Electronic Structure and Reactivity of a Well-Defined Mononuclear

Article pubs.acs.org/IC

Electronic Structure and Reactivity of a Well-Defined Mononuclear Complex of Ti(II) Gayan B. Wijeratne,† Eva M. Zolnhofer,‡ Skye Fortier,§,⊗ Lauren N. Grant,⊥ Patrick J. Carroll,⊥ Chun-Hsing Chen,§ Karsten Meyer,‡ J. Krzystek,∥ Andrew Ozarowski,∥ Timothy A. Jackson,*,† Daniel J. Mindiola,*,§,⊥,∇ and Joshua Telser*,# †

Department of Chemistry, University of Kansas, Lawrence, Kansas 66045, United States Inorganic Chemistry, Department of Chemistry and Pharmacy, Friedrich-Alexander University Erlangen-Nürnberg, 91058 Erlangen, Germany § Department of Chemistry and Molecular Structure Center, Indiana University, Bloomington, Indiana 47405, United States ⊥ Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States ∥ National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32310, United States # Department of Biological, Chemical and Physical Sciences, Roosevelt University, Chicago, Illinois 60605, United States ‡

S Supporting Information *

ABSTRACT: A facile and high-yielding protocol to the known Ti(II) complex trans-[(py)4TiCl2] (py = pyridine) has been developed. Its electronic structure has been probed experimentally using magnetic susceptibility, magnetic circular dichroism, and high-frequency and highfield electron paramagnetic resonance spectroscopies in conjunction with ligand-field theory and computational methods (density functional theory and ab initio methods). These studies demonstrated that trans-[(py)4TiCl2] has a 3 Eg ground state (d1xyd1xz,yz orbital occupancy), which, as a result of spin−orbit coupling, yields a ground-state spinor doublet that is EPR active, a first excited-state doublet at ∼60 cm−1, and two next excited states at ∼120 cm−1. Reactivity studies with various unsaturated substrates are also presented in this study, which show that the Ti(II) center allows oxidative addition likely via formation of [Ti(η2-R2E2)Cl2(py)n] E = C, N intermediates. A new Ti(IV) compound, mer-[(py)3(η2-Ph2C2)TiCl2], was prepared by reaction with Ph2C2, along with the previously reported complex trans(py)3TiNPh(Cl)2, from reaction with Ph2N2. Reaction with Ph2CN2 also yielded a new dinuclear Ti(IV) complex, [(py)2(Cl)2Ti(μ2:η2-N2CPh2)2Ti(Cl)2], in which the two Ti(IV) ions are inequivalently coordinated. Reaction with cyclooctatetraene (COT) yielded a new Ti(III) complex, [(py)2Ti(η8-COT)Cl], which is a rare example of a mononuclear “piano-stool” titanium complex. The complex trans-[(py)4TiCl2] has thus been shown to be synthetically accessible, have an interesting electronic structure, and be reactive toward oxidation chemistry.

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(2,2′-bipyridine).21 These species were all relatively insoluble and likely contain bridging halide ligands. In addition to these examples, there are some Ti(II) synthons or derivatives that are known, which are often stabilized with soft or π-acceptor ligands such as CO,22,23 N2,23−32 alkynes, alkenes,33−37 arenes,38 cyclooctatetraene (COT),39 or cyclopentadienides (Cp−),40,41 as well as Cp variants42 and iminido donors, such as alkyl- or arylamidinates43−46 and porphyrins.47−54 Despite all of these cases being formally Ti(II), the strongly π-accepting nature of one or more of the ligands means that these behave more like Ti(III) or Ti(IV) species. Thus, only a few welldefined mononuclear complexes that unequivocally contain Ti(II) exist. The first examples go back to 1985, when Wilkinson, Girolami and co-workers made a seminal report of the complex [(dmpe)2TiX2] (dmpe = bis(dimethylphosphino)-

INTRODUCTION Titanium(II) systems have been indispensable in organic synthesis with such examples being the Kulinkovich or McMurry-type carbon−carbon coupling reactions,1−5 wherein the active Ti(II) species is usually prepared via treatment of Ti(OiPr)4 or TiCl4 with the corresponding reducing agent (such as Zn, or an alkyllithium or Grignard reagent having βhydrogens). Intramolecular cyclizations have also been achieved using Ti(II) reagents.6−8 Among many other organic reactions, Ti(II) systems have been also implicated in enantioselective pinacol coupling reactions,9,10 and in N2 activation and N-atom transformations resulting in N-heterocycles.11−13 Other ill defined sources of Ti(II) include [TiCl2(THF)n],4,14−16 [TiCl2(NCMe)4],17 or mixed-valent titanium reagents.18−20 Fowles et al. reported an extensive series of complexes of general formula TiX2L2 or TiX2(L-L) where X = Cl− or Br−; L = MeCN, THF (tetrahydrofuran), THP (tetrahydropyran), or py (pyridine); and L-L = phen (1,10-phenanthroline) or bpy © XXXX American Chemical Society

Received: August 7, 2015

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DOI: 10.1021/acs.inorgchem.5b01796 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry ethane, X− = Cl;55 Girolami and co-workers subsequently reported [(dmpe)2TiX2], where X− = Me, η2-BH4, and PhO.56,57 Since then, distinct examples of mononuclear Ti(II) without π-acid ligands have appeared in the literature and include [(TMEDA)2TiCl2] (TMEDA = tetramethylethylenediamine),19 [TiCl2(py)4] (py = pyridine),19,58 (porphyrin)Ti(L)2 (L = THF,48 4-picoline,51 OP(octyl)3),49 the “scorpionate” complex (κ3-Tp)2Ti (Tp− = tris(3,5-dimethylpyrazolyl)hydroborate),59 and the titanocene family [Ti(Cp(Rn))2] (Cp(Rn)− = sterically substituted cyclopentadienide-based ligand).60−67 As noted above, titanium(II) (3d2) is a relatively uncommon oxidation state, and as such its electronic structure remains unknown, even for well-defined examples of its coordination compounds.19,59,68 In terms of simple, purely inorganic species, it is found in binary halides TiX2 (X = Cl,69 Br70) and in TiO,71 all of which have Ti(II) with octahedrally coordinated anions. There are also halide complexes of Ti(II), such as Na 2[TiCl 4],72 in which Ti(II) is likewise octahedrally coordinated with edge-sharing chlorido ligands. TiF2 has been isolated in Ne and Ar matrices and has a bent structure.73 It has an S = 1 ground state, which has been thoroughly investigated by X-band electron paramagnetic resonance (EPR) spectroscopy. The zero-field splitting (zfs) of TiF2 has surprisingly small magnitude (|D| = 0.0784 cm−1, |E| = 0.0020 cm−1; consensus of Ne and Ar results).73 A recent, and surprising development is the observation that Ti metaldissolved in aqueous HF (or other fluoride-containing/producing acids) at very low pH does not give aqueous Ti(III) but rather Ti(II) along with Ti(IV) species. From this solution, a solid with empirical formula TiF3(H2O)2 was isolated, which is actually [Ti(TiF6)(H2O)4], and contains TiIVF6 and TiIIF2(H2O)4 octahedra.74,75 As with matrix-isolated TiF2, this complex is EPR-active at Xband with very small zfs (|D| < 0.003 cm−1). The report by Kölle and Kölle74 on aqueous titanium(II) did spur a number of studies on the redox reactivity of this ion, mainly by Gould and co-workers, as reviewed by Gould.76 In this work, we report a modified protocol for the complex trans-[(py)4TiCl2], originally reported by Gambarotta and coworkers19 and later studied by Cotton and co-workers.58 There has been another report of this complex,77 but the identity of the product is questionable. Their reported synthetic procedure (direct reduction of TiCl4 in the presence of pyridine) is unlikely to give Ti(II), based on numerous examples of Ti(II) chemistry cited above, and the product is referred to as “yellowish”, which is in direct contrast to our findings as well as those of previous workers.19,58 As a continuation of our interest in both the reactivity and the electronic structure of lowoxidation-state early transition metal complexes,78−82 we have selected for an initial foray into Ti(II) chemistry the complex trans-[(py)4TiCl2]. In addition to investigating the reactivity of this complex, and determining independently a crystal structure, we have used physical-inorganic techniques that are suited to such systems with multiple unpaired electrons (i.e., S > 1/2), namely variable-temperature, variable-field magnetic circular dichroism (VTVH-MCD) spectroscopy,83,84 highfrequency and high-field electron paramagnetic resonance (HFEPR) spectroscopy,85,86 and superconducting quantum interference device (SQUID) magnetization methods. We have also used quantum chemistry theory (QCT), including ab initio methods, as well as classical ligand-field theory (LFT) to aid in the interpretation of the experimental results. The result is, to our knowledge, the first complete analysis of the electronic

structure and reactivity of a bona f ide Ti(II) coordination complex.

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EXPERIMENTAL SECTION

General Procedures. Unless otherwise stated, all operations were performed in an M. Braun Lab Master double drybox under an atmosphere of purified nitrogen or using high-vacuum standard Schlenk techniques under a nitrogen atmosphere. Anhydrous benzene, pentane, and toluene were purchased from Aldrich in sure-sealed reservoirs (18 L) and dried by passage through two columns of activated alumina and a Q-5 column. THF and Et2O were distilled, under nitrogen, from purple sodium benzophenone ketyl and stored over sodium metal. Distilled THF and Et2O were transferred under vacuum into thick walled reaction vessels before being carried into a drybox. Deuteriobenzene was purchased from Cambridge Isotope Laboratory, degassed by freeze−pump−thaw cycles, and stored over 4 Å molecular sieves. Celite, alumina, and 4 Å molecular sieves were activated under vacuum overnight at 200 °C. Compounds mer[TiCl3(THF)3],87 N2CPh2,88 and KC889 were prepared following literature procedures. All other chemicals were purchased and used as received. 1H and 13C NMR spectra were recorded on a Bruker DMX 300 MHz or a Bruker DMX 360 MHz spectrometer at 27 °C. 1H and 13 C NMR spectra are reported with reference to residual 1H solvent resonances of C6D6 at 7.16 and 128.1 ppm, respectively. Elemental analyses were conducted in the inorganic chemistry department at the Friedrich-Alexander University Erlangen-Nürnberg, Germany, and also by Robertson Microlit Laboratories, Ledgewood, NJ, USA. Syntheses. Modified Protocol To Prepare trans-[(py)4TiCl2]. In a 250 mL round-bottom flask, 1.000 g (2.70 mmol) of mer[TiCl3(THF)3], prepared as described by Jones et al.,87 was dissolved in 100 mL of THF, giving a pale blue solution. With vigorous stirring, 1.1 mL (13.5 mmol) of pyridine was added, turning the solution dark green instantaneously. A sample of 0.5472 g (4.049 mmol) of KC8 was then added to the flask. The reaction mixture was stirred for 15 min and then filtered through a medium porosity frit layered with a bed of Celite. The Celite was washed with 10 mL of THF, and the combined dark blue filtrate was concentrated in vacuo to ∼20 mL. The supernatant was decanted off, and the dark blue product trans[(py)4TiCl2] was dried in vacuo (yield: 880 mg, 75%, one crop). Attempts to isolate more compound from the filtrate did not produce pure material. Elemental analysis (%), calc for C12H32Cl2N4Ti (MW = 351.18 g/mol): C, 55.20; N, 12.87; H, 4.63. Found: C, 47.27; N, 10.51; H, 4.04. The lower carbon content (as well as H and N) is most likely due to pyridine loss (and possible replacement by oxygen and/or water during handling) and the overall thermally unstable nature of this species. Synthesis of Complex 1. In a 20 mL vial containing a 1/2-in.-long Teflon-coated magnetic stir bar, toluene (4 mL) was added to trans[(py)4TiCl2] (107 mg, 246 μmol), producing a dark blue slurry. In a separate 20 mL vial, toluene (1 mL) was added to PhCCPh (44 mg, 246 μmol), producing a colorless solution. Both vials were left in a freezer at −35 °C for 30 min. The vials were removed from the freezer, and the PhCCPh solution was added dropwise over 5 min to the stirred trans-[(py)4TiCl2] slurry, producing a dark, amber-colored slurry, which was stirred for 1 h at ambient temperature before filtering through a plug of Celite in a pipet, resulting in separation of a small amount of dark green solid from the amber-colored filtrate. The solvent volume was reduced to ∼2 mL in vacuo and n-pentane (10 mL) was added, resulting in the precipitation of a light-green solid. The supernatant was decanted, and the solid was dried in vacuo and isolated as an olive-green microcrystalline powder (99 mg, 75% yield). 1 H NMR (C6D6, 300 MHz, Me4Si, 27 °C): δH 9.28 (ν1/2 = 34 Hz, 6H, o-Hpy), 7.48 (d, 3JHH = 8 Hz, 4H, o-HPh), 7.13−7.02 (5H, Htol) 7.01 (pseudo-t, 3JHH = 8 Hz, 4H, m-HPh), 6.89 (t, 3JHH = 7 Hz, 2H, p-HPh), 6.69 (ν1/2 = 44 Hz, 3H, p-Hpy), 6.42 (ν1/2 = 32 Hz, 6H, m-Hpy), 2.11 (s, 3H, Htol). 13C{1H} NMR (C6D6, 75 MHz, Me4Si, 27 °C): δH 152.0 (ν1/2 = 58 Hz), 141.5 (ν1/2 = 1 Hz), 136.8 (ν1/2 = 41 Hz), 131.9 (ν1/2 = 1 Hz), 128.1 (ν1/2 = 1 Hz), 127.6 (ν1/2 = 1 Hz), 126.5 (ν1/2 = 2 Hz), 123.4 (ν1/2 = 19 Hz). Crystals suited for X-ray diffraction (XRD) B

DOI: 10.1021/acs.inorgchem.5b01796 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

using a Jasco J-815 spectropolarimeter interfaced with an Oxford Instruments SM 4000-8 magneto-optical cryostat. The mull sample was prepared using Fluorolube under an argon atmosphere in a glovebox. The sample was immediately flash frozen in liquid nitrogen upon removal from the glovebox and stored under liquid nitrogen until use. Deconvolution of the MCD spectra of trans-[(py)4TiCl2] was performed using Igor Pro software (version 6.32A). A minimum number of Gaussian functions were used to fit the 2 K, 7 T MCD spectrum from 9300 to 32 000 cm−1. For bands between 9300 and 20 000 cm−1, the widths of the Gaussian functions were constrained to be within 25% of a common value (2000 cm−1). MCD features above 20 000 cm−1 could only be successfully modeled using bands of significantly larger widths (3000−4000 cm−1) or using a greater number of narrower bands. Larger widths for these bands would be consistent with the assignment of these features as charge-transfer transitions that involve larger excited-state distortions relative to the ground-state equilibrium geometry. In our analysis, we chose to include a small set of wider Gaussian functions to model the higherenergy region. In any case, the number of electronic transitions, and the widths of these transitions, is not as well-defined in this spectral region as compared to the lower-energy region. VTVH-MCD data were analyzed following a formalism developed by Neese and Solomon83 and using a fitting and simulation program developed by Riley.96,97 Prior to fitting, the intensities of individual isotherms of a given VTVH-MCD data set were normalized relative to the most intense signal. The VTVH-MCD fits employed ground-state spin Hamiltonian parameters obtained from the SQUID magnetization data. These parameters were fixed during the fitting procedure. Transition moment products (Mxz, Myz, and Mxy) were adjusted so as to minimize the χ2 parameter (χ2 = ∑( f icalc − f iexp)2). To investigate the sensitivity of the VTVH-MCD curves to the magnitude of D, fits were also performed with smaller, but still fixed, D values, as described in the Supporting Information. High-Frequency and High-Field Electron Paramagnetic Resonance Spectroscopy and Data Analysis. HFEPR experiments were performed on multiple, powder samples of trans[(py)4TiCl2] using the Electron Magnetic Resonance Facility at the National High Magnetic Field Laboratory (NHMFL, Tallahassee, FL, USA). The spectrometer employs a Virginia Diodes (Charlottesville, VA, USA) source operating at a base frequency of 12−14 GHz and multiplied by a cascade of multipliers in conjunction with a 15/17 T superconducting magnet. Low-temperature control was provided by an Oxford Instruments (Oxford, UK) continuous-flow cryostat. EPR spectra were simulated using locally written programs available from J. Telser and A. Ozarowski. Magnetometry. Magnetization data of crystalline, finely powdered samples of trans-[(py)4TiCl2] (15−30 mg) restrained within a polycarbonate gel capsule were recorded with a Quantum Design MPMS-XL SQUID magnetometer. DC susceptibility data were collected in the temperature range of 2−300 K under a DC field of 1 T. Values of the magnetic susceptibility were corrected for core diamagnetism of the sample estimated using tabulated Pascal’s constants.98 Data reproducibility was checked by susceptibility measurements on four independently synthesized samples, as shown in Figure S2. Complete field-dependent magnetization studied were made on one of these samples. The program julX written by E. Bill (MPI-CEC, Mülheim/Ruhr, Germany) was used for the fitting of magnetic susceptibility and magnetization data. This program allows the spin Hamiltonian parameters D, E/D, and giso to be independently varied or fixed. Computational Methods. QCT. All quantum chemical calculations were performed using the program ORCA, versions 2.9.1 and 3.0.1.99,100 Two types of geometry optimization were performed, full and constrained. In the constrained optimizations, initial models of trans-[(py)4TiCl2] were constructed using the XRD coordinates reported in this work and the XRD coordinates reported by Araya et al.,58 and only the positions of the hydrogen atoms were geometry optimized. The Cartesian coordinates of all atoms using the XRD and optimized structures are given in Tables S9−S12. These calculations employed the BP functional101−103 and the TZVP (Ti, Cl, and N) and

analysis were obtained by layering a saturated solution in toluene with n-pentane. 1H and 13C NMR spectra for complex 1 are presented in Figure S1a,b. Multiple attempts to obtain satisfactory elemental analysis have been unsuccessful. Synthesis of Complex 2. Complex trans-[(py)4TiCl2] (50.4 mg, 0.116 mmol) was suspended in ∼3 mL of toluene. To this suspension, PhNNPh (14.8 mg, 0.081 mmol) was added as a solid, changing rapidly the color from blue to brown-red with precipitation of a brown solid. The brown solid was filtered, washed with pentane, and then dried under reduced pressure. 1H NMR spectroscopy of the solid in CDCl3 confirmed quantitative formation of complex 2 by comparison with the literature report.90 Synthesis of Complex 3. In a 20 mL vial containing a 1/2-in.-long Teflon-coated magnetic stir bar, toluene (4 mL) was added to trans[(py)4TiCl2] (135 mg, 310 μmol), producing a dark blue slurry. In a separate 20 mL vial, toluene (2 mL) was added to Ph2CN2 (60 mg, 310 μmol), producing a fuchsia-colored solution. The Ph2CN2 solution was added dropwise to the slurry of trans-[(py)4TiCl2] over 2 min, resulting in a color change to orange-red. The mixture was stirred for 15 h and then filtered to remove a small amount of a dark green precipitate, presumably a Ti(III) disproportionation product. The reaction mixture was concentrated to approximately 2 mL in vacuo. To this solution n-pentane (10 mL) was added, resulting in the generation of a precipitate. The supernatant was decanted and the solid was dried in vacuo. The solid was triturated in n-pentane (5 mL) and the supernatant was decanted. The solid was dried in vacuo and isolated as a dark, red residue (76 mg, 21% yield). 1H NMR (C6D6, 300 MHz, Me4Si, 27 °C): δH 9.38 (s/br, 6H, o-HPy), 8.20 (d/br, 4H, oHPh), 7.71 (s/br, 4H, HPh), 7.14−6.78 (12H, HPh), 6.72−6.41 (9H, m,p-HPy). 13C{1H} NMR (C6D6, 75 MHz, Me4Si, 27 °C): δC 136.4, 136.1, 131.6, 131.2, 131.1, 131.0, 129.8, 129.5, 129.1, 128.9, 128.8, 126.8, some resonances for the py and phenyl ligands could not be located. 1H and 13C NMR spectra for complex 3 are presented in Figure S1c,d. Multiple attempts to obtain satisfactory elemental analysis have been unsuccessful. Synthesis of Complex 4 Contaminated with mer-[(py)3TiCl3]. An excess of COT (10−15 equiv) was added at 25 °C to a stirring suspension of trans-[(py)4TiCl2] (100 mg, 0.230 mmol) in toluene (5 mL) resulting in formation of an insoluble brown powder, which was dissolved in a minimum of pyridine (0.1−0.5 mL). The solution was then layered with Et2O and cooled to −35 °C to afford pale-colored crystals of 4 contaminated with pale blue-green crystals of the known complex mer-[(py)3TiCl3].87 Multiple attempts to purify 4 by fractional crystallization failed due to its similar solubility to that of mer-[(py)3TiCl3]. A few crystals of 4 could be separated manually and were used for XRD studies. X-ray Structure Determinations. Suitable single crystals for analysis were placed onto the tip of MiTeGen loop coated in NVH oil and mounted on an Apex Kappa Duo diffractometer. The data collection was carried out at 100−150 K using Mo Kα radiation (graphite monochromator). A randomly oriented region of reciprocal space was surveyed to achieve complete data with a redundancy of 4. Sections of frames were collected with 0.50° steps in ω and ϕ scans. Final cell constants were calculated from the xyz centroids of a particular number of strong reflections for each crystal from the actual data collection after integration (SAINT).91 The intensity data were corrected for absorption (SADABS).92 The space groups were determined based on intensity statistics and systematic absences. The structures were solved using SIR-9293 and refined (full-matrix least-squares) using either SHELXL-9794 or Oxford University Crystals for Windows system.95 A direct-methods solution was calculated, which provided most non-hydrogen atoms from the Emap. Full-matrix least-squares/difference Fourier cycles were performed, which located the remaining non-hydrogen atoms. All nonhydrogen atoms were refined with anisotropic displacement parameters. The hydrogen atoms were placed in ideal positions and refined as riding atoms. Magnetic Circular Dichroism Spectroscopy and Data Analysis. MCD spectra at various temperatures and fields (VTVHMCD) were collected for a solid (mull) sample of trans-[(py)4TiCl2] C

DOI: 10.1021/acs.inorgchem.5b01796 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry SVP (C and H) basis sets104 in conjunction with the corresponding TZVP/J and SV/J auxiliary basis sets,105,106 as required for use with the RI approximation.107 Two separate full optimizations were performed. One was at the BP/TZVP/SVP level. The second also utilized TZVP/SVP basis sets, but featured the B3LYP hybrid functional 108−110 and dispersion corrections developed by Grimme111−113 (B3LYP-D). All geometry optimizations were performed using the spin-unrestricted approach and tightly converged to the S = 1 spin state. Numerical frequency calculations were used to verify that the fully optimized models represent true minima. Electronic transition energies and ground-state splittings were computed for all models by performing complete active space selfconsistent field (CASSCF) calculations in conjunction with N-electron valence perturbation theory (NEVPT2).114−116 The CASSCF/ NEVPT2 calculations were performed on authentic (i.e., nontruncated) structures of trans-[(py)4TiCl2]. These calculations used TZVPP (Ti, Cl, and N) and TZVP (C and H) basis sets, along with the corresponding Coulomb fitting auxiliary basis sets (TZVP/C and SV/C). The CAS(2,5) active space included only the five Ti 3d-based orbitals. This active space is justified by the relatively low covalency between the Ti(II) center and the ligands; the use of a significantly larger CAS(12,10) active space was also explored (vide inf ra). The initial guess orbitals for the CASSCF calculations were a set of quasirestricted orbitals (QROs) from density functional theory (DFT) calculations (at the BP/TZVPP/TZVP level). The “rotate” keyword in ORCA was used to ensure that the frontier QROs contained the Ti(II) 3d-based orbitals. Ten roots for the triplet state (corresponding to the 3 F and 3P free ion terms) and 15 roots for the singlet state (corresponding to the 1D, 1G, and 1S free ion terms) were included. Convergence of the CASSCF wave function required the use of modest level shifting. The NEVPT2 calculations were performed on top of the state-averaged CASSCF calculation, as used previously in the calculation of ligand-field states and zfs parameters for transition metal systems.117−121 NEVPT2 calculations employed both stateaveraged orbitals with state-specific orbital energies and alternatively, canonical orbitals for each state. These two procedures gave similar results (vide inf ra). Zero-field-splitting parameters were calculated using the CASSCF/NEVPT2 wave function using the effective Hamiltonian approach, as described previously.121 LFT. Analysis of the electronic structure of Ti(II) in trans[(py)4TiCl2] was performed with use of two approaches crystal-field parametrization122 and the angular overlap model (AOM).123 Two computer programs were employed, Ligfield,124 written by J. Bendix (Ørsted Institute, Copenhagen, Denmark), and a locally written program, DDN, available from J. Telser. Both programs use the complete d2 weak-field basis set including interelectronic repulsion (Racah parameters: B and C)125 and spin−orbit coupling (SOC)124 and either crystal-field (for DDN, the parameters Dq, Ds, Dt, Dr) or AOM ligand-field bonding parameters (εσ, επ).123 For free-ion Ti(II) (values in cm−1), B = 714, C = 2663 (C = 3.73B),125 and ζ = 118.124 The two programs gave identical results when directly compared. The Ligfield program allows identification of the orbital occupancy and spin progeny of a given energy level (eigenstate). The Tanabe− Sugano diagram was generated using a locally written computer program, DTANSUG, available from J. Telser, that employs the matrix elements tabulated by McClure126 and the strong-field model assignments therein.

treating the dimer [Ti2(μ2-Cl)3Cl2(TMEDA)2] with pyridine.19 Consequently, the difficulty in preparing thermally unstable [(TMEDA)2TiCl2], as well as the low-yield process to the blue species, trans-[(py)4TiCl2], from reduction of Ti(III) prompted us to explore a simpler route or alteration to the original protocol. It is known that trans-[(py)4TiCl2] has a propensity to undergo disproportionation to mer-[(py)3TiCl3] along with other byproducts. As a result, detailed magnetic and spectroscopic studies of the royal-blue crystalline solid trans[(py)4TiCl2] have not been reported, nor has its reactivity been investigated, presumably due to the difficulty in accessing this species in bulk quantities from readily available precursors. This difficulty is not unique to trans-[(py)4TiCl2]; Gambarotta’s related Ti(II) species trans-[(TMEDA)2TiCl2] unfortunately decomposes to mer-[(L)3TiCl3] and the mixed-valent species [Ti2(μ2-Cl)3Cl2(TMEDA)2] in donor solvents, such as L = THF or Et2O, and is unstable even in toluene solution.19 In an attempt to prepare trans-[(py)4TiCl2] in better yield, we discovered that treatment of mer-[TiCl3(THF)3] with a slight excess of KC8 (1.3−1.4 equiv) and a nearly stoichiometric amount of py (5 equiv) in toluene and a minimum of THF resulted in consistently good yields (70−80%) of pure trans[(py)4TiCl2] (Scheme 1). By dissolving completely the starting Scheme 1. Synthesis of Complex trans-[(py)4TiCl2] from mer-[(THF)3TiCl3]

material in a minimum of THF, and by using a nearly stoichiometric amount of py (∼5 equiv), the reaction will result in precipitation of the pure material as blue crystals. The tendency of trans-[(py)4TiCl2] to decompose in THF is also minimized when using less pyridine, i.e., just enough to dissolve the Ti(III) starting material. Thus, pyridine is virtually used as a ligand and not as a solvent because in excess it hastens decomposition of the product. Regrettably, trans-[(py)4TiCl2] cannot be prepared directly from the convenient Ti(IV) reagent trans-[TiCl4(THF)2] using a slight excess of KC8 (2.4 equiv) under similar solvent conditions. This is likely due to the propensity of Ti(IV) and Ti(II) to comproportionate, especially when the second reduction is expected to be sluggish.127 Along these lines, it should also be noted that reports of TiCl2 being prepared from reduction of TiCl4 with Me3SiSiMe3 were incorrect,128 and were instead leading to formation of TiCl3.129 Once prepared, trans-[(py)4TiCl2] is remarkably stable as a solid at low temperature (−35 °C), decaying mostly to mer-[(py)3TiCl3] over the course of 2 days at room temperature. Therefore, we have conducted reactions in toluene so long as the reagent reacts rapidly with the Ti(II) precursor, otherwise disproportionation reactions take place over a period of hours (when using gram quantities). In THF solution, trans-[(py)4TiCl2] readily forms mer-[TiCl3(THF)3] along with an intractable material within 2−3 h at room temperature. Complex trans-[(py)4TiCl2] has limited solubility in arene solvents and Et2O with the latter also promoting rapid disproportionation. Multiple attempts to obtain satisfactory elemental analysis failed due to the thermal instability of this

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RESULTS AND DISCUSSION Synthesis of trans-[(py)4TiCl2]. Complex trans-[(py)4TiCl2] was originally reported by Gambarotta and co-workers in 1991,19 and a structure as well as alternative protocol was later published by Cotton and co-workers in 1995.58 This complex was initially prepared via treatment of [(TMEDA)2TiCl2] with pyridine19 and soon after by reduction of TiCl3 with KC8 in a mixture of pyridine, toluene, and THF.58 Alternatively, trans-[(py)4TiCl2] can be obtained, albeit in lower yield and contaminated with mer-[(py)3TiCl3], by D

DOI: 10.1021/acs.inorgchem.5b01796 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

originally reported. As noted in the previously reported structure, some of the C-atoms forming the pyridine rings have fairly large and elongated thermal parameters, therefore suggesting some disorder in these ligands (the THF solvate was also highly disordered). Given that our data were collected at much lower temperature, we can “freeze-out” such disorder and consequently render the system less symmetric without the pyridine rings being coplanar. Overall, our structure yields a better least-squares R-factor (0.033 vs 0.073) with data being collected at a better resolution (max 2θ was 55° for this work vs 45° for prior structure). Discussion of the metrical parameters of trans-[(py)4TiCl2] will be addressed later (Table 3, vide inf ra). Optical Spectroscopic Studies of trans-[(py)4TiCl2]. To provide a framework for our analysis of spectroscopic data collected for trans-[(py)4TiCl2], we will briefly summarize prior optical spectroscopic investigations of the ligand-field excited states of Ti(II) centers. Such studies of Ti(II) centers are uncommon; however, there are two relevant ones, each instructive in its own way. The UV−vis spectrum of what is purportedly [Ti(H2O)6]2+ exhibits bands at 650 and 430 nm that, by analogy with [V(H2O)6]3+, are assigned (Oh point group symmetry) to 3T1g → 3T2g(F) and 3T1g → 3T2g(P) transitions, respectively.74 From these bands, the authors calculated LFT parameter values of B = 432 cm−1 (only 60% of the free-ion value, 714.3 cm−1124) and Δoct (10Dq) = 11 665 cm−1.74 This analysis is incorrect, as it would yield 3T1g → 3T2g at 10 790 cm−1 (927 nm) and 3T1g → 3T2g(P) at 16 394 cm−1 (610 nm). A correct analysis gives B = 607.1 cm−1 (85% of the free-ion value) and Δoct (10Dq) = 16 623 cm−1, which matches the observed bands within ∼2 cm−1. The parameters determined here are also closer to those reported for [V(H2O)6]3+.130,131 In contrast, rigorously analyzed electronic absorption and detailed luminescence studies were performed for Ti(II) doped into MgCl2 and MgBr2.132 Also useful is a subsequent review of Ti(II)- and Ni(II)-doped halides.133 The relevant results of the original study are summarized in Table 2. For TiII:MgCl2, the lowest-energy spin-allowed ligand-field transition, 3T1g(F) → 3 T2g (using Oh notation), was observed in the near-infrared (NIR) region of the 10 K electronic absorption spectrum at 9260 cm−1. Electronic transitions from the 3T1g(F) ground state to the 3T1g(P) and 3A2g excited states were observed at 16 130 and 19 340 cm−1. The 3T2g and 3T1g(P) excited states of TiII:MgBr2 were observed to be red-shifted relative to TiII: MgCl2 (Table 2), consistent with the weaker-field nature of the bromido ligands. The 3T1g(F) → 3A2g transition was not observed for TiII:MgBr2.132 For visual comparison between TiII:MgCl2 and trans[(py)4TiCl2], we show in Figure 2 a Tanabe−Sugano diagram generated using the LFT parameters derived for the latter complex (vide inf ra). Not all transitions observed by Jacobsen et al. are depicted,132 but the relationship between the two systems in the context of electronic transitions in octahedral d2 complexes can be readily seen. Octahedral symmetry, however, is insufficient to describe either system fully; TiII:MgCl2 exhibits trigonal distortion and trans-[(py)4TiCl2] exhibits tetragonal distortion. This effect in trans-[(py)4TiCl2] will be described next, making use of structural information. Because the present X-ray structure of the trans-[(py)4TiCl2] complex shows C1 symmetry (Figure 1), the 3T(1,2)g ground (t22e0 in a strong-field representation) and excited states (t21e1) in the parent Oh group will each split into three components.

complex, presumably due to its tendency to disproportionate or lose py. Solid-State Structural Studies of trans-[(py)4TiCl2]. Although the solid-state structure of trans-[(py)4TiCl2] has been reported by Cotton and co-workers,58 we re-collected XRD data on a single crystal, at lower temperature than originally performed (100 K versus 213 K) in order to use more precise metrical parameters for the purpose of computational studies. Accordingly, crystals of trans-[(py)4TiCl2]·THF can be obtained from cooling the reaction mixture after separation of graphite at −35 °C. A perspective view of the molecular structure of trans-[(py)4TiCl2] is shown in Figure 1, and

Figure 1. Thermal ellipsoid plot of complex trans-[(py)4TiCl2] at 50% probability level. Hydrogen atoms and one THF solvent molecule have been excluded for clarity.

crystallographic parameters are shown in Table 1. For comparison, Table 1 also lists the corresponding data for the structure reported previously.58 Overall, metrical parameters are quite similar, but not identical, to those observed by Cotton and co-workers.58 Although our complex crystallizes in the same crystal system, the more precise thermal parameters do not render the titanium atom on an inversion center as Table 1. Refinement Data for Reported Structures of trans[(py)4TiCl2]·THF molecular formula formula weight T (K) crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) Z V (Å3) μ (mm−1) dcalc (g/cm3) F(000) crystal size (mm) λ (Å) 2θ range (deg) R1a, wR2b R1, wR2 (all data)

C24H28Cl2N4OTi 507.30 100(1) monoclinic Cc 14.7185(7) 9.2708(4) 17.9821(8) 90 96.803(3) 90 4 2436.42(19) 0.594 1.383 1056 0.26 × 0.14 × 0.04 0.71073 2.28−27.50 0.0329, 0.0706 0.0400, 0.0733

C24H28Cl2N4OTic 507.30 213(2) monoclinic C2/c 14.884(1) 9.317(3) 18.167(2) 90 95.98(2) 90 4 2505.6(5) 0.578 1.345 1260 0.15 × 0.30 × 0.39 0.71073 1.38−30.12 0.073, 0.170 0.087, 0.180

R1 = (|F0|−|Fc|)/ |F0|. bwR2 = [[w(F02 − Fc2)2]/[w(F02)2]]1/2. cSee Araya et al.58

a

E

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Inorganic Chemistry Table 2. Experimental and Calculated Energy Levels (cm−1) for Ti(II) Systems

However, on the basis of the experimental Ti−ligand bond lengths and angles (see Table 3; a more extensive summary of metrical parameters for trans-[(py)4TiCl2] from the several crystal structures58,77 now available is given in Table S1), it is reasonable to assume approximate D4h symmetry for the spectral analysis. The use of an idealized high symmetry for the inner coordination sphere of a metal complex is well established.134 Under this assumption, the 3T1g(F) ground state will split into 3Eg and 3A2g components, with the relative ordering of these states dependent on both the energies of the b2g and eg orbitals that derive from the t2g set (Figure 3) and the electron−electron repulsion energies. The π-donor properties of the chloride ligands will place eg(dxz,dyz) above b2g(dxy), giving a 3Eg ground state with a low-lying 3A2g excited state. In addition, the ordering of the eg-derived a1g(dz2) and b1g(dx2−y) orbitals will determine the nature in which the 3T2g and 3T1g(P) excited states are split. Assuming a splitting of a1g below b1g, as supported by electronic structure computations discussed later, the ligand-field state ordering shown in Figure 3 (right) is expected. The 7 T MCD spectrum of a mull of trans-[(py)4TiCl2] shows a set of signals between 9000 and 32 000 cm−1 that all show an increase in intensity with decreasing temperature (Figure 4). This observed temperature-dependence of the MCD signals is known as C-term behavior,83 which is expected from the paramagnetic, triplet ground state of trans-[(py)4TiCl2]. A Gaussian deconvolution of the 2 K, 7 T MCD spectrum reveals a minimum of 10 transitions between 9000 and 32 000 cm−1 (see Figure 5 and listings in Tables 2 and S2). The NIR region of the MCD spectrum is dominated by a pseudo-A term (derivative-shaped feature) centered at 11 500 cm−1, with positive and negative features that peak at 10 000 and 12 900 cm−1, respectively (the shoulder at ∼11 800 cm−1 has been observed in MCD spectra of unrelated samples and is an artifact of the NIR detector). Considering the previous spectral assignments for TiII:MgCl2 and TiII:MgBr2, this NIR signal must derive from components of the 3T2g excited state (i.e., 3B2g and 3Eg; Figure 3). The strong SOC between the (nearly) degenerate excited states from the 3Eg

trans-[(py)4TiCl2] CASSCF/ NEVPT2

Oh state

Ti:MgCl2a

Ti:MgBr2a

MCDd

3

T1g(F)

0 690−810b

0

0

1

T2g(G)

7 665b

8 477 8 673 8 816

Eg

8 203c 8 232c

10 416 10 726

T2g

9 260

1

3

1

A1g(G)

3

8 220

15 935c

10 000 (1) 12 900 (2)

11 901 14 532 14 725

14 500 (3)

20 216

15 650 (4) 17 500 (5) 19 800 (6)

20 911 21 163 21 470

T1g(P)

16 130

1

T2g(G)

17 612c 17 916c

22 146 24 835 25 042

1

T1g(G)

19 518c 19 573c

26 488 26 765 26 938

19 340

28 157

3

A2g

15 040

0 76 1 337

a

From 10 K electronic absorption experiments described in ref 132. From 10−20 K luminescence experiments described in ref 132. c Determined from a full LFT calculation; see text and ref 132. dThe numbers in parentheses correspond to bands identified in Figure 4. b

Figure 2. Tanabe−Sugano diagram for octahedral Ti(II) complexes, generated using the Racah parameters derived for trans-[(py)4TiCl2] (B = 477 cm−1, C = 1780 cm−1). The electronic transitions observed here for this complex are shown as black arrows. A simplified representation of the numerous transitions observed by Jacobsen et al.132 are shown as gray arrows (see Table 2 for a complete listing). The predominant free-ion parentage of each term is indicated on the right and is color coded. The “g” subscript has been deleted in these labels for clarity. The strong-field representation of each term is also provided. F

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Inorganic Chemistry Table 3. Bond Lengths (Å) and Bond Angles (deg) for trans-[(py)4TiCl2] As Determined by XRD and DFT Energy Minimizations experimental a

ref 58b

this work

B3LYP-Dc

BPc

Ti−N

N1: N2: N3: N4:

2.261 2.265 2.280 2.254

N1: 2.271(4) N2: 2.278(4) N1′: 2.271(4) N2′: 2.278(4)

2.289

2.218

Ti−Cl

Cl1: 2.492 Cl2: 2.498

Cl1: 2.497(1) Cl1′: 2.497(1)

2.521

2.431

Cl1−Ti−Cl2

176.21

180

179.99

180

Cl1−Ti−N

N1: N2: N3: N4:

N1: 90.0(1)

N1: 89.35 N2: 90.66 N1B: 89.35 N2B: 90.66

N1: 89.22 N2: 90.78 N1B: 89.22 N2B: 90.78

N1−Ti−N

N2: 90.44 N3: 178.37 N4: 271.54

N2: 91.0(2) N3: 179.997(1) N4: 89.0(2)

N2: 83.50 N1B: 178.69 N2B: 263.49

N2: 89.19 N1B: 178.45 N2B: 269.22

89.78 92.23 90.89 88.09

a

Data taken from the structure determined in this work. bData taken from the structure of trans-[(py)4TiCl2] with THF solvent reported by Araya et al.58 cThe calculations employed the TZVP (Ti, N, and Cl) and SVP (C and H) basis sets.

Figure 4. Variable-temperature, 7 T MCD spectra for a mull sample of trans-[(py)4TiCl2]. The inset shows an expanded view of the NIR spectral region. Figure 3. Ligand-field (top) and d-orbital (bottom) splitting patterns for Ti(II) in an octahedral field (Oh, left; see also Figure 2) and for the approximate D4h field of trans-[(py)4TiCl2] (right).

these excited states can account for only four of the six MCD features in this spectral window. Several singlet excited states (1A1g(G), 1T2g(G), and 1T1g(G), using Oh notation) could also give rise to some of the observed MCD features. These spinforbidden transitions can gain intensity by SOC with the nearby triplet excited states. The 1A1g excited state is expected to be at lower energy than the 3T1g(P) state, and an excitation to this state could account for the sharp, negative MCD band at ∼14 800 cm−1 (band 3; see Figure 5). Under this assignment, bands 4, 5, and 6 arise from excitation to the components of the 3 T1g(P) parent state (Figure 3). Specifically, band 4 would arise from the 3A2g excited state, with bands 5 and 6, which consist of a pseudo-A term centered at 18 800 cm−1, due to the second 3 Eg excited state. The MCD features above 23 000 cm−1 would

term is expected to give rise to a pseudo-A term in the MCD spectrum;84 therefore, this NIR feature is attributed to the components of this 3Eg state (Table 2). The excited state is blue-shifted relative to the 3T2g excited states of TiII:MgCl2 and TiII:MgBr2, as expected given the stronger donor properties of the pyridine ligand. Transitions to the 3A2g and 3Eg states (derived from 3T1g(P)), as well as the 3B1g state (derived from 3A2g), contribute to the cluster of six transitions from 15 000 to 25 000 cm−1 (bands 3− 8; Figure 5 and Table S2). However, even with C1 symmetry, G

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Inorganic Chemistry

allowed transitions are well fitted by this model (see Table S3), which employs (values in cm−1): B = 439.4 (62% of the free-ion value), C = 1640 (based on the free-ion ratio Racah parameter ratio C/B = 3.73125), Dq = 1380, Ds = −462, Dt = +331. These LFT parameters can be compared to those obtained for Ti: MgCl2: B = 527 (74% of the free-ion value), C = 1983 (C/B = 3.76), Dq = 1018, Dσ = +281, Dτ = −142.132 The greater covalency in the molecular complex, trans-[(py)4TiCl2], as well as its heteroleptic coordination, leads to lower Racah parameters and overall larger magnitude crystal-field parameters with a greater distortion from an octahedral field. The electronic transitions can also be analyzed using the AOM.123 Titanium(II) has not been previously analyzed by the AOM, so there are no bonding parameters available, even for ligands as common as the present ones, chloride and pyridine. However, for CrIII in the relevant coordination environment, trans-MA2B4 (D4h), for B = ethylenediamine and NH3 (values in cm−1), εσ ≈ 7000−7500, for A = Cl−, εσ ≈ 5500, επ ≈ 1000; and for NiII in this geometry, for B = py, εσ = 4670, επ = 570, for A = Cl−, εσ = 2980, επ = 540.135 For Cr(III) with generic pyridine coordination, εσ = 6150, επ = −330.123 Using the above value for 10Dq, one obtains an average εσ ≈ 4600 cm−1, which is in line with the value for Ni(II) in trans-[(py)4NiCl2]. The π-bonding interactions can be complicated and, at present, will be ignored for the py ligands, but include cylindrical πdonation from the chlorido ligands. As described above, this πdonation to the dxz,yz orbitals (eg in D4h) is the key factor in the tetragonal splitting of the 3T1g ground state (see Figure 3). Fitting the observed electronic transitions using the AOM is thus severely hampered because we lack knowledge of this tetragonal splitting, as it likely is in the mid-IR region.136 Accordingly, we have performed fits with B fixed at each of 430, 450, and 470 cm−1, and with επ(Cl) fixed at each of 500 and 1000 cm−1, with initial guesses for εσ(Npy) = 5000 and εσ(Cl) = 3000 cm−1, based on the above examples. The results are summarized in Table S3 and show that estimates of εσ(Npy) = 5000(100) and εσ(Cl) = 3200(100) cm−1 are consistent with either set of επ(Cl) values and provide a reasonable match to the observed electronic transitions. A wealth of options exist to fine-tune these AOM results, such as use of the crystallographic geometry(ies) and including π-bonding of pyridine ligands (with the experimental twist angle, ψ), but the relatively small number of observed transitions make such efforts unwarranted for describing the optical spectra. A related point of interest is that the present structure, unlike the earlier one,58 lacks the linear Cl−Ti−Cl axis, although we have not introduced this effect in the above analysis. This bend might be a manifestation of a pseudo-Jahn−Teller (PJT) effect (“pseudo” because we have no real understanding of what distortions are introduced directly by the ligands). The Jahn−Teller effect in (pseudo)octahedral t21(5)e0(0) systems137 or t22(4)e0(0,2) systems138 is more complicated and less studied than in t23(6)e1(1,3)systems139 and we will not speculate further here. The program Ligfield124 provides orbital occupancies and a representative output file is shown in Table S4. It can be seen that the 3Eg ground state corresponds to d1xyd1xz,yz and the lowlying 3A2g excited state to d0xyd2xz,yz. The 3B2g excited state (derived from 3T2g) corresponds to d1xyd0xz,yzd1z2d0x2−y2 and its 0.5 1 partner 3Eg to d0xyd1xz,yzd0.5 z2 dx2−y2. The excited singlet state, A1g, observed via a spin-forbidden transition, corresponds to 1.5 2 0 3 3 d0.5 xy dxz,yz, i.e., still t2 e , but with a spin-flip. The Eg and A2g 3 excited states, derived from T1g(P), respectively correspond to 0.5 0 1 3 1 0 d0xyd1xz,yzd0.5 z2 dx2−y2 and dxydxz,yzdz2dx2−y2. Lastly, the B1g excited

Figure 5. MCD spectrum of trans-[(py)4TiCl2] at 2 K and 7 T (solid green line). Individual Gaussian bands (dashed black lines) and their sum (dashed red line), obtained from fits of the MCD data, are displayed.

be due to excitations to the 1T2g, 1T1g, and 3A3g states. Given the complexity of this region of the MCD spectrum, as well as the possibly large number of contributing excited states, assignments for the remainder of the MCD features of trans[(py)4TiCl2] cannot be made solely on the basis of the experimental data. We note for comparison that previous electronic absorption spectra of the trans-[(py)2TiCl2] complex reported by Fowles et al. showed a broad absorption envelope centered at 24 400 cm−1 (solid-state, diffuse reflectance) or 25 200 cm−1 (pyridine solution).21 The Ti(II) center in trans-[(py)2TiCl2] was proposed to have a polymeric structure with trans-py and bridging chlorido ligands. In light of that polymeric structure, the absorption features were attributed to transitions to a conduction-type band rather than to d−d transitions. On the basis of our assignments for trans-[(py)4TiCl2], it is reasonable to presume that Ti(II) d−d transitions at least contribute to the broad visible absorption features of trans-[(py)2TiCl2]. Ligand-Field Theory Analysis of Optical Spectroscopy of trans-[(py)4TiCl2]. The Ti:MgCl2 system studied by Jacobsen et al. exhibited a wealth of electronic transitions, observable using absorption and luminescence techniques.132 Moreover, the Ti(II) ion had homoleptic chlorido hexacoordination with only a small magnitude trigonal distortion (calculated as Vtrig = −246 cm−1, using only the 3T1g levels). It was thus possible using LFT to match perfectly all of the excited states of this system. The situation for trans-[(py)4TiCl2] is not so favorable, but we can make use of the electronic transitions seen by MCD (Table 2) to model the electronic structure of this complex using LFT. We first use a model with Oh symmetry, which yields (values in cm−1): 3 T1g(F) → 3T2g at 11 450, 3T1g(F) → 3T1g(P) at 17 650, using B = 477 (67% of the free-ion value) and Dq = 1240. The observed spin-forbidden transition, 3T1g(F) → 1A1g(G), is also well fitted by this model. The fit results are summarized in Table S3; Figure 2 shows the energy levels of the various terms using this description. We then introduce a tetragonal splitting, parametrized by Ds and Dt,122 analogous to the trigonal parameters used by Jacobsen et al.132 The observed spinH

DOI: 10.1021/acs.inorgchem.5b01796 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry state, which was not observed as it corresponds to t20e2, derived from 3A2g(F), thus corresponds to d0xyd0xz,yzd1z2d1x2−y2. The nature of the remaining singlet excited states can also be seen in Table S5. Thus, use of LFT, whether with a crystal-field or AOM parametrization, provides a reasonable explanation of the optical spectra of trans-[(py)4TiCl2], analogous to, albeit less precise than, the crystal-field analysis of Ti:MgCl2.132 Quantum Chemistry Theory (DFT and CASSCF/ NEVPT2) Electronic Structure Computations. We next turn to more sophisticated QCT calculations, the likes of which were unavailable to Jacobsen et al.132 (and likely would still be a challenge for an extended solid system). We first describe geometry optimizations and then turn to calculation of electronic transitions. Geometry Optimizations. We utilized four different models of the trans-[(py)4TiCl2] complex in our electronic structure computations. Two models were derived directly from X-ray coordinates (the present X-ray structure and that reported previously by Araya et al.58), with only the positions of the hydrogen atoms optimized, and the other two models were fully optimized using the BP and B3LYP-D functionals, the latter of which features an empirical van der Waals correction. Although no symmetry constraints were used to optimize the models, the structure optimized at the BP level shows nearly perfect D4h symmetry, with all equivalent Ti−N and Ti−Cl bond lengths and metal−ligand bond angles within 2° of their idealized values (Table 3). The B3LYP-D-optimized structure is close to D4h symmetry, but displays deviations in the equatorial N−Ti−N angles (Table 3). Although these deviations are minor, this structural difference, as well as other minor structural differences between the experimental and geometry-optimized structures, will lead to differences in the calculated energies of the lowest ligand-field states (i.e., the components of the 3T1g(F) state), as will be discussed later. In any case, the structures of both optimized models are in good agreement with the experimental structure, showing bond length and bond angle deviations of less than 0.062 Å and 4°, respectively (Table 3). The largest deviation is in the Cl−Ti− Cl angle, which is 180° in the computed models, but 176.21° in the experimental structure. Notably, the BP and B3LYP-D procedures yield Ti−ligand bond lengths that are too short and too long, respectively (Table 3). Overall, the B3LYP-D method gives smaller deviations with respect to the experimental structure. Electronic Structure. A spin-unrestricted MO energy level diagram of the Ti(II) 3d-based orbitals of trans-[(py)4TiCl2] derived from DFT calculations (Figure 6) is qualitatively similar to that developed from simple LFT considerations (Figure 3). Spin polarization, a consequence of electron exchange interactions, causes the α-spin orbitals to lie at lower energy than their β-spin counterparts. As expected on the basis of ligand-field arguments (vide supra), the b2g(dxy) orbital lies at lowest energy. The DFT calculations predict a Ti−pyridine back-bonding interaction. The eg(dxz,dyz) set are split, as expected for this C1 complex, although the splitting is minor (less than 0.12 eV or 970 cm−1 for the β-spin orbitals). Both MOs of the eg(dxz,dyz) set have π-antibonding interactions with the chloride ligands, although the chloride contributions to these MOs are minor (6.6−0.7%). The b1g(dx2−y2) and a1g(dz2) MOs, which lie highest in energy, show Ti σ-antibonding interactions with the pyridine and chloride ligands, respectively. These MOs are split in energy by only 0.3 eV (2400 cm−1), with b1g(dx2−y2) at higher energy.

Figure 6. Ti 3d orbital splitting pattern for trans-[(py)4TiCl2] based on spin-unrestricted DFT calculations performed using the present Xray structure coordinates. Isosurface plots of corresponding quasirestricted MOs are shown on the right, along with the percent contributions from Ti 3d, Cl 3p, and N 2p orbitals for the β-spin orbitals. The list of orbital contributions for the α-spin orbitals is found in Table 4.

Table 4. Molecular Orbital Symmetry Labels, Energy, and Percent Compositions Based on Spin-Unrestricted DFT Computations Using the Present X-ray Structure Coordinates of trans-[(py)4TiCl2] composition (%) MO 111α 112α 113α 122α 123α 115β 120β 121β 122β 123β

symmetry label (in D4h) b2g(dxy) eg(dxz,dyz) a1g(dz2) b1g(dx2−y2) b2g(dxy) eg(dxz,dyz) a1g(dz2) b1g(dx2−y2)

occupancy

energy (eV)

1.0 1.0 0.0 0.0 0.0

−3.1547 −2.6913 −2.3646 −0.4866 −0.1283

56.8 75.8 75.7 74.5 73.0

0.2 6.6 5.5 10.1 0.8

5.0 2.9 3.7 3.2 6.5

0.0 0.0 0.0 0.0 0.0

−1.6375 −0.6356 −0.5102 0.0437 0.3458

50.9 82.0 71.5 72.0 64.8

3.3 3.2 0.7 8.2 0.4

3.3 2.5 8.2 2.1 4.6

Ti 3da Cl 3pb N 2pc

a

Sum of all Ti 3d contributions to this MO. bSum of all Cl 3p contributions to this MO. cSum of all N 2p contributions to this MO.

Ligand-Field Excited States. Using the trans-[(py)4TiCl2] models described above, CASCF/NEVPT2 calculations were performed to predict the ligand-field transition energies to all triplet and singlet excited states of the Ti(II) center. Neese and co-workers have recently described the CASSCF/NEVPT2 approach,117−121 which incorporates both static and dynamic correlation, as an economical procedure for treating groundand excited-state properties of transition metal complexes. We note that while the multi-reference SORCI (spectroscopically oriented configuration interaction) method has also been shown to be highly accurate in this regard, this method is not economical and is therefore limited to systems with a small number of electrons.140−144 I

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Figure 7. SQUID magnetization data on several, independently synthesized powder samples of trans-[(py)4TiCl2]. Left: variable-temperature effective magnetic moments at an external field of 1 T (two independently synthesized samples, data points indicated respectively by black uppointing and down-pointing triangles), lines represent fits using a spin Hamiltonian with S = 1, D = −62.0 cm−1, |E/D| = 0.05 (yielding E = −3.1 cm−1; value derived from HFEPR, but consistent with independent fits of susceptibility data), with g = 1.820 (red trace; fit of data set given by uptriangles); and with g = 1.765 (green trace; fit of data set given by down-triangles). Right: temperature dependence of the magnetization for a third, independently synthesized sample recorded at four different magnetic fields: 0.1 T (at baseline), 1, 3, and 5 T. Experimental data points (indicated by black squares) are fitted (red traces) using S = 1, D = −62.0 cm−1, |E/D| = 0.09 (yielding E = −5.6 cm−1), g = 1.805.

bands 4, 5, and 6 derive from the 3T1g(F) → 3T1g(P) transition (Table 2). Because of the large number of contributing excited states at higher energy, as well as the modest accuracy in calculated transition energies, the calculations do not provide significant aid in the assignments of the higher energy MCD features (bands 7−10; see Figure 5). Magnetic-Field-Related Measurements. The above experimental (MCD) and theoretical (LFT and QCT) provides a consistent picture of the electronic structure of the ground and excited states of trans-[(py)4TiCl2]. What has not been considered, however, is the effect of SOC on these states, which can be considerable for an orbitally degenerate ground state, such as 3Eg, found here for trans-[(py)4TiCl2]. Jacobsen et al. observed a splitting of the 3A2g (in D3d) ground state of Ti: MgCl2 into two spinor levels: A1g and Eg, separated by 4.7 cm−1.132 Using a spin Hamiltonian model for S = 1, which is appropriate for such an orbitally non-degenerate state, this corresponds to D = +4.7 cm−1, a modest zfs reasonable for such a system.85,146 The situation for trans-[(py)4TiCl2], however, is very different and will be probed inasmuch as possible with use of a variety of magnetic-field-related physical techniques: DC magnetometry, and variable-temperature, variable-field MCD (VTVH-MCD), and HFEPR spectroscopies. Theoretical models will also be applied to understand these results. DC Magnetometry. DC magnetometry measurements were performed on trans-[(py)4TiCl2]. The VT DC magnetic susceptibility curves (Figure 7, left) confirm the S = 1 spin ground state of this complex, with an effective magnetic moment μeff of 2.78 μB at 300 K (based on an average of four independent samples; see Figure S2). This result is consistent with the room temperature magnetic moment measured by the method of Evans reported for this complex (μeff = 2.67 μB, no specific temperature provided).19 Our solid-state experimental μeff value corresponds to a spin-only calculation for S = 1 with g = 1.966, which is quite reasonable for a titanium paramagnet. (For example, Ti(III), 3d1, S = 1/2, which gives conventional (X-band) EPR, exhibits g|| = 2.000, g⊥ = 1.921 (gav = 1.947) in Ti(acac)3;147 g|| = 1.994, g⊥ = 1.896 (gav = 1.929) in [Ti(H2O)6]3+.148) This result indicates that, at least at ∼300 K, trans-[(py)4TiCl2] can be described as a simple S = 1 system. The difficulty lies in describing the low-temperature magnetic behavior. We have available fitting protocol that uses only a spin Hamiltonian (see Experimental Section). Fits to the

Electronic transition energies calculated at the CASSCF/ NEVPT2 level are listed alongside the MCD transition energies in Table 2. For the sake of convenience, we will discuss these excited states by reference to the Oh parent states. While we have included the energies of the components of the 3T1g(F) state in this table for completeness, more appropriate energies require the inclusion of spin-orbit interactions, as described later. Good agreement is observed for the experimental and calculated energies for the components of the 3T2g excited state (Table 2), confirming the assignment of the lowest-energy MCD features. In particular, the energies are within approximately 2000−4000 cm−1 of the experimental values. Use of a significantly larger CAS(12,10) active space, which includes all frontier chlorido-based orbitals, showed only minor changes in excited-state energies (typically less than 300 cm−1; see Table S5). Additionally, two separate sets of NEVPT2 calculations were employed, one using state-averaged orbitals and state-specific orbital energies and the second using canonical orbitals for each state. These different procedures likewise led to only modest differences in state energies (less than 400 cm−1; see also Table S5). Therefore, we consider only the former set of calculations in this work. We note that similar discrepancies between experiment and CASSCF/NEVPT2calculated ligand-field transition energies were reported for a series of Ni(II) complexes and attributed to missing dynamic correlation in the NEVPT2 treatment and/or the use of an incomplete basis set.145 Notably, calculated electronic transition energies for the four models of trans-[(py)4TiCl2] described above (Table 2), show only minor variations of less than 500 cm−1. Thus, the discrepancy between experimental and calculated excited-state energies is likely not due to structural inaccuracies. However, these structural variations between the four DFT models become important when considering groundstate magnetic properties (vide inf ra). As expected on the basis of LFT considerations (vide supra; also see Tables S3 and S4), there is a cluster of transitions predicted by the CASSCF/NEVPT2 calculations between 20 000 and 28 150 cm−1. These arise from transitions to the 1 A1g(G), 3T1g(P), 1T2g(G), 1T1g(G), and 3A2g(F) excited states. Importantly, the 1A1g(G) is predicted to lie just below a set of three transitions arising from the 3T1g(P) excited states. Thus, these calculations lend credence to our previous assignments; i.e., band 3 arises from the 3T1g(F) → 1A1g(G) transition, and J

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by HFEPR.150,151 However, given that Ti(II) (3d2, here S = 1) is a non-Kramers (integer spin number) system, its EPR response strongly depends on the magnitude of zfs.73,85,86 As described above, MCD and magnetometric techniques indicate a very large magnitude zfs, which could make trans-[(py)4TiCl2] “EPR-silent” even at the highest frequencies available to EPR spectroscopists, except in special cases. Nevertheless, we have subjected one batch of powder trans-[(py)4TiCl2] to HFEPR experiments (the corresponding SQUID magnetometry data set is given by up-triangles in Figure 7). No transitions in the geff ≈ 1.9 range (e.g., at ∼7.5 T using ∼200 GHz) were observed, which would correspond to allowed (ΔMS = ±1) transition for a true S = 1 system with small zfs, as suggested by the high-temperature magnetic moment (vide supra). This result is consistent with a large (by EPR standards) energy separation among states, which is defined in a spin Hamiltonian as a large magnitude D value, as described in the magnetometry and MCD sections. However, we did observe an EPR transition at relatively low fields, ∼1.8 T in the 200−220 GHz range of frequencies as shown in Figure 8 (main figure).

susceptibility data with an S = 1 spin Hamiltonian gave an acceptable fit, with an isotropic g value of 1.80(5)not unreasonableand an axial zfs parameter of D = −60(5) cm−1, with a rhombicity factor of |E/D| = 0.055(5) (E = −3.3 cm−1, assigned the same sign as D). This very large magnitude D value (i.e., on the order of magnitude of the SOC constant for Ti(II): ζ = 93 cm−1 in Ti:MgCl2132) is indicative of unquenched orbital angular momentum,149 as expected from the 3Eg ground state described above. Therefore, this axial zfs has no real physical meaning, but nevertheless serves two useful functions: (a) it provides a measure of consistency among different physical techniques, all of which can be fitted with the same spin Hamiltonian, and (b) it gives a crude idea as to the energy separation between the ground state (or, at least, lowlying states) and first excited states as a result of SOC. The magnetization data obtained by field-dependent measurements shows the expected nesting behavior (Figure 7, right). This data set, recorded for only one sample due to experimental constraints, was successfully fitted, consistent with point (a) above, using spin Hamiltonian parameters close to those used for the susceptibility (see Figure 7 caption), albeit with a larger rhombicity, |E/D| = 0.09 (so that E ≈ −5.6 cm−1). Alternatively, roughly the same rhombicity could be used throughout, but with a correspondingly larger magnitude D value in some cases (see Figure S2). In general, rhombicity is both extremely sensitive to small structural variations and notoriously difficult to extract from magnetometry,146 even when a spin Hamiltonian is fully appropriate. The present case is experimentally further complicated by the air and moisture sensitivity of the complex and its documented structural variation. Concern over variations in magnetic fits is a case of “not seeing the forest for the trees” in that the very applicability of the model is questionable. VTVH-MCD. With its physical analogy to magnetometry, VTVH-MCD data can likewise be analyzed using a spin Hamiltonian. However, the appearance of VTVH-MCD saturation curves is also dependent on the transition moment products (Mxy, Mxz, Myz), making such fitting more challenging, even in a case for which a spin Hamiltonian is appropriate. Nevertheless, good fits for two VTVH-MCD data sets on trans[(py)4TiCl2] were obtained using the S = 1 spin Hamiltonian with an E/D value of 0.05 and axial g values (gz = 1.90, gx,y = 1.75; giso = 1.80), as suggested by a combined analysis of DC magnetization and HF-EPR data (vide inf ra). The sensitivity of the VTVH-MCD fits to the magnitude of D were assessed systematically by varying D while optimizing the transition moment products and fixing E/D and g in accordance with either the SQUID or combined SQUID/HF-EPR analyses (Figure S3). From this analysis, it is clear that the VTVH-MCD curves collected at 35 700 cm−1 are largely independent of the magnitude of D as along as D < −20 cm−1. The goodness of fit for curves collected at 13 000 cm−1 are more dependent on E/ D and g (Table S6 and Figure S4), but are uniformly poor when D < −70 cm−1. Overall, the VTVH-MCD fits do not provide a precise value for D but are certainly consistent with the range −70 cm−1 < D < −20 cm−1. Thus, with the acknowledgment that an S = 1 spin Hamiltonian is not appropriate for trans[(py)4TiCl2], we see that both magnetometry and VTVHMCD provide a large magnitude, negative D value. HFEPR. Paramagnetic transition metal complexes have been traditionally successfully investigated by EPR, including both conventional and HFEPR techniques.85,86 The Ti(III) ion (3d1, S = 1/2), as found in heterogeneous catalysts, has been studied

Figure 8. Main plot: HFEPR spectrum (black trace) of powder trans[(py)4TiCl2] recorded at 5 K and 208 GHz, with simulation (red trace) using a spin Hamiltonian: S = 1, D = −62.0 cm−1, E = −3.115 cm−1, g⊥ (gxy) = g|| (gz) = 1.90; single-crystal line width 45 mT (Gaussian, fwhm). Inset: Energy levels for S = 1 spin sublevels (labeled on right) with magnetic field oriented along the z-axis of the zfs tensor, calculated with the spin Hamiltonian parameters used for the red trace. The red arrow indicates the transition observed in the main plot.

As with the other magnetic field-dependent methods, for lack of a better model we use a spin Hamiltonian and describe this as the nominally forbidden ΔMS = ±2 transition.86 Within this model, this transition couples the MS = ±1 spin levels (ΔMS = ±2 and/or 0); and is thus uninformative with regard to the value of D (Figure 8, inset). However, its observation at multiple frequencies allowed us to apply further the spin Hamiltonian and determine by a best fit the parameter |E|, which is equal to 3.10(1) cm−1. Use of this E value (assigning it a negative sign by convention) combined with the D value from magnetometry, gives |E/D| = 0.05, which is consistent with most of the fitting models for magnetometry and VTVH-MCD. Since the resonance observed in Figure 8 corresponds to the parallel turning point of that particular transition, we could also determine the gz value by following its field and frequency dependence, as shown in Figure S5, which equals 1.90(2). This gz (g||) value is higher than the isotropic g value obtained from the spin Hamiltonian fits to field-dependent magnetometry; K

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⟨Sz2⟩ ≈ ±1) and a singlet (with ⟨Sz2⟩ ≈ 0) which thus could be modeled by D = −9.2 cm−1, i.e., in the range of Ti:MgCl2, but here, it is an excited, not the ground state. The 3Eg ground state splits into two closely spaced singlets, separated by ∼1.5 cm−1, i.e., in the range of the rhombic zfs extracted using the S = 1 spin Hamiltonian. The splitting here using LFT is smaller, but we use ideal octahedral geometry and symmetry-equivalent ligands. At ∼55 cm−1 higher in energy is a spin doublet. This energy level corresponds roughly to |D|, but rather than being a singlet, as in the spin Hamiltonian model, it is actually a true doublet. There are then two higher singlet states that have no correspondence whatsoever in the S = 1 model. We currently do not have the ability to apply this LFT to fitting the magnetometric, VTVH-MCD, or HFEPR data, but we can do a calculation that includes an applied external magnetic field. The energy levels are relatively insensitive to external magnetic field, so we use for illustration 0.1 and 1 T. The lowest-energy doublet remains split by ∼1.5 cm−1 at 0.1 T and at 1 T with B0x, but the splitting increases to 1.66 cm−1 at 1 T for B0z. An EPR transition within this doublet is magnetic dipole allowed, with B1⊥B0x and with B1∥B0z. We note that in the HFEPR spectrometer employed here, there is no resonant cavity so that both parallel and perpendicular mode transitions are possible. With 1 T applied field the spin expectation value of the ground-state doublet is ⟨Sz2⟩ ≈ ±0.43, while it is near zero for the doublet at ∼55 cm−1. Transitions from the ground doublet to this first excited doublet would in any event be outside of our HFEPR spectrometer range, requiring ∼1.65 THz. However, the allowed transition within the ground-state doublet would require only ca. 45−50 GHz. We observed transitions at ∼200 GHz (∼6.7 cm−1), so clearly the LFT model employed here understates the ground-state doublet splitting, as already noted above, but at least qualitatively we can explain the observation of a relatively low-field (and low-frequency, by HFEPR standards) signal for trans-[(py)4TiCl2]. Indeed, although a purely heuristic exercise, we found that inclusion of π-bonding from trans-pyridine ligands was a means to increase the ground-state doublet splitting. The interaction must of course be trans-oriented to introduce a rhombic ligand-field splitting; involving all four py has little effect. It is also the case that the επ‑c bonding parameter primarily affects dxz and thus has a much larger rhombic effect than επ‑s, which primarily affects dxy (the py4 plane in our system). To conform to the results of QCT calculations described above, we attribute π-acceptor behavior to the trans-py. This interaction at a very modest level, (επ‑c)py2,4 = −50 cm−1, increased the ground-state doublet splitting to ∼2.5 cm−1 (∼75 GHz). We do not feel that it is worthwhile to search for a parameter set that exactly reproduces the observed HFEPR transition, as we would still have no meaningful (i.e., non-spin Hamiltonian) model for the VTVH-MCD and magnetometry results, but we believe that it is possible to rationalize the spin Hamiltonian fits to these techniques to a more realistic model. Ground-State Splittings from QCT. The CASSCF/NEVPT2 computations can also be used to probe the splitting of the 3Eg ground state and 3A2g excited state in the presence of SOC. The results provide a complementary picture, as seen in Table 5, which provides the energies of the nine lowest-lying energy levels (see Figure 9) following SOC. The exact energies vary by model, but, with the exception of the model optimized at the B3LYP-D level, are quite similar. The lowest-energy B1 and B2 spinor components are split by ∼1 cm−1, and lie ∼50 cm−1

however, if gx,y (g⊥) values that were correspondingly lower were used, e.g., g = [1.75, 1.75, 1.90], then giso = 1.80 would result. Overall, using the spin Hamiltonian parameters as a metric for consistency, the agreement within a given sample preparation is remarkably good, and all batches and techniques taken together give a consistent model for trans-[(py)4TiCl2]. LFT Analysis of SOC Effects. An S = 1 spin Hamiltonian was used to provide a framework for modeling the combination of related physical techniques: magnetometry, VTVH-MCD, and HFEPR. This model yielded a large magnitude, negative axial zfs, D ≈ −60 cm−1 with a small rhombic component (|E/D| ≈ 0.05) and g values that were close enough to the room temperature magnetic moment value to be plausible. The problem is that this model assumes that there is a ground-state spin doublet ⟨S, MS| = ⟨1,±1| which is split by a small amount (2|E|), and then there is an excited-state singlet ⟨S, MS| = ⟨1,0| some 60 cm−1 higher in energy (see Figure 8, inset). This picture is seriously misleading because there are actually six low-energy states, not three, as a result of the 3Eg ground state of trans-[(py)4TiCl2]. A realistic picture can be generated by use of the LFT model developed above, with inclusion of SOC. One must first choose a magnitude of SOC parameter, ζ. Jacobsen et al. had a precise measure of zfs in Ti:MgCl2: D = +4.7 cm−1, which they were able to match closely using ζ = 93 cm−1,132 which is 80% of the free-ion value: 118 cm−1.124 We employed, somewhat arbitrarily, ζ = 95 cm−1, with the numerical results given in Table S7 and shown graphically in Figure 9. Note that since the SOC is relatively small, the

Figure 9. Energy level diagram of trans-[(py)4TiCl2] using the LFT (AOM) parameters given in Table S6. The left side of the diagram is not to scale, but is shown to provide context for the ground-state 3T1g term in octahedral and tetragonal symmetry. The right side of the diagram is roughly to scale, except that the terms derived from the 3A2g excited state are approximately 1000 cm−1 higher in energy, as indicated. The designation of the spinor terms in the D4* double group, as determined by the Ligfield program, is also indicated.

energies of the states corresponding to the experimental electronic transitions, both spin-allowed and forbidden, are still well matched (Table S3, values in parentheses). As seen in Figure 9, the 3T1g ground state in Oh symmetry splits into 3Eg and 3A2g in D4h (repeating Figure 3), then the effect of SOC is shown. The 3A2g state splits into a spin doublet (with L

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Inorganic Chemistry Table 5. Energies (in cm−1) of the Lowest Eigenvectors of the Spin−Orbit Coupling Matrix from CASSCF/NEVPT2 Calculations

Scheme 2. Reactivity Studies (25 °C) of Complex trans[(py)4TiCl2] with Various Unsaturated Substrates To Form Complexes 1−4

XRD Oh 3

Eg

3

A2g

D4*

this work

B1 B2 E

0 0.98 43.8 120.4 162.8 171.7 1389.0 1389.1 1392.9

A1 A2 E A1

a

ref 58b

BPc

B3LYP-Dd

0 1.37 59.1 93.2 150.4 159.2 1467.7 1467.8 1471.3

0.0 0.21 50.4 146.1 194.9 198.2 2857.4 2861.5 2861.5

0.0 2.64 7.55 702.9 709.0 717.5 1425.9 1428.2 1432.2

a

Present structure was used. Positions of the hydrogen atoms were energy-minimized at the BP/TZVP/SVP level. The positions of all other atoms are from the X-ray structure coordinates. bStructure from Araya et al.58 Positions of the hydrogen atoms were energy-minimized at the BP/TZVP/SVP level. The positions of all other atoms are from the X-ray structure coordinates. cAll nuclear coordinates were subjected to full energy minimization using the BP functional and TZVP (Ti, N, and Cl) and SVP (C and H) basis sets. dAll nuclear coordinates were subjected to full energy minimization using the B3LYP functional, the VDW10 keyword, and TZVP (Ti, N, and Cl) and SVP (C and H) basis sets.

solution (C6D6) over several days at room temperature. A solidstate structure analysis of single crystal of 1 reveals a mononuclear, formally titanium(IV) complex having a strained metallacyclopropene framework (Figure 10). The CC distance of the alkyne (1.2966(19) Å) has been substantially reduced in accord with a CC moiety and compares well with other structurally characterized examples of titanium η2-alkyne complexes.35,47,51,154−157 However, doubly reduced alkynes can also serve as 6e− donors, as suggested by Cotton and coworkers.158 To preserve a pseudo-octahedral geometry, the titanium center retains the two chlorides as well as three pyridine ligands, where the latter occupy a mer-configuration.

below the lowest component of the E level. Here the E level is non-degenerate, as the models do not have true D4h symmetry. For completeness, we have also included the corresponding zfs parameters calculated for these systems in Table S8. The anomalous, and erroneous, result for the B3LYP-Doptimized model is nonetheless instructive. In this case, there are three lowest energy levels well below all others. This is exactly the situation expected for an orbitally non-degenerate ground-state, and an example of a system that would be wellmodeled using a spin Hamiltonian. In fact, zfs parameters calculated for this model at the CASSCF/NEVPT2 level are typical (D = −6.23 cm−1, E/D = 0.21), but completely inconsistent with experimental results obtained for trans[(py)4TiCl2]. Presumably, the larger splitting between the ground and lowest excited state for the B3LYP-D-optimized structure is due in part to the deviations of the adjacent equatorial N−Ti−N bond angles from 90° that cause the molecular x- and y-axes to be inequivalent (Table 3). This result is an extreme example of the exquisitely sensitive relationship of ground-state properties to structural parameters,152,153 and highlights the difficulty (or, in some cases, futility) in predicting ground-state magnetic parameters for structurally ill-defined systems. Reactivity Studies of Complex trans-[(py)4TiCl2]. Surprisingly, the reactivity of trans-[(py)4TiCl2] has not been reported, presumably due to its original low-yield reaction from mer-[TiCl3(THF)3], as described above in the synthesis section. Our facile protocol, coupled to the high-yield synthesis of this simple Ti(II) reagent, allowed us to investigate some of its reactivity, in particular how it can engage in two- and oneelectron chemistry. Accordingly, treatment of trans‑[(py)4TiCl2] with 1 equiv of PhCCPh in toluene immediately results in formation of an amber-orange-colored solution from which the complex trans-[Ti(η2-PhCCPh)(Cl)2(py)3] (1) could be isolated as olive-green-colored crystals in 75% yield after workup of the reaction mixture (Scheme 2). Complex 1 has limited solubility and gradually decomposes in

Figure 10. Thermal ellipsoid plot of complex 1 at 50% probability level. Hydrogen atoms have been excluded for clarity. Distances are reported in Å and angles in degrees. Ti1−Cl1, 2.4199(4); Ti1−Cl2, 2.4150(4); Ti1−N1, 2.2486(12); Ti1−N2, 2.3874(12); T1-N3, 2.2486(12); Ti1−C1, 2.0430(14); Ti1−C2, 2.0397(14); C2−C2, 1.2966(19); Cl1−Ti1−Cl2, 160.584(16); N3−Ti1−N1, 167.24(4); C2−Ti1−C1, 37.03(5); N3−Ti1−Cl1, 94.15(3); N3−Ti1−Cl2, 86.22(3); N3−Ti1−N2, 82.83(4); N1−Ti1−N2, 84.43(4); N1− Ti1−Cl1, 84.55(3); N1−Ti1−Cl2, 90.81(3). M

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Figure 11. Thermal ellipsoid plots of complexes 3 (left) and 4 (right) at 50% probability level. Hydrogen atoms and three pyridines (for 3) confined in the asymmetric unit have been excluded for clarity. Distances are reported in Å and angles in degrees. For 3: Ti1−N1, 1.972(2); Ti1−N2, 2.088(3); Ti1−N3, 1.987(3); Ti1−N4, 2.072(3); Ti1−N7, 2.279(3); Ti1−Cl1, 2.3651(11); Ti1−Cl2, 2.3403(10); Ti2−N1, 1.888(3); Ti2−N3, 1.869(2); Ti2−N5, 2.273(3); Ti2−N6, 2.296(3); Ti2−Cl3, 2.3752(10); Ti2−Cl4, 2.3751(11); N1−N2, 1.339(3); N2−C1, 1.305(4); N3−N4, 1.335(3); N4−C14, 1.306(4); Ti1···Ti2, 2.8104(8); Cl2−Ti1−Cl1, 162.20(4); Cl4−Ti2−Cl3, 160.58(4); C1−N2−N1, 128.0(3); C14−N4−N3, 129.3(3); N5−Ti2−N6, 86.25(10); N7−Ti1−Cl1, 81.75(8); N7−Ti1−Cl2, 81.00(8). For 4: Ti−Cl1, 2.49 (disordered); Ti1−N1, 2.3075(12); Ti1−N2, 2.3058(13); Ti1−C1, 2.369(3); Ti1−C2, 2.364(3); Ti1−C3, 2.373(3); Ti1−C4,, 2.378(6); Ti1−C5, 2.387(2); Ti1−C6, 2.377(3); Ti1− C7, 2.376(3); Ti1−C8, 2.372(3); N2−Ti1−N1, 83.42(4).

combines with starting material by displacing two equatorial pyridine ligands. We do not observe any evidence for N2 elimination to form diphenylcarbene ligands in contrast to other diazoarylalkane adducts of titanium.32,162−169 We last turned our attention to COT, a well-known 2e− acceptor molecule, to form planar, aromatic COT2−. Accordingly, treating trans-[(py)4TiCl2] with COT in toluene caused a color change from blue to a pale blue, and after several hours of mixing, workup of the reaction resulted in crystallization of two distinct complexes. One of the two compounds was identified as mer-[TiCl3(py)3]. Separation of most of the blue-greencolored crystals of mer-[TiCl3(py)3] resulted in isolation of another product. X-ray structural analysis of a single crystal revealed the formation of a new complex, namely the Ti(III) three-legged piano-stool complex [Ti(η8-COT)Cl(py)2] (4, Figure 11, right). Although the COT ligand is rotationally disordered, the connectivity of this species can be unambiguously ascertained. The sluggish nature of the reaction, coupled with the formation of Ti(III), suggests a possible mechanism for the formation of 4, namely that the initial 18e− Ti(IV) product, [Ti(η8-COT)(Cl)2(py)2] is reduced by unreacted trans-[(py)4TiCl2] via the following reaction sequence:

This titanium(II) center can also form metal−ligand multiple bonds by using 2 equiv of trans-[(py)4TiCl2] for 4e− reduction chemistry. For example, treatment of trans-[(py)4TiCl2] with 0.5 equiv of PhNNPh results in formation of the known Ti(IV) phenylimido complex, namely trans-[TiNPh(Cl)2(py)3] (2), previously reported by Mountford and coworkers.90,159 This 4e− reductive splitting of azobenzene most likely involves a bimolecular pathway via the intermediate trans[Ti(η2-PhNNPh)(Cl)2(py)3], which has a geometry at titanium analogous to that of 1. Reductive cleavage of azobenzene to form imido complexes of titanium(IV) has been previously documented by Gambarotta and Rothwell and their coworkers.160,161 Mononuclearity is not always preserved when trans-[(py)4TiCl2] reacts with a 2e− oxidant. For example, treatment of trans-[(py)4TiCl2] with N2CPh2 slowly changes the blue suspension to an orange-red-colored solution from which yellow-colored crystals of a new material can be obtained. Due to the highly arylated nature of the reaction product, 1H and 13 C NMR spectra were rather uninformative (Figure S1). In addition, no gas evolution was observed, suggesting the diazo unit in N2CPh2 to be preserved in the product. Ultimately, we relied on single-crystal XRD studies to establish the degree of aggregation as well as the binding mode of the diazoalkane ligand (Figure 11, left). A single-crystal XRD study revealed the dark-colored plate to be the dinuclear complex [(py)2(Cl)2Ti(μ2:η2-N2CPh2)2Ti(Cl)2] (3), resulting from bridging of two diazoalkane ligands via the α-N-atoms of N2CPh2 (Figure 8). Although each titanium center in 3 is pseudo-octahedral and formally Ti(IV), each metal center possesses significantly different ligand environments. Notably, this inequivalence is due to η2-coordination of two diazoalkane ligands to only one metal ion (Scheme 2, Figure 11). The chlorides are nearly trans to each other, while the pyridines on Ti2 are orthogonal at 86.25(10)°. The formation of 3 suggests that two N2CPh2 molecules most likely react rapidly with trans-[(py)4TiCl2] to form the transient [TiCl2(N2CPh2)2(py)2] species, which then

[Ti IICl 2(py)4 ] + COT → [Ti IV(η8‐COT)(Cl)2 (py)2 ] + 2py [Ti IV(η8‐COT)(Cl)2 (py)2 ] + [Ti IICl 2(py)4 ] → [Ti III(η8‐COT)Cl(py)2 ] + [Ti IIICl3(py)3 ] + py

Structurally characterized complexes of titanium with η8-COT, including substituted COT, are few in number (the most recent CSD release lists only 23 such complexes). In the absence of donor ligands such as py or THF, these species exist as tetramers;170 however, examples of sandwich-like Ti(III) compounds such as [CpTi(COT)] (or with substituted Cp− ligands) and mixed-valent Ti(III)−Ti(IV) species with N

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N-η1-NCPh2)2] (3), and a “piano-stool” bis-pyridine Ti(III) complex of COT (4), which resembles a previously reported bpy analogue.178 We hope that our studies will inspire further work in the coordination and organometallic chemistry of trans[(py)4TiCl2] and related low-oxidation-state early transition metal coordination complexes.

substituted COT2− ligands are known.171−176 Even with donor ligands, such complexes can be dinuclear as in the chloride bridged Ti(III) COT complex [Ti(η8-COT)(THF)(μ-Cl)]2.170 Examples of mononuclear three-legged piano-stool COT complexes of Ti(III) are extremely rare, such as the scorpionate complex [Ti(η8-COT)(κ3-Tp)],177 and [Ti(η8-COT)(bpy)Cl] which was prepared from mer-[TiCl3(THF)3] and structurally characterized.178 Lehmkuhl and Mehler reported the electrochemical reduction of TiCl4 in the presence of py and COT to give what was described as [Ti(η8-COT)Cl(py)], but no structure, nor other physical characterization, was given.179 Thus, the structural characterization of 4 is of interest even though attempts to isolate and fully characterize 4 in pure form were marred by contamination with mer-[TiCl3(py)3].

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b01796. Tables S1−S12, giving energy levels from CASSCF/ NEVPT2 and LFT calculations, VTVH-MCD results, and Cartesian coordinates based on crystallography and DFT computations; Figures S1−S5, showing NMR spectra for 2 and 3, magnetic susceptibility data and fit reproducibility, VTVH-MCD results and fits, and HFEPR frequency/field dependence (PDF) X-ray crystallographic data for 1 (CIF) X-ray crystallographic data for 2 (CIF) X-ray crystallographic data for 3 (CIF) X-ray crystallographic data for 4 (CIF)

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CONCLUSIONS Species containing a well-defined mononuclear titanium(II) ion not compromised by strongly π-acidic ligands are relatively uncommon, yet have an important role in organic synthesis. We are interested in the electronic structure and reactivity of coordination complexes that are unequivocally those of Ti(II). The complex trans-[(py)4TiCl2], originally reported by Gambarotta and co-workers19 and later studied by Cotton, Murillo and co-workers,58 is such a system. We have developed an improved, higher yield, and more convenient synthetic route to this complex andfor the purpose of its electronic structure investigationredetermined its crystal structure at lower temperature. The structure is similar, but distinctly different from that reported originally,58 which is suggestive of the lability of such an ion. We have employed a suite of physical methods, SQUID magnetometry and VTVH-MCD and HFEPR spectroscopies in conjunction with classical (LFT) and contemporary (DFT, ab initio) QCT computations. In particular, VTVH-MCD is rarely applied to early transition metal complexes in general, and the present work is its first app lication to Ti(II). These studies show t hat trans‑[(py)4TiCl2] has a 3Eg electronic ground state (in 1 1 idealized D4h symmetry), corresponding to dxy dxz,yzdz02dx02−y2 orbital occupancy. Such an orbitally degenerate (or minimally, nearly degenerate) system is much more complicated to describe than orbitally non-degenerate systems. As a result, the application of a standard S = 1 spin Hamiltonian to describe the magnetic-field-related techniques (magnetometry, VTVHMCD, and HFEPR) is problematic. Lacking suitable alternatives, we forged ahead and applied this model, which did provide a consistent result from all three techniques, namely a large magnitude, negative, nearly axial zfs (D ≈ −60 cm−1). We show using LFT that this is an oversimplified picture for what actually consists of a ground-state doublet that is EPR-active, with two excited (nearly) doublet states lying at ∼60 and ∼120 cm−1 above the ground state. We speculate that this electronic configuration, with unpaired electron density residing in all of the π-interacting 3d orbitals, may facilitate its reactivity with πacceptor molecules. We have indeed documented interesting reactivity of this complex with unsaturated substrates, forming a variety of novel oxidative addition products, each of which has been structurally characterized. We observe the expected 2e− chemistry for trans-[(py)4TiCl2] but due to its thermal instability, reactions are accompanied by disproportionation to form Ti(III) species. The reactions reported here include formation of a new η2-alkyne complex (1), an imido complex (2), previously prepared by a different route,90,159 a novel dinuclear reaction product with diazobenzene, [Ti2Cl4(py)3(μ2-

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Present Addresses ⊗

S.F.: Department of Chemistry, University of Texas, El Paso, TX 79968, USA ∇ D.J.M.: Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104, USA Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been supported by the NHMFL, which is funded by the U.S. National Science Foundation (NSF, Cooperative Agreement DMR 1157490), the State of Florida, and the U.S. Department of Energy (DOE). T.A.J. acknowledges the NSF (CHE-1056470) for support. S.F acknowledges the NSF American Competitiveness in Chemistry Fellowship (CHE1137284) for support. D.J.M. thanks the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Science, Office of Science, U.S. DOE (DE-FG02-07ER15893) for financial support. K.M. and E.M.Z. thank the FriedrichAlexander University of Erlangen-Nürnberg (FAU) for financial support. We thank Prof. Mark J. Riley, University of Queensland, Australia, for the MCD analysis software and Prof. Gregory S. Girolami, University of Illinois at Urbana− Champaign, for helpful comments.

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DOI: 10.1021/acs.inorgchem.5b01796 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.5b01796 Inorg. Chem. XXXX, XXX, XXX−XXX