Titanium(III) Member of the Family of Trigonal ... - ACS Publications

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Titanium(III) Member of the Family of Trigonal Building Blocks with Scorpionate and Cyanide Ligands Andrew Brown,† Mohamed Saber,†,‡ Willem Van den Heuvel,§ Kelsey Schulte,† Alessandro Soncini,§ and Kim R. Dunbar*,† †

Department of Chemistry, Texas A&M University, College Station, Texas 77843, United States School of Chemistry, University of Melbourne, Parkville, Victoria 3010, Australia

§

S Supporting Information *

titanium(III) with cyanide ligands are very rare, with the only previously reported notable examples being [Et4N]3[Ti(CN)6] and [Cp2Ti(CN)]4.37,38 The study of d1 ions with strong-field ligands in an octahedral or distorted octahedral field is of considerable interest in the context of magnetic anisotropy, but there are few such investigations in the literature. The compound [Et4N][Tp*TiCl3] (1) was prepared by the reaction of KTp* with TiCl3 in the presence of [Et4N]Cl. Slow evaporation of this filtrate yielded crystals of 1 and [Tp*TiCl2pz*]pz* (2; pz* = 3,5-dimethylpyrazole), which is a previously noted decomposition product; details can be found in the Supporting Information.39 Further treatment of 2 with [Et4N]Cl yields more of the trichloro-substituted derivative, 1. Substitution of the chloride ligands with cyanide ligands was achieved by treating 1 with a slight excess of [Et4N]CN in acetonitrile. Crystals of the highly air-sensitive compound 3 were obtained from a solution of acetonitrile and diethyl ether. The TiIII ion in compound 1 resides in a trigonally distorted octahedral environment with three N atoms from the Tp* ligand and three terminal chloride ligands (Figure 1), as observed in

ABSTRACT: The titanium(III) cyanide compound [Et4N][Tp*Ti(CN)3] ([Et4N] = tetraethylamonium; Tp* = 3,5-dimethyltrispyrazolylhydroborate) is reported, which exhibits a trigonally distorted geometry. Magnetic data and ab initio calculations verified that the molecule is an S = 1/2 paramagnet and that it exhibits significant temperature-independent paramagnetism. yanide compounds are important players in the field of molecular magnetism research due, in large measure, to their high stabilities, flexible coordination environments, and rich redox properties and the widespread existence of homoleptic analogues throughout the transition-metal series. Prussian blue type architectures with high ordering temperatures were discovered early on in the renaissance of molecular magnets based on coordination chemistry.1,2 Shortly thereafter, the isolation of discrete molecular analogues became a high priority, efforts that continue after more than 15 years. Reducing the number of available binding sites in cyanometalates is important for the controlled synthesis of polynuclear structures.3−9 In this vein, the trispyrazolyborate (Tp) ligand and its derivatives have been highly useful.10 This ligand is appealing because of its trigonal symmetry and ease of substitution, which allows for fine tuning of the electronic and steric properties of the ligand. The Tp ligand and its derivatives have been used with success to isolate the tricyanide anionic building block [TpFeIII(CN)3], which has been used to prepare numerous magnetically interesting polynuclear complexes.11−16 Other 3d transition metals have been employed as well to synthesize trivalent Cr, Mn, and V and divalent Fe tricyanide complexes.17−21 Dicyanide molecules containing divalent Ni, Co, and Cr ions,22 as well as monocyanide molecules with divalent Zn and Hg ions have also been reported.23−27 To the best of our knowledge, no cyanidecontaining molecules with Ti metal ions and Tp, or its derivatives, have been reported to date. In this vein, we elected to study the TiIII d1 ion in the trigonal field of the anionic scorpionate ligand tris(3,5-dimethyl-1pyrazolyl)borohydride (Tp*).28 A perusal of the literature indicates that Tp* has been explored in trivalent titanium chemistry but not with strong-field ligands such as cyanide.29−36 Herein we report the synthesis of the first trivalent titanium cyanide building block in this family, namely, [Et4N][Tp*Ti(CN) 3 ] (3), as well as its chloride precursor and a dichloropyrazolate decomposition product. Compounds of

C

© XXXX American Chemical Society

Figure 1. Molecular structure of 1. Ellipsoids are plotted at the 50% probability level; H atoms are omitted for the sake of clarity.

another salt of the compound reported previously, namely, a trimethylhydrazinium species, [NHMe2NHMe][Tp*TiCl3].39 Compound 1 crystallizes in the space group Cc. The trigonal distortion of the crystal field is evident by elongation of the Tp*Ti− moiety with an average N−Ti−N angle of 84.35(2)° and compression of the −TiCl3 moiety with an average Cl−Ti−Cl angle of 95.08(6)°. The compound also suffers further rhombic Received: October 30, 2016

A

DOI: 10.1021/acs.inorgchem.6b02643 Inorg. Chem. XXXX, XXX, XXX−XXX

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which leads to a linear decrease in χT as the temperature is lowered. In these systems, TIP arises from the coupling of the orbitally nondegenerate 2A ground-state term with the thermally unpopulated 2E excited state via matrix elements of the orbital angular momentum operator Lα.40 In Van Vleck formalism, the α component of the TIP (α = x, y, and z) reads χTIP,αα = NA2∑J≠0|⟨0|Lα|J⟩|2/ΔJ0, where |0⟩ is the ground state and |J⟩ values are the excited states with an energy gap of ΔJ0 from the ground state.41 Note that symmetry selection rules here demand that the TIP must be anisotropic, as ⟨2A|Lα|2E⟩ ≠ 0 only if α = x and y. The TIP value for compound 1 is within the range of values reported in the literature for titanium(III) complexes. Homometallic complexes of titanium(III), e.g., the anion [Ti2Cl9]3− and the neutral compound [(η5-C5H5)2Ti]2(μ-O), exhibit TIP values of 8.0 × 10−4 and 3.57 × 10−4 cm3 mol−1, respectively.42,43 The heterometallic complexes [(η5-C5H5)2TiCl]2ZnCl2·C6H6 and [Mn4Ti4(μ-Cl)2(μ3,η2-L)2(μ,η2-L)10Cl6] exhibit TIP values of 2.6 × 10−4 and 1.3 × 10−4 cm3 mol−1, respectively.44,45 The magnetization plot at 1.8 K for 1 is shown in Figure S3. The saturation value of 0.86 μB is indicative of a g value of the Ti ion at low temperature that is less than 2.0. The magnetization from 0 to 7 T was fit to the Brillouin function, and the best fit occurs for g = 1.77. The χT versus T plot and magnetization plots for 3 are displayed in Figures 3b and S5, respectively. The magnetic properties are similar to those of compound 1. The value of χT at 300 K is 0.38 emu K−1 mol−1, which is close to the expected value (χT = 0.375 emu K−1 mol−1) for an isotropic TiIII ion (S = 1/2, g = 2.0). The decrease of χT upon cooling is due to a TIP of 2.9 × 10−4 cm3 mol−1. The magnetization plot at 1.8 K (Figure S5) saturates at a value of 0.83 μB, which corresponds to a Brillouin function with g = 1.76. To further understand the magnetic properties of 1 and 3, single-point calculations were performed using crystallographic parameters. The electronic configuration of TiIII is d1, which gives rise, in the ligand field, to five spin doublet states. These were obtained from a complete active space self-consistent field (CASSCF) calculation, followed by a second-order perturbation calculation (CASPT2). CASSCF gives wave functions and energies for ground and excited states. CASPT2 corrects the energies for electron correlation effects. The CASSCF/CASPT2 method does not include spin−orbit coupling (SOC), which is introduced in the next step in which the SOC Hamiltonian is diagonalized on the basis of the CASSCF wave functions. In the present case, there are five S = 1/2 wave functions and SOC gives rise to five Kramers doublets. The CASPT2 and SOC energies are listed in Table 1. The spin−orbit wave functions can be used to calculate the g factors and the temperature dependence of the paramagnetic susceptibility. All computations were carried out with the Molcas7.6 software.46

distortions, as indicated by the three different Ti−Cl bond distances and angles for the three terminal chloride ligands [Ti− Cl1 = 2.3954(18) Å, Ti−Cl2 = 2.3923(16) Å, and Ti−Cl3 = 2.3811(17) Å]. The pyrazolyl ligands exhibit a staggered conformation with the chloride ligands, as viewed along the Ti−B axis. Compound 3 crystallizes in the space group Pmn21. Similar to the chloride analogue, the anion (Figure 2), is trigonally distorted

Figure 2. Crystal structure of 3. Thermal ellipsoids are at the 50% probability level; H atoms are omitted for the sake of clarity.

with an average N−Ti−N bond angle of 85.82°, whereas the −Ti(CN)3 moiety is less compressed than 1 with an average C− Ti−C bond angle of 92.59°. A further rhombic distortion is seen in 3 as indicated by the three different Ti−C bond distances and angles (Tables S4 and S5). The average Ti−N bond distance is 2.154(2) Å, and the average Ti−C bond distance is 2.175(2) Å, which lies within the range of previously reported titanium cyanide complexes.37,38 The N−Ti−C distances are 4.326(2), 4.315(3), and 4.326(2) Å, which indicate that one axis is slightly compressed. Magnetic properties of compounds 1 and 3 were measured using a Quantum Design MPMS magnetometer. Susceptibility data were collected under a direct-current field of 1000 Oe. The χT versus T plot for 1 is displayed in Figure 3a. The expected χT value for an S = 1/2 Curie paramagnet is a horizontal line with a y intercept at 0.375 emu K−1 mol−1. The deviation from ideal Curie behavior is due to a Van Vleck temperature-independentparamagnetic (TIP) contribution of 3.0 × 10−4 cm3 mol−1,

Table 1. Calculated Ligand-Field Energies (cm−1) for 1 and 3 state Oh 2

2

2

2

T2g

Eg

Figure 3. Temperature dependence of χT for (a) 1 and (b) 3. The solid red lines correspond to the modeling from ab intio calculations. B

1 C3v A1 2 E E

3

CASPT2

SOC

CASPT2

SOC

0 1220 1310 15900 16410

0 1180 1380 15910 16420

0 1420 1480 22910 23630

0 1380 1550 22920 23640

DOI: 10.1021/acs.inorgchem.6b02643 Inorg. Chem. XXXX, XXX, XXX−XXX

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state. The slopes are 3.1 × 10−4 cm3 mol−1 for 1 and 2.6 × 10−4 cm3 mol−1 for 3. The TIP arises mainly from Zeeman interactions with the states of the first excited-state 2E level. The effect of a weak trigonal ligand field on an octahedral 2T2g ground-state term has been described in the literature by a parametric model.47,48 The parameters are Δ, the splitting of the t2g orbitals into a1 and e components, k, the orbital reduction factor, and ζ, the SOC constant. We can obtain Δ from the CASPT2 energies in Table 1 as the barycenter of the weakly split first 2E level. This gives Δ = 1260 cm−1 for [Tp*TiCl3]− and Δ = 1450 cm−1 for [Tp*Ti(CN)3]−. The k value can be estimated from the matrix elements of the orbital angular momentum in the CASSCF states and is 1.0 for both compounds, which means that the t2g orbitals are almost purely 3d, with negligible covalent mixing with the ligand orbitals. With ζ = 150 cm−1, this model agrees with the computed g values and the TIP for ∼90%, with 10% being due to interactions with the octahedral 2Eg level, which is not considered in this simple model. In summary, the first example of a trigonal titanium(III) cyanide molecule is reported. The magnetic properties of 3 reveal significant temperature-independent paramagnetism. This new building block may lead to anisotropy in higher-nuclearity compounds depending on the degree of distortion imposed by additional metal spin centers bonded through the nitrogen end of the cyanide ligands.

For both compounds, the coordination geometry is approximately octahedral, but the actual symmetry is distorted C3v. The CASPT2 energies in Table 1 indicate that this descent in symmetry leads to smaller ligand-field perturbations. The strongest field is the octahedral field, separating the t2g levels from the eg levels, followed by a weaker trigonal field, which splits t2g into a1 and e orbitals, and finally followed by a low-symmetry C1 field, which further splits the e levels. We can define the octahedral field strength as the energy difference between the barycenters of the 2T2g and 2Eg terms: For [Tp*TiCl3]−, Δ0 is 15300 cm−1, and it is 22300 cm−1 for [Tp*Ti(CN)3]−. The different Δ0 values are in accord with the fact that CN− is a much stronger field ligand than Cl−. The magnetic properties are determined primarily by the trigonal splitting of the 2T2g term which are very similar for the two compounds; the ground state is the orbitally nondegenerate 2 A1, separated by more than 1000 cm−1 from the orbital doublet 2 E (for simplicity, we use the notation of an idealized C3v symmetry). The a1 orbital that is occupied by the single d electron in the ground state is a dz2 orbital oriented along the C3 axis. It can be seen that the introduction of SOC does not dramatically alter the energy-level structure. This is consistent with the SOC constant of TiIII, ζ ≈ 150 cm−1, which is small in comparison to the ligand-field splitting. The important effect of SOC, however, is that the g factors of the ground state are smaller than the spin-only value of g = 2. The computed values are [Tp*TiCl3]− :

[Tp*Ti(CN)3 ]− :

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g = 1.96, g⊥ = 1.71

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02643. Materials, experimental procedures, physical methods, crystallographic details, computational details, and additional figures (PDF) X-ray crystallographic data in CIF format for 1 (CIF) X-ray crystallographic data in CIF format for 2 (CIF) X-ray crystallographic data in CIF format for 3 (CIF)

g = 1.97, g⊥ = 1.77

Note that, because of the distorted trigonal symmetry, two different g⊥ values were obtained for each complex. The difference between the two is very small, however, and we provide only the average g⊥ value for simplicity sake. For isotropic systems, the effective g value for the A terms is well understood as the splitting of energy levels adjusted by second-order SOC: g = 2 − (8λ/Δ0).41 The parallel components of the g tensor calculated from 1 and 3 are similar to the reported values for other TiIII complexes and for the hexacyanotitanate ion, g = 1.971.37 For 1 and 3, however, we find that g∥ > g⊥, which is a signature of the magnetic anisotropy of these compounds, related to the trigonally distorted ligand field. In fact, the origin of this anisotropy of the g tensor is the same as that of the considerable TIP exhibited by these systems, as in ideal trigonal symmetry, g∥ ∼ g − (λ/N)χTIP,zz, while g⊥ ∼ g − (λ/N)χTIP,xx. Thus, the trigonal crystal field splits the 2T2g term into an orbitally nondegenerate ground-state term 2A and a low-lying excited-state term 2E, separated by an energy gap Δ ≪ Δ0. Hence, in contrast to the isotropic case, there exist low-lying accessible excited states. Because, by symmetry, ⟨2A|Lz|2E⟩ = 0 and the matrix elements of Lx and Ly can be nonzero, it follows that in a perfect trigonal symmetry g∥ ∼ g − 2λΣJ≠0|⟨0|Lz|J⟩|2/Δ = g (although this value will be slightly reduced by contributions arising from lower symmetry and by SOC in higher-order perturbation theory), while g⊥ ∼ g − 2λΣJ≠0|⟨0|Lx|J⟩|2/Δ is significantly reduced because of the small gap Δ and the symmetry-allowed in-plane angular momentum transitions. In Figure 3, the computed susceptibility is compared with the experimental data. On the basis of the energies in Table 1, the computations predict that only the ground state is occupied in the temperature range 0−300 K. Consequently, the computed χT in each case is a straight line whose slope is the TIP of the ground

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Willem Van den Heuvel: 0000-0003-4880-4449 Kim R. Dunbar: 0000-0001-5728-7805 Present Address ‡

M.S.: Department of Chemistry, Fayoum University, Fayoum 63514, Egypt. Author Contributions

All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS K.R.D. gratefully acknowledges financial support from the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award DE-SC0012582. K.R.D. also thanks the Welch Foundation (Grant A-1449) for summer salary for A.B. A.S. acknowledges support from the Australian Research Council Discovery Project (Grant DP150103254). C

DOI: 10.1021/acs.inorgchem.6b02643 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.6b02643 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

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DOI: 10.1021/acs.inorgchem.6b02643 Inorg. Chem. XXXX, XXX, XXX−XXX