Exploring Trends in Metal–Metal Bonding, Spectroscopic Properties

Article pubs.acs.org/IC

Exploring Trends in Metal−Metal Bonding, Spectroscopic Properties, and Conformational Flexibility in a Series of Heterobimetallic Ti/M and V/M Complexes (M = Fe, Co, Ni, and Cu) Bing Wu,† Matthew J. T. Wilding,‡ Subramaniam Kuppuswamy,† 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: To understand the metal−metal bonding and conformational flexibility of first-row transition metal heterobimetallic complexes, a series of heterobimetallic Ti/M and V/ M complexes (M = Fe, Co, Ni, and Cu) have been investigated. The titanium tris(phosphinoamide) precursors ClTi(XylNPiPr2)3 (1) and Ti(XylNPiPr2)3 (2) have been used to synthesize Ti/Fe (3), Ti/Ni (4, 4THF), and Ti/Cu (5) heterobimetallic complexes. A series of V/M (M = Fe (7), Co (8), Ni (9), and Cu (10)) complexes have been generated starting from the vanadium tris(phosphinoamide) precursor V(XylNPiPr2)3 (6). The new heterobimetallic complexes were characterized and studied by NMR spectroscopy, X-ray crystallography, electron paramagnetic resonance, and Mössbauer spectroscopy, where applicable, and computational methods (DFT). Compounds 3, 4THF, 7, and 8 are C3-symmetric with three bridging phosphinoamide ligands, while compounds 9 and 10 adopt an asymmetric geometry with two bridging phosphinoamides and one phosphinoamide ligand bound η2 to vanadium. Compounds 4 and 5, on the other hand, are asymmetric in the solid state but show evidence for fluxional behavior in solution. A correlation is established between conformational flexibility and metal−metal bond order, which has important implications for the future reactivity of these and other heterobimetallic molecules.

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Cu)5b heterobimetallic complexes featuring M−M multiple bonds (Figure 1). In addition, we have recently found that a bis(phosphinoamide) ligand system also supports Ti/Co multiple bonding, affording more coordinatively unsaturated complexes that may be better poised for reactivity.5c For example, two Ti−Co triply bonded complexes, tris(phosphinoamide) (THF)Ti(XylNP i Pr 2 ) 3 Co 6 and bis(phosphinoamide) ClTi(XylNPiPr2)2CoPMe3 (Xyl = 3,5dimethylphenyl),5c have nearly identical Ti−Co distances of 2.02 Å, and theoretical investigations indicated Ti−Co triple bonds with (σ)2(π)4(Conb)4 electronic configurations (Figure 1). Lu and co-workers have reported systematic studies of the effects of metal variations in a series of V/M (M = Fe, Co, Ni) and Cr/M (M = Cr, Mn, Fe) bimetallic complexes.7 However, the use of phosphinoamide ligands with different nitrogen and phosphorus substituents in our previous studies has complicated the comparison of complexes featuring different metal− metal combinations. Herein, we synthesize a series of Ti/M and V/M (M = Fe, Co, Ni, Cu) heterobimetallic complexes

INTRODUCTION The field of multiple bonding between transition metals has continued to grow as researchers strive to understand the nature of metal−metal interactions and how to best take advantage of their properties to promote reactivity.1 However, multiple bonding between two different transition metals has been explored to a far lesser extent.2 Our group and others specifically targeted trigonal systems featuring metal−metal bonds between two different transition metals.2b−d,3 The Thomas group’s work in this area began with the synthesis and reactivity of early/late heterobimetallic Zr/Co complexes linked by phosphinoamide ligands.2c,4 Systematic investigations of metal−metal multiple bonding between varying metal combinations have subsequently been carried out using phosphinoamide ligands of the general form [R′NPR2]−, mainly focusing on first-row transition metals.5 Similarly, the Lu group has been exploring metal−metal bonds between firstrow transition metals using heptadentate “double-decker” ligands in which the three bridging ligands are tethered to an amine that binds in the apical coordination site of the early transition metal.2b Using tris(phosphinoamide) ligand frameworks, we have reported Zr/Co,4b Ti/Co,6 V/Fe,5a and Cr/M (M = Fe, Co, © XXXX American Chemical Society

Received: June 28, 2016

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DOI: 10.1021/acs.inorgchem.6b01543 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

solution magnetic moment of complex 3 indicates an S = 1 (μeff = 3.05 μB) ground state. Addition of TiIV precursor 1 to stoichiometric Ni(COD)2 (COD = 1,5-cyclooctadiene) resulted in a color change from orange to dark brown, yielding diamagnetic (η2-iPr2PNXyl)Ti(XylNPiPr2)2NiCl (4, Scheme 2). In contrast to 3, complex 4 adopts an asymmetric geometry with one of the phosphinoamide ligands bound η2 to the titanium center in the solid state (vide inf ra), which was also observed in the previously reported (η2-iPr2PNXyl)Ti(XylNPiPr2)2CoI complex.6 The reaction between 1 and one equivalent of CuI generates a dark red diamagnetic product, (η2-iPr2PNXyl)Ti(XylNPiPr2)2(μ-Cl)CuI (5, Scheme 2). Similar to 4, the solid-state geometry of complex 5 features two bridging phosphinoamide ligands and one phosphinoamide ligand bound η2 to the titanium center, with an additional chloride anion bridging between Ti and Cu (vide inf ra). The 1H NMR spectra of diamagnetic complexes 4 and 5 in C6D6 display five or six very broad resonances between 0 and 7.5 ppm, and their 13C{1H} NMR spectra show eight broadened signals, suggesting C3-symmetric geometries for both complexes in solution on the time scale of these experiments. However, two broad yet distinct resonances were observed in the room-temperature 31P{1H} NMR spectra for both 4 (δ 25.7, 3.41) and 5 (δ 15.9, −5), which is more consistent with asymmetric structures. This suggests that the phosphinoamide ligands are rapidly exchanging on the 1H NMR time scale in solution via reversible phosphine dissociation, resulting in coalescence behavior. The differences in symmetry information presented by the 1H and 31P NMR data can be explained by the difference between the 1H NMR and 31P NMR spectroscopic time scales.8 Due to the larger difference between the two 31P NMR resonance frequencies in 4 and 5, the ligand exchange rate is too slow to observe by 31P NMR spectroscopy but fast enough to show averaged signals by 1 H and 13C NMR spectroscopy at room temperature. Variable-temperature NMR studies of 4 and 5 from −60 to +60 °C were undertaken to further explore the fluxional processes. The 31P NMR spectrum of 5 sharpens as the temperature decreases, resulting in a 31P NMR spectrum with two peaks in a 2:1 integral ratio (Figure S15). The 1H NMR spectrum broadens even further as the temperature is initially decreased, and the peaks coalesce around 5 to −15 °C before re-emerging as a sharp spectrum consistent with a Cs-symmetric molecule at −55 °C (Figures S16 and S17). These data confirm that at low temperature complex 5 adopts an asymmetric structure with two distinct phosphinoamide environments, consistent with the structure in the solid state. As the temperature is increased above room temperature, the two signals in the 31P NMR of 5 coalesce and begin to re-merge as a single broad resonance at 60 °C. The 1H NMR spectrum of 5 sharpens considerably as the temperature is raised, resulting in well-resolved resonances consistent with a C3-symmetric molecule at 60 °C. Thus, an asymmetric structure is favored for complex 5, but we hypothesize that the phosphinoamide ligands are rapidly exchanging via an associative pathway that involves a C3-symmetric intermediate. From the 31P NMR coalescence temperature, the activation energy of this fluxional process can be estimated to be 10 kcal/mol. The variable-temperature NMR spectra for complex 4 bear similarities to 5, with higher coalescence temperatures indicative of a somewhat higher barrier to phosphine exchange (∼13 kcal/mol based on 31P coalescence temperature, Figures

Figure 1. Examples of heterobimetallic Ti/Co5c,6 and V/Fe5a complexes featuring metal−metal multiple bonds. Formal shortness ratio (FSR) is the ratio of the metal−metal interatomic distance to the sum of the single-bond atomic radii of the two metal ions.1a

supported by the same phosphinoamide ligand, [XylNPiPr2]−, and systematically study trends in bonding and electronic structure as metal identity is varied. In addition, we find that the conformational flexibility of the tris(phosphinoamide) ligand framework is tied directly to the strength of the metal−metal interaction. Understanding the driving force for ligand hemilability is an important step in designing reactive heterobimetallic molecules, since we have found that many of the most novel reactions performed by our Zr/Co heterobimetallic complexes involve ligand dissociation to expose a substrate binding site between the two transition metals.4d,f,g

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RESULTS Synthesis and Spectroscopic Characterization of 3− 10. Our initial effort to synthesize a heterobimetallic Ti/Fe complex from a TiIV precursor was unsuccessful; however, we found that a more electron rich TiIII precursor allows the second metalation. As previously reported, the tris(phosphinoamide) titanium(IV) precursor ClTi(XylNPiPr2)3 (1) can be synthesized via salt metathesis from TiCl4 and Li[XylNPiPr2], which is generated in situ via deprotonation of XylNHPiPr2 with nBuLi.6 The TiIV complex 1 can be reduced in situ with excess nBuLi to generate TiIII(XylNPiPr2)3 (2).5c Treatment of 2 with stoichiometric FeBr2 and KC8 resulted in formation of brown paramagnetic Ti(XylNPiPr2)3FeBr (3, Scheme 1). The 1H NMR spectrum of 3 displays four distinct, broad, and paramagnetically shifted resonances between 6 and −4 ppm, suggesting a C3-symmetric structure (isopropyl methine protons are presumably too broad to observe). The Scheme 1

B

DOI: 10.1021/acs.inorgchem.6b01543 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 2

Scheme 3

metric isomer (XylNPiPr2)Ti(XylNPiPr2)2NiCl and the C3s ym m e t r i c i s o m e r s Cl T i ( X y l N P i Pr 2 ) 3 Ni a nd T i(XylNPiPr2)3NiCl were all predicted to be similar in energy (within 2 kcal/mol) and have similar metal−metal bonding interactions (see Table S5 and Figures S35−S38). Interestingly, when complex 4 is dissolved in THF, a color change from dark brown to red is observed. The 1H, 13C, and 31 P NMR resonances of the new species, (THF)Ti(XylNPiPr2)3NiCl (4THF), in THF-d8 are sharp and indicative of a C3-symmetric species. However, when the THF solvent is removed in vacuo and the compound is redissolved in C6D6, the NMR features indicate that 4 is regenerated. The solid-state molecular structure of 4THF was confirmed crystallographically (vide inf ra). Notably, similar solvent-dependent NMR behavior was observed for the analogous Ti/Co compound (η2-iPr2PNXyl)Ti(XylNPiPr2)2CoI, but in this case the paramagnetism of the compound complicated analysis of the spectroscopic data.6 For comparative purposes, a series of V/M complexes was also targeted on the same phosphinoamide ligand platform. A vanadium tris(phosphinoamide) precursor V(XylNPiPr2)3 (6) was synthesized in 84% yield by the addition of VCl3(THF)3 to three equivalents of Li[XylNPiPr2] (generated in situ) in diethyl ether. As an S = 1 complex (μeff = 2.81 μB), 6 was found to have four paramagnetically shifted, broad signals in its 1H NMR spectrum. Complex 6 is a useful synthon for the generation of

S6 and S7). The 1H NMR spectrum of 4 becomes sharp and consistent with a Cs-symmetric structure by −35 °C. However, as the temperature is decreased and the two 31P NMR resonances sharpen, an additional sharp signal appears at 15 ppm and continues to grow in until it comprises ∼30% of the mixture at −35 °C. The appearance of this C3-symmetric species is also apparent in the low-temperature 1H NMR spectra, and in the well-resolved spectrum at −35 °C all of the signals for this species can be tentatively assigned (Figure S8). While there is not enough evidence to conclusively confirm the identity of the C3-symmetric species, we hypothesize that this species could result from the migration of the chloride ligand from nickel to titanium, as shown in Scheme 2, either by a pathway involving Cl− dissociation or an associative process involving a μ-Cl intermediate to the more electrophilic titanium center. The resulting “ClTi(XylNPiPr2)3Ni” isomer differs from the proposed phosphine-exchange intermediate Ti(XylNPiPr2)3NiCl in that the latter compound is zwitterionic. Nonetheless, it can be concluded that the interconversion between this C3-symmetric species and the other isomers of 4 is slow since a sharp 31 P NMR resonance for “ClTi(XylNPiPr2)3Ni” can be observed before the resonances associated with 4 begin to sharpen. Since further experimental confirmation of the fluxional processes at play was not possible, the relative energies of the possible isomers were calculated using density functional theory (DFT). Notably, the asymC

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Figure 2. Displacement ellipsoid (50%) representations of 3, 4, 4THF, and 5. For clarity, all hydrogen atoms and solvate molecules are omitted. Only one of the two independent molecules in the unit cell of 4 is shown.

and a terminal Fe-bromide (Figure 2). Considering the differences in radii among transition metals, the ratio of the metal−metal interatomic distance to the sum of the single-bond atomic radii of the two metal ions (referred to as the “formal shortness ratio”, FSR) is used to normalize the metal−metal bonding between different metals.1a Among the first-row heterobimetallic complexes that have been characterized, a metal−metal double bond usually corresponds to an FSR value close to 0.9, and a metal−metal triple bond usually has an FSR value close to 0.8.2b The Ti−Fe intermetallic distance in 3 is 2.2212(7) Å and corresponds to an FSR of 0.89, suggesting metal−metal double bonding. The intermetallic distance of 3 is longer than that of Lu’s Ti/Fe complex FeTi(N(o(NCH2PiPr2)C6H4)3) (2.0635(6) Å, FSR = 0.83).2b However, considering the difference in overall d-electron count between these two compounds, this is not a direct comparison. The solid-state structure of heterobimetallic Ti/Ni complex 4 was determined and is shown in Figure 2. Despite the fluxionality observed in solution at room temperature, in the solid state, one of the phosphinoamide ligands is bound η2 to the Ti center and two phosphinoamide ligands bridge Ti and Ni. The Ti−Ni distances in the two molecules of 4 in the asymmetric unit are 2.2530(7) and 2.2533(7) Å. The FSR value of 4 (0.91) is similar to that of 3 (0.89), indicating multiple bonding between Ti and Ni. A structure of the THF-bound C3symmetric complex 4THF was also obtained (Figure 2), revealing a slightly shorter Ti−Ni distance of 2.2182(3) Å (FSR = 0.90). A C3-symmetric S = 1/2 Ti−Ni complex, NiTi(N(o-(NCH2PiPr2)C6H4)3), reported by the Lu group features a much longer Ti−Ni distance (2.4118(7) Å, FSR = 0.98) and a Ti−Ni single bond, but this compound features an additional electron that likely populates a metal−metal antibonding orbital.2b Tonks and co-workers synthesized a series of (κ2-NP)M(μ2-NP)3Ni complexes (NP = 2-diphenylphosphinopyrrolide, M = Ti, Zr, or Hf) with three bridging phosphinopyrrolide ligands, and their d10 Ti−Ni complex also possesses a short Ti−Ni bond distance: Ti−Ni 2.2665(5) Å (FSR = 0.91).3b A single crystal of Ti/Cu complex 5 was examined using Xray diffraction. Complex 5 features two bridging phosphinoamide ligands, one phosphinoamide ligand bound η2 to the Ti center, a terminal Cu-bound iodide ligand, and one chloride ligand bridging the two metals. The metal−metal interatomic distance of complex 5 (2.9419(5) Å, FSR = 1.18) is much longer than those of 3 and 4, which suggests no significant interaction between the two metal centers. The absence of an

V/M heterobimetallic complexes. Addition of stoichiometric FeI2 to precursor 6 in the presence of excess Zn powder as an external reductant led to the formation of V(XylNPiPr2)3FeI (7, Scheme 3). The 1H NMR spectrum of 7 displays six welldefined paramagnetically shifted resonances, consistent with a 3-fold symmetric complex, and the solution magnetic moment of 7 (μeff = 1.82 μB) indicates an S = 1/2 ground state. In a similar procedure, metalation of 6 with cobalt was achieved by treating the metalloligand 6 with one equivalent of CoI2 and excess Zn powder. The 1H NMR spectrum of the resulting heterobimetallic V/Co complex (THF)V(XylNPiPr2)3CoI (8, Scheme 3) is broad and exhibits seven paramagnetically shifted resonances with chemical shifts between 53 and −1 ppm. The number of 1H NMR resonances observed for 8 was initially puzzling and suggestive of a more asymmetric structure than complex 7, but the solid-state structure of 8 revealed a THF molecule bound to V, which accounts for the additional resonances. An S = 1 (μeff = 2.84 μB) ground state was confirmed by solution NMR methods. The brown V/Ni complex (η2-iPr2PNXyl)V(XylNPiPr2)2NiI (9) can be synthesized upon treatment of an equimolar ratio of vanadium precursor 6 and NiI2 in the presence of excess Zn powder (Scheme 3). The 1H NMR spectrum of complex 9 shows 10 paramagnetically shifted resonances, indicating an asymmetric structure analogous to the Ti/Ni complex 4. The solution magnetic moment of 9 (μeff = 1.82 μB) suggests an S = 1/2 ground state. Lastly, a V/Cu complex can be synthesized by treating precursor 6 with one equivalent of CuI to afford the blue paramagnetic product (η2-iPr2PNXyl)V(XylNPiPr2)2CuI (10). Complex 10 maintains the S = 1 ground state of monometallic vanadium complex 6, with a solution magnetic moment μeff = 2.78 μB. Much like 9, complex 10 also adopts an asymmetric structure (vide inf ra). Only eight paramagnetically shifted signals were observed in the 1H NMR spectrum of 10, possibly owing to several peaks that are too broad to observe or a fluxional ligand exchange process in solution. X-ray Crystallography of Complexes 2−10. Complexes 2−10 were characterized in the solid state using single-crystal X-ray diffraction. The solid-state structures of monometallic titanium(III) and vanadium(III) complexes 2 and 6 were confirmed crystallographically (Figures S23 and S28). Both structures are Cs-symmetric, with one of the phosphinoamide ligands rotating 180° with respect to the other two ligands. The solid-state structure of 3 features a C3-symmetric core with three bridging phosphinoamide ligands between Ti and Fe D

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the two molecules are 2.7347(8) Å (FSR = 1.14) and 2.7089(8) Å (FSR = 1.13), which are also too long to indicate significant metal−metal interactions.

interaction between Ti and Cu may originate from the large energy difference between the Ti and Cu atomic orbitals and the ligand geometry. The V/Fe complex 7 is C3-symmetric in the solid state with a trigonal planar geometry at the V center and a distorted tetrahedral geometry at Fe if the metal−metal bond is not taken into consideration (Figure 3). The V−Fe bond distances in the two molecules of complex 7 in the asymmetric unit are 2.0186(7) and 2.0195(7) Å, corresponding to FSR values of 0.84. The V−Fe bond distance is shorter than that of the analogous V−Fe complex, V(iPrNPPh2)3FeI (2.0745(5) and 2.0637(5) Å), that was previously reported by our group and assigned a V−Fe triple bond.5a The stronger interaction between V and Fe in complex 7 is the result of the more electron donating phosphine substituents, which render the iron center more electron rich. The V/Fe complex [FeV(N(o(NCH2PiPr2)C6H4)3)]BPh4 reported by the Lu group also adopts an S = 1/2 ground state, and the V−Fe bond length of 1.9791(6) Å (FSR = 0.83) in this complex is slightly shorter than that in 7.7a The better overlap between V and Fe in Lu’s complex may be attributed to the absence of an axial ligand on Fe in Lu’s complex and the ligand geometry, which allows the metals to arrange in closer proximity. In contrast to 7, the C3symmetric V/Co complex 8 adopts a trigonal bipyramidal geometry at the V center with a THF molecule bound in the axial coordination site (Figure 3). The V−Co bond length of 2.6934(3) Å (FSR = 1.12) in 8 is indicative of a weak interaction, if any, between the two metals and is significantly longer than that in the diamagnetic complex [CoV(N(o(NCH2PiPr2)C6H4)3)]BPh4 (1.9930(11) Å, FSR = 0.84) that was reported by the Lu group.7a

Figure 4. Displacement ellipsoid (50%) representations of 9 and 10. For clarity, all hydrogen atoms and solvate molecules are omitted. Only one of two independent molecules in the unit cell of 9 is shown.

Mössbauer Spectroscopy. 57Fe Mössbauer spectroscopy can be used to gain insight into the electronic structure of heterobimetallic complexes containing iron. Mössbauer parameters, particularly isomer shift (δ), are often viewed as unambiguous indicators of oxidation state, but since δ is a measure of the electron density at Fe, the correlation between oxidation number and δ breaks down in complexes with more covalent metal−ligand interactions such as those of low-valent iron.11 We, and others, have demonstrated that metal−metal interactions indeed result in isomer shifts that fall outside the normal ranges expected for low oxidation state Fe complexes.2b,5b With Mössbauer data for a series of related compounds in hand, we are now in an excellent position to draw some firmer conclusions about the factors that lead to increases and decreases in isomer shift. The zero-field 57Fe Mössbauer spectrum collected for Ti/Fe complex 3 features a single quadrupole doublet centered at δ = 0.51 mm/s with very small quadrupole splitting (|ΔEQ| = 0.17 mm/s, Figure 5, Table 2). Since we previously reported the Mössbauer spectrum of V(iPrNPPh2)3FeI,5a Mössbauer data were not collected for the analogous V/Fe complex 7, as it was expected that the parameters would not differ significantly with ligand substituents. V(iPrNPPh2)3FeI was found to have an isomer shift of δ = 0.33 mm/s and quadrupole splitting of |ΔEQ| = 2.01 mm/s (Table 2). For comparison, we also include the Mössbauer parameters for the reduced VIII/Fe0 complex V(iPrNPPh2)3FePMe3 and Lu’s Ti/Fe complex FeTi(N(o(NCH2PiPr2)C6H4)3) in Table 2.2b,5a The isomer shift of 3 is noticeably higher than the other compounds in Table 2 and is the highest isomer shift reported thus far among early/late heterobimetallic compounds with M−Fe multiple bonds. The comparison of 3 to V(iPrNPPh2)3FeI is particularly enlightening, since Fe is in a similar coordination environment and oxidation state, FeI, in these compounds. The key difference between the two compounds is the longer M−Fe distance in 3 (2.22 vs 2.07 Å), leading to the conclusion that stronger metal−metal interactions decrease the Fe d electron density, resulting in diminished shielding and more effective s electron density, therefore decreasing the isomer shift in compounds with stronger metal−metal bonding. This is also apparent when comparing Lu’s FeTi(N(o-(NCH2PiPr2)C6H4)3) and FeV(N(o-

Figure 3. Displacement ellipsoid (50%) representations of 7 and 8. For clarity, all hydrogen atoms and solvate molecules are omitted. Only one of two independent molecules in the unit cell of 7 is shown.

Both heterobimetallic V−Ni complex 9 and V−Cu complex 10 adopt asymmetric structures in the solid state, in which two phosphinoamide ligands bridge between V and Ni, while the third ligand is bound in a η2 fashion to the V center, with an iodide anion terminally bound to the late metal (Figure 4). The interatomic distance between V and Ni in complex 9 is 2.6417(17) Å, corresponding to an FSR value of 1.11 and indicating a weak or negligible metal−metal interaction in this complex. The C 3 -symmetric V/Ni complex NiV(N(o(NCH2PiPr2)C6H4)3) has an S = 1 ground state and a shorter V−Ni bond distance of 2.4873(14) Å (FSR = 1.05) and was described as a dative Ni(0)→V(III) bonded complex. The solid-state structure of complex 10 features two independent molecules in the asymmetric unit, and the V−Cu distances in E

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Inorganic Chemistry Table 2. Comparison of Mössbauer Parameters of Complexes 3 with Closely Related Complexes complex

δ (mm/s)

ΔEQ (mm/s)

Ti(XylNPiPr2)3FeBr (3) FeTi(N(o-(NCH2PiPr2)C6H4)3)2b V(iPrNPPh2)3FeI5a V(iPrNPPh2)3FePMe35a [FeV(N(o-(NCH2PiPr2)C6H4)3)]BPh47a FeV(N(o-(NCH2PiPr2)C6H4)3)7a

0.51 0.35 0.33 0.19 0.25 0.25

0.17 2.01 2.01 1.85 5.97 4.04

at the Fe centers along with differences in oxidation state and metal−metal distance. The differences in the quadrupole splitting parameters between d8 complex 3 and the d9 V/Fe and Ti/Fe complexes may be attributed to the differences in electronic distribution resulting from the different d electron count: The two Fe-based nonbonding orbitals are both singly occupied in 3 (vide inf ra), while the d9 configuration results in the asymmetric occupation of two Fenb orbitals by introducing one more electron in V(iPrNPPh2)3FeI and FeTi(N(o-(NCH2PiPr2)C6H4)3). Electron Paramagnetic Resonance (EPR) Spectroscopy. The S = 1/2 spin states of 7 and 9 were confirmed by Xband EPR spectroscopy (Figure 6). A nearly axial signal that can be simulated with g values of 2.20, 2.09, and 2.08 (Figure 6a) was observed for V/Fe complex 7. The gav of 2.12 is typical for an Fe-based spin, and the absence of large hyperfine splitting is a good indication of an Fe-based SOMO without significant electron density on V (I = 7/2). Electronic structure calculations confirm this hypothesis (vide inf ra). The EPR spectrum of the V/Ni complex 9 is rhombic with g = 2.00, 1.95, and 1.92. Simulation of the spectrum with hyperfine coupling to the 51V nucleus (I = 7/2, A = 37, 253, and 237 MHz) accurately reproduced the distribution of hyperfine lines (Figure 6b). The average g value (gav = 1.96) below 2.00 and the large hyperfine coupling to the 51V nucleus are consistent with a vanadium-localized spin population.12 A theoretical

Figure 5. Solid-state Mössbauer spectrum of 3 at 90 K (black dotted lines). The spectrum of 3 was fit (red) satisfactorily by including a 5% impurity (δ = 0.8 mm/s; |ΔEQ| = 2.57 mm/s), with the following parameters corresponding to complex 3: δ = 0.51 mm/s; |ΔEQ| = 0.17 mm/s.

(NCH2PiPr2)C6H4)3) compounds, where the V/Fe complex has a shorter metal−metal distance and a lower isomer shift.2b,7a The effect of the metal−metal interaction on isomer shift appears to be sufficient to overpower the effect of metal oxidation state. For example, when Lu’s VIII/Fe0 complex FeV(N(o-(NCH2PiPr2)C6H4)3) is oxidized to the VIII/FeI complex [FeV(N(o-(NCH2PiPr2)C6H4)3)]+, the isomer shift does not decrease as might be expected but rather remains constant. This phenomenon can be attributed to the increase in the metal−metal bond distance upon oxidation, which decreases the Fe s electron density and offsets the effects of diminished shielding by the less populated d shell. Meaningful comparisons between the Lu and Thomas compounds are complicated by the differences in primary coordination spheres

Table 1. Metal−Metal Distances, Spin States, and FSRsa of Complexes 3−5 and 7−10 and a Selection of Similar Heterobimetallic Ti/M and V/M Complexes (M = Fe, Co, Ni)

a

compound

core

d e− count

dM−M′ (Å)

FSR

S

MBO9

Ti(XylNPiPr2)3FeBr (3) FeTi(N(o-(NCH2PiPr2)C6H4)3)2b (η2-iPr2PNXyl)Ti(XylNPiPr2)2CoI6 CoTi(N(o-(NCH2PiPr2)C6H4)3)10 (THF)Ti(XylNPiPr2)3CoN26 (THF)Ti(XylNPiPr2)3Co6 (η2-iPr2PNXyl)Ti(XylNPiPr2)2NiCl (4) (THF)Ti(XylNPiPr2)3NiCl (4THF) (κ2-NP)Ti(μ2-NP)3Ni3b NiTi(N(o-(NCH2PiPr2)C6H4)3)2b (η2-iPr2PNXyl)Ti(XylNPiPr2)2(μ-Cl)CuI (5) V(XylNPiPr2)3FeI (7) V(iPrNPPh2)3FeI5a [FeV(N(o-(NCH2PiPr2)C6H4)3)]BPh47a (THF)V(XylNPiPr2)3CoI (8) [CoV(N(o-(NCH2PiPr2)C6H4)3)]BPh47a (η2-iPr2PNXyl)V(XylNPiPr2)2NiI (9) [NiV(N(o-(NCH2PiPr2)C6H4)3)]7a (η2-iPr2PNXyl)V(XylNPiPr2)2CuI (10)

(TiFe)4+ (TiFe)3+ (TiCo)4+ (TiCo)3+ (TiCo)3+ (TiCo)3+ (TiNi)4+ (TiNi)4+ (TiNi)4+ (TiNi)3+ (TiCu)5+ (VFe)4+ (VFe)4+ (VFe)4+ (VCo)4+ (VCo)4+ (VNi)4+ (VNi)3+ (VCu)4+

8 9 9 10 10 10 10 10 10 11 10 9 9 9 10 10 11 12 12

2.2212(7) 2.0635(6) 2.2735(8)/ 2.2734(8) 2.1979(8) 2.2371(3)/ 2.2268(3) 2.0252(5)/2.0271(5) 2.2533(7)/ 2.2530(7) 2.2182(3) 2.2665(5) 2.4118(7) 2.9419(5) 2.0186(7)/ 2.0195(7) 2.0745(5)/ 2.0637(5) 1.9791(6) 2.6934(3) 1.9930(11) 2.6417(17) 2.4873(14) 2.7347(8)/ 2.7089(8)

0.89 0.83 0.92 0.89 0.90 0.81 0.91 0.90 0.91 0.98 1.18 0.84 0.86 0.83 1.12 0.84 1.11 1.05 1.13

1 1/2 1/2 0 0 0 0 0 0 1/2 0 1/2 1/2 1/2 1 0 1/2 1 1

2.13

1.36 1.22

0.25 2.62b

0.47 1.15 0.52

FSR = DM−M′/(R1M + R1M′).1a F

DOI: 10.1021/acs.inorgchem.6b01543 Inorg. Chem. XXXX, XXX, XXX−XXX

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metal−metal bonding predicted based on the computed frontier molecular orbital (MO) diagrams and Mayer bond orders (MBOs)9 correlate very well with the trends in metal− metal interatomic distance in the solid-state structures (Table 1). Detailed molecular orbital diagrams of 3−5 and 7−10 with pictorial representation of the frontier molecular orbitals are shown in Figures S34−S48, and qualitative representations of the trends in electronic configuration across the series of complexes are shown in Figures 7−9. Due to the large differences in atomic orbital energies between the two different metals involved, there is no significant δ-bonding overlap in any of these Ti/M or V/M heterobimetallic complexes, and the existing π interactions are highly polarized. For the C3-symmetric M/Fe complexes 3, 4THF, and 7, the calculated MO diagrams predict both σ and π overlap. The Ti− Fe complex adopts a (σ)2(π)4(Fenb)2 configuration; however, the π overlap between the Ti and Fe dxz and dyz orbitals is relatively weak and highly polarized and the orbitals are substantially more localized on Fe, which results in a Ti−Fe bond order closer to 2 for complex 3. The calculated MBO of 2.13 also agrees with a Ti−Fe double bond. Despite the larger difference in atomic number of Ti and Ni, the metal−metal interactions in complex 4THF were predicted to be quite similar to those of 3, consistent with the similar metal−metal distances observed in the solid state. Although the (σ)2(π)4(Ninb)4 electron configuration would predict a formal bond order of 3, the metal−metal distance and Mayer bond order are more consistent with a double bond as the result of the localized nature of the metal−metal π interactions. Owing to the more well-matched orbital energies of V and Fe, the (σ)2(π)4(Fenb)3 configuration for complex 7 gives rise to a V−Fe triple bond order (MBO = 2.62), a much shorter metal−metal distance, and a smaller FSR. Consistent with the absence of discernible hyperfine coupling to 51V in the EPR spectrum of 7, the SOMO of 7 is predicted to be an Fe-localized orbital that is nonbonding with respect to V. We previously reported the Ti/Co complexes (THF)Ti(XylNPiPr2)3CoN2 and (THF)Ti(XylNPiPr2)3Co, which have short Ti−Co distances (2.23 and 2.03 Å; FSR = 0.90 and 0.81, respectively) and were assigned Ti−Co double and triple bonds.6 The frontier molecular orbital diagrams and metal−

Figure 6. Experimental (black) and simulated (red) X-band EPR spectra for 7 (a) and 9 (b) in toluene glass obtained at 3 K (frequency = 9.38 GHz, modulation to 10 G, power = 0.6325 mW). Simulation parameters: for 7, g = 2.20, 2.09, and 2.08; for 9, g = 2.00, 1.95, and 1.92; A(51V) = 37, 253, and 237 MHz. See the SI for more details.

investigation of complex 9 indicates that the singly occupied molecular orbital is a V−Ni π* orbital, which is more localized on the V site (vide inf ra). Computational Investigations. To better understand the electronic structures and metal−metal interactions in heterobimetallic complexes 3−5 and 7−10, computational investigations were carried out using DFT. The computed metal− metal and metal−ligand distances are similar to those determined crystallographically (see Table S3), and the

Figure 7. Correlation of the frontier molecular orbital diagrams of complexes 3, 4THF, and 7 and related Ti/Co complexes.6 Note: Bond orders depicted are based on calculated Mayer bond orders and are generally lower than the formal bond orders that would be predicted based on the MO diagram as a result of the polar and localized nature of the π bonds. G

DOI: 10.1021/acs.inorgchem.6b01543 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry metal bond orders of 3, 7, and 4THF were found to be largely similar to those of the Ti/Co complexes (Figure 7), with a substantial shortening of the metal−metal distance and increase in metal−metal bond order only in the absence of an axial ligand in (THF)Ti(XylNPiPr2)3Co. Since V and Co are more similar in electronegativity, the S = 1 spin state and long V−Co distance in the d10 analogue 8 were initially surprising (2.6934(3) Å, FSR = 1.12). Examination of the frontier molecular orbital diagram of 8 (Figures 8 and S41) reveals the

unexpected electronic structure of 8, a series of DFT calculations were performed on hypothetical derivatives of 8 in different spin states and in which either the V-bound THF molecule or Co-bound iodide have been removed. Notably, while the triplet state is preferred for complex 8, the singlet states of both V(XylNP i Pr 2 ) 3 CoI and [(THF)V(XylNPiPr2)3Co]+ were predicted to be 4−5 kcal/mol lower in energy than the triplet configuration (Table S6). Both V(XylNPiPr2)3CoI and [(THF)V(XylNPiPr2)3Co]+ are predicted to have much shorter V−Co distances (1.99 and 1.98 Å) as a result of a (σ)2(π)4(Conb)4 configuration similar to the d10 Ti/Co and Ti/Ni complexes (THF)Ti(XylNPiPr2)3CoN2 and 4THF and Lu’s V/Co complex [CoV(N(o-(NCH2PiPr2)C6H4)3)]BPh4 (Figure 8).7a Thus, it can be concluded that the origin of the anomalous long V−Co interaction in 8 is related to the triplet ground state, which is favored by the binding of both axial ligands. Indeed, the higher energy singlet variant of 8 is predicted to have a much shorter V−Co distance (2.04 Å) and an MBO of 2.31 (see Table S6 and Figure S42). The asymmetric structures of M/Ni complexes 4 and 9 break the degeneracy of the nonbonding orbitals on both metals in the heterobimetallic complexes (Figure 9). Ti/Ni complex 4 adopts a (Ninb)2(σ)2(π)4(Ninb)2 configuration with a σ-bonding interaction as well as two weak π interactions for an overall MBO of 1.38. Given the synthetic procedure used to make 4, we assign this compound as a zwitterionic TiIV/Ni0 complex, so electrostatic interactions may also account for the short Ti−Ni distance in 4. Examination of the electronic configuration of V/ Ni complex 9 reveals that, in contrast to 4, the metal−metal πbonding orbitals of 9 are slightly lower in energy than the σbonding orbital. More consequentially, the π* orbitals drop lower in energy than one of the Ni nonbonding orbitals, and population of the two π* orbitals in this (π)4(σ)2(Ninb)2(π*)3 electronic configuration results in a decreased formal bond order of 1.5. The effective bond order is further decreased as a result of substantial metal−ligand orbital mixing, leading to an MBO = 1.15. The localization of the unpaired electron in 9 in a singly occupied π* orbital localized on vanadium is also consistent with the observed EPR spectrum.

Figure 8. Correlation of the frontier molecular orbital diagrams of complex 8 with the hypothetical molecules V(XylNPiPr2)3CoI and [(THF)V(XylNPiPr2)3Co]+.

absence of π interactions between V and Co and a notable splitting in energy of the Co dx2−y2 and dxy orbitals. Both the solid-state structure and the optimized geometry of 8 indicate some distortions from C3 symmetry with variations in the three V−N distances (1.9646(13), 1.9306(13), and 1.9728(13) Å) and Co−P distances (2.3108(4), 2.3277(4), and 2.2979(4) Å). In addition, one of the N−V−N bond angles is significantly larger than the other two (127.10(6)° vs 115.42(6)°, 115.45(6)°), and a variation, albeit smaller, in the P−Co−P angles is also observed (103.380(16)°, 108.606(17)°, and 106.150(16)°). To explore the factors that lead to the

Figure 9. Correlation of the frontier molecular orbital diagrams of complexes 4, 9, and 10 and a related Ti/Co complex.6 Note: Bond orders depicted are based on calculated Mayer bond orders and are generally lower than the formal bond orders that would be predicted based on the MO diagram as a result of the polar and localized nature of the π bonds. H

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itself in an η2 fashion to the early metal allows the tris(phosphinoamide) scaffold to accommodate a very wide range of metal−metal distances and substrate binding modes. The hemilability of the untethered tris(phosphinoamide) ligand framework provides a distinct advantage over the Lu systems and enables the cooperative bond activation processes that are likely at the heart of the enhanced reactivity and unique reaction profiles observed for the tris(phosphinoamide) Zr/Co system.4d,f,g Thus, it would be constructive to understand the factors that drive the phosphinoamide ligand dissociation to guide the rational design of early/late heterobimetallic complexes that are better poised for substrate activation. The compounds reported herein and in previous publications fall into three categories: (1) compounds that adopt C3symmetric geometries in both solution and the solid state (e.g., 3, 4THF, 7, 8, (THF)Ti(XylNPiPr2)3Co6); (2) compounds that adopt asymmetric geometries with two bridging phosphinoamide ligands and one ligand bound η2 to the early metal (e.g., 9, 10, and (η2-iPr2PNXyl)Ti(XylNPiPr2)2(μ-Cl)CoI6); and (3) compounds that adopt asymmetric structures in the solid state but show evidence for ligand exchange through a C3-symmetric intermediate in solution (e.g., 4, 5, and (η2-iPr2PNXyl)Ti(XylNPiPr2)2CoI6). In general, the preferred conformation appears to be driven by the electron density at the early metal, such that when this metal becomes sufficiently electron-poor, the additional donation of an η2-bound phosphinoamide ligand provides the driving force for ligand dissociation from the late metal. This hypothesis is consistent with our previous observation that ClZr(MesNPiPr2)3CoI adopts a C3-symmetric geometry, while its analogue with less electron-rich N-Xyl substituents, (η2-iPr2PNXyl)Zr(XylNPiPr2)2(μ-X)CoX, adopts an asymmetric geometry.4a,13 The correlation of early metal electron density with geometry is directly tied to metal−metal bonding: Since the metal−metal bonds are quite polar and involve the late metal donating electron density to the early metal, stronger metal−metal interactions serve to attenuate the electrophilicity of the early metal. Similarly, one could take the perspective that strong metal−metal interactions decrease the electron density at the late metal and thereby strengthen late metal−phosphine σ-donor interactions, although back-donation from the late metal to phosphorus would also be decreased. In either case, the conclusion is that early/late heterobimetallic compounds with stronger metal−metal interactions are more likely to adopt C3-symmetric geometries, while those with weaker metal−metal interactions are more likely to favor pseudo-Cs-symmetric geometries. Conformationally flexible heterobimetallic compounds lie at the interface of these two extremes and feature moderate metal−metal interactions, and it is complexes in this optimal regime that are the most promising targets for further explorations of early/late heterobimetallic reactivity. For example, an interesting comparison can be made between Ti/Ni complex 4 and V/Ni complex 9: Complex 4 is fluxional, while complex 9 is asymmetric in solution, which initially seems to go against our hypothesis since Ti should be more electrophilic than V. However, differences in the electronic structure of these two compounds lead to population of π* orbitals and weaker metal−metal bonding in 9, leading to a less electron rich early metal center and favoring an asymmetric geometry.14 We note that metal−metal interactions are certainly not the only factor that effects the electrophilicity of the early metal and the corresponding conformational preference. For example, the electrophilicity of the early transition metal may also be diminished by the binding of

DISCUSSION Through the collection of compounds previously reported by the Lu and Thomas groups, it has been established that as the metals in heterobimetallic complexes become more disparate in atomic number, metal−metal interactions become weaker. Consequently, metal−metal multiple bonding has been observed only when the difference between two metals’ group number, ΔN, is ≤5.2b,d However, some complexes discussed in this work present a different scenario. One might expect that the Ti−Ni bimetallics 4 and 4THF with ΔN = 6 possess high bond polarity and intermediate or weak metal− metal bonding. Instead, a bond order of 2 is supported by both the experimental metal−metal distances and electronic structure calculations for these two complexes. Strong interactions between group IV metals and Ni have also been observed by the Tonks group.3b On the other hand, the V−Ni interaction in 9 is weakened by the population of π* orbitals, which results from the relative drop in energy of the π* orbitals from Ti to V as the early metal becomes more electronegative. A similar phenomenon was observed as the early metal was varied from V to Cr in M(iPrNPPh2)3Fe-L (L = halide or PMe3), where a higher spin state and lower bond order was observed in the Cr/Fe case.5a,b The same is also true when comparing Ti/Ni complex 4THF with its d10 V/Co analogue 8. In addition, it is also useful to compare the series of Ti−M and V−M complexes with some of Lu’s double-decker complexes. The most notable differences between our phosphinoamide ligand framework and the double-decker ligand include (1) the hemilability of the phosphinoamide ligands, (2) the existence of an axial donor ligand in the doubledecker ligand, and (3) the different bite angles of the amide and phosphine donors in the two ligands. The double-decker ligand is capable of placing the metals in closer proximity because the methylene linkers between N and P allow the phosphorus lone pairs to orient themselves into a plane parallel to the tris(amide) plane. However, some of the tris(phosphinoamide) complexes reported herein have shorter metal−metal distances compared to their double-decker analogues. For example, the Ti/Ni complexes 4 and 4THF feature a significantly shorter intermetallic distance compared to NiTi(N(o-(NCH2PiPr2)C6H4)3) (2.25 and 2.22 Å vs 2.41 Å).2b Although it seems logical to attribute the elongated Ti−Ni distance to the extra electron in the double-decker complex occupying a π* orbital, this cannot be confirmed until a detailed analysis of the electronic structure of NiTi(N(o-(NCH2PiPr2)C6H4)3) has been reported. In addition, the N2-free Ti−Co complex (THF)Ti(XylNPiPr2)3Co possesses a Ti−Co triple bond, while a double bond was observed for Lu’s CoTi(N(o(NCH2PiPr2)C6H4)3) complex despite their similarities in electronic structure.10 On the contrary, the V−Co distance in complex 8 is nearly 0.7 Å longer than the V−Co distance in Lu’s d10 analogue [CoV(N(o-(NCH2PiPr2)C6H4)3)]BPh4.7a In this case, the difference can be attributed to the difference in spin state (S = 1 vs 0) imposed by apical halide coordination to Co in 8, and it is worth noting that the DFT-predicted V−Co distance in [(THF)V(XylNPiPr2)3Co]+ is predicted to be similar to that in [CoV(N(o-(NCH2PiPr2)C6H4)3)]BPh4. The most obvious difference between Lu’s double-decker platforms and the tris(phosphinoamide) scaffolds in the present study is the conformational flexibility provided by the phosphinoamide ligand set. The ability of one phosphinoamide ligand to dissociate from the late transition metal and anchor I

DOI: 10.1021/acs.inorgchem.6b01543 Inorg. Chem. XXXX, XXX, XXX−XXX

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3.05 μB. UV−vis−NIR (C6H6) λmax, nm (ε, L mol−1 cm−1): 358 (4720), 508 (630), 708 (170). Anal. Calcd for C42H69N3P3FeTiBr: C, 56.52; H, 7.79; N, 4.71. Found: C, 56.38; H, 7.89; N, 4.39. (η2-iPr2PNXyl)Ti(XylNPiPr2)2NiCl (4) and (THF)Ti(XylNPiPr2)3NiCl (4THF). A solution of ClTi(XylNPiPr2)3 (0.316 g, 0.400 mmol) in Et2O (15 mL) was added to solid Ni(COD)2 (0.110 g, 0.400 mmol). The reaction mixture was stirred at room temperature for 16 h to ensure completion of the reaction. The volatiles were removed in vacuo, and the crude materials were extracted with 10 mL of Et2O and filtered through Celite. A concentrated pentane solution of crude material was left at −35 °C for over 12 h to obtain 4 as a brown crystalline solid (0.28 g, 82%). X-ray quality single crystals of 4 were obtained by slow evaporation of a concentrated Et2O solution of 4 at room temperature. Complex 4 readily converts to 4THF upon dissolution in THF, but removal of THF solvent under vacuum regenerates complex 4. X-ray quality single crystals of 4THF were grown at room temperature by dissolving 4 in THF and layering this concentrated solution with pentane. 1H NMR (400 MHz, 298 K, C6D6): δ 7.02 (br s, 6H, o-Ar), 6.56 (s, 3H, p-Ar), 2.82 (br s, 6H, CH(CH3)2, 2.29 (s, 18H, Ar−CH3), 1.25 (br s, 36H, CH(CH3)2). 13 C{1H} NMR (100.63 MHz, 298 K, C6D6): δ 152 (br), 138.8, 124.5, 120.9, 30.5 (br), 21.8, 20.2, 20.0. 31P{1H} NMR (162 MHz, 298 K, C6D6): δ 25.7 (br s, 2P), 3.41 (br s, 1P). 31P{1H} NMR (162 MHz, 238 K, toluene-d8): δ 22.2 (s, 2P), 1.3 (s, 1P); C3-symmetric species also observed: δ 13.8 (s, ∼25%). 1H NMR (400 MHz, 238 K, toluened8): δ 7.57 (s, 2H, o-Ar), 7.03 (s, 2H, p-Ar), 6.80 (s, 4H, o-Ar), 6.54 (s, 1H, p-Ar), 2.75 (m, 4H, CH(CH3)2), 2.26 (s, 12H, Ar−CH3), 2.20 (s, 6H, Ar−CH3), 2.15 (m overlapping, 2H, CH(CH3)2), 1.84 (m, 6H, CH(CH3)2), 1.62 (m, 6H, CH(CH3)2), 1.28 (m, 6H, CH(CH3)2), 1.23 (m, 6H, CH(CH3)2), 0.76 (m, 6H, CH(CH3)2), 0.60 (m, 6H, CH(CH3)2); C3-symmetric species observed (∼30%): δ 6.53 (s, 6H, oAr), 6.44 (3H, p-Ar), 3.21 (m, 6H, CH(CH3)2), 2.43 (s, 18H, Ar-Me), 1.52 (m, 18H, CH(CH3)2, 1.16 (m, 18H, CH(CH3)2). 1H NMR of 4THF (400 MHz, 298 K, THF-d8): δ 6.74 (s, 6H, o-Ar), 6.60 (s, 3H, pAr), 3.02 (m, 6H, CH(CH3)2), 2.22 (s, 18H, Me), 1.73 (m, 18H, CH(CH3)2), 1.36 (m, 18H, CH(CH3)2). 31P{1H} NMR of 4THF (162 MHz, 298 K, THF-d8): δ 9.0 (s). 13C{1H} NMR of 4THF (100.63 MHz, 298 K, THF-d8): δ 153.2, 138.5, 125.6, 125.3, 36.1, 22.1, 21.6, 20.8. UV−vis−NIR (Et2O) λmax, nm (ε, L mol−1 cm−1): 369 (6270), 519 (1100), 700 (310). Anal. Calcd for C42H69NiClN3P3Ti: C, 59.28; H, 8.17; N, 4.94. Found: C, 59.11; H, 9.19; N, 4.70. (η2-iPr2PNXyl)Ti(μ-Cl)(XylNPiPr2)2CuI (5). A solution of ClTi(XylNPiPr2)3 (0.316 g, 0.400 mmol) in Et2O (15 mL) was added to solid CuI (0.076 g, 0.40 mmol). The reaction mixture was stirred at room temperature for 24 h to ensure completion of the reaction. The volatiles were removed in vacuo, and the crude materials were extracted with 15 mL of Et2O and filtered through Celite. A concentrated Et2O solution of crude material was left at −35 °C for over 12 h to obtain 5 as a red crystalline solid (0.22 g, 55%). X-ray quality single crystals of 5 were obtained by slow evaporation of a concentrated Et2O solution of 5 at room temperature. 1H NMR (400 MHz, toluene-d8, 298 K): δ 6.48 (br s, 3H, p-Ar), 6.43 (br s, 6H, o-Ar), 2.22 (br m, 6H, CH(CH3)2), 2.10 (s, 18H, Ar−CH3), 1.26 (br s, 18H, CH(CH3)2), 1.03 (br s, 18H, CH(CH3)2). 1H NMR (400 MHz, toluene-d8, 218 K): δ 6.64 (s, 2H, o-Ar), 6.41 (s, 1H, p-Ar), 6.35 (s, 4H, o-Ar), 6.17 (s, 2H, p-Ar), 2.42 (m, 4H, CH(CH3)2), 2.17 (s, 6H, Ar-Me), 2.13 (m, 2H, CH(CH3)2), 2.04 (s, 12H, Ar-Me), 1,62 (m, 6H, CH(CH3)2), 1.54 (m, 6H, CH(CH3)2), 1.36 (m, 6H, CH(CH3)2), 0.77 (m, 6H, CH(CH3)2), 0.62 (m, 6H, CH(CH3)2), 0.57 (m, 6H, CH(CH3)2). 31 1 P{ H} NMR (162 MHz, C6D6, 298 K): δ 15.9 (br), −10 to 0 (br). 31 1 P{ H} NMR (162 MHz, C6D6, 298 K): 16.8 (s, 2P), −5.03 (s, 1P). 13 C{1H} NMR (100.63 MHz, C6D6, ppm): δ 162.1, 138.5, 124.2, 120.5, 37.0, 21.5, 19.8, 19.6. UV−vis−NIR (Et2O) λmax, nm (ε, L mol − 1 cm − 1 ): 546 (1180), 680 (70). Anal. Calcd for C42H69CuClIN3P3Ti: C, 51.33; H, 7.08; N, 4.28. Found: C, 51.22; H, 7.34; N, 4.45. V(XylNPiPr2)3 (6). A solution of XylNHPiPr2 (3.8 g, 16 mmol) in Et2O (35 mL) was chilled to −35 °C. To this was added dropwise n BuLi (10.2 mL, 1.6 M in hexanes, 16 mmol) over a period of 10 min.

additional ligands, such as the ligated THF in 4 and 8 or the bridging halide in 5.

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CONCLUSION In summary, we have synthesized and characterized a complete series of Ti/M and V/M (M = Fe, Co, Ni, and Cu) complexes. We have shown that metal−metal multiple bonding is available in both the tris(phosphinoamide)-linked C3-symmetric geometry and in more asymmetric compounds with just two bridging phosphinoamide ligands. Multiple bonding between different combinations of first-row transition metals is indicated by the short metal−metal distances in the Ti/M (M = Fe, Co, and Ni) and V/Fe complexes. A careful analysis of Mö ssbauer parameters among a series of M/Fe heterobimetallic complexes has lent new insight into trends in isomer shift: The decrease in isomer shift as a function of increased metal−metal bond orders strongly challenges the notion that Mössbauer isomer shifts can be used as a useful gauge of Fe oxidation state in metal−metal bonded complexes. Systematic variation of one of the metal atoms can effectively tune the metal−metal bonding and also the conformational flexibility of the tris(phosphinoamide) ligand framework. Moving forward, this study provides a guide for choosing appropriate metal−metal combinations for targeting bimetallic bond activation processes. The reactivity of the most promising compounds reported herein will be the focus of further study.

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EXPERIMENTAL SECTION

General Considerations. All manipulations were carried out under an inert atmosphere using a nitrogen-filled glovebox or standard Schlenk techniques unless otherwise noted. All glassware was oven- or flame-dried immediately prior to use. Et2O and THF were obtained as HPLC grade without inhibitors; pentane and benzene were obtained as ACS reagent grade. All protio solvents were degassed by sparging with ultra-high-purity argon and dried via passage through columns of drying agents using a Seca solvent purification system from Pure Process Technologies. THF-d8 and toluene-d8 were dried with CaH2 and degassed before use. All NMR spectra were obtained using a Varian Inova or MR 400 MHz instrument, and all chemical shifts are reported in ppm. 1H and 13C NMR chemical shifts were referenced to residual solvent, and 31P NMR chemical shifts were referenced to 85% H3PO4. For the paramagnetic molecules, the 1H NMR data are reported with the chemical shift, followed by the peak width at halfheight in parentheses (in Hz). iPr2PNHXyl,13 Ti(XylNPiPr2)3 (2),5c and ClTi(XylNPiPr2)3 (1)6 were synthesized using literature procedures. All other reagents and solvents were obtained from commercial sources and used without further purification. IR spectra were recorded on a Varian 640-IR spectrometer controlled by Resolutions Pro software. UV−vis spectra were recorded on a Cary5000 spectrometer using Cary WinUV software. Elemental microanalyses were performed by Complete Analysis Laboratories, Inc. (Parsippany, NJ, USA) and Robertson Microlit Laboratories (Ledgewood, NJ, USA). Ti(XylNPiPr2)3FeBr (3). A solution of Ti(XylNPiPr2)3 (0.307 g, 0.400 mmol) in Et2O (15 mL) was added to solid FeBr2 (0.086 g, 0.40 mmol). The reaction mixture was stirred at room temperature for 30 min, and to this solution was added KC8 (0.054 g, 0.40 mmol). The reaction mixture was stirred for 2 h to ensure completion of the reaction. The insoluble byproducts were removed by filtration. The volatiles were removed in vacuo, and the crude materials were extracted with 15 mL of Et2O and filtered through Celite. A concentrated Et2O solution of crude materials was left at −35 °C for over 12 h to obtain 3 as a brown crystalline solid (0.19 g, 63%). X-ray quality single crystals of 3 were obtained by slow evaporation of a concentrated Et2O solution of 3 at room temperature. 1H NMR (400 MHz, C6D6): δ 5.54 (20), 4.46 (202), 1.70 (24), −2.83 (1060). Evans’ method (C6D6): J

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byproducts were removed via filtration. The volatiles were removed from the filtrate in vacuo, and the crude material was washed with Et2O several times. Upon standing, a concentrated THF solution of 10 at room temperature yielded pale blue single crystals of 10 (0.46 g, 87%). 1 H NMR (400 MHz, C6D6): δ 15.8 (129), 8.8 (346), 6.1(122), 5.7 (110), 3.3 (234), 2.0 (98), −0.08 (932) −3.2 (592). UV−vis−NIR (C6H6) λmax, nm (ε, L mol−1 cm−1): 580 (880), 750 (400), 890 (200). Evans’ method (C6D6): 2.78 μB. Anal. Calcd for C42H69CuIN3P3V: C, 53.08; H, 7.32; N, 4.42. Found: C, 52.97; H, 7.25; N, 4.32. X-ray Crystallography. All operations were performed on a Bruker-Nonius Kappa Apex2 diffractometer, using graphite-monochromated Mo Kα radiation. All diffractometer manipulations, including data collection, integration, scaling, and absorption corrections, were carried out using the Bruker Apex2 software.15 Preliminary cell constants were obtained from 3 sets of 12 frames. Fully labeled diagrams and data collection and refinement details are included in Tables S1 and S2 and on pages S18−S38. Computational Details. All calculations were performed using Gaussian09, revision A.02, for the Linux operating system.16 Density functional theory calculations were carried out using a combination of Becke’s 1988 gradient-corrected exchange functional17 and Perdew’s 1986 electron correlation functional18 (BP86). A mixed basis set was employed, using the LANL2TZ(f) triple-ζ basis set with effective core potentials for titanium, vanadium, iron, cobalt, nickel, and copper,19 the LANL2DZ(p,d) double-ζ basis set and effective core potentials for bromine and iodine,19a,20 Gaussian09’s internal 6-311+G(d) for nitrogen, phosphorus, chlorine, and oxygen, and Gaussian09’s internal LANL2DZ basis set (equivalent to D95 V21) 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 on pages S36−S56. EPR Spectroscopy. X-band EPR spectra were obtained on a Bruker ElexSys E500 EPR spectrometer (fitted with a cryostat for measurements at 3 K). EPR samples were crystalline samples, and spectra were measured as frozen toluene glasses at 3 K. The spectra were referenced to diphenylpicrylhydrazyl (DPPH; g = 2.0037) and modeled using EasySpin for MATLAB. The EPR spectrum of complex 7 was simulated as a nearly axial signal with g values of 2.20, 2.09, and 2.08 using a peak-to-peak Gaussian line width of 5.22 mT. The asymmetric broadening in the spectrum is likely the result of unresolved superhyperfine coupling to either 51V (I = 7/2), 127I (I = 5/2), or 31P (I = 1/2) and was fit by incorporating g strain values of 0.064, 0, and 0.042. The EPR spectrum of complex 9 was simulated as a rhombic signal with g values of 2.00, 1.95, and 1.92 using a combination of Gaussian (4.39 mT) and Lorentzian (0.20 mT) peak-to-peak line widths. The hyperfine coupling in the spectrum was fit using hyperfine coupling exclusively to the 51V (I = 7/2) nucleus, with A = 37.2, 252.6, and 236.8 MHz. Mö ssbauer Spectroscopy. Zero-field 57Fe Mössbauer spectra were measured on a constant acceleration spectrometer (SEE Co, Minneapolis, MN, USA) with a Janis SVT-100 cryostat. Isomer shifts are quoted relative to Fe foil (