtrate in vacuo. The remaining red powder was extracted into benzene (10 mL) and filtered through Celite. The solvent was evaporated from the filtrate in vacuo and the remaining red powder was washed with cold pentane (2 mL) to give 9 as brick red powder (20 mg, 40%). Single crystals suitable for X-ray

changed from bright yellow to dark green. The solution was then quickly filtered through Celite using the prechilled glassware and filtration setup. The resulting green solution was frozen in the glovebox cold well, and the solvent was evaporated in vacuo. The product was washed with thawing pentane (8 mL), and the product was dried in vacuo (in the cold well) to obtain 1 as a gray/green powder (104 mg, 94% yield). Compound 1 is thermally unstable. It rapidly decomposes in solution at and above room temperature. Single crystals suitable for X-ray crystallography were grown from a concentrated Et2O solution at −35 °C. 1H NMR (400 MHz, C6D6): δ 8.8 (br), 8.0 (br s), 6.6 (br), 3.5 (br). μef f (C6D6): 1.61 μB. UV−vis (THF, λmax, nm (ε, L mol−1 cm−1)): 505 (480). Anal. Calcd for C45H51N3P3ClNb: C, 63.20; H, 6.01; N, 4.91. Found: C, 62.93; H, 6.05; N, 4.97. Cl−Nb(iPrNPPh2)3Fe−Br (2). To a freshly prepared solution of 1 (300 mg, 0.35 mmol) in THF (6 mL), solid FeBr2 (76 mg, 0.35 mmol) was added in one portion at room temperature. The solution was stirred for 16 h and then filtered through Celite. The solvent was evaporated from the blue filtrate in vacuo. The resulting powder was extracted into 10 mL benzene, filtered through Celite, and the filtrate was dried in vacuo. The powder was extracted with benzene (10 mL) a second time, and the filtrate was again dried in vacuo. The resulting product was washed with pentane (4 mL) to obtain 2 as an analytically pure blue powder (135 mg, 39%). Crystals suitable for X-ray crystallography were grown by layering a THF solution of 2 with pentane at −35 °C. 1H NMR (400 MHz, C6D6): δ 13.5 (br), 11.5 (br), −1.7 (br), −4.5 (br). μeff (C6D6): 3.04 μB. UV−vis (THF, λmax, nm (ε, L mol−1 cm−1)): 434 (sh), 589 (1550), 693 (1040). Anal. Calcd for C45H51N3P3ClFeBrNb: C, 54.54; H, 5.19; N, 4.24. Found: C, 54.66; H, 5.29; N, 4.00. (O)Nb(iPrNPPh2)3 (3). Solid Nb(O)Cl3(THF)2 (100 mg, 0.28 mmol) was added to a solution of Li(iPrNPPh2)(OEt2) (270 mg, 0.85 mmol) in Et2O (6 mL) at −35 °C, and the solution was stirred for 1 h at room temperature. The brown suspension was filtered through Celite, and the filtrate was evaporated to dryness in vacuo. The product was washed with 4 mL pentane to obtain 3 as a light-brown powder (132 mg, 57% yield). 1H NMR (400 MHz, CD2Cl2): δ 7.36−7.42 (br m, 12H, oPh), 7.24−7.28 (m, 6H, p-Ph), 7.12−7.20 (m, 12H, m-Ph], 3.80 (m, 3H, (CH3)2CH), 1.21 (d, JH−H = 6.2 Hz, 18H, (CH3)2CH). 13C{1H} NMR (100.6 MHz, CD2Cl2): δ 26.5 (s, (CH3)2CH), 54.2 (s, (CH3)2CH), 127.7 (m, m-Ph), 128.5 (s, p-Ph), 133.2 (m, o-Ph), 137.9 (m, ipso-Ph). 31 1 P{ H} NMR (161.8 MHz, CD2Cl2): δ 14.5 (s). Anal. Calcd for C45H51N3NbOP3: C, 64.67; H, 6.15; N, 5.03. Found: C, 64.01; H, 5.99; N, 4.87. [Cl−Nb(iPrNPPh2)3][OTf] (5). To a stirring solution of 3 (170 mg, 0.20 mmol) in Et2O (4 mL), Tf2O (34 μL, 0.20 mmol) was added at −35 °C. The resulting yellow suspension was stirred for 30 min at room temperature. A solution of LiCl (17 mg, 0.4 mmol) in a CH2Cl2/THF mixture (4:3, 10 mL) was added to the yellow suspension at room temperature. The solution was stirred for 12 more hours, and the volatiles were removed in vacuo to give brown/orange solids. The solids were dissolved in CH2Cl2 (6 mL) and filtered through Celite to remove the insoluble LiOTf and LiCl salts. The filtrate was evaporated to dryness, and the remaining brown powder was washed with pentane (2 mL) and Et2O (2 mL) to obtain 5 as a light yellow crystalline powder (91 mg, 45%). Crystals of 5 suitable for X-ray diffraction were grown via slow diffusion of pentane into a concentrated CH2Cl2 solution at room temperature. 1H NMR (400 MHz, CD2Cl2): δ 7.21−7.25 (m, 6H, pPh), 7.12−7.18 (m, 12H, m-Ph), 6.80−7.08 (br m, 12H, o-Ph), 4.88 (m, 3H, (CH3)2CH), 1.37 (d, 18H, JH−H = 6.6 Hz, (CH3)2CH)). 13C{1H} NMR (100.6 MHz, CD2Cl2): δ 26.2 (s, (CH3)2CH), 56.8 (s, (CH3)2CH), 127.8 (m, ipso-Ph), 128.8 (m, o- or m-Ph), 131.8 (s, pPh), 132.3 (m, o- or m-Ph). 31P{1H} NMR (161.8 MHz, CD2Cl2): δ −35 (br). 19F NMR (376 MHz, CD2Cl2): δ −77.7. Multiple attempts to obtain satisfactory elemental analysis were unsuccessful owing to the difficulty in separating excess LiCl and LiOTf from the product. Alternative Synthesis of Cl−Nb(iPrNPPh2)3Fe−Br (2). To a frozen solution of 5 (900 mg, 0.90 mmol) in THF (70 mL), a thawing solution of KC8 (134 mg, 0.99 mmol) in THF (20 mL) was added. Upon the color change from the brown suspension to the homogeneous dark solution, solid FeBr2 (194 mg, 0.90 mmol) was added in one portion and left at room temperature. The blue solution was filtered 9634


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Journal of the American Chemical Society diffraction were grown via slow diffusion of pentane into a concentrated Et2O solution of 9 at room temperature. 1H NMR (400 MHz, C6D6): δ 20.0 (br), 14.8 (br), 0.8 (br), −1.0 (br), −4.2 (br), −15 (very br). UV− vis (THF, λmax, nm (ε, L mol−1 cm−1)): 327 (7700), 415 (6480). μef f (C6D6): 3.07 μB. Anal. Calcd for C51H67FeN4NbP4: C, 60.72; H, 6.69; N, 5.55. Found: C, 60.14; H, 6.38; N, 4.92. Spectroscopic Characterization and Physical Measurements. All NMR spectra were recorded at ambient temperature unless otherwise stated on a Varian Inova or MR 400 MHz instrument. 1H NMR chemical shifts were referenced to residual solvent and are reported in ppm. 31P{1H} NMR chemical shifts (in ppm) were referenced to 85% H3PO4. 19F NMR chemical shifts (in ppm) were referenced to trifluoroacetic acid (−78.5 ppm). UV−vis spectra were recorded on a Cary 50 UV−vis spectrophotometer using Cary WinUV software. Elemental microanalyses were performed by Complete Analysis Laboratories, Inc., Parsippany, NJ and Robertson Microlit Ledgewood, NJ. Solution magnetic moments (μeff) were measured using the Evans method and are reported without taking into account any diamagnetic contributions (Pascal’s constants were not used).37 Cyclic voltammetry measurements were carried out in a glovebox under a dinitrogen atmosphere in a one-compartment cell using a CH Instruments electrochemical analyzer. A glassy carbon electrode and platinum wire were used as the working and auxiliary electrodes, respectively. The reference electrode was Ag/AgNO3 in THF or CH2Cl2. Solutions (THF or CH2Cl2) of electrolyte (0.40 M [nBu4N][PF6]) and analyte were also prepared in the glovebox. All potentials are reported versus the ferrocene/ferrocenium couple by comparison to an internal ferrocene reference added following data collection. 57 Fe Mössbauer spectra were measured on liquid nitrogen-cooled samples at 90 K at zero magnetic field with a constant acceleration spectrometer (SEE Co., Edina, MN). Solid or crystalline samples were prepared as Paratone-N mulls in a drybox and frozen in liquid nitrogen prior to handling in air. Isomer shifts are quoted relative to Fe foil at room temperature. Data was processed, simulated, and analyzed using an in-house package written by Evan R. King for Igor Pro 6 (Wavemetrics, Lake Oswego, OR). The X-band EPR spectrum of 1 was obtained on a Bruker ElexSys E500 EPR spectrometer fitted with a cryostat. The EPR sample was a crystalline sample dissolved in toluene, and the spectrum was measured at 9 K. Computational Details. All calculations were performed using Gaussian09, Revision A.02 for the Linux operating system.38 Density functional theory calculations were carried out using a combination of Becke’s 1988 gradient-corrected exchange functional39 and Perdew’s 1986 electron correlation functional40 (BP86). A mixed-basis set was employed, using the LANL2TZ(f) triple-ζ basis set with effective core potentials for niobium and iron,41 the LANL2DZ(p,d) double-ζ basis set and effective core potentials for bromine,42 Gaussian09’s internal 6311+G(d) for nitrogen, phosphorus, and chlorine and Gaussian09’s internal LANL2DZ basis set (equivalent to D95 V43) for carbon and hydrogen. Using crystallographically determined geometries as a starting point, the geometries were optimized to a minimum, followed by analytical frequency calculations to confirm that no imaginary frequencies were present. XYZ coordinates of optimized geometries are provided in the Supporting Information, pp S32−S44. Single-point NBO44 and Mayer bond order45 calculations were subsequently performed on the optimized geometries of 2, 6, 8, and 9.

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of DFT-optimized geometries computed for complexes 2, 6, 8, and 9 (PDF) Crystallographic data for 1, 2, and 5−9 (CIF)

AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Christine M. Thomas: 0000-0001-5009-0479 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division, under Award No. DE-SC0014151. The authors would like to thank Dr. Bing Wu for insightful discussions and experimental assistance and Prof. Theodore Betley (Harvard University) for providing access to Mössbauer and EPR facilities. The authors are also grateful for access to the Brandeis University high-performance computing cluster.

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REFERENCES

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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/jacs.7b04151. Spectroscopic data for complexes 1−9, CV of complex 5, EPR of complex 1, solid-state structures of complexes 1, 5, and 7, crystallographic data collection and refinement details for 1, 2, and 5−9, additional computational details, frontier molecular orbital diagrams and XYZ coordinates 9635


Article

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Heterobimetallic Complexes Comprised of Nb and Fe: Isolation of a

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