Article pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Magnetism and EPR Studies of Binuclear Ruthenium Hydride Binuclear Species Bearing Redox-Active Ligands Michael E. Noss,† Noah L. Wieder,† Patrick J. Carroll,† Michael J. Zdilla,‡ and Donald H. Berry*,† †
Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323, United States Department of Chemistry, Temple University, Philadelphia, Pennsylvania 19122, United States
‡
S Supporting Information *
ABSTRACT: The binuclear complex {[N3]Ru(H)}2(μ-η1:η1-N2) ([N3] = 2,6(ArylNCMe)2C5H3N and Aryl = mesityl or xylyl) contains two formally Ru(I), d7 centers linked by a bridging dinitrogen ligand, although the odd electrons are substantially delocalized onto the redox non-innocent pincer ligands. The complex exhibits paramagnetic behavior in solution, but is diamagnetic in the solid state. This difference is attributed to intermolecular “πstacking” observed in the solid state, which essentially couples unpaired electrons on each half of the complex to form delocalized 22-center-2-electron covalent bonds. Introduction of a bulky t-butyl group on the ligand pyridine ring prevents this intermolecular association and allows further investigation of the magnetic behavior and electronic structure of the binuclear species. The interaction of the unpaired electrons in the two halves of the complex has been probed with magnetic susceptibility and perpendicular and parallel mode EPR measurements, revealing a weakly antiferromagnetically coupled system with a thermally accessible triplet excited state. In addition, the mixed valent, S = 1/2, {[N3]Ru(H)}(μ-η1:η1-N2){[N3]Ru} system has also been observed via perpendicular mode EPR and was used to quantify the growth of the thermally accessible triplet state of the dihydride complex using parallel mode EPR.
■
INTRODUCTION In 1969, Creutz and Taube first reported on the Creutz−Taube ion, [(NH3)5Ru(pz)Ru(NH3)5]5+ (pz is pyrazine), prompting much research to focus on electron transfer, understanding the effects structural changes have on electron transfer, and electronic structures as a whole in these types of systems.1−5 What was originally thought of as two Ru atoms of different oxidation states, +2 and +3, became generally accepted as two Ru atoms sharing the same non-integer state, +2.5, due to electron delocalization mediated by an organic bridging ligand.6,7 Much work has since been spent on understanding other bridging binuclear systems and the amount of electron delocalization, if any, that exists to better understand and explain the electronic structure of these systems.8−12 Understanding the electronic structure of an organometallic species is important for understanding chemical transformations that it may perform.13−20 The electronic structure of compounds bearing the bis(imino)pyridine ligand has long been a topic of great interest and discussion due to the redox non-innocent properties observed and the prevalence of these systems in ethylene polymerization chemistry.21−28 Our group has previously reported on two reactive binuclear ruthenium species possessing the bis(imino)pyridine ligand and a bridging dinitrogen molecule, {[N3Xyl]Ru}2(μ-η1:η1-N2) (1), where [N3Xyl] = 2,6-(XylNCMe)2C5H3N and Xyl = 2,6-xylyl, and {[N3Xyl]Ru(H)}2(μ-η1:η1-N2) (2). Preliminary observations of solid state magnetism, in attempts to elucidate the electronic structure of paramagnetic 2, were impeded by a weak delocalized bond over 22 atoms between neighboring © XXXX American Chemical Society
molecules in the crystal structure (vide infra). This bonding interaction quenches any intramolecular magnetic susceptibility in contrast to solution magnetic susceptibility measurements.29,30 In this contribution, the electronic structure of an analogue of complex 2 is reported, having successfully blocked the intermolecular bonding interaction. Akin to the Creutz−Taube ion, investigations to probe the amount and type of communication between unpaired electrons on distinct Ru atoms through an organic bridging ligand were performed. Through X-band EPR spectroscopy and solid state magnetism measurements, a better understanding of the electronic structure of the reported analogue is presented. This newly understood electronic structure may shed light on the mechanism for future transformations performed by this family of compounds.
■
RESULTS The previously reported complex 2, synthesized by the addition of H2 to the parent complex 1 (eq 1), contains two formally Ru(I) hydride centers, although substantial electron delocalization onto the redox non-innocent [N3] pincer ligands renders the [N3]−/Ru(II) more apt. Either extreme would result in an unpaired electron on each half of the binuclear complex. Complex 2 is paramagnetic in solution, with resonances in the 1 H NMR spectrum between δ 2 and 20 at 25 °C and exhibiting Received: March 19, 2018
A
DOI: 10.1021/acs.inorgchem.8b00735 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Figure 1. Plot of selected chemical shifts of {[N3Xyl]Ru(H)}2(μ-η1:η1-N2), 2, vs 1/T (302.3−247 K) in THF-d8.
Curie−Weiss behavior as indicated by linear plots of chemical shifts vs 1/T (Figure 1). The observed peaks in the 1H NMR spectrum are indicative of the C2 symmetric binuclear species in solution as opposed to the Cs symmetric geometry expected for a monomeric structure such as [N3]Ru(H)(N2).
of aromatic “π-stacking”. The plane-to-plane separation is extremely short (3.26 Å), suggesting significant molecular orbital overlap. This is supported by DFT calculations (Figure 3), which reveal the interaction is substantially bonding in
No significant paramagnetic moment was observed in the solid state magnetism studies of 2 performed from 2.4 to 130 K, demonstrating the diamagnetic nature of the complex in the solid state. The cause for this disparity between the solid state and solution magnetic behavior can be found in the solid state crystal structure of 2 reported in our initial communication (Figure 2).30 Markedly close intermolecular contacts between metal-pyridine units of neighboring molecules are reminiscent
Figure 3. Molecule orbital surface (left) and contour (right) plots of the HOMO of the model system investigated via DFT to illustrate the bonding interaction observed in complex 2. Figure reproduced from ref 30.
nature, with the odd electrons in each moiety pairing to create a two-electron covalent bond delocalized over 22 atoms.30 This bonding interactionalbeit relatively weakis sufficient to render the complex diamagnetic in the solid state. In order to prevent the bonding interaction observed in the solid state and better probe the interaction between the odd electrons within a single binuclear molecule, the [N3] ligand was modified to block the interaction observed for 2 in the solid state. Substitution of a bulky t-butyl group on the para position of the pyridine ring as well as substituting mesityl for xylyl as the imine aromatic group leads to complex 3 (Figure 4). With this analogue in hand, experiments and DFT calculations were performed to better elucidate the relationship, if any, between the two unpaired electrons in the complex. As expected, binuclear complex 3 is paramagnetic in solution, with a magnetic moment of 2.56 μB determined by the Evans NMR method. Although somewhat less than the 2.83 μB expected for an S = 1 ground state, this is quite close to the value of 2.45 μB expected for a ground state singlet with a
Figure 2. Crystal structure of 2 showing the close stacking interaction between two binuclear species quenching unpaired spins. Figure reproduced from ref 30. B
DOI: 10.1021/acs.inorgchem.8b00735 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
energetically low-lying triplet excited state that is already partially thermally populated at 4.8 K. As such, the signal due to the S = 1 species should increase with temperature compared to an S = 1/2 internal standard. A very convenient standard was generated by mixing 3 with the dinitrogen complex precursor, 4, resulting in an equilibrium distribution of 3, 4, and the S = 1 /2 mixed valence complex 5 (eq 3). Complex 5 could not be isolated from the equilibrium mixture. However, frozen glasses of the mixture provide constant signal for the S = 1/2 complex for comparison with the S = 1 signal in parallel mode (Figure 6). The intensity of the S = 1 signal was compared over
Figure 4. Complex 3 with t-butyl substitution in the pyridine para position and Mes substitution in the imine aryl position.
thermally accessible triplet (vide infra). Unlike the unsubstituted 2, complex 3 is also paramagnetic in the solid state, indicating the increased steric bulk is sufficient to block the intermolecular association observed for 2. Solid state magnetic susceptibility measurements were performed on 3, providing a signal consistent with a paramagnetic species in the solid state (Figure 5). This paramagnetic response suggests the association
Figure 6. Solid: Experimental parallel mode X-band EPR spectra of complex 3 with trace mixed valence complex 5 at 4.8, 16, and 50 K, MW freq. 3.393230 GHz, MW power 6.236 mW, conversion 164.840 ms, time const. 327.680 ms. Dashed: Simulation of 4.8 K spectrum by weighted addition of simulated g = 2 signal (Figure 7) and simulation of g = 4 signal: gx = 4.34, gy = 4.2, gz = 3.2. Figure 5. Plot of χ (diamond) and 1/χ (circles) versus T of complex 3. Solid line is the fit from the Bleaney Bowers equation.
different temperatures by keeping the S = 1/2 standard constant in intensity. At 16 K, the S = 1 signal grows to 1.2 times the intensity of the 4.8 K signal and to 2 times the intensity at 50 K. The S = 1/2 signal of the internal standard observed in perpendicular mode is simulated to have g values of gx = 2.043, gy = 1.9938, and gz = 1.9671 (Figure 7).
observed in 2 is no longer present in the solid state of 3. Repeated attempts to grow suitable single crystals of 3 were unsuccessful; thus it was not possible to confirm the details of the solid state structure. Magnetic susceptibility data of 3 were interpreted using the Heisenberg−Dirac−Van Vleck Hamiltonian, Ĥ = −JSÂ ·ŜB, showing an antiferromagnetic response at low temperatures, illustrating the type and strength of communication between the two unpaired electrons found in 3. The χT vs T data were fit using the Bleaney Bowers equation (eq 2) χ=
2Ng 2B2 kT[3 + e(−J / kT )]
(2) 31,32
where J is the isotropic interaction parameter. This antiferromagnetic response demonstrates a very small energy separation between the singlet ground state and thermally accessible triplet excited state with a J value of −4.1(7) cm−1 and a g value of 1.98(33). At temperatures above 30 K, the triplet state is well populated. A χ vs T plot of an antiferromagnetic interaction usually exhibits a maximum signal; however, due to the weak coupling observed, the maximum signal was not observed within the temperature range measured. No response for a Kramers S = 1/2 system was observed in the low temperature perpendicular mode EPR measurements of 3 in a methylcyclohexane glass at 4 and 77 K, consistent with the magnetism data. Parallel mode EPR, however, does provide an axial response for the non-Kramers S = 1 system at 4.8 K with gx = 4.34, gy = 4.2, and gz = 3.2, as determined by simulation. This parallel mode EPR result is consistent with an
■
DISCUSSION As originally observed in the crystal structure of 2, there exists close intermolecular contact between the pyridyl units of two binuclear species.30 The plane-to-plane separation of ∼3.26 Å was characteristic of a bonding interaction suggesting each monomeric unit of the binuclear species contained a significant amount of delocalized unpaired spin. DFT calculations of the previously investigated modified system (Figure 3) are consistent with this assignment, showing 73% of the unpaired spin density residing on the [N3] ligand.30 This initial observation, however, overshadowed the intramolecular interaction of the unpaired electrons on a lone binuclear species. C
DOI: 10.1021/acs.inorgchem.8b00735 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
may give rise to the weak nature of the coupling observed in this system. The coupling of these two electrons can be described as either ferromagnetic (triplet state) or antiferromagnetic (singlet state). The solid state measurements show a magnetic moment approaching 0 μB at low temperatures and increasing to close to 2.5 μB at temperatures above 30 K, consistent with the singlet ground state and thermal population of the triplet. At higher temperatures, there are essentially equal populations of the four possible microstates: one from the singlet and three from the triplet. The expected moment, χT, in the high temperature limit would be due to 25% S = 0 (χT = 0) and 75% S = 1 (χT = 1), for an average χT = 0.75, which corresponds to an effective moment of 2.45 μB, not the 2.83 expected for a pure S = 1 state. This situationa singlet ground state and very low-lying triplet stateexplains the “low” value for the solution magnetic moment (2.56 μB) observed at room temperature.33−35 Parallel mode EPR measurements at low temperatures provide a signal consistent with a non-Kramers triplet state that is increasingly populated at as the temperature is increased.36,37 These results further support the assignment of a singlet ground state for this molecule with an energetically low-lying triplet state that is thermally accessible, consistent with the small J value of −4.1(7) cm−1. The electronic structure of binuclear complex 2 was examined using DFT methods. The ground state is best described by an unrestricted wave function in which the odd electrons are localized in separate halves of the complex and antiferromagnetically coupled, i.e., an open-shell singlet. Consistent with the very small J value observed experimentally, the triplet state lies extremely close in energy to the open-shell singlet ground state. Calculations using the MN12L38 functional and the Def2-TZVP39,40 basis of Weigend and Ahlichs place the singlet/triplet gapand hence Jat only −58 cm−1 (−0.17 kcal·mol−1). The highest energy singly occupied MOs of the singlet and triplets are shown in Figure 8. Repeating the
Figure 7. Top: Experimental perpendicular mode X-band EPR spectrum of mixed valent complex 5, T = 4.8 K, MW freq. 9.642061 GHz, MW power 0.63 mW, conversion 81.920 ms, time const. 163.840 ms. Bottom: Simulated EPR spectrum, gx = 2.043, gy = 1.9938, gz = 1.9671, ax = 100 MHz.
The magnetism and EPR spectroscopic results obtained above for complex 3 are only possible for a system in which the unpaired electrons on each half of the molecule are capable of communicating with each other. Rather than thinking of a mixed valent system like the Creutz−Taube ion and discussing metal oxidation states as a result of electron delocalization through a bridging ligand, the electron delocalization exhibited in this system allows for the discussion of a singlet/triplet electronic structure as opposed to two independent doublets. Solid state magnetism measurements and parallel mode EPR measurements confirm the coupling of the unpaired electrons on each half of 3. Keeping in mind the established electron delocalization through the π* system of the [N3] ligand, the distance over which the two electrons from each half of the molecule are coupling could potentially be as long as from one pyridine to the other, a distance of up to 14.44 Å as observed in the crystal structure of 2. Another geometric consideration is the orthogonal nature of the two halves of these complexes causing poor π* orbital overlap through the cylindrically symmetrical two nitrogen atom bridge. Both of these factors
Figure 8. Alpha and beta SOMOs of antiferromagnetically coupled singlet of 2 (bottom) and highest two alpha SOMOs of the triplet state (top). D
DOI: 10.1021/acs.inorgchem.8b00735 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry calculations using the popular B3LYP41 functional and the same basis sets yields a similarly negligible value of 5 cm−1. Thus, both functionals yield values of J which are essentially zero, given the accuracy expected from this level of theory, and are consistent with the small value of −4.1(7) cm−1 determined experimentally. Not surprisingly, the closed-shell singlet state determined by a restricted calculation was found to be higher energy than both the open-shell singlet and the triplet, leading to a value of J that is too large (1467 cm−1) and a ground state multiplicity inconsistent with the experimental results. The small value of J is attributed to the perpendicular orientation of the two halves of complex 2. The SOMOs of the antiferromagnetically coupled open-shell singlet show only a small amount (
