Article Cite This: Organometallics XXXX, XXX, XXX−XXX
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Tunable Binding of Dinitrogen to a Series of Heterobimetallic Hydride Complexes Samantha Lau,† Andrew J. P. White,† Ian J. Casely,*,‡ and Mark R. Crimmin*,† †
Department of Chemistry, Imperial College London, South Kensington, London, SW7 2AZ, U.K. Johnson Matthey Technology Centre, Blounts Court, Sonning Common, Reading, RG4 9NH, U.K.
‡
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S Supporting Information *
ABSTRACT: The reaction of [Ru(H)2(N2)2(PCy3)2] (1) with βdiketiminate stabilized hydrides of Al, Zn, and Mg generates a series of new heterobimetallic complexes with either H2 or N2 ligated to the ruthenium center. Changing the main-group fragment of the M·Ru-N2 (M = Al, Zn, Mg) complexes can subtly alter the degree of binding, and therefore activation, of the diatomic ligand, as evidenced by the νNN absorptions in the infrared data. Experimental and computational data rationalize this tunable binding; decreasing the electronegativity of the main group in the order Al > Zn > Mg infers greater ionic character of these M·Ru-N2 complexes, and this in turn results in greater destabilization of the frontier molecular orbitals of ruthenium and therefore greater Ru(4d) → π*(N2) back-donation.
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INTRODUCTION A detailed understanding of the coordination of N2 at transition-metal centers has paved the way for countless discoveries in dinitrogen fixation.1−3 End-on N2 binding can be considered within the Dewar−Chatt−Duncanson model (Figure 1a). Donation of electron density from the filled
The extent of back-donation is a tunable property. It can be regulated by the judicous control of the ligand environment at the transition metal. Modification of the oxidation state, coordination number, or geometry of the transition metal or the electronic or steric properties of the ligands all affects the symmetry and energy of the frontier orbitals (fMOs) of the transition-metal fragment and influences the extent of backdonation. For example, as part of detailed investigations into N2 fixation with carefully designed trigonal-bipyramidal group 8 complexes, Peters and co-workers have shown that the extent of back-donation to N2 is a function of both the metal and the σ-donating properties of the ligand trans to the N 2 coordination site.6 More recently, bimetallic complexes have been explored in dinitrogen fixation. For example, Lu and co-workers have reported a [Co-Co] bimetallic complex as a catalyst for the reductive silylation of N2 with Me3SiCl.7 As part of these studies, it has been shown that N2 binding to [Co-M] bimetallics can be subtly modulated by the choice of the second metal, with νNN decreasing across the first-row period M = V > Cr > Co (Δν = 25 cm−1).8 In related studies, Thomas and co-workers have shown that modification of the early transition metal in [Co-M] complexes from M = Ti to M = Zr has an effect on N2 binding to the late transition metal, again
Figure 1. (a) Dewar−Chatt−Duncanson model for N2 binding and (b) tunable N2 binding to heterobimetallic complexes.
bonding σ orbital to a vacant orbital of suitable symmetry on the transition metal is accompanied by back-donation from the transition metal to the 1πg* antibonding orbitals of the diatomic species. The back-donation is synonomous with lowering of the νNN stretching frequency and is often associated with activation of N2 and catalytic methods for its transformation to NH3 (or related nitrogen-containing molecules at the amine or hydrazine oxidation level).4,5 © XXXX American Chemical Society
Special Issue: Organometallic Complexes of Electropositive Elements for Selective Synthesis Received: May 19, 2018
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DOI: 10.1021/acs.organomet.8b00340 Organometallics XXXX, XXX, XXX−XXX
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reactions have not been fully identified, typically the formation of free ligand and 1 is observed. In addition, for the aluminum analogue, two decomposition products have been identified through single-crystal X-ray diffraction experiments. [Ru(H)(N2)(Cl)(PCy3)2] (see the Figure S1 in the Supporting Information) and a heterobimetallic ruthenium−aluminum hydroxide (see Figure S2 in the Supporting Information) are present in decomposition mixtures and are likely formed from a ligand exchange reaction in which the chloride exchanges from Al to Ru and a hydrolysis reaction, respectively. Exposure of M·Ru-H2 to a N2 atmosphere results in gradual formation of M·Ru-N2 (Scheme 1).14 While ruthenium dinitrogen complexes are well-known15,16 and include [Ru(NH3)5(N2)]2+, the first isolated dinitrogen complex,17,18 those incorporating main-group metals are rare. Despite conducting reactions at modest pressures of N2 (4 atm) and also bubbling of N2 through a solution of M·Ru-H2, in all cases a mixture is formed and the position of the equilibrium could not be forced entirely toward the dinitrogen complexes. The H2:N2 complex ratios after addition of N2 (4 atm) were measured to be 6:1 and 5:1 for the Zn and Mg heterobimetallic complexes, respectively. The direct reaction of 1 with the βdiketiminate-stabilized hydrides also results in the formation of a mixture of M·Ru-N2 and M·Ru-H2, and we ascribe this result to unavoidable decomposition of the hydride species leading to the generation of H2 in situ. A third species was observed in these reactions at low concentration, which has been tentatively assigned as M·Ru. The apical ruthenium site of M·Ru is not occupied by a diatomic ligand, and this is the expected intermediate in a dissociative exchange mechanism between M·Ru-H2 and M·Ru-N2. A further reaction of mixtures of Al·Ru-N2 and Al·Ru-H2 with CO provides additional evidence for ligand exchange occurring primarily with the apical site on ruthenium (for further details see the Supporting Information). Attempts to generate M·Ru cleanly by carrying out reactions under an argon atmosphere or placing M·Ru-N2/H2 mixtures under vacuum led to the onset of the previously mentioned decomposition. M·Ru-N2 species display diagnostic hydride resonances for both bridging (Al, δH −11.95 ppm, fwhm = 113 Hz; Zn, δH −11.40 ppm; Mg, δH −11.36 ppm) and terminal (Al, δH −15.81 ppm, fwhm = 37 Hz; Zn, δH −15.20 ppm; Mg, δH −15.69 ppm) hydrides. Assignment of these hydrides was confirmed by calculation of the NMR spectra by DFT methods. The larger fwhm value for the resonance assigned to the bridging hydride, relative to the terminal hydride, in Al· Ru-N2 is consistent with the bridging hydride coupling to the quadrapolar I = 5/2 27Al nucleus. The T1(min) relaxation times are all long (0.2−0.5 s), as would be expected in the absence of the dihydrogen ligand. The terminal hydride resonance of Mg·Ru-N2 is sharp enough to allow resolution of the cis couplings from the 1H NMR data at 298 K (2JH−H = 9.4 Hz; 2JP−H = 17.9 Hz). 31P{1H} NMR data collected on M·RuN2 provide support for the cis relation between the large PCy3 ligands. At room temperature in toluene-d8 two 31P resonances are observed in Al·Ru-N2 (δP 51.8 and 38.5 ppm at 298 K) due to the chlorine atom on the aluminum center leading to magnetic and chemical inequivalence of the phosphine ligands. At the same temperature, M·Ru-N2 (M = Zn, Mg) display a single 31P NMR resonance (Zn, δP 46.6 ppm; Mg, δP 45.6 ppm). The three-coordinate Zn and Mg imparts a higher symmetry on the complexes and the cis phosphine ligands are now chemically equivalent due to the higher symmetry. At
reported by the change in the N2 stretch in vibrational spectroscopy (Δν = 39 cm−1).9,10 Both of these systems contain defined and quantifiable metal−metal bonds, and N2 coordination occurs at a site in line with the metal−metal bond, maximizing the prospect for bimetallic cooperation. Herein we demonstrate N2 (and H2) binding to a series of Ru- - -M heterobimetallic hydride complexes (M = Al, Zn, Mg).11 These complexes possess weak metal- - -metal interactions at best; the main-group metal sits in the secondary coordination sphere of a pseudooctahedral ruthenium complex. A combination of characterization techniques (single-crystal X-ray diffraction and infrared spectroscopy) and calculations indicates that the variation of the main-group metal has an effect on N2 activation, with the more electropositive ligands resulting in a subtle but measurable increase in back-donation to the diatomic species. We rationalize the results through detailed DFT calculations and ultimately show that the electronegativity of the maingroup metal affects the energy of fMOs on ruthenium and, hence, dinitrogen binding.
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RESULTS AND DISCUSSION Preparation of N2 and H2 Complexes of Ru- - -M Heterobimetallics. Reaction of [Ru(H)2(N2)2(PCy3)2] (1)12 with a series of the β-diketiminate stabilized hydrides of aluminum, zinc, and magnesium under an atmosphere of H2 proceeded at 25 °C over the course of 24 h to generate the new heterobimetallic dihydrogen complexes M·Ru-H2 (M = Al, Zn, Mg, Scheme 1). The new dihydrogen complexes are only stable under an atmosphere of H2, precluding their full characterization in the solid state. Scheme 1. Preparation of Dihydrogen and Dinitrogen Complexes of a Series of Heterobimetallic Hydrides
In solution, the heterobimetallic dihydrogen complexes are characterized by a single broad resonance, integrating to five protons, in the hydride region of the 1H NMR spectrum (Al, δH −10.85 ppm; Zn, δH −10.71 ppm; Mg, δH −10.32 ppm). While low-temperature 1H NMR data acquired at 193 K failed to resolve the fluxional process that interconverts the hydride and dihydrogen ligands of M·Ru-H2, the formulation of these complexes is supported by T1 relaxation times, DFT calculations, and D2 exchange reactions. T1 measurements were taken across a 193−293 K temperature range at 400 MHz, and all three complexes exhibit short T1(min) times (Al, 36 ms; Zn, 35 ms; Mg, 52 ms) indicative of nonclassical hydrides.13 These species, and the related complexes described below, are highly unstable with decomposition beginning at temperatures above 40 °C. While these decomposition B
DOI: 10.1021/acs.organomet.8b00340 Organometallics XXXX, XXX, XXX−XXX
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Figure 2. Crystal structures of M·Ru-N2 (M = Al, Zn, Mg). Two molecules are found within the asymmetric unit of Mg·Ru-N2; both are disordered with a second overlapping orientation, and data are provided for the least disordered fragment.
lower temperature the 31P resonance of M·Ru-N2 decoalesces to two resonances (Zn, δP 47.5 and 43.8 ppm at 253 K; Mg, δP 45.5 and 42.8 ppm at 233 K) consistent with a further fluxional process that renders the phosphine ligands chemically inequivalent. Over the same temperature range the methyl and methine 1H resonances of the isopropyl groups of the main-group fragment decoalesce to a complex series of resonances. While we cannot unambiguously rule out the formation of isomers that differ in the coordination environment about ruthenium, the data are consistent with M·Ru-N2 retaining cis phosphine ligands but adopting a low symmetry due to a locked conformation of the β-diketiminate ligand. Hindered rotation about the N−CAr bond of these ligand systems is common. The main-group fragments in these complexes appear tightly bound across 193−353 K, and no data have been collected that support bimetallic dissociation into the monometallic components.19,20 The dinitrogen ligand was found to display a strong νNN stretching frequency (Al, 2160 cm−1; Zn, 2135 cm−1, Mg, 2130 cm−1) acquired on solid samples of M·Ru-N2. These vibrational frequencies are consistent with that reported for 1, and their assignment as N2 and not H2 stretches was confirmed by preparation of M·Ru-D2, an isotopomer of M· Ru-H2. M·Ru-N2 crystallize upon slow evaporation of concentrated hydrocarbon solutions under an N2 atmosphere (Figure 2). The solid-state structures reflect the solution characterization data, and in the case of Zn·Ru-N2 the data were of sufficient quality to locate the terminal and bridging hydride ligands from the electron density difference map. The geometry at ruthenium is a heavily distorted octahedron with the maingroup metal sitting in the secondary coordination sphere attached by bridging hydride ligands and three-center−twoelectron interactions. In the equatorial plane, the P−Ru−P angles are obtuse (X-ray, 104−107°; DFT, 104−106°) and the Heq−Ru−Heq angles are close to 90° (DFT = 92−94°). The length of the terminal Ru−Hax bond of Zn·Ru-N2 (1.45(3) Å) is shorter than both the bridging Ru−Heq (1.62(3) and 1.70(2) Å) and bridging Zn−H bonds (1.82(3) and 1.88(3) Å). The Ru- - -M distances are short (Al, 2.443(2) Å; Zn, 2.4957(3) Å; Mg, 2.5974(19) Å) and could be interpreted as an indication of a metal- - -metal interaction. Bonding Analysis of M·Ru-N2 . The metal−metal distances in M·Ru-N2 are all within the sum of the singlebond radii as defined by Pauling,21 and only the Ru−Zn distance is longer than the sum of the covalent radii defined by Pyykkö .22 We, and others, have considered normalized metal- - -metal bond distances and the formal shortness ratio
(fsr) as a crude metric to interrogate the nature of metal− metal bonds.11,23−25 The fsr values of M·Ru-N2 determined using the Pauling radii are all ∼1 and could be an indicator of a metal−metal bond (Al, 0.99; Zn, 1.00; Mg, 0.99). The presence of multiple bridging ligands is well-known to contract metal−metal separations, however, to a point that the true nature of the bonding can become opaque. We have previously used the fsr as an indicator of metal−metal bonding, but only in combination with detailed spectroscopic (NMR, infrared) and computational analysis (DFT, QTAIM) to support the conclusion. To interrogate the position of the hydride ligands and gain insight into the electronic structure of M·Ru-N2, a series of calculations were conducted using Gaussian 09. The ωB97x-D functional was employed. Ru, Al, Mg, and Zn centers were described with Stuttgart RECPs and associated basis sets,26−28 the 6-31++G(d,3pd) basis set was used for all hydrides, the 631G* basis set was used to describe C and H atoms, and the 6311+G* basis set was used to describe N, P, and Cl atoms.29−31 The calculations reproduce the position of the heavy atoms in M·Ru-N2 and confirm the location and bond lengths of the hydride ligands in Zn·Ru-N2. The bridging hydride ligands of M·Ru-N2 are located in the equatorial plane of ruthenium and coordinate to the main-group metal in a κ2-binding mode through short Ru−Heq (DFT = 1.6−1.7 Å) and long M−Heq distances (DFT = 1.8−1.95 Å). The location of the maingroup metal is not perfectly within the equatorial plane, and slippage toward the axial hydride on ruthenium and the resulting κ3-binding mode is observed for the whole series. The effect can be quantified by considering both the M−Hax distance from the DFT calculations (Al, 2.1 Å; Zn, 2.55 Å; Mg, 2.4 Å) and the N−Ru−M angle from the X-ray data (Al, 112.15(18)°; Zn, 95.64(8)°; Mg, 107.32(16)°) and is most pronounced for the aluminum analogue. For comparison, the Al−Hax distance is on the upper limit of the range suggested for a significant bonding interaction in analysis of σ complexes of aluminum11 and is slightly longer than Al−H distances (1.9−2.0 Å) in a related rhodium−aluminum heterobimetallic hydride that has been formulated with a κϵ-binding mode (Figure 3).32 NBO analysis in combination with QTAIM calculations reveal that, despite the short distances, metal−metal bonding in M·Ru-N2 is nominal at best. The Wiberg bond indices of the Ru−M interactions are small and decrease across the series Al (0.15) > Zn (0.07) > Mg (0.02). The WBIs of the Ru−Hax bonds are similar to those of the Ru−Heq bonds, suggestive of a similar covalent character despite the bridging interaction. C
DOI: 10.1021/acs.organomet.8b00340 Organometallics XXXX, XXX, XXX−XXX
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Table 1. Experimental and Computational Data on N2 Binding to M·Ru-N2 Ru−N (X-ray) (Å) N−N (X-ray) (Å) N−N (DFT) (Å) WBI (DFT) νNN (IR) (cm−1) νNN (DFT)a (cm−1) Δν (IR) (cm−1) Δν (DFT) (cm−1) a
Al
Zn
Mg
2.004(6) 1.086(7) 1.104 2.76 2160 2342 0 0
2.022(2) 1.107(3) 1.106 2.75 2135 2320 25 21
2.011(6) 1.088(8) 1.107 2.74 2130 2311 30 30
Uncorrected values.
The molecular orbital analysis of some simplified models delivers an explanation for the tunable N2 activation and the trend disclosed above. The frontier molecular orbitals of the square-based-pyramidal transition-metal fragment cis-[Ru(H)3(PMe3)2]− are represented in Figure 4a. The LUMO is preferentially oriented to accept electron density from the 3σg orbital of N2, while the HOMO and HOMO-2 are orientated in a fashion to back-donate to the degenerate and orthogonal 1πg orbitals. The model was expanded to include the maingroup fragment, and similar symmetry fMOs are calculated for model heterobimetallic complexes in which peripheral groups on the ligands have been truncated (see the Supporting Information). Inspection of the energies of the fMOs of suitable symmetry for N2 binding reveals that when the electronegativity of the main group metal is decreased (χm: Al (1.61) > Zn (1.59) > Mg (1.29)) all of the molecular orbitals of the model heterobimetallic complex are destabilized to a small extent (Figure 4b). The orbital destabilization is a direct effect of the second metal on the ruthenium center. This effect is not exerted through a metal−metal bond, nor does it require a trans relationship between the second metal and the binding site; it occurs due to modulation of the degree of ionicity of the metal−hydride bonding and charge localization on the transition-metal center (Figure 3). The heterobimetallic complex with the least electronegative main-group metal, Mg, has the lowest covalent character to the bonding, the largest charge localization on the transition-metal fragment, and hence the most destabilized fMOs. Destabilization of the occupied molecular orbitals on ruthenium raises their energy and increases back-donation to N2, populating the antibonding orbital of the diatomic and lowering the N2 stretching frequency. The degree of back-donation from ruthenium into N2 can be examined by looking at the values obtained from second-order perturbation calculations on the M·Ru-N2 complexes themselves. The most significant Ru(4d) → π*(N−N) donor−acceptor contribution increases in the order Al (20.0 kcal mol−1) < Zn (23.5 kcal mol−1) < Mg (24.6 kcal mol−1), which mirrors the observed trend in the degree activation of N2 in these heterobimetallic complexes.
Figure 3. Calculated bond lengths and angles in M·Ru-N2 along with selected NPA charges and WBI values.
The M−Heq WBIs are much smaller and decrease across the series Al > Zn > Mg, suggestive of decreasing covalent character as the main-group metal becomes more electropositive. NPA charges on both these hydrides and ruthenium are negative, while the main-group metal is, expectedly, highly positive. The combined NPA charge on the ruthenium, hydride, and dinitrogen moieties becomes more negative across the series Mg ≈ Zn > Al. In combination the analysis is consistent with a large ionic component to the bonding, and in the extreme these complexes could be formulated as [M]+[RuN2]− adducts in which the key interaction between the two fragments is via the bridging hydride ligands. For comparison, in 2015 we reported a series of rhodium hydride complexes which possess fsr values similar to those reported herein but with defined Rh−M bonds.23 The metal− metal bonding was supported by both calculations and spectroscopic data, and crucially these complexes possess a four-legged piano-stool geometry at rhodium and long M−H distances (>2.0 Å) that are conserved across a series (M = B, Si, Al, Zn, Mg). More recently Whittlesey, Macgregor, and coworkers isolated a heterobimetallic complex in which Ru and Zn sites are bridged by two hydride ligands in a κ2 fashion.33 The bonding mode and computational analysis are nearly identical with that found for Zn·Ru-N2, and the authors of this study also concluded that no significant direct Ru−Zn interaction is found in this species. Tunable N2 Activation. Experimentally determined νNN stretches for M·Ru-N2 decrease across the series Al > Zn > Mg. The calculated vibrational modes also follow this trend, and while the absolute values are systematically overestimated in comparison to experiment, the differences relative to νNN of Al, Δν, match closely (Table 1). The differences in the experimental NN and Ru−N bond lengths are very small and are well within the measurement error of the X-ray diffraction data. For comparison, the Raman stretch of free N2 is 2331 cm−1 and 1 shows two vibrational modes at 2163 and 2121 cm−1. Comparison of the latter vibrations with the heterobimetallic species is complicated by the presence of IRactive asymmetric and symmetric vibrations of the two coupled N2 ligands in 1; however, broadly it can be seen that inclusion of the main-group fragment only results in a slight perturbation of the N2 stretches, consistent with the conclusion that the heterobimetallic influence is only subtle.
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CONCLUSIONS In summary, we report the synthesis and characterization of a series of ruthenium main-group heterobimetallic complexes with X-ray structures obtained for the M·Ru-N2 complexes (M = Al, Zn, Mg). A small decrease in the νNN stretch, corresponding to a weakening of the N−N bond, was observed from changing the main-group metal of the heterobimetallic species in the order Al < Zn < Mg. This trend can be D
DOI: 10.1021/acs.organomet.8b00340 Organometallics XXXX, XXX, XXX−XXX
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Figure 4. (a) Selected fMOs of cis-[Ru(H)3(PMe3)2]− of relevance for N2 binding. (b) Energies of selected fMOs of a model heterobimetallic complex.
rationalized due to the greater ionic character of the metal− hydride bonding within the M·Ru-N2 complex, which becomes more pronounced with a less electronegative metal. The increased ionic character results in greater destabilization of the fMOs of ruthenium, which in turn allows for increased Ru(4d) → π*(N−N) back-donation. While the net effect on the N2 stretching frequency for the series of complexes reported herein is subtle, it clearly represents an example of a growing number of phenomena in which the binding and reactivity of small molecules at a transition-metal center can be modified by the proximity of a second metal.34−37
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ACKNOWLEDGMENTS
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REFERENCES
We are grateful to the EPSRC and Johnson Matthey for provision of a CASE Award and the Royal Society for a University Research Fellowship (UF090149). Johnson Matthey are also thanked for generous loans of precious metals. Stephen Bennett and Damian Grainger are thanked for productive discussions.
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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.organomet.8b00340. Experimental procedures, details of the DFT studies, single-crystal X-ray data and multinuclear NMR spectra (PDF) Full coordinates for calculated structures (XYZ) Accession Codes
CCDC 1843865−1843869 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail for I.J.C.:
[email protected]. *E-mail for M.R.C.:
[email protected]. ORCID
Mark R. Crimmin: 0000-0002-9339-9182 Notes
The authors declare no competing financial interest. E
DOI: 10.1021/acs.organomet.8b00340 Organometallics XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.organomet.8b00340 Organometallics XXXX, XXX, XXX−XXX
