The Tyranny of Arm-Wrestling Methyls on Iron(II) Spin State in Pseudo-Octahedral [Fe(didentate) 3 ] Complexes

: The connection of a sterically constrained 3-methyl-pyrazine ring to a N -methyl-benzimidazole unit to give the unsymmetrical α , α’ -diimine ligand L5 has been programmed for the design of pseudo-octahedral spin-crossover [Fe( L5 ) 3 ] 2+ units, the transition temperature ( T 1/2 ) of which occurs in between those reported for related facial tris-didentate iron chromophores fitted with 3-methyl-pyridine-benzimidazole in a LaFe helicate ( T 1/2 ~ 50 K) and with 5-methyl-pyrazine-benzimidazole L2 ligands ( T 1/2 ~350 K). A thorough crystallographic analysis of [Fe( L5 ) 3 4 ) 2 by five orders of magnitude observed in going from [M( L2 ) 3 ] 2+ to [M( L5 ) 3 ] 2+ (M = Ni II and Zn II ) is diagnostic for the operation of this effect, which had been not foreseen by the authors.


Introduction
In line with the formulation of the ligand field theory [1,2], or as it was originally called by Bethe, crystal-field theory [3], it was realized that an open-shell metal with at least two valence electrons in a specific chemical environment could exist with either high-spin or low-spin configuration [4]. Following van Vleck's approach to magnetism [5], Pauling perceptively recognized that it would be feasible to obtain systems in which two spin states could be present simultaneously, while their ratio should depend on the energy difference between them [6,7]. The discovery of thermal spin-state equilibria operating in Fe III dithiocarbamate by Cambi et al. [8][9][10] at the same period, indeed confirmed these predictions. Since then, a myriad of metal coordination complexes and polymeric materials have been shown to display spin transitions, often referred to as spin-crossover (SCO) materials. These have been studied in detail and extensively reviewed over the last two decades [11][12][13][14][15][16][17][18][19][20]. Due to the 'on-off' switching of the magnetic properties accompanying the spin transition from the low-spin diamagnetic configuration ( 1 A1 label in octahedral symmetry) to the high-spin paramagnetic form ( 5 T2 label in octahedral symmetry) for d 6 transition metals in pseudo-octahedral geometry (Scheme 1a), the 'magic' [Fe II N6] chromophores, where N is a heterocyclic nitrogen donor atom, have been intensively investigated [11][12][13][14][15][16][17][18][19][20]. Various external stimulations such as changes in temperature [21,22], pressure [23,24], magnetic field [25] or light-irradiation [26,27] can be used for inducing the SCO processes, which makes these microscopic magneto-optical switches very attractive for their introduction into responsive macroscopic materials [12,13,16,[28][29][30][31]. The most common and accepted approach for rationalizing the design of spin-crossover pseudo-octahedral Fe II complexes relies on the energetic balance accessible temperatures. However, the latter statement is misleading and physically unsound since both ∆oct and P change during the spin transition as a result of the population of the antibonding orbitals in the high-spin form. For pseudo-octahedral spin-crossover [Fe II N6] complexes, the Fe-N bond lengths extend by approximately 10% upon the low-spin to high-spin transition and ∆oct consequently decreases according to a 1/r n dependence with n = 5-6 (Equation (1)) [31]. 75% of the free ion values, whereas the dashed green domain corresponds to non-accessible ligand-field strengths (see text) [31].
Taking into account the 10% bond length expansion accompanying the spin transition, the simplistic zero-point energy differences between the two states summarized in Scheme 1a (i.e.,  [31]. Following this theoretical approach, the toolkit of coordination chemists for programming and tuning the thermodynamic spin transition parameters in molecular [Fe II N6] complexes logically relied on the manipulation of ∆oct and B via (i) some controlled distortions of the coordination geometry from a perfect octahedron by using chelating ligands with fixed bite angles [33] and (ii) specific programming of/metal-ligand bonding interactions via ligand design [15,19,34]. Benefiting from the huge amount of experimental data collected during the last decades for [Fe(N ∩ N)3] 2+ complexes, where N ∩ N is an α,α'-diimine chelate ligand possessing two N-heterocyclic donor atoms, it was shown that the connection of a six-membered heterocycle to a five-membered heterocycle in N ∩ N provides favorable ligand-field strengths around Fe II for promoting spin-state equilibria (Equation (5)) with transition temperatures T1/2 = ∆HSCO/∆SSCO (i.e., the temperature at which ∆GSCO = 0 and xhs = xls = 0.5) within the 30-500 K range [14,15,33].
Moving the methyl group bound to the pyridine ring from the 5-position in L1 to the 3-position in L3 and to the 6-position in L4 (Scheme 2) is well-known to stepwise decrease the ligand-field strengths in the resulting [Fe(Lk)3] 2+ complexes because the operation of additional sterical constraints, produced by intra-strand interactions in [Fe(L3)3] 2+ [37] and by inter-strand interactions in [Fe(L4)3] 2+ , extends the Fe-N bond lengths (see Equation (1)) [38][39][40]. The associated trend ∆oct(L1) ≈ ∆oct(L2) > ∆oct(L3) > ∆oct(L4) observed for the isostructural [Ni(Lk)3] 2+ complexes (Scheme 2), for which the determination of ligand field ∆oct and Racah B parameters are not complicated by any SCO behavior, are in line with the observation of pure high-spin configurations for the [Fe(L3)3] 2+ and [Fe(L4)3] 2+ complexes in solution (Scheme 2) [35]. Whereas the connection of methyl groups adjacent to the donor nitrogen atom in the bound 6-methyl-pyridine groups in [Fe(L4)3] 2+ produces such large inter-strand interactions that the contraction accompanying the high-spin to low-spin transition cannot be envisioned [41], the situation with the remote 3-methyl substituted pyridine units in [Fe(L3)3] 2+ is less clear and a sophisticated triple-stranded heterometallic LaFe helicate containing the facial [Fe(L3)3] 2+ chromophore has been shown to display partial SCO behavior at low temperature (T1/2 ~50 K) [36]. Taking into account that (i) the replacement of a pyridine with a pyrazine ring in This effort is justified by our long-term quest for designing a pseudo-octahedral spin-crossover [Fe(Lk)3] 2+ unit that can modulate the luminescence of adjacent emissive lanthanides in (supra)molecular assemblies via energy transfers within a temperature domain (77-150 K) accessible to optical reading and addressing [36]. Finally, since minor structural variations may induce large changes in ligand-field strength, the systematic exploration of unpredictable intermolecular packing interactions [42,43] operating in crystalline samples of [Fe(L5)3]X2 complexes (X-= monoanionic counter-ions) may contribute to the lucky search for some 'ideal' Fe II complexes, which additionally exhibit hysteretic behavior and bistability [17,44,45].

-methyl-2-(3-methylpyrazin-2-yl)-1H-benzo[d]imidazole (L5).
In a typical synthesis, 0.3 mmol (3 eq) of the ligand L5 dissolved in acetonitrile (2 mL) was added to 0.1 mmol (1 eq) of Fe(ClO4)2•6H2O or Fe(CF3SO3)2 or Ni(BF4)2•6H2O or Zn(CF3SO3)2 in acetonitrile (2 mL). The resulting mixture was stirred under an inert atmosphere for 3 h, then evaporated to dryness under vacuum to yield microcrystalline powders of the respective complexes. These powders were dissolved in acetonitrile and allowed to crystallize by evaporation or by slow diffusion of tert-butyl methyl ether to give 64- (Table S1). Single crystals suitable for characterization by x-ray diffraction could be obtained by slow evaporation of acetonitrile solution containing 10 eq of ( n Bu)4NClO4 or Caution! Dry perchlorates may explode and should be handled in small quantities and with the necessary precautions [46,47].

Spectroscopic and Analytical Measurements
1 H and 13 C NMR spectra were recorded at 298 K on a Bruker Avance 400 MHz spectrometer. Chemical shifts are given in ppm with respect to tetramethylsilane. Spectrophotometric titrations were performed with a J&M diode array spectrometer (Tidas series) connected to an external computer. In a typical experiment, 25  inert atmosphere. After each addition of 33 μL, the absorbance was recorded using Hellma optrodes (optical path length 0.1 cm) immersed in the thermostated titration vessel and connected to the spectrometer. Mathematical treatment of the spectrophotometric titrations was performed with factor analysis [48][49][50] and with ReactLab TM Equilibria (previously Specfit/32) [51][52][53]. Pneumatically-assisted electrospray (ESI-MS) mass spectra were recorded from 10 −4 M (ligands) and 10 −3 M (complexes) solutions on an Applied Biosystems API 150EX LC/MS System equipped with a Turbo Ionspray source. Elemental analyses were performed by K. L. Buchwalder from the Microchemical Laboratory of the University of Geneva. Elemental analysis was not conducted for perchlorate salts for security reasons, while crystals of the tetrafluoroborate salts lost their solvent upon separation from the mother liquor and were not further characterized. Electronic spectra in the UV-Vis region were recorded at 293 K from solutions in CH3CN with a Perkin-Elmer Lambda 1050 using quartz cells of a 0.1 or 1.0 mm path length. Solid-state absorption spectra were recorded with a Perkin-Elmer Lambda 900 using capillaries. Solid-state magnetic data were recorded on a MPMS 3 or MPMS 5 QUANTUM DESIGN magnetometers using magnetic fields of 1000-5000 Oe at 1 K/min rates within the 5-300 K range. The magnetic susceptibilities were corrected for the magnetic response of the sample holder and for the diamagnetism of the compounds by using the

X-Ray Crystallography
Summary of crystal data, intensity measurements, and structure refinements for compounds Tables S2-S4. Pertinent bond lengths, bond angles, and interplanar angles are collected in Tables S5-S14 together with ORTEP views and pertinent numbering schemes gathered in Figures S1-S5. The crystals were mounted on MiTeGen kapton cryoloops with protection oil. X-ray data collection was performed with an Agilent SuperNova Dual diffractometer equipped with a CCD Atlas detector (Cu[Kα] radiation). The structures were solved by using direct methods [55,56] or dual-space methods [57]. Full-matrix least-square refinements on F 2 were performed with SHELX2014 [58]. CCDC 1988655-1988659 contained the supplementary crystallographic data. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/. Single and [Zn(L5)3](BF4)2•1.5CH3CN (VI) could also be obtained as inversion twins. The two complexes were isostructural and crystallized in the trigonal system (P3c1 space group) with five independent complexes in the asymmetric unit, all located on three-fold rotation axes (the metal content of the asymmetric unit is 5/3) and Z = 10 (Table S15). Although there is no doubt that the three ligands adopt facial arrangements around the metal to give exclusively fac-[M(L5)3] 2+ cations ( Figures S6-S7), we were only able to locate unambiguously three BF4 -counter-anions in the asymmetric unit. Despite numerous efforts, we were not able to obtain a satisfying model for the last third of a BF4 − counter-anion and gave up to further discuss these structures and to deposit the cif files.

Results and Discussion
Synthesis, characterization, and solid-state structures were obtained for the didentate ligand L5 and its pseudo-octahedral complexes [M(L5)3]X2 (M = Fe, Ni, Zn and X = BF4, ClO4). Compared with pyridine-carboxylic acids, which are easily activated via their transformation into acyl chloride with the help of thionyl chloride or oxalyl chloride [59], the electron-rich pyrazine analogue 1 produced only negligible yield (<1%) of the target amide product 2 under these standard conditions [60]. The 3-methylpyrazine-2-carboxylic acid 1 was thus activated as its anhydride through reaction with either isobutyl chloroformate (left path in Scheme 3a) or pivaloyl chloride (right path in Scheme 3a). Subsequent nucleophilic attack with N-methyl-2-nitroaniline 4 yielded the ortho-nitroamide compound 2 in moderate yield. A subsequent reductive cyclisation reaction provided ligand L5, which was characterized by its 1 H-NMR spectrum (Scheme 3b). The lack of NOE effect observed between the methyl groups in positions 5 and 8 indicates an anti-conformation for the α,α'-diimine chelate unit, which was confirmed by (i) the crystal structure of L5 ( Figure 1a) and (ii) gas-phase calculations predicting a global energy minimum for the planar anti-conformation (interplanar angle between the two aromatic rings α = 180°, Figure 1b) [61]. Given that the same anti-conformations are (i) found in the solid state ( Figure S8) and (ii) predicted in the gas phase for the ligands L2 [60] and L5, their computed EHMO frontiers orbitals are comparable ( Figure S9a) and lead to akin electronic absorption spectra dominated by intense π*π covering the near UV range ( Figure S9b). Gas-phase energy computed for L5 at the MM2 level as a function of the interplanar angle [61].
Interestingly, the gas-phase energy of L5 displayed two additional local energy minima for α = ±16° (Figure 1b), which shifted from α = 0° previously reported for the second local minimum in L2 ( Figure S10) [60]. The larger interplanar angle of 16° in the optimized syn-conformation of L5 was the result of the sterical crowding between the close methyl groups connected to the adjacent aromatic rings (positions 5 and 8 in the numbering of Scheme 3b). Taking the latter conformation as a limiting structural model when L5 is bound to a metal cation provides dN•••N = 2.83 Å between the two nitrogen donor atoms of the α,α'-diimine chelate. According to Phan et al. [33], the latter separation matches the 2.78 ≤ dN•••N ≤ 2.93 Å range for which a diimine ligand might be used to achieve spin-crossover behavior in tris-homoleptic Fe II complexes. On the contrary, the crystal structures of the tetrafluoroborate salts showed the existence of fac-[M(L5)3] 2+ , in which the three didentate ligands adopted the same orientation along the pseudo-threefold axis passing through the metal (Figure 3). Having previously established that the energy gap between the facial (C3-symmetry) and meridional (C1-symmetry) geometries in [Zn(L1)3] 2+ and [Zn(L2)3] 2+ roughly followed a pure statistical (i.e., entropic) trend and does not overcome thermal energy at room temperature [60], we concluded that packing forces, specific to the use of perchlorate or tetrafluoroborate counter anions, are more than enough for the quantitative and selective crystallization of pure meridional, respectively facial isomers.  [60]. Moreover, the shift of the methyl group bound to the pyrazine ring from the 5-position in L2 to the 3-position in L5 has globally no geometric influence on the [ZnN6] coordination sphere, thus leading to Zn-N bond distances surrounding the standard value of Zn-N = 0.74 + 1.46 = 2.20 Å deduced from the effective ionic radii [68]. In other words, the close methyl groups found in the bound didentate ligand L5 do not induce major intramolecular steric constraints in [Zn(L5)3] 2+ and only a slight increase of the interannular intraligand angles can be detected in going from [Zn(L2)3] 2+ (α = 21(13)°) to [Zn(L5)3] 2+ (α = 38(4)°, entry 5 in Table 1). The molecular structures of [Ni(L5)3] 2+ were very similar to those observed for the Zn II analogues (Figures 2 and 3), except for the slightly shorter Ni-N bond distances, a trend in line with the contraction of Shannon's effective ionic radii predicted to be 0.74 Å for six-coordinate Zn 2+
The additional abrupt decrease in the magnetic susceptibility occurring at low temperature (T < 40 K) can be assigned to zero-field splitting (ZFS) of high-spin Fe(II), which was modeled with Equation (7), where D and E are the axial and rhombic ZFS parameters, respectively [70][71][72][73].
The pseudo-threefold axis characterizing the [FeN6] chromophore in [Fe(L5)3](ClO4)2 implies that E can be neglected (E ~ 0). Consequently, the electron-electron interaction splits the S = 2 manifold at zero magnetic field into three energy levels located at 0  The latter magnetic data closely matched those reported for the analogous [Fe(L3)3](CF3SO3)2 complex (g = 2.20(2), D = 0.85(1) cm −1 [36]), and demonstrate that our novel [Fe(L5)3](ClO4)2 complex, in which the 3-methyl-pyridine group of L3 is replaced with a 3-methyl-pyrazine group in L5, is also purely high-spin within the 5-300 K range with no trace of SCO behavior. A careful inspection of the experimental curve around 80 K (Figure 4) showed a very minor deviation from the theoretical model, which could be tentatively assigned to traces of trapped low-spin form as previously reported for fac-[Fe(L3)3] 2+ when it is incorporated into a LaFe triple-stranded helicate [36].
The electronic absorption spectrum recorded for [Fe(L5)3](ClO4)2 (I) in the solid state shows the expected Jahn-Teller split Fe II ( 5 E 5 T2) ligand-field transition (Figure 5a) [31]. A deconvolution using two Gaussian functions gives max A Gaussian deconvolution of the visible part of the absorption spectrum into three peaks yielded two broad bands, diagnostic for the spin-allowed, but parity-forbidden, transitions at 10,672 cm −1 ( 3 T2 3 A2) and 16,763 cm −1 ( 3 T1 3 A2; Table S16), together with a third weaker band at 12,584 cm −1 , which can be ascribed to the spin-forbidden 1 E 3 A2 component (Figure 5b). Subsequent non-linear least-squares fits of the energies of these transitions with Equations (10)-(13) provides a first rough set of ligand field strength Δoct =10,672 cm −1 and Racah parameters B = 760 cm −1 and C = 3413 cm −1 (Table S16). However, the mixing of the spin-allowed 3 T2 3 A2 transition with the spin-forbidden 1 E 3 A2 transition via spin-orbit coupling for apparent ligand field strengths around 11,000-12,000 cm −1 , as found for [Ni(L5)3](BF4)2•H2O, requires further refinements [76]. A detailed analysis of a series of Ni II complexes led Hancock and coworkers to propose three empirical Equations (14)- (16) to obtain more reliable ligand field strengths Δoct and Racah parameters B and C in cm −1 units (ε1 and ε2 are the extinction coefficients at the observed frequencies of the 1 E 3 A2 transition and 3 T2 3 A2 transition, respectively) [76]. The analysis of the experimental absorption spectra using this model gives the corrected parameters gathered in Table 2   complex displayed spin-crossover behavior above room temperature (T1/2 ~400 K in the solid state, T1/2 ~ 350 K in acetonitrile solution [35]). Moreover ∆oct ([Ni(L5)3] 2+ ) = 11,630 cm −1 is compatible with the ligand field range 11,200 ≤ ∆oct (Ni II ) ≤ 12,400 cm −1 established by Busch and coworkers [78] as a reliable and useful benchmark for predicting and rationalizing the spin-crossover of the related Fe II complexes [40]. The absence of SCO behavior depicted by [Fe(L5)3](ClO4)2 is thus difficult to assign to some inadequate electronic properties of the [FeN6] chromophore, but more probably to the impossibility of the coordination sphere to shrink for adopting short-enough Fe-N bonds compatible with low-spin Fe II . This pure sterical limitation can be tentatively assigned to the intraligand sterical constraints programmed to occur between the methyl groups bound to the pyrazine and benzimidazole rings in each coordinated syn-L5 ligand in [Fe(L5)3] 2+ . However, packing forces operating in the solid state may be as important, or even much larger than intramolecular constraints and a definitive assessment requires the extension of our analysis to isolated complexes in solution, where intermolecular interactions are significantly reduced.
According to the site-binding model [82,83], the first stability constant M,  (20)) modulated by a pure entropic contribution ω1,1 = 24 [60] produced by the change in rotational entropies accompanying the transformation of the reactants into products, a parameter often referred to as the statistical factor [84,85].
Applying Equation (20) to the stability constants M, 1,1  Lk collected in Table 3 (entry 3) (18) requires twice the intermolecular metal-ligand affinity, a statistical factor of ω1,2 = 120, which takes into account all the possible geometric isomers [60] and the operation of allosteric cooperativity factors , u LL kk measuring the extra energy cost ( , u LL kk < 1), respectively, energy benefit ( , u LL kk > 1) produced by the binding of two ligands to the same metal (Equation (21)) [82,83,86].
The resulting values of ( )  Table 3 shows that [M(L5)3] 2+ (M = Ni, Zn) corresponds to more than 90% of the distribution at the stoichiometric M:L5 = 1/3 ratio (Figure S12). At a total ligand concentration of 0.1 M in acetonitrile, [Ni(L5)3] 2+ stands for 84% of the ligand speciation and its absorption spectrum closely matches that recorded for related solid state samples ( Figure 5). Repeating the detailed analysis described in the previous section (Equations (10)-(16)) provides ligand-field strengths (∆oct) and Racah parameters (B, C) similar to those found in the solid state (  Figure S13) showed a single set of signals compatible with the exclusive formation of an averaged C3-symmetrical species with no contribution from either blocked facial and meridional isomers or from partial decomplexation to give [Zn(L5)2] 2+ + L5 (equilibrium (19)). These observations are in contrast with the detection at low temperature in CD3CN of two well-resolved spectra characteristics of a slow exchange operating between fac-[Zn(L2)3] 2+ and mer-[Zn(L2)3] 2+ [60], and suggests that the weaker stability constants are accompanied by faster ligand exchange processes around Zn 2+ in [Zn(L5)3] 2+ . This decrease in affinity reaches its paroxysm for the coordination of Fe 2+ since the spectrophotometric titration of L5 with Fe(CF3SO3)2 conducted at submillimolar concentration displays only a minor drift of the absorption spectra with no pronounced end point (Figure 7). Attempts to model these limited variations within the frame of equilibria (17)-(18) only failed. The amount of [Fe(L5)n] 2+ in solution is strongly limited by (very) low cumulative stability constants, a situation produced by the impossibility for Fe II to adopt a compact low-spin configuration in the sterically constrained complex [35]. In the absence of a significant amount of [Fe(L5)3] 2+ complexes in solution, no spin state equilibria could be investigated for this system.

Conclusions
Having established that the shift of the methyl group connected to the pyridine ring in going from the didentate ligand L1 (5-position) to L3 (3-position) was accompanied by a drift of the transition temperature in the associated spin-crossover complexes from T1/2 ~310 K in [Fe(L1)3] 2+ [34] to T1/2 ~50 K in a pure facial version of the [Fe(L3)3] 2+ chromophore [36], we attempted in this work to transpose this trend for pyrazine analogues L2 and L5 with the preparation of the missing member of the series [Fe(L5)3] 2+ (3-methyl-pyrazine), for which an ideal T1/2 ~ 90 K could be naively predicted since T1/2 ~ 350 K in [Fe(L2)3] 2+ (5-methyl-pyrazine). At first sight, this approach appeared to be promising since the molecular structures of pseudo-octahedral [M(L2)3] 2+ and [M(L5)3] 2+ (M = Ni II and Zn II ) were comparable, except for the expected larger interannular twist between the connected aromatic rings produced by the close methyl groups in bound L5. The diagnostic ratio ∆/B = 13.3 was identical for both Ni II complexes, thus pointing to electronic properties also compatible with the induction of SCO behavior in the analogous Fe II complexes. Surprisingly and disappointingly, [Fe(L5)3] 2+ exists as a pure high spin complex within the 5-300 K range with no trace of spin state equilibrium. A thorough analysis of its thermodynamic formation in solution highlights a huge decrease in affinity of the ligand L5, compared with L2, for its binding to M 2+ cations despite the presence of the same nitrogen donor atoms. Compared with the pyridine analogues [M(L3)3] 2+ , which possess similar intra-strand sterical constraints (3-methyl substituents), the much weaker σ-donating N-pyrazine donor atoms were unable to compensate for the additional interstrand constraints required for chelating L5 around M 2+ . This limiting factor, which can be compared to a sort of arm wrestling match, is amplified with small cations and low-spin Fe II cannot be complexed to L5.