Seeking Redox Activity in a Tetrazinyl Pincer Ligand: Installing

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Seeking Redox Activity in a Tetrazinyl Pincer Ligand: Installing Zerovalent Cr and Mo Nicholas A. Maciulis,† Richard N. Schaugaard,† Yaroslav Losovyj, Chun-Hsing Chen, Maren Pink, and Kenneth G. Caulton* Department of Chemistry, Indiana University, Bloomington, Indiana 47405, United States

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S Supporting Information *

ABSTRACT: Reaction of the readily reduced pincer ligand bistetrazinylpyridine, btzp, with the zerovalent metal source M(CO)3(MeCN)3 yields M(btzp)2 for M = Cr, Mo. These diamagnetic molecules show intrapincer bond lengths consistent with major charge transfer from metal to ligand, a result which is further supported by X-ray photoelectron spectroscopy. These molecules show up to five reversible outer-sphere electron transfers by cyclic voltammetry. The electronic structure of neutral M(btzp)2 is analyzed by DFT and CASSCF calculations, which reveal the degree of back-donation from the metal into pincer π* orbitals and also subtle differences in metal−ligand interaction for Mo vs Cr. Near-IR absorptions exhibited by both M(btzp)2 species originate from charge transfer among dif ferently reduced tetrazine rings, which thus further support pincer reduction in these species.

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INTRODUCTION The low-lying LUMO of nitrogen heterocycles (in comparison to hydrocarbon analogues) makes them into redox-active components of its metal complexes: redox-active ligands (RAL).1 The LUMO drops progressively in energy with each replacement of CH in a phenyl ring by N. By the time one carries this to tetrazines (tz, Scheme 1) these rings readily

AgCl). In that work, the reduced species were not isolated and structurally characterized. We wanted next to extend beyond the Fe(II) example by installing an already reduced, low valent metal to the btzp environment and study the distribution of electrons within that assembly/composite/construct. Reacting ortho quinones with metal carbonyls6 is a classic synthesis approach introduced by Pierpont. We report here on the unusual outcome of reaction of btzp with group VI metal carbonyls, demonstrating this to be an effective synthetic strategy, and showing the unusual electron configuration of the resulting product.

Scheme 1. Structures of btzp and H4btzp, Where R = Me, Et

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RESULTS AND DISCUSSION The particular btzp variant chosen has a pendant ethyl group to improve solubility in comparison to the variant5a which had methyl tetrazine substituents; moreover, each such substituent is a useful spectroscopic reporter. Our original goal was to synthesize M(CO)3btzpEt, where M = Mo, Cr, with the expectation that the carbonyl ligands would be labile, to open a site for substrate coordination. In reality M(CO)3(MeCN)3,7 where M = Cr, Mo, reacts instantly with btzpEt to form a dark violet solution and all CO ligands are lost on the time scale of minutes at 25 °C; CO-free products were obtained. We attribute this to the fact that btzp must be considered to be a Lewis base with the paradoxical property of being an oxidant; as metal d electrons are drained away into the btzp LUMO, back-donation to CO becomes so diminished that CO

accept electrons, and their radical anions are persistent, even at 25 °C. We have tried to exploit this as a way to store reducing equivalents in ligand π* orbitals, instead of having electrons occupy metal orbitals; reduction into d orbitals might work against binding a nucleophilic substrate at those orbitals. It has already been shown that this “reduction at ligand” principle applies even to bipyridyl,2 and also to terpyridyl.3 Kaim has demonstrated4 that single tetrazine rings are the site of reduction of a coordinated tetrazine on divalent Ru, on monovalent Cu, and even on zerovalent Mo. We have reported5 on the synthesis of a pincer ligand with two tetrazine rings connected by the two ortho carbons of pyridine, a bistetrazinylpyridine, or btzp. When attached to Fe(II), the resulting Fe(btzp)22+ complex can be reduced by up to four electrons at potentials no more negative than −0.90 V (vs Ag/ © XXXX American Chemical Society

Received: July 3, 2018

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DOI: 10.1021/acs.inorgchem.8b01761 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry dissociates from the metal. This behavior is already evident in the loss of more than one CO when Mo(CO)6 is oxidized by equimolar Br28 and is true of the reaction of o-quinone with M(CO)6.9 In the present case, an unsaturated (btzp)Mo(CO)3 intermediate is highly reactive and rapidly scavenges additional btzp to give the observed product. One product, a black precipitate insoluble in even the most polar solvents employed (MeCN, dioxane, and DMF), was not further studied. The second product was diamagnetic and showed all 1H NMR resonances consistent with a C2symmetric btzp ligand. The 1H NMR spectrum of the product shows, in the 0−10 ppm region of diamagnetic compounds, one ethyl environment and one pyridyl AX2 spin system with intensities appropriate for C2v symmetric btzp ligands. The yield was higher for Cr than for Mo, and we were unable to completely eliminate formation of the insoluble product by any alternative zerovalent metal source (hexacarbonyl, (benzene)M(CO)3, or Cr(benzene)2), reaction temperature, direction of addition, or btzp:M ratio. Since Cr(CO)3(MeCN)3 was later shown to react immediately with equimolar Cr(btzp)2 in CH2Cl2 or benzene at 25 °C, to form the black precipitate, we devised a synthesis keeping the concentration of free Cr(CO)3(MeCN)3 minimal during its addition. The conditions that finally gave M(btzpEt)2 in highest yield and smallest amount of black precipitate was slow addition of a dilute solution of M(CO)3(MeCN)3 to a concentrated btzpEt solution in a polar, weakly coordinating solvent, CH2Cl2, cooled to −78 °C. The ESI+ mass spectrum of the soluble product for M = Mo shows an ion of formula Mo(btzpEt)2H+, where facile protonation is consistent with its electron-rich character. For both metals, dark violet crystals of X-ray diffraction quality contained the molecules M(btzpEt)2, where M = Cr, Mo. While both Cr(btzpEt)2 and Mo(btzpEt)2 give 1H NMR signals that correspond to protons on btzpEt, revealing diamagnetism, the ∼7 Hz vicinal HH coupling in the CH2 1 H NMR signal was often not resolved. This broadening in Cr(btzpEt)2 in CD2Cl2 was not decoalesced at −50 °C and so was not due to fluxionality and diastereotopic character. While Mo(btzp)2 reacts with O2 in CH2Cl2 or MeCN or benzene in the time of addition, the Cr analogue is unaltered by these conditions. Structure of M(btzpEt) 2 , Where M = Cr, Mo. Recrystallization of the dark violet Mo complex from dichloromethane/pentane at −35 °C afforded dark violet crystals which were shown by X-ray diffraction (Figure 1) to have the formula Mo(btzpEt)2. The asymmetric unit contains two molecules, but the two do not differ significantly and the following discussion will quote data for only one molecule. The molecule is six-coordinate, but the MoN6 substructure has significant deviations from octahedral: the Npy−Mo−Npy angle involving the two pyridyl nitrogens is 160° ± 3°, and the MoN3 plane of one btzp makes an angle of 82.8° with the MoN3 plane of the other btzp. The angular distortions away from octahedral observed in Mo(btzpEt)2 occur in both molecules, suggesting that they are intramolecular in origin. In addition, angular distortions are absent in all three isoelectronic M(terpy)2 species (M = Cr, Mo, W),3b and they are absent in the less reduced isoelectronic analogue Fe(btzp)22+.5a This “bent” structural preference reflects some extreme condition in Mo(btzpEt)2, whose origin we must determine.

Figure 1. Mercury view (50% probabilities) of the non-hydrogen atoms of one molecule of Mo(btzpEt)2, showing selected atom labeling. Unlabeled atoms are carbons. Selected bond lengths (Å) and angles (deg): Mo1−N18, 2.025(4); Mo1−N14, 2.085(4); Mo1− N10, 2.059(4); Mo1−N1, 2.057(4); Mo1−N8, 2.094(4); Mo1−N9, 2.043(4); ∠N14−Mo1−N8, 158.02(16).

The chromium compound crystallizes with CH2Cl2 and cyclohexane guests (Figure 2) and with four molecules in the

Figure 2. X-ray crystal structure of Cr(btzpEt)2 with ellipsoids at 50% probability. Selected bond lengths (Å) and angles (deg): Cr1−N10, 1.974(3); Cr1−N14, 2.000(3); Cr1−N18, 1.975(3); Cr1−N1, 1.987(3); Cr1−N5, 1.994(3); Cr1−N9, 1.987(3); ∠N5−Cr−N14, 170.13(13).

asymmetric unit. The molecular packing, hence also intermolecular force, is thus very different from that in Mo(btzpEt)2. One molecule, labeled Cr4, shows some disorder of the pincer ligand rings, consistent with the softness of the conformational energy surface (see below) and will not be considered in the analysis of structural parameters. The X-ray data for Cr(btzpEt)2 shows significant variation from D2d (“octahedral”) symmetry in each of the four molecules in the asymmetric unit, whose Npy−Cr−Npy angles vary from 160.68 to 170.13°. As with the Mo analogue, we must establish the cause of this bending, as well as the variability of the bend angle. For understanding this “bent” structure, we turned to density functional theory. Mo(btzpEt)2. DFT calculations on Mo(btzpEt)2 revealed that dispersion forces must be included to have the bent form more stable than D2d, and M06L, for example (Figure 3), gives an angle of 163.2° in contrast to that for B3LYP of 179.8°. To quantitatively assess the importance of dispersion forces, we augmented the B3LYP functional with the nonlocal correlation B


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Inorganic Chemistry

Figure 3. Functional dependence of geometry optimized Npy−Mo−Npy bond angles.

component of Vydrov and Van Voorhis’ VV10 functional.10 This approach allows evaluation of the dispersion component of the total energy from the electron density self-consistently and separately from the rest of the exchange-correlation functional. This showed that the nonlocal VV10 dispersion correction favors the bent structure by ∼3.4 kcal/mol to the point where it is more stable overall (total energy) than the linear form, but by only 0.5 kcal/mol. The more important conclusion from these studies is that (1) the purely electronic potential energy surface (PES), as judged by a number of functionals, is very soft to deviations from bending, and (2) the dispersion forces are large enough to overcome such a soft surface and lead to bending. Thus, while we can duplicate the observed bent experimental structure, the more important conclusion is that the electronic PES is so soft that dispersion forces become influential on the actual observed Npy−M−Npy angle, with a deviation of as much as 20° from linear. As a result of this bending, the molecular dipole moment deviates from the zero value of the linear structure, but to only 0.17 D, where, for comparison, simply toluene is calculated at 0.44 D; while the dipole moment does not contribute to the (gasphase) DFT PES, it does show the possible influence of the polar solvent or crystal lattice on the detailed structure observed. Finally, the calculated soft PES is fully consistent with the observation that the CH2 protons in bent Mo(bptzEt)2 show no diastereotopic inequivalences by 1H NMR; therefore, any bent structures are rapidly equilibrating in solution, effecting site exchange on the 1H NMR time scale. This softness will also become important in an interpretation of the electronic spectrum. Cr(btzpEt)2. For Cr(btzpEt)2, at the M06L def2-SVP level of theory, three singlets were located: a D2d open-shell singlet, a D2d closed-shell singlet, and a bent (176.8°) open-shell singlet. Of these, the D2d (broken symmetry) open-shell singlet was the most energetically favorable, with the closed-shell and bent structures being 10.6 and 2.0 kcal/mol higher in energy, respectively. The deviation from linearity for the axially distorted structure is only 3.2°. The PES of Cr(btzpEt)2 is significantly less flat than that of Mo(btzpEt)2 with respect to the Npy−M−Npy angle. The more contracted 3d orbitals of Cr, in comparison to the 4d orbitals of Mo, make orbital overlap with the ligand inferior for Cr, resulting in Cr−L bonding energies that are more sensitive to bending. Although our calculations for Cr(btzpEt)2 do not find a bent structure as most stable, calculations cannot match the exact solid-state environment, and the orbitals and the energy of bent and D2d structures are very similar;11 thus, this only reinforces our

conclusion from molybdenum, that angular deviations from 180° are not of major significance for chemical bonding. Comparison to Isoelectronic Fe(btzpEt)22+. In contrast to the Cr(btzpEt)2 and Mo(btzpEt)2 species calculated, none of the minimum energy Fe(btzpEt)22+ structures exhibited significant nonlinearity of the Npy−M−Npy angle and the ground-state wave function in all cases was a closed-shell singlet. Attempts to converge to an open-shell singlet solution invariably collapsed to a closed-shell solution. The lowest energy triplet was found to be 32.95 kcal/mol higher in energy than the singlet, making it unlikely that higher spin states would stabilize the ground-state wave function, and thus the existence of a valid broken-symmetry wave function is doubtful. The favored closed-shell state was characterized by negligible charge delocalization to the btzpEt ligands from the cationic (less easily oxidized) iron center. Without shared π space density between the ligand and metal, the preference for an open-shell singlet does not exist. General Trends of ESCF and ENonlocal. Results from relaxed surface scans along Npy−M−Npy angles from 180 to 120° revealed that the energy of dispersion interactions decreases in a first-order fashion as the Npy−M−Npy angle distorts from linearity. By our calculations the dispersion forces dominate for Mo(btzpEt)2 up to a Npy−M−Npy angle of ∼162° and never dominate for Cr(btzpEt)2 or [Fe(btzpEt)2]2+, which explains the observed angle trend of these species. Computational substitution of btzpEt with btzpH reduces the dispersion forces to the extent that orbital overlap remains the dominant energy term at all Npy−M−Npy angles for complexes of all metals tested and thus distortion from linearity is absent in the calculated results for these species. Results from the Jakubikova group12 showing bent character in the excited states of Fe(terpy)22+, which has D2d symmetry in its ground state, are fully in line with the conclusion here that bending is associated with a soft potential energy surface along the axial L−M−L angle in the presence of reduced, electron-rich pincers. This condition allows dispersion forces to dominate within a range of axial angle values determined by the relative magnitude of the dispersion and conventional bonding interactions. Dispersion-based distortions are thus a potential future RAL diagnostic. Intraligand Distances. Detecting intraligand reduction can be done by noting that the diazabutadiene LUMO contained in a pyridyl tetrazine is antibonding, bonding, and antibonding among its three contiguous NCCN bonds. The data in Table 1 show this trend, supporting reduction of btzp. Overall, the changes are graphically illustrated in Scheme 2. C


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Inorganic Chemistry Table 1. Comparison of Bond Lengths (Å) of btzpMe, M(btzpEt)2, and H4btzpMe (Å)a

label

btzpMe

Cr(btzpEt)2

Mo(btzpEt)2

H4btzpMe

a b c d e f g h

1.336(1) 1.483(2) 1.345(2) 1.320(2) 1.345(2) 1.333(2) 1.329(2) 1.337(2)

1.361(5) 1.457(6) 1.375(5) 1.363(5) 1.334(5) 1.351(5) 1.345(5) 1.320(5)

1.358(6) 1.447(7) 1.386(6) 1.381(5) 1.317(6) 1.349(7) 1.354(6) 1.311(6)

1.336(3) 1.480(4) 1.389(3) 1.441(3) 1.272(4) 1.405(4) 1.431(3) 1.279(3)

Scheme 3. Tetrazine Bond Order Changes Following Reduction and Deprotonation

The Cr(btzpEt)2 data provide eight independent determinations of the bond lengths within the pincer ligand, and all of these show bond lengths which put the chromium compound at an intermediate btzp LUMO occupancy between the free ligand and the molybdenum analogue: tetrazine reduction is greater for Mo than for Cr. The full set of bond lengths is available in the Supporting Information, but for simplicity the data from only one of these molecules are given in Table 1. Together the pattern of M(btzpEt)2 intraligand bond length changes, vs free btzp, maps out the bonding and antibonding character of the LUMO of the free ligand and is thus consistent with experimentally detectable back-bonding in btzp on coordination to Cr or to Mo. The LUMO comparison (Figure 4) of free btzp and the minimally back-donating Fe2+ in Fe(btzp)22+ provide a useful comparison of influences on btzp of a cationic metal with little reducing power. These drawings also show the increased Cpy−CTz bonding character in the Fe LUMO, vs free btzp, and also the increased pyridine CN character, vs free btzp and hence the lengthening, in the Cr and Mo cases, of the pyridine C−N bonds. In summary, the diagrams in Figure 4 and Scheme 2 are mutually consistent. M−N Distances. Table 2 reveals that btzp complexes with metals in higher oxidation states (hence minimal electron

a

Both btzpEt ligands on each M(btzpEt)2 show similar bond distortions, therefore, only the bond lengths of one btzpEt ligand are presented.

Scheme 2. Summary of Change of Bond Lengths in Mo(btzpEt)2 vs Metal-Free btzpa

a

The complexed pincer is s (shorter) or L (longer). The complexed tetrazine thus has two CN double bonds, consistent with the drawing at far right in Scheme 3. See also the Supporting Information.

Table 2. Comparison of M−Npy and M−Ntz Bond Distances (Å) for btzp Complexesa

In Table 1, comparisons of tetrazine rings are made between M(btzpEt)2 vs btzpMe and H4btzpMe (cf. Scheme 1). The pattern of single- and double-bond intra-tetrazine distances in the fully reduced (hydrogenated) bis-dihydrotetrazinyl pincer13 serves as a comparison. Scheme 2 shows anticipated bond length changes, upon reduction. The corresponding bond lengths in the two distinct tetrazine arms of M(btzpEt)2 are identical within 3σ.11 Bonds d and g lengthen for M(btzpEt)2 vs free ligand, while bonds e and h shorten vs free ligand. This pattern of change conforms to the bonding and antibonding character in the LUMO of btzpEt (Figure 4, left). The longest N−C bonds within the tetrazine rings involve the nitrogen bonded to Mo. Two C−N bonds are the shortest (imine), consistent with the Lewis structure in Scheme 3, which also localizes negative charge on the nitrogens bound to Mo; these are amide in character.

complex ZnCl2(btzpMe) CuCl2(btzpMe) CrCl3(btzpEt) [Fe(btzpMe)2] (BF4)2 Cr(btzpEt)2 Mo(btzpEt)2 a

M−Npy

M−Ntz

Δ(MNtz−MNpy)

2.114(1) 1.981(3) 2.038(4) 1.900(1)[1.91]

2.225(2) 2.106(5) 2.104(4) 1.947(7)[1.95]

+0.111 +0.125 +0.066 +0.047

2.000(3)[2.00] 2.085(4)[2.10]

1.975(3)[2.00] 2.058(4)[2.07]

−0.025 −0.027

Calculated DFT values are given in brackets.

transfer to tetrazine) show shorter M−Npy in comparison to M−Ntz bond distances (positive Δ values in far right column), while the M−Ntz and M−Npy bond lengths are reversed for both M(btzp)2: M−Npy is longer than M−Ntz because the tetrazines have been reduced, giving those nitrogens amidic character. The Δ values in this table also show varying degrees of back-donation to the btzp pincer as the π basicity of the metal varies. In comparison to Mo, the Cr−nitrogen distances confirm an intermediate reductive situation, as judged by Δ values. Altogether, the collective evidence is that the degree of population of the pincer π* orbitals in the chromium complex is intermediate between that in an unreduced free ligand and that in the ligand coordinated to molybdenum. The Cpy-Ctz bond length between the pyridine and tetrazine moieties is also diagnostic, since the lowest btzp π* orbital contains significant bonding character between these rings, shortening that bond length. On the basis of these calculated Cpy−Ctz distances, it is clear that the species with the most

Figure 4. Orbital contour diagrams of the LUMO of free btzpEt (left) and of Fe(btzpEt)22+ (right), showing similarities and differences. D


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Inorganic Chemistry

Figure 5. 1H NMR spectra in CDCl3 of btzpEt (top), H4btzpEt (bottom), and various btzpEt complexes. S = solvent.

Figure 6. Stacked 13C NMR spectra of selected btzp moieties. S = solvent.

These data show that every 1H NMR signal is shifted upfield by reduction of the tetrazines. The CH2 protons shift the most. Using these spectra as reference, the chemical shifts of the ethyl peaks of ZnCl2(btzpEt) show a downfield shift, consistent with btzp coordinated to a Lewis acidic metal. In contrast, the M(btzp)2 data show a dramatic upfield shift in the CH2 peaks, consistent with more electron density at the tetrazine ligands, while the CH3 also shows an upfield shift. The btzpEt benzylic proton chemical shift thus correlates with ligand reduction.

reduced ligands in ascending order are Fe(btzp)22+ (1.46 Å) < Cr(btzp)2 (1.45 Å) < Mo(btzp)2 (1.44 Å). This is consistent with the expectations for metal reducing ability on the basis of charge and periodic trends. Spectroscopic Evidence for Ligand Reduction. Figure 5 compares the 1H NMR spectra of relevant complexes together with those of btzpEt and H4btzpEt,13 the latter showing the effect on 1H NMR of tetrazine reduction (hydrogenation). If reduction is occurring at the tetrazine ring in M(btzp)2, the CH2 1H NMR chemical shift should be most sensitive. E


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Inorganic Chemistry The 13C NMR chemical shifts (Figure 6) corresponding to the ring carbons of M(btzpEt)2, where M = Cr, Mo, move upfield relative to free btzpEt and [Fe(btzpEt)2]2+, indicating delocalization of electron density into the ligand framework. Since this is also true of the hydrogenated tetrazine, this further supports reduction occurring on btzpEt coordination to initially zerovalent metals. Orbital Analysis. To look deeper for evidence of weak coupling in the unrestricted DFT wave functions for M(btzp)2, the occupied α and β orbitals were transformed to “corresponding orbitals”.14 Figure 7 shows that, for M = Mo,

weak electron coupling. In summary, these results show backdonation to the pincer π system in both metals but evidence for ionic character for Cr; covalent sharing of electrons present for Mo is diminished for Cr. Interrogation of Charge Distribution in M(btzpEt)2 Using XPS. X-ray photoelectron spectroscopy (XPS) was employed to further probe for evidence of electron transfer within M(btzpEt)2. XPS measures the binding energy (BE) of an ejected core electron, which should correlate with the oxidation state of the metal: high binding energies correlate to higher oxidation states, while lower BEs are consistent with electron-rich atom surroundings. This is confirmed by the literature data at the right in Figures 8 and 9, which also show the XPS doublets of Cr 2p and Mo 3d for M(btzpEt)2. For Cr(btzpEt)2, the Cr 2p3/2 peak at 576.1 eV falls within the range of Cr(II)/Cr(III) and the Mo 3d5/2 peak at 229 eV falls within the range of Mo(III)/Mo(IV).17 For additional comparison, (btzpEt)CrCl3 was synthesized and characterized and its structure determined by X-ray diffraction.11 The XPS data for (btzpEt)CrCl3 showed a Cr 2p3/2 BE of 577.8 eV, fully consistent with the data for Cr(III) in Figure 8 and only 1.7 eV higher than that observed in Cr(btzpEt)2. For comparison, the Cr 2p3/2 BE of anhydrous CrCl3 is 577.7 eV. The CASSCF charges (see below) on M(btzpEt)2 for Cr and Mo are +2.84 and +3.04, respectively, a trend in satisfactory agreement with the XPS binding energies. The orbital picture shows dπ and π* overlap, appearing as extreme back-bonding, leaving the metals depleted of electron richness. The nitrogen 1s binding energy of M(btzp)2 provides complementary information on the location of charge removed from the metal. The N 1s BE of both M(btzp)2 complexes is a single signal at 399.5 eV, which is within 0.5 eV of the value measured for reduced tetrazine on surface-bound PtII and VII species.18 For comparison, the N 1s BE measured for (btzp)CrCl3 is 401.6 eV, significantly higher than that in Cr(btzp)2 and thus further establishing higher electron richness in Cr(btzpEt)2. Given the evidence of spin contamination in the unrestricted DFT calculations of Cr(btzpEt)2, absent for Mo(btzpEt)2, CASSCF calculations of the singlet state of both the Mo and

Figure 7. Unrestricted corresponding orbitals and their overlaps for the valence orbitals of bent Mo(btzpEt)2 (left) and D2d Cr(btzpEt)2 (right) at the def2-TZVPP M06L level of theory. The top line shows α orbitals, and the bottom line shows β orbitals.

three α orbitals most like t2g d orbitals find high overlap with β partners. For M = Cr, the resulting corresponding orbitals have distinctly smaller overlap integrals (0.55−0.77 vs 0.99 for Mo) (Figure 7). Moreover, the α orbitals for Cr have much more ligand character than do those for Mo, and the Cr β orbitals have much more metal character than do their α partners. The π and π* corresponding orbitals of Cr(btzpEt)2 are different, showing polarization indicative of a weak interaction between the metal and ligand through the π space. Further diagnostic of the difference in these unrestricted wave functions, the expectation value for spin operator S2 is 0.00 for Mo but 1.85 for Cr. This indicates that the wave function of Cr is stabilized, within the limits of a single determinant representation, by including components that are derived from higher (S2 > 0) spin states. This is a clear indication of

Figure 8. XPS spectrum of Cr(btzpEt)2 and binding energies of literature 2p3/2 values.15 F


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Inorganic Chemistry

Figure 9. XPS spectrum of Mo(btzpEt)2 and binding energies of literature 3d5/2 values.16

Figure 10. Pie charts showing the total configuration state function (CSF) contributions to the CASSCF wave functions of the species studied, demonstrating significantly less closed-shell character for Cr(btzpEt)2 in comparison to Mo(btzpEt)2.

different tetrazine rings. In contrast, for linear Cr(btzpEt)2 results shown in Figure 11, three Cr dπ orbitals are even more metal localized than the Mo analogue, while the metal contribution in the other orbitals portrayed is even less than in their Mo analogue; this also indicates, in the chromium case, decreased coupling between metal and ligand. The Cr charge, +2.84, is less positive (consistent with more back-donation to pincer in the Mo case), and this involves a small (0.1) electron population change in each pincer ring. The illustrated orbitals yields a picture of the M(btzp)2 complexes as having a negatively charged shell and a positively charged center, very different from that envisioned as neutral ligands around elemental metal. TD-DFT and Electronic Spectra of M(btzpEt)2 Complexes. Since breaking the degeneracy between tetrazine rings by bending angle Npy−M−Npy creates a clear donor/acceptor pair, we investigated the possibility of charge transfer transitions between ligand orbitals, including ligand/ligand charge transfer, LLCT. The electronic spectrum (Figures 12 and 13) of Mo(btzpEt)2 has broader peaks than that of Cr(btzpEt)2, suggesting that more ground-state conformers are present in solution. The assignments of the bands in the UV−vis−NIR spectra were made on the basis of molar absorptivities, control spectra of reduced btzp complexes,11 and DFT calculations. To achieve

Cr species were carried out using a [6,7] active space consisting of the bonding and antibonding combinations of the three dπ orbitals, three ligand π* orbitals, and a ligand π* orbital which was nonbonding with respect to metal dπ orbitals (Figure 10). The closed-shell configuration of the CASSCF wave function accounted for 69% of the total wave function for the Mo species in comparison to only 36% for the Cr species. Configurations containing single excitations contributed (22 + 8)% to the chromium ground-state singlet, which indicated that the wave function as a whole contained significant ionic character. The greater incorporation of excited configurations in the CASSCF solution for Cr vs Mo reflects the same thing about the actual Cr(btzpEt)2 species that the S2 = 1.85 value does in the DFT result. Pierloot’s method of localizing a CASSCF active space to determine the composition of the valence orbitals and formal oxidation state of a complex19 was used to picture the orbitals of bent Mo(btzp)2 found by CASSCF and to determine atom charges from orbital occupancies. Figure 11 shows that there are three singly occupied primarily Mo orbitals and that there are four primarily ligand orbitals. From the orbital occupancy numbers shown in Figure 11, the metal has charge +3.04 while each pincer has charge −1.51 and −1.52. Note also that, since the two tetrazine arms in a given pincer are not symmetry equivalent in this bent structure, the orbitals localize charge in G


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Inorganic Chemistry

Figure 11. Pierloot analysis of metal and ligand charges in D2d Cr(btzpEt)2 (lower) and bent Mo(btzpEt)2 (upper).

Figure 12. Overlay of electronic spectra of M(btzpEt)2 complexes in dichloromethane: (upper) Cr, in blue; (lower) Mo, in purple.

Full details of the assignments are given in the Supporting Information, but an example, based on difference density representation of orbitals emptied and orbitals populated, is shown in Figure 14. The strongest transitions are indeed MLCT, consistent with the abundance of π* orbitals in these multiring species. MLCT, above ∼23000 cm−1, is of lower

an intuitive understanding of the electronic excited states, it is useful to calculate the difference densities (ρ[excited state] − ρ[ground state]) between the ground and excited states (see below). In this way it is possible to understand the spatial transfer of electron density occurring with each transition and assign descriptive labels such as LLCT, MLCT, and LMCT. H


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Inorganic Chemistry

Figure 13. Simulated (TD-DFT) and observed NIR absorption spectra of Cr(btzpEt)2..

Figure 15. Overlay of CV of 0.0044 M Cr(btzpEt)2 (black trace, 12 sweeps) and 0.0029 M Mo(btzpEt)2 (red trace, 7 sweeps) in THF solution containing 0.4 M [NnBu4][PF6] at 1 V/s. The open circuit potential for both is ∼−0.6 V. The Mo CV shows an impurity of btzpEt.

Table 3. E° (V vs Ferrocene) for btzpEt and M(btzpEt)2 Complexes in THF, Where M = Cr, Mo compound btzpEt Cr(btzpEt)2 Mo(btzpEt)2

Figure 14. Difference densities of the lowest energy transitions of Mo(btzpEt)2 with oscillator strengths, f. Red indicates a loss of electron density upon excitation, and blue indicates an increase.

+2/+1 +0.022

+1/0

0/−1

−1/−2

−2/−3

−0.182 −0.031

−1.46 −0.831 −0.868

−1.90 −1.76

−2.32 −2.23

The CV is reproducible over at least 12 cycles of the full +0.4 to −2.7 V range. The reduction of Cr(btzpEt)2 is exceptionally facile, with E°(0/−1) = −0.831 V, and oxidation is also easy, at E°(+1/0) = −0.182 V. Both the low potentials (btzpEt itself is harder to reduce than is neutral Cr(btzpEt)2, E1/2 = −1.46 V in THF) and the reversibility support the variable line broadening seen by 1H NMR, attributed to trace conversion of Cr(btzpEt)2 to its anion or cation due to an adventitious redox impurity. The persistence of the Cr(btzpEt)2 composition over six charge states is attributed to (a) heavy delocalization of charge and spin density at each of the redox states, (b) the chelate effect which makes ligand loss difficult, and (c) steric saturation at this maximum chromium coordination number of 6. In general bond making and breaking are inhibited, favoring reversible redox change. Molybdenum. The CV of Mo(btzpEt)2 in Figure 15 shows three reversible reduction peaks at −0.868, −1.76, and −2.23 V, each with a ΔE = Epc − Epa value of 0.13 V. The three reversible potentials roughly correlate to the same reduction peaks of Cr(btzpEt)2 and can be assigned as the analogous redox couples and each a 1 e process. There is a small shoulder on the E° = −1.76 V peak that is attributed to an impurity. Finally, there is an irreversible oxidation peak at −0.031 eV, which cannot be from ligand loss, since the signal at −1.34 V (corresponding to free btzpEt) does not grow following passage through the −0.03 V voltage. Comparison of the CV potentials of Cr vs Mo in Table 3 shows no simple periodic trend. Chromium is harder to reduce than molybdenum for the −1/−2 and −2/−3 redox processes, but chromium is easier to oxidize than Mo for the +1/0 and +1/+2 processes. On the basis of the DFT results, an explanation is that the bonding between Mo and btzp ligands is more covalent and the metal is acting as a bridge between two btzp ligands, allowing delocalization of electrons through the π system. In contrast, Cr(btzpEt)2 has charge more localized on the four tetrazines, making it harder to add an additional electron due to electron−electron repulsion. In

energy for Mo than for Cr, consistent with generally easier oxidation from Mo orbitals. For both metals, LMCTs are lower in energy than MLCTs, and n → π* transitions are identified,11 but of lower intensity. Charge transfer bands at ∼14500 cm−1 correspond to LLCT (π → π*) and were assigned by DFT calculations. Some of these effect electron transfer between pincers, and some involve tetrazine to pyridine transitions. The lowest energy transition at 6000 cm−1 is an LLCT (π*•‑ → π*): an electron excited from a reduced btzp ligand to the LUMO of the second btzp ligand. TD-DFT calculations on Cr(btzpEt)2 (Figure 13) show three transitions, which were then sought and located experimentally. The effect, on LLCT, of linear vs bent structure is shown for the TD-DFT calculations on Mo(btzpEt)2 in Figure 14, showing higher energy and increased intensity (oscillator strength) in the less symmetric structure. Control spectra of the one-electron-reduced free btzp do not show a NIR band, while Fe(btzpEt)22+ shows growth of an absorption at 9000 cm−1 only upon addition of one and two electrons.11 An absorption in the NIR is thus an indicator that reduction of the ligand has occurred for bis-btzp complexes. Electrochemistry of M(btzpEt)2 Complexes. Does cyclic voltammetry reveal redox activity of the M(btzp)2 species and is it persistent on that time scale? The CV and E° values of M(btzpEt)2 and free ligand are provided in Figure 15 and Table 3. The first reduction potentials for M(btzpEt)2 are all more positive than that of the free ligand, as was observed for isoelectronic Fe(btzpMe)22+. Chromium. The CV of Cr(btzpEt)2 (Figure 15) in THF at a platinum electrode is exceptionally rich and fully reversible at each of five waves. The values of Epc − Epa (in V) for the five waves are 0.15, 0.15, 0.14, 0.15, and 0.15, where ferrocene in this medium has a value of 0.30. Reversibility allows assigning a redox product formula with unchanged composition, and only altered charge, and each wave has a very similar peak current, establishing that every process is a single-electron transfer, further solidifying the species formulas assigned in Figure 15. I


Article

Inorganic Chemistry

the 3d metals is fully consistent with the strongest reducing power being for the early “electropositive” metal cases. Most relevant to Cr(btzp)2 and Mo(btzp)2 are the changing metal oxidation states down group 6: CrIII(terpy2−)(terpy•−), MoIV(terpy2−)2, WV(terpy2−)(terpy•3−). This is in full agreement with the general trend that it becomes harder to reduce a metal as one goes down a group in the transition series, but it is still remarkable to see pentavalent tungsten coexisting with the terpy trianion. A second important principle, in a comparison to M(btzp)2 here, is the location of the charge within the pincer ligand in M(terpy)2: is the charge symmetrically placed, hence equally in both outer arms of the pincer, or is it located in only one pincer arm (and to some extent in the central pyridine)? While the evidence presented here shows examples of each situation, it appears likely that the energetic difference between these two alternatives is exceptionally small and subject to being dictated by local environment: either solvent or, in the solid state, neighbor effects. This is exactly the question of distinguishing between Robin−Day class II and class III situations, where it is already well-established that environmental effects can convert the situation between class II and class III. Computationally (DFT) it was found3b that it was necessary to include (polar) solvent effects to achieve localization of charge in one ring of a terpy, to agree with solid-state structural evidence of such localization (i.e., the second pyridyl arm had bond lengths consistent with no reduction in one terpy•−). A particularly persuasive example of this is for Mn(terpy)2, where there are two molecules in the crystallographic asymmetric unit and one molecule has two unsymmetrical terpy ligands. In all of this previous work, what has not been discussed is deviations of the trans angle N−M−N involving the central pyridyl rings in two different terpy ligands. In fact, for M(terpy)2 all such angles are >171°, except for Mn(terpy)2, where the angles are 162.7 and 168.7°. In summary, our work shows that M(btzp)2 species are highly “in/out” polarized, with the metal (in) having surrendered considerable electron density to the collective assembly of two pincer ligands (out). In the sense of in/out polarization, these are thus crudely analogous to the polarization in excited states of Ru(bipy)32+, but our synthesis, from reduced metal sources and highly electron poor btzp, gives this polarized electronic structure as the ground state. Consistent with the continuous change of oxidizability moving from Mo to Cr, establishing integral oxidation states for M(btzp)2 species is elusive and perhaps should not be expected: redox change is a continuous variable. What is experimentally detectable is spin state, and our experimental work shows this to be uniformly singlet in M(btzp)2, but a single electron conf iguration is no longer sufficient to describe the more complicated electronic structure when M = Cr. The fluidity of charge within these assemblies is evident in the highly congested cyclic voltammograms and in the unusual observed 1H NMR line broadening. The benzylic proton NMR chemical shift is suggested as a useful diagnostic of reduction of tetrazine rings, and this is attributed to incremental and continuous loss of tetrazine aromaticity as M is varied in M(btzp)2. Ring carbon NMR chemical shifts are likewise affected. Observed NIR absorptions are assigned, using the TD-DFT method, to charge transfer among the various rings in the btzp ligands when these are reduced by their attached metals. If they were not reduced, there would be no LL charge transfer absorptions.

other words, redox occurs at the tetrazines, and so metal-based periodic trends (4d harder to reduce and easier to oxidize) do not apply. The CV study of M(btzp)2 shows that a large range of ions is persistent on that time scale, and we used natural population analysis (NPA) of DFT(M06L) geometry optimized calculated species M(btzp)2q to see where the electron goes with each redox step, describing here trends upon adding electrons. NPA11 also reveals differences between Cr vs Mo. Over a range of six different overall charges, the metal charge stays essentially unchanged. The Cr vs Mo charges differ by 0.14 e, and the more positive is again found to be for Mo, consistent with trends in the Pierloot analysis. In every pair, the full charge added in any redox half-reaction resides at the two pincers. A comparison of the slope of the charge for the two pyridines vs that on the four tetrazines shows that 3 times more of the charge is on tetrazines than on pyridines. If these data are instead plotted for pyridine vs for each tetrazine, the slope for tetrazine is 50% larger than for pyridine, again indicating that tetrazine predominates as an electron storage site. However, pyridine is not inert to reduction but is only quantitatively less affected, consistent with its fewer number of nitrogens per ring. Pyridine nitrogen is conjugated with (its ortho) tetrazine, consistent also with experimental trends in C−Npy distances. The largest bond length changes are that the N−N distances lengthen by 0.07 Å over the species change range +3 to −2, fully consistent with the two para nitrogens becoming amidic.11 The wave functions show that molybdenum (vs Cr) accomplishes better conjugation between the two pincers, which contributes to a more facile reduction of the ligands, due to the increased size of the conjugated ligand π system, in contrast to normal periodic trends down a periodic group.

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CONCLUSIONS Attempts to synthesize M(CO)3(btzpEt), where M = Cr, Mo, resulted in the isolation of M(btzpEt)2. Intraligand bond lengths suggest reduction of ligands, and XPS data indicate thatCr and Mo are +II and +III, respectively. Further support for reduction of the btzp ligand by the zerovalent metal is provided by absorption bands in the NIR, with assignments established computationally. These results indicate that btzpEt is so π acidic that it can be termed an oxidizing agent and polarizes M(btzpEt)2 in such a fashion that the ligands are negatively charged and the metal is positively charged. Distortion of the Npy−M−Npy angle observed in the solid state is due to dispersion and crystal-packing effects. This oxidizing property of btzpEt results in CO loss in the synthesis and explains why carbonyl products are not isolated. The cyclic voltammogram shows numerous reversible redox processes, demonstrating the electron storage capacity of these complexes. It is of interest to compare the distances of btzp complexes to those of the classic terpyridyl ligands. The electronic structure of neutral M(terpy) 2 presents a picture of considerable complexity.3,20 The overriding conclusion from the numerous studies is that negative terpy charge states are significantly favored over electrons residing at the metal. This principle is illustrated by the charge states TiIV(terpy2−)2, V IV (terpy 2− ) 2 , Cr III (terpy 2− )(terpy •− ) , Mn II (terpy •− ) 2 , FeII(terpy•−)2, and RuII(terpy•−)2, which show the coexistence of sometimes quite positive formal metal oxidation states in the presence of even dianionic terpy. In general, the trend across J


Article

Inorganic Chemistry Author Contributions

The collective view of these M(btzp)2 species is that they can be useful new outer-sphere electron transfer reagents, protected from metal−ligand bond formation with the reactive partner by their six-coordinate character. Their only potential Achilles heel in this application can be reactivity at the tetrazine rings, especially at tetrazine nitrogen lone pairs or by loss of N2, including by Diels−Alder substitution reactions, although we have not seen these reactions in the chemistry reported here.21 The variety of structures established here in the solid state needs to be taken into account simultaneously with the bending of the trans Npy−M−Npy angle. Although none of the angles are near linearity, the scatter of the angles is to be understood as indicating a soft potential energy surface for such bending: solid-state packing forces readily dictate the variety of angles. It is the low resistance to such bending that leaves the energy of the linear alternative very close to that of the observed (bent) structures. This same subtlety characterizes the (de)localization question for the Creutz−Taube ion.22 The bending does affect the tetrazine rings, in that any bending creates at least two types of tetrazines, and this is exceptionally important in a redox-active ligand situation: the two rings in one pincer oxidize the metal to different extents, and the rings thus take on different charges. The bending of the Npy−M−Npy angle differentiates two tetrazines in a given pincer but does not differentiate one pincer from another in a given C2-symmetric M(btzp)2. This alone encourages LL charge transfer excited states. It also creates a molecular dipole moment (absent in a D2d, rigorously “octahedral” structure); such a moment creates a stabilizing force from interaction with polar solvent. In addition, solvation of an even slightly polar molecule can help to explain why an otherwise “organic” molecular surface, decorated with ethyl groups, has poor solubility in nonpolar solvents and demands halocarbon solvent for most of our solution measurements: dipole moment and especially surface charge at peripherally directed anionic nitrogens of the “ionic” ground state of these ML2 species are influential. Overall, this is apparently a case where small features have big, or at least readily detectable, effects.

■

†

N.A.M. and R.N.S. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Indiana University Office of Vice President for research and the National Science Foundation, Chemical Synthesis Program (SYN), by grant CHE-1362127.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01761. 1

H, 13C, UV−vis−NIR, ESI/MS, and XPS spectra, cyclic voltammetry experiments, and computational information (PDF)

Accession Codes

CCDC 1845405−1845407 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail for K.G.C.: [email protected]. ORCID

Kenneth G. Caulton: 0000-0003-3599-1038 K


Article

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Seeking Redox Activity in a Tetrazinyl Pincer Ligand: Installing

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