Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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Assessing the Metal−Metal Interactions in a Series of Heterobimetallic Nb/M Complexes (M = Fe, Co, Ni, Cu) and Their Effect on Multielectron Redox Properties Brett A. Barden,‡ Gursu Culcu,† Jeremy P. Krogman,†,§ Mark W. Bezpalko,† Gregory P. Hatzis,‡ Diane A. Dickie,†,∥ Bruce M. Foxman,† and Christine M. Thomas*,†,‡ †
Department of Chemistry, Brandeis University, 415 South Street, Waltham, Massachusetts 02454, United States Department of Chemistry and Biochemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio 43210, United States
Inorg. Chem. Downloaded from pubs.acs.org by TULANE UNIV on 12/20/18. For personal use only.
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ABSTRACT: A one-pot synthetic procedure for a series of bimetallic Nb/M complexes, Cl−Nb(iPrNPPh2)3M−X (M = Fe (2), Ni (4), Cu (5)), is described. A similar procedure aimed at synthesizing a Nb/Co analogue instead affords i PrNNb(iPrNPPh2)2(μ-PPh2)Co−I (3) through cleavage of one phosphinoamide P−N bond under reducing conditions. Complexes 4 and 5 are found to have short Nb-M distances, corresponding to unusual metal−metal bonds between Nb and these first row transition metals. For comparison, a series of heterobimetallic ONb(iPrNPPh2)3M−X complexes (M = Fe (7), Co (8), Ni (9), Cu (10)) was synthesized. In these complexes, the NbV center is engaged in sufficient π-bonding to the terminal oxo ligand to remove the driving force for direct metal−metal interactions. A comparison of the cyclic voltammograms of 2 and 4−10 reveals that the presence of a second metal shifts the redox potentials of both Nb and the late metal center anodically, even when direct metal−metal interactions are not present.
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INTRODUCTION Although the ability of bonds to form between metal atoms is a well-established and thoroughly studied concept, metal−metal bonding has become an increasingly important topic in recent years in light of the growing appreciation for the importance of metal−metal cooperativity in the areas of both molecular and enzymatic catalysis.1 While the fundamental aspects of metal− metal bonding in homobimetallic complexes are relatively wellunderstood,2 the synthetic challenges presented by the construction of bonds between two different metals have circumvented similarly extensive studies of heterobimetallic complexes until recently.3 The inherent differences in electronegativity between two different metals and the corresponding disparity in atomic orbital energies lead to weaker overlap and more polar bonds as the metals become further apart on the periodic table. Thus, early/late heterobimetallic complexes present the unique opportunity to assemble metal−metal multiple bonds that are more reactive than their homobimetallic counterparts. Early/late heterobimetallic complexes are particularly well-poised for catalytic processes involving heterolytic bond activation processes owing to the polarity of the metal−metal interaction, leading this field to become an active research area in the latter part of the twentieth century.1d−h © XXXX American Chemical Society
Inspired by the ultimate goal of uncovering new reactivity profiles, the past decade has seen an impressive expansion in the number of early/late heterobimetallic complexes featuring multiple bonds and/or short metal−metal distances. Using a heptadentate “double-decker” ligand framework, Lu and coworkers reported a series of Cr/M,4 V/M,5 and Ti/M complexes (M = Fe, Co, Ni) and systematically explored the changes in metal−metal bonding as a function of both metal− metal combination and redox changes.3c,6 Inspired by earlier studies by Nagashima,7 our group has used bis- and tris(phosphinoamide) ligand frameworks to construct multiply bonded early/late heterobimetallic Ti/M,8 V/M,8c,9 Cr/M,10 and Zr/Co complexes (M = Fe, Co, Ni, Cu).11 The conformational flexibility and coordinative unsaturation at both metal sites in Zr/Co and Ti/Co phosphinoamidesupported heterobimetallics have been shown to lead to unique reactivity patterns facilitated by different modes of metal−metal cooperativity.8a,b,12 In related work by Tonks and co-workers aimed at increasing the Lewis acidity of the early metal center by attenuating the π-donor ability of the N-donor functionality, 2-(diphenylphosphino)pyrrolide ligands have Received: October 17, 2018
A
DOI: 10.1021/acs.inorgchem.8b02960 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
synthesis of Cl−Nb(iPrNPPh2)3Fe−Br (2) and its reduction to Nb(iPrNPPh2)3Fe−PMe3, which features a NbFe triple bond and represents the first example of multiple bonding between Nb and a different metal. Similar to previous studies, we sought to expand the Nb/M series to explore the impact of systematic variation of the late metal on Nb/M interactions. Herein, we report a simplified one-pot synthetic procedure for the synthesis of 2 that can be successfully employed to synthesize Nb/Ni and Nb/Cu complexes. In order to probe the effects of metal−metal interactions on the respective redox properties of the two metals, both through-ligand and via metal−metal bonding, a complete series of ONbV/MI (M = Fe, Co, Ni, Cu) complexes lacking metal−metal interactions has also been synthesized. Since the NbV oxo moiety was found to be redox-inert, we are able to confidently assign redox processes and determine that tethering two metals together decreases electron density at both metal centers even when no direct metal−metal bonding is present, facilitating reduction at both metal centers.
been used to buttress metal−metal bonding in M/Ni (M = Ti, Zr, Hf) and Ti/Fe complexes.13 One fundamental paradigm that has emerged from the combined efforts of researchers on heterobimetallic complexes is that tethering two metals together facilitates multielectron redox processes and provides an opportunity to finely tune redox potentials by varying the metal−metal combination.3b,c For example, the tris(phosphinoamide)-supported Zr/Co complex Cl−Zr(iPrNPPh2)3Co−I undergoes facile two-electron reduction at a potential nearly 1 V more positive than the monometallic Co analogue (Ph2PNHiPr)3Co−I.11a A recent computational mechanistic study by Ess and co-workers has suggested that facilitating two-electron reduction of the Co center is the crucial role that Zr plays in Kumada coupling reactions mediated by heterobimetallic Zr/Co catalysts.14 Questions remain, however, as to the importance of direct metal−metal bonds in tuning redox potentials in heterobimetallics and to what extent through-ligand or through-space interactions can also impact redox processes. Heterobimetallic complexes featuring Nb have been the focus of far fewer recent studies; however, complexes comprised of bonds between CpNb fragments and first row metals were first reported in the literature several decades ago. For example, Labinger, Scheidt, and Wong reported the synthesis and structural characterization of Cp2(CO)Nb(μH)Fe(CO)4 and Cp2(CO)Nb(μ-CO)Co(CO)3, which were found to have Nb−M distances of 3.324(1) Å and 2.986(1) Å, respectively, indicative of metal−metal bonds with differences in metal−metal distance arising from differences in the bridging ligand.15 Likewise, the bridging C−H activated Cp C5H42− ligand in Cp2Nb(H)(σ,π-C5H4)Fe(CO)2 affords a Nb−Fe distance of 2.968(1) Å.16 Sundermeyer and Runge reported the synthesis of Cp2(NtBu)Nb−Fe(CO)2Cp and Cp(NtBu)Nb-{Fe(CO)2Cp}2 via an anion metathesis route, although no structural data or discussion of metal−metal bonding was provided.17 A Nb−Ni distance of 2.759(1) Å was reported in CpNbCl2(μ-SEt)2NiCp and ascribed to a single bond.18 Cp2Nb(CO)HZn(BH4)2 and Cp2NbH2ZnCp were found to have Nb−Zn distances of 2.829 Å and 2.5407(7) Å, indicative of dative and normal covalent Nb−Zn bonds, respectively.19 More recently, Bruno and co-workers reported the isolation of the Nb-M single-bonded complexes (C5H4SiMe3)2Nb(μ-CO)2Fe(CO)Cp, (C5H4SiMe3)2(CO)NbCo(CO)4, and (C5H4SiMe3)2Nb(μ-CO)2NiCp, and structural characterization of the Nb/Fe complex revealed a Nb−Fe separation of 2.8156(6) Å.20 A Nb−Cr single bond was also reported in Cp2Nb(μ-PPh2)(μ-CO)Cr(CO)3(PMe2H).21 In all of these examples, conclusions about metal−metal interactions were drawn solely on the basis of Nb-M interatomic distances. However, computational analysis of the metal−metal bonding in [{(C5H4SiMe3)2(CO)Nb}2(μH)2Cu]+ revealed only weak direct interactions between the metals despite a relatively short Nb−Cu distance of 2.732(2) Å,22 and analysis of the frontier molecular orbitals of Cp 2 Nb(μ-CO)(μ-PPh 2 )Fe(CO) 3 (Nb−Fe distance = 2.884(2) Å) revealed substantial involvement of the semibridging CO ligand in the metal−metal bonding interaction.23 In light of the scarcity of structurally characterized nonmetallocene-based heterobimetallic complexes featuring bonds between Nb and other metals, we recently turned our attention to Nb/M complexes and reported the synthesis and characterization of a series of Nb/Fe complexes supported by tris(phosphinoamide) ligands.24 Most notably, we reported the
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RESULTS AND DISCUSSION Synthesis and Spectroscopic Characterization of Nb/ M Complexes 2−10. We recently reported the synthesis and characterization of Cl−Nb(iPrNPPh2)3Fe−Br (2) through two different synthetic routes involving the isolation of either the NbIV precursor Cl−Nb(iPrNPPh2)3 (1) or NbV species ONb(iPrNPPh2)3 (6).24 While both synthetic routes were successful, the thermal instability of 1 and the extra steps to remove the oxo ligand from 6 rendered neither synthetic pathway ideal for the routine synthesis of 2. Thus, a more efficient and atom-economical synthesis of 2 has now been developed. This new one-pot synthetic methodology has also proven to be a straightforward synthetic route for an expanded collection of Nb/M (M = Fe, Ni, Cu) heterobimetallic complexes. The revised synthetic route to 2 involves the generation of 1 in situ, followed by immediate addition of FeBr2 and excess Zn at low temperature (Scheme 1). Addition of 1 equiv of NbCl4(THF)2 to 3 equiv of Li(iPrNPPh2)(OEt2) in thawing THF (−108.4 °C) results in an immediate transformation of the initial yellow suspension to a dark green homogeneous solution as 1 is formed. Addition of FeBr2 and Zn powder to the solution of 1 at low temperature affords the blue Nb/Fe complex 2 in 84% yield. The spectroscopic features of 2 generated in this fashion were consistent with those reported previously.24 Attempts to generate an analogous Nb/Co complex using a similar procedure were unsuccessful and instead cleanly afforded a NbV imido complex as a result of P−N bond cleavage. The addition of 1 equiv of solid CoBr2 and excess Zn powder to a solution of 1 (generated in situ at low temperature) results in a dramatic color change from green to red. The 1H NMR spectrum of the isolated red powder features nine paramagnetically shifted broad peaks indicative of a product that does not have a C3-symmetric structure similar to 2. The solid state structure obtained using X-ray crystallography (vide infra) confirms this hypothesis, identifying the product as iPrNNb(μ-PPh2)(iPrNPPh2)2Co−Br (3, Scheme 1). Although two phosphinoamide ligands bridge the Nb and Co centers, the P−N bond of the third phosphinoamide ligand has been cleaved into a Nb-bound terminal imido ligand and a diphenylphosphide moiety that bridges the two metal centers. A similar P−N bond cleavage B
DOI: 10.1021/acs.inorgchem.8b02960 Inorg. Chem. XXXX, XXX, XXX−XXX
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In our previous study of Nb/Fe complexes, we found that while dihalide NbIV/FeI complex 2 features a significant dative Fe→Nb interaction, there was essentially no metal−metal interaction in the analogous Nb V /FeI imido complex i PrNNb(iPrNPPh2)3Fe−Br.24 This allowed for rigorous evaluations of the effects of direct metal−metal interactions vs through-ligand metal−metal communication on late metal redox potentials. To allow similar comparisons among the expanded Nb/M series, a series of heterobimetallic ONb/M (M = Fe, Co, Ni, Cu) complexes was targeted, starting from the previously reported tris(phosphinoamide) niobium(V) oxo precursor ONb(iPrNPPh2)3 (6).24 Synthetic efforts began with the ONb/Fe variant, ONb(iPrNPPh2)3Fe−I (7), via addition of 1 equiv of FeI2 to a THF solution of 6 at room temperature in the presence of 0.5 equiv of zinc powder (Scheme 2). After stirring for 16 h,
Scheme 1
Scheme 2
process was reported as a decomposition pathway of the reduced Nb/Fe complex Nb(iPrNPPh2)3Fe-PMe3, although in the previous case the fate of the PPh2− phosphide fragment was not determined.24 The solution magnetic moment of 3 (μef f = 2.83 μB) is indicative of an S = 1 ground state, as would be expected for a NbV/CoI d8 complex. A somewhat different synthetic method was required to obtain a heterobimetallic Nb/Ni complex. In the synthetic procedures for 2 and 3, excess Zn powder was used to reduce the dihalide salts during the metalation of 1, but this method was unsuccessful using NiII precursors. Therefore, we sought to generate a stable NiI synthon in situ by the comproportionation of Ni(COD)2 (COD = cyclooctadiene) and NiCl2(dme) (dme = dimethoxyethane).25 Addition of a 1:1 mixture of Ni(COD)2 and NiCl2(dme) to 1 afforded a dark red solution containing a single diamagnetic product, Cl−Nb(iPrNPPh2)3Ni−Cl (4). The 31P{1H} NMR spectrum of the isolated orange/red powder shows a single peak at −39 ppm, and the 1H NMR spectrum of 4 also supports a C3-symmetric structure. The NiII source was found to be critical for the reaction, as addition of unligated salts such as NiCl2, NiBr2, or NiI2 to Ni(COD)2 resulted in instant formation of nickel mirror. In contrast, the NiI species generated upon addition of NiCl2(dme) to Ni(COD)2 was stable on a longer time scale, resulting in cleaner reactions and higher yields of 4. To complete the Nb/M series, a Nb/Cu complex was synthesized. Addition of 1 equiv of CuBr to 1 (generated in situ) afforded Cl−Nb(iPrNPPh2)3Cu−Br (5) as an orange/ brown powder in high yield. The five relatively sharp, paramagnetically shifted peaks in the 1H NMR spectrum of 5 are in line with a C3-symmetric geometry. The solution magnetic moment of 5 (1.40 μB), although slightly below the expected spin only value (1.73 μB), is consistent with an S = 1/2 spin state and is similar to the low magnetic moment reported for 1 (1.61 μB).24
complex 7 was obtained as a red powder. Complex 7 contains five paramagnetically shifted resonances in its 1H NMR spectrum with a similar pattern to that previously reported for iPrNNb(iPrNPPh2)3Fe−Br,24 suggesting the formation of a C3-symmetric Nb/Fe heterobimetallic complex. The solution magnetic moment of 7 was found to be 3.90 μB, consistent with a d0 NbV and high spin S = 3/2 FeI assignment. The ONb/Co analogue, ONb(iPrNPPh2)3Co−I (8), was synthesized in a similar manner. Treatment of 6 with CoI2 and Zn powder afforded 8 as a blue powder. The 1H NMR spectrum of 8 exhibits four paramagnetically shifted peaks in its 1H NMR spectrum, with a pattern very similar to that reported for tBuNNb(iPrNPPh2)3Co−I.26 The solution magnetic moment of 8 was found to be 2.86 μB, consistent with the S = 1 ground state expected for a tetrahedral d8 CoI ion. As previously described for the synthesis of 4, the tris(phosphinoamide) ONb/Ni complex was synthesized using a NiII/Ni0 comproportionation strategy to generate NiI in situ. NiCl2(dme) and Ni(COD)2 were added to a THF solution of 6, to afford ONb(iPrNPPh2)3Ni−Cl (9) as a yellow powder. The 1H NMR spectrum of 9 features five paramagnetically shifted resonances, as expected for a C3symmetric Nb/Ni heterobimetallic complex. Complex 9 C
DOI: 10.1021/acs.inorgchem.8b02960 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry adopts an S = 1/2 ground state, as verified by the EPR spectrum, which shows a complex superhyperfine coupling pattern that cannot be confidently assigned as coupling to 31P (I = 1/2), 35/37Cl (I = 3/2), or 93Nb (I = 9/2), but is consistent with a Ni-based radical; an oxo-NbIV-based radical would be expected to have a much larger ANb coupling constant (130−270 G)27 than the largest coupling observed in the EPR spectrum of 9 (∼50 G, Figure S20). Lastly, a closed shell ONbV/CuI complex was synthetically targeted. The reaction between equimolar quantities of 6 and CuI produced ONb(iPrNPPh2)3Cu−I (10) as a colorless crystalline solid. Diamagnetic complex 10 has a single broad 31P{1H} NMR resonance at −21 ppm, and its 1H NMR spectrum is consistent with a C3-symmetric geometry in solution. Structural Characterization of Complexes 2−5. The new Nb/M complexes 3−5 and 7−10 were structurally characterized using single crystal X-ray diffraction (Figures 1−3 and Table 1). Complex 3 adopts an approximately Cs-
isopropylimido functionality. The Nb−Co distance in 3 is 2.8147(6) Å, which is relatively short compared to literature precedents but remains too long to signify significant metal− metal bonding. In a previous report, we found that the strong π-donating ability of a terminal imido ligand in tBuN Nb(iPrNPPh2)3Co−I out-competed dative Co→Nb interactions,26 and this is almost certainly the case for complex 3. Nonetheless, prior to this study, the shortest reported Nb−Co distances were 2.8492(4) Å for [tBuNNb(iPrNPPh2)3Co−N2][Na(THF)5]26 and 2.992(1) Å for Cp2(CO)Nb(μ-CO)Co(CO)3.15a The short Nb−Co distance in 3 is, however, most likely the result of the geometric constraints of the bridging phosphido ligand rather than metal−metal bonding. The phosphido ligand bridges the two metals asymmetrically, with a much shorter Co−P bond distance of 2.2896(10) Å compared to the Nb−P distance of 2.5068(10) Å. As indicated by 1H NMR data, the solid state structures of complexes 4 and 5 reveal C3-symmetric tris(phosphinoamide) geometries. Similar to 2, the Nb atoms in 4 and 5 are in the center of a trigonal plane formed by three amide donors (∑N−Nb−N = 360.0° (4) and 357.6° (5)) and the late metal centers adopt distorted trigonal bipyramidal environments. The most interesting feature of complexes 4 and 5 revealed by X-ray crystallography is the short metal−metal distances. Comparison between metal−metal distances among a series of different bimetallic combinations is best accomplished using Cotton’s “formal shortness ratio” (FSR) to normalize interatomic distances by representing the ratio of the metal− metal distance to the sum of the single bond atomic radii (R1, Pauling)28 of the two metal ions.2 The Nb−Ni distance in 4 is 2.2990(3) Å in the solid state, which corresponds to an FSR of 0.92. There has only been one structurally characterized Nb/Ni heterobimetallic complex reported to date, CpNbCl2(μ-SEt)2NiCp, with a Nb−Ni distance of 2.759(1) Å (FSR = 1.10),18 along with several Nb/ Ni clusters with Nb−Ni interatomic distances in the 2.7−3.1 Å range.29 Our group, Tonks, Lu, and Nagashima, have reported Ti/Ni and V/Ni complexes with FSRs in the 0.90−1.05 range, but making direct comparisons between these compounds and 4 is difficult owing to the differences in first vs second row metals and coordination environments.3c,7d,8c,13a The most meaningful comparison is between Tonks’ d10 Zr/Ni complex (κ2-NP)Zr(μ2-NP)3Ni (NP = 2-diphenylphosphinopyrrolide), whose metal−metal distance is longer (2.3724(3) Å) but comparable to 4 when corrected for the larger size of Zr (FSR = 0.91).13a The solid state structure of Nb/Cu complex 5 reveals a metal−metal distance of 2.6572(4) Å, corresponding to an FSR value of 1.05 and indicating a relatively weak interaction between the two metal centers. Nonetheless, Nb−Cu bonds and distances less than 2.7 Å have not been reported to date, rendering complex 5 unique among the relatively large number of Nb/Cu complexes and clusters in the literature (e.g., [{(C5H4SiMe3)2Nb(CO)}2(μ-H)2Cu]+ , 2.700(2) Å and 2.732(2) Å, FSR = 1.07).22 Our group and others have reported the solid state structures of heterobimetallic Zr/Cu, Ti/Cu, V/Cu, and Cr/Cu complexes.7b,8c,10 However, only the Zr/Cu and Cr/Cu complexes exhibit C3-symmetry in the solidstate, allowing for the most meaningful comparisons with 5. The M−Cu distances in Cl−Zr(iPrNPPh2)3Cu−I and Cr(iPrNPPh2)3Cu−I are 2.6854(6) Å (FSR = 1.02) and
Figure 1. Displacement ellipsoid (50%) representation of 3 and relevant bond distances and angles. Hydrogen atoms and solvate molecules have been omitted for clarity.
Figure 2. Displacement ellipsoid (50%) representations of 4 and 5. Hydrogen atoms and solvate molecules have been omitted for clarity.
symmetric geometry with two bridging phosphinoamide ligands, a bridging Ph2P− ligand, and a terminal Nb D
DOI: 10.1021/acs.inorgchem.8b02960 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 3. Displacement ellipsoid (50%) representations of 7−10. Hydrogen atoms and solvate molecules have been omitted for clarity.
Table 1. Selected Interatomic Distances and Angles for Complexes 2, 4, 5, 7−10, and Selected Related Bimetallic Compounds for Comparison Complex
M1−M2 (Å)
FSRa
M1−N (Å, avg)
M2−P (Å, avg)
N−M1−N (°, avg)
P−M2−P (°, avg)
Cl−Nb( PrNPPh2)3Fe−Br (2) ONb(iPrNPPh2)3Fe−I (7) ONb(iPrNPPh2)3Co−I (8) t BuNNb(iPrNPPh2)3Co−I26 Cl−Nb(iPrNPPh2)3Ni−Cl (4) (κ2-NP)Zr(μ2-NP)3Ni13a ONb(iPrNPPh2)3Ni−Cl (9) Cl−Nb(iPrNPPh2)3Cu−Br (5) ONb(iPrNPPh2)3Cu−I (10) Cl−Zr(iPrNPPh2)3Cu−Ib,7b
2.4269(4) 3.0567(5) 3.0178(5) 3.0319(4) 2.2990(3) 2.3724(3) 2.9368(3) 2.6572(4) 2.9380(6) 2.6854(6) 2.6790(6) 2.6419(8) 2.6183(9)
0.97 1.21 1.20 1.21 0.92 0.91 1.18 1.05 1.16 1.01
2.05 2.03 2.03 2.05 2.04 2.21 2.03 2.04 2.04 2.10
2.29 2.34 2.26 2.25 2.23 2.21 2.23 2.31 2.28 2.33
120.0 115.9 115.5 114.8 120.0 n/a 115.5 119.1 115.4 118.9
109.8 104.0 105.2 105.4 112.7 119.8 107.4 108.5 108.0 109.0
1.11
1.87
2.32
119.8
106.0
i
Cr(iPrNPPh2)3Cu−Ib,10 a
FSR = ratio of the metal−metal distance to the sum of the single bond atomic radii (R1, Pauling)28 of the two metal ions.2 bMetal−metal distances are reported for each of two molecules in the asymmetric unit.
become shorter, the late metal center does flatten further into the plane of the three phosphorus donors to allow the metals to approach one another. For example, the average P− M−P angle in 4, which has the shortest Nb−M distance, is 113°, which is smaller than the ideal 120° value of an ideal trigonal bipyramid but larger than expected for a tetrahedral tris(phosphine) complex with no metal−metal interactions (e.g., 7−10, vide inf ra). In the Fe and Cu complexes 2 and 5, where the metal−metal interactions are weaker, the P−M−P angles are much closer to the tetrahedral 109.5° value (Table 1). It is also observed that the distance between the late metal center and the plane of the three phosphorus atoms decreases as intermetallic distances become shorter in the order: 5 (0.80 Å) > 2 (0.75 Å) > 4 (0.61 Å). The ONb(iPrNPPh2)3M−X complexes 7−10 were also structurally characterized by single crystal X-ray diffraction (Figure 3, Table 1). Consistent with 1H NMR data, all four complexes have C3-symmetric geometries in the solid state with Nb-M distances that are greater than 2.9 Å and indicative of no metal−metal bonding in these compounds. Similar to the effect of the imido ligand in complex 3 and the previously reported iPrNNb(iPrNPPh2)3Fe−Br and t BuNNb(iPrNPPh2)3Co−I complexes,24,26 the terminal oxo ligand in 7−10 prevents strong metal−metal interactions by acting as a strong π-donor ligand.
2.6183(9) Å (FSR = 1.11) and were attributed to weak dative metal−metal interactions.7b,10 While metal−metal distances are not necessarily an accurate indicator of bond strength or bond order (vide infra), some additional geometric trends among 2, 4, and 5 can be related back to differences in metal−metal distance. While the Nb−N distances remain essentially constant among the series, the placement of the Nb center with respect to the three amide ligands appears to be dependent on the extent of metal−metal bonding. In 2 and 4, which have more significant metal−metal interactions, the Nb center lies rigorously in the plane of the three amide nitrogen atoms. In contrast, the Nb center in 5 is pushed 0.19 Å out of the trigonal plane, leading to a slight contraction of the average N−Nb−N angle from the ideal 120° expected in a trigonal geometry. This out-of-plane distortion is also observed for other M/Cu (M = Zr, Cr) heterobimetallics with long intermetallic distances,7b,10 suggesting that this phenomenon can be attributed to the weaker metal−metal interactions in these complexes (Table 1). The geometry about the late metal center also varies widely as metal−metal interactions fluctuate across the Nb/M series. Given the geometric constraints of the two-atom phosphinoamide bridging ligands and the sp3-hybridized phosphorus lone pair, the late metal cannot reside in the same plane as the three phosphine donors.3b However, as metal−metal distances E
DOI: 10.1021/acs.inorgchem.8b02960 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 4. Cyclic voltammograms of complexes 2, 4, 5, and 7−10 recorded in 0.30 M [nBu4N][PF6] in THF (scan rate = 100 mV/s).
Table 2. Redox Potentials (vs Fc/Fc+) Measured Using Cyclic Voltammetry for Complexes 1, 2, 4, 5, and 7−10, along along with Several Related Nb/M Complexesa NbV/IV
Complex i
24
Cl−Nb( PrNPPh2)3 (1) Cl−Nb(iPrNPPh2)3Fe−Br (2) Cl−Nb(iPrNPPh2)3Ni−Cl (4) Cl−Nb(iPrNPPh2)3Cu−Br (5) ONb(iPrNPPh2)3Fe−I (7) i PrNNb(iPrNPPh2)3Fe−Br24 ONb(iPrNPPh2)3Co−I (8) t BuNNb(iPrNPPh2)3Co−I26 ONb(iPrNPPh2)3Ni−Cl (9) ONb(iPrNPPh2)3Cu−I (10)
NbIV/III
−1.65 V 0.54 Vb 0.49 Vb −0.52 Vc
−2.55 −1.62 −1.67 −1.69
V V V V
b
MI/0
MII/I
N/A −2.35 Vb
N/A 0.13 Vb 0.07 Vb 0.32 Vb,c −0.37 Vb −0.52 V 0.19 V 0.00 V −0.11 V 0.66 Vb
−1.63 −1.84 −1.97 −2.02 −1.40 −2.74
Vb V V V Vb Vb
a All data was recorded in 0.3 M [nBu4N][PF6] in THF at a scan rate of 100 mv/s, and potentials are E1/2 values unless otherwise noted. bThese redox processes were irreversible, so the oxidation and reduction potentials reported are Epa or Epc, respectively. cAssignment of these two oxidative events to Nb and Cu is more ambiguous, and these assignments are to be considered tentative.
further validated by comparison to the MI/0 potentials of monometallic tris(phosphine) Fe, Ni, and Cu complexes, which are significantly more negative (vide infra).32.33 The NbIV/III reduction is fully reversible in the case of Nb/Cu complex 5 but quasi-reversible in the case of Nb/Ni and Nb/ Fe complexes 4 and 2 (Figure S13 and S12). As previously reported,24 an additional irreversible reduction is observed at −2.35 V in the CV of complex 2, but no additional reductive features are observed for 4 and 5 within the THF solvent window, allowing this feature to be assigned to an FeI/0 redox event. It is worth noting that the CV of complex 2 also features a third poorly defined and smaller reductive feature around −2.01 V, in between the two reductions. Since we previously reported that the chemical reduction of 2 with excess KC8 affords a two-electron reduced product with both metal halides dissociated,24 we hypothesize that the irreversibility of the reduction at −1.62 V and the additional reductive feature at −2.01 V are both related to halide loss following the first reduction. Complexes 2, 4, and 5 also have two oxidative features in their CVs. The oxidation at lower potential can be ascribed to the late metal in complexes 2 and 4 based on comparison with monometallic Fe and Ni complexes. For example, the FeII/I couple of PhB(CH2PPh2)3FeCl was reported as −0.82 V33,31 and the NiII/I couple of PhB(CH2PPh2)NiCl is at −1.20 V.32a The NbIV complexes Cp2Nb(SiMe3)Cl and Cp2Nb(CH2Ph2)2
Electrochemistry. As the late metal in the Nb/M complexes is varied, variations in redox behavior are expected. Further, the isolation of heterobimetallic complexes with and without direct metal−metal interactions provides the unique opportunity to probe the effect of metal−metal bonding on the redox behavior of the two metals involved. The cyclic voltammograms (CVs) of monometallic Nb complex 1 and Nb/Fe complex 2 were previously reported.24 For comparison, the redox properties of the complete series of Nb/M and ONb/M complexes 4, 5, and 7−10 reported herein were also explored using cyclic voltammetry. Redox potentials are reported referenced to the ferrocene/ferrocenium redox couple. The CVs of all three Cl−Nb(iPrNPPh2)3M−X complexes 2, 4, and 5 reveal somewhat reversible reductive processes at nearly identical potentials (−1.62 V to −1.69 V, Figure 4, Table 2). Based on the similarity of the E1/2 values as the late metal is varied, this reduction process can be readily assigned as a Nb-based reduction. A NbIV/III reduction was also observed in the CV of monometallic complex 1, but in this case the redox wave was irreversible and at much more negative potential (−2.55 V). For comparison, the NbIV complexes Cp2Nb(SiMe3)Cl and Cp2Nb(CH2Ph2)2 were reported to undergo reduction at −2.36 V and −2.49 V vs ferrocene/ferrocenium, respectively.30,31 The assignment of the reductive features around −1.6 V in the CVs of 2, 4, and 5 is F
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Inorganic Chemistry were reported to undergo oxidation to NbV at higher potentials (−0.12 V and −0.62 V vs Fc/Fc+, respectively).30,31 The CV of Nb/Ni complex 4 displays a quasi-reversible oxidation at 0.07 V, and the CV of Nb/Fe complex 2 has an irreversible oxidative wave at 0.13 V, which are assigned as FeII/I and NiII/I oxidations, respectively. An irreversible oxidation is observed at −0.52 V for complex 5. The CuII/I oxidations for tris(pyrazolyl)borate and tris(pyrazolyl)methane CuI complexes were reported in the −0.24 to 0.53 V range,34,31 so assignment of the −0.52 V oxidation in the CV of 5 to Cu is less likely. Instead, we tentatively assign the lower potential oxidative event to NbV/IV oxidation, which we suggest is more feasible in the Nb/Cu case due to diminished direct metal−metal interactions (vide infra). The anodic shift in MII/I redox potential as M is varied from Fe and Ni to Cu is consistent with the increasing effective nuclear charge across the series. The second oxidation observed in the CVs of 2 and 4, assigned to oxidation from NbIV to NbV, is also found to shift cathodically from Fe to Ni to Cu and appears to increase in potential as a function of metal−metal distance (Table 1). Since the oxo complexes 7−10 do not contain metal−metal bonds, a comparison between the redox properties of 2, 4, and 5 and those of 7−10 allows direct conclusions to be made about the effect of direct metal−metal bonds on the redox properties of the late metal center. The difference in Fe-bound halide in 2 and 7 and the Cu-bound halide in 5 and 10 complicates direct quantitation of the impact of metal−metal interactions, but general trends are nonetheless evident. Notably, the cyclic voltammogram of monometallic precursor ONb(iPrNPPh2)3 (6) does not show any accessible redox events between 0.3 and −3 V (Figure S15). Moreover, chemical reduction of the analogous NbV imide complexes t BuNNb(iPrNPPh2)3Co−I and iPrNNb(iPrNPPh2)3Fe− Br has been reported to lead to reduced Co−I and Fe0 products, respectively, with no Nb-based redox changes.24,26 Therefore, all of the redox events observed for 7−10 can be readily assigned to the late metal. The CVs of all four Nb oxo complexes reveal a one-electron reduction assigned as MI/0 reduction. In the case of Nb/Fe complex 7, an irreversible reduction is observed at −1.63 V, followed by a second irreversible reduction at −2.05 V. Both of these reductions are presumed to be iron-based since a second reduction is not observed for 8−10. The Nb/Co complex 8 has a single quasi-reversible CoI/0 reduction at −1.97 V in its CV, while the corresponding NiI/0 reduction of Nb/Ni complex 9 is observed at −1.40 V and is electrochemically irreversible. The CV of Nb/Cu complex 10 features an irreversible reduction at a very negative potential (−2.74 V) that is tentatively assigned as a CuI/0 reduction, although reduction of the NbV oxo center at this negative potential is also feasible. In general, a cathodic shift in the MI/0 potential is observed from Fe to Co to Cu, consistent with the increasing number of d-electrons across the series. The Nb/Ni complex likely forms an exception to this trend owing to the stable closed shell configuration that would result upon its oneelectron reduction. In the Fe and Co cases, where analogous NbV imido complexes have been reported, both the MII/I oxidations and MI/0 reductions are shifted to more positive potentials in the Nb oxo complexes, reflecting the moderately stronger donor ability of the imido functionality that renders the imido analogues more electron-rich. Comparing the redox behavior of 2, 4, and 5 with oxo complexes 7−10 and with monometallic analogues where
available, it can be concluded that the presence of a second metal center shifts the redox processes of both metals anodically compared to monometallic analogues even when a direct metal−metal bond is not present. A comparison between the NbV/IV and NbIV/III redox potentials of 1 and metallocene-based monometallic NbIV complexes30 with 2, 4, and 5 reveals that the addition of the late metal results in ∼1 V shifts to more positive potential for both reduction and oxidation of the NbIV center, consistent with significantly diminished electron density at Nb in the presence of a second metal center. A similar phenomenon was observed by our group for a V/Fe system,9 and by Lu and co-workers for a series of Cr/M and V/M complexes.4b,5 An anodic shift in the MI/0 redox couples of complexes 8−10 compared to known monometallic analogues is also observed, demonstrating that the presence of the appended NbV center also decreases the electron density at the late metal even when no metal−metal bond is present. For example, the monometallic complex (iPrNHPPh2)3Co−I was reported to have an irreversible CoI/0 reduction at −2.49 V,11a which is ca. 0.5 V more negative than the reversible CoI/0 couple of either t BuNNb(iPrNPPh2)3Co−I26 or 8. Similarly, the CV of (iPrNHPPh2)3Cu−I was reported to have an irreversible CuI/0 reduction at −2.99 V,32b which is moderately more negative than the CuI/0 reduction observed for Nb/Cu complex 10. Although a monometallic NiI (iPrNHPPh2)3Ni−Cl complex has not been synthesized, preventing a direct comparison to 9, the CVs of the C3 symmetric tris(phosphine) ligated NiI complexes [PhB(CH 2 P i Pr 2 ) 3 ]Ni(PMe 3 ) and [PhB(CH2PiPr2)3]Ni(CNtBu) were investigated by Peters and coworkers,32a revealing reversible NiI/0 couples (−1.95 V and −1.84 V, respectively) ca. 0.5 V more negative than the NiI/0 couple of 9. The observed anodic shifts in the redox potentials of both metals can be attributed to inductive effects that alter the donor properties of the phosphinoamide ligands. For example, the interaction of the three phosphine donors with the late metal likely diminishes the donor properties of the amide moieties, decreasing the electron density at Nb. Likewise, interaction of the amide functionalities with Nb would diminish the electron density at the late metal center by weakening the phosphorus σ-donor strength. This is corroborated by the increased average Co−P distance in complex 8 (2.26 Å) compared to the monometallic tris(phosphinoamine) complex (iPrNHPPh2)3Co−I (2.21 Å).11a One might expect, based on the above trends, that the MI/0 reduction potential would anodically shift even more dramatically in complexes 2, 4, and 5, where direct metal− metal bonding interactions are present. However, the FeI/0 reduction of complex 2 is actually 0.7 V more negative than that of 7, and a MI/0 reduction is not observed within the solvent window for 4 and 5. This initially perplexing observation can be explained by the fact that the MI/0 reductions of complexes 2, 4, and 5 are preceded by a Nbbased reduction and the delocalization imparted by the metal− metal interactions in these complexes necessitates that a Nbbased reduction inherently effects the reduction of the late metal center. Reduction at Nb increases the electron density in the bimetallic system and renders the electrochemically generated [ClNb(iPrNPPh2)3MX]− complex anionic, resulting in a cathodic shift in the reduction potential of the M center in 2 compared to 7. Presumably, the NiI/0 and CuI/0 reductions of 4 and 5 are shifted too far negative to be observed within the solvent window. The metal−metal bonding interactions in 2, 4, G
DOI: 10.1021/acs.inorgchem.8b02960 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry and 5 do, however, shift the MII/I oxidation potentials anodically, as might be expected based on the delocalization of late metal electron density into interactions with the pendent Nb center. Computational Investigations. The electronic structure and bonding in heterobimetallic complexes 4 and 5 was also investigated computationally using density functional theory (DFT). The computed bond metrics, including the metal− metal distances, are in agreement with those determined by Xray crystallography (Table S3). The computed frontier molecular orbital (MO) diagrams of 4 and 5 are similar to that previously reported for Nb/Fe complex 2 (Figures 5 and
having the longest Nb−M distance and with the absence of unpaired electrons on the d10 Cu center to pair with the unpaired electron on the NbIV center. In all cases, although the occupied dxz and dyz orbitals of the two metals are of the right symmetry to overlap, π interactions between Nb and the late metal are weak or absent as a result of the energetic mismatch of the Nb 4d and Fe/Ni/Cu 3d metals. As shown in Figure 5, very weak π overlap is observed in Nb/ Ni complex 4, while the Cu dxz and dyz orbitals remain exclusively nonbonding in 5. The combination of σ and π interactions in 4 provides an explanation for the short Nb−Ni distance observed in this complex. However, from the qualitative MO pictures it is not clear why there would be a significant difference in Nb-M bond order between Nb/Fe complex 2 and Nb/Ni complex 4, as indicated by the smaller FSR of 4. To facilitate a more quantitative comparison between the metal−metal bonding in complexes 2, 4, and 5, Mayer bond order (MBO)35 and natural bond orbital (NBO)36 calculations were performed. The resulting MBOs, Wiberg bond indices (WBIs), and metal atom natural charges are tabulated in Table 3. Despite the smaller FSR of the Nb/Ni complex 4, the MBO Table 3. Computed Mayer Bond Order (MBO), Wiberg Bond Index (WBI), and Natural Charges of the Nb and M Atoms in 2, 4, and 5 Natural charge Complex
FSR
MBO
WBI
Nb
M
224 4 5
0.97 0.92 1.05
1.64 1.44 0.26
1.1 0.93 0.31
0.59 0.56 0.76
−0.74 −0.58 −0.32
and WBI values both indicate a lower metal−metal bond order compared to Nb/Fe complex 2. Weaker metal−metal bonding is expected as the two metal partners become farther apart on the periodic table owing to increasing differences in energy between the overlapping metal-based orbitals.3b,c Thus, the trends in bond order displayed in Table 3 are as expected and consistent with previously reported trends in Ti/M, Cr/M, and V/M bonding (M = Fe, Co, Ni, Cu).3b,c,4b,5,8c,10 However, complex 4 serves as an important cautionary tale that metal− metal distances, even when corrected for atomic size using ratios such as FSR, can be misleading when drawing conclusions about metal−metal bond order.2
Figure 5. Frontier molecular orbital diagrams and graphical representations of selected Kohn−Sham orbitals containing metal− metal interactions.
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CONCLUSIONS In summary, a convenient one-pot pathway has been devised to synthesize a series of heterobimetallic Cl−Nb(iPrNPPh2)3M−X complexes, where M = Fe, Ni, and Cu. Structural characterization of the new Nb/Ni and Nb/Cu complexes reveals Nb−M distances shorter than any others reported to date. Computational studies show that these distances are the result of metal−metal bonding, with very weak, if any, π-bonding contributions on account of the disparate electronegativities and orbital energies of Nb and these late transition metals. In order to fully evaluate the effect of metal−metal bonding on the redox properties of both Nb and the late first row metals, a complete series of ONb(iPrNPPh2)3M-X complexes has also been synthesized (M = Fe, Co, Ni, Cu). In all of these compounds, the ONbV fragment is redox-inactive and the metal−metal distances are too long to signify any
S28−S29).24 In all three cases, a qualitative examination of the pictorial representations of the molecular orbitals reveals σoverlap between the filled dz2 orbitals of the late metal center and the empty Nb dz2 orbital. However, the σ-bonding orbital has substantially more electron density on the late metal centers than on the Nb atom, indicative of dative rather than covalent bonding between the two metals. The bonding between Nb and Ni in complex 4 can also be considered as the pairing of the lone d electron on the NbIV center with a NiIbased unpaired electron in metal−metal σ bonding MO; the resulting σ bond is more centered on the Ni atom owing to a mismatch in orbital energy between the Ni 3d and Nb 4d orbitals. In the case of Nb/Cu complex 5, the molecular orbital corresponding to the Cu dz2 orbital has essentially no contribution from the Nb atom, in line with this complex H
DOI: 10.1021/acs.inorgchem.8b02960 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
resulting red solids were extracted into benzene (8 mL) and filtered through Celite, and the filtrate was dried in vacuo. The remaining powder was extracted with benzene (10 mL) a second time, and the filtrate was again dried in vacuo. The resulting product was washed with hexanes (5 mL) to obtain 3 as a red powder (216 mg, 21%). Crystals suitable for X-ray crystallography were grown via vapor diffusion of hexanes into a concentrated toluene solution of 3 at room temperature. 1H NMR (400 MHz, C6D6): δ 15.9 (br s), 13.8 (br s), 13.4 (br s), 4.2 (br s), 0.89 (br s), −2.6 (br), −4.3 (br s), −6.6 (br s), −7.5 (br s). UV−vis [C6H6, λmax, nm (ε, L mol−1 cm−1)]: 392 (5200), 896 (270), 1114 (210), 1188 (200). μeff (C6D6) = 2.83 μB. Anal. Calcd for C45H51N3P3CoBrNb: C, 56.38; H, 5.36; N, 4.38. Found: C, 56.72; H, 5.56; N, 4.37. Cl−Nb(iPrNPPh2)3Ni−Cl (4). A solution of Li(iPrNPPh2)(OEt2) (128 mg, 0.39 mmol) in THF (2 mL) was frozen. Solid NbCl4(THF)2 (50 mg, 0.13 mmol) was added to the frozen solution in one portion. The solution was allowed to thaw but was kept near the freezing point of THF (−108.4 °C). The yellow suspension was stirred for 2 min until the color changed from bright yellow to dark green. Upon the color change, NiCl2(dme) (14 mg, 0.06 mmol) was added in one portion followed by the addition of solid Ni(COD)2 (18 mg, 0.06 mmol). The resulting red solution was allowed to warm to room temperature, stirred for 4 h, and then filtered through Celite. The solvent was evaporated from the red filtrate in vacuo. The resulting powder was extracted into benzene (10 mL) and filtered through Celite, and then the filtrate was dried in vacuo. The remaining 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 (10 mL) to obtain 4 as an orange/red powder (65 mg, 52%). Crystals suitable for X-ray crystallography were grown from a concentrated THF/Et2O mixture of 4 at −35 °C. 1H NMR (400 MHz, C6D6): δ 7.62 (br m, 12H, o-Ph), 6.89 (t, JH,H = 6.8 Hz, 6H, p-Ph), 6.74 (dd, JH,H = 7.6 Hz, 12H, m-Ph), 4.42 (br m, 3H, (CH3)2CH), 1.61 (d, JH,H = 6.4 Hz, 18H, (CH3)2CH). 13C{1H} NMR (100.6 MHz, C6D6): δ 26.1 (s, (CH3)2CH), 54.1 (s, (CH3)2CH), 128.6 (overlapping with benzene signal, Ph), 129.8 (s, Ph), 133.6 (m, o-Ph), 134.2 (m, ipso-Ph). 31P{1H} NMR (161.8 MHz, C6D6): δ −38.7 (s). UV−vis [THF, λmax, nm (ε, L mol−1 cm−1)]: 677 (640), 745 (620). Anal. Calcd For C45H51N3P3Cl2NbNi: C, 56.93; H, 5.42; N, 4.43. Found: C, 56.77; H, 5.50; N, 4.03. Cl−Nb(iPrNPPh2)3Cu−Br (5). A solution of Li(iPrNPPh2)(OEt2) (128 mg, 0.39 mmol) in THF (2 mL) was frozen. Solid NbCl4(THF)2 (50 mg, 0.13 mmol) was added to the frozen solution in one portion. The solution was allowed to thaw but was kept near the freezing point of THF (−108.4 °C). The yellow suspension was stirred for 2 min until the color changed from bright yellow to green. Upon the color change, solid CuBr (19 mg, 0.13 mmol) was added in one portion. The resulting orange solution was allowed to warm to room temperature, stirred for 16 h, and then filtered through Celite. The solvent was evaporated from the red filtrate in vacuo. The resulting powder was extracted into benzene (6 mL) and filtered through Celite, and then the filtrate was dried in vacuo. The remaining powder was extracted with benzene (6 mL) a second time, and the filtrate was again dried in vacuo. The resulting product was washed with pentane (2 mL) to obtain 5 as an orange/brown powder (110 mg, 83%). Crystals suitable for X-ray crystallography were grown by layering a THF solution of 5 with pentane at −35 °C. 1H NMR (400 MHz, C6D6): δ 10.0 (s), 8.4 (s), 6.8 (s), 5.6 (s), −9.6 (br). μef f (C6D6) = 1.40 μB. UV−vis [THF, λmax, nm (ε, L mol−1 cm−1)]: 326 (1800). Anal. Calcd for C45H51N3P3ClCuBrNb: C, 54.12; H, 5.15; N, 4.21. Found: C, 53.89; H, 5.26; N, 4.38. ONb(iPrNPPh2)3Fe−I (7). FeI2 (37 mg, 0.12 mmol) was added to a solution of 6 (100 mg, 0.12 mmol) in THF (5 mL) at room temperature. Zinc powder (4 mg, 0.06 mmol) was added as a solid. The light brown solution was stirred for 16 h and then filtered through Celite. The solvent was evaporated from the filtrate in vacuo to give a brown solid. The solid was extracted in dichloromethane (10 mL), filtered through Celite, and the solvent was evaporated from the filtrate in vacuo. The remaining solid was washed with hexanes (5 mL), and then dried in vacuo to give a brown solid. The solid was
significant metal−metal bonding. A comparison of the redox properties of the Cl−Nb(iPrNPPh2)3M−X and ONb(iPrNPPh2)3M−X complexes measured using cyclic voltammetry therefore allows (1) the redox events to be definitively assigned to each metal and (2) conclusions to be drawn about the effects of metal−metal cooperative effects on the redox potential of both metals. The redox potentials of both Nb and the late metal center in heterobimetallic complexes were found to shift anodically, even when direct metal−metal interactions are not present. This study, therefore, provides valuable insight into the tuning of redox potential via metal−metal cooperativity that has important implications in catalyst design and in understanding the principles at play in redox-active multimetallic metalloenzymes.
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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 glovebox with an atmosphere of purified nitrogen. The glovebox 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 from Pure Process Technology following literature procedures.37 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. iPrNHPPh2,38 ONb(iPrNPPh2)3 (6),24 NbCl4(THF)2,39 NiCl2(dme),40 and Li(iPrNPPh2)(OEt2)24 were prepared by literature methods. All other chemicals were purchased from Aldrich, Strem, or Alfa Aesar and used without further purification. Cl−Nb(iPrNPPh2)3Fe−Br (2). The following one-pot synthetic procedure is a significantly modified and improved method in comparison to the procedures originally reported, involving fewer steps and a higher yield.24 A solution of Li(iPrNPPh2)(OEt2) (256 mg, 0.79 mmol) in THF (2 mL) was frozen in the glovebox cold well. Solid NbCl4(THF)2 (100 mg, 0.26 mmol) was added to the frozen solution in one portion. The solution was allowed to thaw but was kept near the freezing point of THF (−108.4 °C). The thawing yellow suspension was stirred for 2 min until the color changed from bright yellow to green. Upon the color change, solid FeBr2 (57 mg, 0.26 mmol) and Zn powder (70 mg, 1.0 mmol) were added in one portion to the thawing green solution. The remaining FeBr2 and Zn powder in the vials were transferred into the reaction vessel by rinsing with thawing THF (3 mL). The solution was allowed to warm to room temperature and vigorously stirred for 4 h. The blue solution was then filtered through Celite. The solvent was evaporated from the blue filtrate in vacuo. The resulting powder was extracted into 15 mL of benzene, filtered through Celite, and the filtrate was dried in vacuo. The remaining powder was then extracted with benzene (15 mL) a second time, and the filtrate was again dried in vacuo. The resulting product was washed with pentane (5 mL) to obtain 2 as a blue powder (230 mg, 88%). The spectroscopic features of the product were identical to those reported for the compound in the literature.24 i PrNNb(μ-PPh2)(iPrNPPh 2) 2Co−Br (3). A solution of Li(iPrNPPh2)(OEt2) (1.052 g, 3.20 mmol) in THF (8 mL) was frozen in the glovebox cold well. Solid NbCl4(THF)2 (0.411 g, 1.08 mmol) was added to the frozen solution in one portion. The solution was allowed to thaw but was kept near the freezing point of THF (−108.4 °C). The yellow suspension was stirred for 2 min until the color began to change from bright yellow to green. Once the color change was observed, solid CoBr2 (0.236 g, 1.08 mmol) and Zn powder (0.44 g, 6.7 mmol) were added in one portion. The remaining CoBr2 and Zn powder in the vials were transferred into the reaction vessel by rinsing with thawing THF (3 mL). The resulting red solution was stirred for 45 min at room temperature and then filtered through Celite. The solvent was evaporated from the red filtrate in vacuo. The I
DOI: 10.1021/acs.inorgchem.8b02960 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry then extracted in benzene (5 mL), filtered through Celite, and dried in vacuo to give spectroscopically pure product as a brown powder (110 mg, 88%). Single crystals suitable for X-ray crystallography were grown from a concentrated dichloromethane solution of 7 at −35 °C. 1 H NMR (400 MHz, C6D6): δ 13.0 (br s), 9.9 (s), 3.4 (s), −8.0 (s), −16.6 (br s). μef f (CD2Cl2) = 3.90 μB. UV−vis (THF, λmax, nm (ε, L mol −1 cm −1 )): 361 (1700), 501 (340). Anal. Calcd for C45H51N3P3FeINbO•3CH2Cl2: C, 45.28; H, 4.51; N, 3.30. Found: C, 45.05; H, 4.76; N, 3.21 (Note: Complex 7 crystallizes with three molecules of CH2Cl2 per molecule of 7 in the asymmetric unit, and it appears that these molecules are not removed upon drying the crystals under vacuum). ONb(iPrNPPh2)3Co−I (8). CoI2 (37 mg, 0.12 mmol) dissolved in THF (2 mL) was added to a solution of 6 (100 mg, 0.12 mmol) in THF (5 mL) at room temperature. Zinc powder (4 mg, 0.06 mmol) was added as a solid. The brown solution was stirred for 16 h and then filtered through Celite. The solvent was evaporated from the filtrate in vacuo to give a dark brown solid. The solid was extracted in dichloromethane (10 mL), filtered through Celite, and the solvent was evaporated from the filtrate in vacuo. The remaining solid was washed with hexanes (5 mL) and then dried in vacuo to give a brown solid. The solid was then extracted in benzene (5 mL), filtered through Celite, and dried in vacuo to give a brown/blue solid (79 mg, 65%). Crystals suitable for X-ray crystallography were grown from a concentrated THF solution of 8 at −35 °C. 1H NMR (400 MHz, C6D6): δ 14.4 (br s), 0.4 (br s), −4.5 (br s), −6.1 (br s). UV−vis (THF, λmax, nm (ε, L mol−1 cm−1)): 674 (210). μef f (CD2Cl2) = 2.86 μB Anal. Calcd for C45H51N3P3CoINbO: C, 52.91; H, 5.03; N, 4.11. Found: C, 50.34; H, 5.12; N, 3.77. Repeated attempts to obtain satisfactory elemental analysis on spectroscopically pure samples of 8 resulted in low C, H, and N content, and we hypothesize that this is the result of the air and moisture sensitive nature of the complex. For example, oxidation of all three phosphines would hypothetically lead to C, 50.53; H, 4.81; N, 3.93 for C45H51N3P3CoINbO4, which matches well with the values obtained. ONb(iPrNPPh2)3Ni−Cl (9). NiCl2(dme) (13 mg, 0.06 mmol) was dissolved in THF (2 mL), and Ni(COD)2 (17 mg, 0.06 mmol) was dissolved in THF (2 mL). The NiCl2(dme) solution was added to a solution of 6 (100 mg, 0.12 mmol) in THF (7 mL), followed immediately by the addition of the Ni(COD)2 solution. The resulting brown solution was stirred for 16 h. After stirring, the solvent was evaporated in vacuo. The brown solid was washed with Et2O (2 × 10 mL) and dried. The resulting yellow-orange powder was then washed with hexanes (5 mL) and dried, affording spectroscopically pure product as a yellow-orange powder (42 mg, 38%). Crystals suitable for X-ray crystallography were grown by vapor diffusion of pentane into a concentrated solution of 9 in toluene. 1H NMR (400 MHz, C6D6): δ 11.5 (br s), 3.3 (br s), 2.6 (br s), 1.8 (br s), −0.45 (br s). μeff (C6D6) = 1.57 μB. UV−vis (THF, λmax, nm (ε, L mol−1 cm−1)): 409 (5100), 1078 (220). Anal. Calcd for C45H51N3P3NiClNbO: C, 58.12; H, 5.53; N, 4.52. Found: C, 53.17; H, 5.29; N, 3.65. Repeated attempts to obtain satisfactory elemental analysis on spectroscopically pure samples of 9 resulted in low C, H, and N content, and we hypothesize that this is the result of air and moisture sensitive nature of complex. For example, oxidation of all three phosphines would hypothetically lead to C, 55.27; H, 5.26; N, 4.30 for C45H51N3P3NiClNbO4, which is more consistent with the values obtained. ONb(iPrNPPh2)3Cu−I (10). CuI (23 mg, 0.12 mmol) dissolved in THF (2 mL) was added to a solution of 6 (100 mg, 0.12 mmol) in THF (10 mL) at room temperature. The yellow-orange solution was stirred for 16 h and then filtered through Celite. The solvent was evaporated from the filtrate in vacuo to give a light orange-brown powder. The solid was extracted in dichloromethane (10 mL), filtered through Celite, and concentrated to ∼1 mL. The vial was placed in a −35 °C freezer for 36 h. Colorless crystals were collected and dried in vacuo after decanting the solvent (45 mg, 36%). Crystals suitable for X-ray crystallography were grown from a concentrated dichloromethane solution at −35 °C. 1H NMR (400 MHz, CD2Cl2): δ 7.20− 7.35 (m, 18H, overlapping o-Ph and p-Ph), 7.02 (dd, JH−H = 7.5 Hz,
12H, m-Ph) 3.93 (m, 3H, (CH3)2CH), 1.39 (d, JH−H = 6.4 Hz, 18H, (CH3)2CH). 13C{1H} NMR (100.6 MHz, CD2Cl2): δ 136.3 (s, ipsoPh), 134.9 (s, o-Ph), 131.0 (s, p-Ph), 129.1 (s, m-Ph), 55.2 (s, (CH3)2CH), 28.5 (s, (CH3)2CH). 31P{1H} (161.8 MHz, CD2Cl2): δ −20.9 (s). Anal. Calcd for C45H51N3P3CuINbO: C, 52.67; H, 5.01; N, 4.09. Found: C, 51.06; H, 4.89; N, 3.68. Repeated attempts to obtain satisfactory elemental analysis on spectroscopically pure samples of 10 resulted in low C, H, and N content, and we hypothesize that this is the result of the air and moisture sensitive nature of the complex. For example, oxidation of two phosphines would hypothetically lead to C, 51.08; H, 4.86; N, 3.97 for C45H51N3P3CuINbO3, which matches well with the values obtained. Spectroscopic Characterization and Physical Measurements. All NMR spectra were recorded at ambient temperature unless otherwise stated on a Varian Inova 400 MHz instrument, a Varian MR 400 MHz instrument, or a Bruker DPX 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. UV−vis spectra were recorded on a Cary 5000 UV−vis spectrophotometer using Cary WinUV software. Elemental microanalyses were performed by Midwest Microlab, Indianapolis, IN. 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).41 Cyclic voltammetry measurements were carried out in a glovebox under a dinitrogen or argon 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 a Ag/AgNO3 nonaqueous reference electrode assembled in THF. Solutions (THF) of electrolyte (0.30 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. X-ray Crystallography Procedures. All operations were performed on either a Bruker-Nonius Kappa Apex2 diffractometer or a Bruker D8 Venture PHOTON II CPAD system, using graphitemonochromated Mo Kα radiation. All diffractometer manipulations, including data collection, integration, scaling, and absorption corrections were carried out using the Bruker Apex2 or Apex3 software. Fully labeled diagrams and full data collection and refinement details are included in the ESI. Computational Details. All calculations were performed using Gaussian09, Revision A.02 for the Linux operating system.42 Density functional theory calculations were carried out using a combination of Becke’s 1988 gradient-corrected exchange functional43 and Perdew’s 1986 electron correlation functional44 (BP86). A mixed-basis set was employed, using the LANL2TZ(f) triple-ζ basis set with effective core potentials for niobium, nickel, and copper,45 the LANL2DZ(p,d) double-ζ basis set and effective core potentials for bromine,45a,46 Gaussian09’s internal 6-311+G(d) for nitrogen, phosphorus, and chlorine, and Gaussian09’s internal LANL2DZ basis set (equivalent to D95V47) 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 ESI.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b02960. 1 H and 31P NMR data for complexes 3−5 and 7−10, cyclic voltammetry data for complexes 2 and 4−10, crystallographic data collection and refinement details for 3−5 and 7−10, additional computational details and
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DOI: 10.1021/acs.inorgchem.8b02960 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
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