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Crystals 2016, 6(5), 58; doi:10.3390/cryst6050058

Review
The Effect of Ligand Design on Metal Ion Spin State—Lessons from Spin Crossover Complexes
School of Chemistry, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK
Academic Editor: Martin T. Lemaire
Received: 22 April 2016 / Accepted: 13 May 2016 / Published: 18 May 2016

Abstract

: The relationship between chemical structure and spin state in a transition metal complex has an important bearing on mechanistic bioinorganic chemistry, catalysis by base metals, and the design of spin crossover materials. The latter provide an ideal testbed for this question, since small changes in spin state energetics can be easily detected from shifts in the spin crossover equilibrium temperature. Published structure-function relationships relating ligand design and spin state from the spin crossover literature give varied results. A sterically crowded ligand sphere favors the expanded metal–ligand bonds associated with the high-spin state. However, steric clashes at the molecular periphery can stabilize either the high-spin or the low-spin state in a predictable way, depending on their effect on ligand conformation. In the absence of steric influences, the picture is less clear since electron-withdrawing ligand substituents are reported to favor the low-spin or the high-spin state in different series of compounds. A recent study has shed light on this conundrum, showing that the electronic influence of a substituent on a coordinated metal ion depends on its position on the ligand framework. Finally, hydrogen bonding to complexes containing peripheral N‒H groups consistently stabilizes the low-spin state, where this has been quantified.
Keywords:
transition metals; spin state; spin crossover; structure–function relationships; magnetostructural correlations; magnetic measurements

1. Introduction

In principle, controlling the spin state of a coordinated transition ion through ligand design should be a straightforward problem of coordination chemistry. Undergraduate crystal field theory teaches that octahedral metal ions with d4d7 electron counts can adopt either a high-spin or a low-spin configuration, corresponding to the maximum or minimum possible number of unpaired electrons (Scheme 1) [1]. Whether a compound is high- or low-spin depends on a competition between the crystal field orbital splitting (Δoct) and the intra-orbital electron pairing energy (P), which are both unfavorable energy terms. If Δoct < P it is less costly to populate the eg orbital subshell than to pair electrons in the t2g orbitals, leading to the adoption of a high-spin state. If Δoct > P the converse is true, and a low-spin state is favored (Scheme 1). The magnitude of P is effectively constant for a particular metal ion in a given coordination geometry, being a function of its electron configuration and effective nuclear charge. Hence, all other things being equal, the spin state of a complex is controlled by the strength of the ligand field around the metal ion, as defined by Δoct. The same arguments can be generalized to other coordination geometries, although the d-electron counts that can show a high-spin/low-spin dichotomy vary according to the relevant d-orbital splitting. Among other things, a large Δ and a low-spin state are favored by ligands near the top of the spectrochemical series, most of which have π-acceptor capability; by a tetragonal coordination geometry, which strongly differentiates the on-axis (dx2‒y2/dz2) and off-axis (dxy/dxz/dyz) d-orbitals; and by heavier second and third row metals, with more diffuse valence d-orbitals and stronger metal–ligand covalency.
This apparently simple question has an important bearing on a number of areas of transition metal reactivity [3]. Most simply, the spin state of a metal ion determines its reactivity towards ligand substitution. The high-spin state of a metal ion always contains more electrons in metal-ligand antibonding d-orbitals (the eg subshell in Scheme 1), leading to weaker M‒L bonds and a lower crystal field activation energy than the low-spin state. Thus, high-spin complexes are generally more kinetically labile. Obtaining quantitative data comparing substitution rates of the same metal center in two spin states is a challenging problem [4]. However, a qualitative example is provided by [Fe(3-bpp)2]2+ (Scheme 2), which has a 70% ± 10% low-spin population in aqueous solution at room temperature and is stable in water for a period of days [5,6]. This contrasts with its isomer [Fe(1-bpp)2]2+ (Scheme 2), which is 90% high-spin in solution at room temperature and hydrolyzes spontaneously in contact with water [5]. The difference probably arises from strong N‒H···OH2 hydrogen bonding undertaken by [Fe(3-bpp)2]2+ in aqueous solution, which makes the 3-bpp ligands more electron rich by increasing the Nδ−‒Hδ+ bond polarity (see below). More subtly, the involvement of high-spin intermediates in substitution reactions of low-spin complexes can lead to a change in mechanism and increase in reaction rate [3,7].
Another generalization is that high-spin complexes, with their increased radical character, are more prone towards single-electron reactivity than low-spin metal centers. This is particularly relevant in biological and synthetic oxidation catalysis, which most commonly involves high-valent iron or manganese intermediates whose spin states are malleable, depending on the co-ligands present. The two-step radical rebound mechanism typically adopted by these reactions involves an initial, rate-limiting H atom abstraction from the substrate. That is favored by free-radical catalyst oxidant species, which generate unpaired spin density on the reactive oxo ligand centers [3,8]. The Fe=O (ferryl) intermediates in heme enzymes get round this problem particularly elegantly, by combining a low-spin Fe(IV) ion (for kinetic stability) with a porphyrin cation ligand radical to initiate the radical rebound [9]. Many non-heme enzymes and synthetic complexes adopt the alternative approach of using weak field ligands to generate a high-spin ferryl active oxidant [10]. Comparisons of synthetic iron(IV) ferryl species with S = 1 and S = 2 ground states have shown that the latter are consistently more reactive towards H atom abstraction from a substrate [4,11].
Such considerations can also have a bearing on organometallic catalysis, where first-row transition ions are increasingly playing a role [12]. An important example is provided by the complexes in Scheme 3, which are highly active pre-catalysts for the polymerization of styrene [13]. Activation of the complexes with (2-chloroethyl)benzene affords five-coordinate iron(III) active catalysts [LFeCl2(C2H4Ph)], where L is the α-diimine chelate. Interestingly, the active intermediates formed from A have a high-spin state (S = 5/2), which undergo atom transfer radical polymerization (ATRP) with all the characteristics of a living polymerization reaction. In contrast, the five-coordinate activated species formed from B have a reduced, intermediate spin state (S = 3/2). This strongly disfavors atom transfer chemistry, leading to a much slower catalytic chain transfer process yielding low molecular-weight polymer product [13]. The difference is caused by the inductive effect of the ligand phenyl substituents in B, which promote iron→diimine back bonding sufficiently to induce a spin state flip [14].
A final, rather different consequence of the existence of high- and low-spin states in metal complexes is spin crossover (SCO); that is, a transition between spin states in a molecule or material under the action of a physical stimulus [15,16,17]. This is most often observed upon heating or cooling the sample, but spin state changes can also be produced under high pressure (ca. 1 GPa), by a strong magnetic field, or, in the solid state, by irradiating the sample. Where individual switching centers (molecules) are isolated from each other or are otherwise weakly interacting, as in solution for example, thermal SCO is observed as a gradual Boltzmann-type equilibrium [15,18]. More efficient packing of switching centers in molecular solids or coordination polymers can lead to more cooperative SCO switching, occurring abruptly and/or with thermal hysteresis (which leads to genuine spin state bistability) [19]. Such compounds have added interest because of the changes in their physical properties that accompany SCO [20]. Most obvious is the expected change in magnetic moment, which leads to diamagnetic/paramagnetic switching in iron(II) complexes for example (Scheme 1). However, SCO in different compounds also leads to changes in color; fluorescence; conductivity (in the bulk or at the single molecule level); and/or dielectric constant [15]. It can also lead to a measurable mechanical response in a solid sample [21]. SCO materials can be fabricated at the nanoscale, retaining their switching functionality at least partially in the 101–102 nm size regime [22]. This combination of versatility, multifunctionality, and fabricability has made SCO compounds one of the most popular forms of molecular switch in molecular materials research.
Tuning spin state properties for catalysis or for switchable materials requires an understanding of how the spin state of a complex is influenced by its molecular structure. While it is well understood how factors such as metal oxidation state and coordination geometry control Δoct and spin state (see above), the influence of ligand design on spin state is more subtle and harder to predict. Spin crossover complexes are particularly suited to this question, because small changes in spin state energetics between different compounds are measurable, in principle, via the SCO equilibrium midpoint temperature (T½). For example, a 10 K shift in T½ corresponds to a change of ca. 1 kJmol‒1 in the relative enthalpies of the spin states or ca. 5 Jmol‒1K‒1 in their relative entropies, all other things being equal. While SCO is usually measured in the solid state, such small changes are better studied in solution, where molecules undergo SCO independently of their neighbors [15,18]. That allows their molecular properties to be measured without contamination from lattice effects, which can have a large influence on SCO in solid samples [19]. While the influence of ligand displacement reactions and secondary bonding interactions with a solvent must be guarded against, these are easily detected when measuring SCO in solution [23].
This article surveys the SCO literature to determine how the spin state of a complex is influenced by the nature of its ligands. Solution phase measurements are emphasized, for the reasons in the previous paragraph, but solid state data are also presented where they add to the discussion and are clearly distinguished in the text.

2. Results

2.1. Steric Influence of Ligand Substituents on Metal Ion Spin States

As mentioned previously, high-spin complexes always have more electrons in eg-type d-orbitals, which are metal–ligand antibonding, than the corresponding low-spin forms (Scheme 1). Hence, high-spin complexes would be expected to have longer, and weaker, M‒L bonds. In practice this difference is more pronounced for some metal–ligand combinations than for others. For example, SCO in d5 or d6 metal ions like iron(III) or iron(II), which involves a ΔS = ±2 spin state change, has a greater effect on their molecular structure than the ΔS = ±1 SCO undergone by d4 and d7 centers [manganese(III) or cobalt(II)]. The strong Jahn–Teller elongation distortions exhibited by the high-spin d4 and low-spin d7 configurations also affect the structural changes during SCO in complexes of those ions. Moreover, bonds to harder N, O-donor ligands tend to be more sensitive to metal ion spin state than softer P, S or halide ligand donors [24]. However, in iron(II) SCO complexes with N-donor ligands, for example, the Fe‒L bonds can be up to 10% longer in the high-spin state than the low-spin. Hence, one would expect a sterically crowded ligand sphere to consistently favor the high-spin state of a complex.
There are a number of SCO systems that bear out that assumption. The first clear demonstration was with the [Fe(Py3tren)]2+ podand complex system (Figure 1). The parent complex [Fe(Py3tren)]2+ is low-spin at room temperature, but methylation of the pyridyl C6 positions (which are in close proximity to the iron atom) induces SCO behavior. Notably, the temperature of SCO decreases with an increased number of methyl groups in [Fe(MexPy3tren)][PF6]2, following the trend x = 1 (T½ ≈ 433 K) > x = 2 (290 K) > x = 3 (219 K) in the solid state, and x = 1 (T½ ≈ 430 K) > x = 2 (319 K) >> x = 3 (high-spin) in solution [25]. That is, sequential methylation of the complex consistently stabilizes the high-spin state of the complex. Comparison of their crystal structures clearly shows the steric crowding between the methyl group of one arm of the podand and the pyridyl group of its neighbor, which inhibits contraction of the Fe‒N bonds associated with the transition to a low-spin state (Figure 1). These results have been used more recently to produce SCO metallomesogens based on an [Fe(MexPy3tren)]2+ core [26].
Other studies that have drawn similar conclusions, based on solution measurements, involve the compounds shown in Scheme 4. The SCO equilibrium for [Co(R2impy)2]2+ in (CD3)2CO shifts to lower temperature according to the trend R = Bn > iPr ≈ Tol > tBu (the tertbutyl complex remains high-spin at all temperatures) [29]. The low-spin complex [Fe(bipy)3]2+ becomes fully high-spin when methylated at the pyridyl C6 positions, as in [Fe(mbpy)3]2+ [30] and [Fe(dmbpy)3]2+ [31]. A series of iron(II) complexes based on tetradentate tris-(pyrid-2-ylmethyl)amine (tpa), [Fe(tpa)(NCMe)2]2+, also shows steadily increasing stabilization of the high-spin state as the tpa pyridyl C6 atoms are sequentially methylated [32]. The SCO equilibrium temperatures in the scorpionate complexes [Fe(HB{pz}y{dmpz}3‒y)2] shift progressively to higher temperature as the number of methyl groups in the complex increases, with fully substituted [Fe(HB{dmpz}3)2] being high-spin [33,34,35]. The hexadentate ligand complex [Fe(tpen)][ClO4]2 is SCO-active in solution (T½ = 363 K in dmf), while its dimethylated analogue [Fe(btpen)][ClO4]2 remains high-spin at all temperatures [36].
The unsubstituted macrocyclic complex [Fe(Pytacn)(NCMe)2]2+ is predominantly low-spin at room temperature in CD3CN, but three analogues [Fe(Py6−Rtacn)(NCMe)2]2+ bearing substituents at the pyridyl C6 position are all high-spin under the same conditions [37]. Unsubstituted [Fe(1-bpp)2]2+ (Scheme 2) is SCO-active, with T½ = 248 K in (CD3)2CO [38], but [Fe(R2-1-bpp)2]2+ are fully high-spin in the same solvent when R = Ph or iPr [39]. The same trend is shown by the isomeric 3-bpp complex series, in that [Fe(3-bpp)2]2+ itself (Scheme 2) is SCO-active [5] but four [Fe(R2-3-bpp)2]2+ derivatives with R ≠ H are all fully high-spin [40]. Like most derivatives of [Fe(terpy)2]2+, [Fe(terpy)2]2+ itself is low-spin when R = H, but [Fe(Ph2terpy)2]2+ undergoes SCO (T½ ≈ 300 K in EtCN solution) [41].
Finally, a similar conclusion was reached for a rather different series of compounds, namely tetrahedral [(PhB{MesIm}3)FeN=PR3]. A linear relationship between T½ and the phosphoraminimato PR3 group cone angle was found, with more bulky phosphoraminimate ligands stabilizing the high-spin forms of the complexes and reducing T½ [42]. This trend was also reproduced by DF calculations [43].
In each case, trends observed from solid state data on these compounds mirror the solution phase results [29,30,31,32,36,38,39,40,41,44,45,46,47]. Other derivatives of [Fe(HBpz3)2] [48,49,50], [Fe(terpy)2]2+ [51,52] and other complexes in Scheme 4 [53,54] with sterically significant distal ligand substituents also exhibit stabilized high-spin states in solid phases. Interestingly, however, [Fe(Mes2-1-bpp)2]2+ presents an exception to the above discussion, being fully low-spin at room temperature despite the steric bulk of its distal mesityl substituents [39,51]. The increased basicity of the ligand pyrazolyl donors, reflecting the electron-donating character of the mesityl substituents (as opposed to electron-withdrawing phenyl groups), evidently overcomes any steric repulsion in the metal coordination sphere in that example.
Steric effects can also favor the low-spin state, where ligand substituents are oriented to inhibit lengthening of the metal–ligand bonds. This is apparent in the [Fe(RB{mpz}3)2] scorpionates (Figure 2), which are high-spin for R = H, pz, nBu, iBu, C4H4I-4 or C6H4CCH4. However, when R = tBu, the compound is low-spin at 300 K but undergoes gradual SCO upon heating in the solid state [55]. This reflects steric repulsion between the tertbutyl substituents and the H5 atoms on the pyrazolyl rings, which promotes the low-spin state by disfavoring the expanded metal coordination sphere adopted by the high-spin form (Figure 2). Another example is [Fe(Me4-1-bpp)2]2+, which contains methyl groups at both the pyrazolyl C3 and C5 positions. Intra-ligand steric repulsion between the C5 methyl groups and the pyridyl H3/H5 atoms also force the pyrazolyl ligand donors closer to the iron atom (Figure 2). Hence [Fe(Me4-1-bpp)2]2+ is low-spin [56,57], in contrast to unsubstituted [Fe(1-bpp)2]2+ which is SCO-active below room temperature [38]. In both examples the steric influence of pyrazolyl C3 methyl groups, which should promote a high-spin state as above, is overcome by a sterically imposed ligand conformation favoring a low-spin structure.
Steric repulsion can also influence the spin state in framework materials. A good example is provided by [Fe(3-Xpy)2{M(CN)2}2] (M = Cu or Au) [58,59,60], and [Fe(3-Xpy)2{M(CN)4}2] (M = Ni, Pd and Pt) [61], where 3-Xpy corresponds to 3-fluoro-, 3-chloro-, 3-bromo-, or 3-iodo-pyridine. Although the members of these series are not perfectly isostructural, they all adopt 2D grid-type topologies with the layers being separated by the 3-halopyridine rings. In each case, the high-spin state of the material is stabilized as the size of the halogen substituent increases, with most 3-bromopyridine frameworks and all the 3-iodopyridine frameworks being high-spin. This was attributed to increased congestion of the inter-layer space in the frameworks as the halogens get larger (Figure 3).

2.2. Influence of Ligand Conformation on Metal Ion Spin States

The high-spin form of a complex can be stabilized by a sterically-induced twisted ligand conformation. The best example is provided by the 3,3′-disubstituted bipyridyl ligand complexes in Scheme 5, where solid salts of [Fe(3,3′-dmbpy)2]2+, [Fe(dmbc)2]2+ and [Fe(biq)3]2+ are all SCO-active while the parent complex [Fe(bipy)3]2+ is low-spin [31,62]; [Fe(biq)3]2+ also exhibits SCO in solution [31]. This was attributed to twisting of the bipyridyl ligands, induced by an intra-ligand steric clash between the pyridyl substituents, or between the isoquinolinyl H8 atoms in coordinated biq. The twisted ligand conformations displace the ligand lone pairs away from the metal–ligand vector, thus weakening the ligand field in the complexes. Such twisting is clear in the crystal structure of [Fe(3,3′-dmbpy)2][ClO4]2, where the dihedral angles between the least squares planes of the pyridyl rings in the 3,3′-dmbpy ligands are 36.9–37.1° (Figure 4) [62].
Another example of the same effect occurs in some trans-[Fe(NCS)2L] complexes, where L represents a series of meridional tetrapyridyl ligands which adopt helically twisted conformations when coordinated to a metal ion. While not all were crystallographically characterized, examples that might be expected to adopt more twisted L ligands are high-spin, or exhibit significantly reduced SCO T½ values, in the solid state [63,64]. Comparably distorted ligand conformations, imposed by intermolecular steric contacts rather than by ligand design, may also be responsible for stabilization of the high-spin state in some other crystalline metal complexes [65,66,67,68,69,70].

2.3. Influence of Chelate and Macrocycle Ring Size on Metal Ion Spin States

Expanded chelate ring sizes should also promote the high spin states of complexes, by increasing the ligand conformational strain associated with the more compact low-spin form [71,72]. Thus, [Fe(NCS)2(tpa)] [73] and [Fe(NCS)2(pmea)] [74] are SCO-active with T½ between 150 and 220 K, but [Fe(NCS)2(pmap)] and [Fe(NCS)2(tpea)] are fully high-spin in the solid state (Scheme 5) [72]. Similar results might be expected in iron(III) complexes of polydentate salicyl Schiff base ligands like salten or saltrien [75,76], for example, although that remains to be confirmed by a solution-phase study. It is harder to elucidate trends from those compounds in the solid state, since their SCO is often very gradual and can be influenced by the salicyl ligand conformation in the crystal [68].
For similar reasons, macrocyclic complexes with larger ring sizes will also tend to favor the expanded coordination sphere in the high-spin state [77]. Although there have been few systematic studies, an example is provided by a series of iron(II) complexes of a series of hexadentate ligands based on triaza-macrocyclic skeletons with pyridyl pendant arms. These complexes are low-spin for a nine-membered macrocyclic ligand [78], SCO-active in solution for a 10-membered macrocycle [79], and high-spin for a 12-membered macrocycle [78].

2.4. Influence of Ligand Donor Types on Metal Ion Spin State

This is fundamentally a straightforward question, since ligand donors near the top of the spectrochemical series should favor the low-spin state, and vice versa. However, most SCO complexes are prepared from a limited palette of ligands, mostly N-donors of different types or mixed N/O-donor chelates. Among the field of SCO complexes, the influence of different types of N- or O-donor on metal ions spin states is more subtle, and harder to quantify from available solution-phase data. On one hand, SCO equilibria in [Fe(HBpz3)2] (in thf or a chlorinated solvent [33,34]) and [Fe(HBtz3)2] (in D2O [80]) have both been reported with T½ ≈ 330 K (Scheme 6), which is consistent with the similar basicities of the pyrazole and 1,2,4-triazole heterocycles [81]. In contrast, however, [Fe(3-bpp)2]2+ exhibits SCO with T½ ≈ 248 K in (CD3)2CO [5,82], but [Fe(3-btzp)2]2+ remains fully high-spin across the liquid range of that solvent [83] while [Fe(4-bthp)2]2+ is apparently low-spin [84] (Scheme 6). Hence, the identity of the distal heterocyclic donors has a much greater influence on the spin state of complexes with that ligand geometry.
More consistent data are found with azinyl donor heterocycles, in that [Fe(1-bppyz)2]2+ (T½ = 268 K in {CD3}2CO [85]) exhibits a slightly higher temperature SCO than [Fe(1-bpp)2]2+ (T½ = 248 K [38]); and, T½ for [Fe(paptz)2] (>300 K in CD3OD) is also higher than in [Fe(papt)2] (220 K) [86] (Scheme 6). Thus, replacement of pyridyl by pyrazinyl donors stabilizes the low-spin state of these complexes to a small extent. This is unexpected at first glance, since pyrazine is a less basic heterocycle than pyridine, but it may reflect improved Fe→L backbonding to the more electron-deficient pyrazine heterocycle, as discussed in the next section.
A result relevant to O-donor ligands is that [Fe(naphtrien)]+ (T½ = 332 K in {CD3}2CO [68]) exhibits SCO at slightly higher temperature than [Fe(saltrien)]+ in the same solvent (280 K) [87] (Scheme 6). Since phenol and 2-naphthol have acidic pK as of 10.0 and 9.5, respectively, the less basic phenoxide donors in [Fe(naphtrien)]+ are stabilizing the low-spin state in that system. That is consistent with the observed effect of ligand substituents on the spin state of [Fe(saltrien)]+, as again described below. The same trend was observed in the solid state, in these compounds [68] and in another family of iron(III) complexes with pentadentate Schiff base supporting ligands [88].

2.5. Electronic Influence of Ligand Substituents on Metal Ion Spin State—Inductive vs. Resonance Effects

Relatively few studies have been published relating ligand substituents and metal ion spin state, in systems where contamination from steric influences can be excluded. The first report concerned a series of di-iron complexes [Fe2(NCE)2(pyR)2(μ-bpypz)2] (E = S or BH3), where pyR is a 3- or 4-substituted pyridine terminal ligand, which all exhibit comparably gradual thermal SCO in the solid state involving both iron atoms (Scheme 7). Two groups of complexes with thiocyanato or cyanoborohydride co-ligands, each with six different substituted pyridines, show similar positive linear free energy relationships between the pyridine substituent Hammett parameter [89] and the SCO T½ [90,91]. That is, more electron-withdrawing pyridine substitutents raise T½ and stabilize the low-spin states of the complexes. Similar results were obtained in solution for two series of complexes of polydentate ligands, [Fe(Py4−Rtacn)(NCMe)2]2+ [37,92] and [Fe(stybpR)2]2+ [93] (Scheme 7). In both cases, electron-withdrawing substituents on the 4-pyridyl ligand donors were observed to lower the room temperature magnetic moment or to raise T½, both consistent with stabilization of the low-spin state. Collected data from [Fe(saltrienR)]+ derivatives [68,87], measured in (CD3)2CO solution, also afford a positive linear free energy relationship when T½ is plotted against the “R” substituent Hammett parameter (Figure 5).
These results are consistent with those in Section 2.4. Pyridyl groups with electron-withdrawing substituents, and pyrazinyl donors, are both less electron-rich than an unsubstituted pyridyl ring and lead to higher T½ values in otherwise identical complexes (Scheme 6). Similarly, a phenoxide donor with an electron-withdrawing substituent, and a more conjugated naphthoxide ring, are less basic than an unsubstituted phenoxide and also act to raise T½ in derivatives of [Fe(saltrien)]+.
Conversely, however, some other studies have observed the opposite trend. Most striking are three tetrahedral complexes [(PhB{MesIm}3)FeN=P(C6H4R)3], which afforded a negative free energy relationship between T½ and the aryl “R” substitutents in d8-thf solution (Figure 5) [42]. That is, the low-spin state of these complexes is stabilized by electron-donating, not electron-withdrawing, substituents. Another recent study of different tetrahedral iron(II) complexes [(MeC{CH2PR12}3)FeN=PR23] (Scheme 8) showed a dependence of T½ on both R1 and R2, but deconvolution of steric and electronic substituent effects in that series is not so straightforward [94]. However, data from the solid dinuclear complexes [Fe2(tpa)2(μ-dabqR)][BArF]2 showed T½ decreasing as the electronegativity of ‘R’ increases, in the order X = H > Br > Cl > F [95]. That is a similar trend to the [(PhB{MesIm}3)FeN=P(C6H4R)3] system.
SCO data in methanol solution are also available for three complexes of type [Co(triphos)(O2CC{O}R1R2)] (Scheme 8). However, their behavior within the liquid range of the solvent is too similar to distinguish any differences between them [96].
The contradictory results described above were reconciled in a recent study of solution-phase SCO data from 24 derivatives of [Fe(1-bpp)2]2+ (Figure 6) [97]. Examples bearing pyridyl substituents, [Fe(1-bppR1,H)2]2+, showed a positive linear free energy relationship between T½ and the substituent R1, consistent with the complexes in Scheme 7. In contrast, complexes substituted at the pyrazole donors, [Fe(1-bppH,R2)2]2+, exhibited a weaker negative linear free energy relationship between T½ and the substituent R2. DFT calculations reproduced that trend, and showed it to reflect a competition between metal–ligand σ- and π-bonding effects.
On one hand, electron-withdrawing substituents reduce the ligand field on inductive grounds, by weakening ligand→metal σ-donation. On the other, electron-withdrawing substituents also lower the energy of the heterocycle π * MOs, which promotes metal–ligand π-back-bonding to strengthen the ligand field. Back-bonding is most important in complexes with 4-substituted pyridyl donors like [Fe(1-bppR1,H)2]2+, where the substituents are para to the N-donor atom and can conjugate with the metal ion t2g d-orbitals. However, the R2 substituents in [Fe(1-bppH,R2)2]2+ are meta to the pyrazolyl N-donors, and are unable to undergo π-conjugation to the metal center. Thus, metal–ligand σ-bonding considerations dominate in the [Fe(1-bppH,R2)2]2+ series.
Moreover, when only the subset of complexes bearing weakly π-bonding halogen substituents was considered, the [Fe(1-bppR1,H)2]2+ and [Fe(1-bppH,R2)2]2+ series instead exhibited identical behavior, with a linear correlation between decreasing T½ and increasing halogen electronegativity [97]. That reproduces the trend observed for [Fe2(tpa)2(μ-dabqR)][BArF]2 (Scheme 8) [95], while emphasizing the importance of π-back-bonding to the behavior of the [Fe(1-bppR1,H)2]2+ series.
As well as the pyridyl-substituted complexes in Scheme 7, these arguments explain the behavior of [(PhB{MesIm}3)FeN=P(C6H4R)3] (Figure 5) whose C6H4R substituents cannot conjugate with the iron atom, being separated from it by a sp3-hybridized P atom. The spin state of the complex would then be governed by the δ-inductive properties of the R substituents, as observed in [42]. The positive linear free energy relationship for [Fe(saltrienR)]+ (Figure 5) also implies that iron/phenoxide π-bonding is important to those compounds. In that case, electron-withdrawing substituents would strengthen the ligand field by reducing ligand–metal π-donation.
Surprisingly, derivatives of [Fe(3-bpp)2]2+ (Scheme 2) substituted at the pyrazolyl C5 positions did not follow the same trends as for [Fe(1-bppH,R2)2]2+, since in that study both electron-donating and -withdrawing substituents led to a similar reduction in T½ [98]. It may be relevant that those pyrazolyl substituents are adjacent to the 3-bpp NH groups, which could lead to them influencing hydrogen bonding by these groups. Hydrogen bonding by [Fe(3-bpp)2]2+ has a strong bearing on its SCO, as described in the next section.

2.6. Effect of Secondary Bonding Interactions on Metal Ion Spin State—Hydrogen Bonding

While data from the solid state are inconclusive [19], solution-phase studies have demonstrated that donation of hydrogen bonds by a coordinated ligand acts to strengthen the ligand field in a complex. For example, SCO measurements of [Fe(3-bpp)2]2+ (Scheme 2) [5], [Fe(tacn)2]2+ [99], [Fe(pic)3]2+ [100] and [Fe(bzimpy)2]2+ (Scheme 9) [101] all afforded increased T½ values in more associating solvents. Where it was measured, the highest T½ was always obtained in pure water [5,99,100]. The effect was strongest in [Fe(3-bpp)2]2+, where T½ in D2O (317 K) is 60–70 K higher than in six organic solvents (244–257 K) [5,6]. This was attributed to the more polar pyrazolyl NH groups in 3-bpp, which should form stronger hydrogen bonds than the less acidic amino NH groups in tacn or pic [5]. The same trend was noted in the room temperature magnetic moment of [Fe(saltrien)]+, which is reduced in organic solvents of greater donor number (indicating a greater low-spin population) [87].
Hydrogen bonding to anions has a comparable effect on a complex’s spin state. This has been best studied in a series of complexes [Fe(bip)2L]2+, where L = bip, bipy or another di-imine co-ligand (Scheme 9). Each bip ligand contains two peripheral NH groups oriented to form chelating N‒H···X hydrogen bonds to an X anion guest [102,103,104,105]. Titrating halide or other more associating anions into chloroform solutions of [Fe(bip)2L][BPh4]2, around the SCO temperature for each complex, causes a steady high→low spin conversion associated with anion binding to the bip ligands. A comparable effect was also noted in different salts of [Fe(3-bpp)2]2+ (Scheme 2), whose SCO was shifted by up to 15 K higher temperature in the presence of more associating counterions, at NMR concentrations in a 9:1 (CD3)2CO:D2O solvent mixture [106]. While studies of [Fe(tacn)2]2+ [99] or [Fe(pic)3]2+ [100] reported no measurable response to different anions in solution, that might reflect the aqueous solvents used for those measurements, which would disfavor intermolecular hydrogen-bonding and ion pairing [107].
Although these results have yet to be addressed computationally, it is reasonable that formation of an N‒H···X hydrogen bond to an anion or solvent should increase the dipole on the Nδ‒‒Hδ+ group. That would make the ligand more electron-rich and more basic, thus increasing the ligand field, as observed. The converse effect on T½, of guest-binding to a hydrogen-bond acceptor ligand site, has not yet been studied systematically. However, it is noteworthy that binding of barbiturate derivatives to [Fe(datb)2]2+ causes a low→high spin state change, the opposite of the other compounds in this section [108,109]. The 4,6-diaminotriazinyl receptor sites in [Fe(datb)2]2+ contain a mixture of hydrogen bond donor and receptor functions, making the interaction between substrate binding and spin state harder to rationalize in that system.

3. Materials and Methods

Crystallographic figures in this article were prepared with XSeed [110], using coordinates obtained from the Cambridge Crystallographic Database [111], and graphs were generated using SigmaPlot [112].

4. Conclusions

The steric aspects of ligand design for control of metal ion spin state are clear cut. Bulky substituents in the vicinity of the ligand donor atoms, expanded ligand chelate ring structures, or any other factor that disfavors contraction of the metal–ligand bonds will stabilize the high-spin state of a complex. Steric bulk at the periphery of the ligand sphere can promote the low-spin state if it inhibits expansion of the metal coordination sphere, or the high-spin state if it leads to distortion of the ligand→metal dative interaction. These effects can be reliably understood or predicted in individual compounds computationally [43,50,92] or using molecular models.
The response of a complex’s spin state to changes in the electronic structure of its ligands is more subtle, and requires further study. Remote electron-withdrawing substituents can promote either the high-spin or the low-spin state, depending on two factors: the π-bonding character of the substituent, as expressed by its Hammett parameter [89]; and its position on the ligand framework, which influences the ability of the substituent to conjugate to the metal t2g orbitals [97]. For heterocyclic ligand donors, this can be predicted using physical organic chemistry principles: substituents ortho or para to a reaction center (ligand donor/metal ion) can conjugate to it, but meta substituents cannot.
Finally, the modulation of spin state through supramolecular host:guest binding offers an attractive route to reporter groups for molecular sensors, for example. There are enough proofs-of-principle for a consistent result to emerge that ligands with peripheral NH groups exert an increased ligand field when engaged in strong hydrogen bonding interactions. However, the literature also contains more complicated examples that do not follow that simple trend [98,108], and the synthesis of new supramolecular SCO systems could shed new light on this complicated question.

Acknowledgments

My own work in this area has been supported by the EPSRC (EP/H015639/1, EP/K012568/1), the University of Leeds, and the COST network CM1305 Explicit Control of Spin States in Technology and Biology (ECOSTBio). The author thanks Laurence Kershaw Cook and Rafal Kulmaczewski (University of Leeds), Rob Deeth (University of Edinburgh), and other collaborators mentioned in the reference list for their contributions.

Conflicts of Interest

The author declares no conflict of interest.

Abbreviations

The following abbreviations are used in this manuscript:
1-bpp2,6-di{pyrazol-1-yl}pyridine
1-bppyz2,6-di{pyrazol-1-yl}pyrazine
3-bpp2,6-di{1H-pyrazol-3-yl}pyridine
3-btzp2,6-di{1H-1,2,4-triazol-3-yl}pyridine
3,3′-dmbpy3,3′-dimethyl-2,2′-bipyridine
4-bthp2,6-di{thiazol-4-yl}pyridine
BArFtetrakis(3,5-di{trifluoromethyl}phenyl)borate
Bip2,2′-bi(1,4,5,6-tetrahydropyrimidine)
bipy2,2′-bipyridine
biq2,2′-bi(iso)quinoline
blbpenN N′-bis(6-methylpyrid-2-ylmethyl)-N,N′-bis(pyrid-2-ylmethyl)-1,2-diaminoethane
Bnbenzyl
bpypzH3,5-di(pyrid-2-yl)-1H-pyrazole
bzimpy2,6-bis(1H-benzimidazol-2-yl)pyridine
Cycyclohexyl
dabqH22,5-(phenylamino)-1,4-benzoquinone
dmbcdimethyl 2,2'-bipyridine-3,3'-dicarboxylate
dmbpy6,6′-dimethyl-2,2′-bipyridine
dmpz3,5-dimethylpyrazol-1-yl
impy2,6-bis(carbaldimino)pyridine
iPrisopropyl
mbpy6-methyl-2,2′-bipyridine
Memethyl
MeCNacetonitrile
MePy3trenbis(4-{pyrid-2-yl}-3-aza-3-butenyl)(4-{6-methylpyrid-2-yl}-3-aza-3-butenyl)amine
Me2Py3tren(4-{pyrid-2-yl}-3-aza-3-butenyl)-bis-(4-{6-methylpyrid-2-yl}-3-aza-3-butenyl)amine
Me3Py3trentris(4-{6-methylpyrid-2-yl}-3-aza-3-butenyl)amine
MesIm3-(2,4,6-trimethylphenyl)imidazol-1-yl
Metpabis(pyrid-2-ylmethyl)({6-methylpyrid-2-yl}methyl)amine
Me2tpa(pyrid-2-ylmethyl)-bis({6-methylpyrid-2-yl}methyl)amine
Me3tpatris({6-methylpyrid-2-yl}methyl)amine
Mesmesityl, 2,4,6-triphenylphenyl
Mpz3-methylpyrazol-1-yl
naphtrienH21,8-bis(2-hydroxynaphthaldiminato)-3,6-diazaoctane
paptH2-(pyrid-2-y1amino)-4-(pyridin-2-yl)thiazole
Phphenyl
Pic2-(aminomethyl)pyridine, 2-picolylamine
Pmap(pyrid-2-ylmethyl)-bis(2-{pyrid-2-yl}ethyl)amine
Pmeabis(pyrid-2-ylmethyl)(2-{pyrid-2-yl}ethyl)amine
Pytacn1-(pyrid-2-ylmethyl)-4,7-dimethyl-1,4,7-triazacyclononane
Py3trentris(4-{pyrid-2-yl}-3-aza-3-butenyl)amine
Pypyridine
pzpyrazol-1-yl
pzaptH2-(pyrazin-2-y1amino)-4-(pyridin-2-yl)thiazole
saltenH21,7-bis(salicylaldiminato)-4-azaheptane
saltrienH21,8-bis(salicylaldiminato)-3,6-diazaoctane
stybp4-(cis-styryl)-2,6-di(pyrazol-1-yl)pyridine
SCOSpin crossover
Tacn1,4,7-triazacyclononane
tButertbutyl
thfetrahydrofuran
Tol4-methylphenyl, 4-tolyl
Tpatris(pyrid-2-ylmethyl)amine
tpeatris(2-{pyrid-2-yl}ethyl)amine
tpenN,N,N′,N′-tetrakis-(pyrid-2-ylmethyl)-1,2-diaminoethane
triphos1,1,1-tris(diphenylphosphinomethyl)ethane.

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Scheme 1. High-spin and low-spin electron configurations of d4d7 transition ions in an octahedral ligand field, with their weak-field term symbols and spin quantum numbers [2].
Scheme 1. High-spin and low-spin electron configurations of d4d7 transition ions in an octahedral ligand field, with their weak-field term symbols and spin quantum numbers [2].
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Scheme 2. The structures of [Fe(3-bpp)2]2+, which is stable in aqueous solution, and [Fe(1-bpp)2]2+, which is not.
Scheme 2. The structures of [Fe(3-bpp)2]2+, which is stable in aqueous solution, and [Fe(1-bpp)2]2+, which is not.
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Scheme 3. Structures of alkene polymerization pre-catalysts that proceed by rapid atom radical transfer polymerization (A) or a slow chain transfer reaction (B; R = cyclohexyl or tertbutyl) [13].
Scheme 3. Structures of alkene polymerization pre-catalysts that proceed by rapid atom radical transfer polymerization (A) or a slow chain transfer reaction (B; R = cyclohexyl or tertbutyl) [13].
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Figure 1. [Fe(Py3tren)]2+ and its methylated derivatives (left); and crystallographic space-filling views of [Fe(Py3tren)]2+ (center) and [Fe(Me3Py3tren)]2+ (right) highlighting the steric influence of the methyl substituents in the latter compound [27,28]. The orientation of the space-filling views is indicated in the structure diagram. Color code: C, white; H, pale gray; Fe, green; N, blue.
Figure 1. [Fe(Py3tren)]2+ and its methylated derivatives (left); and crystallographic space-filling views of [Fe(Py3tren)]2+ (center) and [Fe(Me3Py3tren)]2+ (right) highlighting the steric influence of the methyl substituents in the latter compound [27,28]. The orientation of the space-filling views is indicated in the structure diagram. Color code: C, white; H, pale gray; Fe, green; N, blue.
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Scheme 4. Complexes whose high-spin state is stabilized by distal ligand substituents.
Scheme 4. Complexes whose high-spin state is stabilized by distal ligand substituents.
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Figure 2. Structures and crystallographic space-filling views of two complexes exhibiting low-spin states stabilized by the highlighted steric repulsions at the periphery of the molecules [55,57]. The orientation of the space-filling views is similar to the structure diagrams. Color code: C, white; H, pale gray; Fe, green; N, blue.
Figure 2. Structures and crystallographic space-filling views of two complexes exhibiting low-spin states stabilized by the highlighted steric repulsions at the periphery of the molecules [55,57]. The orientation of the space-filling views is similar to the structure diagrams. Color code: C, white; H, pale gray; Fe, green; N, blue.
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Figure 3. Two views of the 2D Hoffman network [Fe(3-Brpy)2{Pt(CN)4}2], plotted from the same orientation [61]. The space-filling view shows the steric influence of the pyridyl bromo substituents in the interlayer space. Color code: C, white; H, pale gray; Br, yellow; Fe, green; N, blue; Pt, cyan.
Figure 3. Two views of the 2D Hoffman network [Fe(3-Brpy)2{Pt(CN)4}2], plotted from the same orientation [61]. The space-filling view shows the steric influence of the pyridyl bromo substituents in the interlayer space. Color code: C, white; H, pale gray; Br, yellow; Fe, green; N, blue; Pt, cyan.
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Figure 4. [Fe(bipy)3]2+ derivatives designed to adopt twisted ligand conformations, and the crystal structure of [Fe(3,3′-dmbpy)3]2+ oriented to emphasize the twisting of the bipyridyl framework [62]. Color code: C, white; H, pale gray; Fe, green; N, blue.
Figure 4. [Fe(bipy)3]2+ derivatives designed to adopt twisted ligand conformations, and the crystal structure of [Fe(3,3′-dmbpy)3]2+ oriented to emphasize the twisting of the bipyridyl framework [62]. Color code: C, white; H, pale gray; Fe, green; N, blue.
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Scheme 5. A series of complexes with sequentially expanded chelate ring sizes.
Scheme 5. A series of complexes with sequentially expanded chelate ring sizes.
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Scheme 6. Comparable complexes, containing different types of heterocyclic N or aromatic O donors, whose SCO has been measured in solution.
Scheme 6. Comparable complexes, containing different types of heterocyclic N or aromatic O donors, whose SCO has been measured in solution.
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Scheme 7. Three series of complexes whose low-spin state is stabilized by electron-withdrawing pyridyl ‘R’ substituents in solution [37,90,93].
Scheme 7. Three series of complexes whose low-spin state is stabilized by electron-withdrawing pyridyl ‘R’ substituents in solution [37,90,93].
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Figure 5. Two families of compounds giving contrasting linear free energy relationships between SCO T½ and their ligand ‘R’ substituent δP Hammett parameters from solution measurements [42,68,87].
Figure 5. Two families of compounds giving contrasting linear free energy relationships between SCO T½ and their ligand ‘R’ substituent δP Hammett parameters from solution measurements [42,68,87].
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Scheme 8. Other families of substituted complexes with available, comparative SCO data [94,95,96].
Scheme 8. Other families of substituted complexes with available, comparative SCO data [94,95,96].
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Figure 6. Linear SCO free energy relationships for [Fe(1-bpp)2]2+ bearing pyridyl R1 (black circles) and pyrazolyl R2 (red squares) substituents. Data were measured in (CD3)2CO or CD3NO2 solution [97]. The data points for R1 = NH2 and NMe2 (white circles) represent upper limits for those T½ values, since those complexes remained fully high-spin within the liquid range of the solvent used.
Figure 6. Linear SCO free energy relationships for [Fe(1-bpp)2]2+ bearing pyridyl R1 (black circles) and pyrazolyl R2 (red squares) substituents. Data were measured in (CD3)2CO or CD3NO2 solution [97]. The data points for R1 = NH2 and NMe2 (white circles) represent upper limits for those T½ values, since those complexes remained fully high-spin within the liquid range of the solvent used.
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Scheme 9. Complexes with peripheral NH functionality that show a response between hydrogen bond formation and metal ion spin state. See also [Fe(3-bpp)2]2+ (Scheme 2).
Scheme 9. Complexes with peripheral NH functionality that show a response between hydrogen bond formation and metal ion spin state. See also [Fe(3-bpp)2]2+ (Scheme 2).
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