Stabilisation of a paramagnetic BH₄⁻-bridged dinickel(II) complex by a macrodinucleating hexaaza-dithiophenolate ligand

Abstract

The first paramagnetic borohydrido-bridged dinuclear nickel(II) complex, [(L)Ni^II₂(μ_1,3-BH₄)]⁺, stabilised by a sterically demanding hexaaza-dithiophenolate macrocycle, has been obtained by the reaction of [(L)Ni^II₂(μ-ClO₄)]⁺ with NⁿBu₄BH₄.

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The search for dinuclear dithiolato-bridged complexes which model key features of the active site of hydrogenase enzymes is an active research area.^1,2 Two main strategies exist to access such compounds. One involves the addition of an electrophilic metal–carbonyl fragment to a nucleophilic metal complex with cis-oriented thiolate functions.³ The resulting [NiFeS₂], or [Fe₂S₂], assemblies are co-ligated with CO and CN⁻ and represent good structural analogues of the proposed active site structures, as demonstrated recently by a number of research groups.^4–7 In the other strategy, macrocyclic dinucleating polyaza-dithiolate ligands are used for the cluster assembly.⁸ Until now this strategy has only allowed for the production of homodinuclear nickel complexes, and it is unclear at present whether these more classical Werner-type coordination compounds will ever be able to bind the biologically relevant co-ligands CO, CN⁻ and H⁻. Herein we provide the first evidence for nickel–hydrogen interactions in such compounds.

Our study was initiated by the recent discovery of Desrochers et al.⁹ who demonstrated that the sterically encumbered hydrotris(3,5-dimethyl-pyrazolyl)borate ligand (Tp*⁻) can stabilize a hydrogen-rich nickel environment in [Tp*Ni^II(μ_1,3-BH₄)]. In order to test whether similar dinickel(II) complexes with a bridging borohydride co-ligand are supported by the dinucleating hexaaza-dithiophenolate ligand (L)²⁻ (Scheme 1),¹⁰ an acetonitrile solution of the chloro-bridged complex [(L)Ni^II₂(μ-Cl)]ClO₄ (1·ClO₄) was treated with NⁿBu₄BH₄ under an argon atmosphere at ambient temperature. Unlike [Tp*Ni^IICl],⁹ however, no reaction occurred. In a second approach, the reaction was carried out using the dark-green perchlorato-bridged complex [(L)Ni^II₂(μ-ClO₄)]ClO₄ (2·ClO₄) which was prepared by Cl⁻ abstraction from 1·ClO₄ with Pb(ClO₄)₂.† This gave a pale-green solution of the desired borohydrido-bridged complex 3, which was isolated as its BPh₄⁻ salt in ca. 75% yield. In the absence of air and protic reagents this compound is stable for weeks, both in the solid state and in solution. This stability is quite remarkable given that nickel(II) complexes of sterically less demanding ligands are readily reduced to nickel boride.¹¹

Scheme 1 Complexes 1–5.

IR measurements of solid 3·BPh₄ showed intense absorption bands at 2390, 2360, 2153 and 2071 cm⁻¹ indicative of terminal B–H and bridging B–H⋯Ni functions.¹² The UV-Vis spectrum recorded in acetonitrile suggested the presence of octahedral Ni(II) ions [λ = 650 (ν₂) and 1074 nm (ν₁)].¹³ Final confirmation came from an X-ray crystal structure determination of 3·BPh₄·2MeCN (Fig. 1).§ As can be seen, the BH₄⁻ ion bridges the two Ni(II) centres in a symmetrical fashion to generate a bioctahedral N₃Ni^II(μ-S)₂(μ-BH₄)Ni^IIN₃ core structure that has never been observed before in nickel–thiolate chemistry. There are no interactions between the MeCN of solvent of crystallization and the [(L)Ni^II₂(μ-BH₄)]⁺ cations. The average Ni–H distance at 1.89(4) Å compares well with that in the mononuclear NiS₄H₂ complex [Ni^II(bmp)₂] (bmp = bis(2-mercapto-1-methylimidazolyl)borate),¹⁴ the only other sulfur-rich Ni(II) complex with B–H⋯Ni interactions that has been structurally characterised.

Fig. 1 ORTEP representation of the structure of complex 3 at 50% probability ellipsoids. Hydrogen atoms, except those of the BH₄⁻ co-ligand, have been omitted for clarity. Selected bond lengths (Å) and angles (°): Ni1–N1 2.219(4), Ni1–N2 2.152(4), Ni1–N3 2.284(4), Ni1–S1 2.437(2), Ni1–S2 2.459(1), Ni1–H1 1.93(4), Ni2–N4 2.271(4), Ni2–N5 2.135(4), Ni2–N6 2.260(4), Ni2–S1 2.439(2), Ni2–S2 2.457(2), Ni2–H2 1.84(4), B1–H1 1.25(4), B1–H2 1.24(4), B1–H3 1.12(4), B1–H4 1.12(4); Ni1⋯Ni2 3.458; Ni1–S1–Ni2 90.33(5), Ni2–S2–Ni1 89.41(5).

Magnetic susceptibility measurements have been carried out to see whether magnetic exchange interactions are present in 3·BPh₄. As can be seen from Fig. 2, the product χ_mT (per dinuclear complex) gradually increases from 2.69 cm³ K mol⁻¹ at 295 K to a maximum of 3.29 cm³ K mol⁻¹ at 28 K, and then decreases rapidly to 2.74 μ_B at 2 K. This behaviour indicates an intramolecular ferromagnetic exchange interaction between the two Ni²⁺ ions in 3.¹⁵

Fig. 2 Plot of χ_mT against T for 3·BPh₄. The solid line represents the best theoretical fit of the magnetic susceptibility data by full-matrix diagonalization of the spin Hamiltonian H = −2JS₁·S₂ + D(S_z1² + S_z2² − 4/3) + gβ(S₁ + S₂)B. Experimental and calculated data are provided as ESI.†

Least-squares fit of the magnetic susceptibility data by full-matrix diagonalization of the appropriate spin Hamiltonian H = −2JS₁·S₂ + D(S_z1² + S_z2² − 4/3) + gβ(S₁ + S₂)B gave J = 27 cm⁻¹, D = +4.3 cm⁻¹, and g = 2.09 (or alternatively J = 27.5 cm⁻¹, D = −3.9 cm⁻¹, and g = 2.09). Interestingly, the J value of 27 cm⁻¹ is by far the largest value that has ever been observed in dinuclear nickel(II) complexes supported by (L)²⁻.¹⁶ It indicates that the ferromagnetic magnetic exchange interactions are not only propagated via the bridging thiolate functions but also through the μ_1,3-bridging borohydride ion. To the best of our knowledge, this property of the BH₄⁻ ion has not been documented previously in the literature. This suggests its use as a building block in the construction of molecular based magnetic materials.

Preliminary results show that 3 reacts with protic reagents HA, such as HCl, H₂O, or HCO₂H with liberation of H₂ and formation of the respective [(L)Ni^II₂(A)]⁺ species (A = Cl⁻1, OH⁻4 and HCO₂⁻5). Complexes 1 and 4 have been reported earlier. The new complex 5 is also readily produced by the reaction of 3 with CO₂. IR measurements of 5·BPh₄ showed two absorption bands at 1602 and 1424 cm⁻¹, attributable to the symmetric and antisymmetric stretching frequencies of a μ_1,3-bridging formate ion. This was also confirmed by an X-ray crystal structure determination of 5·BPh₄ (see ESI†).

In conclusion, we have prepared the first stable dinuclear nickel(II) borohydrido-bridged complex of a macrodinucleating hexaaza-dithiophenolate ligand. Work in progress is directed towards the synthesis of related compounds with bridging hydride ligands by taking advantage of the steric protection offered by the supporting ligand. Such compounds may also aid in understanding the electronic structures and the reactivities of the dinuclear active sites of the hydrogenase enzymes.

This work was supported by the Deutsche Forschungsgemeinschaft (Project KE 585/4-1,2).

Notes and references

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9. P. J. Desrochers, S. LeLievre, R. J. Johnson, B. T. Lamb, A. L. Phelps, A. W. Cordes, W. Gu and S. P. Cramer, Inorg. Chem., 2003, 42, 7945.
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15. In the solid state the dinuclear nickel complexes are well separated by the bulky tetraphenylborate anions. The abrupt decrease in χ_mT below 28 K is therefore most likely due to zero field splitting of Ni^II and not to intermolecular exchange interactions.
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Footnotes

† Electronic supplementary information (ESI) available: Preparation and characterisation data for complexes 2–5. See DOI: 10.1039/b512744k

‡ Present address: Institut für Anorganische und Analytische Chemie, Universität Freiburg, Albertstrasse 21, D-79104 Freiburg, Germany.

§ Crystal data for [(L)Ni^II₂(μ_1,3-BH₄)](BPh₄)·2MeCN (3·BPh₄·2MeCN): C₆₆H₉₄B₂N₈Ni₂S₂, M = 1202.65. Triclinic, space group P1̄, (no. 2), a = 15.992(3), b = 16.080(3), c = 16.515(3) Å, α = 63.82(3), β = 70.97(3), γ = 66.47(3)°, V = 3436(1) ų, Z = 2, D_calcd. = 1.162 g cm⁻³, μ(Mo-Kα) = 0.651 mm⁻¹, T = 210 K. Using Mo-Kα radiation (0.71073 Å), a total of 31051 reflections were collected of which 16023 were independent. Refinement converged to R₁ = 0.0749, wR₂ = 0.2053 [I > 2σ(I)]. Hydrogen atoms for the η²-BH₄ ligand were located from the final difference map and were refined isotropically with U_eq(H) 1.2 times that of the boron to which they are attached. The MeCN solvate molecules were found to be disordered over two positions. CCDC 280004. Crystal data for [(L)Ni^II₂(μ_1,3-O₂CH)](BPh₄) (5·BPh₄): C₆₃H₈₅BN₆Ni₂O₂S₂, M = 1150.72. Monoclinic, space group P2₁/n, a = 18.452(4), b = 34.531(7), c = 18.452(4) Å, β = 92.22(3)°, V = 11748(4) ų, Z = 8, D_c = 1.301 g cm⁻³, μ(Mo-Kα) = 0.761 mm⁻¹, T = 210 K. Using Mo-Kα radiation (0.71073 Å), a total of 74914 reflections were collected of which 28479 were independent. Refinement converged to R₁ = 0.0778, wR₂ = 0.1651 [I > 2σ(I)]. CCDC 280005. For crystallographic data in CIF or other electronic format see DOI: 10.1039/b512744k

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