Ddpd as Expanded Terpyridine : Dramatic Effects of Symmetry and Electronic Properties in First Row Transition Metal Complexes

The 2,2′:6′:2′′-terpyridine ligand has literally shaped the coordination chemistry of transition metal complexes in a plethora of fields. Expansion of the ligand bite by amine functionalities between the pyridine units in the tridentate N,N’-dimethyl-N,N’-dipyridine-2-ylpyridine-2,6-diamine ligand (ddpd) modifies the properties of corresponding transition metal complexes, comprising redox chemistry, molecular dynamics, magnetism and luminescence. The origins of these differences between ddpd and tpy complexes will be elucidated and comprehensively summarized with respect to first row transition metal complexes with d2–d10 electron configurations. Emerging applications of these ddpd complexes complementary to those of the well-known terpyridine ligand will be highlighted.

Variations of the prototypical tpy ligand include substitution at the central and outer pyridines, ring annulation at the outer N-heterocycles, for example, forming quinolines and substitution of pyridines by other heterocycles such as thiophenes or pyrimidines.Expansion of the tpy scaffold by formally inserting a single-atom bridge between the pyridines has been very successfully employed especially in the field of emissive ruthenium(II) complexes and photosensitizers [28][29][30][31][32][33][34][35][36][37][38].
The effect of tpy ligand expansion in other, especially 3d metal complexes has been only scarcely explored and a systematic description and comparison with the prototypical tpy complexes is lacking.This review summarizes all available data on the expanded ligand ddpd (N,N'-dimethyl-N,N'-dipyridine-2-yl-pyridine-2,6-diamine), its homoleptic metal complexes [M(ddpd) 2 ] n+ covering d 2 -d 10 electron configurations of the metal centre and a few heteroleptic complexes MCl n (ddpd) and [M(dcpp)(ddpd)] 2+ (dcpp = 2,6-bis(2-carbonylpyridyl)pyridine).A special focus will be placed on the similarities and differences with respect to analogous terpyridine complexes (Scheme 1) in the fields of stereochemical, dynamic, magnetic, redox and photophysical phenomena.
Similar to tpy [44], ddpd is not pre-organized for metal complexation and the terminal pyridines are oriented outward (Figure 1a,c).This allows for CH . . .N hydrogen bonding interactions and avoids repulsion between the pyridine nitrogen lone pairs and between CH groups (Figure 1a).Terpyridine forms 5-membered rings including a CH . . .N hydrogen bond with the central pyridine nitrogen atom N2 (Figure 1c), while the additional N-Me groups in ddpd enforce 6-membered rings with N2 (Figure 1a).
Although the N-Me group of ddpd should be more basic [45,46], addition of acid protonates one terminal pyridine unit (N1) and both terminal pyridines rearrange to point inward allowing for one short and one medium long N1H1 . . .N2/N3 hydrogen bond with N . . .N distances in the cavity of 2.51, 2.81 and 3.24 Å (Figure 1b).This hydrogen bonding pattern favours the observed pyridine protonation site similar to pre-organized proton sponges with N . . .N distances between 2.3 and 2.6 Å [47].The cavity formed by the three pyridines is somewhat too large for the small proton resulting in two different distorted six-membered chelate rings (Figure 1b).On the other hand, this cavity should perfectly fit to accommodate metal ions.Tpy is protonated at N1 as well (Figure 1d) [48].However, tpy only realizes a single N1H1 . . .N2 hydrogen bond with an N1 . . .N2 distance of 2.65 Å forming a five-membered ring, while the second pyridine (N3) remains oriented outward.The [CF 3 SO 3 ] − counterion additionally binds to the pyridinium site N1H1.
The conformations and protonation sites already point to key differences between tpy and ddpd, namely the different available ring sizes and the proton sponge-like behaviour of ddpd due to its electron-richness and flexibility.
Inorganics 2018, 6, 86 3 of 37 Similar to tpy [44], ddpd is not pre-organized for metal complexation and the terminal pyridines are oriented outward (Figure 1a,c).This allows for CH … N hydrogen bonding interactions and avoids repulsion between the pyridine nitrogen lone pairs and between CH groups (Figure 1a).Terpyridine forms 5-membered rings including a CH … N hydrogen bond with the central pyridine nitrogen atom N2 (Figure 1c), while the additional N-Me groups in ddpd enforce 6-membered rings with N2 (Figure 1a).
Although the N-Me group of ddpd should be more basic [45,46], addition of acid protonates one terminal pyridine unit (N1) and both terminal pyridines rearrange to point inward allowing for one short and one medium long N1H1 … N2/N3 hydrogen bond with N … N distances in the cavity of 2.51, 2.81 and 3.24 Å (Figure 1b).This hydrogen bonding pattern favours the observed pyridine protonation site similar to pre-organized proton sponges with N … N distances between 2.3 and 2.6 Å [47].The cavity formed by the three pyridines is somewhat too large for the small proton resulting in two different distorted six-membered chelate rings (Figure 1b).On the other hand, this cavity should perfectly fit to accommodate metal ions.Tpy is protonated at N1 as well (Figure 1d) [48].However, tpy only realizes a single N1H1 … N2 hydrogen bond with an N1 … N2 distance of 2.65 Å forming a fivemembered ring, while the second pyridine (N3) remains oriented outward.The [CF3SO3] − counterion additionally binds to the pyridinium site N1H1.
The conformations and protonation sites already point to key differences between tpy and ddpd, namely the different available ring sizes and the proton sponge-like behaviour of ddpd due to its electron-richness and flexibility.A computational study of Ni(CO) 3 (κN1-ddpd), Ni(CO) 3 (κN2-ddpd) complexes and other Ni(CO) 3 (L) complexes revealed that the net donor strength (σ + π) of ddpd donor atoms ranks between that of trimethylamine and N-heterocyclic carbene ligands and is larger than that of pyridine, as estimated from the Density Functional Theory (DFT) calculated A 1 carbonyl stretching modes of the Ni(CO) 3 fragments [35].Obviously, ddpd is a stronger Lewis base and a stronger Brønsted base than tpy.
In line with the electron donating character of the N-Me groups, ddpd is very difficult to reduce to its radical anion ddpd• − (peak potential −3.27 V vs. ferrocene in CH 3 CN) [49].On the other hand, tpy is reversibly reduced to tpy• − at −2.55 V versus ferrocene in CH 3 CN [52].The relative ease of reduction causes the typical redox non-innocent behaviour of tpy in many first-row transition metal complexes [23][24][25][26][27].The electron rich ddpd ligand in contrast should behave essentially redox-innocent in transition metal complexes.Oxidation of ddpd's N-Me groups to the radical cations is irreversible with peak potentials at 0.55 V and 1.06 V versus ferrocene in CH 3 CN [49].
Typical synthetic procedures for homoleptic [M(ddpd)  modes of the Ni(CO)3 fragments [35].Obviously, ddpd is a stronger Lewis base and a stronger Brønsted base than tpy.
In line with the electron donating character of the N-Me groups, ddpd is very difficult to reduce to its radical anion ddpd• − (peak potential −3.27 V vs. ferrocene in CH3CN) [49].On the other hand, tpy is reversibly reduced to tpy• − at −2.55 V versus ferrocene in CH3CN [52].The relative ease of reduction causes the typical redox non-innocent behaviour of tpy in many first-row transition metal complexes [23][24][25][26][27].The electron rich ddpd ligand in contrast should behave essentially redoxinnocent in transition metal complexes.Oxidation of ddpd's N-Me groups to the radical cations is irreversible with peak potentials at 0.55 V and 1.06 V versus ferrocene in CH3CN [49].
The rigid five-membered C2N2M chelate rings formed by tpy ligands merely allow for a meridional coordination in homoleptic complexes [M(tpy)2] n+ .In contrast, the more flexible sixmembered C2N3M chelate rings formed by ddpd ligands enable both meridional and facial coordination modes, that is, a pincer-type [1] or a tripodal topology (Scheme 4a).In fact, mer isomers of [M(ddpd)2] n+ complexes appear to be more prevalent but cis-fac isomers have been observed as well.The six-membered chelate rings in the ddpd complexes form boat conformations.The metal centre M and the N-Me group (N7, N9) lie above the mean plane of the ring (Scheme 4a).The rigid five-membered C 2 N 2 M chelate rings formed by tpy ligands merely allow for a meridional coordination in homoleptic complexes [M(tpy) 2 ] n+ .In contrast, the more flexible six-membered C 2 N 3 M chelate rings formed by ddpd ligands enable both meridional and facial coordination modes, that is, a pincer-type [1] or a tripodal topology (Scheme 4a).In fact, mer isomers of [M(ddpd) 2 ] n+ complexes appear to be more prevalent but cis-fac isomers have been observed as well.The six-membered chelate rings in the ddpd complexes form boat conformations.The metal centre M and the N-Me group (N7, N9) lie above the mean plane of the ring (Scheme 4a).The N-M-N bite angles of ddpd are significantly larger (80-89°) than that of tpy with 75-82° (Tables 1 and 2).With the tpy ligand and the five-membered chelates being planar, the point group of [M(tpy)2] n+ complexes is C2v.As ddpd is a non-planar ligand in homo-and heteroleptic complexes, the point group of [M(ddpd)2] n+ complexes is only C2.Consequently, mer-[M(ddpd)2] n+ complexes are chiral with helical chirality (Scheme 4b).Mixed ruthenium(II) complexes [Ru(ddpd)(tpy-X)] 2+ form diastereomeric ion pairs with chiral enantiopure counterions [33].Racemic mer-[Cr(ddpd)2] 3+ complexes can be partially resolved into the corresponding P and M enantiomers.These seem to be configurationally stable on the HPLC timescale [53].The isomeric cis-fac-[M(ddpd)2] n+ complexes are chiral as well (Scheme 4b) and have been isolated only as racemic mixtures so far.In the following, all mer-and cis-fac-[M(ddpd)2] n+ complexes are racemic mixtures and this will not be indicated explicitly.
All homoleptic complexes [M(ddpd)2] n+ and [M(tpy)2] n+ share a [MN6] coordination sphere.The deviation of the experimental coordination geometry from the ideal octahedron can be expressed by a continuous shape measure S(OC-6) as implemented in the program SHAPE 2.1 (free download at http://www.ee.ub.es) [54][55][56][57][58].For an ideal octahedron, this value will be zero and increases with the structural deviation.The higher flexibility and thus higher adaptability of the six-membered chelate rings of ddpd allow for N-M-N angles closer to 90° and for similar M-N distances and thus enable "more octahedral" coordination geometries.The rigid pincer ligand tpy features smaller N-M-N angles and significantly differing M-N distances to the central and terminal pyridines yielding much more distorted coordination polyhedrons.This is clearly obvious from the significantly larger shape measure S(OC-6) of [M(tpy)2] n+ complexes as compared to analogous [M(ddpd)2] n+ complexes (Tables 1 and 2).Interestingly, the S(OC-6) values increase along the series Fe II /Co III < Cr III < Ni II < Co II < Cu II < Zn II in both ligand series, suggesting that this trend is coded in the d electron configuration of the metal ion (low-spin d 6    1 and 2).With the tpy ligand and the five-membered chelates being planar, the point group of [M(tpy) 2 ] n+ complexes is C 2v .As ddpd is a non-planar ligand in homo-and heteroleptic complexes, the point group of [M(ddpd) 2 ] n+ complexes is only C 2 .Consequently, mer-[M(ddpd) 2 ] n+ complexes are chiral with helical chirality (Scheme 4b).Mixed ruthenium(II) complexes [Ru(ddpd)(tpy-X)] 2+ form diastereomeric ion pairs with chiral enantiopure counterions [33].Racemic mer-[Cr(ddpd) 2 ] 3+ complexes can be partially resolved into the corresponding P and M enantiomers.These seem to be configurationally stable on the HPLC timescale [53].The isomeric cis-fac-[M(ddpd) 2 ] n+ complexes are chiral as well (Scheme 4b) and have been isolated only as racemic mixtures so far.In the following, all merand cis-fac-[M(ddpd) 2 ] n+ complexes are racemic mixtures and this will not be indicated explicitly.
All homoleptic complexes [M(ddpd) 2 ] n+ and [M(tpy) 2 ] n+ share a [MN 6 ] coordination sphere.The deviation of the experimental coordination geometry from the ideal octahedron can be expressed by a continuous shape measure S(OC-6) as implemented in the program SHAPE 2.1 (free download at http://www.ee.ub.es) [54][55][56][57][58].For an ideal octahedron, this value will be zero and increases with the structural deviation.The higher flexibility and thus higher adaptability of the six-membered chelate rings of ddpd allow for N-M-N angles closer to 90 • and for similar M-N distances and thus enable "more octahedral" coordination geometries.The rigid pincer ligand tpy features smaller N-M-N angles and significantly differing M-N distances to the central and terminal pyridines yielding much more distorted coordination polyhedrons.This is clearly obvious from the significantly larger shape measure S(OC-6) of [M(tpy) 2 ] n+ complexes as compared to analogous [M(ddpd) 2 ] n+ complexes (Tables 1 and 2).Interestingly, the S(OC-6) values increase along the series Fe II /Co III < Cr III < Ni II < Co II < Cu II < Zn II in both ligand series, suggesting that this trend is coded in the d electron configuration of the metal ion (low-spin d 6 , d 3 , d 8 , high-spin d 7 , d 9 , d 10 ), especially in the occupation of the e g * orbitals and in the presence of Jahn-Teller effects [59].[Cr(L) 2 ] 2+ complexes feature different electron distributions for L = ddpd and tpy as will be discussed below.Specific to ddpd are the bridging three-coordinate nitrogen atoms N7/N8 and N9/N10 and their respective degree of planarization as defined by [43].In ddpd as well as in [H-ddpd] + , these N atoms are in a planar environment with PL = 100%, while in the homoleptic complexes mer-[M(ddpd) 2 ] n+ the degree of planarization varies between 81% and 93% (Table 1).This indicates that metal chelation induces some strain in the ligand.In essence, the main differences between tpy and ddpd are the stronger electron donating and weaker electron accepting character of ddpd in addition to its higher flexibility and larger N-M-N bite angles.In the following chapters, the translation of these key differences of the tridentate oligopyridine ligands into different properties of first row transition metal ddpd and tpy complexes will be elaborated in more detail.
The [Zn(tpy)2] 2+ complex can be reversibly reduced twice at the tpy ligands (Table 4) [20].In contrast, [Zn(ddpd)2] 2+ is only irreversibly reduced analogous to the ddpd ligand itself (Table 4).Due to the positive charge of the complex, these potentials are less negative than in the non-coordinated In essence, the main differences between tpy and ddpd are the stronger electron donating and weaker electron accepting character of ddpd in addition to its higher flexibility and larger N-M-N bite angles.In the following chapters, the translation of these key differences of the tridentate oligopyridine ligands into different properties of first row transition metal ddpd and tpy complexes will be elaborated in more detail.
In the solid state, the coordination geometry of [Zn(ddpd) 2 ] 2+ deviates appreciably from an idealized octahedron with S(OC-6) = 1.18 when compared to other [M(ddpd) 2 ] n+ complexes (Figure 3a, Table 1).However, the distortion of [Zn(tpy) 2 ] 2+ is dramatically larger with S(OC-6) = 4.45, which can be accounted for by the very small N-Zn-N bite angles of 75 • (Table 2).Coordination of Zn 2+ to ddpd shifts the π-π* absorption maxima to higher energy from 274/333 nm to 248/308 nm.In the colourless homoleptic complex, these bands are assigned to ligand-centred transitions essentially from the electron-rich N-Me groups to the pyridine's π* orbitals.Fluorescence is observed at 373 nm in (acid-free) CH2Cl2 with 6.5% quantum yield and 0.6 ns lifetime [49].The diminished fluorescence lifetime as compared to ddpd is likely due to the heavy-atom effect of Zn 2+ , enabling intersystem crossing (ISC) to the triplet state.The tpy-based fluorescence of [Zn(tpy)2] 2+ is found at higher energy with significantly higher quantum yield (λem = 353 nm, Φ = 65%) [81].The N-Me donor groups of ddpd are likely responsible for the slightly lower emission energy of [Zn(ddpd)2] 2+ .The flexibility of the six-membered chelates possibly allow for enhanced non-radiative relaxation in [Zn(ddpd)2] 2+ .
Interestingly, the analogous five-coordinate complex Zn(H2tpda)Cl2 has been reported (Figure 3c) with a fluorescence emission band peaking at 392 nm in MeOH [41].The reported UV/Vis absorption spectrum of Zn(H2tpda)Cl2 in MeOH with a strong 311 nm absorption band [41], however, strongly resembles that of [Zn(ddpd)2] 2+ ions [49].Hence, an analogous equilibrium Coordination of Zn 2+ to ddpd shifts the π-π* absorption maxima to higher energy from 274/333 nm to 248/308 nm.In the colourless homoleptic complex, these bands are assigned to ligand-centred transitions essentially from the electron-rich N-Me groups to the pyridine's π* orbitals.Fluorescence is observed at 373 nm in (acid-free) CH 2 Cl 2 with 6.5% quantum yield and 0.6 ns lifetime [49].The diminished fluorescence lifetime as compared to ddpd is likely due to the heavy-atom effect of Zn 2+ , enabling intersystem crossing (ISC) to the triplet state.The tpy-based fluorescence of [Zn(tpy) 2 ] 2+ is found at higher energy with significantly higher quantum yield (λ em = 353 nm, Φ = 65%) [81].The N-Me donor groups of ddpd are likely responsible for the slightly lower emission energy of [Zn(ddpd) 2 ] 2+ .The flexibility of the six-membered chelates possibly allow for enhanced non-radiative relaxation in [Zn(ddpd) 2 ] 2+ .
The diamagnetic d 10 -[ZnL 2 ] 2+ complexes with L = ddpd and tpy are distorted, substitutionally labile, optically transparent in the visible spectral region and fluoresce only in the UV spectral region.The tpy complex shows reversible ligand-centred reductions, while the ddpd complex is basically redox-inert between −2.3 V and +1.1 V.
Both copper(II) complexes are fluxional at room temperature.The dynamics slow down at lower temperature."Freezing-in" occurs below 77 K for the tpy complex and at ≈100 K for the ddpd complex [60].This suggests a larger Jahn-Teller barrier for [Cu(ddpd) 2 ][BF 4 ] 2 between the two degenerate Jahn-Teller isomers (Figure 4b).In the solid state at 123 K, the [CuN 6 ] coordination sphere of [Cu(ddpd  4).In the intermediate temperature range, the EPR spectra are weighted superpositions of the low-and high-temperature spectra [60].As this temperature-dependent dynamic is essentially independent of the environment (pure copper(II) crystal, diamagnetic host crystal, frozen solution), cooperative effects are absent and the Jahn-Teller barrier is an intrinsic feature of the [Cu(ddpd) 2 ] 2+ cation.
Compared to [Cu(tpy) 2 ] 2+ , [Cu(ddpd) 2 ] 2+ is more difficult to reduce to copper(I), features a larger ligand field and Jahn-Teller splitting and a higher barrier for the Jahn-Teller dynamics.All these phenomena can be accounted for by the stronger σ-donating character of ddpd.
The UV/Vis/NIR absorption spectra of red [Ni(tpy) 2 ] 2+ and pink [Ni(ddpd) 2 ] 2+ are dominated by the two spin-allowed 3 A 2 → 3 T 1 and 3 A 2 → 3 T 2 ligand field transitions at 18,500/12,350 and 19,200/12,700 cm -1 , respectively (Figure 5) [24,61,91].The transitions at lowest energy correspond directly to the respective ligand field splittings ∆ o = 12,350 and 12,700 cm −1 , demonstrating that ddpd induces a stronger ligand field by 350 cm −1 .Nickel(II) polypyridine complexes display the spin-forbidden 3 A 2 → 1 E transition close to the low-energy spin-allowed 3 A 2 → 3 T 2 transition indicating a comparably strong ligand field in all cases.The intensity of the spin-flip band is rather high due to an intensity borrowing mechanism, which scales directly with the square of the spin-orbit coupling parameter λ and inversely with the square of the energy difference ∆E (Figure 5; shoulder at 11,300 cm -1 for [Ni(ddpd) 2 ] 2+ ) [93].The energy differences between the 1 E and 3 T 2 spectroscopic terms are quite small and consequently luminescence from the spin-flip 1 E states is not observed [61].
Inorganics 2018, 6, 86 13 of 37 The UV/Vis/NIR absorption spectra of red [Ni(tpy)2] 2+ and pink [Ni(ddpd)2] 2+ are dominated by the two spin-allowed 3 A2 → 3 T1 and 3 A2 → 3 T2 ligand field transitions at 18,500/12,350 and 19,200/12,700 cm -1 , respectively (Figure 5) [24,61,91].The transitions at lowest energy correspond directly to the respective ligand field splittings ∆o = 12,350 and 12,700 cm −1 , demonstrating that ddpd induces a stronger ligand field by 350 cm −1 .Nickel(II) polypyridine complexes display the spinforbidden 3 A2 → 1 E transition close to the low-energy spin-allowed 3 A2 → 3 T2 transition indicating a comparably strong ligand field in all cases.The intensity of the spin-flip band is rather high due to an intensity borrowing mechanism, which scales directly with the square of the spin-orbit coupling parameter λ and inversely with the square of the energy difference ∆E (Figure 5; shoulder at 11,300 cm -1 for [Ni(ddpd)2] 2+ ) [93].The energy differences between the 1 E and 3 T2 spectroscopic terms are quite small and consequently luminescence from the spin-flip 1 E states is not observed [61].Reduction of [Ni(ddpd)2] 2+ is irreversible (−2.03 V).Presumably, the irreversible reduction is associated with the formation of [Ni I (ddpd)] + species after ligand loss.In the presence of CO2/acetic acid a catalytic current with an onset potential of ca.−1.1 V versus ferrocene is observed (Figure S2).A detailed comparison of this behaviour and that proposed for [Ni(tpy)2] 2+ will be subject to future studies [20].
In summary, the magnetic, optical and redox properties of [Ni(ddpd)2] 2+ are very similar to those of [Ni(tpy)2] 2+ , with the ligand field splitting induced by ddpd being larger by 350 cm −1 .
In summary, the magnetic, optical and redox properties of [Ni(ddpd) 2 ] 2+ are very similar to those of [Ni(tpy) 2 ] 2+ , with the ligand field splitting induced by ddpd being larger by 350 cm −1 .
The second order rate constant for the self-exchange reaction [Co(tpy) 2 ] 3+/2+ has been determined as k ex = 2820-3000 M −1 s −1 (in H 2 O) between low-spin Co II and low-spin Co III [106,107].Much lower rate constants of k ex = 41.7 and 17.5 M −1 s −1 have been reported for the [Co(phen) 3 ] 3+/2+ and [Co(bpy) 3 ] 3+/2+ pairs, respectively [106,107].This can be traced back to the different spin states of [Co(tpy) 2 ] 2+ (mainly low-spin) and [Co(phen) 3 ] 2+ /[Co(bpy) 3 ] 2+ (high-spin) at room temperature in solution.In any case, self-exchange is comparably slow on the 1 H NMR timescale and this holds for the [Co(ddpd) 2 ] 3+/2+ couple as well [62].In addition to the spin-barrier for the ddpd complexes, the Co-N bond lengths Co-N2 and Co-N1/3 decrease by 0.148 and 0.166 Å upon oxidation (Table 1).Similarly, the averaged Co-N2 and Co-N1/N3 bond lengths of the tpy complexes shrink by 0.149 and 0.200 Å, respectively (Table 2).Obviously, the large reorganization energies (irrespective of the Co II spin state) mainly account for the comparably slow electron transfer rates.Even smaller self-exchange rate constants are found for Co III/II complexes of aliphatic amines, such as [Co(en) 3 ] 3+/2+ with k ex = 2 × 10 −5 M −1 s −1 [108].This has been ascribed to the innocent nature of ethylene diamine in contrast to the redox-activity of bpy or phen ligands.The latter enable a strong electronic coupling via π* orbitals of the ligands in the electron transfer step, for example via the ubiquitous "phenyl embraces" of polypyridine complexes (Figure 2) [72].As ddpd is a poor electron acceptor, electron transfer to [Co(ddpd) 2 ] 3+ is retarded by the high-lying π* orbitals of ddpd.
The slow electron transfer kinetics of the reduction of [Co(ddpd) 2 ] 3+ can be beneficial for applications in dye-sensitized solar cells, retarding the undesired back-electron transfer (recombination) at the TiO 2 electrode [102].However, dye-regeneration by [Co(ddpd) 2 ] 2+ is also slow [103].Compared to the standard iodide/triiodide electrolyte, the overall performance of outer sphere cobalt(III/II) electrolytes in dye-sensitized solar cells depends on the dye and the electrode surface treatment among other factors [102,103,109,110].
Refluxing a solution of [Fe(ddpd) 2 ] 2+ in CH 3 CN does not lead to spin-crossover (SCO) to the 5 T 2 high-spin state but to ligand dissociation [64].Under these conditions, the [Fe(tpy) 2 ] 2+ ion is thermally stable but does not undergo SCO either.This suggests a too strong ligand field splitting in both complexes.Substitution of tpy at the sterically unhindered 4,4 positions retains the low-spin ground state [111].On the other hand, substituents at the α positions (6,6 ) (X = F, Cl, Br) lead to a diminished ligand field strength and high-spin ground states (X = Cl, Br) or SCO behaviour (X = F) [114,115].
Replacement of a ddpd ligand by the electron accepting tridentate dcpp ligand give the deep blue-coloured heteroleptic push-pull substituted complex [Fe(dcpp)(ddpd)] 2+ (Scheme 5) [64].The colour arises from a strong charge transfer absorption band peaking at 592 nm and tailing into the near-IR.According to time-dependent DFT calculations, the lowest energy components are of mixed MLCT (iron(II) → dcpp) and LL'CT (ddpd → dcpp) character.The homoleptic complex [Fe(dcpp) 2 ] 2+ prepared by McCusker features charge transfer bands as well but these are of pure MLCT nature [64,117].After excitation of the charge transfer bands of [Fe(dcpp) 2 ] 2+ and [Fe(dcpp)(ddpd)] 2+ , transient absorption spectroscopy reveals a rapid recovery of the ground state with τ = 280 and 548 ps, respectively.This unusually fast relaxation in the picosecond range has been tentatively assigned to a shortened relaxation cascade 1 MLCT → 3 MLCT → 3 MC( 3 T 1 ), omitting the 5 MC( 5 T 2 ) high-spin state with typical lifetimes in the nanosecond range [64,117].It has been proposed that the very large ligand field splitting in these complexes shifts the 5 T 2 state close to or even above the 3 T 1 excited state.However, non-radiative relaxation via the 3 T 1 states is still too fast to allow for a long-lived 3 MLCT state, as has been established recently in polycarbene iron(II) complexes [118][119][120][121][122]. [64,117].After excitation of the charge transfer bands of [Fe(dcpp)2] and [Fe(dcpp)(ddpd)] , transient absorption spectroscopy reveals a rapid recovery of the ground state with  = 280 and 548 ps, respectively.This unusually fast relaxation in the picosecond range has been tentatively assigned to a shortened relaxation cascade 1 MLCT → 3 MLCT → 3 MC( 3 T1), omitting the 5 MC( 5 T2) high-spin state with typical lifetimes in the nanosecond range [64,117].It has been proposed that the very large ligand field splitting in these complexes shifts the 5 T2 state close to or even above the 3 T1 excited state.However, non-radiative relaxation via the 3 T1 states is still too fast to allow for a long-lived 3 MLCT state, as has been established recently in polycarbene iron(II) complexes [118][119][120][121][122]. Scheme 5. Heteroleptic iron(II) complexes of ddpd/R2tpda and dcpp.
All iron(II) and iron(III) complexes of the oligopyridine ligands tpy and ddpd are low-spin complexes due to the large ligand field splitting in all cases.SCO is not observed.Ligand-centred reductions are reversible for the tpy complex but irreversible for the ddpd complex.The iron(III/II) couple is reversible for both complex types.MLCT bands appear in the visible spectral region for [Fe(tpy) 2 ] 2+ due to the low-energy π* orbitals of tpy but in the UV for the ddpd complex.The heteroleptic complex [Fe(dcpp)(ddpd)] 2+ displays MLCT bands with Fe → dcpp and LL'CT bands with ddpd → dcpp character in the Vis-NIR region.

X-ray diffraction of ddpd, [H
Intensity data were collected with a STOE IPDS-2T diffractometer with an Oxford cooling system using Mo-Kα radiation (λ = 0.71073 Å).The diffraction frames were integrated using the Bruker SMART software package [156] and most were corrected for absorption with MULABS [157] of the PLATON software package [158].The structures were solved by direct methods and refined by the full-matrix method based on F 2 using the SHELXL software package [159,160].All non-hydrogen atoms were refined anisotropically, while the positions of all hydrogen atoms were generated with appropriate geometric constraints and allowed to ride on their respective parent atoms with fixed isotropic thermal parameters.Only, the non-hydrogen atoms of co-crystallized acetonitrile in mer-[Fe(ddpd) 2 ]Br 2 ×2CH 3 CN have been refined isotropically.
Crystallographic data for the structure reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no CCDC-1832949 (ddpd), R 1 = 0.3163, wR 2 = 0.3895; largest diff.peak and hole 1.151 and −0.540 e Å −3 .Unfortunately, only a few needle-shaped crystals were suitable for single-crystal X-ray analysis.The investigated needle-shaped crystal was the one of highest quality.Up to now, no crystals with better diffracting power could be obtained and hence no better reflection data.In spite of the weak diffraction quality, the cis-fac coordination mode of the ddpd ligand in this complex salt is unambiguous.
NMR spectra were recorded with a Bruker Avance DRX 400 spectrometer at 400.31 MHz ( 1 H).All resonances are reported in ppm versus the solvent signal as an internal standard (CD 2 Cl 2 : 1 H, δ = 5.32 ppm; s = singlet, d = doublet, t = triplet, m = multiplet) [161].IR spectra were recorded with a BioRad Excalibur FTS 3100 spectrometer as KBr disks or with a Bruker Alpha FT-IR spectrometer with an ATR unit containing a diamond crystal.UV/Vis/near-IR spectra were recorded with a Varian Cary 5000 spectrometer by using 1.0 cm cells (Hellma, Suprasil).ESI + mass spectra were recorded with a Micromass Q-TOF-Ultima spectrometer.FD mass spectra were recorded on a Thermo Fisher DFS mass spectrometer with a LIFDI upgrade.DC magnetic studies were performed with a Quantum Design MPMS-XL-7 SQUID magnetometer on powdered microcrystalline samples embedded in eicosane to avoid orientation of the crystallites under applied field (1 Tesla).Experimental susceptibility data were corrected for the underlying diamagnetism using Pascal's constants.The temperature dependent magnetic contribution of the holder and of the embedding matrix eicosane were experimentally determined and subtracted from the measured susceptibility data.Electrochemical experiments were carried out on a BioLogic SP-50 voltammetric analyser using a platinum working electrode, a platinum wire as a counter electrode and a 0.01 M Ag/AgNO 3 CH 3 CN electrode as reference electrode.The measurements were carried out at a scan rate of 100 mV s −1 for cyclic voltammetry experiments using 0.1 M [ n Bu 4 N] [PF 6 ] as supporting electrolyte and 0.002 M of the sample in acetonitrile.Potentials are given relative to the ferrocene/ferrocenium couple.Elemental analyses were performed by the microanalytical laboratory of the chemical institutes of the University of Mainz.
To account for solvent effects, a conductor-like screening model (CPCM) modelling acetonitrile was used in all calculations [169].Geometry optimizations were performed using Ahlrichs' split-valence double-ξ basis set def2-SVP, which comprises polarization functions for all non-hydrogen atoms [170,171].Atom-pairwise dispersion correction was performed with the Becke-Johnson damping scheme (D3BJ) [172,173].
The six-membered chelate rings of ddpd form larger ligand bite angles and geometries closer to an ideal octahedron than the more strained tpy ligand.The increased flexibility of the six-membered rings allow both pincer type (meridional) and tripodal (facial) coordination of ddpd, for example merand cis-fac-[Co(ddpd) 2 ] 2+ complexes.The meridional isomers [M(ddpd) 2 ] n+ are typically preferred and most often encountered.The bite angles of ddpd close to 90 • enable a better metal orbital-pyridine orbital overlap.This results in generally larger ligand field splittings than in the corresponding [M(tpy) 2 ] n+ complexes (except for high-spin [Co(L) 2 ] 2+ complexes).The ligand field splitting affects both the ground state multiplicity (low-spin/high-spin) and the excited state level ordering, especially in d 3 complexes.Specifically, the very strong ligand field strength of ddpd enables strongly luminescent chromium(III) complexes due to the high energy of the detrimental ligand field states.Electron configurations with degenerate electronic states, especially (e g ) 1 and (e g ) 3  have not yet been discovered with low-spin spin states but might be available with different counter ions or within different matrices.Iron(II) complexes with unsubstituted ddpd ligands are all low-spin.However, lowering the ligand field strength by 6,6 substitution might be a promising strategy, similar to the tpy complexes.
Modification of ddpd to tune the ligand field strength (higher or lower), to allow for conjugation with functional groups such as light antenna or catalysts, for incorporation into supramolecular arrays or for immobilization on surfaces or in nanoparticles are important future perspectives of transition metal ddpd complexes.The deliberate incorporation of ddpd in heteroleptic chelate complexes will further open plentiful tuning possibilities with respect to optical, magnetic and catalytic properties.These future developments will possibly enable applications of [M(ddpd)(L) m ] n+ in magnetic storage devices, optical sensing, catalysis, electrocatalysis, photocatalysis and supramolecular chemistry.

Inorganics 2018, 6 , 86 5 of 37 Scheme 4 .
Scheme 4. (a) Chelate ring size (5 for tpy; 6 for ddpd) and corresponding envelope and boat conformations and the resulting conceivable mer and cis-fac stereoisomers and (b) space-filling models of mer and cis-fac stereoisomers of [M(ddpd)2] n+ with the two ligands color-coded red and blue, respectively.For numbering of nitrogen atoms see Scheme 3. Hydrogen atoms omitted for clarity.
, d 3 , d 8 , high-spin d 7 , d 9 , d 10 ), especially in the occupation of the eg* orbitals and in the presence of Jahn-Teller effects [59].[Cr(L)2] 2+ complexes feature different electron distributions for L = ddpd and tpy as will be discussed below.

Scheme 4 .
Scheme 4. (a) Chelate ring size (5 for tpy; 6 for ddpd) and corresponding envelope and boat conformations and the resulting conceivable mer and cis-fac stereoisomers and (b) space-filling models of mer and cis-fac stereoisomers of [M(ddpd) 2 ] n+ with the two ligands color-coded red and blue, respectively.For numbering of nitrogen atoms see Scheme 3. Hydrogen atoms omitted for clarity.
3 CN and H 2 O, respectively [50,53].These values have been further boosted by the combined action of ligand (Scheme 2) and solvent deuteration to φ = 30% and 22% in D 2 O and CD 3 CN, respectively [42,53].These measures reduce the multiphonon relaxation via CH and OH oscillators and consequently increase the luminescence quantum yield [42,53]. Concomitant with the very high quantum yields, the lifetime of the doublet excited state increases to up to 2.3 ms [42].Similarly, deuteration of the NH groups of the analogous [Cr(H 2 tpda) 2 ] 3+ complex increases the luminescence quantum yield from 8.8% to 12.0% in CH 3 CN due to the removal of the NH oscillators [138].The high quantum yields and excited state lifetimes of [Cr(ddpd) 2 ] 3+ complexes led to their description as "molecular rubies" [42,50,53].Inorganics 2018, 6, 86 22 of 37 with a geometry close to octahedral (S(OC-6) < 0.5, Table 1) enables record luminescence quantum yields of φ = 12% and 11% in CH3CN and H2O, respectively [50,53].These values have been further boosted by the combined action of ligand (Scheme 2) and solvent deuteration to φ = 30% and 22% in D2O and CD3CN, respectively [42,53].These measures reduce the multiphonon relaxation via CH and OH oscillators and consequently increase the luminescence quantum yield [42,53]. Concomitant with the very high quantum yields, the lifetime of the doublet excited state increases to up to 2.3 ms [42].Similarly, deuteration of the NH groups of the analogous [Cr(H2tpda)2] 3+ complex increases the luminescence quantum yield from 8.8% to 12.0% in CH3CN due to the removal of the NH oscillators [138].The high quantum yields and excited state lifetimes of [Cr(ddpd)2] 3+ complexes led to their description as "molecular rubies" [42,50,53].

Table 1 .
Relevant metrical data of [M(ddpd) 2 ] n+ ions in the solid state; mer isomers if not indicated otherwise; [BF 4 ] − salts if not indicated otherwise; distances in Å, PL in % and angles in • .Red and blue color code indicates the different ddpd ligands (see Scheme 3).

Table 2 .
Relevant metrical data of [M(tpy) 2 ] n+ ions in the solid state; [BF 4 ] − salts if not indicated otherwise, distances in Å and angles in • .Red and blue color code indicates the different tpy ligands.