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Electronic Structure of Anilinopyridinate-Supported Ru25+ Paddlewheel Compounds Amanda R. Corcos,† Michael D. Roy, Michelle M. Killian, Stephanie Dillon, Thomas C. Brunold, and John F. Berry* Department of Chemistry, University of WisconsinMadison, 1101 University Avenue, Madison, Wisconsin 53706, United States

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S Supporting Information *

ABSTRACT: The electronic structures of the diruthenium compounds Ru2(ap)4Cl (1, ap = 2-anilinopyridinate) and Ru2(ap)4OTf (2) were investigated with UV−vis, resonance Raman, and magnetic circular dichroism (MCD) spectroscopies; SQUID magnetometry; and density functional theory (DFT) calculations. Both compounds have quartet spin ground states with large axial zero-field splitting of ∼60 cm−1 that is characteristic of Ru25+ compounds having a (π*, δ*)3 electron configuration and a Ru−Ru bond order of ∼2.5. Two major visible absorption features are observed at ∼770 and 430 nm in the electronic spectra, the assignments of which have previously been ambiguous. Both bands have significant charge-transfer character with some contributions from d → d transitions. MCD spectra were measured to enable the identification of d → d transitions that are not easily observable by UV−vis spectroscopy. In this way, we are able to identify bands due to δ → δ* and δ → π* transitions at ∼16 100 and 11 200−12 300 cm−1, respectively, the latter band being sensitive to the π-donating character of the axial ligand. The Ru−Ru stretches are coupled with pyridine rocking motions and give rise to observed resonance Raman peaks at ∼350 and 420 cm−1, respectively.

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in Table 1.36−38,41−45 While some of these assignments are in agreement, others, like those for Ru2(ap)4(CCPh), are contradictory.37,40,44 Given our interest in ap-supported Ru25+ species with nonconjugated, displaceable, anionic axial ligands, particularly π-donor ligands, we were not confident that such electronic assignments would be congruent with those of carbon-based, covalently bound axial ligands, and it was not obvious which of these assignments would be relevant given their contradictory nature. We present here experimental and theoretical electronic structure studies of two Ru25+ compounds that contain monatomic or displaceable, nonconjugated, anionic axial ligands, specifically, Ru2(ap)4Cl (1) and Ru2(ap)4OTf (2). Unlike previous studies, which assigned these transitions on the basis of UV−vis data that are dominated by a small subset of calculated transitions, we present here magnetic circular dichroism (MCD) data that provide additional details about the electronic transitions, especially d → d excitations. Notably, MCD spectra have not been previously reported for any metal−metal bonded Ru2 species. Additionally, we report corresponding Ru−Ru vibrational data collected via resonance Raman measurements, which previously have only been documented for tetracarboxylate-supported Ru2 species.1

INTRODUCTION Metal−metal bonded diruthenium compounds are welldocumented1 and are of current interest due to their magnetic,2−12 structural,13−18 chemical,19−21 electrochemical,22−27 and catalytic properties.28 The most stable and common types of these compounds are Ru25+ species since most are formed from the Ru25+ synthon Ru2(OAc)4Cl.29 The electronic structures of carboxylate-supported Ru25+ species have been studied in depth.30−33 We34,35 and others36−38 have been interested in anilinopyridinate (ap)-supported Ru2 species because the (4,0) geometry of the bound ap ligands, which leaves all of the phenyl rings on one end of the molecule, allows for selective axial ligand modification at a single axial site. Electronic spectra of Ru25+ complexes supported by ap ligands generally feature two broad absorption bands, at ∼430 and 770 nm. Cotton’s original report of the first Ru2 anilinopyridinate species, Ru2(ap)4Cl, does not include assignments for the electronic transitions.39 The following year, Chakravarty and Cotton40 suggested that the low-energy band is due to a [π(N) → π*(Ru2)] transition and that the highenergy band could be due to a [π(axial ligand) → π*/δ*(Ru2)] transition, but acknowledged that “a more definite assignment would require a thorough MO calculation on the system”. Miskowski et al. suggest comparable, albeit still ambiguous, assignments (Table 1).33 Ren and co-workers have extensively studied ap-supported Ru2 complexes with carbon-based, covalently bound axial ligands, some of which are identified © 2017 American Chemical Society

Received: October 5, 2017 Published: November 15, 2017 14662

DOI: 10.1021/acs.inorgchem.7b02557 Inorg. Chem. 2017, 56, 14662−14670

Article

Inorganic Chemistry Table 1. Literature Assignments for Electronic Transitions in Selected ap-Supported Ru25+ Compounds compound Ru2(ap)4Cl

Ru2(ap)4(CCR) R = Ph R = SiMe3, H, CH2OCH3 R = SiiPr3 R = SiMe3 Ru2(ap)4((CC)k-Ph), k = 1−5 Ru2(iBuOap)4(CCR) R = Ph, SiiPr3 Ru2(Xap)4(CC−C6H4R) Ru2(Xap)4(Y-gem-DEE)

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DFT?

ref

not assigned π(N) → π*(Ru2) π(N) → π*(Ru2) or δ*(Ru2)

low-energy band (∼770 nm)

not assigned π(Cl) → π*/δ*(Ru2) not assigned

no no no

39 40 33

π(N) → π*(Ru2) δ(Ru2) → δ*(Ru2) π(axial) → π*/δ*(Ru2) π(Ru−N) → π*/δ*(Ru2) δ(Ru2)/ π(N/Ru2) → δ*(Ru2) δ(Ru2) → δ*(Ru2) π(Ru−N, Ru2) → δ*(Ru2) π(axial) → δ*(Ru2) π(Ru−N)/δ(Ru2) → π*/δ*(Ru2)

π(axial) → π*/δ*(Ru2) π(N) → π*(Ru2)/ π*(axial) π(axial) → π*/δ*(Ru2) π(axial) → π*/δ*(Ru2) π(Ru2/axial) → π*/δ*(Ru2) π(N) → π*(Ru2)/ π*(axial) π(Ru−N, axial) → δ*(Ru2) π(Ru2)/π(Ru−N) → δ*(Ru2) π(N) → π*/δ*(Ru2)/π*(axial)

no no no no no no yes yes yes

40 37 41 36 43 37 44 38 45

RESULTS AND DISCUSSION Compounds 1 and 2 are shown in Scheme 1 and were prepared according to literature procedures.35,39 The absorption spectra

high-energy band (∼430 nm)

conventional absorption methods and is significantly more sensitive to the electronic structure of the complex under study because it measures positive and negative features, which leads to spectra with increased fine structures.46 While the two MCD spectra shown here have similar overall shapes, the features for 1 are shifted to slightly higher energy compared to those in 2, and their relative intensities are distinctly different. These MCD spectral differences reveal an unexpectedly large modulation of the electronic structures of 1 and 2 by their anionic axial ligands (note that increased spin−orbit coupling among the electronic excited states in 1 due to the presence of the Cl atom may also contribute to these differences). The ground spin states of 1 and 2 have been the subject of limited previous investigations. The μeff of 1 was measured as 3.94 μB at 25 °C,39 and an EPR spectrum of 2 showed the main signals at effective g values of ∼4 and ∼2,35 consistent with an S = 3/2 ground state with large axial zero-field splitting (D ≫ hν). Magnetic susceptibility data were collected from 2 to 300 K on 1 and 2 in applied fields of 1000 G to determine the magnitude of the zero-field splitting. As shown in Figure 2, both 1 and 2 exhibit linear χT above ∼150 K, with slightly positive slopes indicative of temperature-independent paramagnetism. The high-temperature limits of 2.1−2.3 emu K mol−1 are consistent with S = 3/2 species.48 Below 150 K, both data sets curve downward due to zero-field splitting. Additionally, VTVH

Scheme 1. Compounds Ru2(ap)4Cl (1) and Ru2(ap)4OTf (2)

of these species show two broad, intense features in the visible region centered at ∼13 000 and 23 000 cm−1 (770 and 430 nm) as shown in Figure 1. Cotton noted that the high-energy band

Figure 1. Room temperature solution phase UV−vis (top)47 and 4.5 K solid state MCD (bottom) spectra of 1 and 2.

in the spectrum of 1 may have contributions from more than one transition,39 and this is more evident here where these data are plotted as a function of energy rather than wavelength. The MCD data for 1 and 2 are also shown in Figure 1 and are more distinguishing. This measurement records the differential absorption of left and right circularly polarized light by 1 and 2 as a function of wavelength in the presence of an external magnetic field parallel to the direction of propagation of the light. MCD has different selection rules compared to

Figure 2. Plot of χT vs T for 1 and 2 measured in an external field of 0.1 T (1000 G). 14663


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Inorganic Chemistry (variable temperature, variable field) reduced magnetization measurements were recorded as shown in Figure 3 and Figure

Table 3. Selected Bond Distances and Angles

a

Ru2(ap)4Cl (1)

Ru2(ap)4OTf (2)

type

expt

calcd

expt

calcd

Ru−Ru (Å) Ru2−L (Å) Ru−Ru−L (deg) Ru−Npyridine (Å) Ru−Naniline (Å) ref

2.275(3) 2.437(7) 180.00(0) 2.104[8] 2.026[8] 39

2.277 2.510 180.0 2.146 2.068 t.w.a

2.2571(7) 2.233(7) 180.0(0) 2.081(4) 2.038(3) 35

2.260 2.410 177.8 2.125 2.078 t.w.

t.w. = this work; B3LYP.

were found to be subpar matches for both the Ru−Ru and Ru2−L distances (see Supporting Information). Previously characterized Ru25+ species were shown to possess a quartet ground state deriving from an electronic configuration of (σ)2(π)4(δ)2(π*)2(δ*)1,1,30,33 in agreement with the magnetic measurements described above. As such, calculations for 1 and 2 were performed in their quartet ground states, and the singularly occupied molecular orbitals in both species were found to be the π* and δ* orbitals, as expected. The relevant αand β-spin Ru2 orbitals for 1 and 2 are shown in Figures 4 and 5, respectively.

Figure 3. Reduced magnetization data for 2. H/T has been scaled by β/k.

S1. For each compound, the three data sets show nesting behavior and saturation below the expected value for an S = 3/2 ground state, indicative of large zero-field splitting.48 Simultaneous fitting of the susceptibility and magnetization data sets was performed, yielding the parameters listed in Table 2. The data for 1 are best fitted using both axial (|D| = 56.9 cm−1) and rhombic (E = 15 cm−1) zero-field splitting, while 2 has only well-defined axial (|D| = 61.6 cm−1) zero-field splitting; including a rhombic component did not significantly improve the fit. To date, the D values of relatively few Ru25+ complexes supported by N,N-donor equatorial ligands have been determined,49−51 in contrast to the many examples of Ru25+ carboxylate compounds that have been examined.33 The |D| values determined here fall within the range of those for reported compounds (∼50−80 cm−1).49−51 Though the sign of D cannot be determined by the methods employed here, we presume these are positive on the basis of the EPR results of all Ru25+ complexes supported by equatorial N,N-donors that have been measured to date.35,50,52−55 The (isotropic) g value of 2.03 for 2 determined here compares reasonably well with the frozen solution-state EPR results of g⊥ = 2.01 and g|| = 1.89 (giso = 1.97).35 Density functional theory (DFT) calculations were performed to gain insight into the electronic transitions for 1 and 2. The B3LYP functional accurately calculates the Ru−Ru bond distances as 2.277 Å in 1 and 2.260 Å in 2 (reported as 2.275(3) Å for 1 and 2.2571(7) Å for 2)35,39 (Table 3). In both cases the DFT calculations slightly overestimate the Ru2−L distances. Given the strong ionic bonding character of these axial ligands, it is not surprising that these gas phase geometryoptimized bond distances differ from the experimental values. Indeed, this phenomenon has been previously documented.56 Calculations were also performed with the BP86 functional but

Figure 4. Computed molecular orbital diagram for 1 and boundary surface plots of select β-spin MOs.

Time-dependent DFT (TD-DFT) calculations were used to predict the electronic absorption spectra of 1 and 2, which are shown along with electron density difference maps (EDDMs) for the relevant transitions in Figures 6 and 7, respectively. These data were used to assign the key features in the experimental absorption and MCD spectra (Tables 4 and 5, respectively). Transition II contributes significantly to the intensity of the low-energy absorption band observed for both 1 and 2 and has predominantly ligand-to-metal [π(Ru−N) → δ*(Ru2)] charge-transfer character but also contains contributions from the [π(N) → π*(Ru2)] and [δ(Ru2) → π*(Ru2)]

Table 2. Fitted Parameters from the Magnetic Data for 1 and 2

1 2

g

|D| (cm−1)

E (cm−1)

zJ (cm−1)

TIP (emu/mol)

residual

2.00 (fix) 2.034(1)

56.9(8) 61.6(5)

15(1) N/A

−0.188(4) −0.016(3)

1.59(2) × 10−3 7.9(1) × 10−4

1.8 × 10−3 2.9 × 10−5

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The computed spectra for 1 and 2 also include weak features associated with transitions that are not discernible in the experimental absorption spectrum but produce significant contributions to the MCD spectrum, which suggests they should be assigned as d → d transitions. These include transitions III−VI and VIII in 1 and III−V in 2, the specific assignments of which are provided Tables 4 and 5, respectively. As expected, the axial ligand in 1 is a better π donor than in 2, and this is reflected in the significant (∼1100 cm−1) red-shift of the features associated with [δ(Ru2) → π*(Ru2)] transitions from 1 to 2 and the large axial chloride contributions to the EDDMs for these transitions (i.e., iiib, iv, v, viib, viii, ix). It is instructive to compare the δ → δ* transitions in 1 and 2, observed at ∼16 000 cm−1, to those of other systems with (δ,δ*)1 or (δ,δ*)3 ground state configurations. The δ → δ* energies are particularly informative in these cases because they correspond directly to the δ−δ* orbital splitting, Δ, unlike the case of (δ,δ*)2 complexes where the δ → δ* energies have contributions from (1) the energy difference Δ, (2) exchange contributions, and (3) a configuration interaction contribution.57 Notably, Δ has been examined for a number of (δ,δ*)1 and (δ,δ*)3 complexes containing Mo25+, Tc25+, Re25+, and W25+ bimetallic centers.33 Complexes bearing π-donating equatorial ligands, such as halides, exhibit δ → δ* transitions between 6500 and 7600 cm−1 .1,58−62 Complexes with equatorial O-donor ligands (e.g., carboxylates) have Δ values of 14 500−18 200 cm−1,63,64 and those with N-donor ligands are similar (∼14 900 cm−1).65 In comparison, the δ → δ* transitions for Ru25+ carboxylate complexes have been observed at ∼1100 nm, yielding Δ ≈ 9000 cm−1.31 The analogous transition in 1 and 2, at 16 190 and 16 099 cm−1, respectively, is therefore significantly higher in energy. The transition energy difference between the Ru25+ carboxylate complexes on one hand and 1 and 2 on the other is most likely due to the increased π-donation of the equatorial ap ligands in the latter, which preferentially increases the energy of the δ* orbitals.

Figure 5. Computed molecular orbital diagram for 2 and boundary surface plots of select β-spin MOs.

excitations. Assigning the high-energy absorption bands displayed by 1 and 2 to a specific transition is more difficult. In the case of 1, our TD-DFT results indicate that the transition associated with the high-energy band VII peaking at 21 400 cm−1 has contributions from multiple d → d excitations, specifically the [π/σ/δ(Ru2) → δ*(Ru2)] transitions, while the other high-energy bands IX and X at ∼24 000 cm−1 can be attributed to ligand-to-metal [π(N/aryl) → σ*/π*/δ*(Ru2)] transitions. In the case of 2, the two transitions associated with the high-energy absorption bands VI and VII both have mixed d → d and ligand-to-metal charge-transfer character (Table 5). It should be noted that while the TD-DFT computed absorption spectra for both 1 and 2 exhibit features associated with [π/δ(Ru2) → δ*(Ru2)] transitions, they are relatively lower in energy in 1 and so afford a visible shoulder not present in 2.

Figure 6. Gaussian deconvolutions and band labels for the room temperature UV−vis (top) and 4.5 K MCD (middle) spectra of 1 with the results of the TD-DFT calculation (bottom) for comparison. Also shown are the EDDMs for the transitions of interest (red and yellow represent the gain and loss of electron density, respectively). 14665


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Figure 7. Gaussian deconvolutions and band labels for the room temperature UV−vis (top) and 4.5 K MCD (middle) spectra for 2 with the results of the TD-DFT calculation (bottom) for comparison. Also shown are the EDDMs for the transitions of interest (red and yellow represent the gain and loss of electron density, respectively).

Table 4. Band Assignments for 1 Based on TD-DFT Results obsd band I

a

obsd energy (cm−1)

calcd band

calcd energy (cm−1)

calcd εb

assignment

12 269

i iia iib iiia iiib iv v vi viia viib viic viii ix xa xb xi xii

10 289 12 455 13 497 16 030 16 330 18 451 19 484 20 030 21 438 21 651 21 967 23 149 24 248 24 404 24 853 26 959 27 314

138 574 30 609 13 5 1021 1140 2136 14 356 103 4884 1 9211 215 118 959 2101

δ(Ru2) → π*(Ru2) π(N) → π*(Ru2) π(Ru−N) → δ*(Ru2) δ(Ru2) → δ*(Ru2) π(N)/δ*(Ru2) → σ*(Ru2/axial) σ(Ru2/axial) → π*(Ru2) π*(Ru2) → σ*(Ru2/axial) δ(Ru2) → π*(Ru2) π(Ru2) → π*(N)/δ*(Ru2) σ(Ru2/axial) → π*(N)/δ*(Ru2) δ(Ru2) → δ*(Ru2) δ(Ru2) → σ*(Ru2/axial) π(N/aryl) → σ*(Ru2/axial) π(N/aryl) → π*(Ru2) π(aryl) → δ*(Ru2) π(aryl) → π*(Ru2) π(N)/δ*(Ru2) → π*(aryl)

II III

14 136 16 190

IV V VI VII

17 139 18 998 20 111 21 826

VIII IX X

22 979 24 040 25 506

XI XII

27 180 29 909

excited statea 4

E E 4 E 4 B2 4 A1 4 E 4 E 4 E 4 E 4 A1 4 B2 4 A1 4 E 4 E 4 E 4 E 4 E 4

Excited state symmetries are given in the C4v point group. bε calculated using a fwhm line width of 500 cm−1.

feature occurs at 421 and 420 cm−1 for 1 and 2, respectively. DFT calculations suggest that the corresponding vibrations entail a mixture of Ru−Ru stretching character and pyridine rocking motions, as shown in Scheme 2. The predicted energies of these vibrations, 334 and 347 cm−1 for 1 and 2, respectively, for the lower-energy band and 407 and 402 cm−1 for 1 and 2, respectively, for the higher-energy band, are in good agreement with the experimental values, lending credence to our assignments. Though of mixed character, the Ru−Ru stretching frequencies in 1 and 2 are certainly higher than those seen in carboxylate-supported Ru25+ species, which range from 301 to 339 cm−1.1 Thus, the switch from carboxylate to anilinopyridinate equatorial ligands increases not only the Ru−ligand bond covalency, but also the strength of the Ru−Ru bond.

It is also useful to compare the δ → π* transitions in 1 and 2 with similar systems. For Ru25+ tetracarboxylates, this transition is suggested to occur at 630 nm (∼15 900 cm−1)31,32,66−68 and is thus higher in energy than in 1 and 2, where it occurs around 12 300 and 11 200 cm−1, respectively. These δ → π* transitions have been documented in Ru24+ oxypyridinates in the region 10 000−13 000 cm−1.69 Thus, ignoring differences in πdonation of oxypyridinates versus anilinopyridinates to the δ(Ru2) orbital, it appears that a change in oxidation state of the Ru2 unit only marginally modulates the energy of this transition. Resonance Raman data were recorded using 514.5 nm (∼19 500 cm−1) excitation to further probe the electronic structures of 1 and 2. As shown in Figure 8, two features are present in the resonance Raman spectra of 1 and 2 that have no counterparts in the CH2Cl2 solvent spectrum. The first feature is seen at 345 cm−1 for 1 and 351 cm−1 for 2, and the second 14666


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Inorganic Chemistry Table 5. Band Assignments for 2 Based on TD-DFT Results obsd band I

a

obsd energy (cm−1)

calcd band

calcd energy (cm−1)

calcd εb

assignment

11 198

ia ib iia iib iii iv va vb via vib vic vid viia viib viic viiia viiib

9805 10 349 12 516 13 209 15 630 17 072 18 732 19 390 20 767 21 073 22 029 22 841 23 593 23 859 24 094 26 315 26 544

171 611 18 21 218 22 1632 1476 124 162 1969 3112 4662 570 367 541 24 653

δ(Ru2) → π*(Ru2) π(N) → π*(Ru2) π(N)/δ*(Ru2) → σ*(Ru2/axial) π(Ru−N) → δ*(Ru2) δ(Ru2) → δ*(Ru2) π*(Ru2) → σ*(Ru2/axial) δ(Ru2) → π*(Ru2) π(Ru2) → π*(Ru2) δ(Ru2) → σ*(Ru2/axial) π(N) → σ*(Ru2/axial) σ(Ru2/axial) → π*(Ru2) π(Ru2) → π*(N)/δ*(Ru2) π(N)/δ*(Ru2) → π*(aryl) δ(Ru2) → δ*(Ru2) π(N/aryl) → π*(Ru2) π(aryl) → δ*(Ru2) π(aryl) → π*(Ru2)

II

13 014

III IV V

16 099 16 597 18 599

VI

21 700

VII

23 957

VIII

26 530

excited statea 4

E E 4 A1 4 E 4 B2 4 E 4 E 4 E 4 A1 4 E 4 E 4 E 4 E 4 B2 4 E 4 E 4 E 4

Excited state symmetries are given in the C4v point group. bε calculated using a fwhm line width of 900 cm−1.

transitions responsible for the single high-energy UV−vis band in the spectrum of 2 have mixed d → d [δ/σ/π(Ru2) → σ*/π*/δ*(Ru2)] and ligand-to-metal [π(N/aryl) → σ*/ π*(Ru2)] charge-transfer character. Thus, our assignments of the low-energy UV−vis bands displayed by 1 and 2 are in agreement with those proposed by Chakravarty and Cotton40 and Miskowski et al.33 The high-energy transitions of 1 and 2 have a significant amount of ligand-to-metal charge-transfer character, but the axial ligand only contributes to those for 1, in particular to the transition responsible for the shoulder on the high-energy UV−vis band. The most informative ligand-field transitions for these compounds, namely, the δ → δ* and δ → π* transitions, are too weak to be observable in the UV−vis spectra but can be identified in the corresponding MCD spectra. While the δ → δ* transitions for 1 and 2 appear at similar energies, the δ → π* transition is higher in energy for 1 than for 2, in agreement with the greater π-donor strength of the axial chloride ligand compared to that of the triflate ligand.

Figure 8. Resonance Raman data for 1 and 2 collected at 77 K in CH2Cl2. Peaks of interest are highlighted by the dashed box.

Scheme 2. Coupled Ru−Ru Stretching and Pyridine Rocking Vibrations Observed for 1 (X = Cl) and 2 (X = OTf)

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EXPERIMENTAL DETAILS

General Procedures and Physical Measurements. Compounds 1 and 2 were prepared according to a literature procedure.35,39 UV−vis data were recorded at room temperature in CH2Cl2 and obtained using a StellarNet Miniature BLUE-wave UV−vis dip probe with a tungsten−krypton light source and a 10 mm path length. Magnetometry. SQUID data were collected on powder samples of 1 and 2 contained in gel capsules using a Quantum Design MPMS 3 SQUID magnetometer. Magnetic susceptibility data were collected in an externally applied magnetic field of 0.1 T (1000 G) from 2 to 300 K, and magnetization data were measured as a function of the applied field (1−7 T) at three different temperatures: 2, 5, and 10 K for 1 and 2, 4, and 8 K for 2. Magnetic susceptibility and magnetization were fit simultaneously using the software program PHI.70 A range of different fitting models were examined, including ones in which the g tensors were refined isotropically or anisotropically, and either axial and rhombic ZFS tensor components or just the axial component were included in the model. Ultimately, we chose a model for each compound that provided the best fit to the data with the smallest number of parameters that yielded well-defined and physically reasonable results. In our final model, the g value was fixed at 2.00 for 1 and was allowed to refine isotropically for 2. The rhombic ZFS component (E) was found to significantly improve the fit of 1, but not

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CONCLUSIONS We present here quantitative electronic structure descriptions for 1 and 2 and an unambiguous identification of the transitions responsible for their dominant visible absorption bands. The analysis of the electronic and vibrational features of 1 and 2 was greatly facilitated by the remarkable agreement between the DFT and TD-DFT computational results and our experimental data. The low-energy band in the UV−vis spectra for both species is primarily due to a ligand-to-metal [π(Ru−N) → δ*(Ru2)] transition, while the high-energy band stems from transitions with contributions from several one-electron excitations. The low-energy shoulder on the high-energy UV−vis band in 1 is due to a d → d [π/σ/δ(Ru2) → δ*(Ru2)] transition, while the main band is due to ligand-tometal [π(N/aryl) → σ*/π*/δ*(Ru 2)] transitions. The 14667


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of 2, so it was left out of the final model for 2. The intermolecular interaction term (zJ) for 1 is an order of magnitude larger than for 2, which is reflected in the increased downward slope for 1 below 10 K in the χT plot. Because of this interaction, the reduced magnetization data for 1 do not overlay for low H/T values. This causes a discrepancy between the data and the fit, as zJ is not incorporated into the fitting model for the reduced magnetization data. Fits that only use the higher-temperature data sets give comparable fitting parameters. Magnetic Circular Dichroism (MCD). Mulls of compounds 1 and 2 for MCD were prepared by grinding solid-state samples with poly(dimethylsiloxane). Mulls were sandwiched between quartz windows and mounted in MCD cells. MCD data were collected at 4.5 K and +7 T with a Jasco J-715 spectropolarimeter in conjunction with an Oxford Instruments SM4000-8T superconducting magnetocryostat. Resonance Raman Spectroscopy. Resonance Raman data were collected on frozen solution samples in a finger Dewar filled with liquid nitrogen. A Coherent I-305 Ar+ ion laser (514.5 nm) was used as the excitation source, and ∼135° backscattered light was dispersed by an Acton Research triple monochromator equipped with 1200 and 2400 grooves/mm gratings. Dispersed light was analyzed by a Princeton Instruments Spec X:100BR deep depletion, back-thinned CCD camera. Data were collected with a laser power of 40 mW at the sample, an integration time of 60 s, and averaged over 3 scans. Computational Methods. The initial coordinates for 1 and 2 were obtained from the corresponding crystallographic data.35,39 Density functional theory (DFT) geometry optimizations and energy calculations were performed on unconstrained structures using the ORCA 3.0.3 software package.71 Geometry optimizations and subsequent time-dependent DFT calculations were performed using Becke’s three-parameter hybrid functional for exchange along with the Lee−Yang−Parr functional for correlation (B3LYP). 72−75 The polarized split-valence def2-SV(P) basis set76 and the def2-SVP/J auxiliary basis set77,78 were used to model C and H atoms. Ahlrich’s polarized triple-ζ-valence def2-TZVP(-f) basis set79 with the def2TZVP/J auxiliary basis set77,78 was used for N, O, Cl, S, and F atoms, and the def2-TZVPP basis set with the def2-TZVPP/J auxiliary basis set77,78 was used for Ru atoms along with the ZORA approximation.80 The RIJCOSX approximation, tight optimization, and tight selfconsistent field convergence were employed along with an integration grid of 302 Lebedev points (Grid 4) for all calculations. Orbital and spectral predictions included the conductor-like screening model (COSMO) for CH2Cl2 solvation.81,82 Löwdin population analysis was used to determine orbital populations.83−85 Frequency calculations were performed following geometry optimizations to ensure minimum-energy structures. The Avogadro program86,87 was used to edit .xyz files. The Jmol program88 was used to view/animate vibrational stretches, and the UCSF Chimera package89 was used for molecular graphics and analyses. Geometry optimizations were also attempted using Becke’s functional for exchange along with Perdew’s functional for correlation (BP86);90,91 however, these models displayed larger deviations from the experimental structures (see Supporting Information for comparison of selected bond distances and angles). Iterative Gaussian deconvolutions of the Abs and MCD spectra were conducted using IGOR Pro. Each transition was modeled as a Gaussian band with a full-width half-maximum of 1500 cm−1.92

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Thomas C. Brunold: 0000-0001-6516-598X John F. Berry: 0000-0002-6805-0640 Present Address †

Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the U.S. Department of Energy, Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, Office of Science (DE-SC0016442). Computational facilities at UWMadison are supported by the NSF (CHE0840494). A.R.C. thanks the National Science Foundation for a Graduate Research Fellowship (DGE-0718123) and Kristine Cheloha and Tzuhsiung Yang for insightful discussions.

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REFERENCES

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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.7b02557. Reduced magnetization data for 1, magnetic measurement expressions, selected calculated bond distances and angles for 1 and 2 using BP86, and geometry-optimized coordinates for 1 and 2 (PDF) 14668


Article

Inorganic Chemistry

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